New class of non-symmetrical homo-dibenzimidazolium salts and their dinuclear Silver(I) di-NHC complexes

Abstract This work describes the synthesis of an uncommon non-symmetrical dibenzimidazolium salts as a precursor for the dinuclear Ag(I) di-NHC complexes with a formula of [Ag2L2]‧2PF6 (L = NHC = bis-N-heterocyclic carbene). Through the reaction of 3-(2-bromoethyl)-1-butylbenzimidazole bromide (i) with n-alkylbenzimidazole (alkyl = methyl, ethyl, propyl, pentyl, hexyl, heptyl, benzyl), seven non-symmetrical dibenzimidazolium bromide salts, 1–3 and 5–8 were obtained. The symmetrical dibenzimidazolium bromide salt 4 was obtained either as a minor product from the reaction resulting i or through the reaction between n-butylbenzimidazole with 1,2-dibromoethane in 2:1 M ratio. Salts 1–8 were then reacted with Ag2O via in-situ deprotonation method to facilitate the formation of dinuclear Ag(I) di-NHC complexes, 9–16. The formation of these complexes is confirmed by the disappearance of both H2’ peaks in 1H NMR and the presence of carbene peaks in the range of 187–191 ppm in the 13C NMR of the complexes. From single crystal X-ray diffraction study, complex 16 is determined to be a dinuclear complex bearing dicarbene ligands which is further stabilized by significant argentophilic interaction with the separation of Ag⋯Ag being 3.358(5) A. All the bis-benzimidazolium salts 1–8 show no activities while all dinuclear Ag(I) di-NHC complexes, 9–16 show medium to higher activities against E. coli (ATCC 25922) and S. aureus (ATCC 12600) compared to the standard antibiotic drug, Ampicillin.