N,N,C Heteroscorpionates as a Platform for Monovalent Ni Chemistry

We report herein the synthesis and characterization of a number of monovalent Ni complexes bound by a class of N,N,C heteroscorpionate ligands and their derivatives. One-electron reduction of Ni2+ complexes supported by less bulky heteroscorpionates led to a thermally unstable Ni+ species that could be trapped as the corresponding PPh3 adduct. In situ reaction of the aforementioned Ni+ complex with a bulky aryl azide affords a transient Ni3+ imide, for which attack of the C donor at the Nimido atom, followed by rapid isomerization, furnishes a three-coordinate Ni+ complex ligated by a new type of “expanded” N,N,C chelate. An N,N,C heteroscorpionate ligand with large Ph2(CH3)C– substituents was able to support a thermally stable, isolable Ni+ complex, in which a phenyl group of one Ph2(CH3)C– unit binds Ni in an η2 fashion. This complex proved to be relatively unreactive toward organic azides. The labile C donor in the former Ni+ complex could be trapped via the addition of B(C6F5)3, affording a zwitterionic Ni+ complex. All Ni+ species were characterized by electron paramagnetic resonance (EPR) spectroscopy, which was corroborated by density functional theory (DFT) calculations. Our results are expected to guide future efforts by ourselves and others in the pursuit of low-valent Ni complexes and Ni imides in general.

本文报道了一类由N,N,C型杂蝎形配体（N,N,C heteroscorpionate ligands）及其衍生物配位的多种一价镍配合物的合成与表征。采用位阻较小的杂蝎形配体稳定的Ni²+配合物经单电子还原后，得到热不稳定的Ni+物种，该物种可被捕获为相应的三苯基膦（PPh3）加合物。将上述Ni+配合物与大位阻芳基叠氮化物进行原位反应，可生成瞬态三价镍酰亚胺中间体；该中间体经碳给体对亚胺镍原子的进攻，随后快速异构化，得到由新型“扩环”N,N,C螯合配体配位的三配位一价镍配合物。带有大位阻Ph₂(CH₃)C–取代基的N,N,C型杂蝎形配体可稳定热稳定且可分离的一价镍配合物，其中一个Ph₂(CH₃)C–单元的苯基以η²配位模式与镍结合。该配合物对有机叠氮化物表现出相对较弱的反应活性。上述一价镍配合物中的不稳定碳给体可通过添加三(五氟苯基)硼烷（B(C₆F₅)₃）进行捕获，得到两性离子型一价镍配合物。所有一价镍物种均通过电子顺磁共振波谱（EPR）进行了表征，并通过密度泛函理论（DFT）计算予以佐证。本研究结果有望为我们及其他研究团队未来开展低氧化态镍配合物及通用型镍酰亚胺相关研究提供指导。