Unexpected Solvent-Induced Cis/Trans Isomerization and Catalytic Application of a Bis-bidentate Nickel(II) Complex with N-Heterocyclic Carbene and Amido Functionalities

A series of bis-bidentate nickel(II) complexes with bifunctional ligands of N-heterocyclic carbene and
amido moieties were prepared from the ligand precursors, [L1H1H2]Cl and [L2H12H2]Cl (H1 = NHCO,
H2 = NCHN), in 48−82% yields. The complexes NiL12 and Ni(L2H1)2 are stable in the air and toward
moisture. They are characterized by NMR (1D and 2D) and single-crystal X-ray diffraction studies.
According to the NMR, crystallographic, and PXRD studies, one of the NiL12 complexes undergoes an
intriguing reversible solvent-induced cis/trans transformation process, producing the cis form with DMSO
and trans with CHCl3. The exchange process is highly specific and can be explained by the higher polarity
of cis-NiL12, which favors the polar DMSO solvent, whereas the less polar trans form due to cancellation
of polarity from the trans disposition of ligands favors the less polar CHCl3 solvent. A DFT calculation
is in support of the experimental findings. Unlike the previous reported palladium analogues, NiL12 and
Ni(L2H1)2, in the presence of PPh3 as cocatalyst, are highly efficient in catalyzing the Suzuki cross-coupling reaction of aryl chlorides with phenylboronic acid. A 3 mol % of NiL12 (Ni:PPh3 = 1:2) was
sufficient to mediate the formation of 4-methoxy-1,1‘-biphenyl from 4-chloroanisole in 95% yield in
12 h.

本研究以[L1H1H2]Cl与[L2H12H2]Cl（其中H1=NHCO，H2=NCHN）为配体前体，合成了一系列兼具氮杂环卡宾（N-heterocyclic carbene, NHC）与酰胺片段的双双齿配体配位镍(II)配合物，产率介于48%至82%之间。配合物NiL12与Ni(L2H1)2在空气与湿气中均表现出良好的稳定性，且通过一维、二维核磁共振（NMR）及单晶X射线衍射完成了表征。结合核磁共振、晶体学与粉末X射线衍射（PXRD）的表征结果可知，其中一种NiL12配合物可发生有趣的溶剂诱导可逆顺反异构转化：以二甲基亚砜（DMSO）为溶剂时得到顺式构型，以三氯甲烷（CHCl3）为溶剂时则得到反式构型。该转化过程具有高度特异性，可通过顺式NiL12的极性更高、更适配极性溶剂二甲基亚砜来解释；而反式构型因配体的反式排布导致极性相互抵消，极性更低，更适配低极性的三氯甲烷溶剂。密度泛函理论（DFT）计算结果也验证了上述实验结论。与此前报道的钯基类似物不同，在三苯基膦（PPh3）作为助催化剂的条件下，NiL12与Ni(L2H1)2可高效催化芳基氯与苯硼酸的铃木交叉偶联反应。当使用3 mol%的NiL12（镍与三苯基膦的摩尔比为1:2）时，即可在12小时内以95%的产率由4-氯苯甲醚合成4-甲氧基-1,1'-联苯。