Salen-Based Covalent Organic Framework - Journal of the American

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Salen-Based Covalent Organic Framework Li-Hua Li, Xiao-Lin Feng, Xiao-Hui Cui, Yun-Xiang Ma, San-Yuan Ding, and Wei Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01523 • Publication Date (Web): 07 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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Journal of the American Chemical Society

Salen-Based Covalent Organic Framework Li-Hua Li,† Xiao-Lin Feng,† Xiao-Hui Cui,† Yun-Xiang Ma,† San-Yuan Ding,*,† and Wei Wang*,†,‡ †State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China ‡Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China

ABSTRACT: Salen unit represents one of the most important ligands in coordination chemistry. We report herein the first example of Salen-based covalent organic framework (COF), in which both the construction of COF structure and the functionalization with Salen moieties have been realized in a single step. Due to its structural uniqueness, the obtained COF material, Salen-COF, possesses high crystallinity and excellent stability. Based on this, a series of metallosalen-based COFs were prepared via metalation for further applications.

N,N’-bis(salicylidene)ethylenediamine (Salen, Scheme 1a) has been considered as one of the most important ligands in the field of coordination chemistry.1 Due to their ability to stabilize metal ions in various oxidation states, Salen complexes have been widely used as efficient catalysts for numerous organic transformations.2 However, Salen-based small molecules are not stable under acidic conditions: even the silica gel of flash chromatography can cause some degree of hydraulic decomposition.3 In addition, Salen complexes are difficult to separate from the reaction mixture and therefore, cannot be reused in homogeneous systems. Accordingly, much effort has been devoted to the construction of Salenbased materials via heterogenization4 into silicas,5 polymers,6 metal organic frameworks,7 and so on. In most of these strategies, non-stable Salen units had to be synthesized in advance and then subject to further immobilization.4b Herein, we report the one-step construction of Salen-based crystalline porous network (Scheme 1b). Covalently constructed from organic building blocks, covalent organic frameworks (COFs)8 are an emerging class of crystalline porous polymers and have shown great potentials in diverse applications.9 However, construction of functional COFs is very difficult because functionality, crystallinity, and porosity must be considered together.10 Taking the advantage of their structural uniqueness, we achieve here the concise synthesis of Salen-functionalized COF material. As shown in Scheme 1a, two salicylaldehyde (1) and one ethylenediamine (2) afford the traditional Salen unit (3) via the simultaneous formation of two imine bonds.1 Coincidently, most COF materials are indeed constructed via multiple imine linkages.8 We therefore reasoned that the co-condensation11 of salicylaldehyde-based C3-geometric monomers, such as 5, with the linear monomer 2 may directly result in the Salen-based crystalline COF through imine-bond formation (Scheme 1b).

Scheme 1. Inspired by the classical synthesis (a) of the Salen unit 3 from 1 and 2, we realized the one-step construction (b) of Salen-based COF, Salen-COF, via the cocondensation of monomers 5 and 2 under solvothermal conditions. Similar to the formation of Co(Salen) 4 from 3, a series of metallosalen-based COFs, M/Salen-COF, were prepared via metalation of Salen-COF. After the careful screening of the reaction conditions, we synthesized the crystalline Salen-based COF material (denoted as Salen-COF) in a single step. The characteri-

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zation results indicated that Salen-COF possesses high crystallinity and excellent stability. Moreover, Salen-COF showed its versatility in coordinating various metal ions for further applications.

Figure 1. Observed PXRD pattern (black), the Pawleyrefined pattern (red), the difference plot (purple), and the simulated pattern (blue) for the eclipsed model (left inset: C, gray; N, blue; O, red; H atoms omitted for clarity) of Salen-COF. Right inset: Expansion of the observed (black) and the simulated (blue) PXRD profiles. The synthesis of Salen-COF was achieved via solvothermolysis of 2 and 5 in the mixed solvents of 1,4dioxane, ethanol, and 3 M aqueous acetic acid (see SI for details). Note that the key monomer 5 can be synthesized from cheap and commercially available compounds. The key to obtain highly crystalline Salen-COF is the careful screening12 of the mixed solvents (Table S3 and Figure S7). Its crystalline structure was determined by powder X-ray diffraction (PXRD) analysis with Cu Kα radiation. The experimental PXRD pattern (Figure 1, black) shows the most intense diffraction peak arising from the (100) facet with the d spacing of 31.84 Å, as well as other diffractions from (200) (16.31 Å), (210) (12.39 Å), and (001) (3.44 Å) facets. The structural simulation suggests that Salen-COF possesses preferably the eclipsed π−π stacking arrangement shown in Figure 1. In this model, the Pawley refined profile (Figure 1, red) matched well with the observed pattern (Rwp = 5.93% and Rp = 4.57%, respectively). Verification of the eclipsed structure was further supported by the pore-size-distribution analysis. The nitrogen adsorption isotherms at 77 K (Figure S10) showed a sharp uptake below P/P0 = 0.05 with a step between P/P0 = 0.05−0.30, which is characteristic for a mesoporous material. Calculated by the nonlocal density functional theory (NLDFT), the pore size distribution of Salen-COF was centered at ca. 2.5 nm (Figure S13), which agrees well with the calculated value of 2.9 nm based on the eclipsed structure. Meanwhile, the Brunauer−Emmett−Teller (BET) surface area was determined as 1366 m2 g-1 (Figure S19) and the total pore volume as 0.73 cm3 g-1 (P/P0 = 0.97). In addition, the 129Xe NMR spectrum showed only one peak for the adsorbed 129Xe atoms (Figure S22), indicating that Salen-COF afforded a

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uniformed porous environment.13 All these data identified the porous structure of Salen-COF with high crystallinity.

Figure 2. 13C CP/MAS NMR spectrum of Salen-COF. Asterisks denote spinning sidebands. The assignments of 13C chemical shifts of Salen-COF were indicated in the chemical structure. The well-resolved 13C NMR peaks indicates the high crystallinity of Salen-COF. The covalent construction of Salen-COF was further verified by FT-IR and 13C solid-state NMR spectroscopy. The FT-IR spectrum of Salen-COF (Figure S1) showed typical imine and hydroxyl bands at 1632 and 3440 cm-1, respectively, the case of which was also found for the model compound 3 (Figure S2). As shown in Figure 2, the 13C cross-polarization magic-angle spinning (CP/MAS) NMR signals of Salen-COF can be explicitly assigned as the proposed structure. Specifically, the typical signal at 166 ppm indicates the successful formation of imine bonds via the condensation of 2 and 5. Furthermore, the well-resolved 13C NMR peaks with narrow line widths confirmed the high crystallinity of Salen-COF. The stability of Salen-COF was thoroughly assessed in organic solvents and in aqueous solutions. Our results indicated that Salen-COF was insoluble and extremely stable in common solvents. Notably, the crystalline structure of Salen-COF was preserved in the aqueous solutions at pH = 1−13 (Figure S40). In contrast, its homogeneous counterpart 3 was completely degradated at pH = 1 (Figure S38). Note that other imine-based COFs, such as COF-LZU1,9l could not survive in the aqueous solution at pH = 1 either. The high stability of Salen-COF should originate from its structural characters: the 2D extended network, hydrogen-bonding, π-π stacking arrangement, and high crystallinity.9h,14 In this regard, Salen-COF may be potentially served as a stable host for coordinating a variety of metal ions. In order to validate this idea, we conducted metalation of Salen-COF with different metal ions (Scheme 1b, see SI for details). The ICP analysis (Figure 3, inset) on the resulting metallosalen-based COFs (denoted as M/SalenCOF) showed that most of the Salen pockets have been occupied by the metal ions. Meanwhile, the metalation of Salen-COF could be easily visualized by its color-change. In addition, the unchanged PXRD patterns of M/SalenCOF verified that the crystalline structure of Salen-COF was preserved after metalation (Figure 3).


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Figure 3. Comparison of the PXRD patterns of SalenCOF and M/Salen-COF. Top inset shows metal contents of M/Salen-COF: the measured values obtained from ICP analysis and the theoretical ones calculated for the quantitative metalation of Salen-COF. Digital photographs of Salen-COF and M/Salen-COF samples are also presented.

ingly, these results identified that Salen-COF can indeed act as a stable and versatile platform for preparing a series of metallosalen-based COFs. We further explored the possible application of metallosalen-based COFs. As an example, the catalytic activity of Co/Salen-COF was examined in the Henry reaction,15 which is an important route for C−C bond formation. Our results indicated that Co/Salen-COF could catalyze the Henry reaction to produce the corresponding products (Table S4) and could be easily separated by centrifugation for the recycle use. The catalytic activity (Table S5) and the crystalline structure (Figure S8) were almost remained after 4 recycles. In conclusion, we develop herein a ʻkill two birds with one stoneʼ16 approach to construct a Salen-functionalized COF, Salen-COF. Due to its structural uniqueness, SalenCOF exhibits better stability than its homogeneous counterpart and other imine-based COFs. Furthermore, a series of metallosalen-based COFs can be facilely obtained by using Salen-COF as a stable, porous, and crystalline host for metalating different metal ions. We expect that our effort will not only lead to the concise synthesis of other Salen-based COFs, but also push forward the applications of Salen-functionalized materials.

ASSOCIATED CONTENT Supporting Information. Detailed synthetic procedures, general procedure for Henry reaction, FT-IR Spectra, PXRD patterns, modeling details and atomic coordinates, gas adsorption, TGA traces, SEM images, TEM images, and XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *W.W. [email protected] *S.-Y.D. [email protected]

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 21425206 and 21632004).

Figure 4. IR spectra of Co/Salen-COF with the Co (blue), 6.9 (purple), and 8.4 The spectra were normalized group.

Salen-COF (black) and content of 1.6 (red), 3.5 wt% (green), respectively. by the band of the alkyne

We applied FT-IR spectroscopy to investigate the strong interaction between Salen-COF and metal ions. For example, upon metalation with Co(OAc)2 (the Co content varied from 1.6 to 8.4 wt%, see Table S1), the typical IR bands at 1632 cm-1 for the imine groups (−C=N−) and at 3440 cm-1 for the hydroxyl groups (−O−H) in Salen-COF (black line) were gradually decreased in Co/Salen-COF (Figure 4). Meanwhile, a new band appeared at 1601 cm-1, indicating the strong coordination between the imine bonds and Co2+. Similar behavior was also observed upon metalation of the corresponding homogeneous counterpart (Figure S2). Accord-

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Salen-Based Covalent Organic Framework - Journal of the American

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