Sulphate binding by a quinolinyl-functionalised tripodal tris-urea receptor

A quinolinyl-functionalised tripodal tris(urea) receptor (L) has been designed for sulphate binding. The neutral receptor formed the 1:1 binding mode with sulphate ion (as tetrabutylammonium salt). However, when L interacted with H ₂SO ₄, a 2:1 (host/guest) complex (HL) ₂SO ₄EtOH12.5H ₂O (1) was isolated. Crystal structural analysis showed that the tertiary amine N atom of L is protonated, and the sulphate ion is located outside the receptor rather than inside the tripodal cleft. ¹H NMR studies revealed that the 2:1 binding ratio was persistent in solution. Interestingly, the protonated receptor displayed an unusual enhanced binding for sulphate ion in aqueous environments because of the stronger electrostatic effect in the presence of water.