The Effect of Specific Solvent–Solute Interactions on Complexation of Alkali-Metal Cations by a Lower-Rim Calix[4]arene Amide Derivative

Complexation
of alkali-metal cations with calix[4]­arene secondary-amide derivative,
5,11,17,23-tetra­(tert-butyl)-25,26,27,28-tetra­(N-hexylcarbamoylmethoxy)­calix­[4]­arene (L),
in benzonitrile (PhCN) and methanol (MeOH) was studied by means of
microcalorimetry, UV and NMR spectroscopies, and in the solid state
by X-ray crystallography. The inclusion of solvent molecules (including
acetonitrile, MeCN) in the calixarene hydrophobic cavity was also
investigated. The classical molecular dynamics (MD) simulations of
the systems studied were carried out. By combining the results obtained
using the mentioned experimental and computational techniques, an
attempt was made to get an as detailed insight into the complexation
reactions as possible. The thermodynamic parameters, that is, equilibrium
constants, reaction Gibbs energies, enthalpies, and entropies, of
the investigated processes were determined and discussed. The stability
constants of the 1:1 (metal:ligand) complexes measured by different
methods were in very good agreement. Solution Gibbs energies of the
ligand and its complexes with Na+ and K+ in
methanol and acetonitrile were determined. It was established that
from the thermodynamic point of view, apart from cation solvation,
the most important reason for the huge difference in the stability
of these complexes in the two solvents lay in the fact that the transfer
of complex species from MeOH to MeCN was quite favorable. That could
be at least partly explained by a more exergonic inclusion of the
solvent molecule in the complexed calixarene cone in MeCN as compared
to MeOH, which was supported by MD simulations. Molecular and crystal
structures of the lithium cation complex of L with the
benzonitrile molecule bound in the hydrophobic calixarene cavity were
determined by single-crystal X-ray diffraction. As far as we are aware,
for the first time the alkali-metal cation was found to be coordinated
by the solvent nitrile group in a calixarene adduct. According to
the results of MD simulations, the probability of such orientation
of the benzonitrile molecule included in the ligand cone was by far
the largest in the case of LiL+ complex. Because
of the favorable PhCN–Li+ interaction, L was proven to have the highest affinity toward the lithium ion in
benzonitrile, which was not the case in the other solvents examined
(in acetonitrile, sodium complex was the most stable, whereas in methanol,
complexation of lithium was not even observed). That could serve as
a remarkable example showing the importance of specific solvent–solute
interactions in determining the equilibrium in solution.