U(IV)/Ln(III) unexpected mixed site in polymetallic oxalato complexes. Part I. Substitution of Ln(III) for U(IV) from the new oxalate (NH4)2U2(C2O4)5·0.7H2O

A new ammonium uranium (IV) oxalate (NH4)2U2(C2O4)5·0.7H2O (1) and three mixed uranium (IV)–lanthanide (III) oxalates, (N2H5)2.6U1.4M0.6(C2O4)5·xH2O (M=Nd (2) and M=Sm (3)), Na2.56U1.44Nd0.56(C2O4)5·7.6H2O (4) and Na3UCe(C2O4)5·10.4H2O (5), have been prepared. The crystal structures of compounds 1, 4 and 5 have been determined by single-crystal X-ray diffraction. The crystal structures were solved by the direct methods and Fourier difference techniques, and refined by a least square method on the basis of F2 for all unique reflections. Compounds 2 and 3 are isotypic with 1. Crystallographic data: 1, hexagonal, space group P63/mmc, a=19.177(3), c=12.728(4) Å, Z=6, R1=0.0575 for 52 parameters with 1360 reflections with I⩾2σ(I); 2, hexagonal, space group P63/mmc, a=19.243(4), c=12.760(5) Å, Z=6; 3, hexagonal, space group P63/mmc, a=19.211(3), c=12.274(4) Å, Z=6; 4, orthorhombic, space group Pbcn, a=18.79(3), b=11.46(1), c=12.77(2) Å, Z=4, R1=0.0511 for 183 parameters with 3026 reflections with image; 5, monoclinic, space group C2/c, a=18.878(6), b=11.684(4), c=12.932(4) Å, β=95.97(1)°, Z=4, R1=0.0416 for 213 parameters with 4060 reflections with I⩾2σ(I). The honeycomb-like structure of the five compounds is built from the same three-dimensional arrangement of metallic and oxalate ions. Similar hexagonal rings of alternating metallic and oxalate ions form layers parallel to the (001) plane that are pillared by another oxalate ion. Indeed, some torsions or rotations of the bridging oxalate ligands led to modifications of the network symmetry. The monovalent cations and the water molecules occupy the hexagonal tunnels running down the [001] direction. Starting from the uranium (IV) compound A2U2(C2O4)5·0.7H2O with A=NH4+ (1), the mixed U(IV)/Ln(III) oxalates are obtained by partial substitution of U(IV) by Ln(III) in a ten-coordinated site, the charge deficit being compensated by intercalation of supplementary monovalent ions within the tunnels. The distortion of the arrangement in the [001] direction for the Na-containing compounds allows the accommodation of a greater number of water molecules that insure an octahedral coordination of the Na atoms.