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 -[Pd 2 L A 2 L B 2 ] by non-statistical self-assembly. Here 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 is shown to modulate the nuclearity, topology and number of distinguishable cavities in a family of four structurally highly diverse supramolecular assemblies. Ligand-dimer L A1 , with flexibly connected backbones, reacts in acetonitrile with its L B counterpart to a tetranuclear back-to-back dimer of heteroleptic cages D1 . In DMSO, however, a trinuclear pseudo-tetrahedral structure T1 is formed. The product of L A2 , with rigidly connected backbones, 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.