Factors That Affect the Nature of the Final Oxidation Products in “Peroxo‐Shunt” Reactions of Iron–Porphyrin Complexes

Abstract

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.

Transition Metal Complexes and the Activation of Dioxygen

“…These results were interpreted to be consistent with the notion that pH-dependent redox equilibria may in fact mask the true identity of the oxidation products, especially if product analysis is the method of identification [85]. Interestingly, further investigation using rapid-scan UV-vis experiments performed at low temperature and under conditions of excess oxidant only showed formation of the Fe(IV)-oxo π* cation-radical in the pH ranges studied (pH ¼ 6.3-11.4), indicating that, for the complexes studied, O-O bond cleavage only proceeds via heterolysis when meta-chlorobenzoic acid (m-CPBA) is the oxidant [86].…”

“…These findings are consistent with the fact that under basic conditions (pH >7) the Fe(IV)-oxo form is electrochemically more stable than the Fe(IV)-oxo π* cation-radical [85]. The authors propose that under basic conditions, reaction of the Fe(IV)-oxo π* cation-radical with RO À (R¼H, CH 3 ) likely generates a reduced Fe(III)-OOR species which can then undergo homolysis to form the observed Fe(IV)-oxo intermediate or lose ROO À to reform the starting Fe(III) precursor [86]. These findings highlight the importance of the nature of the oxidant as well as the reaction conditions in dictating the mechanism of O-O bond cleavage.…”

“…Two more recent studies [85,86] have sought to further characterize the nature of the observed pH dependence of these systems. While it had been postulated that a mechanistic changeover was the result of acid-base effects or changes in speciation of the reacting Fe(III) complexes, it was found that the observed reactivity correlates well with the pH dependence of E '(Fe IV/III ) and E '(P +• /P) (P ¼ porphyrin).…”

Spectroscopic observation of iodosylarene metalloporphyrin adducts and manganese(V)-oxo porphyrin species in a cytochrome P450 analogue

“…Peroxyacids 26,[32][33][34][35] , hydroperoxides [36][37][38][39] or inorganic salts [40][41][42][43][44][45] were generally employed as terminal oxidants in mechanistic studies, and the reactions were always performed in water or mixed solvents (such as CH 3 OH/H 2 O or CH 3 CN/H 2 O). In fact, PhIO is often mechanistically cleaner than these alternatives 46 .…”

Mechanistic studies on versatile metal-assisted hydrogen peroxide activation processes for biomedical and environmental incentives

“…For the mechanism of O-O bond cleavage in (Por)Fe III -OOH, it is crucial to consider the stability and reactivity of the initial product of the peroxo-shunt oxidation of iron(III) porphyrins, since the identity of the catalytically active species does not always reflect the actual mode of O-O bond scission in transient compound 0 models [90,139]. This is an important aspect when considering molecular mechanisms for H2O2 activation, since an excess of H2O2 can act as substrate and react with the primarily formed Compound I (Scheme 2 pathway C) [71,134,138].…”