Postsynthetic Linker Exchange Research Articles

P/n-Type Polyimide Covalent Organic Frameworks for High-Performance Cathodes in Sodium-Ion Batteries.

Covalent organic frameworks (COFs) are viewed as promising organic electrode materials for metal-ion batteries due to their structural diversity and tailoring capabilities. In this work, firstly using the monomers N,N,N',N'-tetrakis(4-aminophenyl)-1,4-phenylenediamine (TPDA) and terephthaldehyde (TA), p-type phenylenediamine-based imine-linked TPDA-TA-COF is synthesized. To construct a bipolar redox-active, porous and highly crystalline polyimide-linked COF, i.e., TPDA-NDI-COF, n-type 1,4,5,8-naphthalene tetracarboxylic dianhydride (NDA) molecules are incorporated into p-type TPDA-TA-COF structure via postsynthetic linker exchange method. This tailored COF demonstrated a wide potential window (1.03.6 V vs Na+/Na) with dual redox-active centers, positioning it as a favorable cathode material for sodium-ion batteries (SIBs). Owing to the inheritance of multiple redox functionalities, TPDA-NDI-COF can deliver a specific capacity of 67 mAh g-1 at 0.05 A g-1, which is double the capacity of TPDA-TA-COF (28 mAh g-1). The incorporation of carbon nanotube (CNT) into the TPDA-NDI-COF matrix resulted in an enhancement of specific capacity to 120 mAh g-1 at 0.02 A g-1. TPDA-NDI-50%CNT demonstrated robust cyclic stability and retained a capacity of 92 mAh g-1 even after 10 000 cycles at 1.0 A g-1. Furthermore, the COF cathode exhibited an average discharge voltage of 2.1 V, surpassing the performance of most reported COF as a host material.

Unraveling Early-Stage Dynamics of Cage-to-Covalent Organic Framework Transformation at Liquid-Liquid Interface.

Postsynthetic linker exchange (PLE) has emerged as an emerging synthetic strategy for constructing high-quality covalent organic frameworks (COFs) from preassembled entities such as linear polymers, amorphous networks, COFs, and porous organic cages by using the principles of dynamic covalent chemistry. The PLE strategy has recently been extended at the liquid-liquid interface to fabricate highly crystalline two-dimensional (2D)-COF membranes at a faster time scale (24 h). Examining the early stages of the interfacial PLE dynamics becomes essential to understanding the expedited COF growth process. In this regard, pendant drop tensiometry has been employed to probe the initial reaction dynamics of the imine cage-to-COF transformation through dynamic interfacial tension (IFT) measurements. The contrasting trends in IFT profiles between PLE-mediated (from cage) and direct COF synthesis (from parent monomers) are in qualitative agreement with the kinetics of bulk-scale interfacial polymerizations. Further, the distinct early-stage interfacial behaviors between the diverse synthetic routes have been experimentally demonstrated using tensiometry, optical microscopy, electron microscopy, and powder X-ray diffraction (PXRD) analysis. The pivotal role of in situ generated imine intermediates (ImIs) and the phenomenon of spontaneous emulsification toward accelerated interfacial COF growth process was delineated. The current study on deploying the pendant drop tensiometric technique to examine early-stage interfacial polymerization dynamics opens up a gripping avenue for mechanistic exploration in PLE-based COF synthesis. The generality of the developed methodology to study the initial COF growth kinetics was extended to a new interfacial PLE-mediated cage-to-COF transformation.

Pushing the Boundaries of Covalent Organic Frameworks through Postsynthetic Linker Exchange

AbstractCovalent organic frameworks (COFs) are a fast‐developing family of porous organic materials that have received substantial research interest during the last two decades. Dynamic covalent chemistry (DCC) is the cornerstone of COF fabrication. DCC is a process that entails reversible bond breaking‐reforming under equilibrium to attain the thermodynamically most stable structure. Due to the reversible nature of the covalent linkages, the building blocks of pre‐synthesized COF or pre‐assembled chemical entities, like network polymers and supramolecular hosts, can be replaced postsynthetically under appropriate reaction conditions. The technique is known as postsynthetic linker exchange (PLE). PLE provides an easy way to introduce functional building blocks into the COF backbone and control its chemical and physical properties. In this article, we have highlighted the recent advancements (from 2017 to 2023) in the postsynthetic linker exchange strategy for constructing highly crystalline and porous COFs that are often unattainable via de novo fabrication. The mechanistic insights of the linker exchange process for transforming various parent entities, such as COFs, amorphous covalent organic networks, linear polymers, and molecular cages to daughter COFs, have been deliberated with fascinating examples. We have also outlined some future avenues for applying the PLE process for the large‐scale fabrication of highly crystalline COFs for real‐time applications.

Linker Exchange as Facile Method for Post-Synthetic Modification of β-Ketoenamine-linked Covalent Organic Frameworks.

Post-synthetic linker exchange performed on covalent organic frameworks (COFs) has become an important method to introduce functional building blocks into their backbone and thus to tune their chemical and physical properties. However, the linker exchange method has so far only been described for COFs with relatively weak linkages like imines. Herein, it is shown that this method can be also used for a post-synthetic linker exchange reaction on a β-ketoenamine linked COF. The time needed to achieve considerable linker exchange is much prolonged compared to other COFs with less stable linkages, however, this enables to achieve very good control on the ratio of the respective building blocks within the framework.

Conferring efficient alcohol dehydration to covalent organic framework membranes via post-synthetic linker exchange

Covalent organic frameworks (COFs) have gained tremendous interest as membrane building blocks owing to rigid, long-range ordered and easily functionalized skeletons. The facile, controllable and task-specific construction of COF membranes lies in the frontier of membrane technology. Herein, we explored a post-synthetic linker exchange (PLE) method to engineer COF membranes for efficient alcohol dehydration. The proposed PLE method, utilizing the reversible breaking-reformation basis between pristine COF membranes and post-introduced linkers, narrowed the effective pore size to intensify molecular sieving property. Moreover, the PLE method also endowed COF membranes with more hydrophilic groups, thus improving the water sorption ability of COF membranes. The resultant COF membranes exhibited superior n-butanol dehydration performances with 3620 of selectivity, 11.35 kg m−2 h−1 of total flux and remained stable in 15-day operation. This PLE method may flourish the library of engineering COF membranes for efficient molecular separations.

Construction of a zeolite A-type multivariate metal–organic framework for selective sensing of Fe3+ and Cr2O72−

A multivariant metal–organic framework (MTV-MOF) BUT-27 with a unique zeolite A structure was synthesized by post-synthetic linker exchange via single-crystal-to-single-crystal transformation.

Metal-Organic Framework Derived Nanozymes in Biomedicine.

Nanozymes, which integrate the advantages of both nanomaterials and natural enzymes, have accumulated enormous research interest over the past decades because of the opportunity they provide to appreciate and further cultivate artificial enzymes with comparable properties. By mimicking the coordination environments of the catalytic sites in natural enzymes, nanozymes with confined nanostructures can serve as substitutes in many catalytic processes with comparable activity and robust stability even in harsh conditions. Since the pioneering report about peroxidase-mimicking ferromagnetic nanoparticles in 2007, nanozymes have been developed as specialized for nanomaterials with intrinsic enzyme-mimicking property. With the rapid development in nanoscience and nanotechnology, nanomaterials with superior advantages, such as large-scale production, desired activity, and robust stability, can bridge the natural enzymes with nanozymes.Metal-organic frameworks (MOFs) and their derivatives hold great promise to serve as direct surrogates of conventional enzymes for enzymatic reactions. According to their chemical nature, MOF-based nanozymes can be divided into three main categories: pristine MOFs, enzyme-encapsulated MOF composites, and MOF-based derivatives. Due to the versatility of metallic nodes and bridging linkers together with the feasibility of postsynthetic engineering and modification, MOFs and their derivatives are envisioned as one of the most appropriate surrogates for this purpose. Using MOFs as precursors or sacrificial templates, multiple MOF-based derivatives including carbon-based nanomaterials (e.g., heteroatom-doped carbon or carbon with M-N-C moiety), metal oxide/carbon nanoparticles, and metal/carbon nanomaterials can be rationally synthesized through one-step direct carbonization/oxidation or indirect post-synthesis treatments of MOFs (e.g., bridging linker-exchange and metallic node-doping). Compared with existing nanozymes, MOF-based derivatives open up a new avenue for constructing mesoporous nanozymes. In this way, the intrinsic mesoporous properties of MOFs can still be maintained, while the stability and activity can be greatly improved. In this Account, we highlight some important research advances in MOF-based derivatives (including M-N-C moieties (M = single metal atom), metal oxide/carbon, metal/carbon, and MOF derivatives obtained through postsynthetic linker exchange and metal doping strategies) with enzyme-mimicking activity. We also demonstrate that, through integrating physicochemical properties of mesoporous nanomaterials and enzymatic activities of natural enzymes, MOF-derived nanozymes can provide multifunctional platforms in biomedical fields such as antibacterial agents, biosensors, imaging, cancer therapy, and environmental protection. Finally, we propose future design principles and possible research approaches for deeper understanding of mechanisms, thus pointing out future research directions to offer more opportunities for the conventional enzyme-engineering industry.

Co-immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal-Organic Framework: In Depth Structural Characterization and Photocatalytic Properties for CO2 Reduction.

The Keggin-type polyoxometalate (POM) PW12O403- and the catalytic complex Cp*Rh(bpydc)Cl2 (bpydc = 2,2'-bipyridine-5,5'-dicarboxylic acid) were coimmobilized in the Zr(IV) based metal organic framework UiO-67. The POM is encapsulated within the cavities of the MOF by in situ synthesis, and then, the Rh catalytic complex is introduced by postsynthetic linker exchange. Infrared and Raman spectroscopies, 31P and 13C MAS NMR, N2 adsorption isotherms, and X-ray diffraction indicate the structural integrity of all components (POM, Rh-complex and MOF) within the composite of interest (PW12,Cp*Rh)@UiO-67. DFT calculations identified two possible locations of the POM in the octahedral cavities of the MOF: one at the center of a UiO-67 pore with the Cp*Rh complex pointing toward an empty pore and one off-centered with the Cp*Rh pointing toward the POM. 31P-1H heteronuclear (HETCOR) experiments ascertained the two environments of the POM, equally distributed, with the POM in interaction either with the Cp* fragment or with the organic linker. In addition, Pair Distribution Function (PDF) data were collected on the POM@MOF composite and provided key evidence of the structural integrity of the POM once immobilized into the MOF. The photocatalytic activity of the (PW12,Cp*Rh)@UiO-67 composite for CO2 reduction into formate and hydrogen were evaluated. The formate production was doubled when compared with that observed with the POM-free Cp*Rh@UiO-67 catalyst and reached TONs as high as 175 when prepared as thin films, showing the beneficial influence of the POM. Finally, the stability of the composite was assessed by means of recyclability tests. The combination of XRD, IR, ICP, and PDF experiments was essential in confirming the integrity of the POM, the catalyst, and the MOF after catalysis.

Metal-Diamidobenzoquinone Frameworks via Post-Synthetic Linker Exchange.

Metal-organic frameworks with amidic linkers often exhibit exceptional physical properties, but, owing to their strong metal-nitrogen bonds, are exceedingly challenging to isolate through direct synthesis. Here, we report a route to access metal-diamidobenzoquinone frameworks from their dihydroxobenzoquinone counterparts via postsynthetic linker exchange. The parent compounds (Me2NH2)2[M2L3] (M = Zn, Mn; H2L = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) undergo linker exchange upon exposure to a solution of monodeprotonated 2,5-diamino-3,6-dibromo-1,4-benzoquinone or 2,5-diamino-3,6-dichloro-1,4-benzoquinone, proceeding through single-crystal-to-single-crystal reactions. The presence of both types of linker in the resulting frameworks is confirmed by a combination of NMR, Raman, and energy-dispersive X-ray (EDX) spectroscopies. Moreover, the extent of linker exchange in the Zn frameworks is quantified using 13C NMR spectroscopy, and spatially resolved EDX spectroscopy reveals the two types of linker to be homogeneously distributed within a crystal. Finally, we propose a tentative mechanism of linker exchange based on pKa measurements, considerations of framework solubility, and powder X-ray diffraction analysis. This work provides the first method to exchange organic linkers with different donor atoms in metal-organic frameworks and in doing so demonstrates exchange between linkers with donor atoms differing in acidity by a remarkable 11 units of pKa. Together, these results offer a potentially general synthetic strategy toward new materials with exotic metal-linker coordination modes.

Strongly visible light-absorbing metal-organic frameworks functionalized by cyclometalated ruthenium(ii) complexes.

Four different ruthenium(ii) complexes were incorporated into the metal–organic framework (MOF) UiO-67 using three different synthetic strategies: premade linker synthesis, postsynthetic functionalization, and postsynthetic linker exchange. One of these complexes was of the type (N–N)3Ru2+, and three of the complexes were of the type (N–N)2(N–C)Ru+, where N–N is a bipyridine-type ligand and N–C is a cyclometalated phenylpyridine-type ligand. The resulting materials were characterized by PXRD, SC-XRD (the postsynthetic functionalization MOFs), N2 sorption, TGA-DSC, SEM, EDS, and UV-Vis spectroscopy, and were digested in base for subsequent 1H NMR analysis. The absorption profiles of the MOFs that were functionalized with cyclometalated Ru(ii) complexes extend significantly further into the visible region of the spectrum compared to the absorption profiles of the MOFs that were functionalized with the non-cyclometalated reference, (N–N)3Ru2+.

Vapor‐Phase Linker Exchange of the Metal–Organic Framework ZIF‐8: A Solvent‐Free Approach to Post‐synthetic Modification

AbstractZeolitic imidazolate frameworks (ZIFs) are a sub‐class of metal–organic frameworks (MOFs). Although generally stable, ZIFs can undergo post‐synthetic linker exchange (PSLE) in solution under mild conditions. Herein, we present a novel, solvent‐free approach to post‐synthetic linker exchange through exposure to linker vapor.

Vapor-Phase Linker Exchange of the Metal-Organic Framework ZIF-8: A Solvent-Free Approach to Post-synthetic Modification.

Zeolitic imidazolate frameworks (ZIFs) are a sub-class of metal-organic frameworks (MOFs). Although generally stable, ZIFs can undergo post-synthetic linker exchange (PSLE) in solution under mild conditions. Herein, we present a novel, solvent-free approach to post-synthetic linker exchange through exposure to linker vapor.

MOF Decomposition and Introduction of Repairable Defects Using a Photodegradable Strut.

Photoswitchable components can modulate the properties of metal organic frameworks (MOFs); however, photolabile building blocks remain underexplored. A new strut NPDAC (2-nitro-1,4-phenylenediacetic acid) that undergoes photodecarboxylation has been prepared and incorporated into a MOF, using post-synthetic linker exchange (PSLE) from the structural analogue containing PDAC (p-phenylenediacetic acid). Irradiation of NPDAC-MOF leads to MOF decomposition and concomitant formation of amorphous material. In addition to complete linker exchange, MOFs containing a mixture of PDAC and NPDAC can be obtained through partial linker exchange. In NPDAC30-MOF, which contains approximately 30 % NPDAC, the MOF retains crystallinity after irradiation, but the MOF contains defect sites consistent with loss of decarboxylated NPDAC linkers. The defect sites can be repaired by exposure to additional PDAC or NPDAC linkers at a much faster rate than the initial exchange process. The photoremoval and replacement process may lead to a more general approach to customizable MOF structures.

Active Role of Methanol in Post-Synthetic Linker Exchange in the Metal–Organic Framework UiO-66

UiO-66 is known as one of the most robust metal–organic framework materials. Nevertheless, UiO-66 has also been shown to undergo postsynthetic exchange of structural linkers with surprising ease in...

High-Flux Zeolitic Imidazolate Framework Membranes for Propylene/Propane Separation by Postsynthetic Linker Exchange.

While zeolitic imidazolate framework, ZIF-8, membranes show impressive propylene/propane separation, their throughput needs to be greatly improved for practical applications. A method is described that drastically reduces the effective thickness of ZIF-8 membranes, thereby substantially improving their propylene permeance (that is, flux). The new strategy is based on a controlled single-crystal to single-crystal linker exchange of 2-methylimidazole in ZIF-8 membrane grains with 2-imidazolecarboxaldehyde (ZIF-90 linker), thereby enlarging the effective aperture size of ZIF-8. The linker-exchanged ZIF-8 membranes showed a drastic increase in propylene permeance by about four times, with a negligible loss in propylene/propane separation factor when compared to as-prepared membranes. The linker-exchange effect depends on the membrane synthesis method.

High‐Flux Zeolitic Imidazolate Framework Membranes for Propylene/Propane Separation by Postsynthetic Linker Exchange

AbstractWhile zeolitic imidazolate framework, ZIF‐8, membranes show impressive propylene/propane separation, their throughput needs to be greatly improved for practical applications. A method is described that drastically reduces the effective thickness of ZIF‐8 membranes, thereby substantially improving their propylene permeance (that is, flux). The new strategy is based on a controlled single‐crystal to single‐crystal linker exchange of 2‐methylimidazole in ZIF‐8 membrane grains with 2‐imidazolecarboxaldehyde (ZIF‐90 linker), thereby enlarging the effective aperture size of ZIF‐8. The linker‐exchanged ZIF‐8 membranes showed a drastic increase in propylene permeance by about four times, with a negligible loss in propylene/propane separation factor when compared to as‐prepared membranes. The linker‐exchange effect depends on the membrane synthesis method.

Uniform distribution of post-synthetic linker exchange in metal-organic frameworks revealed by Rutherford backscattering spectrometry.

Rutherford backscattering spectrometry (RBS) has been used for the first time to study post-synthetic linker exchange (PSE) in metal–organic frameworks. RBS is a non-invasive method to quantify the amount of introduced linker, as well as providing a means for depth profiling in order to identify the preferred localization of the introduced linker. The exchange of benzenedicarboxylate (bdc) by similarly sized 2-iodobenzenedicarboxylate (I-bdc) proceeds considerably slower than migration of I-dbc through the UiO-66 crystal. Consequently, the I-bdc is found evenly distributed throughout the UiO-66 samples, even at very short PSE exposure times.

Photocatalytic carbon dioxide reduction with rhodium-based catalysts in solution and heterogenized within metal-organic frameworks.

The first photosensitization of a rhodium-based catalytic system for CO2 reduction is reported, with formate as the sole carbon-containing product. Formate has wide industrial applications and is seen as valuable within fuel cell technologies as well as an interesting H2 -storage compound. Heterogenization of molecular rhodium catalysts is accomplished via the synthesis, post-synthetic linker exchange, and characterization of a new metal-organic framework (MOF) Cp*Rh@UiO-67. While the catalytic activities of the homogeneous and heterogeneous systems are found to be comparable, the MOF-based system is more stable and selective. Furthermore it can be recycled without loss of activity. For formate production, an optimal catalyst loading of ∼10 % molar Rh incorporation is determined. Increased incorporation of rhodium catalyst favors thermal decomposition of formate into H2 . There is no precedent for a MOF catalyzing the latter reaction so far.

Tetrazine functionalized zirconium MOF as an optical sensor for oxidizing gases.

Dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate was introduced into the chemically stable UiO-66 structure by a postsynthetic linker exchange reaction to create an optical sensor material for the detection of oxidative agents such as nitrous gases. The incorporated tetrazine unit can be reversibly oxidized and reduced, which is accompanied by a drastic colour change from yellow to pink and vice versa. The high stability of the framework during redox reaction was proven by powder X-ray diffraction and nitrogen physisorption measurements.

Assessing Chemical Heterogeneity at the Nanoscale in Mixed‐Ligand Metal–Organic Frameworks with the PTIR Technique

AbstractRecently, the use of mixtures of organic‐building‐block linkers has given chemists an additional degree of freedom for engineering metal–organic frameworks (MOFs) with specific properties; however, the poor characterization of the chemical complexity of such MixMOF structures by conventional techniques hinders the verification of rational design. Herein, we describe the application of a technique known as photothermal induced resonance to individual MixMOF microcrystals to elucidate their chemical composition with nanoscale resolution. Results show that MixMOFs isoreticular to In‐MIL‐68, obtained either directly from solution or by postsynthetic linker exchange, are homogeneous down to approximately 100 nm. Additionally, we report a novel in situ process that enables the engineering of anisotropic domains in MOF crystals with submicron linker‐concentration gradients.