Vanadium(V)-Substitution Reactions of Wells-Dawson-Type Polyoxometalates: From [X2M18O62](6-) (X = P, As; M = Mo, W) to [X2VM17O62](7-)

The formation processes of V(V)-substituted polyoxometalates with the Wells-Dawson-type structure were studied by cyclic voltammetry and by P-31 NMR and Raman spectroscopy. Generally, the vanadium-substituted heteropolytungstates, [P2VW17O62](7-)and [As2VW17O62](7-), were prepared by mixing equimolar amounts of the corresponding lacunary species-[P2W17O61](10-)and [As2W17O61](10-)-and vanadate. According to the results of various measurements in the present study, the tungsten site in the framework of [P2W18O62](6-)and [As2W18O62](6-) without defect sites could be substituted with V(V) to form the [P2VW17O62](7-) and [As2VW17O62](7-), respectively. The order in which the reagents were mixed was observed to be the key factor for the formation of Dawson-type V(V)-substituted polyoxometalates. Even when the concentration of each reagent was identical, the final products differed depending on the order of their addition to the reaction mixture. Unlike Wells-Dawson-type heteropolytungstates, the molybdenum sites in the framework of [P2Mo18O62](6-) and [As2Mo18O62](6-) were substituted with V(V), but formed Keggin-type [PVMo11O40](4-) and [AsVMo11O40](4-) instead of [P2VMo17O62](7-) and [As2VMo17O62](7-), respectively, even though a variety of reaction conditions were used. The formation constant of the [PVMo11O40](4-) and [AsVMo11O40](4-) was hypothesized to be substantially greater than that of the [P2VMo17O62](7-) and [As2VMo17O62](7-).