CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 6 1027-1030
Communications A Novel Chiral Doubly Folded Interpenetrating 3D Metal-Organic Framework Based on the Flexible Zwitterionic Ligand Xian-Wen Wang,*,† Lei Han,*,‡ Tie-Jun Cai,§ Yue-Qing Zheng,*,†,‡ Jing-Zhong Chen,† and Qian Deng§ Faculty of Material Science and Chemical Engineering, China UniVersity of Geoscience, Hubei, Wuhan 430074, China, Faculty of Materials Science & Chemical Engineering, Ningbo UniVersity, Zhejiang, Ningbo 315211, China, and College of Chemistry and Chemical Engeering, Hunan UniVersity of Science and Technology, Hunan, Xiangtan 411201, China ReceiVed December 17, 2006; ReVised Manuscript ReceiVed April 25, 2007
ABSTRACT: A novel chiral 2-fold interpenetrating three-dimensional (3D) porous metal-organic framework (MOF), [PbCl(C7H7N2O4)] (1), has been constructed from the flexible zwitterionic ligand N,N′-diacetic acid imidazolium chloride and Pb(NO3)2. The 3D MOF possesses right-handed helical rectangular channels with 41 helices running along the c-axis. Compound 1 represents the first even number-fold interpenetrating 3D chiral MOF constructed from a symmetrical flexible ligand without any chiral auxiliary. The field of metal-organic frameworks (MOFs) has undergone explosive growth over the past decade1 due to their potential technological applications such as separations and catalysis,2 gas storage,3 and magnetism.4 The designed construction of chiral or homochiral porous MOFs is of great current interest because of their unique applications in asymmetric catalysis, enantioselective separations, biomimetic chemistry, nonlinear optical materials, and magnetic materials.5 Chiral MOFs can be built from chiral ligands or by using achiral ligands under spontaneous resolution without any chiral sources.6 The synthesis of chiral species from achiral ligands is the key issue in studying the genesis of chirality in biological systems. But how to generate chiral units from achiral components and induce the chiral information of the chiral units into higher dimensional chiral MOFs without any chiral auxiliary is the main challenge that should be solved, and there are only a few examples of chiral information in the building units being transmitted into higher dimensionality to produce chiral MOFs. The helix gives a good opportunity to transmit chiral information, and flexible multidentate organic bridging ligands may be efficient candidates to improve the helix elements for generating chiral MOFs.6a,7 Zwitterionic complexes can function as ionic liquid precursors or ionic liquids, which have aroused considerable interest in recent years as potentially green, cleaner, and environmentally friendly reaction media in organic synthesis and catalysis.8,9 However, they have been rarely utilized in the areas of synthetic coordination chemistry, and relatively few studies have investigated the use of ionic liquid ligands or zwitterionic complexes for the purposeful generation and characterization of MOFs.10 Furthermore, lead cations are the most commonly encountered toxic metal pollutant * To whom correspondence should be addressed. E-mail: wxw10108092@ yahoo.com.cn (X.-W.W.);
[email protected] (L.H.);
[email protected] (Y.Q.Z). † China University of Geoscience. ‡ Ningbo University. § Hunan University of Science and Technology.
in the environment, and exploiting the unique coordination properties of Pb(II) provides opportunities for the development of practical ligands as extractants or lead-poisoning treatment agents. The functionalized imidazolium-containing polycarboxylate ionic liquids or ionic liquid precursors may be efficient lead-poisoning treatment candidates to deposit or extract the weight metal Pb(II) ion from environmental solutions. Inspired by the aforementioned considerations, we have attempted to extend our work to study the systematic synthesis of novel MOFs with flexible functionalized imidazolium-containing ionic liquid precursors as bridging ligands. In this communication, we present a novel three-dimensional (3D) chiral MOF [PbCl(C8H7N2O4)] (1), which resulted from self-assembly of the zwitterionic salt N,N′diacetic acid imidazolium chloride and Pb(NO3)2 under mild conditions. Compound 1 represents the first 3D MOF built from a zwitterionic complex ligand; to our best knowledge, it is also the first example of even number-fold interpenetrating 3D chiral MOF generated from a symmetrical flexible ligand without any chiral auxiliary.11 Reaction of the zwitterionic salt N,N′-diacetic acid imidazolium chloride12 with Pb(NO3)2 in aqueous solution at ambient temperature affords colorless prismatic crystals 1 suitable for single-crystal X-ray analyses.13 The crystalline product was characterized by elemental analysis, IR, UV-vis spectroscopy, thermogravimetry (TG-DSC), and X-ray powder diffraction (XRD). The absorptions at 1590 and 1387 cm-1 for 1 are the characteristic νas(CO2-) and νs(CO2-) stretching vibrations of the carboxylate groups. In comparison to the characteristic stretching vibration bands of carboxylate groups of free ligand molecules, the significant blue-shift of both the νas(CO2-) and the νs(CO2-) stretching vibration peaks may be attributed to the coordination interactions. 1 crystallizes in a chiral space group I4122 (No. 98) and is formulated as [PbCl(C8H7N2O4)].14 As shown in Figure 1, each of the zwitterionic N,N′-diacetate imidazolium anion ligands acts as bridge connectors linking four lead atoms, with the carboxylate groups adopting a bidentate chelating plus monodentate bridging
10.1021/cg060922z CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007
1028 Crystal Growth & Design, Vol. 7, No. 6, 2007
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Figure 1. ORTEP view of [PbCl(C8H7N2O4)] with displacement ellipsoids (40% probability) and atomic labeling, showing the coordination environment of Pb(II) atoms, Cl-, and the zwitterionic ligand. (The symmetry codes are #1 ) -y, -x, -z; #2 ) y + 1/2, -x, z - 1/4; #3 ) x, -y - 1/2, -z + 1/4; #4 ) -y, x - 1/2, z + 1/4, z - 1/4; and #5 ) x, -y + 1/2, -z + 1/4.)
mode. Each of the Pb2+ atoms is eight-coordinated with a significantly distorted monocapped pentagonal bipyramidal geometry by two µ2-Cl atoms and six oxygen atoms from four carboxylate groups, which come from four crystallographically different N,N′diacetate acid imidazolium anion ligands. The capping position is occupied by the monoatomically bridging carboxylato O2#1, and the carboxylato O1#1 and Cl1 atoms reside at the axial apexes with a trans angle of 155.26(11)°. The basal plane is defined by one µ2-Cl#2, two bidentate chelating carboxylato oxygen atoms (O1 and O2), and two monoatomically bridging oxygen atoms (O2#2 and O2#3). The Pb1-O bond distances ranging from 2.555(5) to 2.706(4) Å are similar to those observed in the related complex [C18H24PbN2O10].15 The Pb-Cl bond lengths (2.985(2) Å) are comparable to those in the compound Pb5Cl6[(CH3)2NCH2CH2O]4.16 Along c-axis, the monocapped pentagonal bipyramidally coordinated Pb atoms are connected by the N,N′-diacetate imidazolium anion ligands into one-dimensional right-handed helical chains (Figure 2a). Each repeated helical unit includes five Pb atoms and four zwitterionic ligands with a pitch of 15.458(3) Å, and the righthanded helix is generated from the crystallographic 41 axis. The infinitely right-handed helical chains are held together to form a 3D homochiral MOF, where the neighboring Pb atoms of different helical chains are interlinked by the µ2-Cl atoms and bridging carboxylate oxygen atoms into face-shared inorganic [Pb(µ2-Cl)2O6]n chains along the c-axis (Figure S1, Supporting Information). Interestingly, the 3D MOF exhibits tetragonal righthanded helical nanochannels with an opening of 1.1871(2) × 1.1871(2) nm (Figure 2b), which is similar to the two helical nanotubular nickel(II) complexes reported by Lin et al.17 A spacefilling model shows the open rectangular channels in the individual framework (Figure 2c), and the schematic diagram (Figure 2d) shows that the Pb atoms are connected by N,N′-diacetate imidazolium ligands and µ2-Cl atoms into a 3D framework. Compound 1 adopts a doubly folded interpenetrating structure to avoid the formation of very large open cavities (Figure 3a). It should be noted that the interpenetration of the 3D nets, which are topologically equivalent in terms of composition and configuration, results in a homochiral framework (Figure 3b), indicating that spontaneous resolution took place in the course of crystallization. Even after the 2-fold interpenetration, the void spaces still exist in 1 and are clearly shown in the space-filling mode viewed down the b-axis (Figure S2, Supporting Information). Calculations using PLATON18 revealed that the open channels constitute about 30.6% of the crystal volume. Although to date numerous chiral MOFs have been obtained based on the asymmetric ligands and transition metal,6-7,11 the even number-fold interpenetrating chiral 3D frameworks constructed from symmetrical ligands without any chiral auxiliary are quite unusual.11,19 A solid-state circular dichroism (CD) spectrum of 1 was recorded on single crystals with a KBr pellet between 200 and 450 nm
Figure 2. (a) View of the right-handed helical chain in 1. (b) View down from the c-axis showing the nanotube. (c) A space-filling model showing the open rectangular channels in the individual framework. (d) View of the 3D framework (the purple spheres and purple lines indicate Pb atoms and N,N′-diacetate imidazolium ligands, respectively, and the green balls and green bonds suggest chloride atoms and Pb-Cl bonds, respectively).
Figure 3. (a) A space-filling model showing the doubly interpenetrating framework of 1. (b) A schematic illustrating the same handedness of the interpenetrating chains.
(Figure 4a). The CD spectrum exhibits an obvious Cotton effect. The CD signals suggest that the entire bulk sample of 1 is a same handed conformation, which further demonstrates the validity of structural analyses. The electronic spectrum of N,N′-diacetic acid imidazolium chloride shows one maximal peak centered at 215 nm due to the imidazolium ring πf π* transitions. Upon the formation of compound 1, a new intense peak centered at 263 nm appeared, which can be assigned to the ligand to metal LMCT (Cl f Pb(II)) from the extended [PbCl]+ inorganic chains (Figure 4b).20 XRD data of complex 1 was collected on a Rigaku D_max/3b diffractometer with Cu-KR radiation (λ ) 1.5418 Å). The experimental XRD patterns agreed well with the simulated ones generated on the basis of the single-crystal analyses for 1 (Figure 5), suggesting the phase purity of the products. Thermogravimetric
Communications
Crystal Growth & Design, Vol. 7, No. 6, 2007 1029 Natural Science Foundation of China (NSFC 60508012), the Expert Project of Key Basic Research of the Ministry of Science and Technology of China (2003CCA00800), and Ningbo Municipal Natural Science Foundation (2006A610061). The authors are grateful to Dr. Hong-Cheng Pan at Nanjing University for the solidstate CD measurements. Supporting Information Available: Crystallographic data and IR spectrum for 1; Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 4. (a) The solid-state circular dichroism (CD) spectrum of 1; (b) UV-vis spectrum of 1.
Figure 5. The experimental and the simulated XRD patterns based on the structural analysis for 1.
Figure 6. The TGA-DSC diagram for complex 1.
analysis (TGA-DSC) experimental results indicate that 1 shows remarkable thermal stability because of the neutral 3D polymeric structure, with an initial framework decomposition temperature of 250 °C. Then, the sample suffers incessant weight loss until it reaches a temperature of 580 °C. The total mass loss of 43.4% corresponds to the removal of the organic species (calc. 43.0%). The DSC curves of 1 exhibit four exothermic peaks centered at 299, 308, 418, and 486 °C (Figure 6). Inspired by the quick deposition of the reaction of Pb(II) ions with the zwitterionic salt and the resultant compound being insoluble in common solvents, we are attempting to use the zwitterionic ionic liquid precursors and their extended ionic liquids such as [{(CH2)nCOOH}im]BF4 (n ) 1, 2, 3) to extract Pb2+ from aqueous solutions containing other divalent cations by way of inductively coupled plasma MS (ICP-MS). These studies are under investigation. In conclusion, a novel, doubly interpenetrating chiral 3D Pb(II) MOF containing a flexible functionalized zwitterionic complex ligand has been synthesized and structurally characterized. To our best knowledge, it is the first 2-fold interpenetrating chiral 3D MOF constructed from a symmetrical ligand without any chiral auxiliary. This work clearly illustrates the utility of potential ionic liquids or ionic precursors as ligands for the rational design and synthesis of novel functional materials. Acknowledgment. The authors thank the Scientific Research Fund of Ningbo University (019-011090, XK200459, SS2004033),
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1030 Crystal Growth & Design, Vol. 7, No. 6, 2007 hours at reflux temperature, and the pH of the solution was controlled in the range of 10-12 with 5 M KOH solution. Aqueous HCl (1 M) was poured into the resultant reaction solution until the pH ) 2-3. A colorless crystalline sample was obtained by concentration and cooling to room temperature. Yield: >90%; IR data (cm -1, KBr): 3389s, 3145s, 2992s, 2626m, 1736vs, 1625m, 1577m, 1405m, 1354w, 1317w, 1292w, 1225s, 1203s, 1179s, 1088s, 976w, 891w, 833w, 756m, 660m, 624m. (13) Preparation of compound 1: In a typical synthesis, N,N′-diacetic acid imidazolium chlorate (0.220 g, 1.0 mmol) was added to astirred aqueous solution of Pb(NO3)2 (0.331g, 1.0 mmol). The mixture was stirred for a further ca. 30 min and filtered. Colorless block crystals were generated from the filtrate in several hours at room temperature (about 40 °C) in a yield of 85% based on the initial Pb(NO3)2. Anal. Calc. for C7H7PbN2O4Cl (%): C, 19.73; H, 1.64; N, 6.58. Found: C, 19.79; H, 1.55; O, 6.62. IR data (cm -1, KBr): 3477m, 3154m, 3102m, 2988m, 2939w, 1590s, 1437m, 1387s, 1303s, 1180s, 1102w, 1036w, 1028w, 923m, 860w, 790m, 7441w, 701s, 628w, 584w.
Communications (14) Crystal data for C7H7ClPbN2O4 1: tetragol, space group I4122 (No. 98), Mr ) 425.79, a ) 11.9666(17), b ) 11.9666(17), c ) 15.485(3) Å, V ) 2217.4(6) Å3, Z ) 8, T ) 293(2) K, Dc ) 2.551 g cm-3, F(000) ) 1552, Mo KR radiation (λ ) 0.71073 Å), µ ) 15.453 mm-1, R1 ) 0.0204 and ωR2 ) 0.0518, S ) 1.166. The Flack value is -0.003(17). (15) Battistuzzi, G.; Borsari, M.; Menabue, L.; Sal, M. Inorg. Chem. 1996, 35, 4239. (16) Sienkiewicz, A. V.; Kokozay, V. N. Polyhedron 1994, 13, 1439. (17) Cui, Y.; Lee, S. J.; Lin, W. J. Am. Chem. Soc. 2003, 125, 6014. (18) Spek, A. L. PLATON, version 1.62; University of Utrecht: Utrecht, 1999. (19) (a) Sasa, M.; Tanaka, K.; Bu, X.; Shiro, M.; Shionoya, M. J. Am. Chem. Soc. 2001, 123, 10750. (b) Ma, J.-P.; Dong, Y.-B.; Huang, R.-Q.; Smith, M. D.; Su, C.-Y. Inorg. Chem. 2005, 44, 6143. (20) (a) Friedman, H. L. J. Am. Chem. Soc. 1952, 74, 5. (b) Gamlen, G. A.; Jordan, D. O. J. Am. Chem. Soc. 1953, 75, 1435.
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