Covalent Organic Frameworks and Cage Compounds: Design and Applications of Polymeric and Discrete Organic Scaffolds

Porous organic materials are an emerging class of functional nanostructures with unprecedented properties. Dynamic covalent assembly of small organic building blocks under thermodynamic control is utilized for the intriguingly simple formation of complex molecular architectures in one‐pot procedures. In this Review, we aim to analyze the basic design principles that govern the formation of either covalent organic frameworks as crystalline porous polymers or covalent organic cage compounds as shape‐persistent molecular objects. Common synthetic procedures and characterization techniques will be discussed as well as more advanced strategies such as postsynthetic modification or self‐sorting. When appropriate, comparisons are drawn between polymeric frameworks and discrete organic cages in terms of their underlying properties. Furthermore, we highlight the potential of these materials for applications ranging from gas storage to catalysis and organic electronics.     

手性多孔材料在色谱分离分析中的应用

手性多孔有机骨架材料(Chiral porous organic frameworks,CPOFs)具有孔性质优异、比表面积高、稳定性好以及易功能化等诸多优点,已经在手性催化、识别和分离等领域中得到应用。手性多孔有机骨架材料主要有手性金属-有机骨架材料(Chiral metal-organic frameworks,CMOFs)和手性共价有机骨架材料(Chiral covalent organic frameworks,CCOFs)及其他材料,这类材料具有特殊的手性识别、吸附作用,在色谱分离分析领域中已成为研究热点之一。该文综述了手性多孔材料的合成及其在色谱分离和选择性吸附中的应用,展望了未来CPOFs材料可能的应用与发展方向。     

Transforming Porous Organic Cages into Porous Ionic Liquids via a Supramolecular Complexation Strategy

Porous liquids are a type of porous materials that engineer permanent porosity into unique flowing liquids, exhibiting promising functionalities for a variety of applications. Here a Type I porous liquid is synthesized by transforming porous organic cages into porous ionic liquids via a supramolecular complexation strategy. Simple physical mixing of 18‐crown‐6 with task‐specific anionic porous organic cages affords a porous ionic liquid with anionic porous organic cages as the anionic parts and 18‐crown‐6/potassium ion complexes as the cationic parts. In contrast, mixing of 15‐crown‐5 and anionic porous organic cages in a 2:1 ratio gives only solids, while the addition of excess 15‐crown‐5 affords a Type II porous liquid. The permanent porosity in the cage‐based porous liquids has been also confirmed by molecular simulation, positron (e⁺) annihilation lifetime spectroscopy, and enhanced gas sorption capacity compared with pure crown ethers.     

Stabilizing Metal–Organic Polyhedra (MOP): Issues and Strategies

Metal–organic polyhedra (MOPs) are discrete, metal–organic molecular entities composed of edge‐sharing molecular polygons or connected molecular vertices. Unlike the infinite metal–organic coordination networks popularized by metal–organic frameworks (MOFs), spherical MOPs, also known as nanocages, nanospheres, nanocapsules, or nanoballs, are obtained through the self‐organization of metal–carboxylate or metal–pyridine/pyrimidine links to afford cage‐like nanoarchitectures. MOPs offer much promise as porous materials owing to their well‐defined structures and solution processability. However, these advantages become moot if their poor aqueous stability and/or guest‐removal‐induced aggregation handicaps remain unaddressed. The concise premise of this contribution limits our discussion to the design principles in action behind recent developments in stable carboxylate MOPs. To highlight the structure–property relationships between the structural and compositional features of these metal carboxylate polyhedra, related scientific challenges and state‐of‐the‐art research directions for further exploration are presented in brief.     

Interpenetrated Cage Structures

This Review covers design strategies, synthetic challenges, host–guest chemistry, and functional properties of interlocked supramolecular cages. Some dynamic covalent organic structures are discussed, as are selected examples of interpenetration in metal–organic frameworks, but the main focus is on discrete coordination architectures, that is, metal‐mediated dimers. Factors leading to interpenetration, such as geometry, flexibility and chemical makeup of the ligands, coordination environment, solvent effects, and selection of suitable counter anions and guest molecules, are discussed. In particular, banana‐shaped bis‐pyridyl ligands together with square‐planar metal cations have proven to be suitable building blocks for the construction of interpenetrated double‐cages obeying the formula [M₄L₈]. The peculiar topology of these double‐cages results in a linear arrangement of three mechanically coupled pockets. This allows for the implementation of interesting guest encapsulation effects such as allosteric binding and template‐controlled selectivity. In stimuli‐responsive systems, anionic triggers can toggle the binding of neutral guests or even induce complete structural conversions. The increasing structural and functional complexity in this class of self‐assembled hosts promises the construction of intelligent receptors, novel catalytic systems, and functional materials.     

Recent development trends for chiral stationary phases based on chitosan derivatives,cyclofructan derivatives and chiral porous materials in high performance liquid chromatography

The separation of enantiomers by chromatographic methods, such as gas chromatography, high‐performance liquid chromatography and capillary electrochromatography, has become an increasingly significant challenge over the past few decades due to the demand of pharmaceutical, agrochemical, and food analysis. Among these chromatographic resolution methods, high‐performance liquid chromatography based on chiral stationary phases has become the most popular and effective method used for the analytical and preparative separation of optically active compounds. This review mainly focuses on the recent development trends for novel chiral stationary phases based on chitosan derivatives, cyclofructan derivatives, and chiral porous materials that include metal‐organic frameworks and covalent organic frameworks in high‐performance liquid chromatography. The enantioseparation performance and chiral recognition mechanisms of these newly developed chiral selectors toward enantiomers are discussed in detail.     

Porous Solids Arising from Synergistic and Competing Modes of Assembly: Combining Coordination Chemistry and Covalent Bond Formation

Design and synthesis of porous solids employing both reversible coordination chemistry and reversible covalent bond formation is described. The combination of two different linkage modes in a single material presents a link between two distinct classes of porous materials as exemplified by metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). This strategy, in addition to being a compelling material‐discovery method, also offers a platform for developing a fundamental understanding of the factors influencing the competing modes of assembly. We also demonstrate that even temporary formation of reversible connections between components may be leveraged to make new phases thus offering design routes to polymorphic frameworks. Moreover, this approach has the striking potential of providing a rich landscape of structurally complex materials from commercially available or readily accessible feedstocks.     

Water‐Soluble Three‐Dimensional Polymers: Non‐Covalent and Covalent Synthesis and Functions†

Water‐soluble three‐dimensional (3D) polymers are structurally ideal for the construction of ordered porous materials for in‐situ and tunable loading and release of guests. For many years, studies on ordered porous materials have been confined to crystalline solids. Since 2014, self‐assembly has been developed as a robust strategy for the preparation of water‐soluble 3D polymers that possess defined and intrinsic porosity. Through the encapsulation of cucurbit[8]uril for aromatic dimers, ordered diamondoid supramolecular organic frameworks can be assembled from tetrahedral monomers. With [Ru(bipy)₃]²⁺‐derived octahedral complexes as precursors, cubic supramolecular metal‐organic frameworks have been assembled. One supramolecular organic framework has also been utilized to prepare the first homogeneous covalent organic framework through the [2+2] alkene cycloaddition, whereas the quantitative formation of the hydrazone bonds can be utilized to synthesize flexible porous organic frameworks. The new water‐soluble ordered and flexible polymeric frameworks are able to include drugs and biomacromolecules to accomplish in situ loading and intracellular delivery and to enrich photosensitizers and catalysts to enhance discrete visible light‐induced reactions. This review highlights the advances.     

A homochiral porous organic cage with large cavity and pore windows for the efficient gas chromatography separation of enantiomers and positional isomers

Porous organic cages composed of discrete cage molecules have attracted considerable recent attention as gas adsorption materials and separation media. In this study, we report a homochiral porous organic cage CC5 with a large cavity and pore windows as a novel stationary phase for high‐resolution gas chromatographic separations. The capillary column was prepared by a static coating method. A large number of racemic compounds have been resolved on the coated capillary column, including derivatized amino acids, alcohols, alcohol amines, esters, ethers, ketones, and epoxides. It is interesting that the CC5‐coated capillary column exhibits significant chiral recognition complementarity to a commercial β‐DEX 120 column and a previously reported homochiral porous organic cage CC3‐R‐coated column, which could expand the range of the analytes amenable to separation on porous organic cage‐based capillary columns. Moreover, the fabricated column also shows excellent selectivity for the separation of positional isomers, including the challenging ethylbenzene and xylene isomers. Experimental results demonstrate an excellent separation performance and stability of the CC5‐coated column, making it promising for gas chromatography applications.     

Carbonyl and carboxylate crosslinked cyclodextrin as a nanocarrier for resveratrol: in silico,in vitro and in vivo evaluation

Recently, much attention has been devoted towards the development of methods for the capture and separation of inorganic gases and organic compounds with high selectivity and efficiency using nanoporous materials. Unlike metal–organic frameworks and covalent organic network polymer, nanoporous molecular crystals (NMCs) do not have extended network structures through coordination or covalent bonding. Instead, they are composed of discrete organic molecules with only weak noncovalent interactions between them. Calixarenes, used as artificial hosts for molecular recognition, constitute a representative class of NMCs that exhibit “porosity without pores.” Despite the absence of empty-channels, calixarene crystals can absorb various inorganic gases and organic compounds, thereby undergoing a guest-induced structural change. Thus, because of their ability to precisely discriminate between molecules of similar sizes and structures, such NMCs show great potential for application as separation materials. This review summarizes reports on the absorption and inclusion of inorganic gases and organic molecules with crystals of calixarenes and their derivatives and discusses their potential as separation materials.     

Functional Material Systems Based on Soft Cages

Discrete molecular soft cages integrate multiple functionalities in one molecule. They express their functions from the confined space in their cavity, functional groups in the cavity interior wall and exterior wall, and the chelating nodes in many chelating cages. Such functional integrity render cage molecules special applications in material engineering. Increasing applications of cage molecules in material design have been reported in recent years. Compared with other cavity-rich molecular structures such as metal-organic framework (MOF) or covalent organic frameworks (COF), discrete soft cages present the unique advantage of material design flexibility, that they can easily composite with nanoparticles or polymers and exist in materials of various forms. We document the development of cage-based materials in recent years and expect to further inspire materials engineering to integrate contribution from the functionality specificity of cage molecules and ultimately promote the development of functional materials and thus human life qualities.     

Mixed‐Matrix Membranes

Research into extended porous materials such as metal‐organic frameworks (MOFs) and porous organic frameworks (POFs), as well as the analogous metal‐organic polyhedra (MOPs) and porous organic cages (POCs), has blossomed over the last decade. Given their chemical and structural variability and notable porosity, MOFs have been proposed as adsorbents for industrial gas separations and also as promising filler components for high‐performance mixed‐matrix membranes (MMMs). Research in this area has focused on enhancing the chemical compatibility of the MOF and polymer phases by judiciously functionalizing the organic linkers of the MOF, modifying the MOF surface chemistry, and, more recently, exploring how particle size, morphology, and distribution enhance separation performance. Other filler materials, including POFs, MOPs, and POCs, are also being explored as additives for MMMs and have shown remarkable anti‐aging performance and excellent chemical compatibility with commercially available polymers. This Review briefly outlines the state‐of‐the‐art in MOF‐MMM fabrication, and the more recent use of POFs and molecular additives.     

Homochiral metal–organic framework used as a stationary phase for high‐performance liquid chromatography

Metal–organic frameworks are promising porous materials. Chiral metal–organic frameworks have attracted considerable attention in controlling enantioselectivity. In this study, a homochiral metal–organic framework [Co₂(D‐cam)₂(TMDPy)] (D‐cam = d ‐camphorates, TMDPy = 4,4′‐trimethylenedipyridine) with a non‐interpenetrating primitive cubic net has been used as a chiral stationary phase in high‐performance liquid chromatography. It has allowed the successful separation of six positional isomers and six chiral compounds. The good selectivity and baseline separation, or at least 60% valley separation, confirmed its excellent molecular recognition characteristics. The relative standard deviations for the retention time of run‐to‐run and column‐to‐column were less than 1.8 and 3.1%, respectively. These results demonstrate that [Co₂(D‐cam)₂(TMDPy)] may represent a promising chiral stationary phase for use in high‐performance liquid chromatography.     

Metal–Covalent Organic Frameworks (MCOFs): A Bridge Between Metal–Organic Frameworks and Covalent Organic Frameworks

Many sophisticated chemical and physical properties of porous materials strongly rely on the presence of the metal ions within the structures. Whereas homogeneous distribution of metals is conveniently realized in metal–organic frameworks (MOFs), the limited stability potentially restricts their practical implementation. From that perspective, the development of metal–covalent organic frameworks (MCOFs) may address these shortcomings by incorporating active metal species atop highly stable COF backbones. This Minireview highlights examples of MCOFs that tackle important issues from their design, synthesis, characterization to cutting‐edge applications.     

Hydrogen‐Bonded Organic Frameworks (HOFs): A New Class of Porous Crystalline Proton‐Conducting Materials

Two porous hydrogen‐bonded organic frameworks (HOFs) based on arene sulfonates and guanidinium ions are reported. As a result of the presence of ionic backbones appended with protonic source, the compounds exhibit ultra‐high proton conduction values (σ) 0.75× 10^?2 S cm^?1 and 1.8×10^?2 S cm^?1 under humidified conditions. Also, they have very low activation energy values and the highest proton conductivity at ambient conditions (low humidity and at moderate temperature) among porous crystalline materials, such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). These values are not only comparable to the conventionally used proton exchange membranes, such as Nafion used in fuel cell technologies, but is also the highest value reported in organic‐based porous architectures. Notably, this report inaugurates the usage of crystalline hydrogen‐bonded porous organic frameworks as solid‐state proton conducting materials.     

Chiral Isocamphoric Acid: Founding a Large Family of Homochiral Porous Materials

Homochiral metal–organic frameworks (MOFs) are an important class of chiral solids with potential applications in chiral recognition; however, relatively few are available. Of great importance is the availability of low‐cost, racemization‐resistant, and versatile enantiopure building blocks. Among chiral building blocks, d ‐camphoric acid is highly prolific, yet, its trans‐isomer, l ‐isocamphoric acid, has remained unknown in the entire field of solid‐state materials. Its rich yet totally untapped synthetic and structural chemistry has now been investigated through the synthesis of a large family of homochiral metal isocamphorates. The first observation of diastereoisomerism in isostructural MOFs is presented. Isocamphorate has a powerful ability to create framework topologies unexpected from common inorganic building blocks, and isocamphoric acid should allow access to hundreds of new homochiral materials.     

Recent Advances of Covalent Organic Frameworks for Chiral Separation

Covalent organic frameworks(COFs), orderly assembled from the building blocks via covalent bonds, are a novel type of porous materials with rich functional sites and permanent porosity. At present, most of COFs are achiral networks, nevertheless, chiral COFs(CCOFs) have become a research hotspot in recent years, due to their unique chiral sites and microenvironment. As one of the most important applications of CCOFs, chiral separation has attracted huge attention for the convenient, rapid and efficient feature. In this review, recent progresses of covalent organic frameworks for chiral separation are covered. And we also present the challenges and outlooks of CCOFs in the future for this field.     

Metal–Covalent Organic Frameworks (MCOFs): A Bridge Between Metal–Organic Frameworks and Covalent Organic Frameworks

Many sophisticated chemical and physical properties of porous materials strongly rely on the presence of the metal ions within the structures. Whereas homogeneous distribution of metals is conveniently realized in metal–organic frameworks (MOFs), the limited stability potentially restricts their practical implementation. From that perspective, the development of metal–covalent organic frameworks (MCOFs) may address these shortcomings by incorporating active metal species atop highly stable COF backbones. This Minireview highlights examples of MCOFs that tackle important issues from their design, synthesis, characterization to cutting-edge applications.     

多孔材料化学:从无机微孔化合物到金属有机多孔骨架

本文主要从无机微孔化合物和金属有机多孔骨架的合成化学和结构化学这两方面来介绍多孔材料化学的研究进展。多孔材料是一类具有规则孔结构的固态化合物,它们在催化、分离、离子交换等工业领域有着广泛的应用。硅铝酸盐是最为人们所熟知的微孔分子筛,经过半个多世纪的发展,人们又相继开发出磷酸盐、砷酸盐、锗酸盐、亚磷酸盐、硫酸盐、亚硒酸盐以及金属硫化物等类沸石无机微孔化合物。近十多年来,配位聚合物与金属有机多孔骨架开始大量兴起,为微孔化合物的多样化与组成的复杂性增添了新的领域。     

2020 roadmap on pore materials for energy and environmental applications

Porous materials have attracted great attention in energy and environment applications. In this roadmap, several porous materials are discussed for energy storage and conversion. It will help the researchers to obtain them guidance from it in future.