Oxygen-isotope exchange rates for three isostructural polyoxometalate ions

We compare oxygen-isotope exchange rates for all structural oxygens in three polyoxoniobate ions that differ by systematic metal substitutions of Ti(IV) → Nb(V). The [H _xNb ₁₀O ₂₈] ^(6-x)-, [H _xTiNb ₉O ₂₈] ^(7-x)-, and [H _xTi ₂Nb ₈O ₂₈] ^(8-x)- ions are all isostructural yet have different Brãnsted properties. Rates for sites within a particular molecule in the series differ by at least ∼10 ⁴, but the relative reactivities of the oxygen sites rank in nearly the same relative order for all ions in the series. Within a single ion, most structural oxygens exhibit rates of isotopic exchange that vary similarly with pH, indicating that each structure responds as a whole to changes in pH. Across the series of molecules, however, the pH dependencies for isotope exchanges and dissociation are distinctly different, reflecting different contributions from proton- or base-enhanced pathways. The proton-enhanced pathway for isotope exchange dominates at most pH conditions for the [H _xTi ₂Nb ₈O ₂₈] ^(8-x)- ion, but the base-enhanced pathways are increasingly important for the [H _xTiNb ₉O ₂₈] ^(7-x)- and [H _xNb ₁₀O ₂₈] ^(6-x)- structures at higher pH. The local effect of Ti(IV) substitution could be assessed by comparing rates for structurally similar oxygens on each side of the [H _xTiNb ₉O ₂₈] ^(7-x)- ion and is surprisingly small. Interestingly, these nanometer-size structures seem to manifest the same general averaged amphoteric chemistry that is familiar for other reactions affecting oxides in water, including interface dissolution by proton- and hydroxyl-enhanced pathways.