Synthesis, structural investigation and antibacterial studies of non–symmetrically p–nitrobenzyl substituted benzimidazole N–heterocyclic carbene–silver(I) complexes

Abstract

Several new non–symmetrically p–nitrobenzyl substituted benzimidazole–based mono–N–heterocyclic carbene (NHC)–silver(I) acetate (7a–e) and bis–NHC–silver(I) hexafluorophosphate complexes (8a–e) were synthesized by combination of 1–ethyl/1–propyl/1–butyl/1–pentyl/1–hexyl–3–(4–nitrobenzyl)benzimidazolium hexafluorophosphate salts (6a–e) with silver(I) acetate and silver(I) oxide in methanol and acetonitrile, respectively, in good yields. All the benzimidazolium hexafluorophosphate salts (6a–e) and their corresponding NHC–silver(I) complexes (7a–e and 8a–e) were structurally characterized by 1H, 13C NMR, ATR–IR spectrometry and elemental analysis. The solid–state structures of bis–NHC–silver(I) hexafluorophosphate complexes (8c) and (8e) were determined by single crystal X–ray diffraction method. Preliminary in vitro antibacterial potential of all the compounds (6a–e, 7a–e and 8a–e) were studied against six standard bacterial strains; two Gram–positive (Staphylococcus aureus and Bacillus subtilis) and four Gram–negative bacterial species (Escherichia coli, Pseudomonas aeruginosa, Shigella sonnei, and Salmonella typhi) by Kirby–Bauer’s disc diffusion method and their Minimum Inhibitory Concentration (MIC) was determined by broth macrodilution method. All the NHC–silver(I) complexes (7a–e and 8a–e) exhibited moderate to high antibacterial activities with MIC ranging between 16.0 and 128.0µg/mL, whereas benzimidazolium hexafluorophosphate salts (6a–e) demonstrated poor antibacterial potentials.