Structure and selectivity trends in crystalline urea-functionalised anion-binding capsules

A tripodal trisurea receptor (L1) persistently self-assembles with various divalent oxoanion salts M _nX (M=Na, K, Mg, Ca, Cd; X=SO ₄ ^2-, SO ₃ ^2-, SeO ₄ ^2-, CrO ₄ ^2-) into isomorphous series of crystalline frameworks in three different compositions: MX(L1) ₂(H ₂O) ₆ (M=Mg, Ca, Cd) (1), Na ₂X(L1) ₂(H ₂O) ₄(2) and K ₂X(L1) ₂(H ₂O) ₂(3). Single-crystal X-ray structural analysis revealed that all three series of structures adopt a NaCl-type topology, consisting of alternating anionic X(L1) ₂ ^2- capsules and M(H ₂O) ₆ ²⁺, Na ₂(H ₂O) ₄ ²⁺ or K ₂(H ₂O) ₂ ²⁺ hydrated cations. The capsules provide a complementary environment to tetrahedral oxoanions via 12 hydrogen bonds from six urea groups lining the cavities of the capsules. The persistent formation of the capsules facilitated the investigation of structural trends and structure-selectivity relationships across series 1-3. First, it was found that the size of the capsules is relatively unresponsive to the change in the encapsulated anion, resulting in good shape and size recognition in the separation of anions by competitive crystallisations. Second, it was found that the size of the capsules varies linearly with the size of the external cation, which provides a way for tuning the anion encapsulation selectivity. However, no straightforward dependence was found between the size of the capsules and the relative selectivity for different-sized tetrahedral oxoanions in competitive crystallisations.