The kinetics of oxygen exchange between the GeO4Al12(OH)24(OH2)128+(aq) molecule and aqueous solutions

We report rates of oxygen exchange with bulk solution for an aqueous complex, (GeO4Al12)-Ge-IV(OH)(24)(OH2)(12)(8+)(aq) (GeAl12), that is similar in structure to both the (AlO4Al12)-Al-IV(OH)(24)(OH2)(12)(7+) (aq) (Al-13) and (GaO4Al12)-Ga-IV(OH)(24)(OH2)(12)(7+)(aq) (GaAl12) molecules studied previously. All of these molecules have epsilon-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 Al-13 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 O-17-NMR. Apparent rate parameters for exchange of the bound water molecules (eta-OH2) are k(ex)(298) = 200 (+/-100) s(-1), DeltaH(double dagger) = 46 (+/-8) kJ.mol(-1), and DeltaS(double dagger) = -46 (+/-24) J.mol(-1) K-1 and are similar to those we measured previously for the GaAl12 and Al-13 complexes. In contrast to the Al-13 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 mu(2)-OH sites in the GeAl12 molecule exchange at greatly differing rates. The more labile set of mu(2)-OH sites in the GeAl12 molecule exchange at a rate that is faster than can be measured by the O-17-NMR isotopic-equilibration technique. The second set of mu(2)-OH sites have rate parameters of k(ex)(298) = 6.6 (+/-0.2).10(-4) s(-1), DeltaH(double dagger) = 82 (+/-2) kJ.mol(-1), and DeltaS(double dagger) = -29 (+/-7) J.mol(-1) K-1, corresponding to exchanges approximate to40 and approximate to1550 times, respectively, more rapid than the less labile mu(2)-OH sites in the Al-13 and GaAl12 molecules. We find evidence of nearly first-order pH dependence on the rate of exchange of this mu(2)-OH site with bulk solution for the GeAl12 molecule, which contrasts with Al-13 and GaAl12 molecules. Copyright (C) 2002 Elsevier Science Ltd