Facile preparation of an α-Keggin-type [H3W12O40]5− complex: Does it exist in aqueous solution?

The formation processes of α-Keggin-type [H2W12O40]6− and [H3W12O40]5− complexes were investigated in aqueous WVI (0.05–0.50 M) solutions. The formation of [H2W12O40]6− was ascertained by the appearance of a 183W NMR line at −117 ppm, but no evidence was found for the existence of [H3W12O40]5− in the solution at the accessible pH range. The addition of (CH3)4N+ (Me4N+) to the WVI solution directly precipitated the (Me4N)6[H2W12O40] salt. On the other hand, the addition of the larger Bu4N+ cation precipitates the (Bu4N)4.5H0.5[H3W12O40] salt, because a naked proton formed during the crystallization process or in the solid state may enter into the Keggin shell to produce [H3W12O40]5−. This explanation is based on the fact that [H2W12O40]6− is not spontaneously converted into [H3W12O40]5− in acidified aqueous solution. On the basis of their voltammetric properties, a simple diagnostic criterion was developed to distinguish between [H2W12O40]6− and [H3W12O40]5−.

Graphical abstractAlthough no evidence was found for the existence of [H3W12O40]5− in aqueous WVI solutions at the accessible pH range, its Bu4N+ salt was selectively precipitated from the [H2W12O40]6−-containing solution. A simple voltammetric diagnostic criterion was also developed to distinguish between [H2W12O40]6− and [H3W12O40]5−.Figure optionsDownload full-size imageDownload as PowerPoint slide