Oxygen-Isotope Exchange Rates for Three Isostructural Polyoxometalate Ions

摘要

Abstract: We compare oxygen-isotope exchange rates for all structural oxygens in three polyoxoniobatenions that differ by systematic metal substitutions of Ti(IV) f Nb(V). The [HxNb10O28]n(6 x)n,[HxTiNb9O28]n(7 x)n,nand [HxTi2Nb8O28]n(8 x)nions are all isostructural yet have different Brønsted properties. Rates for sites withinna particular molecule in the series differ by at least ∼104, but the relative reactivities of the oxygen sitesnrank in nearly the same relative order for all ions in the series. Within a single ion, most structural oxygensnexhibit rates of isotopic exchange that vary similarly with pH, indicating that each structure responds as anwhole to changes in pH. Across the series of molecules, however, the pH dependencies for isotopenexchanges and dissociation are distinctly different, reflecting different contributions from proton- or base-nenhanced pathways. The proton-enhanced pathway for isotope exchange dominates at most pH conditionsnfor the [HxTi2Nb8O28]n(8 x)nion, but the base-enhanced pathways are increasingly important for then[HxTiNb9O28]n(7 x)nand [HxNb10O28]n(6 x)nstructures at higher pH. The local effect of Ti(IV) substitution couldnbe assessed by comparing rates for structurally similar oxygens on each side of the [HxTiNb9O28]n(7 x)nionnand is surprisingly small. Interestingly, these nanometer-size structures seem to manifest the same generalnaveraged amphoteric chemistry that is familiar for other reactions affecting oxides in water, including interfacendissolution by proton- and hydroxyl-enhanced pathways.