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.