Hydrothermal synthesis and crystal structures of new uranyl oxalate hydroxides: α- and β-[(UO2)2(C2O4)(OH)2(H2O)2] and [(UO2)2(C2O4)(OH)2(H2O)2]·H2O

Two modifications of the new uranyl oxalate hydroxide dihydrate [UO2)2(C2O4)(OH)2(H2O)2] (1 and 2) and one form of the new uranyl oxalate hydroxide trihydrate [(UO2)2(C2O4)(OH)2(H2O)2]·H2O (3) were synthesized by hydrothermal methods and their structures determined from single-crystal X-ray diffraction data. The crystal structures were refined by full-matrix least-squares methods to agreement indices R(wR)=0.0372(0.0842) and 0.0267(0.0671) calculated for 1096 and 1167 unique observed reflections (I>2σ(I)), for α (1) and β (2) forms, respectively and to R(wR)=0.0301(0.0737) calculated for 2471 unique observed reflections (I>2σ(I)), for 3. The α-form of the dihydrate is triclinic, space group image, image, image, image, image, image, image, image, image, β-form is monoclinic, space group image, image, image, image, image, image, image. The trihydrate is monoclinic, space group image, image, image, image, image, image, image. In the three structures, the coordination of uranium atom is a pentagonal bipyramid composed of dioxo UO22+ cation perpendicular to five equatorial oxygen atoms belonging to one bidentate oxalate ion, one water molecule and two hydroxyl ions in trans configuration in 2 and in cis configuration in 1 and 3. The UO7 polyhedra are linked through hydroxyl oxygen atoms to form different structural building units, dimers [U2O10] obtained by edge-sharing in 1, chains [UO6]∞ and tetramers [U4O26] built by corner-sharing in 2 and 3, respectively. These units are further connected by oxalate entities that act as bis-bidentate to form one-dimensional chains in 1 and bi-dimensional network in 2 and 3. These chains or layers are connected in frameworks by hydrogen-bond arrays.