Communication pubs.acs.org/IC
Synthesis of Manganese ZIF‑8 from [Mn(BH4)2·3THF]·NaBH4 Kentaro Kadota,† Easan Sivaniah,†,‡ Sareeya Bureekaew,§ Susumu Kitagawa,*,‡ and Satoshi Horike*,‡,∥ †
Department of Molecular Engineering, Graduate School of Engineering, and ∥Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡ Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Institute for Advanced Study, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan § Department of Chemical and Biomolecular Engineering, School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand S Supporting Information *
is a challenge to construct an open framework.19−22 We first attempted to synthesize Mn-ZIF-8 from a conventional solvothermal reaction. When we utilized Mn(NO3)2·6H2O or Mn(CH3COO)2·4H2O as a precursor, Mn3O4 was the main product because of the thermal decomposition of manganese salts (Figure S3). One literature discussed that [Mn(Im)2· 2(ImH)] (ImH = imidazole) having distorted tetrahedral Mn2+4N (N−Mn−N is 99.86−126.45°) was synthesized from Mn2(CO)10 and melting ImH.23 The reaction produces CO and H2 gases as byproducts along with formation of the kinetically favored framework. Likewise, the metal carbonyl precursors and metal hydride or borohydride also kinetically control the equilibrium of the uncommon coordination geometry.24 Metal borohydrides are often soluble in organic solvents, and we utilized manganese borohydride, [Mn(BH4)2· 3THF]·NaBH4, as a precursor for Mn-ZIF-8 synthesis. Manganese borohydrides were prepared via mechanochemical or metathesis reactions between MnCl2 and alkaline borohydrides.25,26 We synthesized [Mn(BH4)2·3THF]·NaBH4 by a metathesis reaction according to the literature.27 As a next step, an acetonitrile solution of 2-methylimidazole was slowly added dropwise to an acetonitrile solution of [Mn(BH4)2·3THF]· NaBH4 at room temperature inside an Ar-filled glovebox. Consequently, a white powder precipitated immediately following gas generation. We collected and washed the powder, and the product wil be denoted as Mn-ZIF-8 hereafter. Figure 1a shows the powder X-ray diffraction (PXRD) patterns of Zn-ZIF8 and Mn-ZIF-8 at 298 K under Ar. The peaks of Mn-ZIF-8 exhibit a lower angle shift in 2θ, indicating a larger unit cell of Mn-ZIF-8 than that of Zn-ZIF-8. The crystal structure of MnZIF-8 was determined using synchrotron PXRD data collected at 100 K under Ar in Figure 1b. The diffraction pattern was indexed in the cubic system. A Le Bail fitting was performed to extract the refined unit cell parameters and integrate the intensities. We constructed the initial structure model based on the crystal structure of Zn-ZIF-8. Zn2+ was replaced with Mn2+, and the cell parameter was modified by the obtained values. Accordingly, Rietveld refinement against experimental diffraction patterns results in converging refinement and low residual values (Figure 1b; a = 17.52334(4) Å, Rwp = 5.46%, and S = 3.089).
ABSTRACT: Cubic and highly porous [Mn(2-methylimidazolate)2] (Mn-ZIF-8) was synthesized from [Mn(BH4)2·3THF]·NaBH4 under an Ar atmosphere. The structure contains rare Mn2+-4N tetrahedral geometry and has larger cell parameters, resulting in 20% larger amounts of gas uptake compared with [Zn(2-methylimidazolate)2]. A kinetically favored reaction using a reactive metal borohydride precursor is key for the construction of new metal−organic framework systems.
M
etal−organic frameworks (MOFs) or porous coordination polymers (PCPs) are a class of porous materials composed of metal ions linked by organic bridging ligands.1−4 Since the discovery of the gas storage property of these frameworks, a huge library has been constructed to date. Unlimited selection of organic ligands is expanding the library of MOFs; on the other hand, it is also important to synthesize widely studied MOF topologies by using new metal ions. For example, the synthesis of Hf4+- or Ce4+-based [M4O4(OH)4(bdc)12] (UiO-66, where bdc = 1,4-benzenedicarboxylate) frameworks after an earlier report of Zr4+-UiO-66 continues to develop the chemistry and potential application.5−7 As a significant subset of MOFs, zeolitic imidazolate frameworks (ZIFs) are of particular interest given their chemical stability and processing properties.8−11 One of the most popular ZIF structures is [Zn(mIm)2] (Zn-ZIF-8 or MAF-4, where mIm = 2-methylimidazolate), which is a prototypical structure with sodalite topology.9,10 It is intensively studied for gas separation or heterogeneous catalysis.12,13 Although Zn-ZIF-8 is of broad importance, there are limited number of examples of other metalion-based ZIF-8 structures. To date, Co2+- and Cd2+-ZIF-8 have also been reported,9,14,15 and recently we reported Mg-ZIF-8.16 Isostructural zeolitic boron imidazole frameworks containing tetrahedral Li+-4N and B3+-4N are reported.17 The ZIF-8 structure requires a nondistorted tetrahedral M2+-4N coordination geometry,18 and other transition-metal ions such as Mn2+, Fe2+, Ni2+ could form the same topology in principle. However, no reports of frameworks with these metal ions exist. In this work, we tried to synthesize Mn-based ZIF-8 architecture. Only five compounds having a nondistorted tetrahedral Mn2+-4N geometry have been reported according to the Cambridge Crystallographic Data Centre database, and it © XXXX American Chemical Society
Received: May 26, 2017
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DOI: 10.1021/acs.inorgchem.7b01322 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 3. Adsorption (solid circle) and desorption (open circle) of ZnZIF-8 (black) and Mn-ZIF-8 (purple) for N2 at 77 K. Inset: Semilog plot of N2 adsorption.
Zn-ZIF-8 exhibits stepped sorption behavior due to reorientation of the imidazolate linkers.30 On the other hand, Mn-ZIF-8 does not show such steps in the adsorption isotherm (Figure 3). This suggests that Mn-ZIF-8 has a different structural response by gas adsorption compared with Zn-ZIF-8. The N2 uptake amount for Mn-ZIF-8 (507 mL g−1 at 96 kPa) is 20% larger than that of ZnZIF-8 (417 mL g−1 at 92 kPa). The Brunauer−Emmett−Teller surface area of Mn-ZIF-8 is 1852 m2 g−1, whereas that of Zn-ZIF8 is 1684 m2 g−1. Mn-ZIF-8 also exhibits a higher CO2 uptake amount at 195 K (360 mL g−1 at 101 kPa; Figure S7) than ZnZIF-8 (205 mL g−1). The higher uptake amounts of N2 and CO2 result from the unit cell expansion and the replacement of Zn2+ with lighter Mn2+ in Mn-ZIF-8. Mn-ZIF-8 exhibits a negligible uptake amount of O2 at 298 K (Figure S8). We found structural instability of Mn-ZIF-8 against air, and its cubic structure decomposes upon exposure to air for 20 min (Figure S4). This is because of hydrolysis of Mn2+, and the same trend was observed for Mg-ZIF-8.16 To investigate the different reaction properties of Mn-ZIF-8 and Zn-ZIF-8 versus a water molecule, we conducted theoretical calculations using the density functional theory method at the B3-LYP/def2-SVP level of theory. The models chosen to represent Mn- and Zn-ZIF-8 are shown in Figure 4. Four 2-methylimidazoles coordinate with
Figure 1. (a) PXRD patterns of Zn-ZIF-8 (black) and Mn-ZIF-8 (purple) under Ar. (b) Observed (red) and calculated (blue) PXRD patterns as a result of Rietveld refinement.
As shown in Figure 2, a Mn2+ ion is tetrahedrally coordinated by four imidazolate nitrogen atoms, and each imidazolate bridges
Figure 2. (a) Local coordination geometry of Mn2+ and (b) threedimensional packing structure of Mn-ZIF-8.
two Mn2+ ions. The N−Mn2+−N angles are in the range of 107.89 and 110.27°. Mn-ZIF-8 exhibits a longer metal−N distance than Zn-ZIF-8 [Mn2+−N 2.059(2) Å; Zn2+−N 1.966(6) Å]. The bond elongation results in expansion of the unit cell [Mn-ZIF-8 17.52334(4) Å; Zn-ZIF-8 16.8509(3) Å].28 The larger ionic radius of Mn2+ than Zn2+ (Mn2+ 0.66; Zn2+ 0.60 Å) provides larger cell parameters. To study the thermal stability of Mn-ZIF-8, we conducted thermogravimetric analysis (TGA) under an Ar atmosphere. The TGA curve of Mn-ZIF-8 showed no significant weight loss at 300 °C (Figure S6). The thermal stability of Mn-ZIF-8 under an Ar atmosphere is lower than that of Zn-ZIF-8. We activated the powder of Mn-ZIF-8 at 80 °C under vacuum for 12 h to remove all adsorbed solvents and measured the gas adsorption properties. N2 adsorption at 77 K for Mn-ZIF-8 shows typical type I isotherms (Figure 3), indicating a uniform microporosity in the structure. N2 uptake at a higher pressure indicates textural macroporosity constructed by the packing of small particles.29
Figure 4. Fragments employed in the EDA for [Mn(mImH)4]2+ and [Zn(mImH)4]2+.
Mn2+ and Zn2+, resulting in [Mn(mImH)4]2+ and [Zn(mImH)4]2+ complexes with a total charge of 2+. Both complexes were optimized without any symmetry constraints (P1 symmetry). The calculated Mn2+−N bond length is 2.05 Å, which is 0.05 Å longer than Zn−N, which is in good agreement with the different value of 0.07 Å determined experimentally. Energy decomposition analysis (EDA) was also performed for the two fragments [M(mImH)3]2+ and mImH, and the B
DOI: 10.1021/acs.inorgchem.7b01322 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Collaborative Research Program from the Japan Science and Technology Agency.
decomposed energy results are summarized in Table S3. The total interaction energy of [Mn(mImH)4]2+ is smaller by 3.6 kcal mol−1 compared to that of [Zn(mImH)4]2+, indicative of the less stability of Mn-ZIF-8. The individual term (Pauli term) from the calculation suggested that the main contribution making [Mn(mImH)4]2+ less stable than [Zn(mImH)4]2+ is less orbital overlap between the fragments in the Mn2+ complex. To investigate the water stability of these complexes, bond formation between the metal center and the entering water molecule was calculated. [Zn(mImH)4]2+ does not accept the water molecule as an additional ligand, but [Mn(mImH)4]2+ accepts it. [Mn(mImH)4(H2O)]2+ is found in a pseudo-trigonal-bipyramidal configuration (Figure S10). Distortion in the coordination geometry of the Mn2+ complex after binding with water results in collapse of the Mn-ZIF-8 framework. With regard to the instability of the Mn2+-4N tetrahedral geometry, we found one literature trying to incorporate Mn2+ into Zn-ZIF-8 by a postsynthetic modification procedure.31 The attempt allowed only 11% exchange of Mn2+ to Zn2+ in the structure. The low amount of exchange is due to the instability of Mn2+-4N cores as described above. In conclusion, we synthesized Mn-ZIF-8 from [Mn(BH4)2· 3THF]·NaBH4 under an Ar atmosphere. The rapid crystallization process and the reaction only with gaseous byproducts are achieved using metal borohydride as the precursor, and this results in the construction of Mn-ZIF-8 having rare Mn2+-4N tetrahedral geometry. This approach would be widely available for the rational design of MOF/PCP with unique coordination geometry and open metal sites.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01322. Experimental details and additional characterizations (PXRD, TGA, CO2 and O2 sorption, Rietveld refinement, and theoretical calculation) (PDF) Accession Codes
CCDC 1551898 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Satoshi Horike: 0000-0001-8530-6364 Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Challenging Exploratory Research from Japan Society for the Promotion of Science and “Molecular Technology” of Strategic International C
DOI: 10.1021/acs.inorgchem.7b01322 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry (18) The tetrahedral geometry is defined as coordination geometry with a N−M2+−N angle in the range of 105−115° (the ideal tetrahedral angle is 109.5°). (19) Lewis, R. A.; Wu, G.; Hayton, T. W. Stabilizing High-Valent Metal Ions with a Ketimide Ligand Set: Synthesis of Mn(N-CtBu2)4. Inorg. Chem. 2011, 50, 4660−4668. (20) Putzer, M. A.; Pilz, A.; Müller, U.; Neumüller, B.; Dehnicke, K. Amido-Komplexe von Mangan(II). Synthese und Kristallstrukturen von [Mn(NPh2)2(THF)]2 und Na2[Mn(NPh2)4]·2 C7H8. Z. Anorg. Allg. Chem. 1998, 624, 1336−1340. (21) Karadas, F.; Avendano, C.; Hilfiger, M. G.; Prosvirin, A. V.; Dunbar, K. R. Use of a rhenium cyanide nanomagnet as a building block for new clusters and extended networks. Dalton Trans. 2010, 39, 4968− 4977. (22) Milon, J.; Daniel, M.-C.; Kaiba, A.; Guionneau, P.; Brandès, S.; Sutter, J.-P. Nanoporous Magnets of Chiral and Racemic [{Mn(HL)}2Mn{Mo(CN)7}2] with Switchable Ordering Temperatures (TC = 85 K ↔ 106 K) Driven by H2O Sorption (L = N,N-Dimethylalaninol). J. Am. Chem. Soc. 2007, 129, 13872−13878. (23) Lehnert, R.; Seel, F. Darstellung und Kristallstruktur des Mangan(II)- und Zink(II)-Derivates des Imidazols. Z. Anorg. Allg. Chem. 1980, 464, 187−194. (24) Rybak, J.-C.; Hailmann, M.; Matthes, P. R.; Zurawski, A.; Nitsch, J.; Steffen, A.; Heck, J. G.; Feldmann, C.; Götzendörfer, S.; Meinhardt, J.; Sextl, G.; Kohlmann, H.; Sedlmaier, S. J.; Schnick, W.; MüllerBuschbaum, K. Metal−Organic Framework Luminescence in the Yellow Gap by Codoping of the Homoleptic Imidazolate ∞3[Ba(Im)2] with Divalent Europium. J. Am. Chem. Soc. 2013, 135, 6896−6902. (25) Č erný, R.; Penin, N.; Hagemann, H.; Filinchuk, Y. The First Crystallographic and Spectroscopic Characterization of a 3d-Metal Borohydride: Mn(BH4)2. J. Phys. Chem. C 2009, 113, 9003−9007. (26) Schouwink, P.; D’Anna, V.; Ley, M. B.; Lawson Daku, L. M.; Richter, B.; Jensen, T. R.; Hagemann, H.; Č erný, R. Bimetallic Borohydrides in the SystemM(BH4)2−KBH4(M = Mg, Mn): On the Structural Diversity. J. Phys. Chem. C 2012, 116, 10829−10840. (27) Makhaev, V. D.; Borisov, A. P.; Gnilomedova, T. P.; Lobkovskii, É. B.; Chekhlov, A. N. Production of manganese borohydride complexes of manganese solvated with THF, and the structure of Mn(BH4)2 (THF)3. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1987, 36, 1582−1586. (28) Morris, W.; Stevens, C. J.; Taylor, R. E.; Dybowski, C.; Yaghi, O. M.; Garcia-Garibay, M. A. NMR and X-ray Study Revealing the Rigidity of Zeolitic Imidazolate Frameworks. J. Phys. Chem. C 2012, 116, 13307− 13312. (29) Pan, Y.; Liu, Y.; Zeng, G.; Zhao, L.; Lai, Z. Rapid synthesis of zeolitic imidazolate framework-8 (ZIF-8) nanocrystals in an aqueous system. Chem. Commun. 2011, 47, 2071−2073. (30) Fairen-Jimenez, D.; Moggach, S. A.; Wharmby, M. T.; Wright, P. A.; Parsons, S.; Duren, T. Opening the gate: framework flexibility in ZIF8 explored by experiments and simulations. J. Am. Chem. Soc. 2011, 133, 8900−8902. (31) Fei, H.; Cahill, J. F.; Prather, K. A.; Cohen, S. M. Tandem Postsynthetic Metal Ion and Ligand Exchange in Zeolitic Imidazolate Frameworks. Inorg. Chem. 2013, 52, 4011−4016.
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DOI: 10.1021/acs.inorgchem.7b01322 Inorg. Chem. XXXX, XXX, XXX−XXX
