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 P1¯, Z=1Z=1, a=6.097(2)a=6.097(2), b=5.548(2)b=5.548(2), c=7.806(3)Å, α=89.353(5)α=89.353(5), β=94.387(5)β=94.387(5), γ=97.646(5)°γ=97.646(5)°, V=260.88(15)Å3, β  -form is monoclinic, space group C2/cC2/c, Z=4Z=4, a=12.180(3)a=12.180(3), b=8.223(2)b=8.223(2), c=10.777(3)Å, β=95.817(4)β=95.817(4), V=1073.8(5)Å3. The trihydrate is monoclinic, space group P21/cP21/c, Z=4Z=4, a=5.5095(12)a=5.5095(12), b=15.195(3)b=15.195(3), c=13.398(3)Å, β=93.927(3)β=93.927(3), V=1119.0(4)Å3. 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.

Graphical abstractThe two configurations of the U-centered pentagonal bipyramid and the structural polyhedral building units in the three [(UO2)2(C2O4)(OH)2(H2O)2] networks.Figure optionsDownload full-size imageDownload as PowerPoint slide