Bromide-Mediated Low-Energy Ru^IV═O Pathway of Stable Water Oxidation

Mimicking natural photosynthesis to split water into oxygen and hydrogen represents a promising pathway for transitioning from fossil fuels to a sustainable energy future. It is extremely challenging to duplicate the efficient and elegant oxygen evolution complex of photosynthesis II of oxidizing water to O₂ being regarded as the bottleneck of water splitting. Cutting-edge artificial molecular water oxidation catalysts (WOCs) with low overpotentials are highly desirable for efficient water oxidation. Here we report the design of a molecular water oxidation catalyst (WOC) RuN5 (Ru(N5)(pic)₂; N5 = 4-tert-butyl-2,6-di(1',8'-naphthyrid-2'-yl)pyridine, pic = 4-picoline). Following electrochemical activation and bromide mediation, RuN5 achieves a high turnover frequency of 2604 s^-1 with a low overpotential of 363 mV at pH 7. The catalyst is highly stable, maintaining a steady current density of 1.8 mA cm^-2 over 200 h. Mechanistic studies reveal that activation and bromide mediation facilitate O-O bond formation via a ligand-oxidized [Ru^IV═O]²⁺ intermediate through a low energy pathway, distinct from the classical [Ru^V(O)]³⁺ route. This work opens a new avenue for developing efficient molecular WOCs and advancing artificial photosynthesis.