Electrochemically Driven Water Oxidation by a Highly Active Ruthenium-Based Catalyst

The highly active ruthenium-based water oxidation catalyst [Ru^X (mcbp)(OH_n )(py)₂ ] [mcbp^2- =2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine; n=2, 1, and 0 for X=II, III, and IV, respectively], can be generated in a mixture of Ru^III and Ru^IV states from either [Ru^II (mcbp)(py)₂ ] or [Ru^III (Hmcbp)(py)₂ ]²⁺ precursors. The precursor complexes are isolated and characterized by single-crystal X-ray analysis, NMR, UV/Vis, EPR, and FTIR spectroscopy, ESI-HRMS, and elemental analysis, and their redox properties are studied in detail by electrochemical and spectroscopic methods. Unlike the parent catalyst [Ru(tda) (py)₂ ] (tda^2- =[2,2':6',2''-terpyridine]-6,6''-dicarboxylate), for which full transformation into the catalytically active species [Ru^IV (tda)(O)(py)₂ ] could not be carried out, stoichiometric generation of the catalytically active Ru-aqua complex [Ru^X (mcbp)(OH_n )(py)₂ ] from the Ru^II precursor was achieved under mild conditions (pH 7.0) and short reaction times. The redox properties of the catalyst were studied and its activity for electrocatalytic water oxidation was evaluated, reaching a maximum turnover frequency (TOF_max ) of around 40 000 s^-1 at pH 9.0 (from foot-of-the-wave analysis), which is comparable to the activity of the state-of-the-art catalyst [Ru^IV (tda)(O)(py)₂ ].