Impacts of Oxo Interactions on Np(V) Crown Ether Complexes

Intermolecular interactions between the oxo group of an actinyl cation and other metal cations (i.e., cation–cation interactions) are dependent on the strength of the actinyl bond. These cation–cation interactions are prominently observed for the neptunyl cation [Np­(V)­O₂]⁺ and are sufficiently stable enough to explore using a variety of chemical techniques. Herein, we investigate these intermolecular interactions in the neptunyl 18-crown-6 system, because this macrocyclic ligand provides both stable coordination and the proper sterics to engage the oxo group in bonding with both low-valent metal cations and neighboring neptunyl units. We report the structural and spectroscopic characterization of five neptunyl, [Np­(V,VI)­O₂]^+,2+, compounds: <b>Np1a</b> ([NpO₂(18-crown-6)]­ClO₄), <b>Np1b</b> ([NpO₂(18-crown-6)]­AuCl₄), <b>Na–Np</b> ([Np­(V)­O₂­(18-crown-6)­(Na­(H₂O)­(18-crown-6)]­[Np­(VI)­O₂Cl₄], <b>Np–Np</b> ([NpO₂(18-crown-6)]­(NpO₂Cl₂NO₃)], and <b>Np–Cl</b> (NpO₂Cl­(H₂O)_1.75). Each of these compounds were prepared from the ambient reactions of Np­(V) in HX (where X = Cl, NO₃) with the 18-crown-6 ether molecule. Structural information obtained from single-crystal X-ray diffraction data was paired with solid-state and solution Raman spectroscopy to provide information on the interaction of the neptunyl oxo atom with neighboring cations. Neptunyl (NpO) bond lengths are not perturbed upon interaction with the Na⁺ cation (<b>Na–Np</b>), but elongation is observed upon formation of a neptunyl–neptunyl interaction (<b>Np–Np</b>). This is also the first structurally characterized isolated, molecular complex that contains a simple T-shaped neptunyl–neptunyl interaction. Raman spectroscopy indicates little perturbation to the neptunyl bond until the formation of the neptunyl–neptunyl motif, which also results in activation of the ν₃ asymmetric stretch. Additional spectroscopic studies indicated that the neptunyl 18-crown-6 inclusion complexes form in solution and persist in the presence of other low-valence cations

Thermal aging of heteroatom substituted Keggin type aluminum oxo polycation solutions: Aggregation behavior and impacts on dissolved organic carbon and turbidity removal

Coagulation processes within water treatment plays an important role in contaminant removal and aluminum-oxo Keggin polycations are proved to be an effective coagulating agents. Previous work demonstrated that heteroatom substitution within the Keggin-type polycation ε-Al13 to form ε-GaAl12 and ε-GeAl12 can enhance removal of bacteria, DOC, and turbidity from wastewater. Additional hydrolysis of the ε-Al13 species to form larger Al30 species has also been shown to improve coagulation, but this aspect has not been evaluated for the ε-GaAl12 and ε-GeAl12 systems. In the current study, hydrolysis of ε-Al13, ε-GaAl12 and ε-GeAl12 was promoted through hydrothermal aging to evaluate the overall solution stability/behavior and water treatment efficiency. Turbidity measurement of aged solution indicated that Ga substituted aluminum-oxo Keggin polycations remain stable in solution and DLS studies demonstrated greater diversity in particle sizes within the system. Additional thermogravimetric analyses of metal hydroxide precipitates formed from the aging studies indicate that the GaAl12 system behaves more like an amorphous Al(OH)3 phase, which has higher solubility than other aluminum hydroxide phases. Hydrothermal aging did not significantly change %DOC removal as all solution showed high efficiency for removal across a range of pH values. GaAl12 solutions demonstrated good turbidity removal efficiency in all pH range, with enhanced performance at pH 5. The study suggests that larger, relatively stable oligomers do exist within the aged GaAl12 solutions that may contribute to enhanced contaminant removal in a similar manner to what is observed within the PACl-Al30 coagulant

Evaluating Best Practices in Raman Spectral Analysis for Uranium Speciation and Relative Abundance in Aqueous Solutions

Raman spectroscopy is emerging as a powerful tool for identifying hexavalent uranium speciation in situ; however, there is no straightforward protocol for identifying uranyl species in solution. Herein, uranyl samples are evaluated using Raman spectroscopy, and speciation is monitored at various solution pH values and anion compositions. Spectral quality is evaluated using two Raman excitation wavelengths (532 and 785 nm) as these are critical for maximizing signal-to-noise and minimizing background from fluorescent uranyl species. The Raman vibrational frequency of uranyl shifts according to the identity of the coordinating ions within the equatorial plane and/or solution pH; therefore, spectral barcode analysis and rigorous peak fitting methods are developed that allow accurate and routine uranium species identification. All in all, this user’s guide is expected to provide a user-friendly, straightforward approach for uranium species identification using Raman spectroscopy

Formation of the [Ge4O16Al48(OH)108(H2O)24]20+ Tetramer from Condensation of ε-GeAl12 Keggin Polycations

Keggin-type polyaluminum cations belong to a unique class of polyoxometalates (POMs) with their large positive charge, hydroxo bridges, and divergent isomerization/oligomerization. Previously reported oligomerizations of the polyaluminum cations were driven solely by the δ-Keggin isomer, which created Al26, Al30, and Al32 dimeric species. We herein report the isolation of largest ever Keggin-type structure for this system through a unique mode of self-condensation among four ε-GeAl128+ to form [NaGe4O16Al48(OH)108(H2O)24]21+(Ge4Al48). Elemental analysis confirms the Ge4+ substitution, and dynamic light scattering experiments indicated that these larger species exist in the thermally aged solutions. DFT calculations have revealed that a single atom Ge substitution in tetrahedral site of ε-Al137+ is the key for the formation this cluster because it activates the deprotonation at certain octahedral sites to assist self-condensation in a specific mode

Metal Substitution into Metal Organic Nanotubes: Impacts on Solvent Uptake and Stability

Transition metal dopants can be incorporated in metal organic frameworks to change the physical properties of the material. Metal organic nanotubes are a less well studied form of hybrid material, and in this study, transition metals were substituted into U­(VI) metal organic nanotubes (UMON) to investigate changes with water uptake, solvent selectivity, and hydrostability. Single-crystal X-ray analysis, UV/vis spectroscopy, and electron microprobe analysis confirmed the substitution of (VO)²⁺, Co­(II), Ni­(II), Fe­(II), and Cu­(II), with the highest amount of incorporation by Cu­(II). Water uptake and release by the substituted materials were similar to that of the pure UMON sample, with the exception in the Cu­(II)-UMON samples, where less water present in the nanotubular cavities and additional heating were necessary for dehydration. A detailed investigation of the Cu­(II)-UMON material indicated that the overall selectivity of the material was maintained and the hydrostability was drastically enhanced with incorporation. In the presence of ammonia, the pure and doped UMON material degraded to secondary phases

Surface Modification of Al₃₀ Keggin-Type Polyaluminum Molecular Clusters

Keggin-type molecular clusters formed from the partial hydrolysis of aluminum in aqueous solutions have the capacity to adsorb a variety of inorganic and organic contaminants. The adsorptive capability of Keggin-type polyaluminum species, such as Al₁₃ and Al₃₀, lead to their wide usage as precursors for heterogeneous catalysts and clarifying agents for water purification applications, but a molecular-level understanding of adsorption process is lacking. Two model Al₃₀ clusters, whose surface has been modified with chelated metals (Al³⁺ and Zn²⁺) have been synthesized and structurally characterized by single-crystal X-ray diffraction. <b>Al</b>_<b>32</b><b>IDA</b> [(Al­(IDA)­H₂O)₂­(Al₃₀O₈(OH)₆₀­(H₂O)₂₂)]­(2,6-NDS)₄­(SO₄)_2­Cl₄­(H₂O)₄₀, IDA = iminodiacetic acid, 2,6-NDS = 2,6 napthalene disulfonate) crystallize in the triclinic space group, <i>P</i>1̅ with <i>a</i> = 13.952(2) Å, <i>b</i> = 16.319(3) Å, <i>c</i> = 23.056(4) Å, α = 93.31(1)°, β = 105.27(1)°, and γ = 105.52(1)°. <b>Zn</b>_<b>2</b><b>Al</b>_<b>32</b> [(Zn­(NTA)­H₂O)₂­(Al­(NTA)­(OH)₂)₂­(Al₃₀(OH)₆₀(O)₈­(H₂O)₂₀]­(2,6-NDS)₅­(H₂O)₆₄, (NTA = nitrilotriacetic acid), also crystallizes in <i>P</i>1̅ with unit cell parameter refined as <i>a</i> = 16.733(7) Å, <i>b</i> = 18.034(10) Å, <i>c</i> = 21.925(11) Å, α = 82.82(2)°, β = 70.96(2)°, and γ = 65.36(2)°. The chelated metal centers adsorb to the surface of the Al₃₀ clusters through hydroxyl bridges located at the central belt region of the molecule. The observed binding sites for the metal centers mirror the reactivity predicted by previously reported molecular dynamic simulations and can be identified by the acidity and hydration factor of the water group that participates in the adsorption process

Surface Modification of Al₃₀ Keggin-Type Polyaluminum Molecular Clusters

Keggin-type molecular clusters formed from the partial hydrolysis of aluminum in aqueous solutions have the capacity to adsorb a variety of inorganic and organic contaminants. The adsorptive capability of Keggin-type polyaluminum species, such as Al₁₃ and Al₃₀, lead to their wide usage as precursors for heterogeneous catalysts and clarifying agents for water purification applications, but a molecular-level understanding of adsorption process is lacking. Two model Al₃₀ clusters, whose surface has been modified with chelated metals (Al³⁺ and Zn²⁺) have been synthesized and structurally characterized by single-crystal X-ray diffraction. <b>Al</b>_<b>32</b><b>IDA</b> [(Al­(IDA)­H₂O)₂­(Al₃₀O₈(OH)₆₀­(H₂O)₂₂)]­(2,6-NDS)₄­(SO₄)_2­Cl₄­(H₂O)₄₀, IDA = iminodiacetic acid, 2,6-NDS = 2,6 napthalene disulfonate) crystallize in the triclinic space group, <i>P</i>1̅ with <i>a</i> = 13.952(2) Å, <i>b</i> = 16.319(3) Å, <i>c</i> = 23.056(4) Å, α = 93.31(1)°, β = 105.27(1)°, and γ = 105.52(1)°. <b>Zn</b>_<b>2</b><b>Al</b>_<b>32</b> [(Zn­(NTA)­H₂O)₂­(Al­(NTA)­(OH)₂)₂­(Al₃₀(OH)₆₀(O)₈­(H₂O)₂₀]­(2,6-NDS)₅­(H₂O)₆₄, (NTA = nitrilotriacetic acid), also crystallizes in <i>P</i>1̅ with unit cell parameter refined as <i>a</i> = 16.733(7) Å, <i>b</i> = 18.034(10) Å, <i>c</i> = 21.925(11) Å, α = 82.82(2)°, β = 70.96(2)°, and γ = 65.36(2)°. The chelated metal centers adsorb to the surface of the Al₃₀ clusters through hydroxyl bridges located at the central belt region of the molecule. The observed binding sites for the metal centers mirror the reactivity predicted by previously reported molecular dynamic simulations and can be identified by the acidity and hydration factor of the water group that participates in the adsorption process

Crystallization of Keggin-Type Polyaluminum Species by Supramolecular Interactions with Disulfonate Anions

The hydrolysis of aluminum and formation of polynuclear species, such as the Keggin-type polycations, impacts the chemical and physical properties of the resulting aluminum oxide and hydroxide materials. Despite years of study, only a handful of Keggin-type species have been identified, hampering efforts toward a molecular-level understanding of the mechanisms of condensation. To improve the crystallization of Keggin-type polyaluminum cations, a supramolecular approach using 2,6-napthalene disulfonate (2,6-NDS) was proposed herein for the isolation of novel compounds. The present study describes the successful synthesis and structural characterization of three Keggin-type polyaluminum compounds, including (Na­(Al­(μ₄-O₄)­Al₁₂(μ-OH)₂₄(H₂O))₁₂(2,6NDS)₄(H2O)_13.5 (δ-Al₁₃), (Al₂(μ₄-O₈)­(Al₂₈(μ₂-OH)₅₆(H₂O)₂₆)­(2,6NDS)₈Cl₂(H₂O)₄₀ (Al₃₀), and a new polycation, (Al₂(μ₄-O₈)­(Al₂₄(μ₂-OH)₅₀(H₂O)₂₀)­(2,6NDS)₆(H₂O)_12.4 (Al₂₆). Additional chemical characterization of the compounds, particularly ²⁷Al-NMR, suggests that identifying the Al₂₆ polycation in aqueous solutions may be difficult due to structural similarities to the δ-Al₁₃ moiety. The structural characterization of novel Keggin-type aluminum polycations is important for a complete understanding of aluminum hydrolysis in aqueous solutions, and organosulfonates represent a viable approach for the crystallization of new polynuclear species

Crystallization of Keggin-Type Polyaluminum Species by Supramolecular Interactions with Disulfonate Anions

The hydrolysis of aluminum and formation of polynuclear species, such as the Keggin-type polycations, impacts the chemical and physical properties of the resulting aluminum oxide and hydroxide materials. Despite years of study, only a handful of Keggin-type species have been identified, hampering efforts toward a molecular-level understanding of the mechanisms of condensation. To improve the crystallization of Keggin-type polyaluminum cations, a supramolecular approach using 2,6-napthalene disulfonate (2,6-NDS) was proposed herein for the isolation of novel compounds. The present study describes the successful synthesis and structural characterization of three Keggin-type polyaluminum compounds, including (Na­(Al­(μ₄-O₄)­Al₁₂(μ-OH)₂₄(H₂O))₁₂(2,6NDS)₄(H2O)_13.5 (δ-Al₁₃), (Al₂(μ₄-O₈)­(Al₂₈(μ₂-OH)₅₆(H₂O)₂₆)­(2,6NDS)₈Cl₂(H₂O)₄₀ (Al₃₀), and a new polycation, (Al₂(μ₄-O₈)­(Al₂₄(μ₂-OH)₅₀(H₂O)₂₀)­(2,6NDS)₆(H₂O)_12.4 (Al₂₆). Additional chemical characterization of the compounds, particularly ²⁷Al-NMR, suggests that identifying the Al₂₆ polycation in aqueous solutions may be difficult due to structural similarities to the δ-Al₁₃ moiety. The structural characterization of novel Keggin-type aluminum polycations is important for a complete understanding of aluminum hydrolysis in aqueous solutions, and organosulfonates represent a viable approach for the crystallization of new polynuclear species

Crystallization of Keggin-Type Polyaluminum Species by Supramolecular Interactions with Disulfonate Anions

The hydrolysis of aluminum and formation of polynuclear species, such as the Keggin-type polycations, impacts the chemical and physical properties of the resulting aluminum oxide and hydroxide materials. Despite years of study, only a handful of Keggin-type species have been identified, hampering efforts toward a molecular-level understanding of the mechanisms of condensation. To improve the crystallization of Keggin-type polyaluminum cations, a supramolecular approach using 2,6-napthalene disulfonate (2,6-NDS) was proposed herein for the isolation of novel compounds. The present study describes the successful synthesis and structural characterization of three Keggin-type polyaluminum compounds, including (Na­(Al­(μ₄-O₄)­Al₁₂(μ-OH)₂₄(H₂O))₁₂(2,6NDS)₄(H2O)_13.5 (δ-Al₁₃), (Al₂(μ₄-O₈)­(Al₂₈(μ₂-OH)₅₆(H₂O)₂₆)­(2,6NDS)₈Cl₂(H₂O)₄₀ (Al₃₀), and a new polycation, (Al₂(μ₄-O₈)­(Al₂₄(μ₂-OH)₅₀(H₂O)₂₀)­(2,6NDS)₆(H₂O)_12.4 (Al₂₆). Additional chemical characterization of the compounds, particularly ²⁷Al-NMR, suggests that identifying the Al₂₆ polycation in aqueous solutions may be difficult due to structural similarities to the δ-Al₁₃ moiety. The structural characterization of novel Keggin-type aluminum polycations is important for a complete understanding of aluminum hydrolysis in aqueous solutions, and organosulfonates represent a viable approach for the crystallization of new polynuclear species