Mechanistic studies on peroxide activation by a water-soluble iron(III)-porphyrin: implications for O-O bond activation in aqueous and nonaqueous solvents.
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.
Wolak
Maria
M
Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany.
van Eldik
Rudi
R
D002723:Chlorobenzoates / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007462:Iodobenzenes; D007501:Iron / Q000737:chemistry; D008665:Metalloporphyrins / Q000737:chemistry*; D016877:Oxidants / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D010545:Peroxides / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry; D012995:Solubility; D012997:Solvents / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D014867:Water / Q000737:chemistry*
C521848:(meso-tetrakis(sulfonatomesityl)porphyrinato)iron(III); D002723:Chlorobenzoates; D007462:Iodobenzenes; D008665:Metalloporphyrins; D016877:Oxidants; D010545:Peroxides; D011166:Porphyrins; D012997:Solvents; C028565:porphyrin pi cation radical; D014867:Water; D007501:Iron; C000433:3-chloroperbenzoic acid; D010100:Oxygen; C037276:iodosobenzene