Structure of double Keggin-Ti/W-mixed polyanion [(A-β-GeTi3W9O37)2O3]14- and multielectron-transfer-based photocatalyic H2-generation

[(A-beta-GeW9Ti3O37)(2)O-3](14-) (1a) is isolated as K+/Na+/H+ salt, K9NaH4[(beta-GeW9Ti3O37)(2)O-3]center dot 40.5H(2)O (1), and characterized by single-crystal X-ray structural and W-183 NMR spectral analyses in comparison with [(A-alpha-GeW9Ti3O37)(2)O-3](14-) (2a) previously isolated as K9H5[(alpha-GeW9Ti3O37)(2)O-3]center dot 16H(2)O (2). Two isomers of 1a and 2a have local symmetry of D-3h and are stable in aqueous solutions at least at pH values of <= 7. The photocatalytic H-2-formation for the dehydrogenative oxidation of methanol (CH3OH -> H-2 + HCHO) with 1 and 2 is investigated with a help of electronic and ESR spectra and electrochemistry, and is explained in terms of the two-electron reduction of protons at the central Ti-111-O-Ti-111 linkage of the corner-shared TiO6 octahedra connecting two half molecules. (c) 2006 Elsevier B.V. All rights reserved.