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{sup n-} which is either O{sup 2-} or N{sup 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{sub 3}){sub 4}OH]{sup +/2+/3+} (1-3) and trans-[FeN(NH{sub 3}){sub 4}OH]{sup 0/+/2+} (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 Moessbauer 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{sub z{sup 2}} orbital, which is able to mix with Fe 4p{sub z}, allowing an efficient mechanism for the intensification of pre-edge features.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 958606
- Report Number(s):
- SLAC-REPRINT-2009-082; TRN: US201001%%640
- Journal Information:
- Phys.Chem.Chem.Phys.10:4361,2008, Vol. 10, Issue 30; ISSN 1463-9076
- Country of Publication:
- United States
- Language:
- English
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