EXPLORING THE REACTIVITY OF IRIDIUM AND RHODIUM BIS- PHOSPHINE COMPLEXES WITH AMINE-BORANES Amit Kumar A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at the University of Oxford Balliol College March 2016 बड-़े बड ़े नाज़ों स़े मनैं ़े पाली है साकीबाला, कललत कल्पना का ही इसऩे सदा उठाया है प्याला, मान-दलु ारों स़े ही रखना इस म़ेरी सुकुमारी को, विश्ि, तुम्हाऱे हाथों में अब सौंप रहा ह ूँ मधशु ाला।। Madhushala, Harivansh Rai Bachchan. Translation: With immense care, I have raised up my wine-maiden She has always picked up goblets of creativity and innovation Keep my cherished with comfort and affection World, to your hands, I am submitting the tavern. The work presented in this thesis was carried out between October 2012 and March 2016 under the supervision of Professor Andrew S. Weller. All of the work is my own, unless otherwise stated, and has not been submitted previously for any degree at this, or any other, university. Amit Kumar March 2016 i Acknowledgements Andy Thank you for taking a chance on me and offering me an opportunity to be one of the Wellers, so that a young boy could travel 4500 miles from Delhi to dehydrocouple amine-boranes and make some cool Rh-dimers. Thank you for the inspirations and suggestions whenever I got stuck and be a part of the evenings when we all could get drunk Thank you for showering your experiences, intellect and wisdom and teach an eaglet to fly high with the confidence and freedom Thank you for not giving me food instead training me how to hunt; to dive deep into the sea, with the hands empty and not just stroll on an oceanfront. It is not just about a song it is about the lesson-lifelong The experiences from the past three and a half years are much beyond this 250 pages chemistry report and nothing could have ever been possible without your supreme guidance and support. ii Indrek, My hands owe you for teaching me glove box and Schlenk line and these restless lips for introducing me to spirits and wine thank you for teaching me all the coolest lab tricks so that I build this thesis-home with colourful bricks Tom, Thank you for sharing, from authorship to bedrooms, almost like a lab better half and mounting many of my colourless crystals, which often came out to be sodium barf thank you for the advices that opened up windows, the front and the back door and directing my path, whether it is research or a zoo in Singapore. Heather, You were no less than a ‘Senapati’ of an amine-borane force thank you for teaching me all from the dehydrocoupling course and for an excellent company to down pints of cider and then break the dancing floors with the ‘Womanizer’. Amparo, Ahead was the parting, behind the partying; between them lay the chemistry Gracias for the tips to solve the lab problems and resolve my life’s mystery. Thanks a lot to Seb, for the excellent companionship and sharing the wealth of crystallography and Mark for the advices in solving crystal structures, introducing ‘cock or ball’ and hosting house party Gemma, for running GPC, and always being excited that mixed love in the air and Peng, for a great friendship that will be valued for ever Miguel and Rowan for getting my DPhil truck in top gear Rosie, Mark and Becky for a great fun in the first year iii Isobel for being a lovely part-II and exploring the B-phenyl amine-boranes Nick and Amy for the ultimate night outs and being such great companions Annie, I must thank your cheerful presence and baking that keeps everyone happy and Alasdair for the chemdraw advices and a tough competition to swallow chilli Joe, undoubtedly a great friendship, thank you for the epic emails from badminton club Anna and Hannah, thanks for the awesome time whether in tea-breaks or in uni-club Thanks to all the collaborators Dr. A.G. Algarra, Nick Beattie and Prof. S. A. Macgregor for the DFT calculations And Jacob Ishibashi and Prof. S.-Y. Liu for providing cyclic amine-boranes and to all the CRL staffs in particular Dr. Nick Rees for the NMR assistance and offering Bollywood music in the basement Colin for the mass-spec help and Dr. Amber Thompson for the X-ray diffraction training Other than amine-boranes Life has been full of men-boranes and women-boranes; thank you all for giving me infinite love and happiness: Arnab, Vikaran, Krishna, Challenger, Anuradha, Midori, Shiksha, Vikranth, Urmi, Richa, Moni, Harish, Jed, Shifali, Alex, Nikita, Simin, Abhishek, Sudakshina, Aparajita, Sneha, Karishma, Madhav, Akul, Dhruti, Anjul, Shreya, Amogh, Rituraj, Zoubeir,Toli, Natasha and Rami. Sincere gratitude to the Rhodes Trust for offering me the funding and family in Oxford and finally a big thanks to my parents for the unconditional love and support. iv Abbreviations 3C-2e Three-center two-electron atm Atmospheres 2C-2e Two-center two-electron [BArF ]- [B{3,5-C H (CF ) } ]- 4 6 3 3 2 4 Å Angstrom (1 x 10-10 metres) [BArCl ]- [B{3,5-C H Cl } ]- 4 6 3 2 4 Bu Butyl COD Cyclooctadiene Cp Cyclopentadienyl Cy Cyclohexyl δ Chemical shift ΔG Change in Gibb’s free energy ΔH Change in enthalpy ΔS Change in entropy DCM Dichloromethane DFT Density Functional Theory dcype 1,2-Bis(dicyclohexylphosphino)ethane Dipp 2,6-di-isopropylphenyl ESI-MS Electrospray Ionisation Mass Spectrometry Et Ethyl eq Equivalent HBCat Catecholborane GPC Gel Permeation Chromatography Hz Hertz h Hours IMes N,N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene iBu Isobutyl iPr Isopropyl IR Infrared J Coupling constant K Equilibrium constant eq LUMO Lowest Unoccupied Molecular Orbital M Mol dm-3 Me Methyl m/z Mass to charge ratio M Molecular weight w v NBD Norbornadiene NHC N-heterocyclic carbene nBu Normal butyl NMR Nuclear Magnetic Resonance OA Oxidative addition Ph Phenyl ppm Parts per million Pr Propyl t Half life ½ TBE 3,3-Dimetyl-1-butene THF Tetrahydrofuran tBu Tertiary butyl TOF Turnover frequency TON Turnover number Xantphos 4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene vi Abstract This thesis details the reactivity of amine-boranes with iridium and rhodium bis-phosphine complexes and offers mechanistic insights into the catalytic dehydrocoupling of amine- boranes to produce polyamino-boranes [H BNRH] that are isoelectronic to the societally 2 n ubiquitous polyolefins [H CCRH] . 2 n The 18-electron cationic complexes [Ir(H) (PCy )(η2-H ) ][BArF ] (II) and [Rh(PP)(η6- 2 3 2 2 4 C H F)][BArF ] (VI and VII) have been used to study the catalytic dehydrogenation of N- 6 5 4 substituted amine-boranes (H B∙NRH , Chapter 2 and 3), B,N-substituted amine-boranes 3 2 (PhH B∙NRMe , Chapter 4) and cyclic amine-boranes (Chapter 5). 2 2 The stoichiometric (1 equivalent) reactions of metal-complexes (II, III and VI) with amine- boranes result in the formation of metal∙∙∙amine-borane complexes which have either been isolated in the solid state or characterised in situ in the solution state by NMR spectroscopy and ESI-MS. Further addition of amine-boranes to these metal∙∙∙amine- borane complexes results in the dehydrogenation/dehydrocoupling of the amine-boranes. Direct intermediates involved in the catalytic dehydrogenation/dehydrocoupling processes have either been isolated or characterised in situ to gain insight into the dehydrogenation mechanism. Dehydrogenation mechanisms have been postulated based upon the observed intermediates, kinetic studies, and DFT calculations performed by Prof. Stuart A Macgregor (Heriot-Watt University, Edinburgh). Overall, a detailed investigation into the coordination chemistry of metal∙∙∙amine-borane complexes and their subsequent dehydrogenation is reported. vii Contents Declaration i Acknowledgements ii Abbreviations v Abstract vii Contents viii 1 Introduction ............................................................................................................................. 1 1.1 Organotransition metal complexes ...................................................................................... 1 1.2 Phosphine-ligands ............................................................................................................... 1 1.2.1 Introduction .................................................................................................................. 1 1.2.2 Chelating phosphines .................................................................................................. 3 1.3 Weakly coordinating anions ................................................................................................. 5 1.4 Homogeneous organometallic catalysis .............................................................................. 7 1.5 Homogeneous vs Heterogeneous catalysis ........................................................................ 8 1.6 Rhodium: General properties and applications ................................................................. 10 1.7 Iridium: General properties and applications ..................................................................... 11 1.8 Comparison of properties of rhodium and iridium (complexes) ......................................... 13 1.8.1 [RhCl(PPh ) ] vs [IrCl(PPh ) ] towards hydrogenation .................................................... 14 3 3 3 3 1.8.2 [Rh(CO) (I) ]− vs [Ir(CO) (I) ]− towards carbonylation of methanol ................................. 15 2 2 2 2 1.9 Sigma-complexes .............................................................................................................. 17 1.9.1 Dihydrogen sigma-complexes .................................................................................... 19 1.9.2 Borane sigma-complexes .......................................................................................... 22 1.10 Amine-boranes .................................................................................................................. 24 1.11 General pathway for the dehydrocoupling of amine-boranes............................................ 25 1.12 Metal catalysed dehydrocoupling of amine-boranes ......................................................... 27 1.13 Heterogenous catalysts for the dehydrocoupling of amine-boranes ................................. 27 1.14 Homogeneous catalysts for the dehydrocoupling of amine-boranes ................................ 29 1.14.1 Outer sphere mechanism based on metal-ligand cooperativity ................................. 29 1.14.2 Inner sphere mechanism for the dehydrocoupling of amine-boranes ....................... 34 viii