Octahydroxypyridine[4]arene self-assembles spontaneously to form hexameric capsules and dimeric aggregates

In recent years it has been observed that resorcin[4]arenes and pyrogallol[4]arenes form hydrogen-bonded hexameric capsules in nonpolar solvents. In the present study we have used NMR spectroscopy, with an emphasis on diffusion NMR, to investigate the self-assembly and the aggregation mode of solutions of octahydroxypyridine[4]arene (1b) in chloroform. In spectroscopic studies, the hexameric capsule of C-undecylresorcin[4]arene (2 b) was used as a reference compound and in some cases also as an internal reference. The current diffusion NMR spectroscopy study shows, in contrast to a previous report, that compound lb self-assembles spontaneously into hexameric and dimeric aggregates in solutions in chloroform. The ¹H NMR and diffusion NMR spectroscopic studies on a solution of lb in CHCl₃ show the presence of new upfield-shifted peaks, which diffuse with the same diffusion coefficient as the hexameric peaks in the spectrum. Therefore, these new upfield-shifted peaks were attributed to encapsulated CHCl₃ molecules. Interestingly, diffusion NMR measurements showed that the addition of trifluoroacetic acid (6.7 equiv), which had no effect on the hexameric capsules of 2 b, led to the disassembly of the hexamer and the dimer of lb into its monomers. Therefore, we conclude that compound lb self-assembles spontaneously into hexameric capsules in nonpolar organic solvents, as do resorcin[4]arenes 2b and 2c and pyrogallol [4] arenes 3a and 3b.