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

The formation of [(N4Py)FeIV =O]2+ species was accomplished by the reaction of [FeII (N4Py)]2+ with 20 equivalents of tBuO2 H (TBHP, 70 % in H2 O). The temperature, [FeII (N4Py)]2+ -concentration and H2 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)FeII -HOOtBu]2+ to [(N4Py)FeII -OO⊕ HtBu]2+ is the rate-determining step followed by rapid heterolytic O-O bond cleavage of FeII -OO⊕ HtBu to FeIV =O complex. The formation of [(TPA)FeIV =O]2+ is thus revealed to be greatly enhanced by the similar oxidation of [FeII (TPA)]2+ (40 mm) with 10 equivalents of tBuO2 H at -45 °C. These results demonstrate the heterolytic O-O bond cleavage of FeII -alkylperoxo species to form FeIV =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.