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

Abstract

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.

Complexation of Alkali Metal Cations by a Tertiary Amide Calix[4]Arene Derivative in Strongly Cation Solvating Solvents

“…However, most of the reported studies of alkali metal calixarene complexes were focused on a rather limited series of solvents. The most often used solvents are methanol and acetonitrile, [4,5,15,19,20] whereas the strongly cation-solvating media (e.g. N,N-dimethylformamide and dimethyl sulfoxide) were rarely investigated.…”

Synthesis of Fluorescent Diphenylantracene-Based Calix[4]arene Derivatives and their Complexation with Alkali Metal Cations

“…These findings could serve to (at least partly) explain notably more favorable cation complexation in MeCN/CH2Cl2 than MeOH/CH2Cl2 in solvent mixture. [11,24] The computational investigations provided information regarding the possible structures of compound L2 and its complexes. The results of quantum chemical calculations indicated that diphenylanthracene ether oxygen atom was not involved in the coordination of sodium cation in the NaL2MeCN + complex species.…”

“…This is the inclusion of the solvent molecule into the hydrophobic calixarene cavity of the ligand and especially of the complex formed [21,22,24,26,27] whereby the particularly favorable interaction between the acetonitrile methyl protons and the electron rich aromatic rings can be realized. In order to explore such a possibility in the studied MeCN/CH2Cl2 mixture, the molecular dynamics simulations of the ligand and its alkali metal complexes were performed.…”

“…The binding mode of the solvent molecule was similar to that previously found by the MD simulations of different calixarene-methanol adducts. [22,24] The calixarene cavity was occupied with MeOH approximately 2-5 times longer period of time than with CH2Cl2 (Tables S6-S7, Supporting Information). The solvent-ligand interaction energy in metal ion complexes and the corresponding adducts was about twice lower for dichloromethane than methanol (Tables S6-S7, Supporting Information).…”

“…[4,[17][18][19][20][21][22][23][24][25] In this context, the inclusion of solvent molecules in the macrocycle hydrophobic cavity can play an important role in the cation hosting. [20][21][22]24,26,27] The extent of complexation reactions in non-aqueous solvents can also be significantly affected by the process of ion pairing. [4,11] Among the vast variety of calixarene derivatives, those bearing suitable binding sites as well fluorescent functionalities (e.g.…”