1 research outputs found
Electronic Structure of an [FeFe] Hydrogenase Model Complex in Solution Revealed by X‑ray Absorption Spectroscopy Using Narrow-Band Emission Detection
High-resolution X-ray absorption spectroscopy with narrow-band
X-ray emission detection, supported by density functional theory calculations
(XAES-DFT), was used to study a model complex, ([Fe<sub>2</sub>(μ-adt)Â(CO)<sub>4</sub>(PMe<sub>3</sub>)<sub>2</sub>] (<b>1</b>, adt = S–CH<sub>2</sub>–(NCH<sub>2</sub>Ph)–CH<sub>2</sub>–S),
of the [FeFe] hydrogenase active site. For <b>1</b> in powder
material (<b>1</b><sub>powder</sub>), in MeCN solution (<b>1</b>′), and in its three protonated states (<b>1H</b>, <b>1Hy</b>, <b>1HHy</b>; <b>H</b> denotes protonation
at the adt–N and <b>Hy</b> protonation of the Fe–Fe
bond to form a bridging metal hydride), relations between the molecular
structures and the electronic configurations were determined. EXAFS
analysis and DFT geometry optimization suggested prevailing rotational
isomers in MeCN, which were similar to the crystal structure or exhibited
rotation of the (CO) ligands at Fe1 (<b>1</b><sub>CO</sub>, <b>1Hy</b><sub>CO</sub>) and in addition of the phenyl ring (<b>1H</b><sub>CO,Ph</sub>, <b>1HHy</b><sub>CO,Ph</sub>), leading
to an elongated solvent-exposed Fe–Fe bond. Isomer formation,
adt–N protonation, and hydride binding caused spectral changes
of core-to-valence (pre-edge of the Fe K-shell absorption) and of
valence-to-core (Kß<sup>2,5</sup> emission) electronic transitions,
and of Kα RIXS data, which were quantitatively reproduced by
DFT. The study reveals (1) the composition of molecular orbitals,
for example, with dominant Fe-d character, showing variations in symmetry
and apparent oxidation state at the two Fe ions and a drop in MO energies
by ∼1 eV upon each protonation step, (2) the HOMO–LUMO
energy gaps, of ∼2.3 eV for <b>1</b><sub>powder</sub> and ∼2.0 eV for <b>1</b>′, and (3) the splitting
between iron dÂ(<i>z</i><sup>2</sup>) and dÂ(<i>x</i><sup>2</sup>–<i>y</i><sup>2</sup>) levels of ∼0.5
eV for the nonhydride and ∼0.9 eV for the hydride states. Good
correlations of reduction potentials to LUMO energies and oxidation
potentials to HOMO energies were obtained. Two routes of facilitated
bridging hydride binding thereby are suggested, involving ligand rotation
at Fe1 for <b>1Hy</b><sub>CO</sub> or adt–N protonation
for <b>1HHy</b><sub>CO,Ph</sub>. XAES-DFT thus enables verification
of the effects of ligand substitutions in solution for guided improvement
of [FeFe] catalysts