Fine-Tuning Conformational Motion of a Self-Assembled Metal-Organic Macrocycle by Multiple CH⋅⋅⋅Anion Hydrogen Bonds

Adaptive self-assembly and induced-fit transformations of anion-binding metal-organic macrocycles

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.