Complexation of the calcium cation with antamanide: An experimental and theoretical study

By using extraction experiments and ?-activity measurements, the extraction constant corresponding to the equilibrium Ca²⁺(aq) + 1 ·Sr²⁺(nb) 1 ·Ca²⁺(nb) + Sr²⁺(aq) occurring in the two-phase water-nitrobenzene system (1 = antamanide; aq = aqueous phase, nb = nitrobenzene phase) was determined as log K<inf>ex</inf> (Ca²⁺, 1 ·Sr²⁺) = 1.6 ± 0.1. Further, the stability constant of the 1 ·Ca²⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log ?<inf>nb</inf> (1 ·Ca²⁺) = 10.9 ± 0.2. Finally, applying quantum mechanical density functional level of theory calculations, the most probable structure of the cationic complex species 1 ·Ca²⁺ was derived. In the resulting complex, the central cation Ca²⁺ is bound by six strong bonding interactions to the corresponding six carbonyl oxygen atoms of the parent ligand 1. Besides, the whole 1 ·Ca²⁺ complex structure is stabilised by two intramolecular hydrogen bonds. The interaction energy of the considered 1 ·Ca²⁺ complex, involving the Boys-Bernardi counterpoise corrections of the basis set superposition error, was found to be-1219.3 kJ/mol, confirming the formation of this cationic species.