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.