Syntheses and characterization of palladium(II) complexes with a bidentate bis-NHC ligand having methyl and aryl substituents on terminal nitrogen atoms

An imidazolium salt, N-chloromethyl-N'-methylimidazolium bromide was prepared as an intermediate for the syntheses of unsymmetrical bidentate bis-NHC ligands bearing methyl and aryl substituents on the terminal nitrogen atoms. Three bis-NHC ligand precursors and their corresponding palladium(II) complexes were also prepared. H-1 NMR spectra of the imidazolium salts of the bidentate ligands implied that the carbene on a methyl-substituted imidazolylidene ring is a stronger donor than one on an aryl-substituted ring. The crystal structures of the palladium(II) complexes showed a distorted square-planar structure. The complexes demonstrated good catalytic performance in the Heck reaction. (C) 2014 Published by Elsevier B. V.