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 H₂O₂ and presence or absence of sacrificial substrates. The outcome can be switched between: 1)catalysed H₂O₂ 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^IIIO-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.