Site-Selective X-ray Spectroscopy
on an Asymmetric
Model Complex of the [FeFe] Hydrogenase Active Site
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Abstract
The active site for hydrogen production in [FeFe] hydrogenase
comprises
a diiron unit. Bioinorganic chemistry has modeled important features
of this center, aiming at mechanistic understanding and the development
of novel catalysts. However, new assays are required for analyzing
the effects of ligand variations at the metal ions. By high-resolution
X-ray absorption spectroscopy with narrow-band X-ray emission detection
(XAS/XES = XAES) and density functional theory (DFT), we studied an
asymmetrically coordinated [FeFe] model complex, [(CO)<sub>3</sub>Fe<sup>I</sup>1-(bdtCl<sub>2</sub>)-Fe<sup>I</sup>2(CO)(Ph<sub>2</sub>P–CH<sub>2</sub>–NCH<sub>3</sub>–CH<sub>2</sub>–PPh<sub>2</sub>)] (<b>1</b>, bdt = benzene-1,2-dithiolate),
in comparison to iron–carbonyl references. Kβ emission
spectra (Kβ<sup>1,3</sup>, Kβ′) revealed the absence
of unpaired spins and the low-spin character for both Fe ions in <b>1</b>. In a series of low-spin iron compounds, the Kβ<sup>1,3</sup> energy did not reflect the formal iron oxidation state,
but it decreases with increasing ligand field strength due to shorter
iron-ligand bonds, following the spectrochemical series. The intensity
of the valence-to-core transitions (Kβ<sup>2,5</sup>) decreases
for increasing Fe-ligand bond length, certain emission peaks allow
counting of Fe-CO bonds, and even molecular orbitals (MOs) located
on the metal-bridging bdt group of <b>1</b> contribute to the
spectra. As deduced from 3d → 1s emission and 1s → 3d
absorption spectra and supported by DFT, the HOMO–LUMO gap
of <b>1</b> is about 2.8 eV. Kβ-detected XANES spectra
in agreement with DFT revealed considerable electronic asymmetry in <b>1</b>; the energies and occupancies of Fe-d dominated MOs resemble
a square-pyramidal Fe(0) for Fe1 and an octahedral Fe(II) for Fe2.
EXAFS spectra for various Kβ emission energies showed considerable
site-selectivity; approximate structural parameters similar to the
crystal structure could be determined for the two individual iron
atoms of <b>1</b> in powder samples. These results suggest that
metal site- and spin-selective XAES on [FeFe] hydrogenase protein
and active site models may provide a powerful tool to study intermediates
under reaction conditions
