Precisely Embedding Active Sites into a Mesoporous Zr-Framework through Linker Installation for High-Efficiency Photocatalysis.

The pore engineering of microporous metal-organic frameworks (MOFs) has been extensively investigated in the past two decades, and an expansive library of functional groups has been introduced into various frameworks. However, the reliable procurement of MOFs possessing both a targeted pore size and preferred functionality together is less common. This is especially important since the applicability of many elaborately designed materials is often restricted by the small pore sizes of microporous frameworks. Herein, we designed and synthesized a mesoporous MOF based on Zr6 clusters and tetratopic carboxylate ligands, termed PCN-808. The accessible coordinatively unsaturated metal sites as well as the intrinsic flexibility of the framework make PCN-808 a prime scaffold for postsynthetic modification via linker installation. A linear ruthenium-based metalloligand was successfully and precisely installed into the walls of open channels in PCN-808 while maintaining the mesoporosity of the framework. The photocatalytic activity of the obtained material, PCN-808-BDBR, was examined in the aza-Henry reaction and demonstrated high conversion yields after six catalytic cycles. Furthermore, thanks to the mesoporous nature of the framework, PCN-808-BDBR also exhibits exceptional yields for the photocatalytic oxidation of dihydroartemisinic acid to artemisinin.

Switching in Metal-Organic Frameworks.

In recent years, metal-organic frameworks (MOFs) have become an area of intense research interest because of their adjustable pores and nearly limitless structural diversity deriving from the design of different organic linkers and metal structural building units (SBUs). Among the recent great challenges for scientists include switchable MOFs and their corresponding applications. Switchable MOFs are a type of smart material that undergo distinct, reversible, chemical changes in their structure upon exposure to external stimuli, yielding interesting technological applicability. Although the process of switching shares similarities with flexibility, very limited studies have been devoted specifically to switching, while a fairly large amount of research and a number of Reviews have covered flexibility in MOFs. This Review focuses on the properties and general design of switchable MOFs. The switching activity has been delineated based on the cause of the switching: light, spin crossover (SCO), redox, temperature, and wettability.

Functionalization of Zirconium-Based Metal-Organic Layers with Tailored Pore Environments for Heterogeneous Catalysis.

Intriguing properties and functions are expected to implant into metal-organic layers (MOLs) to achieve tailored pore environments and multiple functionalities owing to the synergies among multiple components. Herein, we demonstrate a facile one-pot synthetic strategy to incorporate multiple functionalities into stable zirconium MOLs via secondary ligand pillaring. Through the combination of Zr6 -BTB (BTB=benzene-1,3,5-tribenzoate) layers and diverse secondary ligands (including ditopic and tetratopic linkers), 31 MOFs with multi-functionalities were systematically prepared. Notably, a metal-phthalocyanine fragment was successfully incorporated into this Zr-MOL system, giving rise to an ideal platform for the selective oxidation of anthracene. The organic functionalization of two-dimensional MOLs can generate tunable porous structures and environments, which may facilitate the excellent catalytic performance of as-synthesized materials.

Tuning the Ionicity of Stable Metal-Organic Frameworks through Ionic Linker Installation.

The predictable topologies and designable structures of metal-organic frameworks (MOFs) are the most important advantages for this emerging crystalline material compared to traditional porous materials. However, pore-environment engineering in MOF materials is still a huge challenge when it comes to the growing requirements of expanded applications. A useful method for the regulation of pore-environments, linker installation, has been developed and applied to a series of microporous MOFs. Herein, employing PCN-700 and PCN-608 as platforms, ionic linker installation was successfully implemented in both microporous and mesoporous Zr-based MOFs to afford a series of ionic frameworks. Selective ionic dye capture results support the ionic nature of these MOFs. The mesopores in PCN-608 are able to survive after installation of the ionic linkers, which is useful for ion exchange and further catalysis. To illustrate this, Ru(bpy)32+, a commonly used photoactive cation, was encapsulated into the anionic mesoporous PCN-608-SBDC via ion exchange. Photocatalytic activity of Ru(bpy)3@PCN-608-SBDC was examined by aza-Henry reactions, which show good catalytic performance over three catalytic cycles.

Enhancing Pore-Environment Complexity Using a Trapezoidal Linker: Toward Stepwise Assembly of Multivariate Quinary Metal-Organic Frameworks.

Multicomponent metal-organic frameworks (MOFs) promise the precise placement of synergistic functional groups with atomic-level precision, capable of promoting fascinating developments in basic sciences and applications. However, the complexity of multicomponent systems poses a challenge to their structural design and synthesis. Herein, we show that linkers of low symmetry can bring new opportunities to the construction of multicomponent MOFs. A carbazole-tetracarboxylate linker of  C s point group symmetry was designed and combined with an 8-connected Zr6 cluster to generate a low-symmetry MOF, PCN-609. PCN-609 contains coordinatively unsaturated Zr sites arranged within a lattice with three crystallographically distinct pockets, which can accommodate linear linkers of different lengths. Sequential linker installation was carried out to postsynthetically insert three linear linkers into PCN-609, giving rise to a quinary MOF. Functionalization of each linker from the quinary MOF system creates multivariate pore environments with unprecedented complexity.

Interior Decoration of Stable Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are a diverse class of hybrid organic/inorganic crystalline materials composed of metal-containing nodes held in place by organic linkers. Through a discerning selection of these components, many properties such as the internal surface area, cavity size and shape, catalytic properties, thermal properties, and mechanical properties may be manipulated. Because of this high level of tunability, MOFs have been heralded as ideal platforms for various applications including gas storage, separation, catalysis, and chemical sensing. (1-8) Regrettably, these theoretical possibilities are limited by the reality of constraining conditions for solvothermal synthesis, which typically include high temperatures (usually over 100 °C), the use of specific solvents, and necessary exposure to acidic or basic conditions. In order to incorporate more delicate functionalities, postsynthesis decoration methods were developed. This feature article focuses on developed interior decoration methods for stable MOFs and the dynamic relationship between such methods and MOF stability. In particular, methods to transform organic, inorganic, and organometallic MOF parts as well as combination techniques, the generation of defects, and the inclusion of enzymes are addressed.

Controllable Fluorescence Switching of a Coordination Chain Based on the Photoinduced Single-Crystal-to-Single-Crystal Reversible Transformation of a syn-[2.2]Metacyclophane.

The observation of a reversible chemical transformation corresponding to an external stimulus in the solid state is intriguing in the exploration of smart materials, which can potentially be applied in molecular machines, molecular switches, sensors, and data storage devices. The solid-state photodimerization reaction of 1,3-bis[2-(4-pyridyl)ethenyl]benzene (1,3-bpeb) in a one-dimensional coordination polymer {[Cd2(1,3-bpeb)2(4-FBA)4]·H2O}n (4-FBA = 4-fluorobenzoate) with 365 nm UV light afforded syn-tetrakis(4-pyridyl)-1,2,9,10-diethano[2.2]metacyclophane (syn-tpmcp) in quantitative yield via a single-crystal-to-single-crystal (SCSC) transformation. Upon irradiation with 254 nm UV light, an SCSC conversion from syn-tpmcp to 1,3-bpeb was also achieved in quantitative yield within the syn-tpmcp-supported coordination polymer {[Cd2(syn-tpmcp)(4-FBA)4]·H2O}n. In particular, accompanied by the reversible transformation between 1,3-bpeb and syn-tpmcp, the coordination chain exhibits photocontrollable fluorescence-switching behavior, which makes this intelligent material an appealing candidate for practical applications.

Enzyme-MOF (metal-organic framework) composites.

The ex vivo application of enzymes in various processes, especially via enzyme immobilization techniques, has been extensively studied in recent years in order to enhance the recyclability of enzymes, to minimize enzyme contamination in the product, and to explore novel horizons for enzymes in biomedical applications. Possessing remarkable amenability in structural design of the frameworks as well as almost unparalelled surface tunability, Metal-Organic Frameworks (MOFs) have been gaining popularity as candidates for enzyme immobilization platforms. Many MOF-enzyme composites have achieved unprecedented results, far outperforming free enzymes in many aspects. This review summarizes recent developments of MOF-enzyme composites with special emphasis on preparative techniques and the synergistic effects of enzymes and MOFs. The applications of MOF-enzyme composites, primarily in transferation, catalysis and sensing, are presented as well. The enhancement of enzymatic activity of the composites over free enzymes in biologically incompatible conditions is emphasized in many cases.

Pore-Environment Engineering with Multiple Metal Sites in Rare-Earth Porphyrinic Metal-Organic Frameworks.

Multi-component metal-organic frameworks (MOFs) with precisely controlled pore environments are highly desired owing to their potential applications in gas adsorption, separation, cooperative catalysis, and biomimetics. A series of multi-component MOFs, namely PCN-900(RE), were constructed from a combination of tetratopic porphyrinic linkers, linear linkers, and rare-earth hexanuclear clusters (RE6 ) under the guidance of thermodynamics. These MOFs exhibit high surface areas (up to 2523 cm2 g-1 ) and unlimited tunability by modification of metal nodes and/or linker components. Post-synthetic exchange of linear linkers and metalation of two organic linkers were realized, allowing the incorporation of a wide range of functional moieties. Two different metal sites were sequentially placed on the linear linker and the tetratopic porphyrinic linker, respectively, giving rise to an ideal platform for heterogeneous catalysis.

Formation of a Highly Reactive Cobalt Nanocluster Crystal within a Highly Negatively Charged Porous Coordination Cage.

Earth-abundant first-row transition-metal nanoclusters (NCs) have been extensively investigated as catalysts. However, their catalytic activity is relatively low compared with noble metal NCs. Enhanced catalytic activity of cobalt NCs can be achieved by encapsulating Co NCs in soluble porous coordination cages (PCCs). Two cages, PCC-2a and 2b, possess almost identical cavity in shape and size, while PCC-2a has five times more net charges than PCC-2b. Co2+ cations were accumulated in PCC-2a and reduced to ultra-small Co NCs in situ, while for PCC-2b, only bulky Co particles were formed. As a result, Co NCs@PCC-2a accomplished the highest catalytic activity in the hydrolysis of ammonium borane among all the first-row transition-metals NCs. Based on these results, it is envisioned that confining in the charged porous coordination cage could be a novel route for the synthesis of ultra-small NCs with extraordinary properties.

Control the Structure of Zr-Tetracarboxylate Frameworks through Steric Tuning.

Ligands with flexible conformations add to the structural diversity of metal-organic frameworks but, at the same time, pose a challenge to structural design and prediction. Representative examples include Zr-tetracarboxylate-based MOFs, which afford assorted structures for a wide range of applications, but also complicate the structural control. Herein, we systematically studied the formation mechanism of a series of (4,8)-connected Zr-tetracarboxylate-based MOFs by altering the substituents on different positions of the organic linkers. Different ligand rotamers give rise to three types of structures with flu, scu, and csq topologies. A combination of experiment and molecular simulation indicate that the steric hindrance of the substituents at different positions dictates the resulting MOF structures. Additionally, the controllable formation of different structures was successfully implemented by a combination of linkers with different steric effects at specific positions.

Flexible Zirconium MOFs as Bromine-Nanocontainers for Bromination Reactions under Ambient Conditions.

A series of flexible MOFs (PCN-605, PCN-606, and PCN-700) are synthesized and applied to reversible bromine encapsulation and release. The chemical stability of these Zr-MOFs ensures the framework's integrity during the bromine adsorption, while the framework's flexibility allows for structural adaptation upon bromine uptake to afford stronger host-guest interactions and therefore higher bromine adsorption capacities. The flexible MOFs act as bromine-nanocontainers which elongate the storage time of volatile halides under ambient conditions. Furthermore, the bromine pre-adsorbed flexible MOFs can be used as generic bromine sources for bromination reactions giving improved yields and selectivities under ambient conditions when compared with liquid bromine.

Ylide mediated carbonyl homologations for the preparation of isatin derivatives.

An exceptionally mild method for the preparation of isatin derivatives has been developed using a sulfur ylide mediated carbonyl homologation sequence starting from anthranilic acid precursors. This method proceeds at ambient temperature via a sulfur ylide intermediate without the need for protection of the amine or chromatographic isolation of the intermediate ylide. Gentle oxidation of the sulfur ylides provides isatin derivatives with N-H, N-alkyl, N-aryl substitution, electron-rich and electron-poor aromatic rings, and heterocyclic aromatic systems. We anticipate that this method will greatly expand the accessibility of complex isatin derivatives.

Zirconium metal-organic frameworks incorporating tetrathiafulvalene linkers: robust and redox-active matrices for in situ confinement of metal nanoparticles.

Redox-active metal-organic frameworks (MOFs), with highly ordered porous structures and redox tunability, have attracted research interest in the fields of catalysis, energy storage, and electrochemical sensing. However, the chemical lability has limited the application scope of many redox-active MOFs. Herein, we selected stable Zr6 inorganic nodes and redox-active tetrathiafulvalene (TTF)-based linkers to construct two robust, redox-active MOFs, namely compounds 1 ([Zr6(TTFTB)2O8(OH2)8]) and 2 ([Zr6(Me-TTFTB)1.5O4(OH)4(C6H5COO)6]) (TTFTB = tetrathiafulvalene tetrabenzoate; Me-TTFTB = tetrathiafulvalene tetramethylbenzoate). The structure and topology of the MOFs were controlled by tuning the linker conformation through steric effects, resulting in a variety of pore structures from microporous channels (compound 1) to hierarchically micro/mesoporous cages (compound 2). Compound 2 shows high porosity with a BET surface area of 1932 m2 g-1 and strong chemical stability in aqueous solutions with pH ranging from 1 to 12. Furthermore, the reductive TTF moieties allow for in situ generation and stabilization of ultra-small noble metal (Ag, Pd, and Au) nanoparticles by incubating MOFs in the respective metal salt solution. Single crystal structures, TEM images, and pore size distribution data from N2 adsorption measurements indicated that the metal nanoparticles were mostly placed in the small cubic cavities of hierarchically porous compound 2, leaving the large cages open for substrate diffusion. As a proof of concept, Pd NPs@compound 2 was utilized as a heterogeneous catalyst for aerobic oxidation of alcohols, showing noteworthy activity and recyclability.

High Modulus, Thermally Stable, and Self-Extinguishing Aramid Nanofiber Separators.

Mechanically and thermally robust separators offer an alternative approach for preventing battery failure under extreme conditions such as high loads and temperatures. However, the trade-off between electrochemical performance and mechanical and thermal stability remains an ongoing challenge. Here, we investigate aramid nanofiber (ANF) separators that possess high moduli and self-extinguishing characteristics. The ANF separators are formed from the dissolution of bulk Kevlar fibers and their subsequent vacuum-assisted self-assembly. Thermogravimetric analysis shows a high 5 wt % decomposition temperature of 447 °C, which is over ∼175 °C higher than commercial Celgard separators. The ANF separator also possesses a high Young's modulus of 8.8 GPa, which is ∼1000% higher than commercial separators. Even when dry or when soaked in battery electrolyte, the ANF separators self-extinguish upon exposure to flame, whereas commercial separators melt or drip. We show that these features, although adventitious, present a trade-off with electrochemical performance in which a lithium nickel manganse cobalt (NMC) oxide-based battery possessed a reduced capacity of 123.4 mA h g-1. Considering the separator holistically, we propose that the ANF separator shows an excellent balance of the combined properties of high modulus, flame-resistance, thermal stability, and electrochemical stability and might be suitable for extreme environment applications with further testing.

Photosensitizer-Anchored 2D MOF Nanosheets as Highly Stable and Accessible Catalysts toward Artemisinin Production.

2D metal-organic frameworks (2D-MOFs) have recently emerged as promising materials for gas separations, sensing, conduction, and catalysis. However, the stability of these 2D-MOF catalysts and the tunability over catalytic environments are limited. Herein, it is demonstrated that 2D-MOFs can act as stable and highly accessible catalyst supports by introducing more firmly anchored photosensitizers as bridging ligands. An ultrathin MOF nanosheet-based material, Zr-BTB (BTB = 1,3,5-tris(4-carboxyphenyl)benzene), is initially constructed by connecting Zr6-clusters with the tritopic carboxylate linker. Surface modification of the Zr-BTB structure was realized through the attachment of porphyrin-based carboxylate ligands on the coordinatively unsaturated Zr metal sites in the MOF through strong Zr-carboxylate bond formation. The functionalized MOF nanosheet, namely PCN-134-2D, acts as an efficient photocatalyst for 1O2 generation and artemisinin production. Compared to the 3D analogue (PCN-134-3D), PCN-134-2D allows for fast reaction kinetics due to the enhanced accessibility of the catalytic sites within the structure and facile substrate diffusion. Additionally, PCN-134(Ni)-2D exhibits an exceptional yield of artemisinin, surpassing all reported homo- or heterogeneous photocatalysts for the artemisinin production.

Stable Metal-Organic Frameworks with Group 4 Metals: Current Status and Trends.

Group 4 metal-based metal-organic frameworks (MIV-MOFs), including Ti-, Zr-, and Hf-based MOFs, are one of the most attractive classes of MOF materials owing to their superior chemical stability and structural tunability. Despite being a relatively new field, MIV-MOFs have attracted significant research attention in the past few years, leading to exciting advances in syntheses and applications. In this outlook, we start with a brief overview of the history and current status of MIV-MOFs, emphasizing the challenges encountered in their syntheses. The unique properties of MIV-MOFs are discussed, including their high chemical stability and strong tolerance toward defects. Particular emphasis is placed on defect engineering in Zr-MOFs which offers additional routes to tailor their functions. Photocatalysis of MIV-MOF is introduced as a representative example of their emerging applications. Finally, we conclude with the perspective of new opportunities in synthesis and defect engineering.

Stable metal-organic frameworks as a host platform for catalysis and biomimetics.

Recent years have witnessed the exploration and synthesis of an increasing number of metal-organic frameworks (MOFs). The utilization of stable MOFs as a platform for catalysis and biomimetics is discussed. This Feature Article will provide insights into the rational design and synthesis of three types of stable MOF catalysts on the basis of structural features of MOFs, that is, (i) MOF catalysts with catalytic sites on metal nodes, (ii) MOF catalysts with catalytic sites immobilized in organic struts, and (iii) MOF catalysts with catalytic centres encapsulated in the pores. Then, MOFs used in biomimetics including biomimetic mineralization, biosensors and biomimetic replication are introduced. Finally, a discussion on the challenges that must be addressed for successful implementation of MOFs in catalysis and biomimetics is presented.

Retrosynthesis of multi-component metal-organic frameworks.

Crystal engineering of metal-organic frameworks (MOFs) has allowed the construction of complex structures at atomic precision, but has yet to reach the same level of sophistication as organic synthesis. The synthesis of complex MOFs with multiple organic and/or inorganic components is ultimately limited by the lack of control over framework assembly in one-pot reactions. Herein, we demonstrate that multi-component MOFs with unprecedented complexity can be constructed in a predictable and stepwise manner under simple kinetic guidance, which conceptually mimics the retrosynthetic approach utilized to construct complicated organic molecules. Four multi-component MOFs were synthesized by the subsequent incorporation of organic linkers and inorganic clusters into the cavity of a mesoporous MOF, each composed of up to three different metals and two different linkers. Furthermore, we demonstrated the utility of such a retrosynthetic design through the construction of a cooperative bimetallic catalytic system with two collaborative metal sites for three-component Strecker reactions.

[Ti8Zr2O12(COO)16] Cluster: An Ideal Inorganic Building Unit for Photoactive Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) based on Ti-oxo clusters (Ti-MOFs) represent a naturally self-assembled superlattice of TiO2 nanoparticles separated by designable organic linkers as antenna chromophores, epitomizing a promising platform for solar energy conversion. However, despite the vast, diverse, and well-developed Ti-cluster chemistry, only a scarce number of Ti-MOFs have been documented. The synthetic conditions of most Ti-based clusters are incompatible with those required for MOF crystallization, which has severely limited the development of Ti-MOFs. This challenge has been met herein by the discovery of the [Ti8Zr2O12(COO)16] cluster as a nearly ideal building unit for photoactive MOFs. A family of isoreticular photoactive MOFs were assembled, and their orbital alignments were fine-tuned by rational functionalization of organic linkers under computational guidance. These MOFs demonstrate high porosity, excellent chemical stability, tunable photoresponse, and good activity toward photocatalytic hydrogen evolution reactions. The discovery of the [Ti8Zr2O12(COO)16] cluster and the facile construction of photoactive MOFs from this cluster shall pave the way for the development of future Ti-MOF-based photocatalysts.