Redox Potential and C−H Bond Cleaving Properties of a Nonheme FeIV═O Complex in Aqueous Solution

摘要

Abstract: High-valent iron oxo intermediates have been identified as the key oxidants in the catalyticncycles of many nonheme enzymes. Among the large number of synthetic FeIVdO complexesncharacterized to date, [FeIV(O)(N4Py)]n2 (1) exhibits the unique combination of thermodynamic stability,nallowing its structural characterization by X-ray crystallography, and oxidative reactivity sufficient toncleave C H bonds as strong as those in cyclohexane (DC H 99.3 kcal moln1). However, its redoxnproperties are not yet well understood. In this work, the effect of protons on the redox properties of 1nhas been investigated electrochemically in nonaqueous and aqueous solutions. While the cyclicnvoltammetry of 1 in CH3CN is complicated by coupling of several chemical and redox processes, thenFeIV/IIIncouple is reversible in aqueous solution with E1/2 0.41 V versus SCE at pH 4 and involvesnthe transfer of one electron and one proton to give the FeIIInOH species. This is in fact the first examplenof reversible electrochemistry to be observed for this family of nonheme oxoiron (IV) complexes. C Hnbond oxidations by 1 have been studied in H2O and found to have reaction rates that depend on thenC H bond strength but not on the solvent. Furthermore, our electrochemical results have allowed anDO H value of 78(2) kcal moln1 to be calculated for the FeIIInOH unit derived from 1. Interestingly,nalthough this DO H value is 6 11 kcal moln1 lower than those corresponding to oxidants such asn[FeIV(O)(TMP)] (TMP tetramesitylporphinate), [RuIV(O)(bpy)2(py)]n2 (bpy bipyridine, py pyridine),nand the tert-butylperoxyl radical, the oxidation of dihydroanthracene by 1 occurs at a rate comparablento rates for these other oxidants. This comparison suggests that the nonheme N4Py ligand environmentnconfers a kinetic advantage over the others that enhances the C H bond cleavage ability of 1.