Enabling light-driven water oxidation via a low-energy RuIV=O intermediate.

The discovery of catalysts capable of driving water oxidation at relatively low overpotential is a key challenge for efficient photoinduced water oxidation. The mononuclear Ru(II) polypyridyl complex (1) [Ru(NPM)(H2O)(pic)2](2+) (NPM = 4-tert-butyl-2,6-di-(1',8'-naphthyrid-2'-yl)-pyridine, pic = 4-picoline) has been examined as a catalyst for visible-light-driven water oxidation in a three-component homogeneous system containing [Ru(bpy)3](2+) as a photosensitizer, persulfate as a sacrificial electron acceptor and catalyst 1. In contrast to the well-established water oxidation mechanism via the nucleophilic attack of a water molecule on the high-energy [Ru(V)=O](3+) species, a lower-energy "direct pathway" for O-O bond formation via a [Ru(IV)=O](2+) intermediate was proposed for the first time for the catalyst 1 (Polyansky et al., J. Am. Chem. Soc., 2011, 133, 14649). In this report we successfully demonstrate that this unique proton-coupled low-energy pathway actually takes place with the use of a mild oxidant such as the photogenerated [Ru(bpy)3](3+) (1.26 V vs. NHE) to drive water oxidation. The overall quantum yield of 9%, TOF of 0.12 s(-1) and TON of 103 (limited solely by a drop in pH) were found for photochemical water oxidation with 1 using [Ru(bpy)3](2+) as a photosensitizer and [S2O8](2-) as a sacrificial electron acceptor. These values render catalyst 1 as one of the most active mononuclear ruthenium-based catalysts for light-driven water oxidation in a homogeneous system. The utilization of a pH-dependent pathway for water oxidation is a new and promising direction as a low-energy pathway. Furthermore, the detailed analysis of individual photochemical steps leading to O2 evolution provides benchmarks for future mechanistic studies of photo-induced water oxidation catalysis.