Self-assembly processes of octahedron-shaped Pd6L12 cages

Abstract

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 [Pd_mL_2m]^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.