Folding of coordination polymers into double-stranded helical organization

Self-assembling coordination polymers based on Pd-II and Cu-II metal ions were prepared from complexation of a bent-shaped bispyridine ligand and a corresponding transition metal. These coordination polymers were observed to self-assemble into supramolecular structures that differ significantly depending on the coordination geometry of the metal center. The polymer based on Pd-II self-assembles into a layer structure formed by bridging bispyridine ligands connected in a trans-position of the square-planar coordination geometry of metal center. In contrast, the polymer based on Cu-II adopts a double-helical conformation with regular grooves, driven by interstranded, copper-chloride dimeric interaction. The double-stranded helical organization is further confirmed by structure optimization from density functional theory with aromatic framework, showing that the optimized double-helical structure is energetically favorable and consistent with the experimental results. These results demonstrate that weak metal-ligand bridging interactions can provide a useful strategy to construct stable double-stranded helical nanotubes.