Cyclization or bridging: which occurs faster is the key to the self-assembly mechanism of Pd6L3 coordination prisms†
The self-assembly processes of Pd6L3 coordination prisms consisting of cis-protected Pd(II) complexes and porphyrin-based tetratopic ligands with four 3-pyridyl or 4-pyridyl groups (L) were investigated by experimental and numerical methods, QASAP (quantitative analysis of self-assembly process) and NASAP (numerical analysis of self-assembly process), respectively. It was found that contrary to common intuition macrocyclization takes place faster than the bridging reaction in the prism assembly and that the bridging reaction occurring before the macrocyclization tends to produce kinetically trapped species. A numerical simulation demonstrates that the relative magnitude of the rate constants between the macrocyclization and the bridging reaction is the key factor that determines whether the self-assembly leads to the thermodynamically most stable prism or to kinetically trapped species. Finding the key elementary reactions that largely affect the selection of the major assembly pathway is helpful to rationally control the products under kinetic control via modulation of the energy landscape.