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Keywords:

  • homogeneous catalysis;
  • hydrogen peroxide;
  • iron;
  • O[BOND]O bond homolysis;
  • oxidation;
  • oxo ligands

Abstract

The reaction of [FeII(tpa)(OTf)2] (tpa=tris(2-pyridylmethyl)amine) and its related 5-Me3-tpa complex with hydrogen peroxide affords spectroscopically distinct iron(III)–peroxo intermediates in CH3CN and acetone. The reaction in acetonitrile at −40 °C results in the formation of the previously reported FeIII–OOH intermediate, the end-on hydroperoxo coordination mode of which is established in this paper by detailed resonance Raman isotope-labeling experiments. On the other hand, the reaction in acetone below −40 °C leads to the observation of a different peroxo intermediate identified by resonance Raman spectroscopy to be an FeIII–OOC (CH3)2OH species; this represents the first example of an intermediate derived from the adduct of H2O2 and acetone. The peroxoacetone intermediate decays more rapidly than the corresponding FeIII–OOH species and converts to an FeIV[DOUBLE BOND]O species by O[BOND]O bond homolysis. This decay process is analogous to that observed for [FeIII(tpa)(OOtBu)]2+ and in fact exhibits a comparable enthalpy of activation of 54(3) kJ mol−1. Thus, with respect to their physical properties at low temperature, the peroxoacetone intermediate resembles [FeIII(tpa)(OOtBu)]2+ more than the corresponding FeIII–OOH species. At room temperature, however, the behavior of the Fe(tpa)/H2O2 combination in acetone in catalytic hydrocarbon oxidations differs significantly from that of the Fe(tpa)/tBuOOH combination and more closely matches that of the Fe(tpa)/H2O2 combination in CH3CN. Like the latter, the Fe(tpa)/H2O2 combination in acetone catalyzes the hydroxylation of cis-1,2-dimethylcyclohexane to its tertiary alcohol with high stereoselectivity and carries out the epoxidation and cis-dihydroxylation of olefins. These results demonstrate the subtle complexity of the Fe(tpa)/H2O2 reaction surface.