Cucurbit[6]uril as ligand for the complexation of lanthanide cations in aqueous solution

The complex formation between lanthanide cations and the macrocyclic ligand cucurbit[6]uril has been studied in aqueous solution and in aqueous formic acid using different experimental techniques. With the exception of Nd3+ the stability constants of all complexes formed in aqueous formic acid are nearly identical. The values of the reaction enthalpies abruptly increase from a mean value of 1–3.4 kJ mol−1 going from Eu3+ to Gd3+. The values of the reaction entropies are nearly identical for all lanthanide cations. In pure aqueous solution a slight increase of the stability constants from La3+ to Lu3+ is observed.