Multicomponent Assembly of Boronic Acid-Based Macrocycles, Cages, and Polymers

This work describes the synthesis and characterization of boronic acid-based supramolecular structures. Different reversible interactions such as boronate ester formation, imine condensation and dative B–N bond formation are used to assemble macrocycles, cages and polymers. Multicomponent condensation reactions of different diamines with tetraols and formylphenylboronic acids give macrocycles via formation of imine bonds and boronate esters. The [4+2+2] condensation products are generally preferred but larger macrocycles are observed for some combinations of staring materials. The larger macrocycles are in dynamic equilibrium with the smaller [4+2+2] products. Utilization of a triamine instead of the diamine results in the formation of molecular cages. The syntheses of these cages require the formation of 18 covalent bonds between 11 building blocks, and, interestingly, can be performed in a ball mill. Multicomponent reactions of diboronic acids, catechol derivatives, and different pyridyl ligands are described. The condensation of diboronic acids with catechols gives dioxaboroles. Upon crystallization, the ester aggregates with the N-donor ligands via dative B−N bonds. Depending on the nature of the pyridyl ligand, molecularly defined macrocycles, cages or polymeric structures are obtained. One-dimensional polymers are formed with 4,4-bipyridine and 1,2-di(4-pyridyl)ethylene, whereas a two-dimensional network is obtained with the tetradentate ligand tetra(4-pyridylphenyl)ethylene. A tripyridyl linker results in the formation trigonal prismatic cages. The size of the cages can be varied by changing the diboronic acid building block. The cages are able to encapsulate polyaromatic molecules such as triphenylene or coronene. The results highlight the potential of dative B−N bonds in structural supramolecular chemistry and crystal engineering.