Electronic structure and spectroscopy of “superoxidized” iron centers in model systems: theoretical and experimental trends

Recent advances in synthetic chemistry have led to the discovery of "superoxidized" iron centers with valencies Fe(V) and Fe(VI) [K. Meyer et al., J. Am. Chem. Soc., 1999, 121, 4859–4876; J. F. Berry et al., Science, 2006, 312, 1937–1941; F. T. de Oliveira et al., Science, 2007, 315, 835–838.]. Furthermore, in recent years a number of high-valent Fe(IV) species have been found as reaction intermediates in metalloenzymes and have also been characterized in model systems [C. Krebs et al., Acc. Chem. Res., 2007, 40, 484–492; L. Que, Jr, Acc. Chem. Res., 2007, 40, 493–500.]. These species are almost invariably stabilized by a highly basic ligand Xn− which is either O2− or N3−. The differences in structure and bonding between oxo- and nitrido species as a function of oxidation state and their consequences on the observable spectroscopic properties have never been carefully assessed. Hence, fundamental differences between high-valent iron complexes having either FeO or FeN multiple bonds have been probed computationally in this work in a series of hypothetical trans-[FeO(NH3)4OH]+/2+/3+ (1–3) and trans-[FeN(NH3)4OH]0/+/2+ (4–6) complexes. All computational properties are permeated by the intrinsically more covalent character of the FeN multiple bond as compared to the FeO bond. This difference is likely due to differences in Z* between N and O that allow for better orbital overlap to occur in the case of the FeN multiple bond. Spin-state energetics were addressed using elaborate multireference ab initio computations that show that all species 1–6 have an intrinsic preference for the low-spin state, except in the case of 1 in which S = 1 and S = 2 states are very close in energy. In addition to Mössbauer parameters, g-tensors, zero-field splitting and iron hyperfine couplings, X-ray absorption Fe K pre-edge spectra have been simulated using time-dependent DFT methods for the first time for a series of compounds spanning the high-valent states +4, +5, and +6 for iron. A remarkably good correlation of these simulated pre-edge features with experimental data on isolated high-valent intermediates has been found, allowing us to assign the main pre-edge features to excitations into the empty Fedz2 orbital, which is able to mix with Fe 4pz, allowing an efficient mechanism for the intensification of pre-edge features.