Abstract
The binding of alkali metal cations with two tertiary-amide lower-rim calix[4]arenes was studied in methanol, N,N-dimethylformamide, and acetonitrile in order to explore the role of triazole and glucose functionalities in the coordination reactions. The standard thermodynamic complexation parameters were determined microcalorimetrically and spectrophotometrically. On the basis of receptor dissolution enthalpies and the literature data, the enthalpies for transfer of reactants and products between the solvents were calculated. The solvent inclusion within a calixarene hydrophobic basket was explored by means of 1H NMR spectroscopy. Classical molecular dynamics of the calixarene ligands and their complexes were carried out as well. The affinity of receptors for cations in methanol and N,N-dimethylformamide was quite similar, irrespective of whether they contained glucose subunits or not. This indicated that sugar moieties did not participate or influence the cation binding. All studied reactions were enthalpically controlled. The peak affinity of receptors for sodium cation was noticed in all complexation media. The complex stabilities were the highest in acetonitrile, followed by methanol and N,N-dimethylformamide. The solubilities of receptors were greatly affected by the presence of sugar subunits. The medium effect on the affinities of calixarene derivatives towards cations was thoroughly discussed regarding the structural properties and solvation abilities of the investigated solvents.
Highlights
One of the main goals in supramolecular chemistry is the preparation of efficient, selective, and possibly water-soluble receptors
The data, based on Ph4AsPh4B convention, were taken from [32]. Such analysis for Na+ complexation resulted in the thermodynamic cycle presented in Scheme 2, which explains the differences in standard reaction enthalpies for the binding of Na+ in methanol and N,N-dimethylformamide
The MD simulations of alkali metal cation complexes with L in N,N-dimethylformamide indicated the inclusion of DMF molecule in the hydrophobic cavity of calixarene (Figures S47 and S50, Tables S9 and S10)
Summary
One of the main goals in supramolecular chemistry is the preparation of efficient, selective, and possibly water-soluble receptors. The complex stabilities are lower than in MeCN and MeOH (Figure 3, Table 1) This is not surprising considering the strongly favorable solvation of alkali metal cations in DMF. The titration of receptor l with Cs+ in methanol did not result in measurable enthalpy changes, indicating the low affinity of glycocalixarene for the largest alkali metal cation [16] This is in agreement with previously reported spectrophotometric investigations. The stability constants of alkali metal cation complexes with glycocalixarene l are highly similar to those with the triazole derivative L (Tables 1 and S1). As in methanol, this is a consequence of relatively small differences among ∆rH◦ and ∆rS◦ values for each M+–ligand pair. This qualitative observation can, from the thermodynamic point of view, be rationalized by means of standard transfer functions of receptors l, L, and glucose among the reaction media of interest, which are discussed
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