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Effect of substituent carboxylic group on the self-assembly of cadmium coordination polymers: Syntheses, topological analyses and photoluminescence properties
Inorganic Chemistry Communications 58 (2015) 1–4
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Effect of substituent carboxylic group on the self-assembly of cadmium coordination polymers: Syntheses, topological analyses and photoluminescence properties Ni-Ya Li, Wei Ma, Shu-Jun Wang, Dong Liu ⁎ College of Chemistry and Materials Science, Huaibei Normal University, Huaibei 235000, Anhui, PR China
a r t i c l e
i n f o
Article history: Received 18 January 2015 Received in revised form 5 May 2015 Accepted 14 May 2015 Available online 16 May 2015 Keywords: Coordination polymers Carboxylic group Syntheses Topological analyses Photoluminescence properties
a b s t r a c t Two coordination polymers, formulated as {[Cd(1,4-BDC)(ppene)]·CH3CN}n (1) and [Cd(H2O)(1,2,4HBTC)(ppene)0.5]n (2) have been synthesized through the solvothermal reactions between Cd(NO3)2·4H2O and 4-pyr-poly-2-ene (ppene) in the presence of 1,4-benzenedicarboxylic acid (1,4-H2BDC) or its derivative 1,2,4-benzenetricarboxylic acid (1,2,4-H3BTC). These two compounds were structurally characterized by infrared spectroscopy, elemental analysis, single crystal X-ray diffraction and powder X-ray diffraction. Compound 1 exhibits a three-dimensional two-fold interpenetrating 41263-pcu network. The coordination network of 2 shows a three-dimensional (4,5)-connected framework with rare (4462)(4466) tcs topology. Owing to the linkage of hydrogen bonding interactions between protonated carboxylic groups, 2 further displays a three-dimensional (5,5)-connected supramolecular network with an unprecedented Schläfli symbol (4862)(4664). Thermal stability and photoluminescence properties of 1 and 2 were also investigated. © 2015 Elsevier B.V. All rights reserved.
Self-assembled combinations of metal ions and organic ligands have afforded a wide range of coordination polymers (CPs) and attracted significant attention from chemists because of their interesting structural topologies and extensive potential applications as functional materials in many areas, such as photoluminescence, magnetism, catalysis, nonlinear optics, chemical sensors, gas storage and so on [1–16]. The aim of CP research is to control the architectures of target products and explore the relationships between structures and properties [1,2]. However, controllable synthesis of predictable CPs is still a great challenge since the construction of coordination frameworks are sensitive to many reaction conditions, such as metal ions, ligands, concentrations, counterions, templates, solvents, temperatures, pH values and so on [3–8]. It has been found that the selection of organic linkers is extremely important for the preparation of CPs because the structures and properties of these compounds are usually influenced by the shape, length, flexibility, symmetry and substituent groups of ligands [9–14]. Among the multitude of organic ligands, multidentate carboxylates exist in a wide variety of coordination modes and geometries, which is helpful for the formation of novel CPs with varied structural topologies and properties [9]. On the other hand, multidentate pyridyl ligands can be used together with the multicarboxylates, to meet and satisfy the requirements for coordination geometries of metal ions in the assembly process [10–14]. ⁎ Corresponding author. E-mail address: [email protected] (D. Liu).
http://dx.doi.org/10.1016/j.inoche.2015.05.012 1387-7003/© 2015 Elsevier B.V. All rights reserved.
In view of the various multidentate pyridyl ligands, we found that the coordination chemistry of the conjugated dipyridyl ligand 4-pyr-poly-2-ene (ppene) has been rarely investigated so far [15]. In this paper, as a continuation of our previous work [15–17], we employed ppene as the N-containing linker to react with Cd(NO 3) 2 ·4H2 O and two different multicarboxylic acids 1,4benzenedicarboxylic acid (1,4-H2BDC) or 1,2,4-benzenetricarboxylic acid (1,2,4-H3BTC), respectively. The difference between 1,4-H2BDC and 1,2,4-H3BTC is that one of the four phenyl hydrogen atoms in 1,4H2BDC is replaced by a carboxylic group in 1,2,4-H3BTC. We anticipated that such a carboxylic group could affect the coordination modes of CdII ions and thus afford CPs with different frameworks. Herein, we report the syntheses, topological analyses and photoluminescence properties of two new CPs {[Cd(1,4-BDC)(ppene)]·CH3CN}n (1) and [Cd(H2O)(1,2,4-HBTC)(ppene)0.5]n (2). Solvothermal reaction between Cd(NO3)2·4H2O, ppene and 1,4H2BDC in a 1:1:1 molar ratio in H2O/MeCN (1:1 v/v) at 170 °C for 3 days gave rise to yellow blocks of 1. Then, a 1,4-H2BDC derivative, 1,2,4-H3 BTC was employed to react with Cd(NO 3 )2 ·4H2 O and ppene under the same conditions. Finally, crystals of 2 were obtained. Single-crystal X-ray diffraction analysis reveals that the structure of 1 is a 3D two-fold interpenetrating pcu network while 2 is an unprecedented 3D supramolecular framework based on a rare tcs net. For compounds 1 and 2, the measured PXRD patterns are closely matched with the simulated patterns generated from their single crystal data, indicating the phase purity of the as-synthesized products (Fig. S1).
2
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Compound 1 crystallizes in the triclinic space group Pī [18]. The asymmetric unit contains one CdII cation, two halves of 1,4-BDC anions, one ppene molecule and one solvent MeCN molecule. The Cd1 ion is six-coordinated to give a [CdO4N2] octahedral geometry. Four of the six coordination positions are filled with four carboxylate O atoms from three different 1,4-BDC anions. The remaining two sites are occupied by N atoms from two ppene molecules (Fig. S2a). The average Zn–O bond length (2.3478(3) Å) of 1 is shorter than that found in [Cd(bpe)(CBA)2]n (2.3679(19) Å, bpe = 1,2-bis(4-pyridyl)ethylene, HCBA = 4-chlorobenzoic acid) and the mean Zn–N bond length (2.3165(4) Å) of 1 is also slightly shorter than that of [Cd(bpe)(CBA)2]n (2.3209(19) Å) [16b]. In 1, each pair of Cd ions is binded by one pair of chelating bis-bidentate 1,4-BDC ligands and one pair of bridging bisbidentate 1,4-BDC ligands to form a dinuclear [Cd2(1,4-BDC)4] unit (Fig. S2b). Meanwhile, the adjacent [Cd2(1,4-BDC)4] units are linked with each other through 1,4-BDC ligands to give rise to a 2D (4,4) net of [Cd(1,4-BDC)]n (Fig. S2b). The 2D [Cd(1,4-BDC)]n nets are further interconnected by bidentate ppene ligands to generate a 3D framework of [Cd(1,4-BDC)(ppene)]n (Fig. 1a). From a topological point of view, each [Cd2(1,4-BDC)4] unit can be considered as a 6-connecting node. Therefore, this 3D framework can be simplified into a 41263-pcu network (Fig. 1b) [19]. Such a single 3D framework has 1D channels along the b axis, the window size of each channel is about 11.11 × 16.25 Å2. The channels of this framework are spacious enough to allow the other crystallographically equivalent net to penetrate to produce a two-fold interpenetrating network (Fig. 1b). The remaining void space of this interpenetrating network is further filled by solvent MeCN molecules. Compound 2 crystallizes in the triclinic space group Pī [18]. The asymmetric unit contains one CdII cation, one 1,2,4-HBTC anion, one coordinated H2O molecule and one half of ppene molecule. As shown in Fig. S3, the coordination mode of Cd center is obviously different from that of 1. The Cd1 ion is in a distorted pentagonal pyramidal coordination geometry, surrounded by an [NO6] donor set, generated by five carboxylate O atoms from four different 1,2,4-HBTC ligands, one O atom from coordinated H2O molecule and one N atom from ppene ligand. The Cd–N bond distance (2.348(5) Å) and Cd–O bond lengths (range from 2.260(5) to 2.523(5) Å) are comparable to those of [Cd2(dcpy)2(H2O)] (H2dcpy = 3-(2′,5′-dicarboxylphenyl)pyridine acid) [2d]. It is noted that the 1,2,4-HBTC ligand adopts only two carboxylate groups to coordinate with Cd ions, the other one carboxylic group in the 4-position of 1,2,4-HBTC ligand is still protonated. In 2, each Cd ion is coordinated with a H2O molecule and further connected by bridging bis-bidentate 1,2,4-HBTC ligands to generate a 2D (4,4) net of [Cd(H2O)(1,2,4-HBTC)]n extending in the ab plane (Fig. 2a). Then, each ppene ligand links one Cd ion in a [Cd(H2O)(1,2,4-HBTC)]n net to another Cd ion in a neighboring 2D net, forming a 3D framework of [Cd(H2O)(1,2,4-HBTC)(ppene)0.5]n (Fig. 2b). In such a 3D framework, each pair of adjacent 1,2,4-HBTC ligands is connected to each other through hydrogen bonding interactions (O6⋯O5 2.689(7) Å,
O6–H6A⋯O5 168.5°) between uncoordinated carboxylic groups (Fig. 2b). From the topological perspective, each 1,2,4-HBTC ligand can be considered as a 4-connecting node since each 1,2,4-HBTC ligand is linked with four Cd ions, and each Cd center can be considered as a 5-connecting node because each Cd ion is connected by four 1,2,4-HBTC ligands and one ppene ligand. Therefore, the coordination network of 2 can be simplified as a 3D (4,5)-connected net with rare (4462)(4466) tcs topology (Fig. 2c) [19]. Taking the O6–H6A⋯O5 hydrogen bonding interactions into account, the whole supramolecular framework can further be regarded as a (5,5)-connected net with an unprecedented Schläfli symbol (4862)(4664) (Fig. 2d). Thermogravimetric analysis (TGA) of complex 1 (Fig. S4) exhibits a weight loss of 8.19% from room temperature to 153 °C, corresponding to the loss of one CH3CN molecule per formula unit (calcd. 7.81%). 1 is stable up to about 316 °C and then its framework starts to collapse after that temperature. The weight of the final residue (24.68%) is assumed to be CdO (calcd. 24.42%). For compound 2, a slight weight loss of 4.39% in the region 183–245 °C, which is in good agreement with the removal of one coordinated H2O molecule per formula unit (calcd. 4.07%). The decomposition of the organic components occurs at 307 °C, and the remaining weight of the CdO residue is 29.36% (calcd. 29.01%). The photoluminescence properties of CPs with d10 metal ions and conjugated ligands have attracted intense interest because they are potential candidates for applicable photoluminescent materials such as light-emitting diodes (LEDs), chemical sensors, and electroluminescent display [20]. Thus, the solid-state photoluminescence properties of complexes 1–2 as well as the free ppene ligand were investigated at room temperature (Fig. 3). Free ppene shows emission with a maximum at 549 nm upon excitation at 430 nm in solid state. Compounds 1–2 show emission peaks at 540 nm (λex = 426 nm) and 542 nm (λex = 426 nm), which are close to that of the free ppene ligand. The enhancement of photoluminescence in 1 and 2 may be attributed to ligand chelation to the Cd center which effectively increases the rigidity of the ligand and reduces the loss of energy [21]. The luminescence quantum yield at room temperature is 9.1% with the lifetime of 2.32 ns for compound 1. For 2, the luminescence efficiency is 7.9% and the emission decay lifetime is 2.21 ns. According to the literature, the Cd ion is difficult to be oxidized or reduced owing to its d10 configuration [21]. As a result, the emissions of these CPs are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT). Therefore, they may be assigned to a mixture characteristic of intraligand and ligand to ligand charge transition (LLCT), as reported for other CPs constructed from mixed N-donor and O-donor ligands [21]. The polycarboxylic acids H2BDC (λem = 408 nm, λex = 329 nm) and 1,2,4-H3BTC (λem = 425 nm, λex = 280 nm) can also exhibit fluorescence at room temperature [21c,d]. The emission bands of H2BDC and 1,2,4-H3BTC ligands can be assigned to the π*–n transition as previously reported [21c,d]. In comparison with the free ppene ligand, blue-shifts
Fig. 1. (a) The 3D framework of 1. The cyan, red, blue, gray and green balls represent cadmium, oxygen, nitrogen, carbon and hydrogen atoms, respectively. (b) The topological net of 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. (a) View of the 2D [Cd(H2O)(1,2,4-HBTC)]n net in 2. (b) The 3D framework of 2. The pink dashed lines represent the hydrogen bonding interactions between carboxylic groups. All hydrogen atoms except those from H2O molecules and carboxylic groups have been omitted for clarity. The cyan, red, blue, gray and green balls represent cadmium, oxygen, nitrogen, carbon and hydrogen atoms, respectively. (c) The topological net for the coordination network of 2. (d) The topological net for the supramolecular network of 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of about 9 nm and 7 nm are found for 1 and 2, respectively. This may be attributed to the influence of π*–n transition of polycarboxylate ligands in 1 and 2 [21]. In summary, two new CPs have been constructed under solvothermal conditions by using of ppene with CdII ions and different polycarboxylic acids, respectively. The crystal structure of 1 possesses a 3D two-fold interpenetrating pcu network with the 41263 topology. Compound 2 represents a 3D (5,5)-connected supramolecular framework with an unprecedented Schläfli symbol (4862)(4664) based on a rare tcs coordination network. As the other reaction conditions are the same, the structural difference between 1 and 2 indicates that the polycarboxylic acids used in two reaction systems are the key factors in the formation of corresponding architectures. It seems that the only difference between the two multicarboxylic acids is that one phenyl H atom in 1,4-H2BDC is replaced by one carboxylic group in 1,2,4-H3BTC. However, 1,2,4-H3BTC can be considered as a derivative of either 1,4-H2BDC, 1,3-benzenedicarboxylic acid (1,3-H2BDC) or 1,2-benzenedicarboxylic acid (1,2-H2BDC). Therefore,
the 1,2,4-H3BTC ligands may exist various coordination modes to occupy the coordination sites of metal centers. Actually, the carboxylate ligand in compound 2 can be regarded as a derivative of 1,2-H2BDC because two carboxylate groups in the 1- and 2- positions of 1,2,4HBTC are connected with Cd ions while the other carboxylic group in the 4- position is still protonated. Each 1,2,4-HBTC ligand links its neighboring symmetry-related one through hydrogen bonding interactions between protonated carboxylic groups. Compared with 1,4H2BDC, the 1,2,4-H3BTC ligands not only modify the coordination environment of the Cd ions by extending or blocking the polymerization but also dramatically affect the crystal packing of 2 through hydrogen bonding interactions. As a result, two CPs with obviously different architectures were afforded. This work demonstrates a valuable approach for the construction of novel coordination frameworks through tuning the substituent groups of ligands. It is expected that more and more CPs with interesting structures, topologies and photoluminescence properties will be obtained by this method. Studies on this respect are under way in our laboratory.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 21201068) and the Natural Science Foundation of Anhui Province (Grant Nos. 1208085QB26).
Appendix A. Supplementary data
Fig. 3. Solid state emission spectra of 1–2 and ppene ligand at ambient temperature. Inset: solid state excitation spectra of 1–2 and ppene ligand.
Additional Table, Experimental details, Figures, PXRD patterns for 1–2 and TGA curves for 1–2 in PDF format. CCDC 1038884 and 1038886 contain the supplementary crystallographic data for 1 and 2. The data can be obtained free of charge via http://www.ccdc.cam.ac. uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.inoche.2015.05.012
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Effect of substituent carboxylic group on the self-assembly of cadmium coordination polymers: Syntheses, topological analyses and photoluminescence properties Ni-Ya Li, Wei Ma, Shu-Jun Wang, Dong Liu ⁎ College of Chemistry and Materials Science, Huaibei Normal University, Huaibei 235000, Anhui, PR China
a r t i c l e
i n f o
Article history: Received 18 January 2015 Received in revised form 5 May 2015 Accepted 14 May 2015 Available online 16 May 2015 Keywords: Coordination polymers Carboxylic group Syntheses Topological analyses Photoluminescence properties
a b s t r a c t Two coordination polymers, formulated as {[Cd(1,4-BDC)(ppene)]·CH3CN}n (1) and [Cd(H2O)(1,2,4HBTC)(ppene)0.5]n (2) have been synthesized through the solvothermal reactions between Cd(NO3)2·4H2O and 4-pyr-poly-2-ene (ppene) in the presence of 1,4-benzenedicarboxylic acid (1,4-H2BDC) or its derivative 1,2,4-benzenetricarboxylic acid (1,2,4-H3BTC). These two compounds were structurally characterized by infrared spectroscopy, elemental analysis, single crystal X-ray diffraction and powder X-ray diffraction. Compound 1 exhibits a three-dimensional two-fold interpenetrating 41263-pcu network. The coordination network of 2 shows a three-dimensional (4,5)-connected framework with rare (4462)(4466) tcs topology. Owing to the linkage of hydrogen bonding interactions between protonated carboxylic groups, 2 further displays a three-dimensional (5,5)-connected supramolecular network with an unprecedented Schläfli symbol (4862)(4664). Thermal stability and photoluminescence properties of 1 and 2 were also investigated. © 2015 Elsevier B.V. All rights reserved.
Self-assembled combinations of metal ions and organic ligands have afforded a wide range of coordination polymers (CPs) and attracted significant attention from chemists because of their interesting structural topologies and extensive potential applications as functional materials in many areas, such as photoluminescence, magnetism, catalysis, nonlinear optics, chemical sensors, gas storage and so on [1–16]. The aim of CP research is to control the architectures of target products and explore the relationships between structures and properties [1,2]. However, controllable synthesis of predictable CPs is still a great challenge since the construction of coordination frameworks are sensitive to many reaction conditions, such as metal ions, ligands, concentrations, counterions, templates, solvents, temperatures, pH values and so on [3–8]. It has been found that the selection of organic linkers is extremely important for the preparation of CPs because the structures and properties of these compounds are usually influenced by the shape, length, flexibility, symmetry and substituent groups of ligands [9–14]. Among the multitude of organic ligands, multidentate carboxylates exist in a wide variety of coordination modes and geometries, which is helpful for the formation of novel CPs with varied structural topologies and properties [9]. On the other hand, multidentate pyridyl ligands can be used together with the multicarboxylates, to meet and satisfy the requirements for coordination geometries of metal ions in the assembly process [10–14]. ⁎ Corresponding author. E-mail address: [email protected] (D. Liu).
http://dx.doi.org/10.1016/j.inoche.2015.05.012 1387-7003/© 2015 Elsevier B.V. All rights reserved.
In view of the various multidentate pyridyl ligands, we found that the coordination chemistry of the conjugated dipyridyl ligand 4-pyr-poly-2-ene (ppene) has been rarely investigated so far [15]. In this paper, as a continuation of our previous work [15–17], we employed ppene as the N-containing linker to react with Cd(NO 3) 2 ·4H2 O and two different multicarboxylic acids 1,4benzenedicarboxylic acid (1,4-H2BDC) or 1,2,4-benzenetricarboxylic acid (1,2,4-H3BTC), respectively. The difference between 1,4-H2BDC and 1,2,4-H3BTC is that one of the four phenyl hydrogen atoms in 1,4H2BDC is replaced by a carboxylic group in 1,2,4-H3BTC. We anticipated that such a carboxylic group could affect the coordination modes of CdII ions and thus afford CPs with different frameworks. Herein, we report the syntheses, topological analyses and photoluminescence properties of two new CPs {[Cd(1,4-BDC)(ppene)]·CH3CN}n (1) and [Cd(H2O)(1,2,4-HBTC)(ppene)0.5]n (2). Solvothermal reaction between Cd(NO3)2·4H2O, ppene and 1,4H2BDC in a 1:1:1 molar ratio in H2O/MeCN (1:1 v/v) at 170 °C for 3 days gave rise to yellow blocks of 1. Then, a 1,4-H2BDC derivative, 1,2,4-H3 BTC was employed to react with Cd(NO 3 )2 ·4H2 O and ppene under the same conditions. Finally, crystals of 2 were obtained. Single-crystal X-ray diffraction analysis reveals that the structure of 1 is a 3D two-fold interpenetrating pcu network while 2 is an unprecedented 3D supramolecular framework based on a rare tcs net. For compounds 1 and 2, the measured PXRD patterns are closely matched with the simulated patterns generated from their single crystal data, indicating the phase purity of the as-synthesized products (Fig. S1).
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Compound 1 crystallizes in the triclinic space group Pī [18]. The asymmetric unit contains one CdII cation, two halves of 1,4-BDC anions, one ppene molecule and one solvent MeCN molecule. The Cd1 ion is six-coordinated to give a [CdO4N2] octahedral geometry. Four of the six coordination positions are filled with four carboxylate O atoms from three different 1,4-BDC anions. The remaining two sites are occupied by N atoms from two ppene molecules (Fig. S2a). The average Zn–O bond length (2.3478(3) Å) of 1 is shorter than that found in [Cd(bpe)(CBA)2]n (2.3679(19) Å, bpe = 1,2-bis(4-pyridyl)ethylene, HCBA = 4-chlorobenzoic acid) and the mean Zn–N bond length (2.3165(4) Å) of 1 is also slightly shorter than that of [Cd(bpe)(CBA)2]n (2.3209(19) Å) [16b]. In 1, each pair of Cd ions is binded by one pair of chelating bis-bidentate 1,4-BDC ligands and one pair of bridging bisbidentate 1,4-BDC ligands to form a dinuclear [Cd2(1,4-BDC)4] unit (Fig. S2b). Meanwhile, the adjacent [Cd2(1,4-BDC)4] units are linked with each other through 1,4-BDC ligands to give rise to a 2D (4,4) net of [Cd(1,4-BDC)]n (Fig. S2b). The 2D [Cd(1,4-BDC)]n nets are further interconnected by bidentate ppene ligands to generate a 3D framework of [Cd(1,4-BDC)(ppene)]n (Fig. 1a). From a topological point of view, each [Cd2(1,4-BDC)4] unit can be considered as a 6-connecting node. Therefore, this 3D framework can be simplified into a 41263-pcu network (Fig. 1b) [19]. Such a single 3D framework has 1D channels along the b axis, the window size of each channel is about 11.11 × 16.25 Å2. The channels of this framework are spacious enough to allow the other crystallographically equivalent net to penetrate to produce a two-fold interpenetrating network (Fig. 1b). The remaining void space of this interpenetrating network is further filled by solvent MeCN molecules. Compound 2 crystallizes in the triclinic space group Pī [18]. The asymmetric unit contains one CdII cation, one 1,2,4-HBTC anion, one coordinated H2O molecule and one half of ppene molecule. As shown in Fig. S3, the coordination mode of Cd center is obviously different from that of 1. The Cd1 ion is in a distorted pentagonal pyramidal coordination geometry, surrounded by an [NO6] donor set, generated by five carboxylate O atoms from four different 1,2,4-HBTC ligands, one O atom from coordinated H2O molecule and one N atom from ppene ligand. The Cd–N bond distance (2.348(5) Å) and Cd–O bond lengths (range from 2.260(5) to 2.523(5) Å) are comparable to those of [Cd2(dcpy)2(H2O)] (H2dcpy = 3-(2′,5′-dicarboxylphenyl)pyridine acid) [2d]. It is noted that the 1,2,4-HBTC ligand adopts only two carboxylate groups to coordinate with Cd ions, the other one carboxylic group in the 4-position of 1,2,4-HBTC ligand is still protonated. In 2, each Cd ion is coordinated with a H2O molecule and further connected by bridging bis-bidentate 1,2,4-HBTC ligands to generate a 2D (4,4) net of [Cd(H2O)(1,2,4-HBTC)]n extending in the ab plane (Fig. 2a). Then, each ppene ligand links one Cd ion in a [Cd(H2O)(1,2,4-HBTC)]n net to another Cd ion in a neighboring 2D net, forming a 3D framework of [Cd(H2O)(1,2,4-HBTC)(ppene)0.5]n (Fig. 2b). In such a 3D framework, each pair of adjacent 1,2,4-HBTC ligands is connected to each other through hydrogen bonding interactions (O6⋯O5 2.689(7) Å,
O6–H6A⋯O5 168.5°) between uncoordinated carboxylic groups (Fig. 2b). From the topological perspective, each 1,2,4-HBTC ligand can be considered as a 4-connecting node since each 1,2,4-HBTC ligand is linked with four Cd ions, and each Cd center can be considered as a 5-connecting node because each Cd ion is connected by four 1,2,4-HBTC ligands and one ppene ligand. Therefore, the coordination network of 2 can be simplified as a 3D (4,5)-connected net with rare (4462)(4466) tcs topology (Fig. 2c) [19]. Taking the O6–H6A⋯O5 hydrogen bonding interactions into account, the whole supramolecular framework can further be regarded as a (5,5)-connected net with an unprecedented Schläfli symbol (4862)(4664) (Fig. 2d). Thermogravimetric analysis (TGA) of complex 1 (Fig. S4) exhibits a weight loss of 8.19% from room temperature to 153 °C, corresponding to the loss of one CH3CN molecule per formula unit (calcd. 7.81%). 1 is stable up to about 316 °C and then its framework starts to collapse after that temperature. The weight of the final residue (24.68%) is assumed to be CdO (calcd. 24.42%). For compound 2, a slight weight loss of 4.39% in the region 183–245 °C, which is in good agreement with the removal of one coordinated H2O molecule per formula unit (calcd. 4.07%). The decomposition of the organic components occurs at 307 °C, and the remaining weight of the CdO residue is 29.36% (calcd. 29.01%). The photoluminescence properties of CPs with d10 metal ions and conjugated ligands have attracted intense interest because they are potential candidates for applicable photoluminescent materials such as light-emitting diodes (LEDs), chemical sensors, and electroluminescent display [20]. Thus, the solid-state photoluminescence properties of complexes 1–2 as well as the free ppene ligand were investigated at room temperature (Fig. 3). Free ppene shows emission with a maximum at 549 nm upon excitation at 430 nm in solid state. Compounds 1–2 show emission peaks at 540 nm (λex = 426 nm) and 542 nm (λex = 426 nm), which are close to that of the free ppene ligand. The enhancement of photoluminescence in 1 and 2 may be attributed to ligand chelation to the Cd center which effectively increases the rigidity of the ligand and reduces the loss of energy [21]. The luminescence quantum yield at room temperature is 9.1% with the lifetime of 2.32 ns for compound 1. For 2, the luminescence efficiency is 7.9% and the emission decay lifetime is 2.21 ns. According to the literature, the Cd ion is difficult to be oxidized or reduced owing to its d10 configuration [21]. As a result, the emissions of these CPs are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT). Therefore, they may be assigned to a mixture characteristic of intraligand and ligand to ligand charge transition (LLCT), as reported for other CPs constructed from mixed N-donor and O-donor ligands [21]. The polycarboxylic acids H2BDC (λem = 408 nm, λex = 329 nm) and 1,2,4-H3BTC (λem = 425 nm, λex = 280 nm) can also exhibit fluorescence at room temperature [21c,d]. The emission bands of H2BDC and 1,2,4-H3BTC ligands can be assigned to the π*–n transition as previously reported [21c,d]. In comparison with the free ppene ligand, blue-shifts
Fig. 1. (a) The 3D framework of 1. The cyan, red, blue, gray and green balls represent cadmium, oxygen, nitrogen, carbon and hydrogen atoms, respectively. (b) The topological net of 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. (a) View of the 2D [Cd(H2O)(1,2,4-HBTC)]n net in 2. (b) The 3D framework of 2. The pink dashed lines represent the hydrogen bonding interactions between carboxylic groups. All hydrogen atoms except those from H2O molecules and carboxylic groups have been omitted for clarity. The cyan, red, blue, gray and green balls represent cadmium, oxygen, nitrogen, carbon and hydrogen atoms, respectively. (c) The topological net for the coordination network of 2. (d) The topological net for the supramolecular network of 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of about 9 nm and 7 nm are found for 1 and 2, respectively. This may be attributed to the influence of π*–n transition of polycarboxylate ligands in 1 and 2 [21]. In summary, two new CPs have been constructed under solvothermal conditions by using of ppene with CdII ions and different polycarboxylic acids, respectively. The crystal structure of 1 possesses a 3D two-fold interpenetrating pcu network with the 41263 topology. Compound 2 represents a 3D (5,5)-connected supramolecular framework with an unprecedented Schläfli symbol (4862)(4664) based on a rare tcs coordination network. As the other reaction conditions are the same, the structural difference between 1 and 2 indicates that the polycarboxylic acids used in two reaction systems are the key factors in the formation of corresponding architectures. It seems that the only difference between the two multicarboxylic acids is that one phenyl H atom in 1,4-H2BDC is replaced by one carboxylic group in 1,2,4-H3BTC. However, 1,2,4-H3BTC can be considered as a derivative of either 1,4-H2BDC, 1,3-benzenedicarboxylic acid (1,3-H2BDC) or 1,2-benzenedicarboxylic acid (1,2-H2BDC). Therefore,
the 1,2,4-H3BTC ligands may exist various coordination modes to occupy the coordination sites of metal centers. Actually, the carboxylate ligand in compound 2 can be regarded as a derivative of 1,2-H2BDC because two carboxylate groups in the 1- and 2- positions of 1,2,4HBTC are connected with Cd ions while the other carboxylic group in the 4- position is still protonated. Each 1,2,4-HBTC ligand links its neighboring symmetry-related one through hydrogen bonding interactions between protonated carboxylic groups. Compared with 1,4H2BDC, the 1,2,4-H3BTC ligands not only modify the coordination environment of the Cd ions by extending or blocking the polymerization but also dramatically affect the crystal packing of 2 through hydrogen bonding interactions. As a result, two CPs with obviously different architectures were afforded. This work demonstrates a valuable approach for the construction of novel coordination frameworks through tuning the substituent groups of ligands. It is expected that more and more CPs with interesting structures, topologies and photoluminescence properties will be obtained by this method. Studies on this respect are under way in our laboratory.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 21201068) and the Natural Science Foundation of Anhui Province (Grant Nos. 1208085QB26).
Appendix A. Supplementary data
Fig. 3. Solid state emission spectra of 1–2 and ppene ligand at ambient temperature. Inset: solid state excitation spectra of 1–2 and ppene ligand.
Additional Table, Experimental details, Figures, PXRD patterns for 1–2 and TGA curves for 1–2 in PDF format. CCDC 1038884 and 1038886 contain the supplementary crystallographic data for 1 and 2. The data can be obtained free of charge via http://www.ccdc.cam.ac. uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.inoche.2015.05.012
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