Designable Aluminum Molecular Rings: Ring Expansion and Ligand Functionalization.

Presented here are the Al(III) molecular ring architectures from 8-ring to 16-ring. Although there are numerous reported cyclic coordination compounds based on transition metals, gallium, or lanthanides, so far the Al molecular are less developed due to the fast hydrolysis nature of Al 3+ ion. In this work, with the assistant of different monohydric alcohol, a series of atomic precisely aluminum molecular rings based on benzoates have been successfully synthesized. The ring expansion of these Al-rings from 8-ring to 16-ring is related to the structure-directing agents of monohydric alcohols. Moreover, the organic ligands on the Al-ring can be properly modified by using various benzoate derivatives, which lead to tunable surface properties of these Al-rings from hydrophilicity to ultra-hydrophobicity. Importantly, 4-aminobenzoic acid bridged 16-ring is soluble in organic solvents and exhibits high solution stability revealed by mass spectroscopy. Ligand substitution also can be performed between these Al-rings, which reveal controllable ligand functionalization of these Al-rings. This family of Al-rings with the ability to be post-modified therefore can be considered and expected as interesting and potential lego-like nano-building units for reticular chemistry. Such a model of organic shell modification without alteration of the inorganic core structure opens a new platform to further tune the chemical-property and subsequent utilization of these promising Al-rings. This work not only reveals molecular ring architectures assembled from Al 3+ ions and benzoates but also provides coordination delayed hydrolysis strategy toward the synthesis of crystalline Al(III)-based materials.