Rational Design of Ligand Precursors to Prepare Abnormal (Mesoionic) and Normal Carbene Complexes and Zwitterionic CX-Type Palladacycles (X= C, N)

The preparation of a series of ligand precursors that feature imidazo[1,2-a]pyridine and amido functional groups is reported. The reactions between the new ligand precursors, pyridine, and palladium acetate in DMF afforded monodentate Pd-II abnormal (mesoionic) carbene complexes. An isomeric Pd-II normal carbene complex with a formally identical steric environment to that of the abnormal carbene counterpart was also prepared. The isomeric pairs were characterized by X-ray structural studies. Under different complexation conditions, in which PdCl2 was employed as metal precursor, a zwitterionic CC-type palladacycle formed by the C-H activation at the ortho-N-phenyl and methylene carbon atoms was obtained. The use of a similar ligand precursor with an NH instead of an NMe group produced a zwitterionic palladalactam complex. In contrast to the previous work in the literature, owing to the rigidity of the fused heterocyclic ring, the palladalactam complex did not undergo intramolecular proton transfer through the coordinated N atom. Overall, we demonstrated that proper modification of the structures of ligand precursors and complexation conditions allowed us to obtain a full range of intriguing Pd-II complexes including normal and abnormal carbene complexes, and zwitterionic palladalactam and CC-type palladacycles by means of A-H bond (A = N, C) activations.