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  • Title: Mechanistic investigations of the reaction of an iron(III) octa-anionic porphyrin complex with hydrogen peroxide and the catalyzed oxidation of diammonium-2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate).
    Author: Brausam A, Eigler S, Jux N, van Eldik R.
    Journal: Inorg Chem; 2009 Aug 17; 48(16):7667-78. PubMed ID: 19601585.
    Abstract:
    A detailed study of the effect of pH, temperature, and pressure on the reaction of hydrogen peroxide with [Fe(III)(P(8-))](7-), where P(8-) represents the octa anionic porphyrin, was performed using stopped-flow techniques. Depending on the pH, different high valent iron-oxo species were formed. At pH < 9 formation of a two-electron oxidized species [(porphyrin(+*))Fe(IV)=O] was observed. In contrast, at pH > 9 only the one electron oxidized species [(porphyrin)Fe(IV)=O] was found to be present in solution. Under selected conditions at pH 8 it was possible to determine rate constants for both the coordination of hydrogen peroxide and subsequent heterolytic cleavage of the O-O bond. At pH 11 a composite rate constant for coordination of H(2)O(2) and homolytic cleavage of the O-O bond could be measured. In addition, it was possible to determine the activation parameters for the overall reaction sequence leading to the formation of [(porphyrin)Fe(IV)=O]. Careful analysis of the obtained data supports an associatively activated mechanism for the coordination of hydrogen peroxide. The catalytic properties of [Fe(III)(P(8-))](7-) in the presence of H(2)O(2) were also investigated. Both high valent iron-oxo species turned out to be able to oxidize diammonium-2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) to the radical cation ABTS(+*). At higher hydrogen peroxide concentrations a reduced yield of ABTS(+*) was observed because of increased catalase activity of [Fe(III)(P(8-))](7-). At high pH disproportionation of ABTS(+*) to ABTS and ABTS(2+) occurred, which could be suppressed by an excess of unreacted ABTS. In slightly basic to acidic solutions this reaction did not play a role.
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