Thermodynamics of Phosphate and Pyrophosphate Anions Binding by Polyammonium Receptors

Abstract

The interactions of phosphate and pyrophosphate anions with polyammonium cations deriving from 14 polyamines (five polyazacycloalkanes, four polyazacyclophanes, and five acyclic polyamines) in aqueous solution have been studied by means of potentiometric, microcalorimetric, and NMR measurements in solution. Very stable 1:1 receptor-to-anion complexes are formed. The stability trends of such complexes are not strictly determined by electrostatic forces, hydrogen bond interactions being of considerable importance in the complex formation, the thermodynamic data being consistent with different hydrogen bonding modes. ΔH°−TΔS° compensatory relationships hold for such complexation reactions. The crystal structures of (H4L1)(H2P2O7)2·2H2O and (H4L2)(H2P2O7)2·6H2O (L1 = 1,4,7,10,13,16-hexaazacyclooctadecane, L2 = 1,10-dimethyl-1,4,7,10,13,16-hexaazacyclooctadecane) have been solved by X-ray analysis, evidencing different substrate/anion binding characteristics.