Lithium ionophore VIII as an extraordinarily effective receptor for the strontium cation: Experimental and theoretical study

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Sr2+(aq) + 2A−(aq) + 1(nb) ⇔ 1⋅Sr2+(nb) + 2A−(nb) taking place in the two–phase water–nitrobenzene system (A− = picrate, 1 = lithium ionophore VIII; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (1⋅Sr2+, 2A−) = 7.8 ± 0.1. Further, the extremely high stability constant of the 1⋅Sr2+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log βnb (1⋅Sr2+) = 16.9 ± 0.1. Finally, by using DFT 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 very strong bond interactions to the corresponding six oxygen atoms of the parent ligand 1; the interaction energy of this complex was found to be −959.9 kJ/mol.