Department of Chemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
J Am Chem Soc. 2016 Jul 27;138(29):9234-45. doi: 10.1021/jacs.6b04579. Epub 2016 Jul 18.
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; 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 initially produces unsym-H1, which converts by a first-order pathway to sym-H1. These species have C1 (unsym) and Cs (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 was generated by reduction of H1 with Cp*2Co. Density functional theory (DFT) calculations indicate that H1 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. Whereas H1 does not evolve H2 upon protonation, treatment of H1 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 and external acid or from protonation of the Fe-H bond in H1 to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from H1 initially produces 1, which is reduced by H1. 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.
镍铁配合物催化氢析出反应中还原镍-铁氢化物的中间体得到了实验和计算的验证。除了催化氢析出反应外，高碱性和大体积的(dppv)Ni(μ-pdt)Fe(CO)(dppv) (1)及其氢化物衍生物在光谱、成键和反应性方面得到了详细的表征。1的质子化最初生成非对称-H1，它通过一级途径转化为对称-H1。这些物种分别具有 C1(非对称)和 Cs(对称)对称性，这取决于八面体 Fe 位的立体化学。实验和计算研究都表明H1在硫上质子化。H1用 Cp*2Co 还原生成 S = 1/2 氢化物H1。密度泛函理论(DFT)计算表明，H1最好被描述为 Ni(I)-Fe(II)衍生物，其中 Ni 上有显著的自旋密度，S 和 Fe 上有一定程度的离域。电子顺磁共振(EPR)光谱揭示了H1的动力学和热力学异构体。虽然H1在质子化时不会析出 H2，但用酸处理H1会得到 H2。“远程”金属(镍)的氧化态调节 Fe(II)-H 中心的氢化物性质。正如 DFT 计算所支持的那样，H2 的析出要么直接从H1和外部酸中进行，要么从H1中 Fe-H 键的质子化进行，生成不稳定的双氢配合物。化学计量测试表明，H1质子化诱导的氢析出最初产生1，它被H1还原。我们的结果协调了金属氢化物还原所需的还原活化和金属氢化物对还原的抵抗力。这种二分法通过还原双金属单元的远程(非氢化物)金属得到解决。
