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.
|Number of pages||7|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1992|
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