Porous Aromatic Frameworks for Size-Selective Halogenation of Aryl

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Porous Aromatic Frameworks for Size-Selective Halogenation of Aryl Compounds Yajie Yang,† Xiaoqin Zou,† Peng Cui,‡ Yingxi Zhou,‡ Shuai Zhao,† Lili Wang,† Ye Yuan,† and Guangshan Zhu*,† †

Key Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China ‡ State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China S Supporting Information *

ABSTRACT: Organic halides are vitally important chemical precursors or intermediates in the fields of agrochemical synthesis, molecular recognition, and material science. However, it is difficult to selectively synthesize these compounds due to the multiple reactive sites in aryl fragments. In this work, we prepared the first fully fluorinated porous aromatic framework (PAF). Its −C−F bond and hierarchical porosity have great benefits for PAF functionalization. After being decorated with different cyclodextrins (CDs), CD-PAF materials can incorporate diverse aryl compounds to protect their ortho sites from being attacked to produce parasubstituted molecules. This selectivity obviously increased with a decrease in the substrate size (from 0.97 to 0.41 nm). In addition, the CD-PAFs can undergo long-term use in both chlorination and bromination. KEYWORDS: porous aromatic framework, cyclodextrin, fluorinated bond, size-exclusive effect, selective halogenation (PIMs),27 conjugated microporous polymers (CMPs),28 covalent triazine frameworks (CTF),29 porous aromatic frameworks (PAFs),30 and porous organic polymer (POP),31 and they have attracted attention because they are easily prepared for special functions. PAFs are well-known stable structures with large surface areas.32 Apart from the easily tailored porous environment, their multiple constructing strategies provide even more possibilities for practical applications, including polymeric sieves,33 antibacterial coatings,34 and iodine capture.35 These salient characteristics of PAFs make them a desired platform for selective halogenation. In this work, we prepared a fully fluorinated PAF material (porous aromatic frameworks are series of porous materials. To date, we have published several works of PAFs, such as PAF1,32 PAF-23,35 and PAF-50.34 Also, the fully fluorinated PAF material in this report is denoted as PAF-63). Taking advantage of the −C−F bond, we introduced different CD molecules into the hierarchical channel of PAF-63 to gain CD-PAFs. With the protection of the CD-PAFs, the ortho site of the aryl substrates was protected from halogenation, and this selectivity has a close relationship with the size of the substrates.

1. INTRODUCTION Currently, numerous aryl halides are commercially used as raw materials or chemical intermediates in industrial production, mainly because the carbon−halogen bond possesses a unique reactivity in organic chemistry.1 Because these reagents are so important, many attempts have been made to investigate a strategy for the selective preparation of aryl halogen compounds, such as nucleophilic substitution, transition metal catalysis, and radical reactions.2−7 However, due to the multiple reactive sites in aryl fragments, it is difficult to control the appropriate conditions to prepare the target product. Mimetic halogenase was proposed as an effective route to accomplish this objective. A rationally designed cavity can incorporate aryl substrates to protect the unwanted sites from being attacked to prepare ideal aryl halides.8−11 Porous organic frameworks (POFs) are emerging as novel functional materials.12−14 Their spatial structures and pure organic compositions are of interest for applications in optoelectronics, gas separation, and catalysis.15−18 Currently, POFs can be roughly divided into two categories: crystalline POFs and amorphous POFs. Covalent organic frameworks (COFs)19−23 and porous organic cages (CCs)24,25 are two famous crystalline POFs because their structures are clearly determined. Examples of amorphous POFs include hyper-crosslinked polymers (HCPs),26 polymers of intrinsic microporosity © XXXX American Chemical Society

Received: July 19, 2017 Accepted: August 23, 2017

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DOI: 10.1021/acsami.7b10540 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

2. RESULTS AND DISCUSSION 2.1. Synthesis of the PAF Materials. As shown in Figure 1, the fully fluorinated PAF-63 was synthesized by an

demonstrated that no CD molecules were released from the porous framework. All these results indicate that the CDs are decorated onto the PAF skeleton via covalent bonds. Elemental analysis was performed to determine the elemental content of each sample (Table S1). After the calculations, we primarily concluded that there is one CD molecule per six PAF-63 segments. The powder X-ray diffraction (PXRD) results indicate that the PAF materials are amorphous, probably because of their lack of long-range ordered structures (Figure S4). All PAFs are agglomerated particles (Figure S5) with sizes from 2.0 to 5.0 μm in the scanning electron microscopy (SEM) images. The transmission electron microscopy (TEM) images also agree with the PXRD results, showing that the PAF materials are amorphous structures (Figure S6). The thermal stability of the PAFs was assessed by thermogravimetric analysis (TGA). Almost a 15.0% weight loss occurs in PAF-63 before 100 °C due to the loss of the solution molecules, and when heated over 350 °C, the PAF skeleton begins to decompose (Figure S7). There are almost no residues (800 °C in air), suggesting a high purity without any trace of inorganic components in the PAF63 product. For the CD-PAFs, their thermal stability is lower than that of the parent PAF-63, as evidenced by the lower decomposition temperature (ca. 250 °C). The porous nature of the PAF-63 was studied via N2 physical sorption at 77 K. As illustrated in Figure 2a, PAF-63 possesses a high surface area (2436 m2 g−1) based on the BET model and a widespread pore size distribution from 0.5 to 3 nm. Its pore

Figure 1. Scheme for the preparation of PAF-63 (a) and CD-PAFs (b) and the possible fragments of PAF-63 (c) and CD-PAFs (d) to guide the eye.

ionothermal reaction of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ).36 Then α-CD, β-CD, and γ-CD molecules were added into the dimethylformamide (DMF) solution of PAF-63 and K2CO3.37,38 After heating to 120 °C for 48 h, α-CD-PAF-63, β-CD-PAF-63, and γ-CD-PAF-63 were prepared in high yields. 2.2. Characterization of the PAF Materials. There are obvious changes from F4-TCNQ to PAF-63 in the Fourier transform infrared spectra (FT-IR) (Figure S1). The −CN band at 2233 cm−1 disappears, indicating the occurrence of the coupling reaction. Similarly, the relative intensity of the −OH band (at 3300 cm−1) in the CD-PAFs decreases, apparently due to the CD molecules, proving that the CDs decorate the PAF skeleton.38 The solid-state 13C CP/MAS NMR spectra (Figure S2) show two main peaks at 110−133 and 133−170 ppm, corresponding to the carbons from the triazine ring and aromatic ring in the PAF structure. The appearance of multiple peaks between 0 and 100 ppm are from the CD molecules, which are fixed onto the porous architecture. For comparison, a physical mixture of α-CD and PAF-63 was prepared (named CD@PAF-63) to study the covalent chemistry of CD-PAF-63. As shown in the 13C solid NMR spectra (Figure S3), we could distinguish the series of peaks at 50−106 and 110−170 from the α-CD molecule and PAF-63, respectively, in this noncovalent composite (CD@PAF-63). In contrast, the multiple peaks (0−100 ppm) clearly vary from that of the α-CD molecule in the covalently linked α-CD-PAF-63. Further, the covalent nature was investigated by a leaching experiment. As indicated in Table S1, the CD leaching test of the CD-PAFs

Figure 2. Nitrogen adsorption isotherms for PAF-63 (a) and CDPAFs (b). Pore size distribution of PAF-63 calculated by the NL-DFT method is shown in the inset of a. B


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Figure 3. Cavity diameters of α-CD (a), β-CD (b), and γ-CD (c). Schematic diagram of the incorporated complex (d). Ortho sites are protected, and para sites are available in the incorporated complexes (e). Various aromatic substrates (phenol, anisole, phenyl acetate, acetanilide, benzanilide, and 2-chloro-5-nitro-N-phenyl benzamide) with different tail diameters (f).

volumes are 0.38, 0.52, and 0.39 cm3 g−1, corresponding to pore sizes from 0.5 to 0.7, 0.7 to 2.0, and above 2.0 nm, respectively. This hierarchical porosity is conducive for the integration of CD molecules (outside diameters of α-CD, β-CD, and γ-CD are 1.37, 1.53, and 1.69 nm, respectively) into the PAF-63 channel.39 After reacting with the CDs, the CD-PAFs show almost no porosity (Figure 2b). This phenomenon implies that almost all channels of the PAF-63 are occupied by flexible CD molecules (Figure S8), which also proves the CDs successfully decorated the PAF-63 framework. 2.3. Uptake Experiments with the PAFs. Due to the high reactivity of halogenating agents it is difficult to control the appropriate conditions to selectively prepare aryl halides. To accomplish this objective, fully fluorinated PAF-63 displays several advantages. (1) The high energy of the −C−F bond can prevent unnecessary reactions and ensure the concentration of the halogenating agent, which is evidenced by the intact FT-IR spectra without any band changes for PAF-63 before and after the halogenating reaction (Figure S9). (2) Their hierarchical porosity favors the decoration of functional molecules into the PAF skeleton. Herein, three types of commercial CD molecules with different internal cavities (internal sizes of α-CD, β-CD, and γCD range from 0.57, 0.78, and 0.95 nm in diameter) were chosen for the PAF functionalization (Figure 3a−c).40 CDPAFs with hydrophobic CD internal cavities are good hosts to incorporate aromatic derivatives. As shown in Figure 3f, five aryl compounds (phenol, anisole, acetanilide, benzanilide, and 2-chloro-5-nitro-N-phenyl benzamide) were selected as model compounds to probe the size-selective effect of the PAFs. When the CD-PAFs (50 mg mL−1) were added into the aromatic derivative (1.0 × 10−4 mmol mL−1) solution, it only took a few minutes to form the incorporated complex. According to Figure 4, the capacity of the guest molecules decreases when the diameter of the substituent group is enlarged. The reduced uptake might result in a shrinking affinity for monosubstituted

Figure 4. Uptake experiments of phenol (a), anisole (b), phenyl acetate (c), acetanilide (d), benzanilide (e), and 2-chloro-5-nitro-Nphenyl benzamide (f).

benzene derivatives (head diameter of 0.58 nm and tail diameters increase from 0.41 to 0.97 nm). As observed, the incorporation capability of α-CD-PAF-63 is much better than that of β-CD-PAF-63. 2.4. Selective Halogenation of the PAFs. The selectivity of the PAFs was evaluated by halogenating aromatic substrates, and the results are summarized in Table 1. In this work, we C


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ACS Applied Materials & Interfaces Table 1. Halogenation Product Yields of the Aromatic Derivativesa,b

Pristine material is added into a 10−2 HOCl solution and treated at room temperature for 12 h.41 bPristine material is added into a CCl4 solution of Br2 at room temperature for 10 h.42,43

a

reaction in the presence of β-CD-PAF-63 (Table 1). Both results prove that CD-PAFs (either α or β) can protect the ortho sites of aryl substrates from being accessed by halogenating agents. To further determine the mechanism of selectivity, reference experiments were carried out. As shown in Figure S9, the skeleton of the CD-PAFs cannot be halogenated, which is evidenced by the lack of band changes in α-CD-PAF63 before and after the halogenating reaction in the FT-IR spectra. When pure PAF-63 is employed as the host, the products for o-chloroanisole and p-chloroanisole are 38% and 62%, and these values are similar to those of the blank experiments (40% and 60% for o-chloroanisole and pchloroanisole, respectively). As seen in Table S2, the selectivity of soluble α-CD and β-CD is roughly the same as that of the CD-PAFs. The combined data from the individual experiments using blank, PAF-63, CDs and CD-PAF materials support the conclusion that CD fragments play a central role in the selective halogenation.

prepare the para-substituted molecules in high yield and selectivity, so the molar ratio of CD (in CD-PAFs) and aryl substrate are excessive (∼200%). After α-CD-PAF-63 was added into the system, the substitution reaction occurred with the dominant products being the para species in the chlorination reactions (Table 1). This is mainly because αCD-PAF-63 preferentially blocks the chlorination of ortho sites, which is evidenced by the increased selectivity ratios of the para/ortho products for the same conversion (∼100%). The contents of the para substitutes increase to 97%, 95%, 92%, 82%, and 75% from 54%, 60%, 52%, 69%, and 67% for phenol, anisole, acetanilide, benzanilide, and 2-chloro-5-nitro-N-phenyl benzamide, respectively. In addition, the values in the brominating reaction are 94% (phenol), 92% (anisole), 96% (acetanilide), 89% (benzanilide), and 83% (2-chloro-5-nitro-Nphenyl benzamide), which are higher than those of the blank reference (73%, 86%, 56%, 71%, and 68%, respectively). A similar phenomenon is also observed for the halogenation D



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More interestingly, the contents of the para-substituted products decrease from 97% to 75% during the chlorination process as the tail diameter of the substituent group increases from 0.41 to 0.97 nm. An analogous decreasing trend was also observed in the brominating reaction (Table 1). Both the uptake and the halogenation experiments indicate that the amount of the substrates incorporated by the CD cavities decreases with increases in their sizes, and thus, the protecting effect of the CD-PAFs becomes weaker. This observation is attributed to the size-exclusive effect and could guide practical applications. On the basis of the above observations, we believe that the content of the para derivatives might exceed 90% if the tail diameter of the aryl substitute is smaller than 0.62 nm. We selected phenyl acetate (tail diameter of 0.60 nm) as an example to examine this hypothesis. As shown in Table 1, the values for the para substitution are 90% and 93% for the chlorination and bromination, respectively, which agrees with our prediction. For large aryl molecules (such as benzanilide and 2-chloro-5-nitro-N-phenyl benzamide), we believe that calixarenes with a strong bonding affinity for aromatic molecules might improve the selectivity of the halogenation reaction. γ-CD-PAF-63 possesses a larger internal cavity than the other CD-PAFs (internal sizes of α-CD, β-CD, and γ-CD range from 0.57, 0.78, and 0.95 nm in diameter). In this work, no results from the selective halogenation of aryl substrates with the γ-CD-PAF-63 are found. However, we believe γ-CDPAF-63 might provide the opportunity for selective halogenation of large aryl substrates (naphthalene and pyrene, for example). Due to the insoluble nature of PAFs, several washing procedures are required to remove the reactants or products for cycled uses. After cycling 10 times, the selectivity of the halogenation for anisole slightly decreases to 91% from 95%, indicating the good recyclability of the CD-PAFs (Figure S10). For the sake of comparison, α-CD was physically loaded into the hierarchical porous materials (PAF-63, commercial carbon, SiO2) to use α-CD@PAF-63, α-CD@C, and α-CD@SiO2 for the selective chlorination of anisole (Figure S11). As seen, the contents of p-chloroanisole significantly decrease for α-CD@ PAF-63, α-CD@C, and α-CD@SiO2 after three repetitive uses. This result verifies that the CD moieties are covalently decorated on the PAF-63 matrix to form CD-PAFs, which be used for long-term halogenation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10540. Experimental procedures and characterization of all new materials (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guangshan Zhu: 0000-0001-6841-737X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of the National Natural Science Foundation of China (NSFC Project, grant nos. 21531003, 21302061, and 21604008), National Basic Research Program of China (973 Program, grant nos. 2012CB821700 and 2014CB931804), Major International (Regional) Joint Research project of NSFC (grant no. 21120102034), and Opening Project of Key Laboratory of Polyoxometalate Science of Ministry of Education.

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3. SUMMARY In conclusion, we first prepared a fully fluorinated PAF-63 with a high surface area and hierarchical porosity. In contrast to the −C−H bond in traditional POFs, the −C−F bond in PAF-63 had a higher bond energy that prevented the skeleton from participating in side reactions, and the fluoride group could react with other molecules to realize its functionalization. After the CD molecules were fixed into the PAF skeleton, the CDPAFs exhibited a high selectivity for aryl substrates to form para-substituted products in halogenation reactions. In addition, their selectivity obviously increased as the tail sizes of the substrates decreased from 0.97 to 0.41 nm in both the chlorination and the bromination reactions. In addition to the size-selective halogenation performance, the CD-PAF materials also possessed high stability for repetitive uses. E


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Porous Aromatic Frameworks for Size-Selective Halogenation of Aryl

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