The uranyl cage clusters (UO₂)₂₂(O₂)₁₅(PHO₃)₂₀(H₂O)₁₀^26– (U₂₂) and (UO₂)₂₈(O₂)₂₀(PHO₃)₂₄(H₂O)₁₂^32– (U₂₈) can be probed in aqueous solutions by using a combination of ¹H Diffusion-Ordered Spectroscopy (DOSY) and ¹H-³¹P Heteronuclear-Single Quantum Coherence (HSQC) spectroscopy. This class of clusters is ideal for ¹H NMR analysis in D₂O because of the covalent character of the H–P bond in the phosphonic bridges. ¹H DOSY indicates that the clusters are stable in solution and provides hydrodynamic radii of 9.8 ± 0.4 Å for the U₂₂ and 12.3 ± 0.5 Å for the U₂₈ clusters. Furthermore, ¹H-³¹P HSQC delivers unequivocal signal assignment for both nuclei, which enables solution dynamics to be monitored by variable-temperature experiments, and reveals the presence of phosphonic-bridge conformers. The results provide some of the first dynamic information about steady conformational changes in these enormous actinide macroions.

Nuclear spin relaxation rates of ²H and ¹³⁹La in LaCl₃+²H₂O and La(ClO₄)₃+²H₂O solutions were determined as a function of pressure in order to demonstrate a new NMR probe designed for solution spectroscopy at geochemical pressures. The ²H longitudinal relaxation rates (T₁) vary linearly to 1.6 GPa, consistent with previous work at lower pressures. The ¹³⁹La T₁ values vary both with solution chemistry and pressure, but converge with pressure, suggesting that the combined effects of increased viscosity and enhanced rates of ligand exchange control relaxation. This simple NMR probe design allows experiments on aqueous solutions to pressures corresponding roughly to those at the base of the Earth’s continental crust.

Nuclear spin relaxation rates of ²H and ¹³⁹La in LaCl₃+²H₂O and La(ClO₄)₃+²H₂O solutions were determined as a function of pressure in order to demonstrate a new NMR probe designed for solution spectroscopy at geochemical pressures. The ²H longitudinal relaxation rates (T₁) vary linearly to 1.6 GPa, consistent with previous work at lower pressures. The ¹³⁹La T₁ values vary both with solution chemistry and pressure, but converge with pressure, suggesting that the combined effects of increased viscosity and enhanced rates of ligand exchange control relaxation. This simple NMR probe design allows experiments on aqueous solutions to pressures corresponding roughly to those at the base of the Earth’s continental crust.

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 ¹¹B 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.

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 ¹¹B 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.

The vanadium-containing heteropolyoxoniobate (TMA)₉[V₃Nb₁₂O₄₂]·18H₂O was synthesised by hydrothermal reaction of V₂O₅ and hydrous niobium oxide in tetramethylammonium hydroxide solution. The cluster has an α-Keggin structure with a central VO₄ and two trans-bicapped VO₅. The water-soluble product was characterised by X-ray crystallography, ESI-MS and both liquid- and solid-state ⁵¹V NMR spectroscopy. The solid- and solution-phase ⁵¹V NMR spectra indicate two major peaks corresponding to one VO₄ and two VO₅ sites.

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 ³¹P and ⁵¹V 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.

[ThB₅O₆(OH)₆][BO(OH)₂]·2.5H₂O (Notre Dame Thorium Borate-1, NDTB-1) is an inorganic supertetrahedral cationic framework material that is derived from boric acid flux reactions. NDTB-1 exhibits facile single crystal to single crystal anion exchange with a variety of common anions such as Cl⁻, Br⁻, NO₃⁻, IO₃⁻, ClO₄⁻, MnO₄⁻, and CrO₄²⁻. More importantly, NDTB-1 is selective for the removal of TcO₄⁻ from nuclear waste streams even though there are large excesses of competing anions such as Cl⁻, NO₃⁻, and NO₂⁻. Competing anion exchange experiments and magic-angle spinning (MAS)-NMR spectroscopy of anion-exchanged NDTB-1 demonstrate that this unprecedented selectivity originates from the ability of NDTB-1 to trap TcO₄⁻ within cavities, whereas others remain mobile within channels in the material. The exchange kinetics of TcO₄⁻ in NDTB-1 are second-order with the rate constant k₂ of 0.059 s⁻¹ M⁻¹. The anion exchange capacity of NDTB-1 for TcO₄⁻ is 162.2 mg g⁻¹ (0.5421 mol mol⁻¹) with a maximum distribution coefficient K_d of 1.0534 × 10⁴ mL g⁻¹. Finally, it is demonstrated that the exchange for TcO₄⁻ in NDTB-1 is reversible. TcO₄⁻ trapped in NDTB-1 can be exchanged out using higher-charged anions with a similar size such as PO₄³⁻ and SeO₄²⁻, and therefore the material can be easily recycled and reused.

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 ¹⁷O-NMR-based Swift–Connick method. The compounds were the [Co₄(H₂O)₂(B-α-PW₉O₃₄)₂]¹⁰⁻ and the larger αββα-[Co₄(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 X-ray crystallography, and potentiometry. For [Co₄(H₂O)₂(B-α-PW₉O₃₄)₂]¹⁰⁻ at pH 5.4, we estimate: k²⁹⁸=1.5(5)±0.3×10⁶ 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 cm³ mol⁻¹. For the Wells–Dawson sandwich cluster (αββα-[Co₄(H₂O)₂(P₂W₁₅O₅₆)₂]¹⁶⁻) at pH 5.54, we find: k²⁹⁸=1.6(2)±0.3×10⁶ 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 cm³mol⁻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.

Rates of oxygen-isotope exchange were measured in the tetrasiliconiobate ion [H_2+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∼7, where new peaks began to slowly form without losing the ¹⁷O 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.