Surface Organometallic Chemistry: Hydrogenation of Ethylene with Os₃(CO)₁₀(μ-H)(μ-OSi⋖) and Os₃(CO)₁₀(μ-H)(μ-OPh). Evidence for Cluster Catalysis

The silica-supported cluster Os₃(CO)₁₀(μ-H)(μ-OSi⋖) (1) efficiently catalyzes the hydrogenation of ethylene under mild reaction conditions; the reaction is zero order in ethylene and first order in hydrogen. The reaction kinetics as well as volumetric and IR studies of the interaction of 1 with C₂H₄, H₂, and CO indicate a mechanism involving the intact triosmium framework in all the elementary steps conforming the catalytic cycle. The facile 3e^-⇄1e^- interconversion of surface-oxygen ligands provides the appropriate energy balance for cluster catalysis without fragmentation. The reactivity of the molecular analogue Os₃(CO)₁₀(μ-H)(μ-OPh) (2) toward C₂H₄ and H₂ has also been studied by IR and NMR spectroscopy, and the data are in agreement with the proposed hydrogenation cycle for 1. However, in contrast with 1, 2 is quickly transformed in solution under catalytic conditions with loss of phenol and formation of H₂Os₃(CO)₁₀ and H₄Os₄(CO)₁₂. This difference of behavior indicates the stabilizing influence of the silica support.