A THERMODYNAMIC STUDY OF COMPLEXATION OF ALKALI AND ALKALINE-EARTH METAL-IONS WITH LOW-SYMMETRY CRYPTANDS

Citation
Xx. Zhang et al., A THERMODYNAMIC STUDY OF COMPLEXATION OF ALKALI AND ALKALINE-EARTH METAL-IONS WITH LOW-SYMMETRY CRYPTANDS, Inorganica Chimica Acta, 254(1), 1997, pp. 43-47
Citations number
18
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry Inorganic & Nuclear
Journal title
ISSN journal
0020-1693
Volume
254
Issue
1
Year of publication
1997
Pages
43 - 47
Database
ISI
SICI code
0020-1693(1997)254:1<43:ATSOCO>2.0.ZU;2-A
Abstract
Log K, Delta H and T Delta S values for interactions of two low-symmet ry cryptands, [3.2.1] and [4.2.2], with alkali and alkaline-earth meta l ions have been determined at 25 degrees C by calorimetric titration in a 95:5 (vol./vol.) methanol/water (95% M/W) solvent and an aqueous solution. Cryptand [3.2.1] forms very stable complexes with alkali met al ions (except Li+), Sr2+ and Ba2+ in 95% M/W. The log K values have the sequences of K+ > Rb+ > Na+ > Cs + > Li+ and Ba2+ > Sr2+. Because of its large size, [4.2.2] forms less stable complexes than [2.2.2], [ 3.2.1] and [3.2.2] with Na+, K+ and Ba2+ but a highly stable complex w ith Cs+ is formed. Cryptand [3.2.1] is an isomer of cryptand [2.2.2],b ut it shows cation-binding properties different from those of [2.2.2] due to its low symmetry. [3.2.1] binds Li+ and Cs+ more strongly but N a+, K+, Rb+, Sr2+ and Ba2+ more weakly than does cryptand [2.2.2]. The decrease in log K values for [3.2.1] interactions with Na+, K+, Rb+, Sr2+ and Ba2+ results from decreased enthalpic gains suggesting a larg er steric strain for the [3.2.1] complexes than for the [2.2.2] ones. The K+/Na+ selectivity factors of [3.2.1] and [4.2.2] are lower than t hat of [2.2.2]. The study shows that symmetry is an important factor i n cryptand complexation and selectivity. Cryptands of high symmetry us ually show better complexation and selectivity properties than those o f low symmetry. Thermodynamic quantities give some insight into crypta nd complexation with the metal ions. The log K values for interaction of [3.2.1] with the cations decrease dramatically in aqueous solution, which is caused by large decreases in enthalpic gains.