Complexation of decamethylcucurbit[5]uril with alkali metal ions

Four coordination compounds, namely [Na(H2O)(H2O)2⊂C40H50N20O10](C6H6O2)2Cl·8H2O (1), [K2(H2O)2(H2O)⊂C40H50N20O10](C6H6O2)2Cl2·7H2O (2), [Rb2(H2O)2(H2O)⊂C40H50N20O10](C6H6O2)2Cl2·7H2O (3) and [Cs(H2O)2(H2O⊂C40H50N20O10)](C6H6O2)2Cl·6H2O (4), were obtained by the reactions of the corresponding alkali metal salts with decamethylcucurbit[5]uril (Me10Q[5]) in the presence of hydroquinone, and their structures were determined by single-crystal X-ray diffraction. The results revealed that in compounds 1 and 4 each Me10Q[5] ligand coordinates one Na+ or Cs+ ion to form a molecular bowl structure, while in compounds 2 and 3 each Me10Q[5] ligand coordinates two K+ or Rb+ ions to form a closed molecular capsule structure, and adjacent molecular capsules bridge each other through water molecules to form 1D coordination polymers. In addition, we found that the coordination distances for the metal ions and the height of the metal ions out-of-portal-plane for the four compounds are in the same order, 1 < 2 < 3 < 4, which is attributed to the fact that the radius of alkali cations is in the order Na+ < K+ < Rb+ < Cs+. Although each portal of Q[6] binds with two alkali cations (not including Cs+), the Q[6]-based alkali cations complexes display similar structural trends.

Graphical abstractFour compounds of alkali metal ions (Na+, K+, Rb+ and Cs+) with the macrocyclic ligand Me10Q[5] were synthesized and structurally characterized. The relationship between the structural trends of the four compounds and the ionic radii of the alkali metals has been demonstrated. The differences and similarities of the crystal structures of Me10Q[5] and Q[6]-based alkali cations complexes have also been discussed.Figure optionsDownload full-size imageDownload as PowerPoint slide