Interaction of the thallium cation with 1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6: Experimental and theoretical study

• Stability of the Tl+·1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6 complex was determined.
• The quantum mechanical DFT calculations were carried out.
• Structures A and B of the resulting cationic complex species were predicted.

From 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 = 1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (Tl+, 1·Cs+) = −2.1 ± 0.1. Further, the stability constant of the 1·Tl+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log βnb(1·Tl+) = 11.6 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structures A and B of the cationic complex species 1·Tl+, which are obviously in a dynamic equilibrium, were indicated. In both of these structures of the resulting 1·Tl+ complex, the “central” cation Tl+ is bound by eight strong bond interactions to six oxygen atoms from the 18-crown-6 moiety and to two carbons of the respective two benzene rings of the parent ligand 1 via cation-π interaction.

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