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

Abstract

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 1H, 13C-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.