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

Proton‐coupled electron‐transfer oxidation of a RuII−OH2 complex, having an N‐heterocyclic carbene ligand, gives a RuIII−O. species, which has an electronically equivalent structure of the RuIV=O species, in an acidic aqueous solution. The RuIII−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 RuIII−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 RuIII−O. complex bearing a strong radical character as the active species.