Solution and Solid State Structural Chemistry of Th(IV) and U(IV) 4-Hydroxybenzoates

Leveraging Nitrogen Linkages in the Formation of a Porous Thorium-Organic Nanotube Suitable for Iodine Capture

We report the synthesis, characterization, and iodine capture application of a novel thorium-organic nanotube, TSN-626, [Th₆O₄(OH)₄(C₆H₄NO₂)₇(CHO₂)₅(H₂O)₃]·3H₂O. The classification as a metal-organic nanotube (MONT) distinguishes it as a rare and reduced dimensionality subset of metal-organic frameworks (MOFs); the structure is additionally hallmarked by low node connectivity. TSN-626 is composed of hexameric thorium secondary building units and mixed O/N-donor isonicotinate ligands that demonstrate selective ditopicity, yielding both terminating and bridging moieties. Because hard Lewis acid tetravalent metals have a propensity to bind with electron donors of rival hardness (e.g., carboxylate groups), such Th-N coordination in a MOF is uncommon. However, the formation of key structural Th-N bonds in TSN-626 cap some of the square antiprismatic metal centers, a position usually occupied by terminal water ligands. TSN-626 was characterized by using complementary analytical and computational techniques: X-ray diffraction, vibrational spectroscopy, N₂ physisorption isotherms, and density functional theory. TSN-626 satisfies design aspects for the chemisorption of iodine. The synergy between accessibility through pores, vacancies at the metal-oxo nodes, and pendent N-donor sites allowed a saturated iodine loading of 955 mg g^-1 by vapor methods. The crystallization of TSN-626 diversifies actinide-MOF linker selection to include soft electron donors, and these Th-N linkages can be leveraged for the investigation of metal-to-ligand bonding and unconventional topological expressions.

Gas-phase synthesis and structure of thorium benzyne complexes

The thorium benzyne complex (η²-C₆H₄)ThCl₃^- was synthesized in the gas phase through consecutive decarboxylation and dehydrochlorination from the (C₆H₅CO₂)ThCl₄^- precursor upon collision-induced dissociation. Theoretical calculations suggest that (η²-C₆H₄)ThCl₃^- exhibits a metallacyclopropene structure with two polarized Th-C_benzyne σ bonds. This procedure can be generally extended to the synthesis of a wide range of gas-phase thorium benzyne complexes.

Discrimination and quantitation of halobenzoic acid positional isomers upon Th(IV) coordination by mass spectrometry

A fast and reliable mass spectrometry-based method has been developed to discriminate the positional isomers of o-, m- and p-C₆H₄XCO₂H (X = F, Cl and Br). This is based on the distinct fragmentation patterns of isomeric ThCl₄(C₆H₄XCO₂)^- ions generated by electrospray ionization of the solutions with C₆H₄XCO₂H isomers and ThCl₄. Moreover, the composition of these positional isomers can be conveniently quantified without any pre-treatment according to the proportion of gas-phase fragmentation products.

Pentanuclear Thorium(IV) Coordination Cluster from the Use of Di(2-pyridyl) Ketone

The Th(NO₃)₄·5H₂O/di(2-pyridyl) ketone [(py)₂CO] reaction system gives a pentanuclear cluster containing the doubly deprotonated form of the gem-diol derivative of the ligand. The cluster consists of a tetrahedral arrangement of four Th^IV ions centered on the fifth ion, which is the first characterized Th^IV₅ complex. The analysis of its structure reveals that this is a Kuratowski-type coordination compound.

Amidinate Supporting Ligands Influence Molecularity in Formation of Uranium Nitrides

Uranium nitride complexes are attractive targets for chemists as molecular models for the bonding, reactivity, and magnetic properties of next-generation nuclear fuels, but these molecules are uncommon and can be difficult to isolate due to their high reactivity. Here, we describe the synthesis of three new multinuclear uranium nitride complexes, [U(BCMA)₂]₂(μ-N)(μ-κ¹:κ¹-BCMA) (7), [(U(BIMA)₂)₂(μ-N)(μ-NⁱPr)(K₂(μ-η³:η³-CH₂CHNⁱPr)]₂ (8), and [U(BIMA)₂]₂(μ-N)(μ-κ¹:κ¹-BIMA) (9) (BCMA = N,N-bis(cyclohexyl)methylamidinate, BIMA = N,N-bis(iso-propyl)methylamidinate), from U(III) and U(IV) amidinate precursors. By varying the amidinate ligand substituents and azide source, we were able to influence the composition and size of these nitride complexes. ¹⁵N isotopic labeling experiments confirmed the bridging nitride moieties in 7-9 were formed via two-electron reduction of azide. The tetra-uranium cluster 8 was isolated in 99% yield via reductive cleavage of the amidinate ligands; this unusual molecule contains nitrogen-based ligands with formal 1-, 2-, and 3- charges. Additionally, chemical oxidation of the U(IV) precursor U(N₃)(BCMA)₃ yielded the cationic U(V) species [U(N₃)(BCMA)₃][OTf]. Magnetic susceptibility measurements confirmed a U(IV) oxidation state for the uranium centers in the three nitride-bridged complexes and provided a comparison of magnetic behavior in the structurally related U(III)-U(IV)-U(V) series U(BCMA)₃, U(N₃)(BCMA)₃, and [U(N₃)(BCMA)₃][OTf]. At 240 K, the magnetic moments in this series decreased with increasing oxidation state, i.e., U(III) > U(IV) > U(V); this trend follows the decreasing number of 5f valence electrons along this series.

Structure, assembly mechanism and magnetic properties of heterometallic dodecanuclear nanoclusters DyIII4MII8 (M = Ni, Co)

Two isostructural heterometallic dodecanuclear nanoclusters [Dy4Co8(μ3-OH)8(L)8(OAc)4(H2O)4]·3EtOH·3CH3CN·H2O (1) and [Dy4Ni8(μ3-OH)8(L)8(OAc)4(H2O)4]·3.5EtOH·0.5CH3CN·5H2O (2) with different assembly mechanisms are presented here.

Decisive Role of 5f-Orbital Covalence in the Structure and Stability of Pentavalent Transuranic Oxo [M6O8] Clusters

Actinide metal oxo clusters are of vital importance in actinide chemistry, as well as in environmental and materials sciences. They are ubiquitous in both aqueous and nonaqueous phases and play key roles in nuclear materials (e.g., nuclear fuel) and nuclear waste management. Despite their importance, our structural understanding of the actinide metal oxo clusters, particularly the transuranic ones, is very limited because of experimental challenges such as high radioactivity. Herein we report a systematic theoretical study on the structures and stabilities of seven actinide metal oxo-hydroxo clusters [An^IV₆O₄(OH)₄L₁₂] (1-An; An = Th-Cm; L = O₂CH^-) along with their group 4 (Ti, Zr, Hf, Rf) and lanthanide (Ce) counterparts [M^IV₆O₄(OH)₄L₁₂] (1-M). The work shows the T_d-symmetric structures of all of the 1-An/M clusters and suggests the positions of the -OH functional groups, which are experimentally challenging to determine. Furthermore, by removing six electrons from 1-An, we found that oxidation could happen on the An^IV metal ions, producing [An^V₆O₄(OH)₄L₁₂]⁶⁺ (2-An; An = Pa, U, Np), or on the O^2- and OH^- ligands, producing [An^IV₆(O^•-)₄(OH^•)₂(OH)₂L₁₂]⁶⁺ (3-An; An = Pu, Am, Cm). On the basis of 2-An, we constructed a series of tetravalent and pentavalent actinide metal oxo clusters [An^IV₆O₁₄]^4- (4-An) and [An^V₆O₁₄]²⁺ (5-An), which proves the feasibility of the highly important pentavalent actinyl clusters, demonstrates the f orbital's structure-directing role in the formation of linear [O≡An^V═O]⁺ actinyl ions, and expands the concept of actinyl-actinyl interaction into pentavalent transuranic actinyl clusters.

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.

Metal-Organic Frameworks Based on Group 3 and 4 Metals

Metal-organic frameworks (MOFs) based on group 3 and 4 metals are considered as the most promising MOFs for varying practical applications including water adsorption, carbon conversion, and biomedical applications. The relatively strong coordination bonds and versatile coordination modes within these MOFs endow the framework with high chemical stability, diverse structures and topologies, and interesting properties and functions. Herein, the significant progress made on this series of MOFs since 2018 is summarized and an update on the current status and future trends on the structural design of robust MOFs with high connectivity is provided. Cluster chemistry involving Y, lanthanides (Ln, from La to Lu), actinides (An, from Ac to Lr), Ti, and Zr is initially introduced. This is followed by a review of recently developed MOFs based on group 3 and 4 metals with their structures discussed based on the types of inorganic or organic building blocks. The novel properties and arising applications of these MOFs in catalysis, adsorption and separation, delivery, and sensing are highlighted. Overall, this review is expected to provide a timely summary on MOFs based on group 3 and 4 metals, which shall guide the future discovery and development of stable and functional MOFs for practical applications.

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

Uranium(IV) oxide clusters, colloids, and materials are designed and studied for 1) nuclear materials applications, 2) understanding the environmental fate and transport of actinides, and 3) exploring the complex bonding behavior of open-shell f-elements. U^IV -oxyhydroxsulfate clusters are particularly relevant in industrial processes and in nature. Recent studies have shown that counter-cations to these polynuclear anions differentiate rich structural topologies in the solid-state. Herein, we present nine different structures with wheel-shaped [U₇₀ (OH)₃₆ (O)₆₄ (SO₄ )₆₀ ]^4- (U₇₀ ) linked into one- and two-dimensional frameworks with sulfate, divalent transition metals (Cr^II , Fe^II , Co^II , Ni^II ) and U^V . Small-angle X-ray scattering of these phases dissolved in butylamine reveals differing supramolecular assembly of U₇₀ clusters, controlled primarily by sulfates. However, observed trends in transition metal linking guide future design of U₇₀ materials with different topologies. Finally, U₇₀ linking via U^IV -O-U^V -O-U^IV bridges presents a rare example of mixed-oxidation-state uranium oxides without disorder.

Supramolecular Assembly of U(IV) Clusters and Superatoms with Unconventional Countercations

Superatoms are nanometer-sized molecules or particles that form ordered lattices, mimicking their atomic counterparts. Hierarchical assembly of superatoms gives rise to emergent properties in lattices of quantum dots, p-block clusters, and fullerenes. Here, we introduce a family of uranium-oxysulfate cluster anions whose hierarchical assembly in water is controlled by two parameters: acidity and the lanthanide or transition-metal countercation. In acid, larger Ln^III (Ln = La-Ho) link hexamer (U₆) oxoclusters into body-centered cubic frameworks, while smaller Ln^III (Ln = Er-Lu and Y) promote linking of 14 U₆ clusters into hollow superclusters (U₈₄ superatoms). U₈₄ assembles into superlattices including cubic-closest packed, body-centered cubic, and interpenetrating networks, bridged by interstitial countercations and U₆ clusters. Divalent transition metals (TM = Mn^II and Zn^II) charge-balance and promote the fusion of 10 U₆ and 10 U monomers into a wheel-shaped cluster (U₇₀). Dissolution of U₇₀ in organic media reveals (by small-angle X-ray scattering) that differing supramolecular assemblies are accessed, controlled by TM^II-linking of U₇₀ clusters. Magnetic measurements of these assemblies reveal Curie-Weiss behavior at high temperatures, without pairing of the 5f²-electrons down to 2 K.

On the Aqueous Chemistry of the UIV -DOTA Complex

The 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) aqueous complex of U^IV with H₂ O, OH^- , and F^- as axial ligands was studied by using UV/Vis spectrophotometry, ESI-MS, NMR spectroscopy, X-ray crystallography, and electrochemistry. The U^IV -DOTA complex with either water or fluoride as axial ligands was found to be inert to oxidation by molecular oxygen, whereas the complex with hydroxide as an axial ligand slowly hydrolyzed and was oxidized by dioxygen to a diuranate precipitate. The combined data set acquired shows that, although axial substitution of fluoride and hydroxide ligands instead of water does not seem to significantly change the aqueous DOTA complex structure, it has an important effect on the electronic configuration of the complex. The U^IV /U^III redox couple was found to be quasi-reversible for the complex with both axially bonded H₂ O and hydroxide, but irreversible for the complex with axially bonded fluoride. Intriguingly, binding of the axial fluoride renders the irreversible one-electron U^V /U^IV oxidation of the [U^IV (DOTA)(H₂ O)] complex quasi-reversible, which suggests the formation of the short-lived pentavalent form of the complex, an aqueous non-uranyl chelated U^V cation.

Self-Assembly-Assisted Kinetically Controlled Papain-Catalyzed Formation of mPEG-b-Phe(Leu)x

Self-assembling peptide materials are promising next-generation materials with applications that include tissue engineering scaffolds, drug delivery, bionanomedicine, and enviro-responsive materials. Despite these advances, synthetic methods to form peptides and peptide-polymer conjugates still largely rely on solid-phase peptide synthesis (SPPS) and N-carboxyanhydride ring-opening polymerization (NCA-ROP), while green methods remain largely undeveloped. This work demonstrates a protease-catalyzed peptide synthesis (PCPS) capable of directly grafting leucine ethyl ester (Leu-OEt) from the C-terminus of a methoxy poly(ethylene glycol)-phenylalanine ethyl ester macroinitiator in a one-pot, aqueous reaction. By using the natural tendency of the growing hydrophobic peptide segment to self-assemble, a large narrowing of the (Leu)_x distributions for both mPEG₄₅-b-Phe(Leu)_x and oligo(Leu)_x coproducts, relative to oligo(Leu)_x synthesized in the absence of a macroinitiator (mPEG₄₅-Phe-OEt), was achieved. A mechanism is described where in situ β-sheet coassembly of mPEG₄₅-b-Phe(Leu)_x and oligo(Leu)_x coproducts during polymerization prevents peptide hydrolysis, providing a means to control the degree of polymerization (DP) and dispersity of diblock (Leu)_x segments (matrix-assisted laser desorption time-of-flight (MALDI-TOF) x = 5.1, dispersity ≤ 1.02). The use of self-assembly to control the uniformity of peptides synthesized by PCPS paves the way for precise peptide block copolymer architectures with various polymer backbones and amino acid compositions synthesized by a green process.

Unexpected Roles of Alkali-Metal Cations in the Assembly of Low-Valent Uranium Sulfate Molecular Complexes

The directing effect of coordinating ligands in the formation of uranium molecular complexes has been well established, but the role of counterions in metal-ligand interactions remains ambiguous and requires further investigation. In this work, we describe the targeted isolation, through the choice of alkali-metal ions, of a family of tetravalent uranium sulfates, showing the influence of the overall topology and, unexpectedly, the U^IV nuclearity upon the inclusion of such countercations. Analyses of the structures of uranium(IV) oxo/hydroxosulfate oligomeric species isolated from consistent synthetic conditions reveal that the incorporation of Na⁺ and Rb⁺ promotes the crystallization of 0D discrete clusters with a hexanuclear [U₆O₄(OH)₄(H₂O)₄]¹²⁺ core, whereas the larger Cs⁺ ion allows for the isolation of a 2D-layered oligomer with a less condensed trinuclear [U₃(O)]¹⁰⁺ center. This finding expands the prevalent view that counterions play an innocent role in molecular complex synthesis, affecting only the overall packing but not the local oligomerization. Interestingly, trends in nuclearity appear to correlate with the hydration enthalpies of alkali-metal cations, such that the alkali-metal cations with larger hydration enthalpies correspond to more hydrated complexes and cluster cores. These findings afford new insights into the mechanism of nucleation of U^IV, and they also open a new path for the rational design and synthesis of targeted molecular complexes.

Titanium phosphonate oxo-alkoxide “clusters”: solution stability and facile hydrolytic transformation into nano titania

Oligonuclear Ti(iv) oxo-alkoxide-phosphonate complexes, produced by reaction of tBuPO(OH)₂ with Ti(OR)₄, are easily topotactically hydrolyzed forming intricate nanostructures.

Tetranuclear oxido-bridged thorium(iv) clusters obtained using tridentate Schiff bases

Thorium(iv) complexes are currently attracting intense attention from inorganic chemists due to the development of liquid-fluoride thorium reactors and the fact that thorium(iv) is often used as a model system for the study of the more radioactive Np(iv) and Pu(iv). Schiff-base complexes of tetravalent actinides are useful for the development of new separation strategies in nuclear fuel processing and nuclear waste management. Thorium(iv)-Schiff base complexes find applications in the colorimetric detection of this toxic metal ion and the construction of fluorescent on/off sensors for Th(iv) exploiting the ligand-based light emission of its complexes. Clusters of Th(iv) with hydroxide, oxide or peroxide bridges are also relevant to the environmental and geological chemistry of this metal ion. The reactions between Th(NO₃)₄·5H₂O and N-salicylidene-o-aminophenol (LH₂) and N-salicylidene-o-amino-4-methylphenol (L'H₂) in MeCN have provided access to complexes [Th₄O(NO₃)₂(LH)₂(L)₅] (1) and [Th₄O(NO₃)₂(L'H)₂(L')₅] (2) in moderate yields. The structures of 1·4MeCN and 2·2.4 MeCN have been determined by single-crystal X-ray crystallography. The complexes have similar molecular structures possessing the {Th₄(μ₄-O)(μ-OR')₈} core that contains the extremely rare {Th₄(μ₄-O)} unit. The four Th^IV atoms are arranged at the vertexes of a distorted tetrahedron with a central μ₄-O^2- ion bonded to each metal ion. The H atom of one of the acidic -OH groups of each 3.21 LH^- or L'H^- ligand is located on the imine nitrogen atom, thus blocking its coordination. The Th^IV centres are also held together by one 3.221 L^2- or (L')^2- group and four 2.211 L^2- or (L')^2- ligands. The metal ions adopt three different coordination numbers (8, 9, and 10) with a total of four coordination geometries (triangular dodecahedral, muffin, biaugmented trigonal prismatic, and sphenocorona). A variety of H-bonding interactions create 1D chains and 2D layers in the crystal structures of 1·4 MeCN and 2·2.4 MeCN, respectively. The structures of the complexes are compared with those of the uranyl complexes with the same or similar ligands. Solid-state and IR data are discussed in terms of the coordination mode of the organic ligands and the nitrato groups. ¹H NMR data suggest that solid-state structures are not retained in DMSO. The solid complexes emit green light at room temperature upon excitation at 400 nm, the emission being ligand-centered.

Stabilizing γ-Alkyltin-Oxo Keggin Ions by Borate Functionalization

We report a hierarchical self-assembly engineering of tin-oxo clusters from nanosized hydrophobic clusters to a single-layer film of assembled clusters. These clusters are derivatives of the previously reported Na-centered butyltin Keggin ions, but they are bicapped with butyltin and with borate ligands. The formulas γ-[( n-BuSn)₁₄(OCH₃)₁₀(OH)₃O₉(NaO₄)(HBO₃)₂] and γ-[( n-BuSn)₁₄(OCH₃)₁₀(OH)₃O₉(NaO₄)(PhBO₂)₂] were determined from single-crystal X-ray diffraction and bulk solution characterization including small-angle X-ray scattering, electrospray ionization mass spectrometry, and multinuclear and multidimensional NMR (¹¹⁹Sn, ¹³C, and ¹H). Solution characterization confirms that borate functionalization inhibits the solution-phase β-γ Keggin isomer interconversion that was recognized prior for uncapped butyltin clusters, and in this case, the γ isomer is favored. The assembly of the γ-NaSn₁₄BO₃ clusters into a homogeneous Langmuir-Blodgett monolayer is the first step toward creating nanopatterned films for microelectronic devices.

Structural Snapshots of Cluster Growth from {U6 } to {U38 } During the Hydrolysis of UCl4

Herein we report the assembly of large uranium(IV) clusters with novel nuclearities and/or shapes from the controlled hydrolysis of UCl₄ in organic solution and in the presence of the benzoate ligands. {U₆ }, {U₁₃ }, {U₁₆ }, {U₂₄ }, {U₃₈ } oxo and oxo/hydroxo clusters were isolated and crystallographically characterized. These structural snapshots indicate that larger clusters are slowly built from the condensation of octahedral {U₆ } building blocks. The uranium/benzoate ligand ratio, the reaction temperature and the presence of base play an important role in determining the structure of the final assembly. Moreover, the isolation of different size cluster {U₆ } (few hours), {U₁₆ } (3 days), {U₂₄ } (21 days) from the same solution in a chosen set of conditions shows that the assembly of uranium oxo clusters in hydrolytic conditions is time dependent.

Time-controlled synthesis of the 3D coordination polymer U(1,2,3-Hbtc)2 followed by the formation of molecular poly-oxo cluster {U14} containing hemimellitate uranium(iv)

Two coordination compounds bearing tetravalent uranium were synthesized in the presence of tritopic hemimellitic acid in acetonitrile with a controlled amount of water (H₂O/U ≈ 8) and structurally characterized. Compound 1, [U(1,2,3-Hbtc)₂]·0.5CH₃CN is constructed around an eight-fold coordinated uranium cationic unit [UO₈] linked by the poly-carboxylate ligands to form dimeric subunits, which are further connected to form infinite corrugated ribbons and a three-dimensional framework. Compound 2, [U₁₄O₈(OH)₄Cl₈(H₂O)₁₆(1,2,3-Hbtc)₈(ox)₄(ac)₄] ({U₁₄}) exhibits an unprecedented polynuclear {U₁₄} poly-oxo uranium cluster surrounded by O-donor and chloride ligands. It is based on a central core of [U₆O₈] type surrounded by four dinuclear uranium-subunits {U₂}. Compound 1 was synthesized by a direct reaction of hemimellitic acid with uranium tetrachloride in acetonitrile (+H₂O), while the molecular species ({U₁₄} (2)) crystallized from the supernatant solution after one month. The slow hydrolysis reaction together with the partial decomposition of the starting organic reactants into oxalate and acetate molecules induces the generation of such a large poly-oxo cluster with fourteen uranium centers. Structural comparisons with other closely related uranium-containing clusters, such as the {U₁₂} cluster based on the association of inner core [U₆O₈] with three dinuclear sub-units {U₂}, were performed.

The synthesis of a 3D coordination polymer [U(HL)₂] (L = hemimellitate) and a new poly-oxo cluster {U₁₄} afterwards is presented.