Catalysis Letters https://doi.org/10.1007/s10562-017-2268-1
Dinuclear Silver Complexes for Solvent-Free Catalytic Synthesis of Cyclic Carbonates from Epoxides and CO2 at Ambient Temperature and Pressure Jing‑Jing Chen1 · Zhi‑Liang Gan1 · Xiao‑Yi Yi1,2 Received: 6 September 2017 / Accepted: 2 December 2017 © Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract A well structure-defined dinuclear silver complex [Ag2(PDP)]OTf, where HPDP—bispyridylpyrrole ligand, OTf−—triflato, was used as catalyst for coupling of C O2 with epoxides to generate exclusive cyclic carbonates. Yield up to 87% was obtained for various substrates at a low loading of 0.1 mol%, at ambient temperature and pressure under solvent-free condition. The catalytic reusability of 1·OTf was also studied. Graphical Abstract Although coupling reaction of CO2 with epoxides can be catalyzed by dinuclear silver complexes with low catalyst loading at ambient temperature and pressure, the experiment of reusability of catalyst displays the dinuclear silver complex is decomposed due to the excess Br− in co-catalyst TBAB. The formed silver triphenylphosphine complex or inorganic silver materials provide real catalytic reactivity to the coupling reaction. 1·OTf + TBAB
O R
CO2
O
silver triphenylphosphine complex or inorganic silver materials real catalyst
O
O
R
Keywords Green chemistry · Cycloaddition · CO2 fixation · Dinuclear silver complex
Jing-Jing Chen and Zhi-Liang Gan have contributed equally to this research. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10562-017-2268-1) contains supplementary material, which is available to authorized users. * Xiao‑Yi Yi
[email protected] 1
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, People’s Republic of China
Hunan Provincial Key Laboratory of Efficient and Clear Utilization of Manganese Resources, Central South University, Changsha 410083, Hunan, People’s Republic of China
2
The development of advanced technologies and chemical processes targeting to convert C O2 into valuable products is of great concern to scientists [1–3]. One of the most successful processes for use of C O2 is the coupling with epoxides to generate polycarbonates and/or cyclic carbonates. These chemical products have commercial importance. For example, the cyclic carbonates can be used as fuel additives, electrolytes in batteries, polar aprotic solvents [4, 5]. However, coupling of C O2 and expoxide is a two-component reaction, which does not react spontaneously. A suitable catalyst is required in the transformation. In the last decade, various catalysts, such as porous polymer materials [6, 7], ionic liquid [8], organocatlyst [9–11], and metal complexes [12–17] have been developed to allow the efficient synthesis of cyclic carbonates from CO2 and epoxides. Of these, the metal complexes exhibit some advantages such as
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J.-J. Chen et al. Table 1 Coupling of C O2 and chloromethylethylene oxide catalyzed by dinuclear silver complexes
Scheme 1 The structure of dinuclear silver complexes 1·OTf–4·OTf
easy synthesis, well-tunable structure, high stability to this conversion. Silver complexes are one of the most popular catalysts for chemical fixation of C O2. For example, silver phosphine complexes [Ag(PPh3)2]2CO3 generated from Ag2CO3 and PPh3 can catalyze carboxylative cyclization of propargylic alcohols and C O2 to formation of α-methylene cyclic carbonates in excellent yields [18], in which the silver is bifunctional atom acting as Lewis acid and alkynophilic center to simultaneously activate CO2 and propargylic alcohol, respectively. Recently, we have reported dinuclear silver complex 1·OTf (Scheme 1) with one bispyridylpyrrole and two P Ph3 ligands, which shows excellent catalytic efficiency and usability for three-component coupling reaction of aldehydes, alkynes and amines under mild conditions [19]. In contrast to the catalytic process of [Ag(PPh3)2]2CO3, it is found (i) silver center in 1·OTf only shows the alkynophilic capability; (ii) The framework of 1·OTf permits control of the spatial arrangement for this three-component catalytic process. It seemed fairly natural to think whether 1·OTf acts as Lewis acid center and works in the two-component catalytic process involving in C O2. Therefore, herein we will report the results of a study using 1·OTf as catalyst in the coupling of CO2 with a variety of epoxides. This coupling reaction between C O 2 (1 atm) and 2-(chloromethyl)oxirane was studied first in the presence of 1·OTf (0.1 mol%), co-catalyst tetra-n-butylammonium bromide (TBAB, 10 mol%) in neat at room temperature, as shown in Table 1. Merely 1·OTf can not catalyze the reaction, while co-catalyst TBAB is active to this coupling reaction which is similar with what reported in the other literatures [20–22]. After stirring for 12 h, the cyclic carbonates product was generated with 52% yield. Higher yield (90%) was achieved when the reaction time was extended to even 60 h. The structure analogous 2·OTf–4·OTf gave 74–87% yield of cyclic carbonate product in 48 h, which was comparable to that of 1·OTf (87%). It is clear that complexes 1·OTf–4·OTf have similar catalytic activity. Although the catalytic efficiency of
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Entry
Catalyst
Co-catalyst TBAB (mol%)
Yield (%)
1 2 3
1·OTf – 1·OTf
– 10 10
4 5 6 7a 8b 9c
2·OTf 3·OTf 4·OTf 1·OTf 1·OTf 1·OTf
10 10 10 10 10 10
Trace 54 52 (12 h) 70 (24 h) 87 (48 h) 90 (60 h) 86 74 87 39 95 99
Reaction conditions: reaction run in neat epoxide, catalyst (0.0125 mmol, 0.1 mol%), epoxide (12.5 mmol), TBAB (1.25 mmol), CO2 (0.1 MPa), 25 °C, 48 h. The yields were determined by comparing the ratio of product to substrate in the 1H NMR spectrum of an aliquot of the reaction mixture; a
In CH3CN solvent (1 mL);
b c
In THF solvent (1 mL);
In DMF solvent (1 mL)
metal complex in solvent, such as THF and DMF, was higher than that in neat, we did not consider the reaction in organic solvent from the perspective of green chemical experiment. Thus, we chose 1·OTf as a model for the coupling reaction between CO2 and epoxides in neat. The same loading of 0.1 mmol% catalyst and 10 mol% co-catalyst TBAB was used for the cycloaddtion of various epoxide substrates in neat under atmospheric pressure of CO2. All the reactions proceeded smoothly to give exclusive corresponding cyclic carbonates. The general reaction trend of aliphatic epoxides > styrene oxide. For example, propylene oxide and halogenated propylene oxide gave 85–87% yield, and the longer chain epoxide, such as 1,2-epoxybutane offered slightly lower yield of 63%. The yields were drastically reduced when phenoxymethyloxirane (32%) and styrene oxide (20%) were used. It indicates that steric hindrance group on the ethylene oxide may reduce the activity. This dinuclear silver complexes only show medium efficiency for coupling reaction of CO2 in comparison with what reported in the other metal complexes in solvent-free conditions [3, 21] (Table 2).
Dinuclear Silver Complexes for Solvent-Free Catalytic Synthesis of Cyclic Carbonates from… Table 2 Coupling of CO2 and various epoxides catalyzed by 1·OTf E
S
P
Y
Table 3 Catalytic repeatability of 1·OTf
a
Entry
Yield (%)a
1 2 3 4 5 6
86 70 64 89 91 97
Reaction conditions: reaction run in neat epoxide, 1·OTf (0.0187 mmol, 1.5 mol%), epoxide (1.25 mmol), TBAB (0.125 mmol), CO2 (0.1 MPa), 25 °C, 48 h a
Yields was determined by 1H NMR, average of 2 runs. After one cycle, the catalyst and co-catalyst was separated by precipitation by adding hexane and diethyl ether, and reused directly without further purification for the next cycle
Reaction conditions: reaction run in neat epoxide, 1·OTf (0.0125 mmol, 0.1 mol%), epoxide (12.5 mmol), TBAB (1.25 mmol), CO2 (0.1 MPa), 25 °C, 48 h a
The yields were determined by comparing the ratio of product to substrate in the 1H NMR spectrum of an aliquot of the reaction mixture
To evaluate the robustness of the catalytic system, a typical recycling experiment was usually performed (Table 3). In fact, it was not feasible to isolate the catalyst when only 0.1 mmol% of it was used. Thus we used 1.5 mmol% of catalyst in order to obtain the more reliable results. We carefully recovered the catalyst and co-catalyst by precipitation from hexane and diethyl ether, and reused directly without further purification under identical conditions. It was found that the catalytic activity of 1·OTf was slightly decreased in the first three runs, while catalytic activity was dramatically increased in the run 4–6. The 1H NMR spectrum of recovered catalyst after first run and sixth run as shown in Fig. 1. It was found: (i) 1·OTf disappears and free bispyridinepyrrole ligand in 1·OTf appears after first catalytic run. It is possible that 1·OTf is decomposed in presence of TBAB, and possibly forms a silver triphenylphosphine complex; (ii) after sixth run, the 1H NMR spectrum is completely calm for organic ligands, suggesting that silver triphenylphosphine complex generated in the first run was completely decomposed in
presence of excess TBAB and possibly formed various inorganic silver materials. Based on the information of experiment and spectrum, the catalytic mechanism may be changed through six times recycles. In the first three recycles, the catalytic reaction is due to silver metal complex 1·OTf and/or the regenerated silver triphenylphosphine complex. The loss of catalytic performance may be due to the decrease of active sites or loss of the catalyst and co-catalyst TBAB during sampling process. The similar loss of catalytic performance of metal complexes was observed in the synthesis of cyclic carbonates by Bi(III) porphyrin/TBAI reported by Guo groups [22]. In the last three cycles, no any organic compounds was observed, but the catalytic reaction is still ongoing. Clearly, new catalytic materials is generated. it is possible that 1·OTf is totally decomposed in presence of large excess of Br− in TBAB, to form inorganic salt AgBr or other forms of silver materials. According to the literature, the inorganic salt combined with the co-catalyst, such as Z nI2/NEt3,[23] Ag2WO4/PPh3,[18, 24] AgOAc/DBU [25], AgBF4/MTBD [26] and Ni/Zn/PPh3 [27] could activate CO2 to generate corresponding cyclic carbonate or products of carboxylative cyclizations. Thus, we proposed the catalytic reaction in the last three runs is possible to be due to the formation of inorganic silver materials. A tentative mechanism is shown in Scheme 2 [13–15]. On the basis of the above information, cationic Ag+ as Lewis acid from silver complexes or inorgnaic silver materials acts with O site of epoxide to form A g+·(epoxide) adduct firstly,
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Fig. 1 The 1H NMR of 1·OTf, free HPDP ligand in 1·OTf and catalytic reaction mixture after first and sixth run
complexes act as the catalyst for coupling reaction of C O2 and epoxide. Acknowledgements This work was supported by the National Natural Science Foundation of China (Project 21571190), Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007).
Compliance with Ethical Standards Conflict of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
References Scheme 2 Proposed mechanism for coupling of CO2 and various epoxides catalyzed by 1·OTf
which followed by a nucleophilic attack of B r− in the cocatalyst TBAB on the less sterically hindered β-carbon atom of the epoxide to make epoxide ring-open. Subsequently, CO2 is inserted into the Ag–O bond to form a metal carbonate. Finally, ring closes with a second inversion to yield the cyclic carbonate. In summary, well structure-defined dinuclear silver complexes were used in catalytic coupling reaction of CO2 and epoxides in presence of co-catalyst TBAB. Although this catalytic reaction could be happened with low catalyst loading at ambient temperature and pressure for various epoxide substrates, the experiment of reusability displays dinuclear silver complexes in presence of TBAB is decomposed, possibly to form silver triphenylphosphine complex or inorganic silver materials, which provides real activity to the coupling reaction. Thus, the commonly used co-catalyst such as n-Bu4X (X=Cl, Br, I) had better not to use when silver
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