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Three new coordination polymers based on tripodal flexible ligand: Synthesis, structures and luminescent properties
Inorganica Chimica Acta 412 (2014) 46–51
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Three new coordination polymers based on tripodal flexible ligand: Synthesis, structures and luminescent properties Hui Gao a, Xin-hua Lou b,⇑, Qiao-Tong Li b, Wen-Jun Du b, Chen Xu b,⇑ a b
Department of Environmental Engineering and Chemistry, Luoyang Institute of Science and Technology, Luoyang 471023, People’s Republic of China Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, Henan 471022, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 24 September 2013 Received in revised form 8 December 2013 Accepted 10 December 2013 Available online 21 December 2013 Keywords: Coordination polymer Polycatenated structure Pillar-layered architecture Luminescence
a b s t r a c t Three new coordination polymers, formulated as [(CH3)2NH2]n[Zn(tci)(bpy)0.5]n (1), [Zn(Htci)(bpe)0.5]n n(H2O) (2) and [Zn2Ca(tci)2(DMF)2]n2n(CH3OH)2n(DMF) (3) (H3tci = tri(2-carboxyethyl)isocyanurate, bpy = 4,40 -bipyridine, bpe = 1,2-di(4-pyridyl)ethylene, DMF = N,N0 -dimethylformamide) have been hydrothermally prepared and characterized by single crystal X-ray diffraction, elemental analyses, IR spectra, and thermal analyses. Single crystal X-ray analysis reveals that compound 1 features a 2D ? 3D polycatenated supramolecular structure based on rare (3,4)-connected 2D bilayers, compound 2 features a 3D pillar-layered architecture, and compound 3 represents a rare (3,6)-connected kgd topological network based on heterometallic trinuclear [Zn2Ca(COO)6] clusters. Moreover, the luminescent properties and thermal stabilities of these three compounds have also been studied. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The design and synthesis of novel metal–organic frameworks (MOFs) have drawn considerable attention not only because of their fascinating structural diversities but also because of their significant potential applications in the areas of luminescence, magnetism, catalysis, drug delivery, nonlinear optics, gas storage and so on [1]. However, from a crystal engineering viewpoint, how to rationally design and construct the preferred structures with unique properties remains a far-reaching challenge owing to many unpredictable factors (such as auxiliary ligands, reaction temperature, template agents, solvent system, pH values, metal and ligand stiochiometry and so on) that can exert significant influence on the construction of MOFs [2]. Therefore, a careful investigation for understanding the relationship between the structures of MOFs and various external stimuli may help us better design and construct MOFs with desired structures. Apart from the unpredictable external factors, the organic ligands also play an important role in the construction of MOFs. Thus, the rational selection of organic ligands with suitable shape, functionality, flexibility, and symmetry is of curious important. Currently, multicarboxylate ligands due to their versatile coordination fashions and strong coordination ability have been widely used to construct various multifunctional MOFs [3,4]. Tris(2carboxyethyl)isocyanuric acid (H3tci) as a tripodal symmetrical
carboxylate ligands has three highly flexible carboxylate arms (–CH2–CH2–COOH), which can make it adopt two potential conformations (cis–cis–cis and cis–cis–trans) according to the geometrical requirement of metal ions in the self-assembly process. In view of its various coordination modes and strong coordination abilities, a great number of tci-based MOFs have been previously reported [5]. Particular interesting is that the Kitagawa group have reported two isomorphous tci-based MOFs that exhibit reversible singlecrystal to single-crystal phase transformations triggered by removal or re-obtaining the guest water molecules [6], and Sun and co-workers have reported two new supramolecular isomers induced by different reaction solvents [7]. In order to further explore the coordination chemistry of H3tci and construct new mutifunctional MOFs materials, in this work we also selected flexible H3tci as the organic ligand to construct three new luminescent coordination polymers, namely [(CH3)2NH2]n[Zn(tci)(bpy)0.5]n (1), [Zn(Htci)(bpe)0.5]nn(H2O) (2) and [Zn2Ca(tci)2(DMF)2]n2n (CH3OH)2n(DMF) (3) (H3tci = tri(2-carboxyethyl)isocyanurate, bpy = 4,40 -bipyridine, bpe = 1,2-di(4-pyridyl)ethylene, DMF = N,N0 dimethylformamide) (Scheme 1). Herein, we will report the synthesis, structures and luminescent properties of these three compounds. 2. Experimental 2.1. Materials and measurements
⇑ Corresponding authors. E-mail addresses: [email protected] (X.-h. Lou), [email protected] (C. Xu). 0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2013.12.016
All reagents were purchased commercially and used without further purification. All syntheses were carried out in 23 ml
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Scheme 1. Synthetic procedures of compounds 1–3.
polytetrafluoroethylene lined stainless steel containers under autogenous pressure. Elemental analyses were performed on an EA1110 CHNS-0 CE elemental analyzer. FT-IR spectra were measured as KBr pellets on a Nicolet Magna 750 FT-IR spectrometer in the range of 400–4000 cm1. Powder X-ray diffraction (PXRD) analyses were recorded on a Rigaku Dmax2500 diffractometer with Cu Ka radiation (k = 1.54056 Å) with a step size of 0.05°. The fluorescence spectra were measured on powder samples at room temperature using an Edinburgh FLS920 TCSPC fluorescence spectrophotometer. TGA analyses were carried out on a NETSCHZ STA-449C thermoanalyzer with a heating rate of 10 °C/min under a nitrogen atmosphere. Single-crystal data for compounds 1–3 were collected on an Oxford Xcalibur E diffractometer with graphite monochromated Mo Ka radiation (k = 0.71073 Å). 2.2. Synthesis of complexes [(CH3)2NH2]n[Zn(tci)(bpy)0.5]n (1): A mixture of Zn(NO3)26H2O (0.20 mmol, 0.060 g), H3tci (0.1 mmol, 0.034 g) and bpy (0.1 mmol, 0.016 g) was placed in a 23 mL Teflon liner, 4 mL DMF and 6 mL H2O were then added. The resulting mixture was stirred for 30 min at room temperature, and then the mixture was sealed in a Parr autoclave and kept at 110 °C for 4 days. After being slowly cooled to the room temperature, colorless block crystals of 1 were isolated in 43% yield based on H3tci. Anal. Calc. for C19H24N5O9Zn (531.81): C, 42.87; H, 4.51; N, 13.16. Found: C, 42.85; H, 4.53; N, 13.17%. IR data (KBr pellet): 3448(m), 2428(w), 1684(s), 1632(s), 1467(w), 1382(vs), 1221(w), 1067(w), 834(m), 764(m). [Zn(Htci)(bpe)0.5]nn(H2O) (2): The reaction was carried out in a procedure similar to that for 1, but using bpe (0.10 mmol, 0.018 g) instead of bpy. Colorless block crystals of 2 were obtained in 48% yield based on H3tci. Anal. Calc. for C18H16N4O10Zn (513.74): C, 42.04; H, 3.11; N, 10.90 Found: C, 42.01; H, 3.12; N, 10.89%. IR data (KBr pellet): 3508(m), 3426(m), 2928(w), 2427(w), 1722(m), 1685(s), 1608(m), 1570(m), 1472(m), 1423(m), 1382(s), 1334(w), 1204(m), 768(m), 533(m). [Zn2Ca(tci)2(DMF)2]n2n(CH3OH)2n(DMF) (3): A mixture of Zn(NO3)26H2O (0.20 mmol, 0.060 g), Ca(NO3)24H2O (0.1 mmol, 0.023 g), H3tci (0.2 mmol, 0.069 g) and was placed in a 23 mL Teflon liner, 2 mL DMF and 2 mL CH3OH were then added. The resulting mixture was stirred for 30 min at room temperature, and kept at 80 °C for four days. After being slowly cooled to the room temperature, colorless block crystals of 3 were isolated in 53% yield based on H3tci. Anal. Calc. for C38H60N10O24Zn2Ca (1211.82): C, 37.63; H, 4.95; N, 11.55. Found: C, 37.63; H, 4.92; N, 11.56%. IR data (KBr pellet): 3434(m), 1693(vs), 1650(s), 1573(s), 1469(s), 1412(s), 765(m), 526(m). 2.3. X-ray crystallography Suitable single crystals of 1–3 were carefully selected under an optical microscope and glued to thin glass fibres. Structural mea-
surements were performed on a computer-controlled Oxford Xcalibur E diffractometer with graphite-monochromated Mo Ka radiation (kMo Ka = 0.71073 Å) at T = 293.2 K. Absorption corrections were made using the SADABS program [8]. The structures were solved using the direct method and refined by full-matrix least-squares methods on F2 by using the SHELXL-97 program package [9]. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms attached to carbon atoms were fixed at their ideal positions. And the hydrogen atoms of the free water molecule in compound 2 were not found from difference Fourier maps, which were not included in the final refinement. Crystal data, as well as details of data collection and refinements of compounds 1–3 are summarized in Table 1, and selected bond lengths and angles are given in Tables S1–S3.
3. Result and discussion 3.1. Description of crystal structures 3.1.1. [(CH3)2NH2]n[Zn(tci)(bpy)0.5]n (1) Single crystal X-ray structural analysis revealed that compound 1 crystallizes in monoclinic space group C2/c and features a 2D ? 3D polycatenated supramolecular structure based on rare (3,4)-connected 2D bilayers. The asymmetric unit of 1 contains one Zn(II) ion, one tci ligand, half of bpy ligand and one dimethylamine cation ((CH3)2NH2+) from the hydrolysis of DMF. Compound 1 is an anionic framework which is neutralized by the lattice dimethylammonium cations. The local coordination geometry around Zn(II) ion is depicted in Fig. 1a. The Zn1 ion adopts a distorted tetrahedral coordination sphere that is defined by three carboxylate oxygen atoms (O5, O6a and O9b) from three different tci ligands and a nitrogen (N4) atom from one bpy ligand. The Zn–O distances are in the range of 1.9404(4)–1.961(4) Å, and the Zn–N distance is 2.063(4) Å, which are in the normal range according to previously reported Zn-based polymers [10]. In 1, tci ligand adopts a cis–cis–trans conformation, and acts as a tridentate ligand linking three Zn(II) ions with its three carboxylate groups in uniform monodentate mode. Each tci ligand bridges three Zn(II) ions to form a 2D (6,3) layer of hexagonal meshes extending along crystallographical ab plane (Fig. 1b). Adjacent 2D (6,3) layers are bridged together by the bpy ligands, giving rise to a 2D bilayer structure (Fig. 1c(up)). From the view of topology, if the tci ligands and Zn(II) ions were reduced into 3-, 4-connected nodes, respectively, and bpy ligands were looked as linear linkers, this 2D bilayer can be simplified into a (3,4)-connected topological network with the schläfli symbol of {63}{66} (Fig. 1c (down)). To the best of knowledge, this topological network is unprecedented. Viewing from a axis, there exist rectangular channels with a cross section of approximately 10.96 11.08 Å. The channel is so large that another two equivalent bilayers can be accommodated in that channel. Thus, the most fascinating structural feature is that each (3,4)-connected 2D bilayer is interlocked with two adjacent identical ones, resulting in a 2D ? 3D
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Table 1 Crystallographic data for compounds 1–3.
Empirical formula T (K) Crystal color Fw Crystal system space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc Absorption coefficient (mm1) F(0 0 0) Reflections collections Independent reflections Observed reflections Goodness of fit (GOF) on F2 R1, wR2 [I > 2r(I)] R1, wR2 (all data)
1
2
3
C19H24N5O9Zn 293(2) colorless 531.81 monoclinic C2/c 13.3478(8) 21.9271(12) 18.5134(9) 90 105.261(6) 90 5227.4(5) 8 1.349 0.992 3328 10199 4590[Rint = 0.0266] 3574 1.084 0.0721, 0.2125 0.0880, 0.2259
C18H16N4O10Zn 293(2) colorless 513.74 monoclinic P21/c 12.9695(3) 23.3663(7) 6.6604(2) 90 93.675(2) 90 2014.28(10) 4 1.694 1.287 1048 4639 2520[Rint = 0.0144] 2232 1.023 0.0308, 0.0822 0.0368, 0.0860
C38H60N10O24Zn2a 293(2) colorless 1211.82 monoclinic P21/n 11.1225(5) 12.8850(5) 18.6082(7) 90 91.973(4) 90 2665.22(19) 2 1.510 1.085 1244 9858 4684[Rint = 0.0276] 3815 1.042 0.0463,0.1204 0.0602, 0.1204
Fig. 1. (a) Coordination environment of Zn(II) ion in 1. Symmetry codes: (a) 0.5 + x, 0.5 + y, z; (b) 0.5 + x, 0.5 + y, z. (b) Perspective view of the 2D (6,3) layer constructed by tci ligands and Zn(II) ions. (c) 2D bilayer structure of 1 (up) and schematic representation of the (3,4)-connected topological network for 1 (down). (d) Topological representation of the 2D ? 3D polycatenated supramolecular structure for 1.
parallel entangled supramolecular architecture (Fig. 1d). Upon polycatenation, compound 1 just contains a small solvent accessible void space of 12.6% of the total crystal volume according to a calculation performed using PLATON. Compared to other 2D ? 3D polycatenation systems in parallel/parallel inclined fashion or parallel/ parallel highly undulating fashion, fewer examples of 2D ? 3D parallel entangled structures based on (3,4)-connected 2D bilayer have been observed [11].
3.1.2. [Zn(Htci)(bpe)0.5]nn(H2O) (2) A new framework compound 2 was obtained when bpy ligand was replaced by another bridging bpe ligand under the same conditions. Single crystal X-ray structural analysis reveals that compound 2 crystallizes in monoclinic space group P21/c and features a 3D pillar-layered architecture. There are one Zn(II) ion, one Htci ligand, half of bpe ligand, and one free water molecule in the asymmetric unit of 2. As shown in Fig. 2a, the coordination
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environment of Zn(II) ion is six-coordinated and can be described as a slightly distorted octahedron. Four equatorial plane sites are occupied by four carboxylate oxygen atoms (O5, O3a, O4b and O6b) from four different Htci ligands, and the axial sites are ligated by one carboxylate oxygen atom (O4a) from one Htci ligand and a nitrogen atom (N1) from bpe ligand. The Zn–O bond lengths fall in the range of 1.988(2)–2.354(3) Å, and the Zn–N distance is 2.073(3) Å. Different from 1, H3tci in 2 deprotonated two protons into Htci, and Zn(II) ions in 2 exhibit octahedral coordination geometries rather than tetrahedral coordination geometries, which may be caused by the different alkalinity and space length of the auxiliary ligands (bpy and bpe). The Htci ligand also adopts cis–cis–trans conformation linking four Zn(II) ions with its two deprotonated carboxylate groups in l4-g1:g1:g1:g2 coordination mode. As shown in Fig. S1, crystallographically different Zn(II) ions are bridged by the carboxylate groups of Htci ligands, forming an infinite 1D chain structure extending along crystallographical c axis. Then, these adjacent 1D chains are further connected together by the Htci ligands, giving rise to a 2D layer structure (Fig. 2b). Finally, adjacent 2D sheets with a separation of 10.075 Å are pillared by the linear bpe ligands to generate the 3D pillar-layered framework of 2 (Fig. 2c).
3.1.3. [Zn2Ca(tci)2(DMF)2]n2n(CH3OH)2n(DMF) (3) Compound 3 crystallizes in a monoclinic P21/n space group with the asymmetric unit containing one Zn(II) ion, one Ca(II) ion, one tci ligand, one coordinated DMF molecule, one free DMF molecule, as well as a free CH3OH molecule. As shown in Fig. 3a, the Zn(II) center is four-coordinated by three carboxylate oxygen atoms (O1, O3a and O6b) from three different tci ligands and one coordinated DMF molecule (O10), and has a slightly distorted tetrahedral coordination geometry. The Ca(II) ion locates in an octahedral coordination environment, which is defined by four carboxylate oxygen atoms (O4a, O4c, O5b and O5c) from four different tci ligands occupying the equatorial plane and two carboxylate oxygen (O2 and O2d) atoms from another two tci ligands at the remaining axial sites. The Zn–O and Ca–O bond distances fall in the range of 1.921(3)–1.997(3), 2.271(3)–2.357(2) Å, respectively, which are in
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agreement with previously reported heterometallic Zn(II)–Ca(II) carboxylates [12]. In compound 3, each tci ligand in cis–cis–trans conformation links six metal centers (three Zn(II) and three Ca(II)) with its three carboxylate groups in uniform bis-bridging mode. In the structure of 3, the Ca(II) ion resides on a special inversion center, so that another symmetry-related Zn(II) ion is generated. Two symmetry-related Zn(II) ions and one Ca(II) ion are in a line and bridged by six bis-bridging carboxylate groups from six separated tci ligands, generating a heterometallic trinuclear [Zn2Ca(COO)6] cluster with a Zn Ca distance of 3.7909(4) Å, which is shorter than that of the reported dinuclear [ZnCa(COO)3] cluster [13]. These heterometallic trinuclear [Zn2Ca(COO)6] clusters are further bridged by tci ligands into a 2D layered structure (Fig. 3c). Topologically speaking, each tci ligand linking three separated heterometallic trinuclear [Zn2Ca(COO)6] clusters can be reducede into a 3-connected node, and each heterometallic trinuclear [Zn2Ca(COO)6] cluster surrounded by six different tci ligands can be viewed as 6-connected nodes (Fig. S2), so this 2D sheet can be represented as a (3,6)-connected 2-nodal net with a kgd topology (Fig. 3d). The short schläfli vertex notation of this net can be represented as {43}2{46.66.83} indicated by TOPOS software [14]. Compared with another previously reported tci-based compound, [(Me2NH2)Zn(tci)30.5DMF]n (4), which also represented a kgd topology [7], the obvious difference between these two kgd networks is that the 6-connected nodes in 3 is the heterometallic trinuclear [Zn2Ca(COO)6] clusters, while in 4 the 6-connected nodes are the paddlewheel binuclear [Zn2(COO)4] units.
3.2. Poweder X-ray diffraction and thermal properties The purities of compounds 1–3 were confirmed by X-ray powder diffraction analysis. As shown in Fig. S3, compounds 1–3 show fair agreement between their experimental and calculated X-ray powder diffraction patterns, indicating reasonable phase purity of compounds 1–3. The differences in intensities between the experimental and calculated patterns may be due to the preferred orientation of the crystalline powder samples.
Fig. 2. (a) Coordination environment of Zn(II) in 2. Symmetry codes: (a) 1 + x, y, z; (b) x, 2.5 y, 0.5 + z. (b) Perspective view of the 2D layer structure for 2. (c) 3D pillarlayered framework of 2 (right) and schematic representation of pillar-layered framework of 2.
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Fig. 3. (a) Coordination environments of Zn(II) and Ca(II) ions in 3. Symmetry codes: (a) 0.5 + x, 0.5 y, 0.5 + z; (b) 0.5 + x, 0.5 y, 0.5 + z; (c) 2.5 x, 0.5 y, 0.5 z; (d) 2 x, y, 1 z. (b) Heterometallic trinuclear [Zn2Ca(COO)6] clsuter. (c) Perspective view of the 2D layer structure for 3. (d) Schematic representation of simplified 2D sheet with kgd topology for 3.
To characterize these compounds more fully in terms of thermal stability, thermal gravimetric analysis (TGA) of compounds 1–3 were performed on polycrystalline samples with a heating rate of 10 °C/min under N2 atomsphere in the temperature range of 30– 900 °C (Fig. S4). For compound 1, the TG curve reveals that the (CH3)2NH molecules are gradually released from 30 to 185 °C (obsd = 8.50%, calcd = 8.65%). Subsequently, the skeleton of 1 can be stable up to 350 °C. After that, the continuous weight loss step corresponds to the decomposition of the organic ligands. The remaining weight corresponds to the formation of ZnO (obsd 14.98%, calcd 15.23%). For compound 2, the first weight loss of 3.42% between 50 and 85 °C corresponds to the loss of one lattice water molecule per unit cell (calcd: 3.50%). The plateau region in the temperature range 85–320 °C indicates that the molecular architecture of 2 can be stable up to 320 °C in the absence of guests. When heating beyond 320 °C, dramatically weight loss occurs because of the decomposition of the organic ligands, suggesting the skeleton collapse of 2. For compound 3, the first weight loss observed between 30 and 120 °C corresponds to the release of two free CH3OH molecules and two free DMF molecules per unit cell (obsd:8.75%, cacld:8.66%). The second step of weight loss from 210 to 350 °C corresponds to the release of two coordinated DMF molecules. When upon heating above 425 °C, the third step weight loss occurs owing to the decomposition of the organic ligands, and thus the framework of 3 collapses.
pounds 1–3 and free H3tci ligand were investigated at ambient temperature. As depicted in Fig. 4, free H3tci ligand shows a maximum emission peak at 418 nm with an excitation of 305 nm, which can be probably ascribed to p⁄ ? n or p⁄ ? p orbital transitions, and the observed maximum emission peaks are at 484 nm (kex = 300 nm) for 1, 500 nm (kex = 300 nm) for 2, 534 nm (kex = 300 nm) for 3, respectively. It is noteworthy that the shape of the emission bands for these compounds is similar to that of corresponding free H3tci ligand. Considering that the Zn(II) ion is difficult to oxidize or reduce because of its d10 configuration [16], so the luminescent emissions of compounds 1–3, which are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal trans-
3.3. Photoluminescent properties Coordination polymers constructed from d10 transition metal ions and organic ligands with excellent luminescent properties have attracted more interest because of their potential applications in chemical sensors, photochemistry and electroluminescent display [15]. Therefore, the solid-state luminescent properties of com-
Fig. 4. The photoluminescence of compounds 1–3 and free ligand (H3tci) in the solid-state at room temperature.
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fer (LMCT) in nature, may be ascribed to intraligand fluorescent emission. Compared with the luminescent emission of free ligand (H3tci), not only the emission peak intensities of compounds 1–3 increase, and the emission spectra for compounds 1–3 are redshifted with different level, which may be caused by the synergy effect of the deprotonation of H3tci and the coordination of the tci ligands to metal ions. In addition, the different emission intensities for compounds 1–3 may originate from the difference of coordination mode of the organic ligand, the degree of deprotonation, the coordination environments of central metal ions and the rigidity of solid-state crystal packing [7].
[3]
[4]
4. Conclusions In summary, we synthesized three new metal–organic frameworks based on tripodal flexible ligand for the first time. Compounds 1 and 2 were synthesized under the same temperature and solvents conditions but with different auxiliary ligand, while compound 3 was obtained in the presence of Ca(II) ions at different temperature and solvent conditions. Different auxiliary ligands, solvent systems, and the second metal ions exert a profound influence on the coordination mode of tci ligands and the degree of depronation of H3tci, further resulting in three new polymers with different topological frameworks. Compound 1 features a 2D ? 3D polycatenated supramolecular structure based on rare (3,4)-connected 2D bilayer, compound 2 features a 3D pillar-layered architecture, and compound 3 represents a rare (3,6)-connected kgd topological network based on heterometallic trinuclear [Zn2Ca(COO)6] clusters. In addition, compounds 1–3 exhibit intense luminescence at room temperature.
[5]
[6] [7]
Acknowledgement
[8]
This work was supported by the National Natural Science Foundation of China (Nos. 21272110 and 20902043).
[9] [10]
Appendix A. Supplementary material CCDC 952629–952631 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.ica.2013.12.016.
[11]
[12]
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Three new coordination polymers based on tripodal flexible ligand: Synthesis, structures and luminescent properties Hui Gao a, Xin-hua Lou b,⇑, Qiao-Tong Li b, Wen-Jun Du b, Chen Xu b,⇑ a b
Department of Environmental Engineering and Chemistry, Luoyang Institute of Science and Technology, Luoyang 471023, People’s Republic of China Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, Henan 471022, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 24 September 2013 Received in revised form 8 December 2013 Accepted 10 December 2013 Available online 21 December 2013 Keywords: Coordination polymer Polycatenated structure Pillar-layered architecture Luminescence
a b s t r a c t Three new coordination polymers, formulated as [(CH3)2NH2]n[Zn(tci)(bpy)0.5]n (1), [Zn(Htci)(bpe)0.5]n n(H2O) (2) and [Zn2Ca(tci)2(DMF)2]n2n(CH3OH)2n(DMF) (3) (H3tci = tri(2-carboxyethyl)isocyanurate, bpy = 4,40 -bipyridine, bpe = 1,2-di(4-pyridyl)ethylene, DMF = N,N0 -dimethylformamide) have been hydrothermally prepared and characterized by single crystal X-ray diffraction, elemental analyses, IR spectra, and thermal analyses. Single crystal X-ray analysis reveals that compound 1 features a 2D ? 3D polycatenated supramolecular structure based on rare (3,4)-connected 2D bilayers, compound 2 features a 3D pillar-layered architecture, and compound 3 represents a rare (3,6)-connected kgd topological network based on heterometallic trinuclear [Zn2Ca(COO)6] clusters. Moreover, the luminescent properties and thermal stabilities of these three compounds have also been studied. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The design and synthesis of novel metal–organic frameworks (MOFs) have drawn considerable attention not only because of their fascinating structural diversities but also because of their significant potential applications in the areas of luminescence, magnetism, catalysis, drug delivery, nonlinear optics, gas storage and so on [1]. However, from a crystal engineering viewpoint, how to rationally design and construct the preferred structures with unique properties remains a far-reaching challenge owing to many unpredictable factors (such as auxiliary ligands, reaction temperature, template agents, solvent system, pH values, metal and ligand stiochiometry and so on) that can exert significant influence on the construction of MOFs [2]. Therefore, a careful investigation for understanding the relationship between the structures of MOFs and various external stimuli may help us better design and construct MOFs with desired structures. Apart from the unpredictable external factors, the organic ligands also play an important role in the construction of MOFs. Thus, the rational selection of organic ligands with suitable shape, functionality, flexibility, and symmetry is of curious important. Currently, multicarboxylate ligands due to their versatile coordination fashions and strong coordination ability have been widely used to construct various multifunctional MOFs [3,4]. Tris(2carboxyethyl)isocyanuric acid (H3tci) as a tripodal symmetrical
carboxylate ligands has three highly flexible carboxylate arms (–CH2–CH2–COOH), which can make it adopt two potential conformations (cis–cis–cis and cis–cis–trans) according to the geometrical requirement of metal ions in the self-assembly process. In view of its various coordination modes and strong coordination abilities, a great number of tci-based MOFs have been previously reported [5]. Particular interesting is that the Kitagawa group have reported two isomorphous tci-based MOFs that exhibit reversible singlecrystal to single-crystal phase transformations triggered by removal or re-obtaining the guest water molecules [6], and Sun and co-workers have reported two new supramolecular isomers induced by different reaction solvents [7]. In order to further explore the coordination chemistry of H3tci and construct new mutifunctional MOFs materials, in this work we also selected flexible H3tci as the organic ligand to construct three new luminescent coordination polymers, namely [(CH3)2NH2]n[Zn(tci)(bpy)0.5]n (1), [Zn(Htci)(bpe)0.5]nn(H2O) (2) and [Zn2Ca(tci)2(DMF)2]n2n (CH3OH)2n(DMF) (3) (H3tci = tri(2-carboxyethyl)isocyanurate, bpy = 4,40 -bipyridine, bpe = 1,2-di(4-pyridyl)ethylene, DMF = N,N0 dimethylformamide) (Scheme 1). Herein, we will report the synthesis, structures and luminescent properties of these three compounds. 2. Experimental 2.1. Materials and measurements
⇑ Corresponding authors. E-mail addresses: [email protected] (X.-h. Lou), [email protected] (C. Xu). 0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2013.12.016
All reagents were purchased commercially and used without further purification. All syntheses were carried out in 23 ml
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Scheme 1. Synthetic procedures of compounds 1–3.
polytetrafluoroethylene lined stainless steel containers under autogenous pressure. Elemental analyses were performed on an EA1110 CHNS-0 CE elemental analyzer. FT-IR spectra were measured as KBr pellets on a Nicolet Magna 750 FT-IR spectrometer in the range of 400–4000 cm1. Powder X-ray diffraction (PXRD) analyses were recorded on a Rigaku Dmax2500 diffractometer with Cu Ka radiation (k = 1.54056 Å) with a step size of 0.05°. The fluorescence spectra were measured on powder samples at room temperature using an Edinburgh FLS920 TCSPC fluorescence spectrophotometer. TGA analyses were carried out on a NETSCHZ STA-449C thermoanalyzer with a heating rate of 10 °C/min under a nitrogen atmosphere. Single-crystal data for compounds 1–3 were collected on an Oxford Xcalibur E diffractometer with graphite monochromated Mo Ka radiation (k = 0.71073 Å). 2.2. Synthesis of complexes [(CH3)2NH2]n[Zn(tci)(bpy)0.5]n (1): A mixture of Zn(NO3)26H2O (0.20 mmol, 0.060 g), H3tci (0.1 mmol, 0.034 g) and bpy (0.1 mmol, 0.016 g) was placed in a 23 mL Teflon liner, 4 mL DMF and 6 mL H2O were then added. The resulting mixture was stirred for 30 min at room temperature, and then the mixture was sealed in a Parr autoclave and kept at 110 °C for 4 days. After being slowly cooled to the room temperature, colorless block crystals of 1 were isolated in 43% yield based on H3tci. Anal. Calc. for C19H24N5O9Zn (531.81): C, 42.87; H, 4.51; N, 13.16. Found: C, 42.85; H, 4.53; N, 13.17%. IR data (KBr pellet): 3448(m), 2428(w), 1684(s), 1632(s), 1467(w), 1382(vs), 1221(w), 1067(w), 834(m), 764(m). [Zn(Htci)(bpe)0.5]nn(H2O) (2): The reaction was carried out in a procedure similar to that for 1, but using bpe (0.10 mmol, 0.018 g) instead of bpy. Colorless block crystals of 2 were obtained in 48% yield based on H3tci. Anal. Calc. for C18H16N4O10Zn (513.74): C, 42.04; H, 3.11; N, 10.90 Found: C, 42.01; H, 3.12; N, 10.89%. IR data (KBr pellet): 3508(m), 3426(m), 2928(w), 2427(w), 1722(m), 1685(s), 1608(m), 1570(m), 1472(m), 1423(m), 1382(s), 1334(w), 1204(m), 768(m), 533(m). [Zn2Ca(tci)2(DMF)2]n2n(CH3OH)2n(DMF) (3): A mixture of Zn(NO3)26H2O (0.20 mmol, 0.060 g), Ca(NO3)24H2O (0.1 mmol, 0.023 g), H3tci (0.2 mmol, 0.069 g) and was placed in a 23 mL Teflon liner, 2 mL DMF and 2 mL CH3OH were then added. The resulting mixture was stirred for 30 min at room temperature, and kept at 80 °C for four days. After being slowly cooled to the room temperature, colorless block crystals of 3 were isolated in 53% yield based on H3tci. Anal. Calc. for C38H60N10O24Zn2Ca (1211.82): C, 37.63; H, 4.95; N, 11.55. Found: C, 37.63; H, 4.92; N, 11.56%. IR data (KBr pellet): 3434(m), 1693(vs), 1650(s), 1573(s), 1469(s), 1412(s), 765(m), 526(m). 2.3. X-ray crystallography Suitable single crystals of 1–3 were carefully selected under an optical microscope and glued to thin glass fibres. Structural mea-
surements were performed on a computer-controlled Oxford Xcalibur E diffractometer with graphite-monochromated Mo Ka radiation (kMo Ka = 0.71073 Å) at T = 293.2 K. Absorption corrections were made using the SADABS program [8]. The structures were solved using the direct method and refined by full-matrix least-squares methods on F2 by using the SHELXL-97 program package [9]. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms attached to carbon atoms were fixed at their ideal positions. And the hydrogen atoms of the free water molecule in compound 2 were not found from difference Fourier maps, which were not included in the final refinement. Crystal data, as well as details of data collection and refinements of compounds 1–3 are summarized in Table 1, and selected bond lengths and angles are given in Tables S1–S3.
3. Result and discussion 3.1. Description of crystal structures 3.1.1. [(CH3)2NH2]n[Zn(tci)(bpy)0.5]n (1) Single crystal X-ray structural analysis revealed that compound 1 crystallizes in monoclinic space group C2/c and features a 2D ? 3D polycatenated supramolecular structure based on rare (3,4)-connected 2D bilayers. The asymmetric unit of 1 contains one Zn(II) ion, one tci ligand, half of bpy ligand and one dimethylamine cation ((CH3)2NH2+) from the hydrolysis of DMF. Compound 1 is an anionic framework which is neutralized by the lattice dimethylammonium cations. The local coordination geometry around Zn(II) ion is depicted in Fig. 1a. The Zn1 ion adopts a distorted tetrahedral coordination sphere that is defined by three carboxylate oxygen atoms (O5, O6a and O9b) from three different tci ligands and a nitrogen (N4) atom from one bpy ligand. The Zn–O distances are in the range of 1.9404(4)–1.961(4) Å, and the Zn–N distance is 2.063(4) Å, which are in the normal range according to previously reported Zn-based polymers [10]. In 1, tci ligand adopts a cis–cis–trans conformation, and acts as a tridentate ligand linking three Zn(II) ions with its three carboxylate groups in uniform monodentate mode. Each tci ligand bridges three Zn(II) ions to form a 2D (6,3) layer of hexagonal meshes extending along crystallographical ab plane (Fig. 1b). Adjacent 2D (6,3) layers are bridged together by the bpy ligands, giving rise to a 2D bilayer structure (Fig. 1c(up)). From the view of topology, if the tci ligands and Zn(II) ions were reduced into 3-, 4-connected nodes, respectively, and bpy ligands were looked as linear linkers, this 2D bilayer can be simplified into a (3,4)-connected topological network with the schläfli symbol of {63}{66} (Fig. 1c (down)). To the best of knowledge, this topological network is unprecedented. Viewing from a axis, there exist rectangular channels with a cross section of approximately 10.96 11.08 Å. The channel is so large that another two equivalent bilayers can be accommodated in that channel. Thus, the most fascinating structural feature is that each (3,4)-connected 2D bilayer is interlocked with two adjacent identical ones, resulting in a 2D ? 3D
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Table 1 Crystallographic data for compounds 1–3.
Empirical formula T (K) Crystal color Fw Crystal system space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc Absorption coefficient (mm1) F(0 0 0) Reflections collections Independent reflections Observed reflections Goodness of fit (GOF) on F2 R1, wR2 [I > 2r(I)] R1, wR2 (all data)
1
2
3
C19H24N5O9Zn 293(2) colorless 531.81 monoclinic C2/c 13.3478(8) 21.9271(12) 18.5134(9) 90 105.261(6) 90 5227.4(5) 8 1.349 0.992 3328 10199 4590[Rint = 0.0266] 3574 1.084 0.0721, 0.2125 0.0880, 0.2259
C18H16N4O10Zn 293(2) colorless 513.74 monoclinic P21/c 12.9695(3) 23.3663(7) 6.6604(2) 90 93.675(2) 90 2014.28(10) 4 1.694 1.287 1048 4639 2520[Rint = 0.0144] 2232 1.023 0.0308, 0.0822 0.0368, 0.0860
C38H60N10O24Zn2a 293(2) colorless 1211.82 monoclinic P21/n 11.1225(5) 12.8850(5) 18.6082(7) 90 91.973(4) 90 2665.22(19) 2 1.510 1.085 1244 9858 4684[Rint = 0.0276] 3815 1.042 0.0463,0.1204 0.0602, 0.1204
Fig. 1. (a) Coordination environment of Zn(II) ion in 1. Symmetry codes: (a) 0.5 + x, 0.5 + y, z; (b) 0.5 + x, 0.5 + y, z. (b) Perspective view of the 2D (6,3) layer constructed by tci ligands and Zn(II) ions. (c) 2D bilayer structure of 1 (up) and schematic representation of the (3,4)-connected topological network for 1 (down). (d) Topological representation of the 2D ? 3D polycatenated supramolecular structure for 1.
parallel entangled supramolecular architecture (Fig. 1d). Upon polycatenation, compound 1 just contains a small solvent accessible void space of 12.6% of the total crystal volume according to a calculation performed using PLATON. Compared to other 2D ? 3D polycatenation systems in parallel/parallel inclined fashion or parallel/ parallel highly undulating fashion, fewer examples of 2D ? 3D parallel entangled structures based on (3,4)-connected 2D bilayer have been observed [11].
3.1.2. [Zn(Htci)(bpe)0.5]nn(H2O) (2) A new framework compound 2 was obtained when bpy ligand was replaced by another bridging bpe ligand under the same conditions. Single crystal X-ray structural analysis reveals that compound 2 crystallizes in monoclinic space group P21/c and features a 3D pillar-layered architecture. There are one Zn(II) ion, one Htci ligand, half of bpe ligand, and one free water molecule in the asymmetric unit of 2. As shown in Fig. 2a, the coordination
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environment of Zn(II) ion is six-coordinated and can be described as a slightly distorted octahedron. Four equatorial plane sites are occupied by four carboxylate oxygen atoms (O5, O3a, O4b and O6b) from four different Htci ligands, and the axial sites are ligated by one carboxylate oxygen atom (O4a) from one Htci ligand and a nitrogen atom (N1) from bpe ligand. The Zn–O bond lengths fall in the range of 1.988(2)–2.354(3) Å, and the Zn–N distance is 2.073(3) Å. Different from 1, H3tci in 2 deprotonated two protons into Htci, and Zn(II) ions in 2 exhibit octahedral coordination geometries rather than tetrahedral coordination geometries, which may be caused by the different alkalinity and space length of the auxiliary ligands (bpy and bpe). The Htci ligand also adopts cis–cis–trans conformation linking four Zn(II) ions with its two deprotonated carboxylate groups in l4-g1:g1:g1:g2 coordination mode. As shown in Fig. S1, crystallographically different Zn(II) ions are bridged by the carboxylate groups of Htci ligands, forming an infinite 1D chain structure extending along crystallographical c axis. Then, these adjacent 1D chains are further connected together by the Htci ligands, giving rise to a 2D layer structure (Fig. 2b). Finally, adjacent 2D sheets with a separation of 10.075 Å are pillared by the linear bpe ligands to generate the 3D pillar-layered framework of 2 (Fig. 2c).
3.1.3. [Zn2Ca(tci)2(DMF)2]n2n(CH3OH)2n(DMF) (3) Compound 3 crystallizes in a monoclinic P21/n space group with the asymmetric unit containing one Zn(II) ion, one Ca(II) ion, one tci ligand, one coordinated DMF molecule, one free DMF molecule, as well as a free CH3OH molecule. As shown in Fig. 3a, the Zn(II) center is four-coordinated by three carboxylate oxygen atoms (O1, O3a and O6b) from three different tci ligands and one coordinated DMF molecule (O10), and has a slightly distorted tetrahedral coordination geometry. The Ca(II) ion locates in an octahedral coordination environment, which is defined by four carboxylate oxygen atoms (O4a, O4c, O5b and O5c) from four different tci ligands occupying the equatorial plane and two carboxylate oxygen (O2 and O2d) atoms from another two tci ligands at the remaining axial sites. The Zn–O and Ca–O bond distances fall in the range of 1.921(3)–1.997(3), 2.271(3)–2.357(2) Å, respectively, which are in
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agreement with previously reported heterometallic Zn(II)–Ca(II) carboxylates [12]. In compound 3, each tci ligand in cis–cis–trans conformation links six metal centers (three Zn(II) and three Ca(II)) with its three carboxylate groups in uniform bis-bridging mode. In the structure of 3, the Ca(II) ion resides on a special inversion center, so that another symmetry-related Zn(II) ion is generated. Two symmetry-related Zn(II) ions and one Ca(II) ion are in a line and bridged by six bis-bridging carboxylate groups from six separated tci ligands, generating a heterometallic trinuclear [Zn2Ca(COO)6] cluster with a Zn Ca distance of 3.7909(4) Å, which is shorter than that of the reported dinuclear [ZnCa(COO)3] cluster [13]. These heterometallic trinuclear [Zn2Ca(COO)6] clusters are further bridged by tci ligands into a 2D layered structure (Fig. 3c). Topologically speaking, each tci ligand linking three separated heterometallic trinuclear [Zn2Ca(COO)6] clusters can be reducede into a 3-connected node, and each heterometallic trinuclear [Zn2Ca(COO)6] cluster surrounded by six different tci ligands can be viewed as 6-connected nodes (Fig. S2), so this 2D sheet can be represented as a (3,6)-connected 2-nodal net with a kgd topology (Fig. 3d). The short schläfli vertex notation of this net can be represented as {43}2{46.66.83} indicated by TOPOS software [14]. Compared with another previously reported tci-based compound, [(Me2NH2)Zn(tci)30.5DMF]n (4), which also represented a kgd topology [7], the obvious difference between these two kgd networks is that the 6-connected nodes in 3 is the heterometallic trinuclear [Zn2Ca(COO)6] clusters, while in 4 the 6-connected nodes are the paddlewheel binuclear [Zn2(COO)4] units.
3.2. Poweder X-ray diffraction and thermal properties The purities of compounds 1–3 were confirmed by X-ray powder diffraction analysis. As shown in Fig. S3, compounds 1–3 show fair agreement between their experimental and calculated X-ray powder diffraction patterns, indicating reasonable phase purity of compounds 1–3. The differences in intensities between the experimental and calculated patterns may be due to the preferred orientation of the crystalline powder samples.
Fig. 2. (a) Coordination environment of Zn(II) in 2. Symmetry codes: (a) 1 + x, y, z; (b) x, 2.5 y, 0.5 + z. (b) Perspective view of the 2D layer structure for 2. (c) 3D pillarlayered framework of 2 (right) and schematic representation of pillar-layered framework of 2.
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Fig. 3. (a) Coordination environments of Zn(II) and Ca(II) ions in 3. Symmetry codes: (a) 0.5 + x, 0.5 y, 0.5 + z; (b) 0.5 + x, 0.5 y, 0.5 + z; (c) 2.5 x, 0.5 y, 0.5 z; (d) 2 x, y, 1 z. (b) Heterometallic trinuclear [Zn2Ca(COO)6] clsuter. (c) Perspective view of the 2D layer structure for 3. (d) Schematic representation of simplified 2D sheet with kgd topology for 3.
To characterize these compounds more fully in terms of thermal stability, thermal gravimetric analysis (TGA) of compounds 1–3 were performed on polycrystalline samples with a heating rate of 10 °C/min under N2 atomsphere in the temperature range of 30– 900 °C (Fig. S4). For compound 1, the TG curve reveals that the (CH3)2NH molecules are gradually released from 30 to 185 °C (obsd = 8.50%, calcd = 8.65%). Subsequently, the skeleton of 1 can be stable up to 350 °C. After that, the continuous weight loss step corresponds to the decomposition of the organic ligands. The remaining weight corresponds to the formation of ZnO (obsd 14.98%, calcd 15.23%). For compound 2, the first weight loss of 3.42% between 50 and 85 °C corresponds to the loss of one lattice water molecule per unit cell (calcd: 3.50%). The plateau region in the temperature range 85–320 °C indicates that the molecular architecture of 2 can be stable up to 320 °C in the absence of guests. When heating beyond 320 °C, dramatically weight loss occurs because of the decomposition of the organic ligands, suggesting the skeleton collapse of 2. For compound 3, the first weight loss observed between 30 and 120 °C corresponds to the release of two free CH3OH molecules and two free DMF molecules per unit cell (obsd:8.75%, cacld:8.66%). The second step of weight loss from 210 to 350 °C corresponds to the release of two coordinated DMF molecules. When upon heating above 425 °C, the third step weight loss occurs owing to the decomposition of the organic ligands, and thus the framework of 3 collapses.
pounds 1–3 and free H3tci ligand were investigated at ambient temperature. As depicted in Fig. 4, free H3tci ligand shows a maximum emission peak at 418 nm with an excitation of 305 nm, which can be probably ascribed to p⁄ ? n or p⁄ ? p orbital transitions, and the observed maximum emission peaks are at 484 nm (kex = 300 nm) for 1, 500 nm (kex = 300 nm) for 2, 534 nm (kex = 300 nm) for 3, respectively. It is noteworthy that the shape of the emission bands for these compounds is similar to that of corresponding free H3tci ligand. Considering that the Zn(II) ion is difficult to oxidize or reduce because of its d10 configuration [16], so the luminescent emissions of compounds 1–3, which are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal trans-
3.3. Photoluminescent properties Coordination polymers constructed from d10 transition metal ions and organic ligands with excellent luminescent properties have attracted more interest because of their potential applications in chemical sensors, photochemistry and electroluminescent display [15]. Therefore, the solid-state luminescent properties of com-
Fig. 4. The photoluminescence of compounds 1–3 and free ligand (H3tci) in the solid-state at room temperature.
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fer (LMCT) in nature, may be ascribed to intraligand fluorescent emission. Compared with the luminescent emission of free ligand (H3tci), not only the emission peak intensities of compounds 1–3 increase, and the emission spectra for compounds 1–3 are redshifted with different level, which may be caused by the synergy effect of the deprotonation of H3tci and the coordination of the tci ligands to metal ions. In addition, the different emission intensities for compounds 1–3 may originate from the difference of coordination mode of the organic ligand, the degree of deprotonation, the coordination environments of central metal ions and the rigidity of solid-state crystal packing [7].
[3]
[4]
4. Conclusions In summary, we synthesized three new metal–organic frameworks based on tripodal flexible ligand for the first time. Compounds 1 and 2 were synthesized under the same temperature and solvents conditions but with different auxiliary ligand, while compound 3 was obtained in the presence of Ca(II) ions at different temperature and solvent conditions. Different auxiliary ligands, solvent systems, and the second metal ions exert a profound influence on the coordination mode of tci ligands and the degree of depronation of H3tci, further resulting in three new polymers with different topological frameworks. Compound 1 features a 2D ? 3D polycatenated supramolecular structure based on rare (3,4)-connected 2D bilayer, compound 2 features a 3D pillar-layered architecture, and compound 3 represents a rare (3,6)-connected kgd topological network based on heterometallic trinuclear [Zn2Ca(COO)6] clusters. In addition, compounds 1–3 exhibit intense luminescence at room temperature.
[5]
[6] [7]
Acknowledgement
[8]
This work was supported by the National Natural Science Foundation of China (Nos. 21272110 and 20902043).
[9] [10]
Appendix A. Supplementary material CCDC 952629–952631 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.ica.2013.12.016.
[11]
[12]
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