Quantitative Analysis of Self-Assembly Process of a Pd₂ L₄ Cage Consisting of Rigid Ditopic Ligands

The self-assembly process of a Pd₂ L₄ 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: [Pd₂ L₄ Py*₂ ]⁴⁺ →[Pd₂ L₄ Py*₁ ]⁴⁺ +Py* and [Pd₂ L₄ Py*₁ ]⁴⁺ →[Pd₂ L₄ ]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.