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 H ₂O ₂ and presence or absence of sacrificial substrates. The outcome can be switched between: 1) catalysed H ₂O ₂ 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.