Multi-edge X-ray Absorption Spectroscopy. 1. X-ray Absorption near-Edge Structure Analysis of a Biomimetic Model of FeFe-Hydrogenase

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• 作者：Logan J. Giles ; Alexios Grigoropoulos ; Robert K. Szilagyi
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：December 20, 2012
• 年：2012
• 卷：116
• 期：50
• 页码：12280-12298
• 全文大小：963K
• 年卷期：v.116,no.50(December 20, 2012)
• ISSN：1520-5215

文摘

In this work, we demonstrate the potential of multi-edge X-ray absorption near-edge structure (XANES) analysis in completely defining the ground state electronic structure of a prototypical biomimetic complex of the [2Fe]-subcluster of the catalytic H-cluster of FeFe-hydrogenase. The spectral features at the ionization thresholds for Fe, S, C, and O 1s (K-edge) and Fe 2p (L-edge) core electrons were considered simultaneously to obtain the atomic compositions of the unoccupied frontier molecular orbitals. A systematic error analysis was carried out at the most informative S K-edge for spectra collected by multiple detection methods, at various data collection temperatures, and different sample preparation protocols. As expected for the difference in bonding between bridging and terminal Fe鈥揝(thiolate) coordination, the Fe鈥揝 bond is more covalent in the [2Fe]-biomimetic complex with formally iron(I) centers (36 卤 2% S character per Fe鈥揝 bond) than in the previously described [2Fe鈥?S] clusters (25 卤 3% S character per Fe鈥揝 bond) with formally iron(III) centers. An electron hole-based analysis of the pre-edge features at Fe K-, Fe L-, and S K-edges experimentally defines the composition of the first three frontier unoccupied molecular orbitals to contain 4% Fe 4p, 44% Fe 3d, and 24% S 3p contributions per electron hole, respectively. The complementary CO ligand contribution thus can be defined as 28% per electron hole. These experimental orbital covalency values are important in rationalizing redox properties, electrophilicity of the metals, or nucleophilicity of the ligands, and critically evaluating the absolute accuracy of electronic structure calculations.