Proton NMR probing of stoichiometry and thermodynamic data for the complexation of Na+ and Li+ ions with 15-Crown-5 in acetonitrile–nitrobenzene mixtures

Proton NMR was used to study the complexation reaction of Li+ and Na+ ions with 15-Crown-5 (15C5) in a number of binary acetonitrile (AN)–nitrobenzene (NB) mixtures at different temperatures. In all cases, the exchange between free and complexed 15C5 was fast on the NMR timescale and only a single population average 1H signal was observed. The formation constants of the resulting 1:1 complexes in different solvent mixtures were determined by computer fitting of the chemical shift mole ratio data. There is an inverse relationship between the complex stability and the amount of AN in the solvent mixtures. The enthalpy and entropy values for the complexation reaction were evaluated from the temperature dependence of the formation constants. In all the solvent mixtures studied, the resulting complex is enthalpy stabilized but entropy destabilized. Finally, the experimental results were compared with theoretical ones that were obtained from molecular modeling methods. Based on our results, it is most probable that Li+–15C5 in solvent stays in a rather nesting complex form with greater LogKf values, but Na+–15C5 forms a complete perching complex form with lower LogKf values.

The complex formation reactions between Li+ and Na+ metal cations with the macrocyclic ligand 15-Crown-5 (15C5) were studied in acetonitrile–nitrobenzene binary mixtures at different temperatures using the 1H NMR method. The experimental results were compared with theoretical results obtained from molecular modeling methods. We found similar results to the published theoretical investigation on the same reaction. Based on our results, in all solvents Li+–15C5 has a nesting complex form with greater LogKf values, whilst Na+–15C5 has a complete perching complex form with lower LogKf values.Figure optionsDownload as PowerPoint slideResearch highlights
► The complex formation reactions between Li+ and Na+ with 15-Crown-5 were studied.
► Complexation was studied in acetonitrile–nitrobenzene mixtures, the using 1H NMR method.
► These experimental results were in agreement with theoretical results.
► In all solvents Li+ forms a nesting complex while Na+ forms a perching complex.