Proton-coupled electron-transfer oxidation of a RuII−OH2complex, 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−1and 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.
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