Barium uranyl diphosphonates

Three Ba2+/UO22+ methylenediphosphonates have been prepared from mild hydrothermal treatment of uranium trioxide, methylendiphosphonic acid (C1P2) with barium hydroxide octahydrate, barium iodate monohydrate, and small aliquots of HF at 200 °C. These compounds, Ba[UO2[CH2(PO3)2]·1.4H2O (Ba-1), Ba3[(UO2)4(CH2(PO3)2)2F6]·6H2O (Ba-2), and Ba2[(UO2)2(CH2(PO3)2)F4]·5.75H2O (Ba-3) all adopt layered structures based upon linear uranyl groups and disphosphonate molecules. Ba-2 and Ba-3 are similar in that they both have UO5F2 pentagonal bipyramids that are bridged and chelated by the diphosphonate moiety into a two-dimensional zigzag anionic sheet (Ba-2) and a one-dimensional ribbon anionic chain (Ba-3). Ba-1, has a single crystallographically unique uranium metal center where the C1P2 ligand solely bridges to form [UO2[CH2(PO3)2]2− sheets. The interlayer space of the structures is occupied by Ba2+, which, along with the fluoride ion, mediates the structure formed and maintains overall charge balance.

Illustration of the stacking of the layers in Ba3[(UO2)4(CH2(PO3)2)2)F6]·6H2O viewed along the c-axis. The structure is constructed from UO7 pentagonal bipyramidal units, U(1)O7=gray, U(2)O7=yellow, barium=blue, phosphorus=magenta, fluorine=green, oxygen=red, carbon=black, and hydrogen=light peach.Figure optionsDownload as PowerPoint slideHighlights
► The polymerization of the UO22+ sites to form uranyl dimers leads to structural variations in compounds.
► Barium cations stitch uranyl diphosphonate anionic layers together, and help mediate structure formation.
► HF acts as both a mineralizing agent and a ligand.