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 firstly revealed by a recently developed QASAP (quantitative analysis of self-assembly process), with which the self-assembly process of the coordination assemblies can be investigated by monitoring the time evolution 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 calculation of the TS (transition state) structures for the two steps indicates that the distortion of the trigonal-bipyramidal Pd(II) center at the TS geometries increases the activation free energy of the two steps.