Quantitative Analysis of Self-Assembly Process of a Pd2L4 Cage Consisting of Rigid Ditopic Ligands

The self-assembly process of a Pd2L4 cage complex consisting of rigid ditopic ligands, in which two 3-pyridyl groups are connected to a benzene ring through acetylene bonds and Pd-II ions was revealed by a recently developed quantitative analysis of self-assembly process (QASAP), with which the self-assembly process of coordination assemblies can be investigated by monitoring the evolution with time of the average composition of all the intermediates. QASAP revealed that the rate-determining steps of the cage formation are the intramolecular ligand exchanges in the final stage of the self-assembly: [Pd2L4Py*(2)](4+) -> [Pd2L4Py*(1)](4+)+Py* and [Pd2L4Py*(1)](4+) -> [Pd2L4](4+) + Py* (Py*: 3-chloropyridine, which was used as a leaving ligand on the metal source). The energy barriers for the two reactions were determined to be 22.3 and 21.9 kcal mol(-1), respectively. DFT calculations of the transition-state (TS) structures for the two steps indicated that the distortion of the trigonal-bipyramidal Pd-II center at the TS geometries increases the activation free energy of the two steps.