Heterolytic Cleavage of Peroxide by a Diferrous Compound Generates Metal‐Based Intermediates Identical to Those Observed with Reactions Utilizing Oxygen‐Atom‐Donor Molecules

Under cryogenic stopped-flow conditions, addition of 2-methyl-1-phenylprop-2-yl hydroperoxide (MPPH) to the diiron(II) compound, [Fe(2)(H(2)Hbamb)(2)(NMeIm)(2)] (1; NMeIm=N-methylimidazole; H(4)HBamb: 2,3-bis(2-hydroxybenzamido)dimethylbutane) results in heterolytic peroxide O-O bond cleavage, forming a high-valent species, 2. The UV/Vis spectrum of 2 and its kinetic behavior suggest parallel reactivity to that seen in the reaction of 1 with oxygen-atom-donor (OAD) molecules, which has been reported previously. Like the interaction with OAD molecules, the reaction of 1 with MPPH proceeds through a three step process, assigned to oxygen-atom transfer to the iron center to form a high-valent intermediate (2), ligand rearrangement of the metal complex, and, finally, decay to a diferric mu-oxo compound. Careful examination of the order of the reaction with MPPH reveals saturation behavior. This, coupled with the anomalous non-Arrhenius behavior of the first step of the reaction, indicates that there is a preequilibrium peroxide binding step prior to O-O bond cleavage. At higher temperatures, the addition of the base, proton sponge, results in a marked decrease in the rate of O-O bond cleavage to form 2; this is assigned as a peroxide deprotonation effect, indicating that the presence of protons is an important factor in the heterolytic cleavage of peroxide. This phenomenon has been observed in other iron-containing enzymes, the catalytic cycles of which include peroxide O-O bond cleavage.