Isomeric Palladium Complexes Bearing Imidazopyridine-Based Abnormal Carbene Ligands: Synthesis, Characterization, and Catalytic Activity in Direct C–H Arylation Reaction

A series
of “abnormal” NHC ligand precursors based on imidazopyridines
and their corresponding NHC-palladium diiodide complexes were prepared.
Our study included the first report on a palladium complex with an
abnormal NHC ligand derived from imidazo­[1,5-a]­pyridine.
DFT calculation on the free ligands confirmed the higher electron-donating
properties of the abnormal NHC ligands than their normal NHC isomers.
The structural properties of all the complexes were studied in detail
by single-crystal X-ray diffraction studies, revealing that, in general,
Pd–carbene bond distances in palladium complexes with abnormal
NHCs were longer than those in their isomers with normal NHCs. A significant trans influence is observed in the palladium complexes with
abnormal NHC ligands. But in the complexes with normal NHC ligands,
both Pd–carbene and Pd–N bond distances are short. Catalytic
studies revealed that the palladium complexes were very efficient
in catalyzing direct C–H arylation between 1,2-dimethylimidazole
and aryl halides using a low Pd loading of 0.5 mol %. The difference
in catalytic activity between the normal and abnormal isomers is,
however, small.