A General Synthetic Procedure for Heteropolyniobates

The heteropolyanions of W, Mo, and V, which have found numerous applications, are formed simply by acidification of solutions of their oxoanions. Under similar conditions, these oxoanion precursors are not available for Nb, and Nb-oxo chemistry is dominated by formation of the Lindquist ion [Nb 6 O 19 ] 8– only. However, heteropolyniobate formation is favored in hydrothermal reactions of aqueous, alkaline precursor mixtures. Here we give two examples of heteropolyniobates formed by this general reaction type: K 12 [Ti 2 O 2 ][SiNb 12 O 40 ]·16H 2 O [1], which contains chains of silicododecaniobate Keggin ions, and Na 14 [H 2 Si 4 Nb 16 O 56 ]·45.5H 2 O [ 2 ], a new heteropolyanion structure type.