DYNAMICS OF REACTION OF [5,10,15,20-TETRAKIS(2,6-DIMETHYL-3-SULFONATOPHENYL)-PORPHINATO]IRON(III) HYDRATE WITH TERT-BUOOH IN AQUEOUS-SOLUTION .3. COMPARISON OF REFINED KINETIC-PARAMETERS AND D2O SOLVENT ISOTOPE EFFECTS TO THOSE FOR [5,10,15,20-TETRAKIS(2,6-DICHLORO-3-SULFONATOPHENYL)-PORPHINATO]IRON(III) HYDRATE AND IRON(III) HYDRATE

The decomposition of t-BuOOH in the presence of added ferric ion has been studied in buffered H2O and D2O solutions between pH(D) 1 to 4, by using 2,2'-azinobis(3-ethylbenzthiazolinesulfonate) (ABTS) to trap the oxidizing products. The reaction is first-order in both [t-BuOOH] and [Fe(III)]total. The value of the second-order rate constants in H2O (k2H = 29 M-1 s-1) and in D2O (k2D) = 15 M-1 s-1) is independent of H+(D+) and added buffer species but exhibits a solvent deuterium kinetic isotope effect (k2H/k2D) of 1.9. At pH(D) < 3, the second-order rate constants for reaction of [5,10,15,20-tetrakis-(2,6-dimethyl-3-sulfonatophenyl)porphinato]iron(III) [(1)Fe(III)(H2O)2] with t-BuOOH and the reaction of [5,10,15,20-tetrakis (2,6-dichloro-3-sulfonatophenyl)porphinato]iron(III) [(2)Fe(III)(H2O)2] with H2O2 exhibit the very same independence upon H+(D+) and kinetic isotope effects (k(ly)H/k(ly)D) of 2.7 and 3.0, respectively. Thus, below pH 3 the reactions of Fe(III), (1)Fe(III)(H2O)2, and (2)Fe(III)(H2O)2 with t-BuOOH all display sizable deuterium solvent isotope effects and an independence of rate on [H+] suggesting a common mechanism. We suggest a homolytic mechanism with either H. or H+ transfer rate controlling. In the low pH region, the calculated second-order rate constant for the reaction of ferric ion with t-BuOOH exceeds that for the reaction of (1)Fe(III)(H2O)2 with t-BuOOH by 5-fold. Above pH 3, a plot of the second-order rate constant (k(ly)) vs pH, for reaction of t-BuOOH with H2O and HO- ligated [(1)Fe(III)]+, shows that k(ly), increases and then decreases with increase in pH to form a bell-shaped plot with maximum k(ly) at pH 7.0. In the mid pH range the decomposition of the critical intermediates to products is not associated with a deuterium solvent isotope effect. With further increase in pH, log k(ly) again increases to reach a second maxima. The very same log k(ly) vs pH profile has been seen for the reaction of t-BuOOH with H2O and HO-ligated [(2)Fe(III)]+. On the basis of the percentage yields of (CH3)2C = O and CH3OH the reactions above pH 3 must reflect rate-determining t-BuO-OH bond homolysis. Reaction mechanisms are discussed in terms of the structures of steady-state intermediates and the ground-state structures of oxidized iron porphyrin species as determined in the previous paper in this issue by Kaaret, Zhang, and Bruice. The pH-dependent second-order rate constants (k(ly)) for the decomposition of t-BuOOH by [(2)Fe(III)(X)2, X = OH- or H2O] exceed k(ly) values for the reaction of [(1)Fe(III)(X)2, X = OH- or H2O] with t-BuOOH by at most 2.5-3.8-fold across the entire pH range. The marked influence of ionic strength on the pK(a) associated with (1)Fe(III)(H2O)2 reversible (1)Fe(III)(H2O)(HO) + H+ has been determined, and from the Debye-Huckel equation for a monobasic acid the thermodynamic pK(a) = 7.3 and the charge on the Fe(III)(H2O)(HO) moiety is 2-.