Pillararenes are macrocyclic oligomers of alkoxybenzene akin to calixarenes but tethered at the 2,5-positions via methylene bridges. Benefiting from their unique pillar-shaped architecture favorable for diverse functionalization and versatile host–guest properties, pillararenes decorated with chelating groups worked excellently as supporting platforms to construct extractants or adsorbents for metal ion separation. This feature article provides a detailed summary of pillararenes in Ln/An separation by liquid–liquid extraction and heavy metal separation by solid–liquid extraction. The preorganization effect of the rigid pillararene framework has a profound impact on the extraction of metal ions, and a unique extraction mechanism is observed when employing ionic liquids as solvents. The rich host–guest chemistry of pillararenes enables construction of a wide variety of supramolecular materials as metal ion adsorbents. We also discuss the differences between pillararenes and several well-known macrocycles, with a focus on the metal–ligand coordination and its influencing factors. We hope this review will provide useful information and unleash new opportunities in this field.

Pillar[5]arenes were functionalized with ten diglycolamide (DGA) arms on both sides of the pillar framework and were found to exhibit excellent separation and extraction efficiency towards Am(iii) and Eu(iii) at high acidity.

A new class of pillar[5]arene-based phosphine oxides tethered with ten chelating groups on both rims of the pillar were synthesized via two steps from macrocyclization of 1,4-bis(bromoalkoxy)benzenes with paraformaldehyde, followed by Arbusov reaction with iso-propoxydiphenylphosphine. Solvent extraction of these ligands towards selected lanthanides and actinides was investigated under acidic condition. Compared with acyclic monovalent analogs and classical extractant tri-n-octylphosphine oxide (TOPO), the remarkable efficiency and selectivity for thorium(IV) and uranyl(VI) as observed in these novel extractants bearing a varying spacer length revealed the significance of preorganization of chelating groups on pillararene platform. Log–log plot analysis indicated the 1 : 1 stoichiometry (ligand/metal) for the extracted complex formed between the ligand and thorium(IV) or uranyl(VI). The extraction efficiency was considerably improved with increasing acidity in a range of 0.1–1.5 M HNO₃ , a result that is distinct from the extraction behaviour of calixarene-based phosphine oxides. The extractability increased with enhancing NaNO₃ concentration and high salinity assisted the preferential extraction of U(VI) over Th(IV). These ligands also showed moderate efficiency in differentiating europium(III) and americium(III) at 1 M HNO₃ in the presence of a synergist (Br₆-COSAN).

A unique host–guest recognition process involving a new class of homoditopic CMPO-pillar[5]arenes and lanthanides was revealed to proceed in a stepwise manner, and correlated with the efficient separation of americium(III) and europium(III) under acidic feed conditions.

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 UO₂ ²⁺, PuO₂ ²⁺, 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.