A symmetrical tetra N-heterocyclic carbene binuclear silver(I) complex: synthesis, characterization and anticancer study

The synthesis of dinuclear carbene complexes from tetrakis-N-heterocyclic (NHC) and silver(I) ions has been achieved through molecular self-assembly. The reaction between 1,2,4,5-tetra(bromomethyl)benzene with four equivalents of N-substituted alkyl benzimidazole resulted in formation of various tetra-benzimidazolium salts, [H4L]<middle dot>4Br(-) (1-5). Further reaction of the respective salts with Ag2O yielded mono tetra-NHC dinuclear silver(I) complexes, [Ag2L1]<middle dot>2PF(6) (6-10). The salts and their silver(I) complexes were characterized by H-1 and C-13 NMR, FTIR, elemental analysis, molar conductivity, and UV-Visible spectroscopy. Structure elucidation from single crystal X-ray diffraction analyses data revealed that 10 is a dinuclear complex in a tweezer-like structure containing two silver(I) ions that are bridged by one tetra-carbene ligand with the cyclic aromatic backbone of the ligand perpendicular to the benzimidazolium moieties. Anticancer activities of the salts (1-5) and complexes (6-10) were studied, with Etoposide used as the positive control. Subsequently, 1 showed less activity at 24 h while all salts, 1-5, showed better activity at 48 h. The complexes (6-10) indicated moderate and better activity, which is higher compared to 1-5 and Etoposide, both within 24 and 48 h incubation periods.