¹ Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany
² Stanford Synchrotron Radiation Laboratory, SLAC, Stanford University, Stanford, CA 94309, USA

^* Present address: The University of Wisconsin - Madison, Department of Chemistry, 1101 University Ave. Madison, WI 53706-1322, USA
⁺ Present address: Institut für Physikalische und Theoretische Chemie, Universität Bonn, D-53115 Bonn, Germany

Iron is the most abundant transition element on earth, and is typically found in formal oxidation states of either II or III. However, high valent Fe(IV) and Fe(V) complexes are invoked in the mechanisms of both heme and non-heme enzymes; and Fe(VI) is known to exist in the mineral ferrate.[1] Ferrate is a powerful oxidant, which has been used in soil and wastewater treatment, batteries, and disinfectants; however, it is unstable and often indiscriminately reactive. This has driven chemists to try and synthesize other hexavalent iron species, but until recently this was not possible. In the June 30 issue of Science, John Berry and co-workers reported the synthesis and characterization of the second known compound of Fe(VI) [2].

	Figure 1. Proposed Fe(VI)-nitrido photolysis product (left). Comparison of normalized Fe K-edge XAS spectra of Fe(V) and Fe(VI)-nitrido complexes.

Using an Fe(IV)-azide(Me₃-cyclam-acetato) complex, an Fe(VI) species can be generated by photolysis with 650 nm light, producing a formally Fe(VI)-nitrido complex. This photolysis product has been investigated through detailed spectroscopic studies, combined with DFT calculations. X-ray absorption spectroscopic measurements (conducted at SSRL BL9-3) were key in establishing that this complex is a genuine Fe(VI) complex with a very short (1.57 Å) Fe-N(nitrido) bond. The XAS edge energy increases as the effective nuclear charge on the Fe complex increases. This results in an ~1 eV shift in edge energy for the Fe(VI) complex, as compared to a previously reported Fe(V) complex (Figure 1) [3]. The EXAFS data provide local structural information, providing an experimental measure of the Fe-N(nitrido) bond length.

The results of these experiments provide important spectroscopic markers for high-valent Fe species and should encourage the consideration of high valent intermediates in biological systems, where our proposals are often dictated by what can be synthesized in small molecule systems.

Primary Citation

J. F. Berry, E. Bill, E. Bothe, S. DeBeer George, B. Mienert, B. F. Neese, K. Wieghardt, Science 2006, 312, 1937.

References

a) L. D. Slep, F. Neese, Angew. Chem. Int. Ed. 2003, 42, 2942 and references therein. b) M. Costas, M. P. Mehn, M. P. Jensen, L. Que Jr., Chem. Rev. 2004, 104, 939; c) D. L. Harris, Curr. Op. Chem. Biol. 2001, 5, 724. J. F. Berry, E. Bill, E. Bothe, S. DeBeer George, B. Mienert, B. F. Neese, K. Wieghardt, Science 2006, 312, 1937. N. Aliaga-Alcalde, S. DeBeer George, B. Mienert, E. Bill, K. Wieghardt, F. Neese, Angew. Chem. Int. Ed. 2005, 44, 2908.

| PDF Version |     | Lay Summary |     | Highlights Archive |   SSRL is supported by the Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program, and the National Institute of General Medical Sciences.

 

Last Updated:	20 JULY 2006
Content Owner:	Serena DeBeer George, SSRL
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