Kinetic Analysis of H<sub>2</sub>O<sub>2</sub> Activation by an Iron(III) Complex in Water Reveals a Nonhomolytic Generation Pathway to an Iron(IV)oxo Complex
Christopher J. Miller, Yingyue Chang, Christina Wegeberg, Christine J. McKenzie, T. David Waite
Abstract
[FeIII(OH)(tpena)]+ (tpena– = N,N,N′-tris(2-pyridylmethyl)ethylenediamine-N′-acetate) catalytically activates H2O2 with the concomitant formation of the active oxidants [FeIV(O)(tpena)]+ and HO• in aqueous solutions at pH 8. A kinetic model is used to demonstrate that the activation of [FeIII(OH)(tpena)]+ by H2O2 proceeds by the formation of [FeIII(OOH)(tpena)]+. Two previously unreported reactions of [FeIII(OOH)(tpena)]+, the first with another H2O2 molecule to afford [FeIII(OH)(tpena)]+, O2•–, and HO• and the second, and dominant, with [FeIII(OH)(tpena)]+ to yield 2 equiv of [FeIV(O)(tpena)]+ and H2O, are found to be the major pathways for the formation of HO• and [FeIV(O)(tpena)]+, respectively. The production of HO• was quantified by a chemiluminescence method showing that [FeIV(O)(tpena)]+ is produced in much larger yields than HO•. The generation of HO• compromises the stability of [FeIII(OH)(tpena)]+ unless an external substrate is present that can outcompete [FeIII(OH)(tpena)]+ for HO•. Significantly, we demonstrate that the reaction commonly assumed to occur in the decay of nonheme iron(III)hydroperoxides, homolytic O–O bond cleavage, is of minor significance for the generation of HO• and the iron(IV)oxo complex. The production of both a reactive high-valent iron–oxo species and HO• under mild, aqueous ambient conditions represents a significant contribution to the current state of the art for biomimetic nonheme chemistry in water.