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

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 UO2 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.