Co(II)-embedded covalent organic framework for catalyzing CO₂ fixation to highly valuable N-formamides and 2-oxazolidinones under mild conditions

Abstract

Turning carbon dioxide (CO₂) into useful substances, such as fuel and chemicals, offers a clear way to lower the amount of this greenhouse gas in the environment. In the present scenario, COFs (covalent organic frameworks) provide tremendous potential for CO₂ fixation. Accordingly, a hydroxyl-rich EtDh-COF was constructed through Schiff-base condensation using 4,4′,4″,4‴-(ethene-1,1,2,2-tetrayl)tetraaniline (Et-NH₂) with 2,5 dihydroxyterephthalaldehyde (Dh-CHO). The prepared EtDh-COF possesses an abundant number of imine and hydroxyl groups, facilitating the easy grafting of non-noble Co(II) ions to acquire a Co-COF, which was subsequently applied to formylate various amines to valuable N-formamides utilizing CO₂ as a C-1 feedstock with phenylsilane as the reductant. Moreover, the effective conversion of propargylic amines into 2-oxazolidinones, valuable bioactive commodity chemicals, has also been accomplished by consuming carbon dioxide. Notably, the Co-COF displayed excellent catalytic activity for producing formamides and 2-oxazolidinones, which serve as crucial precursors of pharmaceuticals, agrochemicals, and other bioactive substances. Moreover, the Co-COF can be easily recycled for multiple runs without losing scaffold stability and catalytic activity. Thus, this work demonstrates the sensible design of a Co-COF for the efficient use of CO₂ to produce two useful products, N-formamides and 2-oxazolidinones, under mild conditions.

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Introduction

Over the last few decades, the widespread use of fossil fuels has caused atmospheric CO₂ to rise at a rapid pace, resulting in adverse phenomena like extreme weather patterns, rising sea levels, global climate change, and ocean acidification.¹ In the present scenario, selective carbon capture, storage and utilization (CCSU) is a promising approach for lowering carbon dioxide concentrations in the atmosphere while producing highly valuable substances.^2–13 Of all the transformations of CO₂ reported in the literature, cycloaddition reactions providing cyclic carbonates and α-alkylidene carbonates from epoxides and propargylic alcohols are the most intriguing in recent years.^14–22 Compared to other products of the pathways of CO₂ fixation, the reduction of CO₂ to alternative valuable N-formamides has been less explored.

Notably, N-formamides are widely employed for the manufacturing of medicines, agrochemicals, dyes, gasoline additives, fragrances, and natural products.^23,24 Moreover, these formamides also serve as useful intermediates in the Leuckart and Vilsmeier–Haack reactions, needed for the commercial production of important compounds.^25,26 In most cases, amines are N-formylated using CO₂ with reducing agents such as hydrogen (H₂)^27–31 or phenylsilanes.^32–36 These reductants facilitate the activation of CO₂, enabling its transformation into a formylating species that reacts with amines to produce N-formylated products. The usage of H₂ necessitates relatively harsher reaction conditions, which include high pressure and temperature.^37–39 In contrast, the more polar and weaker Si–H bond of hydrosilanes compared to the H–H bond usually necessitates gentler reaction parameters for amines’ N-formylation with hydrosilanes, making them a more convenient reducing agent for this transformation.⁴⁰ However, as CO₂ is kinetically inert and thermodynamically stable, it makes the process of CO₂ fixation challenging.⁴¹ Hence, an extremely efficient catalyst design is necessary to capture and convert CO₂ to N-formamides. In this regard, various catalytic systems, including noble metal (Pd, Ru, Ir, Pt, and Rh)-based homogeneous^42–47 and heterogeneous^48–51 systems, are reported for N-formylation, often at high temperature or elevated CO₂ pressure. Consequently, developing non-noble metal-based systems effective for N-formylation under ambient conditions is highly desirable.

On the other hand, the transformation of propargylic amines with CO₂ provides a sustainable and atom-economical route to 2-oxazolidinones, contributing to carbon dioxide valorization and green chemistry initiatives.⁵² Notably, these oxazolidinones are a crucial class of heterocyclic substances that serve a major function as commodity chemicals in the production of many organic molecules. Moreover, they serve as crucial components of multiple drugs and agrochemicals.^53–60 Consequently, the formation of oxazolidinones and their derivatives has garnered substantial interest. Traditional synthesis approaches of 2-oxazolidinones typically involve the oxidative carbonylation of 2-amino alcohols with hazardous phosgene and carbon monoxide (CO) or the cycloaddition of ketenes with dipolar molecules.^61–64 However, these approaches are hindered by the use of toxic chemicals. Since CO₂ is a plentiful C1 resource, its chemical fixation has been increasingly explored as a means to produce high-value compounds, contributing to carbon neutrality.^65–67 Despite this, most CO₂-based syntheses of 2-oxazolidinones rely on catalysts containing noble (Ag, Ru, Pd) or toxic (Cd) metals and frequently require high pressure and temperature.^68–75 Thus, it is highly desirable to create efficient heterogeneous catalysts free of precious metals for the mildly reactive synthesis of 2-oxazolidinones.

In this context, COFs offer promise for capturing and utilizing carbon dioxide under mild conditions due to their flexible synthesis, facilitating the rational integration of catalytically active and CO₂-philic sites. Furthermore, the application of reticular synthesis facilitates the rational construction of COFs with desired properties.^76–79 With these objectives in mind, we establish the utilization of a 2D EtDh-COF featuring a high density of hydroxyl groups and imine nitrogen sites, enabling the facile grafting of non-noble Co(II) ions to obtain a Co-COF. Here, Co(II) centers act as catalytically active sites owing to their appropriate Lewis acidity, flexible coordination environment, and proven capability to activate the Si–H bond of silane and the alkyne C–C bond, which are key steps in N-formylation and propargylic amine cyclization reactions.^80,81 Therefore, a Co(II)-embedded COF was utilized for N-formylation of amines with CO₂, providing N-formamides in excellent yields and >99% selectivity. Furthermore, the Co(II)-anchored COF (Co-COF) exhibited remarkable catalytic activity upon employing CO₂ and propargylic amines to produce 2-oxazolidinones, demonstrating exceptional catalytic performance for both the transformations under atmospheric pressure. Furthermore, the catalyst displayed good reusability, sustaining its activity over numerous cycles. This study underscores the potential of a noble-metal-free Co(II)-embedded, hydroxyl-functionalized COF as an effective catalyst for synthesizing two highly valuable products, i.e., N-formamides and 2-oxazolidinones, via efficient CO₂ coupling with amines under mild conditions.

Results and discussion

The EtDh-COF was produced via a condensation reaction of ET-NH₂ and Dh-CHO in 1,4-dioxane at 120 °C (Scheme 1, more details are provided in the SI).⁸² The FT-IR (Fourier-transform-infrared) spectrum of the EtDh-COF showed the absence of stretching bands at 3360 cm⁻¹ and 1660 cm⁻¹, assigned to the free –NH₂ of ET-NH₂ and the –C=O of Dh-CHO, respectively, confirming the use of the initial materials. Moreover, a new peak at 1619 cm⁻¹ emerged, indicating the formation of an imine (–C=N–) linkage between the precursors, suggesting the successful production of the COF (Fig. 1a).⁸² Additionally, the ¹³C solid-state Cross Polarisation Magic Angle Spinning (CP-MAS) NMR (Nuclear Magnetic Resonance) spectrum of the EtDh-COF displayed a characteristic carbon peak at 161 ppm, attributed to the carbon of the –C=N– bond, further validating the production of the COF (Fig. 1b).⁸² Additionally, extra carbon peaks owing to phenolic and ethylene were detected in the 110–155 ppm region (Fig. 1b). Furthermore, the EtDh-COF was modified post-synthetically by anchoring Co(II) onto the imine N and hydroxyl sites to obtain the Co-COF (Scheme 1, more details are provided in the SI).⁸³

Scheme 1 The synthetic route for the formation of EtDh-COF and Co-COF.

Fig. 1 (a) FT-IR spectra of Et-NH₂ (i), Dh-CHO (ii), EtDh-COF (iii), and Co-COF (iv), (b) ¹³C solid-state NMR spectrum of EtDh-COF, (c) comparison of theoretical and experimental P-XRD patterns of EtDh-COF, and (d) TGA plots for EtDh-COF (i) and Co-COF (ii).

The crystalline structures of the EtDh-COF and Co-COF were determined from powder X-ray diffraction (PXRD), structural models, and Pawley refinement. The PXRD patterns of the EtDh-COF and Co-COF showed a strong lower-angle peak at 2.74° attributed to the (100) plane, validating the crystallinity and long-range ordering of COFs, and additional minor peaks were observed at 4.85°, 5.42°, and 8.16° corresponding to the (110), (200), and (300) lattice planes of the EtDh-COF and Co-COF, confirming their high crystallinity (Fig. 1c and Fig. S2). The experimental PXRD patterns of the EtDh-COF and Co-COF fit well with the predicted diffraction data obtained based on the aligned AA stacking conformation with a kagome (kgm) topology in the P6 space group with the unit cell parameters of a = 38.9 Å, b = 37.9 Å, c = 5.2 Å, and angles α = β = 90°, γ = 120° (Fig. S1, Tables S1 and S2).

The FT-IR spectra of the Co-COF revealed similar characteristics to the pristine COF, with the imine peak remaining intact, confirming that the incorporation of Co(II) in the EtDh-COF does not affect its chemical structure (Fig. 1a). Moreover, thermogravimetric analysis (TGA) of the EtDh-COF and Co-COF demonstrated thermal stability up to 450 °C and 430 °C, respectively (Fig. 1d). The structural and thermal stability of the EtDh-COF was further evaluated by heating the sample under vacuum at 50, 100, 150, and 200 °C. The PXRD analysis of the thermally-treated samples confirms the retention of the framework structure, demonstrating the thermal stability of the COF (Fig. S3). Furthermore, MP-AES (Microwave Plasma-Atomic Emission Spectroscopy) revealed a Co(II) content of 1.95 wt% in the COF (Fig. S4). The FE-SEM (field emission-scanning electron microscopy) and HR-TEM (high-resolution transmission electron microscopy) images of the EtDh-COF exhibited an assembly of irregular layered morphology (Fig. S5a and S6a), which was nearly intact even after embedding Co(II) ions (Fig. S5b and S6b). Furthermore, energy dispersive X-ray spectroscopy (EDS) analysis and elemental mapping of the EtDh-COF and Co-COF revealed the existence of constituent elements (Fig. S7–S9).

Gas sorption analysis

The N₂-sorption isotherms were measured to evaluate the porous nature of COFs. Before sorption analysis, both EtDh-COF and Co-COF were degassed overnight at 120 °C. The N₂ sorption isotherms disclosed a type-II plot, with a BET (Brunauer–Emmett–Teller) surface area of 1379 m² g⁻¹ for the pristine COF (Fig. 2a). Moreover, the surface area of the Co-COF was reduced to 889 m² g⁻¹ due to the incorporation of Co(II) ions in the COF (Fig. 2a). The relatively lower BET surface area of the Co-COF arises from partial pore blockage after the incorporation of Co(II) in the EtDh-COF (Fig. S10). Furthermore, the CO₂-sorption plots of the COFs displayed type-I plots, with uptake values of 63.3/40.7 cm³ g⁻¹ for the EtDh-COF and 45.9/32.3 cm³ g⁻¹ for the Co-COF at 273 K/298 K, respectively (Fig. 2b). The Freundlich–Langmuir equation was applied to validate CO₂-sorption plots and accurately predict CO₂ uptake at saturation (Fig. S11 and S12). Additionally, the isosteric heat of adsorption (Q_st) for CO₂ in the EtDh-COF and Co-COF, estimated using the Clausius–Clapeyron equation, was discovered to be 23.88 and 31.05 kJ mol⁻¹, respectively (Fig. S11 and S12). The relatively high Q_st value of the Co-COF can be attributed to the strong CO₂ interaction at Co(II) ions located within the pores of the COF.

Fig. 2 (a) N₂ sorption isotherms (carried out at 77 K) for EtDh-COF (i) and Co-COF (ii). (b) CO₂ sorption plots of EtDh-COF (i and iii) and Co-COF (ii and iv), carried out at 273 K and 298 K, respectively.

X-ray photoelectron spectroscopy (XPS) analysis

XPS investigations were conducted to examine the elemental composition of COFs. The survey scan confirmed the existence of the component elements of both COFs (Fig. 3a). Moreover, the N 1s spectra of the EtDh-COF (Fig. 3b) disclosed a single peak with a binding energy (BE) of 399.1 eV, attributed to the imine N present in the COF. In contrast, the deconvoluted N 1s spectra of the Co-COF (Fig. 3c) revealed an extra BE peak at 398.6 eV, corresponding to imine N coordinated with Co(II). The 0.5 eV shift in BE for N–Co relative to free imine N further supports the interaction of Co(II) at the imine N sites.⁸³ Similarly, the O 1s spectra of the pristine COF (Fig. 3d) featured a single peak at 532.9 eV, associated with the hydroxyl groups in the COF. However, the deconvoluted O 1s spectra of the Co-COF (Fig. 3e) revealed an additional BE peak at 532.3 eV, corresponding to hydroxyl O coordinated with Co(II). The 0.6 eV negative shift in BE for hydroxyl O upon Co(II) coordination further confirms the presence of Co(II) at the hydroxyl sites of the COF.⁸³ The Co 2p XPS spectra displayed two prominent peaks at 781.2 eV and 796.8 eV, corresponding to Co 2p_3/2 and Co 2p_1/2, respectively (Fig. 3f).⁸³ Thus, the XPS studies unambiguously validate the successful affixing of Co(II) at both the imine N and hydroxyl units of the EtDh-COF.

Fig. 3 XPS plot of the COFs (a) Full survey scan of EtDh-COF (i) and Co-COF (ii). (b and c) N 1s spectra of EtDh-COF and Co-COF. (d and e) O 1s spectra of EtDh-COF and Co-COF, and (f) Co 2p spectra of Co-COF.

Reductive functionalization of CO₂ to N-formamides

The co-existence of both basic CO₂-philic sites and Lewis acidic Co(II) sites encouraged us to assess the catalytic performance of the Co-COF for the production of highly valuable N-formamides. The optimization reaction was tuned by adjusting the reaction temperature, duration, and catalyst. Initially, N-methylaniline was employed as a model amine, Co-COF as the catalyst, dry DMF as the solvent, and phenyl silane (PhSiH₃) as the reducing agent (Table S3). The reaction kept at RT for 6 h and 9 h with a CO₂ balloon yielded 51% and 72% N-methylformanilide, respectively (Table S3). However, the increase of temperature to 50 °C within a reaction time of 9 h showed a reduced yield of 55%, suggesting that higher temperatures are responsible for unwanted side reactions, favouring N-methylated product formation (Fig. S13 and S14). In addition, the extension of the reaction time to 12 h at RT showed a quantitative conversion with a 96% isolated yield, confirming that RT and 12 h are the optimal reaction conditions (Table S3). The study of reaction parameters, including the selection of solvents, was also carried out (Table S3). Out of all the solvents studied, DMF showed the highest product formation. A trace amount of N-methylformanilide was obtained from additional control experiments carried out with EtDh-COF as the catalyst, confirming the pivotal role of Co(II) in activating the Si–H bond of phenyl silane and further facilitating the functionalization of CO₂ to form formoxysilane (Table S3). In addition, the formation of N-methylformanilide was reduced to 42% using Co(OAc)₂·4H₂O as the catalyst under optimal reaction conditions, suggesting that the catalytic ability of the Co-COF is much higher than that of its homogeneous counterpart (Table S3). The synergistic effect arising from CO₂-philic sites and catalytically active Co(II) centres makes this easier and gives an effective result for carbon dioxide conversion under ambient conditions. The reaction was performed under an inert N₂ atmosphere to check the crucial role of CO₂, as there was no production of N-methylformanilide, confirming that CO₂ is crucial for the reaction (Table S3).

Another piece of evidence is the importance of phenyl silane as a reducing agent in this process, since no product was observed in its absence (Table S3). Prompted by the excellent catalytic performance of the Co-COF, we explored the N-formylation of various amines, affording exquisite yields under optimal conditions (Table 1, Table S4 and Fig. S15–S32). For further study, the CO₂ fixation ability of the Co-COF was also assessed using diluted gas with a dry flue gas composition (N₂ : CO₂ = 87 : 13%). Indeed, the Co-COF catalyzed the 45% conversion of N-methylaniline to N-methylformanilide within 12 h (Fig. S33), which increased to 92% in 24 h under optimal conditions. This validates the importance of the Co-COF for using CO₂ from dry flue gas. Moreover, a comparative analysis with previously reported catalysts demonstrated the superior catalytic activity of the Co-COF (Table S5), underscoring its potential as a proficient and sustainable non-noble metal green catalyst.

Table 1 Substrate scope for the N-formylation reaction of amines with CO₂ catalyzed by Co-COF^a

a Reaction conditions: amine (1 mmol), PhSiH₃ (2 mmol), catalyst (20 mg), dry DMF (2 mL), RT (25 °C), CO₂ (balloon), time (12 h). b Isolated yield. c TON = moles of product/moles of active metal sites. d Simulated flue gas (N₂ : CO₂ = 87 : 13%). e Time (18 h). f Time (24 h).

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Mechanistic study for the reductive functionalization of CO₂ into N-formamides

Based on the experimental studies and previously reported literature, a mechanistic route for the transformation of amines into N-formamides is illustrated in Scheme 2. The reaction proceeds through the activation of the Si–H bond in PhSiH₃ through coordination interaction in an η²-(Si–H) mode at the Co(II) sites.⁸¹ To establish the viability of this process, the EtDh-COF and Co-COF were exposed to phenyl silane for 3 h with constant stirring, then isolated, and washed extensively with methanol/acetone before vacuum drying overnight.

Scheme 2 A probable mechanistic route for the reductive functionalization of CO₂ into N-formamides.

The FT-IR examination of the silane-treated COFs displayed stretching bands at 2165 cm⁻¹ in the case of the Co-COF, while no similar bands were identified in the EtDh-COF, demonstrating Si–H bond activation at the Co(II) sites (Fig. S34). Furthermore, this interaction weakens the Si–H bond, aiding its cleavage via a hydride shift from phenyl silane to CO₂, forming a stronger Si–O bond upon CO₂ insertion, resulting in the generation of formoxysilane, which is in line with the mechanistic studies reported in the literature (Fig. S35).⁸⁴ Simultaneously, the formoxysilane endures a nucleophilic attack by the amine, leading to the desired formamide product. Finally, the elimination of formamide regenerates the catalyst, permitting its reuse in subsequent catalytic cycles.

Catalytic conversion of propargylic amines to 2-oxazolidinones with the utilization of CO₂

Following the remarkable catalytic performance of the Co-COF for N-formylation, its scope for CO₂ fixation with propargylic amines was studied. Initially, various propargylic amine derivatives were synthesized following a reported procedure (Scheme S1) and confirmed by the ¹H NMR technique (Fig. S36–S45).⁸⁵ The reaction was optimized by modifying the temperature, duration, and catalysts. N-Benzylprop-2-yn-1-amine was chosen as the propargylic amine for the model reaction, Co-COF as a catalyst, DMSO as a solvent, and DBU as a base to aid in the deprotonation of amine (Table S6). Propargylic amine was converted to 2-oxazolidinone by 38% after the reaction was run at RT for 6 h under atmospheric CO₂ pressure (balloon) (Table S6). Additionally, raising the temperature to 40 °C led to a 52% conversion of propargylic amine, whereas prolonging the reaction for 9 h at this temperature produced a 74% conversion (Table S6). The total conversion of propargylic amine could be achieved by further increasing the reaction time to 12 h at a constant temperature of 40 °C, thereby confirming the Co-COF's efficacy in the synthesis of oxazolidinones (Table S6). The impact of reaction conditions was also examined for solvent selection (Table S6). Of all the solvents screened, DMSO yielded the maximum product (Table S6). Further control reactions under optimized conditions involving EtDh-COF as a catalyst resulted in no conversion, thereby confirming that Co(II) plays an essential role in activating alkyne functionality in propargylic amine (Table S6). The use of Co(OAc)₂·4H₂O as a catalyst provided only 47% conversion of propargylic amine under optimized conditions (Table S6). The most likely reason for the Co-COF's increased catalytic activity compared to a metal salt may be attributed to the presence of CO₂-philic and catalytically active Co(II) sites that have a synergistic role in enhancing CO₂ conversion under mild conditions.

Additionally, catalysis was carried out under an inert N₂ atmosphere to further examine the significance of CO₂, as no 2-oxazolidinone could be detected, demonstrating the critical role of carbon dioxide in the reaction (Table S6). Encouraged by the Co-COF's remarkable catalytic activity, its utilization was expanded to test the transformation of several propargylic amines (Table 2). Remarkably, the Co-COF effectively catalyzed several propargylic amines to their corresponding 2-oxazolidinone products, producing high yields under optimized conditions (Table S7 and Fig. S46–S65). Furthermore, the CO₂ fixation ability of the Co-COF was studied using simulated flue gas (N₂ : CO₂ = 87 : 13%). Remarkably, the Co-COF catalyzed 48% conversion of propargylic amine (Fig. S66), which increased to 98% in 24 h under optimized conditions. This emphasizes the significance of the Co-COF in efficiently using dilute CO₂ to yield useful 2-oxazolidinones. Moreover, the catalytic activity of the Co-COF was found to be on par with or superior to previously reported catalysts (Table S8).

Table 2 Cyclic carboxylation of propargylic amines with CO₂ catalyzed by Co-COF^a

a Reaction conditions: propargylic amine (1 mmol), catalyst (20 mg), DBU (0.1 equiv.), DMSO (2 mL), temperature (40 °C), CO₂ (balloon), time (12 h). b Determined by ¹H NMR analysis. c TON = moles of product/moles of active metal sites. d Simulated flue gas (13 : 87% = CO₂ : N₂). e Time (24 h).

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Mechanistic study for the transformation of propargylic amines to 2-oxazolidinones

A probable mechanistic route for the formation of 2-oxazolidinones is illustrated in Scheme 3. The reaction commences with the activation of the alkyne bond in propargylic amine at the Co(II) ions,^80,86 while CO₂ is activated at the basic nitrogen sites within the COF. To assess the viability of this, both EtDh-COF and Co-COF were reacted with amine under constant stirring for 3 h, followed by separating the COFs, which were completely washed with acetone/methanol before vacuum drying overnight. The FT-IR analysis of the amine-treated COFs revealed stretching bands at 2110–2120 cm⁻¹, indicating Co(II) coordinated alkyne, whereas no similar bands were detected in the EtDh-COF, confirming alkyne polarization at the Co(II) sites (Fig. S67). Concurrently, propargylic amine undergoes deprotonation by DBU, leading to CO₂ insertion and the establishment of a carbamate intermediate, which undergoes intramolecular cyclization to yield a 2-oxazolidinone intermediate that takes the proton from the protonated DBU to form the desired 2-oxazolidinone product. Finally, the product is liberated from the Co(II) centre, which regenerates the catalyst for further cycles (Scheme 3).

Scheme 3 A probable mechanistic route for the formation of 2-oxazolidinones.

Catalyst recyclability and a leaching test

To assess the Co-COF's reusability, it was isolated, extensively washed, and vacuum-dried before reuse in subsequent catalytic cycles. Then, the recovered catalyst was weighed to determine the sample yield. The mass recovery of the Co-COF reveals that >95% of Co-COF was recovered in each of the five cycles, verifying that minimal physical loss occurred during handling. The recovered catalyst was activated and reused in subsequent cycles, which remained highly active for five successive cycles with a negligible drop in the yield of N-methylformanilide (Fig. 4a and Fig. S68). In order to confirm the reused catalyst's structural stability, FT-IR, P-XRD, HR-TEM, and XPS analyses of the reused Co-COF were performed, which are in good agreement with the pristine material, confirming preservation of its framework structure (Fig. S69–S72). Furthermore, a leaching test was performed by discontinuing the reaction after 3 h, at which point the catalytic yield reached 22%. The Co-COF was then removed, and the reaction persisted for another 9 h. However, no further increment in N-methylformanilide yield was detected, indicating no leaching of Co into the solution (Fig. 4b). Moreover, to confirm the stability and heterogeneity of this catalyst, MP-AES analysis of the reaction filtrate was performed, which revealed negligible content of Co(II), demonstrating the heterogeneity and robustness of the Co-COF under catalytic conditions.

Fig. 4 (a) Recyclability test and (b) leaching test of the Co-COF for the N-formylation reaction of amines.

Conclusion

In summary, the strategic design of a hydroxyl-functionalized porous covalent organic framework (EtDh-COF) for the facile integration of noble metal-free catalytically active Co(II) ions to form a Co-COF for the efficient functionalization of CO₂ to produce two highly valuable chemicals is presented. The Co-COF exhibited remarkable catalytic efficiency, facilitating the conversion of various amines into highly valuable N-formamides along with the transformation of various propargylic amines into bioactive 2-oxazolidinones. The enhanced catalytic performance of the Co(II)-functionalized COF, compared to its individual components (Co(OAc)₂·4H₂O and EtDh-COF), is attributed to the synergistic outcome between Co(II) and the CO₂-philic sites present inside the pores of the framework. Additionally, the Co-COF demonstrated excellent structural stability and recyclability while retaining catalytic efficacy over multiple cycles. Therefore, this study provides a unique illustration of a sustainable, non-noble metal COF-based catalyst that effectively uses CO₂ to produce two valuable compounds under mild reaction conditions.

Author contributions

Vaibhav Parihar: conceptualization, methodology, and writing – original draft. Shubham Kumar: N₂ and CO₂ adsorption–desorption analysis. Dr Pooja Rani: review and editing draft. Prof. C. M. Nagaraja: supervision and writing – review and editing. This manuscript was written with the contribution of all authors, and they have approved the final version.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

The supporting data of this article have been uploaded as part of the supplementary information (SI). Supplementary information (SI): NMR plots for all compounds and catalysis procedures, FT-IR plots, PXRD patterns, EDS plots, FE-SEM and HR-TEM images. See DOI: https://doi.org/10.1039/d5qi02385h.

Acknowledgements

CMN acknowledges ANRF (CRG/2022/006762) for funding. VP thanks PMRF (PMRF ID-2902502) for the fellowship. SK acknowledges CSIR-UGC for SRF and P. R. acknowledges SERB for the award of NPDF (File no. PDF/2023/000057). The authors thank CRF, IIT Ropar, for solid-state NMR data.

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