Antimicrobial Effects of Silver(I) and Bismuth(III) Complexes with Secnidazole-Derived Schiff Base Ligands: the Role of the Nitro Group Reduction

[Ag(HL)NO3] complexes (1-4) were obtained with (E)-N’-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)acetohydrazide (HL1); (E)-N’-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)benzohydrazide (HL2); (E)-2-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)hydrazinecarbothioamide (HL3) and (E)-N-methyl-2-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)hydrazinecarbothioamide (HL4) secnidazole-derived Schiff bases. In addition, complexes [Bi(HL3)Cl3] (5) and [Bi(HL4)Cl3] (6) were also prepared. The silver(I) complexes (1-4) showed antifungal activity against Candida fungal strains while the uncomplexed ligands and the bismuth(III) complexes (5-6) were inactive, suggesting that the antifungal effects are probably due to the presence of silver. Although the Schiff base ligands and complexes (1-6) revealed to be inactive against Gram-positive and Gram-negative aerobic bacteria, all compounds exhibited potent antimicrobial effects against several anaerobic bacterial strains, indicating that their mode of action probably involves anaerobic bio reduction of the nitro group, with formation of metabolites which are toxic to the microorganisms. Electrochemistry studies showed that bio reduction of the nitro group is favored in complexes (1-6) in comparison to the free ligands, which might, at least in part, explain their increased antimicrobial effects.