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^–(Fe^IIITSPP); CO₂^–(Fe^IIITCPP); and N⁺Me₃(Fe^IIITMAPP)] 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 Fe^IIITSPP is first order in [Bu^tO₂H] and in [Fe^IIITSPP] at all pH values studied (3.8–9.8). At pH ≥ 6.0 the concentration of Fe^IIITSPP 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 Fe^IIITSPP, at pH 9.2, suggest that it is unreactive towards Bu^tO₂H.

The reactivity of Fe^IIITCPP towards Bu^tO₂H at pH 9.2 is essentially the same as that of Fe^IIITSPP. However, Fe^IIITMAPP 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.