Multi-Stimuli-Responsive Metallogel Molded from a Pd2L4-Type

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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

Multi-Stimuli-Responsive Metallogel Molded from a Pd2L4‑Type Coordination Cage: Selective Removal of Anionic Dyes Sudhakar Ganta and Dillip K. Chand* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India S Supporting Information *

ABSTRACT: A self-assembled binuclear coordination cage of the Pd2L4 formulation has been constructed by complexation of Pd(NO3)2 with N,N′-bis(3-pyridylmethyl)naphthalenediimide (L). The cage, i.e., [Pd2(L)4](NO3)4 (1), displayed a further self-assembly phenomenon to afford a gel phase, upon dissolution in either dimethyl sulfoxide or acetonitrile−water (1:1) followed by standing at room temperature. It was observed that a synergy among the metal ion, ligand, counteranion, solvent, and concentration played a vital role for metallogel formation. The morphology of the metallogel as observed from microscopy studies revealed the formation of a rare variety of nanoscale metal−organic particles. Salient features of the gel include the thixotropic (mechanoresponsive) nature, in addition to the reversible chemical-stimuli-responsive behavior. The presence of naphthalenediimide moieties at the backbone of the cage and the cationic nature of the cavity of the cage could be exploited to study the functional aspects of the gel. The porous gel exhibited the abilities to uptake pyrene as a guest and to selectively remove anionic dyes from aqueous solutions. The gel could bind representative anionic dyes like “acid blue 93” and “methyl orange” in the absence or presence of certain cationic dyes, making the material suitable for selective dye removal applications.

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INTRODUCTION The combination of selected metal ions with suitable organic ligands is an elegant method for the construction of discrete selfassembled coordination cages.1−10 In this regard, palladium(II) is one of the choicest metal centers that is usually employed either in its cis-protected or bare form for complexation with bi/ polydentate nonchelating ligands to achieve a variety of exotic complexes.11−14 Bidentate chelating ligands possessing either nitrogen or occasionally phosphorus as the donor atom are typically used for cis protection around a palladium(II) center. Our research effort is directed toward the construction of homoand heteroleptic self-assembled coordination complexes, prepared from a variety of palladium(II) components and nitrogendonor ligands.15−22 Further, we have been systematically investigating the self-assembly of already self-assembled palladium(II)-based coordination complexes in solution, gel, and solid phases. In this context, [2]catenanes were prepared via the solvent-promoted interlocking of already self-assembled coordination rings (in the solution phase);18 a few relevant complexes are also employed as tectons, aiming at intermolecular entanglement (in the gel phase)19 and predefined packing (in the solid phase).16,17,20,22 Complexation of palladium(II) with a wide variety of nonchelating bidentate ligands is known to afford a series of PdmL2m-type self-assembled coordination cages.11 The Pd2L4type compounds are among the simplest yet most utilized cages of this series.11,23−56 Confined cavities of Pd2L4-type cages are © XXXX American Chemical Society

well explored for the encapsulation of guests that are as varied as anionic,23−27 neutral,28−32 radical initiator,33 and drug molecules.34,35 The study of the structural,36−42 photophysical,43−45 electrochemical,46,47 gas adsorption,48 and functionalization49−51 aspects of these complexes has also been quite impactful. However, the potency of the intermolecular interactions in Pd2L4-type cages has not been well tapped. The anion-templated interlocking of two units of a Pd2L4-type cage in order to prepare the corresponding [Pd2L4]2-type [2]catenanes52−55 has been reported. Systems that exemplify such bimolecular interaction in the solution phase are, however, only rare.53−56 Also, surprisingly, there exists only one report that deals with the intermolecular entanglement of P2L4-type cages to prepare a supramolecular soft matter called gel.56 A variety of interactions like hydrogen bonding, π−π stacking, metal−ligand binding, etc., are responsible for the formation of a supramolecular gel.57−61 Gel that responds to its environment or to external inputs in a specific manner is termed a stimuliresponsive gel;62,63 such types of gels have been gaining prominence recently because of their potential applications.64,65 The construction of a supramolecular organogel from lowmolecular-weight gelators is well-known.66 However, metalcontaining supramolecular gels, i.e., metallogels, have received Special Issue: Self-Assembled Cages and Macrocycles Received: August 31, 2017

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DOI: 10.1021/acs.inorgchem.7b02239 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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attention only in recent years. Metallogels are afforded from in situ prepared coordination polymers or from discrete metal complexes wherein the materials are featured with entangled arrangements.67−69 Complexes of palladium(II) are some of the less explored families of metallogelators. 70−80 Discrete palladium(II) compounds known for making metallogels include mononuclear,70,71 binuclear,72,56 trinuclear,19 and the proposed dodecanuclear79,80 coordination complexes. Various properties/ applications of metallogels embrace the fields of catalysis, gas adsorption, guest encapsulation, and in situ synthesis of metal nanoparticles and photoelectric studies.73,77,81−88 Supramolecular gels that are prepared via the entanglement of molecules having well-defined cavities have been pursued by researchers only recently.89−95 Along these lines, conventional macrocyclic cavitands, e.g., cyclodextrins, calixarenes, and cucurbit[n]uril, have been utilized and also reviewed.89,90 However, self-assembled discrete coordination cages are less known for the preparation of gels.19,56,70,72,79,80,89−95 It would thus be interesting to study the properties of gel materials that possess well-defined coordination cages in their building blocks. The porous gel materials thus prepared could be used as hosts for the encapsulation of fitting guest molecules, possibly inside the cavity of the concerned cage, for promising applications. In this work, we report the preparation of a cylindrical-drumshaped self-assembled Pd2L4-type coordination cage (Scheme 1)

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EXPERIMENTAL SECTION

Materials and Methods. 1,4,5,8-Naphthalenetetracarboxylic dianhydride, 3-picolylamine, tetra-n-butylammonium bromide (TBABr), PdCl2, PtCl2, AgBF4, AgClO4, AgOTs, AgPF6, AgSbF6, and rhodamine B (RB) were obtained from Aldrich, whereas AgNO 3 , 4(dimethylamino)pyridine (DMAP), p-toluenesulfonic acid (TsOH), Ni(NO3)2·6H2O, acid blue 93 (AB93), methyl orange (MO), methylene blue (MB), crystal violet (CV), and all of the common solvents were obtained from Spectrochem, India, and used as such without further purification. The deuterated solvents were obtained from Aldrich. NMR spectral data were obtained from a Bruker FT NMR spectrometer (400 MHz for 1H and 100 MHz for 13C and also 500 MHz for 1H and 125 MHz for 13C) in DMSO-d6 using external tetramethylsilane in CDCl3 as the reference. The electrospray ionization (ESI-MS) spectra were obtained from Bruker microOTOF and Agilent LC/Q-TOF mass spectrometers. UV−vis spectral studies were performed using a JASCO V-650 instrument. Field-emission scanning electron microscopy (FESEM) studies were carried out using a FEIQuanta 450 microscope. Transmission electron microscopy (TEM) images were recorded using a JEM3010JEO instrument. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Advance Xray diffractometer using Cu Kα radiation (λ = 1.54178 Å). Dynamic light scattering (DLS) experiments were performed on a Malvern Zetasizer nanoseries at 30 °C, with a path length of 1 cm. The wavelength of the laser used was 632.8 nm, and the scattering angle was kept at 90°. The rheology data were obtained from an Anton Paar Physica MCR 301 rheometer equipped with a cone-shaped plate, and a 0.052 mm gap was employed throughout the experiment. A strain sweep was carried out, at a constant frequency (1 rad s−1), in the 0.1−100% strain range for the gel prepared from dimethyl sulfoxide (DMSO; 1.3% gel) and the 0.1− 1000% strain range for the gel prepared from 1:1 acetonitrile (MeCN)− water (H2O) (1.3% gel). A frequency sweep was done, at a constant strain (5%), in the frequency range of 0.1−100 rad s−1 for both of the gel samples. The thixotropic loop test for the gel (1.3% in DMSO) was performed by applying low (5%) and high (100%) strains in an alternating manner with a constant interval of 90 s, at a constant frequency (1 rad s−1). In the case of the other gel (1.3% in MeCN− H2O), low (5%) and high (750%) strains were applied in an alternating manner with a constant interval of 90 s, at a constant frequency (1 rad s−1). The time interval for the change in the strain from low to high was 30 s. Synthesis of N,N′-Bis(3-pyridylmethyl)naphthalenediimide (L). Ligand L was synthesized by a slight modification of the reported procedure (change of the solvent and use of a Dean−Stark apparatus) to improve the yield.96 A mixture of napthalenetetracarboxylic dianhydride (1.0 g, 0.0037 mol) and 3-(aminomethyl)pyridine (1.6 g, 0.0149 mol) in toluene (50 mL) was stirred under reflux for 36 h in a Dean−Stark apparatus, upon which a dark mixture was formed. The mixture was cooled to room temperature, and the precipitated part was isolated by filtration. The precipitate was washed with methanol (100 mL), followed by drying to obtain the ligand as a brown solid. Yield: 75% (1.25 g). 1H NMR spectrum of the ligand recorded in CDCl3 was found to be comparable with the reported data,96 whereas 13C NMR data in CDCl3 and 1H NMR data in DMSO-d6 are reported in this work. 1H NMR (400 MHz, CDCl3, 298 K): δ 8.83 (s, 2H, Ha), 8.78 (s, 4H, He), 8.52 (d, 2H, J = 3.6 Hz, Hb), 7.89 (d, 2H, J = 6.8 Hz, Hc), 7.26 (Hd peak merged with a CDCl3 residual peak), 5.39 (s, 4H, Hf). 13C NMR (100 MHz, CDCl3, 298 K): δ 161.68, 149.75, 148.26, 136.07, 131.23, 130.33, 125.81, 125.59, 122.47, 40.64. 1H NMR (500 MHz, DMSO-d6, 298 K): δ 9.25 (s, 2H, He), 9.21 (s, 4H, Ha), 9.00 (d, 2H, J = 4.5 Hz, Hb), 8.36 (d, 2H, J = 8 Hz, Hc), 7.89 (t, 2H, J = 4.5 Hz, Hd), 5.83 (s, 4H, Hf). Synthesis of [Pd2L4](NO3)4 (1). Method 1. A solution of PdCl2 (5 mg, 0.028 mmol) was prepared in MeCN (5 mL), to which AgNO3 (10 mg, 0.056 mmol) was added, and the resulting solution was heated for 1 h at 60 °C to ensure complete precipitation of AgCl. The mixture was centrifuged, and the supernatant was decanted to afford a clear solution of Pd(NO3)2. The solution of Pd(NO3)2 so obtained was added to a suspension of ligand L (25 mg, 0.056 mmol) in H2O (5 mL). The

Scheme 1. Synthesis of the Self-Assembled Cage 1

having naphthalenediimide (NDI) moieties in its framework. This cage displayed a further self-assembly phenomenon to afford porous supramolecular gel under the appropriate conditions. Salient features of the gel include thixotropic (mechanoresponsive) nature, in addition to the reversible stimuli-responsive (chemical inputs) behavior. Applications of this porous gel in pyrene uptake and the selective removal of anionic dyes from aqueous solutions are demonstrated to qualify it as an advanced material. B


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ligand L96 (Supporting Information, Figures S1−S9) that is crafted with an NDI spacer was selected for complexation with palladium(II) to afford the envisioned cage. Complexation of cadmium(II), cobalt(II), zinc(II), and mercury(II) with ligand L was explored and resulted in coordination polymers. The crystal structures of these coordination polymers displayed π−π and hydrogen-bonding interactions in their build.96 A variety of organic molecules and metal complexes containing NDI moieties have also been studied for their potential applications in the fields of molecular electronics, biomedical research, thin films, sensors, and nanotechnology.97−103 Synthesis and Characterization of the Cage 1. Complexation of Pd(NO3)2 with the ligand L was carried out in DMSO or MeCN−H2O systems, upon which the Pd2L4-type complex 1 was formed (Scheme 1). The cage could be isolated in the solid state by either precipitation from the reaction mixture (in the case of DMSO) or complete evaporation of the solvent (in the case of MeCN−H2O) followed by drying of the samples in a vacuum. The complex was characterized by NMR and ESI-MS techniques (Supporting Information, Figures S10−S15); however, all efforts to crystallize the cage resulted in gel-like or precipitated materials. The 1H NMR spectrum of complex 1 in DMSO-d6 exhibited a single set of peaks (Figure 1), albeit somewhat broad,

reaction mixture was stirred at room temperature for 5 h to afford a clear solution that was taken in a watch glass and allowed to evaporate by standing at room temperature. Gel formation was observed during evaporation of the solvent. A solid mass was deposited upon complete evaporation of the solvent that was washed with MeCN and finally dried under vacuum to get complex 1 as a brown solid. Yield: 85% (26.8 mg). Mp: >300 °C. 1H NMR (500 MHz, DMSO-d6, 298 K): δ 9.58 (b, 2H, Ha), 9.26 (b, 2H, Hb), 9.28 (s, 4H, He), 8.72 (b, 2H, Hc), 8.18 (b, 2H, Hd), 5.89 (s, 4H, Hf). ESI-MS (time-of-flight, TOF; in DMSO). Calcd for [1 − 4NO3]4+: m/z 501.5694. Obsd: m/z 501.5718. ESI-MS (TOF; in MeCN−H2O). Calcd for [1 − 3NO3]3+: m/z 689.4231. Obsd: m/z 689.4219. Anal. Calcd for C105H67N20O28Pd2(H2O)18: C, 48.62; H, 4.00; N, 10.80. Found: C, 48.75; H, 3.85; N, 11.21. Method 2. Method 1 was slightly modified, and DMSO was used as the solvent in place of MeCN−H2O. However, during the final stage, the solvent was not evaporated, but ethyl acetate (20 mL) was added to the reaction mixture to precipitate out the complex. The solid was separated by filtration, washed with MeCN, and finally dried under vacuum to obtain complex 1 as a brown solid. Yield: 73% (23 mg). Preparation of the Gel. A sample of gel was prepared by dissolving complex 1 (13.5 mg, 0.0059 mmol) in 1 mL of DMSO, and then the solution (10 mM in metal) was kept aside. The solutions turned into opaque gel within 1 h, and the gel was termed 1.3% gel. The 1.3% gel could also be prepared from in situ generated complex 1. Thus, a mixture of Pd(NO3)2 (2.7 mg, 0.0117 mmol) and ligand L (10.5 mg, 0.0235 mmol) in 1 mL of DMSO was stirred for 10 min, and the resulting solution was kept aside for 1 h, upon which an opaque gel was formed. The gel material can also be prepared by dissolving the preprepared complex or in situ preparing the complex in MeCN−H2O (1:1). Pyrene Uptake. Complex 1 (13.5 mg, 0.0059 mmol) and pyrene (1.2 mg, 0.0059 mmol) were added to 1 mL of DMSO or MeCN−H2O (1:1); alternatively, a sample of 1.3% gel prepared from 13.5 mg of complex 1 in 1 mL of either of the solvent systems was shaken for 5−10 min, and then pyrene (1.2 mg, 0.0059 mmol) was added, followed by further shaking of the mixture for 5−10 min and allowing the resultant solution to stand for a period of 5 min. In both cases, a typical red color developed because of the charge-transfer phenomenon. The chargetransfer interaction between 1 and pyrene was studied by an UV−vis spectroscopy technique, however, at a lower concentration such as 100 μm with respect to palladium. Dye Uptake. Stock solutions of the dyes were prepared by dissolving 100 mg of a dye (AB93, CV, or RB) in 100 mL of distilled H2O. However, in the case of MO, only 25 mg of the dye was dissolved in 100 mL of distilled H2O because of the limited solubility. UV−vis spectra of the dye solutions were recorded for further comparison. For a given dye uptake experiment, a sample of the gel was initially prepared by dissolving 13.5 mg of complex 1 in 1 mL of MeCN−H2O (1:1) followed by allowing the mixture to stand as usual for 1 h. A 3 mL portion of a chosen stock solution of a dye (i.e., 3 mg for AB93, CV, and RB; 0.75 mg for MO) was layered over the as-prepared gel. The UV−vis spectrum of the dye solution that was layered over the gel was recorded after it remained in contact with the gel for 1 h. In the case of selective uptake of anionic dye, a portion of 3 mL of a mixed dye solution was prepared by combining 1.5 mL of the anionic AB93 solution (1.5 mg of dye) and 1.5 mL of the cationic RB solution (1.5 mg of dye). The resulting mixed dye solution was layered over a sample of the gel. The UV−vis spectrum of the dye solution was recorded after the sample was allowed to stand for 1 h.

Figure 1. 400 MHz 1H NMR spectra (DMSO-d6) of (i) L and (ii) 1 (the concentration of 1 was 10 mM with respect to the metal).

corresponding to a proposed single discrete product. The broad nature of the signals is ascribed to probable intermolecularinteraction-induced aggregation.19 After the NMR tube was allowed to stand for 1 h, the solution got transformed to a gel that further supported the aggregation behavior. While the signals corresponding to the protons (i.e., Ha, Hb, Hc, and Hd) of the bound pyridine unit were appreciably downfield-shifted (compared to the free ligand L) due to their proximity to the metal center, the other two signals corresponding to the NDI moiety and methylene linkers (i.e., He and Hf) were marginally downfield-shifted. The complexation reaction was also carried out directly in DMSO-d6, and the in situ formed complex was monitored by recording 1H NMR spectra as a function of time. It was observed that the signals corresponding to free ligand L disappeared within 30 min and a new set of signals resembling that of the isolated complex were observed. The 1H NMR spectrum of the complex was recorded in CD3CN−D2O (1:1) and displayed a single set of peaks (also broad) assignable to the protons present in the system (Supporting Information, Figure S13). However, a direct comparison of the positions of the signals with that of the free ligand could not be made in this solvent system because of the limited solubility of the ligand. A solution of complex 1 in CD3CN−D2O also turned to gel after it was allowed to stand for 1 h.

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RESULTS AND DISCUSSION It was intended to prepare a Pd2L4-type coordination cage having NDI moieties in its framework. The NDI fragment was considered suitable because it is capable and known to promote hydrogen-bonding and π−π interactions.96−99 Thus, further selfassembly of the designer self-assembled Pd2L4 cage was anticipated. A host molecule crafted with a π-acidic NDI moiety should promote its interaction with electron-rich guest molecules.97 The pyridine-appended bidentate nonchelating C


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counteranions on metallogel formation is well-known, but the exact reasons for this behavior are not well established.104,105 In this context, a variety of PdX2 (where X stands for BF4− ClO4−, PF6−, SbF6−, and OTs−) were used for complexation with ligand L to prepare a series of complexes. 1H NMR spectra of these samples showed broad and multiple peaks indicating possible aggregation behavior/mixture (Supporting Information, Figure S16). Subsequently, the gelation ability was tested for the samples using DMSO as the solvent. It was noted that the proposed [Pd2L4](BF4)4 (2)/mixture could form a gel, whereas the other samples remained viscous even at high concentrations like 15 mM (with respect to palladium; Supporting Information, Table S2). This observation clearly states that the counteranion plays a key role in the intermolecular entanglement. It is safe to assume that synergy among the metal ion, ligand, counteranion, solvent, and concentration is vital for the gel formation reported in this work, and a proper optimization of these parameters is essential. UV−Vis Spectral Studies. Variable-concentration UV−vis spectra of complex 1 were collected in DMSO to gather further support on its aggregation behavior. However, such experiments could be performed only at lower concentration because of the limitation of the technique. The spectra were obtained in the concentration range of 20−400 μM with respect to palladium (Figure 3). It exhibited two absorption bands (characteristic

The ESI-MS data of complex 1 were recorded to confirm its molecular formula. Data collection was carried out in two solvent systems, namely, DMSO and MeCN−H2O. Isotopic peak patterns at m/z 501.5718, corresponding to the polycation [1 − 4NO3−]4+ for the sample prepared in DMSO, and m/z 689.4219, corresponding to the polycation [1 − 3NO3−]3+ for the sample prepared in MeCN−H2O, supported the binuclear nature of the cage. The experimental and simulated isotopic patterns were found to be comparable (Supporting Information, Figures S14 and S15). Energy-minimized structures of ligand L and complex 1 were obtained by the density functional theory method (Figure 2; see the Supporting Information for details).

Figure 2. Energy-minimized structures of L and the cationic part of the cage 1 (hydrogen atoms are not shown for clarity).

The overall shape of complex 1 resembled that of a cylindrical drum, where the nonbonded distance between two palladium(II) centers was around 13.8 Å. Gelation Test Using Cage 1. Complex 1 upon dissolution in DMSO followed by standing resulted in an opaque gel, as described next. A solution of 1 in DMSO [1.3% (w/v), i.e., 10 mM in metal] was allowed to stand at room temperature (30 °C), upon which the gel (1.3% gel) was molded from the solution within 1 h. However, upon a 2-fold increase in the concentration, the gel could form within 15 min of standing and the gel sample was named 2.6% gel. A simple inversion test of the samples supported the claim of gel formation. The gelation ability of cage 1 was probed in a variety of solvents. While solvent systems such as DMSO and MeCN−H2O (1:1) were found to be suitable for gelation, the complex could not be solubilized in solvents like MeCN, methanol, acetone, isopropyl alcohol, dioxane, tetrahydrofuran, dichloromethane, trichloromethane, and hexane (Supporting Information, Table S1). Dissolution of the complex in N,N-dimethylformamide yielded a viscous solution that remained viscous and did not form gel even after standing for 2 days. Because nickel and platinum belong to the same group as palladium, it was decided to combine Ni(NO3)2 and Pt(NO3)2 separately with ligand L and investigate the possibility of gel formation in DMSO. However, the experiments were unsuccessful with respect to gel formation, and the nature of the complex(es) formed was not established. Complexation of platinum(II) with the ligand resulted in a hazy solution that showed no signals in the 1H NMR spectrum in DMSO-d6, and the ESI-MS study was also unsuccessful. Complexation of nickel(II) with the ligand resulted in an insoluble precipitate, indicating oligomerization/polymerization. Counteranion and the Gel. The counteranion present in complex 1 is NO3−, and the complex is a gelator under appropriate conditions as discussed above. The influence of

Figure 3. Variable-concentration UV−vis spectra of complex 1 in DMSO (the concentration is with respect to the metal).

π−π* transitions) in the 300−500 nm region due to the presence of NDI chromophore, wherein the λmax values of the peaks were located around 361 and 382 nm. A hyperchromic shift was observed with an increase of the concentration due to probably agglomeration of the complex.106,107 Variable-concentration UV−vis spectra of the free ligand L were recorded in DMSO for comparison purposes. The data related to L showed only a proportionate increase of the absorbance of both peaks as the concentration was increased (Supporting Information, Figure S17). Thus, it is assumed that the ligand alone does not agglomerate under the conditions employed. A synergic effect that evolved because of the metal-and-ligand combination is proposed to be responsible for the observed agglomeration or gel formation. Of course, the choice of the anion is vital too, as was already explained in the above section. The UV−vis spectra of complex 1 in MeCN−H2O (1:1) also showed a hyperchromic shift (Supporting Information, Figure S18). D


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Inorganic Chemistry PXRD Studies. The molecular packing pattern of complex 1 (isolated from DMSO) was investigated by using the PXRD technique (Supporting Information, Figure S19a). The observed 2θ values of 24.66°, 27.89°, and 32.17° correspond to interplanar distances (d) of 3.68, 3.29, and 2.75, Å, respectively. These distances are assignable to the occurrence of π interactions108,109 in the bulk state of the material. Thus, we propose the existence of aromatic interactions in the xerogel. Gelation Mechanism. The agglomeration of complex 1 upon dissolution in suitable solvents is supported by UV−vis and PXRD studies. The gelation of complex 1 is proposed to be driven by the agglomeration induced by several noncovalent interactions including intermolecular π−π stacking and hydrogen bonding. The NDI moieties present in the ligand backbone are considered to be the major reason for the observed gelation. Also, anion- and solvent-mediated hydrogen-bonding interactions, anion-mediated electrostatic interactions, and solvent− ligand exchange reactions are probably responsible for the agglomeration. Microscopic Imaging and DLS Studies. The morphology of the gel was examined by FESEM and TEM techniques. The FESEM and TEM images of the sample prepared in DMSO (5 mM in metal) are shown in Figure 4, whereas the images of the

The average hydrodynamic diameter (Dh) of the aggregated particles was examined by DLS experiments. The particle sizes of the samples prepared in DMSO and 1:1 MeCN−H2O (5 mM in metal) are found to be in the ranges of 10−44 and 11−33 nm having average sizes of 27 and 22 nm, respectively (Supporting Information, Figure S21). Rheology. The storage modulus (G′) and loss modulus (G″) of the 1.3% gel (in DMSO) were measured at a constant frequency of 1 rad s−1 at room temperature (30 °C) as a function of strain (Figure 5a). The elastic nature of the gel could be

Figure 4. (a) FESEM and (b) TEM images of the samples prepared from a solution of 1 in DMSO [5 mM in metal, i.e., 0.65% (w/v)].

Figure 5. Plots showing the storage modulus (G′) and loss modulus (G″) of the 1.3% gel at 30 °C (prepared in DMSO) (a) as a function of the strain at a constant frequency of 1 rad s−1 and (b) as a function of the frequency at a constant strain of 5%.

sample in MeCN−H2O (5 mM in metal) are shown in the Supporting Information (Figure S20). The FESEM images of both samples revealed the presence of well-defined nanoscale metal−organic particles (NMOPs)110−112 with the diameter ranging from 10 to 20 nm. The TEM images of the samples also supported the presence of NMOPs in both cases. Potential applications of the NMOPs include drug delivery, sensing, and molecular electronics.113−116

retained up to 40% strain (i.e., G′ > G″), whereas at higher strain, the elastic nature was lost and the material became free-flowing (i.e., G″ > G′). Subsequently, G′ and G″ of the 1.3% gel were measured as a function of the angular frequency at room temperature (30 °C) at such a constant strain where the sample retained its gel nature (e.g., 5%), as shown in Figure 5b. The average G′ value (1385 Pa) was found to be higher than the average G″ value (118 Pa) for the 1.3% gel. G′ and G″ of the 2.6% gel (in DMSO) were measured at room temperature (30 °C) as a E


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Inorganic Chemistry function of the angular frequency at 5% strain, whereupon the values were found to be (5443 Pa) and (407 Pa), respectively (Supporting Information, Figure S22a). The ΔG value that is G′ − G″ can be qualitatively related to the elastic strength of the material, where a higher value indicates higher strength. The ΔG value for the 2.6% gel (5036 Pa) was found to be approximately 4 times higher than that of the 1.3% gel (1267 Pa). This observation indicated augmented strength of the gel upon an increase of the concentration of complex 1. The G′ and G″ values of 1.3% gel were measured as a function of angular frequency at 5% strain at a higher temperature such as 90 °C, whereupon the G′ and G″ values were found to be 4070 and 320 Pa, respectively. The ΔG value was found to be 3750 Pa (Supporting Information, Figure S22b). These preliminary data indicate that the strength of the gel increases upon an increase of the concentration or temperature. Probably the connectivity of the nodes in the gel structure becomes prominent, making the gel stronger with an increase of the concentration/temperature. The elastic properties of the gel prepared in the MeCN−H2O system were also probed under similar conditions (Supporting Information, Figure S23a,b). As expected, the G′ value (8369 Pa) was found to be higher than the G″ value (691 Pa) for the 1.3% gel, as measured by the frequency sweep method at room temperature (30 °C). Interestingly, the ΔG value of the 1.3% gel in MeCN−H2O (7678 Pa) was found to be higher than that in DMSO (1267 Pa). Stimuli-Responsive Gel. Supramolecular gels are wellknown for their multiple stimuli-responsive behaviors.62,63 The gel samples respond to the physical factors (i.e., stimuli to the temperature, light, force, sound, etc.) or chemical factors (i.e., stimuli to the acid−base or pH, ligand, anions, etc.) or combinations thereof. These kinds of gels are potential candidates in applications where on-demand flow of the material is a requirement.64,65 A gel that flows under shearing forces and then resolidifies when the force is removed is termed a thixotropic gel.117 The gel studied here showed thixotropic behavior, as explained below. A preliminary test was carried out by vigorously shaking a vial containing a sample of the 1.3% gel (in DMSO), upon which the gel phase got transformed to a solution phase within 5 min. The solution phase so obtained was allowed to stand, upon which the gel phase was reconstructed within 5 min (Figure 6).

Figure 7. Thixotropic loop test of the 1.3% gel (in DMSO).

90 s and then the strain was suddenly increased to 100%, upon which the material got transformed to a solution (G′ < G″) within a period of 30 s. After the solution was retained for 90 s, the strain was suddenly decreased to 5%, upon which the material returned to the gel phase (G′ > G″) within 30 s. This means that spontaneous reconstruction of the gel could be achieved by removal of the applied strain. These gel-to-sol and sol-to-gel cycle tests were performed for three cycles. It is assumed that such cycles could be repeated several times. The material is thus considered to be a thixotropic gel. The 1.3% gel prepared in MeCN−H2O also displayed a thixotropic nature (Supporting Information, Figure S23c). The cavities of palladium(II)-based coordination cages are well suited for appropriate anionic guests.23−27 Some selected cavities are found to be suitable for the encapsulation of halides as guest entities. However, decomplexation reactions upon the addition of an excess of halides via ligand-exchange reactions are also known.21 Taking advantage of this fact, 5 equiv [with respect to palladium(II)] of a representative halide, i.e., TBABr, was spread over a sample of gel. The gel phase slowly changed to a solution phase at the upper part of the gel, and then the entire gel slowly turned to a solution (Figure 8). The release of free ligand L due to the formation of [Pd(Br)4]2− is proposed in this process. The process was repeated in DMSO-d6, and a 1H NMR spectrum of the resulting solution was recorded that confirmed the release of free ligand L (Supporting Information, Figure S24). Upon the introduction of 5 equiv of AgNO3 to the solution containing L and [Pd(Br)4]2−, the gel phase was reestablished within 15 min.

Figure 6. Illustration for the thixotropic (mechanoresponsive) nature of the 1.3% gel (in DMSO).

The response of the gel toward mechanical force was further examined by the thixotropy loop test (Figure 7). The gel (1.3% DMSO) was subjected to low (5%) and high (100%) strains in an alternating manner with a constant interval of 90 s. At low strain, the material behaved as a gel (G′ > G″), as was already explained in an earlier section. At the onset, the gel was kept at low strain for

Figure 8. Reversible stimuli-responsive phase transition of the gel-tosol/sol-to-gel using TBABr/AgNO3 or DMAP/TsOH pairs as the inputs and the 1.3% gel (in DMSO) as the sample. F


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Inorganic Chemistry This transformation is possible because [Pd(Br)4]2− reacts with AgNO3 to release free palladium(II) in favor of the complexation reaction with the free ligand already available in the same solution. Further, the disassembly and reassembly of complex 1 was established with the help of a DMAP/TsOH reagent pair.35,118 The gel phase changed to afford a solution upon the addition of DMAP (Figure 8), where the release of free ligand L is due to the formation of [Pd(DMAP)4]2+ (Supporting Information, Figure S25). The gel phase was reestablished upon the addition of TsOH because free palladium(II) was released in favor of complexation with the already available free ligand. Pyrene Uptake. Guest encapsulation in the cavity of Pd2L4type cages is well established where noncovalent interactions like electrostatic, hydrogen bonding, π−π, and hydrophobic, etc., are exploited for the binding.23−35 The energy-minimized structure of 1 shown in Figure 2 suggests the existence of a rigid and welldefined cavity in the architecture of 1. It is expected that a rigid cagelike molecule would retain its binding ability even after forming the gel because the cavity is probably not disturbed. The NDI moiety, which possesses π-acid character, is crafted in the framework of 1; thus, it was expected that a good π-base like pyrene should interact with the porous gel. The binding of pyrene with the gel could be carried out by varying the sequence of the addition of the components, keeping the ratio of metal, ligand, and pyrene as 2:4:1 and the targeted gel at 1.3% in DMSO (the overall ratio of complex 1 and pyrene is 1:1). The two methods adopted for the study are (i) the gel was prepared in the presence of pyrene, where the xerogel 1 was added to DMSO followed by pyrene, and (ii) the already prepared gel was shaken for 5−10 min followed by the addition of pyrene, further shaking for 5−10 min, and standing for 5 min. In each case, a typical red color was observed (Supporting Information, Figure S26), whereas the gel as such is brown in color. Interaction between the host and guest was established by UV−vis spectroscopy, however at a lower concentration. A new shoulderlike absorption band around 517 nm was observed as a result of the interaction of pyrene with 1 (Figure 9).119,120 This

ligand and free pyrene. The absorption bands are found to be unchanged in the mixture (Supporting Information, Figure S28), so it is safe to assume that that there is no interaction between the free ligand and pyrene. The optimized structure (PM6) showing the possible interaction of pyrene with the cage 1 is provided in Supporting Information (Figure S29). It is assumed from the optimized structure that the cavity is suitable for accommodating only one unit of pyrene because there is no further space available for another unit. Dye Removal. Dyes are toxic in nature and not easily biodegradable, which makes their presence in water bodies a serious threat to biosystems and ecology. Therefore, the development of methods that would facilitate the removal of dye from samples of aqueous solutions is important. Supramolecular gels are suitable materials that could cater to this requirement.121−126 The porous gel material developed in this work is constructed from a cationic cage with a NDI core. Thus, it was assumed that the gel might uptake anionic dye molecules.97−99 The binding ability of the gel with representative anionic (Figure 10a) as well as cationic (Figure 10b) dye molecules was probed. The dyes used were AB93, MO, MB, CV, and RB. The 1.3% gel prepared from MeCN−H2O (1:1) was

Figure 9. UV−vis spectra in DMSO showing the evolution of a peak due to the host−guest interaction of pyrene (guest) with the gel (host).

band arises because of the charge-transfer interaction between the host and guest molecules. [A similar charge-transfer band was noticed when pyrene was allowed to interact with the gel sample prepared in MeCN−H2O (Supporting Information, Figure S27).] A solution of the ligand (4 equiv, 200 μM) and pyrene (1 equiv, 50 μM) was prepared in DMSO. The UV−vis spectrum of this mixture was recorded and compared with that of the free

Figure 10. Chemical structures of the (a) anionic and (b) cationic dyes. G


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Inorganic Chemistry employed for this purpose. Stock solutions of the dyes were prepared by dissolving 100 mg of the dye (25 mg in the case of the less soluble MO) in 100 mL of distilled H2O. The gel was found to be effective for the removal of anionic dyes (AB93 and MO separately) but not for cationic dyes (MB, CV, and RB). A solution of AB93 (3 mL) or MO (3 mL) was layered over a sample of the gel (13 mg in 1 mL of MeCN−H2O). The color of the anionic dye solution faded with time, indicating an uptake of dye molecules by the gel. The uptake of anionic dyes was monitored by recording UV−vis spectra of the dye solutions that were layered over the gel. The study revealed that the dye present in its aqueous solution could be completely taken by the gel within 1 h (Figure 11a,b). Further, the selective removal of anionic dye by the gel from a mixture of anionic and cationic dyes122,123 was demonstrated (Figure 11c). A dark mixture of 1.5 mL of a AB93 solution (λmax = 307 nm) and 1.5 mL of a RB solution (λmax = 554 nm) was layered over the gel. The color of the solution turned to clear pink (similar to that of RB) after 1 h, indicating that the anionic AB93 was taken by the gel material, leaving behind RB in the solution layered above the gel. The UV−vis spectrum of the pink solution, obtained after 1 h, was recorded and showed the presence of RB and the disappearance of AB93. This confirms the selective uptake of anionic dye by the gel, while the cationic dye remained in solution. The anionic dyes (AB93 and MO) were combined with an equimolar amount of complex 1 in CD3CN−D2O (1:1), and the ensuing interactions were probed by recording the 1H NMR spectra of the samples. While the signals of complex 1 were marginally changed or unchanged, the signals of the dye molecules were found to be broadened and upfield-shifted (Supporting Information, Figures S30 and S31). These data support encapsulation of the dye molecules in the cavity of the complex, and the same phenomenon is expected in the gel state. We have considered the encapsulation of one unit of MO in the cavity of complex 1, and optimization of the host−guest complex was carried out by the PM6 method. Electrostatic and hydrophobic interactions are responsible for encapsulation of the guest, where the anionic part is held by electrostatic interaction with the metal center and NDI moieties (Supporting Information, Figure S32).

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CONCLUSION The production and establishment of functional aspects of a metallogel through self-assembly of the already self-assembled coordination cage of Pd2L4 formulation have been achieved. A synergistic interplay of the metal ion, ligand, counteranion, solvent, and concentration played a vital role in the metallogel formation. Salient features of the gel include thixotropic and reversible chemical-stimuli-responsive behavior. A transition from the gel to solid phase could thus be achieved by employing either physical impacts that disturbed the intermolecular interactions or a suitable chemical input (e.g., Br−/DMAP) that interacted preferentially with the metal center, releasing the ligand. The gel phase could be reachieved by allowing the solid (which was generated by the physical impact) to stand or making the metal ion available by removing bound Br−/DMAP using AgNO3/TsOH. Applications of this porous gel in pyrene uptake and the selective removal of representative anionic dyes (e.g., AB93 and MO) from aqueous solutions have been demonstrated.

Figure 11. UV−vis spectra of the aqueous solutions of selected dyes before and after treatment with the 1.3% gel: (a) AB93; (b) MO; (c) a mixture of AB93 and RB. The inset images show the original dye solution (alone) and the status of the dye solutions layered over the gel and allowed to stand for 1 h (samples 1−3).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02239. 1D and 2D NMR, ESI-MS, and UV−vis spectra, PXRD patterns, rheology data, FESEM and TEM micrographs, DLS data, a gelation study, pyrene and dye uptake studies by the UV−vis technique, and energy minimization studies (PDF) H


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Inorganic Chemistry

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(15) Prusty, S.; Yazaki, K.; Yoshizawa, M.; Chand, D. K. A truncated molecular star. Chem. - Eur. J. 2017, 23, 12456−12461. (16) Panneerselvam, A. P.; Jagan, R.; Chand, D. K. Trinuclear introvertere circular helicate and its columnar hexagonal stacking. Cryst. Growth Des. 2017, 17, 2929−2935. (17) Naranthatta, M. C.; Bandi, S.; Jagan, R.; Chand, D. K. DoubleStranded binuclear helicates and helicity modulation. Cryst. Growth Des. 2016, 16, 6722−6728. (18) Prusty, S.; Krishnaswamy, S.; Bandi, S.; Chandrika, B.; Luo, J.; McIndoe, J. S.; Hanan, G. S.; Chand, D. K. Reversible mechanical interlocking of D-shaped molecular karabiners bearing coordinationbond loaded gates: route to self-assembled [2]catenanes. Chem. - Eur. J. 2015, 21, 15174−15187. (19) Ganta, S.; Chand, D. K. Nanoscale metallogel via self-assembly of self-assembled trinuclear coordination rings: multi-stimuli-responsive soft materials. Dalton Trans. 2015, 44, 15181−15188. (20) Dasary, H.; Jagan, R.; Chand, D. K. Octadecanuclear gear wheels by self-assembly of self-assembled “double saddle”-type coordination entities: molecular “rangoli”. Chem. - Eur. J. 2015, 21, 1499−1507. (21) Bandi, S.; Pal, A.; Hanan, G. S.; Chand, D. K. Stoichiometric controlled revocable self-assembled “spiro” versus quadruple stranded “double-decker” type coordination cages. Chem. - Eur. J. 2014, 20, 13122−13126. (22) Naranthatta, M. C.; Das, D.; Tripathy, D.; Sahoo, H. S.; Ramkumar, V.; Chand, D. K. Consequence of presence and absence of π-clouds at strategic locations of designed binuclear Pd(II) complexes on packing: Self-assembly of self-assembly by intermolecular locking and packing. Cryst. Growth Des. 2012, 12, 6012−6022. (23) Chand, D. K.; Biradha, K.; Fujita, M. Self-assembly of a novel macrotricyclic Pd(II) metallocage encapsulating a nitrate ion. Chem. Commun. 2001, 1652−1653. (24) Clever, G. H.; Tashiro, S.; Shionoya, M. Inclusion of anionic guests inside a molecular cage with palladium(II) centers as electrostatic anchors. Angew. Chem., Int. Ed. 2009, 48, 7010−7012. (25) Han, M.; Michel, R.; He, B.; Chen, Y.-S.; Stalke, D.; John, M.; Clever, G. H. Light-triggered guest uptake and release by a photochromic coordination cage. Angew. Chem., Int. Ed. 2013, 52, 1319−1323. (26) Zhu, R.; Lübben, J.; Dittrich, B.; Clever, G. H. Stepwise halidetriggered double and triple catenation of self-assembled coordination cages. Angew. Chem., Int. Ed. 2015, 54, 2796−2800. (27) Clever, G. H.; Shionoya, M. A pH switchable pseudorotaxane based on a metal cage and a bis-anionic thread. Chem. - Eur. J. 2010, 16, 11792−11796. (28) Kishi, N.; Li, Z.; Yoza, K.; Akita, M.; Yoshizawa, M. An M2L4 molecular capsule with an anthracene shell: encapsulation of large guests up to 1 nm. J. Am. Chem. Soc. 2011, 133, 11438−11441. (29) Yamashina, M.; Yuki, T.; Sei, Y.; Akita, M.; Yoshizawa, M. Anisotropic expansion of an M2L4 coordination capsule: host capability and frame rearrangement. Chem. - Eur. J. 2015, 21, 4200−4204. (30) Liao, P.; Langloss, B. W.; Johnson, A. M.; Knudsen, E. R.; Tham, F. S.; Julian, R. R.; Hooley, R. J. Two-component control of guest binding in a self-assembled cage molecule. Chem. Commun. 2010, 46, 4932−4934. (31) Kishi, N.; Li, Z.; Sei, Y.; Akita, M.; Yoza, K.; Siegel, J. S.; Yoshizawa, M. Wide-ranging host capability of a PdII-linked M2L4 molecular capsule with an anthracene shell. Chem. - Eur. J. 2013, 19, 6313−6320. (32) Bandi, S.; Chand, D. K. Cage-to-cage cascade transformations. Chem. - Eur. J. 2016, 22, 10330−10335. (33) Yamashina, M.; Sei, Y.; Akita, M.; Yoshizawa, M. Safe storage of radical initiators within a polyaromatic nanocapsule. Nat. Commun. 2014, 5, 4662−4668. (34) Lewis, J. E. M.; Gavey, E. L.; Cameron, S. A.; Crowley, J. D. Stimuli-responsive Pd2L4 metallosupramolecular cages: towards targeted cisplatin drug. Chem. Sci. 2012, 3, 778−784. (35) Schmidt, A.; Molano, V.; Hollering, M.; Pöthig, A.; Casini, A.; Kühn, F. E. Evaluation of new palladium cages as potential delivery

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dillip K. Chand: 0000-0003-1115-0138 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Department of Chemistry, IIT Madras, for NMR, PXRD, and elemental analysis and the Department of Materials and Metallurgical Engineering, IIT Madras, for SEM and TEM facilities. We thank Dr. Yoshizawa (TIT, Japan) and Agilent Technologies India Pvt. Ltd. for providing the ESI-MS data. We also thank Prof. A. Deshpande (IIT Madras) and Dr. E. Prasad (IIT Madras) for extending the rheometer and DLS study facilities, respectively. The authors thank the Science and Engineering Research Board (SERB) under DST, Government of India (Project SB/S1/IC-05/2014), for financial support.

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