Organic Linkers Research Articles

Polysilicate Porous Organic Polymers (PSiPOPs), a Family of Porous, Ordered 3D Reticular Materials with Polysilicate Nodes and Organic Linkers

Polysilicate Porous Organic Polymers (PSiPOPs), a Family of Porous, Ordered 3D Reticular Materials with Polysilicate Nodes and Organic Linkers

Construction of defective hydroxyl-rich metal–organic framework for effective capture of borate ion

Developing high-efficiency adsorbents for borate ion still remains up to now. Herein, a facile and green method is used to obtain serial hydroxyl functionalized UiO-66-(OH)2-AA-X (AA, acetic acid; X, AA usage) by using AA as a modulator. The addition of AA can induce the generation of defect units in the adsorbents due to its partial substitution for the organic linkers. It is found that the defect degree as well as the porosity can be neatly regulated by the AA amount. In particular, UiO-66-(OH)2-AA-20 with the most defects exhibits a high adsorption capacity (891.1 mg g−1) towards borate ion, outperforming the most previously reported adsorbents. The adsorption reaches equilibrium at 600 min. The adsorption behavior can be well described by the Langmuir isotherm model and pseudo-second-order model. Besides, UiO-66-(OH)2-AA-20 possesses a wide pH (3–10) tolerance, excellent anti-interference ability, and good reusability. A combination of experimental characterization and density functional theory calculation indicates the free hydroxyl groups serve as the main adsorption sites, and the adsorption-driven forces involve electrostatic, coordination, hydrogen bonds and π–π stacking interactions. On these basis, our work may provide a guideline for constructing high-efficiency defective adsorbents for borate ions via a modulated strategy.

Bridging the Homogeneous and Heterogeneous Catalysis by Supramolecular Metal-Organic Cages with Varied Packing Modes.

Integrating the advantages of homogeneous and heterogeneous catalysis has proved to be an optimal strategy for developing catalytic systems with high efficiency, selectivity, and recoverability. Supramolecular metal-organic cages (MOCs), assembled by the coordination of metal ions with organic linkers into discrete molecules, have performed solvent processability due to their tunable packing modes, endowing them with the potential to act as homogeneous or heterogeneous catalysts in different solvent systems. Here, the design and synthesis of a series of stable {Cu3} cluster-based tetrahedral MOCs with varied packing structures are reported. These MOCs, as homogeneous catalysts, not only show high catalytic activity and selectivity regardless of substrate size during the CO2 cycloaddition reaction, but also can be easily recovered from the reaction media through separating products and co-catalysts by one-step work-up. This is because that these MOCs have varied solubilities in different solvents due to the tunable packing of MOCs in the solid state. Moreover, the entire catalytic reaction system is very clean, and the purity of cyclic carbonates is as high as 97% without further purification. This work provides a unique strategy for developing novel supramolecular catalysts that can be used for homogeneous catalysis and recycled in a heterogeneous manner.

Atomic observation and structural evolution of covalent organic framework rotamers

Dynamic 3D covalent organic frameworks (COFs) have shown concerted structural transformation and adaptive gas adsorption due to the conformational diversity of organic linkers. However, the isolation and observation of COF rotamers constitute undergoing challenges due to their comparable free energy and subtle rotational energy barrier. Here, we report the atomic-level observation and structural evolution of COF rotamers by cryo-3D electron diffraction and synchrotron powder X-ray diffraction. Specifically, we optimize the crystallinity and morphology of COF-320 to manifest its coherent dynamic responses upon adaptive inclusion of guest molecules. We observe a significant crystal expansion of 29 vol% upon hydration and a giant swelling with volume change up to 78 vol% upon solvation. We record the structural evolution from a non-porous contracted phase to two narrow-pore intermediate phases and the fully opened expanded phase using n-butane as a stabilizing probe at ambient conditions. We uncover the rotational freedom of biphenylene giving rise to significant conformational changes on the diimine motifs from synclinal to syn-periplanar and anticlinal rotamers. We illustrate the 10-fold increment of pore volumes and 100% enhancement of methane uptake capacity of COF-320 at 100 bar and 298 K. The present findings shed light on the design of smarter organic porous materials to maximize host-guest interaction and boost gas uptake capacity through progressive structural transformation.

Controlled Growth of Highly Defected Zirconium-Metal-Organic Frameworks via a Reaction-Diffusion System for Water Remediation.

The relentless growth of metal-organic framework (MOF) chemistry is paralleled by the persistent urge to control the MOFs physical and chemical properties. While this control is mostly achieved by solvothermal syntheses, room temperature procedures stand out as more convenient and sustainable pathways for the production of MOF materials. Herein, a novel approach to control the crystal size and defect numbers of a dihydroxy-functionalized zirconium-based metal-organic framework (UiO-66(OH)2) at room temperature is reported. Through a reaction-diffusion method in a 1D system, zirconium salt was diffused into an agar gel matrix containing the organic linker to form nanocrystals of UiO-66(OH)2 with tailored structural features that include crystal size distribution, surface area, and defect number. By variation of the synthesis parameters of the system, hierarchical MOF nanocrystals with an average size ranging from 30 nm up to 270 nm and surface areas between 201 and 500 m2 g-1 were obtained in a one-pot synthetic route. To stress the importance of crystal size, morphology, and structural defects on the adsorption properties of UiO-66(OH)2, the adsorption capacity of the MOF toward methylene blue dye was tested with the largest and most defected crystals achieving the best performance of 202 mg/g. The distinctive structural characteristics including the hierarchical micromesoporous frameworks, the nanosized particles, and the highly defective crystals obtained by our synthesis procedure are deemed challenging through the conventional synthesis methods. This work paves the way for engineering MOF crystals with tunable physical and chemical properties, using a green synthesis procedure, for their advantageous use in many desirable applications.

Two-in-one: Proton-conductive and luminescence properties of one strontium(II)-organic framework bearing imidazole dicarboxylate

Despite the current surge in the production of metal-organic frameworks (MOFs) by crystal engineering, more structurally diverse MOFs must be developed and prepared for the study of their application properties and structural stability. Herein, one novel two-dimensional MOF, [Sr(μ5-p-FPhHIDC)(H2O)]n (1) was solvothermally constructed and structurally characterized by adopting an organic linker, 2-(4-fluorophenyl)-1H-imidazole-4,5-dicarboxylic acid (p-FPhH3IDC), in which the [SrO9] tetrakaidecahedrons were joined by the organic linkers to constitute a beautiful grid structure. The remarkable fluorescence characteristics and proton transfer are due to the strong coordination between the organic ligands and the central metal ions in the framework of 1, as well as the carboxylic acid and coordination water molecules carried on the framework. Extensive tests demonstrated that, at relative humidity of 75–98 %, this MOF displayed a specific proton conductivity (10−7-10−5 S/cm) and significant room-temperature solid-state fluorescence (λem = 406 nm). Proton conduction and luminescence mechanisms were also conjectured. This study is a helpful resource for creating MOF materials with a range of application-specific properties.

Metal-Organic Framework-Based Nanomedicines for Ferroptotic Cancer Therapy.

As an iron-dependent, non-apoptosis, regulated cell death (RCD) modality, ferroptosis has gained growing attention for cancer therapy. With the development of nanomaterials in the biomedical field, ferroptotic cancer nanomedicine is extensively investigated. Amongst various nanomaterials, metal-organic frameworks (MOFs) are hybridized porous materials consisting of metal ions or clusters bridged by organic linkers. The superior properties of MOFs, such as high porosity and cargo loading, ease of surface modification, and good biocompatibility, make them appealing in inducing or sensitizing ferroptotic cell death. There are remarkable achievements in the field of MOF-based ferroptosis cancer therapy. However, this topic is not reviewed. This review will introduce the fundamentals of MOF and ferroptosis machinery, summarize the recent progress of MOF-based ferroptotic anticancer drug delivery, discuss the benefits and problems of MOFs as vehicles and sensitizers for cancer ferroptosis, and provide the perspective on future research direction on this promising field.

Pore-Networked Soft Materials Based on Metal-Organic Polyhedra.

ConspectusThe last two decades have witnessed a tremendous development of crystalline microporous adsorbents in a wide range of applications including molecular adsorption, storage and separation, purification, as well as catalysis. The main players as porous materials that have contributed to the developments are extended molecular frameworks (e.g., metal-organic frameworks, MOFs; covalent-organic frameworks, COFs) or discrete porous molecules (e.g., metal-organic cages, MOCs; porous organic cages, POCs) thanks to the high degrees of freedom in their structural designability and tunability. To overcome the processability issue originating from their powder forms after synthesis, one main strategy is to hybridize the microporous adsorbents as pore-containing fillers with solvents or polymers as processable matrices to produce porous soft materials, such as porous liquids, gels/aerogels, and mixed-matrix membranes, depending on the form of matrix used. Nevertheless, the fabrication of "ideal" hybrid materials relies on the homogeneous distribution of the pore-containing fillers within the matrices. It is still challenging to find a versatile way to solve the aggregation issues of fillers and their insufficient interaction with the matrices, which are concerned with inhibiting the translation of the distinctive properties of microporous adsorbents into the obtained hybrid soft materials.Herein, we describe a new bottom-up approach for the fabrication of "pore-networked soft materials" based on the concept of directly interconnecting the pore-containing fillers into a continuous pore network within the matrices. The advantages of the pore-networking strategy lie in two main aspects: (i) the elimination of the need to struggle with the aggregation issue of fillers due to their overall interconnection throughout the matrices; (ii) the generation of continuous pore networks that guarantee the efficient molecular mass transfer in the materials. In this Account, we summarize our state-of-the-art progress of pore-networked soft materials based on the use of MOCs, alternatively called metal-organic polyhedra (MOPs) herein, as pore units for the pore network construction. The good solubility of MOPs in organic solvents allows them to be feasibly processed in solution, wherein the coordination of MOPs with organic linkers leads to the formation of linked MOP gels featuring not only intrinsic MOP cavities but also tunable extrinsic porosities generated between linked MOPs through the control of MOP/linker structures and network connectivity. Furthermore, the matrix of the linked MOP network, here referred to as the continuous phase with respect to the entire porous MOP network, is not limited to the solvents. We anticipate that the implementation of air, liquids, and polymers as the matrices could result in different forms of pore-networked soft materials like aerogels, foams, gels, monoliths, and membranes. For instance, we demonstrate the fabrication of linked MOP aerogel and permanently porous gel with their potential applications on selective CO2 photoreduction and gas sorption, respectively. We believe that the pore-network strategies will advance the development of porous soft materials featuring unique advantages and properties beyond the current hybrid systems.

Visible-light-active benzothiadiazole-based MOFs as efficient ROS generators for the synthesis of benzimidazoles and benzothiazoles

We provided a convenient approach to integrate the 2,1,3-benzothiadiazole-based organic linker into a UiO-68-type MOF as the first example generating O2˙− and 1O2 for the green synthesis of benzimidazoles and benzothiazoles.

Antibacterial, antibiofilm, and antioxidant activities of two novel metal-organic frameworks (MOFs) based on 4,6-diamino-2-pyrimidinethiol with Zn and Co metal ions as coordination polymers.

In the present era, the increase in free radical species (FRs) and multidrug-resistant (MDR) bacteria represents a major worldwide concern for public health. Biofilm development and the overuse and misuse of antibiotics could lead to the adaptation of bacteria to antimicrobial agents. Consequently, finding novel multifunctional species with antibacterial, antioxidant, and antibiofilm properties has become crucial in the fight against challenging bacterial infections and chronic inflammatory conditions. Metal-organic frameworks (MOFs) with zinc and cobalt metal centers are widely utilized in biological and environmental remediation owing to their versatility. In this study, multifunctional Zn-MOFs and Co-MOFs were successfully synthesized with zinc and cobalt as metal centers and 4,6-diamino-2-pyrimidinethiol as an organic linker using a hydrothermal technique. Numerous characterization techniques were used to fully examine the MOF structure, functionality, chemical makeup, crystalline structure, surface appearance, thermal behavior, and magnetic characteristics; the techniques included XPS, PXRD, FTIR, FESEM, EDX, UV-visible, BET, BJH, TGA/DTG, DSC, and magnetic susceptibility measurement. The antioxidant, antibacterial, and antibiofilm activities of the MOFs were examined, and they demonstrated potent activity in each of these aspects. The proposed mechanisms of antibacterial activity suggest that bacterial cell death results from multiple toxic effects, including electrostatic interaction and lipid peroxidation, when MOFs are attached to bacteria, leading to the formation of reactive oxygen species (ROSs). Zn-MOFs exhibit high antibacterial and antibiofilm efficacy owing to their large surface-to-volume ratio and porous nature, while Co-MOFs exhibit high antioxidant capacity owing to their redox properties.

Mitochondrial topoisomerase 1 targeted anticancer therapy using irinotecan encapsulated mesoporous MIL-101(Fe) synthesized via a vapour assisted method.

Mitochondrial topisomerase 1 (Top1mt) is critical for mtDNA replication, transcription, and energy production. Here, we investigate the carrier-mediated targeted delivery of the anticancer drug irinotecan into the mitochondria to selectively trap Top1mt covalent complexes (Top1mtcc) and its role in anticancer therapeutics. We have designed a biocompatible mesoporous metal-organic framework (MOF) material, namely MIL-101(Fe), as the drug delivery carrier that selectively localizes inside mitochondria. In contrast to the traditional way of synthesising MOFs, here we have employed a vapour-assisted solvothermal method for the synthesis of MIL-101(Fe) using terephthalic acid as the organic linker and Fe(III) as the metal source. The advantage of this method is that it recycles the excess solvent (DMF) and reduces the amount of washing solvent. We demonstrate that MIL-101(Fe)-encapsulated irinotecan (MIL-Iri) was selectively targeted towards the mitochondria to poison Top1mtcc in a dose-dependent manner and was achieved at a low nanomolar drug concentration. We provide evidence that Top1mtcc generated by MIL-Iri leads to mtDNA damage in human colon and breast cancer cells and plays a significant role in cellular toxicity. Altogether, this study provides evidence for a new and effective strategy in anticancer chemotherapy.

Reticular chemistry guided single-linker constructed pillar-layered metal-organic frameworks via an in situ "one-pot" strategy.

In the present work, we report a "two-in-one" strategy to construct single-linker-based pillar-layered metal-organic frameworks (PL-MOFs) guided by reticular chemistry via an in situ "one-pot" approach. Two carboxyl groups and one pyridine group are integrated into one molecular skeleton to form bifunctional organic linkers via the reaction of pyridine-containing aldehyde and bicarboxylate-containing o-phenylenediamine. During the synthesis of organic linkers, two zinc-based PL-MOFs, non-interpenetrated HIAM-3016-op and two-fold interpenetrated HIAM-3017-op, can be simultaneously constructed. The different interpenetrations for these two PL-MOFs can be attributed to the increased length of the pyridine-containing moiety. HIAM-3017-op can be utilized for Cr2O72- detection with excellent sensitivity and selectivity. The present work not only provides a novel insight to design and prepare PL-MOFs with specific structures guided by reticular chemistry, but also indicates the universality of the in situ "one-pot" strategy to construct porous materials.

Synthesis of diverse stable MOFs and their electro catalytic capabilities towards desulfurization, water splitting and various nitrophenol reduction reactions

Metal–organic frameworks are a novel class of crystalline porous materials with enormous application potential, consisting of metal nodes and organic linkers.

An anthraquinone-based bismuth-iron metal-organic framework as an efficient photoanode in photoelectrochemical cells.

Metal-organic frameworks (MOFs) are appealing candidate materials to design new photoelectrodes for use in solar energy conversion because of their modular nature and chemical versatility. However, to date there are few examples of MOFs that can be directly used as photoelectrodes, for which they must be able to afford charge separation upon light absorption, and promote the catalytic dissociation of water molecules, while maintaining structural integrity. Here, we have explored the use of the organic linker anthraquinone-2, 6-disulfonate (2, 6-AQDS) for the preparation of MOFs to be used as photoanodes. Thus, the reaction of 2, 6-AQDS with Bi(iii) or a combination of Bi(iii) and Fe(iii) resulted in two new MOFs, BiPF-10 and BiFePF-15, respectively. They display similar structural features, where the metal elements are disposed in inorganic-layer building units, which are pillared by the organic linkers by coordination bonds through the sulfonic acid groups. We show that the introduction of iron in the structure plays a crucial role for the practical use of the MOFs as a robust photoelectrode in a photoelectrochemical cell, producing as much as 1.23 mmol H2 cm-2 with the use of BiFePF-15 as photoanode. By means of time-resolved and electrochemical impedance spectroscopic studies we have been able to unravel the charge transfer mechanism, which involves the formation of a radical intermediate species, exhibiting a longer-lived lifetime by the presence of the iron-oxo clusters in BiFePF-15 to reduce the charge transfer resistance.

Harnessing a Ti-based MOF for selective adsorption and visible-light-driven water remediation.

In pursuit of universal access to clean water, photocatalytic water remediation using metal-organic frameworks (MOFs) emerges as a strong alternative to the current wastewater treatment methods. In this study, we explore a unique Ti-based MOF comprised of 2D secondary-building units (SBUs) connected via biphenyl dicarboxylic acid (H2bpdc) ligands - denoted as COK-47 - as a visible-light-driven photocatalyst for organic dye degradation. Synthesized via a recently developed microwave-assisted method, COK-47 exhibits high hydrolytic stability, demonstrates a strong dye uptake, and shows noteworthy dye-degradation performance under UV, visible, and solar light, outperforming benchmark TiO2 and MIL-125-Ti photocatalysts. Due to its nanocrystalline structure and surface termination with organic linkers, COK-47 exhibits selective degradation of cationic pollutants while remaining inert towards anionic dyes, thus highlighting its potential for selective oxidation reactions. Mechanistic studies reveal the involvement of superoxide radicals in the degradation process and emphasize the need to minimize the recombination of photogenerated electron-hole pairs to achieve optimal performance. Post-catalytic studies further confirm the high stability and reusability of COK-47, making it a promising photocatalyst for water purification, organic transformation, and water splitting reactions under visible light.

Tetrathiafulvalene-based covalent organic frameworks as high-voltage organic cathodes for lithium batteries

We report a series of 2D tetrathiafulvalene (TTF)-based COFs incorporating different organic linkers between the electroactive moieties that were investigated as p-type organic cathode materials for lithium-organic batteries.

Simulation studies of water adsorption on MIL-101(Cr) revealing the role of inhomogeneous potential field composed of open metal sites and organic linkers

Simulation studies reveal that the capillary condensation of water on MIL-101(Cr) occurs due to the formation of a liquid film that covers the hydrophobic pore surface, driven by the presence of open metal sites.

A highly efficient lanthanide coordination polymer luminescent material for the multi-task detection of environmental pollutants.

It is challenging to explore novel-structure lanthanide coordination polymers (Ln-CPs) for sensing environmental pollutants. Herein, we designed and synthesized an organic bridging linker 3-(carboxymethoxy)-1-(carboxymethyl) pyrazole-4-carboxylic acid (H3ccpc), and then successfully prepared and characterized a novel Ln-CP, namely [Tb2(ccpc)2(H2O)6]·1.5H2O (ccpcTb). Structural analysis indicates that ccpcTb exhibits a two-dimensional structure, in which Tb ions are in an eight-coordinated environment. The photoluminescence performance of ccpcTb was discussed in detail. The ccpcTb displays bright green luminescence and behaves as a multi-responsive luminescent sensor toward Fe3+ ions, Cr2O72- ions and 2,4,6-trinitrophenol with high selectivity and low detection limits. Furthermore, the possible luminescence sensing mechanisms have been addressed in detail. The luminescence quenching mechanism of sensing Fe3+ and Cr2O72- is attributed to the energy competitive absorption, while that of sensing TNP is due to the synergistic effect of energy competitive absorption and photo-induced electron transfer.

Crystal engineering of a new platform of hybrid ultramicroporous materials and their C2H2/CO2 separation properties.

Hybrid ultramicroporous materials (HUMs) comprised of combinations of organic and inorganic linker ligands are a leading class of physisorbents for trace separations involving C1, C2 and C3 gases. First generation HUMs are modular in nature since they can be self-assembled from transition metal cations, ditopic linkers and inorganic "pillars", as exemplified by the prototypal variant, SIFSIX-3-Zn (3 = pyrazine, SIFSIX = SiF6 2-). Conversely, HUMs that utilise chelating ligands such as ethylenediamine derivatives are yet to be explored as sorbents. Herein, we report the structures and sorption properties of two HUMs based upon the chelating ligand N 1,N 2-bis(pyridin-4-ylmethyl)ethane-1,2-diamine (enmepy), [Zn(enmepy)(SiF6)] n (SIFSIX-24-Zn) and [Zn(enmepy)(SO4)] n (SOFOUR-2-Zn). These HUMs are isostructural and exhibit high C2H2 uptakes of 85 cm3 g-1 (3.79 mmol g-1) and 79 cm3 g-1 (3.52 mmol g-1), and C2H2/CO2 IAST selectivities of 7.4 and 8.1 (1 bar, 1 : 1 mixture, 298 K), respectively. Dynamic column breakthrough experiments resulted in separation factors of 5.26 and 2.05, and CO2 effluent purities of 99.991 and 99.989%, respectively. Temperature programmed desorption experiments at 60 °C resulted in rapid desorption of CO2, followed by fuel grade C2H2 (>98%), affording productivities of 9.45 and 7.96 L kg-1 and maximum C2H2 outlet purities of 99.92% and 99.66%, respectively. This study introduces the use of diamine chelating ligands in HUMs for gas separations through two parent sorbents that are prototypal for families of related materials, one of which, SOFOUR-2-Zn, uses the earth-friendly sulfate anion as a pillar.

Designing organic bridging linkers of metal–organic frameworks for enhanced carbon dioxide adsorption

The global rate of anthropogenic carbon dioxide (CO2) emission is rising, which urges the development of efficient carbon capture and storage (CCS) technologies.