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Synthesis and characterization of a novel cadmium(II) photoluminescent coordination polymer based on pyridine-2,3,5-tricarboxylic acid
Journal of Molecular Structure 980 (2010) 257–260
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Synthesis and characterization of a novel cadmium(II) photoluminescent coordination polymer based on pyridine-2,3,5-tricarboxylic acid Ling Qiu a, Jian-Guo Lin a,b,*, Qing-Jin Meng b a b
Key Laboratory of Nuclear Medicine, Ministry of Health & Jiangsu Key Laboratory of Molecular Nuclear Medicine, Jiangsu Institute of Nuclear Medicine, Wuxi 214063, PR China Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
a r t i c l e
i n f o
Article history: Received 9 June 2010 Received in revised form 17 July 2010 Accepted 18 July 2010 Available online 23 July 2010 Keywords: Cadmium(II) coordination polymer Pyridine-2,3,5-tricarboxylic acid Thermal stability Photoluminescence
a b s t r a c t Employing the multidentate ligand pyridine-2,3,5-tricarboxylic acid (H3PTC) as a bridge, a novel cadmium(II) coordination polymer [Cd3(PTC)2(bpy)(H2O)4] (1) was obtained and characterized by single crystal X-ray diffraction. The PTC3- ligand exhibits an interesting coordination mode to form a dense three-dimensional (3-D) framework of 1. The complex displays good thermal stability and strong photoluminescence in the blue region band. Thus it may serve as a potential candidate of thermally stable bluelight-emitting luminescent material. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Rational design and assembly of organic–inorganic hybrid coordination polymers with diverse pyridine-polycarboxylic acids have received increasing attention and been developing rapidly in recent years [1–4]. And the complexes have intriguing potential applications in functional materials and biomolecular sciences based on the diverse pyridinedicarboxylic acids [5–9] and pyridine-2,4,6-tricarboxylic acid [10–14]. In fact, pyridine-tricarboxylic acids are more versatile ligands due to the fact that they have multifarious coordinate configurations in monoatomic and triatomic syn–syn, syn–anti and anti–anti conformations. Compared with the pyridine-dicarboxylic acid, the pyridine-tricarboxylic acid with novel structures and coordination modes are desired to be obtained. In the previous work, several examples based on the pyridine2,4,6-tricarboxylic acid have been reported by our group [15] and others [10–14]. To continue the research in this field, the pyridine-2,3,5-tricarboxylic acid was employed as a multidentate ligand in the self-assembly of coordination polymers. To the best of our knowledge, the coordination polymers constructed from mixed-ligand including H3PTC molecule have been largely unexplored up to date [16]. In order to understand the coordination chemistry of H3PTC in preparing new materials with interesting
* Corresponding author at: Key Laboratory of Nuclear Medicine, Ministry of Health & Jiangsu Key Laboratory of Molecular Nuclear Medicine, Jiangsu Institute of Nuclear Medicine, Wuxi 214063, PR China. Tel.: +86 510 85514482; fax: +86 510 85513113. E-mail address: [email protected] (J.-G. Lin). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.07.028
structural topologies and excellent physical properties, we have recently engaged in the research of polymeric complexes based on this multidentate ligand. In this paper, we report the preparation, single-crystal structural characterization, IR spectrum, thermal stability and photofluorescent property of the title complex. 2. Experimental 2.1. Materials and measurements All analytical reagents were purchased from commercial sources and used without further purification. The ligand pyridine-2,3,5-tricarboxylic acid was synthesized by oxidation of 2,3,5-trimethylpyridine with potassium permanganate similar to the reported method [17]. C, H, and N microanalyses were carried out with a Perkin–Elmer 240 elemental analyzer. The IR spectrum was recorded using KBr discs on a Bruker Vector 22 spectrophotometer in the 4000–400 cm1 region. Thermogravimetric analysis was performed on a simultaneous SDT 2960 thermal analyzer under flowing N2 with a heating rate of 10 °C min1 between ambient temperature and 750 °C. Photoluminescent spectrum for the solid sample was recorded with a Hitachi 850 fluorescence spectrophotometer. 2.2. Synthesis of the complex 1 [Cd3(PTC)2(bpy)(H2O)4] (1). A mixture of equal mole (0.1 mmol) of pyridine-2,3,5-tricarboxylic acid (0.0211 g), bpy (0.0156 g) and
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Table 1 Crystal data and structure refinement details for 1.
Temp./K Formula Mr Cryst. system Space group a/Å b/Å c/Å b/° V/Å3 Z Dc/g cm3 l/mm1 F(0 0 0) Unique reflns. Reflns. obsd. (I > 2r(I)) R1 and wR2 [I > 2r(I)] R1 and wR2 (all data) GooF
Table 2 Selected bond lengths (Å) and bond angles (°) for 1a. [Cd3(PTC)2(bpy)2(H2O)4] (1)
Bond lengths (Å)
291(2) C26H20Cd3N4O16 981.66 Monoclinic P2/c 12.606(2) 11.436(2) 10.2213(19) 109.184(3) 1391.7(4) 2 2.343 2.364 952 2736 2071 0.0399, 0.0689 0.0638, 0.0718 1.044
Cd1–O3ii Cd1–O7 Cd1–N1 Cd1–O1 Cd1–O1iii Cd1–O6iv Cd1–O7vii
2.202(6) 2.204(6) 2.369(7) 2.393(5) 2.422(6) 2.465(5) 2.726(3)
Cd2–N3v Cd2–N2 Cd2–O8 Cd2–O8i Cd2–O4i Cd2–O4
2.225(9) 2.246(9) 2.298(6) 2.298(6) 2.332(5) 2.332(5)
Bond angles (°) N1–Cd1–O1 O3ii–Cd1–O1iii O7–Cd1–O1iii N1–Cd1–O1iii O1–Cd1–O1iii O3ii–Cd1–O6iv O7–Cd1–O6iv N1–Cd1–O6iv O1–Cd1–O6iv O1iii–Cd1–O6iv O3ii–Cd1–O7 O3ii–Cd1–N1 O7–Cd1–N1 O3ii–Cd1–O1 O7–Cd1–O1
68.5(2) 86.2(2) 86.0(2) 135.3(2) 67.5(2) 83.2(2) 90.08(19) 79.6(2) 147.56(18) 144.89(18) 155.6(2) 110.1(2) 91.5(3) 101.8(2) 96.4(2)
N3v–Cd2–N2 N3v–Cd2–O8 N2–Cd2–O8 N3v–Cd2–O8i N2–Cd2–O8i O8–Cd2–O8i N3v–Cd2–O4i N2–Cd2–O4i O8–Cd2–O4i O8i–Cd2–O4i N3v–Cd2–O4 N2–Cd2–O4 O8–Cd2–O4 O8i–Cd2–O4 O4i–Cd2–O4
180.000(1) 87.90(15) 92.10(15) 87.90(15) 92.10(15) 175.8(3) 95.19(13) 84.81(13) 89.6(2) 90.8(2) 95.19(13) 84.81(13) 90.8(2) 89.6(2) 169.6(3)
h i R1 = R||Fo| |Fc||/R|Fo; wR2 ¼ R wðF 2o F 2c Þ2 =R½wðF 2o Þ2 1=2 .
Cd(NO3)24H2O (0.0308 g) and 0.2 mmol NaOH (0.0080 g) in a 10 ml mixed water–ethanol (1:1 v/v) solution was placed in a 23-ml Teflon-lined autoclave. The autoclave is heated under autogenous pressure to 120 °C for 3 days. After the mixture was slowly cooled to room temperature, colorless block-shaped crystals were obtained. (Yield: 45.3%.) Anal. calcd. (%) For: C26H20Cd3N4O16: C, 37.11; H, 2.05; N, 5.70. Found (%): C, 36.97; H, 2.12; N: 5.83. 2.3. Crystal structure determination Single crystal of approximate dimensions 0.28 0.24 0.22 mm for the complex 1 was used for the structural determination on a Bruker SMART APEX CCD diffractometer with graphitemonochromatized Mo Ka radiation (k = 0.71073 Å) at room temperature using the x-scan technique. Lorentz polarization and absorption corrections were applied. The structure was solved by direct method and refined with the full-matrix least-squares technique using the SHELXS-97 [18] and SHELXL-97 [19] programs. All non-hydrogen atoms were refined with anisotropic displacement parameters, except for disordered atoms. To assist the refinement, several restraints were applied for 1, the disordered C9 and C10 atoms of pyridyl ring were restrained in disorder to obtain reasonable thermal parameters. Hydrogen atoms attached to the carbon atoms were calculated and refined with isotropic displacement parameters 1.2 or 1.5 times higher than the values of the corresponding carbon atoms. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data and selected bond lengths for 1 were listed in Tables 1 and 2.
a Symmetry codes: (i) 1 x, y, 0.5 z; (ii) 2 x, y, 1.5 z; (iii) 2 x, 1 y, 2z; (iv) 2 x, y, 2 z; (v) x, 1 + y, z; (vi) x, 1 + y, z; (vii) 2x, y, 2.5z.
PTC3 ligand. Notably, the distance is 2.726 Å between Cd1 and O7vii (symmetry code: vii = 2 x, y, 2.5 z), suggesting a nonnegligible interaction between them. This can be described as a semi-chelating coordination mode [20,21]. Hence, the coordination environment of Cd1 atom can also be regarded as a distorted pentagonal bipyramidal coordination geometry. As for the Cd2 center, it also adopts a distorted octahedral coordination environment which is surrounded by two carboxylate oxygens, two aqua molecules as well as two nitrogen donors from different 4,40 -bipyridine (bpy) ligands. On the whole, the bond distances between Cd1 and the coordinated oxygen atoms are longer than those of Cd2–O bonds. This just originates from the different coordination conformations of cadmium(II) centers. As shown in Fig. 2, the fully deprotonated PTC3 ligand in 1 coordinated to five Cd(II) atoms. Three carboxylates in the PTC3 exhibit diverse coordination modes respectively, i.e. mono-dentate terminal, mono-dentate bridging and bidentate bridging, which is totally different from the previous work [16]. The largest dihedral angle between the carboxylate group and the phenyl ring is 72.4°. This bridging ligand connected the Cd(II) centers to form a 3-D open framework. Along the c axis, there are one-dimensional (1D) parallelogram channels with the effective cavity size of ca. 8.84 8.78 Å in the framework (Fig. 3). Furthermore, the ‘‘second’’ ligand bpy binds to the unsaturated sites of the Cd(II) centers in the 1-D infinite chains, resulting in the final dense 3-D framework with no extra space reserved (see Fig. 4).
3. Results and discussion 3.2. FT-IR spectrum 3.1. Crystal structure of [Cd3(PTC)2(bpy)(H2O)4] (1) Single crystal X-ray structural analysis shows that the structure of complex 1 crystallizes in the space group of P2/c and has a complicated 3-D network. As depicted in Fig. 1, there exist two types of coordination environments around the cadmium ions in the crystal structure. Cd1 possesses a distorted octahedral coordination sphere, where the apical positions are occupied by one carboxylate oxygen atom and one aqua molecule, while the basal plane is completed by two carboxylate oxygen atoms from two different PTC3 ligands, one nitrogen atom and one carboxylate oxygen atom of a
The FT-IR spectrum of compound 1 shows broad peaks in the range of 3550–3300 cm1 due to the existence of water molecules. In the middle range of the IR spectrum, the vibration modes are commonly employed to distinguish the coordination modes of the carboxylate groups [22]. The general correlation between the carboxylate coordination modes and the metal ions can be described according to the difference in the infrared data of the coordination compounds, i.e. [Dm(COO) = mas(COO)–ms(COO)] [23]. As for 1, the stretching vibrations of mas(COO) and ms(COO) occur at 1621, 1563 and 1413, 1395 cm1, respectively. The different
L. Qiu et al. / Journal of Molecular Structure 980 (2010) 257–260
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Fig. 1. Molecular structure of complex 1. Hydrogen atoms are omitted for clarity (symmetry codes are listed in Table 2).
Fig. 2. Coordinaton mode of PTC3 ligand in 1. Fig. 4. The ligand 4,40 -bipyridine connects the unsaturated sites of the Cd(II) centers in the [Cd3(PTC)2] architecture to form a dense 3-D framework. Hydrogen atoms are omitted for clarity.
Fig. 3. Space-filling model for 3-D framework of 1 connected by PTC3 ligand. Hydrogen atoms are omitted for clarity.
Dm(COO) values indicate that the carboxylate groups of the PTC3 ligand exhibit different coordination modes in the complex. In this case, the X-ray diffraction analysis was allowed to assign the binding modes of the carboxylate groups unambiguously. 3.3. Thermogravimetric analysis To study the thermal stability of the complex 1, the thermogravimetric analysis (TGA) on the new crystalline samples was performed under a nitrogen atmosphere in flowing N2 with a heating rate of 10 °C min1, as depicted in Fig. 5. At the first glance, one can see that complex 1 is stable enough at the ambient condition, and there is no weight loss below 134 °C. The first weight loss of 7.21% occurs from 134 to 200 °C (calcd. 7.33%), corresponding to the loss of four coordination water molecules per formula. Subsequently, a
Fig. 5. TGA curve of complex 1.
plateau region is observed over 354 °C. Then, two consecutive decompositions take place in the temperature range of 354– 498 °C, suggesting the total destruction of the framework by the oxidation of the organic component. The residues correspond to the formation of the stoichiometric amount of cadmium(II) oxide. This conclusion is supported by 37.36% remained, which is in agreement with the expected value (calcd. 37.41%). 3.4. Luminescent property The luminescent spectra of 1 and the free ligand H3PTC in the solid state at room temperature were investigated. As depicted in
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5. Supplementary material CCDC 772483 contains the supplementary crystallographic data for 1. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Acknowledgements
Fig. 6. Photofluorescent spectra of complex 1 (solid lines) and free ligand H3PTC (dash lines) in the solid state at room temperature.
The authors are very grateful to the National Natural Science Foundation of China (20801024), Natural Science Foundation of Jiangsu Province (BK2009077), Science Foundation of Health Department of Jiangsu Province (H200963), China Postdoctoral Science Foundation (20080441026) and Jiangsu Planned Projects for Postdoctoral Research Funds (0901002B) for their financial support. References
Fig. 6, upon excitation at 359 nm, a strong blue fluorescent emission band at 411 nm was observed for the complex 1. This emission is neither metal-to-ligand charge transfer (MLCT) nor ligand-tometal charge transfer (LMCT) in nature since the Cd2+ ion is difficult to oxidize or to reduce due to the d10 electronic configuration. It can probably be assigned to the intraligand p–p fluorescent emission because similar emission is observed at 406 nm for the free ligand H3PTC with the excitation at 355 nm [24]. On the other hand, the relative intensity of fluorescent emission enhanced for 1 in comparison with that of the free ligand. This may be ascribed to the increase in the conformational rigidity of the ligands as they are coordinated to the Cd(II) ions, which result in a decrease in the nonradiative decay of intraligand excited states [25]. Therefore, one can conclude that the complex may be a potential candidate of thermally stable and solvent-resistant blue fluorescent materials.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
4. Conclusions In this work, the interesting versatile ligand H3PTC was employed to construct a novel organic–inorganic hybrid Cd(II) coordination polymer. The H3PTC ligand exhibits a novel coordination mode to form a dense 3-D framework of complex 1. The skeleton of 1 is thermally stable up to a higher temperature of 354 °C after losing the guest water molecules. In addition, it exhibits efficient photoluminescence at room temperature and would be a good candidate of blue-light-emitting luminescent material.
[19] [20] [21] [22] [23] [24] [25]
S.K. Ghosh, P.K. Bharadwaj, Inorg. Chem. 44 (2005) 3156. C. Qin, X.L. Wang, E.B. Wang, Z.M. Su, Inorg. Chem. 44 (2005) 7122. R. García-Zarracino, H. Höpfl, Angew. Chem. Int. Ed. 43 (2004) 1507. M.S. Liu, Q.Y. Yu, Y.P. Cai, C.Y. Su, X.M. Lin, X.X. Zhou, J.W. Cai, Cryst. Growth Des. 8 (2008) 4083. B. Zhao, X.Y. Chen, P. Cheng, D.Z. Liao, S.P. Yan, Z.H. Jiang, J. Am. Chem. Soc. 126 (2004) 15394. S.M. Humphrey, P.T. Wood, J. Am. Chem. Soc. 126 (2004) 13236. N. Tancrez, C. Feuvrie, I. Ledoux, J. Zyss, L. Toupet, H.L. Bozec, O. Maury, J. Am. Chem. Soc. 127 (2005) 13474. Y.L. Liu, V.C. Kravtsov, D.A. Beauchamp, J.F. Eubank, M. Eddaoudi, J. Am. Chem. Soc. 127 (2005) 7266. R. García-Zarracino, H. Höpfl, J. Am. Chem. Soc. 127 (2005) 3120. S.K. Ghosh, G. Savitha, P.K. Bharadwaj, Inorg. Chem. 43 (2004) 5495. S.K. Ghosh, P.K. Bharadwaj, Eur. J. Inorg. Chem. (2005) 4886. B. Zhai, L. Yi, H.S. Wang, B. Zhao, P. Cheng, D.Z. Liao, S.P. Yan, Inorg. Chem. 45 (2006) 8471. M.C. Das, S.K. Ghosh, E.C. Sañudo, P.K. Bharadwaj, Dalton Trans. 7 (2009) 1644. M. Andruh, J.P. Costes, C. Diaz, S. Gao, Inorg. Chem. 48 (2009) 3342. J.G. Lin, L.L. Wen, S.Q. Zang, Y. Su, Z.D. Lu, H.Z. Zhu, Q.J. Meng, Inorg. Chem. Comm. 10 (2007) 74. H.S. Wang, J.G. Ma, B. Zhai, Y.Y. Liu, P. Cheng, J Mol. Struct. 829 (2007) 1. L. Syper, K. Kloc, J. Mlochowski, Tetrahedron 36 (1980) 123. G.M. Sheldrick, SHELXS-97, Program for X-ray Crystal Structure Solution, University of Göttingen, Göttingen, Germany, 1997. G.M. Sheldrick, SHELXL-97, Program for X-ray Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997. B. Moulton, M. Zaworotko, Chem. Rev. 101 (2001) 1629. M. Eddaoudi, D.B. Moler, H. Li, B. Chen, T.M. Reineke, M. O’Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319. G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980) 227. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1997. L.L. Wen, D.B. Dang, C.Y. Duan, Y.Z. Li, Z.F. Tian, Q.J. Meng, Inorg. Chem. 44 (2005) 7161. Z.Y. Fu, X.T. Wu, J.C. Dai, S.M. Hu, W.X. Du, H.H. Zhang, R.Q. Sun, Eur. J. Inorg. Chem. (2002) 2730.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Synthesis and characterization of a novel cadmium(II) photoluminescent coordination polymer based on pyridine-2,3,5-tricarboxylic acid Ling Qiu a, Jian-Guo Lin a,b,*, Qing-Jin Meng b a b
Key Laboratory of Nuclear Medicine, Ministry of Health & Jiangsu Key Laboratory of Molecular Nuclear Medicine, Jiangsu Institute of Nuclear Medicine, Wuxi 214063, PR China Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
a r t i c l e
i n f o
Article history: Received 9 June 2010 Received in revised form 17 July 2010 Accepted 18 July 2010 Available online 23 July 2010 Keywords: Cadmium(II) coordination polymer Pyridine-2,3,5-tricarboxylic acid Thermal stability Photoluminescence
a b s t r a c t Employing the multidentate ligand pyridine-2,3,5-tricarboxylic acid (H3PTC) as a bridge, a novel cadmium(II) coordination polymer [Cd3(PTC)2(bpy)(H2O)4] (1) was obtained and characterized by single crystal X-ray diffraction. The PTC3- ligand exhibits an interesting coordination mode to form a dense three-dimensional (3-D) framework of 1. The complex displays good thermal stability and strong photoluminescence in the blue region band. Thus it may serve as a potential candidate of thermally stable bluelight-emitting luminescent material. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Rational design and assembly of organic–inorganic hybrid coordination polymers with diverse pyridine-polycarboxylic acids have received increasing attention and been developing rapidly in recent years [1–4]. And the complexes have intriguing potential applications in functional materials and biomolecular sciences based on the diverse pyridinedicarboxylic acids [5–9] and pyridine-2,4,6-tricarboxylic acid [10–14]. In fact, pyridine-tricarboxylic acids are more versatile ligands due to the fact that they have multifarious coordinate configurations in monoatomic and triatomic syn–syn, syn–anti and anti–anti conformations. Compared with the pyridine-dicarboxylic acid, the pyridine-tricarboxylic acid with novel structures and coordination modes are desired to be obtained. In the previous work, several examples based on the pyridine2,4,6-tricarboxylic acid have been reported by our group [15] and others [10–14]. To continue the research in this field, the pyridine-2,3,5-tricarboxylic acid was employed as a multidentate ligand in the self-assembly of coordination polymers. To the best of our knowledge, the coordination polymers constructed from mixed-ligand including H3PTC molecule have been largely unexplored up to date [16]. In order to understand the coordination chemistry of H3PTC in preparing new materials with interesting
* Corresponding author at: Key Laboratory of Nuclear Medicine, Ministry of Health & Jiangsu Key Laboratory of Molecular Nuclear Medicine, Jiangsu Institute of Nuclear Medicine, Wuxi 214063, PR China. Tel.: +86 510 85514482; fax: +86 510 85513113. E-mail address: [email protected] (J.-G. Lin). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.07.028
structural topologies and excellent physical properties, we have recently engaged in the research of polymeric complexes based on this multidentate ligand. In this paper, we report the preparation, single-crystal structural characterization, IR spectrum, thermal stability and photofluorescent property of the title complex. 2. Experimental 2.1. Materials and measurements All analytical reagents were purchased from commercial sources and used without further purification. The ligand pyridine-2,3,5-tricarboxylic acid was synthesized by oxidation of 2,3,5-trimethylpyridine with potassium permanganate similar to the reported method [17]. C, H, and N microanalyses were carried out with a Perkin–Elmer 240 elemental analyzer. The IR spectrum was recorded using KBr discs on a Bruker Vector 22 spectrophotometer in the 4000–400 cm1 region. Thermogravimetric analysis was performed on a simultaneous SDT 2960 thermal analyzer under flowing N2 with a heating rate of 10 °C min1 between ambient temperature and 750 °C. Photoluminescent spectrum for the solid sample was recorded with a Hitachi 850 fluorescence spectrophotometer. 2.2. Synthesis of the complex 1 [Cd3(PTC)2(bpy)(H2O)4] (1). A mixture of equal mole (0.1 mmol) of pyridine-2,3,5-tricarboxylic acid (0.0211 g), bpy (0.0156 g) and
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Table 1 Crystal data and structure refinement details for 1.
Temp./K Formula Mr Cryst. system Space group a/Å b/Å c/Å b/° V/Å3 Z Dc/g cm3 l/mm1 F(0 0 0) Unique reflns. Reflns. obsd. (I > 2r(I)) R1 and wR2 [I > 2r(I)] R1 and wR2 (all data) GooF
Table 2 Selected bond lengths (Å) and bond angles (°) for 1a. [Cd3(PTC)2(bpy)2(H2O)4] (1)
Bond lengths (Å)
291(2) C26H20Cd3N4O16 981.66 Monoclinic P2/c 12.606(2) 11.436(2) 10.2213(19) 109.184(3) 1391.7(4) 2 2.343 2.364 952 2736 2071 0.0399, 0.0689 0.0638, 0.0718 1.044
Cd1–O3ii Cd1–O7 Cd1–N1 Cd1–O1 Cd1–O1iii Cd1–O6iv Cd1–O7vii
2.202(6) 2.204(6) 2.369(7) 2.393(5) 2.422(6) 2.465(5) 2.726(3)
Cd2–N3v Cd2–N2 Cd2–O8 Cd2–O8i Cd2–O4i Cd2–O4
2.225(9) 2.246(9) 2.298(6) 2.298(6) 2.332(5) 2.332(5)
Bond angles (°) N1–Cd1–O1 O3ii–Cd1–O1iii O7–Cd1–O1iii N1–Cd1–O1iii O1–Cd1–O1iii O3ii–Cd1–O6iv O7–Cd1–O6iv N1–Cd1–O6iv O1–Cd1–O6iv O1iii–Cd1–O6iv O3ii–Cd1–O7 O3ii–Cd1–N1 O7–Cd1–N1 O3ii–Cd1–O1 O7–Cd1–O1
68.5(2) 86.2(2) 86.0(2) 135.3(2) 67.5(2) 83.2(2) 90.08(19) 79.6(2) 147.56(18) 144.89(18) 155.6(2) 110.1(2) 91.5(3) 101.8(2) 96.4(2)
N3v–Cd2–N2 N3v–Cd2–O8 N2–Cd2–O8 N3v–Cd2–O8i N2–Cd2–O8i O8–Cd2–O8i N3v–Cd2–O4i N2–Cd2–O4i O8–Cd2–O4i O8i–Cd2–O4i N3v–Cd2–O4 N2–Cd2–O4 O8–Cd2–O4 O8i–Cd2–O4 O4i–Cd2–O4
180.000(1) 87.90(15) 92.10(15) 87.90(15) 92.10(15) 175.8(3) 95.19(13) 84.81(13) 89.6(2) 90.8(2) 95.19(13) 84.81(13) 90.8(2) 89.6(2) 169.6(3)
h i R1 = R||Fo| |Fc||/R|Fo; wR2 ¼ R wðF 2o F 2c Þ2 =R½wðF 2o Þ2 1=2 .
Cd(NO3)24H2O (0.0308 g) and 0.2 mmol NaOH (0.0080 g) in a 10 ml mixed water–ethanol (1:1 v/v) solution was placed in a 23-ml Teflon-lined autoclave. The autoclave is heated under autogenous pressure to 120 °C for 3 days. After the mixture was slowly cooled to room temperature, colorless block-shaped crystals were obtained. (Yield: 45.3%.) Anal. calcd. (%) For: C26H20Cd3N4O16: C, 37.11; H, 2.05; N, 5.70. Found (%): C, 36.97; H, 2.12; N: 5.83. 2.3. Crystal structure determination Single crystal of approximate dimensions 0.28 0.24 0.22 mm for the complex 1 was used for the structural determination on a Bruker SMART APEX CCD diffractometer with graphitemonochromatized Mo Ka radiation (k = 0.71073 Å) at room temperature using the x-scan technique. Lorentz polarization and absorption corrections were applied. The structure was solved by direct method and refined with the full-matrix least-squares technique using the SHELXS-97 [18] and SHELXL-97 [19] programs. All non-hydrogen atoms were refined with anisotropic displacement parameters, except for disordered atoms. To assist the refinement, several restraints were applied for 1, the disordered C9 and C10 atoms of pyridyl ring were restrained in disorder to obtain reasonable thermal parameters. Hydrogen atoms attached to the carbon atoms were calculated and refined with isotropic displacement parameters 1.2 or 1.5 times higher than the values of the corresponding carbon atoms. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data and selected bond lengths for 1 were listed in Tables 1 and 2.
a Symmetry codes: (i) 1 x, y, 0.5 z; (ii) 2 x, y, 1.5 z; (iii) 2 x, 1 y, 2z; (iv) 2 x, y, 2 z; (v) x, 1 + y, z; (vi) x, 1 + y, z; (vii) 2x, y, 2.5z.
PTC3 ligand. Notably, the distance is 2.726 Å between Cd1 and O7vii (symmetry code: vii = 2 x, y, 2.5 z), suggesting a nonnegligible interaction between them. This can be described as a semi-chelating coordination mode [20,21]. Hence, the coordination environment of Cd1 atom can also be regarded as a distorted pentagonal bipyramidal coordination geometry. As for the Cd2 center, it also adopts a distorted octahedral coordination environment which is surrounded by two carboxylate oxygens, two aqua molecules as well as two nitrogen donors from different 4,40 -bipyridine (bpy) ligands. On the whole, the bond distances between Cd1 and the coordinated oxygen atoms are longer than those of Cd2–O bonds. This just originates from the different coordination conformations of cadmium(II) centers. As shown in Fig. 2, the fully deprotonated PTC3 ligand in 1 coordinated to five Cd(II) atoms. Three carboxylates in the PTC3 exhibit diverse coordination modes respectively, i.e. mono-dentate terminal, mono-dentate bridging and bidentate bridging, which is totally different from the previous work [16]. The largest dihedral angle between the carboxylate group and the phenyl ring is 72.4°. This bridging ligand connected the Cd(II) centers to form a 3-D open framework. Along the c axis, there are one-dimensional (1D) parallelogram channels with the effective cavity size of ca. 8.84 8.78 Å in the framework (Fig. 3). Furthermore, the ‘‘second’’ ligand bpy binds to the unsaturated sites of the Cd(II) centers in the 1-D infinite chains, resulting in the final dense 3-D framework with no extra space reserved (see Fig. 4).
3. Results and discussion 3.2. FT-IR spectrum 3.1. Crystal structure of [Cd3(PTC)2(bpy)(H2O)4] (1) Single crystal X-ray structural analysis shows that the structure of complex 1 crystallizes in the space group of P2/c and has a complicated 3-D network. As depicted in Fig. 1, there exist two types of coordination environments around the cadmium ions in the crystal structure. Cd1 possesses a distorted octahedral coordination sphere, where the apical positions are occupied by one carboxylate oxygen atom and one aqua molecule, while the basal plane is completed by two carboxylate oxygen atoms from two different PTC3 ligands, one nitrogen atom and one carboxylate oxygen atom of a
The FT-IR spectrum of compound 1 shows broad peaks in the range of 3550–3300 cm1 due to the existence of water molecules. In the middle range of the IR spectrum, the vibration modes are commonly employed to distinguish the coordination modes of the carboxylate groups [22]. The general correlation between the carboxylate coordination modes and the metal ions can be described according to the difference in the infrared data of the coordination compounds, i.e. [Dm(COO) = mas(COO)–ms(COO)] [23]. As for 1, the stretching vibrations of mas(COO) and ms(COO) occur at 1621, 1563 and 1413, 1395 cm1, respectively. The different
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Fig. 1. Molecular structure of complex 1. Hydrogen atoms are omitted for clarity (symmetry codes are listed in Table 2).
Fig. 2. Coordinaton mode of PTC3 ligand in 1. Fig. 4. The ligand 4,40 -bipyridine connects the unsaturated sites of the Cd(II) centers in the [Cd3(PTC)2] architecture to form a dense 3-D framework. Hydrogen atoms are omitted for clarity.
Fig. 3. Space-filling model for 3-D framework of 1 connected by PTC3 ligand. Hydrogen atoms are omitted for clarity.
Dm(COO) values indicate that the carboxylate groups of the PTC3 ligand exhibit different coordination modes in the complex. In this case, the X-ray diffraction analysis was allowed to assign the binding modes of the carboxylate groups unambiguously. 3.3. Thermogravimetric analysis To study the thermal stability of the complex 1, the thermogravimetric analysis (TGA) on the new crystalline samples was performed under a nitrogen atmosphere in flowing N2 with a heating rate of 10 °C min1, as depicted in Fig. 5. At the first glance, one can see that complex 1 is stable enough at the ambient condition, and there is no weight loss below 134 °C. The first weight loss of 7.21% occurs from 134 to 200 °C (calcd. 7.33%), corresponding to the loss of four coordination water molecules per formula. Subsequently, a
Fig. 5. TGA curve of complex 1.
plateau region is observed over 354 °C. Then, two consecutive decompositions take place in the temperature range of 354– 498 °C, suggesting the total destruction of the framework by the oxidation of the organic component. The residues correspond to the formation of the stoichiometric amount of cadmium(II) oxide. This conclusion is supported by 37.36% remained, which is in agreement with the expected value (calcd. 37.41%). 3.4. Luminescent property The luminescent spectra of 1 and the free ligand H3PTC in the solid state at room temperature were investigated. As depicted in
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5. Supplementary material CCDC 772483 contains the supplementary crystallographic data for 1. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Acknowledgements
Fig. 6. Photofluorescent spectra of complex 1 (solid lines) and free ligand H3PTC (dash lines) in the solid state at room temperature.
The authors are very grateful to the National Natural Science Foundation of China (20801024), Natural Science Foundation of Jiangsu Province (BK2009077), Science Foundation of Health Department of Jiangsu Province (H200963), China Postdoctoral Science Foundation (20080441026) and Jiangsu Planned Projects for Postdoctoral Research Funds (0901002B) for their financial support. References
Fig. 6, upon excitation at 359 nm, a strong blue fluorescent emission band at 411 nm was observed for the complex 1. This emission is neither metal-to-ligand charge transfer (MLCT) nor ligand-tometal charge transfer (LMCT) in nature since the Cd2+ ion is difficult to oxidize or to reduce due to the d10 electronic configuration. It can probably be assigned to the intraligand p–p fluorescent emission because similar emission is observed at 406 nm for the free ligand H3PTC with the excitation at 355 nm [24]. On the other hand, the relative intensity of fluorescent emission enhanced for 1 in comparison with that of the free ligand. This may be ascribed to the increase in the conformational rigidity of the ligands as they are coordinated to the Cd(II) ions, which result in a decrease in the nonradiative decay of intraligand excited states [25]. Therefore, one can conclude that the complex may be a potential candidate of thermally stable and solvent-resistant blue fluorescent materials.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
4. Conclusions In this work, the interesting versatile ligand H3PTC was employed to construct a novel organic–inorganic hybrid Cd(II) coordination polymer. The H3PTC ligand exhibits a novel coordination mode to form a dense 3-D framework of complex 1. The skeleton of 1 is thermally stable up to a higher temperature of 354 °C after losing the guest water molecules. In addition, it exhibits efficient photoluminescence at room temperature and would be a good candidate of blue-light-emitting luminescent material.
[19] [20] [21] [22] [23] [24] [25]
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