Complexation of Eu3+ with a macrocyclic lactam receptor: Experimental and theoretical study

From extraction experiments and gamma-activity measurements, the extraction constant corresponding to the equilibrium Eu3+(aq) + 3A(-)(aq) + 1 (nb) double left right arrow 1 center dot Eu3+(nb) + 3A(-)(nb) taking place in the two-phase water-nitrobenzene system (A(-) = CF3SO3-; 1 = macrocyclic lactam receptor - see Scheme 1; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K-ex(1 center dot Eu3+, 3A(-)) = -4.1 +/- 0.1. Further, the stability constant of the 1 center dot Eu3+ cationic complex in nitrobenzene saturated with water was calculated for a temperature of 25 degrees C: log beta(nb), (1 center dot Eu3+) = 9.0 +/- 0.1. Finally, by using DFT calculations, the most probable structure of the cationic complex species 1 center dot Eu3+ was derived. In the resulting 1 center dot Eu3+ complex, the "central" cation Eu3+ is bound by five bond 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-pi interaction. (C) 2013 Elsevier B.V. All rights reserved.