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

Herein is described a highly active ruthenium-based water oxidation catalyst [RuX(mcbp)(OHn)(py)2] (5, mcbp2− = 2,6-bis(1- methyl-4-(carboxylate)benzimidazol-2-yl)pyridine; n = 2, 1, and 0 for X = II, III, and IV, respectively), which can be generated in a mixture of RuIII/RuIV states from either [RuII(mcbp)(py)2] (4II) or [RuIII(Hmcbp)(py)2]2+ (4III). Complexes 4II and 4III were isolated and characterized by single crystal X-ray analysis, NMR, UV-vis, FT-IR, ESI-HRMS, EPR, and elemental analysis, and their redox properties were studied in detail by electrochemical and spectroscopic methods. Unlike for the parent catalyst [Ru(tda)(py)2] (1, tda2− = [2,2′:6′,2′′- terpyridine]-6,6′′-dicarboxylate), for which full transformation to the catalytically active species [RuIV(tda)(O)(py)2] (2) could not be carried out — stoichiometric generation of the catalytically active Ru-aqua complex 5 from 4II was achieved under mild conditions (pH 7.0) and short reaction times. The redox properties of the catalys were studied and its activity for electrocatalytic water oxidation was evaluated, reaching TOFmax ≈ 40 000 s−1 at pH 9.0 (from the foot-of- the-wave analysis, FOWA), which is comparable to the activity of the state-of-the-art catalyst 2.