Theoretical study on the mechanism of H₂ activation mediated by two transition metal thiolate complexes: Homolytic for Ir, heterolytic for Rh

The molecular mechanism of H₂ activation by two transition metal thiolate complexes [Cp*M(PMe₃)(SDmp)](BAr^F₄) (M = Ir, Rh) (Ohki, Y; Sakamoto, M; Tatsumi, K. J. Am. Chem. Soc., 2008, 130, 11610–11611) has been investigated using density functional theory calculations. According to our calculations, the reaction of the iridium thiolate complex with H₂ is likely to proceed through the following steps: (1) the oxidative addition of H₂ to the iridium center to generate a dihydride intermediate; (2) the reductive elimination of one Ir-bound hydrogen to produce the hydride thiol product. For the rhodium thiolate complex, its reaction with H₂ is to form the dihydrogen intermediate first, and then the H–H bond is heterolytically cleaved at the Rh–S bond via a four-center transition state to yield the hydride thiol product. The rate-determining step is the oxidative addition step (with a barrier of 18.0 kcal/mol in the solvent) for the iridium complex, and the formation of the dihydrogen complex (with a barrier of 13.9 kcal/mol in the solvent) for the rhodium complex. The calculated free energy profiles for both metal thiolate complexes can reasonably account for the observed reversible H₂ activation by two metal thiolate complexes under mild conditions.