Comparative study of the catalytic activity of [MnII(bpy)2Cl2] and [Mn2III/IV(μ-O)2(bpy)4](ClO4)3 in the H2O2 induced oxidation of organic dyes in carbonate buffered aqueous solution

Abstract

The kinetics of the hydrogen peroxide induced oxidative degradation of the azo dye Orange II in aqueous carbonate buffered solution were studied for the oxo-bridged [Mn₂^III/IV(μ-O)₂(bpy)₄](ClO₄)₃ complex and its mononuclear analogue [Mn^II(bpy)₂Cl₂] as catalysts to reveal the underlying reaction mechanism and reactive intermediates participating in the catalytic cycle. Both catalysts show identical oxidative reactivity when used at equimolar manganese concentration. If a simple Mn(II) salt and a 1 : 2 concentration of bipyridine are added to the substrate and oxidant containing reaction mixture, the same oxidative reactivity as found for both readily prepared catalysts was observed for several investigated substrates. This demonstrates the in situ accessibility of a reactive intermediate and its precursor complex. The crucial role of bicarbonate as co-catalyst was studied. The distinct dependence of the observed rate constant for the oxidation reaction on the total carbonate concentration can be accounted for in terms of in situ generation of peroxycarbonate. EPR and rapid scan UV/Vis measurements of the reaction of hydrogen peroxide in carbonate buffered solution with [Mn₂^III/IV(μ-O)₂(bpy)₄](ClO₄)₃ and [Mn^II(bpy)₂Cl₂], revealed for both catalysts the presence of monomeric Mn(II) and Mn^IV-oxo species as the main intermediates. The proposed reaction mechanism involves two-electron oxidation of a mononuclear Mn(II) precursor complex to a high-valent Mn(IV)O intermediate as catalytically active species. Differences in the activity of in situ prepared catalyst precursors of different metal to ligand ratios are reported. The 1 : 2 complex was found to be the catalytically more active precursor for the oxidation of the selected substrates, whereas the 1 : 3 complex rather catalyzed the disproportion of hydrogen peroxide.