Synthesis, characterization, crystal structure and antibacterial properties of N- and O-functionalized (benz)imidazolium salts and their N-heterocyclic carbene silver(I) complexes

Here, two series of structurally and electronically varied benzonitrile and coumarin substituted imidazolium (2-5) and benzimidazolium (7-10) hexafluorophosphate salts and corresponding NHC silver(I) complexes (11-18) are reported. Substituents on the coumarin ring are varied with an intention of bringing appropriate electronic changes in the complex, which facilitates the targeted activity. Additionally, the structure of two azolium salts (2 and 7) has been established by SCXRD technique. Extended structure of these salts witnessed a series of hydrogen bonds between the azolium cations and the hexafluorophosphate anions along with moderate π-π stacking interactions operating between the adjacent heterocyclic and/or aromatic rings such as coumarin, (benz)imidazole and benzonitrile phenyl systems. Further, a preliminary antibacterial study of the representative salts and silver complexes against a Gram positive (S. aureus, ATCC 29213) and a Gram negative (E. coli, ATCC 25922) bacteria has been carried out. Both the series of silver complexes demonstrated potent antibacterial activity with a MIC of 8/16 μg/mL against E. coli, while there activity against S. aureus found moderate with a MIC in the range 32-128 μg/mL. Furthermore, the salts evidenced no activity in the working concentration range against tested bacteria, indicating that the activity of the complexes is due to the metal center.