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)]center dot 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 (1 > 2 sigma(I)), for alpha (1) and beta (2) forms, respectively and to R(wR) = 0. 0301(0.0737) calculated for 2471 unique observed reflections (I > 2 sigma(I)), for 3. The alpha-form of the dihydrate is triclinic, space group P1, Z = 1, a = 6.097(2), b = 5.548(2), c = 7.806(3) angstrom, alpha = 89.353(5), beta = 94.387(5), gamma = 97.646(5)degrees, V = 260.88(15) angstrom(3), beta-form is monoclinic, space group C2/c, Z = 4, a = 12.180(3), b = 8.223(2), c = 10.777(3) angstrom, beta = 95.817(4), V = 1073.8(5) angstrom(3). The trihydrate is monoclinic, space group P2(1)/c, Z = 4, a = 5.5095(12), b = 15.195(3), c = 13.398(3)angstrom, beta = 93.927(3), V = 1119.0(4) angstrom(3). In the three structures, the coordination of uranium atom is a pentagonal bipyramid composed of dioxo UO(2)(2+)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 I 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 bisbidentate 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. (c) 2005 Elsevier Inc. All rights reserved.