Self-Assembly Drives Rare-Earth Separations in a Family of M4L4 Cages

Rare-earth elements are indispensable to high-tech applications, making efficient rare-earth separations increasingly essential. An emerging approach to these challenging separations is the self-assembly of lanthanide-based supramolecular cages. In this study, we establish design principles for self-assembly-driven rare-earth separations by investigating five ligands with distinct substituents (L-Cbz, L-propyl, L-alkene, L-benzyl, L-NH2), two polymer networks based on these ligands, and a monotopic fragment of the tritopic ligand structure. The ligand L-CBz shows a unique ability to separate lanthanides compared to the monotopic ligand and the polymer networks, underscoring the importance of cage self-assembly to separation performance. We found that L-Cbz forms M4L4 cages with middle-to-late rare-earth ions (Eu3+, Tb3+, Dy3+, Y3+, Ho3+, Lu3+) but a mixture of M2L2/M4L4 species with larger lanthanides (La3+, Pr3+, Nd3+, Sm3+), while L-propyl forms cages with all tested rare-earth ions, with minor La2L2. Single crystal X-ray diffraction shows that pure L-Cbz precipitates from La2(L-Cbz)2 solution, suggesting that M2L2 is an unstable intermediate. In contrast, L-alkene and L-benzyl display size-dependent thresholds for cage formation, beginning at Sm3+ and Pr3+, respectively. Our results suggest that ligand-arm flexibility modulates these thresholds, providing design rules for L-based lanthanide separations. Extended X-ray absorption fine structure (EXAFS) measurements, corroborated by DFT calculations, further reveal a correlation between M–N bond length and a ligand’s propensity for cage formation. Collectively, these insights provide practical guidelines for designing ligands for self-assembly-based rare-earth separations.