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](PF 6 ) 2 ) and bis( N -picolyl)benzimidazoliumyl)methane hexafluorophosphate ([H 2 L2](PF 6 ) 2 ) as the precursors of potentially tetradentate NHC ligands were synthesized in 63–98% yields. Their reactions with Ag 2 O afforded silver−NHC complexes [Ag 3 (L1) 2 (CH 3 CN) 2 ](PF 6 ) 3 ( 1 ) and [Ag 4 (L2) 2 (CH 3 CN) 2 ](PF 6 ) 4 ( 2 ), respectively. Further reactions of these silver complexes as carbene transfer reagents yielded square-planar nickel(II) complexes [NiL1](PF 6 ) 2 ( 3 ) and [NiL2](PF 6 ) 2 ( 4 ), respectively. These silver and nickel complexes have been fully characterized by 1 H and 13 C 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.