Chemists synthesized intermediate-sized metal-organic cages by connecting twisted carbon linkers to palladium atoms. Flat molecular linkers normally assemble into either tiny structures with two to six metals or large spheres with twelve or more metals, leaving medium sizes out of reach. Inserting internal twists and offset attachment points into naphthalene and pyrene molecules forced the binding sites into the exact angles required for intermediate cages.
Flat molecular linkers cannot create the lower symmetry required to close intermediate cage shells. Twisting the chemical backbone acts like angling the corner connectors on a tent frame. The twisted linkers orient their imidazole donor groups to lock palladium atoms into precise corner angles. This structural realignment satisfies the geometric demands of medium-sized cages without collapsing into smaller rings.
The research team paired bisimidazole-naphthalene and bisimidazole-pyrene ligands with palladium ions to test their structural design rules. Crystallographic analysis confirmed target coordination angles of approximately 105.1 degrees for sixteen-ligand complexes and 109.5 degrees for eighteen-ligand complexes. The reactions delivered thermodynamically stable nine-palladium tricapped trigonal prisms alongside multiple rare eight-palladium square antiprisms.
The authors state this strategy establishes a general route to intermediate-sized cages, bridging the gap between small and large structures. Their accompanying three-angle mathematical framework offers predictive guidelines to systematically control cage symmetry and topology in future syntheses.