Selective recognition of tetrahedral dianions by a hexaaza cryptand receptor

A hexaamine cage was synthesised in good yield by a [2+3] Schiff-base condensation followed by sodium borohydride reduction to be used as a receptor for the selective binding of anionic species. The protonation constants of the receptor, as well as its association constants with Cl-, I-, NO3-, AcO-, ClO4-, H2PO4-, SO42-, SeO42- and S2O32- were determined by potentiometry at 298.2 +/- 0.1 K in H2O-MeOH (50 : 50 v/v) and at ionic strength 0.10 +/- 0.01 mol dm(-3) in KTsO. These studies revealed a remarkable selectivity for dianionic tetrahedral anions by the protonated receptor, with association constants ranging 5.03-5.30 log units for the dianionic species and 1.49-2.97 log units for monoanionic ones. Single crystal X-ray determination of [(H(6)xyl)(SO4)(H2O)(6)](SO4)(2)center dot 9.5H(2)O showed that one sulfate anion is encapsulated into the receptor cage sited between the two 2,4,6-triethylbenzene caps establishing three N-H center dot center dot center dot O hydrogen bonds with two adjacent N-H binding sites and additional O-H center dot center dot center dot O hydrogen bonding interactions with six water of crystallization molecules. Four water molecules of the (SO4)(H2O)(6) cluster interact with [H(6)xyl](6+) through N-H center dot center dot center dot O hydrogen bonds. Molecular dynamics simulations (MD) carried out with SO42- and Cl-anions in H2O-MeOH (50 : 50 v/v) allowed the full understanding of anion molecular recognition, the selectivity of the protonated receptor for SO42- and the role played by the methanol and water solvent molecules.