Functionalized Transition Metal Alkyl Complexes

Synthesis, Structures and Reactivities of Sterically Hindered A^-Functionalized Transition Metal Alkyl Complexes

by Hung Kay LEE

A thesis submitted to the Chemistry Division, Graduate School, The Chinese University of Hong Kong in partial fulfilment of the requirements for the degree of Doctor of Philosophy 1995

Thesis Committee:

Dr. Kevin W.-P. LEUNG (Supervisor) Dr. Wai-Kee LI Prof. Thomas C.W. MAK Prof. Michael F. LAPPERT (External Examiner, University of Sussex) Prof. Chi-Ming CHE (Additional External Examiner, University of Hong Kong) ‘(9^

k 12 fsi^sj Table of Contents page Table of Contents i Acknowledgements vi Abstracts vii Abbreviations ix CHAPTER L GENERAL INTRODUCTION 1.1 A Brief Review on the Background of Organometallic Chemistry 1 1.2 Stability of Organotransition Metal Complexes 2 1.3 Decomposition Pathways of Transition Metal a Hydrocarbyls .. 5 1.4 Preparation of Transition Metal a-Alkyl Complexes 10 1.5 A Survey on the Use of Functionalized Alkyl and Aryl Ligands in Synthesis of Transition Metal a-Hydrocarbyls 15 L6 2-(TrimethyIsilyl)methyIpyridine (3) and 2-Bis(trimethylsilyl)- methylpyridine (4) as Ligand Precursors 20 1.7 A Brief Review of Previous Results on Synthesis of Metal Alkyl Complexes Using (1) and (2) as Ligands 23 1.8 Objective of This Work 28 L9 References for Chapter I 29

CHAPTER IL SYNTHESIS, STRUCTURES AND REACTIVITY OF IRON(n) AND COBALT(n) DIALKYL COMPLEXES II.l Introduction II.l.l Synthesis of Iron(II) and Cobalt(II) Alkyl Complexes .. 34 ILLl.l Iron(II) Alkyl Complexes 34 II.l.1.2 Cobalt(II) Alkyl Complexes 41 II. 1.2 Reactions of Transition Metal Alkyl Complexes with Protic Reagents and Halogens 45 11.1.3 The Chemistry of Transition Metal Alkoxides and Thiolates 一 A Brief Review 48

IL2 Results and Discussion 11.2.1 Synthesis of Homoleptic Iron(II) and Cobalt(n) Dialkyl ‘ Complexes [M {<：：识乂63)2。5_-2} 2] (M 二 Fe 55，Co 56) 51 11.2.2 Attempted Reaction of [FeCl2(PPh3)2] with [{R2Li}2] . 52 11.2.3 Attempted Synthesis of Monoalkyliron(n) Complexes .. 53

i 11.2.4 Synthesis of Homoleptic Iron(II) and Cobalt(II) Dialkyl Complexes [IS^{(CHSiMe3)C9H6N-8}2] (M = Fe 57, Co 58) 54 11.2.5 Molecular Structures of [M{C(SiMe3)2C5H4N-2}2] (M =Fe 55，Co 56) and [Co{(CHSiMegjCgHsI^-Sh] (58) 56 11.2.6 Spectroscopic and Magnetic Properties of Compounds 55-58 63 11.2.7 Electrochemistry of [Co(R2)2] (56) and [Co(R3)2 (58) ^ 66 11.2.8 Reactivities of [M{C(SiMe3)2C5H4l^-2}2] (M = Fe 55，

Co 56) 70

11.3 Experimentals for Chapter II 83

11.4 References for Chapter II 86

CHAPTER m. SYNTHESIS AND STRUCTURES OF DIALKYL COMPLEXES OF NICKEL(n) AND PALLADIUM(n) III. 1 Introduction III. 1.1 Nickel(II) Alkyl Complexes 一 A General Survey . 91 III.1.1.1 T]^-Cyclopentadienylnickel Alkyl and Aryl

Complexes 91 III.LI2 Nickel Dialkyls 96 III. 1.2 Palladium(II) Alkyl Complexes — A General Survey 99 III. 1.2.1 Palladium Dialkyls 99 IILl 22 Cyclopalladated Compounds 101

IIL2 Results and Discussion IIL2.1 Synthesis of Nickel(n) Alkyl Complexes 104 1112.1.1 Reactions of [{R^Li(Et20)}2] and [{R^Li}】] with Nickelocenes — Synthesis of 77 ^ - Cyclopentadienylnickelalkyl Complexes (78-80).. 104

1112.1.2 Synthesis of the Substituted Nickelocene [Ni{ri^- C^Hs(SiMes)2}2] (SI) 106 I 11.2.1.3 Spectroscopic Properties of Compounds 78-80 ... 107

ii Ill2.1.4 Molecular Structures of [(rj 5 - C^H^)Ni{C(SiMes)2C^H4N-2}] (78) and [{(”、

C^H^)Ni{CH(SiMes)C^H4N-2}}2] (80) 110 III2.L5 Reactions of [{R^Li}2] with NiCl】 and [NiCliL〗] (L2 = TMEDA, 2PPhs) — Synthesis of Nickel Dialkyl Complex [Ni{C(SiMe^)2C5_-2}2] (82) 115 III2.1.6 Molecular Structure of [Ni{C(SiMe^)2C^H4N-

2} 2] (S2) 118 111.2.1.7 Spectroscopic Properties of [Ni(C(SiMes)2C5H4N-2}2] (82) 121 111.2.1.8 Electrochemistry of [Ni{C(SiMes)2C5H4N-2}2]

(82) 122 1112.1.9 Reactivities of N.i[C(SiMe姊(82) ... 124 in.2.1.10 Reactions of [{RZU}:] with [NiCl2(diphos)]

[diphos = 1,2-bis(diphenylphasphino)ethane] ... 125 in.2.1.11 Molecular Structure of [Ni{C(SiMe^)2C5H4N' 2} {5-(2，-C^H4NC(SiMes)2C3H4N-2 - CH(SiMes)2}Cl] (83) 127 III2 J.12 Reactions of [{R^Li(Et20)}2] and [R^Li] ['R^ 二 -CH2C5H4N-2] withNiCl2 and [NiCl2L2] (L2 二 TMEDA, 2PPhs) 130 III2.1.13 Molecular Structure of [{CH(SiMes)C^H4N- V2] (S6) 136

111.2.2 Synthesis of PaUadium(II) Alkyl Complexes ... 139 III22.1 Reactions of [{R^Li}〗] with [PdXiL〗](L = PPh^, Et2S; X = Cl，Br) — Synthesis of [Pd(R2)2] (88) and [Pd(R^)(PPhs)X] [X 二 Cl 89, Br 90] 139 III.2 2 2 Molecular Structures of Compounds [P'd{C(SiMes)2C 5H 4N-2}2] (8 8)， [Pd{C(SiMes)2C5H在2}(PPhs)Cl] (89)，and [Pd{C(SiMes)2C^H4N-2}(PPhs)Br] (90)…… 140 III.2 2 3 Spectroscopic Properties of Compounds 88-90 146 III.22 A Electrochemistry of [P'd(C(SiMes)2C5H4N-2}2] (88) 147

11• • 1I III22.5 Studies on Stereospecificity of the Reactions between [{R^Li}】] and [PdX2(PPh如]= CI, Br) 150 HI.2.2.6 Attempted Synthesis of [Pd(Rl)2] and [Pd(Rl)(PPhs)Cl] 152 111.2.2.7 Attempted Synthesis of [(Pd(R^)Cl}2] via

Intramolecular C-H Activation 153 III.22.8 Molecular Structure of [{CH(SiMe3)2C^H4N- 2}2PdCl2} (91) 154

in.3 Experimentals for Chapter III 157

III.4 References for Chapter III 155

CHAPTER IV. SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF ZIRCONIUM(IV) AND HAFNIUM(IV) ALKYL COMPLEXES IV. 1 Introduction 169 IV. 1.1 Homoleptic and Heteroleptic Complexes 170 IV. 1.2 Organo-Zirconium(IV) and -Hafnium(IV) Compounds Containing Functionalized Alkyl Ligands 177

IV.2 Results and Discussion IV.2.1 Synthesis of Zirconium(IV) and Hafnium(IV) Dialkyl Dichloride Compounds [M(R2)2C12] (M 二 Zr 103, Hf 104) 181 IV.2.2 Attempted Synthesis of Titanium(IV) Alkyl by the Reaction of TiCl4 with [{R2Li}2] 182 IV.2.3 Attempted Synthesis of Zirconium(IV) and Hafnium(IV) Alkyls with "R 1 [= -CH(SiMe3)C5H4N-2] as Ligand 182 IV.2.4 Molecular Structure of [Zr{C(SiMe3)2C5H4N- 2}2Cl2] (103) 183 IV.2.5 Spectroscopic Properties of [W{C(SiMe3)2C5H4N- ’ 2}2Cl2] (M = Zr 103, Hf 104) 187

IV.3 Experimentals for Chapter IV 188

IV.4 References for Chapter IV 190

iv CHAPTER V. COMPARISON OF BONDING PARAMETERS AMONG METAL ALKYL COMPLEXES WHICH CONTAIN ALKYL LIGANDS -R2 193 References for Chapter V 201

APPENDIX I A. General Procedures 202 B. Physical and Analytical Measurements 202

APPENDIX II

Magnetic Moment Measurements 205

APPENDIX III Table A-1. Selected Crystallographic Data for Compounds 55, 56, 58，and 63 207 Table A-2, Selected Crystallographic Data for Compounds 78, 80, 82 and 83 208 Table A-3. Selected Crystallographic Data for Compounds 86, 88, 89 and 90 209 Table A-4. Selected Crystallographic Data for Compounds 91 and 103..… 210

V Acknowledgements

I wish to express my deepest gratitude to my supervisor, Dr. Kevin W.-P. Leung, for his invaluable guidance, continuous enthusiasm and encouragement throughout the course of my work and the preparation of this thesis.

I would also like to thank Prof. Thomas C.W. Mak, Prof. Z.-Y Zhou, Prof. J.C. Wang, and Prof. B.-S. Luo for their skillful determination of crystal structures.

I am indebted to Prof. Sunney 1. Chan and Dr. Wai-Kee Li for very useful discussions and suggestions.

Thanks also go to Mr. Y.H. Law，K.W. Kwong, and Ms. W.P. Chung for their assistance in measuring all mass spectra and some ^H and ^^C NMR spectra.

Financial support from the Chinese University of Hong Kong in the form of Postgraduate Studentship is gratefully acknowledged.

December, 1994

Hung Kay LEE Department of Chemistry The Chinese University of Hong Kong

.) . VI Abstract

The work presented in this thesis focuses on two areas: (1) synthesis and

characterization of A^-functionalized transition metal alkyl complexes by employing

sterically demanding ligands, and (2) studies on the reactivities of these metal alkyl

complexes towards protic reagents such as phenol and thiophenoL

Chapter 1 begins with a brief introduction to the general background of

organometallic chemistry. Decomposition pathways and common synthetic methods for

organo-transition metal compounds are briefly discussed. A brief review of previous results on the synthesis of metal alkyl complexes involving the bulky A^-functionalized

alkyl ligand "R^ = -C(SiMe3)2C5H4N-2 concludes Chapter 1.

Chapter 2 describes synthetic and structural studies leading to novel homoleptic iron(n) and cobalt(II) dialkyl complexes [M(R")2] ["R^ = -C(SiMe3)2C5H4N-2，M = Fe

(55)，Co (56); -R3 = -CH(SiMe3)C9H6N-8，M = Fe (57)，Co (58)]. Complexes 55，

56 and 58 have been fully characterized (including X-ray diffraction studies). Cyclic voltammetry of 56 and 57 is also reported. In addition, reactivities of complexes 55

and 56 towards a variety of protic reagents and Lewis bases have also been studied.

Compound 55 reacts with the bulky thiophenol ArSH (Ar = 2’4,6」Bu3C6H2) and the

bulky phenol Ar^eOH (ArMe = 4-Me-2,6」Bu2C6H2) to give the corresponding neutral

monomeric iron(II) dithiolate [Fe(SAr)2(R^H)] (63) and diaryloxide

[Fe(OArMe)2(R2H)] (66), respectively. A free molecule of R^H was found to be bound

intact to the iron(II) center through iV-coordination. The cyclic voltammogram of 63

showed that neither reversible oxidation nor reduction occurred. In contrast, 56 is inert

towards these bulky protic reagents.

Chapter 3 deals with the synthetic and structural studies of nickel(II) and

palladmm(II) alkyl complexes. Reactions of the pyridine-functionalized lithium alkyls

[-Rl = -CH(SiMe3)C5H4N-2; "R^ = -C(SiMe3)2C5H4N-2] with nickelocenes NiCp"2

:Cp = C5H5, Cp" = C5H3(SiMe3)2] gave the corresponding T]5-cyclopentadienylnickel

alkyl complexes: [{CpNiRl}〗](80), [CpNiR^] (78), and [Cp'’NiR2] (79). In

vii addition, an improved method of the synthesis of the bulky nickelocene NiCp”2 is also found. Thermally stable homoleptic nickel(II) dialkyl complexes [Ni(R2)2] (82) was accessible by the reaction of [{R^Li}〗] with the appropriate metal dihalide complexes.

Reaction of [NiCl2(diphos)] with [{R2Li}2] gave the novel nickel(II) complex

[NiR2(R2-R2)ci] (83) in which a ”head-to-tail” coupled alkyl ligand remains coordinated to the NiR2(Cl) moiety. With the less bulky alkyl ligands, reductive coupling to Rl-Rl (86) and R5-R5 (87) ["R^ = "CH2C5H4N-2] were observed in an attempt to synthesize the nickel dialkyl complex Ni(R«)2. Reaction of [{R2Li}2] with stochiometric amounts of appropriate palladium dihalides gave the corresponding monoalkylpalladium(II) halide complexes [Pd(R2)(PPh3)X] [X 二 CI (89)，Br (90)] and homoleptic palladium(II) dialkyl complex [Pd(R2)2] (88). Proposed mechanistic pathways leading to 88-90 are proposed. Treatment of the free ligand R^H with PdCl2 or [PdCl42-] gave the bis(ligand)palladium(II) chloride (91). Compounds 78, 80, 82，

83，86，and 88-91 were characterized by single crystal X-ray diffraction studies. In addition, cyclic voltammetry of the dialkyl complexes 82 and 88 was studied.

Chapter 4 begins with a brief review of organo-zirconium(IV) and -hafnium(IV) compounds. The synthesis of novel dialkyl zirconium(IV) and hafnium(IV) dichloride complexes [M(R2)2C12] [M = Zr (103)，Hf (104)] is reported. X-Ray structural determination reveals that 103 is a six-coordinate complex.

Finally, Chapter 5 gives a brief comparison of the structural data for metal alkyl complexes containing "R^ as an alkyl ligand. Correlations between M-C and M-N bond lengths, and the bite angle ZC-M-N with the atomic number of the central metal are discussed.

viii Abbreviations

acac acetylacetonate

At aryl (or 2,4,6-TRI-r^rr-butylphenyl if stated otherwise)

ArMe 2,6-di-rm-butyl-4-methylphenyl atm atmospheric pressure bipy bipyridine

"Bu 〜butyl

^Bu rm-butyl

Cp cyclopentadienyl

Cp" l,3-bis(trimethylsilyl)cyclopentadienyl

Cp* pentamethylcyclopentadienyl

COD 1,5-cyclooctadiene calc calculated cP formal d-electron configuration d doublet (NMR) dd doublet of doublet (NMR) dec decomposed

dt doublet of triplet (NMR)

diphos bis(diphenylphosphino)ethane

Et ethyl

Et^O diethyl ether

Hz Hertz

m multiplet (NMR)

M+ parent peak

Me : methyl

Mes mesityl

Mp., m.p. melting point

MHz megahertz

ix MS mass spectroscopy

NMR nuclear magnetic resonance

Ph phenyl

叩 r ^-propyl

中 r isopropyl

Py pyridine q quartet (NMR)

R alkyl group (or aryl group if stated otherwise) s singlet (NMR) t triplet (NMR)

THF tetrahydrofuran

TMEDA AyV，iVyV'-tetramethylethylenediamine

TMS trimethylsilyl group

•j • X CHAPTER I. GENERAL INTRODUCTION

I.L A BRIEF REVIEW ON THE BACKGROUND OF ORGANOMETALLIC CHEMISTRY

Organometallic chemistry, especially organotransition metal chemistry, has attracted much attention during the last two decades. In general, organometallic compounds are compounds containing direct metal-carbon bonds. Nevertheless, metal complexes having

PH3, PF3 or tertiary phosphines ligands are often studied within the realm of organometallic chemistry.

The preparation of the first organometallic compounds was an unexpected discovery. In 1827, Danish pharmacist W.C. Zeise reported the synthesis and isolation of the first organometallic species [Pt(C2H4)Cl2.KCl.H20] by treating a mixture of PtCl?,

PtCl4, and KCl in ethanol (equation I-l).l

RCI2 + RCI4 + KCl ^^——[R(C2H4)Cl2 -KCl • H2O] (I-l)

Later, structural determination of this compound revealed that it contains an ethene molecule combined with platinum through its double bond.2,3’4

�H 、、CI .->H + ��C K CI—Pt——II • H2O c/ VH H

The compound is now known as Zeise's salt.

The first organometallic compound having a direct metal-carbon a bond was prepared accidentally by E. Frankland in 1849 when he studied the reaction of C2H5I with

Zn.5 The product of the reaction is, in fact, diethyl zinc.

After that, there was a steady increase in the number of new organometallic compounds being reported. This included various Grignard reagents, organolithium

1 compounds, organochromium compounds, and the butadiene complex [Fe(C4H6)(C〇)3].

In 1951，the discovery of ferrocene by two independent groups added a new dimension to

the studies of organometallic compounds.6，7 Then, in 1953, the invention of the Ziegler process for the polymerization of alkenes proves the usefulness of organotransition metal compounds in catalysis.^ The discovery of the Ziegler process was followed by the

discovery of the Wacker Process for converting ethylene into acetaldehyde by employing

Pd(n) compounds as catalyst.^ Since then, considerable interests have been devoted to the

studies of organotransition metal chemistry leading to an explosive growth in the number of organotransition metal complexes reported in literatures‘

1.2 STABILITY OF ORGANOTRANSITION METAL COMPLEXES

Stoichiometric uses of organotransition metal species in organic synthesis and

studies of organometallic catalysis have attracted the interest in the relative ease with which

a-bond between a transition metal and carbon can be made or cleaved. As time lapsed,

changing views on the nature of bond-making and bond-breaking processes had led to

dramatic progress in transition metal chemistry.

Although homoleptic metal a-hydrocarbyl complexes were described for most of

the main group elements in the 60's，none of the transition metal analogues had been

reported at that time. Unsuccessful attempts to synthesize and isolate simple transition

metal c-hydrocarbyl complexes led to the generalization in the early days that transition

metal -carbon a bonds were inherently unstable. This generalization was once supported

by theoretical calculations at that time. 10 Although the evaluation of all terms in the

calculations were difficult, Jaffe and DoaklO could show that covalent metal-carbon bonds

were weak with the use of Mulliken's "magic formula". They also showed that the

difference between electronegativities of metal and carbon was so large that stable ionic

bond between these two elements was impossible. Results of their calculations strongly

2 suggested that the difficulties encountered in the preparation of organotransition metal complexes was owing to the thermodynamic instability of transition metal-carbon a bonds.

The weakness of transition metal-carbon a bonds was also believed to be due to the availability of high-energy d electrons from the metals. Promotion of an electron from the highest filled orbital to a non-bonding or vacant a* anti-bonding orbital led to the cleavage of transition metal-carbons bonds. Hence, suggestion was made during the early 60's to the possibility of stabilizing transition metal-carbon a bonds by incorporating strong field ligands (often with 7r-bonding properties such as CO, T15-C5H5 or PR3) into the coordination sphere of the metal, n It was proposed that these Tt-bonding ligands

"stabilized" the metal-carbon a bonds by lowering the energy of the filled d-orbitals and thereby increasing the energy gap for facile promotion. Since then, numerous organotransition metal a-hydrocarbyl complexes having these "stabilizing ligands" have been prepared and isolated. The hypothesis was once used to rationalize why simple homoleptic transition metal complexes were highly unstable (e.g. TiMe4l2 and ZrMo,^^'^ decomposed at low temperature) but complexes such as [(Et3P)2Pt(Me)Cl]，14 cis-

:(Et3P)2PtMe2]，14 and cw-[(Et3P)2PtEt2]^^ could be stable enough to be isolated.

The successful preparations and isolations of transition metal complexes which did not contain supporting Ti-ligands, such as [PhCH2Cr(OH2)5]2+ or [EtRu(NH3)5]2+,16 have rendered the "supporting 71-ligand" hypothesis untenable. The growing number of reports on successful synthesis and isolation of simple homoleptic transition metal a- hydrocarbyls in the late 60's and early 70's, such as [Ti(CH2Ph)4]，17 [:Zr(CH2Ph)4]’17

[TaMe5],18 [WMe6]’18 and [Ni(CPh3)2]’19 all of which did not contain any supporting ligands further rendered the prevailing hypothesis at that time questionable.

Thermochemical and spectroscopic studies together with bond-length data led to the suggestion that transition metal-carbon bond strength is not as weak as was previously thought. Although the data were only limited until recently, they were just enough to show that transition metal-carbon bond strength are comparable to those main group metal-carbon bond strength.l7’l8，20-24 Results of these studies had really marked the milestone for the development of organotransition metal chemistry in the next decades. Some average bond

3 dissociation energies D of homoleptic transition and non-transition metal alkyls of the type

[MRJ are listed in Table I-l and 1-2 for comparison.

Table I-l. Mean Bond Dissociation Energies D(kJ/mol) of Compounds [MR^] for Non-Transition Metals.20

M-C Bond D (kJ/mol) M-C Bond D (kJ/mol)

Li-Me 209 Sn-Et 193

LiBu 248 Sn-Pr 197

Zn-Et 145 Sn-Ph 195

Hg-Et 101 Pb-Et 129

Al-Et 242 P-Et 258

Ge-Et 237 P-Ph 301

Sn-Me ^ As-Ph ^

Table 1-2. Mean Dissociation Energies D(kJ/mol) of Some

Organotransition Metal Conipounds.20

Compound M-C Bond D (kJ/mol)

[Ti[CH2C(CH3)3]4] Ti-CH2R 170

[TKCH^QHyj Ti-CH2R 240

[Ti[CH2Si(CH3)3]4] Ti-CH2R 250

[Cp2Ti(CH3)2] Ti-CH3 250

[Z:r[CH2C(CH3)3]4] Zr-CH2R 220

[Zr(CH2C6H5)4] Zr-CH2R 380

[Zr[CH2Si(CH3)3]4] Zr-CH2R 225

[(CO)3MnCH3] Mn-CH3 150

,[(CO)5ReCH3] Re-CH3 220

[(Et3P)2Pt(C6H5)2] Pt-QHs 250

[Ta(CH3)5] Ta-CH3 260

[W(CH3)6] W-CH3 160

4 The following statements can be made to summarize the above data:

(1) Transition metal-carbon bonds are of similar strength to main group element-

carbon bonds，i.e. the former are not inherently unstable as was once

suggested.

(2) The D(M-C) values increase from left to right along a transition series and

from top to bottom down a series, exactly the opposite trend for non-

transition metal alky Is.

(3) Stability of metal-carbon bonds decreases in the order D(M-CF3) > D(M-

C6H5) > D(M-CH3)〜D(M-C2H5) > D(M-CH2C6H5).

Although transition metal-carbon bonds are found not to be intrinsically weak as reflected from the above data, a large number of transition metal a-alkyl complexes (e.g. tetramethyltitanium, diethylcobalt, trimethyliron, and dibenzylrhodium) are too unstable to exist under normal conditions. During the 70's, two independent groups25,26 proposed that instability of transition metal alkyls was of kinetic origin. The characteristics of transition metal to possess variable oxidation states and coordination numbers allow some low energy decomposition pathways to exist which are not found in their main group analogues. Examples of these decomposition pathways are the a-elimination, |3- elimination, reductive elimination, and binuclear elimination (vide supra). Therefore, if the ligands are so designed that these low energy decomposition pathways can be blocked, thermally stable transition metal alkyls can be prepared.

1.3 DECOMPOSITION PATHWAYS OF TRANSITION METAL a HYDROCARBYLS

The instability of transition metal alkyl complexes were due to the presence of low

energy decomposition pathways. Among them, a-elimination, p-elimination, reductive

elimination, and binuclear elimination have received much attention.

5 (i) a-elimination:

a-elimination has been recognized only recently. It involves a hydride transfer from the a-carbon of the ligand to the metal with concomitant formation of a hydride-metal carbene complex (equation 1-2).

M-CH2-R „ 一 M (1-2) ^CHR

Schrock et al.l] has discussed this decomposition process in a review. The process is often found in metal alkyl complexes in which p-elimination is prohibited but the alkyl ligands still contain a-hydrogen so that hydride transfer to the metal can occur. Cooper and

Green28 had recognized the following reaction using deuterium-labelled compounds.

^CH2CH2PMe2Ph /W\ 1——• w^ ⑶ 3 CD3 ^ - PMe2Ph

y D further reacts with ^ W、 ^ other reagents leading (1-3) ^^^ 义CD to decomposition

(ii) p-elimination:

Among various decomposition pathways, p-elimination is well-documented. Over-

emphasis of the process gives an erroneous impression that it is the predominate

decomposition pathway in organometallic chemistry. In fact, it is only one of a number of

decomposition pathways to be considered in organotransition metal chemistry. Two

reviews have appeared in discussions of this process.29，30 The reaction involves the

6 abstraction of a hydrogen from the p-carbon of an alkyl group by the metal with concomitant formation of an (Ti2-olefin)-metal hydride complex. Then, loss of the olefin from the complex leads to decomposition of the initial organotransition metal complex

(equation 1-4).

,H、 [_j M � €H2—R ^ • � • / \ch/ -\ CH2 — CH/ 、丨丨 CH2

項 -MH + H2C=CHR (1-4)

The mechanism was confirmed by deuterium-labelling studies.

[(Bu3P)CuCH2CD2Et] [(已U3P)CUD] + HsC^CDEt (1-5)

However, owing to the reversible nature of the reaction, when a fast reverse reaction operates, effective mixing of H and D results. Interestingly, the reverse process is, in fact, insertion of an olefin into an M-H bond, which is very important in olefin metathesis and organometallic catalysis.

In order to prepare thermally stable transition metal alkyl complexes, ligands void of p-hydrogen are employed. These ligands are mainly in the form of benzyl or neopentyl type. Typical examples of these ligands are CH(SiMe3)(C6H5), CH2C(CH3)3, CH2CPh3,

‘•• • • •I 丨 • C(SiMe3)3，CH(SiMe3)2，CH2(SiMe3). The introduction of ligands of these types do have a significant impact on the scope of metal alkyl chemistry. Hence, a number of stable homoleptic metal alkyls such as [Sn[CH(SiMe3)2]2]，31 [Pb[CH(SiMe3)2]2]，3 丄 [Sc[CH2(SiMe3)]3.2THF],32 [Ti[CH2C(CH3)3]4]，33，34 [Zr[CH2C(CH3)3]4],33，34 [Cr[CH2(SiMe3)]4],26b [Zn[CH2(SiMe3)]2],35 [Hg[C(SiMe3)3]2]36 have been successfully synthesized.

One exception to this decomposition process is found in [Cr(CMe3)4].37 Although the r-butyl ligand is very susceptible to p-elimination，this compound is thermally very

7 stable at room temperature (Mp. = 80 °C dec.). The stability of this compound is believed to be due to the tight packing of the methyl groups so that a proper orientation for p- elimination can be prevented.

(Hi) Reductive elimination:

In general, reductive elimination is the reverse of oxidative addition. Being a route for the cleavage of the M-C bonds, it leads to the elimination of two ligands from the coordination sphere of the metal with simultaneous reduction of the formal oxidation state of the metal by two units. A schematic representation of the process is illustrated in equation 1-6.

/X LM • LM + X—Y (1-6) \丫

The process is almost confined to those transition metals which possess stable oxidation states differing by two units and is found especially important for those complexes of late transition metals. Some typical examples of reductive elimination are given as shown below (equation 1-7,1-8,1-9).

[(PhsPjAuMeal [(PhaPjAuMe] + CaHs (1-7)

[(PhMe2P)2PtMe3l] [(PhMe2P)PtMel] + C2H6 (1-8)

[MPh4] ^ [{MPh3(〇Et2)n}m] + Ph-Ph (1-9) M = Ti, Zr, V

Despite the fact that reductive elimination is one of the low energy decomposition pathways for organotransition metal complexes, it is an important chemical transformation in organic synthesis. One such example is the cross-coupling of alkyl halides and Grignard

8 reagents catalyzed by Ni(II) complexes which involves reductive elimination of coupled organic species from an unstable nickel(II) dialkyl or diaryl complexes (Scheme I-l).

R^MgX

[NiX2L2] R MgX- [NiR^2L2]円乂 [NiR^XLs] [NiR^R^Ls]

广

r1-r2 I

Scheme I-l

In order to prepare stable transition metal dialkyl or diaryl complexes, the use of bulky ligands is vital as direct coupling of two bulky alkyl or aryl ligands will become difficult to occur. Hence, neopentyl, bis(trimethylsilyl)methyl, bicyclo(2.2.1)hept-l-yl, 1- adamantylmethyl, and 2，4，6-tri(r-butyl)phenyl ligands are typical examples of bulky ligands being used to stabilize homoleptic transition metal alkyl or aryl complexes.

(iv) Binuclear elimination:

Being an intermolecular reaction, dinuclear elimination involves the formation of metal alkyl or aryl bridges in order to bring into close proximity of leaving groups from different metal atoms. The alkyl groups can then be eliminated as its dimer as illustrated in equation I-10.

2 [{Ph3P)AuMe] ^ 2Au + CaHs + 2PPh3 (MO)

If the alkyl groups contain p-hydrogen, disproportionation product will be obtained

(equation I-11).

9 [(Bu3P)CuD] + [ (BuaPjCuCHsCDsEt] 2Cu + CHsDCDsEt + 2PBU3 (I-ll)

In some other instances, oxidation state of the metals will remain unchanged. In schematic terms, the process may be represented as shown below.

2LMCH2R •LM—CHR—H • (LM)2CHR + RCH3 (1-12) ‘ I I I • I LM——CH2R

The reaction is usually believed to proceed via a four-membered transition state.^la

1.4 PREPARATION OF TRANSITION METAL g-ALKYL COMPLEXES

There are a number of methods used in synthesis of transition metal G-alkyl complexes. Among these methods, alkyl transfer reactions are the most widely employed synthetic approach.

(i) Synthesis by alkyl transfer reactions:

A variety of alkyl compounds of main-group elements such as lithium, magnesium, aluminum, zinc, mercury, and tin serve as alkylating reagents. They are allowed to react with transition metal halides, acetylacetonates or carboxylates to yield the corresponding transition metal alkyl complexes.

M—X + M•—R • M — R + M,—X (1-13)

M: transition metal; M': main group elements; R: alkyl groups; x : halides, acetylacetonates, carboxylates.

Some typical examples of preparation of transition metal alkyls using this method are given as shown below.

10 TiCU + 4 MeLi [TiMe4] (1-14)

TiCU + 3 [(Me3Si)2CHU(Et2〇)] [Ti[CH(SiMe3)2]3CI] (1-15)

ZrCU + 4 PhCH2MgCl [Zr(CH2Ph)4] (1-16)

Ni(acac)2 + AIEt2(0Et) + bipy [NiEt2(bipy)] (1-17)

NbCIs + ZnMe2 [NbMezCIs] (1-18)

Although transition metal acetylacetonates, carboxylates or alkoxides may be used, the use of transition metal halides is the most common one. These reactions probably proceed through bridged intermediates:

/X�� M—X + M'—R tr M M’ -、rZ

，〜M" M. ，、M—R + M'—X (1-19) Y

M: transition metal; M': main group elements; R: alkyl groups; X : halides, acetylacetonates, carboxylates.

In general, the nature of products often depends on the alkylating power of the alkylating reagents employed. For instance, reaction of titanium(IV) chloride with methyl lithium gives tetramethyltitanium whereas trimethylaluminum gives methyl-titanium trichloride complex.讯卯

_imi_^ [TiMe4]

‘ ^cu -/ (1-20)

\__^_^ [MeTiCIa]

11 (ii) Reaction of anionic transition metal complexes with RX:

Reaction of an electron-rich transition metal anionic species with an appropriate alkyl derivative RX (where X is a good leaving group) gives transition metal alkyl complexes.

LnM" + R—X LnM—R + X" (1-21)

This method is particularly useful for preparation of transition metal alkyl complexes containing carbonyl ligands. Some general examples are given below:

[{CpMo(C〇)3}2]圖g • Na+[CpMo(C〇)3]- —~~^ [CpMo(C〇)3R] (1-22)

[Co2(C〇)al Na/Hg • Na+[Co(C〇)4r ——"^ [RCo(C〇)4] (1-23)

Na2[Fe(C〇)4] — ^ Na+[RFe(C〇)4r (1-24)

.1) A [CpFe(C〇)2] ~~- [CpFe(C〇)2(CH2CH2〇H)] 2) H+

1- [CpFe(C〇)2(ri2-CH2=CH2)]+ (1-25)

The most common alkyl derivative RX used for this method are alkyl, aryl, alkenyl, benzyl or allyl halides. As expected, aryl and vinyl halides react considerably more slowly than their corresponding alkyl, benzyl or allyl derivatives. In addition, the nature of leaving group X also has a substantial effect on the rate of the reaction.

(in) Synthesis using insertion reactions:

Formation of transition metal-carbon a bond can be achieved by the insertion of some unsaturated hydrocarbons (e.g. olefins) into a metal-hydrogen bond. Thus, it may be viewed as the reverse of p-elimination of a transition metal alkyl complex.

12 \ y insertion _ | M-H + CzzCQ , 二 M- C—C—H (1-26) , \ p-elimi nation I

Some typical examples are given as shown below.

[Cp2ZrH(Cj)� + /=<( Cp,Zr^

[CpFeH(C〇)2] + ^ [CpFe(C〇)2(CH2CH=CHCH3)] (1-28)

[RhHCl2(PPh3)2] + HC三CH [RhCl2(PPh3)2(CH=CH2)] (1-29)

[HCo(C〇)4] + ,〇\ ^ [H〇CH2CH2CO(C〇)4] (1-30)

AgF + CF3CF=CF2 [AgCFsCFaCFa] (1-31)

Reaction (1-29) is the insertion of an acetylene into the Rh-H bond to give a rhodium alkenyl compound. In reaction (1-30), a transition metal hydroxyethyl compound can be obtained by insertion of an epoxide into a metal-hydride bond. Reaction (1-31) represents a special but interesting example in which an alkene can be "inserted" into a metal-halogen bond.

(iv) Synthesis by oxidative addition reactions:

A typical oxidative addition reaction may be represented in equation (1-32).

A LnM + A-B • Lniv/ (1-32)

13 The process involves the addition of compound A-B to the transition metal complex with cleavage of the A-B bond. Both the oxidation state and coordination number of metal M increase by 2. Some general examples are given below.

[Rh(C〇)(PBu3)2CI] + Mel ^ [MeRh(C〇)(PBu3)2lCI] (1-33)

[lr(C0)(PPh3)CI] + CH3COCI [lr(C〇)(PPh3)(C〇CH3)Cl2] (1-34)

Pt(PPh3)3 + Mel ^ [MePt(PPh3)2il _ • [Me2R(PPh3)2l2] (1-35)

In some instances, both the oxidation state and coordination number of the metal increase

only by 1.

2 [CP2V] + RX ^ [CpaVR] + [CP2VX] (1-36)

R = Me, Et, PhCHa

(v) Synthesis by elimination or de-insertion reactions:

Transition metal alkyls can be prepared by expulsion of a group (e.g. CO, CO2，

SO2 or N2) separating the carbon and metal bonds. Some examples of this process are

given below.

[MeC〇Mn(C〇)5]———~~[MeMn(C〇)5] (1-37) A

〇2N"^^KN=N-R(PEt3)2CI ~Al2〇3 •〇2N"^^>-Pt(PEt3)2CI (1-38)

MeHQKS〇2-lr(C〇)(PPh3)2Cl2 ——j~~MeH^^lrlCOjlPPhJsCIs (1-39)

However, this method is seldom used in synthesis.

14 (vi) Synthesis by attack on coordinated ligands:

Since a number of transition metal complexes contain coordinated ligands such as

alkenes or alkynes, external nucleophilic attack on these coordinated ligands may transform

the metal ji-complexes into metal complexes containing M-C a bonds. Some examples are

given as follows.

^ Me〇

Lj>dCl, + MeOH (I,

CpPd + CH3O" • CpPd (1-41) • ^^ CH2CH2OCH3

1.5 A SURVEY ON THE USE OF FUNCTIONALIZED ALKYL AND ARYL LIGANDS IN SYNTHESIS OF TRANSITION METAL a-

HYDROCARBYLS

Donor ligands and solvents were recognized by several gTOups21,30,40，to have the

ability to stabilize early transition metal homoleptic alkyls. Hence donor ligands such as

bipyridine, tertiary phosphines or bis(dimethylphosphino)ethane were used to stabilize

thermally unstable metal alkyls. Brintzinger et found that reactions of RMgX with

[Cp2TiCl2] gave reactive paramagnetic compounds [CpsTiR:]-. In contrast, if chelating

organo-lithium reagents were used, stable diamagnetic crystalline compound with structure

I was obtained.42

CH2\ Cp2Ti\ PR2 CH2Z I

15 Later work by several other groups in the 1970’s has shown that considerable stability can be attained with the use of chelating functionalized ligands.43-47 a general representation of the metal-ligand interaction is depicted as shown below.

f M E = N，〇，P，S，olefin

The a-carbon and the heteroatom donor form parts of the same ligand. The heteroatom donor may be nitrogen, phosphorus, oxygen, sulphur or even an olefinic group. Over the past two decades, a large number of transition metal a-hydrocarbyls which contained functionalized alkyl or aryl ligands had been reported.43-50 However, the majority of these species were prepared from C-H activation of the corresponding organic moiety by the transition metal involved. In general, it is the o厂r/z6>-hydrogen which is abstracted and eliminated as HX (equation 1-42).

M \ / f HC E + MX • ^M + HX (1-42)

A typical example is palladation of 8-methylquinoline to give the corresponding

alkylpalladium(n) chloride complex.^^

/^v^ . /"^cHa CI J 『丁〕+ PdCi,2 HCI _ W yPd X (1-4刃

CH3 2

Reactions of functionalized organo-lithium and -magnesium with metal halides

often give the corresponding functionalizd transition metal alkyl or aryl complexes. Manzer

16 et a/.52 have reported the synthesis and characterization of a series of paramagnetic early transition metal alkyls or aryls by treating the appropriate iV-functionalized organo-lithium reagents (III, IV) with the corresponding metal halides (Scheme I-la and I-lb). Most complexes thus prepared were confined to those early transition metals such as Ti(III),

Sc(III), V(III), and Cr(III).52 Among the late transition metals, only their Co(III),44

Ni(II)�4 7Pd(II),47 and Pt(II)47 analogues have been synthesized and characterized. Not surprisingly, these ligands are bound to the metals in a C,N-chelating fashion.

Nevertheless, enhancement of thermal stability of these metal alkyls and aryls is achieved through the intramolecular coordination.

R R R

f^"^ r^^

L Z, J CP2M

/ \ / \ / \ 一 J3

M = Sc; R = H (即 M=Ti,V;R = H M = Cr; R = H, feu

Scheme I-la

一一

^^nZ

/ \ / \ / \ _ 」3 L 」3-X (IV) M = Sc, Cr M = Ti, V; x = 2 M=Ti; x = 1

Scheme I-lb

17 Other examples of functionalized ligands include:53-58

^f^f^

Me2

SiMea

M I ^^p, r^VS N 今 M^N Me2 Me2

Pyridine-functionalized ligands of the type (V) were of particular interest over the past few years.纵

x^n^Y-

(V)

In general, X is usually H, aryls, alkyls or halogens whereas Y may be aryls, alkyls,〇,S，

NR or PR (R = alkyls).

r^^

hc3 乂 N 人』、人 z R L o 。 \o

R = H, Ph, SiMea

18 Rothwell et al.^ have studied the chemistry of the pyridine-functionalized ligand 2-

(6-methylpyridyl)methyl (VI).

HsC^N'^CHa-

(VI)

A number of Group 4 metal alkyl complexes have, thus, been prepared and characterized.

These include the following complexes: [CpaZrR〗], [CpsHfR:]，and [(Ar，〇)2ThR2] [R =

2-(6-MeC5H3N)CH2； Ar, = 2，6-彻2匸6只3]. R was shown to act as both C-centered ligand and C,A^-chelating ligand (despite the presence of the resultant strained four- membered metallacycle ring).

H3C> N 入 CH2 HsC^ \ 义/ 严 M M

C-centered ligand C,/V-chelating ligand

In addition, the reaction of [CpaZrR?] with Ar'NC has also been studied (Scheme 1-2).

19 [MCl2(CH2-py-6Me)2l [Cp2M(CH2-py-6Me)2] M = Hf,Th M = Zr, Hf / MCI>\ /CP2MCI2 Ar'OLi/

/ Li[CH2-py-6Me]

[M(〇A02(CH2-py-6Me)2]

[{(C〇D)MCI}2]

[{(COD)M(|a-CH2-py-6Me)}2] M = Rh, Ir

Scheme 1-2

The presence of the aromatic nitrogen donor atom makes ligand (VI) exhibit different bonding modes and coordination patterns.

1.6 2-(TRIMETHYLSILYL)METHYLPYRIDINE (3) AND 2-BIS(TRI- METHYLSILYDMETHYLPYRIDINE (4) AS LIGAND PRECURSORS.

As "-functionalized ligands of the type (V) are known to exhibit unusual but interesting bonding patterns and are capable of stabilizing those otherwise kinetically labile transition metal alkyl complexes, growing interests have been found in the studies of metal alkyl complexes which contain this type of ligand. A number and variety of metal alkyl complexes were prepared and characterized by Raston et a!.59 with the use of the very bulky ligands [2-(Me3Si)CHC5H4N]- ("Rl) (1) and [2-(Me3Si)2CC5H4N]- ("R^) (2).

O H &�SiMe3 1 2

20 The free ligands 3 and 4 can be prepared by a straight forward method in an "one-pot" reaction (Scheme 1-3).59

• + N CHa “ N 八 CH2(SiMe3) ^N^CH(SiMe3)2

3 4

32。C，0.1 mmHg 52。C’ 0.1 mmHg

Scheme 1-3. Reagents and conditions: (i) 2 eq. "BuLi， TMEDA, hexane, 0。（：，1 h. (ii) excess TMSCl, 0。C, then 25 8 h.

Lithiation of a-picoline with /i-butyllithium in hexane, in the presence of TMEDA, at 0 °C followed by quenching of the resultant suspension with excess trimethylsilyl chloride gives a mixture of compounds 3 and 4. The two compounds 3 and 4 can be separated by fractional distillation at 32°C and 52°C, 0.1 mmHg, respectively.

Preparation of the corresponding organo-lithium compounds from ligand precursors 3 and 4 is summarized as shown in Scheme I-4.59a.b it is noteworthy that lithiation of 4 in different solvents yields different products. Lithiation of 3 with n- butyllithium in ether at 0°C gives dimer 5 with one EtzO molecule coordinates to each lithium ion. In contrast, lithiation of 4 in catalytic amount of diethyl ether at 0°C gives dimer 6. Unlike dimer 5, the lithium ions on compound 6 are free from solvent coordination. If compound 4 is lithiated in the absence of ether, but in hexane instead, an

T|3-aza-allyl type compound 7 will be obtained. In compound 7, a free ligand molecule

21 ‘ I^SiMea Li Li MeaSi^ J

/ ^

(iil/ 6 八 Zx = 2 / 八 k N人C'SiMes rpZ Et2〇i"•"“ Li Li —OB. ^ & ^N^CH3-x(SiMe3)x ~⑷ _ | 、SiMe3 I X = 1 x= 2 Li MeaSi^ Y > • 个

4. X = 2 {Me3Si)2HC. N

5 \iv) ^

If^ 、丨么"SiMe3 N^C、siMe3 Li

Scheme 1-4. Reagents and conditions: (i”BuLi，Et〗。，0 (ii) ^BuLi, Et20, 0。C� (Hi). ^BuLi, hexane, 0 T; (iv) ^BuLi, TMEDA，hexane，0。C. coordinates to the lithium ion through its pyridyl nitrogen. One interesting feature is that this free ligand cannot be further lithiated despite the presence of an excess of n- butyllithium. If the lithiation is carried out in the presence of TMEDA, the corresponding alkyl lithium-TMEDA adduct (8) will be obtained.

Alkylpyridines are more acidic than toluene or xylenes. The latter two compounds can only undergo metallation by organolithium reagents in the presence of tertiary amines such as TMEDA or PMDTA. In contrast, alkylpyridines can be lithiated directly with

22 organo-lithium reagents as exemplified by the formation of compounds 5 and 6 from 3 and

4，respectively, as shown above in Scheme 1-4.

The readiness of alkylpyridines to lithiation may be attributed to the ability of the

pyridyl nitrogen to accommodate the negative charge which is delocalized over the aromatic

Ti-system. Thus, the resultant carbanion can be stabilized making the lithiation reaction

thermodynamically favourable. Theoretical calculations on electron densities for these

systems also support this reasoning.68,69

In addition, lithiation of a-picoline derivatives such as 3 and 4 is also facilitated by

a complex-induced proximity effect.69 Coordination of the ring nitrogen to the lithium ion

of an organo-lithium reagent, e.g. n-butyl lithium, increases the carbanionic character of the

metalating alkyllithium. It has been suggested that a six-membered ring intermediate (VII)

may form and the n-butyl group is capable of directing towards the active proton of the free

ligand.69 This greatly assists lithiation of alkylpyridines.

1 9 , 卜 r2 R\ R^ = H, alkyl

T H

"Bu

(VII)

1.7 A BRIEF REVIEW OF PREVIOUS RESULTS ON SYNTHESIS OF METAL ALKYL COMPLEXES USING (1) AND (2) AS LIGANDS

Ligands 1 and 2 are so designed that they are bulky, void of p-hydrogen, and

possessing pyridyl nitrogen. On complexation to metals, the latter can act as a Lewis base

and coordinate to the central metal, acting as a 4-electron donor by chelation or bridging.

These structural features of 1 and 2 are crucial in suppressing their corresponding metal

alkyls from decomposition by various low energy pathways as described in the above

23 section. In addition, the chelating effect provided by these monoanionic bidentate ligands further stabilizes the resultant complexes.

Raston et al，9 have synthesized and characterized a wide variety of metal alkyl complexes using 1 and 2 as ligands. The main synthetic approach was by alkyl transfer reactions, i.e. the reaction of appropriate organo-lithium reagents with the corresponding metal halides. Results show that the two ligands have demonstrated versatile features and the potential to stablilize unusual bonding configuration in some metal alkyls. Scheme I-

5a and I-5b summarize these results.

The two ligands, —Rl (1) and "R^ (2)，coordinate to the central metals mainly as bidentate ligands through C, iV-ligation. However, in some cases, they may also behave as monodentate C-centered or r|3-aza-allyl type ligands. Ligand features of "Rl and _R2 are described briefly as follows.

(1) As C, N-chelating ligands:

Nearly all monomeric homoleptic complexes [M(R2)2] (M= Mg 15,^^d Zn 16，59d Cd

17,59d Sn 1859e,l)； dinuclear complexes [{RlLKEt^O)}?] 5,59b [{MR〗}〗](M = Li

6，59a，b Cu 12，59a,i Ag 13,59g,i Au 1459i); tetrameric complex [CU(R1)]4] (9);59i

heteroleptic complexes [Cp2Zr(Rl)2] (10),59c [M(R2)2C1] (M 二 A1 20,59hJ Qa

2l59j)，[M(R2)C12] (M = As 26,59k sb 27,59k Bi 2859k); and hypervalent

complexes [Si(R2)2XCl2] (X = H 29,59f Me 3059f) exist in such a form that "Rl and

-R2 coordinate either intramolecularly or intermolecularly via C, iV-ligation. The

ability of "R^ and "R^ to form "-donor compounds imposes a "stabilizing" effect to

the stability of these novel complexes. Moreover, the first novel alkyltin(II) chloride

[R^SnCl] 25’59e,l alkyltin(II) amide [R2Sn[N(SiMe3)2]] 24,59e,l and dialkylmetal

cation [(R2)2M]+ (M = A1 22，59h，j Ga 2359j) have also been prepared as thermally

stable complexes in which "R^ was found to coordinate to the central metal in a C, N-

chelating manner.

24 (2) As mo no dentate C-centered ligand:

The only example was found in the mercury(II) dialkyl [Hg(R2)2] (19) in which a

linear C-Hg-C framework was determined (C-Hg-C = 179.5°).59cl

(3) As v^-aza-allyl type ligands:

_R2 bonds to lithium ion in lithium compounds 7 and 8 in an r|3-aza-allyl fashion.59b

Another example of this structure was proposed for "Rl in the Zr(III) species

[Cp2Zr(Rl)].59c

25 y==S H-丨 y H S\Me, (K N ^CU CU一。• M c H C_Cu Cu-4-N �

MEAS/'H I H / (^、 9 、人 crfJMes 令I H

丨丨丨丨丨“ji《‘Et2 __Cp2Zr(Ri)2 Me3Si,C丫 Nv^^ 1 Q

\ cpTi^ - cp、JO / \ / \ H SIME3 H S'ME^ 11 11a

Scheme I-5a. Reagents and conditions: (i) CuCl, THF, 0。C; (ii) 2 eq. [(nS-CsHghZrCy，THF, 0。C; (iii) 1 eq. [(Ti5-C5H5)2ZrCl2], THF, -30。C.

26 CIsZp

MegSi,、SiMe3 ^6381 SiMeg

25 M = As (26), Sb (27), Bi (28)

[(Me3Si)2N]Sn：^ P \ / VV^

Me3Si'、SiMe3 \ (viii) (ix)/ ^ ‘尸、

24 \(vii) \ / (x)X \ / ^ X = H (29), Me (30)

PS f^ ri J V 、Ki"«^。zSiMe3 < ^SiMeo (Me3Si)2 CI N C^ N^C: 3 xN /) i �SiMe 3 I �SiMe3

\ / (SiMe3)2 MegSi、 | MegSi^ | M = AI (20). Ga (21) MeaSi-^^Nc^-^N Me^Sl^ ^^^

,•、 6 M = Cu (12), Ag (13), Au (14) (VI) I

” X - "I + MegSi�S Mei g /=\ MegSi、SiMeg /=\ C, y^N \

\ 厂 ,。、 \=/ MeaSi SiMeg \=/ MegSi SiMe3

MeaSi SiMe. M = Mg (15), Zn (16). M = A. (22). Ga (23) Q^c- Hg d?), MeoSi SiMe3 ~ 3 (iii) 19

I SiMe3 H

Scheme I-5b. Reagents and conditions: (i) MLn (M = Cu, Ln = CI"； M = Ag, .Ln = BF4-; M = All, Ln = (CO)Cl-), THF, 0�C ;(ii) MCI2 (M = Zn, Cd, Sn), THF, -78。C; (iii) 0.5 eq. "Bu化uMg，heptane, 0。C; (iv) HgCl:，THF, 0。C; (V) MCI3 (M = Al, Ga), Et20, 25�C ;(vi) MCI3 (M = Al，Ga), CsHs，25�C; (vii) Sn[N(SiMe3)2]2，Et20, -78。C; (viii) SnCl〗，Et20, -78。C; (ix) MCI3 (M 二 As, Sb, Bi), Et20, -80。C; (x) SiClgX (X = H, Me), THF, -78 °C.

27 1.8 OBJECTIVE OF THIS WORK

Both -Rl (3) and "R^ (4) have been demonstrated to exhibit unusual bonding patterns when they are bound to a variety of main group elements. They also have the potential to stabilize several novel metal alkyl complexes, especially [Sn(R2)2] (18)，59e,l

[{CUR1}4] (9),59i [{CUR2}2] (12)’59a,i and [{AgR^h] (13).59g,i In addition, they also possess the ability to accommodate a wide variation of metal size through different bonding configurations. The ability of the aromatic N donor to form chelating linkages either intramolecularly or intermolecularly also exerts a stabilization effect on the resultant metal alkyl complexes.

The objective of this work is to further explore the potential of —Rl and _R2 to stabilize those otherwise kinetically more labile transition metal alkyl complexes, mainly transition metal Group 4 (Zr, Hf), 8 (Fe), 9 (Co), and 10 (Ni, Pd) alkyls. The main

synthetic approach is by "metal alkyl exchange" reaction: the reaction between the organolithium reagents [{RlLi(Et20)}2] (5) and [{R2Li}2] (6) and the appropriate

transition metal halides, although in the preparation of nickel alkyl complexes reactions

with nickelocenes will also be employed.

The transition, metal alkyl complexes which have been synthesized thus will be

structurally characterized by single X-ray diffraction studies. In addition, spectroscopic

methods such as NMR spectroscopy, mass spectroscopy; and physical methods such as

Evan's method for magnetic susceptibility measurement will also be employed.

Finally, the reactions of these newly synthesized metal alkyl complexes with

different protic reagents such as ArMeQH or ArSH [Ar^e = 4-Me-2，6」Bu2C6H2，Ar =

2，4,6」Bii3C6H2] will be studied.

28 1.9 REFERENCES FOR CHAPTER I

(1) Zeise, W.C. Mag. Pharm. 1830，55, 105.

(2) Jarvis, J.A.; Kilbourn, B.T.; Owston，P.G. Acta Crystallogr., Sect. B 1971, 27,

366.

(3) Hiraishi, J.N. Spectrochim. Acta, Part A 1969,25, 749.

(4) Hall, P.W.; Puddephatt, RJ.; Tipper, C.F.H. J. Organomet. Chem. 1974, 71, 145.

(5) Frankland, E. Ann. 1849, 71, 171.

(6) Kealy, T.J.; Pauson, P.L. Nature (London) 1951,168, 1039.

(7) Miller, S.A.; Tebboth, J.A.; Tremaine, J.F. J. Chem. Soc. 1952, 632.

(8) Ziegler, K.; Holtzkampf, E.; Breil, H.; Martin, H. Angew. Chem. 1955, 67, 543.

(9) Smidt, L; Hafner, W.; Jira, R.; Seldmeier, J.; Sieber, R.; Rutlinger, R.; Kojer, H.

Angew. Chem. 1959, 77, 176.

(10) Jaffe, H.H.; Doak, G.O. J. Chem. Phys. 1953, 21, 196.

(11) Green, M.L.H. Organometallic Compounds, Methuen: London, UK” 1968.

(12) Berhold, J J.; Groh, G. Z. Znorg. AUg. Chem. 1963’ 319, 230.

(13) Berhold, JJ.; Groh, G. Angew. Chem. 1966, 78，495.

(14) Chatt, J.; Shaw, B丄.J. Chem Soc. 1959，705; 1960，1718.

(15) Chatt, 1; Shaw, B丄.J. Chem. Soc. 1959，4020.

(16) (a) Anet, F.A丄.；Leblanc, E. /. Am. Chem. Soc. 1957, 79，2649.

(b) Thomas, K.; Osborn, J.A.; Powell, A.R.; Wilkinson, G. J. Chem, Soc. (A)

1968, 1801.

(17) Lappert, M.F.; Patil, D.S.; Pedley, J.B. J. Chem. Soc. Chem. Commun. 1975,

830.

(18) Connor, J.A. Top. Cur. Chem. 1977, 77，71.

(19) Wilke，G.; Schott, H. Angew. Chem. 1966，78, 592.

(20) Skinner, H.A. Adv. Organometal. Chem. 1964, 2，49.

(21) (a) Davidson, P.J•； Lappert, M.R; Pearce, R. Acc. Chem. Res. 1974, 7，209.

(b) Davidson, PJ.; Lappert, M.R; Pearce, R. Chem. Rev. 1976’ 76, 219.

(22) Taube, R.; Drevs, H.; Steinborn, D. Z. Chem. 1978,18, 425.

29 (23) Skinner, H.A. J. Chem. Thermodyn. 1978, 10, 309.

(24) Pilcher, G.; Skinner, H.A. In The Chemistry of the Metal-Carbon Bond; Hartley,

F.R., Patai, S.，Eds.; Wiley: New York, 1982.

(25) (a) Collier, M.R.; Lappert, M.F.; Truelock, M.M. J, Organomet. Chem. 1970, 25,

C36.

(b) Collier, MR.; Lappert, M.F.; Pearce, R. J. Chem. Soc.’ Dalton Trans. 1973,

445.

(26) (a) Yagupsky, G,; Mowat, W.; Shortland, A.; Wilkinson, G. Chem. Commun.

1970, 1369.

(b) Mowat, W.; Shortland, A.; Yagupsky, G.; Hill, NJ.; Yagupsky, M.; Wilkinson,

G. J. Chem. Soc., Dalton Trans. 1972, 533.

(27) Schrock, R.R. Acc. Chem. Res. 1977’ 12’ 98.

(28) Cooper, W.J.; Green, M.L.H. J. Chem. Soc., Chem. Commun, 1974, 761.

(29) Braterman, P.S.; Cross, RJ. Chem. Soc. Rev. 1973,2, 271.

(30) Schrock, R.R.; Parshall, G.W. Chem, Rev. 1976, 76, 263.

(31) Davidson, PJ.; Lappert, M.F. J. Chem. Soc., Chem. Commun. 1972, 317。

(32) Lappert, M.F.; Pearce, R. J. Chem. Soc” Chem. Commun. 1973, 126.

(33) Davidson, P.J•； Lappert, M.F.; Pearce, R. J. Organomet. Chem. 1973, 57, 269.

(34) Mowat, W.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1973，1120.

(35) Moorhouse, S,; Wilkinson, G. J. Organomet. Chem. 1973, 52, C5.

(36) Bassindale, A.R.; Bowles, AJ.; Cook, M.A.; Eaborn, C; Hudson, A.; Jackson,

R.A.; Jukes, A.E. Chem. Commun. 1970, 559.

(37) Kruse, W. J. Organomet. Chem. 1972, 42, C39.

(38) Berhold, JJ.; Groh, G. Z. Znorg. Allg. Chem. 1963, 319, 230.

(39) Lanovskaya, L.M.; Prarikova, N.A.; Gantmakher, A.R.; Medvelev, S.S. Polym.

.Sci, USSR (Engl Transl) 1969，11, 1315.

(40) Wilkinson, G. Science (Washington, DC) 1974,185, 109.

(41) Brintzinger, H. J, Am. Chem. Soc. 1967, 89, 6871.

(42) Manzer, L.E. Inorg. Chem. 1976,15, 2567.

30 (43) (a) van Koten, G.; Noltes, J.G. J. Organomet. Chem. 1975,102, 551.

(b) van Koten, G.; Leusink, A.J.; Noltes, J.G. J. Organomet. Chem. 1975，85,

105; 84, 117.

(44) Cope, A.C.; Gourley, R.N. J. Organomet. Chem. 1967, 5, 527.

(45) van Koten, G.; Noltes, J.G. J. Organomet. Chem. 1974, 52, C53.

(46) Leusink, AJ.; van Koten, G.; Marsman, J.W.; Noltes, J.G. J. Organomet. Chem.

1973,55, 419.

(47) Longoni G.; Fantucci, P.; Chini, P.; Canziani, F. J. Organomet. Chem. 1972, 39,

413.

(48) Dehand, J.; Pfeffer, M. Coord. Chem. Rev. 1976，18, 327.

(49) (a) Omae, 1. Chem. Rev. 1979, 79, 287.

(b) Omae, 1. Coord. Chem. Rev, 1980, 32, 235.

(50) Cope, A.C.; Siekman, R.W. J. Am. Chem. Soc. 1965，87, 3272.

(51) Hartwell, G.E.; Lawrence, R.V.; Smas, MJ. Chem. Commun. 1970, 912.

(52) Manzer. L.E. J. Am. Chem. Soc. 1978，100, 8086.

(53) Breliere, C.; Carre, F.; Corriu, R.J.P.; Poirier, M.; Royo, G.; Zwecker, J.

Organometallics 1989, <5, 1831.

(54) Tzschach, A.; Nindel, N. J. Organomet. Chem. 1970, 24, 159.

(55) Byrne, L.T.; Engelhardt, L.M.; Jacobsen, G.E.; Leung, W.-P.; Papasergio, R.L;

Raston，C.L.; Skelton, B.W.; Twiss, P.; White, A.H. J. Chem. Soc., Dalton Trans.

1989，105.

(56) Hein, F.; Tille，D. Z. Anorg. Allg. Chem. 1964, 529, 72.

(57) van Koten, G.; Jastrzebski, J.T.B.H.; Noltes, J.G.; Spek, A丄.；Schoone，J.C J.

Organomet. Chem. 1978,148, 233.

(58) (a) Tandura, S.N.; Voronkov, M.G.; Alekseev, N.V. Top. Curr. Chem. 1986,131,

99.

(b) Breliere, C; Carre, F.; Corriu, RJ.P.; Poirier, M.; Royo, G. Organometallics

1986,5, 388.

31 (c) Boyer, J.; Breliere, C.; Carre, F.; Corriu, RJ.P.; Kpoton, A.; Poirier, M.; Royo,

G.; Young, J.C. J. Chem. Soc.’ Dalton Trans. 1989, 43.

(59) (a) Papasergio, R.I.; Raston, C.L.; White, A.H. J. Chem. Soc., Chem. Commun.

1983, 1419.

(b) Colgan, D.; Papasergio, R.L; Raston, C.L.; White, A.H. J. Chem. Soc., Chem.

Commun. 1984, 1708.

Raston, CL.; White, A.H. J. Chem. Soc” Dalton Trans. 1986, 603.

(d) Henderson, M.J.; Papasergio, R.L; Raston, C.L.; White, A.H.; Lappert, M.F. J.

Chem. Soc., Chem. Commun. 1986, 672.

(e) Engelhardt, L.M.; Jolly, B.S.; Lappert, M.F.; Raston, C.L.; White, A.H. J.

Chem. Soc., Chem. Commun. 1988, 336.

(f) van den Ancker, T.; Jolly, B.S.; Lappert, M.F.; Raston, C.L.; Skelton, B.W.;

White, A.H. J. Chem, Soc., Chem. Commun. 1990, 1006.

(g) Papasergio, R.L; Raston, C.L.; White, A.H. /. Chem. Soc., Chem. Commun.

1984，612.

(h) Engelhardt, L.M.; Kynast，U.; Raston, C.L.; White, A.H. Angew. Chem。InL

Ed. Engl. 1987, 26, 7.

(i) Papasergio, R.L; Raston, C.L.; White, A.H. J. Chem. Soc., Dalton Trans.

1987，3085.

(j) Kynast, U.; Skelton, B.W.; White, A.H.; Henderson, M.L; Raston, C.L. J.

Organomet. Chem. 1990, 554, CI.

(k) Cameron, L; Engelhardt, L.M.; Junk, P.C.; Hutchings, D.S.; Patalinghug,

W.C.; Raston, CL.; White, A.H. J. Chem. Soc” Chem. Commun. 1991, 1560.

(1) Jolly, B.S.; Lappert, M.F.; Engelhardt, L.M.; White, A.H.; Raston, C.L. /。

Chem. Soc” Dalton Trans. 1993, 2653.

(60) Engelhardt, L.M.; Jacobsen, G.E.; Junk, P.C.; Raston, C.L; Skelton, B.W. /.

Chem. Soc., Dalton Trans. 1988, 1011.

32 (61) Engelhardt, L.M.; Jacobsen, G.E.; Patalinghug, W.C.; Skelton, B.W.; Raston,

CL.; White, A.H. J. Chem. Soc.，Dalton Trans. 1991, 2859.

(62) Barr, D.; Clegg, W.; Mulvey, R.E.; Snaith, R. J. Chem. Soc., Chem. Commun.

1984, 469.

(63) Ban, D.; Clegg, W.; Mulvey, R.E.; Snaith, R. J. Chem. Soc., Chem. Commun.

1984’ 700.

(64) (a) Beshouri, S.M.; Fan wick, P.E.; Rothwell, LP.; Huffman, J.C. Organometallics

1987,6, 891.

(b) Beshouri, S.M.; Fanwick, P.E.; Rothwell, LP.; Huffman, J.C. Organometallics

1987, 6, 2498.

(65) van der Schaaf, P.A.; Wissing, E.; Boersma, J.; Smeets, WJJ.; Spek, A.L.; van

Koten, G. Organometallics 1993，72, 3624.

(66) Wang, S.; Fackler, LP. Inorg. Chem. 1989, 25, 2615.

(67) (a) Tang, K.; A slam, m.; Block, E.; Nicholson, T.; Zubieta, J. Inorg. Chem. 1987,

26，1488.

(b) Block, E.; Gemon, M.; Kang, H.; Ofori-Okai, G.; Zubieta, J. Inorg. Chem.

1989,25, 1263.

Chem. 1990,29，3172.

(68) Konishi, K.; Takahashi, K. Bull. Chem. Soc, Jpn. 1977, 50, 2512.

(69) Beak, P.; Meyers, A.I. Acc. Chem. Res. 1986,19, 356.

33 CHAPTER II. SYNTHESIS, STRUCTURES AND REACTIVITY OF IRON(II) AND COBALT(II) DIALKYL COMPLEXES

II.l INTRODUCTION

II.l.l Synthesis of Iron(II) and Cobalt(II) Alkyl Complexes

Many organoiron(II) and -cobalt(II) species have been reported as intermediates

which decompose readily to alkanes or their coupled products. As a consequence of their

low stability, very few iron(II) and cobalt(II) dialkyl or diaryl complexes have been fully

characterized. 1-5 The labile iron- and cobalt-carbon a bonds are mainly due to the small

energy difference between the d orbitals of the metal and the antibonding orbitals of the M-

C bond.l Therefore, many organoiron(II) and -cobalt(II) compounds are considered to be

unstable in the absence of appropriate supporting donor ligands (e.g. bipyridines) or n-

bonding ligands (e.g. 715-C5H5, CO, PR3).1，2

II.lJ.l Iron(n) Alkyl Complexes

The vast majority of iron(II) alkyl complexes which have been isolated so far exist

either in the form of [FeR(X)Ln] (VIII) or [FeRaLn] (IX) where R may be alkyl or aryl

groups, X is generally ri^-C^U^, and L is strong field auxiliary ligands such as CO, PR3,

and bipyridines. 1 Some representative examples of these iron(II) alkyl or aryl complexes

are listed in Table 11-1.

The role of the supporting ligands is not fully understood. It may be due to the

presence of 冗* orbitals in some jc-acceptor ligands such as CO, PR3, etc., which

interact with the metal d orbitals and hence increasing the ligand field stabilization energy;

or the occupation of any vacant coordination sites by other ligands such as bipyridines.^ In

the latter case some of the low energy decomposition pathways (e.g. a-elimination and p-

elimination) are blocked. That is the reason why thermally stable complexes of the type

[FeR2(bipy)2] (R = Me 31,9 Et 32,940 "p^ 33,9 "Bu 349) have been successfully isolated

•v Table II-l. Representative Examples of Some Iron(II) Alkyls Stabilized with Supporting Ligands.

Compound m.p.(。C) References

[(7l5-C5H5)Fe(CO)2(Me)] 78-82 6

[(Tl5-C5H5)Fe(CO)2(屯 u)] 72-75 6

[0i5-C5H5)Fe(CO)2(CH2Ph)] 55-57 7

[FeMe2(CO)2(PMe3)2] Subl. at 40。C in vacuo 8

[FeMe2(bipy)2] (31) -- 9

[FeEt2(bipy)2] (32) -- 9，10

[Fe叩r2(bipy)2] (33) -- 9 [Fe"Bii2(bipy)2] (34) - 9 [(CH2CH2CH2CH2)Fe(bipy)2] -- 9 [(CHF2CF2)2Fe(CO)4] - 11 [(CF2CF=CFCF2)Fe(CO)4] - 12

35 ‘ even though some of them do contain p-hydrogens on the alkyl ligands. The metal-carbon bonds may also be stabilized by altering the electronegativity of the a-carbon by using carbon of different hybridization states. In fact, the metal-carbon strength is observed to increase in the order M-C(sp) > M-C{sp^) > M-C(吵勺.1 In addition, the attachment of strongly electronegative substituents (e.g. F, CN) on the a-carbon is also known to impose a stabilization effect, n-13

Iron(II) alkyl and aryl complexes can be prepared by the general synthetic methods which have been dealt with in Chapter I. Reactions between main group alkyls and the appropriate iron(II) halides are one of the most frequently encountered methods. However, the failure to obtain alkyl iron derivatives in the early days was mainly due to the absence of stabilizing ligands on the metal and inappropriate reaction conditions. Reaction of substituted iron(II) iodide [FeI(CO)(PR3)(Ti5-C5H5)] with a Grignard reagent gave the corresponding iron(II) alkyl complex 35 (equation II-1) in which 35 was stabilized by three different supporting ligands. 14

[Fel(C〇)(PR3)(Ti5-C5H5)】+ Me3Si(Ph)CHMgBr

^ [Me3Si(Ph)CHFe(CO)(PR3)(Ti^-C5H5)] (II-1)

Iron(n) dialkyl complex in the form of metallacycle such as the ferracyclopentane

complex 36 in (equation II-2) was prepared from FeCl2(bipy)2 and the appropriate di-

Grignard reagent.^

FeCl2(bipy)2 + BrMg(CH2)4MgBr • Fe(bipy)2 (11-2)

In addition to Grignard and organolithium reagents, the use of organosodium

reagents has also been reported (equation n-3).15

36 r FeCl2(dmpe)2 + sodium naphthalide — [FeH(2-naphthyl)(dmpe)2] (II-3)

Chatt and Shaw have reported the synthesis of the diaryl iron(II) complexes[(C6Cl5)2Fe(PEt2Ph)2] 37 by treatment of [FeCl2(PEt2Ph)4], prepared in situ by stirring ferrous chloride with the phosphine, with the Grignard reagent CsClsMgCl in benzene (equation II-4).3

[FeCl2(PEt2Ph)4] + CeClsMgCI [(C6Cl5)2Fe(PEt2Ph)2] (II-4)

However, the resultant golden-yellow iron(II) complex 37 was thermally unstable and decomposed gradually once after being dissolved in benzene at 25 The analogous complex [Fe(mesityl)2(PEt2Ph)2] have also been prepared by a similar procedure at -40。C.

The bright orange precipitate, probably [Fe(mesityl)2(PEt2Ph)2] decomposed very rapidly on attempted isolation.

In addition to Grignard and organo-alkali metal reagents which are often used， organoaluminum reagents also give the corresponding alkyl iron(II) compounds when react with the appropriate starting materials. Yamamoto et al have reported the synthesis of diethyl bis(a，a'-bipyridine)iron(II) 32 by treating Fe(acac)3 with Et2Al(0Et) in the presence of bipyridine (equation n-5).lO

Fe(acac)3 + 曰2八1(〇日） + bipy [FeEt2(bipy)2] (11-5)

The organoaluminum reagent Et2Al(0Et) acts not only as the source of the organic ligand, but also as reducing agent to reduce Fe(in) to Fe(n). By using a similar procedure, Kochi et al have also prepared the methyl 31 and A2-propyl 33 analogues starting with the commercially available trimethyl- and tri-«-propyl alanes, respectively.^ However, the application of this synthetic method is severely limited by the availability of the corresponding organoaluminum reagents.

37 Kochi et al have also reported an alternative route to dialkyl-iron complexes of the type [FeR2(bipy)2] by direct alkylation of FeCl2(bipy)2 with the appropriate Grignard or organolithium reagents.^ Thus, in addition to compounds 31-33, [Fe叱U2(bipy)2] 34, the ferracyclopentane [CHzCHsCHzCH^FeCbipy)〗]36 (equation II-2) have also been synthesized. However, attempts to synthesize the isopropyl and tert-huiyl analogues were unsuccessful probably due to the low thermal stability of these two complexes. Moreover, reaction of neopentyl-lithium with FeCl2(bipy)2 proceeded very slowly and the corresponding dialkyliron(n) compounds could not be obtained in pure form 9

Homoleptic Complexes.

Despite the fact that iron is the most abundant transition element, examples of simple homoleptic iron dialkyl or diaryl of the type [FeR〕] are rare.lb，2,5 One interesting example is probably tetra( 1 -norbornyl)iron(IV) 38,16 prepared by the reaction of 1- norbomyllithium with iron(in) chloride (equation 11-6).

[FeCl3(〇Et2)] + ^^：：^^ (11-6)

Fe 38

Complex 38 decomposed slowly at room temperature with a half-life of ca. 30 h at 23 Other examples are mainly anionic complexes such as [FeMe4^"]^^ and [Fe(C=CR)64-].l8

[FeCl3(〇Et2)] + MeLi [LialFeMe』（11-7)

[Fe(SCN)2(NH3)4] + RC=CK ~- [K4[Fe(C=CR)6]】（II-8)

‘A few iron(II) diaryls [FeAr〗](Ar = C6H5CH2, p-MeC^HA, p-MQOC^ld4, and p-

NH2C6H4) have also been reported without any structural information. 19

38 Homoleptic iron(II) diaryl complexes which have been well characterized are

[{FeMes2h] 39a (Mes == 2’4，6-Me3C6H2) and [{Fe(Ar^'P02}2] 39b (Ar,Pr 二 2,4,6-

^Pr3C6H2), prepared by treating FeCl〕with MesMgBr or 中r3C6H2MgBr in THF (equation

11-9)20-22

R 人具 2叫 + (II-9)

Li R R论厂

39a: R = Me 39b: R = 'Pr

Compound 39a was first prepared by Machelett in the 70’s.20 An improved synthetic method was reported by Floriani et al?-'^ X-Ray structural analysis has been carried out only recently revealing that compounds 39a is dimeric: the two iron centers are bridged by two mesityl groups and no metal-metal interaction have been observed.^^ In addition to an improved synthesis of 39a, Floriani et al. also prepared the analogous compound 39b which is isostructural with 39a.22b Nevertheless, compound 39a was reported to be less stable than that of 39b and thus the former complex had better be kept at low temperature.

A monomelic form of iron-mesityl 39c can be obtained from the reaction of 39a with pyridine in toluene (equation 11-10) .22b

39 ‘ XX 、、‘口 [{FeMes2h] ？乂論日. J^pe ^ ^ (11-10) toluene . \ 39a / \ 尸N N\ o ^ 39c

Recently, Roriani et al showed that the magnetic moment is 1.71 [x^ per iron for

39a.The same research group also studied the chemistry of this complex where insertion of isocyanide ^BuNC into the iron-carbon a bonds of compound 39a was observed leading to the first homoleptic iron(II) iminoacyl complex 40 (equation 11-11).22

39a + 4 feu4N=C

它u、K,〜Mes 它 u Mes THF , \N、Z \ ,。， II Fe Fe、H (Il-ll) MesZ 它 U Mes

CH3

Mes = Xz H3C H3

In addition, reaction of 39a with benzonitrile gave the corresponding insertion product 41 (equation II-12).22

40 ‘ Mes、 .Ph cz II

39a + 4P.C.N t。l_ , Mes)Fe/N\Fe(NCPh (腿） PhCr/ \ Z ^Mes N II

MesZ \ph

11.1.1.2 Cobalt(II) Alkyl Complexes

Although cobalt-carbon a bonds are thermally labile,23 a number of stable cobalt(n) dialkyl and diaryl complexes which contain supporting ligands such as PR3, P(〇R)3，and bipy can be prepared. The first stable cobalt(II) diaryl complexes of the type trans-

:CoR2(PR’3)2] were synthesized by Chatt and Shaw.3 The reaction of CoBr2(PEt2Ph)2 with the appropriate Grignard reagents RMgX gave the corresponding cobalt(II) diaryl compounds (equation 11-13).

CoBr2(P曰2Ph)2 + 2 RMgX fra/7S-[CoR2(PEt2Ph)2] (2-13)

42: R = mesityl; X = Br 43:R = 2-biphenylyi;X = Br 44: R = 2-methyl-naphthyl; X =已r 45: R = pentachlorophenyl; X = CI

Treatment of CoBr2(PEt2Ph)2 with mesityllithium, also gave the desired product 42.

Compounds 42-45 are golden yellow in colour. The trans square planar geometry around

the central metal was elucidated from dipole moment measurements. Among compounds

42-45, 45 is the most stable one which did not decompose below 220 The phenyl, o-

tolyl, o-chlorophenyl, 6>-bromophenyl, a-naphthyl, and 9-anthryl groups all appeared to

yield the corresponding cobalt(II) diaryl compounds but they decomposed on attempted

isolation.

41 •v Chatt and Shaw also attempted to prepare alkylcobalt(II) derivatives by treating

CoBr2(PEt2Ph)2 with alkyllithium or Grignard reagents but were unsuccessful.^

The synthesis of diethylbis(bipyridyl)cobalt(II) [CoEt2(bipy)2] has been reported by Yamamoto et al. by employing a similar method as in the preparation of the iron(II) analogues.24 However, recent analysis showed that the originally formulated cobalt(II) species was, in fact, a cobalt® compound [CoEt(bipy)2].25 (equation 11-14).

Co(acac)3 + BsAKOEt) + bipy [CoEt(bipy)2] (11-14)

Homoleptic Complexes.

According to Schrauzer and Kohnle,26 and Johnson and co-workers^'^, the presence of cob alt-nitrogen chelating linkages can enhance the stability of aryl-cobalt a- bonded complexes. Thus some thermally stable cobalt aryl complexes have been prepared by the two research groups. All of which contained cobalt-nitrogen chelating linkages.

Cope and Gourley have studied the reaction between 6>-lithio-N,iV-dimethylbenzylamine with anhydrous C0CI2.28 Surprisingly, instead of the corresponding cobalt(n) diaryl, only the cobalt(III) aryl complex 46 and its analogue 47 were obtained which involved an unusual oxidation of Co(n) to Co(in) (equation 11-15).

42 r R R

——^iuU 一 kA^ / ’N� ^N H3C CH3 H3C 一 ^CH3

^—N-CH3 CH3/

46: R = H 47: R = feu

The homoleptic cobalt(II) mesityl complex, presumably the first well-characterized homoleptic dimeric cobalt(n) diaryl complex, have been prepared by Theopold et The reaction of C0CI2.THF with three equivalents of mesityllithium in THF gave compound

48. Subsequent protonation of 48 with one equivalent of HBF4-Et20 yielded the

thermally stable neutral complex bis(M.-mesityl)dimesitylciicobalt 49 (equation 11-16).

43 CoCl2'THF + 3

The reaction of (CF3)2CHCN with cobalt acetate in acetic acid probably afforded the homoleptic complex Co[C(CN)(CF3)2]2. However, evidence of its existence are eqiiivocal.30

44 II.1.2 Reactions of Transition Metal Alkyl Complexes with Protic Reagents and Halogens

(i) Reaction with protic reagents:

Transition metal alkyl complexes often react readily with water, alcohols, thiols, and other protic compounds.

Yamamoto et al. studied the reactions of [FeEt2(bipy)2] 32 with ethanol and water. 10b Ethane was detected by mass spectroscopy as the only gaseous product

(equation 11-17).

C2H5OH [FeEt2(bipy)2] + or [Fe(bipy)2]2+ + CsHe (11-17)

3 2 H2〇

Those other products remaining after the reactions were found to contain [Fe(bipy)2]2+ as confirmed by their visible spectra.

The addition of an equimolar of dry hydrogen chloride to a solution of

[Co(Mes)2(PEt2Ph)2] 42 (Mes = 2，4,6-Me3C6H2) gave [CoCl2(PEt2Ph)2] and unreacted

[Co(Mes)2(PEt2Ph)2] (ca. 50% recovery).^

[Co(Mes)2(PEt2Ph)2l + HCI(g)

4 2

•MesH • CoCl2(PEt2Ph)2 + ’.Co(Mes)2(PEt2Ph)2" (unreacted) (11-18)

Karsch et al. reported that the Co-C a bonds in [CoMe2(PMe3)2] can be cleaved

readily by weak acids such as MeOH，PhOH, and acetylacetone to give polymers of

composition {Co(OR)2}n (OR = OMe, OPh, acac) (Scheme n-l).3l

The reaction of manganese(II) dialkyl [MnR2(THF)2] [R 二 CH(SiMe3)2] with the

bulky phenol Ar^^OR (Ar^e 二 4-Me-2，6」Bu2C6H2) gave the corresponding manganese

aryloxide (equation II-19).

45 ‘ [Co(acac)(Me)(PMe3)]

acacH/

"7coMe2(PMe3)3】""| ^ \

ROH\

\ Me R /PMe3 /〇\ / \ roH MeaP——Co——PMea (MeaHaMeCo CoMe(PMe3)2 "Co(〇R)2"

MeaP \oZ

OR R

Scheme II-1 [MnR2(THF)2] + 2ArMe〇H ^ [Mn(0Ar^®)2] + 2 RH (11-19)

Me A产=A

Interestingly, platinum(IV) and gold(ni) alkyl complexes are most stable towards water and alcohols. Moreover, some complexes of the type [MR^] which contain bulky ligands and devoid of p-hydrogen are also resistant to water and alcohols. A noteworthy example is the vanadium(IV) tetramesityl complex [V(Mes)4].33

(ii) Reaction with halogens

Cleavage of metal-carbon a bond is also caused by reaction with halogens. The reaction can be illustrated schematically in equation 11-20.

MR + X2 MX + RX (11-20)

A typical example can be found in the reaction of [PtPh2(PEt3)2] with 1〗(equation

II-21).34

[PtPh2(PEt3)2] + l2 — ^ [PtPh(l)(PEt3)2] + Phi (n-21)

The reaction of [FeEt2(bipy)2] 32 with iodine was studied by Yamamoto et al.

Coupling of the alkyl groups shown by the release of butane was observed, and a red

reaction product remained which was proved to be Fel2(bipy)2 by microanalysis (equation

1-22).10 Kochi et al. also noted a similar result in the reaction between of 32 with

bromine. 35

47 [FeEt2(bipy)2] + X2 ^ …• C4H10 + FeX2(bipy)2 (11-22) X = Br, I 32

Sometimes halogens may act as oxidizing agent to oxidize the organometal compounds. Karsch et al. have reported that [CoMe2(PMe3)2] reacted with bromine or iodine to give the cobalt(in) alkyl halide complexes as shown in equation 11-23.31

[CoMe2(PEt3)2] + X2 x = 3「I * [CoMe2X(PEt3)2] (11-23)

II. 1.3 The Chemistry of Transition Metal Alkoxides and Thiolates — A Brief Review

The studies of transition metal alkoxides and aryloxides have attracted much interests in the past decades.36 However, there are only a few reports on the synthesis and structural characterization of homoleptic alkoxyl and aryloxyl compounds of late transition metals such as Mn, Fe, and Co, in contrast to their early transition metal analogues of Ti，37

Zr,38 Ta,39 Cr^O Mo,4M2 and W43 which have been thoroughly studied. One of the difficulties which hampers the studies of homoleptic alkoxides and aryloxides of the late transition metals is due to their extensive association and low solubility in hydrocarbon solvents. Reports have shown that the degree of association can be greatly decreased by employing alkoxyl or aryloxyl ligands bearing sterically demanding hydrocarbon substituents such as 1-adamantyl, PhgC，（CgHn)]�（出u)2CH,(出u)3 Cand 2,4,6-

出U3C6H2.44-46 Hence, alkoxyl and aryloxyl compounds of the types [M(〇R)mLn] and

:{M(OR)2}n] where R is an sterically demanding alkyl or aryl group and n = 2 or 3 have been structurally characterized 44-46 A lower degree of association results in a higher solubility of these compounds in hydrocarbon solvents which, in turn, makes their structural characterization feasible. Some representative examples of these complexes are listed in Table 11-2.

48 ‘ Table II-2. Physical Properties of Some Representative Late Transition Metal (Mn, Fe, Co) Alkoxides and Aryloxides»

Compound Colour m.p.(X) References

[{Mn(OAr)2}2] “ colourless 234-240 46d [Mn(OCPh3)2(Py)2] colourless -- 46d

[{Fe(OAr)2}2] yellow 228-235 (dec) 46d

[{Fe(OAr)[N(SiMe3)2]h] yellow 155-160 46d [Fe(OCPh3)2(THF)2] colourless - 46d

[{Co[OC(C6Hn)3]2h] green 210-212 46c [{Co(OCPh3)2}2] green 267 46c

[Co(OCPh3)2(THF)2] red 139-141 46c

[Co(OSiPh3)2(THF)2] purple 194 46c

[Co(Cl)(OC 屯 113)2]-办 blue - 46b

[Co[N(SiMe3)2](OC 屯 U3)2]-c blue -- 46b

[Li[Co[N(SiMe3)2](OCnBu3)2]] green - 46b a: Ar = 2，4，6-屯 113C6H2. b: actual formula: [Co(Cl)(OC尔U3)2.Li(THF)3]. c: actual formula:

[Li(THF)4.5] {Co[N(SiMe3)2](OC 屯 113)2).

49 ‘ In addition to transition metal alkoxides and aryloxides, in recent years, the chemistry of transition metal thiolate complexes have attracted much attention due to their relevance to the structures and properties of active sites in metalloenzymes such as ferredoxins and nitrogenase.47-50 However, like the alkoxyl and aryloxyl analogues, most of transition metal thiolates exist as insoluble polymers hindering their full characterizations. Several reports have been published showing that protolysis of metal amides by thiophenols with bulky alkyl substituents is a convenient route to the preparation of the corresponding metal thiolates with low degree of association.49’50 por instance, with this synthetic approach, the first dimeric transition metal thiolates complexes

:{M(SAr)2}2] [M = Mn 50, Fe 51, Co 52] were synthesized by the reaction of ArSH with the corresponding metal amides [M[N(SiMe3)2]2].49a With the use of the more bulky aryl substituted thiophenol 2,6-Mes2C5H3SH (Mes 二 2’4,6-Me3C6H2), Power et al. have reported the preparation of the monomelic iron(II) thiolate complexes [Fe(SC6H3Mes2-

2，6)[N(SiMe3)2]] 53 and [Fe(SC6H3Mes2-2，6)2] 54 using the same approach.49b Some physical properties of these transition metal thiolates (50-54) are listed in Table 11-3.

Table II-3. Physical Properties of Homoleptic Transition Metal Thiolate Complexes of Mn，Fe，and Co.

Compound Colour m.p.(°C) References

[{Mn(SAr)2}2] (50)t colourless -- 49a

[{Fe(SAr)2h] dark red -- 49a

[Fe(SC6H3Mes2-2，6)[N(SiMe3)2]] (53) yellow 150 49b

[Fe(SC6H3Mes2-2,6)2] (54) red 250-253 49b

[{Co(SAr)2h] (52)t green -- 49a

tAr = 2，4,6-^U3C6H2.

50 ‘ IL2 RESULTS AND DISCUSSION

II.2.1 Synthesis of Homoleptic Iron(II) and Cobalt(II) Dialkyl Complexes 幽C(SiMe3)2C5H4r^-2}2] (M = Fe 55, Co 56)

Treatment of two equivalents of the lithium alkyl [{R2Li}2] =

-C(SiMe3)2C5H4N-2] (6) with metal dichlorides MCI2 (M 二 Fe, Co) in ether leads to the corresponding hydrocarbon soluble homoleptic iron(II) dialkyl [Fe(R2)2] (55) and cobalt(II) dialkyl [CO(R2)2] (56)，respectively (Scheme II-2). No metal, an obvious reduction product, was observed in any stage of the reactions. The insoluble lithium chloride was filtered through Celite and both compounds 55 and 56 could be obtained as crystalline substances by concentrating the resultant ether solutions followed by cooling to

-30 for ca. 18 h. The product yields are good, viz. 80% for 55 and 85% for 56. By carrying out the reactions in other solvent like THF did not have any obvious influence on the yield of reaction.

^^ MC 丨 2

I J = Fe, ^N'^C(SiMe3)2 X X MesSi、>SiMe3 ^^^ i L丨 Z \ P I t \ (MegSOsC^^^^N MegSi, 々SiMe。

M = Fe;L=PPh3 M = Fe 55, Co 56

Scheme II-2

Compound 55 was isolated as an extremely air sensitive yellow crystalline compound whilst 56 as deep red air sensitive crystals. Hence, both compounds should be handled with great care under a stream of high purity nitrogen or argon. The melting points of 55 and 56 are 110-112 and 116-118 respectively. In addition, both compounds are thermally robust and can be sublimed at 112-116。C (55) and 160。C (56) under high

51 ‘ vacuum (10-2 mmHg). The sublimation of these compounds at such high temperatures signifies their thermal stability, in contrast to the general knowledge that homoleptic iron(II) and cobalt(II) dialkyls are thermally unstable. To our knowledge, compounds 55-56 are the first thermally stable mononuclear, homoleptic iron(11) and cobalt(II) dialkyls being fully characterized. Table II-4 summarizes the physical properties of compounds 55 and

56.

Table II-4. Some Physical Properties of Compounds 55 and 56. Sublimation Compound Yield (%) Colour m.p. (°C) temperature (�C) at 10-2 mmHg

55 80 yellow 110-112 116

56 85 deep red 116-118 160

The successful synthesis of compounds 55 and 56 which contain the very bulky ligand ['C(SiMe3)2C5H4N-2] ("R^) is noteworthy. The bulkiness of the ligand "R^ not only protects the central metal from attacks by other reagents or solvents, it also hinders decomposition of the corresponding complexes via reductive coupling of two alkyl ligands from occurring. Both compounds have been structurally characterized by single crystal diffraction analysis (Figures II-1 and 11-2). In both cases, the two alkyl ligands are bonded to the central metal via (7，iV-ligation {vide supra). The nitrogen-metal linkages in a chelating structure accounts for their stability.

II.2.2 Attempted Reaction of [Fe(PPh3)2Cl2] with [{R2Li}2]

Attempts to prepare iron(II) alkyl complex of the type [Fe(PPh3)2R2] by treating

[FeCl2(PPh3)2] with [{R^Li}〗] in THF at room temperature were unsuccessful. It was thought that a compound similar to that of Chatt and Shaw，3 [Fe(PR3)2(R2)2]’ would be

52 ‘ formed in this reaction but only the homoleptic compound 55 and triphenylphosphine were isolated (Scheme II-3).

,�� * Fe(PPh3)2(R2)2

[Fee 丨 2(PPh3)2] <

^ 55 + PPh3

Scheme II-3

It is conceivable that owing to steric congestion of the alkyl ligand R2 and PPh], accommodation of four bulky ligands on the coordination sphere of Fe(II) is difficult to envisage.

II.2.3 Attempted Synthesis of Monoalkyliron(II) Complexes

Another attempts to prepare monoalkyl-iron(II) and -cobalt(II) complex of the type

[{Fe(R2)Cl}x] have so far been unsuccessful (Scheme II-4). Addition of one equivalent of [{R2Li}2] to FeCl2 in ether at -78 °C yielded only the dialkyl complex 55. This may imply that the formation of 55 rather than [{Fe(R^)Cl}x] is thermodynamically more favourable. On the other hand, the addition of one equivalent of [{R2Li}2] to C0CI2 in ether at -78� Cyielded a deep reddish brown solid, probably [{CO(R2)C1}x]. The solid compound decomposed to C0CI2 and [CO(R2)2] 56 on attempted isolation from its mother liquor (Scheme II-4).

1 eq. FeCl2 Et2〇，-78。C^ 55

1 日gc^g^ • -78。Ct0 25。C, ^^ + ooCI,

Scheme II-4

53 ‘ 11-2.4 Synthesis of Homoleptic Iron(II) and Cobalt(II) Dialkyl Complexes []Vl{(CHSiMe3)C9H6N-8}2] (M = Fe 57, Co 58)5o

A new N-functionalized alkyl ligand [-(CHSiMe3)C9H6N-8] ("R^) (59) has been developed recently and shown to stabilize a number of main group metal alkyl complexes.50

CH(SiMe3)

These include the Group 12 metal dialkyls [M(R3)2] (M = Zn, Cd), the alkyl cadmium(n) chloride complex [Cd(R3)(Cl)(TMEDA)]，the tin(II) dialkyl complex [Sn(R3)2]，and the tin(IY) alkyl complex [Ph2Sn(R3)Cl].

The latter three compounds have also been structurally characterized. All of them are monomelic with intramolecular ^-coordination.

The work was extended here to prepare the iron(II) and cobalt(II) analogues

[M(R3)2] (M = Fe, Co) (Scheme II-5).

MCI2 + 2 [R\i(TMEDA)] ^^——[M(r\] + 2 LiCI 25 8 h 6 0 M = Fe 57’ Co 58

Scheme II-5

The reactions of two equivalents of the organolithium reagents [R3Li(TMEDA)] (60) with iron(n) chloride in ether at room temperature for 8 h gave a dark brown solution and a gray precipitate, the latter being most apparently lithium chloride. The solution was filtered and concentrated in vacuo. Cooling to -30 °C for one night yielded the corresponding iron(II) dialkyl complex (57) as dark-brown crystals in 65% yield.

54 An exactly identical procedure gave the corresponding cobalt(II) dialkyl complex

[CO(R3)2] (58) in 70% yield. Compound 58 exists as dark green crystals. Table II-5 summarizes some physical properties of compounds 57 and 58.

Table II-5. Some Physical Properties of Compounds 57 and 58.

Compound Yield (%) Colour m.p. (�Q

57 65 dark brown 161-163 (dec.)

58 70 dark green 183-185 (dec.)

55 ‘ II.2.5 Molecular Structures of [M{C(SiMe3)2C5H4N-2}2] (M = Fe 55，Co 56) and [Co{(CHSiMe3)C9H6N-8}2] (58)*

The molecular structures of compounds 55 and 56 were determined by single crystal X-ray diffraction studies. Projection of the molecules with atom numbering schemes are shown in Figure II-1 and 11-2 for compounds 55 and 56, respectively.

Selected bond distances (A) and angles (。) of both compounds are listed in Table 11-6.

Complex 55 consists of discrete species in a monoclinic space group Pl^lc (No.

14). The iron(II) center has a highly distorted tetrahedral coordination environment with each of the two alkyl ligands acting in a C,N-chelating manner. Coordination from the pyridyl nitrogens is believed to impose a stabilizing effect on the metal complex. The observed Fe-C bond distances are 2.139(7) and 2.154(8) A, which are somewhat longer than the average Fe-C single bond distance of 2.065 A in [FeEt2(bipy)2] (32)9 and 2.024

A in the dinuclear complex [{Fe(Mes)2}2] (39a) 21 They are also somewhat longer than that of 2.083(9) and 2.104(9) A in [{Fe(Ar^.P02}] (39b).22b Interestingly, they are similar o to those of 2.141(5) and 2.156(5) A in the monomeric dimesityl complex [Fe(Mes)2(py)2]

(39c).22b The relatively long Fe-CCsp2) q- bonds in 39c may be attributed to the steric hindrance of the mesityl groups which contain ortho substituted methyl groups.

The Fe-N distances of 2.111(8) and 2.135(5) A are significantly longer than those of 1.937(2) and 1.943(2) A in [FeEt2(bipy)2] (32).9 However, they are only slightly shorter than those of 2.169(8) and 2.179(7) A in the dimesityl complex [Fe(Mes)2(py)2]

(39c)22b.

The bite angle ZC-Fe-N of 67.2(3) and 66.8(2)。in 55 are similar to the corresponding angles of 67.3。，67.1。，59.8。，59.9。in [Mg(R2)2] (15),51 [Zn(R2)2] (16)，5l

[Cd(R2)2] (17)51 and [Sn(R2)2] (18)52 where the four latter complexes adopt a tetrahedral geometry. The large C-Fe-C angle of 160.4(3)。in 55 is probably due to steric repulsion of trimethylsilyl groups on the two a carbons.

Complex 56 exhibits a totally different molecular geometry, a square planar geometry. The two alkyl ligands "R^ coordinate to the cobalt(n) center in a trans fashion.

* Dimeric compounds [{M(R4)2}2] (M = Fe 61, Co 62) have recently been structurally characterized. Please refer to reference 53.

56 ‘ ⑵ 2)

C(3)l NClJ ens) \

Sid) I c(9) €r I c(i9) \ 議）I © C(7) C(21)

• Figure II-1. Molecular Structure of IFetCCSiMcj),^! (55) C(9)Q C^) yP™

/ >JC(6) Cd^ / V cm^f ^ UC(3a)

C⑷ / C(12a) J \c(9o)

cmoi c(8。）

，. Figure 11-2. Molecular Structure of [Co(C(SiMe3)2C5H4N-2) J (56) Table 11-6, Selected Bonds Distances (A) and Angles (°) for Compounds 55 and 56,

[Fe{C(SiMe3)2C5H4仏2h] (55)

Fe-C(l) 2.154(8) Fe-N(l) 2.135(5)

Fe-C(13) 2.139(7) Fe-N(2) 2.111(8)

C ⑴-C(2) 1.489(9) C(2)-N(l) 1.347(10) C(13)-C(14) 1.469(12) C(14)-N(2) 1.340(13)

C ⑴-Si(l) 1.861(8) C(l)-Si(2) 1.841(7)

C(13)-Si(3) 1.843(7) C(13)-Si(4) 1.869(8)

C(l)-Fe-N(l) 66.8(2) C(13)-Fe-N(2) 67.2(3)

C(l)-Fe-C(13) 160.4(3) N(l)-Fe-N(2) 116.3(2)

C(l)-Fe-N(2) 124.1(3) N(l)-Fe-C(13) 125.4(3)

C(l)-C(2)-N(l) 112.7(6) C(13)-C(14)-N(2) 113,6(9)

Si(l)-C(l)-Si(2) 117.1(4) Si(3)-C(13)-Si(4) 119.2(4)

[Co{C(SiMe3)2C5H4N-2} 2] (56)

Co-C(6) 2.092(6) Co-N(l) 1.923(4)

Co-C(l) 2.474(5) C(6)-Si(l) 1.878(6)

C(6)-Si(2) 1.858(5) C(l)-C(6) 1.482(7)

C(l)-N(l) 1.345(8)

C(6)-Co-N(l) 69.3(2) C(6)-Co-C(6a) 180.0

N(l)-Co-N(la) 180.0 Co-C(6)-Si(l) 117.6(2)

Co-C(6)-Si(2) 108.3(3) N(l)-C(l)-C(6) 107.9(4)

Si(l)-C(6)-Si(2) 119.7(3)

59 Square planar structures were also found in a number of heteroleptic cobalt(II) aryl complexes of the type [CoR2(PR丨3)2] where "R is an m/z6»-substituted phenyl group.3

Chart and Shaw^ suggested that the adoption of a square planar geometry for cf organocobalt(II) complexes is attributed to the large gains in ligand field stabilization energy (L.F.S.E.) over other molecular geometry.

/ p 0x2-y2

XT\ 久 y / ^ ^ ^―i 久y c/xz dyz \N \\ \) ^如 ~~\ 八E \ dz2 \\ \\ - Cfx2.y2 Square planar Co^ Tetrahedral Co^

The observed Co-C bond distance is 2.094(6) A which is larger than that of

1.931(5) A in [Co(C6F5)2(ti6-C6H5CH3)]54 but comparable to 1.994(2) A in trans-

:Co(mesityl)2(PPhEt2)2].55 The longer Co-C bond distance in 56 than that of

[Co(C6F5)2(ti6-C6H5CH3)]54 may be attributed to the steric demand of the two bulky

ligands R2 in 56. The observed Co-N distance in 56 is 1.919(4) A.

The biting angles ZC-Co-N of 69.6(2) in 56 is larger than that of compounds

15，51 16,51 1751 and 18.52 We interpret this as a consequence of the adoption of square

planar coordination geometry in our title compound 56.

Attempts to obtain good quality crystals of compound 57 for X-ray analysis has so

far been unsuccessful. On the other hand, recrystallization of compound 58 from warm

toluene yielded large rod-like crystals suitable for X-ray diffraction studies.

The molecular structure of [C'o{ (CHSiMe3)C9H6l^-8 h] (58) with atom numbering

scheme is shown in Figure 11-3. Selected bond distances (A) and angles of 58 are listed in

60 ‘ 、 C(24) ^^^ C(25)

C(13) 衫⑶ 2)

• ， Figure II-3. Molecular Structure of [Co((CHSiMe3)G;H,N-8)2l (58) Table II-7. Selected Bonds Distances (A) and Angles (。）for Compound 58.

[Co {(CHSiMe3)C9H6N-8}2] (58)

Co-C(l) 2.009(4) Co-N(l) 1.930(2)

Co-C(14) 2.041(4) Co-N(2) 1.940(2)

C(l)-Si(l) 1.883(3) C(14)-Si(2) 1.863(3)

C(l)-C(9) 1.476(4) C(14)-C(22) 1.485(4)

C(9)-C(10) 1.395(3) C(22)-C(23) 1.420(3)

C(10)-N(l) 1.390(5) C(23)-N(2) 1.380(5)

a;i)-Co-N(l) 83.4(1) C(14)-Co-N(2) 83.8(1)

N(l)-Co-N(2) 179.5(1) C �-Co-C(14 ) 179.6(1)

C(14)-Co-N(l) 96.7(1) C(l)-Co-N(2) 96.1(1)

C(9)-C(l)-Co 106.7(2) C(22)-C(14)-Co 105.2(2) C �-C(9)-C(10 ) 115.2(3) C(14)-C(22)-C(23) 115.5(3)

C(9)-C(10)-N(l) 114.9(2) C(22)-C(23)-N(2) 115.3(2)

C(10)-N �-C o 113.9(1) C(23)-N �-C o 113.6(1)

62 Table II-7. Compound 58 crystallizes in a monomelic space group Pn (No.7). Like compound 56, the two alkyl ligands [(CHSiMe3)C9H5N-8]" in 58 bound to the cobalt center in a C,N-chelating fashion constituting a square planar coordination geometry around the metal center (S°Co = 360°). The two SiMe] groups, one from each alkyl ligand, are oriented trans to each other. The Co-C bond distances are 2.009(4) and

2.041(4) A which are longer than that of 1.931(5) A in [Co(C6F5)2(ti6-C6H5CH3)]54 but are slightly shorter than that of 2.094(6)人 in 56. These distances are comparable to that of

1.994(2)人 in y^7"s-[Co(mesityl)2(PPhEt2)2]55 and the terminal Co-C bonds of 1.985 A

(av.) in the dimeric complex [{Co(mesityl)()i-mesityl)}2] (49).29

The Co-N distances in 58 are 1.930(2) and 1.940(2) A, slightly longer than that of

I.919(4) A in 56.

The bite angles ZC-Co-N of 83.4(1) and 83.8(1)。in 58 are significantly larger than that of 69.6(2)° in 56. This is attributed to the formation of a less strained five-membered metallacycle ring in 58.

II.2.6 Spectroscopic and Magnetic Properties of Compounds 55-58

Mass Spectra:

The mass spectra of metal dialkyls 55 and 56 (E.L, 70 eV) displays the

mononuclear molecular ion [M(R2)2]+’ indicating that the compounds are monomelic in

vapour phase as they are in solid phase. The mass spectrum of 55 displays the molecular

ion m/z = 528 in low relative abundance (16%) and other major fragement peaks due to

[M-R2]+ (291, 9%), [R2]+ (236, 20%), [R2-CH3]+ (222, 100%), [R2-2CH;3]+ (206’

17%), [SiMe3]+ (73,31%).

The mass spectrum of 56 shows the molecular peak at m/z = 531 in a higher

relative abundance (36%). Other main peaks are ascribed to the molecular fragments which

includes [M-R2]+ (296, 52%), [R2]+ (236, 31%), [R2-CH3]+ (222, 100%), [RZ-SiMeg^

(162, 28%), [SiMe3]+ (73，36%).

63 Both mass spectra of [Fe(R3)2] (57) and [CO(R3)2] (58) shows their respective molecular ion peak at m/z = 484 (4.8%) and 487 (9.7%) indicating that both compounds are monomelic in vapour phase. The other major peaks for 57 are [R3]+ (214, 25%); [R^-

CH3]+ (199，100%); [R3-2CH3]+ (184, 25%); [SiMe3]+ (73，27%) and for 58 [M-

SiMe3]+ (413, 37%); [M-2SiMe3]+ (341，100%); [R3]+ (199, 90%); [SiMe3]+ (73，32%).

NMR Spectra:

The iH NMR spectrum of compound 55 was recorded in benzene solution at ambient temperature. Owing to the paramagnetic nature of the d^ iron(II) compound, the iH NMR signals are broad and exhibited significant isotropical shift to downfield position.

The signals have been assigned on the basis of their relative intensities. Signal due to

SiMe3 group was assigned at 11.04 ppm (broad, 18 H) and those due to the aromatic protons were assigned 27.07 ppm (broad, IH), 32.00 ppm (s，1 H), 71.13 ppm (s, IH), and 84.48 ppm (broad, 1 H).

No iH NMR signals were observed in the case of the cf complex 56 as expected, presumably because of the broadness of the resonance signals.

Broadness of the resonance signals also hampers the recognition of any ^H NMR signals for compounds 57 and 58.

UV-Vis Spectra:

The iron(II) dialkyl 55 with UV -vis bands: [X^^^^ (nm), £ (M-icm_i) in parentheses]

406 (sh, 765), 342 (7.9 x 10^), 272 (1.4 x lO^), and 223 (2.6 x lO” are observed. The first one, with a lower molar absorptivity, is assigned to d-d transition and the latter three are probably due to charge transfer and ligand-ligand transitions.

The deep red coloured cobalt(II) dialkyl 56 has UV bands at 532 (1.5 x lO^), 368

(2.8 X 103)，265 (4.4 x ICP)，and 231 (4.6 x 103). The first two bands are assigned to d-d transitions. A point charge splitting diagram can be constructed for the square planar cobalt(II) complex 56.

64 ；r 々-y2

~\ ^ ^xy ~ 3/5 Ao

-4 《2 〜Vio If If :

Square planar Co^

The electronic configuration is a typical one for most square planar (f complexes. By taking this approximation, the Aq value of 56 estimated from the low energy absorption maximum is 19,000 cnr、This value is consistent with those found in literatures where bands near 20,000 cm-l have been found on electronic spectra of some square planar complexes.56’57

The UV spectrum of compound 57 is similar to compound 55 with absorption bands at 316 (4.6 x ICP)，289 (sh, 4.7 x 103)，275 (4.8 x 103)，and 226 (4.8 x ICP). All bands are probably due to charge transfer and ligand-ligand transition.

Compound 58，which is dark green coloured, has UV-vis bands at 615 (2.8 x

103), 440 (4.1 X 103)，283 (5.4 x ICP)，273 (5.4 x 103)，and 227 (5.6 x ICP). The former two bands may be attributed to d-d transitions of the unpaired electron on the cf metal center.

Magnetic Susceptibility:

Magnetic moment of dialkyl complex 55, as measured by Evan's method in C5D5 solution at 308K, is 4.24 jig. The value for 55 falls somewhat below the normal range of

5.1-5.2 Jig for tetrahedral iron(II) complexes is presumably due to some spin-mixing.

65 For compound 56, magnetic moment measurement, by Evan's method again in

solution at 308K, gives a value of 3.11 m-B- The value is somewhat higher than the normal range of 2.3-2.7 jig for some Co(II) complexes reported in the literature.^ We believe this may be a consequence of spin-orbit contribution. In fact, value as high as 2.9

M-B FOR some planar cobalt(II) complexes with chelate ligands have been reported.58

The magnetic moment of 3.80 iig has been observed for compound 57.

For compound 58, the magnetic moment was found to be 2.40 jig. The value lies within the normal literature values of 2.3-2.7 iig^ implying that spin-orbit coupling is less important in the case of compound 58. Table 11-7 lists selected spectroscopic and magnetic data for 55-58.

Table II-8. Selected Spectroscopic and Magnetic Data of 55-58.

Compound (I^b) UV-vis^ X(s), nm 力

[Fe(R2)2] 55 4.24 223 (2.6 x lO^), 272 (1.4 x lO^), 342 (7.9 X 103), 406 (sh, 765)

[CO(R2)2] 56 3.11 231 (4.6 x ICP)，265 (4.4 x 103)，368 (2.8 X 103)，532 (1.5 X 103)

[Fe(R3)2] 57 3.80 226 (4.8 x 103), 289 (sh, 4.7 x ICP), 316 (4.6 X 103)

[Co(R3)2] 58 2.40 227 (5.6 x 103), 273 (5.4 x lO^), 283 (5.4 X 103), 440 (4.1 X 103)，665 (2.8 x 103)

a: In CgDg solution, Evan's method, b: In THF solution, sh = shoulder.

11.2.7 Electrochemistry of [CO(R2)2] (56) and [CO(R3)2] (58)

The cyclic voltammetry of compound 56 was studied. All potentials were internally referenced to the FeCp2'^/FeCp2 reference redox couple in THF (Appendix I).

The result of the c.v. were reported as anodic peaks potential (Ep^), or cathodic peak

66 potential (Ep。)，for irreversible processes. For reversible or quasi-reversible process, the half wave potential [Ei" 二 l/2(EpC + Ep^)] was reported.

A single sweep c.v. of compound 56 (Figure II-4), sweeping initially in the anodic direction, shows a reversible one electron reduction at Ey2 = -2.59V (at 100 mVs'\ AEp =

90 mV) giving the Co(I) species:

[Co(R2)2] ^ [CO(R2)2]-

An irreversible oxidation peak with Ey2 = 0.86V (at 100 mVs"^) was recorded. The latter is presumably due to the oxidation of 56 to give the unstable Co(III) species:

[Co(R2)2] — [CO(R2)2]+

Sweep-rate dependence shows that the Co(I) species is completely stable on the c.v. time-

scale.

The c.v. of compound 58 shows one reversible one-electron cathodic wave at Ey2

=-2.33 V (at 100 mVs"^) and one anodic peak superimposed on the fringe of background discharge (THF decomposition) (Figure 11-5). The reduction wave corresponds to reduction of 58 to the corresponding Co(I) species:.

[Co(R3)2]2 - [CO(R3)2]2-

The oxidation peak is presumably due to oxidation of 58 to give the corresponding cobalt(ni) species.

Unfortunately, electrochemical study of compound 55 was found to be unsuccessfully. Reaction between 55 and the supporting electrolyte has occurred before

the cyclic voltammetry experiment was performed. No evidence of oxidation or reduction

can be observed on the c.v. of 57 down to the limits of background decomposition.

67 ‘ .1

丨丨丨I丨丨丨I I h I I I I I I

0.4 0 -0.4 -2.4 -2.8 -3.0

E/V versus

Figure 11-4. Cyclic voitammogram of 4 x 10'^ mol dm"^ (56): 0.2 mol dm-^ [NBii4J[BF4j in THF. Sweep rate = 0.100 V s'� : The Co^/Co^ and Co辽/Co工 reactions are shown separately

•：丨；! i j • I 68 : ~T~~I 11 I1 I i i 1 j 1

2.0 1.0 0 -1.0 -2.0 —— w « • £7V versus [FeCTi-qH^)^^/^

Figuren-5. Cyclic voltammogram of 4 x 10.3 mol dm.] (5S). 02 - moi dm-J [NBUJCBF^J in THF. Sweep rate = 0.100 V s'^

69 ‘ II.2.8 Reactivities of [lV^{C(SiMe3)2C5H4N-2}2] (M = Fe 55，Co 56)

(A) Reactivities of [Fe[C(SiMeC5H4N-2}2 ] (55)

(i) Reaction of 55 with ArSH:

Subsequent reaction of [Fe(R2)2] 55 with two equivalents of ArSH (Ar = 2,4,6-

(BU3C6H2) in hexane yielded the monomelic iron(II) dithiolate complex [Fe(SAr)2(R^H)]

63 (Scheme II-6). The reaction was carried out at 0 °C during which a hexane solution of ArSH was added dropwise to a stirred solution of compound 55 in hexane. After the addition, the resultant pale yellowish-brown solution was further stirred at ambient temperature for 8 h. Filtration followed by cooling to -30 "^C gave the dithiolate complex

63 in 65% yield as a yellow crystalline compound which decomposes at >182

55 + 2 RHQ^EH ~~^ -V \eZ 干

.N^CH{SiMe3)2

63: E = S, R = feu 66: E = 〇，R = Me

Scheme 11-6

The reaction of metal amides with thiophenols constitutes a convenient route to the corresponding metal thiolate complexes (vide zVz/hz).49,59 The reaction of 55 with two equivalents of ArSH to give 63 provides an alternative route for the preparation of monomelic metal thiolate compounds via metal alkyls. Unlike [{Cd(SAr)2}2]’59a metal dithiolate complexes 50-52 remain as dimer in solution and do not dissociate to monomers. The Lewis acid center of compounds 50-52 are free from nitrogen coordination in spite of the presence of a strong base NH(SiMe3)2 in solution.49a The mechanism for the formation 63 is not clearly known. However, two proposed routes may be suggested for the formation of 63 (Scheme II-7). In route a, a three-membered

70 /。广 SAr \

/ 64 V^rSH /route a \

,C、 yN、 / \ 2 Z A + ArSH / [Fe(SAr)2(R'H)] �Ze\c J \ / 63 5 5 \ route b / \ � /+ ArSH

^ \ 广\ /N^CH / Fe�

、/^、Ar

N CorR =、人？、;3二，3 ； Ar =

Scheme II-7 intermediate [Fe(R2)(SAr)] 64 may first form, which is followed by the protonation of the second alkyl ligand to give a neutral R^H molecule whose pyridine ring remained coordinated to the iron(II) center to achieve steric congestion around the metal center. On the other hand, an intermediate in which the first protonated ligand (designated as [R^H]^) may still remain coordinated to the Fe(n) center forming a four-coordinated intermediate

:Fe(R2)(SAr)(R2H)I] 65 as proposed in route b. Protonation of the second alkyl ligand with concomitant leaving of [R2H]I from the coordination sphere of Fe(II) leads to 63.

Unfortunately, various attempts to isolate the proposed intermediates 64 or 65 were unsuccessful.

The three bulky ligands (2 ArS" and R^H) are believed to play a vital role in preventing the association of the neighbouring Fe(SAr)2 moieties and thus resulting in the formation of the mononuclear three-coordinated iron(11) dithiolate complex 63.

Recently, the studies of transition metal complexes with low coordination number

(e.g. 2 or 3) have attracted much attention.46，49，59’60 Examples of these complexes include the transition metal amides [{M[N(SiMe3)2]2}2] (M = Mn, Fe, Co);60 the thiolates

[{M(SAr)2h] (M = Mn 50，Fe 51, Co 52) 49a [Fe(SAr’)[N(SiMe3)2]] 53 49b

[Fe(SA02] 54;49b and the alkoxides or aryloxides [{M(0Ar)2h] (Mn, Fe),

[M(OCPh3)2L2] (M = Mn, L = Py; M = Fe, L = THF)46d The impetus of the studies of transition metal thiolate complexes with low coordination number mainly comes from the disclosure of the crystal structure of the nitrogen-fixing FeMo cofactor of Azotobacter vinelandii.^^ The iron centers in the cofactor molecule are ligated by sulphur atoms, which are probably three-coordinated. The unknown ligand Y may be probably an N- or O- containing species (Figure II-6).61

72 ‘ Hisai95 GInaigi . -O2CCH2

/、、、， Fc, / \ \ / 、、、 \/ \ O—C Cys--Sy -Fc/_M-oJ：⑶^CH^CO!. \ /、、、、/V—NSi

Figure II-6. Structural Model of the FeMo-cofactor model.

By employing sterically demanding alkyl substituents, a number of late transition metal thiolate complexes with low coordination number have been successfully synthesized. For instance, the metal centers in dimeric complexes 50-5249a are three-coordinated and complexes 53-5449b are even monomeric two-coordinated species. However, among all late transition metal thiolate complexes which have so far been reported in literatures, only compounds 53 and 54 are neutral monomeric species.49b The others are either anionic

species or neutral species with a high degree of association (2 or above). Being a neutral three-coordinated monomeric iron dithiolate complex, our current compound 63 may be the

second neutral monomeric transition metal thiolate complex next to compounds 53 and 54

and will potentially serve as a model compound and shed light on the understanding of the

structure and reactions of the FeMo cofactor of the nitrogenase.

Molecular Structure of [Fe(SAr)2(R2H)] 63:

The molecular structure of [Fe(SAr)2(R2H)] (63) with the numbering scheme is

shown in Figure 11-7. Selected bond distances (人）and angles (°) are listed in Table II-9.

The dithiolate complex 63 possesses a trigonal planar coordination geometry

around the central metal atom [Z。Fe 359.9(5)]. Owing to their bulkiness, the three ligands

are oriented in such a way that the complex adopts a propeller-like structure. The Fe-S

bond distances in 63 are 2.259(4) and 2.261(3) A. These are similar to the terminal Fe-S

distances of 2.256(3) A, but shorter than the bridging Fe-S distances of 2.365(3) and

73 ‘ C(9) © C(12) ^ cm) P

Figure 11-7. Molecular Structure of [Fe(SQH2^Bu3-2,4,6),(CM(SiMe3),C5ir,N-2],) (63) Table II-9. Selected Bond Distances (A) and Angles (。）for Compound ^

[Fe(SC6H2屯U3-2，4,6)2{NC5H4CH(SiMe3)2-2}] (63)

Fe(l)-S(l) 2.259(4) Fe(l)-N(l) 2.086(9) Fe(l)-S(2) 2.261(3) N(l)-C(l) 1.35(1)

N(l)-C(5) 1.35(1) S(l)-C(13) 1.80(1) S(2)-C(31) 1.80(1)

S(l)-Fe(l)-N(l) 118.7(2) Fe(l)-S(l)-C(13) 113.1(4) S(2)-Fe(l)-N(l) 123.4(2) Fe(l)-S(2)-C(31) 101.0(3) S(l)-Fe(l)-S(2) 117.8(1) Fe(l)-N(l)-C(5) 125.1(6) Fe(l)-N(l)-C(l) 116.5(8)

75 ‘ 2.366(3) A in complex [{Fe(SAr)2}2] 51.49a With comparison to the monomelic thiolate complexes [Fe[N(SiMe3)2](SAr')] 53 and [Fe(SAr')2] 54 [Ar' = 2,6-Mes2C6H3SH, Mes

二 2,4，6-Me3C6H2]，reported by Power et al^^^, the Fe-S distances in the current complex

63 are shorter than that of 2.308(2) and 2.314(2) A in 53 49b but is comparable to that of

2.277(2) and 2.275(2) A in 54.49b The Fe-N distance in 63 is 2.086 A, which only differs from those found in the dialkyl complex [Fe(R2)2] 55 by 0.03 A but longer than that of 1.913(6) and 1.923(5) A in 53.49b The larger N(l)-Fe-S(2) angle of 123.4(2)。 over the remaining two angles in the FeSzN plane viz. 118.7(2)° and 117.8(1)° in 63 is probably due to the repulsion of the C(SiMe3)2 group at the ortho position of the pyridine ring.

feu

A /

a= 117.81。 118.72° c= 123.42�

76 Spectroscopic and Magnetic Properties of Compound 63:

Elemental analysis (C, H, N) data for 63 are consistent with its composition.

However, peak due to the molecular ion was not observed. Only the peak corresponding to molecular ion for (ArS)2 {m/z 554) was observed. It may be inferred that 63 undergoes reductive elimination to the disulphide in the vapour phase. Similar example has been observed on the mass spectrum of [Bi(SAr)3].62 Other major peaks on the mass spectrum of 63 are due to the organic fragments such as [ArS]+，[R2H]+, [R2H-CH3]+, and

[SiMe3]+.

The UV-vis spectrum of 63 shows only one high energy electron transition at 355 nm (e = 1.8 x 10^ M'^cm"^) superimposed on the fringe of the charge transfer band.

Similar to 56, by taking a point charge modelling approach, the electronic distribution diagram for the trigonal planar complex 63 can be constructed as shown below.

=5=4=々-y2 d^y

4——

High spin trigonal planar Fe^

The magnetic moment of compound 63 is 5.11 jig as determined by Evan's method in C5D5 solution at 308K. This value is close to the spin only value of 4.9 iig which indicates that the iron(II) center appears most likely to adopt a high-spin cfi configuration with four unpaired electrons.

77 ‘ Electrochemistry of 63:

The cyclic voltammogram of 63 shows a well resolved but completely irreversible oxidation peak at Ep。二 -0.32 V and a shoulder at Ep。=： 0.67 V (at 100 rnVs"^) (Figure II-

8). The peak at -0.32 V is presumably due to the formation of a Fe(III) species:

[Fe(SAr)2(R2H)] -> [Fe(SAr)2(R2H)]+

(ii) Reaction of 55 with Ar^WH:

In order to further probe the reactions of the iron dialkyl [Fe(R2)2] 55 its reactions with other protic sources were studied. In addition to thiophenol, compound 55 also reacts readily with the bulky phenol ArMeQH (Ar^e 二 4-Me-2,6-屯uzCsH�） inon-coordinatinn g solvent. Inferentially, the product which exists as an off-white, very air sensitive solid may be the corresponding iron(II) diaryloxide 66 (Scheme II-6). Its melting point was found to be 129-131 °C which may prove that the off-white compound should not be the starting materials. Both mass spectrometry and elementary analysis confirm the presence of free ligand molecule (R^H). The free ligand (R^H) was presumed to remain coordinated to the iron(n) center similar to the one observed in the case of the dithiolate complex 63.

78 I \\ 、

1-0 0 -1.0 丨一2.0 Efy versus Figure II-8. Cyclic vol tarn mogram of 4 x 10.3 mol dm"^ (63). 02 moi dm-j [NBu^JPFJ in THF. Sweep rate = 0.100 V s"^.

79 ‘ (HO Reaction of 55 with other reagents:

Reactions of PhgCOH and 出uOH with [Fe(R2)2] 55 did occur but no isolable

compound could be obtained (Scheme II-8).

The corresponding alkoxides probably No reaction polymer

\\Ar(Me)P H PhgCOH/^ / Z)r feuOH

[Fe(R2)2] 55

PhC=N

1 r

No reaction

Scheme II-8

In both cases, only a pale greenish blue precipitate was obtained. The products, probably polymeric in nature, are insoluble in common hydrocarbon solvents making their characterizations difficult to be carried out. The polymeric nature of transition metal alkoxides have been reported by other workers by using sterically less demanding alkyl substituents as discussed in the previous section.36

Reaction of manganese(n) dialkyls (MnR2)n (R = CHaCMe]，n =4; CHsCMe^Ph, n = 2; CH2SiMe3, n = oo ) with 出U2PH was reported to give the corresponding phosphido-bridged dimers [{MnR(|Li」Bii2P)}2] 63 Attempted reaction of iron dialkyl complex 55 with the phosphine ArP(Me)H was unsuccessful even at refluxing hexane and only the starting materials were recovered. The unsuccessful synthesis of the

80 ‘ corresponding iron(II) phosphido complex may be attributed to the steric hindrance of the two starting materials and the low acidity of the P-H bond.

It has been reported that homoleptic iron(II) diaryl [{FeMessh] (39) reacted readily with PhCN to give the insertion product [{(PhCN)(Mes)Fe}2{!^-N=CPhMes)2]

(41) in which the irons were bridged by two diaryl imino groups.22 in contrast, attempted reaction of [Fe(R2)2] (55) with PhCN in benzene was unsuccessful and only compound

55 was recovered after the reaction. We believe that the inertness of compound 55 towards PhCN by insertion is also attributed to the steric effect of the two alkyl ligands R2 on 55.

(B) Reactivities of [C'o{C(SiMe^)2CsH4N•2)2 ] (56)

The reactivities of the cobalt dialkyl [CO(R2)2] 56 towards protic reagents such as

ArMeOH and ArSH are strikingly different from those of the iron analogue 55 and thus are worth mentioning. Compound 56 does not react with ArMeQH，PhgCOH，and ArSH even though Co-C a bonds are generally believed to be vulnerable to attack by protic reagents (Scheme II-9).

81 The corresponding alkoxides, probably 一y; No reaction

^^ or PhaCOH [C0(R2)2]

PPh3

V No reaction Scheme II-9

In each case, the starting materials were recovered quantitatively。The inertness of 56 is related more or less to its structure. According to the molecular structure of 56, two SiMe] groups, one from each ligand R2，are located in the vicinity of the idealized fifth and sixth octahedral sites on both sides of the square planar complex 56. This shields the central cobalt metal from being attacked by bulky reagents such as, the alcohols and thiophenol in our present discussion. With the less bulky ^BuOH, reaction did occur giving a pale blue precipitate which was insoluble in all common solvents. The insolubility of this substance hampers its characterizations. It is conceivable that the solid is the corresponding cobalt(II) alkoxide with composition of {Co(OBu)2}n which exists as polymer.

Attempted addition of the Lewis base PPhg onto the coordination sphere of cobalt(II) in order to achieve the corresponding adduct similar to that of Chatt and Shaw,

:CO(R2)2(PR'3)2]，3 were unsuccessful and only the starting materials were recovered.

82 11.3 EXPERIMENTALS FOR CHAPTER II*

Materials:

Anhydrous C0CI2 was prepared by standard procedure.64 FeCl] (Fluka) was used as purchased. 4-Me-2,6」Bu2C6H20H (Aldrich) was recrystallized from hexane before use. [{R2Li)2], [8-C9H6NCH(SiMe3)Li.TMEDA], and 2,4，6」Bu3C6H2SH were prepared according to literature procedure.50-52，65

Synthesis of Compounds:

Synthesis of [Fe{C(SiMe3)2C5H4r^-2}2]，55. An ether solution (30 ml) of

[{R2Li}2] (1.57 g, 3.2 mmol) was added dropwise to a stirring suspension of FeCl〕（0.41 g，3.2 mmol) in the same solvent at 0 The suspension was further stirred at room temperature for 20 h. The grayish brown precipitate (tested positive for Li+) was filtered and the filtrate was conc. in vacuo followed by cooling to -30 for 1 d yielded 55 as yellow crystals (yield 1.35 g, 80%). Mp: 110-112 MS: m/z (%) = 528 (16) [M]+，291

(9) [M-R2]+, 236 (20) [R2]+，222 (100) [R2-CH3]+. Anal. Found: C，55.82; H，9.05; N,

5.43%. Calc. for C24H44N2Si4Fe: C, 54.51; H, 8.39; N，5.30%. (Owing to the extreme air sensitivity of the compound, reproducible elemental analysis data cannot be obtained).

Synthesis of [Co{C(SiMe3)2C5H4N-2}2], 56. A solution of [{机”之](1.21 g,

5.0 mmol) in 30 ml ether was added to a stirring suspension of C0CI2 (0.32 g, 2.5 mmol) in 20 ml ether at 0 The reaction mixture acquired a deep red colour immediately. After being stirred at room temperature for 8 h, a deep red supernatant solution with grey precipitate was obtained. This was filtered and the filtrate was concentrated in vacuo.

Deep red crystals of the title compound formed at -30 °C (yield 1.13 g, 85%). Mp: 116-

118。C. MS: miz (%) 二 531 (36) [M]+，296 (52) [M-R2]+，236 (31) [R2]+, 222 (100) [R2-

* Please refer to Appendix I for general procedures.

83 CH3]+. Anal. Found: C, 54.13; H, 8.30; N, 5.29%. Calc. for C24H44N2Si4Co: C,

54.24; H, 8.29; N, 5.27%.

Synthesis of [Fe{CH(SiMe3)C9H6N-8}2], 57. To a stirring suspension of FeCl2

(0.38 g, 3.0 mmol) in ether (20 ml) at 0。C was added a solution of [R3Li(TMEDA)] (2.02 g, 6.0 mmol) in ether (40 ml). After the addition has been completed, the dark brown suspension was further stirred at room temperature for 8 h‘ The grey precipitate was filtered and the resultant solution was concentrated in vacuo followed by cooling to -30 °C to give the title compound as dark brown needle-shaped crystals (yield 0.94 g, 65%). Mp:

161-163。C (dec.). MS: m/z (%) = 484 (4.8) |>1]+’ 214 (25) [R3]+, 199 (100) [R3-

CH3]+. Anal. Found: C, 62.73; H, 6.52; N, 5.59%. Calc. for CasHssN^SizFe: C,

64.45; H, 6.66; N, 5.78%.

Synthesis of [Co{CH(SiMe3)C9H6N-8}2], 58. A solution of [R3Li(TMEDA)]

(1.35 g, 4.0 mmol) in ether (40 ml) was added slowly to a stirring suspension of C0CI2

(0.26 g, 2.0 mmol) in ether (20 ml) at 0 The suspension acquired a dark green colour immediately. The solution was allowed to be stirred at room temperature for a further 8 h。

The grey precipitate was filtered and the filtrate was concentrated under reduced pressure.

Cooling of the resultant solution to -30 °C for one night yielded 58 as dark green crystals

(yield 0.68 g, 70%). Mp: 183-185。C (dec.). MS: miz (%) = 487 (9.7) [M]+，413 (37)

[M-SiMes]"^, 341 (100) [M-2SiMe3]+，199 (90) [R3]+. Anal. Found: C, 63.63; H, 6.56;

N, 5.69%. Calc.. for C26H32N2Si2Co: C, 64.04; H, 6.61; N, 5.74%.

Synthesis of [Fe(SQH2^Bu3-2,4,6)2[CH(SiMe3)2C5H4N-2]], 63. A hexane solution (40ml) of 2,4,6-^Bu3C5H2SH (0.41g, 1.47 mmol) was added dropwise to a stirring solution of 55 (0.38g. 0.73 mmol) in the same solvent at 0 °C. After addition had been completed, the resultant pale brown solution was further stirred for 8 h at ambient temperature. It was filtered and the filtrate concentrated in vacuo. Upon cooling to -30 °C for 1 d，yellow crystals of 57 was obtained (yield 0.40g，65%). Mp: 182-185 (dec.).

84 ‘ Anal. Found: C, 67.88; H, 9.70; N, 1.60%. Calc. for CqsHgiNSsSisFe: C, 67.96; H, 9.62; N, 1.65%.

Synthesis of [Fe(OC6H2-4-Me-2，6"Bu2)2[CH(SiMe3)2C5H4N-2]]，66. A solution of 4-Me-2,6」Bu2C6H20H (0.46g, 2.08 mmol) in hexane (30 ml) was added dropwise to a stirring solution of 58 (0.55g, 1.04 mmol) in hexane (20 ml) at 0 °C The resultant clear yellowish-brown solution was kept being stirred at ambient temperature for 8 h. The solution was filtered and then concentrated in vacuo followed by cooling to -30 for one day to give 66 as colourless crystals (yield 0.32g, 41%). Mp: 129-131 Anal.

Found: C，69.75; H，9.96; N, 2.81%. Calc. for C42H69N02Si2Fe: C，68.91; H, 9.50; N，1.91%.

85 II.4 REFERENCES FOR CHAPTER II

(1) (a) Pearson, AJ. In Comprehensive Organometallic Chemistry; Wilkinson, G.;

Stone, F.G.A.; Abel, E.W.，Eds.; Pergamon: Oxford, U.K., 1982; Volume 8, Chapter 58.

(b) The Organic Chemistry of Iron; Koerner von Gustorf, E.A.; Grevels, F.W.;

Fischer, L, Eds.; Academic: London, U.K., 1978; Volume 1.

(2) Johnson, M‘D. In Comprehensive Organometallic Chemistry, Wilkinson, G.; Shone,

F.G.A.; Abel, E.W” Eds.; Pergamon: Oxford, U.K., 1982; Volume 4, Chapter 31.2.

(3) Chatt, J.; Shaw, B丄.J. Chem. Soc. 1961，285.

(4) Denney, D.B.; Davis, W.R. J. Organomet. Chem. 1970，24, 537.

(5) Davidson, PJ.; Lappert, M.R; Pearce, R. Chem. Rev. 1976，76, 219.

(6) Yamamoto, A. J. Organomet, Chem. 1986, 300, 347.

(7) (a) Bibler, LP.; Wojcicki, A. J. Am. Chem. Soc. 1966，88, 4862.

(b) Bruce, M.L Inorg. Nucl. Chem. Lett. 1967, J, 157; J. Organomet. Chem� 1967，10, 495.

(8) (a) Pankowski, M.; Samuel, E.; Bigorgne, M. J, Organomet. Chem. 1975, 97，105。

(b) Pankowski, M.; Samuel, E. J. Organomet. Chem. 1981, 221, C2L

(9) Lau, W.; Huffman, J.C.; Kochi, J.K. Organometallics 1982,1, 155.

(10) (a) Yamamoto, A.; Morifuji, K.; Ikeda, S.; Saito, T.; Uchida, Y.; Misono, A. J.

Am. Chem. Soc. 1965，87, 4652.

(b) Yamamoto, A.; Morifuji, K.; Ikeda, S.; Saito, T.; Uchida, Y‘； Misono, A. J.

Am. Chem. Soc. 1968，90, 1878.

(11) Churchill, M.R. Inorg. Chem. 1967, 6’ 185.

(12) Hitchcock, P.B.; Mason, R. Chem. Commun. 1967, 242.

(13) Su, S.R•； Hanna, LA.; Wojcicki, A. /. Organomet. Chem. 1970，27, 21.

(14) Stanley, K.; Baird, M.C. J. Am. Chem. Soc. 1977, 99，1808.

86 (15) Tolman, C.A.; Ittel, S.D.; English, A.D.; Jesson, J.R J. Am. Chem. Soc. 1978, 100, 4080.

(16) Bower, B.K.; Tennent，H.G. J. Am. Chem. Soc. 1972, 94, 2512.

(17) Spiegl, H.J.; Groh, G.; Berthold, H.J. Z. Anorg. Allg. Chem. 1973, 398, 225.

(18) Nast，R.; Urban, F. Z. Anorg. Allg. Chem. 1956’ 257, 17.

(19) Martynova, V.R; /. Gen. Chem. USSR 1962, 32, 2660; Zh. Obshch. Khim. 1962，32, 2702.

(20) Machelett, B. Z. Chem. 1976，16, 116.

(21) Miiller, H.; Seidel, W.; Gorls, H. J. Organomet. Chem. 1993，445, 133.

(22) Klose, A.; Solari, E.; Ferguson, R.; Floriani, C; Chiesi-Villa, A.; Rizzoli, C.

Organometallics 1993，72, 2414.

(23) Kemmitt, R.D.W.; Russell, D.R. In Comprehensive Organometallic Chemistry,

Wilkinson, a； Shone, RG.A.; Abel, E.W.，Eds.; Pergamon: Oxford, U.K., 1982;

Volume 5, Chapter 34.

(24) Saito, T.; Uchida, Y.; Misono, A.; Yamamoto, A.; Morifuji, K.; Ikeda, S.

Organomet. Chem. 1966, 6, 572.

(25) Komiya, S.; Bundo, M.; Yamamoto, T.; Yamamoto, A. /. Organomet. Chem. 1979, 174, 343.

(26) Schrauzer, G.N.; Kohnle, J. Chem. Ber. 1964, 97，3056.

(27) Clarke, D.A.; Grigg, R.; Johnson, A.W. Chem. Commun. 1966，208.

(28) Cope, A.C.; Gourley, R.N. J. Organomet. Chem. 1967, 8，527.

(29) Theopold, K.H.; Jerome, S.; Byrne. E.K.; Richeson, D.S. Organometallics 1989,

2001. (30) Knunyants, LL.; Aktaev, N.P.; Semenov, N.P.; SokoFskii, G.A. Dokl Chem.

(Engl Transl.) 1975, 279, 771.

(31) Klein, H.-F.; Karsch, H.H. Chem. Ber. 1976,109, 1453.

(32) Hitchcock, P.B.; Lappert, M.F.; Leung, W.-P.; Buttrus, N.H. /. Organomet. Chem. 1990, 394, 57

(33) Seidel, W.; Kreisel, G. Z. Chem. 1976，16, 115.

87 (34) Rieke, R.D. J. Org. Chem. 1979, 44, 3069.

(35) Tsou, T.T.; Kochi, J.K. J. Am. Chem. Soc. 1978,100, 1634.

(36) Bradley, D.C.; Mehrotra, R.C.; Gaur, D.P. Metal Alkoxides; Academic Press: New York, 1978.

(37) (a) Latesky, S.L.; Keddington, J.; McMullen, A.K.; Rothwell, LP.; Huffman, J.C. Inorg. Chem, 1985，24, 995.

(b) Chamberlain, L.; Keddington, J.; Huffman, J.C; Rothwell, LP. J. Chem. Soc” Chem. Commun. 1982, 85.

(38) (a) Lubben，T.V.; Wolczanski, P.T. J. Am. Chem. Soc. 1985,107, 701.

(b) Lubben, T.V.; Wolczanski, P.T.; Van Duyne, G.D. Organometallics 1984, 5, 977.

(39) Chamberlain, L.; Keddington, J.; Rothwell, LP.; Huffman, J.C. Organometallics 1982, 1, 1538.

(40) Horvath, B.; Horvath, E.G. Z. Anorg. Allg. Chem. 1979，457, 51.

(41) Hvoslef, J.; Hope, H.; Murray, B.D.; Power, P.P. J. Chem. Soc” Chem.

Commun. 1983, 1438.

(42) Coffindaffer, T.W.; Rothwell, LP.; Huffman, J.C. Inorg. Chem. 1983,22, 2906;

22, 3179.

(43) Chisholm, M.H.; Hoffman, D.M.; Huffman, J.C. Inorg. Chem. 1983，22, 2903.

(44) Horvath, B.; Moseler, R.; Horvath, E.G. Z. Anorg. Allg. Chem. 1979, 449, 41.

(45) Bochmann, M.; Wilkinson, G.; Young, G.B.; Hursthouse, M.B.; Malik, K.M.A. J.

Chem. Soc., Dalton Trans. 1980, 1863.

(46) (a) Murray, B.D.; Power, P.P. L Am. Chem. Soc. 1984,106, 7011.

(b) Olmstead, M.M.; Power, P.P.; Sigel, G. Inorg. Chem. 1986, 25, 1027.

Chem. 1987，26’ 1773.

(d) Bartlett, R.A.; Ellison, JJ.; Power, P.P.; Shoner, S.C. Inorg. Chem. 1991, 30,

2888.

(47) Dance, I.G. Polyhedron 1986，5，1037.

88 (48) Blower, P.L; Dilworth，LR. Coord. Chem. Rev. 1987, 76, 121.

(48) Krebs, B.; Henkel, G. In Rings，Cluster and Polymers of Main Group Transition

Elements, Roesky, H.W. Eds.; Elsevier: Amsterdam, 1989. (49) (a) Shoner, S.C.; Power, P.P. Angew. Chem. Int. Ed. Engl. 1991, 30, 330.

(b) Ellison, J.J.; Ruhlandt-Senge, K.; Power, P.P. Angew. Chem. Int. Ed. Engl. 1994, 33, 1178.

(50) 8-(CHSiMe3)C9H6N is a newly developed alkyl ligand. Law, L.T.C. M.Phil.

Thesis, The Chinese University of Hong Kong, 1994.

(51) Henderson, MJ.; Papasergio, R.L; Raston, CL.; White, A.H.; Lappert, M.F. J.

Chem. Soc., Chem. Commun. 1986, 672.

(52) Engelhardt, L.M.; Jolly, B.S.; Lappert, M.R; Raston, CL•； White, A.H. J. Chem,

Soc., Chem. Commun. 1988, 336.

(53) Leung, W.-P.; Weng, L.-H.; Mak, T.C.W. unpublished results.

Dimeric compounds [{Fe(R4)2}2] (61) and [{CO(R4)2}2] (62) [-R4 =

"CH(SiMe2^Bu)C5H4N-2] have recently been structurally characterized in our

laboratory. Both compounds 61 and 62 exist as dimer in the solid state. Each metal

center processes a distorted tetrahedral coordination environment. The terminal alkyl

groups are bound to the metal atom in a C^V-Ugating fashion whilst the bridging alkyl

groups spanning over two metal atoms via C,7V-ligation.

它uMegSi、 L

(^BuMe2Si)HC.

y I SiMe2 把 u

M = Fe61,Co 62

(54) Anderson, B.B.; Behrens, C丄•； Radonovich, L.L; Klabunde, K.J. J. Am. Chem.

Soc. 1976, 95, 5390.

(55) Falvello, L.; Gerloch, M. Acta Crystallogr,, Sect. B 1979，2547.

89 (56) Nicolini, M.; Pecile, C; Turco, A. Coord. Chem. Rev. 1966, 1, 133.

(57) Lever, A.B.P.; Lewis, J.; Nyholm, R.S. J. Chem. Soc. 1963，2552.

(58) Figgis, B.N.; Nyholm, R.S. J. Chem. Soc. 1954’ 12; 1959, 338.

(59) (a) Bochmann, M.; Webb, K.; Harman, M.; Hursthouse, M.B. Angew. Chem. Int. Ed. Engl 1990,29, 638.

(b) Bochmann, M.; Webb, K.J.; Hursthouse, MB.; Mazid, M. J. Chem. Soc., Dalton Trans. 1991, 2317.

(d) Bochman, M.; Bwembya, G.C.; Grinter, R.; Lu, J.; Webb, K.J.; Williamson,

D.L; Hursthouse, M.B.; Mazid, M. Inorg. Chem, 1993, 32, 532.

(d) Bochman, M.; Bwembya, G.C.; Grinter, R.; Powell, A.K.; Webb, K.J.;

Hursthouse, M.B.; Abdul Malik, KM.; Mazid, M.A. Inorg. Chem. 1993, 32, 532.

(60) Andersen, RA.; Faegri, K.; Green, J.C; Haaland, A.; Lappert, M.R; Leung, W.-P. Inorg. Chem. 1988，27, 1782.

(61) (a) Kim, J.; Rees, D.C. Nature 1992, 360, 553.

(b) Kim, 1; Rees, D.C Science 1992，257, 1677.

(62) Atwood, D.A.; Cowley, A.H.; Hernandez, R.D.; Jones, R.A.; Rand, L丄.；Bott,

S.G.; Atwood, J.L. Inorg. Chem. 1993，32, 2972.

(63) Jones, R.A.; Koschmieder, S.U.; Nunn，CM. Inorg. Chem. 1988, 27，4524.

(64) Pray, A.R. In Inorganic Synthesis, Moeller, T.，Eds.; McGraw-Hill: New York,

1957; Volumn V, Chapter VIIA.

(65) Rundel, W. Chem. Ber. 1968,101’ 2956.

90 r CHAPTER III. SYNTHESIS AND STRUCTURES OF NICKEL(II) AND PALLADIUM(II) ALKYL COMPLEXES

III.l INTRODUCTION

The studies of Group 10 organo-transition metal complexes have attracted much interest owing to their value as promoters in organic synthesis. This is dictated by a large number of organic reaction which Group 10 metals catalyse. These include olefin oxidation, the oligomerization of olefins and acetylenes, carbonylation，coupling reaction and isomerization, etc.

III.1.1 Nickel(II) Alkyl Complexes — A General Survey

The majority of nickel alkyl and aryl complexes have been prepared in the presence of TT-bonding ligands such as ti5-C5H5, PR3, bipyridines, or C0.1’2 The use of bulky alkyl and aryl ligands which devoid of p-hydrogen also imposes a stabilization effect on the resultant complexes. Hence, a variety of nickel alkyls that have been reported are in the form [Ni(R)XL2]，[NiRsL�]， o[Ni(Ti5_C5H5)(R)Lr ] in which X may be halides，NCS,

CN, acac, etc.; L may be unidentate ligands like PR3 or bidentate ligands like diphos, bipy, etc. The 冗-acceptor ligands tend to stabilize these nickel alkyls and -aryls by interacting with the nickel d orbitals and hence increasing the ligand field stabilization energy. 1

III. 1.1.1 7]^-Cyclopentadienylnickel Alkyl and Aryl Complexes

Owing to its 71-bonding ability, cyclopentadienyl group has been used for a long time to stabilize a large number of nickel alkyl and aryl complexes. l，2 However, complexes of the type [Ni(Ti5-C5H5)R] in which R is an unidentate alkyl or aryl group are still to be isolated. Indeed, [Ni(Ti5-C5H5)Me] has been postulated as an intermediate compound {vide infra) and no example has yet been isolated.^ With stabilization by donor ligands such as

91 ‘ PR3，CO, or RNC, a larger number of Ti-cyclopentadienyl complexes of the type [(t^^-

C5H5)Ni(R)L] (X) (L = PR3, AsRs, SbRs, CO, RNC) have been reported. 1'2,5

Tl^-cyclopentadienylnickel alkyl or -aryl complexes of the type (X) can be prepared by the reaction between [(Ti5-C5H5)Ni(X)L] (X = halide) and Grignard or organolithium reagents (equation in-l).l

+ ^ (iii-i)

(X)

R = alkyl, aryl, vinyl, alkynyl groups;

L=PR3, ASRs. SbRa；

X = CI, Br;

M = U, MgX

Those compounds thus prepared are summarized in Table IH-l.

Reaction of nickelocene with lithium alkyls, such as lithium benzyl, neopentyl，and trimethylsilylmethyl, in the presence of donor ligand L such as trialkylphosphines, also provides an alternative route to the corresponding Ti^-cyclopentadienylnickel alkyls. In contrast, the parent complexes [(Ti5-C5H5)Ni(R)] are unstable and decompose readily to different products which depend on the choice of ligand R.16

For instance, if R = Me，a coupling product, ethane, and tris(Ti5- cyclopentadienyl)(ji3-alkylidyne)trinickel cluster [(NiCp)3C-CH3] were obtained (equation

III-2).16

NiCp2 + CHaLi ^ [(Tl^-C5H5)NiCH3] unstable

CH3CH3 + [(CpNi)3C-CH3] (in-2)

92 Table III-l. Some Representative Examples of [(ri^-CsHsjNiRCL)]. .一

上 L Colour m.p. (^C) References

CH3 P(C6H5)3 green 126-129 (dec) 6-9

CH2CH3 P(C6H5)3 green 118-120 (dec) 7-9

CH2Si(CH3)3 P(C6H5)3 green 130-131 10-12

CH2C6H5 P(C6H5)3 green-brown 129-131 (dec) 7

① C三CC6H5 P(C6H5)3 green 135 (dec) 7,8

C6H5 P(C6H5)3 green 137-139 (dec) 8

As(C6H5)3 green 126-127 (dec) 8

Sb(C6H5)3 green 96-98 (dec) 8

CF3 CO red (liq) - 6，13

C2F5 CO red-purple (liq) - 13

CF2CF=CF2 CO dark red (liq) - 13

C(CH3)=N(C6HII) CN(C6HII) red-brown 72.5-73.5 (dec) 14,15

C(C6H5)=NCBU) CN(^BU) red brown 82-84 (dec) 15 If R IS a higher alkyl derivatives which do not possess p-hydrogen, they may undergo decomposition to form coupling products. 16 A typical example is illustrated as follows:

NiCp2 + RCH2Li _li^- [(Ti'-C5H5)NiCH2R] (III-3a) unstable

2 [(Tl^-C5H5)NiCH2R] \ [{NiCpln] + RCH2-CH2R (III-3b)

3 [{Tl^-CsHslNiCHsR] [(CpNi)3CR] + 2 CH3R (III-3c)

On the other hand, for alkyl groups with p-hydrogen, product of alkenes and alkanes will be formed by the reaction between [Ni(Ti5-C5H5)2] and the alkyllithium compounds. 16

[(n^-C5H5)NiCH2CH2R] [CpNiH] + H2C=CHR (ni-4a)

[(Ti^-C5H5)NiCH2CH2R] + [CpNiH] \ [{NiCpW + CH3CH2R (in-4b)

Pietrzykowski et al}^ reported the reactions of nickelocenes with substituted phenyllithium which gave an unstable complex of the type [(Ti5-C5H5)Ni(C6H5R)]，and reacted further to form coupled bi-aryl as the main product together with a mixture of cyclopentadienylnickel compounds as the minor products.

• 。

NiCp, + '^QkU ：^. CpNi-^^'

• •

^ ~+ [Nicp】（III

94 However, in the presence of ethylene, the corresponding aryUii^-

cyclopentadienyl)(Ti2-ethene)nickel complexes can be obtained at low temperature which

decompose at > -20 °C (equation III-6). 16,17

• I

^^^^ CpNi^Hj^^zR + CH2=CH2 (III-6)

unstable

decomposes further to other substances

Although Ti^-cyclopentadienylnickel alkyls or -aryls of the type [(Ti5-C5H)NiR] are unstable, thermally stable analogues can be isolated if R contains donor atom such as N.

The stability of these complexes is mainly attributed to the formation of 18-electron intramolecular coordination complexes. Recently, Pietrzykowski et al. have reported the synthesis of {2-[(dimethylamino)methyl]phenyl} {ii^-cyclopentadienyl)nickel with no structural data (equation ni-7).16

^^Li + NiCp2 ——^——^ (ni-7) Sn、、N、 Me' Me Me^ Me

Functionalized alkyl or aryl ligands which contain an alkene and a-bonded group that formed part of the same organic ligand have also been reported. 18

95 ‘ NiCPa + J^^^^^Mga •⑴"-。已,^’ （駆）

in.lJ.2 Nickel Dialkyls

Most of the nickel(n) alkyls or -aryls which have been reported were prepared by reacting substituted nickel dihalide complexes [Nil^X〗](L = coordinated ligands) with the appropriate organo-main group metal reagents. Although Grignard reagents were mostly used，the uses of other reagents such as organo-lithium or -aluminum reagents have also been reported. A general synthetic pathway is shown schematically as follows.

[(R'3P)2NiX2] + RM ^ [(R'3P)2Ni(R)X] ^——^ [(R’3P)2NiR2] (111-9)

R = alkyl, aryl group; M = Li, MgX; X = CI, Br

Yamamoto et al. have isolated [NiEt2(bipy)] form a mixed catalyst system which contained Ni(acac)2 and AlEt2(0Et) (equation I-17). 19 The diethyl nickel complex was found to have catalytic activiy towards cyclodimerization of butadiene.

A number of coordinated-ligand-free neutral nickel alkyl and aryl complexes of the type [NiR2] and [NiR(X)] in which X is halide have been reported. 1，2 However, none of these solvent-free species have been identified unequivocally. Two likely examples are bis(trityl)nickel(II)20 (67) and bis(mesityl)nickel(II) (68) have been reported. 1,21 The former was prepared by the reaction of bis(cyclooctadiene)nickel with hexaphenylethane

(equation IE-10) 20

—+ {(C6H5)3C}2 [(C6H5)3C-Ni-C(C6H5)3] (HMO) 67

96 ‘ Compound 67^0 is most likely to have the structure as shown below.

CeHsv ,C6H5 \ /VC6H5 ,C——Ni——C.I CeHs-i••“y \ CsHs' C6H5 67

Nevertheless, it is noteworthy that whether the alkyl ligand (C6H5)3C- was bonded to the central metal as an T^l-carbanionic ligand or as an Ti^-allyl type ligand still remained unclear as the compound have not been structurally authenticated.

Bis(mesityl)nickell’21 (68) was prepared by the reaction of 71-allylnickel with mesitylmagnesium bromide (equation III-11).

[{NiBr(Ti^-C3H5)}2] + 4 ~

• ^ ~^~ + 2 CaHsMgBr (III-11)

The molecular planes of the two mesityl groups in 68 are believed to be perpendicular to each other. The ortho-methyl groups helps to shield the central metal against attacks from other nucleophilic reagents.

The related nickel(II) dialkyl complexes [Ni{ C(SiMe3)3} 2]" and

[Ni{C(SiMe3)(C6H5)2}2]lO，ll have been prepared by reacting nickel(II) bromide with the corresponding organolithium reagents. The successful isolation of these dialkyl complexes is attributed to the bulkiness of the organic ligands which stabilize the kinetically labile complexes.

97 The well-characterized thermally stable homoleptic nickel dialkyl 69 was reported

by Longoni et al.^^ The dialkyl complex was obtained by treating 2-

diphenylphosphinobenzyl potassium with (PEt3)2NiCl2 (equation III-12).

2 11 + [(PEt3)2NiCl2]

^ /Ni I (III-12)

/ \ Ph

An interesting feature of this complex is that the alkyl function and donor ligand form part

of the same system. The chelating behaviour of the organic ligand is believed to have

imposed a stabilization effect to the existence of the dialkyl compound.

By employing a similar synthetic approach, the nickel diaryl compound，bis[2-

(dimethylamino)methylphenyl]nickel (70), has been prepared by the same research group

(equation in-13)23 C、 Me、 ^^^^ …,….� — I^Ni^) ^ (in-13) LI \ \] MeZ、Me Me,、Me 70

Another aryl ligand which contains P donor atom was also reported to give the corresponding nickel diaryl in satisfactory yield (equation EI-14) 24

98 2 I + [NiCl2 -(TMEDA)]

PhzP\ph

III. 1.2 Palladium(II) Alkyl Complexes — A General Survey

III.1.2.1 Palladium Dialkyls

For organopalladium(II) compounds, complexes of the type [Pdl^R〗] are known

for many years.25,26 in these complexes, L is usually a strong field ligand such as PR3,

ASR3，or bipyridines. In general, compounds containing Pd(II)-C(^/?2) a bond are more

stable than those containing a bond.

Organometallic compounds of palladium(II) are commonly prepared by treating complexed palladium(II) dihalides [PdL2X2] with the appropriate Grignard or organolithium reagents, according to equation 111-15.

[PdL2X2] ~~^——-[Pd(R){X)L2] ~~^~~- [PdRaLa] (111-15)

X = CI’Br’l; M = Li, MgX

Normally, the use of Grignard reagents is more favourable as organolithium reagents usually lead to decomposition. Stable organopalladium(II) compounds can be prepared by using alkyl or aryl ligands containing electronegative substituents and in the presence of phosphines as one of the other auxilliary ligands. For instance, a wide variety of organopalladium complexes of the type [Pd(R)(X)L2] (XI) and [PdR2L2] (XII) have been reported.25，26

99 Organopalladium(II) complexes of the type [PdR〗](R 二 alkyl groups) containing exclusively Pd-C a bonds have not been known. The only hitherto reported homoleptic palladium(n) complex is [(N"B 114)2] [Pd(C6F5)4] (equation 111-16)27

frans-[Pd(tht)2Cl2] + 4 LiCeFs ~(问“已—厂• [N"Bu4][Pd(C6F5)4] (III-16)

tht = tetrahydrothiophene

The use of chelating alkyl or aryl ligands have been observed to impose a pronounced stabilization effect on Pd-C a bonds. Thus, palladium(II) dialkyl complexes in the form of palladacycle such as compound 71 has been reported.28

Ph2

[PdCl2(cliphos)] + U(CH2)4Li • Pd (HI-17)

Ph2

7 1

More common examples are the use of functionalized alkyl or aryl ligands containing P or N atom. For example, phosphorus chelation can change the unstable benzylic palladium-carbon a bond to a very stable one. Thus, organometallic compounds of palladium(II) such as 72 and 73 were successfully prepared and characterized.22’23，29 The related compounds containing N donor also show remarkable thermal stability (equation ni-

18 and IE-19).

PPh2

[PdLscy + 2〈\ /)—cHsLi 「丁 T I (ni-18)

Ph2 Ph2

7 2

100 CH2ER2 ^^ ^^

[PdLsCy + 2 ^Y^Pd-"^!^ (Ill-19)

R2 R2

L = PhCN, EtaS; ER2 = NMe2, NEt^，PPh? 73: = pp^^

74: ER2 = NMe2 75: ER2 = NEt2

III.1.2.2 Cyclopalladated Compounds

In 1965, Cope et aL说 reported the reaction of azobenzene with [KaPdCy gave the

cyclopalladated compound 76 (equation 111-20).

八 Z‘N 2 [KaPdCU] + 2 PhN=NPh T Pd I (in-20)

N�� NZ\ / "YS I a Ph 76

Since then, a wide variety of related compounds have been prepared by using similar synthetic approach. Besides metallation of a phenyl ring and nitrogen as the donor, a large number of complexes involving metallation of other groups and/or other donor atoms such as P,31-37 S，38,39 and AS32 have been reported. The mechanism involves electrophilic attack of palladium at the aromatic rings. Subsequent loss of HCl and re-aromatization gives the target compounds.

After extensive studies on the cyclopalladation reactions, Cope et found that three requirements must be met by the ligand involved the viability this reaction. The three requirements are:

(i) the nitrogen must be tertiary;

101 (ii) a planar five-membered metallacyclic ring has to be formed;

(iii) the aromatic carbon atom that attaches to the proton which the metal will displace

is not highly deactivated towards electrophilic attack.

If these requirements are not met, a bis(ligand)palladium dichloride complex of type (XIII) will be obtained.

\ / Pd c/

XIII

Some exceptions to Cope's three requirements have been reported in literatures. For instance, metallation of 8-alkylquinoline in which the metal is bonded to an aliphatic carbon has been reported 41-43 In fact, metallation of an aliphatic carbon through activation of C-H bonds by transition metal complexes is one of the current interests in organometallic chemistry.

R ff^ 八 zvS

yPd Pd、、

I R

R = Me, Et

The chloride on the cyclometallated compounds can sometimes be further substituted with the appropriate organolithium reagents. For instance, a novel palladium diaryl 77 which contains mixed aryl ligands has been reported (equation 111-21).44

102 I BU2

CHpPPhp / t /=< BU2 Ph2 On (\

一 (腳）

103 IIL2 RESULTS AND DISCUSSION

III.2.1 Synthesis of Nicke!(II) Alkyl Complexes

III.2丄 1 Reactions of [{RlLUEtiO)}� ] and 脾叫]with Nickelocenes —Synthesis of t]^-cyclopentadienylnickelalkyl Complexes (78- 80)

Reactions of the pyridine-functionalized lithium alkyls with nickelocenes [Ni(CpR)2] have been studied. The reactions of [{RlLi(Et2〇）}2] (5) and [{R2Li}2] (6) with

[Ni(CpR)2] in THF afforded the thermally stable compounds [{CpRNiRlh] and

[CpRNiR2], respectively (Scheme Ill-la and Ill-lb).

[Ni(CpA)2] + 1/2 二 ’ A

78: R = H 79: R = SiMea

Scheme III-la

[NiCp2] + V2 [{R Yi(Et20)}2] 。二严。h 、 \\i) 25C，8h ^^ 丨\}

r^ ^V^ N^SiMeH o3

80 Scheme Ill-lb

104 The addition of a solution of [{R^Lih] in THF to [NiCp2] in the same solvent at 0

followed by a further stirring of 8 h at room temperature gave a greenish brown solution.

After solvent removal at reduced pressure, the residue was extracted into hexane. The

insoluble LiCp was filtered off and the filtered extract was concentrated in vacuo and set

aside at room temperature to give greenish brown crystals of compound 78. In another

experiment to prepare 70, a small amount of unreacted [NiCp2] was present which was

shown from the ^H NMR spectrum of the product.* The compound was purified by

column chromatography under nitrogen on neutral alumina using hexane as eluent.

The reaction of [{R^Li}?] with the novel nickelocene [Ni{Ti5-C5H3(SiMe3)2h]

which bears the more bulky cyclopentadienyl ligand, yielded the corresponding nickel alkyl

complex 79 which exists as brown coloured crystals. Owing to steric hindrance around the

metal center of the C5H3(SiMe3)2 rings, the rate of the reaction was found to be relatively

lower when compared with the reaction of [{R^Li}〗] with [NiCp2；.

[{RlLi(Et20)}2] reacted readily with [NiCp2] in THF to give dimeric compound

80. The reaction was performed by treating a THF solution of [{RlLi(Et20)}2] with a

solution of [NiCp2] in THF at 0 followed by stimng at room temperature for a further 8 h. The reddish brown solution so obtained was filtered and concentrated to give 80 as dark red crystals (upon cooling to -30 By comparing with the ^H NMR spectrum of the similar compound [Cp2Zr(Rl)Cl], the alkyl ligand "Rl is believed to bond to the nickel metal via an ii^-aza-allyl type bonding in solution, based on the chemical shift of the methine proton {vide infra).

As in the case of compound 78，compounds 79 and 80 can be purified by chromatography under nitrogen on neutral alumina using hexane as eluent.

Compounds 78-80 are unstable towards oxygen in solution although as crystalline materials they can be handled in air for a short period of time.

Table 111-2 summarizes the physical properties of compound 78-80.

* A small amount of paramagnetic NiCp2 present in the product will lead to broadening of its ^H NMR signals.

105 Table III-2. Some Physical Properties of Compounds 78-80.

~C— Yield (%} Cnlnur 释⑵

77 greenish-brown 186-188 (dec.)

79 84 dark brown 99-101

80 72 dark red 170-172 (dec.)

UU 丄 2 Synthesis of the Substituted Nickelocene [Nif^S. C5H3(SiMe3)2}2] (81)

The hitherto unknown nickelocene [Ni(CpR)2] [CpR = Ti5_C5H3(SiMe3)2] was

prepared by the reaction of LiCpR with NiCl^ in IHF at ca. 65 However, the reaction

was relatively slow and gave only a low yield (42%) of the target nickelocene. The product

yield can be improved by the reaction of LiCpR with 'activated' NiBr〗. LiCpR reacted

readily with [NiBr2-2DME] (DME = dimethoxyethane), the latter being prepared by direct

bromination of nickel powder with bromine in dimethoxyethane 45 to give [Ni(CpR)2] at

room temperature. The yield of the reaction has been increased to 81% (Scheme 111-2).

MeaSi '^^^^^-^S^SiMeg _ fl^- [NiBr^. 2DME] 2 LiCpA — ' •E THF，25。C '^siMe3

、SiMe3

8 1 Scheme III-2

[NiCp*2] (Cp* 二 CsMes) can be prepared by direct reaction of NiBr? with Cp*Li at room temperature with satisfactory yield.46 However, during our preparation of the title compound 81’ LiCpR was found to be not quite reactive towards NiCl2 and hence the

106 •activated, form of nickel(II) bromide [NiBr2.2DME] was employed. The preparation of crowded nickelocenes by using the more reactive [NiCl〗.（THF) 1.65] has also been reported.47

III.2.1.3 Spectroscopic Properties of Compounds 78-80

Table ni-3 lists the iH and I3c NMR spectral data for compounds 78-80 together

with that of the free ligand [CH2(SiMe3)C5H4N-2] (RlR) and [CH(SiMe3)2C5H4N-2]

(R2H) for comparison.

Compounds 78-80 are diamagnetic and achieve the eighteen-electron configuration as required by the EAN rule. The compounds were characterized by microanalysis, mass spectroscopy, and ^H and 13c NMR spectroscopy.

Mass spectra of compounds 78-80 all show the molecular ion as the base peak.

The IH NMR spectra of 78 displayed a singlet due to SiMe) groups from the alkyl ligand R2 and another singlet due to the Cp protons. For compound 79, in addition to two singlets due to two types of SiMe] groups: one from the ligand R2 and the other from

C5H3(SiMe3)2，two broad signals due to the CsHgCSiMe])〗ring proton s were observed.

In order to investigate the effect of the SiMe] groups on the rotation of the C5H3(SiMe3)2 ring (Figure 111-1)，variable temperature ^H NMR experiments were carried out.

107 Table III-3. ^H NMR Spectral Data (ppm)^ for [RlH], [R^H], and Compounds 78-80.

MesSi CH CsHsb Aromatic [RlH] 0.04 2.29 - 6.53-6.58 (ddd)，6.61-6.65 (dt)，7.00- 7.07 (dt), 8.43-8.46 (ddd) s [R2H] 0.14 1.68 - 6.47-6.52 (ddd), 6.56-6.59 (dt), 6.97- 7.04 (dt), 8.37-8.40 (ddd)

78 0.30 -- 5.25 5.90-5.96 (m), 6.05-6.09 (m), 6.80-6.87 (dt), 7.11-7.15 (m)

79。 0.49，0.55 - 5.15-5.16 6.17-6.28 (m, 2H), 7.03-7.10 (dt), 7.42- (m)，5.20- 7.44 (dd) 5.21 (m)

0.43 4.39 4.76 6.02-6.05 (m), 6.27-6.30 (m)，6.48-6.55 (m), 8.45-8.46 (m)

a : 250 MHz, CgD^. b : C5H3(SiMe3)2 for compound 79. c : Solvent: C7D8. ~� \ f SiMe3 MegSi~| / Ni MeaSi、^^^^ MeaSi^/ ‘ > c- -Ni-^.N .Me3S, 丫） Meas/ ^

Figure III-1. Top view of compound 79 directly along the Cp^-Ni axis showing possible interaction among the four SiMe] groups.

The results showed that even at low temperature (-80。(：)，significant changes in the

IH NMR spectrum of 79 was not observed, indicating that the bulky ligand R2 does not

interfere with the free rotation of the C5H3(SiMe3)2 ring at the temperature studied.

In the IH NMR spectrum of compound 80’ the chemical shift of 5 4.39 for the

methine proton of 80 is significantly different from that of 5 2.31 for the free ligand

[CH2(SiMe3)C5H4N-2]. The down-field shift suggests an Ti3-aza-allyl type bonding of "Rl to the nickel center in solution state. A similar down-field shift of iR NMR signal in the zirconi蘭 compound [Zr(Ti5-C5H5)2(Rl)(Cl)] have been reported.^S Transition metal alkyl complexes which contain Ti3-aza-allyl type bonding are rare and two novel examples can be found in complexes containing A^-functionalized ligands, viz. [Zr(7i5_C5H5)2(Rl)(Cl)] and

[R2Li(R2H)] (7)48

NiLp H》丄N、 MesSi.z 丫 "s

The 13c{ iH) NMR spectrum of 78 is normal. For compound 79, two signals at 6

1.80 and 3.23 were observed owing to two different types of SiMes groups on the molecule

109 as in the case of its iR NMR spectrum. Three signals arising from three different kinds of

carbon on the C5H3(SiMe3)2 ring were obvious. Signals due to a-carbon of compounds

78and 79 are not observed most probably due to the slow relaxation time of the quaternary carbons.

丄4 Molecular Structures of [(”�c^Hs)Ni{C(SiMe3)2C5H}] (78) and _c5Hs)Ni{CH(SiMe3)CsH4N-2H2] (80)

The molecular structures of 78 and 80 with the atom numbering schemes are shown

in Figure ni-2 and IH-S, respectively. Selected bond distances (A) and bond angles (。）are

included in Table in-4 and III-5. Compound 78 is a ’half-sandwich”，with the pyridine-

functionalized alkyl ligand bonds in a chelate fashion. The five Ni-C(Cp) distances are

not identical, ranging from 2.060 to 2.149 A, revealing that the ti5 coordination is not

symmetrical. Unsymmetrical bonding of nickel to Cp ring was also reported for [(n^-

C5H5)Ni(PPh2)]2.49 The Ni-Cp(centroid) distance is 1.765(2) A，and the average Ni-

C(Cp) distance is 2.124(2) A, significantly shorter than the corresponding distance in

NiCp2.50 The Ni-Ca distance of 2.018(2) A in 78 is longer than the corresponding

distance of 1.89(1) A in aMNi(CH2SiMe3)2(py)2]，51 which contains a pair of the less

bulky alkyl ligands [CHsSiMegr.

The Ni-N(py) distance of 1.856(2) A in 78 is significantly shorter than that of 1.957(8) A in m-[Ni(CH2SiMe3)2(py)2].51

Compound 80 crystallizes as a dimer, in a monoclinic space group of Pl^/n (No.

14). Each molecule is located in a crystallographic inversion center. The alkyl ligand

[CH(SiMe3)C5H4N-2]- joins two nickel atoms via a C^-chelation in such a way that the resulting eight-membered ring adopts a chair-form structure. The mean Ni-Ca and Ni-N distances are 2.002 and 1.923 A, respectively. The mean Ni-Ca distances in 80 is shorter than that of 2.018(2) A in 78 but is longer than the corresponding distance of 1.89(1) A in cz\s-[Ni(CH2SiMe3)2(py)2].51 The shorter Ni-Ca distance in 80 than that in 78 is

110 _ CNA,

• Figure 1II-2. Molecular Structure of [N"!^；；^：^；^^；^^^^^；；^；^；；；^^^^]} �(78) Table ni-4. Selected Bond Distances (A) and Angles (。）for Compound 78.

[(Ti5.C5H5)Ni{C(SiMe3)2C5H4N-2}] 78

Ni(l)-N(l) 1.856⑵ Ni ⑴-C(5) 2.416(2)

Ni ⑴-C(6) 2.018(2) Ni ⑴-C(13) 2.060(3)

Ni ⑴-C(14) 2.137(13) Ni ⑴-C(15) 2.128(2)

Ni ⑴-C(16) 2.149(3) Ni(l)-C(17) 2.147(2)

N ⑴-Ni(l)-C(6) 71.5(1) Ni(l)-N(l)-C(l) 142.2(2)

Ni ⑴-N ⑴-C(5) 96.6(1) Ni ⑴-C(6)-C(5) 85.7(1)

N(l)-Ni(l)-Cp 142.4 C(6)-Ni(l)-Cp 146.1

112 /5^C(12) C(5) C(4) T ^^

I C(8) C(7) \

� 4) C(13) cmiTf

CdS)^^ C(19) C(21) �2 ) (^C(26)

Figure III-3. Molecular Structure of [{Ni(7i^-C5H5){CH(SiMe3)C5H4N-2} )2] (80) Table III-5. Selected Bond Distances (A) and Angles (。）for Compound 80.

[{(Ti5-C5H5)Ni{CH(SiMe3)C5H4N-2) j〗]80

Ni(l)-C(l) 2.159(4) Ni(l)-C(2) 2.071(4)

Ni(l)-C(3) 2.164(5) Ni(l)-C(4) 2.158(5)

Ni(l)-C(5) 2.172(5) Ni ⑴-C(25) 2.000(3)

Ni(l)-N(l) 1.920(3) C(10)-C(ll) 1.473�

C(10)-N � 1‘365(5) C(ll)-Si(l) 1.861 �

Ni ⑵-C(15) 2.169(4) Ni(2)-C(16) 2.058(4)

Ni(2)-C(17) 2.142(5) Ni(2)-C(18) 2.154(5)

Ni(2)-C(19) 2.173(5) Ni(2)-C(l 1) 2.004(4)

Ni(2)-N ⑵ 1.926(3) C(24)-C(25) 1.470(4)

C(24)-N(2) 1.356(4) C(25)-Si(2) 1.870�

N ⑴-Ni(l)-C(25) 94.0(1) Ni(l)-C(25)-C(24) 108.9(2)

C(25)-C(24)-N ⑵ 118.9(3) C(24)-N(2)-Ni(2) 122.2(2)

N(2)-Ni(2)-C(l 1) 93.3(1) Ni ⑵-C(l I)-C(IO) 108.5(2)

C(ll)-C(10)-N(l) 118.6(3) C(10)-N(l)-Ni(l) 122.1(2)

C(10)-C(l l)-Si(l) 120.7(3) C(24)-C(25)-Si(2) 119.5(2)

114 presumably the consequence of the less bulkiness of the alkyl ligand "Rl in the former

complex. The mean Ni-N distance in 80 is slightly longer than that of 1.957(8) A in cis-

[Ni(CH2SiMe3)2(py)2]5l but is much longer than that of 1.856(2) A in 78. The Ni"..Ni

distance is very long, viz. 4.006 人.Hence, any possibility of metal-metal bond formation can be excluded.

Ti^-coordination of the two Cp rings in compound 80 is unsymmetrical, as in the

case of compound 78，with the Ni-C(Cp) distances ranging from 2.058 to 2.1743 A (av.

2.142 A). Both the Ni-C(centroid) distance (1.789 A) and the mean Ni-C(Cp) distance in

compound 80 are slightly longer than those in compound 78 but are still shorter than the corresponding distances in [NiCp2].50

III.2.1.5 Reactions of [{R^Li}!] with NiCl! and [NiCl2L2] (L! = TMEDA, 2 P P h 3) — Synthesis of Nickel Dialkyl Complex _C(SiMe3)2C5H4riJ-2}2] (82)

In addition to nickelocenes, the reactions of [{R^Li}〕] with other nickel(II) compounds have also been investigated. The reaction of [{R^Li}?] with NiCl〗 in THF at room temperature gave a reddish brown solution with black precipitate. The black precipitate, apparently nickel metal, indicates that a substantial amount of reduction have occurred. In fact, direct reaction of NiX〗 with organolithium reagents usually leads to reduction.

The reddish brown solution was filtered and concentrated in vacuo followed by chromatographic separation on silica gel using hexane/CHzCl〗1:1 as eluent. A red crystalline compound was obtained which was identified as the nickel dialkyl complex [Ni(R2)2] (82) (equation III-15).

115 • MegSi/、>SiMe3 /=\

+ ^^^^ yi〉

82 modest yield

However, the yield was modest and it was shown that most of the nickel(II) chloride had been reduced to metallic nickel.

The yield of compound 82 was found to improve substantially by the reactions of

[NiCl2.(TMEDA)] or [Nia2(PPh3)2] with [{R2Li}2]. In both cases, no nickel metal, an obvious sign of reduction, were observed. Nickel dichlorides [NiClzL〗](L2 = TMEDA,

2PPh3) reacted with 2 equivalents of [{R2Li}2] in THF at 25。C to give [Ni(R2)2] (82) in

50% (L2 = TMEDA) and 48% (L2 = 2PPh3) yield (Scheme 111-3).

Me3Si,,.c>SiMe3 /=\ [NiCl2L2] + [{R\i}2] THF，25。C _ _/ ^Ni^ 8h fA,、尸

\=/ Me3Si、、,。、SiMe3

82 L2 = TMEDA, 50% 2 PPha, 48% Scheme III-3

Compound 82 is a thermally and air stable compound which melts at 158-160

[NiCl2-(TMEDA)] was prepared by stimng a suspension of NiCl〗 with 1 molar equivalent of TMEDA in THF at ambient temperature for 8 h. The pale yellow suspension of NiCl2 changed gradually to a pale green suspension which was used in the subsequent steps without isolation.

A solution of [{R^Li}〗] in THF was added to a suspension of [NiCl〕.(TMEDA)] or

[Ni(PPh3)2Cl2] in THF at -40。C. After the addition had been completed, the mixture was

116 allowed to warm back slowly to room temperature. A deep red solution was obtained

without any metallic nickel being observed. After solvent removal in vacuo, the residue

was extracted into hexane. Removal of hexane under reduced pressure gave compound 82

as red crystals. The product can be further purified by subjecting the hexane extract to

column chromatography on silica gel using hexane as eluent. The red fraction was collected

and the solution was evaporated to dryness to give compound 82.

Since organolithium reagents are also known for their reducing properties towards both main group and transition metal compounds, direct reactions of organolithium reagents with metal halides often lead to reduction. Owing to its low reduction potential, nickel(II) compounds are especially easy to be reduced. As we have discussed above, the addition of

[{R2Li}2] to NiCl2 led to a substantial amount of reduction. In order to tackle the problem of reduction during the synthesis of [Ni(R2)2] (82) and improve the reaction yield, nickel(II) dichlorides complexes [Nil^Cy were employed. Issleib et had synthesized a nickel(II) diaryl complex (69) in satisfactory yield by treating [NiCl〗.(TMEDA)] with the corresponding aryl-lithium which contains a 尸-donor (equation HI-16).

PPh2

[NiCIa- (TMEDA)] + 2

Longoni et al.^^ also studied the reaction of [NiCl〗.(TMEDA)] with dimethylbenzylamine which gave the corresponding nickel(II) diaryl complex which probably possessed a trans square planar geometry (equation 111-17).

Ii JL / \ [NiCl2-(TMEDA)] + 2、I 隱2 ^ Ni ] (III-17) ^^Li L / vS \n \ I Me2 ^^^ 70

117 However, the thermal stability of complex 70 is low.

It is noteworthy that Thornton et al.52 had reported the synthesis of 82 by the reaction of [NiCl2.2(PEt3)] with [{R^Lih]. However, the reaction yield was very low

(5%) and attempts to obtain X-ray quality crystals of the same compound 82 were reported to be unsuccessful.

III.2.1.6 Molecular Structure of [Ni[C(SiMej)2C^H4N-2；(82)

The molecular structure with atom numbering scheme for complex 82 are shown in

Figure ni-4. Selected bond distances (A) and angles (。）of all complexes are listed in Table

III-6. Compound 82 crystallizes in a tiiclinic crystal system with space group Pl(No.2).

It contains two independent molecules of nearly the same structure with each being located at a crystallographic inversion center. The coordination sphere of the square planar nickel(II) center of 82 comprises the two alkyl ligands "R^ which are bound to the metal center in a trans CW-chelating manner. In other words, it is isostructural with its cobalt(II) analogues [Co(R2)2] (56).

The average Ni-Ca distances is 2.075(8) A which is slightly longer than the corresponding distances of 2.018(2) A in [(Ti5-C5H5)NiR2] (78) but much longer than that of 1.89(1) A in d^KNi(CH2SiMe3)2(py)2].51 The Ni-N bond distances in 82, average

1.889(2) A, are comparable to the corresponding distance of 1.856(2) A in 78. However, they are significantly shorter than that of 1.957(8) A in cis-[Ni(CH2SiMe3)2(py)2]• ^^ The bite angle C(l)-Ni-N(l) in 82 is 70.6(1)°.

It is noteworthy that the results of this work offer the unique opportunity to compare isostructural paramagnetic (compound 56) and diamagnetic (compound 82) complexes. On going from Co(II) to Ni(II), the M-C a bonds decrease by 0.017 A and M-N bonds decrease by 0.034 A (Table ni-7). Despite the small differences, the shorter Ni-C and Ni-

118 • Q © I S丨2Q A \ k

f kja 严

©C9

C12 €)

, Figure III-4. Molecular Structure of [NTf^^^i^；；?^^^；^^』）】）（82) Table 111-6, Selected Bond Distances (A) and Angles (。）for Compound 82.

[Ni{C(SiMe3)2C5H4^^-2}2] 82

Ni(l)-N(l) 1.887(2) Ni(l)-C(l) 2.078(3)

Ni �- C � 2.443(2) C(l)-C(2) 1.486(3)

N(l)-C(2) 1.350� Si(l)-C(l) 1.867(2)

Si(2)-C(l) 1.872(3)

N(l)-Ni(l)-C(l) 70.6(1) Ni �-N(l)-C(6 ) 142.9(2)

Ni �- N�- C� 96.6(1) Ni(l)-C(l)-C(2) 84.8(2)

N(l)-C(2)-C(l) 108.0(2) Si �- C�-Si(2 ) 120.3(1)

120 • N bonds can be explained in terms of the nuclear charge effect. The bite angle of the two compounds are nearly identical.

Table III-7. Structural Parameters of Compounds 56 and 82.

_Compound 叫 M-C (A) M-N (A) ZC-M-N

[Co(R2)2] (56) 3.11 2.092(6) 1.923(4) 69.3(2)

{Ni(R2)2] (82) 一 2.078(3) 1.887(2) 70.6(1)

人7 Spectroscopic Properties of [Ni{C(SiMeshCsIUN-lh] (82)

In addition to X-ray crystallography, 82 was characterized by mass spectroscopy,

IH and 13c NMR spectroscopy, UV-vis spectroscopy, and microanalysis.

Mass Spectrum:

The nickel dialkyl, [Ni(R2)2] (82)，showed the molecular ion peak [M]+ miz = 530, and the peaks due to the organic fragments such as [R2-CH3]+，[R2-SiMe3]+，and [SiMe3]+.

NMR spectra:

Like most d^ transition metal complexes, compound 82 is also diamagnetic. The X- ray structure of the dialkyl complex 82 indicates that two alkyl ligands which bond to the central metal are equivalent in solid state. This is consistent with the solution NMR spectra of compound 82 which shows only one type of ligand as only one set of ^H and 13c NMR signals are being assigned to this group. The low intensities of 13c NMR signals of the a- carbon and the quaternary carbon of the C5H4N fragment were due to their slow relaxation time.

121 UV-Vis spectra:

Absorption bands at 388 (1.2 x lO^), 367 (1.3 x 104), 268 (1.3 x 104)，and 229

(1.3 X 104) are observed on the UV-vis spectrum of 82. The large absorption coefficients for these bands suggest that they are probably charge transfer or ligand-ligand transition bands.

IU.2.1.8 Electrochemistry of [Ni{C(SiMe3)2CsH4N-2J2] (82)

The cyclic voltammetry of compound 82 shows a fairly reversible wave at Ey2 二

0.99 V (100 mVs-l) superimposed on the fringe of THF decomposition (background discharge) (Figure 111-5). The one electron oxidation process gives rise to a Ni(III) species:

[Ni(R2)2] 4 [Ni(R2)2]+

This indicates that a Ni(III) species stable enough to detect in the c.v. time scale was detected. Studies with different sweep rates from 50-1000 mVs"^ indicated that the oxidized

Ni(m) species decomposes substantially during the time span of a c.v. scan.

Nill/in oxidation in electrochemical study of tetraaza macrocycle complexes ofNi(II) has also been reported by Bernhardt et 议53 The Ni(in) species existed only transiently and the processes gave completely irreversible waves at potentials ranging from +1.11 V to

+1.74 V for these tetraaza macrocycles of Ni(n).

122 • 2.0 1.0 0 versus [Fe(Ti-qHy2]+刃

Figurem-5. Cyclic voltammogram of 4 x 10.3 mol drn'^ (82). 0) moi dm-j [NBu^JCBF^] in THF. Sweep rate = 0.100 V s.厂

123 iJJ'2,1,9 Reactivity of [rh{C(SiMe3)2C5H41^-2)2] (82)

Attempts to prepare [(Ti5-C5H5)NiR2] (78) by the redistribution reaction of a THF solution of [Ni(R2)2] (82) with [Ni(Ti5_C5H5)2]，even at refluxing temperature, were unsuccessful (Scheme III-4). The starting materials were recovered quantitatively.

MegSi/, >SiMe3 /=\ /C\ N 》 + [NiCp2]

\=/N Me3Si、、*“SiMe3

8 2

THF \/ , 八 A MegSi",./ \ MesSi^

78 Scheme III-4

In addition, neither insertion of a diphenylacetylene molecule into the Ni-Ca bond of

82 nor catalytic oligmeriztion of diphenylacetene by the nickel dialkyl (82) has occurred.

The "inertness" of the complex may be related to its molecular structure. In the previous chapter, we have described that the cobalt(II) analogue [Co(R2)2] (56)，which adopts a trans square planar coordination geometry, was observed to have the fifth and sixth octahedral sites on both sides of the molecule being blocked by two SiMe] groups, one from each alkyl ligand. Being isostnictural with [Co(R2)2] (56)，[Ni(R2)2] (82) has the two bulky alkyl ligands completely shielded the central metal from attacks by the surrounding media.

124 • III.2.1.10 Reactions of [{R^Li^] with [NiCl2(cliphos)] [diphos 二 1，2_ Bis(diphenylphosphino)ethane]

Reaction of [NiCl2(diphos)] with 2 equivalents of [{R2Li}2] in Et2〇 at 25。C yielded，instead of compound 82 as would have been expected, the red complex

[Ni(R2)(R2.R2)ci] (83) in 60% yield (Scheme 111-5).

[NiCI,L,] + 曰 2〇，25。C， Me3Si /SiMe3 fl = diphos / \ X \ /r\ ^Ni CH(SiMe3)2

60% Scheme III-5

Interestingly, a "head-to-tail" coupled alkyl ligand [R2-R2] (84) was found to be coordinated to the nickel(II) center. Unlike the dialkyl complex 82，monoalkyl complex 83 shows a lower solubility in saturated hydrocarbon solvents but can be recrystallized from warm toluene. Compound 83 is unstable towards oxygen in solution, although as crystalline materials they can be handled in air for a short period of time.

The formation of the "head-to-tail" coupled alkyl ligand [R2-R2] (84) has been reported by Raston et al in the reaction of [{R^Li}〗] with BiCl3.54a it is conceivable that the presence of two SiMe; groups on the a-C of R2 hampers a "head-to-head" coupling of the R2 groups. However, a 'tail-to-tail' coupling (4,4'-coupling of the pyridyl rings)

(Figure III-6) was reported by the same research group in their studies of the monomeric dialkyl-aluminum and -gallium radical species [M(R2)2]. (M 二 Al，Ga).54b

125 • CH(SiMe3)2

NP^N

(Me3Si)2HC

Figure III-6. "Tail-to-tail" coupling product of RA

A mechanism for the formation of the 'head-to-tail' coupling product was proposed in which a carbanionic alkyl ligand R2 attacked position 5 of the pyridyl ring of a nickelocycle to give the intermediate compound 85 which then re-aromatizes to give the metal species containing the "head-to-tail" coupled product (Scheme III-6).52

Nevertheless, the mechanism for the formation of compound 83 as well as the reason why the formation of the coupled alkyl ligand [R2-R2] (84) was not observed in the reactions of

[{R2Li}2] with [NiCl2-(TMEDA)] and [NiCl2(PPh3)2] still remained unknown. However, one may suggest that [NiCl2(diphos)] is known to be an important catalyst for coupling reactions in organic synthesis.

C(SiMe3)2 N^'5siMe3)2 \a \ Ni^ • Ni

85 rS^

' ‘ ••• I » I III ••• I 11 • I N CH(SiMe3)2 _ =

〜/V “

Scheme III-6

126 111.2.1,11 Molecular Structure of [Ni{C(SiMe3)2C 5H 4N-2 } {5-(2 ’- CsH4NC(SiMe3)2)CsH4N-2-CH(SiMe3)2}Cl] (83)

The molecular structure of compound 83 with the atom numbering scheme is shown in Figure III-7. Selected bond distances (A) and angles (。）are given in Table III-8. The title compound 83 crystallizes in a monoclinic crystal system in the C2/c space group. The coupled ligand is bound to the nickel center via the pyridyl nitrogen N(2) and is trans to

C⑴ of the alkyl ligand -R2. N(2) of the coupled ligand [R2-R2]，C1(1), and the bidentate ligand 'R^ constitute a square-planar coordination geometry round the nickel(II) center with the metal atom being located slightly above the plane formed by the four groups. The Ni-

C(l) and Ni-N(l) bond distances are 2.024(6) and 1.871(4) A，respectively. Ni-N(2) distance is 2.000(5) A which is much longer than that of Ni-N(l). The observed Ni-Cl(l) bond distance is 2.166(2) A. The bite angle C(l)-Ni-N(2) is 70.7(2)。.

The Ni-Ca bond distance of 2.024(6) A is only 0.054 A shorter than that of

[Ni(R2)2] (82) and is comparable to that of [(Ti5-C5H5)NiR2] (78). However, it is also significantly longer than that of 1.89(1) A in d5"-[Ni(CH2SiMe3)2(i)y)2].5l The Ni-N(l) distance of 1.871(4) A for 83 is comparable to that of 1.887(2) A of 82 and 1.856(2) A of

[(Tl5-C5H5)NiR2] (78). However, it is shorter than that of 1.957(8) A in cis-

[Ni(CH2SiMe3)2(py)2].5l The Ni-N(2) distance for the coupled alkyl ligands is 2.000(5) A which is much longer than the Ni-N(l) distance but is comparable to that of cis-

[Ni(CH2SiMe3)2(py)2].5l The shorter Ni-N(l) distances in 82，83，and [(iiS-CsHyNiR〗]

(78) than those for normal pyridine coordination may be due to the presence of the strained four-membered nickelocycle ring.

An interesting feature of complex 83 is a slight distortion can be observed that the metal atom lies slightly out of the plane formed by the C,A^-chelating ligand _R2，the chlorine atom, and the aromatic nitrogen from [R2-R2]. In fact, the maximum overlap between the pyridyl nitrogen and the metal would involve the metal and heterocyclic ring atoms lying coplanar as in the cases of the dialkyl complexes [Co(R2)2] (56) and [Ni(R2)2]

(82). This slight deformation is believed to take place in order to relieve steric strain at the metal center without necessarily decreasing pyridine-metal bonding.

127 • C30學

/ 熟 C25 JA^ /

/ \ C26

r� i C7T C35 “

C32 err^ I r

% _ _ I 办 Table III-8. Selected Bond Distances (A) and Angles (。）for Compound 83.

[Ni{C(SiMe3)2C5H4N-2) {5-(2’-C5H4NC(SiMe3)2)C5H4N-2-CH(SiMe3)2}Cl] 83

Ni �-Cl(l ) 2.166(2) Ni �-C(l ) 2.024(6)

Ni(l)-N(l) 1.871 � Ni �-C(2 ) 2.426(6)

Ni �- N � 2.000(5)

CI �-N �i - C � 99.3(2) CI �-N �i - N� 167.8(2)

C �-Ni(l)-N(l ) 70.7(2) N(l)-Ni �-N(2 ) 97.9 �

CI �-N �i -N(2 ) 91.6(1) Ni �- C�-C(2 ) 86.0 �

Ni �- N�- C� 97.0(3)

129 The bite angle C(l)-Ni-N⑴ of ligand "R^ in the three nickel(II) complexes 78，82，

and 83 are very similar, viz. 71.5(1)。，70.6(1)。，and 70.7(2)。，respectively.

HL2.1.12 Reactions of [{RlLi(Et20)}2] and [R^Li] (R5 = CH2C5H4N-2) NiCh and [NiCl2L2] (L! = TMEDA, IPPh^)

Attempts to prepare dialkyl complexes [Ni(R")2] by the reaction of [{RlLi(Et20)}2]

and [R5Li] ("R^ = -CH2C5H4N-2) with NiCl〕in THF were unsuccessful. Instead, the

•head-to-head' coupled products [Rl-Rl] (86) and [R5-R5] (87) were obtained,

respectively (Scheme III-7).

+ + Ni(0) + Lie.

H R

86: R = SiMea 87: R = H

Scheme III-7

A THF solution of [{RlLi(Et20)}2] and [R^Li] was added to a suspension of NiCl〕in

THF at 0 °C to afford a clear pale brown supernatant solution with black precipitate. The black solid was shown to contain nickel, presumably nickel(O) species, by X-ray fluorescence spectroscopy. After removal of solvent at reduced pressure, the organic products were extracted into hexane and then separated by column chromatography on silica gel. They were characterized by their ^H NMR and 13c NMR spectra, mass spectra, elemental analysis, and for compound 86 a single crystal X-ray diffraction experiment was carried out. It is conceivable that the formation of [R«-R«] is due to intramolecular reductive coupling of the alkyl groups within the thermally unstable nickel dialkyl compounds formed initially. The thermally decomposition of the benzylnickel(II) compound,

130 • [PhCH2Ni(Br)(PPh3)] is known to give the coupled product PhCH^CHaPh in quantitative

yield.55 Furthermore, coupled product of butane and biaryls are the major products from

thermal decomposition reactions of the thermally stable organonickel(II) complexes cis-

[NiEt2(bipy)] and rran^-[Ar2Ni(PEt3)2].l

In the case of the reaction of [{R^Li}^] with NiCl〗 in THF, direct coupling between

the bulky alkyl ligands "R^ in a 'head-to-head' fashion is sterically unfavourable mainly due

to the bearing of two bulky SiMe; groups on the a-carbon. In contrast to [Ni(R2)2]，the

failure to prepare [Ni(Rl)2] and [Ni(R5)2] were owing to their kinetic lability. The less

bulky alkyl ligands "Rl and "R^ are insufficient to stabilize their corresponding nickel

dialkyls towards further decompositions, such as reductive coupling of alkyl ligands.

Moreover, less bulky alkyl ligands do not provide enough shielding to the central metal

making the latter more susceptible to attacks by the surrounding media. This offers a good

example for the use of bulky ligands to stabilize those otherwise kinetically labile transition metal alkyl complexes.

So far, the result has shown some correlations between the bulkiness of an alkyl

group and its coupling product. For the less sterically demanding alkyl ligands "Rl and

direct "head-to-head" coupling was observed. The less bulkiness on the a-carbon of "Rl

and -R5 makes a "head-to-head" approach of the alkyl ligands feasible. In contrast, only

"head-to-tail" and "tail-to-tail” coupling have been observed for the very bulky ligand "R^.

The variety of coupling products obtained from different alkylating reagents and metal halides are given in Table III-9.

The reaction of [{RlLi(Et20)}2] with [NiCl〗.(TMEDA)] in THF gave also the coupled organic product [Rl-Rl] (86), the same product as with NiCl〗. However, treatment of [{RlLi(Et20)}2] with [NiCl2(PPh3)2] in THF gave an unknown air sensitive, reddish brown compound. ^H NMR studies of this compound indicated the absence of the alkyl ligand _Rl and only the peaks due to triphenylphosphine-containing species were observed. The broadness of the signals in the ^H NMR spectrum indicated the paramagnetic nature of the compound. Mass spectroscopic data show only peaks

131 Table III-9. The Variety of Coupling Products from Different Alkylating Reagents and Metal Halides.

Alkylating Reagent Metal Halides Coupling Products References

/N SiMe3 [{RlLi(Et20)}2] NiCl2, [NiCl2-(TMEDA)] 而厂让

M MeaSi N—^ ro

CH(SiMe3)2 [{R2Lih] AICI3, GaCl3 53b

(Me3Si)2HC

^MeaSi

[{R2Li}2] BiCl3 { 53a N T I N CH(SiMe3)2 Table III-9. Cont，d.

A_atin只 Reagent Metal Halides Coupling Products References

MeaSi

[{R'Li}^] [NiCl2(diphos)] This work

N 丁 I N CH(SiMe3)2 M LJ CO i~ fsl [R礼i] NiCl2 This work

N—y corresponding to triphenylphosphine ([M]+ m/z = 262). Unfortunately, owing to its air sensitivity, reproducible elemental analysis data can hardly be obtained which hampering characterization of the compound. Since intramolecular ortho metallations involving P- donor have been reported, it is suggested that a nickel® species with a probable formula of

[Ni(C6H4-o-PPh2)(PPh3)] would have been formed in our current reaction.

Only diphos was recovered from the reaction of [{RlLi(Et2〇）} 2] with

[NiCl2(diphos)], the remaining brownish oil remained uncharacterized and no coupling product has so far been observed.

The reaction products of the iV-functionalized lithium alkyls [{RlLi(Et20)}2],

[{R2Li}2], and [R5Li] with different nickel(II) compounds together with their reaction products are summarized in Table III-10.

134 Table III-IO. Reactions of [{RiLKEtzO)}�]，[{R^Li}^ an] d [R^Li] with Different Nickel(II) Compounds.

[NiCp2p [NiCp、]。 ^ [Ni(PPh3),Cl,] [NiCl,-(TMEDA)] [NiCl,(diphQs)]

+ »RiLi(Etp))2] t{CpNiRi),](80)^ dark green oil P^i则（86户 [rLr!] (86户 diphos . unidenufied

M reddish brown CO Ln compounds^

+ [{R2Li)2] [CpNiR2] (78)办 [Cp"NiR2] (79)" 82 in poor yield [Ni(R2)J (82)。 [Ni(R2),J (82)^ [Ni(R2)(R2-R2)ci] (83)。

+ [R5Li] -- - [R5.R5] (87户 __

Cp= T15-C5H5, Cp" = Ti5_C5H3(SiMe3)2. b: Ref. 12. c: This work. III.2.1.13 Molecular Structure of [{CH(SiMe3)C5H4N-2}2] (86)

The molecular structure of [{CH(SiMe3)C5H4N-2h] (86) with the numbering scheme is shown in Figure 111-8. Selected bond distances (A) and angles (。）are given in

Table III-l 1. 86 is composed of two nearly identical molecules, one of which is located at a center of symmetry. The bond distances and angles in both molecules are normal

136 C�3 ) C(14) C(15)

CdO) c� JS'^)

Oc(18) ^O C(16) C(17)

Figure III-8. Molecular Structure of lCimMt^)C^U^N-2]2 (86) Table III-ll. Selected Bond Diatances (A) and Angles (。）for Compound 86.

[{CH(SiMe3)C5H4N-2}2] 86

Si(l)-C(l) 1.905(8) Si ⑴-C(13) 1.866(9)

Si(l)-C(14) 1.857(7) Si(l)-C(15) 1.882(7)

Si(2)-C ⑵ 1,907(8) N(l)-C(3) 1.345(13)

N(l)-C(7) 1.517(11) C(l)-C(2) 1.585(11)

C ⑴-C(7) 1.517(11) C ⑶-C ⑷ 1.371(13)

C ⑷-C(5) 1.363(16) C(5)-C(6) 1.360(14)

C(6)-C(7) 1.382(10)

C(l)-Si(l)-C(13) 110.9(4) C(2)-Si ⑵-C(16) 108.2(3)

C(3)-N ⑴-C(7) 116.7(7) C(3)-C ⑷-C(5) 119.8(9)

N ⑴-C(7)-C ⑴ 115.3(6) N(2)-C(12)-C(2) 117.3(6)

C ⑵-C(12)-C(ll) 121.5(8)

138 III.2.2 Synthesis of Palladium(II) Alkyl Complexes

上 1 Reactions of [{R^Li}� ] with [PdXiL!] (L = PPhj, Et2S; X = Cl， Bf^) — Synthesis of [Pd(R2)2] (88) and [Pd(R^)(PPh.)X] (X = Cl 89, Br 90) “ The palladium(II) dialkyl [Pd(R2)2] (88) and monoalkyl [Pd(Rl)(PPh3)X] (X = Cl

89’ Br 90) was prepared by the reactions of [{RZu}〗] with palladium(II) dihalides

[PdX2L2] (L = PPh3, Et2S; X 二 Cl, Br) (Scheme III-8).

MegSi, >SiMe3 /=\ ,[{RLih] /)

/ 冊:25。。’8" - /^Z、M / L = PPh3;X = CI, Br \—/ Me3Si、、.C、SiMe3 / L = SEt2; X = CI / 88

[PdX,L2] -/ 1/病

\ THF, 25

\ MegSi, >SiMe3 \ V2[(R^Li}2] _/ \ /X THF, -30 to 25 /Tv 乂 Pd、〈/ N^ PPh3 L = PPh3； X = Cl, Br \——/ 89:X=:CI 90:X = Br

Scheme 111-8

Reactions between [{R2Li}2] and [PdX2(PPh3)2] at room temperature gave the homoleptic palladium(II) dialkyl complex 88 in satisfactory yield (X 二 Cl 49%, Br 43%).

In addition, [{R2Li}2] and [PdCl2(SEt2)2] in THF also gave 88 in 45% yield. The reactions were carried out in THF and the starting materials were allowed to react at 0 initially and then at room temperature. After removal of THF in vacuo, the residue was

139 extracted with hexane and then chromatographed on silica gel using CHsCls/pentane 1:4 as

eluent to give compounds 88 as bright yellow crystals.

The monoalkyl palladium(II) halides (89-90) were prepared by treating a stirring

suspension of [PdX2(PPh3)2] in THF with one equivalent of [{R^Li}〗] in THF at low

temperature (-30。C) initially and then at room temperature for 8 h. The resultant clear

orange yellow solution was evaporated to dryness and the residue was dissolved in a

minimum amount of toluene. The extract was chromatogaphed on silica gel using

pentane/ether 3:1 to give compound 89 and using pure CHCI3 to give compound 90. Both

compounds 89 and 90 are bright yellow crystals.

Further reactions of the monoalkylpalladium(n) halides (89-90) with [{R2Li}2] led

to further substitution of the remaining halides on 89 and 90 to give the dialkyl compound 88.

The use of complexed palladium(II) halides [PdX2L2] is necessary in order to produce a homogeneous solution (as in the case of [PdCl2(SEt2)2]) and to avoid reduction as it is well known that direct reaction of palladium(II) halides PdX〗 with organolithium reagents usually leads to reduction of the metal. The choice of L in our present work is based on their unreactivity with the starting organolithium reagents and, in the case of L =

SEt2，its volatility so that they can be easily removed in vacuo, [PdCl2(NCPh)2] have been used in the synthesis of [Pd(C6H4-o-CH2NMe2)2] (74)29 However, owing to its potential reactivity with organolithium reagents, PhCN was not chosen as L in our work.

7/7.2.2.2 Molecular Structures of Compounds [P'd{C(SiMe3)2CsH参！)2] (88), [P'd{C(SiMe2)2C 5H 4N-2](PPhs)Cl] (89), and [P'd[C(SiMe3)2CsH4N-2)(PPh^)Br] (90)

• The molecular structures with atom numbering schemes for compounds 88-90 are shown in Figure 111-9 to ni-11，respectively. Selected bond distances (A) and angles (°) are listed in Table 111-12.

140 Q /

lA I。） I r pc(3) O C(6)d / C(5) L/

Figure III-9‘ Molecular Structure of [PhQSiMes)��^!!^!^-?}��(88)

--i-- . — -- . - .二一…-.--‘ —— ---- C15

c/^I J- \ ca

J ©C29 iJ �

• C2Figur6 e© III-IO . Molecular Structur/ e C12 (89) � n C(28)

CST""^^ 等 BHI)

C(4) C(5) ^^^^^•"•Qcns)

C(7) UC(9) C(14)Q \ ^ Ocd?)

Figure ni-ll. Molecular Structure of [pSfCfSiMe])?�-!!力-2KPPh3)I3r ](90) Table III-12. Selected Bond Distances (A) and Angles (。）for Compounds 88-90.

[Pd{C(SiMe3)2C5H4N-2} 2] 88

M ⑴-N ⑴ 2.031(2) Pd(l)-C(l) 2.187(2) Pd ⑴-C(2) 2.581(2)

N(l)-Pd ⑴-C ⑴ 66.6(1) N(l)-Pd(l)-N(la) 180.0(10

C ⑴-Pd(l)-N(la) 113.4(1) C(l)-Pd(l)-C(la) 180.0(1)

Pd(l)-C(l)-C(2) 86.8(2) Pd ⑴-N ⑴-C(2) 97.6(2)

[Pd{C(SiMe3)2C5H4N-2}2(PPh3)Cl] 89

Pd ⑴-Cl(l) 2.324(1) Pd ⑴-P(l) 2.367(2)

Pd ⑴-N ⑴ 2.019(4) Pd(l)-C(l) 2.167(6)

Pd(l)-C(2) 2.578(6)

Cl(l)-Pd(l)-P(l) 89.0(1) Cl(l)-Pd(l)-N(l) 166.0(2)

P(l)-Pd ⑴-N ⑴ 104.4(1) Cl(l)-Pd(l)-C(l) 99.9(1)

P(l)-Pd ⑴-C(l) 171.0(1) N ⑴-Pd ⑴-C ⑴ 66.7(2)

Pd ⑴-N ⑴-C ⑵ 98.0(3) Pd(l)-C(l)-C(2) 87.4(3)

[P'd{C(SiMe3)2C5H4N-2}2(PPh3)Br] 90

Pd(l)-Br(l) 2.414(1) Pd(l)-P(l) 2.350(3)

Pd ⑴-N ⑴ 2.023(6) Pd ⑴-C ⑴ 2.149(9)

Br(l)-Pd(l)-P(l) 94.4(1) Br(l)-Pd ⑴-N ⑴ 167.1(2)

P(l)-P(i ⑴-N ⑴ 98.4(2) Br(l)-Pd ⑴-C ⑴ 100.6(2)

P ⑴-Pd(l)-C(l) 165.1(2) N ⑴-Pd(l)-C(l) 66.6(3)

Pd(l)-C(l)-C(2) 87.9(6) Pd(l)-N(l)-C(2) 97.0(5)

144 Palladium dialkyl complex 88 crystallizes in a triclinic Pl(No.2) space group. It is isostructural to that of the nickel analogue 82: The two alkyl ligands "R^ are C^- bound to the central palladium center in trans square planar geometry. A four-membered cyclometalated ring is formed by each -R2 ligand and the central palladium. In fact, organopalladium(II) complexes containing four-membered cyclopalladated ring are rare. To our knowledge, compound 88 is the first homoleptic palladium(II) dialkyl complex which contains highly strained 4-membered cyclopalladacycle rings being structurally characterized. The Pd-Ca and Pd-N(l) distances are 2.187(2) and 2.031(2) A, respectively. The C(l)-Pd-N(l) bite angle is 66.6(1)。which is smaller than that of 70.6(1)。 in [Ni(R2)2] (82).

The palladium atom of complex 89 is four-coordinated and the approximately square planar coordination sites are occupied by the alkyl ligand "R^, the PPh; group, and the CI group. The ligand _R2 is bound to the palladium center in a C,A^-chelating fashion forming a 4-membered cyclopalladacycle ring. The triphenylphosphine is located trans to the a-C of "R^. The observed Pd-Ca and Pd-N(l) distances are 2.167(6) and 2.019(4) A, respectively. An T]3-aza-allyl type ligation of "R^ to the palladium(II) center is excluded as reflected by the long Pd-C(2) distance of 2.578(6) A. The C(l)-Pd-N(l) biting angle is

66.7(2)0. These bond distances and angles are comparable to those of the dialkyl complex

88. Pd-P(l) and Pd-Cl(l) distances are 2.367(2) and 2.324(1) A, respectively.

Compound 90 is isostructural to compound 89 with Br in the former complex substitutes for the position of CI in the latter one. 90 crystallizes in a monoclinic Plilc

(No. 14) space group. The Pd-Ca distance is 2.149(9) A which is slightly shorter than that of 88 and 89. The bond distances Pd-N(l), Pd-P(l), and Pd-Br(l) are 2.023(6),

2.350(3), and 2.414(1) A, respectively. The bite angle C(l)-Pd-N(l) is 66.6(3尸 which is nearly identical to that of 89.

Pd-C isp^) distances usually fall in the range 2,0-2.2 A. This can be compared with that of 2.07 A calculated from the sum of covalent radii of Pd(n) and The Pd-Ca distances of compounds 88，89 and 90 are 2.187(2), 2.167(6) and 2.149(9) A, respectively, falling well within the range. Apparently, the highly strained four-membered

145 • palladacycle rings do not cause a significant change in Pd-C distances among these

complexes. The Pd-N⑴ distances are 2.031(2), 2.019(4) and 2.023(6) A for compounds

88-90，respectively. Pd(II)-N lengths found in square-planar palladi画(II) complexes in

which the Pd-N bond is neither trans to a ligand with strong 广训^influence nor sterically

hindered are in the range of 1.99 - 2.04 人.56 The Pd-N⑴ distance of 2.031(2) A for

[Pd(R2)2] (88) in which each nitrogen is trans to the a-carbon of another _R2 falls at the upper limit of the range although it has been well-known that a-carbon donors are strong rr^zn^-influencing ligands. The Pd-N(l) distances of 2.019(4) and 2.023(6) A for complexes 89 and 90, respectively still fall within this range although the two complexes do bear sterically demanding ligands.

The Pd-P(l) bond lengths of compound 89 and 90 viz. 2.367(2) and 2.350(3) A, are relatively long. This may be probably due to the strong tram effect arising from the a-C donor "R^. The bite angle C(l)-Pd-N(l) of 88-90 are nearly identical, viz. 66.6(1)。，

66.7(2)0 and 66.6(3)°. These angles are smaller than the corresponding angles of the square planar nickel(II) alkyl complexes 82 and 83 as we have discussed in the previous sections.

III.2.2.3 Spectroscopic Properties of Compounds 88-90

Compounds 88-90 were characterized by mass spectroscopy, ^H, 13c and 31p

NMR spectroscopy, UV-vis spectroscopy (for 88)，and elemental analysis in addition to single crystals X-ray diffraction studies.

Mass Spectra:

Both the palladium dialkyl [Pd(R2)2] (88) and the palladium alkyl bromide

[Pd(R2)(PPh3)Br] (90) showed their respective molecular ion peak [M]+ m/z = 578 and

683.

146 NMR Spectra:

Only one set of signal was observed on the ^H and 13c NMR of the dialkyl complex

[Pd(R2)2] (88) indicating that the two alkyl groups are equivalent. Low intensities on the

13c NMR spectrum for peaks due to the a-carbon and the quaternary carbons of the pyridyl ring are the consequence of their long relaxation time as expected.

An interesting feature can be found in the l^c NMR spectra of the complex

[Pd(PPh3)(R2)Cl] 89. '^KP-Ca) coupling is large, namely 73.58 Hz (Figure HI-12). The large phosphorus-carbon coupling suggests that the molecular structure of complex 89 in solution remains identical to that of it in solid state, that is the a-carbon is still trans to the

PPh3 group. In addition, long range phosphorus-carbon coupling, extending up to five bonds, i.e. ^Jp-c, can be observed for the signals of aromatic carbons of the pyridyl rings with coupling constants range from 3.77 to 11.32 Hz. This suggests that delocalization of

7i-electrons via Pd-N dn-pn interaction and Pd-P dn-dn interaction must have contribution.

UV-Vis Spectra:

The UV-vis spectrum of 88 consists of bands at 365 nm (e = 6.5 x 10^ M-^cm"^),

301 nm (sh, e = 4.7 x 10^ M-^cm-^), and 277 nm (e = 5.8 x 10^ M-hm'^), These are assigned to charge transfer and ligand-ligand transition bands.

111.2,2.4 Electrochemistry of [P'd{C(SiMe3)2C^H4N-2h] (88)

The cyclic voltammogram of compound 88 is similar to its nickel(II) analogue 82.

A single sweep c.v. of compound 88，sweeping initially in the anodic direction, shows a reversible one electron oxidation at E1/2 = 1.14 V (at 100 mVs'^) (Figure III-13). In contrast to that of compound 82，sweep-rate dependence of 88 shows that the Pd(III) species formed by oxidation is completely stable on the c.v. time-scale. This strongly suggests that the stability of the Pd(III) species is higher than that of the Ni(in) species.

147 • — __

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Figure 111-12, NMR Spectrum of [P'd{C(SiMe])?。。!!*!^-:}(PPh3)CI] (89)

148 1.0 0 £/V versus [FeCri-C^H^)^]''^^

Figureni-13. Cyclic voitammogram of 4 x lO'^ mol dnT) (88). 02 moi dm-j [NBu^JtBF^] in THF. Sweep rate = 0.100 V s].

149 Studies on Stereospecificity of the Reactions between [[R^Li]2] ^nd [PdX2(PPh3)2] (X = Cl，Br)

Since only compound 89 in which a-carbon of -R2 being located trans to PPhs was

isolated by the reaction between [{RZLi}〕] and [PdCl2(PPh3)2]，it is of interest to study the

relationship between stereochemistry of the starting palladium(II) halides [PdX2(PPh3)2]

and that of the products. Experiments were carried out by starting with d^KPdBr2(PPh3)2]

and rra似-[PdBr2(PPh3)2]. Surprisingly, the products of the reactions in both cases are

identical: only the square planar complex [Pd(R2)(PPh3)Br] in which a-C of -R2 being

施ris to PPh3 were isolated (Scheme III-IO).

Ph3P\ ^PPh3 Pd b/ \已「 ,C Br OR + V2 [{R\i}2] ( PcjZ V / \ Ph3P\ Br N PPh3 Pd 90 Br, \pph3

一 1 ] C N J^ 々 (Me3Si)2CZ、NZ

Scheme III-IO

In order to account for the results, an associative mechanism is proposed herein.

Firstly，the aromatic iV-donor of [{R2Li}2] is believed to coordinate to the fifth octahedral site of the starting complexes forming a five-coordinate intemediate (Scheme III-ll).

150 Ph3P\ Br pZ

as N CLi N、

Z PhaP PhsP ^ Ph3P\ , Br Br Pd z BrZ .\pph3 _PPh3 trans ，, 90 Scheme III-ll

Subsequent attack on the equatorial bromine atom by the carbanion followed by the leaving of PPh3 gives compound 90. It is believed that the product formed in such a way that the

PPI13 group remains located trans to the a-C in order to reduce steric congestion if the two groups were located cis to each other.

As cis configuration had been reported for dialkyl complexes 72-75，one may suppose that further reaction of the alkyl palladium(II) halides 89 and 90 with [{R2Li}2] would probably give a cis a-bonded palladium(II) dialkyl complex. However, treatment of

89 and 90 with one equivalent of [{R^Li}〗] gave the m3AW-palladium(II) dialkyl complex

88 as the only product (Scheme III-12).

^ Pd

trans 88

X = Cl 89, Br 90 / \ I J N 乂 cis

Scheme III-12

151 An associative mechanism is proposed to account for the experimental results

(Scheme III-13). Again the aromatic "-donor is believed to coordinate on to the fifth

octahedral site of the starting complex (89 or 90) followed by subsequent attacks from the

carbanion on the halide forming a five-coordinate intermediate. Leaving of the PPh^ group

gives the dialkyl complex 88. The formation of the trans square planar configuration over

the CIS configuration in the final rearrangement step may be attributed to the alleviation of

steric congestion caused by the two alkyl groups. r\ N CU N"""^

、Z \pph3 X领、N

PPh3 PPhs

-PPh3

88 Scheme 111-13

111.2.2.6 Attempted Synthesis of [Pd(Rl)2] and [Pd(Ri)(PPhj)Cl]

As palladium-carbon a bond is thermodynamic ally more stable than Ni-C a bond, attempts were carried out to synthesize palladium alkyl complexes with the less bulky ligand

-Rl. Unfortunately, efforts were unavailing and "Rl only gave an air sensitive intractable oil with [PdCl2(PPh3)2].

152 III.2.2.7 Attempted Synthesis of [{Pd(R2)Cl}2] via Intramolecular C-H Activation

Attempted synthesis of [{Pd(R2)Cl}2] by treating R^H with a methanol solution of

-K2PdCl4] at room temperature was unsuccessful Only a yellow solid was obtained. The solid was recrystallized from toluene to give pale yellowish brown crystals which was characterized by X-ray crystallography to be bis[2-bis(trimethylsilyl)methylpyridine]- palladium(II) chloride (91). Alternatively, stirring of R^H with PdCl〗 in acetone under nitrogen for 2 days also afforded compound 91 in good yield. In the latter case, the PdCl〗 dissolved gradually and a pale yellowish brown solid of compound 91 was obtained

(Scheme 111-14).

\^CH(SiMe3)2 CN\ CI — /d

(SiMe3)2

Scheme 111-14

Compound 91 is insoluble in common solvents such as hexane and ether, but readily soluble in chloroform, dichloromethane, acetone, benzene, and toluene. The solution should be handled with the exclusion of air as the compound, once being dissolved in solvents, is slightly air sensitive and decomposed to a black solid.

Although the formation of a planar five-membered ring containing the palladium metal is one of the requirements for cyclopalladation reaction involving C-H activation of functionalized ligands, as proposed by Cope et ^z/.,30,40 the occurrence of strained, four-

153 membered palladacycle ring in compounds 88-90 prompts us to look into the possibility of metallation of 2-bis(trimethylsilyl)methylpyridine (R^H) by [K^PcICIa] or PdCl?. However，activation of the benzyl C-H bond on the picoline derivative which would result in the formation of four-membered metallacycle ring was unsuccessful and the requirements proposed by Cope et a/.30,40 still hold in this case.

m Molecular Structure of [{CH(SiMej)2C^H4N-2}2PdCl2] (91)

In Figure 111-14, the molecular structure of compound 91 with the atom numbering scheme is shown. Selected bond distances (A) and angles (。）are tabulated as shown in

Table III-13. The square planar coordination sites are occupied by two A^-coordinated 2- bis(trimethylsilyl)methylpyridine ligands and two chlorine atoms in which a trans geometry is adopted.

An interesting feature of compound 91 is that the two picoline ligands bond to the metal in such a way that the alkyl side chains of the ligands are located on the same side of the N2PdCl2 plane. The expected linear N(l)-Pd-N(2) bond bent slightly upwards apparently due to the steric crowding of the four SiMeg groups [Z N(l)-Pd-N(2)=

171.3(3)。]. The molecular planes of the two pyridyl rings are not coplanar, but twisted in such a way that the crowding of the four SiMe] can be reduced substantially. The Pd-N(l) and Pd-N(2) distances of 2.046(8) and 2.014(8) A, respectively, are comparable to the Pd-

N distance of 2.031(2) A in the dialkyl complex 88, 2.019(4) A in the monoalkyl complex

89，and 2.023(6) A in 90. The Pd-Cl⑴ and Pd-Cl(2) distances in 91 are 2.296(3) and

2.312(3) A, respectively, which is also similar to the Pd-Cl distance of 2.324(1) A in 89.

154

！K C(17,

0C(24) C(9) O

Figure III-14. Molecular Structure of [PdlNC^H,CII(SiMe3V2)2CI,] (91) f Table III-13. Selected Bond Distances (A) and Angles (� fo) r Compound 91.

[Pd{CH(SiMe3)2C5H4N-2} 2CI2] 91

Pd(l)-Cl(l) 2.296(3) Pd ⑴-CI ⑵ 2.312(3)

Pd(l)-N(l) 2.046(8) Pd(l)-N(2) 2.014(8)

N(l)-C(2) 1.34(1) N(l)-C(6) 1.34(1)

N(2)-C(14) 1.35(1) N(2)-C(18) 1.34(1)

CI �-Pd(l)-a(2 ) 176.8(1) CK1)-Pd �-N(l ) 87.8(2)

Cl(2)-Pd(l)-N(l) 92.2(2) Cl(l)-Pd(l)-N(2) 90.4(3) Cl(2)-Pd(l)-N(2) 89.1(2) N(l)-Pd(l)-N(2) 171.3(3) Pd(l)-N(l)-C(2) 128.4(7) Pd(l)-N(l)-C(6) 113.1(6)

Pd(l)-N(2)-C(14) 128.9(5) Pd(l)-N ⑵-C(18) 112.1(7)

156 • III.3 EXPERIMENTALS FOR CHAPTER III

Materials:

Anhydrous NiCl2 was prepared by standard procedure.57 a-picoline,

[PdCl2(PPh3)2]，cz^-[PdBr2(PPh3)2], [PdBr2(PPh3)2]，and [KaPdCl。] was purchased from The Aldrich Chemical Co. and used without further purification.

[{RlLi(Et20)}2],58 [{R2Li}2],58 [Ni(Ti5_C5H5)2]，45 and [PdCl2(SEt2)2]59 were prepared as described in the literatures.

Synthesis of Compounds:

Synthesis of [NUTiS-CsHjCSiMe〕）：}!]，81. Method A, by the reaction of nickel(n) chloride with Li[C5H3(SiMe3)2]. A solution of Li[C5H3(SiMe3)2] (1.84 g, 8.5 mmol) in 25 ml of THF was added to a slurry of nickel(II) chloride (0.55 g，4.24 mmol) in

10 ml THF. The mixture was heated under reflux for 2.5 h, during which the colour changed to dark green. All volatiles were then removed in vacuo and the residue was extracted with hexane. Filtration of the extract followed by concentration and cooling to -30

。C yielded dark green crystals of [Ni{t]5-C5H3(SiMe3)2}2] (yield 0.84 g, 42%).

Method B. by the reaction of 'activated, nickel(II) bromide with Li[C5H3(SiMe3)2].

Bromine (0.53 ml, 10.3 mmol) was added to a stirred suspension of nickel powder (0.57 g，

9.65 mmol) in 30 ml of dimethoxyethane, and the resulting mixture was stirred until the supernatant solution turned colourless. (During this period, a yellow precipitate was observed.) The solvent was then removed in vacuo, 20 ml of THF was added, and a solution of Li[C5H3(SiMe3)2] (4.15 g, 19.2 mmol) in 70 ml THF was added with stirring, the reaction mixture immediately turned green. After 6 h stirring at room temperature, the solvent was removed in vacuo and the residue was Soxhlet-extracted with pentane. The extract was concentrated and dark green crystals were obtained (yield 3.7 g, 81%). Mp:

112-114。C. MS: m/z (%) = 475 (100) [M]+，460 ⑷[M-CH3]+，404 (3) [M-SiMe;]..

Anal. Found: C，55.42; H, 8.57%. Calc. for C22H42Si4Ni: C, 55.33; H, 8.86%.

157 Synthesis of 78. A solution of [Ni(Ti5.

C5H5)2] (0.75g, 4 mmol) in 15 ml THF was added to one equivalent of [{R^Lih] (0.97 g,

4 mmol) in 20 ml of TOP at 0 After 30 min stirring at 0 ^C the solution had turned from dark green to greenish-brown. It was stirred for a further 6 h at room temperature and the solvent was then removed in vacuo. The residue was extracted with hexane and the insoluble LiCp, as a white precipitate, was filtered off. The filtrate was concentrated at room temperature, then set aside to give brownish-green crystals (yield 1.1 g，77%). Mp:

186-188。C (dec.). MS: m/z (%) = 359 (96) [M]+, 344 (35) [M-CH3]+，294 (5.6) [M-

C5H5]+. Anal. Found: C，56.73; H, 7.53; N, 3.97%. Calc. for Ci7H27NSi2Ni: C,

56.67; H, 7.55; N，3.89%. ^H NMR (250 MHz, CeDg): 5 0.30 (s, MegSi，18H), 5.25 (s,

C5H5，5H), 5.90-5.96 (m, pyridyl, IH)，6.05-6.09 (m, pyridyl, IH)，6.80-6.87 (dt, J 二

1.8 and 7.8 Hz, pyridyl, IH), 7.11-7.15 (m, pyridyl, IH). 13c{1h} NMR (62.89 MHz,

C6D6):S 2.20 (MesSi), 89.06 (C5H5), 117.74, 121.96, 134.77，152.63，176.96

(C5H4N).

Synthesis of [{Ti�C5H3(SiMe3)2}叫C(SiMe3)2C5H4r^-2}]，79 . [{R^Li}�]

(0.51 g, 2.12 mmol) in 20 ml THF was added to a stirred solution of [Nifri^- C5H3(SiMe3)2}2] (1.01 g, 2.12mmol) in THF (20 ml) at 0。(：. The colour changed slowly from yellowish-green to brownish-green. It was stirred for a further 10 h and the volatiles was then removed. The brown residue extracted with hexane. The white precipitate was filtered off and the extract concentrated and cooled to -30 °C to give dark brown crystals

(yield 0.9 g, 84%). Mp: 99-101。C. MS: miz (%) = 503 (100) [M]+，488 (15) [M- C5H3(SiMe3)2;r. Anal. Found: C, 54.67; H, 8.48; N，2.64%. Calc. for C23H43NSi4Ni:

C，54.72; H, 8.59; N, 2.78%. ^H NMR (250 MHz, CyDg): 5 0.49 (s, MegSi，18H), 0.55

(s, MegSi，18H), 5.15-5.16 (m, C5H3, IH), 5.20-5.21 (m, C5H3, 2H), 6.17-6.28 (m, pyridyl, 2H), 7.03-7.10 (dt, J = 1.6 and 7.9 Hz, pyridyl, IH), 7.42-7.44 (dd, J 二 0.9 and

4.6 Hz, pyridyl, IH). l^CpH} NMR (62.89 MHz, CyDg): 5 1.80 (MegSi); 3.23 (MegSi);

90.67 (C5H3); 96.78 (C5H3); 111.29 (C5H3); 118.51, 122.70，135.96, 153.27, 178.29

(C5H4N).

158 Synthesis of [{(TiS-C5H5)Ni{CH(SiMe3)C5H4N-2}}2]，80. A solution of

[{RlLi(Et20)}2] (0.72 g, 2.94 mmol) in THF (20 ml) was added with stirring to a solution

of [Ni(Ti5-C5H5)2] ( 0.55 g, 2.94 mmol) in THF (20 ml) at 0 The solution turned from

blue-green to dark brown. After 6 h stirring, the solvent was removed in vacuo and the

residue extracted with hexane. The white precipitate of LiCp was filtered off and the extract

concentrated, to give dark red crystals which were recrystallized from benzene (yield 0.61

g, 72 %)； Mp: 170-172。C (dec.). MS: m/z (%) = 287 (35) [M]+’ 272 (9) [M-CH3]+，214

(23) [M-SiMe3]+. Anal. Found: C, 58.61; H, 6.62; N, 4.85%. Calc. for Ci4Hi9NSiNi:

C, 58.37; H，6.65; N, 4.86%. iR NMR (250 MHz, QD^): 5 0.43 (s，Me^Si，9H), 4.39

(s, CHSi，IH), 4.76 (s, C5H5, 5H), 6.02-6.05 (m, pyridyl, IH), 6.27-6.30 (m, pyridyl,

IH)，6.48-6.55 (m, pyridyl, IH), 8.45-8.46 (m, pyridyl, IH). 13c{1h} NMR (62.89

MHz，C6D6): 5 1.94 (MegSi)，(CH) obscured, 92.03 (C5H5), 113.60，120.80, 134.95，

139.40，156.17 (C5H4N).

Synthesis of [Ni{C(SiMe3)2C5H4N-2}2], 82. Method A. 0.23 g (2 mmol) of

N,N,A^',A^'-tetramethylethylenediamine was stirred with a suspension of NiCl〗(0.26 g, 2 mmol) in 20 ml THF at 25 for 8 h. A solution of [{R2Li}2] (0.49 g, 2 mmol) in 20 ml

THF was added slowly to the resultant suspension at -40 After the addition was completed, the mixture was stirred at ambient temperature for a further 8 h to give a reddish brown solution. The solvent was removed in vacuo and the residue was extracted with hexane. The solution was filtered and concentrated. It was chromatographed on silica gel using pentane as eluent. All the red fraction was collected. The solvent was removed under reduced pressure to give 82 as red crystals (yield 0.53 g, 50%). Mp: 158-160 MS. m/z (%) = 531 (9.3) [M]+, 295 (54) [M-R2]+. Anal. Found: C, 54.22; H, 8.37; N，

5.18%. Calc. for C24H44Si4N2Ni: C, 54.22; H, 8.34; N, 5.27%. iR NMR (250 MHz,

C6D6): 5 0.474 (s, MesSi, 18H), 6.01-6.06 (m, pyridyl, IH), 3.32 (m, pyridyl, IH),

6.73-6.79 (dt, J = 1.7 and 7.9 Hz, pyridyl, IH), 7.56-7.58 (m，pyridyl, IH). 13c{1H}

NMR (62.89 MHz, CeDe)： 5 4.10 (MesSi), not observed (CHSi), 117.64, 125.58,

136.43，149.15, 179.68 (C5H4N).

159 • 腳—B. A solution of [{R^U}2] (0.49 g，2 mmol) in 20 ml THF was added slowly to a suspension of [NiCl2(PPh3)2] (1.31 g, 2 mmol) in 30 ml THF at -40 The resultant mixture was stirred at room temperature for 8 h. The resultant reddish brown solution was worked up by a similar procedure as described in Method A to give 0.51 g (48%) of the title compound.

Synthesis of [Ni{C(SiMe3)2C5H4l<^.2}{5-(2'-C5H4NC(SiMe3)2)C5H4N-2-

CH(SiMe3)2}Cl]，83. A solution of [{R^Li}〗](0.36 g，1.5 mmol) in ether (30 ml) was added dropwise to a slurry of [NiCl2(diphos)] (0.79 g, 1.5 mmol) in ether (20 ml) at 0。C.

After the addition had been completed, the resultant mixture was kept being stirred at 0 °C for an additional 15 min and then at 25 for 8 h. The reddish brown supernatant solution was filtered through Celite. The filtrate was concentrated in vacuo followed by cooling at

-30 for 1 night to give dark reddish brown microcrystalline substance which was recrystallized from toluene to give complex 83 as dark red crystals (yield 0.42g，35%).

Mp. 220-222。C (dec.). MS. m/z (%) == 472 (49) [R2-R2]+, 457 (16) [(R2-R2)-CH3]+，399

(10) [(R2-R2)-SiMe3]+，236 (20) [R2]+，222 (100) [R^-CR^]^. Anal. Found: C, 54.25;

H, 8.18; N，5.05%. Calc. for C^s^^^Cm^Si^Ni: C, 53.81; H，8.28; N，5.23%.

Reaction of [{RlLKEtzO)}!] with NiCh. To a slurry of NiCl〗(0.26 g, 4 mmol) in

20 ml THF at 0。C was added [{RlLi(Et20)}2] (0.98 g, 4 mmol) in THF (25 ml). The colour turned from light brown to dark brown. After two days' stirring, a pale brownish green solution and black precipitate had been formed. The solvent was then removed and the residue extracted with hexane. After filtration of the extract and evaporation of the solvent the residue was chromatographed (silica gel) with 3:1 hexane/ethyl acetate mixture as eluent to give [{CH(SiMe3)C5H4N-2}2] (86) as white crystals (yield 0.51g, 39%). Mp:

130-132 MS: m/z (%) = 328 (5) [M]+, 313 (5) [>1-013]+，254 (40) [M-SiMe〕]..

Anal. Found: C, 65.63; H, 8.49; N，8.34%. Calc. for CisHasN^Sis: C，65.79; H, 8.59;

N，8.52%. IH NMR (250 MHz, CDCI3): 5 -0.43 (s, MegSi, 18H), 3.25 (s, CHSi，2H),

6.96-7.01 (dd, J = 5.1 and 7.4 Hz, pyridyl, 2H), 7.11-7.14 (d, J = 7.7 Hz, pyridyl, 2H),

160 7.49-7.50(5) (m，pyridyl, 2H), 8.48-8.50 (dd, J = 0.8 and 4.8 Hz, pyridyl, 2H).

13C{1H} NMR (62.89 MHz, CDCI3): 5 -1.55 (MegSi)，40.22 (CHSi), 119.87, 123.98,

135.57, 149.11, 164.71 (C5H4N).

Reaction of [{RlLi(Et20)}2] with [NiCl2.(TMEDA)]. 0.25 g (2.15 mmol) of

A^A^Ar，iV'-tetramethylethylenediamine was stirred with a suspension of NiCl? (0.28 g,

2.15 mmol) in THF (20 ml) at 25。C for 8 h. A solution of [{RlLi(Et20)h] ( 1.05 g, 4.30 mmol) in THF (30 ml) was added dropwise to the resultant suspension at -20 °C After the addition had been completed, the resultant mixture was further stirred at 25 °C for 20 h. All volatiles were then removed under reduced pressure and the residue was extracted with hexane. The black precipitate was filtered off and the filtrate was chromatographed on silica gel with hexane/ethyl acetate 3:1 as eluent to give [{CH(SiMe3)C5H4N-2}2] (86) as white crystals (yield 0.27 g, 38 %). Mp: 130-132

Reaction of [R^Li] with NiCl!. A solution of [R^Li], freshly prepared by treating 1.6 ml (16.2 mmol) of a-picoline with 10.6 ml (17.0 mmol) of ^BuLi in a 1:1 mixture of THF and hexane, was added dropwise to a stirred suspension of NiCl〗(1.1 g, 8.1 mmol) in 20 ml of THF at 0 °C. The reaction mixture was stirred at room temperature for two days and the solvent was then removed under reduced pressure and the residue extracted with hexane. The black precipitate was filtered off and the filtrate concentrated and then chromatographed (silica gel) with 3:1 hexane/ethyl acetate mixture as eluent to remove impurities. Final elution with pure ethyl acetate gave [(CHaCsH^N-Z)〗](87) as white crystals (yield 0.15 g，10.1 %). Mp: 39-40。C. MS: mh (%) 二 184 (64) [M]+, 169 (5.1)

[M-CH3]+, 154 (6) [M-2CH3]+，106 (100) [M-CsHaN].. iR NMR (250 MHz, CDCI3): 6

3.21 (s, CH2, 2H), 7.05-7.11 (m, pyridyl, 2H), 7.49-7.56 (dt, J 二 1.8 and 7.6 Hz, pyridyl, IH), 8.51-8.53 (dd, J 二 0.7 and 4.8 Hz, pyridyl, IH).

Synthesis of [P'd{C(SiMe3)2C5H4r^-2}2]，88. Method A, A THF solution (30 ml) of [{R2Li}2] (0.31 g, 1.27 mmol) was added dropwise to a stirring suspension of

161 [PdCl2(PPh3)2] (0.445g，0.635 mmol) in THF (20 ml) at 0 After the addition had

completed, the orange solution was further stirred at 25 for 8 h. The solvent was

removed in vacuo and the orange-red oily residue was extracted with hexane. The solution

was chromatographed on silica gel using CHsCVpentane 1:4 as eluent. The yellow

fraction was collected. Removal of solvent under reduced pressure gave the title compound

as bright yellow crystals (yield 0.18 g, 49 %). Mp: 212-214。C (dec.). FAB-MS: m/z (%)

=578 (29) [M]+，342 (28) [M-R2]+，236 (52) [R2]+. Anal Found: C, 49.89; H，7.68;

N, 4.78%. Calc. for C24H44N2Si4Pd: C, 49.75; H, 7.65; N, 4.83%. ^H NMR (250

MHz, CDCI3): 5 0.11 (s, MesSi, 18H), 6.63-6.66 (m, pyridyl, IH), 6.74-6.79 (m,

pyridyl, IH), 7.43-7.50 (dt, J = 1.7 and 7.8 Hz, pyridyl, IH), 7.94-7.96 (dd，J = 0.7 and

5.4 Hz, pyridyl, IH). l3c{lH} NMR (62.89 MHz, CDCI3): 5 3.54 (Me^Si), 30.41

(CSiMe3)’ 117.67，126.63’ 136.09, 148.63’ 182.34 (C5H4N).

Method B. A similar procedure as described in (A) starting from [{R^Li}〗] (0.378g,

1.54 mmol) and rr咖MPdBr2(PPh3)2] (0.61 g, 0.772 mmol) to give compound 88 as

bright yellow crystals (yield 0.19 g, 43 %).

Method C. [{R2Li}2] (0.26 g, 1.09 mmol) in THF (25 ml) was added slowly to a

solution of [PdCl2(SEt2)2] (0.19 g, 0.54 mmol) in the same solvent (15 ml) at 0。C. The resultant olive solution was stirred at 25 for 8 h. All volatiles was removed in vacuo and

the oily residue was extracted with hexane. The black precipitate was filtered and the olive

solution was concentration followed by cooling of the solution at -30 °C for 1 night to give compound 88 as yellowish-brown crystals (yield 0.14 g, 45 %). Analytically pure sample of 88 can be obtained by recrystallization of the crude product from toluene.

Synthesis of [Pd{C(SiMe3)2C5H4r^-2}(PPh3)Cl]，89. To a stirring suspension of [PdCl2(PPh3)2] (0.45 g, 0.64 mmol) in THF (25 ml) at -40。匸 was added a solution of

[{R2Li}2] (0.31 g, 1.28 mmol) in THF (30 ml) through a dropping funnel. The mixture was allowed to warm back to ambient temperature slowly whilst the yellow [PdCl2(PPh3)2' dissolved gradually to give a clear yellowish brown solution. It was further stirred at room temperature for 8 h. The solution was evaporated to dryness leaving a brown oil which was

162 extracted with toluene. The mixture was chromatographed on silica gel using pentane/ether

3:1 as eluent and the yellow fraction was collected. Removal of the solvents gave

compound 89 as bright yellow crystals (yield 0.22 g, 53 %). Mp: 172-175 (dec.).

Anal. Found: C，56.54; H, 5.90; N，2.06%. Calc. for CsoHs^ClNPSizPd: C, 56.25; H，

5.82; N, 2.19%. IH NMR (250 MHz, CDCI3): 5 0.28 (s，MegSi，18H), 6.39-6.49 (m，

pyridyl，2H), 6.72-6.75 (m, pyridyl, IH), 7.33-7.46 (m, PhgP, 9H), 7.46-7.50 (dt, J =

1.9 and 7.7 Hz, pyridyl, IH), 7.61-7.69 (m, PhgP，6H). 13c{1h} NMR (62,89 MHz,

CDCI3): 5 3.20 (MegSi)，22.37 (d, J 二 73.58 Hz, CSiMe]), 125.84 (d，J = 6.92 Hz,

C5H4N), 128.92 (d, J : 10.69 Hz, C5H4N), 129.18 (d, J = 9.43 Hz, PPh])，130.94

(PPh3，)，132.79 (d, J 二 32.07 Hz, PPh〕)，135.42 (d，J = 12.58 Hz, PPh])，136.46 (d, J =

1L32 Hz, C5H4N), 144.01 (d，J 二 3.77 Hz, C5H4N), 182.82 (d，J = 10.06 Hz, C5H4N).

31P{1H} NMR (101.26 MHz, CDCI3): 5 11.02 (s).

Synthesis of [Pd{C(SiMe3)2CsH4]<^2}(PPh3)Br]，90. Method A, A solution

of [{R2Li}2] (0.36 g, 1.50 mmol) in THF (30 ml) was added slowly to a slurry of trans-

[PdBr2(PPh3)2] (0.59 g, 0.75 mmol) in the same solvent (20 ml) at -40。(：. The resultant clear yellowish brown solution was further stirred at room temperature for 8 h‘ The solution was evaporated to dryness and the residue was extracted with toluene. The solution was chromatographed on silica gel using CHCI3 as eluent to give the title compound as yellow crystals (yield 0.26 g, 51%). Mp: 199-200 (dec.). FAB-MS: m/z

(%) = 683 (26) [M]+, 603 (29) [M-Br]+，236 (52) [R2]+. Anal. Found: C, 52.92; H,

5.49; N, 1.99%. Calc. for CgoHsyBrNPSi^Pd: C，52.60; H, 5.44; N，2.04%.

Method B. An essentially identical procedure as described in (A) gave compound 81 as yellow crystals. Yield 0.23 g (45 %) form 0.59 g (0.75 mmol)aHPdBr2(PPh3)2].

Reaction of [Pd(R2)(PPh3)Cl] (89) with [{R^Li}!]. A solution of [{R^Lih]

(0.10 g, 0.43 mmol) in THF (20 ml) was added to a stirring solution of compound 89

(0.27 g, 0.43 mmol) in THF (20 ml) at 0。C. The resultant yellow solution was further stirred at 25 °C for 6 h. After solvent removed and extraction of the residue with benzene,

163 • the solution was filtered. The filtrate was concentrated in vacuo and cooling to -30 °C for

one night yielded the dialkyl compound 88 as yellow plates (yield 0.14 g, 56 %).

Reaction of [Pd(R2)(PPh3)Br] (90) with [{R^Lih]. A procedure identical with

that described for complex 89 was employed starting with complex 90 (0.25 g, 0.37

mmol) gave the dialkyl complex 88 as yellow plates (yield 0.13 g, 60 %).

Synthesis of [Pcl{CH(SiMe3)2C5H4N-2}2Cl2]，91. Method A. The free ligands CH(SiMe3)2C5H4N-2 (R2H) (0.24 g, 1.0 mmol) was added to a stirring solution of

[K2PdCl4] (0.33 g, 1.0 mmol) in methanol (20 ml) at 25 A yellow precipitate formed immediately and the reddish brown solution faded gradually. The suspension was further for 1 h until the solution had decolourized. The yellow precipitate was filtered followed by washing with cold methanol and hexane and then dried in vacuo. The product can be recrystalHzed from toluene (yield 0.55, 85%). Mp: 182。C (dec.).

Method B. A heterogeneous mixture of CH(SiMe3)2C5H4N-2 (R2H) (1.90 g，8.0 mmol) and PdCl2 (0.71 g, 4.0 mmol) in methanol (40 ml) was stirred under N2 at 25。C. After being stirred for 24 h, all PdCl2 had dissolved and a yellow brown precipitate was obtained.

It was filtered, washed with cold methanol and then hexane and dried in vacuo to give the title compound as yellow brown precipitate (1.86 g, 71%), Mp: 185 (dec.).

164 • III.4 REFERENCES FOR CHAPTER III

(1) Jolly, P.W.; Wilke, G. The Organic Chemistry of Nickel: Volume I and //; Academic

Press, London, UK., 1974.

(2) Jolly, P.W. In Comprehensive Orgnometallic Chemistry; Wilkinson, G.; Shone,

F.G.A.; Abel, E.W., Eds.; Pegamon: Oxford, UK” 1982; Volume 6，Chapter 37.4.

(3) Lehinkuhl, H.; Kriiger, C; Pasynkiewicz, S.; Poplawska, J. Organometallics 1988，

7, 2038.

(4) Pasynkiewicz, S.; Pietrzykowski, A.; Poplawska, M‘ J. Organomet. Chem. 1993, 443, 131.

(5) Yamazaki, H.; Nishido, T.; Matumoto, Y.; Sumida, S.; Hagihara, N. J. Organomet. Chem. 1966, 6, 86.

(6) Rausch, M.D.; Chang, Y.F.; Gordon, H.B. Inorg. Chem. 1969, 5, 1355.

(7) Yamazaki, H.; Nishido, T.; Matsumoto, Y.; Sumida, S.; Hagihara, N. J. Organomet. Chem. 1966, 6, 86.

(8) Yamazaki, H‘； Hagihara, N‘ Bull. Chem. Soc. Jap. 1964，37, 907.

(9) Thomson, 1; Baird，M.C. Can. J. Chem. 1967，48, 3443.

(10) Collier, M.R.; Lappert, M.F.; Truelock, MM. /. Organomet. Chem. 1970，75,

C36.

(11) Collier, M.R.; Kingston, B.M.; Lappert, M.R; Truelock, M.M. {Nat. Res. Develop.

Corp.), South Africa Patent 70-04，922，1971; Chem. Abstr. 1971，75, 129943.

(12) Thomson, J.; Keeney, W.; Baird, M.C.; Reynolds, W.F. J. Organomet. Chem.

1972, 40, 205.

(13) McBride.D.W.; Dudek, E.; Stone, F.G.A. J. Chem. Soc” London 1964, 1752.

(14) Yamamoto, Y.; Yamazaki, H.; Hagihara, N. Bull Chem. Soc. Jap. 1968, 41, 532.

(15). Yamamoto, Y.; Yamazaki, H.; Hagihara, N. J. Organomet. Chem. 1969，18, 189.

(16) Pietrzykowski,A-； Pasynkiewicz, S. J. Organomet. Chem. 1992, 440, 401.

(17) Keil, T. Dissertation, Universitdt-Essen-Gesamthochschule 1987.

165 (18) Lehmkuhl, H.; Rufinska, A.; Benn, R.; Schroth, G.; Mynott, R. J. Organomet.

CHEM. 1980，188, C36; Liebigs Ann. Chem. 1981, 317.

(19) (a) Yamamoto, A.; Morifuji, K.; Ikeda, S.; Saito, T.; Uchida, Y.; Migono, A. J. Am.

Chem. Soc. 1965，57, 4652.

(b) Saito, T.; Uchida, Y.; Misono, A.; Yamamoto, A.; Morifuji, K.; Ikeda, S. J. Am.

Chem. Soc. 1966, 88, 5198.

(20) Wilkie, G.; Schott, H. Angew. Chem. 1966，78, 592.

(21) Bonnemann, H. Dissertation, Technische Hochschule Aachen 1967.

(22) Longoni, G.; Chini, P.; Canziani, F.; Fantucci, P. Chem. Commun. 1971，470.

(23) Longoni, G.; Fantucci, P.; Chini, P.; Canziani, F. J. Organomet. Chem. 1972，39,

413.

(24) Abiht, H.-P.; Issleib, K.; J. Organomet. Chem. 1978’ 149, 209.

(25) Maitlis, P.M. The Organic Chemistry of Palladium: Volume I and 11; Academic Press:

London, UK., 1971.

(26) Mailtis, P.M.; Russell, MJ.H. In Comprehensive Organometallic Chemistry;

Wilkinson, G.; Stone, F.G.A.; Abel, E.W., Eds.; Pegamon: Oxford, UK” 1982;

Volume 6, Chapter 38.4.

(27) (a) Uson, R.; Fornies, J•； Espinet, P.; Navarro, R.; Martinez, F.; Tomas, M. /.

Chem. Soc” Chem. Commun, 1977, 789.

(b) Uson, R.; Fornies, J.; Martinez, F.; Tomas, M. /. Chem. Soc., Dalton Trans.

1980, 888.

(28) Diversi, P.; Ingrosso, G.; Lucherini, A. J, Chem. Soc., Chem. Commun. 1978,

735.

(29) Kasahara, A.; Izumi, T. Bull Chem. Soc. Jap, 1969, 42, 1765.

(30) Cope, A.C.; Siekman, R.W. J, Am. Chem. Soc. 1965，<57，3272.

(31) Shaw, B.L.; Truelock, M.M. J. Organomet. Chem. 1975,102, 517.

(32) Dias, S.A.; Alyea, E.G. Transition Met. Chem. 1979，4, 205.

(33) Hietkamp, S.; Stufkens, DJ.; Vrieze, K. J. Organomet. Chem. 1979,168, 351.

(34) Moulton, C.L; Shaw, B.L. J. Chem. Soc” Dalton Trans. 1976，1020.

166 (35) Al-Salem, N.A.; Empsall, H.D‘； Markham, R.; Shaw, B丄.；Weeks, B. J. Chem.

Soc., Dalton Trans. 1979, 1972.

(36) Bennet, M.A.; Clark, P.W.; Robertson, G.B.; Whimp, P.O. J. Organomet. Chem. 1973, C15.

(37) Bennet, M.A.; Clark, P.W. J. Organomet. Chem. 1976，110, 367. (38) Takahashi, Y.; Tokuda, A.; Sakai, S.; Ishii, Y. J. Organomet. Chem. 1972, 35, 415.

(39) Alper, H. J. Organomet. Chem. 1973，61, C62. (40) Cope, A.C.; Friedrich, E.G. /. Am. Chem. Soc. 1968，90, 909.

(41) Hartwell, G.E.; Lawrence, R.V.; Smas, M.L Chem. Commun, 1970，912.

(42) Sokolov, V.L; Sorokina, T.A.; Troitskaya, L.L.; Soloviewa，L.L; Reutov, O.A. J. Organomet. Chem. 1972, 36, 389.

(43) Sokolov, V.L; Bashilov, V.V.; Musaev, A.A.; Reutor, O.A. J. Organomet. Chem. 1982,225, 57. (44) Abicht, H.P.; Issleib, K. J. Organomet. Chem. 1978,149, 209.

(45) Eisch, JJ.; King, R.B. Organometallic Synthesis', Academic Press: New York;

Volume 1’ 1965.

(46) Werner, H.; Demberger, Th. J. Organomet. Chem. 1980，198, 97.

(47) Kohler, F.H.; Doll, K.H.; Fladerer, E.; Geike, W.A. Transition Met. Chem. 1981，

6，126.

(48) Bailey, S.L; Colgan, D.; Engelhardt, L.M.; Leung, W.-P.; Papasergio, R.L; Raston,

C.L.; White, A.H. J. Chem. Soc” Dalton Trans. 1986, 603.

(49) Jarvis, J.AJ.; Mais, R.H.B.; Owston, G.P.; Thompson, D.T. J. Chem. Soc. A

1970, 1867.

(50) Seller, P.; Dunitz, ID. Acta Crystallogr. 1980，B36, 2255.

(51) Carmona, E.; Gonzalez, F.; Poveda, M.L.; Atwood, J.L.; Rogers, R.D. J. Chem.

Soc” Dalton Trans. 1981，777.

(52) Izod, KJ.; Thornton, P. Polyhedron 1993，12’ 1613.

167 (53) Bernhardt, P.V.; Lawrance, G.A.; Sangster, D.F. Inorg. Chem. 1988，27，4055.

(54) (a) Cameron, L; Engelhardt, L.M.; Junk, P.C; Hutchings, D.S.; Patalinghug, W.C;

Raston，CI.; White, A.H. J. Chem. Soc., Chem. Commun. 1991, 1560.

(b) Kynast, U.; Skelton, B.W.; White, A.H.; Henderson, MJ.; Raston, C.L. J.

Organomet. Chem. 1990, 384, CL

(55) Otsuka, S.; Nakamura, A.; Yoshida，T.; Nanito, M.; Ataka, K. J. Am, Chem. Soc. 1973，95，3180.

(56) Maitlis, P.M.; Espinet, P.; Russell, MJ.H. In Comprehensive Orgnometallic

Chemistry; Wilkinson, G.; Shone, F.G.A.; Abel, E.W., Eds.; Pegamon: Oxford,

UK., 1982; Volume 6, Chapter 38.4.

(57) Pray, A.R. In Inorganic Synthesis, Moeller, T.，Eds.; McGraw-Hill: New York,

1957; Volumn V, Chapter VIIA.

(58) (a) Henderson, MJ.; Papasergio, R.L; Raston, CL.; White, A.H.; Lappert, M.F. /.

Chem. Soc” Chem. Commun. 1986，672.

(b) Engelhardt, L.M.; Jolly, B.S.; Lappert, M.F.; Raston, CI.; White, A.H. J.

Chem. Soc., Chem. Commun. 1988, 336.

(59) Mann, RG.; Purdie, D. J, Chem. Soc. 1935, 1549.

168 • CHAPTER IV. SYNTHESIS AND STRUCTURAL

CHARACTERIZATION OF ZIRCONIUM(IV) AND HAFNIUM(IV) ALKYL COMPLEXES

IV.1 INTRODUCTION

Organometallic chemistry of the early transition metal is relatively new even though the now diversified field of organometallic chemistry had its begining in the nineteenth century. The report of the metallocene(IV) dibromide of titanium and zirconium [MCpsBr〗](M = Ti, Zr) by Wilkinson et al. in the 1950's has been known to be the dawn of the era for the synthesis of organo-Group-4 compounds, l However, organohafnium compounds were still unknown at that time. It was until 1968 that the hafnium analogue [HfCpsBr〗] had been prepared.^ The chemistry of organo-zirconium compounds are much well-developed than that of organo-hafnium analogues which leads to a phenomenon that the total number of publications for zirconium far exceeds that for hafnium.

The largest group of organo-zirconium(IV) and -hafnium(IV) compounds are those containing cyclopentadienyl ligands with the bis(cyclopentadienyl) species forming the majority. The cyclopentadienyl group is a good stabilizing ligands by virtue of their size as they effectively occupy three coordination sites. It also possesses a good electron donating power. That is the reason why complexes of the type [(TI^-

C5H5)mMRnX4_m-n] (M = Ti，Zr’ Hf; R = alkyl or aryl groups; X 二 halide; m,n = 1,2, or 3) are frequently encountered in the literatures. More important，these zirconocene compounds are catalysts in the Zieglar-Natta industrial process such as polymerization of ethylene. The general route to Group 4 metallocene alkyl complexes is by the reaction of [(Ti5-C5H5)nMX4_n] (n = 1, 2, or 3) with the appropriate organolithium or

Grignard reagents (equation IV-1).

169 [Cn5-C5H5)nMX4-n] + RM' ^ [(”5-C5H5)nMR4_n ] (IV-1) n = 1,2, 3 M = Ti, Zr, Hf M' = Li, MgX X = CI, Br

IV.1.1 Homoleptic and Heteroleptic Complexes

After the realization that stability of transition metal alkyl or aryl complexes is of kinetic origin,4 a large number of homoleptic zirconium(IV) and hafnium(IV) have been prepared by using bulky and p-hydrogen stabilized alkyl ligands. Some representative examples of these complexes are included in Table IV-1. The general route to homoleptic [MR4] is by the reaction of MCI4 with the corresponding organolithium reagents (equation IV-2).13

4RLi MCI4 + or [MR4] (IV-2) M=Zr，Hf k 乂 4 RMgX

Although it has been known that RLi and RMgX possess reducing properties, it is not normally a problem with those heavier Group 4 transition metals (Zr, Hf) as it is with titanium. For instance, (Me3Si)2CHLi and TiCl4 gave Ti[CH(SiMe3)2]3，a reduction of Ti(IV) to Ti(ni). In contrast, no reduction was observed with MCI4 (M =

Zr, Hf).14

When very bulky alkyl ligands are employed, it will be difficult to substitute all the halides on MX4 with the alkyl ligands. For instance, RLi (R = CH2Ph,8a,c

CH2CMe3,8e or CH2SiMe3'7) reacts with MCI4 (M = Zr, Hf) to give [MR4] whilst in the case of the more bulky alkyl ligand R' = CH(SiMe3)2 only trialkyl metal chloride

[MR'3C1] was obtained (Scheme IV-1) 6a,l4

170 Table IV-1. Some Representative Examples of Homoleptic Alkyls and Aryls of Zircon- ‘ ium(IV) and Hafniuni(IV) without 兀-Anionic Ligands,

Compound Method^ m.p. (°C) References

[ZrMe4(OEt2)n] A >15 (dec) 5 [Zr(CH2CMe3)4] A 108-111 (dec) 6 [Hf(CH2CMe3)4] A 115-116 6a，c，d [Zr(CH2SiMe3)4] A 10-11 6c, d，7 (b.p. ca 25。C/10-3 mmHg) [Hf(CH2SiMe3)4] A 8-10 6c, 7 二 (b.p. ca 50。C/10-3 mmHg) H [Zr(CH2Ph)4] A 133-134 6d, 8 [Hf(CH2Ph)4] A - 8b, d [Zr(CH2Ph)4(bipy)] B 250 (dec) 9 [Zr(CH2Ph)4(4,4'-bipy)] B 9 [Zr(CH2Ph)4(py)] B 9 [Zr(CH2Ph)4(NC5H4Me-2)] B 250 (dec) 9 [Zr(CH2Ph)4(NC5H4Me-2)2] B 9 [M(CH2C6H40Me-4)4] M = Zr, Hf - - 10 [M(CH2C6H3Cl-2-OMe-4)4] M = Zr, Hf - - 10 [M(CH2C6H3Me-2-OMe-4)4] M = Zr, Hf - - 10 [M(CH2C6H4Me-4)4] M = Zr, Hf - - 10 [M(CH2C6H3Cl-2-Me-4)4] M = Zr, Hf - - 10 Table IV.l - Cont'd.

Compound Method^ m.p.(。匸) References

[Zr(CH2C6H4CH2-o)2(THF)] A 120 (dec) 11

[Hf(CH2C6H4CH2-^)2(THF)] A 130 (dec) 11

[Zr-[CH2(CH2)2NMe2]-4] A 20 12

NJ Reference: Cardin, D.J.; Lappert, M.F.; Raston, C.L. Chemistry of Organo-Zirconium and -Hafnium Compounds., Ellis Horwood Limited: UK., 1986.

t Method A: By salt elimination. Method B: Melalhetical exchange: e.g. [Zr(Tii-C5H5)(Ti5-C5H5)3] + ZrCl^ -> 2[Zr(Ti5-C3H5)2Cy [MR4] . 4RLi ——WU——^ [MR'aCI] R = CHaPh, R_ = CH(SiMe3)2 CH2CMe3, CHaSiMea

M = Zr, Hf

Scheme IV-1

Another example is the reaction of [HfCl2{N(SiMe3)2}2] (92) with

Mg(CH2CMe3)2. Owing to the bulkiness of the entering group "CH2CMe3 and the two amide ligands -N(SiMe3)2 on the hafnium complex (92), complete substitution of two chlorides is not possible and only [Hf(CH2CMe3)Cl{N(SiMe3)2}2] is obtained, although with LiCHzSiMe]，[Hf(CH2SiMe3)2{N(SiMe3)2}2] will be the product

(equation IV-3).18

[HfC丨2{N(SiMe3)2}2] + Mg(CH2CMe3)2 [Hf(CH2CMe3)CI{N(SiMe3)2}2] (IV-3)

Both the reaction yield and purity of the product [MR4] sometimes depend on the solvents and stoichiometry of the starting materials involved. A typical example can be found in the synthesis of [M(CH2SiMe3)4] (M 二 Zr, Hf): a 5% excess of MCI4 was reported to be critical in order to avoid contamination of the product with other by- products such as lithium chloride or magnesium chloride.^

The Grignard reagent (9-C6H4(CH2MgCl)2 reacted with ZrCl4 or HfCl4 to give alkyl complexes of empirical formula {M[(CH2)2C6H4-6>]2(THF)} (M = Zr, Hf)

(equation IV-4).11,16

MCI4 + 2 11 丄 ^ M(CH2)2C6H4-0]2(THF)} (IV-4)

173 However, insolubility of the compound renders its full characterization difficult to be carried out and it is not certain whether the bidentate ligand behaves as chelating,

bridging, or even oxidized as represented in (XII).

(XII)

The complex [Zr{CH2(CH2)2NMe2}4] was obtained from ZrCl4 and Li(CH2)3NMe2 in which the central Zr metal was believed to be eight-coordinated. 12

Although the complex does possess p-hydrogen, the inability to achieve a suitable conformation for hydrogen transfer to the metal, due to the presence of chelate rings, keeps the complex away from p-hydrogen decomposition pathway.

Heteroleptic a-hydrocarbyl complexes of Zr(IV) and Hf(IV) of types [MRY3],

[MR2Y2]’ [MR3Y], [MR2R’Y], and [MRRT2] [Y 二 monomelic a bonded ligands such

as Cr, Br", F, (Me3Si)2N_] have been described in the literatures. Table IV-2

summarizes some typical examples of these heteroleptic organo-zirconium(IV) and

-hafnium(IV) complexes.

The most commonly employed routes to these complexes is by treating MCI4

with appropriate Grignard or organolithium reagents. For example, [ZrPhCy can be

easily accessed by treating ZrCl4 with PhMgCl (equation IV-5).

ZrCU + PhMgCl [ZrPhCb] (IV-5)

In order to control the stoichiometry a less powerful alkyl transfer reagent and

critical reaction conditions are necessary. A typical example is the synthesis of the

monoalkyl complexes [ZrRXg] (R = Me, Et, Pr, X 二 halides) (Scheme IV-2).

174 Table IV-2. Some Representative Examples of Heteroleptic Alkyls and Aryls of Zircon- ium(IV) and Hafnium(IV) without 71-Anionic Ligands.

Compound Method卞 m.p. (°C) References

[ZrMeCls] A -- 17 [ZrMeCl3(OEt2)2] A >80 (dec) 17 [HfMeCl3(OEt2)2] A -- 17 [ZrMe[N(SiMe3)2]3] A 176-177 15 [HfMe[N(SiMe3)2]3] A 188-190 15 二 [Hf(CH2CMe3)a[N(SiMe3)2]2] A 52-54 18 U1 [Hf(CH2SiMe3)Cl[N(SiMe3)2]2] A oil 18 [HfMe(OSiMe3)[N(SiMe3)2]2] A 98-102 18 [Hffit(OSiMe3)[N(SiMe3)2]2] A 116-118 18 [ZrPhCl3(THF)3] A - 19 [ZrPhCl3(bipy)] C - 19 [ZrPha3(NC5H4Me-2)2(THF)] C - 19 [Zr[CH(SiMe3)2]3Cl] A 6a，7，14 [Hf[CH(SiMe3)2]3Cl] A 6a，7, 14

Reference: Cardin, D.J.; Lappert, M.F.; Raston, C.L. Chemistry of Organo-Zirconium and -Hafnium Compounds; Ellis Horwood Limited: Chichester，U.K., 1986.

t Method A: By salt elimination. Dialkyl zinc [ZnR〗](R 二 Me, Et, Pr) reacts with 21X4 (X = CI, Br) in toluene at -10。C gives [ZrRX3] whilst in pyridine at 0。C [ZrRsX〗] is obtained. 17

ZnRp [ZrRXa] PhMe，-10°C R = Me, Et, Pr

ZrX4 X = CI, Br

^^~^~C5H4N’0�- C [ZrR^Xa]——^~~- [ZrR2X2(bipy)L 2 A •]

not isloated

Scheme IV-2

Besides the tetrabenzyl zirconium(IY) complex [Zr(CH2Ph)4],8a Zucchini et al. have also prepared the heteroleptic dibenzyl zirconium(rV) dichloride [Zr(CH2Ph)2Cl2 by direct reaction of [Zr(CH2Ph)4] with a stoichiometric amount of 2 equivalents of hydrogen chloride gas (equation IV-6).8c

[Zr(CH2Ph)4] + 2 HCI (g) ^ [Zr(CH2Pli)2Cl2] (IV-6)

The tribenzyl titanium(rV) and zirconium(IV) analogues were also reported by the same research group (equation IV-7).8a’c

[M(CH2Ph)4] + HX 一 [M(CH2Ph)3X] + C6H5CH3 . (IV-T)

M = Ti, Zr X=F, CI, Br, I, OC2H5

Bis(neopentyl) zirconium(IV) dihalide complexes [ZrR2X2(OEt2)2] [R =

CH2CMe3, X = CI (93)，Br (94)] were easily accessible by treating ZiX^ with 2

176 equivalents of Li(CH2CMe3) in ether.8f Subsequent reaction of 93 or 94 with Lewis

bases gave the corresponding Lewis base adducts (equation IV-8).

2 I iR o I ZrX4 ———~- [ZrR2X2(〇Et2)2]—————-[ZrRsXaLs] (IV-8)

X = CI 93，Br 94

R = CHaCMes X = CI; L = PMea, PMesPh, NEta, 1/2 DMPE, 1/2 TMEDA X= Br; L = PMea, 1/2 TMEDA

Further reaction of [ZrR2Cl2(OEt2)2] (93) with 0.5 equivalents of MgR〗 at low

temperature gave the trialkyl derivative 95 (equation IY-9).8f

[ZrR2Cl2(OEt2)2] + 0.5MgR2 1."8。C’Et2〇• [ZrRaCI] (IV-9) 2. sublime 93 9 5

R = CH2CMe3

On the other hand, reaction of [ZrR^ClaCTMEDA)] with dichloromethane at 50。€ gave

the monoalkyl derivative 96 (equation IV-10).^^

[ZrR2Cl2(TMEDA)]——^！^_^ [ZrRCWTMEDA)] + CMe* (IV-10) 50 10h R = CH2CMe3 9 ®

IV.1.2 Organo-Zirconium(IV) and -Hafnium(IV) Compounds Containing Functionalized Alkyl Ligands

As we have discussed in the previous chapters, the use of functionalized alkyl or

aryl ligands do lead to more stable transition metal alkyl or aryl complexes. Among different functionalized ligands, alkyl pyridine systems have been shown to exhibit different coordination behaviour. Rothwell et al have reported the synthesis of the

177 metallocene dialkyl complex [M(ti5_C5H5)2R2] [M = Zr (97), Hf (98); R = CH2(6-

MeC5H3N)-2] by treating M(7i5_C5H5)2Cl2 with two equivalents of Li[CH2(6-

MeC5H3N)-2] [CH2(6-MeC5H3N)-2 = 2-methyl(6-methylpyridine)] in diethyl ether.^O

The zirconium compound (97) have been structurally characterized by X-Ray diffraction study which showed that the metal atom to be five-coordinate with one of the pyridylmethyl ligands being C,iV-bound while the other remained C-centered resulting in an 18-electron complex (equation IV-11).

[M(Ti^-C5H5)2Cl2] + 2 Li[CH2(6-MeC5H3N)-2]

/ Y

M=Zr’Hf • (“5H5)2M — CH, (IV-11)

97: M = Zr 98: M = Hf

- r^ CH2(6-MeC5H3N)-2 = 丄、丄 HsP 八 N 八CH3

The reaction of Li[CH2(6-MeC5H3N)-2] with MCI4 (M = Hf, Th) in diethyl ether led to the corresponding dialkyl metal dichloride complexes [MR2CI2] [R =

CH2(6-MeC5H3N)-2] which are only sparingly soluble in common hydrocarbon solvents (equation IV-12).2l The low solubility of these complexes and contamination with LiCl hampered full characterization of these compounds.

178 MCU ———~- [R2MCI2] Ar'OLi • [R2M(OAr')2] (IV-12)

M = Hf，Th 99: M = Hf

100: M = Th

R" = I II 八 CH3

Ar'o- 一一 ^vyV 〇_

However, attempts to further substitute the remaining chlorides with RLi were

unsuccessful most probably due to steric hindrance of the alkyl ligand so that

accommodation of up to three or four bulky ligands on the coordination sphere of the

metals will become difficult. The zirconium analogue [ZTRSCI〗] was reported to be

thermally unstable. Only can an intractable oil be obtained by treating ZrCl4 with

Li[CH2(6-MeC5H3N)-2]. Subsequent reactions of [MR2CI2] (M = Hf, Th) with 2

equivalents of Ar'OLi (Ar' = 2，6」Bii2C6H3) gave the corresponding dialkyl metal diaryloxides (99 and 100).

Raston et al have studied the reactions of [Zr(Ti5-C5E[5)2Cl2] with a number of functionalized alkyl ligands (equation IV-13).22 The influence of the functional groups on the nature and stability of their respective d^ reduction products have also been investigated.

179 Zr(Ti^-C5H5)2Cl2 + R"U [Zr(Ti^-C5H5)2(R^)CI] (IV-13)

-1 r^ 11 : R = I N^CH(SiMe3)

101 : "R® = I

102 ： = T

9H{SiMe3)

Compounds 11，101, and 102 have been structurally characterized by single crystal X- ray diffraction studies. In solid state, the ligand Rl was observed to coordinate to the metal center in a C/v'-chelating mode. In contrast, a temperature dependent equilibrium between the C,A^-bond chelating complex 11 and the C-centered species 11a was proposed as evidenced from its variable temperature iR NMR spectrum (Scheme IV- 3).

H、,SiMe3 Hj'Mea^ J) \ Nvc^ \ Cl Cl

11 11 a

Scheme IV-3

Compounds 101 and 102 are four-coordinate (by assuming that each cyclopentadienyl ligand occupys one coordination site).

180 IV.2 RESULTS AND DISCUSSION

IV.2.1 Synthesis of Zirconium(IV) and Hafniuin(IY) Dialkyl Dichloride Compounds [M(R2)2Cl2] (M = Zr 103，Hf 104)

The reaction of [{R2Li}2] with ZrCl4 in diethyl ether at ambient temperature gave a bright yellow crystalline compound of 103 in 45% yield (Scheme IV-4).

(Me3Si)2c C|l

MCi4 + [(R^u), 二6h’ + 2ua

CI C(siMe3)2

103: M = Zr 104: M = Hf

Scheme IV-4

The hafnium analogue [Hf(R2)2Cl2] (104) was prepared by a similar procedure in 40% yield.

Table rV-3 lists some physical properties of compounds 103 and 104.

Table TV-3. Some Physical Properties of Compounds 103 and 104.

Compound Yield (%) Colour m.p. (°C)

103 45 bright yellow 150-152 (dec.)

104 40 yellow 174-176 (dec.)

Only two of the four chlorides on MCI4 have been substituted by "R^ and attempts to further substitute the remaining two chlorides were unsuccessful.

Presumably, this may be due to steric hindrance of the bulky alkyl ligand "R^ so that the accommodation of more than two _R2 groups on the coordination sphere of the metals is difficult. This is similar to the result obtained by Lappert et al. in the synthesis of

181 [M{CH(SiMe3)2}3Cl] (M = Zr, Hf) as we have discussed in the previous section.6a,7,l4

Rothwell et al have also reported a similar observation during their studies on a similar

/V-functionalized alkyl ligand, [CH2(6-MeC5H3N)-2]-, in which only two chlorides of the HfCl4 can be substituted (equation IV-12).22 The zirconium(IV) complex

_Zr{CH2(6-MeC5H3N)-2}2CI2] obtained was reported to be unstable and could not be isolated.

IV.2.2 Attempted Synthesis of Titanium(IV) Alkyl by The Reaction of TiCl4 with [{R2Li}2]

The reaction of [{R^Li}2] with TiCl; in ether have also been studied. A solution of [{R2Li}]2] in ether was added to a solution of TiCl4 in the same solvent. A deep red solution was obtained. Unfortunately, no isolable crystals can be obtained but only a deep red intractable oil remained upon removal of solvent in vacuo (Scheme IY-5).

T1CI4 + [{R^Li}2] 曰2〇 • deep red intracable oil

Scheme IV-5

IV.2.3 Attempted Synthesis of Zirconium(IV) and Hafnium(IV) Alkyls with -Rl [= -CH(SiMe3)C5H4N-2] as Ligand

We have also attempted to synthesize the related metal alkyl chloride complexes by using the less bulky ligand —Rl. Only an intractable oil was obtained which remained uncFiaracterized, though there was evidence that reaction between the organolithium reagent and MCI4 had occurred (Scheme IV-6).

182 MCl4 + [{RVi(Et20)}2] ~J^f^,~• intractable oil M = Zr, Hf

Scheme IV-6

IV.2.4 Molecular Structure of [Zr{C(SiMe3)2C5H4l<^2}2Cl2] (103)

The molecular structure of 103 has been determined by X-ray diffraction. The structure with the atom numbering scheme is shown in Figure IV-1. Selected bond distances (A) and angles (。) are given in Table IV-4. The central zirconium metal is 6- coordinate with the two alkyl ligands bonded to the metal center in a chelate manner.

The molecules contain a crystallographically imposed C〗axis. The coordination environment around the metal center can be approximated to a trigonal prism, with each triangular face being formed by C, N, and CI atoms.

The Zr-Ca distances in 103 [2.399(5) A] is very similar to that of 2.38(1) A in

:Zr(Ti5-C5H5)2(Rl)Cl] (11) in which "Rl being the monosubstituted picoline derivative

[CH(SiMe3)C5H4N-2]-22 The Zr-C„ distances in [Zr(Ti5-C5H5)2{CH2(6-MeC5H3N)-

2)2] are 2.422(4) A (for the C-centered alkyl ligand) and 2.406 A (for the C，iV-bound alkyl ligand).21 The slightly longer distances in 97 may be attributed to the presence of two cyclopentadienyl rings and two alkyl ligands on the coordination sphere of the metal. Furthermore, as "R^ is related to benzyl ligands, comparisons is made for those structural parameters of 103 with other organozirconium compounds which contain benzyl ligands. The Zr-Ca distances in [Zr(Ti5-C5H5)2(CHPh2)2]23 and

C5H5)2{(CHSiMe3)2C6H4-^?}]24 are 2.388 A and 2.374 A, respectively. Only in highly hindered benzyl compounds of zirconium, such as the above two compounds, are the Zr-Ca distances close to that of our current complex 103. The Zr-C a-bond distance of 2.350(4) A in [ZrCnS-CsHs)�！（CHSiMe3)C6H4PPh2-6>)Cl (101] ) are shorter than that of complex 103.

183 "1

I i / (s) »

« ( Vc

) ^ L L c

^ c —:「，-、【v^

./VJl Table IV-4. Selected Bond Distances (A) and Angles (。）for Compound 103,

[Zr{C(SiMe3)2C5_-2} 2CI2] (103)

Zr(l)-Cl(l) 2.437(2) Zr(l)-N(l) 2.264(5)

Zr(l)-C(l) 2.399(5) N(l)-C(2) 1.357(7)

C(l)-C(2) 1.496(9) Si(l)-C(l) 1.909(6)

Si(2)-C(l) 1.895(6)

Cl(l)-Zr(l)-N(l) 82.7(1) Cl(l)-Zr(l)-C(l) 127.0(2)

N(l)-Zr(l)-C(l) 60.1(2) Cl(l)-Zr(l)-Cl(la) 91.0(1)

N(l)-Zr(l)-Cl(la) 137.5(1) C(l)-Zr(l)-Cl(la) 92.9(1)

N(l)-Zr(l)-N(la) 128,4(2) C(l)-Zr(l)-N(la) 95.3(2)

C(l)-Zr(l)-C(la) 124.4(3)

185 Surprisingly, the Zr-N(l) distance in 103 is very short [2.264(5) A]. It is even shorter than those of 2.443(5)-2.347(6) A in 11.22 (Compound 11 was reported to possess surprisingly short Zr-N distances) It is significantly shorter than that of

2.403(1) A in [Zr(NC5H5)(Ti2_C0CH2)(Ti5-C5Me5)2].25 It is also shorter than that found in other Zr(IV)-pyridine derivatives. Nevertheless, it is still longer than that for the anionic iV-centered ligand as in [Zr(Ti5-C5H5)2{N(CH2)4}2] with Zr-N distance being 2.198(6) A.26

The Zr-Cl(l) distance of 2.437 A in 103 is normal. The typical range for Zr-Cl distances in the literature lie within 2.42-2.52 人.13b The value for 103 is very similar to that of 2.438(1) A in [Zr(Ti5_C5H5)2{(CHSiMe3)C6H4PPli2-dCl] (101)22 and

2.454 A in [Zr(Ti5-C5H5)2{CHSiMe3(Ci4H9-9)}Cl] (102).22 However, it is significantly shorter than those of 2.564(2)-2.563(2) A in compound 11.22

The bite angle C(l)-Zr-N(l) of 60.1(2)。in 103 is similar to that of 58.2(2)。in

[Zr(Ti5-C5H5)2(Rl)Cl] (11)22 and 59.3(2)。in [Hf{CH2(6-MeC5H3N)-2}2(OAr')2] (Ar'

=2，6-屯U2C6H3) (99)21 This can also be compared with those values of 67.3。，67.1。，

59.8。，59.9。in [Mg(R2)2] (15)27 [Zn(R2)2] (16)27 [Cd(R2)2] (17)27 and [Sn(R2)2]

(18),28 respectively, where compounds 15-18 all adopt a tetrahedral coordination geometry and are monomelic. It is also comparable to that of 66.67° in the monomelic five-coordinate [Ga(R2)2Cl] (21)29

186 Spectroscopic Properties of []^{C(SiMe3)2C5H4N-2}2Cl2] (M : Zr 103，Hf 104)

Mass Spectra:

No molecular ion peak was observed for the zirconium dialkyl dichloride complex 103. Only daughter peaks and peaks due to its organic fragments were observed: mh = 399 [M-R2]+, 236 [R2]+, 222 [R2-CH3]+.

The hafnium analogue 104 showed its molecular ion peak at [M]+ m/z = 722.

However, the percentage abundance of the peak is very low (5%). This may be

attributed to the low volatility of the compound owing to its large molecular weight.

Other daughter peaks are: m/z = 707 [M-CH3]+，687 [M-C1]+, 649 [M-SiMe3]+, 236

[R2]+，222 [R2-CH3]+.

NMR Spectra:

Only one set of signals was observed on the ^H and ^^C NMR spectra of both compounds indicating that the two alkyl ligands in each complex are being equivalent in solution. However, non-equivalence of the SiMe] groups on each alkyl ligand of compound 103 is obvious form its X-ray crystal structure. Thus, it is conceivable that both compounds 103 and 104 do show dynamic equivalence of these groups owing to a weak metal-nitrogen interactions which allows rotation about metal-carbon bonds in

solution.

187 IV.3 EXPERIMENTALS FOR CHAPTER IV

Materials:

Anhydrous and HfCl4 were purchased from The Aldrich Chemical Co.

and used without further purification. [{R2Li}2] was prepared as described in the

literature.30

Synthesis of Compounds:

Synthesis of [Zr{C(SiMe3)2C5H4N-2}2Cl2]，103. To a suspension of ZrCL

(0.27 g, 1.17 mmol) in ether (20 ml) at 0。C was added slowly with a solution of

[{R2Li}2] (0,57 g, ) in ether (30 ml). The resultant yellow suspension was further

stirred at 25 °C for 16 h. The white precipitate was then filtered and the yellow was

concentrated in vacuo. Upon standing at room temperature for one night, bight yellow

crystals of compound 103 was obtained (yield 0.33 g, 45%). The crystals can be recrystallized from warm toluene. Mp.: 150-152 °C. MS.: m/z (%) = 399 (7) [M-R2]+，

384 ⑷[M-R2-CH3]+, 236 (20) [R2]+, 222 (100) [R2-CH3]+. Anal. Found: C, 45.16;

H, 6.95; N, 4.31%. Calc. for C24H44N2Si4ZrCl2： C, 45.39; H, 6.98; N, 4.41%. iR

NMR (250 MHz, C^D^): 5 0.25 (s, MegSi，18H), 6.09-6.15 (ddd, J 二 1.5，5.6, and

7.0 Hz, pyridyl, IH), 6.74-6.87 (m, pyridyl, 2H), 7.76-7.78 (m, pyridyl, IH).

13C{1H} NMR (62.89 MHZ, CeDg): 6 4.31 (MegSi)，34.32 (CSiMe?)，117.38，

124.71, 140.71, 145.73, 17232 (C5H4N).

Synthesis of [Hf{C(SiMe3)2C；！！斗！^-!}!。!】]，104. A solution of [{R2Li}2]

(1.81 g, 7.47 mmol) in ether (30 ml) was added dropwise to a stirring suspension of

HfCl4 (0.80 g, 2.49 mmol) in the same solvent (10 ml) at 0。C. The resultant light yellow suspension was further stirred at room temperature for 16 h in which a clear pale yellow solution with white precipitate was obtained. The solution was filtered through

Celite and concentrated in vacuo. Upon standing at room temperature for 1 night, large yellow crystals of the title compound was obtained (yield 40%). Mp: 174-176 °C

188 (dec.). MS: m/z (%) = 111 (5) [M]+，707 (32) [M-CH3]+, 687 (52) [M-C1]+. Anal.

Found: C，39.68; H，6.09; N, 3.83%. Calc. for C24H44N2Si4HfCl2： C, 39.91; H,

6.14; N，3.88%. IH NMR (250 MHz, CgDs): 5 0.24 (s, Me^Si, 18H), 6.10-6.16

(ddd, J = 1.2, 5.7, and 7.1 Hz, pyridyl, IH), 6.73-6.80 (ddd, J = 1.8, 7.2, and 8.3

Hz，pyridyl, IH), 6.85-6.88 (dt, J = 1.1 and 7.1 Hz, pyridyl, IH), 7.70-7.72 (m, pyridyl, IH). l^cflR} NMR (62.89 MHZ, C^D^)： 5 3.61 (MegSi), 36.12 (CSiMe;),

116.80，118.20, 139.70’ 144.52, 173.71 (C5H4N).

189 IV.4 REFERENCES FOR CHAPTER IV

(1) Wilkinson, G.; Pauson, P.L.; Birmingham, LM.; Cotton, F.A. J. Am. Chem.

Soc. 1953, 75, 1011.

(2) (a) Dnice, P.M.; Kingston, B.M.; Lappert, M.F.; Spalding, T.R.; Srivastava,

R.C. J. Chem. Soc. (A) 1969, 2106.

(b) Druce, P.M.; Kingston, B.M.; Lappert, M.F.; Srivastava, R.C; Frazer, MJ.;

Newton, W.E. J. Chem. Soc. (A) 1969, 2814.

(3) (a) Collier, M.R.; Lappert, M.F.; Truelock, M.M. J. Organomet. Chem. 1970,

25, C36.

(b) Collier, M.R.; Lappert, M.F.; Pearce, R. J. Chem. Soc., Dalton Trans. 1973,

445.

(4) (a) Yagupsky, G.; Mowat, W.; Shortland, A.; Wilkinson, G. Chem. Commun.

1970, 1369.

(b) Mowat, W.; Shortland, A.; Yagupsky, G.; Hill, NJ.; Yagupsky, M.;

Wilkinson, G. J. Chem. Soc., Dalton Trans. 1972, 533.

(5) Berthold, HJ.; Groh, G. Angew. Chem. Int. Ed. Engl 1966，5, 516.

(6) (a) Davidson, PJ.; Lappert, M.F.; Pearce, R. J. Organomet. Chem. 1973, 57,

269.

(b) Mowat, W.; Wilkinson, G. J. Chem. Soc” Dalton, Trans. 1973, 1120.

271.

(d) Lappert, M.F.; Patii, D.S.; Pedley, LB. J. Chem. Soc., Chem. Commun. 1975, 830.

(7) Collier, M.R.; Lappert, M.F.; Pearce, R. J. Chem. Soc., Dalton Trans. 1973，

445.

(8) (a) Zucchini, U.; Albizzati, E.; Giannini, U. J. Organomet. Chem. 1971, 26,

357. (b) Felten, JJ.; Anderson, W.P. J. Organomet. Chem. 1972, 36’ 87.

190 (c) Zucchini, U.; Albizzati, E.; D'Angelo, R. J. Chem. Soc” Chem. Commun. 1969，1174.

(d) Davies, G.R•； Jarvis, J.AJ.; Kilboum, B.T.; Pioli, AJ.P. J. Chem. Soc.,

Chem. Commun. 1971, 677.

(e) Wengrovius, J.H.; Schrock, R.R. J. Organomet. Chem. 1981, 205, 319.

(f) Said, F.F.; Tuck, D.G. Canad. J. Chem. 1980, 58, 1673.

(9) Clarke, IF.; Fowles，G.W.A.; Rice, D.A. J. Organomet. Chem. 1974, 74, 417.

(10) Pioli, AJ.P.; Hollyhead, W.B.; Todd, P.R Ger. Pat, 1971，2，6>2(5, 032.

(11) Lappert, M.F.; Martin, T.R.; Atwood, J.L.; Hunter, W.E. J. Chem. Soc., Chem.

Commun. 1980, 476.

(12) Langguth, E.; Thu, N.V.; Shakoor, A.; Jacob, K.; Thiele, K.-H. Z. Anorg. Allg.

Chem. 1983，505, 127.

(13) (a) Cardin, DJ.; Lappert, M.F.; Raston, C.L.; Riley, P.L In Comprehensive

Organometallic Chemistry; Wilkinson, G.; Shone, F.G.A.; Abel, E.W., Eds.;

Pergamon: Oxford, U.K., 1982; Volume 3’ Chapter 23.2 and 23.3.

(b) Cardin, DJ.; Lappert, M.F.; Raston, CL. Chemistry of Organo-Zirconium

and -Hafnium Compounds; Ellis Horwood Limited: Chichester, U.K., 1986.

(14) Barker,G,K.; Lappert, M.F.; Howard, J.A.K. J. Chem. Soc., Dalton Trans. 1978’ 734.

(15) Andersen, R.A. Inorg. Chem. 1979，18, 1724.

(16) Bristow, G.S.; Lappert, M.F.; Martin, T.R.; Atwood, J.L.; Hunter, W.E. J.

Chem. Soc” Dalton Trans. 1984, 399.

(17) (a) Thiele, K.-R; Kriiger, J. Z. Anorg. Allg. Chem. 1971, 383, 272.

(b) Thiele, K.-H. Pure AppL Chem. 1972, 30, 575.

(18) Andersen, R.A. J. Organomet. Chem. 1980，192, 189.

(19) Clarke, IF.; Fowles, G.W.A.; Rice, D.A. J. Organomet. Chem. 1974, 76, 349.

(20) Beshouri, S.M.; Fanwick, P.E.; Rothwell, LP.; Huffman, J.C. Organometallics 1987, 6，891.

191 (21) Beshouri, S.M•； Fanwick, P.E.; Rothwell, LP.; Huffman, J.C. Organometallics

1987, 6, 2498.

(22) Bailey, S.L; Colgan, D.; Engelhardt, L.M; Leung, W.-P.; Papasergio, R.L;

Raston，C.L.; White, A.H. J. Chem. Soc., Dalton Trans. 1986, 603.

(23) (a) Atwood, J.L.; Barka, G.K.; Holton, J.; Hunter, W.E.; Lappert, M.F.; Pearce,

R. J. Am. Chem. Soc. 1977, 99, 6645.

(b) Fischer, K.; Seidel, W.; Schmiedeknecht, K. Z. Anorg. Allg. Chem. 1972,

390’ 273.

(24) Lappert, M.F.; Raston, CL.; Skelton, B.W.; White, A.H. J. Chem. Soc., Dalton

Trans. 1984, 893.

(25) Moore, E.L.; Straus, D.A.; Armantrout, J.; Santarsiero, B.D.; Grubbs, R.H.;

Bercaw, J.E. J. Am. Chem. Soc. 1983，105, 2068.

(26) Bynum, R.V.; Hunter, W.E.; Rogers, R.O.; Atwood, J.L. Inorg. Chem. 1980,

19, 2368.

(27) Henderson, MJ.; Papasergio, R.L; Raston, C.L.; White, A.H.; Lappert, M.F. J.

Chem. Soc., Chem. Commun. 1986，672.

(28) (a) Engelhardt, L.M.; Jolly, B.S.; Lappert, M.R; Raston, CI.; White, A.H. J.

Chem. Soc., Chem. Commun. 1988，336.

(b) Jolly, B.S.; Lappert, M.R; Engelhardt, L.M.; White, A.H.; Raston, C.L. J.

Chem. Soc., Dalton Trans. 1993, 2653.

(29) Kynast，U.; Skelton, B.W.; White, A.H.; Henderson, MJ.; Raston, C.L. J.

Organomet. Chem. 1990, 384, CI.

(30) Papasergio, R.L; Raston, C.L.; White, A.H. J. Chem, Soc” Chem. Commun.

1983, 1419.

192

•ft CHAPTER V COMPARISON OF BONDING PARAMETERS AMONG METAL ALKYL COMPLEXES WHICH CONTAIN ALKYL LIGAND "R^

The objective of this work is to explore the potential of sterically hindered N- functionalized alkyl ligands (—Rl，"R^, and 'R^) in the stabilization of those otherwise kinetically labile transition metal alkyl complexes. Among these transition metal alkyl complexes which have been successfully synthesised in this work, only those containing

-R2 have been structurally characterized (except [{(Ti5-C5H5)NI(RL)}2] and [Co(R3)2]).

Those complexes which contain the less bulky alkyl ligand _Rl are, in general, believed to be thermally unstable (except [{(Tj5-C5H5)Ni(Rl)}2]).

A number of novel transition metal alkyl complexes have been successfully prepared with _R2 as the alkyl ligand. These include the first fully characterized iron(n) and cobalt(II) dialkyl complexes, the ri^-cyclopentadienylnickelalkyl complexes, the nickel(II) and palladium(II) dialkyls, and the novel dialkyl zirconium(IV) and hafnium(rV) dichloride complexes. It has been found that "R^ exhibits as C/^-chelating ligand in the above complexes. In addition, the role played by the aromatic N-donor should be important in the stabilization of these novel metal complexes.

Since "R^ is a relative new ligand, it is of particular interest to compare those bonding parameters among both main group and transition metal alkyl complexes which contain "R^. Table V-1 lists selected structural data for these metal alkyl complexes.

Figure V-1 shows the variation of M-C distance (A) with different metal M.

Although M-C distance depends on a number of factors such as coordination environment of the central metal M, charge on M，and steric requirement of the ligands involved. Figure V-1 shows some relationship between the two parameters. M-C distance decreases from Li to Al and from Fe to Cu. This is consistent with the increase in effective nuclear charge from left to right across each period, though differences in these bonding parameters between two consecutive congeners are small.

193 Table V-1. Selected Structural Data^ for Metal Alkyl Complexes Which Contain R2.

Bond length (A) Bond angle (。) Ref.

M-C M-N C-M-N C-M-C N-M-N

[{LiR2)2] (6) 2.713(7) 1.936(6) 一一一 1

[Mg(R2)2] (15) 2.22 2.13 67.3 157.1 117.4 2

[A1(R2)2]+(22) L99 1.92 72.9 152.2 114.6 3

[Fe(R2)2] (55) 2.15 2.12 67.0 161.1 116.8 This work

[Co(R2)2] (56) 2.094(6) 1.919(4) 69.6(2) 180.0 180.0 This work f [CpNi(R2)] (78) 2.018(2) 1.856(2) 71.5(1) — — This work

[Ni(R2)2] (82) 2.08 1.89 70.5 180.0 180.0 This work

[Ni(R2)(R2-R2)ci] (83) 2.024(6) 1.871(4) 70.7(2) — — This work

[{CuR2}2] (12) 1.950(4) 1.910(3) — — — 1,4

[Zn(R2)2] (16) 2.07 2.29 67.1 164.4 167.5 2

[Ga(R2)2Cl] (21) 2.07 2.22 66.7 139.6 162.6 5

[&(R2)2C12] (103) 2.399(5) 2.264(5) 60.1(2) 124.4(3) 128.4 This work

[Pd(R2)2] (88) 2.187(2) 2.031(2) 66.6(1) 180.0 180.0 This work

[Pd(R2)(PPh3)Cl] (89) 2.167(6) 2.019(4) 66.7(2) — 一 This work Table V-1. Cont'd.

Bond length (A) Bond angle (。) Ref.

M-C M-N C-M-N C-M-C N-M-N

[Pd(R2)(PPh3)Br] (90) 2.149(9) 2.023(6) 66.6(3) 一一 This work

[{AgR2)2] (13) 2.154(5) 2.160(5) 一一一 6

[Cd(R2)2] (17) 2.27 2.49 59.8 174.3 108.9 2

[Sn(R2)2] (18) 2.35 2.42 59.9 116.7 136.5 7

[Sn(R2)(N，，)] (24) 2.356(8) 2.299(5) 61.1(2) — — 7

J [Sn(R2)(Cl)] (25) 2.32 2.27 61.6 — — 7

[In(R2)2Cl] 2.26 2.40 62.1 151.9(4) 167.2(3) b

[Sb(R2)Cl2] (27) 2.213(5) 2.371(7) 61.4(2) — 一 8

[{AuR2)2] (14) 2.094(12) 2.081(11) 一一一 4

[Hg(R2)2] (19) 2.16 2.78 56.6 179.53 115.62 2

a: average values are reported, b: Leung, W.-P.; Lee, H.K.; Mak, T.C.W. unpublished result. 2.9-

2.7 - Q

运嘗：：:.

1.9-

1.7 - 1-5 H~I—I~I—r~I~I—I~I~I—I~I—I~I~I—II—I~II—II"""I—I I Li Mg Al Fe Co Nia Nib Nic Cu Zn Ga Zr Pda Pdb Pdc Ag Cd Sna Snb Snc In Sb Au Hg

Melal M

Figure V-1. Variation of M-C Distance (A) with Metal M for Metal Alkyl Complexes which Contain Alkyl Ligand R2 [Nia: 78； Nib: 82; Ni。： 83; Pda: 88; Pdb: 89; Pd。： 90; Sna: 18； Snb： 24; Sn。： 25]. 3

2.8 - j>

呂言：AA^/^v/W

1.8 -

L6H I I I I I I I I I I I~1 1 1 1 1 1 1 1 1 1 1 1 1 Li Mg Al Fe Co Nia Nib Nic Cu Zn Ga & Pda Pdb Pdc Ag Cd Sna Snb Snc In Sb Au Hg

Metal M

Figure V-2. Variation of M-N Distance (A) with Metal M for Metal Alkyl Complexes which Contain Alkyl Ligand R2 [Nia： 78； Nib: 82; Ni。： 83; Pda: 88; Pdb: 89; Pd。： 90; Sna: is； Snb: 24； Sn。： 25]. 75“ 73- o

S ‘ ；;: V

55- 53 H~~I~I~I~II~I~1~I~r—1~I~I~I~~I~~II~~I~~I—I~I~~ Mg Al Fe Co Nia Nib Nic Zn Ga Zr Pcla Pdb Pdc Cd Sna Snb Snc In Sb Hg

Metal M

Figure V-3. Variation of C-M-N C) with Metal M for Metal Alkyl Complexes which Contain Alkyl Ligand R2. [Nia: 78； Nib： 82; Ni。： 83; Pd^： 88; Pdb: 89; Pd。： 90; Sna: is； Snb： 24； Snc： 25]. The M-C distances of [Mg(R2)2] (15)2 are significantly longer than that of

[A1(R2)2]+ (22).3 This is believed to be the consequence of the cationic charge on the latter complex, in addition to the effective nuclear charge effect.

A steady decrease in M-C bond lengths is observed from [Fe(R2)2] (55) to

[Ga(R2)2Cl] (21).5 As all complexes in this series are neutral, the trend should be related to nuclear charge effect. It is noteworthy that M-C distance of 2.018(2) A in [(r|5-

C5H5)NiR2] is much shorter than that of 2.08 A (av.) in the nickel dialkyl [Ni(R2)2].

The most probable reason is that severe bulkiness of the "R2 group makes itself more difficult to approach closer to the metal center in the dialkyl complex than in the monoalkyl complex.

Among the series from Fe to Ga, only the last two members (i.e. Zn and Ga) deviate from the expected trend of decreasing M-C distance with increasing effective nuclear charge for metals across each period.

M-C distance increases abruptly from Ga to Zr as the latter metal belongs to the next period. From [Zr(R2)2Cl2] (103) to [Sb(R2)Cl2] (27),8 no obvious trend on variation of M-C distances is observed. It decreases from Zr to Ag as would be expected. The Pd-C distance of 2.187(2) A in [Pd(R2)2] (88) is only slightly longer than that of 2.167(6) and 2.149(9) in [PdR2(PPh3)Cl] (89) and [PdR2(PPh3)Br] (90) respectively. This may show that the trans influencing effect of the carbanionic ligand

-R2 is slightly larger than that of PPhs. Surprisingly, M-C distance increases from

[Ag(R2)]2 (13) to [Sn(R2)(N”)] (24) and then decreases smoothly to [Sb(R2)Cl2] (27) with a local maximum at M-C = 2.356(8) A for [Sn(R2)(N”)] (24).

The last series contains only [Au(R2)]2 (14)4 and [Hg(R2)2] (19).2 The monomeric mercury dialkyl complex has a longer M-C distance than the dimeric

[AU(R2)]2 (14)2,4

o Figure V-2 shows the variation of M-N distance (A) with different metal M. A first glance at the data showed that no obvious trend on variation of M-N distances is observed. Similar to M-C distance, M-N distance decreases from Fe to Ni for homoleptic iron(II) complex [Fe(R2)2] (55) to nickel(II) complex [Ni(R2)2] (82),

199 consistent with the increase in effective nuclear charge. M-N distances in the three nickel(II) complex are quite similar. Again the slightly longer M-N distance of 1.89 A

(av.) in the dialkyl complex 82 as compared with the distance of 1.856(2) A in the monoalkyl complex 78 is probably due to steric congestion of two bulky ligands around the metal center in 82. The same reasoning can also be applied to account for the slightly longer M-N distance of the palladium dialkyl complex 88 than that of the monoalkyl complex 89 and 90. The M-N distance in [Hg(R2)2] (19) is virtually absent as "R^ only acts as a C-centered ligand in this complex and the mercury center is free from N- coordination 2

When comparing the bite angle ZC-M-N of different metal complexes, steric effect of other ligands on the same metal complex should be taken into consideration.

Thus only the bite angle ZC-M-N of those [MR〗] species will be considered in the following discussion. The bite angle ZC-M-N of all [MR2] species lies within the range of 59.8° to 72.9°. In general, as the atomic size increases down a triad, the angle subtended from the metal center to the C/v^-chelating ligand will decrease. From Table

V-1 and Figure V-3, the bite angles ZC-M-N for metal alkyl complexes of the type

[MR2] from Mg to Zn is larger than those from Pd to Sn which , in turn, larger than those in the last series.

Both angles of ZC-M-C and ZN-M-N are dependent on the coordination environment of the central metal. Thus, for square planar complexes such as [M(R2)2]

(M 二 Co 56, Ni 82，Pd 88) attain the largest value, viz. 180.0°. The drastic decrease in the ZC-M-C angle to 116.7° in [Sn(R2)2] (18) is attributed to the presence of a lone pair on the tin(II) center^ Another noteworthy example can be found in [Zr(R2)2Cl2] (103).

Both ZC-M-C and ZN-M-N angles in 103 are well removed from values of other complexes. It is believed that both angles are being "compressed" in this six-coordinate metal complex.

200 REFERENCES FOR CHAPTER V

(1) Papasergio, R.I.; Raston, C.L.; White, A.H. J. Chem. Soc., Chem. Commun.

1983’ 1419.

(2) Henderson, MJ.; Papasergio, R.L; Raston, C.L.; White, A.H.; Lappert, M.F. J,

Chem. Soc., Chem. Commun. 1986, 672.

(3) Engelhardt, L.M.; Kynast, IL; Raston, C.L.; White, A.H. Angew. Chem. Int. Ed.

Engl. 1987,26, 681.

(4) Papasergio, R.L; Raston, C.L.; White, A.H. J. Chem. Soc., Dalton Trans. 1987,

3085.

(5) Kynast, U.; Skelton, B.W.; White, A.H.; Henderson, MJ.; Raston, C.L. J. Organomet. Chem. 1990, 384, CI.

(6) Papasergio, R.L; Raston, C.L.; White, A.H. J. Chem. Soc., Chem. Commun.

1984, 612.

(7) (a) Engelhardt, L.M.; Jolly, B.S.; Lappert, M.F.; Raston，C.L.; White, A.H. J. Chem. Soc., Chem. Commun. 1988，336.

(b) Jolly, B.S.; Lappert, M.F.; Engelhardt, L.M.; White, A.H.; Raston, C.L J.

Chem. Soc” Dalton Trans. 1993, 2653.

(8) Jones, C; Engelhardt, L.M.; Junk, P.C; Hutchings, D.S.; Patalinghug, W.C.;

Raston, C.L.; White, A.H. J. Chem. Soc” Chem. Commun. 1991，1560.

201 APPENDIX III

A. General Procedures

All experiments were performed under an argon atmosphere using standard Schlenk techniques or in a BRAUN MB 150 M drybox. Solvents were dried over and freshly distilled, under nitrogen, from CaH? (hexane), and sodium benzophenone ketyl (THF, ether, toluene), sodium metal (pentane), and degassed twice by freeze-thaw cycle prior to use.

B. Physical and Analytical Measurements

(i) Spectroscopic Measurements:

iH NMR spectra were recorded at 250 MHz using a Bruker WM-250 spectrometer.

Chemical shifts were referenced to residual solvent protons of 7.15 ppm for C5D5, 2.3 ppm for the benzylic protons of C7D8，and 7.24 ppm for CDCI3.

13 c NMR spectra were recorded at 69.2 MHz using a Bruker WM-250 spectrometer. Chemical shifts were referenced to solvent peaks of 128 ppm for C5D5, 21 ppm for the benzylic carbon of CyDg, and 77 ppm for CDCI3.

31p NMR spectra were recorded at 101.3 MHz using a Bruker WM-250 spectrometer. Chemical shifts were referenced to P(0Me)3 at 0 ppm.

Mass spectra (E.L 70 eV) were obtained on a VG7070F mass spectrometer.

UV spectra were recorded on a Hitachi U-2000 spectrometer with quartz cells of

1cm pathlength. THF was used as solvent extensively except otherwise stated.

(ii) Melting Point Measurements:

Melting points were recorded on an Electrothermal Melting Point Apparatus and were uncorrected.

202 (iii) Microanalysis:

Elemental (C, H, N) analyses were performed by The Institute of Organic Institute at Shanghai, People's Republic of China, or MEDAC Ltd.，Brunei University, UK.

(iv) Electrochemical Measurements (Cyclic Voltammetry):

The - electrochemical experiments (cyclic voltammetry) were carried out using a

EG&G PAR Model 173 Potentiostat, Model 175 Universal Programmer, and Model

RE0089 X-Y Recorder. The electrochemical cell (Figure A-1) was composed of a platinum wire working electrode, a silver wire counter electrode, and a tungsen wire reference electrode.

The electrochemical cell was cleaned before each experiment by cycling its potential between the onset of H2 and O2 evolution in 1 M H2SO4.

All measurements were then manipulated in an argon (or high purity nitrogen) atmosphere using standard Schlenk techniques. All sample solutions (THF) were prepared to be 0.2 M in "BU4NBF4 (supporting electrolyte) and 4 x 10-3 M in sample complexes.

Chemical potentials were internally referenced to FeCp2'^/FeCp2 reference redox system.

203 ^^ copper wire

/ fi\ / I\ 一 Armlet

uu^

\ \ t a\

I u

¥ reference Uj Pt working electrode electrode \ \ \ �

- • / / tungsten auxiliary I ^'''/''l electrode

glass frit J

Figure A-L The Electrochemical Cell Used in This Work.

204 APPENDIX III

Magnetic moments measurements were determined using Evan's NMR Method in

C6D6 solution at room temperature. All measurements were recorded on a Jeol PMX 60si

NMR spectrometer.

The mass susceptibility X^ of the dissolved sample is given by:

？Ig = (3 At) / 271 f c) -

where Av = frequency separation between external and internal TMS signals in Hz;

f = operating frequency of the NMR spectrometer in MHz;

c = concentration of the sample solution in g cm-、

Xq 二 mass susceptibility of the solvent (7 x 10"^ for C5H5 solution).

Then,

入m = MW • \

where MW 二 molecular weight of the sample compound.

The effective magnetic moment (in 吨）at temperature T (in K) can be calculated by:

205 APPENDIX III

Table A-l. Selected Crystallographic Data for Compounds 55, 56，58 and 63. (p. 207)

Table A-2. Selected Crystallographic Data for Compounds 73, 80, 82 and 83. (p. 208)

Table A-3. Selected Crystallographic Data for Compounds 86, 88，89 and 90. (p. 209)

Table A-4. Selected Crystallographic Data for Compounds 91 and 103. (p. 210)

206 Table A-1. Selected Crystallographic Data for Compounds 55，56, 58，and 63. ^ ^ 58 « Molecular formula C24H44N2Si4Fe C24H44N2Si4Co C26H32N2Si2Co CAgHgiNSiaSaFe Molecular weight 528.8 531.99 487.6 848,29 Color and habit dark brown plates dark red plates dark green prism yellow prism Crystal size, mm 0.36 x 0.40 x 0.60 0.26 x 0.24 x 0.34 0.18 x 0.26 x 0.44 0.30 x 0.30 x LOO Crystal system monoclinic triclinic monoclinic monoclinic Space group PI* (No. 14) P\ (No. 2) Pn (No. 7) 尸2i/c (No. 14) a,k 23.882(5) 8.519(2) 6.586(1) 14.585(5) Z?，人 16.820(6) 11.906(6) 8.608(1) 19.894(6) c，人 16.406(4) 16.212(6) 22.196(2) 19.266(5) A deg 107.32(1) 80.74(3) 94.18(1) 111.16(1) to y,人3 6291(3) 1522(1) 1254.9(6) 5213(3) < Z 8 2 2 4 Density, g cm-3 1.117 1.161 1.290 1.081 Abs. coeffs，mm-1 0.645 0.730 0.791 0.44 Transmission factors 0.691 - 0.819 0.771 - 0.836 0.921 - 0.965 Scan type Wyckoff co co co-2G qscan Scan rate, deg min-1 3.08 - 29.3 3.005 - 15.625 4.0 - 26.0 4.19 - 29.3 2 沒max，deg 50 50 45 Unique data measd. 10957 4850 6826 No. of obsd. reflects 4418 3356 5416 3656 No. of variables,/? 559 283 279 487 R 0.0411 0.064 0.036 0.0845 wR 0.0621 0.066 0.051 0.1251 Table A-2. Selected Crystallographic Data for Compounds 73，80，82 and 83. 73 ^ ^ ^ Molecular fomula CivHavNSizNi CagHssNsSiaNi? C24H44N2Si4Ni CssHssNgClSisNi Molecular weight 360.3 576.20 531.7 803.61 Color and habit brown-green prism brown plates red plates red transparent block Crystal size, mm 0.24 x 0.36 x 0.45 0.20 x 0.24 x 0.34 0.48 x 0.42 x 0.38 0.38 x 0.36 x 0.44 Crystal system triclinic monoclinic triclinic monoclinic Space group PI (No. 2) PhU (No. 14) P\(No. 2) C2/c (No. 15) a,k 8.695(1) 8.6288(2) 8,527(2) 27.703(6) b,k 9‘140(1) 17.7724(6) 11.858(2) 9.755(2) c,k 13.679(1) 28.3058(9) 16.200(2) 37.585(8) ^ p, deg 95.45(1) 96.214(2) 80.650(0) 109.25(3) g V, A3 961(1) 4315.3(8) 1519.6(5) 9589(4) Z 2 6 2 8

Density，g cm-3 1.245 1.330 1.162 1.113 Abs. coeffs, mm-l 1.13 1.41 0.810 0.636 Transmission factors 0.744 - 0.883 0.743 - 0.642 0.821 - 0.912 Scan type O)/20 o) co co Scan rate, deg min-l 3.0 - 29.3 8.0 - 29,3 7.00 - 60.00 7.00 - 60.00

2 知ax，deg 60 65 50 2.0 - 48.0 Unique data measd. 5579 7983 5837 8492 No. of obsd. reflects 4130 6116 4684 4700 No. of variables,/? 191 461 284 422 R 0.0309 0.029 0.043 0.0575 wR 0.0403 0.038 0.066 0.0629 Table A-3. Selected Crystallographic Data for Compounds 86，88，89 and 90. 86 ^ ^ ^ Molecular formula CigHisN^Si〗 C24H44N2Si4Pd CsoHsyNClPSi^Pd C3oH37NBrPSi2Pd Molecular weight 330.62 579.36 640.6 685.1

Color and habit colourless transparent block ycllow piism golden yellow yellow prism Crystal size, mm 0.18 x 0.24 x 0.3 0.20 x 0.30 x 0.30 0.48 x 0.42 x 0.42 0.20 x 0.30 x 0.40 Crystal system triclinic triclinic monoclinic monoclinic Space group PI (No, 2) PI (No. 2) P2i (No. 4) P2i/c (No. 14) a,k 8.764⑷ 8.749(2) 10.193(2) 12.319(2) /7，入 12.565(5) 9.251(2) 14.468(3) 17.241(3) c,入 14.270(7) 10.886(2) 11.777(2) 15.819(3) ftdeg 73.36(4) 102.07(3) 104.58(0) 109.99(3) o V，人3 1482.0(1.)) 766.3(3) 1579.3(7) 3158(1)

Z 3 1 2 4

Density，g cm-3 1.11 1.255 1.347 1.441 Abs. coeffs，mm-1 1.80 0.766 0.818 2.00 Transmission factors 1.000 - 0.850 0.710 - 0.983 Scan type co co (O o) Scan rate, deg min-l 4.0 4.00 - 60.00 3.00 - 60.00 2.0 - 60.0

2 0max，deg 45 50 45 50 Unique data measd. 3649 2654 3008 5467 No. of obsd. reflects 1876 2415 2709 2997 No. of variables, p 293 142 288 325

R 0.063 0.030 0.039 0.054

wR 0.055 0.037 0.055 0.054 Table A-4. Selected Crystallographic Data for Compounds 91 and 103. 91 ^ Molecular formula C24H45Cl2N2Si4Pd C24H44Cl2N2Si4Zr Molecular weight 652.28 635.1 Color and habit red prism yellow parallelepipedon Crystal size, mm 0.30 x 0.30 x 0.40 0.20 x 0.30 x 0.40 Crystal system monoclinic monoclinic Space group Flxlc (No. 14) CUc (No. 15) fl，人 16.034(3) 11.024(3) b, A 12.979(3) 15.735(5) c，入 16.613� 19.418(6)

Adeg 103.37(3) 105.38(2) g V,入3 3363(1) 3428(2) Z 4 4

Density, g cm-3 1.288 1.299 Abs. coeffs, mm-1 0.87 0.67 Transmission factors 0.384 - 0.657 Scan type co co Scan rate, deg min-l 3.0 - 60.0 3.0 - 30.0 2 ^max. deg 46 48 Unique data measd. 4512 2525 No. of obsd. reflects 2924 1725 No. of variables, 299 151 R 0.064 0.048 wR 0.065 0.045 . ；^、"；“-“.... wf ,..V，，-:.:;.. 1:.-..-:

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