Towards Building Blocks for Supramolecular Architectures Based on Azacryptates

In this work, we report the synthesis of a new bis(tris(2-aminoethyl)amine) azacryptand L with triphenyl spacers. The binding properties of its dicopper complex for aromatic dicarboxylate anions (as TBA salts) were investigated, with the aim to obtain potential building blocks for supramolecular structures like rotaxanes and pseudo-rotaxanes. As expected, UV-Vis and emission studies of [Cu₂L]⁴⁺ in water/acetonitrile mixture (pH = 7) showed a high affinity for biphenyl-4,4'-dicarboxylate (dfc^2-), with a binding constant of 5.46 log units, due to the best match of the anion bite with the Cu(II)-Cu(II) distance in the cage's cavity. Compared to other similar bistren cages, the difference of the affinity of [Cu₂L]⁴⁺ for the tested anions was not so pronounced: conformational changes of L seem to promote a good interaction with both long (e.g., dfc^2-) and short anions (e.g., terephthalate). The good affinity of [Cu₂L]⁴⁺ for these dicarboxylates, together with hydrophobic interactions within the cage's cavity, may promote the self-assembly of a stable 1:1 complex in water mixture. These results represent a good starting point for the application of these molecular systems as building units for the design of new supramolecular architectures based on non-covalent interactions, which could be of interest in all fields related to supramolecular devices.