The kinetics of oxygen exhange between the

We report rates of oxygen exchange with bulk solution for an aqueous complex, ^IVGeO₄Al₁₂(OH)₂₄(OH₂ )₁₂⁸⁺(aq) (GeAl₁₂), that is similar in structure to both the ^IVAlO₄Al₁₂(OH)₂₄(OH₂ )₁₂⁷⁺(aq) (Al₁₃) and ^IVGaO₄Al₁₂(OH)₂₄(OH₂ )₁₂⁷⁺(aq) (GaAl₁₂) molecules studied previously. All of these molecules have ε-Keggin-like structures, but in the GeAl₁₂ molecule, occupancy of the central tetrahedral metal site by Ge(IV) results in a molecular charge of +8, rather than +7, as in the Al₁₃ and GaAl₁₂. Rates of exchange between oxygen sites in this molecule and bulk solution were measured over a temperature range of 274.5 to 289.5 K and 2.95 < pH < 4.58 using ¹⁷O-NMR. Apparent rate parameters for exchange of the bound water molecules (η-OH₂) are k_ex²⁹⁸ = 200 (± 100) s^-1, ΔH^‡ = 46 (±8) kJ · mol^-1, and ΔS^‡ = -46 (±24) J · mol^-1 K^-1 and are similar to those we measured previously for the GaAl₁₂ and Al₁₃ complexes. In contrast to the Al₁₃ and GaAl₁₂ molecules, we observe a small but significant pH dependence on rates of solvolysis that is not yet fully constrained and that indicates a contribution from the partly deprotonated GeAl₁₂ species. The two topologically distinct μ₂-OH sites in the GeAl₁₂ molecule exchange at greatly differing rates. The more labile set of μ₂-OH sites in the GeAl₁₂ molecule exchange at a rate that is faster than can be measured by the ¹⁷O-NMR isotopic-equilibration technique. The second set of μ₂-OH sites have rate parameters of k_ex²⁹⁸ = 6.6 (±0.2) · 10^-4 s^-1, ΔH^‡ = 82 (±2) kJ · mol^-1, and ΔS^‡ = -29 (±7) J · mol^-1 · K^-1, corresponding to exchanges ≈40 and ≈1550 times, respectively, more rapid than the less labile μ₂-OH sites in the Al₁₃ and GaAl₁₂ molecules. We find evidence of nearly first-order pH dependence on the rate of exchange of this μ₂-OH site with bulk solution for the GeAl₁₂ molecule, which contrasts with Al₁₃ and GaAl₁₂ molecules.