Department of Chemistry, Texas A&M University, College Station, TX 77843.
Department of Chemistry, Texas A&M University, College Station, TX 77843
Proc Natl Acad Sci U S A. 2017 Nov 14;114(46):E9775-E9782. doi: 10.1073/pnas.1710475114. Epub 2017 Oct 30.
The hydrogen evolution reaction, as catalyzed by two electrocatalysts [M(NS)·Fe(NO)], [-Fe] (M = Fe(NO)) and [Ni-Fe] (M = Ni) was investigated by computational chemistry. As nominal models of hydrogenase active sites, these bimetallics feature two kinds of actor ligands: Hemilabile, MNS ligands and redox-active, nitrosyl ligands, whose interplay guides the H production mechanism. The requisite base and metal open site are masked in the resting state but revealed within the catalytic cycle by cleavage of the MS-Fe(NO) bond from the hemilabile metallodithiolate ligand. Introducing two electrons and two protons to [Ni-Fe] produces H from coupling a hydride temporarily stored on Fe(NO) (Lewis acid) and a proton accommodated on the exposed sulfur of the MNS thiolate (Lewis base). This Lewis acid-base pair is initiated and preserved by disrupting the dative donation through protonation on the thiolate or reduction on the thiolate-bound metal. Either manipulation modulates the electron density of the pair to prevent it from reestablishing the dative bond. The electron-buffering nitrosyl's role is subtler as a bifunctional electron reservoir. With more nitrosyls as in [-Fe], accumulated electronic space in the nitrosyls' π*-orbitals makes reductions easier, but redirects the protonation and reduction to sites that postpone the actuation of the hemilability. Additionally, two electrons donated from two nitrosyl-buffered irons, along with two external electrons, reduce two protons into two hydrides, from which reductive elimination generates H.
通过计算化学研究了由两种电催化剂[M(NS)·Fe(NO)]，-Fe和Ni-Fe催化的析氢反应。作为氢化酶活性位点的名义模型，这些双金属具有两种作用配体：半配位的 MNS 配体和氧化还原活性的亚硝酰配体，它们的相互作用指导了 H 生成机制。在静止状态下，必需的碱和金属开放位点被掩盖，但在催化循环中，通过对半配位金属二硫代物配体中的 MS-Fe(NO)键的裂解而暴露出来。向[Ni-Fe]中引入两个电子和两个质子，通过将暂时储存在 Fe(NO)(路易斯酸)上的氢化物和容纳在 MNS 硫醇盐暴露的硫上的质子耦合，从耦合中产生 H。路易斯酸碱对通过硫醇或硫醇结合金属上的还原对硫醇的质子化来启动和保存。这两种操作都调节了该对的电子密度，以防止其重新建立配位键。作为双功能电子储库，电子缓冲亚硝酰的作用更为微妙。在[-Fe]中，更多的亚硝酰会积累在亚硝酰π*-轨道中的电子空间，使还原更容易，但会将质子化和还原引导到推迟半配位作用的位置。此外，两个由两个亚硝酰缓冲铁提供的电子，加上两个外部电子，将两个质子还原成两个氢化物，从中进行还原消除生成 H。
