SEARCH

SEARCH BY CITATION

Keywords:

  • Polyoxometalates;
  • Rhenium;
  • X-ray diffraction;
  • UV/Vis spectroscopy;
  • Photocatalysis

Abstract

The synthesis and crystal structure of a Dawson-type dirhenium(V)-oxido-bridged polyoxotungstate are described. The potassium salt K14[O{ReV(OH)(α2-P2W17O61)}2]·21H2O (K-1) was obtained as analytically pure, homogeneous, black-purple crystals by the reaction of a monolacunary Dawson polyoxotungstate with [ReIVCl6]2– in water, followed by crystallization from an aqueous HCl solution in the dark. Single-crystal X-ray structure analysis revealed that the dimeric dirhenium(V)-oxido-bridged structure has overall C2h symmetry. Characterization of K-1 was also accomplished by elemental analysis, TG/DTA, and FTIR, UV/Vis absorption, ESR, and solution 31P NMR spectroscopy. Furthermore, the black-purple compound K-1 was grafted onto a TiO2 surface by electrostatic binding to give TiO2 with a cationic quaternaryammonium moiety (1-grafted TiO2). Elemental analysis of 1-grafted TiO2 indicated that the seven potassium counterions of K-1 were ion-exchanged with seven cationic ammonium moieties to form {TiO2}5500[≡Si(CH2)3N(CH3)3Cl]7[≡Si(CH2)3N+(CH3)3]7(K7[O{Re(OH)(α2-P2W17O61)}2]7–) and seven molecules of KCl. The diffuse reflectance (DR) UV/Vis spectrum of 1-grafted TiO2 exhibited two sharp bands at 496 nm and 751 nm due to the ReV→WVI intervalence charge transfer (IVCT) band and the d–d band of the rhenium(V) atom, respectively, in the visible light region, which suggestedthat polyoxoanion 1 was isolated on the TiO2 surface. With 1-grafted TiO2, hydrogen evolution from water vapor under irradiation with visible light (>400 nm and >420 nm) was achieved. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)