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 (17)O-NMR-based Swift-Connick method. The compounds were the [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) and the larger αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(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 X-ray crystallography, and potentiometry. For [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) at pH 5.4, we estimate: k(298)=1.5(5)±0.3×10(6) s(-1), ΔH(≠)=39.8±0.4 kJ mol(-1), ΔS(≠)=+7.1±1.2 J mol(-1) K(-1) and ΔV(≠)=5.6 ±1.6 cm(3) mol(-1). For the Wells-Dawson sandwich cluster (αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-)) at pH 5.54, we find: k(298)=1.6(2)±0.3×10(6) s(-1), ΔH(≠)=27.6±0.4 kJ mol(-1) ΔS(≠)=-33±1.3 J mol(-1) K(-1) and ΔV(≠)=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(H(2)O)(6)](2+) monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.

A class of uranyl peroxide clusters was discovered before as nanometer-sized ions that form spontaneously in aqueous solutions. The uranyl(VI) cluster investigated here is approximately 2 nm in diameter, contains 24 uranyl moieties, and 12 pyrophosphate units. NMR spectroscopy shows that the ion has two distinct forms that interconvert in milliseconds to seconds depending on the temperature and the size of the counterions. P blue, O red, U yellow.

A water-soluble tetramethylammonium (TMA) salt of a novel Keggin-type polyoxoniobate has been isolated as TMA9[PV2Nb12O42]·19H2O (1). This species contains a central phosphorus site and two capping vanadyl sites. Previously only a single example of a phosphorus-containing polyoxoniobate, [(PO2)3PNb9O34](15-), was known, which is a lacunary Keggin ion decorated with three PO2 units. However, that cluster was isolated as an insoluble structure consisting of chains linked by sodium counterions. In contrast, the [PV2Nb12O42](9-) cluster in 1 is stable over a wide pH range, as evident by (31)P and (51)V NMR, UV/Vis spectroscopy, and ESI-MS spectrometry. The ease of substitution of phosphate into the central tetrahedral position suggests that other oxoanions can be similarly substituted, promising a richer set of structures in this class.

A non-magnetic piston-cylinder pressure cell is presented for solution-state NMR spectroscopy at geochemical pressures. The probe has been calibrated up to 20 kbar using in situ ruby fluorescence and allows for the measurement of pressure dependencies of a wide variety of NMR-active nuclei with as little as 10 μL of sample in a microcoil. Initial (11)B NMR spectroscopy of the H3BO3-catechol equilibria reveals a large pressure-driven exchange rate and a negative pressure-dependent activation volume, reflecting increased solvation and electrostriction upon boron-catecholate formation. The inexpensive probe design doubles the current pressure range available for solution NMR spectroscopy and is particularly important to advance the field of aqueous geochemistry.

Predicting the reactivity of actinide elements in nature is among the most pressing concerns in environmental geochemistry, yet even some basic measures of reactivity are not well known. Included among these are the mechanisms of ligand exchange in simple complexes. These data are important because the reaction dynamics can be compared to computer simulations to gain confidence for cases where experiments are impossible. Among the key parameters used to describe ligand-exchange mechanisms are activation volumes, which derive from the pressure dependence of the reaction rates. These activation volumes are interpreted to indicate the extents to which the incoming ligand can influence the activated state. In this sense, the UO 2 (CO 3 ) 3 4À (aq) ion is a particularly compelling system because rates of carbonate exchange are apparently independent of free carbonate concentrations.[1]Herein we report high-pressure rate data for the mononuclear uranyl carbonate ion, UO 2 (CO 3 ) 3 4À (aq), which is a dominant species in uranyl-rich, near-neutral pH solutions that are open to atmospheric CO 2 .[1b-d, 2] Bµnyai et al. presented rate equations for two pathways for carbonate exchange [1d] in the mononuclear UO 2 (CO 3 ) 3 4À (aq) species, which were combined into the overall rate equation [Eq. (1)]:where k 1 and k 2 are the rate coefficients, [H + ] is the proton concentration, and [UO 2 (CO 3 ) 3 4À ] is the concentration of the target ion. Pathway 1 (k 1 ) is independent of pH and dominates at high pH (~9.50), while the proton-enhanced pathway 2 (k 2 ) dominates at lower pH (~7.00).The 13 C chemical shifts of bound and free carbonate are less than 1 kHz apart, which presents a unique challenge for highpressure saturation-transfer experiments. The long, soft, Gaussian-shaped pulses used in previous studies [3] were not adequately selective. Others have employed the DANTE [4] pulse sequence to better shape excitation bandwidth.[1d] However, the inherently long 908 durations that are characteristic of highpressure probes make optimization of the DANTE sequence excessively long. Instead, we used the pulse sequence, described below and illustrated in Figure 1, which exploits the principles of null points obtained by means of the DANTE sequence, and is similar to the pulse sequence used by Bodor et al. [5] Magnetization for both sites is tipped into the xy-plane using a hard 908 pulse. A calibrated time, t Prec , is allowed to pass where, in the rotating frame, the off-resonance signal is allowed to precess until it is exactly 1808 out of phase with the on-resonance peak. Here, another hard 908 pulse is used to achieve inversion of magnetization for one site and, by varying the phase of the second pulse, one can manipulate which peak achieves the inversion. We then allow a delay so the two spin states are allowed to mix via chemical exchange. As magnetization between the two sites is transferred, intensity decreases for the positive site, and increases for the inverted (negative) site (Figure 2). Eventually T 1 relaxation do...