Selective recognition of halide anions by calix[4]pyrrole: A detailed thermodynamic study

Several analytical techniques (H-1 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 (Deltadelta) of the pyrrole proton relative to the free ligand resulting from the addition of the anion salts to the ligand in CD3CN 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 Deltadelta 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.