A review on framework (MOF/COF/POP)-based materials for efficient conversion of CO₂ to bio-active oxazolidinones

Abstract

Excessive reliance on fossil fuels has increased atmospheric CO₂ emissions, resulting in the greenhouse effect that endangers global climate stability and human well-being. Consequently, the storage and chemical conversion of CO₂ into sustainable products can play a vital role in reducing anthropogenic emissions. Hence, there is an upsurge in research on selective carbon capture, sequestration and utilization (CCSU) to mitigate the rising atmospheric CO₂ concentration. Carbon capture and utilization (CCU), in particular, has attracted considerable interest because it enables the utilization of CO₂ as a C1 feedstock for generating commodity chemicals and fuels such as cyclic or polycarbonates, cyclic carbamates, oxazolidinones, formamides, methane, methanol and so on. Among these products, oxazolidinones are essential five-membered heterocyclic compounds found in several important pharmaceuticals. Oxazolidinones also function as versatile intermediates and chiral agents in organic synthesis. Thus, developing highly efficient heterogeneous catalysts containing dense basic and catalytic sites is potentially significant for effectively capturing and transforming CO₂ into 2-oxazolidinones under ambient conditions. In this regard, porous framework-based materials viz metal–organic frameworks (MOFs), covalent organic frameworks (COFs) and porous organic polymers (POPs) are excellent candidates owing to their fascinating attributes, like ample active sites, intrinsic porosity and accessible functionalities. These framework-based materials have been exploited as recyclable catalysts in efficient cyclization of CO₂ with aziridines, propargylic amines and alcohols coupled with amines/epoxides to produce oxazolidinones. This review provides a detailed analysis of recent advancements in developing porous framework-based recyclable catalysts for environmentally friendly conversion of CO₂ to oxazolidinones. Furthermore, future considerations and challenges for fabricating efficient framework-based catalysts in transforming CO₂ into value-added oxazolidinones are also discussed.

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Pooja Rani

Dr Pooja Rani earned her bachelor's degree from Kurukshetra University in Kurukshetra, India, in 2015, followed by a master's degree from Panjab University in Chandigarh, India, in 2017. She then completed her PhD at Panjab University under the joint supervision of Prof. K. K. Bhasin and Dr Girijesh Kumar, graduating in 2023. Currently, she is working as a SERB-National Postdoctoral Fellow at the Indian Institute of Technology Ropar. Her research interests are centered on the synthesis of porous functional materials aimed at environmental remediation and carbon capture and conversion.

Rajesh Das

Dr Rajesh Das obtained his B.Sc. degree in Chemistry (Honours) from Utkal University, Odisha, India, in 2016, and his M.Sc. in Inorganic Chemistry from Sambalpur University, Odisha, India, in 2018. He completed his doctoral research under the supervision of Dr C. M. Nagaraja at the Department of Chemistry, IIT Ropar, and earned his Ph.D. in 2023. His doctoral work earned him the CO2 INDIA Best PhD Thesis Award in 2023, conferred by India's carbon capture & utilization network (CO2 India). Currently, he is a postdoctoral researcher at the Department of Chemical & Biomolecular Engineering, National University of Singapore, working with Professor Anthony K. Cheetham and Professor Dan Zhao. His research interests focus on the design of functional metal–organic frameworks for the catalytic conversion of CO2 to value-added chemicals.

C. M. Nagaraja

Dr Nagaraja received his Ph.D. degree in 2007 from the Indian Institute of Science, Bangalore. Subsequently, worked as a postdoctoral researcher and research scientist at Brandeis University, USA, and Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bangalore. Later in 2012, he was appointed as an Assistant Professor at the Indian Institute of Technology (IIT) Ropar. He was promoted to Associate Professor (2018) and Professor (2024) at the Department of Chemistry. His current research interests include the design of framework (MOF/COF) materials for selective capture and utilization of carbon dioxide and hetero-structured nanomaterials for energy and environmental applications. For more details visit: https://www.iitrpr.ac.in/CMNagaraja.

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1. Introduction

The escalating energy demand driven by excessive population progression is primarily fulfilled through the widespread utilization of fossil fuels like natural gas, petroleum and coal combustion, leading to extensive release of greenhouse gases like CO₂, nitrogen oxide and fluorinated gases, etc. (Fig. 1).^1–3 As a result, CO₂ concentration in the atmosphere is increasing with time, from 280 ppm (early 1800s) to 421 ppm (2024) and it continues to rise.^4,5 This rapid escalation in atmospheric CO₂ has resulted in several detrimental environmental issues, such as ocean acidification, climate change, global warming and other related challenges.^6–8 Therefore, it is crucial to establish sustainable procedures to alleviate the increasing CO₂ concentration in the atmosphere. In this context, CO₂ capture and storage/sequestration (CCS) has been employed to mitigate CO₂ emissions from industries and power plants.^9,10 However, the current CCS technology necessitates significant energy for separation, compression, transportation and storage.^11–13

Fig. 1 (a) Global greenhouse gas emission from various processes. (b) Global greenhouse gas emission by economic sector.^1–3

An alternative value-added approach is carbon capture and utilization (CCU), where CO₂ serves as a C1 feedstock for producing value-added compounds,^14–16 such as cyclic^17,18 or polycarbonates,¹⁹ high-value formamides,²⁰ cyclic carbamates,²¹ oxazolidinones^22,23 and so on. Moreover, the generation of C1 (CO, CH₄, HCHO, HCOO⁻, CH₃OH) and C2 (C₂H₅OH) products can be accomplished by the reduction of CO₂ with the application of appropriate catalysts (Fig. 2).^24,25

Fig. 2 Transformation of carbon dioxide to various value-added chemicals and fuels.

Notably, oxazolidinones represent a significant class of crucial five-membered heterocyclic compounds that are vital in various active pharmaceutical agents. These compounds are noteworthy for their development as novel synthetic antibiotics, which have demonstrated remarkable efficacy in the treatment of multiple drug-resistant Gram-positive bacterial infections.²⁶ For instance, tedizolid phosphate is a highly effective medication against Gram-positive bacteria, known for its low incidence of drug resistance and minimal adverse reactions, making it particularly suitable for elderly individuals and children.²⁷ Similarly, linezolid effectively inhibits bacterial protein synthesis and is compatible with other medications, making it a widely utilized treatment for bacterial skin infections (Fig. 3).²⁸ Beyond their applications in medicine, oxazolidinones also hold promise in organic synthesis serving as versatile intermediates and chiral agents in asymmetric synthesis, which is essential for producing a wide range of pharmaceuticals and fine chemicals.

Fig. 3 Important value-added antibiotics and drugs comprising different 2-oxazolidinone derivatives.

This versatility contributes significantly to their economic and medical value on a global scale, as they facilitate the development of new compounds that can lead to more effective therapies and innovations in chemical manufacturing.²⁹ Moreover, the demand for oxazolidinone compounds extends beyond the pharmaceutical sector. They are increasingly sought after in various industries, including agriculture, where they are used in the formulation of pesticides, insecticides³⁰ and dye industry, further broadening their utility.³¹ Given the diverse applications of oxazolidinones and the growing need for sustainable practices in chemical synthesis, developing green and efficient synthesis methods for these compounds has become increasingly important. Therefore, the exploration of oxazolidinones is not only significant for advancing medical treatments but also for fostering economic growth and sustainability in various industrial applications.

Several methods have been developed for synthesizing oxazolidinones, with the CO₂ cycloaddition reaction standing out as particularly challenging yet promising in organic chemistry. Recently, notable advances have been made in this area. CO₂ can be used as a safer alternative to toxic reagents like phosgene to produce cyclic carbamates through coupling reactions with amines. Several efficient synthetic routes have been designed to incorporate CO₂ into cyclic carbamates for the production of oxazolidinones.³² Thus, it is indispensable to develop effective catalytic systems for sustainable synthesis of oxazolidinones by utilization of the greenhouse gas CO₂ under mild conditions. However, the inert nature of CO₂ presents a considerable obstacle to its functionalization under ambient conditions.³³ Consequently, researchers worldwide are dedicating significant efforts in developing high-performance catalytic systems for efficient CCU to produce valuable oxazolidinones.^34,35 To accomplish this goal, the catalyst employed must demonstrate strong CO₂ affinity, excellent catalytic activity and exceptional moisture stability. Moreover, to efficiently utilize CO₂ from direct air or flue gas of industries, the catalyst should possess high CO₂-philic sites for effective capture from low concentrations and catalytic active sites for promising conversion to oxazolidinones.³⁶ In this context, researchers worldwide are investigating the application of various catalysts, such as homogeneous metal complexes,^37,38 inorganic semiconductors,³⁹ ionic liquids⁴⁰ and carbonaceous materials,⁴¹ in CO₂ capture and conversion. However, the exact structure–activity relationship of these catalysts is not well understood. Among various heterogeneous catalytic systems explored for CCU, framework-based materials, particularly MOFs and COFs/POPs, present distinct advantages owing to their modular design featuring customized pore size and functionality.⁴² Furthermore, MOFs⁴³ and COFs⁴⁴ provide versatility in combining various features like introducing high density of CO₂-philic and catalytic sites, which further empowers them with immense potential in selectively capturing and converting CO₂ into oxazolidinones.

For the past two decades, there has been a notable increase in the utilization of MOFs within the realm of CCU.^45,46 Owing to the unique combination of inorganic and organic building blocks, MOFs/PCPs exhibit distinctive characteristics like tunable pore size, exceptionally high surface area and functionality, making them ideal candidates for a range of applications, including selective gas storage,⁴⁷ separation,⁴⁸ catalysis,^49–52 sensing,^53–55 drug delivery,⁵⁶ and so on. MOFs are assembled by connecting metal ions (nodes) or clusters (SBUs) with multidentate organic spacers, resulting in frameworks exhibiting various structural architectures. The network topology and functionality of MOFs/PCPs can be customized by strategically selecting organic spacers to generate infinite 1D, 2D, or 3D network structures.⁵⁷

On the other hand, COFs are a novel class of organic polymers characterized by permanent porosity, high crystallinity and structured architectures akin to MOFs. Moreover, COFs have customizable chemical and physical properties, making them ideal applicants for diverse applications, including gas storage,⁵⁸ separation,⁵⁹ photoelectricity⁶⁰ and catalysis, particularly in CO₂ capture and conversion.⁶¹ Highly effective COFs/POPs can be rationally designed by utilizing organic linkers containing basic sites like azine (C–N–N–C), azo (N=N), imine (C=N) and triazine moieties to proficiently capture CO₂ and transform it into valuable products under mild conditions.⁶² Thus, the strategic development of framework-based materials has seen a rapid increase in establishing an ideal platform for selectively capturing and utilizing CO₂ under mild conditions. This is apparent from the rise in the number of publications in this area over the past decade (Fig. 4).

Fig. 4 The number of publications on MOF and COF-based materials for CO₂ fixation consistent with the SciFinder search using the keyword: CO₂ fixation using MOFs and CO₂ fixation using COFs (statistics from SciFinder dated 31 October 2024).

The current research on CO₂ transformation to value-added products catalyzed by porous framework-based materials is highly advanced. Some excellent reviews focussed on the necessity of CCU and its applications are reported.^63–65 Based on the current knowledge, several high-quality reviews have been published in the scientific literature that highlights the advancements in the progress of MOFs and COFs supported catalysts for CO₂ conversion to cyclic carbonates^66–69 and fuels.⁷⁰ For instance, He and co-workers systematically discussed CO₂ transformations over nanomaterials.⁷¹ Singh and co-workers comprehensively reviewed advancements in porous materials for CO₂ capture and utilization.⁷² Cao and co-workers comprehensively discussed CO₂ transformation to useful products over porous MOFs and COFs via thermo-, electro- and photocatalytic processes.⁷³ However, no comprehensive review has addressed porous framework-based materials for CO₂ transformation to oxazolidinones. Thus, it is advantageous to review the recent advancements in CO₂ conversion using MOFs and COFs/POPs-based catalysts and examine approaches for enhancing the activity of these catalysts towards sustainable synthesis of oxazolidinones.

The present review includes a comprehensive exploration of the progress made in developing recyclable catalysts derived from MOFs and COFs/POPs for environmentally sustainable conversion of CO₂ into oxazolidinones. Various methods of synthesizing oxazolidinones from CO₂ are thoroughly discussed, displaying the advancements in MOF/COF structures on carbon dioxide conversion to oxazolidinones. Also, the underlying mechanism for CO₂ conversion to oxazolidinones has been described. Furthermore, we offer insights into the challenges that must be tackled and potential avenues for future research. We anticipate that the present review will promote a comprehensive understanding of CO₂ transformation reactions to oxazolidinone synthesis over framework-based materials, serving as a treasured resource for researchers working in this domain and will be pivotal for further investigations into effective CO₂ utilization.

2. Rational design of MOFs and COFs/POPs for CO₂ utilization

2.1. Design of MOFs for CO₂ utilization

Over the last two decades, MOFs have become a significant part of porous crystalline reticular frameworks owing to their multi-functional applications. MOFs are constructed by rationally assembling metal nodes and organic linkers (Scheme 1) with desired structural and functional properties. This reticular synthesis allows for the design of frameworks with tailored properties. MOFs feature open metal sites (OMS), Lewis basic sites (LBS), polar functional groups, tunable pore sizes and hydrophobic stability, making them ideal for CO₂ adsorption and conversion reactions. The following discussion includes a brief overview of the rational design and synthesis of CO₂-philic MOFs for efficient transformation of carbon dioxide to value-added oxazolidinones.

Scheme 1 Schematic representation of the linkers containing Lewis basic sites and polar functional groups utilized in the preparation of MOFs for catalytic utilization of CO₂ to prepare oxazolidinones.

2.1.1. MOFs with open metal sites (OMSs). A widely utilized approach to improve CO₂ capture and catalytic performance is by utilizing MOFs composed of unsaturated, active open metal sites or SBUs.⁷⁴ These open metal sites in MOFs are created by employing coordinating solvents, such as water (H₂O), acetonitrile (ACN) and N,N-dimethylformamide (DMF) during framework synthesis. The metal-coordinated solvents are subsequently removed through controlled activation of the frameworks by post-synthetic treatment. The resulting activated frameworks composed of open metal sites, actively support the interaction of various guest molecules with frameworks or serve as catalytic centers. Activated solvent-free MOFs enhance the porosity and performance for gas storage.⁷⁵ These OMSs function as Lewis acidic sites (LASs) that have been demonstrated to bind and coordinate CO₂ molecules. This property gives many MOFs an improved ability to adsorb carbon dioxide, particularly at low concentrations of the adsorbate gas. The concept of designing open metal sites (OMSs) and utilizing them for CO₂ capture was first reported in the late 1990s by Yaghi and co-workers.⁷⁶ Numerous MOFs with open metal sites, such as M-MOF-74, MIL-101 and NU-1000, have been utilized as potential adsorbents for effective CO₂ capture.^77,78 Also, Zhao and co-workers⁷⁹ prepared a Cu-MOF having Lewis acidic open Cu²⁺ sites which exhibited efficient conversion of aziridines to oxazolidinones by utilization of carbon dioxide. Detailed literature is covered in Section 3.1.1.

2.1.2. MOFs with tunable pore size. To improve carbon dioxide selectivity and uptake capacity, materials featuring microporosity are favored over meso- and macroporous ones due to CO₂'s small kinetic diameter of approximately 3.3 Å. In the case of MOFs, the pore sizes can be tailored to induce selective CO₂ capture property by judicious choice of organic linkers or struts. Besides, MOFs featuring self-interpenetration can exhibit highly selective adsorption of CO₂ due to the presence of narrow pore channels with sizes equivalent to the kinetic diameter of carbon dioxide.⁶⁵

2.1.3. MOFs with hydrophobic stability. It is crucial to acknowledge that all practical CO₂ adsorption takes place in the presence of moisture. Consequently, hydrophobic frameworks are well-suited for selective CO₂ capture in humid environments. While water's significant dipole moment leads to stronger interactions with most frameworks over CO₂, the hydrophobicity of certain MOFs enables them to minimize competitive water adsorption. Typically, the hydrophobicity of a MOF arises from bulky hydrophobic organic linkers or hydrophobic functional groups attached to these linkers. For instance, Yang et al. described the engineering of noble-metal-free MOFs decorated with perfluoroalkyl groups, which facilitate an efficient one-pot, four-component coupling involving aldehydes, amines, alkynes and flue gas.⁸⁰ The incorporation of perfluoroalkyl groups within the framework has been found to enhance the framework's affinity for hydrophobic substrates, boosting both activity and selectivity. Additionally, these modifications confer excellent water resistance when the MOFs are exposed to flue gas. More literature is covered in subsequent sections.

2.1.4. MOFs with Lewis basic sites (LBSs) and polar functional groups. Integrating organic linkers composed of Lewis basic sites (LBS) such as pyridyl and free –NH₂/–NH groups into frameworks has rendered MOFs with high CO₂ affinities.⁸¹ Besides, NH₂-functionalized linkers as well as N-comprising linkers such as pyrimidines, triazines and azoles can also be employed to construct MOFs with LBS, facilitating high CO₂ adsorption and conversion to high-value compounds.^82,83 For instance, Sun and co-workers reported a Cd-MOF decorated with free –NH₂ groups from a tripodal imidazole linker.⁸⁴ The –NH₂ groups decorated on framework surface boosted CO₂ interaction within the MOF. Thus, the MOF exhibited excellent efficiency for propargyl amines conversion to oxazolidinones using CO₂ (additional literature discussed in Section 3.2.1). Moreover, introducing polar groups like S, –OH and –SO₃ (Scheme 1) on the organic spacers has also been found to improve the CO₂ adsorption and conversion abilities of MOFs. Here, interactions between the dipole of the polar functional group and the quadrupole of CO₂ can result in high carbon dioxide adsorption and catalytic conversion.^85–87

From the aforementioned discussion, it is apparent that for effective capture and conversion of CO₂ into value-added chemicals under mild conditions, it is essential to rationally design frameworks by incorporating basic and active sites. In this context, Liu et al. rationally designed a linker (Scheme 1, H₂L⁸) having dual functional groups (carboxyl and triazole) wherein the –COOH group forms classical paddlewheel Cu^II₂ clusters and the triazole group supports stabilizing Cu^I clusters having high activity. The synergistic effects of Cu^I and Cu^II clusters boosted the CO₂ conversion efficiency. Further, the application of aminotriazoles as building blocks has resulted in frameworks with abundant LB sites exhibiting high adsorption capacity of CO₂ along with conversion efficiency.⁸⁸

Thus, the rational choice of the linkers facilitates the successful preparation of MOFs with optimal properties of CO₂-philicity and catalytic activity for utilization of carbon dioxide to form oxazolidinones. Various organic linkers employed in constructing MOFs with high-density Lewis basic/polar functional groups and catalytic sites to promote effective capture and transformation of CO₂ to oxazolidinones are shown in Scheme 1.

2.2. Design of COFs for CO₂ utilization

COFs are an emerging class of crystalline porous organic polymers constructed from organic linkers composed of light elements (B, C, N, O, S, and F) through covalent bonding to form extended 2D or 3D framework structures.^89–92 In COFs, the organic building units are generally bonded together by strong covalent bonds. The strong covalent connections within the building units are crucial in forming extended crystalline networks. The distinctive properties of COFs include large surface area, crystallinity, thermal/chemical stability and tailored properties making them ideal catalysts for various applications.^93,94 These applications encompass gas adsorption/separation, sensing, proton conduction, catalysis, energy storage, etc.^95–99 COFs are synthesized using various organic linkers to construct frameworks with diverse architectures. The pore dimensions and functionality can be finely modified by using appropriate linkers, rendering the formation of COFs with 1D, 2D, or 3D structures, as illustrated in Scheme 2a. The 1D COFs are linear or rod-like structures where the covalent connectivity extends primarily in one dimension (Scheme 2).¹⁰⁰ The 2D COFs are planar frameworks with covalent bonds extending in two distinct directions, generating sheet-like networks.¹⁰¹ These layers stack on top of one another through weak van der Waals interactions or π–π stacking. On the other hand, 3D COFs represent the most complex and fully interconnected networks, extending in three spatial dimensions.¹⁰² These 3D covalent linkers render rigid, highly porous structures featuring exceptional stability and large surface area.

Scheme 2 (a) Construction of 1D chain, 2D sheet and 3D framework of COFs connected through different linkers. (b) Schematic representation of amines and aldehydes utilized for the preparation of COFs suitable for catalytic conversion of CO₂ to oxazolidinones.

The synthesis of 1D, 2D or 3D COFs can be achieved by rationally choosing organic building blocks. For instance, a C₂ + C₂ amalgamation of organic building units along a direction results in 1D COF. Whereas a combination of C₃ + C₂ or C₃ + C₃ linkers results in hexagonal COFs and a C₄ + C₂ combination affords tetragonal COFs with an extended 2D structure.⁸⁹ It is important to highlight that the formation of 3D COFs requires a minimum of one tetrahedral (Td) organic unit.⁹² For example, combining a tetrahedral node with a C₂ or C₃ organic building block renders 3D COFs, as depicted in Scheme 2. Developing COFs for specific purposes presents significant challenges. However, altering COF structures to integrate specific active sites within the framework represents an intriguing approach to address this challenge. Therefore, COF backbone design and PSM processes are commonly employed to tailor the structure and functionality of COFs. Further, the catalytic efficiency for converting CO₂ into value-added oxazolidinones can be greatly improved by strategically integrating Lewis acidic metallic active sites into COFs using PSM. The alkynophilic metals like Cu and Ag can be precisely integrated into the framework at free bipyridine or Lewis basic sites to promote high efficacy in catalytic conversions of CO₂ to oxazolidinones.^103,104 Thus, rational design and synthesis of COFs having diverse active sites could provide great opportunities to realize CO₂ conversion to value-added oxazolidinones. Scheme 2 comprises the amines and aldehydes that can be utilized to prepare COFs, which can promote the effective utilization of CO₂ to form oxazolidinones with high yield and selectivity. The detailed literature related to the prepared COFs is covered in the later sections of the review article.

3. Versatile synthetic routes to oxazolidinones employing CO₂ as a C1 building block

Typically, the synthesis of oxazolidinones utilizing CO₂ has been accomplished by following three different strategies: (i) cycloaddition of CO₂ to aziridines, (ii) carboxylative cyclization of CO₂ to propargyl amines and (iii) three-component reaction of CO₂ with propargyl alcohols and primary amines or with primary amines and epoxides (Fig. 5). The following section describes these transformations and provides the in-sight mechanistic details for synthesizing oxazolidinones utilizing framework-based heterogeneous catalysts.^105–107

Fig. 5 Different strategies employed for the synthesis of oxazolidinones by utilizing CO₂.

3.1. Cycloaddition of CO₂ with aziridines

Aziridine, an N-containing analog of epoxide, is a crucial heterocycle present in numerous naturally occurring compounds and pharmaceuticals having biological activity.¹⁰⁸ It is also recognized as a versatile and widely used intermediate for developing many N-containing compounds. An exciting and promising practical use of aziridines is their involvement in preparing 2-oxazolidinone derivatives through coupling with CO₂.

The effective generation of 2-oxazolidinones from aziridines by their activation and further coupling with CO₂ necessitates Lewis/Brønsted acidic sites and a nucleophilic co-catalyst.¹⁰⁹ Literature studies have demonstrated that catalysts incorporating Lewis acidic sites like Zn²⁺ and Cu²⁺ can facilitate the activation of aziridine ring.¹¹⁰ Additionally, nucleophilic halide ions are essential to promote aziridine ring opening.¹¹¹

The general catalytic cycle involves the following steps: the initiation of the catalysis takes place through the interaction of substrate (aziridines) and CO₂ at the acidic and basic sites of the catalyst, respectively. Then, the substrate/aziridine undergoes activation via interaction at the unsaturated metal site. In the second step, aziridine ring-opening occurs by nucleophilic attack of halide (Br⁻), resulting in two distinct pathways labeled as path 1 and 2. Notably, path 1 leads to a more stable carbamate salt intermediate by opening the aziridine ring at the highly substituted carbon, leading to the major product formation. Then, CO₂ insertion at the ‘N’ site of the intermediate results in carbamate salt, which undergoes intramolecular ring closure to yield the corresponding oxazolidinone (Fig. 6). Consequently, an effective catalyst is necessary to promote efficient coupling of CO₂ with aziridine to yield desired oxazolidinone product with excellent selectivity and yield.¹¹²

Fig. 6 General mechanistic route for the MOF/TBAB-catalyzed carboxylative cycloaddition of CO₂ with aziridine.

3.1.1. Metal–organic frameworks-catalyzed cycloaddition of CO₂ with aziridines. MOFs constitute highly competent materials for CO₂ transformation reactions due to their plentiful active sites and modular nature with adaptable pore assembly, among other characteristics.¹¹³ The decorative characteristics of organic ligands and metal centers in the MOFs contribute to numerous Lewis acid/base sites, which influence their catalytic activity. Additionally, their distinctive porous structures create sufficient spaces for substrate enrichment, while the high chemical and thermal stability of MOF materials ensures a strong foundation for material recyclability. MOFs frequently surpass other adsorbents and catalysts, particularly in achieving highly proficient CO₂ storage and conversion processes.¹¹⁴ Their exceptional CO₂ trapping ability is attributed to unsaturated open metal sites and/or basic sites.¹¹⁵ The following section summarizes MOF-based materials utilized for converting aziridines to oxazolidinones utilizing CO₂.

The instability of certain MOFs in wet flue gas conditions is a persistent issue and necessitates pre-drying of flue gas. In this context, Bio-MOFs derived from amino acids hold promise as a potential solution for bridging the gap between CO₂ capture and transformation.¹¹⁶ Thus, amino acid linker-based MOFs (AA-MOFs) can alleviate the reaction parameters by combining Lewis acidic metals, basic sites and H-bonding groups. Further, Zn-glutamate units are recognized for their role as active sites in biological systems, exemplified by the enzyme carboxypeptidases, which facilitate the degradation of peptides. In this direction, Park and co-workers reported the first example of amino acid-based Zn-glutamate MOF, {[Zn(H₂O)(C₅H₇NO₄)]·H₂O}_n (ZnGlu) having Lewis acidic Zn²⁺ sites and basic –NH sites which are employed for synthesis of oxazolidinones from aziridine in presence of TBAB with 90% yield at RT under 1 MPa CO₂ (Scheme 1 and Fig. 7a).¹¹⁷ Notably, enhanced CO₂ cycloaddition was observed using water as a solvent, which supports the feasibility of CO₂ conversion from wet flue gas. Mechanistic investigation revealed that ZnGlu offered both Lewis acid and basic sites. Removal of water from the catalyst opens the pores, facilitating access and interaction with the substrates. Additionally, further removal of water can generate acidic open metal sites. When there is an adequate amount of water, opportunistic catalysis is expected to happen, akin to specific zeolitic imidazolate frameworks, where CO₂ inserts at the labile Zn–OH₂ bonds.¹¹⁸ Zhao and co-workers prepared a novel Cu-MOF {[Cu₂(BCP)(H₂O)₂]·3DMF}_n possessing nano-sized censer-like [Cu₃₀] cages (Fig. 7b). The [Cu₃₀] cage is sectioned into two sub-cages, the larger one is labeled A and the smaller one as cage B (Fig. 7c).⁷⁹ Three Cu₂(O₂CR)₄ paddlewheel units occupied cage A's top and bottom, each with a window size of 8.1 Å and 16 Å (Fig. 7c). Because of Lewis acidic open Cu²⁺ sites in Cu-MOF, it showed promising results in the reaction of CO₂ with 1-ethyl-2-phenylaziridine resulting in 3-ethyl-5-phenyloxazolidin-2-one (major product) with 99% yield and excellent regioselectivity (98%) at 100 °C under 2 MPa CO₂ with recyclability up to ten cycles (Table 1 and Fig. 7d).

Fig. 7 (a) 2D structure of ZnGlu showing 1D pore channels and its application towards CO₂ fixation.¹¹⁷ Copyright 2016, the Royal Society of Chemistry. (b) The 3D framework of Cu-MOF. (c) Nano-sized [Cu₃₀] cage. (d) Carboxylative cyclization of aziridines catalyzed by Cu-MOF.⁷⁹ Copyright 2016, WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Hexagonal channels of MMPF-10. (f) Pentagonal windows of MMPF-10. (g) Carboxylative cyclization of CO₂ to aziridines using MMPF-10 as a catalyst.¹²⁰ Copyright 2018, the Royal Society of Chemistry.

Table 1 Summary of MOF/COF/POP-based materials known for oxazolidinone preparation by utilization of CO₂

MOF/COF/POP	Co-catalyst	Catalyst loading (mol%)	Substrate	Product	Reaction conditions (Temp. (°C)/Pressure (MPa)/Time (h))			Yield (%)	Ref.
MOF-based catalysts for cycloaddition of CO 2 with aziridines
{[Zn(H2O)(C5H7NO4)]·H2O}n (ZnGlu)	TBAB	0.8			25	1	24	94	117
{[Cu2(BCP)(H2O)2]·3DMF}n Cu-MOF	TBAB	10			100	2	12	>99	79
[Cu4(CuTBCPPP)(H2O)4] MMPF-10	TBAB	0.625			100	2	10	>99	120
Zn-MOF	TBAB	2.8			70	2	12	>99	122
PCN-222(Co)	TBAB	1.0			40	0.1	18	82	123
{[M2(XN)2(IPA)2]·2H2O}n (M = Co, Mn, Ni)	TBAB	3.63			30	1	10	89	124
{Na[LnCo(DATP)2(Ac)(H2O)](NO3)·DMA·11H2O}n (Ln = Er and Yb)	TBAB	0.68			70	1	10	94	125
{[NH2(CH3)2][In(CPT)2]·3CH3CN·3DMA}n(In-MOF)	TBAB	1.7			30	1	10	>99	126
{[Ni(DCTP)]·6.5DMF}n (Ni-MOF)	NA	2.4			70	2	10	95	127
{[K1.2Na2.8Zn^I8(HL^1)12]·4H2O}n (Zn-MOF)	TBAB	5.3			70	2	12	99	129
{[H2N(CH3)2]3[Zn3(BTB)2(5-atz)3]· 3EtOH·3H2O·3DMF}n (Zn-MOF)	TBAB	3.2			70	1	10	94	130
{(NH2Me2)[Co3(μ3-OH)(BTB)2(H2O)]·9H2O·5DMF}n (Co-MOF)	TBAB	1.8			25	1	10	99	159
{[Cu2((L^2)^4−)(H2O)2]·3DMF·2H2O}n (Cu-MOF)	TBAB	0.95			60	0.5	12	98	131
[Ce2(DCTP)2(DMA)2(OAc)2]n (Ce-MOF)	TBAB	1.4			70	0.5	10	97	132
[Eu2Cu2I2(IN)6(DMF)4]·4DMF (Eu-MOF)	TBAB	1.0			60	1	12	92	133
COF-based catalysts for cycloaddition of CO 2 with aziridines
2,3-DhaTph and 2,3-DmaTph	TBAI	0.02 mmol			50	2	3	93	136
MOF-catalyzed cyclic carboxylation of propargyl amines with CO 2
[Cd3(L^3)2(BDC)3]2·16DMF (Cd-MOF)	NA	0.4			60	0.5	24	99	84
TNS-Ag8	DBU	1.0			25	0.1	24	99	148
TOS-Ag4								99
TMOF-3-Ag	DBU	0.1			25	0.1	6	>99	196
NiBDP-AgS	DBU	0.5			60	0.1	4	90	85
Ag-MOF-1	DBU	20 mg			25	0.1	24	55	153
Ag27-MOF	DBU	1.0			25	0.1	6	97	154
Ag-MOF	DBU	2.0			25	0.1	5	98	155
[Zn22(Trz)8(OH)12(H2O)9.8H2O]n (Zn-MOF)	TBD	0.27			70	0.1	12	99	156
{[(Cu^I6I5)Cu^II3(L^6)6(DMA)3](NO3)·9DMA} (Cu-MOF)	TEA	5.7			25	0.1	1	93	157
[Mg3Cu2I2(IN)4(HCOO)2(DEF)4]n(Mg-Cu-MOF)	TEA	10 mg			25	0.1	6	93	158
[Cu^II2Cu^I4I4L^8] (Cu-MOF)	NA	0.7			60	0.1	1	79.7	88
(Co-BTB)	TBD	1.8			70	2	5	32	159
(Co-XN)								76
Cu-TSP	DBU	2.0			50	0.1	24	99	160
WYU-11	TMG	1.0			60	0.1	24	99	161
(Cu-TCPP(Fe))	DBU	1.0			50	0.1	24	91	162
[Eu2Cu2I2(IN)6(DMF)4]·4DMF (Eu-MOF)	TBD	0.8			70	0.1	12	98	132
Th-MOF	TEA	10 mg			25	0.1	6	94	163
10.1–20.4–30.5-JNM	DBU	3.0			25	0.1	3	94	164
Cu2O@ZIF-8	DBU	5.0			40	0.1	6	99	167
Cu2O@MIL-101(Cr)-DABCO	DABCO	5.0			25	0.1	12	97.5	168
CuBr@NH2-MIL-101	DBU	5.0			25	0.1	8	97	169
Cu(I)-GSH/ZIF-8	DBU	5.0			60	0.1	6	98	170
COFs catalyzed cyclic carboxylation of propargyl amines with CO 2
Ag@TpPa-1	N-Iodosuccinimide	40 mg			25	0.1	16	78	178
Ag@TpTta								88
Cu-NPs@COF	DBU	30 mg			50	0.1	12	95	179
Pd(II)@TFR-OT	NA	15 mg			25	0.1	10	92	180
Ag@2,6-FPP-TAPT	DBU	0.052			50	0.1	2	99	181
Ag@Pybpy-COF	DBU	0.2			50	0.1	0.5	99	182
Cd-Bpy-COF	DBU	10 mg			60	0.1	12	99.9	183
Cu^ITpBD-COF	DBU	5 mg			80	0.1	6	95	184
POPs catalyzed cyclic carboxylation of propargyl amines with CO 2
Pd@BBA-2	NA	1.12			60	0.1	10	95	188
AgN@COF	DBU	20 mg			55	0.1	10	94	189
Ag@BT-COP	DBU	1.08			60	0.1	14	99.9	190
Ag@NPOPs-1	DBU	1 mg			50	0.1	2	97	191
Ag@NPOPs-1								93
Cu@NHC-1	NA	1.3			50	0.1	7.5	93	192
MOF catalyzed synthesis of oxazolidinones by one-pot three-component reaction
TMOF-3-Ag(I)	PPh3	NA			50	0.1	12	>99	196
MOF-SO3Ag	DBU	0.15			25	0.1	26	99	197
UiO-66–40	NA	70 mg			85	0.1	12	87	198
Ni-MOF	TBAI	60 mg			90	0.1	12	80	202
Cu(I)@NHC–MOF	DBU	0.5			25	0.1	12	99	209
Ag(I)@MOF-NHC	DBU	1.0			25	0.1	12	99	212
CMOF-801(ASP)	NA	50 mg			90	0.1	12	90	213
{[Ni2(tpxn)(oxdz)2(H2O)2]·13H2O}n Ni-MOF	TBAB	2.5			80	0.1	12	85.5	214
MOF catalyzed synthesis of oxazolidinones by one-pot four-component reaction
PCN-(BPY-CuI)-(TPDC-F7)	NA	5.0			75	0.1	24	84	80
Cu@UiO-66-NH2	NA	0.6 mmol			75	0.2	24		215
COF-catalyzed synthesis of oxazolidinones by three-component reaction
Zn@RIO-1	NA	15 mg			80	0.1	12	94	219
TpMA(MC)@Ag	DBU	15 mg			25	0.1	6	95	220
Ag@TFPNDA-COF	NA	15 mg			25	0.1	4	92	221

---

The Cu-MOF catalyzed the cycloaddition reaction with a wide range of aziridines containing various substituents (R₁) at the N-atom and on the phenyl ring (R₂) (Fig. 7d). Mechanistic investigations revealed that 1D channels and nanoscopic [Cu₃₀] cages of the Cu-MOF effectively trapped CO₂ and aziridines and the confined pore environment within nanoscopic [Cu₃₀] cages boosted reactivity between aziridines and CO₂. The Cu₂(O₂CR)₄ paddlewheel unit showed steric crowding near Cu²⁺ ions, effectively hindering the coordination of Br⁻ ions to the metal center. This arrangement promoted a more efficient nucleophilic attack of Br⁻ ions on aziridine, facilitating its activation for coupling with CO₂. The detailed catalytic mechanism is discussed in Section 3.1.

Metal–metalloporphyrin frameworks (MMPFs) represent a significant subset of MOFs that feature mono-, bi- or multi-metallic systems owing to the unique features of porphyrin scaffold supporting the anchoring of diverse metal ions at the pyrrole ring via post-synthetic modification.¹¹⁹ Porphyrin-based linkers are chemically adaptable, facilitating tunable structure and functionality by modification at the meso-positions. In this direction, Ma and co-workers reported a novel metal–metalloporphyrin framework [Cu₄(CuTBCPPP)(H₂O)₄] (MMPF-10) composed of elongated hexagonal channels (25.6 Å × 15.6 Å) running parallel to c-axis (Fig. 7e), generated from four Cu-paddlewheels with a ring of tetra metallated porphyrin ligands.¹²⁰ This arrangement resulted in pentagonal windows with a cavity diameter of 11 Å (Fig. 7f). Owing to the Lewis acidity of Cu(II), MMPF-10 displayed good catalytic presentation for the fixation of CO₂ to oxazolidinones at 100 °C under 2 MPa CO₂ (Table 1 and Fig. 7g). Notably, increasing the molecular size of the substrates did not affect the products yield significantly. An yield of 71% was achieved for isopropyl-substituted aziridine, which is understandable considering the increased steric hindrance caused by the isopropyl group linked to the N-atom. The MMPF designed here from a larger porphyrin linker holds larger pores, facilitating diffusion of substrates to the catalytic sites exposed in the pore channels of the MOF, thereby, resulting in enhanced catalytic activity.¹²¹ This work constitutes the first example of an MMPF-based catalyst that efficiently converts CO₂ to oxazolidinones. In another instance, the development of a multifunctional nanocage-based Zn-MOF containing 24-nuclear Zn-nanocages having pores (5.5 Å) with a pore volume of 0.65 cm³ g⁻¹ has been reported by Zhao's group (Fig. 8a).¹²² The MOF displayed promising CO₂ adsorption capacity (35.6 wt%) at 273 K with a BET surface area of 1151 m² g⁻¹. Owing to the potential adsorbing capacity of CO₂ and plenitude Lewis acidic Zn-active sites in the MOF, it was explored for fixation of CO₂ to form oxazolidinones with TBAB as a co-catalyst at 70 °C under 2.0 MPa CO₂ with up to ten catalytic cycles (Fig. 8b). Aziridines with various substituent groups were examined and the ethyl group substituted substrate exhibited the highest conversion (99%) and selectivity over propyl (91%) and butyl (93%) group containing substrates. This observation has been correlated to reduced steric hindrance of ethyl over propyl and butyl substituents. Besides, the effect of substituent groups (–Cl and –CH₃) on the phenyl ring was also studied to obtain the corresponding aziridines, 1-ethyl-2-(4-chlorophenyl)-aziridine (92%) and 1-ethyl-2-(4-methyl)-aziridine (99%). The lower yield of 1-ethyl-2-(4-chlorophenyl)-aziridine was attributed to the electron-withdrawing effect of the Cl-group (Fig. 8b). The cooperative catalytic effect of abundant Zn-Lewis acidic sites and TBAB, facilitated an effective cyclization reaction between aziridines and CO₂ at atmospheric conditions.

Fig. 8 (a) 3D structural view of Zn-MOF. (b) Cycloaddition of CO₂ with aziridines catalyzed by Zn-MOF.¹²² Copyright 2018, the Royal Society of Chemistry. (c) 3D structure of PCN-222(Co). (d) Synthesis of oxazolidinones using PCN-222(Co) as catalyst.¹²³ Copyright 2019, American Chemical Society.

It is worth noting that, in porphyrin-based frameworks, such as PCN family, the Lewis acidity arises from the presence of Zr₆ cluster nodes and metalloporphyrin within the polymeric matrix rendering them promising catalysts for CO₂ cycloaddition reactions. Expansive channels and pores significantly enhance interactions between the substrate and catalyst, resulting in increased catalytic efficiency. Thus, porphyrin-based materials are utilized to couple CO₂ with aziridines at ambient pressure. In this regard, Martín-Matute and co-workers reported a one-pot, microwave-assisted preparation of pristine PCN-222 and metallated PCN-222(M) (M = Co, Ni, Cu, or Zn) frameworks (Fig. 8c) and studied their catalytic activity.¹²³ Indeed, PCN-222(Co) demonstrated significant catalytic activity, yielding a diverse range of oxazolidinones under mild conditions (Fig. 8d). The yield of the products decreased with an increase in the chain length of the alkyl substituent on the N-atom due to steric hindrance (Fig. 8d). Moreover, the material maintained its catalytic activity and crystallinity across four recycling runs. Kang et al. reported three new isomorphous MOFs {[M₂(XN)₂(IPA)₂]·2H₂O}_n (M = Co, Mn and Ni) possessing 1D pores of 3.74 × 9.90 Å along b-axis (Scheme 1).¹²⁴ Taking into account the presence of unsaturated Lewis-acid active sites on the surface of the MOFs, they were employed as promising catalysts for converting CO₂ into valuable oxazolidinones at 30 °C under 1 MPa CO₂ with TBAB as co-catalyst (Table 1 and Fig. 9a). The MOF exhibited good recyclability for five cycles. Aziridine with an ethyl substitution at the N-atom rendered a higher yield of products with superior selectivity compared to those with propyl and butyl substitutions, owing to the reduced steric hindrance of the ethyl group. Similarly, the yield of 3-ethyl-5-(4-chlorophenyl)oxazolidin-2-one was found to be lower than that of 3-ethyl-5-p-tolyloxazolidin-2-one, which has been attributed to electron-withdrawing effect of –Cl group (Fig. 9a). Mechanistic investigation revealed that stable Co₂(COO)₄ unit's electric neutrality and steric hindrance significantly impede the coordination interaction between Br⁻ and Co²⁺, thereby increasing the chances for Br⁻ to nucleophilically attack aziridines. Thus, the combined effect of Br⁻ from TBAB and the unsaturated Lewis-acid Co²⁺ active sites in Co-MOF was crucial for the CO₂ coupling reaction.

Fig. 9 (a) Fixation of CO₂ to aziridines catalyzed by Co-MOF.¹²⁴ (b) Synthesis of oxazolidinones catalyzed by Er-MOF.¹²⁵ (c) Rhombic pores (16 Å × 18 Å) in In-MOF. (d) 1D circular channels in In-MOF along the b-axis. (e) Preparation of oxazolidinones catalyzed by In-MOF.¹²⁶ Copyright 2021, American Chemical Society.

Zhao's group reported two isostructural heterometallic MOFs, {Na[LnCo(DATP)₂(Ac)(H₂O)](NO₃)·DMA·11H₂O}_n (Ln = Er and Yb). Er-MOF displayed diverse building blocks [Er(COO)₄(Ac)(H₂O)] and [Co(DATP)₂], which are interconnected through H₂DATP ligands to form a 2D layer architecture.¹²⁵ Given the unsaturated Ln³⁺ Lewis acid sites in Er-MOF and Yb-MOF, they served as promising catalysts for the CO₂ fixation reaction to form oxazolidinones at 70 °C under 1 MPa CO₂ with up to ten catalytic cycles (Table 1 and Fig. 9b). The seven-coordinated Ln³⁺ ion, which is unsaturated and has more Lewis acid sites, effectively activated the substrates and CO₂, leading to faster catalytic reactions. It is worth noting that the application of main-group metal-based MOFs for the cycloaddition of CO₂ to aziridines was rarely studied. In this context, in 2021, Zhao and co-workers reported a quadruplex-interpenetrated porous In-MOF, {[NH₂(CH₃)₂][In(CPT)₂]·3CH₃CN·3DMA}_n exhibiting two types of pores, viz a rhombic (16 Å × 18 Å) and circular (10 Å) along a- and b-axis, respectively (Fig. 9c and d).¹²⁶ Notably, despite the quadruplex-interpenetrated structure, the In-MOF possesses a total solvent-accessible volume of 70.2%. Due to its high porosity and Lewis acidic In³⁺ sites, In-MOF was employed as a heterogeneous catalyst for CO₂ coupling with aziridines using TBAB at 30 °C under 1 MPa CO₂ pressure (Table 1 and Fig. 9e). Further, In-MOF displayed a high level of catalytic versatility across a range of aziridines with recyclability up to five cycles. Under mild conditions, the compound 3-ethyl-5-phenyloxazolidin-2-one was obtained with a high yield of 99%. Mechanistic studies revealed that the porous structure of In-MOF efficiently enriches and captures CO₂ molecules, with an isosteric heat of adsorption (Q_st) of approximately 36 kJ mol⁻¹, indicating a notable affinity for CO₂.

It is worth mentioning that, most of the literature discussed in the aforementioned sections involves the requirement of co-catalyst, TBAB for the opening of the aziridine ring. To overcome this issue, Shi et al. reported a 3D Ni-MOF, {[Ni(DCTP)]·6.5DMF}_n exhibiting two distinct 1D channels along the a-axis, with openings measuring 11.2 Å × 10.4 Å (Channel A) and 7.8 Å × 5.5 Å (Channel B), respectively (Fig. 10a and b).¹²⁷ The spacious pores offered a promising opportunity for CO₂ adsorption and catalytic processes. As a result, Ni-MOF efficiently catalyzed the transformation of CO₂ to oxazolidinones without a co-catalyst at 70 °C and 2 MPa CO₂ in 10 h, with recyclability up to five cycles (Table 1 and Fig. 10c). Furthermore, Ni-MOF also showed substantial catalytic activity for the large-scale demonstration. The good catalytic performance demonstrated by Ni-MOF could be attributed to the extensive porous structure formed by DCTP and Ni²⁺ effectively capturing and enriching CO₂ molecules on the surface. The combined catalytic effect of Ni²⁺ ions, acting as Lewis acidic centers and pyridyl-N sites from the DCPT ligand significantly facilitated the cycloaddition reaction between CO₂ and aziridines.

Fig. 10 (a and b) The 3D channels A and B in Ni-MOF. (c) Cycloaddition of CO₂ with aziridines catalyzed by Ni-MOF.¹²⁷ Copyright 2021, Springer Nature. (d) 3D view of Zn-MOF. (e) Preparation of oxazolidinones using CO₂ and aziridines catalyzed by Zn-MOF.¹²⁹ Copyright 2021, the Royal Society of Chemistry.

Notably, polynuclear metal cluster-based MOFs can greatly enhance catalytic efficiency owing to the availability of high-density catalytic sites.¹²⁸ Most of the cluster-based MOFs exhibit common oxidation states. However, those containing uncommon low oxidation sites are rare and offer significant opportunities in catalysis. In this regard, Zhao and co-workers reported a unique cluster-based MOF with unusual multi-centered Zn^I–Zn^I bonds, {[K_1.2Na_2.8Zn^I₈(HL¹)₁₂]·4H₂O}_n (Zn-MOF) (Fig. 10d).¹²⁹ A distinctive Zn₈ cubic cluster is formed by assembling eight zinc ions, where the distance between adjacent zinc ions (Zn^I–Zn^I) measures 2.357 Å. Given the numerous Lewis acidic active sites in Zn-MOF, it was exploited for cycloaddition of CO₂ with aziridines to form oxazolidinones at 70 °C under 2 MPa of CO₂ pressure with recyclability for up to five cycles with broad substrate universality (Table 1 and Fig. 10e). Indeed, this is the first multi-center metal–metal bonded cluster-based MOF utilized as a catalyst for carbon dioxide fixation. In another example of Zn-MOF, Kang and Liu group reported a Zn-MOF, {[H₂N(CH₃)₂]₃[Zn₃(BTB)₂(5-atz)₃]·3EtOH·3H₂O·3DMF}_n featuring a continuous 1D chain along the a-axis, expanding into a 3D pillar-chain framework by inclusion of BTB³⁻ ligands (Scheme 1 and Fig. 11a).¹³⁰ An isosceles triangular channel with an approximate diameter of 13.4 Å can be observed in the Zn-MOF (Fig. 11a). Due to the potent Lewis acidity of unsaturated Zn sites and the Lewis basicity of uncoordinated –NH₂, Zn-MOF demonstrated effective catalysis in transforming CO₂ into oxazolidinones at 70 °C under 1 MPa CO₂ pressure with reusability for at least three cycles (Table 1 and Fig. 11b). The impact of steric hindrance from ethyl, propyl, butyl, or benzyl groups on N-atoms was investigated, revealing a decrease in yield and selectivity with an increase in the size of substituent groups. In contrast, when the R₂ group was substituted by –Cl, –Br, or –CH₃ groups, the yield of the corresponding oxazolidinones followed the trend –Cl(53%), <–Br(79%) < –CH₃ (82%) suggesting that electron-donating groups promote efficient coupling of aziridines with CO₂ (Fig. 11b). The comprehensive mechanistic investigation demonstrated that the unsaturated four-coordinated Zn²⁺, characterized by its strong Lewis acidity, effectively activates the N-atom of aziridine intermediate, thereby accelerating the ring-opening process.

Fig. 11 (a) 3D view of Zn-MOF. (b) Fixation of CO₂ to oxazolidinone catalyzed by Zn-MOF.¹³⁰ Copyright 2022, the Royal Society of Chemistry. (c) The 3D framework of Cu-MOF.¹³¹ Copyright 2023, American Chemical Society. (d) 2D Eu-MOF.¹³³ Copyright 2024, Elsevier.

The application of a Cu-organic framework, {[Cu₂(L²)⁴⁻(H₂O)₂]·3DMF·2H₂O}_n (Cu-MOF) having Cu₂(CO₂)₄ units linked together through the (L²)⁴⁻ ligand possessing nanocage structure with 1.1 nm wide inner cavity has been reported (Fig. 11c).¹³¹ Additionally, the nanocages are interconnected to create a 3D framework characterized by two types of 1D channels (0.70 nm and 0.32 nm) along the c-axis (Fig. 11c). The Cu-MOF exhibited significant selectivity for CO₂ due to its strong interactions with exposed Cu(II) sites and ligands within the framework. The MOF efficiently catalyzed the conversion of CO₂ to oxazolidinones at 60 °C under 0.5 MPa CO₂ pressure with good substrate scope (Table 1). DFT calculations revealed that the adsorbed CO₂ molecules are predominantly situated near the unsaturated dinuclear Cu(II) site, F-site and the adjacent H-site within the benzene ring of the ligand L². This work highlights the importance of nanocage-based MOFs for CO₂ fixation and conversion reactions. In another instance, Liu et al. reported Ce-based lanthanide MOF, [Ce₂(DCTP)₂(DMA)₂(OAc)₂]_n, which was further employed as a heterogeneous catalyst for facilitating the cycloaddition reaction between aziridines and CO₂, resulting in value-added oxazolidinones in 10 h, at 70 °C under 0.5 MPa CO₂ (Table 1).¹³² The Ce-MOF demonstrated effective catalytic activity with various substituent groups under optimal conditions. Mechanistic analysis disclosed that the combination of abundant Ce centers and uncoordinated pyridine ligands within Ce-MOF works synergistically to catalyze the reaction effectively.

Recently, 2D MOFs are gaining significant interest owing to their layered structure with highly exposed catalytic sites. In this regard, Cao and co-workers reported a novel 2D-MOF, [Eu₂Cu₂I₂(IN)₆(DMF)₄]·4DMF (Eu-MOF) composed of Eu³⁺ ions and Cu₂I₂ clusters, exhibiting sql topology (Fig. 11d).¹³³ Through catalytic investigation, it was discovered that Eu-MOF (1 mol%) effectively catalyzed cyclization of CO₂ with aziridines under 1 MPa CO₂ pressure at 60 °C in 12 h (Table 1). The previous discussion displayed that MOF-based catalysts possessing Lewis acidic metal sites and basic (–NH/–NH₂) sites show promising activity for efficiently transforming aziridines into oxazolidinones by utilizing CO₂ under environmentally friendly conditions. It is worth highlighting that, presence of open metal sites (OMSs) within MOFs significantly enhances the activation of CO₂. These OMSs function as Lewis acidic sites, effectively promoting the functionalization of CO₂ with aziridines. In addition to OMSs, the pore size of the MOF plays a critical role in CO₂ activation. Given the small size of CO₂ molecule, microporosity is particularly advantageous. The finer pores allow for better interaction and accommodation of CO₂, leading to more efficient capture and conversion processes.

3.1.2. Covalent organic frameworks catalyzed cycloaddition of CO₂ with aziridines. Literature studies have shown that porphyrin and its metal derivatives, possessing highly conjugated π-electron macrocycles with distinctive photophysical and redox characteristics, have been extensively utilized as crucial molecular building blocks in materials chemistry.¹³⁴ Porphyrin-based COFs with high porosity and chemical stability exhibit remarkable dynamic catalytic sites.¹³⁵ These attributes render them highly effective catalysts for various organic conversions, including conversion of CO₂ to value-added oxazolidinones. Keeping this in mind, in 2016, Banerjee and co-workers reported a remarkably porous and crystalline catechol porphyrin COF containing units of 2,3-DhaTph and 2,3-DmaTph (Scheme 2 and Fig. 12a).¹³⁶ They demonstrated that 2,3-DhaTph unit with a hydrogen bond donor (HBD) group acted as an exceptionally active reusable catalyst for chemically transforming CO₂ into oxazolidinones under atmospheric pressure. High yields and turnover numbers were notably attained during the oxazolidinone synthesis with 2,3-DhaTph, even at milder temperatures and moderate CO₂ pressures (Table 1, Fig. 12b and c). Moreover, 2,3-DhaTph COF exhibited outstanding regioselectivity in synthesizing 5-aryl-2-oxazolidinones under ambient reaction conditions. This work is the first demonstration of COF-catalyzed CO₂ fixation with aziridines for the efficient synthesis of oxazolidinones. Thus, incorporating Lewis acidic and basic sites via PSM is crucial for efficiently synthesizing oxazolidinones.

Fig. 12 (a) Structures of 2,3-DhaTph and 2,3-DmaTph COF. (b) Synthesis of oxazolidinones. (c) Recyclability study of COFs.¹³⁶ Copyright 2016, the Royal Society of Chemistry.

3.2. Carboxylative cyclization of CO₂ with propargyl amines

The utilization of aziridines for the cycloaddition reaction with CO₂ proves beneficial for generating oxazolidinones. However, the synthesis of these aziridine substrates necessitates the use of toxic bromine. Furthermore, aziridines are inherently unstable in atmospheric conditions and susceptible to oxidation or hydrolysis upon exposure to humidity or oxygen.¹³⁷ Another promising approach for synthesizing oxazolidinones is through carboxylative cyclization of propargyl amines with CO₂. N-Propargyl amines represent a highly versatile and distinctive category of heteroatom-containing alkynes with a wide range of reaction patterns.¹³⁸ They are commonly utilized as fundamental building units for the formation of various N-heterocycles, including pyrroles, pyrazines, pyridines, imidazoles, quinolines, lactams and other compounds.¹³⁹ Recently, considerable efforts have been engaged toward transforming these commodity chemicals into value-added oxazolidinones through carboxylative cyclization with CO₂. For an efficient conversion process of propargyl amines to oxazolidinones using CO₂, the catalyst should possess Lewis acidic sites for activating the alkyne group. Additionally, Lewis basic sites are essential for the deprotonation of the amines, thereby enhancing the overall conversion efficiency.

The general catalytic mechanism involves the activation of the alkyne group of propargylic amine by the metal center. This activation is followed by deprotonation of propargylic amine using DBU as a base. Subsequently, the insertion of CO₂ takes place to form a carbamate. The following step involves an intramolecular cyclization process, wherein the carbamate intermediate undergoes rearrangement to form 2-oxazolidinones. This cyclization step is crucial for the synthesis of the desired product. Finally, the formed 2-oxazolidinones are separated from the metal center, regenerating the catalyst and enabling its participation in subsequent catalytic cycles (Fig. 13).¹⁴⁰

Fig. 13 General mechanistic route for the MOF-catalyzed carboxylative cyclization of CO₂ to propargyl amines.

3.2.1. Metal–organic frameworks-catalyzed cyclic carboxylation of propargyl amines with CO₂. MOFs, serving as efficient catalysts, can enable the environmentally friendly conversion of CO₂. However, most research efforts have been directed toward converting epoxides into cyclic carbonates, with the equally vital conversion to oxazolidinones being largely overlooked.¹⁴¹ MOFs are advantageous due to their tunable structure, which can be chemically tailored to feature a homogeneous distribution of various functional sites, demonstrating simultaneous catalytic conversion performance.¹⁴² One efficient method for activating propargyl amines is through π-bond activation and the subsequent formation of oxazolidinones with CO₂ which requires catalytic active sites within the framework. The following section covers the literature on MOF-based catalysts known for preparation of oxazolidinones using propargyl amines and CO₂.

Functionalized flexible MOFs are ideal candidates for advancing artificial switchable catalysts owing to their natural cavities, dynamic characteristics and customizable functional groups within the framework channels.^143,144 The dynamic nature of flexible MOFs enables tailoring of the pore channels through external stimuli for desired applications. This ability to fine-tune the porous structure in situ can significantly influence the catalytic performance of heterogeneous catalysts based on flexible MOFs. However, the potential of flexible MOFs for switchable catalysis remains largely untapped.^145,146 In contrast to other catalysts, the dynamic responses of functionalized flexible MOFs to guest molecules can trigger conformational shifts that contribute to substrate selectivity, similar to complex biological systems.¹⁴⁷ Keeping this in mind, in 2017, Sun and co-workers reported a dynamic, functional Cd-MOF, [Cd₃(L³)₂(BDC)₃]₂·16DMF decorated with –NH₂ groups from tripodal imidazole linker. Cd-MOF displayed a dynamic five-fold interpenetrating structure (Fig. 14a).⁸⁴ The dynamic structure of Cd-MOF was switched on and off through reversible structural transformations. The –NH₂ groups decorated on the channel surfaces boosted CO₂ interaction with the MOF. Thus, the MOF was exploited for carboxylative cyclization of carbon dioxide with propargyl amines at 60 °C under 0.5 MPa CO₂ pressure with good substrate scope and recyclability (Table 1 and Fig. 14b). The promising activity demonstrated by MOF to form oxazolidinones using CO₂ and terminal propargyl amines indicated that small-sized substrates entered the channels and displayed catalytic reactions smoothly. The catalytic mechanism was validated by collecting FT-IR spectra of Cd-MOF after it was immersed in N-methylpropargylamine. Indeed, the FT-IR studies validated the interaction of the amine substrate with the MOF (Fig. 14c). Thus, the free –NH₂ groups in Cd-MOF play a crucial role in the catalytic reaction. These experimental findings demonstrated that Cd-MOF functioned as a flexible and switchable catalytic system with selective properties for substrates, akin to advanced biological systems.

Fig. 14 (a) 3D view of Cd-MOF. (b) Cyclization reaction of CO₂ with propargyl amines catalyzed by Cd-MOF. (c) FTIR spectra of Cd-MOF, MOF after immersed in N-methylpropargylamine and N-methylpropargylamine.⁸⁴ Copyright 2017, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim.

Considering the alkynophilic character of Ag–metal, Duan and co-workers utilized thiourea functionalized MOFs and then embedded Ag nanoclusters to obtain TNS-Ag₈ and TOS-Ag₄ which possess 1D channels of dimension (9.0 × 9.0 Ų) and (10.0 × 10.0 Ų), respectively (Fig. 15a).¹⁴⁸ The surface of the Ag clusters is decorated with H-bond donor (–NH₂ and –NH–) groups and H-bond acceptor (C=N– and S) groups, offering potential H-bonding interactions with substrates. These interactions facilitate synergistic activation and fixation during catalytic transformations.¹⁴⁹ Further, TNS-Ag₈ and TOS-Ag₄ showed high-density catalytic sites with uniformly distributed pores, facilitating them as efficient π-activators for coupling of propargylamine with CO₂ to yield oxazolidinones (99%) at RT under 0.1 MPa CO₂ (Table 1 and Fig. 15b). The combined effect of π-activation and H-bonding interactions promoted trapping of substrates around the catalytic sites, facilitating chemical transformations under mild reaction conditions (Fig. 15c).

Fig. 15 (a) Structure of TOS-Ag₄. (b) Cycloaddition of propargylamine derivatives with CO₂ catalyzed by TOS-Ag₄. (c) Propargylamine-impregnated crystals of 1a@TOS-Ag₄.¹⁴⁸ Copyright 2018, American Chemical Society. (d) Scheme for NiBDP-AgS synthesis.⁸⁵ Copyright 2018, the Royal Society of Chemistry.

From a mechanistic perspective, the substrate molecules are effectively confined within the pores of the framework through H-bonding interactions with the –NH groups. This confinement facilitates the alignment of the trapped molecules in optimal orientations, enabling the Ag atoms to engage with the –C≡C– bond via π coordination, thereby enhancing substrate activation. Simultaneously, CO₂ molecules interact with the –NH groups, promoting a conducive environment for the activated substrates to undergo a nucleophilic attack. This interaction results in the conversion of substrates into products. The synergistic interplay between π-activation and H-bonding interactions significantly accelerates the capture of substrates around the catalytic sites. This dual mechanism not only enhances substrate availability but also catalyzes the chemical transformation under mild reaction conditions, making it an efficient process. The ability to be recycled and achieve high turnover numbers showcases the extensive potential applicability of these engineered materials as catalysts for π activation, making them suitable for real-world applications within the chemical industry. In this regard, pyrazolate-based MOFs have shown significantly improved alkaline stability compared to their carboxylate-based frameworks¹⁵⁰ which are attributed to the high pK_a of N–H bond in pyrazole, resulting in robust M–N coordination.¹⁵¹ Therefore, pyrazolate-based MOFs represent one of the earliest examples of frameworks exhibiting high stability in a concentrated alkaline solution, such as NaOH.¹⁵² In light of this, Fei and co-workers reported an alkali-resistant Ag(I)-anchored MOF (NiBDP-AgS), which was prepared via PSM of a pyrazolate-MOF via thiol functionalization (Fig. 15d).⁸⁵ Owing to its high robustness and strong interaction ability with CO₂ and alkyne-comprised molecules, NiBDP-AgS displayed promising catalytic activity for cyclic carboxylation of propargylic amines to oxazolidinones (Table 1). Further, Duan's group recently reported a new Ag–S-based porous MOF (Ag-MOF-1), having unique structural features that enable efficient cycloaddition of CO₂ with propargyl amines (Fig. 16a and b).¹⁵³ The novel Ag-MOF-1 catalyst, with its distinctive double-helical Ag–S rods and thiosemicarbazide ligands, demonstrated promising activity for oxazolidinones synthesis.

Fig. 16 (a) Structure of Ag-MOF-1. (b) CO₂ conversion to oxazolidinones catalyzed by Ag-MOF-1.¹⁵³ Copyright 2019, American Chemical Society. (c) 2D structure of Ag27-MOF with topology. (d) Preparation of oxazolidinones catalyzed by Ag27-MOF.¹⁵⁴ Copyright 2020, Wiley–VCH GmbH.

The strong Ag–S bonds and inhibition of non-porous Ag NPs formation contribute to the catalyst's efficiency and stability during catalysis. As the size of the aryl substituents increased, the conversion decreased due to factors such as steric hindrance and reduced accessibility of catalytic sites. In a similar approach, Sun and co-workers reported a robust 2D MOF (Ag27-MOF) using pyridyl functionalized porphyrin (TPyP-H₂) ligand, which functioned as a promising catalyst for oxazolidinones synthesis using propargyl amines and CO₂ under atmospheric pressure (Fig. 16c and d).¹⁵⁴ The metallic node in Ag27-MOF is saddle-shaped, offering an accessible platform where densely packed Ag atoms act as π-Lewis acid sites to activate the –C≡C– bond of propargyl amines. Consequently, various sterically restricted alkyne substrates were efficiently activated via π-interactions with cationic Ag centers. This study demonstrated that incorporating high-density Ag centers into a 2D framework as π-Lewis acid catalytic sites not only improves the stability of the 2D MOF but also enhances the activation of organic alkyne substrates, particularly bulky ones, thereby greatly boosting catalytic performance. Thus, 2D MOFs composed of catalytic metal clusters could be convincing catalysts for CO₂ transformation reactions. In another example, Sun and co-workers reported an Ag-MOF and applied it towards the fixation of CO₂ to propargyl amines, resulting in oxazolidinones.¹⁵⁵ The Ag-MOF demonstrated significant flexibility in terms of the range of substrates, its ability to selectively target different sizes, and its ease of recycling as a heterogeneous catalyst at RT and atmospheric pressure conditions.

As discussed in the previous sections, normally noble metal-based catalysts are employed for the coupling of the alkynes with CO₂. However, Zhao and co-workers reported a [Zn₁₁₆] nanocage-based lantern-like 3D architecture, [Zn₂₂(Trz)₈(OH)₁₂(H₂O)_9.8H₂O]_n (Zn-MOF) (Scheme 1), exhibiting large nanocage, assembled by Zn-clusters and Trz (C₄N₁₂O)⁴⁻ linkers as building blocks.¹⁵⁶ The MOF is comprised of two types of clusters: six [Zn₁₄O₂₁] clusters and eight [Zn₄O₄] clusters (Fig. 17a). The distinctive cage features a spacious interior cavity (0.81 × 1.03 nm) with an external edge of about 2.37 × 3.65 nm. This Zn-MOF demonstrated promising catalytic activity for the synthesis of 2-oxazolidinones utilizing CO₂ and propargyl amines under ambient pressure at 70 °C within 12 h with recyclability up to ten cycles (Table 1 and Fig. 17b). Remarkably, the scope of catalysis was extended to various propargylic amine derivatives to obtain high yields of the desired products by transforming terminal propargylic amines with n-butyl or cyclohexyl substituents at the N-position (Fig. 17b). This MOF catalyst, devoid of noble metals, marked a significant step towards the environmentally friendly transformation of CO₂ into oxazolidinones. In another demonstration of noble-metal free alkyne activation, Zhao's group reported a novel Cu^I/Cu^II mixed valence MOF {[(Cu^I₆I₅)Cu^II₃(L⁶)₆(DMA)₃](NO₃)·9DMA} having stp-type topology (Fig. 17c).¹⁵⁷ Thanks to its exceptional chemical and thermal stability, Cu^I/Cu^II active centers and easily accessible free –NH₂ groups exposed in the 1D channels of Cu-MOF rendered effective catalytic performance for cyclization of propargylamine with CO₂ at 30 °C under 1 atm CO₂ pressure within 1 hour, without the need of any co-catalyst or solvent (Table 1 and Fig. 17d). Different propargyl amines with various substituent groups were utilized, resulting in high yields of the corresponding 2-oxazolidinone products, ranging from 86% to 99% yield (Fig. 17d). Further, the abundance of free –NH₂ groups within the spacious channels of Cu-MOF contributed to the alkalinity of the catalytic system, thus, facilitating the ring closure process to form oxazolidinones. The synergistic catalytic effect was enhanced by the ternary active sites comprising [Cu^I₆I₅] nodes, Cu^II paddle wheel nodes, and uncoordinated –NH₂ groups within the MOFs, improving overall catalytic efficiency. Importantly, Cu-MOF effectively catalyzed the synthesis of oxazolidinones from propargylamine reaction with simulated flue gas. This work offered a promising route for directly using waste gases in industrial processes.

Fig. 17 (a) 3D framework of Zn-MOF. (b) The cyclization reaction of CO₂ with propargylic amines catalyzed by Zn-MOF.¹⁵⁶ Copyright 2020, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim. (c) 3D view of Cu-MOF. (d) Cycloaddition of CO₂ with propargylic amines catalyzed by Cu-MOF.¹⁵⁷ Copyright 2021, Wiley–VCH GmbH. (e) 3D view of Mg-Cu-MOF. (f) Preparation of oxazolidinones using Mg-Cu-MOF.¹⁵⁸ Copyright 2021, American Chemical Society.

In a similar demonstration, Wu and co-workers reported noble-metal free 3D framework, [Mg₃Cu₂I₂(IN)₄(HCOO)₂(DEF)₄]_n (Mg-Cu-MOF) (Fig. 17e) exhibiting outstanding stability and well-dispersed active metal sites contributing to its high catalytic activity.¹⁵⁸ Under mild conditions, Mg-Cu-MOF effectively catalyzed the transformation of CO₂ to produce 2-oxazolidinones from propargyl amines using TEA as a base (Table 1 and Fig. 17f), demonstrating recyclability for up to five cycles. The desired products were obtained in high yields using various N-aryl- or N-alkyl-substituted terminal propargylic amines. However, N-phenylprop-2-yn-1-amine exhibited low reactivity, with only trace amounts of the product. This unfavorable outcome may be attributed to weak N-nucleophilicity, which likely impedes the nucleophilic attack on CO₂, preventing the formation of the carbamate intermediate. Li and co-workers in 2022, reported a Cu-MOF with mixed valency, [Cu^II₂Cu^I₄I₄L⁸], consisting of two distinct, paddle–wheel Cu(II) clusters and Cu₄I₄ nodes which acted as a pillar-linker to form a 3D framework having 60.9% solvent-accessible volume.⁸⁸ The pore configuration of Cu-MOF displayed a 3D architecture and retained a larger 1D pore structure (12.1 Å). The walls of the 1D pore channels are adorned with a significant quantity of –NH₂ groups. The substantial size of the pores, along with the abundance of Lewis acidic and basic sites, rendered efficient catalytic activity for converting propargylic amines into oxazolidinones by utilizing CO₂ under ambient pressure without any co-catalyst at 60 °C within 1 h with excellent substrate scope. The mechanism involves the synergistic effect of the –NH₂ groups exposed in the pores and cuprous ions trigger the dehydrogenation of alkynyl amine, leading to a transition state and formation of a protonated amino group. At the same time, CO₂ is activated at the active sites of both divalent and monovalent Cu, resulting in the formation of a carbamate anion which is further converted into an oxazolidinone product.

In an important demonstration of tailoring the Lewis acid–base sites to achieve turn on/off catalysis, Tian et al. reported a Co-MOF, {(NH₂Me₂)[Co₃(μ₃-OH)(BTB)₂(H₂O)]·9H₂O·5DMF}_n (Co-BTB) featuring a rhombic window (13.7 Å × 16.5 Å) along the a-axis (Fig. 18a–d).¹⁵⁹ It was noted that Co-BTB facilitated the fixation of CO₂ to aziridines with a yield of 99%. However, the fixation of CO₂ to propargyl amines resulted in only 32% oxazolidinone, attributed to the absence of Lewis base sites (Fig. 18e). Notably, the catalytic performance of Co-BTB was improved by incorporating ligand, XN (4′-(4′′-pyridyl)2,4′:6′,4′′-terpyridine) with Lewis basic sites into Co-BTB which resulted in MOF, ({(NH₂Me₂)[Co₃(μ₃-OH)(NHMe₂)(BTB)₂(XN)]·8H₂O·4DMF}_n) named as Co-XN having less Lewis acid sites and more basic sites (Fig. 18c and d). Co-XN featured a 3D framework structure composed of rhombus channels (10 Å × 10 Å) (Fig. 18d). The incorporation of Lewis basic sites resulted in 2.4 times rise in the yield of oxazolidinones (Fig. 18e). Moreover, the reaction mechanism validated that Lewis acid sites efficiently facilitated the ring-opening and cycloaddition of aziridines, while the presence of Lewis basic sites accelerated the dehydrogenation of propargyl amines. Thus, controlling Lewis acid–base sites within MOFs on a molecular scale can facilitate achieving tailored catalytic activity for effective CO₂ transformation to desired products. Jing and co-workers reported a 3D Cu-framework, Cu-TSP for coupling propargylic amines with CO₂.¹⁶⁰ Owing to the high porosity and suitable arrangement of Cu centers, Cu-TSP efficiently catalyzed the synthesis of 2-oxazolidinones from CO₂ using DBU as the base at 50 °C under 0.1 MPa CO₂, with recyclability up to five cycles (Table 1) with broad substrate scope.

Fig. 18 (a–d) Synthesis of Co-BTB and Co-XN. (e) Effect of Lewis acid–base sites on the catalytic activity of Co-BTB and Co-XN.¹⁵⁹ Copyright 2022, Wiley–VCH GmbH.

In 2023, Liu and co-workers reported a pyrene-based MOF, [Cd₂(PTTB)(H₂O)₂] (WYU-11), (Fig. 19a), exhibiting remarkable catalytic efficacy for the preparation of 2-oxazolidinones by utilizing CO₂, under mild conditions (60 °C, atmospheric CO₂) with a wide range of propargylamine substrates.¹⁶¹ The steric hindrance effect can be observed from the low yield obtained from the introduction of the –CH₃ group for the N-aryl-substituted group (Fig. 19b). Further, with N-phenylprop-2-yn-1-amine, only a trace amount of the product was formed owing to the weak N-nucleophilicity prohibiting the nucleophilic attack of CO₂. The combined action of WYU-11 and 1,1,3,3-tetramethylguanidine (TMG) synergistically activated propargylic amine substrates and drove the reaction forward (Fig. 19b). The application of a mixed metal Cu–Fe porphyrin-based porous framework (Cu-TCPP(Fe)) composed of Cu-carboxylate layers has been reported recently (Fig. 19c).¹⁶² These layers result in a rhombic-shaped 2D lattice in the ab plane pillared by Fe-TCPP ligands. The large porous structure and appropriate coordination of the Cu centers in the Cu-TCPP(Fe) catalyst facilitated an efficient catalytic transformation of propargylic amines to 2-oxazolidinones using CO₂ under mild reaction conditions (Table 1 and Fig. 19d). Moreover, the numerous H-bonding sites on the ligands strengthened the interaction between the framework and reactants. Additionally, the spacious pores and optimal coordination of the Cu centers facilitated an effective catalytic transformation under mild conditions.

Fig. 19 (a) Structure of WYU-11. (b) CO₂ conversion to oxazolidinones catalyzed by WYU-11.¹⁶¹ Copyright 2023, American Chemical Society. (c) Structure of Cu-TCPP(Fe). (d) CO₂ conversion to oxazolidinones catalyzed by Cu-TCPP(Fe).¹⁶² Copyright 2024, the Royal Society of Chemistry.

The synthesis of a Cu(I)-based Th-MOF, {[Cu₅I₆Th₆(μ₃-O)₄(μ₃-OH)₄(H₂O)₁₀(L)₁₀]·OH·4DMF·H₂O}_n constituted by [Th₆] clusters and [Cu_xI_y] subunits has been reported by Wei and co-workers (Fig. 20a).¹⁶³ Owing to the excellent chemical stability and exposed CuI sites, it was exploited for the utilization of CO₂ along with propargylic amines to form oxazolidinones at RT using TEA as a base. Moreover, the gram scale experiment resulted in an 85% yield of oxazolidinones has been achieved (Fig. 20b).

Fig. 20 (a) Structure of Th-MOF. (b) CO₂ conversion to oxazolidinones catalyzed by Th-MOF.¹⁶³ Copyright 2024, American Chemical Society. (c) Structures of 1_X%–2_0.5−X%–3_0.5-JNM (X = 0, 10, 25, 50). (d) CO₂ conversion to oxazolidinones catalyzed by 1_0.1–2_0.4–3_0.5-JNM.¹⁶⁴ Copyright 2024, American Chemical Society.

The bioactive macromolecule dehydroabietylamine derivative was successfully transformed to the corresponding 2-oxazolidinone with a 91% yield. The high yield was achieved due to the large pore aperture and excellent catalytic activity of Th-MOF. It is worth mentioning that, such transformations of macromolecular substrates efficiently are rarely achieved using MOF-based catalysts. These results highlight the potential of Th-MOF in the synthesis of biological compounds. The widely distributed [Cu_xI_y] subunits within the channels of Th-MOF enabled it to serve as an efficient heterogeneous catalyst for the synthesis of oxazolidinones. Chen et al. recently reported isostructural series of 2D MTV-MOFs, 1_X%–2_0.5−X%–3_0.5-JNM (X = 0, 10, 25, 50), comprising Cu- and Ag-based cyclic trinuclear clusters by varying the ratio of Cu and Ag (Fig. 20c).¹⁶⁴ It was found that when 10% Ag was incorporated into the framework, JNM showed highest catalytic activity for the conversion of propargyl amines to oxazolidinones using CO₂ and DBU as a base. Under solvent-free conditions and atmospheric pressure, a TOF of 243 h⁻¹ was achieved using 1_0.1–2_0.4–3_0.5-JNM as the catalyst (Fig. 20d). This TOF is about 20 times higher than 2_0.5–3_0.5-JNM (10.8 h⁻¹). Further, the catalyst 1_0.1–2_0.4–3_0.5-JNM maintained its activity for the simulated flue gas, resulting in isolated yields of up to 76%. This study offers novel insights for the rational design of synergistic MTV-MOFs as catalysts at the molecular level, aiming to achieve efficient and environmentally friendly chemical conversion of CO₂.

As discussed previously, it is apparent that MOF-based catalysts composed of alkynophilic Ag(0)/Ag(I) and Cu(0)/Cu(I) metals are promising catalytic materials for effective coupling of CO₂ with propargyl amines as starting materials, to form oxazolidinones under ambient conditions. The catalytic sites within MOFs are widely distributed and easily available, promoting the complete utilization of the active sites. Metal nodes within MOFs are interchangeable with other metals and functional groups can be grafted on the organic linkers. This allows for carefully regulating MOF activities tailored to different reactions.¹⁶⁵ A single MOF catalyst can readily incorporate multiple active sites, potentially leading to a synergistic effect that enhances catalytic efficiency.¹⁶⁶ Typically, the interactions between MOF and CO₂ molecules are crucial, as strengthening these interactions enhances the material's ability to capture CO₂, particularly at low-pressure conditions. Achieving this necessitates modification of the inherent characteristics of MOFs through careful design and synthesis using de novo methods or PSM.

The goal is to leverage the chemical compatibility and tunability of the MOFs to increase their affinity for CO₂ molecules. In this regard, Gu et al., prepared Cu₂O@ZIF-8 catalyst by incorporating Cu₂O NPs into robust ZIF-8 with BET surface area of 1343 m² g⁻¹, comparatively lower than the original ZIF-8 (1974 m² g⁻¹) due to substantial encapsulation of Cu₂O NPs (Fig. 21a).¹⁶⁷ Taking advantage of the outstanding CO₂ adsorption capability of robust ZIF-8 and highly active Cu₂O sites, the Cu₂O@ZIF-8 composite efficiently facilitated the conversion of CO₂ to valuable oxazolidinones at 40 °C under atmospheric pressure of CO₂ using CH₃CN as a solvent and DBU as a base with good substrate scope (Table 1 and Fig. 21b). Notably, the conversion of N-phenylprop-2-yn-1-amine was negligible (2%) due to its weak N-nucleophilicity hindering its nucleophilic attack with CO₂, thereby, preventing formation of carbamate intermediate (Fig. 21b). It can be seen that the internal propargylamine substrate produced only a moderate yield of the product (48%) over 12 h (Fig. 21b). The Cu₂O/DBU binary system efficiently promoted the CO₂ fixation, resulting in oxazolidinones through a mechanism that includes H⁺ migration from propargylic amine to base followed by intramolecular cyclization and H-demetalation steps. Similarly, Wu and co-workers reported Cu₂O@MIL-101(Cr)-DABCO, which was meticulously engineered by sequentially incorporating DABCO and Cu₂O into the MIL-101(Cr) MOF using a stepwise assembly approach.¹⁶⁸ Due to the significant confinement of Cu₂O NPs within the MOF and the robust coordination interaction between DABCO and the MOF, Cu₂O@MIL-101(Cr)-DABCO efficiently facilitated the cyclization of CO₂ to propargylic amines with a wide range of substrate scope. This catalyst exhibited good reusability without the need for a co-catalyst or solvent, operating at RT under atmospheric pressure conditions.

Fig. 21 (a) Synthesis of Cu₂O@ZIF-8. (b) Preparation of oxazolidinones catalyzed by Cu₂O@ZIF-8.¹⁶⁷ Copyright 2022, Wiley–VCH GmbH. (c) Synthesis of CuBr@NH₂-MIL-101. (d) Reaction of propargylic amines with CO₂ yielding oxazolidinones catalyzed by CuBr@NH₂-MIL-101.¹⁶⁹ Copyright 2023, Elsevier.

The application of CuBr anchored MOF composite, CuBr@NH₂-MIL-101 (Fig. 21c) for selective CO₂ adsorption has been reported by Hu and co-workers.¹⁶⁹ CuBr@NH₂-MIL-101 displayed excellent results for carboxylative fixation of CO₂ to propargylic amines under ambient conditions (25 °C, DBU, CH₃CN, 0.1 MPa CO₂) (Table 1 and Fig. 21d) with good substrate universality. Recently, Astruc and co-workers reported synthesis of a Cu(I)-GSH/ZIF-8 composite using a biologically essential Cu-glutathione (GSH) redox system.¹⁷⁰ The BET surface and pore volume of ZIF-8 remain the same after the introduction of 0.3 wt% Cu(I)-GSH and 0.45 wt% Cu(I)-GSH, displaying Cu(I)-GSH polymer loading on ZIF-8 surface, not in the pores. Cu(I)-GSH/ZIF-8 was exploited as a heterogeneous catalyst for the carboxylative cyclization of propargyl amines with CO₂ with high yields under atmospheric pressure. Notably, Cu(I)-GSH/ZIF-8 displayed a good substrate tolerance to electron-withdrawing or electron-donating groups on the N-benzyl unit of propargylic amines.

The aforementioned literature studies have shown that PSM of frameworks is a useful tool to integrate CO₂-philic and catalytic sites to achieve catalytic transformations with high yield and selectivity.¹⁷¹ The reaction of propargyl amines with CO₂ to produce 2-oxazolidinone is a vital industrial reaction. However, it typically requires noble-metal catalysts with organic bases under severe conditions. Notably, Cu-containing MOFs exhibit significant catalytic activity in converting CO₂ to oxazolidinones using propargylamine substrates.^169–172 Thus, being an affordable metal, Cu-based catalysts can replace noble metals like Ag, Pt and Au without compromising catalytic proficiency and reducing catalysis costs.

3.2.2. Covalent organic frameworks catalyzed cyclic carboxylation of propargyl amines with CO₂. COFs present a promising alternative to metal–organic frameworks, exhibiting remarkable stability under industrial reaction conditions. Their adaptable structures allow for the integration of molecular catalytic components, such as porphyrin and bipyridyl units that contain active metal sites. The distinctive characteristics of COFs—including their customizable porous surfaces, high heteroatom density, ease of modification and functionalization along with robust chemical and thermal stability—have drawn significant interest from researchers focused on CO₂ conversion reactions.^172,173 The post-modification functionalization of COFs and the development of COF-based functional catalysts with active sites for CO₂ conversion represent exciting avenues for ongoing research. As discussed in the previous sections, metal sites have been widely employed as Lewis acid catalysts to effectively convert CO₂ into various accessible chemicals. Since metal ions and clusters are dispensable in developing COFs.¹⁷⁴ Two primary methods have been devised for incorporating metal sites into COFs viz by using metallated organic monomers or post-modification techniques. Metal-incorporated COFs exhibit exceptional photo/electrocatalytic properties and demonstrate high catalytic activities for CO₂-related reactions.^175,176 When it comes to converting CO₂ into valuable chemicals, metal-incorporated COFs, with their metal coordination bonds, typically exhibit a stronger interaction force than pristine COFs’ hydrogen bonds, this enhanced force helps to activate both substrates and CO₂ molecules.¹⁷⁷

To date, certain metal-incorporated COFs have been effectively employed in transforming CO₂ into a variety of oxazolidinones. For instance, the application of Ag-NPs anchored Tp-based COFs (TpPa-1 and TpTta) for carboxylation of propargylic amines has been reported by Islam and co-workers.¹⁷⁸ The Ag-decorated COFs exhibited remarkable catalytic capabilities in incorporating atmospheric CO₂ into unsaturated amines under neat conditions, producing industrially significant cyclic carbamates and oxazolidinones. Especially, Ag@TpTta facilitated efficient, atom-economical and high-yield synthesis of alkylidene-oxazolidinones under solvent-free conditions utilizing 1 atm CO₂ pressure and temperatures ranging from 40 to 80 °C (Table 1). Later, the same group reported incorporation of Cu-NPs in the 1D pores of triazole-based COF (Cu-NPs@COF).¹⁷⁹ The Cu-NP-embedded COF showed promising catalytic activity for producing 2-oxazolidinones via cycloaddition reaction of CO₂ with propargyl amines under mild conditions. The reaction involving CO₂ and propargylic amines offered the production of highly valuable chemicals. These studies were extended to the incorporation of Pd(II) in the COF to obtain (Pd(II)@TFR-OT COF) and its application for synthesis of oxazolidinone via chemical fixation of CO₂ was demonstrated under sunlight.¹⁸⁰

The encapsulation of Ag NPs in porous pyridine-based COFs has been demonstrated by Bai and co-workers.¹⁸¹ The presence of pyridyl-N in the framework effectively incorporated Ag metal within the pores, resulting in Ag@2,6-FPP-TAPT and Ag@3,5-FPP-TAPT COFs (Scheme 2 and Fig. 22a). The orientation of pyridyl-N determined the placement of Ag sites in diverse pores and constrained them to specific dimensions, greatly influencing their catalytic activity and stability. These materials were utilized as eco-friendly effective catalysts for synthesizing various oxazolidinones using propargylic amines and CO₂ under ambient conditions (50 °C and 1 atm pressure) (Table 1 and Fig. 22b). Detailed investigations revealed that propargylamine substrates could be trapped and interact with extremely small, well-dispersed Ag sites within the pores. Additionally, 2,6-FPP-TAPT could absorb CO₂, enhancing the reaction efficiency by facilitating the rapid conversion of CO₂. In a similar approach, our group recently reported a pyrene-based COF, Pybpy-COF designed to securely anchor catalytic Ag(0) NPs (Fig. 23a) and applied it to synthesize commodity chemicals using CO₂ under standard environmental conditions.¹⁸² Ag@Pybpy-COF catalyzed CO₂ conversion to 2-oxazolidinones using diverse propargylic amines under mild conditions (Table 1 and Fig. 23b). The remarkable catalytic efficacy of Ag@Pybpy-COF is attributed to the abundance of exposed, alkynophilic Ag(0) catalytic sites strategically distributed on the pore surface (787 m² g⁻¹) of Pybpy-COF. This study showcased the potential use of Pybpy-COF as a reliable platform for anchoring Ag NPs and its application for transforming CO₂ into valuable oxazolidinones. Lan and co-workers embedded Cd single-atom sites in a 2,2′-bipyridine based COF (Cd-Bpy-COF).¹⁸³ The strategic placement of bipyridine coordination units within the framework anchored Cd single sites with high loading, exposed substantial active sites to boost catalytic activity (Fig. 24a). The resulting Cd-Bpy-COF demonstrated exceptional performance and remarkable stability in catalyzing the CO₂ conversion to oxazolidinones using propargyl amines under ambient reaction conditions. The fusion of COFs with Cd sites significantly enhanced CO₂ adsorption and its activation, thereby facilitating subsequent cyclization reactions (Fig. 24a).

Fig. 22 (a) Synthetic routes for the Ag@2,6-FPP-TAPT and Ag@3,5-FPP-TAPT catalysts. (b) Preparation of oxazolidinones catalyzed by Ag@2,6-FPP-TAPT.¹⁸¹ Copyright 2022, the Royal Society of Chemistry.

Fig. 23 (a) Preparation of Pybpy-COF and Ag@Pybpy-COF. (b) Synthesis of oxazolidinones using propargylic amines and CO₂ catalyzed by Ag@Pybpy-COF.¹⁸² Copyright 2024, American Chemical Society.

Fig. 24 (a) Preparation of oxazolidinones using propargyl amines and CO₂ catalyzed by Cd-Bpy-COF.¹⁸³ Copyright 2023, the Royal Society of Chemistry. (b) Synthesis of Cu^I-TpBD-COF. (c) Preparation of oxazolidinones catalyzed by Cu^ITpBD-COF.¹⁸⁴ Copyright 2024, the Royal Society of Chemistry.

A proposed mechanism for Cd-Bpy-COF is illustrated in Fig. 24a. The process begins with the activation of propargylic amine at the Cd sites through interactions with its N–H bond and is subsequently activated by DBU. This activation allows for an electrophilic attack by CO₂, resulting in the formation of a carbamate intermediate. Following this, the negatively charged oxygen atom of the carbamate attacks the Cu–C bond, facilitated by activation from Cd, which leads to intramolecular cyclization. Finally, a proton from DBUH⁺ is transferred to the triple bond, yielding the desired product. On the other hand, the construction of Cu^I-anchored β-ketoamine COF (Cu^ITpBD-COF) and its utilization for carboxylative cycloaddition of CO₂ with propargylic amines has been reported (Fig. 24b and c).¹⁸⁴ Cu^I@TpBD-COF displayed proficient catalytic results when simulated flue gas was utilized as a CO₂ source.

The previous discussions signified the application of COF-based frameworks for catalyzing the synthesis of oxazolidinones using propargylamine as substrates. While significant progress has been made in synthesizing oxazolidinones using COF catalysts, most literature reports utilize noble metal (Ag, Pd) incorporated COFs for the effective activation of propargylic hydrogen because of the excellent alkynophilicity of these metals. Thus, research on developing noble-metal-free catalysts is still in its early stages. Meeting industrial demand requires noble-metal-free catalysts, which have garnered significant attention from scientists dedicated to advancing this research. Therefore, creating efficient COF catalysts using non-noble metals is crucial for developing affordable and sustainable methods for environmentally friendly CO₂ conversion into oxazolidinones. However, greater emphasis should be placed on using eco-friendly and economically viable metals for CO₂ conversion reactions.

3.2.3. Porous organic polymers (POPs) as catalysts for cyclic carboxylation of propargyl amines with CO₂. Porous Organic Polymers (POPs) prepared from various monomers via simple and convenient chemical synthetic procedures can act as promising candidates for CO₂ capture and transformation reactions.¹⁸⁵ Various POPs can be synthesized by polycondensation or solid-state condensation reactions using diverse multidentate amines, aldehydes, carboxylic acids or triazine monomers.¹⁸⁶ The importance of CO₂ absorption and conversion reactions is highlighted by their straightforward synthesis, consistent pore size and high surface area featuring key basic sites like –N/–NH/–NH₂ and other heteroatoms. The following section describes the designing of POPs and their application in CO₂ utilization reactions to synthesize value-added oxazolidinones.¹⁸⁷

The synthesis of two microporous POPs, BBA-1 and modified BBA-2 has been reported by Ghosh et al.¹⁸⁸ They decorated Pd NPs on these POPs to produce Pd@BBA-1 and modified Pd@BBA-2 nanomaterials. The presence of Pd NPs on the surface facilitated efficient cyclic carboxylation of a diverse range of propargylamine, resulting in corresponding 2-oxazolidinones under 0.1 MPa CO₂ with DMSO solvent at temperatures ranging from 40 to 80 °C. Remarkably, Pd@BBA-2 exhibited good recyclability for generating oxazolidinones. Similarly, the application of Ag NP anchored COF (AgN@COF) for coupling of CO₂ with terminal propargylic amines and propargylic alcohols, rendering 2-oxazolidinones at 55 °C under 1 atm of CO₂ has also been reported.¹⁸⁹

The application of a thiadiazole-based COP (BT-COP) for anchoring Ag NPs and its catalytic investigation has been reported by Lan and co-workers.¹⁹⁰ Here, the S-atom within the thiadiazole unit served as an anchoring site for Ag NPs growth. The resulting Ag@BT-COP exhibited notable CO₂ adsorption capabilities, along with commendable catalytic performance and reusability for efficiently converting CO₂ and propargylic amines to valuable 2-oxazolidinones at 60 °C under standard 1 atm CO₂ pressure (Table 1). This research work highlighted the potential utility of functionalized COPs in CO₂ fixation, shedding light on the advancement of efficient catalysts for converting CO₂ into appreciated chemicals. In another example, Zhang and co-workers introduced two nitrogen-rich porous organic polymers (NPOPs) comprised of covalent triazine and triazole N-heterocycles. Ag NPs were effectively incorporated into the POPs, resulting in Ag@NPOPs characterized by excellent distribution and small particle size.¹⁹¹ These Ag@NPOPs were then employed in potential conversion reactions involving CO₂, terminal alkynes and propargylic amines under mild reaction conditions (50 °C, 1 atm CO₂). Ag@NPOP-1 demonstrated exceptional catalytic stability and durability, being successfully reproduced and reused five times. This research offered valuable insights for designing and fabricating novel multifunctional catalysts. The design of a noble metal-free Cu-based NHC POP catalyst (Cu@NHC-1) by copolymerization technique, followed by complexation with Cu(OAc)₂, and its utilization for converting low-concentration CO₂ into oxazolidinones has been reported.¹⁹² Owing to its porous structure, N-activation sites and catalytic Cu center working together synergistically, Cu@NHC-1 exhibited exceptional efficiency and selectivity in adsorbing, activating, and converting low-concentration CO₂ (30 vol%) to oxazolidinones. The practical potential of this catalyst is evidenced by its capability to effectively convert CO₂ from lime kiln waste gas into oxazolidinones with satisfactory yields under mild conditions. Moreover, the gram-scale reaction yields the desired product in high quantities, showcasing the catalyst's significant potential for industrial CO₂ conversion. Recently, our research group reported the strategic incorporation of catalytically active Pd(II) into a porous covalent triazine framework (CTF) composed of bipyridine sites (bpy-CTF) via PSM.¹⁹³ The resulting Pd(II)@bpy-CTF catalyst demonstrated exceptional catalytic performance in the cyclization of CO₂ with propargylic amines, producing high-value oxazolidinones under mild conditions. The exceptional catalytic behavior of Pd(II)@bpy-CTF is attributed to a high abundance of N-rich triazine units within the framework and exposed catalytic Pd(II) sites within the 1D channels of the CTF.

Based on the literature studies, it can be concluded that POPs remain highly regarded for their role in the selective capture and utilization of CO₂ into valuable chemicals like oxazolidinones, contributing significantly to combat global warming. The engineering of their pore environments enhances their affinity for CO₂, enabling the development of highly microporous networks rich in heteroatoms that provide abundant catalytic sites. This structural versatility allows POPs to effectively facilitate reactions such as the utilization of CO₂ to prepare oxazolidinones. Their relatively low cost and ease of synthesis compared to other porous materials further underscore their potential in sustainable carbon management strategies.

3.3. Synthesis of oxazolidinones by a one-pot multi-component reaction involving CO₂

As discussed before, the three-component reactions involving CO₂, propargyl alcohol and primary amine represent a highly promising and innovative strategy for synthesizing 3-substituted-2-oxazolidinones. An alternative three-component approach involves epoxides, primary amines and CO₂ resulting in the preparation of oxazolidinones. Literature studies revealed that efficient formation of oxazolidinones through a three-component coupling of alkynes/amines with CO₂ necessitates (Cu(0/I) or Ag(0/I)) metal sites due to the alkynophilicity exhibited by these metals. Here, the reaction begins with the polarization of the alkyne –C≡C– bond at the catalytic sites of Cu/Ag. Then, CO₂ is inserted into the deprotonated alkyne, followed by a ring-closure step, rendering α-alkylidene cyclic carbonate, which reacts with propargylic amines to produce oxazolidinones (Fig. 25b).¹⁹⁴

Fig. 25 (a) Three-component synthesis of oxazolidinones catalyzed by TMOF-3-Ag. (b) Catalytic mechanism of the three-component reaction.¹⁹⁶ Copyright 2018, American Chemical Society.

3.3.1. MOF-based catalysts for preparing oxazolidinones by one-pot, three-component reaction. The three-component reaction involving CO₂ is appealing due to the affordability and easy accessibility of the substrates, as well as the flexibility of functional groups in the product.¹⁹⁵ Yet, owing to the intricate reaction process, these three-component reactions have primarily been carried out using noble-metal catalysts, leading to the inevitable mixing of by-products in the final products. In this direction, MOFs composed of polar sulfonate groups are beneficial in supporting high CO₂-philicity and providing anchoring sites for the incorporation of catalytic metal sites. For instance, Fei and co-workers reported an organosulfonate-based non-interpenetrated MOF [Cu(bpy)₂(1,2,4,5-BTMS)_0.5(H₂O)_0.5]_n (TMOF-3), demonstrating a pcu structure characterized by a consistent porosity of 43%, which was functionalized by incorporating alkynophilic Ag(I) sites, to obtain TMOF-3-Ag catalyst.¹⁹⁶ Owing to the presence of a high density of CO₂-philic sulfonate functional sites and the alkynophilic Ag(I) sites, TMOF-3-Ag(I) exhibited competent catalytic activity for three-component cyclization reaction of primary amine, CO₂ and propargyl alcohol resulting in oxazolidinones under 1 bar pressure at 50 °C (Table 1 and Fig. 25a). The mechanistic path of the reaction is discussed in section 3.3 (Fig. 25b). Notably, this work highlights the potential uses of functionalized and permanently porous sulfonate-group functionalized MOFs with intrinsically high-polarity pore surfaces for CO₂ sequestration and capture technologies.

In another example, the utilization of SO₃H-functionalized MOF (MOF-SO₃H) comprising polar sulfonate group for anchoring alkynophilic Ag(I) sites has been studied (Fig. 26a).¹⁹⁷ The reduction in the BET surface area from 1191 m² g⁻¹ (pristine MOF-SO₃H) to 851 m² g⁻¹ (MOF-SO₃Ag) validates the incorporation of Ag(I) at the –SO₃H groups exposed in the 1D channels of MOF. MOF-SO₃H exhibited a high heat of interaction value of 37.8 kJ mol⁻¹, which enhanced its selective CO₂ adsorption capabilities over other gases, attributed to the presence of polar sulfonate groups exposed in the pores. The combined influence of polar sulfonate groups and catalytically active Ag(I) sites created a favorable environment for catalyzing the three-component reaction of propargylic alcohols and primary amines under 1 atm CO₂ at RT (Table 1 and Fig. 26b). More importantly, the absence of leaching of the catalytic active site, Ag(I) was established with catalytic recyclability for up to five cycles (Fig. 26c).

Fig. 26 (a) Synthesis of MOF-SO₃Ag. (b) Synthesis of oxazolidinones by a three-component reaction catalyzed by MOF-SO₃Ag. (c) Recyclability test of MOF-SO₃Ag.¹⁹⁷ Copyright 2020, American Chemical Society.

Towards the development of noble-metal-free MOF for one-pot synthesis of oxazolidinones, Yamani and co-workers¹⁹⁸ reported a series of UiO-66^199–201 frameworks possessing linker-induced defects. Here, the application of linker-induced defects-rich UiO-66 MOF has demonstrated improved catalytic activity for the reaction of aromatic amines, epoxides and CO₂ over the pristine MOF. A wide range of biologically relevant oxazolidinones were obtained in yields exceeding 90% while maintaining structural rigidity and recyclability for up to 5 cycles. This work opened new doors for linker-induced defects-based MOFs, resulting in the fixation of CO₂ into value-added products. An example of Ni-based MOF prepared from trismic acid linker has been employed as a noble-metal-free catalyst for the synthesis of oxazolidinones (Fig. 27a).²⁰² The MOF displayed a CO₂ uptake of 37 cc g⁻¹ and on thermal activation, it generates open metal sites that acted as Lewis acidic sites in the CO₂ utilization reaction involving aromatic amines and epoxides. This resulted in the production of oxazolidinone at 90 °C and 1 bar pressure using TBAI as a co-catalyst (Table 1 and Fig. 27b).

Fig. 27 (a) Synthesis of Ni-MOF. (b) Preparation of oxazolidinones catalyzed by Ni-MOF.²⁰² Copyright 2021, Elsevier. (c) Synthesis of Cu(I)@NHC-MOF. (d) The three-component CO₂ fixation catalyzed by Cu(I)@NHC-MOF.²⁰⁹ Copyright 2021, the Royal Society of Chemistry.

The detailed literature study indicated that the majority of catalysts used for the transformation of CO₂ using propargylic alcohols to form oxazolidinones incorporate noble metals.^203–206 The application of non-noble metal-based catalysts like Cu(I) for transforming CO₂ into oxazolidinones is important from a green and sustainable chemistry perspective. In this regard, N-heterocyclic carbene (NHC) based linkers provide an appropriate platform for boosting CO₂-philicity and anchoring catalytic metal ions at the carbene carbon center.^207,208 Keeping this in mind, our group²⁰⁹ post-synthetically modified NHC-based MOF²¹⁰ by Cu(I) ions to generate Cu(I)@NHC-MOF²¹¹ as shown in (Fig. 27c). The cooperative involvement of CO₂-attracting NHC and catalytically active Cu(I) sites created a promising platform for transforming CO₂ into oxazolidinones, specifically at RT and atmospheric CO₂ pressure (Table 1 and Fig. 27d). The synthesized hybrid material showed high recyclability for up to ten cycles. Further, PSM of NHC-MOF by incorporation of Ag(I) ions to form Ag(I)@MOF-NHC and its catalytic investigation has also been reported.²¹² The Ag(I)@MOF-NHC displayed significant catalytic efficacy in a one-pot reaction involving primary amines, CO₂ and propargylic alcohol, leading to the production of various oxazolidinones under environmentally friendly conditions, specifically at RT and atmospheric pressure with simulated flue gas.

The construction of a chiral framework, CMOF-801(ASP) and its application for the synthesis of chiral oxazolidinones by asymmetric CO₂ utilization has been reported by Morsali and co-workers. The chiral MOF was generated by substituting fumaric acid with L-aspartic acid (Scheme 1) in MOF-801.²¹³ The CMOF-801(ASP) facilitated asymmetric catalysis for producing valuable products with optimal efficiency in a short time by simultaneously incorporating Zr sites as Lewis acidic sites, –OH and NH₃⁺ sites as Brønsted acid sites and –NH₂ as Lewis base sites. The enhanced catalytic performance of CMOF-801(ASP) stemmed from its ample active sites, created using the missing-cluster defect strategy, which notably eased substrate diffusion and activation. Chiral oxazolidinones with 90% conversion and 98% ee were obtained using the catalyst having three functional catalytic sites without a co-catalyst under solvent-free conditions (90 °C, 12 h, 1 bar CO₂) (Table 1). This research showcased that introducing cluster defects alongside Lewis basic chiral linkers can generate numerous catalytic sites within MOFs. This setup allows for swift and eco-friendly asymmetric catalysis to occur under mild conditions. Recently, Mandal and co-workers reported a Ni-MOF with remarkable polarizing properties, showing significant uptake of CO₂ and water vapor, reaching up to 10.53 cm³ g⁻¹ and 290 cm³ g⁻¹, respectively, at 298 K.²¹⁴ Leveraging its CO₂-philic and catalytic features, Ni-MOF was employed as an excellent heterogeneous catalyst (2.5 mol%) along with a very low concentration of TBAB (0.02 mmol) as a co-catalyst for the preparation of oxazolidinones via a one-pot reaction involving an epoxide, a substituted aniline and CO₂, without use of any solvent.

The literature studies demonstrated that Lewis acidic sites increase the catalytic efficiency of the MOF, resulting in a high yield of oxazolidinones. In addition, Lewis basic sites and CO₂-philic groups enhance the easy activation of carbon dioxide, resulting in efficient catalytic results. It is important to highlight that the interaction between the framework and CO₂ molecules plays a crucial role in CO₂ capture and conversion by the MOF. Strengthening this interaction can significantly increase the CO₂ uptake and conversion capacity of the MOF material, particularly at low loading pressures.

3.3.2. MOF-based catalysts for preparing oxazolidinones by four-component reaction involving CO₂. Recently synthesis of oxazolidinones by a four-component reaction of aldehyde, alkyne, amine and CO₂ is gaining significant interest owing to its eco-friendly nature.^80,215 In this context, Huang and colleagues developed a series of noble-metal-free MOFs decorated with perfluoroalkyl groups, namely PCN-(BPY-CuI)-(TPDC-Fx) and employed them as catalysts for a one-pot, four-component reaction involving aldehyde, alkyne, amine and CO₂ to synthesize 2-oxazolidinones (Fig. 28a).⁸⁰ Here, perfluoroalkyl group enhanced the framework's affinity towards hydrophobic substrates, improving selectivity/activity and providing exceptional hydrophobicity under flue gas conditions. Furthermore, the plentiful –NH₂ groups within the framework enhanced the CO₂-philicity from flue gas owing to a stronger preference for carbamate acid formation. This phenomenon significantly increased the effective CO₂ concentration and facilitated the final cyclization step to produce 2-oxazolidinones.

Fig. 28 (a) Four-component oxazolidinone synthesis from PCN-(BPY-CuI)-(TPDC-F₇). (b and c) Probable mechanism for the one-pot four-component reaction.⁸⁰ Copyright 2023, Wiley–VCH GmbH.

Mechanistic studies indicated that the MOF effectively enriched the substrates along its wall, with the adsorbed amine species playing a crucial role as external binding sites for dilute CO₂ by favoring carbamate acid formation (Fig. 28b and c). This study presents an encouraging approach for the straightforward synthesis of 2-oxazolidinones, utilizing noble metal-free catalysts utilizing flue gas as a CO₂ source and affordable industrial bulk raw materials, all under mild conditions. In a similar approach, our group has recently reported the application of non-noble metal copper nanoparticles (Cu NPs) grafted porous, CO₂-philic MOF for one-pot synthesis of 2-oxazolidinones (Fig. 29a).²¹⁵

Fig. 29 (a) Structure of Cu@UiO-66-NH₂. (b) Four-component oxazolidinone synthesis from Cu@UiO-66-NH₂.²¹⁵ Copyright 2024, American Chemical Society.

This approach enables the straightforward one-pot synthesis of 2-oxazolidinones through a four-component reaction involving phenylacetylene, CO₂, acetone and aliphatic amines. Notably, the embedding of small-size Cu-NPs with an average diameter of 10 nm exhibiting high catalytic activity was achieved. The MOF with anchored Cu NPs demonstrated remarkable catalytic activity for converting CO₂ into valuable 2-oxazolidinones by a one-pot, four-component reaction through C–H bond functionalization of alkynes under ambient conditions (Table 1 and Fig. 29b).

3.3.3. COF-based catalysts for the synthesis of oxazolidinones by one-pot, three-component reaction. The remarkable adaptability of COFs at the atomic and molecular scale provides them with numerous distinctive benefits, including a well-defined porous structure, large surface area, highly ordered system with exceptional stability, structural versatility and straightforward functionalization.²¹⁶ These qualities make them highly promising catalysts for CO₂ capture and conversion to valuable chemicals like oxazolidinones.²¹⁷ Considering the versatility and customizable nature of the COF structure, they can incorporate various functional moieties consistently, including Brønsted/Lewis acidic centers and ionic sites. COFs characterized by extensive surface areas and structured pores can effectively capture and concentrate CO₂ molecules, thereby boosting the chemical conversion of CO₂.²¹⁸ For instance, the synthesis of a nanoporous 3D-COF-supported Zn(II) catalyst (Zn@RIO-1), demonstrating CO₂ conversion to oxazolidinones has been reported.²¹⁹ Oxazolidinones were produced through three-component reactions involving CO₂, aromatic/aliphatic amines and propargylic alcohols at 80 °C, resulting in a high product yield (Table 1). The solvent-free nature of both catalytic methods renders the processes safe and eco-friendly. The Zn(II)-embedded nanoporous 3D-COF catalyst (Zn@RIO-1) was reusable without significant change in the activity. Here the nanoporous 3D-COF served not only as a support for Zn–metal but also as a dynamic matrix that adsorbs CO₂. This research highlights the potential of nanoporous 3D-COF-based materials in catalysis, particularly in CO₂ capture and chemical conversion into valuable fine chemicals. In a similar approach, the incorporation of AgNPs in a porous COF, TpMA(MC)@Ag and its exploitation as a catalyst for producing oxazolidinones from various propargylic alcohols is reported.²²⁰ By incorporating a potent organic DBU base, oxazolidinones were prepared at RT via a three-component reaction involving aromatic/aliphatic amines, CO₂ and propargylic alcohols with good recyclability. Additionally, the nanoporous polymeric material supported the incorporation of Ag metal and exhibited potential for adsorbing CO₂. This study highlights the potential of nanoporous polymeric materials in catalysis, specifically in CO₂ capture and chemical fixation applications. Recently, Ag NP incorporated COF (Ag@TFPNDA-COF) was employed for a one-pot reaction of propargyl alcohol with primary amine and CO₂ under mild conditions.²²¹

Based on the previous discussions, it is evident that COF-based materials possess extensive surface areas and consistent pores, enabling numerous approachable catalytic sites and rapid mass transfer of reactants and products.²²² The structural framework and porous surroundings of COFs can be precisely designed and controlled, offering an effective strategy to improve catalytic performance, including selectivity, reaction kinetics and other factors.²²³ Additionally, the plentiful pores within COFs provide an ideal platform for hosting and enclosing diverse functional species, including metal NPs, organic molecules, fullerenes and ionic liquids. Metal-incorporated COFs and metal NPs incorporated COFs hold significant potential for applications in CO₂ utilization owing to their high density of catalytic metal sites, thus enhancing the catalytic process.

4. Conclusions and future outlook

Recent advancements in framework-based materials have led to the development of novel and superior catalytic materials for CO₂ capture and conversion to oxazolidinones. The key to their effectiveness lies in the distinctive chemistry of the framework architectures. By customizing the frameworks, it is possible to fine-tune the interactions between CO₂ molecules and the internal structure of the MOFs/COFs/POPs, which include active functional groups, open metal sites and microscopic pores.^224–227 This review systematically examines the evolution of framework-based materials as heterogeneous catalysts for the chemical transformation of CO₂ to value-added oxazolidinones, covering literature developments from the earliest examples to the latest research. It is worth emphasizing that framework (MOF/COF/POP) materials provide an exceptional platform for in-depth investigation into the relationship between catalytic performance and the local reaction environment. By analyzing these interactions, researchers can gain critical insights that are essential for optimizing the design of next-generation efficient framework materials for industrial applications.^228,229 Despite significant advancements, substantial challenges remain in developing framework-based catalysts that catalyze CO₂ transformations from flue gas containing competing gases like nitrogen oxides (NOx) and sulfur oxides (SOx).^230,231 Also, achieving catalytic reactions under mild conditions is crucial—specifically, at low temperatures and atmospheric pressure conditions facilitating CO₂ transformation reactions from dilute gas or direct air.^232–234 To enhance the efficiency of these reactions, it is crucial to improve the CO₂ uptake capacity of framework materials.^235,236 Consequently, incorporating a high density of CO₂-philic sites leads to an increase in the local concentration of carbon dioxide around active sites, thereby promoting higher reaction yields.^237,238

Thus, a range of frameworks (MOFs/COFs/POPs) and their composites have been developed with diverse structural characteristics, such as Lewis acidic/basic and nucleophilic sites, to facilitate effective preparation of oxazolidinones under ambient conditions.^239,240 Further, the incorporation of alkynophilic metal NPs with the frameworks having heteroatoms has greatly enhanced both the adsorption of CO₂ and its conversion to oxazolidinones.^241,242 However, there is considerable potential for developing noble metal-free framework-based catalysts aimed at efficiently converting CO₂ into high-value oxazolidinone derivatives. In conclusion, framework-based materials hold a bright future as heterogeneous catalysts for CO₂ conversion to oxazolidinones and other value-added chemicals (Fig. 30). The present review provides valuable information for the future design of novel framework-based materials for effective capture and chemical fixation of CO₂ into high-value chemicals.

Fig. 30 Framework-based catalysts for chemical fixation of CO₂ to oxazolidinones.

Abbreviations

AA-MOFs	Amino acid linker-based MOFs
4-AMBA	4-Aminonethyl benzoic acid
ASP	Aspartic acid
5-atz	5-Amino-1H-tetrazole
BBA	Benzene-benzylamine
H4BCP	5-(2,6-Bis(4-carboxyphenyl)pyridin-4-yl)isophthalic acid
BDC	1,4-Benzenedicarboxylic acid
H2BDP-NH2	2-Amino-[1,4-bis(1H-pyrazol-4-yl)benzene]
BD	Biphenylamine
BPY	2,2′-Bipyridine-5,5′-dicarboxylate
Bpy	4,4′-Bipyridine
Bpy	2,2′-Bipyridien-4,4′-CHO
H3BTB	1,3,5-Tris(4-carboxyphenyl)benzene
1,2,4,5-BTMS	1,2,4,5-Benzenetetramethanesulfonate
H2btz	1,5-Bis(5-tetrazolo)-3-oxapentane
H3-BTC	1,3,5-Benzenetricarboxylic acid
CCSU	Carbon capture, sequestration and utilization
CCU	Carbon capture and utilization
CCS	CO2 capture and storage/sequestration
COP	Conjugated organic polymer
COFs	Covalent organic frameworks
CPT	3,5-Bis(4′-carboxyphenyl)-1,2,4-triazole
CTF	Covalent triazine framework
DABCO	1,4-Diazabicyclo[2.2.2]octane
H2DATP	4′-(3,5-Dicarboxyphenyl)-2,2′:6′,2′′′-terpyridine
DBU	1,8-Diazabicyclo[5.4.0]undec-7-ene
H2DCTP	4′-(3,5-Dicarboxyphenyl)-4,2′:6′,4′′-terpyridine
DEF	N,N-Diethylformamide
DFT	Density functional theory
2,3-Dha	2,3-Dihydroxyterephthalaldehyde
2,3-Dma	2,3-Dimethoxyterephthalaldehyde
DMA	N,N-Dimethylacetamide
DMF	N,N-Dimethylformamide
2,6-FPP	2,6-(4-Formylphenyl)pyridine
3,5-FPP	3,5-(4-Formylphenyl)pyridine
Glu	Glutamic acid
HBD	Hydrogen bond donor
HIN	Isonicotinic acid
IPA	Isophthalic acid
HL^1	Tetrazole monoanion
H4L^2	2′-Fluoro-[1,1′:4′,1′′-terphenyl]-3,3′′,5,5′′-tetracarboxylic acid
L^3	N ^1-(4-(1H-Imidazol-1-yl)benzyl)-N^1-(2-aminoethyl)-ethane-1,2-diamine
H3L^4	(2Z,2′Z,2′′Z)-2,2′,2′′-((((1,3,5-Triazine-2,4,6-triyl)tris(azanediyl))tris(benzene-4,1-diyl))tris(ethan-1-yl-1-ylidene))tris(hydrazine-1-carbothioamide)
H3L^5	(2Z,2′Z,2′′Z)-2,2′,2′′-((((1,3,5-Triazine-2,4,6-triyl)tris(oxy))tris(benzene-4,1-diyl))tris(methanylylidene))tris(hydrazine-1-carbothioamide)
HL^6	5-Aminonicotinic acid
L^7	5-Hydroxynicotinic acid
H2L^8	4,4′-(4-Amino-4H-1,2,4-triazole-3,5-diyl)dibenzoic acid
H2L^9	4,4′-((1E,1′E)-(2-Oxocyclohexane-1,3-diylidene)bis(methanylylidene))dibenzoic acid
MMPF	Metal–metalloporphyrin framework
MOFs	Metal–organic frameworks
NA	Not applicable
NHC	N-heterocyclic carbene
NPs	Nanoparticles
PD	p-Phenylenediamine
PCPs	Porous coordination polymers
PCN	Porous coordination network
POPs	Porous organic polymers
ppm	Parts per million
Ppa	p-Phenylenediamine
H4PTTB	1,3,6,8-Tetrakis(3-carboxyphenyl)pyrene
Py	Pyrene-NH2
RT	Room temperature
SBUs	Secondary building units
TAPT	Tris(4-aminophenyl)triazine
TBAB	Tetrabutylammonium bromide
TBD	1,5,7-Triazabicyclo[4.4.0]dec-5-ene
H10TBCPPP	Tetrakis-3,5-bis[(4-carboxy)phenyl]phenyl porphine
H2TCPP	Tetrakis(4-carboxyphenyl)porphyrin
TEA	Triethylamine
TFR-OT	Triformylresorsinol-O-tolidine
TFP	2,4,6-Trihydroxybenzene-1,3,5-tricarbaldehyde
Tta	4,4′,4′′-(1,3,5-Triazine-2,4,6-triyl)trianiline
TMG	1,1,3,3-Tetramethylguanidine
TON	Turnover number
TPyP-H2	5,10,15,20-Tetra(4-pyridyl)porphyrin
TPDC-NH2	2′-Amino-[1,1′:4′,1′′-terphenyl]-4,4′′-dicarboxylic acid
Tp	Triformylphenol
Tph	5,10,15,20-Tetrakis(4-aminophenyl)-21H,23H-porphine
H4Trz	Tri(1H-tetrazol-5-yl)methanol
TSP	2,2′,2′′-(Benzene-1,3,5-triyltris(ethan-1-yl-1-ylidene))tris(hydrazine-1-carbothioamide)
Tta	4,4′,4′′-(1,3,5-Triazine-2,4,6-triyl)-trianiline
XN	4′-(4-Pyridine)4,2′:2′,4′′-terpyridine
TABH	(E)-2-(4-((E)-(Thioureidomethylene) amino)benzylidene)hydrazinecarbothioamide
Oxdz	4,4′-(1,3,4-Oxadiazole-2,5-diyl)dibenzoate
Tpxn	N,N′,N′′,N′′′-tetrakis(2-pyridylmethyl)-1,4-diaminooxylylene

Data availability

Data will be available on request.

Conflicts of interest

The authors declare that they have no competing financial interests.

Acknowledgements

C. M. N. acknowledges DST-SERB (CRG/2018/001176) for financial support. PR thanks DST-SERB (PDF/2023/000057) for funding in the form of NPDF.

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Footnote

† Current Address: Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585, Singapore.

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