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Electronic structure of an [FeFe] hydrogenase model complex in solution revealed by X-ray absorption spectroscopy using narrow-band emission detection

Leidel, Nils ; Chernev, Petko ; Havelius, Kajsa G.V. LU ; Schwartz, Lennart ; Ott, Sascha and Haumann, Michael (2012) In Journal of the American Chemical Society 134(34). p.14142-14157
Abstract

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 (1powder), 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... (More)

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 (1powder), 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 (1CO, 1HyCO) and in addition of the phenyl ring (1HCO,Ph, 1HHyCO,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 (Kß2,5 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 1powder 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 1HyCO 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.

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publishing date
type
Contribution to journal
publication status
published
in
Journal of the American Chemical Society
volume
134
issue
34
pages
16 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • pmid:22860512
  • scopus:84865632756
ISSN
0002-7863
DOI
10.1021/ja304970p
language
English
LU publication?
no
id
62ef7a04-a9e9-4545-a0aa-a3bf766e917d
date added to LUP
2020-01-15 10:17:49
date last changed
2024-04-17 03:04:09
@article{62ef7a04-a9e9-4545-a0aa-a3bf766e917d,
  abstract     = {{<p>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)(CO) <sub>4</sub>(PMe<sub>3</sub>)<sub>2</sub>] (1, adt = S-CH<sub>2</sub>-(NCH <sub>2</sub>Ph)-CH<sub>2</sub>-S), of the [FeFe] hydrogenase active site. For 1 in powder material (1<sub>powder</sub>), 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<sub>CO</sub>, 1Hy<sub>CO</sub>) and in addition of the phenyl ring (1H<sub>CO,Ph</sub>, 1HHy<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 1<sub>powder</sub> and ∼2.0 eV for 1′, and (3) the splitting between iron d(z<sup>2</sup>) and d(x <sup>2</sup>-y<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 1Hy<sub>CO</sub> or adt-N protonation for 1HHy <sub>CO,Ph</sub>. XAES-DFT thus enables verification of the effects of ligand substitutions in solution for guided improvement of [FeFe] catalysts.</p>}},
  author       = {{Leidel, Nils and Chernev, Petko and Havelius, Kajsa G.V. and Schwartz, Lennart and Ott, Sascha and Haumann, Michael}},
  issn         = {{0002-7863}},
  language     = {{eng}},
  month        = {{08}},
  number       = {{34}},
  pages        = {{14142--14157}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Journal of the American Chemical Society}},
  title        = {{Electronic structure of an [FeFe] hydrogenase model complex in solution revealed by X-ray absorption spectroscopy using narrow-band emission detection}},
  url          = {{http://dx.doi.org/10.1021/ja304970p}},
  doi          = {{10.1021/ja304970p}},
  volume       = {{134}},
  year         = {{2012}},
}