Dynamic Layered Coordination Polymer: Adsorption and Separation

Published as part of the Crystal Growth & Design virtual special issue IYCr 2014 ... The versatile host framework, synthesized from Cd(ClO4)2 and benz...
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Dynamic Layered Coordination Polymer: Adsorption and Separation of Aromatics and I2 by Single Crystals Published as part of the Crystal Growth & Design virtual special issue IYCr 2014 - Celebrating the International Year of Crystallography Gargi Mukherjee and Kumar Biradha* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India S Supporting Information *

ABSTRACT: The versatile host framework, synthesized from Cd(ClO4)2 and benzene-1,3,5-triyltriisonicotinate, has been examined as a potential adsorbent for polycyclic-aromatic hydrocarbon molecules and exhibits adsorptive selectivity based on the shape/size of the molecule in a singlecrystal-to-single-crystal manner. The framework also shows the ability to capture I2 either from its solution or in vapor form.

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adsorptive-based separation has drawn major attention due to its simple, inexpensive, and clean route. Recently, Lang et al. had demonstrated the ability of CPs for the separation of PAHs mixture of naphthalene-anthracene in a single-crystal-to-singlecrystal manner.13 Nishikiori et al. had shown the efficient adsorption and separation of PAHs of a variety of molecules.14 Recently, we have reported a layered CP with pyridine based tripodal ligand, benzene-1,3,5-triyltriisonicotinate (L) with different metal salts, {[M(L)2(H2O)2·(anion)2]·guest·2H2O}n (M = Cu(II), Zn(II), Cd(II), anion = ClO4−, PF6−, SbF6−), that can be prepared in the presence of a wide range of guest molecules such as PAHs, monosubstituted benzenes, and withstands the exchange of metal ions and anions, conserving the structural integrity.15 Interestingly, the same hosts containing volatile guests (CHCl3 and MeOH, Figure 1) were shown to exhibit breathing sorption behavior with N2 and H2 gases.16 Further, these CPs were found to be amenable for fine-tuning the gate opening pressures and uptake capacities in the form of mixed metallic CPs which are synthesized by a solid solution approach. Such a flexibility and solid state structural dynamics of the host CPs will act as an added advantage, over the static hosts, to adsorb and separate variety of guest molecules with different sizes and shapes. Accordingly, in this communication the potential of these materials ({[Cd(L)2(H2O)2]·2(ClO4)·

oordination polymers (CPs) have attracted much attention for their potential applications such as storage,1 separation,2 catalysis,3 molecular sensing,4 and actuators.5 Among these various applications, separation of gases and hydrocarbons are of major interest.6 Adsorption based separation is one of the most important separation technologies extensively used in industries as it can be carried out at ambient conditions and the strategy is free from any side reactions.7 Porous, flexible, and soft coordination polymers, compared to robust and rigid frameworks, are better candidates in this regard.8 Because of their “structural dynamism,” the host framework can act in a switchable way in response to guest molecules.9 However, most of such separations reported to date, using soft frameworks, are restricted to small molecules or benzene analogues.10 CPs capable of separation of larger aromatic molecules such as polycyclic-aromatic hydrocarbons (PAHs) are relatively rare. PAHs are well-known as organic pollutants, which are widely acknowledged for their carcinogenic, mutagenic, and teratogenic properties.11 They are found in air, groundwater, and soil. They are introduced into environment through vehicle exhaust, coal, coal tar, and incomplete combustion of fossil fuels. PAHs are mostly used in dye, plastic, and pesticide industries. In most of the industries, a mixture of these compounds are considered as waste because of the expensive methods for their separation.12 Several methods such as distillation, crystallization, and chromatography, specifically HPLC columns, are designed for their separation, which are not cost-effective due to the amount of eluent and the environmental concerns involved in the process. Implementation of CPs to carry out © 2014 American Chemical Society

Received: May 6, 2014 Revised: June 22, 2014 Published: July 20, 2014 3696

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Figure 1. Illustrations of the crystal structure of 1: (a) structure of layered host network, channels propagating through the layers (b) channel, central phenyl rings (shown in space filling mode) facilitates the guest adsorption through aromatic interactions.

2CHCl3·2CH3OH·4(H2O)}n), 1) for adsorption of PAH and I2 as well as the separation of PAH mixtures will be reported. The crystals of complex 1 gradually lose the solvent molecules when exposed to air; however they are found to be stable and insoluble when dipped in solvents such as benzene, hexane, chloroform, and dichloromethane. Further, the crystal structure of the CHCl3-dipped crystals ({[Cd(L)2(H2O)2]· 2(ClO4)·4CHCl3}n), 2) was determined, and it was found that it has a similar structure but with different proportion of CHCl3 molecules. In a similar manner, the crystals of 1 (20 mg, 0.012 mmol) were immersed into the DCM solutions (3 mL) of pyrene (1a) (0.026 mmol) or triphenylene (1b) (0.026 mmol) to find out the adsorption of the respective PAH molecules into the crystal lattices. The crystals were found to be intact in terms of their quality, size, and morphology, albeit unstable (in air) colorless crystals turned to be stable pale-yellow colored crystals in the case of 1a. The powder X-ray diffraction (PXRD) patterns of the immersed crystals were found to be in agreement with those of respective as-synthesized crystals (Figures 2 and S1). In fact, the single crystal X-ray diffraction of the pyrene-sorbed crystal (1a) confirms the presence of pyrene (highly disordered) in the lattice. Further, the complexes 1a and 1b were characterized by spectroscopic methods (1H NMR, luminescence) (Figure 2). Solid state luminescence study, at the excitation wavelength of 325 nm, was carried out on complexes 1, 1a, and 1b at room temperature (Figure 2e) to notice the differences in their emission properties due to guest inclusion. The host material (1) was found to be nonemissive. However, complex 1a exhibits intense emission bands at 467 and 490 nm. As for complex 1b, a sharp band appears at 435 nm. The luminescent emission of the complexes 1a and 1b are attributed to the presence of pyrene and triphenylene guest molecules in the host crystals. The host/guest ratio for 1a calculated from the 1 H NMR is 1:0.62 (1:0.35 for 1b), which is less than that of assynthesized complex (1:1). The crystal structure of 1a reveals significant contraction of the unit cell from that of 1 (Figure 3, Table S1). Adsorption of pyrene by 1, reduces c-axis length and unit cell volume by 6.50% and 6.92%, respectively, and lengthens the b-axis by 0.87%. Despite the presence of guest molecules as big as pyrene, such a remarkable contraction of caxis and volume can be attributed to the strong interactions between the central phenyl ring of ligand and the guest molecule, which acts as a gluing agent and bring the layers closer. An appreciable change in intensity and shifting of peaks from lowest angle reflection to higher Bragg angles also indicates the contraction of structure upon pyrene inclusion (Figure S1, Supporting Information).17 This observation throws some light onto guest-induced adaptability of the host

Figure 2. 1H NMR spectra of (a) host 1; separation of (b) pyrene from naphthalene−pyrene (1:1) mixture and (c) triphenylene from naphthalene−triphenylene (1:1) mixture. (d) PXRD patterns and (e) luminescence spectra of 1 (dark yellow), 1a (red), and 1b (blue).

Figure 3. Schematic representation of structural change of host, triggered by the encapsulation of pyrene. Elongation and contraction are represented by green and red arrows, respectively.

layers. Since the larger molecules have better interaction with the host, contraction is more feasible than expansion. In addition to the adsorption of large PAH molecules, 1 is suitable for the sorption of small aromatic molecules, as well. Crystals of 1 were found to include other hydrocarbons such as naphthalene, anthracene, phenanthrene, naphthols, and xylene from the CH2Cl2 solution of target molecules, and in all these cases the inclusions were characterized by 1H NMR (Figures S2−S8, Supporting Information). In addition to the remarkable ability of 1 to entrap aromatic molecules of various sizes while maintaining its single crystal character, it was also found to exhibit a very interesting sieving property. Competitive guest encapsulation analyses was conducted by dipping the crystals of 1 in CH2Cl2 solution containing taking two PAH molecules, namely, naphthalene and pyrene or triphenylene. A sample of 1 (20 mg) was immersed into a dichloromethane solution of equimolar (0.026 mmol) mixtures of naphthalene-pyrene, and naphthalenetriphenylene for 1 day at room temperature. The colorless crystals of 1 turned pale yellow for the naphthalene−pyrene solution and remained colorless for the naphthalene− triphenylene solution. Preferential uptake of pyrene/triphenylene over naphthalene was confirmed by 1H NMR spectra (Figures S9−S10, Supporting Information). It is interesting to note here that the host system can also selectively capture pyrene from a 2:1 molar mixture of naphthalene and pyrene (Figure S11, Supporting Information). In order to verify whether the differences in shape and size of the aromatic 3697

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which a sudden weight loss at 50 °C (12%) was observed (Figure S18, Supporting Information). Solid state UV−vis spectra of 3 was recorded at room temperature and compared with that of 1 (Figure 4e). A single peak at 280 nm was observed for complex 1, whereas in the case of complex 3, a broad band appeared at the visible region between 265 and 500 nm with three shoulder peaks at 263, 310, and 369 nm. Similar features in UV−vis spectra were detected earlier for I2 included host materials, which confirms the inclusion of I2 in 3.22 Finally, the crystal of 3 was diffracted coated with paratone oil at 100 K. The iodine molecules were found to be disordered. It is also possible to load I2 in the crystals of complex 1 via vapor sublimation. Pristine crystals of complex 1 were introduced in a sealed vessel containing iodine granules at ambient condition. Loading of I2 was revealed as the colorless crystals gradually turned almost black over time. This experiment suggests that complex 1 is capable of adsorbing guests not only from solvents; even it can adsorb volatile guests also. We addressed the stability of 3 by washing the crystals with different solvents. In most of the reports, the loss of iodine from the systems occurs when the crystals are exposed to the atmosphere or immersed in organic solvents.23 However, crystal 3 is stable in air for months and even in boiling water (Figure S19, Supporting Information). These crystals can even withstand common organic solvents such as hexane, benzene, and chloroform except methanol without any loss of iodine. Solid-state emission spectra of complex 1 and 3 were measured at room temperature, and it was observed that the photoluminescence of 3 was markedly modified upon iodine encapsulation (Figure 4f). Upon excitation at 450 nm, L did not show any emission. Complex 1 showed two emission bands at 425 and 506 nm when excited at 325 nm. The emission spectra of the light violet crystals of 3 showed a red shift of ∼100 nm (from 440 to 544) with respect to 1, and in addition to some small shoulders at 406, 464 nm, a strong new band appeared at 518 nm. The observed red shift of band after the iodine uptake may result from the decrease in the energy gap of HOMO and LUMO from 1 to 3, which indicates to the effective interaction between the host and the guest.23c To conclude, we have synthesized a dynamic host layer, 1 which can act as an aromatic adsorbent with retention of single crystallinity due to structural flexibility. 1 can encapsulate a wide variety of guests by adjusting the channel size. This feature of 1 can be used to modulate its properties just by systematic variation of guest molecules, but the complex preferentially captures the molecule with larger size from mixture of PAHs based on thermodynamic selectivity. This selective capture can be advantageous for separation purpose. It can even be used for effective capture and storage of radioactive iodine from both solution and gaseous states. It is important to note here that the complex does not require any prior activation to perform. The pristine sample can be directly used for the adsorption study. This is the first time that a single host has been used to encapsulate guest molecules including solid (PAH), solvent (xylene), and vapor (I2) by the exploitation of the structural dynamicity of the complex.

hydrocarbons were a reason for the selective adsorption, competitive guest encapsulation analysis was performed with 1 using equimolar mixtures of pyrene and triphenylene in similar way. Remarkably, inclusion of both pyrene and triphenylene was observed by 1H NMR analyses (Figure S12, Supporting Information). From the above studies, it is evident that the different affinities were ascribed to “shape/size responsive fitting” of PAH molecule. The molecule having the larger aromatic plane exhibits stronger interaction with the framework and is preferentially retained by the host lattice. As part of our study with complex 1 to capture the environmentally hazardous organic pollutants, we have also explored its potential for iodine confinement. Iodine (I2) is an extremely volatile gas and a well-known radioactive pollutant (I-129 and I-131).18 I-131 has a half-life of 8 days and causes thyroid problems. The effective capture and storage of radioactive iodine has attracted considerable attention.19 When the as-synthesized crystals (20 mg) of complex 1 was transferred directly from mother liquor into CH2Cl2 solution of I2 in a sealed vial, the colorless crystals gradually turned light violet to dark violet in 10 h and resulted in complex 3 {[Cd(L)2(H2O)2]·(ClO4)2·2(CH2Cl2)·4(H2O)·(I2)} (Figure 4).20 For further characterization, IR, PXRD, and Raman

Figure 4. Photographs of complex 1: (a) before and (b) after I2 exposure, (c) ground crystals of I2 exposed sample (3). (d) PXRD patterns, (e) solid state UV−vis spectra, and (f) luminescence spectra of 1 (dark yellow) and 3 (purple).

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spectra were recorded (Figures 4, S16 and S17, Supporting Information). The PXRD pattern of 3 clearly demonstrated that the crystallinity was maintained even after iodine inclusion, and it differs significantly from that of 1, suggesting the possible adsorption of iodine by the host.21 Further, TGA of 3 was also found to differ from that of 1: in the case of 3, a gradual weight loss (∼16%) over 100−250 °C was observed in contrast to 1, in

ASSOCIATED CONTENT

S Supporting Information *

Synthesis procedures and characterization of complexes by 1H NMR, PXRD patterns. 1H NMR, UV, luminescence and for inclusion of PAH and iodine and separation of PAH. This 3698

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(16) Mukherjee, G.; Biradha, K. Chem. Commun. 2014, 50, 670−672. (17) Henke, S.; Schneemann, A.; Wuts̈ cher, A.; Fischer, R. A. J. Am. Chem. Soc. 2012, 134, 9464−9474. (18) (a) Lee, W. E.; Ojovan, M. I.; Stennett, M. C.; Hyatt, N. C. Adv. Appl. Ceram. 2006, 105, 3−12. (b) Health effects - agency for toxic substances and disease registry. www.atsdr.cdc.gov/toxprofiles/tp158c3.pdf. (19) (a) Krumhansl, J. L.; Nenoff, T. M. Appl. Geochem. 2011, 26, 57−64. (b) Chapman, K. W.; Chupas, P. J.; Nenoff, T. M. J. Am. Chem. Soc. 2010, 132, 8897−8899. (c) Haefner, D. R.; Tranter, T. J. Methods of Gas Phase Capture of Iodine from Fuel Reprocessing Off-Gas: A Literature Survey; INL/EXT-07-12299; Idaho National Laboratory: Idaho Falls, ID, 2007. (20) The formula was interpreted based on thermogravimetry (TG) and single crystal X-ray data collected at low temperature (100 K). (21) Martí-Rujas, J.; Islam, N.; Hashizume, D.; Izumi, F.; Fujita, M.; Kawano, M. J. Am. Chem. Soc. 2011, 133, 5853−5860. (22) Usseglio, S.; Damin, A.; Scarano, D.; Bordiga, S.; Zecchina, A.; Lamberti, C. J. Am. Chem. Soc. 2007, 129, 2822−2828. (23) (a) Lang, J. P.; Xu, Q. T.; Yuan, R. X.; Abrahams, B. F. Angew. Chem., Int. Ed. 2004, 43, 4741−4741. (b) Wang, Z. M.; Zhang, Y. J.; Liu, T.; Kurmoo, M.; Gao, S. Adv. Funct. Mater. 2007, 17, 1523−1536. (c) Zeng, M. H.; Wang, Q. X.; Tan, Y. X.; Hu, S.; Zhao, H. X.; Long, L. S.; Kurmoo, M. J. Am. Chem. Soc. 2010, 132, 2561−2563. (d) Liu, Q.-K.; Ma, J.-P.; Dong, Y.-B. Chem. Commun. 2011, 47, 7185−7187. (e) Zhang, Z.-J.; Shi, W.; Niu, Z.; Li, H.-H.; Zhao, B.; Cheng, P.; Liao, D.-Z.; Yan, S.-P. Chem. Commun. 2011, 47, 6425−6427.

material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +91-3222-282252. Tel: +91-3222-283346. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the DST for the financial support and DST-FIST for the single-crystal X-ray facility. G.M. thanks CSIR for research fellowship.

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