Self-Assembly of a Tris(Urea) Receptor as Tetrahedral Cage for the Encapsulation of a Discrete Tetrameric Mixed Phosphate Cluster (H₂PO₄^–•HPO₄^2–)₂

A tris­(2-aminoethyl)­amine-based tris­(urea) receptor <b>L</b>, in the presence of excess TBA­(H₂PO₄) self-assembled into a tetrahedral molecular cage by encapsulation of a tetrameric tetrahedral mixed phosphate cluster (H₂PO₄^–•HPO₄^2–)₂ via 24 −NH hydrogen bonds [complex (<i>n</i>-TBA)₆{4<b>L</b>(H₂PO₄^–•HPO₄^2–)₂}] (<b>1</b>). Further, in the presence of orthophosphoric acid the receptor <b>L</b> has been observed to be self-assembled with side-cleft bonded tetrameric anion-acid cluster (H₂PO₄^–•H₃PO₄)₂ [complex {(<b>L</b>H)⁺(H₂PO₄^–•H₃PO₄)­DMSO•H ₂O}] (<b>2</b>). In a proof of concept experiment, transformation of salt complex <b>2</b> into neutral phosphate encapsulated complex has been demonstrated in solution state by quantitative ¹H NMR titration with (TBA)­OH in DMSO-<i>d</i>₆ (298 K). Extensive studies with receptor <b>L</b> have also been carried out with other halides in the solid state. Crystal structure elucidation revealed the formation of a pseudodimeric (2 + 2) capsular self-assembly with 1:1 host:guest ratio in fluoride encapsulated complex, (TEA)­[<b>L</b>(F)] (<b>3</b>). 2D-NOESY NMR (DMSO-<i>d</i>₆) experiments were carried out on the phosphate complexes (complexes <b>1</b> and <b>2</b>) to study the binding discrepancies of phosphate anion with the neutral and charged host