DOI: 10.1002/chem.201500457
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& Metal–Organic Frameworks
Ligand Symmetry Modulation for Designing a Mesoporous Metal– Organic Framework: Dual Reactivity to Transition and Lanthanide Metals for Enhanced Functionalization Miao Du,*[a, b] Xi Wang,[a, b] Min Chen,[b] Cheng-Peng Li,[a] Jia-Yue Tian,[b] Zhuo-Wei Wang,[b] and Chun-Sen Liu*[b] Abstract: A promising alternative strategy for designing mesoporous metal–organic frameworks (MOFs) has been proposed, by modifying the symmetry rather than expanding the length of organic linkers. By means of this approach, a unique MOF material based on the target [Zn8(ad)4] (ad = adeninate) clusters and C3-symmetric organic linkers can be obtained, with trigonal microporous (ca., 0.8 nm) and hexag-
Introduction Metal–organic frameworks (MOFs), constructed through the coordination linkages of inorganic (metal ions or clusters) and organic (linkers) structural units, have emerged as a promising class of crystalline materials for their tunable porous nets and unusual properties.[1, 2] The pore sizes for such porous systems can range from ultramicroporous (< 3 æ), microporous (3 æ– 2 nm), even up to mesoporous (2–50 nm) regions. However, among tens of thousands MOFs reported so far, only about 100 examples possess the available mesoporous pores (Supporting Information, Table S1).[3] In fact, the preparation of mesoporous MOFs represents a great challenge for their intrinsic resistance of crystalline lattice extension to form large cavities, especially those close to or even over 3 nm.[3, 4] As a consequence, the development of effective strategy for the design of mesoporous MOFs will be of extreme significance, considering the serious limitations for the applications of microporous MOFs materials caused by their smaller pores that cannot hold larger guest moiety and also are disadvantage to fast molecular diffusion.[4] [a] Prof. M. Du, X. Wang, Dr. C.-P. Li College of Chemistry, Tianjin Key Laboratory of Structure and Performance for Functional Molecules MOE Key Laboratory of Inorganic–Organic Hybrid Functional Material Chemistry, Tianjin Normal University Tianjin 300387 (P.R. China) E-mail: [email protected] [b] Prof. M. Du, X. Wang, Dr. M. Chen, J.-Y. Tian, Z.-W. Wang, Prof. C.-S. Liu Henan Provincial Key Laboratory of Surface and Interface Science Zhengzhou University of Light Industry, Zhengzhou 450002 (P.R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500457. Chem. Eur. J. 2015, 21, 9713 – 9719
onal mesoporous (ca., 3.0 nm) 1D channels. Moreover, the resulting 446-MOF shows distinct reactivity to transition and lanthanide metal ions. Significantly, the transmetalation of CoII or NiII on the ZnII centers in 446-MOF can enhance the sorption capacities of CO2 and CH4 (16–21 %), whereas the impregnation of EuIII and TbIII in the channels of 446-MOF will result in adjustable light-emitting behaviors.
In this context, ligand extension in specific targeting network topologies has been applied to successfully enlarge the pores of MOFs. Early in 2002, Yaghi et al. prepared the first mesoporous MOF (IRMOF-16) with a long linker p-terphenyl4,4’’-dicarboxylic acid, which takes the expected topology of CaB6 adapted by the prototype IRMOF-1.[5] Recently, this concept was again demonstrated by Yaghi et al. for a systematic expansion of MOF-74 to afford an isoreticular series of structures with the pore apertures ranging from 14 to 98 æ (IRMOF74-I to XI).[4b, 6] Another alternative strategy to design mesoporous MOFs involves the utilization of metal-cluster vertexes with bulky sizes as the building units, in which metal-carboxylate clusters, such as [Fe3O(CO2)6],[7] [Zn4O(CO2)6],[6, 8] [Cr3O(CO2)6],[9] and [Zr6O4(OH)4][10] are mostly known. In this context, such metal clusters, normally comprising part of the organic linkers, are formed in situ during the synthesis of MOFs, which thus inevitably leads to their high uncertainty. Inspiringly, Rosi et al. illustrated a design strategy for porous MOFs with metal-adeninate clusters and different dicarboxylate linkers. Analysis of this series of ZnII-MOFs reveals that the large [Zn8(ad)4] (ad = adeninate) units will be invariably formed in the resulting host frameworks, either mesoporous (bio-MOF100[11a] and its isoreticular analogues bio-MOF-101, 102, and 103[11b]) or microporous (bio-MOF-1[12a] and ZJU-48[12b]). Notably, bio-MOF-1 and bio-MOF-100 are constructed with the same dicarboxylate ligand, in which infinite [Zn8(ad)4O]n chains and discrete [Zn8(ad)4] building units are afforded, respectively (Supporting Information, Table S2). Mathematically, the larger channel level will be achieved (Scheme 1) when the channel is transformed from quadrangle (with fourfold symmetry) to hexagon (with threefold or sixfold symmetry). Empirically, the tricarboxylate ligands with C3 symmetry will induce the crystallization of MOFs with hexagonal channels,[13] whereas the linear
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Full Paper dicarboxylate linkers may mostly lead to the quadrangle voids of porous coordination frameworks, just as observed in bioMOF-1.[12a] Thus, we propose here a new strategy for designing mesoporous MOFs with the reliable larger metal clusters by increasing the symmetry of organic linkers. As a result, a unique mesoporous MOF material can be constructed, which shows different trapping mechanisms to bivalent transition metals (cation exchange) and trivalent lanthanide metals (guest inclusion). Remarkably, such modified MOF analogues will show enhanced gas uptake capacities and adjustable light emissions.
group, the trigonal TATAB linker normally takes the planar pinwheel-like configuration in the reported structures.[14] However, in this case, the TATAB ligand adopts the highly distorted Y-like conformation (Supporting Information, Figure S6), in which two branches of the trigonal tecton are close to each other with the distance between their carboxylate carbon atoms of only 6.62 æ and in comparison, the other one is far away from them with such distances of 13.45 and 15.25 æ. As a result of the unidentate coordination of each carboxylate group (Supporting Information, Figure S4 b) and this unique conformation, two coupled TATAB ligands actually act as a linear connector between two adjacent octahedral cages at each open face (Figure 1 b).
Scheme 1. Schematic representation of the design strategy for mesoporous MOFs in this work.
Results and Discussion Structural design and analysis for 446-MOF Solvothermal reaction of [Zn(OAc)2]·2 H2O, adenine, HBF4, and 4,4’,4’’-s-triazine-1,3,5-triyltri-p-aminobenzoic acid (H3TATAB; Scheme S1, Supporting Information) in DMF at 120 8C affords colorless hexagonal-prism crystals for [Zn2(ad)(TATAB)O1/4](Me2NH2)1/2(DMF)6(H2O)4 (446-MOF). Single-crystal X-ray diffraction analysis reveals that 446-MOF crystallizes in primitive hexagonal space group P622, with two types of microporous and mesoporous open channels. The asymmetric unit holds three crystallographically independent ZnII ions (occupancy factors: 1 for Zn1 and 1/2 for Zn2 or Zn3), one 1/4-occupied m4-O2¢ anion, and fully deprotonated adeninate and TATAB. All ZnII centers are coordinated by two nitrogen donors from different adeninates and two oxygen atoms from TATAB or m4-O2¢, taking distorted tetrahedral spheres (Supporting Information, Figure S4 a). As expected, four rigid adeninate tectons are combined to eight ZnII ions to complete a [Zn8(ad)4] octahedral cage (Supporting Information, Figure S5), which is similar to that observed in bio-MOFs series.[11a] Such adjacent [Zn8(ad)4] cages interact with each other through the m4-O2¢ bridging on Zn1 ions, located at the axial sites of octahedra, which affords columnar [Zn8(ad)4O]n secondary building units (SBUs) along the c axis (Figure 1 a). Despite the flexibility of extended imino Chem. Eur. J. 2015, 21, 9713 – 9719
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Figure 1. View of: a) 1D [Zn8(ad)4O]n SBU, and b) coupled TATAB connector between two adjacent [Zn8(ad)4] cages in 446-MOF. c) Space-filling model for 446-MOF with two types of 1D channels along the c axis. d) Comparison of the structural features for microporous bio-MOF-1 and mesoporous 446MOF.
It is quite instructive to compare the structural parameters for bio-MOF-1[11a] and 446-MOF, which consist of the same columnar [Zn8(ad)4O]n SBUs but different carboxylate linkers. For both cases, each [Zn8(ad)4] octahedral cage interacts with further four cages through linear connectors (coupled ligands in 446-MOF or triply ligands in bio-MOF-1) along different directions to produce a distorted cage-connector tetrahedron (Supporting Information, Figure S7). In bio-MOF-1, such a distorted tetrahedral unit has two sets of vertex–center–vertex angles, two of 132.66 8 and four of 99.28 8, whereas all angles should be 109.5 8 for a perfect tetrahedron. In comparison, the tetrahedron in 446-MOF shows a more distorted configuration, in which three pairs of such angles are 78.13, 126.74, and
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Full Paper 127.41 8, because of the flexibility of TATAB ligand (Supporting Information, Figure S7). As a consequence, very large 1D hexagonal mesoporous channels, each of which is surrounded by six trigonal microporous channels, are afforded in 446-MOF (Figure 1 c), with the approximate diameters of 3.0 and 0.8 nm, respectively (considering the van der Waals radii of atoms). Such mesoporous and microporous channels are constituted by sixfold right-handed helices and threefold left-handed helices (Figure 1 c). In contrast to bio-MOF-1 with the square microporous channels, the symmetry is transferred from tetragonal system for bio-MOF-1 to hexagonal system for 446-MOF, clearly confirming our proposed strategy of ligand symmetry modulation for designing mesoporous MOFs with specific vertexes (Figure 1 d). The intricate 3D structure of 446-MOF can be simplified to a uninodal 6-connected network with the point symbol of (48.67), by considering each [Zn8(ad)4] cage as the net node (Supporting Information, Figure S8), which notably represents an unprecedented topological type.[2a]
Porosity for 446-MOF As estimated by PLATON[15] with the 1.8 æ probe radius, the free volume of host framework in 446-MOF is 12 005.5 æ3 (61.4 % per unit cell volume) without consideration of the lattice solvent and dimethylammonium cation. The permanent porosity of 446-MOF can be conventionally confirmed by nitrogen sorption isotherm at 77 K for the desolvated material (the Experimental Section), which exhibits a Type IV isotherm with a saturated N2 uptake of 384.1 cm3 g¢1 (Figure 2 a) and a Brunauer–Emmett–Teller (BET) surface area of 1605 m2 g¢1. Also, 446MOF shows the CO2 and CH4 adsorption capacity of 203.1 and 34.5 cm3 g¢1, respectively, at 195 K (Figure 2 b). The dye-uptake experiments have also been used to visually characterize porous MOFs with large open channels or pores.[16] In this case, by soaking the 446-MOF crystals in a DMF solution of methyl blue (MB), crystal violet (CV), or rhodamine 6G (R6 G), the solution will gradually fade and the colorless crystals of 446-MOF at the bottom of each cuvette accordingly turn to turquoise blue, brilliant blue, and pink, respectively. In a parallel experiment with larger food green 3 (FG3) adsorbate, no obvious change is observed (Figure 3 a and the Supporting Information, Figure S10). The sorption processes of dyes can be monitored by UV/Vis spectra (Supporting Information, Figure S11), which clearly show the decrease of dye concentration for MB, CV, and R6 G at different rates, and as expected, the inalterability for FG3 (Figure 3 b). A proper explanation for this result can be made based on the sizes of dyes (Supporting Information, Figure S10), in which the largest dimension of FG3 (27.4 æ) is close to the available pore size for 446-MOF (30.0 æ). Further evaluation by UV/Vis (Supporting Information, Figures S12–15) reveals that the adsorptivity of 446MOF is 17.0 mg g¢1 for MB, 3.70 mg g¢1 for CV, and 1.28 mg g¢1 for R6 G, also agreeing well with the results observed above. Moreover, the dye-adsorbed 446-MOFs materials still keep their crystallinity (Supporting Information, Figure S16). Chem. Eur. J. 2015, 21, 9713 – 9719
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Figure 2. a) N2 adsorption isotherms of 446-MOF, 446-MOF-Co, 446-MOF-Ni, Eu@446-MOF, and Tb@446-MOF at 77 K. b) CO2/CH4 adsorption isotherms of 446-MOF, 446-MOF-Co, and 446-MOF-Ni at 195 K.
Reactivity of 446-MOF to metal ions Post-synthesis modification by exchanging the original metal sites with other functional metal ions (e.g., active CoII and NiII as well as luminescent EuIII and TbIII), is a potential approach to design and synthesize new MOF materials with desired properties that cannot be achieved by using de novo synthesis.[17] Nevertheless, such examples are relatively rare in the known MOFs.[17d] When the crystals of 446-MOF are immersed in a DMF solution of CoII or NiII ion with heating at 80 8C for one week, the crystals will turn purple or green (Figure 4) without losing their structural integrity (Supporting Information, Figure S2). Moreover, single-crystal X-ray diffraction shows that the framework of 446-MOF-Co or 446-MOF-Ni is the same as that of 446-MOF (Supporting Information, Table S5). The molar ratio of Co/Zn or Ni/Zn in the final product is 0.64:1 or 0.53:1 (Figure 5 a), as monitored by using inductively coupled plasma mass spectrometry (ICP-MS) for the reaction processes. As reported, the Me2NH2 + cations residing as counterions in the pores or channels of MOFs can be easily exchanged with additional metal ions.[18] Moreover, the exposed Zn2 and Zn3 around the 1D channels in 446-MOF can get more contact with the implanting CoII or NiII, compared with Zn1 in the joints between adjacent [Zn8(ad)4] clusters. Thus, it can be concluded that the Me2NH2 + cations and a part of Zn2 and Zn3 are exchanged by CoII (63 %) or NiII (53 %) in this
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Figure 5. The molar ratios of metal/ZnII (ICP-MS) for metal-modified 446MOF samples, in which the metals are: a) CoII or NiII, and b) EuIII or TbIII.
Figure 3. a) 446-MOF samples after dye-adsorption for 6 h; b) percentage of dye in solution monitored with time in the presence of 446-MOF.
Figure 4. Photographs of 446-MOF-Co, 446-MOF, and 446-MOF-Ni samples.
course. The complete exchange of ZnII by CoII or NiII cannot be achieved by further extending the reaction time or increasing the reaction temperature. This may result from the fact that NiII or CoII centers usually favor the octahedral coordination geometry in contrast to the tetrahedral coordination sphere for ZnII ions.[17g] As expected, the N2 sorption isotherm for 446-MOF-Co or 446-MOF-Ni at 77 K is quite similar to that for 446-MOF (Figure 2 a), with a slight change of the saturated N2 uptakes (384.1 cm3 g¢1 for 446-MOF, 391.0 cm3 g¢1 for 446-MOF-Co, and 395.2 cm3 g¢1 for 446-MOF-Ni). However, it is interesting that 446-MOF-Co and 446-MOF-Ni show a significant increase in CO2 uptake, with the amounts of 236.2 and 234.8 cm3 g¢1 (16.3 and 15.6 % compared with that of 446-MOF, Figure 2 b). Similar phenomena are also found for CH4 trapping (Figure 2 b), in which the adsorption amounts of CH4 are promoted to 40.2 cm3 g¢1 for 446-MOF-Co and 41.6 cm3 g¢1 for 446-MOF-Ni (by 16.5 and 20.5 %). The higher sorption capacities of CO2 and Chem. Eur. J. 2015, 21, 9713 – 9719
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CH4 for both metal-modified analogues of 446-MOF can be properly attributed to the stronger interactions between CoII/ NiII ions and gas molecules.[19] It is universally acknowledged that the lanthanide metal ions, such as EuIII and TbIII, are excellent candidates for the preparation of luminescent materials.[20] The color for 446-MOF crystals dipped in a DMF solution of EuIII or TbIII at 80 8C will be unchanged. The ICP-MS results indicate that the molar ratios of Eu/Zn and Tb/Zn are 0.14:1 and 0.13:1, respectively (Figure 5 b). Considering the unmatched charge numbers between LnIII and ZnII, and also the ICP-MS results, it may be deduced that the EuIII and TbIII nitrates are resided in the 1D channels of 446-MOF, in which the Me2NH2 + cation can also be exchanged with EuIII or TbIII. The appearance of a characteristic sharp absorption band for the nitrate anion in IR spectroscopy for Eu@446-MOF or Tb@446-MOF (Supporting Information, Figure S1), and the significant decrease in their N2 sorption isotherms (Figure 2 a) will definitely support this speculation. In fact, the impregnation of EuIII or TbIII in Eu@446-MOF or Tb@446-MOF will not change the structural integrity for 446MOF (Supporting Information, Figure S2), which is also a single-crystal-to-single-crystal transformation (Supporting Information, Table S5), as that observed for 446-MOF-Co or 446MOF-Ni. Moreover, the luminescent behaviors of a series of Lndoped analogues, obtained by immersing the crystals of 446MOF into the DMF solutions of EuIII and TbIII with different molar ratios, can be systematically regulated. Upon the same
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Full Paper excited wavelength (lex = 324 nm), ligand-based luminescent emission will be similarly found with the peak maximum at 362 nm in all these Ln-doped analogues (Figure 6). However, the intensity of typical emission peak features (5D4 !7F6 at 489 nm, 5D4 !7F5 at 545 nm, 5D4 !7F4 at 587 nm, and 5D4 !7F3 at 621 nm) for TbIII is enhanced accompanied by the growth of TbIII content, with the intensity ratio of emissions at 545/ 362 nm changing from 0.85 (Eu/Tb = 9:1) to 2.49 (Tb@446MOF). Upon excitation with a UV lamp (lex = 365 nm), the EuIII and TbIII-doped hybrid materials exhibit their colors of medium purple and bright green, respectively, which can be readily recognized by naked eye. Notably, for the Eu/Tb co-doped materials, when the Eu/Tb ratio is less than 4:1, the sample is aqua blue, whereas further increase of the TbIII content will lead to different shades of green.
Experimental Section General materials and methods With the exception of the bridging ligand H3TATAB, which was prepared according to the literature approach,[22] all starting reagents and solvents were obtained commercially and used as received. Elemental analysis of C, H, and N was performed on a Vario EL III Elementar analyzer. FTIR spectra were measured on a Bruker Tensor 27 OPUS FTIR spectrometer (KBr pellet) in 4000–400 cm¢1. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku (model Ultima IV) diffractometer with a Rigaku D/teX ultrahigh-speed position sensitive detector and CuKa X-ray (40 kV, 40 mA). The intensity data were taken in the step-scan mode with the scan rate of 2 8 min¢1 and step size of 0.02 8. Thermogravimetric analysis (TGA) curves were taken on a PerkinElmer Diamond SII thermal analyzer from room temperature to 600 8C, with a heating rate of 10 8C min¢1 under nitrogen atmosphere. Inductively coupled plasma mass spectrometry (ICP-MS) analysis was conducted by using a PerkinElmer ELAN 9000 instrument after degradation of the samples in HNO3. The emission and excitation spectra of the powder samples were recorded on a Hitachi F-7000 spectrophotometer at room temperature. UV/Vis absorbance measurements were performed on a Hitachi U-3010 spectrophotometer. The gas adsorption isotherms were collected on a Micromeritics 3Flex surface area and pore size analyzer under ultrahigh vacuum in a clean system, with a diaphragm and turbo pumping system. Ultrahigh purity-grade (> 99.999 %) N2, CO2, CH4, and He gases were applied in adsorption measurements. The experimental temperatures were kept by liquid nitrogen (77 K) and dry ice-acetone baths (195 K), respectively.
Syntheses 446-MOF: A mixture of [Zn(OAc)2]·2 H2O (87.8 mg, 0.4 mmol), adenine (27.0 mg, 0.2 mmol), H3TATAB (97.3 mg, 0.2 mmol), and DMF (3 mL) was stirred for 5 min in a vial (10 mL) with the addition of 7 drops of HBF4 (40 %). The vial was sealed into a Teflon-lined stainless steel vessel, which was heated at 120 8C for 48 h in an oven. After cooling to room temperature, colorless hexagonal-prism crystals were obtained in 72 % yield (185.2 mg, based on H3TATAB or adenine). Elemental analysis (%) calcd for C48H73N17.5O16.25Zn2 : C 44.83; H 5.72; N 19.06; found: C 44.81, H 5.21, N 18.48. Figure 6. Top: emission spectra for Eu@446-MOF (1), Tb@446-MOF (11), and Eu/Tb co-doped 446-MOFs possessing different Eu/Tb ratios (0.45:0.05 for 2, 0.4:0.1 for 3, 0.35:0.15 for 4, 0.3:0.2 for 5, 0.25:0.25 for 6, 0.2:0.3 for 7, 0.15:0.35 for 8, 0.1:0.4 for 9, and 0.05:0.45 for 10). Bottom: photographs of Eu/Tb co-doped 446-MOFs illuminated with UV light (365 nm).
446-MOF-Co, 446-MOF-Ni, Eu@446-MOF, and Tb@446-MOF: Colorless crystals of 446-MOF were immersed in a DMF solution of metal nitrate (0.2 mol L¢1 for CoII or NiII ; 0.5 mol L¢1 for EuIII or TbIII) with heating at 80 8C. After cooling to room temperature, the crystalline products were thoroughly washed with DMF and then soaked in fresh DMF for one day (see the Supporting Information for the characterization of modified 446-MOF materials).
Conclusion
EuIII/TbIII co-doped analogues of 446-MOF: Colorless crystals of 446-MOF were immersed in a DMF solution (0.5 mol L¢1) with a mixture of [Eu(NO3)]3·6 H2O and [Tb(NO3)3]·6 H2O (0.45:0.05 for 2, 0.4:0.1 for 3, 0.35:0.15 for 4, 0.3:0.2 for 5, 0.25:0.25 for 6, 0.2:0.3 for 7, 0.15:0.35 for 8, 0.1:0.4 for 9, and 0.05:0.45 for 10). The crystal products were thoroughly washed with DMF and then soaked in fresh DMF for one day.
We present herein a new strategy of ligand symmetry modulation for designing mesoporous MOFs. The dual reactivity to transition and lanthanide metals for the same MOF has also been demonstrated, which can be used to achieve modified materials in the aspect of gas sorption and photoluminescence. Further applications of this strategy for the construction of new mesoporous MOFs based on high symmetry ligands and specific metal clusters are underway.[21]
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Single-crystal X-ray crystallography Single-crystal X-ray diffraction data were collected on an Oxford Xcalibur Gemini Eos diffractometer by using graphite-monochromated CuKa radiation (l = 1.54178 æ) at 295(2) K. Multi-scan absorption corrections were performed with CrysAlisPro[23] and empirical
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Full Paper absorption corrections were carried out with spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. The crystal structure was solved by direct methods, and all non-H atoms were refined anisotropically by full-matrix least-squares method with the SHELXTL crystallographic software package.[24] H atoms of the framework ligands were located in calculated positions and treated in the subsequent refinement as ridings. The guest Me2NH2 + cations and solvents cannot be crystallographically defined for the very large voids. Therefore, the SQUEEZE routine, a part of the PLATON software package,[15] was applied to calculate the solvent disorder areas and remove their diffraction contribution to afford a set of guest free diffraction intensity. The adeninate ligand was treated by using a disorder model, in which pseudo-isotropic (ISOR) restraint was imposed on. Crystallographic data and structural refinement details for 446-MOF are summarized in Table S3 in the Supporting Information. Selected bond lengths and angles are shown in Table S4 in the Supporting Information. CCDC-1036498 (446-MOF) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
Activation of 446-MOF and its metal-modified analogues Each as-synthesized crystalline sample was soaked in DMF for two days, during which the solvent was refreshed three times. After that, the sample was activated by supercritical carbon dioxide drying (SCD) process in a Tousimis Samdri PVT-3D critical point dryer. The sample was placed in the chamber and the solvent was exchanged with liquid CO2. The CO2 filling was kept for one hour and followed by a purge vent. Such a refilling and purging of liquid CO2 recycle was repeated three times. After that, the chamber with the MOF sample and liquid CO2 was heated to 40 8C and retained under the supercritical condition (typically 1300 psi) for one hour. The above activated sample was transferred into a preweighed 12 mm sample tube, evacuated (< 10¢3 Torr) at room temperature overnight prior to gas sorption.
Dye adsorption The as-synthesized 446-MOF crystals (10.0 mg) were transferred into a DMF solution (3 mL, 5 Õ 10¢6 mol L¢1) of methyl blue (MB), crystal violet (CV), rhodamine 6G (R6 G), or food green 3 (FG3) in a cuvette. Then, the upper clear solution was taken out at 0, 1, 2, 3, 4, 5, and 6 h, respectively, for UV/Vis absorbance measurement. The maximum absorbance of original solution for dye (before adsorption) was normalized and the percentage of dye remaining in solution was calculated by comparing its absorbance maximum with that of the original solution for dye.
UV/Vis spectra determination of the dye content The as-synthesized 446-MOF crystals (10.0 mg) were transferred into a DMF solution (3 mL) of MB (1.6 Õ 10¢4 mol L¢1), CV (4.0 Õ 10¢5 mol L¢1), or R6 G (2.25 Õ 10¢5 mol L¢1) in a cuvette. Then, the upper clear solution was taken out for UV/Vis measurement after one day. The concentration of final solution was determined by reference to the standard curve. The dye content was the difference between the original and final solutions. Chem. Eur. J. 2015, 21, 9713 – 9719
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Acknowledgements We are grateful to the support from the National Natural Science Foundation of China (nos. 21031002, 21171151, and 21201154), Program for Innovative Research Team in University of Tianjin (no. TD12-5038), Program for Science and Technology Innovative Research Team in University of Henan Province (no. 15IRTSTHN-002), and Plan for Scientific Innovation Talent of Henan Province. Keywords: cluster compounds · mesoporous materials · metal–organic frameworks · synthetic methods · zinc [1] a) S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem. Int. Ed. 2004, 43, 2334 – 2375; Angew. Chem. 2004, 116, 2388 – 2430; b) H. Furukawa, K. E. Cordova, M. O’Keeffe, O. M. Yaghi, Science 2013, 341, 1230444; c) Z. Zhang, M. J. Zaworotko, Chem. Soc. Rev. 2014, 43, 5444 – 5455; d) Q.-L. Zhu, Q. Xu, Chem. Soc. Rev. 2014, 43, 5468 – 5512; e) J.-P. Zhang, P.-Q. Liao, H.-L. Zhou, R.-B. Lin, X.-M. Chen, Chem. Soc. Rev. 2014, 43, 5789 – 5814. [2] a) S. R. Batten, S. M. Neville, D. R. Turner, Coordination Polymers: Design, Analysis and Application, RSC, Cambridge, 2008; b) M. Schrçder, Functional Metal – Organic Frameworks: Gas Storage, Separation and Catalysis, Springer, Berlin, 2010; c) L. R. MacGillivray, Metal – Organic Frameworks: Design and Application, Wiley, Hoboken, 2010. [3] M. Du, M. Chen, X.-G. Yang, J. Wen, X. Wang, S.-M. Fang, C.-S. Liu, J. Mater. Chem. A 2014, 2, 9828 – 9834. [4] a) Q.-R. Fang, G.-S. Zhu, Z. Jin, Y.-Y. Ji, J.-W. Ye, M. Xue, H. Yang, Y. Wang, S.-L. Qiu, Angew. Chem. Int. Ed. 2007, 46, 6638 – 6642; Angew. Chem. 2007, 119, 6758 – 6762; b) H. Deng, S. Grunder, K. E. Cordova, C. Valente, H. Furukawa, M. Hmadeh, F. Gndara, A. C. Whalley, Z. Liu, S. Asahina, H. Kazumori, M. O’Keeffe, O. Terasaki, J. F. Stoddart, O. M. Yaghi, Science 2012, 336, 1018 – 1023; c) Q.-R. Fang, T. A. Makal, M. D. Young, H.-C. Zhou, Comments Inorg. Chem. 2010, 31, 165 – 195; d) W. Xuan, C. Zhu, Y. Liu, Y. Cui, Chem. Soc. Rev. 2012, 41, 1677 – 1695; e) L. Song, J. Zhang, L. Sun, F. Xu, F. Li, H. Zhang, X. Si, C. Jiao, Z. Li, S. Liu, Y. Liu, H. Zhou, D. Sun, Y. Du, Z. Cao, Z. Gabelica, Energy Environ. Sci. 2012, 5, 7508 – 7520. [5] M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe, O. M. Yaghi, Science 2002, 295, 469 – 472. [6] N. L. Rosi, J. Kim, M. Eddaoudi, B. Chen, M. O’Keeffe, O. M. Yaghi, J. Am. Chem. Soc. 2005, 127, 1504 – 1518. [7] a) D. Yuan, R. B. Getman, Z. Wei, R. Q. Snurr, H.-C. Zhou, Chem. Commun. 2012, 48, 3297 – 3299; b) H. Chevreau, T. Devic, F. Salles, G. Maurin, N. Stock, C. Serre, Angew. Chem. Int. Ed. 2013, 52, 5056 – 5060; Angew. Chem. 2013, 125, 5160 – 5164. [8] a) H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi, A. ©. Yazaydin, R. Q. Snurr, M. O’Keeffe, J. Kim, O. M. Yaghi, Science 2010, 329, 424 – 428; b) Y.-Q. Lan, H.-L. Jiang, S.-L. Li, Q. Xu, Adv. Mater. 2011, 23, 5015 – 5020. [9] a) G. F¦rey, C. Serre, C. Mellot-Draznieks, F. Millange, S. Surbl¦, J. Dutour, I. Margiolaki, Angew. Chem. Int. Ed. 2004, 43, 6296 – 6301; Angew. Chem. 2004, 116, 6456 – 6461; b) G. F¦rey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surbl¦, I. Margiolaki, Science 2005, 309, 2040 – 2042; c) A. Sonnauer, F. Hoffmann, M. Frçba, L. Kienle, V. Duppel, M. Thommes, C. Serre, G. F¦rey, N. Stock, Angew. Chem. Int. Ed. 2009, 48, 3791 – 3794; Angew. Chem. 2009, 121, 3849 – 3852. [10] a) J. B. DeCoste, G. W. Peterson, H. Jasuja, T. G. Glover, Y.-G. Huang, K. S. Walton, J. Mater. Chem. A 2013, 1, 5642 – 5650; b) H.-L. Jiang, D. Feng, K. Wang, Z.-Y. Gu, Z. Wei, Y.-P. Chen, H.-C. Zhou, J. Am. Chem. Soc. 2013, 135, 13934 – 13938. [11] a) J. An, O. K. Farha, J. T. Hupp, E. Pohl, J. I. Yeh, N. L. Rosi, Nat. Commun. 2012, 3, 604; b) T. Li, M. T. Kozlowski, E. A. Doud, M. N. Blakely, N. L. Rosi, J. Am. Chem. Soc. 2013, 135, 11688 – 11691. [12] a) J. An, S. J. Geib, N. L. Rosi, J. Am. Chem. Soc. 2009, 131, 8376 – 8377; b) H. Xu, J. Cai, S. Xiang, Z. Zhang, C. Wu, X. Rao, Y. Cui, Y. Yang, R. Krishna, B. Chen, G. Qian, J. Mater. Chem. A 2013, 1, 9916 – 9921. [13] D. Zhao, D. J. Timmons, D. Yuan, H.-C. Zhou, Acc. Chem. Res. 2011, 44, 123 – 133.
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[19] a) M. Dinca˘, J. R. Long, J. Am. Chem. Soc. 2007, 129, 11172 – 11176; b) X. Song, T. K. Kim, H. Kim, D. Kim, S. Jeong, H. R. Moon, M. S. Lah, Chem. Mater. 2012, 24, 3065 – 3073. [20] J. Rocha, L. D. Carlos, F. A. A. Paz, D. Ananias, Chem. Soc. Rev. 2011, 40, 926 – 940. [21] In fact, the generality of such a designing strategy through ligand symmetry modulation for mesoporous MOFs materials requires further verification by more experimental examples in practice. [22] G. M. Pogosyan, V. N. Zaplishnyi, I. A. Asaturyan, Arm. Khim. Zh. 1977, 30, 342 – 348. [23] CrysAlis CCD and CrysAlis RED, version 1.171.35.21, Oxford Diffraction Ltd., Yarnton, Oxfordshire, 2008. [24] a) G. M. Sheldrick, SHELXTL, version 6.10, Bruker Analytical X-ray Systems, Madison, WI, 2001; b) G. M. Sheldrick, Acta Crystallogr. A 2008, 64, 112 – 122.
Received: February 4, 2015 Published online on May 26, 2015
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& Metal–Organic Frameworks
Ligand Symmetry Modulation for Designing a Mesoporous Metal– Organic Framework: Dual Reactivity to Transition and Lanthanide Metals for Enhanced Functionalization Miao Du,*[a, b] Xi Wang,[a, b] Min Chen,[b] Cheng-Peng Li,[a] Jia-Yue Tian,[b] Zhuo-Wei Wang,[b] and Chun-Sen Liu*[b] Abstract: A promising alternative strategy for designing mesoporous metal–organic frameworks (MOFs) has been proposed, by modifying the symmetry rather than expanding the length of organic linkers. By means of this approach, a unique MOF material based on the target [Zn8(ad)4] (ad = adeninate) clusters and C3-symmetric organic linkers can be obtained, with trigonal microporous (ca., 0.8 nm) and hexag-
Introduction Metal–organic frameworks (MOFs), constructed through the coordination linkages of inorganic (metal ions or clusters) and organic (linkers) structural units, have emerged as a promising class of crystalline materials for their tunable porous nets and unusual properties.[1, 2] The pore sizes for such porous systems can range from ultramicroporous (< 3 æ), microporous (3 æ– 2 nm), even up to mesoporous (2–50 nm) regions. However, among tens of thousands MOFs reported so far, only about 100 examples possess the available mesoporous pores (Supporting Information, Table S1).[3] In fact, the preparation of mesoporous MOFs represents a great challenge for their intrinsic resistance of crystalline lattice extension to form large cavities, especially those close to or even over 3 nm.[3, 4] As a consequence, the development of effective strategy for the design of mesoporous MOFs will be of extreme significance, considering the serious limitations for the applications of microporous MOFs materials caused by their smaller pores that cannot hold larger guest moiety and also are disadvantage to fast molecular diffusion.[4] [a] Prof. M. Du, X. Wang, Dr. C.-P. Li College of Chemistry, Tianjin Key Laboratory of Structure and Performance for Functional Molecules MOE Key Laboratory of Inorganic–Organic Hybrid Functional Material Chemistry, Tianjin Normal University Tianjin 300387 (P.R. China) E-mail: [email protected] [b] Prof. M. Du, X. Wang, Dr. M. Chen, J.-Y. Tian, Z.-W. Wang, Prof. C.-S. Liu Henan Provincial Key Laboratory of Surface and Interface Science Zhengzhou University of Light Industry, Zhengzhou 450002 (P.R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500457. Chem. Eur. J. 2015, 21, 9713 – 9719
onal mesoporous (ca., 3.0 nm) 1D channels. Moreover, the resulting 446-MOF shows distinct reactivity to transition and lanthanide metal ions. Significantly, the transmetalation of CoII or NiII on the ZnII centers in 446-MOF can enhance the sorption capacities of CO2 and CH4 (16–21 %), whereas the impregnation of EuIII and TbIII in the channels of 446-MOF will result in adjustable light-emitting behaviors.
In this context, ligand extension in specific targeting network topologies has been applied to successfully enlarge the pores of MOFs. Early in 2002, Yaghi et al. prepared the first mesoporous MOF (IRMOF-16) with a long linker p-terphenyl4,4’’-dicarboxylic acid, which takes the expected topology of CaB6 adapted by the prototype IRMOF-1.[5] Recently, this concept was again demonstrated by Yaghi et al. for a systematic expansion of MOF-74 to afford an isoreticular series of structures with the pore apertures ranging from 14 to 98 æ (IRMOF74-I to XI).[4b, 6] Another alternative strategy to design mesoporous MOFs involves the utilization of metal-cluster vertexes with bulky sizes as the building units, in which metal-carboxylate clusters, such as [Fe3O(CO2)6],[7] [Zn4O(CO2)6],[6, 8] [Cr3O(CO2)6],[9] and [Zr6O4(OH)4][10] are mostly known. In this context, such metal clusters, normally comprising part of the organic linkers, are formed in situ during the synthesis of MOFs, which thus inevitably leads to their high uncertainty. Inspiringly, Rosi et al. illustrated a design strategy for porous MOFs with metal-adeninate clusters and different dicarboxylate linkers. Analysis of this series of ZnII-MOFs reveals that the large [Zn8(ad)4] (ad = adeninate) units will be invariably formed in the resulting host frameworks, either mesoporous (bio-MOF100[11a] and its isoreticular analogues bio-MOF-101, 102, and 103[11b]) or microporous (bio-MOF-1[12a] and ZJU-48[12b]). Notably, bio-MOF-1 and bio-MOF-100 are constructed with the same dicarboxylate ligand, in which infinite [Zn8(ad)4O]n chains and discrete [Zn8(ad)4] building units are afforded, respectively (Supporting Information, Table S2). Mathematically, the larger channel level will be achieved (Scheme 1) when the channel is transformed from quadrangle (with fourfold symmetry) to hexagon (with threefold or sixfold symmetry). Empirically, the tricarboxylate ligands with C3 symmetry will induce the crystallization of MOFs with hexagonal channels,[13] whereas the linear
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Full Paper dicarboxylate linkers may mostly lead to the quadrangle voids of porous coordination frameworks, just as observed in bioMOF-1.[12a] Thus, we propose here a new strategy for designing mesoporous MOFs with the reliable larger metal clusters by increasing the symmetry of organic linkers. As a result, a unique mesoporous MOF material can be constructed, which shows different trapping mechanisms to bivalent transition metals (cation exchange) and trivalent lanthanide metals (guest inclusion). Remarkably, such modified MOF analogues will show enhanced gas uptake capacities and adjustable light emissions.
group, the trigonal TATAB linker normally takes the planar pinwheel-like configuration in the reported structures.[14] However, in this case, the TATAB ligand adopts the highly distorted Y-like conformation (Supporting Information, Figure S6), in which two branches of the trigonal tecton are close to each other with the distance between their carboxylate carbon atoms of only 6.62 æ and in comparison, the other one is far away from them with such distances of 13.45 and 15.25 æ. As a result of the unidentate coordination of each carboxylate group (Supporting Information, Figure S4 b) and this unique conformation, two coupled TATAB ligands actually act as a linear connector between two adjacent octahedral cages at each open face (Figure 1 b).
Scheme 1. Schematic representation of the design strategy for mesoporous MOFs in this work.
Results and Discussion Structural design and analysis for 446-MOF Solvothermal reaction of [Zn(OAc)2]·2 H2O, adenine, HBF4, and 4,4’,4’’-s-triazine-1,3,5-triyltri-p-aminobenzoic acid (H3TATAB; Scheme S1, Supporting Information) in DMF at 120 8C affords colorless hexagonal-prism crystals for [Zn2(ad)(TATAB)O1/4](Me2NH2)1/2(DMF)6(H2O)4 (446-MOF). Single-crystal X-ray diffraction analysis reveals that 446-MOF crystallizes in primitive hexagonal space group P622, with two types of microporous and mesoporous open channels. The asymmetric unit holds three crystallographically independent ZnII ions (occupancy factors: 1 for Zn1 and 1/2 for Zn2 or Zn3), one 1/4-occupied m4-O2¢ anion, and fully deprotonated adeninate and TATAB. All ZnII centers are coordinated by two nitrogen donors from different adeninates and two oxygen atoms from TATAB or m4-O2¢, taking distorted tetrahedral spheres (Supporting Information, Figure S4 a). As expected, four rigid adeninate tectons are combined to eight ZnII ions to complete a [Zn8(ad)4] octahedral cage (Supporting Information, Figure S5), which is similar to that observed in bio-MOFs series.[11a] Such adjacent [Zn8(ad)4] cages interact with each other through the m4-O2¢ bridging on Zn1 ions, located at the axial sites of octahedra, which affords columnar [Zn8(ad)4O]n secondary building units (SBUs) along the c axis (Figure 1 a). Despite the flexibility of extended imino Chem. Eur. J. 2015, 21, 9713 – 9719
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Figure 1. View of: a) 1D [Zn8(ad)4O]n SBU, and b) coupled TATAB connector between two adjacent [Zn8(ad)4] cages in 446-MOF. c) Space-filling model for 446-MOF with two types of 1D channels along the c axis. d) Comparison of the structural features for microporous bio-MOF-1 and mesoporous 446MOF.
It is quite instructive to compare the structural parameters for bio-MOF-1[11a] and 446-MOF, which consist of the same columnar [Zn8(ad)4O]n SBUs but different carboxylate linkers. For both cases, each [Zn8(ad)4] octahedral cage interacts with further four cages through linear connectors (coupled ligands in 446-MOF or triply ligands in bio-MOF-1) along different directions to produce a distorted cage-connector tetrahedron (Supporting Information, Figure S7). In bio-MOF-1, such a distorted tetrahedral unit has two sets of vertex–center–vertex angles, two of 132.66 8 and four of 99.28 8, whereas all angles should be 109.5 8 for a perfect tetrahedron. In comparison, the tetrahedron in 446-MOF shows a more distorted configuration, in which three pairs of such angles are 78.13, 126.74, and
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Full Paper 127.41 8, because of the flexibility of TATAB ligand (Supporting Information, Figure S7). As a consequence, very large 1D hexagonal mesoporous channels, each of which is surrounded by six trigonal microporous channels, are afforded in 446-MOF (Figure 1 c), with the approximate diameters of 3.0 and 0.8 nm, respectively (considering the van der Waals radii of atoms). Such mesoporous and microporous channels are constituted by sixfold right-handed helices and threefold left-handed helices (Figure 1 c). In contrast to bio-MOF-1 with the square microporous channels, the symmetry is transferred from tetragonal system for bio-MOF-1 to hexagonal system for 446-MOF, clearly confirming our proposed strategy of ligand symmetry modulation for designing mesoporous MOFs with specific vertexes (Figure 1 d). The intricate 3D structure of 446-MOF can be simplified to a uninodal 6-connected network with the point symbol of (48.67), by considering each [Zn8(ad)4] cage as the net node (Supporting Information, Figure S8), which notably represents an unprecedented topological type.[2a]
Porosity for 446-MOF As estimated by PLATON[15] with the 1.8 æ probe radius, the free volume of host framework in 446-MOF is 12 005.5 æ3 (61.4 % per unit cell volume) without consideration of the lattice solvent and dimethylammonium cation. The permanent porosity of 446-MOF can be conventionally confirmed by nitrogen sorption isotherm at 77 K for the desolvated material (the Experimental Section), which exhibits a Type IV isotherm with a saturated N2 uptake of 384.1 cm3 g¢1 (Figure 2 a) and a Brunauer–Emmett–Teller (BET) surface area of 1605 m2 g¢1. Also, 446MOF shows the CO2 and CH4 adsorption capacity of 203.1 and 34.5 cm3 g¢1, respectively, at 195 K (Figure 2 b). The dye-uptake experiments have also been used to visually characterize porous MOFs with large open channels or pores.[16] In this case, by soaking the 446-MOF crystals in a DMF solution of methyl blue (MB), crystal violet (CV), or rhodamine 6G (R6 G), the solution will gradually fade and the colorless crystals of 446-MOF at the bottom of each cuvette accordingly turn to turquoise blue, brilliant blue, and pink, respectively. In a parallel experiment with larger food green 3 (FG3) adsorbate, no obvious change is observed (Figure 3 a and the Supporting Information, Figure S10). The sorption processes of dyes can be monitored by UV/Vis spectra (Supporting Information, Figure S11), which clearly show the decrease of dye concentration for MB, CV, and R6 G at different rates, and as expected, the inalterability for FG3 (Figure 3 b). A proper explanation for this result can be made based on the sizes of dyes (Supporting Information, Figure S10), in which the largest dimension of FG3 (27.4 æ) is close to the available pore size for 446-MOF (30.0 æ). Further evaluation by UV/Vis (Supporting Information, Figures S12–15) reveals that the adsorptivity of 446MOF is 17.0 mg g¢1 for MB, 3.70 mg g¢1 for CV, and 1.28 mg g¢1 for R6 G, also agreeing well with the results observed above. Moreover, the dye-adsorbed 446-MOFs materials still keep their crystallinity (Supporting Information, Figure S16). Chem. Eur. J. 2015, 21, 9713 – 9719
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Figure 2. a) N2 adsorption isotherms of 446-MOF, 446-MOF-Co, 446-MOF-Ni, Eu@446-MOF, and Tb@446-MOF at 77 K. b) CO2/CH4 adsorption isotherms of 446-MOF, 446-MOF-Co, and 446-MOF-Ni at 195 K.
Reactivity of 446-MOF to metal ions Post-synthesis modification by exchanging the original metal sites with other functional metal ions (e.g., active CoII and NiII as well as luminescent EuIII and TbIII), is a potential approach to design and synthesize new MOF materials with desired properties that cannot be achieved by using de novo synthesis.[17] Nevertheless, such examples are relatively rare in the known MOFs.[17d] When the crystals of 446-MOF are immersed in a DMF solution of CoII or NiII ion with heating at 80 8C for one week, the crystals will turn purple or green (Figure 4) without losing their structural integrity (Supporting Information, Figure S2). Moreover, single-crystal X-ray diffraction shows that the framework of 446-MOF-Co or 446-MOF-Ni is the same as that of 446-MOF (Supporting Information, Table S5). The molar ratio of Co/Zn or Ni/Zn in the final product is 0.64:1 or 0.53:1 (Figure 5 a), as monitored by using inductively coupled plasma mass spectrometry (ICP-MS) for the reaction processes. As reported, the Me2NH2 + cations residing as counterions in the pores or channels of MOFs can be easily exchanged with additional metal ions.[18] Moreover, the exposed Zn2 and Zn3 around the 1D channels in 446-MOF can get more contact with the implanting CoII or NiII, compared with Zn1 in the joints between adjacent [Zn8(ad)4] clusters. Thus, it can be concluded that the Me2NH2 + cations and a part of Zn2 and Zn3 are exchanged by CoII (63 %) or NiII (53 %) in this
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Figure 5. The molar ratios of metal/ZnII (ICP-MS) for metal-modified 446MOF samples, in which the metals are: a) CoII or NiII, and b) EuIII or TbIII.
Figure 3. a) 446-MOF samples after dye-adsorption for 6 h; b) percentage of dye in solution monitored with time in the presence of 446-MOF.
Figure 4. Photographs of 446-MOF-Co, 446-MOF, and 446-MOF-Ni samples.
course. The complete exchange of ZnII by CoII or NiII cannot be achieved by further extending the reaction time or increasing the reaction temperature. This may result from the fact that NiII or CoII centers usually favor the octahedral coordination geometry in contrast to the tetrahedral coordination sphere for ZnII ions.[17g] As expected, the N2 sorption isotherm for 446-MOF-Co or 446-MOF-Ni at 77 K is quite similar to that for 446-MOF (Figure 2 a), with a slight change of the saturated N2 uptakes (384.1 cm3 g¢1 for 446-MOF, 391.0 cm3 g¢1 for 446-MOF-Co, and 395.2 cm3 g¢1 for 446-MOF-Ni). However, it is interesting that 446-MOF-Co and 446-MOF-Ni show a significant increase in CO2 uptake, with the amounts of 236.2 and 234.8 cm3 g¢1 (16.3 and 15.6 % compared with that of 446-MOF, Figure 2 b). Similar phenomena are also found for CH4 trapping (Figure 2 b), in which the adsorption amounts of CH4 are promoted to 40.2 cm3 g¢1 for 446-MOF-Co and 41.6 cm3 g¢1 for 446-MOF-Ni (by 16.5 and 20.5 %). The higher sorption capacities of CO2 and Chem. Eur. J. 2015, 21, 9713 – 9719
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CH4 for both metal-modified analogues of 446-MOF can be properly attributed to the stronger interactions between CoII/ NiII ions and gas molecules.[19] It is universally acknowledged that the lanthanide metal ions, such as EuIII and TbIII, are excellent candidates for the preparation of luminescent materials.[20] The color for 446-MOF crystals dipped in a DMF solution of EuIII or TbIII at 80 8C will be unchanged. The ICP-MS results indicate that the molar ratios of Eu/Zn and Tb/Zn are 0.14:1 and 0.13:1, respectively (Figure 5 b). Considering the unmatched charge numbers between LnIII and ZnII, and also the ICP-MS results, it may be deduced that the EuIII and TbIII nitrates are resided in the 1D channels of 446-MOF, in which the Me2NH2 + cation can also be exchanged with EuIII or TbIII. The appearance of a characteristic sharp absorption band for the nitrate anion in IR spectroscopy for Eu@446-MOF or Tb@446-MOF (Supporting Information, Figure S1), and the significant decrease in their N2 sorption isotherms (Figure 2 a) will definitely support this speculation. In fact, the impregnation of EuIII or TbIII in Eu@446-MOF or Tb@446-MOF will not change the structural integrity for 446MOF (Supporting Information, Figure S2), which is also a single-crystal-to-single-crystal transformation (Supporting Information, Table S5), as that observed for 446-MOF-Co or 446MOF-Ni. Moreover, the luminescent behaviors of a series of Lndoped analogues, obtained by immersing the crystals of 446MOF into the DMF solutions of EuIII and TbIII with different molar ratios, can be systematically regulated. Upon the same
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Full Paper excited wavelength (lex = 324 nm), ligand-based luminescent emission will be similarly found with the peak maximum at 362 nm in all these Ln-doped analogues (Figure 6). However, the intensity of typical emission peak features (5D4 !7F6 at 489 nm, 5D4 !7F5 at 545 nm, 5D4 !7F4 at 587 nm, and 5D4 !7F3 at 621 nm) for TbIII is enhanced accompanied by the growth of TbIII content, with the intensity ratio of emissions at 545/ 362 nm changing from 0.85 (Eu/Tb = 9:1) to 2.49 (Tb@446MOF). Upon excitation with a UV lamp (lex = 365 nm), the EuIII and TbIII-doped hybrid materials exhibit their colors of medium purple and bright green, respectively, which can be readily recognized by naked eye. Notably, for the Eu/Tb co-doped materials, when the Eu/Tb ratio is less than 4:1, the sample is aqua blue, whereas further increase of the TbIII content will lead to different shades of green.
Experimental Section General materials and methods With the exception of the bridging ligand H3TATAB, which was prepared according to the literature approach,[22] all starting reagents and solvents were obtained commercially and used as received. Elemental analysis of C, H, and N was performed on a Vario EL III Elementar analyzer. FTIR spectra were measured on a Bruker Tensor 27 OPUS FTIR spectrometer (KBr pellet) in 4000–400 cm¢1. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku (model Ultima IV) diffractometer with a Rigaku D/teX ultrahigh-speed position sensitive detector and CuKa X-ray (40 kV, 40 mA). The intensity data were taken in the step-scan mode with the scan rate of 2 8 min¢1 and step size of 0.02 8. Thermogravimetric analysis (TGA) curves were taken on a PerkinElmer Diamond SII thermal analyzer from room temperature to 600 8C, with a heating rate of 10 8C min¢1 under nitrogen atmosphere. Inductively coupled plasma mass spectrometry (ICP-MS) analysis was conducted by using a PerkinElmer ELAN 9000 instrument after degradation of the samples in HNO3. The emission and excitation spectra of the powder samples were recorded on a Hitachi F-7000 spectrophotometer at room temperature. UV/Vis absorbance measurements were performed on a Hitachi U-3010 spectrophotometer. The gas adsorption isotherms were collected on a Micromeritics 3Flex surface area and pore size analyzer under ultrahigh vacuum in a clean system, with a diaphragm and turbo pumping system. Ultrahigh purity-grade (> 99.999 %) N2, CO2, CH4, and He gases were applied in adsorption measurements. The experimental temperatures were kept by liquid nitrogen (77 K) and dry ice-acetone baths (195 K), respectively.
Syntheses 446-MOF: A mixture of [Zn(OAc)2]·2 H2O (87.8 mg, 0.4 mmol), adenine (27.0 mg, 0.2 mmol), H3TATAB (97.3 mg, 0.2 mmol), and DMF (3 mL) was stirred for 5 min in a vial (10 mL) with the addition of 7 drops of HBF4 (40 %). The vial was sealed into a Teflon-lined stainless steel vessel, which was heated at 120 8C for 48 h in an oven. After cooling to room temperature, colorless hexagonal-prism crystals were obtained in 72 % yield (185.2 mg, based on H3TATAB or adenine). Elemental analysis (%) calcd for C48H73N17.5O16.25Zn2 : C 44.83; H 5.72; N 19.06; found: C 44.81, H 5.21, N 18.48. Figure 6. Top: emission spectra for Eu@446-MOF (1), Tb@446-MOF (11), and Eu/Tb co-doped 446-MOFs possessing different Eu/Tb ratios (0.45:0.05 for 2, 0.4:0.1 for 3, 0.35:0.15 for 4, 0.3:0.2 for 5, 0.25:0.25 for 6, 0.2:0.3 for 7, 0.15:0.35 for 8, 0.1:0.4 for 9, and 0.05:0.45 for 10). Bottom: photographs of Eu/Tb co-doped 446-MOFs illuminated with UV light (365 nm).
446-MOF-Co, 446-MOF-Ni, Eu@446-MOF, and Tb@446-MOF: Colorless crystals of 446-MOF were immersed in a DMF solution of metal nitrate (0.2 mol L¢1 for CoII or NiII ; 0.5 mol L¢1 for EuIII or TbIII) with heating at 80 8C. After cooling to room temperature, the crystalline products were thoroughly washed with DMF and then soaked in fresh DMF for one day (see the Supporting Information for the characterization of modified 446-MOF materials).
Conclusion
EuIII/TbIII co-doped analogues of 446-MOF: Colorless crystals of 446-MOF were immersed in a DMF solution (0.5 mol L¢1) with a mixture of [Eu(NO3)]3·6 H2O and [Tb(NO3)3]·6 H2O (0.45:0.05 for 2, 0.4:0.1 for 3, 0.35:0.15 for 4, 0.3:0.2 for 5, 0.25:0.25 for 6, 0.2:0.3 for 7, 0.15:0.35 for 8, 0.1:0.4 for 9, and 0.05:0.45 for 10). The crystal products were thoroughly washed with DMF and then soaked in fresh DMF for one day.
We present herein a new strategy of ligand symmetry modulation for designing mesoporous MOFs. The dual reactivity to transition and lanthanide metals for the same MOF has also been demonstrated, which can be used to achieve modified materials in the aspect of gas sorption and photoluminescence. Further applications of this strategy for the construction of new mesoporous MOFs based on high symmetry ligands and specific metal clusters are underway.[21]
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Single-crystal X-ray crystallography Single-crystal X-ray diffraction data were collected on an Oxford Xcalibur Gemini Eos diffractometer by using graphite-monochromated CuKa radiation (l = 1.54178 æ) at 295(2) K. Multi-scan absorption corrections were performed with CrysAlisPro[23] and empirical
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Full Paper absorption corrections were carried out with spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. The crystal structure was solved by direct methods, and all non-H atoms were refined anisotropically by full-matrix least-squares method with the SHELXTL crystallographic software package.[24] H atoms of the framework ligands were located in calculated positions and treated in the subsequent refinement as ridings. The guest Me2NH2 + cations and solvents cannot be crystallographically defined for the very large voids. Therefore, the SQUEEZE routine, a part of the PLATON software package,[15] was applied to calculate the solvent disorder areas and remove their diffraction contribution to afford a set of guest free diffraction intensity. The adeninate ligand was treated by using a disorder model, in which pseudo-isotropic (ISOR) restraint was imposed on. Crystallographic data and structural refinement details for 446-MOF are summarized in Table S3 in the Supporting Information. Selected bond lengths and angles are shown in Table S4 in the Supporting Information. CCDC-1036498 (446-MOF) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
Activation of 446-MOF and its metal-modified analogues Each as-synthesized crystalline sample was soaked in DMF for two days, during which the solvent was refreshed three times. After that, the sample was activated by supercritical carbon dioxide drying (SCD) process in a Tousimis Samdri PVT-3D critical point dryer. The sample was placed in the chamber and the solvent was exchanged with liquid CO2. The CO2 filling was kept for one hour and followed by a purge vent. Such a refilling and purging of liquid CO2 recycle was repeated three times. After that, the chamber with the MOF sample and liquid CO2 was heated to 40 8C and retained under the supercritical condition (typically 1300 psi) for one hour. The above activated sample was transferred into a preweighed 12 mm sample tube, evacuated (< 10¢3 Torr) at room temperature overnight prior to gas sorption.
Dye adsorption The as-synthesized 446-MOF crystals (10.0 mg) were transferred into a DMF solution (3 mL, 5 Õ 10¢6 mol L¢1) of methyl blue (MB), crystal violet (CV), rhodamine 6G (R6 G), or food green 3 (FG3) in a cuvette. Then, the upper clear solution was taken out at 0, 1, 2, 3, 4, 5, and 6 h, respectively, for UV/Vis absorbance measurement. The maximum absorbance of original solution for dye (before adsorption) was normalized and the percentage of dye remaining in solution was calculated by comparing its absorbance maximum with that of the original solution for dye.
UV/Vis spectra determination of the dye content The as-synthesized 446-MOF crystals (10.0 mg) were transferred into a DMF solution (3 mL) of MB (1.6 Õ 10¢4 mol L¢1), CV (4.0 Õ 10¢5 mol L¢1), or R6 G (2.25 Õ 10¢5 mol L¢1) in a cuvette. Then, the upper clear solution was taken out for UV/Vis measurement after one day. The concentration of final solution was determined by reference to the standard curve. The dye content was the difference between the original and final solutions. Chem. Eur. J. 2015, 21, 9713 – 9719
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Acknowledgements We are grateful to the support from the National Natural Science Foundation of China (nos. 21031002, 21171151, and 21201154), Program for Innovative Research Team in University of Tianjin (no. TD12-5038), Program for Science and Technology Innovative Research Team in University of Henan Province (no. 15IRTSTHN-002), and Plan for Scientific Innovation Talent of Henan Province. Keywords: cluster compounds · mesoporous materials · metal–organic frameworks · synthetic methods · zinc [1] a) S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem. Int. Ed. 2004, 43, 2334 – 2375; Angew. Chem. 2004, 116, 2388 – 2430; b) H. Furukawa, K. E. Cordova, M. O’Keeffe, O. M. Yaghi, Science 2013, 341, 1230444; c) Z. Zhang, M. J. Zaworotko, Chem. Soc. Rev. 2014, 43, 5444 – 5455; d) Q.-L. Zhu, Q. Xu, Chem. Soc. Rev. 2014, 43, 5468 – 5512; e) J.-P. Zhang, P.-Q. Liao, H.-L. Zhou, R.-B. Lin, X.-M. Chen, Chem. Soc. Rev. 2014, 43, 5789 – 5814. [2] a) S. R. Batten, S. M. Neville, D. R. Turner, Coordination Polymers: Design, Analysis and Application, RSC, Cambridge, 2008; b) M. Schrçder, Functional Metal – Organic Frameworks: Gas Storage, Separation and Catalysis, Springer, Berlin, 2010; c) L. R. MacGillivray, Metal – Organic Frameworks: Design and Application, Wiley, Hoboken, 2010. [3] M. Du, M. Chen, X.-G. Yang, J. Wen, X. Wang, S.-M. Fang, C.-S. Liu, J. Mater. Chem. A 2014, 2, 9828 – 9834. [4] a) Q.-R. Fang, G.-S. Zhu, Z. Jin, Y.-Y. Ji, J.-W. Ye, M. Xue, H. Yang, Y. Wang, S.-L. Qiu, Angew. Chem. Int. Ed. 2007, 46, 6638 – 6642; Angew. Chem. 2007, 119, 6758 – 6762; b) H. Deng, S. Grunder, K. E. Cordova, C. Valente, H. Furukawa, M. Hmadeh, F. Gndara, A. C. Whalley, Z. Liu, S. Asahina, H. Kazumori, M. O’Keeffe, O. Terasaki, J. F. Stoddart, O. M. Yaghi, Science 2012, 336, 1018 – 1023; c) Q.-R. Fang, T. A. Makal, M. D. Young, H.-C. Zhou, Comments Inorg. Chem. 2010, 31, 165 – 195; d) W. Xuan, C. Zhu, Y. Liu, Y. Cui, Chem. Soc. Rev. 2012, 41, 1677 – 1695; e) L. Song, J. Zhang, L. Sun, F. Xu, F. Li, H. Zhang, X. Si, C. Jiao, Z. Li, S. Liu, Y. Liu, H. Zhou, D. Sun, Y. Du, Z. Cao, Z. Gabelica, Energy Environ. Sci. 2012, 5, 7508 – 7520. [5] M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe, O. M. Yaghi, Science 2002, 295, 469 – 472. [6] N. L. Rosi, J. Kim, M. Eddaoudi, B. Chen, M. O’Keeffe, O. M. Yaghi, J. Am. Chem. Soc. 2005, 127, 1504 – 1518. [7] a) D. Yuan, R. B. Getman, Z. Wei, R. Q. Snurr, H.-C. Zhou, Chem. Commun. 2012, 48, 3297 – 3299; b) H. Chevreau, T. Devic, F. Salles, G. Maurin, N. Stock, C. Serre, Angew. Chem. Int. Ed. 2013, 52, 5056 – 5060; Angew. Chem. 2013, 125, 5160 – 5164. [8] a) H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi, A. ©. Yazaydin, R. Q. Snurr, M. O’Keeffe, J. Kim, O. M. Yaghi, Science 2010, 329, 424 – 428; b) Y.-Q. Lan, H.-L. Jiang, S.-L. Li, Q. Xu, Adv. Mater. 2011, 23, 5015 – 5020. [9] a) G. F¦rey, C. Serre, C. Mellot-Draznieks, F. Millange, S. Surbl¦, J. Dutour, I. Margiolaki, Angew. Chem. Int. Ed. 2004, 43, 6296 – 6301; Angew. Chem. 2004, 116, 6456 – 6461; b) G. F¦rey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surbl¦, I. Margiolaki, Science 2005, 309, 2040 – 2042; c) A. Sonnauer, F. Hoffmann, M. Frçba, L. Kienle, V. Duppel, M. Thommes, C. Serre, G. F¦rey, N. Stock, Angew. Chem. Int. Ed. 2009, 48, 3791 – 3794; Angew. Chem. 2009, 121, 3849 – 3852. [10] a) J. B. DeCoste, G. W. Peterson, H. Jasuja, T. G. Glover, Y.-G. Huang, K. S. Walton, J. Mater. Chem. A 2013, 1, 5642 – 5650; b) H.-L. Jiang, D. Feng, K. Wang, Z.-Y. Gu, Z. Wei, Y.-P. Chen, H.-C. Zhou, J. Am. Chem. Soc. 2013, 135, 13934 – 13938. [11] a) J. An, O. K. Farha, J. T. Hupp, E. Pohl, J. I. Yeh, N. L. Rosi, Nat. Commun. 2012, 3, 604; b) T. Li, M. T. Kozlowski, E. A. Doud, M. N. Blakely, N. L. Rosi, J. Am. Chem. Soc. 2013, 135, 11688 – 11691. [12] a) J. An, S. J. Geib, N. L. Rosi, J. Am. Chem. Soc. 2009, 131, 8376 – 8377; b) H. Xu, J. Cai, S. Xiang, Z. Zhang, C. Wu, X. Rao, Y. Cui, Y. Yang, R. Krishna, B. Chen, G. Qian, J. Mater. Chem. A 2013, 1, 9916 – 9921. [13] D. Zhao, D. J. Timmons, D. Yuan, H.-C. Zhou, Acc. Chem. Res. 2011, 44, 123 – 133.
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Received: February 4, 2015 Published online on May 26, 2015
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