Self-assembly processes of octahedron-shaped Pd₆L₁₂ cages

Self-assembly processes of three octahedron-shaped [Pd6L12]12+ cages were investigated by an NMR-based quantitative approach. As to the on-pathway of the Pd6L12 cage assembly, the final intramolecular ligand exchange in an incomplete cage, [Pd6L12Py*]12+ (Py*: 3-chloropyridine, which was used as a leaving ligand), is the rate-determining step in the self-assembly of all the three [Pd6L12]12+ cages. Contrary to the previous finding that the self-assembly of [PdmL2m]2m+ structures (m = 2, 3) and [Pd6L8]12+ capsules from rigid multitopic ligands efficiently takes place without the formation of kinetically trapped species under mild conditions, in the self-assembly of the [Pd6L12]12+ cages, even relatively rigid ditopic ligands co-produced 100 nm-sized kinetic traps through off-pathways, which would be because the energy landscape becomes more complicated by increasing the number of components in the final assembly. It was found that when Py* was used as a leaving ligand in CD3CN, the [Pd6L12]12+ cages were produced in high yield, preventing the formation of the kinetically trapped species, which indicates that the use of Py* as a leaving ligand in CD3CN is effective to obtain the thermodynamically most stable species.