A General Synthetic Procedure for Heteropolyniobates

The heteropolyanions of W, Mo, and V, which have found numerous applica-tions, 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 [Nb6O19] 8– only. However, heteropolyniobate formation is favored in hydrothermal reac-tions of aqueous, alkaline precursor mixtures. Here we give two examples of heteropolyniobates formed by this general reaction type: K12[Ti2O2] [SiNb12O40]16H2O [1], which contains chains of silicododecaniobate Keggin ions, and Na14[H2Si4Nb16O56]45.5H2O [2], a new heteropolyanion structure type. Polyoxometalates (POMs) comprise a ver-satile class of inorganic, anionic clusters that has made a mark on disciplines as diverse as medicine (1), catalysis (2), ma-terials science (3), geochemistry (4), nucle-ar waste processing (5), and applications involving photochemical or electrochemi-cal response (6). POMs can be further sub-categorized as isopolyanions and hetero-polyanions (7). An isopolyanion consists exclusively of an assembly of edge- and corner-sharing octahedra of d0 early transi-tion metals, whereas the heteropolyanions contain one or more central tetrahedra, about which these octahedra are assembled (8). Heteropolyanion chemistry is dominat-ed by heteropolytungstates,-molybdates, and-vanadates and are usually obtained by precipitation from aqueous, acidified solu-tions at ambient temperature and pressure. Furthermore, isopolyanion chemistry is dominated by W, Mo, and V anionic clus-ters obtained from acidified solutions. Isopolyanions of Ta and Nb can also be obtained from basic solutions. Isopolynio-bate chemistry is dominated by the [Nb6O19] 8 – Lindquist ion (5, 9–12), and the formation and structural characterization o