Homogeneous Water Oxidation by Half-Sandwich Iridium(III) N-Heterocyclic Carbene Complexes with Pendant Hydroxy and Amino Groups

Abstract

Herein, we report three Ir^III Cp* complexes with hydroxy- or amino-functionalized N-heterocyclic carbene (NHC) ligands that catalyze efficient water oxidation induced by addition of ceric ammonium nitrate (CAN). The pendant hydroxy or amino groups are very important for activity, and the complexes with heteroatom-functionalized NHC ligands show up to 15 times higher rates of oxygen evolution in CAN-induced water oxidation than a reference Ir^III Cp* complex without heteroatom functionalization. The formation of molecular high-valent Ir intermediates that are presumably involved in the rate-determining step for water oxidation is established by UV/Vis spectroscopy and ESI-MS under turnover conditions. The hydroxy groups on the NHC ligands, as well as chloride ligands on the iridium center are proposed to structurally stabilize the high-valent species, and thereby improve the catalytic activity. The Ir^III complex with a hydroxy-functionalized NHC shows the highest catalytic activity with a TON of 2500 obtained in 3 h and with >90 % yield relative to the amount of oxidant used.

Keywords: N-heterocyclic carbenes; homogeneous catalysis; iridium; water oxidation; water splitting.