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 X(n-) which is either O(2-) or N(3-). 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 Fe=O or Fe=N multiple bonds have been probed computationally in this work in a series of hypothetical trans-FeO(NH(3))(4)OH (1-3) and trans-FeN(NH(3))(4)OH (4-6) complexes. All computational properties are permeated by the intrinsically more covalent character of the Fe=N multiple bond as compared to the Fe=O 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 Fe=N 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 Fe d(z(2)) orbital, which is able to mix with Fe 4p(z), allowing an efficient mechanism for the intensification of pre-edge features.