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(-), NO(3)(-), AcO(-), ClO(4)(-), H(2)PO(4)(-), SO(4)(2-), SeO(4)(2-) and S(2)O(3)(2-) were determined by potentiometry at 298.2 +/- 0.1 K in H(2)O-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)(SO(4))(H(2)O)(6)](SO(4))(2).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-HO hydrogen bonds with two adjacent N-H binding sites and additional O-HO hydrogen bonding interactions with six water of crystallization molecules. Four water molecules of the (SO(4))(H(2)O)(6) cluster interact with [H(6)xyl](6+) through N-HO hydrogen bonds. Molecular dynamics simulations (MD) carried out with SO(4)(2-) and Cl(-) anions in H(2)O-MeOH (50 : 50 v/v) allowed the full understanding of anion molecular recognition, the selectivity of the protonated receptor for SO(4)(2-) and the role played by the methanol and water solvent molecules.