Models of the Ni-L and Ni-SIa States of the [NiFe]-Hydrogenase Active Site

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• 作者：Geoffrey M. Chambers ; Mioy T. Huynh ; Yulong Li ; Sharon Hammes-Schiffer ; Thomas B. Rauchfuss ; Edward Reijerse ; Wolfgang Lubitz
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：January 19, 2016
• 年：2016
• 卷：55
• 期：2
• 页码：419-431
• 全文大小：632K
• ISSN：1520-510X

文摘

A new class of synthetic models for the active site of [NiFe]-hydrogenases are described. The Ni^I/II(SCys)₂ and Fe^II(CN)₂CO sites are represented with (RC₅H₄)Ni^I/II and Fe^II(diphos)(CO) modules, where diphos = 1,2-C₂H₄(PPh₂)₂(dppe) or cis-1,2-C₂H₂(PPh₂)₂(dppv). The two bridging thiolate ligands are represented by CH₂(CH₂S)₂^2– (pdt^2–), Me₂C(CH₂S)₂^2– (Me₂pdt^2–), and (C₆H₅S)₂^2–. The reaction of Fe(pdt)(CO)₂(dppe) and [(C₅H₅)₃Ni₂]BF₄ affords [(C₅H₅)Ni(pdt)Fe(dppe)(CO)]BF₄ ([1a]BF₄). Monocarbonyl [1a]BF₄ features an S = 0 Ni^IIFe^II center with five-coordinated iron, as proposed for the Ni-SI_a state of the enzyme. One-electron reduction of [1a]⁺ affords the S = ¹/₂ derivative [1a]⁰, which, according to density functional theory (DFT) calculations and electron paramagnetic resonance and M?ssbauer spectroscopies, is best described as a Ni^IFe^II compound. The Ni^IFe^II assignment matches that for the Ni-L state in [NiFe]-hydrogenase, unlike recently reported Ni^IIFe^I-based models. Compound [1a]⁰ reacts with strong acids to liberate 0.5 equiv of H₂ and regenerate [1a]⁺, indicating that H₂ evolution is catalyzed by [1a]⁰. DFT calculations were used to investigate the pathway for H₂ evolution and revealed that the mechanism can proceed through two isomers of [1a]⁰ that differ in the stereochemistry of the Fe(dppe)CO center. Calculations suggest that protonation of [1a]⁰ (both isomers) affords Ni^III–H–Fe^II intermediates, which represent mimics of the Ni-C state of the enzyme.