What Singles out the FeO2+ Moiety? A Density-Functional Theory Study of the Methane-to-Methanol Reaction Catalyzed by the First Row Transition-Metal Oxide Dications MO(H2O)p2+, M = V−Cu

Steric hindrance effect of the equatorial ligand on Fe(IV)O and Ru(IV)O complexes: a density functional study

The geometric structures and mechanisms for hydrogen abstraction from cyclohexane for four high-valent complexes, [Fe IV(O)(TMC)(NCMe)]2+ (where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane; 1-NCMe), the inverted isomer [Fe IV(NCMe)(TMC)(O)]2+ (2-NCMe), [Ru IV(O)(TMC)(NCMe)]2+ (the ruthenium analogue of 1-NCMe; 3-NCMe), and the inverted isomer [Ru IV(NCMe)(TMC)(O)]2+ (4-NCMe), were investigated using density functional theory. The axial NCMe ligand was found to be sterically more hindered in 2-NCMe than in 1-NCMe, which is in accord with the calculated results that the Fe-L axial distance is longer in the former. Both 1-NCMe and 2-NCMe are capable of hydrogen abstraction from cyclohexane via two-state reactivity patterns. In contrast, 3-NCMe and 4-NCMe react with cyclohexane by a single-state mechanism. The reaction pathways computed reveal that 2-NCMe is more reactive than 1-NCMe, in agreement with experimental results, whereas the reactivity of 3-NCMe and 4-NCMe shows little dependence on whether the oxo unit is syn or anti to the four N-methyl groups. Our analysis shows that along the reaction pathway for 2-NCMe in the triplet spin state, the NCMe ligand moves away from the iron center, and therefore the energy of the sigma *2 z (alpha-spin) orbital decreases and an electron is transferred to this orbital. Finally, we calculated the kinetic isotope effect and investigated the relationship between this effect and reaction barriers.

Million-fold activation of the [Fe(2)(micro-O)(2)] diamond core for C-H bond cleavage

In biological systems, the cleavage of strong C–H bonds is often carried out by iron centers – such as the methane monooxygenase in methane hydroxylation – through dioxygen activation mechanisms. High valent species with [Fe₂(μ-O)₂] diamond cores are thought to act as the oxidizing moieties, but the synthesis of complexes that cleave strong C–H bonds efficiently has remained a challenge. We report here the conversion of a synthetic complex with a valence-delocalized [Fe^3.5(μ-O)₂Fe^3.5]³⁺ diamond core (1) into a complex with a valence-localized [HO-Fe^III-O-Fe^IV=O]²⁺ open core (4), which cleaves C–H bonds over million-fold faster. This activity enhancement results from three factors: the formation of a terminal oxoiron(IV) moiety, the conversion of the low-spin (S = 1) Fe^IV=O center to a high-spin (S = 2) center, and the concentration of the oxidizing capability to the active terminal oxoiron(IV) moiety. This suggests that similar isomerization strategies might be employed by nonheme diiron enzymes.

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.