Pyromellitamide Aggregates and Their Response to Anion Stimuli

Abstract

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 (rho=0.22). The pyromellitamide binds to a range of small anions with the binding strength decreasing in the order chloride>acetate>bromide>nitrate approximately 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.

Binding Constants and Their Measurement

“…If the host or the guest also aggregate, the assumptions behind Job's method usually also break down. 26 …”

Characterization of Halogen Bonded Adducts in Solution by Advanced NMR Techniques

“…[23] As already noted before, the Ka between the iodotriazole and 4-methylpyridine cannot be estimated through a 19 F NMR titration, as the fluorine atoms are too far from the XB acceptor group, but the use of 1 H, 15 N HMBC NMR spectroscopy led to a value of 1.67 M -1 (Ka = 2.67 M -1 for the I2/4-methylpyridine). The substitution of the para-F with a 3-hydroxyl-pyridine group led to a molecule having both XB acceptor and donor groups on the same side, leading, according to DFT calculations, to a stable doublyhalogen bonded homodimer.…”

“…A partial solution to the problem is the use of the Job's plot, but it is not entirely reliable. [12,15] In both cases, for the structure and the stoichiometry of the adduct, assumptions are generally made on the basis of common sense, or, if the solid-state structure is available, it is just assumed that in solution the adducts have the same structure/stoichiometry. Given the importance of a correct, detailed and accurate characterization of XB adducts, especially when applications in solution are involved, more sophisticated tools should be employed and coupled to classical titrations, to have a thorough description of the system.…”

“…Unfortunately, this method is reliable under two conditions: (i) DmAn is the only adduct present in solution and (ii) the two components do not self-aggregate. [12,15] Sometimes, if the X-ray structure of the adduct is available, it can be assumed that the stoichiometry in solution is the same, but this assumption is not always safe (see later). Clearly, an accurate determination of the stoichiometry is always desirable, since the equation used to fit the experimental data of a titration depends on the stoichiometry of the adduct.…”

Challenges and breakthroughs in recent research on self-assembly

“…Hydrogen-bond-based supramolecular polymers were fabricated through quadruplex-type self-assembly of ditopic guanosine macromonomers [505]. Fibrous nanostructures were found with hydrogen-bond-type self assembly of bis(2,6-diacylaminopyridine) derivatives [506], benzene-1,3,5-tricarboxamides [507], and pyromellitamide [508].…”