Thermodynamics of phosphate and pyrophosphate anions binding by polyammonium receptors

The interactions of phosphate and pyrophosphate anions with polyammonium ca tions deriving from 14 polyamines (five polyazacycloalkanes, four polyazacy clophanes, and five acyclic polyamines) in aqueous solution have been studi ed by means of potentiometric, microcalorimetric, and NMR measurements in s olution. Very stable 1:1 receptor-to-anion complexes are formed. The stabil ity trends of such complexes are not strictly determined by electrostatic f orces, hydrogen bond interactions being of considerable importance in the c omplex formation, the thermodynamic data being consistent with different hy drogen bonding modes. Delta H degrees-T Delta S degrees compensatory relati onships hold for such complexation reactions. The crystal structures of (H( 4)L1)(H2P2O7)(2)(.) 2H(2)O and (H(4)L2)(H2P2O7)(2). 6(2)O (L1 1,4,7,10,13,1 6-hexaazacyclooctadecane, L2 = 1,10-dimethyl-1,4,7, 10,13,1 6-hexaazacycloo ctadecane) have been solved by X-ray analysis, evidencing different substra te/anion binding characteristics.