Isolation and molecular structure of a monomeric, tris[peroxotitanium(IV)]-substituted α-Dawson polyoxometalate derived from the tetrameric anhydride form composed of four tris[titanium(IV)]-substituted α-Dawson substructures and four bridging titanium(IV) octahedral groups

The isolation and structural characterisation of a novel tris[peroxotitanium(IV)]-substituted alpha-Dawson polyoxometalate (POM), [alpha-1,2,3-P2W15(TiO2)(3)O-56(OH)(3)](9-) (1a), containing side-on coordination (i.e. eta2-coordination) of the peroxo group to the titanium(IV) atom are described. A water-soluble, completely inorganic compound of the monomeric POM Nag [alpha-1, 2,3-P2W15(TiO2)(3)O-56(OH3)(3)].16H(2)O (1), obtained as analytically pure, homogeneous orange crystals, was obtained from a reaction in aqueous solution of an excess of 30% aqueous hydrogen peroxide with {[alpha-1,2,3-P2W15Ti3O59(OH)(3)](4) [mu(3)-Ti(OH)(3)](4)Cl}(33-) (2a). The latter has been recently shown to be a giant "tetrapod"-shaped, tetrameric anhydride formed from the tris[titanium(IV)]-substituted Dawson substructures with four bridging Ti octahedral groups. Compound I could not be obtained from the reaction with another giant tetrapod POM, {[alpha-1,2,3-P2W15Ti3O57.5(OH)(3)](4)Cl}(25-) (3a), which does not contain the bridging octahedral Ti groups. The molecular structure of la was successfully determined by a single-crystal X-ray diffraction study and was shown to be a monomeric alpha-Dawson POM derived from the tris[peroxotitanium(IV)] species. Bond valence sum (BVS) calculations suggest that the mu(2)-oxo sites of the three Ti-O-Ti bridges in the Ti-3 cap of the Dawson unit are protonated. Characterisation of 1 was also accomplished by complete elemental analysis, TG/DTA, FTIR and solution (P-31 and W-183) NMR spectroscopy. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).