Sulfate recognition by a hexaaza cryptand receptor

A hexamine macrobicycle with pyrrolyl spacers was evaluated as an anion receptor in its protonated forms. The protonation constants of the receptor, as well as its association constants with Cl^-, NO₃^-, AcO^-, ClO₄^-, H₂PO₄^-, and SO₄^2- were determined by potentiometry at 298.2 ± 0.1 K in H₂O-MeOH (50:50 v/v) and at an ionic strength of 0.10 ± 0.01 M in KTsO. These studies revealed that the H_npyrrⁿ⁺ receptor has a very high effective association constant value for the SO₄^2- at pH 4.0 (logK_eff = 6.42), and it is selective for the uptake of this anion in the presence of the other studied anionic substrates. In particular, the receptor showed very high SO₄^2-/NO₃^- selectivity. Using the indicator-displacement approach the receptor is able to signal the presence of sulfate by a change of color. Single crystal X-ray diffraction determination of [(H₆pyrr)(SO₄)(H₂O)₃](SO₄)₂·9.3H₂O revealed the presence of one sulfate anion inside the receptor cavity and showed that the encapsulation of the anion is favored by an array of nine hydrogen bonding interactions, including N-H⋯O, C-H⋯O and water-mediated ones.