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, [L 1 H 1 H 2 ]Cl and [L 2 H 1 2 H 2 ]Cl (H 1 = N H C O, H 2 = NC H N), in 48−82% yields. The complexes NiL 1 2 and Ni(L 2 H 1 ) 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 NiL 1 2 complexes undergoes an intriguing reversible solvent-induced cis/trans transformation process, producing the cis form with DMSO and trans with CHCl 3 . The exchange process is highly specific and can be explained by the higher polarity of cis-NiL 1 2, 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 CHCl 3 solvent. A DFT calculation is in support of the experimental findings. Unlike the previous reported palladium analogues, NiL 1 2 and Ni(L 2 H 1 ) 2, in the presence of PPh 3 as cocatalyst, are highly efficient in catalyzing the Suzuki cross-coupling reaction of aryl chlorides with phenylboronic acid. A 3 mol % of NiL 1 2 (Ni:PPh 3 = 1:2) was sufficient to mediate the formation of 4-methoxy-1,1‘-biphenyl from 4-chloroanisole in 95% yield in 12 h.