Experimental and theoretical study on the complexation of the strontium cation with nonactin

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Sr 2 + ( aq ) + 2 A - ( aq ) + 1 ( nb ) ⇔ 1 · Sr 2 + ( nb ) + 2 A - ( nb ) taking place in the two-phase water-nitrobenzene system (A− = picrate, 1 = nonactin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex(1·Sr2+, 2A−) = −2.0 ± 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 βnb (1·Sr2+) = 7.1 ± 0.1. Finally, by using DFT calculations, the most probable structure of the cationic complex species 1·Sr2+ was derived. In the resulting complex having S4 symmetry, the “central” cation Sr2+ is bound by eight relatively strong bond interactions to eight oxygen atoms of the parent nonactin ligand 1.