Cd(II)-Schiff-Base Metal–Organic Frameworks: Synthesis, Structure

ARTICLE pubs.acs.org/crystal

Cd(II)-Schiff-Base Metal Organic Frameworks: Synthesis, Structure, and Reversible Adsorption and Separation of Volatile Chlorocarbons Jing Xiao, Cheng-Xia Chen, Qi-Kui Liu, Jian-Ping Ma, and Yu-Bin Dong* College of Chemistry, Chemical Engineering and Materials Science, Key Laboratory of Molecular and Nano Probes, Engineering Research Center of Pesticide and Medicine Intermediate Clean Production, Ministry of Education, Shandong Provincial Key Laboratory of Clean Production of Fine Chemicals, Shandong Normal University, Jinan 250014, People’s Republic of China

bS Supporting Information ABSTRACT: A new Cd(II)-MOF (1) was successfully synthesized on the basis of a new long double Schiff-base ligand and Cd(SCN)2. Its crystal structure was determined by X-ray diffraction. In 1, each pair of Cd(II) nodes is bridged by two inversely related μ-SCN to form infinite inorganic chains with the remaining two trans-positions being occupied by two Schiffbase ligands to generate a 3D framework with rhombus-like channel (∼9  9 Å). 1 can reversibly absorb CH2Cl2 and CHCl3 at different temperature without collapsing of the host framework. Furthermore, it displays a strict size selectivity and can completely separate these volatile chlorocarbons of CH2Cl2 and CHCl3 in liquid and vapor phases.

’ INTRODUCTION As typical porous materials, metal organic frameworks (MOFs) are particularly attractive due to their potential applications in adsorption and separation.1 Among the reported MOFs, they are widely used as molecular vessels for gas adsorption and storage, such as hydrogen, methane, or carbon dioxide and so on.2 In contrast, the efforts focused on the adsorption and separation of organic halocarbons are really rare,3 although a handful MOFs that are able to selectively adsorb and separate organic hydrocarbons have been reported very recently.4 Chlorocarbons, such as CH2Cl2 and CHCl3, are very important materials in organic industrial chemistry. In addition, recent research indicates that halocarbons, as high volatile and trace atmospheric species, can cause chemical and radiative change in the atmosphere.5 Thus, their separation and isolation are of high value.6 Unfortunately, because of their very similar physical and chemical properties, they are very hard to completely separate by the traditional methods such as chromatography and distillation. Organic inorganic hybrid MOFs with well-defined inner cavities and desired functionalized microenvironment have a new chemical phase, which might provide an alternative approach to effectively separate these high volatile chlorocarbons under ambient conditions and, furthermore, provide fundamental data for industrial and environmental application. Our research group has been exploring the reversible adsorption and separation of organic molecules based on porous MOFs and discrete molecular containers.7 Motivated by our interest in molecular adsorption and separation based on MOFs, we have initiated a synthetic research program for preparation of the porous heteroatom-rich MOFs, in which the double Schiff-base bridging organic spacers with ether side arms are chosen as the building blocks. The porous MOFs containing heteroatom-rich rungs would incorporate polar organic species into pores in a r 2011 American Chemical Society

crown ether manner.8 In this contribution, we report a new Cd(II)-MOF based on a new double Schiff-base ligand and Cd(SCN)2, and its reversible adsorption and complete separation of volatile CH2Cl2 and CHCl3 in liquid and vapor phases.

’ EXPERIMENTAL SECTION Materials and Methods. Cd(SCN)2 (Acros) was used as obtained without further purification. Infrared (IR) samples were prepared as KBr pellets, and spectra were obtained in the 400 4000 cm 1 range using a Perkin-Elmer 1600 FTIR spectrometer. Elemental analyses were performed on a Perkin-Elmer model 2400 analyzer. 1H NMR data were collected using an AM-300 spectrometer. Chemical shifts are reported in δ relative to TMS. All fluorescence measurements were carried out on a Cary Eclipse spectrofluorimeter (Varian, Australia) equipped with a xenon lamp and quartz carrier at room temperature. Thermogravimetric analyses were carried out using a TA Instrument SDT 2960 simultaneous DTA-TGA under flowing nitrogen at a heating rate of 10
C/min. XRD pattern was obtained on a D8 ADVANCE X-ray powder diffractometer (XRD) with Cu Kα radiation (λ = 1.5405 Å). Synthesis of L. A mixture of A (4.08 g, 0.017 mol) and K2CO3 (23.46 g, 0.17 mol) in DMF (25 mL) was combined with a mixture of 3-(chloromethyl)pyridine hydrochloride (5.57 g, 0.034 mol) and KI (0.99 g, 0.006 mol) in DMF (5 mL). The reaction solution was stirred at room temperature and monitored by TLC. The product was purified by column to generate L in 44% yield. Mp 153 155
C. IR (KBr pellet, cm 1): 3421 (m), 1602 (s), 1556 (m), 1540 (m), 1462 (s), 1413 (m), 1384 (w), 1251 (m), 1191 (s), 1157 (m), 992 (w), 876 (s), 827 (w), 698 (m). 1H NMR (300 MHz, CDCl3, 25
C, TMS, ppm): 8.75 (s, 2H, C5H4N), 8.63 (d, 2H, C6H4), 8.15 (d, 2H, C5H4N), Received: September 17, 2011 Revised: October 26, 2011 Published: October 28, 2011 5696

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’ RESULTS AND DISCUSSION

7.89 7.86 (q, 2H, C6H4), 7.56 (d, 2H, C5H4N), 7.43 (d, 2H, C6H4), 7.40 (s, 2H, C6H4 ), 7.10 7.09 (q, 2H, C5H4N), 5.18 (s, 4H, CH2 ). Anal. Calcd for C26H22N4O2: C, 73.92; H, 5.25; N, 13.26. Found: C, 73.65; H, 5.11; N, 13.58. Synthesis of 1. A solution of Cd(SCN)2 (1.5 mg, 0.006 mmol) in MeOH (10 mL) was layered onto a solution of L (8.0 mg, 0.019 mmol) in CH2Cl2 (8 mL). The solutions were left for about 1 week at room temperature, and yellow block-like crystals were obtained. Yield, 45%. IR (KBr pellet, cm 1): 3420 (m), 1557 (m), 1491 (s), 1413 (m), 1384 (w), 1251 (m), 1191 (s), 1157 (m), 992 (w), 955 (w), 827 (w), 734 (m), 698 (m). Anal. Calcd for C58H48Cd2Cl4N12O4S4: C, 47.33; H, 3.29; N, 11.42. Found: C, 47.13; H, 3.09; N, 11.22. Single-Crystal Structure Determination. A suitable single crystal of 1 was selected and mounted in air onto thin glass fibers. X-ray intensity data of 1 were measured at 173 K on a Bruker SMART APEX CCD-based diffractometer (Mo Kα radiation, λ = 0.71073 Å). The raw frame data for 1 were integrated into SHELX-format reflection files and corrected for Lorentz and polarization effects using SAINT.9 Corrections for incident and diffracted beam absorption effects were applied using SADABS.9 The crystal showed no evidence of crystal decay during data collection. The structure was solved by a combination of direct methods and difference Fourier syntheses and refined against F2 by the full-matrix least-squares technique. Crystal data, data collection parameters, and refinement statistics for 1 are listed in Table 1. Relevant interatomic bond distances and bond angles for 1 are given in Table 2.

Ligand. The Schiff-base ligand L (1,4-bis(3-phenoxymethylene-(3-pyridyl))-2,3-diaza-1,3-butadiene) was expediently prepared in moderate yield by the combination of double Schiff-base spacer A (1,4-bis(3-phenol)-2,3-diaza-1,3-butadiene) and 3(chloromethyl)pyridine in the presence of K2CO3/KI at room temperature. As shown in Scheme 1, L is a symmetric ligand that contains two flexible ether bridging pyridyl coordination sites. L is soluble in common polar organic solvents such as CH2Cl2, CHCl3, THF, MeOH, and so on, which facilitates the reactions between it and metal ions in solutions. Synthesis and Structure. As shown in Scheme 2, the combination of L with Cd(SCN)2 in a CH2Cl2/MeOH mixed solvent system affords compound 1 ([Cd(L)(SCN)2] 3 CH2Cl2) as yellow rod-like crystals in 45% yield. 1 is insoluble in water or common organic solvents because of its polymeric nature but is dissolved by DMF and DMSO. X-ray single-crystal analysis indicates that compound 1 crystallized in a monoclinic space group C2/c. The local coordination sphere of Cd(II) can be described as a distorted octahedron {CdN4S2} (d(Cd(1) S(1)) = 2.712(1) Å; d(Cd(1) N(1)) = 2.361(4) Å; and d(Cd(1) N(3)) = 2.314(4) Å) completed by two N-donors from L, two S, and two N atoms from thiocyanate anion (Figure 1). Adjacent Cd(II) atoms are linked together by two inversely related μ-1,3-SCN anionic ligands, forming a neutral {Cd(SCN)2}n metal thiocyanate inorganic polymeric chain extended along the crystallographic c axis, which is shown in Figure 2. The adjacent eight-membered (S C N Cd)2 rings are perpendicular to each other. The Cd 3 3 3 Cd distance within the ring is ca. 5.8 Å, and the S C and C N bond lengths are 1.640(4) and 1.156(6) Å, respectively. This observation is consistent with the fact that the SCN ligand is easily polarizable ( S CtNTSdCdN ). These {Cd(SCN)2}n columns are connected to each other by L in the crystallographic ab plane to generate a 3D MOF with a 4-connected uninodal net (diam, 66 topology) (Figure 3a). As shown in Figure 3b, the 3D framework contains rhombuslike channels resulting from the ligand zigzag conformation. The open channel of ∼9  9 Å in dimensions extends along the crystallographic c axis and is filled with CH2Cl2 guest molecules. Notably, the framework is embedded N and O atoms to result in the channels being heteroatom-rich. Such inner surface is perfect for binding polar guest species through hydrogen-bonding interactions. Upon close inspection of the structure, we found that the encapsulated guest CH2Cl2 molecules are stabilized in the

Table 1. Crystallographic Data for 1 C29H24CdCl2N6O2S2, 1 735.96

cryst syst

monoclinic

a (Å) b (Å)

23.712(5) 12.822(3)

c (Å)

10.888(2)

α (deg)

90

β (deg)

112.132(2)

γ (deg)

90

V (Å3)

3066.5(10)

space group

P2(1)/c

Z value Fcalc (g/cm3)

4 1.594

μ (Mo Kα) (mm 1)

1.061

temp (K)

173(2)

reflections collected/unique

7904/2881 [R(int) = 0.0355]

final R indices [I > 2σ(I)]: R, Rw

0.0425, 0.1006

|Fc /∑|Fo|. wR2 = {∑[w(Fo2 )

R1 = ∑ Fo| )

a

empirical formula fw

Fc2)2]/∑[w(Fo2)2]}1/2.

Table 2. Interatomic Distances (Å) and Bond Angles (deg) with esd’s for 1a 1 N(3) Cd(1)#2 Cd(1) S(1) N(3)#3 Cd(1) N(3)#2 N(3)#3 Cd(1) N(1)

a

2.314(4) 2.7124(11)

Cd(1) N(1)#4 N(2) N(2)#5

174.31(19) 85.98(12)

N(3)#3 Cd(1) S(1)#4

96.48(9)

N(1)#4 Cd(1) S(1)#4

172.63(8)

2.361(3) 1.406(6)

N(3)#3 Cd(1) N(1)#4

89.85(12)

N(1)#4 Cd(1) N(1)

85.84(15)

N(3)#2 Cd(1) S(1)#4

87.44(10)

N(1) Cd(1) S(1)#4

90.83(8)

N(3)#3 Cd(1) S(1)

87.44(10)

N(3)#2 Cd(1) S(1)

N(1)#4 Cd(1) S(1)

90.83(8)

N(1) Cd(1) S(1)

S(1)#4 Cd(1) S(1)

93.21(5)

96.48(9) 172.63(9)

Symmetry transformations used to generate equivalent atoms: #1, x + 1, y, z + 1; #2, x + 1, y + 1, z + 1; #3, x, y + 1, z + 1/2; #4, x + 1, y, z + 3/2; #5, x + 1/2, y + 3/2, z + 3. 5697

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Scheme 1. Synthesis of L

Scheme 2. Synthesis of 1

Figure 1. The ORTEP figure of 1.

Figure 2. Side (left) and top (right) views of {Cd(SCN)2}n metal thiocyanate inorganic polymeric chain.

channels through very weak C H 3 3 3 Cl (dH(1) 3 3 3 Cl(1) = 3.051(1) Å, dC(1) 3 3 3 Cl(1) = 3.912(2) Å, and — C(1) H(1) Cl(1) = 151.25
) and C H 3 3 3 O (d H(15B) 3 3 3 O(1) = 2.946(1) Å, dC(15) 3 3 3 O(1) = 3.912(2) Å, — C(15) H(15B) O(1) = 162.35
) hydrogen bonds10 (Figure 4). Thermogravimetric analysis (TGA, Figure 5) revealed that the encapsulated CH2Cl2 could be removed at temperatures ranging from 50 to 150
C (calculated 8.9%, observed 9.9%), which is further demonstrated by the 1H NMR spectrum (Figure 6b). The desolvated sample of 1 was obtained by heating crystals of 1 at 110
C. The XRPD pattern of 1 shows that the shapes and intensities of some reflections are identical to those of the original sample (Figure 6a,b). This means that guest loss does not result in symmetry change or cavity volume collapse.

In addition, the solvent-free framework 1 is able to reabsorb CH2Cl2 in both vapor and liquid phases to re-form the original solvent filled host CH2Cl2⊂Cd(L)(SCN)2. As is shown in Figure 6c, when the desolvated solids 1 are suspended in CH2Cl2 vapor for 4 days at room temperature (the solid samples were dried in air and then dissolved in DMSO-d6 for 1H NMR measurements), the 1H NMR spectra (δ = 5.74 ppm) indicate the CH2Cl2 guest molecules were reincorporated into the framework to regenerate CH2Cl2⊂Cd(L)(SCN)2 (1c). The reabsorption amount in vapor phase is around 79% based on 1H NMR spectrum and elemental analysis. Similarly, when the desolvated solids of 1 are immersed in CH2Cl2 solvent for 4 days, the 1H NMR spectrum performed on the resulted solid sample in DMSO-d6 indicates the CH2Cl2 (δ = 5.74 ppm) guest molecules were reabsorbed into the framework (1d). The restored amount is up to ∼90% in liquid phase under experimental conditions. Furthermore, the framework of 1 could be reused, much like a zeolite, and showed reversible adsorption and desorption of CH2Cl2 guest. Such property would lead to practical applications. The XRPD patterns based on the regenerated samples of 1c,d confirm that the Cd(L)(SCN)2 framework is stable during these reversible adsorption processes (Figure 6c,d). Because 1 is insoluble in CH2Cl2, the possibility of a dissolution recrystallization mechanism to explain the solvent guest reabsorption is unlikely. 5698

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Figure 3. (a) Schematic representation of 3D diamond net of 1. (b) The solid-state crystal structure of 1.

Figure 4. CH2Cl2 guest in the channel is stabilized by weak hydrogenbonding interactions.

Figure 6. 1H NMR spectra (DMSO-d6) and corresponding XRPD patterns recorded at room temperature: (a) The as-synthesized sample of 1. (b) The solvent free solid samples of 1 heated at 110
C. (c) The desolvated solids 1 suspended in CH2Cl2 vapor for 4 days and dried at room temperature. (d) The desolvated solids 1 immersed in CH2Cl2 liquid for 4 days and dried at room temperature. The proton resonance of the encapsulated CH2Cl2 molecule is marked in the 1H NMR spectra. Figure 5. TGA trace of 1. The observed solvent mass loss is 9.9%, and the calculated solvent mass loss is 8.9%.

Equally important to reversible adsorption is guest selectivity for capturing specific guest substrate in the presence of other competitors. To explore the possibility of separation chlorocarbons in

vapor phase, the crystals of 1 were exposed to a mixed vapor that consists of equimolar amounts of CH2Cl2 and CHCl3 at room temperature for 4 days, and only CH2Cl2 was allowed into the pores of 1 to result in CH2Cl2⊂Cd(L)(SCN)2 (Figure 7), which is well supported by 1H NMR spectra. For example, as indicated 5699

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Figure 7. The separation of CH2Cl2 and CHCl3 based on Cd(L)(SCN)2 in both vapor and liquid phases.

Figure 8. 1H NMR spectrum (left) and XRPD pattern (right) of CHCl3⊂Cd(L)(SCN)2.

in Figure 7, no proton resonance related to the CHCl3 is observed. In addition to vapor-phase separation, 1 is still able to completely separate CH2Cl2 and CHCl3 in the liquid phase at room temperature. When the crystals of 1 were immersed in a mixed solvent of CH2Cl2 and CHCl3 (molar ratio 1:1) at room temperature for 4 days, the 1H NMR spectrum clearly evidenced that only the CH2Cl2 was taken up by 1 (Figure 7). Herein, guest dimension might be the dominating factor instead of their vapor presures and polarity. The experiment demonstrates that the empty framework of 1 cannot absorb CHCl3 at room temperature; however, it can trap CHCl3 at higher tempature (∼40
C). The 1H NMR spectrum indicated that only a trace amount of CHCl3 was taken up at ∼40
C (Figure 8). So the adsorption is a dynamic process, and the guest dimension is a deciding factor for this selective sorption. Besides 1H NMR and XRPD, emission spectrum was also used to monitor the adsorption process. In comparison with the emission for free L, the emission of desolvated Cd(L)(SCN)2 might be assigned to the ligand-centered (n π* or π π*) emission because similar emission bands are observed.11 For Cd(L)(SCN)2 (λem = 547 nm), 0.79CH2Cl2⊂Cd(L)(SCN)2 (λem = 548 nm), and CH2Cl2⊂Cd(L)(SCN)2 (λem = 543 nm), the emission colors are in the range of 543 548 nm upon excitation at 466 nm, corresponding to the typical green colors. The emission intensities, however, are different. As shown in Figure 9, the emission intensities are orderly reduced from the empty framework to the full filled framework. As mentioned above, the CH2Cl2 guests are very weakly bound to the framework,

Figure 9. Solid-state emission spectra of empty Cd(L)(SCN)2 (a), 0.79CH2Cl2⊂Cd(L)(SCN)2 (b), and CH2Cl2⊂Cd(L)(SCN)2 (c).

so the observed emission quenching could be attributed to nonrediative energy transition.12 As was indicated in Figure 9, CH2Cl2⊂Cd(L)(SCN)2 exhibits the lowest emission intensity. Logically, the more guest encapsulating would lead to more nonradiative transition, and, consequently, the lower emission intensity.

’ CONCLUSION In summary, a new host guest supramolecular complex CH2Cl2⊂Cd(L)(SCN)2 has been synthesized on the basis of a new symmetric double Schiff-base ligand L and Cd(SCN)2. Compound 1 features a three-dimensional network and contains rhombic channels (∼9  9 Å). Notably, the Cd(L)(SCN)2 host 5700

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Crystal Growth & Design is robust and able to reversibly adsorb volatile chlorocarbons. More importantly, it displays a clear preference for these volatile chlorocarbons (CH2Cl2 > CHCl3) on the basis of dimension and can effectively separate them under mild conditions. In addition, the loaded Cd(L)(SCN)2 complex exhibits guest-driven luminescence, which might be applied as a sensor for volatile chlorocarbons.

’ ASSOCIATED CONTENT

bS

Supporting Information. Crystallographic details (CIF). This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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’ ACKNOWLEDGMENT We are grateful for financial support from the NSFC (grant nos. 91027003 and 21072118), “PCSIRT”, the 973 Program (grant no. 2012CB821705), and the Shangdong Natural Science Foundation (grant no. JQ200803). ’ REFERENCES (1) (a) Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C. B.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2010, 43, 58. (b) Zhao, D.; Timmons, D. J.; Yuan, D.; Zhou, H.-C. Acc. Chem. Res. 2011, 44, 123. (c) Chen, B.; Xiang, S.; Qian, G. Acc. Chem. Res. 2010, 43, 1115. (d) Dinca, M.; Long, J. R. Angew. Chem., Int. Ed. 2008, 47, 6766. (2) (a) Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C. B.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2010, 43, 58. (b) Bourrelly, S.; Llewellyn, P.; Serre, C.; Millange, F.; Loiseau, T.; Ferey, G. J. Am. Chem. Soc. 2005, 127, 13519. (c) Panella, B.; Hirscher, M.; Puetter, H.; Mueller, U. Adv. Funct. Mater. 2006, 16, 520. (d) Liu, Y.; Eubank, J. F.; Cairns, A. J.; Eckert, J.; Kravtsov, V. C.; Luebke, R.; Eddaoudi, M. Angew. Chem., Int. Ed. 2007, 46, 3278. (e) Ma, S.; Zhou, H.-C. Chem. Commun. 2010, 46, 44. (3) So far, there are only a few examples of reversible adsorption and separation of halocarbons based on discrete molecular containers, see: (a) Dong, Y.-B.; Zhang, Q.; Liu, L.-L.; Ma, J.-P.; Tang, B.; Huang, R.-Q. J. Am. Chem. Soc. 2007, 129, 1514. (b) Han, Y.-F.; Jia, W.-G.; Lin, Y.-J.; Jin, G.-X. Angew. Chem., Int. Ed. 2009, 48, 1. (4) (a) Deng, H.; Doonan, C. J.; Furukawa, H.; Ferreira, R. B.; Towne, J.; Knobler, C. B.; Wang, B.; Yaghi, O. M. Science 2010, 327, 846. (b) Wang, X.; Liu, L.; Jacobson, A. J. Angew. Chem., Int. Ed. 2006, 45, 6499. (c) Alaerts, L.; Maes, M.; Giebeler, L.; Jacobs, P. A.; Martens, J. A.; Denayer, J. F. M.; Kirschhock, C. E. A.; De Vos, D. E. J. Am. Chem. Soc. 2008, 130, 14170. (d) Cychosz, K. A.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2008, 130, 6938. (e) Alaerts, L.; Kirschhock, C. E. A.; Maes, M.; van der Veen, M. A.; Finsy, V.; Depla, A.; Martens, J. A.; Baron, G. V.; Jacobs, P. A.; Denayer, J. F. M.; De Vos, D. E. Angew. Chem., Int. Ed. 2007, 46, 4293. (f) Xu, G.; Zhang, X.; Guo, P.; Pan, C.; Zhang, H.; Wang, C. J. Am. Chem. Soc. 2010, 132, 3656. (g) Chen, B.; Liang, C.; Yang, J.; Contreras, D. S.; Clancy, Y. L.; Lobkovsky, E. B.; Yaghi, O. M.; Dai, S. Angew. Chem., Int. Ed. 2006, 45, 1390. (h) Xiang, S.-C.; Zhang, Z.; Zhao, C.-G.; Hong, K.; Zhao, X.; Ding, D.-R.; Xie, M.-H.; Wu, C.-D.; Das, M. C.; Gill, R.; Thomas, K. M.; Chen, B. Nat. Commun. 2011, 2, 204. (5) Prinn, R. G.; Weiss, R. F.; Fraser, P. J.; Simmonds, P. G.; Cunnold, D. M.; Alyea, F. N.; O’Doherty, S.; Salameh, P.; Miller, B. R.; Huang, J.; Wang, R. H. J.; Hartley, D. E.; Harth, C.; Steele, L. P.; 5701

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