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 M 4 L 4 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 M 4 L 1 4 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 self-assembly of tetrahedral M 4 L 4 cages, but not on the M 2 L 3 counterparts. Based on the strong cooperative metal ion self-recognition behavior of M 4 L 4 cages, a supramolecular approach to lanthanide separation is demonstrated, offering a new design principle of next-generation extractants for highly efficient lanthanide separation.