p –Nitrobenzyl-substituted N –heterocyclic carbene in Silver(I) and Gold(I) complexes and their antibacterial activities

The synthesis of the silver(I) N –heterocyclic carbene [Ag(NBMIM)Cl] 2 ( 2 ) from the reaction of imidazolium salt {NBMIM}Cl ( 1a ) (NBMIM = 1–methyl–3–(4–nitrobenzyl)–1 H –imidazol–3–ium) and silver(I) oxide was achieved. Compound {NBMIM}PF 6 ( 1b ) was also obtained by a counteranion exchange reaction. Compound 2 was used as a transmetalation agent to form Au(I) monocarbene complex ( 3a ) and biscarbene complex ( 3b ), by using [AuCl(SMe 2 )]. All compounds were characterized in solution by infrared spectroscopy and by 1 H and 13 C{ 1 H} NMR. The molecular structures of compounds {NBMIM}PF 6 ( 1b ), [Ag(NBMIM)Cl] 2 ( 2 ), and [Au(NBMIM)Cl] ( 3a ) were also determined by single crystal X–ray diffraction studies. The molecular structure of silver complex 2 showed a 1D–staircase structure with Ag⋯Ag and Ag⋯Cl intermolecular interactions. Compounds 2 and 3a were also characterized by elemental analysis, powder X–ray diffraction (PXRD), Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) measurement. In preliminary in vitro antibacterial assays, compounds 2 and 3a were tested against four bacterial strains: two Gram positive ( S. aureus and B. subtilis ) and two Gram negative species ( P. aeruginosa and E. coli ) by Kirby–Bauer’s disk diffusion method. Compound 2 exhibited moderate antibacterial activity against both types of bacteria. Both silver(I) and gold(I) N –heterocyclic carbenes showed from good to high antibacterial activity against S. aureus and P. aeruginosa and moderate activity against B. subtilis and E. coli strains.