Mechanistic Studies on Peroxide Activation by a Water‐Soluble Iron(III)–Porphyrin: Implications for OO Bond Activation in Aqueous and Nonaqueous Solvents

Abstract

The reactions of a water-soluble iron(III)-porphyrin, [meso-tetrakis(sulfonatomesityl)porphyrinato]iron(III), [Fe(III)(tmps)] (1), with m-chloroperoxybenzoic acid (mCPBA), iodosylbenzene (PhIO), and H(2)O(2) at different pH values in aqueous methanol solutions at -35 degrees C have been studied by using stopped-flow UV/Vis spectroscopy. The nature of the porphyrin product resulting from the reactions with all three oxidants changed from the oxo-iron(IV)-porphyrin pi-cation radical [Fe(IV)(tmps(*+))(O)] (1(++)) at pH<5.5 to the oxo-iron(IV)-porphyrin [Fe(IV)(tmps)(O)] (1(+)) at pH>7.5, whereas a mixture of both species was formed in the intermediate pH range of 5.5-7.5. The observed reactivity pattern correlates with the E degrees' versus pH profile reported for 1, which reflects pH-dependent changes in the relative positions of E degrees'(Fe(IV)/Fe(III) ) and E degrees'(P(*+)/P) for metal- and porphyrin-centered oxidation, respectively. On this basis, the pH-dependent redox equilibria involving 1(++) and 1(+) are suggested to determine the nature of the final products that result from the oxidation of 1 at a given pH. The conclusions reached are extended to water-insoluble iron(III)-porphyrins on the basis of literature data concerning the electrochemical and catalytic properties of [Fe(III)(P)(X)] species in nonaqueous solvents. Implications for mechanistic studies on [Fe(P)]-catalyzed oxidation reactions are briefly addressed.

Transition Metal Complexes and the Activation of Dioxygen

“…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].…”

“…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). 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].…”

“…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).…”

Superoxide Dismutase Mimics: Chemistry, Pharmacology, and Therapeutic Potential

“…Although the corresponding Fe and Mn porphyrins have very similar rate constants for O 2 ·À dismutation, all Fe porphyrins studied by us thus far were toxic to Escherichia coli; no aerobic growth was detected in SOD-negative mutants with Fe porphyrins at levels at which analogous Mn porphyrins were fully protective (30). A loss of metal from the metal complexes during redox cycling could occur, whereby ''free'' Fe would give rise, through Fenton chemistry, to highly oxidizing · OH species; Fenton chemistry presumably occurs even if reduced iron is still bound to the porphyrin ligand (338). Thus, we limited our studies to Mn porphyrins as SOD mimics (Fig.…”

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

“…vary the solution acidity) to stabilize the particular high-valent oxo species in order to better comprehend the mechanistic aspects that affect the reactivity of compound I and II models for P450 [24,62,90]. 'Peroxide-shunt' reactions which resemble those of peroxidase enzyme have also been applied to 'catch' P450 compound I in order to uncover the secrets of the mechanism of its formation and its action in substrate oxidation [52,[91][92][93].…”