l-proline modulated zirconium metal organic frameworks: Simple chiral catalysts for the aldol addition reaction

Defect-engineered chiral metal-organic frameworks

Chirality has an important impact on chemical and biological research, as most active substances are chiral. In recent decades, metal-organic frameworks (MOFs), which are assembled from metal ions or clusters and organic linkers via metal-ligand bonding, have attracted considerable scientific interest due to their high crystallinity, exceptional porosity and tunable pore sizes, high modularity, and diverse functionalities. Since the discovery of the first functional chiral metal-organic frameworks (CMOFs), CMOFs have been involved in a variety of disciplines such as chemistry, physics, optics, medicine, and pharmacology. The introduction of defect engineering theory into CMOFs allows the construction of a class of defective CMOFs with high hydrothermal stability and multi-stage pore structure. The introduction of defects not only increases the active sites but also enlarges the pore sizes of the materials, which improves chiral recognition, separation, and catalytic reactions, and has been widely investigated in various fields. This review describes the design and synthesis of various defective CMOFs, their characterization, and applications. Finally, the development of the materials is summarized, and an outlook is given. This review should provide researchers with an insight into the design and study of complex defective CMOFs.

Engineering Porosity and Functionality in a Robust Twofold Interpenetrated Bismuth-Based MOF: Toward a Porous, Stable, and Photoactive Material

Defect engineering in metal-organic frameworks (MOFs) has gained worldwide research traction, as it offers tools to tune the properties of MOFs. Herein, we report a novel 2-fold interpenetrated Bi-based MOF made of a tritopic flexible organic linker, followed by missing-linker defect engineering. This procedure creates a gradually augmented micro- and mesoporosity in the parent (originally nonporous) network. The resulting MOFs can tolerate a remarkable extent of linker vacancy (with absence of up to 60% of linkers per Bi node) created by altering the crystal-growth rate as a function of synthesis temperature and duration. Owing to the enhanced porosity and availability of the uncoordinated Lewis acidic Bi sites, the defect-engineered MOFs manifested improved surface areas, augmented CO2 and water vapor uptake, and catalytic activity. Parallel to this, the impact of defect engineering on the optoelectronic properties of these MOFs has also been studied, offering avenues for new applications.

Merging gold plasmonic nanoparticles and L-proline inside a MOF for plasmon-induced visible light chiral organocatalysis at low temperature

Light-driven asymmetric photocatalysis represents a straightforward approach in modern organic chemistry. In comparison to the homogeneous one, heterogeneous asymmetric photocatalysis has the advantages of easy catalyst separation, recovery, and reuse, thus being cost- and time-effective. Here, we demonstrate how plasmon-active centers (gold nanoparticles - AuNPs) allow visible light triggering of chiral catalyst (proline) in model aldol reaction between acetone and benzaldehyde. The metal-organic framework UiO-66-NH2 was used as an advanced host platform for the loading of proline and AuNPs and their stabilization in spatial proximity. Aldol reactions were carried out at a low temperature (-20 °C) under light illumination which resulted in 91% ee with a closed-to-quantitative yield, 4.5 times higher than that without light (i.e. in the absence of plasmon triggering). A set of control experiments and quantum chemical modeling revealed that the plasmon assistance proceeds through hot electron excitation followed by an interaction with an enamine with the formation of anion radical species. We also demonstrated the high stability of the proposed system in multiple catalytic cycles without leaching metal ions, which makes our approach especially promising for heterogeneous asymmetric photocatalysis.

Raising the Asymmetric Catalytic Efficiency of Chiral Covalent Organic Frameworks by Tuning the Pore Environment

Chiral covalent organic frameworks (COFs) hold considerable promise in the realm of heterogeneous asymmetric catalysis. However, fine-tuning the pore environment to enhance both the activity and stereoselectivity of chiral COFs in such applications remains a formidable challenge. In this study, we have successfully designed and synthesized a series of clover-shaped, hydrazone-linked chiral COFs, each with a varying number of accessible chiral pyrrolidine catalytic sites. Remarkably, the catalytic efficiencies of these COFs in the asymmetric aldol reaction between cyclohexanone and 4-nitrobenzaldehyde correlate well with the number of accessible pyrrolidine sites within the frameworks. The COF featuring nearly one pyrrolidine moiety at each nodal point demonstrated excellent reaction yields and enantiomeric excess (ee) values, reaching up to 97 and 83%, respectively. The findings not only underscore the profound impact of a deliberately controlled chiral pore environment on the catalytic efficiencies of COFs but also offer a new perspective for the design and synthesis of advanced chiral COFs for efficient asymmetric catalysis.

Supracluster Assembly of Archimedean Cages with 72 Hydrogen Bonds for the Aldol Addition Reaction

Clusters that can be experimentally precisely characterized and theoretically accurately calculated are essential to understanding the relationship between material structure and function. Here, we propose the concept of "supraclusters", which aim to connect "supramolecules" and "suprananoparticles" as well as reveal the unique assembly behavior of "supraclusters" with nanoparticle size at the molecular level. The implementation of supraclusters is full of challenges due to the difficulty in satisfying the ordered connectivity of clusters due to their abundant and dispersed hydrogen bonding sites. By solvothermal synthesis under a high catechol (H2 CATs) content, we successfully isolated a series of triangular {Al6 M3 } cluster compounds possessing brucite-like structural features. Interestingly, eight {Al6 M3 } clusters form 72-fold strong hydrogen bonding truncatedhexahedron Archimedean {Al6 M3 }8 supracluster cage (abbreviated as H-tcu). Surprisingly, the solution stability of the H-tcu was further proved by electrospray ionization mass spectrometry (ESI-MS) characterization. Therefore, it is not difficult to explain the reason for assembly of H-tcu into edge-directed and vertex-directed isomers. These porous supraclusters can be obtained by scale-up synthesis and exhibit a noticeable catalysis effect towards the condensation of acetone and p-nitrobenzaldehyde. As an intermediate state of supramolecule and suprananoparticle, the supracluster assembly can enrich the cluster chemistry and bring new structural types.

Revelation of the Electrochemical Chiral Recognition of l-Proline-Tuned Zr-MOFs

Chirality plays an extremely important role in nature. In this work, a highly ordered and non-clustered crystalline material UiO-88-LP was synthesized by using l-proline (l-Pro)-tuning Zr-MOF and the solvothermal method, which was then modified on the glassy carbon electrode (GCE) to construct an electrochemical chiral interface for the recognition of tryptophan (Trp) configuration. UiO-88-LP composites were characterized by scanning electron microscopy, X-ray transmission diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy. After optimization of the experimental conditions, redox peaks for l-Trp and d-Trp were clearly observed at the UiO-88-LP/GCE electrochemical sensing interface with a peak-to-current ratio (IL/ID) of 2.47. The peak current was positively correlated with the concentration of Trp. The electrochemical recognition behavior of l-Trp and d-Trp was investigated by differential pulse voltammetry. The electrochemical characterization showed that UiO-88-LP/GCE had an enantiomeric resolution of amino acids. The recognition mechanism showed that l-Pro entering the UiO-66 molecular cage provided a site for the system to be recognized, so the purpose of recognition was achieved. The relevant data provide theoretical support for the practical application of UiO-88-LP in electrochemical sensors.

Metal-Organic Frameworks as Formose Reaction Catalysts with Enhanced Selectivity

The formose reaction is an autocatalytic series of aldol condensations that allows one to obtain monosaccharides from formaldehyde. The formose reaction suffers from a lack of selectivity, which hinders practical applications at the industrial level. Over the years, many attempts have been made to overcome this selectivity issue, with modest results. Heterogeneous porous catalysts with acid-base properties, such as Metal-Organic Frameworks (MOFs), can offer advantages compared to homogeneous strong bases (e.g., calcium hydroxide) for increasing the selectivity of this important reaction. For the very first time, four different Zeolite Imidazolate Frameworks are presented in this work as catalysts for the formose reaction in liquid phase, and their catalytic performances were compared with those of the typical homogeneous catalyst (i.e., calcium hydroxide). The heterogeneous nature of the catalysis, the possible contribution of leached metal or linkers to the solution, and the stability of the materials were investigated. The porous structure of these solids and their mild basicity make them suitable for obtaining enhanced selectivity at 30% formaldehyde conversion. Most of the MOFs tested showed low structural stability under reaction conditions, thereby indicating the need to search for new MOF families with higher robustness. However, this important result opens the path for future research on porous heterogeneous basic catalysts for the formose reaction.

The development of chiral metal-organic frameworks for enantioseparation of racemates

MIL-101(Cr), an achiral metal-organic framework, made up of a terephthalic acid ligand and a metal chromium ion was selected as a template. Its structural features are unsaturated Lewis acid sites that can be easily activated and it has an extremely high specific surface area, big pore size, and good thermal/chemical/water stability. This achiral framework was modified to introduce chirality within the structure to develop chiral metal-organic frameworks (CMOFs). Here, natural chiral ligands, amino acids (l-proline, l-thioproline and l-tyrosine), were selected for post synthetic modification (PSM) of MIL-101(Cr). This is a very simple, clean and facile methodology with respect to the reactants and reaction conditions. CMOFs 1-3 abbreviated as MIL-101-l-proline (CMOF-1), MIL-101-l-thioproline (CMOF-2) and MIL-101-l-tyrosine (CMOF-3) were prepared by introducing l-proline, l-thioproline and l-tyrosine as chiral moieties within the framework of (Cr). These CMOFs were characterized by FTIR, PXRD, SEM, and thermo gravimetric analysis. Chirality within these CMOFs 1-3 was established by circular dichroism (CD) and polarimetric methods. These three CMOFs 1-3 showed enantioselectivity towards RS-ibuprofen, RS-mandelic acid and RS-1-phenylethanol to varying extents. Their enantioselectivity towards racemates was studied by chiral HPLC and polarimetry.

CuCo Nanoparticle, Pd(II), and l-Proline Trifunctionalized UiO-67 Catalyst for Three-Step Sequential Asymmetric Reactions

Metal-organic frameworks (MOFs) have become a promising support for different active sites to construct multifunctional and heterogeneous catalysts. However, the related investigation mainly focuses on introducing one or two active sites into MOFs and trifunctional catalysts have been very rarely reported. Herein, non-noble CuCo alloy nanoparticles, Pd²⁺, and l-proline, as encapsulated active species, functional organic linkers, and active metal nodes, respectively, were successfully decorated to UiO-67 to construct a chiral trifunctional catalyst by the one-step method, which was further applied to asymmetric three-step sequential oxidation of aromatic alcohols/Suzuki coupling/asymmetric aldol reactions with excellent oxidation and coupling performance (yields up to 95 and 96%, respectively), as well as good enantioselectivities (ee_anti value up to 73%) in asymmetric aldol reactions. The heterogeneous catalyst can be reused at least five times without obvious deactivation due to the strong interaction between the MOFs and the active sites. This work provides an effective strategy to construct multifunctional catalysts via the introduction and combination of three or more of active sites, including encapsulated active species, functional organic linkers, and active metal nodes, into stable MOFs.

Chiral Metal-Organic Frameworks

In the past two decades, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) assembled from metal ions or clusters and organic linkers via metal-ligand coordination bonds have captivated significant scientific interest on account of their high crystallinity, exceptional porosity, and tunable pore size, high modularity, and diverse functionality. The opportunity to achieve functional porous materials by design with promising properties, unattainable for solid-state materials in general, distinguishes MOFs from other classes of materials, in particular, traditional porous materials such as activated carbon, silica, and zeolites, thereby leading to complementary properties. Scientists have conducted intense research in the production of chiral MOF (CMOF) materials for specific applications including but not limited to chiral recognition, separation, and catalysis since the discovery of the first functional CMOF (i.e., d- or l-POST-1). At present, CMOFs have become interdisciplinary between chirality chemistry, coordination chemistry, and material chemistry, which involve in many subjects including chemistry, physics, optics, medicine, pharmacology, biology, crystal engineering, environmental science, etc. In this review, we will systematically summarize the recent progress of CMOFs regarding design strategies, synthetic approaches, and cutting-edge applications. In particular, we will highlight the successful implementation of CMOFs in asymmetric catalysis, enantioselective separation, enantioselective recognition, and sensing. We envision that this review will provide readers a good understanding of CMOF chemistry and, more importantly, facilitate research endeavors for the rational design of multifunctional CMOFs and their industrial implementation.

Vapor-assisted crystallization of in situ glycine-modified UiO-66 with enhanced CO2 adsorption

Vapor consisting of DMF and HCl promotes crystallization of in situ glycine-modified UiO-66.

Catalytic Synthesis of Methacrolein via the Condensation of Formaldehyde and Propionaldehyde with L-Proline Intercalated Layered Double Hydroxides

Aldol condensation reactions are very important C–C coupling reactions in organic chemistry. In this study, the catalytic performance of layered double hydroxides (LDHs) in the aldol condensation reaction of formaldehyde (FA) and propionaldehyde (PA) was investigated. The MxAl-LDHs (denoted as re-MxAl–LDHs; M = Ca and Mg; X = 2–4), as heterogeneous basic catalysts toward the aldol condensation reaction, were prepared via a two-step procedure. The catalyst exhibited a high PA conversion (82.59%), but the methacrolein (MAL) selectivity was only 36.01% due to the limitation of the alkali-catalyzed mechanism. On this basis, the direct intercalation of L-proline into LDHs also has been investigated. The influences of several operating conditions, including the temperature, reaction time, and substrate content, on the reaction results were systematically studied, and the optimized reaction conditions were obtained. The optimized Mg3Al–Pro-LDHs catalyst exhibited a much higher MAL selectivity than those of re-MgxAl–LDHs.

Creation of Exclusive Artificial Cluster Defects by Selective Metal Removal in the (Zn, Zr) Mixed-Metal UiO-66

The differentiation between missing linker defects and missing cluster defects in MOFs is difficult, thereby limiting the ability to correlate materials properties to a specific type of defects. Herein, we present a novel and easy synthesis strategy for the creation of solely "missing cluster defects" by preparing mixed-metal (Zn, Zr)-UiO-66 followed by a gentle acid wash to remove the Zn nodes. The resulting material has the reo UiO-66 structure, typical for well-defined missing cluster defects. The missing clusters are thoroughly characterized, including low-pressure Ar-sorption, iDPC-STEM at a low dose (1.5 pA), and XANES/EXAFS analysis. We show that the missing cluster UiO-66 has a negligible number of missing linkers. We show the performance of the missing cluster UiO-66 in CO₂ sorption and heterogeneous catalysis.

Generating Catalytic Sites in UiO-66 through Defect Engineering

UiO-66 is regarded as an epitome of metal-organic frameworks (MOFs) because of its stability. Defect engineering has been used as a toolbox to alter the performance of MOFs. UiO-66 is among the most widely explored MOFs because of its capability to bear a high number of defects without undergoing structural collapse. Several representative works in the field of MOF-based defect engineering are available based on UiO-66. In this review, more emphasis is given toward the construction of catalytic sites by engineering defects in UiO-66 as a representative including all the detailed synthesis procedures for inducing defects, and the characterization techniques used to analyze these defects in UiO-66 are discussed. Furthermore, a comprehensive review for the defects themselves and the support using defects in catalysis is provided to accentuate the importance of defect engineering.

Role of additives and solvents in the synthesis of chiral isoreticular MOF-74 topologies

Chiral induction is a simple and inexpensive approach to synthesise chiral metal–organic frameworks, even when using achiral building-blocks. The challenge lies in selecting the proper chiral inductor. This can only be achieved upon understanding the mechanism behind the chirality transfer from the chiral guest to the achiral MOF. In this work, the role of two types of chiral additives and different solvents was investigated in the crystallization of isoreticular MOF-74. We show that pyrrolidone-based solvents can interact with the framework walls and influence the thermal stability of the MOF. The role of the different chiral additives is related to the strength of their interaction with the MOF. Unlike cinchona alkaloids that have weak interactions with the framework, l- or d-trans-4-hydroxyproline (l- or d-Hyp) can strongly bind to the Zn²⁺ metal centres and cause the twisting of the organic linker. Moreover, l- and d-Hyp additives can affect the IRMOF-74 nucleation process depending on their concentration and handedness.

Strongly interacting chiral additives play a complex role during the crystallisation of chiral isoreticular MOF-74. They can coordinate to the open sites of the metal ions and induce strain on the framework, leading to a local twisting of the organic linker.

An advancement in the synthesis of nano Pd@magnetic amine-Functionalized UiO-66-NH2 catalyst for cyanation and O-arylation reactions

The magnetic MOF-based catalytic system has been reported here to be an efficient catalyst for synthesis of benzonitriles and diarylethers of aryl halides under optimal conditions. The MOF catalyst was built based on magnetic nanoparticles and UiO-66-NH₂ which further modified with 2,4,6-trichloro-1,3,5-triazine and 5-phenyl tetrazole at the same time and the catalyst structure was confirmed by various techniques. This new modification has been applied to increase anchoring palladium into the support. Furthermore, the products' yields were obtained in good to excellent for all reactions under mild conditions which result from superior activity of the synthesized heterogeneous catalyst containing palladium. Also, the magnetic property of the MOF-based catalyst makes it easy to separate from reaction mediums and reuse in the next runs.

Defect-Engineered Chiral Metal-Organic Frameworks for Efficient Asymmetric Aldol Reaction

By employment of a mixed truncated chiral ligand synthetic strategy, a defect-engineered chiral metal-organic framework with hierarchical micro/mesoporous structure was prepared, and it exhibited efficient heterogeneous catalytic activity and enantioselectivity for asymmetric aldol reaction.

Aqueous Nd3+ capture using a carboxyl-functionalized porous carbon derived from ZIF-8

A porous graphitic carbon was obtained via the pyrolysis of a zeolite imidazolate framework (ZIF-8) under Ar atmosphere. Then, the carbon was functionalized with carboxylic groups and applied for separation of neodymium ions (Nd³⁺) from water. The adsorbent (denoted as C-ZDC) was characterized by X-ray diffraction, N₂ adsorption-desorption isotherms, infrared spectroscopy, X-ray photoelectron spectroscopy, scanning and transition electron microscopies, thermogravimetric analysis, and Boehm titration. A practical adsorption equilibrium was attained within 4 h, and the adsorption isotherm at 25 °C revealed a maximum adsorption capacity of 175 mg/g, which is one of the highest values reported for different kinds of adsorbents. The adsorption kinetics and equilibrium isotherms were modeled, and the selectivity for Nd³⁺ over other metal ions was examined. From the effect of solution pH on the adsorption and material characterization results before and after adsorption, the high adsorption capacity of C-ZDC was ascribed to the formation of coordination bonds between Nd³⁺ ions and the -COOH groups. Further, the material was reusable for at least four adsorption-desorption cycles after a simple step of acid washing.

Synthesis of Chiral MOF-74 Frameworks by Post-Synthetic Modification by Using an Amino Acid

The synthesis of chiral metal-organic frameworks (MOFs) is highly relevant for asymmetric heterogenous catalysis, yet very challenging. Chiral MOFs with MOF-74 topology were synthesised by using post-synthetic modification with proline. Vibrational circular dichroism studies demonstrate that proline is the source of chirality. The solvents used in the synthesis play a key role in tuning the loading of proline and its interaction with the MOF-74 framework. In N,N'-dimethylformamide, proline coordinates monodentate to the Zn2+ ions within the MOF-74 framework, whereas it is only weakly bound to the framework when using methanol as solvent. Introducing chirality within the MOF-74 framework also leads to the formation of defects, with both the organic linker and metal ions missing from the framework. The formation of defects combined with the coordination of DMF and proline within the framework leads to a pore blocking effect. This is confirmed by adsorption studies and testing of the chiral MOFs in the asymmetric aldol reaction between acetone and para-nitrobenzaldehyde.

Chiral Nanoceramics

The study of different chiral inorganic nanomaterials has been experiencing rapid growth during the past decade, with its primary focus on metals and semiconductors. Ceramic materials can substantially expand the range of mechanical, optical, chemical, electrical, magnetic, and biological properties of chiral nanostructures, further stimulating theoretical, synthetic, and applied research in this area. An ever-expanding toolbox of nanoscale engineering and self-organization provides a chirality-based methodology for engineering of hierarchically organized ceramic materials. However, fundamental discoveries and technological translations of chiral nanoceramics have received substantially smaller attention than counterparts from metals and semiconductors. Findings in this research area are scattered over a variety of sources and subfields. Here, the diversity of chemistries, geometries, and properties found in chiral ceramic nanostructures are summarized. They represent a compelling materials platform for realization of chirality transfer through multiple scales that can result in new forms of ceramic materials. Multiscale chiral geometries and the structural versatility of nanoceramics are complemented by their high chiroptical activity, enantioselectivity, catalytic activity, and biocompatibility. Future development in this field is likely to encompass chiral synthesis, biomedical applications, and optical/electronic devices. The implementation of computationally designed chiral nanoceramics for biomimetic catalysts and quantum information devices may also be expected.

Assessing Crystallisation Kinetics of Zr Metal-Organic Frameworks through Turbidity Measurements to Inform Rapid Microwave-Assisted Synthesis

Controlling the crystallisation of metal-organic frameworks (MOFs), network solids of metal ions or clusters connected by organic ligands, is often hindered by the significant number of synthetic variables inherent to their synthesis. Coordination modulation, the addition of monotopic competing ligands to solvothermal syntheses, can allow tuning of physical properties (particle size, porosity, surface chemistry), enhance crystallinity, and select desired phases, by modifying the kinetics of self-assembly, but its mechanism(s) are poorly understood. Herein, turbidity measurements were used to assess the effects of modulation on the solvothermal synthesis of the prototypical Zr terephthalate MOF UiO-66 and the knowledge gained was applied to its rapid microwave synthesis. The studied experimental parameters-temperature, reagent concentration, reagent aging, metal precursor, water content, and modulator addition-all influence the time taken for onset of nucleation, and subsequently allow microwave synthesis of UiO-66 in as little as one minute. The simple, low cost turbidity measurements align closely with previously reported in situ synchrotron X-ray diffraction studies, proving their simplicity and utility for probing the nucleation of complex materials while offering significant insights to the synthetic chemist.

Modulated self-assembly of metal-organic frameworks

Exercising fine control over the synthesis of metal-organic frameworks (MOFs) is key to ensuring reproducibility of physical properties such as crystallinity, particle size, morphology, porosity, defectivity, and surface chemistry. The principle of modulated self-assembly - incorporation of modulator molecules into synthetic mixtures - has emerged as the primary means to this end. This perspective article will detail the development of modulated synthesis, focusing primarily on coordination modulation, from a technique initially intended to cap the growth of MOF crystals to one that is now used regularly to enhance crystallinity, control particle size, induce defectivity and select specific phases. The various mechanistic driving forces will be discussed, as well as the influence of modulation on physical properties and how this can facilitate potential applications. Modulation is also increasingly being used to exert kinetic control over self-assembly; examples of phase selection and the development of new protocols to induce this will be provided. Finally, the application of modulated self-assembly to alternative materials will be discussed, and future perspectives on the area given.

Engineering a Highly Defective Stable UiO-66 with Tunable Lewis- Brønsted Acidity: The Role of the Hemilabile Linker

The stability of metal-organic frameworks (MOFs) typically decreases with an increasing number of defects, limiting the number of defects that can be created and limiting catalytic and other applications. Herein, we use a hemilabile (Hl) linker to create up to a maximum of six defects per cluster in UiO-66. We synthesized hemilabile UiO-66 (Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate (PSBA) as the hemilabile linker. The PSBA acts not only as a modulator to create defects but also as a coligand that enhances the stability of the resulting defective framework. Furthermore, upon a postsynthetic treatment in H₂SO₄, the average number of defects increases to the optimum of six missing BDC linkers per cluster (three per formula unit), leaving the Zr-nodes on average sixfold coordinated. Remarkably, the thermal stability of the materials further increases upon this treatment. Periodic density functional theory calculations confirm that the hemilabile ligands strengthen this highly defective structure by several stabilizing interactions. Finally, the catalytic activity of the obtained materials is evaluated in the acid-catalyzed isomerization of α-pinene oxide. This reaction is particularly sensitive to the Brønsted or Lewis acid sites in the catalyst. In comparison to the pristine UiO-66, which mainly possesses Brønsted acid sites, the Hl-UiO-66 and the postsynthetically treated Hl-UiO-66 structures exhibited a higher Lewis acidity and an enhanced activity and selectivity. This is further explored by CD₃CN spectroscopic sorption experiments. We have shown that by tuning the number of defects in UiO-66 using PSBA as the hemilabile linker, one can achieve highly defective and stable MOFs and easily control the Brønsted to Lewis acid ratio in the materials and thus their catalytic activity and selectivity.

The synthesis and applications of chiral pyrrolidine functionalized metal–organic frameworks and covalent-organic frameworks

Proline based ligands show versatile functionality to construct chiral MOFs and COFs; meanwhile, the resulted frameworks are potential materials for enantioselective adsorption and asymmetric catalysis.

Cooperative acid–base bifunctional ordered porous solids in sequential multi-step reactions: MOF vs. mesoporous silica

Two different catalytic platforms, MOF and mesoporous silica, were compared as porous support for basic amino groups to promote sequential multi-step reactions.

An easy and low-cost method of embedding chiral molecules in metal–organic frameworks for enantioseparation

A facile post-synthetic modification method has been demonstrated here to prepare chiral metal–organic frameworks for enantioseparation.

Heterogeneous Prolinamide-Catalyzed Atom-Economical Synthesis of β-Thioketones from Bio-Based Enones

Sulfur-containing compounds are a class of important motifs extensively applied in pharmaceutical and pesticidal industries as well as in electrochemistry, toxicant separation, and organic syntheses. Herein, we describe a novel and efficient metal-free catalytic strategy for the rapid synthesis of β-thioketones from sustainable enone derivatives and thiols via thia-Michael addition enabled by heterogeneous prolinamide. At room temperature, up to 98% yield of β-thioketones could be obtained over the solid UiO-66-NH-proline catalyst facilely prepared by the covalent immobilization of proline onto UiO-66-NH₂ (a well-known metal-organic framework) via a stable amido linkage. A cooperative effect of proline (amino group) and UiO-66-NH₂ (in situ-derived amide species) was observed to play a promotional role in the proceeding of thia-Michael addition, resulting in a high TOF value of 1124.3 h^-1. A three-component "iminium" intermediate was illustrated to the key species approaching the product β-thioketone. Moreover, the UiO-66-NH-proline could be easily recovered from the reaction mixture and recycled for at least five times with a slight loss of activity.

A new approach to enhancing the CO2 capture performance of defective UiO-66 via post-synthetic defect exchange

Zirconium-based metal-organic frameworks (Zr-MOFs) are a subclass of MOFs known for their remarkable stability, especially in the presence of water. This makes them extremely attractive for practical applications, including CO₂ capture from industrial emission sources; however, the CO₂ adsorption capacity of Zr-MOFs is moderate compared to that of the best performing MOFs reported to date. Functionalization of Zr-MOFs with amino groups has been demonstrated to increase their affinity for CO₂. In this work, we assessed the potential of post-synthetic defect exchange (PSDE) as an alternative approach to introduce amino functionalities at missing-cluster defective sites in formic acid modulated UiO-66. Both pyridine-containing (picolinic acid and nicotinic acid) and aniline-containing (3-aminobenzoic acid and anthranilic acid) monocarboxylates were integrated within defective UiO-66 with this method. Non-defective UiO-66 modified with linkers bearing the same amino groups (2,5-pyridinedicarboxylic acid and 2-aminoterephthalic acid) were prepared by classical post-synthetic ligand exchange (PSE), in order to compare the effect of introducing functionalities at defective sites versus installing them on the backbone. PSDE reduces the porosity of defective UiO-66, but improves both the CO₂ uptake and the CO₂/N₂ selectivity, whereas PSE has no effect on the porosity of non-defective UiO-66, improving the CO₂ uptake but leaving selectivity unchanged. Modification of defective UiO-66 with benzoic acid reveals that pore size reduction is the main factor responsible for the observed uptake improvement, whereas the presence of nitrogen atoms in the pores seems to be beneficial for increasing selectivity.

An effective strategy for creating asymmetric MOFs for chirality induction: a chiral Zr-based MOF for enantioselective epoxidation

A simple and rapid procedure was used to prepare chiral NU-1000 as a robust Zr-based MOF without complexity. The functionalization of NU-1000 was performed by utilizing chirall-(+)-tartaric acidviasolvent-assisted linker incorporation, resulting in [C-NU-1000]. A Mo-complex was immobilized onto chiral NU-1000 for enantioselective epoxidation.

Ferrocenecarboxylic acid: a functional modulator for UiO-66 synthesis and incorporation of Pd nanoparticles

FcCOOH was found to be an efficient modulator for UiO-66 synthesis and a functional group for incorporation of Pd nanoparticles.