Unveiling the formation 1 : 2 supramolecular complexes between cucurbit[7]uril and a cationic calix[4]arene derivative

The formation of host-guest complexes between cucurbit[7]uril (CB7) and a tetracationic calix[4]arene derivative in the so-called cone conformation was investigated by ¹H NMR, DOSY NMR, isothermal titration calorimetry and ESI-MS. The results point to the formation of 1 : 1, 1 : 2 and 2 : 1 CB7 : calixarene complexes with binding constants of 3 × 10⁶ M^-1, ≈2 × 10² M^-1, and 9 × 10⁴ M^-1 respectively. The study demonstrates, on one hand, that despite having four potential recognition sites, the calixarene only binds two CB7 molecules and, on the other, that for sterically crowded binding motifs that prevent CB7 inclusion with optimized hydrophobic and ion-dipole interactions, the formation of 1 : 2 complexes can be observed most likely due to formation of external binding of the cationic moieties to the CB7 carbonyl portals.