Remarkable Influence of Structures around Rhenium (V) Centers Constructed in Polyoxotungstates for Methanol Dehydrogenation under Visible Light Irradiation

Abstract

The synthesis and characterization of α1-Dawson-type mono-rhenium (V)-substituted polyoxotungstate, [α1-P2W17ReVO62]7- (1), are described. The dimethylammonium salt of 1, [Me2NH2]7[α1-P2W17ReVO62]•9H2O (Me2NH2-1), was obtained as analytically pure homogeneous black-blue crystals by reacting mono-lacunary α1-Dawson polyoxotungstate with [ReIVCl6]2- in CH3COOH/CH3COOLi buffer, followed by crystallization via a vapor diffusion from acetonitrile/ethanol. Characterization was also accomplished by X-ray crystallography, elemental analysis, TG/DTA, FTIR, UV-vis, and solution 31P NMR spectroscopy; these results showed that the polyoxoanion 1 was a monomeric α-Dawson structure, in which the rhenium(V) ion was coordinated to the monovacant site of [α1-LiP2W17O61]9-, resulting in an overall C1 symmetry. The polyoxoanion 1, [O{ReV(OH)(α2-P2W17O61)}2]14- (2), and [α-PW11ReVO40]4- (3) showed the hydrogen evolution from methanol under visible light irradiation (≥400nm) in the presence of titanium dioxide. The activities were remarkably depended on the rhenium (V) sites in 1–3; polyoxoanion 2 possessing the dirhenium(V)-oxido-bridged site showed the highest activities among these complexes.