Sterically modulated silver(I) complexes of coumarin substituted benzimidazol–2–ylidenes: Synthesis, crystal structures and evaluation of their antimicrobial and antilung cancer potentials

Abstract

In this contribution, a series of sterically–encumbered coumarin substituted benzimidazole–based N–heterocyclic carbene (NHC) precursors (1−12) and their silver(I)–NHC complexes (13–24) are reported. Molecular structure of NHC precursors 8 and 12 and cationic complexes 15 and 16 was established by single crystal X–ray diffraction method. The silver(I) complexes demonstrated various significant intramolecular agostic–like interactions operating between the metal center and the hydrogen atoms of the substituents alongside a variety of feeble π–π stacking interactions. A distorted linear coordination geometry is documented at the silver(I) center with the anti–arrangement of the ligands. Further, the complexes demonstrated promising antibacterial properties against Gram positive and Gram negative bacterial strains, especially complex 18 displayed a minimum inhibitory concentration (MIC) of 2 and 4 μg/mL against S. aureus and E. coli, and P. aeruginosa, respectively. Furthermore, complexes 14, 15, 16 and 18 were found cytotoxic against the human lung cancer cell lines A549 and H1975 with the IC50 (concentration of the test sample required to kill 50% of the cell population) value under 10 μM, while mono–NHC complex 20 displayed a potential drug window with the IC50 of 13.7 ± 2.70 and 14.5 ± 1.20 μM against the cancer cell lines H1975 and A549, respectively. Notably, these complexes displayed relatively lesser cytotoxic behaviour against the normal skin fibroblast cell line, Hs68. All the NHC precursors displayed significantly lower biological activities compared with their respective complexes, indicating the utility of silver(I) ions in antimicrobial and antilung cancer applications.