An oxyanion-binding hexaurea cage

The selective binding of phosphate (PO₄³⁻) by synthetic receptors remains a formidable challenge due to its extremely high charge density and hydration energy. Here, we report a hexaurea covalent cage 1 synthesized via an anion-templated stepwise strategy, achieving up to 95% yield and gram-scale preparation. Cage 1 exhibits selective charge-dense oxyanion binding over charge-diffusion anions, with a selective PO₄³⁻ binding affinity of 3.8 × 10⁷ M⁻¹ in DMSO-d₆, the highest ever reported for a charge-neutral synthetic receptor. Single-crystal structures reveal that the C₃-symmetric cavity perfectly complements PO₄³⁻ through twelve N–H⋯O hydrogen bonds, whereas HPO₄²⁻ and SO₄²⁻ anions induce structural symmetry-breaking at the supramolecular complexation level. Moreover, cage 1 can reversibly capture CO₂ at room temperature and release it upon heating over 40 °C through a combination of carbonate anion binding and protonation of the terminal amines. This work establishes a versatile cage platform that integrates preorganized hydrogen-bonding cavities with charge complementarity, offering a promising route for anion recognition and acidic gas capture.