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.