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Product studies on the oxidation of two 1-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy acids and by tert-butyl hydroperoxide catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin in solution have been carried out. In single turnover experiments at pH 11.88, 9.30 and 6.93, dye bleaching by peroxy acids is a two-step process; an initial fast reaction, during which the oxoiron(IV) porphyrin is also formed, is followed by a slow phase involving the regeneration of the iron(III) porphyrin from the oxoiron(IV) porphyrin. The peroxy acid∶dye stoichiometry is 1∶1 for the initial fast dye bleaching and 2∶1 for the slow phase; the latter is equivalent to a 4∶1 stoichiometry of oxoiron(IV) porphyrin to dye. In multiple turnover reactions the dye bleaching occurs in two sequential oxidations by oxoiron(IV) porphyrin π-cation radical, each with a peroxy acid∶substrate stoichiometry of 1∶1.Evidence is presented to show that, in multi-turnover reactions at pH 11.88 where the substrate is the dye anion, the initial product arises from the hydroxylation of the 1-position of the naphthol ring which in the second step is oxidised further to give an azoxy compound. At pH 9.30, where the dyes exist predominantly as the hydrazone tautomers, the initial oxidation generates an azoxy compound which following a Wallach rearrangement is further oxidised on the naphthol ring. More extensive oxidation results in the cleavage of the N–N bond.The iron(III) porphyrin-catalysed dye oxidation with excess of tert-butyl hydroperoxide at pH 6.93, which occurs by hydrogen atom-abstraction, has a hydroperoxide∶dye stoichiometry approaching 2∶1 and gives the same azoxy product as obtained at pH 11.88 with the peroxyacids.Mechanisms to account for the observed products and stoichiometries are discussed.

The oxidation of 1-(substituted phenylazo)-2-hydroxynaphthalene-6-sulfonate dyes by oxoiron(IV) tetra(2,6-dichloro-3-sulfonatophenyl)porphyrin (OFeIVTDCSPP) 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, kobs, which shows a minimum between pH 8.5 and 9.5, can be simulated using two pH-dependent equilibria [OFeIVTDCSPP(OH2)/OFeIVTDCSPP(OH) and dye/dye anion] and the rate constants for three oxidations, dye by OFeIVTDCSPP(OH2) and OFeIVTDCSPP(OH) and dye anion by OFeIVTDCSPP(OH). The fourth combination, the oxidation of the dye anion by OFeIVTDCSPP(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 OFeIVTDCSPP(OH) whereas at neutral pH the major reaction is hydrogen atom-abstraction by OFeIVTDCSPP(OH2) from the azo tautomer of the dye. The mechanism at the pH–rate minimum, between OFeIVTDCSPP(OH) and dye, is less clear and alternatives are discussed.

The complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin with vinyl acetic acid and a selection of α-halo and α-hydroxycarboxylic acids, α-,β- and γ-ketoacids, β-dicarboxylic acids and polyacrylic acid have been photolysed (λ > 390 nm) in anaerobic aqueous solution. UV–VIS spectroscopy shows that the primary reaction is photoreduction of the iron(III) atom by the axial carboxylate ligand to give a solvent-caged carboxyl radical and iron(II) porphyrin.The rate of iron(II) formation is determined by the competitive reactions of the carboxyl radical in the solvent cage: recombination with the iron(II) porphyrin to regenerate the starting complex versus decarboxylation. EPR spin-trapping methods have been used to detect the carbon radicals arising from decarboxylation.Vinylacetic and α-hydroxycarboxylic acids lead to a rapid rate of iron(II) porphyrin formation; with the latter this is partly due to the one-electron reduction of the iron(III) porphyrin by the α-hydroxyalkyl radicals from the photo-decarboxylation. By contrast the build-up of iron(II) porphyrin from the photoreactions of the α-haloacids is slow and does not go to completion since the iron(III) porphyrin is regenerated by reaction of the halosubstrate with the iron(II) compound.Ketoacids show an unusual variety of reactions, depending on structure. The α-ketoacids give acyl and, by decarboxylation, alkyl radicals and the iron(II) porphyrin CO complex, whereas with β-ketoacids the photoreaction is catalytic, since, following decarboxylation of the carboxyl radical, the iron(III) porphyrin is rapidly regenerated by oxidation of the iron(II) porphyrin by the α-keto radical. γ-Ketoacids behave in a similar fashion to alkylcarboxylic acids.The photoreactions of β-dicarboxylic acids resemble those of their monocarboxylic analogues. With polyacrylic acid the rate of iron(II) formation is slow and this is attributed to the polyacid encouraging recombination of iron(II) porphyrin and carboxyl radical.