Surface-assisted transfer hydrogenation catalysis on a γ-Al2O3-supported Ir dimer

Abstract

A novel oxide-supported Ir dimer, which was found to be active for transfer hydrogenation of aromatic ketones, was prepared on a γ-Al₂O₃ surface from an Ir dimer complex [Ir₂{η⁵-C₅(CH₃)₅}₂(μ-CH₂)₂] (Ir₂) with an IrIr bond. Detailed characterization of the γ-Al₂O₃-supported Ir dimer (Ir₂/γ-Al₂O₃) revealed that the structure of Ir₂ consisted of an Ir dimer with an Ir–Ir bond attached to the γ-Al₂O₃ surface by two bridged Ir–(OAl)₂–Ir bonds. The supported Ir₂/γ-Al₂O₃ dimer with bridged Ir–(OAl)₂–Ir bonds acted as an efficient catalyst for transfer hydrogenation (turnover number of acetophenone = 699 (24 h)), while homogeneous Ir₂, SiO₂- and MgO-supported Ir₂ were much less active. A structural transformation at the interface of the Ir dimer and the γ-Al₂O₃ surface was suggested to assist the transfer hydrogenation catalysis via the formation of an Ir₂–H₂ species on the γ-Al₂O₃ surface (Ir₂–H₂/γ-Al₂O₃) as a key intermediate in the transfer hydrogenation. The present study deepened the understanding of the role and dynamic behaviour of the oxide surface in the hydrogen transfer catalysis on the supported Ir dimer.