Three-Component Transformation of CO ₂ , Propargyl Alcohols and Secondary Amines into β-Oxopropylcarbamates Promoted by a Noble Metal-Free Metal–Organic Framework Catalyst

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES5 Feb 2024Three-Component Transformation of CO2, Propargyl Alcohols and Secondary Amines into β-Oxopropylcarbamates Promoted by a Noble Metal-Free Metal–Organic Framework Catalyst Xiao-Lei Jiang†, Fang-Yu Ren†, Ying Shi, Yao Xie, Sheng-Li Hou and Bin Zhao Xiao-Lei Jiang† Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry (Ministry of Education), MOE, Renewable Energy Conversion and Storage Center (RECAST), Nankai University, Tianjin 300071 College of Chemistry and Chemical Engineering, Yantai University, Yantai 264010 , Fang-Yu Ren† Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry (Ministry of Education), MOE, Renewable Energy Conversion and Storage Center (RECAST), Nankai University, Tianjin 300071 , Ying Shi Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry (Ministry of Education), MOE, Renewable Energy Conversion and Storage Center (RECAST), Nankai University, Tianjin 300071 , Yao Xie Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry (Ministry of Education), MOE, Renewable Energy Conversion and Storage Center (RECAST), Nankai University, Tianjin 300071 , Sheng-Li Hou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry (Ministry of Education), MOE, Renewable Energy Conversion and Storage Center (RECAST), Nankai University, Tianjin 300071 and Bin Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry (Ministry of Education), MOE, Renewable Energy Conversion and Storage Center (RECAST), Nankai University, Tianjin 300071 https://doi.org/10.31635/ccschem.024.202303476 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The three-component reaction of CO2, propargyl alcohols, and secondary amines frequently applies noble metal or homogeneous catalysts, therefore it is necessary and important to seek a recyclable catalyst without noble metals for this reaction, despite the multitude of challenges. Herein, a unique framework {[Ce3(Cu4I4)2(CNA)8(DMF)8](NO3)}n ( CuCe-CNA) with two kinds of metal cluster nodes [Ce3] and [Cu4I4] was synthesized, displaying high thermal and chemical stabilities. CuCe-CNA-300 was obtained by removing the coordinated DMF molecules with [Ce3] cluster at 300 °C, in which more metal sites are exposed and the pores are enlarged, but the three-dimensional framework still remains intact. CuCe-CNA-300 can effectively catalyze the three-component reaction of 2-methyl-3-butyn-2-alcohol, CO2, and pyrrolidine to β-oxopropylcarbamates with a yield of 98% under mild conditions, and the catalytic activity is higher than those of noble metal catalysts. Additionally, CuCe-CNA-300 exhibits high catalytic efficiency for various kinds of propargyl alcohols and secondary amines, and also shows good recyclability. Mechanistic studies suggest that the α-alkylidene cyclic carbonates derived from the cycloaddition of propargyl alcohols and CO2 are the intermediates, which further react with secondary amines through ammonolysis reaction to generate β-oxopropylcarbamates. Importantly, this work may open a new avenue for the one-pot transformation of CO2 with two or more components into value-added chemicals activated by noble metal-free metal–organic frameworks catalysts. Download figure Download PowerPoint Introduction With the frequent daily activities of human beings and the increasing energy demand, global CO2 emissions have sharply increased, resulting in the melting of glaciers, the frequent occurrence of catastrophic climate, and other environmental problems which threaten human survival and development.1,2 Meanwhile, as a C1 raw material, CO2 possesses the advantages of safety, easy availability, and low cost.3,4 Therefore, converting CO2 into fuels or chemicals is becoming an important and effective strategy to solve the environmental problems derived from CO2.5–7 Until now, various high value-added chemicals have been synthesized by utilizing CO2, and many impressive results have been well documented.8–10 Nevertheless, it is noted that most of the reported chemical fixations of CO2 belong to the two-component reaction of CO2 with another substrate,11–13 and comparably, the three-component conversion reactions of CO2 with two types of substrates are rarely investigated due to the thermal inertia and chemical stability of CO2 (ΔG = −394.38 kJ/mol).14,15 Recently, explorations on β-oxopropylcarbamates synthesized by the three-component reaction of CO2, propargyl alcohols, and secondary amines,16,17 have attracted intense attention of many research groups because of their wide applications in the fields of medicine, pesticide, fabric finishing, and resin modification.18,19 However, such a three-component reaction was mainly performed by noble metal and/or homogeneous catalysts under harsh conditions like high temperature/pressure and special cocatalyst.20–27 Therefore, it is crucial and necessary to pursue a noble metal-free heterogeneous catalyst that can effectively activate three-component reactions under mild conditions, but it is challenging. As a kind of porous material, metal–organic frameworks (MOFs) have been applied to hydrogenation,28–30 hydration,31 coupling,32,33 and cyclization reaction34,35 that benefit from their large surface area, designable channel structure, highly dispersed active sites, and so on.36–39 In recent years, the chemical fixation of CO2 catalyzed by MOFs also attracts wide attention, such as transformation of CO2 with epoxides substrates,40,41 propargyl alcohols,42 aziridines,43 propargyl amines,44,45 and so on. But it has not been reported yet for MOFs catalysts to catalyze the three-component reaction of propargyl alcohols, CO2, and secondary amines in preparing β-oxopropylcarbamates. This has led us to design novel MOF materials with Cu(I) and Ln(III) sites to catalyze such three-component reactions with the following considerations: (1) the channel in MOFs can enrich substrates, and the highly dispersed catalytic sites in MOFs ensure substrates sufficiently contact with the active center. (2) Cu(I) ions have high activity in catalyzing CO2 with propargyl alcohols,46,47 and Ln(III) ions can activate CO2 and facilitate the ring-opening reactions.48–51 (3) Different metal sites Cu(I) and Ln(III) in one framework may bring synergistic effects for the highly efficient synthesis of β-oxopropylcarbamates under mild conditions. With this idea, a novel three-dimensional (3D) cationic framework {[Ce3(Cu4I4)2(CNA)8(DMF)8](NO3)}n ( CuCe-CNA) (CNA: 5-chloronicotinic acid) with two kinds of metal cluster nodes, [Ce3] and [Cu4I4], was synthesized and characterized. By removing the coordinated dimethylformamide (DMF) molecules on Ce centers in CuCe-CNA at 300 °C, the obtained CuCe-CNA-300 not only can open the 1D channel and maintain the stability of the framework, but can also expose more catalytic sites than that of CuCe-CNA. CuCe-CNA-300 exhibits excellent efficiency for the three-component reaction of 2-methyl-3-butyn-2-alcohol, CO2, and pyrrolidine, and the yield of β-oxopropylcarbamate can reach 98% under mild conditions. Moreover, CuCe-CNA-300 as a recycle catalyst can also effectively catalyze various kinds of propargyl alcohols and secondary amines. Mechanistic studies show that the primary catalytic sites in the reaction are [Cu4I4] clusters, and [Ce3] nodes act as synergistic catalysts in improving the catalytic efficiency. This is the first example that MOFs catalyst can catalyze the three-component reaction of propargyl alcohols, CO2, and secondary amines to β-oxopropylcarbamates. Experimental Methods Synthesis of {[Ce3(Cu4I4)2(CNA)8(DMF)8](NO3)}n (CuCe-CNA) Ce(NO3)3·6H2O (30 mg) and CNA (16 mg) were dissolved in 2 mL CH3CN and 3 mL DMF separately, then transferred to vials containing CuI (15 mg) and stirred. After adding 30 μL formic acid as regulator, the reaction system was closed and the vials were heated to 120 °C for 3 days. The obtained yellowish octahedron crystal was washed with DMF three times and dried in a vacuum drying oven. Synthesis of CuCe-CNA-300 300 mg CuCe-CNA was placed in a tubular furnace, which was then closed and continuously supplied with argon to empty the air in the tubular furnace. After 30 min, the temperature was raised to 300 °C at a rate of 2 °C per minute for 30 min, while argon gas was continuously pumped in until the temperature of the reaction chamber cooled naturally to room temperature. The brown crystal CuCe-CNA-300 was obtained. Three-component carboxylic cyclization of propargyl alcohol, CO2, and secondary amine In a general experiment, 20 mg CuCe-CNA-300 catalyst (0.049 mmol based on Cu), 168 mg (2 mmol, about 200 μL) 2-methyl-3-butyn-2-alcohol, 150 mg (2.1 mmol, about 180 μL) pyrrolidin, and 25 μL 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were added to a Teflon-lined stainless autoclave. Then the autoclave was purged with CO2 and charged in 3 bar CO2. Subsequently, the autoclave was placed in an oil bath at 70 °C and stirred magnetically for 7 h. After the reaction, the catalyst was separated by centrifugation, and the mixed solution after the reaction with 1,3,5-trimethoxybenzene as the internal standard was analyzed by proton nuclear magnetic resonance (1H NMR). Finally, the product was redetermined by the gas chromatography–mass spectrometry. The recycle ability for catalytic reaction of CuCe-CNA-300 In a typical experiment, 20 mg catalyst CuCe-CNA-300 and 2 mmol 2-methyl-3-butyn-2-alcohol, 2.1 mmol pyrrolidin, and 25 μL DBU were added to a Teflon-lined stainless autoclave and charged in 3 bar CO2. Then the autoclave was placed in an oil bath and reacted under optimal conditions at 70 °C for 7 h. Then the mixture was separated by centrifugation, and the liquid was calculated the product yield by 1H NMR, and the solid was transferred to a new autoclave. Then, the next cycle of experiments was operated as the initial run. Results and Discussion Yellowish octahedron-shaped crystals (compound CuCe-CNA) were synthesized by solvothermal reaction of CuI, Ce(NO3)3, and the ligand CNA at 120 °C for 3 days. Single crystal X-ray diffraction analysis confirms that CuCe-CNA crystallizes in the orthorhombic system with Ccce space group ( Supporting Information Table S1). The asymmetric unit consists of two [Cu4I4] clusters, one [Ce3] node, eight CNA, and eight coordinated DMF molecules ( Supporting Information Figure S1). Four copper ions are linked by four μ3-bridging iodide anions to form a [Cu4I4] cluster (Figure 1a), which is further linked by four pyridine nitrogen atoms from four ligands (Figure 1b). The [Ce3(COO)8(DMF)8] moiety contains three Ce ions, and all of them are octa-coordinated. The Ce1 and Ce1a ions are bridged with the central Ce2 ion through four carboxyl groups, and coordinated with four DMF molecules, respectively (Figure 1c). The ligand connects with one Cu ion and two Ce ions (Figure 1b), forming a 3D framework (Figure 1d). Along the b-axis, two [Cu4I4] and two [Ce3(COO)8(DMF)8] are bridged through four CNA to form a 1D channel, which is occupied by the coordinated DMF molecules. Considering the [Cu4I4] cluster as a 4-connected node and [Ce3(COO)8(DMF)8] moiety as an 8-connected node, the 3D framework can be simplified to a binodal (4,8)-connected net with flu-type topology (Figure 1e). Furthermore, the oxidation states of metal ions were characterized by X-ray photoelectron spectroscopy (XPS). The peaks at 932.7 and 952.8 eV are consistent with the binding energy of Cu(I) 2p3/2 and 2p1/2, respectively52–54 ( Supporting Information Figure S2b). The peaks at 882.3 and 885.9 eV can be assigned to the binding energy of Ce(III) 3d5/2, and the other peaks at 904.1 and 900.6 eV are consistent with the binding energy of Ce(III) 3d3/255–57 ( Supporting Information Figure S2c). The above results indicate that compound CuCe-CNA is a cationic framework, and the counter anion is demonstrated by Fourier transform infrared (FT-IR) spectroscopy. The characteristic peak of NO3− is found at 1407 cm−1 in the FT-IR spectra ( Supporting Information Figure S3), and the binding energy of N from NO3− is also observed at 406.6 eV in the XPS spectra ( Supporting Information Figure S2d); these results prove that the counteranion in pores is NO3–.58–60 Figure 1 | (a) [Cu4I4] cluster in compound CuCe-CNA. (b) The 3D framework of CuCe-CNA along the b-axis. (c) The flu topology of CuCe-CNA. (d) Coordination environment of CNA and Cu(I)/Ce(III) ions. (e) Coordination environment of [Ce3] unit in CuCe-CNA. Download figure Download PowerPoint The powder X-ray diffractometry (PXRD) patterns ( Supporting Information Figure S4) exhibit that the as-synthesized peaks of CuCe-CNA are consistent with the simulated one from the single crystal data, implying that the as-synthesized samples possess a high phase purity. Then the thermal stability of compound CuCe-CNA was tested by thermogravimetric analysis (TGA) ( Supporting Information Figure S5). There is no loss of weight in the TGA curve before 200 °C, which is ascribed to the absence of free solvent in the channel of compound CuCe-CNA. And the first weight loss (15.5%) between 200 and 300 °C can be attributed to the departure of the coordinated DMF molecules, which also matches well with theoretical calculations (15.2%). Subsequently, the TGA curve stays in a plateau until 325 °C, after that the 3D framework of CuCe-CNA began to collapse, indicating that CuCe-CNA has high thermal stability. According to previous results, metal coordination sites occupied by solvent molecules can be activated to expose metal active sites, which are conducive to their catalytic application.61–63 Compound CuCe-CNA in this experiment is difficult to be used for catalytic reactions because the channels and the metal sites are occupied by coordinated DMF molecules (Figure 2a,c). To remove the coordinated DMF molecules, 300 °C was selected as the activation temperature according to the TGA results, and CuCe-CNA-300 was obtained under the condition of argon gas (Figure 2b,d). The TGA curve of CuCe-CNA-300 shows that no weight loss is observed before 325 °C, and the carbonyl peak of DMF at 1680 cm−1 in the FT-IR spectra ( Supporting Information Figure S6) disappeared.64,65 In addition, the 13C peaks at δ = 40.5 ppm, which belong to the methyl carbon of DMF in the solid-state 13C NMR, also disappeared ( Supporting Information Figure S7). These results suggest that DMF molecules were successfully removed from the as-synthesized CuCe-CNA-300. To determine whether the compound CuCe-CNA can remain stable after activation, CuCe-CNA-300 was tested by thermogravimetry-mass spectrometer (TG-MS), XPS, PXRD, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The TG-MS curves of compound CuCe-CNA showed that the weight loss from 150 to 300 °C corresponds to the molecular-ion mass peaks of DMF (m/z = 73), (CH3)2N+ (m/z = 44), CO2 (m/z = 44), CH2O (m/z = 30), ·CHO (m/z = 29), and CO (m/z = 28), which are attributed to the departure of the coordinated DMF molecules and the fragmentation ion peaks from DMF decomposition ( Supporting Information Figure S8). The XPS spectra of CuCe-CNA-300 are consistent with the data of CuCe-CNA, confirming that the oxidation states of Cu(I) and Ce(III) in CuCe-CNA-300 are unchanged after heating ( Supporting Information Figure S9). The PXRD patterns of CuCe-CNA-300 display the emergence of two new peaks at 2θ = 8.7 and 12.4°, which may attribute to the removal of coordinated DMF molecules. Hence, CuCe-CNA-300 was resoaked in DMF and kept at 90 °C for 2 days, and the PXRD patterns of obtained sample are well matched with CuCe-CNA, indicating that the peaks in CuCe-CNA-300 were caused by the removal of coordinated DMF molecules (Figure 3a). As shown in Figure 3b, the Brunauer–Emmett–Teller (BET) surface area of compound CuCe-CNA is 4.02 m2/g. Owing to the removal of DMF molecules during the preparation of CuCe-CNA-300, the amount of N2 uptake is increased to 120 cm3/g, and the BET surface area of CuCe-CNA-300 is increased to 325.9 m2/g. The average micropore diameter of CuCe-CNA-300 is 0.61 nm, which is consistent with the pore sizes of CuCe-CNA after removing DMF in channels. In addition, mesoporous pores were also generated during calcination at 300 °C, and the average mesopore diameter of CuCe-CNA-300 is 4.05 nm. In addition, SEM and TEM (Figure 4a–j) results display that CuCe-CNA-300 can maintain its original morphology without collapse of framework, which further demonstrate that the framework can remain stable during the thermal treatment. Subsequently, the solvent stability test of CuCe-CNA-300 shows that it also can remain stable in DMF, N,N-dimethylacetamide, CH3CN, MeOH, EtOH, and other organic solvents ( Supporting Information Figure S10). Figure 2 | The 3D framework of CuCe-CNA along the c-axis (a) before and (b) after removing the coordinated DMF molecule. Coordination environment of Ce(III) ions (c) before and (d) after removing the coordinated DMF molecule. Download figure Download PowerPoint By comparison, CuCe-CNA was treated at different activation temperatures of 200, 250, and 350 °C, respectively, and the obtained CuCe-CNA-200, CuCe-CNA-250, and CuCe-CNA-350 were characterized. SEM and TEM ( Supporting Information Figures S11–S14) images show that the materials can maintain their original morphology and no new substances were formed before 300 °C. But the collapse of framework is observed in SEM images of CuCe-CNA-350 ( Supporting Information Figure S14a–h), and nanoparticles are also observed in the TEM images of CuCe-CNA-350 ( Supporting Information Figure S14i,j), indicating that compound CuCe-CNA was decomposed at 350 °C, which is accordance with TGA results ( Supporting Information Figure S5). The lattice spacing d = 3.46 Å and d = 2.14 Å in CuCe-CNA-350 are obtained based on the TEM images, which match with the (1 1 1) and (2 2 0) crystal planes of CuI, respectively, suggesting that the nanoparticles in CuCe-CNA-350 are CuI. In addition, the peaks at 2θ = 25.50, 29.52, 42.22, 49.98, 52.36, 61.26, 67.44, 69.44, and 77.22 in PXRD patterns are also consistent with that of CuI ( Supporting Information Figure S15). All these results indicate that the framework of CuCe-CNA has collapsed at 350 °C, and CuCe-CNA can remain stable before 300 °C. Figure 3 | (a) The PXRD patterns of CuCe-CNA, CuCe-CNA-300, and CuCe-CNA-300 resoaked in DMF. (b) N2 adsorption/desorption isotherms of CuCe-CNA and CuCe-CNA-300. Download figure Download PowerPoint Figure 4 | (a) SEM image and energy dispersive X-ray spectroscopy mapping for (b) C, (c) I, (d) Cl, (e) Ce, (f) Cu, (g) N, and (h) O of compound CuCe-CNA-300. (i, j) TEM images of CuCe-CNA-300 (The scale bar on the images of (a–h) is 200 μm, on the image of (i) is 100 nm, and on the image of (j) is 5 nm). Download figure Download PowerPoint β-Oxopropylcarbamates, as a class of crucial organic synthesis intermediates, have broad application prospects in the fields of medicine, pesticide, fabric finishing, and resin modification. However, phosgene or CO usually act as the C1 source to react with amine compounds to synthesize β-oxopropylcarbamates.66,67 These methods have a low yield of product, complex reaction process, high toxicity of raw materials and by-products, and they are easy to cause accidents and environmental pollution. Considering the activation capacity of Cu(I) for alkynyl groups, the good stability, and open metal sites in framework, CuCe-CNA-300 was used in the three-component reaction of propargyl alcohols, CO2, and secondary amines to synthesize β-oxopropylcarbamates. To explore the optimal conditions of the catalytic reaction, 2-methyl-3-butyn-2-alcohol and pyrrolidine were chosen as the model substrates. And the reaction conditions are listed in Supporting Information Table S2. Under conditions of 3 bar of CO2, and DBU as cocatalyst, the yield of β-oxopropylcarbamate can reach 98% at 70 °C after 7 h ( Supporting Information Table S2, entry 5). At the same time, the CuCe-CNA-X ( CuCe-CNA-200, CuCe-CNA-250, CuCe-CNA-350) catalysts were also used for this reaction under optimal conditions. As shown in Supporting Information Table S2 (Entries 5 and 14–16), with the increase of activation temperature, the catalytic activity of CuCe-CNA-X displays a trend of first increasing and then decreasing, among which CuCe-CNA-300 had the best catalytic effect, indicating that the catalytic efficiency can be enhanced after activating the framework at proper temperature. The kinds of propargyl alcohols and secondary amines were further changed to explore the universality of this three-component reaction catalyzed by CuCe-CNA-300 under the optimal conditions (3 bar, DBU as cocatalyst, 70 °C and 7 h). As shown in Table 1, when the R1 or R2 groups of propargyl alcohols are replaced by electron-donating groups, such as the ethyl group and isopropyl group, and exhibit good yields (3aa: 98%; 3ba: 95%; 3ca 96%; 3da 96%; 3ea: 97%; 3fa: 83%; 3ga: 97%) with the increasing substrate size, the yields of the products decrease. While the R1 or R2 groups are replaced by benzene ring (3ha: 46%; 3ia: 0.5%), the reaction yield is significantly reduced. It can be attributed to the electron withdrawing effect and the large steric hindrance of benzene ring, which are unfavorable for the reaction. Subsequently, the R3 or groups of secondary amines were for ethyl and CuCe-CNA-300 showed high catalytic activity for the catalytic reaction. While R3 or is replaced by a benzene ring or group of large size, their products be and These experiments that as-synthesized CuCe-CNA-300 possesses good general for propargyl alcohols and secondary amines, and has a Table 1 | of Propargyl CO2, and Secondary 2 mmol of propargyl alcohols, 2.1 mmol of secondary amines, 20 mg of CuCe-CNA-300, 25 μL of 3 bar of CO2, 70 °C, 7 and solvent The yields were by 1H with 1,3,5-trimethoxybenzene as the internal The of these compounds are in Supporting Information Figures ability is also for heterogeneous catalysts in Therefore, the of CuCe-CNA-300 was further and are listed in the Supporting The results showed that the catalytic effect of CuCe-CNA-300 not significantly after three catalytic ( Supporting Information Table S2, and Meanwhile, the catalytic reaction after removing CuCe-CNA-300, and the experiment shows that CuCe-CNA-300 is a heterogeneous catalyst ( Supporting Information Figure The stability after three catalytic was also of PXRD, and spectroscopy results are consistent with the data before the catalytic cycle ( Supporting Information Figures and and Meanwhile, XPS shows that Cu and Ce ions in CuCe-CNA-300 still remain the oxidation of and after the catalytic respectively, which that CuCe-CNA-300 can remain stable after three ( Supporting Information Figure The effects of Cu(I) and Ce(III) on the reaction were through experiments the yield of β-oxopropylcarbamates is without CuCe-CNA-300 0.5%), suggesting CuCe-CNA-300 is the catalyst for this reaction. Subsequently, the experiments were with Ce(NO3)3, CNA, or CuI, and all of them have low yield of the β-oxopropylcarbamates indicating that the porous 3D framework has a effect on this reaction The catalytic effect of CuI is than that of and the yields of and as catalysts were and respectively (Entries and that Cu(I) is the primary catalytic active in this reaction. Moreover, the catalytic yield of CuI is higher than CuI suggesting that is a synergistic catalytic effect between Cu and Ce ions for this reaction. The between Cu(I) and its Ce(III) are and ( Supporting Information Figure The between the of and carbon of in the substrate is ( Supporting Information Figure the reaction, the between the activated and carbon of in the is ( Supporting Information Figure According to these results, the Cu(I) and Ce(III) sites in the MOF activate one and they an during this three-component reaction. Hence, CuCe-CNA-300 is a which is also a strategy for reactions to synthesize highly complex To the catalytic effect of other ions, a of MOFs with different ions were synthesized and activated at 300 °C in argon to the and ( Supporting Information Figures and The experiment results under the same conditions display that the product yields catalyzed by and are and respectively, which are than CuCe-CNA-300, indicating that Ce ion is more efficient than other ions for this reaction. In addition, this MOF also exhibits higher activity than typical MOF = of and entry and most reported catalysts ( Supporting Information Table suggesting CuCe-CNA-300 is a unique noble metal-free catalyst that possesses the advantages of high activity and good Table 2 | of of Propargyl CO2, and Secondary Catalyst 1 CuCe-CNA-300 2 3 CNA 2 4 CNA 4 5 2 CuI 7 CuI CuI CNA CuCe-CNA 2 mmol of 2-methyl-3-butyn-2-alcohol, 2.1 mmol of pyrrolidine, mmol of 25 μL of 3 bar of CO2, 70 °C, 7 and solvent yields were by 1H with 1,3,5-trimethoxybenzene as internal And the was listed in the Supporting It has been that catalysts can effectively activate the alkynyl group and the cycloaddition of propargyl alcohols with CO2 through and Therefore, theoretical calculations were to the catalyzed by [Ce3] sites in this reaction. The activities of catalysts CuCe-CNA after and removal of DMF molecules were selected for According to the results, after removal of a of the coordinated DMF molecules, the activation energy for the of group in propargyl is eV (Figure And the activation energy is to eV after removal of all the coordinated DMF molecules (Figure In addition, the activation energy for the ring reaction is also from eV (Figure to eV (Figure These results indicate that the [Ce3] sites can catalyze the cycloaddition of propargyl alcohols with CO2. Furthermore, for the ring-opening reaction of by secondary the activation energy by CuCe-CNA-300 is eV (Figure which is than that of activated catalysts (Figure Hence, [Ce3] sites in MOF CuCe-CNA is an active sites for

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