Spectroscopic and Quantum Chemical Studies on Low-Spin FeIVO Complexes: Fe−O Bonding and Its Contributions to Reactivity

Abstract

High-valent Fe^IVO 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^IVO (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^IVO (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^IVO (S = 1) model complexes resulting in a detailed understanding of the Fe−O bond and its contributions to reactivity.