Reactivities of Fe(IV) complexes with oxo, hydroxo, and alkylperoxo ligands: An experimental and computational study

In a previous paper [Jensen et al. J. Am. Chem. Soc. 2005, 127, 10512], we reported the synthesis of the turquoisecolored intermediate [Fe ^IV(βBPMCN)(OO^tBU)(OH)]²⁺ (Tq; BPMCN = N,N'-bis(2-pyridylmethyl)-N,N'-dimethyltrans-1,2-diaminocyclohexane). The structure of Tq is unprecedented, as it represents the only synthetic example to date of a nonheme Fe^IV complex with both alkylperoxo and hydroxide ligands. Given the significance of similar highvalent Fe intermediates in the mechanisms of oxygenase enzymes, we have explored the reactivity of Tq at -70°C, a temperature at which it is stable, and found that it is capable of activating weak X-H bonds (X = C, O) with bond dissociation energies ≤∼80 kcal/mol. The Fe^IV-OH unit of Tq, and not the alkylperoxo moiety, performs the initial H-atom abstraction. However at -45°C, Tq decays at a rate that is independent of substrate identity and concentration, forming a species capable of oxidizing substrates with stronger C-H bonds. Parallel reactivity studies were also conducted with the related oxoiron(IV) complexes [Fe^IV(β-BPMCN)(O)(X)]²⁺ (3-X; X = pyridine or nitrile), thereby permitting a direct comparison of the reactivity of Fe ^IV centers with oxo and hydroxide ligands. We found that the H-atom abstracting ability of the Fe^IV-O species greatly exceeds that of the Fe^IV-OH species, generally by greater than 100-fold. Examination of the electronic structures of Tq and 3-X with density functional theory (DFT) provides a rationale for their differing reactivities.