Featuring research from Donglin Jiang’s laboratory / Department of Materials Molecular Science, Institute for Molecular Science, Okazaki, Japan
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Catalytic covalent organic frameworks via pore surface engineering We report a synthetic strategy for construction of the first example of organocatalytic covalent organic frameworks via pore surface engineering. The COF catalyst combines a number of striking features, including enhanced activity, broad applicability, good recyclability, and high capability to perform catalytic transformation under continuous flow. See Donglin Jiang et al., Chem. Commun., 2014, 50, 1292.
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Cite this: Chem. Commun., 2014, 50, 1292 Received 19th November 2013, Accepted 29th November 2013
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Catalytic covalent organic frameworks via pore surface engineering† Hong Xu,a Xiong Chen,a Jia Gao,a Jianbin Lin,a Matthew Addicoat,b Stephan Irleb and Donglin Jiang*a
DOI: 10.1039/c3cc48813f www.rsc.org/chemcomm
We report a synthetic strategy for construction of the first example of organocatalytic covalent organic frameworks via pore surface engineering. The COF catalyst combines a number of striking features, including enhanced activity, broad applicability, good recyclability, and high capability, to perform catalytic transformation under continuous flow conditions.
Covalent organic frameworks (COFs) are a class of crystalline porous polymers that enable the atomically precise integration of building blocks into two- or three-dimensional (2D or 3D, respectively) periodicities.1 In 2D COFs, the building blocks for the vertices and edges are covalently linked to form extended 2D polygon sheets that stack to constitute layered frameworks, giving rise to two structural characters, i.e., periodic p arrays and ordered one-dimensional (1D) channels. The size and shape of 1D pores can be tailored and they are useful for exploring gas adsorption and guest encapsulation.1–10 If catalytic sites can be successfully integrated into the frameworks, the highly ordered skeletal alignment and open-channel structure of 2D COFs provide an intriguing motif for exploring well-defined nanoreactors. Two strategies can be used for constructing catalytic COFs. Incorporating building blocks that possess catalytic sites constitutes the direct method but requires a tedious solvothermal synthesis. Particularly, if the catalytic site is bulky, forming a crystalline COF structure becomes difficult. Another methodology involves the postsynthetic integration of catalytic sites into a crystalline COF skeleton. This approach can reduce the influence of the bulky catalytic sites on the COF crystallinity and the undesired effect of harsh solvothermal conditions on the catalytic sites. The COF-based catalyst is limited to only one example that is based on a Pd-loaded COF for the Suzuki
a
Department of Materials Molecular Science, Institute for Molecular Science, National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan. E-mail: [email protected] b WPI-Research Initiative-Institute of Transformative Bio-Molecules and Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan † Electronic supplementary information (ESI) available: Synthetic methods, IR and gas sorption analysis, computational results, and cycle test. See DOI: 10.1039/c3cc48813f
1292 | Chem. Commun., 2014, 50, 1292--1294
cross-coupling reaction.10 The construction of a covalently linked, yet highly active catalyst remains a synthetic challenge in the field.11 Here we report the pore surface engineering strategy for the construction of covalently linked and highly active organocatalytic COFs (Scheme 1). Pore surface engineering has been demonstrated using boronate-linked COFs based on alkyne–azide5 and thiol–ene7 reactions. Recently, we explored a novel strategy for engineering the pore surface of stable imine-linked COFs and developed the first example of a covalently linked COF catalyst. We demonstrate this strategy by highlighting the controlled integration of organocatalytic sites into the pore walls to synthesize organocatalytic COFs that exhibit enhanced activity in asymmetric Michael addition reactions while retaining stereoselectivity. This COF catalyst combines a number of striking features, including broad applicability, good recyclability, and high capability to perform catalytic transformation under continuous flow conditions. We utilized a mesoporous imine-linked porphyrin COF as a scaffold in which the porphyrin units are located at the vertices and the phenyl groups occupy the edges of tetragonal polygon frameworks.6 A three-component reaction system consisting of 5,10,15,20-tetrakis(40 -tetraphenylamino) porphyrin as the vertices and a mixture of 2,5-bis(2-propynyloxy)terephthalaldehyde (BPTA) and 2,5-dihydroxytere-phthalaldehyde (DHTA) at varying molar ratios (X = [BPTA]/([BPTA] + [DHTA]) 100 = 0, 25, 50, 75, 100) as the edge units was developed to synthesize the COFs, by allowing the integration of ethynyl units of varying content into the edges (Scheme 1, [HCRC]X-H2P-COFs, X = 25, 50, 75, and 100 (X = 0: H2P-COF)). These reactions exhibited similar isolated yields to each other, indicating that the reactivity of BPTA and DHTA is similar under the solvothermal conditions (Table S1, ESI†). The click reaction of the ethynyl units with the azide compounds takes place quantitatively and anchors the desired groups to the pore walls at the desirable contents (Scheme 1, [Pyr]X-H2P-COFs). Therefore, this engineering methodology features catalytic sites on the pore walls whose density can be systematically designed and synthetically controlled (Scheme 1B). Infrared (IR) spectroscopy provides direct evidence for the presence of ethynyl units in [HCRC]X-H2P-COFs with characteristic CRC and H–CRC vibration bands at 2120 and 3290 cm1,
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Scheme 1 (A) The general strategy for the pore surface engineering of imine-linked COFs via a condensation reaction and click chemistries (the case for X = 50 was exemplified). (B) A graphical representation of [Pyr]X-H2P-COF with different densities of catalytic sites on the pore walls (gray: carbon, red: nitrogen, green: oxygen, purple: carbon atoms of the pyrrolidine unit; hydrogen is omitted for clarity).
respectively, which were absent in the spectrum for H2P-COF (Fig. S1, ESI†).5 The intensities of these two bands increased with increasing ethynyl content, indicating the successful integration of ethynyl units at different contents onto the pore walls of [HCRC]X-H2P-COFs. Elemental analysis reveals that the content of ethynyl units is close to the theoretical value (Table S2, ESI†). Pyrrolidine derivatives are well-known organocatalysts for Michael addition reactions, while the derivatives with bulky substituents give high stereoselectivity.12,13 In this study, we utilized the simplest pyrrolidine unit for demonstrating the effectiveness of a click reaction in synthesizing heterogeneous organocatalysts with active sites on the 1D channel walls; the simplest pyrrolidine unit has a relatively low steroselectivity. The click reaction of [HCRC]X-H2P-COFs with pyrrolidine azide in the presence of a CuI catalyst in toluene–tert-butanol at 25 1C quantitatively yielded the corresponding [Pyr]X-H2P-COFs (Scheme 1A, and ESI†).5 After the click reaction, the IR spectra demonstrated the disappearance of the vibration bands at 2120 and 3290 cm1 that were attributed to the H–CRC units (Fig. S1, ESI†), indicating that all the CRC units are clicked to bear the catalytic sites. The X-ray diffraction (XRD) intensity tended to decrease with increasing ethynyl content (Fig. 1A). The H2P-COF exhibited the highest XRD intensity of 1.47 104 counts, with 1.05 103 counts for the [HCRC]100-H2P-COF. This decrease in intensity is caused by the large number of amorphous ethynyl chains on the pore walls. A similar trend was previously reported for boronate-linked COFs upon pore surface engineering with amorphous units5 and Pd-imine
Fig. 1 XRD patterns of (A) H2P-COF (black) and [HCRC]X-H2P-COFs, and (B) [Pyr]X-H2P-COFs (orange: Pawley refined XRD curve of [Pyr]50H2P-COF, black: their difference).
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coordinated COFs.10 The presence of ethynyl groups does not create new XRD peaks. Because the [HCRC]X-H2P-COFs exhibit similar XRD patterns as H2P-COF, [HCRC]X-H2P-COFs possess the same crystalline structure as the H2P-COF.6 As more pyrrolidine units were integrated into the pore walls, the XRD intensity decreased (Fig. 1B); a similar phenomenon was observed for [HCRC]X-H2P-COFs (Fig. 1A). Similarly, the XRD peak positions remained unaffected, indicating that the crystalline skeleton was retained. For example, both [Pyr]50-H2P-COF (Fig. 1B, blue) and H2P-COF (Fig. 1A, black) exhibited peaks at 3.51, 4.91, 7.01, and 231, which are assignable to 100, 110, 200, and 001 facets, respectively. The Pawley refined XRD pattern (Fig. 1B, orange) reproduced the experimental curve (blue), confirming the aforementioned peak assignment, as is evident from their negligible difference (black). The density-functional tight-binding method including Lennard-Jones dispersion was employed to determine the optimal stacking isomer structures and revealed that these COFs adopt a 0.8 Å-slipped AA stacking structure (Table S3, ESI†). As the ethynyl content increased, [HCRC]X-H2P-COFs exhibited a decrease in the Brunauer–Emmett–Teller (BET) surface area (Fig. S2A, ESI†). The BET surface area was 1126, 1092, 859, 324, and 206 m2 g1 (Table S4, ESI†) as the ethynyl content increased from 0 to 25, 50, 75, and 100, respectively, whereas their pore sizes decreased from 2.2 to 2.0, 1.9, 1.5, and 1.5 nm (Fig. S3, ESI†). After the click reactions, much bulky groups were introduced into the pore walls of the COFs. Thus, [Pyr]X-H2P-COFs exhibited a distinct decrease in their BET surface areas (Fig. S2B, ESI†). The [Pyr]25-H2P-COF had a surface area of 960 m2 g1, which decreased to 675, 86, and 63 m2 g1 for [Pyr]50-H2P-COF, [Pyr]75-H2P-COF, and [Pyr]100-H2P-COF, respectively (Table S4, ESI†). Their pore sizes decreased from 1.9 to 1.6, 1.4, and 1.4 nm, respectively. The pore size distribution profile revealed that only one pore type existed (Fig. S3, ESI†), indicating that the catalytic units are homogeneously engineered onto the walls. The similar pore sizes of COFs with 75% and 100% engineered surfaces are likely related to the slipped AA structure. We investigated the catalytic activities of [Pyr]X-H2P-COFs in a Michael addition reaction in aqueous solutions, using (S)-4-(phenoxymethyl)-1-(pyrrolidin-2-ylmethyl)-1H-1H-1,2,3-triazole as a control (ESI†), which has an active structure identical to the pyrrolidine catalytic site of [Pyr]X-H2P-COFs. The pyrrolidine control yields a homogeneous system that required 3.3 h to achieve 100% conversion
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Table 1 Comparison of the pyrrolidine control, amorphous nonporous polymers, and COFs as catalysts for a Michael addition reaction
Control [Pyr]25-H2P-COF [Pyr]50-H2P-COF [Pyr]75-H2P-COF [Pyr]100-H2P-COF Amorphous polymer 1 Amorphous polymer 2
Time for 100% conversion (h)
dr
ee (%)
3.3 1 2.5 5 9 43 65
60/40 70/30 70/30 70/30 65/35 70/30 65/35
49 49 50 51 44 48 46
with ee and dr values of 49% and 60/40, respectively (Table 1). The steroselectivity of pyrrolidine derivatives is highly dependent on their substitutions; a large substituent gives a high stereoselectivity, while the simplest pyrrolidine unit that we employed in this study yields moderate steroselectivity.12,13 Remarkably, the activity was significantly enhanced when the pyrrolidine units were integrated to the pore walls of COFs. For example, the reaction time was shortened to only 1 h when [Pyr]25-H2P-COF was used as a heterogeneous catalyst, yielding ee and dr values of 49% and 70/30, respectively. Similarly, the reaction in the presence of [Pyr]50-H2P-COF required 2.5 h for 100% conversion and resulted in 50% ee and 70/30 dr. Therefore, the organocatalytic COFs have significantly higher catalytic activity than the monomeric catalyst while retaining the stereoselectivity. The 1D channels of the COFs could accommodate the reactant and substrate, which cannot be dissolved in a water–ethanol mixture. The catalytic activity depends upon the density of the active sites on the pore walls. [Pyr]75-H2P-COF with 75% active sites on the walls requires 5 h to complete the reaction. When each edge is anchored with catalytic sites, [Pyr]100-H2P-COF requires 9 h to reach 100% conversion. Therefore, highly dense pyrrolidine units in the pores cause a steric congestion and impede the mass transport through the channels. The crystallinity and porosity of COFs play a vital role in determining their catalytic activities. Amorphous and nonporous polymers 1 and 2 (Fig. S2C, ESI†), analogues to [Pyr]25-H2P-COF and [Pyr]50-H2P-COF, exhibited sluggish reactions, requiring 43 and 65 h to complete, respectively. The amorphous and nonporous polymers can drastically decrease the catalytic activity because only catalytic sites exposed to the particle surface are effective for the reaction. We further investigated seven different substrates to confirm the generality of the reaction system. As indicated in Table S5 (ESI†), the COF catalysts are widely effective for different substrates with similar activities and ee and dr values. [Pyr]X-H2P-COFs are easily separated from the reaction mixture via centrifugation. Thus, [Pyr]25-H2P-COFs can be reused at least four times (Table S6, ESI†). The slight decrement in activity could be attributed to the channels becoming blocked upon repetitive use, as evidenced by the decreased BET surface area after recycling (Fig. S4, ESI†). We explored the possibility of a continuous flow reaction using a columnar setup. We prepared a column consisting of a vertically mounted Teflon pipe loaded with silica gel (as a plug
1294 | Chem. Commun., 2014, 50, 1292--1294
Fig. 2 Representative chart for the flow reaction system based on the organocatalytic COF column.
to prevent COFs from flowing out) at the bottom and [Pyr]25H2P-COF (10 mg, 10 mm bed height) atop the silica gel (Fig. 2). After optimizing the parameters for the continuous-flow process, the device was found to work well at room temperature and yielded an optimal conversion when a solution of trans-4chloro-b-nitrostyrene (8.3 mM) and propionaldehyde (83 mM) in a mixture of water–EtOH (1/1 v/v) was passed through at a flow rate of 18 mL min1. The column maintained a 100% conversion and its stereoselectivities (44% ee, 65/35 dr) for more than 48 h under flow conditions. In summary, we have developed a pore surface engineering strategy that allows the molecular design of COF skeletons, controls the density and composition of the functional groups on the pore walls, and offers a general principle for designing catalytic COFs. Engineering pyrrolidine units onto the pore walls creates aqueous organocatalytic COFs, which combine a number of striking catalytic features, including significantly enhanced activity, good recyclability, and high capability to perform transformation under continuous flow conditions while retaining stereoselectivity. With this major advancement, we envisage that this work will initiate the exploration of various catalytic systems based on COFs via the structural engineering of both pores and skeletons.
Notes and references 1 X. Feng, X. Ding and D. Jiang, Chem. Soc. Rev., 2012, 41, 6010–6022. ¨ck, M. O’Keeffe and 2 F. J. Uribe-Romo, J. R. Hunt, H. Furukawa, C. Klo O. M. Yaghi, J. Am. Chem. Soc., 2009, 131, 4570–4751. 3 R. W. Tilford, S. J. Mugavero III, P. J. Pellechia and J. J. Lavigne, Adv. Mater., 2008, 20, 2741–2746. 4 S. Wan, J. Guo, J. Kim, H. Ihee and D. Jiang, Angew. Chem., Int. Ed., 2008, 47, 8826–8830. 5 A. Nagai, Z. Guo, X. Feng, S. Jin, X. Chen, X. Ding and D. Jiang, Nat. Commun., 2011, 2, 536. 6 X. Chen, M. Addicoat, S. Irle, A. Nagai and D. Jiang, J. Am. Chem. Soc., 2013, 135, 546–549. 7 D. N. Bunck and W. R. Dichtel, Chem. Commun., 2013, 49, 2457–2459. 8 N. L. Campbell, R. Clowes, L. K. Ritchie and A. I. Cooper, Chem. Mater., 2009, 21, 204–206. 9 P. Kuhn, M. Antonietti and A. Thomas, Angew. Chem., Int. Ed., 2008, 47, 3450–3453. 10 S. Y. Ding, J. Gao, Q. Wang, Y. Zhang, W. G. Song, C. Y. Su and W. Wang, J. Am. Chem. Soc., 2011, 133, 19816–19822. 11 Z. Xie, C. Wang, K. E. deKrafft and W. Lin, J. Am. Chem. Soc., 2011, 133, 2056–2059. 12 D. W. MacMillan, Nature, 2008, 455, 304–308. 13 C. A. Wang, Z. K. Zhang, T. Yue, Y. L. Sun, L. Wang, W. D. Wang, Y. Zhang, C. Liu and W. Wang, Chem.–Eur. J., 2012, 18, 6718–6723.
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As featured in:
Catalytic covalent organic frameworks via pore surface engineering We report a synthetic strategy for construction of the first example of organocatalytic covalent organic frameworks via pore surface engineering. The COF catalyst combines a number of striking features, including enhanced activity, broad applicability, good recyclability, and high capability to perform catalytic transformation under continuous flow. See Donglin Jiang et al., Chem. Commun., 2014, 50, 1292.
www.rsc.org/chemcomm Registered charity number: 207890
Published on 02 December 2013. Downloaded by MEDICAL RESEARCH COUNCIL LABORATORY OF MOLECULAR BIOLOGY on 23/10/2014 10:06:28.
ChemComm COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 1292 Received 19th November 2013, Accepted 29th November 2013
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Catalytic covalent organic frameworks via pore surface engineering† Hong Xu,a Xiong Chen,a Jia Gao,a Jianbin Lin,a Matthew Addicoat,b Stephan Irleb and Donglin Jiang*a
DOI: 10.1039/c3cc48813f www.rsc.org/chemcomm
We report a synthetic strategy for construction of the first example of organocatalytic covalent organic frameworks via pore surface engineering. The COF catalyst combines a number of striking features, including enhanced activity, broad applicability, good recyclability, and high capability, to perform catalytic transformation under continuous flow conditions.
Covalent organic frameworks (COFs) are a class of crystalline porous polymers that enable the atomically precise integration of building blocks into two- or three-dimensional (2D or 3D, respectively) periodicities.1 In 2D COFs, the building blocks for the vertices and edges are covalently linked to form extended 2D polygon sheets that stack to constitute layered frameworks, giving rise to two structural characters, i.e., periodic p arrays and ordered one-dimensional (1D) channels. The size and shape of 1D pores can be tailored and they are useful for exploring gas adsorption and guest encapsulation.1–10 If catalytic sites can be successfully integrated into the frameworks, the highly ordered skeletal alignment and open-channel structure of 2D COFs provide an intriguing motif for exploring well-defined nanoreactors. Two strategies can be used for constructing catalytic COFs. Incorporating building blocks that possess catalytic sites constitutes the direct method but requires a tedious solvothermal synthesis. Particularly, if the catalytic site is bulky, forming a crystalline COF structure becomes difficult. Another methodology involves the postsynthetic integration of catalytic sites into a crystalline COF skeleton. This approach can reduce the influence of the bulky catalytic sites on the COF crystallinity and the undesired effect of harsh solvothermal conditions on the catalytic sites. The COF-based catalyst is limited to only one example that is based on a Pd-loaded COF for the Suzuki
a
Department of Materials Molecular Science, Institute for Molecular Science, National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan. E-mail: [email protected] b WPI-Research Initiative-Institute of Transformative Bio-Molecules and Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan † Electronic supplementary information (ESI) available: Synthetic methods, IR and gas sorption analysis, computational results, and cycle test. See DOI: 10.1039/c3cc48813f
1292 | Chem. Commun., 2014, 50, 1292--1294
cross-coupling reaction.10 The construction of a covalently linked, yet highly active catalyst remains a synthetic challenge in the field.11 Here we report the pore surface engineering strategy for the construction of covalently linked and highly active organocatalytic COFs (Scheme 1). Pore surface engineering has been demonstrated using boronate-linked COFs based on alkyne–azide5 and thiol–ene7 reactions. Recently, we explored a novel strategy for engineering the pore surface of stable imine-linked COFs and developed the first example of a covalently linked COF catalyst. We demonstrate this strategy by highlighting the controlled integration of organocatalytic sites into the pore walls to synthesize organocatalytic COFs that exhibit enhanced activity in asymmetric Michael addition reactions while retaining stereoselectivity. This COF catalyst combines a number of striking features, including broad applicability, good recyclability, and high capability to perform catalytic transformation under continuous flow conditions. We utilized a mesoporous imine-linked porphyrin COF as a scaffold in which the porphyrin units are located at the vertices and the phenyl groups occupy the edges of tetragonal polygon frameworks.6 A three-component reaction system consisting of 5,10,15,20-tetrakis(40 -tetraphenylamino) porphyrin as the vertices and a mixture of 2,5-bis(2-propynyloxy)terephthalaldehyde (BPTA) and 2,5-dihydroxytere-phthalaldehyde (DHTA) at varying molar ratios (X = [BPTA]/([BPTA] + [DHTA]) 100 = 0, 25, 50, 75, 100) as the edge units was developed to synthesize the COFs, by allowing the integration of ethynyl units of varying content into the edges (Scheme 1, [HCRC]X-H2P-COFs, X = 25, 50, 75, and 100 (X = 0: H2P-COF)). These reactions exhibited similar isolated yields to each other, indicating that the reactivity of BPTA and DHTA is similar under the solvothermal conditions (Table S1, ESI†). The click reaction of the ethynyl units with the azide compounds takes place quantitatively and anchors the desired groups to the pore walls at the desirable contents (Scheme 1, [Pyr]X-H2P-COFs). Therefore, this engineering methodology features catalytic sites on the pore walls whose density can be systematically designed and synthetically controlled (Scheme 1B). Infrared (IR) spectroscopy provides direct evidence for the presence of ethynyl units in [HCRC]X-H2P-COFs with characteristic CRC and H–CRC vibration bands at 2120 and 3290 cm1,
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Scheme 1 (A) The general strategy for the pore surface engineering of imine-linked COFs via a condensation reaction and click chemistries (the case for X = 50 was exemplified). (B) A graphical representation of [Pyr]X-H2P-COF with different densities of catalytic sites on the pore walls (gray: carbon, red: nitrogen, green: oxygen, purple: carbon atoms of the pyrrolidine unit; hydrogen is omitted for clarity).
respectively, which were absent in the spectrum for H2P-COF (Fig. S1, ESI†).5 The intensities of these two bands increased with increasing ethynyl content, indicating the successful integration of ethynyl units at different contents onto the pore walls of [HCRC]X-H2P-COFs. Elemental analysis reveals that the content of ethynyl units is close to the theoretical value (Table S2, ESI†). Pyrrolidine derivatives are well-known organocatalysts for Michael addition reactions, while the derivatives with bulky substituents give high stereoselectivity.12,13 In this study, we utilized the simplest pyrrolidine unit for demonstrating the effectiveness of a click reaction in synthesizing heterogeneous organocatalysts with active sites on the 1D channel walls; the simplest pyrrolidine unit has a relatively low steroselectivity. The click reaction of [HCRC]X-H2P-COFs with pyrrolidine azide in the presence of a CuI catalyst in toluene–tert-butanol at 25 1C quantitatively yielded the corresponding [Pyr]X-H2P-COFs (Scheme 1A, and ESI†).5 After the click reaction, the IR spectra demonstrated the disappearance of the vibration bands at 2120 and 3290 cm1 that were attributed to the H–CRC units (Fig. S1, ESI†), indicating that all the CRC units are clicked to bear the catalytic sites. The X-ray diffraction (XRD) intensity tended to decrease with increasing ethynyl content (Fig. 1A). The H2P-COF exhibited the highest XRD intensity of 1.47 104 counts, with 1.05 103 counts for the [HCRC]100-H2P-COF. This decrease in intensity is caused by the large number of amorphous ethynyl chains on the pore walls. A similar trend was previously reported for boronate-linked COFs upon pore surface engineering with amorphous units5 and Pd-imine
Fig. 1 XRD patterns of (A) H2P-COF (black) and [HCRC]X-H2P-COFs, and (B) [Pyr]X-H2P-COFs (orange: Pawley refined XRD curve of [Pyr]50H2P-COF, black: their difference).
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coordinated COFs.10 The presence of ethynyl groups does not create new XRD peaks. Because the [HCRC]X-H2P-COFs exhibit similar XRD patterns as H2P-COF, [HCRC]X-H2P-COFs possess the same crystalline structure as the H2P-COF.6 As more pyrrolidine units were integrated into the pore walls, the XRD intensity decreased (Fig. 1B); a similar phenomenon was observed for [HCRC]X-H2P-COFs (Fig. 1A). Similarly, the XRD peak positions remained unaffected, indicating that the crystalline skeleton was retained. For example, both [Pyr]50-H2P-COF (Fig. 1B, blue) and H2P-COF (Fig. 1A, black) exhibited peaks at 3.51, 4.91, 7.01, and 231, which are assignable to 100, 110, 200, and 001 facets, respectively. The Pawley refined XRD pattern (Fig. 1B, orange) reproduced the experimental curve (blue), confirming the aforementioned peak assignment, as is evident from their negligible difference (black). The density-functional tight-binding method including Lennard-Jones dispersion was employed to determine the optimal stacking isomer structures and revealed that these COFs adopt a 0.8 Å-slipped AA stacking structure (Table S3, ESI†). As the ethynyl content increased, [HCRC]X-H2P-COFs exhibited a decrease in the Brunauer–Emmett–Teller (BET) surface area (Fig. S2A, ESI†). The BET surface area was 1126, 1092, 859, 324, and 206 m2 g1 (Table S4, ESI†) as the ethynyl content increased from 0 to 25, 50, 75, and 100, respectively, whereas their pore sizes decreased from 2.2 to 2.0, 1.9, 1.5, and 1.5 nm (Fig. S3, ESI†). After the click reactions, much bulky groups were introduced into the pore walls of the COFs. Thus, [Pyr]X-H2P-COFs exhibited a distinct decrease in their BET surface areas (Fig. S2B, ESI†). The [Pyr]25-H2P-COF had a surface area of 960 m2 g1, which decreased to 675, 86, and 63 m2 g1 for [Pyr]50-H2P-COF, [Pyr]75-H2P-COF, and [Pyr]100-H2P-COF, respectively (Table S4, ESI†). Their pore sizes decreased from 1.9 to 1.6, 1.4, and 1.4 nm, respectively. The pore size distribution profile revealed that only one pore type existed (Fig. S3, ESI†), indicating that the catalytic units are homogeneously engineered onto the walls. The similar pore sizes of COFs with 75% and 100% engineered surfaces are likely related to the slipped AA structure. We investigated the catalytic activities of [Pyr]X-H2P-COFs in a Michael addition reaction in aqueous solutions, using (S)-4-(phenoxymethyl)-1-(pyrrolidin-2-ylmethyl)-1H-1H-1,2,3-triazole as a control (ESI†), which has an active structure identical to the pyrrolidine catalytic site of [Pyr]X-H2P-COFs. The pyrrolidine control yields a homogeneous system that required 3.3 h to achieve 100% conversion
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Table 1 Comparison of the pyrrolidine control, amorphous nonporous polymers, and COFs as catalysts for a Michael addition reaction
Control [Pyr]25-H2P-COF [Pyr]50-H2P-COF [Pyr]75-H2P-COF [Pyr]100-H2P-COF Amorphous polymer 1 Amorphous polymer 2
Time for 100% conversion (h)
dr
ee (%)
3.3 1 2.5 5 9 43 65
60/40 70/30 70/30 70/30 65/35 70/30 65/35
49 49 50 51 44 48 46
with ee and dr values of 49% and 60/40, respectively (Table 1). The steroselectivity of pyrrolidine derivatives is highly dependent on their substitutions; a large substituent gives a high stereoselectivity, while the simplest pyrrolidine unit that we employed in this study yields moderate steroselectivity.12,13 Remarkably, the activity was significantly enhanced when the pyrrolidine units were integrated to the pore walls of COFs. For example, the reaction time was shortened to only 1 h when [Pyr]25-H2P-COF was used as a heterogeneous catalyst, yielding ee and dr values of 49% and 70/30, respectively. Similarly, the reaction in the presence of [Pyr]50-H2P-COF required 2.5 h for 100% conversion and resulted in 50% ee and 70/30 dr. Therefore, the organocatalytic COFs have significantly higher catalytic activity than the monomeric catalyst while retaining the stereoselectivity. The 1D channels of the COFs could accommodate the reactant and substrate, which cannot be dissolved in a water–ethanol mixture. The catalytic activity depends upon the density of the active sites on the pore walls. [Pyr]75-H2P-COF with 75% active sites on the walls requires 5 h to complete the reaction. When each edge is anchored with catalytic sites, [Pyr]100-H2P-COF requires 9 h to reach 100% conversion. Therefore, highly dense pyrrolidine units in the pores cause a steric congestion and impede the mass transport through the channels. The crystallinity and porosity of COFs play a vital role in determining their catalytic activities. Amorphous and nonporous polymers 1 and 2 (Fig. S2C, ESI†), analogues to [Pyr]25-H2P-COF and [Pyr]50-H2P-COF, exhibited sluggish reactions, requiring 43 and 65 h to complete, respectively. The amorphous and nonporous polymers can drastically decrease the catalytic activity because only catalytic sites exposed to the particle surface are effective for the reaction. We further investigated seven different substrates to confirm the generality of the reaction system. As indicated in Table S5 (ESI†), the COF catalysts are widely effective for different substrates with similar activities and ee and dr values. [Pyr]X-H2P-COFs are easily separated from the reaction mixture via centrifugation. Thus, [Pyr]25-H2P-COFs can be reused at least four times (Table S6, ESI†). The slight decrement in activity could be attributed to the channels becoming blocked upon repetitive use, as evidenced by the decreased BET surface area after recycling (Fig. S4, ESI†). We explored the possibility of a continuous flow reaction using a columnar setup. We prepared a column consisting of a vertically mounted Teflon pipe loaded with silica gel (as a plug
1294 | Chem. Commun., 2014, 50, 1292--1294
Fig. 2 Representative chart for the flow reaction system based on the organocatalytic COF column.
to prevent COFs from flowing out) at the bottom and [Pyr]25H2P-COF (10 mg, 10 mm bed height) atop the silica gel (Fig. 2). After optimizing the parameters for the continuous-flow process, the device was found to work well at room temperature and yielded an optimal conversion when a solution of trans-4chloro-b-nitrostyrene (8.3 mM) and propionaldehyde (83 mM) in a mixture of water–EtOH (1/1 v/v) was passed through at a flow rate of 18 mL min1. The column maintained a 100% conversion and its stereoselectivities (44% ee, 65/35 dr) for more than 48 h under flow conditions. In summary, we have developed a pore surface engineering strategy that allows the molecular design of COF skeletons, controls the density and composition of the functional groups on the pore walls, and offers a general principle for designing catalytic COFs. Engineering pyrrolidine units onto the pore walls creates aqueous organocatalytic COFs, which combine a number of striking catalytic features, including significantly enhanced activity, good recyclability, and high capability to perform transformation under continuous flow conditions while retaining stereoselectivity. With this major advancement, we envisage that this work will initiate the exploration of various catalytic systems based on COFs via the structural engineering of both pores and skeletons.
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