Amine Borane Dehydrocoupling with d Complexes - Opus

332 Pages·2014·4.87 MB·English

Amine Borane
Dehydrocoupling with
0
d Complexes

Peter Bellham
A thesis submitted for the degree of Doctor of Philosophy
University of Bath
Department of Chemistry
June 2014

COPYRIGHT
Attention is drawn to the fact that copyright of this thesis rests with its author. A
copy of this thesis has been supplied on condition that anyone who consults it is
understood to recognise that its copyright rests with the author and they must not
copy it or use material from it except as permitted by law or with the consent of the
author.
Publications Resulting from this Thesis

Alkylstrontium diamidoboranes: beta-hydride elimination and Sr-C insertion.
Bellham, P., Hill, M. S., Liptrot, D. J., MacDougall, D. J. and Mahon, M. F., Chem.
Commun., 2011, 47 (32), 9060-9062.

Alkaline earth alkyl insertion chemistry of in situ generated aminoboranes.
Bellham, P., Hill, M.S., Kociok-Köhn, G. and Liptrot, D.J., Dalton Trans., 2013, 42
(3), 737-745.

Bespoke synthesis of unsymmetrical diaminoboranes by alkaline earth catalysis.
Bellham, P., Hill, M.S., Kociok-Köhn, G. and Liptrot, D.J., Chem. Commun., 2013,
49 (19), 1960-1962.

I
Contents
1 Introduction 1
1.1 Alkaline Earth Chemistry 1
1.1.1 Group 2-Centred Hydroamination 6
1.1.2 Group 2-Centred Hydrosilylation 8
1.1.3 Group 2-Centred Dehydrogenative Silicon-Nitrogen Coupling 9
1.1.4 Hydrocarbon-Soluble Alkaline Earth Metal Hydrides 12
1.2.1 Hydrogen Storage and Amine Boranes 15
1.2.2 Hydrogen Evolution Via Thermolysis 17
1.2.3 Metal-Assisted Hydrogen Release 18
1.2.4 Dehydrocoupling of Amine Boranes with Frustrated Lewis Pairs 22
1.3 Dehydrocoupling of Amine Boranes by d0 Complexes 23
1.4 Amine Boranes Beyond Hydrogen Storage 34
1.5 Aims of The Research Described In This Thesis 37
1.6 Chapter 1 References 41

2 Stoichiometric Reactions of Alkaline Earth Dialkyls with Amine 64
Boranes
2.1 Stoichiometric Reactions Between Alkaline Earth Dialkyls VII and 64
Secondary Amine Boranes
2.2 Kinetic NMR Experiments on the Reaction of Strontium Dialkyl VIISr with
DMAB
2.3 Kinetic NMR Experiments on the Reaction of Magnesium- and Calcium- 77
Dialkyls (VIIMg, VIICa) with DMAB
2.4 Stoichiometric Reaction of a Strontium Dialkyl VIISr with a Primary 82
Amine Borane
2.5 Conclusion for Chapter 2 88
2.6 General Experimental Procedures for Chapter 2 90
2.6.1 Typical Procedure for Stoichiometric NMR-Scale Reactions 90
2.6.2 Experimental Procedures for Stoichiometric Reaction of Secondary 92
Amine Boranes with VIISr
2.6.3 Experimental Procedures for Thermolysis Reactions of Secondary 92
Amine Boranes with VIIMg, VIICa and VIISr
II
2.6.4 Reaction of VIISr with Primary Amine Borane, TBAB 94
2.6.5 Typical Procedure for Kinetic Experiments 96
2.7 References for Chapter 2 97

3 Mechanistic Investigation of d0 Complex Dehydrocoupling of 98
Amine Boranes
3.1.1 Consideration of Alkaline Earth Pre-catalyst for Amine Borane 99
Dehydrocoupling: Limitations of Previously Studied Pre-catalysts
3.1.2 Stoichiometric Reactions Between Calcium and Magnesium Anilido- 100
Imine Reagents, IX-Ca and IX-Mg, and Secondary Amine Boranes
3.1.3 Catalytic Dehydrocoupling of Secondary Amine Boranes by Calcium and 104
Magnesium Anilido-Imine Precatalysts IX-Ca and IX-Mg
3.1.4 Stoichiometric Reactivity of Strontium Anilido-Imine IX-Sr with 106
Secondary Amine Boranes and Anilido-Imine Ligand Stability
3.1.5 Selection of Alkaline Earth Precatalyst for Amine Borane 114
Dehydrocoupling
3.2.1 Stoichiometric Reactivity of Group 1 bis(trimethylsilyl)amides 115
MN(SiMe ) LIX with DMAB
3 2
3.2.2 Catalytic Dehydrocoupling of DMAB By Group 1 121
Bis(trimethylsilyl)amides LIX
3.2.3 Stoichiometric Reactivity of β-diketiminato-Magnesium Amidoborane 122
3.2.4 Heavier Alkaline Earth M[NMe BH NMe BH ] Complexes 125
2 2 2 3 2
3.2.5 Stoichiometric Reactivity of β-diketiminato-Calcium Amidoboranes 126
3.2.6 Reactivity of Alkaline Earth Reagents with a Primary Amine Borane 138
3.3 Kinetic Analysis of the Stoichiometric Reactivity of Alkaline Earth 150
Amidoboranes
3.4 Chapter 3 Conclusion 162
3.5 Experimental Procedures for Chapter 3 168
3.5.1 Reactions of Anilido-imine Species IX with Amine Boranes 168
3.5.2 Stoichiometric Reactions of Group 1 Reagents with Amine Boranes 182
3.5.3 Stoichiometric Reactions of β-diketiminato-Magnesium Species 185
with Amine Boranes

III
3.5.4 Synthesis of Heavier Alkaline Earth [M(NMe BH NMe BH ) ] 186
2 2 2 3 2
Complexes
3.5.5 Stoichiometric Reactions of β-diketiminato-Calcium Amidoboranes 188
3.5.6 Catalytic NMR-Scale Reactions Between Group 1 192
Bis(trimethylsilyl)amides [MN(SiMe ) ] (M = Li, Na, K) LIX and
3 2
Me NH.BH
2 3
3.5.7 NMR-Scale Reactions Between Calcium Reagents and tBuH N.BH 193
2 3
3.5.8 Kinetic Characterisation of Dehydrocoupling of Me HN.BH with d0 197
2
Reagents
3.5.9 Kinetic Characterisation of Stoichiometric Reactions of d0 199
Amidoboranes
3.6 References for Chapter 3 202

4 Kinetic Analysis of Catalytic Dehydrocoupling of Amine Boranes 206
by d0 Precatalysts
4.1 Kinetic Analysis of Catalytic Dehydrocoupling of Amine Boranes by Group 207
3 Precatalysts [Sc{N(SiHMe ) } (THF) ] XL and [Y{N(SiMe ) } ] XLI
2 2 3 2 3 2 3
4.2 Kinetic Analysis of Catalytic Dehydrocoupling of Amine Boranes by 213
Calcium Precatalysts VI and V-Ca
4.2.1 Kinetic Analysis of the Dehydrocoupling of the Primary Amine 228
Borane Tert-Butylamine Borane
4.3 Kinetic Analysis of Catalytic Dehydrocoupling of DMAB by Magnesium 230
Precatalysts XV and V-Mg
4.4 Comparison of Reactivity of Group 1 Precatalysts LIX for Catalytic 235
Dehydrocoupling of Amine Boranes
4.5 The Trend in Reactivity of Group 1, 2 and 3 Precatalysts for Catalytic 237
Dehydrocoupling of Amine Boranes
4.6 Chapter 4 Conclusion 240
4.7 Experimental Procedures for Chapter 4 241
4.7.1 Kinetic Characterisation of Catalytic Dehydrocoupling of 245
[Me HN.BH ] by Group 3 Species
2 3

IV
4.7.2 Kinetic Characterisation of Catalytic Dehydrocoupling of 250
[Me HN.BH ] by Calcium Precatalysts [β-diketiminato.Ca{N(SiMe ) }]
2 3 3 2
VI and [Ca{{N(SiMe ) }] V-Ca
3 2
4.7.3 Kinetic Characterisation of Catalytic Dehydrocoupling of 258
[Me HN.BH ] by Magnesium Precatalysts [β-diketiminato.MgnBu] XV
2 3
and [Mg{N(SiMe ) }] V-Mg
3 2
4.7.4 Kinetic Characterisation of Catalytic Dehydrocoupling of 260
[Me HN.BH ] by 5 mol % (i) [Li{N(SiMe ) }] LIX-Li, (ii)
2 3 3 2
[Na{N(SiMe ) }] LIX-Na and (iii) [K{N(SiMe ) }] LIX-K at 323 K
3 2 3 2
Using the Method of Initial Rates
4.8 References for Chapter 4 261

5 Alkaline Earth Mediated Reactions of Amine Boranes With 263
Amines
5.1 Catalytic Insertion Reactions with Amines 264
5.2 Stoichiometric Reactions Between Calcium Amidoborane and Amine 274
5.3 Conclusion for Chapter 5 277
5.4 Experimental Procedures for Chapter 5 279
5.4.1 Experimental Procedures for Catalytic Amine Insertion Reactions 279
5.4.2 Preparative-Scale Synthesis of [{tBuHN}BH{NPh }] 60 286
2
5.4.3 Experimental Procedures for Uncatalysed Amine Insertion 287
Reactions
5.4.4 Kinetic Dehydrocoupling of Me HN.BH and Pyrrolidine by d0 290
2
Species
5.4.5 Stoichiometric Reactions Between β-diketiminato Calcium 291
dimethylamidoborane LXV and Amines
5.5 References for Chapter 5 292

6 Conclusions 293
6.1 References for Chapter 6 298

V
Acknowledgements
Undertaking a PhD is a very personal project and journey, testing your resolve in so
many ways and, as those around me will testify, I have usually gone about
overcoming those things in a typically unique way. This journey would not have
been possible without Mike, whose faith in giving me the opportunity to undertake
this PhD, combined with enthusiasm about chemistry and style of supervision have
all been greatly appreciated.
I am grateful for the support of my parents, family and friends who continually ask
me how everything is going. Whilst they may not understand what it is that I’m
doing I know that I have all of their support. The non-chemist who knows most
about my thesis is my wife Michelle, whose support, belief in me and listening ear
for me to grumble about kinetics and general amine borane based woes has been
constant and unwavering. It has been reassuring to have someone by my side who
knows what it’s like to undertake a PhD and know without me saying.
Within the chemistry department I wish to thank Gabriele Kociok-Köhn for all of the
crystallography, John Lowe for all of his NMR help and allowing me to run so many
NMRs on the 300 MHz spectrometer, Carlo Di Iulio and Rhodri Owen for their help
with GC MS and Stephen Boyer at London Metropolitan University for CHN
microanalysis. I also extend my gratitude to my colleagues in the Hill, Johnson and
Molloy groups and the University of Bath for funding.
My PhD moved me away from friends and family, but brought me to the lovely city
of Bath, a place which has changed my life in so many ways and somewhere I will
greatly miss.

VI
Abstract
As the developed world seeks to reduce the dependence upon fossil fuels, amine
boranes have received increased interest as a clean, stable hydrogen source. This
thesis sought to scrutinise the mechanism of amine borane dehydrocoupling
proposed by Hill and provide quantitative evidence for the dependence of
dehydrocoupling activity upon identity of the metal centre. This work led to a
refined mechanism, shown in Figure A1, incorporating proton-assisted β-hydride
elimination steps, occurring via potentially concerted processes. A pathway to
account for the previously unreported reactivity of [R N-BH ] to ultimately afford
2 2 2
[HB(NR ) ] is also included.
2 2

Figure A1: Proposed mechanism of amine borane dehydrocoupling by alkaline earth
elements (M = Mg, Ca, Sr, Ba).
A dependence upon the identity of the metal centre on the dehydrocoupling activity
was found, identifying a decrease in reactivity with increasing ionic radius and
decreasing cation charge density as each group is descended. This thesis showed
that the mechanism of secondary amine borane dehydrocoupling by d0 reagents is
complex, suggesting that a generic mechanism which fully describes
dehydrocoupling of all amine boranes would be too much of an oversimplification.
VII
Abbreviations Used in This Thesis
β-diketiminate Dipp-nacnac ligand
AB Ammonia borane
br. Broad NMR resonance
Dipp 2,6-di-iso-propylphenyl
DMAB Dimethylamine borane
DMAP 4-Dimethylaminopyridine
FLP Frustrated Lewis Pair
GC MS Gas Chromatography Mass Spectrometry
HMBC Heteronuclear Multiple Bond Correlation
HMQC Heteronuclear Multiple Quantum Correlation
HSQC Heteronuclear Single Quantum Correlation
iPr Iso-propyl
L Supporting ligand
KIE Kinetic Isotope Effect
Me Methyl
nBu Normal butyl
NMR Nuclear Magnetic Resonance
OTf Triflate
PB Pyrrolidine borane
Ph Phenyl
TBAB Tert-butylamine borane
THF Tetrahydrofuran
tBu Tert-butyl
TOCSY Total Correlation Spectroscopy

VIII
Table 1: Literature Compounds Referred to in This Thesis

I [M{P(SiMe ) } (THF) ]
3 2 2 4
II [M{N(SiMe ) } (THF) ]
3 2 2 2
III [Ba{C(Ph) (CH CH Ph)} (THF) ]
2 2 2 2 2
IV [M(DMAT) (THF) ]
2 2
[M[N(SiMe ) ] ]
3 2 2 2
V-Mg, M = Mg
V
V-Ca, M = Ca
V-Sr, M = Sr
VI

[M{CH(SiMe ) } (THF) ]
3 2 2 2
VIIMg, M = Mg
VII
VIICa, M = Ca
VIISr, M = Sr
VIII

IX