Tetrameric, Trititanium( IV )‐Substituted Polyoxotungstates with an α ‐Dawson Substructure as Soluble Metal‐Oxide Analogues: Molecular Structure of the Giant “Tetrapod” [( α ‐1,2,3‐P 2 W 15 Ti 3 O 62 ) 4 { μ 3 ‐Ti(OH) 3 } 4 Cl] 45−

The preparation and structural characterization of the novel polyoxoanion [( α ‐1,2,3‐P 2 W 15 Ti 3 O 62 ) 4 { μ 3 ‐Ti(OH) 3 } 4 Cl] 45− ( 1 a ; abbreviated to {TiO 6 } 16 ; FW ∼16 000) which consists of four tri‐Ti IV ‐1,2,3‐substituted α ‐Dawson substructures, four Ti(OH) 3 bridging groups, and one encapsulated Cl − ion, are described. A water‐soluble, all‐inorganic composition compound of the tetrameric Ti‐O‐Ti‐bridged anhydride form, Na x H 45− x [ 1 a ] ⋅ y  H 2 O ( 1 ; x =16–19, y =60–70), which was afforded by the reaction of the tri‐lacunary Dawson polyoxotungstate Na 12 [B‐ α ‐P 2 W 15 O 56 ] ⋅ 19 H 2 O with an excess of TiCl 4 in aqueous solution, was obtained as analytically pure, homogeneous colorless crystals. Single‐crystal X‐ray structure analysis revealed that 1 a was an inorganic, giant “tetrapod”‐shaped molecule (inscribed to a sphere with a diameter of ∼32 Å) with approximately T d symmetry, in which the 16 edge‐ and/or corner‐shared TiO 6 octahedra were contained. This number of TiO 6 octahedra was larger than that found in other titanium( IV )‐substituted polyoxotungstates. Complex 1 was characterized by complete elemental analysis, TG/DTA, FTIR, UV/Vis absorption, and solution ( 31 P and 183 W) NMR spectroscopy. The longest wavelength band in the UV/Vis absorption spectra of 1 in water was attributed to the O→Ti IV ligand‐to‐metal charge‐transfer (LMCT) transition: the wavelength of the LMCT band increased linearly as the number of TiO 6 octahedra contained in the Keggin and Dawson polyoxoanions increased. The Ti n chromophores formed in the Keggin and Dawson polyoxotungstates are water‐soluble analogues of solid TiO 2 or SrTiO 3 as light‐semiconductors and photocatalysts.