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 (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.