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 turquoise-colored intermediate [Fe(IV)(beta-BPMCN)(OO(t)Bu)(OH)](2+) (Tq; BPMCN = N,N'-bis(2-pyridylmethyl)-N,N'-dimethyl-trans-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 high-valent Fe intermediates in the mechanisms of oxygenase enzymes, we have explored the reactivity of Tq at -70 degrees 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 < or = approximately 80 kcal/mol. The Fe(IV)-OH unit of Tq, and not the alkylperoxo moiety, performs the initial H-atom abstraction. However at -45 degrees 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)(beta-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.