Mechanism and Catalytic Impact of Ir–Ta Heterobimetallic and Ir–P Transition Metal/Main Group Interactions on Alkene Hydrogenation

Abstract

Transition metal heterobimetallic catalysts provide an alternative to classic transition metal ligand catalyst design. The resurgence in popularity of heterobimetallic complexes prompted our use of density functional theory to examine the mechanism and reactivity of alkene hydrogenation catalyzed by the transition metal heterobimetallic complex Cp2Ta(CH2)2Ir(CO)(PPh3) and the transition metal/main group complex Ph2P(CH2)2Ir(CO)(PPh3). Calculations indicate that the Ir–Ta and Ir–P catalysts operate by different mechanisms. For the Ir–Ta complex, initial H2 oxidative addition to the Ir metal center followed by reductive elimination of an Ir–H and μ-CH2 bridge transforms the starting heterobimetallic complex into an active Ir–H catalyst. This catalyst precursor transformation occurs because the cationic Cp2Ta group provides a low activation barrier for reductive elimination. This transformation does not occur for the Ir–P catalyst because the reductive elimination activation barrier is significantly higher in energy. The active heterobimetallic Ir–H likely catalyzes multiple turnovers of alkene hydrogenation before reforming the original heterobimetallic Ir–Ta complex. The Ir–H catalytic cycle involves a series of classic organometallic reaction steps: alkene migratory insertion, H2 oxidative addition, and reductive elimination. In the Ir–P mechanism, the Ph2P(CH2)2 group remains as a spectator ligand throughout the active catalytic cycle. The Ir–P catalytic cycle involves H2 oxidative addition, phosphine ligand dissociation, ethylene migratory insertion, and reductive elimination.