Metal-macrocyclic framework featuring adaptive cavity for precise palladium recognition

The rational synthesis of advanced porous materials with performances mimicking those of biological proteins remains challenging. Evidently, metal-organic frameworks with dense, rigid aromatic motifs hinder the creation of adaptable cavities with superior guest-to-framework interactions. Herein, a zirconium macrocyclic framework, named metal-macrocyclic framework (MMCF)-6, featuring an adaptive cavity, was constructed using a cyclam-based linker. The vacant yet adaptable cyclam cavity in MMCF-6 renders it capable of precisely recognizing Pd2+ in aqueous solution with a high uptake capacity of 326 mg g-1 and extraordinary recovery efficiency of >99.99%. The characteristic proteomimetic behavior of MMCF-6, that is, guest-induced fit during the Pd2+ recovery, was unveiled by combined studies of X-ray crystallography, extended X-ray absorption fine structure, and density functional theory, which provided unambiguous confirmation of the cyclam macrocycle's allosteric behavior accountable for the excellent Pd capture performance. Our work herein opens a new avenue by incorporating semi-rigid cavities into framework materials for efficient host-guest chemistry.