Anion Recognition‐Induced Selective Separation of Phosphates from Complex Aqueous Matrices by Self‐Assembled Molecular Capsule

Abstract

Selective recognition of an oxoanion necessitates the design and synthesis of hydrogen bond donor receptors capable of discriminating between anions of comparable size, shape, and basicity. A urea‐functionalized second‐generation tripodal anion receptor has been demonstrated to selectively encapsulate and separate phosphate ions (PO₄^3‐, and HPO₄^2‐) from aqueous matrices of high anionic and cationic complexity. Competitive liquid–liquid extraction (LLE) experiments reveal exclusive formation of phosphate complexes in the presence of an excess of competing oxoanions and halide, which is a prerequisite for efficient phosphate recovery from highly eutrophic aquatic systems and also important for therapeutic applications as an effective phosphate binder. Tetrabutylammonium hydroxide has served as a receptor solubilizer and an anion‐exchange agent, facilitating phosphate transfer from the aqueous to the organic phase (dichloromethane) in exchange for hydroxide ions. The identity and purity of the LLE‐derived anion complexes are confirmed by nuclear magnetic resonance (¹H, ¹³C, and ³¹P) and mass spectroscopy, establishing phosphate selectivity of the receptor. Single‐crystal X‐ray diffraction analysis reveals phosphate encapsulation within a π‐stacked dimeric capsular assembly of the receptor (2:1 host–guest complex).