A versatile tripodal amide receptor for the encapsulation of anions or hydrated anions via formation of dimeric capsules

Abstract

A bowl-shaped tripodal receptor with an appropriately positioned amide functionality on the benzene platform and electron-withdrawing p-nitrophenyl terminals (L¹) has been designed, synthesized, and studied for the anion binding properties. The single-crystal X-ray crystallographic analysis on crystals of L¹ with tetrabutylammonium salts of nitrate (1), acetate (2), fluoride (3), and chloride (4) obtained in moist dioxane medium showed encapsulation of two NO₃⁻, [(AcO)₂(H₂O)₄]²⁻, [F₂(H₂O)₆]²⁻, and [Cl₂(H₂O)₄]²⁻ respectively as the anionic guests inside the staggered dimeric capsular assembly of L¹. The p-nitro substitution in the aryl terminals assisted the formation of dimeric capsular assembly of L¹ exclusively upon binding/encapsulating above different guests. Though L¹ demonstrates capsule formation upon anion or hydrated anion complexation for all of the anions studied here, its positional isomer with the o-nitro-substituted tripodal triamide receptor L² selectively formed the dimeric capsular assembly upon encapsulation of [F₂(H₂O)₆]²⁻ and noncapsular aggregates in the cases of other anions such as Cl⁻, NO₃⁻, and AcO⁻. Interestingly, structural investigations upon anion exchange of the complexes revealed that both isomers have selectivity toward the formation of a [F₂(H₂O)₆]²⁻ encapsulated dimeric capsule. In contrast, solution-state ¹H NMR titration studies of L¹ and L² in DMSO-d₆ with AcO⁻ indicated 1:3 (host:guest) binding.