Design and Crystallization of Water-Stable Uranyl Phosphonates Using a Metalloligand Strategy

The uranyl ion can form strong metal–ligand bonds with phosphonate groups, making it an excellent choice for constructing water-stable MOFs. However, reactions of uranyl ion and phosphonate ligands often occur too quickly, resulting in powders rather than single crystals. In this work, we employed a metalloligand strategy and synthesized four coordination polymers with layered structures, (UO2)Fe(notpH)·0.5H2O (1), (UO2)Fe2(notpH2)2·0.75H2O (2), (UO2)Co(notpH)(H2O)·5H2O (3), and (UO2)2Co2(notpH)2(H2O)2·7H2O (4), by reacting metalloligands MIII(notpH3) [M = Co, Fe; notpH6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid)] with UO2(OAc)2 under hydrothermal conditions. By optimizing the synthesis conditions, we obtained pure phases of compounds 1, 3, and 4 and studied their stability in water. Compounds 1 and 3 were stable even in boiling water, whereas compound 4 converted to 3 after 2 days in boiling water. We also investigated the proton conductive properties of compounds 1 and 3.