Computational studies into the rhodium catalysed dehydrocoupling of amine- and phosphine-boranes

Abstract

This thesis details the use of density functional theory to study the mechanisms of dehydrocoupling of amine- and phosphine-boranes using a variety of Rh catalysts in collaboration with the group of Professor Andrew Weller at the University of Oxford. The dehydrocoupling of phosphine-boranes with pre-catalysts [Rh(Ph2P(CH2)2(PPh2)(η6-C6H5F)]+ (Chapter 3) and [Rh(Me)(CH2Cl2)(PMe3)(η5-Cp*)]+ (Chapter 4) were studied. For [Rh(Ph2P(CH2)2(PPh2)(η6-C6H5F)] + the computed mechanism involves facile P-H activation, B-H activation and a rate-limiting B-P coupling process. A functional and basis set study was conducted to benchmark against experimental activation parameters. Furthermore, the differences in reaction of pre-catalyst [Rh(Me)(CH2Cl2)(PMe3)(η5-Cp*)]+ with H3B-PPh2H and H3BPtBu2H, which yield [Rh(PMe3)(η5-Cp*)(PPh2BH3)]+ and [Rh(H)(η5-Cp*)(PtBu2BH2PMe3)]+ respectively were rationalised computationally. The dehydropolymerisation of monomethylamine-borane H3B-NMeH2 using a range of alkylXantphos Rh catalysts: neutral [Rh(mer-κ3-P,O,P-Xantphos-iPr)H], and [Rh(mer-κ3-P,O,PXantphos-tBu)H], and cationic [Rh(mer-κ3-P,O,P-Xantphos-iPr)(H)2(η1-H3B-NMeH2)]+, and [Rh(mer-κ3-P,O,P-Xantphos-tBu)H2]+ is discussed in Chapter 5. The neutral catalysts were found to proceed via different outer-sphere dehydrogenation pathways. [Rh(mer-κ3-P,O,P-XantphosiPr)H] forms free H2B=NMeH and a tri-hydride intermediate while [Rh(mer-κ3-P,O,P-XantphostBu)H] proceeds through a novel process to form free H2B=NMeH, H2 and regenerate the catalyst in one step. A head-to-tail propagation mechanism would then form polyamino-borane [H2BNMeH]n. Dehydropolymerisation mechanisms have also been explored for the cationic catalysts with [Rh(mer-κ3-P,O,P-Xantphos-tBu)H2]+ being postulated to follow a dehydrogenation mechanism involving the formation of boronium cations [(NMeH2)2BH2]+. In Chapter 6 a study of the electronic structure and bonding of boron-containing Rh-dimers [{Rh(dipp)}2(H)(BH2NH2)]+, [{RhH}2(μ-BNMe2)(μ-dpcm)2(μ-H)]+, and [{Rh(κ3-P,O,P-XantphosiPr)}2B]+ was conducted. All of these species are formed during amine-borane dehydrocoupling catalysis.