Ether and coumarin–functionalized (benz)imidazolium salts and their silver(I)–N–heterocyclic carbene complexes: Synthesis, characterization, crystal structures and antimicrobial studies

Abstract

To explore the impact of different coumarin substituted N–heterocyclic carbene (NHC) ligand backbones on the biological applications of corresponding silver(I) complexes, a series of structurally related ether–functionalized imidazolium (3–5) and benzimidazolium (6–8) hexafluorophosphate salts bearing 6–methylcoumarin, 6–chlorocoumarin and 5,6–benzannulated coumarin substituents, have been reported. These salts have been employed to react with silver(I) oxide at mild reaction conditions to obtain corresponding ionic, bis–NHC coordinated silver(I) hexafluorophosphate complexes (9–14) in excellent yields following in situ deprotonation method. Further, the bromide counterparts of the salts have been treated with silver(I) oxide in dichloromethane to afford neutral mono–NHC coordinated bromido silver(I) complexes (15–20). Both, azolium salts and their silver(I)–NHC complexes, have been thoroughly characterized by 1H and 13C NMR, ATR–IR and elemental analyses. The structure of a benzannulated coumarin substituted imidazolium hexafluorophosphate salt 5 and a silver complex 12 of benzimidazole–based NHC bearing methylcoumarin substituent have been studied through single crystal X–ray diffraction technique. In the case of complex 12, the metal center lies at the inversion center adopting linear coordination geometry with anti– arrangement of the NHC ligands. Feeble π–π stacking interactions between adjacent coumarin rings have been evident in the extended complex structure along with the hydrogen bonding interactions. In the preliminary antibacterial evaluations, silver complexes displayed promising activity with the MIC values in the range 8–64 μg/mL against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Salmonella typhi, while azolium salts displayed almost no activity.