Synthesis, structural characterization, crystal structures and antibacterial potentials of coumarin–tethered N–heterocyclic carbene silver(I) complexes

Abstract

A series of sterically–tuned coumarin–tethered imidazolium bromide (1–7) and hexafluorophosphate (8–14) salts and their mono– and bis–N–heterocyclic carbene (NHC) silver(I) complexes (15–28) of the type [NHCAgOAc] and [NHCAgNHC]+PF6−, respectively, has been reported. The imidazolium salts and silver complexes have been prepared by following the successive N–alkylation and in–situ deprotonation methods, respectively. Both, the imidazolium salts and their silver(I) complexes, have been characterized by 1H and 13C NMR and ATR–IR spectroscopic and elemental analyses. A preliminary stability study of silver complex 18 has been performed using 1H NMR spectroscopy and it is found that the complex is stable in solution for five days upon exposure to sunlight. Molecular structures of bis–NHC coordinated cationic silver(I) hexafluorophosphate complexes, 16 and 17, have been unambiguously established by single crystal X–ray diffraction method. Further, both the salts and their silver(I)NHC complexes have been evaluated for their potential antibacterial properties against a panel of bacterial strains namely, S. aureus, B. subtilis, E. coli, P. aeruginosa, S. typhi and S. sonnei. The results show that silver complexes are effective against E. coli and P. aeruginosa with moderate to high activity, and their minimum inhibitory concentrations ranging from 8 to 16 μg/mL.