An interlocked coordination cage based on aromatic amide ligands

An interlocked M₄L₈ coordination cage was synthesized by coordination-driven self-assembly of palladium(II) ions with aromatic amide bidentate ligands. The reaction of the ligand and the metal at 2:1 ratio led to the monomeric M₂L₄ cage as the kinetic product, while the thermodynamic product M₄L₈ cage was obtained by prolongating the reaction. This conversion and the interlocked structure was clearly revealed by using ¹H NMR, mass spectrometry and X-ray crystallography. The driving force of interlocking was mainly attributed to the interactions (hydrogen bonding, aromatic stacking and electrostatic interaction) arising from the aptitude of flexibility of the amide ligand.