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Tetrahedral tetrazolate frameworks for high CO2 and H2 uptake† Fei Wang, Duan-Chuan Hou, Hui Yang, Yao Kang and Jian Zhang*
DOI: 10.1039/c3dt53269k
Three tetrahedral tetrazolate frameworks with two different 4-connected topologies including lonsdaleite (lon, for 1) and diamond (dia, for 2 and 3) have been synthesized, and the lon-type framework with high
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CO2 and H2 uptake capacity can irreversibly transform to the dia-type framework via solvent-exchange.
Introduction Metal–organic frameworks (MOFs) are a new class of network solids that have great potential in specific applications in clean energy, most significantly as storage media for gases such as hydrogen and methane, and as high-capacity adsorbents to meet various separation needs.1–3 Among numerous known MOFs, a family of materials that closely resemble zeolite topologies, namely zeolitic imidazolate frameworks (ZIFs), are constructed by the tetrahedrally coordinated divalent cations (M2+ = Zn2+ or Co2+) and the uninegative imidazolate ligands (im−).4,5 Compared to those imidazole derivatives, the tetrazole derivatives can adopt a similar μ2-bridging coordination mode to link tetrahedral divalent metal centers (Scheme 1). Of particular interest is that there are two uncoordinated N-heteroatom sites for each μ2-tetrazole ligand, which are potential functional sites for gas sorption or catalysis. Although various tetrazole-based MOFs have been reported,6,7 it is still an ongoing challenge to create zeolite-like metal–tetrazole frameworks with uncoordinated N-donor sites for gas sorption and separation. In this work, we report four tetrahedral tetrazolate frameworks with the typical framework formula Zn(5-MT)2·(solvent) (1–3; 5-MT = 5-methyltetrazole). Different solvents used for the synthesis lead to three distinct framework structures. Compound 1 with both tetrahydro-1,3-dimethyl-2(1H) pyrimidine (DMPU) and ethanol as the guest molecules exhibits a porous framework with 4-connected lonsdaleite (lon) topology.
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: TGA, emission spectra of 1–3, CO2 and H2 sorption isotherms of 1a estimated by virial equation, adsorption data and isosteric heat data for 1a and CIF files. CCDC 961654–961657. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt53269k
3210 | Dalton Trans., 2014, 43, 3210–3214
Scheme 1 tetrazole.
The similar μ2-bridging coordination mode of imidazole and
Compounds 2 and 3 showing a dia-type framework structure are synthesized by using N,N-dimethylacetamide (DMA) or N-methylpyrrolidone (NMP) as the solvent, respectively. Remarkably, the lon-type framework of 1 can irreversibly transfer to the dia-type frameworks of 2 and 3 through solventexchange. Noteworthy, the activated framework of 1 (denoted: 1a) shows high CO2 and H2 adsorption and remarkable selectivity of CO2 over N2 under ambient conditions.
Experimental Materials and general methods All chemicals were obtained from commercial sources and used without further purification. Elemental analyses of C, H and N were performed on a Vario MICRO E III elemental analyzer. The IR spectra (KBr pellets) were recorded on a Magna 750 FT-IR spectrophotometer. Powder X-ray diffraction data were recorded on a Rigaku MultiFlex diffractometer with a scan speed of 5° min−1. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer under N2 (30–800 °C) at a heating rate of 10 °C min−1. Fluorescence spectra were measured using a HORIBA Jobin-Yvon FluoroMax-4 spectrometer. The gas adsorption isotherms were measured by using ASAP-2020 volumetric adsorption equipment.
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Synthesis of Zn(5-MT)2·0.5(DMPU·ethanol) (1). The mixture of Zn(NO3)2·6H2O (0.5 mmol, 0.150 g), 5-HMT (0.083 g, 1 mmol), tetrahydro-1,3-dimethyl-2(1H) pyrimidine (2 ml) and ethanol (2 ml) was sealed in a 20 ml vial and heated to 120 °C for 3 days. After cooling to room-temperature, the colourless crystals were obtained in pure phase (yield: 84%). Anal. Calcd for C8H15N9OZn (318.65): C, 30.16; H, 4.74; N, 39.56. Found: C, 30.10; H, 4.71; N, 40.02. IR (KBr cm−1): 3127 (s), 1608 (vs), 1572 (m), 1524 (s), 1371 (vs), 1304 (m), 1071 (s), 966 (w), 783 (m), 660 (w), 504 (w). Synthesis of Zn(5-MT)2·(DMA) (2). 2 was obtained by the same method as described for 1 except for using N,N-dimethylacetamide (DMA) instead of DMPU (yield: 80%). Anal. Calcd for C8H15N9OZn (318.66): C, 30.16; H, 4.74; N, 39.56. Found: C, 30.60; H, 4.51; N, 40.02. IR (KBr cm−1): m = 1618 (s), 1580 (m), 1535 (s), 1370 (vs), 1305 (m), 1071 (s), 966 (w), 783 (m), 660 (w), 500 (w). Synthesis of Zn(5-MT)2·(NMP) (3). 3 was obtained by the same method as described for 1 except for using N-methyl pyrrolidone (NMP) instead of DMPU (yield: 74%). Anal. Calcd for C9H15N9OZn (330.67): C, 32.67; H, 4.57; N, 38.12. Found: C, 33.05; H, 4.51; N, 38.02. IR (KBr cm−1): m = 1620 (s), 1581 (m), 1540 (s), 1372 (vs), 1310 (m), 1070 (s), 966 (w), 783 (m), 660 (w). Synthesis of Zn(5-MT)2·0.8(methanol) (1a). The single crystal samples of 1 were soaked in methanol solvent in an icewater bath for more than one month. Powder X-ray and TG were used to monitor guest-exchange. Anal. Calcd for C8H15N9OZn (318.65): C, 30.16; H, 4.74; N, 39.56. Found: C, 30.10; H, 4.71; N, 40.02. IR (KBr cm−1): 3127 (s), 1608 (vs), 1572 (m), 1524 (s), 1371 (vs), 1304 (m), 1071 (s), 966 (w), 783 (m), 660 (w). Single-crystal structure determination Single X-ray diffraction intensities of all crystals were collected on a CCD diffractometer at 293(2) K. All diffractometers were
Table 1
equipped with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and expanded with Fourier techniques. All calculations were performed with a SHELXL-97 package. Pertinent crystallographic data and structural refinements are listed in Table 1.
Results and discussion X-ray single crystal structural determination reveals that compound 1 crystallizes in the orthorhombic noncentrosymmetric space group Ama2. Second harmonic generation experimental result shows that compound 1 has SHG intensity of 1/3 versus that of technologically useful potassium dihydrogen phosphate (KDP), which confirms its acentric symmetry.8 In the structure of 1, each Zn2+ site displays a tetrahedral coordination environment by four N atoms from four different 5-MT ligands. Each μ2-5-MT ligand adopts imidazole-type coordination mode to link two adjacent Zn centers. The tetrahedral Zn centers are connected by the μ2-5-MT ligands into a 3D framework with 4-connected lon topology (Fig. 1a). The free spaces were occupied by the structurally ordered DMPU and partly disordered ethanol solvent molecules. A total of 48.3% of solvent accessible volume for 1 was found. Compounds 2 and 3 are isostructural and crystallize in the monoclinic noncentrosymmetric space group Cc, which are also isostructural to the previously reported compound ZTF-1 disregarding the guests.6a In the structure of 2, the tetrahedral Zn centers are crosslinked by the μ2-5-MT ligand into a 3D framework with typical dia topology (Fig. 1b). The free spaces were occupied by the structurally ordered DMA solvent molecules, correspondingly, which were occupied by the structurally ordered NMP molecules in 3 (Fig. 1c). Interestingly, compound 1 can be irreversibly transformed to another two compounds 2 and 3 with dia-type frameworks
The crystal data for compounds 1, 2, 3 and 1a
Compound reference
1
2
3
1a
Chemical formula Formula mass Crystal system a/Å b/Å c/Å α/° β/° γ/° Unit cell volume/Å3 Temperature/K Space group No. of formula units per unit cell, Z No. of reflections measured No. of independent reflections Rint Final R1 values (I > 2σ(I)) Final wR(F2) values (I > 2σ(I)) Final R1 values (all data) Final wR(F2) values (all data) Goodness of fit on F2
C4H6N8Zn·0.5(C6H12N2O)·0.5(C2H6O) 318.65 Orthorhombic 15.1635(4) 17.9739(4) 10.0176(3) 90.00 90.00 90.00 2730.27(12) 293(2) Ama2 8 10 046 2459 0.0320 0.0339 0.0890 0.0390 0.0918 1.213
C4H6N8Zn·C4H9NO 318.66 Monoclinic 13.2925(11) 13.6600(7) 9.5631(9) 90.00 126.840(13) 90.00 1389.68(30) 293(2) Cc 4 2455 1618 0.0194 0.0528 0.1460 0.0545 0.1482 1.053
C9H15N9OZn 330.67 Monoclinic 12.989(2) 13.8218(9) 9.2766(12) 90.00 124.43(2) 90.00 1373.6(3) 293(2) Cc 4 4747 1835 0.0891 0.0420 0.0835 0.0658 0.0960 0.817
C4H6N8Zn·(CH4O) 263.57 Orthorhombic 15.6799(14) 18.1616(12) 9.6615(9) 90.00 90.00 90.00 2751.3(4) 293(2) Ama2 8 8178 2091 0.0735 0.0564 0.1440 0.0718 0.1516 0.970
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Fig. 3 Fig. 1 The lon-type cage with guest molecules in 1 (a); view of the 3D framework of 1 along the c-axis (b); the dia-type cage with guest molecule in 2 (c) and 3 (d).
The powder XRD patterns of 1 to 3.
Fig. 2 Structural transformation from the lon net of 1 (a) to the dia net of 2 (b).
obtained from the thermogravimetric analysis (TGA) curve and elemental analysis (EA) results, which indicate 0.8 methanol molecule per Zn(5-MT)2 unit. The TGA curve of 1a indicated that a weight loss of 9.92% occurred in the temperature range of 25–145 °C, corresponding to the loss of 0.8 methanol molecule (expected 9.95%) (Fig. S2†). The permanent porosity of the desolvated form of 1a, which is activated at room temperature under vacuum for 5 hours, was established by N2 sorption experiments at 77 K (Fig. 4a and b). The Langmuir and BET surface areas were 1097 m2 g−1 and 792 m2 g−1, respectively, which are much higher than that of ZTF-1.6a A single data point at relative pressure 0.98 gives a pore volume of 0.39 cm3 g−1 by the Horvath–Kawazoe equation.
(Fig. 2). After the samples of 1 were dipped in DMA or NMP and heated at 120 °C for several hours, the powder X-ray diffraction (PXRD) measurements demonstrated the obvious structural transformation from 1 to 2 or 3 (Fig. 3). However, it is difficult to get single crystals after the solvent-exchange process, because all samples of 1 become powdered. It should be noted that compounds 1–3 are all sensitive to methanol solvent at room temperature and higher temperatures. In fact, compounds 1–3 are easily changed to one unidentified compound after dipping them into methanol solvent for several hours, which is proven by the powder XRD results (Fig. 3 and S1†). To activate the framework of 1, low temperature (ca. 10 °C) was chosen to perform the methanol-exchange, and a new methanol-exchanged phase Zn(5-MT)2·0.8(methanol) (1a) was obtained. Compound 1a was also characterized by singlecrystal X-ray diffraction and exhibited the same framework as 1, although the methanol guest and one methyl group of 5-MT in 1a were disordered. The contribution of the disordered solvent molecules was subtracted from the reflection data by the SQUEEZE method as implanted in PLATON program.9 An accurate determination of solvent molecules in the structure is
Fig. 4 The sorption isotherms of 1a. The N2 adsorption (a) and desorption (b) at 77 K; the H2 adsorption (c) and desorption (d) at 77 K; the H2 adsorption (e) and desorption (f ) at 87 K; the CO2 adsorption (g) and desorption (h) at 273 K; the CO2 adsorption (i) and desorption ( j) at 298 K; the N2 adsorption (k) and desorption (l) at 273 K; the N2 adsorption (m) and desorption (n) at 298 K; the H2 adsorption at 273 K (o) and at 298 K ( p), respectively.
3212 | Dalton Trans., 2014, 43, 3210–3214
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The adsorption isotherms of CO2 for 1a were measured up to 1 bar at 273 K and 298 K, respectively (Fig. 4g and h). The CO2 uptake values were 124.7 cm3 g−1 (5.57 mmol g−1) at 273 K, and 76.7 cm3 g−1 (3.42 mmol g−1) at 298 K, respectively. The CO2 uptake (5.57 mmol g−1 at 273 K) of 1a compares well with that of the ZTF-1 (5.6 mmol g−1), and outperforms other ZIFs and most of the well-known MOFs.1c The isosteric heats of adsorption (Qst) for 1a was calculated using the adsorption data collected at 273 K and 298 K. At zero coverage, the enthalpy of CO2 adsorption for 1a is 25.68 kJ mol−1 (Fig. S4 and S5†), almost equal to that of the ZTF-1. In addition, the N2 adsorption isotherms of 1 were also measured up to 1 bar at 273 K and 298 K, respectively. It is clear that N2 is hardly adsorbed at all at 273 K (4.77 cm3 g−1) and 298 K (4.55 cm3 g−1) (Fig. 4). We employed the Henry’s constants to estimate the adsorption selectivity of 1 for CO2/ N2, which had been shown to be valid for calculating the gas selectivity of MOFs. 1 is calculated to exhibit an adsorption selectivity of 43 for CO2 over N2 at 273 K and 28 at 298 K, respectively (Fig. S8†). The results indicate that 1 has excellent CO2/N2 adsorption selectivity under ambient conditions. Low-pressure H2 adsorption isotherms for 1a were also measured up to 1 bar at 77 K and 87 K, respectively (Fig. 4c–f ). The H2 uptake values were 1.92 wt% (215 cm3 g−1) at 77 K and 1.54 wt% (173 ml g−1) at 87 K, which are higher than that of ZTF-1 (1.6 wt%) and other ZIFs, as well as CPF-6, a polytetrazole-based porous MOF.10 It may also be attributed to the possible interaction of H2 with remaining uncoordinated N-heteroatom of each 5-MT ligand in the pores. At zero coverage, the enthalpy of H2 adsorption for 1a is 7.22 kJ mol−1 (Fig. S6 and S7†). This value of the isosteric heat of adsorption is moderate compared with the many famous porous materials, such as ZIF-8, MIL-100, PCN-10, HKUST-1, MOF-5, IRMOF-1 and IRMOF-8.2c The fraction of the pore volume filled by liquid H2 (ρH2 = 0.0708 g cm−3) at 1 bar and 77 K for 1a is 0.69 (Table S4†), suggesting that H2 is highly compressed within the pores of 1a even at 1 bar. This result is much larger than those of many famous porous MOFs, such as IRMOFs, HKUST-1, MOF-74, and our recent result.11
Conclusions In conclusion, three tetrahedral tetrazolate frameworks with different topologies were successfully prepared by employing different solvents. The lon-type framework of 1 can irreversibly transform to dia-type frameworks of 2 and 3 by solventexchange. Furthermore, the activated 1a with lon topology shows high H2 and CO2 uptake and remarkable selectivity of CO2 over N2 under ambient conditions.
Acknowledgements We thank the support of this work by 973 program (2011CB932504 and 2012CB821705), NSFC (21221001,
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21103189, 21173224, 91222105) and the specialized fund from the Youth Innovation Promotion Association of CAS.
Notes and references 1 (a) R. B. Getman, Y.-S. Bae, C. E. Wilmer and R. Q. Snurr, Chem. Rev., 2012, 112, 703; (b) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724; (c) H. Wu, Q. Gong, D. H. Olson and J. Li, Chem. Rev., 2012, 112, 836; (d) J.-R. Li, J. Sculley and H.-C. Zhou, Chem. Rev., 2012, 112, 869. 2 (a) M. P. Suh, H. J. Park, T. K. Prasad and D.-W. Lim, Chem. Rev., 2012, 112, 782; (b) E. D. Bloch, W. L. Queen, R. Krishna, J. M. Zadrozny, C. M. Brown and J. R. Long, Science, 2012, 335, 1606; (c) F. Luo, M.-S. Wang, M.-B. Luo, G.-M. Sun, Y.-M. Song, P.-X. Li and G.-C. Guo, Chem. Commun., 2012, 48, 5989; (d) Q. Lin, T. Wu, S. Zheng, X. Bu and P. Feng, Chem. Commun., 2011, 47, 11852; (e) S. Yang, X. Lin, W. Lewis, M. Suyetin, E. Bichoutskaia, J. E. Parker, C. C. Tang, D. R. Allan, P. J. Rizkallah, P. Hubberstey, N. R. Champness, K. M. Thomas, A. J. Blake and M. Schröder, Nat. Mater., 2012, 11, 710. 3 (a) T. A. Makal, J. R. Li, W. Lu and H.-C. Zhou, Chem. Soc. Rev., 2012, 41, 7761; (b) Y. He, W. Zhou, R. Krishna and B. Chen, Chem. Commun., 2012, 48, 11813; (c) S. Bourrelly, P. L. Llewellyn, C. Serre, F. Millange, T. Loiseau and G. Férey, J. Am. Chem. Soc., 2005, 127, 13519; (d) D. Yuan, D. Zhao, D. Sun and H.-C. Zhou, Angew. Chem., Int. Ed., 2010, 49, 5357; (e) Y. Peng, V. Krungleviciute, I. Eryazici, J. T. Hupp, O. K. Farha and T. Yildirim, J. Am. Chem. Soc., 2013, 135, 11887. 4 (a) Y. Tian, Y. Zhao, Z. Chen, G. Zhang, L. Weng and D. Zhao, Chem.–Eur. J., 2007, 13, 4146; (b) X.-C. Huang, Y.-Y. Lin, J. P. Zhang and X.-M. Chen, Angew. Chem., Int. Ed., 2006, 45, 1557. 5 (a) R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe and O. M. Yaghi, Science, 2008, 319, 939; (b) A. Phan, C. Doonan, F. J. Uribe-Romo, C. B. Knobler, M. O’keeffe and O. M. Yaghi, Acc. Chem. Res., 2009, 43, 58; (c) T. Wu, X. Bu, R. Liu, Z. Lin, J. Zhang and P. Feng, Chem.–Eur. J., 2008, 14, 7771. 6 (a) T. Panda, P. Pachfule, Y. Chen, J. Jiang and R. Banerjee, Chem. Commun., 2011, 47, 2011; (b) W. Ouellette, B. S. Hudson and J. Zubieta, Inorg. Chem., 2007, 46, 4887. 7 (a) J.-P. Zhang, Y.-B. Zhang, J.-B. Lin and X.-M. Chen, Chem. Rev., 2012, 112, 1001; (b) X. S. Wang, Y. Z. Tang, X. F. Huang, Z. R. Qu, C. M. Che, P. W. H. Chan and R.-G. Xiong, Inorg. Chem., 2005, 44, 5278; (c) X. W. Wang, J.-Z. Chen and J.-H. Liu, Cryst. Growth Des., 2007, 7, 1227. 8 The powder second-harmonic generation test was carried out on the sample by the Kurtz and Perry method. Secondharmonic generation intensity data were obtained by placing a sieved (80–100 mesh) powder sample in an intense fundamental beam from a Q-switched Nd:YAG
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laser of wavelength 1064 nm. The output (λ 532 nm) was filtered first to remove the multiplier and was then displayed on an oscilloscope. This procedure was then repeated using standard NLO material (microcrystalline KDP), and the ratio of the second-harmonic intensity outputs was calculated.
3214 | Dalton Trans., 2014, 43, 3210–3214
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9 P. Van der Sluis and A. L. Spek, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 1990, 46, 194. 10 Q. Lin, T. Wu, S.-T. Zheng, X. Bu and P. Feng, J. Am. Chem. Soc., 2012, 134, 784. 11 Y.-X. Tan, Y.-P. He and J. Zhang, Chem. Commun., 2011, 47, 10647.
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Cite this: Dalton Trans., 2014, 43, 3210 Received 20th November 2013, Accepted 27th November 2013
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Tetrahedral tetrazolate frameworks for high CO2 and H2 uptake† Fei Wang, Duan-Chuan Hou, Hui Yang, Yao Kang and Jian Zhang*
DOI: 10.1039/c3dt53269k
Three tetrahedral tetrazolate frameworks with two different 4-connected topologies including lonsdaleite (lon, for 1) and diamond (dia, for 2 and 3) have been synthesized, and the lon-type framework with high
www.rsc.org/dalton
CO2 and H2 uptake capacity can irreversibly transform to the dia-type framework via solvent-exchange.
Introduction Metal–organic frameworks (MOFs) are a new class of network solids that have great potential in specific applications in clean energy, most significantly as storage media for gases such as hydrogen and methane, and as high-capacity adsorbents to meet various separation needs.1–3 Among numerous known MOFs, a family of materials that closely resemble zeolite topologies, namely zeolitic imidazolate frameworks (ZIFs), are constructed by the tetrahedrally coordinated divalent cations (M2+ = Zn2+ or Co2+) and the uninegative imidazolate ligands (im−).4,5 Compared to those imidazole derivatives, the tetrazole derivatives can adopt a similar μ2-bridging coordination mode to link tetrahedral divalent metal centers (Scheme 1). Of particular interest is that there are two uncoordinated N-heteroatom sites for each μ2-tetrazole ligand, which are potential functional sites for gas sorption or catalysis. Although various tetrazole-based MOFs have been reported,6,7 it is still an ongoing challenge to create zeolite-like metal–tetrazole frameworks with uncoordinated N-donor sites for gas sorption and separation. In this work, we report four tetrahedral tetrazolate frameworks with the typical framework formula Zn(5-MT)2·(solvent) (1–3; 5-MT = 5-methyltetrazole). Different solvents used for the synthesis lead to three distinct framework structures. Compound 1 with both tetrahydro-1,3-dimethyl-2(1H) pyrimidine (DMPU) and ethanol as the guest molecules exhibits a porous framework with 4-connected lonsdaleite (lon) topology.
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: TGA, emission spectra of 1–3, CO2 and H2 sorption isotherms of 1a estimated by virial equation, adsorption data and isosteric heat data for 1a and CIF files. CCDC 961654–961657. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt53269k
3210 | Dalton Trans., 2014, 43, 3210–3214
Scheme 1 tetrazole.
The similar μ2-bridging coordination mode of imidazole and
Compounds 2 and 3 showing a dia-type framework structure are synthesized by using N,N-dimethylacetamide (DMA) or N-methylpyrrolidone (NMP) as the solvent, respectively. Remarkably, the lon-type framework of 1 can irreversibly transfer to the dia-type frameworks of 2 and 3 through solventexchange. Noteworthy, the activated framework of 1 (denoted: 1a) shows high CO2 and H2 adsorption and remarkable selectivity of CO2 over N2 under ambient conditions.
Experimental Materials and general methods All chemicals were obtained from commercial sources and used without further purification. Elemental analyses of C, H and N were performed on a Vario MICRO E III elemental analyzer. The IR spectra (KBr pellets) were recorded on a Magna 750 FT-IR spectrophotometer. Powder X-ray diffraction data were recorded on a Rigaku MultiFlex diffractometer with a scan speed of 5° min−1. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer under N2 (30–800 °C) at a heating rate of 10 °C min−1. Fluorescence spectra were measured using a HORIBA Jobin-Yvon FluoroMax-4 spectrometer. The gas adsorption isotherms were measured by using ASAP-2020 volumetric adsorption equipment.
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Synthesis of Zn(5-MT)2·0.5(DMPU·ethanol) (1). The mixture of Zn(NO3)2·6H2O (0.5 mmol, 0.150 g), 5-HMT (0.083 g, 1 mmol), tetrahydro-1,3-dimethyl-2(1H) pyrimidine (2 ml) and ethanol (2 ml) was sealed in a 20 ml vial and heated to 120 °C for 3 days. After cooling to room-temperature, the colourless crystals were obtained in pure phase (yield: 84%). Anal. Calcd for C8H15N9OZn (318.65): C, 30.16; H, 4.74; N, 39.56. Found: C, 30.10; H, 4.71; N, 40.02. IR (KBr cm−1): 3127 (s), 1608 (vs), 1572 (m), 1524 (s), 1371 (vs), 1304 (m), 1071 (s), 966 (w), 783 (m), 660 (w), 504 (w). Synthesis of Zn(5-MT)2·(DMA) (2). 2 was obtained by the same method as described for 1 except for using N,N-dimethylacetamide (DMA) instead of DMPU (yield: 80%). Anal. Calcd for C8H15N9OZn (318.66): C, 30.16; H, 4.74; N, 39.56. Found: C, 30.60; H, 4.51; N, 40.02. IR (KBr cm−1): m = 1618 (s), 1580 (m), 1535 (s), 1370 (vs), 1305 (m), 1071 (s), 966 (w), 783 (m), 660 (w), 500 (w). Synthesis of Zn(5-MT)2·(NMP) (3). 3 was obtained by the same method as described for 1 except for using N-methyl pyrrolidone (NMP) instead of DMPU (yield: 74%). Anal. Calcd for C9H15N9OZn (330.67): C, 32.67; H, 4.57; N, 38.12. Found: C, 33.05; H, 4.51; N, 38.02. IR (KBr cm−1): m = 1620 (s), 1581 (m), 1540 (s), 1372 (vs), 1310 (m), 1070 (s), 966 (w), 783 (m), 660 (w). Synthesis of Zn(5-MT)2·0.8(methanol) (1a). The single crystal samples of 1 were soaked in methanol solvent in an icewater bath for more than one month. Powder X-ray and TG were used to monitor guest-exchange. Anal. Calcd for C8H15N9OZn (318.65): C, 30.16; H, 4.74; N, 39.56. Found: C, 30.10; H, 4.71; N, 40.02. IR (KBr cm−1): 3127 (s), 1608 (vs), 1572 (m), 1524 (s), 1371 (vs), 1304 (m), 1071 (s), 966 (w), 783 (m), 660 (w). Single-crystal structure determination Single X-ray diffraction intensities of all crystals were collected on a CCD diffractometer at 293(2) K. All diffractometers were
Table 1
equipped with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and expanded with Fourier techniques. All calculations were performed with a SHELXL-97 package. Pertinent crystallographic data and structural refinements are listed in Table 1.
Results and discussion X-ray single crystal structural determination reveals that compound 1 crystallizes in the orthorhombic noncentrosymmetric space group Ama2. Second harmonic generation experimental result shows that compound 1 has SHG intensity of 1/3 versus that of technologically useful potassium dihydrogen phosphate (KDP), which confirms its acentric symmetry.8 In the structure of 1, each Zn2+ site displays a tetrahedral coordination environment by four N atoms from four different 5-MT ligands. Each μ2-5-MT ligand adopts imidazole-type coordination mode to link two adjacent Zn centers. The tetrahedral Zn centers are connected by the μ2-5-MT ligands into a 3D framework with 4-connected lon topology (Fig. 1a). The free spaces were occupied by the structurally ordered DMPU and partly disordered ethanol solvent molecules. A total of 48.3% of solvent accessible volume for 1 was found. Compounds 2 and 3 are isostructural and crystallize in the monoclinic noncentrosymmetric space group Cc, which are also isostructural to the previously reported compound ZTF-1 disregarding the guests.6a In the structure of 2, the tetrahedral Zn centers are crosslinked by the μ2-5-MT ligand into a 3D framework with typical dia topology (Fig. 1b). The free spaces were occupied by the structurally ordered DMA solvent molecules, correspondingly, which were occupied by the structurally ordered NMP molecules in 3 (Fig. 1c). Interestingly, compound 1 can be irreversibly transformed to another two compounds 2 and 3 with dia-type frameworks
The crystal data for compounds 1, 2, 3 and 1a
Compound reference
1
2
3
1a
Chemical formula Formula mass Crystal system a/Å b/Å c/Å α/° β/° γ/° Unit cell volume/Å3 Temperature/K Space group No. of formula units per unit cell, Z No. of reflections measured No. of independent reflections Rint Final R1 values (I > 2σ(I)) Final wR(F2) values (I > 2σ(I)) Final R1 values (all data) Final wR(F2) values (all data) Goodness of fit on F2
C4H6N8Zn·0.5(C6H12N2O)·0.5(C2H6O) 318.65 Orthorhombic 15.1635(4) 17.9739(4) 10.0176(3) 90.00 90.00 90.00 2730.27(12) 293(2) Ama2 8 10 046 2459 0.0320 0.0339 0.0890 0.0390 0.0918 1.213
C4H6N8Zn·C4H9NO 318.66 Monoclinic 13.2925(11) 13.6600(7) 9.5631(9) 90.00 126.840(13) 90.00 1389.68(30) 293(2) Cc 4 2455 1618 0.0194 0.0528 0.1460 0.0545 0.1482 1.053
C9H15N9OZn 330.67 Monoclinic 12.989(2) 13.8218(9) 9.2766(12) 90.00 124.43(2) 90.00 1373.6(3) 293(2) Cc 4 4747 1835 0.0891 0.0420 0.0835 0.0658 0.0960 0.817
C4H6N8Zn·(CH4O) 263.57 Orthorhombic 15.6799(14) 18.1616(12) 9.6615(9) 90.00 90.00 90.00 2751.3(4) 293(2) Ama2 8 8178 2091 0.0735 0.0564 0.1440 0.0718 0.1516 0.970
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Fig. 3 Fig. 1 The lon-type cage with guest molecules in 1 (a); view of the 3D framework of 1 along the c-axis (b); the dia-type cage with guest molecule in 2 (c) and 3 (d).
The powder XRD patterns of 1 to 3.
Fig. 2 Structural transformation from the lon net of 1 (a) to the dia net of 2 (b).
obtained from the thermogravimetric analysis (TGA) curve and elemental analysis (EA) results, which indicate 0.8 methanol molecule per Zn(5-MT)2 unit. The TGA curve of 1a indicated that a weight loss of 9.92% occurred in the temperature range of 25–145 °C, corresponding to the loss of 0.8 methanol molecule (expected 9.95%) (Fig. S2†). The permanent porosity of the desolvated form of 1a, which is activated at room temperature under vacuum for 5 hours, was established by N2 sorption experiments at 77 K (Fig. 4a and b). The Langmuir and BET surface areas were 1097 m2 g−1 and 792 m2 g−1, respectively, which are much higher than that of ZTF-1.6a A single data point at relative pressure 0.98 gives a pore volume of 0.39 cm3 g−1 by the Horvath–Kawazoe equation.
(Fig. 2). After the samples of 1 were dipped in DMA or NMP and heated at 120 °C for several hours, the powder X-ray diffraction (PXRD) measurements demonstrated the obvious structural transformation from 1 to 2 or 3 (Fig. 3). However, it is difficult to get single crystals after the solvent-exchange process, because all samples of 1 become powdered. It should be noted that compounds 1–3 are all sensitive to methanol solvent at room temperature and higher temperatures. In fact, compounds 1–3 are easily changed to one unidentified compound after dipping them into methanol solvent for several hours, which is proven by the powder XRD results (Fig. 3 and S1†). To activate the framework of 1, low temperature (ca. 10 °C) was chosen to perform the methanol-exchange, and a new methanol-exchanged phase Zn(5-MT)2·0.8(methanol) (1a) was obtained. Compound 1a was also characterized by singlecrystal X-ray diffraction and exhibited the same framework as 1, although the methanol guest and one methyl group of 5-MT in 1a were disordered. The contribution of the disordered solvent molecules was subtracted from the reflection data by the SQUEEZE method as implanted in PLATON program.9 An accurate determination of solvent molecules in the structure is
Fig. 4 The sorption isotherms of 1a. The N2 adsorption (a) and desorption (b) at 77 K; the H2 adsorption (c) and desorption (d) at 77 K; the H2 adsorption (e) and desorption (f ) at 87 K; the CO2 adsorption (g) and desorption (h) at 273 K; the CO2 adsorption (i) and desorption ( j) at 298 K; the N2 adsorption (k) and desorption (l) at 273 K; the N2 adsorption (m) and desorption (n) at 298 K; the H2 adsorption at 273 K (o) and at 298 K ( p), respectively.
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The adsorption isotherms of CO2 for 1a were measured up to 1 bar at 273 K and 298 K, respectively (Fig. 4g and h). The CO2 uptake values were 124.7 cm3 g−1 (5.57 mmol g−1) at 273 K, and 76.7 cm3 g−1 (3.42 mmol g−1) at 298 K, respectively. The CO2 uptake (5.57 mmol g−1 at 273 K) of 1a compares well with that of the ZTF-1 (5.6 mmol g−1), and outperforms other ZIFs and most of the well-known MOFs.1c The isosteric heats of adsorption (Qst) for 1a was calculated using the adsorption data collected at 273 K and 298 K. At zero coverage, the enthalpy of CO2 adsorption for 1a is 25.68 kJ mol−1 (Fig. S4 and S5†), almost equal to that of the ZTF-1. In addition, the N2 adsorption isotherms of 1 were also measured up to 1 bar at 273 K and 298 K, respectively. It is clear that N2 is hardly adsorbed at all at 273 K (4.77 cm3 g−1) and 298 K (4.55 cm3 g−1) (Fig. 4). We employed the Henry’s constants to estimate the adsorption selectivity of 1 for CO2/ N2, which had been shown to be valid for calculating the gas selectivity of MOFs. 1 is calculated to exhibit an adsorption selectivity of 43 for CO2 over N2 at 273 K and 28 at 298 K, respectively (Fig. S8†). The results indicate that 1 has excellent CO2/N2 adsorption selectivity under ambient conditions. Low-pressure H2 adsorption isotherms for 1a were also measured up to 1 bar at 77 K and 87 K, respectively (Fig. 4c–f ). The H2 uptake values were 1.92 wt% (215 cm3 g−1) at 77 K and 1.54 wt% (173 ml g−1) at 87 K, which are higher than that of ZTF-1 (1.6 wt%) and other ZIFs, as well as CPF-6, a polytetrazole-based porous MOF.10 It may also be attributed to the possible interaction of H2 with remaining uncoordinated N-heteroatom of each 5-MT ligand in the pores. At zero coverage, the enthalpy of H2 adsorption for 1a is 7.22 kJ mol−1 (Fig. S6 and S7†). This value of the isosteric heat of adsorption is moderate compared with the many famous porous materials, such as ZIF-8, MIL-100, PCN-10, HKUST-1, MOF-5, IRMOF-1 and IRMOF-8.2c The fraction of the pore volume filled by liquid H2 (ρH2 = 0.0708 g cm−3) at 1 bar and 77 K for 1a is 0.69 (Table S4†), suggesting that H2 is highly compressed within the pores of 1a even at 1 bar. This result is much larger than those of many famous porous MOFs, such as IRMOFs, HKUST-1, MOF-74, and our recent result.11
Conclusions In conclusion, three tetrahedral tetrazolate frameworks with different topologies were successfully prepared by employing different solvents. The lon-type framework of 1 can irreversibly transform to dia-type frameworks of 2 and 3 by solventexchange. Furthermore, the activated 1a with lon topology shows high H2 and CO2 uptake and remarkable selectivity of CO2 over N2 under ambient conditions.
Acknowledgements We thank the support of this work by 973 program (2011CB932504 and 2012CB821705), NSFC (21221001,
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21103189, 21173224, 91222105) and the specialized fund from the Youth Innovation Promotion Association of CAS.
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laser of wavelength 1064 nm. The output (λ 532 nm) was filtered first to remove the multiplier and was then displayed on an oscilloscope. This procedure was then repeated using standard NLO material (microcrystalline KDP), and the ratio of the second-harmonic intensity outputs was calculated.
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