A 3D Metal–Organic Framework with Rare 3-Fold Interpenetrating Dia-g Nets Based on Silver(I) and Novel Tetradentate Imidazolate Ligand: Synthesis, Structure, and Possible Ferroelectric Property Li Song,†,‡ Shao-Wu Du,*,† Jian-Di Lin,† Hui Zhou,†,‡ and Tao Li† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian, Fuzhou 350002, P. R. China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2268–2271
ReceiVed May 17, 2007; ReVised Manuscript ReceiVed October 2, 2007
ABSTRACT: A novel 3D metal–organic framework [Ag8(L)4](NO3)8 · 4H2O (1) has been solvothermally synthesized by self-assembly of
a new tetradentate imidazolate ligand L (L ) bis(3,5-bis((1H-imidazol-1-yl)methyl)-2,4,6-trimethylphenyl)methane) and silver(I) salt. X-ray crystallography reveals that 1 contains rare 3-fold interpenetrating dia-g (or 4.142) nets. Each single dia-g net comprises linked Ag–L meso-helices. The possible ferroelectric behavior and luminescence property of the title compound have also been studied. Metal–organic frameworks (MOFs) have been extensively studied because of their rich structural diversity and potential applications in gas adsorption,1 enantioselective separation,2 catalysis,3 nonlinear optics,4 and so on.5 In the attempt to obtain new MOFs for functional materials, various organic bridging ligands have been designed and employed, especially those containing Nor O-donors. Among the N-donor linkers, the imidazole-containing tectons are particularly appealing because of their versatile ligand types and variational ligand conformations compared to the rigid N-donor ligands. In general, the flexibility of imidazolate spacers usually resulted in the formation of novel network topologies because the flexible ligands can adopt diverse conformations to meet the different coordination requirements of the metal ions. For example, the bi-,6,7 tri-,8,9 and hexa-imidazole10 ligands have been widely used to combine with various metal centers to generate interesting supramolecular architectures such as polygons,6a zeolitelike topology,6b polyrotaxanes,6c trigonal-prismatic cages,8a,b,9a and many other unprecedented polymeric structures. However, in the family of multiconnecting imidazolate ligands, the tetradentate linkers are rare and the only known example is 1,2,4,5-tetrakis(imidazol-1-ylmethyl)benzene, from which a 2D zinc polymer, [Zn3(HPO4)3(H2PO4)(C22H22N8)0.5(C22H24N8)0.5], has been synthesized.11 Ferroelectric materials are receiving great attention because of their promising applications as multifarious technological electric devices, such as dynamic random access memories and nonvolatile binary data storage media.12 Nevertheless, the studies on ferroelectric materials of MOFs are still inchoative, and only a few ferroelectric MOFs have been reported up to now.13,14 Furthermore, the reported ferroelectric MOFs are mostly built upon chiral organic tectons, whereas the proper use of achiral ligands by spontaneous resolution are scarce.13b Herein, we describe the synthesis, structure, and preliminary investigation of the possible ferroelectric property of a novel three-dimensional (3D) MOF, [Ag8(L)4](NO3)8 · 4H2O (1). The ligand L was prepared as described in Scheme 1. It is soluble in common organic solvents such as CH3OH, CH3CN, CHCl3, etc. Light-yellow plate-shaped crystals of 1 were obtained in ca. 42% yield (based on Ag) when AgNO3 was reacted with L in a 1:1 CH3OH:H2O mixed solvent in a Teflon-lined stainless steel reactor at 120 °C for 48 h (see the Supporting Information). Thermogravimetric analysis (see the Supporting Information, Figure S2) reveals that 1 is stable up to 315 °C, from which the organic components * To whom correspondence should be addressed. Fax: (86) 591-83709470. E-mail:
[email protected]. † Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
Figure 1. Coordination environment of silver and L. The hydrogen atoms are omitted for clarity. Symmetry codes: (A) –x + 1/4, y + 1/4, z – 3/4; (B) –x, –y, z; (C) –x + 1/4, y – 1/4, z + 3/4.
Figure 2. (a) Single 3D network in 1 viewed along the [013] axis with the meso-helical chains shown in red circle and ellipse part. A and B are both meso-helices of the same construction. A extends along the [013] axis, whereas B is along the [0–13] direction. Orange, Ag; blue, N; green, C. Hydrogen atoms and methyl groups are omitted for charity. (b) Side view of the meso-helical chain constructed by Ag1 and two imidazole groups of L. The other two imidazoles of L are omitted for clarity.
10.1021/cg070458d CCC: $37.00 2007 American Chemical Society Published on Web 10/23/2007
Communications
Crystal Growth & Design, Vol. 7, No. 11, 2007 2269 Scheme 1. Synthetic Route of ligand L
Figure 3. Schematic representation of the single 4.142 net along the c axis. Highlighted are a single helix (yellow) and one 4-membered and two 14-membered shortest circuits (purple). The handedness of the helices are illustrated as orange arrows, and the center dot defines the direction as out of the page.
are burnt. The initial weight loss before 110 °C could be assigned to the escape of the water solvent molecules (calcd, 1.9%; found, 1.9%). X-ray crystallography15 reveals that 1 is made of 3-fold interpenetrating three-dimensional (3D) coordination networks, nitrate ions, and water solvent molecules. As shown in Figure 1, each Ag(I) center is in a linear geometry by coordinating to two N atoms from different L, with the N1–Ag1–N8A and N6–Ag2–N4B angles of 175.1(2) and 177.7(3)°, respectively. The Ag–N bond distances are typical values for the Ag–N coordination bonds.8a,9b The distance between Ag2 · · · Ag2B is 3.044(1) Å, which is markedly shorter than the sum of the van der Waals radii of two silver atoms (3.44 Å),16 indicating a significant intramolecular Ag · · · Ag interaction. The nanometer-sized ligand L acts as a tetradentate building block with the N1 · · · N8 distance being 12.455(8) Å. On the basis of one phenyl ring of L, the two imidazole groups and the –CH2– that links the two phenyl rings display the cis, cis, cis conformation in 1. The carbon atom of the –CH2– linker shows a distorted tetrahedral geometry with the angle C7–C12–C19 being 120.3(5)°. In addition, the dihedral angle between two phenyl rings of L is 82.1°. A single 3D network of 1 consists of Ag1–L meso-helices bridged by L and Ag2 atoms, exhibiting vaselike nanometer-sized channels (ca. 16.3 × 15.4 Å2) along the [013] axis (Figure 2a). As
shown in Figure 2b, each Ag1 atom coordinates to N1 and N8 donors from two imidazole groups of L, creating an uncommon meso-helical chain. As highlighted in Figure 2(a), the meso-helices extend along the [013] (A) and [0–13] (B) axes, respectively. A stack of meso-helices B is linked with a meso-helix A through the Ag2–N4 and Ag2–N6 connections to form a 3D framework. As the relative of the helix, the meso-helix is little known and characteristic of its linked right- and left-handed helix with two points of contraflexure (see Figure 2b).17 To the best of our knowledge, only a few meso-helices have been reported and the examples are usually 1D chains or 2D layers.17,18 Compound 1 is the first 3D polymer built upon meso-helices up to now. If Ag atoms are omitted due to their linear coordination geometry, the ligand L can be viewed as linked three-coordinate nonplanar nodes with triangular-pyramidal geometry. A single 3D framework could be extended to a rare 4.142 net, as displayed in Figure 3. The extended Schläfli symbol of this net is 4.1412.1412, which is assigned to the dia-g net. An interesting feature of this 4.142 net is the presence of the separated octagonal single helices running along the c axis, which is highlighted in Figure 3. Each octagonal helix connects to four adjacent helices by sharing a common edge. As the left- and right-handed helices are alternatively arranged, the whole net is racemic. The present net is strongly distorted and differs from the regular dia-g net, whose symmetrical configuration is normally chiral I4122 of the tetragonal cell. To the best of our knowledge, only a few compounds of the 4.142 net have been reported.19–21 It should be emphasized that the formation of large void spaces along the [013] axis in the single 3D network, which results from the coordination mode of the nanometer-sized ligand and the linear coordination geometry of silver atoms, greatly requires the accommodation of the guest molecules. Thus, instead of forming an open,
Figure 4. View of the 3-fold interpenetrating networks along the (a) c and (b) a axes in 1.
2270 Crystal Growth & Design, Vol. 7, No. 11, 2007
Communications 6). The emission of 1 may be attributed to the intraligand emission states, as reported for other silver(I) complexes with N-donor ligands.23 The red shift of the emission from the free ligand to 1 may be due to the existence of the intramolecular Ag · · · Ag interaction.24 In conclusion, we have successfully synthesized a novel threedimensional MOF built upon silver(I) and a new tetradentate flexible ligand L with rare 3-fold interpenetrating dia-g (or 4.142) nets and carried out the preliminary investigation of its possible ferroelectric property. Further research work on exploring the formation of functional materials by the use of L is in progress.
Acknowledgment. We thank the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), the National Basic Research Program of China (973 Program, 2007CB815306) and the National Science Foundation of China (20333070 and 20673117) for financial support. Figure 5. Electric hysteresis loop of 1, observed for a powdered sample in the form of a pellet on a ferroelectric tester at room temperature.
Supporting Information Available: Crystallographic data for 1 (CIF); syntheses details, ferroelectric and luminescence measurements details, XRD pattern, TGA curve, frequency dependence of permittivity, and other supported figures (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 6. Solid-state emission spectra for ligand L and compound 1 at room temperature.
nanoporous structure, the potential voids are filled via mutual interpenetration of three identical 4.142 nets (see Figure 4). These 4.142 nets are 3-fold parallel interpenetrating along the c axis and interlocked to each other, with the inter-net Ag · · · Ag distance being 9.467 Å. The nitrate anions and water solvent molecules are embedded in the voids viewed along the c axis (see the Supporting Information, Figure S3). As far as know, compounds with 4.142 nets are either 2-fold20,21 or 5- or 6-fold interpenetrating.19,22 The 3-fold interpenetrating 4.142 nets have not been seen before. The noncentrosymmetric space group Fdd2 is associated with the point group C2V, one of the 10 polar point groups (C1, C2, Cs, C2V, C4, C4V, C3, C3V, C6, C6V) required for ferroelectricity. Experimental results indicate that compound 1 probably exhibits ferroelectric behavior. Figure 5 clearly shows that there is an electric hysteresis loop that is a typical ferroelectric feature with a remanent polarization (Pr) of ca. 0.027 µC cm-2 and coercive field (Ec) of 7 kV cm-1. The saturation spontaneous polarization (Ps) of 1 is ca. 0.066 µC cm-2. We also studied the behavior of permittivity (ε) ) ε1(ω) – iε2(ω), where ε1(ω) and iε2(ω) are the real (dielectric constant) and imaginary (dielectric loss) parts, respectively. As shown in Figure 4 (see the Supporting Information), Figure S4, the frequency dependence of the dielectric constant ε1 at room temperature (20 °C) indicates that ε1 rapidly decreases with the increase of frequency, and from 8000 Hz to higher frequencies, ε1 remains nearly unchanged. The free ligand shows a fluorescence emission band at λmax ) 466 nm (λex ) 380 nm), whereas complex 1 displays a fluorescence emission at λmax ) 495 nm with the excitation at 364 nm (Figure
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CG070458D
