Benzoxazole and dioxolane substituted benzimidazole–based N–heterocyclic carbene–silver(I) complexes: Synthesis, structural characterization and in vitro antimicrobial activity

Abstract

A novel series of 1,3–benzoxazole and 1,3–dioxolane substituted benzimidazole–based N–heterocyclic carbene (NHC) precursors (6a–b and 11a–b) and their corresponding NHC–silver(I) acetate (12a–b and 14a–b) and bis–NHC–silver(I) hexafluorophosphate complexes (13a–b and 15a–b) were synthesized and characterized by 1H NMR, 13C NMR, ATR-IR spectroscopic methods and elemental analysis techniques. Also, the molecular structure of benzimidazolium salt (6a) and bis–NHC–silver(I) hexafluorophosphate complex (13b) were unambiguously established by single crystal X–ray diffraction analysis. Fascinatingly, bis–NHC–silver(I) hexafluorophosphate complex (13b) was revealed to be dinuclear in nature. All the azolium salts (6a–b and 11a–b) as well as the corresponding silver(I) complexes (12a–b, 13a–b, 14a–b and 15a–b) were evaluated for their antimicrobial potential against Gram positive (Staphylococcus aureus), and a set of Gram negative (Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa) bacterial strains and two fungi, Candida albicans and Cryptococcus neoformans. Antimicrobial studies revealed the comparable activities of benzimidazolium salts (6a–b and 11a–b) and the silver(I) complexes (12a–b, 13a–b and 15a–b). Whereas, the 1,3–dioxolane substituted benzimidazole–based silver(I) acetate complexes (14a–b) demonstrated excellent potentials with both bacterial and fungal growth inhibition above 95%.