Backbone-Bridging Promotes Diversity in Heteroleptic Cages

The combination of shape-complementary bis-monodentate ligands L^A and L^B with Pd^II cations yields heteroleptic cages cis-[Pd₂ L^A₂ L^B₂ ] by self-sorting. Herein, we report how such assemblies can be diversified by introduction of covalent backbone bridges between two L^A units. Together with solvent and guest effects, the flexibility of these linkers can modulate nuclearity, topology, and number of cavities in a family of four structurally diverse assemblies. Ligand L^A1 , with flexible linker, reacts in CH₃ CN with its L^B counterpart to a tetranuclear dimer D1. In DMSO, however, a trinuclear pseudo-tetrahedron T1 is formed. The product of L^A2 , with rigid linker, looks similar to D1, but with a rotated ligand arrangement. In presence of an anionic guest, this dimer D2 transforms and a hexanuclear prismatic barrel P2 crystallizes. We demonstrate how controlling a ligand's coordination mode can trigger structural differentiation and increase complexity in metallo-supramolecular assembly.