Mechanism of the oxidation of ferrocene and its derivatives with hydrogen peroxide: A new effect

A new effect was recorded during the studies of the mechanism of ferrocene oxidation with hydrogen peroxide, namely, a shift of λmax of the absorption band of the ferricinium cation (ABFC) toward the long-wave region and its broadening during the reaction. The shiftΔλmax increased with the excess of the H2O2 concentration with respect to the ferrocene concentration and reached 90 nm and more at H2O2/Fc ≈ 80–100. Similar changes in ABFC took place during the oxidation of a series of ferrocene derivatives. A shift of ABFC was not revealed at comparable concentrations of the metal complex and H2O2. During the oxidation of ferrocene with other peroxides (t-C4H9OOH or (PhCOO)2), there were no changes in the spectrum of the ferricinium cation at any ratios of reagent concentrations. The observed effect is based on the formation of a {ferricinium cation + .OH} radical pair during the primary interaction of the metallocomplex with H2O2 and subsequent reaction between the radicals of the radical pair according to the radical substitution mechanism, which leads to the formation of the hydroxy derivatives of ferrocene and their cations. Sequential accumulation of OH substituents in the metallocomplex and the corresponding ferricinium cations caused a continuous shift of ABFC toward the long-wave region.