Dinuclear silver(I)- and gold(I)-N-heterocyclic carbene complexes of N-alkyl substituted bis-benzimidazol-2-ylidenes with aliphatic spacer: Synthesis, characterizations and antibacterial studies

Abstract New series of dinuclear Ag(I)-NHC and Au(I)-NHC complexes (NHC = N-heterocyclic carbene) bearing n-alkyl homologous series with propylene spacers were synthesized. Initially, bis-benzimidazolium salts, 1-5 were synthesized via two steps n-alkylation reaction. The salts were then subsequently deprotonated with basic metal source (Ag2O) using in-situ deprotonation method to obtain the Ag(I)-NHC complexes 6-10, respectively. The Au(I)-NHC complexes,11-15were synthesized via transmetalation reaction from their respective Ag(I)-NHC complexes. All compounds were fully characterized by elemental analyses, FT-IR, 1H and 13CNMR spectroscopy. The antibacterial studies of these compounds were evaluated against E. coli and S. aureus using disc diffusion method. The Ag(I)-NHC complexes exhibit better activities with inhibition zone of11 ± 1-20 ± 1mm and 9 ± 0-16 ± 1mm against E. coli and S. aureus respectively, while Au(I)-NHC complexes show inhibition zone of 7 ± 0-14 ± 1mmfor both E. coli and S. aureus bacteria. All the respective benzimidazolium salts were found to be inactive against the bacterial strains. Herein, we found that the antibacterial activity is enhanced by the degree of bond strength due to the match between the hardness and softness of the metal centre (Lewis acid) and NHC moiety (Lewis base). In this point of view, the Ag(I)-C bond was found to be stronger bond to NHC ligand (hard acid-hard base combination) than that of Au(I)-C (soft acid-hard base combination) which resulted the slow release of silver ions for better activity. This is due to the fact that the antibacterial activities of complexes are related with delaying in releasing the metal ions (Ag+ and Au+) into the cell membrane.