Synthesis and Structural Characterization of Nickel(II) Complexes Supported by Pyridine-Functionalized N-Heterocyclic Carbene Ligands and Their Catalytic Acitivities for Suzuki Coupling

Abstract

The imidazolium salts bis(N-pyridylimidazoliumyl)methane hexafluorophosphate ([H2L1](PF6)2) and bis(N-picolyl)benzimidazoliumyl)methane hexafluorophosphate ([H2L2](PF6)2) as the precursors of potentially tetradentate NHC ligands were synthesized in 63–98% yields. Their reactions with Ag2O afforded silver−NHC complexes [Ag3(L1)2(CH3CN)2](PF6)3 (1) and [Ag4(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 1H and 13C 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.