Isolation of a Ru(iv) side-on peroxo intermediate in the water oxidation reaction

The electrons that nature uses to reduce CO2 during photosynthesis come from water oxidation at the oxygen-evolving complex of photosystem II. Molecular catalysts have served as models to understand its mechanism, in particular the O–O bond-forming reaction, which is still not fully understood. Here we report a Ru(iv) side-on peroxo complex that serves as a ‘missing link’ for the species that form after the rate-determining O–O bond-forming step. The Ru(iv) side-on peroxo complex (η2-1–OO) is generated from the isolated Ru(iv) oxo complex (1=O) in the presence of an excess of oxidant. The oxidation (iv) and spin state (singlet) of η2-1–OO were determined by a combination of experimental and theoretical studies. 18O- and 2H-labelling studies evidence the direct evolution of O2 through the nucleophilic attack of a H2O molecule on the highly electrophilic metal–oxo species via the formation of η2-1–OO. These studies demonstrate water nucleophilic attack as a viable mechanism for O–O bond formation, as previously proposed based on indirect evidence. Obtaining mechanistic data after the rate-determining step of a chemical reaction is difficult but essential for its understanding. Now, a Ru(iv) side-on peroxo complex has been isolated following the rate-determining step of the water oxidation reaction (O–O bond formation) carried out using a Ru-based molecular catalyst.