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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry