[Co3]-Cluster Based Metal-Organic Framework Enables “Two Birds with One Stone” in Efficient Transformation of CO2 to Oxazolidinones

Zhuohao Jiao,Xinyuan Zhao,Jian Zhao,Yao Xie,Shengli Hou ,Bin Zhao

College of Chemistry,Key Laboratory of Advanced Energy Material Chemistry,Ministry of Education,Renewable Energy Conversion and Storage Center,Nankai University,Tianjin 300071,China.

Abstract: CO2,as a greenhouse gas,has excessive emissions that lead to many environmental problems and is a rich and cheap C1 resource.Effective utilization and transformation of CO2 has become an important means of achieving carbon neutrality.Oxazolidinones are important intermediates in pharmaceutical chemistry that can be synthesized by carboxylation cyclization of CO2 with propargyl amines or cycloaddition of CO2 with aziridines.Owing to CO2’s high stability,these reactions typically require harsh conditions,such as high temperatures or pressures.It is desirable,but challenging,to find a catalyst that can catalyze these two types of reactions under relatively mild conditions.Metal-organic frameworks (MOFs) are an emerging class of heterogeneous catalysts that with great potential in the catalytic conversion of CO2 to value-added products because of their attractive features,such as abundant metal active sites,inherent porosity,and easy functionalities.Herein,a unique three-dimensional (3D) MOF,{(CH3NH2CH3)2[Co3(BCP)2]·6H2O·4DMF}n (1) (H4BCP: 5-(2,6-bis(4-carboxyphenyl)pyridin-4-yl) isophthalic acid; DMF: N,N'-dimethylformamide),was synthesized using carboxylic acid ligands and cobalt salts via a solvothermal method.According to structural analysis,[Co3] clusters as secondary building units (SBU) are bridged by BCP4- ligands,forming an anion framework with flu topology,and dimethylamine cations act as counter ions in the pores.The framework has rectangular channels of approximately 0.4 nm × 0.9 nm along the a-axis direction,exhibiting its porous property.Infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) characterizations proved the coordination interaction between the carboxyl groups in the ligands and the metal ions.The powder X-Ray diffraction (PXRD) test further confirmed the phase purity of the synthesized samples.PXRD and thermogravimetry (TG) analyses indicated that 1 possessed good solvent and thermal stabilities.The catalytic experiments revealed that 1 could effectively catalyze CO2 with aziridines or propargyl amines to prepare oxazolidinones.In the cycloaddition of CO2 with aziridines,1 can facilitate the reaction under relatively mild conditions compared to other reported MOF-based catalysts.It shows excellent universality for substrates with various substitutions on the N atom or benzene ring.Investigation of the mechanism indicated that the coordination interaction of cobalt metal sites with the nitrogen atoms of aziridines can activate the substrates.For the carboxylative cyclization of CO2 with propargylic amines,this catalyst also has a broad substrate scope.Control experiments and nuclear magnetic resonance (NMR) tests suggest that Lewis acid metal sites are responsible for the high catalytic efficiency achieved by activating the alkyne groups.Moreover,1 showed good reusability in both reactions.Compound 1 represents a new catalyst that enables “two birds with one stone” in the catalytic synthesis of oxazolidinones using CO2.

Key Words: Metal-organic framework; Bifunctional catalyst; CO2 utilization; Oxazolidinone; Reusability

1 Introduction

With the proposal of carbon neutrality,the emission reduction and effective utilization of CO2have become the focus of researches.CO2as an inexpensive and readily available C1 feedstock,can be converted into a variety of high value-added products,including urea,formic acid,methanol,cyclic carbonates,oxazolidinones,and polymers through photocatalysis,electro-catalysis,or thermos-catalysis1–11.Among them,oxazolidinones as heterocyclic compounds have been widely used in synthesizing antibacterial drugs such as tedizolid,radezolid,and linezolid12.And they also play an important role in serving as intermediates in organic synthesis and chiral auxiliary reagents13,14.Heretofore,diverse methods have been proposed for the preparation of oxazolidinones,among which two of the most common and efficient approaches are the cycloaddition of CO2with aziridines or carboxylation cyclization of CO2with propargyl amines due to their high atom economy and simple procedure15.Some catalysts have been applied in preparation of oxazolidinones by these two approaches,including amino acids16,haloids17,noble metal complexes18,etc.However,high cost or complicated postprocessing procedures limit their large-scale industrial applications,and these studies only focus on one kind of reaction on account of the high thermodynamic stability and kinetic inertness of CO2.Hence,it is important to construct a heterogeneous catalyst that can catalyze these two kinds of reactions.

Metal-organic frameworks (MOFs),as an emerging class of heterogeneous catalysts,have gained increasing attention due to their abundant metal active sites19–22,inherent porosity23–25,and easy functionalities26–29.In recent years,researches focused on MOFs for catalyzing the formation of oxazolidinones from CO2with aziridines or propargyl amines,and many exciting works have been reported successively30,31.However,these catalysts are usually used to catalyze only one type of these reactions because these two reactions are Lewis-acid and Lewis-base preferred reactions,respectively.In addition,due to the high stability of CO2,these reactions usually require harsh conditions such as high temperature or pressure.Hence,the construction of one MOF that can simultaneously catalyze these two types of reactions under mild conditions is desirable but challenging.

The Co2+ion has been used to construct a variety of MOFs structures due to its diverse coordination modes,and its Lewis acidity also allows it to apply in numerous catalytic reactions32–37.In this contribution,we synthesized a unique 3D MOF{(CH3NH2CH3)2[Co3(BCP)2]·6H2O·4DMF}n(1) by combining Co2+and a carboxylate ligand 5-(2,6-bis(4-carboxyphenyl)pyridin-4-yl) isophthalic acid (H4BCP) through solvothermal method,which has a large 1D channel with a size of 0.4 nm ×0.9 nm.Experiment results reveal that 1 shows excellent catalytic performance for promoting the conversion of both aziridines and propargylic amines with CO2into oxazolidinones,and it can be reused for five cycles and maintains high catalytic activity.In particular,1 enables “two birds with one stone” in the synthesis of oxazolidinones by CO2,which represents a rarely reported catalyst for preparing oxazolidinones through two pathways.

2 Experimental and computational section

2.1 Preparation of {( CH3NH2CH3)2[Co3(BCP)2]·6H2O·4DMF}n (1)

Co(NO3)2·6H2O (99.99%,15 mg) and H4BCP (20 mg) were mixed withN,N'-dimethylformamide (DMF,99.5%,2.5 mL)and deionized water (0.5 mL).Then 200 μL of nitric acid(65.0%–68.0%) was dropped in the mixture to adjust the pH.Subsequently,the mixture was transferred to a 25 mL Teflonlined stainless-steel vessel and heated at 145 °C for 72 h under autogenous pressure.After cooling to room temperature naturally,dark purple lamellar crystals were collected.The resultant crystals were washed with DMF and H2O several times to remove the residual metal salts or ligands.Elemental analysis of crystal 1,calculated results: C 51.33%,H 4.83%,N 6.84%.Tested results: C 51.93%,H 4.79%,N 6.89%.CCDC number:2160038.

2.2 Catalyst characterization

Powder X-ray diffraction (PXRD) patterns were measured on an Ultima IV diffractometer (Rigaku,Japan) using CuKαas radiation sources with a 5 (°)·min-1scanning speed.X-ray photoelectron Spectrometer (XPS) analysis was carried out on an ESCALAB 250Xi spectrometer (ThermoFisher,USA) using Mono AlKαradiation,and the internal standard carbon is 284.8 eV.1H NMR chemical shifts were measured on 400 MHz spectrometer (ASCEND 400,Bruker,Germany) using CDCl3as internal standard.13C NMR chemical shifts were carried out on a AVANCE III HD 400 spectrometer (101 MHz,Bruker,Germany).The metal content was tested by Aglient 5110 (USA)inductively coupled plasma optical emission spectrometer (ICPOES).Typically,100 μL of reaction mixture obtained by centrifugation is decomposed by aqua regia and then diluted to 5 mL by deionized water.The resultant sample was measured by ICP-OES.Infrared spectra (IR) were measured by a Bruker Tensor 27 (Germany) spectrophotometer.The thermogravimetric analysis (TGA) data were measured on thermal gravimetric analyzer (NETZSCH TG209,Germany) under air atmosphere.Single-crystal X-ray diffraction was performed on a SuperNova Single Crystal diffractometer (Rigaku,Japan) with MoKαradiation (λ= 0.71073 ?,1 ? = 0.1 nm).The obtained data was analyzed by Olex2 software to get the structure38.Due to the highly disordered state of solvent molecules in frameworks,we use PLATON/SQUEEZE programs to remove the disordered solvent molecules39,and the number of free solvent molecules was determined by elemental analysis and TGA.Detail of single crystal data and structure refinement for 1 were recorded in Table S1 in Supporting Information.

2.3 Catalytic experiments

Cycloaddition of CO2with aziridines: Compound 1 (10 mg),aziridine substrates (0.5 mmol) and co-catalyst TBAB (32.4 mg,0.1 mmol) were sealed in an autoclave.After purging with 0.5 MPa CO2,the autoclave was placed in water bath of 30 °C and stirred for 9 h.When the reaction was completed,the catalyst was separated by centrifugation and filtration.The filtrate was dissolved in 0.5 mL CH2Cl2(99.9%),and then the yields of corresponding products (2x and 3x) were analyzed by1H NMR with 1,3,5-trimethoxybenzene (99.9%) as an internal standard.

Carboxylation cyclization of CO2with propargylic amines:Compound 1 (10 mg),propargylic amines (0.5 mmol) and additive 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU,98%,19 mg,0.125 mmol) were mixed in CH3CN (99.7%,0.5 mL).After adding the mixture in a 15 mL Schlenk tube and purging 0.1 MPa of CO2,the tube was placed in oil bath at 80 °C and kept stirred for 10 h.The mixture was separated by centrifugation and filtration when the reaction was completed,and the filtrate was analyzed by1H NMR with 1,3,5-trimethoxybenzene as an internal standard.

Cycle experiments: The filter residue was washed with fresh CH3CN three times and dried under vacuum at 60 °C.The recovered catalyst was used for the next catalytic experiments.

3 Results and discussion

3.1 Characterization

According to single-crystal X-ray diffraction analysis,compound 1 crystallizes in a triclinic system withP-1 space group.The asymmetric unit contains one and a half Co2+ions,one independent BCP4-ligand,and one dimethylamine cation([(CH3)2NH2]+).There are two types of Co ions (denoted as Co1 and Co2),and both of them are coordinated with six oxygen atoms from the carboxyl group of ligands (O1,O1A,O3,O3A,O7,O7A for Co1,and O3,O5,O9 O12,O13,O16 for Co2) (Fig.S1).One Co1 and two Co2 ions are bridged by the carboxyl group to form a [Co3] cluster (Fig.1a).Each ligand coordinates with four [Co3] clusters,expanding into a 3D anion framework(Fig.1b,c).[(CH3)2NH2]+acts as counterion to balance the charge which may generate from the decomposition of DMF and its existence was confirmed by structural analysis40.By considering the [Co3] clusters and H4BCP ligands as nodes,the structure can be rationalized to a 4,8-connected net withflutopology (Fig.1d).Along thea-axis direction,a rectangular channel about 0.4 nm × 0.9 nm is present in the framework,and the total potential pore volume of the structure is estimated to be 39.6% by PLATON software (Fig.1e).

Fig.1 (a) The 8-connected [Co3] cluster SBU; (b) coordination mode of H4BCP; (c) 3D structure of the framework along the a-axis; (d) flu topology of 1; (e) the rectangular channel of 1 along the a-axis direction.

According to the analysis of infrared spectroscopy (IR),the ―OH vibration peak of the carboxyl group in the ligand is located at 2940–3300 cm-1,while it disappears in the spectrum of 141.Meanwhile,the peak at 1672.8 cm-1which belongs to the stretching vibration of C＝O shifted to 1656.4 cm-1in that of 1,which demonstrates the coordination of carboxyl groups to the Co ions (Fig.S2)42.

XPS analysis shows the fitting peak at the binding energy of 531.78 eV in O 1sspectrum corresponds to ―C―O―Co in the framework,conforming the coordination interaction between carboxyl groups and Co ions (Fig.S3c)43.In Co 2pspectrum,the peaks located at 781.7 and 797.4 eV belong to Co 2p3/2and Co 2p1/2of Co2+,while the binding energy of 786.2 and 802.0 eV is in accordance with the satellite peaks of Co2+(Fig.S3d)44.The PXRD peaks of 1 are identical to those of the simulated one,demonstrating the satisfactory phase purity of synthesized crystals (Fig.S4a).After immersing the crystals in several common solvents (DMF,N,N'-dimethylacetamide,methanol,ethanol,acetonitrile,dichloromethane,dioxane,Nmethylpyrrolidone) for 24 h,the patterns still agree with that of synthesized samples and standard patterns,which confirms the excellent solvent stability of 1 (Fig.S4b).In addition,the thermal stability was explored by performing the thermogravimetric analysis (TGA) at 30–700 °C (Fig.S5).As the temperature rises,there is a weight loss about 25.05% before 330 °C,which is assigned to the escape of free solvent molecules(H2O and DMF,calculated 25%).Then the framework begins to gradually collapse and decompose.Meanwhile,PXRD test was performed under different temperatures (Fig.S6).Combined with TGA and PXRD results,1 can maintain its structure integrity under 200 °C,which indicates that this compound has good thermal stability.The favorable thermal and solvent stabilities provide a guarantee for compound 1 to be used for thermal catalysis.

3.2 Catalytic conversion of CO2

3.2.1 Cycloaddition with aziridines

Benefiting from the porous nature of the MOF material,the catalyst may enrich the substrates and CO2.Hence,we tested the CO2adsorption capacity of the activated samples at 298 K (Fig.S7).The adsorption-desorption isotherms display that the CO2absorption is 10.68 cm3·g-1at 298 K,which indicates that 1 possesses the ability to adsorb CO2in thermal catalysis.Owing to the attractive stability and adsorption ability of this compound,we conducted to study the MOF-catalysed conversion of CO2to oxazolidinones.Initially,we explored the performance of 1 in catalytic reaction of CO2with aziridines,and 1-ethyl-2-phenylaziridine (1a) was chosen as an example to explore the optimal reaction conditions.The effect of pressure was screened at first and the target product 3-ethyl-5-phenyloxazolidin-2-one (2a) was obtained with 19% NMR yield under atmospheric pressure.By increasing pressure to 0.5 MPa,the substrate could be quantitatively converted to the desired compound (entries 1–3,Table 1).As the temperature decreased,the substrate could be completely converted at 40 or 30 °C,but when the temperature dropped to room temperature (25 °C),the yield could only reach 70% (entries 4–6,Table 1).With increasing reaction time,the yield gradually increased,and 99% yield could be obtained in 9 h (entries 7 and 8,Table 1).Therefore,the optimal reaction condition is considered as 10 mg of catalyst 1 with 0.1 mmol of TBAB as co-catalyst,0.5 mmol of substrates,and 0.5 MPa CO2for 9 h at 30 °C in solvent-free conditions.Compared to the reported MOF-based catalysts,1 can facilitate the reaction under relatively mild conditions.(Table S2).

Table 1 Catalytic reaction of CO2 with 1-ethyl-2-phenylaziridine under different conditions a.

The universality of 1 with different aziridines was discussed on the basis of the optimal reaction conditions.For the alkyl substituents from methyl ton-pentyl on the N atom,the yields of corresponding products decreased from 99%–79%,which is mainly attributed to the increase of size for R1 group hindering the proceeding of the reaction (entries 1,2,4–6,Table 2).In addition,when benzyl substituent with larger steric hindrance was introduced on the N atom,the reaction yield decreased to 83%,which further proves the steric hindrance effect for the reaction (entry 7,Table 2).Besides,isopropyl substituent led to a relatively lower yield (70%),probably because the larger bulk of the N-atom-connected propyl group hinders the coordination of metal sites with N atoms (entry 3,Table 2)45.As for the substituents at theparaposition of the benzene ring,the electron-donating substituent ―CH3had almost no effect on the yield and still gave excellent yields.In contrast,the electronwithdrawing substituents such as chloro (―Cl) and bromo (―Br)reduced the reaction efficiency to some extent,resulting in the corresponding product 5-(4-chlorophenyl)-3-ethyloxazolidin-2-one (2j) and 5-(4-bromophenyl)-3-ethyloxazolidin-2-one (2k) of 78% and 75% yields,respectively (entries 8–10,Table 2).To sum up,compound 1 can successfully prepare oxazolidinonesviathe pathway of cycloaddition of aziridines and CO2with broad substrate scope.

Table 2 Exploration of substrate universality of catalyst 1 under optimal conditions a.

Recyclability is of crucial importance for heterogeneous catalysts in practical applications.By simple centrifugation and filtration,the catalyst was recovered and applied to the next catalytic experiment.1 preserved high catalytic activity and only a slight decrease in yield was observed after five continuous reaction cycles (Fig.2).The catalyst recovered after five cycles was analysed by PXRD.The pattern agreed well with that of the original sample,indicating that 1 can maintain stable during the reaction (Fig.S8,The ICP-OES tests showed that only a small amount of Co ions leached into the reaction mixture (calculated to be 1.39% leakage of Co2+),which could be caused by the destruction of trace amounts of MOF during vigorous stirring(Table S3)46.All the results above show that 1 maintains its efficiency and robustness during cyclic reactions.

Fig.2 The cycling performance of compound 1 in catalytic reactions.

To explore the possible mechanism,we conducted control experiments with different conditions (Table S4).It is apparent that the ligand H4BCP could not make the experiment proceed,and the cobalt nitrate and co-catalyst TBAB could only obtain 30% or 48% yield,respectively.The mechanical mixture of these three components also only yielded 68% of desired products,which is much lower than that of 1.The results indicate that the binding effect between free Co2+and Br-may inhibit the nucleophilicity of Br-,thereby weakening the ability of Br-to attack the aziridines.Combining the above findings and literature reports30,47,a possible mechanism is proposed for the cycloaddition reaction of the aziridines with CO2.Firstly,the porous structure of 1 enriches CO2and substrates,and cobalt metal sites coordinate with nitrogen atoms of the aziridines to activate the substrates; secondly,the nucleophilic attack of Br–at the C atoms of aziridines leads to ring-opening by two pathways which are presented as I and II; thirdly,CO2is inserted into the N―Co bond to form a carbamate salt intermediate;finally,the desired oxazolidinone product is obtained by an intramolecular nucleophilic substitution ring-closure reaction with the regeneration of 1 and TBAB (Scheme 1).As for regioselectivity,path I is the preferred one,mainly because Br–tends to attack the aziridines from the more substituted carbon with the generation of a more stable carbamate salt than path II.Thus,the 5-substituted oxazolidinones can be obtained in excellent yields and regioselectivity when 1 is used as the catalyst.

Scheme 1 Mechanistic proposal for cycloaddition reaction of CO2 and aziridines.

3.2.2 Carboxylative cyclization with propargylic amines

Then we turned to investigate whether 1 could act as a bifunctional catalyst to promote the carboxylative cyclization of propargylic amines with CO2.At the outset,we choseN-benzyl-2-propynylamine (A1) as the model compound and the reaction was carried out using 1 (10 mg) as catalyst and DBU (0.25 mmol) as base in CH3CN at 40 °C under atmospheric CO2pressure.Under given conditions,the target product 3-benzyl-5-methyleneoxazolidin-2-one was detected with an NMR yield of 19% (entry 1,Table S5).Subsequently,the influence of reaction temperature (40,60 and 80 °C) was explored and the results indicated that the rise of temperature significantly promoted the reaction,with a higher yield of 76% at 80 °C (entries 2 and 3,Table S5).Then the influence of time was examined,and when the reaction time was extended to 10 h,the substrate was almost quantitatively converted to the desired product (entry 4,Table S5).Under the above-optimized conditions,the reaction still proceeds smoothly with an excellent yield (97%) after reducing the amount of DBU to 0.125 mmol (entry 5,Table S5).Noticeably,when the reaction performed without solvent,it still achieved a satisfactory yield up to 85% (entry 6,Table S5).Therefore,we choose 10 mg of catalyst 1 with 0.125 mmol of DBU as base at 80 °C and 1 atm (1 atm = 101325 Pa) of CO2in 0.5 mL CH3CN for 10 h as the optimal reaction conditions.In addition,control experiments show that compound 1 exhibits superior catalytic performance compared with the mixture of Co(NO3)2and H4BCP,suggesting that the framework of 1 plays an important role in this catalysis (Table S6).

Similarly,the catalytic activity of 1 was examined with various propargyl amines (Table 3).The substrates bearing electron-donating substituent methoxy (―OCH3) or electron-withdrawing groups like chloro (―Cl) and bromo (―Br) at thepara-position of the benzyl group could smoothly transform to corresponding oxazolidinone products with excellent yields (B2 93%,C2 99%,D2 91%,entries 2–4,Table 3).However,whentert-butyl was introduced on the benzene ring,there was a significant decrease in yield to 64%,probably caused by the large steric hindrance effect oftert-butyl group (entry 5,Table 3).In addition,when there are alkyl substituents such as isopropyl,n-butyl,and cyclohexyl on the N atom of propargyl amines,the corresponding products were detected with 92%–95% yield(entries 6–8,Table 3).Thus,1 also exhibits satisfactory substrate universality for the carboxylative cyclization of propargylic amines with CO2.

Table 3 Preparation of various oxazolidinones with CO2 and propargylic amines a.

Using the same recycling procedures above,we investigated the recycling performance of 1 for this approach.1 could be recovered after 5 cycles of reactions and maintained its catalytic activity for catalysing the reaction in good yields (Fig.2).The PXRD of the catalyst still matched with the simulated data,illustrating the integrity of the framework (Fig.S7).Furthermore,ICP tests showed that only about 1.16% Co leaked into the reaction mixture after five continuous catalytic runs(Table S3).These results demonstrate that 1 can act as a heterogeneous catalyst for carbonylative cyclization of CO2and propargyl amines with good recyclability and stability.

To investigate the reaction mechanism,we attempted to monitor the activation of the propargyl amines by 1 and DBU through1H NMR and13C NMR spectroscopy.The N―H signal of A1 at 2.63 disappeared when DBU was added,which indicates that DBU can activate the hydrogen proton of the N―H group in substrates (Fig.S9)48.At the same time,the strength of the13C signals atδ= 73.7 and 82.8 assigned to the alkyne group was obviously weakened after the addition of 1,suggesting that the C≡C bond of propargyl amine can be activated by 1 (Fig.S10).Based on the NMR results and literature research49,50,a possible mechanism is proposed for the carboxylative cyclization of propargylic amines with CO2.Initially,the imino and alkyne groups of propargyl amine A1 are activated by DBU and 1,respectively; Then the activated substrate further reacts with CO2to generate the carbamate intermediate; Subsequently,the negatively charged oxygen ion undergoes an intramolecular ring closure reaction through nucleophilic attack; Finally,proto-demetallation occurs with the help the proton carrier to obtain the desired product A2,accompanied by the regeneration of DBU and 1 (Scheme S1).

4 Conclusions

In summary,a unique 3D Co-based MOF 1 with 1D channels was successfully synthesized.Compound 1 features good solvent and thermal stability.The results of catalytic experiments indicated that 1 can serve as an effective heterogeneous catalyst for promoting the cycloaddition of CO2with aziridines or carboxylative cyclization of CO2with propargylic amines to prepare oxazolidinones.The catalyst presents broad substrate universality for both reactions.Besides,it can be reused for five cycles without significant decrease in catalytic efficiency.Compound 1 represents a rare catalyst that enables “two birds with one stone” in catalytic synthesis of oxazolidinones by CO2,which may expand the application of MOFs in the field of catalysis and the potential of MOFs to catalyse multiple reactions simultaneously.

Supporting Information: Materials,control experiments details,and IR,TGA,PXRD,CO2adsorption-desorption,1H NMR and13C NMR spectra (PDF); X-ray crystallographic data of 1 (CIF) available free of chargeviathe internet at http://www.whxb.pku.edu.cn.