New Three-Fold Interpenetrated Uranyl Organic Framework Constructed by Terephthalic Acid and Imidazole Derivative

Energy-structure-property relationships in uranium metal-organic frameworks

Located at the foot of the periodic table, uranium is a relatively underexplored element possessing rich chemistry. In addition to its high relevance to nuclear power, uranium shows promise for small molecule activation and photocatalysis, among many other powerful functions. Researchers have used metal-organic frameworks (MOFs) to harness uranium's properties, and in their quest to do so, have discovered remarkable structures and unique properties unobserved in traditional transition metal MOFs. More recently, (e.g. the last 8-10 years), theoretical calculations of framework energetics have supplemented structure-property studies in uranium MOFs (U-MOFs). In this Perspective, we summarize how these budding energy-structure-property relationships in U-MOFs enable a deeper understanding of chemical phenomena, enlarge chemical space, and elevate the field to targeted, rather than exploratory, discovery. Importantly, this Perspective encourages interdisciplinary connections between experimentalists and theorists by demonstrating how these collaborations have elevated the entire U-MOF field.

Progress in solid state and coordination chemistry of actinides in China

In the past decade, the area of solid state chemistry of actinides has witnessed a rapid development in China, based on the significantly increased proportion of the number of actinide containing crystal structures reported by Chinese researchers from only 2% in 2010 to 36% in 2021. In this review article, we comprehensively overview the synthesis, structure, and characterizations of representative actinide solid compounds including oxo-compounds, organometallic compounds, and endohedral metallofullerenes reported by Chinese researchers. In addition, Chinese researchers pioneered several potential applications of actinide solid compounds in terms of adsorption, separation, photoelectric materials, and photo-catalysis, which are also briefly discussed. It is our hope that this contribution not only calls for further development of this area in China, but also arouses new research directions and interests in actinide chemistry and material sciences.

Noncomplexed Cucurbituril-Mediated Structural Evolution of Layered Uranyl Terephthalate Compounds

Template synthesis is one of the most feasible ways to explore new uranyl compounds with intriguing structures and properties. Here we demonstrate the preparation of six novel "sandwichlike" uranyl coordination polymers (UCPs) based on two-dimensional uranyl-terephthalate acid (H₂TP) networks using CBn (n = 5, 6, 8) as template ligands in the presence of different cations (Na⁺, K⁺, Cs⁺, or H₂N(CH₃)₂⁺). Compound 1 ([UO₂(TP)₂][Na₂(CB5)(H₂O)](H₂O)₅) is composed of layered uranyl-TP networks with the complex of CB5 and sodium cations as template ligands. In compound 2 ([(UO₂)₂(TP)₃]₂(CB6)(H₂O)₁₀), CB6 located between uranyl-TP networks contacts them by π-π interactions and hydrogen bonds. Compound 3 ([(UO₂)₂(TP)₃]₂[Na₂(H₂O)₁₀(CB6)]) is the same as compound 2 except for sodium cations bonding with CB6. Similarly in compound 4 ([(UO₂)₂(TP)₃][Cs(H₂O)₃(CB6)]), CB6 is a capsulelike structure capped with two cesium cations and interacts with uranyl-TP networks through π-π and C-H···π interactions. Compound 5 ([(UO₂)₂(TP)₃(HCOO)₂][K(H₂O)₂(CB5)]₂[H₂N(CH₃)₂]₂(CB6)(H₂O)₆) consists of both templates of CB6 and CB5 in which each CB5 is capped with one potassium cation while the H₂N(CH₃)₂⁺ cation is held at CB6 portals. In compound 6 ([(UO₂)₂(TP)₃]₂[UO₂(TP)₂(H₂O)₂][Cs(CB8)₃(H₂O)₄](H₂O)₁₆), CB8 ligands are connected by cesium cations to form a triangle motif and are further located between the uranyl-TP networks as template agents. All of the 2D layered structures with free CBn or cation-anchored CBn intercalate into the laminates of uranyl-terephthalate and shows a cucurbituril-mediated structural evolution. The regulating role of CBn as structure-directing template agents for the construction of layered UCPs through outer-surface interactions with layers of uranyl terephthalate is demonstrated, especially for the case of CB6 with contractive interlayer spacing.

Structural Diversity of Bipyridinium-Based Uranyl Coordination Polymers: Synthesis, Characterization, and Ion-Exchange Application

As well-known functional groups with excellent electro/photochromic and ion-exchange properties, bipyridinium motifs have been used in functionalized metal-organic coordination polymers, but they are still rarely applied to construct actinide coordination polymers. In this work, we utilized a bipyridinium-based carboxylic acid, 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium bis(chloride) ([H₂bcbp]Cl₂), as the organic ligand to assemble with uranyl cations. By the introduction of different kinds of auxiliary ligands and adjustment of the pH, five novel uranyl coordination compounds, 1-5, have been synthesized through hydrothermal reactions. Starting from uranyl ions and terephthalic acid (H₂TP) and H₂bcbp ligands, [(UO₂)₂(bcbp)(TP)₂]·3H₂O (1) has a wave-shaped two-dimensional (2D) structure consisting of dinuclear units connected by terephthalate linkers and further supported by the longer H₂bcbp ligands. [(UO₂)₂(bcbp)(PA)₂]·4H₂O (2) has a zigzag chain of dimeric uranium units, and [(UO₂)₂(bcbp)(bpdc)₂]·5H₂O (3) forms a one-dimensional ribbonlike structure. The 2D structures of [(UO₂)(bcbp)(OH)(H₂O)]·Cl (4) and [(UO₂)(bcbp)Cl]·Cl (5) are similar, both of which are constructed from dinuclear uranyl units and bcbp^2- ligands. Furthermore, the performance for perrhenate removal of compound 4 with a cationic framework is assessed, and we found that compound 4 can efficiently remove ReO₄^- from an aqueous solution in a wide range of pH values. This work extends the library of viologen derivative-based uranyl coordination polymers, provides to some extent broader insights into actinide coordination chemistry of functionalized ligands, and may facilitate the ion-exchange applications of related coordination polymers.

Hydrothermal synthesis, crystal structure and properties of a two-dimensional uranyl coordination polymer based on a flexible zwitterionic ligand

The interaction between the uranyl cation, (UO₂)²⁺, and organic species is of interest due to the potential applications of the resulting compounds with regard to nuclear waste disposal and nuclear fuel reprocessing. The hydrothermal reaction of various uranyl compounds with flexible zwitterionic 1,1'-[1,4-phenylenebis(methylene)]bis(pyridin-1-ium-4-carboxylate) dihydrochloride (Bpmb·2HCl) in deionized water containing drops of H₂SO₄ resulted in the formation of a novel two-dimensional uranyl coordination polymer, namely poly[tetraoxido{μ₂-1,1'-[1,4-phenylenebis(methylene)]bis(pyridin-1-ium-4-carboxylate)}di-μ₃-sulfato-diuranium(VI)], [(UO₂)₂(SO₄)₂(C₂₀H₁₆N₂O₄)]_n, (1). Single-crystal X-ray diffraction reveals that this coordination polymer exhibits a layered arrangement and the (UO₂)²⁺ centre is coordinated by five equatorial O atoms. The structure was further characterized by FT-IR spectroscopy, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA). The polymer shows high thermal stability up to 696 K. Furthermore, the photoluminescence properties of (1) has also been studied, showing it to exhibit a typical uranyl fluorescence.

Structural Consequences of 1,4-Cyclohexanedicarboxylate Cis/Trans Isomerism in Uranyl Ion Complexes: From Molecular Species to 2D and 3D Entangled Nets

trans-1,4-Cyclohexanedicarboxylic acid (t-1,4-chdcH₂) or the commercially available mixture of the cis and trans isomers (c,t-1,4-chdcH₂) has been used in the synthesis of a series of 14 uranyl ion complexes, all obtained under solvohydrothermal conditions, some in the presence of additional metal cations and/or 2,2'-bipyridine (bipy). With its two isomeric forms having very different shapes and its great sensitivity to the experimental conditions, 1,4-chdc^2- appears to be suitable for the synthesis of uranyl ion complexes displaying a wide range of architectures. Under the conditions used, the pure trans isomer gives only the complexes [UO₂(t-1,4-chdc)(H₂O)₂] (1) and [UO₂(t-1,4-chdc)] (2), which crystallize as one- and two-dimensional (1D and 2D) species, respectively. Complexes containing either the cis isomer alone or mixtures of the two isomers in varying proportion were obtained from the isomer mixture. The neutral complexes [UO₂(c-1,4-chdc)(DMF)] (3) and [UO₂(c-1,4-chdc)(bipy)] (4) are 2D and 1D assemblies, respectively, while all the other complexes are anionic and include various counterions. [C(NH₂)₃]₃[H₂NMe₂][(UO₂)₄(c-1,4-chdc)₆]·H₂O (5) crystallizes as a three-dimensional (3D) framework with {10³} topology. While [H₂NMe₂]₂[(UO₂)₂(c-1,4-chdc)₂(t-1,4-chdc)]·DMF·2H₂O (6) is a 1D ladderlike polymer, [H₂NMe₂]₂[(UO₂)₂(c-1,4-chdc)(t-1,4-chdc)₂]·2H₂O (7), which differs in the cis/trans ratio, is a 3-fold 2D interpenetrated network with {6³} honeycomb topology. The related [H₂NMe₂]₂[(UO₂)₂(c,t-1,4-chdc)₃]·2.5H₂O (8), with one disordered ligand of uncertain geometry, is a 3-fold 3D interpenetrated system. The two isomorphous complexes [Co(bipy)₃][(UO₂)₂(c-1,4-chdc)₃]·1.5H₂O (9) and [Cd(bipy)₃][(UO₂)₂(c-1,4-chdc)₃]·1.5H₂O (10) form 3D frameworks with the {10³} srs topological type. In contrast, [Ni(bipy)₃]₂[(UO₂)₄(c-1,4-chdc)₂(t-1,4-chdc)(NO₃)₆]·2H₂O (11) is a molecular, tetranuclear complex due to the presence of terminal nitrate ligands. A 2-fold 3D interpenetration of frameworks with {10³} ths topology is observed in [Cu(bipy)₂]₂[(UO₂)₂(c-1,4-chdc)₂(t-1,4-chdc)]·2H₂O (12), while [Zn(bipy)₃][(UO₂)₂(c-1,4-chdc)₃]·4H₂O (13) crystallizes as a 2D net with the common {4.8²} fes topological type. The additional Pb^II cation is an essential part of the 3D framework formed in [UO₂Pb₂(c-1,4-chdc)(t-1,4-chdc)₂(bipy)₂] (14), in which uranyl and its ligands alone form 1D subunits. Together with previous results, the solid-state uranyl emission properties of seven of the present complexes evidence a general trend, with the maxima for the complexes with O₆ equatorial environments being blue-shifted with respect to those for complexes with O₅ environments.

Dicyanoaurate-based heterobimetallic uranyl coordination polymers

The first series of uranyl ([UO₂]²⁺)-dicyanoaurate coordination polymers and molecular complexes has been synthesized. Reactions of [A][Au(CN)₂] (A = [ⁿBu₄N]⁺ or [(Ph₃P)₂N]⁺ ([PPN])) and uranyl nitrate in alcoholic solvents in ambient light led to [A]₂[(UO₂)₂(μ-η²:η²-O₂)(NO₃)₂(μ-Au(CN)₂)₂], which incorporates peroxo ligands into a one-dimensional ladder topology with alternating aurophilic and peroxo rungs. Conducting the reaction with non-alcoholic solvents formed two polymorphs of a one-dimensional chain, [PPN][UO₂(NO₃)₂Au(CN)₂], from acetone, and a molecular analogue, [PPN]₂[UO₂(NO₃)₂(Au(CN)₂)₂], from acetonitrile, none of which exhibited aurophilic interactions. The addition of 2,2'-bipyridine to the initial reaction resulted in [UO₂(bipy)(MeO)(MeOH)]₂[(μ-Au(CN)₂)(Au(CN)₂)], a one-dimensional structure which propagates via a series of linear aurophilic bonds with pendant uranyl complexes; methanol and methoxy ligands provide additional connections through hydrogen bonding. The addition of 5,5'-dimethyl-2,2'-bipyridine using solvothermal conditions resulted in the one-dimensional ladder [UO₂(Me₂bipy)Au(CN)₂]₂[(μ-OH)₂], generated through aurophilic bonds and hydroxide ligands. The incorporation of 2,2':6',2''-terpyridine (terpy) using solvothermal conditions resulted in [[UO₂(terpy)]₂(μ-NO₃)(μ-O)][Au(CN)₂], a molecular salt with no aurophilic interactions. Emission spectra attributable to aurophilic interactions are observed in [ⁿBu₄N]₂[(UO₂)₂(μ-η²:η²-O₂)(NO₃)₂(μ-Au(CN)₂)₂], while all others only show emission typical of the uranyl cation.

Recent advances in structural studies of heterometallic uranyl-containing coordination polymers and polynuclear closed species

A survey is given of recent original structural results on heterometallic species incorporating uranyl ions, particularly with carboxylate ligands.

An Unprecedented Two-Fold Nested Super-Polyrotaxane: Sulfate-Directed Hierarchical Polythreading Assembly of Uranyl Polyrotaxane Moieties

The hierarchical assembly of well-organized submoieties could lead to more complicated superstructures with intriguing properties. We describe herein an unprecedented polyrotaxane polythreading framework containing a two-fold nested super-polyrotaxane substructure, which was synthesized through a uranyl-directed hierarchical polythreading assembly of one-dimensional polyrotaxane chains and two-dimensional polyrotaxane networks. This special assembly mode actually affords a new way of supramolecular chemistry instead of covalently linked bulky stoppers to construct stable interlocked rotaxane moieties. An investigation of the synthesis condition shows that sulfate can assume a vital role in mediating the formation of different uranyl species, especially the unique trinuclear uranyl moiety [(UO2 )3 O(OH)2 ](2+) , involving a notable bent [O=U=O] bond with a bond angle of 172.0(9)°. Detailed analysis of the coordination features, the thermal stability as well as a fluorescence, and electrochemical characterization demonstrate that the uniqueness of this super-polyrotaxane structure is mainly closely related to the trinuclear uranyl moiety, which is confirmed by quantum chemical calculations.

Counter-ion control of structure in uranyl ion complexes with 2,5-thiophenedicarboxylate

Various counterions containing d-block metal ions and N-donating chelators were used to generate one- and two-dimensional uranyl-2,5-thiophenedicarboxylate species, one of them displaying inclined polycatenation.

The templated synthesis of a unique type of tetra-nuclear uranyl-mediated two-fold interpenetrating uranyl–organic framework

A novel type of tetra-nuclear uranyl-mediated two-fold interpenetrating uranyl–organic framework has been hydrothermally synthesized as a couple of supramolecular isomers from a semi-rigid carboxylate in the presence of an organic base as the template.