Water Trapping of Metal-Organic Cages with Endohedral Variation

A series of lantern-shaped M2L4 metal organic cages was synthesized by coordination-driven self-assembly of palladium(II) centers with aromatic amide bidentate ligands, and their crystal structures were investigated. Amide-functionalized ligands provide the complex with polar internal environment and the ability to encapsulate water. Tuning the size of the internal substituent allows to adjust the conformation of the ligand as well as to operate the pattern of counter-anions via steric effect and, further, to influence the arrangement of the encapsulated water guests. Contrary to expectations, the cage decorated with fluorine atoms only encapsulates two water molecules, whereas the cage substituted with larger methoxy groups can bind six water molecules with a chair arrangement. The enlarged capacity was attributed to the extrusion of two nitrate counter-anions from the cavity.