Calix[4]arene-bis(t-octylbenzo-18-crown-6) as an extraordinarily effective macrocyclic receptor for the univalent thallium cation

Abstract

From extraction experiments and $$ \gamma $$ -activity measurements, the exchange extraction constant corresponding to the equilibrium Tl+ (aq) + 1·Cs+ (org) ⇔ 1·Tl+ (org) + Cs+ (aq) taking place in the two-phase water–phenyltrifluoromethyl sulfone (abbrev. FS 13) system (1 = calix[4]arene-bis(t-octylbenzo-18-crown-6); aq = aqueous phase, org = FS 13 phase) was evaluated as log K ex (Tl+, 1·Cs+) = 1.7 ± 0.1. Further, the extraordinarily high stability constant of the 1·Tl+ complex in FS 13 saturated with water was calculated for a temperature of 25 °C: log β org(1·Tl+) = 13.1 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1·Tl+ was derived. In the resulting 1·Tl+ complex, the “central” cation Tl+ is bound by eight bond interactions to six oxygen atoms from the respective 18-crown-6 moiety and to two carbons of the corresponding two benzene rings of the parent receptor 1 via cation–π interaction.