Mechanistic insight into catalytic oxidations of organic compounds by ruthenium(iv)-oxo complexes with pyridylamine ligands

A series of Ru(iv)-oxo complexes (4-6) were synthesized from the corresponding Ru(ii)-aqua complexes (1-3) and fully characterized by ¹H NMR and resonance Raman spectroscopies, and ESI-MS spectrometry. Based on the diamagnetic character confirmed by the ¹H NMR spectroscopy in D₂O, the spin states of 5 and 6 were determined to be S = 0 in the d⁴ configuration, in sharp contrast to that of 4 being in the S = 1 spin state. The aqua-complexes 1-3 catalyzed oxidation of alcohols and olefins using (NH₄)₂[Ce^IV(NO ₃)₆] (CAN) as an electron-transfer oxidant in acidic aqueous solutions. Comparison of the reactivity of electrochemically generated oxo-complexes (4-6) was made in the light of kinetic analyses for oxidation of 1-propanol and a water-soluble ethylbenzene derivative. The oxo complexes (4-6) exhibited no significant difference in the reactivity for the oxidation reactions, judging from the similar catalytic rates and the activation parameters. The slight difference observed in the reaction rates can be accounted for by the difference in the reduction potentials of the oxo-complexes, but the spin states of the oxo-complexes have hardly affected the reactivity. The activation parameters and the kinetic isotope effects (KIE) observed for the oxidation reactions of methanol indicate that the oxidation reactions of alcohols with the Ru^IVO complexes proceed via a concerted proton-coupled electron transfer mechanism.