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.