Pillar(5)arene-based ligands as extractants for separation of actinides and lanthanides

As a new class of macrocyclic compounds akin to calixarenes, pillar(n)arenes have received considerable attention since the first report in 2008. The unique pillar architectures with readily funactionalizable rims provide the possibility of using them as a macrocyclic platform for preorganizing chelating groups. We have recently revealed that pillar(5)arenes functionalized with phosphine oxides (PAPO) are able to differentiate UO₂²⁺ and Th⁴⁺ more efficiently and selectively than their acyclic analogues from acidic media using molecular solvents. Preorganizing diglycolamide (DGA) moieties on the pillararene platform resulted in high efficiency towards partitioning of Eu(III) over Am(III) in highly acidic media. When attaching CMPO moieties to pillar(5)arenes, these homoditopic molecules exhibited efficient and selective extraction of Am(III) over Eu(III) under acidic feed conditions. A bimetallic complex was found to form in a stepwise manner via a host-guest recognition process, which is correlated to the separation process. Since we initiated the investigation of pillararene-based extractants for actinide and lanthanide separation in 2013, our follow-up reports are still limited to traditional solvent extraction systems. Our recent work demonstrated that RTIL (C₈mimNTf₂) can serve as an excellent solvent extraction system for actinide separation with pillar(5)arene-based diglycolamides (P5DGAs, L-I, L-II and L-III) as extractants. The high distribution ratios and favorable separation of Am³⁺ and Pu⁴⁺ in nitric acid media were observed. Particularly, a unique competitive displacement mechanism associated with host-guest interactions in the extraction process was proposed based on NMR technique and DFT calculations. These results suggest the promising use with pillar(5)arene-based extractants for separation of actinides and lanthanides at high acidity. (author)