An Integrated Approach (Thermodynamic, Structural, and Computational) to the Study of Complexation of Alkali-Metal Cations by a Lower-Rim Calix[4]arene Amide Derivative in Acetonitrile

The calix[4]­arene secondary-amide derivative <b>L</b> was synthesized, and its complexation with alkali-metal cations in acetonitrile (MeCN) was studied by means of spectrophotometric, NMR, conductometric, and microcalorimetric titrations at 25 °C. The stability constants of the 1:1 (metal/ligand) complexes determined by different methods were in excellent agreement. For the complexation of M⁺ (M = Li, Na, K) with <b>L</b>, both enthalpic and entropic contributions were favorable, with their values and mutual relations being quite strongly dependent on the cation. The enthalpic and overall stability was the largest in the case of the sodium complex. Molecular and crystal structures of free <b>L</b>, its methanol and MeCN solvates, the sodium complex, and its MeCN solvate were determined by single-crystal X-ray diffraction. The inclusion of a MeCN molecule in the calixarene hydrophobic cavity was observed both in solution and in the solid state. This specific interaction was found to be stronger in the case of metal complexes compared to the free ligand because of the better preorganization of the hydrophobic <i>cone</i> to accept the solvent molecule. Density functional theory calculations showed that the flattened <i>cone</i> conformation (<i>C</i>₂ point group) of <b>L</b> was generally more favorable than the square <i>cone</i> conformation (<i>C</i>₄ point group). In the complex with Na⁺, <b>L</b> was in square cone conformation, whereas in its adduct with MeCN, the conformation was slightly distorted from the full symmetry. These conformations were in agreement with those observed in the solid state. The classical molecular dynamics simulations indicated that the MeCN molecule enters the <b>L</b> hydrophobic cavity of both the free ligand and its alkali-metal complexes. The inclusion of MeCN in the <i>cone</i> of free <b>L</b> was accompanied by the conformational change from <i>C</i>₂ to <i>C</i>₄ symmetry. As in solution studies, in the case of M<b>L</b>^<b>+</b> complexes, an allosteric effect was observed: the ligand was already in the appropriate square <i>cone</i> conformation to bind the solvent molecule, allowing it to more easily and faster enter the calixarene cavity