Unusual encapsulation of two anions in the cavity of neutral macrocyclic octalactam - Preliminary communication

A new neutral anion receptor - macrocyclic octalactam 2, possessing a large 36-membered ring, was synthesized, and its unusual property of encapsulation of two Cl- ions and two H2O molecules in the cavity was confirmed by X-ray crystallography (see Figure). The octalactam, consisting of four 2,6-dicarbamoylpyridine and four ethylene moieties, exhibits a symmetrical, fully expanded conformation in the complex [(Cl)(2)(2)(H2O)(2)](2-) (obtained as 2 . 2 Bu4NCl . 4H(2)O . 2CH(2)Cl(2)). Inside the cavity, encapsulated species - two Cl- ions and two H2O molecules - are connected to each other by H-bonds. This kind of complex-in-complex system is formed by means of twenty intra-ring H-bonds and additionally four from the 'outside' H2O molecules. Considering size and symmetry. it can be stated that the planar (H2O . Cl-)(2) dimer fits well into the cavity of octalactam 2. The ligand has an optimal arrangement of binding sites to form H-bonds to every corner of the dimer.