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

Abstract

A tris(2-aminoethyl)amine-based tris(urea) receptor L, in the presence of excess TBA(H2PO4) self-assembled into a tetrahedral molecular cage by encapsulation of a tetrameric tetrahedral mixed phosphate cluster (H2PO4–•HPO42–)2 via 24 −NH hydrogen bonds [complex (n-TBA)6{4L(H2PO4–•HPO42–)2}] (1). Further, in the presence of orthophosphoric acid the receptor L has been observed to be self-assembled with side-cleft bonded tetrameric anion-acid cluster (H2PO4–•H3PO4)2 [complex {(LH)+(H2PO4–•H3PO4)DMSO•H 2O}] (2). In a proof of concept experiment, transformation of salt complex 2 into neutral phosphate encapsulated complex has been demonstrated in solution state by quantitative 1H NMR titration with (TBA)OH in DMSO-d6 (298 K). Extensive studies with receptor L 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)[L(F)] (3). 2D-NOESY NMR (DMSO-d6) experiments were carried out on the phosphate complexes (complexes 1 and 2) to study the binding discrepancies of phosphate anion with the neutral and charged host.