Elaine Nam - Investigation of the Mechanism of Formation of a Thiolate-Ligated Fe(III)-OOH

Version 1

      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
      States

      Inorg. Chem. 2011, 50,   1592–1602
      DOI: 10.1021/ic101776m

      Abstract:

      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.

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