Statistical Distribution of Binary Ligands within Rhodium-Organic Octahedra Tunes Microporosity in Their Assemblies.

Structure-porosity relationships for metal-organic polyhedra (MOPs) are hardly investigated because they tend to be amorphized after activation, which inhibits crystallographic characterization. Here, we show a mixed-ligand strategy to statistically distribute two distinct carbazole-type ligands within rhodium-based octahedral MOPs, leading to systematic tuning of the microporosity in the resulting amorphous solids.

Unexpected Self-Assembly Pathway to a Pd(II) Coordination Square-Based Pyramid and Its Preferential Formation beyond the Boltzmann Distribution.

Experimental and theoretical investigations of the self-assembly process of a Pd(II) coordination M6L4 square-based pyramid (SP) were conducted. It was found that the probable self-assembly pathway, in which the dimerization of M2L2 with two M leads to SP, expected from the connectivity of the building blocks is not a major self-assembly pathway to the M6L4 SP. Whether the M6L4 SP is assembled or M2L2 is trapped is determined by an inter- or intramolecular reaction in a chain-like M2L2X, where X is a leaving ligand. The kinetically trapped state where the M6L4 SP is produced from M2L2 beyond the Boltzmann distribution was realized by a concentration-induced process and was kept for at least 2 months at 298 K.

Towards kinetic control of coordination self-assembly: a case study of a Pd3L6 double-walled triangle to predict the outcomes by a reaction network model.

Numerical analysis of self-assembly process (NASAP) was performed for a [Pd3L6]6+ double-walled triangle (DWT) complex. With a chemical reaction network and a parameter set of the reaction rate constants obtained from a numerical search in an eighteen-dimensional parameter space to obtain a good fit to the data from the experimental counterpart (quantitative analysis of self-assembly process, QASAP), a refined calculation resulted in a detailed time evolution of each molecular species. Analysis based on those clues revealed dominant self-assembly pathways and a balance between inter- and intramolecular reactions, and enabled prediction of the reaction outcomes depending on the initial stoichiometric ratio under kinetic control.

Navigated Self-Assembly of a Pd2L4 Cage by Modulation of an Energy Landscape under Kinetic Control.

Kinetic control of molecular self-assembly remains difficult because of insufficient understanding of molecular self-assembly mechanisms. Here we report the formation of a metastable [Pd2L4]4+ cage structure composed of naphthalene-based ditopic ligands (L) and Pd(II) ions in very high yield (99%) under kinetic control by modulating the energy landscape. When self-assembly occurs with anionic guests in weakly cooordinating solvent then suitable intermedites and the metastable cage is formed. These conditions also prevent further transformation into the thermodynamically decomposed state. The cage formation pathways under kinetic control and the effect of the anions encapsulated on the self-assembly processes were investigated by QASAP (quantitative analysis of self-assembly process) and NASAP (numerical analysis of self-assembly process). It was found that the self-assembly with a preferred guest (BF4-) proceeds through intermediates composed of no more components than the cage ([PdaLbXc]2a+ (a ≤ 2, b ≤ 4, X indicates a leaving ligand)) and that the final intramolecular cage-closure step is the rate-determining step. In contrast, a weaker guest (OTf-) causes the transient formation of intermediates composed of more components than the cage ([PdaLbXc]2a+ (a > 2, b > 4)), which are finally converted into the cage.

Multiple Pathways in the Self-Assembly Process of a Pd4L8 Coordination Tetrahedron.

The self-assembly of a Pd418 coordination tetrahedron (Tet) from a ditopic ligand, 1, and palladium(II) ions, [PdPy*4]2+ (Py* = 3-chloropyridine), was investigated by a 1H NMR-based quantitative approach (quantitative analysis of self-assembly process, QASAP), which allows one to monitor the average composition of the intermediates not observed by NMR spectroscopy. The self-assembly of Tet takes place mainly through three pathways and about half of the Tet structures were produced through the reaction of a kinetically produced Pd3L6 double-walled triangle (DWT) and 200-nm-sized large intermediates (IntL). In two of the three pathways, the leaving ligand (Py*), which is not a component of Tet, catalytically assisted the self-assembly. Such a multiplicity of the self-assembly process of Tet suggests that molecular self-assembly takes place on an energy landscape like a protein-folding funnel.

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.

Self-assembly processes of octahedron-shaped Pd6L12 cages.

Self-assembly processes of three octahedron-shaped [Pd6L12]12+ cages were investigated by an NMR-based quantitative approach. As to the on-pathway of the Pd6L12 cage assembly, the final intramolecular ligand exchange in an incomplete cage, [Pd6L12Py*]12+ (Py*: 3-chloropyridine, which was used as a leaving ligand), is the rate-determining step in the self-assembly of all the three [Pd6L12]12+ cages. Contrary to the previous finding that the self-assembly of [PdmL2m]2m+ structures (m = 2, 3) and [Pd6L8]12+ capsules from rigid multitopic ligands efficiently takes place without the formation of kinetically trapped species under mild conditions, in the self-assembly of the [Pd6L12]12+ cages, even relatively rigid ditopic ligands co-produced 100 nm-sized kinetic traps through off-pathways, which would be because the energy landscape becomes more complicated by increasing the number of components in the final assembly. It was found that when Py* was used as a leaving ligand in CD3CN, the [Pd6L12]12+ cages were produced in high yield, preventing the formation of the kinetically trapped species, which indicates that the use of Py* as a leaving ligand in CD3CN is effective to obtain the thermodynamically most stable species.

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.