Design and Crystallization\nof Water-Stable Uranyl\nPhosphonates Using a Metalloligand Strategy

The\nuranyl ion can form strong metal–ligand bonds with phosphonate\ngroups, making it an excellent choice for constructing water-stable\nMOFs. However, reactions of uranyl ion and phosphonate ligands often\noccur too quickly, resulting in powders rather than single crystals.\nIn this work, we employed a metalloligand strategy and synthesized\nfour coordination polymers with layered structures, (UO<sub>2</sub>)­Fe­(notpH)·0.5H<sub>2</sub>O (<b>1</b>), (UO<sub>2</sub>)­Fe<sub>2</sub>(notpH<sub>2</sub>)<sub>2</sub>·0.75H<sub>2</sub>O (<b>2</b>), (UO<sub>2</sub>)­Co­(notpH)­(H<sub>2</sub>O)·5H<sub>2</sub>O (<b>3</b>), and (UO<sub>2</sub>)<sub>2</sub>Co<sub>2</sub>(notpH)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>·7H<sub>2</sub>O (<b>4</b>), by reacting metalloligands M<sup>III</sup>(notpH<sub>3</sub>) [M = Co, Fe; notpH<sub>6</sub> = 1,4,7-triazacyclononane-1,4,7-triyl-tris­(methylenephosphonic\nacid)] with UO<sub>2</sub>(OAc)<sub>2</sub> under hydrothermal conditions.\nBy optimizing the synthesis conditions, we obtained pure phases of\ncompounds <b>1</b>, <b>3</b>, and <b>4</b> and studied\ntheir 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\ninvestigated the proton conductive properties of compounds <b>1</b> and <b>3</b>.