Pillared and open-framework uranyl diphosphonates

The hydrothermal reactions of uranium trioxide, uranyl acetate, or uranyl nitrate with 1,4-benzenebisphosphonic acid in the presence of very small amount of HF at 200 °C results in the formation of three different uranyl diphosphonate compounds, [H3O]2{(UO2)6[C6H4(PO3)(PO2OH)]2[C6H4(PO2OH)2]2[C6H4(PO3)2]}(H2O)2 (Ubbp-1), [H3O]4{(UO2)4[C6H4(PO3)2]2F4}·H2O (Ubbp-2), and {(UO2)[C6H2F2(PO2OH)2(H2O)}2·H2O (Ubbp-3). The crystal structures of these compounds were determined by single crystal X-ray diffraction experiments. Ubbp-1 consists of UO7 pentagonal bipyramids that are bridged by the phosphonate moieties to form a three-dimensional pillared structure. Ubbp-2 is composed of UO5F2 pentagonal bipyramids that are bridged through the phosphonate oxygen atoms into one-dimensional chains that are cross-linked by the phenyl spacers into a pillared structure. The structure of Ubbp-3 is a three-dimensional open-framework with large channels containing water molecules with internal dimensions of approximately 10.9×10.9 Å. Ubbp-1 and Ubbp-2 fluoresce at room temperature.

Graphical AbstractIllustration of the three-dimensional open-framework structure of {(UO2)[C6H2F2(PO2OH)2(H2O)}2·H2O viewed along the c-axis. The structure is constructed from UO7 units, pentagonal bipyramids=green, oxygen=red, phosphorus=magenta, carbon=black, hydrogen=white.Figure optionsDownload as PowerPoint slideHighlights
► The influence of the uranyl salt anions and pH were critically examined in relation to structural variation.
► The acetate and nitrate counter ions of uranyl may be acting as structure directing agents.
► The use of rigid phenyl spacer yield a three-dimensional network of pillared structures of uranyl diphosphonates that fluoresce.
► The fluorination of the phenyl ring under hydrothermal condition.
► The large voids in this structure are suggestive of potential applications in sorption, separation of gases and in catalytic processes.