Novel symmetrically p-benzyl-substituted 4,5-diaryl-imidazole N-heterocyclic carbene-silver(I) acetate complexes – Synthesis and biological evaluation

Symmetrically substituted N-heterocyclic carbene (NHC) precursors 1a–e and 3a–e were synthesised by reacting 4,5-bisaryl-1H-imidazole with 2 equivalents of 4-benzyl bromide, 4-methylbenzyl bromide, 4-methoxybenzyl chloride, 4-methoxycarbonylbenzyl bromide or 4-cyanobenzyl bromide to give the 1,3-bis(p-benzyl substituted)-4,5-bisaryl-imidazolium halides. The NHC-silver(I) acetate complexes (1,3-bisbenzyl-4,5-bisphenyl-imidazole-2-ylidene) silver(I) acetate (2a), (1,3-bis(4-methylbenzyl)-4,5-bisphenyl-imidazole-2-ylidene) silver(I) acetate (2b), (1,3-bis(4-methoxybenzyl)-4,5-bisphenyl-imidazole-2-ylidene) silver(I) acetate (2c), (1,3-bis(4-methoxycarbonylbenzyl)-4,5-bisphenyl-imidazole-2-ylidene) silver(I) acetate (2d), (1,3-bis(4-cyanobenzyl)-4,5-bisphenyl-imidazole-2-ylidene) silver(I) acetate (2e), (1,3-bisbenzyl-4,5-bis(4-methoxyphenyl)-imidazole-2-ylidene) silver(I) acetate (4a), (1,3-bis(4-methylbenzyl)-4,5-bis(4-methoxyphenyl)-imidazole-2-ylidene) silver(I) acetate (4b), (1,3-bis(4-methoxybenzyl)-4,5-bis(4-methoxyphenyl)-imidazole-2-ylidene) silver(I) acetate (4c), (1,3-bis(4-methoxycarbonylbenzyl)-4,5-bis(4-methoxyphenyl)-imidazole-2-ylidene) silver(I) acetate (4d) and (1,3-bis(4-cyanobenzyl)-4,5-bis(4-methoxyphenyl)-imidazole-2-ylidene) silver(I) acetate (4e) were yielded by reacting these NHC precursors with silver(I) acetate. The silver(I) acetate complexes 2d, 2e and 4c were characterised by single crystal X-ray diffraction. Qualitative antibacterial studies against the Gram-negative bacteria Escherichia coli and the Gram-positive bacteria Staphylococcus aureus, using the Kirby–Bauer disc diffusion method were carried out on the ten NHC-silver(I) acetate complexes 2a–e and 4a–e. Also the IC50 values of these ten complexes were determined by an MTT-based assay against the human renal cancer cell line Caki-1 and the human breast cancer cell line MCF-7. The complexes 2a–e and 4a–e revealed the following IC50 values in μM against Caki-1: 14 (±1), 3.6 (±1.0), 4.2 (±0.5), 33 (±2), 59 (±4), 21 (±1), 21 (±2), 21 (±1), 34 (±2), 46 (±2) and against MCF-7: 5.8 (±0.6), 3.5 (±0.4), 5.4 (±0.3), 28 (±1), 25 (±2), 11 (±2), 5.0 (±0.3), 6.5 (±0.4), 17 (±1), 13 (±1); respectively.