Supramolecular Properties of a Monocarboxylic Acid-Functionalized "Texas-Sized" Molecular Box

A new carboxylic acid-functionalized "Texas-sized" molecular box TxSB-CO₂H has been prepared by combining two separate building blocks via an iodide-catalyzed macrocyclization reaction. A single-crystal X-ray diffraction analysis revealed a paired "clip-like" dimer in the solid state. Concentration-dependent behavior is seen for samples of TxSB-CO₂H as prepared, as inferred from ¹H NMR spectroscopic studies carried out in DMSO- d₆. However, in the presence of excess acid (1% by weight of deuterated trifluoracetic acid; TFA- d₁), little evidence of aggregation is seen in DMSO- d₆ except at the highest accessible concentrations. In contrast, the conjugate base form, TxSB-CO₂^-, produced in situ via the addition of excess triethylamine to DMSO- d₆ solutions of TxSB-CO₂H acts as a self-complementary monomer that undergoes self-assembly to stabilize a formal oligomer ([TxSB-CO₂^-] _n) with a degree of polymerization of approximately 5-6 at a concentration of 70 mM. Evidence in support of the proposed oligomerization of TxSB-CO₂^- in solution and in the solid state came from one- and two-dimensional ¹H NMR spectroscopy, X-ray crystallography, dynamic light scattering (DLS), and scanning electron microscopy (SEM). A series of solution-based analyses carried out in DMSO and DMSO- d₆ provide support for the notion that the self-assembled constructs produced from TxSB-CO₂^- are responsive to environmental stimuli, including exposure to the acetate anion (as its tetrabutylammonium, TBA⁺, salt), and changes in overall concentration, temperature, and protonation state. The resulting transformations are thought to reflect the reversible nature of the underlying noncovalent interactions. They also permit the stepwise interconversion between TxSB-CO₂H and [TxSB-CO₂^-] _n via the sequential addition of triethylamine and TFA- d₁. The present work thus serves to illustrate how appropriately functionalized molecular box-type macrocycles may be used to develop versatile stimuli-responsive materials. It also highlights how aggregated forms seen in the solid state are not necessarily retained under competitive solution-phase conditions.