Synthesis and characterization of uranyl chromate sheet compounds containing edge-sharing dimers of uranyl pentagonal bipyramids

Eight uranyl chromates have been crystallized from aqueous solution and characterized: Mg(H2O)6[(UO2)2(CrO4)2(OH)2](H2O)3 (1), (NH4)2[(UO2)2(CrO4)2(OH)2](H2O)3, Rb2[(UO2)2(CrO4)2(OH)2](H2O)3 (3), Cs[(UO2)(CrO4)(OH)]H2O (4), Rb[(UO2)(CrO4)(OH)]H2O (5) Co(H2O)4(Co(H2O)6)2[(UO2)4(CrO4)6(OH)2](H2O)4 (6), Li2[(UO2)2(CrO4)3](H2O)7 (7), and Zn(H2O)6[(UO2)2(CrO4)3](H2O)3 (8). The structural units of 1 through 8 each consist of a sheet of uranyl pentagonal bipyramids and (Cr(VI)O4)2− tetrahedra. In each case two uranyl pentagonal bipyramids share an equatorial edge, giving a dimer that is linked into the sheet through vertex sharing with (Cr(VI)O4)2− tetrahedra. The sheets are based upon three distinct sheet anion topologies, and the sheets based on a given anion topology can differ in the orientations of the non-bridging O atoms of (CrO4)2− tetrahedra. The interlayers of these compounds contain either monovalent or divalent cations, as well as H2O groups that are either bonded to the interlayer cation or are held in place by H bonding only. We explore the relationships between sheet topologies and interlayer configuration in these compounds.