DOI: 10.1021/cg9015096
Assembly of Two Metal-Organic Frameworks with Intrinsic Chiral Topology from Achiral Materials
2010, Vol. 10 492–494
Xuemin Jing,† Lirong Zhang,† Tianliang Ma,† Guanghua Li,† Yang Yu,† Qisheng Huo,† Mohamed Eddaoudi,‡ and Yunling Liu*,† †
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China and ‡Department of Chemistry, University of South Florida, 4202 East Fowler Avenue (SCA400), Tampa, Florida 33620
Received December 2, 2009
ABSTRACT: The assembly of Cu(CH3COO)2 3 H2O and pyridine-4,5-imidazoledicarboxylic acid (H3PyImDC) under solvothermal conditions in the presence of different organic templates yielded two three-dimensional chiral frameworks, |(C3H10N2)(C3H7NO)0.5(H2O)2|[CuIIC10H5O4N3] (1) and |(C4H9NO)0.5(H2O)8|[CuII2CuI(C10H5O4N3)2(C10H6O4N3)] (2). Both of the compounds crystallized in the cubic, P4332 space group and possessed open helical channels in the frameworks. To the best of our knowledge, compounds 1 and 2 present the first synthesized MOFs with a bmn and lcy-a net. Further characterization of the two compounds has been performed, including X-ray powder diffraction, inductively coupled plasma analysis, CHN, IR spectra, fluorescence spectra, X-ray photoelectron spectroscopy spectrum and thermogravimetric-differential thermal analysis. Metal-organic frameworks (MOFs) have been recognized for their potential applications in gas storage, carbon dioxide capture, renewable catalysts, and drug delivery.1 Chiral MOFs, a special class of functional porous materials, are of particular interest due to the pressing demand in areas pertaining to enantioselective separation, chiral sensing, and nonlinear optics applications.2 Although chirality is ubiquitous in nature, its occurrence in MOFs is particularly scarce. The search for novel strategies to implement chirality in solids in general, and MOFs in particular, remains an ongoing challenge in crystal chemistry. Three synthetic strategies can be employed for the construction of chiral materials: (1) enantioselective synthesis, introduction of enantiopure ligand owing to the specific coordination arrangements;3 (2) use of racemic ligands that coordinate with metal ions in a particular geometry to construct extended chiral frameworks;4 (3) spontaneous resolution, induced by local distortion of the achiral organic ligand during the assembly process; numerous examples are known in which achiral components crystallize in chiral symmetry, and it has been proven as a constructive approach for the synthesis of chiral materials.5 Nevertheless, it is still a challenge to design an organic ligand, combining both flexibility and directionality, that permits the construction of the desired chiral materials under controlled synthesis conditions. Inspired by the impact of the (10,3)-a chiral network in the field of MOFs,6 as it can be predictably targeted and constructed from 3-connected organic linkers and metal nodes, herein we report the potential use of organic amines as structure-directing agents (SDA) to assemble two chiral MOFs, based on H3PyImDC as a 3-connected linker, with highly selective, nitrogen-oxygen chelating, substrate-binding ability and directionality.7 Reactions of H3PyImDC and Cu(CH3COO)2 3 H2O under solvothermal conditions have afforded the synthesis of blue crystals of 1 (see Figure 1a) and dark green crystals of 2 (see Figure 2a). The assynthesized samples were characterized and formulated by elemental analysis, IR spectra, thermogravimetric analysis, and single-crystal X-ray diffraction studies as |(C3H10N2)(C3H7NO)0.5(H2O)2|[CuIIC10H5O4N3] 18 and |(C4H9NO)0.5(H2O)8|[CuII2CuI(C10H5O4N3)2(C10H6O4N3)] 2.9 The as-synthesized compounds were insoluble in water and common organic solvents. The phase purity of the as-synthesized samples was confirmed by the evident *To whom correspondence should be addressed. Fax: þ86-431-85168624. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 12/31/2009
Figure 1. A description of the structure of |(C3H10N2)(C3H7NO)0.5(H2O)2|[CuIIC10H5O4N3] 1. Color scheme: carbon=gray, nitrogen= blue, oxygen = red, copper = green, and some of the H atoms were omitted for clarity. (a) Optical image of 1. (b) Illustration of copper centers, viewed as a 3-connected node. (c) Stick model of the 3D framework with right-helical channel and left-helical tube. (d) Space-filling representation of two helices. (e) Space-filling model as viewed along the [111] direction. (f) A schematic representation of the bmn net.
similarity between the calculated and experimental X-ray powder diffraction patterns. In the crystal structure of 1 (see Supporting Information, Figure S1), there is one crystallographically independent CuII ion and one (HPyImDC)2- ligand. The Cu ions adopt distorted r 2009 American Chemical Society
Communication
Figure 2. A description of the structure of |(C4H9NO)0.5(H2O)8|[CuII2CuI(C10H5O4N3)2(C10H6O4N3)] 2. Color scheme: carbon = gray, nitrogen = blue, oxygen = red, copper = green, and some of the H atoms were omitted for clarity. (a) Optical image of 2. (b) Illustration of copper sites, viewed as a 4-connected node. (c) Stick model of the framework with right-handed tube and left-handed channel. (d) Space-filling model of two helices. (e) A schematic representation of the lcy-a net. (f) Polyhedral view of two kinds of helical channels along the [100] and [111] directions.
trigonal bipyramidal geometries with O1, O3A (Cu-O = 2.033(8), 2.183(9) A˚), and N3A atoms (Cu-N = 2.035(7) A˚) at the equatorial positions, N1 and N2A atoms (Cu-N = 1.928(6), 1.944(6) A˚) at the axial position. The (HPyImDC)2- serves as tridentate ligand bridging three Cu ions through N1, N2A, N3A, O1, and O3A atoms and the Cu 3 3 3 Cu distance is in the range of 6.022-7.733 A˚. Three (HPyImDC)2- ligands link four Cu ions to form a 3-connected triangle node (Figure 1b), and the 3-connected nodes are further linked to construct a three-dimensional (3D) open framework (Figure 1c). The distortion of the (HPyImDC)2- ligands leads to the crystallization of 1 in the chiral space group P4332 which possesses two types of helical channels with opposite chirality. Both helical channels can be viewed as four {CuII 3 3 3 CuII 3 3 3 CuII} centers arranged in clockwise/anticlockwise direction with 90° span to generate two helical chains with a pitch of 24.0 A˚ (Figure 1d). In contrast to the righthelical channel, the Cu ions and (HPyImDC)2- ligands are arranged in the center of the left-helical channel. More interestingly, there are two types of right-helical chains (one large helix and three small helices) interwoven to form an extra large righthelical channel (see Figure 1e and Supporting Information, Figure S6) along the [111] direction, approximately 11 11 A˚2 in diameter. Calculation performed using PLATON10 reveals a total solvent-accessible volume equal to 8732.9 A˚3 per unit cell, which counts for 63.0% of the cell volume, offering possibilities for chiral separation. To better understand the structure of 1, the Cu sites and the connecting ligands (HPyImDC)2- can be regarded as topologically equivalent 3-connected nodes and thus leading to a framework simplification into a uninodal three-periodic bmn net
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Figure 3. The X-ray powder diffraction patterns for the simulated, as-synthesized samples and C2H5OH-, CH2Cl2-, CH3CN-exchanged samples (a for compound 1, b for compound 2).
(Figure 1f), as determined by Systre.11 To the best of our knowledge, compound 1 is the first synthesized MOF with a bmn net, the three code letters referring to the net in the RCSR database (Supporting Information, Table S1). In the structure of 2 (Supporting Information, Figure S1), there are two crystallographically independent CuII and CuI ions; the oxidation states of Cu ions are further confirmed by X-ray photoelectron spectroscopy (XPS) (Supporting Information, Figure S3). The CuII ions are 5-coordinated (see Figure 2b) as in 1, and the Cu-O bond distances range from 1.918(7) to 2.308(7) A˚, whereas the Cu-N distances vary from 1.967(7) to 2.004(7) A˚. The CuI ions are 2-coordinated with N2 and N2A atoms (Cu-N = 1.879(6) A˚) of the (HPyImDC)2- ligands. The (H2PyImDC)- serves as a tridentate ligand linking CuII and CuI through O1, N1, N2, and N3 atoms, while (HPyImDC)2- acts as a bridging ligand through N4 and O5 centers. Accordingly, five CuII ions, two CuI ions, and five ligands can be viewed as a 4-connected tetrahedron node (see Figure 2b), which are further linked to form two types of helical channels in 2 (see Figure 2c). The left-helical channel can be constructed as follows: the CuII ions and (H2PyImDC)- ligand are interlinked in an anticlockwise direction with 90° span to form the left-helical chain. On the other hand, the CuII, CuI, (HPyImDC)2-, and (H2PyImDC)- units are interconnected to construct the right-helical channel with fillers in it, and the pitch is 27.3 A˚ (Figure 2d). Calculation using PLATON shows a total solvent-accessible volume of 12473.2 A˚3 per unit cell, equivalent to 61.4% of the cell volume. The best approach to interpret and understand the structure of 2 is to consider all CuI ions as an integral part of the metalloligand, and CuII sites as 4-connected nodes linked to four other surrounding CuII sites. Such simplification allows the determination of the network topology of 2 as lcy-a net (see Figure 2e), an unprecedented topology in crystal chemistry according to the
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RCSR database. In this aforementioned net, six CuII nodes can be viewed as a triangular metaprism with a face symbol (32.43) (Supporting Information, Figure S7 and Table S1); the triangular metaprisms are further connected through the vertices to form a helix. There are two types of helical channels constructed by the helices: three left-helices are interwoven to form the left-helical channel along the [100] direction, and two right-helices are also interwoven to construct the right-helical channel along the [111] direction (Figure 2f). The as-synthesized samples were immersed in C2H5OH, CH2Cl2 and CH3CN solvent for 24 h and the phase purity of the guest-exchanged samples (including C2H5OH-, CH2Cl2-, CH3CN-exchanged samples) were confirmed by the evident similarity between the simulated and experimental X-ray powder diffraction patterns (Figure 3). The IR spectra (Figure S4, Supporting Information) for the as-synthesized and guestexchanged samples have further confirmed that the frameworks keep their stability after the guest molecules are exchanged. The gas-sorption experiments are under investigation. In summary, two open MOFs with novel intrinsic chiral nets (bmn and lcy-a) have been successfully assembled under solvothermal conditions by using different SDAs. Work is in progress to assemble novel chiral MOFs based on H3PyImDC ligands and other metals.
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Acknowledgment. We gratefully acknowledge the financial support of the Natural Science Foundation of China (Grant No. 20671041, No. 20701015, No. 20788101). Supporting Information Available: ORTEP view, IR data, XPS spectra, TGA curves, structure figures, and X-ray crystallographic data are available free of charge via the Internet at http://pubs. acs.org.
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