Thermodynamics of Complexation of Alkali Metal Cations by a Lower-Rim Calix[4]arene Amino Acid Derivative |
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Authors: | Josip Požar Tajana Preočanin Leo Frkanec Vladislav Tomišić |
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Institution: | (1) Laboratory of Physical Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia;(2) Laboratory of Supramolecular and Nucleoside Chemistry, Department of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Zagreb, Croatia; |
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Abstract: | Complexation of alkali metal cations with 5,11,17,23-tetra-tert-butyl-26,28,25,27-tetrakis(O-methyl-d-α-phenylglycylcarbonylmethoxy)calix4]arene (L) in methanol and acetonitrile was studied by means of direct and competitive microcalorimetric titrations at 25 °C. The thermodynamic
parameters of complexation reactions showed that all the reactions investigated were enthalpically controlled. In both solvents
the reaction enthalpy was most favorable for Na+ binding with L leading to the highest affinity of the examined calix4]arene derivative towards this cation. The solubilities (and consequently
the solution Gibbs energies) of the ligand were determined, as were the corresponding solution enthalpies and entropies. No
significant difference was observed between the solution thermodynamic quantities of L in the two solvents, whereas the transfer of complex species from methanol to acetonitrile was found to be quite favorable.
The interactions of solvent molecules with the free and the complexed ligand were investigated by 1H NMR spectroscopy. It was concluded that in both cases inclusion of an acetonitrile molecule into the hydrophobic cavity
of L occurred, which significantly affected the cation complexation in this solvent. The thermodynamic data were discussed regarding
the structural properties of the ligand, the free and the complexed cations as well as the solvation abilities of the solvents
examined. In this respect, the specific solvent-solute interactions and the intramolecular NH⋅⋅⋅O=C hydrogen bonds at the
lower rim of L were particularly addressed. |
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