Conformational changes of the pyrophosphate (Pp)-functionalized uranyl peroxide nanocluster [(UO2)24(O2)24(P2O7)12]48- ({U24Pp12}), dissolved as a Li/Na salt, can be induced by the titration of alkali cations into solution. The most symmetric conformer of the molecule has idealized octahedral (Oh) molecular symmetry. One-dimensional 31P NMR experiments provide direct evidence that both K+ and Rb+ ions trigger an Oh-to-D4h conformational change within {U24Pp12}. Variable-temperature 31P NMR experiments conducted on partially titrated {U24Pp12} systems show an effect on the rates; increased activation enthalpy and entropy for the D4h-to-Oh transition is observed in the presence of Rb+ compared to K+. Two-dimensional, exchange spectroscopy 31P NMR revealed that magnetization transfer links chemically unique Pp bridges that are present in the D4h conformation and that this magnetization transfer occurs via a conformational rearrangement mechanism as the bridges interconvert between two symmetries. The interconversion is triggered by the departure and reentry of K (or Rb) cations out of and into the cavity of the cluster. This rearrangement allows Pp bridges to interconvert without the need to break bonds. Cs ions exhibit unique interactions with {U24Pp12} clusters and cause only minor changes in the solution 31P NMR signatures, suggesting that Oh symmetry is conserved. Single-crystal X-ray diffraction measurements reveal that the mixed Li/Na/Cs salt adopts D2h molecular symmetry, implying that while solvated, this cluster is in equilibrium with a more symmetric form. These results highlight the unusually flexible nature of the actinide-based {U24Pp12} and its sensitivity to countercations in solution.

Invited for the cover of this issue are Anna Oliveri from Southern Oregon University, USA, and co-workers. The cover image shows the outer surface of a nanoscale U28 cage cluster.

The conformational dynamics of nanometer-sized actinide ions are exceptionally sensitive to the choice of counter-cations. A new means of following these dynamics in solution is presented that follows 1H NMR signals. The direct bond between hydrogen and phosphorus atoms in the bridging phosphonic groups of the [(UO2)28(O2)20(PHO3)24(H2O)12]32– (U28) cluster allows unparalleled recording of the orientations of these bridges in situ. The µ3-PHO3 bridges are organized into two supersets of conformers (facing inward vs. outward from the cluster), but each of these supersets additionally have four subsets that can be identified based upon orientations of the lone pairs of electrons in the associated oxygen atoms. The ensemble of these subsets changes over days or weeks depending upon bulk solution chemistry, temperature, and pressure. They also reflect cations trapped within the molecule. The 1H NMR spectra at room temperature indicate that the molecular orientation of this enormous ion tunes itself in response to solution composition, which suggests a strategy for selecting host–guest combinations.

Understanding the stability fields and decomposition products of various metal- and actinide-oxide nanoclusters is essential for their development into useful materials for industrial processes. Herein, we explore the spontaneous transformation of the sulfate-centered, phosphate functionalized uranyl peroxide nanocluster {U₂₀P₆} to {U₂₄} under aqueous ambient conditions using time-resolved small-angle X-ray scattering, Raman, and ³¹P NMR spectroscopy. We show that the unusual μ-η¹:η² bridging mode of peroxide between uranyl ions observed in {U₂₀P₆} may lead to its rapid breakdown in solution as evidenced by liberation of phosphate groups that were originally present as an integral part of its cage structure. Remarkably, the uranyl peroxide moieties present after degradation of {U₂₀P₆} undergo cation-mediated reassembly into the {U₂₄} cluster, demonstrating the propensity of the uranyl peroxide systems to preserve well-defined macro-anions.

Herein, we report a new salt of a pyrophosphate-functionalized uranyl peroxide nanocluster {U₂₄Pp₁₂} (1) exhibiting O_h molecular symmetry both in the solid and solution. Study of the system yielding 1 across a wide range of pH by single-crystal X-ray diffraction, small-angle X-ray scattering, and a combination of traditional ³¹P and diffusion-ordered spectroscopy (DOSY) NMR affords unprecedented insight into the amphoteric chemistry of this uranyl peroxide system. Key results include formation of a rare binary {U₂₄}·{U₂₄Pp₁₂} (3) system observed under alkaline conditions, and evidence of acid-promoted decomposition of {U₂₄Pp₁₂} (1) followed by spatial rearrangement and condensation of {U₄} building blocks into the {U₃₂Pp₁₆} (2) cluster. Furthermore, ³¹P DOSY NMR measurements performed on saturated solutions containing crystalline {U₃₂Pp₁₆} show only trace amounts (∼2% relative abundance) of the intact form of this cluster, suggesting a complex interconversion of {U₂₄Pp₁₂}, {U₃₂Pp₁₆}, and {U₄Pp_4-x} ions.

Technetium-99, a prevalent by-product of the nuclear fuel cycle, is a transition metal with 9 accessible oxidation states and a half-life of 2.1 × 105 years. 99TcVII has been found in the environment surrounding National Lab sites and fuel reprocessing centers, and its separation and long-term storage is complicated by its extensive redox chemistry. In our prior work we have reported the synthesis and reduction of TcVO to TcIVO in α1- and α2-Wells–Dawson P2W17O6110– polyoxometalates and reported on the polyoxometalate features that promote reduction and stabilization. In this work we report on the oxidation state stabilization of TcVO, TcVIO and TcVIN within the α-Keggin PW11O397– polyoxometalate. Both Density Functional Theory and experimental results show that Tc in TBA4[TcVIN-PW11O39] can be reduced to TcV, but any subsequent electrons reduce the tungsten framework of the polyoxometalate. TBA4[TcVO-PW11O39] however, can be theoretically reduced down to TcI before reduction of the polyoxometalate occurs.

Tetrahedrally coordinated aluminate Al(OH)₄^- and dialuminate Al₂O(OH)₆^2- anions are considered to be major species in aluminum-rich alkaline solutions. However, their relative abundance remains difficult to spectroscopically quantify due to local structure similarities and poorly understood effects arising from extent of polymerization and counter-cations. To help unravel these relationships here we report detailed characterization of three solid-phase analogues as structurally and compositionally well-defined reference materials. We successfully synthesized a cesium salt of the aluminate monomer, CsAl(OH)₄·2H₂O, for comparison to potassium and rubidium salts of the aluminate dimer, K₂Al₂O(OH)₆, and Rb₂Al₂O(OH)₆, respectively. Single crystal and powder X-ray diffraction methods clearly reveal the structure and purity of these materials for which a combination of ²⁷Al MAS-NMR, Al K-edge X-ray absorption and Raman/IR spectroscopies was then used to fingerprint the two major tetrahedrally coordinated Al species. The resulting insights into the effect of Al-O-Al bridge formation between aluminate tetrahedra on spectroscopic features may also be generalized to the many materials that are based on this motif.

Boehmite (γ-AlOOH) and gibbsite (α-Al-(OH)₃) are important archetype (oxy)hydroxides of aluminum in nature that also play diverse roles across a plethora of industrial applications. Developing the ability to understand and predict the properties and characteristics of these materials, on the basis of their natural growth or synthesis pathways, is an important fundamental science enterprise with wide-ranging impacts. The present study describes bulk and surface characteristics of these novel materials in comprehensive detail, using a collectively sophisticated set of experimental capabilities, including a range of conventional laboratory solids analyses and national user facility analyses such as synchrotron X-ray absorption and scattering spectroscopies as well as small-angle neutron scattering. Their thermal stability is investigated using in situ temperature-dependent Raman spectroscopy. These pure and effectively defect-free materials are ideal for synthesis of advanced alumina products.