Hydrazinium lanthanide oxalates: synthesis, structure and thermal reactivity of N2H5[Ln2(C2O4)4(N2H5)]·4H2O, Ln = Ce, Nd

Abstract

New hydrazinium lanthanide oxalates N₂H₅[Ln₂(C₂O₄)₄(N₂H₅)]·4H₂O, Ln = Ce (Ce-HyOx) and Nd (Nd-HyOx), were synthesized by hydrothermal reaction at 150 °C between lanthanide nitrate, oxalic acid and hydrazine solutions. The structure of the Nd compound was determined from single-crystal X-ray diffraction data, space group P2₁/c with a = 16.315(4), b = 12.127(3), c = 11.430(2) Å, β = 116.638(4)°, V = 2021.4(7) ų, Z = 4, and R₁ = 0.0313 for 4231 independent reflections. Two distinct neodymium polyhedra are formed, NdO₉ and NdO₈N, an oxygen of one monodentate oxalate in the former being replaced by a nitrogen atom of a coordinated hydrazinium ion in the latter. The infrared absorption band at 1005 cm⁻¹ confirms the coordination of N₂H₅⁺ to the metal. These polyhedra are connected through μ2 and μ3 oxalate ions to form an anionic three-dimensional neodymium-oxalate arrangement. A non-coordinated charge-compensating hydrazinium ion occupies, with water molecules, the resulting tunnels. The N–N stretching frequencies of the infrared spectra demonstrate the existence of the two types of hydrazine ions. Thermal reactivity of these hydrazinium oxalates and of the mixed isotypic Ce/Nd (CeNd-HyOx) oxalate were studied by using thermogravimetric and differential thermal analyses coupled with gas analyzers, and high temperature X-ray diffraction. Under air, fine particles of CeO₂ and Ce_0.5Nd_0.5O_1.75 are formed at low temperature from Ce-HyOx and CeNd-HyOx, respectively, thanks to a decomposition/oxidation process. Under argon flow, dioxymonocyanamides Ln₂O₂CN₂ are formed.