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

Proton‐coupled electron‐transfer oxidation of a Ru II −OH 2 complex, 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 −1 and 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.