HgII Bimetallic Gel

Article pubs.acs.org/crystal

Steric Environment Triggered Self-Healing CuII/HgII Bimetallic Gel with Old CuII−Schiff Base Complex as a New Metalloligand Kasturi Sarmah,† Gopal Pandit,† Amit Baran Das,‡ Bipul Sarma,† and Sanjay Pratihar*,† †

Department of Chemical Sciences and ‡Department of Food Engineering and Technology, Tezpur University, Napaam, Asaam 784028, India S Supporting Information *

ABSTRACT: The present work reports an easy to synthesize CuII−Schiff base complex (C-2), in which tunable access of a vacant coordination site for a guest metal via stereoelectronic modulation of a substituent is possible, which might be of great use in the supramolecular self-aggregation process and thus utilized as a metalloligand for the formation of a heterobimetallic CuII/HgII gel in methanol. The gelation property of complex C-2 is found to be dependent upon HgII content, and the initial yellow gel transformed into green with a higher equivalent of Hg(OAc)2. The coordination of HgII to complex C-2 is perceived to cause significant charge polarization around the coordinated imine and ultimately lead to corresponding green 2-hydroxy benzaldehyde coordinated CuII complex (C3) after the hydrolysis, which is responsible for yellow to green gel transformation. The role of anions, metal salts, ligand, and solvents on the gelation property of complex C-2 along with the effect of various external chemical stimuli on the gel and its selfsustainability were studied in detail. The as-synthesized hetero-bimetallic CuII/HgII gel shows an excellent ability as a reusable material for the adsorption of various cationic as well as anionic dyes.

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dyes, and toxic pollutants from water bodies.61−64 By using a tetrazole derivative as a ligand, Yan et al. showed the synthesis of supramolecular heterometallic self-healing CoII/NiII gels with the synergistic feature of the constituent metal ions.65−69 On the other hand, the success of metalloliganda for their utilization in various homo- or hetero-multimetallic complexes relies mainly on their unique stereoelectronic properties, selective and strong binding affinity to other metals, which can be suitably tuned by different substituents appended in the ligand frame. At the same time, various metalloligands with an accessible vacant coordination site for other host metals to generate bimetallic complexes have been successfully utilized.70−74 In the present work, an easy to synthesize CuII− Schiff base complex was designed to explore its potential toward various heterometallic complexes or supramolecular metallogel networks. The model CuII-complexes were chosen because of the following reasons: (i) tunable accessibility of a vacant coordination site is possible via stereoelectronic modulation of a substituent (R), (ii) the access of weak versus strong coordination of model complexes with guest metal ions is possible through variable R substituents, which will directly control the imine hydrolysis, (iii) the weak versus strong

INTRODUCTION The triggering of coordination of a metal ion to a suitably designed molecule by tuning appropriate binding sites or its stereoelectronic property could lead to various self-assembled structures, which might even ultimately lead to metallogels in certain cases.1−6 In this regard, elegant design strategies and intriguing architectural plans have been implemented to construct various supramolecular organogels,7−12 hydrogels,13−18 and metallogels.19,20 In an early demonstration, Biradaha et al. showed that the structure and binding site of the ligand play an important role toward the formation of a metallogel using bis(benzimidazole) as the ligand.21−24 Simultaneously, various ligands based on amino acid, peptide, pyridyl bis(urea), terpyridyl, and pyridyl derivatives have been successfully utilized for the synthesis of metallogels with various metal salts.25−40 Very recently, Diaz and Banerjee showed the utilization of oxalic acid to form robust supramolecular metallogel networks in the presence of metal salts, which further displayed an unprecedented ability to impart selfhealing properties to other gel networks lacking this capacity.41−44 To date, although various work based on metal−organic gels (MOGs) or metallogels has been explored,45−60 a heterometallic gel in which the metal complex acts as a ligand (metalloligand) for a second metal is relatively rare and rather unexplored. In this regard, Pal et al. introduced vanadate ions as gelators for Ag+ ions to form pure inorganic metallogels without any carbon and applied to sequestering gas, © XXXX American Chemical Society

Received: December 1, 2016 Revised: December 1, 2016

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DOI: 10.1021/acs.cgd.6b01545 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Known and proposed model of metal organic gel.

coordination might be of great use in the supramolecular selfaggregation process (Figure 1). Herein, we report the utilization of complex C-2 as a metalloligand for the formation of heterometallic CuII/HgII gel in methanol. The gelation property of complex C-2 is found to increase with HgII content, and the initial yellow gel transformed into green after the addition of 8 equiv of HgII. The coordination of metalloligand (C-2) to a guest HgII metal ion is perceived to cause significant charge polarization around the coordinated imine and favors its hydrolysis, which ultimately leads to green 2-hydroxy benzaldehyde coordinated CuII complex (C-3). In this report, the role of acetate, metal salts, ligand, and solvents on the gel formation and the effect of various external chemical stimuli on its self-sustainability were studied in detail. The synthesized hetero-bimetallic CuII/HgII gel is also found to be suitable for the adsorption of various dyes.

complex C-2 was also checked with various metal salts in methanol and found to be inactive in most of the cases.75 However, to our delight a yellow gel was observed after the addition of 4 equiv of Hg(OAc)2 to C-2 in methanol (Figure 2).

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RESULTS AND DISCUSSION The Schiff base ligands were prepared upon reacting 6-amino pyridine derivative with salicyldehyde in the presence of a catalytic amount of ambertlite IR-120 under refluxing toluene. The corresponding CuII-complexes were synthesized from the reaction between CuCl2·2H2O and corresponding ligands in the presence of sodium acetate in methanol at room temperature. Initially, complex C-1 was chosen for the study, and its gelation was tried with various metal salts. For gelation study, 40 mL of methanol stock solution of C-1 (80 mg) was prepared and distributed in 20 different vials. To these vials, 1 mL of methanol solution of different metal salts (5 equiv with respect to C-1) was added, and vials were closed with a screw cap and left at room temperature. In all the cases, C-1 was found to be inactive for the formation of a gel with any of the metal salts. However, a green solution was observed in some cases, which upon standing at room temperature afforded a green crystal after 2−3 days. Further, structural analysis of the green crystal confirmed 2-hydroxy benzaldehyde coordinated CuII complex (C-3). So, the coordination of metalloligand (C-1) to guest metal ions was perceived to cause significant charge polarization around the coordinated imine in complex, which favors the hydrolysis to produce complex C-3. Therefore, hydrolysis should be stopped to get a hetero bimetallic complex. Toward this, complex C-2 was synthesized, in which close proximity of a methyl group and pyridine nitrogen may trim down the imine hydrolysis by dropping the successive interaction between metalloligand (C-2) and guest metal. The gelation property of

Figure 2. Formation of gel in the presence of C-3 with mercury acetate.

Interestingly, after the addition of 7 equiv of Hg(OAc)2, the yellow gel transformed into green, and it is likely to be observed even with the addition of up to 60 equiv of Hg(OAc)2. Effect of Solvent, Anions, and Other Metal Salts. Next, to check the effect of other solvents on the gelation property, 8 mg of complex C-2 and 90 mg of Hg(OAc)2 (Cu/Hg = 1:15, BMG-3) taken in a glass vial and to it 3 mL of the desired organic solvent was added and sonicated for 10 min to get a homogeneous solution, and the mixture was kept at room temperature for setting of the gel. Interestingly, apart from methanol, no other alcoholic solvent such as ethanol, propanol, isopronal, and water was found to be effective for gel formation. Other organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and dichloromethane (DCM) was also found to be not suitable for gelation of complex C-2. However, BMG-3 gel sets again even after the addition followed by shaking of up to 1 mL of different solvents such as DMF, DMSO, THF, DCM, ethyl acetate, toluene, and hexane to it. This effect is also consistent in other green gels such as BMG-4 (Cu/Hg = 1:20), BMG-6 (Cu/Hg = 1:30), and BMG-10 (Cu/Hg = 1:50).76 Further, to check the effect of water on BMG-3, 1 mL of water was added to it and sonicated for 10 min to get a homogeneous solution, which also B



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Figure 3. Comparative PXRD plot of (a) BMG-1, (b) BMG-3, (c) C-2 (experimental and simulated), and (d) C-3 (experimental and simulated).

corresponding green gel in 5 mL of methanol, and beyond that ratio, a precipitate was observed.81 The formation of BMG-3 was also tried in variable amounts of methanol and found to be suitable in up to 6 mL of methanol; after that a green color solution was observed.82 Mechanism. Next, to check the mechanism, powder X-ray diffraction (PXRD) of complexes C-2 and C-3, xerogels of BMG-1 and BMG-3 were recorded and depicted in Figure 3. PXRD patterns of both complexes C-2 and C-3 match well with their simulated ones which confirm that both products obtained are in their pure crystalline forms. The PXRD pattern of xerogel of BMG-1 reveals an amorphous nature. However, the presence of complex C-2 in a yellow BMG-1 gel matrix is found to be observed as both PXRD patterns are well matched with each other. On the other hand, the PXRD pattern of xerogel BMG-3 was not matched with that of complex C-2. However, the matching in PXRD pattern between BMG-3 xerogel and C-3 is further evident of Hg(OAc)2 mediated hydrolysis of coordinated imine of C-2 and subsequent generation of complex C-3.83 Next, Fourier transform infrared (FT-IR) spectra of complex C-2 in methanol were monitored after gradual addition of Hg(OAc)2. The complex C-2 shows peaks at 1635 cm−1 for the coordinated azomethine group (νCN), 1430 cm−1 for C−O, 1026 cm−1 for C−C, and 600 cm−1 for Cu−O. When we compare the FT-IR spectrum of C-2 and BMG-1, the CN stretching frequency shifted to the lower region by 7 cm−1, which is indicative of secondary interaction between C-2 and Hg(OAc)2.84 Upon gradual addition of Hg(OAc)2 in methanol solution of C-2, the peaks for CN at 1635 cm−1 slowly disappear with the generation of three new peaks at 1410, 1578, and 1708 cm−1. The peaks at 1410 and 1578 cm−1 were typical for metal acetate. The newly generated FT-IR peak at 1708 cm−1 upon addition of 5 equiv Hg(OAc)2 is ascribed to the

found to set again after keeping it at room temperature for 1 h. The gelation of complex C-2 with Hg(OAc)2 was also tried in a mixture of solvent. Interestingly, a 1:1 mixture of different solvent combinations including DCM/methanol, MeCN/ MeOH, H2O/MeOH, and toluene/MeOH were found to be effective for the formation of BMG-3.77 To check the effect of other mercury salts on the gelation property of complex C-2, various salts such as HgCl2, Hg2Cl2, and HgClO4 were tried, but they failed to get a corresponding gel.78 The gelation property of C-2 with HgCl2 in the presence of other coordinating anions such as chloride, acetate, nitrate, sulfate, and non-coordinating anions such as PF6−, BF4−, ClO4− were also tried, but these attempts failed to get any green or yellow gel in each of the cases.79 On the other hand, gelation between C-2 and Hg(OAc)2 in methanol was also found to be inactive in the presence of both coordinating and non-coordinating anions. The formation of BMG-3 from the reaction between C2 and Hg(OAc)2 in the presence of different main group metal salt such as NaCl, CaCl2, MgSO4, NaOAc or different color transition metal salt such as CoCl2·6H2O, Co(NO3)2, NiCl2· 6H2O, CuSO4·5H2O, FeCl3·6H2O also failed. However, the formation of BMG-3 from the reaction between C-2 and Hg(OAc)2 in the presence of 1 equiv of Zn(OAc)2 or Cu(OAc)2 was found to be observed in methanol.80 Further, to prepare magnetic gel, magnetic Fe3O4 nanomaterial was added to a mixture of C-2 and Hg(OAc)2 (1:15) in 3 mL of methanol and sonicated for 15 min and kept at room temperature for 2 h. During this time, a green gel set in the glass vial, in which deposition of magnetic material at the bottom of the vial was observed. Further, to check the maximum HgII gelation capacity of C-2, the amount of Hg(OAc)2 was varied by keeping a fixed amount of C-2 at 8 mg in 5 mL of methanol. Interestingly, C-2 is likely to be able to absorb up to 60 equiv (360 mg) of Hg(OAc)2 and form a C



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Figure 4. Proposed structure of yellow and green gel and their transformation at different conditions.

coordinated CN of pyridine to HgII.85 The new peak for coordinated pyridine to HgII as well as disappearance of stretching vibration of coordinated CN of C-2 directly confirm the hydrolysis of coordinated imine to form complex C-3. Although yellow to green gel transformation is found to be detected after the hydrolysis of complex C-2 with a larger amount of Hg(II), in situ generation of green gel from the corresponding hydrolyzed species (C-3 and 2-amino-6-picoline) and Hg(OAc)2 in methanol failed and afforded a green color precipitate (Figure 4). This observation justifies the need for complex C-2 to generate the supramolecular gel assembly in a controlled way. Effect of External and Chemical Stimuli. To check the effect of external stimuli on green BMG-3 gel, the observed green gel was heated at 50 °C for 30 min to get a solution, which upon standing for 1 h at room temperature regenerates the green BMG-3 gel. On the other hand, reversible transformation between sol and gel is also achieved even after sonication or shaking. On pulling out the BMG-3 gel from vial, a deformed shape is observed, which on standing reverts back to its original shape. The BMG-3 gel is found to be chemoresponsive, and the addition of a few drops of aqueous NH3 solution to the gel resulted in an instant destruction of the gels into a green transparent solution due to the breaking of the gel and formation of a metal amine complex. A similar type of green color solution was also detected after the addition of ethylene diamine (en) to BMG-3 due to the copper complex with en. The addition of hydrazine hydrate (N2H4·2H2O) to BMG-3 leads to a black color solution (Figure 5). Further, the addition of HCl resulted in a transparent solution with white precipitate. However, addition of HNO3 to BMG-3 generates a yellow solution with white precipitate. Furthermore, addition of the metal trapping agent EDTA in crystalline form does not allow penetration of the gel and it remained unchanged at the top of the vial even after one month. However, addition of methanol solution of EDTA to BMG-3 gradually transferred the gel into a green solution with white precipitate due to the destruction of a supramolecular gel network with EDTA (Figure 5). UV−vis and Microscopy Study. The methanol solution of complex C-2 shows two distinct peaks at 270 and 380 nm with

Figure 5. Effect of different chemical stimuli (a) and external stimuli (b) on BMG-3.

extinction coefficients (ε, mol−1 L cm−1) of 5 × 104 and 4.1 × 103 respectively. Upon gradual addition of mercuric acetate [Hg(OAc)2] to complex C-2 (200 μM) in methanol, the absorbance at 395 nm decreases with gradual red shifting of the existing band to 405 nm with the appearance of a new band at 440 nm due to the interaction between C-2 and mercury (Figure 6). Gradual addition of Hg(OAc)2 results a decrease in absorbance of both the band and complete disappearance of both the band observed after addition 10 equiv of Hg(OAC)2. Interestingly, no peaks were found after dissolving the synthesized BMG-1 and BMG-3 in methanol, which also correlates well with the above observation. We have also carried out the cyclic voltametry study of complex C-2 which indicates one irreversible reduction at 0.57 V for CuII/CuI.86 The gels have been analyzed using various types of microscopy to get some visual insights into their morphology. Supramolecular three-dimensional (3D) aggregation of the gel fiber was thoroughly investigated with field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The phase image (Figure 7a) and 3D view of the gel (Figure 7b) and clearly shows the helical nature of the BMG-3. The 3D view of AFM image with respect to height profile clearly illustrated the fibrous helical nature of the gel. From the cross sectional analysis, it was observed that the helical gel D



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Figure 6. UV−vis spectra of C-2 after the gradual addition of Hg(OAc)2 in methanol (a); comparative UV−vis spectra of C-2, BMG-1, and BMG-3 gel in methanol (b).

Figure 7. AFM phase image (a); 3D view (b); height profile and height vs. distance (c, d); cross section and analysis (e, f) of BMG-3 gel.

fibers have an average height of 3.8−4.2 nm and width 15−20 nm (Figure 7f). The average pitch calculated from the corresponding images found to be 40−45 nm for BMG-3. The FESEM images of BMG-3 indicate an entangled supramolecular rope-like morphology having a width of 50− 100 nm and length of several micrometers (Figure 8). The ropelike structures are twisted at several places indicating their tendency to form coiled structures.87,88 To confirm the presence of mercury in the gel, Energy-dispersive X-ray analysis (EDAX) along with atom mapping was done for BMG-1 and BMG-3 xerogels. This clearly indicates the presence of Cu and Hg in the gel along with the C, N, and O atom. The ratio of Cu and Hg is observed to be 1:6 and 1:25 in BMG-1 and BMG-4 xerogel respectively, which also is in accordance with the synthesized stoichiometric ratio of Cu and Hg in the material.89 Rheology Study. The mechanical properties using rheology have been studied for synthesized BMG-1, BMG-6, and BMG-10 gel material to check the effect of mercury content in its mechanical property. Typical curves of storage (G′) and loss (G″) modulus of gel as a function of frequency

(ω) are shown in Figure 9. Mercury equivalent led to a remarkable change in oscillatory rheological behaviors of the gel. Storage moduli of the gel were plotted against the angular frequency ranging from 0 to 100 rad/s in Figure 9a. The storage modulus (G′) of the gel (BMG-10) showed linear viscoelastic behavior, whereas BMG-2 and BMG-6 gel, in which the mercury content was low compared to BMG-10, showed nonlinear viscoelastic behavior. The linear viscoelastic behavior of BMG-10 revealed a lower dependence of moduli with frequency which elucidates that there was a more rigid system than a fluid. It may be due to the incorporation of a higher amount of mercury which enhances the supramolecular aggregation in gel and thereby enhances the pasting capacity. Moreover, for BMG-10, at any given point, G′ is greater than G″, indicating the true gel-like nature of the material. Low mercury gel (BMG-2 and BMG-6) had shown the frequency dependence (nonlinear) behavior of storage G′(ω) and loss modulus G″(ω) (Figure 9a). The frequency dependence of G′ can be an indication of the presence of a weak network structure in gels. A linear behavior in G′ was observed between E



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Figure 8. SEM images of BMG-3.

Figure 9. Plot of storage modulus (G′) loss modulus (G″) with frequency (a); plot of complex viscosity (η*) with frequency (b).

revealed the frequency dependence of the complex viscosity (η*) of gels. This result shows that the elastic properties of gels in terms of complex viscosity decreased with an increase in frequency due to the decrease in the rate of chain rearrangement, which tends to form a more ordered structure or crystalline structure. Self-Sustaining Nature of the Synthesized BMG. Selfsustaining nature is the ability of the gel to rest by itself without any support and without changing the shape and defines the stability of the gel with encapsulated solvent in it. Selfsustaining nature of all the synthesized BMGs with a variable concentration of mercury was tested by carefully pulling/ pushing out gel from glass vials.90−93 The synthesized yellow BMG failed to retain its original shape while being taken out

0 and 50 rad/s for BGM-2 and BGM-6 gels, and this suggested that structural rearrangement was indeed taking place during the mechanical test. Under continuous mechanical action, above a certain frequency (ω) (50 rad/s) a declining pattern of G′ was observed (Figure 9a). The decrease in storage modulus means the viscosity of the gel decreased and the G″ was elevated which was clearly observed in Figure 9a. Therefore, this phenomenon confirmed that at low mercury levels the supramolecular aggregation was less, which enhances the disintegration of the intermolecular bond at a high frequency level. Figure 11b shows complex viscosity (η*) of gels (BMG-2, BMG6, and BMG-10), plotted against frequency (ω). It was observed that the magnitudes of η* of gels were parallel to each other and decayed linearly with an increase in frequency, which F



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for color removal from industrial effluents. Toward the adsorption of color dyes, various smart material viz. dendritic polymers, clays, and supramolecular gels were shown to act as efficient dye adsorbing agents.100−103 In this regard, we also wanted to check the dye adsorption behavior of our synthesized gel. Initially, a 10 mL of 10−1 (M) aqueous solution of methylene blue (MB) was prepared and from that a 1 mL solution was poured into 3 g of synthesized BMG-3 gel and kept at room temperature. After 1 h, the complete adsorption of dye solution into BMG-3 resulted in the formation of a bluish-green gel. To check the effect of other dyes and their charge on the adsorption process, cationic dye viz. rhodamine B (RhB), crystal violet (CV), methyl violet (MV), anionic dye viz. methyl orange (MO), congo red (CR), and neutral dye viz. rose Bengal (RB), methyl red (MR) have been chosen for the study. For dye adsorption, 3 g of BMG-3 gel was taken in a glass vial and to it 1 mL of 10−1 (M) aqueous solution the representative dye was poured into it and kept at room temperature. Gratifyingly, the as-synthesized BMG-3 gel was found to be active for the complete adsorption of the aqueous solution within 6 h in the case of all four cationic dyes, while adsorption of methyl orange took place in 12 h, which indicates a slower adsorption rate in the case of anionic dye. However, BMG-3 was found to be not suitable for the adsorption of neutral dye such as rose Bengal and methyl red even after 3 days. The higher adsorption activity of BMG-3 in the case of cationic dye may be ascribed due to the more electrostatic interaction between negatively charge surface and positively charge dye molecule. Further, to check the adsorption of gel on the surface of the gel, 10 mg of dried gel was taken and to it 10−5 M MB solution was added and monitored under UV−vis spectrometer. The absorbance at 642 nm gradually decreases with time, and almost 90% adsorption of MB is found to be observed after 48 h. The synthesized 1 g BMG-3 xerogel was also applied for the adsorption of other dyes and it was found to be capable of adsorption of 55 mg of MB, 45 mg of RhB, 50 mg of CV, 65 mg of MV, and 40 mg of MO, respectively. Next, for practical applications of BMG-3 xerogel, the lifetime of the materials and their reusability are very important factors. For this, the adsorption of aqueous MB solution promoted by BMG-3 xerogel was studied. After the completion of the first reaction, the xerogel was washed with acetone and then methanol 3−4 times to remove the adsorbed dye from the xerogel and dried for reutilization as an adsorbent. After that, a

from the vials. However, among all the synthesized green BMGs, BMG-3 to BMG-12 showed excellent self-sustainability, and any shape can be made by using either the gelation vessels with different shapes or by carefully cutting them. On the other hand, the synthesized BMGs containing less than 12 equiv of Hg(OAc)2 failed to retain their original shape that was formed in the vial. At the same time, self-sustainability of the synthesized BMG increases with an increase in the amount of mercury content in the gel. Further, when different gel blocks of synthesized BMG-3 were kept in close contact along the cut surfaces without any external stimuli, they recombined within 2 h, whereas the gel blocks of synthesized BMG-6 healed into a one block after keeping these blocks in close contact for 40 min. Further, construction of an approximately 3 cm long bridge by connecting four alternate dye-doped (rhodamine B) and undoped gel blocks of synthesized BMG-3 was made to understand the self-healing nature (Figure 10). The diffusion of

Figure 10. Self-sustaining nature of BMG-3.

color from dye-doped block to undoped block was found to be increased over time and almost transformed into one red block after 12 h, which clearly indicates the self-healing property of the synthesized BMG.94−99 Dye Adsorption Study. The various dyes and chemicals contained in wastewater can cause deterioration in water quality and sometimes causes food chain contamination resulting in adverse effects on biodiversity. To date, various chemical and physical processes, such as chemical precipitation and separation of pollutants, coagulation, electrocoagulation, elimination by adsorption on activated carbon, etc. are applied

Figure 11. BMG-3 xerogel promoted adsorption of methylene blue (left); % of adsorption of MB versus no. of cycles plot for BMG-3 xerogel (right). G



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Figure 12. Adsorption of different dyes with BMG-3 gel (a); dye concentration versus amount of mercury plot for the adsorption of MB, RB, and MV to BMG-3 (b); amount of water versus mercury content plot for different BMG gels (c).

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CONCLUSIONS In summary, an easy to synthesize CuII−Schiff base complex (C-2), in which tunable access of a vacant coordination site for a guest metal is possible via stereoelectronic modulation of substituents, which helps in the supramolecular self-aggregation process and forms a hetero-bimetallic CuII/HgII gel. The gelation property of complex C-2 is found to increase with HgII content, and the initial yellow gel transformed into green with a higher equivalent of Hg(OAc)2 due to significant charge polarization around the coordinated imine, which ultimately leads to a corresponding green 2-hydroxy benzaldehyde coordinated CuII-complex (C-3). The role of anions, metal salts, ligand, and solvents on the gelation property of complex C-2 along with the effect of various external and chemical stimuli on gels, their self-sustainability, and transformation mechanisms were studied with the help of various spectroscopic and microscopic techniques. Finally, an as-synthesized hetero-bimetallic CuII/HgII gel is found to be a reusable material for the adsorption of various cationic and anionic dyes.

new reaction was performed using a fresh MB solution under similar reaction conditions. In terms of adsorption activity, the synthesized BMG-3 xerogel could be reused at least 3 times without any change in activity. However, a considerable drop down in the adsorption activity in xerogel was observed in fourth cycles (Figure 11). Next, to check the chemi-adsorption of dye molecules into the supramolecular gel assembly, dye adsorbed BMG-3 material was dried and an FT-IR spectrum was recorded. The FT-IR spectrum of all the dye adsorbed material shows the presence of dye in the material, as most of the peaks are well matched with the standard FT-IR spectrum of dye molecule.104 PXRD spectra of xerogel of BMG-3 were recorded before and after the dye adsorption, and no change was observed between the two materials.105 Further, to check the effect of mercury content in the gel material toward the amount of adsorption of different dyes, other material with variable amounts of mercury were applied for the adsorption of MB as a representative dye. For this 3 g of gel having different concentrations of mercury was taken in different vials and to it MB was added slowly until full adsorption occurred. The resulting graph between the amount of MB versus the amount of mercury shows a gradual increment in the adsorption of MB with an increase in the concentration of mercury in the gel. A similar observation has been made with rhodamine B as well as with methyl violet (Figure 12). During the dye adsorption study, the synthesized BMG-3 gel was found to be active for the adsorption of water in its supramolecular network. Further, to check the role of mercury content, different synthesized BMG gels with variable concentrations of mercury were tried for the adsorption of water. The adsorption of water gradually increased with the concentration of mercury and showed a linear relationship up to BMG-5 (Figure 12c). Interestingly, 3 g of BMG-5 can entrap up to 4 mL of water, and after that, a slow enhancement in the adsorption of water was observed and it almost got saturated in BMG-8.

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EXPERIMENTAL SECTION

Synthesis of Ligands. In a 250 mL round-bottom flask, 2-amino 6-methylpyridine-2-amine (30 mmol) and 2-hydroxy benzaldehyde (30 mmol) was taken in 150 mL of toluene. To this, 50 g of molecular sieves (4 Å) and 300 mg of Ambertlite IR-120 resin were added and stirred for 10 min at room temperature. After that, the reaction mixture was refluxed at 150 °C for 24 h using a Dean−Stark apparatus. During the course of the reaction, an appropriate amount of water generated from the reaction was collected. After the completion of the reaction, the remaining amount of toluene was evaporated in reduced pressure and an as-obtained yellow condensed product was dried under reduced pressure and collected for further analysis. Yield (L2) = 5.6 g (88%). IR (KBr, cm−1): ν = 3058, 2939 (C−H), 1627 (CN). δH(400 MHz; CDCl3): 2.58 (s, 3H, CH3), 6.96 (t, 1H, J = 7.5 Hz), 7.03 (d, 1H, J = 8.0 Hz), 7.12 (d, 1H, J = 7.8 Hz), 7.15 (d, 1H, J = 7.8 Hz), 7.40 (d, 1H, J = 7.8 Hz,), 7.52 (d, 1H, J = 7.6 Hz,), 7.70 (t, 1H, J = 7.8), 9.47 (s, 1H, HC = N), 13.52 (s, 1H, OH). A similar procedure H



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was applied for the synthesis of ligand L-1. Yield (L2) = 4.8 g (80%). IR (KBr, cm−1): ν = 3048, 2945 (C−H), 1623 (CN).

Table 1. Crystal Data Parameter for Complexes C-2 and C3111−113 formula unit formula wt crystal system T [K] a [Å] b [Å] c [Å] α [°] β [°] γ [°] volume [Å3] space group Z Dcalc [g cm−3] μ/mm−1 reflns collected unique reflns observed reflns R1 [I > 2σ(I)], wR2 GOF instrument X-ray CCDC reference no.

General Procedure To Synthesize the Complexes. In a typical procedure, 2 equiv of L2 (1.06 g, 5.00 mmol) and Na(OAc)·3H2O (0.680 g, 5.00 mmol) dissolved in 20 mL of methanol and stirred at room temperature for 30 min. After that, 5 mL of methanol solution of CuCl2·2H2O (0.425 g, 2.5 mmol) was added to the reaction mixture and stirred for 6 h at room temperature. During the reaction the color of the solution changed from yellow to brown. After the completion of the reaction, the solvent was evaporated in reduced pressure, and the as-obtained solid was washed with diethyl ether 3−4 times and dried under a vacuum and collected for further analysis. The suitable crystal for structure determination was isolated by the slow evaporation of methanol solution of complex C-2. Yield (C-2) = 0.380 g (78%) and pH is 6. IR (KBr, cm−1): ν = 3070, 2969 (C−H), 1614 (CN). A similar procedure was applied for the synthesis of complex C-1. Yield (C-1) = 0.320 g (70%). IR (KBr, cm−1): ν = 3070, 2969 (C−H), 1647 (C = N). To check the effect of NaOAc on the complexation process, the reaction between L2 and CuCl2 was tried with a variable amount of NaOAc, keeping all other reaction conditions unchanged. The usage of 0.5 or 0.25 equiv of NaOAc with respect to ligand produced the desired complex C-2 in 62% and 48% yield, respectively. Interestingly, in both cases the reaction solution turned into green, which upon standing at room temperature produced complex C-3. Further, pH of the reaction mixture is found to be 4.9, 5.5, and 6.3 after usage of 0.25, 0.5, and 1.0 equiv of NaOAc, respectively. So, the acidic pH in a lower equivalent of NaOAc further drives the hydrolysis and produced hydrolyzed product C-3 in higher yield and thus lowering the yield of desired C-2.106

C-3

C-2

C19H20O2Cu 280.35 monoclinic 296 8.7075(3) 6.2286(2) 12.3105(4) 90 118.164(2) 90 588.61(3) P21/c 2 1.725 1.861 5088 1351 1102 0.0374; 0.0956 1.095 Bruker APEX-II MoKα; λ = 0.71073 1502837

C26H22CuN4O2 486.02 triclinic 296 7.8693(6) 12.0662(9) 12.4840(10) 69.010(4) 89.977(5) 89.898(5) 1106.73(15) P1̅ 2 1.458 1.019 5566 3064 971 0.0563; 0.1552 0.985 Bruker APEX-II MoKα; λ = 0.71073 1502838

atmosphere was created by the addition of methanol around the gel material. Finally, dye-doped and undoped gel blocks were put in direct contact along the cut surfaces alternately at room temperature without any external stimuli. Selective Dye Adsorption and Desorption Study. The dye adsorption experiments were performed using methylene blue (MB), rhodamine B (RB), crystal violet (CV), methyl violet (MV), methyl orange (MO), congo red (CR), rose Bengal (RB), and methyl red (MR). For dye adsorption in synthesized BMG-3 gel, 3 g of BMG-3 was taken in a glass vial and to it 1 mL of 10−1(M) aqueous solution of the representative dye was poured into it and kept at room temperature. For BMG xerogel promoted dye adsorption, 3 mL of 5 × 10−5 (M) dye solution was taken in a beaker. After that, 10 mg of synthesized BMG xerogel was added to it, and progress of the reaction was monitored by UV−visible spectroscopy at room temperature. The desorption studies of the BMGs were carried out in acetone/methanol solution and determined by UV−visible spectroscopy. UV−vis Study. Absorption spectra were recorded in a Dynamica Halo DB-30 double beam digital spectrophotometer (Switzerland) attached with a Lab Companion RW-0525G chiller and also in SHIMADZU UV 2550 spectrophotometer with quartz cuvette with quartz cuvette. UV−vis spectra of the synthesized complexes were recorded in methanol. The UV−vis spectra of both the yellow and green gel were recorded by dissolving the corresponding gel in methanol. For reaction monitoring between C-2 and HgII, initially complex C-2 was dissolved in methanol and to it methanol solution of Hg(OAc)2 was added, and the corresponding change was monitored with UV−vis spectrometer. Field Emission Scanning Electron Microscope (FESEM). The morphologies of the synthesized BMGs were characterized using a field emission scanning electron microscope (ZEISS EVO 60 with oxford EDS detector) operating at 5−10 kV. For sample preparation, diluted sample of BMG was put into the thin aluminum sheet by using capillary tube and then allowing it to dry in air. The sample was also coated with a thin layer of Au before the experiment to minimize sample charging. For SEM-EDS analysis, both BMG-1 and BMG-3 gels were dried in air and used for analysis without Au coating. Atomic Force Microscope (AFM). Surface morphologies of the BMGs were characterized by AFM (Agilent 5500) in tapping mode.

X-ray Crystallography. X-ray reflections were collected on a Bruker APEX-II CCD diffractometer using Mo Kα (λ = 0.71073 Å) radiation. Data reduction was performed using Bruker SAINT Software.107 Intensities for absorption were corrected using SADABS. Structures were solved and refined using SHELXL-2014 with anisotropic displacement parameters for non-H atoms. Hydrogen atom on O was experimentally located in the crystal structure. All C− H atoms were fixed geometrically using the HFIX command in SHELX-TL.108 A check of the final CIF file using PLATON did not show any missed symmetry.109,110 The crystallographic parameters for all structures are summarized in Table 1. Typical Procedure for the Synthesis of Gel. In a typical gel formation reaction, a required amount of complex C-2 and Hg(OAc)2 was dissolved in 3 mL of methanol in a 5 mL vial and sonicated for 5 min to get a clear solution. Depending upon the Hg(OAc)2 stoichiometry, the initial brown solution turned into either yellow or green. After that, the solution was kept undisturbed until the gel was formed. Then, the formation of gel was confirmed by the glass vial inversion method at room temperature. Self-Healing Experiment. Initially BMG-3 green gel blocks were prepared using a 1:15 ratio of C-2 and Hg(OAc)2 in 3 mL of methanol in a beaker. For clear visualization, we also prepared dye-doped (rhodamine B) gel blocks. After that, each of the synthesized dyedoped and undoped gel blocks was cut into two pieces by using a knife. To protect the BMG-3 from drying, the persistent methanol I



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Figure 13. ORTEP with 50% probability ellipsoid of complexes C-2 and C-3. For AFM sample preparation, a diluted sample of BMG was put into the glass slide by using a capillary tube and then allowed to dry for overnight before imaging. PXRD. The PXRD patterns were recorded on a Philips PW1710 Xray diffractometer (40 kV, 20 mA) using CuKα radiation (k1/4 1.5418°A) in the 2θ range of 10−60° at a scanning rate of 0.5°/min−1. The PXRD analysis of both the complex C-2 and C-3 was done with their dry powder sample. For PXRD of BMG-1 and BMG-3 xerogel, first the gels were dried in air until all the solvents were evaporated and then ground properly to get a powder sample. Finally, the dry powder samples of both the xerogels were used for analysis. FT-IR. All the samples for FTIR study were properly washed and then dried under a vacuum. Finally, samples for the FT-IR spectra were recorded using an IMPACT 410 Thermo-Nicolet instrument from a thin transparent KBr pellet. Rheological Study. The dynamic rheological properties of starches were assessed using a rotational rheometer (Antron Paar, Physical MCR 301, PP 50) equipped with a plate and plate geometry with a diameter of 50 mm and a gap between plates of 0.1 mm. For analysis, samples were used on the stationary plate and excess sample was removed carefully by using a sharp razor blade. A thin layer of silicon oil was used on the exposed surface of the sample to prevent drying during testing. Frequency sweep tests (mechanical spectra) from 0 to 200 rad/s were performed at room temperature (30 °C).

The storage modulus (G′), loss modulus (G″), and complex viscosity (η*) were calculated for each samples. The data were analyzed using Rheoplus Version 3.61 software.

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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.cgd.6b01545. Experimental data (PDF) Accession Codes

CCDC 1502837−1502838 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. J



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(18) Xu, Y.; Sheng, K.; Li, C.; Shi, G. Self-Assembled Graphene Hydrogel via a One-Step Hydrothermal Process. ACS Nano 2010, 4, 4324−4330. (19) Tam, A. Y.-Y.; Wong, K. M.-C.; Yam, V. W.-W. Unusual Luminescence Enhancement of Metallogels of Alkynylplatinum(II) 2,6- Bis(N-alkylbenzimidazol-2′-yl)pyridine Complexes upon a Gel-toSol Phase Transition at Elevated Temperatures. J. Am. Chem. Soc. 2009, 131, 6253−6260. (20) Piepenbrock, M.-O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Metal- and Anion-Binding Supramolecular Gels. Piepenbrock, M.O.M.; Lloyd, G.O.; Clarke, N.; Steed, J.W. Chem. Rev. 2010, 110, 1960−2004. (21) Banerjee, K.; Biradha, K. Two-dimensional coordination of symmetrical and unsymmetrical dipyridyl b-diketones: luminescence, dye absorption and mechanical properties. New J. Chem. 2016, 40, 1997−2006. (22) Samai, S.; Biradha, K. Chemical and Mechano Responsive Metal−Organic Gels of Bis(benzimidazole)-Based Ligands with Cd(II) and Cu(II) Halide Salts: Self Sustainability and Gas and Dye Sorptions. Chem. Mater. 2012, 24, 1165−1173. (23) Samai, S.; Dey, J.; Biradha, K. Amino acid based low-molecularweight tris(bis-amido) organogelators. Soft Matter 2011, 7, 2121− 2126. (24) Roy, S.; Katiyar, A. K.; Mondal, S. P.; Ray, S. K.; Biradha, K. Multifunctional White-Light-Emitting Metal−Organic Gels with a Sensing Ability of Nitrobenzene. ACS Appl. Mater. Interfaces 2014, 6, 11493−11501. (25) Adarsh, N. N.; Dastidar, P. A New Series of ZnII Coordination Polymer Based Metallogels Derived from Bis-pyridyl-bis-amide Ligands: A Crystal Engineering Approach. Cryst. Growth Des. 2011, 11, 328−336. (26) Sahoo, P.; Kumar, D. K.; Trivedi, D. R.; Dastidar, P. An easy access to an organometallic low molecular weight gelator: a crystal engineering approach. Tetrahedron Lett. 2008, 49, 3052−3055. (27) Sahoo, P.; puranik, V. G.; Patra, A. K.; Sastry, P. U.; Dastidar, P. Ferrocene based organometallic gelators: a supramolecular synthon approach. Soft Matter 2011, 7, 3634−3641. (28) Nath, K.; Husain, A.; Dastidar, P. Metallogels and Silver Nanoparticles Generated from a Series of Transition Metal-Based Coordination Polymers Derived from a New Bis-pyridyl-bis-amide Ligand and Various Carboxylates. Cryst. Growth Des. 2015, 15, 4635− 4645. (29) Husain, A.; Parveen, R.; Dastidar, P. New Series of ZnII/CdII Mixed Ligand Coordination Polymers: Toward the Design of Metallogels. Cryst. Growth Des. 2015, 15, 5075−5085. (30) Adarsh, N. N.; Sahoo, P.; Dastidar, P. Is a Crystal Engineering Approach Useful in Designing Metallogels? A Case Study. Cryst. Growth Des. 2010, 10, 4976−4986. (31) Banerjee, S.; Adarsh, N. N.; Dastidar, P. A crystal engineering rationale in designing a CdII coordination polymer based metallogel derived from a C3 symmetric tris-amide-tris-carboxylate ligand. Soft Matter 2012, 8, 7623−7629. (32) Liu, Y.; He, L.; Zhang, J.; Wang, X.; Su, C. Y. Evolution of Spherical Assemblies to Fibrous Networked Pd(II) Metallogels from a Pyridine-Based Tripodal Ligand and Their Catalytic Property. Chem. Mater. 2009, 21, 557−563. (33) Zhang, J.; Su, C.-Y. Metal-Organic Gels: From Discrete Metallogelators to Coordination Polymers. Coord. Chem. Rev. 2013, 257, 1373−1408. (34) Zheng, S.-R.; Yang, Q. Y.; Liu, Y.-R.; Zhang, J. Y.; Tong, Y.-X.; Zhao, C.-Y.; Su, C.-Y. Assembly of CdI2-type coordination networks from triangular ligand and octahedral metal center: topological analysis and potential framework porosity. Chem. Commun. 2008, 356−358. (35) Zhang, J.; Wang, X.; He, L.; Chen, L.; Su, C.-Y.; James, S. L. Metal−organic gels as functionalisable supports for catalysis. New J. Chem. 2009, 33, 1070−1075. (36) Huang, J.; He, L.; Zhang, J.; Chen, L.; Su, C.-Y. Dynamic functionalised metallogel: An approach to immobilised catalysis with improved activity. J. Mol. Catal. A: Chem. 2010, 317, 97−103.

Sanjay Pratihar: 0000-0002-0229-735X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work by DST-New Delhi (to S.P. for Inspire Grant No. IFA-12/CH-39 and K.S. for DST-Inspire fellowship) is gratefully acknowledged. S.P. is highly grateful to Professor M. K. Chaudhuri and Professor T. K. Maji for all the help, support, and inspiration.

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REFERENCES

(1) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels. Chem. Rev. 1997, 97, 3133− 3160. (2) Abdallah, D. J.; Weiss, R. G. Organogels and Low Molecular Mass Organic Gelators. Adv. Mater. 2000, 12, 1237−1247. (3) Mallia, V. A.; Weiss, R. G. Self-assembled fibrillar networks and molecular gels employing 12-hydroxystearic acid and its isomers and derivatives. J. Phys. Org. Chem. 2014, 27, 310−315. (4) Weiss, R. G. The Past, Present, and Future of Molecular Gels. What Is the Status of the Field, and Where Is It Going? J. Am. Chem. Soc. 2014, 136, 7519−7530. (5) Randall, J.; Meador, M. A. B.; Jana, S. C. Tailoring mechanical properties of aerogels for aerospace applications. ACS Appl. Mater. Interfaces 2011, 3, 613−626. (6) Meador, M. A. B.; Scherzer, C. M.; Nguyen, B. N.; Quade, D.; Vivod, S. L. Epoxy reinforced aerogels made using a streamlined process. ACS Appl. Mater. Interfaces 2010, 2, 2162−2168. (7) Ajayaghosh, A.; Praveen, V. K. π-Organogels of Self-Assembled pPhenylenevinylenes: Soft Materials with Distinct Size, Shape, and Functions. Acc. Chem. Res. 2007, 40, 644−656. (8) Rao, M. R.; Sun, S.-S. Supramolecular Assemblies of AmideDerived Organogels Featuring Rigid π-Conjugated Phenylethynyl Frameworks. Langmuir 2013, 29, 15146−15158. (9) Shen, Y.-T.; Li, C.-H.; Chang, K.-C.; Chin, S.-Y.; Lin, H.-A.; Liu, Y.-M.; Hung, C.-Y.; Hsu, H.-F.; Sun, S.-S. Synthesis, Optical, and Mesomorphic Properties of Self-Assembled Organogels Featuring Phenylethynyl Framework with Elaborated Long-Chain Pyridine-2,6Dicarboxamides. Langmuir 2009, 25, 8714−8722. (10) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M. Two Component Dendritic Gel: Effect of Spacer Chain Length on the Supramolecular Chiral Assembly. Langmuir 2004, 20, 7070−7077. (11) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C.; George, S. J. Molecular Wire Encapsulated into π Organogels: Efficient Supramolecular Light-Harvesting Antennae with Color-Tunable Emission. Angew. Chem., Int. Ed. 2007, 46, 6260−6265. (12) Zhong, Q.; Metwalli, E.; Kaune, G.; Rawolle, M.; BivigouKoumba, A. M.; Laschewsky, A.; Papadakis, C. M.; Cubitt, R.; MüllerBuschbaum, P. M. Switching kinetics of thin thermo-responsive hydrogel films of poly(monomethoxy-diethyleneglycol-acrylate) probed with in situ neutron reflectivity. Soft Matter 2012, 8, 5241− 5249. (13) Overstreet, D. J.; Dutta, D.; Stabenfeldt, S. E.; Vernon, B. L. Injectable Hydrogels. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 881− 903. (14) Estroff, L. A.; Hamilton, A. D. Water Gelation by Small Organic Molecules. Chem. Rev. 2004, 104, 1201−1217. (15) Yang, Z.; Liang, G.; Xu, B. Enzymatic Hydrogelation of Small Molecules. Acc. Chem. Res. 2008, 41, 315−326. (16) Zhao, F.; Ma, M. L.; Xu, B. Molecular Hydrogels of Therapeutic Agents. Chem. Soc. Rev. 2009, 38, 883−891. (17) Zhang, Y.; Yang, Z.; Yuan, F.; Gu, H.; Gao, P.; Xu, B. Molecular Recognition Remolds the Self-Assembly of Hydrogelators and Increases the Elasticity of the Hydrogel by 106-Fold. J. Am. Chem. Soc. 2004, 126, 15028−15029. K



Article

(37) Li, L.; Xiang, S.; Cao, S.; Zhang, J.; Ouyang, G.; Chen, L.; Su, C.-Y. A synthetic route to ultralight hierarchically micro/mesoporous AlIII-carboxylate metal-organic aerogels. Nat. Commun. 2013, 4, 1774. (38) Tam, A.Y.-Y.; Wong, K. M.-C.; Wang, G.; Yam, V. W.-W. Luminescent metallogels of platinum(II) terpyridyl complexes: interplay of metal-metal, p−p and hydrophobic−hydrophobic interactions on gel formation. Chem. Commun. 2007, 2028−2030. (39) Zhao, L.; Wong, K. M.-C.; Li, B.; Li, W.; Zhu, N.; Wu, L.; Yam, V. W.-W. Luminescent Amphiphilic 2,6-Bis(1-alkylpyrazol-3-yl)pyridyl Platinum(II) Complexes: Synthesis, Characterization, Electrochemical, Photophysical, and Langmuir−Blodgett Film Formation Studies. Chem. - Eur. J. 2010, 16, 6797−6809. (40) Tam, A. Y.-Y.; Yam, V. W.-W. Recent Advances in Metallogels. Chem. Soc. Rev. 2013, 42, 1540−1567. (41) Feldner, T.; Häring, M.; Saha, S.; Esquena, P.; Banerjee, R.; Díaz, D. D. Supramolecular Metallogel that imparts Self-Healing Properties to other Gel Networks. Chem. Mater. 2016, 28, 3210−3217. (42) Aiyappa, H. B.; Saha, S.; Garai, B.; Thote, J.; Kurungot, S.; Banerjee, R. A Distinctive PdCl2-Mediated Transformation of FeBased Metallogels into Metal−Organic Frameworks. Cryst. Growth Des. 2014, 14, 3434−3437. (43) Saha, S.; Bachl, J.; Kundu, T.; Diaz Diaz, D.; Banerjee, R. Dissolvable metallohydrogels for controlled release: evidence of a kinetic supramolecular gel phase intermediate. Chem. Commun. 2014, 50, 7032−7035. (44) Saha, S.; Bachl, J.; Kundu, T.; Diaz Diaz, D.; Banerjee, R. Amino acid-based multiresponsive low-molecular weight metallohydrogels with load-bearing and rapid self-healing abilities. Chem. Commun. 2014, 50, 3004−3006. (45) Basak, S.; Nanda, J.; Banerjee, A. Multi-Stimuli Responsive Self Healing Metallohydrogels: Tuning of the Gel Recovery Property. Chem. Commun. 2014, 50, 2356−2359. (46) Adhikari, B.; Nanda, J.; Banerjee, A. Pyrene-Containing Peptide Based Fluorescent Organogels: Inclusion of Graphene into the Organogel. Chem. - Eur. J. 2011, 17, 11488−11496. (47) Samanta, S. K.; Pal, A.; Bhattacharya, S.; Rao, C. N. R. Carbon Nanotube Reinforced Supramolecular Gels with Electrically Conducting, Viscoelastic and Near-Infrared Sensitive Properties. J. Mater. Chem. 2010, 20, 6881−6890. (48) Nandi, G.; Titi, H. M.; Thakuria, R.; Goldberg, I. Solvent Dependent Formation of Metallogels and Single-Crystal MOFs by La(III) and Ce(III) Connectors and 3,5-Pyridinedicarboxylate. Cryst. Growth Des. 2014, 14, 2714−2719. (49) Krumm, M.; Pueyo, C. L.; Polarz, S. Monolithic Zinc Oxide Aerogels from Organometallic Sol-Gel Precursors. Chem. Mater. 2010, 22, 5129−5136. (50) Chen, K.; Jin, X.; Tang, L. Supramolecular Metallogels. Prog. Chem. 2010, 22, 1094−1101. (51) Beck, J. B.; Rowan, S. J. Multistimuli, Multiresponsive MetalloSupramolecular Polymers. J. Am. Chem. Soc. 2003, 125, 13922−13923. (52) Liu, Z.-X.; Feng, Y.; Zhao, Z.-Y.; Yan, Z.-C.; He, Y.-M.; Luo, X.J.; Liu, C.-Y.; Fan, Q.-H. A New Class of Dendritic Metallogels with Multiple Stimuli Responsiveness and as Templates for the In Situ Synthesis of Silver Nanoparticles. Chem. - Eur. J. 2014, 20, 533−541. (53) Sheikhi, A.; van de Ven, T. G. M. Trapping It Softly: Ultrasoft Zirconium Metallogels for Macromolecule Entrapment and Reconfiguration. ACS Macro Lett. 2016, 5, 904−908. (54) Howlader, P.; Mukherjee, P. S. Face and edge directed selfassembly of Pd12 tetrahedral nano-cages and their self-sorting. Chem. Sci. 2016, 7, 5893−5899. (55) Tatikonda, R.; Bhowmik, S.; Rissanen, K.; Haukka, M.; Cametti, M. Metallogel formation in aqueous DMSO by perfluoroalkyl decorated terpyridine ligands. Dalton Trans. 2016, 45, 12756−12762. (56) Compel, W. S. Metallogels through glyme coordination. Dalton Trans. 2016, 45, 4509−4512. (57) Karan, C. K.; Bhattacharjee, M. Self-Healing and Moldable Metallogels as the Recyclable Materials for Selective Dye Adsorption and Separation. ACS Appl. Mater. Interfaces 2016, 8, 5526−5535.

(58) Ma, X.; Li, X.; Cha, Y.-E.; Jin, L.-P. Highly Thermostable One Dimensional LanthanideIII Coordination Polymers Constructed from Benzimidazole-5,6-dicarboxylic Acid and 1,10-Phenanthroline: Synthesis, Structure, and Tunable White-Light Emission. Cryst. Growth Des. 2012, 12, 5227−5232. (59) Ma, M.-L.; Ji, C.; Zang, S.-Q. Syntheses, Structures, Tunable Emission and White Light Emitting Eu3+ and Tb3+ Doped Lanthanide Metal−Organic Framework Materials. Dalton Trans. 2013, 42, 10579− 10586. (60) Zhang, Y.; Zhang, B.; Kuang, Y.; Gao, Y.; Shi, J.; Zhang, X. X.; Xu, B. A Redox Responsive, Fluorescent Supramolecular Metallohydrogel Consists of Nanofibers with Single-Molecule Width. J. Am. Chem. Soc. 2013, 135, 5008−5011. (61) Mondal, C.; Ganguly, M.; Pal, J.; Sahoo, R.; Sinha, A. K.; Pal, T. Pure inorganic gel: a new host with tremendous sorption capability. Chem. Commun. 2013, 49, 9428−9430. (62) Sarkar, S.; Dutta, S.; Bairi, P.; Pal, T. Redox-Responsive Copper(I) Metallogel: A Metal−Organic Hybrid Sorbent for Reductive Removal of Chromium(VI) from Aqueous Solution. Langmuir 2014, 30, 7833−7841. (63) Sarkar, S.; Dutta, S.; Chakrabarti, S.; Bairi, P.; Pal, T. RedoxSwitchable Copper(I) Metallogel: A Metal−Organic Material for Selective and Naked-Eye Sensing of Picric Acid. ACS Appl. Mater. Interfaces 2014, 6, 6308−6316. (64) Dutta, S.; Ray, C.; Mallick, S.; Sarkar, S.; Sahoo, R.; Negishi, Y.; Pal, T. A Gel-Based Approach To Design Hierarchical CuS Decorated Reduced Graphene Oxide Nanosheets for Enhanced Peroxidase-like Activity Leading to Colorimetric Detection of Dopamine. J. Phys. Chem. C 2015, 119, 23790−23800. (65) Yan, L.; Shen, L.; Lv, M.; Yu, W.; Chen, J.; Wang, S.; Fu, X.; Ye, Z. Self-Healing Supramolecular Heterometallic Gels Based on the Synergistic Effect of the Constituent Metal Ions. Chem. Commun. 2015, 51, 17627−17629. (66) Yan, L.; Gou, S.; Ye, Z.; Zhang, S.; Ma, L. Self-healing and moldable material with the deformation recovery ability from selfassembled supramolecular metallogels. Chem. Commun. 2014, 50, 12847−12850. (67) Lee, J. H.; Kang, S.; Lee, J. Y.; Jung, J. H. A tetrazole-based metallogel induced with Ag+ ion and its silver nanoparticle in catalysis. Soft Matter 2012, 8, 6557−6563. (68) Park, J.; Lee, J. H.; Jaworski, J.; Shinkai, S.; Jung, J. H. Luminescent Calix[4]arene-Based Metallogel Formed at Different Solvent Composition. Inorg. Chem. 2014, 53, 7181−7187. (69) Metallogel of bis(tetrazole)-appended pyridine derivative with CoBr2 as a chemoprobe for volatile gases containing chloride atom Lee, J. H.; Baek, Y. E.; Kim, K. Y.; Choi, H.; Jung, J. H. Supramol. Chem. 2016, 28, 870−873. (70) Wheatley, N.; Kalck, P. Structure and Reactivity of Early-Late Heterobimetallic Complexes. Chem. Rev. 1999, 99, 3379−3419. (71) Matsunaga, S.; Shibasaki, M. Recent advances in cooperative bimetallic asymmetric catalysis: dinuclear Schiff base complexes. Chem. Commun. 2014, 50, 1044−1057. (72) Buchwalter, P.; Rosé, J.; Braunstein, P. Multimetallic Catalysis Based on Heterometallic Complexes and Clusters. Chem. Rev. 2015, 115, 28−126. (73) Das, D.; Mohapatra, S. S.; Roy, S. Recent advances in heterobimetallic catalysis across a “transition metal−tin” motif. Chem. Soc. Rev. 2015, 44, 3666−3690. (74) Biswas, S.; Saha, R.; Ghosh, A. Copper(II)−Mercury(II) Heterometallic Complexes Derived from a Salen-Type Ligand: A New Coordination Mode of the Old Schiff Base Ligand. Organometallics 2012, 31, 3844−3850. (75) Please see Figures S2 and S3 in Supporting Information for details. (76) Please see Figure S8 in Supporting Information for corresponding images. (77) For other mixture of solvent combinations, please see Figure S9 in Supporting Information. We are thankful to one of the esteemed reviewers for suggesting this. L



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(78) Throughout the experiment the ratio between complex C-2 and HgII/HgI salt maintained at 1:15 in 3 mL of methanol. (79) For details please see Figure S6 in Supporting Information. (80) Please see Figure S5 in Supporting Information for corresponding images. (81) Please see Figure S10 in Supporting Information for details. (82) For details please see Figure S11 in Supporting Information. (83) The presence of Hg(OAC)2 in BMG-3 gel matrix may influence the shifting of peaks. (84) Please see the Figure S12a in Supporting Information for comparative FT-IR spectra. (85) Although S7 shows the peak at 1708 for Hg(II) coordination with pyridine. However, the corresponding peak at 1708 cm−1 may also be ascribed due to 2-hydroxy benzaldehyde coordinated Cu(II) complex. (86) Please see the Figure S12b in Supporting Information. Shaju, K. S.; Joby, T. K.; Raphael, V. P.; Kuriakose, N. Spectral and Cyclic VoltammetricStudies on Cu (II)-Schiff Base Complex Derived from Anthracene-9(10 H)-one. IOSR. IOSR J. Appl. Chem. 2014, 7, 64−68. (87) Hirst, A. R.; Smith, D. K. Two-Component Gel-Phase MaterialsHighly Tunable Self-Assembling Systems. Chem. - Eur. J. 2005, 11, 5496−5508. (88) Dixit, M. K.; Pandey, V. K.; Dubey, M. Alkali base triggered intramolecular charge transfer metallogels based on symmetrical A− p−D-chiral-D−p−A type ligands. Soft Matter 2016, 12, 3622−3630. (89) Please see the Figures S13 and S14 of Supporting Information for SEM-EDAX and atom mapping of BMG-1 and BMG-4. (90) An, S. Y.; Arunbabu, D.; Noh, S. M.; Song, Y. K.; Oh, J. K. Recent Strategies to Develop Self-Healable Crosslinked Polymeric Networks. Chem. Commun. 2015, 51, 13058−13070. (91) Yan, X.; Xu, D.; Chen, J.; Zhang, M.; Hu, B.; Yu, Y.; Huang, F. A Self-Healing Supramolecular Polymer Gel with Stimuli-Responsiveness Constructed by Crown Ether Based Molecular Recognition. Polym. Chem. 2013, 4, 3312−3322. (92) Bekas, D. G.; Tsirka, K.; Baltzis, D.; Paipetis, A. S. Self-Healing Materials: A Review of Advances in Materials, Evaluation, Characterization and Monitoring Techniques. Composites, Part B 2016, 87, 92− 119. (93) Wang, Q.; Mynar, J. L.; Yoshida, M.; Lee, E.; Lee, M.; Okuro, K.; Kinbara, K.; Aida, T. High-Water-Content Mouldable Hydrogels by Mixing Clay and a Dendritic Molecular Binder. Nature 2010, 463, 339−343. (94) Cho, S. H.; Braun, P. V.; White, S. R. Self-Healing Polymer Coatings. Adv. Mater. 2009, 21, 645−649. (95) Wang, X.; Liu, F.; Zheng, X.; Sun, J. Water-Enabled Self-Healing of Polyelectrolyte Multilayer Coatings. Angew. Chem., Int. Ed. 2011, 50, 11378−11381. (96) South, A. B.; Lyon, L. A. Autonomic Self-Healing of Hydrogel Thin Films. Angew. Chem., Int. Ed. 2010, 49, 767−771. (97) Burnworth, M.; Tang, L.; Kumpfer, J. R.; Duncan, A. J.; Beyer, F. L.; Fiore, G. L.; Rowan, S. J.; Weder, C. Optically Healable Supramolecular Polymers. Nature 2011, 472, 334−337. (98) Shen, Z.; Jiang, Y.; Wang, T.; Liu, M. Symmetry Breaking in the Supramolecular Gels of an Achiral Gelator Exclusively Driven by π−π Stacking. J. Am. Chem. Soc. 2015, 137, 16109−16115. (99) Araujo, M.; Diaz-Oltra, S.; Escuder, B. Triazolyl-Based Molecular Gels as Ligands for Autocatalytic ‘Click’ Reactions. Chem. - Eur. J. 2016, 22, 8676−8684. (100) Karadag, E.; Uzum, O. B.; Saraydin, D. Swelling equilibria and dye adsorption studies of chemically crosslinked superabsorbent acrylamide/maleic acid hydrogels. Eur. Polym. J. 2002, 38, 2133−2141. (101) Ekici, S.; Isikver, Y.; Saraydın, D. Poly(Acrylamide-Sepiolite) Composite Hydrogels: Preparation, Swelling and Dye Adsorption Properties. Polym. Bull. 2006, 57, 231−241. (102) Wu, Z.; Ahn, I. S.; Lee, C. H.; Kim, J. H.; Shul, Y. G.; Lee, K. Enhancing the organic dye adsorption on porous xerogels. Colloids Surf., A 2004, 240, 157−164. (103) Krieg, E.; Shirman, E.; Weissman, H.; Shimoni, E.; Wolf, S. G.; Pinkas, I.; Rybtchinski, B. Supramolecular Gel Based on a Perylene

Diimide Dye: Multiple Stimuli Responsiveness, Robustness, and Photofunction. J. Am. Chem. Soc. 2009, 131, 14365−14373. (104) Please see Figure S15b in Supporting Information for corresponding FTIR plot for dye doped BMG-3 material. (105) Please see Figure S15a in Supporting Information for corresponding PXRD plot for dye doped BMG-3 material. (106) We are thankful to one of the esteemed reviewer for raising this point. (107) SAINT Plus, v 6.14; Bruker AXS Inc.: Madison, WI, 2008. (108) Bruker AXS Inc.: Madison, WI, 2008. (109) PLATON, A multipurpose crystallographic tool; Spek, A. L. Utrecht University: Utrecht, Netherland, 2002. (110) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7−13. (111) Bevan, J. A.; Graddon, D. P.; McConnell, J. F. Crystal Structures of Bis-(Salicylaldehydato)-copper(II) and Bis-(8-Hydroxyquinolinato)-copper(II). Nature 1963, 199, 373. (112) McKinnon, A. J.; Waters, T. N.; Hall, D. The Colour Isomerism and Structure of Copper Co-ordination Compounds. Part VII. The Crystal Structure of Bissalicylaldehydatocopper(II). J. Chem. Soc. 1964, 3290−3294. (113) Castineiras, A.; Castro, J. A.; Duran, M. L.; Garcia-Vazquez, J. A.; Macias, A.; Romero, J.; Sousa, A. The Electrochemical Synthesis of Neutral Copper(II) Complexes of Schiff Base Ligands: The Crystal Structures of Bis-(N-[2-(3-Methylpyridyl)]-5methoxysalicylideneiminato}Copper(II) and Bis-{ N-[2-(6-Methylpyridyl)]- salicylideneiminato}Copper(II). Polyhedron 1989, 8, 2543− 2549.

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HgII Bimetallic Gel

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