DYNAMICS OF THE REACTION OF [5,10,15,20-TETRAKIS(2,6-DIMETHYL-3-SULFONATOPHENYL)PORPHINATO]IRON(III) HYDRATE WITH T-BUOOH IN AQUEOUS-SOLUTION .5. INFLUENCE OF NITROGEN BASE LIGATION ON THE RATE OF REACTION OF [5,10,15,20-TETRAKIS(2,6-DIMETHYL-3-SULFONATOPHENYL)-PORPHINATO]IRON(III) HYDRATE WITH T-BUOOH IN AQUEOUS-SOLUTION

In aqueous solution, the axial ligands of [5,10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato)iron(III) are water and/or hydroxide ion [(1)FeIII(OH2)2, (1)FeIII(OH2)(OH), and (1)FeIII(OH)2; (pK(a1) = 6.75, pK(a2) congruent-to 10.81. Hydroperoxide reduction occurs via axial ligation of hydroperoxide to form the complexes (1)FeIII(OH2)(HOOR), (1)FeIII-(OH2)(OOR), and (1)Fe(III)(OH)(OOR). In the pH ranges 4-8 with CF3CH2NH2 (pK(a) = 5.70) and 6-10 with NH3 (pK(a) = 9.24) and imidazole (pK(a) = 6.95), the nitrogen base ligated iron(III) porphyrin species are (1)FeIII(OH2)(Am), (1)FeIII(Am)2, and (1)Fe(III)(OH)(Am) (Scheme I). We have established that mono nitrogen base ligation to form the complex (I)FeIII(OH2)(Am) results in a rate enhancement, whereas bis ligation to form the complex (1)FeIII(Am)2 inhibits thc reaction. Thus hydroperoxide reduction by iron(III) porphyrin must occur via axial ligation of hydroperoxide. As the extinction coefficients for the mono nitrogen base ligated species [epsilon(1N) for (1)FeIII(OH2)(Am) and epsilon2N for (1)Fe(III)(OH)(Am)] are unknown, the equilibrium constants for the formation of nitrogen base ligated iron(III) porphyrin were calculated using a wide range of plausible values for epsilon1N and epsilon2N. The calculated equilibrium constants for formation when Am = NH3 are K(N1) = (0.1-1.5) X 10(2) M-1 for (1)-FeIII(OH2)(Am) and beta = K(N1)K(N2) = (4.4-8.4) X 10(4) M-2 for (1)FeIII(Am)2. When Am = imidazole, K(N1) = (1-5) X 10(3) M-1 and beta = (5-8) X 10(8) M-2. With CF3CH2NH2 equilibrium constants are much lower, so that sufficient concentrations of CF3CH2NH2 for the determination of K(N1) and K(N2) could not be reached. Kinetic studies of the pH dependence for the reaction of the iron(III) porphyrin with t-BuOOH in the presence of CF3CH2NH3+/CF3CH2NH2, (NH4+/NH3, and imidazole. H+/imidazole were carried out using disodium 2,2'-azinobis(3-ethylbenzthiazolinesulfonate) as a trap to follow the turnover of the iron(III) porphyrin catalyst. It was established that NH3 does not serve as a substrate for oxidation by iron(IV)-oxo porphyrin species. The equilibrium constant K(N1) for the formation of (1)FeIII(OH2)(imidazole) is 10(1)-10(2)-fold larger than for formation of (1)FeIII(OH2)(NH3), while the equilibrium constant K(N1)K(N2) for ligation of imidazole to form the inert (1)FeIII(imidazole)2 is 10(4)-fold greater than K(N1K)K(N2) to form (1)FeIII(NH3)2. Thus when Am = NH3 or CF3CH2NH2, (1)FeIII(OH2)(Am) accumulates to a kinetically significant extent at low [Am] before enough of the unreactive (1)FeIII(Am)2 builds up to inhibit the reaction, but when Am = imidazole the converse is true. Therefore, addition of low concentrations of NH3 or CF3CH2NH2 enhances the reaction of the iron(III) porphyrin with t-BuOOH, while the addition of imidazole does not produce a rate enhancement. The ratio of the second-order rate constants for the reactions of (1)FeIII(OH2)(NH3)/(1)FeIII(OH2)2 with t-BuOOH was calculated to be between 1.4 X 10(3) and 1 X 10(5), while that for the reactions of (1)-FeIII(OH2)(NH3)/(1)FeIII(OH2)(OH) with t-BuOOH was calculated to be between 10 and 10(3), dependent upon the choice of epsilon1N and epsilon2N. These ratios represent, in part, the favorable electron donor capacity of ammonia as compared to water.