Xiangyang Yuan

Highly efficient extraction of actinides with pillar[5]arene-derived diglycolamides in ionic liquids via a unique mechanism involving competitive host–guest interactions

Actinide partitioning is considered as one of the most challenging issues in nuclear waste remediation. Herein, we unravel a novel extraction mode pertinent to the competitive host-guest interactions for highly efficient actinide extraction. The host-guest recognition event involves binding of a room temperature ionic liquid (RTIL), 1-n-octyl-3 methylimidazolium bis(trifluoromethane)sulfonamide (C8mimNTf2), as both the guest and the solvent by the hosts pillar[5]arene-based diglycolamides (P5DGAs) and the subsequent displacement of the guest by a metal ion. This two-step process suggests a unique competitive ion-mediated displacement mechanism for the metal ion partitioning in the extraction process. The supramolecular extraction system is evaluated for its extraction abilities towards actinide ions such as UO22+, PuO22+, Pu4+, Am3+, and fission product elements such as Eu3+, Sr2+, Cs+. The results demonstrate the exceedingly high distribution ratios and favorable separation of Am3+ and Pu4+ in nitric acid media. All the three P5DGAs form 1 : 1 complexes with Am3+. Time resolved laser fluorescence spectroscopic (TRLFS) studies reveal a strong complexation involving no inner-sphere water molecules in the Eu3+-P5DGA complexes when C8mimNTf2 is used as the diluent. With high efficiency in the extraction of actinides and a quantitative back extraction outcome, the RTIL-based solvent systems containing pillar[5]arene-DGA ligands developed in this work hold potential as promising candidates for nuclear waste remediation in a more sustainable fashion.

CMPO-calix[4]arenes with spacer containing intramolecular hydrogen bonding: effect of local rigidification on solvent extraction toward f-block elements.

To understand intramolecular hydrogen bonding in effecting liquid-liquid extraction behavior of CMPO-calixarenes, three CMPO-modified calix[4]arenes (CMPO-CA) 5a-5c with hydrogen-bonded spacer were designed and synthesized. The impact of spacer rotation that is hindered by introduction of intramolecular hydrogen bonding upon extraction of La(3+), Eu(3+), Yb(3+), Th(4+), and UO2(2+) has been examined. The results show that 5b and 5c containing only one hydrogen bond with a less hindered rotation spacer extract La(3+) more efficiently than 5a containing two hydrogen bonds with a more hindered rotation spacer, demonstrating the importance of local rigidification of spacer in the design of extractants in influencing the coordination environment. The large difference in extractability between La(3+) and Yb(3+) (or Eu(3+)) by 5b (or 5c), and the small difference by 5a, suggests intramolecular hydrogen bonding do exert pronounced influence upon selective extraction of light and heavy lanthanides. Log-log plot analysis indicates a 1:1, 2:1 and 1:1 stoichiometry (ligand/metal) for the extracted complex formed between 5b and La(3+), Th(4+), UO2(2+), respectively. Additionally, their corresponding acyclic analogs 7a-7c exhibit negligible extraction toward these metal ions. These results reveal the possibility of selective extraction via tuning local chelating surroundings of CMPO-CA by aid of intramolecular hydrogen bonding.

Ion-pair recognition of amidinium salts by partially hydrogen-bonded heteroditopic cyclo[6]aramide

Convergent heteroditopic cyclo[6]aramide demonstrates efficient ion-pair recognition of amidinium salts in 10% methanolic chloroform (>104 M−1) as confirmed by NMR and conductivity experiments. As predicted by density functional theory (DFT) method simulations, cyclo[6]aramide 1a is able to bind amidinium salts G1–G5 with varying binding affinity in 1 : 1 stoichiometry through hydrogen bonding interactions involving both anion-recognizing amide H-atoms and cation-binding amide carbonyl O-atoms. Particularly, the binding affinities for G1, G2 and G3 are found to decrease with increasing the size of substituents in the amidinium ion in the order of G1 > G2 > G3. Moreover, the association ability for simultaneous binding of cationic and anionic guest species depends considerably on the counterions. Among the four formamidinium salts (Cl−, Br−, I− and BPh4−) examined, formamidinium chloride is best encapsulated as a contact ion-pair species in the macrocycle. The reduced association constants with increasing the size of counterions in the order of G1 > G6 > G7 > G4 underscore the importance of ion paring in effecting the host–guest interaction. Comparative conductivity studies provide a convenient approach to differentiate between contact and loose ion pairs for these amidinium complexes. This work provides a rare example of binding biologically important types of amidinium cations as ion pairs by a synthetic receptor.

Highly efficient extraction of tetra- and hexavalent plutonium using DGA functionalized pillar[5]arene in RTIL: Understanding speciation, thermodynamics and radiolytic stability

ABSTRACT DGA functionalized pillar[5]arene (P5DGA) in ionic liquid was demonstrated as highly efficient system for the extraction of plutonium from acidic aqueous solution in tetravalent and hexavalent oxidation state. The extraction followed ‘cation-exchange’ mechanism via [Pu.P5DGA]4+ and [PuO2.P5DGA]2+, as extracted species for Pu4+ and PuO22+, respectively. Evaluation of thermodynamic parameters (ΔG, ΔH and ΔS) showed the feasibility and spontaneity of the extraction process. The process was exothermic and primarily ‘enthalpy driven’, since entropy change was found negative. P5DGA-RTIL solvent system showed good radiolytic stability even at 1000 kGy of gamma dose.