Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides

详细信息    查看全文

• 作者：Olbelina A. Ulloa ; Mioy T. Huynh ; Casseday P. Richers ; Jeffery A. Bertke ; Mark J. Nilges ; Sharon Hammes-Schiffer ; Thomas B. Rauchfuss
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：July 27, 2016
• 年：2016
• 卷：138
• 期：29
• 页码：9234-9245
• 全文大小：628K
• 年卷期：0
• ISSN：1520-5126

文摘

The intermediacy of a reduced nickel–iron hydride in hydrogen evolution catalyzed by Ni–Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(μ-pdt)Fe(CO)(dppv) ([1]⁰; dppv = cis-C₂H₂(PPh₂)₂) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1]⁰ initially produces unsym-[H1]⁺, which converts by a first-order pathway to sym-[H1]⁺. These species have C₁ (unsym) and C_s (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1]⁺ protonates at sulfur. The S = 1/2 hydride [H1]⁰ was generated by reduction of [H1]⁺ with Cp*₂Co. Density functional theory (DFT) calculations indicate that [H1]⁰ is best described as a Ni(I)–Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1]⁰. Whereas [H1]⁺ does not evolve H₂ upon protonation, treatment of [H1]⁰ with acids gives H₂. The redox state of the “remote” metal (Ni) modulates the hydridic character of the Fe(II)–H center. As supported by DFT calculations, H₂ evolution proceeds either directly from [H1]⁰ and external acid or from protonation of the Fe–H bond in [H1]⁰ to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1]⁰ initially produces [1]⁺, which is reduced by [H1]⁰. Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit.