The pH dependence of the mechanism of reaction of hydrogen peroxide with a nonaggregating, non-mu-oxo dimer-forming iron (III) porphyrin in water.

The reaction of hydrogen peroxide with 5, 10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato- iron(III) hydrate [(P)FeIII(H2O)] has been investigated in water between pH 1 and pH 12. The water-soluble (P)FeIII(H2O) neither aggregates nor forms a mu-oxo dimer. The pH dependence and rate-limiting second-order rate constants (kly) for oxygen transfer from H2O2 and HO2- to the iron(III) porphyrin were determined by trapping of the resultant higher-valent iron-oxo porphyrin species with 2,2'-azinodi(3-ethylbenzthiazoline)-6-sulfonate (ABTS). Reactions were monitored spectrophometrically by following the appearance of the radical ABTS.+. From a plot of the logarithm of the determined second-order rate constants for reaction of hydrogen peroxide with iron(III) porphyrin vs. pH, the composition of the transition states can be assigned for the three reactions that result in oxygen transfer to yield a higher-valent iron-oxo porphyrin species. The latter not only reacts with ABTS to provide ABTS.+ in a peroxidase-type reaction but also reacts with hydrogen peroxide to provide O2 in a catalase-type reaction. The nitrogen base 2,4,6-collidine serves as a catalyst for oxygen transfer from hydrogen peroxide to the (P)FeIII-(H2O) and (P)FeIII(HO) species. The preferred mechanism involves a 1,2-proton shift concerted with heterolytic cleavage of the peroxide O-O bond. An analogous mechanism is believed to occur in the peroxidase enzymes.