Influence of the cavity dimension on encapsulation of halides within the capsular assembly and side-cleft recognition of a sulfate–water cluster assisted by polyammonium tripodal receptors

Abstract

The p-nitrophenyl and p-bromophenyl functionalized tris-polyamine receptors, L₁ and L₂, have formed capsular assembly with halide ions in an encapsulated fashion through efficient hydrogen-bonding. On the other hand, the positional isomer of L₁, the m-nitrophenyl functionalized tripodal amine receptor L₃, displays a rather flat-open conformation and is unable to bind halide anions in an encapsulated form. The presence of a smaller cavity in these receptors hinders the binding of larger oxyanions like sulfate. As a result, the protonated tripodal scaffold encapsulates small solvent molecules and helps in side-cleft binding of the larger sulfate anion. Herein, we report the design, synthesis and characterization of tren-based polyammonium receptors L₁, L₂ and L₃ and their complexation as well as binding discrepancy with several anions in the presence of acid. The solid state crystal structure of the anion complexes with L₁, L₂ and L₃ reveal that the anions are recognized via stable N–H⋯A, C–H⋯A, anion–π interactions with the protonated receptor molecule in a unimolecular fashion either inside or outside the cavity. The sulfate–water complexes of receptors L₁, L₂ and L₃ are stabilized by (NH)⁺⋯O type H-bonding and electrostatic interactions among sulfate, water and ammonium groups. The polyammonium based tripodal scaffold with positional variation of the functional group shows significant difference in anion binding fashion through either capsular or non-capsular complex formation.