Synthesis and structural characterization of nickel(II) complexes of 20-membered macrocyclic rings bearing chelating bis(N-heterocyclic carbene) ligands

by Thapa, Rajesh; Kilyanek, Stefan M.

20-Membered macrocyclic rings (L-benz and L-imid) containing two amine moieties and two N-heterocyclic carbene (NHC) moieties coordinate in a kappa(2) coordination mode to Ni(II) through the NHC moieties. Complexes 3a and 3b were synthesized by generating free carbenes in situ from bis-benzimidazolium bromide salts and subsequent addition of one equivalent of [Ni(DME)X-2] (X = Cl, Br) at 25 degrees C. Free carbenes of 20-membered macrocyclic ligands L-benz and L-imid can be generated in situ in THF-d(8) and characterized by H-1 and C-13 NMR spectroscopy. 3b can also be synthesized by treatment of silver(I)-bis(NHC) complex 2a with one equivalent of [NiBr2(DME)] in refluxing methylene chloride for 20 h affording yields of up to70%. Complexes 3a and 3b were characterized by X-ray diffraction. [NiI2(L-benz)] 5c was prepared by halogen exchange of 3a or 3b with NaI at 25 degrees C. Ni complexes of L-imid can be prepared and characterized in situ but cannot be isolated cleanly from the reaction mixtures. The cis and trans isomers of [NiCl2(L-imid)] (4a and 4b) and the bromide derivative 4c were characterized by X-ray crystallography by judicious selection of individual, high-quality crystals obtained from crude reaction mixtures.

Journal of Organometallic Chemistry
1872-8561; 0022-328X