New antimonato polyoxovanadates based on the [V14IVSb8IIIO42(H2O)]4− cluster type

Two new antimonato polyoxovanadates with compositions (enH2)2[V14Sb8O42(H2O)]∙3H2O (1) (en = ethylenediamine) and (ppzH2)2[V14Sb8O42(H2O)] (2) (ppz = piperazine) were synthesized under solvothermal conditions. Compound 1 crystallizes in the monoclinic space group P21/n with a = 13.6969(8) Å, b = 11.9183(10) Å, c = 19.0413(12) Å, β = 108.346(7), V = 2950.4(4) Å3 and compound 2 crystallizes in the monoclinic space group P21/c with lattice parameters a = 13.7114(10) Å, b = 11.9476(5) Å, c = 19.9391(14) Å, β = 109.043(8), V = 3087.6(3) Å3. The central structural motif of both structures can be derived from the {V18O42} archetype cluster replacing four VO5 square pyramids by four Sb2O5 moieties yielding two perpendicular oriented eight-membered rings composed of edge-sharing VO5 polyhedra. According to bond valence sum calculations the electronic situation in the clusters may be formulated as [VIV14SbIII8O42(H2O)]4−. In compound 1 the cluster anions are arranged along the b-axis in a …ABAB… fashion, whereas the anions in 2 are stacked along the c-axis in a …AAA… mode. In both compounds neighbored clusters exhibit relatively short Sb–O separations indicating weak inter-cluster interactions, leading to a layer-like arrangement of the [VIV14SbIII8O42(H2O)]4− anions. In the structure of 1 the charge balancing organic ammonium ions are fully disordered whereas the organic cations in 2 are ordered. Further characterization of compound 1 and 2 revealed that the initially used N,N,N′,N′-tetramethylethylenediamine was decomposed to ethylenediamine in 1 and the applied amine 1-methylpiperazine was fragmented to piperazine in 2. Syntheses with the latter amine led to crystallization of compound 1. But compound 2 could not be prepared applying piperazine in the reaction slurry.