Sophisticated Design of Covalent Organic Frameworks with Controllable Bimetallic Docking for a Cascade Reaction

Cascade Electrocatalytic Reduction of Nitrate to Ammonia Using a Heterobimetallic Covalent Organic Framework Composed of Cu-Porphyrin and Co-Bipyridine

The electrocatalytic reduction of nitrate (NO3-) to ammonia (NH3) not only offers an effective solution to environmental problems caused by the accumulation of NO3- but also provides a sustainable alternative to the Haber-Bosch process. However, the conversion of NO3- to NH3 is a complicated process involving multiple steps, leading to a low Faradaic efficiency (FE) for NH3 production. The structural designability of covalent organic frameworks (COFs) renders feasible and precise modulation at the molecular level, facilitating the incorporation of multiple well-defined catalytic sites with different reactivities into a cohesive entity. This promotes the efficiency of the overall reaction through the coupling of multistep reactions. Herein, heterobimetallic CuP-CoBpy was prepared by postmodification, involving the anchoring of cobalt ions to the CuP-Bpy structure. As a result of the cascade effect of the bimetallic sites, CuP-CoBpy achieved an outstanding NH3 yield of 13.9 mg h-1 mgcat.-1 with a high FE of 96.7% at -0.70 V versus the reversible hydrogen electrode and exhibited excellent stability during catalysis. A series of experimental and theoretical studies revealed that the CuP unit facilitates the conversion of NO3- to NO2-, while the CoBpy moiety significantly prompts the reduction of NO2- to NH3. This study demonstrates that tailoring the structural units for the construction of COFs based on each step in the multistep reaction can enhance both the catalytic activity and product selectivity of the overall process.

Metalated covalent organic frameworks as efficient catalysts for multicomponent tandem reactions

Multicomponent tandem reactions have become indispensable synthetic methods due to their economic advantages and efficient usage in natural products and drug synthesis. The emergence of metalated covalent organic frameworks (MCOFs) has opened up new opportunities for the advancement of multicomponent tandem reactions. In contrast to commonly used homogeneous transition metal catalysts, MCOFs possess regular porosity, high crystallinity, and rich metal chelation sites that facilitate the uniform distribution and anchoring of metals within their cavities. Thus, they show extremely high activity and have recently been widely employed as catalysts for multicomponent tandem reactions. It is timely to conduct a review of MCOFs in multicomponent tandem reactions, in order to offer guidance and assistance for the synthesis of MCOF catalysts and their application in multicomponent tandem reactions. This review provides a comprehensive overview of the design and synthesis of MCOFs, their application and progress in multicomponent tandem reactions, and the primary challenges encountered during their current development with the aim of contributing to the promotion of the field.

Control over Charge Separation by Imine Structural Isomerization in Covalent Organic Frameworks with Implications on CO2 Photoreduction

Two-dimensional covalent organic frameworks (COFs) are an emerging class of photocatalytic materials for solar energy conversion. In this work, we report a pair of structurally isomeric COFs with reversed imine bond directions, which leads to drastic differences in their physical properties, photophysical behaviors, and photocatalytic CO2 reduction performance after incorporating a Re(bpy)(CO)3Cl molecular catalyst through bipyridyl units on the COF backbone (Re-COF). Using the combination of ultrafast spectroscopy and theory, we attributed these differences to the polarized nature of the imine bond that imparts a preferential direction to intramolecular charge transfer (ICT) upon photoexcitation, where the bipyridyl unit acts as an electron acceptor in the forward imine case (f-COF) and as an electron donor in the reverse imine case (r-COF). These interactions ultimately lead the Re-f-COF isomer to function as an efficient CO2 reduction photocatalyst, while the Re-r-COF isomer shows minimal photocatalytic activity. These findings not only reveal the essential role linker chemistry plays in COF photophysical and photocatalytic properties but also offer a unique opportunity to design photosensitizers that can selectively direct charges.

A Nickel Anchored Covalent Organic Framework as Unimolecular Metallaphotocatalyst for Visible Light Driver C-P Bond Coupling Reaction

The urgent need to develop a sustainable and environmentally friendly method for synthesizing organophosphine compounds is underscored by their extensive applications in organic synthesis, coordination chemistry, medicinal chemistry, and photoelectric materials. Metalated covalent organic frameworks (MCOFs), which seamlessly integrate the inherent photo properties of COF with the catalytic capabilities of metal ions, offer an optimal material for efficient transformation of organics sustainably. In this study, we introduce a simple COF with nickel anchorages (Bpy-COF-NiCl2 ) as a unimolecular metallaphotocatalytic system for effective C-P bond formation. This heterogeneous photocatalyst exhibits superior catalytic performance, achieving yields of up to 95 %, and demonstrates broad substrate tolerance and functional group reactivity. Notably, the metallaphotocatalytic system has demonstrated the capability to process aryl bromides to produce the desired product, a feat not previously reported. Finally, the production and reusability test at the gram scale attests to its superior practicality for designing future organic cross-coupling reactions.

Topology-Selective Manipulation of Two-Dimensional Covalent Organic Frameworks

The manipulation of topological architectures in two-dimensional (2D) covalent organic framework (COF) materials for different applications is promising but remains a great challenge. Here, we first report the topology-selective synthesis of two distinct varieties of 2DCOFs, imine-based HT-COFs and benzimidazole-fused BI-HT-COFs, by simply altering acid catalysts. To HT-COFs, a superlattice of 1D channel with a persistent triangular shape is formed via Schiff base reaction, while to BI-HT-COFs, a hexagonal lattice structure with a highly conjugated structure and imidazole linkages is constructed due to an imine-based cyclization reaction. The two COFs exhibited marked differences in their bandgap, chemical stability, molecular adsorption, and catalytic activity, which make them have different fields of application. This work not only diversifies the hexaaminotriphenylene-based 2DCOF topologies but also provides vivid examples of structure-property relationships, which would facilitate fundamental research and potential applications of 2DCOFs.

Large-Scale Synthesis of Covalent Organic Frameworks: Challenges and Opportunities

Connecting organic building blocks by covalent bonds to design porous crystalline networks has led to covalent organic frameworks (COFs), consequently transferring the flexibility of dynamic linkages from discrete architectures to extended structures. By virtue of the library of organic building blocks and the diversity of dynamic linkages and topologies, COFs have emerged as a novel field of organic materials that propose a platform for tailor-made complex structural design. Progress over the past two decades in the design, synthesis, and functional exploration of COFs in diverse applications successively established these frameworks in materials chemistry. The large-scale synthesis of COFs with uniform structures and properties is of profound importance for commercialization and industrial applications; however, this is in its infancy at present. An innovative designing and synthetic approaches have paved novel ways to address future hurdles. This review article highlights the fundamental of COFs, including designing principles, coupling reactions, topologies, structural diversity, synthetic strategies, characterization, growth mechanism, and activation aspects of COFs. Finally, the major challenges and future trends for large-scale COF fabrication are outlined.

Cu-α-diimine Compounds Encapsulated in Porous Materials as Catalysts for Electrophilic Amination of Aromatic C-H Bonds

Electrophilic amination has emerged as a more environmentally benign approach to construct arene C-N bonds. However, heterogeneous catalysts remain largely unexplored in this area, even though their use could facilitate product purification and catalyst recovery. Here we investigate strategies to heterogenize a Cu(2,2'-bipyridine) catalyst for the amination of arenes lacking a directing group with hydroxylamine-O-sulfonic acid (HOSA). Besides immobilization of Cu on a metal-organic framework (MOF) or covalent organic framework (COF) with embedded 2,2'-bipyridines, a ship-in-a-bottle approach was followed in which the Cu complex is encapsulated in the pores of a zeolite. Recyclability and hot centrifugation tests show that zeolite Beta-entrapped Cu^II(2,2'-bipyridine) is superior in terms of stability. With N-methylmorpholine as a weakly coordinating, weak base, simple arenes, such as mesitylene, could be aminated with yields up to 59%, corresponding to a catalyst TON of 24. The zeolite could be used in three consecutive runs without a decrease in activity. Characterization of the catalyst by EPR and XAS showed that the active catalytic complex consisted of a site-isolated Cu^II species with one 2,2'-bipyridine ligand.

Linker Engineering of 2D Imine Covalent Organic Frameworks for the Heterogeneous Palladium-Catalyzed Suzuki Coupling Reaction

Covalent organic frameworks (COFs) are an emerging class of porous organic polymers that have been utilized as scaffolds for anchoring metal active species to act as heterogeneous catalysts. Though several examples of such COFs exist, a thorough experimental and computational analysis on such catalysts is limited. In this work, a series of two-dimensional (2D) imine COFs (TTA-DFB COF (N), TTA-TBD COF (N∧O), and TTA-DFP COF(N∧N)) were synthesized by using suitable building units to obtain three different coordination sites (N, N∧O, and N∧N). These were post-modified with Pd(II) to catalyze the Suzuki-Miyaura coupling reaction. Pd@TTA-DFB COF, where Pd(II) was coordinated to N sites, showed the fastest reactivity and lower stability. Pd@TTA-DFP COF showed highest stability but slowest reactivity. Pd@TTA-TBD COF was the best among the three with both high stability and fast reactivity. By combining both experimental and computational results, we conclude that the Pd(II) to Pd(0) reduction is a key step in the difference between the catalytic reactivities of the three COFs. This study demonstrates the importance of the building block approach to design COFs for efficient heterogeneous catalysis and to understand the fate of the reaction profile.

Imine and imine-derived linkages in two-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) are porous crystalline polymers that result from the formation of covalent bonds between precisely assembled organic units. Linkage chemistry is a crucial factor in the controllable synthesis and resulting physicochemical properties of COFs. Imine linkages are popular in the formation of polyfunctional two-dimensional (2D) COFs because they are formed easily with structural and functional diversity. There has been much recent interest in expanding beyond this to COFs with imine-derived linkages. This review highlights the development of chemistry to modify and prepare derivatives of imines within 2D COFs. We discuss the derivation of imine bonds via covalent and noncovalent bonding and the properties and potential applications of the resulting materials in order to provide a better understanding of the relationship between covalent linkages and overall performance for 2D COF materials.

Immobilization of Ionic Liquid on a Covalent Organic Framework for Effectively Catalyzing Cycloaddition of CO2 to Epoxides

Transforming CO₂ into value-added chemicals has been an important subject in recent years. The development of a novel heterogeneous catalyst for highly effective CO₂ conversion still remains a great challenge. As an emerging class of porous organic polymers, covalent organic frameworks (COFs) have exhibited superior potential as catalysts for various chemical reactions, due to their unique structure and properties. In this study, a layered two-dimensional (2D) COF, IM4F-Py-COF, was prepared through a three-component condensation reaction. Benzimidazole moiety, as an ionic liquid precursor, was integrated onto the skeleton of the COF using a benzimidazole-containing building unit. Ionization of the benzimidazole framework was then achieved through quaternization with 1-bromobutane to produce an ionic liquid-immobilized COF, i.e., BMIM4F-Py-COF. The resulting ionic COF shows excellent catalytic activity in promoting the chemical fixation of CO₂ via reaction with epoxides under solvent-free and co-catalyst-free conditions. High porosity, the one-dimensional (1D) open-channel structure of the COF and the high catalytic activity of ionic liquid may contribute to the excellent catalytic performance. Moreover, the COF catalyst could be reused at least five times without significant loss of its catalytic activity.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Nanopores of a Covalent Organic Framework: A Customizable Vessel for Organocatalysis

Covalent organic frameworks (COFs) as crystalline polymers possess ordered nanochannels. When their channels are adorned with catalytically active functional groups, their highly insoluble and fluffy powder texture makes them apt heterogeneous catalysts that can be dispersed in a range of solvents and heated to high temperatures (80-180 °C). This would mean very high catalyst density, facile active-site access, and easy separation leading to high isolated yields. Different approaches have been devised to anchor or disperse the catalytic sites into the nanospaces offered by the COF pores. Such engineered COFs have been investigated as catalysts for many organic transformation reactions. These range from Suzuki-Miyaura coupling, Heck coupling, Knoevenagel condensation, Michael addition, alkene epoxidation, CO₂ utilization, and more complex biomimetic catalysis. Such catalysts employ COF as a "passive" support that merely docks catalytically active inorganic clusters, or in other cases, the COF itself participates as an "active" support by altering the electronics of the inorganic catalytic sites through the redox activity of its framework. Even more, catalytic organic pockets or metal complexes have been directly tethered to COF walls to make them behave like single-site organocatalysts. Here, we have listed most COF-based organic transformations by categorizing them as metal-free non-noble-metal@COF and noble-metal@COF. The initial part of this review highlights the advantages of COFs as a component of a heterogeneous catalyst, while the latter part discusses all of the current literature on this topic.

A conveniently synthesized redox-active fluorescent covalent organic framework for selective detection and adsorption of uranium

Uranium is a key element in the nuclear industry and also a global environmental contaminant with combined highly toxic and radioactive. Currently, the materials based on post-modification of amidoxime have been developed for uranium detection and adsorption. However, the affinity of amidoxime group for vanadium is stronger than that of uranium, which is the main challenge hindering the practical application of amidoxime-based adsorbents. Herein, we synthesized a fluorescent covalent organic framework (TFPPy-BDOH) through integrating biphenyl diamine and pyrene unit into the π-conjugated framework. TFPPy-BDOH has an excellent selectivity to uranium due to the synergistic effect of nitrogen atom in the imine bond and hydroxyl groups in conjugated framework. It can achieve ultra-fast fluorescence response time (2 s) and ultra-low detection limit (8.8 nM), which may be attributed to its intrinsic regular porous channel structures and excellent hydrophilicity. More excitingly, TFPPy-BDOH can chemically reduce soluble U (VI) to insoluble U (IV), and release the binding site to adsorb additional U (VI), achieving high adsorption capacity of 982.6 ± 49.1 mg g^-1. Therefore, TFPPy-BDOH can overcome the challenges faced by current amidoxime-based adsorbents, making it as a potential adsorbent in practical applications.

Bimetallic docked covalent organic frameworks with high catalytic performance towards coupling/oxidation cascade reactions

Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers that make these materials suitable for use as excellent scaffold in heterogeneous catalysis. Here we synthesize a layered two-dimensional (2D) COF (TADP–COF) through the condensation reaction between four-branched 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAPP) and linear 2,5-dihydroxyterephthalaldehyde (Dha) and 1,4-phthalaldehyde (PA) building blocks. Porphyrin units, imine and hydroxyl groups together with imines can provide wide coordination sites for metal docking. Using a programmed synthetic procedure, Cu(ii) ions first coordinated with the imine groups in conjunction with their adjacent hydroxyl groups, and porphyrin units and subsequently added Pd(ii) ions occupied the remaining imine sites in the space between adjacent COF layers. The bimetallic Pd(ii)/Cu(ii)@TADP–COF showed high catalytic activity in a one-pot coupling/oxidation cascade reaction in water. The high surface area, one-dimensional (1D) open channel structure and predesigned catalytic active sites of this material make it ideal candidate for use as heterogeneous catalyst in a wide range of catalytic reactions.

Cu(ii) and Pd(ii) ions were selectively coordinated within an imine-linked 2D COF that exhibited good catalytic performance towards a one-pot Suzuki coupling/oxidation cascade reaction.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Photoredox Heterobimetallic Dual Catalysis Using Engineered Covalent Organic Frameworks

The functionalization of an imine-based layered covalent organic framework (COF), containing phenanthroline units as ligands, has allowed the obtention of a heterobimetallated material. Photoactive Ir and Ni fragments were immobilized within the porous structure of the COF, enabling heterogeneous light-mediated Csp3-Csp2 cross-couplings. As radical precursors, potassium benzyl- and alkoxy-trifluoroborates, organic silicates, and proline derivatives were employed, which brings out the good versatility of Ir,Ni@Phen-COF. Moreover, in all the studied cases, an enhanced activity and stability have been observed in comparison with analogous homogenous systems.

Single-Pore versus Dual-Pore Bipyridine-Based Covalent-Organic Frameworks: An Insight into the Heterogeneous Catalytic Activity for Selective CH Functionalization

Exponential growth in the field of covalent-organic frameworks (COFs) is emanating from the direct correlation between designing principles and desired properties. The comparison of catalytic activity between single-pore and dual-pore COFs is of importance to establish structure-function relationship. Herein, the synthesis of imine-linked dual-pore [(BPyDC)]_{x %} -ETTA COFs (x = 0%, 25%, 50%, 75%, 100%) with controllable bipyridine content is fulfilled by three-component condensation of 4,4',4″,4'″-(ethene-1,1,2,2-tetrayl)tetraaniline (ETTA), 4,4'-biphenyldialdehyde, and 2,2'-bipyridyl-5,5'-dialdehyde in different stoichiometric ratio. The strong coordination of bipyridine moieties of [(BPyDC)]_{x %} -ETTA COFs with palladium imparts efficient catalytic active sites for selective functionalization of sp² CH bond to CX (X = Br, Cl) or CO bonds in good yield. To broaden the scope of regioselective CH functionalization, a wide range of electronically and sterically substituted substrates under optimized catalytic condition are investigated. A comparison of the catalytic activity of palladium decorated dual-pore frameworks with single-pore imine-linked Pd(II) @ Py-2,2'-BPyDC framework  is undertaken. The finding of this work provides a sporadic example of chelation-assisted CH functionalization and disclosed an in-depth comparison of the relationship between superior catalytic activity and core properties of rationally designed imine linked frameworks.

Probe metal binding mode of imine covalent organic frameworks: cycloiridation for (photo)catalytic hydrogen evolution from formate

Metalation of covalent organic frameworks (COFs) is a critical strategy to functionalize COFs for advanced applications yet largely relies on the pre-installed specific metal docking sites in the network, such as porphyrin, salen, 2,2'-bipyridine, etc. We show in this study that the imine linkage of simple imine-based COFs, one of the most popular COFs, readily chelate transition metal (Ir in this work) via cyclometalation, which has not been explored before. The iridacycle decorated COF exhibited more than 10-fold efficiency enhancement in (photo)catalytic hydrogen evolution from aqueous formate solution than its molecular counterpart under mild conditions. This work will inspire more functional cyclometallated COFs to be explored beyond catalysis considering the large imine COF library and the rich metallacycle chemistry.

N-Heterocyclic Carbene Functionalized Covalent Organic Framework for Transesterification of Glycerol with Dialkyl Carbonates

The development of a heterogeneous catalyst through the combination of novel carrier and powerful catalytic active sites is of particular interest. Herein, the successful integration of an N-Heterocyclic carbene (NHC) moiety into a covalent organic framework (COF) was achieved by coupling 4,4′,4′′,4′′′-(pyrene-1,3,6,8-tetrayl) tetraaniline (PyTTA) and equimolar 4,7-bis(4-formylphenyl)-1-methyl-1H-benzimidazole (IM) and 2′3′5′6′-tetrafluoro-[1,1′:4′,1′′-terphenyl]-4,4′-dicarbaldehyde (4F) followed by ionization with 1-bromobutane (C4H9Br) and then deprotonation upon addition of a base. The resulting material exhibited promising heterogeneous catalytic activity towards transesterification reaction of glycerol with dialkyl carbonate. Moreover, good recyclability granted no substantial loss of activity upon five cycles. Combination of COFs and NHCs might synergize their characteristics, thus providing more possibilities for creating new patterns of catalytic reactivity.

Pt(II)-Decorated Covalent Organic Framework for Photocatalytic Difluoroalkylation and Oxidative Cyclization Reactions

A new covalent organic framework (COF) based on imine bonds was assembled from 2-(4-formylphenyl)-5-formylpyridine and 1,3,6,8-tetrakis(4-aminophenyl)pyrene, which showed an interesting dual-pore structure with high crystallinity. Postmetallation of the COF with Pt occurred selectively at the N donor (imine and pyridyl) in the larger pores. The metallated COF served as an excellent recyclable heterogeneous photocatalyst for decarboxylative difluoroalkylation and oxidative cyclization reactions.

Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications

Covalent organic frameworks offer a molecular platform for integrating organic units into periodically ordered yet extended 2D and 3D polymers to create topologically well-defined polygonal lattices and built-in discrete micropores and/or mesopores.

Structural Approaches to Control Interlayer Interactions in 2D Covalent Organic Frameworks

The ability to design and synthesize monomers can affect fundamental aspects in 2D covalent organic frameworks, such as dimensionality, topology, and pore size. Besides this, the structure of the monomers can also affect interlayer interactions, which provide an additional means to influence crystallinity, layer arrangement, interlayer distances, and exfoliability. Herein, some of the effects that the structure of monomers can have on the interlayer interactions in 2D covalent organic frameworks and related materials are illustrated.

Nanoscale covalent organic frameworks as theranostic platforms for oncotherapy: synthesis, functionalization, and applications

Cancer nanomedicine is one of the most promising domains that has emerged in the continuing search for cancer diagnosis and treatment. The rapid development of nanomaterials and nanotechnology provide a vast array of materials for use in cancer nanomedicine. Among the various nanomaterials, covalent organic frameworks (COFs) are becoming an attractive class of upstarts owing to their high crystallinity, structural regularity, inherent porosity, extensive functionality, design flexibility, and good biocompatibility. In this comprehensive review, recent developments and key achievements of COFs are provided, including their structural design, synthesis methods, nanocrystallization, and functionalization strategies. Subsequently, a systematic overview of the potential oncotherapy applications achieved till date in the fast-growing field of COFs is provided with the aim to inspire further contributions and developments to this nascent but promising field. Finally, development opportunities, critical challenges, and some personal perspectives for COF-based cancer therapeutics are presented.

Enhanced Proton Conductivity of Imidazole-Doped Thiophene-Based Covalent Organic Frameworks via Subtle Hydrogen Bonding Modulation

Anhydrous proton-conductive materials have attracted great attention in recent years. Doping imidazole as a proton carrier in porous materials, especially pure organic crystalline covalent organic frameworks (COFs), is a promising solution. However, the influence of the hydrogen donor ability of imine functional groups in COFs on the proton conduction has largely been unexplored. Herein, a series of iso-reticular thiophene-based COFs has been synthesized with a similar pore structure and surface area. Different amounts of imidazole were infiltrated to the COFs by vapor diffusion in a highly controlled manner. The introduction of thiophene rings increases the hydrogen bonding donation ability of the imine linker, which resulted in an enhanced proton conductivity of the imidazole-doped COFs by one order of magnitude. The formation of hydrogen bonding between the imine group and imidazole was demonstrated by Fourier transform infrared spectroscopy and density functional theory calculations.

Covalent Organic Frameworks: Design, Synthesis, and Functions

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Solving the COF trilemma: towards crystalline, stable and functional covalent organic frameworks

Strategies in covalent organic frameworks and adjacent fields are highlighted for designing stable, ordered and functional materials.

Post-synthetic modification of imine linkages of a covalent organic framework for its catalysis application

Post-synthetic modification has been the most powerful strategy for covalent organic frameworks (COFs) for their functionalization in many fields. This strategy is typically achieved through the quantitative reaction between existing reactive sites on the linkers (building units) and incoming functional groups. However, usage of linkages (bonds formed to construct COFs) for the post-synthetic modification still remains limited. Herein, we develop a new post-synthetic modification route that is based on the modification of linkages. With this strategy, the imine linkages of a two-dimensional (2D) COF, TFPPy–PyTTA–COF, have been transformed into amine linkages to give the amine-linked isostructure with retention of crystallinity and porosity. The subsequent aminolysis of the amine linkages with 1,3-propane sultone and further metathetical reaction with cobalt acetate [Co(OAc)₂] enable the introduction of cobalt alkyl sulfonate to the one-dimensional (1D) channel walls of the COF. The resulting ionic COF with coupled Co²⁺ in the frameworks shows excellent catalytic activity and good recyclability towards the cycloaddition reactions of epoxides and CO₂. This strategy is of interest as it opens a way to use linkage modification for exploring the potential of COFs for different applications.

A new approach for post-synthetic modification of covalent organic frameworks has been developed based on the modification of the linkages and the resulting COF exhibited excellent catalytic performance towards cycloaddition of epoxides and CO₂.

Postsynthetic functionalization of covalent organic frameworks

Covalent organic frameworks (COFs) have been at the forefront of porous-material research in recent years. With predictable structural compositions and controllable functionalities, the structures and properties of COFs could be controlled to achieve targeted materials. On the other hand, the predesigned structure of COFs allows fruitful postsynthetic modifications to introduce new properties and functions. In this review, the postsynthetic functionalizations of COFs are discussed and their impacts towards structural qualities and performances are comparatively elaborated on. The functionalization involves the formation of specific interactions (covalent or coordination/ionic bonds) and chemical reactions (oxidation/reduction reaction) with pendant groups, skeleton and reactive linkages of COFs. The chemical stability and performance of COFs including catalytic activity, storage, sorption and opto-electronic properties might be enhanced by specific postsynthetic functionalization. The generality of these strategies in terms of chemical reactions and the range of suitable COFs places them as a pivotal role for the development of COF-based smart materials.

Covalent Organic Frameworks: Chemical Approaches to Designer Structures and Built-In Functions

A new approach has been developed to design organic polymers using topology diagrams. This strategy enables covalent integration of organic units into ordered topologies and creates a new polymer form, that is, covalent organic frameworks. This is a breakthrough in chemistry because it sets a molecular platform for synthesizing polymers with predesignable primary and high-order structures, which has been a central aim for over a century but unattainable with traditional design principles. This new field has its own features that are distinct from conventional polymers. This Review summarizes the fundamentals as well as major progress by focusing on the chemistry used to design structures, including the principles, synthetic strategies, and control methods. We scrutinize built-in functions that are specific to the structures by revealing various interplays and mechanisms involved in the expression of function. We propose major fundamental issues to be addressed in chemistry as well as future directions from physics, materials, and application perspectives.

Pore surface engineering of covalent organic frameworks: structural diversity and applications

Connecting molecular building blocks by covalent bonds to form extended crystalline structures has caused a sharp upsurge in the field of porous materials, especially covalent organic frameworks (COFs), thereby translating the accuracy, precision, and versatility of covalent chemistry from discrete molecules to two-dimensional and three-dimensional crystalline structures. COFs are crystalline porous frameworks prepared by a bottom-up approach from predesigned symmetric units with well-defined structural properties such as a high surface area, distinct pores, cavities, channels, thermal and chemical stability, structural flexibility and functional design. Due to the tedious and sometimes impossible introduction of certain functionalities into COFs via de novo synthesis, pore surface engineering through judicious functionalization with a range of substituents under ambient or harsh conditions using the principle of coordination chemistry, chemical conversion, and building block exchange is of profound importance. In this review, we aim to summarize dynamic covalent chemistry and framework linkage in the context of design features, different methods and perspectives of pore surface engineering along with their versatile roles in a plethora of applications such as biomedical, gas storage and separation, catalysis, sensing, energy storage and environmental remediation.

Self-Assembly of Nanoparticles in a Modular Fashion to Prepare Multifunctional Catalysts for Cascade Reactions: From Simplicity to Complexity

One-pot cascade reactions can simplify the synthetic route and reduce the use of solvents and energy. The critical part of the completion of the cascade reaction is the preparation of multifunctional catalysts. In this work, a novel and simple pathway was developed to construct multifunctional catalysts with acidic, basic, and magnetic properties at the same time. Mesoporous silica materials modified with different metal oxides were used as catalytic elements. Microspheres that assembled with catalytic components have a diameter of 150 μm and a specific surface area larger than 400 m² g^-1 and can be used as catalysts for cascade reactions. The yield of the final product in the deacetalization-Knoevenagel reaction is 92%. Microspheres integrated with Fe₃O₄ nanoparticles have a magnetic susceptibility of 7.2 emu g^-1 and can be easily removed from the reaction system by applying an external magnetic field. This multimodule assembly method fully reflects the enormous power of complexity resulting from simplicity. This method provides a reference and practical technical support for the preparation of other multifunctional materials.

Covalent Organic Framework Decorated with Vanadium as a New Platform for Prins Reaction and Sulfide Oxidation

Chemical functionalization or docking of transition-metal ions in covalent organic frameworks (COFs) is of importance for calibrating properties and widening potential applications. In this work, we demonstrate the successful decoration of COF with vanadium as exemplified in the context of post-synthetically modifying two-dimensional COF that features eclipsed stacking structure, large pores, hydroxyl functionalities, high thermal and chemical stability using vanadyl acetylacetonate. The potent catalytic behavior of vanadium-decorated COF was systematically investigated in the reactions of Prins condensation and sulfide oxidation, which revealed its excellent catalytic performances in terms of efficacious activity, preservation of framework crystallinity and reusability. Our work not only contributes the first ever report of vanadium-decorated COF-catalyzed Prins reaction and sulfide oxidation but paves a new way for docking COF with metals for a broad range of applications.

Post-synthetic modification of covalent organic frameworks

This review is aimed at providing an in-depth understanding of the potential of post-synthetic strategies for the modification of covalent organic frameworks.

Covalent Organic Frameworks: Promising Materials as Heterogeneous Catalysts for C-C Bond Formations

Covalent organic frameworks (COFs) are defined as highly porous and crystalline polymers, constructed and connected via covalent bonds, extending in two- or three-dimension. Compared with other porous materials such as zeolite and active carbon, the versatile and alternative constituent elements, chemical bonding types and characteristics of ordered skeleton and pore, enable the rising large family of COFs more available to diverse applications including gas separation and storage, optoelectronics, proton conduction, energy storage and in particular, catalysis. As the representative candidate of next-generation catalysis materials, because of their large surface area, accessible and size-tunable open nano-pores, COFs materials are suitable for incorporating external useful active ingredients such as ligands, complexes, even metal nanoparticles deposition and substrate diffusion. These advantages make it capable to catalyze a variety of useful organic reactions such as important C-C bond formations. By appropriate pore-engineering in COFs materials, even enantioselective asymmetric C-C bond formations could be realized with excellent yield and ee value in much shorter reaction time compared with their monomer and oligomer analogues. This review will mainly introduce and discuss the paragon examples of COFs materials for application in C-C bond formation reactions for the organic synthetic purpose.