Supramolecular aggregates between carboxylate anions and an octaaza macrocyclic receptor

Abstract

The 28-membered octaazamacrocycle Me₂[28]py₂N₆ was used as a receptor for the molecular recognition of aromatic and aliphatic carboxylate substrates. The receptor-substrate binding behaviour of (H₆Me₂[28]py₂N₆)⁶⁺ with an aliphatic (⁻O₂C(CH₂)_nCO₂⁻, n = 0 to 4) and an aromatic (phthalate, isophthalate, terephthalate, 4,4′-dibenzoate, benzoate, 3- and 4-nitrobenzoate) series of carboxylate anions was evaluated by ¹H NMR spectroscopy (carried out in DMSO-d₆ at 300 K). Two association constants were found for most of the studied cases, except for 3- and 4-nitrobenzoate for which only K₁ was determined. For oxalate, malonate, benzoate and dibenzoate anions only the β₂ constants could be obtained. The values of the first association constant cover a range from 2.86 to 3.69 (log units), and the second stepwise constant from 2.15 to 2.89 (also in log units). No special selectivity was found but the highest values were determined for adipate and the lowest for the monoprotic 3- and 4-nitrobenzoates. Single crystal X-ray structures of H₆Me₂[28]py₂N₆⁶⁺ with terephthalate, 1, and 4,4′-dibenzoate (2) were determined showing supramolecular entities with general formula (H₆Me₂[28]py₂N₆)·(substrate)₂(PF₆)₂·4H₂O. These anions are the building blocks of an extensive 3-D network of hydrogen bonds.