Complexation of the strontium cation with a macrocyclic lactam receptor: Experimental and theoretical study

Abstract

On the basis of 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=macrocyclic lactam receptor — see Scheme 1; aq=aqueous phase, nb=nitrobenzene phase) was evaluated as log Kex (1·Sr2+, 2A−)=−4.0±0.1. Further, the stability constant of the 1·Sr2+ cationic complex in nitrobenzene saturated with water was calculated for a temperature of 25°C: log βnb (1·Sr2+)=5.1±0.1. Finally, applying quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1·Sr2+ was derived. In the resulting complex, which is most energetically favored, the “central” cation Sr2+ is bound by five bonding interactions to two ethereal oxygen atoms and two carbonyl oxygens, as well as to one carbon atom of the corresponding benzene ring of the parent macrocyclic lactam receptor 1 via cation–π interaction.