Electrochemical and spectroscopic properties of manganese tetra(4-NN′N″-trimethylanilinium)porphyrin

Abstract

The electrochemistry of the manganese(III)/manganese(II) tetra(4-NN′N″-trimethylanilinium)-porphyrin couple (abbreviated as Mn^III/Mn^IITMAP) has been investigated in aqueous solutions in the entire pH range by means of cyclic voltammetry and thin-layer coulometry. The spectroscopic characteristics of the oxidized and reduced porphyrin in the 350–700 nm range have been studied using a gold optical transparent electrode. A displacement reaction of Mn²⁺ from Mn^IITMAP by hydrogen ions was identified at pH < 3.6. The effect of adsorption of MnTMAP on the electrode process is described. The absorbed reactant is reduced at a potential close to that of the dissolved porphyrin; E_1/2 is –0.375 V, independent of the pH of the solution. The catalytic aspects of Mn^IIITMAP for the reduction of molecular oxygen to hydrogen peroxide are also presented in terms of cyclic voltammetry. The overpotential is reduced by ca. 300 mV. The role of an oxygen adduct formed between molecular oxygen and the metal atom of the macrocycle is postulated to explain the difference between the redox potential for the Mn^III/Mn^IITMAP couple and the potential at which the catalytic reduction of oxygen occurs.