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

A novel oxide-supported Ir dimer, which was found to be active for transfer hydrogenation of aromatic ketones, was prepared on a gamma-Al2O3 surface from an Ir dimer complex [Ir-2{eta(5)-C-5(CH3)(5)}(2)(mu-CH2)(2)] (Ir-2) with an Ir=Ir bond. Detailed characterization of the gamma-Al2O3-supported Ir dimer (Ir-2/gamma-Al2O3) revealed that the structure of Ir-2 consisted of an Ir dimer with an Ir-Ir bond attached to the gamma-Al2O3 surface by two bridged Ir-(OAl)(2)-Ir bonds. The supported Ir-2/gamma-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 Ir-2, SiO2- and MgO-supported Ir-2 were much less active. A structural transformation at the interface of the Ir dimer and the gamma-Al2O3 surface was suggested to assist the transfer hydrogenation catalysis via the formation of an Ir-2-H-2 species on the gamma-Al2O3 surface (Ir-2-H-2/gamma-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.