Imidazo[1,5-a]pyridine-3-ylidenes and dipyridoimidazolinylidenes as ancillary ligands in Palladium allyl complexes with potent in vitro anticancer activity

The synthesis and characterization of silver and palladium allyl complexes bearing imidazo[1,5-a]pyridine-3-ylidene and dipyridoimidazolinylidene ligands is reported. NMR analysis of the starting azolium salts revealed the weaker σ-donor character of these NHC ligands compared to the most common imidazol-2-ylidenes and imidazolin-2-ylidenes. All cationic palladium allyl complexes showed remarkable antitumoral activity toward a panel of cisplatin-sensitive and cisplatin-resistant cell lines, with IC50 in the micro- and nano-molar range. Among the tested derivatives, those with a pyridine arm combine good anticancer activity with a poor cytotoxicity against normal cells. Finally, all palladium complexes, especially the bisNHC species, showed good photoluminescence properties, with emission in the blue/green region.