Anion-anion proton transfer in hydrogen bonded complexes.

Complexation of dihydrogen phosphate by an anion receptor containing six hydrogen bond donor groups has been shown to reduce the pK(a) of the bound anionic species to such an extent that addition of further aliquots of dihydrogen phosphate result in deprotonation of the bound species with the resultant formation of a monohydrogen phosphate receptor complex. X-ray crystallographic studies confirm monohydrogen phosphate complex formation in the solid state. In this way, this study explains the formation of complexes with unusual stoichiometries when investigating the binding of dihydrogenphosphate anion to hydrogen-bonding receptors.