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

Abstract

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(η²-H₃B·NMe₃)][BAr^F₄] (M = Rh, Ir; R = Cy for experiment; R = Me, Cy for computation; Ar^F = 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)₂(η²-H₃B·NMe₃)][BAr^F₄]. 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.