Porous Uranium Diphosphonate Frameworks with Trinuclear Units Templated by Organic Ammonium Hydrolyzed from Amine...

By varying solvent systems, the solvothermal treatment of uranyl nitrate and methylenediphosphonic acid (H₄PCP) afforded three new porous uranyl-organic frameworks (UOFs). All were structurally characterized by single-crystal X-ray diffraction and formulated as (Et₂NH₂)₂[(UO₂)₃(PCP)₂](H₂O)_2.5 (1), (MeNH₃)(H₃O)[(UO₂)₃(PCP)₂(H₂O)₃] (2), and [Na(H₂O)₄](H₃O)[(UO₂)₃(PCP)₂(H₂O)₂](H₂O)₅ (3). These compounds crystallize with three-dimensional anionic frameworks containing U(VI) and distinct cationic species due to in situ solvent hydrolysis. The solvent systems diethylformamide (DEF), N-methyl-2-pyrrolindone (NMP), and the additive sodium vanadate (Na₃VO₄) significantly impact the resultant structures, affording diethyl ammonium, methyl ammonium, and sodium cations captured in channels of the anionic frameworks of 1-3. In 1, a trinuclear U₃O₁₈ unit formed by three uranyl polyhedra that share edges is connected into a three-dimensional framework. Compound 2 has a three-dimensional framework formed from a uranyl-methylenediphosphonate layer that is pillared by UO₇ pentagonal bipyramids. With the inclusion of sodium cations, 3 is a porous framework containing UO₇ pentagonal bipyramids within a layer, with sodium cations and UO₆ square bipyramids linking the adjacent layers. Compounds 1-3 feature the uranyl/ligand ratio of 3:2, but present diverse structural building units ranging from edge-shared trinuclear to heteronuclear assemblies. The compounds have been characterized by infrared (IR), Raman, and UV-vis spectroscopies, X-ray diffraction, and thermogravimetric analysis.