Acid-Tolerant Sulfate Tetrahedral Cages from Anion-Coordination-Driven Assembly

Supramolecular cages have been constructed by anion-coordination-driven assembly (ACDA) in recent years and have shown unique host-guest interactions. However, most of the reported cages are made of the phosphate ion (PO₄³⁻); other anions have rarely been explored. Here we show for the first time that the sulfate ion (SO₄²⁻) is also able to form the A₄L₄ tetrahedral motif with tris-bis(urea) ligands, but this is dependent on the stoichiometry of the sulfate ion (in solution). Notably, the sulfate cages display enhanced resistance for both Brønsted (pH as low as 4.3 in acetone containing 15 % water) and Lewis acids (metal complexes) compared to the corresponding phosphate cages, and thus could find applications where an acidic (proton or metals) environment is required.