Flexible Ligand, Structural, and Topological Diversity: Isomerism in Zn(NO3)2 Coordination Polymers Zhengfang Tian, Jianguo Lin, Yang Su, Lili Wen, Yangmei Liu, Huizhen Zhu, and Qing-Jin Meng*
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1863-1867
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed March 21, 2007; ReVised Manuscript ReceiVed June 24, 2007
ABSTRACT: Two new metal-organic frameworks (MOFs), [Zn(p-BDC)(m-bix)0.5][Zn(p-BDC)(m-bix)]‚H2O (1) and [Zn(p-BDC)(m-bix)] (2) (p-BDC ) terephthalate, m-bix )1,3-bis(imidazol-1-ylmethyl)benzene), were synthesized and characterized structurally. Both of the MOFs feature the interpenetrated two-dimensional (2D) f three-dimensional (3D) network motifs. In particular, complex 1 represents the second example of intertwining of different 2D coordination motifs, and complex 2 shows a 3-fold parallel interpenetrated framework. In addition, the spectroscopic, thermal, and fluorescence properties of complexes 1 and 2 are investigated. Introduction Various intriguing architectures and potential properties as functional solid materials are two interests in the research of coordination polymer frameworks.1,2 Many factors, involving building modules and their compatibilities, synthetic conditions, and methods, influence intensively the construction process of meta--organic frameworks (MOFs), so prediction or control over the molecular packing in the solid-state remains challenging to chemists.3 Polymorphism and topological isomerism are common phenomena in MOFs because coordination modes of metal ions, flexibility in ligands, and subtle environmental changes all influence the resulting structures.4 First, the diversity and indeterminacy of coordination modes of metal ions could make it hard to predict the structure of the products. Second, conformational flexibility and different binding modes could enhance the possibility of forming supramolecular isomers.5 Finally, subtle environmental changes (such as temperature, solvent, pH) could lead to further complication of the products.6 However, it providesan opportunity to understand the relationships between the influence factors and the diversity of structure. Moreover, experimental and theoretical analysis of the network topology as a powerful tool enhances the understanding of the resulting crystal structures.7 Many supramolecular isomers that contain the flexible ligands (such as bispyridyl, bisimidazol) have been synthesized.8 We have been investigating the use of flexible ligands based on bisimidazol [such as bix (bix ) 1,4-bis(imidazol-1-ylmethyl)benzene), bbi (bbi ) 1,1′-(1,4-butanediyl)bis(imidazole)] linked to arms to build coordination polymer arrays.9 To further understand the coordination chemistry of the flexible ligand and to prepare novel materials with beautiful architectures and good physical properties, herein we employ the flexible ligand m-bix (m-bix )1,3-bis(imidazol-1-ylmethyl)benzene). With p-BDC(p-BDC ) terephthalate) and Zn2+, [Zn(p-BDC)(mbix)0.5][Zn(p-BDC)(m-bix)]‚H2O (1) and [Zn(p-BDC)(m-bix)] (2) have been obtained. Two new MOFs feature the interpenetrated two-dimensional (2D) f three-dimensional (3D) network motifs.4e In particular, complex 1 represents the second example of intertwining of different 2D coordination motifs, and complex 2 shows a 3-fold parallel interpenetrated framework. In addition, * To whom correspondence should be addressed. Fax: +86-2583314502. Tel: +86-25-83597266. E-mail:
[email protected].
the spectroscopic, thermal, and fluorescence properties of complexes 1 and 2 are investigated here. Experimental Section Materials and General Techniques. The reagents and solvents employed were commercially available and used as received without further purification. The m-bix ligand was prepared according to a procedure described in the literature.10 The C, H, and N microanalyses were carried out with a PerkinElmer 240 elemental analyzer. The IR spectra were recorded on KBr disks on a Bruker Vector 22 spectrophotometer in the 4000-400 cm-1 region. Luminescence spectra for the solid samples were recorded with a Hitachi 850 fluorescence spectrophotometer. Thermogravimetric analyses were performed on a simultaneous SDT 2960 thermal analyzer under flowing N2 with a heating rate of 10 °C/min between ambient temperature and 750 °C. Synthesis of [Zn(p-BDC)(m-bix)0.5][Zn(p-BDC)(m-bix)]‚H2O (1). A mixture of Zn(NO3)2‚6H2O (0.1 mmol), p-BDC (0.1 mmol), m-bix(0.1 mmol), five drops of Et3N/CH3CN mixture (1:9), CH3CN (1 mL), and H2O (9 mL) was placed in a Parr Teflon-lined stainless steel vessel (25 cm3), and then the vessel was sealed and heated at 120 °C for 3 days. After the mixture was slowly cooled to room temperature, colorless crystals of 1 were obtained (yield: 67% based on Zn). Anal. Calcd for C74H60N12O17Zn4: C, 53.84; H, 3.66; N, 10.18. Found: C, 53.80; H, 3.60; N, 10.20%. IR spectrum (cm-1): 3193 (m), 1589 (vs), 1523 (m), 1364(vs), 1235 (m), 1109 (s), 953 (w), 828 (m), 749 (s), 654 (m), 583 (w). Synthesis of [Zn(p-BDC)(m-bix)] (2). The synthesis of compound 2 was similar to compound 1 except that Cd(NO3)2‚4H2O (0.1 mmol) was added and colorless crystals of 2 were obtained (yield: 89% based on Zn). Anal. Calcd for C22H18N4O4Zn: C, 56.49; H, 3.88; N, 11.98. Found: C, 56.47; H, 3.80; N, 12.01%. IR spectrum (cm-1): 3424 (w), 3124 (m), 1629 (vs), 1524 (s), 1342 (vs), 1247 (m), 1107 (s), 1085 (m), 817 (m), 746 (s), 731(s), 655 (m). X-ray Crystal Structural Determinations for 1 and 2. Complexes 1 and 2 were collected on a Siemens SMART-CCD diffractometer with graphite monochromatic Mo KR radiation (λ ) 0.71073 Å) using the SMART and SAINT programs. The structures were solved by direct methods and refined on F2 using full matrix least-squares methods with SHELXTL version 5.1.11 Anisotropic thermal parameters were refined for the non-hydrogen atoms. Hydrogen atoms attached to carbon and nitrogen were positioned geometrically (C-H ) 0.93 Å) and included in the refinement in a riding model approximation with an isotropic thermal displacement parameter fixed at 1.2 times Ueq of the atom to which they are attached. Crystallographic data and other pertinent information for all of complexes are summarized in Table 1. Selected bond lengths and bond angles with their estimated standard deviations are listed in Table 2.
10.1021/cg070274z CCC: $37.00 © 2007 American Chemical Society Published on Web 08/17/2007
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Table 1. Crystal Data and Structure Refinement Information for 1 and 2
empirical formula Mr T (K) λ (Å) crystal size (mm) crystal system space group a/Å b/Å c/Å R/° β/° γ/° U (Å3) Z dcalcd (g cm-1) µ (mm-1) θmin, θmax (°) reflns collected independent reflns Rint no. params, restraints final R1, wR2 [I > 2σ(I)] R1, wR2 (all data)
1
2
C74H60N12O17Zn4 1650.8 298(2) 0.71073 0.30 × 0.25 × 0.24 triclinic P1h 9.584(1) 12.563(2) 16.810(2) 76.78(1) 87.43(1) 68.21(1) 1827.7(3) 1 1.500 1.374 2.41, 17.74 7029 4903 0.0272 514, 0 0.0975, 0.1290 0.0633, 0.1223
C22H18N4O4Zn 467.8 298(2) 0.71073 0.30 × 0.30 × 0.25 triclinic P1h 9.601(3) 9.690(3) 11.815(3) 108.37(1) 103.74(1) 91.99(1) 1006.1(5) 2 1.544 1.259 2.20, 23.80 3482 2801 0.0764 280, 0 0.0478, 0.1005 0.0590, 0.1036
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1 and 2 N(1)-Zn(1) O(3)-Zn(1) N(5)-Zn(2) O(7)-Zn(2) O(3)-Zn(1)-O(1) O(1)-Zn(1)-N(4ii) O(1)-Zn(1)-N(1) O(7)-Zn(2)-N(5) N(5)-Zn(2)-O(5)
Complex 1a 2.031(4) N(4)-Zn(1i) 1.925(3) O(1)-Zn(1) 1.988(4) O(5)-Zn(2) 1.938(3) 110.26(16) O(3)-Zn(1)-N(4ii) 101.82(16) O(3)-Zn(1)-N(1) 120.28(14) N(4ii)-Zn(1)-N(1) 119.06(15) O(7)-Zn(2)-O(5) 106.76(15)
Zn(1)-O(1) Zn(1)-N(4A) O(1)-Zn(1)-O(3) O(3)-Zn(1)-N(4i)
Complex 2b 1.938(2) Zn(1)-O(3) 1.994(3) Zn(1)-N(1) 115.16(12) O(1)-Zn(1)-N(4i) 113.51(11) O(1)-Zn(1)-N(1)
1.973(4) 1.942(3) 2.008(3) 121.51(16) 102.80(15) 101.07(18) 125.05(14)
1.951(3) 2.057(3) 115.33(12) 92.15(12)
a Symmetry codes: i ) x, y - 1, z - 1. b Symmetry codes: i ) x - 1, y + 1, z; ii ) x + 1, y - 1, z.
Results and Discussion Crystal Structure of (1). A single-crystal X-ray diffraction study reveals that complex 1 crystallizes in the triclinic space group P1h and features an inclined interpenetrated 2D f 3D network motif. In complex 1, the asymmetric unit contains two parts and one free water molecule, as shown in Figure 1a. Part 1 contains one Zn(II) ion, one p-BDC molecule, and one m-bix ligand. The Zn1(II) ion is in a distorted tetrahedron coordination sphere that is defined by two nitrogens from the m-bix nitrogen donors and two oxygen atoms from the carboxyl groups of p-BDC ligands. Part 2 contains one Zn(II) ion, one p-BDC molecule, and half of one m-bix ligand. The Zn2(II) ion has a slightly distorted trigonal coordination geometry, coordinated by two oxygen atoms from the carboxyl groups of p-BDC ligands and one nitrogen atom from the m-bix nitrogen donor. The coordination mode of Zn2 (II) is less common,12 and the Zn2(II) is located under (0.346 Å) the mean-molecular-plane defined by oxygen and nitrogen atoms, the bond angles of O5Zn2-O7, O5-Zn2-N5, and O7-Zn2-N5 are respectively 125.1(2)°, 106.8(2)°, 119.1(2)°. For the m-bix ligand of part 1, two terminal imidazole groups assume trans conformation and their planes are steeply titled 65.2° and 89.2° with respect to the average plane of the phenyl
ring. The angles of N2-C4-C5 and N3-C11-C9 (N2 and N3 separately from the two imidazole groups, C4 and C11 from the CH2 groups between the phenyl ring and imidazole rings, C5 and C9 from the phenyl ring) are 117.5° and 110.6°. For the m-bix ligand of part 2, two terminal imidazole groups also assume trans conformation, but the phenyl ring is disordered and the dihedral angle between the imidazole ring plane and the plane of phenyl ring is 83.0°. The angle of N6-C26-C27 (N6 from the imidazole group, C26 from the CH2 group between the phenyl ring and the imidazole ring, C27 from the phenyl ring) is 97.6°. For part 1, the Zn1(II) ions are connected to the undulated planes by p-BDC and m-bix ligands and can be classified topologically as a (4,4) network (type I). For part 2, the Zn2(II) ions are interlinked to the planes by the p-BDC and m-bix ligands and can be classified topologically as a (6,3) network (type II) (see Figure 1b and layer networks in different directions as shown in Figure 1c). Both types of networks are parallel with interplane distances the type I and II of 8.75 and 5.38 Å, respectively, and the two sets intercross at an angle of ca. 79°; thus they interpenetrate to give a 3D array, which displays small pores that contain the guest H2O molecules. It is noteworthy that each grid (either square or hexagonal) is interpenetrated by an infinite number of sheets of the other type (see Figure 1d). However, each window of the hexagonal sheets is penetrated by three square sheets, and each window of each square sheet is penetrated by two of the hexagonal sheets. This interpenetration represents the second example of intertwining of different 2D coordination motifs.13 Crystal Structure of (2). When Cd(NO3)2‚6H2O was added to the admixture of Zn(NO3)2‚6H2O, p-BDC, and m-bix ligand with a molar ratio of 1:1:1:1, we obtained complex 2. Complex 2 also crystallizes in the triclinic space group P1h and features a 3-fold parallel interpenetrated 2D f 3D network motif. As depicted in Figure 2a, the asymmetric unit contains one Zn(II) ion, one p-BDC molecule, and one m-bix ligand. The Zn(II) ion has the same coordination geometry to the Zn1(II) ion of compound 1, and Zn(II) ion occupies the center of the distorted tetrahedron which is composed of two nitrogens from the m-bix nitrogen donors and two oxygen atoms from the carboxyl groups of p-BDC. By contrast, two terminal imidazole groups of the m-bix ligand assume cis conformation and have their planes steeply titled 71.1° and 72.5° with respect to the average plane of phenyl ring. The angles of N2-C12-C13 and N3-C19C15 (N2 and N3 separately from the two imidazole groups, C12 and C19 from the CH2 groups between the phenyl ring and the imidazole ring, C13 and C15 from the phenyl ring) are 116.2° and 111.2°. Four Zn atoms are interlinked by the two m-bix and the two p-BDC ligands, thus affording a square grid with a large window of 11.0 × 12.7 Å2, which are further connected together into a 2D puckered 44 net (see Figure 2b). The interesting feature of 2 is the occurrence of a parallel interpenetrating motif consisting of three identical single nets (see Figure 2c). However, upon interpenetration, the PLATON calculation shows that the unit cell contains no residual solvent-accessible void. Comparing 1 and 2, two kinds of 2D f 3D network motifs show tremendous diversity. It should be attributed to the following contributions: (1) The coordination modes of Zn(II) ions: two kinds of modes exist in complex 1, and one kind of mode exists in complex 2. (2) The highly flexible m-bix ligand: two methylene (CH2) groups between the phenyl and the imidazol rings can rotate freely, endows itself with high flexibility to take on a variety of conformation, and finally gives
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Figure 1. (a) ORTEP view of compound 1. Thermal ellipsoids are drawn at the 50% probability level. (b) The two types of layers present in compound 1: type I left, and II right. (c) View of the two types of layers in different direction: type I left, and II right. (d) A schematic view of the inclined interpenetration of the two types of layers. Hydrogen atoms and free water molecule have been omitted for clarity.
two interpenetrated frameworks.14 (3) Upon addition of Cd(II) ions, there is a possible competition between Cd(II) and Zn(II) ions, which modifies the coordinated geometry of the Zn(II) ion and the trigonal coordination geometry of the Zn(II) ion has not formed; further work is planned to support this hypothesis. Thermal Properties. Results of thermogravimetric analysis (TGA) for frameworks 1 and 2 measured under a N2 atmosphere are shown in Figure 3. For 1, the weight loss of 1.60% from 150 to 262 °C (calcd 1.10%) corresponds to the loss of one free water molecule per formula. After the loss of water molecules, a plateau region is observed from 262 to 350 °C that the 3D framework of complex 1 may still stand. A rapid
weight loss can be detected from 360 to 439 °C, which is attributed to the complete decomposition of the m-bix and p-BDC ligand. In comparison with 1, compound 2 is slightly more stable up to 360 °C, and the 3D framework of complex 2 begans to collapse from 360 to 440 °C. Luminescent Properties. The emission spectra of complexes 1 and 2 in the solid state at room temperature are investigated, as depicted in Figure 4. Complex 1 exhibits strong fluorescent emission bands at ca. 384, 403, and 425 nm upon excitation at ca. 353 and 360 nm. Complex 2 displays strong fluorescent emission bands at ca. 395 and 413 nm, respectively, upon excitation at ca. 360 nm. Because the Zn(II) ion is difficult to oxidize or to reduce due to its d10 configuration, these emissions
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Figure 3. TGA curves for 1 (black) and 2 (red).
Figure 4. The excitation and emission spectra of complex 1 and 2. 1em (black), 1ex (red), 2em (green), and 2ex (blue).
Figure 2. (a) ORTEP view of compound 2. Thermal ellipsoids are drawn at the 50% probability level. (b) View of a single 2D puckered grid representing the four-connected (4, 4) topology. (c) Schematic representation of the interpenetration of complex 2. Hydrogen atoms have been omitted for clarity.
can probably be assigned to intraligand (π-π*) fluorescent emission.15 At the same time, the free m-bix exhibits weak fluorescent emission bands at ca. 466 nm upon excitation at ca. 368 nm, which further supports this interpretation. The enhancement of the emissions for 1 and 2 compared with those of the free ligands may be ascribed to the increase in the ligand conformational rigidity due to their coordination to the Zn(II) ion resulting in a decrease in the nonradiative decay of intraligand excited states. They appear to be good candidates for novel hybrid inorganic-organic photoactive materials.
Conclusion In summary, we have successfully synthesized and characterized two interpenetrated entanglement systems based on the connectivity co-effect between the benzenedicarboxylate and the highly flexible m-bix ligand. Complex 1 represents the second example of intertwining of different 2D coordination motifs, and complex 2 shows a 3-fold parallel interpenetrated framework. This work will not only deepen our systemic understanding of the structural functionality of the conformational flexibility of the m-bix ligand molecule but also excite our interest in the chemical topology. Moreover, complexes 1 and 2 display strong emissions at room temperature; therefore, they appear to be good candidates for novel hybrid inorganicorganic photoactive materials. Acknowledgment. We thank the National Nature Science Foundation of China (Grant No. 20490218), Jiangsu Science & Technology Department, and the Center of Analysis and Determining of Nanjing University for financial support. Supporting Information Available: X-ray crystallographic files in CIF format and X-ray power diffraction diagram. This material is available free of charge via the Internet at http://pubs.acs.org.
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