Synthesis, cytotoxicity and antibacterial studies of symmetrically and non‐symmetrically benzyl‐ or p‐cyanobenzyl‐substituted N‐Heterocyclic carbene–silver complexes

From the reaction of 1H‐imidazole (1a), 4,5‐dichloro‐1H‐imidazole (1b) and 1H‐benzimidazole (1c) with p‐cyanobenzyl bromide (2), symmetrically substituted N‐heterocyclic carbene (NHC) [(3a–c)] precursors, 1‐methylimidazole (5a), 4,5‐dichloro‐1‐methylimidazole (5b) and 1‐methylbenzimidazole (5c) with benzyl bromide (6), non‐symmetrically substituted N‐heterocyclic carbene (NHC) [(7a–c)] precursors were synthesized. These NHCprecursors were then reacted with silver(I) acetate to yield the NHC‐silver complexes [1,3‐bis(4‐cyanobenzyl)imidazole‐2‐ylidene] silver(I) acetate (4a), [4,5‐dichloro‐1,3‐bis(4‐cyanobenzyl)imidazole‐2‐ylidene] silver(I) acetate (4b), [1,3‐bis(4‐cyanobenzyl)benzimidazole‐2‐ylidene] silver(I) acetate (4c), (1‐methyl‐3‐benzylimidazole‐2‐ylidene) silver(I) acetate (8a), (4,5‐dichloro‐1‐methyl‐3‐benzylimidazole‐2‐ylidene) silver(I) acetate (8b) and (1‐methyl‐3‐benzylbenzimidazole‐2‐ylidene) silver(I) acetate (8c) respectively. The four NHC‐precursors 3a–c, 7c and four NHC–silver complexes 4a–c and 8c were characterized by single crystal X‐ray diffraction. The preliminary antibacterial activity of all the compounds was studied against Gram‐negative bacteria Escherichia coli, and Gram‐positive bacteria Staphylococcus aureus using the qualitative Kirby‐Bauer disc‐diffusion method. All NHC–silver complexes exhibited medium to high antibacterial activity with areas of clearance ranging from 4 to 12 mm at the highest amount used, while the NHC‐precursors showed significantly lower activity. In addition, all NHC–silver complexes underwent preliminary cytotoxicity tests on the human renal‐cancer cell line Caki‐1 and showed medium to high cytotoxicity with IC50 values ranging from 53 ( ± 8) to 3.2 ( ± 0.6) µM.