Self-Assembly of a Pd4L8 Double-Walled Square Takes Place through Two Kinds of Metastable Species.

The self-assembly process of a Pd4L8 double-walled square (DWS) was investigated. As was seen in Pd2L4 cages, Pd(II)-linked coordination self-assembly processes are generally affected by the rigidity of multitopic ligands. However, the self-assembly of a Pd4L8 DWS from rigid ditopic ligands took place with the formation of two kinds of metastable species [submicrometer-sized species and a Pd3L6 double-walled triangle (DWT)]. This result suggests that the self-assembly process of the DWS is largely affected by the geometry of the final product and not by that of the ditopic ligand.

How does chiral self-sorting take place in the formation of homochiral Pd6L8 capsules consisting of cyclotriveratrylene-based chiral tritopic ligands?

The chiral self-sorting process during the self-assembly of homochiral Pd6L8 capsules from cyclotriveratrylene (CTV)-based chiral tritopic ligands (L) and (Py*: 3-chloropyridine) was investigated by an NMR-based approach (QASAP: quantitative analysis of the self-assembly process). From the beginning to the formation of the immature capsules (ICs), enantiomeric ligands are distributed in the intermediates in a non-self-sorting manner, which leads to the isomers of heterochiral ICs over 99% yield. The mismatch of the chirality in the heterochiral ICs prevents intramolecular ligand exchanges in ICs to form the heterochiral capsules. The correction of the chirality in the heterochiral ICs (chiral self-sorting) takes place very slowly to finally lead to the homochiral capsules. The reason why the chiral self-sorting took place in the late stage of the self-assembly (after the formation of the heterochiral ICs) would be due to the relatively high flexibility of the CTV-based ligand.

Two dominant self-assembly pathways to a Pd3L6 double-walled triangle.

An NMR-based quantitative analysis of self-assembly process (QASAP) for the self-assembly of a Pd3L6 double-walled triangle (DWT) from PdPy*4(BF4)2 (Py* indicates 3-chloropyridine) and V-shaped ditopic ligands (L) revealed that DWT was assembled through trinuclear single-walled chains, in which the intramolecular macrocyclization and the formation of double-walls took place leading to the final assembly.

Energy-Landscape-Independent Kinetic Trap of an Incomplete Cage in the Self-Assembly of a Pd2 L4 Cage.

A kinetic trap is the metastable species that is transiently or constantly produced during the reaction by trapping in a deep energy well. In most cases, the reactivity of kinetically trapped species is relatively low under the reaction conditions. Herein, we report another type of kinetically trapped species that is an incomplete cage (IC) intermediate produced during the self-assembly of a Pd2 L4 cage from ditopic ligand (L) and PdII ions with a certain lifetime, although IC has a high enough reactivity to be converted into the cage with the reaction of free L, which was confirmed by the reaction of the isolated IC and L under the self-assembly conditions. IC was kinetically trapped not because IC lies on the bottom of a deep energy well but because the conversion of the intermediates essential for the conversion of IC to the cage preferentially takes place; IC was kinetically trapped independently of the shape of the energy landscape of the self-assembly.

Flexibility of components alters the self-assembly pathway of Pd2L4 coordination cages.

The self-assembly process of a Pd2L4 cage consisting of flexible ditopic ligands and Pd(ii) ions was revealed by QASAP (quantitative analysis of self-assembly process), which enables one to obtain information about the intermediates transiently produced during the self-assembly as the average composition of all the intermediates. It was found that the dominant pathway to the cage is the formation of a submicrometre-sized sheet structure, which was characterized by dynamic light scattering (DLS) and scanning transmission electron microscopy (STEM), followed by the addition of free ditopic ligands to the Pd(ii) centres of the sheet structure to trigger the cage formation. This assembly process is completely different from that of a Pd2L4 cage composed of rigid ditopic ligands, indicating that the flexibility of the components strongly affects the self-assembly process.

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

Self-Assembly Process of a Pd2 L4 Capsule: Steric Interactions between Neighboring Components Favor the Formation of Large Intermediates.

The effect of molecular interactions between the components on the self-assembly process of Pd2 L4 structures was investigated by a 1 H NMR-based quantitative approach (QASAP: quantitative analysis of self-assembly process). Although the self-assembly of the Pd2 L4 cage without interactions between the bent ligands took place, mainly producing small intermediates, the self-assembly of the Pd2 L4 capsule composed of bent ligands with anthracene panels tends to produce large intermediates containing more components than the capsule. This is ascribed to steric interactions between the panels.

Quantitative Analysis of the Self-Assembly Process of a Pd12 L24 Coordination Sphere.

The self-assembly process of a Pd12 L24 sphere was revealed by a quantitative approach (quantitative analysis of self-assembly process: QASAP) quantifying all the substrates, the products, and the observable intermediates, indicating that the Pd12 L24 sphere is produced through several pathways. Firstly, Pdn L2n (n=6, 8, and 9), which are perfectly closed structures smaller than the Pd12 L24 sphere, and a mixture of intermediates not observed by NMR (Int) were produced. Next, the sphere was assembled from intra-/intermolecular reaction of a certain class of Int (path A) and from the coordination of free pyridyl groups in Int to the PdII center of Pdn L2n (n=6, 8, and 9) (path B). While capping the free pyridyl groups in Int with PdII ions perfectly inhibited the sphere formation, the addition of free L to Int accelerated the formation of the sphere.

The Effect of Solvent and Coordination Environment of Metal Source on the Self-Assembly Pathway of a Pd(II)-Mediated Coordination Capsule.

The effect of reaction environment on the self-assembly process of an octahedron-shaped Pd6L8 capsule was investigated. Quantitative analysis of self-assembly process with 1H 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 of the 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.

Homochiral Self-Sorted and Emissive IrIII Metallo-Cryptophanes.

The racemic ligands (±)-tris(isonicotinoyl)-cyclotriguaiacylene (L1), or (±)-tris(4-pyridyl-methyl)-cyclotriguaiacylene (L2) assemble with racemic (Λ,Δ)-[Ir(ppy)2 (MeCN)2 ]+ , in which ppy=2-phenylpyridinato, to form [{Ir(ppy)2 }3 (L)2 ]3+ metallo-cryptophane cages. The crystal structure of [{Ir(ppy)2 }3 (L1)2 ]⋅3BF4 has MM-ΛΛΛ and PP-ΔΔΔ isomers, and homochiral self-sorting occurs in solution, a process accelerated by a chiral guest. Self-recognition between L1 and L2 within cages does not occur, and cages show very slow ligand exchange. Both cages are phosphorescent, with [{Ir(ppy)2 }3 (L2)2 ]3+ having enhanced and blue-shifted emission when compared with [{Ir(ppy)2 }3 (L1)2 ]3+ .