Stable Mg-Metal–Organic Framework (MOF) and Unstable Zn-MOF

Communication pubs.acs.org/crystal

Stable Mg‑Metal−Organic Framework (MOF) and Unstable Zn-MOF Based on Nanosized Tris((4-carboxyl)phenylduryl)amine Ligand Yan-Ping He, Yan-Xi Tan, and Jian Zhang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

ABSTRACT: By employment of a nanosized tris[(4carboxyl)-phenylduryl]amine ligand (L) to assembly with the Zn2+ or Mg2+ ions, two non-interpenetrating microporous metal−organic frameworks (MOFs) constructed from chainshaped building units are presented here. The Zn-MOF formulated as ((CH3)4N)(Zn4L3)·28DMF (FIR-4; DMF = N,N-dimethylformamide, FIR denotes Fujian Institute of Research) is a nanoporous anionic framework, but it is unstable after the removal of guest molecules. In contrast, the Mg-MOF formulated as Mg3L2(H2O)2(DMA)2·2.5DMA (FIR-5; DMA = N,N-dimethylacetamide) features a neutral framework with (3,8)-connected tfz topology derived from kgd subnets and has high permanent porosity with a Langmuir surface area of 1457 m2·g−1.

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building units, respectively. FIR-4 is a nanoporous anionic framework, which is unstable after removing guest molecules. In contrast, FIR-5 features a neutral framework and has a high permanent porosity with a Langmuir surface area of 1457 m2·g−1. FIR-4 and FIR-5 were solvothermally synthesized by employing different mixed solvents.11 It is also notable that additional tetramethylammonium bromide was used for the successful synthesis of FIR-4, because the tetramethylammonium cations may act as structure-directing agents for the construction of this anionic framework. In comparison with the previously reported FIR-1 which has a neutral framework, FIR4 presented here shows an anionic host framework with chargebalancing tetramethylammonium cations. Single-crystal X-ray diffraction reveals that FIR-4 crystallizes in orthorhombic space group Pna21.12 There are four independent Zn2+ ions in the asymmetric unit. The local coordination geometry around each Zn2+ center is depicted in Figure 1b. The Zn1, Zn2, and Zn3 atoms are all coordinated by five carboxylate oxygen atoms from distinct L ligands, respectively, while the Zn4 atom is only coordinated by four carboxylate oxygen atoms and shows a distorted tetrahedral coordination geometry. Interestingly, four independent Zn centers are bridged by the carboxylate groups from L ligands to form a metallic tetramer Zn4(COO)8. Each Zn4(COO)8 tetramer is further linked by two carboxylate groups, giving rise to an infinite anionic chain along the a axis (Figure 1b). The intrachain Zn···Zn distances are 3.69 Å, 3.72 Å, 3.53 Å, and 4.84 Å for Zn1···Zn2, Zn2···Zn3, Zn3···Zn4, and Zn4···Zn1*, respectively. In the structure of FIR-4, three

etal−organic frameworks (MOFs) have attracted great attention because of their potential applications in the fields of photochemical areas,1 gas adsorption and separation,2 and heterogeneous catalysis.3 Recently, a number of porous MOFs based on nanosized polycarboxylate ligands and metal ions have been reported.4 For example, Yaghi and co-workers have reported MOF-177, which is composed of the node [Zn4O(CO2)6] and the ligand 1,3,5-tris(4-carboxyphenyl)benzene.5 Lin et al. reported a 2-fold interpenetrating porous MOF formed from the [Cu2(O2CR)4] units and methanetetra(biphenyl-p-carboxylic acid).6 As a nanosized N-centered carboxylate ligand, tris((4-carboxyl)phenylduryl)amine (H3L) was also used to synthesize several new MOFs, and the common feature presented in these MOF structures is interpenetration.7 Actually, the great mass of porous MOFs based on large carboxylate ligands are apt to form interpenetrating structures.4a,e,5−8 How to avoid this interpenetrating phenomenon is challenging and significant because framework interpenetration often blocks the pores. Some traditional methods, including the use of large template molecules or short linkers, decreasing reaction temperature and concentration, have been considered to avoid interpenetration.9 However, these factors are hard to react on porous MOFs based on above large ligands. In fact, chainshaped metal-carboxylate secondary building units (SBUs) provide a means to access MOFs that do not interpenetrate due to the intrinsic packing arrangement of such rods in the crystal structure.10 Yet the analogous MOFs involving infinite rodshaped SBUs remain largely unexplored. In this work, the solvothermal assembly between tris((4carboxyl)phenylduryl)amine ligand with Zn2+ or Mg2+ ions leads to two non-interpenetrating MOFs, namely, ((CH3)4N)(Zn4L3)·28DMF (FIR-4) and Mg3L2(H2O)2(DMA)2·2.5DMA (FIR-5), which are constructed from infinite chain-shaped © XXXX American Chemical Society

Received: September 16, 2012 Revised: November 28, 2012

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Communication

Figure 1. (a) The 3D framework of FIR-4; (b) the anionic chain; (c) the channel in FIR-4.

Figure 2. (a) The 3D framework of FIR-5 with coordinating DMA molecules occupying the B channels along the a axis; (b) the metalcarboxylate chain with the terminal H2O and DMA molecules extending outside; (c) the resulting tfz net derived from kgd subnets.

adjacent parallel chains are joined by the L ligands. Finally, a non-interpenetrated three-dimensional (3D) anionic architecture is created by repeating these joints. This open framework exhibits a honeycomb lattice of hexagonal channels along the a axis (Figure 1a), and the aperture for each hexagonal channel is approximately 16.5 Å (excluding van der Waals radii) (Figure 1c). Remarkably, no interpenetration was observed for this framework. The solvent-accessible volume (including chargebalancing cations) of FIR-4 is estimated by the PLATON program to be about 66.2% of the total crystal volume. The free spaces are occupied by the structurally disordered DMF solvent molecules and tetramethylammonium cations. Thermogravimetric analyses (TGA) and powder X-ray diffraction pattern (PXRD) measurements were carried out to examine the thermal stability of this nanoporous framework. The TGA curve of FIR-4 reveals a weight loss of 48.69% before 250 °C, corresponding to the release of guest molecules, and no further weight loss is observed until 400 °C (Figure S7, Supporting Information). The TGA of CH2Cl2-exchanged sample (FIR-4a), prepared by soaking FIR-4 in CH2Cl2, shows that all DMF molecules can be completely exchanged by CH2Cl2. For gas adsorption studies, FIR-4a was activated by liquid and supercritical CO2. The PXRD pattern of desolvated solid FIR-4a-ht shows that some main peaks disappear and a few peaks also shift, compared to the PXRD pattern of the assynthesized sample (Figure S8, Supporting Information). This result indicates that the host framework of FIR-4 is unstable and may collapse after the removal of the guests. Single-crystal X-ray diffraction reveals that FIR-5 crystallizes in the triclinic space group P1̅.11 The asymmetric unit contains one and a half Mg(II) atoms, one L linker, one coordinated H2O and one DMA molecule. The Mg1 atom is coordinated by five carboxylate oxygen atoms from four L ligands and one DMA oxygen atom, showing distorted octahedral coordination geometry (Figure 2b). The Mg2 atom also adopts octahedral coordination geometry, but it is coordinated by four carboxylate

oxygen atoms of four different L ligands at the equatorial sites and two H2O molecules at the axial positions. Every two Mg centers is fixed by two carboxylate groups, and such connectivity leads to a S-shaped chain with the terminal H2O and DMA molecules extending outside (Figure 2b). The metal centers are arrayed in the order Mg2*···Mg1*···Mg1···Mg2 with separated Mg···Mg distances of 3.85 and 4.74 Å, respectively. In comparison with FIR-4, this S-shaped chain in FIR-5 is a neutral one. Each resulting chain is further linked by the L linkers along the b and c axis to six neighboring ones, thus generating a neutral 3D framework with 1D channels along the a axis (Figure 2a). There are two types of channels labeled as A (pore dimension: 3 × 8 Å2) and B (pore dimension: 2.5 × 4.5 Å2, and 6 × 13 Å2 for without the coordinated solvent molecules) in Figure 2a. The coordinated DMA molecules protrude into channel B, and channel A might be occupied by the structurally disordered guest molecules. The void volume of FIR-5 without any guest solvent molecules is 20.0% of the cell volume, while the void volume increases to 36.1% on removal of the coordinated DMA and H2O molecules, as estimated by PLATON. To better understand the structure of FIR-5, the topology for FIR-5 was analyzed in detail by using the TOPOS program. If the Mg2 atoms can be neglected in advance, a binuclear SBU formed by adjacent Mg1 and Mg1* atoms connects other six binuclear SBUs by three L ligands to form a layer with (3,6)connected kgd topology (Figure S5, Supporting Information). Through the Mg2 centers, adjacent kgd-type layers are coordinatively pillared into a 3D framework with (3,8)connected tfz topology (Figure 2c). TGA curve of FIR-5 reveals a weight loss of 25.4% before 350 °C, corresponding to the release of the coordinating DMA and H2O molecules as well as the guest molecules. No obvious weight loss is observed from 350 to 490 °C (Figure S9, B

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respectively. The measured pore volume from the N2 sorption is 0.082 cm3·g−1. These pore parameters are greatly lower than those estimated from the single crystal structure, which further demonstrates that the host framework is unstable and easy to collapse after removal of guest molecules. For FIR-5a-ht, the N2 sorption isotherm shows a reversible type I sorption behavior. The adsorption behavior between 0 and 0.01 bar is thus explained by the filling of the microporous, whereas at 0.01 bar the phenyl rings rotate under pressure, leading to an increase in the observed pore volume. Subsequently, a gradual, rather than an abrupt, change in the N2 uptake from 0.01 to 0.15 bar is observed. The similar adsorption behavior has also been reported.13 The N2 uptake capacity of FIR-5a-ht reaches 305.1 cm3·g−1 at 1 bar. The BET and Langmuir surface areas are estimated to be 950.8 cm2·g−1 and 1457.3 m2·g−1, respectively, which are much larger than those of FIR-4a-ht. Remarkably, the measured pore volume of 0.472 cm3·g−1 is higher than the estimated value (0.310 cm3·g−1) from the single crystal structure, which further proves the above statement. The H2 sorption experiments at 77 K for FIR-4a-ht and FIR5a-ht were also measured. As shown in Figure 3b, the adsorption and desorption isotherm curves of FIR-4a-ht do not overlap with each other and show significant hysteresis. The H2 uptake capacity reaches 95.2 cm3·g−1 (0.85 wt %) at 77 K and 1 bar. In contrast, FIR-5a-ht has a fully reversible H2 uptake of 109.6 cm3·g−1 (0.98 wt %) at 77 K and 1 bar, a value surpassing that of the most favorable zeolite ZSM-5 (0.7 wt %) and closing to those of recently reported MOFs at the same condition.4a,14 In addition, the CO2 uptake of FIR-4a-ht and FIR-5a-ht are 33 cm3·g−1 and 42 cm3·g−1 at 273 K and 1 bar, respectively (Figure 3c). In summary, by employing a large tris((4-carboxyl)phenylduryl)amine ligand (L) to assembly with Zn2+ or Mg2+ ions, two microporous MOFs FIR-4 and FIR-5 were successfully synthesized and structurally characterized. Both of them are non-interpenetrating 3D frameworks constructed from metal-carboxylate chains linked by the nanosized L ligands. It is interesting that FIR-4 with large honeycomb-like channels is not stable after the removal of guest molecules, while FIR-5 with small channels is much more stable and exhibits high permanent porosity with a Langmuir surface area of 1457 m2·g−1.

Supporting Information), indicating a comparatively high degree of thermal stability. The TGA of methanol-exchanged sample (FIR-5a), prepared by soaking FIR-5 in methanol, shows that all guest molecules and the coordinated solvent molecules can be completely exchanged by methanol. Furthermore, the methanol molecules can be easily moved from the pores by heating FIR-5a at 100 °C for 12 h under a vacuum, and then a desolvated solid FIR-5a-ht is obtained. Compared to the PXRD pattern of FIR-5, the peak positions in the PXRD pattern of FIR-5a-ht have no obvious change, although some peaks are distinctly weakened and broadened (Figure S10, Supporting Information). These results indicate that the host framework of FIR-5 can be retained after the removal of the coordinated molecules and guest molecules. To investigate the permanent porosity of desolvated FIR-4 and FIR-5, the adsorption experiments of N2 were measured at 77 K (Figure 3a). The uptake of N2 for FIR-4a-ht is only 52.9 cm3·g−1 at 1 bar. The BET and Langmuir surface areas of FIR4a-ht were calculated to be 122.6 m2·g−1 and 185.4 m2·g−1,

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures, TGA, powder X-ray diffraction patterns, and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the support of this work by 973 program (2011CB932504 and 2012CB821705), NSFC (21073191, 21221001, 91222105), NSF of Fujian Province (2011J06005), and CAS (XDA07070200).

Figure 3. Gas adsorption isotherms for FIR-4a-ht and FIR-5a-ht: (a) N2 at 77 K; (b) H2 at 77 K; (c) CO2 at 273 K. C

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D.; O’Keeffe, M.; Yaghi, O. M.; Kim, J. Angew. Chem., Int. Ed. 2012, 51, 8921−8925. (10) (a) Rosi, N. L.; Eddaoudi, M.; Kim, J.; O’Keeffe, M.; Yaghi, O. M. Angew. Chem., Int. Ed. 2002, 114, 294−297. (b) Rosi, N. L.; Kim, J.; Eddaoudi, M.; Chen, B. L.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 1504−1518. (c) Li, L. N.; Wang, S. Y.; Chen, T. L.; Sun, Z. H.; Luo, J. H.; Hong, M. C. Cryst. Growth Des. 2012, 12, 4109−4115. (11) Synthesis of ((CH3)4N)(Zn4L3)·28DMF (FIR-4): H3L (60 mg, 0.1 mmol), Zn(NO3)2·6H2O (60 mg, 0.2mmol) and tetramethylammonium bromide (70 mg, 0.45 mmol) were dissolved in DMF/EtOH (3:1, v/v), which were placed in a small vial. The mixture was heated at 100 °C for 24 h and then cooled to room temperature. Yellow crystals of the product were formed and collected by filtration and washed with DMF several times. Yield: 70% (based on L). Synthesis of Mg3L2(H2O)2(DMA)2·2.5DMA (FIR-5): H3L (60 mg, 0.1 mmol) and Mg(NO3)2·6H2O (51 mg, 0.2mmol) were dissolved in DMA/H2O (3:2, v/v), which were placed in a small vial. The mixture was heated at 120 °C for 48 h and then cooled to room temperature. Yellow crystals of the product were formed and collected by filtration and washed with DMA several times. Yield: 65% (based on L). (12) The diffraction data for both compounds were collected on an Oxford Xcalibur diffractometer equipped with a graphite−monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 293(2) K. Crystal data for FIR-4: space group Pna21, orthorhombic, a = 30.1777(17) Å, b = 21.4903(16) Å, c = 33.5299(18) Å, α = β = γ = 90.00°, V = 21745(2) Å3, Z = 4, 32620 reflections measured, 19170 independent reflections (Rint = 0.0676). The final R1 value was 0.0681 (I > 2σ(I)). The final wR(F2) value was 0.1583 (I > 2σ(I)). The goodness of fit on F2 was 0.910. Crystal data for FIR-5: space group P1̅, triclinic, a = 8.4319(9) Å, b = 12.8251(13) Å, c = 20.924(4) Å, α = 73.46°, β = 84.69°, γ = 78.31°, V = 2122.6(5) Å3, Z = 1, 14746 reflections measured, 8199 independent reflections (Rint = 0.0294). The final R1 value was 0.0969 (I > 2σ(I)). The final wR(F2) value was 0.3712 (I > 2σ(I)). The goodness of fit on F2 was 0.913. The structures were solved by the direct method and refined by the full-matrix least-squares on F2 using the SHELXTL−97 program. (13) Yang, W. B.; Davies, A. J.; Lin, X.; Suyetin, M.; Matsuda, R.; Blake, A. J.; Wilson, C.; Lewis, W.; Parker, J. E.; Tang, C. C.; George, M. W.; Hubberstey, P.; Kitagawa, S.; Sakamoto, H.; Bichoutskaia, E.; Champness, N. R.; Yang, S. H.; Schröder, M. Chem. Sci. 2012, 3, 2993−2999. (14) (a) Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C. B.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2010, 43, 58−67. (b) Higuchi, M.; Nakamura, K.; Horike, S.; Hijikata, Y.; Yanai, N.; Fukushima, T.; Kim, J.; Kato, K.; Takata, M.; Watanabe, D.; Oshima, S.; Kitagawa, S. Angew. Chem., Int. Ed. 2012, 124, 8494−8497. (c) Jayaramulu, K.; Reddy, S. K.; Hazra, A.; Balasubramanian, S.; Maji, T. K. Inorg. Chem. 2012, 51, 7103−7111.

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