Antimicrobial Activity of Anionic Bis(N-Heterocyclic Carbene) Silver Complexes

The antimicrobial properties of a series of anionic bis(carbene) silver complexes Na₃[Ag(NHC^R)₂] were investigated (2a-2g and 2c', where NHC^R is a 2,2'-(imidazol-2-ylidene)dicarboxylate-type N-heterocyclic carbene). The complexes were synthesized by the interaction of imidazolium dicarboxylate compounds with silver oxide in the presence of aqueous sodium hydroxide. Complexes 2f,g were characterized analytically and spectroscopically, and the ligand precursor 1f and complexes 2c and 2g were structurally identified by X-ray diffraction methods. The anions of 2c and 2g, [Ag(NHC^R)₂]^3-, showed a typical linear disposition of C_carbene-Ag-C_carbene atoms and an uncommonly eclipsed conformation of carbene ligands. The antimicrobial properties of complexes 2a-g, which contains chiral (2b-2e and 2c') and non-chiral derivatives (2a,f,g), were evaluated against Gram-negative bacteria, Escherichia coli and Pseudomonas aeruginosa, and a Gram-positive bacterium, Staphylococcus aureus. From the observed values of the minimal inhibitory concentration and minimal bactericidal concentration, complexes 2a and 2b showed the best antimicrobial activity against all strains. An interesting chirality-antimicrobial relationship was found, and eutomer 2c' showed better activity than its enantiomer 2c against the three bacteria. Furthermore, these complexes were investigated experimentally and theoretically by ¹⁰⁹Ag nuclear magnetic resonance, and the electronic and steric characteristics of the dianionic carbene ligands were also examined.