Structures and syntheses of framework triuranyl diarsenate hydrates

Two homeotypic hydrated uranyl arsenates, (UO2)[(UO2)(AsO4)]2(H2O)4, <f>UAs4</f>, and (UO2)[(UO2)(AsO4)]2(H2O)5, <f>UAs5</f> were synthesized by hydrothermal methods. Intensity data were collected at room temperature using <f>MoKα</f> X-radiation and a CCD-based area detector. Their crystal structures were solved by direct methods and refined by full-matrix least-squares techniques on the basis of <f>F2</f> to agreement indices (<f>UAs4</f>, <f>UAs5</f>) <f>wR2=0.116</f>, 0.060, for all data, and <f>R1=0.046</f>, 0.033, calculated for 3176, 5306 unique observed reflections (<f>|Fo|>4σF</f>) respectively. <f>UAs4</f> is monoclinic, space group <f>P21/c</f>, <f>Z=4</f>, <f>a=11.238(1)</f>, <f>b=7.152(1)</f>, <f>c=21.941(2) </f>A˚, <f>β=104.576</f>(2)°, <f>V=1706.8(1) </f>A˚3, <f>Dcalc=4.51</f> g/cm3. <f>UAs5</f> is orthorhombic, space group <f>Pca21,</f> <f>Z=4</f>, <f>a=20.133(2)</f>, <f>b=11.695(1)</f>, <f>c=7.154(1) </f>A˚, <f>V=1684.4(1) </f>A˚3, <f>Dcalc=4.65</f> g/cm3. Both structures contain sheets of arsenate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO2)(AsO4)]1− and the uranophane sheet anion-topology. The sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with an arsenate tetrahedron on each of two adjacent sheets, resulting in open-frameworks with isolated H2O groups in the larger cavities of the structures. The uranyl arsenate sheet in <f>UAs4</f> is relatively planar, and is topologically identical with the uranyl phosphate sheet in (UO2)[(UO2)(PO4)]2(H2O)4. The uranyl arsenate sheet in <f>UAs5</f> is the same geometrical isomer as in <f>UAs4</f>, but is highly corrugated, exhibiting approximately right angle bends of the sheet after every second uranyl arsenate chain repeat. [Copyright &y& Elsevier]