Intrinsic affinity, size match and solvent hindrance during complexation of cryptands 221 and 222 with Na+ and K+ cations; a computational study

Molecular recognition plays a key role in many biochemical processes, in the design of new materials, and in the development of new procedures for analytical chemistry [1].Host-guest interactions, which are usually noncovalent, have been modeled in many simple systems using the premises of supramolecular chemistry [2,3]. The 221 and 222 cryptands (see Figure 1a) are synthetic macrocyclic multidentate ligands (hosts) that exhibit high selectivity towards metal ions (guests), and have been used as prototype formolecular recognition by complexation. The experimental observations reveal that 221 is selective for Na+ over K+ and 222 is selective for K+ over Na+. Herein, we report atheoretical study on complexation of cryptands 221 and 222 with Na+ and K+ cations, at BP86/def2-TZVP level of theory. The calculations on the interaction energies in the gasphase show that the interaction of both hosts with sodium ion is larger than that with potassium and this means that 221 and 222 have intrinsically larger affinity for sodiumion. However, the results of calculations on the formation of complexes in solution are in agreement with experimental observations (see Figure 1b). Indeed, in the solution there is a competition between the host and solvent molecules to catch the metal cation