Enabling light-driven water oxidation via a low-energy Ru^IVO 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)(H ₂O)(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 ^VO]³⁺ species, a lower-energy "direct pathway" for O-O bond formation via a [Ru^IVO]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.