Journal of Physical Chemistry A, Vol.108, No.35, 7324-7330, 2004
Recognition of biologically and environmentally important phosphate anions by calix[4]pyrrole: Thermodynamic aspects
The complex formation of calix[4]pyrrole (1) and nonspherical anions in acetonitrile and N,N-dimethylformamide at 298.15 K has been investigated. H-1 NMR in CD3CN at 298 K showed that, among the anions considered (dihydrogen phosphate, hydrogen pyrophosphate, dihydrogen pyrophosphate, hydrogen sulfate, perchlorate, nitrate, and trifluoromethane sulfonate), 1 interacts with H2PO4- and HP2O73- in these solvents through hydrogen-bond formation. However, H2P2O72- is not sufficiently soluble in these solvents. Therefore, extraction experiments that were performed in the water: dichloromethane solvent system demonstrated that calix[4]pyrrole also interacts with this anion. Conductometric titrations were used to (i) determine the composition of the H2PO4- and HP2O73- complexes with 1 and (ii) establish the concentration range at which the free and complex anion salts are predominantly in their ionic forms in acetonitrile and N,N-dimethylformamide. Titration microcalorimetry was used to determine the thermodynamics of complexation of these systems in these solvents. It is shown that, although 1 is able to form 1:1 (ligand:anion) complexes in acetonitrile and N,N-dimethylformamide, 2:1 complexes are formed with H2P2O72- in these solvents. The same complex stoichiometry is found for H2P2O72- and this ligand in water-saturated dichloromethane. The binding properties of the ligand for the H2PO4-are assessed in terms of the transfer thermodynamics of the reactants and the product from acetonitrile to N,N-dimethylformamide. Final conclusions are given.