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 [(H₆xyl)(SO₄)(H₂O)₆](SO₄)₂·9.5H₂O 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 (SO₄)(H₂O)₆ cluster interact with [H₆xyl]⁶⁺ through N–HO 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.