Interaction of the univalent thallium cation with antamanide: Experimental and theoretical study

On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Tl+(aq) + 1·Na+(nb) ⇔ 1·Tl+ (nb) + Na+(aq) occurring in the two-phase water–nitrobenzene system (1 = antamanide; aq = aqueous phase, nb = nitrobenzene phase) was determined as log Kex (Tl+, 1·Na+) = 0.7 ± 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+) = 4.5 ± 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 complex, the “central” cation Tl+ is bound by four bond interactions to the corresponding four carbonyl oxygen atoms of the parent ligand 1. Besides, the whole 1·Tl+ complex structure is stabilized by two intramolecular hydrogen bonds. The interaction energy of the considered 1·Tl+ complex was found to be −359.0 kJ/mol, confirming also the formation of this cationic species.