Simultaneous Presence of Open Metal Sites and Amine Groups on a 3D Dy(III)-Metal–Organic Framework Catalyst for Mild and Solvent-Free Conversion of CO2 to Cyclic Carbonates

Copper-Based Bio-MOF/GO with Lewis Basic Sites for CO2 Fixation into Cyclic Carbonates and C-C Bond-Forming Reactions

Several measures, including crude oil recovery improvement and carbon dioxide (CO2) conversion into valuable chemicals, have been considered to decrease the greenhouse effect and ensure a sustainable low-carbon future. The Knoevenagel condensation and CO2 fixation have been introduced as two principal solutions to these challenges. In the present study for the first time, bio-metal-organic frameworks (MOF)(Cu)/graphene oxide (GO) nanocomposites have been used as catalytic agents for these two reactions. In view of the attendance of amine groups, biological MOFs with NH2 functional groups as Lewis base sites protruding on the channels' internal surface were used. The bio-MOF(Cu)/20%GO performs efficaciously in CO2 fixation, leading to more than 99.9% conversion with TON = 525 via a solvent-free reaction under a 1 bar CO2 atmosphere. It has been shown that these frameworks are highly catalytic due to the Lewis basic sites, i.e., NH2, pyrimidine, and C═O groups. Besides, the Lewis base active sites exert synergistic effects and render bio-MOF(Cu)/10%GO nanostructures as highly efficient catalysts, significantly accelerating Knoevenagel condensation reactions of aldehydes and malononitrile as substrates, thanks to the high TOF (1327 h-1) and acceptable reusability. Bio-MOFs can be stabilized in reactions using GO with oxygen-containing functional groups that contribute as efficient substitutes, leading to an expeditious reaction speed and facilitating substrate absorption.

Two-Dimensional Cu(II)-MOF with Lewis Acid-Base Bifunctional Sites for Chemical Fixation of CO2 and Bioactive 1,4-DHP Synthesis via Hantzsch Condensation

Five- and six-membered heterocycles containing nitrogen or oxygen have been considered as privileged scaffolds in organic chemistry and the chemical industry because of their usage in high-value commodities. Herein, we report a two-dimensional (2D) Cu(II)-based MOF catalyst, IITKGP-40, via the strategic employment of ample Lewis acid-base bifunctional sites (open metal nodes and free pyrazine moieties) along the pore wall. IITKGP-40 could convert toxic CO2 to cyclic carbonates in an atom-economical manner under solvent-free conditions and aromatic aldehyde to bioactive 1,4-DHPs via Hantzsch condensation. Exceptional catalytic performance (99%) and turnover number under mild reaction conditions for CO2 fixation using sterically hindered styrene oxide, and good-to-excellent yields for a wide range of aromatic aldehydes toward 1,4-dihydropyridines (1,4-DHPs) make IITKGP-40 promising as a multipurpose heterogeneous catalyst. Moreover, to demonstrate the practical utility of the catalyst, two biologically important drug molecules, diludine and nitrendipine analogue, have also been synthesized. IITKGP-40 is recyclable for at least three consecutive runs without significant loss of activity, making it promising for real-time applications.

Functionalization of Magnetic UiO-66-NH2 with a Chiral Cu(l-proline)2 Complex as a Hybrid Asymmetric Catalyst for CO2 Conversion into Cyclic Carbonates

In this study, a chiral [Cu(l-proline)2] complex-modified Fe3O4@SiO2@UiO-66-NH2(Zr) metal-organic framework [Fe3O4@SiO2@UiO-66-NH-Cu(l-proline)2] via multifunctionalization strategies was designed and synthesized. One simple approach to chiralize an achiral MOF-structure that cannot be directly chiralized using a chiral secondary agent like 4-hydroxy-l-proline. Therefore, this chiral catalyst was synthesized with a simple and multistep method. Accordingly, Fe3O4@SiO2@UiO-66-NH2 has been synthesized via Fe3O4 modification with tetraethyl orthosilicate and subsequently with ZrCl4 and 2-aminoterephthalic acid. The presence of the silica layer helps to stabilize the Fe3O4 core, while the bonding between Zr4+ and the -OH groups in the silica layer promotes the development of Zr-MOFs on the Fe3O4 surface, and then the surfaces of the synthesized magnetic MOFs composite are functionalized with 1,2-dichloroethane and Cu(II) complex with 4-hydroxy-l-proline, [Cu(l-proline)2] to afford the magnetically chiral nanocatalyst. Multiple techniques were employed to characterize this magnetically chiral nanocatalyst such as Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), powder X-ray diffraction (PXRD), circular dichroism (CD), inductively coupled plasma (ICP), thermogravimetric analysis (TGA), vibrating-sample magnetometry (VSM), and Brunauer-Emmett-Teller (BET) analyses. Moreover, a magnetically chiral nanocatalyst shows the asymmetric CO2 fixation reaction under solvent-free conditions at 80 °C and in ethanol under reflux conditions with up to 99 and 98% ee, respectively. Furthermore, the reaction mechanism was illustrated concerning the total energy of the reactant, intermediates and product, and the structural parameters were analyzed.

A Multivariate 2D Metal-Organic Framework with Open Metal Sites for Catalytic CO2 Cycloaddition and Cyanosilylation Reactions

This work reports the synthesis of a dual functional 2D framework, {[Zn2(4-tpom)2(oxdz)2]·4H2O}n (1), at room temperature, where a bent dicarboxylate, oxdz2- (4,4'-(1,3,4-oxadiazole-2,5-diyl)dibenzoate), and a neutral flexible N-donor linker, 4-tpom (tetrakis(4-pyridyloxymethylene)methane), are utilized. Its single-crystal X-ray analysis indicated a 2-fold interpenetrated 2D framework having tetracoordinated Zn(II) centers and dangling pyridyl groups. Its further characterization was carried out with elemental microanalysis, FTIR spectroscopy, TG analysis, and powder X-ray diffraction. The tetracoordinated Zn(II) centers as active Lewis acidic sites and the N atoms of 4-tpom as Lewis basic sites in 1 are explored for its functioning as a heterogeneous catalyst in two important reactions, (i) cycloaddition of CO2 with various epoxides and (ii) cyanosilylation reaction under solvent-free conditions. We could successfully show the cycloaddition of styrene oxide with CO2 (99% conversion) under balloon pressure with low catalyst (0.2-0.3 mol %) and cocatalyst (0.5-0.75 mol %) loadings, which is otherwise difficult to achieve. It was observed that all the substrates (aromatic and aliphatic), irrespective of their sizes, showed conversion percentage >99%. In the cyanosilylation reaction, a conversion of 96% was obtained with 1.5 mol % of 1 at room temperature for 12 h. This framework emerged as an excellent recyclable catalyst for both the reactions.

Fluorinated Metal-Organic Frameworks with Dual-Functionalized Linkers to Enhance Photocatalytic H2 Evolution and High Water Adsorption

Photocatalytic H2 evolution has recently attracted much attention due to the reduction of nonrenewable energy sources and the increasing demand for renewable sustainable energies. Meanwhile, metal-organic frameworks (MOFs) are emerging potential photocatalysts due to their structural adaptability, porous configuration, several active sites, and a wide range of performance. Nevertheless, there are still limitations in the photocatalytic H2 evolution reaction of MOFs with higher charge recombination rates. Herein, a copper-organic framework with dual-functionalized linkers {[Cu2(L)(H2O)2]·(5DMF)(4H2O)}n (fluorinated MOF(Cu)-NH2; H4L = 3,5-bis(2,4-dicarboxylic acid)-4-(trifluoromethyl)aniline) and with a rare 2-nodal 4,12-connected shp topology has been synthesized by a ligand-functionalization strategy and evaluated for the photocatalytic production of H2 to overcome this issue. According to the photocatalytic H2 evolution results, fluorinated MOF(Cu)-NH2 showed a hydrogen evolution rate of 63.64 mmol·g-1·h-1 exposed to light irradiation, indicating values 12 times that of the pure ligand when cocatalyst Pt and photosensitizer Rhodamine B were present. In addition, this MOF showed a maximum water absorption of 205 cm3·g-1. When dual-functionalized linkers are introduced to the structure of this MOF, its visible-light absorption increases considerably, which can be associated with nearly narrower energy band gaps (2.18 eV). More importantly, this MOF contributes to water absorption and electron collection and transport, acting as a bridge that helps to separate and transfer photogenerated charges while shortening the electron migration path because of the functional group in its configuration. The current paper seeks to shed light on the design of advanced visible-light photocatalysts with no MOF calcination for H2 photocatalytic production.

Crystal structure and photocatalytic activity of luminescent 3D-Supramolecular metal organic framework of dysprosium

A 3D supramolecular metal organic framework of dysprosium has been fabricated through a facile hydrothermal procedure with the ligand, 2,6-naphthalene disulphonic acid and the co-ligand, 4,4'-bipyridine. The MOF has been characterized as [C60H81DyN8O30S4] by routine analytical procedures. SXRD studies of the MOF show the existence of a hydrogen-bonded 3D supramolecular structure with high porosity. It crystallizes in monoclinic space group P21/n with unit cell parameters, a = 16.5424(6) Å, b = 37.0052(14) Å, c = 24.4361(9) Å, β = 100.7410°, α = γ = 90°. The Dy-MOF has eight coordinated water molecules around the metal centre and exhibits square anti-prismatic geometry. The band gap is 3.11 eV. The degradation experiments under visible light confirmed that Dy-MOF can act as a photocatalyst. Addition of hydrogen peroxide remarkably increases the degradation efficiency of the MOF through an advanced oxidation process. The newly synthesized MOF produced sharp emission peaks characteristic of dysprosium ion.

Ratiometric Fluorescence Thermometry, Quantitative Gossypol Detection, and CO2 Chemical Fixation by a Multipurpose Europium (III) Metal-Organic Framework

A lanthanide-based molecular crystalline material endows metal-organic frameworks (MOFs) with many fascinating applications such as fluorescence detection and CO2 chemical fixation. Herein, we describe and study a multipurpose europium(III) MOF with the formula of {[Eu2(TATAB)2]·2.5H2O·2DMF}n (Eu-MOF) (where H3TATAB is 4,4',4″-((1,3,5-triazine-2,4,6-triyl)tris(azanediyl))tribenzoic acid ligand) for photoluminescence sensor matrix and CO2 chemical fixation. This Eu-MOF features 1D square channels along the c direction with a pore size of ca.14.07 Å × 14.07 Å, occupied by lattice water and DMF molecules. The obtained Eu-MOF can achieve simultaneous luminescence of the H3TATAB ligand and Eu3+ ions, which can be developed as the sensor matrix for ratiometric fluorescence thermometry. The luminescence of the Eu-MOF demonstrates an obvious color change from red to yellow as temperature rises from 303 to 373 K and the Eu-MOF has a satisfying relative sensitivity of 3.21% K-1 and a small temperature uncertainty of 0.0093 K at 333 K. Moreover, sensitive detection of gossypol was achieved with a quenching constant Ksv of 1.18 × 105 M-1 and a detection limit of 4.61 μM. A combination of the competitive absorption and photoinduced electron transfer caused by host-guest interactions and strengthened π-π packing effect synergistically between gossypol molecules and the Eu-MOF skeleton realizes the "turn-off" sensing of gossypol. Importantly, the nature of the Eu-MOF allows showing CO2 chemical fixation under mild conditions. Thus, the Eu-MOF can be utilized as a multipurpose material for ratiometric fluorescence thermometry, quantitative gossypol detection, and CO2 chemical fixation.

Fixing CO2 under Atmospheric Conditions and Dual Functional Heterogeneous Catalysis Employing Cu MOFs: Polymorphism, Single-Crystal-to-Single-Crystal (SCSC) Transformation and Magnetic Studies

New copper compounds, [Cu(C14H8O6)(C10H8N2)(H2O)] (1), [Cu(C14H8O6)(C10H8N2)(H2O)]·(C3H7ON)2 (2), [Cu(C14H8O6)(C10H8N2)(H2O)2]·(C3H7ON) (3), [Cu(C14H8O6)(C10H8N4)] (4), and [Cu(C14H8O6)(C10H8N4)]·(H2O) (5), were prepared employing 2,5-bis(prop-2-yn-1-yloxy)terephthalic acid (2,5-BPTA) as the primary ligand and 4,4'-bipyridine (1-3) and 4,4'-azopyridine (4-5) as the secondary ligands. Single-crystal studies indicated that compounds 1-4 have two-dimensional layer structures and compound 5 has a three-dimensional structure. Compounds 1-3 were isolated from the same reaction mixture but by varying the time of reaction. The framework structures of compounds 1-3 are similar and may be considered as polymorphic structures. Compounds 4 and 5 can also be considered polymorphic with a change in dimensionality of the structure. Compounds 1-3 can be formed through a single-crystal-to-single-crystal transformation under a suitable solvent mixture. The Cu center was explored for the Lewis acid-catalyzed cycloaddition reaction of epoxide and CO2 under ambient conditions in a solventless condition and also for the synthesis of propargylamine derivatives by three-component coupling reactions (A3 coupling) in a DCM medium. The Lewis basic functionality of the MOF (-N═N- group) has been explored for the Henry reaction (aldol condensation) in a solventless condition. In all of the catalytic reactions, good yields and recyclability were observed. The magnetic studies indicated that compounds 1 and 4 have antiferromagnetic interactions and compound 5 has ferromagnetic interactions. The present studies illustrated the rich diversity that the copper-containing compounds exhibit in extended framework structures.

Thermocatalytic Conversion of CO2 to Valuable Products Activated by Noble-Metal-Free Metal-Organic Frameworks

Thermocatalysis of CO2 into high valuable products is an efficient and green method for mitigating global warming and other environmental problems, of which Noble-metal-free metal-organic frameworks (MOFs) are one of the most promising heterogeneous catalysts for CO2 thermocatalysis, and many excellent researches have been published. Hence, this review focuses on the valuable products obtained from various CO2 conversion reactions catalyzed by noble-metal-free MOFs, such as cyclic carbonates, oxazolidinones, carboxylic acids, N-phenylformamide, methanol, ethanol, and methane. We classified these published references according to the types of products, and analyzed the methods for improving the catalytic efficiency of MOFs in CO2 reaction. The advantages of using noble-metal-free MOF catalysts for CO2 conversion were also discussed along the text. This review concludes with future perspectives on the challenges to be addressed and potential research directions. We believe that this review will be helpful to readers and attract more scientists to join the topic of CO2 conversion.

A Dual-Purpose Ce(III)-Organic Framework with Amine Groups and Open Metal Sites: Third-Order Nonlinear Optical Activity and Catalytic CO2 Fixation

The present work focuses on the synthesis and properties of a novel multifunctional cerium(III) MOF, [Ce2(data)3(DMF)4]·DMF (data2-: 2,5-diaminoterephthalate), abbreviated as NH2-Ce-MUM-2. Its crystal structure reveals an intricate 3D 4,5-connected framework with a xah topology. This MOF features unique properties, such as open metal sites, presence of free amino groups, and high stability. Two main applications of NH2-Ce-MUM-2 were investigated: (i) as a heterogeneous catalyst in the CO2 fixation into cyclic carbonates and (ii) as a material with third-order nonlinear optical activity. As a model reaction, the cycloaddition of CO2 to propylene oxide to give the corresponding cyclic carbonate was explored under mild conditions, at the atmospheric pressure of carbon dioxide and in the absence of cocatalyst and added solvent. Various reaction parameters were investigated toward optimization and exploration of substrate scope, revealing up to 99% product yields of cyclic carbonate products. Besides, the structure of NH2-Ce-MUM-2 is highly stable, permitting its recyclability and reusability in further catalytic experiments. The significant contributions of free amino groups and open metal sites within this catalyst were particularly considered when proposing a potential mechanism for the reaction. Z-Scan measurements were used to evaluate the nonlinear optical (NLO) properties of NH2-Ce-MUM-2 at various laser intensities. A high two-photon absorption (TPA) under greater incident intensities shows that NH2-Ce-MUM-2 might be applicable in optical switching devices. Besides, the self-focusing effects of NH2-Ce-MUM-2 under various incident intensities were highlighted by the nonlinear index of refraction (n2). By reporting the synthesis and characterization of a novel MOF, along with its highly promising catalytic and NLO behavior, the current study introduces an additional example of multifunctional material into a growing family of metal-organic frameworks.

Thermally activated bipyridyl-based Mn-MOFs with Lewis acid-base bifunctional sites for highly efficient catalytic cycloaddition of CO2 with epoxides and Knoevenagel condensation reactions

Metal-organic frameworks (MOFs) have become preferred heterogeneous catalytic materials for many reactions due to their advantages such as porosity and abundant active sites. Here, a 3D Mn-MOF-1 [Mn₂(DPP)(H₂O)₃]·6H₂O (DPP = 2,6-di(2,4-dicarboxyphenyl)-4-(pyridine-4-yl)pyridine) was successfully synthesized under solvothermal conditions. This Mn-MOF-1 possesses a 3D structure constructed by the combination of a 1D chain and the DPP^4- ligand and features a micropore with a 1D drum-like shaped channel. Interestingly, Mn-MOF-1 can maintain the structure unchanged by the removal of coordinated and lattice water molecules, whose activated state (denoted as Mn-MOF-1a) contains rich Lewis acid sites (tetra- and pentacoordinated Mn²⁺ ions) and Lewis base sites (N_pyridine atoms). Furthermore, Mn-MOF-1a shows excellent stability, which can be used to catalyze CO₂ cycloaddition reactions efficiently under eco-friendly, solvent-free conditions. In addition, the synergistic effect of Mn-MOF-1a resulted in its promising potential in Knoevenagel condensation under ambient conditions. More importantly, the heterogeneous catalyst Mn-MOF-1a can be recycled and reused without an obvious decrease of activity for at least 5 reaction cycles. This work not only paves the way for the construction of Lewis acid-base bifunctional MOFs based on pyridyl-based polycarboxylate ligands but also demonstrates that Mn-based MOFs hold great promise as a heterogeneous catalyst toward both CO₂ epoxidation and Knoevenagel condensation reactions.

A highly robust cluster-based indium(III)-organic framework with efficient catalytic activity in cycloaddition of CO2 and Knoevenagel condensation

The efficient catalytic performance displayed by MOFs is decided by an appropriate charge/radius ratio of defect metal sites, large enough solvent-accessible channels and Lewis base sites capable of polarizing substrate molecules. Herein, the solvothermal self-assembly led to a highly robust nanochannel-based framework of {[In₄(CPDD)₂(μ₃-OH)₂(DMF)(H₂O)₂]·2DMF·5H₂O}_n (NUC-66) with a 56.8% void volume, which is a combination of a tetranuclear cluster [In₄(μ₃-OH)₂(COO)₁₀(DMF)(H₂O)₂] (abbreviated as {In₄}) and a conjugated tetracyclic pentacarboxylic acid ligand of 4,4'-(4-(4-carboxyphenyl)pyridine-2,6-diyl)diisophthalic acid (H₅CPDD). To the best of our knowledge, NUC-66 is a rarely reported {In₄}-based 3D framework with embedded hierarchical triangular-microporous (2.9 Å) and hexagonal-nanoporous (12.0 Å) channels, which are shaped by six rows of {In₄} clusters. After solvent exchange and vacuum drying, the surface of nanochannels in desolvated NUC-66a is modified by unsaturated In³⁺ ions, N_pyridine atoms and μ₃-OH groups, all of which display polarization ability towards polar molecules due to their Lewis acidity or basicity. The catalytic experiments performed showed that NUC-66a had high catalytic activity in the cycloaddition reactions of epoxides with CO₂ under mild conditions, which should be ascribed to its structural advantages including nanoscale channels, rich bifunctional active sites, large surface areas and chemical stability. Moreover, NUC-66a, as a heterogeneous catalyst, could greatly accelerate the Knoevenagel condensation reactions of aldehydes and malononitrile. Hence, this work confirms that the development of rigid nanoporous cluster-based MOFs built on metal ions with a high charge and large radius ratio will be more likely to realize practical applications, such as catalysis, adsorption and separation of gas, etc.

Highly Robust {In2}-Organic Framework for Efficiently Catalyzing CO2 Cycloaddition and Knoevenagel Condensation

To improve the catalytic performance of metal-organic frameworks (MOFs), creating higher defects is now considered as the most effective strategy, which can not only optimize the Lewis acidity of metal ions but also create more pore space to enhance diffusion and mass transfer in the channels. Herein, the exquisite combination of scarcely reported [In₂(CO₂)₅(H₂O)₂(DMF)₂] clusters and 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (H₅BDCP) under solvothermal conditions generated a highly robust nanoporous framework of {[In₂(BDCP)(DMF)₂(H₂O)₂](NO₃)}_n (NUC-65) with nanocaged voids (14.1 Å) and rectangular nanochannels (15.94 Å × 11.77 Å) along the a axis. It is worth mentioning that an In(1) ion displays extremely low tetra-coordination modes after the thermal removal of its associated four solvent molecules of H₂O and DMF. Activated {[In₂(BDCP)](Br)}_n (NUC-65Br), as a defective material because of its extremely unsaturated metal centers, could be generated by bromine ion exchange, solvent exchange, and vacuum drying. Catalytic experiments proved that the conversion of epichlorohydrin with 1 atm CO₂ into 4-(chloromethyl)-1,3-dioxolan-2-one catalyzed by 0.11 mol % NUC-65Br could reach 99% at 65 °C within 24 h. Moreover, with the aid of 5 mol % cocatalyst n-Bu₄NBr, heterogeneous NUC-65Br owns excellent universal catalytic performance in most epoxides under mild conditions. In addition, NUC-65Br, as a heterogeneous catalyst, exhibits higher activity and better selectivity for Knoevenagel condensation of aldehydes and malononitrile. Hence, this work offers a fresh insight into the design of structure defect cationic metal-organic frameworks, which can be better applied to various fields because of their promoted performance.

Exploiting a Multi-Responsive Oxadiazole Moiety in One Three-Dimensional Metal-Organic Framework for Remedies to Three Environmental Issues

Multifunctional metal-organic frameworks (MOFs) rely on the properties of metal centers (nodes) and/or linkers (struts) for their diverse applications in the emerging field of research. Currently, there is a huge demand for MOF materials in the field of capture/fixation/sensing of air pollutants, harmful chemical effluents, and nuclear waste. However, it is a challenging task to utilize one MOF for providing remedies to all these issues. On the basis of our current research activities, we have identified that an oxadiazole moiety-a five-membered ring with two different heteroatoms (O and N)-in a carboxylate linker can be the key to generating such MOF materials for its (a) inherent polarizable nature and molecular docking ability and (b) photoluminescence properties. In this work, we report a 3D MOF {[Co₂(oxdz)₂(tpbn)(H₂O)₂]·4H₂O}_n (1), self-assembled at room temperature from a three-component reaction, with an oxadiazole moiety (where H₂oxdz = 4,4'-(1,3,4-oxadiazole-2,5-diyl)dibenzoic acid and tpbn = N,N',N,"N″'-tetrakis(2-pyridylmethyl)-1,4-diaminobutane). The inherent polarizable nature of the oxadiazole moiety in 1 has been efficiently exploited for (i) multimedia iodine capture and (ii) fixation of CO₂ under solvent-free and ambient conditions. On the other hand, the luminescent nature of the framework is found to be an efficient, highly preferred turn-on sensor for the ultra-fast detection of ketones with a limit as low as parts-per-trillion (mesitylene oxide: 447 ppt; cycloheptanone: 4.7 ppb; cyclohexanone: 17.2 ppb; acetylacetone: 18 ppb).

Coordination Polymers Constructed from an Adaptable Pyridine-Dicarboxylic Acid Linker: Assembly, Diversity of Structures, and Catalysis

4,4'-(Pyridine-3,5-diyl)dibenzoic acid (H2pdba) was explored as an adaptable linker for assembling a diversity of new manganese(II), cobalt(II/III), nickel(II), and copper(II) coordination polymers (CPs): [Mn(μ4-pdba)(H2O)]n (1), {[M(μ3-pdba)(phen)]·2H2O}n (M = Co (2), Ni (3)), {[Cu2(μ3-pdba)2(bipy)]·2H2O}n (4), {[Co(μ3-pdba)(bipy)]·2H2O}n (5), [Co2(μ3-pdba)(μ-Hbiim)2(Hbiim)]n (6), and [M(μ4-pdba)(py)]n (M = Co (7), Ni (8)). The CPs were hydrothermally synthesized using metal(II) chloride precursors, H2pdba, and different coligands functioning as crystallization mediators (phen: 1,10-phenanthroline; bipy: 2,2'-bipyridine, H2biim: 2,2'-biimidazole; py: pyridine). Structural networks of 1-8 range from two-dimensional (2D) metal-organic layers (1-3, 5-8) to three-dimensional (3D) metal-organic framework (MOF) (4) and disclose several types of topologies: sql (in 1), hcb (in 2, 3, 5), tfk (in 4), 3,5L66 (in 6), and SP 2-periodic net (6,3)Ia (in 7, 8). Apart from the characterization by standard methods, catalytic potential of the obtained CPs was also screened in the Knoevenagel condensation of benzaldehyde with propanedinitrile to give 2-benzylidenemalononitrile (model reaction). Several reaction parameters were optimized, and the substrate scope was explored, revealing the best catalytic performance for a 3D MOF 4. This catalyst is recyclable and can lead to substituted dinitrile products in up to 99% product yields. The present study widens the use of H2pdba as a still poorly studied linker toward designing novel functional coordination polymers.

Atomically Dispersed Iron Sites on the Hollow Nitrogen-Doped Carbon Framework with a Highly Efficient Performance on Carbon Dioxide Cycloaddition

The exploration of efficient and low-consumption catalysts for carbon dioxide (CO₂) conversion is desirable yet remains a great challenge. Herein, a novel catalyst composed of a hollow nitrogen-doped carbon framework (HNF) enriched with high-loading (9.8 wt %) atomically dispersed iron sites (defined as Fe_SAs/HNF) has been fabricated by a polymer-assisted strategy. As a result, Fe_SAs/HNF has an excellent performance on the cycloaddition reactions of CO₂ with epoxides (the conversion >96%) under milder conditions because of its ultrahigh loading of atomically dispersed iron sites. This study not only provides an advanced catalyst for driving CO₂ cycloaddition but also furnishes a novel perspective on the rational design of superior catalysts with high-loading active sites for diverse heterogeneous catalytic reactions.

Construction of Bimetallic Metal-Organic Frameworks with the Nanosheet-Assembled Hierarchical Hollow Structure for CO2 Fixation

Incorporating secondary metal nodes with functionality into organic ligand nodes to form a bimetallic metal-organic frameworks (MOFs) would facilitate an enhancement in properties and broaden applied areas of MOFs. Hierarchical tubular Cu/Zn-MOF-74 assembled by nanosheet arrays is synthesized at ambient temperature and pressure by phase transformation of Cu-based precursor MOF in immersion solution with Zn²⁺. The content of Zn in Cu/Zn-MOF-74 can be controlled by adjusting the concentration of Zn²⁺ in immersion solution, and it can reach a maximum of 36.4%. Moreover, the catalytic activity toward cycloaddition of CO₂ with styrene oxide of Cu/Zn-MOF-74 is improved significantly compared with that of monometallic Cu-MOF-74. Meanwhile, the advanced hierarchical tubular structure contributing to enhancement in catalytic activity enables Cu/Zn-MOF-74 to present higher conversion toward this cycloaddition of CO₂ than traditional rod-like Cu/Zn-MOF-74. This templated synthesis would provide an opportunity for designing various bimetallic MOFs or MOF-based compounds with improved performances in multiple applications.

Coordination Polymers from Biphenyl-Dicarboxylate Linkers: Synthesis, Structural Diversity, Interpenetration, and Catalytic Properties

The present work explores two biphenyl-dicarboxylate linkers, 3,3'-dihydroxy-(1,1'-biphenyl)-4,4'-dicarboxylic (H4L1) and 4,4'-dihydroxy-(1,1'-biphenyl)-3,3'-dicarboxylic (H4L2) acids, in hydrothermal generation of nine new compounds formulated as [Co2(μ2-H2L1)2(phen)2(H2O)4] (1), [Mn2(μ4-H2L1)2(phen)2]n·4nH2O (2), [Zn(μ2-H2L1)(2,2'-bipy)(H2O)]n (3), [Cd(μ2-H2L1) (2,2'-bipy)(H2O)]n (4), [Mn2(μ2-H2L1)(μ4-H2L1)(μ2-4,4'-bipy)2]n·4nH2O (5), [Zn(μ2-H2L1)(μ2-4,4'-bipy)]n (6), [Zn(μ2-H2L2)(phen)]n (7), [Cd(μ3-H2L2)(phen)]n (8), and [Cu(μ2-H2L2) (μ2-4,4'-bipy)(H2O)]n (9). These coordination polymers (CPs) were generated by reacting a metal(II) chloride, a H4L1 or H4L2 linker, and a crystallization mediator such as 2,2'-bipy (2,2'-bipyridine), 4,4'-bipy (4,4'-bipyridine), or phen (1,10-phenanthroline). The structural types of 1-9 range from molecular dimers (1) to one-dimensional (3, 4, 7) and two-dimensional (8, 9) CPs as well as three-dimensional metal-organic frameworks (2, 5, 6). Their structural, topological, and interpenetration features were underlined, including an identification of unique two- and fivefold 3D + 3D interpenetrated nets in 5 and 6. Phase purity, thermal and luminescence behavior, as well as catalytic activity of the synthesized products were investigated. Particularly, a Zn(II)-based CP 3 acts as an effective and recyclable heterogeneous catalyst for Henry reaction between a model substrate (4-nitrobenzaldehyde) and nitroethane to give β-nitro alcohol products. For this reaction, various parameters were optimized, followed by the investigation of the substrate scope. By reporting nine new compounds and their structural traits and functional properties, the present work further outspreads a family of CPs constructed from the biphenyl-dicarboxylate H4L1 and H4L2 linkers.

Reversible Structural Transformation and Catalytic Potential of Lanthanide-Azobenzenetetracarboxylates

Inspired by the catalytic potential of lanthanide coordination polymers of 3,3',5,5'-azobenzenetetracarboxylic acid (H₄abtc), two new isostructural [Ln₂^III(Habtc)₂(DMSO)₄]·DMSO·H₂O (Ln^III = Sm^III (I), Eu^III = (II), DMSO = dimethyl sulfoxide) were synthesized and characterized. Their single-crystal structures were elucidated and described. Structural transformations of II in the solid state prompted by ligand substitution and thermal treatment were studied, from which genuine reversible transformation of II to [Eu^III(Habtc)(H₂O)₄]·3H₂O (II') and [Eu^III(Habtc)(H₂O)₂]·2H₂O (II″) was revealed. This illustrates the rare case of reversible transformation in lanthanide coordination polymers. The transformation between II' and II″ was also investigated. Structural transformations among these frameworks are discussed with regard to the coordination environment of Eu^III, coordination modes of Habtc^3-, and similarities and disparities in framework architecture and registration. In addition, the catalytic performance of II with and without the prior activation in CO₂ cycloaddition reaction with epichlorohydrin was studied in comparison with II' and II″. The excellent performance of II disregarding the activation process has been demonstrated with the maximum turnover number and turnover frequency of 7682 and 1921 h^-1, respectively, for the activated II and 7142 and 1786 h^-1, respectively, for the nonactivated II. The maintenance of the catalytic efficiency over 10 cycles of the catalysis and the regeneration process is illustrated and discussed with respect to structural transformation.

Non-Calcined Layer-Pillared Mn0.5Zn0.5 Bimetallic-Organic Framework as a Promising Electrocatalyst for Oxygen Evolution Reaction

Electrocatalytic generation of oxygen is of great significance for sustainable, clean, and efficient energy production. Multiple electron transfer in oxygen evolution reaction (OER) and its slow kinetics represent a serious hedge for efficient water splitting, requiring the design and development of advanced electrocatalysts with porous structures, high surface areas, abundant electroactive sites, and low overpotentials. These requisites are common for metal-organic frameworks (MOFs) and derived materials that are promising electrocatalysts for OER. The present work reports on the synthesis and full characterization of a heteroleptic 3D MOF, [Zn₂(μ₄-odba)₂(μ-bpdh)]_n·nDMF (Zn-MUM-1), assembled from 4,4'-oxydibenzoic acid and 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene (bpdh). Besides, a series of heterometallic MnZn-MUM-1 frameworks (abbreviated as Mn_0.5Zn_0.5-MUM-1, Mn_0.66Zn_0.33-MUM-1, and Mn_0.33Zn_0.66-MUM-1) was also prepared, characterized, and used for the fabrication of working electrodes based on Ni foam (NF), followed by their exploration in OER. These noble-metal-free and robust electrocatalysts are stable and do not require pyrolysis or calcination while exhibiting better electrocatalytic performance than the parent Zn-MUM-1/NF electrode. The experimental results show that the Mn_0.5Zn_0.5-MUM-1/NF electrocatalyst features the best OER activity with a low overpotential (253 mV at 10 mA cm^-2) and Tafel slope (73 mV dec^-1) as well as significant stability after 72 h or 6000 cycles. These excellent results are explained by a synergic effect of two different metals present in the Mn-Zn MOF as well as improved charge and ion transfer, conductivity, and stability characteristics. The present study thus widens the application of heterometallic MOFs as prospective and highly efficient electrocatalysts for OER.

Topologically Driven Pore/Surface Engineering in a Recyclable Microporous Metal-Organic Vessel Decorated with Hydrogen-Bond Acceptors for Solvent-Free Heterogeneous Catalysis

The use of metal-organic frameworks (MOFs) comprising custom-designed linkers/ligands as efficient and recyclable heterogeneous catalysts is on the rise. However, the topologically driven bifunctional porous MOFs for showcasing a synergistic effect of two distinct activation pathways of substrates (e.g., involving hydrogen bonding and a Lewis acid) in multicomponent organic transformations are very challenging. In particular, the novelty of such studies lies in the proper pore and/or surface engineering in MOFs for bringing the substrates in close proximity to understand the mechanistic aspects at the molecular level. This work represents the topological design, solid-state structural characterization, and catalytic behavior of an oxadiazole tetracarboxylate-based microporous three-dimensional (3D) metal-organic framework (MOF), {[Zn₂(oxdia)(4,4'-bpy)₂]·8.5H₂O}_n (1), where the tetrapodal (4-connected) 5,5'-(1,3,4-oxadiazole-2,5-diyl)diisophthalate (oxdia^4-), the tetrahedral metal vertex (Zn(II)), and a 2-connected pillar linker 4,4'-bipyridine (4,4'-bpy) are unique in their roles for the formation, stability, and function. As a proof of concept, the efficient utilization of both the oxadiazole moiety with an ability to provide H-bond acceptors and the coordinatively unsaturated Zn(II) centers in 1 is demonstrated for the catalytic process of the one-pot multicomponent Biginelli reaction under mild conditions and without a solvent. The key steps of substrate binding with the oxadiazole moiety are ascertained by a fluorescence experiment, demonstrating a decrease or increase in the emission intensity upon interaction with the substrates. Furthermore, the inherent polarizability of the oxadiazole moiety is exploited for CO₂ capture and its size-selective chemical fixation to cyclic carbonates at room temperature and under solvent-free conditions. For both catalytic processes, the chemical stability, structural integrity, heterogeneity, versatility in terms of substrate scope, and mechanistic insights are discussed. Interestingly, the first catalytic process occurs on the surface, while the second reaction occurs inside the pore. This study opens new ways to catalyze different organic transformation reactions by utilizing this docking strategy to bring the multiple components close together by a microporous MOF.

Four series of lanthanide coordination polymers based on the tetrabromobenzene-1,4-dicarboxylate ligand: structural diversity and multifunctional properties

Four series of lanthanide-based coordination polymers (LnCPs), namely [Ln(Br₄bdc)_1.5(MeOH)₃] (1Ln; Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy), [Ln₂(Br₄bdc)₂(NO₃)₂(MeOH)₄] (2Ln; Ln = Ce, Pr, Nd, Sm), [Ln(Br₄bdc)(NO₃)(MeOH)] (3Ln; Ln = Gd, Tb, Dy), and [Ln₂(Br₄bdc)₃(H₂O)_2.3(MeOH)_2.7] (4Ln; Ln = Gd, Tb, Dy) have been synthesized by reacting hydrated lanthanide(III) salts with tetrabromobenzene-1,4-dicarboxylic acid (H₂Br₄bdc) in different solvents under solvothermal conditions. The structural diversity found in the system mainly resulted from the effects of anions, solvents, and the variation in the ionic radii of the lanthanide(III) ions. Compounds in series 1Ln feature a two-dimensional (2D) layered structure with sql topology based on {(Ln(COO)₂)₂(μ-COO)₂} secondary building units (SBUs). Compounds in series 2Ln and 3Ln comprise, respectively, infinite uniform and alternate chains of {Ln(COO)₂}_n SBUs that are assembled into a similar network topology to 1Ln. Meanwhile, compounds in series 4Ln feature 3D coordination networks of a pcu α-Po topological net consisting of binuclear {Ln₂(COO)₃} SBUs. The formation of polymeric networks in series 1Ln-4Ln is facilitated by the numerous coordination sites of the ligand Br₄bdc^2- and the fact that its bromine atoms can participate in the formation of various types of intermolecular interactions. The solid-state photoluminescence studies on Eu- (1Eu) and Tb- (1Tb, 3Tb, 4Tb) containing compounds indicate that the Br₄bdc^2- ligands can efficiently sensitize Eu³⁺ and Tb³⁺ emission. Notably, such compounds exhibit highly sensitive fluorescence sensing for acetone, water, and Fe³⁺ ions via the fluorescence quenching effect. As the representatives of the series, activated 1Eu, 2Pr, 3Tb, and 4Tb show the maximum CO₂ uptake capacities of 170.4, 273.7, 255.3, and 303.5 cm³ g^-1, respectively, at 50 bar and 298 K with good repeatability of the adsorption-desorption properties. Magnetic studies indicate that the Gd- and Dy-based compounds 1Gd, 1Dy, 3Gd, 3Dy, and 4Gd show simple paramagnetic behaviours, whereas compound 4Dy exhibits weak ferromagnetic interactions.

Metal-Organic Framework-Based Selective Molecular Recognition of Organic Amines and Fixation of CO2 into Cyclic Carbonates

Synthesis and structural depiction of two new metal-organic frameworks (MOFs), namely, [{Zn(L)(oba)}·4H₂O]_α (Zn-MOF-1) and [{Cd_1/2(L)_1/2(nipa)_1/2(H₂O)_1/2}·(DMF)_1/2(H₂O)]_α (Cd-MOF-2) (where L = N²,N⁶-di(pyridin-4-yl)naphthalene-2,6-dicarboxamide, 4,4'-H₂oba = 4,4'-oxybisbenzoic acid, and 5-H₂nipa = 5-nitroisophthalic acid) are reported. Both Zn-MOF-1 and Cd-MOF-2 have been prepared by reacting ligand L and coligand 4,4'-H₂oba or 5-H₂nipa with the respective dihydrates of Zn(OAc)₂ and Cd(OAc)₂ (OAc = acetate). Crystal structure X-ray analysis discloses that Zn-MOF-1 displays an overall 2D → 3D interpenetrated framework structure. The topological analysis by ToposPro suggests a (4)-connected uninodal sql topology with a point symbol of {4⁴·6²} having 2D + 2D parallel polycatenation. However, crystal packing of Cd-MOF-2 adapted a porous framework architecture and was topologically simplified as (3,4)-connected binodal 2D net. In addition, both Zn-MOF-1 and Cd-MOF-2 were proved to be multifunctional materials for the recognition of organic amines and in the fixation of CO₂ to cyclic carbonates. Remarkably, Zn-MOF-1 and Cd-MOF-2 showed very good fluorescence stability in aqueous media and have shown 98 and 97% quenching efficiencies, respectively, for 4-aminobenzoic acid (4-ABA), among all of the researched amines. The mechanistic study of organic amines recognition proposed that fluorescence quenching happened mainly through hydrogen-bonding and π-π stacking interactions. Additionally, cycloaddition of CO₂ to epoxide in the presence of Zn-MOF-1 and Cd-MOF-2 afforded up to 96% of cyclic carbonate within 24 h. Both Zn-MOF-1 and Cd-MOF-2 exhibited recyclability for up to five cycles without noticing an appreciable loss in their sensing or catalytic efficiency.

Efficient synthesis of cyclic carbonates from CO2 under ambient conditions over Zn(betaine)2Br2: experimental and theoretical studies

It is very interesting to synthesize high value-added chemicals from CO₂ under mild conditions with low energy consumption. Here, we report that a novel catalyst, Zn(betaine)₂Br₂, can efficiently promote the cycloaddition of CO₂ with epoxides to synthesize cyclic carbonates under ambient conditions (30 °C, 1 atm). DFT calculations provide important insights into the mechanism, particularly the unusual synergistic catalytic action of Zn²⁺, Br^- and NR⁴⁺, which is the critical factor for the outstanding performance of Zn(betaine)₂Br₂. The unique features of the catalyst are that it is cheap, green and very easy to prepare.

Mixed Metal Fe2Ni MIL-88B Metal-Organic Frameworks Decorated on Reduced Graphene Oxide as a Robust and Highly Efficient Electrocatalyst for Alkaline Water Oxidation

The development of cost-effective and efficient oxygen evolution reaction (OER) catalysts has found increasing popularity due to the sluggish kinetics of OER, which has hampered the H₂ production by H₂O electrolysis. In this study, Fe₂Ni MIL-88 (denoted FeNi) was composited by reduced graphene oxide (rGO, denoted R). Owing to the high porosity and abundant active sites of bimetallic MOF, proper conductivity of rGO, and the synergistic impact of Ni and Fe, the optimal composite (R@FeNi (1:1)) offered remarkable OER activity in alkaline environments. The obtained composite was employed in the OER, which led to a low overpotential of 264 mV at a current density of 10 mA cm^-2 with a Tafel slope of 62 mV dec^-1. Also, the bimetallic Fe₂Ni MIL-88 nanorods grown on rGO led to a reduction in the onset potential of the OER. These findings exceeded the results of standard IrO₂-based catalysts; they are also comparable or even better than the previously reported MOF-based catalysts.

Hypercrosslinked Ionic Polymers with High Ionic Content for Efficient Conversion of Carbon Dioxide into Cyclic Carbonates

The effective conversion of carbon dioxide (CO2) into cyclic carbonates requires porous materials with high ionic content and large specific surface area. Herein, we developed a new systematic post-synthetic modification strategy for synthesizing imidazolium-based hypercrosslinked ionic polymers (HIPs) with high ionic content (up to 2.1 mmol g−1) and large specific surface area (385 m2 g−1) from porous hypercrosslinked polymers (HCPs) through addition reaction and quaternization. The obtained HIPs were efficient in CO2 capture and conversion. Under the synergistic effect of high ionic content, large specific surface area, and plentiful micro/mesoporosity, the metal-free catalyst [HCP-CH2-Im][Cl]-1 exhibited quantitative selectivities, high catalytic yields, and good substrate compatibility for the conversion of CO2 into cyclic carbonates at atmospheric pressure (0.1 MPa) in a shorter reaction time in the absence of cocatalysts, solvents, and additives. High catalytic yields (styrene oxide, 120 °C, 8 h, 94% yield; 100 °C, 20 h, 93% yield) can be achieved by appropriately extending the reaction times at low temperature, and the reaction times are shorter than other porous materials under the same conditions. This work provides a new strategy for synthesizing an efficient metal-free heterogeneous catalyst with high ionic content and a large specific surface area from HCPs for the conversion of CO2 into cyclic carbonates. It also demonstrates that the ionic content and specific surface area must be coordinated to obtain high catalytic activity for CO2 cycloaddition reaction.

Structures, Fluorescence and Magnetism of a Series of Coordination Polymers Driven by Tricarboxypyridine Ligand

In this study, a tricarboxypyridine ligand 3-((5-carboxypyridin-3-yl)oxy)phthalic acid (H3cppa), that combines three distinct types of functional groups (COOH, O-ether and N-pyridyl) was used to construct metal complexes by hydrothermal reaction,...

A reusable fluorescence Eu-based coordination polymer for the detection of Zr4+ ions in aqueous media

A fluorescence coordination polymer [Eu(L)1.5(phen)(H2O)] (1, H2L = 9,10-anthracenedicarboxylic acid) was prepared. 1 represents a fluorescence turn-off sensing material for Zr4+ ions with an excellent detection performance.

Quick removal of metronidazole from aqueous solutions using metal–organic frameworks

Two MOFs were assembled, characterized and investigated in detail as efficient adsorbents for removal of the metronidazole antibiotic. Adsorption isotherms and kinetic features were also studied.