Publication Details (including relevant citation information):
Elaine Nam, Pauline E. Alokolaro, Rodney D. Swartz, Morgan C. Gleaves, Jessica Pikul, and Julie A. Kovacs*
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United
Inorg. Chem. 2011, 50, 1592–1602
Kinetic studies aimed at determining the most probable mechanism for the proton-dependent [FeII(SMe2N4(tren))]+ (1) promoted reduction of superoxide via a thiolate-ligated hydroperoxo intermediate [FeIII(SMe2N4(tren))(OOH)]+ (2) are described. Rate laws are derived for three proposed mechanisms, and it is shown that they should conceivably be distinguishable by kinetics. For weak proton donors with pKa(HA) > pKa(HO2) rates are shown to correlate with proton donor pKa, and display first-order dependence on iron, and half-order dependence on superoxide and proton donor HA. Proton donors acidic enough to convert O2 - to HO2 (in tetrahydrofuran, THF), that is, those with pKa(HA) < pKa(HO2), are shown to display first-order dependence on both superoxide and iron, and rates which are independent of proton donor concentration. Relative pKa values were determined in THF by measuring equilibrium ion pair acidity constants using established methods. Rates of hydroperoxo 2 formation displays no apparent deuterium isotope effect, and bases, such as methoxide, are shown to inhibit the formation of 2. Rate constants for p-substituted phenols are shown to correlate linearly with the Hammett substituent constants σ-. Activation parameters ((ΔHq = 2.8 kcal/mol, ΔSq = -31 eu) are shown to be consistent with a low-barrier associative mechanism that does not involve extensive bond cleavage. Together, these data are shown to be most consistent with a mechanism involving the addition of HO2 to 1 with concomitant oxidation of the metal ion, and reduction of superoxide (an “oxidative addition” of sorts), in the rate-determining step. Activation parameters for MeOH- (ΔHq = 13.2 kcal/mol and ΔSq =-24.3 eu), and acetic acid- (ΔHq = 8.3 kcal/mol and ΔSq = -34 eu) promoted release of H2O2 to afford solvent-bound [FeIII(SMe2N4(tren))(OMe)]+ (3) and [FeIII(SMe2N4(tren))(O(H)Me)]+ (4), respectively, are shown to be more consistent with a reaction involving rate-limiting protonation of an Fe(III)-OOH, than with one involving rate-limiting O-O bond cleavage. The observed deuterium isotope effect (kH/kD = 3.1) is also consistent with this mechanism.