An ¹⁷O NMR Study of Hydrolyzed Nb(V) in Weakly Acidic and Basic Aqueous Solution

Time-dependent ¹⁷O NMR spectra of basified decaniobate (Nb₁₀O₂₈^6-) solutions displayed intense resonances assigned to the well-known protonated hexaniobate anion (Nb₆O₁₉^8-) and two other species identified as heptaniobate (Nb₇O₂₂^9-) and protonated tetracosaniobate (Nb₂₄O₇₂^24-) anions. The decaniobate ion showed no sign of protonation from pH 6 - 10, in contrast with the hexaniobate ion which was protonated at doubly-bridging oxygen sites at pH 10-13. Most (> 90%) of the heptaniobate formed 1 h after basification was transformed into other species after 3 weeks. Tetracosaniobate was formed reversibly from decaniobate, but only when KOH, NaOH and [(CH₃)₄N]OH were employed; none was observed after basification with [(n-C₄H₉)₄N]OH. Moreover, far more tetracosaniobate was formed from KOH than from [(CH₃)₄N]OH. This effect was attributed to a tetracosaniobate cation binding site that binds K⁺ more readily than (CH₃)₄N⁺ but is too small to accommodate (n-C₄H₉)₄N⁺.