Catalyst Activation by Loss of Cyclopentadienyl Ligands in Hydrogen Transfer Catalysis with Cp*IrIII Complexes

The activity of the two related complexes [Cp*Ir(IMe)(2)X]BF4 (X = Cl (1), H (2)) in transfer hydrogenation from isopropyl alcohol to acetophenone was investigated. The results suggest that the commonly accepted mono hydride mechanism for transfer hydrogenation mediated by cyclopentadienyl iridium species does not apply to chloride 1. We have found evidence that, although the two monodentate NHC ligands are retained in the coordination sphere, the Cp* ligand is completely released under mild conditions in a precatalytic activation step. Synthesis of modified versions of the initial precatalyst 1 with different cyclopentadienyl and NHC ligands demonstrated that increasing the steric pressure around the iridium center facilitates precatalyst activation and thus enhances the catalytic performance. Study of five new iridium(III) complexes bearing mono- or diphosphines helped us monitor Cp* ligand loss under mild conditions. An unusual P-C bond cleavage was also noted in a 1,2-bis(dimethylphosphino)methane (dmpm) ligand. On the basis of these findings, a novel catalyst activation mechanism is proposed for [(eta(5)-C5R5)Ir] transfer hydrogenation based on the lability of the cyclopentadienyl ligand.