A Ruthenium(III)-Oxyl Complex Bearing Strong Radical Character.

Abstract

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.