Ligand Isomerism in Palladium(II)-Based Multi-Cavity Discrete Coordination Cages

The term ligand isomerism stands for two or more isomeric coordination complexes having regioisomeric ligands coordinated around the metal center. Single-cavity discrete coordination cages (SCDCCs) and multi-cavity discrete coordination cages (MCDCCs) are exotic class of self-assembled complexes that should be suitable for exploration of ligand isomerism. This work describes rare varieties of double-cavity tetranuclear, triple-cavity pentanuclear and quadruple-cavity hexanuclear MCDCCs to exemplify ligand isomerism. Square planar Pd(II) and pyridine-based bis-, tris- and tetrakis-monodentate ligands are employed as the modular building blocks for constructing the cages. The frameworks of all the ten cages studied here (four reported and six new) contain trinuclear Pd₃L₆ type double-walled triangular core (or sub-framework) that is decorated with one, two and three units of Pd₂L₄ type entity or sub-framework resulting in tetra, penta and hexanuclear MCDCCs, respectively. Suitable incorporation of isomeric arms as part of the double-walled trinuclear core by sourcing from the basket of regioisomeric ligands would offer ligand isomerism in the MCDCCs. Our ligand design afforded four members for the tetra or pentanuclear and two for the hexanuclear architectures to demonstrate ligand isomerism in MCDCCs.