Nickel pincer systems have recently attracted much attention for applications in various organometallic reactions and catalysis involving small molecule activation. Their exploration is in part motivated by the presence of nickel in natural systems for efficient catalysis. Among such systems, the nickel-containing metalloenzyme carbon monoxide dehydrogenase (CODH) efficiently and reversibly converts CO₂ to CO at its active site. The generated CO moves through a channel from the CODH active site and is transported to a dinuclear nickel site of acetyl-coenzyme A synthase (ACS), which catalyzes organometallic C–S and C–C bond forming reactions. An analogous C–S bond activation process is also mediated by the nickel containing enzyme methyl-coenzyme M reductase (MCR). The nickel centers in these systems feature sulfur- and nitrogen-rich environments, and in the particular case of lactate racemase, an organometallic nickel pincer motif revealing a Ni–C bond is observed. These bioinorganic systems inspired the development of several nickel pincer scaffolds not only to mimic enzyme active sites and their reactivity but also to further extend low-valent organonickel chemistry. In this Account, we detail our continuing efforts in the chemistry of nickel complexes supported by acridane-based PNP pincer ligands focusing on our long-standing interest in biomimetic small molecule activation. We have employed a series of diphosphinoamide pincer ligands to prepare various nickel(II/I/0) complexes and to study the conversion of C₁ chemicals such as CO and CO₂ to value-added products. In the transformation of C₁ chemicals, the key C–O bond cleavage and C–E bond (E = C, N, O, or S, etc.) formation steps typically require overcoming high activation barriers. Interestingly, enzymatic systems overcome such difficulties for C₁ conversion and operate efficiently under ambient conditions with the use of nickel organometallic chemistry. Furthermore, we have extended our efforts to the conversion of NO_x anions to NO via the sequential deoxygenation by nickel mediated carbonylation, which was applied to catalytic C–N coupling to produce industrially important organonitrogen compound oximes as a strategy for NO_x conversion and utilization (NCU). Notably, the rigidified ^acriPNP pincer backbone that enforces a planar geometry at nickel was found to be an important factor for diversifying organometallic transformations including (a) homolysis of various σ-bonds mediated by T-shaped nickel(I) metalloradical species, (b) C–H bond activation mediated by a nickel(0) dinitrogen species, (c) selective CO₂ reactivity of nickel(0)–CO species, (d) C–C bond formation at low-valent nickel(I or 0)–CO sites with iodoalkanes, and (e) catalytic deoxygenation of NO_x anions and subsequent C–N coupling of a nickel–NO species with alkyl halides for oxime production. Broadly, our results highlight the importance of molecular design and the rich chemistry of organonickel species for diverse small molecule transformations.

中文翻译：

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在 acriPNP 钳形支持的镍位点进行小分子激活


镍钳系统最近因在涉及小分子活化的各种有机金属反应和催化中的应用而引起广泛关注。他们的探索部分是由于天然系统中存在用于高效催化的镍。在这些系统中，含镍金属酶一氧化碳脱氢酶 （CODH） 在其活性位点有效且可逆地将 CO₂ 转化为 CO。产生的 CO 通过 CODH 活性位点的通道移动，并被转运到乙酰辅酶 A 合酶 （ACS） 的双核镍位点，该位点催化有机金属 C-S 和 C-C 键形成反应。类似的 C-S 键激活过程也由含镍酶甲基辅酶 M 还原酶 （MCR） 介导。这些系统中的镍中心具有富含硫和氮的环境，在乳酸消旋酶的特殊情况下，观察到揭示 Ni-C 键的有机金属镍钳子基序。这些生物无机系统激发了几种镍钳支架的开发，不仅用于模拟酶活性位点及其反应性，而且还进一步扩展了低价有机镍化学。在本账户中，我们详细介绍了我们在基于吖烷的 PNP 钳形配体支持的镍络合物化学方面的持续努力，重点关注我们对仿生小分子活化的长期兴趣。我们采用了一系列二膦酰胺钳形配体来制备各种镍 （II/I/0） 配合物，并研究了 CO 和 CO₂ 等 C₁ 化学物质向增值产品的转化。在 C₁ 化学品的转化中，关键的 C-O 键裂解和 C-E 键（E = C、N、O 或 S 等。）形成步骤通常需要克服高活化障碍。有趣的是，酶系统克服了 C₁ 转化的困难，并使用镍有机金属化学在环境条件下高效运行。此外，我们还将工作扩展到通过镍介导的羰基化顺序脱氧将 NO_x 阴离子转化为 NO，这被应用于催化 C-N 偶联，以产生具有工业重要性的有机氮化合物肟，作为 NO_x 转化和利用 （NCU） 的策略。值得注意的是，在镍处执行平面几何形状的刚性 ^acriPNP 钳形主干被发现是有机金属转变多样化的重要因素，包括 （a） 由 T 形镍 （I） 金属自由基物种介导的各种 σ 键的均解，（b） 由镍 （0） 二氮物种介导的 C-H 键激活，（c） 选择性 CO₂镍 （0）-CO 物种的反应性，（d） 在低价镍（I 或 0）-CO 位点与碘烷烃形成 C-C 键，以及 （e） NO_x 阴离子的催化脱氧和随后镍-NO 物种与烷基卤化物的 C-N 偶联以产生肟。从广义上讲，我们的结果强调了分子设计和有机镍种类的丰富化学性质对于各种小分子转化的重要性。