Zr-based metal organic framework containing squaramide group as well as preparation method and application of Zr-based metal organic framework

Abstract

The invention discloses a Zr-based metal organic framework containing squaramide groups and a preparation method and application thereof, and the preparation method comprises the following steps: adding tetra-coordinated carboxylic acid ligands H4HPA, H4TPCB or H4NDTB, ZrCl4 and benzoic acid into N, N-dimethylformamide, and carrying out solvothermal reaction to generate crystalline products Zr-MOF-1, Zr-MOF-2 and Zr-MOF-3; adding the obtained Zr-MOFs crystals into a DMF (Dimethyl Formamide) solution containing SQ ligands, and standing in a drying oven at 120 DEG C for two days to enable the SQ ligands to be inserted into unsaturated coordination sites of Zr6 clusters of the three MOFs, so as to obtain Zr-MOF-1-SQ, Zr-MOF-2-SQ and Zr-MOF-3-SQ; according to the present invention, the synthesized Zr-MOF-1-SQ, Zr-MOF-2-SQ and Zr-MOF-3-SQ have advantages of excellent chemical stability, accurately controllable catalytic activity center and excellent heterogeneous catalytic performance, and can maintain the structural integrity after multiple catalytic cycles, and the synthesis method is simple, convenient and controllable so as to solve the problem of poor stability of the traditional transition metal-based squaramide MOFs.

Description

Technical Field

[0001] This invention relates to the synthesis of metal-organic frameworks and the field of organocatalysis, specifically to a Zr-based metal-organic framework containing a squaramide group, its preparation method and application, and is particularly suitable for heterogeneous organocatalytic reactions mediated by hydrogen bond donors (HBD). Background Technology

[0002] Hydrogen-bonded donor (HBD) catalysts, with their ability to activate electrophiles through synergistic hydrogen bonding and promote nucleophilic addition reactions, have become important alternatives to Lewis acid catalysts, exhibiting excellent performance in various organic synthesis processes such as Friedel-Crafts and Diels-Alder reactions. However, the simultaneous presence of strong hydrogen bond donor and acceptor structures in HBD catalyst molecules makes them prone to self-association via intermolecular hydrogen bonds, leading to reduced or deactivated catalytic activity. Confining HBD molecules within a long-range ordered framework material can effectively suppress intermolecular hydrogen bond formation and improve catalytic performance.

[0003] Metal-organic frameworks (MOFs), as a class of crystalline porous materials, possess advantages such as tunable structure, high specific surface area, and convenient functionalization, providing an ideal platform for the immobilization of HBD catalysts. In recent years, research on introducing HBD groups such as squamamides into MOFs has made some progress, but many challenges remain: squamamide-based MOFs based on transition metals such as Cu and Zn exhibit poor chemical stability; existing highly stable Zr-based MOFs (such as UiO3-type) have limitations in controlling pore shape, size, and spatial distribution of catalytic active centers, making it difficult to achieve precise directional arrangement of active centers, thus restricting the improvement of HBD-mediated organocatalytic efficiency. Therefore, developing synthetic strategies for squamamide-based Zr-MOFs with high stability and precisely tunable active centers has become an urgent research need. Summary of the Invention

[0004] To address the issues of insufficient stability and limited structural control in existing technologies, this invention synthesizes a series of scu topological Zr-based metal-organic frameworks containing squamamide groups using a strategy combining ligand design and post-synthetic modification. This series of Zr-based metal-organic frameworks exhibits excellent chemical stability, precisely tunable catalytic active centers, and superior heterogeneous catalytic performance, all synthesized using a simple and controllable method.

[0005] To solve the above problems, the technical solution adopted by the present invention is as follows: A method for synthesizing Zr-MOFs containing squaramide groups includes the following steps: (1) Synthesis of ligands: The four-coordinate carboxylic acid ligand H4HPA (4',5'-bis(4-carboxyphenyl)-3',6'-dimethyl-[1,1':2',1''-terphenyl]-4''-dicarboxylicacid) was designed and synthesized. At the same time, the linear dicarboxylic acid ligand SQ (1,2-di(4-carboxyphenylamino)-3,4-dioxycyclobutane) containing the squaric acid group was synthesized. Its synthesis was achieved by reacting 4-aminobenzoic acid with squaric acid ester.

[0006] (2) Preliminary synthesis of scu topological Zr-MOFs: 10-15 mg of tetracoordinate carboxylic acid ligands (H4HPA, H4TPCB, or H4NDTB), 20 mg of ZrCl4, and 0.5-0.6 g of benzoic acid were added to a 5 mL glass bottle containing 3 mL of N,N-dimethylformamide (DMF). After stirring thoroughly to dissolve the reactants, the mixture was placed in a 120 °C oven for a solvothermal reaction for 72 h to generate crystalline products Zr-MOF-1, Zr-MOF-2, and Zr-MOF-3. The products were collected by centrifugation and washed three times each with fresh DMF and acetone to obtain scu topological Zr-MOFs with unsaturated coordination pockets.

[0007] (3) Installation of SQ ligands: 0.05 g of Zr-MOFs crystals obtained in step (2) were added to a DMF solution containing SQ ligands. The solution was placed in an oven at 120°C for two days to allow the SQ ligands to insert into the unsaturated coordination sites of the Zr6 clusters of the three MOFs, resulting in Zr-MOF-1-SQ, Zr-MOF-2-SQ, and Zr-MOF-3-SQ. The crystals were collected by centrifugation, soaked in acetone for 3 days for solvent exchange, and then heated under vacuum at 120°C for 6 h to obtain the solvent-de-saturated activated products. The precise structure of the products was determined using three-dimensional electron diffraction (3D Electron diffraction) and X-ray single crystal diffraction (X-ray single crystal diffraction) techniques.

[0008] The metal-organic framework containing squaramide groups described in this invention can be applied to the field of heterogeneous organic catalysis, especially Friedel-Crafts alkylation reactions. The invention will be further described below with reference to specific embodiments, but the scope of protection of this invention is not limited thereto.

[0009] S1: Activate the freshly prepared MOF catalyst under vacuum at 120℃ for 6-12 h to remove solvent molecules from the framework.

[0010] S2: In the catalytic reaction system, 5-10 mol% of activated MOFs were weighed and added to a dichloromethane (DCM) solution containing N-methylpyrrole / N-methylindole and β-nitrostyrene derivatives. The catalyst was uniformly dispersed in a constant-temperature shaker. The reaction was carried out at 50℃ for 48 h. 1 ¹H NMR spectroscopy was used to quantitatively analyze the product conversion rate. In the cycle stability test, the MOF catalyst after reaction was centrifuged, washed with acetone, dried, weighed, and then fed into the next cycle reaction.

[0011] The method for synthesizing Zr-MOFs containing squaramide groups prepared in this invention is controllable and highly stable, and can be used for HBD-mediated heterogeneous organic catalytic reactions. The series of Zr-MOFs prepared have precisely tunable active center distribution and excellent catalytic efficiency, and are a class of excellent heterogeneous catalysts.

[0012] Compared with the prior art, the present invention has the following advantages: 1. This invention employs a combination of pre-synthetic ligand design and post-synthetic ligand mounting, achieving for the first time the directional introduction of square amide groups and precise control of the spatial distribution of active centers in scu topological Zr-MOFs, laying the foundation for the in-depth application of square amide MOFs in the field of HBD catalysis.

[0013] 2. The synthesized Zr-MOFs (Zr-MOF-1-SQ, Zr-MOF-2-SQ and Zr-MOF-3-SQ) have excellent chemical stability and maintain structural integrity after multiple catalytic cycles, which solves the problem of poor stability of traditional transition metal-based squaramide MOFs.

[0014] 3. Zr-MOF-3-SQ constructed through a ligand length extension strategy (such as H4NDTB) achieves the dual HBD catalytic site function of "low loading and high conversion", requiring only 5 mol% catalyst loading to achieve near-quantitative conversion (>99%), which is significantly improved in catalytic efficiency compared with existing UiO type square amide MOFs.

[0015] 4. This study is the first to apply 3D ED technology to the structural characterization of square amide-based Zr-MOFs, providing an effective means for the accurate structural analysis of complex framework materials and facilitating in-depth research on structure-property relationships. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the steps involved in the synthesis of H4HPA ligands in this invention. Figure 2 This is a schematic diagram of the steps involved in the synthesis of SQ ligands in this invention. Figure 3 This is a schematic diagram of the structure of the scu topology Zr-MOFs (Zr-MOF-1, Zr-MOF-2 and Zr-MOF-3) in this invention. Figure 4 This is a schematic diagram of the structure of Zr-MOFs (Zr-MOF-1-SQ, Zr-MOF-2-SQ and Zr-MOF-3-SQ) after SQ ligand installation in this invention. Figure 5 This is a schematic diagram of the topological structure of Zr-MOFs containing squaramide groups synthesized in this invention. Figure 6 This is the XRD pattern of the Zr-MOFs containing squaramide groups synthesized in this invention. Detailed Implementation

[0017] The technical solution of the present invention will be further described below with reference to the accompanying drawings. In the embodiments of the present invention, Zr-MOF-1, Zr-MOF-2, and Zr-MOF-3 are initially synthesized using three four-coordinate carboxylic acid ligands H4HPA, H4TPCB, or H4NDTB, respectively. Furthermore, the SQ ligand is inserted into the unsaturated coordination sites of the Zr6 clusters of Zr-MOF-1, Zr-MOF-2, and Zr-MOF-3 to obtain Zr-MOF-1-SQ, Zr-MOF-2-SQ, and Zr-MOF-3-SQ. In the embodiments of the present invention, ligand H4HPA is obtained through the examples of the present invention. Ligands H4TPCB or H4NDTB can achieve the purpose of the present invention and complete the subsequent synthesis of Zr-MOFs using ligands in the prior art. The present invention does not limit the use of ligands H4TPCB or H4NDTB.

[0018] Example 1: Synthesis of H4HPA ligands.

[0019] Step 1: Weigh 1.69 g of 1,2,4,5-tetrabromo-3,6-dimethylbenzene, 1.16 g of (4-ethoxycarbonylphenyl)boric acid, 2.60 g of cesium carbonate, and 0.12 g of tetratriphenylphosphine palladium into a 500 mL three-necked flask. Vacuum the flask under vacuum, then purge with nitrogen three times. Transfer 300 mL of degassed 1,4-dioxane to the system. Heat at 85 °C for 72 h under N2 atmosphere. After cooling to room temperature, rotary evaporate 1,4-dioxane at low pressure. Extract with dichloromethane (3 × 50 mL), and filter the organic layer after drying with anhydrous magnesium sulfate. Then evaporate the dichloromethane to obtain the crude product. Sonicate the crude product in 200 mL of anhydrous ethanol for 10 min, and filter to remove excess (4-(ethoxycarbonyl)phenyl)boric acid and other organic byproducts. Remove palladium by silica gel column chromatography with dichloromethane to obtain a white powder. The first step product was a white solid (1.78 g, with 1,2,4,5-tetrabromo-3,6-xylene as the starting material, yield 76%).

[0020] Step 2: Weigh 4.90 g of Product 1 and dissolve it in a 500 mL flask. Add 100 mL of tetrahydrofuran and 150 mL of an aqueous solution containing 5.61 g of NaOH. Heat the mixture at 85 °C for 48 h. After the reaction is complete, evaporate the tetrahydrofuran under reduced pressure to obtain a white solid. Redissolve the white solid in 100 mL of water and acidify with 6 M HCl (until the pH of the mixture reaches 2) to obtain a white powder. Filter the powder and wash it with 3 × 80 mL of water to remove NaCl. Dry the powder in an oven at 60 °C for 8 hours (3.86 g, 94%).

[0021] Example 2: Synthesis of SQ ligands.

[0022] 4-Aminobenzoic acid (8.45 g) was mixed with diethyl succinate (4.32 mL, 28.3 mmol) and Zn(OTf)₂ (2.15 g, 5.8 mmol) in 95 mL of toluene and 5.0 mL of n-methyl-2-pyrrolidone. The mixture was heated to 100°C for 24 hours. The solid was separated by filtration. The solid was boiled in 20 mL of methanol for 5 minutes. The boiling solution was filtered. The product was washed twice with methanol, yielding a yellow solid (99% yield). Example 3: Synthesis of Zr-MOF-1.

[0023] 10 mg of H4HPA ligand, 20 mg of ZrCl4, and 0.60 g of benzoic acid were weighed and added to a 5 mL glass bottle containing 3 mL of DMF. The mixture was ultrasonically dispersed for 10 min and then stirred until completely dissolved. The glass bottle was sealed and placed in an oven at 120 °C for 72 h. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The crystals were collected by centrifugation, washed three times with fresh DMF, and then washed three times with acetone. The crystals were then dried under vacuum to obtain 15 mg of white crystalline Zr-MOF-1 (yield 65%). Example 4: Synthesis of Zr-MOF-1-SQ.

[0024] Weigh 0.05 g of Zr-MOF-1 crystals and add them to 10 mL of DMF solution containing 0.05 g of SQ ligand. Let stand in an oven at 120 °C for 48 h. Collect the crystals by centrifugation, wash them three times with DMF, and then transfer them to acetone for 3 days (changing the acetone daily). Then heat them under vacuum at 120 °C for 6 h to obtain 0.052 g of the yellow activated product Zr-MOF-1-SQ (yield 92%). Example 5: Synthesis of Zr-MOF-2 and Zr-MOF-2-SQ.

[0025] Following the methods of Examples 3 and 4, Zr-MOF-2 crystals and activated product Zr-MOF-2-SQ were synthesized using H4TPCB as the initial ligand, with yields of 68% and 96%, respectively.

[0026] Example 6: Synthesis of Zr-MOF-3 and Zr-MOF-3-SQ.

[0027] Following the methods of Examples 3 and 4, Zr-MOF-3 crystals and activated product Zr-MOF-3-SQ were synthesized using H4NDTB as the initial ligand, with yields of 64% and 93%, respectively.

[0028] Example 7: Catalytic performance test of Friedel-Crafts alkylation reaction.

[0029] 10 mol% of Zr-MOF-1-SQ, Zr-MOF-2-SQ, and 5 mol% of Zr-MOF-3-SQ catalysts were weighed and added to 2 mL of DCM solution containing 0.02 mmol N-methylpyrrole / N-methylindole and 0.1 mmol β-nitrostyrene-R (R=F,Cl,OMe,OH), respectively, and reacted in a constant temperature shaker at 50 °C for 48 h. 1The reaction process was monitored by ¹H NMR. After the reaction, the catalyst was separated by centrifugation, and the filtrate analysis showed the product conversion rate. The conversion rate of Zr-MOF-1-SQ was low (<70%), while Zr-MOF-2-SQ and Zr-MOF-3-SQ achieved near-quantitative conversion (>99%). The recovered catalyst was washed three times with acetone, vacuum dried, and used for the next cycle. The conversion rates of Zr-MOF-2-SQ and Zr-MOF-3-SQ remained above 95% after three consecutive cycles.

[0030] Comparative Example 1: In the literature Inorg. Chem. Front., 2022, 9, 1897, Neogi et al. introduced -NH groups into CSMCRI-12 using a post-synthetic modification method. With a catalyst loading of 10 mol%, they achieved only 5% conversion in the reaction of N-methylindole, which is significantly less efficient than the catalyst in this invention.

[0031] Comparative Example 2: In the literature ChemCatChem. 2017, 9, 1172−1176, Mandal et al. reported the synthesis of [Zn4O(L1)(DMF)2]·3DMF. Due to the limitation of its pore structure, a catalyst loading of 22 mol% is required to achieve a conversion rate of 90% in the catalytic reaction. Compared with the Zr-MOF-3-SQ catalyst of this invention, it has obvious disadvantages in both catalytic efficiency and loading. The provided figures illustrate the following issues or effects: Appendix Figure 1 The specific steps for synthesizing the H4HPA ligand in this invention are described, and the specific molecular structures of compound 1 and the H4HPA ligand mentioned in Example 1 are given. Appendix Figure 2 The specific steps for synthesizing the SQ ligand in this invention are described, and the specific molecular structure of the SQ ligand in Example 2 is given. Appendix Figure 3 The structures of the scu topologies Zr-MOF-1, Zr-MOF-2, and Zr-MOF-3 in this invention are explained, clearly demonstrating the coordination mode of the ligands with the Zr6 cluster. Appendix Figure 4 This invention illustrates the structure of the SQ ligand mounted in Zr-MOFs, demonstrating the precision of post-synthetic modification. Appendix Figure 5 The topological structure of Zr-MOFs containing squaramide groups in this invention was described, verifying the successful construction of the framework. Appendix Figure 6This demonstrates that the series of Zr-MOFs containing squaramide groups synthesized in this invention have good crystallinity and phase purity, and are in high agreement with the simulated XRD patterns.

[0032] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for synthesizing Zr-MOFs containing squaric amide groups, characterized by Comprising the following steps: (1) Synthesis of ligands: design and synthesis of tetra-coordinated carboxylic acid ligand H4HPA; synthesis of linear dicarboxylic acid ligand SQ containing squaramide groups at the same time; (2) Preliminary synthesis of scu topology Zr-MOFs: 10-15 mg of tetra-coordinated carboxylic acid ligand H4HPA, H4TPCB or H4NDTB, 20 mg of ZrCl4 and 0.5-0.6 g of benzoic acid are added to a glass bottle containing N,N-dimethylformamide, the reaction is stirred to dissolve the reactants, and then placed in a 120°C oven for solvothermal reaction for 72 h to generate crystalline products Zr-MOF-1, Zr-MOF-2 and Zr-MOF-3, respectively. The products are collected by centrifugation, washed with fresh DMF and acetone for 3 times to obtain scu topology Zr-MOFs with unsaturated coordination pockets; (3) Installation of SQ ligand: the Zr-MOFs crystals obtained in step (2) are added to a DMF solution containing SQ ligand, and the SQ ligand is inserted into the unsaturated coordination sites of the Zr6 clusters of the three MOFs by placing the mixture in a 120°C oven for two days to obtain Zr-MOF-1-SQ, Zr-MOF-2-SQ and Zr-MOF-3-SQ; the crystals are collected by centrifugation, and solvent exchange is performed by soaking in acetone for 3 days, followed by heating at 120°C under vacuum for 6 h to obtain the activated products after solvent removal, and the precise structure of the products is analyzed by three-dimensional electron diffraction and X-ray single crystal diffraction techniques.

2. The method of synthesis of Zr-MOFs containing squaric amide groups according to claim 1, characterized in that The synthesis of H4HPA ligand comprises: First step: weigh 1,2,4,5-tetrabromo-3,6-dimethylbenzene, (4-ethoxycarbonylphenyl)boronic acid, cesium carbonate and tetrakis(triphenylphosphine)palladium into a three-necked flask, evacuate the three-necked flask, then fill it with nitrogen three times, then transfer the degassed 1,4-dioxane into the system, heat at 85°C for 72 h under N2 atmosphere, cool to room temperature, then low-pressure rotary evaporation of 1,4-dioxane, extract with dichloromethane, dry the organic layer with anhydrous magnesium sulfate, then filter, then evaporate dichloromethane to obtain the crude product; ultrasonic the crude product in anhydrous ethanol for 10 min, filter to remove organic by-products, then remove palladium by silica gel column chromatography with dichloromethane to obtain product 1; Second step: dissolve product 1 in a flask, add tetrahydrofuran and NaOH aqueous solution, heat at 85°C for 48 h, then evaporate tetrahydrofuran under reduced pressure to obtain white solid, redissolve the white solid in water, acidify with 6M HCl until the pH of the mixture reaches 2 to obtain white powder, filter, then wash with water to remove NaCl, dry in an oven at 60°C for 8 hours to obtain the final product.

3. The method of synthesis of squaramide group containing Zr-MOFs according to claim 1, characterized in that The synthesis of SQ ligand comprises: Mix 4-aminobenzoic acid with squaric diester and Zn(OTf)2 in toluene and n-methyl-2-pyrrolidone, heat the mixture to 100°C for 24 hours, separate the solid by filtration, put the solid into methanol and boil for 5 minutes, filter the boiling solution, then repeat the washing with methanol for two more times to obtain the final product.

4. Use of Zr-MOFs containing squaramide groups in HBD-mediated heterogeneous organic catalytic reactions.

5. The use according to claim 4, characterized in that: S1: freshly prepared MOFs catalyst is activated under vacuum at 120℃ for 6-12 h to remove solvent molecules in the framework; S2: in the catalytic reaction system, 5-10 mol% of activated MOFs is weighed and added to a dichloromethane (DCM) solution containing N-methylpyrrole / N-methylindole and β-nitrostyrene derivatives, a constant temperature shaker is used to make the catalyst uniformly dispersed, the reaction is carried out at 50℃ for 48 h, in the cycle stability test, the reacted MOFs catalyst is centrifuged, washed with acetone and dried, and then weighed and put into the next cycle reaction.

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