Monodentate Palladium Complexes Bearing Abnormal and Normal Carbene Ligands with a Formally Identical Steric Environment

We report a series of novel isomeric Pd­(II) complexes with monodentate abnormal and normal carbene ligands, possessing a formally identical steric environment around the metal center. The corresponding ligand precursors were prepared via the key steps based on Pd-catalyzed direct C–H arylation between imidazo­[1,2-a]­pyridine and aryl bromides and Cu-catalyzed C–N coupling reactions between benzoimidazole and aryl bromides. The new complexes were characterized by 1D and 2D NMR spectroscopy, X-ray crystallography, and elemental analysis. The isomeric pairs of ligands show minor differences among their steric properties, which were examined by calculating their buried volume. The abnormal carbene Pd­(II) complexes were found to be more efficient in catalyzing Mizoroki–Heck coupling, direct C–H arylation, and decarboxylative coupling reactions, which can be clearly attributed to a stronger electronic donating effect of the abnormal carbene ligands.