A combined experimental and computational study of fluxional processes in sigma amine-borane complexes of rhodium and iridium

Andrés G. Algarra, Laura J. Sewell, Heather C. Johnson, Stuart A. Macgregor*, Andrew S. Weller

*Corresponding author for this work

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A combined experimental and computational study on the fluxional processes involving the M-H and B-H positions in the sigma amine-borane complexes [M(PR3)2(H)22-H 3B·NMe3)][BArF 4] (M = Rh, Ir; R = Cy for experiment; R = Me, Cy for computation; ArF = 3,5-(CF3)2C6H3) is reported. The processes studied are: B-H bridging/terminal exchange; reaction with exogenous D2 leading to exchange at M-H; and intramolecular M-H/B-H exchange. Experimentally it was found that B-H bridging/terminal exchange is most accessible and slightly favoured for Rh; D2/M-H exchange occurs at qualitatively similar rates for both M = Rh and Ir, while M-H/B-H exchange is the slowest overall, with the Ir congener having a lower barrier than Rh. Evidence for the isotopic perturbation of equilibrium is also reported for the BH/BD isotopologues of [Ir(PCy3)2(H)22-H3B·NMe3)][BArF 4]. DFT calculations using model complexes (R = Me) qualitatively reproduce the relative rates of the various exchange processes for both M = Rh and Ir, i.e. barriers for B-H bridging/terminal exchange are less than those for M-H/H2 exchange, which in turn are less than those for M-H/B-H exchange. Which metal promotes these processes more effectively depends upon the nature of the rate-limiting transition state, which can change between Rh and Ir. Computational analysis of the full experimental system (R = Cy) reveals similar overall trends in terms of the relative ease of the various exchange processes. However, there are differences in the details, and these are discussed. This journal is

Original languageEnglish
Pages (from-to)11118-11128
Number of pages11
JournalDalton Transactions
Issue number29
Publication statusPublished - 7 Aug 2014

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