A Ruthenium(III)–Oxyl Complex Bearing Strong Radical Character

Proton-coupled electron-transfer oxidation of a Ru^II−OH₂complex, having an N-heterocyclic carbene ligand, gives a Ru^III−O^.species, which has an electronically equivalent structure of the Ru^IV=O species, in an acidic aqueous solution. The Ru^III−O^.complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru−O bond showed a lower-energy Raman scattering at 732 cm⁻¹and the Ru−O bond length was estimated to be 1.77(1) Å. The Ru^III−O^.complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C−H abstraction from the formyl group by the Ru^III−O^.complex bearing a strong radical character as the active species.