The significant role of the solvent in high selectivity of symmetrical calix[4]tubes for potassium ion in solution: A DFT study

In this work the formation of p-tert-butylcalix[4]tube (1) and p-H-calix[4]tube (3) complexes with Na+, K+, Rb+ and Cs+ cations are studied theoretically. Two different modes of complexation, poly-ether tube (p.e.t.) and benzene-rings pocket (b.r.p.), were investigated and the results showed that the complexes of p.e.t. form are significantly more stable. Whereas K+ selectivity is observed for calix[4]tubes 1 and 3 in aqueous environments, calculations show that both calix[4]tubes have larger bonding interaction with Na+. Thus, the K+ selectivity of calix[4]tubes in solution is not an intrinsic property of the calix[4]tube itself. Indeed, our calculations show that the selectivity is reversed for Na+ and K+ because the K+ leaves much easier the solvent molecules and has a greater chance to go into the calix[4]tubes.