Selective Recognition of Halide Anions by Calix[4]pyrrole:  A Detailed Thermodynamic Study

Abstract

Several analytical techniques ( 1 H NMR, conductance measurements, and titration calorimetry) have been used to assess the interaction of calix [4]pyrrole and halide anions in dipolar aprotic solvents (acetonitrile and N,N-dimethylformamide). Solubility data for calix [4]pyrrole in various solvents at 298.15 K were determined. These data were used to calculate the standard Gibbs energies of solution. Taking acetonitrile as the reference solvent, the transfer Gibbs energies of this ligand to various solvents were calculated. Chemical shift changes (∆δ) of the pyrrole proton relative to the free ligand resulting from the addition of the anion salts to the ligand in CD 3 CN at 298 K follow the sequence F -> Cl -> Br -> I -. Conductance measurements were performed (i) to establish the stoichiometry of the anionic calix[4]pyrrole complexes and (ii) to assess the range of concentration at which the free and complex anion salts are predominantly in their ionic forms in acetonitrile and N,N-dimethylformamide at 298.15 K. Titration calorimetry was used to determine the stability constant, K s , (hence the standard Gibbs energy) and the enthalpy. Combination of Gibbs energy and enthalpy data yields the entropy of complexation. A linear correlation is found between the log K s and the ∆δ values. The results show that calix [4]pyrrole is able to recognize selectively the halide anions in these solvents. The selectivity of the ligand for one anion relative to another is quantitatively evaluated through the calculation of the selectivity factor. It is shown that the ligand behavior is representative of flexible ligands in which calix[4]pyrrole compete successfully with the solvent for the anion to an extent that the higher selectivity of the ligand is for the smallest anion (fluoride). The thermodynamics of complexation is discussed in terms of the solvation properties of the reactants and the products in acetonitrile and N,N-dimethylformamide.

Calix[4]pyrrole macrocycle: Extraction of fluoride anions from aqueous media

“…In the present work, the results (E = 10 kJ/mol) implied that the uptake process is chemical adsorption. This result agrees with 1 H NMR studies [11]. The values of R 2 provide an indication of how well the experimental data fits with the isotherm models.…”

“…Danil de Namor has previously demonstrated (through 1 H NMR, conductance and calorimetric measurements) that calixpyrrole forms a 1:1 complex with fluoride anion via Hbond formation with the NH groups of this ligand [11]. The selectivity trend for this ligand for anions was established to be as follows:…”

“…Given that calixpyrrole is completely insoluble in water [11], the ligand was used in solid-phase removal of fluoride anions from aqueous media. The effect of the amount of calixpyrrole used to explore the retention properties of calixpyrrole has been studied by varying the amount of material and keeping other parameters (pH, temperature, contact time, and the initial concentration of anion (19 ppm)) constant ( Figure 1).…”

“…Calixpyrroles, a class of hetero-calixarenes, are a topic of considerable current interest in supramolecular chemistry because of their ability to act as receptors for anions [11][12][13][14][15][16]. Key features of supramolecular chemistry [17][18][19] involve firstly recognition, where the macrocycle shows selectivity toward one substrate relative to another.…”

“…Calix [4]pyrroles are known for their anion binding properties as these are able to make hydrogen bonds through the pyrrolic -NH-group [23]. Thermodynamic investigations involving calixpyrrole and anions have demonstrated the selectivity of this ligand for the fluoride anion among the halides in solution [11,12]. Recently, Sessler et al have developed the first polymeric systems containing a calixpyrrole anion receptor directly appended to a polymeric backbone which could be readily optimized for use in a range of ion-binding and extraction applications [24].…”