Fabrication of Self‐Expanding Metal–Organic Cages Using a Ring‐Openable Ligand

• Published in: Angewandte Chemie International Edition Vol. 63; no. 17; pp. e202404155 - n/a
• Main Authors: Nishijima, Ami, Osugi, Yuto, Uemura, Takashi
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 22.04.2024
• Edition: International ed. in English
• Subjects: Benzene; Cage compounds; Cages; Coordination numbers; Fabrication; Holes; Ligands; Macrocyclic ligands; Molecular recognition; Ozonolysis; Stilbene; Tethering; Topology; Ozonolysis; Macrocyclic ligands; Molecular recognition; Cage compounds
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202404155

Metal–organic cages (MOCs), which are formed via coordination‐driven assembly, are being extensively developed for various applications owing to the utility of their accessible molecular‐sized cavity. While MOC structures are uniquely and precisely predetermined by the metal coordination number and ligand configuration, tailoring MOCs to further modulate the size, shape, and chemical environment of the cavities has become intensively studied for a more efficient and adaptive molecular binding. Herein, we report self‐expanding MOCs that exhibit remarkable structural variations in cage size and flexibility while maintaining their topology. A cyclic ligand with an oligomeric chain tethering the two benzene rings of stilbene was designed and mixed with RhII ions to obtain the parent MOCs. These MOCs were successfully transformed into expanded MOCs via the selective cleavage of the double bond in stilbene. The expanded MOCs could effectively trap multidentate N‐donor molecules in their enlarged cavity, in contrast to the original MOCs with a narrow cavity. As the direct synthesis of expanded MOCs is impractical because of the entropically disfavored structures, self‐expansion using ring‐openable ligands is a promising approach that allows precision engineering and the production of functional MOCs that would otherwise be inaccessible. The framework of a metal–organic cage can be expanded drastically with its topology retained (self‐expansion). A rigid RhII6 cage with an oligomer‐functionalized, stilbene‐type dicarboxylic ligand transformed into a flexible cage upon cleavage of C=C double bond and simultaneous ring‐opening. The dramatic increase in the cavity size enabled the effective trapping of multidentate N‐donor guest molecules.