An Aluminum-Based Metal-Organic Cage for Cesium Capture

Metal-organic cages are a class of supramolecular structures that often require the careful selection of organic linkers and metal nodes. Of this class, few examples of metal-organic cages exist where the nodes are composed of main group metals. Herein, we have prepared an aluminum-based metal-organic cage, H₈[Al₈(pdc)₈(OAc)₈O₄] (Al-pdc-AA), using inexpensive and commercially available materials. The cage formation was achieved via solvothermal self-assembly of solvated aluminum and pyridine-dicarboxylic linkers in the presence of a capping agent, acetic acid. The obtained supramolecular structure was characterized by single-crystal X-ray diffraction (SCXRD), thermogravimetric analysis, and NMR spectroscopy. Based on crystal structure and computational analyses, the cage has a 3.7 Å diameter electron-rich cavity suitable for the binding of cations such as cesium (ionic radius of 1.69 Å). The host-guest interactions were probed with ¹H and ¹³³Cs NMR spectroscopy in DMSO, where at low concentrations, Cs⁺ binds to Al-pdc-AA in a 1:1 ratio. The binding site was identified from the crystal structure of CsH₇[Al₈(pdc)₈(OAc)₈O₄] (Cs⁺⊂Al-pdc-AA), and a binding affinity of ∼10⁶-10⁷ M^-1 was determined from NMR titration experiments. The Al-pdc-AA showed improved selectivity for cesium binding over alkali metal cations (Cs⁺ > Rb⁺ > K⁺ ≫ Na⁺ ∼ Li⁺). Collectively, the study reports a novel aluminum cage that can serve as a promising host for efficient and selective cesium removal.