Electrochemical approach of the reductive activation of O2 by a nonheme FeII complex. Some clues for the development of catalytic oxidations

Abstract

We report an in-depth study of the reductive activation of O₂ by the nonheme [Fe^II(L²₅)(MeCN)]²⁺ complex carried out by cyclic voltammetry. Experimental evidence is obtained for the slow coordination of dioxygen to the ferrous center yielding an Fe^II/O₂ adduct with a strong Fe^II–O₂ character rather than an Fe^III–superoxo one. Electron injection in the Fe^II–O₂ species occurs at a potential of ca. −700 mV vs. SCE, i.e. 200 mV above the O₂ to O₂˙⁻ reduction, leading to the formation of a Fe^III–peroxo intermediate and then Fe^III–hydroperoxo upon protonation by residual water. The experimental CVs recorded at variable scan rate or variable Fe^II concentration are well simulated taking into account a detailed mechanism initiated by the competitive reduction of O₂ and the Fe^II–O₂ adduct. Analysis of the concentration of the reaction intermediates generated as a function of the applied potential indicates that the Fe^III–peroxo intermediate significantly accumulates at a potential of −650 mV. Oxidative bromination of anisole is assayed under electrolytic conditions at this potential to yield bromoanisole products. The low faradaic yields observed reveal that deleterious reactions such as direct reduction of reaction intermediates likely occur. Based on the detailed mechanism elucidated, a number of improvements to achieve more efficient catalytic reactions can be proposed.