transition metal complexes and main group frustrated lewis pairs for stoichiometric ... ·...

Transition Metal Complexes and Main Group Frustrated

Lewis Pairs for Stoichiometric and Catalytic P-P and H-H

Bond Activation

by

Stephen Joseph Geier

A thesis submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Department of Chemistry

University of Toronto

© Copyright by Stephen Joseph Geier 2010

ii

Transition Metal and Main Group Frustrated Lewis Pairs for Stoichiometric and Catalytic P-P

and H-H Bond Activation

Stephen Joseph Geier

Doctor of Philosophy

Department of Chemistry

University of Toronto

2010

Abstract

Stoichiometric and catalytic small molecule activation reactions are vital for the synthesis

of new materials. The activation of phosphorus-hydrogen or phosphorus-phosphorus bonds

allows for the facile synthesis of new phosphorus-containing molecules for a wide variety of

applications.1

An investigation of the P-H dehydrocoupling reaction was undertaken utilizing two

rhodium(I) based catalysts. Over the course of this investigation it was found that the Rh(I)

systems were also active catalysts for the reverse reaction: phosphorus-phosphorus bond

hydrogenation (and hydrosilylation). This reaction was exploited for the synthesis of novel

phosphines from P-P bound species. Molecules with P-P bonds were reacted in a stoichiometric

fashion with the catalyst precursor, producing a variety of novel species with interesting bonding

features which shed some light on the reaction mechanism.

iii

Following the discovery in 2006 that a linked phosphine-borane system could reversibly

activate hydrogen2 a tremendous effort has been put forth to understand and expand this

unprecedented reactivity.3,4

This new archetype for metal-free small molecule activation,

containing a bulky Lewis acid and Lewis base which are unable to bond directly due to steric

repulsion, has been termed a “frustrated Lewis pair” (FLP).3,4

The FLP concept is expanded to include bulky P-P bound species, pyridines and P-O

bound Lewis bases as partners for B(C6F5)3. In some cases small molecule activation produced

ion pairs or zwitterions related to those found for reactions with tertiary phosphines,3,4

but in

others novel reaction pathways were discovered including phosphorus-phosphorus bond

cleavage, catalytic hydrogenations and the formation of novel intramolecular FLPs. An

unexpected situation was observed for the pair of 2,6-lutidine with B(C6F5)3, where adduct

formation was observed along with free Lewis acid and base, but H2 activation by the FLP

proceeded smoothly.

Covalently bound phosphinoboranes of the general formula R2PB(C6F5)2 are synthesized.

While systems with small R groups dimerized, monomers existed for cases with bulkier R

groups. These monomers were found to exhibit extraordinarily short phosphorus-boron bonds

yet were still capable of H2 activation analogous to bimolecular phosphine-borane systems.

These systems also showed unique reactivity with Lewis acids and Lewis bases.

This work further demonstrates the broad and general utility of the FLP concept in the

synthesis of new materials and in catalytic transformations.

iv

All synthetic and characterization work was performed by the candidate, with the

valuable assistance of others (see Acknowledgements), with the exception of Appendix A, a

significant portion of which was performed by Eva Ouyang (an undergraduate student working

under the candidate’s guidance) and portions of the work presented in section 5.3.6 which was

part of a collaboration with Jonathan Webb, Veronique Laberge and Dr. Cathleen Crudden of

Queen’s University.

Portions of this work have been previously discussed in the following publications:

Chapter 2: Geier, S.J.; Stephan, D.W. Chem. Commun. 2008, 99. Geier, S.J.; Stephan, D.W.

Chem. Commun. 2008, 2779.

Chapter 3: Geier, S.J.; Dureen, M.A.; Ouyang, E.Y.; Stephan, D.W. Chem.-Eur. J. 2010, 16, 988.

Geier, S.J.; Stephan, D.W. Chem. Commun. 2010, 1026.

Chapter 4: Geier, S.J.; Gilbert, T.M.; Stephan, D.W. J. Am. Chem. Soc. 2008, 130, 12632.

Chapter 5: Geier, S.J.; Stephan, D.W. J. Am. Chem. Soc. 2009, 131, 3476. Geier, S.J.; Gille,

A.L.; Gilbert, T.M.; Stephan, D.W. Inorg. Chem. 2009, 48, 10466. Webb, J.D.; Laberge, V.S.;

Stephan, D.W.; Crudden, C.M. Chem-Eur. J. 2010, 16, 4895. Geier, S.J.; Chase, P.A.; Stephan,

D.W. Chem. Commun. 2010, In Press. Birkmann, B.; Voss, T.; Geier, S.J.; Ullrich, M.; Kehr,

G.; Erker, G.; Stephan, D.W. Organometallics. 2010, In Press.

Appendix A: Neu, R.C.; Ouyang, E.Y.; Geier, S.J.; Zhao, X.; Ramos, A.; Stephan, D.W. Dalton

Trans. 2010, 39, 4285.

v

Acknowledgments

I would like to take this opportunity to thank the many people who have helped me in my

time at the University of Windsor and at the University of Toronto. In particular I would like to

thank Jenny McCahill, Greg Welch and Preston Chase who were invaluable mentors and friends

during my “formative years” in graduate school and also for assistance with the preparation of

this thesis. I would also like to thank Dr. Charles MacDonald, Dr. Samuel Johnson, Dr. Ulrich

Fekl and Dr. Robert Morris for serving on my committees and offering helpful advice. Next, I

would like to thank Mike Fuerth and Dr. Robert Schurko for setting up several 2D NMR

experiments and for help with the analysis of some complex NMR spectra. Special thanks to

Alan Lough for invaluable assistance with X-ray crystallography. Thanks to Eva Ouyang for her

contributions to the phosphite/phosphinite work presented in Appendix A.

I would like to thank Megan Mitton for the support throughout my graduate studies,

feigning interest in my work and particularly for keeping me ground and not letting my

frustration get the best of me.

Thanks to Steve Westcott, who deserves much of the credit (or blame) for starting me

down this path.

Finally, I would like to thank Doug Stephan for guidance, encouragement, financial

support and offering the freedom to pursue different avenues of research during my 4+ years of

graduate studies.

vi

Table of Contents

Acknowledgments ........................................................................................................................... v

Table of Contents ........................................................................................................................... vi

List of Tables ................................................................................................................................. xi

List of Figures .............................................................................................................................. xiii

List of Abbreviations, Nomenclature and Symbols ................................................................ xix

Chapter 1: Introduction ................................................................................................................... 1

1.1: Phosphines in Coordination and Materials Chemistry ........................................................ 1

1.2: P-P Bond Activation ........................................................................................................... 3

1.3: Lewis Acid/Lewis Base Chemistry ..................................................................................... 3

1.4: Frustrated Lewis Pairs: Reactivity ...................................................................................... 5

1.4.1: Nucleophilic Aromatic Substitution ......................................................................... 5

1.4.2: THF Ring-Opening ................................................................................................... 6

1.4.3: H2 Activation by FLPs .............................................................................................. 6

1.4.4: Addition to Alkenes and Alkynes ............................................................................. 8

1.4.5: Other Small Molecules ........................................................................................... 10

1.4.6: Catalytic Hydrogenations ....................................................................................... 10

Chapter 2: Stoichiometric and Catalytic P-P Bond Activation by Rh(I) Complexes ................... 12

2.1: Introduction ....................................................................................................................... 12

2.2: Experimental ..................................................................................................................... 15

2.2.1: General Considerations ........................................................................................... 15

2.2.2: Synthesis of Rh(Ph2PPPh2) Complexes .................................................................. 15

2.2.3: General Catalytic Procedures .................................................................................. 16

2.2.4: Characterization of Related Species ....................................................................... 17

2.2.5: Synthesis of RhNacNac(P5R5) Complexes ............................................................. 18

vii

2.2.6: X-Ray Data Collection, Reduction, Solution and Refinement ............................... 19

2.3: Results and Discussion ...................................................................................................... 23

2.3.1: Catalyst Selection and Initial Screening ................................................................. 23

2.3.2: Stoichiometric Reactions of Catalyst Precursors with Ph2PH and Ph2PPPh2 ......... 24

2.3.3: Catalytic Hydrogenation and Hydrosilylation Reactions of Ph2PPPh2 .................. 26

2.3.4: Mechanistic Insight into the Catalytic Activation of P-P Bonds ............................ 28

2.3.5: Heterodehydrocoupling of Silanes with Diphenylphosphine ................................. 31

2.3.6: Reactions of Catalyst Precursors with Additional Phosphines and Biphosphines . 31

2.3.7: Reactions with Cyclic Polyphosphines ................................................................... 33

2.4: Conclusions ................................................................................................................ 39

Chapter 3: Frustrated Lewis Pair Reactivity of Bulky Catena-Polyphosphines with B(C6F5)3 ... 40

3.1: Introduction ....................................................................................................................... 40

3.2: Experimental ..................................................................................................................... 43


3.2.2: Generation of a Phosphonium Borate Zwitterion through Nucleophilic

Aromatic Substitution ........................................................................................... 43

3.2.3: Synthesis of Alkenyl-bridged Phosphonium Borate Zwitterions via Activation

of Terminal Alkynes ............................................................................................. 44

3.2.4: Synthesis of a Phosphonium Borate Ion Pair via H2 Activation ............................. 45

3.2.5: Hydrogenation and Hydrosilylation of P5Ph5 ......................................................... 45

3.2.6: X-Ray Data Collection, Reduction, Solution and Refinement .............................. 48


3.3.1: Stoichiometric Reactions of Bulky Polyphosphines with B(C6F5)3 ....................... 51

3.3.2: Nucleophilic Aromatic Substitution (NAS) Reactions ........................................... 51

3.3.3: Synthesis of Alkenyl-Bridged Phosphonium Borate Zwitterions by Terminal

Alkyne Activation ................................................................................................. 54

3.3.4: Activation of H-H and Si-H Bonds by Polyphosphosphine/Borane FLPs ............. 56

viii

3.3.5: Scope of Reactivity in Terms of Bulk at Silicon and Lewis Acidity of the

Borane ................................................................................................................... 62

3.4: Conclusions ....................................................................................................................... 63

Chapter 4: Frustrated Lewis Pairs: Synthesis and Reactivity of Covalently-Bound

Phosphinoboranes .................................................................................................................... 64

4.1: Introduction ....................................................................................................................... 64

4.2: Experimental ..................................................................................................................... 66


4.2.2: Synthesis of Phosphinoboranes R2PB(C6F5)2 ......................................................... 66

4.2.3: Synthesis of Secondary Phosphine Adducts of HB(C6F5)2 .................................... 68

4.2.4: Reactions of R2PB(C6F5)2 with 4-tert-butylpyridine .............................................. 71

4.2.5: Synthesis of Dimers (R2PBCl2)2 and ClB(C6F5)2 by Reaction of BCl3 with

R2PB(C6F5)2 .......................................................................................................... 72

4.2.6: X-Ray Data Collection, Reduction, Solution and Refinement ............................... 72


4.3.1: Synthesis and Characterization of Phosphinoboranes R2PB(C6F5)2 ....................... 77

4.3.2: Reactions of Phosphinoboranes with H2 and Independent Synthesis of

Phosphine-Borane Adducts R2(H)PB(H)C6F5)2 ................................................... 82

4.3.3: Reactions of Phosphinoboranes with Lewis Acids and Lewis Bases ..................... 86

4.4: Conclusions ....................................................................................................................... 92

Chapter 5: Frustrated Lewis Pairs: Reactions of Pyridines and Other Nitrogen-Containing

Heterocycles with B(C6F5)3 ...................................................................................................... 93

5.1: Introduction ....................................................................................................................... 93

5.2: Experimental Section ........................................................................................................ 95


5.2.2: Synthesis of Pyridine-B(C6F5)3 adducts: ................................................................ 95

5.2.3 Synthesis of Pyridinium Borate Ion Pairs through H2 Activation by Pyridine-

Borane FLPs .......................................................................................................... 99

5.2.4: Synthesis of a Pyridinium Borate Zwitterion via THF Ring-Opening ................. 101

ix

5.2.5: Reactions of Diethyl-2,6-dimethyl-3,5-pyridinedicarboxylate with B(C6F5)3 ..... 101

5.2.6: Reactions of Bulky Substituted Quinolines with B(C6F5)3 ................................... 102

5.2.7: Metal-Free Catalytic Hydrogenations ................................................................... 103

5.2.8: Reaction of Aminopyridines with Fluoroarylboranes .......................................... 103

5.2.9: X-Ray Data Collection, Reduction, Solution and Refinement ............................. 107

5.3: Results and Discussion .................................................................................................... 114

5.3.1: Reactions of Alkyl or Aryl-Substituted Pyridines with B(C6F5)3 ......................... 114

5.3.2: Frustrated Lewis Pairs of Pyridines with B(C6F5)3 ............................................... 117

5.3.4: Small Molecule Activation by Pyridine-Borane FLPs ......................................... 119

5.3.5: FLPs of Bulky Pyridines with Other Lewis Acids ............................................... 122

5.3.6: Reaction of Diethyl-2,6-dimethyl-3,5-pyridinedicarboxylate with B(C6F5)3 ....... 123

5.3.7: Reactions of Substituted Quinolines with B(C6F5)3 ............................................. 125

5.3.8: Reactions of 2-Aminopyridines with Fluoroarylboranes ...................................... 129

5.4: Conclusions ..................................................................................................................... 134

Chapter 6: Summary and Conclusions ........................................................................................ 136

Appendix A: Frustrated Lewis Pairs Derived From P(OR)nR3-n and B(C6F5)3 .......................... 138

A.1: Introduction .................................................................................................................... 138

A.2: Experimental .................................................................................................................. 139

A.2.1: General Considerations ........................................................................................ 139

A.2.2: Lewis Acid-Base Adducts of Phosphites with B(C6F5)3 ...................................... 139

A.2.3: Synthesis of Phosphinites tBu2POR ..................................................................... 140

A.2.4: Generation of a Phosphine-Oxide Adduct of B(C6F5)3 ........................................ 141

A.2.5: Generation of Phosphonium Borate Ion Pairs by H2 Activation ......................... 141

A.2.6: X-Ray Data Collection, Reduction, Solution and Refinement ............................ 142

A.3: Results and Discussion ................................................................................................... 144

A.3.1: Reactions of Phosphites with B(C6F5)3 ................................................................ 144

x

A.3.2: Reactions of RnP(OtBu)3-n with B(C6F5)3 ............................................................ 147

A.3.3: FLP Reactions of tBu2POAr with B(C6F5)3 ......................................................... 149

A.4: Conclusions .................................................................................................................... 151

References ................................................................................................................................... 152

xi

List of Tables

Table 2.1: Selected crystallographic data for compounds 2-3, 2-4 and 2-11 ............................... 21

Table 2.2: Selected crystallographic data for compounds 2-12, 2-13 and 2-14 ........................... 22

Table 2.3: Silylation of Ph2PPPh2 using 10 mol% 2-1 at 100°C in toluene ................................. 28

Table 2.4: Results of heterodehydrocoupling reactions of Ph2PH with silanes using 5 mol% 2-1

at 50°C in toluene.......................................................................................................................... 29

Table 3.1: Selected crystallographic data for compounds 3-1, 3-2, 3-3 and 3-9 .......................... 50

Table 3.2: Selected NMR spectroscopic data and yields for adducts 3-6 – 3-11 ......................... 60

Table 4.1: Selected crystallographic data for compounds 4-1, 4-2 and 4-3 ................................. 73

Table 4.2: Selected crystallographic data for compounds 4-4, 4-6 and 4-7 ................................. 74

Table 4.3: Selected crystallographic data for compounds 4-8, 4-12 and 4-13 ............................. 75

Table 4.4: Selected crystallographic data for compound 4-14 ..................................................... 76

Table 4.5: Selected NMR spectroscopic data for compounds 4-1 to 4-5 ..................................... 80

Table 4.6: Selected metrical parameters for monomeric phosphinoboranes ................................ 82

Table 4.7: Hydrogenation of phosphinoboranes 4-3 and 4-4 ....................................................... 83

Table 4.8: Selected NMR spectroscopic data for phosphine-borane adducts 4-6 to 4-10 ............ 84




Table 5.4: Selected crystallographic data for compounds 5-13, 5-14 and 5-19 ......................... 111

xii

Table 5.5: Selected crystallographic data for compounds 5-25, 5-26 and 5-27 ......................... 112

Table 5.6: Selected crystallographic data for compounds 5-29 and 5-30 ................................... 113

Table 5.7: Selected NMR spectroscopic and X-ray crystallographic data obtained for pyridine-

borane adducts 5-1 to 5-8 ............................................................................................................ 116

Table 5.8: Catalytic hydrogenation of quinolines (reactions were conducted under 4 atm H2 in

toluene in a sealed Teflon-capped Schlenk bomb) ..................................................................... 128

Table A.1: Selected crystallographic data for compounds A-1, A-6 and A-7 ............................ 143

Table A.2: Comparison of Me3P and (MeO)3P and their B(C6F5)3 adducts ............................... 145

Table A.3: Cone angles and basicities of phosphorus bases and reactivity with B(C6F5)3 ........ 146

xiii

List of Figures

Figure 1.1: Wilkinson’s Catalyst (left) and Grubbs’ Catalyst (right) ............................................. 1

Figure 1.2: Generation of the catalytically active species from Wilkinson’s catalyst (R=Ph) ....... 1

Figure 1.3: Formation of metallacyclobutane intermediate in olefin metathesis ............................ 2

Figure 1.4: Examples of phosphorus-containing polymers: polyphosphazenes (left) and

polyphosphinoboranes (right). ........................................................................................................ 2

Figure 1.5: Catalytic dehydrocoupling of PhPH2 to generate P5Ph5 ............................................... 2

Figure 1.6: Formation of a Lewis acid-base adduct (B(C6F5)3 – Lewis acid, PMe3 – Lewis base) 3

Figure 1.7: Rankings of pyridine basicity (top, pKa’s in nitrobenzene) and nucleophilicity

(bottom) ........................................................................................................................................... 5

Figure 1.8: Resonance forms of B(C6F5)3 and resulting para-NAS by a phosphine ...................... 6

Figure 1.9: Para-NAS by a bulky phosphine on trityl cation ([B(C6F5)4]- is the counterion) ........ 6

Figure 1.10: THF ring-opening by a phosphine-borane pair .......................................................... 6

Figure 1.11: Reversible H2 activation by a linked phosphine-borane ............................................ 7

Figure 1.12: H2 activation mechanisms: Top: Papai and co-workers; Bottom: Grimme and co-

workers ............................................................................................................................................ 8

Figure 1.13: Alkene activation by a frustrated Lewis pair .............................................................. 9

Figure 1.14: Terminal alkyne activation by frustrated Lewis pairs (left-deprotonation, right-

addition) .......................................................................................................................................... 9

Figure 1.15: Addition of a linked phosphine-borane to norbornene ............................................... 9

Figure 1.16: Activation of N2O and CO2 by FLPs ........................................................................ 10

xiv

Figure 1.17: Imine hydrogenation catalyzed by B(C6F5)3 ............................................................ 11

Figure 2.1: Scheme for homonuclear catalytic dehydrocoupling reactions .................................. 12

Figure 2.2: Mechanism for the catalytic dehydrocoupling of HPPh2 by Rh(I)-based catalysts.

Left: Cp*Rh (Brookhart and Bohm), right: (dippe)Rh (Tilley and Han) ..................................... 13

Figure 2.3: Rh(I) Catalyst Precursors 2-1: RhNacNac(COE)(N2) (Ar=2,6-di-iso-propylphenyl)

and 2-2: (n5-C9H7)Rh(COE)2 ........................................................................................................ 23

Figure 2.4: Equilibria involving mono-, bis- and tris-diphenylphosphine-rhodium complexes

(L=NacNac or n5-C9H7) ................................................................................................................ 24

Figure 2.5: POV-Ray depictions of 2-3 (left) and 2-4 (right) ....................................................... 25

Figure 2.6: Formation of Rh-phosphide dimer 2-3 ....................................................................... 26

Figure 2.7: Rh-catalyzed hydrosilylation of P2Ph4. ...................................................................... 28

Figure 2.8: Structure of 2-10 ......................................................................................................... 30

Figure 2.9: Proposed catalytic cycles for hydrogenation (left) and hydrosilylation (right) of

Ph2PPPh2 ....................................................................................................................................... 30

Figure 2.10: Proposed mechanism for dehydrocoupling of Ph2PH with silanes .......................... 31

Figure 2.11: POV-Ray depiction of 2-11 (left) and 2-12 (right) .................................................. 33

Figure 2.12: Cyclic Polyphosphines: A) P4R4 (R=Cy, tBu) B) P5R5 (R=Ph, Et) ........................ 34

Figure 2.13: 31

P NMR Spectrum (top, left) and POV-Ray depiction of 2-13-0.5 C6H14 ............. 35

Figure 2.14: 31

P NMR Resonances (top) and POV-Ray depiction (bottom) of 2-14 ................... 38

Figure 3.1: Nucleophilic aromatic substitution (NAS) at the para-position of a C6F5 ring by

phosphines on B(C6F5)3 ................................................................................................................ 40

Figure 3.2: Activation of alkenes (left) and alkynes (right) by a frustrated Lewis pair: PR3 +

B(C6F5)3 ........................................................................................................................................ 41

xv

Figure 3.3: Multinuclear NMR Spectra for 3-1 in CD2Cl2: A: 31

P (resonance for cationic

phosphorus centre), B: 31

P (other 4 phosphorus resonances), C: 19

F, D: 11

B ............................... 52

Figure 3.4: POV-Ray depiction of 3-1-C6H6 ................................................................................ 53

Figure 3.5: Formation of alkyne addition products 3-2 and 3-3 ................................................... 54

Figure 3.6: 31

P{1H} NMR spectrum (left) and POV-Ray depiction of 3-2 (right) ....................... 55

Figure 3.7: POV-Ray depiction of 3-3 .......................................................................................... 56

Figure 3.8: Formation of the phosphonium borate ion pair 3-4 .................................................... 56

Figure 3.9: Multinuclear NMR spectra for 3-4 in CD2Cl2: 1H – showing PH and BH peaks (top

left), 11

B (top right), 19

F (bottom left), 31

P[1H} (bottom right) ..................................................... 57

Figure 3.10: Formation of 3-5 from the reaction of P5Ph5 with B(C6F5)3 and H2. ....................... 58

Figure 3.11: Nucleophilic attack by PMe3 on the cationic phosphorus centre of P4Cy4Me+ ....... 58

Figure 3.12: Hydrogen activation by the frustrated Lewis pair P5Ph5/B(C6F5)3 and subsequent

rearrangement to form (Ph)H2P-B(C6F5)3 ..................................................................................... 59

Figure 3.13: Mechanism for B(C6F5)3 catalyzed hydrosilylation of imines ................................. 59

Figure 3.14: Lewis acid promoted hydrosilylation of P5Ph5. ........................................................ 60

Figure 3.15: POV-Ray depiction of 3-9 ........................................................................................ 61

Figure 3.16: Synthesis of 3-11 ...................................................................................................... 62

Figure 4.1: Dehydrogenation of ammonia borane by a nickel(0)carbene catalyst ....................... 64

Figure 4.2: Potential Reactivity of R2PB(C6F5)2: dimerization (top), H2 activation (bottom) ..... 65

Figure 4.3: Synthesis of 4-1 to 4-5 (LiCl is removed upon workup) ............................................ 77

Figure 4.4: POV-Ray depictions of phosphinoborane dimers 4-1 (left) and 4-2–CH2Cl2 (right).

....................................................................................................................................................... 78

xvi

Figure 4.5: Multinuclear NMR spectra for 4-3. A: 1H, B:

11B, C:

19F, D:

31P{

1H} ..................... 79

Figure 4.6: POV-Ray depictions of phosphinoboranes 4-3 (left) and 4-4 (right) ......................... 81

Figure 4.7: Resonance forms of phosphinoboranes 4-3 to 4-5 ..................................................... 82

Figure 4.8: Synthesis of 4-8 to 4-10 through H2 activation (left) or Lewis acid-base adduct

formation (right) ............................................................................................................................ 83

Figure 4.9: POV-Ray depictions of 4-6 and 4-7 ........................................................................... 85


Figure 4.11: Newman projection along the B-P bond of 4-6 (R-Et) and 4-7 (R=Ph) (left); and 4-8

(right) as determined by X-ray crystallography ............................................................................ 86

Figure 4.12: Formation of adducts 4-11 and 4-12 ........................................................................ 87

Figure 4.13: POV-Ray depiction of 4-12-C7H8 ............................................................................ 87

Figure 4.14: POV-Ray depiction of 4-13 ...................................................................................... 89

Figure 4.15: Proposed formation of 4-13 from 4-11 (L=4-(tBu)C5H4N) ...................................... 90

Figure 4.16: Formation of dimers 4-14 and 4-15 .......................................................................... 90


Figure 5.1: Brown’s observation of a surprising lack of reactivity between 2,6-lutidine and BMe3

....................................................................................................................................................... 93

Figure 5.2: Amine-based reducing agents: Hantzsch’s Ester (A), acridan (B) and

tetrahydroquinoline (C) ................................................................................................................. 94

Figure 5.3: Proposed scheme for catalytic transfer hydrogenation through Hantzsch’s ester. ..... 94

Figure 5.4: Synthesis of Lewis acid-base adducts 5-2 – 5-7; R=Me (5-2), Et (5-3), R=N(H)(2-

C5H4N) (5-4), Ph (5-5), 2-C5H4N (5-6) ...................................................................................... 114

xvii

Figure 5.5: POV-Ray Depictions of 5-3 (left) and 5-4 (right) .................................................... 115

Figure 5.6: POV-Ray Depictions of 5-5-0.5 C7H8 (left) and 5-7 (right) .................................... 115

Figure 5.7: Equilibrium observed in solution between FLP and Lewis acid-base adduct 5-8 ... 117


Figure 5.9: Synthesis of ion pairs 5-9 (R=R1=Me), 5-10 (R=

tBu, R

1=H) and 5-11 (R=R

1=Ph) 119

Figure 5.10: Multinuclear NMR spectra for 5-9 in CD2Cl2: A: 1H, B:

11B and C:

19F. .............. 120


Figure 5.12: H2 loss from ion pairs 5-9 – 5-11 ........................................................................... 121

Figure 5.13: Formation of zwitterion 5-13 by THF ring-opening .............................................. 121

Figure 5.14: POV-Ray depiction of 5-13 .................................................................................... 122


Figure 5.16: Partial hydrogenation of adduct 5-14 ..................................................................... 125

Figure 5.17: Generation of 5-15 and 5-16 by hydride abstraction from Hantzsch’s Ester. ........ 125

Figure 5.18: Equilibria involving the formation of adducts 5-17 and 5-18 ................................ 126

Figure 5.19: POV-Ray depiction 5-19 (one of two crystallographically independent molecules)

..................................................................................................................................................... 127

Figure 5.20: Formation of zwitterion 5-25 ................................................................................. 129

Figure 5.21: POV-Ray depictions of 5-25 and 5-26 (one of two crystallographically independent

molecules) ................................................................................................................................... 130


Figure 5.23: Synthesis of linked pyridine-borane 5-28 .............................................................. 131

xviii

Figure 5.24: Multinuclear NMR spectra for 5-28 in CDCl3. A: 1H, B:

11B, C:

19F ................... 132

Figure 5.25: POV-Ray depictions of 5-29 and 5-30 ................................................................... 133

Figure 5.26: Formation of linked pyridine-borane 5-31 ............................................................. 134

Figure 5.27: Formation of Lewis acid-base adducts 5-32, 5-33 and 5-34 .................................. 134

Figure A.1: Formation of phosphite-borane adducts A-1 and A-2 ............................................. 144

Figure A.2: POV-Ray depiction of A-1 ...................................................................................... 144

Figure A.3: Phosphite-based FLPs and attempted hydrogen activation ..................................... 145


Figure A.5: 1H NMR spectrum showing formation of iso-butene, along with adduct A-9 ........ 148

Figure A.6: Proposed mechanism for the formation of A-9 and iso-butene from reaction of A-3

with B(C6F5)3 .............................................................................................................................. 149

Figure A.7: FLP reactivity of phosphinites A-5 and A-6 with B(C6F5)3 .................................... 149

Figure A.8: Multinuclear NMR spectra for A-7 in CD2Cl2. A: 1H, B:

11B, C:

19F and D:

31P ... 150


xix

List of Abbreviations, Nomenclature and Symbols

Å Angstrom

atm atmospheres

br m broad multiplet

Bu n-butyl (C4H9)

calcd calculated

CCD charge coupled device

COD cyclooctadiene (C8H12)

COE cis-cyclooctene (C8H14)

Cp cyclopentadienyl (C5H5)

Cp* pentamethylcyclopentadienyl (C5(CH3)5)

Cy cyclohexyl (C6H11)

C degrees Celsius

Dcalc calculated density

d doublet

dippe 1,2-bis-(di-iso-propylphosphino)ethane

eq equivalents

Et ethyl (C2H5)

FLP frustrated Lewis pair

xx

g grams

GOF goodness of fit

HOMO Highest Occupied Molecular Orbital

hr hour

Hz Hertz

iPr iso-propyl (CH(CH3)2)

IR infrared

J coupling constant

K degrees Kelvin

kcal kilocalories

kJ kilojoules

L liter

LUMO Lowest Unoccupied Molecular Orbital

m multiplet

M mol L-1

m meta

Me methyl (CH3)

Mes mesityl (2,4,6-(CH3)3C6H2)

mg milligram

MHz megahertz

xxi

min minute

mL milliliter

mmol millimole

NacNac HC{CN(2,6-iPr2C6H3)}2

NAS nucleophilic aromatic substitution

NMR nuclear magnetic resonance

o ortho

ORTEP Oak Ridge thermal ellipsoid plot

p para

Ph phenyl (C6H5)

POV-Ray Persistence of Vision Raytracer

ppm parts per million

R residual

Rw weighted residual

RT room temperature

s singlet, seconds

sept. septet

t triplet

tBu tert-butyl (C(CH3)3)

THF tetrahydrofuran (C4H8O)

xxii

TMS trimethylsilyl (Si(CH3)3)

wt % weight percent

μmol micromole

1

Chapter 1: Introduction

1.1: Phosphines in Coordination and Materials Chemistry

Over the past century phosphines have become ubiquitous as ligands in inorganic

chemistry, owing to their tunability and their strong and generally predictable coordination

mode.5 Some of the most commercially important transition metal catalysts, including

Wilkinson’s Catalyst and Grubbs’ Catalyst, incorporate phosphines as ligands (Figure 1.1).5

Figure 1.1: Wilkinson’s Catalyst (left) and Grubbs’ Catalyst (right)

The tunability of phosphines is key to the mechanism of activation for both of these

species. One of the most important mechanisms for the hydrogenation of olefins utilizing

Wilkinson’s catalyst involves initial oxidative addition of dihydrogen (H2). The oxidative

addition of H2 to the rhodium centre generates an octahedral species with a phosphine ligand

trans to a hydride. As a result of the strong trans-effect of the hydride ligand, this phosphine can

dissociate from the metal centre, allowing for the coordination of olefin (Figure 1.2) and

subsequent hydrogenation.6 The PEt3 analogue of Wilkinson’s catalyst was found to be inactive

for the hydrogenation catalysis but the octahedral Rh(III) dihydride species could be isolated.5

The smaller size and stronger electron-donating ability of PEt3 compared to PPh3 makes

dissociation of the phosphine much less favourable. These results illustrate the advantage of

being able to easily tune the steric and electronic properties of phosphine ligands.

Figure 1.2: Generation of the catalytically active species from Wilkinson’s catalyst (R=Ph)

2

Phosphine dissociation is also a vital step for the activation of Grubbs’ Catalyst. The size

and strong trans-effect of the PCy3 groups results in facile loss of one phosphine substituent

upon coordination of the olefin, generating the metallacyclobutane intermediate (Figure 1.3).7

Again, the easy tunability of phosphines allowed for selection of a ligand of appropriate size and

electron-donating ability.

Figure 1.3: Formation of metallacyclobutane intermediate in olefin metathesis.7

Recently there has been much work on the incorporation of phosphorus into polymeric

materials, due to their ability to render materials resistant to fire, their optical electronics

properties and their ability to provide Lewis basic sites for the potential coordination of transition

metals (Figure 1.4).8,9

One approach to synthesizing new materials containing phosphorus is the

dehydrocoupling of P-H bonds, either with other P-H bonds (homodehydrocoupling), or with

other E-H bonds (heterodehydrocoupling).10

Figure 1.4: Examples of phosphorus-containing polymers: polyphosphazenes11

(left) and

polyphosphinoboranes10

(right).

The homodehydrocoupling reaction has been used to create P-P bound monomers and

macrocycles:12-14

however, long reaction times and harsh conditions are required to attain

reasonable yields (an example is shown in Figure 1.5).

Figure 1.5: Catalytic dehydrocoupling of PhPH2 to generate P5Ph512

3

1.2: P-P Bond Activation

The hydrogenation of P-P bonds, the reverse reaction of P-P dehydrocoupling, is an

important process, as current industrial synthesis of organophosphines involves the reaction of

white phosphorus, P4, with chlorine gas to form PCl3, which is subsequently reacted further to

yield organophosphines.15

Circumventing the use of chlorine gas is highly desirable on health,

environmental and economic grounds. 16-26

There has been much work done on the activation of white phosphorus, P4, involving

stoichiometric use of carbenes,25,26

or transition metals18,20

with the primary goal of fragmenting

P4 cleanly into P1 units thus avoiding the use of chlorine gas in the synthesis of

organophosphines.

While other P-P bound species, such as P5Ph5, P4Cy4 and P2Ph4, are well known in the

literature,27

little work has been dedicated to the possibility of controlled fragmentation of these

molecules to P1 units. Such pathways may allow for the synthesis of previously inaccessible

molecules while also providing the opportunity to discover new modes of reactivity which may

be useful in reactions of P4.

1.3: Lewis Acid/Lewis Base Chemistry

In addition to their ability to coordinate to transition metals, phosphines (and other

molecules with pairs of available valence electrons) have long been known to coordinate to main

group electron acceptors. The resulting compounds, referred to as Lewis adducts or donor-

acceptor adducts, were first explained by G.N. Lewis in 1923.28

Lewis defined acids as

compounds which can accept a pair of electrons and bases as compounds which can donate

electrons. Thus, when these compounds are added to one another, the Lewis base can donate a

pair of electrons to the Lewis acid, forming a new dative bond between the molecules, as shown

in Figure 1.6.

Figure 1.6: Formation of a Lewis acid-base adduct (B(C6F5)3 – Lewis acid, PMe3 – Lewis base)29

4

Owing to its strong Lewis acidity and relatively strong B-C bonds, B(C6F5)3 is a very

commonly used species for Lewis acid catalyzed organic transformations and is used as a co-

catalyst for α-olefin polymerization.30,31

Halogenated boranes such as BF3 and BCl3 exhibit

comparable Lewis acidity to B(C6F5)3, however, they suffer from instability due to redistribution

of the halides and loss of HX when reacted with protic acids, while trialkyl boranes are also

unstable and are significantly less Lewis acidic than B(C6F5)3.30

B(C6F5)3 is also known to form

adducts with a wide variety of Lewis bases including phosphines, amines, imines, pyridines,

nitriles, aldehydes, ketones and esters. 30,31

Recently, the Stephan group has focused on systems which have been termed “frustrated

Lewis pairs” (FLPs).3,4

A FLP is defined as a Lewis acid and base pair which does not form a

Lewis acid-base adduct due to steric conflict. While this chemistry has only recently garnered

much attention in the literature, the first reported observation of a frustrated Lewis pair dates

back to 1942 when H.C. Brown and co-workers noted that 2,6-lutidine failed to form a Lewis

adduct with BMe3.32

In addition to this observation, Brown cited a number of results in reactions

of Lewis acids and Lewis bases which proceeded not to the electronically favoured product, but

to the sterically favoured product. Many of these results were compiled in an article titled

“Chemical Effects of Steric Strains.”33

For example, Brown noted that while the basicity of a

series pyridines increased from pyridine to 2-methylpyridine to 2-tert-butylpyridine to 2,6-

lutidine, the series was nearly reversed in terms of nucleophilicity (pyridine>2-

methylpyridine>2,6-lutidine>2-tert-butylpyridine). This trend was attributed to the steric effects

of the substituents of the pyridines (Figure 1.7).

5

Figure 1.7: Rankings of pyridine basicity (top, pKa’s in nitrobenzene) and nucleophilicity

(bottom) 33

While Brown did observe a lack of interaction between Lewis acid and Lewis base in

solution, the potential reactivity of these systems was not explored. However, these FLPs seen

by Brown include weak Lewis acids that may not have been capable of the small molecule

activation reactions recently discovered with FLPs, pioneered by the Stephan group.

1.4: Frustrated Lewis Pairs: Reactivity

1.4.1: Nucleophilic Aromatic Substitution

Among the first alternate reactivity attributed to FLPs is the para-nucleophilic aromatic

substitution (NAS) by large Lewis bases on aromatic rings of the Lewis acids B(C6F5)3 and

[CPh3]+.2,34-38

This reaction occurs due to steric bulk preventing the reaction of the Lewis base

with the most Lewis acidic centre (boron for B(C6F5)3 and the central cationic carbon for

[CPh3]+). For the smallest phosphines, reaction with B(C6F5)3 results in adduct formation, while

no reaction takes place for the largest phosphines. The reaction of some intermediate-sized

phosphines (PCy3 for example) with B(C6F5)3 occurs at an alternate Lewis acidic centre in the

molecule: the para-carbon.36

While these phosphines are too bulky to interact directly with the

boron centre, they are still nucleophilic enough to attack the molecule at a more accessible site.

Resonance forms for these electrophiles show that both ortho- and para-carbons of the aromatic

rings are also electron-deficient. As the para-carbons are less sterically hindered, substitution

occurs at this site (Figure 1.8).

6

Figure 1.8: Resonance forms of B(C6F5)3 and resulting para-NAS by a phosphine

In the case of B(C6F5)3, fluoride transfer to boron is always observed, while for trityl

cation both the intermediate cyclohexadienyl compound (Figure 1.9 A) and the final para-NAS

(Figure 1.9 B) product can be isolated.35

Figure 1.9: Para-NAS by a bulky phosphine on trityl cation ([B(C6F5)4]- is the counterion)

1.4.2: THF Ring-Opening

While the THF adduct of B(C6F5)3 is well-known, the bound THF molecule is generally

known to be displaced by a stronger base.30

When the stronger base is too sterically encumbered

to form a traditional adduct with B(C6F5)3, nucleophilic ring-opening of THF may occur.39,40

Coordination of the oxygen of THF to B(C6F5)3 renders the α–carbons electrophilic and

susceptible to nucleophilic attack by the Lewis base (Figure 1.10).

Figure 1.10: THF ring-opening by a phosphine-borane pair

1.4.3: H2 Activation by FLPs

FLP chemistry took a giant leap when a report by the Stephan group showed that a linked

phosphine-borane system could reversibly activate H2 (Figure 1.11),2 a reaction which is

common for transition metals but virtually unprecedented for a main group system.41,42

The

7

reaction proceeds via heterolytic cleavage of the dihydrogen molecule, with H+ going to the

Lewis basic phosphorus centre and H- to the Lewis acidic boron centre.

Figure 1.11: Reversible H2 activation by a linked phosphine-borane

Subsequent studies showed that simple bimolecular phosphine-borane Lewis pairs such

as tBu3P or Mes3P and B(C6F5)3 were capable of activating H2.

43 The Stephan group and others

have since shown that pairs of carbenes,44,45

imines46

or amines46,47

with B(C6F5)3 are capable of

activating H2. While several other Lewis acids have been utilized, the scope for hydrogen

activation seems to be limited to B(C6F5)3 and related fluoroaryl boranes3,4,48-50

(while BPh3 was

shown to activate H2 with tBu3P, poor yields were obtained).

38 Interestingly, neither Lewis acid

nor Lewis base alone were found to react with H2 in solution. Several other related linked

systems have also emerged in literature, containing a bulky Lewis basic centre and a 3-

coordinate fluoroaryl boron centre which are unable to coordinate to one another due to steric

factors.51,52

A detailed kinetic study of the hydrogen activation reaction has proven to be very

difficult due to a lack of control over the concentration of H2 in solution. Studies of the reverse

reaction (hydrogen loss) have been hampered by the more favourable nature of the H2 activation

reaction, leading to inconsistent and unclear results.53

The activation of H2 by FLPs has undergone several computational studies. As

termolecular reactions are entropically very unlikely to occur, two of the molecules (Lewis acid,

Lewis base and H2) must interact prior to reaction with the third component. Interaction of

borane or phosphine with H2 has computational precedent in the literature: both BH354-57

and

phosphines58,59

have been calculated to interact independently with H2. While either of these

intermediates seem reasonable, recent computational studies on FLPs suggest that it is the Lewis

acid and base that come together prior to activation of H2.

8

In theoretical studies, Papai and co-workers suggested that the borane and phosphine

form an “encounter complex” where the 2 molecules are held together by secondary interactions

(for example weak hydrogen bonding between alkyl groups and fluorine atoms).60-62

H2 is then

proposed to enter the space between the phosphorus and boron centres and is subsequently

cleaved by interaction of the phosphorus lone pair with the LUMO of H2 and of the vacant p-

orbital at boron with the HOMO of H2 (Figure 1.12, top). Similar transition states were proposed

for other related systems involving a carbene/B(C6F5)3 pair45

and amine-borane FLPs.47,52

Grimme and co-workers have since proposed that the proximity of the Lewis acid and

Lewis base creates an electric field which cleaves H2 in a nonlinear fashion, with B-H bond

formation slightly preceding P-H bond formation.63

This mechanism suggests that a molecular

orbital argument in terms of the FLP is not necessary to explain the H2 activation reaction, but

the mere presence of Lewis acid and base creates an electric field of sufficient strength to cleave

the H-H bond (Figure 1.12, bottom). Previous theoretical work has also suggested that H2

activation by an electric field is indeed possible.64-66

Figure 1.12: H2 activation mechanisms: Top: Papai and co-workers;60-62

Bottom: Grimme and

co-workers63

1.4.4: Addition to Alkenes and Alkynes

Concurrent with the discovery of the H2 activation reaction, work in the Stephan lab

showed that FLPs could also add to alkenes, creating alkyl-bridged phosphonium borate

zwitterions (Figure 1.13).67

Again neither Lewis acid nor Lewis base were found to react with

olefins independently.

9

Figure 1.13: Alkene activation by a frustrated Lewis pair

Terminal alkynes could also be activated by FLPs, forming the addition product and/or an

ion pair in ratios dependant on the strength of the base used (Figure 1.14).68

Stronger bases tend

to deprotonate the alkyne rather than adding to it. In cases of intermediate base strength,

mixtures are observed.

Figure 1.14: Terminal alkyne activation by frustrated Lewis pairs (left-deprotonation, right-

addition)

A recent experimental and computational study by Grimme and co-workers has

suggested that the activation of alkenes by the linked phosphine-borane FLP

Mes2PCH2CH2B(C6F5)2 proceeds via an essentially concerted mechanism, with B-C bond

formation slightly preceding P-C bond formation.69

This concerted pathway is supported

experimentally by the fact that addition of this FLP to norbornene produces only one new

species, resulting from endo-cis-addition across the double bond (Figure 1.15).

Figure 1.15: Addition of a linked phosphine-borane to norbornene

If B-C bond formation occurred significantly prior to P-C bond formation a mixture of

products would be expected from this reaction as a result of a series of rapid Wagner-Meerwein

rearrangements and hydride shifts of the norbornyl cation intermediate. Such products are not

10

observed in the reaction, thus the addition of the phosphine is rapid enough to preclude this

pathway.

1.4.5: Other Small Molecules

CO270

and N2O71

can also be activated by FLPs (Figure 1.16). In the case of CO2 this has

been extended to afford the sub-stoichiometric conversion of CO2 to methanol.72

This work

suggests that catalytic transformations of these materials may be possible. A catalytic cycle

converting these problematic greenhouse gases to useful organic precursors would be of

tremendous importance.

Figure 1.16: Activation of N2O and CO2 by FLPs

1.4.6: Catalytic Hydrogenations

Following the discovery of the hydrogen activation reaction, FLPs were studied for

catalytic hydrogenation activity, traditionally an area dominated by transition metals.42,73

The

common use of stoichiometric borohydride reducing agents suggested this should be possible.42

Addition of hydrogen by a FLP creates a borohydride reagent in situ, which could then transfer

hydride to the substrate. Indeed, the linked system 4-(Mes2PH)-C6F4-B(H)(C6F5)2 was found to

be effective for the hydrogenation of imines.73

Subsequent work showed that B(C6F5)3 alone

was effective as a catalyst for this reaction (the imine itself acts as the Lewis base in the FLP

activation of H2, Figure 1.17).46

In this manner, protected nitriles could be hydrogenated and

aziridines could be ring-opened.46

Other systems have since been shown to effect the catalytic

hydrogenations of imines,52,74

enamines74,75

and silyl enol ethers.76

11

Figure 1.17: Imine hydrogenation catalyzed by B(C6F5)346

These results demonstrate that certain combinations of main group elements can be

exploited for chemistry that has been traditionally confined to transition metals and furthermore,

can be used for completely novel reactivity. Studies of the relatively unexplored P-P bond

activation reaction utilizing both transition metals and FLPs will be presented in this thesis. A

further exploration of the FLP concept, including the synthesis and use of novel FLPs for small

molecule activation reaction, will also be presented.

12

Chapter 2: Stoichiometric and Catalytic P-P Bond Activation by Rh(I) Complexes

2.1: Introduction

There has been much recent interest in the field of “inorganometallics,” which involves

the melding of the chemistry of main group elements, other than carbon and hydrogen, with

transition metals.77

Among the most active fields is the study of inorganic polymers, which

includes polymers containing main group and/or transition metals.78,79

Many inorganic

oligomers and polymers can be synthesized by using a transition-metal catalyzed

dehydrocoupling reaction, which involves the formation of a E-E bond through loss of

dihydrogen (H2) from 2 E-H bonds (E=B, Si, Ge, Sn, P, Sb),10

shown in Figure 2.1.

Heterodehydrocoupling reactions can also be achieved by using 2 different element-hydrogen

bonds. This synthetic methodology has been used perhaps most elegantly by Manners and co-

workers in the heterodehydrocoupling of boranes with amines or phosphines to form polymers

and oligomers containing N-B or P-B bonds.80-97

This reactivity has also been used for the

homocoupling of B-H,98,99

P-H,12-14,100-102

Ge-H,103,104

Si-H,105-109

Sn-H110,111

and Sb-H112

bonds.

The homocoupling of two P-H bonds to form a new P-P bond is of utmost interest given

the high natural abundance of phosphorus and its common use in coordination chemistry.113

While P-P bonds can also be formed by reduction of phosphine chlorides or through salt

metathesis,27

these methods are not atom-economical and generate large amounts of salt by-

products.

Previous work in the Stephan group has shown that the catalytic dehydrocoupling of

primary phosphines can be accomplished using a zirconocene trihydride species to form P5R5

rings.12

Work by the groups of Tilley13

and Brookhart14

has shown that the dehydrocoupling of

secondary phosphines can be accomplished using a Rh(I) catalyst precursor, forming the

biphosphines R2PPR2.

Figure 2.1: Scheme for homonuclear catalytic dehydrocoupling reactions

13

While Brookhart’s proposed catalytic cycle involved dual P-H bond activation resulting

in a Rh(V) intermediate (Figure 2.2, left), Tilley’s catalytic cycle invoked the Rh(III) oxidation

state (Figure 2.2, right). Logically, as both cycles require oxidative addition of at least one P-H

bond followed by elimination of P2R4, these reactions should work best with bulky, electron-rich

catalyst precursors. The steric bulk should hinder aggregation often seen with PR2 fragments

and encourage dissociation of the product, while the increased electron density facilitates

oxidative addition at the metal by making the higher oxidation state more readily accessible.

Figure 2.2: Mechanism for the catalytic dehydrocoupling of HPPh2 by Rh(I)-based catalysts.

Left: Cp*Rh (Brookhart and Bohm)14

, right: (dippe)Rh (Tilley and Han)13

Rhodium(I) is an obvious candidate for this type of reaction as oxidative addition of P-H

bonds and the +3 oxidation state are well-known. In addition, the +5 oxidation state has also

been previously invoked for Rh-based catalysts (for example see Figure 2.2, left). Given the

importance of new phosphorus-containing materials, we sought to gain some insight into the

mechanism of the dehydrocoupling reaction and improve upon current catalyst options. To date,

Rh(I) systems for catalytic dehydrocoupling reactions have shown high activities for only a few

substrates and only under forcing conditions (extended reaction times and high

temperatures).13,14

15

2.2: Experimental

2.2.1: General Considerations

All preparations were done under an atmosphere of dry, O2-free N2 employing both Schlenk line

techniques and an Innovative Technologies or Vacuum Atmospheres inert atmosphere glove box.

Solvents (pentanes, hexanes, toluene, and methylene chloride) were purified employing a

Grubbs’ type column systems manufactured by Innovative Technology and stored over

molecular sieves (4 Å). Molecular sieves (4 Å) were purchased from Aldrich Chemical Company

and dried at 140 ºC under vacuum for 24 hours prior to use. Deuterated solvents were dried over

Na/benzophenone (C6D6, C7D8). All common organic reagents were purified by conventional

methods unless otherwise noted. 1H,

13C,

29Si, and

31P nuclear magnetic resonance (NMR)

spectroscopy spectra were recorded on a Bruker Avance-300 spectrometer at 300K unless

otherwise noted. 1H,

13C and

29Si NMR spectra are referenced to SiMe4 using the residual solvent

peak impurity of the given solvent. 31

P NMR experiments were referenced to 85% H3PO4.

Chemical shifts are reported in ppm and coupling constants in Hz as absolute values.

Combustion analyses were performed in house employing a Perkin Elmer CHN Analyzer.

Et2PH, Cy2PH, Ph2PH and Ph2PPPh2 were purchased from Aldrich Chemical Company and used

as received. Silanes were purchased from Strem Chemicals and used as received. tBu2PLi, and

Ph2PLi were prepared by treating the corresponding phosphine with 1 equivalent of tBuLi in

toluene and collecting the precipitate. Et2PPEt2 was prepared by reaction of the lithium

phosphide and the phosphine chloride. RhNacNac(COE)N2 (2-1)114

and Rh(n5-C9H7)(COE)2

(COE=cis-cyclooctene) (2-2),115

P5Ph5 and P5Et527

were prepared as previously reported.

2.2.2: Synthesis of Rh(Ph2PPPh2) Complexes

[Rh(η5-C9H7)( μ-PPh2)]2 (2-3) - Tetraphenyl biphosphine (Ph2PPPh2) (22 mg, 0.061 mmol) was

added to a solution of Rh(η5-C9H7)(COE)2 (50 mg, 0.11 mmol) in toluene (5 mL). The solution

was transferred to a Schlenk bomb and was heated at 80 ºC for two hours. The solvent was

removed in vacuo and the resulting solid was washed with hexanes (2 mL) leaving a dark solid

Yield: 32 mg (73%). Anal. Calcd. for RhP2C41H34 (%) C: 62.55, H: 4.25; found: C: 62.91, H:

4.71.

16

1H NMR (C6D6) δ: 5.19 (m, 4H), 5.49 (m, 2H), 6.88-7.08 (m, 28H).

31P NMR (C6D6) δ: 152.9

(t, 1JP-Rh=153 Hz).

13C{

1H} NMR (C6D6) δ: 78.7, 86.2, 111.4, 120.2, 122.3, 126.8-128.6 (m,

obscured by C6D6), 133.8.

RhNacNac(Ph2PPPh2) (2-4) - To a solution of RhNacNac(COE)N2 (50 mg, 0.076 mmol) in

toluene (5 mL) was added a solution of Ph2PPPh2 (30 mg, 0.081 mmol) in toluene (5 mL). The

mixture was allowed to stir overnight after which the solvent was removed in vacuo. The dark

red residue was taken up in 10 mL cold pentane (-35°C) and filtered through Celite. Cooling

overnight at -35ºC gave the product as a red solid. Yield: 35 mg (50%). X-ray quality crystals

were grown by slow evaporation from a pentane solution. Anal. Calcd. for RhP2N2C53H61 (%)

C: 71.21, H: 7.22, N: 3.13; found: C: 70.71, H: 7.22, N: 2.72.

1H NMR (C6D6) δ: 0.84 (d,

3JH-H=7 Hz, 6H), 1.20 (d,

3JH-H=7 Hz, 6H), 1.71 (s, 6H), 4.45 (4H,

sept, 3JH-H=7 Hz), 5.15 (1H, s), 6.73-7.19 (26 H, m).

31P NMR (C6D6) δ: -51.43 (d,

1JP-Rh=140

Hz). 13

C NMR (C6D6) δ: 24.0, 24.4, 28.6, 98.0, 124.0, 124.1, 127.5-128.6 (m, obscured by

C6D6), 128.9, 134.9 (app. t, J=8 Hz), 157.7, 159.6.

2.2.3: General Catalytic Procedures

Hydrogenation of Ph2PPPh2 – RhNacNac(COE)N2 (3.5 mg, 0.0055 mmol, 10 mol%) was

added to a solution of 20 mg Ph2PPPh2 (20 mg, 0.055 mmol) in toluene-d8 (0.75 mL). The

solution was transferred to a J. Young’s NMR Tube, subjected to 3 freeze-pump-thaw cycles and

placed under a H2 atmosphere at 77 K. The tube was allowed to thaw to room temperature and

was subsequently heated at 323 K for 12 hours.

Silylation of Ph2PPPh2 – A procedure similar to that of the hydrogenation was carried out,

however five equivalents of silane was added instead of 4 atm hydrogen (See Table 2.3 for

reaction conditions).

General procedure for the heterodehydrocoupling of silanes and phosphines – 20 mg of

phosphine in toluene-d8 (0.75 mL) was added to 5 equivalents of silane and RhNacNac(COE)N2

(1.7 mg, 0.0027 mmol, 5 mol% relative to phosphine). The reaction was heated to 50°C or

100°C and monitored by 31

P NMR spectroscopy (see Table 2.4 for reaction conditions).

17

NMR Data for silyl phosphines:

Data for Ph2PSi(H)Ph2 (2-5),116

Ph2PSiPh3 (2-8),117

and Ph2PSiEt3 (2-9)118

were as previously

reported.

Ph2MeSiPPh2 (2-6) - 1H NMR (C6D6) δ: 0.62 (3H, d,

3JH-P=3 Hz), 6.93-7.16, 7.32-7.53 (m, 20

H). 31

P NMR (C6D6) δ: -59.2 (s). 29

Si NMR (C6D6) δ: -8.2 (d, 1JSi-P=25 Hz).

PhMe2SiPPh2 (2-7) - 1H NMR (C6D6) δ: 0.35 (6H,

3JH-P=4 Hz), 6.95-7.17, 7.33-7.56 (m, 15 H).

31P NMR (C6D6) δ: -58.2 (s).

29Si NMR (C6D6) δ: -3.8 (d,

1JSi-P=23 Hz)

2.2.4: Characterization of Related Species

RhNacNac(H)(HPPh2)(HSiPh2) (2-10) - To a solution of RhNacNac(COE)N2 (20 mg, 0.030

mmol) in toluene (2 mL) was added Ph2SiH2 (12 mg, 0.064 mmol). The solution was cooled to

-35ºC upon which Ph2PH (6 mg, 0.03 mmol) was added. The solution was allowed to warm to

room temperature, the solvent was removed in vacuo and the residue was washed with pentane

(2 x 2 mL), leaving crude 2-10. Yield: 12 mg (43%). Rapid decomposition precluded elemental

analysis.

1H NMR (C6D6) δ: -13.5 (dd,

2JH-P=28 Hz,

1JH-Rh=15 Hz, 1H), 0.23 (d,

3JH-H=7 Hz, 3H), 0.62

(3d, 3JH-H=7 Hz, 3H), 0.71 (d,

3JH-H=7 Hz, 3H), 1.04 (d,

3JH-H=7 Hz, 3H), 1.06 (d,

3JH-H=7 Hz,

3H), 1.11 (d, 3JH-H=7 Hz, 3H), 1.43 (d,

3JH-H=7 Hz, 3H), 1.47 (d,

3JH-H=7 Hz, 3H), 1.65 (s, 3H),

1.88 (s, 3H), 2.76 (sept, 3JH-H=7Hz, 1H), 2.82 (sept,

3JH-H=7 Hz, 1H), 4.07-4.16 (ov sept,

3JH-H=7

Hz, 2H), 5.05 (d, 1JP-H=361 Hz, 1H), 5.00 (d,

2JH-Rh=36 Hz, 1H), 5.34 (s, 1H), 6.35-7.61 (26 H,

ov m). 31

P NMR (C6D6) δ: 47.4 (dd, 1JP-H = 361 Hz,

1JP-Rh=137 Hz).

29Si NMR ( C6D6) δ: 21.4

(dd, J=24 Hz, J=34 Hz).

RhNacNac(P(H)Cy2)(N2) (2-11) - To a solution of RhNacNac(COE)N2 (100 mg, 0.152 mmol) at

-35°C in toluene (5 mL) was added Cy2PH (30 mg, 0.15 mmol). The mixture was allowed to stir

overnight, and the solvent was removed in vacuo. Pentane (5 mL) was added and the solution

was cooled overnight to -35ºC and decanted to give the product as a dark red crystalline solid.

Yield: 90 mg (77%). A second recrystallization from the filtrate yielded X-ray quality crystals.

Anal. Calcd. for RhPN4C41H54 (%) C: 65.94, H: 8.64, N: 7.50; found: C: 66.35, H: 8.24, N:

7.12.

18

1H NMR (C6D6)δ: 1.10-1.84 (m, 20 H, broad peaks obscured by iso-propyl signals), 1.23 (d,

3JH-

H=6 Hz, 6H), 1.36 (d, 3JH-HJ=7 Hz, 6H), 1.59 (d,

3JH-H=7 Hz, 6H), 1.66 (d,

3JH-HJ=7 Hz, 6H), 1.70

(s, 3H), 1.91 (s, 3H), 2.20 (br m, 2H), 3.00 (dm, 1JH-Rh=333 Hz), 3.91 (sept,

3JH-HJ=7 Hz, 2H),

4.07 (sept, 3JH-H=7 Hz, 2H), 5.22 (s, 1H), 7.17-7.32 (m, 12 H).

31P NMR (C6D6) δ: 44.9 (

1JP-

H=333 Hz, 3JP-RhJ=158 Hz).

13C NMR ( C6D6) δ: 23.6, 24.1, 24.2, 24.5, 24.7, 25.4, 26.2, 27.3

(J=9 Hz), 27.7 (d, J-10 Hz), 27.9, 28.4, 30.9, 32.7, 33.2 (d, J-22 Hz), 97.0, 123.4, 125.2, 125.4,

141.5, 141.7, 151.4, 157.9, 158.6.

RhNacNac(Et2PPEt2) (2-12) - To a solution of RhNacNac(COE)N2 (85 mg, 0.13 mmol) in 5 mL

of pentane was added a solution of Et2PPEt2 (23 mg, 0.13 mmol) in 5 mL of pentane. The

mixture was allowed to stir overnight, filtered through Celite and cooled overnight at -35ºC.

Solvent was decanted to give the product as an orange crystalline solid. Yield: 30 mg (37%).

Recrystallization of the pentane wash at -35ºC yielded X-ray quality crystals. Anal. Calcd. for

RhP2N2C37H55 (%) C: 63.32, H: 9.19, N: 3.99; found: C: 63.55, H: 9.24, N: 4.12.

1H NMR (C6D6) δ: 0.74 (m,

3JH-H=8 Hz, 8H), 1.26 (td,

3JH-H=7 Hz,

3JH-P=3 Hz, 12H), 1.35 (d,

3JH-H=7 Hz, 12H), 1.63 (d,

3JH-H=7 Hz, 12H), 1.84 (s, 6H), 4.19 (sept.

3JH-H=7 Hz, 4H), 5.19 (1H,

s), 7.15-7.29 (m, 6H). 31

P NMR (C6D6) δ: -64.51 (d, 1JP-Rh=127 Hz).

13C NMR ( C6D6) δ: 9.8,

12.9, 22.4, 23.9 (d, J=10 Hz), 28.3, 97.0, 123.2, 123.8, 127.3-130.5 (m, obscured by C6D6),

140.3, 156.7, 159.4.

2.2.5: Synthesis of RhNacNac(P5R5) Complexes

RhNacNac(P5Ph5) (2-13) - P5Ph5 (86 mg, 0.16 mmol) was added to a solution of

RhNacNac(COE)N2 (100mg, 0.152 mmol) in toluene (5 mL). The mixture was allowed to stir

overnight at room temperature. Volatiles were removed and cold pentane (5 mL, -35 °C) was

added. The solution was filtered and 2-13 was isolated as a red powder. Yield: 120 mg (73%).

Anal. Calcd. for RhP5N2C59H66 (%) C: 66.79, H: 6.27, N: 2.64; found: C: 66.65, H: 6.47, N:

2.42.

1H NMR (C6D6) δ: 0.49 (d,

3JH-H=7 Hz, 3H), 1.01 (d,

3JH-H=7 Hz, 3H), 1.16 (d,

3JH-H=7 Hz, 3H),

1.35 (d, 3JH-H=7 Hz, 3H), 1.40 (d,

3JH-H=7 Hz, 3H), 1.51 (s, 3H), 1.55 (s, 3H), 1.78 (d,

3JH-H=7

Hz, 3H), 2.18 (d, 3JH-H=7 Hz, 3H), 2.82 (sept.,

3JH-H=7 Hz, 1H), 3.76 (sept.,

3JH-H=7 Hz, 1H),

4.46 (sept., 3JH-H=7 Hz, 1H), 4.83 (sept.,

3JH-H=7 Hz, 1H), 5.04 (s, 1H), 6.20 (d, J=8 Hz, 1H),

19

6.29 (t, J=8 Hz, 2H), 6.33 (t, J=7 Hz, 2H), 6.41 (t, J=7 Hz, 1H), 6.45-6.51 (m, 2H), 6.60 (m, 3H),

6.74-6.85 (m, 6H), 6.90-7.03 (m, 3H), 7.11 (1d, J=7 Hz, 1H), 7.21 (d, J=8 Hz, 1H), 7.24 (t, J=7

Hz, 1H), 7.36 (t, J=7 Hz, 2H), 7.69 (t, J=9 Hz, 4H), 9.51 (t, J=8 Hz, 2H). 31

P NMR (C6D6) δ: -

16.1 (ddddd, J=201 Hz, J=213 Hz, J=18 Hz, J=12 Hz, J=6 Hz), -1.4 (ddddd, J=364, J=265 Hz,

J=9 Hz, J=18 Hz, J=9 Hz), 15.5 (ddddd, J=265 Hz, J=198 Hz, J=12 Hz, J=9 Hz, J=2 Hz), 31.2

(ddddd, J=201 Hz, J=198 Hz, J=150 Hz, J=9 Hz, J=3 Hz), 49.3 (ddddd, J=364 Hz, J=213 Hz,

J=157 Hz, J=9 Hz, J=3 Hz).

RhNacNac(P5Et5) (2-14) - P5Et5 (28 mg, 0.079 mmol) was added to a solution of

RhNacNac(COE)N2 (50 mg, 0.076 mmol) in toluene (5 mL). The mixture was allowed to stir

for two weeks. Volatiles were removed in vacuo and pentane (0.5 mL) was added to the residue.

This solution was stored at -35 °C for three days and the solvent was decanted, leaving the

product as a dark red powder. Yield: 28 mg (43%). Anal. Calcd. for RhP5N2C39H66 (%) C:

57.07, H: 8.11, N: 3.41; found: C: 57.31, H: 8.47, N: 3.18.

1H NMR (C6D6) δ: 0.38 (m, 2H), 0.52 (dt, J=17 Hz, J=7 Hz, 3H), 0.96-1.03 (m, 6H), 1.06 (d,

3JH-H=7 Hz, 3H), 1.10 (d,

3JH-H=7 Hz, 3H), 1.11-1.35 (m, 12H), 1.32 (d,

3JH-H=7 Hz, 3H), 1.37 (d,

3JH-H=7 Hz, 3H), 1.39 (d,

3JH-H=7 Hz, 3H), 1.70 (d,

3JH-H=7 Hz, 3H), 1.72 (s, 3H), 1.73 (d,

3JH-

H=7 Hz, 3H), 1.78 (s, 3H), 1.81 (d, 3JH-H=7 Hz, 3H), 2.25 (m, 1H), 2.48 (m, 1H), 3.12 (sept.,

3JH-

H=7 Hz, 1H), 3.57 (sept., 3JH-H=7 Hz, 1H), 3.61 (sept.,

3JH-H=7 Hz, 1H), 3.65 (sept.,

3JH-H=7 Hz,

1H), 4.94 (s, 1H), 7.02 (dd, J=7 Hz, J=1 Hz, 1H), 7.12 (t, J=8 Hz, 1H), 7.15-7.21 (obscured by

C6D6, 2H), 7.30 (dd, J=8 Hz, J=2 Hz, 1H). 31

P NMR (C6D6) δ: -110.0 (dddd, J=259 Hz, J=174

Hz, J=85 Hz, J=36 Hz), -15.9 (dd, J=147 Hz, J=75 Hz), 18.5 (dd, J=212 Hz, J=167 Hz), 20.9 (dd,

J=174 Hz, J=147 Hz), 79.7 (dddd, J=168 Hz, J=61 Hz, J=39 Hz, J=7 Hz).

2.2.6: X-Ray Data Collection, Reduction, Solution and Refinement

Single crystals were mounted in thin-walled capillaries either under an atmosphere of dry N2 in a

glove box and flame sealed or coated in paratone-N oil. The data were collected using the

SMART software package on a Siemens SMART System CCD diffractometer using a graphite

monochromator with Mo Κα radiation (λ = 0.71073 Å). A hemisphere of data was collected in

1448 frames with 10 second exposure times unless otherwise noted. Data reductions were

performed using the SAINT software package and absorption corrections were applied using

SADABS. The structures were solved by direct methods using XS and refined by full-matrix

20

least-squares on F2 using XL as implemented in the SHELXTL suite of programs. All non-H

atoms were refined anisotropically. Carbon-bound hydrogen atoms were placed in calculated

positions using an appropriate riding model and coupled isotropic temperature factors.

Phosphorus-bound hydrogen atoms were located in the electron difference map and their

positions refined isotropically. Single crystal X-ray structures were obtained for 2-3, 2-4, 2-11,

2-12, 2-13 and 2-14. Selected crystallographic data are included in Tables 2.1 and 2.2.

Diagrams and selected bond lengths and angles are provided in Figures 2.5, 2.11, 2.13 and 2.14.

21

Table 2.1: Selected crystallographic data for compounds 2-3, 2-4 and 2-11

Crystal 2-3 2-4 2-11

Formula C53H61N2P2Rh C42H34P2Rh2 C41H64N4PRh

Formula weight 890.89 806.45 746.85

Crystal system Monoclinic Orthorhombic Monoclinic

Space group P21/n Pbca C2/c

a(Å) 10.8591(11) 10.454(4) 23.1939(5)

b(Å) 35.296(4) 18.159(6) 11.7596(4)

c(Å) 12.6312(13) 35.832(13) 29.7579(9)

(o) 90.0 90.00 90.00

( o) 94.769(2) 90.00 93.5886(17)

( o) 90.0 90.00 90.00

V (Å3) 4824.6(9) 6802(4) 8100.6(4)

Z 4 8 8

d(calc) g cm-1

1.227 1.575 1.225

Abs coeff, , cm-1

0.456 1.094 0.493

Data collected 8472 4882 9146

Data Fo2>3(Fo

2) 6281 4203 5189

Variables 525 415 428

Ra 0.0484 0.0446 0.0547

Rwb 0.1099 0.1097 0.1159

Goodness of Fit 1.053 1.124 1.002

These data were collected at 293 K with Mo Kα radiation (λ = 0.71069 Å).

aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

22


Crystal 2-12 2-13-(0.5 C6H14) 2-14

Formula C37H61N2P2Rh C62H73N2P5Rh C39H66N2P5Rh

Formula weight 698.73 1103.98 820.70

Crystal system Triclinic Monoclinic Monoclinic

Space group P-1 P21/c P21/n

a(Å) 10.566(3) 12.652(2) 12.7417(15)

b(Å) 11.676(4) 23.285(4) 17.733(2)

c(Å) 17.636(6) 20.121(4) 19.684(2)

(o) 102.412(4) 90.0 90.0

( o) 93.045(4) 105.382(2) 103.2450(10)

( o) 114.125(4) 90.0 90.0

V (Å3) 1914.7(11) 5715.3(18) 4329.3

Z 2 4 4

d(calc) g cm-1

1.212 1.283 1.259

Abs coeff, , cm-1

0.555 0.479 0.607


Data Fo2>3(Fo

2) 6106 7128 6478


Ra 0.0459 0.0568 0.0579

Rwb 0.1233 0.1090 0.1480

Goodness of Fit 0.969 1.022 1.020


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

23

2.3: Results and Discussion

2.3.1: Catalyst Selection and Initial Screening

To explore the mechanism and scope of the dehydrocoupling of P-H bonds, the

secondary phosphine Ph2PH was used as the test substrate. The dehydrocoupling activity with

the catalyst precursors RhNacNac(COE)(N2) (2-1) (NacNac=HC{CN(2,6-iPr2C6H3)}2, COE=cis-

cyclooctene) and (n5-C9H7)Rh(COE)2 (2-2) (Figure 2.3) was examined. These catalysts were

chosen according to the criteria previously mentioned: steric bulk and an electron-donating

anionic ligand. Both Rh species present unique possibilities for reactivity as compared to the

previously used systems. 2-1 is a 16-electron precursor with two labile ligands, while 2-2 is an

18-electron precursor with 2 labile ligands, which can allow for facile coordination or oxidative

addition of 2 equivalents of HPPh2. NacNac ligands such as that seen in 2-1 have attracted much

recent attention due to their facile synthesis and ease of tunability (both sterically and

electronically).119

Additionally, the indenyl ligand of 2-2 presents the possibility of ring slippage

from η5 to η

3 to open up an additional coordination site.

120,121

Figure 2.3: Rh(I) Catalyst Precursors 2-1: RhNacNac(COE)(N2) (Ar=2,6-di-iso-propylphenyl)

and 2-2: (n5-C9H7)Rh(COE)2

Initial dehydrocoupling reactions with Ph2PH using both Rh species showed promise as

the product, Ph2PPPh2, was observable in 35% yield after 24 hours. However, additional

reaction time, increasing reaction temperature to 100°C, and use of coordinating solvents such as

THF all failed to improve the yield significantly.

24

2.3.2: Stoichiometric Reactions of Catalyst Precursors with Ph2PH and Ph2PPPh2

In light of these results, stoichiometric reactions of the catalyst and substrate (Ph2PH)

were performed. The addition of two equivalents of Ph2PH to each catalyst precursor resulted in

equilibria between mono-, bis- and tris-diphenylphosphine Rh species (Figure 2.4) and the

formation of small amounts of the dehydrocoupled product: Ph2PPPh2. In each case the bis-

coordinated complexes were favoured. It should be noted that Brookhart and co-workers

observed a similar equilibrium in the reaction of Ph2PH with their Cp*Rh(I) based catalyst

precursor.14

Figure 2.4: Equilibria involving mono-, bis- and tris-diphenylphosphine-rhodium complexes

(L=NacNac or n5-C9H7)

The addition of one equivalent of Ph2PH to each precursor resulted in significantly

different outcomes. In the case of 2-1, equilibrium between starting material and mono- and bis-

diphenyl phosphine species was observed by 31

P NMR spectroscopy. In contrast, when 2-2 was

employed quantitative formation of a new species was observed after 24 hours. This new

compound exhibited a unique signal in the 31

P{1H} NMR spectrum, a triplet appearing at 152.9

ppm with 1

JP-Rh=153 Hz. The 31

P NMR resonance showed no large 1-bond coupling to

hydrogen, thus it was proposed that the large coupling constant is due to coupling to two

equivalent Rh nuclei. The species was formulated as [(n5-C9H7)Rh(µ-PPh2)]2. This connectivity

was further confirmed by X-ray crystallography (Figure 2.5, left). The molecular structure of 2-

3 exhibits a butterfly-type conformation, with two formally Rh(II) centres bound to each other

(as the compound remains diamagnetic). This arrangement is fairly typical of other phosphide-

bridged rhodium complexes known in the literature.122-130

While these related species are

known, most are made by reacting the transition metal chloride with the corresponding lithium

phosphide. It appears that this is the first time such a compound has been made through P-H

bond activation and loss of H2. The Rh-Rh bond length is fairly short at 2.7272(8) Ǻ,

significantly shorter than the related compound (Cp*)Rh(µ-PPh2)(µ-PMe2)Rh(Cp*) at 2.7952(4)

25

Ǻ,129

presumably a result of reduced steric repulsion for 2-3, and similar to [(CO)3Fe(µ-CO)(µ-

PPh2)Rh(µ-PPh2)]2 at 2.723(2) Ǻ.124

The analogous Cp* compound has been synthesized by

reaction of Cp*RhCl2 with HPPh2, followed by treatment with diethylamine, although silica gel

column chromatography was required to separate this product from (Cp*Rh)2(µ-PPh2)(µ-Cl).129

Figure 2.5: POV-Ray depictions of 2-3 (left) and 2-4 (right). Carbon: black, Nitrogen: blue,

Rhodium: pink, Phosphorus: orange. Hydrogen atoms are omitted for clarity. Selected metrical

parameters (Distances: Å, Angles: °): 2-3: Rh1-Rh1a 2.7272(8), Rh1-P1 2.2295(16), Rh1-P1a

2.2326(16), Rh1a-P1a 2.2224(16), Rh1a-P1 2.2292(17), P1-Rh1-P1a 83.65(6), P1-Rh1a-P1a

83.89(5), Rh1-P1-Rh1a 75.42(5), Rh1-P1a-Rh1a 75.49(5). 2-4: Rh1-N1 2.065(3), Rh1-N2

2.054(3), Rh1-P1 2.2300(10), Rh1-P2 2.2424(10), P1-P2 2.1390(14), N1-Rh1-N2 90.52(11), N1-

Rh1-P2 162.38(9), N1-Rh1-P1 107.38(9), N2-Rh1-P1 160.62(8), N2-Rh1-P2 106.23(8), P1-Rh1-

P2 57.14(14).

Compound 2-3 was found to be an active catalyst for the dehydrocoupling reaction of

Ph2PH, thus its potential formation during the dehydrocoupling reaction is not responsible for the

low activity observed. Some other factor must be at play in the dehydrocoupling reaction,

resulting in the low activity observed.

Finally, reactions of the catalyst precursors 2-1 and 2-2 with Ph2PPPh2, the product of the

dehydrocoupling reaction, were attempted. Though the dehydrocoupling reactions had been

previously conducted using Rh(I) precursors, to our knowledge the stoichiometric reaction with

Ph2PPPh2 has not been previously examined.

26

Reaction of 2-2 with one equivalent of Ph2PPPh2 overnight at room temperature did not

show any observable reaction. However, heating at 80 °C for 2 hours produced 2-3, consuming

only 0.5 equivalents of Ph2PPPh2 (Figure 2.6).

Figure 2.6: Formation of Rh-phosphide dimer 2-3

Reaction of 2-1 with one equivalent of Ph2PPPh2 yielded a new product 2-4, showing a

doublet resonance in the 31

P{1H} NMR spectrum at -51.4 (

1JP-Rh=140 Hz). Given the nature of

the reactants and the observed NMR data, species 2-4 was identified as RhNacNac(Ph2PPPh2).

X-ray crystallography showed that the P-P bond remained intact (Figure 2.5, right). As might be

imagined this compound shows a rather unique pseudo-square planar geometry, with an

extraordinarily tight bite angle of 57.14(4)° for the P-P ligand on rhodium. The P-P bond length

of 2.1389(14) Ǻ is reasonable for the intact P-P bond and is in fact substantially shorter than the

bond length determined for the starting material, Ph2PPPh2 at 2.217(1) Ǻ131

and other simple

P2R4 species (R=Me, 2.212(1) Ǻ;132

R=Cy, 2.215(3) Ǻ;133

R=Mes, 2.260(1) Ǻ134

). The Rh-P2

plane is twisted 14.8° from the Rh-N2 plane to minimize crowding between the β-diketiminate

ligand and the phenyl groups of the Ph2PPPh2 ligand. Compound 2-4 is also unique in that

previous reactions of rhodium complexes with biphosphines resulted in either monodentate

complexes,125,127,135-137

or bridging species127,135,138-141

in which the P2 fragment remains intact. It

is perhaps not surprising that bridging compounds were not formed given the bulk of the NacNac

ligand, while a monodentate species is also unlikely due to the highly labile nature of both the

cis-cyclooctene and N2 ligands.

2.3.3: Catalytic Hydrogenation and Hydrosilylation Reactions of Ph2PPPh2

Given the facile formation of 2-3 and 2-4, transformations involving the P2 fragment

were examined. The first reaction attempted was the catalytic hydrogenation of Ph2PPPh2 to see

if this reverse reaction was a possible explanation for the low yields observed in the initial

27

dehydrocoupling reaction (section 2.3.1). Indeed, addition of hydrogen to a solution of

Ph2PPPh2 and 10 mol % of the catalyst precursor 2-1 or 2-2 resulted in hydrogenation of

Ph2PPPh2 to yield two equivalents of the secondary phosphine, Ph2PH. Using 2-1, 95%

conversion was observed over 24 hours at 50°C. Repeating the reaction using 2-2 as the catalyst

precursor showed significantly slower conversion, reaching a maximum of 82% conversion over

36 hours under identical conditions. This is possibly due to the reduced steric bulk of 2-2, which

does not prompt the loss of Ph2PH as efficiently as 2-1. Precursor 2-1 was chosen for further

investigation due to its increased activity. Over the course of the reaction, the peak at -13 ppm

(Ph2PPPh2) in the 31

P NMR spectrum disappears and the peak at -39 ppm (HPPh2) grows in. The

initial Rh species in the reaction (by 31

P NMR spectroscopy) is 2-4 and as the reaction proceeds

more RhNacNac(HPPh2)2 becomes visible by 31

P NMR spectroscopy.

Given the promising results of the hydrogenation of P-P bonds and the analogy of Si-H

and H-H bonds,142

similar reactions were attempted with silanes. Reaction of Ph2PPPh2 with two

equivalents of H2SiPh2 in the presence of 5 mol% 2-1 generated the silyl phosphine

Ph2(H)SiPPh2 in 74% yield, along with the byproduct Ph2PH in 23% yield. Yields of this

relatively slow reaction could be improved significantly by using 5 equivalents of the silane and

heating to 100°C for 48 hours which ensures that any decomposition of the silane by

homodehydrocoupling or reaction with trace moisture is not a factor in the rate of reaction.

Analogous reactions were performed with a series of secondary and tertiary silanes, forming the

corresponding silylphosphines 2-5 - 2-9, with the general trend being that smaller, electron-

deficient silanes provided the best results and that trialkyl silanes proved to be very poor

reactants (Table 2.3). This is likely due to the increased polarization of the Si-H bond in electron

poor silanes, which can facilitate oxidative addition. In all cases, the dominant Rh species in

solution appears to be RhNacNac(HPPh2)2. Notably, uncatalyzed reactions of Ph2PPPh2 with

any of the tested silanes did not proceed.

28

Table 2.3: Silylation of Ph2PPPh2 using 10 mol% 2-1 at 100°C in toluene

Silane (5 equivalents) Time (hours) Yield P-Si (%) Yield P-H (%)

Ph2SiH2 48 98 (2-5) <1

PhMe2SiH 48 95 (2-6) 5

Ph2MeSiH 48 76 (2-7) 17

Ph3SiH 48 72 (2-8) 25

Et3SiH 24 16 (2-9) 44

iPr3SiH 24 - 29

% Yields determined using 31

P NMR spectroscopy

2.3.4: Mechanistic Insight into the Catalytic Activation of P-P Bonds

The silylation of Ph2PPPh2 fragments is proposed to proceed in two steps. The initial

step would provide equal amounts of Ph2PH and Ph2(H)SiPPh2 and the second step must convert

Ph2PH to H2SiPPh2 through reaction with a second equivalent of silane (Figure 2.7).

Figure 2.7: Rh-catalyzed hydrosilylation of P2Ph4

The second step of this reaction, the heterodehydrocoupling of secondary phosphines

with silanes is much faster than the first since only trace amounts of Ph2PH are observed over the

course of the reaction. The dehydrocoupling of diphenylphosphine with a series of silanes was

examined and results are summarized in Table 2.4. Similar reactions involving the

dehydrocoupling of Ph2PH with silanes have been previously reported by Harrod and co-workers

with titanocene catalyst precursors.116

29

Table 2.4: Results of heterodehydrocoupling reactions of Ph2PH with silanes using 5 mol % 2-1

at 50°C in toluene

Silane (5 eq) Time (hours) P-Si P-P

Ph2SiH2 18 99 (2-5) -

PhMe2SiH 18 84 (2-6) -

Ph2MeSiH* 18 85 (2-7) <1

Ph3SiH* 18 40 (2-8) 10

Et3SiH* 18 <1 (2-9) 29

iPr3SiH* 18 - 29

*Reactions were performed at 100°C as reactions at 50°C had very low conversions

In an effort to garner mechanistic insight, further stoichiometric reactions were

undertaken. Addition of hydrogen and excess silane to 2-4 did not show any reaction by

multinuclear NMR spectroscopy. Based on this result it was proposed that the oxidative addition

to Rh(III) must be reversible and that in the absence of excess substrate the equilibrium lies in

favour of 2-4. A similar intermediate can be proposed for the hetero-dehydrocoupling of

phosphines and silanes. The stoichiometric reaction of 2-1 with one equivalent of Ph2PH and

then one equivalent of H2SiPh2 (all conducted at -35°C to slow any potential dehydrocoupling)

produced a new rhodium complex assigned as RhNacNac(Si(H)Ph2)(H)(P(H)Ph2) (2-10) based

on multinuclear NMR spectroscopic data (Figure 2.8). Compound 2-10 was a short-lived

species, decomposing within 2 hours in solution, thus further analysis was not possible. The

NMR data was quite conclusive as a Rh-hydride resonance was observed at -13.5 ppm (2JH-P=28

Hz, 1JH-Rh=15 Hz) and a P-H resonance was observed at 5.09 ppm (

1JP-H=361 Hz) in the

1H NMR

spectrum. Resonances split into doublets of doublets are also observed in the 29

Si{1H} and

31P

NMR spectra. This compound is reminiscent of NacNacIr(H)2(PR3) species reported by Chirik

and co-workers143

and also closely related to Cp*Rh(H)(Si(H)Ph2)(PR3) species synthesized by

Marder and co-workers.144

30

Figure 2.8: Structure of 2-10

From these results, a catalytic cycle was proposed for P2 bond activation which entails

partial dissociation of the P2 ligand, opening up the metal centre for oxidative addition of the H-

H or Si-H bond. This Rh(III) intermediate eliminates Ph2PH, generating a 14-electron Rh(III)

species which rapidly eliminates Ph2PH (or Ph2PSi(H)Ph2) in favour of another P2 unit, restarting

the catalytic cycle (Figure 2.9). A cycle involving a Rh(V) intermediate should not be dismissed

as Rh(V) was proposed as an intermediate in Brookhart’s catalytic cycle (Figure 2.2) and the

Ir(I) analogue of 2-1 has been shown to undergo oxidative addition of two equivalents of H2 by

Chirik and coworkers.145

Figure 2.9: Proposed catalytic cycles for hydrogenation (left) and hydrosilylation (right) of

Ph2PPPh2

31

2.3.5: Heterodehydrocoupling of Silanes with Diphenylphosphine

The reactions of secondary phosphines (the second part of the P2 activation reaction) with

H2 or Si-H bonds is proposed to proceed in a very similar fashion as the P-P bond activation.

Here, the initial loss of a Ph2PH molecule from NacNacRh(PHPh2)2 is required for the

subsequent H-H or Si-H oxidative addition at Rh(I). Elimination of H2, possibly through a

Rh(V) intermediate, leads to a 14-electron Rh(III) species which rapidly picks up two Ph2PH

molecules and reductively eliminates the silyl-phosphine product (Figure 2.10).

Figure 2.10: Proposed mechanism for dehydrocoupling of Ph2PH with silanes

In a related reaction, a solution of Ph2PH and 5 mol% 2-1 was reacted with excess D2,

achieving 85% conversion to Ph2PD in 24 hours at room temperature. This reaction, providing

easy access to Ph2PD without the use of LiAlD4, is expected to proceed in an analogous fashion

to the catalytic cycle shown in Figure 2.10 for the heterodehydrocoupling reaction with silanes.

2.3.6: Reactions of Catalyst Precursors with Additional Phosphines and Biphosphines

In efforts to expand the scope of the reaction, efforts were made to carry out catalytic

reactions with Cy2PH and P2Et4. Unfortunately these substrates were not found to be as reactive

as Ph2PH or Ph2PPPh2. Heterodehydrocoupling reactions of Cy2PH with silanes produced small

amounts of product but conversion was only slightly above stoichiometric in Rh for Ph2SiH2 and

32

PhMe2SiH (21% conversion) and sub-stoichiometric for other silanes. It should also be noted

that heating reactions to 100°C was required to achieve even this minimal conversion, where

heating to only 50°C provided excellent yields using Ph2PH. The low catalytic activity in these

cases is further support for the proposed mechanism since the dissociation of these more basic

phosphines and biphosphines is less facile. While catalytic reactions with Cy2PH were not

successful, stoichiometric reaction provided some insight into reactions of Ph2PH with 2-1, as

reaction of one equivalent of phosphine with 2-1 resulted in formation of the

RhNacNac(P(H)Cy2)(N2) (2-11). The X-ray crystal structure of this species confirmed

connectivity (Figure 2.11, left). The Rh-N2 bond length at 1.951(4) Å is similar to that of the

starting material 2-1, 1.943(4) Å. Surprisingly, the N-N bond length observed in 2-11 of

1.029(4) Å, is substantially shorter than that of 2-1, 1.091(6) Å. The relative stability of this

species compared to the Ph2PH analogue is due to increased basicity and bulk of

dicyclohexylphosphine which reduces the chances of dissociation and bis-coordination,

respectively.

Stoichiometric reaction of Et2PPEt2 with 2-1 resulted in rapid formation of 2-12, a

species which looked spectroscopically quite similar to 2-4, with a doublet resonance seen in the

31P NMR spectrum at -64.5 ppm (

1JP-Rh=127 Hz). X-Ray crystallography confirmed that 2-12

possessed a structure very similar to 2-4 (Figure 2.11, right). 2-12 shows a P-P bond length of

2.1254(14) Ǻ, which results in a Rh-P-P bite angle of 56.77(4)° at rhodium, even tighter than

that observed in 2-4. A combination of reduced steric bulk and increased electron-donating

ability oppose the partial dissociation of the P2Et4 ligand believed to be required for the catalytic

cycle.

33

Figure 2.11: POV-Ray depiction of 2-11 (left) and 2-12 (right). Carbon: black, Nitrogen: blue,

Rhodium: pink, Phosphorus: orange, Hydrogen: white. Carbon-bound hydrogen atoms are

omitted for clarity. Selected metrical parameters (Distances: Å, Angles: °): 2-11: Rh1-N1

1.951(4), Rh1-N3 2.041(3), Rh1-N4 2.0.87(3), Rh1-P1 2.2705(11), N1-N2 1.029(4), N3-Rh1-N4

89.36(12), N1-Rh1-N3 176.83(13), N1-Rh1-N4 90.97(13), N4-Rh1-P1 175.68(9), Rh1-N1-N2

178.6(4). 2-12: Rh1-N1 2.068(2), Rh1-N2 2.064(2), Rh1-P1 2.2465(10), Rh1-P2 2.2231(11),

P1-P2 2.1250(14), N1-Rh1-N2 90.30(9), N1-Rh1-P1 108.03(7), N2-Rh1-P1 160.10(7), N2-Rh1-

P2 105.36(7), P1-Rh1-P2 56.77(4).

2.3.7: Reactions with Cyclic Polyphosphines

Cyclic polyphosphines of the general formula P4R4 and P5R5 are well-known in the

literature27

and are also bulky species containing P-P bonds and may be suitable for P-P bond

activation by 2-1. Thus, polyphosphines A and B (Figure 2.12) were reacted with 2-1. Once

again, while catalytic reactions were unsuccessful, investigations of the stoichiometric reactions

proved to be very interesting. The P4R4 fragments showed no reaction with 2-1, presumably due

to the large bulk or the trans-R groups on adjacent phosphorus centres blocking access of any

lone pair to the Rh centre. In contrast, the reactions of 2-1 with P5Ph5 and P5Et5, resulted in

products which showed very clean, unique patterns in the 31

P NMR spectra after 1 day and 2

weeks at room temperature, respectively.

34

Figure 2.12: Cyclic Polyphosphines: A) P4R4 (R=Cy, tBu) B) P5R5 (R=Ph, Et)

The product of the reaction of 2-1 with P5Ph5 showed 5 remarkably well-resolved

phosphorus resonances from 60 to -20 ppm in the 31

P NMR spectrum (Figure 2.13). Relative

integration of the 1H NMR spectrum confirmed a 1:1 reaction between the P5 fragment and 2-1.

This 1:1 metal:polyphosphine ratio is not common for the P5R5 unit, as most reactions show

either fragmentation of the P5 ring or coordination to multiple metal centres.146,147

Further

examination of the 31

P NMR spectrum (including the use of a 31

P-31

P NMR correlation

spectroscopy) and NUMMRIT148

NMR simulation performed using Spinworks149

revealed that

each resonance was coupled to the other 4 phosphorus centres, as well as to rhodium, with

coupling constants ranging from 2-364 Hz. The 1H NMR spectrum showed 8 inequivalent iso-

propyl-methyl resonances, 2 inequivalent backbone methyl resonances and 4 inequivalent iso-

propyl-methyne resonances, suggesting that 2-13 is asymmetric above and below the RhN2 plane

and each side of the NacNac ligand is also inequivalent. This suggests that the P5 unit is bound

in an unsymmetrical and likely polydentate fashion and there is restricted rotation within the

molecule.

35

The structure of 2-13 was determined crystallographically (Figure 2.13) and this

connectivity is consistent with spectroscopic data obtained in solution. The P5Ph5 ring has not

-15-10-555 50 45 40 35 30 25 20 15 10 5 0 ppm

Figure 2.13: 31

P NMR Spectrum (top, left) and POV-Ray depiction of 2-13-0.5 C6H14. Carbon: black,

Nitrogen: blue, Rhodium: pink, Phosphorus: orange. Solvent and hydrogen atoms are omitted for

clarity. Selected metrical parameters (Distances: Å, Angles: °): Rh1–P1 2.2488(12), Rh1–P2

2.2739(12), Rh1–N2 2.079(3), Rh1–N1 2.105(4), P1–P3 2.1791(16), P1–P5 2.2554(17), P2–P3

2.2088(17), P2–P4 2.2452(17), P4–P5 2.2160(18); N2–Rh1–N1 89.99(14), N2–Rh1–P1 98.58(10),

N1–Rh1–P1 168.65(11), N2–Rh1–P2 166.80(10), N1–Rh1–P2 100.91(11), P3–P1–Rh1 93.39(6),

Rh1–P1–P5 110.23(6), P1–Rh1–P2 71.69(4), P3–P2–Rh1 91.92(5), P4–P2–Rh1 108.21(6), P3–P1–P5

102.32(7), P3–P2–P4 106.99(7), P1–P3–P2 74.26(6), P5–P4–P2 94.17(6), P4–P5–P1 96.65(6).

a

a b c d e

b

c d

e

36

only coordinated to the Rh(I) centre in a bidentate fashion, but also one phosphorus atom has

inverted (in the product, 4 Ph groups are on the same side of the P5 ring, whereas only 3 are cis

in the starting material). While the inversion barrier of phosphorus is generally considered rather

high, experimental studies on cyclic phosphines have shown that the barrier to inversion can be

dramatically lower than barriers for inversion of acyclic phosphines.27,150

It should also be noted

that metal-phosphide complexes27,151-155

and compounds containing Lewis acids proximate to

phosphorus156

have also been shown to have lowered barriers to inversion. This suggests another

possibility: oxidative addition, followed by inversion at the phosphide centre, and then finally

reductive elimination to give 2-13.

The crystal structure provides insight into the nature of the 1H NMR signal observed at

9.51 ppm, integrating for 2 hydrogen atoms. At first glance, this appears to be a simple triplet.

However, in a 1H{

31P} NMR spectrum the resonance is simplified to a doublet, thus these atoms

are coupled to phosphorus with a coupling constant of 9 Hz, characteristic of ortho positions on a

phenyl ring bound to phosphorus. The crystal structure of 2-13 reveals an ortho-hydrogen of the

phenyl ring on P5 lies only 2.780 Ǻ from rhodium. Previous studies have noted similar 1H NMR

chemical shifts based on proximity to d8 metals.157

In solution, rotation about the P-C bond must

be very rapid as even at -80°C this signal did not resolve into two separate peaks. A 31

P-1H

HETCOR experiment was conducted and revealed that resonance “c” in Figure 2.13 was coupled

to these hydrogen atoms. This assignment was supported by comparison of coupling constants

and bond lengths for P4 and P5 (“bottom” P’s) which also suggested that resonance “c” was P5.

Having associated one signal in the 31

P NMR spectrum with the atom in the crystal structure, the

remaining resonances could be assigned using coupling constants (a:P2; b:P1; c:P5; d:P4; e:P3).

Interestingly, the 31

P NMR spectrum of the reaction between P5Et5 and 2-1 after 2 weeks

appeared significantly different from that of 2-13. Again, 5 inequivalent phosphorus centres are

observed, this time spread over a much wider range (80 to -120 ppm, see Figure 2.14) and the 1H

NMR spectrum reveals a lack of symmetry among the iso-propyl groups of the NacNac ligand.

31P NMR spectroscopy of the reaction in progress showed the initial appearance of a species with

resonances very similar to 2-13. This seems to suggest that the reaction proceeds through an

intermediate analogous to 2-13 but continues to a different product. Again, X-ray

crystallography was necessary to elucidate the connectivity of 2-14 (Figure 2.14).

37

In the case of 2-14, oxidative addition of a P-P bond has taken place and the resulting

complex is a five coordinate pseudo-square pyramidal Rh(III) species. Two coordination sites

are occupied by phosphide donors, another is occupied by a neutral phosphine donor and the

other two are occupied by the NacNac ligand. The Rh-phosphide bond lengths are 2.2993(12) Ǻ

for Rh-P1 and 2.2721(11) Ǻ for Rh-P5, while the Rh-phosphine bond length is only slightly

longer, at 2.3189(12) Ǻ. A combination of less steric requirements and stronger donor ability of

the P5Et5 fragment must contribute to the formation and stabilization of the 5-coordinate Rh(III)

complex.

38

Figure 2.14: 31

P NMR resonances (top) and POV-Ray depiction (bottom) of 2-14. Carbon: black,

Nitrogen: blue, Rhodium: pink, Phosphorus: orange. Hydrogen atoms are omitted for clarity.

Selected metrical parameters (Distances: Å, Angles: °): Rh1–P1 2.2993(12) Rh1–P3 2.2721(11),

Rh1–P5 2.3189(12), Rh1–N2 2.152(3), Rh1–N1 2.140(3), P1–P2 2.2226(16), P2–P3 2.1890(16),

P3–P4 2.1858(15), P4–P5 2.2243(15); N2–Rh1–N1 89.93(13), N1–Rh1–P3 68.16(9), N2–Rh1–

P3 101.93(9), N1–Rh1–P1 92.26(9), N2–Rh1–P1 115.67(10), P3–Rh1–P1 78.85(4), N1–Rh1–P5

95.22(9), N2–Rh1–P5 152.56(10), P3–Rh1–P5 77.36(4), P1–Rh1–P5 91.31(4), N1–Rh1–P3

168.16(9), P2–P1–Rh1 97.05(5), P3–P2–P1 82.30(6), P4–P3–P2 106.23(6), P3–P4–P5 81.17(5),

P4–P5–Rh1 99.39(5).

39

2.4: Conclusions

While the present catalyst precursors were relatively ineffective for P-H dehydrocoupling

reactions, a novel catalytic P-P bond activation pathway was discovered allowing for facile,

atom-economical synthesis of novel silyl phosphines. Stoichiometric reactions helped elucidate

a potential reaction pathway involving oxidative addition of Si-H bonds to the Rh(I) centre.

Reaction of the bulky Rh(I) complex 2-1 with cyclic P5R5 (R=Ph, Et) species provided clean

products in a rare 1:1 ratio. These products showed dramatically different connectivity as 2-13

shows simple coordination at 2 phosphorus centres accompanied by inversion at another

phosphorus centre, while 2-14 shows oxidative addition resulting in a tridentate P5R5 dianion.

40

Chapter 3: Frustrated Lewis Pair Reactivity of Bulky Catena-Polyphosphines with B(C6F5)3

3.1: Introduction

As introduced in Chapter 1, Frustrated Lewis Pairs (FLPs) are combinations of Lewis

acids and bases which do not form conventional donor-acceptor Lewis adducts due to steric

constraints. The ability for strong Lewis acids and bases to co-exist without quenching one

another through adduct formation has been exploited in a wide variety of small molecule

activations, including H2,43

olefins,67

THF,39,40

acetylenes,68

N2O71

and CO2.69

To date, this

reactivity has largely focused on combinations of bulky Lewis basic phosphines and the Lewis

acid B(C6F5)3. In addition to small molecule activation reactions in combination with B(C6F5)3,

phosphines have also been shown to carry out nucleophilic aromatic substitution at the para-

position of a C6F5 ring on B(C6F5)3 (Figure 3.1).2,36,37

As discussed in section 1.4.2, these

reactions can be explained in terms of FLP chemistry as the phosphine attacks at a para-carbon

rather than the more Lewis acidic, but more sterically hindered, boron centre. Phosphines have

proven to be particularly good Lewis bases for FLP chemistry as they are good nucleophiles and

sterically bulky derivatives are available commercially at low cost. In particular FLPs of

B(C6F5)3 in combination with the phosphines tBu3P and Mes3P are used extensively in small

molecule activation reactions.3,4

Figure 3.1: Nucleophilic aromatic substitution (NAS) at the para-position of a C6F5 ring by

phosphines on B(C6F5)3

The para-nucleophilic aromatic substitution reactions have even been extended to even

include some smaller phosphines (e.g. Et3P and Cy2PH) which form adducts with B(C6F5)3 at

room temperature.36

At elevated temperatures, these can dissociate in solution, generating free

41

phosphine and borane, and subsequently execute nucleophilic aromatic substitution on the Lewis

acid (Figure 3.1, bottom), forming zwitterionic phosphonium borates.

To date, only mono-phosphines and 1-8-bis(diphenyphosphino)naphthalene,76

a bis-

phosphine, have been utilized in FLP chemistry. The use of catena-polyphosphines represents a

new class of Lewis base used in these reactions, and could allow for unique reactivity based on

relatively weak P-P bonds.27

Despite the reduced basicity of these polyphosphines compared to

most tertiary phosphines, their large steric bulk makes them promising candidates for the

generation of FLPs.

The synthesis of catena-cyclopolyphosphinophosphonium cations, species in which a

formally cationic phosphorus centre is bound to another phosphorus centre, has drawn much

recent attention.158-171

These systems are of fundamental interest due to the diagonal relationship

between carbon and phosphorus and the ubiquitous nature of carbon-carbon bonds in organic

chemistry.172

While an extensive assortment of these cations has been synthesized, the vast

majority of these species have an additional alkyl or aryl group on a PxRx ring or P2R4 moiety.

Catena-polyphosphinophosphonium cations with a pendant borane or borate moiety are

attractive synthetic targets. Such materials may possess interesting structural properties and the

potential for further reactivity unavailable with the current library of catena-

polyphosphinophosphonium cations, allowing for further derivatization. For example, alkene or

alkyne activation reactions, which have been previously examined with tertiary phosphines

(Figure 3.2), could generate alkyl- or alkenyl-bridged polyphosphinophosphonium borates.

Figure 3.2: Activation of alkenes (left) and alkynes (right) by a frustrated Lewis pair: PR3 +

B(C6F5)3

The functionalization of white phosphorus (P4) has attracted recent interest, largely

focusing on the possibility of converting P4 cleanly into small organophosphines. Currently, the

vast majority of organophosphines, which are important in catalysis, pharmaceuticals, materials

and other applications, stem from PCl3, which is derived from P4 and chlorine gas.15

A method

42

for the synthesis of organophosphines avoiding the use of highly corrosive and toxic chlorine gas

is highly desirable.16-23

The functionalization of polyphosphines via FLP reactivity would

provide insight into potential pathways for the functionalization of P4 and the synthesis of

organophosphines from P-P bound species.

43

3.2: Experimental




Solvents (pentane, hexanes, toluene, and methylene chloride) were purified employing a Grubbs’

type column systems manufactured by Innovative Technology and stored over molecular sieves

(4 Å). Molecular sieves (4 Å) were purchased from Aldrich Chemical Company and dried at 140

ºC under vacuum for 24 hours prior to use. Deuterated solvents were dried over

Na/benzophenone (C6D6, C7D8) or CaH2 (CD2Cl2, CDCl3) and distilled prior to use. All common

organic reagents were purified by conventional methods unless otherwise noted. 1H,

13C,

11B,

19F

and 31

P nuclear magnetic resonance (NMR) spectroscopy spectra were recorded on a Bruker

Avance-400 spectrometer at 300K unless otherwise noted. 1H and

13C NMR spectra are

referenced to SiMe4 using the residual solvent peak impurity. 11

B and 19

F NMR experiments

were referenced to 15% BF3-Et2O in CDCl3 and 31

P NMR experiments were referenced to 85%

H3PO4. Chemical shifts are reported in ppm and coupling constants in Hz as absolute values.

Combustion analyses were performed in house employing a Perkin Elmer CHN Analyzer.

Silanes were purchased from Strem Chemicals and used as received. Phenyl acetylene was

purchased from Aldrich Chemicals and used as received. H2 was passed through a dririte

column prior to use. tBu2PLi was prepared by treating the corresponding phosphine with 1

equivalent of tBuLi in toluene and collecting the precipitate by vacuum filtration. R2PPR2 was

prepared by reaction of the corresponding lithium phosphide and phosphine chloride. P5Ph527

and P4Cy427

were prepared as previously reported. B(C6F5)3 was generously donated by Nova

Chemicals.

3.2.2: Generation of a Phosphonium Borate Zwitterion through Nucleophilic Aromatic Substitution

P5Ph5-C6F4-B(F)(C6F5)2 (3-1) – P5Ph5 (300 mg, 0.55 mmol) was added to a solution of B(C6F5)3

(283 mg, 0.55 mmol) in toluene (20 mL) in a teflon-capped reaction bomb. The solution was

heated at 120°C for 6 days. Volatiles were removed in vacuo and the residue was washed with

hexanes (2 x 5 mL). Yield: 554 mg (95%). X-Ray quality crystals were grown from a layered

44

solution of CH2Cl2/C6H6/pentane. Anal. Calcd. for C48H35BF15P5 (%) C: 54.78, H: 2.39; found:

C: 55.16; H: 3.02.

-1H NMR (CD2Cl2) δ: 7.12-7.55 (m, 18H), 7.63 (t, J=7 Hz, 1H), 7.74 (t, J=8 Hz, 2H), 7.87 (t,

J=7 Hz, 2H), 7.94 (t, J=8 Hz, 2H). 19

F NMR (CD2Cl2) δ: -127.6 (d, 3

JF-P=68 Hz, 2F, C6F4), -

130.3 (s, 2F, C6F4), -135.4 (m, 4F, o-C6F5), -161.9 (t, 3JF-F=20 Hz, 2F, p-C6F5), -166.9 (m, 4F, m-

C6F5), -193.1 (br s, 1F, B-F). 31

P NMR (CD2Cl2) δ: 13.3 (tm, 1JP-P=359 Hz, 1P), -19.3 (ddm,

1JP-

P=359 Hz, 1JP-P=78 Hz, 1P), -28.8 (dd,

1JP-P=343 Hz,

1JP-P=107 Hz, 1P), -37.8 (m, 2P).

11B NMR

(CD2Cl2) δ: -2.7 (d, 1JB-F=52 Hz).

3.2.3: Synthesis of Alkenyl-bridged Phosphonium Borate Zwitterions via Activation of Terminal Alkynes

E-P3Cy3(PCy)(Ph)C=C(H)(B(C6F5)3) (3-2) - To a cold (-35°C) solution of B(C6F5)3 (25 mg,

0.049 mmol) and P4Cy4 (22 mg, 0.048 mmol) in CH2Cl2 (5 mL) was added phenyl acetylene (20

mg, 0.20 mmol) dropwise. The pale yellow solution was allowed to stir overnight, the solvent

was removed in vacuo and the residue was washed with pentane (2 x 2 mL), leaving a yellow

powder. Yield: 41 mg (78%). Anal. Calcd. for C50H50BF15P4 (%): C, 56.09%; H, 4.71% ; found:

C, 56.26; H, 4.94.

1H NMR (CDCl3) δ: 1.00 (m, 2H), 1.10-1.37 (m, 18H), 1.55-1.95 (m, 21H), 2.05 (m, 1H), 2.34

(m, 2H), 6.88 (d, 3JH-H=7 Hz, 2H, o-C6H5), 7.11 (t,

3JH-H=7 Hz, 2H, m-C6H5), 7.19 (tm,

3JH-H=7

Hz, 1H, p-C6H5), 8.30 (d, 3JP-H=37 Hz, C=C-H.

19F NMR (CDCl3) δ: -130.6 (d,

3JF-F=24 Hz, o-

C6F5), -161.7 (t, 3JF-F=22 Hz, p-C6F5), -165.9 (t,

3JF-F=23 Hz, m-C6F5).

31P NMR (CDCl3) δ:

20.4 (t, 1JP-P=247 Hz, 1P), -47.2 (dd,

1JP-P=247 Hz,

1JP-P=123 Hz, 2P), -59.4 (t,

1JP-P=123 Hz, 1P).

11B NMR (CDCl3) δ: -15.9 (br s).

E-P4Ph4(PPh)(Ph)C=C(H)(B(C6F5)3) (3-3) - To a solution of B(C6F5)3 (55 mg, 0.11 mmol) and

P5Ph5 (50 mg, 0.093 mmol) in CH2Cl2 (5 mL) was added phenyl acetylene (20 mg, 0.20 mmol).

The pale yellow solution was allowed to stir overnight, the solvent was removed in vacuo and

the residue was washed with pentane (2 x 2 mL), leaving an off-white powder. Yield: 105 mg

(91%). Anal. Calcd. for C56H31BF15P5 (%): C, 58.26; H, 2.71; found: C, 58.37; H, 2.94. X-Ray

quality crystals were grown from a layered solution of CDCl3/pentane.

45

1H NMR (CDCl3) δ: 5.98 (d,

3JH-H=9 Hz, 2H, o-C6H5), 6.51 (t,

3JH-H=8 Hz, 2H, m-C6H5), 6.76 (t,

3JH-H=8 Hz, 1H, p-C6H5), 7.01-7.74 (m, 25H, C6H5), 8.59 (dd,

3JP-H=42 Hz,

4JP-H=4 Hz, 1H,

C=CH). 19

F NMR (CDCl3) δ: -131.0 (d, 3

JF-F=24 Hz, 6F, o-C6F5), -163.1 (t, 3JF-F=22 Hz, 3F, p-

C6F5), -167.3 (t, 3JF-F=19 Hz, 6F, m-C6F5).

31P NMR (CDCl3) δ: 20.9 (t,

1JP-P=300 Hz, 1P), -22.5

(m, 1P), -30.8 (m, 1P), -35.6- -38.9 (m, 2P). 11

B NMR (CDCl3) δ: -15.5 (br s).

3.2.4: Synthesis of a Phosphonium Borate Ion Pair via H2 Activation

[tBu2PP(H)

tBu2]

+[HB(C6F5)3]

- (3-4) - B(C6F5)3 (50 mg, 0.20 mmol) was added to tetra-tert-

butylbiphosphine (28 mg, 0.20 mmol) in toluene (2 mL). The pink solution was placed in a

Schlenk bomb sealed with a teflon cap, subjected to 3 freeze-pump-thaw cycles and backfilled

with H2 at 77 K (generates ~4 atm at room temperature). The solution was allowed to stir

overnight and the solvent was removed in vacuo. The solid was washed with pentane (2 x 2

mL) and all volatiles were removed in vacuo to give a white solid. Yield: 64 mg (74%). Anal.

Calcd for C34H38BF15P2: C, 50.77%; H, 4.76%. Found: C, 50.34%; H, 4.68%. X-Ray quality

crystals were grown from a layered solution of CDCl3/pentane

1H NMR (CDCl3) δ: 1.46 (dd,

3JP-H=14 Hz,

4JP-H=2 Hz, 18H, C(CH3)3), 1.57 (dd,

3JP-H=15 Hz,

4JP-H=1 Hz, 18H, C(CH3)3), 3.67 (q,

1JB-H=91 Hz, 1H, BH), 5.27 (dd,

1JP-H=395 Hz,

2JP-H=8 Hz,

1H, PH). 19

F NMR (CDCl3) δ: -133.1 (br d, 3JF-F=23 Hz, 6F, o-C6F5), -164.0 (t,

3JF-F=20 Hz, 3F,

p-C6F5), -166.9 (tm, 3JF-F=23 Hz, 6F, m-C6F5).

31P{

1H} NMR (CDCl3) δ: 35.0 (d,

1JP-P=464 Hz),

69.7 (d, 1JP-P=464 Hz)

11B NMR (CDCl3) δ: -25.3 (d,

1JB-H=91 Hz).

13C NMR (CDCl3) δ: 31.4

(dd, JP-C=15 Hz, JP-C=6 Hz, C(CH3)3), 32.6 (dd, JP-C=15 Hz, JP-C=7 Hz, C(CH3)3), 36.8 (dd, JP-

C=7 Hz, JP-C=7 Hz, C(CH3)3), 37.2 (dd, JP-C=11 Hz, JP-C=7 Hz, C(CH3)3), 125.2 (br m, BC),

136.3 (dm, 1JF-C=240 Hz, C-F), 137.7 (dm,

1JF-C=240 Hz, CF), 148.3 (dm,

1JF-C=240 Hz, CF).

3.2.5: Hydrogenation and Hydrosilylation of P5Ph5

H2(Ph)P-B(C6F5)3 (3-5) – To a solution of P5Ph5 (22 mg, 0.041 mmol) in toluene (4 mL) was

added B(C6F5)3 (100 mg, 0.20 mmol). This solution was placed in a Schlenk bomb, subjected to

3 freeze-pump-thaw cycles and exposed to 1 atm of H2 at 77 K (generates ~4 atm at room

temperature). The mixture was allowed to stir overnight whereupon the solvent was removed in

vacuo and the residue was recrystallized from hexanes (2 mL) at -35°C. Analytical data matched

that previously published.173

Yield: 110 mg (91%)

46

Ph(Et2(H)Si)(H)P-B(C6F5)3 (3-6) – To a solution of P5Ph5 (11 mg, 0.020 mmol) in CH2Cl2 (4

mL) was added B(C6F5)3 (50 mg, 0.10 mmol). To this solution was added Et2SiH2 (36 mg, 0.20

mmol). The mixture was allowed to stir overnight, whereupon the solvent was removed in

vacuo. The residue was washed with pentane and all solvent was removed in vacuo to give a

white solid. Yield: 57 mg (82%). Anal. Calcd. for C28H17BF15PSi (%): C, 47.48; H, 2.42; found:

C, 47.22; H, 2.82.

1H NMR (C6D6) δ: 0.30 (m, 4H, CH2CH3), 0.51 (t,

3JH-H=8 Hz, 6H, CH2CH3), 3.83 (d,

2JP-H=29

Hz, 1H, SiH), 4.73 (d, 1JP-H =357 Hz, 1H, PH), 6.55 (td,

3JH-H=8 Hz,

4JP-H=2 Hz, 2H, m-C6H5),

6.69 (m, 3H, o-C6H5, p-C6H5). 19

F NMR (C6D6) δ: -129.8 (br s, 6F, o-C6F5), -156.2 (t, 3JF-F=21

Hz, 3F, p-C6F5), -163.5 (td, 3JF-F=21 Hz,

4JF-F=5 Hz, 6F, m-C6F5).

31P NMR (C6D6) δ: -53.7 (br

s); 11

B NMR (C6D6) δ: -12.8 (br s). 13

C{1H} NMR (C6D6) partial δ: 2.5 (d,

2JP-C =4 Hz), 3.3 (d,

J=6 Hz), 7.3 (d, J=3 Hz), 7.7 (d, J=3 Hz), 129.7 (d, 2JC-P=10 Hz, o-PC6H5), 131.1 (d,

4JC-P=3 Hz,

p-PC6H5), 133.4 (d, 3JC-P=7 Hz, m-PC6H5).

Ph(Ph2(H)Si)(H)P-B(C6F5)3 (3-7) – To a solution of P5Ph5 (22 mg, 0.041 mmol) in CH2Cl2 (4

mL) was added B(C6F5)3 (100 mg, 0.20 mmol). To this solution was added Ph2SiH2 (75 mg,

0.41 mmol). The mixture was allowed to stir overnight whereupon the solvent was removed in

vacuo and the residue was recrystallized from pentane at -35°C over one week to give a white

solid. Yield: 147 mg (94%). Anal. Calcd. for C36H17BF15PSi (%): C, 53.75; H, 2.13; found: C,

54.20; H, 2.81.

1H NMR (C6D6) δ: 5.46 (dd,

2JP-H=27 Hz,

3JH-H=5 Hz, 1H, SiH), 5.63 (1H, dd,

1JP-H=363 Hz,

3JH-

H=5 Hz, 1H, PH), 6.81 (td, 3JH-H=8 Hz, J=2 Hz, 2H), 6.95 (m, 2H), 7.09 (t, J=7 Hz, 2H), 7.29 (m,

8H), 7.70 (dd, 3JH-H=8 Hz, J=2 Hz, 2H), 7.75 (1H, dd,

3JH-H=8 Hz, J=2 Hz, 1H), 7.82 (dd,

3JH-H=8

Hz, J=2 Hz, 1H). 19

F NMR (C6D6) δ: -129.4 (d, 3JF-F=21 Hz, 6F, o-C6F5), -156.4 (br s, 3F, p-

C6F5), -163.3 (br s, 6F, m-C6F5). 31

P NMR (C6D6) δ: -47.1 (br s). 11

B NMR (C6D6) δ: -12.8 (br

s). 13

C{1H} NMR (CD2Cl2)partial δ: 128.9 (d,

2JC-P=10 Hz, o-PC6H5), 131.1 (d,

4JC-P=3 Hz, p-

PC6H5), 133.8 (d, 3JC-P=7 Hz, m-PC6H5), 135.5 (d, J=17 Hz, o-SiC6H5), 135.5 (d, J=17 Hz, o-

SiC6H5).

Ph(PhMe(H)Si)(H)P-B(C6F5)3 (3-8) – To a solution of P5Ph5 (11 mg, 0.020 mmol) in CH2Cl2 (4

mL) was added B(C6F5)3 (50 mg, 0.10 mmol). To this solution was added PhMeSiH2 (20 mg,

0.16 mmol). The mixture was allowed to stir overnight whereupon the solvent was removed and

47

the residue was washed with pentane (2 x 2 mL). Yield: 64 mg (96%). Anal. Calcd. for

C31H15BF15PSi (%): C, 50.16; H, 2.04; found: C, 50.18; H, 2.26.

1H NMR (CDCl3) δ: 0.50 (dd,

3JP-H=7 Hz,

2JH-H=4 Hz, 3H, CH3-major), 0.67 (dd,

3JP-H=6 Hz,

2JH-H

=4 Hz, 3H, CH3-minor), 4.86 (d, 2JP-H=29 Hz, 1H, Si-H), 5.03 (d,

1JP-H =357 Hz, 1H, P-Hminor),

5.13 (d, 1JP-H=363 Hz, P-Hmajor), 6.92 (dd,

3JP-H=11 Hz, J=8 Hz, 1H, o-PC6H5), 7.07 (dd,

3JP-H

=11 Hz, J=8 Hz, 1H), 7.12-7.48 (m, 8 H). 19

F NMR (CDCl3) δ: -130.0 (br s, 6F, o-C6F5), -156.6

(t, 3JF-F=27 Hz, 3F, p-C6F5 - minor), -156.7 (t,

3JF-F=27 Hz, 3F p-C6F5 - minor), -163.5 (br d,

3JF-F=17

Hz, 6F, m-C6F5). 31

P NMR (C6D6) δ: -40.9 (minor), -41.6 (major). 11

B NMR (CDCl3) δ: -13.1

(br s). 13

C{1H} NMR (CDCl3) partial δ: 128.9 (d, J=21 Hz), 129.1 (d, J=10 Hz), 129.5 (d, J=10

Hz), 131.3 (d, J=3 Hz), 131.6 (d, J=3 Hz), 131.7 (d, J=1 Hz), 131.9 (d, J=1 Hz), 135.0 (d, J=10

Hz).

Ph(Et3Si)(H)P-B(C6F5)3 (3-9) – To a solution of P5Ph5 (22 mg, 0.041 mmol) in CH2Cl2 (4 mL)

was added B(C6F5)3 (100 mg, 0.20 mmol). To this solution was added Et3SiH (50 mg, 0.43

mmol). The mixture was allowed to stir overnight whereupon the solvent was removed. X-Ray

quality crystals were grown from hexanes at -35°C. Yield: 142 mg (99%). Anal. Calcd. for

C30H21BF15PSi (%): C, 48.93; H, 2.87; found: C, 49.07; H, 3.02.

1H NMR (C6D6) δ: 0.54 (m, 15 H, CH2CH3), 4.72 (d,

1JP-H=345 Hz, 1H, PH), 6.66 (td,

3JH-H=7

Hz, 4JP-H=2 Hz, 2H, m- C6H5), 6.77 (td,

3JH-H=7 Hz, J=2 Hz, 1H, p-C6H5), 6.85 (ddd,

3JH-P=10

Hz, 3JH-H=7 Hz,

4JH-H=2 Hz, 2H, o-C6H5).

19F NMR (C6D6) δ: -129.8 (br s, 6F, o-C6F5), -156.5

(br s, 3F, p-C6F5), -163.7 (br s, 6F, m-C6F5). 31

P NMR (C6D6) δ: -46.6 (br s). 11

B NMR (C6D6)

δ: -12.3 (br s). 13

C{1H} NMR (C6D6) partial δ: 4.4 (d,

2JC-P=8 Hz, Si-CH2CH3), 6.8 (d,

3JC-P=3

Hz, Si-CH2CH3), 128.9 (d, 2JC-P=9 Hz, o-C6H5), 130.7 (d,

4JC-P=3 Hz, p-C6H5), 133.6 (d,

2JC-P=7

Hz, m-C6H5).

(Ph3Si)(Ph)HPB(C6F5)3 (3-10) – To a solution of P5Ph5 (22 mg, 0.041 mmol) in CH2Cl2 (4 mL)

was added B(C6F5)3 (100 mg, 0.20 mmol). To this solution was added 1.05 equivalents of

Ph3SiH (51 mg, 0.21 mmol). The mixture was allowed to stir overnight whereupon the solvent

was removed and the residue was washed with pentane (2 x 2 mL). Yield: 164 mg (95%). Anal.

Calcd. for C42H21BF15PSi (%): C, 57.29; H, 2.40; found: C, 57.34; H, 2.57.

48

1H NMR (C6D6) δ: 5.59 (d,

1JP-H =347 Hz, 1H, PH), 6.49 (td,

3JH-H=8 Hz, J=2 Hz, 2H, o- C6H5),

6.66 (td, 3JH-H=7 Hz, J=2 Hz, 1H, p-CH), 6.78 (ddd,

3JH-H=8 Hz,

3JH-H=10 Hz, J=2 Hz, 2H, m-

CH), 6.94 (t, 3JH-H=7 Hz, 6H, m-C6H5), 7.05 (t,

3JH-H=7 Hz, 3H, p-C6H5), 7.40 (dd,

3JH-H=8 Hz,

4JP-H=1 Hz, 6H, o-C6H5).

19F NMR (C6D6) δ: -129.2 (br s, 6F, o-C6F5), -155.1 (br s, 3F, p-C6F5),

-163.0 (br s, 6F, m-C6F5). 31

P NMR (C6D6) δ: -45.3 (br s). 11

B NMR (C6D6) δ: -11.7 (br s).

13C{

1H} NMR (C6D6) partial δ: 122.2 (d, J=40 Hz), 128.4, 130.6 (d, J=3 Hz), 131.3, 134.6 (d,

J=6 Hz), 136.4 (d, J=1 Hz).

1,4-[SiMe2(Ph)(H)PB(C6F5)3]2C6H4 (3-11) – To a solution of P5Ph5 (22 mg, 0.041 mmol) in

CH2Cl2 (4 mL) was added B(C6F5)3 (100 mg, 0.20 mmol). To this solution was added 1,4-

(Me2SiH)2C6H4 (19 mg, 0.41 mmol). The mixture was allowed to stir overnight whereupon the

solvent was removed and the residue was washed with pentane (2 x 2 mL). Yield: 134 mg

(96%). Anal. Calcd. for C58H28B2F30P2Si2 (%): C, 48.56; H, 1.97; found: C, 48.46; H, 2.18.

1H NMR (CDCl3) δ: 0.44 (d,

3JP-H=6 Hz, 6H, CH3), 0.50 (d,

3JP-H=6 Hz, 6H, CH3), 4.88 (2H, d,

1JP-H=354 Hz, 2H, PH), 6.87 (m, 4H), 7.12 (t, J=7 Hz, 4H), 7.18 (d, J=2 Hz, 4H), 7.30 (t, J=7 Hz,

2H). 19

F NMR (CDCl3) δ: -130.0 (br s, 6F, o-C6F5), -156.7 (t, 3JF-F=22 Hz, 3F, p-C6F5), -163.5

(t, 3

JF-F=21 Hz, 6F, m-C6F5). 31

P NMR (CDCl3) δ: -37.0 (br s). 11

B NMR (CDCl3) δ: -15.3 (br

s). 13

C{1H} NMR (CDCl3) partial δ: 0.1 (d, J=11 Hz), 0.7 (d, J=10 Hz), 0.8 (d, J=10 Hz), 125.1

(d, J=5 Hz), 124.6 (d, J=5 Hz), 132.4 (d, J=10 Hz), 134.5 (d, J=3 Hz), 136.9, 137.1 (d, J=6 Hz),

140.3 (dm, 1JCF=247 Hz, CF), 151.3 (dm,

1JCF=244 Hz, CF).


Single crystals were mounted in paratone-n oil on a Teflon-tipped fiber. The data were collected

using the SMART software package on a Bruker SMART Apex II System CCD diffractometer

using a graphite monochromator with Mo Κα radiation (λ = 0.71073 Å). Data collection

strategies were determined using Bruker Apex software and optimized to provide >99.5%

complete data to a 2θ value of at least 55°. 10 second exposure times were used unless otherwise

noted. Data reductions were performed using the SAINT software package and absorption

corrections were applied using SADABS. The structures were solved by direct methods using

XS and refined by full-matrix least-squares on F2 using XL as implemented in the SHELXTL

suite of programs. All non-H atoms were refined anisotropically. Carbon-bound hydrogen atoms

were placed in calculated positions using an appropriate riding model and coupled isotropic

49

temperature factors. Phosphorus-bound hydrogen atoms were located in the electron difference

map and their positions refined isotropically. Single crystal X-ray structures were obtained for 3-

1, 3-2, 3-3, 3-9. Selected crystallographic data are included in Table 3.1. Diagrams and selected

bond lengths and angles are provided in Figures 3.4, 3.6, 3.7 and 3.15.

50

Table 3.1: Selected crystallographic data for compounds 3-1, 3-2, 3-3 and 3-9

Crystal 3-1-0.25 C6H6c 3-3 3-2 3-9

Formula C49.5H26.5BF15P5 C50H50BF15P4 C56H31BF15P5 C30H21BF15PSi

Formula weight 1071.87 1070.59 1154.47 736.34

Crystal system Monoclinic Monoclinic Triclinic Triclinic

Space group C2/c P21/n P-1 P-1

a(Å) 35.645(4) 15.6548(11) 12.8543(9) 9.5286(7)

b(Å) 15.6446(15) 16.7208(10) 12.9762(10) 9.8618(7)

c(Å) 25.300(3) 19.0352(12) 18.0053(12) 17.2444(13)

(o) 90.00 90.00 74.201(4) 95.866(3)

( o) 134.5620(10) 100.341(2) 76.183(4) 99.447(3)

( o) 90.00 90.00 61.367(4) 105.895(2)

V (Å3) 10052.2(17) 4901.7(5) 2515.5(3) 1518.74(19)

Z 8 4 2 2

d(calc) g cm-1

1.417 1.451 1.524 1.610

Abs coeff, , cm-1

0.272 0.247 0.278 0.245

Data collected 8858 11237 11491 5223

Data Fo2>3(Fo

2) 4672 4840 7793 3831

Variables 622 631 694 438

Ra 0.0765 0.0719 0.0449 0.0675

Rwb 0.2695 0.1375 0.1072 0.1828

Goodness of Fit 1.047 0.968 1.024 1.070


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

c These data were collected at 293 K with Mo Kα radiation (λ = 0.71069 Å).

51


3.3.1: Stoichiometric Reactions of Bulky Polyphosphines with B(C6F5)3

The bulky polyphosphines P2tBu4, P5Ph5, P4Cy4 and P4

tBu4 showed no sign of reaction

with B(C6F5)3 after 24 hours stirring at room temperature in a toluene solution. The 11

B, 19

F and

31P NMR spectra obtained were consistent with the presence of only starting materials, thus these

combinations are considered FLPs. The mixture of P2tBu4 and B(C6F5)3 was pink, such colour

change was previously observed for the mixture of PMes3 and B(C6F5)3 and has been attributed

to weak charge transfer from the Lewis base to the Lewis acid, facilitated by close approach of

the two species without adduct formation.43

3.3.2: Nucleophilic Aromatic Substitution (NAS) Reactions

Under harsh conditions and extended reaction times (up to 7 days at 120 °C) , no

evidence of NAS was observed to occur for solutions of P2tBu4, P4

tBu4 or P4Cy4 with B(C6F5)3.

This lack of reactivity can likely be attributed to the extreme bulk and rigidity of these bases.

However, over 6 days at 120°C in toluene, P5Ph5 proved to be capable of quantitative

nucleophilic aromatic substitution at the para-carbon of one of the aromatic rings of B(C6F5)3.

This reaction quantitatively produced the zwitterionic phosphonium borate 1-(P5Ph5)-(C6F4)-4-

B(F)(C6F5)2 (3-1) over 6 days at 120°C in toluene, isolated in 95% yield. The 11

B and 19

F NMR

data for this product were typical of related compounds,36

most notably the signal in the 11

B

NMR spectrum is a doublet at -2.7 ppm (1JB-F=52 Hz) and a broad signal at -193.1 ppm observed

in the 19

F NMR spectrum, characteristic of the B-F resonance (Figure 3.3). The 31

P NMR

spectrum showed a triplet resonance with very large 1 bond P-P coupling at 13.3 ppm (1JP-P=359

Hz) (Figure 3.3). This signal is typical of the resonance arising from the cationic phosphorus

centre in related [P5Ph5R+] cations.

169 3-1 was additionally characterized by X-ray

crystallography (Figure 3.4).

52

Figure 3.3: Multinuclear NMR Spectra for 3-1 in CD2Cl2: A: 31

P (resonance for cationic

phosphorus centre), B: 31

P (other 4 phosphorus resonances), C: 19

F, D: 11

B

C6F4

o-C6F5

p-C6F5

m-C6F5

BF

A B

C D 1JB-F=52 Hz

53

Figure 3.4: POV-Ray depiction of 3-1-C6H6. Carbon: black, Boron: yellow-green, Fluorine:

deep pink, Phosphorus: orange. Solvent and hydrogen atoms are omitted for clarity. Selected

metrical parameters (distances: Å, angles: °):P1-P2 2.216(2); P2-P3 2.242(2); P3-P4 2.238(2);

P4-P5 2.228(2); P5-P1 2.200(2); P1-C16 1.808(6); P1-C19 1.780(6); P2-C25 1.831(7), P3-C31

1.826(6), P4-C37 1.838(7), P5-C43 1.834(6), C16-P1-C19 107.7(3); P5-P1-P2 103.45(9); P1-P2-

P3 98.42(9); P2-P3-P4 101.85(9); P3-P4-P5 92.00(8); P4-P5-P1 89.16(8).

Structural parameters determined for the borate moiety were analogous to those of related

zwitterionic phosphonium borates.36

The catena-polyphosphinophosphonium fragment of the

molecule exhibited parameters similar to other [P5Ph5R]+ cations.

169 The ring adopts a twist

conformation in the solid state with the cationic phosphorus centre (P1) as the first of three co-

planar atoms (P1-P2-P3). The newly formed P-C bond length (P1-C16=1.808(5) Å) is similar to

those observed for the para-nucleophilic aromatic substitution of tertiary and secondary

phosphines on B(C6F5)3.36

This bond length is also similar to P-C bond lengths observed for

previously studied [P5Ph5R]+ cations. One notable difference is that the bond between the

cationic phosphorus centre and the ipso-carbon of the phenyl ring (P1-C19=1.780(6) Å) has

shortened quite dramatically in comparison to the P-C bonds for the other four phosphorus

centres of 3-1 and cationic phosphorus centres of other related compounds.169

This observation

is consistent with the increased electron-withdrawing ability of the fluoroaryl borate substituent

versus aryl or alkyl groups.

54

3.3.3: Synthesis of Alkenyl-Bridged Phosphonium Borate Zwitterions by Terminal Alkyne Activation

Due to the fact that P4Cy4 and P4tBu4 do not react with B(C6F5)3, and that zwitterion 3-1

formed only under forcing conditions it is conceivable that these Lewis bases could participate in

other reactions typical of FLPs. Accordingly, to probe further reactivity, phenyl acetylene was

added to each FLP. While the use of the P4tBu4\B(C6F5)3 FLP showed only decomposition of the

borane, the use of FLPs of both P4Cy4 and P5Ph5 with B(C6F5)3 produced quantitative

conversions to products 3-2 and 3-3, respectively. Multinuclear NMR spectroscopy indicated

that 3-2 and 3-3 were alkyne addition products (Figure 3.5).

Figure 3.5: Formation of alkyne addition products 3-2 and 3-3

The 19

F NMR spectrum showed meta-para gaps of 4.2 ppm for both 3-2 and 3-3,

characteristic of a borate anion, while the 11

B NMR spectra showed resonances at -15.9 and -15.5

ppm, respectively. The 1H NMR spectrum showed characteristic alkene C-H resonances, which

exhibit coupling to phosphorus, observed at 8.30 and 8.59 ppm, respectively. These data are

consistent with those found for the products of alkyne addition by tertiary phosphines.34

The 31

P

NMR spectra exhibited resonances for the formally cationic phosphorus centres at 20.4 and 20.9

ppm, respectively, appearing as triplets, typical of related systems.169-171

The 31

P NMR spectrum

for 3-2 exhibited three well-resolved resonances in a 1:2:1 ratio (Figure 3.6), while the spectrum

for 3-3 was much more complex due to the lack of symmetry and severe second-order effects for

all neutral phosphorus centres.

Compounds 3-2 and 3-3 were characterized crystallographically. Metrical parameters for

both were similar to those of FLP alkyne addition products34

and of [PxRxR′]+ cations.

169-171 In

3-3, the P5 ring adopts an envelope conformation, with P2 at the vertex (Figure 3.7). As is the

case with addition of other FLPs to alkynes, the FLP adds to produce the E-isomer, with the

Lewis base adding to the more substituted alkynyl carbon, which is attributed to the orientation

55

of the proposed transition state where, by analogy with the alkene activation reaction, B-C bond

formation occurs prior to P-C bond formation. Thus, B-C bond formation occurs at the more

sterically accessible terminal alkynyl carbon.69

Figure 3.6: 31

P{1H} NMR spectrum (left) and POV-Ray depiction of 3-2 (right). Carbon: black,

Boron: yellow-green, Fluorine: deep pink, Phosphorus: orange. Hydrogen atoms are omitted for

clarity. Selected metrical parameters (distances: Å, angles: °): P1-P2 2.2137(8); P2-P3 2.2190(8),

P3-P4 2.2227(9), P4-P5 2.22188(8), P5-P1 2.2077(8), P1-C20 1.821(2), P1-C27 1.801(2), P2-

C33 1.836(2); P3-C39 1.844(2); P4-C45 1.850(2); P5-C51 1.830(2); C20-P1-C27 108.32(10),

P5-P1-P2 106.83(3); P1-P2-P3 97.17(3); P2-P3-P4 103.42(3); P3-P4-P5 107.68; P4-P5-P1

101.19(3).

56

Figure 3.7: POV-Ray depiction of 3-3. Carbon: black, Boron: yellow-green, Fluorine: deep pink,

Phosphorus: orange. Hydrogen atoms are omitted for clarity. Selected metrical parameters

(distances: Å, angles: °): P1-P2 2.2106(15); P2-P4 2.2331(16); P1-P3 2.1990(15); P3-P4

2.2266(16); P1-C20 1.818(4); P1-C27 1.830(4); P2-C39 1.861(4); P3-C33 1.864(4); P4-C45

1.869(4); C20-P1-C27 111.90(19); P3-P1-P2 87.84(6); P1-P2-P4 84.39(6); P1-P3-P4 84.81(6);

P2-P4-P3 86.60(6).

3.3.4: Activation of H-H and Si-H Bonds by Polyphosphosphine/Borane FLPs

The heterolytic cleavage of H2 was investigated using FLPs based on bulky

polyphosphines with B(C6F5)3. In a typical reaction, a toluene solution of P2tBu4 and B(C6F5)3

was pressurized to 4 atm H2. Stirring overnight at room temperature resulted in quantitative

conversion to the ion pair [HP2tBu4][HB(C6F5)3] (3-4) (Figure 3.8) as determined by

multinuclear NMR spectroscopy. The 1H NMR spectrum is particularly diagnostic with 2

doublet resonances for the tert-butyl groups, a P-H resonance at 5.27 ppm which showed

coupling to both phosphorus centres (1JP-H=395 Hz,

2JP-H=8 Hz), and a quartet B-H resonance at

3.67 ppm (1JB-H=91 Hz), attributed to the HB(C6F5)3 anion.

Figure 3.8: Formation of the phosphonium borate ion pair 3-4

57

Figure 3.9: Multinuclear NMR spectra for 3-4 in CD2Cl2: 1H – showing PH and BH peaks (top

left), 11

B (top right), 19

F (bottom left), 31

P[1H} (bottom right)

The analogous reaction was conducted with the P5Ph5/B(C6F5)3 pair. Surprisingly, this

generated one equivalent of (Ph)H2P-B(C6F5)3 (3-5) and left 0.8 equivalents of the P5Ph5

polyphosphine ring in solution. This observation suggests that either only 1/5 of the P5Ph5

molecules were completely consumed, or that one phosphorus centre was abstracted from each

P5Ph5 molecule and the P4Ph4 product rapidly rearranged to P5Ph5. This was indicative of a new

reaction pathway being followed. To determine if this transformation could fully consume the

P5Ph5, the reaction was repeated using 5 equivalents of B(C6F5)3. The known primary phosphine

adduct 3-5 was formed quantitatively in situ and subsequently isolated in 91% yield (Figure

3.10).

A B 1JB-H=91Hz

PH

BH

C D

o-C6F5

p-C6F5 m-C6F5

[tBu2PP(H)

tBu2]

+

1JP-P=464 Hz

[tBu2PP(H)

tBu2]

+

1JP-P=464 Hz

58

Figure 3.10: Formation of 3-5 from the reaction of P5Ph5 with B(C6F5)3 and H2.

This unique reaction can be rationalized through a multi-step mechanistic pathway, with

literature precedent for related reactions at each stage. The H2 activation by the FLP has been

extensively demonstrated by the Stephan group3 and is assumed to be the first step. Nucleophilic

attack, a possible pathway for abstracting the cationic phosphorus centre from a catena-

cyclopolyphosphinophosphonium cation, has been demonstrated by Burford and co-workers with

PMe3 and [P4Cy4Me][OTf], (Figure 3.11).174

As the P5Ph5H+ cation is unprecedented in the

literature, even though P5Ph5 is generated in the presence of excess HCl, this cation is likely

unstable and could potentially decompose in this fashion.

Figure 3.11: Nucleophilic attack by PMe3 on the cationic phosphorus centre of P4Cy4Me+.174

Proposed steps for this reaction are shown in Figure 3.12. The reaction begins with H2

activation by the P5Ph5/B(C6F5)3 FLP, generating the catena-cyclopolyphosphinophosphonium

borohydride. The borohydride then acts as a nucleophile, abstracting the cationic centre from the

ring. One notable difference from the work by Burford and co-workers is that in the current

chemistry, the remaining cyclopolyphosphine either rearranges back to the thermodynamically

preferred 5-membered ring, or the resulting P4 ring reacts until it is consumed. As P5Ph5,

B(C6F5)3 and H2 remain present in solution the reaction can continue until one of the reagents is

completely consumed.

59

Figure 3.12: Hydrogen activation by the frustrated Lewis pair P5Ph5/B(C6F5)3 and subsequent

rearrangement to form (Ph)H2P-B(C6F5)3. As P4Ph4 is not observed in solution it must either

rearrange to form P5Ph5 or rapidly react further until it is consumed.

Given the success of the H2 activation reaction, other E-H bond activation reactions were

pursued. B(C6F5)3 is well known to catalyze the hydrosilylation of ketones,175-177

and imines178

through a proposed interaction between the borane and Si-H bond, which promotes attack by the

imine nitrogen (or carbonyl oxygen) at silicon. The resulting borohydride then attacks the alpha-

carbon, resulting in net hydrosilylation (Figure 3.13). This mechanism for the hydrosilylation

reaction is very similar to the current hydrogen activation reaction and thus a similar reaction

could be imagined with P5Ph5 as the nucleophile attacking the silane, instead of oxygen or

nitrogen. The resulting ion pair may then undergo the nucleophilic attack resulting in

phosphonium abstraction, as with the H2 reaction.

Figure 3.13: Mechanism for B(C6F5)3 catalyzed hydrosilylation of imines178

Initial reactions of the P5Ph5/B(C6F5)3 with silanes were probed using the commercially

available HSiEt3. The results observed were analogous to the reaction of the FLP with H2 as

formation of the silyl phosphine (Et3Si)(H)PhP-B(C6F5)3 (3-9) observed. When a stoichiometric

amount of borane is used, one equivalent of silylphosphine-borane adduct is formed, while the

conversion is quantitative when five equivalents of borane are used (Figure 3.14). A series of

silanes were screened, and silylphoshine borane adducts 3-6 to 3-11 were isolated in excellent

yields (Table 3.2).

60

Figure 3.14: Lewis acid promoted hydrosilylation of P5Ph5

Table 3.2: Selected NMR spectroscopic data and yields for adducts 3-6 – 3-11

Product P-H (

1JP-H)

1H

NMR δ:

11B

NMR δ:

31P NMR

δ:

Δm-p 19

F

NMR δ:a

Isolated

Yield (%)

(H)(Et2HSi)PhP-

B(C6F5)3 (3-6) 4.73 (357 Hz) -12.8 -53.7 7.3 82

(H)(Ph2HSi)PhP-

B(C6F5)3 (3-7) 5.63 (363 Hz) -12.8 -47.1 6.9 94

(H)(MePhHSi)PhP-

B(C6F5)3 (3-8)

5.03 (357 Hz)

5.13 (363 Hz)

-13.1 -40.9

-41.6

6.9

5.9

96

(H)(Et3Si)PhP-

B(C6F5)3 (3-9) 4.72 (345 Hz) -12.3 -46.6 7.2 99

(H)(Ph3Si)PhP-

B(C6F5)3 (3-10) 5.59 (347 Hz) -11.7 -45.3 7.9 95

1,4-[(C6F5)3B-

PPh(H)]2-C6H4 (3-11) 4.88 (354 Hz) -15.3 -37.0 6.8 96

aThis value is the difference in chemical shift between the meta and para-fluorines, and has been

noted to be characteristic for different bonding environments at boron in fluoroarylboranes (the

shortest m-p gaps are generally found for 4 coordinate fluoroaryl borates, while the largest m-p

gaps are found for neutral 3 coordinate fluoroaryl boranes).179,180

Adduct 3-9 was characterized crystallographically (Figure 3.15). The metrical

parameters were as expected, with the P-B34,38

and Si-P181-185

bond lengths comparable to those

observed in secondary phosphine-B(C6F5)3 adducts and silyl phosphines, respectively.

61

Worthy of note is that these reactions create a new chiral centre at phosphorus. While no

enantioselectivity could be promoted in the present reactions, the proposed mechanism indicates

that the use of a chiral Lewis acid may allow for enantioselective reactions. This would produce

chiral phosphines which could be extremely useful for asymmetric catalysis.186-190

For adduct 3-

8 where a chiral centre is created at silicon and phosphorus, slight diastereoselective

enhancement is observed at room temperature, as the 2 products are observed in a 5:4 ratio by

1H,

19F and

31P NMR spectroscopy.

Figure 3.15: POV-Ray depiction of 3-9. Carbon: black, Hydrogen: white, Boron: yellow-green,

Fluorine: deep pink, Phosphorus: orange, Silicon: pink. Carbon-bound hydrogen atoms are

omitted for clarity. Selected metrical parameters (distances: Å, angles: °): P1-B1 2.093(6); P1-

Si1 2.333(2).

Adducts 3-6 to 3-8 retain one Si-H bond; however this Si-H bond is not capable of further

reaction to form a silyl-bisphosphine. This would have provided an attractive route towards a

new family of silyl-bridged phosphines which could be used as bidentate ligands. To

demonstrate multifunctional reactivity in these systems, the reaction with 1,4(Me2SiH)2C6H4

with 2 equivalents of B(C6F5)3 and 2/5 P5Ph5 produces the bisborane adduct of the 1,4-bis-

silylphosphine (3-11) (Figure 3.16). This suggests that while this reactivity is dependent on

steric bulk, the methodology can be used in the synthesis of new bisphosphines and potentially

new phosphorus-containing macrocycles or oligomers (or potentially even polymers) through the

use of bis-silanes and bis-boranes in a similar reaction.

62

Figure 3.16: Synthesis of 3-11

3.3.5: Scope of Reactivity in Terms of Bulk at Silicon and Lewis Acidity of the Borane

The B(C6F5)3-promoted oxidative addition to phosphorus(I) is fundamentally interesting,

however catalytic transformation of this polyphosphine would be especially useful, as this would

provide an efficient method for the synthesis of novel phosphines which could be used in a wide

variety of applications. The major obstacle to achieving catalytic reactivity is the strong Lewis

acid-base adduct formed between the secondary silylphosphine and B(C6F5)3. In efforts to

circumvent this issue, bulkier silanes iPr3SiH and

tBu2SiH2 were used with the hope that the

increased bulk would prevent or at least hinder the ability of the silylphosphine product to

coordinate to B(C6F5)3. This could allow for the free borane to turn over catalytically. Studies

employing these silanes showed no reaction with P5Ph5 and B(C6F5)3 over 24 hours at room

temperature as their steric bulk must prevent this reactivity. This further supports the notion that

adducts 3-6 – 3-8 will not react with a second equivalent of phosphine and borane due to steric

congestion. Another potential avenue to hinder the silylphosphine-borane adduct formation is to

diminish the Lewis acidity at boron. This was investigated by employing BEt3, BPh3 and BMes3

as the Lewis acid. These boranes showed no interaction at room temperature in toluene with

P5Ph5 by multinuclear NMR spectroscopy, however no reaction was observed upon addition of

H2 or Et3SiH to these FLPs. This observation suggests that these boranes are not Lewis acidic

enough to activate the H-H or Si-H bond, thus preventing reactivity.

63

3.4: Conclusions

Frustrated Lewis pair chemistry utilizing polyphosphines can be exploited to synthesize

novel catena-polyphosphinophosphonium borate zwitterions by known reactions of nucleophilic

aromatic substitution, by addition to alkynes and by H2 activation. In addition, a new class of

reactivity for these species has been uncovered: the controlled fragmentation of P5Ph5 to primary

phosphine and silyl phosphine-B(C6F5)3 adducts. The zwitterionic phosphonium borates 3-1 – 3-

3 possess similar properties to the observed moieties in related species, however, they

incorporate B-F (3-1) or alkenyl (3-2 – 3-3) functional groups, previously unavailable in catena-

cyclopolyphosphinophosphonium cations. The adducts 3-5 – 3-11 were isolated in excellent

yields through the straightforward fragmentation pathway involving P5Ph5, 5 equivalents of

B(C6F5)3 and excess silane (or H2). The use of a chiral borane may allow for the enantioselective

synthesis of these chiral secondary silyl phosphines which would otherwise be difficult to access.

The unique fragmentation pathway observed here may be useful in related chemistry, possibly

with white phosphorus.

64

Chapter 4: Frustrated Lewis Pairs: Synthesis and Reactivity of Covalently-Bound Phosphinoboranes

4.1: Introduction

Combinations of group 13 Lewis acids and group 15 Lewis bases are of significant

interest. In addition to unique multiple bonding modes, their potential use as hydrogen storage

materials has become increasingly attractive in light of current issues with fossil fuels and the

quest for alternative energies. Much of this research has been focused on ammonia borane, H3N-

BH3, due to its high hydrogen content by mass, room temperature stability and commercial

availability.191

Ammonia-borane and other related amine-borane or phosphine-borane adducts

have been shown to liberate varying amounts of H2 under thermal duress192

or catalytically,

using transition metal,10,80-97,193-196

Lewis acid191

or Lewis base catalysts197

(see Figure 4.1 for an

example).

Figure 4.1: Dehydrogenation of ammonia borane by a nickel(0)carbene catalyst196

One of the main barriers to the use of such compounds in hydrogen storage is the

thermodynamically downhill pathway for the loss of H2,191

meaning that the hydrogenation of

these products would be strongly endothermic. Thus, the hydrogenation of dehydrogenated

ammonia borane has proven to be challenging and while several reports exist illustrating

potential steps in the reaction,191,198-200

none can hydrogenate these spent materials efficiently

and cost-effectively. Catalytic hydrogenation of these materials could potentially introduce

detrimental impurities into the hydrogen storage material in addition to increased costs.

While these problems are difficult to surmount for the addition of H2 to dehydrogenated

ammonia borane, perhaps judicious modification of the hydrogen storage material could yield a

65

compound which, after dehydrogenated, could readily re-add hydrogen. The un-catalyzed

addition of H2 to a main group compound has been previously observed for germynes, giving a

mixture of products including primary germanes.201

As discussed in section 1.4.2, main group frustrated Lewis pairs have been shown to

activate hydrogen and even act as catalysts in catalytic hydrogenation reactions (section 1.4.7).

In light of this unprecedented reactivity, other main group element systems were envisioned to

continue the fundamental study of these systems in an effort to effect novel metal free bond

activation and potential hydrogen storage systems. Previous computational and experimental

studies on directly bound phosphinoboranes suggest that there is limited interaction between the

lone pair at phosphorus and the vacant p-orbital on boron due to incompatible orbital energy and,

in some cases, geometry.202,203

The lack of reaction between the Lewis acidic and Lewis basic

sites is a key element in the reactivity of FLPs.4 A covalently-bound phosphine-borane system

would require some similarity to these systems, in that substituents at P and B would have to be

large enough to prevent dimerization or the formation of larger aggregates by Lewis acid-base

adduct formation. This system would have to also be “electronically frustrated” in order to

activate H2. In other words, the lone pair on phosphorus and the vacant p-orbital cannot fully

quench each other intramolecularly through π-bonding (Figure 4.2). This P-B π-bonding

interaction should be relatively weak due to the relatively poor orbital overlap between 2nd

and

3rd

row elements discussed above.

Figure 4.2: Potential Reactivity of R2PB(C6F5)2: dimerization (top), H2 activation (bottom)

66

4.2: Experimental








Na/benzophenone (C6D6, C7D8) or CaH2 (CD2Cl2, CDCl3) and distilled prior to use. All common

organic reagents were purified by conventional methods unless otherwise noted. 1H,

13C,

11B,

19F

and 31

P nuclear magnetic resonance (NMR) spectroscopy spectra were recorded on a Bruker

Avance-300 or Avance-400 spectrometer at 300K unless otherwise noted. 1H and

13C NMR

spectra are referenced to SiMe4 using the residual solvent peak impurity of the given solvent. 11

B

and 19

F NMR experiments were referenced to 15% BF3-Et2O in CDCl3 and 31

P NMR

experiments were referenced to 85% H3PO4. Chemical shifts are reported in ppm and coupling

constants in Hz as absolute values. Combustion analyses were performed in house employing a

Perkin Elmer CHN Analyzer. Silanes were purchased from Strem Chemicals and used as

received. Phenyl acetylene was purchased from Aldrich Chemicals and used as received. H2

was passed through a dririte column prior to use. R2PLi (R=Et, Ph, tBu, Cy, Mes) were prepared

by treating the corresponding phosphine with 1.1 equivalents of tBuLi in toluene and collecting

the precipitate.

4.2.2: Synthesis of Phosphinoboranes R2PB(C6F5)2

[Et2PB(C6F5)2]2- (4-1) - To a slurry of Et2PLi (51 mg, 0.53 mmol) in toluene (5 mL) was added a

solution of (C6F5)2BCl (200 mg, 0.53 mmol) in toluene (5 mL) at -35°C. The mixture was

allowed to stir overnight and was then run through celite. The filtrate was concentrated to ~2 mL

and stored at -35 °C overnight. The solution was dried in vacuo and washed with cold pentane

(2 x 2 mL). Et2PB(C6F5)2 was isolated as a colourless polycrystalline solid. Yield: 192 mg

(84%). Anal. Calcd. for C16H10BF10P: C, 44.28; H, 2.32; Found: C, 44.66; H, 2.64. Crystals

were grown from a pentane solution at room temperature.

67

1H NMR (CD2Cl2) δ: 1.07 (6H, dt,

3JP-H=16 Hz,

3JH-H=8 Hz, CH3), 2.16 (4H, m, CH2).

19F NMR

(CD2Cl2) δ: -125.0 (d, 3JF-F=20 Hz, 4F, o-C6F5), -153.5 (t,

3JF-F=23 Hz, 2F, p-C6F5), -160.3 (t,

3JF-

F=20 Hz, 4F, m-C6F5). 31

P NMR (CD2Cl2) δ: -23.4 (br m). 11

B NMR (CD2Cl2) δ:-12.9 (t, 1J P-B

=72 Hz). 13

C{1H} NMR (CD2Cl2) partial δ: 8.3 (CP), 16.2 (m, CH3), 137.4 (dm,

1JC-F=248 Hz,

CF), 140.4 (dm, 1JC-F =209 Hz, CF), 147.3 (dm,

1JC-F =227 Hz, CF).

[Ph2PB(C6F5)2]2 (4-2) - To a slurry of Ph2PLi (101 mg, 0.53 mmol) in toluene (5 mL) was added

a solution of (C6F5)2BCl (200 mg, 0.53 mmol) in toluene (5 mL) at -35°C. The mixture was

allowed to stir for 3 hours and was then run through celite. The filtrate was concentrated to ~2

mL and 2 mL of hexanes were added. The solution was decanted and the precipitate was dried

in vacuo. Yield: 184 mg (65%). Anal. Calcd. for C24H10BF10P: C, 54.38; H, 1.90; Found: C,

55.32; H, 2.30. Crystals were grown by slow evaporation from a 1:1 dichloromethane:pentane

solution.

1H NMR (CD2Cl2) δ: 7.26 (4H, t,

3JH-H=8 Hz, m-C6H5), 7.41 (6H, m, o-C6H5, p-C6H5).

19F

NMR (CD2Cl2) δ: -121.3 (s, 4F, o-C6F5) -156.3 (t, 3JF-F=20 Hz, 2F, p-C6F5), -164.2 (m, 4F, m-

C6F5). 31

P NMR (CD2Cl2) δ: -0.8 (s). 11

B NMR (CD2Cl2) δ:-2.2 (t, 1

J P-B=66 Hz). 13

C{1H}

NMR (CD2Cl2) partial δ: 127.8 (m, C6H5), 129.2 (d, 1JP-C=32 Hz, PC), 130.8 (C6H5), 134.3

(C6H5), 137.0 (dm, 1JC-F =228 Hz, CF), 140.3 (dm,

1JC-F =242 Hz, CF), 146.8 (dm,

1JC-F=239 Hz,

CF).

tBu2PB(C6F5)2 (4-3) - To a slurry of

tBu2PLi (80 mg, 0.53 mmol) in toluene (5 mL) was added a

solution of (C6F5)2BCl (200 mg, 0.53 mmol) in toluene (5 mL). The mixture was allowed to stir

for 3 hours and was then run through celite. The filtrate was stored at -35 °C overnight. The

solution was decanted and dried in vacuo. Yield: 160 mg (61%). Anal. Calcd. for C20H18BF10P:

C, 49.01; H,3.70; Found: C, 48.16; H, 3.54. Crystals were grown from hexanes at -35°C.

1H NMR (CD2Cl2) δ: 1.40 (d,

3JP-H=15 Hz).

19F NMR (CD2Cl2) δ: -130.7 (s, 4F, o-C6F5), -156.0

(t, JF-F=23 Hz, 2F, p-C6F5), -163.4 (m, 4F, m-C6F5). 31

P NMR (CD2Cl2) δ: 120.7 (br m). 11

B

NMR (CD2Cl2) δ: 41.8 (d, 1J P-B=150 Hz).

13C{

1H} NMR (CD2Cl2) partial δ: 32.9 (CH3), 39.6

(d, 1JP-C=23 Hz, PC), 115.9 (m, BC), 137.5 (dm,

1JC-F =253 Hz, CF), 140.9 (dm,

1JC-F =253 Hz,

CF), 143.0 (dm, 1JC-F =239 Hz, CF).

68

Cy2PB(C6F5)2 (4-4) - To a slurry of Cy2PLi (107 mg, 0.53 mmol) in toluene (5 mL) was added a

solution of (C6F5)2BCl (200 mg, 0.53 mmol) in toluene (5 mL). The mixture was allowed to stir

for 18 hours and was then run through celite. The solution was decanted and the solid was dried

in vacuo and washed with cold hexanes (-35°C, 2 mL). Yield: 235 mg (83%). Anal. Calcd. for

C24H22BF10P: C, 53.16; H, 4.09; Found: C, 52.28; H, 4.20. Crystals were grown from hexanes at

-35°C.

1H NMR (CD2Cl2) δ: 1.15 (tt, J=13, J=3, 2H), 1.25 (m, J=13 Hz, J=3 Hz, 4H), 1.49 (m, J=13 Hz,

JP-H=5 Hz, J=3 Hz, 4H), 1.64 (d, J=13 Hz, 2H), 1.75 (dd, J=13 Hz, JP-H=3 Hz, 4H), 1.99 (d, J=13

Hz, 4H), 2.33 (dtt, J=13 Hz, JP-H=9 Hz,, J=3 Hz, 2H). 19

F NMR (CD2Cl2) δ: -130.78 (s, 4F, o-

C6F5), -155.46 (t, 3JF-F=20 Hz, 2F, p-C6F5), -163.51 (m, 4F, m-C6F5).

31P NMR (CD2Cl2) δ: 92.1

(br m). 11

B NMR (CD2Cl2) δ: 39.5 (d, 1JB-P=142 Hz).

13C{

1H} NMR (CD2Cl2) δ: 25.4 (C6H11),

26.8 (d, 2JC-P=34 Hz, C6H11), 33.7 (d,

3JC-P=4 Hz, C6H11), 35.0 (d,

1JC-P=27 Hz, PC), 113.1 (BC),

137.6 (dm, 1JC-F =260 Hz, CF), 141.0 (dm,

1JC-F=264 Hz, CF), 145.2 (dm,

1JC-F =247 Hz, CF).

Mes2PB(C6F5)2 (4-5) - To a slurry of Mes2PLi (145 mg, 0.53 mmol) in toluene (5 mL) was added

a solution of (C6F5)2BCl (200 mg, 0.53 mmol) in toluene (5 mL). The mixture was allowed to

stir for 18 hours and was then run through celite. The solution was dried in vacuo leaving a

yellow oil which subsequently crystallized. The yellow crystals were washed with cold hexanes

(-35°C, 2 mL). Yield: 220 mg (68%). Anal. Calcd. for C30H22BF10P: C, 58.66; H, 3.61; Found:

C, 57.51; H, 3.53.

1H NMR (CD2Cl2) δ: 2.25 (s, 6H, p-CH3), 2.29 (s, 12H, o-CH3), 6.89 (d,

4JP-H=6 Hz, 4H, CH).

19F NMR (CD2Cl2)δ: -131.2 (s, 4F, o-C6F5), -154.6 (t,

3JF-F=20 Hz, 2F, p-C6F5), -163.5 (m, 4F,

m-C6F5). 31

P NMR (CD2Cl2, 121 MHz) δ: 29.3 (br m). 11

B NMR (CD2Cl2) δ: 40.1 (br m).

13C{

1H} NMR (CD2Cl2) partial δ: 20.9 (p-CH3), 22.6 (d,

3JC-P =7.7 Hz, o-CH3), 123.1 (d, JC-P

=72 Hz, PC), 129.3 (d,3 JC-P =11 Hz, CH), 137.2 (

1JC-F=246 Hz, CF), 141.2 (d,

1JC-F=252 Hz,

CF), 141.4 (d, 4JC-P =3 Hz, p-CCH3), 143.6 (d,

2JC-P=7 Hz, o-CCH3), 146.0 (d,

1JC-F=248 Hz, CF).

4.2.3: Synthesis of Secondary Phosphine Adducts of HB(C6F5)2

Et2(H)PB(H)(C6F5)2 (4-6) - A solution of Et2PH (12 mg, 0.15 mmol) in toluene (1 mL) was

added to (C6F5)2BH (50 mg, 0.15 mmol) in hexanes (2 mL). The mixture was stirred for 1 hour,

then stored at -35°C for 2 hours. The solution was decanted and the white precipitate was dried

69

in vacuo. Yield: 48 mg (77%). Anal. Calcd. for C16H12BF10P: C, 44.07; H, 2.77; Found: C,

43.80; H, 2.73. Crystals were grown from hexanes at -35°C.

1H NMR (CD2Cl2) δ: 1.17 (dt,

3JH-P=17 Hz,

2JH-H =8 Hz, 6H, CH3), 1.84 (dq,

2JP-H =24 Hz,

2JP-

H=8 Hz, 4H, CH2), 3.43 (br m, 1H, BH), 4.95 (dm, 1JP-H=388 Hz, 1H, PH).

19F NMR (CD2Cl2)

δ: -131.7 (s, 4F, o-C6F5), -159.2 (t, 3JF-F=20 Hz, 2F, p-C6F5), -164.7 (m, 4F, m-C6F5).

31P NMR

(CD2Cl2) δ: -4.6 (br m). 11

B NMR (CD2Cl2) δ: -30.0 (d, 1JB-P=65 Hz).

13C{

1H} NMR (CD2Cl2)

partial δ: 8.5 (d, 1JC-P =6 Hz, CP), 10.6 (d,

2JC-P =38.5 Hz, CH3), 137.1 (dm,

1JC-F =208 Hz, CF),

140.3 (dm, 1JC-F =235 Hz, CF) 148.0 (dm,

1JC-F=233 Hz, CF).

Ph2(H)PB(H)(C6F5)2 (4-7) - A solution of Ph2PH (28 mg, 0.15 mmol) in toluene (1 mL) was

added to (C6F5)2BH (50 mg, 0.15 mmol) in toluene (1 mL). The mixture was stirred for 1 hour,

upon which hexanes (2 mL) was added and the solution was decanted. The white precipitate was

washed with hexanes (2 x 2 mL) and the remaining solid was dried in vacuo. Yield: 58 mg

(74%). Anal. Calcd. for C24H12BF10P: C, 54.17; H, 2.27; Found: C, 53.82; H, 2.25. Crystals

were grown from 1:1 dichloromethane:hexanes at -35°C.

1H NMR (CD2Cl2) δ: 3.94 (br m, 1H, BH), 6.81 ( ddm,

1JP-H=409 Hz,

3JH-H=15 Hz, 1H, PH),

7.44 (ddd, 3JH-H=8 Hz,

3JH-H=6 Hz,

4JH-P =1 Hz, 4H, m-C6H5), 7.54 (tt,

3JH-H=6 Hz,

4JH-H =2 Hz,

2H, p-C6H5), 7.60 (ddd, 3JH-P=12 Hz,

3JH-H =8 Hz,

4JH-H =2 Hz, 3H, o-C6H5).

19F NMR (CD2Cl2)

δ: -131.2 (s, 4F, o-C6F5), -158.8 (t, 2F, 3JF-F J=20 Hz, p-C6F5), -164.7 (m, 4F, m-C6F5).

31P

NMR (CD2Cl2,) δ: -1.5 (br m). 11

B NMR (CD2Cl2) δ: -28.5 (d, 1JP-B=71 Hz).

13C{

1H} NMR

(CD2Cl2) partial δ: 122.9 (d, 1JC-P=65 Hz, CH), 129.5 (dm,

2JC-P=164 Hz, CH), 133.7(CH), 135.3

(CH) 136.9 (dm, 1

JC-F=248 Hz, CF), 140.7 (dm, 1JC-F=254 Hz, CF), 148.0 (dm,

1JC-F=235 Hz,

CF).

tBu2(H)PB(H)(C6F5)2 (4-8) - A solution of

tBu2PH (22 mg, 0.15 mmol) in hexanes (2 mL) was

added to (C6F5)2BH (50 mg, 0.15 mmol) in hexanes (2 mL). The mixture was stirred for 1 hour,

upon which the solution was concentrated to 2 mL and stored at -35°C overnight. The solution

was decanted and the remaining solid was dried in vacuo. Yield: 48 mg (65%). Anal. Calcd.

for C20H20BF10P: C, 48.81; H, 4.10; Found: C, 48.42; H, 3.90. Crystals were grown from the

hexane wash layer.

70

Alternate synthesis A: An J Young’s tube was charged with tBu2PB(C6F5)2 (20 mg, 0.041 mmol)

and tol-d8 (0.75 mL) the solution was subjected to 3 freeze-pump-thaw cycles and 1 atm of H2

was added at 77 K (~4 atm at room temperature). 80% conversion to tBu2(H)P-B(H)(C6F5)2 was

achieved in four weeks at 25°C, while quantitative conversion was achieved over 48 hours at

60°C.

Alternate synthesis B: An NMR tube was charged with of tBu2PB(C6F5)2 (20 mg, 0.041 mmol),

Me2NH-BH3 (2 mg, 0.04 mmol) and CD2Cl2 (0.75 mL) After 15 minutes at 25°C, 1H,

11B,

31P

and 19

F NMR spectroscopy revealed quantitative conversion to tBu2(H)P-B(H)(C6F5)2 and 0.5

(Me2N-BH2)2.

1H NMR (CD2Cl2) δ: 1.27 (d,

3JH-P =14 Hz, 18H, CH3), 3.48 (br m, 1H, BH), 4.84 (dd,

1JH-P=375

Hz, 3JH-H=11 Hz, 1H, PH).

19F NMR (CD2Cl2) δ: -129.7 (s, 4F, o-C6F5), -159.4 (t,

3JF-F=20 Hz,

2F, p-C6F5), -164.7 (m, 4F, m-C6F5). 31

P NMR (CD2Cl2) δ: 32.0 (br m). 11

B NMR (CD2Cl2) δ:

-30.0 (d, 1JP-B=48 Hz).

13C{

1H} NMR (CD2Cl2) δ: 29.2 (CH3), 33.0 (d,

1JC-P =29 Hz, CP), 117.9

(br m, BC), 137.1 (dm, 1JC-F =255 Hz, CF), 139.6 (dm,

1JC-F=250 Hz, CF), 148.0 (dm,

1JC-F =239

Hz, CF).

Cy2(H)PB(H)(C6F5)2 (4-9) - A solution of Cy2PH (29 mg, 0.15 mmol) in hexanes (2 mL) was

added to (C6F5)2BH (50 mg, 0.15 mmol) in hexanes (2 mL). The reaction was stirred for 1 hour,

upon which the solution was decanted and the white precipitate was washed with hexanes (2 x 2

mL) and the remaining solid was dried in vacuo. Yield: 61 mg (77%). Anal. Calcd. for

C24H24BF10P: C, 52.97; H, 4.45; Found: C, 52.50; H, 4.56. Crystals were grown from hexanes

at -35°C.

Alternate synthesis A: An J Young’s tube was charged with 20 mg Cy2PB(C6F5)2 (20 mg, 0.037

mmol) and tol-d8 (0.75 mL), the solution was subjected to 3 freeze-pump-thaw cycles and 1 atm

of H2 was added at 77 K (~4 atm at room temperature). Quantitative conversion to Cy2(H)P-

B(H)(C6F5)2 was achieved in two weeks at 25°C or 48 hours at 60°C.

Alternate synthesis B: An NMR tube was charged with Cy2PB(C6F5)2 (20 mg, 0.037 mmol),

Me2NH-BH3 (2 mg, 0.04 mmol) and 0.75 mL CD2Cl2 (0.75 mL). After 15 minutes at 25°C, 1H,

11B,

31P and

19F NMR spectroscopy revealed quantitative conversion to Cy2(H)P-B(H)(C6F5)2

and 0.5 (Me2N-BH2)2.

71

1H NMR (CD2Cl2) δ: 1.15 (m, 2H, PC6H11), 1.20-1.29 (br m, 6H, PC6H11), 1.37 (m, 2H,

PC6H11), 1.68 (br d, 2JH-H=13 Hz, 2H, PC6H11), 1.75-1.84 (br m, 6H, PC6H11), 1.89 (m, 2H,

PC6H11), 2.00 (m, 2H, PC6H11), 3.33 (1H, br m, BH), 4.78 (ddm, 1JP-H=381 Hz,

3JH-H=13 Hz,1H,

PH). 19

F NMR (CD2Cl2) δ: -131.0 (s, 4F, o-C6F5), -159.4 (t, 3JF-F=20 Hz, 2F, p-C6F5), -164.7 (t,

3JF-F=17 Hz, 4F, m-C6F5).

31P NMR (CD2Cl2) δ: 7.1 (br m).

11B NMR (CD2Cl2) δ: -28.1 (d,

1JP-

B=68 Hz). 13

C{1H} NMR (CD2Cl2) partial δ: 25.4(CH2), 26.7 (m, CH2), 29.2, (d, J=17 Hz, CH2),

29.7 (d, J=35 Hz, CH), 136.6 (dm, 1JC-F=185 Hz, CF), 148.0 (dm,

1JC-F=237 Hz, CF).

Mes2(H)PB(H)(C6F5)2 (4-10) - A solution of Mes2PH (40 mg, 0.15 mmol) in hexanes (2 mL)

was added to (C6F5)2BH (50 mg, 0.15 mmol) in hexanes (2 mL). The reaction was stirred for 1

hour, upon which the solution was concentrated to 2 mL and stored at -35°C overnight. The

solution was decanted and the remaining solid was dried in vacuo. Yield: 70 mg (78%). Anal.

Calcd. for C30H24BF10P: C, 58.47; H, 3.93; C, 57.82; H, 3.91

1H NMR (CD2Cl2) δ: 2.24 (s, 12H, o-CH3), 2.26 (s, 6H, p-CH3), 6.88 (s, 4H, CH), 7.06 (dd,

1JH-P

=402 Hz, 3JH-H=13 Hz, 1H, PH).

19F NMR (CD2Cl2) δ: -131.4 (s, 4F, o-C6F5), -158.7 (t,

3JF-F=20

Hz, 2F, p-C6F5), -164.8 (m, 4F, m-C6F5). 31

P NMR (CD2Cl2) δ: -39.0 (br m). 11

B NMR

(CD2Cl2) δ: -25.4 (d, 1JP-B =48 Hz).

13C{

1H} NMR (CD2Cl2) partial δ: 20.7 (CH3), 21.6 (CH3),

118.0 (d, 1JC-P =58 Hz, CP), 130.5 (d, J=9 Hz, CH), 136.9 (dm,

1JC-F=245 Hz, CF), 142.4 (p-C-

CH3), 143.0 (d, J=8 Hz, o-C-CH3), 148.4 (dm, 1JC-F=240 Hz, CF).

4.2.4: Reactions of R2PB(C6F5)2 with 4-tert-butylpyridine

A solution of R2PB(C6F5)2 (25 mg) in CDCl3 (0.75 mL) was added to 4-tert-butylpyridine (one

equivalent). The solution was monitored by multinuclear NMR. Using toluene (1 mL) as the

solvent gave X-ray quality crystals for R=Cy. Rapid decomposition precluded further

characterization by elemental analysis and 13

C NMR spectroscopy. While 1H,

19F and

31P NMR

spectra for R=tBu indicated that little reaction had taken place, the clear emergence of a new

peak at -1.3 ppm in the 11

B NMR spectrum indicated the presence of B-N adduct 4-11 in at least

trace amounts. For R=Mes, no clear evidence of adduct formation was observed and the reaction

showed numerous unidentifiable products after several days at room temperature.

72

1H NMR (CDCl3) δ: 0.41-2.00 (m, 22H, C6H11), 1.41 (s, 9H, C(CH3)3), 7.32 (d,

3JH-H=7 Hz, 2H,

CH), 9.05 (br s, 2H, CH). 19

F NMR (CDCl3) δ: -128.0 (br s, 4F, o-C6F5), -157.2 (br s, 2F, p-

C6F5), -162.2 (m, m-C6F5). 31

P NMR (CDCl3) δ: -28.3 (br s). 11

B NMR (CDCl3) δ: -1.3.

4.2.5: Synthesis of Dimers (R2PBCl2)2 and ClB(C6F5)2 by Reaction of BCl3 with R2PB(C6F5)2

BCl3 (one equivalent) is added to a solution of R2PB(C6F5)2 (25 mg) in toluene. The solution

was allowed to stir for 3 hours. Volatiles were then removed in vacuo and the residue was taken

up in CDCl3 for NMR spectroscopy. Resonances attributed to ClB(C6F5)2 corresponded to those

previously reported in the literature.204,205

In the case of 4-13, trace evidence of monomer

tBu2PBCl2 was observed in the

11B NMR spectrum.

4-13 - 1H NMR (CDCl3) δ: 1.33 (d,

3JP-H=15 Hz, 9H, C(CH3)3).

31P NMR (CDCl3) δ: -14.8

(sept, 1JP-B=90 Hz).

11B NMR (CDCl3) δ: 4.2 (t,

1JP-B=90 Hz, (

tBu2PBCl2)2), 2.7 (d,

1JP-B=135

Hz, tBu2PBCl2). 4-14 -

1H NMR (CDCl3) δ: 1.11-2.58 (m, 22H, Cy).

31P NMR (CDCl3) δ: -14.5

(sept, 1JP-B=99 Hz).

11B NMR (CDCl3) δ: -0.5 (t,

1JP-B=99 Hz).













positions refined isotropically. Single crystal X-ray structures were obtained for 4-1, 4-2, 4-3, 4-

4, 4-6, 4-7, 4-8, 4-12, 4-13 and 4-14 Selected crystallographic data are included in Tables 4.1 to

4.4 . Diagrams and selected bond lengths and angles are provided in Figures 4.5, 4.7, 4.10, 4.11,

4.14, 4.15 and 4.18.

73


Crystal 4-1 4-2-CH2Cl2 4-3

Formula C32H20P2F20B2 C50H22P2F20B2Cl2 C20H18BF10P

Formula weight 868.04 1131.10 490.12

Crystal system Triclinic Triclinic Orthorhombic

Space group P-1 P-1 Pbca

a(Å) 9.699(2) 10.3197(16) 12.194(9)

b(Å) 9.703(2) 12.0982(19) 18.331(13)

c(Å) 10.404 20.926(3) 19.774(14)

(o) 67.072(2) 74.492(2) 90.00

( o) 80.770(3) 76.700(2) 90.00

( o) 67.852(2) 68.458(2) 90.00

V (Å3) 835.1(3) 2316.1(6) 4420(5)

Z 1 2 8

d(calc) g cm-1

1.726 1.622 1.473

Abs coeff, , cm-1

0.269 0.327 0.212


Data Fo2>3(Fo

2) 2274 3801 2640


Ra 0.0390 0.0573 0.0553

Rwb 0.0942 0.1347 0.1889

Goodness of Fit 1.035 0.919 1.059


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

74


Crystal 4-4 4-6 4-7

Formula C24H22BF10P C16H12BF10P C24H22BF15P

Formula weight 542.20 436.04 542.20

Crystal system Triclinic Triclinic Triclinic

Space group P-1 P-1 P-1

a(Å) 9.4503(13) 9.198(6) 8.9820(6)

b(Å) 10.4530(14) 12.989(8) 10.3690(6)

c(Å) 13.7545(19) 15.681(9) 12.4150(6)

(o) 110.6870(10) 83.854(6) 106.381(3)

( o) 99.036(2) 87.955(7) 94.690(3)

( o) 95.566(2) 75.462(6) 96.226(3)

V (Å3) 1238.1(3) 1803.0(19) 1095.11(1)

Z 2 4 2

d(calc) g cm-1

1.454 1.606 1.644

Abs coeff, , cm-1

0.197 0.249 0.223


Data Fo2>3(Fo

2) 2870 5186 3199


Ra 0.0631 0.0493 0.0573

Rwb 0.2154 0.1449 0.1834

Goodness of Fit 1.043 1.050 1.078


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

75


Crystal 4-8c 4-12 4-13

Formula C20H20BF10P C40H43BF10NP C29H31BF10NP

Formula weight 492.14 769.53 625.33

Crystal system Triclinic Triclinic Monoclinic

Space group P-1 P-1 Cc

a(Å) 9.5420(17) 9.8554(5) 19.0354(6)

b(Å) 12.872(2) 12.3799(6) 11.9222(4)

c(Å) 19.447(3) 16.4730(8) 27.8012(12)

(o) 96.656(2) 72.596(2) 90.00

( o) 95.583(2) 78.921(2) 106.466(2)

( o) 109.144(2) 86.032(2) 90.00

V (Å3) 2217.9(2) 1881.95(16) 6050.6(4)

Z 4 2 8

d(calc) g cm-1

1.474 1.358 1.373

Abs coeff, , cm-1

0.212 0.153 0.172


Data Fo2>3(Fo

2) 2777 13490 7979


Ra 0.0949 0.0441 0.0498

Rwb 0.2839 0.1333 0.1195

Goodness of Fit 0.952 1.038 1.001


cThese data were collected at 293 K with Mo Kα radiation (λ = 0.71069 Å).

76

Table 4.4: Selected crystallographic data for compound 4-14

Crystal 4-14

Formula C16H36B2Cl4P2

Formula weight 453.81

Crystal system Monoclinic

Space group P21/n

a(Å) 8.8289(3)

b(Å) 14.3784(5)

c(Å) 9.0538(3)

(o) 90.00

( o) 92.991(1)

( o) 90.00

V (Å3) 1147.77(7)

Z 2

d(calc) g cm-1

1.313

Abs coeff, , cm-1

0.654

Data collected 2633

Data Fo2>3(Fo

2) 2416

Variables 115

Ra 0.0277

Rwb 0.0770

Goodness of Fit 1.050


77


4.3.1: Synthesis and Characterization of Phosphinoboranes R2PB(C6F5)2

A series of phosphinoboranes R2PB(C6F5)2 (R=Et, Ph, tBu, Cy, Mes) were synthesized

from (C6F5)2BCl and the corresponding lithium phosphide. The smaller derivatives: R=Et (4-1)

and R=Ph (4-2), exist as dimers [R2P-B(C6F5)2]2, while the R=tBu (4-3), R=Cy (4-4) and R=Mes

(4-5) phosphinoboranes are monomers in solution (Figure 4.3).

Figure 4.3: Synthesis of 4-1 to 4-5 (LiCl is removed upon workup)

Dimers 4-1 and 4-2 are characterized by triplet resonances at -12.9 (1JP-B=72 Hz) and -2.2

(1JP-B=68 Hz) ppm respectively in the

11B NMR spectra. Further NMR spectroscopic evidence

supported these assignments, including gaps of 6.85 and 7.82 ppm between the meta- and para-

fluorine signals in the 19

F NMR spectra for 4-1 and 4-2 respectively, which are typical of neutral,

4 coordinate boron centres.179,180

These structural assignments were additionally supported by

X-ray crystallography (Figure 4.4).

78

Figure 4.4: POV-Ray depictions of phosphinoborane dimers 4-1 (left) and 4-2–CH2Cl2 (right).

Carbon: black, Boron: yellow-green, Fluorine: deep pink, Phosphorus: orange. Solvent and

hydrogen atoms are omitted for clarity. Selected metrical parameters (distances: Å, angles: °)

4-1: B1-P1 2.056(3), B1-P1a 2.058(3), B1-P1-B1a 92.30(10), P1-B1-P2 87.70(10). 4-2–

CH2Cl2: P1-B1 2.096(5), P1-B2 2.088(5), P2-B1 2.022(5), P2-B2 2.080(5), B1-P1-B2

92.65(19), B1-P2-B2 95.05(18), P1-B1-P2 86.57(18), P1-B2-P2 85.32(18)

The crystal structure of 4-1 reveals a symmetric dimer with only one half of the dimer in

the asymmetric unit. The P-B distances are 2.056(3) and 2.058(3) Ǻ, while the B-P-B and P-B-P

bond angles are 92.30(10)° and 87.70(10)° respectively, the remaining metrical parameters are

unexceptional. The structure of the diphenylphosphino analog 4-2 shows similar metrical

parameters, with average P-B bond lengths of 2.072 Ǻ, average P-B-P angle of 85.95° and

average B-P-B angle of 93.87°. The data for these dimers are consistent for those reported for

the related species [(Et2P)2B(µ-PEt2)]2,206

[Et2B(µ-PtBu2)]2

207 and 1,1

1-ferrocene[B(CH3)(µ-

PPh2)]2208

with the exception of P-B bond lengths. The bond lengths are slightly longer for the

compounds reported here, presumably due to crowding resulting from the larger substituents at

boron.

Reaction of the more sterically demanding lithium phosphides (R=tBu, Cy and Mes) with

(C6F5)2BCl resulted in the generation of phosphinoborane monomers of the general form

R2PB(C6F5)2 (Figure 4.3). While related compounds of the general form R2PBMes2 have been

previously synthesized,202,209

these novel compounds were expected to provide different

reactivity and structural features due to the highly electron-withdrawing nature of the fluoroaryl

groups on boron.

79

The NMR spectroscopic data for compounds R2PB(C6F5)2 (4-3, R=tBu; 4-4, R=Cy; 4-5,

R=Mes) were consistent with the proposed formulations and are summarized in Table 4.5, while

spectra for 4-3 are shown in Figure 4.5. The gaps between para- and meta-fluorine resonances

lie closer to the range for typical 4-coordinate boranes, suggesting that there is substantial

electron density being donated into the vacant p-orbital on boron from the lone pair at

phosphorus. The 11

B NMR spectra shows an upfield doublet resonance, typical of 3-coordinate

boranes, which exhibits coupling to phosphorus (1JB-P~150 Hz), while the

31P NMR resonances

are also upfield and significantly broadened due to coupling to the quadrupolar boron centre.

Figure 4.5: Multinuclear NMR spectra for 4-3. A: 1H, B:

11B, C:

19F, D:

31P{

1H}

C

B A

D

o-C6F5

p-C6F5

m-C6F5

80

Table 4.5: Selected NMR spectroscopic data for compounds 4-1 to 4-5

Compound 19

F NMR δ (o, p, m): 19

F NMR

Δm-pa

31P NMR

δ:

11B NMR δ:

[(C6F5)2B(PEt2)]2 (4-1) -125.0, -153.5, -160.3 6.8 -12.9 -23.4

[(C6F5)2B(PPh2)]2 (4-2) -121.3, -156.3, -164.2 7.9 -0.8 -2.2

(C6F5)2B(PtBu2) (4-3) -130.7, -156.0, -163.4 7.4 120.7 41.8

(C6F5)2B(PCy2) (4-4) -130.8, -155.5, -163.5 8.0 92.1 39.5

(C6F5)2B(PMes2) (4-5) -131.2, -154.6, -163.5 8.9 29.3 40.1

a This value is the difference in chemical shift between the meta and para-fluorines, and has been




The structures of 4-3 and 4-4 were confirmed by X-Ray crystallography (Figure 4.6).

The crystal structure of compound 4-3 appeared as anticipated from other analytical data.

Though both phosphorus and boron centres are planar, with sums of angles at 359.07° and

360.0°, respectively, they are not coplanar. This geometry is a result of steric repulsion between

the bulky tert-butyl groups on phosphorus and C6F5 groups on boron. This repulsion manifests

itself in C-P-B-C torsion angles of 21.6° and 7.4°. Compound 4-3 does, however, show a

significantly shorter P-B bond length (1.786(4) Å) than the sterically similar compound

Mes2BPtBu2 (1.841 Ǻ –average of 2 molecules in asymmetric unit) suggesting the electron-

withdrawing effect of the C6F5 group results in a contraction of the P-B bond length.

Compound 4-4 shows essentially coplanar phosphorus and boron centres and a very

short P-B bond length of 1.762(4) Ǻ, significantly shorter than even that observed for 4-3. The

shorter bond length of 4-4 is the result of a stronger P-B π–bond, due to the better overlap on

account of the smaller substituents on phosphorus which allow the phosphorus and boron atoms

to be essentially coplanar. The sum of angles at phosphorus and boron are 359.85° and 359.97°

respectively, while the C-P-B-C torsion angles are 1.1° and 5.6°.

81

Figure 4.6: POV-Ray depictions of phosphinoboranes 4-3 (left) and 4-4 (right). Carbon: black,

Boron: yellow-green, Fluorine: deep pink, Phosphorus: orange. Hydrogen atoms are omitted for

clarity. Selected metrical parameters (distances: Å, angles: °). 4-3: P1-B1 1.786(4), P1-C13

1.862(3), P1-C17 1.865(3), B1-C1 1.580(4), B1-C7 1.590(4), C13-P1-C17 117.08, B1-P1-C13

120.85(14), B1-P1-C17 121.14(15), C1-B1-C7 113.3(2), C1-B1-P1 124.0(2), C7-B1-P1

122.7(2). 4-4: P1-B1 1.762-(4), P1-C13 1.820(4), P1-C19 1.835(4), B1-C1 1.585(5), B1-C7

1.596(5), C13-P1-C19 112.6(2), B1-P1-C13 126.4(2), B1-P1-C19 120.88(19), C1-B1-C7

117.0(3), C1-B1-P1 119.9(3), C7-B1-P1 123.1(3).

The bond lengths observed for 4-3 and 4-4, along with the torsion angles, suggest a

significant π-bonding interaction between the lone pair at phosphorus and the vacant p-orbital on

boron (Figure 4.7). These bonds are much shorter than those observed for the analogous

R2PBMes2 compounds (Table 4.6). This interaction is aided by the bulky, electron-donating R-

groups at phosphorus and the bulky electron withdrawing groups at boron, which both serve to

enforce planarity and encourage electron donation from phosphorus to boron. In fact, values

suggested for P-B double bonds in the literature are 1.79-1.84 Å, while P-B single bonds range

from 1.90-2.00 Å.210

Using these values as a guideline, compounds 4-3 and 4-4 can be

considered as having significant P-B multiple bonding character (in fact, the bond lengths are

shorter than the range attributed to P-B double bonds).

82

A DFT study of 4-3 undertaken in collaboration with Thomas M. Gilbert at the

University of Northern Illinois showed that the π-bonding HOMO was highly polarized, with

74% of this molecular orbital derived from the phosphorus atom, with only 26% from the boron

atom.211

Figure 4.7: Resonance forms of phosphinoboranes 4-3 to 4-5

Table 4.6: Selected metrical parameters for monomeric phosphinoboranes

Compound P-B (Ǻ) Torsion Angles

(C-P-B-C)

Sum of Angles

(B)

Sum of Angles

(P)

4-3 1.786(4) 21.6, 7.4 359.07 360.0

4-4 1.762(4) 1.1, 5.4 359.97 359.85

Mes2BPtBu2

202 1.839(8)

1.843(8)

2.2, 41.2

13.8, 29.8

359.4

359.8

352.0

359.2

Mes2BPMes2202

1.839(8) 4.7, 4.7 360.0 360.0

Mes2BPPh2209

1.859(3) 33.3, 28.5 359.3 339.4

4.3.2: Reactions of Phosphinoboranes with H2 and Independent Synthesis of Phosphine-Borane Adducts R2(H)PB(H)C6F5)2

Though the experimental data for compounds 4-3 to 4-5 suggests significant interaction

between the lone pair at phosphorus and the vacant orbital at boron, the computational data

suggests that they may be highly polarisable. This may allow them to participate in Lewis

acid/Lewis base chemistry. The analogous nature of these species and bimolecular FLPs of

83

phosphines and boranes, suggests that activation of H2 may be possible for these compounds. As

such, compounds 4-1 to 4-5 were exposed to 4 atm H2 at both room temperature (25°C) and

60°C. Indeed, hydrogenation of compounds 4-3 and 4-4 was observed (these results are

summarized in Table 4.6). Compounds 4-1 and 4-2 showed no signs of reaction while 4-5 only

showed trace amounts of reactions after several weeks. The hydrogenations of 4-3 and 4-4 at

60°C are significantly accelerated versus those at room temperature (Table 4.7), suggesting that

while the reaction of related (CF3)2BPR2 species with H2 has been calculated as being

exothermic,203

there may be a relatively large kinetic barrier in the present case. These reactions

produce the compounds R2(H)P-B(H)(C6F5)2, which are secondary phosphine adducts of the

known borane HB(C6F5)2.204,205

These compounds were synthesized independently from

reactions of the phosphines R2PH and the borane HB(C6F5)2 (Figure 4.9). NMR spectroscopic

data for the compounds R2(H)PB(H)(C6F5)2 (4-6, R=Et; 4-7, R=Ph; 4-8, R=tBu; 4-9, R=Cy; 4-

10, R=Mes) are summarized in Table 4.8.

Figure 4.8: Synthesis of 4-8 to 4-10 through H2 activation (left) or Lewis acid-base adduct

formation (right)

Table 4.7: Hydrogenation of phosphinoboranes 4-3 and 4-4

Compound Temperature Time (days) Conversion (by 19

F NMR)

4-3 25 14 100

4-3 60 2 100

4-4 25 28 80

4-4 60 2 100

84

Table 4.8: Selected NMR spectroscopic data for phosphine-borane adducts 4-6 to 4-10

Compound 19

F NMR δ (o, p, m): 31

P NMR δ: 11

B NMR δ:

(C6F5)2HBPHEt2 (4-6) -131.7, -159.2, -164.7 -4.6 -30.0

(C6F5)2HBPHPh2 (4-7) -131.2, -158.8, -164.7 -1.5 -28.5

(C6F5)2HBPHtBu2 (4-8) -129.7, -159.4, -164.7 32.0 -30.0

(C6F4)2HBPHCy2 (4-9) -131.0, -159.4, -164.7 7.1 -28.1

(C6F5)2HBPHMes2 (4-10) -131.4, -158.7, -164.8 -39.0 -25.4

Crystal structures were obtained for compounds 4-6, 4-7 (Figure 4.9) and 4-8 (Figure

4.10). Species 4-6 has the shortest P-B bond length (1.950(3) Å), while 4-7 and 4-8 show

identical P-B bond lengths (1.966(3) Å and 1.966(9) Å, respectively). These lengths are

significantly shorter than those for the R2(H)P-B(C6F5)3 relatives: R=Cy (2.0270(14) Ǻ),212

R=cyclopentyl (2.0243(3) Ǻ), R=Et (2.036(8)Ǻ) and R=Ph (2.098(3)Ǻ),34

as a result of reduced

steric bulk of HB(C6F5)2 compared to B(C6F5)3.

85

Figure 4.9: POV-Ray depictions of 4-6 and 4-7. Carbon: black, Hydrogen: white; Boron:

yellow-green, Fluorine: deep pink, Phosphorus: orange. Carbon-bound hydrogen atoms are

omitted for clarity. Selected metrical parameters (distances: Å, angles: °). 4-6 (one of two

crystallographically independent molecules): P1-B1 1.950(3), H1-B1-P1-H2 176.76. 4-7: P1-B1

1.966(3), H1-B1-P1-H2 178.98.


Fluorine: deep pink, Phosphorus: orange. Carbon-bound hydrogen atoms are omitted for clarity.

Selected metrical parameters (distances: Å, angles: °). 4-8: P1-B1 1.966(9); H1-B1-P1-H2

166.55.

86

In compounds 4-6 and 4-7 the P-H and B-H hydrogen atoms are trans to one another, in a

typical staggered conformation (H-P-B-H=176.76° and 178.98°, respectively), while 4-8 shows

more twisting in the solid state from the staggered conformation (H-P-B-H=166.55°). This

twisting (Figure 4.11) appears to be a solid-state effect as there is no evidence of inequivalent

C6F5 or tert-butyl groups by solution NMR spectroscopy.

Figure 4.11: Newman projection along the B-P bond of 4-6 (R-Et) and 4-7 (R=Ph) (left); and 4-8

(right) as determined by X-ray crystallography

Efforts were made to initiate the loss of H2 from these compounds, however, heating 4-6

to 4-10 (2d, 140°C), even in the presence of a smaller base, which has been shown to accelerate

the loss of H2 from related systems,213

showed no loss of H2. These reactions often showed signs

of dissociation and subsequent decomposition, likely due to trace moisture in solution. These

observations are in contrast to other phosphine borane adducts which can lose hydrogen under

thermal duress,91,195

likely due to the presence of electron-withdrawing groups at boron in the

present case.

A DFT study of this system by Thomas M. Gilbert showed that H2 activation by 4-3 is

exothermic (-43 kcal/mol),211

while the limiting step for the reaction was found to be the first

step: attack of H2 at boron, with a barrier of 22 kcal/mol. The highly exothermic nature of the H2

activation makes the loss of H2 very unfavourable, consistent with the experimental observations.

4.3.3: Reactions of Phosphinoboranes with Lewis Acids and Lewis Bases

To demonstrate the ambiphilic nature of these monomeric P-B compounds, further

reactions were conducted with the goal to coordinate various Lewis acids and bases to the Lewis

basic and acidic sites, respectively, within the monomeric P-B complexes.

These species do react with relatively small Lewis bases, such as 4-tert-butylpyridine,

showing evidence for the formation of Lewis acid-base adducts 4-11 and 4-12, within 20 minutes

87

in CDCl3 at room temperature (Figure 4.12). In the case of 4-12, adduct formation is

quantitative, while only a trace amount of 4-11 is formed initially by multinuclear NMR. Both

reactions show the appearance of multiple products over 24 hours at room temperature in CDCl3.

The molecular structure of 4-12-C7H8 was determined by X-ray crystallography (Figure 4.13).

Figure 4.12: Formation of adducts 4-11 and 4-12

Figure 4.13: POV-Ray depiction of 4-12-C7H8. Carbon: black, Boron: yellow-green, Fluorine:

deep pink, Nitrogen: blue, Phosphorus: orange. Hydrogen and solvent atoms are omitted for

clarity. Selected metrical parameters (distances: Å, angles: °). B1-N1 1.6332(11), B1-P1

2.0329(9), N1-B1-P1 106.04, N1-B1-C1 101.81(6), N1-B1-C7 109.06(6), C1-B1-C7 113.53(7),

P1-B1-C1 115.87(5), P1-B1-C7 109.77(5), B1-P1-C13 101.27(4), B1-P1-C19 109.43(4), C13-

P1-C19 104.20(4).

The crystal structure reveals a dramatic lengthening of the P-B bond to 2.0329(9) Ǻ as a

result of the loss of the π-bonding interaction between the formerly vacant orbital on boron and

88

the lone pair on phosphorus. The B-N bond of 1.6332(11) Ǻ is slightly longer than that reported

for the analogous B(C6F5)3 adduct of 4-tert-butyl pyridine (5-1, 1.618(2) Ǻ). This longer bond

length is a result of diminished Lewis acidity at boron and increased steric crowding caused by

the adjacent phosphorus centre. As expected, the boron centre has become tetrahedral, with

bond angles varying no more than 8 degrees from the idealized value of 109.5°. The N-B-C

angles average 105.44° while the P-B-C angles average 112.82°, indicative of greater steric

repulsion on the B(C6F5)2 fragment from the PCy2 unit than from the coordination of the

pyridine. The phosphorus centre has also pyramidalized with the sum of angles now totalling

314.9°. This is somewhat surprising since the primary reason given for the planarity at

phosphorus in related systems was steric repulsion from the groups on boron.202

The

pyramidalization at phosphorus in 4-12 despite a similar steric environment suggests that the

planarity observed in 4-4 is also aided by a significant electronic factor.

In the case of the reaction of 4-3 with 4-tert-butylpyridine after one day, two broad

upfield peaks typical of B-F resonances were observed in the 19

F NMR spectrum, while 1H,

11B

and 31

P NMR spectra showed a complex mixture of products. The 19

F NMR resonances suggest

that nucleophilic aromatic substitution (NAS) has occurred at one of the C6F5 rings. 4-13, one

component of this complex mixture of products, was characterized crystallographically (Figure

4.14).

89

Figure 4.14: POV-Ray depiction of 4-13. Carbon: black, Boron: yellow-green, Fluorine: deep

pink, Nitrogen: blue, Phosphorus: orange. Hydrogen atoms are omitted for clarity. Selected

metrical parameters (distances: Å, angles: °): B1-N1 1.619(5), B1-C1 1.637(5), B1-C7 1.662(5),

B1-F12 1.395(4), P1-C12 1.854(4), N1-B1-F12 105.1(3), C1-B1-F12 107.6(3), N1-B1-C1

109.8(3), N1-B1-C7 107.0(3), C12-P1-C22 102.27(18), C12-P1-C26 107.59(17), C22-P1-C26

111.27(19). -

4-13 proved to be the product of an unexpected ortho-nucleophilic aromatic substitution

reaction. Such reactions have not been previously reported for fluoroarylboranes and could

potentially be used to generate novel linked FLPs. Optimization of this reaction by tuning of the

size and strength of the Lewis base, could provide cleaner formation of species related to 4-13,

which could transformed into ortho-linked FLPs by abstraction of the base (pyridine in this case)

by a stronger Lewis acid. The formation of 4-13 likely involves initial formation of 4-12, the

loss of π–bonding in 4-12 weakens the P-B bond and makes the phosphine much more

nucleophilic than in 4-3, resulting in rearrangement to 4-13 (Figure 4.15). The P-B bonding in

the starting material places the phosphorus centre close to the ortho-carbons and thus more likely

to attack there than at the para-carbon. While a bimolecular reaction is possible, it would likely

form the more commonly observed para-nucleophilic aromatic substitution product.

90

Figure 4.15: Proposed formation of 4-13 from 4-11 (L=4-(tBu)C5H4N)

Reactions of phosphinoboranes 4-3 and 4-4 with the small Lewis acid BCl3 over 30

minutes in toluene produced dimers 4-14 and 4-15 respectively, [R2PBCl2]2, along with

ClB(C6F5)2 (Figure 4.17). For 4-14 a trace amount of monomer was observed by 11

B NMR

spectroscopy, this observation is consistent with equilibria observed for the related [tBu2PBMe2]2

and [((CH3)3Si)2PBMe2]2 dimers and their respective monomers.214

4-5 did not react with BCl3

under similar conditions, suggesting that the phosphorus centre is not nucleophilic enough to

coordinate to the incoming Lewis acid (the mesityl groups of 4-5 are both bulkier and less

electron-donating than the tert-butyl or cyclohexyl groups of 4-3 and 4-4, resulting in a less

nucleophilic phosphorus centre).

The formation of these species suggests that coordination of BCl3 at phosphorus leads to

a sufficient increase in the Lewis acidity at the fluoroarylborane centre to cause chloride transfer

from BCl3 to the B(C6F5)2 boron (Figure 4.16). Dissociation of ClB(C6F5)2 from the very bulky

phosphine allows dimerization of transient phosphinoborane monomer R2PBCl2. This reaction is

thermodynamically favourable as it results in the formation of 2 P-B bonds while only breaking

one such bond. 4-14 was characterized crystallographically (Figure 4.17).

Figure 4.16: Formation of dimers 4-14 and 4-15

91

Figure 4.17: POV-Ray depiction of 4-14. Carbon: black, Boron: yellow-green, Chlorine:

aquamarine, Phosphorus: orange. Phosphorus: orange; Fluorine: deep pink; Boron: yellow-

green; Chlorine: aquamarine; Carbon: black. Hydrogen atoms are omitted for clarity. Selected

metrical parameters (distances: Å, angles: °): P1-B1 2.0552(15), P1-B1a 2.0557(15), B1-Cl1

1.8409(15), B1-Cl2 1.8453(15), B1-P1-B1a 89.01(6), P1-B1-P1a 90.99(6).

Metrical parameters for 4-14 were similar to those of 4-1 and 4-2, with the P-B bond

lengths again falling towards the long end of the range for related P-B dimers206-208

due to the

bulky groups at phosphorus.

In an effort to react compounds 4-3 and 4-4 with small Lewis acids and bases

simultaneously and possibly trap intermediates in the known Lewis acid catalyzed

dehydrogenation of these species,191

reactions with H3NBH3 and Me2(H)NBH3 were conducted.

While no adduct formation was observed, compounds 4-3 and 4-4 abstracted H2 from the amine-

borane adducts to form compounds 4-8 and 4-9, respectively. The nature of the byproducts from

the reaction with H3NBH3 was not clear due to poor solubility and broad peaks in solvents

compatible with 4-3 and 4-4, while reaction with Me2(H)NBH3 produced the well-known dimer

(Me2N-BH2)2.83

These experiments demonstrate the greater affinity for H2 shown by compounds

4-3 and 4-4 when compared to amine-borane adducts. This is due to enhanced Lewis acidity at

boron in 4-3 and 4-4 compared to the boron centres of Me2(H)NBH3. The proton transfer from

nitrogen to phosphorous is aided by the enthalpically favourable formation of B=N double bond,

while the sterically small B=N species subsequently oligomerize.

92

4.4: Conclusions

Monomeric phosphinoboranes of the general formula R2PB(C6F5)2 can be synthesized,

provided the R groups are large enough to prevent dimerization (tBu, Cy and Mes for example).

These phosphinoboranes do exhibit significant π–bonding as the P-B bond lengths are extremely

short. This bond, however, is sufficiently polarisable for these species to react with H2, or a

variety of small Lewis acids or bases. Lewis acid-base adducts of these ambiphilic

phosphinoboranes are rather unstable due to the steric repulsion and weakening of the P-B bond

caused by the loss of the π-bonding component of the interaction. These reactions could

potentially be optimized to allow synthesis of novel ortho-substituted FLPs related to 4-13.

These results also illustrate the potential of covalently bound group 13-group 15 species

to add hydrogen under mild conditions without a catalyst. This knowledge could aid in the

design of hydrogen storage materials which can be readily hydrogenated following loss of H2.

93

Chapter 5: Frustrated Lewis Pairs: Reactions of Pyridines and Other Nitrogen-Containing

Heterocycles with B(C6F5)3

5.1: Introduction

Hydrogen activation is a tremendously important aspect of modern chemistry. From the

Haber-Bosch process to recent asymmetric hydrogenation reactions, hydrogen activation

chemistry has been dominated by transition metals.215

The area is generally confined to

transition metals due to their ability to undergo oxidative addition reactions with H2.215

As

discussed in section 1.4.3, in recent years heterolytic H2 activation has been found to be possible

utilizing main group frustrated Lewis pairs (FLPs).2 With much recent work focusing on this

reaction,3 efforts are being made to make use of cheaper, lighter Lewis bases in order to expand

the chemistry to more commercially viable applications. Of the currently unexplored options

available, pyridines are a particularly interesting group of compounds. There are a wide variety

of readily available monodentate and bidentate pyridines and potential catalytic hydrogenation

could yield dihydropyridines, which are extremely useful as organic reducing agents.216

Knowing that imines46,73

and amines46,47

are capable of H2 activation with B(C6F5)3, the similar

basicity of pyridines suggests this chemistry should be possible. While pyridine is well known to

form a stable adduct with B(C6F5)3,217

pyridines substituted at the 2 or 6 position should offer the

opportunity for FLP chemistry due to crowding at nitrogen. Also promising is the report by

Brown et al. that 2,6-lutidene does not form an adduct with BMe3 (Figure 5.1).32

Figure 5.1: Brown’s observation of a surprising lack of reactivity between 2,6-lutidine and

BMe332

An interesting aspect of this chemistry is the potential to catalytically generate a

Hantzsch Ester, a powerful pyridine-based source of H2 utilized in a number of organic

reductions,216

from the spent starting material – which is a bulky pyridine. Related compounds

94

such as acridan and tetrahydroquinolines have also been shown to act as stoichiometric reducing

agents (Figure 5.2) 218-223

and again could potentially be regenerated by catalytic hydrogenation

of the corresponding quinoline.

Figure 5.2: Amine-based reducing agents: Hantzsch’s Ester (A), acridan (B) and

tetrahydroquinoline (C)

The regeneration of the spent Hantzsch’s Ester or acridan would require initial H2

activation by the FLP, followed by hydride transfer from boron to the para-carbon of the

pyridine (Figure 5.3). This catalytic hydrogenation pathway is identical to that seen in the

B(C6F5)3 catalyzed hydrogenation of imines,46

except here the hydride attack would take place at

the 4-position of the nitrogen-containing heterocycle. This route could allow for reductions not

possible directly with B(C6F5)3, while maintaining the generally reduced cost and toxicity

compared to transition-metal catalysts.

Figure 5.3: Proposed scheme for catalytic transfer hydrogenation through Hantzsch’s ester.

95

5.2: Experimental Section







ºC under vacuum for 24 hours prior to use. Deuterated solvents were dried over CaH2 (CD2Cl2,

CDCl3) and vacuum distilled prior to use. All common organic reagents were purified by

conventional methods unless otherwise noted. All liquid pyridines were stored over 4 Å

molecular sieves. 1H,

13C,

11B, and

19F nuclear magnetic resonance (NMR) spectroscopy spectra

were recorded on a Bruker Avance-400 spectrometer at 300K unless otherwise noted. 1H and

13C

NMR spectra are referenced to SiMe4 using the residual solvent peak impurity of the given

solvent. 11

B and 19

F NMR experiments were referenced to 15% BF3-Et2O in CDCl3.Chemical

shifts are reported in ppm and coupling constants in Hz as absolute values. Combustion analyses

were performed in house employing a Perkin Elmer CHN Analyzer. B(C6F5)3 was generously

donated by NOVA Chemicals Corporation.

5.2.2: Synthesis of Pyridine-B(C6F5)3 adducts:

(4-tBu)C5H4NB(C6F5)3 (5-1), (2-Me)C5H4NB(C6F5)3 (5-2), (2-Et)C5H4NB(C6F5)3 (5-3), (2-

C5H4N)NH(2-C5H4N)B(C6F5)3 (5-4) (2-Ph)C5H4NB(C6F5)3 (5-5), (2-C5H4N)C5H4NB(C6F5)3 (5-

6), C9H7NB(C6F5)3 (5-7) - These compounds were prepared in a similar fashion and thus only

one preparation is detailed. B(C6F5)3 (100 mg, 0.20 mmol) was added to a solution of 4-tert-

butylpyridine (26 mg, 0.20 mmol) in toluene (2 mL). The solution was stirred for 4 hours,

hexanes (2 mL) was added and the solution was stored at -35 °C overnight. The solution was

decanted from the resulting white precipitate. The precipitate was washed with hexanes (2 mL)

and dried in vacuo.

5-1 - Yield: 98 mg (78%). Anal. Calcd for C27H13BF15N: C, 50.11%; H, 2.02%; N, 2.16%.

Found: C, 50.27%; H, 2.08%; N, 2.16%. Crystals were grown from the hexane wash layer at -

35°C. 1H NMR (CD2Cl2) δ: 1.40 (s, 9H, CH3), 7.64 (d,

3JH-H = 7 Hz, 2H), 8.46 (d,

3JH-H = 7 Hz,

96

2H). 19

F NMR (CD2Cl2) δ: -132.2 (d, 3JF-F = 19 Hz, 6F, o-C6F5), -158.2 (t,

3JF-F = 20 Hz, 3F, p-

C6F5), -164.6 (dd, 3JF-F = 19 Hz,

3JF-F = 20 Hz, 6F, m-C6F5).

11B NMR (CD2Cl2) δ: -4.1 (br s).

13C NMR (CD2Cl2) δ: 30.0 (CH3), 36.1 (C-CH3), 123.0, 137.4 (dm,

1JC-F = 241 Hz, CF), 140.3

(dm, 1JC-F = 260 Hz, CF), 146.3, 148.1 (dm,

1JC-F = 248 Hz, CF), 168.8.

5-2 - Yield: 89%. Anal. Calcd for C24H7BF15N: C, 47.64%; H, 1.17%; N, 2.31%. Found: C,

48.05%; H, 1.38%; N, 2.26%. Crystals were grown from a layered solution of CDCl3/pentane at

-35°C. 1H NMR (CDCl3) δ: 2.51 (s, 3H, CH3), 7.43 (m, 2H), 7.99 (td,

3JH-H = 7 Hz,

4JH-H = 2 Hz,

1H), 8.62 (m, J = 6 Hz, 1H). 19

F NMR (CDCl3) δ: -126.3 (t, 3JF-F = 22 Hz, 1F, o-C6F5), -128.9

(m, 1F, o-C6F5), -132.4 (d, 3JF-F = 22 Hz, 1F, o-C6F5), -133.2 (m, 1F, o-C6F5), -133.4 (m, 1F, o-

C6F5), -137.7 (m, 1F, o-C6F5), -155.6 (t, 3JF-F = 22 Hz, 1F, p-C6F5), -156.2 (t,

3JF-F = 22 Hz, 1F,

p-C6F5), -157.7 (t, 3JF-F = 22 Hz, 1F, p-C6F5), -161.9 (td,

3JF-F = 21 Hz,

4JF-F = 9 Hz, 1F, m-C6F5),

-162.9 (td, 3JF-F = 22 Hz,

4JF-F = 10 Hz, 1F, m-C6F5), -163.8 (td,

3JF-F = 22 Hz,

4JF-F = 9 Hz, 1F,

m-C6F5), -163.9 (td, 3JF-F = 21 Hz,

4JF-F = 9 Hz, 1F, m-C6F5), -164.2 (td,

3JF-F = 22 Hz,

4JF-F = 9

Hz, 1F, m-C6F5), -164.5 (td, 3JF-F = 22 Hz,

4JF-F = 8 Hz, 1F, m-C6F5).

11B NMR (CDCl3) δ: -3.6.

13C NMR (CDCl3) (partial) δ: 14.3, 122.6, 129.3, 142.3, 147.9, 159.8.

5-3 - Yield: 88%. Anal. Calcd. for C25H9BF15N: C, 48.50%; H, 1.47%; N, 2.26%. Found: C,

48.25%; H, 1.58%; N, 2.26%. Crystals were grown from the pentane wash layer at room

temperature. 1H NMR (CD2Cl2) δ: 0.80 (t,

3JH-H = 8 Hz, 3H, CH2CH3), 2.99 (dq,

2JH-H = 23 Hz,

3JH-H = 8 Hz, 1H, CH2CH3), 3.05 (dq,

2JH-H = 23 Hz,

3JH-H = 8 Hz, 1H, CH2CH3), 7.51 (t,

3JH-H =

7 Hz, 1H), 7.63 (d, 3JH-H = 8 Hz, 1H), 8.15 (td,

3JH-H = 8 Hz,

4JH-H = 1 Hz, 1H), 8.67 (q, J = 6 Hz,

1H). 19

F NMR (CD2Cl2) δ: -126.5 (t, 3JF-F = 22 Hz, 1F, o-C6F5), -129.6 (m, 1F, o-C6F5), -132.4

(d, 3JF-F = 22 Hz, 1F, o-C6F5), -133.7 (m, 1F, o-C6F5), -134.7 (m, 1F, o-C6F5), -137.3 (td,

3JF-F =

24 Hz, 4JF-F = 9 Hz, 1F, o-C6F5), -157.0 (t,

3JF-F = 21 Hz, 1F, p-C6F5), -157.3 (t,

3JF-F = 20 Hz, 1F,

p-C6F5), -159.2 (t, 3JF-F = 20 Hz, 1F, p-C6F5), -163.2 (td,

3JF-F = 22 Hz,

4JF-F = 9 Hz, 1F, m-C6F5),

-164.0 (td, 3JF-F = 22 Hz,

4JF-F = 10 Hz, 1F, m-C6F5), -164.8 (td,

3JF-F = 22 Hz,

4JF-F = 9 Hz, 1F,

m-C6F5), -165.1 (td, 3JF-F = 21 Hz,

3JF-F = 9 Hz, 1F, m-C6F5), -165.4 (td,

3JF-F = 22 Hz,

4JF-F = 9

Hz, 1F, m-C6F5), -165.6 (td, 3JF-F = 22 Hz,

3JF-F = 8 Hz, 1F, m-C6F5).

11B NMR (CD2Cl2) δ: -3.6.

13C NMR (CD2Cl2) (partial) δ: 13.2, 27.4, 122.6, 127.6, 142.8, 165.5.

5-4 - Yield: 86%. Anal. Calcd for C28H9BF15N3: C, 49.23%; H, 1.33%; N, 6.15%. Found: C,

49.59%; H, 1.69%; N, 6.13%. X-ray quality crystals were grown by slow evaporation from

97

CD2Cl2. 1H NMR (CD2Cl2) δ: 6.44 (d,

3JH-H = 8 Hz, 1H), 6.96 (dd,

3JH-H = 7 Hz,

3JH-H = 5 Hz,

2H), 7.02 (td, 3JH-H = 7 Hz,

4JH-H = 1 Hz, 1H), 7.55 (td,

3JH-H = 8 Hz,

4JH-H = 2 Hz, 1H), 7.67 (br s,

NH), 7.91 (ddd, 3JH-H = 9 Hz,

3JH-H = 7 Hz,

4JH-H = 2 Hz, 1H), 8.21 (m, 2H), 8.58 (d,

3JH-H = 9 Hz,

1H). 19

F NMR (CD2Cl2) δ: -127.0 (m, 1F, o-C6F5), -128.1 (m, 1F, o-C6F5), -131.6 (d, 3JF-F = 23

Hz, 1F, o-C6F5), -133.0 (m, 1F, o-C6F5); -135.7 (m, 1F, o-C6F5), -137.1 (m, 1F, o-C6F5), -157.0

(t, 3JF-F = 20 Hz, 3F, p-C6F5), -163.0 (td,

3JF-F = 22 Hz,

4JF-F = 8 Hz, 1F, m-C6F5), -163.9 (tt,

3JF-F

= 22 Hz, 4JF-F = 9 Hz, 2F, m-C6F5), -164.0 (td-,

3JF-F = 21 Hz,

4JF-F = 7 Hz, 1F, m-C6F5), -164.2

(td, 3JF-F = 22 Hz,

4JF-F = 8 Hz, 1F, m-C6F5), -165.1 (td,

3JF-F = 22 Hz,

4JF-F = 8 Hz, 1F, m-C6F5).

11B NMR (CD2Cl2) δ: -5.1.

13C NMR (CD2Cl2) (partial) δ: 139.0, 142.8, 144.1 (m), 148.2,

151.2, 152.4 (m).


51.77%; H, 1.71%; N, 2.32%. Crystals were grown from toluene at room temperature. 1H NMR

(CDCl3) δ: 7.05 (br s, 2H), 7.25 (t, 3JH-H = 8 Hz, 1H), 7.38 (d,

3JH-H = 8 Hz, 1H), 7.40 (br s, 1H),

7.65 (t, 3JH-H = 8 Hz, 1H), 7.85 (br s, 1H), 8.13 (t,

3JH-H = 8 Hz, 1H), 8.93 (s, 1H).

19F NMR

(CDCl3) δ: -125.4 (br s, 1F, o-C6F5), -128.6 (br s, 1F, o-C6F5), -131.2 (br s, 1F, o-C6F5), -131.9

(d, 3JF-F = 18 Hz, 1F, o-C6F5), -133.9 (br s, 2F, o-C6F5), -155.4 (t,

3JF-F = 20 Hz, 1F, p-C6F5), -

157.7 (br s, 2F, p-C6F5), -162.0 (t, 3JF-F = 23 Hz, 1F, m-C6F5), -162.9 (t,

3JF-F = 23 Hz, 1F, m-

C6F5), -164.5 (br s, 2F, m-C6F5), -165.1 (br m, 1F, m-C6F5), -165.6 (br s, 1F, m-C6F5). 11

B NMR

(CDCl3) δ: -2.9. 13

C NMR (CDCl3) (partial) δ: 124.3, 128.0, 128.5, 129.8, 131.6, 142.3, 148.5.

5-6 - Yield: 79%. Anal. Calcd for C28H8BF15N2: C, 50.33%; H, 1.21%; N, 4.19%. Found: C,

49.87%; H, 1.44%; N, 4.32%. Crystals were grown from toluene at -35°C. 1H NMR (CD2Cl2) δ:

6.68 (d, 3JH-H = 8 Hz, 1H), 7.16 (ddd,

3JH-H = 8 Hz,

3JH-H = 5 Hz,

4JH-H = 1 Hz, 1H), 7.43-7.48 (ov

m, 2H), 7.72 (ddd, 3JH-H = 8 Hz,

3JH-H = 6 Hz,

4JH-H = 2 Hz, 1H), 8.17 (ddd,

3JH-H = 5 Hz,

4JH-H =

2 Hz, 4JH-H = 1 Hz, 1H), 8.23 (td,

3JH-H = 8 Hz,

4JH-H = 2 Hz, 1H), 8.82 (br s, 1H).

19F NMR

(CD2Cl2) δ: : -125.2 (m, 1F, o-C6F5), -130.9 (m, 1F, o-C6F5), -131.5 (m, 1F, o-C6F5), -133.1 (m,

2F, o-C6F5), -135.6 (d, 3JF-F=21 Hz, 1F, o-C6F5), -156.7 (t,

3JF-F = 19 Hz, 1F, p-C6F5), -158.3 (m,

1F, p-C6F5), -160.0 (t, 3JF-F = 21 Hz, 1F, p-C6F5), -160.5 (m, 1F, m-C6F5), -163.3 (t,

3JF-F = 21

Hz, 1F, m-C6F5), -163.8 (t, 3JF-F = 21 Hz, 1F, m-C6F5), -166.0 (t,

3JF-F = 20 Hz, 1F, m-C6F5), -

166.5 (m, 1F, m-C6F5), -167.6 (m, 1F, m-C6F5). 11

B NMR (CD2Cl2) δ: -2.7. 13

C NMR (CD2Cl2)

(partial) δ: 123.7, 124.0, 125.0, 130.8, 136.4, 142.8, 148.5, 149.3, 153.6, 158.9.

98


50.23%; H, 0.98%; N, 2.35%. 1H NMR (CDCl3) δ: 7.77 (m, 2H), 7.84 (m, 1H), 8.09 (dd,

3JH-H =

8 Hz, 4JH-H = 2 Hz, 1H), 8.51 (d,

3JH-H = 9 Hz, 1H), 8.72 (d,

3JH-H = 8 Hz, 1H), 9.19 (q,

3JH-H = 5

Hz, 1H). 19

F NMR (CDCl3) δ: -126.6 (t, 3JF-F = 27 Hz, 1F, o-C6F5), -128.8 (br m, 1F, o-C6F5), -

131.9 (br m, 1F, o-C6F5), -132.9 (br d, 3JF-F = 36 Hz, 1F, o-C6F5), -133.3 (br m, 1F, o-C6F5), -

133.7 (m, 1F, o-C6F5), -155.1 (tt, 3JF-F = 20 Hz,

4JF-F = 4 Hz, 1F, p-C6F5), -156.2 (tt,

3JF-F = 20

Hz, 4JF-F = 3 Hz, 1F, p-C6F5), -157.2 (tt,

3JF-F = 20 Hz,

4JF-F = 3 Hz, 1F, p-C6F5), -161.2 (td,

3JF-F

= 21 Hz, 4JF-F = 8 Hz, 1F, m-C6F5), -162.3 (td,

3JF-F = 23 Hz,

4JF-F = 10 Hz, 1F, m-C6F5), -163.2

(td, 3JF-F = 22 Hz,

4JF-F = 8 Hz, 1F, m-C6F5), -163.7 (td,

3JF-F = 22 Hz,

4JF-F = 9 Hz, 1F, m-C6F5), -

163.8 (m, peaks overlapping, 1F, m-C6F5), -163.9 (m, peaks overlapping, 1F, m-C6F5). 11

B

NMR (CDCl3) δ: -3.2. 13

C NMR (CDCl3) (partial) δ: 120.2, 122.4, 128.6, 129.6, 130.1, 133.1,

142.6, 145.0, 150.4.

(2,6-Me2C5H3N)B(C6F5)3 (5-8) - B(C6F5)3 (100 mg, 0.20 mmol) was added to 2,6-lutidine (21

mg, 0.20 mmol) in 2 mL of toluene. The solution was allowed to stir for 4 h and pentane (2 mL)

was added. The solution was stored at -35°C. X-ray quality crystals precipitated from solution

and were washed with pentane (2 x 2 mL) and dried in vacuo. Yield: 60 mg (51%). NMR data

were acquired at -10°C.

1H NMR (CD2Cl2) δ: 2.58 (s, CH3), 7.36 (d, 2H,

3JH-H = 8 Hz, m-CH), 7.89 (t, 1H,

3JH-H = 8 Hz,

p-H). 19

F NMR (CD2Cl2) δ: -131.4 (br s, 2F, o-C6F5), -132.4 (br s, 2F, o-C6F5), -133.0 (d, 2F,

3JF-F = 18 Hz, o-C6F5), -157.6 (t, 1F,

3JF-F = 20 Hz, p-C6F5), -158.7 (t, 2F,

3JF-F = 20 Hz, p-C6F5),

-164.4 (t, 2F, 3JF-F = 21 Hz, m-C6F5), -165.2 (m, 4F, m-C6F5).

11B NMR (CD2Cl2) δ: -3.9.

99

van’t Hoff Plot for the Equilibrium Between Adduct 5-8 and the Separated Lewis Acid and

Lewis Base (values for 2 separate reactions are plotted, K was calculated based on 1H and

19F

NMR spectroscopy)

ΔG= ΔH - TΔS= -RTlnK

lnK= - ΔH/(RT) + ΔS/R (y = mx + b)

ΔH0= -mR= -(5062.4)(8.31451 J/(mol*K))= -42(1) kJ/mol

ΔS0= bR= (-15.809) (8.31451 J/(mol*K))= -131(5) J/(mol*K)

ΔG0= ΔH

0-TΔS

0= (-42.1 kJ/mol)-(298.15 K)(8.31451 J/(mol*K))= -2.9 kJ/mol

5.2.3 Synthesis of Pyridinium Borate Ion Pairs through H2 Activation by Pyridine-Borane FLPs

[2,6-Me2C5H3NH][HB(C6F5)3] (5-9), [(2,6-Ph2)C5H3NH][HB(C6F5)3] (5-10), [(2-

tBu)C5H4NH][HB(C6F5)3] (5-11) - These compounds were prepared in a similar fashion and thus

only one preparation is detailed. B(C6F5)3 (100 mg, 0.20 mmol) was added to 2,6-lutidine (21

mg, 0.20 mmol) in toluene (10 mL). The solution was subjected to 3 freeze-pump-thaw cycles

and backfilled with H2 at 77 K (~4 atm). The solution was allowed to stir overnight at room

temperature and dried in vacuo. The solid was washed with pentane (2 x 2 mL) and again dried

in vacuo. 5-9 - Yield: 105 mg (87%). Anal. Calcd for C26H11BF15N: C, 48.34%; H, 1.78%; N,

2.25%. Found: C, 48.49%; H, 2.06%; N, 2.43%. X-Ray quality crystals were grown by slow

evaporation of a toluene solution. 1H NMR (CD2Cl2) δ: 2.61 (s, 6H, CH3), 3.55 (q,

1JB-H = 88

y = 5062.4x - 15.809R² = 0.9824

0

1

2

3

4

5

6

7

8

0.0032 0.0034 0.0036 0.0038 0.004 0.0042 0.0044 0.0046

1/T (K-1)

lnK

100

Hz, B-H), 7.53 (d, 3JH-H = 8 Hz, 2H, m-CH), 8.22 (t, 1H,

3JH-H = 8 Hz, p-CH), 12.01 (br s, 1H, N-

H). 19

F NMR (CD2Cl2) δ: -136.8 (br d, 6F, 3JF-F = 18 Hz, o-C6F5), -165.8 (t, 3F

3JF-F = 20 Hz, p-

C6F5), -169.3 (br t, 6F, 3JF-F = 20 Hz, m-C6F5).

11B NMR (CD2Cl2) δ: -24.7 (d,

1JB-H = 88 Hz).

13C NMR (CD2Cl2) (partial) δ: 19.9, 125.5, 136.7 (dm,

1JC-F = 245 Hz CF),138.4 (dm,

1JC-F =

249 Hz, CF), 147.2, 148.2, (dm, 1JC-F = 238 Hz, CF), 153.8.

5-10 - Yield: 82%. Anal. Calcd for C35H15BF15N: C, 56.40%; H, 2.03%; N, 1.88%. Found: C,

56.19%; H, 2.18%; N, 2.09%. 1H NMR (CD2Cl2) δ: 3.35 (q,

1JB-H = 92 Hz, B-H), 7.57-7.62 (m,

4H), 7.64-7.68 (m, 2H), 7.74-7.77 (m, 4H), 8.05 (d, 3JH-H = 8 Hz, 2H, m-CH), 8.51 (t,

3JH-H = 8

Hz, 1H, p-CH), 11.27 (br s, 1H, N-H). 19

F NMR (CD2Cl2) δ: -134.3 (br d, 3JF-F = 21 Hz, 6F, o-

C6F5), -164.8 (t, 3JF-F = 20 Hz, 3F, p-C6F5), -167.7 (br t,

3JF-F = 20 Hz, 6F, m-C6F5).

11B NMR

(CD2Cl2) δ: -24.6 (d, 1JB-H = 92 Hz).

13C NMR (CD2Cl2) (partial) δ: 122.6, 126.9, 127.5, 130.2,

132.4.

5-11 - Yield: 105 mg (83%). Anal. Calcd for C27H15BF15N: C, 49.95%; H, 2.33%; N, 2.16%.

Found: C, 49.76%; H, 2.22%; N, 2.06%. 1H NMR (CD2Cl2) δ: 1.51 (s, 9H, C-CH3), 3.66 (q,

1JB-

H = 88 Hz, 1H, B-H), 7.80 (t, 3JH-H = 7 Hz, 1H), 7.96 (d,

3JH-H = 8 Hz, 1H), 8.45 (dd,

3JH-H = 8

Hz, 4JH-H = 2 Hz, 1H), 8.48 (d,

3JH-H = 7 Hz, 1H), 12.13 (br s, 1H, N-H).

19F NMR (CD2Cl2) δ: -

134.7 (br d, 3JF-F = 22 Hz, 6F, o-C6F5), -163.6 (t,

3JF-F = 21 Hz, 3F, p-C6F5), -167.1 (m, 6F, m-

C6F5). 11

B NMR (CD2Cl2) δ: -24.7 (d, 1JB-H = 87 Hz).

13C NMR (CD2Cl2) (partial) δ: 28.8,

37.1, 125.2, 125.3, 136.8 (dm, 1JC-F = 254 Hz, CF), 140.7, 147.7 148.2 (dm, CF,

1JC-F = 240 Hz).

[(2,3,5,6-Me4C4N2H)][HB(C6F5)3] (5-12) – In a J. Young-type NMR tube, B(C6F5)3 (20 mg,

0.039 mmol) was added to tetramethylpyrazine (5 mg, 0.04 mmol) in toluene-d8 (0.75 mL). The

solution was subjected to 3 freeze-pump-thaw cycles and backfilled with 1 atm H2 at 77 K (~4

atm at ambient temperature). The tube was sealed and warmed to room temperature. The

solution was monitored by multinuclear NMR; the reaction proceeded to completion over 18

hours. The product decomposed to several other species upon attempted workup and could not

be isolated.

1H NMR (tol-d8) δ: 2.04 (s, 12H, CH3), 3.84 (q,

1JB-H=94 Hz, B-H), 13.98 (br s, 1H, N-H).

19F

NMR (tol-d8): -133.3 (br d, 3JF-F=22 Hz, 6F, o-C6F5), -162.0 (t,

3JF-F=19 Hz, 3F, p-C6F5), -165.8

(br t, 3JF-F=21 Hz, 6F, m-C6F5).

11B NMR (tol-d8) δ: -24.9 (d,

1JB-H=90 Hz).

101

5.2.4: Synthesis of a Pyridinium Borate Zwitterion via THF Ring-Opening

2,6-Me2C5H3N(CH2)4OB(C6F5)3 (5-13) - 2,6 lutidine (25 mg, 0.23 mmol) was added to a

solution of B(C6F5)3 (100 mg, 0.20 mmol) in THF (2 mL). The solution was stirred for 3 days.

Pentane (2 mL) was added to ensure complete precipitation of the product, the solvent was

decanted and the resulting solid washed with pentane (2 x 2 mL). Yield: 120 mg (89%). X-Ray

quality crystals were grown from CH2Cl2 at room temperature. Anal. Calcd. for C29H17BF15NO:

Calcd: C, 50.39; H, 2.48; N, 2.03. Found: C, 50.30; H, 2.66; N, 2.17. 1H NMR (THF-d8) δ: 1.62

(m, 2H, CH2), 1.90 (m, 2H, CH2), 2.74(s, 6H, CH3), 3.18 (t, 3JHH=8 Hz, 2H, CH2), 4.68 (m, 2H,

CH2), 7.65 (d, 3JHH = 8 Hz, m-CH), 8.09 (t,

3JHH = 8 Hz, p-CH).

19F NMR (THF-d8) δ:-132.5 (d,

6F, 3JFF = 21 Hz, o-C6F5), -163.3 (t, 3F,

3JFF=21 Hz, p-C6F5), -166.6 (t, 6F

3JFF = 19 Hz, m-C6F5).

11B NMR (THF-d8) δ: -6.3.

13C NMR (THF-d8) (partial) δ: 22.0, 28.7, 30.2, 55.5, 65.4, 129.8,

138.5(dm, 1JCF = 244 Hz, CF), 140.3 (dm,

1JCF = 244 Hz, CF), 145.7, 150.1 (dm,

1JCF = 241 Hz,

CF), 158.0.

5.2.5: Reactions of Diethyl-2,6-dimethyl-3,5-pyridinedicarboxylate with B(C6F5)3

2,6-(CH3)2-3,5-(COOCH2CH3)2C5HN-B(C6F5)3 (5-14) - B(C6F5)3 (100 mg, 0.20 mmol) was

added to of diethyl-2,6-dimethyl-3,5-pyridinedicarboxylate (48 mg, 0.20 mmol) in toluene (2

mL). The solution was allowed to stir for 4 hours and then dried in vacuo. The solid was

washed with pentane (2 x 2 mL) and again dried in vacuo. Yield: 110 mg (74%). X-Ray quality

crystals were grown from pentane at -35°C. Anal. Calcd for C31H17BF15NO4: C, 48.78%; H,

2.24%; N, 1.84%. Found: C, 48.59%; H, 2.17%; N, 1.85%. Cooling to -60°C resulted in only

broadening of the peaks, not in resolution.

1H NMR (CD2Cl2) δ: 1.40 (t,

3JH-H=8 Hz, CH2-CH3), 2.75 (s, C-CH3), 4.47 (q,

3JH-H=8 Hz, CH2-

CH3), 8.48 (s, CH). 19

F NMR (CD2Cl2): -131.5 (br d, 3JF-F=17 Hz, 6F, o-C6F5), -150.5 (br s, 3F,

p-C6F5), -162.9 (br s, 6F, m-C6F5). 11

B NMR (CD2Cl2) δ: 42.2. 13

C NMR (CD2Cl2) (partial) δ:

13.9, 24.4, 64.0, 122.5, 137.7 (dm, 1JC-F=252 Hz, CF), 140.3, 148.1 (dm,

1JC-F=252 Hz, CF),

162.1, 168.3 (m).

[2,6-(CH3)2-3,5-(COOCH2CH3)2C5HNH][HB(C6F5)3] (5-15) and 2,6-(CH3)2-3,5-

(COOCH2CH3)2-2-H-C5H2NH-B(C6F5)3 (5-16) - 5-14 (20 mg, 0.027 mmol) in 0.75 mL toluene-

d8 was exposed to 4 atm H2 in a J. Young tube. The reaction was monitored by multinuclear

102

NMR spectroscopy and compared to independent syntheses of the products 5-15 and 5-16

through reaction of B(C6F5)3 with Hantzsch’s Ester.224

5.2.6: Reactions of Bulky Substituted Quinolines with B(C6F5)3

C13H9N-B(C6F5)3 (5-17) – Acridine (18 mg, 0.10 mmol) was added to a solution of B(C6F5)3 (50

mg, 0.10 mmol) in CDCl3 (0.75 mL) . In addition to the signals reported for the adducts, peaks

for free B(C6F5)3 were observed in the 19

F and 11

B NMR spectra. Only one set of peaks was

observed in the 1H NMR spectrum, as the resonances for the adduct are averaged with those for

free acridine. From the 19

F NMR spectrum at 25°C in CDCl3, Keq=19.1 M-1

.

1H NMR (CDCl3) δ: 7.64 (br s, 2H), 7.81(t,

3JH-H=8 Hz, 2H), 8.13 (s, 2H), 8.44 (br s, 2H), 9.10

(v br s, 1H). 19

F NMR (CDCl3) δ: -130.4 (br s, 6F, o-C6F5), -157.0 (br s, 3F, p-C6F5), -163.6 (br

s, 6F, m-C6F5). 11

B NMR (CDCl3) δ: -3.2.

2-(CH3)C9H6N-B(C6F5)3 (5-18) – 2-methylquinoline (14 mg, 0.10 mmol) was added to a solution

of B(C6F5)3 (50 mg, 0.10 mmol) in CDCl3 (0.75 mL). In addition to the signals reported for the

adduct, peaks for free B(C6F5)3 were observed in the 19

F and 11

B NMR spectra. Only one set of

peaks was observed in the 1H NMR spectrum, as the resonances for the adduct are averaged with

those for free 2-methylquinoline. From the 19

F NMR spectrum at 25°C in CDCl3, Keq=26.7 M-1

.

1H NMR (CDCl3) δ: 2.86 (s, 3H, CH3), 7.46 (br s, 1H), 7.63 (m, 2H), 7.90 (d,

3JH-H=6 Hz, 1H),

8.30 (br s, 2H). 19

F NMR (CDCl3) δ: -129.9 (br s, 3F, o-C6F5), -130.9(br s, 3F, o-C6F5), -156.3

(br s, 1F, p-C6F5), -156.9 (br s, 2F, p-C6F5), -162.7(br s, 2F, m-C6F5), -163.8 (br s, 2F, m-C6F5).

11B NMR (CDCl3) δ: -3.2.

1,10-C12H8N2-B(C6F5)3 (5-19) – 1,10-phenanthroline (7 mg, 0.04 mmol) was added to B(C6F5)3

(20 mg, 0.039 mmol) in CH2Cl2 (2 mL). The solution was allowed to stir for 2 hours, dried in

vacuo and the resulting solid was washed with pentane (2 x 2 mL). The resulting white solid

was again dried in vacuo. X-Ray quality crystals were grown from a layered solution of

CDCl3/pentane. Yield: 26 mg (96%). Anal. Calcd, for C30H8BF15N2: C, 52.06%; H, 1.16%; N,

4.05%. Found: C, 51.78%; H, 1.19%; N, 4.23%.

1H NMR (CDCl3) δ: 7.77 (m, 2H), 7.84 (m, 1H), 8.09 (dd,

3JH-H=8 Hz,

4JH-H=2 Hz,1H), 8.51 (d,

3JH-H=9 Hz, 1H), 8.72 (d,

3JH-H=8 Hz, 1H), 9.19 (q,

3JH-H=5 Hz, 1H).

19F NMR (CDCl3) δ: -

103

124.9 (br s, 1F, o-C6F5), -130.1 (br s, 1F, o-C6F5), -131.5 (br s, 1F, o-C6F5), -132.5 (br s, 1F, o-

C6F5), -134.9 (br s, 1F, o-C6F5), -155.8 (br s, 1F, p-C6F5), -159.0 (br s, 1F, p-C6F5), -161.7 (br s,

1F, p-C6F5), -162.2 (br s, 1F, m-C6F5), -163.0 (br s, 1F, m-C6F5), -166.0 (br s, 1F, m-C6F5), -

166.3 (br s, 1F, m-C6F5), -166.8 (br s, 1F, m-C6F5), -167.7 (br s, 1F, m-C6F5). 11

B NMR

(CDCl3) δ: -3.2. 13

C NMR (CDCl3) (partial) δ: 123.2 (dm, 1JC-F =227 Hz, CF), 125.9, 130.9

(dm, 1JC-F=296 Hz, CF), 136.5, 146.0 (dm,

1JC-F=279 Hz, CF), 153.2.

5.2.7: Metal-Free Catalytic Hydrogenations

A solvent bomb charged with 100 mg of substrate and the appropriate mass of B(C6F5)3 in

toluene (5 mL) was subjected to 3 freeze-pump-thaw cycles and exposed to 1 atm H2 at 77 K.

The solution was stirred under the prescribed conditions and then allowed to cool. After cooling,

5 mL of ethyl acetate was added and the solution was run through a plug of silica gel. Volatiles

were removed in vacuo. Analytical data for products 5-20,225

5-21,18,226,227

5-22,227

5-23226

and

5-24228

matched that previously published.

5.2.8: Reaction of Aminopyridines with Fluoroarylboranes

(5-Me)C5H3NH(2-NH)B(C6F5)3 (5-25) - 2-amino-6-picoline (4.5 mg, 0.04 mmol) was added to a

solution of B(C6F5)3 (20 mg, 0.039 mmol) in CH2Cl2 (2 mL), The solution was allowed to stand

for two hours, then all volatiles were removed and the residue was washed with pentane (2 x 2

mL). The resulting white solid was dried in vacuo. Yield: 23 mg (96%). X-Ray quality crystals

were grown from a layered solution of CDCl3/pentane at room temperature. Anal. Calcd for

C24H8BF15N2: C, 46.48%; H, 1.30%; N, 4.52%. Found: C, 46.30%; H, 1.18%; N, 5.02%.

1H NMR (CDCl3) δ: 2.13 (s, 3H, CH3), 6.07 (br s, 1H, amide N-H), 6.23 (dm,

3JH-H = 7 Hz, 1H),

6.55 (br d, 3JH-H = 9 Hz, 1H), 7.38 (dd,

3JH-H = 9 Hz,

3JH-H = 7 Hz), 8.65 (br s, 1H, pyridinium N-

H). 19

F NMR (CDCl3) δ: -133.7 (d, 3JF-F = 20 Hz, 6F, o-C6F5), -157.0 (t,

3JF-F = 19 Hz, 3F, p-

C6F5), -163.0 (br s, 6F, m-C6F5). 11

B NMR (CDCl3) δ: -11.1. 13

C NMR (CDCl3) (partial) δ:

19.3, 109.5, 114.5, 137.0 (dm, 1JC-F = 256 Hz, CF), 141.8, 142.3 (dm,

1JC-F = 242 Hz, CF), 148.1

(dm, 1JC-F = 240 Hz, CF), 155.2.

(5-Me)C5H3NH(2-NH)BCl(C6F5)2 (5-26) - 2-amino-6-picoline (28 mg, 0.26 mmol) was added to

a solution of ClB(C6F5)2 (100 mg, 0.29 mmol) in CH2Cl2 (5 mL). The solution was allowed to

stir overnight, the solvent was removed in vacuo and the residue was washed with hexanes (2 x 2

104

mL). X-Ray quality crystals were grown from the hexane wash layer. Yield: 119 mg (91%).

Anal. Calcd. for C18H8BClF15N2 (%) C: 44.25, H: 1.65, N: 5.73; found C: 44.19, H: 1.94, N:

5.55.

1H NMR (CDCl3) δ: 2.39 (s, 3H, CH3), 6.38 (d,

3JH-H=7 Hz, 1H), 6.46 (br s, 1H, NH), 6.58 (d,

3JH-H=9 Hz, 1H), 7.48 (dd,

3JH-H=9 Hz,

3JH-H=7 Hz, 1H, p-CH), 11.10 (br s, 1H, NH) ;

19F NMR

(CDCl3) δ: -133.5 (dd, 3JF-F=23 Hz,

4JF-F=8 Hz 6F, o-C6F5), -155.9 (t,

3JF-F=21 Hz, 3F, p-C6F5), -

162.3 (td, 3JF-F=21 Hz,

4JF-F=8 Hz, 6F, m-C6F5).

11B NMR (CDCl3) δ: -2.8.

13C NMR (CDCl3)

partial δ: 19.6, 110.4, 114.4, 142.5, 144.8, 155.2.

(5-Me)C5H3NH(2-NH)BH(C6F5)2 (5-27) - 2-amino-6-picoline (31 mg, 0.28 mmol) was added to

a solution of HB(C6F5)2 (100 mg, 0.29 mmol) in CH2Cl2 (5 mL). The solution was allowed to

stir overnight, the solvent was removed in vacuo and the residue was washed with hexanes (2 x 2

mL). Yield: 119 mg (91%). X-Ray quality crystals were grown from the hexane wash layer.

Anal. Calcd. for C18H9BF15N2 (%) C: 47.61, H: 2.00, N: 6.17; found C: 47.26, H: 2.38, N: 6.36.

1H NMR (CDCl3) δ: 2.37 (s, 3H, CH3), 3.85 (q,

1JH-B=94 Hz, 1H, BH), 6.18 (d,

3JH-H=7 Hz, 1H),

6.26 (br s, 1H, NH), 6.47 (d, 3JH-H=9 Hz, 1H), 7.32 (dd,

3JH-H=9 Hz,

3JH-H=7 Hz, 1H, p-CH), 9.85

(br s, 1H, NH) . 19

F NMR (CDCl3) δ: -135.4 (d, 3JF-F=23 Hz, 6F, o-C6F5), -159.4 (t,

3JF-F=20

Hz, 3F, p-C6F5), -164.1 (tm, 3JF-F=20 Hz, 6F, m-C6F5).

11B NMR (CDCl3) δ: -18.3 (d,

3JH-B=94

Hz). 13

C NMR (CDCl3) partial δ: 19.9, 108.7, 114.3, 141.5, 144.1, 155.2.

(5-Me)C5H3N(2-NH)B(C6F5)2 (5-28) – Iso-propylmagnesiumchloride (0.657 mL of a 2.0 M

solution in diethyl ether, 1.31 mmol) was added dropwise to a solution of 5-26 (642 mg, 1.31

mmol) in diethyl ether (10 mL). The cloudy solution was allowed to stir for 2 hours, hexanes (10

mL) was added and the solution was filtered. The filtrate was dried in vacuo. Yield: 543 mg

(92%). Anal. Calcd. for C18H7BF10N2 (%) C: 47.82, H: 1.56, N: 6.20; found C: 47.44, H: 1.98,

N: 6.23.

1H NMR (CDCl3) δ: 2.40 (s, 3H, CH3), 6.55 (d,

3JH-H=8 Hz, 1H), 6.94 (d,

3JH-H=8 Hz, 1H), 7.46

(t, 3JH-H=8 Hz, 1H, p-CH), 7.79 (br s, 1H, NH).

19F NMR (CDCl3) δ: -131.7(m, 4F, o-C6F5), -

148.9 (t, 3JF-F=21 Hz, 1F, p-C6F5), -152.1 (t,

3JF-F=20 Hz, 1F, p-C6F5), -160.9 (td,

3JF-F=21 Hz,

4JF-F=8 Hz, 2F, m-C6F5), -161.5 (td,

3JF-F=22 Hz,

4JF-F=8 Hz, 2F, m-C6F5).

11B NMR (CDCl3) δ:

36.0 (br s). 13

C NMR (CDCl3) partial δ: 24.0, 111.0, 120.1, 138.5, 152.3, 158.1.

105

(5-CF3)C5H3NH(2-NH)B(C6F5)3 (5-29) - 2-amino-6-(trifluoromethyl)pyridine (32 mg, 0.20

mmol) was added to a solution of B(C6F5)3 (100 mg, 0.20 mmol) in CH2Cl2 (5 mL). The

solution was allowed to stir overnight, the solvent was removed in vacuo and the residue was

washed with hexanes (2 x 2 mL). Yield: 124 mg (95%). X-Ray quality crystals were grown

from the hexane wash layer. Anal. Calcd. for C24H5BF18N2 (%) C: 42.76, H: 0.75, N: 4.16;

found C: 42.73, H: 0.95, N: 4.24.

1H NMR (CDCl3) δ: 6.74 (br s, 1H, NH), 6.87 (d,

3JH-H=7 Hz, 1H, CH), 7.01 (d,

3JH-H=9 Hz, 1H,

CH), 7.64 (dd, 3JH-H=8 Hz,

3JH-H=7 Hz, 1H, CH), 8.96 (br s, 1H, NH).

19F NMR (CDCl3) δ: -

67.8 (s, 3F, CF3), -133.3 (d, 3JF-F=22 Hz, 6F, o-C6F5), -155.3 (t,

3JF-F=21 Hz, 3F, p-C6F5), -164.1

(tm, 3JF-F=22 Hz, 6F, m-C6F5).

11B NMR (CDCl3) δ: -10.9.

13C NMR (CDCl3) partial δ: 108.5,

122.0, 137.4, (dm, 1JC-F=255 Hz, CF), 140.0, 148.1, (dm,

1JC-F=239 Hz, CF), 154.6.

(5-CF3)C5H3NH(2-NH)BCl(C6F5)2 (5-30) - 2-amino-6-(trifluoromethyl)pyridine (25 mg, 0.15

mmol) was added to a solution of ClB(C6F5)2 (59 mg, 0.15 mmol) in CH2Cl2 (5 mL). The

solution was allowed to stir overnight, the solvent was removed in vacuo and the residue was

washed with hexanes (2 x 2 mL). Yield: 73 mg (87%). X-Ray quality crystals were grown

from the hexane wash layer. Anal. Calcd. for C18H9BF15N2 (%) C: 39.85, H: 0.93, N: 5.16;

found C: 40.12, H: 0.93, N: 5.00.

1H NMR (CDCl3) δ: 7.07 (br m, 3H), 7.79 (t,

3JH-H=8 Hz,

3JH-H=7 Hz, 1H, p-CH), 11.59 (br s,

1H, NH). 19

F NMR (CDCl3) δ: -66.9 (s, 3F, CF3), -133.3 (br d, 3JF-F=19 Hz, 4F, o-C6F5), -

154.7 (br s, 2F, p-C6F5), -161.9 (br s, 4F, m-C6F5). 11

B NMR (CDCl3) δ: 0.4. 13

C NMR

(CDCl3) partial δ: 109.9, 117.8, 120.6, 121.3, 137.4 (dm, 1JC-F=255 Hz, CF), 140.6 (dm,

1JC-

F=255 Hz, CF), 140.9, 147.8, (dm, 1JC-F=244 Hz, CF), 154.9.

(5-CF3)C5H3N(2-NH)B(C6F5)2 (5-31) - 2-amino-6-(trifluoromethyl)pyridine (25 mg, 0.15 mmol)

was added to a solution of HB(C6F5)2 (53 mg, 0.15 mmol) in CH2Cl2 (5 mL). The solution was

allowed to stir overnight, the solvent was removed in vacuo and the residue was washed with

hexanes (1 mL). Yield: 58 mg (76%). Anal. Calcd. for C18H4BF13N2 (%) C: 42.72, H: 0.80, N:

5.54; found C: 42.34, H: 1.05, N: 5.83.

1H NMR (CDCl3) δ: 7.02 (d,

3JH-H=8 Hz, 1H), 7.46 (d,

3JH-H=8 Hz, 1H), 7.74 (br s, 1H, NH),

7.82 (t, 3JH-H=8 Hz, 1H, p-CH).

19F NMR (CDCl3) δ: -67.9 (s, 3F, CF3), -130.9 (br s, 4F, o-

106

C6F5), -147.6 (br s, 1F, p-C6F5), -150.7 (br s, 1F, p-C6F5), -160.2 (br s, 4F, m-C6F5). 11

B NMR

(CDCl3) δ: 37.0 (br s). 13

C NMR (CDCl3) partial δ: 116.8, 117.0, 119.4, 122.1, 137.6 (dm, 1JC-F

=255 Hz, CF), 139.9, 142.8 (dm, 1JC-F=264 Hz, CF), 147.4, (dm,

1JC-F=246 Hz, CF), 153.7.

C9H6N(8-NH2)B(C6F5)3 (5-32) - 8-aminoquinoline (28 mg, 0.19 mmol) was added to a solution

of B(C6F5)3 (100 mg, 0.19 mmol) in CH2Cl2 (5 mL). The light brown solution was allowed to

stir for 3 hours, the solvent was removed in vacuo and the residue was washed with hexanes (2 x

2 mL) and again dried in vacuo. Yield: 111 mg (87%). Anal. Calcd. for C27H8BF15N2 (%) C:

49.42, H: 1.23, N: 4.27; found C: 48.92, H: 1.23, N: 4.08.

1H NMR (CDCl3) δ: 7.55 (m, 2H), 7.67 (d,

3JH-H=7 Hz, 1H), 7.82 (d,

3JH-H=8 Hz, 1H), 8.25 (d,

3JH-H=8 Hz, 1H), 8.39 (br s, 2H, NH2), 8.84 (d,

3JH-H=4 Hz, 1H).

19F NMR (CDCl3) δ: -133.0

(br s, 6F, o-C6F5), -156.4 (t, 3JF-F=20 Hz, 3F, p-C6F5), -163.1 (t,

3JF-F=20 Hz, 6F, m-C6F5).

11B

NMR (CDCl3) δ: -5.8. 13

C NMR (CDCl3) partial δ: 122.4, 122.8, 126.4, 128.0, 131.4, 136.7,

149.9.

C9H6N(8-NH2)-BCl(C6F5)2 (5-33) - 8-aminoquinoline (25 mg, 0.17 mmol) was added to a

solution of ClB(C6F5)2 (66 mg, 0.17 mmol) in CH2Cl2 (5 mL). White precipitate was

immediately visible, the bright orange solution was allowed to stir for 3 hours, the solvent was

removed in vacuo and the residue was washed with hexanes (2 x 2 mL) and again dried in vacuo.

Yield: 78 mg (86%). Anal. Calcd. for C21H8BClF10N2 (%) C: 48.08, H: 1.54, N: 5.34; found C:

47.91, H: 1.35, N: 5.08.

1H NMR (CDCl3) δ: 6.76 (d,

3JH-H=7 Hz, 1H), 6.98 (d,

3JH-H=7 Hz, 1H), 7.50 (t,

3JH-H=8 Hz, 1H),

7.65 (dd, 3JH-H=7 Hz,

3JH-H=5 Hz , 1H), 8.43 (d,

3JH-H=8 Hz, 1H), 8.88 (d,

3JH-H=5 Hz, 1H).

19F

NMR (CDCl3) δ: -134.9 (br s, 6F, o-C6F5), -156.3 (br s, 3F, p-C6F5), -163.1 (br s, 6F, m-C6F5).

11B NMR (CDCl3) δ: 2.7 (br s).

13C NMR (CDCl3) was not obtained due to poor solubility.

C9H6N(8-NH)B(C6F5)2 (5-34) - 8-aminoquinoline (25 mg, 0.17 mmol) was added to a solution of

HB(C6F5)2 (60 mg, 0.17 mmol) in CH2Cl2 (5 mL). The bright red solution was allowed to stir

for 5 days, the solvent was removed in vacuo and the residue was washed with cold hexanes (2

mL) and again dried in vacuo. Yield: 64 mg (76%). Anal. Calcd. for C21H7BF10N2 (%) C:

51.68, H: 1.45, N: 5.74; found C: 51.44, H: 1.68, N: 5.60.

107

1H NMR (CDCl3) δ: 4.85 (br s, 1H, NH), 6.74 (d,

3JH-H=7 Hz, 1H), 6.97 (d,

3JH-H=8 Hz, 1H),

7.65 (t, 3JH-H=8 Hz, 1H), 8.42 (d,

3JH-H=8 Hz, 1H), 8.87 (d,

3JH-H=5 Hz, 1H).

19F NMR (CDCl3)

δ: -134.9 (dd, 3JF-F=21 Hz,

4JF-F=8 Hz, 6F, o-C6F5), -155.9 (t,

3JF-F=21 Hz, 3F, p-C6F5), -162.0

(tm, 3JF-F=21 Hz,

4JF-F=8 Hz, 6F, m-C6F5).

11B NMR (CDCl3) δ: 2.4 (br s).

13C NMR (CDCl3)

partial δ: 105.8, 108.9, 122.4, 129.1, 133.2, 140.4, 142.2, 147.9.













positions refined isotropically. Single crystal X-ray structures were obtained for 5-1, 5-2, 5-3, 5-

4, 5-5, 5-6, 5-7, 5-8, 5-9, 5-13, 5-14, 5-19, 5-25, 5-26, 5-27, 5-29 and 5-30 Selected

crystallographic data are included in Tables 5.1-5.6. Diagrams and selected bond lengths and

angles are provided in Table 5.7 and Figures 5.5, 5.6, 5.8, 5.11, 5.14, 5.15, 5.19, 5.21, 5.22 and

5.25.

108


Crystal 5-1 5-2-0.5 CHCl3 5-3

Formula C27H13BF15N C24.5H7.5BF15N C25H9BF15N

Formula weight 647.19 664.80 619.14

Crystal system Monoclinic Monoclinic Triclinic

Space group P21/n P21/n P-1

a(Å) 11.5466(4) 9.2940(8) 9.7020(9)

b(Å) 13.3174(5) 14.3404(14) 11.5301(11)

c(Å) 16.5172(6) 18.4081(18) 12.1581(11)

α(o) 90.00 90.00 105.149(5)

β( o) 92.0820(10) 98.874(4) 94.518(5)

γ( o) 90.00 90.00 113.003(4)

V (Å3) 2538.18(16) 2424.1(4) 1183.10(19)

Z 4 4 2

d(calc) g cm-1

1.694 1.822 1.738

Abs coeff, μ, cm-1

0.176 0.347 0.185


Data Fo2>3(Fo

2) 4924 3159 5857


Ra 0.0392 0.0528 0.0427

Rwb 0.0938 0.1270 0.1238

Goodness of Fit 1.020 1.007 1.093


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

109


Crystal 5-4 5-5-0.5 C7H8 5-6

Formula C28H9BF15N3 C32.5H13BF15N C28H8BF15N2

Formula weight 763.27 713.25 668.17


Space group P-1 P21/n P21/c

a(Å) 9.6501(10) 9.0823(18) 10.706(2)

b(Å) 9.7264(8) 15.684(3) 16.332(3)

c(Å) 14.9925(11) 20.242(4) 14.368(3)

γ(o) 107.830(3) 90.00 90.00

β( o) 102.077(4) 101.05(3) 91.27(3)

γ( o) 98.166(3) 90.00 90.00

V (Å3) 1277.46(19) 2830.0(10) 2511.8(9)

Z 2 4 4

d(calc) g cm-1

1.776 1.674 1.767

Abs coeff, μ, cm-1

0.182 0.167 0.182


Data Fo2>3(Fo

2) 4246 4163 2499


Ra 0.0427 0.0522 0.0504

Rwb 0.1017 0.1506 0.1409

Goodness of Fit 1.020 1.045 1.006


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

110


Crystal 5-7 5-8 5-9

Formula C27H7BF15N C25H9BF15N C25H11BF15N

Formula weight 641.15 619.14 621.16


Space group P-1 P21/n P21/c

a(Å) 9.9848(9) 12.8108(4) 17.8525(12)

b(Å) 10.9162(10) 13.4663(4) 9.8407(7)

c(Å) 11.6538(11) 13.3937(4) 15.2883(10)

α(o) 107.046(5) 90.00 90.0

β( o) 94.178(5) 100.156(2) 115.010(30)

γ( o) 101.061(5) 90.00 90.0

V (Å3) 1180.52(19) 2274.40(12) 2430.8(3)

Z 2 4 4

d(calc) g cm-1

1.804 1.808 1.697

Abs coeff, μ, cm-1

0.189 0.192 0.180


Data Fo2>3(Fo

2) 4093 6118 4499


Ra 0.0549 0.0494 0.0516

Rwb 0.0903 0.1475 0.1680

Goodness of Fit 1.017 1.055 1.005


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

111


Crystal 5-13 5-14 5-19

Formula C29H17BF15NO C31H17BF15NO4 C30H8BF15N2

Formula weight 691.25 763.27 692.19

Crystal system Orthorhombic Monoclinic Triclinic

Space group Pna21 P21/c P-1

a(Å) 17.4334(9) 11.3933(4) 12.4522(10)

b(Å) 10.6793(6) 18.7049(8) 12.8361(10)

c(Å) 14.4679(8) 15.0463(6) 16.6764(13)

α(o) 90.00 90.00 79.106(4)

β( o) 90.00 108.309(2) 79.436(4)

γ( o) 90.00 90.00 85.941(4)

V (Å3) 2693.6(3) 3044.2(2) 2571.3(4)

Z 4 4 4

d(calc) g cm-1

1.705 1.665 1.788

Abs coeff, μ, cm-1

0.175 0.170 0.182


Data Fo2>3(Fo

2) 10991 6553 7640


Ra 0.0383 0.0375 0.0444

Rwb 0.1146 0.1030 0.1166

Goodness of Fit 1.046 1.026 1.037


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

112


Crystal 5-25 5-26 5-27

Formula C24H8BF15N2 C18H8BClF10N2 C18H9BF10N2

Formula weight 620.13 488.52 454.08

Crystal system Triclinic Monoclinic Triclinic

Space group P-1 P21/c P-1

a(Å) 9.8535(13) 24.9904(18) 8.4351(16)

b(Å) 11.1355(16) 13.1642(11) 10.3150(19)

c(Å) 11.5291(16) 11.2738(8) 10.496(2)

α(o) 74.467(8) 90.00 100.424(9)

β( o) 73.964(7) 91.412(4) 91.554(9)

γ( o) 80.673(7) 90.00 109.413(8)

V (Å3) 1166.1(3) 3707.7(5) 843.3(3)

Z 2 8 2

d(calc) g cm-1

1.697 1.750 1.788

Abs coeff, μ, cm-1

0.189 0.313 0.183


Data Fo2>3(Fo

2) 7552 3644 5014


Ra 0.0416 0.0585 0.0396

Rwb 0.1383 0.1254 0.1210

Goodness of Fit 1.007 0.945 1.067


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

113

Table 5.6: Selected crystallographic data for compounds 5-29 and 5-30

Crystal 5-29 5-30

Formula C24H5BF18N2 C18H5BClF13N2

Formula weight 674.11 542.50

Crystal system Triclinic Monoclinic

Space group P-1 P21/n

a(Å) 9.8877(10) 10.2702(3)

b(Å) 11.1457(12) 31.6155(10)

c(Å) 12.4221(12) 12.3047(4)

α(o) 74.787(5) 90.00

β( o) 70.681(6) 92.436(2)

γ( o) 77.611(5) 90.00

V (Å3) 1234.3(2) 3991.7(2)

Z 2 8

d(calc) g cm-1

1.814 1.805

Abs coeff, μ, cm-1

0.203 0.321

Data collected 5606 9116

Data Fo2>3(Fo

2) 2902 6762

Variables 414 647

Ra 0.0448 0.0413

Rwb 0.1082 0.1024

Goodness of Fit 0.958 1.024


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

114


5.3.1: Reactions of Alkyl or Aryl-Substituted Pyridines with B(C6F5)3

While pyridine forms a strong adduct with B(C6F5)3, a detailed survey of the effects of

increased steric bulk of pyridine on its reactions with B(C6F5)3 has not been undertaken. Such a

study should reveal a limit between adduct formation and the generation of frustrated Lewis pairs

(FLPs). A series of pyridines of varying steric and electronic properties were added to B(C6F5)3.

Pyridine-borane adducts of 4-tert-butylpyridine (5-1), 2-methylpyridine (5-2), 2-ethylpyridine

(5-3), 2,2′-dipyridylamine (5-4), 2-phenylpyridine (5-5), 2,2′-dipyridyl (5-6) and quinoline (5-7)

were formed rapidly and quantitatively within 4 hours in CH2Cl2 (Figure 5.4). The adducts with

a substituent at the 2-position showed 15 different resonances in the 19

F NMR spectrum,

indicative of restricted rotation of the N-B and B-C bonds, as a result of crowding and/or

intramolecular H-F interactions which can also serve to restrict rotation.212

Crystal structures

were obtained for adducts 5-1 – 5-7. Selected metrical parameters are compared in Table 5.7 and

depictions of selected structures are presented in Figures 5.5 and 5.6. Remote substitution shows

mainly electronic effects on B-N bond length as the B(C6F5)3 adducts of 4-(Me2N)C5H4N and 4-

(CH3)3C-C5H4N (5-1) exhibited shorter B-N bond lengths than analogous adduct of the

unsubstituted pyridine.

Figure 5.4: Synthesis of Lewis acid-base adducts 5-2 – 5-7; R=Me (5-2), Et (5-3), R=N(H)(2-

C5H4N) (5-4), Ph (5-5), 2-C5H4N (5-6)

115

Figure 5.5: POV-Ray Depictions of 5-3 (left) and 5-4 (right). Carbon: black, Boron: yellow-

green, Fluorine: deep pink, Nitrogen: blue. Hydrogen atoms are omitted for clarity. Selected

metrical parameters (distances: Å, angles: °): 5-3: B1-N1 1.638(2), B1-C1 1.642(2), B1-C7

1.648(2), B1-C13 1.643(2), N1-B1-C1 102.91(11), N1-B1-C7 110.97(11), C1-B1-C7

112.54(12), C1-B1-C13 116.30(12), C13-B1-C7 103.00(11). 5-4: B1-N1 1.629(2), B1-C1

1.651(3), B1-C7 1.650(3), B1-C13 1.651(3), N1-B1-C1 112.04(13), N1-B1-C7 110.77(14), N1-

B1-C13 102.54(14), C1-B1-C7 102.95(14), C1-B1-C13 112.06(15), C7-B1-C13 116.80(14).

Figure 5.6: POV-Ray Depictions of 5-5-0.5 C7H8 (left) and 5-7 (right). Carbon: black, Boron:

yellow-green, Fluorine: deep pink, Nitrogen: blue. Solvent and hydrogen atoms are omitted for

clarity. Selected metrical parameters (distances: Å, angles: °) 5-5: N1-B1 1.650(3), C1-B1

1.659(3), B1-C7 1.641(3), B1-C13 1.646(3), N1-B1-C1 111.01(16), N1-B1-C7 112.23(17), N1-

116

B1-C13 103.23(16), C1-B1-C7 101.78(17), C1-B1-C13 113.60(17), C7-B1-C13 115.30(18). 5-6:

N1-B1 1.641(2), B1-C1 1.646(2), B1-C7 1.651(2), B1-C13 1.651(2), N1-B1-C1 111.79(12), N1-

B1-C7 110.28(13), N1-B1-C13 102.70(12), C1-B1-C7 102.56(12), C1-B1-C13 116.39(13), C7-

B1-C13 113.35(12).

Table 5.7: Selected NMR spectroscopic and X-ray crystallographic data obtained for pyridine-

borane adducts 5-1 to 5-8

Pyridine Substituent(s) 11

B NMR

δ:

Δ m-p 19

F

NMR δ:a

N-B (Å) Σ C-B-C (°)

None31,229

-3.6 6.4 1.628(2) 333.84

4-NMe2230

-5.3 5.9 1.602(6) 333.4

4-tert-butyl (5-1) -4.1 6.4 1.618(2) 334.43

2-methyl (5-2) -3.6 7.0 1.639(4) 332.1

2-ethyl (5-3) -3.6 6.8 1.638(2) 331.84

2,2′-pyridylamine (5-4) -5.1 7.0 1.629(2) 331.81

2-phenyl (5-5) -2.9 7.2 1.651(4) 330.7

2,2′-pyridine (5-6) -2.7 6.3 1.649(5) 330.9

quinoline (5-7) -3.2 6.8 1.641(2) 332.30

2,6-dimethyl (5-8) -3.9 6.7 1.661(2) 328.30

a This value is the difference in chemical shift between the meta and para-fluorines, and has been




Substitution at the 2-position, however, shows a substantial effect on B-N bond lengths.

The addition of a single small inductive donor at this position, such as methyl or ethyl, results in

noticeable elongation in B-N bond length compared to pyridine-B(C6F5)3, suggesting that in this

117

case the steric effect dominates the electronic effect. The B-N bond length of the 5-4 is slightly

shorter than that of pyridine-B(C5F5)3 reflecting slightly increased nucleophilicity of the pyridyl

nitrogen due to electron donation from the amine nitrogen to the pyridine ring. The flexibility of

the substituent allows it to point away from the C6F5 groups, minimizing steric conflict. The

presence of electron-withdrawing substituents at the 2-position results in a more dramatic

elongation of the B-N bond as exemplified by adducts 5-5 - 5-7.

As shown in Table 5.7, 11

B NMR chemicals shift and meta-para gaps in the 19

F NMR

spectrum are relatively constant throughout the series of adducts, while B-N bond lengths and

the sum of C-B-C bond angles vary in a fairly consistent fashion. As discussed above, the B-N

bond lengths can be rationalized in terms of bulk and basicity, while the sum of C-B-C bond

angles decreases with increasing bulk of the pyridine. Bulkier pyridines force the C6F5 groups

closer together, resulting in a more idealized tetrahedral geometry at boron (tetrahedral geometry

would have the sum of C-B-C angles at 328.5, which is nearly exactly what is observed for 5-8).

5.3.2: Frustrated Lewis Pairs of Pyridines with B(C6F5)3

The reaction of 2,6-lutidine with B(C6F5)3 presented a unique scenario. At room

temperature the solution combination of 2,6-lutidine with B(C6F5)3 gave a 19

F NMR spectrum

with extremely broad peaks, including signals characteristic of B(C6F5)3, suggesting that an

equilibrium exists between adduct and the free Lewis acid and Lewis base (Figure 5.7). The 1H

NMR spectrum showed resonances corresponding to 2,6-lutidine as well as a new set of

resonances attributed to a Lewis acid-base adduct, confirming that there is an equilibrium

between adduct 5-8 formation and free Lewis acid and Lewis base at room temperature (Keq=3.3

M-1

in CD2Cl2). A crystal structure was obtained for the adduct 5-8 confirming the anticipated

connectivity (Figure 5.8).

Figure 5.7: Equilibrium observed in solution between the FLP and Lewis acid-base adduct 5-8

118

Figure 5.8: POV-Ray depiction of 5-8. Carbon: black, Boron: yellow-green, Fluorine: deep pink,

Nitrogen: blue. Hydrogen atoms are omitted for clarity. Selected metrical parameters

(distances: Å, angles: °) N1-B1 1.661(2), B1-C1 1.660(2), B1-C7 1.643(2), B1-C13 1.651(2),

N1-B1-C1 111.11(12), N1-B1-C7 104.00(11), N1-B1-C13 113.84(12), C1-B1-C7 115.59(13),

C1-B1-C13 99.30(12), C7-B1-C13 113.41(12).

Monitoring the equilibrium by variable temperature NMR spectroscopy yielded

thermodynamic data for the process: ΔH = -42(1) kJ/mol; ΔS = -131(5) J/(mol*K); ΔG° = -3

kJ/mol (see experimental section for van’t Hoff plot). These data indicate that the reaction is

only slightly exothermic at room temperature. The crystal structure of 5-8 supports the

experimental data suggesting that formation of adduct 5-8 is not as favourable as formation of

the adducts formed with smaller pyridines. 5-8 showed the longest B-N bond length of all

crystallized pyridine-B(C6F5)3 adducts.31

The pyridines that either did not react with B(C6F5)3 or only showed weak interaction

were those substituted at both the 2- and 6-positions (2,6-lutidine and 2,6-diphenylpyridine) or

with a single very bulky substituent at the 2-position (2-tert-butylpyridine).

These results are consistent with the trends in nucleophilicity observed by Brown and co-

workers, who concluded that while the order of basicity of a series of substituted pyridines was

2,6-lutidine > 2-methylpyridine > 2-tert-butylpyridine > pyridine; the order of nucleophilicity

was pyridine > 2-methylpyridine > 2,6-lutidine > 2-tert-butylpyridine (see Figure 1.3).33

119

5.3.4: Small Molecule Activation by Pyridine-Borane FLPs

To explore the reactivity of the novel FLPs, the activation of H2 at room temperature was

attempted. The pyridine-borane FLPs were exposed to 4 atm H2 and stirred overnight.

Multinuclear NMR spectroscopy revealed quantitative formation of the novel ion pairs 5-9, 5-10,

and 5-11 (Figure 5.9, see Figure 5.10 for NMR spectra of 5-9).

Figure 5.9: Synthesis of ion pairs 5-9 (R=R1=Me), 5-10 (R=

tBu, R

1=H) and 5-11 (R=R

1=Ph)

A closer study of the activation of H2 revealed that, in all cases, quantitative reaction

occurred in 2 hours at 1 atm H2. Ion pair 5-9 was further characterized by X-ray crystallography

(Figure 5.11). The structure revealed a short N-H - - - H-B contact of 1.862 Å. This contact is

substantially shorter than those observed in related phosphonium borate species.38,43

This type of

short N-H - - - H-B contact has been noted in other related species, primarily amine-borane

adducts, and has been proposed to contribute to facile loss of H2 in these species.231

120

Figure 5.10: Multinuclear NMR spectra for 5-9 in CD2Cl2: A: 1H, B:

11B and C:

19F.


Fluorine: deep pink, Nitrogen: blue. Carbon-bound hydrogen atoms are omitted for clarity.

Selected metrical parameters (distances: Å, angles: °) B1-C1 1.640(3), B1-C7 1.643(3), B1-C13

1.637(3), H1 - - H1a 1.862, C1-B1-C7 112.59(16), C1-B1-C13 116.46(16), C7-B1-C13

107.71(16), N1-H1a-H1 169.09, B1-H1-H1a 150.48.

A B

NH

p-CH

m-CH

BH

CH3

o-C6F5

C

1JB-H=92 Hz

p-C6F5

m-C6F5

121

The donor 2,3,5,6-tetramethylpyrazine also showed no reaction with B(C6F5)3 at room

temperature and this FLP was capable of H2 activation, however, the resulting ion pair 5-12

could not be isolated. This species showed decomposition to multiple products. 1,2-

hydrogenation, resulting from borohydride attack at the 2-position of the pyridinium cation of 5-

12, is one suspected reaction pathway, however this could not be determined conclusively.

In an effort to promote loss of H2, ion pairs 5-9 to 5-11 were subjected to controlled

heating. After 6 hours at 80°C, ~35% loss of H2 was observed in all cases. The reaction stopped

at this point as presumably the opposite reaction, the activation of H2, prevents further loss of H2

from solution. Addition of 1 equivalent of a smaller base, pyridine in this case, in order to

quench the free borane after loss of H2 to prevent the hydrogen activation reaction,213

results in

quantitative loss of H2 over 10 hours at 80°C in toluene (Figure 5.12).

Figure 5.12: H2 loss from ion pairs 5-9 – 5-11

To probe the utility of 2,6-lutidine as a nucleophile in FLP chemistry, B(C6F5)3 and 2,6-

lutidine were stirred in a THF solution. The FLP was able to effect THF ring-opening,

analogous to that previously observed with phosphines and B(C6F5)340

(Figure 5.13). The

product, 5-13, was fully characterized, including by X-ray crystallography (Figure 5.14). The

newly-formed B-O bond length of 1.4584(14) Å is similar to the phosphonium borate zwitterions

formed by ring-opening THF while the new N-C bond at 1.4843(16) Å, as expected, is

significantly shorter than the related P-C bonds found in the analogous phosphonium borate

zwitterions.40

Figure 5.13: Formation of zwitterion 5-13 by THF ring-opening

122


pink, Nitrogen: blue, Oxygen: red. Hydrogen atoms are omitted for clarity. Selected metrical

parameters (distances: Å, angles: °) B1-O1 1.4584(14), B1-C1 1.6711(16), B1-C7 1.6575(15),

B1-C13 1.6670(15), N1-C22 1.4843(16), O1-C19 1.4122(13). O1-B1-C1 111.57(9), O1-B1-C7

108.53(8), O1-B1-C13 106.65(9), C1-B1-C7 102.68(8), C1-B1-C13 114.30(8), C7-B1-C13

113.07(8), C24-N1-C28 122.05(10), C24-N1-C22 118.86(10), C28-N1-C22 119.09(10), C19-

O1-B1 119.01(8).

5.3.5: FLPs of Bulky Pyridines with Other Lewis Acids

In an effort to expand the scope of the H2 activation reactions, other Lewis acids were

coupled with the bulky pyridines. Based on the lack of reaction of 2,6-lutidine with BMe3,32

BEt3 was used as it is sterically and electronically similar to BMe3 but exists as a liquid at room

temperature, while BMe3 is a highly reactive gas. As expected, there was no adduct formation

with 2,6-lutidine or 2-tert-butylpyridine. However, charging solutions of these FLPs with 4 atm

H2 resulted in no reaction. Presumably, the Lewis acidity of the trialkyl borane is insufficient to

allow the cleavage of H2 by the pair, however, an equilibrium favouring H2 loss cannot be ruled

out. The notion that this pair lacks the potential for hydrogen activation was later confirmed

computationally by Papai and co-workers.50

123

5.3.6: Reaction of Diethyl-2,6-dimethyl-3,5-pyridinedicarboxylate with B(C6F5)3

Diethyl-2,6-dimethyl-3,5-pyridinedicarboxylate, the product of loss of H2 from

Hantzsch’s Ester, is very sterically similar to 2,6-lutidine at the pyridine nitrogen but exhibited

dramatically different reactivity with B(C6F5)3. The new species formed in the reaction, 5-14,

showed 19

F NMR resonances with a meta-para gap of 12.4 ppm, a relatively large value for a

Lewis acid-base adduct, suggesting the dative interaction is fairly weak.179,180

The high

symmetry of this spectrum, where only one three resonances were observed, suggests that the

bonding does not take place at nitrogen (analogous with 5-8). Based on steric factors, the most

likely site of adduct formation is a carbonyl oxygen atom of one of the ester groups. The 1H

NMR spectrum showed equivalent methyl and ethyl groups suggesting that there is a rapid

equilibrium, if the boron centre is oxygen-bound, with the borane switching between ester

groups. X-ray analysis confirmed that the borane does indeed bind a carbonyl group (Figure

5.15). The B-O bond length of the 5-14 is comparable to other bond lengths of carbonyl-

B(C6F5)3 adducts.30,232

For example, the B-O bond length of 1.5877(16) Ǻ observed in the

crystallographically-determined structure for 5-14 is slightly shorter than that found in the very

similar ester-bound adduct C6H5COOEt-B(C6F5)3 at 1.594(6) Ǻ.232

124


pink, Nitrogen: blue, Oxygen: red. Hydrogen atoms are omitted for clarity. Selected metrical

parameters (distances: Å, angles: °) O1-B1 1.5885(16), O1-C25 1.2473(15), B1-C1 1.6441(18),

B1-C7 1.6331(18), B1-C13 1.6281(18), C25-O1-B1 140.80(10), O1-B1-C1 101.20(9), O1-B1-

C7 110.35(10), O1-B1-C13 106.31(9), C1-B1-C7 114.70(10), C1-B1-C13 109.02(11), C7-B1-

C13 114.17(10).

Adduct 5-14 was exposed to 4 atm H2 at room temperature in toluene-d8. Over several

days a mixture of two products and the starting material was observed by multinuclear NMR

(Figure 5.16). The major product, at 51%, showed several 1H NMR resonances characteristic of

1,2-hydrogenation product (in particular a doublet at 1.10 ppm for CH-CH3) 5-16, while 9%

showed the characteristic NMR signals for ion pair 5-15 (particularly the NH peak at 13.05 ppm)

and the remainder was unreacted starting material. This reactivity suggests that H2 activation by

5-14 is not favourable, likely due to electron-withdrawing substituents on the pyridine and

competing adduct formation at the ester groups. In addition, the strong electron-withdrawing

groups also make the pyridyl ring quite electrophilic, which prompts hydride transfer from

B(C6F5)3.

125

Figure 5.16: Partial hydrogenation of adduct 5-14

To further explore the reactivity of ion pair 5-15, another approach to its synthesis was

undertaken. Hydride abstraction by B(C6F5)3 on Hantzsch’s Ester showed predominantly

formation of 5-15 at -20°C, which rapidly rearranged to 5-16 at room temperature (Figure

5.17).224

Figure 5.17: Generation of 5-15 and 5-16 by hydride abstraction from Hantzsch’s Ester

The observed instability of the 5-15 at room temperature led to attempts at conducting the

H2 activation at lower temperatures. At -30°C, addition of 4 atm H2 to 5-14 showed no

reactivity, while at -15°C a similar product distribution was observed after several weeks. These

observations suggest that the activation energy required for H2 activation by the FLP is higher

than the energy required for hydride insertion, resulting in very little of 5-15 being observed in

solution from the H2 activation reaction. No evidence of 1,4-hydrogenation was observed under

any condition, suggesting that the 2-position is more electrophilic than the 4-position and the

methyl group is not sufficiently bulky to hinder hydride transfer to this carbon.

5.3.7: Reactions of Substituted Quinolines with B(C6F5)3

Next, substituted quinolines were examined for FLP reactivity. These substrates were

chosen for a number of reasons. For example, hydrogenated quinolines are of great interest in

natural product synthesis233

and can be used, similarly to Hantzsch’s Ester, as stoichiometric

126

sources of H2.218-223

Quinolines have been hydrogenated by a variety of methods including Birch

reduction,225,234

transition metal catalyzed hydrogenation,18,235-238

transfer hydrogenation,239,240

stoichiometric reduction using a borohydride source241,242

and H2 transfer from Hantsch’s

Ester.243,244

As quinoline was found to form a strong adduct with B(C6F5)3 (5-7), which did not

react with H2, the steric bulk at nitrogen must be increased. This could be accomplished with

substitution at the 2 or 8 positions. 8-methylquinoline, 2-methylquinoline, 2-phenylquinoline,

1,10-phenanthroline and acridine were chosen due to their steric bulk and commercial

availability.

Stoichiometric reaction of these species with B(C6F5)3 showed a range of reactivity. The

donors 8-methylquinoline and 2-phenylquinoline showed no interaction with B(C6F5)3 while 2-

methylquinoline and acridine showed evidence of adduct formation (5-17 and 5-18, respectively,

Figure 5.18) and free B(C6F5)3 characteristic of equilibrium between adduct and frustrated Lewis

pair (Keq~26.7 M-1

and 19.1 M-1

, respectively, by 19

F NMR spectroscopy at room temperature in

CDCl3). These equilibrium constants are significantly higher than that observed in the case of

2,6-lutidine, suggesting that reduced steric hindrance in the quinoline derivatives results in more

favourable B-N bonding. 1H NMR spectra for both of these reactions showed only one set of

very broad peaks suggesting rapid exchange between adduct and free Lewis acid and base.

Figure 5.18: Equilibria involving the formation of adducts 5-17 and 5-18

Reaction of B(C6F5)3 with 1,10-phenanthroline in CH2Cl2 after 4 hours showed

quantitative formation of the Lewis acid-base adduct 5-19. In this case 15 inequivalent

resonances were observed in the 19

F NMR spectrum, typical of asymmetrically substituted

127

pyridines. Adduct 5-19 was also characterized crystallographically (Figure 5.19) and revealed an

extraordinarily long B-N bond length (1.691(3) and 1.692(3) Å for two independent molecules),

significantly longer than that observed in 5-9, indicative of decreased nucleophilicity of 1,10-

phenanthroline versus 2,6-lutidine.

Figure 5.19: POV-Ray depiction 5-19 (one of two crystallographically independent molecules).

Carbon: black, Boron: yellow-green, Fluorine: deep pink, Nitrogen: blue. Hydrogen atoms are

omitted for clarity. Selected metrical parameters (distances: Å, angles: °): N1-B1 1.692(3), B1-

C1 1.663(3), B1-C7 1.626(3), B1-C13 1.636(3), C1-B1-C7 100.24(17), C1-B1-C13 112.25(17),

C7 B1 C13 118.89(17).

These pairs of bulky quinolines with B(C6F5)3 were exposed to H2 and, surprisingly,

these did not exhibit typical NMR spectroscopic data for clean pyridinium borate ion pairs (such

as 5-8 to 5-11). Rather, in all cases, some degree of hydrogenation of the quinoline backbone

was seen. As free B(C6F5)3 was generally observed in solution during the reaction, catalytic

hydrogenations were attempted.

Optimal conditions were found for the catalytic hydrogenations of acridine, 2-

methylquinoline, 2-phenylquinoline, 8-methylquinoline and 1,10-phenanthroline; and good

yields of products 5-20 to 5-24 were easily isolated by flash chromatography eluting with 1:1

toluene:ethyl acetate (Table 5.8).

128

Table 5.8: Catalytic hydrogenation of quinolines (reactions were conducted under 4 atm H2 in

toluene in a sealed Teflon-capped Schlenk bomb)

Substrate Mol %

B(C6F5)3

Time

(h)

Temp.

(°C) Product

% yield

(isolated)

Acridine

5 2 25

80

2-Methylquinoline

5 16 50

74

2-phenylquinoline

5 4 25

80

8-methylquinoline

10 6 50

88

1,10-

phenanthroline

5 3 80

84

Acridine was reduced under very mild conditions, with quantitative conversion to 5-20

seen in only 2 hours at room temperature in toluene under 4 atm H2. 1,10-phenanthroline

required the highest temperature for conversion, perhaps due to the stronger adduct formation

observed in the stoichiometric reaction with B(C6F5)3. Interestingly, only one of the pyridyl

rings of 1,10-phenanthroline could be reduced by this method, even under harsh conditions

(toluene, 4 atm H2, 100°C, 7 days).

129

5.3.8: Reactions of 2-Aminopyridines with Fluoroarylboranes

2-amino-6-picoline is an intriguing Lewis base for reactions with boranes. While the

steric bulk at the more basic (pyridyl) nitrogen is almost identical to that of 2,6-lutidine, the more

accessible nitrogen centre is the significantly less basic (arylamino) nitrogen centre. This raises

the question of whether the Lewis acid will react with the more basic (pyridyl) or more

accessible (arylamino) nitrogen centre.

Reaction of 2-amino-6-picoline with B(C6F5)3 showed quantitative formation of a new

product, 5-25. The 1H NMR spectrum for 5-25 proved to be particularly diagnostic as 2 separate

N-H peaks were observed at 6.07 and 6.85 ppm. The 11

B and 19

F NMR spectra were

characteristic of a 4 coordinate anionic borate. Together, these data suggest that while the initial

coordination has occurred at the amine nitrogen, the more basic pyridyl nitrogen subsequently

deprotonated the amine centre (Figure 5.20). X-ray crystallography confirmed the anticipated

connectivity of the molecule (Figure 5.21, left).

Figure 5.20: Formation of zwitterion 5-25

Based on the facile formation of 5-25, other reactions were envisioned between 2-amino-

6-picoline and fluoroaryl boranes. Reaction of 2-amino-6-picoline with ClB(C6F5)2 and

HB(C6F5)2 cleanly produced the zwitterions 5-26 and 5-27, respectively (Figures 5.21 and 5.22).

Connectivity of these species was also confirmed by X-ray crystallography. Compound 5-27

shows a N-H - - - H-B contact of 2.095 Å, which is shorter than those observed for phosphonium

borates38,43

but longer than that observed for 5-9.

B-N bond lengths for compounds 5-25 – 5-27 range from 1.536(6) to 1.5596(12) Å,

similar to those observed for other amido-fluroarylborate anions44,245,246

and significantly shorter

than those seen in amine-borane adducts.31

130

Figure 5.21: POV-Ray depictions of 5-25 and 5-26 (one of two crystallographically independent

molecules). Carbon: black, Hydrogen: white, Boron: yellow-green, Chlorine: aquamarine,


Selected metrical parameters (distances: Å, angles: °). 5-25: B1-N2 1.5596(12), N1-C19

1.3601(13), C19-N2 1.3240(13), B1-N2-C19 127.24(8), N1-N2 119.71(8). 5-26: N2-B1

1.536(6), B1-Cl1 1.945(4), H1 - - - Cl1 2.553, B1-N2-C13 129.6(4), N2-C13-N1 120.0(4), B1-

Cl1-H1 70.18, N1-H1-Cl1 131.82.



Selected metrical parameters (distances: Å, angles: °). B1-N2 1.5524(13), N2-C13 1.3572(13),

C13-N1 1.3572(12), H1- - - H1a 2.084, B1-H1-H1a 96.13, N1-H1a-H1 123.84, B1-N2-C13

123.72(8), N1-C13-N2 118.82(8).

131

Compound 5-27 is a linked pyridinium borohydride, which could be the product of

hydrogen activation by a linked pyridine-borane FLP. To determine if 5-27 could lose H2, to

produce a linked pyridine-borane FLP, 5-27 was heated to 80°C for 3 hours. Unfortunately, a

complex mixture of products was observed.

Another approach was taken to obtain the desired linked FLP. Adduct 5-27 was reacted

with one equivalent of iPrMgCl. This reaction showed clean and quantitative loss of HCl from

5-27, forming the desired linked pyridine-borane 5-28 (Figure 5.23).

Figure 5.23: Synthesis of linked pyridine-borane 5-28

The multinuclear NMR spectra were diagnostic for this species (Figure 5.24), particularly

the 19

F NMR spectrum which showed a large meta-para gap of 10.7 ppm typical of a weak

Lewis acid-base adduct (donation by the lone pair of N reduces the meta-para gap),179,180

along

with separate resonances for meta and para fluorines from each C6F5 ring, indicative of restricted

rotation about the B-N bond. This restricted rotation is expected due to the ability of the amide

nitrogen to donate electron density to the vacant p-orbital of the boron centre.

132

Figure 5.24: Multinuclear NMR spectra for 5-28 in CDCl3. A: 1H, B:

11B, C:

19F

In an effort to reduce the basicity at the pyridyl nitrogen and possibly allow for direct H2

loss upon reaction of the pyridylamine with HB(C6F5)3, reactions of the base 2-NH2-6-

(CF3)C5H3N with fluoroarylboranes were examined.

In reactivity analogous to that observed with 2-amino-6-picoline, reaction of 2-NH2-6-

(CF3)C5H3N with B(C6F5)3 and ClB(C6F5)2 produced zwitterions 5-29 and 5-30, respectively

(Figure 5.25). The B-N bond lengths of 5-29 and 5-30 at 1.564(3) and 1.534(3) Å, respectively

are very similar to those observed for the 2-amino-6-picoline analogs 5-25 and 5-26.

A B

C

NH

CH

CH3

o-C6F5

p-C6F5

m-C6F5

133

Figure 5.25: POV-Ray depictions of 5-29 and 5-30. Carbon: black, Hydrogen: white, Boron:

yellow-green, Chlorine: aquamarine, Fluorine: deep pink, Nitrogen: blue. Carbon-bound

hydrogen atoms are omitted for clarity. Selected metrical parameters (distances: Å, angles: °).

5-29: B1-N2 1.564(3), N2-C19 1.318(3), N1-C19 1.359(3), C19-N2-B1 127.4(2), N1-C19-N2

119.4(2). 5-30: B1-N2 1.534(3), B1-Cl1 1.929(2), N2-C13 1.325(3), N1-C13 1.355(3), Cl1 - - -

H1 2.457, N2-B1-Cl1 105.45(14), B1-N2-C13 125.82(18), N2-C13-N1 119.62(19), B1-Cl1-H1

70.52, N1-H1-Cl1 134.77.

Reaction of 2-NH2-6-(CF3)C5H3N with HB(C6F5)3 over the course of 24 hours in CH2Cl2

showed clean conversion to the linked pyridine-borane 5-31. The intermediate zwitterionic

pyridinium borate is observed by multinuclear NMR spectroscopy over the course of the reaction

(Figure 5.26). As is the case with 5-28, inequivalent C6F5 rings are observed in the 19

F NMR

spectrum of 5-31 due to restricted rotation about the N-B bond.

This reaction is reminiscent of work by Piers and co-workers, who demonstrated that the

ortho-substituted ammonium borate 1-(Ph2HN)-2-(BH(C6F5)2)C6H4 rapidly loses H2 to form the

linked amine-borane 1-(Ph2N)-2-(B(C6F5)2)C6H4.247

134

Figure 5.26: Formation of linked pyridine-borane 5-31

8-aminoquinoline was reacted with B(C6F5)3 and ClB(C6F5)2. Interestingly, in these

reactions, the proton transfer reaction observed in the formation of compounds 5-25, 5-26, 5-29

and 5-30 was not seen. Instead, 8-aminoquinoline formed the classical Lewis acid-base adducts

5-32 and 5-33 (Figure 5.27). Reaction of 8-aminoquinoline with HB(C6F5)2 over 2 days in

CH2Cl2 gave a bright red solution with NMR data consistent with loss of H2 and the formation of

intramolecular Lewis acid-base adduct 5-34 (Figure 5.27). This reaction likely occurs due to the

thermodynamically favourable formation of the 5-membered ring. The analogous N-B-O species

has been previously synthesized by reaction of 8-hydroxyquinoline with ClB(C6F5)2 with

concomitant loss of HCl.248

Figure 5.27: Formation of Lewis acid-base adducts 5-32, 5-33 and 5-34

5.4: Conclusions

Mixtures of pyridines with B(C6F5)3 exhibit a range of reactivity from simple adduct

formation, through equilibrium between adduct and FLP, to classical FLPs (where no interaction

between Lewis acid and Lewis base is observed). Despite the reduced basicity of pyridines

compared to most amines, as a result of the sp2 hybridization at nitrogen, FLP-type hydrogen

activation is possible. THF ring-opening has also been seen with a pyridine borane FLP,

producing a zwitterionic pyridinium borate. Substituted quinolines are capable of hydrogen

activation; however, the resulting ion pairs react further to reduce the pyridyl ring and catalytic

hydrogenation can be performed with a number of these species. These represent the first metal-

free catalytic hydrogenations which involve the addition of two equivalents of H2 to an

135

unprotected substrate and could be of considerable interest commercially as tetrahydroquinolines

are important intermediates in natural product synthesis.

Reaction of bulky amino-pyridines with B(C6F5)3 or ClB(C6F5)2 can result in

coordination at the amine nitrogen and proton transfer to the pyridyl nitrogen. The analogous

reaction with HB(C6F5)2 can result in the same proton transfer or loss of H2, depending on the

basicity of the pyridyl nitrogen. This creates a linked pyridinium borate or pyridine borane in a

single step, offering an easy synthesis of linked frustrated Lewis pairs. The proximity of the

nitrogen and boron atoms to one another may allow for more facile activation of small

molecules. This close relationship could also encourage loss of H2 from the pyridinium borate

which could facilitate the catalytic hydrogenation of less polar substrates than those previously

hydrogenated using FLPs.

136

Chapter 6: Summary and Conclusions

A series of small molecule activations have been conducted utilizing both transition metal

catalysis and novel frustrated Lewis pairs. These reactions have been studied in detail and a

number of new reaction modes have been discovered. This has allowed for the discovery of

novel synthetic routes to potentially valuable compounds.

A study of the Rh-catalyzed dehydrocoupling of P-H bonds revealed a new pathway

involving P-P bond activation. This reaction has been exploited in the synthesis of new silyl

phosphines. Reaction of P5Ph5 or P5Et5 with NacNacRhCOE(N2) resulted in complexes

involving bidentate and tridentate coordination modes of the P5 fragment. As is often the case

with phosphorus-based ligands, the substituents at phosphorus profoundly affect the nature of the

product.

The work presented in this thesis demonstrates that the FLP concept is not confined to

tertiary phosphine-borane pairs. The concept is in fact quite general and applies to a wide range

of nucleophiles of varying size and basicity. Bimolecular FLPs of polyphosphines and pyridines

and with B(C6F5)3 can be utilized to effect small molecule activation reactions. Perhaps most

interesting is the observation that covalently-bound phosphinoboranes are also capable of the

activation of H2.

FLPs composed of catena-polyphosphines and B(C6F5)3 have been shown to activate H2

and undergo para-nucleophilic aromatic substitution in a similar fashion to tertiary

phosphine/B(C6F5)3 pairs, yielding zwitterionic phosphonium borates. The FLP P5Ph5/B(C6F5)3

showed new reactivity with H-H and Si-H bonds, forming adducts of the general form

H(E)(Ph)P-B(C6F5)3 (E=H, SiR3) through P-P bond cleavage. This methodology could

potentially be exploited in the synthesis of chiral phosphines and in the activation of white

phosphorus.

Phosphinoborane monomers of the general form R2PB(C6F5)2 can be synthesized,

provided the substituents are bulky enough to prevent dimerization. The monomers show very

short P-B bonds, yet are found to react with H2, Lewis acids and Lewis bases.

137

Bulky pyridines and other nitrogen-containing heterocycles can act as partners for

B(C6F5)3 in FLPs. For the 2,6-lutidine/B(C6F5)3 pair, both adduct and free borane and pyridine

were observable in solution. This represents the border between classical Lewis acid-base

adduct and FLP chemistry. This pair was found to activate H2 and ring-open THF. Other bulky

alkyl- or aryl-substituted pyridine-borane FLPs were found to activate H2 in a fashion analogous

to the previously reported phosphine-borane systems. Several bulky quinolines were found to

undergo B(C6F5)3-catalyzed hydrogenations under mild conditions. The reaction of amine-

substituted pyridines with boranes was found to provide a facile synthesis for linked pyridine-

borane FLPs.

The small molecule activation pathways presented here, in particular the catalytic

hydrogenation reactions, could find utility in various academic and industrial applications. Rh

catalyzed P-P bond cleavage opens a new route to phosphine synthesis. This work serves to

expand the FLP concept as a whole, exploring novel metal-free small molecule activations, and

serves to demonstrate the potential of more environmentally benign main-group systems to

perform chemistry previously confined to transition metals.

138

Appendix A: Frustrated Lewis Pairs Derived From P(OR)nR3-n and B(C6F5)3

A.1: Introduction

While frustrated Lewis pairs derived from trialkyl and triaryl phosphines in combination

with B(C6F5)3 have been examined extensively,3 a detailed study of the cone angles and

basicities needed for hydrogen activation has not been undertaken. The phosphonium borates

derived from these reactions release H2 only under prolonged heating in the presence of a smaller

base.213

The use of less basic phosphorus centres, such as phosphites (which are less basic due to

the adjacent highly electronegative oxygen atoms), will allow testing of the limits of basicity and

cone angles required for the FLP hydrogen activation reaction. A better understanding of these

limits would help target bases which are basic enough to activate H2 with B(C6F5)3, but not so

basic that the reaction is irreversible. This knowledge will be helpful for the targeting of

potential hydrogenation catalysts.

139

A.2: Experimental

A.2.1: General Considerations







Na/benzophenone (C6D6, C7D8) or CaH2 (CD2Cl2, CDCl3). All common organic reagents were

purified by conventional methods unless otherwise noted. 1H,

13C,

11B,

19F and

31P nuclear

magnetic resonance (NMR) spectroscopy spectra were recorded on a Bruker Avance-400

spectrometer at 300K unless otherwise noted. 1H and

13C NMR spectra are referenced to SiMe4

using the residual solvent peak impurity of the given solvent. 31

P NMR experiments were

referenced to 85% H3PO4, while 19

F and 11

B NMR experiments were referenced to 85% BF3-

Et2O in CDCl3. Chemical shifts are reported in ppm and coupling constants in Hz as absolute

values. Combustion analyses were performed in house employing a Perkin Elmer CHN

Analyzer. B(C6F5)3 was generously donated by NOVA Chemicals Corporation. tBu2PCl,

P(OCH3)3 and P(OPh)3 were purchased from Aldrich Chemicals, P(O-2,4-tBu2C6H3)3 was

purchased from Strem Chemicals and used as received. P(O-2,6-Me2C6H3)3 was prepared as

previously reported. 249

A.2.2: Lewis Acid-Base Adducts of Phosphites with B(C6F5)3

(RO)3PB(C6F5)3 (R = Me, A-1; R = Ph, A-2) - These compounds were prepared in a similar

fashion and thus only one preparation is reported. A clear solution of B(C6F5)3 (0.100 g, 0.19

mmol) and P(OCH3)3 (0.024 g, 0.19 mmol) in CH2Cl2 (2 mL) was stirred for one hour at room

temperature. The solvent was removed in vacuo. To the remnants was added pentane (2 mL)

resulting in a clear supernatant and a white solid layer. The supernatant was removed by pipette

and the solid was dried in vacuo.

A-1 - Yield: 96 mg (77 %). Anal. Calcd. for C21H9F15BPO3: C, 39.65; H, 1.43. Found: C, 39.71;

H, 1.24. X-Ray quality crystals were grown by slow evaporation from a CH2Cl2 solution. 1H

140

NMR (CDCl3) δ: 3.80 (d, 3JP-H=10 Hz, 9H, CH3).

19F NMR (CDCl3): -131.1 (d,

3JF-F=22 Hz,

6F, o-C6F5), -156.6 (t, 3JF-F=20 Hz, 3F, p-C6F5), -164.3 (td,

3JF-F=20 Hz,

4JF-F=6 Hz, 6F, m-C6F5).

31P NMR (CDCl3) δ: 75.5 (br m).

11B NMR (CDCl3) δ: -15.5 (d,

1JP-B=141 Hz).

13C{

1H} NMR

(CDCl3) partial δ: 55.7 (d, JP-C=12 Hz).

A-2 - Yield: 75 mg (93 %). Anal. Calcd. for C36H15F15BPO3: C, 52.58; H, 1.84. Found: C, 52.08;

H, 2.07. 1H NMR (CD2Cl2) δ: 7.23 (t,

3JH-H=7 Hz, 6H, o-C6H5), 7.20 (t,

3JH-H=7 Hz, 3H, p-

C6H5), 6.90 (dd, 3JH-H=8 Hz, J=1 Hz, 6H, m-C6H6).

19F NMR (CD2Cl2) δ: -129.8 (s, 6F, o-C6F5),

-156.2 (s, 3F, p-C6F5), -164.3 (s, 6F, m-C6F5). 31

P NMR (CD2Cl2) δ: 62.0 (br s). 11

B NMR

(CD2Cl2) δ: -8.9 (br s). 13

C{1H} NMR (CD2Cl2) partial δ: 150.9 (d, JP-C=14 Hz), 148.8 (d,

1JF-

C=248 Hz, o-C6F5), 140.7 (d, 1JF-C=256 Hz, p-C6F5), 137.3 (d,

1JF-C=252 Hz, m-C6F5), 130.2 (d,

JP-C=1 Hz), 126.2 (d, JP-C=1 Hz), 120.3 (d, JP-C=4 Hz).

A.2.3: Synthesis of Phosphinites tBu2POR

tBu2POR (R =

tBu, A-3; Ph, A-4; 2,6-Me2C6H3, A-5) - These compounds were prepared by

reaction of tBu2PCl with the appropriate potassium alkoxide or aryloxide (generated in situ by

reaction of the corresponding alcohol with KH) over 4 h in THF, followed by removal of the

solvent, extraction into pentane and filtration through celite (similar to the method previously

described for A-4).250

Careful removal of the solvent in vacuo left the products as colourless

oils. If excess alcohol remained, the product was redissolved in pentane and run through a plug

of alumina.

A-3 - Yield: 76 %. 1H NMR (C6D6) δ: 1.14 (d,

3JP-H=11 Hz, 18H,

tBu), 1.26 (d,

4JP-H=1 Hz,

OtBu, 9H).

31P{

1H} NMR (C6D6) δ: 136.7 (s).

13C{

1H} NMR (C6D6) δ: 28.7 (d, JP-C=16 Hz, P-

C(CH3)3), 31.4 (d, JP-C=7 Hz, OC(CH3)3), 34.6 (d, JP-C = 27 Hz, PCMe3), 75.5 (d, JP-C=11 Hz,

OCMe3).

A-4 - Yield: 83 %. 1H NMR (C6D6) δ: 1.08 (d,

3JP-H=12 Hz, 18H,

tBu), 6.78 (t,

3JH-H=7 Hz, 1H,

p-C6H5), 7.07 (t, 3JH-H=8 Hz, 2H, m-C6H5), 7.25 (dm,

3JH-H=8 Hz, 2H, o-C6H5);

31P{

1H} NMR

(C6D6) δ: 153.5 (s); 13

C{1H} NMR (C6D6) δ: 27.4 (d, JP-C=16 Hz), 35.6 (d, JP-C=26 Hz), 118.6 (d,

JP-C=11 Hz), 121.6 (d, JP-C=1 Hz), 129.6 (d, JP-C=1 Hz), 160.3 (d, JP-C=10 Hz).

141

A-5 - Yield: 78 %. 1H NMR (C6D6) δ: 1.08 (d,

3JP-H=11 Hz, 18 H,

tBu), 2.29 (s, 3H, CH3), 2.59

(s, 3H, CH3), 6.76 (t, 3JH-H=7 Hz, 1H, p-C6H3), 6.88 (d,

3JH-H=7 Hz, 2H, m-C6H3).

31P NMR

(C6D6) δ: 162.3 (s). 13

C{1H} NMR (C6D6) δ: 27.5 (d, JP-C=16 Hz), 36.4 (d, JP-C = 33 Hz), 122.0,

154.9 (d, JP-C = 2 Hz).

A.2.4: Generation of a Phosphine-Oxide Adduct of B(C6F5)3

tBu2(H)POB(C6F5)3 (A-6) - A clear solution of B(C6F5)3 (0.050 g, 0.10 mmol) and A-3 (0.030 g,

0.10 mmol) in CH2Cl2 (2 mL) was prepared. The solvent was removed in vacuo. To the remnants

was added pentane (2 mL) resulting in a clear supernatant and a white solid layer. The

supernatant was removed by pipette and the solid was dried in vacuo. Yield: 55 mg (82%).

Anal. Calcd. for C26H19BF15OP (%) C: 46.32, H: 2.84; found: C: 46.23, H: 2.97. 1H NMR

(C6D6) δ: 0.57 (d, 3JP-H=16 Hz, 18H, PC(CH3)3), 5.33 (d,

1JP-H=452 Hz, 1H, PH).

19F NMR

(C6D6) δ: -132.3 (d, 3JF-F=23 Hz, 6F, o-C6F5), -157.2 (t,

3JF-F=21 Hz, 3F, p-C6F5), -163.8 (tm,

3JF-

F = 23 Hz, 6F, m-C6F5). 31

P{1H} NMR (C6D6) δ: 77.0 (s).

11B{

1H} NMR (C6D6) δ: -0.30 (br s).

13C{

1H} NMR (C6D6) partial: 24.6 (br s, C(CH3)3), 33.7 (d, JP-C=57 Hz, C(CH3)3), 137.5 (dm, JC-

F=256 Hz, CF), 14 8.2 (dm, JC-F=242 Hz, CF).

A.2.5: Generation of Phosphonium Borate Ion Pairs by H2 Activation

[tBu2PH(OAr)][HB(C6F5)3] (Ar=Ph, A-7; 2,6-Me2C6H3, A-8) – Compounds A-7 and A-8 were

prepared in a similar fashion, thus only one preparation is described. A-4 (47 mg, 0.20 mmol)

was added to a solution of B(C6F5)3 (100 mg, 0.20 mmol) in toluene (5 mL) in a sealed Teflon-

capped reaction vessel. The solution was subjected to three freeze-pump-thaw cycles and

subsequently exposed to an atmosphere of H2 at 77 K. On warming to room temperature this is

equivalent to approximately 4 atm H2. The solution was allowed to stir overnight at room

temperature. The solution was removed and the bomb was washed with CH2Cl2 (2 x 2 mL).

Volatiles were removed in vacuo and pentane was added to the resulting thick oil. The product

crystallized over 3 days, producing X-Ray quality crystals.

A-7 - Yield: 142 mg (97%). Anal. Calcd. for C32H25BF15PO (%) C: 51.09, H: 3.35; found: C:

51.21; H: 3.59. 1H NMR (CD2Cl2) δ: 1.43 (d,

3JP-H=19 Hz, 18 H, C(CH3)3), 3.55 (q,

1JB-H=93 Hz,

1H, BH), 6.98 (d, 1JP-H=474 Hz, 1H, PH), 7.08 (d, J=8 Hz, 2H, CH), 7.26 (br s, 1H, CH), 7.37 (t,

J=8 Hz, 2H, CH). 19

F NMR (CD2Cl2) δ: -134.2 (d, 3JF-F=24 Hz, o-C6F5), -164.8 (t,

3JF-F=23 Hz,

142

p-C6F5), -167.8 (tm, 3JF-F=24 Hz, m-C6F5).

31P NMR (CD2Cl2) δ: 98.5 (dm,

1JP-H=474 Hz,

3JP-

H=19 Hz); 11

B NMR (CD2Cl2) δ: -25.6 (d, 1JB-H=93 Hz);

13C{

1H} NMR (CD2Cl2) partial δ: 25.4

(d, J=2 Hz), 36.4 (d, J=42 Hz), 115.4, 117.6 (d, J=6 Hz), 131.5.

A-8 - Yield: 147 mg (96%). Anal. Calcd. for C34H29BF15PO (%) C: 52.33, H: 3.75; found: C:

52.33; H: 3.98. 1H NMR (CD2Cl2) δ: 1.44 (d,

3JP-H=20 Hz, 18H, C(CH3)3), 2.28 (s, 6H, Ar-

CH3), 3.51 (q, 1JB-H=93 Hz, BH), 7.02 (d,

1JP-H=483 Hz, 1H, PH), 7.02-7.13 (m, 3H, CH);

19F

NMR (CD2Cl2) δ: -134.3 (d, 3JF-F=23 Hz, o-C6F5), -165.0 (t,

3JF-F=20 Hz, p-C6F5), -167.9 (tm,

3JF-F=20 Hz, m-C6F5).

31P NMR (CD2Cl2) δ: 97.5 (d,

1JP-H=483 Hz,

3JP-H=20 Hz);

11B NMR

(CD2Cl2) δ: -25.3 (d, 1JB-H=93 Hz);

13C{

1H} NMR (CD2Cl2) partial δ: 8.3 (CP), 16.2 (m, CH3),

137.4 (dm, 1JC-F=248 Hz, CF), 140.4 (dm,

1JC-F =209 Hz, CF), 147.3 (dm,

1JC-F =227 Hz, CF).

A.2.6: X-Ray Data Collection, Reduction, Solution and Refinement



SMART software package on a Bruker SMART Apex II System CCD diffractometer using a

graphite monochromator with Mo Κα radiation (λ = 0.71073 Å). Data collection strategies were

determined using Bruker Apex software and optimized to provide >99.5% complete data to a 2θ

value of at least 55°. 10 second exposure times were used unless otherwise noted. Data

reductions were performed using the SAINT software package and absorption corrections were

applied using SADABS. The structures were solved by direct methods using XS and refined by

full-matrix least-squares on F2 using XL as implemented in the SHELXTL suite of programs. All

non-H atoms were refined anisotropically. Carbon-bound hydrogen atoms were placed in

calculated positions using an appropriate riding model and coupled isotropic temperature factors.


positions refined isotropically. Single crystal X-ray structures were obtained for A-1, A-6 and A-

7. Selected crystallographic data are included in Table A.1. Diagrams and selected bond lengths

and angles are provided in Figures A.2, A.4 and A.9.

143

Table A.1: Selected crystallographic data for compounds A-1, A-6 and A-7

Crystal A-1 A-6 A-7

Formula C21H9BF15O3P C26H19BF15OP C34H29BF15OP

Formula weight 636.06 674.19 780.35

Crystal system Triclinic Monoclinic Orthorhombic

Space group P-1 Pc Pbca

a(Å) 10.1164(4) 10.6660(5) 18.3248(13)

b(Å) 10.9283(4) 9.8862(4) 18.6811(13)

c(Å) 10.9799(4) 16.6408(6) 19.3253(15)

(o) 70.753(2) 90.00 90.00

( o) 87.539(2) 129.863(2) 90.00

( o) 81.346(2) 90.00 90.00

V (Å3) 1132.96(8) 1346.88(10) 6615.6(8)

Z 2 2 8

d(calc) g cm-1

1.865 1.662 1.567

Abs coeff, , cm-1

0.271 0.347 0.197


Data Fo2>3(Fo

2) 8039 4386 5442


Ra 0.0362 0.0325 0.0393

Rwb 0.1120 0.0537 0.1045

Goodness of Fit 1.020 0.791 1.010


aR=Σ(Fo-Fc)/ΣFo

bRw=(Σ[w(Fo

2-Fc

2 )

2] /Σ[w(Fo)

2])

½

144

A.3: Results and Discussion

A.3.1: Reactions of Phosphites with B(C6F5)3

Stoichiometric reactions of several phosphites with B(C6F5)3 were initially investigated.

The relatively smaller phosphites, such as trimethyl- and triphenylphosphite form strong adducts

with B(C6F5)3 (A-1 and A-2, respectively).

Figure A.1: Formation of phosphite-borane adducts A-1 and A-2

A-1 was also characterized by single-crystal X-ray diffraction (Figure A.2). Of particular

interest in this case is the extremely short P-B bond length of 2.0209(11) Å. This bond length is

even shorter than that of the adduct formed between Me3P and B(C6F5)3 (2.061(4)Å). The

shorter bond length of the phosphite compared to the phosphine is likely caused by the reduced

cone angle of the phosphite as the phosphine is considerably more basic (Table A.2).

Figure A.2: POV-Ray depiction of A-1. Carbon: black, Boron: yellow-green, Fluorine: deep

pink, Oxygen: red, Phosphorus: orange. Hydrogen atoms are omitted for clarity. Selected

metrical parameters (Distances: Å, Angles: °): P1-B1 2.0209(11).

145

Table A.2: Comparison of Me3P and (MeO)3P and their B(C6F5)3 adducts

Base pKa251

cone angle ()252

dP-B (Å)

(MeO)3P 2.60 107 2.0209(11)

Me3P 8.65 118 2.061(4)

29

The relatively larger phosphites, tris(2,6-dimethylphenyl)phosphite and tris(2,4-di-tert-

butylphenyl)phosphite show no reaction with B(C6F5)3 by multinuclear NMR spectroscopy.

Based on this observation, solutions of these phosphites with B(C6F5)3 are FLPs. Unfortunately

these FLPs showed no reactivity upon addition of H2 (Figure A.3), THF or olefins to solutions of

these acid/base pairs. These results would seem to suggest that, while these phosphites have

sufficiently large cone angles to prevent adduct formation, their low basicity prevents them from

acting as bases or nucleophiles in any of the present FLP small molecule activations (see Table

A.3 for a comparison of phosphines and phosphites).

Figure A.3: Phosphite-based FLPs and attempted hydrogen activation

146

Table A.3: Cone angles and basicities of phosphorus bases and reactivity with B(C6F5)3

Base Cone Angle (°)252

pKa (conjugate

acid)253

Product of reaction with

B(C6F5)3 at 25°C213

(MeO)3P 107 2.6 Adduct (A-1)

Me3P 118 8.6 Adduct

(PhO)3P 128 -2.0 Adduct (A-2)

Bu3P 132 8.7 Adduct

Ph3P 145 2.7 Adduct

iPr3P 160 9.3 Para-NAS

Cy3P 170 9.7 Para-NAS

(tBuO)3P 172 4.5 Multiple Products

[2,6-(CH3)2C6H3-O]3P 182 -0.4 No reaction

tBu3P 182 11.4 No reaction

[2,4-tBu2C6H3-O]3P

190

254 N/A No reaction

[o-(CH3)C6H4)3P 194 3.1 No reaction

Mes3P 212 N/A No reaction

The reactivity of B(C6F5)3 with phosphites found in this study fit the trend of the

previously obtained data for tertiary phosphines. Bases with cone angles of 182° or larger do not

react with B(C6F5)3 under ambient conditions, while the weakest base found to activate H2 in a

FLP with B(C6F5)3 remains tri(ortho-tolyl)phosphine with a pKa of 3.1. These results stress the

147

importance of not only utilizing a large base (i.e. poor nucleophile) to prevent adduct formation,

but also that there is a threshold of base strength required for activation of H2 with B(C6F5)3.

A.3.2: Reactions of RnP(OtBu)3-n with B(C6F5)3

As [o-(CH3)C6H4]3P (cone angle 194°, pKa=3.1) is capable of hydrogen activation in

tandem with B(C6F5)3,49

electronically, tri-tert-butylphosphite (cone angle 172°, pKa=4.5)

should have been capable of similar reactivity. However, based on the similar cone angle to

tricyclohexylphosphine (cone angle 170°) para-nucleophilic aromatic substitution on a C6F5 ring

of the borane is also a possibility.38

Unfortunately, the reaction of (tBu3O)3P with B(C6F5)3

produced several products by multinuclear NMR spectroscopy, none of which could be

conclusively identified.

To probe the possible reactions caused by interaction of the OtBu substituent with

B(C6F5)3, the phosphinite tBuOP

tBu2 (A-3) was synthesized cleanly by reaction of

tBu2PCl with

KOtBu (KCl is removed upon workup). Interestingly, A-3 reacts with B(C6F5)3, and following

workup the peak in the 1H NMR spectrum corresponding to the oxygen-bound tert-butyl group

had disappeared. This observation, along with data from the 11

B, 19

F and 31

P NMR spectra,

suggested the formation of the adduct tBu2(H)P=O-B(C6F5)3 (A-9). This assignment was

confirmed by X-ray crystallography (Figure A.4). Monitoring the reaction by 1H NMR

spectroscopy revealed that the oxygen-bound tert-butyl group is lost as iso-butene upon reaction

with B(C6F5)3 (Figure A.5).

148

Figure A.4: POV-Ray depiction of A-6. Carbon: black, Hydrogen: white, Boron: yellow-green,

Fluorine: deep pink, Oxygen: red, Phosphorus: orange. Carbon-bound hydrogen atoms are

omitted for clarity. Selected metrical parameters (Distances: Å, Angles: °): P1-O1 1.5340(14);

O1-B1 1.524(2); P1-O1-B1 139.55(13).

Figure A.5: 1H NMR spectrum showing formation of iso-butene, along with adduct A-9

The proposed mechanism for this reaction, illustrated in Figure A.6, involves initial co-

ordination of the Lewis acid to the oxygen centre. This initial reaction makes the tert-butoxy

hydrogen atoms acidic, which makes them susceptible to abstraction by the nearby basic

phosphorus centre. The reaction is presumably driven by the formation of strong P=O and C=C

bonds (~500 kJ/mol, and ~733 kJ/mol, respectively).255

PH 1JP-H=452 Hz

CH2=C(CH3)2

CH2=C(CH3)2 PC(CH3)3 3JP-H=16 Hz

149

Figure A.6: Proposed mechanism for the formation of A-9 and iso-butene from reaction of A-3

with B(C6F5)3

A.3.3: FLP Reactions of tBu2POAr with B(C6F5)3

In order to avoid the potential loss of alkene from these species, the phosphinites

tBu2POPh and

tBu2PO(2,6-Me2C6H3), A-4 and A-5 respectively, were synthesized. Loss of the

aryl group in this fashion would have to be accompanied by loss of a benzyne derivative, which

is highly unlikely. These bases did not form adducts with B(C6F5)3 at room temperature. Thus,

FLP reactivity was examined. Both phosphinites proved to be capable of H2 activation in

tandem with B(C6F5)3, quantitatively forming ion pairs A-7 and A-8, respectively, by NMR

spectroscopy (Figure A.7, A.8). The structure of A-8 was confirmed through X-ray

crystallography (Figure A.9).

Figure A.7: FLP reactivity of phosphinites A-5 and A-6 with B(C6F5)3

150

Figure A.8: Multinuclear NMR spectra for A-7 in CD2Cl2. A: 1H, B:

11B, C:

19F and D:

31P

Figure A.9: POV-Ray depiction of A-8. Carbon: black, Hydrogen: white, Boron: yellow-green,

Fluorine: deep pink, Oxygen: red, Phosphorus: orange. Hydrogen atoms are omitted for clarity.

Selected metrical parameters (Distances: Å, Angles: °): B1-C1 1.638(3), B1-C7 1.645(3), P1-O1

1.5757(13), P1-C20 1.8290(19), P1-C24 1.827(2), P-H - - - H-B 2.348, C1-B1-C7 114.07(15),

C1-B1-C13 115.27(15), O1-P1-C20 113.75(8), O1-P1-C24 120.32 (9), P1-O1-C28 134.12(12).

PC(CH3)3 3JP-H=19 Hz

C6H5

PH 1JP-H=474 Hz

BH 1JB-H=93 Hz

1JB-H=93 Hz

A B

C D

o-C6F5

p-C6F5

m-C6F5

PH 1JP-H=474 Hz

151

A.4: Conclusions

In summary, the use of phosphites as donors in FLPs with B(C6F5)3 proved to be

unsuccessful for H2 activation, however, new reaction pathways were discovered. The bulky

phosphites such as P(O-2,6-Me2C6H3)3 and P(O-2,4-tBu2C6H3)3 proved to not be basic enough to

activate H2 in combination with B(C6F5)3. This observation is consistent with the computational

study by Papai and co-workers that suggests that a cumulative acid/base strength is required for a

FLP to activate H2.50

The tert-butoxy substituent of tBuOP

tBu2 was found to react with B(C6F5)3

to form iso-butene and the corresponding phosphine oxide. The aryl-oxy functional group could

be incorporated into the base component of FLP’s capable of H2 activation through the use of

tBu2P(OAr) as the Lewis base.

152

References

(1) Greenberg, S.; Stephan, D. W. Chem. Soc. Rev. 2008, 37, 1482.

(2) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124.

(3) Stephan, D. W.; Erker, G. Angew. Chem. Int. Ed. 2010, 49, 46.

(4) Stephan, D. W. Org Biomol Chem 2008, 6, 1535.

(5) Crabtree, R. H. The organometallic chemistry of the transition metals; 4th ed.; Wiley-

Interscience: Hoboken, N.J., 2005.

(6) Meakin, P.; Tolman, C. A.; Jesson, J. P. J. Am. Chem. Soc. 1972, 94, 3240.

(7) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc. 1997, 119, 3887.

(8) Gates, D. P. Top. Curr. Chem. 2005, 250, 107.

(9) Baumgartner, T.; Reau, R. Chem. Rev. 2006, 106, 4681.

(10) Clark, T. J.; Lee, K.; Manners, I. Chem.-Eur. J. 2006, 12, 8634.

(11) Allcock, H. R. Chemistry and applications of polyphosphazenes; Wiley-Interscience:

Hoboken, N.J., 2003.

(12) Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1995, 117, 12645.

(13) Han, L. B.; Tilley, T. D. J. Am. Chem. Soc. 2006, 128, 13698.

(14) Bohm, V. P. W.; Brookhart, M. Angew. Chem. Int. Ed. 2001, 40, 4694.

(15) Thompson, R.; Royal Society of Chemistry (Great Britain). Inorganic Chemicals Group.;

Royal Society of Chemistry (Great Britain). Fine Chemicals and Medicinals Group.; Royal

Society of Chemistry (Great Britain). Industrial Division. North West Region. Speciality

inorganic chemicals : the proceedings of a symposium organised by the Inorganic Chemicals

Group, the Fine Chemicals and Medicinals Group, and the N.W. Region of the Industrial

Division of the Royal Society of Chemistry, in association with the Dalton Division, University of

Salford, September 10th-12th 1980; The Royal Society of Chemistry: London, 1981.

(16) Cossairt, B. M.; Cummins, C. C. Angew. Chem. Int. Ed. 2008, 47, 8863.

(17) Piro, N. A.; Cummins, C. C. J. Am. Chem. Soc. 2008, 130, 9524.

(18) Fox, A. R.; Clough, C. R.; Piro, N. A.; Cummins, C. C. Angew. Chem. Int. Ed. 2007, 46,

973.

(19) Figueroa, J. S.; Cummins, C. C. Dalton Trans. 2006, 2161.

153

(20) Piro, N. A.; Figueroa, J. S.; McKellar, J. T.; Cummins, C. C. Science 2006, 313, 1276.

(21) Stephens, F. H.; Johnson, M. J. A.; Cummins, C. C.; Kryatov, O. P.; Kryatov, S. V.;

Rybak-Akimova, E. V.; McDonough, J. E.; Hoff, C. D. J. Am. Chem. Soc. 2005, 127, 15191.

(22) Back, O.; Kuchenbeiser, G.; Donnadieu, B.; Bertrand, G. Angew. Chem. Int. Ed. 2009.

(23) Masuda, J. D.; Schoeller, W. W.; Donnadieu, B.; Bertrand, G. J. Am. Chem. Soc. 2007,

129, 14180.

(24) Cummins, C. C. Angew. Chem. Int. Ed. 2006, 45, 862.

(25) Masuda, J. D.; Schoeller, W. W.; Donnadieu, B.; Bertrand, G. J. Am. Chem. Soc. 2007,

129, 14180.

(26) Masuda, J. D.; Schoeller, W. W.; Donnadieu, B.; Bertrand, G. Angew. Chem. Int. Ed.

2007, 46, 7052.

(27) Baudler, M.; Glinka, K. Chem. Rev. 1993, 93, 1623.

(28) Lewis, G. N. Valence and the Structure of Atoms and Molecules; Chemical Catalogue

Company, Inc.: New York, 1923.

(29) Chase, P. A.; Parvez, M.; Piers, W. E. Acta Crystallogr. E 2006, 62, O5181.

(30) Piers, W. E. Adv. Organomet. Chem. 2005, 52, 1.

(31) Focante, F.; Mercandelli, P.; Sironi, A.; Resconi, L. Coord. Chem. Rev. 2006, 250, 170.

(32) Brown, H. C.; Schlesinger, H. I.; Cardon, S. Z. J. Am. Chem. Soc. 1942, 64, 325.

(33) Brown, H. C. J. Chem. Soc. 1956, 1248.

(34) Welch, G. C.; Prieto, R.; Dureen, M. A.; Lough, A. J.; Labeodan, O. A.; Holtrichter-

Rossmann, T.; Stephan, D. W. Dalton Trans. 2009, 1559.

(35) Cabrera, L.; Welch, G. C.; Masuda, J. D.; Wei, P.; Stephan, D. W. Inorg. Chim. Acta

2006, 359 3066.

(36) Welch, G. C.; Holtrichter-Roessmann, T.; Stephan, D. W. Inorg. Chem. 2008, 47, 1904.

(37) Doring, S.; Erker, G.; Frohlich, R.; Meyer, O.; Bergander, K. Organometallics 1998, 17,

2183.

(38) Welch, G. C.; Cabrera, L.; Chase, P. A.; Hollink, E.; Masuda, J. D.; Wei, P. R.; Stephan,

D. W. Dalton Trans. 2007, 3407.

(39) Chivers, T.; Schatte, G. Eur. J. Inorg. Chem. 2003, 3314.

(40) Welch, G. C.; Masuda, J. D.; Stephan, D. W. Inorg. Chem. 2006, 45, 478.

154

(41) Kubas, G. J. Science 2006, 314, 1096.

(42) de Vries, J. G.; Elsevier, C. J. The Handbook of Homogeneous Hydrogenation; Wiley-

VCH: Weinheim, 2007.

(43) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.

(44) Chase, P. A.; Stephan, D. W. Angew. Chem. Int. Ed. 2008, 47, 7433.

(45) Holschumacher, D.; Bannenberg, T.; Hrib Cristian, G.; Jones Peter, G.; Tamm, M.

Angew. Chem. Int. Ed. 2008, 47, 7428.

(46) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008, 1701.

(47) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela, M.; Repo, T.; Rieger, B. Angew. Chem. Int.

Ed. 2008, 47, 6001.

(48) Neu, R. C.; Ouyang, E. Y.; Geier, S. J.; Zhao, X.; Ramos, A.; Stephan, D. W. Dalton

Trans. 2010, 39.

(49) Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 52.

(50) Rokob, T. A.; Hamza, A.; Papai, I. J. Am. Chem. Soc. 2009, 131, 10701.

(51) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Froehlich, R.; Grimme, S.; Stephan, D. W.

Chem. Commun. 2007, 5072.

(52) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskela, M.; Repo, T.;

Pyykko, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117.

(53) Otten, E.; Stephan, D. W. Unpublished Results.

(54) Tague, T. J.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 4970.

(55) Jursic, B. S. J. Mol. Struct. 1999, 492, 97.

(56) Watts, J. D.; Bartlett, R. J. J. Am. Chem. Soc. 1995, 117, 825.

(57) Schreiner, P. R.; Schaefer, H. F.; Schleyer, P. V. J. Chem. Phys. 1994, 101, 7625.

(58) Moroz, A.; Sweany, R. L.; Whittenburg, S. L. J. Phys. Chem. 1990, 94, 1352.

(59) Moroz, A.; Sweany, R. L. Inorg. Chem. 1992, 31, 5236.

(60) Rokob, T. A.; Hamza, A.; Stirling, A.; Soos, T.; Papai, I. Angew. Chem. Int. Ed. 2008,

47, 2435.

(61) Hamza, A.; Stirling, A.; Rokob, T. A.; Papai, I. Int. J. Quantum Chem 2009, 109, 2416.

(62) Rokob, T. A.; Hamza, A.; Stirling, A.; Papai, I. J. Am. Chem. Soc. 2009, 131, 2029.

155

(63) Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. Angew. Chem. Int. Ed. 2010, 49, 1402.

(64) Rozas, I.; Alkorta, I.; Elguero, J. Chem. Phys. Lett. 1997, 275, 423.

(65) Hugas, D.; Simon, S.; Duran, M.; Guerra, C. F.; Bickelhaupt, F. M. Chem-Eur J 2009,

15, 5814.

(66) Meir, R.; Chen, H.; Lai, W. Z.; Shaik, S. Chemphyschem 2010, 11, 301.

(67) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem. Int. Ed. 2007, 46, 4968.

(68) Dureen, M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 8396.

(69) Momming, C. M.; Fromel, S.; Kehr, G.; Frohlich, R.; Grimme, S.; Erker, G. J. Am.

Chem. Soc. 2009, 131, 12280.

(70) Momming, C. M.; Otten, E.; Kehr, G.; Frohlich, R.; Grimme, S.; Stephan, D. W.; Erker,

G. Angew. Chem. Int. Ed. 2009, 48, 6643.

(71) Otten, E.; Neu, R. C.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 9918.

(72) Menard, G.; Stephan, D. W. J. Am. Chem. Soc. 2010, 132, 1796.

(73) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem. Int. Ed. 2007, 46,

8050.

(74) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Fröhlich, R.; Erker, G. Angew. Chem.

Int. Ed. 2008, 47, 7543.

(75) Axenov, K. V.; Kehr, G.; Fröhlich, R.; Erker, G. J. Am. Chem. Soc. 2009, 131, 3454.

(76) Wang, H. D.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2008, 5966.

(77) Fehlner, T. P. Inorganometallics; Plenum Press: New York, 1992.

(78) Chandrasekhar, V. Inorganic and organometallic polymers; Springer: Berlin ; New York,

NY, 2005.

(79) Mark, J. E.; Allcock, H. R.; West, R. Inorganic polymers; 2nd ed.; Oxford University

Press: New York, 2005.

(80) Dorn, H.; Singh, R. A.; Massey, J. A.; Lough, A. J.; Manners, I. Angew. Chem. Int. Ed.

1999, 38, 3321.

(81) Dorn, H.; Jaska, C. A.; Singh, R. A.; Lough, A. J.; Manners, I. Chem. Commun. 2000,

1041.

(82) Dorn, H.; Singh, R. A.; Massey, J. A.; Nelson, J. M.; Jaska, C. A.; Lough, A. J.; Manners,

I. J. Am. Chem. Soc. 2000, 122, 6669.

156

(83) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Chem. Commun. 2001, 962.

(84) Jaska, C. A.; Lough, A. J.; Manners, I. Inorg Chem 2004, 43, 1090.

(85) Jaska, C. A.; Bartole-Scott, A.; Manners, I. Dalton Trans. 2003, 4015.

(86) Jaska, C. A.; Dorn, H.; Lough, A. J.; Manners, I. Chem-Eur J 2003, 9, 271.

(87) Jaska, C. A.; Manners, I. J. Am. Chem. Soc. 2004, 126, 1334.

(88) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Phosphorus Sulfur and Silicon and

the Related Elements 2004, 179, 733.




(92) Bartole-Scott, A.; Jaska, C. A.; Manners, I. Pure Appl. Chem. 2005, 77, 1991.

(93) Clark, T. J.; Rodezno, J. M.; Clendenning, S. B.; Aouba, S.; Brodersen, P. M.; Lough, A.

J.; Ruda, H. E.; Manners, I. Chem.-Eur. J. 2005, 11, 4526.

(94) Jaska, C. A.; Clark, T. J.; Clendenning, S. B.; Grozea, D.; Turak, A.; Lu, Z.-H.; Manners,

I. J. Am. Chem. Soc. 2005, 127, 5116.

(95) Jaska, C. A.; Clark, T. J.; Clendenning, S. B.; Grozea, D.; Turak, A.; Lu, Z. H.; Manners,

I. J. Am. Chem. Soc. 2005, 127, 5116.

(96) Clark, T. J.; Russell, C. A.; Manners, I. J. Am. Chem. Soc. 2006, 128, 9582.

(97) Clark, T. J.; Jaska, C. A.; Turak, A.; Lough, A. J.; Lu, Z. H.; Manners, I. Inorg Chem

2007, 46, 7394.

(98) Corcoran, E. W., Jr.; Sneddon, L. G. J. Am. Chem. Soc. 1985, 107, 7446.

(99) Corcoran, E. W., Jr.; Sneddon, L. G. J. Am. Chem. Soc. 1984, 106, 7793.

(100) Hoskin, A. J.; Stephan, D. W. Angew. Chem. Int. Ed. 2001, 40, 1865.

(101) Etkin, N.; Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1997, 119, 2954.

(102) Roering, A. J.; MacMillan, S. N.; Tanski, J. M.; Waterman, R. Inorg. Chem. 2007, 46,

6855.

(103) Aitken, C.; Harrod, J. F.; Malek, A.; Samuel, E. J. Organomet. Chem. 1988, 349, 285.

(104) Choi, N.; Tanaka, M. J. Organomet. Chem. 1998, 564, 81.

157

(105) Aitken, C.; Harrod, J. F.; Samuel, E. J. Organomet. Chem. 1985, 279, C11.

(106) Aitken, C. T.; Harrod, J. F.; Samuel, E. J. Am. Chem. Soc. 1986, 108, 4059.

(107) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22.

(108) Corey, J. Y.; Zhu, X. H.; Bedard, T. C.; Lange, L. D. Organometallics 1991, 10, 924.

(109) Rosenberg, L.; Davis, C. W.; Yao, J. J. Am. Chem. Soc. 2001, 123, 5120.

(110) Babcock, J. R.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 12481.

(111) Neale, N. R.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 3802.

(112) Waterman, R.; Tilley, T. D. Angew. Chem. Int. Ed. 2006, 45, 2926.

(113) Emsley, J. The 13th element : the sordid tale of murder, fire, and phosphorus; John

Wiley & Sons, Inc.: Chichester ; New York, 2000.

(114) Masuda, J. D.; Stephan, D. W. Can. J. Chem. 2005, 83, 324.

(115) Garon, C. M.; McIsaac, D. I.; Vogels, C. M.; Decken, A.; Williams, I. D.; Kleeberg, C.;

Marder, T. B.; Westcott, S. A. Dalton Trans. 2009, 1624.

(116) Shu, R.; Hao, L.; Harrod, J. F.; Woo, H.-G.; Samuel, E. J. Am. Chem. Soc. 1998, 120,

12988.

(117) Antoniadis, A.; Kunze, U. Z. Anorg. Allg. Chem. 1979, 34, 116.

(118) Hayashi, M.; Matsuura, Y.; Watanabe, Y. Tetrahedron Lett. 2004, 45, 1409.

(119) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002, 102, 3031.

(120) Foo, T.; Bergman, R. G. Organometallics 1992, 11, 1811.

(121) Rerek, M. E.; Basolo, F. J. Am. Chem. Soc. 1984, 106, 5908.

(122) Arif, A. M.; Jones, R. A.; Seeberger, M. H.; Whittlesey, B. R.; Wright, T. C. Inorg.

Chem. 1986, 25, 3943.

(123) Haines, R. J.; Steen, N. D. C.; English, R. B. J. Chem. Soc., Dalton Trans. 1983, 1607.

(124) Haines, R. J.; Steen, N. D. C. T.; English, R. B. J. Chem. Soc., Chem. Commun. 1981,

587.

(125) Hieber, W.; Kummer, R. Chem. Ber. 1967, 100, 148.

(126) Klingert, B.; Werner, H. J. Organomet. Chem. 1987, 333, 119.

(127) Klingert, B.; Werner, H. J. Organomet. Chem. 1983, 252, C47.

158

(128) Tejel, C.; Sommovigo, M.; Ciriano, M. A.; Lopez, J. A.; Lahoz, F. J.; Oro, L. A. Angew.

Chem. Int. Ed. 2000, 39, 2336.

(129) Werner, H.; Klingert, B.; Rheingold, A. L. Organometallics 1988, 7, 911.

(130) Yasufuku, K.; Yamazaki, H. Bull. Chem. Soc. Jpn. 1973, 46, 1502.

(131) Dashti-Mommertz, A.; Neumuller, B. Z. Anorg. Allg. Chem. 1999, 625, 954.

(132) Mundt, O.; Riffel, H.; Becker, G.; Simon, A. Z. Anorg. Allg. Chem. 1988, 43, 952.

(133) Richter, R.; Kaiser, J.; Sieler, J.; Hartung, H.; Peter, C. Acta Crystallogr., Sect. B 1977,

B33, 1887.

(134) Baxter, S. G.; Cowley, A. H.; Davis, R. E.; Riley, P. E. J. Am. Chem. Soc. 1981, 103,

1699.

(135) Werner, H.; Klingert, B. J. Organomet. Chem. 1981, 218, 395.

(136) Klingert, B.; Werner, H. Chem. Ber. 1983, 116, 1450.

(137) Zhang, S. Y.; Yemul, S.; Kagan, H. B.; Stern, R.; Commereuc, D.; Chauvin, Y.

Tetrahedron Lett. 1981, 22, 3955.

(138) Bitterwolf, T. E.; Spink, W. C.; Rausch, M. D. J. Organomet. Chem. 1989, 363, 189.

(139) Crumbliss, A. L.; Topping, R. J.; Szewczyk, J.; McPhail, A. T.; Quin, L. D. J. Chem.

Soc., Dalton Trans. 1986, 1895.

(140) Thompson, D. T.; (Imperial Chemical Industries Ltd.). Application: GB

GB, 1969, p 4 pp.

(141) Werner, H.; Klingert, B.; Zolk, R.; Thometzek, P. J. Organomet. Chem. 1984, 266, 97.

(142) Butts, M. D.; Bryan, J. C.; Luo, X. L.; Kubas, G. J. Inorg. Chem. 1997, 36, 3341.

(143) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Organometallics 2005, 24, 4367.

(144) Campian, M. V.; Harris, J. L.; Jasim, N.; Perutz, R. N.; Marder, T. B.; Whitwood, A. C.

Organometallics 2006, 25, 5093.

(145) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2004, 764.

(146) Ang, H.-G.; Ang, S.-G.; Zhang, Q. J. Chem. Soc., Dalton Trans. 1996, 3843.

(147) Bai, G.; Wei, P.; Das, A. K.; Stephan, D. W. Dalton Trans. 2006, 1141.

(148) Quirt, A. R.; Martin, J. S. J Magn Reson 1971, 5, 318.

159

(149) Marat, K. Spinworks 3.1, University of Manitoba 2009.

(150) Kesanli, B.; Mattamana, S. P.; Danis, J.; Eichhorn, B. Inorg. Chim. Acta 2005, 358, 3145.

(151) Charles, S.; Danis, J. A.; Fettinger, J. C.; Eichhorn, B. W. Inorg. Chem. 1997, 36, 3772.

(152) Couret, C.; Escudie, J.; Saint-Roch, B.; Andriamizaka, J. D.; Satge, J. J. Organomet.

Chem. 1982, 224, 247.

(153) Dixon, D. A.; Arduengo, A. J.; Fukunaga, T. J. Am. Chem. Soc. 1986, 108, 2461.

(154) Rogers, J. R.; Wagner, T. P. S.; Marynick, D. S. Inorg. Chem. 1994, 33, 3104.

(155) Wicht, D. K.; Kovacik, I.; Glueck, D. S.; Liable-Sands, L. M.; Incarvito, C. D.;

Rheingold, A. L. Organometallics 1999, 18, 5141.

(156) Fryzuk, M. D.; Giesbrecht, G. R.; Rettig, S. J. Inorg. Chem. 1998, 37, 6928.

(157) Yao, W. B.; Eisenstein, O.; Crabtree, R. H. Inorg. Chim. Acta 1997, 254, 105.

(158) Burford, N.; Losier, P.; Mason, S.; Royan, B. W.; Spence, R. E. v. H.; Bakshi, P. K.;

Borecka, B.; Cameron, T. S.; Richardson, J. F.; Rogers, R. D. Phosphorus Sulfur and Silicon and

the Related Elements 1993, 76, 277.

(159) Burford, N.; Losier, P.; Bakshi, P. K.; Cameron, T. S. J. Chem. Soc., Dalton Trans. 1993,

201.

(160) Burford, N.; Losier, P.; Macdonald, C.; Kyrimis, V.; Bakshi, P. K.; Cameron, T. S. Inorg

Chem 1994, 33, 1434.

(161) Burford, N.; Cameron, T. S.; Ragogna, P. J.; Ocando-Mavarez, E.; Gee, M.; McDonald,

R.; Wasylishen, R. E. J. Am. Chem. Soc. 2001, 123, 7947.

(162) Burford, N.; Ragogna, P. J. J. Chem. Soc., Dalton Trans. 2002, 4307.

(163) Burford, N.; Ragogna, P. J.; McDonald, R.; Ferguson, M. J. J. Am. Chem. Soc. 2003, 125,

14404.

(164) Burford, N.; Ragogna, P. J.; McDonald, R.; Ferguson, M. J. Chem. Commun. 2003, 2066.

(165) Burford, N.; Dyker, C. A.; Decken, A. Angew. Chem., Int. Ed. 2005, 44, 2364.

(166) Burford, N.; Phillips, A. D.; Spinney, H. A.; Lumsden, M.; Werner-Zwanziger, U.;

Ferguson, M. J.; McDonald, R. J. Am. Chem. Soc. 2005, 127, 3921.

(167) Dyker, C. A.; Burford, N.; Lumsden, M. D.; Decken, A. J. Am. Chem. Soc. 2006, 128,

9632.

(168) Weigand, J. J.; Burford, N.; Decken, A.; Schulz, A. European Journal of Inorganic

Chemistry 2007, 4868.

160

(169) Dyker, C. A.; Riegel, S. D.; Burford, N.; Lumsden, M. D.; Decken, A. J. Am. Chem. Soc.

2007, 129, 7464.

(170) Dyker, C. A.; Burford, N.; Menard, G.; Lumsden, M. D.; Decken, A. Inorg Chem 2007,

46, 4277.

(171) Dyker, C. A.; Burford, N. Chem.-Asian J. 2008, 3, 28.

(172) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus : the carbon copy : from

organophosphorus to phospha-organic chemistry; Wiley: Chichester ; New York, 1998.

(173) Denis, J.-M.; Forintos, H.; Szelke, H.; Toupet, L.; Pham, T.-N.; Madec, P.-J.; Gaumont,

A.-C. Chem. Commun. 2003, 54.

(174) Burford, N.; Dyker, C. A.; Lumsden, M.; Decken, A. Angew. Chem. Int. Ed. 2005, 44,

6196.

(175) Blackwell, J. M.; Morrison, D. J.; Piers, W. E. Tetrahedron 2002, 58, 8247.

(176) Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090.

(177) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440.

(178) Blackwell, J. M.; Sonmor, E. R.; Scoccitti, T.; Piers, W. E. Org. Lett. 2000, 2, 3921.

(179) Blackwell, J. M.; Piers, W. E.; Parvez, M. Org. Lett. 2000, 2, 695.

(180) Horton, A. D.; deWith, J. Organometallics 1997, 16, 5424.

(181) Boere, R. T.; Masuda, J. D. Can. J. Chem. 2002, 80, 1607.

(182) Okazaki, M.; Jung, K. A.; Tobita, H. Organometallics 2005, 24, 659.

(183) Petrie, M. A.; Power, P. P. J. Chem. Soc., Dalton Trans. 1993, 1737.

(184) Tardif, O.; Hou, Z. M.; Nishiura, M.; Koizumi, T.; Wakatsuki, Y. Organometallics 2001,

20, 4565.

(185) Tardif, O.; Nishiura, M.; Hou, Z. M. Tetrahedron 2003, 59, 10525.

(186) Hayashi, T. J. Organomet. Chem. 2002, 653, 41.

(187) Marinetti, A.; Voituriez, A. Synlett 2010, 174.

(188) Ohff, M.; Holz, J.; Quirmbach, M.; Boerner, A. Synthesis 1998, 1391.

(189) Pietrusiewicz, K. M.; Zablocka, M. Chem. Rev. 1994, 94, 1375.

(190) Vedejs, E.; Daugulis, O.; MacKay, J. A.; Rozners, E. Synlett 2001, 1499.

161

(191) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007, 2613.

(192) Niedenzu, K.; Dawson, J. W. Boron-Nitrogen Compounds; Academic Press, Inc: New

York, 1965.

(193) Clark, T. L.; Rodezno, J. M.; Clendenning, S. B.; Aouba, S.; Brodersen, P. M.; Lough, A.

J.; Ruda, H. E.; Manners, I. Chem-Eur J 2005, 11, 4526.

(194) Jaska, C. A.; Bartole-Scott, A.; Manners, I. Phosphorus Sulfur and Silicon and the

Related Elements 2004, 179, 685.

(195) Dorn, H.; Vejzovic, E.; Lough, A. J.; Manners, I. Inorg Chem 2001, 40, 4327.

(196) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007, 129, 1844.

(197) Himmelberger, D. W.; Yoon, C. W.; Bluhm, M. E.; Carroll, P. J.; Sneddon, L. G. J. Am.

Chem. Soc. 2009, 131, 14101.

(198) Hausdorf, S.; Baitalow, F.; Wolf, G.; Mertens, F. O. R. L. Int. J. Hydrogen Energy 2008,

33, 608.

(199) Ramachandran, P. V.; Gagare, P. D. Inorg. Chem. 2007, 46, 7810.

(200) Davis, B. L.; Dixon, D. A.; Garner, E. B.; Gordon, J. C.; Matus, M. H.; Scott, B.;

Stephens, F. H. Angew. Chem. Int. Ed. 2009, 48, 6812.

(201) Spikes, G. H.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2005, 127, 12232.

(202) Pestana, D. C.; Power, P. P. J. Am. Chem. Soc. 1991, 113, 8426.

(203) Gilbert, T. M.; Bachrach, S. M. Organometallics 2007, 26, 2672.

(204) Parks, D. J.; Spence, R. E. V. H.; Piers, W. E. Angew. Chem. Int. Ed. 1995, 34, 809.

(205) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17, 5492.

(206) Noeth, H. Z. Anorg. Allg. Chem. 1987, 555, 79.

(207) Karsch, H. H.; Hanika, G.; Huber, B.; Riede, J.; Mueller, G. J. Organomet. Chem. 1989,

361, C25.

(208) Jakle, F.; Mattner, M.; Priermeier, T.; Wagner, M. J. Organomet. Chem. 1995, 502, 123.

(209) Feng, X.; Olmstead, M. M.; Power, P. P. Inorg. Chem. 1986, 25, 4615.

(210) Paine, R. T.; Noth, H. Chem. Rev. 1995, 95, 343.

(211) Geier, S. J.; Gilbert, T. M.; Stephan, D. W. J. Am. Chem. Soc. 2008, 130, 12632.

162

(212) Lancaster, S. J.; Mountford, A. J.; Hughes, D. L.; Schormann, M.; Bochmann, M. J.

Organomet. Chem. 2003, 680, 193.

(213) Welch, G. C., University of Windsor, 2008.

(214) Groshens, T. J.; Higa, K. T.; Nissan, R.; Butcher, R. J.; Freyer, A. J. Organometallics

1993, 12, 2904.

(215) Clarke, M. L.; Roff, G. J.; Vries, J. G. d., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim,

2007, p 413.

(216) You, S. L. Chem.-Asian J. 2007, 2, 820.

(217) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.

(218) Singh, S.; Chhina, S.; Sharma, V. K.; Sachdev, S. S. J. Chem. Soc., Chem. Commun.

1982, 453.

(219) Sridharan, V.; Avendano, C.; Carlos Menendez, J. Tetrahedron 2009, 65, 2087.

(220) Sahin, A.; Cakmak, O.; Demirtas, I.; Okten, S.; Tutar, A. Tetrahedron 2008, 64, 10068.

(221) Fadel, F.; Titouani, S. L.; Soufiaoui, M.; Ajamay, H.; Mazzah, A. Tetrahedron Lett.

2004, 45, 5905.

(222) Yunnikova, L. P.; Tigina, O. V. Zh. Org. Khim. 1993, 29, 651.

(223) Friebolin, H.; Ruchardt, C. Liebigs Ann 1995, 1339.

(224) Webb, J. D.; Laberge, V. S.; Geier, S. J.; Crudden, C. M.; Stephan, D. W. Chem.-Eur. J.

2010, 16, 8.

(225) Nandi, P.; Dye, J. L.; Jackson, J. E. J. Org. Chem. 2009, 74, 5790.

(226) Murahashi, S.; Imada, Y.; Hirai, Y. Bull. Chem. Soc. Jpn. 1989, 62, 2968.

(227) Pitts, M. R.; Harrison, J. R.; Moody, C. J. J. Chem. Soc., Perkin Trans. 1 2001, 955.

(228) Bianchini, C.; Dal Santo, V.; Meli, A.; Moneti, S.; Psaro, R.; Sordelli, L.; Vizza, F. Inorg.

Chim. Acta 2008, 361, 3677.

(229) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1966, 5, 218.

(230) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B.; Cazenobe, I.; Ledoux, I.;

Zyss, J.; Thornton, A.; Bruce, D. W.; Kakkar, A. K. Chem. Mater. 1998, 10, 1355.

(231) Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, T. B.; Crabtree, R. H. J.

Am. Chem. Soc. 1999, 121, 6337.

163

(232) Parks, D. J.; Piers, W. E.; Parvez, M.; Atencio, R.; Zaworotko, M. J. Organometallics

1998, 17, 1369.

(233) Katritzky, A. R.; Rachwal, S.; Rachwal, B. Tetrahedron 1996, 52, 15031.

(234) Rabideau, P. W.; Marcinow, Z. Organic Reactions (Hoboken, NJ, United States) 1992,

42, No pp given.

(235) Radivoy, G.; Alonso, F.; Yus, M. Tetrahedron 1999, 55, 14479.

(236) Rosales, M.; Vallejo, R.; Bastidas, L. J.; Gonzalez, B.; Gonzalez, A. React. Kinet. Catal.

Lett. 2007, 92, 99.

(237) Rosales, M.; Castillo, J.; Gonzalez, A.; Gonzalez, L.; Molina, K.; Navarro, J.; Pacheco,

I.; Perez, H. Transition Met. Chem. 2004, 29, 221.

(238) Hoenel, M.; Vierhapper, F. W. J. Chem. Soc., Perkin Trans. 1 1980, 1933.

(239) Voutchkova, A. M.; Gnanamgari, D.; Jakobsche, C. E.; Butler, C.; Miller, S. J.; Parr, J.;

Crabtree, R. H. J. Organomet. Chem. 2008, 693, 1815.

(240) Fujita, K.-i.; Kitatsuji, C.; Furukawa, S.; Yamaguchi, R. Tetrahedron Lett. 2004, 45,

3215.

(241) Srikrishna, A.; Reddy, T. J.; Viswajanani, R. Tetrahedron 1996, 52, 1631.

(242) Ranu, B. C.; Jana, U.; Sarkar, A. Synth. Commun. 1998, 28, 485.

(243) Rueping, M.; Theissmann, T.; Antonchick, A. P. Catalysts for Fine Chemical Synthesis

2007, 5, 170.

(244) Rueping, M.; Antonchick, A. P.; Theissmann, T. Angew. Chem. Int. Ed. 2006, 45, 6751.

(245) Mountford, A. J.; Clegg, W.; Coles, S. J.; Harrington, R. W.; Horton, P. N.; Humphrey,

S. M.; Hursthouse, M. B.; Wright, J. A.; Lancaster, S. J. Chem.-Eur. J. 2007, 13, 4535.

(246) Kehr, G.; Roesmann, R.; Frohlich, R.; Holst, C.; Erker, G. Eur. J. Inorg. Chem. 2001,

535.

(247) Roesler, R.; Piers, W. E.; Parvez, M. J. Organomet. Chem. 2003, 680, 218.

(248) Ugolotti, J.; Hellstrom, S.; Britovsek, G. J.; Jones, T. S.; Hunt, P.; White, A. J. Dalton

Trans. 2007, 1425.

(249) Naumann, D.; Butler, H.; Gnann, R. Z. Anorg. Allg. Chem. 1992, 618.

(250) Stewart, A. P.; Trippett, S. J. Chem. Soc., C 1970, 1263.

(251) Rahman, M. M.; Liu, H. Y.; Prock, A.; Giering, W. P. Organometallics 1987, 6, 650.

164

(252) Tolman, C. A. Chem. Rev. 1977, 77, 313.

(253) Rahman, M. M.; Liu, H. Y.; Eriks, K.; Prock, A.; Giering, W. P. Organometallics 1989,

8, 1.

(254) Steyl, G.; Roodt, A. Acta Crystallogr. C 2004, 60, m324.

(255) Lide, D. R.; Frederikse, H. P. R. CRC handbook of chemistry and physics : a ready-

reference book of chemical and physical data; 76th ed / ed.; CRC: Boca Raton, Fla. ; London,

1995.

transition metal complexes and main group frustrated lewis pairs for stoichiometric ... ·...

Documents