A Switch from Mechanistic Competition Mediated by a Combination of Temperature and Concentration Effects in the Oxidation Reaction of [Fe^II(N4Py/TPA)](OTf)₂

Abstract

The formation of [(N4Py)Fe =O] species was accomplished by the reaction of [Fe (N4Py)] with 20 equivalents of tBuO H (TBHP, 70 % in H O). The temperature, [Fe (N4Py)] -concentration and H O-concentration in anhydrous TBHP (5.5 m in decane) dependences of its yields and rates were analyzed to indicate that the proton migration from [(N4Py)Fe -HOOtBu] to [(N4Py)Fe -OO HtBu] is the rate-determining step followed by rapid heterolytic O-O bond cleavage of Fe -OO HtBu to Fe =O complex. The formation of [(TPA)Fe =O] is thus revealed to be greatly enhanced by the similar oxidation of [Fe (TPA)] (40 mm) with 10 equivalents of tBuO H at -45 °C. These results demonstrate the heterolytic O-O bond cleavage of Fe -alkylperoxo species to form Fe =O originating from the direct reaction of iron(II) complexes/TBHP. The observation of concentration and temperature effects leads to the hypothesis that O-O bond homolysis is a kinetic control pathway and O-O bond heterolysis is a thermodynamic control pathway.

Inspiration from Nature: Influence of Engineered Ligand Scaffolds and Auxiliary Factors on the Reactivity of Biomimetic Oxidants

“…92,121,321−328 In addition, subsequent studies on nonheme iron complexes investigated the environmental effects such as solvent and temperature on structure and reactivity. 329,330 Early DFT studies conducted by de Visser, Shaik, and their co-workers suggested that the chemoselectivity originates from the activation energy (E a ) for epoxidation and allylic hydroxylation of iron(IV)oxo-porphyrin π-cation radical 46,331 and that the enthalpy of activation (ΔH ‡ ) dominates over the entropy of activation (ΔS ‡ ) parameter. Moreover, a strong effect on the regioselectivity on the choice of the axial ligand and polarity of the solvent was found.…”

“…As discussed above, the cytochromes P450 form a reactive Cpd I species in their catalytic cycle that performs the oxygen atom transfer reaction to substrates. Interestingly, the reaction of P450 with cyclohexene produces both an epoxidation product and an allylic hydroxylation product. ,, Synthetic iron-porphyrin models have been studied over the years to understand the chemoselectivity of these enzymatic systems which was found to be dependent on external factors including the choice of axial ligands, solvated ions, solvents, and temperature. ,,− In addition, subsequent studies on nonheme iron complexes investigated the environmental effects such as solvent and temperature on structure and reactivity. , Early DFT studies conducted by de Visser, Shaik, and their co-workers suggested that the chemoselectivity originates from the activation energy ( E a ) for epoxidation and allylic hydroxylation of iron­(IV)­oxo-porphyrin π-cation radical , and that the enthalpy of activation (Δ H ‡ ) dominates over the entropy of activation (Δ S ‡ ) parameter. Moreover, a strong effect on the regioselectivity on the choice of the axial ligand and polarity of the solvent was found.…”