DOI: 10.1021/cg100121n
Dimensionality Trends in Metal-Organic Frameworks Containing Perfluorinated or Nonfluorinated Benzenedicarboxylates
2010, Vol. 10 2041–2043
Zeric Hulvey,*,† Joshua D. Furman,‡,† Sara A. Turner,† Min Tang,† and Anthony K. Cheetham*,‡ †
Materials Research Laboratory, University of California, Santa Barbara, California 93106-5121, and Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K.
‡
Received January 26, 2010; Revised Manuscript Received April 1, 2010
ABSTRACT: A new family of hybrid inorganic-organic materials has been synthesized using a combination of flexible bis-pyridyl ligands in conjunction with a perfluorinated or nonfluorinated benzenedicarboxylate ligand. A significant difference between the carboxylate torsion angles of the fluorinated and nonfluorinated ligands leads to the formation of markedly different structures for the two groups of materials. No isostructural phases were found, and the use of perfluorinated ligands tends to increase the dimensionality of the resulting frameworks. Metal-organic frameworks containing fluorinated ligands are receiving increasing attention due to reports of interesting H2 adsorption in these materials.1-4 However, few have been synthesized to date and little is known of the structural chemistry of perfluorinated ligands in hybrid framework materials as compared to their nonfluorinated analogues.5-13 Our current work involves developing a better understanding of the manner in which perfluorinated benzenedicarboxylates incorporate into hybrid structures and examining structural trends of the resulting materials. We have previously reported structures containing perfluorinated carboxylates in combination with nonfluorinated coligands such as imidazole,14 triazole,3 and both 2,20 - and 4,40 bipyridine.15,16 Here we extend this work to include the similar but longer and more flexible 1,2-bis(4-pyridyl)ethane (bpe) and 1,3-bis(4-pyridyl)propane (bpp) ligands in conjunction with tetrafluoroterephthalate (tftpa) and tetrafluoroisophthalate (tfipa). In contrast to our previous work, there are few reports of materials containing bpe or bpp and the nonfluorinated analogues of these dicarboxylates (tpa and ipa) for comparison. We have therefore undertaken a synthetic study of these materials as well. There are two important differences in the chemistry of perfluorinated benzenedicarboxylates as compared to their nonfluorinated analogues. First is their significantly enhanced acidity, which may contribute to the inability of compounds containing only transition metals and perfluorinated benzenedicarboxylates to crystallize, as hybrid frameworks are typically not obtained under strongly acidic conditions. This could explain the relative ease with which perfluorinated benzenedicarboxylates incorporate into hybrid materials when other basic ligands such as triazole and bipyridines are present in the reaction. The second difference involves the effect that the fluorine atoms have on the torsion angle by which the carboxylate groups are twisted out of the plane of the benzene ring. In structures containing tpa and ipa, the carboxylate group typically remains roughly in plane with the benzene ring to which it is attached (i.e., torsion angle near 0°). However, in structures containing tftpa and tfipa, the carboxylate groups are typically rotated between 45° and 60° with respect to the benzene ring.12 This can be attributed both to an electrostatic repulsion between the highly electronegative fluorine atoms on the ring and the lone-pair containing oxygen atoms, and a decrease in aromatic character of the carboxylate group itself due to the electron-withdrawing nature of the fluorine atoms.
Table 1. Formulae, Dimensionalities, and Carboxylate Torsion Angles for 1-20a
a Products from non-fluorinated dicarboxylates are shown in orange, while those from perfluorinated ligands are shown in blue. Connectivities according to the InOm nomenclature of Cheetham et al.17 are shown, where n=the degree of inorganic connectivity between metal centers and m = the degree of organic connectivity between metal centers.
*Corresponding authors: (Z.H.) Phone: (805) 893-5988. Fax: (805) 8938502. E-mail:
[email protected]. (A.K.C.) Phone: þ44 (0) 1223 767061. Fax: þ44 (0) 1223 334567. E-mail:
[email protected].
Here we present a family of 20 new structures synthesized from combinations of four divalent transition metals, plus bpe or bpp, and either a perfluorinated or nonfluorinated benzenedicarboxylate. All the materials were synthesized hydrothermally under similar conditions. Metal acetates or chlorides were reacted with the appropriate dicarboxylic acid ligand and bipyridyl coligand, typically in a 1:1:1 ratio, in water at temperatures between 90 and 125 °C. Specific reaction ratios, temperatures, and solution pH were optimized to obtain single crystals or pure samples of the respective phases. All structures
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Published on Web 04/07/2010
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were solved using single crystal X-ray diffraction, and where pure samples were obtained, elemental and thermogravimetric analyses were performed to confirm sample purity. Extensive descriptions of synthesis conditions, details of structure refinement, and discussion of the structures of 1-20 can be found in the Supporting Information. Table 1 contains the formulae for all 20 compounds and the crystallographically unique torsion angles observed in their respective structures, and a distribution of torsion angles for 1-20 is shown in Figure 1. The histogram was normalized such that the contribution from each compound was equal. The fluorinated structures (blue) show a clustering of torsion angles between 40° and 60° and none below 30°, whereas the torsion angles from the nonfluorinated structures (orange) are largely contained in the 0°-20° range and do not extend above 40°. This preference for high torsion angles in the fluorine-containing dicarboxylates and planar nonfluorinated dicarboxylates is expected for the reasons described above and confirms the observations of Zaworotko and others.12 Notably, in this family of materials there are no structures containing fluorinated dicarboxylates that are isostructural to any structures containing the respective nonfluorinated analogues, which is almost certainly attributable to the significant difference in torsion angles. However, the large number of structures examined in the present work enables us to discern a new trend in which the use of the perfluorinated dicarboxylates as compared to the use of nonfluorinated dicarboxylates generally leads to structures with higher dimensionalities (Table 1). This trend can be nicely illustrated by comparing compound 2,
Figure 1. Histogram of torsion angles for compounds 1-20.
Hulvey et al. containing Co2þ, bpe, and ipa, to compound 15, containing Co2þ, bpe, and tfipa (Figure 2). The only synthetic difference between 2 and 15 is the use of the fluorinated tfipa ligand. The planarity of the nonfluorinated ipa ligand in 2 results in a ribbon-like motif (Figure 2, left) in which the only additional metal coordination sites are those trans across the metal octahedron and adjacent metal centers are aligned parallel to one another. These two axial sites on the metal octahedron are occupied by bridging bpe ligands which can only increase connectivity in one direction, roughly perpendicular to the ribbon, precluding connectivity in more than two dimensions. In 15, the twisting of the carboxylate groups causes a zigzag chain to form (Figure 2, right) in which adjacent metal centers are canted at an angle to each other and there is a third coordination site available. The axial sites are occupied by bridging bpe ligands as in 2; however, because the metal polyhedra are tilted with respect to each other, the Co-tfipa chain weaves between the Co-bpe chains at such an angle that connectivity is completed in three dimensions. The additional coordination site is occupied by another bpe ligand which only further increases the amount of interchain connections. Another reason for the structural difference arises because the ribbon-like motif in 2 is further stabilized by a hydrogen-bonding interaction between the hydrogen on the carbon atom between the carboxylates (ortho position) and the opposite carboxylate oxygen. The fluorine atom in that position on the tfipa ligand would cause too much repulsion and steric hindrance for the ribbon-like feature to form in 15, regardless of the differences in torsion angles. We have previously shown that the dimensionalities of metal dicarboxylate frameworks can be influenced by a range of factors, including reaction temperature,18 pH and time,19 ligand flexibility,20 the use of chiral versus racemic ligands,21 the disposition of the carboxylate groups around a ring,22,23 and the size of the metallic cation.24 The present work demonstrates some of these trends, such as the increase in dimensionality from compound 14 to compound 17 (2D to 3D), which arises from heating an identical combination of metal and ligands for 6 days as opposed to 20 h. However, since the synthesis conditions are largely identical for the crystallization of all 20 structures, the most striking pattern evident here is that the presence of fluorine rather than hydrogen in an aromatic ring favors the formation of framework structures with higher dimensionalities, providing another tool whereby structures of a particular type can be targeted in synthesis. Acknowledgment. This work was funded by the U.S. Department of Energy (DE-FC36-50GO15004) and made use of the MRL Central Facilities supported by the National Science Foundation (DMR05-20415). J.D.F. thanks the Mitsubishi Chemical Center for Advanced Materials for funding, S.A.T. was supported by the RISE program of the MRL at UCSB, and
Figure 2. Metal-carboxylate chain of Co(bpe)(ipa) (2, left) and Co2(bpe)3(tfipa)2 (15, right).
Communication
Crystal Growth & Design, Vol. 10, No. 5, 2010
M.T. was supported by the CISEI program of the MRL at UCSB. A.K.C. thanks the European Research Council for an Advanced Investigator Award. We thank Simon J. Teat for assistance with data collection of structure 7 at the Advanced Light Source at Lawrence Berkeley Laboratory, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Supporting Information Available: Crystallographic files in CIF format, complete descriptions of synthesis conditions, details of structure solution, extensive structure descriptions, and thermogravimetric analysis data. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Yang, C.; Wang, X.; Omary, M. A. J. Am. Chem. Soc. 2007, 129, 15454. (2) Yang, C.; Wang, X.; Omary, M. A. Angew. Chem., Int. Ed. 2009, 48, 2500. (3) Hulvey, Z.; Falcao, E. H. L.; Eckert, J.; Cheetham, A. K. J. Mater. Chem. 2009, 19, 4307. (4) Uemura, K.; Maeda, A.; Maji, T. K.; Kanoo, P.; Kita, H. Eur. J. Inorg. Chem. 2009, 2329. (5) Pan, L.; Sander, M. B.; Huang, X.; Li, J.; Smith, M.; Bittner, E.; Bockrath, B.; Johnson, J. K. J. Am. Chem. Soc. 2004, 126, 1308. (6) Kitaura, R.; Iwahori, F.; Matsuda, R.; Kitagawa, S.; Kubota, Y.; Takata, M.; Kobayashi, T. C. Inorg. Chem. 2004, 43, 6522. (7) Awaleh, M. O.; Badia, A.; Brisse, F. Cryst. Growth Des. 2005, 5, 1897.
2043
(8) Liu, S. Q.; Konaka, H.; Kuroda-Sawa, T.; Maekawa, M.; Suenaga, Y.; Ning, G. L.; Munakata, M. Inorg. Chim. Acta 2005, 358, 919. (9) Chun, H.; Dybtsev, D. N.; Kim, H.; Kim, K. Chem.-Eur. J. 2005, 11, 3521. (10) Chen, B.; Yang, Y.; Zapata, F.; Qian, G.; Luo, Y.; Zhang, J.; Lobkovsky, E. B. Inorg. Chem. 2006, 45, 8882. (11) Yoon, J. H.; Choi, S. B.; Oh, Y. J.; Seo, M. J.; Jhon, Y. H.; Lee, T.-E.; Kim, D.; Choi, S. H.; Kim, J. Catal. Today 2007, 120, 324. (12) Wang, Z.; Kravtsov, V. C.; Walsh, R. B.; Zaworotko, M. J. Cryst. Growth Des. 2007, 7, 1154. (13) Chen, S.-C.; Zhang, Z.-H.; Chen, Q.; Gao, H.-B.; Liu, Q.; He, M.-Y.; Du, M. Inorg. Chem. Commun. 2009, 12, 835. (14) Hulvey, Z.; Wragg, D. S.; Lin, Z.; Morris, R. E.; Cheetham, A. K. Dalton Trans. 2009, 1131. (15) Hulvey, Z.; Ayala, E.; Furman, J. D.; Forster, P. M.; Cheetham, A. K. Cryst. Growth Des. 2009, 9, 4759. (16) Hulvey, Z.; Ayala, E.; Cheetham, A. K. Z. Anorg. Allg. Chem. 2009, 635, 1753. (17) Cheetham, A. K.; Rao, C. N. R.; Feller, R. K. Chem. Commun. 2006, 4780. (18) Forster, P. M.; Burbank, A. R.; Livage, C.; Ferey, G.; Cheetham, A. K. Chem. Commun. 2004, 368. (19) Forster, P. M.; Stock, N.; Cheetham, A. K. Angew. Chem., Int. Ed. 2005, 44, 7608. (20) Forster, P. M.; Cheetham, A. K. Microporous Mesoporous Mater. 2004, 73, 57. (21) Kam, K. C.; Young, K. L. M.; Cheetham, A. K. Cryst. Growth Des. 2007, 7, 1522. (22) Kim, D. S.; Forster, P. M.; de Delgado, G. D.; Park, S.-E.; Cheetham, A. K. Dalton Trans. 2004, 3365. (23) Dan, M.; Cheetham, A. K.; Rao, C. N. R. Inorg. Chem. 2006, 45, 8227. (24) Falcao, E. H. L.; Naraso; Feller, R. K.; Wu, G.; Wudl, F.; Cheetham, A. K. Inorg. Chem. 2008, 47, 8336.
