Time-resolved kinetic study of the electron-transfer reactions between ring-substituted cumyloxyl radicals and alkylferrocenes. Evidence for an inner-sphere mechanism

A time-resolved kinetic study of the reactions of ring-substituted cumyloxyl radicals (4-X-CumO(?): X = OMe, t-Bu, Me, Cl, CF(3)) with methylferrocenes (Me(n)Fc: n = 2, 8, 10) has been carried out in acetonitrile solution. Evidence for an electron transfer (ET) process has been obtained for all radicals and an increase in reactivity has been observed on decreasing the oxidation potential of the ferrocene donor and on going from electron-releasing to electron-withdrawing ring substituents. Computations predict the formation of strongly bound π-stacked 4-X-CumO(?)/DcMFc complexes, characterized by intracomplex π-π distances around 4 ?. These findings point toward a (nonbonded) inner-sphere ET mechanism for the reactions of the 4-X-CumO(?)/Me(n)Fc couples.