PEROXO COMPLEXES OF THE EARLY TRANSITION METALS
by
Andrew Charles Dengel B.Sc. A.R.C.S
A Thesis presented
in partial fulfilment of the requirements
for the Degree of Doctor of Philosophy
of the University of London
Inorganic Chemistry Laboratories Department of Chemistry Imperial College London
December 1987 abstract
The work described in this thesis concerns the peroxo chemistry of the transition metals in Groups IVa-VIa. As well as the preparation, characterisation and study of the chemistry of new peroxo complexes of the second and third row members of these groups, the work includes study of the aqueous peroxo chemistry and some known peroxo complexes of the metals. The work has been undertaken both from an academic viewpoint and to investigate existing and new methods for the recovery, extraction and separation of the early second and third row metals; the latter process is notoriously difficult due to the effects of the lanthanide contraction.
2 New carboxylato n -peroxo complexes K2[M0(02)2(L)J .2H20 (M = Mo, W;
L = glycollate, tartrate, malate, gluconate), K2[Mo0(02)2(L)].2H20 (L = tartronate, quinate), and A[M0(0)(CH0)).4H04 22 24 426 2 (A = K, NH. 4 ; M = Mo, W; C 4 H 2 0 o = tetraionised tartrate) have been prepared, and char- acterised using infrared, Raman, and 13C and 95Mo n.m.r. spectroscopy.
X-Ray crystal structures are presented for K2CMo0(02)2(glyc)].2H20 and K [Mo 0 (0o),(C H 0 ) ].4H_0. The new organic-soluble peroxo complex 4 i c 2 4 4 2 b 2 (Ph,P)„CMo(MO ) (ox)] is shown to oxidise activated alcohols to carbonyl compounds. Studies on the aqueous peroxo chemistry of molybdenum and tungsten include investigation of the action of hydrogen peroxide on tungsten carbide and molybdenum carbide, and study of the known ^M203*02*4^2~ = Mo< w * sPecies. New organic-soluble complexes *Ph4P*2^M2°3*°2*4^ = M°* an
1 2 New Group Va carboxylato n -peroxo complexes )3(L )] (H = Nb,
Ta ; L = tartrate, glycollate, citrate), l<3[Nb(02)3(malate)],
KtM(0 ) (dipicolinate)(H 0)3.HO (M = Nb. Ta), and Ke[Mo(0o)e(CfHo0e)] c c c Z o Z 2 b 4 Z b (M = Nb, Ta) have been prepared and characterised by infrared, Raman and 13 C n.m.r. spectroscopy. The aqueous peroxo chemistry of niobium and tantalum has been studied using Raman spectroscopy, and the
2 - vibrational spectra of the [Nb(0_)F_32 5 anion re-investigated in detail. 2 New Group IVa n -peroxo complexes K [M( M ox) 3.2H 0 and 2 2 2 2 K2 ^ ° 2 * 2 * c*trate^ *H2° = Zr’ have also been prepared, and characterised by infrared and Raman spectroscopy.
2 U SLJtf-CQNTENTS
Page
ABSTRACT i
LIST OF CONTENTS 3
LIST OF TABLES 11
LIST OF FIGURES 13
ABBREVIATIONS AND SYMBOLS 16
ACKNOWLEDGEMENTS 18
DEDICATION 19
CHAPTER ONE : THE R2 -PER0X0 LIGAND AND ITS COMPLEXATION WITH 20 TRANSITION METALS
Section 1.1 HISTORICAL AND GENERAL BACKGROUND 20
Section 1.2 CLASSIFICATION OF DIOXYGEN COMPLEXES 24
Section 1.3 THE q2-PEROXO LIGAND 26
1.3.1 Electronic Structure and Bonding Theory 26
1.3.2 Spectroscopic Properties 28
Section 1.4 FORMATION OF n2“PEROXO COMPLEXES OF TRANSITION 29 METALS
Section 1.5 REACTIVITY OF n2-PEROXO COMPLEXES OF TRANSITION 31 METALS
Section 1.6 APPLICATION OF PEROXO CHEMISTRY TO THE SEPARATION 33 AND EXTRACTION OF EARLY TRANSITION METALS
3 Ptflt
CHAPTER TWO : PEROXO COMPLEXES OF MOLYBDENUM(VI) AND TUNGSTEN(VI) 35
Section 2.1 INTRODUCTION 35
2.1.1 General Background 35
2.1.2 Simple Peroxo and Oxoperoxo Complexes of 37
Molybdenum(VI) and Tungsten(VI)
2.1.3 Heteroligand Peroxo Complexes of Molybdenum(VI) 44
and Tungsten(VI)
(a) Haloperoxo Complexes 44
(b) Peroxo Complexes Containing Ligands Derived 47
from Carboxylic Acids and Nitrogen Bases
(c) Miscellaneous Heteroligand Peroxo Complexes 54
2.1.4 Reactivity and Applications of Peroxo Complexes 56
of Molybdenum(VI) and Tungsten(VI)
2.1.4.1 Reactivity 56
(a) General Reactivity 56
(b) Stoichiometric Oxidation 57
(c) Catalytic Oxidation 65
2.1.4.2 Applications of PeroxoChemistry to the 70
Extraction and Separation of Molybdenum and Tungsten
4 Section 2.2 CARBOXYLATO PEROXO COMPLEXES OF MOLYBDENUM(VI) 73 AND TUNGSTEN(VI)
2.2.1 Preparation of 1:1 Metal:Carboxylate Peroxo 73
Complexes
2.2.1.1 Introduction 73
2.2.1.2 Formation of Complexes 74
(a) Oxalato Peroxo Complexes 74
(b) Citrato Peroxo Complexes 78
(c) New Carboxylato Peroxo Complexes 78
2.2.1.3 Structure and Vibrational Spectra of 85
CM0(02)2(L)]2" (M = Mo, W); The X-Ray Crystal
Structure of K2[Mo0(02)2(glyc)].2H20
2.2.1.4 Use of 95Mo and 13C N.M.R. Spectroscopy to 97
2- Investigate the Structure of [MO(02>2(L)3
Species in Solution
2.2.2 Preparation of 2:1 Metal:Tartrate Peroxo 106
Complexes; The X-Ray Crystal Structure of
K4CMo202(02)^(C^H206)].4H20
2.2.3 Reactivity of Carboxylato Peroxo Complexes 114
of MolybdenumtVI)
(a) Preparation of Organic-Soluble Carboxylato 114 Peroxo Complexes
(b) Attempted Reactions with SO,,2 and C0„2 115 (c) Oxidative Reactivity Towards Alkenes and 117
Alcohols
5 JEgflg
Section 2.2.4 Differences Between the Carboxylato Peroxo 122
Chemistry of MolybdenumtVI) and TungstentVI):
Separational Possibilities
(a) The 1:1 M:Glycollate System (M = Mo, W) 124
(b) The 2:1 M:Tartrate System (M = Mo, W) 125
Section 2.3 0X0PER0X0 AND PEROXO MOLYBDATES(VI) AND 128 TUNGSTATES(VI) IN THE SOLID STATE AND AQUEOUS SOLUTION
2.3.1 Vibrational Spectra of .2H20 128
(M = Mo, W) : 2H- and 180- Substitution Studies
(a) Comparison of Calculated and Experimental 129 18 Shifts in M^O Frequencies upon 0-Substitution 2 (b) Effect of H-Substitution upon the Vibrational 132
Spectra
2.3.2 The Caesium Molybdate - Hydrogen Peroxide 132
System
2.3.3 Vibrational Spectra of Known Oxoperoxo 136
Molybdates(VI)
(a)
2.3.4 Oxidative Reactivity of 139
Species (M = Mo, W)
6 P»g«
Section 2.3.4 (a) Nature of the species Responsible for Molyb- 139
date- and Tungstate-Catalysed Oxidation of
Alkenes and Alcohols by H„0„,2 2 (b) Preparation of Organic-Soluble Salts of 142
tM2°3(02,*IH20,2l2' (cl Use of (Ph4P)2tM203 C02> + 3 (H = Ho, H) and H 3 (Bu ^N)2[Mo 203 (02)^] as Stoichiometric Oxidants Section 2.4 THE ACTION OF HYDROGEN PEROXIDE ON TUNGSTEN 153 CARBIDE AND MOLYBDENUM CARBIDE 2.4.1 Introduction 153 2.4.2 The Action of Hydrogen Peroxide on Tungsten 156 Carbide (a) Recovery of Tungsten as CaWO^ 157 (b) Raman Spectrum of the WC/H202/0xalic Acid 158 Solution (c) Attempted Isolation of PeroxotungstatetVI) 165 Species Present in the WC/H„0„/0xalic2 2 Acid Solution 2.4.3 The Action of Hydrogen Peroxide on Molybdenum 167 Carbide 2.4.4 Conclusions 170 7 flfll Section 2.5 FLUORO CARBOXYLATO PEROXO COMPLEXES OF 171 MOLYBDENUMSVI) AND URANIUM(VI) 2.5.1 Introduction 171 2.5.2 Attempted Preparation of Fluoro Carboxylato 172 Peroxo Complexes of Molybdenum (VI) and Uranium(VI) Section 2.6 EXPERIMENTAL 174 CHAPTER THREE : PEROXO COMPLEXES OF THE GROUP Va AND IVa 210 TRANSITION METALS Section 3.1 INTRODUCTION 210 3.1.1 General Background 210 3.1.2 Peroxo Complexes of Group Va 211 3.1.2.1 Vanadium 211 3.1.2.2 Niobium and Tantalum 215 (a) Aqueous Peroxo Chemistry 216 (b) Heteroligand Peroxo Complexes 219 (c) Reactivity and Applications 227 3.1.3 Peroxo Complexes of Group IVa 229 3.1.3.1 Titanium 229 3.1.3.2 Zirconiumand Hafnium 234 8 Plfll Section 3.2 CARBOXYLATO PEROXO COMPLEXES OF NIOBIUM(V) 240 AND TANTALUM(V) 3.2.1 Introduction 240 3.2.2 Preparation of Starting Materials 241 3.2.3 Preparation of New 1:1 Metal:Carboxylate 242 Peroxo Complexes 3.2.3.1 Formation of Complexes 242 3.2.3.2 Vibrational and 13C N.M.R. Spectra of the 247 Complexes 3.2.4 Preparation of New 2:1 Metal:Tartrate 251 Peroxo Complexes Section 3.3 AQUEOUS PEROXO CHEMISTRY OF NIOBIUM AND TANTALUM 253 3.3.1 Raman Spectra of Peroxidic Solutions of 253 Niobium(V) and Tantalum(V) 3.3.2 The Action of Hydrogen Peroxide on Niobium 256 Carbide and Tantalum Carbide Section 3.4 VIBRATIONAL SPECTRA OF THE CNb(0o)Fe]2"2 5 ANION 258 3.4.1 Introduction 256 3.4.2 The Vibrational Spectra of [Nb(0n)F_]2”Z 5 259 (a) K2[Nb(02)F5].H20 259 (b) "Na2tNb(02)F5r 267 (C) 9 Section 3.5 PREPARATION OF NEW CARBOXYLATO PEROXO COMPLEXES 270 OF ZIRCONIUM(IV) AND HAFNIUM(IV) 3.5.1 Introduction 270 3.5.2 Formation of Complexes 271 3.5.3 Vibrational Spectra of the Complexes 272 Section 3.6 MISCELLANEOUS TITANIUM(IV) AND VANADIUM(V) PEROXO 275 CHEMISTRY 3.6.1 AColorimetric Method for the Distinction 275 between Coordinated and Perhydrate Peroxide using Titanium(IV) 2 3.6.2 The Effect of H-Substitution on the 277 Vibrational Spectra of (NH ) HCV 0 (0 ) ].H 0 Section 3.7 EXPERIMENTAL 279 APPENDIX : GENERAL EXPERIMENTAL 291 Physical Measurements Reagents Analyses REFERENCES 293 PUBLICATIONS 317 10 L IS T OF TABLES I»bl« 1.1 Structural Classification of Dioxygen Complexes 25 2.1 Vibrational Data for 1:1 Metal:Carboxylate Molybdenum(VI) 89 and Tungsten(VI) Carboxylato Peroxo Complexes 2.2 Bond Lengths (A) and Angles (°) in K2CMo0(02)2(glyc) ].2^0 96 95 2.3 Mo N.M.R. Data for Molybdenum!VI) Carboxylato Peroxo 98 Complexes 2.4 13C N.M.R. Data for Molybdenum(VI) and Tungsten(VI) 101 Carboxylato Peroxo Complexes 13 2.5 C N.M.R. Data for Free Carboxylic Acids 103 2.6 Bond Lengths (A) and Angles ( ) in 110 K*4 tMoo°o<2224426 ) J c,«o0e)].4H 20 2.7 Vibrational Data for 2:1 Metal:Carboxylate Molybdenum!VI) 113 and Tungsten!VI) Tartrato Peroxo Complexes 2.8 Stoichiometric Oxidation of Alcohols with 121 2- 2.9 Vibrational Data for Dimeric [M203 !02 )4 2.10 Vibrational Data for Organic-Soluble IH 0 (0 ) ]2‘ 144 Species of Molybdenum and Tungsten 1 1 Page 2.11 Stoichiometric Oxidation of Alcohols with 147 (Ph ,P)JM 0 (0 ),] (M = Mo. W) 4 2 2 3 2 4 2.12 Stoichiometric Oxidation of Alcohols with 149 (Bu N) [Ho 0 (0 ) 3 4 2 2 3 2 4 2.13 Atomic Coordinates of the Non-Hydrogen Atoms (x1oS 181 for K2CMo0(02)2(glyc)].2H20 2.14 Atomic Coordinates of the Non-Hydrogen Atoms (x10*) 184 for K4EMo 202(02 )4(C4H206 )3.4H20 3.1 Vibrational Data for Niobium(V) and Tantalum(V) 249 Carboxylato Peroxo Complexes 2- 3.2 Vibrational Data Previously Reported for [Nb(0.)F_]2 5 261 3.3 Vibrational Data for Ko[Nb(0JFc]2 2 5 .Ho0 2 262 3.4 Vibrational Data for "Nao[Nb(0o)Fc]“2 2 5 268 3.5 Vibrational Data for Zirconium(IV) andHafnium(IV) 274 Carboxylato Peroxo Complexes 12 Ull_fi£_£l£Ji££& figure Page 1.1 The Molecular Orbitals of Molecular Oxygen 26 2 1.2 Molecular Orbital Diagram Diagram for a n -Peroxo 28 Complex 2 1.3 Vibrational Bands Arising from a n “Peroxo Group 29 2.1 Equilibria in the [MoO^]2 — H2°2— HC1 System *1 2.2 Equilibria in the [WO ]2” — H () — HC1 System 43 4 2 2 2.3 Structure of the [Mo0(02)2(cit)]2 Anion 49 2.4 Proposed Mechanism for the Epoxidation of Alkenes by 59 Covalent Molybdenum(VI) Peroxo Complexes [Mo0(0 ) (L) (L') ] 2.5 Possible Cyclic Transition States in Epoxidation of 61 Alkenes by a Molybdenum(VI) Oxo Peroxo Complex as 2.6 Mo N.M.R. of Oxalato Peroxo Molybdate(VI) Species 77 in DgO Solution 2.7 Carboxylate Ligands Capable of 5-Membered Ring Formation 80 in Peroxo Complexes of MolybdenumtVI) and Tungsten(VI) 2.8 Carboxylate Ligands Incapable of Forming Peroxo Complexes 82 with MolybdenumtVI) and Tungsten(VI) 2.9 Vibrational Bands Expected for an Oxoperoxo Metal Complex 87 13 -Figure EA21 2.10 Structure of the [MoO(OjJglyc)]2“ Anion 94 2 2 2.11 The Unit Cell of K2[Mo0(02)2(glyc)].2H20 95 2.12 Quinic Acid 100 . 4- 2.13 Structure of the [Mo.0_(0.).(C.H_Oe )] Anion 109 Z Z Z 4 4 Z o 2. H pH - Time Plots for the Formation of 127 AtCM2°Z(02 ,4(C4H206 ,1 ^ H20 (A * Na, K i M = Mo, W) 2.15 Raman Spectra of Solids Isolated from Cs2CMo0^3 — H202 135 Solution at pH 9.4 and at pH 10.7 2- 2.16 Raman Spectra of CMoO^J / H2(>2 Solution (pH 3.0) 141 at 25°C and 75°C 2.17 Key to Structures of Alcohols 148 2.18 Structure of the tW 0.(0o)_(C0_)]6” Anion 156 4 o Z o 3 2.19 Raman Spectrum of the WC — H202 -Oxalic Acid Solution 159 2.20 Raman Spectra of Tungsten Solution Species 160 2.21 Raman Spectrum of the W — H„0„ — Oxalic Acid Solution 164 2 2 2.22 Raman Spectrum of the Mo2C — H202 Solution 168 . . . . 3- ♦ 3.1 Equilibria in the [VO^] — Excess ^ O g — H System 213 3.2 Coordination of Dipicolinic Acid 244 14 f lf lMr.t Pane 3.3 Raman Spectra of Solid and Aqueous K^CNbCO^)^].O.SH^O 254 3.4 The Infrared Spectrum of Solid K^NbCO^Fg] .H20 263 3.5 The Raman Spectrum of Solid Ko[Nb(0_)F_].H_0 264 Z Z 5 Z 3.6 The Raman Spectrum of Aqueous Ko[Nb(0.)F_].H_0 265 c Z 5 c 3.7 Typical Calibration Graph for Colorimetric 276 Determination of Free H„0„ 2 2 15 ABBREVIATIONS.AND SYHBOLS Bidentate Carboxvlate Uganda ox oxalate n c o j J 2" 2 2 cit citrate [(c h 2c o 2h )2c (o )c o 2]2“ glyc glycollate c h 2c (o )c o 2i2' mal malate [H02CCH2.Cfl(0)C02]2" glucon gluconate [H0CHo{CH(0H)KCH(0)C0o]2" tartron tartronate [H02C.Crt(0)C02]2" tart tartrate [HO CCH(OH) .CH{0)C0_]2- 2 2 quin quinate 1-hydroxo-3,4,5-tetrahydroxy- cyclohexane carboxylate pic picolinate [CgH4N-2-(C02 )]’ (pyridine-2-carboxylate) Other Ligands dipic dipicolinate [C5H3N-2,6-(C02)2]2" (pyridine-2,6-dicarboxylate) bipy 2,2'-dipyridyl phen 1,1O-phenanthroline py pyridine acac acetylacetone (2,4-pentanedione) Miscellaneous CM u X Q o X o 0> CM Me methyl, CH3 Bu butyl, CM (-) X n i Et ethyl, CM Ph phenyl, C6Hg TMS SiMe4 16 Infrared(IR)and^Raman(R)Spectra V stretching mode s strong band 6 bending mode m medium band s symmetrical w weak band as asymmetrical br broad band P polarised sh shoulder band dp depolarised V very Raman intensities are given in parentheses : strongest band "10", and others quoted as a ratio of this band. N.H.R. Spectra 6 chemical shift (p.p.m.) t triplet br broad m multiplet 17 ACKNOWLEDGEMENTS I would like to thank my supervisor Dr. Bill Griffith for his encouragement, advice and patience over the last three (and a bit) years, and for his many replenishments of my beer glass in that time. Grateful thanks must also go to Drs. David Mobbs and Diana Anderson, my industrial supervisors at Interox Chemicals Ltd., and Prof. Dennis Evans and Dr. Les Pratt of this department, for helpful technical discussions. I would also like to thank Dr. Andrzej Skapski and Richard Powell for production of the X-ray crystal structures, Sue Johnson and Dick Sheppard for running the 9 5 Mo and some of the 19 C n.m.r. spectra, and Bill Ali, Colin Robinson and Roger Lincoln for technical assistance. I am grateful to Debbie Dobson for her help in printing out this thesis. The members of the WPG group have made lab. 539 a live ly place to work - thanks to John Flanagan, Allen Fyfe, Sarah Greaves, Chris Leahy, Richard Powell and Ahmed El-Hendawy. The last three years would not have been as enjoyable and rewarding without my friends in the chemistry department, rugby, football and cricket teams, and college as a whole. I would like to mention Simon Anderson, Jim Brannigan, Katy Brown, Kev Buckley, Phil Clapp, Bryan Driscoll, Mick Gaffney, Rob Kelly, Steve Ley, Jim Mulligan, Vince Mole, Ged O'Shea, Erica Parkes, Prem Sagoo, Judith Taylor, Tom Wright, Brent Young, and Mick and Phil at Harlington. Special thanks must go to Luiza Antonioni, Sandy Monaghan, Paul Savage, Andy White and especially Debbie Dobson and Angela Morris for their friendship and humour, and my parents for their love and support. I gratefully acknowledge Interox Chemicals Ltd. and the S.E.R.C. for the provision of a grant. 18 To Mum, Dad, Simon, Nan, and the rest of my family. 19 Time stand Still- Summer's going fast Nights growing colder Children growing up Old friends growing older Freeze this moment a little bit longer Hake this sensation a little bit stronger Experience slips away... (Neil Peart) CHAPTER ONE THE n2"PEROXO LIGAND AND ITS COMPLEXATION WITH TRANSITION METALS CHAPTER ONE THE n2-PEROXO LIGAND AND ITS.COHPLEXATION WITH JPANSlIl.QH.ilEIALS 1.1 HISTORICAL AND GENERAL BACKGROUND The study of dioxygen complexes of transition metals is widely accepted to have started with the work of Fremy, which concerned the oxygenated ammoniacal salts of cobalt, some one hundred and thirty-five years ago.^ It is fair to say that the characteristic colour reactions occurring when hydrogen peroxide is added to transition metal compounds will have been noted some time before that. The intrinsic interest associated with transition metal dioxygen derivatives prompted much work on the isolation of such species in solid form in the latter years of the nineteenth century. As the present century progressed, it became clear to the chemical fraternity that the dioxygen complexes of the transition metals were also of great interest with respect to the catalysis of oxidations involving hydrogen peroxide or molecular oxygen, the storage and use of oxygen in biological systems, and the catalytic decomposition of hydrogen peroxide itself. The auto-oxidation of metal ions and study of synthetic oxygen carriers received a lot of attention in the years leading up to the Second World War, and the first model to explain the interaction of dioxygen with 2 iron in haemoglobin was presented by Pauling and Coryell in 1936. More recent work on the dioxygen derivatives of transition metals has been assisted by the increasing availability of physical methods for the characterisation of the species. In particular, the advent of X-ray 20 crystallography has done much to clarify the nature of the species formed by the complexation of dioxygen with transition metals. More than one mode of coordination exists, and the four main ones were conveniently classified in terms of molecular geometry by Vaska in 3 1976. (see Section 1.2 ). The study of dioxygen transition metal complexes has proven to be an ever-expanding area of chemistry, with the main areas of interest being those concerning biological systems and the catalysis of auto-oxidation reactions. The bonding of a dioxygen ligand to a transition metal involves transfer of charge density to the ligand, and this serves to reduce the kinetic barrier to changes of spin that is inherent in the reactions of molecular oxygen. The dioxygen ligand becomes basic, or nucleophilic, in character, and its reactivity is strictly governed by the metal to which it is coordinated. Other areas of current interest concerning metal-dioxygen complexes include their possible use as catalysts for the reduction of 0^ in fuel cells*, and their use in the extraction and separation of early transition metals; the latter area of dioxygen chemistry has prompted much of the work presented in this thesis. In the course of the work described here, one particular class of 2 metal-dioxygen complex is encountered - namely the n -peroxo species. The rest of this introductory chapter comprises an explanation of the classification of dioxygen complexes, followed by a brief introduction to the structure, properties and reactivity of n2-peroxo complexes of transition metals. 21 Understandably, several reviews dealing with the chemistry of transition metal dioxygen complexes have appeared in the literature. Some of these merit special mention here. The work on dioxygen complexes of all transition metals known to form them (including several of the Actinides) up to 1964 was thoroughly reviewed by Connor and 5 3 Ebsworth. An important publication by Vaska in 1976 , in which all metal-dioxygen complexes known at the time were grouped into four categories according to molecular geometry, has already been mentioned; this did much to unify thinking on this branch of chemistry. A more 6 recent review by Gubelmann and Williams provides an excellent account of the molecular structure, electronic structure, and reactivity of dioxygen complexes of the transition metals. Also, very recently, Mimoun has presented a review of the role of peroxo complexes in organic oxidation7 , and has a more extensive account of the same in press7*11^ (part of a general review of metal complexes in oxidation), in which the subject matter includes superoxo complexes. Since early transition metal peroxo complexes are invariably prepared from the reaction of a high-valent metal oxo complex with hydrogen peroxide, it is fitting to mention the latter chemical at this stage, since the work presented in this thesis thus revolves around its use. Hydrogen peroxide was first synthesised and recognised as a chemical compound by Thenard in 18188. it being produced from the action of an acid upon an alkali peroxide or alkaline earth peroxide. Early indus trial manufacture of H2 <32 was by an electrolytic process, based on the conversion of sulphate to peroxodisulphate, followed by hydrolysis of 22 the product. Modern day industrial synthesis of is by the auto oxidation of an alkyl anthraquinol in the presence of 0^, which produces a 20 or 40Z solution of and the corresponding anthraquinone (the latter being purified and reduced back to the anthraquinol using over g a supported palladium catalyst) . The process, using ethyl anthraquinol as an example, is summarised in Scheme 1.1. The is purified by solvent extraction. + h *o 2 Pure H2°2 *s 3 colour*ess liquid (m.p. -0.43°C ; b.p. 150.2°C) of strong oxidising nature, which readily decomposes in the presence of traces of heavy metal ions by equation (1). This reaction, which may be 2H 0 ------» 2H 0 + 0„ AH -99 kJmol-1 (1) 2 2 2 2 considered as a self oxidation, occurs most rapidly in basic solution. The structure of liquid HgOg is a skew-chain one, in which the 0-0 bond o (1.49 A) has a low barrier to internal rotation. The extensive hydrogen bonding in liquid H202 makes it more associated in this state than is water. 23 1.2 CLASSIFICATION OF DIOXV6EN COMPLEXES 3 The work of Vaska in 1976 involved classification of all the dioxygen complexes to have been structurally classified by that time, in terms of molecular geometry. The four categories are first divided into "superoxo" (Type I) complexes, in which the 0-0 bond distance is - o proximate to that of the superoxide ion, (0^) (ca. 1.3 A), and "peroxo” (Type II) complexes, in which the 0-0 distance is close to that reported for H 0 and the peroxide ion, (0 )2~ (ca. 1.48 A). Each of these two 2 2 2 types is subdivided further into Type a complexes, in which the dioxygen is bound to one metal atom, and Type b complexes, where it is bound to two metal atoms. In Table 1.1, whose first four entries depict the four Vaska categories, and the rest of this work, we have also used the "hapto" nomenclature employed by Gubelmann and Williams6, which signifies the number of oxygen atoms of the dioxygen ligand bound to the, or each, metal. Since 1976, two further modes of dioxygen coordination have been structurally characterised (see the last two entries in Table 1.1). 2 2 4-10 The n :H dioxygen complexes are exemplified by [(UO^Cl^J^O^I , in which a side-on dioxygen bridges two metal atoms. The only report of a 1 2 . 11 H :H dioxygen complex is that of [RhCl(02 )(PPh3)^ , in which each dioxygen ligand binds to one rhodium atom via both of its oxygen atoms, and to the second rhodium atom via just one of its oxygen atoms. In the course of this work, we are only concerned with the peroxo (Vaska Type II) complexes of transition metals, and in particular the 2 f) -peroxo (Vaska Type Ila) complexes. In these species the dioxygen 24. TABLE 1.1 Structural C la s sific a tio n o f Dioxvgen Complexes 3 Structure Type Structural Designation Vaska C la s sific a tio n Example o y H1 dioxygen Type la (superoxo) [Co(CN)c0 ]3 d c M H2 dioxygen Type Ila (peroxo) (Ph3P)2Pto2 toui M—0 \ 5 + 0 —M H1: H1 dioxygen Type lb (superoxo) C(H3N)gCo02Co(NH3)5] M—0 1 1 . 4 + q :H dioxygen Type lib (peroxo) C(H3N)5Co02Co(NH3)5] 2 2 4- H :H dioxygen C(U02C13 )202] / r) 1 :R 2 dioxygen [(Ph3P)2ClRh(02)]2 N j — M molecule is bound in side-on triangular fashion to one metal atom through both of its oxygen atoms. The rest of the discussion in this introductory chapter will therefore involve only the properties and reactivity. . of n 2 -peroxo complexes. 1.3 t h e V- p e r o x o l i g a n d 1.3.1 Electronic Structure_and_ Bonding Theory The triplet ground state of oxygen ( Eg ) has the electronic 1 structure shown in Fig.1.1. The lowest excited singlet states, and , are reached by pairing the spins of the two electrons in the 9 * ¥ orbital. 9 T' 3ou - h - f - ¥ Fig.1.1 The Molecular Orbitals 9 of Molecular Oxygen ¥ - f t - - H - U 3o - t t - 9 The triplet ground state provides a sizeable kinetic barrier to auto-oxidation of normally diamagnetic organic molecules, where reactions involving change of spin are generally slow. Apart from photochemical activation to the lowest excited state and reaction with radical species, this kinetic barrier can be surmounted by reactions with a transition metal; the greater spin-orbit coupling reduces the barrier to change of spin, and the energy of formation of 26 the metal-dioxygen complex may be sufficient itself to pair the spins. It is now widely accepted that the metal-Og bond, though polarised with the dioxygen ligand carrying at least partial negative charge, has appreciable covalent character and is best discussed in terms of 6 molecular orbital theory. The bonding of HL (where L are other n n ligands coordinated to the metal) with 0^ involves the interaction of * the v molecular orbitals of 0 with the frontier metal d-orbitals of 9 2 2 ML . In practice, all known n -peroxo complexes show effective donation n g of two electrons from metal d -orbitals to the M-0„ bond. The early o 2 transition. . metals form o 2 -peroxo complexes m high oxidation states 0 1 (d or d ), whereas in low oxidation states such complexes are rendered unstable by the breakage of the 0-0 bond due to electron transfer from * metal d-orbitals to the 3oy orbital of dioxygen. In the case of the later transition metals, formation of a peroxo complex only occurs when the metal d-orbital interacting with dioxygen is appreciably destabilised by other ligands bound to the metal. The fact that stabilisation of a particular peroxo complex relies on the donor powers of the other ligands coordinated to the metal is explained by the role of the ligands in the destabilisation of the d-orbitals of the metal. The other ligands may also act as electron- donors to the peroxo ligand themselves, with the electronic charge being delocalised on to the ligand via the metal. It is interesting that the high electronegativity of the dioxygen entity makes it act more as an electron acceptor than as an electron 27 donor. The reactivity of the bound peroxo ligand relies heavily on the metal to which it is coordinated (primarily relying on the energy of the metal d-orbitals), as well as upon the influence of other ligands co ordinated to the metal. 1.3.2 Spectroscopic Properties 2 If we consider the molecular orbital diagram for a n -peroxo complex in which the metal donates two electrons (Fig. 1.2), we can see that two ligand to metal charge transfer (LMCT) bands are possible. The transition from the i * orbital of dioxygen, which interacts with the H 0 2 Fig.1.2 Molecular,Orbital Diagram for a 2 n -Peroxo Complex 12 metal d-orbital to form a bond, is found to be of high energy. The * * tt (X) orbital, perpendicular to tt , has 6-symmetry with respect to the 9 y M-Og bond, and interacts very little with the metal, so that the LMCT band due to the transition from this dioxygen orbital is very weak. 2 The triangularly-bound n -peroxo ligand has C2v symmetry (assuming both M-0 bonds to be of equal length, which is generally the case), and shows three distinct vibrational bands due to the peroxo stretch 28 [v(O-O)] and symmetric and metal-peroxo stretches CvSiH(0^)> and vas{M(0^))] (see Fig. 1.3). All three bands are infrared and Raman £ active, with the v(0-0) and v (MIO^)) bands being polarised in the v (0-0) v S{M(02)} vaS{M(02)} Fig.1.3 Vibrational Bands Arising from a n 2-Peroxo Group Raman spectra of the solutions, and the v 3 S {MC02)} depolarised. The 0-0 stretch is strong in both the infrared and Raman, and lies in the region 950-800cm - 1 for 2 n -peroxo complexes, while the metal-peroxo stretches lie in the region 650-500cm \ with the symmetrical stretch generally appearing at somewhat higher frequency than the asymmetric 6 7 13 14 one. ' ' As well as yielding information on the strength of the 0- 0 bond itself, the v(0-0) frequency correlates with the electronic 15 LMCT bands. Lever and co-workers have recently found that v(0-0) falls as the energy of the charge transfer band increases, indicating greater charge transfer to dioxygen in the ground state. The advent of resonance Raman spectroscopy has provided a means of helping to identify LMCT bands coupled with the 0-0 stretching frequencies. 1 6 1- * FORMATION OF n2-PER0X0 COMPLEXES OF TRANSITION METALS 2 q -Peroxo complexes are the most widely known dioxygen complexes and have been reported for several transition metals. As well as the early transition metals in Groups IVa, Va and Via, all of the Group VIII 29 metals form a wide range of n -peroxo complexes, as do copper, manganese, and certain Actinides, notably uranium and thorium. 2 In the case of Group VIII metals, the n -peroxo complexes are formed as the result of direct interaction of dioxygen with the reduced two-electron donor metal complex Mn+, to give M*n+2*+(02), in which * i-bonding from the filled metal d-orbitals to the orbital of dioxygen predominates.7**1 In the case of the early transition metals, with which we are 2 concerned in this work, the n -peroxo complexes are synthesised by the reaction of hydrogen peroxide with high-valent metal oxo complxes, via an oxo-hydroperoxo intermediate [equation (2)]. Notionally, the 2- reaction takes place between dioxygen in its reduced [peroxide, (02 ) ] form and a d° metal in high oxidation state. It is interesting to note that despite differing modes of formation, 2 the early and Group VIII transition metal n -peroxo complexes share many similar properties. Indeed, some complexes may be prepared by either IV route. This is illustrated by the complex [(TPP)Ti (0^)3 (TPP = tetra- phenylporphyrin), which can be prepared either from the reaction of [ (TPP)TiIV0] with H202< or 2[ (TPP)TiH I F] with (>2.17,18 (see Section 3.1.3.1). 30 17 18 Various 0~ and 0-labelling experiments have shown that the peroxo group of H^0^ is substituted intact into the metal oxo complex upon peroxo complex formation, and that the oxo ligand is lost as water.19-21 1.5 REACTIVITY OF n2-PER0X0 COMPLEXES OF TRANSITION,HETAIS 2 H -Peroxo complexes are potential oxygen donors to organic substrates, but not all prove to be reactive. Amongst the many factors that govern their reactivity are the nature of the metal, the geometry of the complex, and the availability of a vacant or releasable coordination site on the metal, facilitating complexation of a 2 nucleophilic substrate. Generally, n -peroxo metal complexes act as 1.3- dipolar reagents M*-0-0 upon opening of the peroxo ring. They are thus able to react with both electrophilic and nucleophilic substrates. 22 The reaction with electrophiles such as cyano-alkenes proceeds by 1.3- dipolar cycloaddition, via the formation of five-membered cyclic, . 2 peroxidic adducts. A characteristic reaction of Group VIII n -peroxo 23 complexes is that with polar bonds within small molecules such as SO^ Zk and COg , yielding chelated sulphato and carbonato complexes respect- 2 ively. Early transition metal n -peroxo complexes show little reactivity in this respect. 6 2 H -Peroxo complexes are also able to oxidise a large variety of nucleophilic substrates such as alkenes, alcohols, enolates, phoshines, sulphides, amides, etc. The oxidation of phosphines to the corresponding phosphine oxides is a fairly general reaction for all 31 n -peroxo complexes. However, when it comes to the reaction with 2 alkenic double bonds, Group VIII n -peroxo complexes tend only to epoxidise the most electrophilic alkenes, whereas the early transition 2 metal n -peroxo complexes, particularly those of Group Via, are able to epoxidise a variety of nucleophilic alkenes as well.6,7*** The mechanism is thought to proceed via initial complexation of the alkene, followed by intramolecular 1,3-dipolar cycloaddition of the peroxidic oxygen, forming a 5-membered peroxo-metallocyclic intermediate which decomposes to an oxo complex and the epoxide (see Fig. 2.4). 2 The role of Group IVa-VIa n -peroxo complexes in stoichiometric oxidations is reviewed in more detail in Sections 2.1.4 (molybdenum and tungsten), 3.1.2.1 (vanadium), 3.1.2.2(c) (niobium and tantalum) and 3.1.3.1 (titanium), along with the study of their role in, and in many cases their direct use in, the catalysis of organic oxidations by H^O^ and 0^. The chemical industry produces millions of tons of oxygenated products every year, and many of the oxidation processes responsible involve the use of peroxo complexes as catalysts, or their generation in situ as catalysts, in both homolytic and heterolytic processes. Dioxygen also plays a crucial role in the biological systems of living cells, in which it is either carried by respiratory pigments and released at active sites, or activated by enzymes known as oxygenases. The selective oxidation brought about by these oxygenases invariably involves a transition metal at the active centre.7**1* The formation of metal-dioxygen complexes has the effect of polarising the 0-0 bond and facilitating its cleavage, and oxidation of various substrates, a 32 process which often involves complexation of dioxygen and the substrate to the metal at the same time. 1.6 APPLICATION OF, PEROXCLCHEMISTRV TO THE SEPARATION AND EXTRACTION OF EARLY TRANSITION METALS The importance of metal peroxo complexes in the oxidation of organic substrates, both in industrial and biological systems, has already been discussed. Another way in which the reactions of hydrogen peroxide with transition metals have been used is in the separation and extraction of the early transition metals from their ores and from base metals. The work in this thesis has been undertaken both from an academic viewpoint, and in order to try to discover new methods of effecting such separations and extractions using peroxo chemistry. In one case, an existing method of recovery of a metal using hydrogen peroxide has been studied. As far as separation of the metals is concerned, we are interested in the three pairs of second and third row transition metals in Groups IVa, Va and Via, namely zirconium/hafnium, niobium/tantalum and moly- bdenum/tungsten, each of which are notoriously difficult to separate due to the effects of the lanthanide contraction. As the 4f shell of the elements lanthanum to lutetium is filled, the effective nuclear charge experienced by each 4f electron increases, since due to the shape of the 4f atomic orbitals the shielding of one 4f electron by another is imperfect, and the result is contraction of the entire shell. As a result of the overall lanthanide contraction, the 33 second and third row elements of each group of the transition metal series have similar atomic and ionic radii, and as a consequence of this a marked similarity in chemistry. It is this similarity in chemistry that makes the pairs of elements hard to separate, and has prompted the work in this thesis. The fact that all six metals under consideration display extensive peroxo chemistries in solution offers possibilities for the separation, extraction and recovery of the metals using hydrogen peroxide. Some examples of existing methods for such processes are given in the appropriate review sections. 34 CHAPTER TWO PEROXO COMPLEXES OF MOLYBDENUM!VI) AND TUNGSTEN!VI) ■CHAEIEF. TWQ PEROXO COMPLEXES OF MOLYBDENUM(VI) AND TUNGSTEN(VI) 2.1 INTRODUCTION 2.1.1 General Background The peroxo chemistry of the Group Via transition metals follows the general trend of chemistry within the group; that is to say that first row chromium behaves differently to second row molybdenum and third row tungsten. Due to the effects of the lanthanide contraction (discussed in Chapter 1), the atomic and ionic sizes of molybdenum and tungsten are very similar, and so are their chemical properties. For instance, the higher oxidation states are generally much more stable for the two VI heavier elements in the group. This is borne out by the fact that Cr VI VI species are strong oxidising agents, while Mo and W species are much less oxidising, not easily reduced, and prone to form a plethora of polynuclear oxo anions on acidification. All three metals display an extensive peroxo chemistry. Chromium, in keeping with all first row members of the transition metal groups, shows a less complicated aqueous chemistry than the second and third row members of its group. Its aqueous peroxo chemistry proves to be no exception to this rule; indeed, the peroxides of chromium are probably better known and characterised as a whole than those of any other 5 25 IV V VI transition metal. ' Peroxo complexes of Cr , Cr and Cr are V known, and generally fall into four categories: A3[Cr C02 )^3 (A = alkali metal); AH[CrVI02(02)2] (A = alkali metal); [CrVI0t02)2B] (B = nitrogen base, e.g. pyridine); and [CrIV(02)2D3] (D = nitrogen base, e.g. NH3 » or 35 A^CCr (O^lgKN)^] (A = alkali metal). Examples of each of the four main categories are K g C C r ( O ^ ] 26, K H C C r O ^ O ^ ] 27, CCr0(02)2(bipy) 328. 29 and [Cr(02 )2(NH3)3]. There are of course other examples of chromium peroxo complexes, and recent research has included synthesis of fluoro- IV IV peroxo chromates (IV) of types A J [Cr 0(0 b ) b F].H 40 and A v [Cr (0 4) F] J 30 (A = alkali metal). As mentioned earlier, a main objective of this work is to concentrate on the peroxo chemistry of the second and third row element pairs of Groups IVa, Va and Via, with a view to finding possible chemical differences between the members of each pair. For this reason, along with the fact that the peroxo chemistry of chromium has already been extensively studied, the work described in this chapter (both the work in the literature reviewed in this section, and our work that follows) concerns only the peroxo chemistry of molybdenum and tungsten. The known peroxo complexes of molybdenum and tungsten are all VI VI apparently derived from the Mo and W oxidation states. Unlike that of chromium, the aqueous peroxo chemistry of molybdenum!VI) and tungsten(VI) is most complicated. Addition of hydrogen peroxide to solutions of molybdate(VI) or tungstate!VI) greatly reduces the degree of polymerisation therein, but despite this the chemistry is still 2- complex. Species with (02 ) :M ratios of 4:1, 3:1, 2:1 and 1:1 are known, along with less well-defined ones displaying other or non integral ratios. Peroxo-polymolybdates and tungstates are also known, as well as an ever increasing range of peroxo and oxoperoxo complexes containing heteroligands such as fluorides, chlorides, carboxylates, phosphates, sulphates, amino-acids, and ligands derived from nitrogen 36 VI bases. Each of these types of heteroligand acts to stabilise the Mo VI or W peroxo complex; simple transition metal peroxo complexes often explode spontaneously or are sensitive to shock or heat, while many of their heteroligand derivatives may be stored for long periods, heated in solution, or even impacted without decomposing. The ligands may impart this stability in various ways, for example by destabilising certain d-orbitals of the metal and so favouring electron transfer to the peroxide ligand, or by acting as electron donors themselves - the charge c imparted being delocalised on to the peroxide ligand via the metal. The work described later in this chapter concerns both the study of VI VI simple peroxo and oxoperoxo complexes of Mo and W , and of peroxo complexes containing heteroligands such as carboxylates and fluoride It also concerns the reactivity and applications of these complexes For the purpose of clarity, the review that follows is divided into three sections as above. 2.1.2 Simple Peroxo and Oxoperoxo Complexes of Molybdenum!VI) and Tunqsten(VI) A marked feature of the chemistry of molybdenum(VI) and tungsten (VI) is the formation of numerous polymolybdate(VI) and polytungstate (VI) acids and their salts. For tungsten, the simplest species are tungstate, [WO,]2", and hexatungstate, [HW.O..]5". 31*32 The latter n b Z 1 species polymerises further when acidified; no di- or tri-tungstates are formed. The aqueous chemistry of molybdenum(VI) is less clear, with the simplest species being molybdate, [MoO^] , acid molybdate, 37 CHMoO^] , and heptamolybdate, [ M O j O ^ 6 . 33 The polyanions consist primarily of octahedral MoOc or W0e units. b 6 The reasons for the ability of certain metals, such as molybdenum and tungsten, to form such species is not entirely clear, but it is certain that a large degree of metal and oxygen orbital overlap to give Tr-bonding is required. The base strength of the metal atoms is also important, as is the expansion of the coordination sphere (by 34 coordination of water molecules) of the initial [MO^tOH)] species. There has been much work on the isopoly species formed when solutions of molybdate!VI) or tungstate!VI) are acidified, and many techniques applied. The addition of hydrogen peroxide, though decreasing the degree of polymerisation markedly, gives rise to a complex aqueous peroxo chemistry, on which there are only scattered, and 5 in many cases conflicting, data in the literature. The techniques used for study of the species involved, which include those with 4:1, 2- 3:1, 2:1, 1:1 and other (0^) :M ratios, include Raman spectroscopy . * 35-37 and n.m.r. spectroscopy. A number of peroxo and oxoperoxo molybdates(VI) have been characterised by X-ray crystal structure determination, as follows. [Zn(NH^)^]CHo(0^)^3 contains a distorted dodecahedral tetraperoxomolyb- 3 date (VI) anion 8 (similar in configuration to that found in K3 £Cr(Og)^3) with a mean 0-0 distance of 1.55 A. The dimeric species K2CMO203(02,4(H20,2],2H2°39 and (pyH}2[Mo2°3(°214(H2°’2]^° b° th contain complex anions containing two pentagonal bipyramidal MoOtOgJg units linked by an equatorial p-oxo bridge. The equatorial pentagon 38 is completed by two peroxo groups (mean 0-0 distance 1.48 A), and the axial positions are occupied by a terminal oxo ligand and an aquo ligand. The X-ray crystal structure of the di-p-hydroperoxo oxoperoxo dimer (pyH)2CMo202(00H)2(02)^]40 reveals a complex anion containing o two MoOtO^)^ moieties (mean 0-0 distance 1.47 A) bridged by two o hydroperoxide ligands (mean 0-0 distance 1.46 A). Stomberg and co workers have determined the X-ray structures of a series of peroxopoly- molybdates(VII: (NH;);(Mo307(02 ) 11 .2H2041 j K^CMo^O^IO^]42; K6CMO5° 10l02 ,8l-5H2 0*3! K6[Mo7022(02 )23.nH20 (n = 6 <2,U ; n = 10 55|; K5tMO7021 (0HM02 12]-6H2°U : ,NH4,8CMO10°22(02l12l-16H20 -'5 Only two simple oxoperoxo tungstates(VI) have been structurally characterised by X-ray. K2^W2°3*°2*4*H2°*2^*2H2° is found to be 46 isostructural with its molybdenum analogue. The novel oxoperoxo- carbonato tungstate Ke[W,0o(0o 4 8 Z )o (CO J )].6H Z 0 has been reported very recently by Stomberg.47 The complex anion contains a bidentate bridging carbonato ligand, and its structure is discussed in more detail in Section 2.4.2 (and depicted in Fig. 2.18). The vibrational spectra (in solid state and in aqueous solution) and the 95Mo <183W where possible) n.m.r. spectra of known species of these types have been used to study the species present in solution when molybdatetVI) and tungstate(VI) are acidified in the presence of hydrogen peroxide.48 The tungstate(VI) - H202 system is the simpler of the two. and its study benefits from the greater stability of its solutions. 39 However, the yellow solutions above pH 9 are too unstable for Raman ?- work. In the pH range 9-7, the main species seems to be [WtO^)^] , especially when an excess of peroxide is present. Raman bands are found -1 48 at 858, 596 and 574cm , and they persist down to pH 5. These frequencies are close to those found due to the side-bonded W(02) unit in CW(0 ^ ) The existence of CW(02)^]2 in tungstate (VI) — H2^2 50 51 solutions of high pH has also been noted by Bogdanov et al. ’ The main species in solution at pH 7-1.3 is clearly the dimeric 2- complex [W203 (0^)^(H20)2I ; indeed it is present in traces above pH 7 when a small excess of peroxide is present. The solutions in the 7-1.3 pH range show Raman bands at 960, 850, 620 and 560cm1 consistent with the v(W=0), v(0-0), vS[W(0^)] and vaSCW(0^)3 vibrational modes of 2- 183 tW203 (02 )4(H20 )2 ^ , and a single peak at 6 -699 p.p.m. in the W n.m.r. spectrum as seen in aqueous solutions of the same dimeric 48 species. These data substantiate earlier evidence for the existence of the £W2°3*°2*4*H2012^2 sPecies in ac*d tungstate(VI) — H202 solutions above pH 2.51,52 Below pH 1.3, the solutions begin to lose oxygen, and the metal-oxo stretch at 960cm * 1 shifts to 970cm • 1 . 4 8 This could be due to the 2- 2- monoperoxo tungstate(VI) species "[W0_] " (i.e. [W0.(0o)3 ) claimed by 0 3 2 53 Bogdanov et al. However, there is no splitting of the W=0 band in 54 the Raman spectra as would be expected for cis-WO units , and under 4 8 the conditions used in these laboratories (e.g. using HC1 for acidification) the known species CW0(02)C1^I is probably the main species present. A simplified diagram of the equilibria occurring in peroxotungstate(VI) solutions at various pH, based on that of 40 48(i) Campbell , is shown in Fig. 2.1. below. CW(02 ) 4 32 2- tW2°3l02 V W pH 14-7 pH 7-1.3 V [W0(02 )C1^]2 pH < 1.3 Fig. 2.1 Equilibria in the [WO^I2^— H O — HC1 Svstem48(i) Turning now to the molybdatetVI) — H2 02 system, the situation is a little more complicated. Again, at pH greater than 9, the solutions 95 (dark-red in colour) are too unstable for Raman or Mo n.m.r. spectroscopy. There have been claims of di- or tri-peroxo molybdate(VI) 56 2- 2 - species formed at high pH , though such “[MoO ] " and "[MoO ] 6 7 complexes are more likely to be mixtures or decomposition products of 2- [Mo(02)^ 3 . The latter species is clearly the main one in the pH range 4 8 9-7. The Raman spectral data coincide with infrared data measured for solid alkali metal salts of [Mo(02)4]2’. 48,57 The 95Mo n.m.r. spectrum of the solutions shows a single sharp resonance at 5 -426 p.p.m., as well as a broad one at 6 -496 p.p.m. (the latter probably 41 due to a decomposition product). A sharp peak would be expected for 2- 00 CMo (02)^3 on account of its high D2d symmetry. 95 In the pH range 7-5, a broad Mo resonance appears at 5 -266 p.p.m. (cf. 6 -263 p.p.m. for aqueous K2 CMo2°3(°2 )4(H20 )23 ■2 H2 0), along with 2 _ the characteristic Raman bands due to the £M°203*02*4*H20 *2^ species, 4 4 which is clearly the principal one at this pH. 8 However, Campbell 8 95 also found Mo n.m.r. (a resonance at 6 -492 p.p.m.) and Raman (bands at 959, 872 and 539cm 1) evidence for the triperoxo complexes claimed 56.58 by Bogdanov et al. 2- The CMo2 03(02 )^(H2 0)23 dimer is clearly the principal species in the pH range 5-2.5, but below pH 2.5 the broad resonance at 8 -263 p.p.m. in the 95Mo n.m.r. spectrum shifts upfield to 6 -251 p.p.m.*8 This suggests a protonation process similar to that shown in peroxo- 51 59 vanadates by an upfield shift in the V n.m.r. resonance. It has been shown by X-ray crystal structure determination in these laboratories that there is protonation of the p-oxo bridge in 4- 3- 60 [ V O (0)3 to give [V O (OH)(0 ),3 . It is therefore likely 2324 2224 that the protonated [Mo202 (0H)(02 )^(H20)23 species exists, as proposed 61 before by Wendling et al. 95 A new Mo resonance at 6 -155 p.p.m. in the n.m.r. spectrum of 2- + [MoO^I — H2®2— ^ solutions below pH 2, along with changes in the 4 Raman spectrum 8 , could indicate the formation of diperoxo molybdate (VI) species of type [Mo02 (02 )2 32". as claimed by Bogdanov et al.58, but no splitting of the Mo=0 vibrational band is oserved, as would be expected for a cis-MoO species. Other species reported in the low pH 42 j. go 9- 63 range include CMo03(02)3 and [Mo02 (02)2] Under the 4 8 conditions used in these laboratories , the main species formed at 2 - very low pH m HCl-containing solutions is likely to be [Mo 0(02 )C1^3 The equilibria occurring in peroxomolybdatetVI) solutions at various pHs and in the presence of HC1 are shown diagrammatically in Fig. 2.2 (a simplified version of that of Campbell*8 ^ ^ ). A recent kinetic VI study of the equilibria present in acidic perchlorate solutions of Mo 64 and H2 02 cites mononuclear diperoxo complexes, “MoOtO ) ^ as the main peroxidic species; it is proposed that hydrolysis of this species occurs, to give [MoO(OH)(02)2] . tMol02)432' - [Mo203 (02)t (H2012] 2 pH 14-7 pH 7-1.7 /] A V v [Mo 0(02)3]2" ^ ± [Mo202(0H)(02)4(H20)23 pH 9-5 pH 1.7-0.8 A CMo 0(02)C1;12 pH < 1.7 Fig. 2.2 Equilibria in the CMoO 4 ]2~— “H 2“2o ------— HC1 System*8*------*** 43 In conclusion, it can be said that there is still little agreement on the precise nature of the species present when aqueous molybdate(VI) or tungstate(VI) solutions are acidified in the presence of hydrogen peroxide. "Isopolyperoxo" species do not seem to play an important role in either system according to the studies discussed above, but clearly exist and are isolable in peroxomolybdate(VI) solutions left to evaporate for some period of time, as signally demonstrated by Stomberg. 2.1.3 Heteroligand Peroxo Complexes of Molybdenum!VI) and TunqstenfVI) A wide variety of heteroligands have been incorporated into peroxo complexes of molybdenum!VI) and tungsten!VI). In most cases their inclusion seems to enhance the stability of the peroxo complex, and to change the reactivity of the peroxo group(s). (a) Halooeroxo Complexes There are several examples of molybdenum!VI) and tungsten!VI) peroxo complexes containing monodentate halide ligands in the literature. Fluoroperoxo-molybdates!VI) and -tungstates(VI) were first prepared by . 65 Piccini in 1892 by reaction of the corresponding oxofluoro compounds with hydrogen peroxide, and formulated as 2AF.Mo03F2 .H2 0, 2AF.W03F2 .H20 !A = alkali metal), and 3NH^F.MoO^F2 . The difference in stoichiometry achieved by using an alkali metal or ammonium counter-cation has since been confirmed by X-ray crystal structure determinations on 66 C7 Kg[MoO^02 )F^]•HgO and WH^^FCMoOCO )F ]. Grandjean and 44 Weiss found the former species to contain a pentagonal bipyramidally coordinated molybdenum atom, with the peroxo group (0-0 distance 1.44 %) and three fluorine atoms in the equatorial plane and the axial positions occupied by the oxo ligand and the fourth fluorine atom. Stomberg67 found a similar geometry in the ammonium salt, but a significantly o shorter 0-0 distance (1.36 A). 6 8 Ruzic-Toros et al. have since determined the X-ray crystal structure of bis(8-hydroxyquinolonium) oxoperoxotetrafluoro tungstate(VI), (C 9 H o NO) Z CWO(0 Z )F,]. 4 In this case, the tungsten atom is reported to adopt octahedral geometry, with one corner of the polyhedron at the centre of the 0-0 bond, whose dimension is extremely short (1.20 t) - a phenomenon explained by the authors as being a result of peroxidic decomposition of the crystal. The infrared and Raman data first reported by Griffith for K [Mo0(0 )F^3.H 0 were conflicting, with the v(0-0) vibration being 69 -I placed at 933cm “ 1 in the first publication , and at 876 and 856cm in the second.1* The reason for this disagreement was later found by Nakamoto and co-workers7^ to be due to decomposition of the complex on exposure to air. A fresh sample of KgCMoOfO^F^] .H20 was reported to give infrared bands at 966 [v(Mo=0)3, 930 [v(0-0)3, 603 [vS{Mo(02 )}3, as and 562 [v CMo(02 )>3 cm -1 ; a sample of the compound left to stand in the laboratory for one month was found to lose the 930cm”1 band in favour of two at 877 and 855cm 1. The authors suggest that the change in v (0-0) frequency could be due to reaction of coordinated peroxide with water to form a perhydrate. Similar results are reported for (NH4)3 CHo0(02 )F4 3. 45 19 The F n.m.r. spectrum of aqueous CWO(°2 ^ 3.H20 was measured in these laboratories by Evans, Griffith and Pratt71, and consists of three shifted bands of relative intensities 1:2:1, thus indicating a 2 - pentagonal bipyramidal structure for [WOtO^F^] in solution (three fluorine environments). The molybdenum analogue proved too insoluble 19 for similar study. More recent Russian F n.m.r. work on aqueous (NH,)_F[Mo0(0_)F,] has shown that several species with different numbers b J c b of fluorine atoms in the coordination sphere exist in solutions of this salt.72'73 The difluoro oxodiperoxo molybdate(VI) species K2[Mo0(02)2F2] was 69 prepared by Griffith by recrystallisation of K2CMo0(02)F^].H20 from 19 H2°2* The F n.m.r. spectrum of this species shows an AX pattern due to two non-equivalent, non-exchanging fluorine environments, indicative of a pentagonal bipyramidal structure with two peroxo groups and a fluorine atom in the equatorial positions, and an oxo ligand and the other fluorine atom in the axial positions.71 The trifluoro oxoperoxo tungstate(VI), (Et^N)CW0(02)F3(H20)], prepared by Calves and Guerchais 74 was reported to show an AX2 pattern 19 in its F n.m.r. spectrum, which suggests pentagonal bipyramidal coordination around the tungsten atom, with three fluorine atoms and the peroxo group in the equatorial positions. The same authors have reacted the complex with various heteroligands. Various monodentate pyridine- N-oxides (L) coordinate to the metal at the expense of the aquo ligand to form complexes (Et^N)CWO(02 )F3 (L )3.75 Bidentate ligands (L*) such as various diketones, RCOCHgCOR (e.g. R = Me, Ph)76, and 2,2-dipyridyl77 displace the aquo ligand and a fluorine to form [W0(0 )F (L*)]. 2 2 46 The chloro ligand has also been found to co-exist with the peroxo moiety in complexes of molybdate(VI) and tungstate!VI). The tetrachloro oxoperoxo species [M0{02 )C14]2‘ (M = Mo, W) have been studied by vibrational spectroscopy14,61,78,79, UV/visible spectroscopy80,81, 61 B1 8 2 potentiometry , and radiocrystallography. ' These species are again seven-coordinate, and show v(M=0), v(0-0), vs[M(02)] and as - 1 v CMC02)] bands at around 950, 900, 600 and 550cm respectively. (b) PeroxoComplexes Containing Ligands Derived from Carboxylic Acids Nitrogen Bases The inclusion of the bidentate oxalato ligand into oxoperoxo complexes of molybdenum!VI) and tungsten(VI) to give [M 0(02 )2(ox)]2’ 83 84 (M * Mo, W) was reported as long ago as 1908 by Mazzucchelli. 85 Rodriguez later showed that use of a stoichiometric amount of hydrogen peroxide in preparation yielded the dioxo monoperoxo species 2 - Cm 02102 ^ OX^ H2 ° ^ = M°' W1* The x“ray crystal structure of the oxalato oxodiperoxo molybdate(VI) species K2[Mo0(02)2(ox)] was 8 6 determined by Stomberg in 1970. The molybdenum atom has pentagonal bipyramidal coordination, with the two peroxo ligands and one of the deprotonated carboxylate groups of the oxalato ligand in the equatorial positions, and the other deprotonated carboxylate and the oxo ligand in the two axial positions. The axial Mo-0(carboxylate) bond is found to be considerably longer than the equatorial one, due to a trans-influence from the oxo ligand. Stomberg has since reported the X-ray crystal structure of the tungsten analogue K2[W0(02)2(ox)]87, and found it to be isomorphous in structure with the molybdenum species. The mean 0-0 bond lengths (1.46 A for the molybdenum complex, 1.50 A for the tungsten 47 complex) are in the range expected for molybdenum(VI) and tungsten(VI) peroxo complexes.7 ^ The KgCMoOf0^) (ox) ] salt, isolated from a solution containing molybdenum trioxide, KOH, hydrogen peroxide and either malic or malonic acid, has also been structurally determined with greater refinement by 8 8 Djordjevic and co-workers [see Section 2.2.1(c)]. A complex oxidative decarboxylation of the malonate or malate ligand in transient carboxylato peroxo complexes is proposed. The salts K^CMOfO^J^tox)] (M = Mo. W) show infrared and Raman bands at around 970 [vM=0], 880 [v0-0], 650 Cv S{HC02)}3 and 590 [v{M(02)>] -1 13 90 cm . ' The monoperoxo salts K2^M°2*°2^OX^ H20 ^ *H2° (M = Mo, W) display bands at around 970 [vS(M=0)], 910 [vaS(M=0)3, 870 [v(0-0)], 650 [vS{M(02)}1 and 590 [v3SfM(02)>3 cm 13 A theoretical study of 2 - g i the [MoO(02)2(ox)] anion has been reported by Brown and Perkins. 92 Sljukic and co-workers have reported the characterisation of the oxalato oxodiperoxo species Na2CM0(02 )2(ox)].3H20 (M = Mo, W) and A2CMo0(02)2(ox)] (A = Rb, Cs, NH^) using infrared and X-ray techniques. 93 Brown and Forsyth have presented polarimetric, potentiometric and spectrometric evidence for the formation of tungsten(VI) peroxo complexes with d-tartaric acid, which seems to indicate that the [WtOg)^]* and [WgOg(Og)^ ((^0)g ] s p e c i e s form complexes with free tartaric acid rather than with the tartrate ion. Similar techniques later indicated the formation of 2:1 complexes of mandelic and glycollic acids with the CW203 (02 )4 (H20 )232” anion over a limited pH range.94 48 The first citrato peroxo complex, K [Mo0(0 ) (cit)3.0.5H 0 .3H 0 was 2 2 2 2 2 2 isolated and characterised by vibrational spectroscopy and X-ray crystal structure determination in these laboratories by Griffith, Wiggins and 95 co-workers. The anion has pentagonal bipyramidal geometry, with a bidentate citrate ligand binding through its hydroxo atom and the 2- central carboxylate group. As with CMoOfO^fox)] , the two peroxo groups occupy equatorial positions, joined in this case by the deproto- nated hydroxyl oxygen of citrate, which prefers to form the shorter, stronger bond possible in this position. The oxygen atom from the deprotonated central carboxylate group of citrate and the terminal oxo ligand assume the axial positions (see Fig. 2.3 below). o Fi9' 2 3 Structure of the rMofHn.) . lcit) 12~ Aninn95 49 The vibrational spectra show the expected metal-oxo, peroxo and metal- peroxo stretches, as well as a band at 855cm”1 likely to be due to the v(0-0) stretch of the hemiperhydrate. Peroxo complexes of molybdenum!VI} and tungsten(VI) containing bidentate and tridentate ligands derived from picolinic acid (2-pyridine carboxylic acid) and dipicolinic acid (2,6-pyridine dicarboxylic acid) respectively have been isolated and characterised. The stable hepta- coordinate complexes CMO(02)(dipic)(H^O)] (M = Mo, W), containing the tridentate dipicolinato ligand, were first prepared and characterised by 1 96 infrared and H n.m.r. spectroscopy by Guerchais et al. The X-ray crystal stucture of CMoOtO^)(dipic)(H2<))] was determined by Mares and co-workers in 1978, along with that of the diperoxo picolinato species 97 HCMoCMO^Jgfpic) ]. (H^pic^.H^O. The dipicolinato complex contains the oxo and aquo ligands in the axial positions of the pentagonal bipyramid, with the two deprotonated carboxylate oxygens and nitrogen atom of the tridentate dipicolinato ligand and the peroxo ligand constituting the equatorial pentagonal plane. The picolinato complex also has pentagonal bipyramidal geometry, in which the nitrogen atom of the bidentate picolinato ligand and the two peroxo groups occupy the equatorial positions. The vibrational spectra of both types of complex (the tungsten analogue of the molybdenum picolinato species was also isolated) are very much as expected, showing characteristic metal-oxo, peroxo and metal-peroxo bands. The 13 C n.m.r. spectra of the complexes show a downfield shift in the resonances due to the carboxylic carbon!s) upon complexation - as expected for a carboxyl group bonded to a metal of 50 high valence. Hares and co-workers have used these picolinato and di- picolinato complexes for the stoichiometric oxidation of cyclic ketones and alcohols (see Section 2.1.4.1). The work of Edwards et al., in which both fluoride and dipicolinato ligands are incorporated into molybdenum(VI) and tungsten(VI) peroxo complexes, by way of substitution of the labile aquo ligand (trans to the molybdenum-oxygen double bond) in the complexes [Mo 0(02 )(dipic)CHgO)] (M = Mo. W), is of great interest. In the presence of cations such as Cs+ , Rb+ , K+, Na+ , (He^N)+ and (Et^N)+, ~ 98 dimeric species of formula [Mo,0(0) (dipic) F] are formed. The 2 2 2 2 2 99 X-ray crystal structure of (Et^N)[Mo^O^(02 )2(dipicJ^F] reveals that the molybdenum unit has pentagonal bipyramidal coordination, and that the two units are axially linked by a symmetric Mo-F-Mo bridge. The Mo-F distance is very long due a trans-influence imparted by the axial oxo ligand. The peroxo group and the tridentate dipicolinato ligand occupy the equatorial positions. It is interesting that, in the presence of the ammonium cation, or chloride as the halogen, a mono nuclear species is formed. The X-ray crystal structure of 100 (NH^)[Mo 0(02 )(dipic)F] shows pentagonal bipyramidal geometry for the anion, with the fluoride ligand joining Mo=0 in the axial positions. Covalent molybdenum(VI) and tungsten(VI) peroxo compounds of general formula [M0(02 )2(L)(L‘)] have been prepared with a wide variety of basic ligands L and L ’, including phosphoramides, amine oxides, pyridines, quinolines and aromatic amines (ref. 7 and references therein). They have been used extensively in the field of organic oxidations, due to their solubility in organic media and often the fact that their 51 possession of vacant or releasable coordination sites on the metal atom allows coordination of the substrate in the oxidation mechanism7 (see Section 2.1.4.1). These covalent peroxo compounds are exemplified by the hexamethyl- phoshoramide (HMPA), [(Me2N)3P0], complexes [MoOlO^fHMPA) (H20) ] and [MoO(0^) (HMPA)(py)3 first reported by Mimoun and co-workers.101 The X-ray crystal structures of both HMPA complexes have been determined by Le Carpentier et al. 102 Both complexes contain a pentagonal bipyramidally coordinated molybdenum atom; the oxygen atom of the monodentate HMPA ligand and the two peroxo groups form the equatorial pentagonal plane, with Mo=0 and either the aquo or pyridine ligand in the axial positions. 103 Schlemper and co-workers have recently determined the X-ray crystal structure of the oxodiperoxo 2,2-dipyridyl (bipy) molybdate(VI) complex [Mo0(02)2(bipy)], first synthesised by Mimoun and co workers.101 Once again, the anion exhibits distorted pentagonal bipyramidal coordination, with the molybdenum displacement in the direction of the strongly bonded axial oxo ligand. Other work worthy of mention involving peroxo complexes of molybdenum!VI) and tungsten!VI) with organic nitrogen bases as co ligands is as follows. Beiles and Beiles have prepared, but not fully characterised, complexes of peroxo-molybdate(VI) or -tungstate!VI) with quinoline, 8-hydroxyquinoline and dipyridyls10*, and substituted p y n d m e s .105 52 106 Weiss and co-workers have more recently reported the X-ray crystal structure of a trans-diperoxo molybdatetVI) porphyrin complex, [Mo(0g)g(TpTP)], (TpTP = tetra-p-tolyl porphyrin). This species, a rare example of a heteroligand peroxo complex of molybdenum(VI) or tungsten- (VI) not containing an oxo group as co-ligand, contains an eight- coordinate molybdenum atom, with two peroxo groups and four nitrogen atoms of the porphyrin involved - thus giving the molecule S. symmetry. The mean 0-0 distance of 1.399 A is significantly shorter than that found in most oxoperoxo molybdate(VI) compounds.7 * 107 Taradfer and co-workers have very recently described the synthesis and infrared spectra of a variety of molybdenum(VI) and tungsten(VI) peroxo complexes of types [M0(02)(L)2] (M = Mo, W), H[Mo02(02)(L)(H^O)]+, and CMo0(02)(L*)], where L = oxoquinolino (I), aniline-2-carboxylato (II), 2-aminophenoxido (III), picolinato, 2-carboxylatoquinolino (IV), and L’ = N-(2-oxophenyl)salicyliden- imino (V). The complexes were prepared by addition of acetone solutions EE I 53 of the organic moiety to a suspension of MoO in H O at 0°C. All gave characteristic infrared spectra, with v(M=0) 960-940cm~1, v(0-0) 900- 825cm \ vSCM(0^)3 680-640cm *, and vas[M(02 )] 610-540cm * reported. The complexes were also found to oxidise allyl alcohol to the glycidol, and PPh3 and AsPh^ to their respective oxides. . . 108 D^ord^evic and co-workers have prepared a series of molybdenum(VI) peroxo compounds using a-amino acids as co-ligands, of general formula [Mo0(02 )2(HAA)(HgO)], (HAA = glycine, a-alanine, valine, proline, leucine, serine). The X-ray crystal structures of [Mo0(02)2(HAA)] (HAA = glycine, proline) show that the amino-acids coordinate in monodentate fashion through the carboxyl group, taking part in pentagonal bipyramidal coordination, in which the aquo ligand is trans-axial to Mo=0. The complexes display characteristic v(Mo=0) (978-970cm S and v(0-0) (880-870cm 1) vibrational bands in their infrared spectra. The X-ray crystal structure of the optically-active (S )-dimethyl- lactamido [(S)-MeCH(0)C0NMe2, or (S)-DML] oxodiperoxo molybdate(VI) 109 compound [MoO(02) ( S)-DML}] has been reported by Winter et al. The bidentate lactamido ligand is coordinated through the carbonyl oxygen (equatorial) and the deprotonated hydroxyl oxygen (axial, trans to Mo=0). The compound was earlier used for the asymmetric epoxidation of simple olefins(see Section 2.1.4.1).110 54 Oshima and co-workers have reported the synthesis of the oxo- peroxo bis(N-phenyl-hydroxamato) molybdate(VI), [Mo0(02)(PhC0N(Ph)0>23, from the reaction of Mo02(acac)2 with hydroxamic acid [PhC0N(Ph)OH3 followed by hydrogen peroxide. The X-ray crystal structure of the compound reveals a pentagonal bipyramidal configuration, in which the equatorial positions are occupied by the peroxo group, the deprotonated hydroxyl groups of both bidentate N-phenylbenzohydroxamato ligands, and the carbonyl group of one of them. The oxo ligand and the other carbonyl group occupy the axial positions. A high degree of crystallo graphic positional disorder accounts for the short 0-0 distance of o 1.212 A. The compound has been used for the oxidation of alcohols to carbonyl compounds and the epoxidation of allylic alcohols (see Section 2.1.4 .1). Although the heteroligand peroxo chemistry of molybdenum(VI) and tungsten(VI) has been widely studied, the use of the more simple carboxylates as ligands has been sparsely researched with the exception of oxalate and latterly citrate. A substantial part of our work described in this chapter is therefore devoted to the isolation of such complexes, their characterisation and reactivity, and their possible application to the problem of the separation of molybdenum and tungsten. 55 2.1.4 Reactivity and Applications of Peroxo,Complexes of Holvbdenum(VI) and Tungsten(vn 2.1.4.1 Reactivity (a) general Reactivity 2 As mentioned in the general introduction to the reactivity of n -peroxo complexes (see Chapter 1), the bonding of a peroxide ligand to a transition metal has the effect of transferring charge density to the ligand. This reduces the kinetic barrier to changes of spin that is experienced in the reactions of molecular oxygen, and imparts basic (or nucleophilic) character to the peroxide ligand. Clearly the reactivity of the peroxide ligand is governed by the metal to which it is coordinated; indeed, the extent to which this is the case makes the peroxide ligand exceptional. Peroxo complexes typically react with diamagnetic electrophiles, such as small molecules (e.g. SO^, CO, CO^) or molecules containing carbon-carbon multiple bonds. Group VIII peroxo complexes show marked reactivity with a variety of small molecules, for example CO, CO^, HO, 0 H02 , S02 , whereas Group VI peroxo complexes are not noted for such reactivity. Conversely, Group VIII peroxo complexes will only react with carbon-carbon multiple bonds in cases where they are made very electrophilic by electron-withdrawing substituents (e.g. the cyano groups in tetracyanoethylene), while Group VI peroxo complexes readily C react with carbon-carbon multiple bonds of varying electrophilicity. 56 The reaction of Group Via peroxo complexes with carbon-carbon and other multiple bonds makes this class of compound very useful in organic oxidations, acting both stoichiometrically and catalytically. The use of chromium(VI), molybdenum!VI) and tungsten(VI) peroxo complexes in organic oxidations has been reviewed recently, but somewhat briefly, by Mimoun.7 A more detailed review of the use of molybdenum!VI) and tungsten!VI) peroxo complexes in organic oxidations would be of benefit here in the light of our work on such oxidations reported later in this chapter. The use of chromium(VI) peroxo complexes in this field will not be considered, since, as explained previously, our work does not concern these species. (b) Stoichiometric Oxidation The molybdenum!VI) and tungsten(VI) peroxo complexes capable of organic oxidation fall into the heterolytic category, that is they react via the formation of metallocyclic intermediates, by the complexation of the organic substrate to the metal.7 They tend to be used mainly as alkene epoxidation reagents, but are also known to oxidise enolisable and cyclic ketones, as well as alcohols. The reactivity of any one peroxo complex towards organic substrates clearly depends on several factors, of which the following seem to be most important:- (1) The solubility of the complex in organic solvents. (2) The overall charge of the complex. (3) The nature and geometry of the metal. (4) The availability of vacant or releasable coordination sites on the metal for complexation of the substrate. 57 (5) The nature and reactivity of the particular type of substrate involved. For example, in the epoxidation of alkenes, the electrophilicity of the carbon-carbon double bond is critical. The molybdenum(VI) and tungsten(VI) peroxo complexes most commonly employed as stoichiometric oxidants are those of general formula [MO(Og)gU-)(L ')] (M = Mo, W), where L and L* are organic bases or water [see Section 2.1.3(b)]. A good example of this class of compound is [Mo0{0 2 ) 2 (HMPA)(HO)],2 which is found to effect stoichiometric epoxidation of alkenes in aprotic solvents at room temperature in excellent yield.112 114 The epoxidations display a series of character istics which help to propose a mechanism.7 It is seen that the process is stereoselective, that is cis-alkenes yield cis.-epoxides, etc. The oxygen atoms interacting with the alkene are exclusively peroxidic in nature, as shown by the 1 8 O-substitution 113 studies carried out by Sharpless and co-workers. The epoxidation is hindered by the presence of o-donor ligands such as DMF which hinder or 112 prevent complexation of the alkene. Furthermore, the alkene clearly requires a releasable coordination site adjacent to the peroxo group for epoxidation to occur. Hence, the complexes [Mo0(02)(dipic)(HgO)] and [Mo0(02)Cl(pic)(HMPA)], which possess anionic and/or strongly-complexing polydentate ligands, are not reactive due to the inability of the alkene to displace such ligands. The mechanism shown in Fig. 2.4 has therefore been proposed by Mimoun112 for the epoxidation of alkenes by molybdenum(VI) and tungsten(VI) peroxo complexes. 58 Fig. 2.4 Proposed Mechanism for the Epoxidation of Alkenes by Covalent Molybdenum (VI) Peroxo Complexes [m o O( o 2)( l )( l')] The mechanism consists of reversible complexation of the alkene with displacement of the equatorial HMPA ligand, followed by irreversible oxygen-transfer to the coordinated alkene. The five-membered metallocyclic intermediate (MC) formed by the attack of peroxidic oxygen 115 on the electrophilic alkenic double bond, has theoretical support. Furthermore, a dioxo-metallocyclic adduct has been isolated from the reaction of [Ti(O^)(pic)^(HMPA)] with tetracyano-ethylene (see also Section 3.1.3.1).22 Tungsten(VI) peroxo complexes, for example [W0(02)2(HMPA)(H^O)]101, are also effective oxidants, but afford reactions at much slower rates than their molybdenum(VI) counterparts. 113 18 Sharpless and co-workers used exclusive O-substitution of the oxo oxygen of the dehydrated form of the molybdenum(VI) HHPA peroxo complex, [Mo0(02 ) (HMPA)], to prove that only the peroxidic oxygen of the complex is transferred to the alkene during epoxidation. This piece 112 of evidence clearly supports the mechanism proposed by Mimoun (Fig. 2.4); however, the authors presented some evidence against the Mimoun mechanism in the form of structure/rate correlations for the epoxidation of certain cyclic alkenes. For example the HMPA peroxo complex was found to epoxidise norbornene 1.94 times faster than it did cyclohexene, at least two orders of magnitude less a ratio than that observed for other epoxidising agents known to react via a five-membered cyclic transition state (e.g.osmium tetraoxide*13•1*6, phenyl azide1*7). The norbornene:cyclohexene reactivity ratio of 1.94 was seen to be much closer to that found for species thought to epoxidise alkenes via a three-membered cyclic transition state, for example perlauric acid 60 <1 -2117) and chromium(VI) C5.5118). On the basis of this evidence, Sharpless proposed transfer of one of the peroxidic oxygen atoms to the alkene via three-membered cyclic transition states, whose form could range between the extremes shown in Fig. 2.5 below, but conceded that the data could fit Mimoun’s mechanism if the formation of the alkene- metal complex were the rate-determining step. Fig. 2.5 Possible Cyclic Transition States in Epoxidation of Alkenes 113 bv a Molvbdenum(VI) Oxoperoxo Complex Arakawa et al.11* later provided kinetic support for the mechanism of Mimoun (Fig. 2.4), using G.C. methods to measure the coordination constant K (k^/k_^) of the alkene to molybdenum (Reaction 1 below) and the rate constant of decomposition of the peroxomolybdenum-alkene complex (Reaction 2 below) in the [Mo0(02)^(HMPA)] epoxidation of twenty-two different alkenes. The results, as well as underlining the k 1 Mo 0(02 )2 (HMPA) ♦ alkene ^ Ho0(02)2(HMPA)(alkene) (1) k-1 k2 Mo 0(02 )2(HMPA)(alkene) ------» Ho02(02 )(HHPA) ♦ epoxide (2) 61 marked effect the nature of alkene substituents has on the epoxidation reaction, showed that k2 generally increases with K, which can be explained in terms of Mimoun's mechanism. Another stoichiometric oxidation effected by molybdenum(VI) and tungsten(VI) peroxo complexes is the o-hydroxylation of enolisable 119 ketones , using for example [Mo0(02 )2(HMPA)(py)] (also known as MoOPH) as shown in Scheme 2.1 below. R MoOPH i r c h 2- * RCH=CR ------* RCH-C-OH l \ / 0 0 ‘ ✓ R RCH-C l Scheme 2.1 OH Frimer 120 reported an interesting oxidation of dihydropyran (A) to a cleavage product, 4-formyloxybutanal (E), using [Mo0(02)2(HMPA)(H20)3 in inert solvents (Scheme 2.2, overleaf). The expected epoxidation of (A) to the corresponding epoxy-ether (B) is followed by reaction of the latter with the oxo-monoperoxo species (C) to yield the cleavage product (E) via a six-membered metallocyclic intermediate (0). 62 Scheme 2.2 The stoichiometric oxidation of secondary alcohols to ketones using the complexes [M0(02)2(HMPA) (H20) ], H[M0(02)2(pic)] .H20 4 and CMO(02)(dipic)(H20)] (M = Mo, W) has been carried out by Mares and co- 121 workers. The HMPA complexes decompose to "blue metal oxides" during the course of the reaction and give low yields of ketone, while the dipicolinato complexes are inactive. Only those peroxo complexes containing the bidentate picolinato ligand give good yields of ketone. In stoichiometric oxidation the complexes H[M0(02)2(pic)].HgO yield up to 1.7mol. of ketone per 1mol. of complex, suggesting that both of the peroxo groups are active, as seen in the oxidation of dihydropyran Scheme 2.2, above). A catalytic procedure using H[W0(02) 2(pic)].H20 in 63 the hydrogen peroxide oxidation of alcohols (with catalytic turnover of up to 20) is also reported. Oshima et al.111 reported the oxidation of primary and secondary alcohols to aldehydes and ketones using the N-phenylbenzohydroxamato peroxo complex [Mo0(02 )2{PhC0N(Ph)0}2] in good yields. Although the complex is loath to react with simple alkenes, it is shown to epoxidise allylic alcohols to a,0-epoxy alcohols in reasonable yield, both stoichiometrically and catalytically (using t-butyl hydroperoxide as co-oxidant. The dimethyl-lactamide peroxo complex CMo0(02)2{(S)-DML}] 109 recently prepared by Winter et al. has since been used to effect 110 stoichiometric asymmetric epoxidation of simple alkenes. Venturello and co-workers 1 22 have very recently reported the crystal structure of the novel peroxo-heteropolytungstate(VI) salt ^ C6H13^4N^3^°4^W0^°2^2^4^ ' and use as a st°ichiometric and stereo specific epoxidant of unactivated alkenes in aprotic solvents. The readiness of this species to epoxidise alkenes is surprising from a mechanistic point of view. The fact that there are no releasable co ordination sites on the metal would seem to prohibit the metallo- 112 cyclisation mechanism of Mimoun discussed earlier. The anionic nature of the species would seem to rule out the Sharpless mechanism 113 involving external attack by the alkene on the peroxo group. The second mechanism seems more favourable in this case, since the electrophilic nature of the peroxide group is bound to be greatly enhanced by the nature and unusual arrangement of ligands about the tungsten atom. 64 The use of [Mo0(02)2(HMPA)]-type peroxo complexes for the oxidation 123 124 of aluminium and boron alkyls to the corresponding alcoholates (which yield alcohols in good yield upon hydrolysis) has been reported by Schmitt et al. The mechanism is thought to involve radical steps, but can be represented simply (Scheme 2.3). R l + AL - R l OR Scheme 2.3 ROH (c) C atalytic. Oxidation Molybdenum(VI) and tungsten(VI) peroxo complexes are very important reactive intermediates in a wide range of catalytic oxidations and epoxidations involving hydrogen peroxide, molecular oxygen, or alkyl hydroperoxides as the oxygen source. The mechanisms involved in such processes are the subject of much debate, although it is clear that they must differ somewhat from those occurring in the stoichiometric oxidations discussed above. 65 As far as epoxidation is concerned, molybdenum(VI) and tungsten(VI) peroxo complexes are undoubtedly involved in the oxidation of water- soluble alkenes by H202 the Presence °* a molybdate( VI) or tung- state(VI) catalyst. An example is the H202 oxidation of maleic (Scheme , 125 2.4) and fumanc acids in the presence of sodium tungstate. Na2W04 Scheme 2.4 / = \ + Hl 0 HOX CO,H h 2o 2 h o 2c c o 2h 65°C 77% In general, tungsten catalysts are more selective andactive than molybdenum ones in aqueous conditions.7 ^^^ Water-insoluble alkenes require polar co-solvents (e.g. alcohols, dioxane), but these inhibit the reaction. In anhydrous conditions, molybdenum catalysts prove the more active, as expected from the comparative stoichiometric activity of Mo 0(02)2 and W0(02)2 complexes.7*li* Water formed by the epoxidation tends to hydrolyse the epoxides formed to glycols, and its removal by 126 azeotropic distillation has been shown to improve yields (Scheme2.5). JW003 H202 Scheme 2.5 dioxane 70°C + H20 az. dist 95% 66 The aqueous tungstate-catalysed epoxidation of a, p-unsaturated acids 125 by H^O^ ^aS *n 9c,ieme 2.4) pioneered by Payne and Williams was 127 improved considerably by Kirshenbaum and Sharpless in 1985. An increase in amount of catalyst used and an increase of pH from 4-5.5 to 5.8-6.8 is found to improve the process and increase its synthetic utility. It appears that the oxidising peroxo species acts as an electrophile, since increasing the nucleophilicity of the alkenic double bond increases reaction rate. Some of the latest work on epoxidation of alkenes by H^O^ with molybdenum(VI) and tungsten(VI) catalysts has been performed by 128 Oi Funa, Modena and co-workers in Italy. Epoxidation of nucleo philic alkenes by under phase-transfer conditions (1,2-dichloro- ethane as co-solvent) in the presence of molybdatetVI) or tungstatetVI) catalyst is accomplished with high yields and selectivity. The fact that yields and rates are increased upon addition of a monodentate ligand such as HMPA or pyridine-N-oxide lends weight to the argument that epoxidation occurs by "external" oxygen-transfer to the alkene 113 (Sharpless ) rather than by the five-membered metallocyclisation 112 mechanism proposed by Mimoun , since such an addition may co- ordinately saturate the hydrated C M o O ^ 23 species proven to be present by isolation. Venturello et al.129 have demonstrated that use of a tungstate- phosphate mixture as catalyst under phase-transfer and acidic (pH < 3) conditions enables epoxidation of a variety of alkenes using very dilute 130 (<10Z) H2°2’ M*moun et al- have successfully used quaternary phosphonium or ammonium oxoperoxo tungstate(VI) catalysts to epoxidise 67 alkenes at 30-50°C under dilute HgOg/1.2-dichloroethane phase-transfer conditions. Use of a [Ph3(CH2Ph)P]2[W203(02)^] catalyst at pH 5-6 affords the best results, with the epoxidation seemingly occurring in the organic phase — into which the catalyst is able to enter because of its lipophilic [Ph3 (CH2Ph)P]* group. A few of the more recent catalytic oxidations, other than epoxidations, using molybdenum!VI) or tungsten(VI) catalysts with peroxide are worthy of mention. The Baeyer-Villiger lactonisation of cyclic ketones by H202 in the presence of the dipicolinato peroxo complex [MoOtO^)(dipic)(H^O)] as catalyst has been performed by Hares et al. 131 (e.g. lactonisation of cyclopentanone, see Scheme 2.6). The reaction is thought to proceed via a five-membered trioxometallocycle. Scheme 2.6 132 Ogata and Tanaka have used catalytic amounts of sodium tungstate to oxidise dimethyl sulphoxide (DMSO) to the sulphone with dilute H^02 at pH 4. A mechanism involving attack of peroxotungstic acids (CHWO ] and [HWO ]” ) on DMSO as the rate determining step is o 8 2 _ proposed, though species such as the dimer are more likely to exist under these conditions (see Section 2.1.2). 133 Di Funa, Modena and co-workers have also been able to oxidise primary and secondary alcohols to the corresponding carbonyl compounds with H2°2' again usin9 sodium tungstate or molybdate as catalyst, under phase-transfer conditions (with 1,2-dichloroethane as co-solvent). High o yields and selectivity are attainable using temperatures of 60-75 C, and optimum pH (adjusted using sulphuric acid) of 3.0 for molybdate and 1.4 2- for tungstate. Again anionic peroxo species such as [M203(02)^(H20)2] (M = Mo, W) are likely to be the principal species in solution [see Sections 2.1.2 and 2.3.4(a)], and this process underlines the usefulness of phase-transfer conditions for effecting organic oxidations involving such moieties. 134 Finally, the very recent work of Ishii, Ogawa et al. has shown that isolation of the peroxo complex responsible for catalysis in a metalate-H202 system can lead to acquisition of a stoichiometric oxidant in its own right. Tris(cetylpyridinium)-12-molybdophosphate (CMP), [ir-CgHgNtCHg)15CH3][PMo 120^q ], was previously shown to catalyse the epoxidation of allylic alcohols by H202 in chloroform.135 Treatment of CMP with H202 yields a yellow peroxo complex PCMP, characterised by IR spectroscopy, capable of the stoichiometric oxidation of alkenes and allylic alcohols to epoxides, and of alcohols to carbonyl compounds. 69 The use of transition metal peroxo complexes in stoichiometric and catalytic oxidation of organic substrates is ever increasing, and the peroxo complexes of molybdenum(VI) and tungsten(VI) play a large part. This along with the emergence of use of anionic peroxo species as well as covalent ones for oxidation, has prompted us to investigate the reactivity of some of our new peroxo species, and some known ones, in this respect. 2.1.4.2 Applications of Peroxo Chemistry to the Extraction and Separation of Molybdenum and Tungsten The separation of industrially-important molybdenum and tungsten from common ores is essential, especially since the presence of molybdenum impurity in tungsten lowers the ductility of the latter. Two of the most common tungsten-bearing minerals, scheelite. CaWO^, and Wolframite, (FeMn)WO^, contain molybdenum, and separation is essential. It is not an easy process however, due to the similarity in the chemistry of the two elements (a result of the lanthanide contraction between the second and third rows of transition elements discussed previously). One area that has proven effective in separating the two metals is peroxo chemistry. Addition of hydrogen peroxide to the metals yields peroxomolybdates(VI) and peroxotungstates(VI), as reviewed in Section 2.1.2. Formation of such species can clearly lead to separation of the two metals when a complexing agent is used to extract one of the metals into an organic phase preferentially, by variation of the pH and contact 70 time between the phases. An example of such a solvent-extraction separational method is that of Zelikman, Voldman, et al.136, in which an 8-stage extraction of molybdenum from a 10:1 Mo:W peroxidic solution is achieved using tributyl phosphine oxide as complexing agent at pH 0.45. Similar work has been carried out by Ozensoy at this 137 college. Extraction of the individual metals from base metals is often needed, and solvent extraction methods for extracting molybdenum and tungsten from their peroxidic solutions have been reported in the literature. For example, molybdenum can be extracted from HgOg-CCl^ solution using trioctylamine at pH 0.3-6.0, reportedly as 3NH^2~ 138 [Mo02(02)2]. Tungsten has been extracted similarly using trialkyl- 139 methyl ammonium sulphates at pH 2.5-6.0. Extraction of molybdenum from its H^O^-HhK^ so*ut*ons *s also reported using tributyl phosphate, 140 B1J3PO4 . The extracted species was characterised by infrared and UV spectroscopy, and formulated as [(H^O)(HgO)^(Bu^PO^)][ M o ^ (H20 )2]. Another example of extraction of a Group Via metal using hydrogen peroxide is the recovery of tungsten from tungsten carbide. The action of hydrogen peroxide on tungsten carbide has been highlighted in recent literature by Kudo, who proposes that a novel 12-heteropolyacid of tungsten, with carbon as the heteroatom, is formed.*** **^ Recent work 143 at Interox Chemicals Ltd. has led to the development of a multi stage batch process for near-quantative recovery of tungsten from tungsten carbide scrap using H^O^ in the presence of nitric or sulphuric acid, and small amounts of a carboxylate additive (see Section 2.4.1). 71 The work we present later in this chapter on the peroxo chemistry of molybdenumtVI) and tungsten(VI) was undertaken with the separational and extractional problems in mind; frequent mention is therefore made of differences in behaviour of the two metals when they occur. The work includes a study of the species present in solution when hydrogen peroxide reacts with tungsten carbide and molybdenum carbide, since their exact nature is not known. 72 2.2 CARBQXYLATQ- PEROXO COMPLEXES OF HOLYBDENUH(VI) AND TUNGSTEN(VI) 2.2.1. Pr_eg&ration_o.f 1:1 Metal: Carboxvlato Peroxo Complexes 2.2.1.1 Introduction As mentioned in Section 2.1.3(b), the only peroxo complexes of molybdenumtVI) or tungsten(VI) containing simple carboxylate ligands to be isolated previous to this work are the oxalato peroxo complexes CHO(02)2(ox)32_ (M = Mo, W)83,84, [M02(02 )(o x )(H20)]2” (M = Mo. W)85, 2- 95 and the citrato peroxo complex [Mo0(02)2(cit)] . All contain pentagonal bipyramidal coordinated metal atoms, and show characteristic vibrational spectra. Both carboxylate ligands impart stability to the metal-peroxo moiety, since the complexes do not explode on heating or impact and can be stored indefinitely. The work presented in this section of the thesis consists of the further study of these known complexes and of the analogous citrato peroxo complex K2CW0(02)2(cit)].0.5H202.3H20, but primarily the study of new peroxo complexes containing different carboxylate ligands such as tartrate, malate, glycollate and gluconate. Many of this class of ligands, notably tartrate93,144, malate145, lactate148, and 94 glycollate , have been studied when incorporated into oxo complexes of molybdenumtVI) and tungsten!VI), or non-isolated peroxo complexes of the free acids (in the case of tartaric acid93 and glycollic 73 2.2.1.2 Formation_of Complexes (a) Oxalato Peroxo Complexes The oxalato oxodiperoxo species K2[M0(02)2(ox)3, (M = Mo, W), are prepared by addition of excess to a solution of the appropriate 83 84 potassium metalate, K.CMO.J,Z 4 and oxalic acid. * We find that use of a 1:1 reactant ratio Mtoxalic acid leads to formation of traces of the 2- tetraperoxomolybdate(VI), CMo(°2)4] (red colour in solution), or 2 - tetraperoxotungstate(VI), [ W I O ^ ] (yellow colour in solution) species. Even small amounts of such species are clearly unwanted in samples of the oxodiperoxo complexes, not least because of their explosive tendencies when dry. We therefore report a slight improvement to the method of 83 84 Mazzucchelli ' for preparation of the complexes. Use of a small excess (~ 1.2:1) of oxalic acid, still with a large excess of H202* keeps the pH of the solution low enough to prohibit formation of the tetraperoxometalate(VI) species which exist at high pH (above pH 5 for 4 8 molybdenum, and above pH 7 for tungsten ). The infrared and Raman spectra of the pure K2CM0(02)2(ox)] species have been remeasured (Table 2.1). The oxalate ligand also forms oxalato dioxo monoperoxo complexes, 2- [M02(02)(ox)(H20)3 , with molybdenum!VI) and tungsten(VI), a phenomenon which turns out to be unique amongst the carboxylate ligands used in this work. The complexes CM02(02)(ox)(H20)]2", (M = Mo, W), are ft R isolated as potassium salt monohydrates by the method of Rodriguez , 74 in which a stoichiometric amount of H^O,,2 2 is added to a solution containing 1:1 M:oxalic acid. We find that failure to isolate the salts from solution within a few hours of formation - by precipitation with a minimum quantity of ethanol - results in crystallisation of the oxalato oxodiperoxo species K2[M0(02)2(ox)]. Our adaptation of the original method of preparation for the dioxo monoperoxo species is therefore simple - it involves immediate precipitation of the potassium salt using ethanol. That the monoperoxo species decompose in solution to give the diperoxo species is borne out by the Raman spectra (Table 2.1) of the the aqueous K^CMO^CO^)(ox)(H^O)].H^O (M = Mo. W) salts, measured here for the first time. Two metal-oxo stretches, as expected for cis-dioxo 54 complexes , are seen m the infrared and Raman spectra of the solid s as -1 monoperoxo salts at around 980 [v (M=0)] and 910 [v (M=0)] cm . Upon dissolution of each of the complexes in water, just one broad M=0 band is seen in the Raman spectra of the complexes, close to that seen in the Raman spectra of aqueous solutions of the oxalato oxodiperoxo species -1 2- (ca. 965cm ). We surmised that in solution the CM02(°2)(ox)(H20)3 2- species gives way to the more stable CM0(02)2(ox)] species,and another non-peroxo metalate species, probably the known oxalato oxo species CM03(ox)]2 . 95 We were later able to prove this to be the case by Mo n.m.r. g c spectroscopy for 020 solutions of K2CMo02(02)(ox)(H20)] .HgO. The Mo n.m.r. spectrum of the dioxo monoperoxo species in 020 shows two sharp resonances at 6 -229.7 and *4.4 p.p.m. (Table 2.3). The first of these corresponds with the sharp single resonance shown by a 020 solution of 75 K^CMoOtOg(ox)3; the second corresponds to the sharp resonance given by a D20 solution of K2tMo03 (ox)]•2H2°' a known oxalato oxo molybdate(VI) complex.Thus we can state that, in aqueous solution, the conversion of the dioxo oxalato monoperoxo species to the oxalato oxodi- peroxo oxalato and oxalato trioxo species clearly occurs (Scheme 2.7):- 2[Mo02 (02)(ox)(H20)]2 CHo0(02)2(o x )]2 + [Mo03(ox)]2 ♦ 2 H2 0 Scheme 2.7 Addition of H202 to an aqueous solution of the oxalato dioxo mono- 95 peroxo complex gives rise to one sharp Mo resonance corresponding to 2- the oxalato oxodiperoxo complex, the CMo03 (ox)] having been peroxidised. These results are diagrammatically summarised in Fig. 2.6. 95 This simple but elegant experiment underlines the usefulness of Mo n.m.r. spectroscopy as a tool for investigating the stability of molybdenum complexes in aqueous solution, and shows that the oxalato di oxo monoperoxo species, while stable in the solid state, does not possess the stability in aqueous solution shown by its oxalato oxo diperoxo counterpart. 2 - Our preparation of organic-soluble salts of [Mo0(02)2(ox)] in order to investigate the reactivity of the anion in organic media is reported later (Section 2.2.3). 76 Fig. 2.6 ^Mo N.m.r. Spectra of Oxalato Peroxo Molybdate (VI) Species in D20 Solution. - 229-7 [ M0 O 2 (^2 )(^2^4)(^2^) ^ [m o o ( o 2 )2 ( c 2o 4)]2“ (or the above complex with excess H20 2 ) +5-1 [Mo 0 3 (C 20 4 )]2 A 6 ( ppm) 2- 2- Reaction 2 [Mo02(02)( c 20 4 )( h 20)] [m o o ( o 2)2( c 204)] 2" + [Mo 0 3 ( c 204)] + 2H20 (b) Citrato Peroxo Complexes We have re-prepared the crystalline citrato oxodiperoxo molyb- date(VI) complex K^CMoOtOgJ^tcit)].0.5H202.3H20, first prepared in these 95 laboratories by Wiggins , and prepared and fully characterised its tungsten analogue, K [W0(0 ) (cit)].0.5H 0 .3H 0, in order to 2 2 2 2 2 2 investigate the stability of these complexes in aqueous solution using 13 95 Raman, C and Mo n.m.r. spectroscopy. The results are included in Tables 2.1, 2.3 and 2.4 respectively, and are discussed in Sections 2.2.1.3/4). The solid dipotassium citrato oxodiperoxo tungstate(VI) shows expected vibrational bands at around 960 Cv(W=0)], 870 [v(O-O)], 650 CvS{W (0^)>3. and 590 CvaSCW(0^)}3 cm \ as well as an extra v(0-0) band at about 850cm”1 due to the hemiperhydrate (a similar band is found 95 -1 in the molybdenum analogue at 855cm , and in cis. trans-PtCl^(OH)^- (2-NH2Pr)2.0.5H202 at 860cm”1 U 8 ). (c) New Carboxvlato Peroxo Complexes We have been able to isolate the new 1:1 metal:carboxylato peroxo 2- complexes CHO(02>2(L)3 (M = Mo, W; L = glycolate, malate, tartrate, 2- gluconate), and [Mo0(02) (L )] (L = tartronate, quinate) as potassium salts. The first features to become apparent in the preparation of these complexes, using 1:1 molar ratios of metal (as K.CMoO.] or K_[W0,]) to 2 4 2 4 carboxylate and excess H202 , is that their formation depends heavily upon three main factors:- (i) The structure of the carboxylate ligand. (ii) The ability of the carboxylate to compete as a ligand with oxo 78 J> _ and peroxo ligands intent on forming species such as [MtO^)^] and CM203 (02 )4(H20)2J2-. (iii) The pH of the reaction solution. Infact, all three of these factors are closely inter-related. The fact that heteroligands such as carboxylates can stabilise peroxo complexes of transition metals is not in doubt; chelation to the metal partially releases the strain on the peroxo group(s), with the heteroligand either donating electrons to the peroxo group(s) via the metal, or destabilising certain d-orbitals on the metal and precipit ating charge transfer to the peroxo group(s). The carboxylate ligands found by us to form peroxo complexes with molybdenum(VI) and tungstentVI), namely oxalate, citrate, glycollate, tartrate, malate, gluconate, tartronate and quinate, all possess the ability to form a five-membered ring with the metal by way of their bidentate coordination through either deprotonated carboxylate or hydroxyl oxygen atoms (see Fig. 2.7). That coordination is possible by deprotonated hydroxyl oxygen atoms in these complexes was proven by the 95 X-ray crystal structure of K2tHoO(02)2(cit)].0.5H202.3H20 ; the driving force originates from the fact that the hydroxyl oxygen can form a short, strong equatorial bond with molybdenumtVI) in the pentagonal bipyramidal complex anion. We find that carboxylate ligands often found in coordination comp ounds but unable to form a five-membered ring with the metal by coord inating in bidentate fashion do not form oxoperoxo complexes with Mo(VI) 79 0 0 V / M M M ° Y s / \ / \ / \ \ 0 0 0 0 V, 0 0 L. h ----4 >>— ' v / HO 0 0 0 o c h ( o h )c o 2h OH oxalate glycollate tartrate quinate o / X H---- — 0 M MM HO — — H / \ / \ / \ H — —OH 0 0 O 0 0 0 MU' M }— ( 0 COjH 0^ ^CH2C0 2H ^---- ^-CHj COj H CH2CO j H c h 2o h tartronate malate citrate gluconate Fig. 2.7 Carboxylate Ligands Capable of 5-Membered Ring Formation in Peroxo Complexes of Molybdenum and Tungsten and W (VI). Examples are malonic acid (six-membered ring), salicylic and sulphosalicylic acids (six-membered rings), and succinic acid (seven-membered ring) (see Fig. 2.8). The fact that optimum stability may be imparted to a metal peroxo moiety by formation of a five-membered ring with a bidentate carboxylate ligand has been noted previously by 149 Jere. The peroxo titanate(IV) complexes CTi(02)(OH)2], £Ti(°2)(ox)], [Ti(Og)(malonate)] and [Ti(0^)(maleate)] were prepared and subjected to thermal decomposition studies. Jere found that all of the complexes suffered decomposition of the peroxo group at raised temperatures, with the carboxylato peroxo complexes surviving to higher temperatures than the hydroxo peroxo complex, and of these the oxalato peroxo complex proving most stable. The five-membered ring formed by the oxalate ligand is clearly more stable than the six- or seven-membered rings formed by malonate and maleate respectively. Returning to our molybdenum!VI) and tungsten(VI) species, the competition for formation between carboxylato peroxo complexes and non- 2- 2- carboxylato species such as CM(0 Z ) 4 3 (high pH) and [H_0.(0_), 232422 (H_0) 3 (low pH) is demonstrated by the formation of the latter species in our attempted preparation of malonato, succinato and adipato peroxo complexes, and an explosive (when dry) mixture of the two in our attempts to isolate a salicylato peroxo complex. The competition inherent in these systems is evidenced further by the failure of the unsaturated carboxylic acids mandelic (phenyl- glycollic) acid, atrolactic [methyl(phenyl)glycollic) acid, and 3-phenyl lactic acid, all capable of five-membered ring formation, to form 1:1 peroxo complexes with molybdenum(VI) or tungsten(VI) at any pH or 81 r^^N-COjH H 0 2C. CH 2.C02H H02C.(cHa)a.C02H \I ^ ^ O H salicylic acid malonic acid succinic acid H Me H OH CH2—| C02H 1 C 0 2H I C02H J °„ mandelic acid atrolactic acid a 3-phenyIIactic acid Fig. 2.8 CARBOXYLATE LIGANDS INCAPABLE OF FORMING PEROXO COMPLEXES WITH MOLYBDENUM(VI) and TUNGSTEN(VI) peroxide concentration. The structures of these carboxylic acids are shown in Fig. 2.8. The electron-withdrawing properties of the aromatic rings present in these carboxylates clearly detract from their ability 2 - to coordinate with the [MO(02)23 unit enough for CM203(02>4(H20)2] to be formed preferentially, for it was the potassium salt of this dimer that we obtained in all three cases. The cases of lactic acid, MeCHtOHICO^H, and 2-hydroxybutyric acid, EtCHtOHJCO^H, are even more fascinating, since both carboxylates are able to form a five-membered ring with a metal, do not possess strongly electron-withdrawing aromatic substituents, yet still do not form 1:1 peroxo complexes with molybdenum!VI) or tungsten!VI). All synthetic strategies have failed, including attempted recrystallisation of known CM02(lactate)2]2 (M = Mo, W) complexes1*6 from HgOg. In all cases, 2- the 1H2°^2^ dimers are formed. This again emphasises the stringent criteria required of a carboxylate ligand for incorporation into a peroxo complex of this type. In all cases the formation of the carboxylato peroxo complex depends markedly on the pH of the reaction mixture. In general, high pH ( >7-8) 2- leads to formation of CH(02 >^3 , while low pH ( <2-3 ) leads to 2- formation of the very stable [M2°3*°2^4^H20 ^21 dimers. A relatively small pH window therefore exists for the formation of pure carboxylato peroxo complexes, and hence, as we have seen, the structure and coordinating ability of the carboxylate are of paramount importance. The oxalato, citrato, tartrato, malato, glycollato and tartronato 1:1 peroxo complexes are all formed at the natural pH of the reaction 83 solutions - in all cases the molybdenum analogue being formed at a faster rate and in better yield than the tungsten one. There are obviously slight differences in the pH of formation between the molybdenum and tungsten complexes for any one carboxylate, with the pH being lower for the tungsten complexes. This factor, along with the far superior speed and quality of crystallisation of the molybdenum analogue, makes the 1:1 glycollate system a possibility in terms of separating the two metals [see Section 2.2.4(a)]. In the case of tartronate, the tungsten analogue does not form in isolable yield at all. The quinato and gluconato oxoperoxo molybdate!VI) complexes both form as yellow oils at around pH 3.5, but only after the reaction solution has been acidified from the natural pH of above 7 - to preclude 2 - the presence of [MotO^) ] . The intense red colour of the tetraperoxomolybdate(VI) species acts as an effective indicator in . . . 2 - acidifying solutions to the correct pH for [Mo0(0 ) (L)] formation, and the "endpoint" when orange turns to yellow is very sharp. The gluconato oxodiperoxo tungstate(VI) complex is the only case of a tungsten complex needing pH adjustment for formation. The natural pH 2- sees a yellow solution due to the CW(02> ^3 anion; pH adjustment to 2- about pH 2.5 allows formation of [W0(0^) (glucon)] . In the case of quinate, the tungsten complex does not form at any pH in isolable yield. This system, as is the case with tartronate, could form the basis for a separational method for molybdenum and tungsten, were it not for the high cost of the carboxylate ligand. 84 Having set out the general factors involved in the formation of the 2 - 1:1 complexes CHO, a detailed account of their structure, stability, spectroscopic properties and reactivity will now follow. 2.2.1.3 Structure and Vibrational Spectra of C M Q t O ^ d ) 12~ (M = Ho.W); The X-Ray Crystal Structure of K CMotO,)_(give)1.2H 0 2 2 2 2 T The carboxylato oxodiperoxo molybdate(VI) and tungstate(VI) complexes are isolated as potassium salts in the form of yellow crystals or powders (molybdenum) or white crystals or powders (tungsten). The exceptions are the salts K2[M0(02)2(glucon)].2H20 (M = Mo, W) and K2CMo0(02 )2(quin)].2H20, which are precipitated from solution as sticky oils, which can be dried in a vacuum dessicator and crushed to homogenous but hygroscopic powders. All of the salts analyse to dihydrates, except for the citrato species, which analyse as hemiperhydrate trihydrates, by elemental analysis, peroxide titration (both iodometric titration of coordinated peroxide, and colorimetric titration of perhydrated peroxide - see 95 Section 3.5.1 for latter), and X-ray crystal structure determination. The complexes are stable in air and light, stable to impact, do not explode in a naked flame, and may be stored indefinitely in a refridgerator (they tend to slowly lose oxygen if stored at room temperature). All are extremely water-soluble and in general stable in aqueous solution (see Section 2.2.1.4). 85 The vibrational spectra of the peroxo salts display the character istic infrared and Raman bands expected for oxo-peroxo complexes of early transition metals (see Fig. 2.9). In addition to the bands arising from the carboxylate ligands, bands are seen due to the M=0 group [v(M=0)3, and the MtO^) group [three modes, as expected for a C2v triangular bidentate M(02) unit7*1*’13114 : the 0-0 stretch, vO(O-O), and symmetric and asymmetric metal-peroxo stretches, vS{M(02)> and 3 S v {M(0 )}]. Of these vibrations, the v(M=0) and v(0-0) stretches are strong in both infrared and Raman spectra, and polarised in Raman spectra of aqueous solutions; the vs(H(02 )> stretch is stronger in the infrared than the Raman, and polarised in Raman spectra of aqueous solutions; conversely, the v {M(02 )} stretch is stronger in the Raman than in the infrared, and depolarised in the Raman spectra of aqueous solutions. The frequency of the v(M=0) stretch varies from 985 to 930cm \ depending on the nature of the carboxylate ligand; the strength of the M=0 bond is reliant on the effect of the heteroligand on the metal, especially as one of the coordinated oxygen atoms of the ligand is trans-axial to it (see later). The peroxo and metal-peroxo stretches again vary with the carboxylate, as the heteroligand affects the delocalisation of electronic charge on the peroxo group. The v(0-0), vS{M(02)} and v3S{MC02 )> stretches lie in the ranges 880-830, 660-620 and 610-565cm_1 respectively. The Raman spectra of aqueous solutions of the complexes, apart from yielding information on the polarisation of the bands, offer a crude guide to whether the [M0(02 )2(L)3^ moiety retains its solid state 86 1. METAL 0X0 STRETCH 2. PEROXO STRETCH 3. M ETAL PEROXO S T R E T C H E S M(02) '0m=o ^ 0 - 0 t M / a. Symmetric /• II MI \ / 0 ^— Ml V 1 -1 - 1 980- 910 cm 8 9 0 - 8 4 0 cm 6 5 0 - 620cm -1 Polarised in Raman of soln. Polarised in Raman of soln. (polarised) f M . , M * b. Asymmetric y// * \ /// \A * 0 0 0 o fM S sym. asym. -1 6 0 0 - 560cm Two bands seen for dioxo species (depolarised) F ig .2.9 Vibrational Bands (approx, frequencies apply to Group IVa-Vla metals) structure in solution. The value of this technique has already been discussed in relation to the detection of the conversion of oxalato monoperoxo species to oxalato diperoxo species in aqueous solution. Of the new carboxylato peroxo complexes prepared, only the citrato 2- complexes [M0(02)2(cit)] (M = Mo, W) show signs of appreciable variation of structure using Raman spectroscopy of their aqueous solutions. The basic structure obviously persists, but a large degree of broadening of bands (there is obviously some broadening in all cases) indicates probable interactions of the free carboxylic groups of the citrate ion with the metal. This is discussed in more detail in Section 2.2.1.4. There is also an appreciable broadening of Raman bands 2- in aqueous solutions of the [M0(02)2(tart)] complexes, indicating more than one species in solution (see Sections 2.2.1.4 and 2.2.2). Full vibrational data for solids and their aqueous solutions appear in Table 2.1. The oxodiperoxo species all presumably involve bidentate co ordination of the heteroligand through the carboxylate oxygen donor site and the hydroxyl oxygen donor site o to it. The ability of a-hydroxy carboxylic acids to coordinate in this way was demonstrated by the X-ray 95 crystal structure of K2CMo0(02)2(cit)].0.5H202.3H20 , in which the hydroxyl and central carboxylate groups of citric acid engage in co ordination. The formation of a strong equatorial Mo-0 bond by the hydroxyl oxygen allows this unusual mode of coordination to occur. ICQ Citrate normally functions as a multidentate ligand , often being able to lose all four ionisable protons to coordinate in tetradentate fashion.150 2 In only one previous case had such bidentate 88 2,1 VibraUor«l.Dit«- for_t:1_Het«l:C>rboxvl«te MolvbdenumCvn andTungstenfVI) Carboxvlato Peroxo Complexes Complex Vibrational Data ( c m 1)3 v(M=0) v (0-0) v SCM(02 )] vaS[M(0 K*CMo0(0 ) (ox)] IR 972vs 872s 661m 606m c c i R 965(10) 880(9) 655(5) 587(7) R 968(10) 876(8) 653(8) 588(7) K2tW0(02 )2(ox)] IR 985s 871m 661s 614m 855m R 970(10) 885(9) 645(4) 592(7) R 961(10) 856(9) 649(4) 595(6) K2CMo O (0 )(o x )(H20)3.H20 IR 9758(b) 870s 656m 601m 915m(c) R 962(8)(b) 877(6) 653(4) 581(5) 909(7)(c) R 964(10) 875(9) 640(6) 584(7) MW0-(CJ )(OX)(H-0)].H 0 IR 985vs(b) 869m 659m 613m 2 2 2 2 2 919m(c) 853m R 966(9)(b) 877(8) 655(6) 592(7) 908(4)(c) R 967(10) 876(8) 650(5) 596(7) K [HoO (ox)].2H 0 IRd 903s(b) c 3 Z 869vs(c ) Rd 901(b) 869(C) (Ph P)_[Mo0(0CM CM ) CM (ox)] IR 962s 871s 648s 585s 851s R 942(9) 874(4) 616(6) 588(5) R6 943(8) 870(5) 616(6) 583(4) cont./ 89 Table 2.1 (cont.) — 1 o Complex Vibrational Data (cm ) v(M=0) v(0-0) v S[M(02)3 vaSCM(02)3 K2[Mo0(02)2(cit)3.0.5H202. IR 957s 875m 653m 603m 861s 3H 0 R 962(10) 877(8) 652(6) 590(7) c R 959(10) 870(9) 640(6) 583(7) K2tWO(02 )2(cit)].0.5H202 . IR 963s 875m 655w 602w 862w 3H,0 R 952(10) 848(9) 637(7) 580(8) c R 918(10) 842(8) 629(5) 574(6) K2CHo0(02)2(glyc)].2H20 IR 937vs 849s 635m 584m 922vs R 943(10) 864(9) 642(5) 596(6) 925(7) R 958(10) 868(9) 639(5) 581(6) 930(7) K2CW0(02)2(glyc)].2H20 IR 940s 84 1m 624s 581m 918s 827s R 938(9) 840(8) 620(4) 572(6) 922(7) R 957(10) 854(9) 632(6) 565(7) 932(8) K2CMo0(02 )2(tart)3.2H20 IR 937 vs 871s 656s 608m 859s R 936(9) 874(8) 637(6) 600(5) R 954(10) 862(9) 640(6) 602(5) Ko[W0(0_)_(tart)3.2H_0 IR 939s 859w 641m 603w 2 2 2 2 R 934(10) 880(8) 635(6) 598(5) K_[Mo0(0 ) (mal)3.2H 0 IR 930s 855s 2 2 2 2 633m 584m R 949(10) 873(8) 638(6) 590(7) R 965(10) 875(8) 641(5) 580(6) cont./ 90 Ttbltt (cont.) Complex Vibrational Data (cm V v(M=0) v(0-0) vSCM(02)3 vaS[M(0 K IWOIO ) (mal)].2H.0 IR 925s 830s 625m 575w ‘ 2 Z t R 945(10) 845(9) 625(6) 565(8) K_[MoO(0_) (glucon)].2H 0 IR 940s 851s 633s 585m c 2 2 ‘ R 942(10) 864(8) 634(5) 579(7) R 958(10) 870(9) 636(4) 577(7) K2CW0(02)2(glucon)].2H20 IR 936br 878m 621m 579w 831m R 943(10) 876(5) 624(2) 556(7) 846(8) R 952(10) 849(7) 628(2) 565(5) K [MoO(0 ) (quin) ].2H.0 IR 935s 848s 627m 580m i 2d ‘ R 946(10) 868(7) 631(4) 588(7) R 952(10) 870(8) 640(6) 585(7) K_[Mo0(0_)_(tartron)].2H 0 IR 947vs 855vs 640m 584s 2 2 2 d R 958(10) 874(8) 640(6) 585(7) R 968(10) 875(8) 640(6) 585(7) a Data for solids or (underlined) for aqueous solutions; relative Raman intensities given in parentheses. d Ref. 147 e In dichloromethane solution. 91 coordination been crystallographically established153 (in this case due to other sites being blocked by a triethylene tetramine ligand). The _Xrrav Crystal Structure of K^lttoiO^tgivc)] .2H20 We have been able to obtain the glycollato oxodiperoxo molybdate(VI) salt K2CHo 0(02 )2(glyc)3.2H20 in a crystalline form suitable for X-ray crystal structure determination. The discussion of the structure that follows may be taken as a model for that of the other new carboxylato peroxo complexes prepared, K2CM0(02)2(L)].2H20 (L = tart, mal, glucon, quin, tartron), in which the heteroligand must also bond in bidentate fashion through adjacent carboxylate and hydroxyl groups (see Fig. 2.7) as part of a pentagonal bipyramidal structure. There have been several X-ray crystal structure determinations on 15* glycollato complexes . but this is the first glycollato peroxo complex to be so characterised. The structure (determined by R.D. Powell and Dr.A.C. Skapski of Imperial College Chem. Dept.) proves that the unusual mode of bonding through a hydroxyl group a to the carboxylate group also involved in bonding is again observed, as discussed for the citrato peroxo complex earlier. Fig. 2.10 shows the structure of the 2- [Mo0(02)2(glyc)3 anion. Bond lengths and angles are given in Table 2.2. A representation of the unit cell of the salt is depicted in Fig. 2.11. The coordination about the molybdenum atom is essentially pentagonal bipyramidal. The axial positions are occupied by the terminal oxo © ligand [Mo-0 1.686(6) A] and the oxygen atom from the deprotonated 92 carboxylate group of the glycollate ligand [Mo-0 2.239(6) A]. The equatorial positions are occupied by the two slightly asymmetrically- 2 o bound n -peroxo ligands [mean Mo-0 1.927(4) and 1.966(4) A ; mean 0-0 1.471(7) £] and the deprotonated hydroxyl group of the glycollate [mean o Mo-0 1.991(5) A]. The 0-0 distance is typical for an early transition metal n^-peroxo complex.^^ The deprotonated hydroxyl group clearly assumes the equatorial rather than axial position in order to form a strong bond. The long axial Mo-O(carboxylate) distance could be due to the trans-influence of the oxo ligand. The crystal structure is held together by ionic attraction between the potassium cations and the complex anions, and hydrogen-bonding involving the water molecules. One of the potassium cations K (1) has eight oxygen near neighbours at distances 2.73-2.90 A, while K(2) is e surrounded by nine oxygen atoms at 2.81-3.12 A. Each of the two water molecules of hydration forms two OH— 0 hydrogen bonds to oxygen atoms of the complex anion. The similarity in the physical state of the salts K2[M0(02)2- (glucon ) ]. 2H20 (M = Mo, W) , and K^MoO (02 ) (quin) ]. 2H20 , all of which are isolated as sticky oils which dry to hygroscopic solids, can be attributed to the fact that both the gluconate and quinate ligands contain a number of free hydroxy groups not involved in coordination to the metal. These groups probably participate in extensive hydrogen bonding, making the complexes hygroscopic. All the other oxodiperoxo complexes are in the form of non-hygroscopic powders or crystals. 93 0 2 Fig-2.11 The Unit Cell of K2[Mo0(02)2(g|yC)].2H20 Table 2.2 BontiLLengthf (A) and Analea t°\ in K^tHoOlQ. (estimated standard deviations (o.S«di-*.) in Pfrrgnth& I&tl 2.239(6) Ho(1l-OC11 1.686(6) Mo(1)—0(7) Mo(1)-0(4) 1.961(4) Mo(11-0(2) 1.970(4) on CM CO «*■ Mo(1)-0(3) 1.926(4) Mo(1)-0(5) Mo(1)-0(6) 1.991(5) 1.472(7) 0 (2)-0(3) 1.469(7) 0(4 )-0(5) C (2 ) -0(7) 1.279(9) C (2) — 0(10) 1.231(11) 1.528(9) C(1) -0(6) 1.413(11) C (1 )-C(2) 0(1)-Mo(1)-0(2) 99.9(2) 0(1)-Mo(1)-0(3) 101.6(2) 102.4(2) 0(1)-Mo(1)-0(4) 99.8(2) 0(1)-Mo(1)-0(5) 87.0(2) 0(1)-Mo(1)-0(6) 92.8(2) 0(3)-Mo(1)—0(5) 44.5(2) 0(2)-Mo(1)-0(3) 44.3(2) 0(4)-Mo(1)—0(5) 89.9(2) 0(2)-Mo(1)-0(6) 89.3(2) 0(4)-Mo(1)—0(6) 75.4(2) 0(7)-Mo(1)-0(1) 168.2(2) 0(7)-Mot 1)-0(6 ) 0(7)-Mo(1)-0(3) 8 6 . 6 ( 2 ) 0(7)-Mo(1)-0 (2) 79.9(2) 0(7)-Mo(1)-0(5) 86.4(2) 0(7)-Mo(1)-0(4) 80.8(2) Mo(1)-0(3)-0(2) 69.5(2) Mo(1)-0(2 )-0(3 ) 66.3(2) Mo (1)—0(5)—0(4) 68.9(2) Mo (1) -0 ( 4 )-0(5) 66.6(2) Mo(1)-0(7)-C(2) 115.8(4) Mo(1)-0(6)-C(1) 120.3(3) 0(7)-C(2)-C(1) 114.9(7) 0 (6 )-C(1)-C(2) 111.6(6) 0(7)-C (2)-0(10) 124.9(6) 0 ( 10)-C(2)-C(1 ) 120.2(7) 96 2-2-1.* ilsj_Pf.95No and 13C N.H.R. Spectroscopy to Investigate the JLtr.ustMry.pf [HO(02)2(l)l2~ Species in_Aqueous Solution 95 The technique of Mo n.m.r. spectroscopy has proven a useful and sensitive tool for the study of molybdenum peroxo complexes in aqueous solution (Ref.48 and references therein). Its use in the study of carboxylato peroxo molybdate(VI) complexes proves to be no exception to this rule. Changes in the carboxylate ligand (L) in the complexes 2- g5 [MoO(0^)2 (L)] result in appreciable shifts of the Mo resonance (see Table 2.3). The sensitivity of 95Mo n.m.r. shifts towards change of co- 155 ordinated ligand has been noted before. Information can also be obtained from the band widths. The spectra of the glycollato (6 -220.6 p.p.m., relative to 2- [Mo0^3 ), tartronato (6 -227.7 p.p.m.). malato (6 -233.0 p.p.m.), and quinato (5 -245.0 p.p.m.) oxodiperoxo molybdate(VI) complexes show single relatively sharp resonances. This is indicative of one molybdenum environment existing in solution, and the pentagonal bipyramidal structure of the complex anion being retained. The far broader resonances obtained for the citrato (ca. 6 -247 p.p.m.) and gluconato (ca. 6 -234 p.p.m.) complexes suggests some change in structure and larger variations in molybdenum environment, possibly due to interaction of free carboxylate or hydroxy groups respectively. The situation regarding the [Mo0(02) (tart)]2" species is worthy of special note. Two broad resonances at 6 -231.7 and -252.0 p.p.m. are seen, clearly suggesting the existence of more than one discreet species in solution. This evidence, allied with the failure to obtain pure 97 Table 2.3 95f4o N.H.R, Data for Holvbdenum (VI) Carboxvlato Peroxo Comolexea 95 a Complex Mo n.m.r. resonance (p.p.m.) K [Ho0(0 ) (ox)J -228.3 K2CHo 02 (02)(o x )(H20)].H20 -229.7 ♦ 4.4 K2CMo 03 (o x )].2H20 ♦ 5.1 K2CMo0(02 )2(cit)].0.5H202.3H20 -247br K2[Mo0(02 )2(giyc)].2H20 -220.6 K2CMo0(02)2(tart)].2H20 -252.0 -231.7 K2tMo0(02)2(mal)].2H20 -233.0 K2CMo0(02)2(glucon)].2H20 -234br K2[Mo0(02)2(quin)].2H20 -245.0 K2tMo0(02)2(tartron)].2H20 -227.7 -235.0 K4[Mo 202(02 ,*,W « ' 1-*M2° a In OgO solution; shifts given relative to [MoO^]2- br Broad resonance 98 samples of the potassium salts of the molybdenum (and tungsten) tartrato 13 oxodiperoxo complexes, and C n.m.r. (see below) and Raman evidence for more than one tartrato peroxo species in solution, prompted us to consider the possibility of a dimeric species existing. By altering the M:tartaric acid ratio we were able to obtain a pure dimeric peroxo- molybdate(VI) species containing a bridging tartrate ligand (see Section 2.2.2). Formation of such a species is clearly responsible for 95 the double Mo resonance seen in aqueous [Mo0(02 )2 (tart)].2H20. Unfortunately, the insensitivity of the 183 W nucleus has prevented us from obtaining useful n.m.r. spectra of the carboxylato peroxo complexes of tungsten(VI). We have not measured n.m.r. spectra 13 either, since they are likely to be complicated in some cases, and C n.m.r. is much more informative. 13 The C n.m.r. spectra of the carboxylato oxodiperoxo molybdate(VI) and tungstate(VI) complexes (data presented in Table 2.4) provide information on the degree to which the species retain their structure in aqueous solution, and on which groups of the carboxylate are involved in coordination to the metal. All resonances of carbon atoms proximate to the metal are generally shifted downfield upon complexation to the metal d° system, as the ligand donates electrons and the carbon nuclei deshield. The largest shifts are obviously experienced by the carbon atoms directly adjacent to the oxygen atoms involved in coordination. Downfield shifts of this type are reported for picolinato and dipicolinato peroxo complexes of molybdenum(VI) and tungsten(VI) by 97 Mares and co-workers. 99 The C n.m.r. spectra of aqueous solutions of K2 [M0(02 )2 (glyc)].- 2H^0 (M = Ho, W) are consistent with retention of solid state structure in solution, with resonances due to coordinated carboxylate (6 186.6 p.p.m. for M = Mo ; 187.5 p.p.m. for M = W) and coordinated hydroxyl bearing carbon (6 74.9 p.p.m. for M = Mo ; 75.3 p.p.m. for M = W). 2- The aqueous [Mo0(02 )2 (mal)3 species demonstrates the difference in 13 C n.m.r. shift for coordinated and non-coordinated carboxylate carbon atoms (6 186.3 and 178.6 p.p.m. respectively). The coordinated hydroxyl-bearing carbon appears at 6 82.0 p.p.m., and the (-CH2~) carbon at 6 44.2 p.p.m. The way in which the 13 C n.m.r. shifts of carbon atoms not immediately adjacent to coordinated oxygen atoms move downfield to some extent as well as those that are is well illustrated by comparison of 13 the C n.m.r. data from aqueous solutions of K2 CMo0(02 )^(quin)].2H20 and K2 [M0(02 )2 (glucon)].2H20 (M = Mo, W) (Table 2.4) with those from the respective free acids in solution (Table 2.5). The resonances seen for aqueous quinic acid [6 180.6 (Cj) ; 78.6 (C2) ; 77.9. 73.0, 69.4 C4-C5) *• *3.2, 39.7 H C X X pO H 2 Fig. 2.12 Quinic Acid 13C N«m.r. Dat«_for Holvbdanum (VI) and Tungsten (VI) Carboxvlato Peroxo Complexes 13 a Complex C n.m.r. resonance (p.p.m.) K [MoOC0 ) (ox)3 170.7 2 2 2 169.0 K_[MoO(0 ) (glyc)].2H 0 186.6 74.9 2 2 2 2 K [WO(0 ) (glyc)].2H 0 187.5 75.3 2 2 2 2 K [MoO(0 ) (cit)].0.5H_0.3HO c.179 88.2 47.1 2 2 2 2 2 2 K [Mo0(0 ) (mal)].2H 0 186.3 82.0 44.2 2 2 2 2 178.6 K2[Mo0(02)2(tart)3.2H20 185.9 89.5 185.5 88.8 k4cho 2o 2(o 2)4(c4h2o 6)].4h2o 186.0 89.0 (NH, ),[Mo 0 JOJIC HO)].4H 0 185.0 84.9 44 2224426 2 K,[W„0 (0 1 ICH0)].4H,0 175.1 88.5 4 2 2 24 426 2 cont./ Table 2.4 (cont.) 13 a Complex C n.m.r. resonance (p.p.m.) Kg[MoO(0 )g(quin)].2HgO 187.6 91.1 45.2 78.5 41.3 73.8 70.1 K [MoO(0 ) (tartron)3.2H 0 181.0 70.3 2 2 2 171.1 Kg[Mo0(0 ) (glucon)].2H 0 185.3 86.4 2 ■ 75.0 74.4 73.9 65.3 Kg[WO(Og tglucon)].2H 0 173.6 86.3 2 2 75.2 74.5 73.5 65.9 In DgO solution; shifts relative to external DSS 102 13 H M U E.g C-JLjn,r, iU.ti f o r Free Carboxylic Acid» 13 a Acid C n.m.r. resonance (p.p.m.) Glycollic acid 178.8 62.0 (♦)-Tartaric acid 176.8 74.4 0,L-Malic acid 178.6 69.3 40.8 176.7 Quinic acid 180.6 78.6 43.2 77.9 39.7 73.0 69.4 Gluconic acid 181.2 76.9 75.6 74.6 74.2 65.8 a In 0^0 solution; shifts relative to external DSS b Aqueous potassium gluconate, to give free acid 103 2- [MoO(Og)g(quin)] species [6 187.6 (C^ ; 91.1 (C2) ; 78.5, 73.8, 70.1 (C4-C6 ) ; 45.2, 41.3 (C^. C^) p.p.m.]. A key to the carbon numbering scheme appears in Fig. 2.12. 13 The C n.m.r. spectrum of an aqueous solution of the citrato peroxo compound K^CMoOCO^)^(cit)].O.SH^O^.SH^O shows just one very broad resonance, as well as those expected for the coordinated hydroxyl bearing carbon atom and (-CH2-) carbon atoms. This is probably due to the interaction of the terminal free carboxylate groups with the moly bdenum atom, which is to be envisaged in the light of the fact that citrate often loses all four ionisable protons in solution, and can co- 150-2 ordinate in tetradentate fashion. As discussed earlier, the mononuclear tartrato oxodiperoxo species 95 K„[Mo0(0,J„(tart)].2H„0 shows at least two resonances in the Mo n.m.r. 2 2 2 2 spectrum of its aqueous solution. The aqueous species also shows at 13 least two C resonances in each of the carboxylate (6 185-186 p.p.m.) and hydroxyl-bearing carbon (6 88-90 p.p.m.) regions of the n.m.r. spectrum. This indicates that the bridged-tartrate peroxo complex described in Section 2.2.2 is forming to some extent in solution, from 2- the mononuclear [MoO(0^)2(tart)] species with the release of free tartrate. Gillard and co-workers1** have observed 13C shifts of 6 183.8 and 88.4 p.p.m. for carboxylate and hydroxyl-bearing carbons in the double-tartrate-bridged oxo species [Mo20^{(+)-C^H20g}2]*". The behaviour of the malato oxalato molybdate(VI) and tungstate(VI) compounds K2CM0(02 )?(mal)].2H20 (M = Mo, W) in solution, and indeed their formation at all. merits further consideration in the light of a 104 88 recent paper by Djordjevic and co-workers , in which conversion of malate and malonate to oxalate in aqueous peroxomolybdatetVI) solutions is reported. The publication describes how K^CMoOfO^^ox)] is formed in 30Z and crystalline yield from a solution of Mo03, KOH, malonic acid (1:1 Mormalonic acid) and excess at pH 3. Indeed, the product was characterised by X-ray crystal structure determination with somewhat 8 better refinement than that achieved by Stomberg 6 in 1970. Similar results are reported using malic acid, and a mechanism involving several intermediates (including malonato and malato peroxo molybdate(VI) complexes) and rearrangements of a complex oxidative decarboxylation is mooted. We find that isolation of malato oxodiperoxo molybdate(VI) salts K2CMO( ) 2(mal)3.2H20 (M = Ho, W) is possible using the relevant potassium metalate, K^CHO^], malic acid and excess H^O^, provided that the salts are precipitated from solution using ethanol almost immediately. The pH of solution in our preparation is slightly higher than that encountered using Mo03 , KOH, and malic acid (3.4, as opposed to 3.0). Nevertheless, we find that upon leaving a solution of K^CMoO^], malic acid and excess to stand, or upon recrystallisation of K2CMoO(02)2(mal) ]. 2H20 from H2°2‘ crystals 0,f K2CHoO(02)2(ox)] indeed appear, indicating that a slow and complicated rearrangement and de carboxylation of the malate ligand has occurred. As far as the malonato system is concerned, we find that a solution \ equimolar in K_[MoO 1 and malonic acid with excess H.O gives 2 4 2 2 K2[Mo203 (02)4 (H20)2].2H20 when ethanol is added immediately, but eventually yields crystalline KglMofOg) (ox)] if left to stand. 105 2.2.2 Prepration of 2:1 Hetal:Tartrate Peroxo_Complexes : The X-Ray Crystal Structure of K4CHo202(02)2(C4H20£n^4H20 As reported in Section 2.2.1, efforts to isolate pure mononuclear tartrato molybdate(VI) and tungstate(VI) salts, K C MO(0 ) {C H 0.)].2H 0 2 2 2 4 4 6 2 drew attention to the fact that a species of different stoichiometry was present in small quantities. Evidence for such a species lies in the slightly errant elemental and peroxide analyses obtained for the salts, but more so from the spectroscopic properties of their aqueous 13 95 solutions. The Raman, C n.m.r. and Ho n.m.r. spectra all indicate the existence of more than one tartrato peroxo species in solution. Our thoughts that a dimeric tartrate-bridged peroxo species could exist proved correct. Reaction of KgCMoO^] and (+)-tartaric acid (in a 2:1 Mo:tartrate ratio) with excess aqueous hydrogen peroxide gives rise to a deep-red 2- solution, clearly containing a predominance of CHotO^)^] . Adjustment of the pH to 4.0 yields a light-yellow solution, from which recrystall- o isation with ethanol at 0 C yields yellow crystals analysing to K4CMo202(02)4(C4H206)].4H20. Interestingly, if the deep-red solution is left to stand for around 2 - five minutes the transition from red CMo(02 ) 43 to yellow [Mo202 (02)4 IC4H206)]*“ occurs automatically without acidification, as pH adjusts itself to around 4 [see Section 2.2.4(b)for details of pH study]. 106 The X-Ray Crystal,Structure of [Mo^0^(0^(C^H^O^)3_,4H^0 We report here the X-ray crystal structure of the novel tartrate- bridged peroxomolybdatetVI) complex K [Mo 0 (0 ) (C H 0 )].4H_0 (see t 2 2 2 4 4 2 6 2 also Ref.156). The structure was determined by R.D. Powell and Dr.A.C. Skapski of Imperial College Chem. Dept.). This complex is not only the first tartrato peroxo complex to be structurally determined by X-ray, but also the first complex to contain a single tetradentate bridging tartrato ligand. The tartrate ligand has two sets of adjacent carboxylate and hydroxyl groups each capable of co ordination to the metal. In this complex, each set bonds in bidentate fashion to an oxodiperoxomolybdate(VI), [MoOfOglg], moiety - giving pentagonal bipyramidal coordination to each of the molybdenum atoms, and making the tartrate ligand tetradentate overall. The result is a bridged structure that is symmetrical about the central C-C bond of the bridging tartrate ligand. The structure of the £Mo2°2*02^4 *C4H206 ^ * anion is shown in Fig. 2.13, and the bond lengths and angles are listed in Table 2.6. The coordination found about each molybdenum atom is essentially that of a pentagonal bipyramid, similar to that recently found in the complexes K2CMo0(02 )2(cit)] .0.5H202.3H2095. and K2CM<\(°2} 2(9lyc’ ] * 2H2° (this wcrkJ see Section 2.2.1.3 and ref.157). The axial positions are occupied by the terminal oxo ligand [mean Mo-0 1.685(8) X] and the oxygen atom from the deprotonated carboxylate group of the tartrate ligand [mean Mo-0 2.248(7) X]. The equatorial positions are occupied by the two slightly 2 o asymmetrically bound n -peroxo ligands [mean Mo-0 1.938(8) A and 107 © 0 . 1.973(8) A ; mean 0-0 1.481(11) A), with the deprotonated hydroxyl group of the tartrate ligand making up the distorted pentagon [mean Mo-0 © 2 1.962(7) A]. The 0-0 distance is typical for a n -peroxo complex of an early transition metal.The deprotonated hydroxyl group takes up the equatorial position rather than the axial position so as to form a short, strong bond. Again, there is a long Mo-O(carboxylate) distance, probably due to a trans-influence from the oxo ligand. The crystal structure is held together by the ionic attraction between the K* cations and the complex anion, and by hydrogen bonding involving the waters of crystallisation. 108 4 4 c n CM o c n 4 4 1 • r — 0 0 c n CO CO COCO cn cn cn cn cn cn cn cn cn cn «*•cn cn cn cn ** cn M* *# CO CO •>4- men an cn an CO CO « M « CO r — i n r — i n i n o c n CT» d— c o i n an a t p- an CM CO *4- -«r co o cn in co an *- r~- cn cn CM p- p- cn * -a n o co -4- *- p- o — d X T o ir> CO CO 4 4 i n o > C O i n i n CM d— CO 4 J CM c n COCM c n an O ) c n CM m4 - CM CM i n CM CO ** m cn an CM cn o CO in cn cn p- co cn «*T- CO in o c o o co cn •4> o »_ p- C cn CM co co co o CO COCM co CO o co CM co cn co in cn CM co P - CO * - CM ®— r»CO • CM d *“ CM . d— «M d"" d “ ®*» r* XF- »“ r* z 1 H ■ * a u a 4I «**M» «**M -O-M, **■M — Mi — * 4*M *—M — »M — Mi — i» c •H *4" 4*"M cn — M. 4»*M* in i n *»M —M m CMCM CM cn CM CM cn ' t , •4- cn in T- i n *4- CM «<• •4- cn cn CM CMCMCM — Mi — »• T- *- CM CM a M-* V“* m m * MM* *M* CM MM* m m * CM CMCMCM MM* •M— m m * «W* CM m m * Mm * m m *— m. — — M — Mi <*^ — k MM* m m m m * w <*-*> H 4 4 * o O V O o o o O W MM* m m * o o O o MM* O o O c n *• CM cn CM ««■ o l O 1 o i o l O 1 1 o 1 l o o o O 1 1 l 1 u U l 1 1 ’ w Mm * m m * m m * w * 1 o l • o «-M» — k —Mk —M* l «*M» 1 — M* 1 4*-M I *—M 1 — M. 1 1 1 I 4—M — 'M *—«* •— * 1 1 —M — — o o O o U o — Mi CM —M* 4 d“ * —M. — k 4 CM c n «** •*■* d— 4— . V* **•* d— — M. CM CM — Mi — Mi CM CM CM CM — *• — *■ CM cn cn i i 1 1 1 1 CM o MCM V i n 4 CM CM cp-> a ■ o IL O c 1 * < < • ■D 1 3 C C M • 1 4 J « c n CM CM —— d— 4 i n — k — k — •k — — Mk 4— M — k. — (+)-tartaric acid with excess H ^ . Traces of yellow [W(0 ) ]2“ disappear after around 15 minutes of stirring, leaving a clear solution from which a white crystalline complex was precipitated, analysing to K4 ^W2°2 ^°2^4 ^C4**2^6^ * *H2^‘ A detailed pH study of the formation of this complex, and its molybdenum analogue, as well as of the sodium salts of both the molybdenum and tungsten complexes, appears in Section 2.2.4(b). The vibrational data for K4£M2°2*°2*4*C4H2 ° 6 ^ ,4H2° (M = Mo, W) and (NH^^4^m °2 ®2 ^ 2 ^4*C4H2 °6^ ’*H2 ° ’ latter salt having been isolated from a 2:1 (NH,)„[MoO.]:tartrate solution in excess H <) at pH 1-2, are 4 2 4 2 2 listed in Table 2.7. The potassium salts show infrared and Raman bands at around 930cm * [v(M=0), polarised in Raman spectra of aqueous -1 - 1 s solutions], 850cm [v(O-O), polarised], 630cm [v {M(0^)>, polarised], — 1 as and 580cm [v {H (0^)>, depolarised] as expected for oxoperoxo transition* * metal * , complexes. , 7 (i), 13,14 The Raman spectra of the aqueous complexes indicate retention of the bridged dimeric structure in solution, since the bands listed above 95 remain relatively sharp. The Mo n.m.r. evidence confirms beyond doubt e«itlr that the novel symmetrical structure^in solution, since spectra of 0^0 solutions of [Mo202 (O2 )4(C4H2 O0)].4H2 O show a solitary, sharp resonance at 6 -235.0 p.p.m. (relative to [MoO^]2-), synonymous with a single molybdenum environment, and one symmetrical binuclear species. The C n.m.r. spectra of aqueous solutions of the bridged dimeric species K4CMo2°2<02>4 The preparation of an organic-soluble tetraphenylphosphonium salt, (PhfP ) CMo.O-(0_) (C.H-0.)], in order to investigate the reactivity of 4 4 C c C h 4 Z o the dimeric tartrato peroxo species in organic media, is reported in Section 2.2.3. Vibrational data for this compound appear in Table 2.7. 112 ***** ? a Yibr«tlon»l D»t» for 2i1 H«t»l;C»rboxtfl«f Holvbdenum (VII «nd Tuno»t»nlVI1 T«rtr«tn Pt o x o Complax«« Complex Vibrational Data (cm V V(M=0) v (0-0) vSCH(02)l vaS[M(0 V m V 2 <°2 V W 6)].- IR 929s 857s 638s 578m 847s 4H.0 R 935(10) 857(7) 640(4) 592(6) c R 954(10) 869(8) 639(5) 582(7) K4tw 0 (o ) (c H 0 IR 924s 831s 622m 570w C % % C 0 R 957(10) 852(9) 628(5) 571(6) ;h 2o R 952(10) 852(9) 628(7) 566(8) 'Ph P) tMo 0 (0 1 (C H 0 )] IR 962vs 862VS 629m 598m ^ C C C h ^ £ 0 R 960(7) 873(6) 616(4) 561(5) ,NHtlttMo202(02)4(C4H206)].- IR 976m 856s 640s 580m 952s *h 2o R 974(7) 869(10) 638(5) 585(6) 952(8) a Data for solids or (underlined) for aqueous solutions; relative Raman intensities given in parantheses 113 2-2.3 teaglivitv of Carboxvlato Peroxo Complexes of MolvbdenumfVI> 2 - In order to investigate the reactivity of the tHoO10^)2(ox)] and [ M o ^ ^ O ^ (C^H^Og) species towards small molecules (e.g. SO^ CO^, carbon-carbon double bonds (i.e. epoxidation of alkenes), and alcohols (i.e. oxidation to carbonyl compounds), we have prepared organic-soluble tetraphenylphosphonium salts of both species. 2.2.3(a) Preparation of Organic-Soluble Carboxvlato Peroxo Complexes The (Ph^P)2[Mo(02)2(ox)) salt, prepared by reaction of the potassium salt with tetraphenylphosphonium chloride, is a light-yellow solid, and reasonably soluble in organic solvents such as dichloromethane, acetone, and, to a greater extent, in acetonitrile. It shows vibrational (infrared and Raman) bands at around 950, 870, 640 and 590cm 1 due to the v(Mo=0), v(0-0), vS[Mo(02)] and vaS[Mo(02)] modes of the oxalato oxodiperoxo molybdate(VI) complex anion (see Table 2.1), as well as ♦ bands due to the (Ph^P) cation. The Raman spectrum of a dichloro methane solution of the salt has the same profile as that of the solid, indicating retention of structure in organic media. 2 - We were unable to form tetrabutylammonium salts of [HoO(02)2(ox)] 2 - or [Mo0(02)2(glyc)] using either tetrabutylammomum chloride or tetra butylammonium hydrogen sulphate. The (Ph.P)„[Mo_0_(0_)f(C#H_0_)] salt can only be formed as an impure 422224426 light-yellow solid, and as a consequence is only sparingly soluble in organic solvents; the best solubility is attained using acetonitrile. 114 The salt shows characteristic vibrational bands at 960 Cv(Mo=0)]( 660 Cv(O-O)], 630 [v S{Mo (02)H, and 580 [vaS{Mo(02)}] cm”1. The known organic-soluble dipicolinato oxoperoxomolybdate(VI) 97 complex [Mo0(02 )(dipic)(H20)] was prepared in order to measure its Raman spectrum, which revealed bands 980. 910, 602 and 578cm \ assigned as above. 2.2.3(b) Attempted Reactions with SO^ and CO^ Group VIII metal peroxo complexes, for example (Ph P) Pt(0 ), are 3 2 2 e known to react with a variety of small molecules . such as with 23 sulphur dioxide to form a chelated sulphato complex (Scheme 2.8), 24 and with carbon dioxide to form a chelated carbonato complex (via a 18 158 cyclic peroxocarbonate intermediate, as confirmed by O-labelling ) (Scheme 2.9). S02 LV /°NC,° s\\ Scheme 2.8 c Q L O r Ls o Scheme 2.9 Ls / ° N Pt. ,C = 0 L ' V' 115 We find that there is no reaction when C02 is bubbled through a dichloromethane solution of (Ph^P)2CMo0(02)2(ox)]. The reaction of the same peroxo complex with S02 in dichloromethane yields a yellow oil, which dries under vacuum to a fluffy yellow solid, analysed to contain 55Z carbon, 4Z hydrogen and 6.6Z sulphur. The infrared spectrum of the product is different from that of the parent complex in that new bands appear at 946, 915 (broad) and 790cm \ and the bands due to the peroxo 2- groups disappear. Formation of a [Mo02(SO^)(ox)] complex is a possibility, but the strong vibrational bands expected for a species 2- containmg (SO^) coordinated to a transition metal are not observed. It is more likely that a mixture of (Ph^P)2S0^ and an oxo complex of molybdate(VI) is formed. When the reaction was repeated with the tetraphenylphosphonium salt of oxalato dioxo monoperoxo molybdatetVI), (Ph^P)2CHo02 (02)(ox)(H203, a fluffy yellow solid with an identical infrared spectrum to that described above was formed. The lack of reactivity towards small molecules shown by our carboxylato peroxo molybdate(VI) complexes is in keeping with the general trend found for Group Via peroxo complexes, which are far more reactive towards another type of diamagnetic electrophile, namely g carbon-carbon multiple bonds. On the other hand, Group VIII peroxo complexes react readily with a variety of small molecules, but only with the most electrophilic carbon-carbon double bonds. 116 2.2.3(c) Oxidative Reactivity Towards Alkenes and Alcohols Epoxidation of Alkenes There are many examples of covalent molybdenum(VI) and tungsten(VI) peroxo complexes, of general formula [MoO(0)(L)(L*)], effecting stoichiometric epoxidations of alkenes7 (see also Section 2.1.4) in the literature. In order to study the possibility of epoxidation using our organic-soluble complexes (Ph^P) ^CMoO(02)2(ox) ] and (PhPl^CMoO,,- H 0 1] we first synthesised the well-tried epoxidising agent 2 4 4 2 6 [Mo0(02 ) (HMPA)(H20)]101'112 and carried out its epoxidation of cyclohexene as a model. The complex is a fluffy yellow solid, exhibiting IR bands at 966 [v(Mo=0)], 870,869 [v(0-0)]. 645 [vS{Mo(02) H . and 582 tv8S{Mo(02)H e m " 1 We have for the first time measured the Raman spectra of the complex in solid state and acetonitrile solution. The bands assigned above are seen at 970, 873, 647 and 574 cm 1 respectively in the Raman spectrum of the solid, and at 974, 884, 650 and 567cm ^ respectively in the spectrum of the dichloromethane solution - indicating retention of structure (pentagonal bipyramidal102). We confirmed that [MoO(02)2(HMPA)(H20 )] epoxidises cyclohexene completely (using one mole of oxidant for every two moles of substrate) 2 in 24hr. in d -dichloromethane. The epoxidation is easily discernable by n.m.r. spectroscopy, with the triplet due to the alkenic protons of cyclohexene (6 5.66 p.p.m.) disappearing in favour of a triplet due to the epoxidic protons of cyclohexene oxide (6 3.05 p.p.m.). 117 For our experiments with the organic-soluble oxalato and bridged 3 tartrato peroxo complexes we used d -acetonitrile as solvent. Neither complex accomplished detectable epoxidation of cyclohexene in reaction runs of up to a week. The failure of these species to carry out epoxidation is understandable if one considers the mechanism for epoxidation of alkenes by covalent molybdenum peroxo complexes, [Mo0(0 ) (L)(H 0)], proposed by Mimoun7 (see Fig. 2.4). This involves reversible complexation of the alkene to the metal with displacement of the equatorial heteroligand, followed by irreversible electron transfer to the coordinated alkene, via a five-membered metallocyclic intermediate. The mechanism is clearly favoured in cases where the molybdenum peroxo complex possesses a vacant or easily- releasable coordination site (for complexation of the alkene substrate) and neutral overall charge. Our anionic carboxylato oxoperoxo complexes are without either of these requisites. The strongly-bound bidentate oxalate and tetradentate tartrate ligands are by no means releasable, in contrast to the easily releasable monodentate organic base ligands such as HMPA. Furthermore, the oxalato and tartrato peroxo complex anions possess overall negative charges of 2- and 4- respectively ; this alone will clearly hinder the attack of an electron-rich double bond on the metal centre. Oxidation of Alcohols There are several methods for the oxidation of alcohols to carbonyl compounds using molybdenum*VI) and tungsten(VI) peroxo complexes reported in the literature. Stoichiometric oxidations using the peroxo 118 species CMO(0 ) (HMPA)(H 0) ], [M0(0 )(dipic)(H 0)], H[Mo(00)„(pic) c c. c Z Z 2 2 2 H^O (M = Mo, W ) ^ \ and [MoOt'O^) {PhCONtPhJO}^] ^ ^ are reported. Good yields of carbonyl compounds are only obtained using the picolinato (pic) and N-phenylbenzohydroxamato complexes. This is interesting, since the former type of complex is anionic in nature, and both types contain only oxo, peroxo and bidentate hetero ligands, with no vacant or easily-releasable coordination sites. The fact that overall charge of the peroxo complex and the availability of releasable coordination sites may not be as critical here as they are in the epoxidation of alkenes provides opportunity for the use of our organic-soluble carboxylato peroxo complexes in the oxidation of alcohols. The mechanism of oxidation is more likely to involve homolytic dissociation of the peroxo group and a bimolecular radical process in this case; in other words, there is "external" rather than "internal" attack of the substrate by peroxidic oxygen. We find that at in dichloromethane at room temperature, the complex (Ph^P)2CMo0(02)2(ox)] is not capable of the oxidation of p-anisyl alcohol (4-methoxybenzyl alcohol), an activated benzyl alcohol which is very susceptible to oxidation, to the corresponding aldehyde. However, using reflux conditions and a reaction time of 3hr., a 96Z yield of p-anisaldehyde (determined by derivatisation to the 2 ,4-dinitrophenyl- hydrazone) is obtained using a 1:1 oxidant:substrate ratio in dichloro methane. A blank reaction run involving the reflux of p-anisyl alcohol alone in dichloromethane for 3hr. has shown that no determinable oxidation of the alcohol by air occurs. The residue of the oxidation is practically white, and has been shown to consist mainly of impure 119 (Fh^P)2[Ho03 (ox)] by elemental analysis and infrared spectroscopy. The need for reflux conditions in the oxidation of even the most reactive alcohols by (Ph^PJ^CMoOtO^J^tox)] severely restricts the possible synthetic usefulness of the complex. We have therefore restricted our studies of the use of the complex to two other simple oxidations. It will also oxidise benzyl alcohol to benzaldehyde, and vanillyl alcohol (4-hydroxy 3-methoxy benzyl alcohol) to vanillyl- aldehyde in 87 and 92Z yields respectively. The results are summarised in Table 2.8. Although it requires heat to effect the oxidations, the oxalato peroxo molybdate(VI) complex can be said to behave as a mild oxidant, since it does not oxidise primary alcohols through to the carboxylic acid. Mechanistically, this could well be due to the fact that water is excluded from the oxidations, since it is widely accepted that the oxidation of an aldehyde to a carboxylic acid requires a mole of water 159 in almost all cases. The oxidation of benzaldehyde to benzoic acid by chromic acid in the presence of water is shown to proceed via an aldehyde hydrate ester. 120 Stoichiometric. Oxidation, of Alcohola^with (Ph;PI;;[Ho0(0;);lox)]1 Substrate Product Time/hr. Yield(Z)b p-Anisyl alcohol p-anisaldehyde 3 96 Benzyl alcohol benzaldehyde 3 87 Vanillyl alcohol vanillylaldehyde 3 92 a All oxidations carried out in dichloromethane under reflux conditions b All yields obtained by derivatisation to the 2,4-dinitrophenylhydrazone 121 2.2.4 Differences Between the Carboxvlato Peroxo Chemistry of HoXyt?denum(VU and Tungsten(VI) : Separations! Possibilities We have seen that the behaviour of molybdenum and tungsten with regards to most of the carboxylato peroxo systems studied is very similar. However, there are some subtle, and indeed some more distinct, differences that offer opportunity for development of separational methods for the two metals. Generally speaking, the carboxylato peroxo complexes of molybdenum are formed at a much faster rate than those of tungsten, and in significantly higher yield. The complexes of molybdenum are generally yellow, while those of tungsten are white. All of these features are expected, since molybdenum is a second row element and tungsten a third row one, despite the effects of the lanthanide contraction. As far as individual vibrational frequencies and n.m.r. shifts are concerned, the small differences between analogous molybdenum and tungsten complexes are no more than is to be expected for such a change in metal atom. The most striking differences occur in the formation of tartronato and quinato peroxo complexes of molybdenum and tungsten. As discussed in Section 2.2.1.2, formation of carboxylato peroxo-molybdate(VI) and -tungstatetVI) complexes clearly depends on fulfilment of an exacting set of criteria, namely structure of the carboxylate (ability to form a five-membered ring with the metal if bound in bidentate fashion, and whether electron-withdrawing substituents affect its donor ability), pH of the reaction media, and competition for formation with non- carboxylato peroxo species such as [ M t O ^ ] 2" and fM2°3^02*4*H2°^2^2~‘ 122 The need for several factors to be suitable is underlined by the cases of the tartronato (2-hydroxy malonato) and quinato ligands, which only form isolable 1:1 metal:carboxylate peroxo complexes with 2 - molybdenum. The complexes [MoOtO^^U-)] = tartron, quin) are formed, at natural pH in the case of tartronate. and with lowering pH to 3.3 in the case of quinate. The analogous tungsten complexes can not be isolated at any pH, W:L ratio, or W:peroxide ratio. Thus the metal involved is also an important factor, even in the case of chemically- similar second and third row pairs such as molybdenum and tungsten. Unfortunately, the high cost of tartronic and quinic acids precludes their commercial use for separating the two metals. We have found that pH of reaction solution is of crucial importance in the formation of all carboxylato peroxo complexes of molybdenum and tungsten. In the cases of [M0{02 )2(L)]2~ (M = Mo, W ; L = ox, cit, 2- glyc, mal) and CM02(02(ox)(H^O)) (M = Mo, W), complex formation occurs at the natural pH (generally 2-4) of the reaction solution. The 2- gluconato complexes [M0(02)2(glucon)] form after acidification to pH 3.6 (M = Mo) or pH 2.6 (M = W). Of these systems, only the glycollate one merits further study, since the small pH difference in formation of the molybdenum and tungsten peroxo complexes is offset by the fact that the molybdenum complex crystallises rapidly and in good yield , with minimal need for precipitation by ethanol, while the non-crystalline tungsten complex appears in poor yield and with the need for copious ethanol addition. 123 The other system worthy of more detailed pH study is the 2:1 H:tartrate one. The tMo2<)2(02 )^ (C^H20g) 3* complex is formed after acidification to pH 4, while the tungsten analogue forms at natural pH (2-3). Both glycollic and tartaric acids are cheaply commercially available. We have therefore undertaken a more detailed pH study of these two systems when the ligand is added in equimolar (glyc) or half molar (tart) quantities to solutions of A2[M0^] (A = K, Na ; M = Ho, W) in the presence of excess hydrogen peroxide. 2.2.4(a) The 1:1 H : Glvcollate System (H = Ho. W) Formation of yellow A2[Mo0(02)2(glyc)3.2H20 is preceded by formation of red CMo (02 )^32” (pH 6-7) for about 10s., with pH stabilising at 3.9 (A = K) or 3.3 (A =Na). In the tungsten system, the solution is 2- initially yellow due to the presence of [W(02)^3 (pH 5.0-5.5), and then becomes colourless due to formation of A2CW0(02)2(glyc)3.2H20 in around 3-5min., with pH gradually decreasing and stabilising at 3.8 (A =K) or 3.2 (A = Na). The pH difference between the molybdenum and tungsten systems is clearly too small for separations on this basis. Nevertheless, the difference in rate and quality of formation described above demands that trial separations are worthwhile. Reaction of equimolar glycollic acid and mixed Na2CM0^3 (50:50 molZ M=Mo : M=W) in the presence of excess H202 , *?0-^0Wec* by minimal ethanol addition and storage in a refridgerator yields near-pure samples of crystalline Na2CMo0(02)2 (glyc)3.2H20, corresponding to an 85Z recovery of molybdenum 124 by weight. However, the samples obtained were not analytically pure, and soon became sticky, probably due to traces of the typically oily 13 Na2[W0(02)2(glyc)3.2H20. The C n.m.r. spectrum of the bulk product shows two peaks in the carboxyl region (6 187.5 and 186.6 p.p.m.) and two peaks in the hydroxyl region (8 76.0 and 74.9 p.p.m.), indicating a mixture of molybdenum and tungsten species. Similar results are obtained using an 80:20 molZ Mo:W ratio. Refinement of this method, with access to molybdenum and tungsten analysis and other techniques such as solvent extraction, is beyond the scope of this thesis, but an encouraging possibility. 2.2.4(b) The 2:1 M : Tartrate System (H = Ho. W) Detailed pH study of the formation of A^CM202 ^(C^H^Og)3.4H20 (A = K, Na ; M = Mo, W) has shown that the molybdenum complexes infact form without the need for pH adjustment if given time, with the red 2- solutions of [Mo(0-),] changing independently to the yellow tartrato Z b peroxo complex. The full results are depicted in the pH-time plots shown in Fig. 2.14. In the molybdenum systems, initial pH of 7-8 slowly decreases to a final pH of 4.2 (A = K) or 4.3 (A = Na) over a period of 4-6min. In the case of tungsten, the process is slower but similar, with the initial yellow solutions (pH 5-5.5) gradually becoming colourless in 10-20min., and with final pH 3.2 (A = K, Na). In both cases, the tetraperoxo- metalate(VI) species is superceded by the bridged-tartrato peroxo complex in solution, with an accompanying drop in pH. 125 The pH difference (approx. 1) between the two metals may be enough for separations on a solvent extraction basis, but hopes of effecting separation on the basis of preferential crystallisation are not high. Trial separations on 50:50, molybdenum-rich and tungsten-rich solutions of the two metalates in the presence of half-molar tartaric acid and excess have yielded mixed crystalline products. Molybdenum and tungsten analyses (atomic absorption method, carried out by Interox Chemicals Ltd.) confirmed that appreciable amounts of both metals were also left in solution in each case. The difference in ease of crystallisation of molybdenum and tungsten analogues is clearly not as acute in the tartrate system as it is in the glycollate system. 126 pH pH Fig. 2.14 pH-Time Plots for formation Of A4 [m 20 2(02)2(C4H206)].4H20 a. M = M0 ; A = K b. M = Mo i A = Na z < (0 C. M = W ; A = K d. M = W ; II 127 2.3 flXQPERQXQ AND PEROXO MOLYBDATES(VI) AND TUNGSTATES(VI) IN THE S O U P STATE AND AQUEOUS SOLUTION 2-3.1 yjJ)ra_tional Spectra of K^CMo^ t O ^ l H ^ l ^ H ^ O (H = Ho. W) ; 2H- and *80-Substitution Studies As discussed in Section 2.3.1, the dimeric oxoperoxo complexes 2- ^ 2 ° 3 ^ 2 ^ 4 *H2°*2^ = M°* W * play an imPortant role in the aqueous peroxo chemistry of molybdenum!VI) and tungsteniVI), particularly in the low pH regions. The stable structure of the species, which involves a symmetrical M-O-M bridge between two pentagonal bipyramidal moieties, makes it the dominant species in the pH range 7-2 for aqueous molybdate!VI) and tungstate!VI) solutions in the presence of hydrogen peroxide. Furthermore, the work on carboxylato peroxo complexes 2- presented in Section 2.2 shows that the [M203!02)^CH^O) dimers are often formed in preference to certain carboxylato peroxo complexes where the carboxylate heteroligand can not impart the same stability as found in their structure. 48 Campbell prepared the salts K2CM203(0^)4 128 We have now calculated the shifts for the v(M20) frequencies of the complexes using the secular equations given for this purpose by Wing and 160 Callahan. We have also re-prepared the K [H 0 (0 ) (H 0) ].2H 0 £ £ O £ % £ £ 4 salts, remeasured their infrared and Raman spectra in the solid state, and deuteriated them for the first time - in order to investigate the 2 effects of H-substitution upon the spectra. 2.3.1(a) .Comparison of,Calculated and Experimental Shifts in M20 Frequencies upon 18 0-Substitution For K2[Mo203(02 (H^O)2).2H20, Campbell*8 reported shifts of v(Mo=0) Raman frequencies from 972 and 958cm 1 to 926 and 914cm 1 (cf. calculated values of 925 and 911cm 1), and of 6(Mo=0) Raman frequencies from 350 and 320cm"1 to 341 and 311cm"1 (cf. calculated values of 333 -1 18 and 304cm ) upon 0-substitution (see Table 2.9). There was no shift in any of the vibrational frequencies of the peroxo group. In the case 18 of 0-substitution shifts the unsplit v(W=0) Raman frequency from 960 to 905cm 1 (cf. calculated value 910cm 1), and the 6(W=0) band from 321 to 312cm 1 (cf. calculated value 305cm *). The infrared bands in the spectra of the molybdenum and tungsten dimers at 715 (M = Mo) and 765 (M = W) cm 1 were assigned to v3S(M20), and shift to 675 and 715cm * 1 respectively upon 18 0-substitution. The bands assigned to vs(M20) at 454 (M = Mo) and 450 (M = W) cm"1 shift to 435 and 405cm“1 respectively. The study showed the value of *®0- substitution in assignment of M=0, M-O-M and peroxo-group vibrational bands in such species. We have calculated the expected shifts for the MgO bands to fully ratify their assignment as such, using the secular 129 equations given for calculation of M20 vibrational frequencies by Wing ^ . ,, . 160 and Callahan: AaS = t Mm ♦ M0 (1 ♦ cosip) J t k * kH0H) A* = t Mm * M0(1 - cosipl 3 (k - kM0Ml -7 2 where A = (5.889 x 10 )v k = bridge M-0 force constant = M-0-M bridge kM0M interaction constant = reduced masses m m ,p o s 18 For the molybdenum dimer, the shift of v (Mo20) upon 0- substitution is calculated to be 454 to 430cm * (experimentally -1 as -1 435cm ) while that of v (Mo^O) is calculated as being 715 to 690cm -1 s (experimentally 675cm ). For the tungsten dimer, v (W20) is calculated • j . j g § to shift from 450 to 426cm (experimentally 405cm ), and v (WgO) from 765 to 732cm 1 (experimentally 715cm ^). It can be seen that, with the exception of v (Mo20), the experimental shifts of the vibrational frequencies are somewhat larger than that expected by calculation, and do not agree as well as the M=0 shifts. However, no other bands are reported to shift upon ^80- substitution*8 , so that the assignments of vs(M=0) and vas(M=0) must be correct. 130 i i i i i i 48 n 48 This n This work work This This Ref. work n i i i i ) i i i i i i i i i t 259(1 255(1) 256(1) work i i 260(1) 255(1) 8H20 i i in i i i i i i i i i i i 312(2) 311m 310(2) 317(2) 330m 339(2) 325w i i 335m 344(4) 341(4) 340m vM(0H2) i i i t i i i i i i i i i i i i *M20 405(1) 447w 445(1) 450(1) 448(1) 450w t - 0 5 w 435(1) i i 454w 454(1) v )* i i n i i n i i i i (cm'1 i i i i i i i i i i **M(02 ) n DATA 550 vs 550 562(7) 547s 565(7) 557(5) 530(4) v i i i i n n i i i i i i i i i i i i i i VIBRATIONAL i i 620(2) 620(2) 622(3) 615m 560s 615m 575s 531m 582(5) 533(4) 595s 530m i i v8M(02) i i i i i i i i "* i i 2 i i i i *#M20 i i 762(2) n 715m 765s 715s v i i i i i i i i i i i i n i i 853s 850(8) 853s 853vs 722s 578s 532m 866(9) 718(2) 588(5) 867(7) 584(7) 535(8) 863s 675s 855vs 867(9) 723(2) vO-O i i i t n it it n ii i i i i 966vs 852s 761s 615m 956(10) 850(8) 761(2) 905(10) 853(7) 715w 955(10) 905s 958(10) 949(10) 914(10) 960vs 961 vs 961 959(10) 950(9) 927vs 926(8) i i vM=0 i t II II p II ir i r II II II II D i II II II II p II p ].2H,180 IR L II o II II C C ] . 2H ° II II 180) II o °) °) II 0) 0) ].22H,° IR h II II |2 ( H II h C c c ♦ ^ II II C. C. II II (° ) (° ) II o c b c c, c II ° II II II II II Oata for solids; relative Raman intensities give in parentheses. J c 3 c. c £ J chic C chic J £ c 6 2[H° 2[H° II K2tM° ° (0 ) K2tM° ° (0 ) MW,0,(0,)£(2H,0),].22H,0 K K 0 CW <0 ) (H,0),].2H,0 IR 964vs K9CW9180,{0,)£ (H K9CW9180,{0,)£ K K CHo 180 (0 ) ,80) (H ].2H ,8° IR k Species II TABLE 2.9 Vibrational Data for Dimeric [H203(02);(H20)2] "species of Molybdenum and Tungsten 131 2-3.1(b) fffj.Q.t.,9f 2Jir.Sub$UtuUpn Upon the Vibrational Spectra We have re-prepared the salts ,2H2° (M = Mo, W) and deuteriated them by simple recrystallisation from The infrared and Raman data obtained from both the normal and deuteriated species in the solid state appear in Table 2.9. In the case of both the molybdenum and tungsten dimeric species, none of the vibrational bands are shifted upon deuteriation except that seen at 335cm * in the infrared spectrum of the molybdenum complex (344cm 1 in the Raman), and 330cm 1 in the infrared spectrum of the -1 2 tungsten complex (317cm in the Raman). Upon H-substitution. these bands are shifted to 325cm 1 (infrared) and 339cm 1 (Raman) in the vibrational spectra of the molybdenum complex, and to 311cm 1 (infrared) and 310cm * in the vibrational spectra of its tungsten analogue. We therefore tentatively assign these bands to a M-0H2 stretching vibration [v(M-OH2)] of the aquo ligand to metal bond in the dimeric tM2°3(02,4(H20,232~ sPecies* 2.3.2 The Caesium Molvbdate - Hydrogen Peroxide System Work concerning the aqueous chemistry of molybdate(VI) when acidi fied in the presence of hydrogen peroxide performed in these laboratories by Campbell 4 8 and described elsewhere in the literature 132 is summarised in Section 2.1.2. The majority of evidence suggests that the main species present are [ M o t O ^ ] 2" at pH H-7, [M o ^ (02)4 (H20)232" at pH 7-1.7, and the protonated species [M o^ lO H) (02) 4(HgO)' bei°w 2 - pH 1.7 (as well as the chloro peroxo species [MoO(0_)Cl,32 4 below pH 1.7 when HC1 is used for acidification). However, there is some Raman and 95 Mo n.m.r. spectroscopic evidence for the presence of an oxotriperoxo 2 - 4 8 species, probably [Mo0(02 )3] , in the pH region 9-5. Isolation of caesium salts of the [Mo0(02)33 2- species, albeit at high pH, and other diperoxo, triperoxo and tetraperoxo species, is 56,58 reported by Bogdanov et al. All of the complexes are reported to have been isolated from a saturated solution of caesium molybdate with 102 H2°2 at alkal:>-ne PH : PH 9.4 Cs2tMo(02)4].nH20 (I) pH 10.7 Cs2CMo0(02)31.nH20 (II) pH 11.4 Cs2CMo02(02)23.nH20 (III) pH 11.8 (IV) CS4CMO205(02 ,3]-nH2° The infrared spectra are reported for (II) (bands at 920, 860, 740, 620, 570, 495 -460cm 1), (m) (bands at 940, 920, 860, 760, 580 -520, 495-460cm ^), and (IV) (bands at 860, 640, 590, 460cm ^). We regard this work with a little suspicion, since such small pH changes in the Cs 2[Mo 043-H20-H202 system are unlikely to result in the formation of four discreet complexes as listed above. We would expect mixtures to exist at pH 11.8-9.4, with the main species being tMo(02)^]2” and 2- 2- CMo203 (02 )4(H20)2] , with traces of [Mo0(02)3] at the lower end of the pH range. It is also interesting that no Mo=0 bands are reported in 133 the infrared spectrum of the "Cs_[Mo_0_(0_)_].nH_0“2 2 5 2 3 2 species. We have therefore repeated the work in order to clarify the situation. We find that treatment of a fresh solution of caesium molybdate (prepared from MoO^ and CsOH) with excess hydrogen peroxide affords a yellow solution of pH 10. If the solution is raised to pH 11 using caesium hydroxide, it becomes orange-red, extremely effervescent, and eventually loses all peroxidic oxygen to become colourless. The main species present is clearly the unstable Cs2[MoO^], and isolation of CsoCMoO (0 ) 3.nH 0 and Cs,[Mo 0 (o„)„] .nH 0 at pH 11.4 and 11.8 2 222 2 42523 2 56 58 respectively as claimed by Bogdanov et al. ' seems improbable. At pH 10.7 and 9.4, yellow solids can be isolated by ethanol addition from yellow solutions whose Raman spectra are strikingly similar, and display the Mo=0 (ca. 960cm 1), 0-0 (ca. 850cm 1), Mo20 (ca. 760cm 1) and MotO^) (ca. 570, 530cm 1) vibrations expected for an aqueous solution of C s ^ M o ^ (02) (H20) . 2H20, as well as various other bands in the metal-oxo, peroxo and metal-peroxo regions, indicating the presence of other oxoperoxo or peroxo species in solution. The solids (A) and (B) isolated from the Cs^tHoO^]-H202 solutions at pH 10.7 and 9.4 respectively have very similar infrared and Raman spectra. The Raman spectrum of solid (B) for instance shows bands in the metal-oxo region (960, 931, 908cm”1), Mo-0-Mo regions (782, 756, 317cm 1), peroxo region (879, 841cm"1), and metal-peroxo regions (636, 609, 568, 530cm 1), and other bands at 504, 392, 348, 299 and 262cm’1. The close similarity in profiles of the Raman spectra of solids (A) and 134 ). ) b ( a ( and at pH 10*7 Solution at pH 9-4 2 O 2 -H 4 O 0 M 2 Fig.2.15 Raman Spectra of Solids Isolated from CS 135 95 (B) is shown in Fig. 2.15). The Mo n.m.r. spectra of DgO solutions of both (A) and (B) show a very broad resonance centred at around 6 -245 p.p.m (cf. 6 -268.8 p.p.m. for K2CMo203(02)4(H20 )g].2H2048) and an even broader one at around 6 -5 p.p.m. (probably due to molybdate). Caesium analyses were too high (ca. 55Z) and peroxide analyses too low (ca. 13Z) for the solids to be pure Cs2[Mo203(02)4(H2<))23.2H2<) tea. 38Z 2 - caesium and 18Z (02 ) ]. All this evidence suggests that the yellow solids isolated at pH 10.7 and 9.4 consist of similar mixtures of largely Cs^[Mo«0^() - (H20»23.2H20, along with traces of C s ^ M o t O ^ ] , Cs^MoO^], and other oxoperoxo molybdates(VI). The latter could possibly include triperoxo species such as Cs2[Mo0(02 )3), and higher peroxomolybdate(VI) species 2- similar to those isolated pure from [MoO^] ~H2°2 solutions at high pH by Stomberg - for example K6[Mog0 10(02)8] .5H20 (pH 8.1)43, and (NH^) CMo 307(02)4].2H20 (pH 8.3-9.2)** (see Section 2.3.3). Certainly, we are not able to isolate pure oxoperoxo molybdate(VI) salts from the Cs^MoO^I-H^C^ solutions at high pH as Bogdanov et al. claim to have done. 2.3.3 Vibrational Spectra of Known Oxoperoxo Molybdates(VI) We have recorded the infrared and Raman spectra (the latter in the solid state and in aqueous solution) of some of the oxoperoxo molyb- dates(VI) recently prepared and structurally characterised by Stomberg (see Section 2.1.2), as vibrational data are not reported in the literature.• * * * Spectra of isolated complexes can help when attempting to identify the nature of peroxo molybdates(VI) in aqueous 136 solution (for example in the CSgtMoO^l-H 0 solutions dealt with in Section 2.3.2). 2.3.3(a) lNH;lflIHo t Q022 (02 ) , 21.16^0 The title peroxomolybdate(VI) compound is prepared from a solution of ammonium heptamolybdate, (NH,)etMo_0_.].4H„0,» b f Z4 2 and hydrogen peroxide 45 at pH 2.8-3.0. The infrared spectrum of the solid displays bands attributable to Mo=0 stretches (953, 947, 925cm"1), 0-0 sketches (891, 872, 853cm 1), and Mo(02) stretches (620 and 555cm S, as well as other “1 as bands, of which those at 686 and 335cm are probably due to v (Mo^O) and 6(Mo^0) vibrations respectively. The complexity of the spectrum reflects the complicated structure of the complex anion, shown to contain both Mo0g(02 ) and Mo03 (02)2 polyhedra by X-ray structure 45 determination. The Raman spectrum of the complex is similar; it retains a similar profile in aqueous solution also, indicating that the structure of the oxoperoxo molybdate(VI) anion is retained upon dissolution in water. The relatively weak peroxo bands seen in the Raman spectrum of the solid could well be due to loss of peroxidic oxygen in the laser beam. The 95 Mo n.m.r. spectrum of the aqueous solution shows one broad resonance at 5 -247 p.p.m., reflecting the fact that more than one molybdenum environment exists in the complex anion. 137 2.3.3(b) The infrared and Raman spectra of this oxoperoxo molybdate(VI) complex (isolated from a solution of ammonium heptamolybdate and hydrogen peroxide at pH show the complexity of a species in which two MoO(0^)g moieties are doubly-bridged by oxygen atoms, one of which is also bonded to a Mo0„ unit. The vibrational bands in the infrared 3 spectrum may be assigned to Mo=0 stretches (953, 918cm *), 0-0 stretches (885, 841cm S . Mo-O-Mo stretches and bends (796, 314cm *) and HotO^) stretches (616, 586, 559cm~1) with reasonable certainty. The Raman spectra of the complex in the solid state and of its aqueous solution are similar in profile, indicating retention of structure in solution. 2.3.3(c) jCgIjlo7ggglfiglgUUtgil This oxoperoxo molybdate(VI), prepared from a solution containing potassium molybdate and hydrogen peroxide at pH 6.2, has five MoOg octahedra and two pentagonal bipyramidal Mo0_(0_) units in its complex 5 2 44 anion structure. The infrared spectrum of the solid shows a very broad Mo=0 band at 900cm 1 due to the "heptamolybdate" oxo groups, and a sharper band at 935cm ' due to the terminal Mo=0 ligands on the MoOgtO^) units. Peroxo stretches at 840 and 825cm \ and metal-peroxo stretches at 650 and 580cm 1 are also evident. The Raman spectrum shows further splitting of the bands, but is basically the same in profile - both in the solid state and in aqueous solution. 138 2 -3 t fly-AdH.UV 9 Re?.tUVi,ty pf [H.fljifl.^.lH.0).]2' Species IH * Mo. Ml 2.3.4(a) Nature of the Peroxo Species Responsible for the Molybdate- and Xung$:tate-Catalvsed Oxidation of Alkenes and Alcohols bv The role of molybdenum and tungsten peroxo complexes as reactive intermediates in the catalytic oxidation of organic substrates by H^O 2 2 has been briefly reviewed in Section 2.1.A.1(c). Since in many examples the catalyst used is molybdate or tungstate at medium to low pH, there is a strong possibility that the stable dimeric complexes of formula 2- = Mo* are resP°nsib^e *or the oxidation, for which their four peroxo groups per molecule equip them well. Epoxidation of alkenes can be accomplished by use of molybdate or tungstate catalysts and aqueous hydrogen peroxide****1, or more effectively with the same catalysts using reasonably dilute hydrogen peroxide under phase-transfer conditions (using a co-solvent such as 125-129 1,2-dichloroethane). Indeed, Mimoun has directly used a pertungstate catalyst formulated as CPh^(PhCHft)P] fW 0^((X ),] at 30-50°C 3 2 2 2 3 2 4 under dilute H^O — 1,2-dichloroethane phase-transfer conditions (at pH 5-6) to epoxidise alkenes.130 133 Oi Furia, Modena and co-workers have recently used similar conditions to selectively oxidise primary and secondary alcohols to the corresponding carbonyl compounds using higher temperature (60-75°C) and a sodium molybdate (pH 3.01 or sodium tungstate (pH 1.4) catalyst. This work has prompted us to confirm the existence of the [M«0-(0«).(H«0)«)2- % J C ^ b 4 species in hydrogen peroxide solutions of molybdate or tungstate at low 139 pH and high temperature, and thus to prove it to be the active peroxo intermediate in the oxidations. We find that a solution of potassium molybdate and hydrogen peroxide at pH 3 (the pH used in the oxidations described above) warmed to 75°C yields analytically-pure upon cooling. Raman and infrared spectra of the complex obtained confirm that there is no decomposition of the dimeric structure in solution at these temperatures. A solution of potassium tungstate and hydrogen peroxide at pH 1.5 similarly afforded K2^W2°3*°2*4*H2°*2^*2H2° upon coolin9 from a temperature of 75°C. 2- We have been able to show that [Mo203(02)^(H20)2] persists in aqueous solutions in a more elegant manner by actually measuring the Raman spectrum of a solution of potassium molybdate and hydrogen o peroxide at pH 3 and 75 C in a spinning solution cell. The solution, in a spinning cell at room temperature, gives the Raman spectrum expected 2- due to the main species in solution, [Mo203 ***H2°^2^ ' with bands at 970 [v(Mo=0H, 873 [v(0-0H, 575 [vS {Mo (0£) } ], 540 [ vaS{Mo (02) } ) , and 320 [6(Mo 20)3 clearly visible. When the solution is heated to approximately 75°C in the cell using a clamped hair-drier, the Raman spectrum of the solution remains similar in profile, though all bands become somewhat weaker and broader (see Fig. 2.16). Upon cooling to room temperature, the spectrum retains its original appearance, and crystals of a yellow complex, found to be K2[Mo203 (02)^ (HgO)2).2H20 , appear in the cell after a few hours (after spinning has stopped). This confirmation of the fact that the CM 0 (0 ) (H 0) ]2" species 2 3 2 4 2 2 140 ). 2 ( and 75°C ) ) 1 ( at at 25°C 3-o) Solution (pH 2 0 2 J / H 4 O 0 Raman Spectra of [M 2.16 Fig . 141 exist in solution at 75 C and are therefore probably responsible for the oxidations of alcohols carried out by di Furia, Modena et al.133 prompted us to prepare organic-soluble salts of the molybdenum and tungsten species and test their use for the stoichiometric oxidation of various alcohols. 2.3.4(b) PrepjuratioiL-OfLQrganic-Soluble Salts of [H (^(Q ) (H 0) I2" (H *_ Ho. W) (Ph^P)^CMo^Os(0^)^] is formed at pH 5.3 by addition of tetraphenyl- phosphonium chloride to an aqueous solution of the potassium salt of the dimeric complex, immediately and in good yield. It is fairly soluble in dichloromethane, acetone and acetonitrile, and, as well as the bands due to the (Ph^P)+ ion, displays vibrational bands at around 960 [v(Mo=0)I, 870 [v(0-0)], 720 CvaS(Mo20)], 590 CvS{Mo(02> >3, 530 [v3S{Mo(02)}], s -1 and 450 [v (Mo20)] cm in the infrared and Raman spectra of its solid state (see Table 2.10). The Raman spectrum of an acetonitrile solution of the complex has a similar profile to that of the solid, showing that the structure is retained upon dissolution in organic media. Disappear ance of the bands due to water in the infrared spectrum and elemental analyses indicate that the tetraphenylphosphonium salt of the dimer contains no water - be it coordinated water or water of hydration. It is interesting that the analogous tungsten(VI) peroxo species (Ph^P^2^2°3^ 2 ^4^ *orms PH 6.3 much more slowly and in far poorer yield than the molybdenum(VI) peroxo species. The salt is off-white in colour, soluble in organic solvents, and possesses vibrational bands at 960, 861, 722, 614, 576, 457 and 326cm ^ (assigned as above) in its 142 infrared and Raman spectra (see Table 2.10). Treatment of aqueous *2 H2° with either tetra- butylammonium chloride or hydrogensulphate affords immediate precipit ation of (Bu^N)2 CMo 2 03(02 )^3, a pale-yellow solid whose solubility in organic solvents is very much higher than its tetraphenylphosphonium counterpart. The salt may be recrystallised in pure form from dichloro- methane, and shows vibrational bands at around 960 [v(Mo=0)3, 870 [v (0-0)], 720 tvas(Mo2 0)3, 590 [vs{Mo(02 )>3, 540 [vas{Mo(02)}], and 450 [v (Mo^O)] cm in its infrared and Raman spectra (the Raman spectrum of its acetonitrile solution has a similar profile to that of the solid) (see Table 2.10). However, reaction of K2 ^w2 °3() 4(H^O)2 1.2H20 with tetrabutyl- ammonium reagents does not give an isolable product. This acute difference in chemistry could prove to be the crude basis for a moly- bdenum/tungsten separational method. 2.3.4(c) Use of (Ph4P)2 [Mo2 03(02)4] (M = Ho. W) and (Bu4N)zIMo2 03(0z);3 as Stoichiometric Oxidants 2- We have seen that the ^M2 °3^°2 ^4^H2 °^2 ^ species is almost certainly involved in catalytic oxidations of alkenes and alcohols by H,0 at medium to low pH in the presence of [M0 ]2- (M = Mo, W) 2 2 4 catalysts, or, in one case, a [Ph3(PhCH2 )P3[W2 03(02)^3 catalyst. As far as epoxidation is concerned, the nature of the mechanism of peroxo complex involvement when the complex is involved as a catalyst for epoxidation by H202 must be different from that involved in stoichio- 143 - 251(3) 250(3) 6M20 *M20 - 448(3) 457w 454w 457w v *8M(02) 558(6) 559(5) 560(5) v (cm”1)* -- 8M(02) 593(3 ) 593(3 594(4) 614m 576m 586(4 ) 586(4 556(6) 593m 534m v - asu _ VIBRATIONAL DATA 715(1) 726(2) 720m 722s 592m 533s 722s v v M20 - 866sh 861m 873s 875(7) 875(6) 725(1)) 606(4 865s vO-O - 960(10) 878(7) 710(3) 960(10) 946s 963(10) 880(6) 967s 965(10) 962s 960s vM=0 R R R R Rb IR (PhtP)2CMo203 (02l4] IR (Ph^PJ^CW^O-, (0?)43 (Ph^PJ^CW^O-, (Bu4N)2tMo203 (02)4l IR b b Sample decomposed in laser beam. a a Data for solids or (underlined) for acetonitrile solutions; relative Raman intensities given in parentheses. Species TABLE 2.10 Vibrational Data for Organic-Soluble Species 144 metric epoxidation by that complex, as noted by Mimoun. 161 The £M2°3*°2*4^ species present in our organic-soluble oxoperoxo complex salts does have an overall negative charge, which discourages attack of an electrophilic carbon-carbon double bond, but more importantly it possesses a vacant coordination site on each of its molybdenum atoms, due to the loss of an aquo ligand, which could be used for complexation of an alkene as part of a heterolytic mechanism. As far as the oxidation of alcohols is concerned, we have already 2- seen from our work that the species CMo0(02)2(ox)] , both negatively- charged and without a vacant or releasble coordination site, effects the stoichiometric oxidation of reactive alcohols, albeit under reflux conditions. The fact that the (Ph^P)2[M203 (02)^] (M = Mo, W) and (Bu ^N)2CMo 203 (02)^3 salts prepared by us possess four peroxo groups per molecule, and that they are more soluble in organic media than the (Ph,P)_[Mo0(0_)_(ox)] species, makes study of their possible use as h C c L stoichiometric oxidants of alcohols a must, especially as the mechanism involved does not seem to involve complexation of the substrate to the metal. It is more likely to involve "external" oxygen-transfer to the substrate by peroxidic oxygen. The study of stoichiometric epoxidation by these organic-soluble oxoperoxo molybdate and tungstate complexes is considered out of the scope of this thesis, but we have undertaken a preliminary study into whether there is competing epoxidation taking place in their oxidation of unsaturated alcohols. 145 5toi.chiom.etidlc_Q)<3L0»tiQn.$ UsiH5-(Ph;PJ2 CHo2 03(0z);] (M = Ho. U) Both (Ph^P)2 [Mo2 03(02)^] and (Ph^P)2 CW2 03(°2 )4^ oxidise p-anisyl alcohol to p-anisaldehyde at room temperature in greater than 95Z yield (yield obtained by derivatisation to the 2 ,4-dinitrophenylhydrazone) using a 1:1 oxidant:substrate molar ratio and dichloromethane as solvent. The fact that stoichiometric oxidation is effected at room temperature (and with a reaction time of only 2-3 hr.), without the need for external heat, makes these peroxo complexes much more attractive as 2- useful oxidants than the [Mo0(02 )2 (ox)3 species. Moreover, by using 2- longer reaction times, one mole of the tM2 °3(02)^] complexes may be used to oxidise four moles of p-anisyl alcohol in 85Z yield, indicating that all four peroxo groups are active in oxidation, although successive oxidations by each peroxo group seem to become slower. We have gone on to attempt oxidations of various primary, secondary, unsaturated and cyclic alcohols with the molybdenum complex only, using 1:1 oxidant:substrate ratios, thin layer chromatography to gauge completion of reaction, and 2,4-DNPH derivatisation for quantification of yields. The results are summarised in Table 2.11, where the alcohols are listed using trival names. Structures of the more obscure alcohols used are shown in Fig. 2.17. The reactive primary benzylic alcohols (vanillyl, piperonyl, benzyl) are all oxidised to the respective aldehydes in good yield. The secondary cyclic alcohol a-tetralol is oxidised to a-tetralone in 60Z yield after a reaction time of 15 hr., after which time the reaction is found to proceed no further. This secondary alcohol is clearly not as 146 T»bl« 2.11 Stolchlom«trlc Oxlditlon of Alcohol! with [Pht).[H;03 (0.)t1. (H « Ho. Ml* Substrate Product0 Time/hr. Yield(Z) p-Anisyl alcohol A 3 97 b p-Anisyl alcohol A 3 96 Vanillyl alcohol A 3 89 Benzyl alcohol A 3 85 Piperonyl alcohol A 3 92 a-Tetralol K 15 60 Cyclooctanol K 24 20 Geraniol A® 15 54 Cinnamyl alcohol A6 15 45 Citronellol A6 15 32 a All oxidations carried out using (Ph4P)2[Mo203(02)43 at R. and in dichloromethane except b b Using (Ph^P)2CW203(02)^] at R.T. and in dichloromethane c A = aldehyde product ; K = ketone product d All yields obtained by derivatisation to the 2,4-dinitro- phenylhydrazone e Product formed, but competing epoxidation probable 147 oe-TETRALOL PIPERONYL ALCOHOL CINNAMYL ALCOHOL GERANIOL CITRONELLOL CYCLO-OCTANOL Fig. 2.17 Key to Structures of Alcohols 1 48 T e b le 2.12 Stoichiometric Oxidation of Alcoholt with b Substrate Product Time/hr. Yield(Z)C p-Anisyl alcohol A 3 97 Piperonyl alcohol A 3 94 a-Tetralol K 15 63 Geraniol 15 47 a All oxidations carried out at room temperature in dichloromethane b A = aldehyde product ; K = ketone product c All yields obtained by derivatisation to the 2,4-dinitro- phenylhydrazone d Product formed, but competing epoxidation probable 149 susceptible to oxidation as the primary alcohols already discussed. This 2 - i s not unprecedented, since the efficient catalytic ruthenatet[RuO^] )- potassium persulphate system developed recently in these laboratories only oxidises a-tetralol in 84Z yield. 162 The very low yield of cyclooctanone (20 Z) obtained from the 24 hr. oxidation of cyclooctanol again reflects the relative lack of reactivity of this secondary alcohol. With regards to the unsaturated alcohols geraniol, citronellol and cinnamyl alcohol, the low yields of carbonyl product apparently gained (54, 32 and 45Z respectively) suggest that competing epoxidation of the alkenic double bonds is taking place. This we have proven to be the case using the more organic-soluble (Bu^NIgtMOgO^tC^)^] complex (see below), and this lack of selectivity clearly restricts the use of the 2- [M^OatO^)^] species as stoichiometric oxidants. However, the fact that the oxidants are mild with respect to oxidising no further than the aldehyde or ketone is in their favour. Stoichiometric Oxidation of .Alcohols bv (Bu^ N ^ t H o ^ T Q ^ I and Investigation of Competing Epoxidation The (Bu ^N)2 CMo 2 03(02)^] salt proves to be much more soluble in organic solvents than the tetraphenylphosphonium salts discussed above. Oxidations can therefore be carried out using solutions rather than part-suspensions. We have carried out oxidations of p-anisyl alcohol, piperonyl alcohol, a-tetralol and geraniol using the tetrabutylammonium peroxomolybdate complex, and obtained similar yields of the respective carbonyl products as with the tetraphenylphosphonium peroxomolybdate 150 complex (see Table 2.12). The low yield and poor melting point obtained for the 2,4-DNPH derivative of geranial again indicate competition from epoxidation of the alcohol's alkenic double bonds. The high solubility of the (Bu 4 N) 2 [ M o2 . 3 O J 2 O 4J J species lends itself to an n.m.r. experiment to investigate the possibility of epoxidation by the complex. A CD3CN solution equimolar in cyclohexene and the peroxo complex was left stirring for 24 hr., by which time around 90Z [approximate yield calculated from the ratio of integrals of alkenic (6 5.5) and epoxidic (5 3.0) proton peaks] of the cyclohexene had been epoxidised to the cyclohexene oxide. The significance of this result is two fold. Firstly, the (Bu^N^CMo^O^O,^] complex, despite its overall negative charge, but presumably because of its vacant coordination sites and wealth of peroxidic oxygen, is potentially a good stoichiometric and catalytic epoxidising agent. Secondly, the result means that the 2- CM°2 ®3^^2 ^4^ sPec^es* while being an effective and mild stoichiometric oxidant for saturated primary and secondary alcohols, is not selective with respect to the oxidation of unsaturated alcohols, and therefore limited in its use therein. We consider the further study of epoxidation beyond the scope of this thesis, but appreciate the possible worth of this or similar systems in the future. The constraints of time have prevented further work on the oxidising 2 - powers of the £M2 °3*°2 *4^ = Mo' W * sPecies being presented in this work. Nevertheless, the way is paved for further work on the use of the oxidants for alkene epoxidation, and work^making both their oxidation of alcohols and epoxidation of alkenes catalytic using co-oxidants such as NMO (4-methylmorpholine-N-oxide) and t-butyl hydroperoxide. 151 Of the three complexes studied the ( B u ^ N ^ C M o ^ O ^ O ^ ] moiety is the most promising due to its high solubility in organic media. Its yellow solutions can be seen to decolourise markedly as the stoichiometric oxidation reactions proceed with loss of bound peroxide, thus providing a crude but useful way of following the reactions. 152 2.4 THE ACTION OF HYDROGEN PEROXIDE OH TUNGSTEN CARBIDE AMD MOLYBDENUM CARBIDI- 2.4.1 Introduction The amount of tungsten recycled from scrap is ever increasing, and the majority of recyclable scrap consists of cemented tungsten carbide. The interstitial carbide, WC, is prepared by carburisation of tungsten metal powder with "carbon black" in a carbon tube or open furnace under hydrogen, followed by cooling. Cemented carbide pieces, shaped for use as cutting tools, dies, and so on, normally consist of WC and small amounts of other carbides such as Tie and TaC, all bound together with a cobalt binder. Several processes exist for the recovery of tungsten from tungsten carbide, some of which involve the use of hydrogen peroxide. Direct treatment of WC with strong acid yields only the stable yellow tungstic oxide, W03 , but the presence of hydrogen peroxide in acid solution has the effect of complexing the tunjrten and keeping it in solution. 143 Some of the latest work at Interox Chemicals Ltd. has shown that over 95Z recovery of tungsten from cemented carbide scrap is possible by treating the latter with hydrogen peroxide in the presence of nitric or sulphuric acid, and small amounts of a carboxylate as an additive (e.g. oxalic acid, salicylic acid). The leaching operation is carried out as a multi-stage batch process, with the peroxide added in portions, and affords hexavalent tungsten in the acid solution. This is later converted to synthetic scheelite, CaWO . or pure WO . 4 3 153 The exact nature of the peroxotungstate(VI) complex or complexes formed during the process is not clear; nor is the question of how small amounts of carboxylate additive improve the efficiency of the recovery method. Tungsten is known to form a variety of peroxo complexes in acid solution, with or without the presence of carboxylate, and their study has formed a large part of the work presented in this chapter. We have therefore gone on to study the species formed in solution when commercially-available WC is treated with hydrogen peroxide at low pH and in the presence of small amounts of oxalic acid by Raman spectroscopy. Unfortunately we have not been able to complement this 183 technique with that of W n.m.r. spectroscopy, due to the insensitivity of the nucleus. We have endeavoured, however, to isolate the species present using a variety of countercations, and have also looked briefly at the molybdenum carbide - H202 system us*n9 Raman 95 and Ho n.m.r. spectroscopy. The study of the peroxotungstate(VI) species in solution is made more interesting due to the recent work by Kudo and co-workers.141"2 141 The first paper describes a new 12-heteropolyacid of tungsten, with carbon as the heteroatom in a Keggin-like structure, obtained from the dissolution of WC (commercial powder) in 15Z ^2^2 ' Thermogravimetric, gas chromatographic and IR spectroscopic evidence is presented for the existence of the heteropolyacid, represented as ^^ 0 ) ^ H ^ W ^ 20^g]. nH20 or ^h 5° 2 ^ H3CWi2°40^'nH2° by Kudo* and said to be comprised of four W3013 units (each consisting of three W0g octahedra with common edges) linked by common corners to give a tetrahedral cavity. If phosphorus were the heteroatom, a PO^ group would be expected to fill this cavity, but infrared and Raman bands detected by Kudo seem to indicate the 154 presence of a more familiar C(>3 group rather than C O . Bands at 970, 880, 1350-1400 and 640cm**1 are assigned to the v ^ E ’) and v^(E') vibrational modes of the C03 group; other bands below 1000cm 1 are assigned to the W0g unit. 142 The second paper describes a similar polytungstic acid, bereft of the carbon heteroatom, prepared by the reaction of metallic tungsten with h 2^2 * and heteropolytungstic acid described above, are said to be isostructural as far as elemental composition and anionic 183 framework are concerned. Both show a single W n.m.r. reosonance at around -186 p.p.m., and similar IR spectra except that the isopolyacid shows none of the C03 bands. Oespite there being no mention of this in the first paper1*1, it is also reported that the two species contain similar amounts of peroxide per tungsten atom; they are reformulated as C02.12W03 .7H202>nH20 and 1 2W03 .7H202 .m^O (n,m = 20-25). Support for the existence of a carbonato peroxopolytungstate species such as that postulated by Kudo has come with the publication of the X-ray crystal structure determination of K[W0(0)(CO )].6H 0 by o 4 8 Z d 3 Z 47 + 2- 2- Stomberg. The species is isolated from the K /[WO.] /(C_0,) / h Z H H2°2 system at pH 9 * or from the K+/[W0^]2 /*c03)2~/H2°2 system at pH 10. The structure of the complex anion is shown in Fig. 2.18. The four tungsten atoms form a slightly distorted paralellogram, including a bidentate carbonato ligand bridgiing two of them. All the tungsten atoms are pentagonal bipyramidal in geometry, with one terminal oxo ligand apiece, as well as either two peroxo proups and two shared oxygens,or one peroxo group and four shared oxygens. 155 Fig.2.18 Structure of the [W 0 (0 ) (C01n 6" Anion47 * o Z o 3 2.4.2 The_Action of Hydrogen Peroxide on Tungsten Carbide In our Raman study of the species present when WC (as commercial powder) is treated with we have used approximately the optimum 143 WC:H2°2 rat*° ,found in work at Interox Chemicals Ltd. We have found that very low pH and high recovery of tungsten as CaWO. is attainable at laboratory scale without the need for a mineral acid, but have added small amounts of oxalic acid. The work done by Interox has proven that additions of small amounts of various carboxylic acids, of which oxalic acid is one of the most effective, improve recovery of 156 tungsten from WC in the H^O^-leaching process, presumably by aiding the complexation of tungsten by the peroxo ligand. The presence of oxalate is also of interest in view of the fact that Stomberg first isolated K6CW408(02)6(C03 )].6H20 from a K+/[W04]2"/( ) 2"/H202 system47, and that oxalic acid has been observed to aid the dissolution of tungsten 163 carbide in hydrogen peroxide in the literature previously. Tungsten carbide/H202/oxalic acid solutions for the purpose of our experiments were prepared by suspending tungsten carbide (commercial powder, ca. 5g) in water and adding oxalic acid (0.25g) followed by 3 60Z h2°2 ^13cm ^ over a period of a few minutes with stirring. Application of heat to the solution is not required on an industrial scale, due to the presence of mineral acid and the exotherms provided by constant additions of hydrogen peroxide. On a laboratory scale, heating o to around 70-80 C is required for efficient digestion of tungsten carbide. After 4-5 hours of heating and stirring, the result is a yellow suspension (due to some precipitation of finely-divided W03) of pH 1-1.5, and a black residue of carbon. Only traces of free peroxide are left, and the suspension may be centrifuged to yield a yellow solution. This solution, which must contain the the peroxo- and/or oxo- tungstate(VI) species responsible for maintaining tungsten(VI) in solution, was used for a series of experiments. 2.4.2(a) Recovery of Tungsten as CaWO^ To ensure that a substantial proportion of tungsten is recovered from tungsten carbide by our laboratory-scale method, the yellow suspension obtained from an accurately-weighed portion of tungsten 157 carbide was filtered and washed into a sodium hydroxide solution. The resultant solution of sodium tungstate was treated with calcium chloride to afford synthetic scheelite, CaWO,. The scheelite was dried at 105°C and found to represent a 98Z recovery of tungsten. 2.4.2(b) Raman $pectrum_of the WC/H^O^/Oxalic Acid Solution Raman spectroscopy of the yellow solution from the WC/H202/oxalic acid mixture produces unsatisfactory results using a capillary tube, due to glass diffraction and bubbling of the solution due to traces of free peroxide. However, use of a quartz-glass spinning solution cell and the yellow 568.2nm line from a krypton-ion laser gives the Raman spectrum shown in Fig. 2.19. The bands apparent are 973 ( 10)p, 965( 10)p , 889(2)p . 721(5)dp. 568(4)p, and 300(1)dp cm * (where relative Raman intensities are given in parentheses, p = polarised, dp = depolarised). There is also a broad band at around 800cm 1 due to a certain amount of glass diffraction. At pH 1.2 Campbell*8 *** reports bands at 998 ( 1 ), 975 ( 1 0 ), 949 ( 2 ), 888(2), 649(3), 617(2) and 515(weak) cm ^ in the Raman spectrum of 1.0M 2 - [WO^] solution, but it must be remembered that hydrochloric acid, used for acidification, was present in this case. Campbell further reports the Raman spectrum of 1.0M [W0;]2'/2H202/H+ at pH 1.7 to display bands at 959(9.5)p, 853(9), 740(weak). 619(3)dp. 559(10)p. 440(weak), and 323(7)dp cm , while 1.0M CWO^] at pH 0.8 and in the presence of H+ and excess H202 is reported to show bands at 966 ( 1 0)p, 883(3)p . 620(2)dp, 560(7)p, and 323(4)dp cm . For ease of comparison, the Raman 158 “ Oxalic Acid Solution. 2 Fig.2.19 Raman Spectrum of WC - H2P 159 Fig. 2.20 Raman Spectra of Tungsten Solution Species. A 1-OM [ W 04] at pH 1*2 B 1-OM [W 04] 2" /xs.H 2 02 /H+ at pH 0-8 C W C-H 2 0 2 -oxalic acid soln. (pH 1 - 1 .5 ) 160 spectra recorded for W o / ’/H* at pH ,.2 and for « o / - / „ ’/exe... at pH 0.8 by Campbell, and the Raman spectrum recorded here for the WC/H^O^/oxalic acid solution are shown together in Fig. 2.20. Our spectrum of the WC/H^O^/oxalic acid solution shows the strong polarised bands in the v(W=0) region (at 973 and 965cm”1) that are typical for tungstate in acid solution in the absence or presence of hydrogen peroxide. It seems that the main species present in non- 2- 4- peroxidic solutions of tungstate at low pH are [Wc0._] and [W4_0,_] 619 1032 since the Raman spectra of such solutions very much resemble those of 48 solid salts of these oxotungstate(VI) species. In the presence of 2- H^O^. the main species at low pH seems to be [W0(02)C1^] when HC1 is . . . 2 - used for acidification, along with traces of [W 0 (0 ) (H 0) ] (see fc J t ^ C 6 Section 2.1.2(a), and Ref. 48). The WC/H^O^/oxalic acid solution shows the polarised bands at around 890 and 560cm 1 also present in the 2- + [WO,] /H-0 /H solutions at low pH, very possibly due to peroxo and 4 i c symmetric metal-peroxo stretches respectively, as well as the W=0 bands mentioned above. However, the spectrum also shows a broad depolarised band at around 720cm 1 in the region expected for asymmetric W-0-W vibrations of poly-oxotungstate species - too strong and low in 2- frequency to be due to the asymmetric W_0c stretch of [W_0o(0_),(H_0) cockle ] (seen at around 760cm”1, see Section 2.3.1 and Ref. 48). From this Raman information, we can conclude that tungsten is probably present in solution as a variety of species. It is almost certain that one or more peroxotungstate(VI) species exist, since 45 several of the peroxomolybdates(VI) isolated by Stomberg have been isolated at low pH, and show solution Raman spectra similar to that of 161 the WC/H^O^/oxalic acid mixture. There is also the possibility of oxotungstate(VI) species existing, due to the broad band at 720cm’1, but this is less likely since they would presumably convert to WO^ in acid solution if no peroxide were available for complexation. As far as the possibility of peroxotungstate(VII species containing 141-2 a carbon heteroatom, as postulated by Kudo , is concerned, it is possible that such a species could exist, but it would prove very hard to detect it by Raman spectroscopy, especially in solution. The Raman bands reported by Kudo at 960, 890 and 640cm’1 for a W C / H ^ solution and at 970, 880 and 630cm 1 for the yellow solid obtained on its evap oration are far more likely to be due to v(W=0), v(0-0) and vCWtO^)] vibrations of peroxomolybdate(VI) species than those of a C03 unit incorporated in a polytungstate as he reports. We find that the yellow, glassy solid obtained from evaporation of our solutions has a strong Raman band at 966cm 1 and broad bands at around 680 and 850cm V This spectrum is similar to that of solid 4 8 oxotungstate salts such as (Bu 4 N) Z [W 6 0 19 ] (which do not, of course, show the 850cm 1 band in the peroxo-stretch region) and to the spectra of solid peroxomolybdates(VI) (see Section 2.3.3). There is no sign of extra carbonato bands in the 1000 or 1400cm 1 regions (CaC03 shows Raman bands at 1084 and 1460cm -1 due to the and v3 modes of (C03)2 - respectively. The only way of positively identifying inclusion of a carbon heteroatom in any of the species present in the WC-H202-oxalic acid system would be by an X-ray crystal structure; our attempts to isolate 162 a crystalline salt of such a species are catalogued in the next section. Certainly, the Raman spectrum of the solution obtained by digesting tungsten powder in H202 in the presence of oxalic acid (see Fig. 2.21) is virtually identical to that obtained when using tungsten carbide, indicating that Raman spectroscopy can not be used to pin down such a heteroatom. We attempted to prepare Stomberg's carbonato peroxo- tungstate, KeCW.0o(0o).(C0ob+otbJ )].6H.0,c from a solution of Ko[W0,3 Z* and KoC0„ 23 and H2 solution at pH 1.5 is very similar to that found by Campbell*8*1 * for a solution of 1.0M [W04]2 /2H202/H+ at pH 1.7 (see Fig. 2.20). The exact role of oxalic acid in improving efficiency of the tungsten-recovery method is not clear. There is no doubt that intermediate oxalato peroxo species of tungsten(VI) are involved in the complexation processes at work. The small amounts of oxalic acid required to have effect indicate that the oxalic acid is acting in a catalytic manner. The fact that carboxylic acids such as salicylic acid, not capable of forming isolable peroxo complexes with tungsten due to their inability to form a five-membered ring when bonded to the metal in bidentate manner (see Section 2.2.1.2), also improve the efficiency H 3 of the system further indicates that the role of carboxylato peroxo tungstate(VI) complexes is transient in nature. However, it is likely that using oxalic acid, there are small amounts of the CW0(02)2(ox)32" are present in the solution at any one time, along with the higher peroxotungstates(VI) mooted previously. The dissolution of WC in H 2 02 without oxalic acid takes a little longer than in its presence, but the Raman spectra obtained are identical. 163 Fig. 2.21 Raman Spectrum of W - ’"OxalicH202 Acid Solution 164 2.4.2(c) Attempted Isolation of PeroxotungstatotVI LSpecies Present in the WC/H^O-/Oxalic Acid Solution The solid isolated by simple evaporation of the WC/H^Og/oxalic acid solution is by no means crystalline, which is not unexpected due to the presence of a high proportion of WO . The tendency for WO to be formed on concentration of the solution is not surprising since this is the most stable form of tungstate at very low pH. In an effort to isolate in crystalline form the main peroxotungstate(VI) species present in the WC-H202-oxalic acid solution, we have used a number of organic cations of various charge. Those cations used in the literature to isolate oxo-, oxoperoxo- and peroxo-molybdate(VI) species include:- pyridinium e.g. CpyH)2[Mo20,t02>4IH20)2l anilinium e.g * CBHeH1 *IM0«°2 «3-2M2 o 1 8 4 o o tetraammine zinc(II) e.g. [Zn(NH3 )4)[Mo(02)4]J 8-hydroxyquinolinium e.g. (c 9h 8n o )[w o (o 2)f 4i74 tetrahexylammonium e.g. C(C6H 13,4N]3[P04{W0(02,2} tetrabutylammonium e.g. (Bu 4n )2e w 6019j 165 1 c c tetraethylammonium e.g. (EW H2W - 9H2° Use of all of these cations has proven unsuccessful in isolating a + crystalline species, as has the use of tetraphenylphosphonium, (Ph.P) , b and melaminium, [C3N3 (NH2)2(NH3 )]+ cations. However, some of the results are worthy of discussion. Addition of tetraphenylphoshonium chloride or iodide to the yellow WC/H202/oxalic acid solution gives off-yellow and orange solids respect 165 ively. Both solids contain around 25Z carbon and 21 hydrogen; they exhibit similar infrared spectra, showing bands at 960, 887 and 794cm” \ as well as those characteristic of the (Ph^P)+ moiety. The Raman spectra of the solids show bands at 955, 900, 800 and 680cm~1. The evidence therefore suggests that a crude form of a tetraphenylphos- phonium salt of an oxo- or peroxo-tungstate species is formed, with WO^ impurity imparting the colour to the solids. Similar results are obtained from the reaction of tetrabutylammonium chloride with the solution. The orange-yellow solid precipitated contains around 15Z carbon, 2.5Z hydrogen and 1Z nitrogen, and shows vibrational bands at 960, 880, 780 and 680cm *. Attempts to recrystallise all three solids from various organic solvents have ended in failure. The reaction of the WC/HgOg/oxalic acid solution with aniline is worthy of consideration. Anilinium 0-octamolybdate(VI) dihydrate, (C_HeN),[Mo00-_].2H.0,bo k o co i is prepared by addition of aniline to a solution o ] 64 of Mo 03 at 80 C and pH 2.5. We have re-prepared this species, and added aniline to our WC-H„0 -oxalic acid solution under similar 2 2 conditions (80°C and pH 1.5). There is no crystallisation in the latter case. When anilinium hydrochloride, CeH_NH_.HCl, is added to the b 3 c solution at room temperature or 80°C the result is a yellow suspension, which if left to stand for several days in a refridgerator changes colour to green, blue and finally violet. Such colour changes are symptomatic of the so called "heteropoly blues" by reduction of hetero polytungstate anions present in the solution. For such species to form, a heteroatom, presumably carbon, would be required in the polytungstate anions, which lends possible support to the findings of Kudo.1*1”2 166 The reaction of the WC-HgO^-oxalic acid solution with tetraethyl ammonium chloride, tetrahexylammonium bromide, 8-hydroxyquinoline, tetraammine zinc(II), melamine, and ammomium chloride in no case provides an isolable salt. 2.4.3 The Action of Hydrogen Peroxide on Molybdenum Carbide Since molybdenum monocarbide, MoC, does not exist at room 167 temperature , we have used the commercial dimolybdenum carbide, which approximates to Mo^C, in our brief study of the action of hydrogen peroxide on molybdenum carbide, for which the additional technique of 95 Ho n.m.r. is available. Using the same metal to H^O^ molar ratio as employed in the tungsten carbide experiments, slow addition of H^Og to an aqueous suspension of dimolybdenum carbide at room temperature provokes a violent exotherm, with the suspension turning green before being forcibly ejected from its r» reaction vessel. With cooling (using an ice-bath) and stirring, and dropwise addition of H^O^, all of the dimolybdenum carbide (ca. 2.5g) 3 is digested by 60Z H^O^ (6*Ocm ) in approximately 2-3 hours. A green- brown suspension and black carbon residue result, the former of which affords an orange-yellow solution (pH 1.6) on filtration. The Raman spectrum of the solution (Fig. 2.22) shows bands at 97 0 sh(7 ) , 948( 10), 91 Obr( 6 ), 870(7), 580sh( 2 ), 560 (4 ), and 380(2) cm"1. This spectrum is very similar in profile to that obtained by Campbell48 for 1.0M [MoO^] /H^O^/H at pH 1.7 (using HC1 for acidification), in which the main species are clearly [ M o ^ (0 ^ (H2°)2]2" and the chloro 167 168 2- species [Mo 0(02)C1^] . Our belief that the main molybdenum species 2- present in the Mo C/H 0 solution is therefore [Mo 0 (0 ) (H 0) 3 is confirmed by the addition of tetraphenylphosphonium chloride. A yellow solid is immediately precipitated, whose infrared and Raman spectra bear close resemblance to that of (Ph.P)_[Mo4 Z Z 0.(0.) 3 Z 4 3 [see Section 2.3.4(b)], as do its elemental and peroxide analyses to a lesser extent. This is not to say that other peroxomolybdate(VI) species do not exist 95 m solution m small amounts. The Mo n.m.r. spectrum of the solution proves difficult to measure due to the presence of free peroxide, but shows broad bands at 6 -196, -260, and +30 p.p.m., which may be attributed to the protonated dimeric species [Mo_00(OH)(0.).(Ho0)0] , 2 2 2 * 2 2 2- the dimeric species [M o^ O^ tO ^^ O^ O^] , and polymolybdate species respectively. Similar results are obtained when a small amount of oxalic acid is added to the Mo2C^H2°2 solution. The possibility that the solution may also contain iso- or hetero polymolybdate species is raised by the existence of the broad Mo=0 band in its Raman spectrum. Moreover, if the solution is left to stand for a period of seven days, it turns blue in colour, and its Raman spectrum loses the bands due to peroxo complexes - displaying only a sharp band at 880cm 1 due to free hydrogen peroxide as well as bands at 979(2), 951(10), 900br(6), 805(1), 435(3), and 370(2) cm”1. This may well be due to formation of “heteropoly blues", which would again seem to indicate the inclusion of a carbon heteroatom in the polymolybdate species and loss of bound peroxide. 169 2.4.A Conclusions The chemistry of the Ho^C/H^O^ system is seemingly made simpler than that of the WC/H2°2 system by the absence of problems with Mo03 precipi- 2- tation. The solution very probably contains [Mo.O,(0.)Z 3 Z 4 (H_0)1 Z Z and tH°202(0H)(02)4(H20)2] as its main species, as expected for a molyb- date-H^O^ solution at pH 1.6, along with other higher peroxo- molybdates(VI) and possibly oxomolybdates(VI), the latter of which feature more when the solution is left to stand. The precipitation of W03 at low pH and laboratory scale helps to make the WC/H2°2 system less clear-cut, and in this case the dimeric 2 - ^ z V ^ V W species does not seem to be the main one in solution, as shown by Raman spectroscopy and the fact that it is not precipitated with tetraphenylphoshonium chloride. One or more peroxo- tungstate(VI) species are probably present, again with oxotungstates(VI) forming after some time, and perhaps with a carbon heteroatom present in some of these species. If one dominant species were present, we would have expected it to have formed a pure salt with at least one of the many counter-cations added. The role of oxalic acid in improving the effectiveness of the WC/H202 recovery process seems to be catalytic. 170 2.5 Fluoro_Carboxvlato Peroxo Complexes of HolvbdenumfVl) and Uranium!VI) 2.5.1 Introduction Several fluoroperoxo complexes of molybdenum!VI) and tungsten(VI) are known (see Section 2.1.3). Similarly, carboxylato peroxo complexes of the two metals have been reported previously (see Section 2.1.3) and in this work. However, the only fluoro carboxylato peroxo complexes of molybdenum!VI) or tungsten(VI) to be reported are A[{Mo0(02)(dipic))2F] (A = K, Na, Rb. Cs, NMe , NEt.), and (NH,)[MoO(OJ(dipic)F).96~7 The former type of complex is exemplified by (NEt^)[{Mo0(02)(dipic)J^F], 97 whose X-ray crystal structure shows two pentagonal bipyramidal {Ho0(02)(dipic)} units linked by a symmetrical Ho-F-Mo bridge. The X-ray crystal structure of the monomeric ammonium fluroperoxo 9 6 dipicolinato complex also shows the anion to have pentagonal bi pyramidal geometry. We have attempted to prepare fluoro oxalato peroxo molybdate(VI) complexes by two routes: reaction of K2[Mo0(02)2 ICO peroxo and carboxylato complexes , but none with all three types of ligand incorporated. 171 2.5.2 Attempted. Preparation s Fluoro Carboxvlato Peroxo Complexes of Molvbdenum(VI) and Uranium(VI) The reaction of potassium oxalato oxodiperoxo molybdate(VI), K^CMoOtO^^tox)], with potassium hydrogen difluoride, KHF2 , a mild fluorinating agent, yields only unreacted K^MoOfO^^tox)] either at room temperature or 50°C. Use of 48Z hydrogen fluoride also has no effect on the oxalato peroxo complex at room temperature or 50°C. The stability of the complex K2tMo0(02)2(ox) ] has been noted in this work and elsewhere, as have the stabilising qualities of oxalate as a bidentate ligand (capable of five-membered ring formation) in the 1 49 presence of the peroxo ligand. It is therefore not surprising that fluoride does not replace either of the peroxo ligands or the oxalato ligand in K2[Mo0(02 )2(ox)]. The tetrafluoro oxoperoxo molybdate(VI) salt K2[Mo0(02)F^].H20 was prepared, and reacted with excess oxalic acid at room temperature, and under reflux conditions. There was no apparent reaction in either case, except that prolonged reflux of the mixture caused peroxide to be lost from solution, as it turned from yellow to colourless. Again, the 2 - stable [Mo0(0 )F.] species persists, with no benefit to be gained from 2 4 replacement of two or more fluoro ligands with an oxalato ligand. The fluoro oxalato uranyl(VI) salts K3[U02 (ox)2F(HgO)].H20 and Na3 CU02(ox)F3(H20)].5H20 were prepared by literature methods from uranyl 169 oxalate. The infrared spectrum of the former complex shows a broad uranyl stretch at 890cm"1, as well as broad U-F vibrations at 492 and 416cm 1. The latter complex shows similar infrared bands at 903, 535 172 and 498cm . Both are yellow solids. Recrystallisation of K3CU02(ox)2F(H20)].H20 from 30Z hydrogen peroxide yields a yellow solid with markedly lower carbon content than the parent compound (5.6Z as opposed to 7.5Z), and new bands at 860 and 666cm 1 in its infrared spectrum. The uranyl stretch is shifted from 890cm 1 to two bands of higher frequency at 917 and 928cm’1. The evidence points to partial replacement of one of the bidentate oxalato ligands by a peroxo ligand. A small amount of peroxidic oxygen is found in the new solid by iodometric titration. It is therefore possible that a crude fluoro oxalato peroxo uranyl(VI) salt has been prepared. The 19 single resonance seen in the F n.m.r. spectrum of K^CUOA(ox)^F(H^O)].- 3 2 2 2 H20 shifts slightly downfield in the new "peroxo" complex due to donation of charge to coordinated peroxide by the fluoro ligand. Recrystallisation of Na3[U02(ox)F3 (HgO)].5H20 from 30Z hydrogen peroxide yields a yellow solid with trace or nil carbon content, a uranyl stretch shifted from 903 to 930cm 1, and containing peroxide. It seems that in this case, peroxide replaces the oxalate ligand in the uranyl species, and that the fluoro oxalato peroxo uranyl(VI) species is not formed. This shows that fluoride is more resistant to displacement than is oxalate in the context of the fluoro oxalato complex of 19 uranium(VI) employed. The F n.m.r. spectra of aqueous solutions of both the original fluoro carboxylato species, and the fluoro peroxo species formed, presumably [U02(02)F3{H20 )]^", were of poor quality (solubility problems) and complicated; however, the resonances were shifted downfield by the inclusion of peroxide, due to the donation of electrons to the peroxo moiety by fluoride. 173 2.6 EXPERIMENTAL SECTION 2.2.1 Preparation of K2[M0(02)2(ox)] (M = Mo. W) This method is an improvement of that reported by Mazzucchelli and 83 4 co-workers. ’ Potassium molybdate, KgCMoO^], (I.Og, 4.2 mmol.) was 3 dissolved m water (20cm ), and a slight excess of oxalic acid dihydrate (0.63g, 5.0 mmol.) added with stirring. The solution was treated with 3 an excess of 301 Addition of ethanol precipitated a yellow microcrystalline solid, which was filtered off, washed with ethanol and air-dried. The tungsten analogue was prepared similarly. FOUND : C, 7.0 ; K, 23.1 ; (02) , 18.6. C K Mo0„ requires c, 7.0 ; K, 22.9 ; t O ^ 2' 18.7Z 2 2 9 FOUND : C, 5.9 ; K, 18.1 ; (02)2’, 14.6. CoKo0oW requires c, 5.6 ; K. 18.2 ; (02 )2“, 14.9Z. 2 2 9 Preparation of K. [MO,. (0 ) (ox) (H_0) ]. H.O (M * Mo, W) 2 2 2 2 2 8 5 The method used is essentially that of Rodriguez , with modification of the isolation procedure. Potassium molybdate (I.Og, 4.2 mmol.) and oxalic acid dihydrate (0.53g, 4.2 mmol.) were dissolved 3 in water (10cm ), and the solution stirred and treated with 30Z H202 3 (0.35cm , 4.2 mmol.). A yellow microcrystalline solid was obtained by immediate addition of a few drops of ethanol and cooling to -5°C. The solid was filtered off, washed with ethanol, and air-dried. The tungsten analogue was prepared similarly. FOUND : C, 7.0 ; H. 0.6 ; K, 22.2 ; (02)2" , 9.5. C H K MoO requires C, 6.6 ; H, 1.1 ; K, 21.6 ; (0 )2‘ , 8.8Z. 2 4 2 10 2 174 FOUND : C, 5.5 ; H. 0.4 ; K, 17.0 ; , 7.8. C2H4K2°10W re(*uires c * 5 *3 5 H * °*9 J K. 17.4 ; (02)2", 7.11. Preparation of Kg[Mo03(oxn.2H20 Potassium molybdate (I.Og. 4.2 mmol.) and oxalic acid dihydrate 3 (0.53g, 4.2 mmol.) were dissolved in water (20cm ). Ethanol was added to produce a flocculent white precipitate, which was filtered off. washed with ethanol and air-dried. FOUND : C. 7.3 ; H, 0.8 ; K, 23.2. C2H4*2Mo09 reclu^res 6 *9 • 1*2 : K, 22.6Z. Preparation of KgCMOIO^gfcit)].0.5H202.3H202 (M = Mo. M) The method used for both salts is that described by Griffith. 95 Wiggins et al. previously for the molybdenum complex. An excess of 3 o 30Z H202 (15cm ) was added at 0 C to a solution of potassium molybdate 3 (1.0g, 4.2 mmol.) and citric acid (0.88g, 4.2 mmol.) in water (15cm ). Addition of ethanol produced yellow crystals, which were filtered off, washed with ethanol and air-dried. FOUND : C, 14.0 ; H, 2.2 ; K, 15.3 ; (02)2”, 15.6. C6H13K2MO°16 re C6H13K2°16W recluires C * 12‘° S H * 2 -2 S K - 13*° • (02 )2“ . 13.3Z. Preparation of K2CMo0(02 )2(L)].2H20 {L * tart(A), mal(B), glyc(C), tartron(D)} and K2CW0(02 )2(L)].2H20 (L * tart(E), mal(F), glyc(G)>. These complexes were prepared by addition of an excess of 30Z H202 to solutions equimolar in CMO^]2” (M = Mo, W) and the carboxylic acid, and isolated in the manner of the oxalato and citrato complexes. 175 The malato complexes were isolated immediately by precipitation with ethanol (see later). (A) FOUND : C, 11.2 ; H. 1.6 ; K, 18.2 ; (02)2". 14.1. C HaK-Ho04_ requires C, 11 .0 ; H, 1.8 ; K, 17.8 ; (02 )2", 14.6Z 4 8 2 13 (B) FOUND : C, 11.1 ; H, 1.2 ; K, 18.8 ; (02)2", 15.3. C , H K M00,o requires C, 11 .4 ; H, 1.9 ; K, 18.5 ; (02)2", 15.2Z 4 8 2 12 (C) FOUND : C, 6.6 ; H, 1.5 ; K, 21.4 ; (02)2", 17.7. C0HeK0MoO,n requires C, 6. 6 ; H, 1.7 ; K, 21.5 ; (Og)2". 17.6Z. 2 6 2 10 (D) FOUND : C, 8.2 ; H, 0.5 ; K, 19.6 ; (02)2‘, 16.8. C H K M°0 requires C, 8.8 ; H, 1.5 ; K, 19.2 ; ( O ^ 2". 15.7Z. 3 6 2 12 (E) FOUND : C. 8.8 ; H, 1.0 ; K, 14.4 ; (02)2‘, 12.7. C H K 0 W requires C, 9.1 ; H, 1.5 ; K, 14.9 ; (02)2", 12.2Z. 4 8 2 13 (F) FOUND : C. 9.7 ; H, 1.1 ; K. 15.0 ; (02)2'. 12.9. C H K 0 W requires C, 9.4 ; H. 1.6 ; K, 15.3 ; ( O ^ 2” , 12.6Z. 4 8 2 12 (G) FOUND : C, 5.6 ; H, 0.8 ; K. 17.0 ; (02)2", 14.7. CoHcK 0 Preparation of K2[Mo0(02 )2(glucon)].2H20 Potassium molybdate (1.0g, 4.2 mmol.) and potassium gluconate, 3 HOCH^CCHtOH)l^CO^K, (0.98g, 4.2 mmol.) were dissolved in water (20cm ), 3 and the solution treated with an excess of 30Z H202 (5cm ). The dark- red solution was treated with dilute HC1 dropwise until the colour just turned light-yellow (pH 3.65). Storage at -5°C, after addition of ethanol, produced a yellow oil, which was filtered off, washed with ethanol, air-dried and crushed to a yellow powder. The complex was found to be hygroscopic, and best stored over silica gel in a desiccator. FOUND : C. H. 4 ; H, 2.1 ; K, 15.7 ; (Og)2", 12.8. C6H14K2Mo017 rec*uires C * 14*9 ; H * 2 ‘9 ; K * 16-1 *. (02)2-, 13.2Z. 176 Preparation of K2tW0(02)2(glucon)].2H20 Potassium tungstate (1.25g, 3.83 mmol.) and potassium gluconate (0.90g, 3.83 mmol.) were dissolved in water, and the solution treated 3 with an excess of 30Z H2°2 The yellow solution (pH 7.8) was treated with dilute HC1 dropwise until colourless (pH 2.6). Addition of o ethanol and storage at -5 C afforded an off-white sticky oil, which was filtered off, washed with ethanol, air-dried and crushed to a white powder. The compound was found to be hygroscopic, and best stored over silica gel in a desiccator. FOUND : C, 12.0 ; H, 1.9 ; K, 13.4 ; (02)2“ , 10.6. C H K 0 W requires C, 12.6 ; H, 2.5 ; K. 13.7 ; (0 )2". 11.2Z. 6 14 Z 17 2 Preparation of K2tHo0(02)2(quin)].2H20 Potassium molybdate (1.0g, 4.2 mmol.) and quinic acid (0.81g, 3 4.2 mmol.) were dissolved in water (10cm ). Addition of excess 30Z H202 gave a dark-red solution, to which dilute HC1 was added dropwise until o the colour just changed to light-yellow (pH 3.3). Storage at -5 C for 24hr., after addition of ethanol, produced a yellow oil; this was filtered off, washed with ethanol, dried in a vacuum desiccator, and crushed to a yellow powder. FOUND : C, 16.3 ; H, 2.1 ; K. 15.3 ; ( O ^ 2’ , 14.4. C7H14K2MO°13 rec>uires C * 17‘5 5 H * 2 '9 ; K * 16’3 ; t02 ,2~* 13-3*- Attempted Preparation of Other Carboxylato Peroxo Complexes of Holybdenum(VI) and Tungsten(VI) Preparation of Mo(VI) and W(VI) oxoperoxo complexes was also attempted using the carboxylates listed below, using equimolar mixtures of K2[M0^) (M = Mo, W) and the carboxylate, and both excess and a 177 stoichiometric amount of H^Og at various pH values. (a) malonic acid, HO^C.CH^CO^H (b) succinic acid, HO^CfCH^JgCO^ (c) adipic acid, H02C(CH2)^C02H (d) salicylic acid, 2-(H0).C H CO H D 4 2 (e) sulphosalicylic acid, S-fHO^S).CgH3-2-(0H)C02H (f) mandelic acid [phenylglycollic acid], PhCH(0H)C02H (g) atrolactic acid [methyKphenyUglycollic acid], PhC(OH)(Me)C02H (h) 3-phenyllactic acid, PhCH2CH(0H)C02H (i) 2-hydroxybutyric acid, EtCH(0H)C02H (j) 2-ethyl 2-hydroxybutyric acid, (Et)2CH(0H)C02H (k) lactic acid. MeCH(0H)C02H In cases (b)-(j), the product of the reaction was the corresponding K2^M2°3 *°2 *4 *H2°* 2^ ‘ 2H2° = Mo‘ salt'» in some cases this was formed at the natural pH of the solution, in others the pH was lowered with 2- dilute HC1 to remove traces of CM(02)^] present at high natural pH. In the case of malonic acid, (a), immediate ethanol addition afforded K2^M2°3^°2*4*H2^2^ ,2H2°' but ** so^uti°ns were allowed to stand, crystals of the oxalato complex salts KgCMOJOg)2(ox)] appeared, as found 8 8 by Djordjevic. Solutions of KgCMO^] (M = Mo, W) and lactic acid, (k), in the presence of H2<)2 gave K2*‘M2°3*°2*4^H2°*2^ *2H20, as did recryst allisation of the known oxo complex [Mo02(lactate)2] f r o m H2°2‘ Studies on the Malato Peroxo Molybdate(VI) System Addition of excess H2<)2 to a solution equimolar in KgCMoO^] and malic acid (pH 3.4), and immediate precipitation with ethanol yielded 178 KgCMoOfO^)2(mal)].2 H2 O (see above). However, if the solution was allowed to stand without ethanol addition, yellow crystals of the oxalato complex K2[Mo0(02)2(ox)] were formed [FOUND : C, 7.1 ; H, nil ; K, 23.2 ; ( O ^ 2 . 18.9. C2K2Mo0g requires C, 7.0 ; H, nil ; K, 22.9 ; (02 )2", 18.77.] . - 3 Addition of excess 307 H2°2 *5cm 1 *° a soluti°n of molybdenum trioxide, Mo03 , (2.0g, 13.9 mmol.), potassium hydroxide pellets (1.56g, 27.8 mmol.) and malic acid (1.86g, 13.9 mmol.), as performed by Djordjevic 88 , yielded a yellow solution of pH 3.0. On standing, yellow crystals of K2[Mo0(02)2(ox)] appeared. [FOUND : C, 7.0 ; H, nil ; K, 23.1 ; (02 ) 2- , 18.6Z ; see above for required values]. X-ray Crystal Structure of K2[Mo0(02)2(glyc)].2H20 Yellow crystals of the title complex were prepared as described above. The crystal selected for intensity data collection was an elongated prism, approximately 0.25 x 0.04 x 0.03 mm. Measurements were carried out on a Nicolet R3m/Eclipse S140 diffractometer system, using graphite-monochromated Cu-K radiation, by Richard Powell and Dr. a Andrzej Skapski of Imperial College Chem. Dept. Unit-cell dimensions were determined by least-squares refinement of the angular settings of seventeen automatically-centred reflections. CRYSTAL DATA : c 2H0K2Mo O 1O' M = 364.21, triclinic, space group PT, a = 7.222(4), b = 7.950(6), c = 10.450(5) A, a s 67.81(4), 0 = 73.72(7), Y = 64.84(4)°, U = 497.7 l3 at 20°C, 1 * 2. D » 2.43 gem"3, F(000) = 351.9. A(Cu-K ) = 1.5418 A, p(Cu-K ) = 191.2 cm"1. 179 Integrated intensities in one hemisphere were measured using the u)-scan technique. Two reflections (110 and 1TT) were monitored every 50 measurements, and these decreased by ca.6Z over the period of data collection (1 day). A total of 1518 independent reflections were measured (to 8 = 57°), of which 49 were judged to be “unobserved" [I < 3a (I)3. The data were scaled using the reference reflections and were corrected for Lorentz and polarisation effects. At a later stage an empirical absorption correction was applied170, based on 3G psi-scan measurements for each of 8 representative reflections. All calculations and drawings were made using the SHELXTL program system170, and atomic scattering factors and anomalous dispersion corrections were taken from Ref. 171. The coordinates of the Ho atom were derived from an original Patterson synthesis, and the positions of all the other atoms were found from subsequent Fourier difference syntheses. Least-squares refinement was by the block-cascade method, typical of the SHELXTL system. All non-hydrogen atoms were refined with anisotropic thermal parameters, while the positions of the glycollate hydrogen atoms were tied to those of the parent carbon atom; those of the water hydrogen atoms were fixed and their isotropic thermal parameters were allowed to refine. A 2 2 weighting scheme was applied so that w = 1/[o(F o ) + 0.0007(F o ) ] for the last cycle; R reduced to 0.035, and R ‘ = [Ew|AF|2/Ew|AF 130 *5 o was 0.038. Fractional coordinates of the non-hydrogen atoms are listed in Table 2.13, overleaf. 180 Table 2.13 Atomic,coordinates,of the non-hydrogen atomt (x 1041 for tfliy s l]1 2H2Qj*i-th .9 «it..dif... in pargnthm? Atom X y z Mo 1540(1 ) 3577(1) 2794(1) K (1 ) 2999(2) 2043(2) 6405(2) K (2) 2958(2) 8129(2) 649(2) 0(1 ) 1247(7) 5270(7) 3509(5) 0(2) 1549(8) 1367(7) 4493(5) 0(3) 3624(7) 1333(6) 3780(5) 0(A) 1656(8) 5100(7) 816(5) 0(5) 3706(7) 3857(7) 1247(5) 0(6) -1498(6) 4328(6) 2931(5) 0(7) 1215(6) 1559(6) 1919(4) 0(8) 3148(7) 2067(6) 9131(5) 0(9) 3410(8) 7520(7) 3695(6) 0(10) -1075(7) 499(6) 1741(5) C( 1 ) -2298(10) 3104(9) 2747(7) C (2) -618(10) 1585(9) 2079(6) 181 Preparation of K4tM202l02>4 lC*H206n . 4 H 20 (M * Mo, W) Potassium molybdate (I.Og, 4.2 mmol.) and (+)-tartaric acid (0.31g, 3 2.1 mmol.) were dissolved in water (10cm ). Addition of excess 30Z 2 2 3 (10cm ) produced a dark-red solution, which was adjusted to pH 4.0 to give a light-yellow colour by dropwise addition of dilute HC1. The yellow complex was precipitated with ethanol, and recrystallised from water-ethanol at -5°C as yellow needles. FOUND : C, 6.8 ; H, 1.3 ; K, 21.8 ; (02)2“. 17.8. C4H10K4MO2°20 requires C* 6'6 5 H’ ; K* 21‘5 5 17.62. The tungsten complex was similarly prepared, except that the solution (pH 2-3) needed no pH adjustment, and gave extremely thin white crystals. FOUND : C, 5.6 ; H, 0.9 ; K. 16.8 ; (Og)2", 15.6. C4H10K4°20W2 requires C * 5 ‘3 ; H * 1*1 S K * 17.3 ! (02,2~' u *27* Preparation of (NH I CMo 0 10 ) IC.H O.J1.4H 0 ♦ ♦ Z Z Z 4 4 Z o Z Ammonium molybdate (1.0g, 5.1 mmol.) and (+)-tartaric acid (0.38g, 3 2.55 mmol.) were dissolved in water (10cm ). Addition of 302 H2°2 3 (5cm ) lowered the pH from 2.0 to 1.5, and formed a yellow solution. from which a microcrystalline solid was precipitated with ethanol. The solid was filtered off, washed with ethanol and air-dried. FOUND : C, 8.0 ; H, 3.0 ; N. 8.8 ; (02 )2“. 20.8. C4H26N4MO2°20 requires c » 7 .5 ; H, 4.0 ; N, 8.7 ; <02)2“, 19.92. 182 The X-ray Crystal Structure of K^CHo OgtOg) (C H^O^) ] .4«2<> Yellow crystals of the complex were prepared as described above. The crystal selected for intensity data collection was of dimensions ca. 0.25 x 0.25 x 0.05 mm. The structure and calculations were carried out by Richard Powell and Dr. Andrzej Skapski of Imperial College Chem. Dept, using the same instruments and techniques as for the structure determination of K2[Mo0(02)2(glyc)].2H20 (outlined above). CRYSTAL DATA : q K2MO2°20 ’ orthorhomb^c • space group P2^2^2^, £ * 1 1.079(3 ), b = 11.341(3), c = 15.328(5) A , 11 = 1925.9 A3 at 20°C, Z = 4, F(000) = 1399.62, A(Cu-K ) = 1.5418 A, p(Cu-K ) s 197.65cm’1. a a A total of 1496 independent reflections were measured (to 8 = 57°), of which 56 were judged to be "unobserved". The structure was solved 170 using the SHELXTL program system, and after absorption correction, least-squares refinement gave R = 0.044. Fractional coordinates of the non-hydrogen atoms are listed in Table 2.14, overleaf. Preparation of Potassium Tungstate, K2CW0^] Potassium tungstate is no longer easily commercially available. The salt was prepared from sodium tungstate, Na^WO^].2H20 , by acidifi cation of an aqueous solution until no more flocculent yellow W03 was precipitated, followed by addition of K0H until the pH reached 7-8 and all the W03 dissolved. White crystals appeared after ca. 24hr., and were filtered off and washed with cold water (3 x 10cm3). 183 Table 2.14 Atomic,Coordinates of the non-hydrogen atom»(x 104) for A 4IMa2^2lQ;J.4JX;ii2^ cil^4W20 with e.a.d.a. in parentheses Atom X y z Mo(1) 176(1) 12449(1) 9301(1) Mo (2) 1295(1) 8192(1 ) 8950( 1 ) KM) 5140(2) 9386(2) 9592(2) K(2) 5571(3) 9961(2) 2374(1) K( 3) 2998(3) 9087(3) 6109(2) K C 4) 6767(3) 6932(3) 6662(2) 0(5) 675(7) 8020(6) 10367(4) 0(3) -281(7) 11139(5) 10086(4) 0(25) 279(7) 6767(6) 8840(5) 0(2) -1521(7) 11764(6) 8739(4) 0(4) -214(7) 9066(6) 9004(4) 0(12) 849(8) 11335(6) 8433(5) 0(14) -484(9) 14030(6) 9116(5) 0(21 ) 1566(8) 8530(7) 7898(4) 0(13) 703(8) 12538(6) 8112(5) 0(11 ) 1456(8) 12745(7) 9850(5) 0(7) 6834(12) 9333(9) 7181(6) 0(24) 1573(9) 6500(7) 9006(5) 0(1) -2892(7) 10317(7) 8854(5) 0(23) 2849(7) 8399(7) 9506(6) 0(6) -590(9) 8860(7) 11309(4) C (2 ) -1287(11) 10440(8) 9916(6) C(1) -1980(9) 10870(9) 9107(6) C (4 ) -247( 1 1 ) 8665(9) 10562(8) C (3 ) -923( 1 1 ) 9167(9) 9772(6) 0(15) -946(8) 13504(6) 9944(5) 0(8) 726(9) 12778(8) 12059(6) 0(22) 2134(8) 9505(7) 9503(5) 0(9) 2540(11) 8484(10) 1615(6) 0(10) -782(11) 5399(9) 12107(6) 184 SECTION 2.2.3 Preparation of (Ph^P^tMoOtO^lox)] and ( P h ^ [H o ^ C O ^ t C ^ O g U Tetraphenylphoshonium chloride (2.19g, 5.8 mmol.) was added to a solution of Kg(Mo0(0 ) (ox)] (I.Og, 2.9 mmol., in 25cm^ of water), and the mixture stirred for 24hr. The yellow product was filtered off, 3 washed with water (3 x 10cm ), and dried in a vacuum desiccator over silica gel. FOUND : C, 61.9 ; H, 4.3 ; P, 6.8. C50H40MO°9P2 reC|uires C * 63-7 '* H * 4 -3 '• p * 6 -6*- The tartrate salt was prepared similarly by adding tetraphenylphos- phonium chloride (1.03g, 2.75 mmol.) to K ^ t M o ^ l O ^ I C ^ O g ) ] .4H20 3 (0.50g, 0.69 mmol.) in water (25cm ). FOUND : C, 62.7 ; H, 3.9 ; P, 6.1. C100H82MO2°16P4 rec>uires C * 64‘7 5 H * 4 *5 S p * 6 -7*- Preparation of CMo0(02)(dipic)(H^O)]97 A solution of molybdenum trioxide, MoO , (1.82g, 12.65 mmol.) in 3 o 30Z ^2^2 ^3®cm ^ was Seated 60 C. The solution was treated with dipicolinic acid, C_H_N(C0_H)_, (1.79g, 10.7 mmol.) and turned orange. o J it Heating was stopped after 7hr.; a flocculent bright-orange precipitate was formed on standing overnight, and was filtered off, washed with cold 3 water (3 x 10cm ), and dried in a vacuum desiccator over silica gel. An iodometric titration of the complex was possible by stirring a sample of it in dilute (with potassium iodide added) for Ihr. before titration against thiosulphate. 185 FOUND : C, 26.0 ; H, 1.4 ; N, 4.2 ; i O ^ 2" . 9.9. CjHgMoNOg requires C, 25.7 ; H. 1.5 ; N, 4.3 ; C02)2~. 9.8Z. IR (nujol mull) : 975vs, 904s, 594s, 573m cm"1. RAMAN (solid, KBr disc) : 980(10), 910(5), 602(3), 578(8) cm"1. Attempted Reaction of (Ph^P^CMoOlO^tox)] with SO^ and C<>2 A saturated solution of (Ph^P)^CMoOtO^Jgtox)] in dichloromethane was placed in a sealed round bottom flask. The flask was flushed with nitrogen, and S02 bubbled through the solution for 30 min. Hexane was added to the solution, whereupon a yellow oil was formed. The super natant solution was decanted off, and the oil dried in vacuo to yield a fluffy light-yellow solid. FOUND : C, 55.5 ; H, 3.8 ; S, 6.6Z. IR (nujol mull) : 1107s. 995m. 946m, 915m,br, 790m, 752w. 721s . 689m. 529s cm 1, [bands due to (Ph^P)+ are underlined]. Similar treatment with C02 gas yielded an unreacted sample of (Ph4P) [Mo0(0 ) (ox)]. Preparation of [MoOIO^IHMPA) (H20)]101 Molybdenum trioxide, Mo03 , (2.0g, 13.9 mmol.) was dissolved in excess 30Z H2°2 ^ 0cm3* *°°c * The solution was transferred to a 10°C ice/water bath and was treated with hexamethylphosphoramide, HMPA, (2.49g, 13.9 mmol.). The yellow precipitate was filtered off, washed with diethyl ether (5 x 10cm3 ), and air-dried. FOUND : C, 18.7 ; H, 4.7 ; N, 11.1 ; P, 10.0. C6H20MoN3°7P recluires c * 18*9 ' H * 5*3 • N * 11 • 0 ; P, 10.32. 186 IR (nujol mull) : 966s, 870s, 860s, 645m, 582s, 533s cm'1. RAMAN (solid. K6r disc) : 970(10), 873(8), 647(2), 584(5), 549(9) cm'1 RAMAN (dichloromethane solution) : 974(10), 884(8), 650(2), 567(7)cm'1 Stoichiometric Epoxidation of Cyclohexene Using [MoOtO^t^lHMPA)(H^O)] The 'h n.m.r. spectra of cyclohexene and cyclohexene oxide were 2 H H recorded in d -dichloromethane. Cyclohexene [6 5.66, t, ; 6 1.97, m, 1.60, m, cyclohexyl (CH^) protons ; p.p.m. rel. to SiMe^]. Cyclohexene oxide [6 3.05, t, ; 8 1.80, m, 1.30, m, cyclohexyl (CH^) protons ; p.p.m. rel. to SiMe^]. [MoO(0^)2 (HMPA)(H^O)] (38mg, 0.103 mmol.) was placed in a 5mm n.m.r. tube containing cyclohexene (17mg, 0.207 mmol.) in d^-dichloromethane 3 1 (1cm ). The H n.m.r. spectrum was measured after 2hr., and epoxidation found not to be complete. The spectrum was remeasured after 24hr. [6 3.05, t, ; 8 1.80, m, 1.30, m, cyclohexyl (CH^) protons ; p.p.m. rel. to SiMe^]. Attempted Epoxidation of Cyclohexene Using (Ph^P)2CMoO(02 )2(ox)3 and 'eW ’Vz'OzWzV 1 (Ph4P)2[MoO(02)2(ox)] (38mg, 0.04 mmol.) and cyclohexene (80mg, 3 3 0.10 mmol.) were dissolved in d -acetonitrile (1cm ) in a 5mm n.m.r. H H tube and left for 24hr. before n.m.r. spectroscopy [6 5.66, t, ; 8 1.97, m, cyclohexyl (CH^) protons ; p.p.m. rel. to SiMe^]. The spectrum was observed to be the same, with no evidence of epoxidation, after a period of 7 days. Similarly, ( P h ^ P J ^ M o ^ f t y ^ C ^ O g ) ] (llOmg. 0.06 mmol.) and cyclohexene (lOmg, 0.12 mmol.) were dissolved in d 3 -acetonitrile (1cm 3 ) in a 5mm n.m.r. tube. There was no sign of epoxidation after 24hr. or 187 H H 7 days [in both cases : 6 5.65, t. y = < ; 5 1.96, in. 1.60, m, cyclohexyl (CH2 ) protons ; p.p.m. rel. to SiMe^]. Stoichiometric Oxidation of Alcohols with (Ph^PJ^HoOIO^lox)] The oxidation procedure for p-anisyl alcohol was typical. p-Anisyl alcohol (24mg, 0.174 mmol.) and (Ph^P)2£Mo0(02)2(ox)] (164mg, 0.174 mmol.) were dissolved in dichloromethane (50cm3) in a round bottom flask. The solution was refluxed with stirring for 3hr., then evaporated to dryness using a rotary evaporator, and the residues 3 extracted with diethyl ether (2 x 25cm ). The combined ethereal extracts were evaporated to dryness, and the aldehydic product dissolved 3 in minimum volume of ethanol. This solution was treated with 10cm of an acidic, saturated solution of 2,4-dinitrophenylhydrazine (see below) to yield an orange-red precipitate of the corresponding 2,4-dinitro- phenylhydrazone. This was filtered off, washed with cold water (3 x 3 5cm )and dried to constant weight. p-anisylalcohol : Yield of 2,4-DNPH derivative = 53mg Theoretical yield = 55mg l Yield of p-anisaldehyde = 96Z o o172 Melting point of derivative = 230-232 C (lit. 233 C ) benzyl alcohol : used substrate (23mg, 0.213 mmol.) and oxidant (200mg, 0.213 mmol. Yield of 2,4-DNPH derivative = 53mg Theoretical yield = 61mg l Yield of benzaldehyde = 872 Melting point of derivative = 239-242°C (lit. 237°C172) 188 vanillyl alcohol : used substrate (19mg. 0.123 mmol.) and oxidant (116mg, 0.123 mmol.) Yield of 2,4-ONPH derivative = 38mg Theoretical yield = 41mg Z Yield of vanillylaldehyde = 92Z Melting point of derivative = 262-265°C (lit. 269°C172) Preparation of 2,4-Dinitrophenylhydrazine Solution 3 2,4-Dinitrophenylhydrazine (4.0g) was dissolved in methanol (250cm ) and the suspension treated with sulphuric acid dropwise until all of the hydrazine just dissolved. 189 SECTION 2.2.4 Detailed pH Study of the Formation of [M0(02)g(glyc)]2~ (M s Ho, W) Kg[MoO^] (2.Og, 8.4 mmol.) and glycollic acid (0.64g, 8.4 mmol.) 3 3 were dissolved in water (15cm ). Treatment with 30Z H2°2 *5cm * 9ave a dark-red solution (pH 7-8) which turned yellow (final pH 3.9) in less than 10s. Use of Na2CMoO^].2H20 (2.03g, 8.4 mmol.) gave identical results (final pH 3.3). K2CW0^] (0.5g, 1.53 mmol.) and glycollic acid (0.12g, 1.53 mmol.) 3 3 were dissolved in water (15cm ). Treatment with 3QZ H.0 (5cm ) gave 2 2 a yellow solution (pH ~ 5) which became colourless (final pH 3.8) in about 6min. Use of Na2[WO^].2H20 (0.5g, 1.53 mmol.) gave similar results (final pH 3.2, after ca. 4min.). All complexes were isolated and checked for purity. Attempted Separation of Molybdenum and Tungsten Using the 1:1 Metal:61ycollate System Na2[Mo0^].2H20 (2.42g, 10.0 mmol.) and Na^WO^]. 2H20 (3.3 mmol.) 3 were dissolved in water (20 cm ) to give a solution equimolar in molyb denum and tungsten. Glycollic acid (1.52g, 20.0 mmol.) was added, 3 3 followed by 30Z H202 (5cm ) with stirring. Ethanol (- 20cm ) was added, and the solution was placed in a freezer overnight. A yellow crystalline material was precipitated, which was filtered off, washed with ethanol and air-dried (Yield 2.8g, equivalent to 84.3Z recovery of molybdenum if sample were pure Na2CMo0(02)2(glyc)].2H20). The sample 13 became sticky with time, indicating impurity, as confirmed by C n.m.r. spectroscopy and elemental analysis. 13C N.M.R. : 187.5, 186.6, 76.0, 74.9 p.p.m. (relative to DSS). 190 FOUND : C, 5.4 ; H, 1.6 ; ( O ^ 2", 17.3. Na2CMoO(02)2(glycJ].2«20 requires C, 7.2 ; H, 1.8 ; <02)2“ . 19.3Z. Na2[W0(02)2(glyc)].2H20 requires C, 5.7 ; H, 1.4 ; (02)2“, 15.2Z. An experiment with a molybdenum-rich solution [using Na2[MoO^].2H20 (1.94g, 8.0 mmol.), N a ^W O^ ]. 2H20 (0.66g, 2.0 mmol.) and glycollic acid (0.76g, 10.0 mmol.)] also gave a sticky yellow solid (Yield 2.3g, equivalent to a 87.3Z molybdenum recovery if sample were pure Na CHo0(02)2(glyc)].2H20). FOUND : C, 5.6 ; H, 1.6 ; (02)2“ , 18.0Z. Detailed pH Study of the Formation of CM Z 0 Z (0 Z ) 4 (C ( H Z 0 6 ) ] * ” (H s Mo, W) KgCMoO^] (2.0g, 8.4 mmol.) and (+)-tartaric acid (0.62g, 4.2 mmol.) 3 were dissolved in water (15cm ). A pH electrode was placed in the solution, and pH readings taken every 30s after addition of 30Z H202 3 (5cm ), with constant stirring. The pH became stable at 4.2 (ca. 6min.). A pH-time plot appears in Fig. 2.14a. The procedure was repeated using Na2[Mo0^].2H20 (2.03g, 8.4 mmol.) instead of KgCMoO^], in which case a final pH of 4.3 was reached in approximately 4min. (see Fig. 2.14b). *2*W04^ 3 *07 mmol.) and (+)-tartaric acid (0.23g, 1.53 mmol.) 3 were dissolved in water (20cm ). The solution was treated with 30Z 3 (5cm ) and the pH monitored as above; final pH was 3.2 (ca. lOmin.) (see Fig. 2.14c). The procedure was repeated using Na„[W0,].2H„0 z * z (1.01g, 3.07 mmol.) instead of KgCWO^]; final pH was 3.25 (ca. 2Qmin.) (see Fig. 2.14d). All complexes were isolated and checked for purity. 191 Attempted Separation of Molybdenum and Tungsten Using the 2:1 Metal:Tartrate System Na2[Mo043.2H20 (2.42g, 10.0 mmol.) and Na [WO ]. 2H20 (3.30g, 3 10.0 mmol.) were dissolved in water (20cm ) to give a solution equimolar in molybdenum and tungsten. (+)-Tartaric acid (1.50g, 10 mmol.) was 3 added, followed by 30Z H2°2 ^5cm The solut;*-on turned dark-red, and 3 then to yellow in around 5min. (pH 3.25). Addition of ethanol (15cm ) o and storage at -5 C overnight yielded a crystalline product, which was filtered off, washed with ethanol and air-dried. On inspection, the product proved to consist of both yellow and white crystals, indicating that both molybdenum and tungsten tartrato peroxo complexes had formed. X-ray Diffraction analysis (performed by the analytical department of Interox Chemicals Ltd., Widnes) of the yellow solution showed that molybdenum and tungsten had both been removed in substantial amounts. Analysis of the bulk crystalline material obtained confirmed its impurity. FOUND : C, 6.5 ; H, 1.4 ; (02 )2“ , 17.1. Na4[Mo202(02)4(C4H20g )].4H20 requires C, 7.3 ; H. 1.5 ; (02)2", 19.3Z. Na4CW202(02)4 (C4H206)].4H20 requires C. 5.7 ; H. 1.2 ; (02)2', 15.3Z. 192 SECTION 2.3.1 Preparation of K2CMo203 (02)4(H20)23.2H2039 A solution of potassium molybdate (5.0g, 21.0 mmol.) in water 3 (100cm ) was placed in an ice/water bath. Treatment with 30Z H2°2 3 (10cm ) gave a dark-red solution, to which dilute hydrochloric acid was added dropwise until the colour just turned bright-yellow, at pH o 4-5. A yellow crystalline solid appeared after 24hr. at 5 C, and was filtered off, washed with ethanol, and air-dried. FOUNO : H, 1.3 ; K. 14.9 ; (02)2", 24.4. H8K2M02°15 re<^uires C * 1’6 • K * 15-1 > <02>2-, 24.71. Preparation of K.[Mo 0 (0.),(2H_OI_3.22H_0 2 2324 22 2 2 K2^°2°3^°2^4 ^H2°^2^ ,2H2° was c,^ssolvecl *-n minimum volume of H2<), and the solution left at 5°C overnight. The yellow crystals formed were 3 filtered off, washed with ethanol (2 x 5cm ), and dried in a vacuum desiccator over silica gel. That deuteriation had been effected was confirmed by the appearance of the following bands in the infrared spectrum : 2595, 2490 [v(0-2H)3, 1195 [6(2H-0-2H)3 cm’1. Preparation of (H 0) J. 2Ho0*6 2232422 2 3 A solution of potassium tungstate (2.0g, 6.1 mmol.) in water (15cm ) 3 was treated with 30Z H202 (10cm ). The yellow solution formed was treated with dilute hydrochloric acid dropwise until it just turned colourless (pH 2.5). White crystals appeared on standing at 5°C for 24hr., and were filtered off, washed with ethanol, and air-dried. FOUND : H. 1.0 ; K, 11.0 ; C02)2”, 18.6. H8K2°15W2 re(luires H * 1 2 S K * I1-3 : (02)2“, 18.4Z. 193 Preparation of K2CW203*°2,4(2H20I23.22H20 K2*W2°3*°2*4*H2012* *2H2° Was recrystall*se<1 'Fro,n 2h2° at 5°C. The 3 white crystals were filtered off, washed with ethanol (2 x 5cm ), and dried in a vacuum desiccator over silica gel. The following new bands in the infrared spectrum confirmed that deuteriation had taken place : 2620, 2480 [v(0-2H)3, 1210 [6(2H-0-2H)3 cm"1. .$ ECU OIL?,?, 2 Preparation of Cs^CMoO^-HgO-HgO^ Solutions Dilute HC1 was added to a saturated solution of potassium molybdate until the flocculent precipitate of HoO^ ceased to form. This precipi tate was dissolved with caesium molybdate to give a fresh solution of caesium molybdate, Cs^CMoO^, to which excess 102 was added to give a yellow solution (pH 10). Aliquots of this solution were adjusted to higher pH using caesium hydroxide, and to lower pH using dilute HC1. Above pH 11, the solutions became orange-red and effervescent, and gradually lost oxygen to leave a colourless solution. At pH 10.7, a yellow solid, (A), was isolated by ethanol addition. Similarly at pH 9.4, a yellow solid, (B), was isolated using ethanol. The solids were filtered off, washed with ethanol, and air-dried. (A) FOUND : Cs, 57.0 ; (Og)2", 12.8Z. VIBRATI0NA1 DATA (cm'1): INFRARED (nujol mull) : 956s, 938s, 932s, 904m, 874sh, 857s, 843s, 760br.ni, 691m, 665s, 627m, 607m, 572s,532m RAMAN (solid,KBr disc): 959(10), 930(3), 908(7), 892(5), 879(5), 840(2), 812(1), 783(1), 755(1), 636(3), 609(2), 568(5), 530(8), 504(2), 392(1), 348(2), 312(6), 298(2), 262(1) 194 (6) FOUND : Cs. 55.7 ; (Og)2", 13.2Z. VIBRATIONAL DATA (cm-1): INFRARED (nujol mull) : 956s, 938s, 933s, 904m, 874s, 855s. 841s, 756br,m, 691m, 665m, 628m, 608m, 570s, 533m RAHAN (solid,KBr disc): 960(10), 931(4), 908(7), 879(5), 841(4). 782(2), 756(1), 636(4), 609(3), 568(5), 530(8), 504(2), 392(1), 348(3), 317(5), 299(2), 262(1) 95 The Mo n.m.r. spectra of (A) and (B) show broad resonances at 2- 6 -245 and 6 -5 p.p.m. in D^O solution (relative to ChoO^] ). SECUPH-g .3 J Preparation of (NH4 IgCMOjQ022(02)12).16«2045 Ammonium heptamolybdate, (NH ) [Mo 0..3.4H 0. (3.72g, 3.0 mmol.) was 4 6 7 24 2 3 dissolved in water (1.5cm ), and the solution treated with 307 H202 3 (2.8cm ). The pH of the solution was adjusted to 2.8-3.0 using nitric acid (8M). The dark-yellow solution yielded yellow crystals after ethanol addition and storage at 5°C; these were filtered off. washed with ethanol, and air-dried. FOUND : H, 2.4 ; N. 5.3 ; (02)2\ 17.6. H64MO10N8°62 rec»uires H * 3*° S N, 5.3 ; (Og)2', 18.0Z. VIBRATIONAL DATA (cm"1): INFRARED (nujol mull) 953vs, 947s, 927s, 891vs, 872s, 849s, 730br, 686m, 622s, 554m, 472w, 335w RAMAN (solid.KBr di,c : ns(if! • siiTlV: i§mi:§§8(11: 111(11: 516(4), 474(2), 366(1), 334(1), 309br RAMAN (aq. solution) 970(10), 943(8), 872(4), 660(2)br, 565(4). 302(2) MOLYBDENUM-95 N.M.R 6 -247 p.p.m.(broad), relative to [MoO^] 2- 195 Preparation of ( N H ^ t M o 0?(02) ].2«2041 A solution of ammonium heptamolybdate (15.45g, 12.5 mmol.) in water 3 3 (25cm ) was treated with 60Z H202 (0.52cm , 12.5 mmol.). The bright- yellow solution (pH 5.5) was adjusted to pH 6 using ammonia solution. The solution was placed in a refridgerator, whereupon yellow crystals formed overnight. The crystals were filtered off, washed with ethanol, and air-dried. FOUND : H, 2.6 ; N, 7.9 ; (Og)2", 19.7. H20MO3N*°17 recluires H * 3 *2 • N * 8 -8 ; 20-1*. VIBRATIONAL DATA (cm-1): INFRARED (nujol mull) : 953s, 918s, 885s, 841m, 820w. 796w, 670br, 616m, 586w, 559m, 470m, 315w RAMAN (solid,KBr disc) : 958(8), 936(10), 900(8), 884(6), 864(4), 619(2), 587(3), 566(5), 556sh, 400(2), 363(4), 336(4), 314(4), 237sh. 219(7) RAMAN (aq. solution) : 957(7), 934(10), 897(9), 874sh, 850(2), 580sh, 558(4), 353(1), 317(3), 306(4), 210( 2 ) Preparation of Kg[Mo7022(02)23.8H20^^ Potassium molybdate (12.38g, 52.0 mmol.) was dissolved in water 3 3 (40cm ), and the solution treated with 60Z H2°2 ^2cm The ®'F'Ferves" 3 cent yellow solution was treated with concentrated HC1 (ca. 3.7cm ) until pH was 6.2. Addition of ethanol and storage in a freezer at -5°C afforded a yellow solid, which was filtered off, washed with ethanol, and air-dried. FOUND : H, 0.9 ; K, 15.6 ; (02)2”,4.2. H1BK6Mo 7°34 recluires 18*° 1 (02)2~, 4.4Z. VIBRATIONAL DATA (cm"1): INFRARED (nujol mull) : 935s, 900vbr, 840s, 825m, 650vbr, 580s, 470m, 368w 196 RAMAN (solid.KBr disc) : 930(10). 921(4). 910(6). 898(8). 892(8). 872(6). 844(2). 830(1). 632(1). 588(1), 557(3). 467(1), 451(2). 410(2), 366(5), 336(3), 312(4), 225(8) RAMAN (aq. solution) : 945(10), 939sh, 926(5), 908(4), 897(5), 870br, 600br, 569(2), 372(3), 353(2), 306(2), 238sh, 212(2) SECTION. 2.3.4 The Raman Spectrum of K^CMoO.]/H„0„ at pH 3.0 and 75°C 2 4 2 2 3 Potassium molybdate (1.0g, 4.2 mmol.) was dissolved in water (10cm ) 3 and the solution treated with excess 30Z H2°2 *10cm cooling from an ice/water bath. Dilute HC1 was added until the solution reached pH 3.0. This yellow solution was placed in a solution cell for Raman spectroscopy at room temperature, and at approximately 75°C (the heat was applied to the spinning cell using a clamped hair-drier). RAMAN (aq. solution at 25°C) : 971(10), 940sh, 873(5), 630(2). 565sh, 540(7)cm~' RAMAN (aq. solution at 75°C) : 970(10), 937(9), 872(6), 620(1), 567(5), 542(8)crn Upon cooling and standing, the solution yielded yellow crystals of K2^Mo2°3^02^4^H2°^2^‘2H20, wh*ch were filtered off, washed with ethanol and air-dried. FOUND : H, 1.4 ; K, 14.9 ; (Og)2”, 24.1. HKMo O c requires H. 1.6 ; K. 15.1 ; (0 )2‘, 24.7Z. o i c 10 2 VIBRATIONAL DATA (cm"1): INFRARED (nujol mull) : 957vs, 853vs. 720s, 615w, 580s, 537m, 452w, 335w RAMAN (solid,KBr disc) : 957(10), 949(9), 865(9), 722(2), 627(1), 578(6). 542(4), 448(1), 351(3), 342(1), 261(1) 197 Preparation of fph4P*2*Mo2°3*°2*4* K2CMo 203 (02)4(H20)2].2H20 (I.Og, 1.93 mmol.) was dissolved in water 3 (50cm ). Addition, with stirring, of tetraphenylphosphonium chloride (1.45g, 3.86 mmol.) afforded a flocculent yellow-white precipitate immediately (pH 5.4). The solid was filtered off, washed with water 3 (3 x 10cm ), and dried in a vacuum desiccator over silica gel. FOUND : C, 56.0 ; H. 3.8 ; (0 )2", 12.5 ; P. 6.1. 2 C48H40MO2°11P2 re(*uires C * 5 5 1 * H - 3 *9 ; (02,2~* 12*2 5 P * 5 *9Z* VIBRATIONAL DATA (cm-1): INFRARED (nujol mull) : 1108vs, 997s, 967s, 900m, 873s, 866sh, 769s, 765s, 744m, 722s, 690s, 592m, 562m, 533vs, 457w, 333w RAMAN (solid.KBr disc) : 1108(2), 1096(4), 1028(5), 1001(10), 960(5), 912(4), 875(4), 726(4). 678(4), 616(4), 594(1), 559(4), 326(2), 251(3) RAMAN (acetonitrile solution) : 965(10), 875(6), 725(1), 606(4), 560(5), 250(3) Preparation of (Ph^P^CWgOjfOg)^] .(Hw0) ]^H^O (0.50g, 0.72 mmol.) was dissolved in water 2232422 2 3 (20cm ). Addition of tetraphenylphosphonium chloride (0.54g, 1.44 mmol.) with stirring afforded a clear solution (pH 6.3), from which an off- white solid precipitated after 30min. The solid was filtered off, 3 washed with water (3 x 10cm ), and dried in a vacuum desiccator over silica gel. FOUND : C, 47.0 ; H, 3.2 ; (02)2~, 10.1 ; P, 4.9. C48H40°11P2W2 recluires c » *7.2 ; H, 3.3 ; (Og)2', 10.5 ; P, 5.1Z. VIBRATIONAL DATA (cm-1): INFRARED (nujol mull) : 1106vs. 997s, 962s, 924m, 861m, 836s, 760s, 722vs, 986s, 614m, 576m, 457w. 326w 198 RAMAN (solid,KBr disc) : Sample decomposed in the laser beam. Preparation of (Bu^NJ^tMo^O^O^)^] KgCMo^O^OgJ^O^O^] .2H20 M.Og, 1.93 mmol.) was dissolved in water 3 (30cm ), and the solution treated with tetrabutylammonium hydrogen- sulphate (1.31g , 3.86 mmol.) with stirring. The flocculent yellow 3 precipitate was filtered off, washed with water (3 x 15cm ) and dried in a vacuum desiccator over silica gel. Recrystallisation from dichloromethane afforded a crystalline sample of the complex. FOUND : C, 44.2 ; H. 7.2 ; N, 2.9 ; (02)2” , 15.6. C32H72M02N2°11 re(*uires C * *5-1 ; H ' 8*5 ; N * 3*3 5 15*0Z* VIBRATIONAL DATA (cm”1): INFRARED (nujol mull) : 960s, 946s, 665s, 720m, 593m, 534m. 454w, 332w RAMAN (solid,KBr disc) : 963(10), 880(6), 715(1), 586(4), 556(6), 448(3), 330(1) RAMAN (acetonitrile solution) : 960(10), 878(7), 710(3), 593(3), 558(6) The complex is similarly prepared from tetrabutylammonium chloride, but the analogous tungsten complex can not be isolated using either tetra butylammonium reagent. Stoichiometric Oxidation of Alcohols Using (Ph^P)2CM203 (02 )^] (M =Mo, N) The oxidation procedure for p-anisyl alcohol using the molybdenum version of the complex is typical. p-Anisyl alcohol (24.7mg. 0.179 mmol.) and (Ph4P)2[Mo203 (02)^] (187mg, 0.179 mmol.) were dissolved in dichloromethane (50cm3 ) in a round bottom flask. The solution was stirred for 3hr., after which TLC 199 (50:50 diethyl ether : petroleum ether) showed the reaction to be complete. The solution was evaporated to dryness, and the residue 3 extracted with diethyl ether (2 x 25cm ). The combined ethereal extracts were evaporated to dryness, and the product dissolved in the 3 minimum volume of ethanol. The solution was treated with 10cm of an acidic, saturated of 2,4-dinitrophenylhydrazine in methanol to yield the hydrazone derivative. This was filtered off, washed with cold water, and dried to constant weight. The yields obtained for the oxidation of several alcohols, and reaction times, are tabulated overleaf. Investigation of Competing Epoxidation : The Reaction of Cyclohexene with (Bu^N)2CMo203 (02)43 Cyclohexene (7.5mg, 0.091 mmol.) and (Bu^N)2[Mo203<02) ( 7 7 . 6 m g , 3 0.091 mmol.) were dissolved in CD3CN (2cm ) and the mixture stirred under nitrogen for 2*hr., after which time the solution had changed from dark-yellow to light-yellow. The proton n.m.r. spectrum was measured [6 5.50, t, ; 6 3.00, t, ; 6 1.78, m, 1.30, m, cyclohexyl (-CHg-) protons ; p.p.m. relative to SiMe^]. The ratio of integrals of the 6 5.50 triplet to the 6 3.00 triplet was approx. 1:1*. 200 Stoichiometric Oxidation of Alcohols with fph4P)2^H2°3*°2*4^ s Mo,w* Alcohol 2,4-DNPH Derivative Time/hr Z Yield of Carbonyl Cpd Melting Point/°C b Actual Lit. p-anisyl (M=Mo) 226-228 233 3 97 p-anisyl (M=W) 225-228 233 3 96 benzyl (M=Mo) 233-236 237 3 85 vanillyl (M=Mo) 264-266 269 3 89 piperonyl (M=Ho) 261-262 265 3 92 a-tetralol (M=Mo) 248-253 256° 15 60 cyclooctanol (M=Mo) 24 20 cinnamyl (M=Mo) 239-245 255 15 45 geraniol (M=Mo) 67-71 79-80d 15 54 citronellol (M=Mo) 124-129 134-5® 15 32 a All oxidations carried out in dichloromethane, and at room temperature, with a 1:1 substrate : oxidant molar ratio. b Ref. 172, except c, d, e, Refs. 283 (i),(ii),(iii) respectively 201 Stoichiom etric Oxidation of Alcohols with (Bu^Nl^tMo^O^CO^)^] The oxidation procedure was the same as described for the tetra- phenylphosphonium oxidants above. All oxidations were carried out in dichloromethane at room temperature (1:1 substrate:oxidant molar ratio) Alcohol 2,4-DNPH Derivative Time/hr Z Yield of Carbonyl Cpd Melting Point/°C Actual Lit.. 4 . a p-anisyl 229-231 233 3 97 piperonyl 259-263 265 3 94 a-tetralol 251-255 256b 15 63 geraniol 66-69 79-80° 15 47 a Ref. 172. b Ref. 283(i ) c Ref. 283(ii) 202 SECTION 2.4 Preparation of NC/H^O^/Oxalic Acid Solution Commercial WC powder (5.3g) was suspended in water (40cm3) in a conical flask, and oxalic acid dihydrate (0.25g) added. The mixture was 3 stirred, placed in a water-bath, and treated with 601 H2<)2 (13.0cm ) over a period of 5-10min. Heat was applied to the solution after H202 addition; the solution reached an exotherm at around 90°C, and was kept o stirring at 80 C for 4-5hr. The suspension turned from black to green, and finally to bright-yellow (with a black residue of carbon) during the course of the reaction. The yellow suspension was centrifuged, and the supernatant yellow solution removed for Raman study and further use, leaving a yellow residue of WOg. RAMAN (spinning solution cell) : 9?3(10)p, 965(10)p, 889(2)p, 721(5)dp, 568(4)p, 300(1)dp cm-1 Evaporation of the solution yielded a yellow, glassy solid : RAMAN (solid, KBr disc) : 966(10), 850(6)br, 680(5) cm"1 Recovery of Tungsten as CaNO^ A yellow suspension prepared as above was filtered, and washed into a sodium hydroxide solution,to convert all tungsten to sodium tungstate Addition of CaCl2 (5.0g) to the solution and standing for 2-3hr. afforded a white precipitate of CaWO^, which was filtered, washed well with water, and dried at 105°C to constant weight. Yield of CaWO^ = 7.65g (equivalent to 981 recovery of tungsten). 203 Addition of Ph^PCl, P^PI and Bi^NCl to WC/H^/Oxalic Acid Solution Ph^PCl (0.10g) was added to an aliquot of the yellow WC/H202/oxalic acid solution with stirring. The off-yellow precipitate formed was filtered off, washed with water and dried in a desiccator. FOUND : C, 23.3 ; H. 1.6Z. VIBRATIONAL DATA (cm”1): INFRARED (nujol mull) : 1108vs, 995m, 964sh, 952br,s, 883br,s, 790s, 742w, 720m, 687m, 528s, 395w RAMAN (solid,KBr disc) : 1102(7), 1028(8), 1002(10), 955(4)br, 890(2), 685(4), 680(4), 620(3), 540(2) Similarly, addition of Ph^Pl afforded an orange-yellow powder, which was filtered off, washed with water and dried in a desiccator. FOUND : C. 27.2 ; H, 2.QZ. VIBRATIONAL DATA (cm”1): INFRARED (nujol mull) : 1108vs, 996m, 960br,s, 887br,s, 852w, 794s. 750w, 721s, 689s, 529vs RAMAN (solid,KBr disc) : 1101(7), 1028(7), 1002(10), 950(4)br, 885(3)br, 806(2), 680(4), 615(4), 538(4) Addition of BuNCl (0.10g) to an aliquot of the WC/H^O^/oxalic acid 4 2 2 solution yielded a yellow-brown solid, which was filtered off, washed with water and dried in a desiccator. FOUND : C, 14.1 ; H. 2.7 ; N. 1.1Z. VIBRATIONAL DATA (cm"1): INFRARED (nujol mull) : 1150w, 1105w, 1064w, 960br,s, 879br,s, 780br,m, 680w, 570m RAMAN (solid,KBr disc) : 960(10)br, 882(4), 785(5), 682(3), 566(2) 204 Attempted Preparation of KgIW40flf ° 2 V C03*I,6H20*7 3 Solutions of potassium tungstate (0.82g, 2.52 mmol, in 20cm of 3 water) and potassium carbonate (0.35g, 2.53 mmol, in 10cm of water) were mixed, and treated with 30Z (2.5cm3) at 5°C (pH 10.5). The white precipitate formed was filtered off, washed with ethanol and air-dried. FOUND : C. NIL ; H. 0.4 ; K. 13.0 ; (Og)2-. 16.5. CH12K6°29W4 requires C ' ° ‘8 5 H * °*8 ; K * 16‘1 ; (02 ,2~* 13-2*- VIBRATIONAL DATA (cm-1): INFRARED (nujol mull) : 953m, 936s, 924s, 890s, 871vs, 845s, 790br, 699s,br, 620w, 538m, 504s, 440w, 385sh RAHAN (solid,KBr disc) : 958(10), 936(5), 899(4), 884(4), 867(2), 828(3), 665(3). 650(3), 569(2), 428(2), 321(3) Preparation of W/H202/0xalic Acid Solution 3 Tungsten powder (5.0g) was slurried in water (40cm ), and oxalic acid dihydrate (0.25g) added. The solution was placed in a water-bath, 3 stirred and treated with 60Z H202 (13.0cm ) over a period of 5-10min. o The suspension was stirred at 80 C for 4-5hr., after which time a green- yellow suspension had formed. The suspension was centrifuged, and the light-yellow supernatant liquid removed for Raman spectroscopy. RAHAN (spinning solution cell) : 985( 10)p, 890(5)p , 725(3)dp, 560(5)p , 304(1)dp Preparation of Mo2C/H202 Solution A suspension of Mo2C (2.5g) in water (25cm3) was cooled using an ice/water bath, and 60Z adc,ed dropwise over a period of 5min. The solution was kept stirring at 10-15°C for 3hr., by which time it had turned to a yellow-brown suspension with a black residue of carbon. 205 Filtration of this suspension yielded an orange-yellow solution of 95 pH 1.6, which was subjected to Raman and Mo n.m.r. spectroscopy. RAMAN (spinning solution cell) : 979(7 )sh, 948(10), 910(6)br, 870sh, 580(2)sh, 560(4), 380(2) MOLYBDENUM-95 N.M.R. : 6 +30, -196, -260 p.p.m. (rel to [MoO^]2") All resonances broad. Addition of tetraphenylphosphonium chloride (1.76g, 4.71 mmol.) was added to one-sixth of the orange-red solution (assumed 100Z digestion of 3 Mo2C in calculation of amount of Ph4PCl) and water (15cm ). A yellow precipitate was formed immediately; this was filtered off, washed with 3 cold water (3 x 10cm ) and dried in a desiccator over silica gel. FOUND : C, 42.5 ; H. 2.7 ; (Og)2”, 9.9Z. VIBRATIONAL DATA (cm”1): INFRARED (nujol mull) : 1109s, 998m, 970s, 954vs, 914s, 083m, 869s, 802br, 754m, 723s, 690s, 642m, 612m, 598m, 532vs, 452w, 330w RAMAN (solid,KBr disc) : 1098(4), 1028(5), 998(10), 969(5), 965br(4), 910(3), 866(3), 677(4), 614(3), 576br(3). 560(2), 314(2), 286(2) 206 Reaction of KgtMoOtO^tox) J with KHF^ and HF A solution of K^CMoOtOgJ^tox)] (0.5g# 1.46 mmol.) in water (30cm**) was stirred with potassium hydrogen difluoride, KHF^, (0.11g, 1.46mmol). Unreacted K [Mo0(0 ) (ox)] was found to crystallise from the solution 2 2 2 after reaction at either room temperature or 50°C. Use of an excess of KHF2 produced no reaction either. Similarly, unreacted K2CMo0(02)2(ox)] was recovered after its treatment with 481 HF at room temperature or 50°C. Preparation of K^MoOtOgJF^].HgO 3 Potassium molybdate (I.Og, 4.2 mmol.) was dissolved in water (10cm ) 3 and 48Z HF (10cm ) in a polypropylene beaker, and the solution treated with 30Z HgOg (2cm3 ). Yhe yellow solution was heated to 50°C, at which temperature a flocculent yellow precipitate was formed. The solid was 3 filtered off, washed copiously with ethanol (5 x 10cm ) and air-dried. 2- FOUND : K, 25.2 ; (02) , 9.8. F4H2K2MO°4 requires 2**7 5 (02>2~. 10.1Z. VIBRATIONAL DATA (cm"1): INFRARED (nujol mull) : 957vs, 930sh, 903vs, 734s, 560vbr,s, 460vbr,s, 378m, 295m, 254m RAMAN (solid,KBr disc) : 964(10), 935(4), 910(6), 565(3)br, 384(4), 296(5) Reaction of K^MoOtO^F^l.H20 with Oxalic Acid The reaction of K^MoOtO^F^].H20 (I.Og, 3.16 mmol.) and oxalic acid dihydrate (0.50g, 3.97 mmol.) in water (20cm3 ) at room temperature yielded only unreacted K2CMo0(02)F4I.H20. as did reflux of the complex 207 with excess oxalic acid dihydrate for 0.5hr. Further refluxing resulted in decolorisation of the solution, due to loss of peroxidic oxygen. Preparation of Uranyl Oxalate, CUOgfox)],3H20 173 Uranyl nitrate, U02(N03) .6H20 (5.02g, 10.0 mmol.) was dissolved in 3 water (25cm ), and the solution warmed and treated with a solution of 3 oxalic acid dihydrate (1.26g, 10.0 mmol., in 15cm of water). The yellow solution yielded a yellow precipitate of CU02(ox)].3H20 upon cooling, which was filtered off, washed with ethanol, and air-dried. FOUND : C, 5.8 ; H, 1,3. CHOU requires C. 5.8 ; H, 1.5Z. Z 6 9 INFRARED (nujol mull) : 1317s, 845vs, 810s, 653br, 497s, 383vs cm 1 ICQ Preparation of K3CU02(ox)2F(H20)l.H20 Uranyl oxalate (2.75g, 6.67 mmol.), potassium fluoride (Q.62g, 6.67 mmol.), and potassium oxalate (1.23g, 6.67 mmol.) were mixed intimately with mortar and pestle. The mixture was dissolved in water 3 (15cm ), and thi solution heated to boiling. Yellow crystals appeared on cooling, and were filtered off, washed with ethanol, and air-dried. FOUND : C, 7.5 ; H, 0.5 ; K, 18.6. C4FH4K3°12U recluires C * 7 '8 S H * °*7 : K * 19-07- VIBRATIONAL DATA (cm"1): INFRARED (nujol mull) : 890vs, 835w, 793s, 780s, 720w, 492br,s, 416m, 365w, 275s RAMAN (solid,KBr disc) : 902(10), 501(8), 486(8) 169 Preparation of Na3tU02(ox)F3(H20)].5H20 Uranyl oxalate (2.0g, 4.85 mmol.) and sodium fluoride (0.61g , 14.6 mmol.) were mixed intimately with mortar and pestle. The mixture 208 3 was dissolved in water (10cm ), and the solution heated to boiling. The solution was filtered while hot, and the filtrate left to cool. Addition of a little ethanol afforded a yellow microcrystalline solid, which was filtered off, washed with ethanol and air-dried. FOUND : C, 4.4 ; H. 1.2. C2F3H12Na3°12U re(*uires c * *-1 '• H * 2-0*- VIBRATIONAL DATA (cm"1): INFRARED (nujol mull) : 903vs, 798s, 721w, 585br, 535vbr, 498s, 366s, 330m, 286m, 271m RAMAN (solid,KBr disc) : 902(10), 816(4), 585(7), 500br(8), 310(6) Recrystallisation of K V [1)0 4 (ox) b F(H fc0)].H ZO and Na3CU02 (ox)F3 (H20)].5H20 from *3^U1J21o ><12F1H20 ^ ,H2° WaS d^ssolved minimum 307 H2°2 and t0 recrystallise at 5°C. The yellow microcrystalline solid obtained was filtered off, washed with ethanol and air-dried. FOUND : C, 5.6 ; H, 0.4 ; (Og)2", 3.8Z. INFRARED (nujol mull) : 928sh, 917vs, 898s, 860m, 835w, 800s, 741s, 721m, 666w cm" Na3[U02(ox)F3(H20)].SHgO was similarly recrystallised from H202 , and the yellow solid obtained filtered off, washed with ethanol and dried. FOUND : 0.4 ; H, 1.5 ; (02 )2“, 5.2Z. INFRARED (nujol mull) : 930br,vs. 884s, 861w, 722br,s, 666w cm"1 209 CHAPTER THREE PEROXO COMPLEXES OF THE GROUP Va AND IVa TRANSITION METALS .CHAPTER THREE PSROXO. COMPLEXES OF THE GROUP Va AND GROUP IVa TRANSITION METALS 3.1 INTRODUCTION 3.1.1 General Background In each of Groups Va and IVa, the chemistry of the first row members (vanadium and titanium respectively) is again considerably different from that of the second and third row element pairs (niobium/tantalum and zirconium/hafnium respectively). The effects of the lanthanide contraction (discussed in Chapter One) render the chemistry of each pair of second and third row metals remarkably similar, as seen in the case of molybdenum and tungsten. In fact, the chemistries of zirconium and hafnium are closer in nature than any other pair of second and third row elements. Their atomic and ionic radii are practically identical (1.45 and 1.44 A for Zr and Hf ; 0.74 and 0.75 A for Zr*+ and Hf* + ) due to the 1 74 lanthanide contraction. The peroxo chemistry exhibited by the metals of each group is no exception to this trend. Vanadium, in the (V) oxidation state, and titanium, in the (IV) oxidation state, show extensive peroxo chemistry, and their peroxo complexes have been widely used in organic oxidation processes.7 The peroxo chemistry of niobium and tantalum is much less studied and understood than that of vanadium, and that of zirconium and hafnium has received relatively scant consideration in the scientific literature. 210 For these reasons, and due to the fact that we are particularly interested in finding differences in the peroxo chemistry of the two second/third row element pairs, the work presented in this chapter is mainly concerned with new peroxo chemistry of niobium, tantalum, zirconium and hafnium. A few topics of more academic interest involving vanadium and titanium peroxo compounds are included. Consequently, the review of the literature that follows is fairly comprehensive for niobium, tantalum, zirconium and hafnium peroxo chemistry, whereas the known peroxo chemistry of vanadium and titanium is dealt with relatively briefly. 3-1.2 Peroxo Complexes of Group Va 3 -1.2.1 Vanadium The vanadium(V) oxo species present in aqueous solution over the full range of pH have been thoroughly studied using a wide range of ^8 175 51 techniques, most notably Raman spectroscopy ' , V n.m.r. spectro- 176-7 17 176 scopy and 0 n.m.r. spectroscopy. At very high pH, the only 3- species present is the orthovanadate ion [VO.3 , but as the solution 4 is acidified, the situation becomes more complex, involving dimers, tetramers and decamers.5* 170~9 At very low pH (<1), the dioxovanadium(V) ion, [V023 , is the main species, and this complexes if suitable anions are present; for instance, if HC1 is present the complex anion becomes trans-rvn cl ]3~. 40 2 4 211 As we saw in Section 2.1.2, addition of hydrogen peroxide to the aqueous CMoO^] or [WO^] systems upon acidification reduces the degree of polymerisation, but leads to a complex peroxo chemistry none the less. The same applies to the CVO^33 - H 202—H + system, on which there is generally reliable data in the literature. Peroxo complexes of 2- 4:1, 3:1, 2:1 and 1:1 C(02 ) ]:V stoichiometry are known to be formed, 5 as reviewed by Connor and Ebsworth. 4 8(i ) 51 The recent work of Campbell in these laboratories used V n.m.r. and Raman spectroscopy to identify the main peroxo species present over the full pH range. Of considerable help was the comparison of Raman spectra of solutions with those of known peroxovanadates(V), examples of which are (NH ) [V 0_(0.) ]180 (NH ) [V (OHIO (0o),].6° 4 4 c J C 4 4 3 Z c C h The results obtained differ from those presented in the only other 51 182 V n.m.r. study on the system , but only in the intermediate pH range - in the formulation of the oxodiperoxo species present in this range. Campbell proposes the scheme shown in Fig. 3.1 on the basis of his study. It can be seen that at very high pH, the tetraperoxo vanadate(V) 3- species £V(0_).] is predominant, while various monomeric diperoxo and Z 4 triperoxo species and dimeric tetraperoxo species exist at intermediate + pH, and the monoperoxo species tVOC02)3 appears at very acidic pH. A wide range of vanadium(V) peroxo complexes in which the dioxygen ligand is stabilised by heteroligands, ranging from halides to carboxy- lates, have been reported in the literature. 212 + H CV(02 ) ^ 3 CV0(0 ) ]3 2 3 pH 14-13 pH 14-13 K H OH M CV02(°2,2]3 Chvo(o2)3]2 pH 14-10 pH 10-4.5 K H N M 3- c h v o 2(o 2)2]2 C H W ^ V pH 7.4-6.7 pH 6.7-4.5 + H N [vo(o2)2] pH 6.7-1 . 1 K + H OH N CVO(02)D pH < 1.0 Fig.3.1 Equilibria Present in the [V04]3~-Excess h ^ - h * SvstemA8(i) 213 The syntheses of several haloperoxo vanadates(V) have been reported recently, in the main by Chaudhuri and co-workers; the recent work on these, and on vanadium(V) peroxo complexes containing the carbonato ? *! ligand, has been recently reviewed by Chaudhuri. X-ray crystal structures have been reported for a number of peroxo vanadate(V) complexes containing carboxylate, amino-polycarboxylate and nitrogen donor ligands. In general, the complexes tend to include oxo ligands and adopt seven-coordinate pentagonal bipyramidal geometries - as is the case for molybdenum!VI) and tungsten(VI) peroxo complexes including heteroligands. A notable exception is (NH,)CV0(0_)„(NH_)], 4 2 2 3 183 which has six-coordinate pentagonal pyramidal geometry. Some of the most interesting work is being performed by Djordjevic, using amino- 184 polycarboxylate heteroligands such as IDA (iminodiacetate) and 185 NTA (nitrilotriacetate) ; work on heteroligand vanadium(V) peroxo 186 complexes up to 1982 has been reviewed by Diordjevic. The oxalato and 2,2-dipyridyl peroxo complexes of vanadium(V) have provoked much disagreement in the literature. The X-ray crystal 187 structure of l<3[VO(0^) (ox)].H20 appeared as early as 1975. Later, 188 claims were made for the complexes A3[V(0^) (ox)].2H20 (A = Na, K, NH^). Although there is evidence for triperoxo vanadate(V) species in solution^8^ ^ , isolated triperoxo complexes are rare and only found for niobium and tantalum.*89-191 The potassium salt of the "triperoxo" 188 species prepared by the literature method in these laboratories was shown by X-ray structure analysis to be K [V0(0 ) (ox)].H 0 ,192 3 2 2 2 2 Also, the (NH^)[V(02)3(bipy)].4H20 complex earlier claimed by 188 Guerchais and Sala-Pala was shown by Campbell to be the diperoxo 214 192 species (NH^KVOtO^I^Ibipy)] .^H^O by X-ray structure determination. Covalent vanadium(V) peroxo species of formula [V0(0 )(pic)(L)(L‘)] 193 194 (for example L, L' = H^O or L, L' = bipy ), and anionic peroxo 1 93 species of formula AtVOtO^)(pic)^] (for example A = H ) have been synthesised. These complexes can be used for the stoichiometric oxidation of hydrocarbons, as reviewed by Mimoun.^^ The most common oxidation achieved is epoxidation of alkenes, and the mechanism by which the vanadium(V) peroxo complexes effect this differs completely from . • 193-5 that exhibited by molybdenum(VI) and tungsten(VI) peroxo complexes. It seems that epoxidation results from homolytic, bimolecular electro philic attack of a radical terminal oxygen atom (from a peroxo group) on the alkene. The easy regeneration of vanadium(V) peroxo species from oxo species in makes the former species useful as catalysts in and alkylhydroperoxide oxidations.*^ 3.1.2.2 Niobium and Tantalum Niobium and tantalum are metals, but in the (V) oxidation state, from which all their known peroxo complexes are apparently derived, they have chemistries in many ways similar to non-metals, in that they have little cationic chemistry, but a wealth of anionic chemistry. In solution, niobate and tantalate isopolyanions are obtained by fusion of the respective oxides with alkali hydroxide or carbonate and dissolving the melt in water, but are only stable at high pH. Addition of hydrogen peroxide can lead to the formation of 4:1, 3:1, 2:1 and 1:1 (O.)2 :M peroxo-niobate(V) and -tantalatetV) species.5 Unlike vanadium, 215 niobium and tantalum tend to form peroxo species without attendant oxo ligands. The review of niobium(V) and tantalum(V) peroxo chemistry that follows is subdivided into three sections : aqueous peroxo chemistry; peroxo complexes containing heteroligands; and finally reactivity and application of the niobium(V) and tantalum(V) peroxo complexes. 3.1.2.2(a) Aqueous Peroxo Chemistry Before considering the effect of peroxide addition on aqueous solutions of niobate(V) or tantalate(V), let us briefly discuss the basic aqueous chemistry of the metalates. Solutions containing iso polyanions of the metals, obtained from fusion of the oxide with alkali hydroxide or carbonate, are only stable at high pH, with precipitation 196 occurring below -*7 for niobates and -10 for tantalates. The only 0 " X species that appear to be present are [H M_0 ] (x = 0,1,2). The X o 19 8 — X-ray crystal structures of potassium salts of CMgO ig] (M = Nb, Ta) 197 have been determined , and show the isopolyanions to be made up of M0g octahedra. The Raman and infrared spectra of solid Kg[Mg0ig].1BH^O (M = Nb, Ta) 198*9 have been measured , and show four infrared and eight Raman bands (three less than calculated in each case) ranging from around 220cm 1 [terminal 6(M=0)] to around 870cm * [in-phase v(M-O)]. The Raman spectra of solutions of the hexaniobate and hexatantalate species are very similar to those of the respective solids , indicating that the isostructural [HO] anions retain their structures in solution. D 19 216 Other isopolyniobates have recently been isolated and structurally c_ 200 a_ 201 determined, notably [Nb 0 ] and [Nb O . J . The decaniobate 1U co cc 59 anion has been structurally determined by X-ray analysis in two salts, (NMe^,6[Nb10°283*6H2° and (NMe4,4Na2CNbio0283,8H2°‘0*5MGOH* in b°th °f which it is found to exist as an isolated group of ten condensed NbOe octahedra. Crystals of Cs.CNbo_0e_] and RbA[Nb_*0__J are found to be 8 cc 59 8 22 59 isomorphous, and the structure of the isopolyanion consists of NbO 6 polyhedra linked together through corners and edges. When hydrogen peroxide is added to an aqueous solution of niobate(V) or tantalate(V) at high pH, white crystalline solids containing the 3- CM(02)^] tetraperoxo anion can be isolated, as first reported by Balke and Smith. 202 Salts of this type of anion are generally stable in air at room temperature, and dissolve in water without decomposition; the tantalum version of the species is generally more stable and less 5 soluble. The potassium salts K fM(0 ),].nH„0 (H = Nb, Ta) were first 3 2 4 2 203 isolated and characterised by Gngor eva and Selezneva , while Guerchais and co-workers later studied the ammonium salts (NH ) CHC0 ) ] 204-6 (H = Nb, Ta) using radiocrystallography and infrared spectroscopy , and reported vibrational bands at around 815-800cm 1 [0-0 stretch] and 575-520cm 3 (metal peroxo stretches). The first tetraperoxo niobate(V) or tantalate(V) to be structurally 207 determined by X-ray diffraction techniques was KMg[Nb(0g)^].7Hg0. 3_ The [NbtOg^] anion was shown to be dodecahedral, with near Dg^ 26 symmetry, and therfore isostructural with chromium(VI) and molyb- 38 denum(VI) tetraperoxo species; the mean 0-0 distance was found to 217 be 1.50 t. Other salts of the [ M I O ^ ] 3 (M = Nb. Ta) species that have been isolated are too numerous to mention, though the organic-soluble A [Ta(0,),] [A = guanidinium, (Bu.N)208} and (Me.N).[Nb(0q).]209 ■3 c k k kick examples are of particular interest. The thermal decomposition of [M(02)4]3 (M = Nb, Ta) species has recently been studied by Jere.21° Addition of dilute acid to solids or solutions containing the tetra- peroxo niobate(V) or tantalate(V) anion affords a gelatinous precipitate of the monoperoxo species CMO2(02)3 (M = Nb, Ta), as first shown by 202 203 Balke and Smith , and later by Grigor’eva and Selezneva. The niobium species is a light-yellow colour, the tantalum species is white. More recent evidence for the [Nb02(0^)3 species was presented by 211. Spinner et al. , using UV-spectroscopic and cryoscopic data from ,6- solutions of [Nb 0 ] and hydrogen peroxide; evidence for triperoxo 6 19 2- species such as CNb(02)33 and [HNbO(02)33 was also reported in this publication. 212 More recently, Bogdanov and co-workers have reported the mono peroxo tantalate(V) compounds ACTaO^lO,^)] (A = K, Rb, Cs), prepared from the reaction of TaClg with H202 and the appropriate alkali hydroxide or carbonate; the compounds were characterised using infrared spectroscopy and thermal analysis. Other non-heteroligand peroxo niobate(V) and tantalate(V) species 213 209 claimed include Rb3CNb0(02 )3] , [Nb202(02)2(0H)2] , and the cationic species CNb(H202)(OH) ]+ . 21* 218 3.1.2.2(b) Heteroliqand Peroxo Complexes Incorporation of heteroligands into the peroxo complexes of niobium(V) and tantalum(V) has the effect of generally enhancing the stability of and changing the reactivity of those complexes, as with the peroxo complexes of the other early transition metals. The hetero ligands used range from halides to carboxylates, amino-polycarboxylates and nitrogen donor ligands. Fluoroperoxo Complexes A wide range of fluoroperoxo niobate(V) and tantalate(V) complexes have been reported in the literature, the most common of which are those 2- of formula [M(0_)F_] (M = Nb, Ta). Fluoroperoxo niobates(V) of this 2 5 stoichiometry were first prepared and characterised by Balke and Smith 202 as long ago as 1908. 2- Several salts of the [Nb(0«)F_] anion have been characterised by C 3 X-ray crystal structure determination : Na2[Nb(02)F5].H20 215 Na2tNb(02)F5].2H20 216 Na3(HF2)[Nb(02)F53 217 ) CNb(() ) FI 218 {(C H N ) = 1 ,10-phenanthrolinium} 12 10 2 2 5 12 10 2 c d 2 1 Q (CgHgN0)2CNb(02 )F5] * {(CgHQN0) = 8-hydroxyquinolinium} All contain a pentagonal bipyramidal anion, in which the side-bonded 1 9 peroxo ligand occupies two of the equatorial positions. Early F n.m.r. studies on the aqueous [Nb(02)Fg] species in these labora- 219 tories proved inconclusive, but later work indicated retention of the anion's pentagonal bipyramidal structure in solution. 220 Infrared and Raman studies on solid K_[Nb(0_)F_].H„02 2 5 2 by Jere and co-workers 221 revealed a distinct 0-0 stretch at around 890cm -1 and NbtO^) stretches at around 620 and 560cm as well as various Nb-F vibrations, all of which are assigned. Recent infrared and Raman 222-3 studies by Nour and co-workers on solid "Na-[Nb(0_)F_]"2 2 5 (quotation marks by A.C.O., since there are three possible sodium pentafluoroperoxo 215-7 niobate species, as found by Stomberg , and the authors of this later work do not report analysis of the sample used for spectroscopy) have produced completely different results (see Section 3.4 for more detail). 19 The F n.m.r. spectrum of Ko[Ta(0„)F_]2 2 5 measured in these laboratories71 indicated a capped octahedral structure for the complex anion in aqueous solution, (I), due to the AX^ pattern observed. 0-- 0 F / (I) F The X-ray crystal structure of the double salt K CTa(0 )F_3.KHF was 2 2 5 2 224 2- reported by Ruzic-Toros et al. in 1976. The CTa(0_)F_) anion was c 5 established to have an irregular octahedral geometry, with one corner in 220 the middle of the 0-0 bond of a capping peroxo ligand. However, problems with crystal decomposition were mentioned, and the 0-0 distance . • 2- found was substantially longer (1.64 A) than that found in [Nb(0.)Fe] 2 5 21 5 " 8 salts , and the structure was redetermined by Stomberg six years 225 later. The salt crystallised in this case as K_(HF_)[Ta(0_)F-], and 3 2 2 5 was found to contain a pentagonal bipyramidal complex anion (0-0 distance 1.443 A at 170K). 3 - The [M(0_)_F,] (M = Nb, Ta) species have also been isolated using 2 i 4 several counter cations. The salts A3[M(0^)^F^].HgO (M = Nb, Ta ; A = K, Na, NH, ) were first properly characterised by Djordjevic and Vuletic191'226. The X-ray crystal structures of (NH^)3CTa(02)2F^] and K^LTafO^^F^] have since been determined by Schmidt, Pausewang and Massa227, and that of (NH^)3[Nb(02)2F^] by Ruzic-Toros et al.228 In all 3 - cases, the ^ ( 0 ) F ] anion displays octahedral coordination, with two of the corners of the polyhedron at the centre of the 0-0 bonds of the peroxo ligands. This is interesting in view of the fact that eight- coordinate oxalato peroxo complexes of niobium(V) and tantalum(V) assume 227 distorted dodecahedral geometry (see below). Schmidt et al. also present infrared and Raman data for the A^CTa(0_)_F,3 (A = K, NH.) species; these, and the fact that K_[Ta(0.)_F.]J 2 2 4 thermally decomposes to tetragonal K_[TaO F ] with cis-standing oxygen atoms, indicate cis 3 2 4 positions for the peroxo groups in the octahedral structure. Shchelokov et al.229 have claimed the isolation of the hexafluoro monoperoxo complexes A3 [Nb(02)Fg] and difluoro triperoxo complexes A3CNb(02)3F23 (A = K, NH^), and Massa and Pausewang have determined the 230 X-ray crystal structure of K6^Ta3 (°2 ,30F13]-H2 0 - The unit cel1 of the 221 latter species contains two different and independent complex anions : 2- a pentagonal bipyramidal [Ta(02)Fg3 anion, and a dimeric anion, 4 _ [Ta 0(0 ) Fq] , consisting of two pentagonal bipyramidal {Ta(0.JF,} c c c b 2 4 units linked by a Ta-O-Ta bridge. The mean 0-0 distance in the dimeric anion (1.70 A) is abnormally long for a fluoroperoxo niobate(V) complex, 2- o while that in the [Ta((>2 )Fg] anion is 1.39 A, a more usual length. The difference in 0-0 distances in the peroxo ligands is reflected in the vibrational spectra, where two bands are seen for each of the v (0-0), vs[Ta(0 )] and vas[Ta(0 )J modes. 2 2 Several fluoroperoxo complexes of niobium(V) and tantalum(V) also containing another type of heteroligand have been reported. Guerchais 231 and co-workers first reported the (Et.N)[Ta(0.)F.(H_0)34 Z 4 2 complex , and later its reaction with 2-methylpyridine-N-oxide to give the complex 232 (Et 4 N)[Ta(0 Z )F 4 (C o H 7 NO)]. The X-ray crystal structure of the latter complex was determined, and revealed a pentagonal bipyramidal structure, in which the bidentate peroxo ligand, two fluorine atoms and the oxygen atom of the 2-methylpyridine-N-oxide assume the equatorial positions. The same authors have also reported the preparation of 0-keto-enolato fluoroperoxo complexes of type (Et,N)[Ta(0_)F.(diket)]4 Z 3 (diket = bidentate diketonate, e.g. acac), from the reaction of diketones with 233 (EtN)[Ta(0)F (H 0)3. 4 i h i Trifluoro monoperoxo complexes containing the bidentate o-phen- anthroline (o-phen) and monodentate triphenylarsine ligands are also reported by Guerchais and co-workers for niobium(V) and tantalumtV)77. formulated as [M(02)F3 (o-phen)] and CM(02)F3 (Ph3AsO)23 (M = Nb, Ta). The same paper also reports dimeric 2,2-dipyridyl (bipy) complexes 222 (bipy(M = Nb, Ta) containing a M-O-M bridge between two ■C H (0 g) Fg (bipy)} moieties, with analytical and vibrational data to support their formulation. Chloroperoxo Complexes Chloroperoxo complexes of niobium(V) and tantalum(V) formulated as 234 A2[Nb(02)Cl53 (A = K, Cs, Rb, NH^) and AgCTa(02JClg] (A = Cs, Rb, 235 NH ) have been isolated by Guerchais and co-workers using the 4 reaction of the metal(V) chloride with concentrated HC1 and hydrogen peroxide, in the presence of gaseous HC1. The complexes show distinct v(0-0) bands at around 650cm \ and those due to metal-peroxo stretches in the 600-550cm * region. Additional crystallographic data for these chloroperoxo niobate(V) and tantalate(V) species was later presented by 236 Wendling. Guerchais and co-workers followed up their work on {3-diketonato fluoroperoxo tantalate(V) complexes (see above) with the preparation of analogous chloroperoxo species for niobium(V), (Et^N)[Nb(0)Cl(diket)3, 237 [diket = acac, or dibenzoyl methane (PhCO.CH2C0.Ph) for example]. As found in the tantalum fluoroperoxo complexes, replacement of the oxo ligand in [NbOCl^(diket)3 by a peroxo ligand has a marked effect on the stereolability of the complex, and annuls the 1 H (and 19 F for the fluoro complexes) n.m.r. exchanges seen in the oxo complex. In other words, the peroxo ligand imparts rigidity on the structure of the molecule in solution. 223 Peroxo Complexes Containing Ligands Derived from Carboxvlates and Nitrogen Bases The first oxalato peroxo complex of niobium(V) or tantalum(V) to be reported was (NH 4 ) 3 [Nb(0 2 )(ox) 3 ].H 20, prepared from the oxo species 206 (NH ) CNbO(ox) ].H 0 by Guerchais and Spinner , and characterised by 4 3 3 2 elemental analysis and infrared spectroscopy. The bisoxalato diperoxo complexes KgCMfOj^fQx),,] .H20 (M = Nb, Ta) were later isolated by 18 9 Djordjevic and Vuletic , and shown by chemical and spectroscopic means to contain bidentate peroxo and oxalato ligands - thus suggesting eight coordination. Weiss and co-workers determined the X-ray crystal structure of the 23 8 - 9 oxo complex (NH^)3[NbO(ox)3].HgO , found to contain a pentagonal bipyramidal anion, and that of the bisoxalato diperoxo complex 238 240 (NH^)3CNb(02)2(ox)21.H20 ' , found to contain a dodecahedral anion, O with cis peroxo groups. The mean 0-0 distance of 1.48 A is of the magnitude expected for a Group Va metal peroxo complex.7^ The oxalato triperoxo niobate(V) species A3[Nb(02)3(ox)] (A = K, Cs, 241 Rb, NH.) have also been claimed by Shchelokov et al. , and characterised by infrared spectroscopy - showing characteristic v(0-0) bands at around 850cm \ More recently, the same authors have reported the preparation of the bis-sulphato oxalato peroxo niobate(V) complex, Cs [Nb(0 )(ox)(SO ) 1, by the reaction of (NH ) [Nb(0 ) (ox) ].H 0 with 3 2 4 2 432222 caesium sulphate, and its characterisation by thermal decomposition and 242 infrared studies. 224 ^ • 189 Djordoevic and Vuletic first reported the synthesis of eight- coordinate triperoxo niobate(V) and tantalate(V) complexes with bidentate o-phenanthroline and 2,2-dipyridyl ligands, formulated as KtMtO^J^to-phen)] and KCMtO^(t>ipy)3 (M = Nb, Ta) respectively, from the reaction of the potassium metalate with H2<)2 and the nitrogen base. These complexes display v{0-0) bands in the 870-860cm~1 region. A neutral complex, [Nb(02)2(OH)(H20)(bipy)], was also reported to be formed by hydrolysis of K 3 CNb(0 2 ) 4] in the presence of 2,2-dipyridyl, though the analogous tantalum complex was not reported to be formed. Diperoxo 8-hydroxyquinolinato complexes have also been prepared and 190 characterised by Djordjevic and Vuletic , and formulated as KCMC02)^(L)2]. (M = Nb, Ta ; HL = 8-hydroxyquinoline). Weiss and co-workers later published the X-ray crystal structure of KCNb(0^)^ (o-phen)3.3H^0, and that of an analogous perhydrate compound. KCNb(02)3(o-phen)].H202 .3H20, the latter being formed by the recrystal- 243 lisation of the former complex from H202. Both species contain a distorted dodecahedral complex anion, in which the mean 0-0 distance in the peroxo groups is 1.50 A. Djordjevic and co-workers preceded their work on vanadium(V) peroxo complexes containing amino polycarboxylate ligands by work concerning the preparation of ethylenediamine tetra-acetato (EDTA) triperoxo niobate(V) and tantalate(V) complexes of general formula A3[M(02,3(E0TA,3*nH2° (A = K * nh4 i M = Nb. Ta ; n = 0,1).244 The complexes are found to be isomorphous for niobium and tantalum, as are those recently prepared using the nitrilotriacetate (NTA) ligand, K2tM(02 )2(NTAIJ.2H20 225 spectra of the oxo species K[M0{0H)(NTA)].2H^0 with those of the corresponding peroxo-HTA complexes confirm the appearance of a strong 0-0 doublet (mass sensitive - at 870-860cm 1 for M = Nb, and 855-845cm” 1 for M = Ta) due to coordinated peroxide. Mimoun has recently prepared the bis-picolinato diperoxo niobate(V) 195 and tantalate(V) complexes (Ph^P)CM(02)2(pic)2] (M = Nb, Ta) , and investigated their activity in the catalytic oxidation of hydrocarbons [see Section 3.1.2.2(c)]. The complexes are prepared from the potassium metalates, and display 0-0 stretches (850, 835cm"1 for M = Nb ; 865, 850cm 1 for M = Ta) and M (02) stretches (600-550cm *) in their infrared spectra. Oroanometallic Peroxo Complexes Finally, the synthesis of a new type of organometallic chloroperoxo . . 5 complex of niobium containing n -bonded cyclopentadienyl rings is worthy 5 of mention. The complexes [(R-Cp)_Nb(0o)C1], (R-Cp = Q -C_H,R , and z z 0 4 R = H, Me), have been prepared by the reaction of (R-Cp)2NbCl2 with 24 6 hydrogen peroxide in deoxygenated dichloromethane solution , and show infrared bands at around 870cm 1 Cv(0-0)] and 550, 530cm 1 [v{Nb(02)}3. 5 247 The X-ray crystal structure of [(n -C_H_)_Nb(0.)Cl]5 5 Z Z reveals that the 5 niobium atom is rj -bonded to two CgHg rings, and also to a side-bonded 5 peroxo group and a chlorine atom in a plane bisecting the (q -CgH5)2Nb bent sandwich system. The 0-0 bond distance is 1.47 A, not unusual for a niobium peroxo species. These organometallic complexes have been 246 employed as stoichiometric and catalytic oxidants [see Section 3.1.2.2(c) below]. 226 3.1.2.2(c) Reactivity and Applications As far as stoichiometric and catalytic oxidation of organic substrates is concerned, the known peroxo complexes of niobium(V) and tantalum(V) show little reactivity. Even the organic-soluble picolinato complexes, (Ph^P) [IKO^^fpic)^] (M = Nb, Ta), prepared by Mimoun prove 195 to show no reactivity towards hydrocarbons. 5 246-7 The niobium peroxo complex E(n -CgHg)2Nb(02)Cl]£ , a rare example of an organometallic peroxo complex containing a metal-carbon ir-bond, has been shown to oxidise triphenylphosphine to its oxide, and sulphur 5 dioxide to coordinated sulphate to form C(n -C_H_)_Nb(SO.)C1].5 5 2 4 However, this complex is unreactive towards alkenes. This is probably due to the fact that there are eighteen electrons in the metal valence orbitals of the complex; such coordinate saturation would prevent coordination of the alkene substrate during the epoxidation mechanism. The only recent example of niobium(V) or tantalum(V) peroxo species acting as oxidation catalysts is that concerning the oxidation of the 24 8 iodide ion by hydrogen peroxide, as reported by Alekseeva et al. Niobium and tantalum, as explained at the beginning of this intro ductory section, are notoriously hard to separate from ores or solutions containing them both, due to the similarity in their chemistries. In some cases, extraction of either metal on its own, or of both metals together, is also hard to accomplish. 227 The possibility of finding methods for separation of the two metals using hydrogen peroxide systems is a distinct one. Extraction of niobium and tantalum from sulphate solutions as peroxo complexes with dibutyl phosphate, Bu2HP0^, in benzene has been reported by Babko et 24 9 al. Extractions of up to 99Z niobium and 91Z tantalum are cited; some of the tantalum is thought to remain in polymerised form. Such a 250 method is also reported by Serafimov et al. , in which niobium and tantalum are thought to be extracted into the organic (benzene) phase as "[NbO(H 0)(HO)(DBP)]" and u[Ta(OH) (HO)(DBP)]" complexes respect- 2 2 2 4 2 2 ively (DBP = dibutyl phosphate). The need for new separational and extraction techniques for niobium and tantalum has prompted us to keep differences between the peroxo chemistries of the two metals in mind during the work described in this chapter. 228 3.1.3 Peroxo Complexes of Group IVa 3.1.3.1 Titanium The development of an intense orange colour when hydrogen peroxide is added to an acid solution of titanium(IV) is one of the most characteristic reactions of titanium in this oxidation state. The reaction can be used for the colorimetric titration of either H„0 or 2 2 titanium (see Section 3.6.1). The colour of the solution varies with pH, becoming yellow at around pH 8, and colourless in strongly-alkaline solution. Early work on the species present in solutions containing titanium(IV) and H202< including the isolation of several solid peroxo 5 derivatives, has been reviewed up to 1964 by Connor and Ebsworth. There is little agreement about many of the species involved. Above pH 10, there is evidence for colourless diperoxo species, formulated 2- 251 as by spectrophotometric techniques. Isolation of solid tetraperoxo titanates(IV) of formula A,[Ti(0J,].6H 0 (A = alkali 4 2 4 2 metal) by reaction of AOH with a H202~titanium(IV) solution at pH 8.6 252 has been reported by Schwarz and Geise. As far as the yellow peroxo species found in neutral solutions of titanium(IV) and H2 O2 is concerned, the literature agrees that it is an uncharged monoperoxo complex, proven to be so by spectrophotometric 251 means ; it is variously characterised as [TiO(02)].2H20, and often 5 referred to as "peroxytitanium hydrate**. 229 The red species present in acid solutions of titanium(IV) and 2 2 have been widely studied, not least due to their importance in analysis. The only agreement in the literature regarding their composition is that the species contain one peroxo group per titanium atom. Between pH 3 + 2 + and 6, species claimed include [Ti(02)(OH)(H20) a n d CTi(02)3 , while below pH 3, the proposed formulations include CTi(OH) (H 0 )]2+, 2 2 2 [TitO^)(HgO)^]2* and -5 Detailed studies show that below pH 1, the main species present in solution is probably the monoperoxo + 253 complex CTi(02 )(OH)(H20)n] . More recent work has shown that the peroxo complexes of titanium(IV) are best characterised in the presence of chelating heteroligands such as sulphate, fluoride, oxalate, dipicolinate and EDTA. In all cases, 2- the ratio of Ti : (0^) is found to be 1:1. Some of the most notable examples follow. The red amorphous solid obtained from the addition of sulphuric acid to the titaniumtIVJ-HgO^ system has been widely formulated in the 5 literature as Ti(0_)(SO.).3H_0 , and notably as the six-coordinate Z 4 Z 254 monohydrate [Ti(02)(SO^)(H ^ O ) .H20 by Patel. The potassium salt of a charged bis-sulphato peroxotitanate(IV), K2[Til02)ISO^)2I.5H20, was 255 first prepared by Schwarz and Geise. The infrared and Raman spectra 4 9 69 of this compound were first reported by Griffith ' , and more recent 256 2- IV resonance Raman studies (using the 425nm (02) — » Ti charge transfer transition) identify the v(0-0), vs[Ti(02)] and vas[Ti(02 )] vibrational bands at approximately 890, 610 and 535cm"1 respectively. 230 The earliest report of an oxalato peroxo titanate(IV) complex, formulated as ( I ^ C ^ , Ti03 * 2* C2°3'2H2°* W3S Cited by Mazzucchelli in 257 1907. This species was characterised by infrared and Raman spectroscopy and reformulated to K2CTi(02)(ox)2].3H20 by Griffith.14,69 The stability imparted to titanium peroxo complexes by the oxalate ligand (which can form a five-membered ring with the metal if bound in bidentate fashion) had earlier been seen in thermal decomposition 149 studies carried out by Jere. It was found that the six- and seven- membered rings attainable by malonate and maleate ligands respectively imparted less stability on the peroxo group. A major breakthrough in titanium(IV) peroxo chemistry came in 1970 when G. Schwarzenbach and co-workers oserved that mononuclear peroxo titanate species condense between pH 1 and 3 to dimeric species 2+ 258 containing the [Ti20(02 >2] unit. The mononuclear species (below pH 1) were shown to form complexes with chelating ligands such as dipicolinate, EDTA and NTA, [Ti(02)(dipic)].4H20, K2[Ti(02)(EDTA)].H20 and K2[Ti(02)(OH)(NTA)].3H20 . The dimeric species (pH 1-3) was also reported to form a dipicolinato complex, K2[Ti20(02)2(dipic)2].5H20, 259 whose X-ray crystal structure was determined by D. Schwarzenbach. This showed two approximately pentagonal bipyramidal {Ti(02)(dipic)} units linked by an axial bridging oxygen (the remaining axial position is occupuied by water). The complex contains peroxo groups with a mean 0-0 distance of 1.45 A; this short bond length leads to a high v(0-0) frequency (around 880cm"1). Mimoun has recently prepared a series of covalent titanium(IV) peroxo compounds [TitO^)(A-A) (L)], where (A-A) is a bidentate ligand 231 such as picolinate or 8-hydroxyquinolinate, and L is a basic ligand such 22 as HMPA. The X-ray crystal structure of CTi(02)(pic>2(HMPA)] shows a pentagonal bipyramidal geometry about the titanium atom. The equatorial positions of the distorted pentagonal bipyramid are taken by the nitrogen atoms of the picolinate ligands, the oxygen atom of the HMPA ligand and the peroxo group, and the deprotonated carboxylate oxygen atoms of the picolinate ligands bond axially. The complexes, unusually stable for peroxo complexes of titanium(IV), will not oxidise simple alkenes, allylic alcohols or cyclic ketones. However, the complexes slowly, stoichiometrically oxidise triphenyl phosphine to the phosphine oxide (being converted to [TiOtA-A)^] in the process), and react with tetracyanoethylene in dichloromethane to form metallocyclic complexes; thus CT±(0^)(pic)^(HMPA)] reacts to give the cyclic peroxo moiety, (II) - identified by analysis, v(CN) absorptions at 2170 and 2205cm \ and disappearance of titanium-peroxo vibrational bands. HMPA NC'C C CN CN CN Peroxo titanium porphyrin complexes of formula CTKOg)(porph)] [porph = octaethylporphinato(OEP), triphenylporphinato(TPP), meso-phenyl octaethylporphinato(MPOEP)] were first synthesised by Marchon and co- workers17, from the reaction of the corresponding oxo complexes, 232 [TiO(porph)3, with HgOg. The X-ray crystal structure of CTi(02)(OEP)] shows hexa-coordination around the titanium atom involving the four porphinato nitrogen atoms and a symmetrical peroxo group (0-0 distance 1.445 £). The complex displays infrared bands at 895cm 1 [v(0-0)] and 645, 600cm 1 [v{Ti(0 )}]. N.m.r. (1H and 13C) studies on the molecule were reported, showing it to be fluxional : the peroxo group undergoes fast exchange between two sites in each of which the peroxo oxygen atoms eclipse a pair of nitrogen atoms. It has since been shown using infrared and n.m.r. 18 0- and 17 0-labelling experiments that the peroxo group of H202 is substituted intact into an oxoporphinato titanium(IV) 18,19 complex, with the release of the oxo oxygen as water. Furthermore, it has been shown that [Ti(0g)(TPP)3 may be synthesised by the reaction of [TiF(TPP) ] with molecular oxygen.17 Examples of these peroxo titanium(IV) porphinato complexes have been shown to oxidise S02 to coordinated sulphate to form [Ti(SO^)(porph)] 260 (porph = OEP, TPP) , but are found to be unreactive towards hydro carbons, notably alkenes.^61 Finally, a number of fluoroperoxo titanate(IV) complexes have been 3- reported, starting with the yellow solid containing the [TitO^JF,.] anion, obtained by the reaction of "Ti0(02 ).2H20" (“peroxytitanium hydrate") with NH OH and NH F by Piccini in 1895.The X-ray 4 4 crystal structures of the pentafluoroperoxo titanate(IV) species (NH, ) ITi(0rt)F,_]263“* and K [Ti (0^) FI 265 have more recently been 4325 325 determined, and show a pentagonal bipyramidal complex anion. This is isostructural with that found in [Nb(02)F,_] ~ , containing the peroxo group in the equatorial plane. The X-ray crystal structure of 233 266 "K^CTiCO^JF^].HgO" has also been obtained, by Pausewang et al. , and reveals the existence of a di(p-fluoro)-bridged CTi complex anion, in which each titanium atom has pentagonal bipyramidal geometry. Chaudhuri has recently reported the direct synthesis of AoCTi(0-)Fc] 3 Z 0 (A = Na, K, NH^) from titanium dioxide, HF and the corresponding alkali 267 hydroxide (AOH) and at pH 6 , and the synthesis of the new fluoroperoxo titanate(IV) complex K[Ti(02)F3].3H20 from a similar reaction mixture at pH 8-9. 268 Chaudhuri has reviewed his recent 25 research on the synthesis of fluoroperoxo titanate(IV) complexes , which relies solely on elemental analyses and vibrational spectroscopy for characterisation of new complexes. 3.1.3.2 Zirconium and Hafnium The effects of the lanthanide contraction make zirconium and hafnium remarkably similar chemically. As mentioned at the start of Section 3.1, their atomic and ionic sizes are virtually identical. The chemistry of zirconium and hafnium is very different from that of titanium, presumably a direct result of the larger atoms and ions. Hence, the second and third row elements have more basic oxides, a more extensive aqueous chemistry, and an increased tendency to attain higher coordination numbers, seven and eight. With regard to peroxo chemistry, that of zirconium, and to a greater extent hafnium, has not been widely studied, and in particular very few isolated peroxo species have been fully characterised. Before the known peroxo chemistry of the metals is discussed, the non-peroxo aqueous 234 behaviour of the zirconium!IV) and hafnium(IV) oxidation states is worthy of mention. Zirconium!IV) and hafnium(IV) oxides, MO^, are more basic than TiO^ L + and when in solution, the aqueous chemistries of the M species are extensive because of the resistance to complete hydrolysis. Some hydro lysis does occur, and the solution chemistry that results is not at all 269 simple. The dominant species in solution seems to be a polymeric 2 + form of the [HO] ion (the "zirconyl" or “hafnyl" ion), as suggested 270 by n.m.r. measurements of hydration numbers. In very acid 3 + solutions of pH less than 1, the CM(OH)] ions are shown to exist by 271 potentiometnc methods. Reaction of a zirconium!IV) or hafnium(IV) salt with hydrogen peroxide in acid or alkaline solution affords a white precipitate, formulated as [ZrO(0^)].nH20 (n varies from 3 to 5) for zirconium as 272 273 early as 1885 by Cleve and 1886 by Bailey , and as [HfOtO^J].2H20 252 274 for hafnium in 1928 by Schwarz and Geise. Work by Pissarjewsky 275 and Hauser showed the peroxo zirconate!IV) hydrate to be a di hydrate, [Zr0(02)].2H20 . Reaction of this compound with cold alkali metal hydroxide in the presence of excess H^O^ was shown to produce a 4 + white solid containing the tetraperoxo zirconate(IV) ion, CZr(0o),]Z 4 by Pissarjewski276 and later by Schwarz and Geise.252 The exact nature of the species present when zirconium!IV) reacts 277 with alkaline H202 may well depend on the conditions of formation , and the species are probably salts of polyacids formed by the condens ation of highly-peroxidised species. At pH 12-14, evidence has been 235 2“ 278 provided for [ZrOtO^Jg] species , with anionic complexes having 2- Zr:(02) ratios of 2:1 and 1:1 appearing as the pH is reduced; these species are most probably condensed polyacid derivatives. Reaction of H202 an ic®-®01** solution of zirconium(IV) sulphate was first noted to produce a peroxidic white solid by Schwarz and 252 Geise. The compound formed was later formulated as Zr 0e(S0 ).8H_0 2 6 4 2 279 by Tikhomirov et al. , who suggested the peroxosulphate structure, (III), shown below. Further evidence, in the form of thermogravimetric, 280 infrared and Raman studies,was presented by Jere and Gupta in 1970. 2 1 1 Raman and infrared evidence for both triangularly-linked (q ) and q :r| bridging peroxo groups and a bidentate bridging sulphate group, and thermogravimetric studies revealing a coordination number of six for the zirconium atom, led to the structure, (IV). below being proposed. The complex is formed at very low pH (0.1-0.7). O O— Ov o \Zr / \ Zr / . 8H,0 >/ \ S04 / \ ' (ill) (IV) Zirconium(IV) and hafnium(IV) form carboxylato oxo complexes, such 2 — 281 282 as [MOfox^J (M = Zr, Hf) and [ZrO(pic)2J • It therefore follows 236 that carboxylato peroxo complexes of zirconium(IV) and hafnium(IV) should be formed. However, only two reports of isolable zirconium peroxo complexes containing carboxylate ligands or ligands derived from nitrogen bases have appeared in the literature. The oxalato peroxo zirconateUV) complex CZrtOg) (ox) (H^0)^3 .nH2<) 284 (n = 3-4) has been prepared and characterised by Jere and Gupta. The complex is prepared from zirconyl chloride and oxalic acid, their solution being acidified to pH 0.3 using HC1 and then treated with an excess of H.0 . The white ethanol-soluble solid shows distinct bands 2 2 due to a triangularly-bound n 2 -peroxo group at 860cm -1 [v(0-0)3 and 600, 500cm~1 Cv-C2rC0^)} 1 in its infrared and Raman spectra. There is said to be little evidence of zirconyl polymerisation (i.e. absence of vibrational bands in the 950-1000cm~1 region), suggesting that such polymerisation is suppressed due to the extremely low pH (noted before 279 by Tikhomirov et al. ) and presence of a hard complexing ligand 285 (noted before by Ermakov et al. ). A series of novel, covalent peroxo complexes of zirconium!IV) containing ligands derived from carboxylates and nitrogen bases have 286 recently been reported by Taradfer and Miah. The complexes [Zr(0 )(L) ] (L = picolinato; 8-hydroxyquinolinato; aniline-2-carboxy- lato; 2-aminophenoxido} and [Zr(0^)(L')(Hg0)] {L' = N-(2-carboxyphenyl) salicylideneaminato} are prepared by boiling zirconium nitrate dihydrate and the relevant organic compound with H202 in acetone. The structures of the more complex ligands used are shown in Section 2.1 (p. 53). The complexes [Zr(0^)(L')(L“)] {L’= dipicolinate; N-(2-carboxyphenyl)- 237 salicylideneaminato ; L" = Ph3P0; Ph3AsO; pyridine-N-oxide} were prepared under reflux conditions in 1:1 acetone:methanol, in a substitution reaction of the aquo ligand by another ligand. All the complexes are shown to be six-coordinate and to contain one peroxo group per molecule, with infrared bands at 865-830 [v(0-0)], 695-650 [vS{Zr(02)}] and 600-580 CvaS{Zr(02 )>3 cm \ Carbon-13 n.m.r. data from the picolinato and dipicolinato complexes show downfield shifts in the carbon resonances of the ligands upon complexation. The complexes CZrtO^)(L)^! are found to oxidise Ph3P and Ph3As to their respective oxides, but do not react with allyl alcohol or cyclo hexanone. This is thought by the authors to be due to resistance of zirconium towards formation of a seven-coordinate intermediate (needed for complexation of a substrate in the oxygen-transfer process). This is in contrast to analogous six-coordinate thorium(IV) peroxo complexes, which Taradfer and Westland had earlier used for stoichiometric and 287 catalytic (H^O ) epoxidation of trans-stilbene. Six-coordinate thorium can expand its coordination shell readily. Several fluoroperoxo complexes of zirconium(IV), but very few for hafnium(IV), have been reported in the last two decades. Griffith and 14 o Wickins prepared the pentafluroperoxo zirconate salt (NH^)3[Zr(02 )F,.] by dissolving freshly-precipitated zirconium oxide in HF and H2<)2, and then adding ammonium hydroxide solution at -5°C until pH reached 7. The X-ray crystal structure of this complex was determined by Pausewang 264 et al. as recently as 1986. A revised disorder model was used to show the complex anion to be pentagonal bipyramidal, and isomorphous with that found by the same authors in A_[Ti(0_)F_] (A = K, NH )265 3 2 5 4 238 2S3 and by Stomberg in (NH,)_[Ti(0_)F_]. The infrared and Raman 4 3 2 5 264 >1 spectra are reported to show a v(0-0) band at around 840cm (as found by Griffith and Wickins1*) and zirconium-peroxo bands at 550 and 470cm 1. Jere and Santhamma have recently synthesised fluoroperoxo zirconates(IV) of formula A [Zr (0 ) FJ.nH 0 (A = K, Cs, Rb, NH ; and J Z c c 7 c 4 n = 0.6-2)288"9 and K rZr.,(0o) F ].nHo0 (n = 4-5)288 from zirconyl c deco d chloride, excess H2 02 and the alkali metal {or ammonium) fluoride. The A3[Zr2 (02 )2 F7].nHgO species are the subject of detailed infrared and 289 2 Raman spectroscopic studies , which indicate the presence of n - peroxo groups and a Zr-F-Zr bridge, as well as terminal Zr-F moieties. The complex anion is therefore postulated to contain two [ZrtOgJF^] units linked by a symmetric Zr-F-Zr bridge. Gerasimova et al. have presented a series of papers on fluoroperoxo zirconates(IV) and hafnates(IV), which report the isolation and 19 characterisation (by vibrational and F n.m.r. spectroscopy, thermo- gravimetry and X-ray powder diffraction) of various species from 3- solution at different pH. Formation of [Zr^O^F^] . along with small 2- amounts of the hydroperoxo species [Zr(00H)F_] , is claimed at neutral D 29 0 pH (5-7). At low pH (2-3), the [Zr(02 )F2 ].2H20 species was isolated 29 1 from solution , while at alkaline pH (9.3) the hydroperoxo and peroxo species [M(00H)Fg]3- and [MgtC^^lOHJFg]3- are reported for both 292-3 zirconium and hafnium. 239 3.2 ,CARBOXYiATQ__PEROXO_,COHPlEXES OF NIOBIUM(V) AND_TANTALUMtV) 3.2.1 Introduction As we have seen, the only known peroxo complexes of niobium(V) and tantalum(V) to contain simple carboxylate or hydroxy-carboxylate 3- 206 co-ligands are the oxalato peroxo complexes [NbtO^)(ox)33 , CM(0 ) (ox) ]3“ (M = Nb. Ta)189, [Nb(0 ) (ox)]3" 24\ and the oxalato 2 2 2 2 3 1 — ? 4 ? sulphato complex [Nb(0~)(ox)(SO.)~] . Of these, only the bis- t •* c A oxalato^peroxo niobate(V) species has been the subject of X-ray crystal structure determination; the salt (NH^)3CNb(02 )2 (ox)2 ].H20 is reported 238,240 to contain a dodecahedral complex anion. Djordjevic and co-workers have described the synthesis of niobium(V) and tantalum(V) peroxo complexes containing polycarboxylate ligands such 244 . . . ,245 195 as EDTA and NTA (nitnlotnacetate) , and Mimoun has reported picolinato peroxo complexes for both metals, but peroxo complexes incorporating simpler carboxylate ligands such as citrate, tartrate, and glycollate have not been isolated. We have studied the reaction of niobium(V) and tantalum(V) with such ligands in the presence of hydrogen peroxide, since the existence of niobium(V) and tantalum(V) oxo complexes containing the same carboxylate ligands has been cited in the literature, and thus the corresponding peroxo complexes should be formed. Just as the oxalato oxo complex 3- 238-9 [Nb0(ox)3] is known to exist , so do niobium(V) oxo complexes of 294 295 tartaric acid and gluconic acid for example. There are several structural possibilities for heteroligand peroxo complexes of 240 niobium(V) and tantalum(V), since peroxo complexes with the following geometries have been characterised by X-ray crystal structure analysis: dodecahedral {e.g. [Nb(02 )2 (ox)2]3” 238*2*°< [Nb(02 )3(o-phen)]~ 2*3}; 2 “ 68 215-9 pentagonal bipyramidal {e.g. CNb(0.)F_]Z 0 ' }; and capped octa- 3- 228 227-8 hedral {e.g. CM(02 )2 F^3 (M = Nb ; Ta )}. Monoperoxo, diperoxo and triperoxo heteroligand complexes have been reported, but the only complex in which oxo and peroxo ligands have been proven to co-exist is [Ta3(02 )3OF13]°” , in which the oxo ligand is a bridging one. We have restricted our studies to the citrate, tartrate, glycollate, malate, and dipicolinate ligands, and have mostly used the tetraperoxo- 3- metallate(V) species [MtOg)^] as starting materials (see below). 3.2.2 Preparation of Starting Materials The most commonly used starting materials employed for the preparation of niobium(V) and tantalum(V) peroxo species are the metal pentoxides, M_2 0 5_, and the potassium metallates, generally of formula K 8 CM 6 0 19 ].16H 2 0. The oxides are extremely insoluble in hydrogen peroxide and, even with the application of heat, do not provide a means of getting sufficient of either metal into solution for our purposes. We find that commercially-available potassium niobate or tantalate, presumably as a result of their polymeric structures, vary greatly in solubility from batch to batch, and at best are only sparingly soluble in hydrogen peroxide. We therefore decided to use the well-known and 3 _ characterised tetraperoxo niobate(V) and tantalate(V) species [M(02)^3 241 as starting materials for our studies. The tetraperoxometalates were prepared, either in situ, or more usefully as crystalline solids, by two similar methods. The first is an adaptation of that found in Brauer's 296 Handbook of Preparative Inorganic Chemistry , in which alkali fusion of metal pentoxide is followed by addition of H202 to the cooled melt. We find that yields are improved by using fresh M O formed by the 2 5 addition of MC1_5 to aqueous potassium hydroxide. The melt obtained from fusion of M 0 with KOH (or NaOH) is treated with a minimum quantity of 2 5 water and a little H2®2’ ancl ’*?*lterec* w^en hot. The crystalline tetra- peroxo metalates(V), K3CHCO^)^!.0.58^0, form on cooling. The white tetraperoxo tantalate(V) proves to be less water-soluble, but generally 5 more stable, than the light-yellow tetraperoxo niobate(V). An alternative method involves fusion of the potassium niobate or potassium tantalate with alkali metal hydroxide and similar treatment of the cooled melt. 3.2.3 Preparation of New 1:1 Metal;Carboxvlate Peroxo Complexes 3.2.3.1 Formation of Complexes Most of the reported niobium(V) and tantalum(V) peroxo complexes reported in the literature have been synthesised by the reaction of H202 with a metal oxo species, such as M_0_Z 5 or the potassium metallate - for 3. instance the oxalato peroxo species [Nb(02 )2 tox)2 ^ was prepared from potassium niobate and HgOg. 1®9 We have chosen to prepare new carboxy- lato peroxo complexes by substituting carboxylate ligands into the tetra- 3_ peroxometalate(V) species CMC02)^3 (M = Nb, Ta). 242 By reacting carboxylates with the tetraperoxo metalate(V) species in the presence of H^Q^. in 1:1 metal:carboxylate ratio and at natural pH, we have been able to form the new carboxylato peroxo complexes CH(02)3(tart)]3", [M(02)3(cit)]3‘, [M(02)3(glyc)]3' (M = Nb, Ta), [Nb(0 ) (mal)]3', and CM(0^) (dipic){H 0)3“ (M = Nb, Ta) as the 2 3 2 2 2 potassium salts. The presence of excess PreParati°n the complexes acts to increase the yield, despite the fact that it is a peroxo ligand (or a pair of peroxo ligands in the case of the dipicolinato complexes) which makes way for the carboxylate ligand. Once again, the stability imparted to the peroxo complexes by the carboxylate co-ligand must be the driving force for the reaction, and it is only those carboxylates capable of forming five-membered rings with the metals that are able to form complexes. As seen in the case of carboxylato peroxo complexes of molybdenum and tungsten (Chapter 2.2), we have found that those ligands only capable of forming a six-membered ring when bonded in bidentate fashion do not form stable carboxylato peroxo complexes with niobium and tantalum, nor do those containing an unsaturated phenyl ring, which can act to impair the electron-donating ability of the ligands. Thus, while tartaric, citric, glycollic and malic acids form peroxo complexes (they can all form five-membered rings - see Fig. 2.7), malonic acid and salicylic acid do not form complexes (they can only form six-membered rings - see Fig. 2.8), and nor does mandelic acid due to the effects of its phenyl ring (see Fig. 2.8 for structure). The tridentate dipicolinato ligand is found to form a peroxo complex with both metals, and this is explained by the fact that in coordination it forms two five-membered rings (see Fig. 3.2 overleaf). 243 o Fig. 3.2 Coordination of The tartrato, citrato, glycollato and malato complexes prepared are of formula K3CM(02 )3(L )] (M = Nb, Ta ; L = tart, cit, glyc) and K^[Nb(0 ) (mal)]. The reaction of malic acid and potassium tetraperoxo tantalate(V) gave only the latter species in unreacted form. Triperoxo heteroligand complexes of niobium and tantalum, in which no oxo ligands exist, and a bidentate carboxylate or nitrogen base ligand makes up the eight-coordination, are well characterised in the literature (ref. 189, 238-9, 241, 244). The tetraperoxo niobate(V) complex KMg[Nb(0_) ].7H.0 2 4 2 has been found to contain a dodecahedral complex anion by X-ray 207 crystallography , as have the phenanthrolinato peroxo complexes 243 K[Nb(0_)(o-phen)].3H0 and KCNb(O) (o-phen)3.3HO.HO_ , and the 2 3 2 2 3 2 2 2 2 3 8 — 9 bisoxalato diperoxo complex (NH,)_[Nb(0_)_(ox)„].H„0. It is there- 4 J Z 2 2 2 fore valid to assume that our new carboxylato triperoxo species also contain dodecahedral complex anions, with the bidentate carboxylate 2 ligand and three n -peroxo ligands making up the polyhedron. In the case of the dipicolinato diperoxo complexes, the dodecahedron could be completed by an aquo ligand, as elemental analyses suggest, but it is possible that the complex anions adopt the pentagonal bipyramidal geometry seen in salts of [Nb(0 )F ]2~. 68«215”9 2 5 244 The complexes have been isolated as the potassium salts in the form of non-crystalline or poor quality crystalline solids, with the niobium ones light-yellow in colour and the tantalum ones white. They are stable to impact and do not explode in a bunsen flame. Unfortunately we have been unable to obtain any of the compounds in a crystalline form of sufficient quality for X-ray crystallography. The elemental and peroxide analyses of the tartrato, citrato and glycollato complexes, while strongly indicating the formulation K [Nb(0 ) (L)], are in some cases slightly errant, a fact that reflects their inherent impurity. There is a strong possibility that some of the complexes may be polymerised to a certain extent, since weak bands in the M=0 region (1000-900cm~1) of their vibrational spectra are apparent. There is also infrared evidence for a certain amount of water of crystallisation, though we have not attempted to quantify this without the aid of X-ray crystallography. The dipicolinato complexes contain an appreciable amount of water of crystallisation, of which one molecule is probably coordinated to the metal to complete eight-coordination. We have been able to formulate these complexes as [M(0„)„(dipic){H„0)].H„0 on the 2 2 2 2 grounds of their analyses. The impure nature of the complexes, even upon recrystallisation, renders them only sparingly soluble in water, a factor which has made 13 our proposed studies on their solutions using Raman and C n.m.r. spectroscopy very difficult (see Section 3.2.3.21. However, we have been able to further characterise the complexes on the basis of their peroxo and metal-peroxo vibrational bands in the solid state (see Section 3.2.3.21. 245 The pH of the reactant solutions is again of importance when considering the formation of niobium(V) and tantalum(V) carboxylato peroxo complexes. The complexes are formed at the natural pH of the K3^M ( ° 2 ^ • 0• SH^/carboxylate/H^z solutions (generally pH 9-11). High pH is clearly required for the formation of the complexes, since below pH 10 for tantalum and pH 7 for niobium there are problems with precipitation. In the absence of peroxide, aqueous solutions of 0 _ w niobium(V) and tantalum(V) are found to precipitate [H M O ] x 6 19 (x = 0,1,2) species below these pH values, and the presence of H202 , though reducing polymerisation, does not circumvent precipitation. We find (see Section 3.3) that below pH 7 (niobium) or pH 10 (tantalum) peroxidic solutions of niobate(V) or tantalate(V) afford gelatinous precipitates, generally accepted to be mainly composed of the monoperoxo species CMrun02 ( 02 )] it" . 202-3,211 The oxalate ligand, as well as forming the triperoxo species ru(. . , ..3- 241 , ^ x 1 89,238,240 CMtOgl^fox)] , also forms the bisoxalato diperoxo 206 and trisoxalato monoperoxo species by further substitution of the peroxo ligands. However, we find that addition of excesses of tartaric, citric, glycollic and malic acids does not result in the further complexation of these ligands beyond the monocarboxylato species. This is perhaps understandable due to the fact that these are larger ligands, and not as strong as complexing ligands as oxalate. The tartrate ligand, as was emphasised in Chapter 2, possesses two adjacent sets of hydroxyl and carboxyl groups capable of bidentate coordination to a metal, and use of a 2:1 metal:tartrate ratio with niobium and tantalum produces binuclear bridged peroxo complexes, as 246 with molybdenum and tungsten. These complexes are discussed in Section 3.2.4, and the possibility of their existence was first indicated by the 3- broad nature of the vibrational bands seen for the (tart)] species in solution (see Section 3.2.3.2). We have not been able to form organic-soluble forms of the triperoxo species, probably due to the fact that it is impossible to pack three bulky organic cations around the triply negatively charged complex anions, and that no suitable "3*" counter cations are available. The dipicolinato complexes are very slightly organic soluble, but we did not study the reactivity of their organic soluble forms, since similar 195 complexes of picolinic acid, K[M(02 )2 (pic)2] (M = Nb, Ta) , were found to be unreactive towards hydrocarbons with respect to oxidation. 13 3.2.3.2 Vibrational and C N.H.R. Spectra of the Complexes The vibrational spectra of the carboxylato peroxo complexes show the characteristic bands expected for peroxo complexes of the early transition metals. In addition to the bands arising from the carboxylate ligands, three bands are seen due to the vibrations of the C2v triangular bidentate M(02 ) unit, namely the peroxo stretch v(0-0), s and symmetric and asymmetric metal-peroxo stretches v CM(02)] and vaSCM(02)],7(l^ *13,1* The 0-0 stretch varies in frequency from BBS-BSScm"1, as a result of the variation in metal and the carboxylate ligand. The nature of the latter affects the delocalisation of electronic charge on the peroxo ligand, and therefore the frequency of the 0-0 vibration. The band is 247 strong in both the Raman and infrared spectra. The metal-peroxo stretches also vary with the metal and carboxylate. with the symmetric one lying in the region 640-600cm *, and the asymmetric one appearing between 595-560cm ^. The symmetric stretch is stronger in the infrared than the Raman and polarised in the Raman spectra of the aqueous solutions; the symmetric stretch is stronger in the Raman than in the infrared and depolarised in the Raman of the aqueous solutions. The appearance of weak bands in the 1000-900cnf1 region of the infrared and Raman spectra indicates the possibility of a certain degree of polymerisation. The Raman spectra of the solids and their aqueous solutions where it was possible to obtain a satisfactory spectrum for the latter (due to solubility problems) show similar band positions and profiles, indicating that the dodecahedral structure of the complexes is retained in solution. The bands are found to broaden upon dissolution, and most 3 - markedly so in the case of the complexes [M(02 )3(tart)] ; this observation is explained by considering the existence of 2:1 metal:tartrate peroxo species in solution as well as the 1:1 complex (see Section 3.2.4). Full vibrational data are given in Table 3.1. The poor solubility of the complexes has made the measurement of 13 C n.m.r. spectra very difficult. We were only able to record satisfactory spectra for the glycollato complexes CM(02 )3(glyc)]3_ (M = Nb, Ta) and the 1:1 metal:tartrato complex [Ta(02) (tart)33”. The 13 niobium glycollato peroxo complex shows C n.m.r bands at 6 182.3 and 73.0 p.p.m., and the tantalum analogue shows bands at 5 183.4 and 73.7 p.p.m. The carboxylate resonances at around 6 180 p.p.m. represent 248 3t1 .Viftr»tioo«L_Data, for Niobium(V) and Tantalum(V) Carboxvlat? Peroxo Complexes Complex Vibrational Data (cm *)a v(0-0) v S{M(02)> vaS{M(02 K[Nb(0)(tart)3 IR 855s 647m 594m J 2 3 R 862(10) 638(2) 570(7) R 860(10) 634(2) 582(8) K_[Ta( .)-(tart)] IR 847s 642m 586m j 0 z 3 R 855(10) 636(3) 593(8) R 854(10) 640(2) 59217) IR 861m 638m 591m V V ^ V W V 1 849s R 867(10) 628(2) 575(8) R 860(10) 625(2) 580(7) K CTa,(0,)R(C.H,0 )J IR 850s 641m 575m b Z Zb k c b R 843(10) 633(3) 555(7) R 841(10) 636(2) 562(8) IR 855s 625m 594m K J CNb(0 Z ) 3(giyc)3 R 852(10) 618(3) 589(7) K [Ta(0 ) (glyc)] IR 838s 624m 580m 3 2 3 R 854(10) 619(2) 569(8) K [Nb(0 ) (cit)] IR 859s 648m 594m 3 2 3 R 860(10) 632(3) 566(8) M T a ( 0 ) (cit)3 IR 853s 645m 3 2 3 592m R 857(10) 632(3) 597(7) cont./ 249 Table 3.1 (cont) _ 1 a Complex Vibrational Data (cm ) v (0-0) v s MNb(OJJmal)] IR 849s 639m 576m 3 2 3 R 863(10) 635(3) 577(7) K[Nb(02)2(dipic)(H20)3.H20 IR 851s 61 8m 589m 845s 576m R 848(10) 611(4) 586(8) 836(8) 573(6) K[Ta(02 )2 (dipic)(H20)].H20 IR 848s 616m 566m 847(10) 604(3) 561(7) a Data for solids or, (underlined), for aqueous solutions. 250 a downfield shift from that in free glycollic acid (6 178.8 p.p.m.), as do the hydroxyl bearing carbon resonances at around 6 73 p.p.m. (cf. 62.0 p.p.m. for free glycollic acid). This is the expected effect for carboxylate and hydroxyl groups involved in complexation to a metal peroxo moiety, in which case electrons are donated to the metal peroxo system with accompanying deshielding of the carbon nuclei, and a downfield shift in resonance. 13 3 - The C n.m.r. spectrum of the CTa(0^) (tart)3 complex shows a group of resonances centred at around 6 180 p.p.m. and another group centred at around 6 80 p.p.m., due to the carboxylate and carboxyl 13 carbons respectively. Free tartaric acid shows C n.m.r. bands at 6 176.8 and 74.4 p.p.m., which indicates that complexation has taken place. The presence of more than one resonance in each case suggests that more than one carbon environment is present for each of the carboxylate and hydroxyl groups, and therefore that more than one species is present in solution. This is confirmed by the formation of 6 - [Ml°o)ctc/Ho°ccomplexes (see Section 3.2.4), traces of which must 2 Zb * 2 o 3- be formed in solutions of [M(02 )3(tart)] with release of free tartrate. 3.2.4 Preparation of New 2:1 Metal:Tartrate Peroxo Complexes The formation of bridged tartrato peroxo complexes [H_0.(0o ) - (C^HgOg)]4- (M = Mo, W) by using 2:1 ratios of (M0^]2_:tartrate was described in Section 2.2.2. We were convinced that a similar type of complex could be formed with niobium(V) and tantalum(V) in the light of 13 the Raman and C n.m.r. evidenced discussed above. We have been 251 able to synthesise tartrato complexes analysing to K [M (0 ) (C HO)] 6 2 2 6 4 2 6 (M = Nb, Ta) by reaction of the tetraperoxo metalate(V) and ( + )- tartaric acid in 2:1 ratio and in the presence of HgOg. The complex anion can be envisaged as consisting of two dodecahedral ) ]~ moieties bridged by a single tetradentrate tartrate ligand, (C.H.O.)* , 4 2 6 with a deprotonated carboxylate oxygen and a deprotonated hydroxyl oxygen of the latter making up the dodecahedron of each metal unit. Unfortunately, we have not been able to obtain the complexes in crystalline form using sodium or potassium as counter-cation, and they have been isolated as light-yellow and white powders for niobium and tantalum respectively. Dissolution of these sparingly soluble compounds in water yields 13 suspensions that are unsuitable for C n.m.r. spectroscopy. However, the analyses are close to those calculated for K„[Mo(0«) ( C,H„0„)], and 6 Z 2 6 4 2 6 the vibrational spectra display the bands expected for carboxylato peroxo complexes of niobium(V) and tantalum(V). Peroxo, symmetric- and asymmetric-metal-peroxo bands are seen at around 860, 635 and 580 -1 cm respectively for the niobium complex, and around 845, 635 and 565cm 1 respectively for the tantanum complex (see Table 3.1). The positions of the Raman bands are very similar for the complexes in solid state and aqueous solution, indicating that there is retention of of structure in solution. 252 3.3 AQU£$m£..P£ROXQ—CHEHISTRt^flf—NIQ-BIUHfV) AND TANTALUM!V) 3.3.1 Raman_3c>ctr« of PToxldic Solutlont of Niobium(V) and Tantalum!V) Campbell*8^ used the technique of Raman spectroscopy to great effect in the investigation of the species present when H^O^ is added to aqueous solutions of molybdatetVI), tungstate(VI) or vanadate(V) over a range of pH. We have gone on to study the species present in peroxidic solutions of niobium(V) and tantalum(V) by Raman spectroscopy. The high concentrations of required to dissolve the metal oxides, or potassium metalates, K [M 0 3.16H 0, preclude the use of Raman 8 6 19 2 spectroscopy due to the instability of the resultant solutions. The addition of to aqueous solutions of Nb(V) or Ta(V) at high pH is 3- known to form the tetraperoxo metalates(V), , as first 202 reported by Balke and Smith and many times since [see Section 3.1.2.2(a)]. We have therefore used the aqueous solutions of the potassium tetraperoxo metalates, K CM(0_) 3.0.5H 0 (prepared as J b ^ C described in Section 3.2.2) for the study. The pH of the aqueous potassium tetraperoxo metalates in the presence of a small amount of H202 is approximately 11. We have measured Raman spectra at this pH, and then acidified with dilute HC1 in order to investigate the spectra of solutions at lower pH. The Raman spectrum of solid KgCNbtOg)^].0.5H20 [Fig. 3.3(A)] shows strong 0-0 stretches at 841 and 813cm accompanied by metal-peroxo stretches at around 600 and 550cm ^. The Raman spectrum of the aqueous solution of the salt in the presence of a little added H 0 shows 2 2 253 ~1 1 200 ------600,------,------1000 Fig. 3.3 Raman Spectra of K3[Nb(o2)«].V z H f i A Solid B,C,D Aq. solution + H202,a t pH 11,9 and 7 respectively 2 5 4 retention of the v(0-0) band at around 845cm"1, and a broad, shouldered metal peroxo band centred at around 540cm"1 [see Fig. 3.3(B)]. When the pH is lowered to 9, these bands are retained in the Raman spectrum [Fig. 3.3(C)]. Between pH 8 and 7, a gelatinous yellow-white precipitate begins to appear. This is universally attributed to the formation of the dioxo monoperoxo species [NbO (0 )]” in the 2 2 202-3 211 literature. ' At pH 7, the Raman of the supernatant solution [Fig. 3.3(D)] shows broader and weaker bands at around 845 and 540cm 1 due to some peroxo niobate species in solution - probably consisting of traces of the tetraperoxo niobate(V) species and other monoperoxo, diperoxo or even polyperoxo niobate species. Below pH 7, precipitation becomes complete, and the solution obtained from centrifugation of the yellow-white suspensions shows no bands of interest in the Raman spectrum. Turning to the tantalum(V)-peroxo system, the problems due to precipitation are even greater, as noted for non-peroxo solutions of 196 tantalum below pH 10. The Raman spectrum of aqueous potassium tetraperoxo tantalate(V) hemihydrate in the presence of a little excess H2°2 *pH 1 0 shows broad peroxo and metal-peroxo bands at approximately 850 and 550cm 1, close to those found in the Raman spectrum of the solid salt. However, below pH 10, flocculent precipitation of a white solid, presumably largely composed of [TaO^tO^)] , makes further Raman study pointless, since little or no tantalum appears to remain in solution. In conclusion, we can say that even in the presence of hydrogen peroxide, niobium(V) and tantalum(V) only exist appreciably in solution 255 at high pH, especially in the case of the latter. The main species are 3- clearly the tetraperoxo metalates(V), CM(0^)^3 , with precipitation of the metals mainly as [MO^fOg)]- taking place below pH T (niobium) and pH 10 (tantalum). Clearly all peroxo chemistry of these Group Va metals must be carried out at high pH. as seen in our formation of carboxylato peroxo niobate(V) and tantalate(V) species in the previous section. This is in sharp contrast to vanadium(V) peroxo chemistry; as discussed in Section 3.1.2.1, vanadium(V) peroxo species are found in solution over virtually the whole pH range. 3.3.2 The Action of Hydrogen Peroxide on Niobium Carbide and Niobium and tantalum form interstitial carbides of approximate formulation NbC and TaC. We have studied the action of H^O^ on commercial samples of the two carbides. The reaction of NbC with in the approximate molar ratio of 143 carbide-.H^Og used at Interox Chemicals Ltd. for WC-dissolution did not accomplish appreciable digestion of the carbide. However, addition of a large excess of ^2^2 and stirr;*-n9 f°r 2* hr. 9ave a 9r®y suspension (pH 1.8) and a black residue of carbon. Centrifugation of the former yielded a light-yellow solution whose only notable feature in the Raman spectrum was a strong band at 880cm“1 due to free H2°2‘ 0n standing, a yellow-white precipitate formed in the solution - clearly the same as that which, mixed with unreacted NbC, formed the grey suspension mentioned earlier. It seems that H ^ does break down the niobium carbide, as shown by the carbon deposits, but that the low pH of the 256 reaction mixture leads to precipitation of most of the niobium as insoluble peroxo, oxoperoxo or oxo niobate species such as [NbOgtOg)] . From the relative reactivities of molybdenum and tungsten carbides described in Section 2.4, we would expect tantalum carbide to react much more slowly with than does niobium carbide. This proves to be the case; in fact prolonged reaction of commercial TaC with an excess of 601 o H2°2 at 80-90 c yields °nly unreacted TaC and a clear solution, devoid of bands in its Raman spectrum (apart from that due to free H202^‘ No residue of carbon is observed. It would seem therefore that use of hydrogen peroxide for the recovery of tantalum from its carbide is not feasible. 257 3.< VIBRATIONAL SPECTRA OF THE I N b jt y F^]2' ANION 3.4.1 Introduction 2- Pentafluoroperoxo nxobates(V) [Nb(02 )Fg] were first prepared by 297 202 Piccini and by Balke and Smith at the turn of the century. Since then a number of X-ray crystal structure determinations on salts of this complex anion have been reported in the literature. The results using the salts Na2CNb(02)Fg].nH20 {n=1215 or n=2216}; Na3(HF2 )CNb(02 )Fg3217; 218 CO 040 {C12H10N2,CNb(°2,F5] and (C9H8N0,2CNb(02,F5] 5,11 indicate pentagonal bipyramidal coordination around the niobium atom, with the peroxo group occupying one of the equatorial positions. The assumption 2- of overall C cV symmetry for the [Nb(0_)F_] 2 5 anion is therefore fully justified. The infrared and Raman data for solid K2CNb<02)FgJ.HgO, prepared by 297 1469 the method of Piccini , were recorded by Griffith ’ ,but later re investigated, and presented with full assignments and similar 221 information on K_CTa(0_)F_].HO, by Jere and co-workers in 1982. Z c 5 Z 2- e The [Nb(0 )F_] anion has overall C0 symmetry, and therfore 2 5 2v A possesses 18 vibrational modes, spanning the following irreducible representations : 7A^ ■ ♦ 2A2 ♦ 4B^ ♦ 5B2> All bands are expected to be infrared and Raman active, except the A2 modes, which are infrared inactive. Jere used force constant calculations and comparison with vibrational spectra of related compounds to assign the spectra of K2CNb(02)F53.H20 (assignments based on those of Jere are used in Tables 3.2 - 3.4). Jere reports a v(0-0) stretch at 890cm"1 (infrared and 258 S -1 Raman), a v CNb(02 )3 stretch at 620cm (infrared and Raman), and a vas[Nb(02)] stretch at 560cm * (infrared only). Other bands are assigned to various Nb-F stretches, and F-Nb-F. F-Nb-0 and skeletal bends. 222-3 Two more recent papers by Nour and co-workers have since appeared, in which the infrared and Raman spectra of solid sodium salts 2- of the [NbfO^JFg] anion are reported and assigned (the authors report preparation of Na CNb(0 )F_].HO and Na(HF)[Nb(0)F„] by the methods 2 252 32 25 n 1 c 017 of Stomberg ’ , but do not report analyses, or for which salt data are presented). Since the spectra and assignments of Nour differ quite 1469 221 substantially from those reported by Griffith ' and Jere for the solid potassium salt, we have remeasured the infrared and Raman spectra 2- of the solid potassium and sodium salts of CNb(0.)F_] , and for the 2 5 first time measured the Raman spectra of their aqueous solutions. 3.4.2 The Yibrational Spectraof CNbfO^F,.]2" 3.4.2(a) & 2.tHfeAQglEgJLligl) 298 K.[Nb(0_)F_]. Ho0 is prepared by the method of Jere , based on z z 5 z 202 that of Balke and Smith , from niobium pentoxide, HF, excess H202, and potassium hydroxide. It is a light-yellow microcrystalline solid, sparingly soluble in water. We list our infrared and Raman for solid K_[Nb(0„)F_].H-0 and Raman Z 2 5 2 data for its aqueous solution in Table 3.3. Infrared and Raman data 221 222-3 reported previously by Jere and Nour appear for comparison in 259 Table 3.2. Our infrared and Raman spectra of solid K_[Nb(0 )F l.H 0 are 2 2 5 2 depicted in Figs. 3.* and 3.5 respectively, and the Raman spectrum of its aqueous solution in Fig. 3.6. Our infrared and Raman data for solid K [Nb{0_)F ].H 0 are quite 2 2 5 2 222-3 different from those of Nour , but are in close agreement with 221 -1 those of Jere. The strong band at 890cm is clearly the 0-0 stetch of coordinated peroxide, as observed in other side-bonded metal peroxo 7 7(il 13 230 264 -1 (H ) complexes. ' ' The bands at 620 and 560cm~ in the Raman spectrum (615 and 560cm * in the infrared) we tentatively assign to vs[Nb(02)] and vas[Nb(02)], the symmetric and asymmetric stretches 2- of the side-bonded (0,,) ligand against the metal. The various other bands are assigned to Nb-F stretching, F-Nb-F bending and skeletal 221 bending modes, according to Jere. The strong v(Nb-F) modes in the — 1 e 620-590cm region make assignment of the v CNb(02)3 mode difficult. The close similarities in profiles and band positions of the Raman spectra of solid K_[Nb(0o)F_].H_0 and its aqueous solution suggest £ £ D £ retention of the pentagonal bipyramidal structure in aqueous solution. In the spectrum of the aqueous solution, there is a strong, polarised v(0-0) band at 885cm *, and symmetric and asymmetric NblO^) stretches at 626 (polarised) and 560cm * respectively, as well as the various Nb-F bands. Fluorine-19 n.m.r. data indicating retention of structure 2 - 220 19 in solution by the [NbfC^lFg] anion has been reported. The F 2- n.m.r. spectrum of aqueous [Ta(02)Fg] has been shown to display the AX^ pattern expected for a capped octahedral coordination**, while the 224 X-ray crystal structures of K2[Ta(02)Fg].KHF2 and its close analogue 225 K3 (HF2)CTa(02 )F5] have revealed capped octahedral and pentagonal 260 Table 3,2 Vibrational Data Previously Reported for CNbtO^lF^2 RAMAN(cm V INFRARED(cm”1)3 u 222-3 ,221 kl 222-3 . 221 Nour Jere Nour Jere Assignment0 890s 890s 0-0 stretch 955m A i v i 880m 620vs 620s vS Nb(0£ ) V 2 595vs 590vs Nb-F stretch 696s V3 650w 500w Nb-F stretch 660s V4 450m Nb-F stretch 608m V5 290s F-Nb-F bend 215s V6 F-Nb-F bend 195s V7 305w F-Nb-F bend A2 V8 230vw 0-Nb-F bend V9 730s 570s Nb-F stretch 728sh V 10 335sh F-Nb-F bend 330m V 11 497m 250w skeletal bend V 12 F-Nb-F bend V 13 61 Ovs 900w CD v3S Nb(02) r o VH 730s 560s Nb-F stretch V 15 330m 335sh F-Nb-F bend V 16 270s 481m 265m skeletal bend V 17 F-Nb-F bend v 18 a Nour data for solid "Na2CNb(02 X o to Jere data for solid K2[Nb(02 )F CSI b Assignments based on those of Jere (ref. 221 ) 261 T?b*gJk3 Vi bra tig n a l_J?_a ta^for_K ^ [ Nb (0^) Fg I... Hg Q RAMAN(cm”1)3 RAMAN(cm”1)^ INFRARED(cm”1)a Assignment0 890(8) 885(7)p 890s 0-0 stretch Ai V 1 620(10) 626sh,p 61 5sh vS Nb(02) V2 610 (10 ) p Nb-F stretch 611(7) V3 597(10) 597(5)p 594s V4 Nb-F stretch Nb-F stretch 59 A sh V5 329(1) 324(1/2) 330w F-Nb-F bend V 6 262m F-Nb-F bend 271(1) V7 F-Nb-F bend *2 V8 O-Nb-F bend V9 570sh Nb-F stretch B, V 10 306m F-Nb-F bend V 11 280w skeletal bend 271(1) V1 2 F-Nb-F bend V 13 560(1) 560br 560m v3S Nb(02) B2 V H 503vs Nb-F stretch V 1 5 4 50m F-Nb-F bend V 16 292w skeletal band V 17 230w F-Nb-F bend V18 r—» . u X o a Data for solid K2[Nb(02 in CM b Data for aqueous solution of K_[Nb(0_)Fe].H„0Z Z 5 2 c Assignments based on those of Jere (ref. 221) 262 Infrared Absorption Fig. 3.4 Fig. h Ifae Setu f N(2F]H2 (solid) 20 [Nb(02)F5]"H 2 K of Spectrum Infrared The \ ------« ------1 ------' ------i 600 400 200cm-1 K2[Nb(02)F5].H20 800 — ,— I T ------j 1000 ------> Fig.3.5 The Raman Spectrum of ------i 1200 264 [Nb(02)F5].H20 2 Fig. 3.6 The Raman Spectrum Of Aqueous K 200 200 600 cm 1000 265 bipyramidal geometries for the anion respectively. Certainly, in solution the two geometries would prove easily interconvertable, as can be shown using models. 222-3 The infrared and Raman spectra reported by Nour for solid "Na [Nb(0 )F 3“ (quotation marks by A.C.D., due to ambiguity over which 2 2 5 salt was actually used) feature bands at 955cm 1 (Raman), 900cm 1 (infrared) and 880cm 1 (Raman), incorrectly assigned to the v(0-0), vs[Nb(02)] and vas[Nb(02)3 modes respectively, as well as various other bands assigned to v(Nb-F), 6(F-Nb-F) and 6(F-Nb-0) modes, and combin ations. For the sake of this work, we are only concerned with the first three bands. It seems likely that the salt examined by Nour was a decomposition product of a sodium fluoroperoxo niobate(V); analytical data are not given in the papers. The presence of bands at 955 and 900cm 1 in the spectra of Nour‘s material suggests that Na2CNbOFg].H^O was one of the constituents of the decomposed fluoroperoxo niobate; the band at 880cm 1 was probably due to some unchanged peroxidic material. 299 The infrared spectrum of K2[NbOF,J .H20, reported by Keller , displays distinct bands at 930 and 738cm \ That of Nour's material shows bands at 900 and 730cm 1. We have attempted to simulate decomposition of Ko[Nb(0«)F_3.H.OZ Z 5 2 by heating samples of the salt in a test-tube in a Bunsen flame, and in an o 2- oven at 200 C (the temperature at which [NbOF-3 is formed from D [Nb(02)F5]2' according to the thermal studies of Jere298). In both cases, a white solid is formed, which shows bands at around 960 (Raman only), 940, 925, 900 (Raman only), 735 (infrared only), 670 (Raman only), 600, and 500 (infrared only) cm * in its vibrational spectra. 266 The similarities with the spectral data of Nour can be seen on inspection of his data for "Na_[Nb(0o)F.3“ in Table 3.2. We therefore 222-3 conclude that differences in the spectra of Nour compared with 14 69 221 those of Griffith * and Jere are due to decomposition of the fluoroperoxo niobate sample in the former case. This conclusion is vindicated by the results of our studies on the sodium salt of the [Nb(0_)F_32Z 5 anion. 3.4.2(b) ! M zLmiSlzlL5ll 2- The sodium salt of [Nb(0^)Fs3 is known to exist in at least three different forms, as shown by the X-ray crystal structure analyses of 215-7 Stomberg .and is therefore difficult to obtain pure. The analyses of our sample reflect this, and since we did not use a crystalline sample of a sodium pentafluoroperoxo niobate(V), we shall continue to refer to that used in our vibrational studies as “Na„CNb(0«)F^3“. The sample is probably a mixture of at least three species, but this should not unduly detract from our spectra and assignments. The infrared and Raman spectra of solid "Na.[Nb(0_)F_]",2 2 5 as well as the Raman spectrum of its aqueous solution (whether prepared directly or in situ by adding sodium perchlorate to an aqueous solution of the potassium salt), are very similar in terms of profiles and band positions to those of K_[Nb(0_)F_3.H_0. The aqueous Raman spectra are £ Z O Z of better quality due to the greater solubility of the sodium salt. Data for all spectra are presented in Table 3.4. The main features of the solid spectra are the v(0-0), vs[Nb(02 )3 267 T»b3iJ-3.t+ Vibrational,Data for "Na2[Nb(02 ) t ^ m RAMAN(cm”1)3 RAMAN(cm"1)b INFRARED(cm”1)3 Assignment0 892(9) 883 ( 7 ) p 891 vs A^ 0-0 stretch 623(10) 625sh,p 626s v S Nb(°2) 614(8) 609( 10) p 612m v3 Nb-F stretch 604(7) 596sh,p 600s Nb-F stretch 596sh v_ Nb-F stretch 5 338(3) 334(1) 336w v„ F-Nb-F bend 6 275(2) 265m F-Nb-F bend A„ v« F-Nb-F bend 2 8 vg O-Nb-F bend 576s B1 V10 stretch 310m F-Nb-F bend 275(2) 291w skeletal bend v 13 F-Nb-F bend 564(1) 564br 558sh B2 v u vas Nb(02) 503m v._ Nb-F stretch 1 0 455s F-Nb-F bend 300m v 17 skeletal bend 250m v <0 F-Nb-F bend 1 0 a Data for solid "Na2[Nb(02)Fg]" b Data for aqueous solution of "Na_CNb(0_)F_]' Z 2 5 c Assignments based on those of Jere (ref. 221) 268 8 8 • 1 and v CNb€02)3 bands at 692, 625 and 564cm respectively. As with the potassium salt, the Raman spectrum of aqueous "Na_[Nb(0.)F_]" is very similar to that of the solid, indicating retention of the pentagonal bipyramidal structure of the complex pentafluoroperoxo niobate(V) anion in solution. The heating of a sample of "Na_[Nb(0.)F1.3", as described for the z c o potassium salt, afforded a white solid showing broad bands at 940 and 740cm 1 in its vibrational spectra, indicative of the presence of Na [NbOF ] as one of the decomposition products, and reminiscent of the c 5 222-3 spectra presented for "Na.CNb(0.)F_]" by Nour. C Z 3 3.4.2(e) (PhtP)2tNb(0; )F!;] 2 - The tetraphenylphosphonium salt of the CNb(0«)F_] anion was Z 5 prepared in impure form by addition of Ph^PCl to a solution of either the potassium or sodium salt. Though displaying similar vibrational spectra to the potassium and sodium salts (with additional bands due to the Ph^P+ cation), the inherent impurity of the compound makes it only very sparingly soluble in organic solvents such as acetone, acetonitrile and dichloromethane, and a Raman spectrum of its solution has proven to be unobtainable. 269 3.5 PREPARATION OF NEH CARBOXYLATO PEROXO COMPLEXES OF ZIRCONIUMIIV) AND HAFNIUM1IVI 3.5.1 Introduction The peroxo chemistry of zirconium(IV) and hafnium!IV) has been quite sparsely studied, and only a handful of peroxo species have been characterised for the two metals. The dominant species in peroxidic solutions appears to be a polymeric form of the [HO] 2 + ion (the 269 "zirconyl- or "hafnyl" ion). The tendency for polymerisation has made the isolation of discrete heteroligand peroxo species difficult, and the only such zirconium and hafnium species to be characterised are the sulphato peroxo complex CZr^CO^)3CSO^){H^O)^3.6H2<) ’ , the 284 oxalato peroxo complex [ZrtO^)(ox)(H^O)^].nH^O, and the series of zirconium peroxo complexes with picolinato, 8-hydroxyquinolinato, aniline-2-carboxylato and other carboxylato ligands reported by Taradfer 286 and Miah. In all cases, the effects of zirconyl polymerisation seem to be suppressed by the use of low pH and of hard, complexing ligands. We have attempted to extend the heteroligand peroxo chemistry of zirconium(IV) and hafnium!IV) to anionic carboxylato peroxo species by reacting the metal oxychlorides, M0C12.8H20, with potassium carboxylates and excess H202. We have restricted our studies in this area to the oxalate, citrate, tartrate and glycollate ligands, all ligands of proven complexing ability and capable of forming five membered rings with the metals in question. 270 3.5.2 formation of Complexes The obvious starting materials for preparation of carboxylato peroxo complexes are the soluble oxychlorides M0C1 .8H 0. Addition of 2 2 potassium oxalate to solutions of these salts (with 1:2 metal:oxalate ratio) in the presence of excess affords the new bisoxalato monoperoxo complexes CM(02)(ox)2].2H20 as white solids. The pH of the reactant solution is around 3 in the case of zirconium and about 2 for hafnium. These pH values are clearly low enough to suppress zirconyl or hafnyl polymerisation, since the complexes analyse very closely to the above formulation, and show very little in the way of M=0 bands in their vibrational spectra (see Section 3.5.3). The complexes are found to be slightly hygroscopic, a fact which points to their tendency to polymerise into zirconyl or hafnyl polymers in air. They are only sparingly soluble in water. Reaction of zirconyl and hafnyl oxychlorides with potassium citrate and excess H^O^, using metal:citrate ratios of both 1:1 and 1:2, yields off-white solids which analyse closely to K2[M(02)2(cit)].H20, but bear the hallmarks of greater degree of polymerisation than their oxalato counterparts. The bonding of just one citrate ligand in bidentate manner is to be expected due to the bulk of the ligand, and the desired six-coordination around the metal is completed by two peroxo ligands. The pH of formation (about 2.5 for zirconium and 2 for hafnium) and lesser complexing strength of the citrate ligand may account for the presence of zirconyl or hafnyl polymerisation, betrayed by the appearance of M=0 bands in the 1000-900cm"1 region of the vibrational spectra of the solids, and their highly hygroscopic nature. 271 The addition of potassium tartrate to peroxidic solutions of zirconium(IV) and hafnium(IV) only provides crystals of potassium hydrogen tartrate. The failure of the Group IVa metals to complex with peroxide and a tartrate co-ligand is indeed puzzling. The reaction of glycollic acid, KOH and M0C12.8H2<> (M = Zr, Hf) in the ratio 1:2:1 in the presence of excess H202 affords highly hygro scopic white solids upon ethanol addition, whose analyses seem to fall between the formulations K2CM(02)(glyc)2].2H20 and K2[H{02)2(glyc)],H20 and whose vibrational spectra reveal a high degree of zirconyl or hafnyl polymerisation. These compounds are probably glycollato peroxo polymers including oxygen- or hydroxy-bridged metal units. The high degree of polymerisation is clearly due to the high pH of the reaction mixture; attempts to acidify the mixtures yielded intractable, sticky white products. The tendency for polymerisation in these carboxylato peroxo species of zirconium and hafnium makes the isolation of pure complexes very difficult, and effectively prohibits the production of crystalline peroxo derivatives. Indeed this malaise would seem to affect all zirconium and hafnium peroxo chemistry, for the only peroxo complex of* either metal to have been characterised by X-ray crystallography to date 264 is (NH^)3[Zr(02 )Fg] , prepared at pH 7, but containing the very hard, complexing fluoride ligand. 3.5.3 Vibrational..Spectra of the Complexes The oxalato and citrato peroxo complexes K^CMIO^lox)^ .2H2<) and ^[ MI O ) (eit)] *H2° s Zr* show the three vibrational bands 272 expected for transition metal n^-peroxo complexes7***'13'U in their infrared spectra : a peroxo stretch in the region 860-840cm \ a symmetric metal-peroxo stretch in the region 635-620cm \ and an asymmetric metal-peroxo stretch in the region 560-540cm \ all due to the triangularly-bound C peroxo group. These frequencies compare with those observed by Taradfer and Miah for their series of monoperoxo carboxylato zirconiumtIV) complexes [M(0„)(L).J and 2 2 CM(02)(L)(H^O)] (see Section 3.1.3.2 for details of the carboxylate ligands, L, employed) at BBS-esOcm"1 [v(0-0)]t BSS-eSOcm'1 Cvs{H(02 )>J and 600-580cm_1 Cvas{H(02)>3. In the Raman spectra of the solid complexes, the 0-0 stretch is clearly seen, but only the asymmetric metal-peroxo stretch is strongly active enough for detection. The low solubility of the complexes makes Raman spectroscopy of their aqueous solutions difficult. Nevertheless, all four complexes show a broad band in the 860-840cm 1 region due to the peroxo stretch, and a very weak feature in the 550cm * area due to vas{M(o2 n . The citrato peroxo complexes show broad bands in the 1000-900cm * area due to a certain degree of polmerisation. In the case of the glycollato peroxo species, the broad band in this M=0 region is extremely strong in both the Raman and infrared spectra, appearing alongside an equally broad peroxo band at around 850cm” 1. Full vibrational data are given in Table 3.5. 273 Table 3^5 Vibrational_Pata__for Zirconium(IV)_and Hafnium(IV) Carboxvlato _Peroxo Complexes 1, a Complex Vibrational Data (cm ) v(M=0)b v(0-0) vs {m (o 2)} vaS K0[Zr(00 )(ox).].2H,0 IR Weak 640s 620m 560m 2 Z Z Z R Weak 848(10) 565(2) R 855(10) K-[Hf(0,)(ox)9].2H90 IR Weak 846s 623m 555m Z c Z Z R Weak 856(10) 572(3) R 860(10) K[Zr(0)(cit)].HO IR 960m.br 855s 638w 548m 2 2 2 2 R 853br(10) 550(3) R 862(10) 842(8) KoCHf(0_)9(cit)].H_0 IR 955m.br 858s 635w 552m 2 2 Z Z R 868br(10) 554(3) R 872(10) c Zr glycollato complex IR 948m,br 846s 555br R 950br(5) 851(10) c Hf glycollato complex IR 940m.br 849s 530br R (d) a Data for solids, or (underlined), for aqueous solutions. b Bands in the M=0 region due to zirconyl or hafnyl polymerisation. c Formulation discussed in the text, d Sample decomposed in the laser beam. 274 3.8 MISCfUANEOUS-TITANIUHtIV) AND VAMADIUH(V) PEROXO CHEMISTRY 3.6.1 A-NtW-CQlprlm8tri.C-Mg.thod-for tha Distinction Between Coj>£dinattd.and Perhvdrate Peroxide u»inq Titanium(IV) The development of an intense orange colour on addition of to an acid solution containing Ti(IV) is a characteristic reaction of the metal ion, and the species responsible has been identified as a mono- 5 peroxo titanate(IV) complex of various formulations (for example 2* CTi(0 )(H O) ] ). The complex has an absorption band at 405nm z z n (e * 770dm3mol ^cm 1) in the UV/visible spectrum300, and its formation has been used for the colorimetric determination of both titanium (IV) and «202. 300 The work of Sheriff in these laboratories saw the development of a method for extracting free hydrogen peroxide into ethyl acetate (the partition co-efficient of H202 between water and ethyl acetate is 301 5/2 ) from an aqueous solution, and then colonmetrically titrating the amount of by adding an acidic titanium(IV) solution to the ethyl acetate phase, and measuring the absorption at 405nm. The titanium(IV) solution is prepared using a modification of the procedure 302 . . . reported by Egerton , from a heated solution of potassium titanium oxalate in concentrated H2S0^. We surmised that since only free peroxide, and not coordinated peroxide, should be extracted into ethyl acetate, this should prove a convenient method for the colorimetric determination of peroxide present in complexes as perhydrate. The two types of peroxide are indiscernable 275 Fig. 3.7 Typical Calibration Graph for Colorimetric Determination of Free H202 using conventional titrimetric methods, but our simple method solves this problem. The procedure involves extraction of a solution of a weighed amount of sample with ethyl acetate twice, and shaking of the combined organic extracts with an excess of titanium(IV) solution. The aqueous layer is then run into a cell for UV/visible spectroscopy and measured for absorption at 405nm. A calibration line is plotted using eight known amounts of (see Fig. 3.7), and used to extrapolate the amount of free peroxide in the sample. The method was tested on two perhydrates to have been characterised by X-ray crystallography. The complex K2[Mo0(02)2(ox)].3H20.0.5H202 was found to contain 2.9Z free peroxide (calculated 3.1Z) by the procedure, while the value for free peroxide content in K [V0(0 ) (ox)].H 0 was J 6 b b C found as 8.4Z (calculated 8.6Z). In the case of the latter complex, our investigation has further disproved the claims in the literature for the 3- oxalato triperoxo species [V(02)3(ox)] (see Section 3.1.2.1). The method was further tested using peroxo complexes containing exclusively coordinated peroxide : thus the complexes K2[Mo0(02)2(ox)] and K3CV0(02)2(bipy)3.4H20 give zero free peroxide as expected. 2 3.6.2 The Effect of H-Substitution on the Vibrational Spectra Q.£.UiH4l3lY202.< .QHU02-U:Lii2& The ammonium peroxovanadate(V) species (KH.) [HV.O ].2H 0 prepared ^ J b 11 b 303 by Wieghardt and Quilitzsch was later shown by Campbell, Griffith and 60 co-workers to be the species (NH ) [H0{V0(0 ) } ].HO, featuring a ^ J b b b Z 277 protonated oxygen bridge, by X-ray crystallography. Campbell earlier 18 used O-substitution of the species to help assignments of its vibrational data, but could only tentatively assign vS(V-O-V) and vas(V-O-V) on the grounds of elimination of the bands that did not shift . «... . . 48 (i) upon substitution. We have deuteriated the complex in order to help further with the assignments of the V-O-V vibrational bands, which are really V-OH-V bands, and should shift upon deuteriation. The infrared and Raman spectra of the normal and deuteriated species are strikingly similar, with the exception of the appearance of v( 2 H-0),6( 2 H-0- 2 H) and 6(V-0- 2 H) bands at around 2380/2340, 1080 and 755cm 1 respectively, shifted from around 3400, 1600 and 1030cm 1 in the unsubstituted compound. The bands assigned by Campbell as v3S(V-0-V) and vS(V-0-V) at around 640 and 455cm 1 in the vibrational spectra do not appear to move upon deuteriation, indicating that these assignments could well be erroneous. as 2 It is possible that the v (V-0 H-V) band is obscured by the very S — 1 strong v [V(02 )] band centred at around 610cm . 278 3.7 JEXE££1MENTAI Section 3.2 Preparation of K3[Nb(02)43.0.5H20 A sample of fresh Nb.O_ was obtained by the addition of NbCl to an z o 5 aqueous solution of potassium hydroxide at 0°C. The Nbo0_ was filtered 2 5 off, washed with cold water and dried, before being fused (2.0g) with KOH pellets (3.4g) in a nickel crucible. The cooled melt was taken up 3 in a minimum of water, and treated with 30Z *5cm ** The cloudy solution was heated for 15min. at 75°C in a water-bath, and filtered when hot. On cooling, the filtrate gave yellow-white crystals of K3[Nb(02 )^].0.5H20, which were filtered off, washed with ethanol and air-dried. FOUND : K, 33.6 ; (02)2” , 37.4. HK,_Nb0a _ requires K, 33.9 ; (0o)2 , 37.OZ. VIBRATIONAL DATA (cm-1): INFRARED (nujol mull) : 813vs, 598s, 548s, 463br, 295m RAMAN (solid,KBr disc) : 841(10), 815(2), 600(1), 548(8), 521(3), 459(2). 300(2), 271(5) A similar sample was obtained by the alkali fusion of potassium niobate. The sodium salt was prepared by using NaOH instead of KOH. Preparation of K3CTa(02)^3.0.5H20 Fresh Ta205 (2.0g - prepared by the addition of TaCls to an aqueous solution of KOH) and KOH pellets (2.0g) were fused in a nickel crucible. The cooled melt was taken up in minimum water, treated with 30Z H202 and heated to 75°C for 15min. The cloudy suspension was filtered when 279 hot. On cooling at room temperature, the filtrate afforded white micro crystals of K3[Ta(02)^].O.SH^O, which were filtered off, washed with ethanol and air-dried. FOUND : K, 26.2 ; ( O ^ 2". 28.9. HK_Oft _Ta requires K. 26.9 ; (0o)2“, 29.4Z. J O * D 2 VIBRATIONAL DATA (cm"1: INFRARED (nujol mull) : 847m, 808vs, 621m, 594m, 560m, 531vs, 441s. RAMAN (solid,KBr disc) : 836(10), 815(4), 633(2), 577(3), 558(4), 340(2) A similar sample was obtained by alkali fusion of potassium tantalate and the same work-up. The sodium salt was prepared by using NaOH instead of KOH. Preparation of K^IMtO^)^!tart)] (M =Nb, Ta) Potassium tetraperoxo niobate(V) hemihydrate, K [NbtO ) ].0.5H 0, v fc % 6 (0.5g, 1.44 mmol.) and (+)-tartaric acid (0.22g, 1.44 mmol.) were 3 dissolved in water (25cm ). The solution was treated with excess 30Z 3 3 *10cm * with stirring. Addition of ethanol (20cm ) and storage at o 5 C yielded a light-yellow solid, which was filtered off, washed with ethanol and air-dried. FOUND : C, 11.7 ; H, 1.5 ; K, 24.2 ; (Og)2", 20.1. C4H4K3Nb012 rec*uires C * 10*6 S H - °*9 1 25-9 1 t02)2". 21.2Z. The tantalum analogue was similarly prepared, and isolated as a white crystalline solid. FOUND : C, 8.5 ; H, 0.8 ; K, 21.2 ; lOg)2", 18.8. C4H4K3°12Ta re(luires C ' 8-8 ; H * °-7 i K* 21-7 S < 02) 2“, 17.8Z. 280 Preparation of K [Nb(0 ) CUJ (L * glyc(A), cit(B), mal(C)) and 3 Z 3 K3CTl(02,3a,] {L 8 citCEIl All complexes were prepared in the general manner described above for the tartrato species. (A) FOUND : C, 5.2 ; H, 0.7 ; K, 29.1 ; ( O ^ 2", 27.1. c H K Nb° requires C, 6.3 ; H, 0.5 ; K, 30.8 ; (0 )2“ , 25.21. & b J *# » (B) FOUND : C, 13.8 ; H. 1.6 ; K, 22.7 ; ( O ^ 2', 18.6. CcHeK NbO requires C, 14.5 ; H, 1.2 ; K, 23.6 ; (0_)2~, 19.3Z. 66313 c (C) FOUND : C, 10.2 ; H. 1.4 ; K, 24.2 ; (02)2“# 20.0. C4H4K3Nb°11 re<*uires C ' 11*° ; H ' °*9 ; K ’ 26*8 ; (02)2"' 21*9** (D) FOUND : C, 4.0 ; H, 0.7 ; K. 22.9 ; (02)2', 21.7. C N K 0 Ta requires C. 5.1 ; H, 0.4 ; K. 25.0 ; (0 )2~, 20.5Z. C b W 9 fa (E) FOUND : C, 12.6 ; H,f.4 ; K, 19.2 ; ( O ^ 2” , 15.2. CcHeK,0._Ta requires C. 12.3 ; H, 1.0 ; K 20.1 ; ( 0 J 2~, U-.+ Z 66313 ' Z Preparation of KCMIO^tdipic) (HgO)] .HgO (M = Nb, Ta) Potassium tetraperoxo niobate(V) hemihydrate (0.50g, 1.48 mmol.) and dipicolinic acid (pyridine-2.6-dicarboxylic acid) (0.25g, 1.48 mmol.) 3 were dissolved in water (15cm ), and the solution stirred and treated with 30Z ^2^2 Addition of ethanol and storage at 5°C afforded a yellow solid, which was filtered off, washed with cold ethanol, and air-dried. FOUND : C, 20.2 ; H, 1.1 ; N, 2.9 ; K, 10.8 ; ( O ^ 2", 14.9. C?H7KNNb010 requires C, 21.1 ; H, 1.8 ; K, 9.8 ; N. 3.5 ; (02)2” , 16.1Z The tantalum analogue was prepared similarly and isolated as a white solid. FOUND : C. 16.3 ; H, 0.9 ; K, 9.2 ; N, 2.4 ; (Og)2", 12.1. C7H7KN010Ta re<*uires c * ,7-3 : «, 1-5 ; K. 8.1 ; N, 2.9 ; (02)2”, 13.2Z 281 Preparation of K o CH Z to Z I o fC 4 H Z 0_13 6 (M * Nb, Ta) Potassium tetraperoxo niobate(V) hemihydrate (1.0g, 2.88 mmol.) and 3 (+)-tartaric acid (0.22g, 1.44 mmol.) were dissolved in water (30cm ), 3 3 and 30Z (10cm ) added with stirring. Addition of ethanol (25cm ) and storage at -5°C yielded a light-yellow solid, which was filtered off, washed with ethanol and air-dried. FOUNO : C, 7.4 ; H. 0.9 ; K, 29.9 ; (<>2)2', 24.5. C4H2K6Nb2018 requires C. 6.3 ; H. 0.3 ; K. 30.9 ; ( O ^ 2". 25.3Z. The tantalum analogue was similarly prepared, and was isolated as a white solid. FOUND : C, 4.5 ; H. 0.4 ; K. 23.9 ; ( O ^ 2", 19.4. C.H„Ke0,Ja, requires C. 5.1 ; H. 0.2 ; K. 25.1 ; (0„)2“ , 20.5Z. 4 2 b I o i Z Attempted Preparation of Other Carboxylato Peroxo Complexes The reactions of malonic, salicylic and mandelic acids with K3CM(02)4].0.5H20 (M = Nb, Ta) in the presence of H202 each yielded only the unreacted potassium tetraperoxometallate(V) upon precipitation with ethanol. Raman Study of the Aqueous Peroxo Chemistry of Niobium and Tantalum An aqueous solution of K^CNbfO^^] ,0.5H20 was treated with a few drops of H202 (pH 11) and subjected to Raman spectroscopy [845(10), 540(8), 528sh cm’1]. The Raman spectrum was remeasured after acidific ation to pH 9 using dilute HC1 [875sh(2), 845(10), 825(2), 545(9) cm'1]. Further acidification saw precipitation of a light-yellow precipitate at pH 8. The Raman spectrum of the supernatant solution at pH 7 was 282 measured [870sh, 845br(8), 550br(10) cm’1]. Below pH 7. only a cloudy suspension was obtainable. The tantalum analogue was dissolved in HgO and a little HgOg to give a solution of pH 11 [Raman 850br(10), 550br(8) cm-1]. Acidification below pH 10 gave heavy precipitation of a gelatinous white solids Preparation and Study of the Niobium Carbide-HgOg Solution Addition of 60Z HgOg to a suspension of niobium carbide (2.5g) in water (25cm3) in a water-bath at 5°C, followed by stirring at room temperature for 3hr. did not accomplish appreciable dissolution of the 3 3 niobium carbide. Addition of further 60Z HgOg (9cm , 15cm in all) and stirring for 12hr. afforded a grey suspension and a black residue of carbon. Filtration of this suspension gave a clear pale-yellow solution of pH 1.8, which was subjected to Raman spectroscopy immediately. Precipitation of a gelatinous yellow-white precipitate was observed after about one hour. Preparation and Study of the Tantalum Carbide-HgOg Solution 3 Tantalum carbide (2.1g) was slurried in water UOcm ), and 60Z H,0 2 2 o o added dropwise at 5 C. The suspension was stirred and heated at 85 C for 6hr., during which time no appreciable dissolution of the carbide took place. Addition of further HgOg and heat gave similar negative results. The clear supernatant liquid showed only a band at 880cm”1 due to free HgOg in its Raman spectra, and there was no sign of a carbon residue. 283 Section 3.4 oq g Preparation of K2CNb(02)F53.H20 3 Niobium pentoxide (I.Og, 3.76 mmol.) and HF (48Z, 20cm ) were placed in a polypropylene beaker and heated on a steam-bath with occasional 3 stirring. Addition of 30Z H202 (15cm ) turned the solution yellow. 3 A solution of KOH pellets (0.429, 7.52 mmol.) in water (5cm ) was added. 3 and the solution heated for a further 10mm. Ethanol (50cm ) was added to the cooling solution to give a flaky light-yellow precipitate, which 3 was filtered off, washed with ethanol (3 x 15cm ), and dried in vacuo. FOUND : F, 29.5 ; K, 24.4 ; (Og)2", 10.3. FcHoKoNb0_5 2 2 3 requires F, 30.1 ; K, 24.7 ; (0o)2“ 2 , 10.1Z. VIBRATIONAL DATA (cm**1): INFRARED (nujol mull) : 690s, 615sh, 594s, 570sh, 560m, 503vs, 450m, 330w, 306m, 292w, 260w, 262m, 230w RAMAN (solid,KBr disc) : 890(8), 620(10), 611(7), 597(10). 594sh, 560( 1 ) , 329( 1 ) , 271 ( 1 ) RAMAN (aqueous soln.) : 885 ( 7 )p. 626sh,p, 610(10)p , 597(5)p, 560br, 324(1/2) Thermal Decomposition of K2[Nb(02)F53.HgO K.[Nb(0o)F_].H„02 2 5 2 was heated in a test-tube in the flame of a bunsen burner. Water of crystallisation was lost, and heating was stopped when the compound had lost all of its yellow coloration to become white. A second sample of K_[Nb(0.)F_].H*02 2 5 2 was left in an oven overnight at about 200°C, the temperature at which Jere and co-workers reported 298 optimum formation of K 2 [Nb0Fc]. 9 A white solid was obtained. The two solids were subjected to infrared and Raman spectroscopy, and gave virtually identical spectra (data listed below). 284 VIBRATIONAL DATA (cm’1): INFRARED (nujol mull) : 936s, 924s, 735s,b, 598sh, 550s, 487m,sh, 409m, 255s RAMAN (solid,KBr disc) : 962w, 938s, 924s, 899m, 671m, 657s, 596s, 296m Preparation of "Na_[Nb(0_)F_]"Z Z 5 A sample of "Na2[Nb(02)F^]“ was prepared using exactly the same method used for preparation of the potassium salt above, except that NaOH (0.30g, 7.52 mmol.) was used instead of KOH. FOUND : (02 )2"), 10.6*. Na [Nb(0 )F 3. H O requires (0_)2’ , 11.31. i i D c c Na (HF )CNb(0 )F ] requires (0o)2~, 9.87. J i c D C VIBRATIONAL DATA (cm”1): INFRARED (nujol mull) : 891vs, 626s, 612m, 600s, 576s, 558sh, 503m, 455s, 336w, 310m, 300m, 291w, 265m, 250m RAMAN (solid,KBr disc) : 892(9), 623(10), 614(8), 604(7), 596sh, 564(1), 338(3), 275(2) RAMAN (aqueous soln.) : 883(7)p , 625sh,p, 609(10)p, 596sh,p, 564br, 334(1). Preparation of Aqueous "Nao[Nb(0_)F_]"Z Z 5 from Aqueous K.CNb(0«)F_3.H.O Z Z 5 Z K [Nb(0 )F ].H O (0.5g, 1.68 mmol.) was dissolved in water (10cm3). i i d z Sodium perchlorate, NaClO^. (0.45g, 3.20 mmol.) was added, and the solution stirred for 48hr. A yellow supernatant solution of aqueous "Na2INb(02)F53" and a white precipitate of potassium perchlorate was the result. The former was centrifuged off and subjected to laser Raman spectroscopy. RAMAN (cm’1) : 935(10), 884(7), 622sh, 612(9), 594sh, 567br(2), 335(1) 285 Thermal Decomposition of "Na [Nb(0„)F_J* Z 2 5 White solids were obtained by heating "Na_[Nb(0.)F_]“ in a test-tube Z Z 5 in a bunsen flame, or heating in an oven at 200°C. Their vibrational spectra were identical. VIBRATIONAL DATA (cm"1): INFRARED (nujol mull) : 940s,br, 740s.br, 600sh, 560s, 480m RAMAN (solid,KBr disc) : 939s, 892m, 605br, 592m Preparation of (Ph.P)-[Nb(0o)F_J S c Z D K [Nb(0 )F ].H O (0.44g, 1.48 mmol.) was dissolve! in water (25cm3). c c o c * Tetraphenylphosphonium chloride (1.10g, 2,96 mmol.) was added with stirring. A yellow-white precipitate appeared, which was filtered off, 3 washed with cold water (3 x 20cm ), and dried in vacuo. FOUND : C, 61.8 ; H, 4.6 ; P,6.6. C48F5H40NbO2P2 recluires C * 64-1 ! H - 4*5 i p * 6.9Z. VIBRATIONAL DATA (cm'1): INFRARED (nujol mull) : 1108vs, 1028m, 998s, 893s, 868br,s, 756s, 725s, 690s, 621s, 579w, 530vs, 450w, 332w, 311w RAMAN (solid,KBr disc) : 1000(10), 880(3), 728(3), 680(6), 616(6), 601(6). 562(1) The solid was too insoluble in organic solvents for a satisfactory solution Raman spectrum to be obtained. Section 3.5 Preparation of K2IM(02 )(ox)2J.2H20 (M * Zr, Hf) Zirconyl chloride, Zr0Cl2 .8H20, (I.OOg, 3.10 mmol.) and potassium oxalate (1.14g, 6.20 mmol.) were dissolved in water (15cm3). Addition 286 3 of 30Z HgOg (10cm ) with stirring, followed by storage overnight at o 5 C, afforded a white solid, which was filtered off, washed with cold 3 ethanol (3 x 10cm ), and dried in a vacuum desiccator over silica gel. The solid proved to be slightly hygroscopic, and was therefore stored in a desiccator permanently. FOUND : C, 11.6 ; H, 0.7 ; K, 18.5 ; (Og)2", 7.5. C H K 0 Zr requires C, 11.6 ; H, 1.0 ; K, 18.9 ; (0 )2~, 7.7Z. 4 4 2 10 2 The analogous hafnium compound was similarly prepared and isolated as a hygroscopic white solid by the reaction of hafnyl chloride, HfOClg.SHgO. with potassium oxalate and excess HgOg. FOUND : C, 9.1 ; H, 0.9 ; K, 15.3 ; (02)2", 6.1. C4H4HfK2O10 requires C, 9.6 ; H, 0.8 ; K. 15.6 ; (02 )2", 6.4Z. Preparation of K2£H(02)2(cit)].H20 (M « Zr, Hf) Zirconyl chloride (I.OOg, 3.10 mmol.) and potassium citrate (I.OOg, 3 3.10 mmol.) were dissolved in water (25cm ), and the solution treated 3 with 30Z H202 (10cm ) with stirring. Addition of ethanol and storage o at 5 C gave a white solid, which was collected, washed with cold ethanol, and dried in a desiccator. The solid proved to be extremely hygroscopic, and therefore best stored over silica gel in a desiccator. FOUND : C, 16.6 ; H. 1.9 ; K, 16.4 ; (02 )2~ , 14.1. 2- CcHoKo°««Zr requires C. 16.3 ; H, 1.8 ; K, 17.7 ; (0 ) , 14.5Z. b o c 1 i Z The analogous hafnium complex, isolated as a hygroscopic white powder, was similarly prepared, but not with the same purity as its zirconium counterpart. FOUND : C. 14.9 ; H. 1.6 ; K. 16.8 ; (Og)2", 10.3. C6H8HfK2°11 recluires C - 13*6 *• H * 1-5 : 14.8 ; (0 )2~. 12.1Z. 287 Preparation of Glycollato Peroxo Complexes of Zr(IV) and Hf(IV) Zirconyl chloride (I.Og, 3.10 mmol.) and glycollic acid (0.A7g( 3 3 6.2 mmol.) were dissolved in water (15cm ) and 30Z (5cm ) added. 3 A solution of KOH (0.35g, 6.20 mmol.) in water (5cm ) was added. A yellow-white solid appeared at 5°C, and was collected, washed with cold ethanol and dried in a desiccator. FOUND : C. 8.5 ; H, 2.3 ; K. 21.3 ; ( O ^ 2'. 10.1. A similar compound was obtained usinja 1:1 zirconium:glycollate ratio. Similarly, the analogous hafnium compound was prepared using 2:1 or 1:1 ratios of hafnium:glycollate, being isolated as a white solid. FOUND : C, 7.9 ; H, 2.2 ; (02), 11.2Z. Section 3.6 303 Preparation of (NN^IgtVgOgCOH) 10^I.H20 A solution of ammonium metavanadate (2.35g, 20 mmol.) in water 3 3 (80cm ) was treated with concentrated ammonia (10cm ), and the solution 3 cooled in an ice-bath before addition of 30Z H^Og (10cm ). Yellow o crystals appeared overnight at 5 C, and were filtered off, washed with ethanol and air-dried. FOUND : H, 4.0 ; N, 11.1 ; ( O ^ 2", 35.4. H 17N3°13V2 rec*uires H* 4-7 * N* 11*4 ; (02 ,2~* 34*77* VIBRATIONAL DATA (cm"1): INFRARED (nujolmull) : 1030m,br, 970s, 948vs, 883s, 868s, 861sh, 722w, 639s, 612vs, 527m, 507m, 456w RAMAN (solid.KBr disc): 965(8), 940(6), 925(4), 894(1), 881(7), 877(7). 860(1), 643(2), 604(5), 564(1), 519(10), 495(3), 452(1/2), 320(2) 288 Preparation of ( 0 ^ 1 .2H2<> (NH4)3[V202 (0H)(02 )43.H20 was dissolved in minimum 2H20 and allowed to recrystallise at 5°C. The yellow crystals were filtered off and dried over silica gel in a desiccator. Deuteriation was confirmed by the appearance of v( 2 H-0) bands at 2380 and 2340cm -1 . and a 6( 2 H-0- 2 H) band at 1080cm 1 in the infrared spectrum. VIBRATIONAL DATA (cm’1): INFRARED (nujol mull) : 970s, 945vs, 883s, 86Bvs, 861sh, 755w, 721w, 638s, 610vs, 525w, 505w, 455w RAMAN (solid,KBr disc): 971(6), 940(4), 884(5), 880(5), 863(1), 649(1), 621sh, 610(5), 522(10), 508sh, 362(1), 327(2) Colorimetric Determination of Free Peroxide Using Titanium(IV) The titanium(IV) solution was prepared by dissolving potassium 3 titanium oxalate (10g) in concentrated sulphuric acid (55cm ), and heating the resulting solution for about 30-60 min., until all 3 effervescence had ceased, followed by cooling and dilution to 500cm . The ethyl acetate extraction procedure was as follows. An aqueous solution of either a weighed amount of H202 (for calibration purposes) or of the sample for analysis was extracted twice with ethyl acetate 3 (10cm ) in a separating funnel. The combined ethyl acetate extracts 3 were then added to the acidic titanium(IV) solution (10cm ) and the mixture shaken. The lower aqueous layer was then run into a 1cm cell (or diluted first if necessary) for measurement of its absorption at 405nm. In a typical experiment, a blank was run, and a calibration graph plotted using up to eight known amounts of *see Fri9* 3,7 for an example). Each compound analysed was also titrated iodometrically for total peroxide content (see Appendix). Results of the analyses are given in tabulated form overleaf. 289 Compound Total Peroxide(Z) Free Peroxide(Z) Calc. Found Calc Found K2CMo0(02)2(cit)].0.5H202.3H20 15.5 15.2 3.1 2.9 K [VO(0 ) (ox)].H 0 25.9 25.6 8.6 8.4 3 2 2 2 2 [Mo0(02)2(ox)] 18.7 18.6 NIL NIL K3[VO(°2)2(bipy )]•^H2° 16.8 17.0 NIL NIL 290 APP-ENm 6ENERAL EXPERIMENTAL Physical Measurements Infrared spectra of solids were measured on Perkin-Elmer 683 and Perkin-Elmer 1720IFTIR) instruments, as mulls in liquid paraffin between potassium bromide or caesium iodide plates, or as pressed potassium bromide pellets. Raman spectra were measured on a Spex Ramalog V spectrometer and Spex Datamate computer control unit, using the exciting lines at 568.2 and 530.9nm from a Coherent model 52 krypton-ion laser, at 568.2nm from a Coherent Innova 90 krypton-ion laser, and 514.5nm from a Coherent Innova 70 argon-ion laser. Spectra of solids were taken as spinning potassium bromide discs, and of solutions in capillary tubes or spinning quartz-glass solution cells. 95 13 The Mo and C n.m.r. spectra were measured on a Bruker WM250 Fourier-transform spectrometer at 16.3 and 62.9 MHz respectively, 2 using 10mm tubes, and H^O for locking purposes (internally for aqueous 1 19 solutions, or in a 5mm insert otherwise). The H and F n.m.r. spectra were recorded on a Jeol FX 90Q Fourier-transform instrument at 89.55 and 2 84.27 MHz respectively, using 5mm tubes, and H^O or a deutenated organic solvent for locking purposes. Ultraviolet/visible spectra were recorded using quartz-glass cells (cell-path 1cm) on a Perkin-Elmer 551 spectrometer with a deuterium power supply. 291 Reagents All reagents were used as supplied. Hydrogen peroxide (30Z and 60Z w/v) used was of "Analar" grade. Ethanol was of "absolute" or "96" grade, and dichloromethane, diethyl ether, petroleum ether, methanol and hexane were of "reagent" grade. Analyses Carbon, hydrogen, nitrogen, phosphorus and sulphur microanalyses were performed by Hr. K . Jones of the Chemistry Department Micro- Analytical Laboratory, Imperial College. The fluorine analysis on K^tNbtO^JFj.].H^O was performed by Hikroanalytisches Labor Pascher, Germany, and XRD molybdenum and tungsten analyses in the separational experiments by the analytical department of Interox Chemicals Ltd. Peroxide analyses were carried out by iodometric titration; a 3 weighed amount of sample was dissolved in 2M H^SO^ (100cm ), potassium iodide (0.25g) added, and the solution titrated against 0.1H sodium thiosulphate using fresh starch solution (added when the solution was straw-coloured) as indicator. Potassium analyses were performed gravimetrically. Enough sample to 3 contain 2Q-30mg of potassium was dissolved in water (100cm ) and the solution acidified to pH 4-6 using acetic acid. Sufficient 0.1M tetra- phenylboron sodium solution was added to give a slight (ca. 1Z) excess. o This addition was performed with both solutions at 60 C. After cooling to room temperature with occassional stirring, the precipitate of tetra- phenylboron potassium was collected on a glass sinter (no. 4), washed 3 with 50cm of wash solution C1Z acetic acid; 0.1Z tetraphenylboron sodium] and water (5cm3), and dried at 105°C to constant weight. Caesium analyses were performed similarly. 292 REflRfNCES 1. E. 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The Vibrational Spectra of CNb(0_)F_]2 . 2 0 A.C. Dengel and W.P. Griffith, Spectrochim.Acta. 1987, 43A. 1173. 317 X-Ray Structure of KJMozC^fOzMC^OelMh^O, A Novel Peroxo Complex containing a Single Tetradentate Bridging Tartrate Andrew C. Dengel, William P. Griffith/ Richard D. Powell, and Andrzej C. Skapski* Chemical Crystallography and Inorganic Chemistry Laboratories, Imperial College, London SW7 2AY, U.K. The single crystal X-ray structure of the title complex (1) shows that two MoO(02)2 moieties are spanned by a tartrato ligand which is bidentate at each end, giving pentagonal bipyramidal co-ordination to the molybdenum atoms; spectroscopic data (13C, 95Mo n.m.r., Raman) show that this structure is maintained in aqueous solution. Reprinted from the Journal of The Chemical Society Chemical Communications 1986 J. CHEM. SOC., CHEM. COMMUN., 1986 556 cm-1, with the first three being polarised. On the basis of their tively}; therefore, this tartrato complex possesses unusual positions and state of polarisation3 we assign the bands stability in solution. respectively to v(Mo= O), v(O-O), vsym. {Mo(02)}, and vasym. Although some molybdenum peroxo complexes epoxidise {Mo (0 2)}. The 13C n.m.r. spectrum of (1) in D20 shows two alkenes,5 we find that (PPh4)4[Mo20 2(02)4(C4H20 6)] in singlets at 6 89.0 and 186.0, assigned to the carbon atoms of acetonitrile does not react with alkenes such as cyclohexene. the carbonyl and-CH(O) groups respectively [in {Mo20 4(+)- We thank Interox Chemicals Ltd. and the S.E.R.C. for (C4H20 6)2}4_, for which a double tartrato bridge has been grants to two of us (A.C.D. and R.D.P.) and the S.E.R.C. for proposed, such shifts are observed at 5 88.4 and 183.84]. It the diffractometer system. K4[Mo 20 2(0 2)4(C4H20 6)H H 20 Received, 20th December 1985; Com. 1798 (1) References K2[Mo O(O2)2(C6H6O7)]-0.5H2O2-3H2O 1 G. M. Sheldrick, ‘SHELXTL—an Integrated System for Solving, (2) Refining and Displaying Crystal Structures from Diffraction Data,’ revision 4, January 1983, Nicolet Instruments Ltd, Warwick, thus seems clear that the tartrato ligand retains its bridging England. character in solution. Furthermore the 95Mo n.m.r. spectrum 2 J. Flanagan, W. P. Griffith, A. C. Skapski, and R. W. Wiggins, of the complex in D20 shows a sharp singlet at 5 - 235 p.p.m. Inorg. Chim. Acta, 1985, 96, L23. 3 W. P. Griffith, J. Chem. Soc. A, 1968, 397. relative to [Mo04]2~, suggesting that only one species is 4 A. M. V. Cavaleiro, J. D. Pedrosa de Jesus, R. D. Gillard, and present. We find that 95Mo n.m.r. spectroscopy is a sensitive P. A. Williams, Transition Met. Chem., 1984, 9, 81; A. M. V. technique for detecting dissociation of carboxylato peroxo Cavaleiro, V. M. S. Gil, J. D. Pedrosa de Jesus, R. D. Gillard, and complexes in solution {thus [Mo02(02)(C20 4)(H20)]2_ P. A. Williams, ibid., p. 62. gives the two bands at 6 -230 and +4.4 p.p.m. due to 5 H. Mimoun, ‘The Chemistry of Peroxides,’ ed. S. Patai, Wiley, [Mo O( 0 2)2(C20 4)]2- and [Mo03(C20 4)(H20)]2- respec New York, 1983, p. 469. J. CHEM. SOC. DALTON TRANS. 1987 991 Studies on Transition-metal Peroxo Complexes. Part 7.t Molybdenum(vi) and Tungsten(vi) Carboxylato Peroxo Complexes, and the X-Ray Crystal Structure of K2[M oO(02)2(glyc)].2H2Ot Andrew C. Dengel, William P. Griffith,* Richard D. Powell, and Andrzej C. Skapski* Chemical Crystallography and Inorganic Chemistry Laboratories, Imperial College, London SW7 2AY New complexes [M oO (02)2L]2_ [L = malate, tartrate (tart), tartronate, quinate, orglycolate (glyc)], [W 0 (0 2)2L]2_ (L = citrate, tartrate, malate, orglycolate), and [W202(02)4(tart)]4_ are reported, and modified preparations given for [M 0 (0 2)2L]2_ and [M 0 2(02)L(H20)]2~ (M = Mo or W; L = oxalate). Raman and i.r. spectra of the solid complexes, and Raman, 13C and ^Mo n.m.r. spectra of their aqueous solutions, have been measured and are used to suggest the structures of the species in the solid state and solution. The X-ray crystal structure of K2[M o 0 (0 2)2(glyc)]*2H20 has been determined: triclinic, space group P\ , with a = 7.222(4), b = 7.950(6), c = 10.450(5) A, a = 67.81 (4), p = 73.72(7), y = 64.84(4)° at 20 °C, and Z = 2; the structure has been refined to R = 0.035. We have previously prepared and characterised the oxalato (ox) peroxo complex depends markedly on the pH of the peroxo complexes K2[MO(02)2(ox)], K2[M 02(02)(ox)- reaction mixture: some complexes (oxalate, citrate, non- (H20)]*H20 (M = Mo or W),1 the citrato (cit) (2-hydroxy- bridged tartrate, tartronate, malate, and glycolate) are propane-1,2,3-tricarboxylate) complex K2[M oO (02)2(cit)]« formed at the natural pH of the metal-carboxylate-peroxide 3H200.5H20 2,2 and the tartrato (tart) complexes K4[M20 2- mixture, while others [bridged tartrate, quinate (quin)] need (02)4(tart)]*4H20 (M = Mo or W).3 A-Ray crystal structures pH adjustment for complex formation. Generally high pH of the complexes K2[MoO(02)2(ox)],4 K2[WO(02)2(ox)],5 (>7—8) leads to the formation of the unstable [M(02)4]2_ K2[MoO(02)2(cit)]*3H200.5H20 2,2 and K4[Mo20 2(02)4- species, while at low pH (< 2—3) formation of the very (tart)]*4H20 3 have been reported. stable [M20 3(02)4(H20)2]2- is favoured rather than the In this paper, we report a number of new, stable carboxylato carboxylato peroxo complex. Rates of formation of peroxo complexes and present spectroscopic data for them in carboxylato peroxo complexes are markedly greater for the solid state and for their aqueous solutions, as well as the first molybdenum than for tungsten. X-ray crystal structure of a glycolato (glyc) peroxo complex, In a recent paper it was shown that reaction of M o03 and namely K2[MoO(02)2(glyc)]*2H20. The work has been KOH with malic acid in the presence of an excess of hydrogen undertaken in order to investigate further the chemistry of peroxide gave K2[MoO(02)2(ox)] rather than a malato peroxo carboxylato peroxo complexes, as our earlier experiments on complex.6 We find however that a malato (mal) complex, oxalato species indicate that carboxylates rival fluorides as co K2[MoO(02)2(mal)]*2H20, can be made from [Mo04]2_, ligands to impart chemical and thermal stability to peroxo malic acid, and an excess of hydrogen peroxide at pH 3.4, a complexes of the Group 4—6 transition metals. higher pH than that (2.9) obtained by using M o03 and KOH. We find that recrystallisation of the malato complex from an excess of H20 2 does however give K2[M oO(02)2(ox)], Results and Discussion indicating that a slow, complicated rearrangement and (a) Formation of Complexes.—We find that carboxylato decarboxylation of the malate ligand has occurred. ligands capable of forming five-membered rings with the metal Although in general the carboxylato peroxo complexes are [e.g. oxalate, citrate, tartrate, malate, tartronate, glycolate, formed under very similar conditions for molybdenum and quinate (1,3,4,5-tetrahydroxycyclohexanecarboxylate)] gener tungsten, there are significant differences for K4[Mo20 2(02)4- ally give stable complexes with peroxo co-ligands. Using the (tart)]-4H20 and K2[MoO(02)2(quin)]-2H20. The molyb same methods we find that carboxylates which do not form denum tartrate complex was formed only when the pH was such rings (e.g. acetate, adipate, succinate, salicylate, and adjusted to 4, while the corresponding tungsten complex was thiosalicylate) do not form such complexes. Unsaturated acids formed at pH 2; the molybdenum quinate was formed at pH 3.3, capable of forming five-membered rings {e.g. mandelic while the tungsten analogue could not be isolated at all. (phenylglycolic), atrolactic [methyl(phenyl)glycolic], and 3- Several covalent molybdenum(vi) complexes are known to phenyl-lactic} do not form stable complexes, presumably due epoxidise simple alkenes in good yield.7 Using the well to the electron-withdrawing properties of their aromatic documented epoxidation of simple alkenes by the complex rings. We also find that the formation of some of the [M oO(02){P(0)(NMe2)3}]7_1° as a model, we used n.m.r. carboxylato peroxo complexes can depend on the reacting spectroscopy to determine whether the complexes [PPh4]2- metal :carboxylate ratio, as with the oxalate and tartrate [MoO (02)2(ox)] and [PPh4]4[Mo20 2(02)4(tart)] in acetoni systems, or on the amount of peroxide present, as with the trile would similarly epoxidise alkenes. In no case was oxalate system. In all cases the formation of the carboxylato epoxidation observed, suggesting it is inhibited by the residual negative charge of these anionic molybdenum(vi) peroxo complexes. t Part 6 is ref. lb. + Dipotassium (glycolato-00')oxodi(peroxo-0O')molybdate(vi) dihy (b) X-Ray Crystal Structure of K2[M oO(02)2(glyc)]« drate. 2H20.—Although there have been A-ray crystal structure Supplementary data available: see Instructions for Authors, J. Chem. determinations on several glycolato complexes,11 this is the first Soc., Dalton Trans., 1987, Issue 1, pp. xvii—xx. peroxo glycolato complex to be so characterised. The glycolate 992 J. CHEM. SOC. DALTON TRANS. 1987 group is of interest in the context of peroxo chemistry in that it A]. As in complexes (1) and (2), it appears that the deprotonated has a hydroxyl donor atom as well as the carboxylate donor hydroxyl group assumes the equatorial rather than the axial sites; we wished to establish whether the unusual mode of position, so as to form a strong bond; the long axial Mo-O bonding involving both hydroxyl and carboxylate oxygen distance could be due to the traits influence of the oxo ligand. atoms found in K2[MoO(O2)2(cit)]*3H2O*0.5H2O22 was The crystal structure is held together by ionic attraction followed in this case as well. The Figure shows the structure of between the K + ions and the complex anion, and by hydrogen the [MoO(02)2(glyc)]2_ anion, and bond lengths and angles are bonding involving the water molecules. One of the potassium given in Table 1. ions K(l) has eight oxygen near neighbours at distances 2.73— The co-ordination about the molybdenum atom is essentially 2.90 A, while K(2) is surrounded by nine oxygen atoms at 2.81— pentagonal bipyramidal, similar to that found by us in the 3.12 A. Each of the two water molecules of hydration forms two complexes K.2[MoO(02)2(cit)]*3H200.5H20 2 (l)2 and K4- OH • • • O hydrogen bonds to oxygen atoms on the complex [Mo20 2(02)4(tart)]*4H20 (2).3 The axial positions are anion. occupied by the terminal oxo ligand [Mo-O 1.686(6) A] and the oxygen atom from the deprotonated carboxylate group (c) Infrared and Raman Data.—The i.r. and Raman data are [M o-O 2.239(6) A]. The equatorial positions are occupied by summarised in Table 2. In addition to the bands arising from the the two slightly asymmetrically bound peroxo ligands [mean carboxylato ligands, all the complexes show strong i.r. and Mo-O 1.927(4) and 1.966(4); mean 0 - 0 1.471(7) A], and the Raman bands near 950 cm-1 due to v(M=0), polarised in the deprotonated hydroxyl group of the glycolate [Mo-O 1.991(5) Raman spectra of the aqueous solutions. For the dioxo complexes [M 02(02)(ox)(H20)]2_, M = Mo or W, two such Table 1.Bond lengths (A) and angles(°) in K2[M oO(02)2(glyc)]*2H20 with estimated standard deviations (e.s.d.s) in parentheses M o(l)-0(l) 1.686(6) Mo(l)-0(7) 2.239(6) Mo(l)-0(2) 1.970(4) Mo(l)-0(4) 1.961(4) Mo(l)-0(3) 1.926(4) Mo(l)-0(5) 1.928(4) Mo(l)-0(6) 1.991(5) 0(4)-0(5) 1.472(7) 0(2)-0(3) 1.469(7) C(2)-O(10) 1.231(11) C(2)-0(7) 1.279(9) C(l)-C(2) 1.528(9) c(\yo(6) 1.413(11) 0(l)-Mo(l)-0(2) 99.9(2) 0(l)-Mo(l)-0(3) 101.6(2) 0(l)-Mo(l>-0(4) 99.8(2) 0( 1 )-Mo( 1 )-0(5) 102.4(2) 0(l)-Mo(l)-0(6) 92.8(2) 0(3)-Mo(l)-0(5) 87.0(2) 0(2)-Mo( 1 )-0(3) 44.3(2) 0(4)-Mo(l)-0(5) 44.5(2) O(2)-M0( 1 )-0(6) 89.3(2) 0(4)-M o( 1 )-0(6) 89.9(2) 0(7)-Mo(l)-0(l) 168.2(2) 0(7)-Mo(l)-0(6) 75.4(2) 0(7)-Mo(l)-0(2) 79.9(2) 0(7)-Mo( 1 )-0(3) 86.6(2) 0(7)-Mo(l)-0(4) 80.8(2) 0(7)-Mo(l)-0(5) 86.4(2) Mo(l)-0(2)-0(3) 66.3(2) Mo(l)-0(3)-0(2) 69.5(2) Mo(l)-0(4)-0(5) 66.6(2) Mo( 1 )-0(5)-0(4) 68.9(2) Mo(l)-0(6)-C(l) 120.3(3) Mo(l)-0(7)-C(2) 115.8(4) 0(6)-C(l)-C(2) 111.6(6) 0(7)-C(2)-C(l) 114.9(7) Figure. Structure of the [M o0(02)2(glyc)]2~ anion. Thermal vibration O(10)-C(2)-C(l) 120.2(7) O(7)-C(2)-O(10) 124.9(6) ellipsoids are scaled to enclose 50% probability Table 2. Vibrational and n.m.r. data Vibrational data (cm ')“ N.m.r. data6 Complex v(M=0) v(O-O) vsym[M (02)] Vasym[M(02)] 95Mo 13c' K2[M oO(02)2(ox)] I.r. 972vs 872s 661m 606m -228.3 R 965(10) 880(9) 655(5) 587(7) R 968(10) 876(8) 653(6) 588(7) K2[WO(02)2( ox )] I.r. 985s 871m 661s 614m 855m R 970(10) 885(9) 645(4) 592(7) R 961(10) 856(9) 649(4) 595(6) K2[M o 0 2(0 2)(ox )(H20)]*H20 I.r. 975sc 870s 656m 601m -229.7 915m" + 4.4 R 962(8)c 877(6) 653(4) 581(5) 909(7)" R 964(70) 875(9) 640(6) 584(7) K2[W 02(02)(ox )(H20)].H20 I.r. 985vsc 869m 659m 613m 919m" 853m R 966(10)c 877(8) 655(6) 592(7) 908(4)" R 967(10) 876(8) 650(5) 596(7) K2[MoO(O2)2(cit)]-3H2O-0.5H2O2 I.r. 957s 875m 653m 603m — 247br ca. 179 861s 88.2 R 962(10) 877(8) 652(6) 590(7) 47.1 R 959(10) 870(9) 640(6) 583(7) J. CHEM. SOC. DALTON TRANS. 1987 993 Table 2 (continued) Vibrational data (cm-1)" N.m.r. data6 ______A.______Complex ' v(M=0) v(O-O) VSym[M(02)] vasym[M (02)j ' 95Mo 13C K2[WO(O2)2(cit)]-3H2O-0.5H2O2 I.r. 963s 875m 655w 602w R 952(10) 848(9) 637(7) 580(8) K4[Mo20 2(0 2)4(tart)]-4H20 I.r. 929s 857s 638s 578m -235.0 186.0 847s 89.0 R 935(10) 857(7) 640(4) 592(6) R 954(10) 869(8) 639(5) 582(7) K4[W20 2(02)4(tart)]-4H20 I.r. 924s 831s 622m 570w 175.1 R 957(10) 852(9) 628(5) 571(6) 88.5 R 952(10) 852(9) 628(7) 566(8) K2[MoO(02)2(mal)]*2H20 I.r. 930s 855s 633m 584m -233.0 186.3 178.6 R 949(10) 873(8) 638(6) 590(7) 82.0 44.2 R 965(10) 875(8) 641(5) 580(6) K2[W 0(02)2(mal)]-2H20 I.r. 925s 830s 625m 575w R 945(10) 845(9) 625(6) 565(8) K2[MoO(02)2(tron)]-2H20 I.r. 947vs 855vs 640m 584s -227.7 R 958(10) 874(8) 640(6) 585(7) R 968(10) 875(8) 640(6) 585(7) K2[M oO(02)2(glyc)]*2H20 I.r. 937vs 849s 635m 584m -220.6 186.6 922vs 74.9 R 943(10) 864(9) 642(5) 596(6) 925(7) R 958(10) 868(9) 639(5) 587(6) 930(7) K2[MoO(02)2(quin)]-2H20 I.r. 935s 848s 627m 580m -245.0 R 946(10) 868(7) 631(4) 588(7) R 952(10) 870(8) 640(6) 585(7) K2[MoO(02)2(tart)]-2H20 I.r. 937vs 871s 656s 608m -252.0 185.9 859s -231.7 185.5 89.5 R 936(9) 874(8) 637(6) 600(5) 88.8 R 954(10) 862(9) 640(6) 602(5) K2[W 0(02)2(tart)].2H20 I.r. 939s 859w 641m 603w R 934(10) 880(8) 635(6) 598(5) [PPh4]2[MoO(02)2(ox)] I.r. 962s 871s 648s 585s 851s R 942(9) 874(4) 616(6) 588(5) Re 943(8) 870(5) 616(6) 583(4) [PPh4]4[Mo20 2(0 2)4(tart)] I.r. 962vs 862vs 629m 598m R 960(7) 873(6) 616(4) 561(5) K 2[M o 0 3(ox )]-2H20 I.r/ 903ss 5.1 869vs^ Rf 901c 869" K2[W 0(02)2(glyc)].2H20 I.r. 940s 841m 624s 581m 187.5 918s 827s 75.3 R 938(9) 840(8) 620(4) 572(6) 922(7) R 957(10) 854(9) 632(6) 565(7) 932(8) “ Data for solids or (italicised) aqueous solutions; relative Raman intensities given in parentheses.6 In p.p.m. vs. SiMe4 for l3C and i>s. [M o04]2 for 95Mo. c vsym. d vasym. e In dichloromethane solution. 1W. P. Griffith and T. D. Wickins, J. Chem. Soc. A, 1968, 400. bands are observed, as in other c/s-dioxo complexes,12 near 980 composition to [Mo0(02)2(ox)]2 and [Mo03(ox)]2 [see and 915 cm-1; these are assigned to vsym(M 02) (polarised in the section (H20 (M = Mo or W). The gel (Found: C, 61.9; H, 4.3; P, 6.8. C50H40MoO9P2 requires C, method used was essentially that of Rodriguez,16 with 63.7; H, 4.3; P, 6.6%). modification of the isolation procedure. Potassium molybdate (1.0 g, 4.2 mmol) and oxalic acid dihydrate (0.53 g, 4.2 mmol) General Experimental.—Infrared spectra were measured on were dissolved in water (10 cm3), and the solution stirred and a Perkin-Elmer 683 spectrometer, as mulls in liquid paraffin treated with 30% H2Oz (0.35 cm3, 4.2 mmol). Yellow crystals between potassium bromide plates. Raman spectra were were obtained by addition of a few drops of ethanol and cooling measured on a Spex Ramalog V instrument and Spex Data- to 5 °C, filtered off, and washed with ethanol. The tungsten mate computer control unit, using the exciting lines at 568.2 analogue was prepared similarly [Found: C, 7.0; H, 0.6; K, 22.2; and 530.9 nm from a Coherent model 52 krypton-ion laser. (0 2)2-, 9.5. C2H4K2Mo O 10 requires C, 6.6; H, 1.1; K, 21.6; Spectra of solids were taken as spinning KBr discs, and of (0 2)2-, 8.8. Found: C, 5.5; H, 0.4; K, 17.0; (0 2)2~, 7.8. aqueous solutions in capillary tubes. The 95Mo and 13C n.m.r. C2H4K2O 10W requires C, 5.3; H, 0.9; K, 17.4; (0 2)2~, 7.1%]. spectra were recorded on a Bruker WM250 Fourier-transform K2[MO(O2)2(cit)]-3H2O-0.5H2O2 (M = Mo or W). The spectrometer. Proton n.m.r. spectra in the epoxidation J. CHEM. SOC. DALTON TRANS. 1987 995 SHELXTL program system,17 and atomic scattering factors Table 3. Atomic co-ordinates of the non-hydrogen atoms ( x 104) with and anomalous dispersion corrections were taken from ref. 18. e.s.d.s in parentheses The co-ordinates of the molybdenum atom were derived from an initial Patterson synthesis and the positions of all the other Atom X y z atoms were found from subsequent Fourier difference syntheses. Mo 1 540(1) 3 577(1) 2 794(1) Least-squares refinement was by the block-cascade method, K(l) 2 999(2) 2 043(2) 6 405(2) typical of the SHELXTL system. All non-hydrogen atoms were K(2) 2 958(2) 8 129(2) 649(2) refined with anisotropic thermal parameters, while the positions 0(1) 1 247(7) 5 270(7) 3 509(5) of the glycolate hydrogen atoms were tied to those of the parent 0(2) 1 549(8) 1 367(7) 4 493(5) 0(3) 3 624(7) 1 333(6) 3 780(5) carbon atom; those of the water hydrogens were fixed and their 0(4) 1 656(8) 5 100(7) 816(5) isotropic thermal parameters were allowed to refine. A 0(5) 3 706(7) 3 857(7) 1 247(5) weighting scheme was applied so that w = l/[cr(F0)2 + 0(6) -1 498(6) 4 328(6) 2 931(5) 0.0007Fo2] for the last cycle; R reduced to 0.035, and R' = 0(7) 1 215(6) 1 559(6) 1 919(4) [2 h’|AF|2/ I iv|AF0|2]* was 0.038. 0(8) 3 148(7) 2 067(6) 9 131(5) Fractional co-ordinates of the non-hydrogen atoms are listed 0(9) 3 410(8) 7 520(7) 3 695(6) in Table 3. O(I0) -1 075(7) 499(6) 1 741(5) C(l) - 2 298(10) 3 104(9) 2 747(7) C(2) -618(10) 1 585(9) 2 079(6) Acknowledgements We thank Miss Sue Johnson for the 95Mo and 13C n.m.r. spectra, Interox Chemicals and the S.E.R.C. for grants (to A. C. D. and R. D. P.), and the S.E.R.C. for the diffractometer experiments were measured on a JEOL FX 90Q Fourier- system. We thank Dr. David Mobbs and Diana Anderson for transform spectrometer. Microanalyses were performed by Mr. helpful discussions. K. Jones of the Imperial College Microanalytical Department, peroxide analyses by iodometric titration, and potassium References analyses gravimetrically. 1 (a) W. P. Griffith and T. D. Wickins, J. Chem. Soc. A, 1967, 590; (b) Absolute ethanol and other reagents were used as supplied, ibid., 1968, 397. except for potassium tungstate which was prepared by 2 J. Flanagan, W. P. Griffith, A. C. Skapski, and R. W. Wiggins, Inorg. acidification of sodium tungstate to W 03, followed by the Chim. Acta, 1985, 96, L23. 3 A. C. Dengel, W. P. Griffith, R. D. Powell, and A. C. Skapski, J. addition of KOH until pH 7—8. Chem. Soc., Chem. Commun., 1986, 555. 4 R. Stomberg, Acta Chem. Scand., 1970, 24, 2024. X-Ray Crystal Structure of K2[MoO(02)2(glyc)]*2H20.— 5 R. Stomberg and S. Olsen, Acta Chem. Scand., Ser. A, 1985, 39, 79. Yellow crystals of the complex were prepared as described in the 6 C. Djordjevic, K. J. Covert, and E. Sinn, Inorg. Chim. Acta, 1985,101, Experimental section. That selected for intensity data collection L37. was an elongated prism, approximately 0.25 x 0.04 x 0.03 7 H. Mimoun, ‘The Chemistry of Peroxides,’ ed. S. Patai, Wiley, New mm. Measurements were carried out on a Nicolet /?3m/Eclipse York, 1983, p. 469 and refs, therein. SI40 diffractometer system with graphite-monochromated Cu- 8 H. Mimoun, I. Seree de Roch, and L. Sajus, Tetrahedron, 1970,26,37. Ka radiation. Unit-cell dimensions were determined by least- 9 H. Arakawa, Y. Moro-Oka, and A. Ozaki, Bull. Chem. Soc. Jpn., squares refinement of the angular settings of 17 automatically 1974, 47, 2958. 10 K. B. Sharpless, J. M. Townsend, and D. R. Williams, J. Am. Chem. centred reflections. Soc., 1972, 94, 295. Crystal data. C2H6K2M oO10, M = 364.21, triclinic, space 11 See, for example, N. W. Alcock, T. J. Kemp, S. Sostero, and O. group Pi, a = 7.222(4), b = 7.950(6), c = 10.450(5) A, Traverso. J. Chem. Soc., Dalton Trans., 1980, 1182. a = 67.81(4), p = 73.72(7), y = 64.84(4)°, U = 497.7 A3 at 12 W. P. Griffith, J. Chem. Soc. A, 1969, 211. 20 °C, Z = 2, Dc = 2.43 g cm'3, F(000) = 351.9, X(Cu- 13 M. V. Capparelli, B. Piggott, S. D. Thorpe, S. F. Wong, and R. N. KJ = 1.541 8 A,n(Cu-Ka) = 191.2 cm"1. Sheppard, Inorg. Chim. Acta, 1984, 106, 19. Integrated intensities in one hemisphere were measured 14 A. Mazzucchelli and G. Inghilleri, Atti. Accad. Naz. Lincei. Cl. Sci. using the ©-scan technique. Two reflections (110 and ill) Fis., Mat. Nat. Rend., 1908, 17, 30. were monitored every 50 measurements, and these decreased 15 A. Mazzucchelli and G. Zangrelli, Gazz. Chim. Ital., 1910, 40, 49. by ca. 6% over the period of data collection (1 d). A total of 16 M. M. Rodriguez, Anales Fis. Quim. (Madrid), 1944, 40, 1270. 1 518 independent reflections were measured (to 0 = 57°), of 17 G. M. Sheldrick, ‘SHELXTL, an Integrated System for Solving Refining and Displaying Crystal Structures from Diffraction Data,’ which 49 were judged to be ‘unobserved’ [/ < 3a(/)]. The data Nicolet Instruments Ltd., Warwick, Revision 4, January 1983. were scaled using the reference reflections and were corrected 18 ‘International Tables for X-Ray Crystallography,’ Kynoch Press, for Lorentz and polarisation effects. At a later stage an Birmingham, 1974, vol. 4. empirical absorption correction was applied,17 based on 36 psi-scan measurements for each of eight representative reflec tions. All calculations and drawings were made using the Received 15th May 1986; Paper 6/936 Spectrochimica Acta, Vol. 43A, No. 9, pp. 1173-1175, 1987. 0584-8539/87 $3.00 + 0.00 Printed in Great Britain. © 1987 Pergamon Journals Ltd. The vibrational spectra of [Nb(02)F5]2 A. C. D engel and W. P. Griffith* Inorganic Chemistry Laboratories, Imperial College, London SW7 2AY, U.K. (Received 16 December 1986; in final form 12 March 1987; accepted 30 March 1987) Abstract—The Raman and i.r. spectra of solid K2[Nb(02)F5] • H20 and the Raman spectrum of its aqueous solution are reported, and assignments for the vibrations of the anion proposed. INTRODUCTION (02)2", 10.1%. The sodium salt, as Na3(HF2)[Nb(02)F5], was prepared by Stomberg ’s method [7J. In two recent papers [1, 2], one in this journal, the Raman spectra were measured as spinning discs (to minim ise thermal decomposition) on KBr supports, or, as solutions, Raman (100-1200cm-1) and i.r. (250-1800cm-1) in capillaries and in a spinning solution cell, using the 5145-A spectra of solid Na2[Nb(02)F5] • H20 were reported excitation line from a CRL Innova 70 argon-ion laser with a by N our et al., and assignments proposed for the Spex Ramalog 5 spectrometer. Infrared spectra were vibrations of the complex anion. Since their spectra measured as liquid paraffin mulls between Csl plates and also and assignments differ substantially from those re as KBr discs using a Perkin-Elmer 683 instrument. ported by Jere et al. [3] and also from those published by ourselves some years ago for the solid potassium RESULTS AND DISCUSSION salt [4], we have remeasured the Raman and i.r. spectra It is clear from X-ray crystal structure determi of K2[Nb(02)F5] -H20 and have for the first time nations on Na3(HF2)[Nb(02)Fs] [7], Na2[Nb(02)- measured the Raman spectrum of the species in F5] H20 [8], and Na2[Nb(02)F5]*2H20 [9] that aqueous solution. these all contain pentagonal bipyramidally co ordinated niobium, the peroxo groups being almost EXPERIMENTAL symmetrically side-bonded in the equatorial plane. The assumption of overall C2„ symmetry for the anion The potassium salt was prepared by the method of J e r e et [1-4] is therefore justified. al. [5], based on the method of Balke and Smith [6], from niobium pentoxide, HF, excess H20 2 and KOH, followed by We list our Raman and i.r. data for solid recrystallisation from ethanol. Found: F, 29.5; K, 24.4; K2[N b(02)F5] ■ H20 in Table 1 (as well as the data of (02)2", 10.3%. F5H2K2Nb03 requires F, 30.1; K, 24.7; N our and Jere for comparison) and reproduce the Table 1. Raman and i.r. bands for the [Nb(02)F5]2 anion (cm *) Raman Infrared N our [1, 2] Jere [3]* This work* This work* Nour [1, 2] Jere [3]* This work* Assignment [3] 955 m 890 s 890(8) 885(7) p 890 s 890 s A! vt 0 -0 stretch 880 m 620 vs 620(10) 626 sh, p 620 s 615 sh v2 vsNb(02) 696 s 595 vs 611(7) 610(10) p 590 vs v3 Nb-F stretch 660 s 597(10) 597(5) p 650 w 500 w 594 s v4 Nb-F stretch 608 m 594 sh 450 m v5 Nb-F stretch 215 s 329(1) 324(1/2) 290 s 330 w v6 F-Nb-F bend 195 s 271(1) 262 m v7 F-Nb-F bend 305 w A2 v„ F-Nb-F bend 230 vw v9 O-Nb-F bend 728 sh 730 s 570 s 570 sh Bj v10 Nb-F stretch 330 m 335 sh 306 m vu F-Nb-F bend 271(1) 497 m 250 w 280 w v12 Skeletal bend Vj 3 F-Nb-F bend 610 vs 560(1) 560 br 900 w 560 m B2 v14vasNb(02) 730 s 560 s 503 vs Vis Nb-F stretch 330 m 335 sh 450 m v)6 F-Nb-F bend 270 s 481 m 265 m 292 s v17 Skeletal bend 230 w Vis F-Nb-F bend Data for “Na2[Nb(02)F5] H 20 ”. "■ Data for K2[Nb(02)F5] • HzO. s—strong; br—broad; m—medium; sh—shoulder; w—weak; p—polarised. Relative Raman intensities for this work given in parentheses. 1173 1174 A. C. D engel and W. P. G riffith Wavonumber (cm-1) Fig. 1. The Raman spectrum of solid K2[Nb(02)F5]-H20. Wavenumber (cm-1) Fig. 2. The i.r. spectrum of solid K2[Nb(02)F5] • H20. spectra in Figs 1 and 2. The Raman spectrum of the very similar. The strong band at 890 cm " 1 is clearly the aqueous solution has been measured for the first time 0 -0 stretch of co-ordinated peroxide, as observed in (an aqueous solution of the sodium salt, made by other side-bonded metal peroxo complexes [4,10,11], Stom berg ’s method [7], gave similar spectra). and recently identified in (NH4)3[Zr(02)F5] [12]. The Our Raman and i.r. data for solid bands at 620 and 560 cm - 1 in the Raman spectra (615 K2[Nb(02)F5] ■ H20 are in general quite different and 560 cm-1 in the i.r.) we tentatively assign to from those of N our et al. [1, 2], but are in close vsNb(Oa) and vasNb(02) respectively, the symmetric agreement with those of j E R E e f al. [3]. The profiles of and asymmetric stretches of the side-bonded (02)2 - the spectra of the sodium and potassium salts were ligand against the metal. Such bands are found in other The vibrational spectra of [Nb(02)Fj]2- 1175 peroxo complexes near these values [10,11,13]. These Acknowledgements—We thank the SERC and Interox bands are absent in the product of the thermal Chemicals Ltd for a CASE award to one of us (A.C.D). decomposition of K2[Nb(02)F5] • HzO. The close similarities in profiles and band positions of the Raman spectra of solid K2[Nb(02)F5] • H20 and its aqueous solution suggest retention in solution of the pentagonal bipyramidal structure of the anion REFERENCES (however, early 19F NMR data suggest that the [1] E. M. N our , A. B. E l-Sayed and F. A bdel -Rehim, analogous tantalum anion [Ta(02)F5]2~ may have a Pakistan J. sci. ind.R es. 29, 9 (1986). capped octahedral rather than a pentagonal bipyr [2] E. M. N our , A. B. E l-Sayed and F. A bdel -Rehim, amidal structure in aqueous solution [14]). In solution Spectrochim. Acta 41A, 865 (1985). [3] L. SURENDRA, D. N. SATHYANARAYANAand G. V. JERE, there is a strong, polarised v(O-O) band at 885 cm-1 Spectrochim. Acta 38A, 1097 (1982). and symmetric and asymmetric Nb(02) stretches at [4] W. P. G riffith and T. D. Wickins , J. chem. Soc. A, 397 626 (polarised) and 560 cm " 1 respectively, as well as (1968). the various Nb-F modes. [5] G. V. J ere, L. Surendra , S. M. Kaushik and M. K. It seems likely that the salt examined by N our et al. G upta , Thermochim. Acta 42, 115 (1980). [6] C. W. BALKEand E. F. Smith, J. Am. chem. Soc. 30,1637 [1,2] was a decomposition product of a sodium (1908). peroxofluoroniobate (no analytical data were given). [7] R. Stomberg , Acta chem. Scand. A35, 389 (1981). The latter salt, as shown by Stomberg [7-9] and [8] R. Stomberg , i4cfa chem. Scand. A34, 193 (1980). verified by us, is difficult to obtain in a pure state [9] R. Stomberg , Acta chem. Scand. A35, 489 (1981). [10] W. P. G riffith and T. D. Wickins , J. chem. Soc. A, 590 and exists in three forms as listed at the beginning (1967). of this section. The potassium salt on the other hand [11] W. Massa and G. Pausewang , Z. anorg. allg. Chem. is easy to prepare, crystallise and characterise, and its 456, 169 (1979). spectra are free from interference of bands due to the [12] R. Schmidt , G. Pausewang and W. Massa , Z. anorg. (HF2)“ ion. The presence of a band at 955 cm-1 in allg. Chem. 535, 135 (1986). [13] A. C. Dengel , W. P. G riffith, R. D. PowELLand A. C. the Raman spectrum of NoUR’s material suggests that Skapski , J. chem. Soc., Dalton Trans., 99 (1987). Na2[NbOF5] was one of the constituents of the [14] D. F. Evans , W. P. G riffith and L. Pratt , J. chem. decomposed peroxofluoroniobate. Soc. 2182 (1965).