Cucurbit[5]uril, decamethylcucurbit[5]uril and cucurbit[6]uril.: Synthesis, solubility and amine complex formation

A simple way to prepare cucurbit[5]uril is described. The macrocycles of the cucurbituril type are nearly insoluble in water. The solubilities of cucurbit[5]uril, decamethylcucurbit[5]uril and cucurbit[6]uril in hydrochloric acid, formic acid and acetic acid of different concentrations have been investigated. Due to the formation of complexes between cucurbit[n]urils and protons the solubility increases in aqueous acids. The macrocyclic ligands are able to form complexes with several organic compounds. Thus, the complex formation of the cucurbituril macrocycles with different amines has been studied by means of calorimetric titrations. The reaction enthalpy gives no evidence of the formation of inclusion or exclusion complexes. H-1-NMR measurements show that in the case of cucurbit[5]uril and cucurbit[6]uril the organic guest compound is included within the hydrophobic cavity. Decamethylcucurbit[5]uril forms only exclusion complexes with organic amines. This was confirmed by the crystal structure of the decamethylcucurbit[5]uril-1,6-diaminohexane complex.