CO₂ conversion efficiently catalyzed under ambient conditions by new self-supported and recyclable Cu₂S

Abstract

The catalytic activation of CO₂ under mild conditions to form value-added heterocyclic products is of paramount interest. Copper sulfides are a large class of materials, many of which are found as minerals in nature. They often show remarkable physical properties, and further studies of their catalytic properties need to be conducted. The simple compound Cu₂S was designed, its synthesis was improved, and its catalytic activities in CO₂ activation are disclosed here. Synthesized Cu₂S is shown to be an efficient heterogeneous catalyst for the carboxylation of propargylamines under ambient temperature and pressure conditions and the carboxylation of terminal alkynes under one atm CO₂ at 80 °C. CO₂ is transformed into high-value-added chemicals with a wide range of substrates with excellent yields, and the catalyst is easily reused at least 5 times without significant activity reduction. The catalytic and recycling performances are significantly better for synthesized Cu₂S compared to those of its commercial analogue. In summary, Cu₂S is a new catalyst that is simple, non-toxic, self-supported, recyclable, and practical. These findings open a general route to the catalytic properties of Cu₂S toward, not only mild CO₂ activation applications, but also a variety of other useful reactions.

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Introduction

Cu₂S, as an important semiconductor material, has shown properties and application potential in photocatalysis and electrocatalysis in recent years.^1–3 Cu₂S is a narrow bandgap semiconductor^4–6 with high conductivity involving fast electron transfer, and it performs well in electrocatalytic reduction and oxidation reactions.^7–9 The abundant active sites on the surface of Cu₂S (such as Cu⁺, S²⁻, etc.) help with adsorbing reactant molecules and reducing activation energy, thereby improving catalytic efficiency.^10–12 In this work, Cu₂S is used to catalyze the carboxylation of propargylamines and terminal alkynes with CO₂. Fixation of propargylamines and terminal alkynes with CO₂ produces oxazolidinone derivatives and propiolic acids, respectively, which are fine chemicals for the pharmaceuticals industry.¹³

Most of the known catalysts for carboxylation reactions are based on noble metals,^14,15 but, in order to achieve widespread application, the use of such expensive and rare noble metals should as much as possible be avoided.^16–18 In recent years, researchers have explored the application of various catalysts with propargylamines and in other carboxylation reactions.^19–23 For example, Qiao²⁴ developed a Cu₆–NH₂ nanocluster with a distorted octahedral Cu₆ core and NH₂-functionalized ligands, which catalyzes the cyclization of propargylamines with CO₂ under mild conditions. Cu₆–NH₂ exhibits good catalytic activity, due to its accessible active sites. The nanocluster also exhibits excellent stability, good recrystallization, and reusability, with no significant decrease in catalytic performance over five catalytic cycles. However, there are problems such as complex synthesis process and possible reaction of NH₂ sites with impurities. Kong²⁵ developed a long-term stable copper catalyst coordinated with sugar acetate (acetylglucose) and 2-methylimidazoline. This catalyst has multiple active sites, enabling it to efficiently capture and simultaneously activate CO₂ and propargylamines. Its turnover frequency is as high as 880 h⁻¹, which is about 180 times that of CuCl₂, and the catalytic activity of Im–Cu A did not decrease after 20 repeated reactions. However, there is a potential problem of uneven distribution of active sites.^26–28 Qiu²⁹ proposed an efficient and stable Cu^I-anchored porous covalent organic framework Cu^I@TpBD-COF, in which TpBD is a β-ketoamine with carbonyl and amine group (–NH₂, –NHR or –NR₂), with these two groups separated by two carbon atoms. This organic framework is used to effectively catalyze the carboxylic cyclization reaction of propargylamines with CO₂ and to synthesize various functionalized oxazolidinone derivatives under mild conditions with yields of up to 99% and a turnover frequency (TOF) of 1058 h⁻¹. In addition, when simulated flue gas was used as the CO₂ source, Cu^I@TpBD-COF also showed good catalytic activity. However, the synthesis cost of COF materials is high.^30–32 Gu³³ loaded nano-Cu₂O on a ZIF-8 support to prepare Cu₂O@ZIF-8 catalyst, which can effectively catalyze the cyclization of propargylalcohols and propargylamines with CO₂ to produce valuable α-alkylene cyclic carbonates and oxazolidinones, with turnover number (TON) values of 12.1 and 19.6, respectively, and this catalyst was recycled at least 5 times. However, Cu₂O is toxic, and its TON is low.^34–36

Despite the performances of these Cu-based systems in the cyclization of propargylamines with CO₂, these catalysts often suffer from structural complexity, high synthesis costs, toxicity issues (e.g., Cu₂O), instability under ambient conditions, and limited scalability.^37–40 To address these challenges, we have developed a Cu₂S catalyst that offers a promising alternative. Cu₂S is a low-cost, environmentally friendly, structurally stable, and easy to synthesize material under mild conditions. More importantly, it provides easily accessible active sites, exhibiting high catalytic efficiency and recyclability in the cyclization of propargylamines with CO₂ even under mild conditions. This work highlights the role of Cu₂S as a powerful and sustainable copper-based catalyst platform for green CO₂ conversion chemistry. The synthesis and their effectiveness in promoting this environmentally beneficial transformation will be discussed. By harnessing the potential of such a catalyst system, this approach represents a significant step forward in the utilization of CO₂ as a valuable chemical feedstock.

Results and discussion

The preparation procedure for Cu₂S, which is based on a reported hydrothermal method^41,42 with some improvements, is described below in the Experimental section. For the identification of the Cu₂S product, XRD analysis was performed. The resultant XRD pattern is shown in Fig. 1. The diffraction pattern aligns well with the standard card of Cu₂S (ICDD PDF# PDF 00-002-1292).⁴³ The characteristic peaks located at approximately 27°, 32° and 46° 2θ are consistent with the standard data. No peaks of Cu(II) impurities (such as CuO or CuS) were detected, confirming that the obtained product is pure Cu₂S. It is worth noting that the diffraction peaks are relatively broad, which is indicative of the small crystallite size and the nanostructured nature of the synthesized Cu₂S. The absence of other sharp peaks confirms the high purity of the phase.

Fig. 1 XRD pattern of the synthesized Cu₂S.

To investigate the valence state of copper in the present catalyst, which is crucial for the rationalization and understanding of the catalytic activity,^44–48 XPS measurements were performed, and the results are shown in Fig. 2. The Cu 2p XPS of synthesized Cu₂S both before and after the catalytic carboxylation reaction, as presented in Fig. 2(a) and (b), respectively, show two spin–orbit doublets with Cu 2p_3/2 components at 932.4 eV (bulk monovalent Cu) and 933.2 eV (surface monovalent Cu), which indicate that Cu in the synthesized Cu₂S exclusively exists in the form of Cu(I).^49–52 Emergence of the surface Cu(I) is ascribed to the presence of surface-confined Cu-deficient Cu₂S domains wherein Cu–S covalency is weakened. The XPS data also obtained after the catalytic carboxylation reaction show that copper remains in the Cu(I) state. However, for commercial Cu₂S (Fig. 2c and d), it was found that, in addition to the two bands around 932.5 eV and 952.4 eV for Cu(I), there were also two discernible Cu 2p spin–orbit doublets indicating the presence of Cu(0) and Cu(II) after catalysis. Therefore, the XPS results thoroughly demonstrate that the synthesized Cu₂S has a high Cu(I) purity, including after recovery from the catalytic reaction, while having a good recycling performance. The Cu XPS analysis results are summarized in Table S1.

Fig. 2 High-resolution Cu 2p XPS of synthesized Cu₂S before (a) and after (b) the catalysis, and of commercial Cu₂S before (c) and after (d) catalysis. Atomic contributions of Cu, Cu⁺, and Cu²⁺ to Cu 2p are reported in each panel.

Fig. S1(a) shows the valence band XPS of synthesized and commercially available Cu₂S before and after catalysis. Only the profiles of synthesized Cu₂S and recycled synthesized Cu₂S resemble the profile of the reference Cu₂S.⁵³ Fig. S1(b) shows the Auger signals for Cu, Cu LMM. Table S2 lists the Auger parameters of synthesized and commercially available Cu₂S before and after catalysis. Analysis of this parameter and inspection of peak shapes allow for assignment of copper species.⁵⁴ The synthesized Cu₂S and recycled synthesized Cu₂S respectively retained Auger parameters of 1849.93 eV and 1849.96 eV, values that are consistent with the reference Cu₂S with its Auger parameter of 1849.84 eV,⁵⁴ indicating that Cu is in the monovalent state, Cu(I), in the synthesized Cu₂S and recycled synthesized Cu₂S. However, the other entries reveal values that differ from that of Cu₂S. To demonstrate the chemical state similarities between the reference Cu₂S and synthesized Cu₂S of this work, a Cu 2p_3/2 vs. Cu LMM Wagner (chemical state) plot is presented in Fig. S1(c). Only synthesized Cu₂S and recycled synthesized Cu₂S fall within the vicinity of the reference Cu₂S and along the diagonal line corresponding to the constant Auger parameter of 1850 eV. This confirms the sole presence of Cu(I) in synthesized Cu₂S and recycled synthesized Cu₂S. The shift of positions along the 1850 eV diagonal line can be attributed to changes in local charge density, particularly in the case of recycled synthesized Cu₂S due to presence of some nitrogen species with total atomic content of 3.15%. Another crucial difference is revealed from the analysis of the S 2p XPS, as shown in Fig. S2 and summarized in Table S3. The synthesized Cu₂S before and after catalysis retains a much larger proportion of lattice S compared to S oxide. The contrary situation is observed in the case of commercial Cu₂S before and after catalysis, where the majority of S is in the form of S oxide. Furthermore, from the S 2p XPS analysis of synthesized Cu₂S and recycled synthesized Cu₂S, we were able to probe two lattice S 2p spin–orbit doublets, corresponding to bulk and surface lattice sulfides. The bulk sulfide components have characteristically smaller full-width at half maximum (FWHM) and binding energy values when compared to the surface sulfide components. The bulk lattice sulfide exists within more crystalline domains, with higher electron delocalization, stronger Cu–S covalency, and stronger electron (back-)donation due to the higher coordination state of sulfur in the bulk as compared to the lower coordination state of sulfur in surface-bound Cu-deficient Cu₂S. It is important to note that the bulk lattice sulfide contribution increases by 10.3 atomic%, while the surface lattice sulfide contribution decreases by 7.2 atomic% after recycling the synthesized catalyst. In addition, the Cu(I) bulk contribution increases by 16.3 atomic%, while the Cu(I) surface contribution decreases by 16.3 atomic% after recycling the synthesized catalyst. This indicates that the catalytic and recycling processes diminish the surface Cu-deficient layer bringing the catalyst closer to the stoichiometric Cu-to-S atomic ratio (Cu/S) of 2 (note the Cu/S values in Table S2). This can also serve as an explanation for the shift of recycled synthesized Cu₂S along the 1850 eV Auger parameter diagonal line shown in Fig. S1(c). The collective analysis of the Auger signal, valence band, and sulfur chemical environment, indicates that commercial Cu₂S is more susceptible to ambient oxidation and hydrolysis, both before and after catalysis, than the synthesized Cu₂S.

Fig. 3 shows the (HR-)TEM/STEM/EDS results of the synthesized Cu₂S. The sample exhibits a strong tendency to aggregate as observed in Fig. 3(a and b). Characteristic Cu₂S diffraction signals arising from distinct lattice planes were probed by HR-TEM as demonstrated in Fig. 3(c–h).⁵⁵ Cu₂S is prone to surface oxidation/contamination and this is evidenced by the presence of an ∼2.3 nm amorphous layer covering crystalline grains/cores, as seen in Fig. 3(g). The STEM image shown in Fig. 3(i) reveals an aggregate of synthesized Cu₂S and EDS elemental maps of Cu and S are presented in Fig. 3(j and k). The elemental mapping shows uniform distribution of Cu and S in the sample. Values of Cu/S are seen to fluctuate between 1.9 and 2.0 averaging to 1.93 as determined from the EDS line scan, shown in Fig. 3(l). This supports the XPS findings by confirming the presence of a surface Cu-deficient Cu₂S layer.

Fig. 3 (a and b) TEM images of a cluster of synthesized Cu₂S at different magnifications. (c–h) HR-TEM images revealing crystalline grains with discernible characteristic lattice planes (111), (200), and (220) with corresponding estimated lattice interplanar spacings shown as insets, obtained using fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) operations. (g) A thin amorphous layer is evident, attributed to spontaneous surface oxidation and/or contamination. (i) HAADF-STEM image of the synthesized Cu₂S cluster with corresponding EDS elemental maps for Cu (j) and S (k), and (l) an EDS line scan along the white dashed line overlaid on the HAADF-STEM image in (i), with the round head indicating the origin of the line-scan profile.

The oxazolidinones obtained from the chemical transformation of propargylamines and CO₂ are important intermediates in the syntheses of heterocyclic compounds in organic chemistry and in the development of antibacterial drugs in medicinal chemistry.⁵⁶ Therefore, the potential application of Cu₂S in the catalytic cyclizing carboxylation of propargylamine with CO₂ was explored.^24,29 To optimize catalytic conditions, N-2-propyn-1-ylbenzylamine was initially selected as a model substrate (Table 1). The heterogeneous reaction with this substrate proceeded smoothly at room temperature (25 ± 1 °C) in the absence of a cocatalyst, yielding the target product, 3-benzyl-5-methylene-2-oxazolidinone, in 99% yield within 8 hours (Table 1, entry 1). Substituting TEA or cesium carbonate as the base for DBU reduced the yield to trace amounts and 50%, respectively (Table 1, entries 2 and 3). Changing the solvent from CH₃CN to DMF, CH₃OH, or DMSO reduced the yield to 80%, 50%, and 30%, respectively (Table 1, entries 4, 5, and 6). Shortening the reaction time to 4 hours reduced the yield to 78% (Table 1, entry 7). Changing the substrate dosage from 0.5 mmol to 0.1 mmol and 1 mmol, respectively, yields reached 98% and 90%, respectively (Table 1, entries 8 and 9). In the absence of catalyst or base, no target product was produced (Table 1, entries 10 and 11). Subsequently, under the same catalytic conditions, commercially available Cu₂S was used as the catalyst, achieving a product yield of 80% (Table 1, entry 12). The results demonstrate that the synthesized Cu₂S is a highly efficient catalyst for the carboxylation of propargylamine with CO₂ at room temperature and under atmospheric pressure without the need for a cocatalyst.

Table 1 Control experiments for carboxylation of N-benzylprop-2-yn-1-amine with CO₂ catalyzed by Cu₂S^a

Entry	Catalyst	Substrate (mmol)	Base	Solvent	t (h)	Yield^b (%)
a Reaction conditions: N-benzylprop-2-yn-1-amine (0.5 mmol), Cu2S (0.005 mmol), DBU (0.5 mmol), CH3CN (3 mL), CO2 (balloon), room temperature.b The yields were determined by ^1H NMR analysis using mesitylene as the internal reference.
1	Cu2S	0.5	DBU	CH3CN	8	99
2	Cu2S	0.5	TEA	CH3CN	8	Trace
3	Cu2S	0.5	Cs2CO3	CH3CN	8	50
4	Cu2S	0.5	DBU	DMF	8	80
5	Cu2S	0.5	DBU	CH3OH	8	50
6	Cu2S	0.5	DBU	DMSO	8	30
7	Cu2S	0.5	DBU	CH3CN	4	78
8	Cu2S	0.1	DBU	CH3CN	8	98
9	Cu2S	1	DBU	CH3CN	8	90
10	None	0.5	DBU	CH3CN	8	0
11	Cu2S	0.5	None	CH3CN	8	0
12	Commercial Cu2S	0.5	DBU	CH3CN	8	80

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This study further explores the universality of the cyclization of propargylamines to yield oxazolidinone compounds.⁵⁷ Using several typical propargylamines with different substituents as raw materials, the reactions were carried out at room temperature (25 ± 1 °C) for 8 h, and the yields of the target products are shown in Table 2. These propargylamines efficiently generate the corresponding oxazolidinone compounds in high yields of at least 90% upon carboxylation reactions using CO₂ under atmospheric pressure.

Table 2 Substrate scope for the carboxylic cyclization of propargylic amines with CO₂ catalyzed by Cu₂S^a

Substrate	Product	Yield^b (%)	TON	TOF/h^−1
a Reaction conditions: 1 mmol substrate, 0.01 mmol catalyst, 1 atm CO2 balloon, 1 mmol DBU and 3 mL CH3CN.b Yields were determined by ^1H NMR with mesitylene as the internal standard.
		99%	99	12.4
		97%	97	12.1
		94%	94	11.8
		98%	98	12.3
		97%	97	12.1
		93%	93	11.6
		95%	95	11.9
		94%	94	11.8
		93%	93	11.6
		96%	96	12.0

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To explore the recyclability of the Cu₂S catalyst, it was collected after the propargylamine transformation to oxazolidinone by reaction with CO₂ and reused under the same catalytic conditions. The data are shown in Fig. 4. On the one hand, the synthesized Cu₂S maintains its catalytic performance during the cyclization process and this only slightly decreases after five consecutive uses. On the other hand, the commercial Cu₂S catalyst shows an obvious activity decrease after five uses. This indicates that synthesized Cu₂S, unlike commercial Cu₂S, shows high stability in the catalytic reaction. To further confirm that the reaction is catalyzed by the solid Cu₂S surface rather than the leached copper species, a leaching test was performed (see Fig. 4c).⁵⁸ The reaction mixture was filtered at approximately 48% conversion to remove the solid catalyst. The filtrate was then stirred under the same reaction conditions for an additional 4 hours. We observed that the reaction proceeded negligibly after the removal of the solid catalyst. This result strongly supports the heterogeneous nature of the catalytic system.

Fig. 4 Recycling performance of synthesized Cu₂S (a) and commercial Cu₂S (b) in the cyclization reaction of N-2-propyn-1-ylbenzenemethanamine with CO₂. (c) Leaching test.

To investigate the solubility of copper species under stoichiometric conditions, a mechanistic experiment was conducted using 0.3 mmol of substrates and 0.3 mmol of Cu₂S (1 : 1 ratio). After the reaction, the supernatant was separated and concentrated to dryness without work-up to ensure full retention of all organic and inorganic components. ICP-MS analysis data, as shown in Table 3, revealed a total leached copper mass of 2.15 mg. Relative to the total copper input (38.13 mg), this corresponds to a leaching percentage of 5.6%. This result indicates that even when the catalyst is used in stoichiometric amounts relative to the substrate, the dissolution of active copper species remains suppressed (<6%).

Table 3 ICP-MS analysis of copper leaching in a stoichiometric mechanistic study^a

Sample	Mass of residue (mg)	Total Cu detected^a (mg)	Cu content (wt%)	Total Cu input (mg)	Leaching percentage (%)
a Reaction conditions: propargylamine (0.3 mmol), DBU (0.3 mmol), Cu2S (0.3 mmol), CH3CN.b Theoretical mass of copper in 0.3 mmol Cu2S.
D1	34.95	1.11	3.17	—	—
D2	23.41	0.75	3.18	—	—
D3	9.18	0.29	3.12	—	—
Total	67.54	2.15	3.18 (avg.)	38.13^b	5.64

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Table 4 presents a comparative analysis of the catalytic efficiency of the developed system in the carboxylation reaction relative to previously reported catalytic systems. Notably, the preparation of the Cu₂S catalyst does not rely on expensive or sensitive organic ligands, resulting in a simpler, lower-cost process, minimized waste generation, and reduced energy consumption, further aligning with the concept of green chemistry. This catalytic system achieves efficient conversion under mild conditions while maintaining high yields. Unlike conventional methods with poor catalyst recyclability, the Cu₂S catalyst synthesized here can be easily recovered and reused at least five times, making it more suitable for large-scale or industrial applications.

Table 4 Comparison of this catalytic system with previously reported methods^29,59–61

Entry	Catalyst	Base	CO2 pressure (MPa)	Solvent	Temperature (°C)	Time (h)	Yield (%)	Ref.
1	Cd-Bpy-COF	DBU	Balloon	CH3CN	60	12	99.9	53
2	Cu2O@MOF	DBU	1.0	CH3CN	70	12	94	26
3	Ag@2,6-FPP-TAPT	DBU	Balloon	CH3CN	50	2	99	54
4	Ag@Pybpy-COF	Cs2CO3	1.0	DMF	50	0.5	99	55
5	Cu2S	DBU	Balloon	CH3CN	RT	8	99	This work

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Based on the above results and previous reports, a possible mechanism for the carboxylic cyclization of CO₂ with propargylamine is proposed (Fig. 5).^62–66 First, the amine reacts with CO₂ with concomitant deprotonation by DBU forming the DBUH⁺ salt of the carbamate, which coordinates Cu through the N atom, generating intermediate A. Then, an oxygen atom of the carboxylic acid group coordinates to Cu, which shifts from N-coordination to π-alkyne coordination and activation in intermediate B. This is followed by a π to σ shift of the Cu atom, leading to isomerization to the Cu-alkenyl heterocycle C. Finally, protonation of C by DBUH⁺ provides the metal-free target heterocyclic product, while regenerating the Cu(I) catalyst and DBU.

Fig. 5 Proposed reaction mechanism for the carboxylative cyclization of N-benzylprop-2-yn-1-amine catalyzed by synthesized Cu₂S.

In recent years, the carboxylation of simple alkynes with CO₂ catalyzed by inorganic catalysts to form propiolic acid derivatives has attracted extensive attention.^67,68 For example, Yu's group^69–72 has been focusing on the application of CO₂ in organic synthesis. In addition to copper catalysis, these authors also used visible light photoredox catalysis to achieve efficient carboxylation cyclization to prepare propiolic acid compounds. Propiolic acid is not only a basic building block in medicinal chemistry but also a major component of bioactive compounds and conductive polymers.⁷³

Herein, exploring the potential application of Cu₂S in the green catalytic carboxylation of terminal alkynes with CO₂ is also considered.^74–76 To optimize the catalytic conditions, phenylacetylene is selected as a model substrate (Table 5). The heterogeneous reaction proceeded smoothly at 80 °C, yielding the target product, phenylpropiolic acid, in 82% yield within 6 hours (Table 5, entry 1). Substituting TEA or DBU for cesium carbonate reduced the yield to trace amounts (Table 5, entries 2 and 3). Changing the solvent from DMF to CH₃CN or DMSO reduced the yield to trace amounts and 51%, respectively (Table 5, entries 4 and 5). Shortening the reaction time to 3 hours reduced the yield to 50% (Table 5, entry 6). After 6 hours of reaction at room temperature, trace amounts of the target product, phenylpropiolic acid, were afforded (Table 5, entry 7). Lewis acids are known to promote the carboxylation of alkynes by binding to one or both oxygen atoms of CO₂, further promoting C–C bond formation with CO₂ (see SI, Fig. S3). Adding the Lewis acid cocatalyst GaBr₃ under the conditions of entry 1 further improved the yield to 99% (Table 5, entry 8). Changing the substrate dosage from 0.5 mmol to 0.1 mmol and 1 mmol resulted in yields of 99% and 92%, respectively (Table 5, entries 9 and 10). In the absence of catalyst or base, only trace amounts of the desired product were produced (Table 5, entries 11 and 12). Subsequently, under the same catalytic conditions as entry 8 (including the presence of GaBr₃), commercially available Cu₂S catalyzed the reaction, yielding only 81% of the product (Table 5, entry 13). These results indicate that the synthesized Cu₂S is an efficient catalyst (more efficient than commercial Cu₂S) and achieves the green conversion of phenylacetylene with CO₂. The efficiency of the reaction, which is already very good in the absence of Lewis acid co-catalyst, is even increased under the action of the co-catalyst GaBr₃.

Table 5 Control experiments for the carboxylation of phenylacetylene with CO₂ catalyzed by Cu₂S^a

Entry	Catalyst	Substrate (mmol)	Base	Solvent	t (h)	T (°C)	Yield^b (%)
a Reaction conditions: phenylacetylene (0.5 mmol), Cu2S (0.005 mmol), GaBr3 (0.005 mmol), Cs2CO3 (0.75 mmol), DMF (3 mL), CO2 (balloon).b Isolated yield.
1	Cu2S	0.5	Cs2CO3	DMF	6	80	82
2	Cu2S	0.5	TEA	DMF	6	80	Trace
3	Cu2S	0.5	DBU	DMF	6	80	Trace
4	Cu2S	0.5	Cs2CO3	CH3CN	6	80	Trace
5	Cu2S	0.5	Cs2CO3	DMSO	6	80	51
6	Cu2S	0.5	Cs2CO3	DMF	3	80	50
7	Cu2S	0.5	Cs2CO3	DMF	6	25	Trace
8	Cu2S/GaBr3	0.5	Cs2CO3	DMF	6	80	99
9	Cu2S/GaBr3	0.1	Cs2CO3	DMF	6	80	99
10	Cu2S/GaBr3	1	Cs2CO3	DMF	6	80	92
11	None	0.5	Cs2CO3	DMF	6	80	Trace
12	Cu2S/GaBr3	0.5	None	DMF	6	80	Trace
13	Commercial Cu2S/GaBr3	0.5	Cs2CO3	DMF	6	80	81

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The carboxylation of terminal alkynes toward the synthesis of propiolic acid derivatives was further explored. Several typical terminal alkynes with different substituents were used as raw materials and reacted at 80 °C for 6 h. The yields of the target products are shown in Table 6. Most yields are higher than 90%. This result indicates that most terminal alkynes efficiently generate the corresponding propiolic acid derivatives and in high yield under these conditions.

Table 6 Substrate scope for the carboxylic cyclization of terminal alkynes with CO₂ catalyzed by Cu₂S^a

Substrate	Product	Yield^b (%)	TON	TOF/h^−1
a Reaction conditions: 1 mmol substrate, 0.01 mmol Cu2S, GaBr3 (0.005 mmol), 1 atm CO2 balloon, 1.5 mmol Cs2CO3 and 3 mL DMF.b Isolated yield.
		99%	99	16.5
		91%	91	15.2
		90%	90	15.0
		91%	91	15.2
		88%	88	14.7
		89%	89	14.8
		88%	87	14.7
		97%	97	16.2
		94%	94	15.7

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Based on the above results and some previous reports,^69,72,75,76 a possible mechanism of action of the Cu₂S catalyst is proposed (Fig. S3). First, the terminal alkyne coordinates with Cu(I), and Cs₂CO₃ deprotonates the terminal alkyne to generate Cu(I) acetylide intermediate B. At the same time, CO₂ inserts into the carbon–copper bond, while Ga(III) coordinates with the oxygen atom to generate copper propiolate intermediate C. The copper propiolate intermediate C then reacts with free Cs ions to release cesium propiolate. At the end of the reaction, cesium propiolate is acidified to obtain the propiolic product.

Experimental

General data

Propargyl bromide, the amines, terminal alkynes, 2,3,4,6,7,8,9,10-octahydropyrimido[1,2-a]azepine (DBU) and commercial Cu₂S were purchased from BLD Pharm. 3-Mercaptopropionic acid (MPA) and CuSO₄·5H₂O were purchased from Thermo Scientific. H₂O₂, HNO₃ and HCl were purchased from Fisher Chemical. GaBr₃ was purchased from abcr GmbH. All commercial chemicals were used without further purification.

¹H NMR and ¹³C spectra were recorded at 25 °C using a Bruker AC 300 MHz instrument. All chemical shifts are reported in parts per million (δ, ppm) and calibrated using residual protons in deuterated solvents (CDCl₃ at 7.26 ppm for ¹H NMR and 77.16 ppm for ¹³C NMR; CD₃OD at 3.31 ppm for ¹H NMR and 49.00 ppm for ¹³C NMR). The contents of carbon, hydrogen, oxygen, nitrogen, and sulfur were determined using a FlashSmart™ Elemental Analyzer (Thermo Scientific™).

The powder X-ray diffraction data were collected at room temperature on a PANalytical X'Pert MPD diffractometer (45 kV, 40 mA) equipped with a fast PIXcel 1D detector with Cu Kα_1,2 radiation in the range 6° < 2θ < 60° 2θ, a step size of 0.013° 2θ and a counting time of 38 s per step. Powdered samples were spread on a glass sampling holder. The data have been compared with the PDF5+ database from ICDD.

The supernatant after the catalytic reaction is dried under vacuum to remove the solvent. The sample was divided into three aliquots of varying masses. Each portion was weighed into a clean, sterile 15 mL polypropylene centrifuge tube (Fisherbrand™ Easy Reader™). Reagents were added sequentially to each tube under a fume hood: 375 µL of H₂O₂, 525 µL of HNO₃, and 100 µL of HCl (total volume: 1.0 mL). Upon addition of HNO₃, the solution became clear, and a tawny (HEX #C68346) precipitate was observed, indicating the presence of copper. This precipitate disappeared upon the addition of HCl. The digestion process lasted over 96 hours. At this point, all samples were completely digested, and the solutions were clear with no visible precipitate. The final digests were analyzed using a iCAP-Q ICP-MS equipment (Thermo Scientific, Bremen, Germany) equipped with an autosampler ASX-500 (CETAC Technologies, Omaha, USA).

XPS spectra were recorded using the PHI XPS VersaProbe III energy spectrometer equipped with a monochromatic 1486.6 eV Al-Kα radiation source. A focused X-ray source with X-ray beam size of 100 μm, power of 25 W, and e-beam energy of 15 kV was used. Charge neutralization was possible by using a complementary dual-beam charge neutralization method. The C 1s peak at 284.8 eV was used as the reference signal to calibrate all acquired spectra. A pass energy of 13 eV was used for the acquisition of all high-resolution spectra.

For the High-Resolution Transmission Electron Microscopy (HR-TEM) studies, samples were dispersed in hexane. Before drop casting on ultrathin carbon film placed on lacey carbon Au-coated grids (Ted Pella Inc., USA), dispersions were subjected to soft sonication until they became homogeneous. The grids were left to dry under ambient conditions prior to investigation. HR-TEM experiments were conducted on a JEOL JEM-2100F UHR electron microscope (200 kV) equipped with an Energy Dispersive X-ray (EDS) detector (Oxford UltimMax) and two Scanning Transmission Electron Microscopy (STEM) detectors: Bright-Field (BF) and High-Angle Annular Dark-Field (HAADF). Flash column chromatography was performed using silica gel (300–400 mesh).

Synthesis of Cu₂S

Cu₂S was prepared by a reported hydrothermal method,³⁸ with some improvements. The specific steps are as follows (Fig. 6). In a three-necked flask, 1 mmol of CuSO₄·5H₂O was dissolved in 95 mL of distilled water and stirred for 15 minutes. 135 µL of mercapto-propionic acid (MPA) was added to the solution under continuous stirring. A yellow precipitate formed within minutes, signaling the creation of a Cu-MPA complex. The pH of the solution was then adjusted from 2 to 10 by adding 1 M NaOH solution. The mixture was subsequently heated to 100 °C, followed by the addition of 5 mL of 0.2 M Na₂S₂O₃ aqueous solution. The solution was refluxed at 100 °C for 7 h, resulting in the formation of a dark-brown precipitate. The mixture of Cu₂S and CuS is obtained by adjusting the pH to 5 with a small amount of dilute hydrochloric acid. After the reaction, the nanocrystal suspension was centrifuged, and the precipitate was washed three times with distilled water and three times with ethanol. The obtained Cu₂S/CuS mixture was dried under vacuum for 24 h. To eliminate trace CuS, 30 mg of the dried powder was dispersed in 10 mL of dry THF and reduced with 10 mg of NaBH₄ (excess) under an argon atmosphere. The reaction mixture was refluxed at 66 °C for 5 h. Finally, the product was washed three times with deionized water and ethanol, followed by drying under vacuum.

Fig. 6 Scheme of the preparation of Cu₂S.

Typical procedure for the cyclization of the propargylic amines with CO₂

A mixture of Cu₂S and propargylic amine was added into a 10 mL Schlenk flask that was vacuumed and refilled 3 times with CO₂ (99.9% CO₂). DBU (0.2 equiv.) in 3 mL of acetonitrile was added into the tube. Then, the reaction mixture was stirred at room temperature (25 ± 1 °C) for 8 h. After reaction completion, the yield was determined by ¹H NMR analysis using mesitylene as an internal standard. To obtain the isolated product, the solvent was first removed by evaporation, then 20 mL of ethyl acetate was added to the mixture, and the mixture was washed three times (3 × 20 mL) with H₂O. The organic phase was extracted using brine water, dried over anhydrous Na₂SO₄, and filtered. The ethyl acetate solvent was removed under vacuum evaporation, and the crude product was further purified by column chromatography on silica gel (petroleum ether/ethyl acetate = 9 : 1 as an eluent) to afford the pure product. Finally, the pure products were characterized by ¹H NMR and ¹³C NMR. In the recycling experiments, the catalyst is separated by centrifugation and washed three times with CH₃CN. Thereafter, the catalyst is dried at 60 °C overnight before its next use.

Catalysis of the carboxylation reaction of the terminal alkynes with CO₂

A mixture of Cu₂S (0.01 mmol), GaBr₃(0.01 mmol) and Cs₂CO₃ (1.5 mmol) was added into a 10 mL Schlenk flask that was vacuumed and refilled with the CO₂ balloon (99.9% CO₂) rapidly, and this operation was repeated twice more before leaving the flask connected to the CO₂ balloon. Finally, the terminal alkyne (1 mmol) in 3 mL DMF was injected into the tube. The reactor was transferred into an oil bath at the desired temperature (80 °C) with magnetic stirring (1400 rpm) for 6 h. It was then cooled down to room temperature, followed by removal of the CO₂ balloon. Then, deionized water (15 mL) was added to the reaction mixture that was stirred for 30 min. The solid catalyst was separated from the reaction mixture by centrifugation (8000 rpm, 5 min). The liquid phase obtained was washed with dichloromethane (3 × 10 mL) in a 125 mL separation funnel to keep the aqueous phase. Then, the aqueous phase was acidified to pH = 1 with concentrated aqueous HCl solution at 0 °C. The solution containing the product was then extracted with ethyl acetate. TLC was used to monitor the aqueous phase during the extraction process until no product remains. The organic phases were collected, dried over anhydrous sodium sulphate, and filtered. Then, the solvent was removed by rotary evaporation to obtain the propiolic acid. The actual mass was weighed and compared with the theoretical mass to calculate the separation yield. The final product was characterized by ¹H NMR. In the recycling experiments, the catalyst was separated by centrifugation and washed twice with DMF, and twice with methanol. Thereafter, the catalyst was dried at 60 °C overnight before its next use.

Conclusion

Cu₂S, a non-toxic material of high Earth abundance, which is known as an efficient p-type semiconductor for solar energy conversion,^38,39 is shown here to be a simple and efficient self-supported heterogeneous catalyst for CO₂ fixation under ambient temperature and pressure conditions in carboxylation reactions of a variety of propargylamines. In addition, it is also active for simple alkyne carboxylation under one atm CO₂. For all these CO₂ fixation reactions, the yields exceed 90% and, compared to previously reported inorganic catalysts, Cu₂S exhibits excellent stability, even in the absence of a support. It provides atom economy, one of the major rules of green chemistry, and its use therefore represents a new strategy for the resource utilization of abundant CO₂. Interestingly, the analogous compound Cu₂O is also an active catalyst for such reactions,³⁰ but its toxicity is well-known.⁷⁷ In conclusion, this work not only expands the application boundaries of copper-based catalysts toward C–C bond formation and heterocyclic chemistry, but also opens up a new direction for the development of green carbon capture technology.

Author contributions

H. Wang: investigation, data curation, methodology, software, validation, writing – original draft; T. Wang: contribution to the investigation; M. Berlande: contribution to the investigation and resources; A. Subrati: investigation, data curation, software, validation, writing – original draft, review and editing; S. Moya: methodology, validation, resources, funding acquisition, writing – review and editing; L. Salmon: contributions to the investigation, data curation, software; N. Daro: data curation, software; N. Audebrand: contribution to the investigation, data curation, software, validation; J.-R. Hamon, writing – review and editing; H.Yu: methodology, software, data curation, validation, resources, funding acquisition, writing – review and editing; J.-L. Pozzo: conceptualization, methodology, resources, project administration, writing – review and editing; D. Astruc: conceptualization, resources, project administration, writing – review and editing.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI): general procedure for the synthesis of propargylic amines; Table S1: Analysis summary of the high-resolution Cu 2p XPS; Fig. S1: High-resolution valence band XPS and Cu Auger signals; Table S2: Analysis summary of the Cu Auger parameter; Fig. S2: High-resolution S 2p XPS; Table S3: Analysis summary of the high-resolution S 2p XPS; Fig. S3: Proposed mechanism for the carboxylative cyclization of terminal alkyne catalyzed by Cu₂S; ¹H NMR and ¹³C NMR data and spectra for the substrates and the targeted products. See DOI: https://doi.org/10.1039/d5qi02546j.

Acknowledgements

The financial support from the China Scholarship Council (CSC, PhD grants to H. W. and T. W.), the Centre National de la Recherche Scientifique (CNRS), the Universities of Bordeaux, Rennes and Toulouse III, and CIC biomaGUNE is gratefully acknowledged.

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