Synthesis, characterization, and crystal structures of uranyl compounds containing mixed chromium oxidation states

The mixed-valence chromium uranyl compounds Li5[(UO2)4(Cr(V)O5)(Cr(VI)O4)4](H2O)17 (1), (Mg(H2O)6)5[(UO2)8(Cr(V)O5)2(Cr(VI)O4)8] (2) and (NH4)5[(UO2)4(Cr(V)O5)(Cr(VI)O4)2]H2O11 (3) have been synthesized and characterized. Each contains an identical sheet of cation-centered polyhedra. Central to the connectivity of the sheet are four uranyl pentagonal bipyramids that share some of their equatorial vertices, giving a four-membered ring. The Cr(V) cation located near the center of this ring is coordinated by O atoms in a square pyramidal arrangement. The Cr(VI) is tetrahedrally coordinated by O atoms, and these tetrahedra link the four-membered rings of bipyramids. The mixed-valence nature of the sheet was verified by XANES, an EPR spectrum, and bond-valence analysis. Low-valence cations and H2O groups reside between the sheets of uranyl and chromate polyhedra, where they provide linkages between adjacent sheets.

Three uranyl chromate compounds contain both pentavalent and hexavalent chromium. The unusual topology of the uranyl chromate sheet contains unusual pentavalent chromium in a square pyramidal coordination environment.Figure optionsDownload as PowerPoint slideHighlights
► The first uranyl compounds with mixed Cr valences are reported.
► A sheet of uranyl polyhedra stabilizes pentavalent chromium.
► Uranyl and chromate polyhedra form a topologically novel sheet.