Polymeric Frameworks Constructed from a Metal-Organic Coordination Compound, in 1-D and 2-D Systems: Synthesis, Crystal Structures, and Fluorescent Properties
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 341-346
Xin Shi,† Guangshan Zhu,† Xiaohui Wang,‡ Guanghua Li,† Qianrong Fang,† Xiaojun Zhao,† Gang Wu,† Ge Tian,† Ming Xue,† Renwei Wang,† and Shilun Qiu*,† State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Department of Chemistry, Jilin University, Changchun 130012, People’s Republic of China, and Electric & Electron Engineering Institute, Chang Chun University of Technology, Changchun 130012, People’s Republic of China Received March 27, 2004;
Revised Manuscript Received November 13, 2004
ABSTRACT: Three novel metal-organic coordination polymers, Cd(BDC)(phen)2‚(C2H5OH)(H2O) (1) (H2BDC ) benzene-1,4-dicarboxylic acid), Cd(1,3-BDC)(phen) (2) (1,3-H2BDC ) benzene-1,3-dicarboxylic acid), and Cd2(fma)2(phen)2 (3) (H2fma ) fumaric acid), have been synthesized by using the metal-organic coordination compound Cd(phen)2(NO3)2 as the metal source instead of metal salts under different conditions. Compound 1 is a 1-D zigzag chain. The presence of two phen ligands chelating to one cadmium atom in 1, which is rare in similar polymers because of the severe steric hindrance for bulky phen groups, is mainly due to these two phen ligands entirely coming from metal source Cd(phen)2(NO3)2. Compound 2 is an interesting twisted double chain with a 1-D channel along the c axis, which crystallizes in a monoclinic system, P2/c space group. And compound 3 is a 2-D undulating network, in which fma ligands adopt three different coordination fashions. Additionally, these three compounds show strong fluorescence in the solid state at room temperature. Introduction The design of metal-organic coordination frameworks is very topical for their various intriguing structural topologies and their potential applications as functional materials.1-3 Metal-organic coordination polymers have provided a fertile ground for design of single-crystalline solids, and many efforts have been made toward the rational design of functional metal-organic crystalline materials in the past decade.4-6 Various polymers with intriguing molecular topologies and crystal packing motifs have been obtained, and these polymers exhibit versatile physical and chemical properties.7-9 Lately, by using nonporous 1-D metal-organic complexes as precursors, a 3-D nanoporous organometallic material with a similar (better) performance in sorption and catalysis as compared to some of the most common zeolites has been synthesized.10 Our recent efforts have been made toward the rational design of metal-organic coordination frameworks with novel topologies by using metal-organic coordination compound Cd(phen)2(NO3)2 as the metal source. And it is demonstrated that novel topologies can be obtained by using a metal-organic coordination compound as the metal source.11 The novel topologies mainly result from the metal-organic coordination compound as a reactant providing not only metal ions but also segmental organic ligands in the synthesis system. Many metal-organic coordination polymers have already been constructed from bicarboxylate ligands and * To whom correspondence should be addressed. Fax: (+86) 4315168589. E-mail:
[email protected]. † Jilin University. ‡ Chang Chun University of Technology.
phen ligands with metal salts.12-14 Here we report three novel fluorescent polymeric frameworks, Cd(BDC)(phen)2‚(C2H5OH)(H2O) (1), Cd(1,3-BDC)(phen) (2), and Cd2(fma)2(phen)2 (3), in 1-D and 2-D systems, which are constructed from metal-organic coordination compound Cd(phen)2(NO3)2 instead of metal salts with benzene1,4-dicarboxylic acid, benzene-1,3-dicarboxylic acid, and fumaric acid, respectively, under different synthetic conditions. Experimental Section General Remarks. The elemental analysis was performed on a Perkin-Elmer 2400 element analyzer and inductively coupled plasma (ICP) analysis on a Perkin-Elmer Optima 3300DV ICP spectrometer. The IR spectrum was obtained on a Nicolet Impact 410 FTIR spectrometer. Thermogravimetric analysis was performed on a Perkin-Elmer TGA7 unit in air and at 1 atm pressure at a heating rate of 10 °C min-1. Fluorescence spectra were obtained on a LS 55 fluorescence/ phosphorescence spectrophotometer at room temperature. Preparation. (a) Compound 1. Preparation was performed under mild conditions by allowing Cd(phen)2(NO3)2 (0.179 g, 0.3 mmol) and H2BDC (0.025 g, 0.15 mmol) to react in a solution of N,N′-dimethylformamide (DMF) (10.0 mL), absolute ethanol (2.0 mL), and distilled water (3.0 mL) at 55 °C for 5 days to produce large colorless block crystals in 48% yield (0.050 g). Anal. Calcd for C34H28CdN4O6 (701.00): C, 58.26; H, 4.026; N, 7.992. Found: C, 58.02; H, 4.039; N, 8.005. IR (KBr): ν ) 3444, 3053, 2971, 1970, 1666, 1563, 1384, 1222, 1097, 848 cm-1. (b) Compound 2. Synthesis was performed by allowing Cd(phen)2(NO3)2 (0.158 g, 0.25 mmol) and 1,3-H2BDC (0.041 g, 0.25 mmol) to react in a solution of DMF (4.0 mL), absolute ethanol (2.0 mL), and distilled water (3.0 mL) at 140 °C for 3 days. The resultant yellow block crystals were filtered off, washed with absolute ethanol, and dried at room temperature.
10.1021/cg049884e CCC: $30.25 © 2005 American Chemical Society Published on Web 12/13/2004
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Table 1. Crystal Data and Structure Refinement for Complexes 1-3 1 formula formula weight crystal system space group a/Å b/Å c/Å β/deg V/Å3 Z Fcalc/g cm-3 µ/mm-1 reflections collected independent reflctns (Rint) R1, wR2 [I > 2δ(I)]
2 C20H12CdN2O4 456.72 monoclinic P2/c 8.3440(3) 10.6057(4) 18.9374(7) 100.796(2) 1646.18(11) 4 1.843 1.357 2877
C32H20Cd2N4O8 813.32 monoclinic P21/c 11.191(10) 20.100(17) 13.856(10) 107.59(4) 2971(4) 4 1.818 1.492 5030
3760 (0.0509)
2189 (0.0490)
3062 (0.0626)
0.0396, 0.0899
0.0270, 0.0650
0.0570, 0.1489
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1
N(4)-Cd(1)-O(3) N(4)-Cd(1)-N(2) O(3)-Cd(1)-N(2) N(4)-Cd(1)-N(3) O(3)-Cd(1)-N(3) N(2)-Cd(1)-N(3) N(4)-Cd(1)-O(1) O(3)-Cd(1)-O(1) N(2)-Cd(1)-O(1) N(3)-Cd(1)-O(1) N(4)-Cd(1)-O(4) O(3)-Cd(1)-O(4) N(2)-Cd(1)-O(4) N(3)-Cd(1)-O(4)
Table 3. Selected Bond Distances (Å) and Angles (deg) for 2a
3
C34H28CdN4O6 701.00 monoclinic P21/n 11.537(3) 16.865(4) 15.613(4) 96.652(4) 3017.4(12) 4 1.543 0.777 5290
Cd(1)-N(4) Cd(1)-O(3) Cd(1)-N(2) Cd(1)-N(3)
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Bond Distances 2.439(4) Cd(1)-O(1) 2.437(4) Cd(1)-O(4) 2.451(4) Cd(1)-N(1) 2.460(4) Cd(1)-O(2)
2.458(4) 2.470(4) 2.482(4) 2.508(5)
Bond Angles 131.76(14) O(1)-Cd(1)-O(4) 139.49(12) N(4)-Cd(1)-N(1) 80.65(13) O(3)-Cd(1)-N(1) 68.27(13) N(2)-Cd(1)-N(1) 159.68(14) N(3)-Cd(1)-N(1) 83.34(12) O(1)-Cd(1)-N(1) 123.05(14) O(4)-Cd(1)-N(1) 80.54(17) N(4)-Cd(1)-O(2) 79.98(15) O(3)-Cd(1)-O(2) 84.44(15) N(2)-Cd(1)-O(2) 79.70(13) N(3)-Cd(1)-O(2) 52.28(13) O(1)-Cd(1)-O(2) 123.23(14) O(4)-Cd(1)-O(2) 147.95(13) N(1)-Cd(1)-O(2)
115.35(16) 84.19(13) 94.42(16) 67.41(13) 90.87(13) 147.39(15) 84.72(15) 76.01(15) 96.55(18) 130.98(14) 84.59(15) 51.61(15) 88.83(17) 159.98(15)
The yield was about 52% (0.059 g). Anal. Calcd for C20H12CdN2O4 (456.72): C, 52.60; H, 2.648; N, 6.133. Found: C, 52.47; H, 2.660; N, 6.109. IR (KBr): ν ) 3048, 1959, 1596, 1545, 1386, 1101, 848, 723 cm-1. (c) Compound 3. Synthesis was performed under solvothermal conditions. The reaction of Cd(phen)2(NO3)2 (0.179 g, 0.3 mmol) and fumaric acid (0.017 g, 0.15 mmol) in a solution of DMF (3.0 mL), absolute ethanol (3.0 mL), and distilled water (3.0 mL) at 160 °C for 4 days produced large colorless block crystals in 71% yield (0.087 g). Anal. Calcd for C32H20Cd2N4O8 (813.32): C, 47.26; H, 2.478; N, 6.889. Found: C, 46.98; H, 2.496; N, 7.002. IR (KBr): ν ) 3060, 1622, 1565, 1512, 1422, 1384, 1344, 1208, 1195, 1143, 1101, 982, 846, 728 cm-1. Single-Crystal Structure Determination. Crystallographic data for 1-3 were collected at 293(2) K on a BrukerAXS Smart CCD diffractometer with Mo KR (λ ) 0.71073 Å). The structures were solved by the direct methods18 and refined by the full-matrix least-squares method against F2 (SHELXL97).19 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of organic ligands were located geometrically. The crystallographic data for 1-3 are listed in Table 1, and selected bond lengths and angles for 1-3 are presented in Tables 2-4, respectively.
Results and Discussion Synthesis. Compound 1 was synthesized in a mixture solution of DMF/C2H5OH/H2O at 55 °C, which is similar to the method of solvent evaporability. But 1 cannot be obtained at room temperature until the
Bond Distances 2.231(3) Cd(1)-N(2) 2.288(3) Cd(1)-N(1) 2.317(3) Cd(1)-O(2)
Cd(1)-O(4)#1 Cd(1)-O(3)#2 Cd(1)-O(1) O(4)#1-Cd(1)-O(3)#2 O(4)#1-Cd(1)-O(1) O(3)#2-Cd(1)-O(1) O(4)#1-Cd(1)-N(2) O(3)#2-Cd(1)-N(2) O(1)-Cd(1)-N(2) O(4)#1-Cd(1)-N(1) O(3)#2-Cd(1)-N(1)
2.361(4) 2.372(4) 2.437(3)
Bond Angles 99.70(13) O(1)-Cd(1)-N(1) 89.71(13) 100.67(12) N(2)-Cd(1)-N(1) 70.81(14) 94.75(12) O(4)#1-Cd(1)-O(2) 98.98(12) 101.03(14) O(3)#2-Cd(1)-O(2) 147.13(12) 116.99(13) O(1)-Cd(1)-O(2) 55.21(11) 137.37(12) N(2)-Cd(1)-O(2) 85.38(12) 169.63(13) N(1)-Cd(1)-O(2) 86.96(12) 79.16(13)
a Symmetry transformations used to generate equivalent atoms: #1 x, -y + 1, z - 1/2; #2 -x + 1, -y + 1, -z + 1.
Table 4. Selected Bond Distances (Å) and Angles (deg) for 3 Cd(1)-O(5) Cd(1)-O(1) Cd(1)-N(2) Cd(1)-N(1) Cd(1)-O(2) Cd(1)-O(3) Cd(1)-C(1) Cd(2)-O(4) O(5)-Cd(1)-O(1) O(5)-Cd(1)-N(2) O(1)-Cd(1)-N(2) O(5)-Cd(1)-N(1) O(1)-Cd(1)-N(1) N(2)-Cd(1)-N(1) O(5)-Cd(1)-O(2) O(1)-Cd(1)-O(2) N(2)-Cd(1)-O(2) N(1)-Cd(1)-O(2) O(5)-Cd(1)-O(3) O(1)-Cd(1)-O(3) N(2)-Cd(1)-O(3) N(1)-Cd(1)-O(3) O(2)-Cd(1)-O(3) O(4)-Cd(2)-O(8) O(4)-Cd(2)-O(5) O(8)-Cd(2)-O(5) O(4)-Cd(2)-O(8′) O(8)-Cd(2)-O(8′) O(5)-Cd(2)-O(8′) O(4)-Cd(2)-N(4)
Bond Distances 2.277(7) Cd(2)-O(8) 2.335(8) Cd(2)-O(5) 2.345(9) Cd(2)-O(8′) 2.355(9) Cd(2)-N(4) 2.374(8) Cd(2)-N(3) 2.415(7) Cd(2)-O(7) 2.673(11) Cd(2)-O(7′) 2.253(8) Bond Angles 102.5(3) O(8)-Cd(2)-N(4) 107.3(3) O(5)-Cd(2)-N(4) 142.2(4) O(8′)-Cd(2)-N(4) 95.7(3) O(4)-Cd(2)-N(3) 127.9(3) O(8)-Cd(2)-N(3) 71.9(3) O(5)-Cd(2)-N(3) 153.5(3) O(8′)-Cd(2)-N(3) 55.6(3) N(4)-Cd(2)-N(3) 98.9(3) O(4)-Cd(2)-O(7) 89.1(3) O(8)-Cd(2)-O(7) 83.1(3) O(5)-Cd(2)-O(7) 79.2(3) O(8′)-Cd(2)-O(7) 81.8(3) N(4)-Cd(2)-O(7) 152.0(3) N(3)-Cd(2)-O(7) 104.4(3) O(4)-Cd(2)-O(7′) 81.3(7) O(8)-Cd(2)-O(7′) 91.3(3) O(5)-Cd(2)-O(7′) 133.7(8) O(8′)-Cd(2)-O(7′) 81.3(5) N(4)-Cd(2)-O(7′) 29.0(8) N(3)-Cd(2)-O(7′) 104.8(6) O(7)-Cd(2)-O(7′) 93.4(4)
2.43(3) 2.313(7) 2.276(13) 2.338(9) 2.362(9) 2.31(3) 2.49(2)
81.6(8) 144.6(3) 110.6(6) 141.1(3) 128.8(8) 83.2(3) 137.3(5) 71.4(3) 126.0(8) 55.3(9) 97.0(6) 45.0(9) 108.2(7) 92.9(9) 132.0(5) 51.5(9) 113.8(6) 53.7(6) 88.3(6) 84.4(5) 20.1(6)
solvent was entirely evaporated. Appropriate heat may be beneficial to deprotonation of H2BDC and the reaction in the synthesis system. The synthesis of 2 and 3 was performed under solvothermal conditions, which are influenced by temperature and the molar ratio of reactants. Description of Crystal Structure. (a) Cd(BDC)(phen)2‚(C2H5OH)(H2O) (1). A single-crystal X-ray analysis reveals that 1 is a 1-D zigzag polymeric coordination chain. In the asymmetric unit of 1, there are one cadmium atom, one BDC ligand, two phen ligands, one free C2H5OH molecule, and one free H2O molecule (Figure 1a). The cadmium atom adopts an eight-coordinate manner by coordinating to four oxygen atoms (O1, O2; O3, O4) of two chelating carboxylate groups from two BDC ligands and four nitrogen atoms (N1, N2; N3, N4) from two chelating phen ligands. Each BDC ligand bridges two cadmium atoms (Cd1 and Cd1A) in bis-bidentate fashion to form a 1-D infinite zigzag chain as shown in Figure 1b. Solvent molecule
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Figure 1. (a) The building unit of 1; thermal ellipsoids are drawn at the 50% probability level; all hydrogen atoms are omitted for clarity. (b) The 1-D zigzag polymeric chain.
C2H5OH is bonded to the 1-D chain through strong hydrogen bond interactions (O5-H5‚‚‚O1: 3.235 Å, 136.6°. O5-H5‚‚‚O3: 3.024 Å, 116.2°).15 Interestingly, two phen groups chelate to the center cadmium atom in the structure of 1, which is rare and largely different from that in other similar polymers.12-14 Commonly, phen ligands are prone to form polymers in which only one phen ligand chelates to the center metal ion to avoid severe steric hindrance resulting from the bulky phen group, although phen is an effective organic ligand in the synthesis system. In addition, phen groups as capping ligands are inclined to form zero-dimensional frameworks when two phen ligands chelate to one center metal ion in the structure. Here the Cd-O bond distances in the structure of 1 are in the range of 2.437(4)-2.508(5) Å, longer than those in other cadmium bicarboxylate coordination polymers.12a The distortion of Cd-O bonds obviously showed that there is steric hindrance in the structure of 1. Although there is certain steric hindrance, two phen ligands still chelate to one cadmium atom in 1 due to the phen ligands in the structure entirely coming from metal source Cd(phen)(NO3)2. At the same time, it is to avoid much more severe steric hindrance that two adjacent ligands are almost perpendicular to each other in the structure of 1. (b) Cd(1,3-BDC)(phen) (2). X-ray crystallography reveals that 2 is an interesting twisted double chain with a 1-D channel along the c axis. Cd1 adopts a distorted octahedral geometry by coordinating to two nitrogen atoms (N1, N2) from one chelating phen ligand and four oxygen atoms (O1, O2; O3; O4) of one chelating carboxylate group and two dimonodentate carboxylate groups from two 1,3-BDC ligands (Figure 2a). The bond lengths and angles of Cd-Ocar are similar to those in other cadmium-carboxylate coordination polymers.20 Cd1 and Cd1A are bridged by two dimonodentate carboxylate groups separately from two crystallographi-
Figure 2. (a) The coordination environment of Cd1 in 2; thermal ellipsoids are drawn at the 50% probability level; all the hydrogen atoms are omitted for clarity. (b) The 1-D twisted double chain formed by distorted octahedrons constructed from Cd1 and Cd1A. (c) View of the 1-D twisted double chain along the (001) direction.
cally equivalent 1,3-BDC ligands. Thus each of the two distorted octahedrons, which are formed by six-coordinate Cd1 and Cd1A, are extended into the 1-D twisted double chain through 1,3-BDC ligands along the c axis as shown in Figure 2b. Viewed from the (010) direction, there are fourmembered ring pores with dimensions of 3.447 × 7.469 Å, which are enclosed by two double cadmium clusters and two 1,3-BDC ligands, in the 1-D twisted double chain. Of particular interest is that these four-membered ring pores linked by double cadmium clusters overlap to form a 1-D channel with a cavity of about 3 × 3 Å along the (001) direction, as shown in Figure 2c. There are aromatic π-π stacking interactions in the structure of the twisted double chain (intrachain π-π stacking interactions). The plane-plane distance between two adjacent phen ligands in a twisted double chain is 3.628 Å, which is reasonable and may be crucially important in the formation of the bimetallic units in 2. And there are also aromatic π-π stacking interactions between neighboring twisted double chains (interchain π-π stacking interactions). The planeplane distance between two phen ligands separately from two neighboring twisted double chains is 3.702 Å.16 At the same time, C atoms with hydrogen atoms (C10H10) of phen ligands with carboxylate oxygen atoms (O2) from adjacent chains can form C10-H10‚‚‚O2
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Coordination Fashions of fma
(3.257 Å, 137.1°) hydrogen bonds.15 The interchain C-H‚‚‚O hydrogen bonds along with the interchain π-π interactions extend the 1-D twisted double chains into a 2-D supramolecular network. (c) Cd2(fma)2(phen)2 (3). A single-crystal X-ray analysis reveals that 3 is a 2-D undulating network. In the asymmetric unit of 3, there are two cadmium atoms, two fma ligands, and two phen ligands. For 3, the fma ligand has three coordination fashions. In one fashion, the ligand contains two chelating bidentate carboxylate groups (Chart 1a). In the second fashion, the ligand contains two bridging dimonodentate carboxylate groups (Chart 1b), and in the third fashion the ligand contains two µ2-O monodentate carboxylate groups with the other two oxygen atoms uncoordinated (Chart 1c). The IR spectrum of 3 exhibits strong bands at 1622, 1565, and 1512 cm-1 and 1422, 1384, and 1344 cm-1, which can be attributed to the antisymmetric and symmetric stretching vibrations of the carboxylate group, respectively. The difference (∆ ) 181 cm-1) between νasym(CO2) (1565 cm-1) and νsym(CO2) (1384 cm-1) is similar to that of an ionic carboxylate, which shows that there are bridging carboxylate groups in the structure of 3. The difference (∆ ) 278 cm-1) between νasym(CO2) (1622 cm-1) and νsym(CO2) (1344 cm-1) is much more than that of an ionic carboxylate, which proves the existence of monodentate carboxylate groups. And the difference (∆ ) 90 cm-1) between νasym(CO2) (1512 cm-1) and νsym(CO2) (1422 cm-1) is much less than that of an ionic carboxylate, which indicates that bidentate carboxylate groups exist in the structure.17 The coordination fashions of fma ligands given by the IR data are in good agreement with those in the crystal structure. Cd1 adopts a distorted octahedral geometry by coordinating to two nitrogen atoms (N1, N2) from one chelating phen ligand and four oxygen atoms (O1, O2; O3; O5) of one chelating carboxylate group (Chart 1a), one dimonodentate carboxylate group (Chart 1b), and one µ2-O monodentate carboxylate group with the oxygen atom (O6) uncoordinated (Chart 1c), from three different fma ligands, respectively. The coordination environment of Cd2 is the same as that of Cd1, as shown in Figure 3a. Here oxygen atoms (O7 and O8) from the disordered carboxylate group, respectively, split into two different positions (O7 and O7′; O8 and O8′) with 50% occupancies for each. Two distorted octahedrons, which are separately formed by Cd1 and Cd2, are bridged by one dimonodentate carboxylate group (Chart 1b) and one µ2-O monodentate carboxylate group (Chart 1c) from two different fma ligands to generate a double cadmium cluster. Thus each double cadmium cluster is linked by four fma ligands, which adopt three different coordina-
tion fashions, to lead to a 2-D undulating network along the ac plane (Figure 3b). There are obviously aromatic π-π stacking interactions in 3, intralayer and interlayer π-π stacking interactions. The plane-plane distance between two adjacent phen ligands in a undulating network (the intralayer plane-plane distance) is 3.534 Å, which is within reasonable ranges and also may be of importance in the formation of the bimetallic units in 3. And the plane-plane distance between two adjacent phen ligands separately from two neighboring undulating networks (the interlayer plane-plane distance) is 3.740 Å.16 At one time, C-H‚‚‚O hydrogen bonds also exist between adjacent 2-D undulating networks. C atoms with hydrogen atoms (C10-H10, C14-H14, C16-H16, C22H22, C25-H25, C28-H28, C29-H29) of phen ligands can form C-H‚‚‚O hydrogen bonds (C10-H10‚‚‚O7′, C14-H14‚‚‚O8, C16-H16‚‚‚O8′, C22-H22‚‚‚O3, C24H24‚‚‚O1, C26-H26‚‚‚O7′, C29-H29‚‚‚O2) with carboxylate oxygen atoms (O7′, O8, O8′, O3, O1, O7′, O2) from neighboring undulating networks. The C‚‚‚O distance (3.133-3.512 Å) and C-H‚‚‚O angles (136.5165.0°) of C-H‚‚‚O hydrogen bonds are both within acceptable ranges.15 Along the (100) direction, the 2-D undulating networks are ultimately extended into a 3-D supramolecular structure through interlayer C-H‚‚‚O hydrogen bonds along with interlayer π-π interactions. In the undulating network, there are eight-membered ring pores with a diameter of ca. 10.134 × 11.273 Å (based on the Cd-Cd distance), which are enclosed by four double cadmium clusters and four fma ligands. Each eight-membered ring pore is divided into three parts by two phen ligands acting as baffles instead of fully occupied by phen ligands like that in other similar polymers.12a The packing of 2-D undulating networks is shown in Figure 3c. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was performed to gauge the thermal stability of these compounds. For 1, there are two separate weight loss steps. The first weight loss of 9.4% occurs at about 100 °C, which is attributed to the evacuation of free C2H5OH and H2O molecules (calcd, 9.1%). The second two-step weight loss of 71.8% between 380 and 450 °C corresponds to the loss of coordinated phen and BDC ligands. The remaining weight of 18.8% corresponds to the percentage (18.3%) of the Cd and O components, indicating that the final product is CdO. The TGA curve of 1 indicates that the 1-D chain formulated as Cd(BDC)(phen)2 is stable below 380 °C after the solvent molecules are removed. For 2, there is only one-step weight loss. The weight loss of 72.2% occurred between 400 and 500 °C, corresponding to coordinated phen and 1,3-BDC ligands. The remaining
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Figure 4. Solid-state fluorescent emission spectra of (a) Cd(phen)2(NO3)2, (b) Cd(BDC)(phen)2‚(C2H5OH)(H2O), (c) Cd(1,3BDC)(phen), and (d) Cd(fma)2(phen)2 at room temperature.
Figure 3. (a) Perspective view of the coordination environment of Cd1 and Cd2 in 3; thermal ellipsoids are drawn at the 50% probability level; all hydrogen atoms are omitted for clarity (the split positions [O7 and O8] of the fma unit are not shown for clarity). (b) The 2-D undulating network formed by distorted octahedrons constructed from Cd1 and Cd2 along the (010) direction. (c) View of the packing structure of the 2-D undulating network along the (100) direction.
weight of 27.8% corresponds to the final product CdO (calcd, 28.1%). TGA of 2 showed the 1-D twisted double chain does not decompose and is stable below 400 °C. For 3, there is only two-step weight loss. The weight loss of 67.6% between 400 and 500 °C corresponds to coordinated phen and fma ligands. The remaining weight of 32.4% corresponds to the percentage (31.6%) of the final product 2CdO. The result of TGA of 3 showed the 2-D network is stable below 400 °C. TGA results of these compounds above show that they are stable below 400 °C, unusually high for coordination polymers, which indicates their good thermal stability.
Fluorescent Properties. At room temperature, compounds 1-3 exhibit strong fluorescence in the solid state. The reactant Cd(phen)2(NO3)2 also fluoresces in the solid state. Of the emission bands for Cd(phen)2(NO3)2 at 366, 389, and 410 nm, the first two obviously can be assigned to the intraligand π* f π transition of the coordinated phen ligand for their similarity to the emission of free phen‚H2O. And the band at 410 nm for Cd(phen)2(NO3)2, which resulted from the planar configuration of excimeric phen molecules maintained by the cadmium ion, still can be assigned to the intraligand charge transfer of coordinated phen ligands11 (Figure 4a). Solid-state carboxylate ligands, H2BDC, 1,3-H2BDC, and H2fma, can also exhibit fluorescence at room temperature. The emission bands of these carboxylate ligands can be assigned to the π* f n transition as previously reported.21 Fluorescent emission of carboxylate ligands resulting from the π* f n transition is very weak compared with that of the π* f π transition of the phen ligand, and the strong electron withdrawing group, the carboxy group, results in fluorescence quenching, so carboxylate ligands almost have no contribution to the fluorescent emission of as-synthesized polymers. The main emission bands of these three polymers almost locate at the same position but with significantly different band shape, which still might be attributed to the π* f π transition of coordinated phen ligands (Figure 4b-d).20-22 The different band shape might be due to the different structural topologies, which can be suggested by the different band shape of fluorescent emissions of Cd(BDC)(phen)2‚(C2H5OH)(H2O) (1-D zigzag chain) in this work and Cd(BDC)(phen)(H2O)11(1-D zigzag chain) and Cd(BDC)(phen)‚DMF23 (2-D undulating network) in our previous work, although these three polymers are all constructed from metal source Cd(phen)2(NO3)2 and the H2BDC ligand. Conclusions Three metal-organic coordination polymers with novel topologies were obtained by using metal-organic
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compound Cd(phen)2(NO3)2 as the metal source instead of a metal salt. The presence of novel topologies is mainly due to not only the phen ligands in the structure entirely coming from Cd(phen)2(NO3)2 but also carboxylate ligands exhibiting more various coordination fashions resulting from the metal-organic compound as the metal source. Additionally, they exhibit strong fluorescence in the solid state at room temperature and their fluorescent emissions are somewhat related to their structural topologies.
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Acknowledgment. We are grateful for the financial support of the State Basic Research Project (G2000077507) and the National Nature Science Foundation of China (Grant Nos. 29873017, 20101004, and 20371020).
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