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A novel porous material constructed from linear helical cadmium(II) coordination polymer based on in-situ ligand reaction
Inorganic Chemistry Communications 35 (2013) 42–44
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A novel porous material constructed from linear helical cadmium(II) coordination polymer based on in-situ ligand reaction Jing Xiang ⁎, Ya Luo, Lu-Lu Fu, Tian-Tian Yang College of Chemistry and Environmental Engineering, Yangtze University, JingZhou 434020, HuBei, PR China
a r t i c l e
i n f o
Article history: Received 16 March 2013 Accepted 24 April 2013 Available online 2 May 2013 Keywords: Helix Porous structure In-situ ligand reaction Luminescence
a b s t r a c t A novel helical cadmium(II) coordination polymer cis,cis,trans-{[Cd(HIDC)(H2O)2]·1/3H2O}n (1) has been obtained by the solvothermal reaction of hydrated CdCl2 with 2H-imidazole-4,5-dicarbonitrile (HIMDN) in the presence of NH3·H2O. The packing of the six neighboring polymeric helical chains of the complex leads to a 1-D channel, which is occupied by lattice water molecules. The solid-state luminescence properties of 1 have been investigated at room temperature. © 2013 Elsevier B.V. All rights reserved.
In the past decade, the design and synthesis of metal–organic frameworks (MOFs) are of great interest because of their various intriguing architectures and their potential applications in catalysis, magnetism, gas adsorption, and luminescence [1]. However, it is not yet possible to prepare fully predictable MOFs by rational design and synthesis, because many factors including pH value of reaction solution, polarity of solvents, reaction pressure, temperature, the functionality and flexibility/rigidity of ligands, and the coordination preference of metal ions must be carefully considered. The in-situ ligand reaction in the presence of metal ion, especially under hydro/ solvothermal condition, is an uncontrolled method to get the MOFs; however, it was proved to be an effective and powerful technique for forming extended coordination polymers [2]. A large number of unexpected coordination polymers bearing interesting structure architecture or functions have been obtained by means of in-situ ligand reaction. For example, Xiong and the other groups have synthesized a series of MOFs based on tetrazole ligands in-situ generated from the CN-containing precursor [3]. Lin et al. reported a series of acentric MOFs of metalcarboxylate based on in-situ ligand reaction [4]. Hetero-ring carboxylate ligand, imidazole-4,5-dicarboxylic acid (H3IDC) is an excellent candidate for assembling new polymeric networks owing to the flexible coordination modes and ability to form H-bonding interaction [5]. In particular, H3IDC can be partially or fully deprotonated to give H2IDC −, HIDC 2 − and IDC 3− species at different pH values. A series of coordination compounds bearing various interesting properties on the basis of this ligand have been constructed. Even so, it is interesting to note that Sun has previously reported a novel complex [H2N(CH3)2]4[Zn8O(IDC)6] with {Zn8O13} as ⁎ Corresponding author. Fax: +86 716 8060650. E-mail address: [email protected] (J. Xiang). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.04.033
secondary building blocks (SBUs), where the ligand IDC 3− is in-situ formed from the hydrolysis of HIMDN (Scheme S1). It is interesting to note that the compound could not be regenerated from direct reaction of H3IDC with corresponding hydrated salts. This result strongly indicates that the hydrolysis of nitrile may be activated by Zn ion and the obtained product is highly dependent on the in-situ ligand reaction [6]. It inspires us to further extend the in-situ ligand reaction. Herein, the HIMDN was also employed as a starting material. The reaction of HIMDN with hydrated CdCl2 in basic water medium afforded a novel 1-D helical complex cis,cis,trans-{[Cd(HIDC)(H2O)2] 1/3H2O}n (1). Compounds with helicity are of great interest, because helices are ubiquitous in nature and are essential to various biological functions and are also the central structural motifs in biological molecules [7]. Solvothermal reaction of HIMDN, hydrated CdCl2 in the presence of NH3·H2O led to the formation of colorless block crystals of cis,cis, trans-{[Cd(HIDC)(H2O)2]·1/3H2O}n (1). In the IR spectrum of 1 the strong peaks at 1555 and 1482 cm−1 that are assigned to the asymmetric and symmetric stretching bands of ν(COO) group (Δν = 73 cm−1), respectively support the formation of the resulted hydrolytic product. It is well known that the hydrolysis of organonitrile could be promoted by the addition of base in the reaction system. The presence of NH3·H2O is crucial for this reaction, because the reactions occurring in neutral or in the acidic environment do not give 1. On the other hand, direct reaction of H3IDC with hydrated CdCl2 in the similar reaction condition has been conducted. Compound 1 could not be regenerated by this method, but it gives another reported compound {[Cd3(IDC)2(H2O)2]·H2O}n [8]. Moreover, some cadmium complexes bearing H3IDC ligand have been reported [9]. However, their structures are significantly different from 1. The diversity strongly indicates that the in-situ ligand reaction is crucial for the formation of 1.
J. Xiang et al. / Inorganic Chemistry Communications 35 (2013) 42–44
Compound 1 crystallizes in trigonal space group R-3c and the structural fragment exhibiting the coordination environment of Cd(II) is shown in Fig. 1a. The structure refinement and selected bond parameters of this compound are summarized in Tables S1 and S2, respectively. There are two C2 symmetry axes. One pass through the C1 atom and midpoint of C2 and C2 i atoms of the HIDC 2− ligand and the other pass through the Cd(II) atom. Thus, the asymmetric unit of 1 consists of half occupancy Cd(II), half of HIDC 2− ligand, an aqua ligand, and 1/6 lattice water molecule. The main framework is isomeric to the compound trans,trans, trans-[Cd(HIDC)(H2O)2]n [9]; however, it is very interesting to note that subtle alterations of coordination environments around the metal centers result in two thoroughly different extended structures. The Cd(II) center in 1 is octahedrally coordinated by four O donors and two N donors from two symmetry-related HIDC2− ligands and two aqua ligands. The N2O2 four donors from two HIDC2− ligands occupy the equatorial positions and two aqua ligands occupy in the axial positions of the coordination geometry. The trans-configuration of two aqua ligands is similar to that in trans,trans,trans-[Cd(HIDC)(H2O)2]n. However, the octahedral geometry is seriously distorted, since there is a very acute trans-O1\Cd\O1 i (symmetry code: (i) x − y + 1/3, −y + 2/3, −z + 1/6) bond angle with a value of 102.78°, which is far from the idealized linear angle (180°) and also much smaller than that in trans,trans,trans-[Cd(HIDC)(H2O)2]n (153.52(11)°) [9]. The strong H-bonding interaction between two carboxylate groups is found, which is indicated by the shorter O3–O3 ii separation of 2.665 Å (symmetry code: (ii) y + 2/3, x − 2/3, −z − 1/6). The two carboxylate groups are well coplanar with the attached imidazole ring and form a very small dihedral angle with a value of 2.4°. The Cd1\N1 bond length is 2.253(4) Å. The Cd1\O2 bond length is 2.549(3) Å, which is much longer than that of Cd1\O1 (2.268(4) Å) and the normal Cd\O bond distances [9–11]. Such a longer Cd\O2 bond length results in the obvious distortion of coordination geometry. The dihedral angle of two coordinated HIDC2− ligands is 39.7°. The bite angle of N1\Cd\O2 is 68.28(12)°, with the Cd\N\C\C\O chelating ring being perfectly co-planar with the imidazole ring. The ligand HIDC2− actually acts as a bis(bidentate) ligand bridging two distinct Cd(II) ions with a Cd⋯Cd distance of 6.627 Å, as shown in Fig. 1b. Such a bridging behavior leads to an infinite helical chain running along c axis. A pair of 1:1 left- and right-handed helices exists in the complex. The helix is generated around the crystallographic 31 axis. Each set of three crystallographically equivalent cadmium(II) centers constitutes a single revolution of the helix with a distance of 7.078 Å (Fig. 2a). Each helical chain aligns in a parallel fashion and further interlinks with six adjacent helices to give a periodically ordered 3D framework. The inter-chain O\H⋯O hydrogen bond contacts between coordinated aqua O\H bonds and the oxygen atoms of carboxylate groups are found (Fig. S2), as they are indicated by the shorter O⋯O
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separations (Table S3). Interestingly, the remarkable feature of compound 1 is the presence of the hexagonal channels with side length approximately 6.622 Å along the polymer axis (Fig. 2b). It is interesting to note that the packing of six neighboring 1-D helical chains could result in the 1-D channel. There are two previous examples of 1-D Ni(II) zigzag chain [12,13], in which the 1-D open channels are formed by the packing of the polymeric chains or within 1-D polymer coils. Recently, Suh has also reported a novel 1-D chain, where the 1-D channels are formed by the packing of chains in cross-linked way [14]. However, the 1-D channels that are formed by packing of the six neighboring polymeric helical chains are never reported. The trans-coordinated aqua ligands direct into the channels, which construct a hydrophilic environment for the guest molecules. The PLATON [15] program reveals that the voids in complex 1 occupy 8.5% of the crystal volume (Fig. S3). Thermal gravimetric analysis (TGA) was carried out in N2 condition from room temperature to 600 °C for examining the properties of dehydration and stability of 1 (Fig. S4). The first stage of weight loss is 2.10% in the temperature range 50–149 °C, corresponding to the loss of 1/3 lattice water molecule per empirical formula (calc. 1.98%). The second weight loss of 11.23% occurs from 152 to 245 °C corresponding to the loss of two coordinated aqua ligand per empirical formula (calc. 11.66%). Finally a sharp weight loss occurred above 324 °C due to the decomposition of framework structure. PXRD experiment was carried out for compound 1. The PXRD experimental and computer-simulated patterns of the compound 1 are shown in Fig. S5, and they show that the bulk synthesized materials and the measured single crystals of 1 are the same. The photoluminescence of complex 1 was studied in the solid state at room temperature and the measurement of emission spectrum was excited at a wavelength of λex = 305 nm. An intense emission band was observed at λem = 418 nm in the blue region (Fig. S6). As we know, it is usually believed that the energy transition of d10 compounds can be assigned to metal-to-ligand charge transfer, intra-ligand emission (π → π*), and ligand-to-metal charge transfer. The studies on the luminescence properties of free ligand H3IDC show that no obvious emission band was found upon excitation in the range of 180– 480 nm [16,17]. Thus, the possibility of ligand-centered fluorescence could be eliminated. The photo-luminescence of 1 is tentatively attributed to ligand-to-metal charge transfer (LMCT) [18]. In conclusion, a novel linear helical cadmium(II) coordination compound, cis,cis,trans-{[Cd(HIDC)(H2O)2] 1/3H2O}n (1), has been obtained from the hydrothermal reaction of hydrated CdCl2 with HIMDN in the presence of NH3·H2O. The ligand HIDC 2− is in-situ formed from the hydrolysis of the cyano groups of HIMDN. By comparison with the direct reaction of HIMDN with hydrated CdCl2 in similar condition, it showed that the obtained product is highly
Fig. 1. (a) The structural fragment of 1 showing the coordination geometry of cadmium(II) center (Symmetry code: (i) x − y + 1/3, −y + 2/3, −z + 1/6); (b) a single revolution of the helix coil constructed by three crystallographically equivalent cadmium(II) centers.
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J. Xiang et al. / Inorganic Chemistry Communications 35 (2013) 42–44
Fig. 2. (a) The space-filling models showing a pair of 1:1 left- and right-handed helices; (b) The space-filling model showing 1-D channel along the c axis.
dependent on the in-situ ligand reaction, which may be associated with the transition state in that metal ion activates hydrolysis of organonitrile. Our experiment demonstrates again that in-situ reaction may represent a promising strategy for obtaining new multi-functional materials. Acknowledgment The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (21201023) and thank the scientific research fund of HuBei Provincial Education Department (D20131202). Appendix A. Supplementary material Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.inoche.2013.04.033.
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Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A novel porous material constructed from linear helical cadmium(II) coordination polymer based on in-situ ligand reaction Jing Xiang ⁎, Ya Luo, Lu-Lu Fu, Tian-Tian Yang College of Chemistry and Environmental Engineering, Yangtze University, JingZhou 434020, HuBei, PR China
a r t i c l e
i n f o
Article history: Received 16 March 2013 Accepted 24 April 2013 Available online 2 May 2013 Keywords: Helix Porous structure In-situ ligand reaction Luminescence
a b s t r a c t A novel helical cadmium(II) coordination polymer cis,cis,trans-{[Cd(HIDC)(H2O)2]·1/3H2O}n (1) has been obtained by the solvothermal reaction of hydrated CdCl2 with 2H-imidazole-4,5-dicarbonitrile (HIMDN) in the presence of NH3·H2O. The packing of the six neighboring polymeric helical chains of the complex leads to a 1-D channel, which is occupied by lattice water molecules. The solid-state luminescence properties of 1 have been investigated at room temperature. © 2013 Elsevier B.V. All rights reserved.
In the past decade, the design and synthesis of metal–organic frameworks (MOFs) are of great interest because of their various intriguing architectures and their potential applications in catalysis, magnetism, gas adsorption, and luminescence [1]. However, it is not yet possible to prepare fully predictable MOFs by rational design and synthesis, because many factors including pH value of reaction solution, polarity of solvents, reaction pressure, temperature, the functionality and flexibility/rigidity of ligands, and the coordination preference of metal ions must be carefully considered. The in-situ ligand reaction in the presence of metal ion, especially under hydro/ solvothermal condition, is an uncontrolled method to get the MOFs; however, it was proved to be an effective and powerful technique for forming extended coordination polymers [2]. A large number of unexpected coordination polymers bearing interesting structure architecture or functions have been obtained by means of in-situ ligand reaction. For example, Xiong and the other groups have synthesized a series of MOFs based on tetrazole ligands in-situ generated from the CN-containing precursor [3]. Lin et al. reported a series of acentric MOFs of metalcarboxylate based on in-situ ligand reaction [4]. Hetero-ring carboxylate ligand, imidazole-4,5-dicarboxylic acid (H3IDC) is an excellent candidate for assembling new polymeric networks owing to the flexible coordination modes and ability to form H-bonding interaction [5]. In particular, H3IDC can be partially or fully deprotonated to give H2IDC −, HIDC 2 − and IDC 3− species at different pH values. A series of coordination compounds bearing various interesting properties on the basis of this ligand have been constructed. Even so, it is interesting to note that Sun has previously reported a novel complex [H2N(CH3)2]4[Zn8O(IDC)6] with {Zn8O13} as ⁎ Corresponding author. Fax: +86 716 8060650. E-mail address: [email protected] (J. Xiang). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.04.033
secondary building blocks (SBUs), where the ligand IDC 3− is in-situ formed from the hydrolysis of HIMDN (Scheme S1). It is interesting to note that the compound could not be regenerated from direct reaction of H3IDC with corresponding hydrated salts. This result strongly indicates that the hydrolysis of nitrile may be activated by Zn ion and the obtained product is highly dependent on the in-situ ligand reaction [6]. It inspires us to further extend the in-situ ligand reaction. Herein, the HIMDN was also employed as a starting material. The reaction of HIMDN with hydrated CdCl2 in basic water medium afforded a novel 1-D helical complex cis,cis,trans-{[Cd(HIDC)(H2O)2] 1/3H2O}n (1). Compounds with helicity are of great interest, because helices are ubiquitous in nature and are essential to various biological functions and are also the central structural motifs in biological molecules [7]. Solvothermal reaction of HIMDN, hydrated CdCl2 in the presence of NH3·H2O led to the formation of colorless block crystals of cis,cis, trans-{[Cd(HIDC)(H2O)2]·1/3H2O}n (1). In the IR spectrum of 1 the strong peaks at 1555 and 1482 cm−1 that are assigned to the asymmetric and symmetric stretching bands of ν(COO) group (Δν = 73 cm−1), respectively support the formation of the resulted hydrolytic product. It is well known that the hydrolysis of organonitrile could be promoted by the addition of base in the reaction system. The presence of NH3·H2O is crucial for this reaction, because the reactions occurring in neutral or in the acidic environment do not give 1. On the other hand, direct reaction of H3IDC with hydrated CdCl2 in the similar reaction condition has been conducted. Compound 1 could not be regenerated by this method, but it gives another reported compound {[Cd3(IDC)2(H2O)2]·H2O}n [8]. Moreover, some cadmium complexes bearing H3IDC ligand have been reported [9]. However, their structures are significantly different from 1. The diversity strongly indicates that the in-situ ligand reaction is crucial for the formation of 1.
J. Xiang et al. / Inorganic Chemistry Communications 35 (2013) 42–44
Compound 1 crystallizes in trigonal space group R-3c and the structural fragment exhibiting the coordination environment of Cd(II) is shown in Fig. 1a. The structure refinement and selected bond parameters of this compound are summarized in Tables S1 and S2, respectively. There are two C2 symmetry axes. One pass through the C1 atom and midpoint of C2 and C2 i atoms of the HIDC 2− ligand and the other pass through the Cd(II) atom. Thus, the asymmetric unit of 1 consists of half occupancy Cd(II), half of HIDC 2− ligand, an aqua ligand, and 1/6 lattice water molecule. The main framework is isomeric to the compound trans,trans, trans-[Cd(HIDC)(H2O)2]n [9]; however, it is very interesting to note that subtle alterations of coordination environments around the metal centers result in two thoroughly different extended structures. The Cd(II) center in 1 is octahedrally coordinated by four O donors and two N donors from two symmetry-related HIDC2− ligands and two aqua ligands. The N2O2 four donors from two HIDC2− ligands occupy the equatorial positions and two aqua ligands occupy in the axial positions of the coordination geometry. The trans-configuration of two aqua ligands is similar to that in trans,trans,trans-[Cd(HIDC)(H2O)2]n. However, the octahedral geometry is seriously distorted, since there is a very acute trans-O1\Cd\O1 i (symmetry code: (i) x − y + 1/3, −y + 2/3, −z + 1/6) bond angle with a value of 102.78°, which is far from the idealized linear angle (180°) and also much smaller than that in trans,trans,trans-[Cd(HIDC)(H2O)2]n (153.52(11)°) [9]. The strong H-bonding interaction between two carboxylate groups is found, which is indicated by the shorter O3–O3 ii separation of 2.665 Å (symmetry code: (ii) y + 2/3, x − 2/3, −z − 1/6). The two carboxylate groups are well coplanar with the attached imidazole ring and form a very small dihedral angle with a value of 2.4°. The Cd1\N1 bond length is 2.253(4) Å. The Cd1\O2 bond length is 2.549(3) Å, which is much longer than that of Cd1\O1 (2.268(4) Å) and the normal Cd\O bond distances [9–11]. Such a longer Cd\O2 bond length results in the obvious distortion of coordination geometry. The dihedral angle of two coordinated HIDC2− ligands is 39.7°. The bite angle of N1\Cd\O2 is 68.28(12)°, with the Cd\N\C\C\O chelating ring being perfectly co-planar with the imidazole ring. The ligand HIDC2− actually acts as a bis(bidentate) ligand bridging two distinct Cd(II) ions with a Cd⋯Cd distance of 6.627 Å, as shown in Fig. 1b. Such a bridging behavior leads to an infinite helical chain running along c axis. A pair of 1:1 left- and right-handed helices exists in the complex. The helix is generated around the crystallographic 31 axis. Each set of three crystallographically equivalent cadmium(II) centers constitutes a single revolution of the helix with a distance of 7.078 Å (Fig. 2a). Each helical chain aligns in a parallel fashion and further interlinks with six adjacent helices to give a periodically ordered 3D framework. The inter-chain O\H⋯O hydrogen bond contacts between coordinated aqua O\H bonds and the oxygen atoms of carboxylate groups are found (Fig. S2), as they are indicated by the shorter O⋯O
43
separations (Table S3). Interestingly, the remarkable feature of compound 1 is the presence of the hexagonal channels with side length approximately 6.622 Å along the polymer axis (Fig. 2b). It is interesting to note that the packing of six neighboring 1-D helical chains could result in the 1-D channel. There are two previous examples of 1-D Ni(II) zigzag chain [12,13], in which the 1-D open channels are formed by the packing of the polymeric chains or within 1-D polymer coils. Recently, Suh has also reported a novel 1-D chain, where the 1-D channels are formed by the packing of chains in cross-linked way [14]. However, the 1-D channels that are formed by packing of the six neighboring polymeric helical chains are never reported. The trans-coordinated aqua ligands direct into the channels, which construct a hydrophilic environment for the guest molecules. The PLATON [15] program reveals that the voids in complex 1 occupy 8.5% of the crystal volume (Fig. S3). Thermal gravimetric analysis (TGA) was carried out in N2 condition from room temperature to 600 °C for examining the properties of dehydration and stability of 1 (Fig. S4). The first stage of weight loss is 2.10% in the temperature range 50–149 °C, corresponding to the loss of 1/3 lattice water molecule per empirical formula (calc. 1.98%). The second weight loss of 11.23% occurs from 152 to 245 °C corresponding to the loss of two coordinated aqua ligand per empirical formula (calc. 11.66%). Finally a sharp weight loss occurred above 324 °C due to the decomposition of framework structure. PXRD experiment was carried out for compound 1. The PXRD experimental and computer-simulated patterns of the compound 1 are shown in Fig. S5, and they show that the bulk synthesized materials and the measured single crystals of 1 are the same. The photoluminescence of complex 1 was studied in the solid state at room temperature and the measurement of emission spectrum was excited at a wavelength of λex = 305 nm. An intense emission band was observed at λem = 418 nm in the blue region (Fig. S6). As we know, it is usually believed that the energy transition of d10 compounds can be assigned to metal-to-ligand charge transfer, intra-ligand emission (π → π*), and ligand-to-metal charge transfer. The studies on the luminescence properties of free ligand H3IDC show that no obvious emission band was found upon excitation in the range of 180– 480 nm [16,17]. Thus, the possibility of ligand-centered fluorescence could be eliminated. The photo-luminescence of 1 is tentatively attributed to ligand-to-metal charge transfer (LMCT) [18]. In conclusion, a novel linear helical cadmium(II) coordination compound, cis,cis,trans-{[Cd(HIDC)(H2O)2] 1/3H2O}n (1), has been obtained from the hydrothermal reaction of hydrated CdCl2 with HIMDN in the presence of NH3·H2O. The ligand HIDC 2− is in-situ formed from the hydrolysis of the cyano groups of HIMDN. By comparison with the direct reaction of HIMDN with hydrated CdCl2 in similar condition, it showed that the obtained product is highly
Fig. 1. (a) The structural fragment of 1 showing the coordination geometry of cadmium(II) center (Symmetry code: (i) x − y + 1/3, −y + 2/3, −z + 1/6); (b) a single revolution of the helix coil constructed by three crystallographically equivalent cadmium(II) centers.
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Fig. 2. (a) The space-filling models showing a pair of 1:1 left- and right-handed helices; (b) The space-filling model showing 1-D channel along the c axis.
dependent on the in-situ ligand reaction, which may be associated with the transition state in that metal ion activates hydrolysis of organonitrile. Our experiment demonstrates again that in-situ reaction may represent a promising strategy for obtaining new multi-functional materials. Acknowledgment The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (21201023) and thank the scientific research fund of HuBei Provincial Education Department (D20131202). Appendix A. Supplementary material Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.inoche.2013.04.033.
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