Cu(I)-<i>N</i>-heterocyclic carbene complexes bearing a naphthyl substituent: synthesis, characterization, and antimicrobial activities

A series of 1-(alkyl)-3-(1-naphthyl)imidazolinium bromides (2a-d) as N-heterocyclic carbene (NHC) precursors were synthesized from 3-(1-naphthyl)-1H-imidazolinium chloride (1). By reacting these NHC precursors with fresh CuCl, Cu(I)-NHC complexes (3a-d) were synthesized. The structures of the compounds were elucidated using H-1, C-13-NMR, FT-IR, ESI-MS, X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis, and elemental analysis techniques. The thermal properties of selected complexes were determined. Salts (1, 2a-d) and Cu(I)-NHC complexes (3a-d) were investigated against test microorganisms to determine their antimicrobial potency. The imidazolinium salts 2b and 2d displayed remarkable antibacterial efficacy, particularly against gram-positive bacteria; however, the corresponding Cu(I)-NHC derivatives (3a-d) did not exhibit a similar level of potency probably due to limited solubility. Overall, it can be concluded that incorporation of N-substituent species with O-heteroatoms, instead of benzyl, allyl, or hexyl groups on NHC, can enhance the activity against microbial derivatives.