The effect of triphenylmethyl cations on the electroreduction of O-2 in nitrobenzene or acetonitrile

The electroreduction of O-2 in two aprotic solvents (nitrobenzene and aceto nitrile) was examined in the absence and in the presence of the very strong Lewis acid, triphenylmethyl cation, phi(3)C(+) (phi = phenyl). The additio n of phi(3)C(+) causes the electroreduction of O-2 to change from the one-e lectron reduction to O-2(-.) into the two-electron reduction to phi(3)CO(2) C phi(3) at a potential that is 1 V more positive than that where O-2 is re duced to O-2(-.). This large positive shift in potential is used to estimat e a value of the equilibrium constant for the association between O-2(2-) a nd two phi(3)C(+) cations (K = 3 x 10(26) M-2). The mechanism of the reduct ion of O-2 to phi(3)CO(2)C phi(3) can be regarded as an inner-sphere electr on-transfer reaction between phi(3)C(.) and O-2 or, equivalently, as a radi cal addition to O-2. (C) 2000 Elsevier Science S.A. All rights reserved.