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

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)(2)(eta(2)-H3B center dot NMe3)][BAr4F] (M = Rh, Ir; R = Cy for experiment; R = Me, Cy for computation; Ar-F = 3,5-(CF3)(2)C6H3) is reported. The processes studied are: B-H bridging/terminal exchange; reaction with exogenous D-2 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; D-2/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)(2)(eta(2)-H3B center dot NMe3)][BAr4F]. 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/H-2 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.