Azo dye oxidation with hydrogen peroxide catalysed by manganese 1,4,7-triazacyclononane complexes in aqueous solution

Abstract

A kinetic and mechanistic study is reported of the oxidation of a number of azonaphthol dyes with hydrogen peroxide in aqueous solution, catalysed by some mono and dinuclear manganese(IV) complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃TACN). The results of UV-Vis investigations, augmented by EPR and ESI-MS studies, are described for a series of experiments in which concentrations, pH and ionic strength have been varied. The reactions are characterised by an induction period followed by a relatively rapid oxidation. For the dinuclear manganese complex 2, these are consistent with an initial perhydrolysis of the manganese complex involving both the dye anion and HO₂⁻, to give mononuclear manganese species and the operation of a catalytic cycle incorporating Mn^IIIL(OH)₃, OMn^VL(OH)₂ and Mn^IVL(OH)₃ (L = Me₃TACN) (cf. the reactions of peroxidase enzymes). ESI-MS results provide evidence for the formation and reaction (with the dye) of Mn^IVL(OH)₃. With the mononuclear manganese complex Mn^IVL(OMe)₃, there is a short lag-phase attributed to perhydrolysis by HO₂⁻ followed by the same catalytic cycle.