A Dawson‐Type Dirhenium(V)‐Oxido‐Bridged Polyoxotungstate: X‐ray Crystal Structure and Hydrogen Evolution from Water Vapor under Visible Light Irradiation

Abstract

AbstractThe 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)