A Robust Pyrazolate Metal-Organic Framework for Efficient Catalysis of Dehydrogenative C-O Cross Coupling Reaction.

Construction of robust heterogeneous catalysts with atomic precision is a long-sought pursuit in the catalysis field due to its fundamental significance in taming chemical transformations. Herein, we present the synthesis of a single-crystalline pyrazolate metal-organic framework (MOF) named PCN-300, bearing a lamellar structure with two distinct Cu centers and one-dimensional (1D) open channels when stacked. PCN-300 exhibits exceptional stability in aqueous solutions across a broad pH range from 1 to 14. In contrast, its monomeric counterpart assembled through hydrogen bonding displays limited stability, emphasizing the role of Cu-pyrazolate coordination bonds in framework robustness. Remarkably, the synergy of the 1D open channels, excellent stability, and the active Cu-porphyrin sites endows PCN-300 with outstanding catalytic activity in the cross dehydrogenative coupling reaction to form the C-O bond without the "compulsory" ortho-position directing groups (yields up to 96%), outperforming homogeneous Cu-porphyrin catalysts. Moreover, PCN-300 exhibits superior recyclability and compatibility with various phenol substrates. Control experiments reveal the synergy between the Cu-porphyrin center and framework in PCN-300 and computations unveil the free radical pathway of the reaction. This study highlights the power of robust pyrazolate MOFs in directly activating C-H bonds and catalyzing challenging chemical transformations in an environmentally friendly manner.

Exceptionally High Perfluorooctanoic Acid Uptake in Water by a Zirconium-Based Metal-Organic Framework through Synergistic Chemical and Physical Adsorption.

Perfluorooctanoic acid (PFOA) is an environmental contaminant ubiquitous in water resources, which as a xenobiotic and carcinogenic agent, severely endangers human health. The development of techniques for its efficient removal is therefore highly sought after. Herein, we demonstrate an unprecedented zirconium-based MOF (PCN-999) possessing Zr6 and biformate-bridged (Zr6)2 clusters simultaneously, which exhibits an exceptional PFOA uptake of 1089 mg/g (2.63 mmol/g), representing a ca. 50% increase over the previous record for MOFs. Single-crystal X-ray diffraction studies and computational analysis revealed that the (Zr6)2 clusters offer additional open coordination sites for hosting PFOA. The coordinated PFOAs further enhance the interaction between coordinated and free PFOAs for physical adsorption, boosting the adsorption capacity to an unparalleled high standard. Our findings represent a major step forward in the fundamental understanding of the MOF-based PFOA removal mechanism, paving the way toward the rational design of next-generation adsorbents for per- and polyfluoroalkyl substance (PFAS) removal.

Aqueous Sol-Gel Synthesis and Shaping of Covalent Organic Frameworks.

The intrinsic fragility and insoluble nature of covalent organic frameworks (COFs) have strongly impeded their processability for practical applications. Herein, an aqueous-based sol-gel synthetic strategy is reported for the synthesis and shaping of COFs with task-specific applications that satisfy the principles of green chemistry for gram-scale production of crystalline materials. Our successful approach involves three pivotal aspects: the "prodrug mimic" design of water-soluble monomers, the utilization of hydrolyzable bonds, and the manipulation of reaction kinetics. The generality of the method is demonstrated by the successful preparation of representative high-surface area two-dimensional (2D) COFs with several commonly used amines. By virtue of this strategy, a COF colloidal dispersion is achieved and can be formulated into processable fluids, structured films, and COF monoliths. Remarkably, the obtained lightweight (∼0.020 g cm-3) and robust aerogels displayed outstanding adsorption capacity (exceeding 57 times its own weight) toward a variety of organic solvents and exhibited superior thermal insulating properties compared to the widely used sponge and cotton. This work demonstrates a versatile strategy for the synthesis and shaping of processable COF materials in water that will contribute to the development of COF monoliths for advanced applications.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis.

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Boosting Hydrostability and Carbon Dioxide Capture of Boroxine-Linked Covalent Organic Frameworks by One-Pot Oligoamine Modification.

Boron-based covalent organic frameworks (COFs) are susceptible to nucleophilic attack by water at the electron-deficient boron sites and even slightly humid air could destroy the integrity of their porous frameworks within hours. Such instability is a major limitation to the practical applications of boron-based COFs. Herein we report a significant enhancement of hydrostability of boroxine-linked COFs (COF-1 as representative) by modification with an oligoamine (tetraethylenepentamine, TEPA), which leads to survival of the modified COF in water and long-time stability under humid atmosphere. Meanwhile, the TEPA modification also results in a considerable increase in CO2 adsorption capacity up to 13 times and a dramatic improvement in CO2 /N2 selectivity in low pressure region, which make the modified COF suitable for capturing CO2 from flue gas. This work provides a facile, efficient, and scalable method to greatly improve hydrostability of boroxine-linked COFs and reshape them into high-performance CO2 adsorbents.

Converting an amorphous covalent organic polymer to a crystalline covalent organic framework mediated by a repairing agent.

We herein report a new approach to converting an amorphous covalent organic polymer to a crystalline heteropore covalent organic framework (COF), which is promoted by using an additive for structure repair. This provides a new method for the construction of COFs from cross-linked polymers.

A Heteromeric Carboxylic Acid Based Single-Crystalline Crosslinked Organic Framework.

The development of large pore single-crystalline covalently linked organic frameworks is critical in revealing the detailed structure-property relationship with substrates. One emergent approach is to photo-crosslink hydrogen-bonded molecular crystals. Introducing complementary hydrogen-bonded carboxylic acid building blocks is promising to construct large pore networks, but these molecules often form interpenetrated networks or non-porous solids. Herein, we introduced heteromeric carboxylic acid dimers to construct a non-interpenetrated molecular crystal. Crosslinking this crystal precursor with dithiols afforded a large pore single-crystalline hydrogen-bonded crosslinked organic framework HC OF-101. X-ray diffraction analysis revealed HC OF-101 as an interlayer connected hexagonal network, which possesses flexible linkages and large porous channels to host a hydrazone photoswitch. Multicycle Z/E-isomerization of the hydrazone took place reversibly within HC OF-101, showcasing the potential use of HC OF-101 for optical information storage.

Two-dimensional covalent organic frameworks with hierarchical porosity.

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers assembled by connecting organic building units via covalent bonds. They are characterized as extended two-dimensional (2D) or three-dimensional (3D) frameworks with precise spatial structures and building block distribution. A key feature of COFs is their inherent porosity originating from their well-ordered nanopores which are designable, tunable and modifiable through pore engineering. This review describes the pore engineering of 2D COFs based on their framework topologies. It begins with a brief summary of the pore design principles of 2D COFs which are composed of uniform micropores or mesopores. Then the state-of-the-art progress achieved in a new branch of 2D COFs, that is, heteropore COFs, which possess multiple-pore skeletons and thus exhibit hierarchical porosity, is comprehensively reviewed, including their design strategies, synthesis, characterization, properties and applications. In the last part, personal perspectives on this emerging class of 2D polymers with complex structures and hierarchical porosity are discussed.

Fabricating Organic Nanotubes through Selective Disassembly of Two-Dimensional Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are an emerging class of crystalline porous organic polymers with potential for innovative applications. Here we report the use of COFs as precursors for the fabrication of well-defined tubular nanomaterials. A proof-of-concept study is presented for the controllable fabrication of organic nanotubes through selective disassembly of two-dimensional heteropore COFs. Two dual-pore COFs are constructed based on orthogonal reactions. Each COF possesses two different kinds of pores, which are formed by linking all-hydrzaone-bonded nanopores with boroxines. Selectively hydrolyzing boroxine rings in the COFs while keeping hydrazone linkages untouched gives rise to organic nanotubes with diameters and shapes corresponding to the nanochannels of the COFs.

Rational design of crystalline two-dimensional frameworks with highly complicated topological structures.

Constructing two-dimensional (2D) polymers with complex tessellation patterns via synthetic chemistry makes a significant contribution not only to the understanding of the emergence of complex hierarchical systems in living organisms, but also to the fabrication of advanced hierarchical materials. However, to achieve such tasks is a great challenge. In this communication we report a facile and general approach to tessellate 2D covalent organic frameworks (COFs) by three or four geometric shapes/sizes, which affords 2D COFs bearing three or four different kinds of pores and increases structural complexity in tessellations of 2D polymers to a much higher level. The complex tessellation patterns of the COFs are elucidated by powder X-ray diffraction studies, theoretical simulations and high-resolution TEM.

A gaseous hydrogen chloride chemosensor based on a 2D covalent organic framework.

A tetraphenylethene-based 2D covalent organic framework (COF) has been synthesized. It exhibits a very fast response and high sensitivity to the presence of gaseous HCl by way of distinct changes in fluorescence emission and color, which makes the COF a good chemosensor for spectroscopic and naked-eye detection of gaseous HCl.

Self-sorted pore-formation in the construction of heteropore covalent organic frameworks based on orthogonal reactions.

Two heteropore COFs have been constructed by taking advantage of orthogonal dynamic covalent bonds. And an unprecedented self-sorted pore-formation in the polymerization process was observed, from which micropores with distinctive bonding manners were produced.

Regulating the topology of 2D covalent organic frameworks by the rational introduction of substituents.

The topology of a covalent organic framework (COF) is generally believed to be dictated by the symmetries of the monomers used for the condensation reaction. In this context, the use of monomers with different symmetries is usually required to afford COFs with different topologies. Herein, we report a conceptual strategy to regulate the topology of 2D COFs by introducing alkyl substituents into the skeleton of a parent monomer. The resulting monomers, sharing the same C2 symmetry, were assembled with a D2h symmetric tetraamine to generate a dual-pore COF or single-pore COFs, depending on the sizes of the substituents, which were evidenced using PXRD studies and pore size distribution analyses. These results demonstrate that the substituent is able to exert a significant influence on the topology of COFs, which is crucial for their application.

Construction of 2D covalent organic frameworks by taking advantage of the variable orientation of imine bonds.

A model system has been established to construct two-dimensional (2D) covalent organic frameworks (COFs) by taking advantage of the variable orientation of imine bonds. During the assembly process, the imine bonds adopt an unprecedented heterodromous orientation to facilitate the formation of the COFs.

Construction of Covalent Organic Frameworks Bearing Three Different Kinds of Pores through the Heterostructural Mixed Linker Strategy.

It is very important to create novel topologies and improve structural complexity for covalent organic frameworks (COFs) that might lead to unprecedented properties and applications. Despite the progress achieved over the past decade, the structural diversity and complexity of COFs are quite limited. In this Communication, we report the construction of COFs bearing three different kinds of pores through the heterostructural mixed linker strategy involving the condensation of a D2h-symmetric tetraamine and two C2-symmetric dialdehydes of different lengths. The complicated structures of the triple-pore COFs have been confirmed by powder X-ray diffraction and pore size distribution analyses.