Extraction and theoretical study on complexation of the strontium cation with antamanide

Abstract

By using extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Sr²⁺(aq) + 2A⁻(aq) + 1(nb) ⇔ 1·Sr²⁺(nb) + 2A⁻(nb) occurring in the two-phase water–nitrobenzene system (A⁻ = picrate, 1 = antamanide; aq = aqueous phase, nb = nitrobenzene phase) was determined as log K _ex (1·Sr²⁺, 2A⁻) = −0.3 ± 0.1. Further, the stability constant of the 1·Sr²⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β _nb (1·Sr²⁺) = 8.8 ± 0.1. Finally, applying quantum mechanical density functional level of theory calculations, the most probable structure of the cationic complex species 1·Sr²⁺ was derived. In the resulting complex, the “central” cation Sr²⁺ is bound by six bond interactions to the corresponding six oxygen atoms of the parent ligand 1. The interaction energy of the considered 1·Sr²⁺ complex was found to be −1,114.9 kJ/mol, confirming the formation of this cationic species as well.