Spectroscopic and Quantum Chemical Studies on low-spin FeIV=O complexes: Fe-O bonding and its contributions to reactivity

High-valent FeIVO 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 FeIVO (S = 1) model complexes, [FeIV(O)(TMC)(NCMe)]2+, [FeIV(O)(TMC)(OC(O)CF3)]+, and [FeIV(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 [FeIV(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 th...