A supramolecular lanthanide separation approach based on multivalent cooperative enhancement of metal ion selectivity

Multivalent cooperativity plays an important role in the supramolecular self-assembly process. Herein, we report a remarkable cooperative enhancement of both structural integrity and metal ion selectivity on metal-organic M4L4 tetrahedral cages self-assembled from a tris-tridentate ligand (L-1) with a variety of metal ions spanning across the periodic table, including alkaline earth (Ca-II), transition (Cd-II), and all the lanthanide (Ln(III)) metal ions. All these M4L41 cages are stable to excess metal ions and ligands, which is in sharp contrast with the tridentate (L-2) ligand and bis-tridentate (L-3) ligand bearing the same coordination motif as L-1. Moreover, high-precision metal ion self-sorting is observed during the mixed-metal selfassembly of tetrahedral M4L4 cages, but not on the M2L3 counterparts. Based on the strong cooperative metal ion self-recognition behavior of M4L4 cages, a supramolecular approach to lanthanide separation is demonstrated, offering a new design principle of next-generation extractants for highly efficient lanthanide separation.