One to Find Them All: A General Route to Ni(I)–Phenolate Species

The past 20 years have seen an extensive implementation of nickel in homogeneous catalysis through the development of unique reactivity not easily achievable by using noble transition metals. Many catalytic cycles propose Ni­(I) complexes as potential reactive intermediates, yet the scarcity of nickel­(I) precursors and the lack of a general, non-ligand-specific protocol for their synthesis have hampered progress in this field of research. This has in turn also limited the access to novel, well-defined Ni­(I) species for the development of new catalytic reactions. Herein, we report a simple, general route to access a wide variety of Ni­(I)–phenolate complexes via an unusual example of an olefinic Ni­(I) complex, [Ni­(COD)­(OPh*)] (COD = 1,5-cyclooctadiene, OPh* = O­(tBu)3C6H2). This route has proven to be highly efficient for several coordination numbers and ligand classes enabling access to the following complexes: [Ni­(IPr)­(OPh*)] (IPr = 1,3-bis­(2,6-diisopropyl­phenyl)­imidazol-2-ylidene), [Ni­(dcype)­(OPh*)] (dcype = 1,2-bis­(dicyclohexyl­phosphino)­ethane), [Ni­(dppe)­(OPh*)] (dppe = 1,2-bis­(diphenyl­phosphino)­ethane), and [Ni­(terpy)­(OPh*)] (terpy = 2,2′:6′,2″-terpyridine). Moreover, reacting [Ni­(dcype)­(OPh*)] with trimethylsilyl triflate has led to the isolation of a unique example of a cationic binuclear Ni­(I)–arene complex. All these complexes have been characterized by single-crystal X-ray, DFT, and EPR analyses, thus providing crucial experimental and theoretical information about their coordination environment and confirming a d9 electronic structure for all complexes involved. Overall, this new synthetic approach offers exciting opportunities for the discovery of new stoichiometric and catalytic reactivity as well as the mechanistic elucidation of Ni-based catalytic cycles.

过去二十年间，镍在均相催化（homogeneous catalysis）领域得到了广泛应用，这得益于其开发出了稀有过渡金属难以实现的独特反应活性。诸多催化循环均提出一价镍（nickel(I)）配合物作为潜在的反应中间体，但由于一价镍前驱体稀缺，且缺乏通用的、非配体特异性的合成方案，该研究领域的发展受到了极大阻碍。这进而也限制了人们获取结构明确的新型一价镍物种，以开发新的催化反应。本文报道了一条简便通用的合成路径，可通过一例特殊的烯烃配位型一价镍配合物[Ni(COD)(OPh*)]（COD = 1,5-环辛二烯（1,5-cyclooctadiene），OPh* = 三(叔丁基)苯氧基（O(tBu)3C6H2)），制备多种一价镍-苯氧基配合物。该路径对多种配位数和配体类型均展现出极高的合成效率，可得到如下配合物：[Ni(IPr)(OPh*)]（IPr = 1,3-双(2,6-二异丙基苯基)咪唑-2-亚基（1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene））、[Ni(dcype)(OPh*)]（dcype = 1,2-双(二环己基膦基)乙烷（1,2-bis(dicyclohexylphosphino)ethane））、[Ni(dppe)(OPh*)]（dppe = 1,2-双(二苯基膦基)乙烷（1,2-bis(diphenylphosphino)ethane））以及[Ni(terpy)(OPh*)]（terpy = 2,2′:6′,2″-三联吡啶（2,2′:6′,2″-terpyridine））。此外，将[Ni(dcype)(OPh*)]与三氟甲磺酸三甲基硅酯（trimethylsilyl triflate）反应，成功分离得到一例独特的阳离子双核一价镍-芳烃配合物。所有上述配合物均通过单晶X射线衍射（single-crystal X-ray diffraction）、密度泛函理论（Density Functional Theory，DFT）以及电子顺磁共振（Electron Paramagnetic Resonance，EPR）分析进行了表征，从而为其配位环境提供了关键的实验与理论信息，并确认了所有涉及配合物的d9电子结构。总体而言，这一新的合成方法为发现新的化学计量及催化反应活性，以及阐明镍基催化循环的反应机理提供了极具前景的机遇。