Discrete polyoxovanadate cluster into an organic free metal-oxide-based material: syntheses, crystal structures, and magnetic properties of a new series of lanthanide linked-POV compounds [{Ln(H₂O)₆}2As₈V₁₄O₄₂(SO₃)]·8H₂O (Ln = La³⁺, Sm³⁺, and Ce³⁺)

Abstract

This article describes the linking propensity of the sulfite encapsulated polyoxovanadate (POV) anion, [As₈V₁₄O₄₂(SO₃)]^6-, with aqua-lanthanide complex cations [Ln(H₂O)₆]³⁺ in a controlled wet synthesis resulting in a series of organic free metal-oxide-based materials [{Ln(H₂O)₆}₂As₈V₁₄O₄₂(SO₃)]·8H₂O, Ln = La³⁺ (1), Sm³⁺ (2), and Ce³⁺ (3). The title compounds have been characterized by elemental analyses, IR, diffuse reflectance, electron paramagnetic resonance, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), and single-crystal X-ray diffraction studies. All three compounds crystallize in the monoclinic space group P2₁/n. Crystal data for 1: α = 13.4839(7), b = 12.3388(6), c = 18.3572(10) Å, β = 108.2570 (10)°, V = 2900.4(3) Å3. Crystal data for 2: α = 13.4156(3), b = 12.2588(3), c = 18.2501(4) Å, β = 108.049(3)°, V = 2853.8(10) ų. Crystal data for 3: α = 13.4934(3), b = 12.3983(3), c = 18.3992(4) Å, β = 108.025 (3), V = 2927.0(10) ų. Crystal structure shows that each cluster is surrounded by six [Ln(H₂O)₆]³⁺ complex cations, and each [Ln(H₂O)₆]³⁺ cation is coordinated to three surrounding POV cluster anions. The electron spin resonance spectra of compounds 1-3 show a typical single line (g = 1.9671 for 1, g = 1.9669 for 2, and g = 1.9704 for 3), characteristic for a V⁴⁺ (d⁴) ion; in addition, a supplementary signal appears for compound 2 at g = 5.9238 due to the presence of the Sm³⁺ (f⁵) ion. All vanadium atoms exit in +4 oxidation states that have been confirmed by bond valence sum calculations. Variable-temperature magnetic studies for all three compounds 1-3 are performed and are discussed in terms of antiferromagnetic coupling interactions, giving importance to linking/assembling the {V₁₄} cluster anions. TGA/mass analyses of compounds 1-3 (linked system) have been compared with that of the starting precursor [NH₄]₆[As₈V₁₄O₄₂(SO₃)] (discrete building unit). Interestingly, the evolution of SO₂ gas takes place for the discrete cluster compound [NH₄]₆[As₈V₁₄O₄₂(SO₃)] in a temperature range of 480-520 °C with the decomposition of the POV cluster anion, whereas the same evolution occurs at 520-580 °C for compounds 1-3. These comparative TGA/mass studies help to understand how the organic free linker elevates the thermal stability of the sulfite encapsulated POV cluster anion in going from a discrete cluster anion to the linked system (molecule to material). It has also been demonstrated that the stability of the sulfite anion increases to a greater extent when it is included in the cluster cage. The powder XRD studies of compounds 1-3 confirm that these are isostructural materials and provide information about the phase purity.