COORDINATION COMPLEXES OF CROWN

ETHER CATION AND GROUP 13 HYDRIDE ANIONS

by

Lenuta Onut

B.Eng. (Honours), Politehnica University of Timisoara, Faculty of Industrial

Chemistry and Environmental Engineering, Romania, 2002

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS FOR THE DEGREE OF

Master of Science

In the Graduate Academic Unit of Chemistry

Supervisor: Gerard Sean McGrady, D. Phil., Chemistry Department

Examining Board: Jack Passmore, Ph.D., Chemistry Department

External Examiner: Aurora Nedelcu, Ph.D., Biology Department

This thesis is accepted

Dean of Graduate Studies

THE UNIVERSITY OF NEW BRUNSWICK

May 2007

©Lenuta Onut, 2007 Library and Archives Bibliotheque et 1*1 Canada Archives Canada Published Heritage Direction du Branch Patrimoine de I'edition

395 Wellington Street 395, rue Wellington Ottawa ON K1A 0N4 Ottawa ON K1A 0N4 Canada Canada

Your file Votre reference ISBN: 978-0-494-56471-4 Our file Notre reference ISBN: 978-0-494-56471-4

NOTICE: AVIS:

The author has granted a non­ L'auteur a accorde une licence non exclusive exclusive license allowing Library and permettant a la Bibliotheque et Archives Archives Canada to reproduce, Canada de reproduire, publier, archiver, publish, archive, preserve, conserve, sauvegarder, conserver, transmettre au public communicate to the public by par telecommunication ou par I'lnternet, preter, telecommunication or on the Internet, distribuer et vendre des theses partout dans le loan, distribute and sell theses monde, a des fins commerciales ou autres, sur worldwide, for commercial or non­ support microforme, papier, electronique et/ou commercial purposes, in microform, autres formats. paper, electronic and/or any other formats.

The author retains copyright L'auteur conserve la propriete du droit d'auteur ownership and moral rights in this et des droits moraux qui protege cette these. Ni thesis. Neither the thesis nor la these ni des extraits substantiels de celle-ci substantial extracts from it may be ne doivent etre imprimes ou autrement printed or otherwise reproduced reproduits sans son autorisation. without the author's permission.

In compliance with the Canadian Conformement a la loi canadienne sur la Privacy Act some supporting forms protection de la vie privee, quelques may have been removed from this formulaires secondaires ont ete enleves de thesis. cette these.

While these forms may be included Bien que ces formulaires aient inclus dans in the document page count, their la pagination, il n'y aura aucun contenu removal does not represent any loss manquant. of content from the thesis.

1*1 Canada DEDICATION

This thesis is dedicated to my family.

ii ABSTRACT

This thesis describes the preparation and structural characterization of complexes formed between alkali metal-crown ether cations and Group 13 hydride anions. Crown ethers

coordinated alkali metal cations have proven utility in stabilizing a range of metal hydride anions, producing high quality crystalline products. In our study, we have

+ explored the coordination chemistry of [M(crown)] (M = Li, Na and K) with EH4 (E =

B, Al and Ga). The complexes have been characterized by single crystal X-ray diffraction and NMR spectroscopy. Cation-anion interactions occur through hydride bridges of the form M—H-E. The hapticity of the anion depends on the size of the Group 13 element and on the size and acidity of the alkali metal cation.

Replacing 15-crown-5 and 18-crown-6 with their azacrown analogues and comparing the

structures adopted by the BH4 complexes reveals that the azacrown systems display pronounced proton-hydrogen H-bonding of the form N-H—H—B, which leads to a chain- type arrangement of adjacent cation-anion pairs and results in these systems adopting a

different space group from that of their simple crown ether counterparts.

111 ACKNOWLEDGEMENTS

I would like to express my special thanks and deep appreciation to my supervisor Dr. Sean McGrady for his guidance and knowledge, and for being a constant source of ideas. This thesis has been possible only due to his constant support and encouragement. I would also like to thank him for the nice atmosphere and great moments (barbeque and curry nights).

I would also like to thank Chris Willson, president of HSM Systems, for financial support and also for all the wonderful conversations that we have had.

I am grateful to past (Mathi Kandiah - Sri Lanka; Aled Jones - England; Prof. Balaji Jagirdar - India; Reyna Ayabe - Hawaii; Ven Reddy - India) and present (Peter Sirsch - Germany; Richard Burchell - Wales; Henrietta Langmi - Cameroon; Terry Humphries - England; Uncharat Setthanan - Thailand; Ben Tardiff -Canada; Ranga Santhanan - India) members of the McGrady Group for teaching me experimental techniques and chemical principles and for sharing pleasant moments inside and outside the lab. Special thanks go to Dr. Peter Sirsch and Richard Burchell for their theoretical calculations.

I extend my gratitude to all the faculty members, especially to Dr. Jeff Banks - Supportive Advisor; Dr. Andreas Decken - X-ray Crystallography; and Dr. Larry Calhoun - NMR Spectroscopy. My sincere thanks also go to the following staff members: Brian Malcolm - Glassblower; Dave Green - Technical Officer; Adam Fowler - Electronics Technician; Ed Goodfellow - Stores Manager; Gilles Vautour - Computer Officer; and Crystal Cavanaugh and Krista Coy - Departmental Secretaries.

Thanks are due also to the members of my Reading Committee: Dr. Thomas Whidden, Dr. Ghislain Deslongchamps and Dr. John Neville; as well as the Examining Committee: Dr. Jack Passmore and Dr. Aurora Nedelcu.

Furthermore, I would like to thank to all the instructors for whom I have been a Teaching Assistant: Dr. Valerie J. Reeves, Dr. Alyson Goodfellow, Dr. Peter Penner, Dr. Andreas Decken and Dr. Noureddin E. Kassimi.

Finally, I would like to take the opportunity to express my infinite gratitude to my family: my husband - Vio, my mother - Anica and my brother - Daniel for their constant love, caring and encouragements throughout my studies.

IV TABLE OF CONTENTS

DEDICATION II

ABSTRACT III

ACKNOWLEDGEMENTS IV

TABLE OF CONTENTS V

LIST OF TABLES VIII

LIST OF FIGURES X

LIST OF SCHEMES XIV

LIST OF SYMBOLS, NOMENCLATURE OR ABBREVIATIONS XV

CHAPTER 1 INTRODUCTION 1

1.1 Hydride Complexes of Boron and Aluminum 1

1.2 Boron Hydrides as Ligands 4

1.2.1 Tetrahydroborate , BH4~ 4

1.2.2 The octahydrotriborate ion, B3H8 12

1.3 Aluminum Hydrides as Ligands 14

1.4 Crown Ethers 23

1.4.1 Alkali Metal-Crown Ether Complexes 24

1.4.2 Alkali Metal Crown ether Complexes with Transition Metal Hydrides 28

CHAPTER 2 RESULTS AND DISCUSSION 31

2.1 General Aims of the Project 31

2.2 Specific Aims of this Research 32

2.3 Synthetic Routes to [M(crown)][EH4] Complexes 32

2.4 Synthesis and Characterization of [M(crown)][EH4] Complexes 35

2.4.1 [Li2(12-crown-4)3][AlH4]2 35

V 2.4.2 [Li2(12-crown-4)3][GaH4]2 36

2.4.3 [Na(15-crown-5)][BH4] 37

2.4.4 [Na(15-crown-5)][AlH4] 38

2.4.5 [K(18-crown-6)][BH4] 39

2.4.6 [K(THF)(18-crown-6)][AlH4] 40

2.4.7 [K(THF)(18-crown-6)][GaH4] 41

2.5 Analysis of the Structures Obtained by X-Ray Diffraction 42

2.5.1 [M(crown)][BH4] Structures 42

2.5.2 [M(crown)][AlH4] Structures 45

2.5.3 [M(crown)][GaH4] Structures 48

2.5.4 [Na(15-crown-5)][EH4] Structures 49

2.5.5 [K(18-crown-6)][EH4] Complexes 52

2.6 [M(azacrown)][BH4] Complexes 57

2.7 Supramolecular Structures 61

2.7.1 [Na(15-crown-5)][BH4] vs. [Na(15-azacrown-5)][BH4] 62

2.7.2 [K(18-crown-6)][BH4] vs. [K(18-azacrown-6)][BH4]/ [K(THF)(18-azacrown-6)][BH4] 66

2.8 Charge Density Analysis of [Na(15-crown-5)][EH4] Complexes 71

2.8.1 DFT Geometry Optimization 73

2.8.2 Density Map 74

2.8.3 Gradient Vector Field 75

2.8.4 AIM Atomic Charges 77

2.8.5 The Laplacian of the Electron Density 78

2.8.6 Topological Parameters 79

CHAPTER 3 EXPERIMENTAL DETAILS 82

3.1 General 82

3.1.1 Solvents 83

3.1.2 General Reagents 84 vi 3.2 Instrumentation 84

3.2.1 NMR Spectroscopy 84

3.2.2 Single Crystal X-Ray Diffraction 85

3.2.3 Theoretical Calculations 85

3.3 Synthesis and Characterization 86

3.3.1 [Li2(12-crown-4)3][AlH4]2 86

3.3.2 [Li2(12-crown-4)3][GaH4]2 86

3.3.3 [Na(15-crown-5)][BH4] 87

3.3.4 [Na(15-crown-5)][AlH4] 87

3.3.5 K(18-crown-6)][BH4] 88

3.3.6 [K(THF)(18-crown-6)][AlH4] 89

3.3.7 [K(THF)(18-crown-6)][GaH4] 89

3.3.8 [Na(15-azacrown-5)][BH4] 90

3.3.9 [K(18-azacrown-6)][BH4] 90

3.3.10 [K(THF)(18-azacrown-6)][BH4] 91

3.4 Crystallographic Data 92

CHAPTER 4 OVERVIEW AND CONCLUSIONS 96

BIBLIOGRAPHY 99

VITAE 104

Vll List of Tables

Table 1.1: Ionic radii of alkali metal cations and corresponding cavity sizes of crown ethers (A), log K and

AH(kJ/mol) 25

Table 2.1: Complexes of alkali metal-crown ether complexes and Group 13 hydride anions obtained in

crystalline form 34

Table 2.2: B-H-M distances (A) with estimated standard deviations for [M(crown)][BH4] complexes (M

= NaandK) , 43

Table 2.3: M-B—H, angles (°), the distance of the metal cation from the crown plane (A) and the B—M

distances (A) with estimated standard deviations for [M (crown)] [BH4] complexes 44

Table 2.4: B-H distances (A) with estimated standard deviations for [M(crown)][BH4] complexes 44

Table 2.5: Al-H—M distances (A) with estimated standard deviations for [M(crown)][AlH4] complexes (M

= NaandK) 46

Table 2.6: M—Al-H, angles (°), the distance of the metal cation from the crown plane (A) and A1--M

distances with estimated standard deviations for [M(crown)][AlH4] complexes 47

Table 2.7: Al-H distances (A) with estimated standard deviations for [M(crown)][AlH4] complexes 47

Table 2.8: Ga—M distances and the distance of the metal cation from the crown plane (A) with estimated

standard deviations for [M(crown)][GaH4] complexes 49

Table 2.9: Ga-H distances (A) with estimated standard deviations for [Li2(12-crown-4)3][GaH4]

complex 49

Table 2.10: Na-O distances (A) with estimated standard deviations for [Na(15-crown-5)][EH4] 51

Table2.11: E-H-M and E—Na distances (A) with estimated standard deviations for [Na(15-crown-

5)][EH4] complexes 52

Table 2.12: K-0 distances (A) with estimated standard deviations for [K(18-crown-6)][EH4]

complexes 55

vni Table 2.13: E-H—K and E—K distances (A) with estimated standard deviations for [K(18-crown-6)][EH4]

complexes 57

Table 2.14: B-H'Na and B "Na distances (A) with estimated standard deviations for [Na(15-azacrown-

5)][BH4] 58

Table 2.15: K-O and K-N distances (A) with estimated standard deviations for [K(18-azacrown-

6)][BH4] 61

Table 2.16: B-H"Na and B--Na distances (A) with estimated standard deviations for [K(18-azacrown-

6)][BH4] complexes 61

Table 2.17: B—Na distances and the distance of the metal cation from the crown plane (A) with estimated

standard deviations for [Na(crown)][BH4] complexes 63

Table 2.18: B-H—Na distances (A) with estimated standard deviations for [Na(crown)][BH4]

complexes 63

Table 2.19: Table B—K distances and the distance of the metal cation from the crown plane (A) with

estimated standard deviations for [K(crown)][BH4] complexes 67

Table 2.20: B-H—K distances (A) with estimated standard deviations for [K(crown)][BH4] complexes...68

Table 2.21: Calculated AIM charges (q) for the atoms involved in the bonding in [Na(15-crown-5)][EH4]

complexes and in the free [EH4] moieties 77

Table 2.22: Topological parameters at the BCPs for [Na(15-crown-5)][EH4] complexes at the [B3LYP/6-

311G(d,p)] level of theory 80

Table 2.23: Calculated derealization indices (5) for [Na(15-crown-5)][EH4] complexes at the [B3LYP/6-

311G(d,p)] level of theory 80

IX List of Figures

5 r 5 1 Figure 1.1: Molecular structure of (a) [Pd{-n -C5H3(2,5-CH2P Bu2}2Fe(Ti -C5H5)(Ti -

1 BH4)] and (b) [FeH(dmpe)2('ri -BH4)] as determined by X-ray diffraction. All non-hydride hydrogen

atoms are omitted for clarity 7

1 l Figure 1.2: (a) Molecular structure of (a) [Cu(triphos)(Ti -BH4)] and (b) trans-[RuH(R,R)-Pnor)2(r\ -BU4)]

as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity 8

2 Figure 1.3: Molecular structure of (a) [A1(BH4)3] and (b) [Li(PMDTA)(n -BH4)] 9

Figure 1.4: Molecular structure of dimeric [NaBH4'PMDTA] as determined by X-ray diffraction. All non-

hydride hydrogen atoms are omitted for clarity 10

2 2 Figure 1.5: Molecular structure of (a) [CoH(PCy3)2(n -BH4)] and (b) [Ni(triphos)(u -BH4)] as determined

by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity 11

3 3 Figure 1.6: Molecular structure of (a) [Tp*Ni(n -BH4)] and (b) [(ArO)3Ti(r] -BH4)] as determined by X-

ray diffraction. All non-hydride hydrogen are omitted for clarity 11

Figure 1.7: Molecular structure of [(triphos)Fe(u,n4:r|4-BH6)Fe(triphos)]+as determined by X-

ray diffraction. All non-hydride hydrogen are omitted for clarity 12

Figure 1.8: Molecular structure of (a) [(Co)3MnB3H8] and (b) [Cr(B3H8)2(Et20)2] com-plex as determined

by X-ray diffraction. All non-hydride H atoms are omitted for clarity 13

5 6 Figure 1.9: Molecular structure of [Oi -C5H5)2Nb(B3H8)] and (b) [

by X-ray diffraction. All non-hydride are omitted for clarity 14

Figure 1.10: Molecular structures of (a) wer-[(Me3P)3HRu(u-H)2BH2] and (b) /ac-[(Me3P)3HRu(u-

H)2A1H2] as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted for

clarity 17

Figure 1.11: Molecular structure of (a) [(MePh2P)2H6ReAlMe2] and (b) [MePh2P)3H4Re 18

Figure 1.12: Molecular structure of [Mn(AIH4)(dmpe)2]2 as determined by X-ray diffraction. All non-

hydride hydrogen atoms are omitted for clarity 20

x Figure 1.13: Molecular structure of [(r^-CsMej^Tir^Alt^k as determined by X-ray diffraction. All non-

hydride hydrogen atoms are omitted for clarity 21

Figure 1.14: Molecular structure of [(r^-C^l-^Nbl^An-^k as determined by X-ray diffraction. All non-

hydride hydrogen atoms are omitted for clarity 21

Figure 1.15: Molecular structure of {[(PMe3)3WH3]2AlH5} as determined by X-ray diffraction. All non-

hydride hydrogen atoms are omitted for clarity. The three terminal hydrides on each tungsten atom

were not located 22

D Figure 1.16: Molecular structure of (a) [(r| -C5Me5)2Ru2(u-dppm)(u-AlH5)] and (b) {[(rf-

C5H5)2Ti2]2AlH4BH4} as determined by X-ray diffraction. All non-hydride hy-drogen atoms are

omitted for clarity 23

+ 4 Figure 1.17: Molecular structure of (a) Rb(18-crown-6)] Rb~ and [Rb(18-crown-6)(CH3NH2)] Na' as

determined by X-ray diffraction 28

Figure 1.18: Molecular structure of (a) [K(18-crown6)[W(PMe3)3H5] and (b) [Na(15-crown-

5)][W(PMe3)3H5] ] as determined by X-ray diffraction. All non-hydride hydrogen atoms are

omitted for clarity 29

Figure 1.19: Molecular structure of (a) [K(18-crown-6)][IrH4(PPh3)2] and (b) [K(THF)(18-crown-

6)][OsH3(PPh3)3] as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted

for clarity 30

Figure 2.1: Molecular structure of [Li2(12-crown-4)3][AlH4]2 complex as determined X-ray diffraction. All

non-hydride hydrogen atoms are omitted for clarity 35

Figure 2.2: Molecular structure of [Li2(12-crown-4)3][GaH4]2 complex as determined by X-ray diffraction.

All non-hydride hydrogen atoms are omitted for clarity 36

Figure 2.3: Molecular structure of [Na(15-crown-5)][BH4] complex as determined X-ray diffraction. All

non-hydride hydrogen atoms are omitted for clarity 37

Figure 2.4: Molecular structure of [Na(15-crown-5)][AlH4] complex as determined by X-ray diffraction.

All non-hydride H atoms are omitted for clarity 38

XI Figure 2.5: Molecular structure of [K(18-crown-6)][BH4] complex as determined by X-ray diffraction. All

non-hydride hydrogen atoms are omitted for clarity 39

Figure 2.6: Molecular structure of [K(THF)(18-crown-6)][AlH4] complex as determined X-ray diffraction.

All non-hydride hydrogen atoms are omitted for clarity 40

Figure 2.7: Molecular structure of [K(THF)(18-crown-6)][GaH4] complex as determined by X-ray

diffraction. All non-hydride hydrogen atoms are omitted for clarity 41

Figure 2.8: Local structure of (a) [Na(15-crown-5)][BH4] and (b) [K(18-crown-6)][BH4] complexes,

showing the cation-anion interaction. All carbon, oxygen and non-hydride hydrogen atoms are

omitted for clarity 43

Figure 2.9: Local structure of (a) [Na(15-crown-5)][AlH4] and (b) [K(THF)(18-crown-6)][AlH4]

complexes, showing the cation-anion interaction All carbon, oxygen and non-hydride hydrogen

atoms are omitted for clarity 46

Figure 2.10: Local structure of (a) [Na(15-crown-5)][BH4] and (b) [Na(15-crown-5)][AlH4] complexes,

showing the cation-anion interaction. All carbon atoms and non-hydride hydrogen atoms are omitted

for clarity 50

Figure 2.11: Local structure of (a) [K(18-crown-6)][BH4], (b) [K(THF)(18-crown-6)][AlH4] and (c)

[K(THF)(18-crown-6)][GaH4] complexes, showing the cation-anion interaction. All carbon and non-

hydride hydrogen atoms are omitted for clarity 53

Figure 2.12: Molecular structure of [Na(15-azacrown-5)][BH4] as determined by X-ray diffraction. All

non-hydride hydrogen atoms are omitted for clarity 58

Figure 2.13: Molecular structure of [K(18-azacrown-6)][BH4] as determined by X-ray diffraction. All non-

hydride H atoms are omitted for clarity 59

Figure 2.14: Molecular structure of [K(18-azacrown-6)][BH4] as determined by X-ray diffraction. All non-

hydride H atoms are omitted for clarity 60

Figure 2.15: Molecular structure of (a) [Na(15-crown-5)][BH4] and (b) [Na(15-azacrown-5)][BH4] as

determined by X-ray diffraction All non-hydride hydrogen atoms are omitted for clarity 62

Xll Figure 2.16: The alternating up-down chain arrangement adopted by neighbouring ion pairs in the [Na(15-

crown-5)][BH4] complex 64

Figure 2.17: A portion of the linear chain adopted by [Na(15-azacrown-5)][BH4]. The polymeric structure

is held together through unconventional N-H—H-B interactions, which are represented by a dotted

line 66

Figure 2.18: Molecular structure of (a) [K(18-crown-6)][BH4] and (b) [K(18-azacrown-6)][BH4] and (c)

[K(THF)(18-azacrown-6)][BH4] as determined by X-ray diffraction. All non-hydride H atoms are

omitted for clarity 67

Figure 2.19: The alternating, up-down chain arrangement adopted by neighbouring ion pairs in the [K(18-

crown-6)] [BH4] complex 68

Figure 2.20: The zigzag chain of [K(18-azacrown-6)][BH4]. The polymeric structure is held together

through unconventional N-H—H-B interactions, which are represented by a dotted line 69

Figure 2.21: The zigzag chain of [K(THF)(18-azacrown-6)][BH4]. The polymeric structure is held together

through unconventional N-H—H-B interactions, which are represented by a dotted line. The THF

molecule is omitted for clarity 70

Figure 2.22: DFT optimized structures, atomic AIM charges (boxed), and geometrical parameters of (a)

[Na(15-crown-5)][BH4] and (b) [Na(15-crown-5)][AlH4] [B3LYP/6-31 lG(d,p)]; all atoms of the

crown ethers are omitted for clarity 73

Figure 2.23: Contour maps of the electron density p(r) of (a) [Na(15-crown-5)][BH4] and (b) [Na(15-

crown-5)][A1H4] in a plane containing Na, B and Al, respectively, and a bridging hydrogen atom...74

Figure 2.24: Gradient vector field, Vp(r) for (a) [Na(15-crown-5)][BH4] and (b) [Na(15-crown-

5)][A1H4] 76

2 Figure 2.25: Calculated V p(r) for the Na-Hb-E moiety of (a) [Na(15-crown-5)][BH4] and (b) [Na(15-

crown-5)][AlH4] derived at the denoted levels of theory. The solid lines represent negative values of

V2p(r) 78

Xlll List of Schemes

Scheme 1.1: (a) Reduction of organic functional groups by (a) LiAlH4 and (b)NaBH4 3

Scheme 1.2: Coordination modes for BH4 5

Scheme 1.3: Schematic molecular orbital energy diagram of BH4 6

Scheme 1.4: Connectivity in (a) M(U-H)2A1H(U-H)2A1H(U-H)2M and (b) M(u-H)2AlH(u-H)2M

complexes 19

Scheme 1.5: Structural representation of (a) 12-crown-4, (b) 15-crown-5, and (c) 18-crown-6 24

Scheme 1.6: Structural representation of (a) [Li (12-crown-4)]+, (b) [Na(15-crown-5)]+, and (c)

[K(18-crown-6]+ 26

XIV List of Symbols, Nomenclature or Abbreviations

Tl Descriptor of hapticity, superscript designates number of bridging sites

Hb Bridging hydrogen

Ht Terminal hydrogen MO Molecular orbital H Descriptor for bridging IR Infrared v(IR) Stretching mode E Group 13 (B, Aland Ga) M Group 1 (Li, Na and K) NMR Nuclear Magnetic Resonance s(NMR) Singlet m(NMR) Multiplet J (NMR) Coupling constant ppm Parts per million ESR Electron Spin Resonance THF Tetrahydrofuran CSD Cambridge Structural Database d day h hour DFT Density Functional Theory AIM Atoms in Molecules Theory BCP Bond critical point P(r) Electron density distribution Vp(r) Gradient vector of field of p(r) V2p(r) Laplacian of p(r)

8 Bond ellipticity

xv CHAPTER 1

INTRODUCTION

1.1 Hydride Complexes of Boron and Aluminum

The complex metal hydrides of boron and aluminum rank among the most useful reagents in both organic and inorganic synthesis. Along with the binary hydrides which are widely used by both inorganic and organic chemists, commercially available aluminum hydride and lithium, and borohydrides are effective and versatile reagents in reduction or metathesis reactions. The relatively easily prepared magnesium aluminum hydride and lithium gallium hydride have also been investigated in these applications, albeit less widely.1

Most interest in the complex metal hydrides in general, and in the aluminum and boron systems in particular, has focused on the reduction of organic compounds. Even though the reducing action of these substances is limited in most cases to aldehydes, ketones, acid and a few other functional groups, this limitation is desirable for selective reductions. Lithium aluminum hydride has been utilized for the reduction of organic compounds to a greater extent than any other complex metal hydride. Lithium, sodium and potassium borohydrides have also found considerable application, especially when selective reduction is desired.

1 The hydride ion, H~, is the simplest conceivable nucleophile. Because of the low solubility of alkali metal hydrides (LiH, NaH and KH) in organic solvents and their strong intrinsic basicity, these reagents3 cannot be used as sources of nucleophilic FT.

Instead, complex metal hydrides are used, which do not have a simple hydride structure.

In these complex hydrides, the hydrogen atoms bear partial negative charges and are covalently bonded to a main group metal or non-metal atom. This arrangement renders the hydride a better nucleophile, at the same time lowering its basicity.4

The two most common sources of hydride are sodium borohydride, NaBLLt, and lithium aluminum hydride, L1AIH4.2 The discovery of these hydrides in the 1940's brought about a radical change in procedures for the reduction of functional groups in organic chemistry.3 Sodium borohydride is representative of the alkali metal borohydrides, and it is the most common reducing agent in organic chemistry. Lithium aluminum hydride accomplishes many otherwise tedious and difficult reductions in organic chemistry, as well as being a useful inorganic reductant. Lithium aluminum hydride is a strong reducing agent capable of reducing practically all organic functional groups; the metal- hydrogen bonds are more polar than the corresponding ones bonds in NaBH4. LiAlLL; is usually used to reduce only compounds such as carboxylic acids, esters and amides that cannot be reduced by a milder reagent.6'7 See Scheme 1.1(a).

Lithium aluminum hydride displaces the halide from most organic halides, in what is effectively a metathesis reaction. Conversion of carboxylic acids to alcohols is the most difficult reduction to achieve, but can be accomplished with LiAlH4. Metal hydride

2 reduction of esters is difficult to stop at the intermediate aldehyde stage, and usually results in further reduction to the alcohol. Either LiAlH4 or LiBFLi (but not NaBH4, which will not reduce esters) is the reagent of choice for this transformation.8 Reduction of a nitrile with UAIH4 gives a primary amine, just as reduction of an ester gives a primary alcohol.9

OH R C OH S—8= =R R C —OH 4 k

R C CI NH, C R, ,R C OH

-C C=R. R C=0 N^C R, II H H ./ laBHj

NH, C R R C OH H, ci c—c—R R C R' II II NH, C R R C OH o O

R C OR'

R C R' H2C C —R HO R' + R C OH I I OH (a) OH (b)

Scheme 1.1: (a) Reduction of organic functional groups by (a) L1AIH4 and (b) NaBtU.

On the other hand, sodium borohydride is a relatively mild reducing agent that reacts readily only with aldehydes, ketones and acid chlorides. Consequently, both LiAlH4 and

NaBH4 reduce aldehydes, ketones and acyl halides, but L1AIH4 is not generally used for this purpose since NaBFLi is safer and easier to use. Sodium borohydride is extensively employed in the reduction of aldehydes and ketones to primary and secondary alcohols.

3 The reactions take place in a wide variety of solvents, including alcohols, ethers, and water. The yields are generally excellent. See Scheme 1.1(b).

1.2 Boron Hydrides as Ligands

Complexes involving interactions of B-H bonds with transition metal centres were among the earliest electron-deficient compounds to be prepared and isolated, and more recently reported systems have been shown to be genuine a-bond complexes.10 The tetrahydroborate (BH4 ) and octahydrotriborate (B3H8 ) are the most commonly used borane anions in such systems.

1.2.1 Tetrahydroborate Ion, BH4"

The tetrahydroborate ion, BH4 , is the simplest known complex anionic hydride, and in addition to forming ionic compounds it can react by ligand displacement to form covalently bonded complexes through 3-centre, 2-electron bonds, B-H->M (where M = main group metal, transitional metal, lanthanide or actinide). A unique feature of this interaction is that the BH4 , unit may be ligated to the metal in monohapto (n1), dihapto

(n2) or trihapto (n3) fashion (Scheme 1.3).11 The coordination mode is typically established by single-crystal X-ray and/or neutron diffraction analysis. The hapticity may also be ascertained by examining the B-Hb and B-Ht stretching modes in the IR spectrum.

4 4 M—H—B —H M B—H M^,, xxW,i»?B H

H H ^H ri1 ri2 if

Scheme 1.2: Coordination modes for BH4 .

The chemical and physical properties of such complexes have been of interest to inorganic and organometallic chemists for several decades. In particular, the way in which the BH4 unit is linked to the metal centre, and the variability of this coordination in the light of the facile fluxionality commonly displayed by metal borohydride complexes, have been the subject of numerous studies.11 The number of varieties of complexes involving direct metal-boron interactions has increased at an exponential rate over the past few decades. The other three major types of metal-boron compounds; viz. borides, metallaboranes, and 71-complexes with boron-containing ligands, are each characterized by electron-precise two-centre two-electron bonds between boron and the metal centre. According to this classification scheme, a number of different coordination modes for boron-containing ligands have been established, permitting a systematic

1 7 classification of those compounds into borane, boryl, and borylene complexes.

The hapticity of BH4 is influenced both by the steric requirement of the co-ligands (e.g.

PMePli2 is less demanding than PPI13), and by the size of the central metal atom (e.g. Zr can accommodate simultaneous ligation by 12 H atoms).11 In monohapto, dihapto and trihapto situations, BH4 has four filled molecular orbitals (MOs) available (containing a total of eight ) that can donate electron density to the metal ion. In each case, these consist of two orbitals with appropriate symmetry for forming metal-ligand a (lai and 2ai MOs) bonds and two appropriate for forming n bonds (lti and 2ti MOs). See

Scheme 1.3. The modes in which the ligand MO interacts with the frontier orbitals of the metal fragment depend on the symmetries and energies of these metal-based orbitals.

1a,

BH4 4H

Scheme 1.3: Schematic molecular orbital energy diagram of BH4 .

The dihapto and trihapto coordination modes occur most frequently, whereas monohapto

1 coordination is observed rather rarely. Koridze et al. were the first to characterize r| -BH4 coordination in the ferrocene-based pincer complex [PdfT^-CsHs^S-CtfeP'B^^Fe^3-

CsHsXV-BFU)].13 The palladium centre has a distorted square-planar geometry, in which three coordination sites are occupied by the P-C-P ligand, and the fourth site is occupied by the bridging hydrogen atom of the borohydride ligand. See Figure 1.1(a).

6 (a) (b)

Figure 1.1: Molecular structure of (a) [Pd^XBu^PCF^CsHbJFeCsF^Ti1-

BH4)] and (b) [FeHtdmpe^Cn'-BHU)] as determined by X-ray diffraction. All non-

hydride hydrogen atoms are omitted for clarity.

It should be noted that the Pd—B distance in Figure 1.1(a) is smaller (2.61 A) than the

corresponding distances in other n'-BFLj metal complexes.11 For example, the Fe—B

1 14 distance in [FeH(dmpe)2(Ti -BH4)], [dmpe - (MezPCHbCF^PMes^] is 2.84 A. See

Figure 1.1(b).

The capacity of the bulky ligand triphos, [MeC(CH2PPh2)3], to form stable complexes

with 3d metals in low oxidation states has been well documented.12 It has been shown11

to stabilize the +1 of cobalt, copper and nickel. Reaction of the halide precursors [M(triphos)X] (M= Cu, Co or Ni; X= CI or Br) with NaBH4 produces the

corresponding metal tetrahydroborate complexes [M(triphos)BH4]. In [Cu^riphos)^1-

BH4)] the copper atom is linked to the three phosphorus donors of the triphos ligand and

7 one bridging hydrogen atom of the borohydride moiety in a distorted tetrahedral environment;15 Figure 1.2(a).

(a) (b)

1 Figure 1.2: (a) Molecular structure of (a) [Cu(triphos)(T| -BH4)] and (b) trans-

1 [RuH(i?r/?)-Pnor)2(ri -BH4)] as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

Another monodentate tetraborohydride complex was reported by Morris et al. In trans-

1 [RuH(i?,R>Pnor)2(ji-H)(Ti -BH4)], [(l,2)-Pnor = PPh2CHPhCHMeNH2], the Ru centre is pentacoordinated (2N, 2P, 1H), with one of the four H atoms of the borohydride group bridging to the Ru atom, as depicted in Figure 1.2(b).

A textbook example of dihapto (II) borohydride coordination is the octahedral aluminum complex [Al(n2-BH4)3] [Figure 1.3(a)],17 which was not only the first covalent borohydride to be characterized early in the 1940s, but also the first ever compound in which the phenomenon of fluxionality was observed; all 12 H atoms were observed to be equivalent on the NMR timescale.

8 (a) (b)

2 Figure 1.3: Molecular structure of (a) [A1(BH4)3] and (b) [Li(PMDTA)(rj -BH4)]

as determined by X-ray diffraction. All non-hydride hydrogen atoms for (b) are

omitted for clarity.

Alkali metal tetrahydroborates, and sodium tetrahydroborate in particular, belong to an important class of hydridic reducing reagents with wide applicability. NaBH4 is tractable in aqueous or alcoholic solutions owing to its good solvolytic stability in neutral and alkaline environments, in contrast to L1BH4 which requires an ether solvent. Noth et al. reported lithium and sodium tetrahydroborate complexes with ligands such as PMDTA

18 (Me2NCH2CH2N(Me)CH2CH2NMe2). The lithium atom is pentacoordinated (3N, 2H) or tetracoordinated (3N, IB) if the BH4 group is treated as a single ligand [Figure 1.3(b)].

In contrast, the sodium congener18 is octacoordinated (3N, 5H) or pentacoordinated (3N,

2B) if the BH4 group is considered a single ligand. Three of the four H atoms of each

9 BHu group coordinate to the sodium atoms; two form a double bridge to two sodium atoms while the third one is bonded only to one Na centre; see Figure 1.4.

Figure 1.4: Molecular structure of dimeric [NaBH^PMDTA] as determined by X-ray

diffraction. All non-hydride hydrogen atoms are omitted for clarity

The first transition metal borohydride complexes found to display dihapto borohydride coordination were prepared in 1949 by Hoekstra and Katz, by allowing metal MCU (M =

19 Ti, Hf or Th) to react with LiBH4. Since this initial report, dozens of other dihapto complexes have been reported. Two examples20'21 of transition metal borohydride complexes involving dihapto coordination are depicted below [Figure 1.5(a) and (b)]. In each complex the metal atom is pentacoordinated, with two bridging hydrides from each

BH4 moiety.

An example of a trihapto borohydride complex is [NiTp*(n3-BH4)],22 where Tp* is the nitrogen donor ligand hydrotris-(3,5-dimethylpyrazolyl)borato. The six-coordinate nickel atom centre is bound by three nitrogen atoms of Tp* and three bridging hydrides of

BH4~ Figure 1.6(a).

10 (a) (b)

2 2 Figure 1.5: Molecular structure of (a) [CoH(PCy3)2(r| -BH4)] and (b) [Ni(triphos)(n -

BH4)] as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted

for clarity.

Another example of a trihapto borohydride ligand is the titanium complex [Ti(ArO)3(r|3-

BH4)]23 [Figure 1.6(b)], where Ar is 2,6-diisopropylphenyl. The hexacoordinated titanium centre is connected to the three oxygen atoms of the Ar ligands and the three bridging hydrides of the borohydride group.

3 3 Figure 1.6: Molecular structure of (a) [NiTp*(n -BH4)] and (b) [Ti(ArO)3(ri -BH4)] as

determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

11 The BH4 anion is known to display at least six coordination modes, because in addition

to the three types depicted in Scheme 1.2 above, there exist also situations in which the

2 3 borohydride moiety bridges between two metal centres, such as (J.(TJ2, n ), u(n ), and

u.(n4), as exemplified by the cationic complex [(triphos)Fe(u,n4:n4-BH6)Fe(triphos)]+

(Figure 1.7).24

4 4 + Figure 1.7: Molecular structure of [(triphos)Fe(u^n :n -BH6)Fe(triphos)] as determined

by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

1.2.2 The octahydrotriborate ion, B3H8

The octahydrotriborate anion, B3H8 is known to act as either bidentate (TJ2) or a tridentate (r|3) ligand towards a number of metals. The first example of tridentate binding

2D is attributed to Gaines et al. In the complex [(Co)3MnB3H8] [Figure 1.8(a)], the B3H8~

12 unit is bound to the manganese centre through three Mn-H-B bridge hydrogen bonds; one bridge bond for each boron atom. The carbonyl units have afac orientation, as do the

Mn-H-B units. Each boron atom has a single terminal hydrogen atom bound to it. The other two hydrogen atoms present in the molecule bridge between boron atoms. Girolami et al.26 reported the to-octahydrotriborate complex [Cr(B3Hg)2(Et20)2], in which the two

B3H8 units and the two ether ligands occupy mutually trans sites at the Cr centre; see

Figure 1.8(b).

Figure 1.8: Molecular structure of (a) [Mn(Co)3B3H8] and (b) [Cr(B3H8)2(Et20)2] as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

Two examples of bidentate octahydrotriborate coordination, involving two M-H-B bridges to the metal centre, are [NbOf-CsHsMBsHg)]27 and [RuCKr^-CeMeeXBsHjj)]28

[Figure 1.9(a) and (b)].

13 (a) (b)

6 Figure 1.9: Molecular structure of [NbOf-CsHs^CBaHs)] and (b) [RuCl(Ti -C6Me6)

(B3H8)] as determined by X-ray diffraction. All non-hydride hydrogen atoms are

omitted for clarity.

1.3 Aluminum Hydrides as Ligands

Whereas a large number of BH4 derivatives of the transition metals have been prepared and characterized, comparatively few A1H4 complexes have been reported. The structural properties of aluminum hydrides differ from those of the corresponding boron hydrides in several ways; whereas boron hydrides are almost invariably four-coordinate and tetrahedral, the majority of aluminum hydrides are five-coordinate with a trigonal

14 bypiramidal aluminum atom. This ability of aluminum to become five-coordinate arises in part from the greater size of Al as compared to B (covalent radii are 1.35 A and 0.83

A, respectively),29 as well as from the greater number of valence orbitals available to the second-row element Al, for which hybridization, involving one of the low lying 3d orbitals, along with the 3s and 3p orbitals, can be invoked.

Boron hydrides have been found to form a variety of bridges of the type [M(u,-H)nB (n =

1, 2 or 3)], but aluminum hydrides generally bridge through two [M(u-H)A1] units, even when additional terminal M-H moieties are present. For aluminum hydrides, the presence of three bridging hydrides occurs when the ligand is precluded from forming a dimer by terminal alkyl groups. The hydrides in the [M(u.-H)A1] bridges are asymmetric, reflecting the difference between axial and equatorial sites on the aluminum, as opposed to those in borohydride complexes, which are generally symmetric, being derived from tetrahedral

BH4".

30 As of 1980, the structures of complexes of the type [LnM(AlH4)m] had been investigated far less extensively, and mainly by indirect methods (IR, NMR and ESR spectroscopy), than their more stable boron analogues, [LnM(BH4)m]. Whereas the structures of transition-metal borohydride complexes have been investigated by a variety of techniques, including gas-phase electron diffraction and single crystal X-ray and neutron diffraction as well as by NMR spectroscopy, the aluminum hydrides have only been studied by the most common solid-state structural technique, single-crystal X-ray diffraction. For aluminum hydride complexes it is more difficult to assign the Al-H modes in the IR spectrum on account of serious overlap of different types of stretching

15 modes. In general, for terminal hydrides, v(M-H) modes appear in the range 1500-

1700cm"1, while bridging hydrides are characterized by stretching vibrations in the 1550-

1650cm"1 range (M-H-Al). A useful tool in the structural characterization of transition metal aluminum hydride complexes has been 27A1 NMR spectroscopy. Though 27A1 possesses a large quadrupole moment, narrow lines can often be obtained in the NMR spectra. The line width depends on the geometry of the substituents around the Al atom.

To date, all aluminum hydride complexes studied show broad singlets in the region 50-

70 ppm, with linewidths of 4000-5000 Hz.31

Differences also exist in the type of fluxional behaviour displayed by BH4 and AIH4 complexes. In the boron hydrides, the bridging [M((x-H)B] and terminal B-H hydrides commonly undergo rapid intermolecular exchange, but rarely undergo intramolecular exchange with any terminal M-H hydrides. Only at elevated temperatures does such exchange occur, for example in the complex mer-[(Me3P)3HRu(u-Ff)2BH2], Figure

1.10(a). By contrast, the terminal M-H and bridging M(fx-H)A1 hydrogen atoms mfac-

[(Me3P)3HRu(u-H)2AlH2] undergo rapid exchange at room temperature, as confirmed by

NMR spectroscopy.

The very facile Ru-H and [Ru(u-H)2A1] exchange, Figure 1.10(b), could be a consequence of the/ac-octahedral configuration about ruthenium, which allows exchange without rearrangement at the transition metal centre. Another consequence of this configuration is that the 31P-{'H} NMR spectrum shows a sharp singlet at room temperature indicating equivalent P environments.

16 Figure 1.10: Molecular structures of (a) OT

[(Me3P)3HRu(u-H)2AlH2] as determined by X-ray diffraction. All non-hydride

hydrogen atoms are omitted for clarity.

The proposed mer-octahedral geometry of the borohydride analogue [(Me3P)3HRu(u-

H)2BH2] was confirmed by X-ray diffraction; the two Ru-H bonds to the chelating BH4

ligand are equivalent, lying trans to the terminal Ru-H bond and one PMe3 ligand. The

Ru-P distance is 0.045 A shorter than those of the other two phosphines, which are trans to each other.

The dimeric nature of most aluminohydride complexes allows for an exchange between terminal Al-H and bridging [Al(u-H)A1] hydrides, whereas no analogous exchange in

"X 1 metal borohydride complexes has been reported. The majority of known transition- metal aluminum hydride complexes are stabilized by tertiary phosphines (mono- and bidentate), or by r)5-cyclopentadienyl ligands (including those with alkyl substituents). 17 However, relatively few monomeric aluminohydride complexes, [M(AlH4)n], have been

reported.

Aluminohydride compounds with alkyl groups attached to the aluminum atom show no proclivity to dimerize and remain as monomers. For example, the X-ray crystal structure

34 of [ReH6(MePh2P)2AlMe2] [Figure 1.11(a)] reveals a planar P2-Re-Al framework. Of

the six metal-bound H atoms, two bridge between the rhenium and aluminum centres and the remaining four H atoms are terminal hydrides on Re. The aluminum atom adopts a tetrahedral geometry, with the Me-Al-Me plane close to perpendicular (98.2°) to the

34 H-Re-H plane. In the closely related complex [ReH4(MePh2P)3AlMe2] [Figure

1.11(b)], the aluminum atom is five-coordinate, with three bridging [Re([x-H)A1] hydrides. The coordination geometry of rhenium is pentagonal bipyramidal.

Figure 1.11: Molecular structure of (a) [ReH6(MePh2P)2AlMe2] and (b) [ReH4MePh2P)3

AlMe2] as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted

for clarity.

18 The numerous examples of monomeric transition-metal borohydride complexes may display one of three types of coordination (see Section 1.2); the bridging and terminal B-

H hydrides either undergo rapid intramolecular exchange, or may exchange with terminal

M-H moieties that are also present in the complex. By contrast, X-ray crystallographic studies have shown aluminohydride complexes to contain bridging AIH4 units. This is an example of the general tendency of aluminum to become five- or six-coordinate (an arrangement not usually possible for boron), with M(u-H)2AlH(^-H)2AlH(u-H)2M or

M((j,-H)2AlH(fx-H)2M units in their structures33 as depicted in Scheme 1.5.

H H H .H. .H, .H, ,H> ,H, M: :M M: :A\' :M 'H H ^^H' 'H" 'H'

(a) (b)

Scheme 1.4: Connectivity in (a) M(U-H)2A1H(U-H)2A1H(U-H)2M and (b) M(u-H)2AlH(u- tfhM complexes.

As described above, transition metal complexes of AIH4 frequently contain a bridging aluminohydride moiety. The first such example was [Mn(dmpe)2AlH4]2, reported in 1983 by Girolami et al.36 (Figure 1.12), which displays the bonding arrangement shown in

Scheme 1.4(a). In the solid state the compound exists as a centrosymmetric dimer due to the formation of a Mn(^-H)2AlH(u-H)2AlH(u-H)2Mn bridge. The aluminum atoms adopt distorted trigonal bipyramidal geometries, with one bridging [Al(u-H)A1] and one bridging Mn(u-H)A1 hydride occupying the axial positions around each aluminum. The

19 dihydride bridges are all formed from one axial and one equatorial hydrogen atom on each aluminum atom. The Mn-H distances in the Mn((u-H)A1 unit are equal, and shorter than the two Al-H distances.

Figure 1.12: Molecular structure of [Mn(dmpe)2AlH4]2 as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

A range of similar complexes has since been reported; these include titanocene aluminohydride compounds such as [TiF^r^-CiMes^AlHbk-37 The crystals of this complex contain centrosymmetric dimeric units, in which the [TiH^rf-CsMes^AlHy moieties are linked to each other via two three-centre Al-H bridges. The linkage between the titanium and aluminum atoms is accomplished via the TiHbAl double hydrogen bridge, whereas the dimmer is held together by AIH2AI bridges. The coordination geometry around the aluminum atom is again distorted trigonal bipyramidal. However, unlike the manganese complex [Mn(dmpe)2AlH4]2 (Figure 1.12), the Ti([X2-H)2Al unit in

[Ti^O^-CsMej^Al^^ is asymmetric, with one hydrogen nearer the Al atom than the other; Figure 1.13.

20 Figure 1.13: Molecular structure of [TitfeCrf-CsMes^AlFfek as determined by

X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

A related complex is [NbP^rf-CsHs^AlFbk (Figure 1.14).38 In the solid state this also exists as a centrosymmetric dimer, owing to the formation of a Nb(u-H)2AlH(u-

H)2AlH(u-H)2Nb bridge. The aluminohydride units are associated via two hydride bridges, with each unique aluminum centre being five-coordinate. The coordination geometry at aluminum can thus be described as pseudo-square pyramidal; Figure 1.24.

Figure 1.14: Molecular structure of [NbFfeCrf-CsHs^Alfbk as determined by

X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

The first example of a complex of the type shown in Scheme 1.4(b) was [WH3(PMe3)3

39 (u.-H)2AlH(u.-H)2WH3(PMe3)3], reported in 1986 by Barron et al. The hydrogen atoms attached to the aluminum atom (both terminal and bridging) were located in the X-ray

21 structure and refined, but the terminal hydrogen atoms were difficult to identify and none was included. Here, the coordination geometry at aluminum is best described as trigonal bipyramidal; Figure 1.15.

Figure 1.15: Molecular structure of {[WH3(PMe3)3]2AlH5} as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity. The three terminal hydrides on each tungsten atom were not located.

40 [Ru2(rf-C5Me5)2(u-dppm)(H-AlH5)] (dppm = Ph2PCH2PPh2), shown in Figure 1.16(a), is representative of the type of bridged aluminohydride complex described in Scheme

1.4(b). This complex consists of two ruthenium centres bridged by the dppm ligand and

2- 2- an A1H5 unit that is best described as (U-H)2A1H(U-H2) . Each ruthenium atom is also

2— r|3-bonded to a pentamethylcyclopentadienyl ligand. The AIH5 unit adopts a distorted square pyramidal geometry. The X-ray structure shows that four of the hydrogen atoms in 2_ the AIH5 group bridge to the ruthenium atoms, and one remains as a terminal Al-H entity.

A most interesting example of a complex of the type shown in Scheme 1.4(b) is the binuclear {[Ti^rf-CjHs^AlFLtBFLt},41 Figure 1.16(b), whose crystal structure shows

22 four centres linked by hydride bridges, with a connectivity more accurately described as

5 5 [(r) -C5H5)2Ti((x-H)2Al(^-H2BH2)(|x-H)2Ti(ri -C5H5)2]. As expected, the borohydride moiety shows bidentate coordination. The coordination geometry at the aluminum atom is distorted octahedral, reminiscent of that in A1(BH4)3 rather than the more usual trigonal bypiramid. The Al-B distance of 2.27 A in {[^(rf-CaHs^kAlfLjBFLi} is greater than the corresponding distance in A1(BH^ (2.14 A).

(a) (b)

Figure 1.16: Molecular structure of (a) [Ru2(rf-C5Me5)2(u-dppmXu-AlH5)] and (b)

5 {[(r| -C5H5)2Ti2]2AlH4BH4} as determined by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

1.4 Crown Ethers

Crown ethers were discovered serendipitously by Charles J. Pedersen in 1967. Two decades later he was awarded the Nobel Prize for the discovery of the synthetic routes to,

23 and binding properties of, crown ethers. These cyclic polyethers are called crown ethers on account of their shape, as they are based on repeating -OCH2CH2- units. The common name of these crown ethers includes a number as a prefix to designate the total number of atoms in the ring, and a suffix to designate the number of oxygen atoms. Structures of the three most commonly used crown ethers are given in Scheme 1.5.

(a) (b) (c)

Scheme 1.5: Structural representation of (a) 12-crown-4, (b) 15-crown-5, and

(c) 18-crown-6.

1.4.1 Alkali Metal-Crown Ether Complexes

The chemistry of the crown ethers was first reviewed in 1974 by Izatt,42 and since this time it has developed at an exponential rate. One of the aspects that has made crown ether chemistry so popular and useful is their ability to selectively bind metal ions of a characteristic size within the cavity of the macrocycle. They are a special category of polydentate ligand in which the ligating atoms arrange themselves about a metal ion, and the remainder of the molecule adopts a puckered arrangement that resembles a crown.

The oxygen atoms of the cyclic poly ether point inwards towards the metal ion.

24 Crown ethers form particularly stable complexes with alkali metal cations. This special stability is related to the cavity radius of the ligand, which fits well with the ionic radii of the alkali metal ions, as detailed in Table l.l.43 Cavity radii for the crown ethers were computed originally from different types of molecular models such as Corey-Pauling-

Kolton and Fisher-Hirschfelder-Taylor. X-ray crystallographic studies have made possible the accurate determination of the positions of the interacting atoms and ions in the complexes as well as their dimensions in the free state. The best fit for the X-ray data is found with CPK models.44

Table 1.1: Ionic radii of alkali metal cations and corresponding cavity sizes of crown ethers (A), log K and AH (kJ/mol).

Alkali . __ . Ioniw c Meta A l „ .. Crown Ether „ a™ y LogK AH „, . Radius Radius Catio A n Li+ 1.36 12-crown-4 1.2-1.5 -3.3 Na+ 1.90 15-crown-5 1.7 — 2.2 4.99 -20.88 K+ 2.66 18-crown-5 2.6-3.2 6.06 -32.2

Table 1.1 also reports the stability constants and enthalpies of formation (methanol; 25

°C) of the crown ether complexes with alkali metal cations. The reliability of these values is confirmed by the excellent agreement for several systems with log K values derived by

Frensdorff in the early 1970s. Values of log K reported for the reactions reflect the stability of the cationic complexes formed.

25 Although they were discovered some four decades ago, alkali metal-crown ether complexes are still a topical area of research, as shown by regular review articles published over this period.44'43

(a) (b) (c)

Scheme 1.6: Structural representation of (a) [Li (12-crown-4)]+, (b) [Na(15-crown-5)]+, and(c)[K(18-crown-6)]+.

Some of the most intriguing complexes were reported by Dye et al. in 1989; these contain an alkali metal anion (alkalide) as counter-ion, such as Na , K , Rb ,and Cs . In an elegantly conceived series of experiments, Dye analyzed the thermodynamics of charge separation in the alkali metals and realized that the enhanced stability of the

[M(crown)]+ cation could actually favour electron transfer to permit formation of the alkalide anion. These anions have closed ns shells and therefore are expected to have spherical shapes and nondirectional interactions with their surroundings. They should be very polarizable because of their large sizes.

The observation in 1970 that crown ethers are metastable47 to reduction by M and

26 e~Soiv, opened up the study of metal solutions in amines and ethers solvents. The behaviour of electropositive metals in amines and ethers solvents in the absence of complexing agents can be adequately described by the following series of equilibria:

+ 2M(s) <-> M soiv + M soiv Equation 1.1

+ M soiv <-• M soiv + 2e~soiv Equation 1.2

+ M soiv + e sow <-• MSoiv Equation 1.3

The effect of adding crown ethers to these systems is striking. For example, the solubility of Na in ethylamine in the presence of 18-crown-6 is > 0.1 M at 0 °C. Since its solubility in pure ethylamine is less than 10"6 M, solubility enhancement of at least five orders of magnitude results from the presence of the crown ether.

When metal is reacted with 18-crown-6 in THF solution, an alkalide complex is obtained, in which the Rb+ cation is coordinated by the six ether oxygen donors on one face, and is essentially in contact with the Rb anion on the other face; Figure 1.17(a).46

Almost a decade after the initial report of this structure, Dye et al. synthesized [Rb(18- crown-6)(CHaNH2)]+Na ].48 Here, the coordination environment of Rb+ consists of six oxygen atoms of the crown ether, the Na anion, and one molecule of the solvent,

CH3NH2. This structure [Figure 1.17(b) is different from that shown in Figure 1.17(a), in which Rb+ and Rb ions are in contact. In this case, the smaller size of the alkalide anion permits coordination of Rb+ to both the solvent, CH3NH2 and a Na ion, each of which are on the same side of the [Rb(18-crown-6)]+ cation.

27 (a) (b)

Figure 1.17: Molecular structure of (a) Rb(18-crown-6)]+Rb and [Rb(18-crown-

6)(CH3NH2)]^a" as determined by X -ray diffraction.

1.4.2 Alkali Metal Crown ether Complexes with Transition Metal Hydrides

Despite the fact that transition metal hydrides have been known for several decades they remain topical systems in such varied fields as hydrogen storage, supramolecular chemistry and organometallic chemistry. One of their key advantages is that they contain at least one hydrogen atom with tunable electronic characteristics.

Transition metal hydrides are involved in unique wide range of bonding types. In addition to the classical M-H moieties, there exist also dihydrogen complexes containing the r| -

H2 ligand, and an intriguing class of polyhydrides which display quantum mechanical site exchange. The interaction between the [M(crown)]+ cation and the M'-H hydride moiety of an anionic polyhydride complex usually results in the formation of a 1-dimensional chain in the solid state, with crystallographically well-defined proton-hydride bonds.

28 This new strategy is providing materials for the study of the influence of the ancillary ligands and the metal on these unique H—H interactions.

Green et al.49 reported in the 1991 the structures of [M(crown)]+ cations coordinated to polyhydride anions such as [W(PMe3)3H5] . Treatment of [W(PMe3)3H6] with 18-crown-

6 and an excess of KH gave crystals of [K(18-crown-6)][W(PMe3)3H5] [Figure 1.18(a)], while 15-crown-5 and an excess of NaH gave crystals of [Na(15-crown-5][W(PMe3)3H5]

[Figure 1.18(b)].

Figure 1.18: Molecular structure of (a) [K(18-crown6)[W(PMe3)3H5] and (b) [Na(15-

crown-5)] [W(PMe3)3H5] ] as determined by X-ray diffraction. All non-hydride hydro­

gen atoms are omitted for clarity.

Morris et al.3° subsequently reported several structures of polyhydride anions of iridium with alkali metal crown-ether counter-ions, including [K(18-crown-6)][IrH4(PPri3)2], as

shown in Figure 1.19(a).

29 More recently, McGrady et al. studied the analogous chemistry of complexes of

osmonium. The hydrido rra-phosphine complex rOsH4(PPh3)3] reacts with KH in THF in

the presence of 18-crown-6 to form rK(THF)(18-crown-6)irOsH3(PPh3)3l [Figure

1.19(b). The structure of [OsFf4(PPh3)3l determined by X-ray crystallography shows

three hydrides point toward the potassium cation, in an arrangement similar to that

reported by Morris et al. for the hydrido 6/5-phosphine complex [K(18-crown-

6)][IrH4(PPh3)2].

(a) (b)

Figure 1.19: Molecular structure of (a) [K(18-crown-6)][IrH4(PPh3)2] and (b)

[K(THF)(18-crown-6)][OsH3(PPh3)3] as determined by X-ray diffraction. All non- hydride hydrogen atoms are omitted for clarity.

30 CHAPTER 2

RESULTS and DISCUSSION

2.1 General Aims of the Project

The aims of this project were to study and to understand the structure and reactivity of

several different types of hydride complex of boron, aluminum and gallium, and to make

a significant contribution to the large but incomplete body of knowledge of Group 13 hydride coordination chemistry.

Alkali metal complexes of the Group 13 hydrides have been considered as potential

hydrogen storage materials for more than a decade; in particular, LiAlH4, NaAlH4, LiBH4

and NaBH4. This interest received a fillip in 1998, when it was shown that NaAlH4 doped with small amounts of Ti could reversibly store around 5% hydrogen by weight over many cycles.52'53 However, in spite of a large body of research carried out to date on the hydrides of boron and aluminum, the potential of gallium hydrides for hydrogen storage has never been investigated in detail. As part of a wider effort in the McGrady group to explore the potential of these gallium systems, one aim of the research presented in this thesis was to prepare, stabilize and study novel complexes of [GaH4]".

We also intended to explore the coordination chemistry and supramolecular bonding potential of the anionic Group 13 hydride complexes EH4 (E = B, Al or Ga), with alkali metal-crown ether counterions, as these were expected to form crystalline products

31 amenable to structural study by X-ray diffraction. The cation-anion interactions in the

resulting products should allow us to learn about the interplay of a range of competing

interactions, including electrostatic binding and weaker interactions such as conventional

and unconventional hydrogen bonds.

2.2 Specific Aims of this Research

This research project focued on the preparation, characterization, and reactivity of alkali

metal-crown ether cations with group 13 hydride anions. The objectives were threefold:

(i) to synthesize and fully characterize a wide range of alkali metal-crown

ether cations with Group 13 hydride complexes;

(ii) to observe how the hapticity of the complexes depends of the size of the

alkali metal and Group 13 element, and to explore the nature of the

M—E interaction; and

(iii) to investigate the competing effects of proton-hydride H-bonding

interactions when a crown ether is replaced with its azacrown analogue.

2.3 Synthetic Routes to [M(crown)] [EH4] Complexes

Crown ethers coordinated alkali metal cations have a proven utility in stabilizing a range of metal hydride anions, producing high quality crystalline products.42'45'31 In our study, we focused on the complexes formed between these cations and Group 13 complex hydride anions.

32 There are two main routes for the preparation of complexes of the type [M(crown)][EH4],

(where M = Li, Na or K and E = B, Al or Ga). The first method (Equations 2.1 and 2.2), involves the reaction in solution of a halide with the crown ether and alkali metal hydride.

This is an impractical route because it involves two steps and has a low overall yield.

MCI + crown • [M(crown)Cl] Equation 2.1

[M(crown)Cl] + M'EH4 • [M(crown)][EH4] + M'Cl Equation 2.2

The second method (Equation 2.3) involves direct addition of the crown ether to the alkali metal complex hydride, and is a more practical and efficient one. The desired complexes can be obtained in high yields from commercial reagents.

MEH4 + crown • [M(crown)][EH4] Equation 2.3

Different crown ethers are used with each of the alkali metals to accommodate the various metal cations. In this study, 12-crown-4, 15-crown-5, and 18-crown-6 were used for Li+, Na+, and K+, respectively. The ionic radii of the alkali metal cations and the approximate cavity size of the complementary crown ethers in an idealized binding conformation are listed in Table 1.1, Section 1.4.1.

During the course of this research we managed to prepare and structurally characterize seven of the possible nine combinations of complexes formed between [M(crown)]+ and

[EH4J~as detailed in Table 2.1. Of these, four structures were known previously: [Li2(12-

54 55 56 57 crown-4)3][AlH4]2, [Na(15-crown-5)][BH4], ' [Na(15-crown-5)][AlH4], [K(18-

58 crown-6)][BH4].

33 Table 2.1: Complexes of alkali metal-crown ether complexes and Group 13 hydride anions obtained in crystalline form.

Metal Crown Group 13 ianio n Cation Ether BH4" A1H4 GaH4" Li+ 12-crown-4 0 0 Na+ 15-crown-5 0 0

K+ 18-crown-6 0 0 0

Complexes gallium hydride, MGaH4 are synthesized by metathesis involving a hydride source (MH, M = Li, Na and K) and a halide (GaCl3); Equation 2. 4.

GaCl3 + 4MH • MGaH4 + 3MLi Equation 2.4

KEH4 (where E = Al) complex is also synthesized by metathesis involving a hydride source (LiEFLj) and a halide (KC1). Metathesis is one of the four general strategies for forming main-group hydrides60 (Equation 2. 5).

KC1 + MEH4 • KEH4 + MCI Equation 2.5

Owing to their proclivity to decompose into MH, H2 gas and Ga metal at ambient temperatures, all MGal-L; salts and their complexes were stored at -40 °C until needed.

The remaining MEH4 (LiBH4, LiAlELi, NaBELt, KBH4) alkali metal complex hydride are available commercially.

34 2.4 Synthesis and Characterization of [M(crown)] [EH4] Complexes

All complexes presented in this Section have been obtained in crystalline form. They are

air and moisture sensitive, very soluble in THF, only sparingly soluble in toluene and

decompose slowly in chlorinated solvents. The relevant crystallographic data and NMR

details are presented in Sections 3.3 and 3.4, respectively.

2.4.1 [Li2(12-crown-4)3][AlH4]2

This was synthesized by reacting LiAlH4 with a solution of 12-crown-4 in THF. The

single crystal X-ray diffraction analysis confirmed that the product was the known

[Li2(12-crown-4)3][AlH4]2 complex3 (Figure 2.1). X-ray diffraction was carried out on a

single crystal obtained from THF at 5 °C.

Figure 2.1: Molecular structure of [Li2(12-crown-4)3][AlH4]2 complex as determined

by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

35 2.4.2 [Li2(12-crown-4)3][GaH4]2

This was synthesized by reacting LiGaH4 with a solution of 12-crown-4 in THF. A

combination of H and Ga NMR spectroscopy and elemental analysis confirmed that the product was [Li2(12-crown-4)3][GaH4J2 (Figure 2.2). X-ray diffraction was carried

out on a single crystal obtained from a THF solution at -40 °C.

Figure 2.2: Molecular structure of [Li2(12-crown-4)3][GaH4]2 complex as determined

by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

36 2.4.3 [Na(15-crown-5)][BH4]

This was synthesized by reacting NaBH4 with a solution of 15-crown-5 in THF. A combination of H and B NMR spectroscopy and elemental analysis confirmed that the

55 56 product was the known [Na( 15 -crown- 5)][BH4] complex ' (Figure 2.3). X-ray diffraction was carried out on a single crystal obtained from THF at 5 °C.

Figure 2.3: Molecular structure of [Na(15-crown-5)][BH4] complex as determined

by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

37 2.4.4 [Na(15-crown-5)][AlH4]

This was synthesized by reacting NaAlH4 with a solution of 15-crown-5 in THF. A

1 27 combination of H and Al NMR spectroscopy and elemental analysis confirmed that the

57 product was the known [Na(15-crown- 5)][A1H4] complex (Figure 2.4). X-ray diffraction was carried out on a single crystal obtained from THF at 5 °C.

Figure 2.4: Molecular structure of [Na(15-crown-5)][AlH4] complex as determined by

X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

38 2.4.5 [K(18-crown-6)][BH4]

This was synthesized by reacting KBH4 with a solution of 18-crown-6 in THF. A

combination of H and B NMR spectroscopy and elemental analysis confirmed that the product was the known [K(18-crown-6)][BH4] complex38 (Figure 2.5). X-ray diffraction

was carried out on a single crystal obtained from THF at 5 °C.

Figure 2.5 Molecular structure of [K(18-crown-6)][BFf4] complex as determined by

X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

39 2.4.6 [K(THF)(18-crown-6)][AlH4]

This was synthesized by reacting KAIH4 with a solution of 18-crown-6 in THF. A

1 27 combination of H and Al NMR spectroscopy and elemental analysis confirmed that the product was [K(THF)(18-crown-6)][AlH4] (Figure 2.6). X-ray diffraction was carried out on a single crystal obtained from THF at 5 °C.

Figure 2.6: Molecular structure of [K(THF)(18-crown-6)][AlH4] complex as deter­

mined by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

40 2.4.7 [K(THF)(18-crown-6)][GaH4]

This was synthesized by reacting KGaH4 with a solution of 18-crown-6 in THF. A

1 71 combination of H and Ga NMR spectroscopy and elemental analysis confirmed that the product was [K(THF)(18-crown-6)][GaH4] (Figure 2.7). X-ray diffraction was carried out on a single crystal obtained from THF at 5 °C. AGa

Figure 2.7: Molecular structure of [K(THF)(18-crown-6)][GaH4] complex as determined

by X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

41 2.5 Analysis of the Structures Obtained by X-Ray Diffraction

The structures obtained for the various complexes we prepare and crystallize can be

discussed and compared in two ways; either by keeping the [EH4]~ ion constant and

observing the effect of varying the [M(crown)]+ counter - ion, or vice versa.

Accordingly, we take both approaches in the following analysis.

2.5.1 [M(crown)][BH4] Structures

During the course of this research, we managed to prepare and characterize two

[M(crown)][BH4] complexes, viz. [Na(15-crown-5)][BH4] and [K(18-crown-6)][BH4]

(Figure 2.8). The structure of [Na(15-crown-5)][BH4] was reported for the first time by a

Russian group in 1985,3:> however the same structure was also reported in

56 Crystallographic Structural Database (CSD) in 2006, and that of [K(18-crown-6)][BH4]

38 by a French group in 2006. The structure of [Na(15-crown-5)][BH4] reported in 1985 is

inferior to our structure because the hydride H atoms were not found in the Fourier

difference map and the R value is lower than ours. The cation-anion interaction is very

similar in each system, with a tridentate BH4 anion attached to the metal centre (Table

2.2). This interaction draws the alkali metal cation out of the plane of the crown ether by

0.659(1) to 0.885(1) A, with B-M distances in the range 2.614(2) to 2.972(5) A, (Table

2.3) but has no significant effect on the length of the B-Hb"M and B-Ht"M bonds*, at

least within the accuracy of the X-ray experiment.

*Here and in all the subsequent places throughout this thesis, E—H—M denotes the distance between the hydrogen atom H and the metal centre M.

42 (a) (b)

Figure 2.8: Local structure of (a) [Na(15-crown-5)][BH4] and (b) [K(18-crown-6)][BH4] complexes, showing the cation-anion interaction. All carbon, oxygen and non-hydride hydrogen atoms are omitted for clarity.

Table 2.2: B-H—M distances (A) with estimated standard deviations for

[M(crown)][BH4] complexes (M = Na and K).

Complex B-Hb-M B-Hb»M B-Hb"M B-Ht"M

[Na(15-crown-5)][BH4] 2.37(2) 2.43(2) 2.59(2) 3.71(2)

56 [Na(15-crown-5)][BH4] 2.38 2.38 2.37 3.70

[K(18-crown-6)][BH4] 2.75(5) 2.90(4) 2.69(4) 4.11(3) 58 [K(18-crown-6)][BH4] 2.73 2.80 2.81 4.10

As expected, the M-B—Ht angle involving the terminal (uncoordinated) H atom in these

0 [M(crown)][BH4] complexes is nearly linear, with an average value of ca.175 . The structure of these complexes shows that the alkali-metal cation occupies a direct co­ ordination at boron (cf. the sum of the van der Waals radii).

43 Table 2.3: M-B—Ht angles (°), the distance of the metal cation from the crown plane (A) and the B---M distances (A) with estimated standard deviations for [M (crown)] [BH4] complexes.

1YI—crown Complex MB H , B-M (B-M) _ t plane vdW

[Na(15-crown-5)][BH4] 174.4(8) 0.885(1) 2.614(2) 3.44 56 [Na(15-crown-5)][BH4] 179 0.875 2.608 3.44

[K(18-crown-6)][BH4] 175.7(14) 0.659(1) 2.972(5) 3.80 58 [K(18-crown-6)][BH4] 176 0.642 2.947 3.80

The length of the B-H bonds in the borohydride anion ranges from 1.09(2) to 1.13(2) A in [Na(crown)][BH4] and 1.14(3) to 1.22(6) A in [K(crown)][BH4]; Table 2.4, a statistically insignificant variation. It is interesting to consider whether the bonding between the boron and the metal centre is mainly ionic or substantially covalent, i.e. whether two-electron, three-centre M-H-B bonds. See Section 2.8

Table 2.4: B-H distances (A) with estimated standard deviations for [M(crown)] [BH4] complexes.

Complex B-H(l) B-H(2) B-H(3) B-H(4)

[Na(15-crown-5)][BH4] 1.10(18) 1.09(17) 1.10(16) 1.13(2) 56 [Na(15-crown-5)][BH4] 1.08 1.04 1.15 1.08

[K(18-crown-6)][BH4] 1.14(3) 1.14(3) 1.14(4) 1.22(6) 58 [K(18-crown-6)][BH4] 1.14 1.15 1.15 1.15

44 2.5.2 [M(crown)][AlH4] Structures

Three [M(crown)] [AIH4] complexes were prepared and structurally characterized, viz.

[Li2(12-crown-4)3][AlH4]2, [Na(15-crown-5)][AlH4] and [K(THF)(18-crown 6][A1H4].

The structure of [Li2(12-crown-4)3][AlH4]2, was first time reported in CSD in 2004.3

The overall structure of [Li2(12-crown-4)3][AlH4], (Figure 2.1) consists of a sandwich type cation, in which the unique vacant 12-crown-4 molecule bridges the other two

occupied 12-crown-4 units with a Li-0 bond in each direction [1.94(2) A]. The smaller

crown ethers, and particularly 12-crown-4, have been shown to form closely similar

sandwich arrangements with both alkali and alkaline earth metal cations, demonstrating

how the small internal diameter of the 12-crown-4 ligand is unsuitable for inclusion of

metal ions except Li+ into the macrocyclic cavity.39 The structure of [Li2(12-crown-

4)3][GaH4] reveals that the cation does not occupy a direct co-ordination site at

alluminum, as evidenced by the average Al—Li distances of 5.93(2) A, which is higher

than the sum of the van der Waals radii for Li and Al (4.11 A).

The structure of [Na(15-crown-5)][AlH4] and [K(THF)(18-crown-6)][AlH4] (Figure 2.9)

display an n2 [AIH4] interaction with the cationic [M(crown)]+ centre (Table 2.5). The

37 structure of [Na(15-crown-5)][AlH4] was reported for the first time in CSD in 2005.

45 (a) (b)

Figure 2.9: Local structure of (a) [Na(15-crown-5)][AlH4] and (b) [K(THF)(18-crown-

6)] [AIH4] complexes, showing the cation-anion interaction All carbon, oxygen and non- hydride hydrogen atoms are omitted for clarity.

Table 2.5: Al-H—M distances (A) with estimated standard deviations for

[M(crown)][AlH4] complexes (M = Na and K)

Complex Al-Hb M Al-Hb M Al-Ht M Al-Ht M

[Na(15-crown-5)][AlH4] 2.43(4) 2.45(5) 4.03(5) 4.30(5)

57 [Na(15-crown-5)][AlH4] 2.46 2.49 4.07 4.30

[K(THF)(18-crown-6)][AlH4] 3.67(4) 2.84(5) 4.74(3) 5.09(3)

This interaction draws the alkali metal cation out of the plane of the crown ether by

0.754(1) and 0.331(1) A, with Al-M distances of 3.119(2) and 3.885(2) A, respectively

# but appears to have no significant effect on the length of the Al-Hb "M and Al-Ht—M

2 distances. Owing to the r\ nature of the anion binding mode, the M-Al"-Ht angles are far from linear, with an average value of around 124° (Table 2.6). The alkali metal cation

46 occupies a direct co-ordination site at Al, as judged by the sum of the van der Waals radii of the two metals.

Table 2.6: M*"A1-Ht angles (°), the distance of the metal cation from the crown plane

(A) and Al-M distances with estimated standard deviations for [M(crown)] [AIH4] complexes.

^ 1 i»« », M-crown ., ,_ , ,^ Complex M-Al-H TT, , A1HVI (Al-M)A1 plane vd_W_ 129.6(18), [Na(15-crown-5)][AlH4] 0.754(1) 3.119(2) 3.96 117.2(18) 132.2 [Na(15-crown-5)][AlH4]37 0.750 3.11 3.96 118.1 [K(THF)(18-crown- 116.7(3), 0.331(1) 3.885(2) 4.31 6)][A1H4] 136.1(11)

The length of the Al-H bonds in the [AIH4] anion ranges from 1.53(5) to 1.58(4) A in

[Na(15-crown-5)][AlH4] and 1.48(4) to 1.57(3) A in [K(THF)(18-crown-6)][AlH4]

(Table 2.7). The nature of the bonding between the anion and the alkali metal cation is answered in Section 2.8.

Table 2.7: Al-H distances (A) with estimated standard deviations for [M(crown)][AlH4] complexes.

Complex Al-H(l) Al-H(2) Al-H(3) Al-H(4)

[Na(15-crown-5)][AlH4] 1.55(4) 1.56(4) 1.53(5) 1.58(5)

57 [Na(15-crown-5)][AlH4] 1.60 1.53 1.54 1.54

[K(THF)(18-crown-6)][AlH4] 1.57(3) 1.55(4) 1.48(4) 1.52(3)

47 2.5.3 [M(crown)][GaH4] Structures

Two [M(crown)][GaH4] complexes were synthesized and structurally characterized, viz. [Li2(12-crown-4)3][GaH4]2 and [K(THF)(18-crown-6)][GaH4]. The overall structu­ re of [Li2(12-crown-4)3][GaH4], (Figure 2.2) consists of a sandwich type cation, in which the unique vacant 12-crown-4 molecule bridges the other two occupied 12-crown-4 units with a Li-0 bond in each direction [1.95(2) A]. The smaller crown ethers, and particularly 12-crown-4, have been shown to form closely similar sandwich arrangements with both alkali and alkaline earth metal cations, demonstrating how the small internal diameter of the 12-crown-4 ligand is unsuitable for inclusion of metal ions except Li+ into

3 the macrocyclic cavity. The structure of [Li2(12-crown-4)3][GaH4] reveals that the cation does not occupy a direct co-ordination site at gallium, as evidenced by the average

Ga*"Li distances of 7.18(2) A, which is almost twice the sum of the van der Waals radii for Li and Ga (4.11 A).

The structure of [K(THF)(18-crown-6)][GaH4J (Figure 2.7) is more conventional, with the expected cation—anion arrangement. In this case, the potassium ion occupies a direct co-ordination site at gallium, (Ga—K distance 3.757(2) A; cf. sum of the van der Waals radii 4.19 A). None of the hydride hydrogen atoms were located in Fourier difference maps. The cation-anion interaction draws the alkali metal out of the plane of the crown ether by 0.818(4) (Li+) or 0.361(1) (K+) A (Table 2.8).

48 Table 2.8: Ga—M distances and the distance of the metal cation from the crown plane

(A) with estimated standard deviations for [M(crown)] [GaH4] complexes.

Complex M-crown plane Ga—M (Ga"'M)vdW

[Li2(12-crown-4)3][GaH4]2 0.818(4) 7.18(2) 4.11

[K(THF)(18-crown-6)][GaH4] 0.361(1) 3.757(2) 4.19

The hydride H atoms were not located for [K(THF)(18-crown-6)][GaH4], but were found for [Li2(12-crown-4)3][GaH4]2: the Ga-H bond lengths in this complex range from

1.44(4) to 1.53(4) A (Table 2.9).

Table 2.9: Ga-H distances (A) with estimated standard deviations for [Li2(12-crown-

4)3][GaH4] complex.

Complex Ga-H(l) Ga-H(2) Ga-H(3) Ga-H(4)

[Li2(12-crown-4)3][GaH4] 1.53(4) 1.51(5) 1.45(6) 1.44(4)

2.5.4 [Na(15-crown-5)][EH4] Structures

Two [Na(15-crown-5)][EH4] complexes were synthesized and structurally characterized during the course of this research, viz. [Na(15-crown-5)][BH4] and [Na(15-crown-

5)][A1H4] (Figure 2.10). The structure of [Na(15-crown-5)][BH4] conforms well to an

+ ionic complex containing cationic [Na(15-crown-5)] and anionic [BH4] centres. The anion displays close-to-regular tetrahedral geometry.

49 Figure 2.10: Local structure of (a) [Na(15-crown-5)][BH4] and (b) [Na(15-crown-

5)][A1H4] complexes, showing the cation-anion interaction. All carbon and non-hydride

hydrogen atoms are omitted for clarity.

The Na+ ion is located at the centre of the 15-crown-5 unit and is bound to five O atoms of the crown ether and to a tridentate borohydride group. On account of this interaction with the anion, Na+ is drawn out of the plane of the crown ether towards the boron [Na- crown centroid = 0.885(1) A]. All Na-0 distances are very similar, with an average value of 2.44 (2) A (Table 2.10). The structure shows that Na+ occupies a direct co-ordination site at boron, as indicated by the short B—Na distance of 2.641(2) A, (cf. the sum of the van der Waals radii, 3.44 A) (Table 2.11). The anion coordinates in a fairly symmetrical n3 manner, with B-H-Na distances in the range 2.37(2) to 2.60(2) A, whereas the Na-

B—Ht angle involving the terminal uncoordinated hydrogen atom is nearly linear, with a value of 174.4(8)°.

50 In a likewise manner, [Na(15-crown-5)][AlH4] contains cationic [Na(15-crown-5)]+ and anionic [AIH4] centres. The anion here also displays close-to-regular tetrahedral geometry. The Na+ ion is located at the centre of the 15-crown-5 unit and is bound to five

O atoms of the crown ether and to a bidentate aluminum hydride group. Once again, the anion attracts the Na+ ion out of the plane and towards the aluminum [Na-crown centroid

= 0.754(1) A]. All Na-0 distances are very similar, with an average value of 2.42(2) A

(Table 2.10). The Na+ ion occupies a direct co-ordination site at aluminum, as indicated by the short Al—Na distance of 3.119(2) A, (cf. the sum of the van der Waals radii, 3.96

A). The [AIH4] ligand coordinates in a fairly symmetrical n2 manner, with Al-H—Na distances of 2.43(13) to 2.45(5) A (Table 2.11). On account of this n2 coordination, the

Na--Al-H angles involving the terminal uncoordinated H atoms are far from linear

[129.6(18) and 117.2(18)°].

Table 2.10: Na-0 distances (A) with estimated standard deviations for [Na(15-crown-

5)][EH4].

Complex Na-O(l) Na-0(4) Na-0(7) Na-O(10) Na-0(13)

[Na(15-crown-5)][BH4] 2.431(2) 2.456(2) 2.396(2) 2.439(2) 2.493(2)

[Na(15-crown-5)][AlH4] 2.407(2) 2.417(2) 2.413(2) 2.425(2) 2.412(2)

In summary, the structures adopted by [Na(15-crown-5)][BH4] and [Na(15-crown-

5)][A1H4] differ mainly in the hapticity of the [EtLt]- anion. The structure of [Na(15-

51 crown-5)][BH4] determined by X-ray crystallography, clearly shows three hydrides to

point toward the sodium cation vs. two hydrides towards the similar cation in [Na(15-

crown-5)][AlH4], whilst the internal tetrahedral geometry of the [EH4] moiety is retained

in each case. The ionic interaction between Na+ and BH4 in NaBFU is stronger than that

between Na+ and AIH4 in NaAlH4.60 This has an impact on the hapticity of the

complexes. The increase in ionic radius between boron and aluminum is reflected in an

increased E-Na distance, 2.6147 (17) vs. 3.1190 (16) A (Table 2.11).

Table 2.11: E-H—M and E—Na distances (A) with estimated standard deviations for

[Na(15-crown-5)][EH4] complexes.

Complex E-H(l)-NaE-H(2)-NaE-H(3)-NaE-H(4)-Na E-Na [Na(15-crown-5)] 2.37(10) 2.43(16) 2.60(19) 3.71(16) 2.614(2) [BH4] [Na(15-crown-5)] 2.43(13) 2.45(5) 4.05(5) 4.30(5) 3.119(2) [AIH4]

2.5.5 [K(18-crown-6)][EH4] Complexes

Three [K(18-crown-6)][EH4] complexes were synthesized and structurally characterized

during the course of this research, viz. [K(18-crown-6)][BH4], [K(THF)(18-crown-

6)][A1H4] and [K(THF)(18-crown-6)][GaH4] (Figure 2.11).

52 (a) (b) (c)

Figure 2.11: Local structure of (a) [K(18-crown-6)][BH4], (b) [K(THF)(18-crown-

6)][A1H4] and (c) [K(THF)(18-crown-6)][GaH4] complexes, showing the cation-anion interaction. All carbon and non-hydride hydrogen atoms are omitted for clarity.

The structure of [K(18-crown-6)][BH4] accords with an ionic complex, in which the anionic [BH4] binds in a tridentate fashion to one face of the [K(18-crown-6)]+ cation.

The anion displays close-to-regular tetrahedral geometry. The K ion is located centrally in the 18-crown-6 unit, all K-0 distances are very similar, with an average value of

2.86(2) A, and is bound to the six O atoms of the crown ether and to the tridentate borohydride group. Once again, the cation is attracted out of the plane of the crown and towards the boron, [K-crown centroid = 0.659(1) A]. The structure shows that the K+ ion occupies a direct co-ordination site at boron, as indicated by the B—K distance, 2.972 (5)

A (cf. the sum of the van der Waals radii, 3.80 A). The B-K distance of 2.972 (5) A is shorter than in KBFL;,39 with a value of 3.36 A. The [BH4]" ligand coordinates in a fairly symmetrical n3 manner, with B-H—K distances in the range 2.69(2) to 2.90(2) A.

53 Accordingly, the K—B-H angle involving the terminal uncoordinated H atom is nearly linear, with a value of 175.7(14) °.

The complex [K(THF)(18-crown-6)][AlH4] contains the same [K(18-crown-6)]+ cation, but coordinated by two different ligands. The K+ ion is located at the centre of the 18- crown-6 unit and is bound to the six O atoms of the crown ether. All K-0 distances are very similar, with an average value of 2.82(2) A (Table 2.12). Each of the open faces of the [K(18-crown-6)]+ cation is coordinated by an additional ligand, viz. a bidentate

[AIH4] moiety and a THF solvent molecule. The antagonistic interplay between these two competing ligands means that the K ion is drawn only slightly out of the plane of the crown ether towards the anion [K-crown centroid = 0.331(1) A]. The THF molecule is located on the concave side of the crown ether, with a K-0 bond length of 2.82(2) A,

in good agreement with the mean value of 2.70(15) A for K-0 (THF) bonds in the CSD

(221 hits).58

The K ion occupies a direct co-ordination site at aluminum, as indicated by the A1---K distance, 3.885(2) A (cf. the sum of the van der Waals radii, 4.31 A). The anion coordinates in a fairly symmetrical r\2 manner, with A1-H'"K distances in the range

2.84(5) to 3.67(4) A (Table 2.13); the K"A1-H angles involving the terminal uncoordinated H atoms are correspondingly far from linear at [116.7(13) and

136.1(11)°].

Although the hydride H atoms were not located in the structure of [K(THF)(18-crown-

6)][GaH4], it is isomorphous with the Al analogue [K(THF)(18-crown-6)][AlH4], giving

54 us good reason to expect that the cation-anion interaction will be similar. The overall structure of this complex consists of cationic [K(THF)(18-crown-6)] and anionic

[GaH4] centres.

The K ion is located at the centre of the 18-crown-6 unit and is bound to the six O atoms. All K-0 distances are very similar, with an average value of 2.82 (2) A (Table

2.12). The opposing faces of the [K(18-crown-6)] moiety are further coordinated with a

THF solvent molecule and the [GaFy anion. As is the case with [K(THF)(18-crown-

6)][A1H4], the metal ion is drawn only slightly out of the plane of the crown ether towards the anion [K-crown centroid = 0.361(1) A]. The THF molecule is located on the concave side of the crown ether, with a K-0 bond length of 2.824(3) A in good agreement with the mean value for K-0 (THF) bonds in the CSD (q.v.). The K ion occupies a direct co-ordination site at gallium, as indicated by the Ga-K distance of

3.757(2) A (cf. the sum of the van der Waals radii, 4.31 A.

Table 2.12: K-0 distances (A) with estimated standard deviations for [K(18-crown-

6)][EH4] complexes.

Complex K-O(l) K-0(4) K-0(7) K-O(IO) K-0(13) K-0(16)

[K(18-crown-6)][BH4] 2.822(2) 2.890(2) 2.861(2) 2.905(2) 2.759(2) 2.926(2) [K(THF)(18-crown-6)] 2.792(2) 2.833(2) 2.809(2) 2.900(2) 2.822(2) 2.816(2) [A1H4] [K(THF)(18-crown-6)] 2.818(3) 2.890(3) 2.800(3) 2.828(3) 2.783(3) 2.825(3) [GaH4]

55 In summary, the structure of [K(THF)(18-crown-6)][GaH4J is isomorphous with that of

[K(THF)(18-crown-6)][AlH4], implying that the EH4 bonding to the K centre is similar in the two complexes. The Al and Ga atoms are located at crystallographic centres of inversion in their respective anions. The K ion is positioned at the centre of the 18- crown-6 unit and is bound to the six O atoms of the crown ether and to a bidentate aluminum and (presumably) gallium hydride ligand, with further coordination to a THF molecule occurring on the opposing face of the cation.

As appears to be the general case, [K(THF)(18-crown-6)][AlH4] differs from [K(18- crown-6)][BH4] in the hapticity displayed by the anion. [K(18-crown-6)][BH4] shows three B-H moieties to point towards K , compared to two Al-H ones in [K(THF)(18- crown-6)][AlFl4]. The increase in ionic radius between boron and aluminum leads to a corresponding change in E---K distance, from 2.972(5) to 3.8856(10) A, on moving from

B to Al. In each case, the K+ ion is attracted out of the best plane of the six oxygen atoms and towards the Group 13 hydride anion, but the extent of this distortion is moderated where there is a THF ligand counterbalancing the effect on the opposing face of the cation. The average E-Hb"-K distance in [K(18-crown-6)][BH4] is 2.78 A compared to a value of 2.854 A in KH. In each case, the E—K distance implies a covalent bonding interaction (Table 2.13). This possibility is explained in Section 2.8.

56 Table 2.13: E-H—K and E—K distances (A) with estimated standard deviations for

[K(18-crown-6)][EH4] complexes.

Complex E-H(l)-K E-H(2)-K E-H(3)-K E-H(4)-K E-K

[K(18-crown-6)][BH4] 2.69(4) 2.75(5) 2.90(4) 4.11(3) 2.972(5)

[K(THF)(18-crown-6)][AlH4] 2.84(5) 3.67(4) 4.74(3) 5.09(3) 3.885(2)

[K(THF)(18-crown-6)][GaH4] 3.757(2)

2.6 [M(azacrown)] [BH4] Complexes

Three [M(azacrown)][BH4] complexes were synthesized and structurally characterized

during the course of this research, viz. [Na(15-azacrown-5)][BH4], [K(18-azacrown-

6)][BH4] and its solvate congener [K(THF)(18-azacrown-6)][BH4].

[Na(15-azacrown-5)][BH4] (Figure 2.12) contains a cationic [Na(15-azacrown-5)] bound

3 to the anionic [BH4] centre in an n manner. The anion displays close-to-regular

tetrahedral geometry. The Na ion is located at the centre of the 15-crown-5 unit and is

bound to the four O and one N donor atom of the crown ether and to the tridentate

borohydride group. There is a particularly strong interaction drawing Na out of the plane

and towards the boron [Na-crown centroid = 1.208(1) A]. All Na-0 distances are very

similar, with an average value of 2.57(2) A.

57 Figure 2.12: Molecular structure of [Na(15-azacrown-5)][BH4] as determined by

X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

The Na ion occupies a direct co-ordination site at boron, as indicated by the B—Na distance of 2.734(3) A (cf. the sum of the van der Waals radii, 3.44 A). The X-ray structure of the complex reveals that the borohydride ligand coordinates in a fairly symmetrical n manner, with B-H"'Na distances in the range 2.35(2) to 2.59(2) A (Table

2.14); the Na'"B-H angle involving the terminal uncoordinated H atom is nearly linear, with a value of 169(3)°.

Table 2.14: B-H "Na and B"'Na distances (A) with estimated standard deviations for

[Na(l 5-azacrown-5)][BH4].

Complex B-Hb-Na B-Hb-Na B-Hb -Na B-Ht-Na B-Na

[Na(l 5-azacrown-5)] [BH4] 2.35(2) 2.59(3) 2.59(3) 3.67(6) 2.734(3)

58 The complex [K(18-azacrown-6)][BH4] (Figure 2.13) consists of a [K(18-azacrown-6)] cation coordinated by an n3 [BH4] ligand, which displays close-to-regular internal tetrahedral geometry. The metal centre, K ion is located at the centre of the 18-crown-6 unit and is bound to the five O and one N atoms of the azacrown ether, and to the tridentate borohydride group on one of the open faces. There is a modest interaction attracting the metal ion out of the plane of the crown ether and towards the boron, [K- crown centroid = 0.312(1) A]. All K-0 distances are very similar, with an average value of 2.84(2) A (Table 2.15). The structure reveals that K occupies a direct coordination

site at boron, as indicated by the B—K distance of 2.979(2) A (cf. the sum of the van der

58 Waals radii, 3.80 A). The B-K distance is shorter than in KBH4 (3.36 A). The borohydride ligand coordinates to the metal centre in a fairly symmetrical n3 manner, with B-H-K distances in the range 2.73(19) to 2.82(19) A (Table 2.16), and the K-B-H angle involving the terminal uncoordinated H atom is nearly linear, with a value of

176.8(10)°.

Figure 2.13: Molecular structure of [K(18-azacrown-6)][BH4] as determined by

X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity.

59 [K(THF)(18-azacrown-6)][BH4], (Figure 2.14) contains a cationic [K(18-azacrown-6)] centre coordinated to anionic [BH4] and a neutral THF ligand on opposite faces. The anion displays close-to-regular internal tetrahedral geometry. The K+ ion is located at the centre of the 18-crown-6 unit and is bound to the six O atoms of the crown ether. All K-

O distances within the ring are very similar, with an average value of 2.85(2) A, (Table

2.15). The tridentate borohydride anion and the THF molecule occupy the two opposing faces of the [K(18-azacrown-6)]+ moiety. The metal ion is drawn out of the plane of the azacrown ligand towards the [BH4] , [K-crown centroid = 0.476(1) A]. It is noteworthy that this distortion is greater than that observed in the solvent-free congener [K(18- azacrown-6)][BH4]. The THF molecule is located on the concave side of the crown ether, with a K-0 bond length of 3.269(7), in good agreement with the mean value of 2.70(15) for K-0 (THF) bonds in the CSD. The K ion occupies a direct coordination site at boron, as indicated by the B-K distance of 3.008(4) A (cf. the sum of the van der Waals radii, 4.31 A).

Figure 2.14: Molecular structure of [K(THF)(18-azacrown-6)][BH4] as determined by

X-ray diffraction. All non-hydride hydrogen atoms are omitted for clarity. 60 The X-ray structure reveals the borohydride ligand to coordinate in a fairly symmetrical rf manner, with B-H-K distances of 2.76(3) - 2.97(6) A, whereas the K-B-H angle involving the terminal uncoordinated H atom is nearly linear, with a value of 170.7(19) °.

Table 2.15: K-0 and K-N distances (A) with estimated standard deviations for [K(18- azacrown-6)] [BH4].

Complex K-Q(4) K-Q(7) K-O(IO) K-Q(13) K-Q(16) K-N [K(l 8-azacrown-6)] 2.784(2) 2.863(2) 2.839(2) 2.907(2) 2.808(2) 2.950(2) [BH4] [K(THF)(18- 2.816(3) 2.957(2) 2.826(2) 2.863(3) 2.800(3) 2.911(3) azacrown-6)] [BH4]

Table 2.16: B-H-Na and B--Na distances (A) with estimated standard deviations for

[K(l 8-azacrown-6)][BH4] complexes.

Complex B-Hb-K B-Hb-K B-Hb-K B-Ht-K B-K [K( 18-azacrown-6)] 2.73(19) 2.80(16) 2.82(19) 4.10(18) 2.979(2) [BH4] [K(THF)(18-azacrown-6)] 2.76(3) 2.97(4) 2.92(6) 4.16(4) 3.008(4) [BH4]

2.7 Supramolecular Structures

61 Replacing 15-crown-5 or 18-crown-6 with its azacrown analogue leads to the substitution of one ether oxygen atom in the ring by an N-H moiety. This provides an acidic centre in the ring that can potentially compete with the alkali metal cation for electron density from the anion. The structures adopted by azacrown ether complexes often differ significantly from those of their regular crown ether counterparts on account of this chemically active

N-H moiety. ' In this study, we sought to answer the following two questions:

(i) does the N-H group affect the alkali metal-anion interaction in any

significant way?

(ii) is the supramolecular structure of the crystalline material affected by the

presence of the N-H moiety?

2.7.1 [Na(15-crown-5)][BH4] vs. [Na(15-azacrown-5)][BH4]

The first question was addressed by analyzing the hapticity and geometry of the [BH4]" interaction with [Na(15-crown-5)]+ and [Na(15-azacrown-5)]+ (Figure 2.15).

Figure 2.15: Molecular structure of (a) [Na(15-crown-5)][BH4] and (b) [Na(15-

azacrown-5)][BH4] as determined by X-ray diffraction All non-hydride hydrogen atoms

are omitted for clarity.

62 Both structures conform to an ionic complex containing cationic [Na(crown)]+ and anionic [BH4] centres. The Na+ ion occupies a direct co-ordination site at boron, as indicated by the short B-Na distances of 2.641(2) and 2.734(3) A in the crown and azacrown complexes respectively, (cf. the sum of the van der Waals radii, 3.44 A). The cation-anion interaction draws the Na+ ion out of the plane crown by an average of

1.046 A (Table 2.17).

The anion coordinates in a fairly symmetrical n3 manner in each case, with B-H--Na distances in the range 2.35(2) to 2.60(2) A, (Table 2.18). The Na-B-H angle involving the terminal uncoordinated hydrogen atom is nearly linear, with values of 174.4(8) and

169(3)°. We conclude that the cation-anion interaction in [Na(15-crown-5)][BH4J and

[Na(15-azacrown-5)][BH4] is identical within the limits of experimental accuracy.

Table 2.17: B—Na distances and the distance of the metal cation from the crown plane

(A) with estimated standard deviations for [Na(crown)][BH4] complexes.

/-•i n XT sn *T \ Na-crown Complex B—Na (B—Na) , vdW plane [Na(15-crown-5)][BH4] 2.614(2) 3.44 0.885(1) [Na(15-azacrown-5)][BH4] 2.734(3) 3.44 1.208(1)

Table 2.18: B-H—Na distances (A) with estimated standard deviations for t [Na(crown)][BH4] complexes.

Complex B-Hb~Na B-Hb~Na B-Hb-Na B-Ht-Na

[Na(15-crown-5)][BH4] 2.37(2) 2.43(16) 2.60(19) 3.72(16)

[Na(15-azacrown-5)][BH4] 2.35(2) 2.59(3) 2.59(3) 3.67(6)

63 Turning now to address the second question, we analyzed the chain-type arrangement of

cation-anion pairs in [Na(15-crown-5)][BH4] and [Na(15-azacrown-5)][BH4]. The

arrangement in the former complex (Figure 2.16) alternates in an up-down manner,

similar to that found for [K(18-crown-6)][BH4] (q.v.), and leading to the adoption of

space group P2(l)2(l)2(l).

Figure 2.16: The alternating up-down chain arrangement adopted by neighbouring

ion pairs in the [Na(15-crown-5)][BH4] complex.

Whereas the cation-anion interactions in [Na(15-crown-5)][BH4] are similar to those in the corresponding azacrown ether system, the supramolecuar arrangement of ion pairs is

significantly different. This arises from the presence of the N-H moiety in the azacrown

system, which encourages the development of N-H—H-B proton-hydride H-bonds

between the cation of one ion pair (N-H) and the anion of a neighbouring ion pair (B-H).

64 Intramolecular N-H—H-M interactions have been recently identified by many researchers; Morris and co-workers showed that they lead to the adoption of a chain

62 structure in the complexes [K(18-azacrown-6)][RuH3(CO)(P'Pr3)2] , [K(18-azacrown-

i 63 i 64 6)][IrH4(P Pr3)2], and [K(18-azacrown-6)][OsH5(P Pr3)2].

The H—H distances observed in such systems are typically 1.7-2.2 A, significantly shorter than the sum of the van der Waals radii for two hydrogen atoms, 2.4 A. Crabtree et al. found strong proton-hydride H-bonding in solid state for the simple system ammonia borane, BH3NH3, with H—H distances of 2.02 A, as determined by neutron diffraction.61

Proton-hydrogen H-bonding of the form N-H—H-B leads to a chain-type arrangement of adjacent cation-anion pairs and results in the [Na(15-azacrown-5)][BH4] complex adopting a different space group (Pnma) from [Na(15-crown-5)][BH4], in which a linear chain-type arrangement of ion pairs is formed. As displayed in Figure 2.17, the terminal

(uncoordinated) hydride atom of [BH4] interacts with the acidic N-H moiety of the azacrown. The H—H distance in N-H—H-B moiety in [Na(15-azacrown-5)][BH4] is 2.03

A, a value that compares closely with those described above from the work of Morris and

Crabtree.

65 Figure 2.17: A portion of the linear chain adopted by [Na(15-azacrown-5)][BH4]. The polymeric structure is held together through unconventional N-H—H-B interactions, which are represented by a dotted line.

2.7.2 [K(18-crown-6)] [BH4] vs. [K(18-azacrown-6)] [BH4]/ [K(THF)(18-azacrown-

6)][BH4]

The hapticity and geometry of the [BH4] interaction with [K(18-crown-6)]+, [K(18- azacrown-6)]+ and [K(THF)(18-azacrown-6)]+ is depicted in Figure 2.18.

All three structures conform to an ionic complex containing cationic [K(crown)]+ and anionic [BH4]" centres. K+ occupies a direct co-ordination site at boron, as indicated by the short B-K distance of 2.972(5) A in crown ether and 2.979(2) A and 3.008(4) A, respectively in azacrown, (cf. the sum of the van der Waals radii, 3.80 A) (Table 2.19).

The cation-anion interaction draws the K+ ion out of the plane of the crown ether by around 0.482 A.

66 Figure 2.18: Molecular structure of (a) [K(18-crown-6)][BH4] and (b) [K(18-azacrown-

6)][BH4] and (c) [K(THF)(18-azacrown-6)][BH4] as determined by X-ray diffraction. All non-hydride H atoms are omitted for clarity.

The anion coordinates in a fairly symmetrical n3 manner, with B-H—K distances in the

range 2.69(4) to 2.92(6) A (Table 2.20) whereas the K-B-H angle involving the terminal

uncoordinated H atom is nearly linear, with values of 165(2), 175.7(14) and 176.8(10)°.

Accordingly, there is no distinguishable difference between the three structures in terms

of the cation-anion interaction.

Table 2.19: Table B---K distances and the distance of the metal cation from the crown

plane (A) with estimated standard deviations for [K(crown)] [BH4] complexes.

K-crown Complex B-K (B-K) vdW plane

[K(18-crown-6)][BH4] 2.972(5) 3.80 0.659(1)

[K(l 8-azacrown-6)][BH4] 2.979(2) 3.80 0.312(1)

[K(THF)(18-azacrown-6)][BH4] 3.008(4) 3.80 0.476(1)

67 Table 2.20: B-H--K distances (A) with estimated standard deviations for

[K(crown)][BH4] complexes.

Complex B-Hb-K B-Hb-K B-Hb-K B-Ht»K

[K(18-crown-6)][BH4] 2.69(4) 2.75(5) 2.90(4) 4.11(3)

[K(18-azacrown-6)][BH4] 2.73(19) 2.80(16) 2.82(19) 4.10(18)

[K(THF)(18-azacrown-6)] [BH4] 2.76(3) 2.97(4) 2.92(6) 4.16(4)

However, as with its Na-15-crown-5 analogue, replacement of 18-crown-6 with 18-

azacrown-6 leads to a marked change in the supramolecular structure adopted by adjacent

ion pairs. The arrangement in [K(18-crown-6)][BH4J is an alternate, up-down motif,

similar to that found for [Na(15-crown-5)][BH4] (Figure 2.19), and leads to the adoption

of the same space group P2( 1)2(1)2(1).

Figure 2.19: The alternating, up-down chain arrangement adopted by neighbouring ion pairs in the [K(18-crown-6)][BH4] complex.

68 The arrangement of ion pairs in [K(18-azacrown-6)][BH4] is different from that in both

[K(18-crown-6)][BH4] and [Na(15-azacrown-5)][BH4] (see Section 2.7.1). In this case, proton-hydrogen H-bonding of the form N-H—H-B leads to a chain-type arrangement of adjacent cation-anion pairs, and results in the adoption of space group R3c. This time the chain-type arrangement is zigzag. The acidic N-H group of the azacrown cation, interacts not with the terminal B-H moiety of the borohydride anion, but rather with one of the bridging B-H entities, as shown in Figure 2.20. The N-H--H-B distance in [K(18- azacrown-6)][BH4] is 2.25 A, slightly longer than that found for [Na(15-azacrown-

5)][BH4], but within the range expected for such an interaction.

Figure 2.20: The zigzag chain of [K(18-azacrown-6)][BH4]. The polymeric structure

is held together through unconventional N-H--H-B interactions, which are represen­

ted by a dotted line.

69 The arrangement of ion pairs in [K(THF)(18-azacrown-6)][BH4] is similar to that in

[K(18-azacrown-6)][BH4]. The proton-hydrogen H-bonding of the form N-H—H-B leads to a chain-type arrangement of adjacent cation-anion pairs, and results in the adoption of space group P2( 1)2(1)2(1). The chain-type arrangement is zigzag (Figure

2.21). The acidic N-H group of the azacrown cation, interacts with the bridging B-H moiety of the borohydride anion. The H'"H distance in the N-H--H-B moiety in

[K(THF)(18-azacrown-6)][BH4] is 2.39 A, slightly longer than that found for [K(18- azacrown-6)][BH4], but within the range expected for such an interaction (q.v) Figure

2.20.

Figure 2.21: The zigzag chain of [K(THF)(18-azacrown-6)][BH4]. The polymeric structure is held together through unconventional N-H—H-B interactions, which are represented by a dotted line. The THF molecule is omitted for clarity.

70 In summary, the cation-anion interactions in the azacrown complexes [Na(15-azacrown-

5)][BH4], [K(18-azacrown-6)][BH4] and [K(THF)(18-azacrown-6)][BH4] resemble closely those of their conventional crown ether analogues, with a fairly symmetrical n interaction between the metal ion and the borohydride ligand, which draws the cation out of the plane of the crown ether. Rather surprisingly, this distortion is markedly more evident in [K(THF)(18-azacrown-6)][BH4] than in its solvent-free congener [K(18- azacrown-6)][BH4]. However, the overall structure adopted by these complexes varies significantly from that found for their conventional crown ether analogues, on account of the significant N-H--H-B interactions that exist in the azacrown systems. These impose an different packing arrangement on neighbouring in pairs, leading to the adoption of completing different in the normal and azacrown systems.

2.8 Charge Density Analysis of [Na(15-crown-5)][EH4] Complexes

Alkali metal cations complexed with crown ethers provide an acidic centre to which a number of transition metal hydride anions have been shown to coordinate. In contrast, there is a dearth of such examples involving a main group hydride anion. Studies of these complexes have focused mainly on the hapticity of the ligand, rather than on the nature of the overall interaction. Accordingly, we have carried out the first theoretical charge density study on [Na(15-crown-5)][EH4] complexes (E = Al and B). The goal of this analysis was to gain a deeper insight into the electronic structure and the bonding between the alkali metal and aluminum hydride and borohydride, respectively.

71 Charge density analysis allows us to observe directly the electrons responsible for chemical bonding in a molecule. The method has two distinct stages: Experimental and/or theoretical electron density distribution may be obtained from a high resolution X- ray diffraction experiment and quantum chemical Density Functional Theory (DFT) calculations, respectively. These data are processed using Bader's Atom in Molecules

(AIM) theory,63 a rigorous topological and quantum mechanically approach which allows us to gain a direct insight into the bonding displayed in a molecule.

With advances in X-ray diffraction technology, experimental electron density, p(r), studies have become an increasingly powerful tool in bonding analysis. The electron density is the key to the bonding and geometry of a molecule because the nuclei are held by attractive forces between the electrons and the repulsions between the nuclei. AIM theory is applied to the calculated or experimentally derived p(r), and from this analysis we obtain meaningful information on the chemical interactions that hold together on molecule of interest.

In the following, we will discuss our initial findings we obtained by analyzing and interpreting the theoretical charge density distribution of the complexes mentioned above.

This was done by using a local version of the AIMPAC software package.66An experimental charge density study of at least one of these systems is planned for the near future.

72 2.8.1 DFT Geometry Optimization

The geometries of both complexes were fully optimized without imposing any symmetry restraints at the [B3LYP/6-31 lG(d,p)] level of approximation using Gaussian03 (Figure

2.22). The geometries thus obtained are in good agreement with X-ray experimental values (see Table 2.17 and 2.18). The Na-B distance of 2.501 A is only slightly smaller than the 2.614 A determined by X-ray diffraction, while the Na-Al distance is comparable in experiment and theory (3.119 A vs. 3.058 A, respectively).

(a) (b)

Figure 2.22: DFT optimized structures, atomic AIM charges (boxed), and geometrical parameters of (a) [Na(15-crown-5)][BH4] and (b) [Na(15-crown-5)][AlH4] [B3LYP/6-

31 lG(d,p)]; all atoms of the crown ethers are omitted for clarity.

The experimental observation of different hapticities in these two complexes is also reflected in our theoretical findings: Three H atoms point towards the Na cation in

[Na(15-crown-5)][BH4], whereas only two H atoms point towards the Na cation in

[Na(15-crown-5)][AlH4].

73 2.8.2 Electron Density Map

One of the common ways to represent the electron density distribution is a contour map, analogous to a topographic contour map representing the relief of a part of the earth's surfaces. The electron density is most concentrated at the nuclear positions and becomes more diffuse as one moves away from the nuclei, which are centres of attractions of the electron density. The molecule can also be viewed by a two-dimensional projection, called a two-dimensional contour map.

At first glance, the electron density appears to show very little chemical information itself, as depicted in Figure 2.23 while contour maps of p(r) for the [Na(15-crown-

5)][EH4] complexes: High densities around Na and Al or B dominate over the low density associated with the bridging hydrogen atom, Hb.

(a) (b)

Figure 2.23: Contour maps of the electron density p(r) of (a) [Na(15-crown-5)][BH4] and (b) [Na(15-crown-5)][AlH4] in a plane containing Na, B and Al, respectively, and a bridging hydrogen atom.

74 2.8.3 Gradient Vector Field

The first stage of any AIM analysis is to generate the gradient field (first derivative) from

electron density; this is a set of field lines representing routes of steepest ascent. The gradient vector is denoted by the symbol Vp(r); it is a directional quantity and points in the direction of the largest increase in electron density; accordingly it must have a start

and an end point. Hence, a path must originate at a minimum or saddle point in electron

density and terminate at a maximum or saddle point. The majority of field lines terminate

at nuclei, and there are bundles of these that form an atomic basin for each atom in the molecule. The surfaces between two atomic basins are called interatomic surfaces or zero-flux surfaces and, in general, these cannot be crossed by any gradient. The only points where field lines are able to cross these surfaces are termed bond critical points

(BCPs) or saddle points in electron density. These points represent minima in the

direction along an axis defined by the two atoms involved in the interaction. In the two

directions perpendicular to this, however, it is a maximum. These three axes are called principal curvatures. There is a trajectory in electron density that starts at these BCPs and

runs to each nucleus. This is called a bond path. Bond paths are always found between

every pair of atoms in a molecule that we usually consider to be bonded to each other,

and not between atoms that are not bonded together. The existence of a bond path

between nuclei of two atoms that share an interatomic surface constitutes the criterion by

which we judge whether two atoms are bonded.

75 Figure 2.24 shows a few of the infinite number of gradient paths that lie in the plane of

interest (as specified in Figure 2.23). The green lines, representing the atomic surfaces,

define the atomic basins associated with each atom, whereas the blue lines are the bond

paths between each of the atoms. As can be seen from Figure 2.24(a) and (b), there are two distinct bonding situations in both systems: In the case of [Na(15-crown-5)][BH4], a

bond path between Na and B is observed, which is severely distorted from linearity,

whereas no bond path is observed between Na and the bridging hydrogen atom. In

contrast, the Al analogue displays no bond path between Na and Al, and the interaction

seems to be dominated by Na'"H interactions, instead.

(a) (b)

Figure 2.24: Gradient vector field, Vp(r) for (a) [Na(15-crown-5)][BH4] and (b)

[Na(15-crown-5)][AlH4].

76 2.8.4 AIM Atomic Charges

By integrating the electron density in the atomic basins, as defined by the interatomic

surfaces we can obtain the electron population and hence the charge of the corresponding

atoms, since the latter is the difference between the electron density within the basin and

the atomic number.

Table 2.21: Calculated AIM charges (q) for the atoms involved in the bonding in [Na(15-

crown-5)][EH4] complexes and in the free [EH4] moieties.

Complex q(B~ q(Al) q(Hb) q(Ht) [Na(15-crown-5)][BH4] +1.68 -0.65 -0.62 +1.67 -0.66 Free [BH4]"

[Na(15-crown-5)][AlH4] +2.23 -0.79 -0.77 +2.22 -0.80 Free [A1H4]"

The results listed in Table 2.21 show that the atomic charges are not altered significantly

by coordination. This suggests that the interaction between the Na+ cation and the

coordinating EH4 unit is mainly electrostatic in nature. The hydrogen atoms bound to Al

carry a significantly greater negative charge (-0.79) compared with those bound to boron

(-0.65), which can be explained in terms of the calculated for differing electronegativities

of B and Al, which are also reflected in the AIM charges B and Al (+1.68 and +2.23, respectively). Furthermore, all bridging hydrogen atoms carry a slightly higher charge than the terminal ones. This is caused by the electrostatic attraction of the Na cation.

77 2.8.5 The Laplacian of the Electron Density

Another useful tool to analyze the topology of the electron density is its second derivative, called the Laplacian or V p(r), which accentuates small variations in p(r).

More precisely, V p(r) < 0 indicates a charge higher than the average; the electron density is locally concentrated. Conversely, V p(r) > 0 indicates a charge lower than the average; the electron density is locally depleted.

In Figure 2.25, we can see that the different electronic shells are revealed, along with the distribution of electron density in the valence shell. The interaction between Na+ and the

AIH4 unit appears to be dominated by direct Na--Hb interaction, whereas in the case of

BH4 the electron density of the whole B-H moiety seems to be involved in the interaction.

(a) (b)

Figure 2.25: Calculated V2p(r) for the Na-Ht,-E moiety of (a) [Na(15-crown-5)]

[BH4] and (b) [Na(15-crown-5)][AlH4] derived at the denoted levels of theory.

The solid lines represent negative values of V2p(r). 78 2.8.6 Topological Parameters

In order to understand the differences in the bonding of AIH4 and BH4 to Na+, we evaluated the properties of the electron density at the BCPs. According to Bader,63 this provides a measure of quantitative information about the degree of ionic or covalent character of bonding between two atoms connected by a bond path. Selective topological parameters describing the E-Ht,—Na unit for the [Na(15-crown-5)][EH4] complexes are presented in Table 2.22. The first parameter, p(r) at the BCP, represents a measure of the strength of the corresponding bond. The parameter V2p(r) specifies the type of interaction and when V2p(r) > 0 there is an ionic, closed shell interaction or V2p(r) < 0 there is a covalent, open shell interaction. The ellipticity (s) gives information about the anisotropy of p(r) around the chemical bond. If s = 0, the bond is symmetric with respect to rotation (o bond). If s > 0, the bond has asymmetric character around the chemical bond: for example in a 71 bond. The final parameter H, the total energy, is another necessary qualifier - in addition to a negative value for V p(r) - for a covalent interaction.

79 Table 2.22: Topological parameters at the BCPs for [Na(15-crown-5)][EH4] complexes at the [B3LYP/6-31 lG(d,p)] level of theory.

Complex p [eA3"] V2p(r)[eA5"] £ H [ha A3" ]__ Na-B 0.11 +1.96 1.81 +0.02

[Na(15-crown-5)][BH4] B-Hb 0.98 -0.67 0.05 -0.91

B-Ht 1.08 -3.11 0.01 -1.06

Na-Hb 0X)9 + 1.33 0.29 +0.01

[Na(15-crown-5)][AlH4] Al-Hb 0.43 +5.15 0.03 -0.10

Al-Ht 0.50 +5.94 0.00 -0.13

As can be seen in the Table 2.22, all B-H bonds are highly covalent, as revealed by comparably large values of p(r), negative Laplacians, and negative total energies (H). In contrast, the Al-H bonds are far more ionic in character, with lower values of p(r) and positive Laplacians, which is also in agreement with the AIM charges previously discussed. As expected, all bridging E-H bonds are weaker [smaller p(r)] and slightly distorted (e > 0) compared to the terminal bonds, since they are involved in bonding to

Na+ cation. All bonds in which Na+ is a participant can be described as weak and ionic in character.

Table 2.23: Calculated derealization indices (5) for [Na(15-crown-5)][EH4] comple­ xes at the [B3LYP/6-31 lG(d,p)] level of theory.

Complex 5 (Na, E) 5 (Na, Hb) 5 (E, Hb) 5 (E, Ht)

[Na(15-crown-5)][BH4] 0.023 (X036 0.523 0.566

[Na(15-crown-5)][AlH4] 0.009 0.064 0.306 0.392

80 Table 2.23 shows the value of 5, the derealization index, which corresponds to the number of electron pairs shared by two atoms. Our previous conclusion that the bridging bonds are weaker than the terminal ones is corroborated by the lower values of 8 from the bridging moieties. This is also true for the weak ionic interaction with the Na+ cation, reflected by extremely low values of 8. However, the values of 8 are the key to understanding the different bonding scenarios in the two systems. For [Na(15-crown-

5)][BH4], the Na, B and Na, Hb values are of the same order of magnitude. This might explain the unusual curvature of the bond path in this system, displaying a sharp change in direction when approaching the B-H unit. In the case of [Na(15-crown-5)][AlH4],

8(Na, Hb), is more than six times larger than 8(Na, Al) and consequently there is only a bond path between Na and Hb, with none between Na and Al.

81 CHAPTER 3

EXPERIMENTAL DETAILS

3.1 General

All operations were carried out under strictly inert atmosphere conditions by use of

Schlenk and/or glove box techniques.68 A vacuum line constructed from Pyrex glass

operated at a vacuum of 10" - 10" Torr of residual pressure, as measured by a Pirani

gauge, and was connected to a supply of argon dried by passage through molecular sieves

pre-activated by heating at 200 °C under vacuum. The vacuum line contained a mixture

of greased glass stopcocks and greaseless Teflon valves (J. Young). Powders and other

solid materials were manipulated by use of a glovebox (MBraun, Labmaster 130). This

consisted of a nitrogen source catalytically purged of moisture and oxygen (BTX catalyst;

BASF), neoprene gloves, a sealed working/storage space constantly kept under an excess pressure of nitrogen, a -35 °C refrigerator, and two ante-chambers of different sizes

capable of performing purging cycles by alternation of high-vacuum and nitrogen

refilling. These provisions guaranteed a level of oxygen and moisture in the chamber below 10 ppm. Standard Schlenk vessels for the synthesis, manipulation and storage of

sensitive starting materials, intermediates, and products were constructed from Pyrex

glass and equipped with greased joints (valves and stoppers) and/or with Teflon valves (J.

Young). NMR tubes (5 mm) were sealed with Teflon valves and could be connected to

82 the vacuum line by use of a conical joint made of Pyrex glass. Prior to use, all glassware was oven-dried and then evacuated using the vacuum line or the glove box ante-chamber.

Separation of solid products from a saturated solution was done under Ar by transferring the solution through a cannula, which relies on a pressure difference to filter a mixture using a double-ended needle (cannula). The cannula was fitted with a filter paper over one end, which was then secured with a length of Teflon tape and inserted into the flask containing the mother liquor (B). A septum cap was then inserted into the open end of the receiver flask (A) and pierced it with a sharp needle. By inserting an exit needle through the septum cap on the receiver flask (A) and closing the valve on this vessel, the solution is transferred under positive gas pressure through the filter when the cannula is lowered below the meniscus in the flask B Figure 3.1.

Needle

Figure 3.1: Filtration by using a double-ended needle (cannula).

3.1.1 Solvents

All solvents, with the exception of pentane, were purified using a Grubbs apparatus6 9 provided by the Seca Solvent System (aka GlassContour). The Grubbs apparatus

83 incorporates a vacuum pump and an argon supply, and it is used to deliver dried,

degassed solvent under a blanket of argon. After collection, the solvents were degassed to

completion by vigorous passage of argon, and stored over activated molecular sieves. For

NMR experiments, deuteriated solvent (ds-THF) was degassed, and then stored over

molecular sieves.

3.1.2 G eneral Reagents

Unless stated otherwise, all reagents were purchased from Sigma-Aldrich. LiAlH4,

LiBH4, NaAlH4, NaBHi and KBH4 were received as powders, and were transferred to the

glove box for storage and manipulation. LiGaFU, NaGaFLj, KGaFLt and KAIH4 were

synthesized by metathesis reactions. LiH, KH and NaH were obtained as suspensions in

mineral oil and stored in the glove-box. The inorganic salts KC1 and NaCl were dried in

an oven before use. Crown ethers and azacrown ethers were thoroughly dried before used

by dissolving them in THF and storing the resulting solution over dried molecular sieves.

3.2 Instrumentation

3.2.1 NMR Spectroscopy

Routine !H, 27A1, 71Ga and UB NMR spectra were recorded using either a 300 MHz

Varian INOVA instrument or a 400 MHz Varian UNITY spectrometer. All NMR

samples were measured in dg-THF solvent with 0.05% tetramethylsilane (TMS) as an

internal reference at room temperature.

84 3.2.2 Single Crystal X-Ray Diffraction

Crystals suitable for diffraction studies were grown by cooling a saturated solution in

THF to either 5 °C or -40 °C. A hemisphere of data was collected on a Bruker AXS

P4/SMART 1000 diffractometer using a scans width of 0.3 and 30 s or 10 s exposure times. The detector distance was 5 cm. The data were reduced (SAINT) and corrected for absorption (SADABS). The structure was solved by direct methods and subjected to full-matrix least squares refinement using F2(SHELX-TL).70 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were found in Fourier difference maps and refined isotropically. POV-Ray was used for the graphical representation of the results.71

3.2.3 Theoretical Calculations

Density functional theory (DFT) calculations were performed by two members of the

McGrady Group, Dr. Peter Sirsch and Richard Burchell. All data in Section 2.8 were calculated using Bader' Atom in Molecule (AIM) theory. All the calculations were performed at the [B3LYP/6-31 lG(d,p)] level of approximation using Gaussian03.

85 3.3 Synthesis and Characterization

3.3.1 [Li2(12-crown-4)3][AlH4]2

A 50 mL flask was charged with 0.092 g (24.2 mmol) of LiAlH4 and 13.5 mL (27.0 mmol) 2 M solution of 12-crown-4 and THF (20.0 mL) was stirred for 2 h at RT under an

Ar atmosphere. After stirring a gray precipitate was observed at the bottom of the flask, which was presumably unreacted starting material. The saturated solution was then separated from the solid by filtration (see Section 3.1). The clear mixture volume was reduced in volume in vacuo before crystallizing at 5 °C. Colourless crystals were obtained after 3 d. X-ray crystallography and NMR spectroscopy revealed these to be the

: 4 known product [Li2(12-crown-4)3][AiH4]. '

'H NMR (THF-dg, 300 MHz, 298 K): 5 3.6 (s, 48H, crown), 3.8-3.7 (q, 4H, A1H4").

27 x A1 (THF-d8, 79 MHz, 298 K) 8 94.4-103.3 (qn, 4H, A1H4", J/,m = 180 Hz).

3.3.2 [Li2(12-crown-4)3][GaH4]2

A mixture of 0.5 g (6.0 mmol) LiGaH4 (prepared from 1.77 g, 0.01 mol GaCl3 and

1.32 g, 0.166 mol LiH), and 4.0 mL (10.0 mmol) 2 M solution of 12-crown-4 and THF

(20.0 mL) was stirred for 4 h at RT under an Ar atmosphere. A solid observed at the bottom of the flask was removed by filtration (see section 3.1) and the solution was transferred to a Schlenk tube, concentrated to half volume in vacuo, and stored in the freezer at -40 °C. Colourless crystals were obtained after 5 d. X-ray crystallography identified these as the novel complex [Li2(12-crown-4)3][GaH4]2.

86 Elemental analysis: Calculated for C24H52GaLi2Oi2: C, 46.75%; H, 8.44%; Ga, 11.36%.

Elemental analysis and NMR spectroscopy were not performed for this complex because of its high air-sensitivity and thermal fragility.

3.3.3 [Na(15-crown-5)][BH4]

NaBH4 (0.075 g, 2.0 mmol) was dissolved in ethanol (10 ml), 1.1 mL (2.0 mmol) 2 M solution of 15-crown-5 was added and the mixture was refluxed in THF (20.0 mL) at

72°C for 3 h. A solid observed at the bottom of the flask was separated from the saturated

solution by filtration (see Section 3.1). The clear mixture volume was reduced in vacuo before crystallizing at 5 °C. Colorless crystals were obtained after 5 d. X-ray crystallography and NMR spectroscopy revealed these to be the known product [Na(15-

crown-5)][BH4].:>:>':>6 The reaction has been done a several times following the same procedure because of the poor solubility of the NaBH4.

Elemental analysis: Calculated for CioH24BNa05: C, 46.51%; H, 7.75%; B, 4.26%.

Found: C, 46.19%; H, 9.78%; B, 3.25%.

! H NMR (THF-d8, 300 MHz, 298 K): 5 2.4 (s, 20H, crown), [-0.8-(-0.3)] (q, 4H, BH4").

U B (THF-dg, 97 MHz, 298 K) 8 [-43.7-(-40.4)], (qn, 4H, BH4~, VBH = 77 Hz).

3.3.4 [Na(15-crown-5)][AlH4]

A 50 mL flask was charged with 0.75 g (13.8.2 mmol) of NaAlH4 and 8.0 mL (16.0 mmol) 2 M solution of 15-crown-5 and THF (20.0 mL) was stirred for 2 h at RT under an

Ar atmosphere. A gray precipitate observed at the bottom of the flask was separated from

87 the saturated solution by filtration (see Section 3.1). The clear mixture was reduced in volume in vacuo before crystallizing at 5 °C. Colourless crystals were obtained after 2 d.

X-ray crystallography and NMR spectroscopy identified these as the known complex

57 [Na(15-crown-5)][AlH4].

Elemental analysis: Calculated for CioH24AlNa05: C, 43.79%; H, 8.75%; Al, 9.85%.

Found: C, 41.81%; H, 8.61%; Al, 9.54%;

[ H NMR (THF-d8, 300 MHz, 298 K): 5 3.7 (s, 20H, crown), 3.4-3.2 (q, 4H, A1H4~).

27 x Al (THF-d8, 79 MHz, 298 K) 5 93.2-102.1 (qn, 4H, A1H4~, Jm = 173 Hz).

3.3.5 K(18-crown-6)][BH4]

A 50 mL flask was charged with 0.20 g (6.3 mmol) of KBH4 and 1.82 mL (6.3 mmol) 2

M solution of 18-crown-6 and THF (20.0 mL) was stirred for 6 h at RT under an Ar atmosphere. A solid observed at the bottom of the flask was separated from the saturated solution by filtration (see Section 3.1). The clear mixture volume was reduced in vacuo before crystallizing at 5 °C. Colourless crystals were obtained after 5 d. X-ray crystallography and NMR spectroscopy revealed these to be the known product [K(18-

58 crown-6)][BH4].

Elemental analysis: Calculated for Ci2H28B06: C, 45.28%; H, 8.80%; B, 3.45%. Given the lowest yield of the product the elemental analysis had not been performed.

! H NMR (THF-dg, 300 MHz, 298 K): 5 3.5 (s, 24H, crown), [-0.9-(-0.2)] (q, 4H, BH4").

n B (THF-dg, 97 MHz, 298 K) 8 [-42.1-(-38.7)], (qn, 4H, BH4", VBH = 78 Hz).

88 3.3.6 [K(THF)(18-crown-6)][AlH4]

A mixture of 0.35 g (5.0 mmol) KAIH4 salt (prepared from 0.33 g, 5.0 mmol KC1 and

0.19 g, 5 mmol LiAlH4), and 2.5 mL (5.0 mmol) 2 M solution of 18-crown-6 and THF

(20.0 mL) was stirred for 4 h at RT under an Ar atmosphere. A gray precipitate observed at the bottom of the flask was separated from the saturated solution by filtration (see

Section 3.1). The product was transferred to a Schlenk tube and stored in the fridge at 5

°C. Colorless crystals were obtained after 3 d. X-ray crystallography and NMR spectroscopy identified these as the novel complex [K(THF)(18-crown-6)][AiH4].

Elemental analysis: Calculated for CieHseAlKO?: C, 47.29%; H, 8.86%; Al, 6.65%.

Found: C, 27.24%; H, 5.83%; Al, 11.4%;

'H NMR (THF-d8, 300 MHz, 298 K): 8 3.6 (s, 24H, crown), 3.8-3.7 (q, 4H, AlFLf).

27 A1 (THF-d8, 79 MHz, 298 K) 5 93.6-102.4 (qn, 4H, A1H4 , VBH = 174 Hz).

3.3.7 [K(THF)(18-crown-6)][GaH4]

A mixture of 0.01 g (0.013 mmol) KGaH4 salt (prepared from 0.0079 g, 0.123 mmol KC1 and 0.01 g, 0.0123 mmol LiGaH4, and 0.06 mL (0.123 mmol) 2 M solution of 18-crown-

6 and THF (20.0 mL) was stirred for 5 h under an Ar atmosphere. A gray precipitate observed at the bottom of the flask was separated from the saturated solution by filtration

(see Section 3.1). This solution was transferred to a Schlenk tube and stored in the freezer at -40 °C. Colourless crystals were obtained after 5 d. X-ray crystallography identified these as the novel complex [K(THF)(18-crown-6)][GaH4].

Elemental analysis: Calculated for CieHseGaKOy: C, 42.76%; H, 8.01%; Ga, 15.59%.

89 Elemental analysis and NMR spectroscopy were not performed for this because of its high air-sensitivity and thermal fragility.

3.3.8 [Na(15-azacrown-5)][BH4]

NaBH4 (0.023 g, 0.62 mmol) was dissolved in ethanol (10 ml), 2.5 mL (0.62 mmol) 2 M solution of 15-crown-5 was added and the mixture was refluxed in THF (20.0 mL) at

72°C for 3 h. A solid observed at the bottom of the flask was separated from the saturated solution by filtration (see Section 3.1). The clear mixture volume was reduced in vacuo before crystallizing at 5 °C. Colorless crystals were obtained after 2 d. X-ray crystallography and NMR spectroscopy identified these as the novel complex [Na(15- azacrown-5)][BH4]. The reaction has been done a several times following the same procedure because of the poor solubility of the NaBH4.

Elemental analysis: Calculated for CioH25BNNa04: C, 46.49%; H, 9.72%; N, 5.44%.

Found: C 45.23%; H 9.15%; N 4.41%. l H NMR (THF-d8, 300 MHz, 298 K): 6 2.4 (s, 20H, crown), 2.0-1.5 (q, 4H, BH4~)

U B (THF-d8, 97 MHz, 298 K) 8 [-42.5-(-39.1)], (qn, 4H, BH4~, %H = 135 Hz).

3.3.9 [K(18-azacrown-6)][BH4]

A 50 mL flask was charged with 0.034 g (0.62 mmol) of KBH4 and 2.5 mL (0.62 mmol)

2 M solution of 18-crown-6 and THF (20.0 mL) was stirred for 12 h under an Ar atmosphere. A solid observed at the bottom of the flask was separated from the saturated solution by filtration (see Section 3.1). The product was transferred to a Schlenk tube and

90 stored in the fridge at 5 °C. Colourless crystals were obtained after 3 d. X-ray crystallography identified these as the novel complex [K(18-azacrown-6)][BH4].

Elemental analysis: Calculated for C12H25KBNO5: C, 45.42%; H, 7.88%; N, 4.41%.

Found: C 45.63%; H 9.08%; N 5.22%.

Given the lower yield of the product the NMR spectroscopy had not been performed.

3.3.10 [K(THF)(18-azacrown-6)] [BH4]

In a procedure identical to the preparation from Section 3.3.9, a mixture of KBH4 (0.034 g; 0.62 mmol), a 0.25M solution of 18-azacrown-6 (2.5 mL; 0.62 mmol) and THF (20.0 mL) was stirred for 12 h under an Ar atmosphere. A solid observed at the bottom of the flask was separated from the saturated solution by filtration (see Section 3.1). The product was transferred to a Schlenk tube and stored in the fridge at 5 °C. Colourless crystals were obtained after 3 d. The complex had been done once more for getting a better yield. Unexpectedly, a novel complex had been identified by X-ray crystallography and NMR spectroscopy as [K(THF)(18-azacrown-6)][BH4].

Elemental analysis: Calculated for C16H37KBNO5: C, 49.35%; H, 9.50%; N 3.35%.

Elemental analysis had not been performed because of the lower yield.

'H NMR (THF-d8, 300 MHz, 298 K): 8 3.0 (s, 24H, crown), [0.5-(-0.5)] (q, 4H, BH4 ). n B (THF-dg, 97 MHz, 298 K) 5 [-39.6-(-36.2)], (qn, 4H, BH4", %H = 88 Hz).

91 3.4 Crystallographic Data

Crystal data for [Li2(12-crown-4)3][GaH4]2: C^HsgGaLiOe, M = 345.00 g/mol, colourless block, 0.40 x 0.40 x 0.20 mm3, monoclinic space group P2(l)/c , a = 7.7439(4)

A, b = 14.3020(8) A, c = 15.4270(8) A, a = 90°, (3 = 90°, y = 90°, V = 1456.2(2) A3, Z =

3 4, Dc = 1.349 Mg/m , F00o = 728, Bruker AXS P4/SMART, X = 0.71073 A, T = 173(1) K,

26max = 1.95 to 27.49°, 11417 reflections collected, 3777 unique (Rint = 0.0181). Final

GooF = 1.117, Rl = 0.0385, wR2 = 0.1105, R indices based on 3777 reflections with I >

2sigma(I) (refinement on F ), 293 parameters. Lp and absorption corrections applied, (i =

1.637 mm"1.

Crystal data for [Na(15-crown-5)][BH4]: CioH24BNa05, M = 258.09 g/mol, colourless block, 0.275 x 0.20 x 0.20 mm3, orthorhombic space group P2(l)2(l)2(l), a = 7.8336(6)

A, b = 11.5497(9) A, c = 16.0950(13) A, a = 90°, (3 = 90°, y = 90°, V = 1698.42(16) A3,

3 Z = 4, Dc = 1.177 Mg/m , F00o = 560, Bruker AXS P4/SMART, X = 0.71073 A, T =

198(1) K , 20max = 2.17 to 27.49°, 9983 reflections collected, 3233 unique (Rint = 0.0406).

Final GooF = 1.059, Rl = 0.0293, wR2 = 0.0658, R indices based on 3233 reflections with I >2sigma(I) (refinement on F2), 250 parameters. Lp and absorption corrections applied, \i = 0.114 mm"1.

Crystal data for [Na(15-crown-5)][AlH4]: CioH24AlNa05,M = 274.26 g/mol, colourless block0.30 x 0.30 x 0.20 mm3, monoclinic space group P2(l)/c, a = 9.7904(18) A, b =

10.2580(18) A, c = 15.926(3) A, a = 90°, (3 = 97.395(3)°, y = 90°, V = 1586.2(5) A3, Z =

3 4, Dc = 1.148 Mg/m , F00o = 592, Bruker AXS P4/SMART, X = 0.71073 A, T = 173(1) K,

92 26max = 22.10 to 27.50°, 10418 reflections collected, 3527 unique (Rint = 0.0259). Final

GooF =1.099, Rl =0.0615, wR2 =0.1825, R indices based on 3527 reflections with I >

2sigma(I) (refinement on F ), 250 parameters. Lp and absorption corrections applied, u. =

0.161 mm-1.

Crystal data for [K(18-crown-6)][BH4]: Ci2H28BK06, M = 318.25 g/mol, colourless

block 0.70 x 0.10 x 0.01 mm3, orthorhombic space group P2(l)2(l)2(l), a = 8.266(3) A,

b = 12.113(5) A, c = 18.102(7) A, a = 90°, (3 = 9o°, y = 90°, V = 1812.4(13) A3, Z = 4,

3 Dc = 1.166 Mg/m , Fooo = 688, Bruker AXS P4/SMART, X = 0.71073 A, T = 198(1) K,

26max = 2.02 to 27.45°, 11846 reflections collected, 3960 unique (Rint = 0.0477). Final

GooF = 1.082, Rl = 0.0385, wR2 = 0.0540 indices based on 3960 reflections with I

>2sigma(I) (refinement on F2), 305 parameters. Lp and absorption corrections applied, \i

= 0.311 mm-1.

Crystal data for [K(THF)(18-crown-6)][AlH4]: CieHseAlKOy, M = 406.53 g/mol,

colourless block 0.45 x 0.25 x 0.20 mm3, monoclinic space group Cc, a = 14.1101(11) A,

b = 9.9824(11) A, c = 16.9575(15) A, a = 90°, |3 = 107.520(2)°, y = 90°, V = 2277.7(4)

3 3 A , Z = 4, Dc = 1.185 Mg/m , F00o = 880, Bruker AXS P4/SMART, X = 0.71073 A, T =

173(1) K, 26max = 2.52 to 27.48°, 7615 reflections collected, 4434 unique (Rint = 0.0194).

Final GooF = 1.064, Rl = 0.0314, wR2 = 0.0715, R indices based on 4434 reflections with I > 2sigma(I) (refinement on F2), 370 parameters, 2 restraints. Lp and absorption corrections applied, \x = 0.301 mm"1.

93 Crystal data for [K(THF)(18-crown-6)][GaH4]: CieF^GaKOy, M = 433.27 g/mol, colourless block 0.45 x 0.20 x 0.125 mm3, monoclinic space group Cc, a = 14.1118(13)

A, b = 9.9582(13) A, c = 16.9873(17) A, a = 90°, (3 = 107.536(2)°, y = 90°, V =

3 3 2276.3(4) A , Z = 4, Dc = 1.264 Mg/m , F00o = 920, Bruker AXS P4/SMART, X =

0.71073 A, T =198(1) K, 26max = 2.51 to 27.50°, 7494 reflections collected, 4384 unique

(Ri„t = 0.0459). Final GooF = 1.007, Rl = 0.0411, wR2 = 0.0985, R indices based on

4384 reflections with I > 2sigma(I) (refinement on F2), 226 parameters, 2 restraints. Lp and absorption corrections applied, [A = 1.415 mm"1.

Crystal data for [Na(15-azacrown-5)][BH4]: QoFfeBNNaCU, M = 257.11 g/mol, colourless block 0.475 x 0.45 x 0.25 mm3, orthorhombic space group Pnma, a =

13.736(4) A, b = 13.794(4) A, c = 7.692(2) A, a = 90°, (3 = 90°, y = 90°, V = 1457.4(7)

3 3 A , Z = 4, Dc = 1.172 Mg/m , F00o = 560, Bruker AXS P4/SMART, X = 0.71073 A, T =

173(1) K, 20max = 1.48 to 27.50°, 9910 reflections collected, 1839 unique (Rint =0.0394).

Final GooF = 1.105, Rl = 0.0327, wR2 = 0.0798, R indices based on 1839 reflections with I > 2sigma(I) (refinement on F2), 176 parameters. Lp and absorption corrections applied, n = 0.111 mm"1.

Crystal data for [K(18-azacrown-6)][BH4]: C12H29BKNO5, M = 317.27 g/mol, colourless block 0.55 x 0.50 x 0.40 mm3, rhombohedral space group R3c, a = 32.502(5)

A, b = 32.502(5) A, c = 8.6389(17) A, a = 90°, (3 = 90°, y = 120°, V = 7904(2) A3, Z =

3 18, Dc = 1.200 Mg/m , F0oo = 3096, Bruker AXS P4/SMART, X = 0.71073 A, T = 173(1)

K, 26max = 1.25 to 27.50°, 17783 reflections collected, 3796 unique (Rint = 0.0305). Final

GooF = 1.087, Rl = 0.0219, wR2 = 0.0544, R indices based on 3796 reflections with I >

94 2sigma(I) (refinement on F2), 297 parameters, 1 restraint. Lp and absorption corrections applied, ^ = 0.318 mm"1.

Crystal data for [K(THF)(18-azacrown-6)][BH4]: CieHayBKNOe, M = 389.38 g/mol, colourless block 0.40 x 0.40 x 0.30 mm3, orthorhombic space group P2(l)2(l)2(l), a =

8.1695(15) A, b = 13.874(3) A, c = 19.967(4) A, a = 90°, (3 = 90°, y = 90°, V = 2263.1(7)

3 3 A , Z = 4, Dc = 1.143 Mg/m , F00o = 848, Bruker AXS P4/SMART, X = 0.71073 A, T =

173(1) K, 29max = 1.79 to 27.50°, 15697 reflections collected, 5093 unique (Rint =0.0390).

Final GooF = 1.005, Rl =0.0531, wR2 = 0.1430, R indices based on 5093 reflections with I > 2sigma(I) (refinement on F2), 342 parameters, 2 restraints. Lp and absorption

corrections applied, JJ, = 0.261 mm" .

95 CHAPTER 4

OVERVIEW and CONCLUSIONS

The aim of the research presented in this thesis was to explore in detail the structure and bonding adopted by complexes formed between alkali metal-crown ether cations and complex Group 13 hydride anions. Alkali metal-Group 13 hydride complexes are very important and versatile reagents in organic and inorganic chemistry, and the balance of covalent and ionic bonding in such systems has an important bearing on their reactivity and selectivity. The research combined an experimental study using X-ray diffraction and

NMR spectroscopy with a theoretical quantum chemical investigation using DFT calculations and a topological analysis. During the course of this work, we managed to prepare and crystallize seven complexes with akali metal-crown ether cations and Group

13 hydride anions, viz. [Li2(12-crown-4)3][AlH4]2, [Li2(12-crown-4)3][GaH4]2 [Na(15- crown-5)][BH4], [Na(15-crown-5)][AlH4], [K(18-crown-6)][BH4], [K(THF)(18-crown-

6)][A1H4], [K(THF)(18-crown-6)][GaH4] and a further three azacrown analogues:

[Na(l 5-azacrown-5)][BH4], [K( 18-azacrown-6)][BH4], [K(THF)(18-azacrown-6)][BH4].

Six of these ten complexes were hitherto unreported.

Chapter 1 introduced several aspects of the project. It described the chemical properties of boron and aluminum hydrides as reducing agents (Section 1.1) and as ligands

(Sections 1.2 and 1.3), and the coordination chemistry of crown ethers with alkali metal and with transition metal cations (Section 1.4). Chapter 2 presented the synthesis and

96 characterization of the complexes and a comparative discussion of the structures that they adopt. Two approaches were taken to analyze trends in these structures: firstly the anion was held constant and the alkali metal-crown ether counter ion was systematically varied; then the cation was held constant and the anion changed in turn.

The structure and bonding of the complexes [Na(15-crown-5)][BH4] and [Na(15-crown-

5)][A1H4] were subjected to a DFT study and subsequent charge density analysis. This was carried out using Bader's Atoms in Molecules (AIM) theory, which is the most direct method for analyzing subtle interactions that are difficult or impossible to study through any other physical technique (Section 2.8). In an extension of the main project, the azacrown compexes [Na(15-azacrown-5)][BH4], [K(18-azacrown-6)][BH4] were also prepared and their structures compared with those of their regular crown ether analogues

(Section 2.6 and 2.7).

[Li(12-crown-4)]+ prefers to coordinate an extra crown ether in preference to the weakly basic Group 13 hydride anions, as revealed clearly in the complexes [Li2(12-crown-

4)3][A1H4J2, and [Li2(12-crown-4)3][GaH4]2. 12-crown-4 ether is a small crown ether and has been shown previously to prefer such sandwich arrangements with both alkali and alkaline earth metal cations, demonstrating how the small internal diameter of the 12- crown-4 ligand is unsuitable for inclusion of metal ions except Li+ into the macrocyclic cavity.39 [M(crown)][BH4] complexes display a tridentate coordination mode for the borohydride anion, whereas their [M(crown)][AlH4] counterparts show exclusively bidentate bonding of the aluminohydride anion to the metal cation. This difference in preferential coordination mode is reflected in the wide range of BH4" and A1H4~

97 complexes described in Sections 1.2 and 1.3, and can be rationalized by the significant

difference in size and electronegativity of B and Al. This difference in electronegativity renders the B-H bonds more covalent and less polar than their Al-H counterparts, which in turn makes the A1-H--M interactions more electrostatic than the B-H—M ones, and the smaller size of B allows three rather than two E-H moieties to approach the cationic metal centre without compromising the important M—E interaction. These conclusions

are borne out by our charge density analysis of [Na(15-crown-5)][BH4] and [Na(15-

crown-5)][AlH4], which shows the B-H bonds to be highly covalent, in contrast to the

Al-H ones, which are far more ionic. The B-H—Na and Al-H—Na component of the

bonding in these complexes is weak and ionic in character.

98 Bibliography

(1) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, Pergamon Press,

Oxford, 1984.

(2) Gaylord, N. G. Reduction with Complex Metal Hydride, Interscience Publishers, New

York, 1956.

(3) Fox, M. A.; Whitesell, J. K. Organic Chemistry, Jones and Bartlett Publishers, 2004.

(4) Wade Jr., L. G. Organic Chemistry, Pearson Education, 2003.

(5) Yoon, N. M. Pure & Appl. Chem. 1996, 68, 843.

(6) Bruice, Y. P. Organic Chemsitry; Pearson Education, 2004.

(7) Smith, J. G. Organic Chemistry, 1st Ed., McGraw-Hill Companies, 2006.

(8) Maitland, J. Organic Chemistry, W. W. Norton & Company, 2000.

(9) Graham Solomons, T. W. Organic Chemistry, John Wiley & Sons, 2004.

(10) Ephritikhine, M. Chem. Rev. 1997, 97, 2193.

(11) Marks, T. B.; Kolb, J. R. Chem. Rev. 1977, 77, 264.

(12) Braunschweig, H.; Cooling, M. C. Chem. Rev. 2001, 223, 1.

(13) Koridze, A. A.; Kuklin, S. A.; Sheloumov, M. V.; Dolgushin, F. M.; Ezernitskaya,

M. G.; Petrovskii, P. V.; Vorontsov, E. V. Russ. Chem. Bui. 2003, 52, 2759.

(14) Bau, R.; Yuan, H. S. H.; Baker, M. V.; Field, L. D. Inorg. Chim. Acta. 1986, 114,

L27.

(15) Ghilardi, C. A.; Midollini, S.; Orlandini, A. Inorg. Chem. 1982, 21, 4096.

(16) Guo, R.; Morris, R. H.; Song, D. J. Am. Chem. Soc. 2005, 127, 516.

99 (17) (a) Schlesingher, H. I.; Brown, H. C; Hyde, E. K. J. Am. Chem. Soc, 1953, 75, 209;

(b) Aldridge, S.; Blake, A. J.; Downs, A. J.; Gould, R. O.; Parsons S.; Pulham, C. R. J.

Chem. Soc. Dalton. 1997, 1007.

(18) Giese, H. H.; Habereder, T.; Noth, H.; Ponikwar, S. T.; Warchhold, M. Inorg. Chem.

1999,35,4188.

(19) Hoekstra, H. R.; Katz, J. J. J. Am. Chem. Soc. 1949, 71, 2488.

(20) Nakajima, M; Kobayashi, A.; Sasaki, Y. J. Chem. Soc, Dalton Trans. 1977, 385.

(21) Kandiah, M.; McGrady, G. S.; Decken, A.; Sirsch, P. Inorg. Chem. Commun. 2005,

44, 8650.

(22) Desrochers, P. J.; LeLievre, S.; Johnson, R. J.; Lamb, B. T.; Phelps, A. L.; Cordes,

A. Weiwei, W. G.; Cramer, S. P. Inorg. Chem. 2003, 42, 7945.

(23) Noth, H.; Schmidt, M. Organometallics 1995, 14, 4601.

(24) Guilera, G.; McGrady, G. S.; Steed, J. W.; Kaltsoyannis, N. New J. Chem. 2004, 28,

444.

(25) Hildebrandt, S. J.; Gaines, D. F.; Calabrese, J. C. Inorg. Chem. 1978, 4, 790.

(26) Goedde, D. M.; Girolami, G. S. J. Am. Chem. Soc. 2004, 126, 12230.

(27) Grebenik, P. D.; Leach, J. B.; Pounds, J. M.; Green, M. L. H.; Mountford, P. J.

Organomet. Chem. 1990, 382, CI.

(28) Bown, M.; Ingham, S. L.; Norris, G. E.; Waters, J. M. Acta Crystallogr., Sec C:

Cryst. Struct. Commun. 1995, 51, 1503.

(29) Cambridge Structural Database. Available at:

http: //www, cede. cam. ac. uk/products/csd/radii/table .php4#name

(30) Lobkovskii, E. B.; Soloveichik, G. L.; Sizov, A. I.; Bulychev, B. ML; Gusev, A. I.;

100 Kirillova,N. I. J. Organomet. Chem. 1984, 265, 161.

(31) Barrron, A. R.; Wilkinson, G. Polyhedron 1986, 5, 1897.

(32) Slater, J. A.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse, B. J. Chem. Soc. Dalton

Trans. 1986, 1731.

(33) Barron, A. R.; Wilkison, G. J. Chem. Soc. Dalton Trans. 1986, 287.

(34) Stupinski, W. A.; Huffman, J. C; Bruno, J. W.; Caulton, K. G. J. Am. Chem. Soc.

1984, 106, 8128.

(35) Barron, A. R.; Salt, J. E.; Wilkison, G. J. Chem. Soc. Dalton Trans. 1986, 1329.

(36) Girolami, G. S.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse, B. J. Am. Chem.

Soc. 1983, 105, 6752.

(37) Bel'sky, V. K.; Sizov, A. I.; Bulychev, B. M.; Soloveichik, G. L. J. Organomet.

Chem. 1985, 280, 67.

(38) Nikonov, G. I.; Kuzmina, L. G.; Howard, J. A. K. J. Chem. Soc. Dalton Trans.

2002, 3037.

(39) Barron, A. R.; Lyons, D.; Wilkison, G. J. Chem. Soc. Dalton Trans. 1986, 279

(40) Lin, W.; Wilson, S. R.; Girolami, G. S. Organometallics 1997, 16, 2987.

(41) Sizov, A. I.; Molodnitskaya, I. V.; Bulychev, B. M. J. Organomet. Chem. 1988, 344,

185.

(42) Christensen, J. J.; Eatough, D. J.; Izatt, R. M. Chem. Rev. 1974, 74, 351.

(43) Lamb, J. D.; Izatt, R. M.; Swain, C. S.; Christensen, J. J. J. Am. Chem. Soc. 1980,

102, 475.

(44) Izatt, E. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J.; Sen, D.

Chem. Rev. 1985, 85, 271.

101 (45) Steed, J. Coord. Chem. Rev. 2001, 215, 171.

(46) Huang, R. H.; Dye, J. L. J. Am. Chem. Soc. 1989, 111, 5707.

(47) Dye, J. L. Prog. Inorg. Chem. 1984, 32, 329.

(48) Huang, R. H.; Huang, S. Z.; Dye, J. L. J. Coord. Chem. 1998, 46,13.

(49) Berry, A.; Green, M. L. H.; Bandy, J. A.; Prout, K. J. Chem. Soc. Dalton. 1991,

2185.

(50) Landau, S. E.; Groh, K. E.; Lough, A. J.; Morris, R. H. Inorg. Chem. 2002, 41, 2995.

(51) Aldrige, S.; Downs, A. J. Chem. Rev. 2001, 101, 3305.

(52) Bogdanovic, B.; Schwickardi, M. J Alloys Comp. 1997, 253, 1.

(53) Grochala, W.; Edwards, P. P. Chem. Rev. 2004, 104, 1283.

(54) Bollmann, M.; Olbrich, F. Private Communication. 2004.

(55) Gorbunov, A. I.; Storozhenko, P. A.; Ivakina, L. V.; Bulychev, B. M.; Gusev, A. I.

Doklady Akademii Nauk SSSR. 1985, 285, 129.

(56) Trzaska, S.; Olbrich, F. Private Communication 2006.

(57) Trzaska, S.; Olbrich, F. Private Communication 2005.

(58) Villiers, C; Thuery, P.; Ephritikhine, M. Acta Cryst. 2006, C62, m275.

(59) Junk, P. C; Smith, M. K.; Steed, J. W. Polyhedron 2001, 20, 2979.

(60) Yoshino, M.; Komiya, K.; Takahashi, Y.; Shinzato, Y.; Yukawa, H.; Morinaga, M.

J. Alloys Compd. 2005, 404, 185.

(61) Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, T. B.; Crabtree, R.

H. J. Am. Chem. Soc. 1999, 121, 6337.

(62) Gusev, D. G.; Logh, A. J.; Morris, R. H. J. Am. Chem. Soc. 1998, 120, 13138.

(63) Landau, S. E.; Groh, K. E.; Lough, A. J.; Morris, R. H. Inorg. Chem. 2002, 41, 2995.

102 (64) Rashid, K. A.; Gusev, D. G.; Lough, A. J.; Morris, R. H. Organometallics 2000, 19,

834.

(65) Bader, R. F. W., Atoms in Molecules: A quantum theory, Clarendon, New York,

1990.

(66) Biegler-Konig, F. W.; Bader, R. F. W.; Tang, T. J. Comput. Chem. 1982, 5, 317.

(67) Frisch, M. J. et al. GAUSSIAN03, revision B.05; Gaussian Inc.; Pittsburgh, PA,

2003.

(68) Erington, R. J. Advanced Practical Inorganic and Metallorganic Chemistry, Nelson

Thornes, 1997.

(69) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J.

Organometallics 1996, 15, 1518.

(70) (a) SAINT 6.02; Bruker AXS, Inc.: Madison, WI, 1997 - 1999; (b) SADABS

Sheldrick, G., Bruker AXS, Inc.: Madison, WI, 1999; (c) SHELXTL 6.14, Bruker

AXS, Inc.: Madison, WI, 2000 - 2003.

(71) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565

103 Vitae

Candidate's full name: Lenuta Onut

Born: 7 August 1978, Hunedoara, Romania

Universities attended:

Sept 1997- July 2002 Politehnica University of Timisoara,

Faculty of Industrial Chemistry and Environmental Engineering,

Department of Organic Chemistry and Technologies,

Timisoara, TM, Romania,

B.Eng. (Honours).

Jan 2005-Present University of New Brunswick,

Department of Chemistry,

Fredericton, NB, Canada,

M.Sc. Chemistry.

Conference presentation:

L. Onut, G. S. McGrady and A. Decken. "Coordination

Complexes of Alkali Metal-Crown Ether Cations with Group

13 Hydride Anions" Maritime Inorganic Discussion Weekend,

Mount Allison University, Sackville, NB, March, 2006.

L. Onut, G. S. McGrady and A. Decken. "Coordination

Complexes of Alkali Metal-Crown Ether Cations with Group

13 Hydride Anions" 89th Canadian Chemistry Conference,

Halifax, NS, May, 2006. L. Onut, G. S. McGrady and A. Decken. "Coordination and

Supramolecular Chemistry of Alkali Metal-Crown Ether

Cations with Group 13 Hydride Anions" 90th Canadian

Chemistry Conference, Winnipeg, MB, May, 2007.