Rates of ligand exchange between >FeIII-OH2 functional groups on a nanometer-sized aqueous cluster and bulk solution

Variable-temperature O-17 NMR experiments were conducted on the nanometer-sized Keplerate MO72Fe30 cluster, with the stoichiometry [MO72Fe30O252(CH3COO)(12)[MO2O7(H2O)](2)[H2MO2O8(H2O)(91)]center dot similar to(H2O)91]center dot similar to 150H(2)O. This molecule contains on its surface 30 Fe(H2O) groups forming a well-defined icosidodecahedron, and we estimated the rates of exchange of the isolated >Fe-III-OH2 waters with bulk aqueous solution, Both longitudinal and transverse O-17-relaxation times were measured, as well as chemical shifts, and these parameters were then fit to the Swift-Connick equations in order to obtain the rate parameters. Correspondingly, we estimate: k(ex)(298) = 6.7(+/- 0.8) x 10(6) s(-1), which is about a factor of similar to 4 x 10(4) times larger than the corresponding rate coefficient for the Fe(OH2)(6)(3+) ion of k(ex)(298) = 1.6 x 10(2) S-1 (Grant and Jordan, 1981; Inorg. Chem. 20, 55-60) and Delta H double dagger and Delta S double dagger are 26.3 +/- 0.6 kj mol(-1) and -26 +/- 0.9 J mol(-1) K-1, respectively. High-pressure 110 NMR experiments were also conducted, but the cluster decomposed slightly under pressure, which precluded confident quantitative estimation of the AV. However, the increase in the reduced transverse-relaxation time with pressure suggests a dissociative character, such as a D or Id mechanism. The enhanced reactivity of waters on the MO72Fe30 cluster is associated with an increase in the Fe-III-OH2 bond length in the solid state of similar to 0.1 angstrom relative to the Fe(OH2)(6)(3+) ion, suggesting that a correlation exists between the Fe-III-OH2 bond length and k(ex)(298). Although there are only few high-spin Fe(III) complexes where both exchange rates and structural data are available, these few seem to support a general correlation.