In traditional supramolecular chemistry the selectivity is mainly characterized by the hole-size fitting concept. However nowadays, we know that the selectivity depends on several factors like - role etc. 1 Following our interest in predicting the formation constants of metal complexes of macrocyclic and macroacyclic ligands2 4 we studied the origin of the selectivity of some 3D hosts for alkaline cations or halide anions.5 8 Two types of host-guest systems were studied: (i) protonated azacryptands with halide anions; (ii) calix4 tubes with alkali metal cations (see Fig. 1). In all cases, our calculations showed that the solvent molecules play an important role in the formation of these complexes. Indeed, always there is a solvent hindrance against the desolvation of host and guest ions that occurs during the formation of host-guest complex in solution. However, whenever the process of desolvation of isolated host and guest ions is not very endothermic and solvation of new host-guest complex is exothermic enough, then the solvent hindrance against the host-guest complexation decreases and formation constant of corresponding complex increases. Thus, in order to design a good host for a special guest, in addition to all important parameters such as a good host-guest interaction, hole size fitting and a good degree of preorganization, we must also think about its structure/charge and strength of its interaction with solvent molecules.
