Solution 31P NMR Study of the Acid-Catalyzed Formation of a Highly Charged {U24Pp12} Nanocluster, [(UO2)24(O2)24(P2O7)12]48–, and Its Structural Characterization in the Solid State Using Single-Crystal Neutron Diffraction

Critical Conditions Regulating the Gelation in Macroionic Cluster Solutions

The critical gelation conditions observed in dilute aqueous solutions of multiple nanoscale uranyl peroxide molecular clusters are reported, in the presence of multivalent cations. This gelation is dominantly driven by counterion-mediated attraction. The gelation areas in the corresponding phase diagrams all appear in similar locations, with a characteristic triangle shape outlining three critical boundary conditions, corresponding to the critical cluster concentration, cation/cluster ratio, and the degree of counterion association with increasing cluster concentration. These interesting phrasal observations reveal general conditions for gelation driven by electrostatic interactions in hydrophilic macroionic solutions.

Woven, Polycatenated, or Cage Structures: Effect of Modulation of Ligand Curvature in Heteroleptic Uranyl Ion Complexes

Combining the flexible zwitterionic dicarboxylate 4,4'-bis(2-carboxylatoethyl)-4,4'-bipyridinium (L) and the anionic dicarboxylate ligands isophthalate (ipht^2-) and 1,2-, 1,3-, or 1,4-phenylenediacetate (1,2-, 1,3-, and 1,4-pda^2-), of varying shape and curvature, has allowed isolation of five uranyl ion complexes by synthesis under solvo-hydrothermal conditions. [(UO₂)₂(L)(ipht)₂] (1) and [(UO₂)₂(L)(1,2-pda)₂]·2H₂O (2) have the same stoichiometry, and both crystallize as monoperiodic coordination polymers containing two uranyl-(anionic carboxylate) strands united by L linkers into a wide ribbon, all ligands being in the divergent conformation. Complex 3, [(UO₂)₂(L)(1,3-pda)₂]·0.5CH₃CN, with the same stoichiometry but ligands in a convergent conformation, is a discrete, binuclear species which is the first example of a heteroleptic uranyl carboxylate coordination cage. With all ligands in a divergent conformation, [(UO₂)₂(L)(1,4-pda)(1,4-pdaH)₂] (4) crystallizes as a sinuous and thread-like monoperiodic polymer; two families of chains run along different directions and are woven into diperiodic layers. Modification of the synthetic conditions leads to [(UO₂)₄(LH)₂(1,4-pda)₅]·H₂O·2CH₃CN (5), a monoperiodic polymer based on tetranuclear (UO₂)₄(1,4-pda)₄ rings; intrachain hydrogen bonding of the terminal LH⁺ ligands results in diperiodic network formation through parallel polycatenation involving the tetranuclear rings and the LH⁺ rods. Complexes 1-3 and 5 are emissive, with complex 2 having the highest photoluminescence quantum yield (19%), and their spectra show the maxima positions usual for tris-κ²O,O'-chelated uranyl cations.

Structural Chemistry of Giant Metal Based Supramolecules

The review presents a bird-eye view on the state of research in the field of giant nonbiological discrete metal complexes and ions of nanometer size, which are structurally characterized by means of single-crystal X-ray diffraction, using the crystal structure as a common key feature. The discussion is focused on the main structural features of the metal clusters, the clusters containing compact metal oxide/hydroxide/chalcogenide core, ligand-based metal-organic cages, and supramolecules as well as on the aspects related to the packing of the molecules or ions in the crystal and the methodological aspects of the single-crystal neutron and X-ray diffraction of these compounds.

Prediction of Solution Behavior via Calorimetric Measurements Allows for Detailed Elucidation of Polyoxometalate Transformation

The solution behavior of a polyoxometalate cluster, LiNa-U₂₄Pp₁₂ (Li₂₄Na₂₄[(UO₂O₂)₂₄(P₂O₇)₁₂]) that consists of 24 uranyl ions, peroxide groups, and 12 pyrophosphate linkers, was successfully predicted based on new thermodynamic results using a calorimetric method recently described for uranyl peroxide nanoclusters (UPCs), molybdenum blues, and molybdenum browns. The breakdown of LiNa-U₂₄Pp₁₂ and formation of U₂₄ (Li₂₄[UO₂O₂OH]₂₄) was monitored in situ via Raman spectroscopy using a custom heating apparatus. A combination of analytical techniques confirmed the simultaneous existence of U₂₄Pp₁₂ and U₂₄ midway through the conversion process and U₂₄ as the single end product. The application of a molecular weight filter resulted in a complete and successful separation of UPCs from solution and, in conjunction with DOSY results, confirmed the presence of large intermediate cluster building blocks.

Cavity Formation in Uranyl Ion Complexes with Kemp's Tricarboxylate: Grooved Diperiodic Nets and Polynuclear Cages

Kemp's triacid (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid, H₃kta) was reacted with uranyl nitrate under solvo-hydrothermal conditions in the presence of diverse counterions or additional metal cations to give eight zero- or diperiodic complexes. All the coordination polymers in the series, [PPh₃Me][UO₂(kta)]·0.5H₂O (1), [PPh₄][UO₂(kta)] (2), [C(NH₂)₃][UO₂(kta)] (3), [Cd(bipy)₃][UO₂(kta)]₂ (4), and [Zn(phen)₃][UO₂(kta)]₂·2H₂O (5) (bipy = 2,2'-bipyridine, phen = 1,10-phenanthroline) crystallize as networks with the hcb topology, the ligand being in the chair conformation with the three carboxylate groups equatorial, except in 3, in which the axial/diequatorial boat conformation is present. Various degrees of corrugation and different arrangements of neighboring layers are observed depending on the counterion, with complexes 4 and 5, in particular, displaying cavities containing the bulky cations. [Co(en)₃]₂[(UO₂)₂(kta)(Hkta)₂]₂·2NMP·10H₂O (6) (en = 1,2-ethanediamine; NMP = N-methyl-2-pyrrolidone) contains a metallatricyclic, tetranuclear anionic species, displaying two clefts in which the cations are held by extensive hydrogen bonding, and with the ligands in both triaxial chair and axial/diequatorial boat conformations. [(UO₂)₃Pb(kta)₂(Hkta)(H₂O)]₂·1.5THF (7) (THF = tetrahydrofuran) and [(UO₂)₂Pb₂(kta)₂(Hkta)(NMP)]₂ (8) are two heterometallic cage compounds containing only the convergent, triaxial chair form of the ligand, which have the same topology in spite of the different U/Pb ratio. These complexes are compared to previous ones also involving Kemp's triacid anions, and the roles of ligand conformation and of counterions in the formation of cavities, either in cage-like species or as grooves in diperiodic networks, is discussed.

Dynamics of Cation-Induced Conformational Changes in Nanometer-Sized Uranyl Peroxide Clusters

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 (O_h) molecular symmetry. One-dimensional ³¹P NMR experiments provide direct evidence that both K⁺ and Rb⁺ ions trigger an O_h-to-D_4h 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 D_4h-to-O_h 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 D_4h 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 O_h symmetry is conserved. Single-crystal X-ray diffraction measurements reveal that the mixed Li/Na/Cs salt adopts D_2h 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.

Stability of Solid Uranyl Peroxides under Irradiation

The effects of radiation on a variety of uranyl peroxide compounds were examined using γ-rays and 5 MeV He ions, the latter to simulate α-particles. The studied materials were studtite, [(UO₂)(O₂)(H₂O)₂](H₂O)₂, the salt of the U₆₀ uranyl peroxide cage cluster, Li₄₄K₁₆[(UO₂)(O₂)(OH)]₆₀·255H₂O, the salt of U₆₀Ox₃₀ uranyl peroxide oxalate cage cluster, Li₁₂K₄₈[{(UO₂)(O₂)}₆₀(C₂O₄)₃₀]·nH₂O, and the salt of the U₂₄Pp₁₂ (Pp = pyrophosphate) uranyl peroxide pyrophosphate cage cluster, Li₂₄Na₂₄[(UO₂)₂₄(O₂)₂₄(P₂O₇)₁₂]·120H₂O. Irradiated powders were characterized using powder X-ray diffraction, Raman spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, and UV-vis spectroscopy. A weakening of the uranyl bonds of U₆₀ was found while studtite, U₆₀Ox₃₀, and U₂₄Pp₁₂ were relatively stable to γ-irradiation. Studtite and U₆₀ are the most affected by α-irradiation forming an amorphous uranyl peroxide as characterized by Raman spectroscopy and powder X-ray diffraction while U₆₀Ox₃₀ and U₂₄Pp₁₂ show minor signs of the formation of an amorphous uranyl peroxide.

Evidence for non-electrostatic interactions between a pyrophosphate-functionalized uranyl peroxide nanocluster and iron (hydr)oxide minerals

The terminal oxygen atoms of the pyrophosphate groups in the uranyl peroxide nanocluster U24Pp12 ([(UO2)24(O2)24(P2O7)12]48-) are not fully satisfied by bond valence considerations and can become protonated. This functionality could allow for specific interactions with mineral surfaces, as opposed to the electrostatically-driven interactions observed between non-functionalized uranyl peroxide nanoclusters and mineral surfaces. The sorption of U24Pp12 to goethite and hematite was studied using batch sorption experiments as a function of U24Pp12 concentration, mineral concentration, and pH. A suite of spectroscopic techniques, scanning electron microscopy, and electrophoretic mobility measurements were used to examine the minerals before and after reaction with U24Pp12, leading to a proposed conceptual model for U24Pp12 interactions with goethite. The governing rate laws were determined and compared to those previously determined for a non-functionalized uranyl peroxide nanocluster. The rate of uranyl peroxide nanocluster sorption depends on the charge density and functionalized component of the uranyl peroxide cage. Electrophoretic mobility and attenuated total reflectance Fourier transform infrared spectroscopy analyses show that an inner-sphere complex forms between the U24Pp12 cluster and the goethite surface through the terminal pyrophosphate groups, leading to a proposed conceptual model in which U24Pp12 interacts with the triply-coordinated reactive sites on the (110) plane of goethite. These results demonstrate that the behavior of U24Pp12 at the iron (hydr)oxide-water interface is unique relative to interactions of the uranyl ion and non-functionalized uranyl peroxide nanoclusters.

Effects of H2O2 Concentration on Formation of Uranyl Peroxide Species Probed by Dissolution of Uranium Nitride and Uranium Dioxide

Dissolution of uranium materials in alkaline aqueous conditions containing H₂O₂ results in uranyl peroxide species in solution, including anionic uranyl peroxide cage clusters. Uranyl peroxide cage clusters are generally highly soluble in water, where they persist as aqueous macroanions. Previous studies indicate that uranyl cluster speciation and dissolution of uranium materials is impacted by the concentration of alkali metal in solution, but in these studies, high concentrations of H₂O₂ were used. Herein, the role of hydrogen peroxide concentration is examined relative to the dissolution of powdered UN and UO₂. Lower initial H₂O₂ concentrations reduce dissolution of UO₂ and UN and tend to produce simple (small) uranyl peroxide species rather the highly soluble uranyl peroxide clusters. H₂O₂ availability will have implications for uranyl speciation and solubility where spent nuclear fuel is in contact with water and where alkaline peroxide conditions are used in dissolution of nuclear material.

Closed Uranyl-Dicarboxylate Oligomers: A Tetranuclear Metallatricycle with Uranyl Bridgeheads and 1,3-Adamantanediacetate Linkers

In the presence of NH₄⁺ and either PPh₄⁺ or PPh₃Me⁺ cations, 1,3-adamantanediacetic acid (H₂ADA) reacts with uranyl ions under solvo-hydrothermal conditions to give the complexes [NH₄]₂[PPh₄]₂[(UO₂)₄(ADA)₆] (1) and [NH₄]₂[PPh₃Me]₂[(UO₂)₄(ADA)₆] (2), both of which contain a tetranuclear metallatricycle built from two 2:2 rings including convergent ligands, linked by two additional ligands in an extended conformation defining a third, larger ring. While the ammonium cations are closely associated with the 2:2 rings through triple hydrogen bonding, the large PPh₄⁺ or PPh₃Me⁺ cations are more loosely bound to each of the two faces of the larger ring. In contrast, the complex [H₂NMe₂][PPh₃Me][(UO₂)₂(ADA)₃]·H₂O (3), in which dimethylammonium replaces ammonium cations, crystallizes as a two-dimensional network with honeycomb {6³} topology, albeit with very distorted, elongated hexagonal cells. These and previous results show that both NH₄⁺ and PPh₄⁺ or PPh₃Me⁺ cations are essential to the formation of the metallatricycle. The role of the flexibility imparted to ADA^2- by the acetate arms, in comparison to the more rigid 1,3-adamantanedicarboxylate (ADC^2-), is also discussed. All three complexes are luminescent, with quantum yields of 0.06, 0.06, and 0.09 for 1-3, respectively. The vibronic fine structure apparent on the emission spectra gives peak positions typical of species in which the uranyl ion is chelated by three carboxylate groups.

An unprecedented nanocage-like and heterometallic [MoO4]-polyoxomolybdate hybrid

The solvothermal oxidation of [Mo₃O₂(O₂CCH₃)₆(H₂O)₃]·ZnCl₄ has been established as a general route toward [Mo⁴⁺₃O₄]-incorporated polyoxomolybdates (Mo^IV-POMs). Two unprecedented family members: the first Mo⁴⁺-Mo⁵⁺-Mo⁶⁺ nanocage cluster, Na[MoMoMoO₄₃(OH)Py₁₂]·11H₂O (1) and the first heterometallic hybrid, [MoMoZn(PO₄)₄(OH)₂O₃₁py₃]·2(C₅H₆N)·3(C₅H₅N)·2H₂O (2) have been prepared and characterized by X-ray crystallography, elemental and DFT theoretical analyses, XPS, mass spectroscopy, cyclic voltammetry, and IR spectroscopy.

Captivation with encapsulation: a dozen years of exploring uranyl peroxide capsules

Uranyl peroxide cages are an extensive family of topologically varied self-assembling nanoscale clusters with fascinating properties and applications.

Single-Crystal Time-of-Flight Neutron Diffraction and Magic-Angle-Spinning NMR Spectroscopy Resolve the Structure and 1H and 7Li Dynamics of the Uranyl Peroxide Nanocluster U60

Single-crystal time-of-flight neutron diffraction has provided atomic resolution of H atoms of H₂O molecules and hydroxyl groups, as well as Li cations in the uranyl peroxide nanocluster U₆₀. Solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to confirm the dynamics of these constituents, revealing the transportation of Li atoms and H₂O through cluster walls. H atoms of hydroxyl units that are located on the cluster surface are involved in the transfer of H₂O and Li cations from inside to outside and vice versa. This exchange occurs as a concerted motion and happens rapidly even in the solid state. As a consequence of its large size and open hexagonal pores, U₆₀ exchanges Li cations more rapidly compared to other uranyl nanoclusters.