Heimann Jessica E, Bernskoetter Wesley H, Guthrie Julie A, Hazari Nilay, Mayer James M
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut, 06520, USA.
Department of Chemistry, University of Missouri, Columbia, Missouri, 65211, USA.
Organometallics. 2018 Nov 12;37(21):3649-3653. doi: 10.1021/acs.organomet.8b00593. Epub 2018 Oct 22.
The insertion of CO into metal-element σ-bonds such as M-H, M-OH, and M-NH is often proposed to be a key step in catalytic CO conversion. Nevertheless, there are few experimental studies measuring the kinetics of CO insertion or comparing insertion into LM-E (E = H, OH, or NH) complexes with the same ancillary ligands. Here, we use a rapid mixing stopped-flow instrument to measure the rate of CO insertion into (PCP)Ni(OH) (PCP = 2,6-CH(CHPBu)) and show that it proceeds at a faster rate than insertion into the corresponding metal hydride. We also demonstrate that the rate of CO insertion into (PCP)Ni(OH) can generally be increased by using solvents with higher acceptor numbers or by modifying the ligand scaffold to reduce steric bulk or increase electron donation to the metal center. We also attempt to measure the rate of CO insertion into (PCP)Ni(NH); however, this reaction is too fast to measure with our methodology. This work enables the direct comparison of CO insertion into analogous LM-E (E = H, OH, or NH) systems and provides quantitative experimental evidence that the rate of insertion into a metal-element σ-bond correlates with the nucleophilicity of E.
一氧化碳插入金属元素的σ键（如M-H、M-OH和M-NH）通常被认为是催化一氧化碳转化的关键步骤。然而，很少有实验研究测量一氧化碳插入的动力学，或比较一氧化碳插入具有相同辅助配体的LM-E（E = H、OH或NH）配合物的情况。在这里，我们使用快速混合停流仪器测量一氧化碳插入(PCP)Ni(OH)（PCP = 2,6-CH(CHPBu)）的速率，结果表明其反应速率比插入相应金属氢化物的速率更快。我们还证明，通过使用具有更高受体数的溶剂，或通过修饰配体骨架以减少空间位阻或增加对金属中心的电子给予，可以普遍提高一氧化碳插入(PCP)Ni(OH)的速率。我们还试图测量一氧化碳插入(PCP)Ni(NH)的速率；然而，该反应太快，无法用我们的方法进行测量。这项工作使得能够直接比较一氧化碳插入类似的LM-E（E = H、OH或NH）体系，并提供了定量实验证据，即插入金属元素σ键的速率与E的亲核性相关。
