Alkenone-enol-alkenone [2+2+2] Cyclotrimerization Producing

5 days ago - Synopsis. The [2+2+2] cycloaddition reaction is first introduced into the construction of functional CPs decorated by carbonyl and hydrox...
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Alkenone-enol-alkenone [2+2+2] Cyclotrimerization Producing Functional Coordination Polymers with Excellent Adsorption Performance Wenjuan Xu,†,# Zhichao Shao,†,# Chao Huang,‡ Ruixue Xu,† Bingzhe Dong,† and Hongwei Hou*,† †

The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan 450001, P. R. China Center for Advanced Materials Research, Zhongyuan University of Technology, Zhengzhou 450007, P. R. China

‡

Inorg. Chem. Downloaded from pubs.acs.org by WASHINGTON UNIV on 03/06/19. For personal use only.

S Supporting Information *

ABSTRACT: [2+2+2] cycloaddition reactions are one of the most elegant routes for the construction of six-membered rings. Here, we report initially the alkenone-enol-alkenone [2+2+2] cycloaddition reaction and introduce the cycloaddition into the system of in situ building complexes, where three novel coordination polymers (CPs) with functional groups, namely {[CdCl2(L1)]}n (1), {[CdCl2(L2)]}n (2), and {[CdCl2(L3)]}n (3), have been obtained. Particularly, the new 1,2,3,4,5pentasubtituted cyclohexanols ligands L1, L2, and L3 are created from the starting chalcone derivatives via the [2+2+2] cyclotrimerization. Cadmium chloride not only plays the role of constructing CPs but also acts as a catalyst to promote the cycloaddition reaction. In addition, benefiting from numerous exposed carbonyl and hydroxyl function groups, CPs 1−3 are applied to the adsorptive removal of dyes (congo red (CR) and methyl orange (MO)) from aqueous solutions. As a result, 1−3 show excellent dye adsorption capacity. 1 exhibits maximum CR adsorption capacity of 485.4 mg g−1, and 3 has ultrahigh MO uptake capacity of 492.6 mg g−1. Experimental results suggest that the dye removal effect derives from the interactions between dye molecules and the exposed carbonyl and hydroxyl groups.

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INTRODUCTION

In the context of the strong demand for simple functionalization methods, in situ synthesis has been paid more and more attention because functional target CPs can be obtained directly by one-step reactions.27,28 Compared with the PSM process, in situ synthesis not only can avoid the loss of crystal state of CP materials but also can save the resource consumption for separation and purification of functional target ligands.29,30 An appropriate organic reaction is the key to in situ synthesis for the one-step construction of CP skeletons, in which the [2+2+2] reaction is a simple direct type to obtain six-membered ring compounds. If such cyclization can be successfully introduced into in situ synthesis of CP materials, the gorgeous CPs can be directly synthesized by using simple organic ligands. Importantly, the alkenes with different functional groups can be good candidates to form CPs loaded functional groups through this one-step synthesis strategy,31 which will provide a novel idea for the design and development of functional adjustable CP materials. Based on the above discussions, the chalcone derivatives (chal-1, chal-2, and chal-3 seen in Scheme S1) were selected as the substrates for the in situ [2+2+2] reaction, and three carbonyl and hydroxyl decorated cadmium-based CPs 1−3

In recent years, functional coordination polymer (CP) materials have been greatly developed due to their unique structures, interesting topology, and potential applications involving ferroelectrics, catalysis, magnetism and luminescence, etc.1−10 Especially, the adsorption and separation characteristics of CPs provide a good strategy and platform for energy storage, water pollution control, and purification of natural products.11,12 According to present reports, the loading of functional groups is a good way to improve the performance of CP materials.13−15 CPs modified with neutral groups such as thiol, thioether, or amino groups can remove different heavy metal ions (Pb 2+ , UO 2+ , Cd 2+ , Hg 2+ ) from aqueous solutions,16−19 and CPs with carbonyl, hydroxyl, and carboxyl groups can easily combine with some organic molecules via hydrogen bonds and selectively capture organic pollutants.20−22 Therefore, the CPs’ functionalization is important for adjusting material performances and extending application prospects. And many strategies have been devoted to modify the functionalization of CPs. Among them, postsynthetic modification (PSM)23−26 can effectively introduce functional groups directly into the skeletons to realize the functionalization of CP materials, but strict reaction requirements have greatly reduced the popularity and application. © XXXX American Chemical Society

Received: January 7, 2019

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DOI: 10.1021/acs.inorgchem.9b00037 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

C35H29CdCl2N3O3: C, 58.51; H, 4.04; N, 5.81. The experimental value: C, 58.87; H, 4.21; N, 3.69. IR: 3296s, 1682s, 1607s, 1419m, 1337w, 1224s, 1065s, 1011m, 933w, 826s, 661m. Synthesis of 2. The preparation of 2 is similar to that of 1 except that chal-1 was replaced by chal-2 (0.05 mmol, 0.013 g). Single crystals appear as canary clumps with a yield of 72% based on chal-2. The theoretical value of elemental analysis (%) for C43H33CdCl2N3O4: C, 61.63; H, 3.85; N, 5.01. The experimental value: C, 60.87; H, 3.21; N, 5.69. IR: 3418s, 1684s, 1608s, 1420s, 1345w, 1224m, 1067s, 974w, 799s, 690w. Synthesis of 3. The preparation of 3 is similar to that of 1 except that chal-1 was replaced by chal-3 (0.07 mmol, 0.023 g). Single crystals appear as yellow clumps with a yield of 87% based on chal-3. The theoretical value of elemental analysis (%) for C55H37CdCl2N3O4: C, 66.92; H, 3.78; N, 4.26. The experimental value: C, 68.87; H, 3.21; N, 3.69. IR: 3445s, 1688s, 1604s, 1417m, 1350w, 1222m, 1065w, 845s, 718w, 681w Crystal Structure Determination. The structures of 1, 2, and 3 were tested on a Bruker D8 VENTURE diffractometer with Mo Kα radiation (λ = 0.71073 Å). The SAINT program33 was used to control the integration of the diffraction data, polarization effects, and the intensity corrections for the Lorentz. We used the SADABS program34 to perform semiempirical absorption correction and solved structures by immediate ways and refined by a full matrix least-squares technique relied on F2 with the SHELXL-2014 software package.35 The hydrogen atoms were generated geometrically and refined isotropically using the riding model. In the crystal structures of 1, 2, and 3, the unit cell includes a large region of disordered solvent molecules, which could not be modeled as discrete atomic sites. We employed PLATON/SQUEEZE to calculate the diffraction contribution of the solvent molecules and to produce a set of solvent-free diffraction intensities. The final formulas were determined by combing element analyses and the electron count of the SQUEEZE results, about one H2O molecule per asymmetric unit for 1, two H2O molecules per asymmetric unit for 2, and four H2O molecules and five MeCN molecules per asymmetric unit for 3. The summary of crystallographic data for these compounds is listed in Table S4. Corresponding bond lengths (Å) and bond angles (deg) are provided in Table S5. Dye Adsorption Determination. CR and MO were selected to explore the adsorption abilities of 1, 2, and 3 for dyes in water at 25 °C. Stock solutions of CR and MO (1000 ppm) were prepared by dissolving solid CR (C32H22N6Na2O6S2, MW: 696.68) and MO (C14H14N3NaO3S, MW: 327.33) in deionized water. Working solutions of CR and MO were prepared by sequential dilution of the stock solution with deionized water. The adsorption rate experiments were performed by adding freshly prepared CPs (10 mg) in 10 mL solution containing organic dye, and the dye solutions containing the adsorbents were mixed well with magnetic stirring. After a predetermined time, the solution was separated from the adsorbent with a centrifuge. The UV−vis absorption spectra were recorded periodically to monitor the process. The maximum adsorption experiments were carried out by adding adsorbent (10 mg) into the prepared solution (15 mL) with a known dye concentration between 20 and 700 ppm, and the solution achieved adsorption equilibrium under shaking for 10 h. The final dye concentration was calculated by comparing the UV−vis absorbance to the appropriate calibration curve, and samples were diluted before analysis. The adsorption quantity (mg g−1) and the removal efficiency (R) were measured from the calibration curve using the equations

were synthesized. Benefiting from the structural features, CR and MO were selected as the model representatives of organic pollutants to study the adsorption performance of CPs 1−3. The studies on adsorption isotherms indicate 1 has maximum CR adsorption amount of 485.4 mg g−1 and 3 shows 492.6 mg g−1 adsorption capacity for MO, which is higher than the most known materials. Meanwhile, the in situ reaction mechanisms are studied by capturing intermediates 1,2,3,4,5-pentasubstituted cyclohexanols (ligands L1, L2, and L3 seen in Scheme S2), involving the [2+2+2] cycloaddition of two chalcone derivatives and one 4-acetylpyridine (enol form). This work not only provides excellent adsorbents for organic dye contaminants but also offers a novel and ingenious strategy of building functionalized complexes.

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EXPERIMENTAL SECTION

Materials and Apparatus. All chemicals used here were of analytical grade quality and obtained from commercial sources without further purification, chalcone derivatives chal-1, chal-2, and chal-3 were synthesized as previously reported.32 Deionized water (distilled) was used throughout the experiments. Powder X-ray diffraction (PXRD) patterns were carried out on a Rigaku D/MAX-3 with Cu Kα (λ = 1.5418 Å) irradiation in a 2θ range of 5−50° at room temperature. The element content (C, N, and H) was obtained by a FLASH EA 1112 elemental analyzer. LC-MS was performed with the Agilent Triple Quad LC/MS using the full scan method and APCI ion source. All UV−vis absorption spectra were performed by JASCO-750 UV−vis spectrophotometer using 3 mL quartz cuvette (10 × 10 mm) cells at 25 °C. The FT-IR spectra were obtained on a Bruker Tensor 27 spectrophotometer. Energy-dispersive spectroscopy (EDS) was measured with a scanning electron microscope (SEM, S4000). Synthesis of L1. 0.005 mmol CdCl2·2.5H2O (1.4 mg) and 0.1 mmol chal-1 (21 mg) were added into acetonitrile (7 mL) and H2O (1 mL). The system was placed in a 10 mL glass vial at 100 °C for 12 h. The white precipitates were purified by washing three times with acetonitrile. 1H NMR (400 MHz, DMSO) δ 8.42 (d, 2H), 8.33 (t, 4H), 7.57 (d, 2H), 7.29 (dd, 4H), 7.23 (d, 2H), 7.11 (t, 2H), 7.00 (s, 3H), 6.88 (d, 3H), 5.75 (s, 1H), 5.16 (d, 1H), 4.68 (t, 1H), 4.44 (dd, 4.7 1H), 4.05 (dd, 1H), 1.87 (dd, 1H). HRMS (ESI+), m/z calculated for C35H29N3O3 [M + H]+ 540.2242, found 540.2279. Synthesis of L2. 0.005 mmol CdCl2·2.5H2O (1.4 mg) and 0.1 mmol chal-2 (26 mg) were added into acetonitrile (7 mL) and H2O (1 mL). The system was placed in a 10 mL glass vial at 100 °C for 12 h. The white precipitates were purified by washing three times with acetonitrile. 1H NMR (400 MHz, DMSO) δ 8.89 (d, 1H), 8.78 (d, 1H), 8.45 (dd, 2H), 8.39 (d, 2H), 7.92−7.76 (m, 6H), 7.70 (d, 2H), 7.55 (dt, 3H), 7.46 (d, 1H), 7.39 (dd, 2H), 7.24−7.13 (m, 3H), 7.05−6.96 (m, 1H), 6.25 (s, 1H), 6.07 (dd, 2H), 5.76 (s, 2H), 5.60 (dt, 2H), 5.14 (d, 1H), 4.87 (t, 1H), 3.69 (t, 1H), 2.01 (dd, 1H). HRMS (ESI+), m/z calculated for C43H33N3O3 [M + H]+ 640.2555, found 640.2590. Synthesis of L3. 0.005 mmol CdCl2·2.5H2O (1.4 mg) and 0.1 mmol chal-3 (33 mg) were added into acetonitrile (7 mL) and H2O (1 mL). The system was placed in a 10 mL glass vial at 100 °C for 12 h. The white precipitates were purified by washing three times with acetonitrile. 1H NMR (400 MHz, DMSO) δ 9.01 (d, 1H), 8.72−8.64 (m, 3H), 8.31 (ddd, 8H), 8.22 (d, 2H), 8.17 (d, 1H), 8.09−7.96 (m, 6H), 7.93 (dd, 2H), 7.86 (d, 1H), 7.72 (d, 2H), 7.14 (dd, 1.5 Hz, 2H), 6.66 (dd, 2H), 6.35 (s, 1H), 5.63 (t, 1H), 5.56−5.40 (m, 2H), 5.34−5.21 (m, 1H), 2.17 (dd, 1H), 2.08−1.81 (m, 1H). HRMS (ESI +), m/z calculated for C55H37N3O3 [M + H]+ 788.2868, found 788.2906. Synthesis of 1. A total of 0.07 mmol CdCl2·2.5H2O (0.016 g) and 0.04 mmol chal-1 (0.0084 g) were added into acetonitrile (7 mL) and H2O (1 mL). The system was placed in a 10 mL glass vial at 100 °C for 12 h. Single crystals appear as colorless clumps with a yield of 84% based on chal-1. The theoretical value of elemental analysis (%) for

qe =

(C0 − Ce)V m

(1)

R=

C0 − Ct × 100% Ct

(2)

where C0 (mg L−1) and Ce (mg L−1) are the initial and equilibrium concentrations, Ct is the solution concentration (mg L−1) at time t B

DOI: 10.1021/acs.inorgchem.9b00037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) View of the coordination environment in 1. (b) View of 1D structure in 2. (c) View of 2D layer structure in 3. (d) Structures of threecomponent cycloaddition ligands L1, L2, and L3. (Hydrogen atoms are omitted for clarity.) (min), V (L) is the volume of the solution, and m (g) is the weight of adsorbent.

synthesis strategy and loading of functional groups for the subsequent studies on adsorption properties. The structures of 2 and 3 are similar to 1, in which the substrate chal-1 was replaced by chal-2 and chal-3 (Figure S1). In the frame of 1, the dihedral angle between two phenyl groups is 114.14°. A similar dihedral angle between two naphthalene groups decreases to 103.79° in 2. For 3, the dihedral angle for two pyrenyl increases to 121.86°. The change of the angle results from the transformation of the pyrenyl group due to the steric hindrance effect. In addition, the two carbonyl groups in 3 exhibit the cis-conformation, which is different from that of 1 and 2. This characteristic can easily lead to different binding ways for the guest molecules and the adsorption selectivity of the CPs. Investigation of [2+2+2] Cyclotrimerization Reaction. Admittedly, we are glad to get the CPs decorated with carbonyl and hydroxy groups, but the mechanism of the situ cycloaddition is vague. In fact, from the single crystal structures we found that the [2+2+2] cyclotrimers are not produced by three chalcone derivatives, and probably from two chalcone derivatives and one enol-type 4-acetylpyridine (Figure 1d). To gain insight into the origin of the cycloaddition reaction, a series of exploratory experiments were carried out. According to the literature on [2+2] cycloaddition,36−40 UV light (λ = 365 or 254 nm) response experiments were conducted. Unfortunately, the products of [2+2+2] cycloaddition cannot be obtained by lighting condition. Then, possible reaction conditions were screened as shown in Table S1. When aromatic aldehydes and 4-acetylpyridine were used as raw materials, the cycloaddition could not take place regardless of CdCl2 addition (entries 1 and 2). CdCl2 played a key role for cycloaddition when chalcone derivatives were used as starting substances (entries 3 and 4). To investigate the effect of CdCl2, CP 3 was selected as the research object of the control experiments. The ratio of CdCl2·2.5H2O to chal-3 was

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RESULTS AND DISCUSSION Crystal Structure Description. Single-crystal X-ray analysis reveals that CP 1 crystallizes in the monoclinic space group P2(1)/c. As shown in Figure 1, the asymmetric unit contains one Cd2+, one L1, and two coordinated Cl−. For Cd1, the coordination environment shows a distorted octahedral geometry. The equatorial plane is occupied by a N atom from the pyridine ring of L1 and three Cl atoms, and two N atoms from two pyridine rings appear at the axial site. The Cd−N bond lengths fall in the range of 2.342−2.433 Å, and the lengths of Cd−Cl were 2.641 and 2.560 Å, respectively. On the basis of the coordination mode, Cd1 and the symmetrical Cd1a are linked together by Cl atoms to give a binuclear [Cd2Cl4] structure unit. The different binuclear units are interconnected with each other via L1 to afford a 2D framework (Figure S1c). In the overall framework, L1 can be considered as 2-connecting nodes, and binuclear [Cd2Cl4] units were simplified as 4-connected nodes. Then the structure of 1 turns into a complicated 2D (2,4)-connected network. L1 is composed of three components by in situ [2+2+2] cyclotrimerization reaction, and the structure analysis shows that the six-element ring has a chair-type configuration, and different substituent groups are attached to the five carbon atoms of the chair ring. It is worth noting that the distance between C24 in the six-membered ring and nearby oxygen atom O2 is 1.412 Å. This length is longer than the other two C−O bonds (1.212 and 1.209 Å), inferring the C24−O2 is a single bond, where O is a hydroxyl group, not a carbonyl. This fact provides important evidence for subsequent studies on the cycloaddition mechanism. Importantly, carbonyl and hydroxyl groups are successfully functionalized into the structure of 1, which mark the successful implementation of our in situ C

DOI: 10.1021/acs.inorgchem.9b00037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

788.30). The appearance of L3 in the yellow solutions indicated that the starting chalcone derivatives formed L3. Since APCI-MS is typically only appropriate for lower molecular weight compounds, the presence of CP 3 cannot be eliminated in the mixed solutions. The CPs are difficult to dissolvable and precipitate readily from the solutions, and there may be very little of CPs in the solutions. Consequently, it is sure that only 1-pyrenecarboxaldehyde, 4-acetylpyridine, and chal-3 are in the solutions without CdCl2. When CdCl2 was added to the reaction system, in addition to the 1pyrenecarboxaldehyde, 4-acetylpyridine, chal-3, and L3, there may be a small amount of CP 3 in the solutions. However, from the control experiment, when the amount of CdCl2 is relatively small, only L3 precipitates out; when CdCl2 exceeds the catalytic amount, the CPs start to precipitate out. Therefore, we are more inclined to believe the process of first generating L3 and then CP 3. It should be clarified that the starting materials were all purified, so 4-acetylpyridine and 1-pyrenecarboxaldehyde were impossible from impurities. In the reaction system of acetonitrile and water, the CC bond of some chal-3 could be cleaved to give 1-pyrenecarboxaldehyde and 4-acetylpyridine at high temperatures and pressures.41 Combined with the APCI-MS analysis, we can determine that chal-3 decomposed into 1-pyrenecarboxaldehyde and 4-acetylpyridine from the appearance of molecular ion peaks (m/z) 231.10 and 122.00. The keto form is easily transformed into enol form in acidic condition.42 In our reaction system, cadmium chloride provided an acidic environment, leading to 4-acetylipridine in the form of enol. One CC bond from the enol form of 4-acetylpyridine and the other two CC bonds from chalcone derivatives undergo a alkenone−enol−alkenone [2+2+2] cycloaddition and resulted in the cyclohexanol ligands. In Scheme 1, the

strictly controlled (other reaction conditions are all the same): chal-3 was fixed at 0.1 mmol, and CdCl2·2.5H2O was kept at 0 mmol, 0.001 mmol, 0.005 mmol, 0.008 mmol, 0.01 mmol, and 0.03 mmol, respectively. The photographs of the control experiment are shown in Figure S2, and the specific situation is summarized in Table 1. There exist 1-pyrenecarboxaldehyde, Table 1. Controlled Experimenta of 3 Entry

CdCl2·2.5H2O (mmol)

Chal-3 (mmol)

1 2 3 4 5 6

0 0.001 0.005 0.008 0.01 0.03

0.1 0.1 0.1 0.1 0.1 0.1

Phenomena Yellow clear solutionsb White precipitates (L3), brilliant solutionsc Yellow slags (3), white precipitates (L3), lemon yellow solutionsc Yellow crystals (3), white precipitates (L3), yellow solutionsc

Reaction conditions: MeCN/H2O (V:V = 7:1/mL) at 100 °C for 12 h. bThe solutions including 1-pyrenecarboxaldehyde, 4-acetylpyridine, and chal-3. cThe mixed solutions including 1-pyrenecarboxaldehyde, 4-acetylpyridine, chal-3, and L3. a

4-acetylpyridine, and chal-3 in the yellow clear solutions by APCI-MS analysis in the absence of CdCl2·2.5H2O. With the addition of 0.005 or 0.001 mmol CdCl2·2.5H2O, the white precipitates were produced at the bottom and confirmed as L3. And there are 1-pyrenecarboxaldehyde, 4-acetylpyridine, chal3, and L3 in the brilliant yellow solution. When the amounts of CdCl2·2.5H2O exceed 0.008 mmol (catalytic amounts), a small amount of yellow slags (or crystals) and white precipitates appear in the bottles. SCXRD, PXRD, and APCI-MS determination showed yellow slags and crystals were CP 3 and white precipitates were L3. And 4-acetylpyridine, 1pyrenecarboxaldehyde, chal-3, and L3 exist in the lemon yellow solutions. Based on the above, we can conclude that CdCl2·2.5H2O facilitates the cycloaddition reaction and 1−3 were generated in situ with CdCl2 exceeding the catalytic amounts. In Figure 2, we noticed that 1-pyrenecarboxaldehyde, 4acetylpyridine, chal-3 and L3 in the lemon yellow solutions were clearly present by the APCI-MS analysis: 1-pyrenecarboxaldehyde (m/z Calcd: 231.08; Found: 231.10), 4-acetylpyridine (m/z Calcd: 122.06; Found: 122.00), chal-3 (m/z Calcd: 334.12; Found: 334.20) and L3 (m/z Calcd: 788.29; Found:

Scheme 1. Suggested [2+2+2] Cycloaddition Reaction Mechanism for the L3

formation of L3 was taken as an example to illustrate the mechanism. APCI-MS analysis suggests that the same thing happens to L1 and L2 (Figure S3). Notably, the present work is totally different from the synthesis of Kostanecki’s triketone in a strong base environment.43,44

Figure 2. APCI-MS characterization for mixed solutions of 1pyrenecarboxaldehyde, 4-acetylpyridine, chal-3, and L3. D

DOI: 10.1021/acs.inorgchem.9b00037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. For CP 3, (left) the photographs show the colors of CR solutions and the powder before and after adsorption, UV−vis spectra of CR in water at various time intervals; (right) the photographs show the colors of MO solutions and the powder before and after adsorption, UV−vis spectra of MO in water at various time intervals.

Figure 4. (a) Adsorption isotherm and Langmuir adsorption mode fitting for CR by 1. (b) Adsorption isotherm and Langmuir adsorption mode fitting for MO by 3. (c) EDS analysis result for 1 after adsorption of CR.

Adsorption of Dyes. CPs 1, 2, and 3 contain numerous functional carbonyl and hydroxyl groups, so the CPs should exhibit potential application for the removal of organic pollutants in water. The water stabilities of 1−3 provide a guarantee for the removal of organic pollutants. In fact, the three compounds were soaked in aqueous solutions with different pH values. CPs 1−3 were soaked in aqueous solutions with different pH values for 12 h, and then the solid was

removed to perform powder X-ray diffraction (PXRD). PXRD patterns demonstrated that the structural integrity of CPs 1−3 retained well in pH = 3−9 aqueous solutions (Figure S4). Moreover, the weights of the remaining solids after the treatment of pH = 3 and 9 solutions were characterized. The results showed that there was almost no loss of the mass of the remaining solid and the morphology of CP 3 was monitored and maintained intact (Figure S5). In addition, the ICP test of E

DOI: 10.1021/acs.inorgchem.9b00037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. (a) FTIR spectra of 1, 1@CR and CR; (b) FTIR spectra of 3, 3@MO and MO.

Furthermore, the adsorption isotherms were investigated to estimate the maximum adsorption capacity of 1 for CR and 3 for MO by different initial dye concentrations from 20 to 700 ppm. The standard adsorption curves of CR and MO solutions with various concentrations were obtained (Figures S12−13). The equilibrium adsorption isotherm data was fitted by the Langmuir model (Figure 4a,b), yielding the relative correlation coefficient of 0.9906 and 0.9936, respectively. The maximum adsorption capacity of 1 for CR is up to 485.4 mg g−1, and that for 3 for MO is up to 492.6 mg g−1, respectively, which closely matched with the experimental equilibrium values. Their adsorption capacities are higher than most reported materials (Tables S2−3),22,47−64 demonstrating that the CPs can be used for the removal of dyes. The excellent adsorption capacity may stem from the highly accessible carbonyl and hydroxyl groups densely populated throughout the frameworks of the CPs. Dyes Removal Mechanism and Recyclability Performance of CPs. As we know, the hydroxy and amide groups are both hydrogen-bonding donors and acceptors, which can easily form intermolecular hydrogen bonds with special molecules.65,66 Therefore, it is possible that 1, 2, and 3 with carbonyl and hydroxyl groups bind to amino and sulfonate groups containing CR and MO by hydrogen bonding. To reveal the intermolecular action between CR/MO and 1−3, Fourier transform infrared spectra (FTIR) were recorded and depicted. The peaks at 3466 and 3296 cm−1 (Figure 5a) are assigned to N−H and O−H vibrations, respectively. After the adsorption of CR, the peak of −OH from 3296 cm−1 shifted to 3415 cm−1, indicating that CR is loaded onto 1 via hydrogen bonds.20 The double peaks between 1600 and 1700 cm−1 derived from CO vibrational coupling merge into a broad peak after CR adsorption. Meanwhile, the peak at 1224 cm−1 assigned to the C−O vibration shifts to 1247 cm−1. The results suggest hydrogen bonds among the −NH2, −OH, and CO groups play a key role for removing CR. For 2, the peak of O− H shifted to 3421 cm−1 from 3418 cm−1 and the peak at 1144 cm−1 of the C−O vibration is presented at 1146 cm−1 after CR adsorption, and the vibration peaks of CO had no change (Figure S14a). We believe that the presence of a hydroxyl group promotes 2’s adsorption for CR. The peak at 3445 cm−1 of O−H shifted to 3439 cm−1, and the peak at 1141 cm−1 of C−O vibration increases to 1147 cm−1 after CR adsorbed on 3. The two peaks at 1603 and 1686 cm−1 of the CO vibration have slight changes (Figure S14b), indicating that carbonyl groups have also made a certain contribution for 3 to capture CR.

the solutions also showed that the concentrations of cadmium atoms are 10 ppm order of magnitude. So the stability of the CPs 1−3 in these pH solutions can be trusted. CR and MO were selected as the model representatives of organic pollutants to study the adsorption ability of CPs 1−3. The adsorption performance was first tested by exposing 10 mg 1 (2 or 3) in 5 mL aqueous solutions of CR (20 ppm) or MO (20 ppm) under stirring for 4 h. The supernatant of CR all appeared colorless (Figure 3 and Figure S6), while only 3 made the supernatant of MO become colorless. The above experiments were repeated three times, and the results were the same. We roughly judged that CPs 1, 2, and 3 all showed good adsorption properties for CR, and only 3 exhibited excellent adsorption capacity for MO. Curiously, why can 3 adsorb MO rather than 1 and 2? Following structure−property relationship principles, we found the two carbonyl groups in the coordination unit of 3 exhibited cis-conformations, but the two carbonyls in 1 or 2 were in the opposite position. And the dihedral angle for two pyrenyls in L3 is distinct from that for two naphthyls in L2 or phenyls in L1. The greater degree of πconjugation could be responsible for the difference in adsorption behavior of 3. Subsequently, the kinetics of the adsorption was examined and a series of time-dependent UV−vis adsorption experiments were recorded. The insoluble composite precipitates were separated by centrifuge, and the solutions were detected by UV−vis absorption spectroscopy at certain time intervals. UV−vis spectra (Figure 3 for CP 3 and Figure S7 for CP 1 and 2) showed that the concentration of CR decreased to nearly zero after a period of time. 1 displayed a faster adsorption rate for CR than 2 and 3, whose removal rate reached to 77% from aqueous solutions at 2 min and about 97% at 5 min. The adsorption results of 3 toward MO suggest that the removal rate reached to 51.53% from aqueous solutions at 2 min, and about 92.87% was removed at 8 min. Energy-dispersive spectroscopy (EDS) analysis of 1, 2, and 3 after adsorbing dyes confirmed the sulfur elemental exists in the structure, which indicates the successful uptake of CR or MO (Figure 4c and Figure S8). In addition, the reflectance spectra before and after dyes adsorption also showed that the organic dyes were adsorbed on the CPs 1−3 (Figure S9). The dyes removal experiment in the dark (Figure S10) ruled out the possibility of photodegradation.45,46 Since 1 and 2 only absorbed CR, no effect on MO, we investigated their separation ability for CR and MO by immersing 1 or 2 in the mixed solution of CR and MO. As shown in Figure S11, almost all CR was selectively removed from the mixture, which manifest 1 and 2 can be used as potential selective separation materials for CR and MO. F

DOI: 10.1021/acs.inorgchem.9b00037 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry ORCID

It can be found from Figure 5b that the O−H vibration peak at 3445 cm−1 shifts to 3449 cm−1 and the C−O vibration peak at 1144 cm−1 is observed at 1147 cm−1 after 3 adsorbs MO. It is shown that the −OH plays key roles in adsorbing MO. In addition, the pattern of two peaks at 1603 and 1686 cm−1 changed after capturing MO, declaring the carbonyl groups have also made contributions. In general, 1−3 demonstrate excellent uptake capacity for organic dye, which profits from their frameworks decorated with carbonyl and hydroxy groups. The application of an adsorbent depends not only on the adsorptive capacity but also on the regeneration and reuse. Thus, it is necessary for an adsorbent that the adsorbed organic pollutants should be easily desorbed under suitable conditions. In this work, the release experiment was performed by immersing the samples of CR@1, CR@2, CR@3, and MO@ 3 in methanol solution, respectively. The methanol solution of CR@1 or CR@2 quickly turned red (Figure S5), indicating that CR was released from the solid sample. Such adsorption and release processes were operated for three continuous cycles, and crystalline structures were well preserved (Figure S15), demonstrating the reversibility and recyclability of 1 and 2. The UV−vis tests showed that the removal rate of 1 remained 81% and 2 remained 45% at the third cycle (Figure S16). However, the deadsorption capacity of methanol to CR@3 and MO@3 is invalid; even acetone shows a weak deadsorption capacity to CR@3 and MO@3, which may be due to the stronger interaction between 3 and dyes.

Chao Huang: 0000-0001-8576-8024 Hongwei Hou: 0000-0003-4762-0920 Author Contributions #

Wenjuan Xu and Zhichao Shao contributed equally to the work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation (21671174), Thousand Talents Program of Zhongyuan, and the Natural Science Foundation of Henan province (182300410008).

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CONCLUSIONS In summary, we first introduced the [2+2+2] cycloaddition reaction into the construction of functional CPs and obtained CPs 1, 2, and 3 decorated by carbonyl and hydroxyl. The catalysis and coordination action of cadmium chloride reveal that the two chalcone derivatives and a 4-acetylpyridine (enol form) form new cyclohexanol ligands through in stiu [2+2+2] cyclotrimerization, which lead to the generation of the CPs. Benefiting from its sufficient adsorption sites and strong affinity, the CP materials display excellent uptake capacity for CR and MO. We believe that our work demonstrates a great potential in designing and synthesizing functional CP materials and provides potential sorbent materials for the capture of pollutant species.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00037. Additional experimental details and supporting figures (PDF) Accession Codes

CCDC 1884026−1884028 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. G

DOI: 10.1021/acs.inorgchem.9b00037 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b00037 Inorg. Chem. XXXX, XXX, XXX−XXX