Polycatenanes Formed of Self-Assembled Metal-Organic Cages

Poly-[n]-catenanes (PCs) self-assembled of three-dimensional (3D) metal organic cages (MOCs) (hereafter referred to as PCs-MOCs) are a relatively new class of mechanically interlocked molecules (MIMs) that combine the properties of MOCs and polymers. The synthesis of PCs-MOCs is challenging because of the difficulties associated with interlocking MOCs, the occurrence of multiple weak supramolecular electrostatic interactions between cages, and the importance of solvent templating effects. The high density of mechanical bonds interlocking the MOCs endows the MOCs with mechanical and physical properties such as enhanced stability, responsive dynamic behavior and low solubility, which can unlock new functional properties. In this Minireview, we highlight the benefit of interlocking MOCs in the formation of PCs-MOCs structures as well as the synthetic approaches exploited in their preparation, from thermodynamic to kinetic methods, both in the solution and solid-states. Examples of PCs-MOCs self-assembled from various types of nanosized cages (i.e., tetrahedral, trigonal prismatic, octahedral and icosahedral) are described in this article, providing an overview of the research carried out in this area. The focus is on the structure-property relationship with examples of functional applications such as electron conductivity, X-ray attenuation, gas adsorption and molecular sensing. We believe that the structural and functional aspects of the reviewed PCs-MOCs will attract chemists in this research field with great potential as new functional materials in nanotechnological disciplines such as gas adsorption, sensing and photophysical properties such as X-ray attenuation or electron conductivity.

Square-planar Tetranuclear Cluster-based Alkaline Earth Metal-organic Frameworks with Enhanced Proton Conductivity

Alkaline earth (AE) metal complexes have garnered significant interest in various functional fields due to their nontoxicity, low density, and low cost. However, there is a lack of systematic investigation into the structural characteristics and physical properties of AE-metal-organic frameworks (MOFs). In this research, we synthesized isostructural MOFs consisting of AE4(μ4-Cl) clusters bridged by benzo-(1,2;3,4;5,6)-tris(thiophene-2'-carboxylic acid) (BTTC3-) ligands. The resulting structure forms a truncated octahedral cage denoted as [AE4(m4-Cl)]6(BTTC)8, which further linked to a porous three-dimensional framework. Among the investigated AE ions (Ca, Sr, and Ba), the Ca4-MOF demonstrated good chemical stability in water compared to Sr4-MOF and Ba4-MOF. The N2 adsorption and solid-state UV-vis-NIR absorption behaviors were evaluated for all AE4-MOFs, showing similar trends among the different metal ions. Additionally, the proton conduction study revealed that the Ca4-MOF exhibited ultra-high proton conductivity, reaching 3.52×10-2 S cm-1 at 343 K and 98 % RH. Notably, the introduction of LiCl via guest exchange resulted in an improved proton conduction of up to 6.36×10-2 S cm-1 under similar conditions in the modified LiCl@Ca4-MOF. The findings shed light on the regulation of physical properties and proton conductivity of AE-MOFs, providing valuable insights for their potential applications in various fields.

Kinetic trapping of 2,4,6-tris(4-pyridyl)benzene and ZnI2 into M12L8 poly-[n]-catenanes using solution and solid-state processes

Here, we show that in a supramolecular system with more than 20 building blocks forming large icosahedral M₁₂L₈ metal-organic cages (MOCs), using the instant synthesis method, it is possible to kinetically trap and control the formation of interlocking M₁₂L₈ nanocages, giving rare M₁₂L₈ TPB-ZnI₂ poly-[n]-catenane. The catenanes are obtained in a one-pot reaction, selectively as amorphous (a1) or crystalline states, as demonstrated by powder X-ray diffraction (powder XRD), thermogravimetric (TG) analysis and ¹H NMR. The 300 K M₁₂L₈ poly-[n]-catenane single crystal X-ray diffraction (SC-XRD) structure including nitrobenzene (1) indicates strong guest binding with the large M₁₂L₈ cage (i.e., internal volume ca. 2600 ų), allowing its structural resolution. Conversely, slow self-assembly (5 days) leads to a mixture of the M₁₂L₈ poly-[n]-catenane and a new TPB-ZnI₂ (2) coordination polymer (i.e., thermodynamic product), as revealed by SC-XRD. The neat grinding solid-state synthesis also yields amorphous M₁₂L₈ poly-[n]-catenane (a1'), but not coordination polymers, selectively in 15 min. The dynamic behavior of the M₁₂L₈ poly-[n]-catenanes demonstrated by the amorphous-to-crystalline transformation upon the uptake of ortho-, meta- and para-xylenes shows the potential of M₁₂L₈ poly-[n]-catenanes as functional materials in molecular separation. Finally, combining SC-XRD of 1 and DFT calculations specific for the solid-state, the role of the guests in the stability of the 1D chains of M₁₂L₈ nanocages is reported. Energy interactions such as interaction energies (E), lattice energies (E*), host-guest energies (E_host-guest) and guest-guest energies (E_guest-guest) were analysed considering the X-ray structure with and without the nitrobenzene guest. Not only the synthetic control achieved in the synthesis of the M₁₂L₈ MOCs but also their dynamic behavior either in the crystalline or amorphous phase are sufficient to raise scientific interest in areas ranging from fundamental to applied sides of chemistry and material sciences.

Experimental X-ray and DFT Structural Analyses of M12L8 Poly-[n]-catenanes Using exo-Tridentate Ligands

Despite their potential applications in host-guest chemistry, there are only five reported structures of poly-[n]-catenanes self-assembled by elusive M₁₂L₈ icosahedral nanocages. This small number of structures of M₁₂L₈ poly-[n]-catenanes is because self-assembly of large metal-organic cages (MOCs) with large windows allowing catenation by means of mechanical bonds is very challenging. Structural reports of M₁₂L₈ poly-[n]-catenanes are needed to increase our knowledge about the self-assembly and genesis of such materials. Poly-[n]-catenane (1·p-CT) self-assembly of interlocked M₁₂L₈ icosahedral cages (M = Zn(II) and L = 2,4,6-tris-(4-pyridyl)benzene (TPB)) including a new aromatic guest (p-chlorotoluene (p-CT)) is reported by single-crystal XRD. Despite the huge internal M₁₂L₈ voids (> 2500 ų), p-CT is ordered, allowing a clear visualization of the relative host-guest positions. DFT calculations have been used to compute the electrostatic potential of the TPB ligand, and various aromatic guests (i.e., o-dichlorobenzene (o-DCB), p-chloroanisole (p-CA), and nitrobenzene (NBz)) included (ordered) within the M₁₂L₈ cages were determined by single-crystal XRD. The computed maps of electrostatic potential (MEPs) allow for the rationalization of the guest's inclusion seen in the 3D X-ray structures. Although more crystallographic X-ray structures and DFT analysis are needed to gain insights of guest inclusion in the large voids of M₁₂L₈ poly-[n]-catenanes, the reported combined experimental/DFT structural analyses approach can be exploited to use isostructural M₁₂L₈ poly-[n]-catenanes as hosts for molecular separation and could find applications in the crystalline sponge method developed by Fujita and co-workers. We also demonstrate, exploiting the instant synthesis method, in solution (i.e., o-DCB), and in the solid-state by neat grinding (i.e., without solvent), that the isostructural M₁₂L₈ poly-[n]-catenane self-assembled with 2,4,6-tris-(4-pyridyl)pyridine (TPP) ligand and ZnX₂ (where X = Cl, Br, and I) can be kinetically synthesized as crystalline (yields ≈ 60%) and amorphous phases (yields ≈ 70%) in short time and large quantities. Despite the change in the aromatic nature at the center of the rigid exo-tridentate pyridine-based ligand (TPP vs TPB), the kinetic control gives the poly-[n]-catenanes selectively. The dynamic behavior of the TPP amorphous phases upon the uptake of aromatic guest molecules can be used in molecular separation applications like benzene derivatives.

Benzotrithiophene-based MOFs: interchromophoric interactions affected Ln(III) crystallization selectivity and optoelectronic properties

Metal-organic frameworks (MOFs) provide an ideal platform for the assembly of chromophores and thus show wide potential applications in optoelectronic devices. The spatial arrangement and interaction of the incorporated chromophores play a key role in the generation of coherent optical and electronic properties. In this work, two series of benzo-(1,2;3,4;5,6)-tristhiophene (BTT) based Ln-MOFs (Ln-1s and Ln-2s) were synthesized. These two series of MOFs present different assembly states of BTT chromophores, that is, BTT-containing ligands exist as separated monomers in Ln-1s but gather as dimers in Ln-2s. From the comparison between these two series of MOFs and theoretical calculations, we show for the first time that this chromophore assembly state difference could affect the crystallization selectivity of MOFs towards different Ln³⁺ ions. In addition, the interaction between BTT chromophores in the dimer also leads to the red-shifted photoluminescence and enhanced photocurrent of Ln-2s relative to those of Ln-1s. The results of this work demonstrate the multiple functions of interchromophoric interactions in the structures and optoelectronic properties of MOFs.

An anionic zeolite-like metal–organic framework (AZMOF) with a Moravia network for organic dye absorption through cation-exchange

We have synthesized a new anionic zeolite-like metal–organic framework (AZMOF) with a twisted partially augmentedthenet, known as the “Moravia” net which is able to selectively adsorb organic cationic dyes (BR, RB, CV and MB) through cation-exchange.