Multivalent cooperativity induced by self-assembly for f-element separation

Preorganization is an effective strategy for f-element separation, but the complexity of extractant synthesis hinders large-scale application. Here the authors discuss an alternative strategy induced by in situ self-assembly that borrows principles of multivalent cooperativity from Nature to separate f-elements.

the ligands are to synthesize. This greatly limits the large-scale production of extractants for industrial application, promoting the search for alternative methods for f-block separation.

Recent advances
Recently, a strategy based on self-assembled systems was proposed for lanthanide or actinide separation [16][17][18] . Distinct from the preorganization strategy, where multiple ligating groups are covalently incorporated into a molecular platform, the new strategy involves in situ self-assembly of multiple components during the extraction process, forming a multinuclear complex, in contrast to the mononuclear complex in most traditional systems. The advantage of such self-assembled systems lies in that the tiny differences in single metal-ligand interactions can be amplified by the multivalent cooperativity effect, which is beneficial in the separation of metal ions with similar properties. Our early work has demonstrated the successful separation of many lanthanide pairs through selective crystallization of lanthanide borates 16 . The magnification of the subtle bonding difference during crystallization is believed to be the main driving force for the high selectivity.
Sun and co-workers reported a significant cooperative enhancement of metal ion selectivity on metal-organic cages selfassembled from a tris-tridentate ligand (L I ) with a variety of metal ions (Fig. 2a) 17 . A tetrahedral cage in which all metal centers adopt nine-coordinate tricapped trigonal prismatic geometry is formed when reacting one equivalent of ligand with metal salt. This versatile ligand shows high discrimination between metal ions with identical coordination geometries and extremely small differences in ionic radii (Fig. 2b). For example, complete binding tendency toward Eu 3+ occurs when treating one equivalent of L I with La 3+ /Eu 3+ (1/1, molar ratio), a pair with only 0.1 Å radius difference. In general, a preference for smaller metal ions is found and the selectivity increases with the increase of ionic radii difference. The authors rationalize the results as a result of the supramolecular multivalent cooperativity, the structural stability of the cage, and favorable coordination sites. Referring to the picture in monophase reaction, the theoretical separation factor (SF) of most lanthanide pairs should  reach several hundred in ideal conditions, as probed by nuclear magnetic resonance spectroscopy. However, affected by the poor water stability of the cage, the actual SF values (2.1-87.7, Fig. 2c) in extraction experiments are relatively lower than expected. Although comparable to most traditional solvent extraction systems 18 , the present results imply a big margin for improvement by employing more stable tetrahedral frameworks. Control experiments applying mono-and bis-tridentate analogs were performed to construct similar cages but resulted in poor performance in stability and selectivity, which highlights the significance of the ligand design in such an exquisite self-assembled system. This study shows how to make use of multivalent cooperativity in a self-assembled system for metal ion separation and provides new insights into the design of next-generation extractants for lanthanide separation.
Actinide separation is crucial for recovery and recycling of key radionuclides in the used nuclear fuel. Shi, Mei, and co-workers proposed a nanoextraction strategy for efficient separation of actinides by multivalent assembly of well-defined metal-organic nanoclusters 19 . The macrocyclic ligand pyrogallol [4]arene (PgC n ) can chelate with uranyl ions to afford a hexameric coordination cage [(UO 2 ) 24 (H 2 O) 24 (PgC n ) 6 ] ([U 24 (PgC n ) 6 ], Fig. 3a) both in solution and the solid state. Generally, the exquisite multinuclear assembly imposes strict requirements on both the molecular geometries of the ligands and their coordination preference for the targeted metal ions and thus biases the desired metal ions and excludes the undesired ones. Along this path, comprehensive biphasic extraction studies were carried out. Notably, the PgC 9 shows two orders of magnitude higher distribution coefficient toward uranyl than most of the trivalent lanthanides (Fig. 3b) under optimized conditions, and the extracted species are characterized to be identical to the hexameric cage [U 24 (PgC n ) 6 ]. This result indicates that the solvent extraction mechanism follows the in situ assembly of multi-components to precise uranyl-polyphenolic clusters, and efficient separation is achieved without the necessity for preassembly of the clusters. It is worth mentioning that such a giant entity is fabricated simply by mixing the uranyl salt with pre-designed PgC n under solvothermal condition. As a comparison, traditional organic synthesis would demand much more efforts to reach this level of complexity. A representative case comes from Rebek's highly preorganized ligand 20 that challenges researchers by its overcomplicated stereochemistry architecture but, in effect, contains only six valid binding sites. This work offers excellent paradigm for how to construct complex framework by small units in a self-assembled way.

Outlook
The assembly of elaborate nano-systems with a precision analogous to Nature examples has long been a topic for extensive investigation. In comparison to the bio-assemblies, the selfassembly-induced multivalent cooperativity shown here are still in the infancy stage. Nevertheless, the above cases guide the way on how to build the complex from the simple and lever the big by the small. Moreover, this idea might also be applicable to separate other metal ions or chemical feedstocks. Future efforts are expected in optimizing the ligand design for improved efficiency and creating architectures of even greater complexity.