Hydrogen-Bonded Hexameric Capsules of Resorcin[4]arene, Pyrogallol[4]arene and Octahydroxypyridine[4]arene are Abundant Structures in Organic Solvents: A View from Diffusion NMR

Hydrogen-bond molecular capsules have attracted much interest in the last decade. In the present paper, we wish to describe new insights obtained from diffusion NMR when studying the self-assembly of resorcin[4]arenes (2), pyrogallol[4]arenes (3) and octahydroxypyridine[4]arenes (4) in solution. These diffusion NMR studies demonstrate that compounds 2 and 3 self-assemble spontaneously into hexameric capsules in organic solvents. For compound 4, both hexameric capsules and dimeric aggregates were identified. Diffusion NMR was found to be very useful in evaluating the relative stability of these capsules as well as in determining the role played by water molecules in the self-assembly of such systems. Moreover, diffusion NMR enabled us to establish whether the self-assembly of these capsules proceeds with self-sorting. We could demonstrate that the hexamers of 3 are more stable than the hexameric capsules of 2 and that the formation of such hexamers proceeds with self-sorting and no hetero-hexamers are formed when macrocycles of different classes are mixed. The hexameric capsules of 2 were found to self-assemble with eight water molecules, whereas water molecules are not required for the formation of the hexameric capsules of 3. Diffusion NMR helped in demonstrating that many of the previously evoked 1:1 and 1:2 host–guest complexes of 2 are in fact hexamers encapsulating a multiplicity of guests. This supports the notion that hexameric capsules of these systems are much more abundant species in organic solvents than previously thought. These studies also demonstrate that diffusion NMR is an extremely useful tool for studying and characterising supramolecular systems and molecular capsules in solution.