Open Chain Tetrabenzimidazolium Salts as Ligand Precursors for Silver(I)-<i>N</i>- Heterocyclic Carbene Complexes: Synthesis, Crystal Structure and Antibacterial Studies

The synthesis of four silver (I)-N-heterocyclic carbene (NHC) complexes bearing tetrabenzimidazol-2-yl ligands is described. The ligand precursors, open-chain tetrabenzimidazolium salts 1-4 was synthesized by the reaction between 3-(2-bromoethyl)-1-alkylbenzimidazole bromide (alkyl = ethyl, n-propyl, n-butyl, n-benzyl), i-iv with 1,2-ethylbisbenzimidazole in 2:1 molar ratio. Furthermore, their respective silver (I)-NHC complexes Ag1-Ag4 were synthesized via in-situ deprotonation method of the salts with silver oxide in 1:4 molar ratio. The synthesis of all salts and complexes were suggested by melting point, elemental analysis, FTIR studies, 1H and 13C NMR spectra. The reported silver (I)-NHC complexes with tetrabenzimidazol-2-ylidene ligands, Ag5 did not form the expected tetranuclear silver (I)-NHC complexes with the formula of [Ag4(μ4-NHC)2]∙4PF6, but the dinuclear silver (I)-NHC complexes with the formula of [Ag2(μ-NHC)]∙2PF6 was obtained, even after the changes in the molar ratio of the tetrabenzimidazolium salts and metal source. Benzimidazolium salt used as a precursor to synthesize Ag5 is similar to the structures of benzimidazolium salts 1-4, only with the different middle linker; butylene instead of ethylene chain for 1-4. Hence, we proposed that the structure of Ag1-Ag4 is similar to the structure of Ag5, suggested by spectral and elemental studies. From antibacterial study against E. coli (ATCC 25922) and S. aureus (ATCC 12600), all silver (I)-NHC complexes, Ag1-Ag4, show medium to higher activities against both bacteria compared to the standard antibiotic drug, ampicillin, while complexes Ag3 possessed the highest activity among all.