THE DYNAMICS OF THE REACTIONS OF METHYL DIPHENYLHYDROPEROXYACETATE WITH [MESO-TETRAKIS(2,6-DIMETHYL-3-SULFONATOPHENYL)PORPHINATO]-MANGANESE(III) HYDRATE AND [MESO-TETRAKIS(2,6-DICHLORO-3-SULFONATOPHENYL)PORPHINATO]-MANGANESE(III) HYDRATE AND IMIDAZOLE COMPLEXES - COMPARISON OF THE REACTIONS OF MANGANESE(III) AND IRON(III) PORPHYRINS

The reactions of (Ph)2(MeOCO)COOH with two water-soluble non-mu-oxo dimer-forming manganese(III) porphyrins, [meso-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato]manganese(III) hydrate, [(1)Mn(III)(X)2], and [meso-tetrakis(2,6-dichloro-3-sulfonatophenyl)porphinato]manganese(III) hydrate [(2)Mn(III)(X)2, where X = HO- or H2O], have been examined in aqueous solution [30-degrees-C; mu = 0.2 (with NaNO3); pH 5.3-12.6] and compared to the reaction with the iron porphyrin (1)Fe(III)(X)2. Kinetic studies were carried out in the presence and absence of imidazole, with the sodium salt of 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) as a trap for higher valent manganese porphyrin intermediates. Reactions were found to be first-order in both [(porph)Mn(III)(X)2]i and [(Ph)2(MeOCO)COOH]i and independent of [ABTS]i and buffer concentrations {H2PO4-/HPO4(2-) (pH 7.3), HCO3-/CO3(2-) (pH 9.8), and 2,4,6-trimethylpyridine.H+/2,4,6-trimethylpyridine (pH 8.4 and 7.5)}. Thus, the reactions are not subject to general base or general acid catalysis. A plot of the log of the pH-dependent second-order rate constant (k(ly)) vs pH may be fit by an equation (eq 8) derived by assumption of steady state in the reactive species (porph)Mn(III)(OH)((Ph)2(MeOCO)COOH)/(porph)Mn(III)(H2O)((Ph)2(MeOCO)COO) and [(porph)Mn(III)(OH)((Ph)2(MeOCO)COO)]-. Product analysis in the absence of ABTS provided 9% yield of (Ph)2CO and 81% yield of (Ph)2(MeOCO)COH. In the presence of the ABTS trap (Ph)2CO is formed in 12% yield, and the main product is again (Ph)2(MeOCO)COH (55%). In reactions with (1)Mn(III)(X)2 and (2)Mn(III)(X)2 (pH 5.3-12.6) a high spin, d3, manganese(IV) intermediate (mu = 3.90-mu-B at 295 K) was observed. A comparison of the log k(ly) vs pH profiles for the reaction of (1)Mn(III)(X)2 and (1)Fe(III)(X)2 with (Ph)2(MeOCO)COOH showed that k(ly) values are comparable at high pH while at lower pH, (1)Fe(III)(X)2 is far more reactive. This difference in log k(ly) vs pH profiles for (1)Mn(III)(X)2 vs (1)Fe(III)(X)2 finds explanation in the differing pH dependence of potentials for le- reduction of Mn(IV) and Fe(IV) species. Addition of imidazole resulted in an enhancement in the rate of reaction. Apparent equilibrium constants were determined at various values of pH for monoligation of imidazole, and the dependence of rate on pH was employed to show that the reactive intermediates are (i) low pH, [(1)Mn(III)(ImH)((Ph)2(MeOCO)COOH)]+; (ii) intermediate pH, (1)Mn(III)(Im)((Ph)2(MeOCO)COOH)/(1)Mn(III)(ImH)((Ph)2(MeOCO)COO); and (iii) high pH, [(1)Mn(III)(Im)((Ph)2(MeOCO)COO)]-. Imidazole ligation provided at most a rate enhancement of approximately 100-fold.