Assembly and Adsorption Properties of Seven Supramolecular Compounds with Heteromacrocycle Imidazolium.

Seven supramolecular compounds comprising multivalent imidazolium macrocycles and metal halides, {[MC-IM][Ag2I4]} n (1), {[PC-IM]2[Ag7I11]} n (2), {[ODC-IM][Ag3I7]} (3), {[ODC-IM][Bi2I10]} (4), {[MDC-IM][Bi2I10]} (5), {[PDC-IM][Bi2I10]} (6), and {[MDC-IM][HgI4]} (7), have been synthesized by solvothermal reactions and structurally characterized by IR spectroscopy, thermogravimetric analysis, and single-crystal X-ray diffraction. Notably, the three tetravalent imidazolium macrocycles were introduced for the first time and the extended anion structures are featured with three-dimensional coordination networks, one-dimensional chains, or zero-dimensional oligomers. This new study attempts to not only fill the gap in this supramolecular hybrid area that has been neglected but also enrich the type of imidazolium cyclophane. It is important that good efforts were devoted to study the adsorption properties of supramolecular compounds. Compound 5 exhibited great adsorption performance for organic dyes methylene blue, methyl orange, and rhodamine B (RhB) and can be evaluated as a potential candidate for industrial wastewater treatment.

Introduction

Over the past few decades, much work has been done in developing imidazolium cyclophane complexes and exploring their potential applications. The properties of macrocyclic compounds mainly include host–guest chemistry,[1−3] self-assembly,[4] selective catalysis,[5] and ion channel.[6,7] In an earlier study, Thummel’s group prepared a series of 2,2′-bibenzimidazolium salts by N,N′-bridging using dihaloalkanes.[8] They found that these salts may be reduced by one or two electrons to the corresponding cation radical or neutral 2,2′-bibenzimidazolinylidene. Many studies have shown that the hydrogen in 2-C in the imidazole ring is strongly acidic. Under alkaline conditions, protons can be removed and coordinated with metals to form carbene complexes. More research about this come from Baker’s group.[9−15] However, research on the use of imidazolium cyclophanes and metal salts to construct a supramolecular system is rare. We concentrated on the use of the imidazolium cation as the template because it has a positive charge on its own.[16−18]

Nowadays, organic dyes are closely related to our daily life, such as for the dyeing of silk, cotton, linen, and orlon fibers, as well as in the papermaking industry. Dyes that are electrically neutral, positive, or negative are usually difficult to degrade. Therefore, dyes must be removed before discharge. Recently, metal–organic frameworks (MOFs) have been widely used in the domain of selective adsorption and separation of dyes.[19] Most of them contain approximately sized channels. Many applications in degradation of organic dyes by MOF have been explored, but supramolecular compounds are rarely used to absorb or isolate dye molecules.[20]

It was rarely reported that imidazolium cyclophanes as the cationic template can be used to synthesize supramolecular compounds with metal halides.[21] In this contribution based on two divalent imidazolium macrocycles (MC-IM·Br2 and PC-IM·Br2) and three novel tetravalent imidazolium macrocycles: di-orthocyclo{l,4}(1,3)imidazolophanium dibromide (abbreviated as ODC-IM·Br4), di-metacyclo{l,4}(1,3)imidazolophanium dibromide (abbreviated as MDC-IM·Br4), and di-paracyclo{l,4}(1,3)imidazolophanium dibromide (abbreviated as PDC-IM·Br4) (shown in Scheme ), we prepared seven new inorganic–organic hybrid supramolecules {[MC-IM][Ag2I4]} (1), {[PC-IM]2[Ag7I11]} (2), {[ODC-IM][Ag3I7]} (3), {[ODC-IM][Bi2I10]} (4), {[MDC-IM][Bi2I10]} (5), {[PDC-IM][Bi2I10]} (6), and {[MDC-IM][HgI4]} (7). They not only enrich the type of imidazole ring but also can be used for dye adsorption research to achieve the purpose of purifying water.

Scheme 1

Bisimidazolium and Tetraimidazolium Macrocycle Cations Used in This Paper

Results and Discussion

Description of Crystal Structures

Crystal Structure of {[MC-IM][Ag2I4]}(1)

The asymmetric unit of 1 consists of an electron-deficient meta-imidazole ring [MC-IM]2+ and an anion [Ag2I4]2– (Figure a). There are two crystallographically independent Ag (I) atoms, both of which are four-coordinated in similar coordination environments. The anion [Ag2I4]2– is partially polymerized into a three-dimensional (3D) iodoargentate network structure (as shown in Figure b). It can be seen from the compound 1 packing diagram in Figure c that the imidazole ring is interspersed in the 3D [Ag2I4]2 anionic tunnel by electrostatic force. Figure d shows the crystal-stacked diagram of compound 1 viewed along the a-axis direction (like bowls tied with a straw).

Figure 1

(a) Asymmetric unit of compound 1; (b) anion structure diagram of compound 1; (c) stacked diagram of compound 1 in the c-axis direction; and (d) stacked diagram of compound 1 in the a-axis direction.

Crystal Structure of {[PC-IM]2[Ag7I11]}(2)

Figure a shows that the characteristic feature of compound 2 is the one-dimensional (1D) iodoargentate chain and cation. Seven Ag atoms have different coordination environments, which are coordinated with four I atoms; there exists a metal–metal bond between Ag2–Ag3 and Ag5–Ag6. A schematic of [Ag4I6]2 and [Ag3I5]2 anionic polymerization is shown in Figure b, where [Ag4I6]2– forms the cubic structure with the smallest building unit hexahedron and [Ag3I5]2– forms the minimum structure unit of tetrahedron. The asymmetric unit is connected by Ag–Ag bonds (the Ag–Ag bond length of 2.942–3.283 Å is less than twice the van der Waals radius of Ag 1.72 Å, which indicates the presence of Ag–Ag metal bonds). By this connection, two different anion polymeric chains are formed. Figure c shows the stacking of heterocyclic cation [PC-IM]2+ in the b-axis direction, and the anion chains [Ag4I6]2 and [Ag3I5]2 are surrounded and interacted with cations by electrostatic interaction. Viewed along the b-direction, this iodoargentate chain looks like a beautiful Chinese knot in Figure c.

Figure 2

(a) Asymmetric unit of compound 2; (b) anion structure diagram of compound 2; and (c) stacked diagram of compound 2 along the b-direction.

Crystal Structure of [ODC-IM][Ag3I7] (3)

Compound 3 was synthesized by the solvothermal reaction from AgI, KI, and [ODC-IM]4+, at a certain molar ratio. Its asymmetric unit in Figure a contains Ag (I) atoms with different coordination environments. Each Ag adopts a four-coordination mode. Ag1 is not only connected to Ag2 and Ag3 through μ2-I but also coordinated with two t-I. Ag3 interacted with Ag2 to form Ag–Ag metal bonds and also coordinated with a t-I and three μ2-I. The Ag–I bond lengths ranged from 2.7310 to 3.1176 Å, and the I–Ag–I bond angles ranged from 55.00 to 131.82°. Figure b,c shows the stacked view of compound 3 under different visions. The anion-building unit Ag3I7 is located in the middle of two cationic [ODC-IM]4+ macrocycles interacted via the electrostatic force forming a sandwichlike structure.

Figure 3

(a) Asymmetric unit of compound 3; (b) anion structure diagram of compound 3 in the b-axis direction; and (c) stacked diagram of compound 3 in the a-axis direction.

Crystal Structures of [ODC-IM][Bi2I10] (4), [MDC-IM][Bi2I10] (5), and [PDC-IM][Bi2I10] (6)

The structures of compounds 4–6 are featured by the binuclear iodobismuthates. Their crystal structures are very similar (except for some differences in the stacked patterns), and they are three isomers. Therefore, the structure of compound 4 is taken as an example here for a detailed description. The asymmetric unit of compound 4 contains 1/2 [ODC-IM]4+, a positive Bi atom, and five negative I atoms (Figure a). As is shown in Figure b, in isomorphic compounds 4–6, Bi(1) and Bi(2) atoms are bonded to four t-I and μ2-I (the Bi atoms are coplanar with μ2-I, and the plane is named β-plane). It can be seen from Figure c that the inorganic moiety [Bi2I10]4– is in the middle of the two cationic [ODC-IM]4+ macrocycles. The anion and cation form a sandwichlike structure and construct a unit model (abstracted in Figure d) by electrostatic force and weak C–H···I bonds (where the bond length of C–H···I in [ODC-IM]4+ is around 3.2 Å, the bond length of C–H···I in [MDC-IM]4+ is around 3.01 Å, and C–H···I of [PDC-IM]4+ is about 3.03 Å). Figure e shows a stacked view of compounds 4–6. It can be seen from the stacking diagram that induced by different macrocyclic cations the [Bi2I10]4– anions are arranged along a different direction. Taking the β-plane as a reference, there existed an angle between the β-plane and the [ODC-IM]4+ macrocyclic plane in compound 4, while it is substantially parallel to the macrocyclic cation plane of [MDC-IM]4+ and [PDC-IM]4+ in compounds 5 and 6, respectively. The phenomenon is probably caused by the presence of different C–H···I bonds.

Figure 4

(a) Structural unit diagram of compound 4; (b) anionic β-face of compound 4; (c) hydrogen bond diagram (blue dash lines) of compound 4; (d) model diagram of compounds 4–6; and (e) stacked diagram of compounds 4–6.

Crystal Structure of {[MDC-IM][Hg2I8]} (7)

As shown in Figure a, its asymmetric unit consists of two parts of 1/2 [MDC-IM]4+ and a mononuclear [HgI4]2– anion. In the anion, the Hg atom adopted a tetracoordinate mode bonded to four I atoms. The bond lengths of the Hg–I bond are between 2.7675 (12) and 2.8416 (12) Å. The bond angles of I–Hg–I are between 102.59 (4) and 113.28 (4)°. Figure b shows the spatial arrangement between anions and cations, and Figure c shows the sandwichlike packing unit.

Figure 5

(a) Asymmetric unit of compound 7; (b) stacked diagram of compound 7; and (c) model diagram of compound 7.

Single-crystal X-ray analysis reveals that compound 1 crystallizes in a monoclinic system with space group P212121, compounds 2 and 7 crystallize in a monoclinic system with space group P21/c, compounds 3–5 crystallize in a monoclinic system with space group P1̅, and compound 6 crystallizes in a monoclinic system with space group P21/c. The anion structures of compounds 1–7 are featured by the 3D iodoargentate network (for 1), 1D iodoargentate chain (for 2), trinuclear iodoargentate (for 3), binuclear iodobismuthates (for 4–6), and mononuclear iodomercute (for 7). The anion and organic cation interacted by electrostatic interaction, van der Waals’ interaction, or hydrogen bond (Figures –5). As shown in Figures –5, the macrocycle of the cation is larger and the anion structure of the supramolecule is simpler. On the contrary, macrocycles of the cation are the smaller, supramolecular anion structures, which are easily polymerized.[22]

Thermal Stability of Compounds 1–7

We studied the thermal stability of compounds 1–7 through the NETZSCH STA 449C synchronous thermal analyzer (Figure S6). The experimental conditions were as follows: N2 gas atmosphere (gas flow rate 30 cm3 min–1), heating rate was 5 K min–1, experimental temperature is from 25 to 800 °C. The weight loss curve of the compounds can be clearly observed from the thermogravimetric analysis chart in Figure S6. The thermal stability of compounds 1 and 2 is similar, and the tendency of weight loss of compounds 3–7 is similar. They are stable below 320 °C. After 320 °C, the weight loss of the sample mainly consists of two stages. When the temperature is 320–450 °C, it is mainly caused by the decomposition of the cations. After 450 °C, the weight loss of the sample is caused by the decomposition of the inorganic structure. As shown in Figure S6, seven compounds are stable up to the temperature of around 300 °C. Therefore, the stabilities of compounds 1–7 are relatively good.

Adsorption Properties of Compounds 1–7

To explore the ability that compounds 1–7 purify organic dyes in wastewater, we selected three common dyes, methylene blue (MB), methyl orange (MO), and rhodamine B (RhB), to investigate the adsorption properties of compounds 1–7. The concentrations of MB, MO, and RhB solutions (100 mL volumetric flask) were 1.0 × 10–5, 5.0 × 10–5, and 2.0 × 10–5 mol L–1. Taking the adsorption of MB by compound 1 as an example, the experiment process is as follows: take 20 mL of MB solutions in a dark place and then take about 3 mL of the solution each time to measure its initial absorbance by ultraviolet adsorption spectrum (MB, A0 at 663 nm). Next, 10 mg of compound 1 was added to one of the MB solutions, and the other was a blank control. They were measured during the appropriate time interval. Figures S7–S13 show the adsorption of compounds 1–7 on MB, MO, and RhB and the UV spectra of their respective blank controls. Taking C/C0 (Lambert Beer’s law A/A0 = C/C0) as the ordinate and the illumination time as the abscissa, the relationships between the C/C0 of the adsorption of MB, MO, and RhB and their respective experiments are shown in Figures –8 and S14–S17. As is shown in Figures –8, compound 5 shows extraordinary adsorption capacity for organic dyes RhB and MB, which is much better than that of some MOFs,[23] and the adsorption effect can approach 100%.[24] The adsorption effect of compounds 4 and 6 on MO is also relatively good. The adsorption of the MB, MO, and RHB dyes by compounds 1–7 may be caused by the hydrogen bond, the electrostatic force, or the van der Waals force between the solid surface and the dye. Compounds 4–6 exhibited different adsorption properties for MO and MB, which may be attributed to the different adsorption sites on the different macrocycles. There is a large amount of C–H···I hydrogen bonds between the inorganic anion and the organic cation in compounds 4–6. It can accelerate dye adsorption.[20] The content of the leaking Ag+ ion after one cycle of adsorption was measured by inductively coupled plasma mass spectrometry (ICP-MS). The residual organic matter in the solution is digested by nitric acid.[24b] The content of Ag+ measured by ICP-MS is 4.67 × 10–5 mol L–1. This result indicates that the content of the leaking Ag+ ion after the first cycle of adsorption is rather small and can be neglected. The color changes of the adsorbed dye before and after the adsorption are provided in Figures S7–S13.

Figure 6

Photographs are relevant solutions at different times. (a–c) Adsorption of MB solution (a), MO solution (b), and RhB solution (c) with the use of compound 4 and blank experiment.

Figure 8

Photographs are relevant solutions at different times. (a–c) Adsorption of MB solution (a), MO solution (b), and RhB solution (c) with the use of compound 6 and blank experiment.

Photographs are relevant solutions at different times. (a–c) Adsorption of MB solution (a), MO solution (b), and RhB solution (c) with the use of compound 5 and blank experiment.

One of the most important factors affecting the availability of dye adsorption in the industry is its recycling performance. In the second recycling experiment, 4 and 5 could be readily recovered from the catalytic system via centrifugation and deionized water rinsing. Reuse of the catalyst was then investigated (Figure S18). The second adsorption rates of compound 5 for MB decreased by 15%. The second adsorption rates of compounds 4 and 5 for MO decreased by 10 and 4%. They also have an obvious adsorption effect.

We explored the reason for the adsorption of MB, MO, and RhB in aqueous solution by compounds 1–7. Taking compound 5 as an example, its specific surface area and pore size distribution were determined by the Brunauer–Emmett–Teller (BET) method. As shown in Figure , the N2 adsorption/desorption isotherms obtained from compound 5 conform to adsorption type II, which reflects the typical physical adsorption process. BET data show that the specific surface area of compound 5 is 8.4264 m2 g–1, the pore diameter is 3.986 nm, and the N2 amount adsorbed is 1.936 cm3 g–1. This result indicated that compound 5 is a mesoporous material and the related process belongs to external surface adsorption.[25]

Figure 9

Gas sorption isotherms of N2 adsorption and desorption at 77 K.

Conclusions

In this article, we successfully synthesized seven novel inorganic–organic hybrid supramolecular polymers using an imidazole ring macrocyclic divalent organic cation template, which consist of a three-dimensional structure of compound 1, a one-dimensional structure of compound 2, a trinuclear structure of compound 3, a binuclear structure of compounds 4–6, and a mononuclear structure of compound 7. Three tetravalent cation imidazolium macrocycles that have not been previously reported were successfully introduced. It is found that the metal anions and the cation ligands mutually influence the final extended structures. We have investigated the thermostability and dye adsorption. The result indicated that compounds 4–6 can readily and quickly absorb MO and MB from water. We will continue to explore these supramolecular compounds for solving environmental pollution problems.

Experimental Section

Materials and Methods

The imidazolium cationic derivative compounds MC-IM·Br2, PC-IM·Br2, ODC-IM·Br4, MDC-IM·Br4, and PDC-IM·Br4 were synthesized according to the reported synthetic method.[26−28] MC-IM·Br2 or PC-IM·Br2 was synthesized by mixing and refluxing 1,4-bis(imidazol-1-yl)butane (5 mmol) and α,α′-dibromo-o-xylene (or 1,4-dibromomethylbenzene, 5 mmol) in acetonitrile for 20 h. After the reaction is completed, the mixture was filtered while hot, to yield a clear yellow filtrate. The solvent was removed from the filtrate under reduced pressure to yield a white solid.[26] ODC-IM·Br4 (MDC-IM·Br4 or PDC-IM·Br4) was synthesized by 1,4-bis(imidazol-1-yl)butane (380 mg, 2 mmol) and 1,2-bis(bromomethyl)benzene (1,3-bis(bromomethyl)benzene or 1,4-bis(bromomethyl)benzene) (264 mg, 2 mmol) dissolved in acetonitrile. The latter was slowly added dropwise to a solution of 1,4-bis(imidazol-1-yl)butane under magnetic stirring (with a feeding time of not less than 40 min). The mixture was allowed to reflux for 20 h. After the reaction was completed, the solvent was removed under reduced pressure to yield a white solid.[27,28] Other chemicals were of reagent grade and used without further purification. Experimental and instrumental conditions in this article referenced the reported literature.[16,27]

Compound Synthesis

{[MC-IM][Ag2I4]} (1)

A mixture of MC-IM·Br2 (0.010 g), AgI (0.010 g), and KI was dissolved in 3 mL of CH3CN.[21] White rod crystals suitable for X-ray diffraction analysis were obtained in a yield of 6% based on silver ions. Anal. calc. (%) for C18H22Ag2I4N4: N, 5.32%; C, 21.32; H, 2.08; found: N, 5.5%; C, 21.22; H, 2.16; IR(KBr): 3441(w), 3128(m), 3041(m), 2932(s), 1623(m), 1557(m), 1441(m), 1342(s), 1151(w), 815(s), 737(m), 616(s) cm–1.

{[PC-IM]2[Ag7I11]} (2)

The preparation method of 2 is similar to that of 1 except that MC-IM·Br2 is replaced with PC-IM·Br2. The reaction gives yellow block crystals. After the reaction, the product was washed with anhydrous ethanol. The yield is 7%. Anal. calc. (%) for C36H44Ag7I11N8: N, 4.12%; C, 15.68; H, 1.59; found: N, 4.09%; C, 15.77; H, 1.61; IR(KBr): 3448(w), 3121(m), 3086(m), 2938(s), 2072(s), 1630(s), 1550(m), 1449(m), 1313(s), 1143(w), 871(s), 786(w), 643(m) cm–1.

[ODC-IM][Ag3I7] (3)

The preparation method of 3 is similar to that of 2 except that PC-IM·Br2 is replaced with ODC-IM·Br4. White rod crystals were prepared, washed, and obtained in a yield of 8.5%. Anal. calc. (%) for C38H47Ag3I7N9: N, 6.51%; C, 24.91; H, 2.65; found: N, 6.81%; C, 24.76; H, 2.55; IR(KBr): 3434(w), 3158(m), 3064(m), 2870(s), 2073(s), 1632(s), 1566(w), 1454(m), 1159(w), 1062(w), 741(m), 521(s) cm–1.

{[ODC-IM][Bi2I10]} (4)

The preparation method of 4 is similar to that of 3 except that AgI is replaced with BiI3. Red block crystals were washed and obtained in a yield of 9%. Anal. calc. (%) for C36H44Bi2I10N8: N, 5.12%; C, 18.76; H, 1.83; found: N, 4.92%; C, 18.98; H, 1.93; IR(KBr): 3441(w), 3093(m), 3062(m), 2920(s), 1638(s), 1557(m), 1438(m), 1352(s), 1147(m), 828(s), 733(m), 638(m) cm–1.

{[MDC-IM][Bi2I10]} (5)

The preparation method of 5 is similar to that of 4 except that ODC-IM·Br4 is replaced with MDC-IM. Red block crystals of compound 5 were obtained with a yield of 12%. Anal. calc. (%) for C36H44Bi2I10N8: N, 5.12%; C, 18.76; H, 1.83; found: N, 4.92%; C, 18.98; H, 1.93; IR(KBr): 3439(w), 3126(s), 3096(s), 2933(s), 1617(m), 1557(m), 1442(m), 1355(s), 1151(m), 825(s), 733(m), 616(s) cm–1.

{[PDC-IM][Bi2I10]} (6)

The preparation method of 6 is similar to that of 5 except that PC-IM-Br2 is replaced with ODC-IM·Br4. Red block crystals of compound 6 were obtained with a yield of 7.7%. Anal. calc. (%) for C36H44Bi2I10N8: N, 5.12%; C, 18.76; H, 1.83; found: N, 5.01%; C, 18.92; H, 1.90; IR (KBr): 3440(w), 3094(m), 3057(m), 2944(s), 1613(s), 1552(m), 1436(m), 1349(s), 1146(w), 816(m), 723(m), 608(m) cm–1.

{[MDC-IM][HgI4]2} (7)

The preparation method of 7 is similar to that of 5 except that BiI3 is replaced with HgI2. Light yellow rod-shaped crystals of compound 7 were obtained with a yield of 5.3%. Anal. calc. (%) for C36H44Hg2I8N8: N, 5.64%; C, 21.67; H, 2.13; found: N, 5.59%; C, 21.55; H, 2.19; IR (KBr): 3440(w), 3127(m), 3092(m), 2939(s), 1609(s), 1558(w), 1437(m), 1361(s), 1150(w), 815(s), 733(s), 638(m) cm–1.