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Rates of oxygen-isotope exchange were measured in the tetrasiliconiobate ion [H2+xSi4Nb16O56]((14-x)-) to better understand how large oxide ions interact with water. The molecule has 19 nonequivalent oxygen sites and is sufficiently complex to evaluate hypotheses derived from our previous work on smaller clusters. We want to examine the extent to which individual oxygen atoms react independently with particular attention given to the order of protonation of the various oxygen sites as the pH decreases from 13 to 6. As in our previous work, we find that the set of oxygen sites reacts at rates that vary over approximately 104 across the molecule at 6 < pH < 13 but with similar pH dependencies. There is NMR evidence of an intra-or intermolecular reaction at pH similar to 7, where new peaks began to slowly form without losing the O-17 isotopic tag, and at pH <= 6 these new peaks formed rapidly. The oxygen atoms bonded to silicon atoms began to isotopically exchange at pH 9 and below. The 17O NMR peak positions also vary considerably with pH for some, but not all, nonequivalent oxygen sites. This variation could be only partly accounted by electronic calculations, which indicate that oxygen atoms should shift similarly upon protonation. Instead, we see that some sites change enormously with pH, whereas other, similarly coordinated oxygen atoms are less affected, suggesting that either some protons are exchanging so rapidly that the oxygen sites are seeing an averaged charge, or that counterions are modulating the effect of the coordinated protons.

Polyoxometalate ions are used as ligands in water-oxidation processes related to solar energy production. An important step in these reactions is the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co-II atoms in two polyoxotungstate sandwich molecules using the O-17-NMR-based Swift-Connick method. The compounds were the [Co-4(H2O)(2)(B-alpha-PW9O34)(2)](10-) and the larger alpha beta beta alpha-[Co-4(H2O)(2)(P2W15O56)(2)](16-) ions, each with two water molecules bound trans to one another in a Co-II sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, EPR, FT-IR, UV-Vis, and EXAFS spectroscopy, ESI-MS, single-crystal Xray crystallography, and potentiometry. For [Co-4(H2O)(2)(B-alpha-PW9O34)(2)](10-) at pH 5.4, we estimate: k(298) = 1.5(5) +/- 0.3 x 10(6) s(-1), Delta H-not equal = 39.8 +/- 0.4 kJ mol(-1), Delta S-not equal = + 7.1 +/- 1.2 J mol(-1)K(-1) and Delta V-not equal = 5.6 +/- 1.6 cm(3)mol(-1). For the Wells-Dawson sandwich cluster (alpha beta beta alpha-[Co-4(H2O)(2)(P2W15O56)(2)](16-)) at pH 5.54, we find: k(298) = 1.6(2) +/- 0.3 x 10(6)s(-1), Delta H-not equal = 27.6 +/- 0.4 kJ mol(-1) Delta S-not equal = -33 +/- 1.3 J mol(-1)K(-1) and Delta V-not equal = 2.2 +/- 1.4 cm(3)mol(-1) at pH 5.2. The molecules are clearly stable and monospecific in slightly acidic solutions, but dissociate in strongly acidic solutions. This dissociation is detectable by EPR spectroscopy as S=3/2 Co-II species (such as the [Co(H2O)(6)](2+) monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.

The synthesis and structure of the first discrete peroxotitanoniobate cluster, [N(CH3)(4)](10)[Ti12Nb6O38(O-2(I))(6)], is described. (C) 2010 Elsevier B.V. All rights reserved.

Our understanding of mineral and glass dissolution has advanced from simple thermodynamic treatments to models that emphasize adsorbate structures. This evolution was driven by the idea that the best understanding is built at the molecular level. Now, it is clear that the molecular questions cannot be answered uniquely with dissolution experiments. At the surface it is unclear which functional groups are present, how they are arranged, and how they interact with each other and with solutes as the key bonds are activated. An alternative approach has developed whereby reactions are studied with nanometre-sized aqueous oxide ions that serve as models for the more complicated oxide interface. For these ions, establishing the structure is not a research problem in itself, and bond ruptures and dissociations can be followed with much confidence. We review the field from bulk-dissolution kinetics to the new isotope-exchange experiments in large oxide ions.

A new water-soluble polyoxometalate, [TiNb9O28](7-), completes an isostructural and homologous decametalate series from the decaniobate to the dititano-octaniobate ion.

A robust and easily reproducible one-pot synthetic method for the preparation of [N(CH3)(4)](6)[Nb10O28] center dot 6H(2)O is introduced.

We examine oxygen-isotope exchanges in a nanometer-size oxide molecule in water and, separately, both its rates of dissociation and molecular products. This molecule, the decaniobate ion ([H(x),Nb(10)O(28)]((6-x)-)), is at the same size scale as geochemically interesting features on minerals, such as surface polymers and kink sites on growth steps, although it is structurally quite dissimilar. Unlike mineral surface structures, however, we have complete confidence in the aqueous structure of this molecule and it yields a clear spectroscopic signature as it reacts. We thus can follow proton-enhanced isotope exchanges and base-induced dissociation in unprecedented detail and clarity. The results are surprising and require new thinking about geochemical reactions at the molecular scale. For example, base-induced dissociation of the molecule, which is unprotonated, causes rates of oxygen-isotope exchanges of all structural oxygens to accelerate dramatically. Similarly, protonation of the molecule causes sets of oxygens to react, although protonation is limited. In general, all reactions are via concerted motions of many atoms and the reactivities vary as though the entire structure was responding to changes in solution composition. The site reactivities could not be inferred from the stable structure of the decaniobate molecule because so much of the structure is involved in each exchange event. Thus, computational models must be structurally faithful to an extraordinary degree, and inherently dynamic, or they will miss the essential chemistry.