Novel derivatives of the antibiotic NHC–Ag(I) drug candidate SBC3: Synthesis, biological evaluation and 109Ag NMR studies

The synthesis of six novel N-heterocyclic carbene silver(I) acetate complexes, three symmetrical and three non-symmetrical, were achieved using 4,5-diphenylimidazole to produce intermediate imidazolium salts and then obtain the corresponding silver(I) complexes through complexation with silver acetate via the Youngs’ method. In vitro biological testing, using the Kirby–Bauer disk diffusion method, was conducted against Methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli, with a NHC–silver(I) acetate compound, SBC3, and Tetracycline as standards. Silver(I) acetate complex 7 resulted in a 4 mm clearance against MRSA, showing the highest antibiotic activity of the novel derivatives. Crystallographic data revealed similar bond lengths and angles to previously reported NHC–silver(I) acetate complexes, with complex 8 showing interesting η2-coordination between the silver atom and acetate oxygens. 109Ag NMR studies were conducted, highlighting the effects of the substituents of the imidazole ring on the silver atom shown by the corresponding shifts in the 109Ag NMR spectra. The incorporation of isopropyl groups to several of the novel complexes resulted in larger upfield 109Ag NMR shift values compared to all other substituents