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)

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.

Mechanistic Insight into Peroxo‐Shunt Formation of Biomimetic Models for Compound II, Their Reactivity toward Organic Substrates, and the Influence of N‐Methylimidazole Axial Ligation

“…This behaviour reflects the much higher reactivity of OOH À than H 2 O 2 , since no pH-dependent process related with (TMPS)Fe III is observed in this pH range. [20,35] The pK a value determined from the pH dependence shown in Figure 3 is 11.26 AE 0.03, which is in good agreement with the acid dissociation constant of hydrogen peroxide (pK a = 11.6). The dependence of the observed rate constant for the formation of (TMPS)Fe IV =O(OH) on pH allowed the determination of the limiting rate constants for the reaction of (TMPS)Fe III (OH) with H 2 O 2 (k 1 = 8.9(AE0.5) 10 2 m À1 s À1 ) and OOH À (k 2 = 4.8(AE0.1) 10 4 m À1 s À1 ).…”

The reactivity of the active metal oxo and hydroxo intermediates and their implications in oxidations

“…20 ABTS (2,2 0 -azino-di-(3-ethylbenzthiazoline-6-sulphonic acid) is a classic reagent to probe the electron transfer reaction, because after one electron transfer, the generated ABTS + demonstrates a distinctly different UV-visible absorbance from that of original ABTS (Scheme 12). Eldik and co-workers found that both Fe IV QO + and Fe IV QO intermediates, the analogs of compound I and II in peroxidases, 21 can be generated by oxidizing the water-soluble octa anionic porphyrin Na 7 [(P 8 (2)), where Z is the collision frequency, K b is the Boltzmann constant, T is the absolute temperature, and l is the reorganization energy of ET. The l value is determined to be 2.37 eV, which is similar to those determined for electron transfer reduction of nonheme iron(IV) oxo complexes, indicating that one electron reduction of high-valent M n+ QO moieties generally requires large reorganization energy, probably due to significant elongation of M n+ QO bonds upon one electron reduction.…”

Trace Oxygen Sensitive Material Based on Two Porphyrin Derivatives in a Heterodimeric Complex

“…The structure of these intermediates, partially confirmed by our UV-vis experiments (as seen in Figure S12 from supplementary file), are according to Scheme 1: Oxoferryl species (por)-Fe IV = O [or its protonated form (por)Fe IV –OH] formed by homolysis of the peroxide and oxoperferryl species (por •+ )Fe IV = O formed by heterolysis [49,50,51,52,53]. The UV-vis study (Figure S12) demonstrates the existence of both oxo-iron (IV) porphyrin species (the shifted band at 520 nm) and of the oxo-iron-porphyrin radical cation species (the isosbestic points at 450 nm and for a few number of curves at 505 nm).…”