Kinetics and mechanism of the manganese(ii) catalysed Calmagite dye oxidation using in situ generated hydrogen peroxide

Abstract

The kinetics and mechanism for the bleaching of Calmagite (H₃CAL, 3-hydroxy-4-(2-hydroxy-5-methylphenylazo)naphthalene-1-sulfonic acid) in aqueous solution at pH 8.00 and 23 ± 1 °C using in situ generated H₂O₂ is described. Complete mineralisation of H₃CAL results with turnover frequencies (TOF = moles of H₃CAL bleached per mole of manganese per hour) of 40 h⁻¹. The monohydroxy azo dyes Me–H₂CAL, Orange G and Orange II are not bleached which indicates that a requirement of dye bleaching is the coordination of the dye to the Mn centre. Spectroscopic studies show the formation of Mn(CAL)₂ and Mn(CAL) species but in the presence of Tiron (1,2-dihydroxybenzene-3,5-disulfonate, disodium salt, monohydrate, Na₂TH₂·H₂O), [Mn(CAL)(T)] is formed. It is proposed that a Mn(III)–hydroperoxide species is generated, [Mn(O₂H)(CAL)(TQ)] from the in situ generated H₂O₂, where TQ represents the o-quinone form of Tiron, and this is the active species in the bleaching of coordinated CAL; the formation of this hydroperoxide species is supported by UV/VIS and ESI-MS data. The formation of a Mn(III) species is supported by EPR studies which also show some evidence for the presence of a labile d⁵ Mn(II) species in the presence of the reducing substrate hydroxylamine (NH₂OH). This would enable rapid ligand exchange for both in situ H₂O₂ generation and dye bleaching to occur; there is no evidence for the presence of Mn^IVO species. The virtue of low local concentrations of in situ generated H₂O₂ is shown to be important in preventing over oxidation of the catalyst and thus contributing to a robust catalytic system.