The effect of triphenylmethyl cations on the electroreduction of O2 in nitrobenzene or acetonitrile

The electroreduction of O2 in two aprotic solvents (nitrobenzene and acetonitrile) was examined in the absence and in the presence of the very strong Lewis acid, triphenylmethyl cation, φ3C+ (φ=phenyl). The addition of φ3C+ causes the electroreduction of O2 to change from the one-electron reduction to O2− into the two-electron reduction to φ3CO2Cφ3 at a potential that is 1 V more positive than that where O2 is reduced to O2−. This large positive shift in potential is used to estimate a value of the equilibrium constant for the association between O22− and two φ3C+ cations (K=3×1026 M−2). The mechanism of the reduction of O2 to φ3CO2Cφ3 can be regarded as an inner-sphere electron-transfer reaction between φ3C and O2 or, equivalently, as a radical addition to O2.