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 (<xref rid="Sec17" ref-type="sec">η²-1^IV-OO</xref>) is generated from the isolated Ru(IV) oxo complex (<xref rid="Sec14" ref-type="sec">1^IV=O</xref>) in the presence of an excess of oxidant. The oxidation (IV) and spin state (singlet) of <xref rid="Sec17" ref-type="sec">η²-1^IV-OO</xref> 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 <xref rid="Sec17" ref-type="sec">η²-1^IV-OO</xref>. These studies demonstrate water nucleophilic attack as a viable mechanism for O-O bond formation, as previously proposed based on indirect evidence.