The Dynamics of Reaction of a Water-Soluble and Non-μ-Oxo Dimer Forming Iron(III) Porphyrin with tert-Butyl Hydroperoxide in Aqueous Solution. 1. Studies Using a Trap for Immediate Oxidation Products

Article abstract of DOI:10.1021/ja00230a022

A kinetic and product study has been carried out in aqueous solution for the reaction of t-BuOOH with the water-soluble and non-μ-oxo dimer forming (5,10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato)iron(III) hydrate ((1)Fe^III(X); where X=H2O or HO^-).Reactions were studied at 30 deg C and μ=0.22 (with NaNO3) between pH 2.22 and 12.96, and the course of reaction was followed by employing the water-soluble 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) as a trap for oxidant intermediates.One-electron oxidation of ABTS provides the chromophoric radical cation ABTS^.+ (λ_max 660 nm).Reactions were carried out under the pseudo-first-order conditions of >> >> <(1)Fe^III(X)> using between 10- and 100-turnovers of the iron(III) porphyrin catalyst.The reaction is first order in both _i and <(1)Fe^III(X)>_i, and both initial and first-order rate constants are independent of _i, ionic strength, and buffer concentrations (buffers employed and pH values for buffer dilution experiments: ClCH2COOH/ClCH2COO^- (pH 3.45), CH3COOH/CH3COO^- (pH 4.60), H2PO4^-/HPO4^2- (pH 6.87), H3BO3/H2BO3^- (pH 8.66), and HCO3^-/CO3^2- (pH 9.13), collidine/collidine*H⁺ (6.18, 6.98, 8.15)).It follows that the rate-determining step occurs after ligation of available alkyl hydroperoxide species (t-BuOOH and t-BuOO^-, pK_a=12.8) with iron(III) porphyrin species ((1)Fe^III(H2O)2 and (1)Fe^III(H2O)(OH), pK_a=7.2), and that these reactions are not subject to either general-acid nor general-base catalysis.A plot of the log of the pH dependent second-order rate constant (k_obsd/<(1)Fe^III(X)>) vs pH may be fit by an equation (eq 3) containing the sum of four terms (A, B, C, D).The equation contains four apparent acid dissociation constants.There are, however, only two acid dissiciation constants associated with the reactants.The pH dependence of the reaction may be explained by taking into account the acid dissociations of t-BuOOH when ligated to both (1)Fe^III(H2O) and (1)Fe^III(OH).With Ph(CH3)2C-OOH as the hydroperoxide the products are Ph(CH3)C=O and CH3OH.The product Ph(CH3)2C=O establishes the formation of Ph(CH3)2C-O^. from alkyl hydroperoxide.With t-BuOOH the products are (CH3)2C=O and CH3OH.When ABTS is used as a trap for intermediate oxidants, the yield of ABTS^.+ is virtually constant (70 percent) between pH 4 and 10, assuming two ABTS molecules undergo 1e^- oxidation for each tert-butyl hydroperoxide moiety reacted.With ABTS the products of tert-butyl hydroperoxide decomposition are t-BuOH, (CH3)2CO, and CH3OH.In the absence of the ABTS trap (CH3)2CO (90 percent) is the predominant product.Increasing concentrations of ABTS result in the yield of (CH3)2CO decreasing asymptotically to 15 percent and the yield of t-BuOH increasing to 84 percent.The following alternate mechanistic proposals have been advanced: (i) The immediate product is the solvent caged species <(1)Fe^IV(X)(OH)-t-BuO^.> and ca. 15 percent of the t-BuO^. fragments within the solvent cage to provide (CH3)2CO and CH3^..The t-BuO...