A Soluble Phosphorus‐Centered Keggin Polyoxoniobate with Bicapping Vanadyl Groups

A water‐soluble tetramethylammonium (TMA) salt of a novel Keggin‐type polyoxoniobate has been isolated as TMA 9 [PV 2 Nb 12 O 42 ]⋅19H 2 O ( 1 ). This species contains a central phosphorus site and two capping vanadyl sites. Previously only a single example of a phosphorus‐containing polyoxoniobate, [(PO 2 ) 3 PNb 9 O 34 ] 15− , was known, which is a lacunary Keggin ion decorated with three PO 2 units. However, that cluster was isolated as an insoluble structure consisting of chains linked by sodium counterions. In contrast, the [PV 2 Nb 12 O 42 ] 9− cluster in 1 is stable over a wide pH range, as evident by 31 P and 51 V NMR, UV/Vis spectroscopy, and ESI‐MS spectrometry. The ease of substitution of phosphate into the central tetrahedral position suggests that other oxoanions can be similarly substituted, promising a richer set of structures in this class.