A first example of polyoxotungstate-based giant molecule. Synthesis and molecular structure of a tetrapod-shaped Ti–O–Ti bridged anhydride form of Dawson tri-titanium(iv)-substituted polyoxotungstate

Abstract

The preparation and structural characterization of a giant “tetrapod”-shaped inorganic molecule, [(α-1,2,3-P₂W₁₅Ti₃O_60.5)₄Cl]³⁷⁻ ,1a (abbreviated to {TiO₆}₁₂; FW ∼15700), consisting of four tri-Ti^IV-1,2,3-substituted α-Dawson substructures and one encapsulated Cl⁻ ion, are described. A water-soluble, all-inorganic composition compound of the tetrameric Ti–O–Ti bridged anhydride form, Na_xH_33−xK₄[1a]·yH₂O, 1 (x = 21–26, y = 60–70), being afforded by a stoichiometric reaction in aqueous solution of tri-lacunary Dawson polyoxotungstate Na₁₂[B-α-P₂W₁₅O₅₆]·19H₂O with 3 equiv of Ti(SO₄)₂·4H₂O in HCl-acidic aqueous solution followed by exchanging the countercation, was obtained as analytically pure, homogeneous colorless plate crystals. Single-crystal X-ray structure analysis revealed that the polyoxoanion 1a was an inorganic, giant “tetrapod”-shaped molecule (described as a sphere with a diameter of ∼31.2 Å) with approximately T_d symmetry. Characterization of 1 was also accomplished by complete elemental analysis, TG/DTA, FTIR, UV-vis absorption and solution (³¹P and ¹⁸³W) NMR spectroscopy.