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 PdII 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 PdII center at the TS geometries increases the activation free energy of the two steps.