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
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).
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:
References:
|
Last Updated: | 13 JUN 2005 |
Content Owner: | Serena DeBeer George and Frank Neese |
Page Editor: | Lisa Dunn |