Water oxidation utilizing a ruthenium complex featuring a phenolic moiety inspired by the oxygen-evolving centre (OEC) of photosystem II

Abstract

In this manuscript, we present the synthesis of a ruthenium complex [1]²⁺ bearing a phenolic moiety, of which design draws inspiration from the oxygen-evolving centre (OEC) of photosystem II. In comparison to its analogue, [Ru(OH₂)(bpy)(trpy)]²⁺ ([2]²⁺, where bpy = 2,2′-bipyridine and trpy = 2,2′:6′,2′′-terpyridine), which lacks the phenolic moiety, [1]²⁺ exhibits a 50 mV reduction in onset-overpotential at pH 10, an augmented turnover number (TON = 1900) and frequency (TOF = 0.54 s⁻¹) for electrochemical water oxidation at pH 9, clearly indicating the promoting influence of the phenolic moiety on catalytic activity. Electrochemical and electron paramagnetic resonance (EPR) experiments have elucidated the formation of a Ru^IVO species with either a phenol cation radical or a phenoxyl radical under oxidative conditions. Theoretical investigations provided robust support for the water oxidation mechanism, predicated upon a nucleophilic attack by OH⁻ onto Ru^IVO, concomitant with electron transfer to the phenol cation radical. This electron transfer effectively lowers the activation energy associated with the nucleophilic attack of OH⁻ onto Ru^IVO.