Theoretical Insights on Improving Amidoxime Selectivity for Potential Uranium Extraction from Seawater.

Extraction of uranium from seawater is one of the important ways to solve the shortage of terrestrial uranium resources. Thereinto, the competition between uranyl and vanadium cations is a significant challenge in the commonly used amidoxime-based adsorbents for extracting uranium from seawater. An in-depth understanding of the extraction behaviors of modified amidoxime groups with uranyl and vanadium ions is one of the effective means to design and develop efficient adsorbents for selective uranium sequestration. In this work, we have designed and systematically investigated the alkyl and amino functionalized amidoxime, (Z)-2-amino-N'-hydroxy-N,N-dimethylbenzimidamide (L1), and its phenyl and methoxy derivatives ((Z)-3-amino-N'-hydroxy-N,N-dimethyl-2-naphthimidamide (L2) and (Z)-2-amino-N'-hydroxy-4-methoxy-N,N-dimethylbenzimidamide (L3)) by quantum chemistry calculations. In the uranyl complexes, the amidoxime groups prefer to act as η2-coordinated ligands as the amidoximes increase, and there exist substantial hydrogen bond interactions, which are different from the vanadium complexes. Various bonding analyses show that the L1 ligand possesses a stronger binding affinity to UO22+, and the -C6H5 and -CH3O substituent groups seem to have no effect on the improvement of extraction ability. Thermodynamic analysis confirms that the L1 ligand has a stronger extraction capability to uranyl ion compared to L2 and L3. According to the calculations of the vanadium (V) (VO2+ and VO3+) complexes with the L1 ligand, L1 is more likely to react with [H2VO4]- and [HVO4]2- to form VO2+ complexes. Expectantly, thermodynamic analysis displays a higher extraction capacity for uranyl ions than vanadium ions. Therefore, these alkyl and amino functionalized amidoxime ligands demonstrate high selectivity for uranyl over vanadium ions, which is mainly due to the coordination mode changes of these ligands toward vanadium in conjunction with the considerable hydrogen bonds in the uranyl complexes. These results are expected to afford useful clues for the design of efficient adsorbents for uranium extraction from seawater.

Mixed-Ligand Uranyl Polyrotaxanes Incorporating a Sulfate/Oxalate Coligand: Achieving Structural Diversity via pH-Dependent Competitive Effect.

A mixed-ligand system provides an alternative route to tune the structures and properties of metal-organic compounds by introducing functional organic or inorganic coligands. In this work, five new uranyl-based polyrotaxane compounds incorporating a sulfate or oxalate coligand have been hydrothermally synthesized via a mixed-ligand method. Based on C6BPCA@CB6 (C6BPCA = 1,1'-(hexane-1,6-diyl)bis(4-(carbonyl)pyridin-1-ium), CB6 = cucurbit[6]uril) ligand, UPS1 (UO2(L)0.5(SO4)(H2O)·2H2O, L = C6BPCA@CB6) is formed by the alteration of initial aqueous solution pH to a higher acidity. The resulting 2D uranyl polyrotaxane sheet structure of UPS1 is based on uranyl-sulfate ribbons connected by the C6BPCA@CB6 pseudorotaxane linkers. By using oxalate ligand instead of sulfate, four oxalate-containing uranyl polyrotaxane compounds, UPO1-UPO4, have been acquired by tuning reaction pH and ligand concentration: UPO1 (UO2(L)0.5(C2O4)0.5(NO3)·3H2O) in one-dimensional chain was obtained at a low pH value range (1.47-1.89) and UPO2 (UO2(L)(C2O4)(H2O)·7H2O)obtained at a higher pH value range (4.31-7.21). By lowering the amount of oxalate, another two uranyl polyrotaxane network UPO3 ((UO2)2(L)0.5(C2O4)2(H2O)) and UPO4 ((UO2)2O(OH)(L)0.5(C2O4)0.5(H2O)) could be acquired at a low pH value of 1.98 and a higher pH value over 6, respectively. The UPO1-UPO4 compounds, which display structural diversity via pH-dependent competitive effect of oxalate, represent the first series of mixed-ligand uranyl polyrotaxanes with organic ligand as the coligand. Moreover, the self-assembly process and its internal mechanism concerning pH-dependent competitive effect and other related factors such as concentration of the reagents and coordination behaviors of the coligands were discussed in detail.

Theoretical insights into selective extraction of uranium from seawater with tetradentate N,O-mixed donor ligands.

The competition of uranium and vanadium ions is a major challenge in extracting uranium from seawater. In-depth exploration of the complexation of uranium and vanadium ions with promising ligands is essential to design highly efficient ligands for selective recovery of uranium. In this work, we systematically explored the uranyl and vanadium extraction complexes with three tetradentate N,O-mixed donor analogues including the rigid backbone ligands 1,10-phenanthroline-2,9-dicarboxylic acid (PDA, L1) and 5H-cyclopenta[2,1-b:3,4-b']dipyridine-2,8-dicarboxylate acid (L3), as well as the flexible ligand [2,2'-bipyridine]-6,6'-dicarboxylate acid (L2) using density functional theory (DFT). These ligands coordinate to the uranyl cation in a tetradentate fashion, while L1 and L3 act as tridentate ligands toward VO2+ due to the smaller ionic radius of VO2+ and larger cleft sizes of L1 and L3. Bonding analyses show that the metal-ligand bonding orbitals of the uranyl complexes [UO2L(CO3)]2-, [UO2L(OH)]-, and [UO2L(H2O)] mainly arise from the interactions of the U 5f, 6d orbitals and N, O 2p orbitals. Because of the rigid structure and more suitable chelate ring size, the L1 ligand possesses a stronger complexing ability for uranyl ions than other ligands, while the L3 ligand has weaker binding affinity than L1 and L2. All these ligands prefer to coordinate with the uranyl cation rather than vanadium ion, indicating the selectivity of these ligands to [UO2(CO3)3]4- over H2VO4- and HVO42- in seawater. This is mainly attributed to the metal ion size-based selectivity and structural preorganization of the ligands. These results demonstrate that the backbone of these ligands affect their extraction behaviors. It is expected that this work might prove useful in designing efficient ligands for uranium extraction from seawater.

Temperature-induced reversible single-crystal-to-single-crystal isomerisation of uranyl polyrotaxanes: an exquisite case of coordination variability of the uranyl center.

The first reversible solid-state single-crystal-to-single-crystal isomerisation mediated by the change of uranyl-ligand coordination modes, that is from seven-coordinated uranium(vi) of α-UP to six-coordinated uranium(vi) of the supramolecular isomer, ß-UP, has been achieved in the uranyl polyrotaxane system by a temperature-induced strategy.