The self-assembly process of a Pd L cage complex consisting of rigid ditopic ligands, in which two 3-pyridyl groups are connected to a benzene ring through acetylene bonds and Pd 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: [Pd L Py* ] →[Pd L Py* ] +Py* and [Pd L Py* ] →[Pd L ] +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 , respectively. DFT calculations of the transition-state (TS) structures for the two steps indicated that the distortion of the trigonal-bipyramidal Pd center at the TS geometries increases the activation free energy of the two steps.

The effect of reaction environment on the self-assembly process of an octahedron-shaped PdL capsule was investigated. Quantitative analysis of self-assembly process with H NMR spectroscopy revealed that the self-assembly pathway of the capsule was altered by solvent and a leaving ligand coordinating to the metal source, which are not the components of the final self-assembly. Solvents definitively determine the pathway ofthe self-assembly at a very early stage of the self-assembly. Contrary to the expectation that the weaker the coordination ability of the leaving ligand is, the faster the formation of the final assembly becomes, a leaving ligand with weak coordination ability tends to generate a kinetically trapped species to prevent the capsule formation under mild conditions.

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