Directing a Non-Heme Iron(III)-Hydroperoxide Species on a Trifurcated Reactivity Pathway

The reactivity of [Fe^III (tpena)]²⁺ (tpena=N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate) as a catalyst for oxidation reactions depends on its ratio to the terminal oxidant H₂ O₂ and presence or absence of sacrificial substrates. The outcome can be switched between: 1) catalysed H₂ O₂ disproportionation, 2) selective catalytic oxidation of methanol or benzyl alcohol to the corresponding aldehyde, or 3) oxidative decomposition of the tpena ligand. A common mechanism is proposed involving homolytic O-O cleavage in the detected transient purple low-spin (S=1/2 ) [(tpenaH)Fe^III O-OH]²⁺ . The resultant iron(IV) oxo and hydroxyl radical both participate in controllable hydrogen-atom transfer (HAT) reactions. Consistent with the presence of a weaker σ-donor carboxylate ligand, the most pronounced difference in the spectroscopic properties of [Fe(OOH)(tpenaH)]²⁺ and its conjugate base, [Fe(OO)(tpenaH)]⁺ , compared to non-heme iron(III) peroxide analogues supported by neutral multidentate N-only ligands, are slightly blue-shifted maxima of the visible absorption band assigned to ligand-to-metal charge-transfer (LMCT) transitions and, corroborating this, lower Fe^III /Fe^II redox potentials for the pro-catalysts.