A kinetic study of the electron-transfer reaction of the phthalimide-N-oxyl radical (PINO) with ferrocenes

A kinetic study of the one-electron oxidation of a series of ferrocenes (FcX: X = H, CO2Et, CONH2, CH2CN, CH2OH, Et, and Me2) by PINO generated in CH3CN by reaction of N-hydroxyphthalimide (NHPI) with the cumyloxyl radical produced by 355 nm laser flash photolysis of dicumyl peroxide has been carried out. Ferrocenium cations were formed, and the reaction rate was determined by following the decay of PINO radical at 380 nm as a function of the FcX concentration. Rate constants were very sensitive to the oxidation potential of the substrates and exhibited a good fit with the Marcus equation, from which a lambda value of 38.3 kcal mol(-1) was calculated for the reorganization energy required in the PINO/ferrocenes electron-transfer process. Knowing the ferrocene/ferrocenium self-exchange reorganization energy it was possible to calculate a value of 49.1 kcal mol(-1) for the PINO/PINO- self-exchange reaction in CH3CN. Moreover, from the Marcus cross relation and the self-exchange rates of ferrocene and dimethylferrocene, the intrinsic reactivity of PINO in electron-transfer reactions has been calculated as 7.6 x 10(2) M(-1) s(-1). The implications of these values and the comparison with the electron-transfer self-exchange reorganization energies of peroxyl radicals are briefly discussed.