Chemical and photochemical water oxidation catalyzed by novel ruthenium complexes comprising a negatively charged NC^NHCO ligand

Incorporation of negatively charged moieties into complexes is a promising strategy to develop water oxidation catalysts with high stability and low over-potentials. We reported here the synthesis of a novel pincer-type ligand precursor, 1-(pyridin-2-ylmethyl)-3-methylcarboxylate-imidazolium (1). Deprotonation of C2-H of 1 resulted in formation of NC^NHCO, a negatively charged asymmetric N-heterocyclic carbene (NHC). Complexes [Ru(NC^NHCO)(terpy)]⁺ (2) and RuCl(NC^NHCO)(DMSO)(pic) (3) were prepared and capable of decomposing water chemically and photo-chemically. Under acidic conditions, 4-picoline/H₂O exchange in 3 was detected by UV-vis and ¹H NMR, forming the corresponding Ru-aqua complex. In a typical three-component light-induced water oxidation, a turnover number of 273 was attained employing 3, which demonstrated catalytic ability comparable to those state-of-the-art catalysts.