A novel layered structure of the heterometallic oxalate compound [NH2(CH3)2]2[NaFe(C2O4)3]·0.33NH(CH3)2·0.33H2O: synthesis, crystal structure and thermal decomposition.

The synthesis, single-crystal X-ray structure determination and thermal analysis are reported for a novel heteronuclear oxalate compound synthesized from a mixture of Fe and Na salts, oxalic acid and N,N-dimethylformamide (DMF) in aqueous solution. The new metallooxalate compound was obtained and identified as a dimethylammonium tris(oxalato)ferrate(III), namely, poly[[bis(dimethylammonium) [tri-μ-oxalato-sodium(I)iron(III)]]-dimethylamine-water (3/1/1)], [NH₂(CH₃)₂]₂[NaFe(C₂O₄)₃]·0.33NH(CH₃)₂·0.33H₂O, which crystallizes in the orthorhombic noncentrosymmetric space group C222₁. In this novel structure, each Fe atom is hexacoordinated by three non-equivalent bidentate oxalate ligands, while the four Na atoms adopt different coordination numbers, i.e. 6, 7 and 8. The structure consists of bimetallic anionic A layers, {[NaFe(C₂O₄)₃]^2-}_n, displaying a layered structure with infinitely linked Fe₂Na₂ tetramers on the ab plane. Two kinds of bimetallic parallel layers (A1 and A2) are present alternately and are found to be staggered, while only the A2 layer is crossed by a twofold axis parallel to the a axis through two Na atoms. The [NH₂(CH₃)₂]⁺ (HDMA) cations occupy the voids between the anionic layers, while the free molecules, i.e. NH(CH₃)₂ (DMA) and H₂O, are located between two different anionic layers. In addition to ionic bonds, the stability of the structure is ensured by hydrogen-bond interactions involving the oxalate ligands and the nitrogenous and water molecules. The layered structure appears to be different in the family of oxalate-bridged Na^I-Fe^III compounds. It is in agreement with the predicted 2D or layered structure of bimetallic complexes containing anionic tris(oxalato)metallate(III) with the [XR₄]⁺ counter-ion template (X = N, P or S, and R = alkyl group or H). The thermal decomposition of the compound shows the final residual product to be NaFeO₂.