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 M^III(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.