Container-molecules are attractive to chemists due to their unique structural characteristics comparable to enzymes and receptors in nature. We report here a family of artificial self-assembled macrocyclic containers that feature induced-fit transformations in response to different anionic guests. Five metal-organic macrocycles with empirical formula of M_nL_2n (M=Metal; L=Ligand; n=3, 4, 5, 6, 7) are selectively obtained starting from one simple benzimidazole-based ligand and square-planar palladium(II) ions, either by direct anion-adaptive self-assembly or induced-fit transformations. Hydrogen-bonding interactions between the inner surface of the macrocycles and the anionic guests dictate the shape and size of the product. A comprehensive induced-fit transformation map across all the M_nL_2n species is drawn, with a representative reconstitution process from Pd₇L₁₄ to Pd₃L₆ traced in detail, revealing a gradual ring-shrinking mechanism. We envisage that these macrocyclic molecules with adjustable well-defined hydrogen-bonding pockets will find wide applications in molecular sensing or catalysis.

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 (L¹) 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 M₄L¹₄ cages are stable to excess metal ions and ligands, which is in sharp contrast with the tridentate (L²) ligand and bis-tridentate (L³) ligand bearing the same coordination motif as L¹. Moreover, high-precision metal ion self-sorting is observed during the mixed-metal self-assembly 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.