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).
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.
Brausam
Ariane
A
Department of Chemistry and Pharmacy, University of Erlangen-Nurnberg, Egerlandstrasse 1, 91058 Erlangen, Germany.
Eigler
Siegfried
S
Jux
Norbert
N
van Eldik
Rudi
R
D052160:Benzothiazoles / Q000737:chemistry*; D002384:Catalysis; D005290:Ferric Compounds / Q000737:chemistry*; D006861:Hydrogen Peroxide / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000737:chemistry; D007700:Kinetics; D008665:Metalloporphyrins / Q000737:chemistry*; D010084:Oxidation-Reduction; D011312:Pressure; D013451:Sulfonic Acids / Q000737:chemistry*; D013696:Temperature; D014867:Water / Q000737:chemistry
C545844:(5(4),10(4),15(4),20(4)-tetra-tert-butyl-5(2),5(6),15(2),15(6)-tetrakis(2,2-bis(carboxylato)ethyl)-5,10,15,20-tetraphenylporphyrin)iron(III); D052160:Benzothiazoles; D005290:Ferric Compounds; D006878:Hydroxides; D008665:Metalloporphyrins; D013451:Sulfonic Acids; D014867:Water; C002502:2,2'-azino-di-(3-ethylbenzothiazoline)-6-sulfonic acid; C031356:hydroxide ion; D006861:Hydrogen Peroxide