Synthesis of Ruthenium Complexes with a Nonspectator Si,O,P-Chelate Ligand: Interconversion between a Hydrido(η2-silane) Complex and a Silyl Complex Leading to Catalytic Alkene Hydrogenation

A ruthenium complex bearing a new phosphine(η2-silane) chelate ligand connected by a xanthene backbone, Ru{κ4(Si,H,O,P)-tBu2xantSiP(H)}(H)Cl(PPh3) (2), was synthesized by a ligand substitution reaction of Ru(H)Cl(PPh3)3 with 2,7-di-tert-butyl-4-dimethylsilyl-5-diphenylphosphino-9,9-dimethylxanthene (1, tBu2xantSiP(H)). Dehydrogenation reaction of 2 with styrene, a hydrogen acceptor, gave a 16-electron phosphine(silyl) complex Ru{κ3(Si,O,P)-tBu2xantSiP}Cl(PPh3) (3) together with ethylbenzene. Complex 2 was reproduced quantitatively by exposure of 3 to H2 (1 atm) at room temperature. Thus, hydrido(η2-silane) complex 2 and silyl complex 3 are interconvertible through alkene hydrogenation (from 2 to 3) and dihydrogen addition to the Ru–Si bond (from 3 to 2) in which tBu2xantSiP functions as a nonspectator ligand by reversibly releasing and accommodating a hydrogen atom. Complex 2 was also found to catalyze hydrogenation of alkenes via this interconversion.