Article pubs.acs.org/crystal
Synthesis and Structure of Two Acentric Heterometallic Inorganic− Organic Hybrid Frameworks with Both Nonlinear Optical and Ferroelectric Properties Fenglei Du,†,‡ Huabin Zhang,†,‡ Chongbin Tian,†,‡ and Shaowu Du*,† †
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *
ABSTRACT: Solvothermal reactions of Cd(NO3)2·4H2O with aromatic polycarboxylic acids in the presence of sodium nitrate led to two acentric three-dimensional (3D) heterometallic inorganic−organic hybrid frameworks, namely, [Me2NH2][Cd2Na3(2,4-PYDC)4]·2H2O (1) and [Me2NH2] [CdNa(OH-m-BDC)2(H2O)2]·2H2O (2) (2,4-H2PYDC = 2,4-pyridinedicarboxylic acid, OH-m-H2BDC = 5-hydroxyisophthalic acid). The framework of 1 is constructed by a 3D inorganic Cd−Na connectivity, which resembles a concrete reinforcement structure and features a {CdNa}n rod-shaped chain, a {CdNa2}n helical chain, and a 20-membered {Cd6Na14} ring. Compound 2 is built up by one-dimensional inorganic {CdNa}n rod-shaped chains which are further connected by OH-m-BDC2− ligands, affording a 3D polymeric framework. Compounds 1 and 2 crystallize in acentric space groups, and both display powder second harmonic generation efficiencies approximately 0.8 and 0.7 times, respectively, than that of the potassium dihydrogen phosphate (KDP) powder. In addition, they also exhibit luminescence and potential ferroelectric properties.
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auxiliary ligands to induce chirality on the frameworks.5 Meanwhile, spontaneous resolution through self-assembly without involving any expensive enantiopure precursors provides another good opportunity to generate chiral frameworks.6 For example, Lin et al. has demonstrated that some chiral frameworks can be achieved by using bipyridyl ligands and linear metal-connecting points.7 Chen et al. also reported some chiral compounds by employing unsymmetric ligands generated in situ under hydrothermal conditions.8 More recently, Cui et al. described a chiral octupolar framework generated from a simple achiral building block, showing a large NLO effect and the cation-dependent NLO behavior.9 However, assembly of metal−organic molecules into a chiral bulk solely from achiral building blocks still presents a significant challenge since most complexes are inclined to crystallize in a centrosymmetric space group, and the formation of chiral products is difficult to predict in most cases. Our synthetic approach to acentric metal−organic hybrid frameworks is based on mixed metal carboxylates. We have focused our attention on mixed metal Cd/M (M = alkali or alkaline earth ions) carboxylate systems for they are more likely to crystallize in acentric or chiral space groups even though the
INTRODUCTION Second-order nonlinear optical (NLO) and ferroelectric materials have received increasing attention due to their wide application in the areas such as telecommunications, electricoptical devices, light modulators, and information storage.1 Traditionally, studies of second-order NLO and ferroelectric materials were mainly focused on inorganic compounds such as potassium dihydrogen phosphate (KDP), lithium niobate (LiNO3), and barium titanate (BaTiO3).2 Inorganic−organic hybrid frameworks, which combine the high NLO coefficients of the organic molecules with excellent physical properties of the inorganics, appear to be promising candidates for NLO and ferroelectric materials.3 However, the development of NLO and ferroelectric materials based on these compounds largely depends on the rational design and synthesis of acentric frameworks since fundamentally material for second-order NLO should be orientationally non-centrosymmetric to be functional. In addition, inorganic−organic hybrid frameworks with both second-order NLO and ferroelectric properties are much more difficult to obtain because such compounds must also crystallize in acentric space groups belonging to the 10 polar point groups.4 The general strategy to construct acentric inorganic−organic hybrid frameworks includes the coordination of chiral organic ligands to metal ions or the use of chiral templates, such as enantiopure solvents of crystallization, countercations and © 2013 American Chemical Society
Received: January 11, 2013 Revised: February 27, 2013 Published: March 5, 2013 1736
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temperature. CrystalClear software11 was used for data reduction and empirical absorption correction. The structures were solved by direct methods using SHELXTL and refined by full-matrix least-squares on F2 using SHELX-97 program.12 Metal atoms in each compound were located from the E-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses. All the non-hydrogen atoms were refined anisotropically. The organic hydrogen atoms were positioned geometrically, whereas those of the water molecules and the nitrogen atom of (Me2NH2)+ in 1 were not added. Crystal data as well as details of the data correction and refinement for compounds are summarized in Table 1, and selected bond lengths and angles are
underlying causes of non-centrosymmetry (or chirality) have not been fully elucidated.6e For example, in our previous studies, we have successfully prepared several Cd/M (M = Li, Ca, Sr) non-centrosymmetric frameworks through the assembly of Cd(II) ions with a variety of polycarboxylic acids under solvothermal conditions. In these reactions, the Li(I), Ca(II), and Sr(II) ions not only function as templates but also serve as nodes of dimensional grids to extend the product structure.6a,b In our continuing efforts to synthesize multifunctional metal− organic hybrid materials, we have utilized polycarboxylic acids to assemble with Cd(II) ions in the presence of sodium nitrate, which afforded two acentric three-dimensional (3D) heterometallic inorganic−organic hybrid frameworks, formulated as [Me2NH2][Cd2Na3(2,4-PYDC)4]·2H2O (1) and [Me2NH2][CdNa(OH-m-BDC)2(H2O)2]·2H2O (2) (2,4-H2PYDC = 2,4pyridinedicarboxylic acid, OH-m-H2BDC = 5-hydroxyisophthalic acid). In this paper, we report the synthesis, structural characterization, NLO, and ferroelectricity of these two compounds. In addition, their luminescent properties were investigated.
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Table 1. Crystallographic Data for Compounds 1 and 2
EXPERIMENTAL SECTION
Materials and Methods. All the chemicals were commercially available and used without further purification. Thermogravimetric analyses were performed on a TGA/NETZSCH STA449C instrument heated from room temperature to 1000 °C under a nitrogen atmosphere at a heating rate of 10 °C/min. Powder X-ray diffraction (PXRD) patterns of crushed single crystals were collected in the 2θ range of 5−50° on a PANalytical X′pert PRO X-ray Diffractometer using Cu−Kα radiation. The Fourier transform infrared (FT-IR) spectra using KBr pellets were recorded on a Spectrum-One FT-IR spectrophotometer in the range of 4000−450 cm−1. Elemental analyses (C, H, and N) were measured with an Elemental Vairo EL III Analyzer. The UV absorption and optical diffuse-reflectance spectra were measured at room temperature with a PE Lambda 900 UV− visible spectrophotometer. The absorption spectrua were calculated from the reflectance spectra using the Kubella-Munk function:10 α/S = (1 − R)2/(2R), where α is the absorption coefficient, S is the scattering coefficient, which is practically wavelength-independent when the particle size is larger than 5 μm, and R is the reflectance. The NLO properties of 1 and 2 were tested on the powder samples by the Kurtz and Perry method using an Nd:YAG laser (1064 nm) with an input pulse of 350 mV. Synthesis of [Me2NH2][Cd2Na3(2,4-PYDC)4]·2H2O (1). Cd(NO3)2·4H2O (0.40 mmol, 0.12 g), 2,4-pyridinedicarboxylic acid (1.00 mmol, 0.19 g), and NaNO3 (0.50 mmol, 0.04 g) in a mixedsolvent (10 mL) of DMF and isopropyl alcohol (V/V = 1:1) were placed in a 20 mL of Teflon-lined stainless steel vessel. The mixture was heated during 4 h to 160 °C and maintained at this temperature for two days. The system was then cooled to room temperature in two days. Light yellow crystals of 1 were collected, washed several times with ethanol, and dried in air (yield 50% based on Cd). Anal. Calc (%). for 1 CdNa1.5C15H12N2.5O9 (518.16): C 34.74, H 2.31, N 6.76. Found: C 34.33, H 2.51, N 6.50. IR (KBr, cm−1): 3444w, 1630s, 1598s, 1551s, 1485s, 1439s, 1387s, 1255w, 1092w, 1019w, 952w, 833w, 785m, 731s, 705m, 687w. Synthesis of [Me2NH2][CdNa(OH-m-BDC)2(H2O)2]·2H2O (2). Colorless crystals of 2 were isolated in a way similar to that described for 1 except that 5-hydroxyisophthalic acid (1.00 mmol, 0.18 g) was used instead of 2,4-pyridinedicarboxylic acid (yield 75% based on Cd). Anal. Calc (%) for 2 CdNaC18H24NO14 (613.77): C 35.19, H 3.91, N 2.28%. Found: C 35.32, H 4.10, N 2.15%. IR (KBr, cm−1): 3431w, 1681s, 1629m, 1594m, 1550s, 1477m, 1416s, 1384s, 1273m, 1223m, 1103m, 1002w, 977w, 903w, 812m, 785m, 772m, 732m, 665w. Single-Crystal Structure Determination. Single-crystal X-ray diffraction data were collected on a Rigaku diffractometer equipped with a Mercury CCD area detector (Mo Kα; λ = 0.71073 Å) at room
a
compound
1
2
formula formula mass crystal system space group a/Å b/Å c/Å V/Å3 Z μ/mm−1 Dcalcd/g cm−3 F(000) reflections measured independent reflections observed reflections R1a [I > 2σ(I)] wR2b [I > 2σ(I)] GOF on F2
C15H12CdN2.5Na1.5O9 518.16 orthorhombic Fdd2 13.3441(10) 43.503(5) 13.1326(10) 7623.6(12) 16 1.233 1.806 4096 14324 4016 (Rint = 0.0475) 3793 (>2σ(I)) 0.0487 0.1044 1.133
C18H24CdNNaO14 613.77 orthorhombic Pca2(1) 18.6294(10) 9.9345(5) 14.7424(8) 2728.4(2) 4 0.879 1.494 1240 20742 6187 (Rint = 0.0287) 5751 (>2σ(I)) 0.0601 0.1774 1.048
R1 = ∑||Fo| − |Fc||/∑ |Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]0.5.
given in Table S1 in the Supporting Information. Further details of the crystal structure determination have been deposited with the Cambridge Crystallographic Data Centre. CCDC numbers for 1 and 2 are 918252 and 918253, respectively.
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RESULTS AND DISCUSSION Synthesis of Compounds 1 and 2. In our previously study, we have demonstrated that without using any chiral source, the solvothermal reactions of Cd(II) salt with a range of aromatic polycarboxylic acids in the presence of Li(I) ions resulted in the formation of several acentric or chiral heterometallic frameworks. The main feature of these compounds is that they are all anionic frameworks in which the Cd(II) center is eight-coordinate, chelated by four carboxylate groups in a severely distorted monocapped pentagonal bipyramidal geometry, while the Li(I) ion acts as a linkage by sharing carboxylate oxygen atoms with Cd(II). This structural motif is different from that commonly observed in the centrosymmetric Cd(II)/carboxylate frameworks where the octahedral Cd(II) center is chelated by two carboxylate groups, leaving two coordination sites to be occupied by solvent molecules (water in most cases) or nitrogen atoms from auxiliary ligands.13 In order to obtain cadmium compounds with such a structural motif, an appropriate solvent system, such as a mixed solvent of DMF and methanol should be chosen. The role of DMF in the synthesis of these compounds is to automatically control the base/acid balance of the solution, and at the same time, produce (Me2NH2)+ under hydrothermal conditions that can neutralize the overall charge in the anionic 1737
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framework and serve as a cation template.14 Besides, the presence of the Li(I) ion is also indispensable for the formation of the products. This approach to the synthesis of acentric metal−organic frameworks makes us wonder if it also works for the cadmium compounds with other alkali metal ions. With this in mind, we have investigated analogous reactions by using Na(I), K(I), Rb(I), and Cs(I) instead of Li(I). Unfortunately, most of the products isolated from this system are neutral frameworks and are no longer acentric compounds. The cause for this difference is not yet clear, but it seems that the anionic framework is more likely to crystallize in an acentric space group. Besides, some of these compounds are extremely unstable when exposed to air. We find that on going from Li(I) to Cs(I) the coordination number of alkali metal ions increases, and they are likely to be coordinated by additional solvent molecules such as DMF and methanol, which readily exchange with water molecules in the atmosphere, leading to the collapse in the crystalline structure. However, in the case of the Cd(I)/ Na(I) system, we are able to isolate two stable frameworks which crystallize in the acentric space groups. Crystal Structure of [Me 2 NH 2 ][Cd 2 Na 3 (2,4-PYDC)4]·2H2O (1). Compound 1 crystallizes in the orthorhombic acentric space group Fdd2. Its asymmetric unit consists of one Cd(II) ion, one and a half Na(I) ions, two independent 2,4PYDC2− dianions, half (Me2NH2)+ cation, and one lattice water molecule. The Cd(II) center is octa-coordinated by two chelating carboxylate groups (O1A, O2A, O7B and O8B) from two different 2,4-PYDC2− ligands (Cd−O = 2.353(5)− 2.718(5) Å), two terminally bonded carboxylate oxygen atoms (Cd−O3 = 2.377(5) and Cd−O8 = 2.434(5) Å), and two nitrogen atoms (Cd−N1 = 2.318(5) and Cd−N2 = 2.347(6) Å) from two different 2,4-PYDC2− ligands, showing a distorted monocapped pentagonal bipyramid geometry (Figure 1). The
Chart 1. Coordination Modes of the 2,4-PYDC2− and OH-mBDC2− Ligands
the former is a new coordination mode, while the latter was only observed in a barium polymeric framework [Ba(2,4PYDC)]n.16 In 1, the adjacent metal centers are linked by the oxygen and nitrogen atoms of 2,4-PYDC2− ligands to form a 3D framework. The most striking feature of this structure is the 3D Cd−Na inorganic connectivity in which the adjacent metal polyhedra can share faces and corners. As shown in Figure 2a, each Cd1
Figure 1. View of the coordination environment of metal ions in 1. Hydrogen atoms and the labels for carbon atoms are not shown for clarity. Symmetry codes: (A) 0.5 + x, y, − 0.5 + z; (B) − 0.25 + x, 0.25 − y, − 0.25 + z; (C) − 0.5 + x, y, 0.5 + z; (D) − x, − y, z; (E) − 1 + x, y, z; (F) − 0.75 + x, 0.25 − y, 0.25 + z; (G) 0.5 + x, y, 0.5 + z; (H) 0.5 − x, − y, 0.5 + z; (I) 1− x, − y, z.
Figure 2. (a) A rod-shaped {CdNa}n chain. (b) Schematic view showing how the Na2 polyhedra connect to a {CdNa}n chain. (c) View of the 3D inorganic connectivity of 1 along the b-axis.
Na1 atom is hepta-coordinated with seven carboxylate oxygen atoms in a distorted pentagonal bipyramidal geometry, while Na2 is tetra-coordinated with four carboxylate oxygen atoms in a deviated tetrahedral geometry. The Na−O bond lengths fall in a normal range from 2.302(6) to 2.809(6) Å.15 The distances between the Cd1 atom and the nearest neighboring Na1 atoms are 3.380(3) and 3.490(3) Å, and those between the Na1 atom and the nearest neighboring Na2 atoms are 4.055(3) and 4.056(3) Å, respectively. In 1, there exist two independent 2,4PYDC2− dianions that display (k1-μ2)-(k1-k1-μ3)-μ6 and (μ2μ2)-(k1-k1-μ2)-μ6 coordination modes, respectively (Chart 1, mode a and b). These μ6-2,4-PYDC2− coordination modes are uncommon for metal−organic coordination polymers. In fact,
polyhedron (CdO4N2) shares faces with the two neighboring Na1 octahedra (NaO6), generating {CdNa}n rod-shaped chains that are distributed on a set of parallel layer planes with equal distance separation. In each plane, the chains lie side-by-side and are aligned parallel to a direction which is almost mutually perpendicular to the direction of the chains in adjacent planes above and below. In addition, the chains are staggered in every other layer. In each {CdNa}n chain, the Na1 polyhedra are corner-shared with Na2 polyhedra (NaO4) at intervals of a Cd1 polyhedron, and alternately point in opposite directions (Figure 2b). The {CdNa}n chains are thus interwoven by Na2 1738
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polyhedra to generate a 3D inorganic skeleton that looks like a concrete reinforcement structure as seen along the b-axis direction (Figure 2c).17 When viewed down the a-axis (or caxis), this 3D inorganic network can be also described as being composed of left-handed and right-handed {CdNa2}n helixes in the sequence of {−Cd1−Na1−Cd1−Na1−Na2−Na1−} (Figure 3a,b). Each left-handed helix is surrounded by two left-
Figure 4. (a) Schematic view of a {Cd6Na14} ring (red) edge-shares with 14 adjacent rings. (b) Perspective views of three different types of ring linkages. (c, d) Schematic representation of topology and the tiling of 1.
that are linked by multifunctional organic ligands have been extensively studied, little progress has been made on compounds with an inorganic connectivity. Recently, some low dimensional extended inorganic−organic hybrids InOm (n = 1, 2 and m = 1, 2) have been successfully synthesized.18 By comparison, compounds with 3D inorganic building blocks are still very limited especially those with a mixed metal 3D inorganic connectivity. This is not surprising because the prerequisite for constructing a 3D inorganic connectivity is that the polyhedron around central atom must share corners or edges with at least three adjacent polyhedra toward three separated directions. As for the heterometallic frameworks, it may be even more difficult for metal ions with different coordination environments to join together into a 3D connectivity via oxygen bridges. On the other hand, there are only just a few Cd−Na frameworks in which the Na(I) ions act both as countercations and nodes for the expansion of network structures.19 Unlike 1, these compounds contain either a oneor two-dimensional (1D or 2D) Cd−Na inorganic connectivity. Among them, there is only one example which crystallizes in a non-centrosymmetric space group.19e To the best of our knowledge, the 3D Cd−Na polyhedral network as well as the Cd−Na helix and nanoscale Cd−Na metallo-ring found in 1 have not been observed in these types of compounds. Crystal Structure of [Me 2 NH 2 ][CdNa(OH-mBDC)2(H2O)2]·2H2O (2). Single-crystal X-ray diffraction analysis shows that 2 crystallizes in the orthorhombic acentric space group Pca2(1) and has a 3D porous architecture with dimethylamine cations, which counterbalance the charge of the system. The asymmetric unit of 2 consists of one Cd(II) ion, one Na(I) ion, two independent OH-m-BDC2− dianions, one (Me2NH2)+ cation, two coordinated water molecules, and two lattice water molecules. The Cd(II) ion is hepta-coordinated and chelated by three carboxylate groups (O1, O2, O3A, O4A, O8B, and O9B) and terminally bonded to one oxygen atom (O6) from another carboxylate group (Figure 5). The Cd−O bond lengths fall within a wide range of 2.221(5)−2.762(5) Å, showing a severely distorted pentagonal bipyramidal geometry
Figure 3. (a, b) View of a left- and a right-handed {CdNa2}n helix. Turquoise: Cd1, violet: Na1, blue: Na2. (c, d) View of a left-handed helix (red) that shares edge with a left- and a right-handed helix (green and blue, respectively). The shared edges {Na1−Na2−Na1} and {Na1−Cd1−Na1} are shown in turquoise and yellow, respectively. (e) View of helixes from the a-axis.
handed and four right-handed helixes (and vice versa) by sharing part of its segment, (Na1−Na2−Na1) and (Na1− Cd1−Na1) respectively (Figure 3c,d), affording a 3D framework (Figure 3e). Another interesting feature of this structure is that it contains the shortest {Cd6Na14} nanoring with a dimension of ca. 26.0 × 9.4 Å. The 3D inorganic framework of 1 can be regarded as being constructed by a {Cd6Na14} ring that shares an edge in three different ways with 14 adjacent ones (6 rings and 8 rings respectively along two nearly orthogonal directions) (Figure 4a,b). Better insight of this complicated 3D inorganic net can be achieved by topology analysis. Topologically, each Na1 is linked by two Cd1 and one Na2, so it can be simplified into a three-connected node, while Cd1 and Na2 can be viewed as linkages between nodes. Thus, the 3D inorganic connectivity can be described as a (103) ths net (Figure 4c). When analyzed with 3dt and systre softwares, it can be seen that there are 104 tiles in the topological view. As illustrated in Figure 4d, which shows how the tiles are accumulated in 1, a tile is surrounded by four tiles from four faces. Although a large number of metal−organic frameworks composed of isolated metal ions or polynuclear metal clusters 1739
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a 1D {CdNa}n chain (Figure 6b), which is connected to adjacent four chains by the OH-m-BDC2− ligands (Figure 6a). To further understand the structure of 2, topological analysis by reducing the multidimensional structure to a simple nodeand-linker net was performed. From a topological point of view, the OH-m-BDC2− ligand and the inorganic connectivity {−O− Na−O−} can be considered as connectors, while the Cd(II) ion can be regarded as a node. Thus, the anionic framework of 2 can be abstracted into a 6-connected network (Figure 6c). The topological network of 2 is analyzed by Topos program.22 Its Schläfli symbol can be described as (412.63), which is named a pcu net. A structurally related Cd−Na framework [CdNa(mBDC)2]·[NH2(CH3)2] (m-H2BDC = 1,3-benzenedicarboxylic acid) has been reported previously. However, it crystallizes in a centrosymmetric space group, and the Cd(II) center in this compound is four-coordinate by four carboxylate oxygen atoms, exhibiting a tetrahedral coordination geometry.19c Spectra and Thermal Characterization. Powder X-ray diffraction (PXRD), obtained at room temperature, has been utilized to check the phase purity of the bulk samples in the solid state. For compounds 1 and 2, the experimental PXRD patterns closely match the simulated patterns generated from the results of single-crystal diffraction data (Figure S1 in the Supportiong Information), indicative of pure products. In their IR spactra, the absence of strong absorption associated with the carboxyl group at around 1701 cm−1 indicates that the carboxylic ligands are completely deprotonated. Their asymmetric and symmetric stretching of carboxylates appear in the range of 1485−1387 and 1680−1550 cm−1, respectively, being shifted to higher frequencies compared with that of carboxyl for the free ligands (Figure S2 in the Supporting Information). The ν(O−H) band from water molecules and the ν(N−H) band from (Me2NH2)+ are observed, as evidenced by broad peaks at around 3445 cm−1 in the spectrum of 1 and 3431 cm−1 in the spectrum of 2. These results are also confirmed by single-crystal structure analysis. The UV absorption spectra of 1 and 2 are shown in Figure S3 in the Supporting Information. A weak and broad absorption band appears in the range of 360−441 nm for 1, while a narrow absorption band appears in the range of 273− 334 nm for 2. The optical diffuse-reflectance study reveals optical band gaps of 3.84 eV for 1 and 1.84 eV for 2 (Figure S4 in the Supporting Information). Compounds 1 and 2 exhibit luminescence in the solid state at room temperature, with maxima at 425 nm (λex = 322 nm) for 1 and 441 nm (λex = 382 nm) for 2 (Figure S5 in the Supporting Information). According to the reported literature, the free 2,4-H2PYDC and OH-m-H2BDC ligands emit luminescence emissions at 320 and 390 nm, respectively.23 Thus, the emission bands of 1 and 2 may be attributed to the intraligand (π → π*) transition, and the red-shifts compared with free ligands are probably caused by mitigation of ligand vibrational emission loss due to the metal−ligand coordination.23 At 10 K, both 1 and 2 exhibit a far stronger emission than that at room temperature (Figure S5 in the Supporting Information). In order to examine the stability of the framework, thermal gravimetric analysis (TGA) for 1 and 2 was carried out in nitrogen gas from 30 to 800 °C (Figure S6 in the Supporting Information). In the TGA curve for 1, the first weight-loss step (3.00%) in the temperature range of 70−231 °C is consistent with the removal of one lattice water molecule (calcd. 3.47%). Then the collapse of the network of 1 occurs. For 2, the TGA curve shows a weight-loss step of 10.74% over a broader
Figure 5. View of the coordination environment of metal ions in 2. The hydrogen atoms and labels for carbon atoms are not shown for clarity. Symmetry codes: (A) x, 1 + y, z; (B) −0.5 + x, 2 − y, z; (C) −1 − x, 2 − y, −0.5 + z; (D) −0.5 − x, y, −0.5 + z.
of the Cd(II) ion. It should be pointed out that the distances of Cd1−O2 and Cd1−O4A are 2.719(5) and 2.762(5) Å, which are more reasonable to be regarded as semicoordination bonds.20 The Na(I) ion is hexa-coordinated and located in a distorted octahedral coordination environment, surrounded by four carboxylate O atoms (O1, O4A, O7C and O9D, Na2−O = 2.274(7)−2.536(5) Å) and two O atoms from water molecules (Na2−O11 = 2.423(9) and Na2−O12 = 2.377(7) Å). The OH-m-BDC2− ligands adopt two coordination modes, (k2-μ2)(k2-μ2)-μ4 and (k2-μ2)-(k1-k1)-μ4 (Chart 1, mode c and d), to link Cd(II) and Na(I) ions into a porous 3D anionic framework with (Me2NH2)+ cations being accommodated in the cavities (Figure 6a). Although a large number of coordinaion polymers based on OH-m-H2BDC have been reported to date, there are only a few examples with these μ4-OH-m-BDC2− coordination modes. 6b,18e,21 In the 3D framework of 2, each CdO 7 polyhedron is connected with neighboring NaO6 polyhedra by edge- and corner-sharing O atoms, respectively, to generate
Figure 6. (a) Perspective view of the 3D network in 2 along the c-axis. Turquoise: Cd, violet: Na, red: O (b) View of the 1D {CdNa}n chain. (c) Topological presentation (left) and natural tiling (right) of the pcu net found in 2. 1740
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Figure 7. Electric hysteresis loops of 1 (a) and 2 (b).
temperature range (20−316 °C), assigned to the weight loss of the lattice and coordinated water molecules (calcd. 11.74%). Then it begins to decompose upon further heating. SHG and Ferroelectric Properties. SHG measurements on powder samples of 1 and 2 were carried out to confirm their acentricity as well as to evaluate their potential as a secondorder NLO material. Approximate estimation was performed on a pulsed Q-switched Nd:YAG laser at a wavelength of 1064 nm. The intensities of the green light (frequency-doubled output: λ = 532 nm) produced by the powder samples of 1 and 2 are about 0.8 and 0.7 times, respectively of that produced by a potassium dihydrogen phosphate (KDP) powder. Besides, both 1 and 2 are air-stable and isoluble in water and common organic solvents, so they are potentially good candidates for second-order NLO materials. Because 1 and 2 crystallize in the noncentrosymmetric space groups Fdd2 and Pca2(1), which fall in the 10 point groups (C1, Cs, C2, C2v, C3, C3v, C4, C4v, C6, C6v) required for ferroelectric materials,24 their ferroelectric properties were also examined. The hysteresis loops of the electric polarization were obtained for 1 and 2, as shown in Figure 7. At room temperature, the remanent polarization (Pr) of 1 is ca. 0.0393 μC cm−2 with a coercive field (Ec) of ca. 561.495 V cm−1. The saturation spontaneous polarization (Ps) of 1 is ca. 0.045 μC cm−2. The values of Pr, Ec, and Ps of 2 can be estimated to be 0.0125 μC cm−2, 377.500 V cm−1, and 0.025 μC cm−2 respectively. Both the Ps values of 1 and 2 are much smaller than that of the typical ferroelectric KDP (Ps: 5.0 μC cm−2). We also studied the behavior of permittivity (ε) = ε1(ω) − iε2(ω) of 1 and 2, where ε1(ω) and iε2(ω) are the real (dielectric constant) and imaginary (dielectric loss) parts, respectively. Frequency dependence of the dielectric constants ε1 of 1 and 2 at room temperature indicates that their ε1 rapidly decrease with the increase of frequency. From 20 000 Hz (for 1) or 5000 Hz (for 2) to higher frequencies, their ε1 remain almost unchanged (Figure S7 in the Supporting Information).
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shaped {CdNa}n chain, a {CdNa2}n helix, and a nanoscale 20membered {Cd6Na14} metallo-ring. The unique network architecture of 1 has not been observed in the area of metal−organic frameworks. Moreover, compounds 1 and 2 crystallize in acentric space groups and display both SHG responses and potential ferroelectric properties.
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ASSOCIATED CONTENT
S Supporting Information *
X-ray crystallographic data in CIF format, tables for selected bond lengths and angles, PXRD patterns, TGA curve, UV−vis and optical diffuse reflectance spectra, luminescent and IR spectra and frequency dependence of permittivity. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Basic Research Program of China (973 Program, 2012CB821702), the National Natural Science Foundation of China (21233009 and 21173221), and the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences for financial support.
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REFERENCES
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CONCLUSION
Two new 3D heterometallic metal−organic hybrid frameworks have been successfully isolated under solvothermal conditions by the reactions of polycarboxylic ligands with Cd(II) salt in the presence of NaNO3. The Na(I) ions in these compounds not only function as countercations but also serve as nodes of dimensional grids to extend the product structure. In particular, compound 1 is constructed by a 3D heterometallic inorganic connectivity, which resembles a reinforced concrete structure and exhibits several unusual structural motifs, such as a rod1741
dx.doi.org/10.1021/cg400060t | Cryst. Growth Des. 2013, 13, 1736−1742
Crystal Growth & Design
Article
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