Novel Supramolecular Thixotropic Metallohydrogels Consisting of

Nov 8, 2013 - thixotropy are still rare14−16 because most of them are extremely sensitive to mechanical stress. The entangled solvents in the gel ma...
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Novel Supramolecular Thixotropic Metallohydrogels Consisting of Rare Metal−Organic Nanoparticles: Synthesis, Characterization, and Mechanism of Aggregation Weiwei Fang, Zheming Sun, and Tao Tu* Department of Chemistry, Fudan University, 220 Handan Road, 200433 Shanghai, China S Supporting Information *

ABSTRACT: Even without obvious sticky sites, novel supramolecular thixotropic metallohydrogels consisting of rare metal−organic nanoparticles (MNPs) have been readily accessible from simple structured pincer-type terpyridine Cu(II) complexes at the gelator concentration as low as 0.25 wt %. The obtained soft materials have been fully characterized by using a combination of experimental techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), X-ray diffraction (XRD), highresolution mass spectrometry (HR-MS), and rheology measurements. Based on these studies, hypothesized molecular assembly mechanisms for distinctly different morphologies observed in different solvents were proposed. Besides general heating−cooling gel preparation procedures, surprisingly, the metallohydrogel was formed by simply stirring the mixture of pincer ligand and Cu salts in water directly, further indicating thixotropic property and self-healing ability of the resulting metallohydrogel under external stress. This stirring approach is highly anion and phase selective with various potential applications. Furthermore, the infrequent observation of the crystal growth from metallogels in situ readily reavealed the self-assembly mechanism that π-stacking and metal−metal interactions along with hydrogen-bonding interactions between gelator and guest molecules are responsible for the gel formation, which is further confirmed by the control gel collapse experiment via external ligand substitution. All these results indicated that pincer organometallic complexes not only can function as a new type of hydrogelators but also are readily to fabricate useful thixotropic materials with various applicability based on their morphologies and assembly mechanism studies.

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INTRODUCTION Thixotropic materials, which readily alter their physical properties under mechanical stress, represent an intriguing class of soft matter with self-healing abilities and have gained considerable attention from a broad range of research fields.1,2 One of the most efficient ways to fabricate self-healing materials is using monomer units which can further assemble and crosslink through noncovalent interactions, such as hydrogen bonds, π−π stacking, van der Waals interaction, metal−ligand coordination, ionic, and hydrophobic interactions, to form a thixotropic matrix.3−13 In this respect, molecular gels are envisaged to be intriguing candidates to produce thixotropic materials. However, the examples of molecular gels exhibiting thixotropy are still rare14−16 because most of them are extremely sensitive to mechanical stress. The entangled solvents in the gel matrix were generally banished irreversibly from the networks and further resulted in a solid suspension after removing the external force. Therefore, the development of simple structured molecules to immobilize solvents to form nanoscale thixotropic materials based on a bottom-up strategy currently represents one of exciting and challenging tasks, especially in the aqueous medium.17,18 © 2013 American Chemical Society

To date, most reported hydrogelators are peptides and their derivatives, in which the amide group and its analogues can function as sticky sites responsible for hydrogen bonding leading to gel formation. Therefore, in general, molecules without obvious sticky sites are presumed as inefficient gelators for aqueous solvents and worse candidates for thixotropic materials preparation. Despite the important role that organometallics plays in chemistry and material sciences, metallogelators have been somewhat neglected, although, incorporation of metal ions constitutes a fascinating means to impart the properties of the metal (such as redox, optical, catalytic behaviors) into the gels.19−27 Ascribed to their air and moisture sensitivity and their solubility problems under aqueous conditions, metallohydrogelators are less explored. As a class of unique skeletons in organometallics, pincer type metal complexes are both robust and active and reveal aggregation properties with potential applications in various research areas.27−33 Recently, we have developed several pyridineReceived: October 2, 2013 Revised: October 31, 2013 Published: November 8, 2013 25185

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bridged metal N-heterocyclic carbene and metallacycles pincer complexes as efficient metallogelators for a variety of organic solvents and ionic liquids even in extremely low gelator concentrations; these systems also demonstrated their potential applications in catalysis, solar cells, and visual chiral recognition.34−39 However, all these metallacycles hardly gelate aqueous solvents. Following our recent interests in developments of metal complexes with potential applications in soft matter and catalysis,34−45 herein, we demonstrated simple structured pincer-type Cu(II) complexes without obvious sticky sites as rarely explored building blocks to fabricate novel metallogels and further study their thixotropic properties, morphologies, and a possible assembly process as well as their potential applications.

1 h and allowed to cool at room temperature. The product was isolated after filtration upon precipitation by addition of diethyl ether. The solid was washed three times with diethyl ether and dried under vacuum to afford the complex 3 as a light-green solid powder (134 mg, 85%). HR-MS (ESI): m/z 330.5707 (calcd [M−2NO3]2+), 330.5728 (found [M−2NO3]2+).

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RESULTS Synthesis of Metallogelators and Their Gelation Behaviors. Pincer terpyridine ligand 1a is readily accessible from inexpensive commercial furfural and 2-acetylpyridine in a moderate yield.46 Further reaction with CuCl2·2H2O in methanol under ambient conditions affords green powder 2a in almost a quantitative yield (Figure 1). Similarly, the

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EXPERIMENTAL SECTION General Methods. All commercial reagents and solvents were used directly without further purification. 1H and 13C NMR were recorded on Jeol ECA-400 and Bruker 500 DRX spectrometers. ESI-MS spectra were recorded on a Bruker micrOTOF II instrument. Powder X-ray diffraction (PXRD) patterns were measured using a Bruker D8 powder diffractometer with Cu Kα radiation (λ = 1.5406 Å). Data collections were carried out on a Bruker Smart APEX diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source (Mo Kα radiation, λ = 0.710 73 Å) operating at 50 kV and 30 mA at 293 K. SEM experiments were carried out on a Philips XL30 microscope operated at 20 kV. TEM experiments were carried out on a JEOL JEM-2010 transmission electron microscope. DSL was measured on a Zetasizer Nano Serie ZS apparatus from Malvern. AFM images were performed by using a Veeco Multimode Nanoscope (Bruker) with tapping mode. All reactions were carried out under air unless otherwise noted. Rheological measurements were carried out on freshly prepared gels using a controlled stress rheometer (Malvern Bohlin Gemini HR nano). Ligands 4′-(2-furyl)2,2′:6′,2″-terpyridine (1a)46 and 4′-(2-thienyl)-2,2′:6′,2″-terpyridine (1b)47 were synthesized according to the literature procedures. Syntheses of Cu(II)−Pincer Complexes 2a−c and 3. Cu(II)−Pincer Complex 2a. Ligand 1a (2.99 g, 10 mmol) and CuCl2·2H2O (2.05 g, 12 mmol) were added to CH3OH (200 mL) and stirred at rt for 24 h. The green solid was collected after filtration, washed with CH3OH (3 × 30 mL) three times, and then dried under vacuum to give complex 2a as a green solid (4.31 g, 99%). 1H NMR (D2O, 400 MHz, 353 K): δ = 10.68 (bs, 6H), 10.24 (bs, 2H), 10.06 (bs, 3H), 6.91 (bs, 3H). HR-MS (ESI): m/z 397.0043 (calcd [M−Cl]+), 397.0078 (found [M−Cl]+). Cu(II)−Pincer Complex 2b. A similar synthetic protocol for complex 2a was applied; complex 2b was provided as a green solid (0.408 g, 91%). HR-MS (ESI): m/z 412.9815 (calcd [M− Cl]+), 412.9827 (found [M−Cl]+). Because of the poor solubility and paramagnetic property of complex 2b, it is hard to get satisfactory NMR spectra. Cu(II)−Pincer Complex 2c. A similar synthetic protocol for complex 2a was applied; complex 2c was obtained as a green solid (0.435 g, 83%). HR-MS (ESI): m/z 442.9518 (calcd [M− Br]+), 442.9551 (found [M−Br]+). Cu(II)−Pincer Complex 3.48 To a solution of complex 1a (120 mg, 0.4 mmol) in 100 mL of methanol a solution of 0.05 mmol of Cu(NO3)2·3H2O (48 mg, 0.2 mmol) in 8 mL of methanol was added. The mixture was then heated to reflux for

Figure 1. Synthesis of Cu(II) complexes 2a−c and corresponding metallohydrogels formation.

thiophene analogue 2b is prepared in an excellent yield. To our surprise, complex 2a is hardly soluble in all selected organic protic and aprotic solvents (see the Supporting Information, Table S1), and the solubility of 2b is even worse. Upon heating to reflux in water, diols, or glycerin and subsequent cooling to room temperature, within a few minutes complex 2a forms thermoreversible transparent green hydrogels even without obvious sticky sites. These observations highlight that 2a behaves not only as a hydrogelator but also as a rare amphiphilic gelator. Because of its lower solubility in most selected solvents, complex 2b only can gelate glycerin. Furthermore, the critical gelator concentration (CGC) of complex 2a in pure water was explored; even at a gelator concentration as low as 0.25 wt % (w/v; 5.79 × 10−4 M), a stable gel is still formed, which is relatively low compared to that of most reported metallogelators.49−51 In general, the heating−cooling sequence is considered as a typical procedure for gel preparation. To our delight, upon shaking the hydrogel obtained from complex 2a was reverted to a sol which can readily re-form the gel after resting within an hour. The transition between two phases is infinitely repeatable by the shaking−resting cycles, which demonstrates a thixotropic hydrogel with self-healing ability. Therefore, we further investigated the possibility of the hydrogel formation by simply mixing ligand 1a with various copper salts in water directly (Figure S1). Although ligand 1a was hardly dissolved in pure water, applying 1:1 molar ratio of ligand and Cu salt in water (1 wt %), the white solid 1a gradually dissolved during stirring via coordination to CuCl2 to form a homogeneous clear green sol within an hour, which readily formed a stable gel upon standing 25186

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for several minutes (Figure 1). With a higher gelator concentration (>1 wt %), the gel even formed during stirring. The molar ratio of ligand and CuCl2 strongly impacts the gel formation: when the ratio is 2, a greenish precipitation is obtained; reducing the ratio to 1.25, a stable green hydrogel is beginning to form. Further decreasing the ratio to 0.5, a turbid green suspension is then observed. To our surprise, the gel formation is highly anion-selective. For other selected Cu(II) salts (such as CuBr2, CuSO4, Cu(NO3)2, Cu(OAc)2, and Cu(OTf)2), no gel formation was found, and green or blue aqueous solutions or precipitation were observed even at higher gelator concentrations, indicating that counter anions also play a key role during the gel assembly. In the case of CuCl, only black precipitation was observed. Direct Crystal Growth from Metallogels in Situ. Although it is hard to obtain stable hydrogel by direct stirring protocol of ligand 1a and CuBr2 in water (Figure S1), complex 2c readily forms a thermoreversible metallohydrogel upon heating to reflux in water and subsequently cooling to room temperature within a few minutes. Interestingly, when the metallohydrogel was allowed to stand for 30 days, the gel gradually transforms into green rodlike single crystals, which were suitable for single crystal diffraction analysis (Figure 2).

gel cannot recuperate to a sol upon heating, and a rubbery matter was obtained instead. Phase-Selective Gelation Behaviors. In order to further extend the gelation scope of this simple structured novel metallogelators, aqueous solutions with various hydrophilic and hydrophobic organic solvents were screened in the gelation tests. In cases of hydrophilic organic solvents, with the help of 50% (v/v) water complex 2a can entrap all of the selected organic solvents (such as various alcohols, acetone, acetic acid, dimethylacetamide, dimethyl sulfoxide, 1,2-dimethoxyethane, and N-methyl-2-pyrrolidone; see Table S2) to form transparent or turbid gels by using heating−cooling and/or shaking−resting gelation procedures. To our delight, selective hydrogel formations were observed when hydrophobic organic solvents were involved in the gelation tests. For example, when complex 2a was put into a cuvette containing water and chloroform (TCM) mixture in 1:1 ratio, after heating−cooling or shaking− resting procedure, metallogelator 2a only selectively immobilized water phase and sealed TCM layer on the top of the cuvette (Figure 3). Similar phase-selective gelation was also

Figure 2. Direct crystal growth from metallohydrogel 2c/H2O (1 wt %) in situ.

Figure 3. Phase-selective gelation by metallogelator 2a.

observed for other gelation tests with aqueous mixtures containing hydrophobic solvents and hydrocarbons, such as 1,2-dichoroethane (DCE), dichloromethane (DCM), benzene (BEN), ethyl acetate (EA), cyclohexane (c-Hex), n-hexane (nHex), petroleum ether (PE), benzonitrile (PhCN), phenyl chloride (PhCl), n-octane (n-Oct), tetrahydrofuran (THF), toluene (Tol), silicone oil (Si-Oil), mineral oil (M-Oil), soybean oil (SB-Oil), pump oil (P-Oil), and even for gasoline (Gas). Therefore, metallogelator 2a represents a new type of phase-selective gelator. 53−55 Oil spill brings about an irrecoverable damage to marine environment, and phaseselective gelation of oil−water mixtures has been regarded as one of the best options for oil-spill recovery.56−64 The phaseselective gelation behavior further indicates another potential application of metallogelator 2a. Morphological of the Metallogels Obtained from Complex 2a. Generally, the properties and behaviors of materials are rationalized by its structure. Therefore, the morphologies of xerogel networks obtained from metallogels 2a with the respective solvents were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The representative images are shown in Figure 4. Surprisingly, unlike routine cross-linked nanofiber networks observed for the most reported hydrogels, the gel 2a/ H2O (1 wt %) obtained by using heating−cooling and stirring−

The crystal formation depends on temperature and gelator concentration: low temperature or higher concentration retards the transformation and gel collapsing process. To the best our knowledge, this is the first example that metallohydrogels can convert into single crystals in situ, which may help us to further understand the mechanism of the molecular assembly and gelation process. Additionally, a similar transformation is also observed for gel 2a/ethylene glycol (1 wt %), in which green rodlike single crystals were obtained; longer resting time (12 months) is required for gel collapse (Figure S3). Gel−Sol Phase Transition Temperatures. With these exciting results in hand, various experiments were carried out to further characterize the gels. The gel−sol phase-transition temperatures (Tg) were determined by the “test-tube-inversion method”.52 In comparison with the organogels from alcohols, metallohydrogel 2a/H2O at 1 wt % gelator concentration exhibited much higher thermostability, as demonstrated by higher Tg value (74 vs 32−36 °C, see Table S1). In addition, the stability of the hydrogels steadily increased with the gelator concentration, as established for a series of metallogels 2a/H2O with a concentration varying from 0.5 to 2.4 wt % (Figure S4). When the concentration was 2.4 wt %, the Tg was beyond the boiling point of water. Further increasing the concentration, the 25187

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Figure 4. Selected SEM images of metallogels: (a) hydrogel 2a/H2O (0.5 wt %); (b) gel 2a/trimethylene glycol (1 wt %); (c) gel 2a/glycerol (1 wt %); and TEM images of metallohydrogel (d) 2a/H2O (0.5 wt %).

Figure 5. Selected AFM height images of (a) freshly prepared hydrogel 2a (1 wt %); (b) collapsed gel/sol 2a upon shaking; (c) rehealed gel after resting for 12 h; and (d) DLS study on nanoparticle size distribution of the sol (0.25 wt %) diluted from metallohydrogel 2a (0.5 wt %).

resting both gelation procedures revealed spherical nanoparticle (NP) morphologies (ca. 250 nm in diameter; Figure 4a and Figure S8), which is rare in hydrogel formation.65 The TEM image provided detailed information (Figure 4d): the large NPs are apparently formed through aggregation of uniform spherical small NPs (ca. 30 nm). EDX analysis indicated that the NPs

consist of Cu, Cl, O, and C elements, which belong to the organic molecule 2a (Figure S6). The small uniform particles may be formed by agglomeration of molecular fibers,65−67 which subsequently merged into a single uniform spherical particle under hydrophobic conditions. To the best of our knowledge, there have been no reports of metal−organic 25188

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Figure 6. Amplitude sweep rheometry data for (a) the gel 2a/H2O (2 wt %) and (b) the rehealed gel 2a/H2O (2 wt %) after resting for 24 h at 25 °C (frequency: 6.283 rad s−1; strain: 0.001−1); (c) dynamic frequency sweep rheometry data for the hydrogel 2a (2 wt %): (angular frequency from 0.005−100 rad s−1; strain kept at 0.5% without deformation; η*: viscosity); and (d) yield stress test of stress sweep rheology of hydrogel 2a (2 wt %): viscosity vs shear stress.

re-formation (Figure 5c). For longer aging time, similar AFM height images like picture from freshly prepared gel were observed. This phenomenon is further confirmed by the direct gel re-forming test from xerogel: when 0.5 mL of H2O was added into a cuvette containing 10 mg of xerogel 2a, after several hours standing, the xerogel imbibed H2O, resulting in a transparent gel (Figure S27). This rare gelation capability under ambient conditions may be valuable for potential applications in areas such as pollutant removal, adhesion, or target delivery system.69 Dilution Behavior and DLS Study. The unusual MNPs observed in the gels were further investigated by a dynamic light scattering (DLS) study, which constitutes a common and practical technique to determine the diameter of small particles in dilute sol. Initially, we tried to measure the sizes of the primary dispersion MNPs in the metallohydrogels directly; however, such experiments were hard to carry out. After dilution of the gel 2a/H2O (0.5 wt %) with additional water, no conspicuous dispersion of water molecules into the gel matrix was observed even after resting for several days. After gentle shaking, the gel was diluted to 0.25 wt % and readily broke into fragments, resulting in a clear sol. With DLS study, the dispersed particles in the sol represented a narrow particle size distribution with on average size of 160−250 nm (Figure 5d), which is in accordance with the results from SEM, TEM, and AFM. It should be pointed out that the experiments of DLS study do not prove that dispersion of MNPs in sol is fully complete into the minimum particles, which are prone to aggregate and remain in the final diluted dispersion. Additionally, the minimum particles may also be partially or completely dissolved under the extremely diluted case. Therefore, the results represented approximate characteristics of the disperse MNPs in the original metallohydrogel 2a/H2O (0.5 wt %). Thixotropic-Responsive and Self-Healing Properties of the Metallohydrogels. Although there is no obvious impact on the molecular level by simple mechanical agitation,70 the conspicuous morphologies alternation of the hydrogels after

nanoparticles (MNPs) that support the formation of molecular hydrogels, although a few examples on the gel formation by metal−organic particles in micrometer dimensions with colloids and polymers have been reported so far.17,65−68 In the case of metallogel 2c/H2O (1 wt %), morphologies consisting of large irregular particles (200−500 nm, see Figure S21a) were observed with freshly prepared hydrogels, which may be aroused from merging process of molecular particles possibly quenched in an intermediate state when gelator 2c was inclined to crystal growth. After aging the hydrogel for several hours, to our astonishment, straight rectangular crystals (ca. 5 μm wide and 20 μm long, Figure S21b) were found, which is consistent with the aforementioned visual transformation from gels to crystals in Figure 2. In contrast, the morphologies of organogels formed by complex 2a with alcohols are distinctly different; thick and twisted nanofibers with tens of micrometers long were observed (Figure 4b,c). In order to gain further insight into the rare MNPs morphologies and thixotropic properties of the metallohydrogels, samples obtained from hydrogel 2a were studied by using atomic force microscopy (AFM). A representative height image of the freshly prepared gel 2a/H2O (1 wt %) revealed the presence of adjoining MNPs with the base size of ca. 30 nm (Figure 5a), which was in accordance with what observed in the SEM and TEM morphologies. The heights of the nanoparticles corresponded to the bases (the height to diameter ratio is ca. 1) and further confirmed the spherical NPs morphologies presented in the gel. Images of the sol that immediately formed upon shaking are depicted in Figure 5b, in which individual elliptical bulky particles with the base size from 150 to 500 and 50 nm in height were observed. Along with these irregular particles, there were obvious water vestiges in the AFM morphologies, indicating deformation of the spherical particles to release the water molecules entrapped by gel during shaking. Upon resting for 20 h, the individual elliptical bulky particles reabsorbed water molecules and became smaller to regenerate the MNPs of 30 nm in diameter leading to the gel 25189

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Figure 7. (a) Molecular structure of 2a·MeOH; (b) the interactions between the molecules; (c) the networks in the crystal; and (d) XRD studies of xerogel 2a/H2O (5 wt %), 2c/H2O (1 wt %), and crystal 2a·MeOH.

the furan ring and the center pyridine ring to form an unit (Figure 7b); (2) the stacking (3.87 Å) between the side pyridines, along with Cu−Cu interaction (3.98 Å) the units further assembled into a linear fiber-type aggregates (Figure 7b); (3) the π-stacking distance between furan ring and one side of the pyridine (which belong two adjacent fiber aggregates, respectively) ring is 3.62 Å (Figure S36). Besides these interactions, the hydrogen bonding between Cl atoms and hydrogen atoms of the furan or center pyridine rings (from 2.38 to 2.83 Å, Figure S37) offered another force to hold the two columns to form the networks (Figure 7c). These dimensions all correspond to the reflection data obtained from X-ray diffraction (XRD) study on the xerogel 2a/H2O (Figure 7d), and the most predominant reflections are at 2θ = 7.4°, 9.8°, 14.8°, 19.8°, and 27.1°, in which 2θ = 27.1° (d110) with a typical distance of 3.29 Å, indicating that π−π stacking and metal−metal interactions play the crucial roles during the gel formation. In addition, the MeOH guest molecule was entrapped by the fiber aggregates via two types of hydrogen bonding interactions: one is the interaction between hydrogen atoms of furan ring and alcohol Me group (2.54 Å); the other is between the methanol OH group and erected Cl atom (2.38 Å). In addition, the crystal of 2a·(CH2OH)2 was also formed in situ via transformation from gel 2a/(CH2OH)2 (1 wt %) after 12 months; similar packing was observed (Figures S38−S41). The distances of π-stacking between the fiber aggregates are 3.70 and 3.90 Å, respectively. The Cu−Cu distance is 4.10 Å, slightly larger than what observed in the crystal of 2a·MeOH. In the case of 2c·H2O crystal obtained directly from gel 2c/ H2O (1 wt %), the dimension of molecule 2c is similar to its chloride analogue. The columnar assembly was held by π−π interactions for the aggregate fiber formation (Figures S42− S45). The π-stacking between the furan and central pyridine ring is 3.66 Å, which bounds two molecules together to form a unit and can further extend to a fiber-type aggregate through metal−metal interaction (4.14 Å) and additional π-stacking (4.11 Å) between the side pyridines with the second unit. The interactions between the two units are weaker than these observed in the crystal 2a·MeOH, which may be hard for further aggregates elongation in one-dimensional manner to form extended fibers, resulting in the easy conversion of the gel 2c/H2O into crystalline forms. In addition, there are strong hydrogen bonding between the guest molecule (H2O) and

shaking and self-repairing inspired us to quantify their thixotropic behavior and its relationship with the MNPs; therefore, a series of shear stress loop tests were performed with original and rehealed gels 2a/H2O (2 wt %). In the case of the shear strain less than 0.6% (a value of yield point), the elastic storage modulus (G′, ca. 45 Pa) was much greater than the loss modulus (G″, ca. 21 Pa) in both cases; when the shear strain was further increased greater than 5.6%, G″ was lower than G′ (Figure 6a,b). Then, the shear strain of 0.5% was selected to characterize viscoelastic behavior of the hydrogel with increasing angular frequency. The dynamic frequency sweep rheometry data are shown in Figure 6c, in which a kind of classic shear viscosity curve is presented. The slightly increasing values of G′ and G″ along with the steadily increasing frequency suggested that the energy storage process occurred without obvious energy dissipation during the test. The inconspicuous increased gap between G′ and G″ indicated the hydrogel possessed better viscid property than elasticity. The complex viscosity (η*) was decreased along with the increasing frequency, highlighting the shearing thinning phenomena occurred during the tests and further confirmed the thixotropic character of the hydrogel. A plot of viscosity (Pas) vs shear stress (Pa) is shown in Figure 6d. When the value of the shear stress was lower than 10.8 Pa, the gel was a solidlike material without deformation; when the value of the shear stress was between 10.8 and 22.5 Pa, the material exhibited sol behaviors. Further increasing the shear stress, homogeneous solution property was manifested. The stress value is relatively low, which may be attributed to the low gelator concentration (2 wt %) and also reflected it is hard for MNPs even with high surface area to maintain strong surface contacts in polar media upon agitation. Nevertheless, the ability of these MNPs to form a stable hydrogel clearly accounted that the physical contacts and cross-links between the MNPs are responsible for the gel thioxtropy and exhibits better viscosity than elasticity, unlike previous reports of the thixotropic gels consisting of nanofibers. Single Crystal and X-ray Diffraction (XRD) Studies. To gain further insight into the self-assembly process, the structures of complexes 2a·MeOH, 2a·(CH2OH)2, and 2c· H2O were studied by single crystal diffraction. In the crystal 2a· MeOH, the dimension of molecule 2a is 11.0 × 11.3 Å2 (Figure 7a). Pincer molecule 2a assembles to form a columnar structure organized in bilayers which are held together by three kinds of π−π stacking: (1) intermolecular interactions (3.71 Å) between 25190

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observed in the metallohydrogel from pincer complexes 2a and 2c can be easily rationalized, which also further supported the conclusion of the thixotropic metallohydrogel exhibits better viscosity than elasticity from the rheology study. As a well-known knowledge for gelation that the gel formation actually is the process of partially microcrystals growth of gelator molecules in the selected solutions, however, it is hard to directly observe the crystals growth from organogels or hydrogels in vivo, especially the crystal growth from the same molecule in both organic and water media. Therefore, crystals growth observation of the metalloorganogels and hydrogels prepared from pincer Cu(II) complexes 2a and 2c not only supports our plausible assembly mechanism but also confirms that two different morphologies observed in the metallogels derived from alcohols and water actually originate from the same fiber aggregates, which could further aggregate together to form nanofibers or compress into MNPs in the case of hydrophobic condition of the selected solvents. In addition, structural modification of a pincer molecule is a suitable strategy to handicap π-stacking and metal−metal interactions between the units (Figure 8c),36 which may lead a gel collapse and further confirm our plausible assembly mechanism of pincer complexes described in Figure 8. In the case of five-coordination Cu(II) complexes, the chloride ligands were easily substituted by external N-containing ligands; therefore, various N-containing ligands were involved in the control tests.71 Although the metallohydrogels prepared from 2a and 2c are sensitive to inorganic base like NaOH, the substitution by using monodentate organic bases like pyridine and its derivatives hardly realized the gel collapsing even with 2 equiv of selected ligands. These results indicated that less hindered N-containing ligands may be not bulky enough to block the π-stacking and metal−metal interactions between the units formed by gelators. Therefore, bulkier N-containing ligand like terpyridine was involved in the control test. To our delight, the addition of 1 equiv of terpyridine ligand 1a into the metallohydrogel 2a/H2O (1 wt %) led to gel collapse of the hydrogel 2a/H2O, resulting in greenish precipitation (Figure 9a), which can also explain why there is no gel formation when ligand 1a and CuCl2 were mixed in water in a 2:1 molar ratio via the stirring procedure. HR-MS study further confirmed two Cl ligands substituted by terpyridine and the six-coordination adduct formation (Figure S59). From the crystal structure of six-coordination Cu(II) complex 3, which was readily accessible from ligand 1a and Cu(NO3)2 (Figure 9b), besides the π−π stacking (3.53 Å) between the central furan ring and one of side pyridine rings, there are no obvious π-stacking and Cu−Cu interaction between the units to support further molecular assembly leading to nanofiber formation, which was apparently blocked by steric hindrance of the bulkier terpyridine ligand 1a. Additional SEM study further confirmed this observation: unlike the MNPs observed in the metallogel 2a/H2O (Figure 9a vs Figure 4a), only irregular bulky crystalline rods (several micrometers long and wide) were found, which hardly encaged the additional solvent resulting in the gel collapse. Following this result, we may fabricate molecular switches based on this novel metallohydrogel system in combination with their thixotropic properties and collapse ability with external switchable ligands, and this work is currently under investigation in our laboratory.

pincer molecular 2c (2.42−2.67 Å), which hold two adjacent fiber aggregates together.

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DISCUSSION Proposed Gelator Assembly Mechanism. In consideration of similar XRD patterns observed with crystals and xerogels, we hypothesized the assembly process of gelator molecules in Figure 8, which may further illustrate the distinctly

Figure 8. Proposed different gel assembly mechanisms in alcohols and water.

different morphologies formed in different types of solvents. At first, the pincer complex readily forms an assembly unit via a strong π−π interaction between the furan and central pyridine rings, which can further assemble to a fiber-type aggregate in assistance with metal−metal interaction and additional πstacking between two side pyridine rings. In the presence of alcohols (Figure 8a), these aggregates can further elongate and bind together, resulting in routine long and twisted nanofibers through additional interactions such as hydrogen-bonding interaction between gelator and guest molecules, which can entangle additional solvents leading to metalloorganogel formation. In the case of water as solvent (Figure 8b), the fiber aggregates may intend to shrink to rare NMPs due to the hydrophobic property of pincer molecules without sticky sites. Because of the larger surface areas of the NMPs and coherence ability of the water molecule, the NMPs are bound together via physical contacts to form bigger nanoclusters (200−300 nm in Figure 4d), which are able to further cage the water molecules by the strong surface contacts to form stable hydrogels. With the plausible assembly mechanism, the rare NMPs morphology 25191

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Figure 9. SEM images of the sol formed by hydrogel 2a/H2O (1 wt %) (a) with 1 equiv of ligand 1a and molecular structures of (b) complex 3; NO3− was omitted for clarity.

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CONCLUSION

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (No. 21172045, 91127041, and 20902011), the Changjiang Scholars and Innovative Research Team in University (IRT1117), the Shanghai Shuguang and Pujiang Programs (No. 11SG04), the Shanghai Leading Academic Discipline Project (B108), and Department of Chemistry, Fudan University is gratefully acknowledged.

In summary, even without obvious sticky sites, simple structured pincer-type terpyridine Cu(II) complexes showed efficient gelation ability toward water, glycol, and glycerol even at the gelator concentration as low as 0.25 wt %. In assistance with water, the gelation ability of pincer complexes for various organic solvents was broadly extended. Based on SEM, TEM, AFM, DLS, XRD, HR-MS, and rheology studies, the hypothesized molecular assembly mechanism for distinctly different morphologies (nanofibers and MNPs) observed in different types of solvents were proposed. Besides the general gel preparation procedure, interestingly, this thixotropic metallohydrogels with self-healing ability are readily accessible by simply stirring the mixture of pincer ligand and Cu salts, and this approach is highly anion and phase selective. The rare observation of the crystal growth from metallogels in situ further supported the self-assembly mechanism that π-stacking and metal−metal interactions along with hydrogen-bonding interactions between gelator and guest molecules are responsible for the gel formation, which is further confirmed by the control gel collapse experiment via external bulky ligand substitution. With these results in hand, further study on the potential application of this novel metallohydrogel in catalysis and molecular switches is being carried on in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedures and characterization data for new compounds, gelation test, and morphology studies of gels formed from different solvents, crystal growth from metallohydrogels in situ, viscoelasticity, DLS, XRD, and gel collapse studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.T.). Notes

The authors declare no competing financial interest. 25192

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