Calix[4]arene-(2,3-naphthylene-crown-6,crown-6) as an extraordinarily efficient macrocyclic receptor for the univalent thallium cation

Abstract

On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Tl+ (aq) + 1·Cs+ (nb) 1·Tl+ (nb) + Cs+ (aq) taking place in the two-phase water–nitrobenzene system (1 = calix[4]arene-(2,3-naphthylene-crown-6,crown-6); aq = aqueous phase, nb = nitrobenzene phase) was determined as log Kex(Tl+,1·Cs+) = 0.3 ± 0.1. Further, the extremely high stability constant of the 1·Tl+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log βnb(1·Tl+) = 10.0 ± 0.2. Finally, applying quantum mechanical calculations, the most probable structure of the cationic complex species 1·Tl+ was derived. In the resulting complex, the ‘central’ cation Tl+ is bound by eight strong bonds to six oxygen atoms of the crown-6 moiety and to two carbons from the respective two benzene rings of the parent ligand 1 via cation-π interaction. The interaction energy, E(int), of the considered 1·Tl+ complex was found to be −305.7 kJ/mol, also confirming the formation of this cationic species.