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

The synthesis and crystal structure of a Dawson‐type dirhenium(V)‐oxido‐bridged polyoxotungstate are described. The potassium salt K 14 [O{Re V (OH)(α 2 ‐P 2 W 17 O 61 )} 2 ] · 21H 2 O (K‐ 1 ) was obtained as analytically pure, homogeneous, black‐purple crystals by the reaction of a monolacunary Dawson polyoxotungstate with [Re IV Cl 6 ] 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 C 2 h symmetry. Characterization of K‐ 1 was also accomplished by elemental analysis, TG/DTA, and FTIR, UV/Vis absorption, ESR, and solution 31 P NMR spectroscopy. Furthermore, the black‐purple compound K‐ 1 was grafted onto a TiO 2 surface by electrostatic binding to give TiO 2 with a cationic quaternaryammonium moiety ( 1 ‐grafted TiO 2 ). Elemental analysis of 1 ‐grafted TiO 2 indicated that the seven potassium counterions of K‐ 1 were ion‐exchanged with seven cationic ammonium moieties to form {TiO 2 } 5500 [≡Si(CH 2 ) 3 N(CH 3 ) 3 Cl] 7 [≡Si(CH 2 ) 3 N + (CH 3 ) 3 ] 7 (K 7 [O{Re(OH)(α 2 ‐P 2 W 17 O 61 )} 2 ] 7– ) and seven molecules of KCl. The diffuse reflectance (DR) UV/Vis spectrum of 1 ‐grafted TiO 2 exhibited two sharp bands at 496 nm and 751 nm due to the Re V →W VI 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 TiO 2 surface. With 1 ‐grafted TiO 2 , 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)