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 [L2H1 2H2]Cl (H1 = NHCO, H2 = NCHN), in 48−82% yields. The complexes NiL1 2 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 NiL1 2 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-NiL1 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 CHCl3 solvent. A DFT calculation is in support of the experimental findings. Unlike the previous reported palladium analogues, NiL1 2 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 NiL1 2 (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.

A new imidazolinium ligand precursor [L(2)H]Cl (2) was prepared in 86 % yield. Compared with its imidazolium counterpart, [L(1)H]Cl (1), 2 is very sensitive to moisture and can undergo ring-opening reactions very readily. Palladium complexes with the ring-opened products from imidazolinium salts were isolated and characterized by X-ray crystallography. Theoretical studies confirmed that the imidazolinium salt has a higher propensity for the ring-opening reaction than the imidazolium counterpart. New mixed phosphine/carbene palladium complexes, cis-[PdCl(2)(L)(PR(3))] (L=L(1) and L(2); R=Ph, Cy), were successfully prepared. These complexes are highly robust as revealed by variable-temperature NMR spectroscopic studies and thermal gravimetric analysis. The structural and electronic properties of the new complexes on varying the carbene group (imidazol-2-ylidene group (unsaturated carbene) vs. imidazolin-2-ylidene (saturated carbene)) and the phosphine group (PPh(3) vs. PCy(3)) were studied in detail by X-ray crystallography, X-ray photoelectron spectroscopy, and theoretical calculations. The catalytic study reveals that cis-[PdCl(2)(L(2))(PCy(3))] is a competent Pd(II) precatalyst for Suzuki coupling reactions, in which unreactive aryl chlorides can be applied as substrates.

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In the structure of the title compound, [PdCl2(C5H5N)2], the PdII atom is located on an inversion centre and the pyridine rings are coplanar. There is inter­molecular π–π stacking between the pyridyl rings, with a centroid-to-centroid separation of 3.916 (1) Å. The structure is a new polymorph of two previously determined structures [Viossat, Dung & Robert (1993 ▶). Acta Cryst. C49, 84–85; Liao & Lee (2006 ▶). Acta Cryst. E62, m680–m681].

Key indicatorsSingle-crystal X-ray study T = 150 K Mean (C-C) = 0.005 Å R factor = 0.024 wR factor = 0.065 Data-to-parameter ratio = 19.3For details of how these key indicators were automatically derived from the article, see