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. 1 H NMR in CD 3 CN 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 H 2 PO 4 - and HP 2 O 7 3- in these solvents through hydrogen-bond formation. However, H 2 P 2 O 7 2- 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 H 2 PO 4 - and HP 2 O 7 3- 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 H 2 P 2 O 7 2- in these solvents. The same complex stoichiometry is found for H 2 P 2 O 7 2- and this ligand in water-saturated dichloromethane. The binding properties of the ligand for the H 2 PO 4 - are assessed in terms of the transfer thermodynamics of the reactants and the product from acetonitrile to N,N- dimethylformamide. Final conclusions are given.