Issue 21, 2013

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(PMe3)]2 (pdt = SC1H2C2H2C3H2S), and their bridging-hydride (Hy) derivative, [1Hy]+++ = [(μ-H)(μ-pdt)[Fe(CO)2 (PMe3)]2]+ (BF4), 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) Kβ1,3, Kβ′, and Kβ2,5 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 Oh–Fe1 and C4v–Fe2 states for 1, but to Oh–Fe1,2 states for [1Hy]+++. (2) Electron-pairing of formal Fe(d7) 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 (x2y2) − (z2) 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.

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

Supplementary files

Article information

Article type
Paper
Submitted
19 Dec 2012
Accepted
11 Feb 2013
First published
12 Feb 2013

Dalton Trans., 2013,42, 7539-7554

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

N. Leidel, C. Hsieh, P. Chernev, K. G. V. Sigfridsson, M. Y. Darensbourg and M. Haumann, Dalton Trans., 2013, 42, 7539 DOI: 10.1039/C3DT33042G

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