Bisindole 3arenes—Indolyl Macrocyclic Arenes Having Significant Iodine Capture Capacity

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2022Bisindole 3arenes—Indolyl Macrocyclic Arenes Having Significant Iodine Capture Capacity Xingke Yu, Wanhua Wu, Dayang Zhou, Dan Su, Zhihui Zhong and Cheng Yang Xingke Yu Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, State Key Laboratory of Biotherapy and Healthy Food Evaluation Research Center, Sichuan University, Chengdu 610064 , Wanhua Wu *Corresponding authors: E-mail Address: email protected E-mail Address: email protected Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, State Key Laboratory of Biotherapy and Healthy Food Evaluation Research Center, Sichuan University, Chengdu 610064 , Dayang Zhou Comprehensive Analysis Center, ISIR, Osaka University, Mihogaoka, Ibaraki 567-0047 , Dan Su Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, State Key Laboratory of Biotherapy and Healthy Food Evaluation Research Center, Sichuan University, Chengdu 610064 , Zhihui Zhong Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, State Key Laboratory of Biotherapy and Healthy Food Evaluation Research Center, Sichuan University, Chengdu 610064 and Cheng Yang *Corresponding authors: E-mail Address: email protected E-mail Address: email protected Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, State Key Laboratory of Biotherapy and Healthy Food Evaluation Research Center, Sichuan University, Chengdu 610064 https://doi.org/10.31635/ccschem.021.202101036 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Several novel macrocyclic arenes that are composed of six indole subunits, so-called bisindole3arenes (BID3s), were conveniently synthesized by the aluminum trichloride-catalyzed one-pot condensation of bisindole derivatives and paraformaldehyde in dichloromethane at room temperature. Their macrocyclic structures were demonstrated by X-ray single-crystal studies, and the presence of the macrocyclic cavities made it possible to accommodate specific small organic molecules. The BID3s have exceptionally high iodine adsorption ability due to the strong and synergic interaction of indole units toward iodine, exhibiting significant morphology changes upon adsorption and desorption of iodine. Iodine uptake capacity of up to 5.12 g·g−1 was found with MeBID3, which is the highest value ever reported for macrocyclic arenes. Download figure Download PowerPoint Introduction The importance of fission reactors in nuclear power plants as energy resources has been continuously growing. One of the major environmental pollutants of nuclear plants is the volatile radioactive iodine, including 131I and 129I, which quickly diffuses into the air and creates safety concerns due to the extremely long radioactive half-life of 129I (1.57 × 107 years) or short-lived but accumulable and metabolism-influencing 131I.1–4 The capture of radioactive iodine with metal-exchanged zeolites is the prevalent technique in the industry, which suffers from low uptake capacity.5,6 Various novel type absorbents, such as metal–organic frameworks,7–10 covalent organic frameworks,11–15 macrocyclic compounds, activated carbon,16,17 and zeolites,18,19 have recently emerged. In particular, macrocyclic compounds have attracted increasing attention due to their favorable aromatic ring-iodine complexation, their thermal and chemical stability, and the feasibility of processing them.20–25 Recent representative examples include pillar6arene23 biphenylnarenes,24 and functionalized leaning-tower6arenes,25 which showed an iodine adsorption capacity of 0.25, 0.67, and up to 2.08 g·g−1, respectively. These studies demonstrate the promising potential of macrocyclic arenes for iodine adsorption besides their significant roles in supramolecular and host–guest chemistry.26–30 The charge-transfer interaction of iodine with aromatic compounds plays a significant role in the uptake of iodine by macrocyclic arenes.31,32 On the other hand, studies on exploring new macrocyclic arenes have recently grown rapidly, producing a variety of new macrocyclic compounds, such as leaning-pillar6arene,33 prismarenes,34 pagoda4arene,35 diphenylaminenarenes,36 and pagoda5arenes.37 Most known macrocyclic arenes are based on electron-enriched aromatic hydrocarbon units, such as phenol derivatives, while only a few were composed of heteroaromatic rings-based subunits.38–42 Indole is a π-electron-rich heteroaromatic ring with a unique ring structure and electron donor–acceptor properties, which forms a charge-transfer complex with iodine and stability constants overwhelmingly higher than common aromatic hydrocarbons.31,43 As a continuation of our studies on supramolecular self-assembly,44–48 we launched the construction of novel indole-based macrocyclic arenes and report herein their significantly high absorption ability toward iodine. Experimental Methods In this study, all reagents were purchased commercially and used without further purification unless otherwise noted. Iodine (99.99%) was purchased from ADAMAS-BETA (Shanghai, China). 1H and 13C NMR spectra were recorded at room temperature on a Bruker AMX-400 (Bruker, Germany) (operating at 400 MHz for 1H NMR and 101 MHz for 13C NMR) with tetramethylsilane (TMS) as the internal standard. High-resolution mass spectrometry (HRMS) data were measured with a Waters Q-TOF Premier (Waters, USA) electrospray ionization mass spectrometry or a matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) spectrometer. UV–vis spectra were recorded by using a JASCO V650 (JASCO, Japan) spectrometer. Fluorescence spectra were taken on a JASCO FP-8500 (JASCO, Japan) spectrofluorometer, and the bandwidth for the measurement was not fixed. Fourier transform infrared (FT-IR) spectra were recorded as KBr pellets on a NEXUS 670 (Thermo Nicolet, USA) FTIR spectrometer. Thermogravimetric analysis (TGA) was carried out using a Shimadzu DTG-60 (Shimadza, Japan) instrument, and the samples were heated under nitrogen gas at a rate of 10 °C/min. Powder X-ray diffraction (PXRD) data were measured with a powder X-ray diffractometer (X' Pert Pro MPD DY129, Malvern Pananalytical, Shanghai, China) at a range 5–60° of 2θ. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) investigations were carried out on JSM-5900LV (JEOL, Tokyo, Japan). N2 adsorption isotherms measurements were performed with a QUADRASORB (Anton Paar, USA) instrument at 87 K. Melting points were measured with a digital melting point apparatus (WRS-1B, Yidian, Shanghai, China). More experimental details and characterization of products are available in the Supporting Information. Results and Discussion Synthesis and characterizations of BID3s The attempt to synthesize indolyl macrocycles by reacting indole with paraformaldehyde yielded cyclic trimers condensed at the indole′s 2′ and 3′ positions, which bore only a small, bowl-like cavity ( Supporting Information Figures S7 and S8).49 However, condensation of bisindole derivatives 1a–1c (Scheme 1, Supporting Information Figures S1–S6), synthesized by reacting corresponding indole derivatives with dibromomethane or methylene chloride in the presence of sodium hydride, with paraformaldehyde under the catalysis of aluminum trichloride, led to the bisindole3arenes (BID3s) in yields of 4.8–10.3%. Other Lewis acid catalysts such as ferric chloride and boron trifluoride ether were also tried, but no macrocyclic compound was formed. BID3, MeBID3, and MeOBID3 (Scheme 1) showed the M–H− peaks at m/z 773.3948, 857.4564, and 953.4167, respectively, in the MALDI-TOF MS spectra ( Supporting Information Figures S28–S30). All BID3s contained six indole units, but only 5, 4, and 4 kinds of aromatic protons were observed in 1H NMR spectra of BID3, MeBID3, and MeOBID3 ( Supporting Information Figures S15–S20), respectively, which coincided with the C3v-symmetry structures of BID3s. Scheme 1 | The synthesis of 1a–1c, BID3, MeBID3, and MeOBID3. Download figure Download PowerPoint By the solvent-evaporation method in the mixed solvents of chloroform and methylbenzene or single solvent of N,N-dimethylformamide, the single crystals of the macrocycles BID3 and MeBID3 were successfully grown, respectively. The X-ray single-crystal analyses revealed that the six indole units of BID3 (Figures 1a and 1c, Supporting Information Figures S36) alternatively pointed upward and downward relative to the annulus plane of the macrocycle. The distances between the carbon or nitrogen atom of the symmetrical indole units were 7.44–9.22 Å, suitable for accommodating a small organic molecule. On the contrary, the methylindole units in MeBID3 were arranged in an orderly petal-like fashion, and the cavity shrunk significantly as all methyl groups inclined directly toward the cavity (Figures 1b and 1d, Supporting Information Figures S37). CH···π interactions played an important role in the packing structure of BID3, showing CH···aromatic plane distances of 2.59 and 2.67 Å, respectively ( Supporting Information Figures S38). On the contrary, the unit cell in the crystal structure of MeBID3 showed a relatively disorganized arrangement (Figure 1d). The CH···π interaction (2.68 Å) and intermolecular π···π interaction (4.49 Å) are also shown in the packing diagram of MeBID3 ( Supporting Information Figures S39). Figure 1 | Crystal structures of (a) BID3 and (b) MeBID3 in top and side view, and the packing mode of (c) BID3 and (d) MeBID3. C, gray; H, white; N, blue; solvent and the hydrogen atoms were omitted for clarity.a Download figure Download PowerPoint Iodine vapor capture by BID3s The TGA revealed that there was no apparent weight loss from 20 to 267 °C for MeBID3. BID3 and MeOBID3 were stable even at a temperature higher than 300 °C ( Supporting Information Figures S49–S51). Moreover, BID3s showed high melting points of >300, 266.2, and 281.8 °C for BID3, MeBID3, and MeOBID3, respectively, while all indole subunits had melting points lower than 60 °C.50–52 This should allow BID3s to be used in the high-temperature environment of the nuclear power plant. The uptake of iodine vapor with BID3s was thus examined by placing the solid powder of BID3s in preweighed vials and then exposing the powder vials to iodine vapor in a sealed container in an oven at 75 °C. After exposure to iodine vapor, the BID3s powders gradually deepened in color over time (Figure 2b). The adsorption iodine over time was weighed after cooling the iodine-loaded samples down to room temperature. As illustrated in Figure 2a, the uptake values of iodine increased with time, and BID3, MeBID3, and MeOBID3 reached saturation adsorption at ca. 25, 12, and 10 h, respectively. Excitingly, all three BID3s showed strong absorption of iodine vapor, and uptake ratios (wt/wt) of 4.49 4.73, and 5.12 g·g−1 were observed for MeOBID3, BID3, and MeBID3, respectively, which is by far the largest unit iodine uptake capacity by macrocyclic compounds ever reported ( Supporting Information Figures S69). Meanwhile, such uptake ratios mean that on average 2.41, 2.67 and 2.81 iodine molecules were attached per indole unit for BID3, MeBID3 and MeOBID3, respectively, demonstrating that the iodine molecules should not just be complexed at the inner cavity of BID3s but also on the exterior side of the cavity, presumably through the exterior wall synergy. Moreover, compared to the indole-derived polymers, which showed an uptake capacity up to 0.66 g·g−1 by virtue of the electron-rich frameworks,53 the iodine uptake capacity with MeBID3 was approximatively eight times higher, further confirming the importance of macrocyclic synergism. Figure 2 | (a) Time-dependent I2 vapor uptake profiles of BID3, MeBID3, and MeOBID3 at 75 °C. (b) The photographs of BID3s upon I2 vapor uptake at varying time intervals, and FT-IR spectra of (c) BID3, (d) MeBID3, and (e) MeOBID[3 before (black line) and after adsorption of iodine vapor (red line). Download figure Download PowerPoint The high uptake values were exceptional compared to the values reported with known macrocyclic arenes.20–24 To shed light on the mechanism of the significant uptake capacity, we examined the pore performance of macrocycles by the physical adsorption of nitrogen at 77 K ( Supporting Information Figures S46–S48). The gradual increase and decrease of the adsorption and desorption curves for the adsorption and desorption processes, respectively, indicated the type-III adsorption isotherm. The specific surface areas were primarily low, showing 5.18 m2·g−1 for BID3, 7.79 m2·g−1 for MeBID3, and 1.78 m2·g−1 for MeOBID3. These results revealed that the BID3s are hardly porous in the crystalline state, which is similar to pillarnarenes in the solid-state.23 The initial nonporous structures of BID3s should change significantly to adaptively accommodate the iodine molecules. FT-IR spectroscopy of BID3s varied visibly after the uptake of iodine (Figures 2c–2e). For MeOBID3 as an example, the skeletal vibrations of the indole rings (1618.75 and 1579.79 cm−1) and the stretching of the C–N bonds (1285.12 and 1166.56 cm−1) vanished or became weak after the uptake of iodine (Figure 2e). This suggested that the indole subunits exist in diverse circumstances after the adsorption of iodine, which is also supported by the PXRD studies (Figures 3a, 3c, and 3e). The PXRD profiles of BID3 and MeBID3 showed sharp diffraction peaks (Figures 3a and 3c), indicating good crystallinity in the solid-state. On the contrary, MeOBID3 appeared to be amorphous in the solid-state because its diffractogram was a rather broad peak centralized around 20° (Figure 3e). However, after the uptake of iodine, the PXRD of all three BID3s showed no clear diffraction peaks assignable to BID3s or iodine, demonstrating the loss of the crystalline structure of BID3s and good dispersion of iodine molecules in I2⊂BID3s. Figure 3 | X-ray powder diffractogram (upper planes) and TGA profiles (lower planes) of BID3 and I2⊂BID3 (a and b), MeBID3 and I2⊂MeBID3 (c and d), and MeOBID3 and I2⊂MeOBID3 (e and f). Download figure Download PowerPoint TGA analyses of the iodine-saturated I2⊂BID3s samples indicated that the adsorbed iodine could be released upon heating. The I2⊂BID3 and I2⊂MeOBID3 exhibited a mass loss of 81.5% and 81.0%, respectively, after heating to 300 °C, and I2⊂MeBID3 showed a mass loss of 82.4% at 265 °C (Figures 3b, 3d, and 3f). The results suggested an incomplete release of iodine. Indeed, the EDS measurements also revealed the residue of a certain amount of iodine from the heated I2⊂BID3s ( Supporting Information Figures S75–S77). These results further demonstrated the strong interaction of iodine with BID3s. SEM studies indicated that BID3 and MeBID3 are microcrystals in the shape of rod and sheet, respectively, while MeOBID3 appeared non-crystalline (Figures 4a, 4d, and 4g), which is consistent with the PXRD study results. It is interesting to find that upon iodine adsorption, the powder of BID3s showed apparent changes in its morphology (Figures 4b, 4e, and 4h). The fine powders of BID3s were transformed into small blocky solids. In particular, I2⊂MeBID3 became a sticky liquid-like lump after the uptake of iodine (Figure 4e), suggesting that MeBID3 and iodine interdisperse to thus break the initial crystalline structure. However, MeBID3 showed a porous morphology after the desorption of iodine from I2⊂MeBID3 (Figure 4f). Such a significant morphology change due to absorption and desorption of iodine (Figure 4) has not been reported with macrocyclic arenes, for which the breakup of the initial BID3's stacking by the strong and synergic interaction of indole units toward iodine should be responsible. In addition, the blue emission of MeOBID3 in the solid-state was completely quenched after the uptake of iodine, presumably due to the charge transfer interaction between indole and iodine (Figure 5a, Supporting Information Figure S35). Figure 4 | SEM images of BID3s, I2⊂BID3s and after the release of iodine from I2⊂BID3s for BID3 (a–c), MeBID3 (d–f), and MeOBID3 (g–i). Download figure Download PowerPoint Iodine release from iodine-loaded macrocyclic arenes All BID3s are insoluble in water, which is applicable to the extraction of iodine from an aqueous iodine solution. As shown in Figure 5c, the solid sample of MeBID3 (5.0 mg) was suspended in the yellow aqueous iodine solution (1.0 mM), and the solution became colorless upon stirring over 0–370 minutes. The time-dependent UV-vis spectra of iodine in water (1.0 mM) gradually decreased with time in the presence of MeBID3 (Figures 5b and 5c). The residual iodine in water measured after 21 h was 10, 26, and 32 ppm, respectively, for MeBID3, BID3, and MeOBID3 ( Supporting Information Figures S52 and S53). Also, the adsorbed iodine could be primarily removed (>84%) after heating the sample at 100 °C under vacuum for 4 h. Such uptake/release could be carried out for several cycles without losing the performance (Figure 5d). On the other hand, the adsorbed iodine could be completely released by putting the I2⊂BID3s in a methanol solution within 50–100 min (Figures 5eand 5f, Supporting Information Figures S70 and S71), and BID3s could be primarily recycled due to the poor solubility of BID3s in methanol ( Supporting Information Figure S78). Figure 5 | (a) Photos of MeOBID3 and I2⊂MeOBID3 under a UV lamp (365 nm) in the light and dark. (b) Time-dependent UV/vis spectra of aqueous I2 solution (1.0 mM) after the addition of MeBID3 (5.0 mg). (c) Color change of the aqueous I2 solution (1.0 mM) after the addition of MeBID3 powder (5.0 mg). (d) I2 adsorption/desorption cycles of MeBID3. (e) Time-dependent UV/vis spectral changes after adding 0.5 mg I2⊂MeBID3 in 10.0 mL methanol. (f) Photographs of iodine release of 10.0 mg I2⊂MeBID3 in 5.0 mL methanol. Download figure Download PowerPoint Conclusion In this study, we have designed and synthesized several brand-new indole-based macrocyclic arenes through aluminum trichloride-catalyzed one-pot condensation. The BID3s showed extremely high iodine uptake capabilities at either the aqueous solution or the vapor phase, with MeBID3 showing an uptake capacity of up to 5.12 g·g−1, the highest value hitherto reported with macrocyclic arenes. Significant morphology changes were observed before and after iodine adsorption and desorption. The electron-enriched indoles and the well-confined macrocyclic structure provide strong interaction, and intermolecular and intramolecular synergy for binding iodine is presumably responsible for the exceptional uptake behavior. This study provides an ingenious method to construct new macrocyclic arenes and opens up new possibilities for efficient iodine adsorbent with macrocyclic arenes. Footnote a CCDC 2039778 and 2054303 contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Center. Supporting Information Supporting Information is available and includes experimental details, 1H and 13C NMR spectra of the compounds, UV–vis and fluorescence spectra, TGA, FT-IR spectra, single-crystal X-ray data (PDF), X-ray data of BID3 and MeBID3 (CIF), PXRD data, SEM and EDS investigations. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (nos. 21871194, 21971169, and 21572142) and National Key Research and Development Program of China (no. 2017YFA0505903). Acknowledgments The authors gratefully acknowledge the test platform of the specialized laboratory, College of Chemistry, Sichuan University, and the Analytical and Testing Center of Sichuan University for instrumental measurements. 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The authors also acknowledge Prof. L.-J. Ma and Dr. S. Liu of the College of Chemistry, and Prof. P. Wu of the Analytical and Testing Center, Sichuan University. times