Spectroscopic and quantum chemical studies on low-spin Fe^IV=O complexes: Fe-O bonding and its contributions to reactivity

High-valent Fe^IV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes and have been structurally defined in model systems. Variable-temperature magnetic circular dichroism (VT-MCD) spectroscopy has been used to evaluate the electronic structures and in particular the Fe-O bonds of three Fe^IV=O (S = 1) model complexes, [Fe^IV(O)(TMC)(NCMe)]²⁺, [Fe ^IV(O)(TMC)(OC(O)CF₃)]⁺, and [Fe ^IV(O)(N4Py)]²⁺. These complexes are characterized by their strong and covalent Fe-O π-bonds. The MCD spectra show a vibronic progression in the nonbonding → π* excited state, providing the Fe-O stretching frequency and the Fe-O bond length in this excited state and quantifying the π-contribution to the total Fe-O bond. Correlation of these experimental data to reactivity shows that the [Fe^IV(O)(N4Py)] ²⁺ complex, with the highest reactivity toward hydrogen-atom abstraction among the three, has the strongest Fe-O π-bond. Density functional calculations were correlated to the data and support the experimental analysis. The strength and covalency of the Fe-O π-bond result in high oxygen character in the important frontier molecular orbitals (FMOs) for this reaction, the unoccupied β-spin d(xz/yz) orbitals, that activates these for electrophilic attack. An extension to biologically relevant Fe^IV=O (S = 2) enzyme intermediates shows that these can perform electrophilic attack reactions along the same mechanistic pathway (π-FMO pathway) with similar reactivity but also have an additional reaction channel involving the unoccupied α-spin d(z²) orbital (σ-FMO pathway). These studies experimentally probe the FMOs involved in the reactivity of Fe^IV=O (S = 1) model complexes resulting in a detailed understanding of the Fe-O bond and its contributions to reactivity.