Chiral receptors for phosphate ions
The binding tendencies of the enantiomeric forms, R,R and S,S, of the neutral receptor 1 towards anions were investigated through UV–vis and 1H NMR titration experiments in DMSO. Both enantiomers form stable H-bond complexes with carboxylates and phosphates. In particular, receptor 1 strongly binds two H2PO4− ions according two stepwise equilibria, in which logK2 is higher than logK1. Such an unusual cooperativity effect is to be ascribed to the formation of strong H-bond interactions between the two H2PO4− anions, when bound to the two urea subunits of the receptor, as demonstrated by the crystal and molecular structures of the 1 : 2 complex salt: [Bu4N]2[R,R-1⋯(H2PO4)2]. The S,S enantiomer forms an H-bond complex with the biologically relevant D-2,3-diphosphoglycerate anion, whose association constant is twice that of the R,R complex. Such an effect is ascribed to the different structural features of the two diastereomeric complexes in solution, as shown by 31P NMR studies.