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

Abstract

AbstractThe isolation and structural characterisation of a novel tris[peroxotitanium(IV)]‐substituted α‐Dawson polyoxometalate (POM), [α‐1,2,3‐P2W15(TiO2)3O56(OH)3]9− (1a), containing side‐on coordination (i.e. η2‐coordination) of the peroxo group to the titanium(IV) atom are described. A water‐soluble, completely inorganic compound of the monomeric POM Na9[α‐1,2,3‐P2W15(TiO2)3O56(OH)3]·16H2O (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 {[α‐1,2,3‐P2W15Ti3O59(OH)3]4[μ3‐Ti(OH)3]4Cl}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 1 could not be obtained from the reaction with another giant tetrapod POM, {[α‐1,2,3‐P2W15Ti3O57.5(OH)3]4Cl}25− (3a), which does not contain the bridging octahedral Ti groups. The molecular structure of 1a was successfully determined by a single‐crystal X‐ray diffraction study and was shown to be a monomeric α‐Dawson POM derived from the tris[peroxotitanium(IV)] species. Bond valence sum (BVS) calculations suggest that the μ2‐oxo sites of the three Ti−O−Ti bridges in the Ti3 cap of the Dawson unit are protonated. Characterisation of 1 was also accomplished by complete elemental analysis, TG/DTA, FTIR and solution (31P and 183W) NMR spectroscopy. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)