Rates of Oxygen Exchange between the [H_xNb₆O₁₉]^8-x_(aq) Lindqvist Ion and Aqueous Solutions

Oxygen-isotope-exchange rates were measured between sites in the Lindqvist-type [H_xNb₆O₁₉]^8-x_(aq) polyoxoanion and aqueous solution as a function of pH and temperature. The ion has a central μ₆-O that is inert to exchange, 12 μ₂-O(H), and 6 η-O. The potassium salt of this ion is recrystallized in ¹⁷O-enriched water to ¹⁷O-label the anion, which is then redissolved into isotopically normal water so that the ¹⁷O NMR signals from structural oxygens can be followed as a function of time. Because the central μ₆-O retains its ¹⁷O signal throughout the experiments, it is clear that the polyoxoanion remains intact during isotopic equilibration of the other structural oxygens. At pH conditions where the [HNb₆O₁₉]^7- ion predominates, the μ₂-O(H) sites isotopically exchange with solution about an order of magnitude more rapidly than the η-O sites. Yet, we observe that the terminal and bridging oxo sites react at nearly the same rates when the ion is coordinated to 2−3 protons and possibly when it is unprotonated. On the basis of molecular models and experimental kinetic data, we propose metastable polymorphs of the hexaniobate structure where four of the μ₂-O(H) and η-O sites are temporarily equivalent and bonded to a coordinatively unsaturated Nb(V). This hypothesized intermediate allows facile access to bulk water molecules for exchange but cannot fully explain the kinetic results and additional experiments on other Lindvist ions are required.