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 di. erences 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 di. erences 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(NH3)(4)OH](+/2+/3+) (1-3) and trans-[FeN(NH3) 4OH](0/2+/3+) (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 di. erence is likely due to di. erences in Z* between N and O that allow for better orbital overlap to occur in the case of the FeQN 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 Mossbauer parameters, g-tensors, zero-field splitting and iron hyper. ne couplings, X- ray absorption Fe K pre-edge spectra have been simulated using time- dependent DFT methods for the. rst time for a series of compounds spanning the highvalent 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(z2) orbital, which is able to mix with Fe 4(pz), allowing an e. cient mechanism for the intensi. cation of pre-edge features.