Mechamism of oxidation of azo dyes by a sterically hindered anionic oxoiron(IV) porphyrin in aqueous solution

Abstract

The oxidation of 1-(substituted phenylazo)-2-hydroxynaphthalene-6-sulfonate dyes by oxoiron(IV) tetra(2,6-dichloro-3-sulfonatophenyl)porphyrin (OFe^IVTDCSPP) in aqueous solution is first-order in the concentration of the dye and of the oxoiron(IV) species over the pH range 6.93–12.68. The pH dependence of the second-order rate constant, k_obs, which shows a minimum between pH 8.5 and 9.5, can be simulated using two pH-dependent equilibria [OFe^IVTDCSPP(OH₂)/OFe^IVTDCSPP(OH) and dye/dye anion] and the rate constants for three oxidations, dye by OFe^IVTDCSPP(OH₂) and OFe^IVTDCSPP(OH) and dye anion by OFe^IVTDCSPP(OH). The fourth combination, the oxidation of the dye anion by OFe^IVTDCSPP(OH₂), makes an insignificant contribution to the overall rate of reaction. The mechanisms of the three oxidation processes have been studied using Hammett correlations of substituent effects and by comparisons with the oxidations of isomeric 1-aryl-4-hydroxynaphthalenesulfonate dyes, of an O-methylated dye and of a deuterated 1-phenylazo-2-hydroxynaphthalene-6-sulfonate dye, bearing in mind that the dyes in aqueous solution exist as an equilibrium between azo and hydrazone tautomers.

In strongly basic solution the dominant reaction is an electron-transfer oxidation between the dye anion and OFe^IVTDCSPP(OH) whereas at neutral pH the major reaction is hydrogen atom-abstraction by OFe^IVTDCSPP(OH₂) from the azo tautomer of the dye. The mechanism at the pH–rate minimum, between OFe^IVTDCSPP(OH) and dye, is less clear and alternatives are discussed.