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

Abstract

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±2.1μM.