133Cs NMR Study of Cs+ Ion Complexes with Aza-18-crown-6, Diaza-18-crown-6 and Dibenzyldiaza-18-crown-6 in Binary Acetonitrile-Nitromethane Mixtures

Cesium-133 nuclear magnetic resonance spectroscopy was used as a sensitive probe to investigate the stoichiometry and stability of Cs+ ion complexes with aza-18-crown-6 (A18C6), diaza-18-crown-6 (DA18C6) and dibenzylediaza-18-crown-6 (DBzDA18C6) in different binary acetonitrile-nitromethane mixtures. In all cases, the exchange between free and complexed cesium ion was fast on the NMR time scale and only a single population average resonance was observed. The Cs-133 chemical shift-mole ratio data indicated that the cesium ion forms 1:1 cation-ligand complexes with the investigated azacrowns in all acetonitrile-nitromethane mixtures. The formation constants of the resulting complexes were evaluated from computer fitting of the chemical shift-mole ratio data. The stability of the resulting 1:1 complexes with Cs+ were found to vary in the order A18C6 > DBzDA18C6 > DA18C6. In all cases, there is the inverse relationship between the complex stability constants and the amount of acetonitrile in the mixed solvent.