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.