Bridging-hydride influence on the electronic structure of an [FeFe] hydrogenase active-site model complex revealed by XAES-DFT

Abstract

Two crystallized [FeFe] hydrogenase model complexes, 1 = (μ-pdt)[Fe(CO)(2)(PMe(3))](2) (pdt = SC1H2C2H2C3H2S), and their bridging-hydride (Hy) derivative, [1Hy](+) = [(μ-H)(μ-pdt)[Fe(CO)(2) (PMe(3))](2)](+) (BF(4)(−)), were studied by Fe K-edge X-ray absorption and emission spectroscopy, supported by density functional theory. Structural changes in [1Hy](+) compared to 1 involved small bond elongations (<0.03 Å) and more octahedral Fe geometries; the Fe–H bond at Fe1 (closer to pdt-C2) was ~0.03 Å longer than that at Fe2. Analyses of (1) pre-edge absorption spectra (core-to-valence transitions), (2) Kβ(1,3), Kβ', and Kβ(2,5) emission spectra (valence-to-core transitions), and (3) resonant inelastic X-ray scattering data (valence-to-valence transitions) for resonant and non-resonant excitation and respective spectral simulations indicated the following: (1) the mean Fe oxidation state was similar in both complexes, due to electron density transfer from the ligands to Hy in [1Hy](+). Fe 1s→3d transitions remained at similar energies whereas delocalization of carbonyl AOs onto Fe and significant Hy-contributions to MOs caused an ~0.7 eV up-shift of Fe1s→(CO)s,p transitions in [1Hy](+). Fed-levels were delocalized over Fe1 and Fe2 and degeneracies biased to O(h)–Fe1 and C(4v)–Fe2 states for 1, but to O(h)–Fe1,2 states for [1Hy](+). (2) Electron-pairing of formal Fe(d(7)) ions in low-spin states in both complexes and a higher effective spin count for [1Hy](+) were suggested by comparison with iron reference compounds. Electronic decays from Fe d and ligand s,p MOs and spectral contributions from Hys,p→1s transitions even revealed limited site-selectivity for detection of Fe1 or Fe2 in [1Hy](+). The HOMO/LUMO energy gap for 1 was estimated as 3.0 ± 0.5 eV. (3) For [1Hy](+) compared to 1, increased Fed (x(2) − y(2)) − (z(2)) energy differences (~0.5 eV to ~0.9 eV) and Fed→d transition energies (~2.9 eV to ~3.7 eV) were assigned. These results reveal the specific impact of Hy-binding on the electronic structure of diiron compounds and provide guidelines for a directed search of hydride species in hydrogenases.

Electrochemical investigation into the electron transfer mechanism of a diiron hexacarbonyl complex bearing a bridging naphthalene moiety

“…Complex 1 in reduced and neutral states were fully optimized in gas phase without any symmetry constraints at the (U)BP86/ TZVP level of theory [26,27], which has been proved to be suitable for investigating [FeFe]-hydrogenase models [14,28]. The atomic coordinates of complex 1 in neutral state are derived from the crystal structure and for complex 1 in all different reduced states, the initial geometries were approximately established using crystal structure parameters of complex 1.…”

Electronic and molecular structures of the active-site H-cluster in [FeFe]-hydrogenase determined by site-selective X-ray spectroscopy and quantum chemical calculations

“…X-Ray absorption and emission spectroscopy was carried out at the undulator beamline ID26 of the European Synchrotron Radiation Facility (ESRF) at Grenoble (France) as previously described. [72][73][74] Samples were held in a laboratory-built liquid-He cryostat at 20 K. The incident energy was set by an Si[311] double-crystal monochromator (energy bandwidth $0.2 eV at the Fe K-edge). Conventional Ka-detected XAS spectra were collected using a scintillation detector 80 ($20 cm 2 area, placed at 90 to the incident X-ray beam and at $1 m to the sample), which was shielded by 10 mm Mn foil against scattered incident X-rays.…”

A Practical Guide to High‐resolution X‐ray Spectroscopic Measurements and their Applications in Bioinorganic Chemistry

“…For example, resonant excitation into a pre‐edge transition, corresponding to a 1s to 3d excitation, changes the number of 3d electrons, and alters the multiplet structure of the resultant emission. We note, however, that a detailed, quantitative description of resonant XES is still emerging, and while two‐dimensional resonant XES/RIXS data have been reported for several bioinorganic systems, including the oxygen‐evolving complex of photosystem II and for iron hydrogenases,, the added information content from these experiments has been, in our view, somewhat limited.…”