Strategic Capture of the {Nb7} Polyoxometalate

Group V Nb‐polyoxometalate (Nb‐POM) chemistry generally lacks the elegant pH‐controlled speciation exhibited by group VI (Mo, W) POM chemistry. Here three Nb‐POM clusters were isolated and structurally characterized; [Nb14O40(O2)2H3]14−, [((UO2)(H2O))3Nb46(UO2)2O136H8(H2O)4]24−, and [(Nb7O22H2)4(UO2)7(H2O)6]22−, that effectively capture the aqueous Nb‐POM species from pH 7 to pH 10. These Nb‐POMs illustrate a reaction pathway for control over speciation that is driven by counter‐cations (Li+) rather than pH. The two reported heterometallic POMs (with UO22+ moieties) are stabilized by replacing labile H2O/HO−Nb=O with very stable O=U=O. The third isolated Nb‐POM features cis‐yl‐oxos, prior observed only in group VI POM chemistry. Moreover, with these actinide‐heterometal contributions to the burgeoning Nb‐POM family, it now transects all major metal groups of the periodic table. Nimble niobates: Nb‐polyoxometalate (POM) chemistry lacks reactive building units that can be rationally manipulated. A route was devised to generate metastable heptaniobate building units in situ and capture them in three structures (see figure). The inert uranyl moiety harnesses heptaniobate in two structures, and peroxide stabilizes a third structure. This strategy provides a path to intricate architectures already demonstrated in Mo and W POMs.