Multielectron oxidation of anthracenes with a one-electron oxidant via water-accelerated electron-transfer disproportionation of the radical cations as the rate-determining step

The six-electron oxidation of anthracene and the four-electron oxidation of 9-alkylanthracene occur with [RU(bpy)(3)](3+) (bpy = 2,2-bipyridine) in acetonitrile (MeCN) containing H2O to yield anthraquinone and 10-yield-10-hydroxy-9(10H)anthracenone, respectively. The direct detection of radical cations of anthracene and its derivatives formed in the multielectron oxidation with [Ru(bpy)(3)](3+) and the extensive kinetic analysis are performed with the use of a stopped-flow technique. Both the rates of decay of anthracene radical cations and the formation of [Ru(bpy)(3)](2+) obey the second-order kinetics. The kinetic deuterium isotope effects and the dependence of the rates on the concentrations of [RU(bpy)(3)](3+), anthracenes, and H2O have revealed that the six-electron oxidation of anthracene and the four-electron oxidation of alkylanthracene proceed via the rate-determining electron-transfer disproportionation of radical cations of anthracene and alkylanthracene, which is accelerated by H2O due to the complex formation between the corresponding dications and H2O. The electron-transfer disproportionation of anthracene radical cations is followed by the facile nucleophilic attack of H2O on the resulting dication leading to six-electron oxidized product, i.e., anthraquinone associated with rapid electron transfer from [Ru(bpy)(3)](3+) and anthracene radical cation in the presence of more than 6 equiv of [Ru(bpy)(3)](3+) and less than 1 equiv of [Ru(bpy)(3)](3+), respectively. The reorganization energy for the self-exchange between 9,10-dimethylanthracene and the radical cation in MeCN is also determined by analyzing line width variations of the ESR spectra at different concentrations of 9,10-dimethylanthracene. The reorganization energy is used to evaluate the rate constant of electron-transfer disproportionation of 9,10-dimethylanthracene radical cations in light of the Marcus theory of electron transfer, which agrees with the experimental value determined from the second-order decay of the radical cations.