Coumarin substituted 4–aryl–1,2,4–triazolium salts and their silver(I) N–heterocyclic carbene complexes: Effects of counterions on the antioxidant and antihaemolytic properties

Abstract Although numerous silver complexes have been found to target human derived leukemia cells, the nature and reactivity of such species with normal human red blood cells and targeting oxidants remain unexplored. In this contribution, we report the synthesis and characterization of three series of structurally varied silver complexes derived from biologically active coumarin substituted 1,2,4–triazol–5–ylidine ligands. 1,2,4–Triazolium salts (6–13) that serve as NHC precursors have been prepared by treating 4–(2,6–dimethylphenyl) triazole with substituted 4–bromomethylcoumarin derivatives, and corresponding silver NHC complexes (14–25) by in situ deprotonation method using appropriate silver sources. Complexes 14, 15 and 18 have been studied for their structure following single crystal X–ray diffraction technique, which adjudicates the presence of silver atom in an almost linear coordination geometry. Complexes 21 and 22 displayed promising antioxidant potential with the IC50 (a concentration of test sample required to neutralize DPPH radicals by 50%) value of 7.49 ± 0.702, 7.42 ± 0.125 μM, respectively; while the triazolium salt displayed no activity in the working concentration range (10–250 μM). Likewise, at a concentration of 100 μM the silver complexes showed significantly less percentage lysis of human red blood cells in the range 2.36 ± 0.59 to 8.49 ± 0.27% (except 18, 24 and 25).