Efficient Transfer-Dehydrogenation of Alkanes Catalyzed by Rhodium Trimethylphosphine Complexes under Dihydrogen Atmosphere

RhL₂Cl(CO) (1; L = PMe₃), a known catalyst for the photodehydrogenation of alkanes, is found to catalyze the highly efficient thermal (nonphotochemical) transfer-dehydrogenation of alkanes under high-pressure hydrogen atmosphere. The proposed mechanism involves addition of H₂, loss of CO, and transfer of H₂ to a sacrificial acceptor, thereby generating RhL₂Cl, the same catalytically active fragment formed by photolysis of 1. Consistent with this proposal, we report that photochemically inactive species, RhL₂ClL′ (L′ = P′Pr₃, PCy₃, PMe₃) and [RhL₂Cl]₂ are also thermochemical catalyst precursors. These species demonstrate much greater catalytic activity than RhL₂Cl(CO), particularly under moderate hydrogen pressures (ca. 500 times greater under 800 Torr of H₂ at 50 °C). The dependence of the turnover rates on hydrogen pressure is consistent with the proposed role of hydrogen, i.e., displacement of L′ from the four-coordinate complexes or fragmentation of H₂Rh₂L₄Cl₂, giving H₂RhL₂Cl, which is dehydrogenated by olefin to give RhL₂Cl. Selectivity studies provide further support for the characterization of the active fragment.