Self-Assembly of Pseudorotaxane Structures from a Dicopper(II) Molecular Cage and Dicarboxylate Axles

In this work, we employed for the first time a dinuclear bis[tris(2-aminoethyl)amine] cryptate to obtain the self-assembly of pseudorotaxane structures in an aqueous solution. The goal was achieved by exploiting the well-known affinity of the dicopper azacryptate with diphenyl spacers for the terephthalate anion. In particular, a series of molecular threads were synthesized by appending either alkyl or polyoxyethylene chains on both sides of the terephthalate benzene ring. The obtained dicarboxylic acids were precipitated as sodium salts, and their affinity toward the dicopper azacryptate was determined in a methanol/water mixture (pH 7). Experimental investigations showed that the chains' length and nature have a small impact on the 1:1 binding constants, whose values range between 4.98 and 5.18 log units. Computational studies indicated that the molecular axle is threaded through the azacryptate cavity, with the terephthalate group wedged between the two copper ions, coordinating both of them in the apical position (the one that, in the free azacryptate, is occupied by a water molecule). Compared to the inclusion complex with the plain terephthalate anion, a slight strain was found in the pseudorotaxane structure, induced by the inner chain of the thread inside the cavity. These results may be of great interest in all of the fields of science and technology in which host-guest recognition and molecular cages are applied.