Electron-Transfer Properties of Short-Lived N-Oxyl Radicals. Kinetic Study of the Reactions of Benzotriazole-N-oxyl Radicals with Ferrocenes. Comparison with the Phthalimide-N-oxyl Radical

A kinetic study of the one-electron oxidation of a series of substituted ferrocenes (FcX: X = H, COCH3, CO2Et, CH2OH, Et, and Me2) by the benzotriazole-N-oxyl radical (BTNO) and of ferrocene (FcH) by a series of ring-substituted benzotriazole-N-oxyl radicals (Z-BTNO: Z = H, 6-CF3, 6-Cl, 6-Me, 6-MeO) has been carried out in CH3CN. N-Oxyl radicals were produced by hydrogen abstraction from 1-hydroxybenzotriazoles (Z-HBT) by the cumyloxyl radical produced after 355 nm laser flash photolysis of a solution of dicumyl peroxide in CH3CN. In both systems, the rate constants exhibited a satisfactory fit with the Marcus equation allowing us to determine self-exchange reorganization energy values for the BTNO/BTNO− couple, which resulted in good agreement: 34.7 kcal mol−1 from the oxidation of ferrocenes by BTNO and 30.5 kcal mol−1 from the oxidation of ferrocene by aryl-substituted Z-BTNO. From the average value of 32.6 kcal mol−1 it is possible to calculate a self-exchange rate for the BTNO/BTNO− couple of 7.6 × 105 M−1 s−1, which is 3 orders of magnitude higher than that determined for the PINO/PINO− couple. The difference in the intrinsic barrier between the two oxidants is so large that it overcomes the thermodynamic factor and the oxidation of ferrocene by BTNO results significantly faster than that by PINO in spite of the higher reduction potential of the latter N-oxyl radical. The higher reactivity of BTNO with respect to PINO in an electron-transfer process contrasts with what is observed in hydrogen atom transfer processes where PINO is always more reactive than BTNO due to the higher NO−H bond dissociation energy in N-hydroxyphthalimide (HPI) than in HBT (88 vs. 85 kcal mol−1, respectively). Thus, the relative reactivity of PINO and BTNO radicals might represent a criterium to help in the distinction of ET and HAT reactions promoted by these transient N-oxyl radicals.