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, ([Fe₂(μ-adt)(CO) ₄(PMe₃)₂] (1, adt = S-CH₂-(NCH ₂Ph)-CH₂-S), of the [FeFe] hydrogenase active site. For 1 in powder material (1_powder), in MeCN solution (1′), and in its three protonated states (1H, 1Hy, 1HHy; H denotes protonation at the adt-N and Hy 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 (1_CO, 1Hy_CO) and in addition of the phenyl ring (1H_CO,Ph, 1HHy_CO,Ph), 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^2,5 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 1_powder and ∼2.0 eV for 1′, and (3) the splitting between iron d(z²) and d(x ²-y²) 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 1Hy_CO or adt-N protonation for 1HHy _CO,Ph. XAES-DFT thus enables verification of the effects of ligand substitutions in solution for guided improvement of [FeFe] catalysts.