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

By using extraction experiments and gamma-activity measurements, the extraction constant corresponding to the equilibrium Ca2+(aq) + 1 center dot Sr2+(nb) [GRAPHICS] 1 center dot Ca2+(nb) + Sr2+(aq) occurring in the two-phase water-nitrobenzene system (1 = antamanide; aq = aqueous phase, nb = nitrobenzene phase) was determined as logK(ex) (Ca2+, 1 center dot Sr2+) = 1.6 +/- 0.1. Further, the stability constant of the 1 center dot Ca2+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 degrees C: log beta(nb) (1 center dot 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 center dot 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 ligand1. Besides, the whole 1 center dot Ca2+ complex structure is stabilised by two intramolecular hydrogen bonds. The interaction energy of the considered 1 center dot 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.