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Title: POP-type rhodium complexes in amine borane dehydropolymerisation
Author: Adams, Gemma
ISNI:       0000 0004 7653 5554
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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This thesis explores the catalytic dehydropolymerisation of N-methylamine borane, H3B·NMeH2, to form N-methylpolyaminoborane, (H2BNMeH)n, using POP-type rhodium complexes based upon the {Rh(Xantphos-iPr)}+ and {Rh(DPEphos)}+ fragments. A comparison of these metal-ligand frameworks provides a structure/activity relationship in order to gain mechanistic insight into the H3B·NMeH2 dehydropolymerisation process. Chapter 2 describes the synthesis of a suitable {Rh(Xantphos-iPr)}+ precatalyst for H3B·NMeH2 dehydropolymerisation. Its reactivity with H3B·NMeH2 is examined by NMR spectroscopy, and independent synthetic routes to the complexes observed are described. From this, a dimetalloborylene is isolated and crystallographically characterised. The kinetics of H3B·NMeH2 dehydrogenation and (H2BNMeH)n growth using the {Rh(Xantphos-iPr)}+ fragment are reported in Chapter 3, which combined with the speciation studies in Chapter 2, allows for a mechanistic proposal for dehydropolymerisation. The formation and subsequent deprotonation of a boronium cation is postulated to lead to the monomer, H2B=NMeH, which forms (H2BNMeH)n via an on-metal end-chain propagation mechanism. In Chapter 4 the focus shifts to synthesise a suitable {Rh(DPEphos)}+ precatalyst for H3B·NMeH2 dehydropolymerisation. Here, speciation studies of the {Rh(DPEphos)}+ fragment with H3B·NMeH2 enables the observation of bimetallic complexes, one of which is considered an amidodiboryl complex. An in-depth dehydropolymerisation kinetic study using the {Rh(DPEphos)}+ system is undertaken in Chapter 5, and strong dependence of (H2BNMeH)n molecular weight on H2 pressure and catalyst loading indicates that an on-metal coordination/dehydrogenation/insertion mechanism operates, contrasting that proposed for the {Rh(Xantphos-iPr)}+ system. The bimetallic amidodiboryl complex described in Chapter 4 and NMeH2-derived {Rh(DPEphos)}+ species are also tested in catalysis, and appear close in identity to the true active catalytic species, due to the lack of induction period observed prior to H2 evolution.
Supervisor: Weller, Andrew Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Inorganic Chemistry ; Chemistry ; Organometallic Chemistry