Synthesis and structural characterization of nickel(II) complexes supported by pyridine-functionalized N-heterocyclic carbene ligands and their catalytic acitivities for Suzuki coupling

The imidazolium salts bis(N-pyridylimidazoliumyl)methane hexafluorophosphate ([H(2)L1](PF6)(2)) and bis(N-picolyl)benzimidazoliumyl)methane hexafluorophosphate ([H(2)L2](PF6)(2)) as the precursors of potentially tetradentate NHC ligands were synthesized in 63-98% yields. Their reactions with Ag2O afforded silver-NHC complexes [Ag-3(L1)(2)(CH3CN)(2)](PF6)(3) (1) and [Ag-4(L2)(2)(CH3CN)(2)](PF6)(4) (2), respectively. Further reactions of these silver complexes as carbene transfer reagents yielded square-planar nickel(II) complexes [NiL1](PF6)(2) (3) and [NiL2](PF6)(2) (4), respectively. These silver and nickel complexes have been fully characterized by H-1 and C-13 NMR spectroscopy and X-ray diffraction analysis. Both 3 and 4 catalyzed Suzuki-type cross-coupling of aryl chlorides and bromides containing electron-withdrawing and electron-donating substituents. Complex 3 is a more active catalyst under mild conditions.