Soluble [PMo₁₂O₄₀]³⁻ salts of the Group 1 (Li⁺ and Na⁺), Group 2 (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) and Group 13 (Al³⁺) metals have been synthesized and thoroughly characterized by chemical analysis, TGA/DTA, IR spectroscopy, X-ray powder diffraction (XRD), and by both solid-state and solution ³¹P NMR spectroscopy. The structure of Ba₃[PMo₁₂O₄₀]₂·(55.3)D₂O at 150 K has also been determined by neutron powder diffraction. The compounds may be prepared by combination of H₃[PMo₁₂O₄₀] and the carbonates or hydroxide/carbonates in the appropriate stoichiometry or, in the case of Al³⁺, by addition of Al₂(SO₄)₃ and subsequent removal of the SO₄²- as insoluble BaSO₄ through the addition of BaCO₃. All soluble (Mⁿ⁺)₃/n[PMo₁₂O₄₀] compounds, where Mⁿ⁺ is a cation of a Group 1, 2 or 13 element, crystallize from solution with about 27–31 water molecules per [PMo12O40]³⁻ ion. The crystallized solids are cubic (all with a ≈ 23.3 Å), space group Fd3m, and are isostructural with H₃[PMo₁₂O₄₀]·(29–30)H₂O. Neutron and X-ray powder diffraction show that the cations and water molecules of crystallization are effectively disordered in the interanionic voids (channels) in the unit cell, and cannot be located. There is no evidence for any co-crystallization of hydrolysed species, based on examination of the crystallized solids by IR spectroscopy and both solid-state and solution 31P NMR spectroscopy.
Journal of Molecular Catalysis A: Chemical Vol. 219, Issue 2, p. 327-342