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

Abstract

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.

Energy‐Landscape‐Independent Kinetic Trap of an Incomplete Cage in the Self‐Assembly of a Pd2L4 Cage

“…(2)] is more destabilized by the rigidity of IC. [28,29] It is true that the reactivity of IC is lower than that of PdPy* 4 ,b ut the significantly slow conversion of IC in the self-assembly of the cage in Equation (1) (3 days) compared with the reactiono ft he isolated IC and 1 in Equation (2) (7 h) indicates that the slightly low reactivity of IC alone is not the reasonf or the kinetic trap of IC.…”

Guest Exchange Mechanisms in Mono‐Metallic PdII/PtII‐Cages Based on a Tetra‐Pyridyl Calix[4]pyrrole Ligand

“…22 kcal mol À1 in abanana-shaped [L 4 ·Pd 2 ] 4+ cage. [15] In contrast, the octahedral [L 4 ·Pt 6 ] 12+ cage does not show signs of dissociative processes at room temperature (rt). [16] Raymond and co-workers investigated the guest exchange mechanisms of at etrahedral [L 6 ·Ga 4 ] 12À cage.S mall cationic guests,s uch as NEt 4 + ,are bound reversibly to [L 6 ·Ga 4 ] 12À via as lippage mechanism (i.e., deformation of ac agesf ace) featuring exchange rate constants k = 10 À3 to 10 s À1 at rt.…”

Self‐Assembly Processes of Pd(II)‐ and Pt(II)‐Linked Discrete Self‐Assemblies Revealed by QASAP

“…However, in some cases we observed almost constant value during the self‐assembly even though the production of the final self‐assembly continued. This special case indicates that the species whose ( n , k ) value is mainly exists as the intermediate and that the conversion of this species is the rate‐determining step in the self‐assembly process (see section 3.1) ,,…”

“…QASAP for the self‐assembly of Pd 2 1 4 cage from rigid ditopic ligand 1 and PdPy* 4 (Figure a) indicates that value stayed around (1.65, 0.5) during the production of the cage, suggesting that Pd 2 1 4 Py* 2 (( n , k )=(1.5, 0.5)) and Pd 2 1 4 Py* (( n , k )=(1.75, 0.5)) are the main intermediates and that both intermediates were consumed keeping their existence ratio constant . Based on these findings, the energy barriers of the intermolecular ligand exchanges in Pd 2 1 4 Py* 2 (Pd 2 1 4 Py* 2 Pd 2 1 4 Py*+Py*) and Pd 2 1 4 Py* (Pd 2 1 4 Py* Pd 2 1 4 +Py*) were determined to be 22.3 and 21.9 kcal mol −1 , respectively.…”

“…Considering this ligand exchange mechanism, the transition structures produced in the late stages of the self‐assembly of the cage should be highly distorted. Indeed, DFT calculations of the transition structures of the two intramolecular ligand exchanges indicate distortion around the Pd(II) centers with a trigonal‐bipyramidal coordination geometry …”