A Versatile Tripodal Amide Receptor for the Encapsulation of Anions or Hydrated Anions via Formation of Dimeric Capsules

A bowl-shaped tripodal receptor with an appropriately positioned amide functionality on the benzene platform and electron-withdrawing p-nitrophenyl terminals (L) has been designed, synthesized, and studied for the anion binding properties. The single-crystal X-ray crystallographic analysis on crystals of L-1 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-), [F-2(H2O)(6)](2-), and [Cl-2(H2O)(4)](2-) respectively as the anionic guests inside the staggered dimeric capsular assembly of L-1, The p-nitro substitution in the aryl terminals assisted the formation of dimeric capsular assembly of L-1 exclusively upon binding/encapsulating above different guests, Though L-1 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 L-2 selectively formed the dimeric capsular assembly upon encapsulation of [F-2(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 [F-2(H2O)(6)](2-) encapsulated dimeric capsule. In contrast, solution-state H-1 NMR titration studies of L-1 and L-2 in DMSO.d(6) with AcO- indicated 1:3 (host:guest) binding.