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Received 00th January 20xx, Accepted 00th January 20xx
Entrapment of a chiral cobalt complex within silver: a novel heterogeneous catalyst for asymmetric carboxylation of benzyl bromides with CO2
DOI: 10.1039/x0xx00000x
Heng-Pan Yang, Ying-Na Yue, Qi-Long Sun, Qiu Feng, Huan Wang* and Jia-Xing Lu**
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A novel way to accommodate heterogeneous catalysis, CO2 fixation and asymmetric synthesis on one catalyst is reported. [Co]@Ag composite was prepared for the first time and used for asymmetric carboxylation of benzyl bromides with CO2. All the procedure were perforemed under mild conditions. Moreover, [Co]@Ag composite has terrific stability and reusability. 1-3
CO2 is well known as greenhouse gas. On the other hand, CO2 is also a kind of clean, non-expensive and abundant C1 feedstock, which has great potential for large-scale industrial application.4-5 In recent years, electro-catalytic fixation of CO2 into valuable, economically competitive products has attracted much attention, which could generate large range of products including hydrocarbons, alcohols, CO, formic acid, carboxylic acids and carbonate with various values.6-11 Asymmetric synthesis is another important issue in modern society. The growing demand for optically active compounds in the pharmaceutical and agrochemical industry has stimulated an increased interest in asymmetric synthesis.12-15 It is obvious fact that combing CO2 fixation and asymmetric synthesis could bring extra economic benefits because optically active compounds are much higher value-added chemicals. This feasibility was investigated in our previous work, we reported a possible electrochemical route for the asymmetric carboxylation of achiral 1-phenylethyl chloride with CO2 in the presence of the chiral cobalt complex, optically active 2phenylpropionic acid with 37% yield and 83% ee was obtained.16 This is a homogeneous system, and the asymmetric inducer, chiral cobalt complex, would be added into reaction solution during every catalytic process, which is
expensive and hardly recyclable, just like many other homogeneous asymmetric systems. To solve this problem and immobilize the chiral cobalt complex, organically doped metals was introduced, which belongs to a new series of metallo-organics hybrid materials. In these composites, metal crystallites act as a porous cage which forms around the dopant, allows the diffusion of substrate and product molecules in and out the catalytic 17-25 material, yet not allowing the dopant to leach out. Given their special nature, we synthesized alkaloid@Ag composite by the entrapment of alkaloids within silver nanoparticles and successfully employed it as heterogeneous catalyst for asymmetric hydrogenation of α-ketoesters, 60% ee value and 93% yield were obtained under mild conditions and 26 alkaloid@Ag composite was highly stable and reusable. In addition, Avnir and co-workers also made great achievements in the preparation and application of organically doped metals, such as alkaloid@Pd composites for the partial enantioselective hydrogenation of ketones27 and [Rh]@Ag for the hydrogenation of styrene.28 In a word, organically doped metals might be an ideal heterogeneous support for homogeneous catalysts.
Fig. 1 Asymmetric carboxylation of benzyl bromides with CO2 on [Co]@Ag.
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Under the guideline of these strategies, we further explored the application of organically doped metals in asymmetric carboxylation (Fig. 1). [Co]@Ag composite was synthesized by the entrapment of a chiral cobalt complex within silver nanoparticles for the first time. It was pressed into coin and used as catalyst for the asymmetric carboxylation of benzyl bromides with CO2, which could realize heterogeneous catalysis, asymmetric synthesis, and fixation of CO2 on one catalyst. Optically active phenylpropionic acids were obtained, which are very important intermediates for Nonsteroidal Antiinflammatory Drugs (NSAIDs), like Ibuprofen, Naproxen, Ketoprofen and Flurbiprofen. Both the synthesis of catalyst and asymmetric carboxylation procedure were performed under mild conditions. The synthesis of [Co]@Ag composite involved reduction of AgNO3 by zinc powder in a chiral cobalt complex aqueous solution and chiral cobalt complex molecules were entrapped within silver nanograins during this procedure; CoII-(R, R)(salen) (Fig. S1) was used as model dopant. Fine powder was achieved after filtration and drying (Fig. S2). Pure Ag nanoparticles (Ag NPs) were prepared in the same way except for the absence of chiral cobalt complex in reducing solution (Fig. S3). An interesting phenomenon was observed that Ag NPs have characteristic metal colour and shine after pressing; while [Co]@Ag composite is relatively dark and lustreless.17, 20 Adsorption on the surface and the entrapment within metal are fundamentally different processes.19, 20, 26 After filtration, trace amount of chiral cobalt complex was detected by high performance liquid chromatography (HPLC) in filtrate, revealing that most of the dopant was entrapped. In contrast, pure Ag NPs was added into a same chiral cobalt complex solution, the concentration only fell by 1%. Moreover, the adsorbed cobalt complex could be detected by FT-TR, but the entrapped dopant couldn’t (Fig. S4).
A
nanometers, which results in the porous characteristics that is important for the functioning of this material as a catalyst. Porous structure was also observed in pure Ag NPs, aggregated from silver nanograins in the ~100 nm size range (Fig. S5). The X-rays diffraction (XRD) patterns of the [Co]@Ag composite is shown in Fig. 2D, typical diffraction peaks of metallic Ag are observed in XRD patterns. Note that the XRD patterns show no signs of Zn, silver oxide or metallic Co crystals. Moreover, shifting of the peaks compared with pure silver nanoparticles is almost negligible. This finding is one of the evidence that the chiral cobalt complex is not immobilized within the silver lattice but between the small metallic cages of the aggregated silver nanocrystals.23 The presence of the chiral cobalt complex was confirmed by EDAX and ICP-AES analysis. EDAX spectrum showed signals corresponding to silver, cobalt, carbon and oxygen (Fig. S6, up). The presence of zinc probably originated from trace amount of zinc in the entrapment process, since these are not seen by XRD, they are apparently residual. EDX mapping of a [Co]@Ag composite (Fig. S6, down) shows a homogenous dispersion of cobalt complex in the composite, as indicated by the different colours for silver (green) and cobalt (red). According to ICP-AES, Co and Ag have a molar ratio of 1:140, consistent with the initial ratio in preparation solution.
A
B
B 5 µm
1 µm
C
D
Fig. 3 X-ray photoelectron spectra of the Ag3d regions (A) of [Co]@Ag (red) and pure Ag NPs (black), Co2p regions (B) of [Co]@Ag (red) and pure cobalt complex (black).
500 nm Fig. 2 FE-SEM patterns of [Co]@Ag composite with different magnification, A (10k), B (50k), C (100k); D: XRD patterns of pure Ag NPs (a) and [Co]@Ag (b).
The FE-SEM patterns of the composite reveal that these elementary particles gather into particles varying in size from 40 to 60 nm, and further agglomerate into hundred
XPS reveal the valence state of Ag and Co in the composite. The Ag3d spectrum (Fig. 3A) shows the peak at the binding energy of 368.3 eV. This peak confirms that silver is present in the metallic form. The Co2p spectrum (Fig. 3B) shows the peaks at the binding energy of 780.0 eV and 795.5 eV, corresponding to Co2+ state in pure cobalt complex.
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Further characterization of the porous structure is provided by the shape of the nitrogen adsorption-desorption isotherm (Fig. 4, left) which is typical of a mesoporous material, an important feature that allows substrate molecules to penetrate and reach the entrapped chiral cobalt catalyst and product molecules to diffuse out. The BJH-calculated average 3 pore diameter is ~11 nm and total pore volume is 0.0212 cm -1 2 -1 g . The calculated BET surface area is 7.7 m g . These values are significantly higher than those for pure silver nanoparticles 3 -1 2 -1 (7 nm, 0.00282 cm g and 1.3 m g , respectively). Apparently, the doping process has a significant impact on the particle growth and agglomeration, leading to materials with smaller particles, more pores and higher surface areas.23, 27 An important observation is that the compliance to the BET equation is excellent (Fig. 4, right), which indicates again the homogeneous dispersion of the dopant in the metal.
achieved, far lower than [Co]@Ag. If no chiral inducer was added, we only detected racemic product (Table 1 entry 1), indicating that chiral cobalt complex was crucial in this chiral induction process, which also indirectly demonstrate the existence of chiral cobalt complex in the composite. Moreover, during an electro-catalytic asymmetric reaction, chiral inducer would gather around the electrode surface and create a chiral 25, 29 atmosphere for the asymmetric induction. Apparently, more chiral inducer on the cathode surface is in favour of higher ee value. As mentioned above, about 1% chiral cobalt complex could be adsorbed on Ag NPs surface, which made the adsorbed amount under 0.00033 g. It is also notable that solvent could easily wash away the adsorbed chiral cobalt complex. On the other hand, we pressed 2 g [Co]@Ag composite into a coin. So each [Co]@Ag cathode contains about 0.047 g chiral cobalt complex, hardly leaching into solvent. Therefore, the entrapment amount was nearly 150 times more than the adsorbed amount of chiral cobalt complex. Since this chiral induction process was conducted on the cathode surface, it could explain why ee value on [Co]@Ag was far higher than those on Ag NPs.
-
Br
+ CO2
[Co]@Ag
+
R
OH
+
O
1a
Fig. 4 Adsorption-desorption isotherms of nitrogen reveal the mesopososity of [Co]@Ag (left). The compliance to the BET equation is indicative of the homogeneity of the dopant (right)
OH
Mg
O
2a
S
Scheme 1
Table 1 Asymmetric carboxylation of 1a with CO2 on different cathodesa.
Besides the preparation and characterization of a new organically doped composite, we pay more attention to the applicability in asymmetric carboxylation. To test the catalytic activity, powder of [Co]@Ag was pressed into coin and used as cathode for the asymmetric carboxylation of benzyl bromides with CO2. 1-phenylethyl bromide (1a, Scheme 1) was used as a model substrate. A typical asymmetric carboxylation was carried out in a mixture of 0.05 M 1a, 0.1 M tetraethylammonium iodide (TEAI) in an undivided glass cell, with a [Co]@Ag composite cathode and sacrificial magnesium (Mg) anode. No chiral cobalt complex was put in the reaction solution, [Co]@Ag composite acted as both cathode for electrochemical reaction and heterogeneous catalyst for asymmetric synthesis. Optically 2-phenylpropionic acid (2a) was obtained with a 73% ee and a 58% yield detected by HPLC (Table 1 entry 3). The ee value was slightly lower but the yield was significantly higher than the best result we achieved in the homogeneous catalytic system (glassy carbon as cathode, not 16 silver). S-[Co]@Ag was prepared the same way as [Co]@Ag II except for the utilization of Co -(s, s)(salen) as dopant (Fig. S1), similar yield and ee value were achieved, but the configuration was opposite as expectation. No by-product was detected by HPLC (or below the limit of detection), asymmetric carboxylation of 1a on [Co]@Ag composite shows excellent selectivity. To compare the heterogeneous and homogeneous catalysis in our current system, pure Ag NPs were used as cathode with chiral cobalt complex in reaction solution and other conditions were the same as [Co]@Ag, 8% ee value was
Entry
Cathode
1c 2d 3 4e
Ag NPs Ag NPs [Co]@Ag S-[Co]@Ag
Yieldb (%) 83 71 58 56
Selectivityb (%) 99 98 99 >99
R-eeb (%) 8 73 67(S)
a Anode: Mg, 20 mL CO2-saturated MeCN, 0.05 M 1a, supporting electrolyte: 0.1 M TEAI, current density: 5 mA cm-2, charge: 2 F mol-1, CO2 pressure: 1 atm, room temperature. b Determined by HPLC with a chiral column. c No chiral cobalt complex. d Chiral Cobalt complex in reaction solution, [Co]/substrate (molar ratio) =0.1. e Dopant: CoII-(s, s) (salen).
The major disadvantage of homogeneous catalysis involves the utilization of valuable and hardly recyclable catalysts. As contrast, no extra catalysts were needed in our reaction solution. The recyclability of the [Co]@Ag composite was subsequently examined in the asymmetric carboxylation of 1a, under reaction conditions of Table 1, entry 3. After the previous reaction, the composite catalyst was thoroughly washed with MeCN and then reused for a next reaction run under identical conditions with fresh reagents. As shown in Fig. 5, the composite showed no appreciable reduction of activity even after seven runs, and the 2a yield and ee value could maintain around 55% and 70%, respectively. The selectivity was always equal to or more than 99%, not listed in Fig. 5. To further investigate the stability of [Co]@Ag electrode, it was characterized by FE-SEM before and after using for 7 times (Fig. S7). Porous structure of [Co]@Ag composite was preserved
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after reuse, although the nanoparticles agglomerate into large ones to some extent. The heterogeneous nature of [Co]@Ag composite was verified by HPLC and ICP-AES. No trace (below the detection limit) of chiral cobalt complex was detected in the supernatant solution. Furthermore, the molar ratio of [Co] to Ag remained 1:140 before and after reusing for 7 times analysed by ICP-AES, indicating that dopant would not leach from the composite during the catalysis procedure, although the weight of composite slightly reduced. According to EDAX patterns, chiral cobalt complex could also remain its homogenous dispersion in the composite (Fig. S8). All the experiments demonstrated that [Co]@Ag composite we prepared is highly stable and reusable. This could be attributed to metallic silver cage forming around the dopant, which effectively prevented the entrapped chiral cobalt complex from leaching during the reaction.23, 25
could be achieved even after recycle for 7 times. Hence, this composite might be an ideal heterogeneous support for the immobilization of homogeneous catalyst. On account of its large surface area, remarkable stability, entrapment of flexible dopant and high conductivity, organically doped metals could also be a perfect choice for chiral modified electrode.
Notes and references
Financial support from National Natural Science Foundation of China (21173085, 21203066, 21373090, 21473060) is gratefully acknowledged. 1 J. Kim, T. A. Johnson, J. E. Miller, E. B. Stechel, C. T. Maravelias, Energy Environ. Sci., 2014, 5, 8417–8429. 2 Q. Lu, J. Rosen, Y. Zhou, G. S. Hutchings, Y. C. Kimmel, J. G. Chen, F. Jiao, Nat. Commun., 2014, 5, 4242/1-4242/6. 3 S. Ma, Y. Lan , G. M. J. Perez, S. Moniri, P. J. A. Kenis, ChemSusChem, 2014, 7, 866 – 874. 4 C. Costentin, S. Drouet, M. Robert, J. M. Saveant, Science, 2012, 338, 90–94. 5 K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo, Energy Environ. Sci., 2012, 5, 7050–7059. 6 C. W. Li, M. W. Kanan, J. Am. Chem. Soc., 2012, 134, 72317234. 7 C. W. Li, J. Ciston, M. W. Kanan, Nature, 2014, 508, 504-507. 8 L. Zhang, D. F. Niu, K. Zhang, G. R. Zhang, Y. W. Luo, J. X. Lu, Green Chem., 2008, 10, 202-206. 9 L. X. Wu, H. Wang, Y. Xiao, Z. Y. Tu, B. B. Ding, J. X. Lu, Electrochem. Commun., 2012, 25, 116-118. 10 R. Reske, H. Mistry, F. Behafarid, B. Roldan Cuenya, P. Strasser, J. Am. Chem. Soc., 2014, 136, 6978– 6986. 11 K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo, Energy Environ. Sci., 2012, 5, 7050– 7059. 12 M. Rueping, T. Theissman, Chem. Sci., 2010, 1, 473-476. 13 J. Calleja, A. B. Gonzalez-Perez, A. R. de Lera, R. Alvarez, F. J. Fig. 5 Reuse of [Co]@Ag cathode. Reaction conditions as Table 1, entry 3. Fananas, F. Rodriguez, Chem. Sci., 2014, 5, 996-1007. 14 G. Bergonzini, L. Gramigna, A. Mazzanti, M. Fochi, L. Bernardi, A. Ricci, Chem. Commun., 2010, 46, 327-329. Encouraged by excellent results obtained with 1-phenylethyl 15 G. Dagousset, J. P. Zhu, G. Masson, J. Am. Chem. Soc., 2011, bromide (1a), the preparative scope of substrates was further 133, 14804-14813. studied of this catalytic system. Using reaction conditions of 16 B. L. Chen, H. W. Zhu, Y. Xiao, Q. L. Sun, H. Wang, J. X. Lu, Electrochem. Commun., 2014, 42, 55–59. Table 1, entry 3, wide range of substituted benzyl bromides including both electron-withdrawing and electron-donating 17 H. Behar-Levy, D. Avnir, Chem. Mater., 2002, 14, 1736-1741. 18 H. Behar-Levy, D. Avnir, Adv. Funct. Mater., 2005, 15, 1141groups could be asymmetric electrolyzed with CO2 to 1146. phenylpropionic acids in moderate to good yields and ee 19 I. Yosef, D. Avnir, Chem. Mater., 2006, 18, 5890-5896. values. As is obvious from the results summarized in Table S1, 20 G. Nesher, G. Marom, D. Avnir, Chem. Mater., 2008, 20, 4425–4432. [Co]@Ag composite could be applicable to the asymmetric carboxylation with wide range of substrates. In addition, 21 H. Behar-Levy, O. Neumann, R. Naaman, D. Avnir, Adv. Mater., 2009, 19, 1207–1211. [Co]@Ag composite was also effective for the asymmetric 22 R. Ben-Knaz, R. Pedahzur, D. Avnir, Adv. Funct. Mater., 2010, carboxylation of 1-phenylethyl chloride. 70% yield and 20% ee 20, 2324–2329. value (Table S1 entry 8) could be obtained using the same 23 S. Krackl, A. Company, Y. Aksu, D. Avnir, M. Driess, ChemCatChem, 2011, 3, 227-232. reaction condition as 1-phenylethyl bromide. In conclusion, [Co]@Ag composite was synthesized for the 24 D. Avnir, Acc. Chem. Res., 2013, 47, 579-592. 25 A. Indra, M. Greiner, A. K. Gericke, R. Schlçgl, D. Avnir, M. first time. It was successfully introduced to asymmetric Driess, ChemCatChem, 2014, 6, 1935 – 1939. carboxylation of benzyl bromides with CO2, which could realize 26 H. P. Yang, D. H. Chi, Q. L. Sun, W. W. Sun, H. Wang, J. X. Lu, heterogeneous catalysis, CO2 fixation and asymmetric Chem. Commun., 2014, 50, 8868-8870. synthesis on one catalyst. Both the preparation of composite 27 I. Yosef, R. Abu-Reziq, D. Avnir, J. Am. Chem. Soc., 2008, 130, 11880-11882. and catalysis procedure were performed under mild conditions, 28 L. D. Pachon, I. Yosef, T. Z. Markus, R. Naaman, D. Avnir, G. without the utilization of high temperature, pressure or Rothenberg, Nat. Chem., 2009, 1, 160−164. additive catalyst in reaction solution. Moreover, [Co]@Ag 29 H. P. Yang, H. Wang, J. X. Lu, Electrochem. Commun., 2015, shown remarkable stability and reusability, similar results 55, 18-21.
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This article can be cited before page numbers have been issued, to do this please use: H. Yang, Y. Yue, Q. Sun, Q. Feng, H. Wang and J. Lu, Chem. Commun., 2015, DOI: 10.1039/C5CC04554A.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Received 00th January 20xx, Accepted 00th January 20xx
Entrapment of a chiral cobalt complex within silver: a novel heterogeneous catalyst for asymmetric carboxylation of benzyl bromides with CO2
DOI: 10.1039/x0xx00000x
Heng-Pan Yang, Ying-Na Yue, Qi-Long Sun, Qiu Feng, Huan Wang* and Jia-Xing Lu**
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A novel way to accommodate heterogeneous catalysis, CO2 fixation and asymmetric synthesis on one catalyst is reported. [Co]@Ag composite was prepared for the first time and used for asymmetric carboxylation of benzyl bromides with CO2. All the procedure were perforemed under mild conditions. Moreover, [Co]@Ag composite has terrific stability and reusability. 1-3
CO2 is well known as greenhouse gas. On the other hand, CO2 is also a kind of clean, non-expensive and abundant C1 feedstock, which has great potential for large-scale industrial application.4-5 In recent years, electro-catalytic fixation of CO2 into valuable, economically competitive products has attracted much attention, which could generate large range of products including hydrocarbons, alcohols, CO, formic acid, carboxylic acids and carbonate with various values.6-11 Asymmetric synthesis is another important issue in modern society. The growing demand for optically active compounds in the pharmaceutical and agrochemical industry has stimulated an increased interest in asymmetric synthesis.12-15 It is obvious fact that combing CO2 fixation and asymmetric synthesis could bring extra economic benefits because optically active compounds are much higher value-added chemicals. This feasibility was investigated in our previous work, we reported a possible electrochemical route for the asymmetric carboxylation of achiral 1-phenylethyl chloride with CO2 in the presence of the chiral cobalt complex, optically active 2phenylpropionic acid with 37% yield and 83% ee was obtained.16 This is a homogeneous system, and the asymmetric inducer, chiral cobalt complex, would be added into reaction solution during every catalytic process, which is
expensive and hardly recyclable, just like many other homogeneous asymmetric systems. To solve this problem and immobilize the chiral cobalt complex, organically doped metals was introduced, which belongs to a new series of metallo-organics hybrid materials. In these composites, metal crystallites act as a porous cage which forms around the dopant, allows the diffusion of substrate and product molecules in and out the catalytic 17-25 material, yet not allowing the dopant to leach out. Given their special nature, we synthesized alkaloid@Ag composite by the entrapment of alkaloids within silver nanoparticles and successfully employed it as heterogeneous catalyst for asymmetric hydrogenation of α-ketoesters, 60% ee value and 93% yield were obtained under mild conditions and 26 alkaloid@Ag composite was highly stable and reusable. In addition, Avnir and co-workers also made great achievements in the preparation and application of organically doped metals, such as alkaloid@Pd composites for the partial enantioselective hydrogenation of ketones27 and [Rh]@Ag for the hydrogenation of styrene.28 In a word, organically doped metals might be an ideal heterogeneous support for homogeneous catalysts.
Fig. 1 Asymmetric carboxylation of benzyl bromides with CO2 on [Co]@Ag.
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Under the guideline of these strategies, we further explored the application of organically doped metals in asymmetric carboxylation (Fig. 1). [Co]@Ag composite was synthesized by the entrapment of a chiral cobalt complex within silver nanoparticles for the first time. It was pressed into coin and used as catalyst for the asymmetric carboxylation of benzyl bromides with CO2, which could realize heterogeneous catalysis, asymmetric synthesis, and fixation of CO2 on one catalyst. Optically active phenylpropionic acids were obtained, which are very important intermediates for Nonsteroidal Antiinflammatory Drugs (NSAIDs), like Ibuprofen, Naproxen, Ketoprofen and Flurbiprofen. Both the synthesis of catalyst and asymmetric carboxylation procedure were performed under mild conditions. The synthesis of [Co]@Ag composite involved reduction of AgNO3 by zinc powder in a chiral cobalt complex aqueous solution and chiral cobalt complex molecules were entrapped within silver nanograins during this procedure; CoII-(R, R)(salen) (Fig. S1) was used as model dopant. Fine powder was achieved after filtration and drying (Fig. S2). Pure Ag nanoparticles (Ag NPs) were prepared in the same way except for the absence of chiral cobalt complex in reducing solution (Fig. S3). An interesting phenomenon was observed that Ag NPs have characteristic metal colour and shine after pressing; while [Co]@Ag composite is relatively dark and lustreless.17, 20 Adsorption on the surface and the entrapment within metal are fundamentally different processes.19, 20, 26 After filtration, trace amount of chiral cobalt complex was detected by high performance liquid chromatography (HPLC) in filtrate, revealing that most of the dopant was entrapped. In contrast, pure Ag NPs was added into a same chiral cobalt complex solution, the concentration only fell by 1%. Moreover, the adsorbed cobalt complex could be detected by FT-TR, but the entrapped dopant couldn’t (Fig. S4).
A
nanometers, which results in the porous characteristics that is important for the functioning of this material as a catalyst. Porous structure was also observed in pure Ag NPs, aggregated from silver nanograins in the ~100 nm size range (Fig. S5). The X-rays diffraction (XRD) patterns of the [Co]@Ag composite is shown in Fig. 2D, typical diffraction peaks of metallic Ag are observed in XRD patterns. Note that the XRD patterns show no signs of Zn, silver oxide or metallic Co crystals. Moreover, shifting of the peaks compared with pure silver nanoparticles is almost negligible. This finding is one of the evidence that the chiral cobalt complex is not immobilized within the silver lattice but between the small metallic cages of the aggregated silver nanocrystals.23 The presence of the chiral cobalt complex was confirmed by EDAX and ICP-AES analysis. EDAX spectrum showed signals corresponding to silver, cobalt, carbon and oxygen (Fig. S6, up). The presence of zinc probably originated from trace amount of zinc in the entrapment process, since these are not seen by XRD, they are apparently residual. EDX mapping of a [Co]@Ag composite (Fig. S6, down) shows a homogenous dispersion of cobalt complex in the composite, as indicated by the different colours for silver (green) and cobalt (red). According to ICP-AES, Co and Ag have a molar ratio of 1:140, consistent with the initial ratio in preparation solution.
A
B
B 5 µm
1 µm
C
D
Fig. 3 X-ray photoelectron spectra of the Ag3d regions (A) of [Co]@Ag (red) and pure Ag NPs (black), Co2p regions (B) of [Co]@Ag (red) and pure cobalt complex (black).
500 nm Fig. 2 FE-SEM patterns of [Co]@Ag composite with different magnification, A (10k), B (50k), C (100k); D: XRD patterns of pure Ag NPs (a) and [Co]@Ag (b).
The FE-SEM patterns of the composite reveal that these elementary particles gather into particles varying in size from 40 to 60 nm, and further agglomerate into hundred
XPS reveal the valence state of Ag and Co in the composite. The Ag3d spectrum (Fig. 3A) shows the peak at the binding energy of 368.3 eV. This peak confirms that silver is present in the metallic form. The Co2p spectrum (Fig. 3B) shows the peaks at the binding energy of 780.0 eV and 795.5 eV, corresponding to Co2+ state in pure cobalt complex.
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Further characterization of the porous structure is provided by the shape of the nitrogen adsorption-desorption isotherm (Fig. 4, left) which is typical of a mesoporous material, an important feature that allows substrate molecules to penetrate and reach the entrapped chiral cobalt catalyst and product molecules to diffuse out. The BJH-calculated average 3 pore diameter is ~11 nm and total pore volume is 0.0212 cm -1 2 -1 g . The calculated BET surface area is 7.7 m g . These values are significantly higher than those for pure silver nanoparticles 3 -1 2 -1 (7 nm, 0.00282 cm g and 1.3 m g , respectively). Apparently, the doping process has a significant impact on the particle growth and agglomeration, leading to materials with smaller particles, more pores and higher surface areas.23, 27 An important observation is that the compliance to the BET equation is excellent (Fig. 4, right), which indicates again the homogeneous dispersion of the dopant in the metal.
achieved, far lower than [Co]@Ag. If no chiral inducer was added, we only detected racemic product (Table 1 entry 1), indicating that chiral cobalt complex was crucial in this chiral induction process, which also indirectly demonstrate the existence of chiral cobalt complex in the composite. Moreover, during an electro-catalytic asymmetric reaction, chiral inducer would gather around the electrode surface and create a chiral 25, 29 atmosphere for the asymmetric induction. Apparently, more chiral inducer on the cathode surface is in favour of higher ee value. As mentioned above, about 1% chiral cobalt complex could be adsorbed on Ag NPs surface, which made the adsorbed amount under 0.00033 g. It is also notable that solvent could easily wash away the adsorbed chiral cobalt complex. On the other hand, we pressed 2 g [Co]@Ag composite into a coin. So each [Co]@Ag cathode contains about 0.047 g chiral cobalt complex, hardly leaching into solvent. Therefore, the entrapment amount was nearly 150 times more than the adsorbed amount of chiral cobalt complex. Since this chiral induction process was conducted on the cathode surface, it could explain why ee value on [Co]@Ag was far higher than those on Ag NPs.
-
Br
+ CO2
[Co]@Ag
+
R
OH
+
O
1a
Fig. 4 Adsorption-desorption isotherms of nitrogen reveal the mesopososity of [Co]@Ag (left). The compliance to the BET equation is indicative of the homogeneity of the dopant (right)
OH
Mg
O
2a
S
Scheme 1
Table 1 Asymmetric carboxylation of 1a with CO2 on different cathodesa.
Besides the preparation and characterization of a new organically doped composite, we pay more attention to the applicability in asymmetric carboxylation. To test the catalytic activity, powder of [Co]@Ag was pressed into coin and used as cathode for the asymmetric carboxylation of benzyl bromides with CO2. 1-phenylethyl bromide (1a, Scheme 1) was used as a model substrate. A typical asymmetric carboxylation was carried out in a mixture of 0.05 M 1a, 0.1 M tetraethylammonium iodide (TEAI) in an undivided glass cell, with a [Co]@Ag composite cathode and sacrificial magnesium (Mg) anode. No chiral cobalt complex was put in the reaction solution, [Co]@Ag composite acted as both cathode for electrochemical reaction and heterogeneous catalyst for asymmetric synthesis. Optically 2-phenylpropionic acid (2a) was obtained with a 73% ee and a 58% yield detected by HPLC (Table 1 entry 3). The ee value was slightly lower but the yield was significantly higher than the best result we achieved in the homogeneous catalytic system (glassy carbon as cathode, not 16 silver). S-[Co]@Ag was prepared the same way as [Co]@Ag II except for the utilization of Co -(s, s)(salen) as dopant (Fig. S1), similar yield and ee value were achieved, but the configuration was opposite as expectation. No by-product was detected by HPLC (or below the limit of detection), asymmetric carboxylation of 1a on [Co]@Ag composite shows excellent selectivity. To compare the heterogeneous and homogeneous catalysis in our current system, pure Ag NPs were used as cathode with chiral cobalt complex in reaction solution and other conditions were the same as [Co]@Ag, 8% ee value was
Entry
Cathode
1c 2d 3 4e
Ag NPs Ag NPs [Co]@Ag S-[Co]@Ag
Yieldb (%) 83 71 58 56
Selectivityb (%) 99 98 99 >99
R-eeb (%) 8 73 67(S)
a Anode: Mg, 20 mL CO2-saturated MeCN, 0.05 M 1a, supporting electrolyte: 0.1 M TEAI, current density: 5 mA cm-2, charge: 2 F mol-1, CO2 pressure: 1 atm, room temperature. b Determined by HPLC with a chiral column. c No chiral cobalt complex. d Chiral Cobalt complex in reaction solution, [Co]/substrate (molar ratio) =0.1. e Dopant: CoII-(s, s) (salen).
The major disadvantage of homogeneous catalysis involves the utilization of valuable and hardly recyclable catalysts. As contrast, no extra catalysts were needed in our reaction solution. The recyclability of the [Co]@Ag composite was subsequently examined in the asymmetric carboxylation of 1a, under reaction conditions of Table 1, entry 3. After the previous reaction, the composite catalyst was thoroughly washed with MeCN and then reused for a next reaction run under identical conditions with fresh reagents. As shown in Fig. 5, the composite showed no appreciable reduction of activity even after seven runs, and the 2a yield and ee value could maintain around 55% and 70%, respectively. The selectivity was always equal to or more than 99%, not listed in Fig. 5. To further investigate the stability of [Co]@Ag electrode, it was characterized by FE-SEM before and after using for 7 times (Fig. S7). Porous structure of [Co]@Ag composite was preserved
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after reuse, although the nanoparticles agglomerate into large ones to some extent. The heterogeneous nature of [Co]@Ag composite was verified by HPLC and ICP-AES. No trace (below the detection limit) of chiral cobalt complex was detected in the supernatant solution. Furthermore, the molar ratio of [Co] to Ag remained 1:140 before and after reusing for 7 times analysed by ICP-AES, indicating that dopant would not leach from the composite during the catalysis procedure, although the weight of composite slightly reduced. According to EDAX patterns, chiral cobalt complex could also remain its homogenous dispersion in the composite (Fig. S8). All the experiments demonstrated that [Co]@Ag composite we prepared is highly stable and reusable. This could be attributed to metallic silver cage forming around the dopant, which effectively prevented the entrapped chiral cobalt complex from leaching during the reaction.23, 25
could be achieved even after recycle for 7 times. Hence, this composite might be an ideal heterogeneous support for the immobilization of homogeneous catalyst. On account of its large surface area, remarkable stability, entrapment of flexible dopant and high conductivity, organically doped metals could also be a perfect choice for chiral modified electrode.
Notes and references
Financial support from National Natural Science Foundation of China (21173085, 21203066, 21373090, 21473060) is gratefully acknowledged. 1 J. Kim, T. A. Johnson, J. E. Miller, E. B. Stechel, C. T. Maravelias, Energy Environ. Sci., 2014, 5, 8417–8429. 2 Q. Lu, J. Rosen, Y. Zhou, G. S. Hutchings, Y. C. Kimmel, J. G. Chen, F. Jiao, Nat. Commun., 2014, 5, 4242/1-4242/6. 3 S. Ma, Y. Lan , G. M. J. Perez, S. Moniri, P. J. A. Kenis, ChemSusChem, 2014, 7, 866 – 874. 4 C. Costentin, S. Drouet, M. Robert, J. M. Saveant, Science, 2012, 338, 90–94. 5 K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo, Energy Environ. Sci., 2012, 5, 7050–7059. 6 C. W. Li, M. W. Kanan, J. Am. Chem. Soc., 2012, 134, 72317234. 7 C. W. Li, J. Ciston, M. W. Kanan, Nature, 2014, 508, 504-507. 8 L. Zhang, D. F. Niu, K. Zhang, G. R. Zhang, Y. W. Luo, J. X. Lu, Green Chem., 2008, 10, 202-206. 9 L. X. Wu, H. Wang, Y. Xiao, Z. Y. Tu, B. B. Ding, J. X. Lu, Electrochem. Commun., 2012, 25, 116-118. 10 R. Reske, H. Mistry, F. Behafarid, B. Roldan Cuenya, P. Strasser, J. Am. Chem. Soc., 2014, 136, 6978– 6986. 11 K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo, Energy Environ. Sci., 2012, 5, 7050– 7059. 12 M. Rueping, T. Theissman, Chem. Sci., 2010, 1, 473-476. 13 J. Calleja, A. B. Gonzalez-Perez, A. R. de Lera, R. Alvarez, F. J. Fig. 5 Reuse of [Co]@Ag cathode. Reaction conditions as Table 1, entry 3. Fananas, F. Rodriguez, Chem. Sci., 2014, 5, 996-1007. 14 G. Bergonzini, L. Gramigna, A. Mazzanti, M. Fochi, L. Bernardi, A. Ricci, Chem. Commun., 2010, 46, 327-329. Encouraged by excellent results obtained with 1-phenylethyl 15 G. Dagousset, J. P. Zhu, G. Masson, J. Am. Chem. Soc., 2011, bromide (1a), the preparative scope of substrates was further 133, 14804-14813. studied of this catalytic system. Using reaction conditions of 16 B. L. Chen, H. W. Zhu, Y. Xiao, Q. L. Sun, H. Wang, J. X. Lu, Electrochem. Commun., 2014, 42, 55–59. Table 1, entry 3, wide range of substituted benzyl bromides including both electron-withdrawing and electron-donating 17 H. Behar-Levy, D. Avnir, Chem. Mater., 2002, 14, 1736-1741. 18 H. Behar-Levy, D. Avnir, Adv. Funct. Mater., 2005, 15, 1141groups could be asymmetric electrolyzed with CO2 to 1146. phenylpropionic acids in moderate to good yields and ee 19 I. Yosef, D. Avnir, Chem. Mater., 2006, 18, 5890-5896. values. As is obvious from the results summarized in Table S1, 20 G. Nesher, G. Marom, D. Avnir, Chem. Mater., 2008, 20, 4425–4432. [Co]@Ag composite could be applicable to the asymmetric carboxylation with wide range of substrates. In addition, 21 H. Behar-Levy, O. Neumann, R. Naaman, D. Avnir, Adv. Mater., 2009, 19, 1207–1211. [Co]@Ag composite was also effective for the asymmetric 22 R. Ben-Knaz, R. Pedahzur, D. Avnir, Adv. Funct. Mater., 2010, carboxylation of 1-phenylethyl chloride. 70% yield and 20% ee 20, 2324–2329. value (Table S1 entry 8) could be obtained using the same 23 S. Krackl, A. Company, Y. Aksu, D. Avnir, M. Driess, ChemCatChem, 2011, 3, 227-232. reaction condition as 1-phenylethyl bromide. In conclusion, [Co]@Ag composite was synthesized for the 24 D. Avnir, Acc. Chem. Res., 2013, 47, 579-592. 25 A. Indra, M. Greiner, A. K. Gericke, R. Schlçgl, D. Avnir, M. first time. It was successfully introduced to asymmetric Driess, ChemCatChem, 2014, 6, 1935 – 1939. carboxylation of benzyl bromides with CO2, which could realize 26 H. P. Yang, D. H. Chi, Q. L. Sun, W. W. Sun, H. Wang, J. X. Lu, heterogeneous catalysis, CO2 fixation and asymmetric Chem. Commun., 2014, 50, 8868-8870. synthesis on one catalyst. Both the preparation of composite 27 I. Yosef, R. Abu-Reziq, D. Avnir, J. Am. Chem. Soc., 2008, 130, 11880-11882. and catalysis procedure were performed under mild conditions, 28 L. D. Pachon, I. Yosef, T. Z. Markus, R. Naaman, D. Avnir, G. without the utilization of high temperature, pressure or Rothenberg, Nat. Chem., 2009, 1, 160−164. additive catalyst in reaction solution. Moreover, [Co]@Ag 29 H. P. Yang, H. Wang, J. X. Lu, Electrochem. Commun., 2015, shown remarkable stability and reusability, similar results 55, 18-21.
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