Proximity Effect in Uranyl Coordination of the Cucurbit[6]uril-Bipyridinium Pseudorotaxane Ligand for Promoting Host-Guest Synergistic Chelating.

In the present work, we proposed regulating uranyl coordination behavior of cucurbituril-bipyridinium pseudorotaxane ligand by utilizing meta-functionalized bipyridinium dicarboxylate guest. A tailored pseudorotaxane precursor involving 1,1'-(hexane-1,6-diyl)bis(3-cyanopyridin-1-ium) bromide (C6BPCN3) and cucurbit[6]uril (CB[6]) has designed and synthesized. Through in situ hydrolysis of the pseudorotaxane ligands and their coordination assembly with uranyl cations, seven new uranyl-rotaxane coordination polymers URCP1-URCP7 have been obtained under hydrothermal conditions in the presence of different anions. It is demonstrated that the variation of carboxylate groups from para- to meta-position greatly affected the coordination behaviors of the meta-functionalized pseudorotaxane linkers, which are enriched from simple guest-only binding to host-guest simultaneous coordination and synergistic chelating. This effective regulation on uranyl coordination of supramolecular pseudorotaxane can be attributed to the proximity effect, which refers to the meta-position carboxyl group being spatially closer to the portal carbonyl group of CB[6]. Moreover, by combining other regulation methods such as introducing competing counterions and modulating solution acidity, the nuclearity of the uranyl center and the coordination patterns of the pseudorotaxane ligand can be diversely tuned, which subsequently exert great influence on the final dimensionality of resultant uranyl compounds. This work presents a large diversity of uranyl-based coordination polyrotaxane compounds with fascinating mechanically interlocked components and, most importantly, provides a feasible approach to adjust and control the metal coordination behavior of the pseudorotaxane ligand that might expand the scope of application of such supramolecular ligands.

Temperature-Triggered Structural Dynamics of Non-Coordinating Guest Moieties in a Fluorescent Actinide Polyrotaxane Framework.

We present here the synthesis of a novel fluorescent actinide polyrotaxane compound URCP1 through the utilization of an end-cutting pseudorotaxane precursor with only the cucurbit[6]uril (CB[6]) macrocyclic components acting as linking struts. The non-coordinating guest motif in the obtained polyrotaxane, with increased freedom and structural flexibility, can display intriguing temperature-triggered conformational variations inside the cavity of CB[6], which was clearly evidenced by crystallographic snapshots at different temperatures. Notably, this observation of temperature-triggered structural dynamics in URCP1 represents the first report of actinide polyrotaxane with such feature in solid-state. Moreover, URCP1 has a high photoluminescence quantum yield (PLQY) of 49.8 %, comparable to other luminescent uranyl compounds, and can work as a fluorescent probe to selectively detect Fe3+ over other eight competing cations in aqueous solution, with the limit of detection being as low as 4.4×10-3  ppm.

Kinked-Helix Actinide Polyrotaxanes from Weakly Bound Pseudorotaxane Linkers with Variable Conformations.

The incorporation of a mechanically interlocked molecule such as pseudorotaxane into metal-organic coordination polymers has afforded plenty of new hybrid materials with special structures and unique properties. In this work, we employ a weakly bound cucurbit[6]uril (CB[6])-bipyridinium pseudorotaxane as a supramolecular precursor to assemble with uranyl, aiming to construct uranyl-rotaxane coordination polymers (URCPs) with intriguing structures. By adjusting the synthetic conditions, a new kinked-helix uranyl rotaxane compound (URCP3), together with three other compounds URCP1, URCP2, and URCP4 varying from 1D chains to 2D interwoven networks, was obtained. Detailed structural analyses indicate that the pseudorotaxane ligand (C8BPCA@CB[6]) shows great configuration diversity in the construction of URCPs, which is most probably due to the weak binding strength between the host and guest molecules. Specifically, based on the monodentate coordination of the end carboxyl groups of C8BPCA forced by the surrounding unilaterally-chelated oxalate, the entire flexible pseudorotaxane linker will be more likely to undergo conformational change, thereby binding to the uranyl center from both sides of the uranyl equatorial plane and promoting the formation of a kinked helix structure of URCP3 that is shaped like a Chinese knot along [001]. This work enriches the library of actinide-rotaxane compounds and provides a new approach to construct metal-organic compounds with complicated structures using weakly bonded pseudorotaxanes as well.

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 bcbp2- 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.

Bipyridine-Directed Syntheses of Uranyl Compounds Containing Semirigid Dicarboxylate Linkers: Diversity and Consistency in Uranyl Speciation.

Bipyridine organic bases are beneficial to the synthesis of novel uranyl-organic hybrid materials, but the relationship between their molecular structures and specific roles as structure-directing agents, especially for the semirigid dicarboxylate systems, is still unclear. Here we demonstrate how the bipyridine ligands direct the coordination assembly of uranyl-organic compounds with a semirigid dicarboxylate linker, 4,4'-dicarboxybiphenyl sulfone (H2dbsf), by utilizing a series of bipyridine ligands, 1,10-phenanthroline (phen), 2,2'-bipyridine (2,2'-bpy), 5,5'-dimethylbipyridine (5,5'-dmbpy), 4,4'-bipyridine (4,4'-bpy), or 1,3-di(4-pyridyl)propane (bpp). Under hydrothermal conditions, eight uranyl-organic coordination polymers (UCPs), four of which [[UO2(dbsf)(phen)] (1), [UO2(dbsf)(phen)]·H2O (1'), [U4O10(dbsf)3]2[H2bpp]2 (6), and [U4O10(dbsf)3]2[H2bpp] (6')] were reported previously, were synthesized and divided into two types based on the chelate or template effect of these bipyridine ligands. 1, 1', [UO2(dbsf)(2,2'-bpy)] (2), and [(UO2)2(dbsf)2(5,5'-dmbpy)2] (3) are springlike triple helices with bipyridine ligands (phen, 2,2'-bpy, or 5,5'-dmbpy) as chelate ligands, while [U4O10(dbsf)3][H2(4,4'-bpy)] (4), [U4O10(dbsf)3]2[H(4,4'-bpy)]2[Ni(H2O)6] (5), 6, and 6' are tetranuclear uranyl-mediated 2-fold-interpenetrating networks with 4,4'-bpy or bpp as template ligands and charge-balancing agents. The participation or not in uranyl coordination of different bipyridine ligands promotes not only diversity in uranyl speciation and final topological structures among different classes of organic bases but also consistency for the same types of bipyridine ligands, which thus endows the possibility of the rational design of UCPs based on semirigid dicarboxylate ligands with the aid of cautiously selected bipyridine ligands.

Anion-adaptive crystalline cationic material for 99TcO4- trapping.

Efficient anion recognition is of great significance for radioactive 99TcO4- decontamination, but it remains a challenge for traditional sorbents. Herein, we put forward a tactic using soft crystalline cationic material with anion-adaptive dynamics for 99TcO4- sequestration. A cucurbit[8]uril-based supramolecular metal-organic material is produced through a multi-component assembly strategy and used as a sorbent for effective trapping of TcO4-. Excellent separation of TcO4-/ReO4- is demonstrated by fast removal kinetics, good sorption capacity and high distribution coefficient. Remarkably, the most superior selectivity among metal-organic materials reported so far, together with good hydrolytic stability, indicates potential for efficient TcO4- removal. The structure incorporating ReO4- reveals that the supramolecular framework undergoes adaptive reconstruction facilitating the effective accommodation of TcO4-/ReO4-. The results highlight opportunities for development of soft anion-adaptive sorbents for highly selective anion decontamination.

Uranyl Compounds Involving a Weakly Bonded Pseudorotaxane Linker: Combined Effect of pH and Competing Ligands on Uranyl Coordination and Speciation.

Pseudorotaxane-type ligands with tunable structural dynamics offer an opportunity in the exploration of new actinide hybrid materials. In this work, we utilized a weakly bonded pseudorotaxane ligand involving CB[6] and 1, 1'-(heptane-1, 7-diyl)bis(4-(ethoxycarbonyl)pyridin-1-ium) bromides ([C7BPCEt]Br2@CB[6]) to assemble with uranyl ion, and we systematically investigated the effect of different factors including pH and competing ligands on the hydrothermal synthesis of URCPs. Nine uranyl-rotaxane coordination polymers (URCPs) with diversity in coordination mode and topological structure were successfully prepared (two previously reported complexes, URCP1 and URCP2 are also included). The results indicate that sulfate, bromide, CB[6], and C7BPCA (the hydrolyzate of [C7BPCEt]Br2) show a combined influence on the obtained URCPs. At low pH, both CB[6] and C7BPCA can bond with uranyl centers and produce interwoven structures in URCP1, URCP2, and URCP6; at high pH, C7BPCA and competing anions (sulfate and bromide) have priority to coordinate with uranyl ions in URCP3-URCP5 and URCP7-URCP9. Notably, for the first time, bromide anion with lower affinity to uranyl ions is also observed in solid-state uranyl coordination polymer (URCP7-URCP9), which has been demonstrated by both energy dispersive X-ray spectroscopy and single-crystal X-ray structure analysis. In addition, a spontaneously single-crystal-to-single-crystal transformation from URCP3 to URCP4, which is driven by thermodynamics, was observed and explained by computational study. Moreover, it reveals that sulfate with stronger coordination ability can inhibit the hydrolysis of uranyl ion to some extent with only a rarely reported pentanuclear uranyl center found in URCP5 obtained at pH 5.67. These results indicate that the combined effect of competing ligands and pH has great significance in the formation of URCPs in terms of uranyl coordination and speciation and can be an alternative way to design and synthesize uranyl coordination polymers with new topologies.

Uranyl-Organic Coordination Compounds Incorporating Photoactive Vinylpyridine Moieties: Synthesis, Structural Characterization, and Light-Induced Fluorescence Attenuation.

The fluorescence of uranyl originated from electronic transitions (S11-S00 and S10-S0v, v = 0-4) of the ligand-to-metal charge transfer (LMCT) process is an intrinsic property of many uranyl coordination compounds. However, light-induced regulation on fluorescence features of uranyl hybrid materials through photoactive functional groups is less investigated. In this work, the photoactive vinyl group-containing ligands, ( E)-methyl 3-(pyridin-4-yl)acrylate and ( E)-methyl 3-(pyridin-3-yl)acrylate, have been used in the construction of uranyl coordination polymers in the presence of 1,10-phenanthroline (phen). Five compounds (UO2)3(µ3-O)(µ2-OH)2(L1)2( phen)2(1), (UO2)3(µ3-O)(µ2-OH)3(L1)( phen)2 (2), (UO2)3(µ3-O)(µ2-OH)3(L2)( phen)2 (3), [(UO2)2(µ2-OH)2(L2)2( phen)2]·2H2O (4), and (UO2)Zn(SO4)(phen)(H2O)(OH)2(5) were obtained under hydrothermal conditions. Compounds 1-4 are polynuclear uranyl structures with abundant π-π interactions and hydrogen bonds contributed to the 3D crystal packing of them. As model compounds, 1 and 3 are selected for exploring photoresponsive behaviors. The emission intensities of these two compounds are found to decrease gradually over the exposure time of UV irradiation. X-ray single crystal structural analysis suggests that the fluorescence attenuation can be explained by the slight rotation of pyridinyl groups around the carbon-carbon double bond during UV irradiation, which is accompanied by the change of weak interactions, i.e., π-π interactions and hydrogen bonds in strength and density. This feature of light-induced fluorescence attenuation may enable these two compounds to act as potential photoresponsive sensor materials.

Releasing Metal-Coordination Capacity of Cucurbit[6]uril Macrocycle in Pseudorotaxane Ligands for the Construction of Interwoven Uranyl-Rotaxane Coordination Polymers.

As an emerging type of actinide hybrid material, uranyl-rotaxane coordination polymers (URCPs) with new coordination patterns and topological structures are still desired. In this work, we propose a new strategy to construct URCPs by promoting the simultaneous coordination of both the wheel and axle moieties in pseudorotaxane linkers with metal nodes. Starting from a series of cucurbit[6]uril (CB[6])-based pseudorotaxane ligands, C nBPCA@CB[6] [C nBPCA = 1,1-(α,ω-diyl)bis[4-(ethoxycarbonyl)pyridin-1-ium] bromides, where n = 5-8] with slightly deformed CB[6], four new URCPs (URCP1, URCP3, URCP4, and URCP5) with interwoven network structures, as well as another noninterwoven polymer(URCP2), have been successfully prepared. According to single-crystal structure analysis, we attribute the interwoven structures of the URCPs to the distortion of CB[6] in pseudorotaxane ligands with shorter or longer spacers (C5, C7, and C8). This indicates that the deformation could effectively diminish the steric hindrance around the portals, thus endowing the "inert" CB[6] host with coordination ability like the string molecule. Besides, the participation of water molecules and sulfate anions in the uranyl coordination sphere is also found to have a great influence on the final structures of the obtained URCPs. The successful preparation of interwoven URCPs in this work gives some new insights into the metal coordination of supramolecular entities and could facilitate other new applications of CB[6]-based pseudorotaxane ligands. Most importantly, the strategy proposed in this work provides some hints in the controllable design of metal-organic rotaxane frameworks with unique topologies.