Pyrazole-Based Metallogels Showing an Unprecedented Colorimetric

Article pubs.acs.org/crystal

Pyrazole-Based Metallogels Showing an Unprecedented Colorimetric Ammonia Gas Sensing through Gel-to-Gel Transformation with a Rare Event of Time-Dependent Morphology Transformation Sudeshna Bhattacharya, Satirtha Sengupta, Sukhen Bala, Arijit Goswami, Sumi Ganguly, and Raju Mondal* Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, West Bengal, India S Supporting Information *

ABSTRACT: A series of novel pyrazole-based bis- and tris-amide molecules have been synthesized and characterized which were found to gel copper chloride only among other first row transition metals or other copper salts. All the gels have been thoroughly characterized using infrared spectroscopy, UV−visible spectroscopy, scanning electron microscopy, transmission electron microscopy, and rheological studies. The gels were composed of an infinitely long nanofibrillar network which displayed an interesting morphological transformation to nanoscale metal−organic particles (NMOPs), further corroborated by microscopic techniques and X-ray powder diffraction data. Although NMOPs, hybrid organic−inorganic nanoparticles, are widely known for their immense applications in the field of drug discovery and medicine, NMOP-based metallogels are not so common. Furthermore, this is a very rare example, to the best of our knowledge, in which such a transformation is observed in metallogelation. Gel 3 shows another extremely rare and fascinating external chemical stimuli responsive gel-to-gel transformation with a potential application of colorimetric ammonia gas sensing.

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INTRODUCTION Over the past decade or so, the development in the field of metallogels, a fast expanding branch of supramolecular gels, has attracted increasing interest.1 Generating stimuli-responsive functional materials with improved physicochemical properties such as magnetic, spectroscopic, or catalytic properties remains the major driving force behind the incorporation of metal ions in these materials.2 Metallogels are formed when a discrete metal complex self-assembles via noncovalent interactions giving rise to a highly cross-linked, intertwined three-dimensional network while immobilizing a large amount of solvent molecules within the framework.3 However, immobilizing a large amount of solvent molecules within the metal complex framework using hydrogen bond and various weak interactions is a formidable obstacle preventing usual “inorganic” ligand molecules from being a supramolecular gelator. Until very recently, strong metal−ligand coordinative interactions are usually accepted as the all-important structure determining interactions in traditional inorganic chemistry and accordingly maneuvered and manipulated for designing targeted frameworks, while other weak interactions such as π−π stacking, halogen bonding, or C−H···X interactions merely appear on an ornamentary basis. On top of that, obtaining a not-too-ordered system, a major prerequisite for gel formation, in order to prevent typical precipitation/crystallization again offers a serious challenge to the manipulation of metal−ligand interactions.4 In general, by virtue of their well-defined directional nature, metal−ligand coordination bonds introduce a higher order of symmetry in © 2014 American Chemical Society

the system. Notwithstanding these challenges, many fascinating results on metallogels have been reported with the inclusion of functional groups such as urea, amide, aromatic rings, which are capable of immobilizing solvent molecules by forming various weak interactions such as H-bond or π−π stacking.5 However, even a cursory inspection would reveal under-exploitation of rich and diverse metal−ligand interactions in the area of molecular gels, as most of the strategies adopted for metallogelation are based on either urea or amide-containing bipyridyl molecules.6 Despite the major role that 1-H pyrazole-based molecules play in traditional inorganic chemistry,7 biomolecules,8 and drugs,9 it is surprising that gelation properties of 1-H pyrazole derivatives is unexplored to date, except for a very few random examples of metallogelation.10 The 1-H pyrazole ring possesses a rich supramolecular chemistry and can offer a broad scope of study on hydrogen bonding as well as metal coordination. The deprotonated pyrazolate anion is also well-known as a bichelating ligand and can lead to the metallogelation with the formation of multinuclear complexes.11 On the other hand, the 1-H pyrazole group can play a dual role in metal coordination as well as hydrogen bond formation with an inbuilt hydrogen bonding site (N−H), which could be useful in immobilizing solvent molecules.12 As a part of our research program, we have been developing a range of new 1-H Received: January 16, 2014 Revised: March 28, 2014 Published: April 9, 2014 2366

dx.doi.org/10.1021/cg5000827 | Cryst. Growth Des. 2014, 14, 2366−2374

Crystal Growth & Design

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gelators even at different pH ranges. However, ligand 1 can form a sufficiently strong metallogel in the presence of varying amounts of copper chloride after sonication in a DMF/H2O (1:4) solvent system. Metallogels were also formed in MeOH/ H2O (7:3) solvent systems. Interestingly, the ligands 1 and 3 were specific toward copper chloride only as no gel formed with other copper salts or any other first row transition metal chlorides. Furthermore, the presence of water in the solvent systems appeared to be a determining factor in gel formation as nonaqueous solvents systems led to either precipitation or crystallization. In order to gain insight into the nature and properties of the gels formed by the ligands, detail microscopic and spectroscopic experiments were performed. Fourier transformed infrared (FTIR) spectroscopy was performed to identify the nature of participation of functional groups present in the ligand and in their corresponding xerogel as well as in crystalline form. The amide CO stretching band at 1649 cm−1 in free ligand split into 1677 and 1618 cm−1 in the xerogel of ligand 1 indicates ambiguous behavior of the carbonyl groups in the ligand. Moreover, amide N−H stretching and pyrazole pyrazole N−H stretching bands which appeared at 3450 cm−1 and 3282 cm−1 respectively in the free ligand reduced to 3375 and 3240 cm−1 in the xerogel indicating the participation of these groups in hydrogen bonding. The lowering of the CO stretching band generally indicates metal coordination or hydrogen bonding; however an increase in the stretching frequency indicates that the hydrogen bonding is decreased in the gel state compared to that present in the free ligand. This is possible only when both the carbonyl carbon of the ligand behave differently with one participating in metal coordination or hydrogen bonding while the second one remains inactive. These observations were consistent with that obtained from the IR performed with the crystalline form where two different kinds of carbonyl stretching have been observed. While the CO band obtained at 1612 cm−1 corresponds to metal coordinated amide group, the other appeared at 1654 cm−1, which suggests similar behavior of amide groups in the gel state as in the solid state. The UV−visible spectra of ligand 1 showed a broad absorption band at about 267 nm, suggesting intraligand π−π* transition (Figure S11a, Supporting Information). Interestingly, the absorption maximum showed a blue shift for the gel form, probably due to H-type aggregation of the polymeric network. Furthermore, an ILCT band which appeared at about 350 nm for the free ligand shifted to slightly higher absorption region in the gel state. Furthermore, in order to get a detailed picture of the metal coordination sphere, an X-band electron paramagnetic resonance (EPR) spectrum was performed with the corresponding metallogel at 77 K (Figure S12, Supporting Information). A typical 4-fold hyperfine splitting in g∥ region was observed, indicating that the copper ion is coordinated with four nitrogen atoms. Again, as g∥ (2.309) > g⊥ (2.092) > (2.0023), the metal center is expected to be octahedral in geometry. Furthermore, the absence of half-field signal due to ΔM = ± 2 transition at around 1600 G indicates that there is no significant amount of metal−metal interaction in the gel state. Considering all these observations, we can predict the network of the gel depicted in Figure S15. In order to study the morphology of the metallogel, FESEM and TEM were performed on the xerogels. A fresh sample of the xerogel prepared from 1 displayed similar SEM and TEM

pyrazole-based molecules for use as ligands in coordination polymers, extended supramolecular networks, and gelating abilities of some of these ligands.13 In the present work, we report the synthesis, characterization of three new amide-based 1-H pyrazole ligands with benzene core, and evaluation of their usefulness as gelator molecules. The ligand molecules show strikingly different gelation behavior in the presence of CuCl2. The 1,3- and 1,3,5substituted ligands (1 and 3) with angular disposition of pyrazole moieties generate strong gels, whereas, under same experimental conditions, linear 1,4-substituted ligand 2 forms a solid precipitate. We also report herein a rare phenomenon of morphology transformation for the metallogel of compound 3 supported by microscopic techniques (field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM)) and X-ray powder diffraction techniques. Furthermore, responses of the gels with various external chemical stimuli including an unprecedented metal stimuli coordination mediated gel to gel transformation with potential application in colorimetric ammonia gas sensing have also been described.

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RESULTS AND DISCUSSION The ligands were prepared by reacting 3-amino-5-methyl-1Hpyrazole with corresponding acid chlorides in the presence of triethylamine under nitrogen atmosphere using dried acetonitrile as solvent. Ligands 1 and 2 are positional isomers and differ only in the relative disposition of pyrazole-amide composites in the central benzene core. Our intention was to study the subtle structural variations and molecular motifs that dictate the supramolecular self-assembly of a particular compound. In other words, this should help us to draw a correspondence between angular disposition of the hydrogen bonding motifs as well as metal coordination sites and the resultant self-assembly. Ligand 3, on the other hand, is a C3-symmetric extended molecule. It is worth mentioning here that tripodal compounds, especially with a 1,3,5-substituted benzene core, have been shown to promote the formation of metallogels.14 Moreover, compared to ligands 1 and 2, ligand 3 contains one extra amide group and pyrazole moiety making it a more suitable candidate to form strong metallogels through expansion of its three arms. Considering the fact that our primary goal was to assess the gel-forming aptitude of 1-H pyrazole based compounds, we started our investigation by testing organogelation in different organic solvents media. The ligands themselves were not 2367



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micrographs consisting of a fibrillar network with a diameters of almost 15−30 nm (Figure 1a,b).

One of the interesting features of ligand 1 is that it forms metallogels with copper chloride only in the presence of water, while nonaqueous solvent systems led to crystalline solid. Fortunately, we have been successful in growing single crystals suitable for diffraction with copper chloride in methanol/DMF solvent system. The X-ray single crystal structure of 1 reveals the formation of a discrete dinuclear complex15 (Figure 2). Two ligand 1 forms a closed loop by coordinating to two copper atoms resulting in a M2L2 (M = metal, L = ligand) type metallocylic motif, with two terminally coordinated chloride atoms. Interestingly, the amide group adopts a trans conformation with respect to the benzene ring and shows markedly different structural features. While one of them coordinates to the metal center in a chelating fashion along with the pyrazole moiety and results in a five-membered ring formation, the other one remains free and forms hydrogen bonds with solvent molecules. This is in excellent agreement with the IR spectra showing two different bands for the amide groups. The 3D supramolecular structure nicely demonstrates the usefulness of a pyrazole based bisamide molecule in immobilizing solvent molecules with optimal utilization of hydrogen bonds, π−π stacking, and weak interactions. Formation of one-dimensional pore channels running along the axis with π−π stacked pyrazole moieties is the essence of the supramolecular framework. Interestingly, all the functional groups that are capable of forming hydrogen bonds, weak

Figure 1. (a) SEM micrograph of the gel formed by ligand 1. (b) TEM micrograph of the same gel.

Stress sweep rheometry was performed keeping the frequency constant at 1 Hz and temperature at 298 K. Viscous modulus and elastic modulus were measured as a function of increasing strain amplitude from 0.01% to 200%. The elastic modulus (G′) was found to be greater than the viscous modulus (G′′) by 1 order of magnitude typically observed for gel-like materials. The gel showed a yield stress of 30 Pa, which suggest moderately strong gels were formed Figure 4.

Figure 2. (a) Coordination environment of Cu(II) in the metal complex, (b) hydrogen bonding motif of the solvent molecules (red and green) in the crystal, (c) crystal packing of the compound showing encapsulation of solvent molecules. 2368



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Figure 3. SEM micrograph of (a) freshly prepared gel of ligand 3, (b and c) concurrent occurrence of fiber and particle showing the transformation of fibers to NMOP, (d) nanoscale metal organic particles, (e) TEM micrograph of freshly prepared gel of ligand 3, (f) TEM micrograph of fibers along with NMOPs.

interactions including halogen bonds point into the pore channel and actively participate in immobilizing solvent molecules inside the channel. The results obtained from ligand 1 were encouraging and prompted us to explore metallogelation with other pyrazolebased bisamide ligands. However, even after numerous attempts with various solvent systems and stoichiometry, ligand 2 did not form a metallogel with copper chloride. This could be attributed to the almost linear orientation of the amide-pyrazole moiety, capable of forming more or less ordered directional hydrogen bonding and metal coordination. Noting that bent conformations of amide-pyrazole arms have a tendency to undergo metallogelation, we shift our focus to ligand 3, a C3 symmetric tripodal ligand, in order to evaluate its gelation ability. We were successful beyond our most-optimistic expectations in obtaining not only copper metallogels with ligand 3, but also a rare event of morphology transformation was observed. 3 can form sufficiently strong metallogels in the presence of a varying amount of hydrated copper(II) chloride after sonication in a DMF/H2O (1:1) solvent system. Similar to ligand 1, ligand 3 forms metallogel exclusively with copper chloride and in aqueous solvent systems. In order to gain a better insight into the metallogel of ligand 3, detailed spectroscopic and rheological analyses were carried out, which interestingly showed features almost similar to that of ligand 1. The UV−visible spectroscopic results of ligand 3 closely resemble that of ligand 1 and show a broad absorption band at about 270 nm and an ILCT band at the 350 nm range. The absorption maxima also shifted to a higher frequency region in the gel state, indicating H-type aggregation (Figure S11b, Supporting Information). On the other hand, rheological analysis of gel 3 was performed in similar way as gel 1. As illustrated in Figure 4, G′ for gel 3 is almost 1 order of magnitude higher than G′′, suggesting gel formation with a yield stress of 32 Pa (Figure 4).

The morphological study using microscopic techniques revealed the most fascinating and unique feature of gel 3. A sample of freshly prepared gel displayed similar SEM and TEM micrographs (Figure 3) consisting of a fibrillar network with each fibril being infinitely long, and their diameters were ca. 20−30 nm. However, to our utter amazement, we observed a morphological transformation from fibers to particles of diameters ranging from 50 to 200 nm over a period of 3 weeks or so. The time-dependent transformation of morphology is in itself a rare phenomenon in the field of gel chemistry and is seldom observed in metallogelation.16 Naturally, it immediately drew our attention and prompted us to study it in detail. The SEM micrographs of gel 3, taken after certain time intervals, further reinforce the notion of gradual transformation of morphology. The coexistence of a fibrillar network along with particles in the SEM micrographs taken after one month (Figure 3) also supported our supposition. Furthermore, appearance of deforming fibrillar network in aged gels vis-àvis normal fibril network in fresh gels only seems to confirm that this is a dynamic transformation process rather than mere coexistence of two morphologies. Moreover, the appearance of isolated particles, quite a long distance away from the fibers, rules out possibility of templated synthesis of nanoparticles. This clearly indicates that the initially formed disordered fibrous network transformed structurally over time to a more stable ordered morphology. Noting further that there are few well-documented examples of metal nanoparticle formation which simply coagulate from the gel state over a time period, we put our emphasis on confirming the exact nature of the particles.17 An energy dispersive X-ray (EDX) study conducted on these particles revealed that they were made of copper along with carbon, nitrogen, and oxygen and hence can be regarded as nanoscale metal organic particles (NMOPs). The gradual morphological transformation from fiber to NMOP can be envisaged as direct 2369



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certain time intervals, show an emergence of some peaks and subsequently increment in intensity with time. This clearly argues in favor of an increase in the crystallinity in the gel, which in turn, provides us with a concrete proof of morphology transformation from amorphous fiber to crystalline NMOPs. Response of the Gels with External Chemical Stimuli. Experiments related to the responsiveness of the gel are highly beneficial in understanding the stability of the gels and can provide valuable insight into the delicate balance, between the dynamic solution phase and solid aggregate, which take place during metallogelation. In order to understand the responsiveness of the gels under the influence of the external chemical stimuli, the gels were treated with EDTA, ascorbic acid, and NH3. Response to Metal Trapping Agent. To study the effect of metal trapping agents on the gel, the gels were treated with a widely used multidentate chelating ligand, ethylene diamine tetraacetic acid (EDTA), which is also well-known to form a highly stable copper complex.20 Interestingly, gel 1 and gel 3 showed different behaviors after the addition of 1 equiv (with respect to the metal salt in the gel) of solid crystalline EDTA over the gel. Gel 1 disrupted in the presence of EDTA due to the formation of a light-blue colored copper−EDTA complex. As illustrated in (Figure S13b, Supporting Information), a slow breakdown of gel 1 was observed after the addition of EDTA. Since the solvent system (water and DMF in a 4:1 ratio) used for gelation contains mostly water with a tiny amount of DMF (for dissolving ligand) and considering the fact that the ligand is insoluble in water, one can safely conclude that the mother liquor is nothing but water. This simply rules out the possibility of the reverse reaction to get back the gel or gelation test of any remaining ligand in the mother liquor. Indeed, no gel formation takes place after the addition of copper salt. However, gel 3 did not show any response in this test. EDTA separated out copper, from the upper layer of the gel, but even after standing for a prolonged time, it did not penetrate the gel. This unique selectivity suggests a higher stability of the metal− ligand network than the the metal−EDTA network in gel 3 but not in gel 1. Redox Property. Incorporation of copper(II) ions in the metallogels also give rise to the possibility of generating redoxresponsive functional materials, using Cu(II)/Cu(I) redox chemistry.21 We choose ascorbic acid to explore the redox property of the gel, both in aerobic as well as anaerobic condition to the gels. Ascorbic acid has biochemical significance, and its oxidation can be catalyzed by cupric ions. In both the cases, Cu(II) of gel 1 and gel 3 was reduced to Cu(I) in the presence of solid ascorbic acid. Addition of solid ascorbic acid on the top of the gel gradually transforms the bright green-colored gel to colorless sol (Figure S13c,d, Supporting Information). This was further corroborated with the SEM experiments (Figure S13e, Supporting Information) showing complete disruption of fibrous morphology of the gels. Furthermore, in UV−visible spectroscopy the corresponding d−d band at 580 nm also disappeared after the reduction. Interestingly, this redox transformation turned out to be reversible in nature. After standing in oxygen atmosphere, they again turned green slowly indicating the oxidation of Cu(I) to Cu(II) in the presence of aerial oxygen. The experiments showing reversible redox responsiveness was also repeated with xerogels using ascorbic acid solution. Colorimetric Ammonia Gas Sensing through Gel-to-Gel Transformation. Gel 3 shows another fascinating yet extremely

manifestation of an Ostwald ripening-type process. In other words, NMOPs are formed when low concentrations of gelators or gelator−metal complex species are dissolved in the liquid phase of the gel and then reprecipitated as nanocrystalline materials, which would have a total potential energy lower than the amorphous gel strands. NMOPs have gained considerable attention in recent years due to the importance in the field of medicine and nanotechnology, with various potential applications ranging from drug delivery, chemo- and biosensing, and contrast agents in MRI to molecular electronics.18 Notwithstanding, entrapment of solvent molecules by using NMOPs for metallogel formation is extremely rare, and to date there are only two previous reported examples of NMOPs-based gel formation.19 As if to make our finding even more interesting, selected area electron diffraction (SAED) results revealed that the resultant NMOPs were crystalline in nature, which are in stark contrast to the amorphous nature of fibrillar networks of freshly prepared gel. As such, this also provided us with a unique opportunity to probe the gradual transformation of morphologies using diffraction techniques. We were gratified to note that our assumption was correct, and indeed, there is a gradual change in the powder X-ray diffraction (PXRD) profiles with time (Figure 5). The overlay of PXRD patterns, taken after

Figure 4. Plot of elastic modulus (G′, black squares) and viscous modulus (G″, red circles) for the gel formed by ligand 1 and elastic modulus (G′, pink squares) and viscous modulus (G″, blue circles) for the gel formed by ligand 3.

Figure 5. Powder XRD pattern of the xerogel of freshly prepared gel, after 20 days, after 60 days, and after 90 days (from bottom to top).

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Figure 6. EPR spectra of (a) the gel formed by ligand 3, (b) transformed gel after the addition of ammonia, (c) photographs of the gel showing the gradual transformation, (d) SEM micrograph of the ammoniated gel showing fibrous morphology, (e) plot of elastic modulus (G′, pink squares) and viscous modulus (G″, blue circles) for the gel formed by ligand 3 and elastic modulus (G′, green squares) and viscous modulus (G″, brown circles) for the ammoniated gel.

of the newly formed gel was confirmed by SEM micrographs. The UV−visible spectra showed a red shift from the band 270 to 312 nm, but there was a little amount of change in the d−d band. We performed EPR spectroscopy with the ammoniated gel to investigate if there is any change in coordination mode of the copper ion. The spectral pattern changed and a wellresolved hyperfine splitting was obtained after the transformation of gel 3. The significant change in EPR spectra clearly demonstrates the change of the coordination environment of copper ion after the transformation of the gel. The Xband EPR spectrum of gel 3 is displayed in Figure 6a. At 77 K the complex or gel shows an intense, almost isotropic, featureless resonance at g = 2.078 due to S = 1/2 spin state of the Cu(II) species, while the spectrum of ammoniated gel in similar conditions reveals a well-resolved three of four hyperfine pattern characteristic of an unpaired electron being coupled to a copper nuclear spin (63,65Cu, I = 3/2). The spectrum is displayed in Figure 6b, corresponding to a single unpaired electron, giving g∥ and g⊥ values of 2.3 and 2.01, respectively, and A∥ = 188 × 10−4 cm−1.23 The observed values of complex indicate that g∥ > g⊥ > 2.00, which suggests the fact that the unpaired electrons lie predominantly in the dx2−y2 orbital23 characteristic of square pyramidal or octahedral geometry for the copper(II) complex. The gav value for the complex is greater

rare external chemical stimuli responsive gel-to-gel transformation with possible application in colorimetric gas sensing. The observation was intriguing and deserves to be described in a little more detail. With an intention to study the stimuli responsiveness of the gel, we added a few drops of ammonia on the surface of the bright green-colored gel. To our astonishment, instead of usual disruption of the gel network, the gel resisted the sol formation by retaining the gel network. However, the color of the upper surface of gel, which was in contact with ammonia, now turned into a deep blue, while the lower portion retained the original green color of the mother gel. Interestingly, upon addition of a couple more drops, ammonia penetrated into the gel media and resulted in a slow gradual change in the color of the gel from original green to deep blue. A tube inversion test was successfully performed to confirm that the deep blue substance is indeed a gel. Even after the complete transformation of the gel, it was able to hold its weight when the tube was inverted. It is important to stress here that stimuli responsive gel-to-gel transformation is an extremely rare phenomenon and to date, to the best of our knowledge, there is only one published report in the literature on metallogelation.22 The spectroscopic findings further confirmed that the chemical responsiveness of the gel is the major reason behind this transformation. Fibrillar morphology 2371



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2010 high-resolution microscope instrument for TEM images. Rheology experiments were performed in SDT Q Series Advanced rheometer AR 2000. X-ray single-crystal data and X-ray powder diffraction (XRPD) patterns were collected using Mo Kα (λ = 0.7107 Å) radiation on a Bruker APEX-2 CCD diffractometer and a Bruker AXS D8 Advance Powder (Cu Kα1 radiation, λ = 1.5406 Å) X-ray diffractometer, respectively. Synthesis of Ligands. All the ligands were prepared by reacting 3amino-5-methyl-1H-pyrazole dissolved in dried acetonitrile with the corresponding acid chlorides in a stoichiometric ratio and refluxing overnight under nitrogen atmosphere. The precipitate so obtained was filtered and dissolved in methanol, and excess triethyl amine was added. After being stirred over 2 h at room temperature, solid compound was obtained, filtered, washed with cold water, and dried in air. Ligand 1. Yield: 2.4 g (75%) with respect to isophthalolyl chloride (2.03 g, 10 mmol); IR (cm−1, KBr) 3450(b), 3282(b), 1649(s), 1598(s), 1550(m), 1496(m), 1429(m), 1309(m); 1H NMR (400 MHz, DMSO-d6, 25 °C. TMS): 12.14 (s, 2H, pzNH), 10.65 (s, 2H, amide NH), 8.54(s, 1H; aromatic H), 8.1 (m, 2H; aromatic H), 7.61 (m, 1H; aromatic H), 6.43 (s, 2H; pz H), 2.23 (s, 6H; methyl H); 13C NMR (75 MHz, DMSO-d6, 25 °C, TMS): 163, 147, 138, 134, 131, 128, 126, 96, 10. Elemental Analysis. Calculated for C16H16N6O2: C 59.19; H 4.9; N 25.89. Found C 58.44; H 4.2, N 25.22. Ligand 2. Yield: 2.1 g (66%) with respect to terephthalolyl chloride (2.03 g,10 mmol); IR (cm−1, KBr): 3280(br), 3141(br), 1649(s), 1595(s), 1537(s), 1488(w), 1317(s), 1120(w), 1010(w), 800(w); 1H NMR (400 MHz, DMSO-d6, 25 °C, TMS): 13C NMR (75 MHz, DMSO-d6, 25 °C, TMS). Elemental Analysis. Calculated for C16H16N6O2: C 59.19; H 4.9; N 25.89. Found C 59.68; H 4.6, N 25.57. Ligand 3. Yield: 2.7 g (61%) with respect to trimesololyl chloride (2.65 g, 10 mmol) IR (cm−1, KBr): 3236(b), 3114(b), 1660(s), 1589(s), 1550(s), 1488(s), 1307(m), 1253(m); 1H NMR (400 MHz, DMSO-d6, 250 °C, TMS): 12.18 (s, 3H; pz NH), 10.78 (s, 3H; amide NH), 8.65 (s, 3H; aromatic H), 6.45 (3H; pz H), 2.24 (9H; methyl H); 13C NMR (75 MHz, DMSO-d6, 25 °C, TMS): 165, 147, 138, 134, 129, 96, 10. Elemental Analysis. Calculated for C21H21N9O3: C 56.31; H 4.69; N 28.15. Found C 56.85; H 4.58; N 27.95. Gelation Test. For gelation studies ligand 1 was dissolved in methanol or DMF in 1 wt %, and aqueous metal salt solutions were then added to it in varying concentration from 0.1 equiv to 1.5 equiv with respect to ligand. The same test was performed by dissolving the ligand in hot methanol. For ligand 3, 2 wt % compound was mixed in DMF, and the metal salt was added in similar way. They were sonicated for 30 min. Ligand 1 formed a gel after the completion of sonication, whereas ligand 3 produced a transparent jelly-like aggregation; on standing almost 2−3 h a strong gel was formed showing a positive result in tube inversion test. For ligand 1 gels were formed from 0.7 equiv to 1.2 equiv of metal salt addition, while ligand 3 showed a positive result in the presence of 0.5 equiv to 1.5 equiv of metal salt. Synthesis of Coordination Compound. A total of 16.2 mg (0.05 mmol) of L1 was dissolved in 0.9 mL of DMF, and to it an aqueous solution of CuCl2·2H2O (8.5 mg, 0.05 mmol, 2.1 mL) was added dropwise with stirring. After overnight stirring, the solution was filtered, and the clear solution was kept undisturbed. Block-shaped, bright green-colored crystals were obtained, filtered, and air-dried. Yield: 51%; FTIR (cm, KBr): 3512 (br), 3274 (br), 2925 (br), 1654 (s), 1612 (s), 1581 (s), 1556 (s), 1504 (m), 1440 (m), 1386 (w), 1259 (m), 1049 (w), 717 (w). Spectroscopic Studies of Gels. For FTIR studies spectra were collected with samples prepared as KBr pellets. UV−visible studies were performed taking 1 × 10−5 ligand solution prepared by 1:1 DMF/H2O solution. For gels, 10.3 mg (for gel 1) and 10.1 mg (for gel 3) gel were dissolved in 10 mL of DMF/H2O mixture and diluted 4 times to get the spectra.

than 2 indicating the covalent nature of the metal−ligand bond. Thus, providing the suitable coordination site for ammonia molecule a chemoresponsive gel can be achieved. Coordination of ammonia to the metal should have a profound effect on the stability of the gel state, considering the fact that ammonia with two hydrogen donor site will enhance the supramolecular interactions with the solvent molecules. This is indeed reflected in the rheological data with enhanced yield stress. The yield stress of the ammoniated gel increased to 52 Pa from 32 Pa of the original gel (Figure 6e) confirming a higher number of supramolecular interactions. As the ammonia evaporated out after drying the gel, we can conclude that the ammonia molecule is very loosely bound to the metal center, which is supportive to the fact that ammonia is coordinated in an elongated axial position of the metal atom. Interestingly, the gel-to-gel transformation was found to be reversible in nature. The original bright green can be revived from the ammoniated deep blue gel simply by overnight heating at 60 °C. This reversible gel-to-gel transformation can be repeated many times. Furthermore, the reversible nature of the transformation is also corroborated with the spectroscopic data, with reverted UV−visible and EPR spectra. We were gratified to note that these gel-to-gel transformations can also be performed using ammonia vapor, confirming that ammonia vapor can diffuse through the gel media. This enhances the potential of this system as colorimetric sensing of ammonia gas. It is noteworthy here that gel-based gas sensors are rare, and, to the best of our knowledge, this is the first such example of colorimetric ammonia gas sensing.

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CONCLUSION In conclusion we have reported a series of novel pyrazole-based benzene bis-amide and tris-amide molecules which were found to gelate copper chloride in an aqueous environment. The ligands offer great selectivity toward copper chloride as no gels were obtained with other copper counter-anions or other first row transition metal chlorides. Moreover, this is the first time, to the best of our knowledge, that we report metallogels where a morphological transformation from fibers to nanoscale metal−organic particles have been observed over a period of few weeks. It is worth mentioning here that the crystallinity of nanomaterials often plays a crucial role in controlling materials’ mechanical, catalytic, magnetic as well as electronic properties. Therefore, metallogelation may be a promising technique for generation of crystalline soft materials and thus opening a new avenue in the ever-increasing exploration of improved, valueadded targeted nanoparticles, while gel 3 shows a remarkable metal-ammonia coordination driven gel-to-gel transformation which can be used for colorimetric ammonia gas sensing.

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EXPERIMENTAL SECTION

All chemicals were commercially available (Aldrich) and used without further purification. FT-IR spectra were performed on a Nicolet MAGNA-IR 750 spectrometer with the KBr pellets containing the samples. 1H and 13C NMR spectra were recorded on Bruker spectrometers operating at 300 and 75 MHz respectively dissolving the compounds in DMSO-d6 solvent. Mass spectra were collected from Micromass Q-Tof Micro instrument, and UV−visible studies were performed in PerkinElmer Lambda 950 UV/vis instrument. The elemental analyses were carried out using a Perkin−Elmer 2400 SeriesII CHN analyzer. EPR spectra were recorded on a JEOL instrument. Electron microscopic studies were made using a JEOL, JMS-6700F field emission scanning electron microscope (FESEM) and JEOL JEM 2372



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To perform EPR studies gels were prepared in the EPR tubes, and spectra were collected. Microscopic Studies. SEM: A drop of gel was placed in a glass coverslip and dried under a vacuum and mounted to the machine. TEM: 10−3 M gel solution was dropped on a 300-mesh carboncoated copper grid and mounted on a microscope after drying in a vacuum for TEM imaging operating at an accelerating voltage of 200 kV. Rheological Study. A cone and plate measuring system was used for the rheological study. 2 wt % (w/v) of both the gel were prepared and left undisturbed overnight at room temperature for the measurement. The plate was 40 mm in diameter, and the cone of angle was 4 deg. The distance between them was adjusted, and there was no air gap between the plate and cone. The gel was transferred on the plate, and the stress sweep experiment was done on the gels by applying stress from 0.1 to 1000 Pa at a constant temperature of 25 °C. Powder X-ray Diffraction Study. Xerogels were prepared by drying the gels (1 wt %, 1 equiv for gel 1 and 2 wt %, 1 equiv for gel 3) in a vacuum. Powder X-ray diffraction was carried out scanning the sample at a 2θ value ranging from 50 to 400 and operating the machine at 35 kV voltage and 30 mA current using Ni-filtered Cu Kα radiation.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic file in CIF format for compound 1, IR, UV and EPR spectra, schematic diagrams showing probable self-assembly, additional pictures of gel and rheology graphs. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.M. gratefully acknowledges Science and Engineering Research Board (SERB) (Project No. SR/S1/IC-65/2012) India, for financial assistance. Sudeshna Bhattacharya, SSG, Sukhen Bala, and A.G. are thankful to CSIR, India, for Research Fellowships. Sudeshna Bhattacharya also acknowledges Anindita Das and Priyadarshi Chakraborty of Polymer Science Unit, IACS, for their assistance in rheology experiments.

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REFERENCES

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