Chemistry (Weinheim an der Bergstrasse, Germany)

Factors that affect the nature of the final oxidation products in "peroxo-shunt" reactions of iron-porphyrin complexes.

PMID 19714686


The present study focuses on the oxidation of the water-soluble and water-insoluble iron(III)-porphyrin complexes [Fe(III)(TMPS)] and [Fe(III)(TMP)] (TMPS = meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphyrinato, TMP = meso-tetrakis(2,4,6-trimethylphenyl)porphyrinato), respectively, by meta-chloroperoxybenzoic acid (m-CPBA) in aqueous methanol and aqueous acetonitrile solutions of varying acidity. With the application of a low-temperature rapid-scan UV/Vis spectroscopic technique, the complete spectral changes that accompany the formation and decomposition of the primary product of O-O bond cleavage in the acylperoxoiron(III)-porphyrin intermediate [(P)Fe(III)-OOX] (P = porphyrin) were successfully recorded and characterized. The results clearly indicate that the O-O bond in m-CPBA is heterolytically cleaved by the studied iron(III)-porphyrin complexes independent of the acidity of the reaction medium. The existence of two different oxidation products under acidic and basic conditions is suggested not to be the result of a mechanistic changeover in the mode of O-O bond cleavage on going from low to high pH values, but rather the effect of environmental changes on the actual product of the O-O bond cleavage in [(P)Fe(III)-OOX]. The oxoiron(IV)-porphyrin cation radical formed as a primary oxidation product over the entire pH range can undergo a one- or two-electron reduction depending on the selected reaction conditions. The present study provides valuable information for the interpretation and improved understanding of results obtained in product-analysis experiments.