Synthesis, crystal structure, and DNA-binding studies of different coordinate binuclear silver(I) complexes with benzimidazole open-chain ether ligands

Three new binuclear silver(I) complexes, namely, [Ag2(Meobb)2](pic)2·2H2O (1), [Ag2(Etobb)2(pic)](pic)·(CH3CN) (2) and [Ag2(Bobb)2(pic)2] (3) (Meobb = 1,3-bis(1-methylbenzimidazol-2-yl)-2-oxapropane, Etobb = 1,3-bis(1-ethylbenzimidazol-2-yl)-2-oxapropane, Bobb = 1,3-bis(1-benzylbenzimidazol-2-yl)-2-oxapropane, pic = picrate), have been synthesized and characterized by elemental analyses, IR spectroscopy and X-ray single crystal diffraction. Complex 1 displays an Ag2(Etobb)2 dimeric structure. Each silver(I) ion is coordinated to two nitrogen atoms that adopt a distorted linear configuration. In complex 2, the coordination environment of silver(I) atoms is different. The Ag1 ion is coordinated in a T-shaped tri-coordinated geometry and the Ag2 is best described to be in a distorted tetrahedron. Complex 3 exhibits a discrete di-silver metallacyclic framework. One silver atom (Ag1) is present in a distorted tetrahedral geometry, whereas the other silver atom (Ag2) is five-coordinated with two nitrogen atoms, oxygen atoms and the Ag1 to form a distorted square-based pyramidal configuration. The interactions of the three complexes with calf thymus DNA (CT-DNA) has been investigated by electronic absorption titration, fluorescence spectroscopy and viscosity measurements, and the modes of CT-DNA binding to the complexes have been proposed. Experimental results suggest that the silver(I) complexes bind to DNA in an intercalation mode, and their binding affinity for DNA follows the order 1 > 2 > 3. The DNA-binding studies demonstrate that steric hindrance has a large influence on the binding ability to DNA. The complex that has smaller steric hindrance binds more strongly to DNA.