Structural diversity of the lanthanide oxalates: Condensation of neodymium oxygen polyhedra under hydrothermal conditions

New neodymium hydroxo-oxalate and oxalate [Nd6(H2O)6(C2O4)7(OH)4]·4H2O (1) and [Nd2(H2O)4(C2O4)3]·2H2O (2) were synthesized by hydrothermal reaction at 150 °C between neodymium nitrate and oxalic acid solutions at pH = 10–11 obtained by adding various monoamines. The structures were determined from single-crystal X-ray diffraction data. The two compounds crystallize in the monoclinic system with space group P21/c and a = 17.4384 (11), b = 8.1717 (5), c = 12.9929 (7), β = 94.66 (1)°, V = 1845.38 (19) Å3, Z = 2 for 1 and a = 9.8249 (2) Å, b = 8.2487 (2) Å, c = 10.1911 (3) Å, β = 99.09 (1), V = 815.53 (4) Å3, Z = 2 for 2. Full matrix least-squares refinement yielded R1 = 0.0365 and 0.0267 for 6033 and 3382 independent reflections for 1 and 2 respectively. In 2, the three-dimensional neodymium–oxalate arrangement results from dimeric units of edge shared NdO9 polyhedra connected through oxalate ions acting as bis-bidentate. In 1, the neodymium atoms are connected through μ2-OH and μ3-OH ions to form a hexanuclear inorganic core [Nd6(OH)4(H2O)6] with an unprecedently reported geometry leading to a hexanuclear polyhedra block. The blocks are connected through an O–O bridge involving two oxygen atoms of two oxalate ions to build a centipede-like ribbon. The ribbons are further connected through oxalate ions to form a three dimensional neodymium oxalate arrangement. In 1, oxalates adopt four distinct bridging modes of coordination, μ2, μ3, μ4 and μ5.