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.