A series of compounds containing various (oxalato)tantalate(v) complex anions – synthesis, properties and the mixed-metal oxide formation via thermal decomposition

Abstract

Four new ionic, oxalate containing complexes, [Hphen][Ni(phen)₃][Ta(C₂O₄)₄]·6.5H₂O (1), [Ni(phen)₃][Ta(OH)(C₂O₄)₃]·8H₂O (2), [Ni(phen)₃][Ta(OC₂H₅)(C₂O₄)₃]·H₂O (3) and [Ni(phen)₃][Ta(OCH₃)(C₂O₄)₃]·H₂O (4) (phen = 1,10-phenanthroline) were obtained by using an (oxalato)tantalate(V) solution as a source of complex anions. Crystal structures of all four compounds were solved. The complexes contain the same [Ni(phen)₃]²⁺ cation but different (oxalato)tantalate(V) complex anions. The nickel(II) atom in the cation is coordinated with six nitrogen atoms in a trigonally distorted octahedral environment. The coordination sphere around the tantalum atom is subject to change due to the solvent change. In the complex anion of 1 the tantalum(V) atom is octacoordinated by eight oxygen atoms from the four oxalate groups, whereas in the anions of 2, 3 and 4 the coordination polyhedron around tantalum is a distorted pentagonal bipyramid comprising six oxygen atoms from the three oxalate groups and another oxygen atom from the additional oxygen ligand. Different complex anions in 1–4 give rise to the variability of the structural architectures. Crystallization water molecules and the (oxalato)tantalate(V) anions direct diverse hydrogen bonding patterns: from one-dimensional hydrogen-bonded chains in 3 and 4, to an extended three-dimensional framework in 2. Compound 1 contains two different cations, [Hphen]⁺ and [Ni(phen)₃]²⁺, that are involved in a two-dimensional stacking arrangement. The compounds were studied also by TG/DTA analysis, and the composition of the oxides, as the final decomposition product, was checked by the powder X-ray diffraction method. The results indicate that the new compounds are prospective and applicable Ni–Ta precursors for the synthesis of nickel tantalate, NiTa₂O₆.