Synthesis and crystal structure of [Nd(H2O)2]2(C2O4)3

Some crystals of Nd(H₂O)₂]₂(C₂O₄)₃ were synthesized hydrothermally by heating at 200°C for 8 days an aqueous suspension of neodymium oxalate decahydrate in presence of terephthallic acid and guanidinium carbonate. They crystallize in the orthorhombic system, space group P2₁2₁2₁, with a = 8.6702(7) Å, b = 9.558(2) Å, and c = 17.009(2) Å. The structure of this complex is built up by two independent neodymium atoms, three bischelating oxalate ligands, and four water molecules forming a rectangle building unit of 6-membered ring, Ln(H₂O)₂(C₂O₄)]₆. The packing of these units leads to a layer parallel to the plane (001). However, the neodymium atoms of two neighbor layers share an edge of oxalato oxygen atoms thus giving a double-layer. The three dimensionality between these double-layers is insured by hydrogen bonds of water molecules which are bound to the neodymium atoms. There is no zeolitic water molecule. The two neodymium atoms are nine-coordinated. In both cases, the coordination polyhedron can be described as a distorted tricapped trigonal prism.