DYNAMICS OF THE REACTION OF HYDROGEN-PEROXIDE WITH A WATER-SOLUBLE NON MU-OXO DIMER FORMING IRON(III) TETRAPHENYLPORPHYRIN .2. THE REACTION OF HYDROGEN-PEROXIDE WITH 5,10,15,20-TETRAKIS(2,6-DICHLORO-3-SULFONATOPHENYL)-PORPHINATOIRON(III) IN AQUEOUS-SOLUTION

The reaction of hydrogen peroxide with 5,10,15,20-tetrakis(2,6-dichloro-3-sulfonatophenyl)porphinatoiron(III) hydrate [(2)FeIII(H2O)(X), where X = H2O or HO-, dependent on pH] has been studied in water between pH 0.5 and 12.25 and in D2O between pD 3.3 and 10.70 (30 °C, μ = 0.2 with NaNO3) by using 2,2'-azinobis(3-ethylbenzthiazoline)sulfonate (ABTS) as a trap for oxidizing intermediates. The reaction is first-order in both (2)FeIII(H2O)(X) and hydrogen peroxide and zero-order in ABTS. The pH (pD) dependence of the log of the second-order rate constant (kly) exhibits three plateau regions showing that there are three reaction paths. Transition-state compositions are at low pH, (2)FeIII(H2)(H2O2); around neutrality, (2)FeIII(HO-)(H2O2); and at high pH, (2)FeIII(HO-)(HO2-). Comparison of the pH dependence of log klyH in H2O to the pD dependence of log klyD in D2O allows the determination of the deuterium solvent kinetic isotope effect for the decomposition of (2)FeIII(H2O)(H2O2)/(2)FeIII(D2O)(D2O2) (k1H/k1D = 3) and (2)FeIII(HO-)(H2O2)/(2)FeIII(DO-)(D2O2) (k2H/k2D = 1.2). A solvent kinetic isotope effect of 3 is in accord with H-O bond breaking accompanying O-O bond breaking (general-base catalysis by lyate species), while an isotope effect of 1.2 is to be anticipated as a solvent effect accompanying transfer from H2O to D2O. Carboxylic acid/carboxylate and other oxygen acid/oxygen base buffers do not serve as general catalysts at any pH. In contrast the sterically hindered nitrogen bases 2,6-dimethylpyridine, 2,4,6-trimethylpyridine, and 4-methoxy-2,6-dimethylpyridine act as general-base catalysts for the decomposition of the critical species formed at low pH (i.e, (2)-FeIII(H2O)(H2O2)). By using 2,4,6-trimethylpyridine/2,4,6-trimethylpyridine-H(D)+ buffers in H2O and D2O, the deuterium solvent isotope effect for nitrogen base catalysis was determined as 3.3. Thus, H2O and nitrogen bases act as general base catalysts at low pH with isotope effects around 3. The Bronsted constant s was determined to be ~0.1 for the nitrogen bases (B:). Various mechanisms are considered. Previous studies have shown that an iron(IV)-oxo porphyrin does not epoxidize 3-cyclohexene-1-carboxylic acid. In this study we show that (2)FeIII(H2O)2 does not catalyze the epoxidation of 3-cyclohexene-1-carboxylic acid (1.0 M) by H2O2 (0.1 M) in water at pH 7.0. Lack of general catalysis in the decomposition of the peroxide ligated species at intermediate pH ((2)FeIII(HO-)(H2O2)) requires explanation based upon the distal ligand being HO- rather than H2O. We propose that the more basic HO- ligand increases the electron density on iron allowing le- transfer to ligated H2O2 without proton removal. The pKa of (2)FeIII(H2O)2 → (2)FeIII(HO-)(H2O) + H+ has been determined as 7.85 (8.68 in D2O). Comparison of this to the pKa (7.25) for proton dissociation from 5,10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinatoiron(III) hydrate (i.e., (1)FeIII(H2O)2) shows that the field effect of eight methyl groups surrounding the ligated water molecules is acid strengthening compared to eight chloro substituents. © 1990, American Chemical Society. All rights reserved.