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A comprehensive study of alkali metal cations complexation by lower-rim calix[4]arene amide derivatives

Abstract

Complexation of alkali metal cations by lower-rim N,N-dihexyl¬acetamide (L1) and newly synthesized N-hexyl-N-methylacetamide (L2) calix[4]arene tertiary-amide derivatives was thoroughly studied at 25 °C in acetonitrile (MeCN), benzonitrile (PhCN), and methanol (MeOH) by means of direct and competitive microcalorimetric titrations, as well as UV and 1H NMR spectroscopies. In addition, by measuring ligands solubilities, solution (transfer) Gibbs energies of the ligands and their alkali metal complexes were obtained. The inclusion of solvent molecules in the free and complexed calixarene hydrophobic cavity was also investigated. Computational (classical molecular dynamics) investigations of the studied systems were carried out as well. The obtained results were compared with those previously obtained by studying complexation abilities of N-hexylacetamidecalix[4]arene secondary-amide derivative (L3). The stability constants of 1:1 complexes were determined in all solvents used (the values obtained by different methods being in excellent agreement), as were the corresponding complexation enthalpies and entropies. Almost all of the examined reactions were enthalpically controlled. The most striking exceptions were reactions of Li+ with both ligands in methanol, for which entropic contribution to the reaction Gibbs energy was substantial due the entropically favourable desolvation of the smallest lithium cation. Thermodynamic stabilities of the complexes were quite solvent dependent (stability decreased in the solvent order: MeCN > PhCN >> MeOH), which could be accounted for by considering the differences in the solvation of the ligand as well as free and complexed alkali metal cations in the solvents used. Comparison of the stability constants of ligands L1 and L2 complexes clearly revealed that higher electron-donating ability of the hexyl with respect to methyl group is of considerable importance in determining the equilibria of the complexation reactions. Additionally, the quite strong influence of intramolecular hydrogen bonds formation in compound L3 (not present in ligands L1 and L2) as well that of inclusion of solvent molecules into the calixarene hydrophobic cone were shown to be of great importance in determining the calixarene-cation complex thermodynamic stability. The experimental results were fully supported by those obtained by MD simulations.

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Publication details

The article was received on 12 Jun 2017, accepted on 09 Aug 2017 and first published on 09 Aug 2017


Article type: Paper
DOI: 10.1039/C7CP03920D
Citation: Phys. Chem. Chem. Phys., 2017, Accepted Manuscript
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    A comprehensive study of alkali metal cations complexation by lower-rim calix[4]arene amide derivatives

    G. Horvat, L. Frkanec, N. Cindro and V. Tomisic, Phys. Chem. Chem. Phys., 2017, Accepted Manuscript , DOI: 10.1039/C7CP03920D

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