Mono- and bis-N-heterocyclic carbene silver(I) and palladium(II) complexes: Synthesis, characterization, crystal structure and in vitro anticancer studies

The benzimidazolium salt 1, with butyl and allyl substituents, demonstrated two types of coordination behavior with the silver(I) center, in which the bis-NHC complex 2 was formed at room temperature, while the mono-NHC complex 3 was formed at an elevated temperature. However, the reaction of analogous salts with silver(I) oxide under the same reaction conditions did not result in the formation of mono-NHC complexes, instead bis-NHC complexes were formed. A series of palladium(II)-NHC complexes, 4, 5, 10 and 11, were obtained by the treatment of the corresponding silver complexes with a palladium(II) source. All new compounds were characterized by various spectroscopic and analytical techniques. Additionally, the structure of the mono-NHC silver complex 3 was unambiguously established following single crystal X-ray diffraction method. The in vitro comparative evaluation of the salt 1, and the silver complexes 2 and 3, and all the palladium complexes for their anticancer properties against human colon cancer cells indicated that the silver complexes 2 and 3 and palladium complexes 10 and 11 were the most effective antiproliferative compounds and were found able to promote a growth inhibitory effect with IC50 values in the range 10.6 ± 1.8–21.4 ± 2.1 μM.