Communication pubs.acs.org/IC
Aldehyde-Tagged Zirconium Metal−Organic Frameworks: a Versatile Platform for Postsynthetic Modification Fu-Gui Xi, Hui Liu, Ning-Ning Yang, and En-Qing Gao* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, College of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, P. R. China S Supporting Information *
prepared by direct synthesis from the aldehyde-tagged linker.10 The MOF known as UiO-67 is among a series of zirconium MOFs (Zr-MOFs) based on Zr6 clusters and dicarboxylates [biphenyl-4,4′-dicarboxylate (bpdc) for UiO-67].12 Here we report the synthesis of aldehyde-tagged UiO-67 MOFs by different methods. With these MOFs, we are able to perform versatile PSM through C−C and C−N coupling. Tandem PSM and a representative catalytic study are also included. A catalytic study of a basic MOF is included as a representative application. Three methods were investigated for the synthesis of aldehyde-tagged MOFs. First, UiO-67-CHO was synthesized by the direct solvothermal reaction between ZrCl4 and 2formalbiphenyl-4,4′-dicarboxylic acid (H2bpdc-CHO) in N,Ndimethylformamide (DMF; method I). Owing to the chemical lability of aldehyde, the reaction temperature is crucially important for UiO-67-CHO. The initial synthetic trials were performed at 120 °C, as widely adopted for UiO-67.12 The powder X-ray diffraction (PXRD) pattern suggests that the products are really analogous to UiO-67 (Figure 1a). However, the IR spectra show no evident band assignable to aldehyde ν(CO) (Figure 1b). After digestion in HF(aq)/dimethyl-d6 sulfoxide, the 1H NMR signals are inconsistent with H2bpdc-
ABSTRACT: Aldehyde-tagged UiO-67-type metal−organic frameworks (MOFs) have been synthesized via the direct solvothermal method or postsynthetic ligand exchange. Various functionalities have been introduced into the MOFs via postsynthetic modification (PSM) employing C−N and C−C coupling reactions of the aldehyde tag. Tandem PSM has also been demonstrated. An amino-functionalized MOF obtained by PSM is shown to be an efficient, heterogeneous, and recyclable catalyst for Knoevenagel condensation.
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etal−organic frameworks (MOFs) are being actively studied for various applications including gas adsorption,1 catalysis,2 sensing,3 and others.4 It is becoming increasingly important to develop MOFs possessing functionality that can modify the pore or bring in sophisticated properties. Postsynthetic modification (PSM) represents an attractive strategy of functionalization.5 Particularly attractive, the organic component of MOFs can be prefabricated to contain a specific reactive group (tag), and then rational covalent PSM may be performed via the diverse organic reactions developed by organic chemists. Great success has been achieved in PSM of MOFs tagged with amino, azido, alkynyl, or sulfonic groups.6 More approaches of PSM are still being sought to enrich the diversity and complexity of MOFs and to achieve better performance and new functions.7 However, there is a dilemma in the study: on the one hand, the group chosen to tag a MOF should not coordinate the metal ion and should be stable enough to survive the synthetic conditions of the MOF; on the other hand, because most MOFs have limited chemical stability, the tag should be active enough to allow PSM under mild conditions without destroying the MOF structure. The aldehyde group is a versatile platform for organic transformations but with weak coordinating ability to metal, promising a good tag for PSM. However, only a few aldehydetagged MOFs have been reported, including three zeolitic imidazolate frameworks (ZIF-90, ZIF-93, and SIM-1),8 a MOF5-type framework,9 and a mixed-linker zinc(II) adeninate based MOF.10 The reactions for PSM were limited to Schiff-base condensation,10,11 aldehyde reduction,8a and a 1,3-dipolar cyclization.9b A possible limitation for the study is the chemical lability of the aldehyde group, which could be incompatible with the synthetic reactions for MOFs. For example, the abovementioned aldehyde-tagged zinc(II) adeninate based MOF was obtained by postsynthetic ligand exchange and could not be © XXXX American Chemical Society
Figure 1. (a) PXRD, (b) IR, and (c) 1H NMR profiles of MOFs synthesized by different methods. Received: March 9, 2016
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DOI: 10.1021/acs.inorgchem.6b00598 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry CHO (Figure 1c). These results disappointingly indicate that the aldehyde group did not survive the reaction at 120 °C. This phenomenon is the subject of a further study that is underway in the laboratory. After many trials, we succeeded in avoiding the undesired reaction of aldehyde by lowering the reaction temperature to 80 °C. As shown in Figure 1, the product shows a strong band at 1690 cm−1, characteristic of aldehyde ν(CO). The NMR spectrum after digestion is in perfect agreement with that for H2bpdc-CHO. The PXRD profile is typical of a UiO-67 crystalline phase. Notably, once formed, UiO-67-CHO is stable. IR, NMR, and PXRD analyses after heating the MOF in DMF at 120 °C or in air at 200 °C suggested that the aldehyde group and the UiO-67 framework were both retained (Figure S1). According to thermogravimetric analysis (Figure S2), the material releases solvent molecules (weight loss, 40%) upon heating to 200 °C. The framework starts to decompose at about 350 °C. Postsynthetic exchange (PSE) of linkers, also known as solvent-assisted linker exchange (SALE),10,13 is a powerful method for incorporating functional linkers into MOFs, which usually leads to mixed-linker (ML) MOFs. We applied this method to tag UiO-67 with aldehyde groups (method II). In a typical test, UiO-67 was soaked in a DMF solution containing an equivalent amount of H2bpdc-CHO (with respect to bpdc in UiO-67) at room temperature (RT) for 24 h. The material isolated (denoted as UiO-67-CHO-ML-PSE) shows an unchanged PXRD profile and the characteristic IR band of aldehyde (Figure 1). NMR analysis confirmed that 39% of the bpdc linkers are exchanged to bpdc-CHO. An alternative approach to mixed-linker MOFs is the one-pot reaction of metal precursors with a mixture of linkers. Compared with PSE, the one-pot synthesis has the advantage of easy and time-saving workup. Encouraged by the successful synthesis of UiO-67-CHO, we sought to prepare UiO-67-CHO-MLs under similar solvothermal conditions (method III), with the fraction of bpdc-CHO varied from 33 to 50 and 67%. The product also shows the IR band of aldehyde, with the intensity increasing with the fraction of bpdc-CHO used (Figure 1). According to NMR analysis, the molar fractions of bpdc-CHO in the MOFs are 21, 40, and 55%, which are somewhat lower than the starting values. The active aldehyde group in the MOFs provides a versatile and convenient “handle” for PSM (Scheme 1). As an example of
PSM via Schiff base condensation, UiO-67-CHO was reacted with excessive ethylenediamine (en) in DMF at RT. Imine formation was evidenced by the IR spectrum of the solid isolated (denoted as UiO-67-en; Figure 2): the aldehyde ν(CO) band disappears, while the new band at 1640 cm−1 is attributable to ν(CN). Complete conversion of aldehyde was confirmed by NMR analysis after digestion.
Figure 2. IR (left) and 1H NMR (right) spectra of UiO-67-en (II) and UiO-67-diamine (III). The IR spectrum of UiO-67-CHO (I) is also shown for comparison.
PSM of UiO-67-CHO via Schiff base condensation was extended to 2-picolylamine (pca), 2-aminophenol (apo), and thiosemicarbazide (tsc). The compounds were chosen because they contain additional functional groups besides amino. NMR analyses (Figures S4−S6) indicated that the modification ratios are 94%, 85%, and 89% for UiO-67-pca, UiO-67-apo, and UiO67-tsc, respectively. PSM of UiO-67-CHO-ML-PSE was also performed with pca and apo under similar conditions, for which almost complete conversion of the aldehyde groups was demonstrated (Figures S4 and S5). To extend the scope of the aldehyde-based PSM, two C−C coupling reactions, Knoevenagel condensation and cyanosilylation, were studied for UiO-67-CHO. The former reaction was performed with malononitrile in DMF and the latter with trimethylsilyl cyanide in CH2Cl2. Both reactions proceeded readily at RT in the presence of Et3N as the catalyst. About 98% and 83% of the aldehyde groups were transformed via Knoevenagel condensation and cyanosilylation, respectively, according to NMR analysis (Figure S7). All MOFs after PSM retain the UiO-67 framework, as confirmed by PXRD (Figure S8). The porosity was evaluated by N2 adsorption measurements (Figures 3 and S9). They exhibit the type I isotherms typical of microporosity. The Brunauer− Emmett−Teller surface areas of UiO-67-CHO and UiO-67CHO-ML-PSE are 1590 and 1797 m2/g, respectively. The difference is simply because the aldehyde groups in the former occupy a larger portion of the pore space. For a similar reason,
Scheme 1. PSM of UiO-67-CHO via Different Routes
Figure 3. Left: N2 adsorption isotherms for selected MOFs. Right: Conversion versus time plots for Knoevenagel condensation of BA with malononitrile in DMF. B
DOI: 10.1021/acs.inorgchem.6b00598 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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the surface area is reduced by 11−25% after PSM (1210−1324 m2/g for MOFs from UiO-67-CHO and 1464−1590 m2/g for those from UiO-67-CHO-ML-PSE; Table S1). Tandem PSM is an approach to further increase the functional diversity and complexity of MOFs.5 To illustrate the feasibility of this approach, we carried out further PSM on UiO-67-en. Considering that the MOF bears both imino and amino groups, three reactions were tested (Scheme 1): reduction of imino with NaBH4 (in methanol, RT), Schiff-base condensation of amino with salicylaldehyde (in DMF, RT), and chelating metalation through both groups with PdCl2(CH3CN)2 (in CH3CN, 65 °C). According to NMR analyses, the modification ratio for reductive PSM is 96% (Figure 2; the material is denoted as UiO-67diamine), while only 15% of the amino groups underwent the second-step Schiff-base condensation (Figure S10). Energydispersive spectrometry on the metalated MOF revealed a Zr:Pd ratio of 1:0.43, indicating that nearly half of the imine−amino moieties are coordinated. PXRD confirmed that the framework is stable enough to withstand tandem PSM (Figure S8). As an example of applications, the basic MOF obtained by tandem Schiff-base condensation and reduction, UiO-67diamine, was studied as a catalyst for Knoevenagel condensation of benzaldehyde (BA) with malononitrile. With the catalyst (6 mol %, with respect to BA), the reaction proceeded smoothly in DMF at RT to give 2-benzylidenemalononitrile. Conversion of BA reached 81% after 30 min and was almost complete after 2 h (Figure 3). For comparison, the reaction without any catalyst needed 24 h to reach 97% conversion; the reaction over UiO-67CHO gave a conversion of only 8% after 2 h. Therefore, the MOF is an efficient basic catalyst for Knoevenagel condensation at RT. To confirm that the activity is not from any species leached into the liquid, the catalyst was filtered off after reacting for 5 min. The reaction in the filtrate was much slower, with the rate being similar to that of the blank reaction (Figure 3). Therefore, the active species is in the solid phase, and the catalysis is heterogeneous. To examine the recyclability, the solid after the catalytic reaction was reused for two successive runs. The conversions were about 96% after 2 h, suggesting retention of the activity. PXRD indicated that the UiO-67 framework is intact after catalytic use (Figure S11). In conclusion, we succeeded in synthesizing aldehyde-tagged Zr-MOFs by direct solvothermal methods and PSE. The reactivity of the aldehyde tag and the stability of the UiO-67 framework have allowed for PSM of the MOFs via C−N and C− C coupling reactions. Tandem PSM has also been demonstrated. A catalytic study suggests that the amino-functionalized MOF is an efficient and recyclable heterogeneous catalyst for Knoevenagel condensation. The work further demonstrates the great potential and versatility of the PSM strategy to functionalize MOFs. Studies along that line will engender MOFs with further complexity and new properties that are important for realizing their full potential in technological applications.
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Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21173083 and 21471057). REFERENCES
(1) De Coste, J. B.; Peterson, G. W. Chem. Rev. 2014, 114, 5695−5727. (2) García-García, P.; Muller, M.; Corma, A. Chem. Sci. 2014, 5, 2979− 3007. (3) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Chem. Rev. 2012, 112, 1105−1125. (4) (a) He, C.; Liu, D.; Lin, W. Chem. Rev. 2015, 115, 11079−11108. (b) Wang, C.; Liu, D.; Lin, W. J. Am. Chem. Soc. 2013, 135, 13222− 13234. (5) Cohen, S. M. Chem. Rev. 2012, 112, 970−1000. (6) (a) Morris, W.; Briley, W. E.; Auyeung, E.; Cabezas, M. D.; Mirkin, C. A. J. Am. Chem. Soc. 2014, 136, 7261−7264. (b) Klinkebiel, A.; Reimer, N.; Lammert, M.; Stock, N.; Luening, U. Chem. Commun. 2014, 50, 9306−9308. (c) Bonnefoy, J.; Legrand, A.; Quadrelli, E. A.; Canivet, J.; Farrusseng, D. J. Am. Chem. Soc. 2015, 137, 9409−9416. (d) Wittmann, T.; Siegel, R.; Reimer, N.; Milius, W.; Stock, N.; Senker, J. Chem. - Eur. J. 2015, 21, 314−323. (e) Jiang, H.-L.; Feng, D.; Liu, T.-F.; Li, J.-R.; Zhou, H.-C. J. Am. Chem. Soc. 2012, 134, 14690− 14693. (f) Li, B.; Gui, B.; Hu, G.; Yuan, D.; Wang, C. Inorg. Chem. 2015, 54, 5139−5141. (g) Gui, B.; Meng, X.; Xu, H.; Wang, C. Chin. J. Chem. 2016, 34, 186−190. (7) Gui, B.; Meng, X.; Chen, Y.; Tian, J.; Liu, G.; Shen, C.; Zeller, M.; Yuan, D.; Wang, C. Chem. Mater. 2015, 27, 6426−6431. (8) (a) Morris, W.; Doonan, C. J.; Furukawa, H.; Banerjee, R.; Yaghi, O. M. J. Am. Chem. Soc. 2008, 130, 12626−12627. (b) Morris, W.; Leung, B.; Furukawa, H.; Yaghi, O. K.; He, N.; Hayashi, H.; Houndonougbo, Y.; Asta, M.; Laird, B. B.; Yaghi, O. M. J. Am. Chem. Soc. 2010, 132, 11006−11008. (c) Baias, M.; Lesage, A.; Aguado, S.; Canivet, J.; Moizan-Basle, V.; Audebrand, N.; Farrusseng, D.; Emsley, L. Angew. Chem., Int. Ed. 2015, 54, 5971−5976. (9) (a) Burrows, A. D.; Frost, C.; Mahon, M. F.; Richardson, C. Angew. Chem., Int. Ed. 2008, 47, 8482−8486. (b) Williams, D. E.; Dolgopolova, E. A.; Pellechia, P. J.; Palukoshka, A.; Wilson, T. J.; Tan, R.; Maier, J. M.; Greytak, A. B.; Smith, M. D.; Krause, J. A.; Shustova, N. B. J. Am. Chem. Soc. 2015, 137, 2223−2226. (10) Liu, C.; Luo, T.-Y.; Feura, E. S.; Zhang, C.; Rosi, N. L. J. Am. Chem. Soc. 2015, 137, 10508−10511. (11) (a) Thompson, J. A.; Brunelli, N. A.; Lively, R. P.; Johnson, J. R.; Jones, C. W.; Nair, S. J. Phys. Chem. C 2013, 117, 8198−8207. (b) Jose, T.; Hwang, Y.; Kim, D.-W.; Kim, M.-I.; Park, D.-W. Catal. Today 2015, 245, 61−67. (c) Canivet, J.; Aguado, S.; Daniel, C.; Farrusseng, D. ChemCatChem 2011, 3, 675−678. (12) (a) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. J. Am. Chem. Soc. 2008, 130, 13850− 13851. (b) Yuan, S.; Chen, Y.-P.; Qin, J.; Lu, W.; Wang, X.; Zhang, Q.; Bosch, M.; Liu, T.-F.; Lian, X.; Zhou, H.-C. Angew. Chem., Int. Ed. 2015, 54, 14696−14700. (c) Schaate, A.; Roy, P.; Godt, A.; Lippke, J.; Waltz, F.; Wiebcke, M.; Behrens, P. Chem. - Eur. J. 2011, 17, 6643−6651. (13) (a) Karagiaridi, O.; Bury, W.; Mondloch, J. E.; Hupp, J. T.; Farha, O. K. Angew. Chem., Int. Ed. 2014, 53, 4530−4540. (b) Kim, M.; Cahill, J. F.; Su, Y.; Prather, K. A.; Cohen, S. M. Chem. Sci. 2012, 3, 126−130.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00598. Experimental details, Table S1, and Figures S1−S11 (PDF) C
DOI: 10.1021/acs.inorgchem.6b00598 Inorg. Chem. XXXX, XXX, XXX−XXX
