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

Abstract

Proton-coupled electron-transfer oxidation of a Ru^II -OH₂ 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.