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Jun 21, 2017 - coordination polymers (CPs) in the efficient capture of inorganic pollutants ... The sorption of chromate/dichromate by CPs via anion e...
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Two-Dimensional Coordination Polymers with “X”-Shaped Cavities as Adsorbents of Oxoanion Pollutants and Toxic Dyes Karabi Nath, Kartik Maity, and Kumar Biradha* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India S Supporting Information *

ABSTRACT: The role of cationic and neutral two-dimensional (2D) coordination polymers (CPs) in the efficient capture of inorganic pollutants (chromate and dichromate) and toxic dyes has been explored. Two cationic CPs with Ag(I) salts of BF4− and ClO4− anions and one neutral CP with Cd(NO3)2 of bis(pyridylcarboxamide) ligands have been structurally characterized, and it was found that all of them have isostructural 2D CPs. The sorption of chromate/dichromate by CPs via anion exchange mechanism was shown to depend not only on the nature of the network but also on the nature of the anion present in the as-synthesized host. Among three CPs, the BF4− containing Ag(I) CP exhibited a better ability of sorbing chromate and dichromate from very dilute solution of sorbates (10−4 M). On the other hand, neutral Cd(II) CP containing NO3− ion were found to have a better ability to sorb chromate/dichromate from somewhat concentrated (10−1 M) solutions. Further, BF4− and NO3− containing CPs exhibit selective sorption of chromate from the solution containing mixture of CrO42−, NO3−, SO42−, and BF4− in equimolar concentrations. The structure of chromate sorbed material was determined with the help of the as-synthesized Cd(II) CP of chromate. Further, Ag(I) CP containing BF4− ion and Cd(II) CP containing NO3− ion have shown the ability for selective dye sorption and luminescence based detection of dichromate ions, respectively.

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INTRODUCTION

demand for the development of novel materials with high performance ion exchange capabilities.10,11 Metal−organic frameworks (MOFs) or coordination polymers (CPs), being a class of structurally diverse porous materials, offer an interesting alternative method for remediation of oxyanion/cation from water.12 Given their amenability to tune the pore size and surface area, CPs have been explored in a wide area of applications including sensing,13 gas-storage,14 catalysis,15 adsorption,16 and chemical separations.17 Cationic MOFs, constructed out of neutral nitrogencontaining organic ligands and metal ions, bears charge balancing uncoordinated anions that show their tendency to capture pollutant anions through an anion exchange process. Very recently, a few studies have been reported where the cationic MOFs exhibit chromate or dichromate trapping through ion exchange.18−22 However, two-dimensional (2D) CPs acting as anion exchange host materials for the removal of both oxoanions and biodegradable dyes with considerable adsorption capacity have not been reported so far. In furtherance with our efforts in synthesizing and exploring the properties of CPs containing bis-pyridyl amide functionalities, in this manuscript the remarkable utility of these CPs for anion exchange and oxoanion removal has been explored. The versatility of network geometries of neutral or cationic CPs with various bis-pyridyl rigid/flexible ligands is well documented in

In recent times, environmental pollution has become a global issue, and several systematic studies are being conducted to address this problem.1 The majority of the pollutants are generated by industrial waste, use of fertilizers and pesticides,2 burning of fossil fuels, and radioactive waste produced from nuclear power stations.3 Remarkably, many of these waste materials contain hazardous inorganic pollutants, in the form of oxoanions, listed by the U.S. Environmental Protection Agency (EPA) as priority pollutants. Various processes such as ion exchange,4 adsorption,5 and photocatalytic degradation6 have been established for their removal. Among these, the ion exchange process is considered the most preferred pathway on account of its cost-effective, ecofriendly, and simplistic execution strategies.7 Selective entrapment of hexavalent chromium Cr(VI), that is produced out of high-level radioactive waste and several other industrial processes including chromium plating, leather tanning, and pigment manufacturing, has been of current interest owing to its hazardous effects on both environment and human health.8 Ion exchangers, categorized as inorganic and organic types, depending on the pore size for exchange and stability, bears the potency to trap these oxoanions from wastewater.9 Organic exchangers such as polymer resins with cationic groups show remarkable adsorption efficiency in comparison to inorganic counterparts in layered double hydroxides (LDH); however, they lags behind in terms of stability. Therefore, there is high © 2017 American Chemical Society

Received: June 4, 2017 Published: June 21, 2017 4437

DOI: 10.1021/acs.cgd.7b00771 Cryst. Growth Des. 2017, 17, 4437−4444

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the literature.23,24 The introduction of amide functionalities in between the pyridine moieties enhances the hydrophilicity of the cavities and thereby enhances the anion binding ability in the cavities.25 The bis-pyridyl ligands containing the amides and aromatic and/or aliphatic spacers are of our interest given their ability to form CPs with hydrogen bonds that define their overall three-dimensional (3D) structure. In these efforts, we have synthesized a new bis-pyridyl ligand L1 containing double bond, −CH2−, and two amide functionalities as spacers. The ligand L1 was found to have a propensity to from cationic CPs with Ag(I) salts which exhibited a potential to trap oxoanion pollutants (chromate and dichromate) from aqueous solutions. In order to understand the role of cationic nature of the network and Ag(I) in such a phenomenon, a neutral CP of Cd(II) and L2,26 reported by us earlier,27 has been explored for chromate ion removal from aqueous solutions. The selective anion exchange by CPs has also been investigated with anions such as NO3−, SO42−, BF4−, and ClO4−. In addition to this, selective dye sorption and luminescence based detection of dichromate ions in aqueous solutions by the CPs have also been studied.

The asymmetric unit is composed of a half unit of L1, Ag(I) with 0.25 occupancy and corresponding anion (BF4− or ClO4−) with 0.25 occupancy. The Ag(I) ion adopts a distorted tetrahedral geometry with four equatorially coordinated L1 molecules, and Ag−N bond distances range from 2.358(2) Å to 2.362(2) Å. The geometry of L1 is found to be divergent in terms of the placement of the N atoms of the pyridine groups, which illustrates the formation of a 2D open network with (4,4)-topology through swastika-shaped Ag(L1)4 building blocks. The layers pack on each other via β-sheet hydrogen bonds in a slipped fashion, with an interlayer separation of ∼4.8−4.9 Å, generating 3D networks with continuous channels along the c-axis. These channels are occupied by the respective anions, such as BF4− in 1 and ClO4− in 2 assisted by weak hydrogen bonding and several other intermolecular interactions (Figure 1). The crystal structure of 3 also contains a similar 2D open network, but it is neutral and contains octahedral Cd(II) and does not contain a β-sheet hydrogen bonding network between the layers. The presence of uncoordinated anions in the channels of these 2D-layered CPs prompted us to investigate their potential to carry out selective trapping of oxoanion pollutants (CrO42−and Cr2O72−) via anion exchange reactions. The bulk purity and thermal stability of CPs were verified by powder Xray diffraction (PXRD) (Figure S11) and thermogravimetric analysis (TGA) (Figure S17) analysis, respectively. For chromate removal from aqueous solutions, as-synthesized crystals of 1 (20 mg) were immersed into a 10 mL (10−4 M) potassium chromate solution, and the vials were kept static under ambient conditions. The rate of absorption of CrO42− was monitored by solution UV−visible spectroscopy, based on the typical absorption of CrO42− at 372 nm, at different time intervals. The intensity of the characteristic absorption peak of CrO42− (372 nm) was found to decrease gradually, indicating the substantial removal of chromate ions from solution via ion exchange with tetrafluoroborate anions that are present in the channels. As indicated by UV−vis, the concentration of chromate ion in solution was found to decrease by 72.8% and 90% at the time intervals of 12 and 48 h, respectively (Figure 2). No further decrease in the intensity of the peak was detected after 48 h, demonstrating the completion of the exchange phenomenon. The adsorption capacity of chromate by 1 was found to be 81.9 mg/g, which is higher than the reported values for 1D CP of Ag(I) (SLUG-21).29 Moreover, we note that the chromate adsorption capacity of 1 is much higher than that of uncalcined LDHs and calcined LDHs under similar conditions (6 and 17 mg/g, respectively).30 The discoloration of the chromate solution (10−4 M) and the change of the color of the crystals from white to yellow after submerging them into the solution clearly demonstrate the phenomenon of trapping oxoanionic species in the cationic framework. Interestingly, the crystalline nature of the material was found to be retained throughout the exchange process with retention of the framework integrity as indicated by the PXRD patterns of the exchanged crystals and as-synthesized ones (Figures S13 and S14). Furthermore, Fourier transform infrared (FTIR) and energy-dispersive X-ray (EDX) spectra supports the homogeneous distribution of chromate in the exchanged solid material (Figure 3). The emergence of the characteristic band for Cr−O stretching modes at around 930 cm−1 in the IR spectrum indicates the incorporation of chromate.

Scheme 1. Structural Drawings of L1 and L2

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RESULTS AND DISCUSSION Ligands L1 and L2 were synthesized from the condensation reaction of respective diamines and dicarboxylic acids in the presence of P(OPh)3.28 The complexation reactions of L1 with Ag(I) salts of BF4− and ClO4− in a MeOH−DCM mixture resulted in the formation of single crystals of complexes {[Ag(L1)2](BF4)}n, 1 and {[Ag(L1)2](ClO4)}n, 2 respectively. Recently, it was reported by us that the network geometries of Cd(II) CPs of L2 are greatly influenced by the anions.27 Tetrahedral and octahedral anions such as perchlorate, sulfate, and hexafluorosilicate formed cationic complexes, while nitrate with planar geometry resulted in the formation of a neutral complex {[Cd(L2)2(NO3)2]·(MeOH)2}n, 3. The complexation reactions of L2 with several Ag(I) salts resulted in precipitates instead of crystalline solids despite several trials. Similarly, the reactions of L1 with Cd(II) salts also resulted in the amorphous precipitates. Interestingly, irrespective of the nature of the cations, complexes 1 and 3 were found to possess the ability to adsorb oxoanion pollutants with different capacities from different concentrations of aqueous solutions of chromate and dichromate. Single crystal X-ray diffraction analyses of 1 and 2 reveal that they are isomorphous and crystallize in the P4/n space group. 4438

DOI: 10.1021/acs.cgd.7b00771 Cryst. Growth Des. 2017, 17, 4437−4444

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Figure 1. Illustrations for the crystal structure of 1: (a) 2D network with (4,4)-topology formed by the Ag(I) ion and L1 molecule; (b) packing of successive 2D-layers via β-sheet hydrogen bonding, shown in the encircled region; (c) presence of one-dimensional (1D) continuous channels, occupied by BF4− anions, in the hydrogen bonded 3D framework.

CrO42− for 48 h. The adsorbed solid material has been analyzed for the presence of various anions by IR (Figures S7−S9) and UV−visible spectroscopy (Figure S2), which reveals the absorption of only chromate ion by complex 1. In IR spectra, Cr−O stretch at 930 cm−1 was obtained along with B−F stretching at 1040 cm−1, and the characteristic bands (N−O: 1385 cm−1 and S−O:1265 cm−1) for other competing ions were absent. Further, the adsorption capacity was found to be the same as before (81.9 mg/g) indicating that the competing ions do not compete with adsorption of chromate ions into the cationic framework. However, the ClO4− ion was found to be a competitor to the CrO42− ion and it stops the adsorption of chromate by 1 from aqueous solution containing ClO4−/ CrO42−. The recyclability of the absorbent was also verified by extracting the chromate into aqueous solution by dipping the chromate absorbed CP material (20 mg) in NaBF4 aqueous solution (0.01 M). The material was further recycled for chromate adsorption and was found to be operational for three cycles with lowered efficiency of adsorption capacity (Figure 5). The observed selective binding of chromate and perchlorate over the other anions (BF4−, NO3−, and sulfate) in the CPs could be due to the shape similarities and better hydrogen bonding capabilities within the channels of the CPs. Interestingly, the neutral framework 3 was also found to generate the chromate incorporated material 3′ when immersed

Similarly, the crystals of 1 also were found to exhibit excellent ability for the removal of another oxoanion Cr2O72−. Accordingly, as-synthesized crystals of 1 (20 mg) were immersed in an aqueous solution of Cr2O72− (10−4 M), and the subsequent absorption process was monitored via UV− visible spectroscopy. Cr2O72− removal by 1 was found to be comparatively slower than CrO42−; about 61 and 80% removal was observed in 12 and 24 h, respectively. The high adsorption capacity and selectivity of the framework toward specific oxoanion depend on the stronger interactions of the corresponding anions with the framework. The presence of the oxoanionic species in the exchanged crystalline material has been confirmed by FTIR, EDX analysis, and their elemental mapping profiles (Figure S1). However, the corresponding trapping experiments with 2 turned out to be ineffective, probably due to the better binding of the ClO4− ion in the channels via hydrogen bonds than that of the BF4− ion in 1. This emphasizes the important role of the interactions of the anion with the host framework in the anion exchange process. Therefore, the selective capture of CrO42− anions by complex 1 was studied with a solution containing a mixture of anions such as NO3−, BF4−, SO42−, and CrO42− in equal concentrations. In a typical experiment, 20 mg of the assynthesized crystals of 1 was immersed in a 10 mL aqueous solution containing 10−4 M each of NO3−, BF4−, SO42−, and 4439

DOI: 10.1021/acs.cgd.7b00771 Cryst. Growth Des. 2017, 17, 4437−4444

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Figure 4. Monitoring of dichromate sorption by 1 using UV−visible absorption spectra; notice the decrease in concentration of dichromate in terms of a decrease in its absorption intensity from a solution of 10−4 M dichromate.

succeeded in obtaining suitable single crystals of {[Cd(L2)2(CrO3(OMe))2]}n, 4, with Cd(NO3)2, L2 and K2Cr2O7 in a similar crystallization attempt as 3 in the MeOH−H2O solvent system. Single crystal X-ray diffraction analysis of 4 reveals that it is isostructural with that of 3 and crystallized in the P1̅ space group. The asymmetric unit of 4 is composed of Cd(II) with half occupancy, two half units of L2, and one unit of anion CrO3(OMe)−. The central Cd(II) ion exhibits a distorted octahedral geometry with four equatorially coordinated L2 molecules (Cd−N: 2.314(5) Å, 2.340(6) Å, and 2.399(2) Å, 2.373(3) Å) and two axially coordinated anions (Cd−O: 2.368(4) Å in 3 and Cd−O: 2.287(4) Å in 4) (Figure 6). The formation of complex 4 demonstrates the selective inclusion of chromate over nitrate from a solution containing equimolar concentrations of both NO3− and Cr2O72− ions. We note here the in situ esterification of chromate ion converts divalent anion (Cr2O72−) to monovalent anion (CrO3(OMe)−). Furthermore,

Figure 2. Illustrations for chromate sorption by 1: (a) Notice the change in the color of crystals from white to yellow and color of the solution from yellow to colorless; (b) UV−visible absorption spectra of the supernatant solution at various time intervals; notice the decrease in the amount of chromate in solution in terms of the decrease in the absorption intensity from 10−4 M of aqueous chromate solution.

in a more concentrated solution of K2Cr2O7 (0.1 M). The exchange process was found to be unsuccessful in dilute solutions preventing UV−vis studies. The chromate exchanged material in both the cases of 1 and 3 were not suitable for single crystal X-ray analysis, despite repeated attempts. Therefore, complexation reactions were carried with L1/L2 and Ag(I)/ Cd(II) salts in the presence of K2Cr2O7 to obtain assynthesized chromate incorporated crystals. However, we

Figure 3. Analysis of chromate absorption by 1: (a) EDX spectra showing qualitative distribution of elements present in 1 before and after chromate exchange (inset: corresponding crystal images); (b) elemental mapping profile of 1 after anion exchange. 4440

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Figure 5. Illustrations for the desorption of chromate from chromate absorbed 1: (a) notice the increase in the concentration of chromate in terms of absorption intensity in UV spectra, with time, of chromate absorbed 1 dipped in 0.01 M NaBF4 solution; (b) bar-graph showing the efficiency of 1 with the number of adsorption−desorption cycles.

Figure 6. 2D-layers containing anion coordination at an epical position in (a) CP-3 (nitrate) and (b) CP-4 (chromate); (c) chromate sorption by CP-3 when immersed into 0.1 M K2Cr2O7; notice the color change of solid from white to yellow; (d) PXRD patterns of 3, 3′, 4: notice matching of the patterns in 3′ and 4.

PXRD patterns of 4 were found to be same as that of chromate exchanged material 3′, indicating that structure 3′ could be same as that of 4. The exchange of nitrate ion with chromate ions in 3 was found to have a profound influence on their photophysical properties. Optical properties of complexes 3 and 3′ were measured in terms of their solid state UV and luminescence

spectra at room temperature. The solid state UV−visible spectrum of complex 3 shows an absorption band in the region 250−350 nm, while that of complex 3′ covers a much broader range from 250 to 550 nm. Furthermore, luminescence measurements, at an excitation wavelength of 350 nm, display a broad emission band, centered at around 415 nm for complex 3. This emission may be attributed to ligand-to-metal charge 4441

DOI: 10.1021/acs.cgd.7b00771 Cryst. Growth Des. 2017, 17, 4437−4444

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Figure 7. (a) Solid state UV−visible spectra; (b) emission spectra upon excitation at 350 nm for nitrate incorporated CP-3 and chromate absorbed CP-3′.

Figure 8. (a) UV−visible spectra representing uptake of CR by 1; (b) UV−visible spectra showing selective uptake of MO from a mixture of MO +MB by 1, at various time intervals.

transfer transitions.31 However, the luminescent emission dropped rapidly for complex 3′ (Figure 7). This rapid fading of the luminescence emission after anion exchange may be because the electron transfer transitions of CrO42− may decrease the energy transfer from L2 to Cd.32 Notably, the photophysical properties of the as-synthesized material 4 were found to be the same as those of 3′, further signifying the sameness of these two materials.

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whereas for 2, it was found to be 61.44 mg/g (CR) and 34.894 mg/g (MO). Furthermore, 1 was also found to selectively adsorb anionic dye (MO) from a solution containing the mixture of anionic dye (MO) and cationic dye methylene blue (MB) (Figure 8).

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CONCLUSIONS In conclusion, the study highlights the importance of CPs 1−3 in dealing with pollutants that are difficult to remove through established techniques.5 All the complexes were found to have a (4,4) topological 2D networks with smaller cavities given the swastika-shaped building blocks due to the presence of flexible −CH2− groups in L1 and L2. The cationic complexes 1 and 2 incorporate anions (BF4− and ClO4−) in the channels that are formed across the packing of the layers. Interestingly, the packing of the layers in 1 and 2 is governed by β-sheet hydrogen bonds such that they have AA type packing. We note that BF4− and ClO4− ions are not interfered in the amide-toamide hydrogen bonds but form weak C−H···F and CH···O hydrogen bonds, respectively. However, the neutral complexes 3 and 4 do not exhibit such amide-to-amide hydrogen bonding as the anions involved in N−H···O hydrogen bonding with amides. As a result, the 3 and 4 exhibit offset packing (AB type). The complexes 1 and 3 were shown to be good for dye sorption as well as chromate and dichromate sorption from the corresponding aqueous solutions. The efficiency of sorption found to depend on the nature of the anion binding in the assynthesized material. Interestingly, the hydrogen bonded anion

DYE ADSORPTION STUDIES

Additionally, the ability of CPs 1 and 2 for selective removal of non-biodegradable dyes, hazardous water pollutants, from their respective aqueous solutions has been explored.33 With the growing development in the industrial sector, the intensity of pollution is at its peak. Dyes are a major portion of these pollutants and impose severe problems to both aquatic environments and human health. Given the characteristic nature of charge in these dyes, several ionic MOFs and CPs have been established as a promising way out for their removal.34,35 The cationic frameworks (1 and 2) were found to have an ability to adsorb anionic dyes congo red (CR) and methyl orange (MO). In a typical dye sorption experiment, crystals of 1 (20 mg) were immersed in an aqueous solution of dye (CR and MO) (10−4 M) and kept in dark for a while to reach equilibrium. The change in the concentration of respective dye in solution was monitored by UV−visible spectroscopy at different time intervals. The uptake capacity of 1 was found to be 98.08 mg/g for CR and 37.88 mg/g for MO, 4442

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(ClO4− in 2) was found to be more difficult to be exchanged with chromate or dichromate than the anion that is coordinately bound (nitrate in 3) to metal. The complex 1 that has weakly bonded anion in the cavity was found to have a higher tendency for exchange with chromate/dichromate/ anionic dye. Further, the matching of the PXRD patterns of 3′ and 4 indicates that the chromate absorption by 3 is also happening through the in situ esterification of chromate and also into the crystal lattice but not on the surface. This phenomenon has also been testified by IR spectra of 3′ and 4. Lastly, compound 3 has the potential to act as a luminescence sensor for chromate ions as its luminescence is quenched by the absorption of chromate ions.

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where Ci and Ce are the initial and equilibrium concentrations of Cr(VI) in the solution. V is the volume of Cr(VI) solution and W is the mass of the adsorbent. Selective Dye Adsorption. The selective dye adsorption experiment was performed using the cationic dye MB and anionic dye MO. In a typical dye sorption experiment, 10 mg of crystals of 1 was immersed in 3 mL of solution of dye mixture at room temperature. 1 was also found to selectively adsorb the anionic dye MO.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00771. Geometrical parameters of hydrogen bonds, IR spectra, PXRD patterns, and other experimental data (PDF)

EXPERIMENTAL SECTION

General. All commercially available reagents and solvents were used as received. Fumaric acid and 3-picolyl amine were purchased from Sigma-Aldrich, and chemicals such as methyl orange, methylene blue, and various metal salts were purchased from local chemical suppliers. FTIR spectra were recorded with a PerkinElmer instrument. PXRD data were recorded with a Bruker D8-advance diffractometer at room temperature. The solution state absorbance spectra were recorded with the use of Shimadzu (model no. UV-2450) UV−vis spectrophotometer. The solid-state luminescence spectra were collected with a Spex Fluorolog-3 (model FL3-22) spectrofluorimeter. The diffuse reflectance spectra (DRS) of the solid samples were recorded with a Cary model 5000 UV−visible-NIR spectrophotometer. Field-emission scanning electron microscopy (FESEM) was performed on ZEISS EVO 60 with oxford EDS detector, operated at an accelerating voltage of 5−10 kV. Thermogravimetric analysis (TGA) data were recorded under nitrogen atmosphere at a heating rate of 5 °C/min with a PerkinElmer instrument, Pyris Diamond TG/ DTA. Crystal Structure Determinations. All the single crystal data were collected on a Bruker-APEX-II CCD X-ray diffractometer that uses graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at low temperature (100 K) by the hemisphere method. The structures were solved by direct methods and refined by least-squares methods on F2 using SHELX-2014.36 Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were fixed at calculated positions and refined using a riding model. The H atoms attached to the O atom or N atoms are located wherever possible and refined using the riding model. Synthesis of Complexes 1−4. The complexes 1, 2, and 4 were synthesized by direct mixing. Complexation reaction was carried out by mixing methanolic solution of the metal salt (0.05 mmol) to a stirred solution of L1 (14.8 mg, 0.05 mmol) in DCM−methanol. A white precipitate appeared immediately that was dissolved by adding 2−3 drops of NH4OH. The vial was then covered with aluminum foil and left for slow evaporation. These reactions resulted in the formation of crystals of complexes 1 and 2 with yields of 75% and 72% respectively. Complex 4 was synthesized by direct mixing of the MeOH solution (5 mL) of L2 (0.1 mmol, 0.0346 g) with the aqueous solution (5 mL) of the metal salt (0.05 mmol) in the presence of K2Cr2O7 (0.05 mmol). Procedure for Anion Exchange. Twenty milligrams of crystals of 1 were immersed into 10 mL of 10−4 (M) aqueous solution of potassium chromate, and the mixture was kept static at room temperature to reach equilibrium conditions. The anion exchange process was then monitored via UV−visible spectroscopy based on the typical absorption (λmax = 372 nm) for chromate. The intensity of the peak gradually decreases with the decolorization of the chromate solution indicating the adsorption phenomenon. The adsorption capacity of 1 for chromate was determined further by measuring the decolorization rate using the formula,

qe =

ASSOCIATED CONTENT

Accession Codes

CCDC 1542644−1542646 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-3222282252. Tel: +91-3222-283346. ORCID

Kumar Biradha: 0000-0001-5464-1952 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge DST (SERB), New Delhi, India, for financial support and DST-FIST for the single crystal X-ray diffractometer, K.N. thanks IIT KGP, and K.M. thanks UGC for research fellowships.

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REFERENCES

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DOI: 10.1021/acs.cgd.7b00771 Cryst. Growth Des. 2017, 17, 4437−4444

Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.7b00771 Cryst. Growth Des. 2017, 17, 4437−4444