Synthesis, characterization, crystal structure and biological studies of silver(I) complexes derived from coumarin-tethered N-heterocyclic carbene ligands

Abstract

Silver(I) complexes bearing functionalized N-heterocyclic carbene (NHC) ligands have been comprehensively explored for their biological properties, but there are few reports on biological properties of this type of organometallics having heterocyclic O-functionalized NHCs. A series of sterically-tuned benzimidazolium hexafluorophosphate derivatives 11–15 with chlorocoumarin substituents were prepared by the reaction of 1-alkyl/benzyl-benzimidazole with 4-bromomethyl-6-chlorocoumarin followed by salt metathesis reaction using potassium hexafluorophosphate. Corresponding bis-NHC silver complexes 16–20 were prepared in good yields by the reaction of salts 11–15 with silver(I) oxide under dark following in situ deprotonation protocol. All new compounds were characterized using spectral techniques such as 1H and 13C NMR, ATR-IR and analytical methods. Structure of the allyl-substituted silver(I) ​complex 16 was unequivocally established by single crystal X-ray diffraction method. Pronounced antibacterial effects were noted for all silver complexes, along with comparable minimal inhibitory concentration (MIC) with that of internal standard drug, ampicillin. Disc diffusion studies showed that few of the complexes evidenced higher activity against Escherichia coli bacteria than that of the standard drug.