Pyromellitamide Aggregates and Their Response to Anion Stimuli

The N,N‘,N‘‘,N‘‘‘-1,2,4,5-tetra(ethylhexanoate) pyromellitamide is found to be capable of both
intermolecular aggregation and binding to small anions. It is synthesized by aminolysis of pyromellitic
anhydride with ethanolamine, followed by a reaction with hexanoyl chloride. The single-crystal X-ray structure
of the pyromellitamide shows that it forms one-dimensional columnar stacks through an intermolecular
hydrogen-bonding network. It also forms self-assembled gels in nonpolar solvents, presumably by a
hydrogen-bonding network similar to the solid-state structure as shown by IR and XRD studies. Aggregation
by intermolecular hydrogen bonding of the pyromellitamide is also observed by NMR and IR in solution.
Fitting of NMR dilution data for pyromellitamide in d6-acetone to a cooperative aggregation model gave KE
= 232 M-1 and positive cooperativity of aggregation (ρ = 0.22). The pyromellitamide binds to a range of
small anions with the binding strength decreasing in the order chloride > acetate > bromide > nitrate ≈
iodide. The data indicate that the pyromellitamide binds two anions and that it displays negative cooperativity.
The intermolecular aggregation of the pyromellitamide can also be altered using small anion stimuli; anion
addition to preformed self-assembled pyromellitamide gels causes their collapse. The kinetics of anion-induced gel collapse are qualitatively correlated to the binding affinities of the same anions in solution. The
cooperative anion binding properties and the sensitivity of the self-assembled gels formed by pyromellitamide
toward anions could be useful in the development of sensors and switching/releasing devices.