Building [UIV 70 (OH)36 (O)64 ]4- Oxocluster Frameworks with Sulfate, Transition Metals, and UV

Aluminum Molecular Ring Meets Deep Eutectic Solvents: Adaptive Assembly and Optical Behavior

Different from the previous neutral reaction solvent system, this work explores the synthesis of Al-oxo rings in ionic environments. Deep eutectic solvents (DESs) formed by quaternary ammonium salts hydrogen bond acceptor (HBA) and phenols hydrogen bond donor (HBD) further reduce the melting point of the reaction system and provide an ionic environment. Further, the quaternary ammonium salt was chosen as the HBA because it contains a halogen anion that matches the size of the central cavity of the molecular ring. Based on this thought, five Al8 ion pair cocrystals were synthesized via "DES thermal". The general formula is Q+ ⊂ {Cl@[Al8(BD)8(μ2-OH)4L12]} (AlOC-180-AlOC-185, Q+ = tetrabutylammonium, tetrapropylammonium, 1-butyl-3-methylimidazole; HBD = phenol, p-chlorophenol, p-fluorophenol; HL = benzoic acid, 1-naphthoic acid, 1-pyrenecarboxylic acid, anthracene-9-carboxylic acid). Structural studies reveal that the phenol-coordinated Al molecular ring and the quaternary ammonium ion pair form the cocrystal compounds. The halogen anions in the DES component are confined in the center of the molecular ring, and the quaternary ammonium cations are located in the organic shell. Such an adaptive cocrystal binding pattern is particularly evident in the structures coordinated with low-symmetry ligands such as naphthoic acid and pyrene acid. Finally, the optical behavior of these cocrystal compounds is understood from the analysis of crystal structure and theoretical calculation.

Metal-Oxo Cluster Formation Using Ammonium and Sulfate to Differentiate MIV (Th, U, Ce) Chemistries

Isolating isostructural compounds of tetravalent metals M^IV (Zr, Hf, Ce, Th, U, Pu, Np) improves our understanding of metal hydrolysis and coordination behavior across the periodic table. These metals form polynuclear clusters typified by the hexamer [M^IV₆O₄(OH)₄]¹²⁺. Exploiting the ammonium M^IV-sulfate (Ce^IV, Th^IV, and U^IV) phase space targeting rapid crystallization, we isolate the common hexamer [M^IV₆(OH)₄(O)₄]¹²⁺ but with different numbers of capping sulfates and water molecules for Ce^IV, Th^IV, and U^IV. These phases allowed a direct comparison of bonding trends across the series. Upon cocrystallization with the hexamers, higher complex structures can be identified. Thorium features assemblies with monomer-linked hexamer chains. Uranium features assemblies with sulfate-bridged hexamers and the supramolecular assembly of 14 hexamers into the U₈₄, [U₆(OH)₄(O)₄)₁₄(SO₄)₁₂₀(H₂O)₄₂]^72-. Last, cerium showcases the isolation from monomers to the Ce₆₂, [Ce₆₂(OH)₃₀(O)₅₈(SO₄)₇₁(H₂O)_33.25]^41-. Furthermore, small-angle X-ray scattering (room temperature) shows ammonium-induced cluster assembly for Ce^IV but minimal reactivity for U^IV and Th^IV. In this study, because the phases crystallized at elevated temperature demonstrates favorable cluster assembly, these solution phase results were surprising and suggest some other characteristics such as Ce's facile redox behavior, contributes to its solution-phase speciation.

Lanthanides Singing the Blues: Their Fascinating Role in the Assembly of Gigantic Molybdenum Blue Wheels

Molybdenum blues (MBs) are a distinct class of polyoxometalates, exhibiting versatile/impressive architectures and high structural flexibility. In acidified and reduced aqueous environments, isopolymolybdates generate precisely organizable building blocks, which enable unique nanoscopic molecular systems (MBs) to be constructed and further fine-tuned by hetero elements such as lanthanide (Ln) ions. This Review discusses wheel-shaped MB-based structure types with strong emphasis on the ∼30 Ln-containing MBs as of August 2021, which include both organically hybridized and nonhybridized structures synthesized to date. The spotlight is thereby put on the lanthanide ions and ligand types, which are crucial for the resulting Ln-patterns and alterations in the gigantic structures. Several critical steps and reaction conditions in their synthesis are highlighted, as well as appropriate methods to investigate them both in solid state and in solution. The final section addresses the homogeneous/heterogeneous catalytic, molecular recognition and separation properties of wheel-shaped Ln-MBs, emphasizing their inimitable behavior and encouraging their application in these areas.

Snapshots of Ce70 Toroid Assembly from Solids and Solution

Crystallization at the solid-liquid interface is difficult to spectroscopically observe and therefore challenging to understand and ultimately control at the molecular level. The Ce₇₀-torroid formulated [Ce^IV₇₀(OH)₃₆(O)₆₄(SO₄)₆₀(H₂O)₁₀]^4-, part of a larger emerging family of M^IV₇₀-materials (M = Zr, U, Ce), presents such an opportunity. We elucidated assembly mechanisms by the X-ray scattering (small-angle scattering and total scattering) of solutions and solids as well as crystallizing and identifying fragments of Ce₇₀ by single-crystal X-ray diffraction. Fragments show evidence for templated growth (Ce₅, [Ce₅(O)₃(SO₄)₁₂]^10-) and modular assembly from hexamer (Ce₆) building units (Ce₁₃, [Ce₁₃(OH)₆(O)₁₂(SO₄)₁₄(H₂O)₁₄]^6- and Ce₆₂, [Ce₆₂(OH)₃₀(O)₅₈(SO₄)₅₈]^14-). Ce₆₂, an almost complete ring, precipitates instantaneously in the presence of ammonium cations as two torqued arcs that interlock by hydrogen boding through NH₄⁺, a structural motif not observed before in inorganic systems. The room temperature rapid assemblies of both Ce₇₀ and Ce₆₂, respectively, by the addition of Li⁺ and NH₄⁺, along with ion-exchange and redox behavior, invite exploitation of this emerging material family in environmental and energy applications.

CeIV 70 Oxosulfate Rings, Frameworks, Supramolecular Assembly, and Redox Activity*

M^IV molecular oxo-clusters (M=Zr, Hf, Ce, Th, U, Np, Pu) are prolific in bottoms-up material design, catalysis, and elucidating reaction pathways in nature and in synthesis. Here we introduce Ce₇₀ , a wheel-shaped oxo-cluster, [Ce^IV ₇₀ (OH)₃₆ (O)₆₄ (SO₄ )₆₀ (H₂ O)₁₀ ]^4- . Ce₇₀ crystallizes into intricate high pore volume frameworks with divalent transition metals and Ce-monomer linkers. Eight crystal-structures feature four framework types in which the Ce₇₀ -rings are linked as propellers, in offset-stacks, in a tartan pattern, and as isolated rings. Small-angle X-ray scattering of Ce₇₀ dissolved in butylamine, with and without added cations (Ce^IV , alkaline earths, Mn^II ), shows the metals' differentiating roles in ring linking, leading to supramolecular assemblies. The large acidic pores and abundant terminal sulfates provide ion-exchange behavior, demonstrated with U^IV and Nd^III . Frameworks featuring Ce^III/IV -monomer linkers demonstrate both oxidation and reduction. This study opens the door to mixed-metal, highly porous framework catalysts, and new clusters for metal-organic framework design.

Bridging the Transuranics with Uranium(IV) Sulfate Aqueous Species and Solid Phases

Isolating isomorphic compounds of tetravalent actinides (i.e., Th^IV, U^IV, Np^IV, and Pu^IV) improve our understanding of the bonding behavior across the series, in addition to their relationship with tetravalent transition metals (Zr and Hf) and lanthanides (Ce). Similarities between these tetravalent metals are particularly illuminated in their hydrolysis and condensation behavior in aqueous systems, leading to polynuclear clusters typified by the hexamer [M^IV₆O₄(OH)₄]¹²⁺ building block. Prior studies have shown the predominance and coexistence of smaller species for Th^IV (monomers, dimers, and hexamers) and larger species for U^IV, Np^IV, and Pu^IV (including 38-mers and 70-mers). We show here that aqueous uranium(IV) sulfate also displays behavior similar to that of Th^IV (and Zr^IV) in its isolated solid-phase and solution speciation. Two single-crystal X-ray structures are described: a dihydroxide-bridged dimer (U₂) formulated as U₂(OH)₂(SO₄)₃(H₂O)₄ and a monomer-linked hexamer framework (U-U₆) as (U(H₂O)_3.5)₂U₆O₄(OH)₄(SO₄)₁₀(H₂O)₉. These structures are similar to those previously described for Th^IV. Moreover, cocrystallization of monomer and dimer and of dimer and monomer-hexamer phases for both Th^IV (prior) and U^IV (current) indicates the coexistence of these species in solution. Because it was not possible to effectively study the sulfate-rich solutions via X-ray scattering from which U₂ and U-U₆ crystallized, we provide a parallel solution speciation study in low sulfate conditions, as a function of the pH. Raman spectroscopy, UV-vis spectroscopy, and small-angle X-ray scattering of these show decreasing sulfate binding, increased hydrolysis, increased species size, and increased complexity, with increasing pH. This study describes a bridge across the first half the actinide series, highlighting U^IV similarities to Th^IV, in addition to the previously known similarities to the transuranic elements.