Metal-mediated self-assembly of pyridylcalixarenes: prevention of intramolecular metal chelation is essential in constructing molecular capsules.

To construct calixarene-based molecular capsules utilizing the pyridyl-Pd(II) interaction, reactions of cone-pyridylcalix[4]arene 3, cone-pyridylcalix[5]arene 13, and cone-pyridylcalix[4]arene bis-crown 16 with square-planar Pd(II) complex 7 were investigated. Because of the coexistence of intermolecular binding and chelate-forming intramolecular binding, the reactions of cone-pyridylcalix[4]arene 3 or cone-pyridylcalix[5]arene 13 with cis-Pd(II) complex 7 yield complicated, structure-unknown oligomers. The short dioxyethylene bridges on the lower rim of pyridylcalix[4]arene bis-crown 16 rigidify the cone conformation and thus prohibit 16 from the intramolecular binding with a metal component. Thus, two cone-tetrapyridylcalix[4]arene bis-crown 16 and four cis-Pd(II) complex molecules self-assemble into molecular capsules that exist as a parallel/antiparallel conformer mixture in a nearly 1:1 ratio. The results demonstrated that to prevent entropically favorable intramolecular binding is essential is constructing higher capsule-like structures with calixarene building blocks by self-assembling.