Bimetallic Iridium–Carbene Complexes with Mesoionic Triazolylidene Ligands for Water Oxidation Catalysis


  • Dedicated to Professor Ernesto Carmona on the occasion of his 65th birthday

  • In admiration of his ground-breaking contributions to bond activation and organometallic chemistry in general


Two new diiridium–triazolylidene complexes were prepared as bimetallic analogues of established mononuclear water oxidation catalysts. Both complexes are efficient catalyst precursors in the presence of cerium ammonium nitrate (CAN) as sacrificial oxidant. Up to 20000:1 ratios of CAN/complex, the turnover limitation is the availability of CAN and not the catalyst stability. The water oxidation activity of the bimetallic complexes is not better than the monometallic species at 0.6 mM catalyst concentration. Under dilute conditions (0.03 mM), the bimetallic complexes double their activity, whereas the monometallic complexes show an opposite trend and display markedly reduced rates, thereby suggesting a benefit of the close proximity of two metal centers in this low concentration regime. The high dependence of catalyst activity on reaction conditions indicates that caution is required when catalysts are compared by their turnover frequencies only.