A kinetic study of the reaction of tert-butyl hydroperoxide with iron(III) 5, 10, 15, 20-tetra(4-sulfonatophenyl)porphyrin and related compounds in aqueous solution
Abstract
The kinetics of the reaction of tert-butyl hydroperoxide with three iron(III) tetra(4-X-phenyl)porphyrins [X = SO3–(FeIIITSPP); CO2–(FeIIITCPP); and N+Me3(FeIIITMAPP)] in aqueous solution have been investigated using an excess of 2,2′-azinobis(3-ethyl-2,3-dihydrobenzothiazole-6-sulfonate) as a one-electron oxidant trap. The reaction with FeIIITSPP is first order in [ButO2H] and in [FeIIITSPP] at all pH values studied (3.8–9.8). At pH ≥ 6.0 the concentration of FeIIITSPP has to be lowered to eliminate the problem of µ-oxo dimer formation and allow the kinetics of the monomer to be studied. The measured second order rate constant shows a complex dependence on pH that is very similar to those reported previously for sterically hindered iron(III) tetra(sulfonatophenyl)porphyrins. Attempts to measure the rate of reaction of the µ-oxo dimer of FeIIITSPP, at pH 9.2, suggest that it is unreactive towards ButO2H.
The reactivity of FeIIITCPP towards ButO2H at pH 9.2 is essentially the same as that of FeIIITSPP. However, FeIIITMAPP reacts significantly more slowly both at pH 9.2 and at 4.8. This is attributed to the greater tendency of this iron(III) porphyrin to form µ-oxo dimers.