Periphery-Functionalized Porous Organic Cages

Programmable synthesis of organic cages with reduced symmetry

Integrating symmetry-reducing methods into self-assembly methodology is desirable to efficiently realise the full potential of molecular cages as hosts and catalysts. Although techniques have been explored for metal organic (coordination) cages, rational strategies to develop low symmetry organic cages remain limited. In this article, we describe rules to program the shape and symmetry of organic cage cavities by designing edge pieces that bias the orientation of the amide linkages. We apply the rules to synthesise cages with well-defined cavities, supported by evidence from crystallography, spectroscopy and modelling. Access to low-symmetry, self-assembled organic cages such as those presented, will widen the current bottleneck preventing study of organic enzyme mimics, and provide synthetic tools for novel functional material design.

Chiral Covalent Organic Cages: Structural Isomerism and Enantioselective Catalysis

Covalent organic cages are a prominent class of discrete porous architectures; however, their structural isomerism remains relatively unexplored. Here, we demonstrate the structural isomerism of chiral covalent organic cages that renders distinct enantioselective catalytic properties. Imine condensations of tetra-topic 5,10-di(3,5-diformylphenyl)-5,10-dihydrophenazine and ditopic 1,2-cyclohexanediamine produce two chiral [4 + 8] organic cage isomers with totally different topologies and geometries that depend on the orientations of four tetraaldehyde units with respect to each other. One isomer (PN-1) has an unprecedented Johnson-type J26 structure, whereas another (PN-2) adopts a tetragonal prismatic structure. After the reduction of the imine linkages, the cages are transformed into two amine bond-linked isomers PN-1R and PN-2R. After binding to Ni(II) ions, both can serve as efficient catalysts for asymmetric Michael additions, whereas PN-2R affords obviously higher enantioselectivity and reactivity than PN-1R presumably because of its large cavity and open windows that can concentrate reactants for the reactions. Density-functional theory (DFT) calculations further confirm that the enantioselective catalytic performance varies depending on the isomer.

Design and assembly of porous organic cages

Porous organic cages (POCs) represent a notable category of porous materials, showing remarkable material properties due to their inherent porosity. Unlike extended frameworks which are constructed by strong covalent or coordination bonds, POCs are composed of discrete molecular units held together by weak intermolecular forces. Their structure and chemical traits can be systematically tailored, making them suitable for a range of applications including gas storage and separation, molecular separation and recognition, catalysis, and proton and ion conduction. This review provides a comprehensive overview of POCs, covering their synthesis methods, structure and properties, computational approaches, and applications, serving as a primer for those who are new to the domain. A special emphasis is placed on the growing role of computational methods, highlighting how advanced data-driven techniques and automation are increasingly aiding the rapid exploration and understanding of POCs. We conclude by addressing the prevailing challenges and future prospects in the field.

Homochiral organic molecular cage RCC3-R-modified silica as a new multimodal and multifunctional stationary phase for high-performance liquid chromatography

In this work, homochiral reduced imine cage was covalently bonded to the surface of the silica to prepare a novel high-performance liquid chromatography stationary phase, which was applied for the multiple separation modes such as normal phase, reversed-phase, ion exchange, and hydrophilic interaction chromatography. The successful preparation of the homochiral reduced imine cage bonded silica stationary phase was confirmed by performing a series of methods including X-ray photoelectron spectroscopy, thermogravimetric analysis, and infrared spectroscopy. From the extracted results of the chiral resolution in normal phase and reversed-phase modes, it was demonstrated that seven chiral compounds were successfully separated, among which the resolution of 1-phenylethanol reached the value of 3.97. Moreover, the multifunctional chromatographic performance of the new molecular cage stationary phase was systematically investigated in the modes of reversed-phase, ion exchange, and hydrophilic interaction chromatography for the separation and analysis of a total of 59 compounds in eight classes. This work demonstrated that the homochiral reduced imine cage not only achieved multiseparation modes and multiseparation functions performance with high stability, but also expanded the application of the organic molecular cage in the field of liquid chromatography.

Metal Organic Polygons and Polyhedra: Instabilities and Remedies

The field of coordination chemistry has undergone rapid transformation from preparation of monometallic complexes to multimetallic complexes. So far numerous multimetallic coordination complexes have been synthesized. Multimetallic coordination complexes with well-defined architectures are often called as metal organic polygons and polyhedra (MOPs). In recent past, MOPs have received tremendous attention due to their potential applicability in various emerging fields. However, the field of coordination chemistry of MOPs often suffer set back due to the instability of coordination complexes particularly in aqueous environment-mostly by aqueous solvent and atmospheric moisture. Accordingly, the fate of the field does not rely only on the water solubilities of newly synthesized MOPs but very much dependent on their stabilities both in solution and solid state. The present review discusses several methodologies to prepare MOPs and investigates their stabilities under various circumstances. Considering the potential applicability of MOPs in sustainable way, several methodologies (remedies) to enhance the stabilities of MOPs are discussed here.

A triple-diazonium reagent for virus crosslinking and the synthesis of an azo-linked molecular cage

The first bench-stable triple-diazonium reagent (TDA-1) was rationally designed and synthesized for coupling and crosslinking. The three reactive sites of TDA-1 can react with phenol-containing molecules as well as plant viruses in aqueous buffers efficiently. In addition, a new-type azo-linked cage was constructed by the direct reaction of TDA-1 with a triple-phenol molecule and was characterized by X-ray crystallography.

Purely Covalent Molecular Cages and Containers for Guest Encapsulation

Cage compounds offer unique binding pockets similar to enzyme-binding sites, which can be customized in terms of size, shape, and functional groups to point toward the cavity and many other parameters. Different synthetic strategies have been developed to create a toolkit of methods that allow preparing tailor-made organic cages for a number of distinct applications, such as gas separation, molecular recognition, molecular encapsulation, hosts for catalysis, etc. These examples show the versatility and high selectivity that can be achieved using cages, which is impossible by employing other molecular systems. This review explores the progress made in the field of fully organic molecular cages and containers by focusing on the properties of the cavity and their application to encapsulate guests.

Accelerating Solvent Selection for Type II Porous Liquids

Type II porous liquids, comprising intrinsically porous molecules dissolved in a liquid solvent, potentially combine the adsorption properties of porous adsorbents with the handling advantages of liquids. Previously, discovery of appropriate solvents to make porous liquids had been limited to direct experimental tests. We demonstrate an efficient screening approach for this task that uses COSMO-RS calculations, predictions of solvent pK_a values from a machine-learning model, and several other features and apply this approach to select solvents from a library of more than 11,000 compounds. This method is shown to give qualitative agreement with experimental observations for two molecular cages, CC13 and TG-TFB-CHEDA, identifying solvents with higher solubility for these molecules than had previously been known. Ultimately, the algorithm streamlines the downselection of suitable solvents for porous organic cages to enable more rapid discovery of Type II porous liquids.

Controlled Synthesis of Palladium Nanoparticles with Size-Dependent Catalytic Activities Enabled by Organic Molecular Cages

Particle size plays a key role in the performance of metal nanoparticles (MNPs). However, the size-controlled synthesis of MNPs still represents a challenging task. In this work, we revealed a strong solvent effect on the growth of palladium nanoparticles (PdNPs), which was directed by a porous [2 + 3] organic molecular cage (OMC, Phos-cage) containing triphenylphosphine moieties. PdNPs with different average diameters of 0.8, 1.2, and 3.3 nm supported by Phos-cage were obtained by simply varying the reaction media. The catalytic performance of such ultrafine PdNPs in the reduction of p-nitrophenol and a Suzuki-Miyaura coupling reaction has been studied, which clearly shows size-dependent catalytic activity and stability. The knowledge gained in this study, controlling the size of PdNPs supported by the OMC template in different solvents, will open new possibilities for size-controlled synthesis of ultrafine MNPs with high catalytic activity and stability.

Gas Storage in Porous Molecular Materials

Molecules with permanent porosity in the solid state have been studied for decades. Porosity in these systems is governed by intrinsic pore space, as in cages or macrocycles, and extrinsic void space, created through loose, intermolecular solid-state packing. The development of permanently porous molecular materials, especially cages with organic or metal-organic composition, has seen increased interest over the past decade, and as such, incredibly high surface areas have been reported for these solids. Despite this, examples of these materials being explored for gas storage applications are relatively limited. This minireview outlines existing molecular systems that have been investigated for gas storage and highlights strategies that have been used to understand adsorption mechanisms in porous molecular materials.

Amphiphilic Organic Cages: Self-Assembly into Nanotubes and Enhanced Anion-π Interactions

An amphiphilic organic cage was synthesized and used as self-assembly synthon for the fabrication of novel functional supramolecular structures in solution. The transmission electron microscopy (TEM) results showed that this amphiphilic cage self-assembled in aqueous solution into unilamellar nanotubes with a diameter of 29±4 nm at a concentration of 0.05 mg mL^-1 . Interestingly, the self-assembly of this cage significantly enhanced the anion-π interactions as indicated by a remarkable increasement of association constant (K_a ) between Cl^- and this amphiphilic cage after self-assembly. In specific, K_a was increased from 223 M^-1 for discrete cages in methanol to 6800 M^-1 for aggregated cages after self-assembly in water at the same concentration of 2.26×10^-5  M. A mechanism based on a synergistic effect was proposed in order to explain this self-assembly process through enhanced anion-π interactions.

Atomically Precise Structure Determination of Porous Organic Cage from Ab Initio PXRD Structure Analysis: Its Molecular Click Postfunctionalization and CO2 Capture Application

A novel porous organic cage (POC) was prepared via condensation reaction between 1,3,5-triformylbenzene (TFB) and (1R,2R)-4-cyclohexene-1,2-diamine (CHEDA). This POC could pack in either an amorphous structure or a crystalline one. Atomically precise structure determination of POC was achieved through ab initio powder X-ray diffraction (PXRD) structure analysis in the chiral trigonal space group R3. The same atomically precise structure determination of POC from single-crystal X-ray diffraction (SXRD) structure analysis could be obtained independently with a slight difference in cell parameter, indicating that the refinement method through ab initio PXRD structure analysis is reliable and may serve as an essential method for atomically precise structure determination. The cage could adsorb up to 8 mmol/g CO₂ at 298 K and 1 bar. Furthermore, 1-thioglycerol and 1-octadecanethiol were chosen to prove that postmodification of this POC was flexible. After post-synthetic modification (PSM) via highly efficient photoinitiated thiol-ene click reaction, the products still kept porous with relatively higher special surface area (337 m²/g of 5T and 156 m²/g of 5O) than mostly reported cages via the reduced-amine approach.

A Coordinative Solubilizer Method to Fabricate Soft Porous Materials from Insoluble Metal-Organic Polyhedra

Porous molecular cages have a characteristic processability arising from their solubility, which allows their incorporation into porous materials. Attaining solubility often requires covalently bound functional groups that are unnecessary for porosity and which ultimately occupy free volume in the materials, decreasing their surface areas. Here, a method is described that takes advantage of the coordination bonds in metal-organic polyhedra (MOPs) to render insoluble MOPs soluble by reversibly attaching an alkyl-functionalized ligand. We then use the newly soluble MOPs as monomers for supramolecular polymerization reactions, obtaining permanently porous, amorphous polymers with the shape of colloids and gels, which display increased gas uptake in comparison with materials made with covalently functionalized MOPs.

Protection strategies for directionally-controlled synthesis of previously inaccessible metal–organic polyhedra (MOPs): the cases of carboxylate- and amino-functionalised Rh(ii)-MOPs

Herein we report that strategic use of protecting groups in coordination reactions enables directional inhibition that leads to synthesis of metal–organic polyhedra (MOPs) highly functionalized with carboxylic acid and amine groups.

Porous Molecular Solids and Liquids

Until recently, porous molecular solids were isolated curiosities with properties that were eclipsed by porous frameworks, such as metal-organic frameworks. Now molecules have emerged as a functional materials platform that can have high levels of porosity, good chemical stability, and, uniquely, solution processability. The lack of intermolecular bonding in these materials has also led to new, counterintuitive states of matter, such as porous liquids. Our ability to design these materials has improved significantly due to advances in computational prediction methods.

PdII2L4-type coordination cages up to three nanometers in size

The utilization of large ligands in coordination-based self-assembly represents an attractive strategy for the construction of supramolecular assemblies more than two nanometers in size. However, the implementation of this strategy is hampered by the fact that the preparation of such ligands often requires substantial synthetic effort. Herein, we describe a simple one-step protocol, which allows large bipyridyl ligands with a bent shape to be synthesized from easily accessible and/or commercially available starting materials. The ligands were used to construct PdII2L4-type coordination cages of unprecedented size. Furthermore, we provide evidence that these cages may be stabilized by close intramolecular packing of lipophilic ligand side chains. Packing effects of this kind are frequently encountered in protein assemblies, but they are seldom used as a design element in metallasupramolecular chemistry.

Porous organic cage membranes for water desalination: a simulation exploration

A proof-of-concept simulation study is reported for water desalination through porous organic cage membranes.