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

We report an in-depth study of the reductive activation of O2 by the nonheme [FeII(L52)(MeCN)]2+ complex carried out by cyclic voltammetry. Experimental evidence is obtained for the slow coordination of dioxygen to the ferrous center yielding an FeII/O2 adduct with a strong FeII–O2 character rather than an FeIII–superoxo one. Electron injection in the FeII–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 FeIII–peroxo intermediate and then FeIII–hydroperoxo upon protonation by residual water. The experimental CVs recorded at variable scan rate or variable FeII concentration are well simulated taking into account a detailed mechanism initiated by the competitive reduction of O2 and the FeII–O2 adduct. Analysis of the concentration of the reaction intermediates generated as a function of the applied potential indicates that the FeIII–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.