Boron Macrocycles Having a Calix-Like Shape. Synthesis, Characterization, X-ray Analysis, and Inclusion Properties

The syntheses, structure, and inclusion properties of trinuclear boron compounds having a calix-like shape are described. The compounds have been obtained via self-assembly reactions between salicylaldehyde derivatives and 3-aminophenylboronic acid, whereby the formation of three N → B coordination bonds favored the oligomerization. The products have high melting points (>370 °C), are stable to moisture, and have good solubility in organic solvents; the latter property is useful for host−guest recognition experiments. The structural analysis by X-ray diffraction revealed that diverse conformations are possible because of the presence of two different units of aromatic rims. A cone−cone (double-cone) conformation is observed for three of these compounds, while the remaining one has a cone−partial cone conformation. An analysis of the molecular packing showed that the molecules are stacked in columns in two different orientations in relation to the organization of the macrocycles when referred to the N−B bonds. The inclusion properties toward primary amines and ammonium chlorides were analyzed by titration experiments and monitored by UV spectroscopy, whereby association constants on the order of 102−103 M-1 were determined.