Characterization of a Genuine S=1/2
Fe(V) Complex

Núria Aliaga-Alcalde,1 Serena DeBeer George,2 Bernd Mienert,1 Eckhard Bill,1 Karl Wieghardt1 and Frank Neese1

1Max Planck Institut für Bioanorganische Chemie, Mülheim an der Ruhr, Germany
2Stanford Synchrotron Radiation Laboratory, SLAC, Stanford University, Stanford, CA

High-valent Fe(IV) and Fe(V) intermediates are invoked in the catalytic mechanisms of both heme and non-heme iron enzymes.[1, 2] Over the last several years, many Fe(IV)-oxo complexes have been synthesized and spectroscopically

Figure 1. Fe K-edge XAS spectra of the Fe(III)-azide precursor (black) and the Fe(V)-nitrido complex (red).

characterized, providing important markers for the characterization of Fe(IV) species.[3] In contrast, much less is known about Fe(V) species.

Recently, Frank Neese, Karl Wieghardt and co-workers have examined an Fe(V)-nitrido complex, which is generated by photolysis of an Fe(III)-azide precursor.[4] Mössbauer parameters for the Fe-nitrido complex are consistent with the an Fe(V) oxidation state assignment. However, further experimental data were needed to corroborate this result. For this reason, Fe K-edge X-ray Absorption (XAS) spectroscopy was used, in conjunction with electronic structure calculations, to obtain further insight into the Fe(V) complex.

The Fe K-edge is sensitive to changes in the effective nuclear charge on the metal, showing an ~1 eV increase in the edge per oxidation state. Comparison of the Fe(V)-nitrido complex to the Fe(III) precursor shows an ~2 eV shift, consistent with the formulation of this complex as an Fe(V) species. The appearance of the pre-edge feature at 7114.2 eV (using a reference calibration point of 7111.2 eV for the first inflection point of an Fe foil) is ~1 eV higher than that of reported Fe(IV) species and provides an important reference for future XAS characterization of Fe(V). In addition, there is a dramatic increase in pre-edge intensity, consistent with the formation of a much shorter Fe-N bond in the nitrido species. EXAFS data were used to determine the first shell distances for the Fe(III) precursor and the Fe(V)-nitrido complex. It was found that the Fe(V) complex contained a short 1.60 Å Fe-N component, which was not present in the precursor.

Density functional theory (DFT) calculations on the Fe(V) species were carried out, allowing for either a doublet (S=1/2) or quartet (S=3/2) ground state. As the Fe(V) species is d3 and isoelectronic with Cr(III) and Mn(IV), an S=3/2 ground state would be expected. Surprisingly, the results favor an unprecedented doublet ground state. In addition, the calculations show a rather large difference in the short Fe-N bond distance, giving optimized Fe-N bond distances of 1.61 Å and 1.74 Å for the doublet and quartet states, respectively. Hence the computational results, coupled with the EXAFS analysis, support a doublet ground state in the Fe(V) species.

The combined experimental and computational results provide strong evidence for the formulation of the Fe-nitrido complex as a genuine S=1/2 Fe(V) complex. This study also provides important experimental markers for the characterization of other Fe(V) species, including high-valent iron intermediates within protein systems .

Primary Citation:
N. Aliaga-Alcalde, S. DeBeer George, B. Mienert, E. Bill, K. Wieghardt, F. Neese, Angew. Chem. Int. Ed. 2005, 44, 2908-2912.

  1. L. D. Slep, F. Neese, Angew. Chem. Int. Ed. 2003, 42, 2942 and references therein.
  2. a) M. Costas, M. P. Mehn, M. P. Jensen, L. Que Jr., Chem. Rev. 2004, 104, 939; b) D. L. Harris, Curr. Op. Chem. Biol. 2001, 5, 724.
  3. a) J.-U. Rohde, J.-H. In, M. H. Lim, W. W. Brennessel, M. R. Bukowski, A. Stubna, E. Münck, W. Nam, L. Que Jr., Science 2003, 299, 1037; b) A. Decker, J.-U. Rhode, L. Que, Jr., E. I. Solomon, J. Am. Chem. Soc. 2004, 126, 5378.
  4. N. Aliaga-Alcalde, S. DeBeer George, B. Mienert, E. Bill, K. Wieghardt, F. Neese, Angew. Chem. Int. Ed. 2005, 44, 2908-2912.

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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: 13 JUN 2005
Content Owner: Serena DeBeer George and Frank Neese
Page Editor: Lisa Dunn