One-electron oxidation of ferrocenes by short-lived N-oxyl radicals. The role of structural effects on the intrinsic electron transfer reactivities

Abstract

A kinetic study of the one electron oxidation of substituted ferrocenes (FcX: X = H, COPh, COMe, CO₂Et, CONH₂, CH₂OH, Et, and Me₂) by a series of N-oxyl radicals (succinimide-N-oxyl radical (SINO), maleimide-N-oxyl radical (MINO), 3-quinazolin-4-one-N-oxyl radical (QONO) and 3-benzotriazin-4-one-N-oxyl radical (BONO)), has been carried out in CH₃CN. N-oxyl radicals were produced by hydrogen abstraction from the corresponding N-hydroxy derivatives by the cumyloxyl radical. With all systems, the rate constants exhibited a satisfactory fit to the Marcus equation allowing us to determine self-exchange reorganization energy values (λ_NO˙/NO⁻) which have been compared with those previously determined for the PINO/PINO⁻ and BTNO/BTNO⁻ couples. Even small modification of the structure of the N-oxyl radicals lead to significant variation of the λ_NO˙/NO⁻ values. The λ_NO˙/NO⁻ values increase in the order BONO < BTNO < QONO < PINO < SINO < MINO which do not parallel the order of the oxidation potentials. The higher λ_NO˙/NO⁻ values found for the MINO and SINO radicals might be in accordance with a lower degree of spin delocalization in the radicals MINO and SINO and charge delocalization in the anions MINO⁻ and SINO⁻ due to the absence of an aromatic ring in their structure.