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 (<b>1</b>), (UO₂)­Fe₂(notpH₂)₂·0.75H₂O (<b>2</b>), (UO₂)­Co­(notpH)­(H₂O)·5H₂O (<b>3</b>), and (UO₂)₂Co₂(notpH)₂(H₂O)₂·7H₂O (<b>4</b>), 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 <b>1</b>, <b>3</b>, and <b>4</b> and studied their stability in water. Compounds <b>1</b> and <b>3</b> were stable even in boiling water, whereas compound <b>4</b> converted to <b>3</b> after 2 days in boiling water. We also investigated the proton conductive properties of compounds <b>1</b> and <b>3</b>.