Coordination polymers (CPs),featured with versatile structure and intriguing properties,exhibit potential applications in a wide field covering magnetism,gas storage and separation,catalysis and sensing [1, 2, 3, 4]. Currently,rational design and synthesis of target coordination polymers with unique structure and function still remains a long-term challenge [5]. From the view of synthetic strategy,CPs structures can be modulated by choosing appropriate organic ligands and metal centers as well as adjusting reaction conditions such as temperature,pH value,solvent and reactant stoichiometry [6, 7]. In particular,choosing organic ligands with appropriate geometric configuration,rigidity/flexibility,functionality,and coordinating group is an effective method to manipulate the structure and properties of CPs [8, 9]. Carboxylate has various coordination modes and has been proven to be a good coordination group by its wide application in the construction of coordination polymer. In addition,mixed ligand strategy by incorporating multicarboxylate ligands withN-donor ligands has been increasingly adopted in the construction of CPs [10],in order to satisfy and mediate the geometric requirement of metal centers.
Taking the above discussion into account,we focus on the application of tris(2-carboxyethyl)isocyanurate (H3tci) in preparing CPs,together with or withoutN-donor ligands. H3tci ligand is a promising ligand for the construction of novel metal-organic polymers based on its following attributes: (i) three flexible - CH2-CH2- arms; (ii) three freely rotatable carboxyl groups and (iii) the existence of two potential conformations,cis-cis-cis and cis-cis-transthat is capable of transformation between each other with a small energy barrier [11]. Herein,we report two coordination polymers based on H3tci ligand,{[Cu3(tci)2(DMAc)3]·6DMAc·2H2O}n (1) and {[Cu3(tci)2(tpt)2(H2O)2]·2DMAc·2H2O}n (2). Meanwhile, magnetism study reveals that there exists antiferromagnetic behavior for 1. 2. Experimental 2.1. Materials and general methods
All the solvents and reagents for synthesis were obtained commercially and used as received. IR spectra were measured on a TENSOR 27 OPUS (Bruker) FT-IR spectrometer using KBr pellets in 4000-500 cm-1 range. Elemental analyses (C,H and N) were performed on a Perkin-Elmer 240C analyzer. Thermogravimetric analyses (TGA) were carried out on a Rigaku standard TG-DTA analyzer under N2 at a heating rate of 10°C min-1,using an empty Al2O3 crucible as reference. The room-temperature powder X-ray diffraction (PXRD) spectra were recorded on a Rigaku D/Max-2500 diffractometer with a Cu-target tube and a graphite monochromator at 40 kV,100 mA. Simulation of the PXRD pattern based on single-crystal data was carried out by diffraction-crystal module of the Mercury (Hg) program available free of chargeviathe Internet at http://www.iucr.org. All fluorescence measurements were performed on a Hitachi F-4500 fluorescence spectrophotometer equipped with a plotter unit. UV-vis absorption spectra were measured with a Hitachi U-3010 UV-vis spectrophotometer (Hitachi,Japan). Magnetic data were collected using crystals of the samples on a Quantum Design MPMS-XL-7 SQUID magnetometer. The data set was corrected using Pascal’s constants to calculate the diamagnetic susceptibility and experimental correction for the sample holder were applied. 2.2. Syntheses of polymers
Synthesis of {[Cu3(tci)2(DMAc)3]·6DMAc·2H2O}n (1): A mixture of Cu(NO3)2·3H2O (0.070 mmol),H3tci (0.035 mmol) in 3 mL DMAc was sealed in a 10 mL vial and heated at 95°C for 72 h. The reaction vessel was cooled to room temperature and green crystals were collected with ca 40% yield based on H3tci. FT-IR (KBr pellets, cm-1 ): 3446 w,2935 m,1697 s,1606 m,1558 m,1471 s,1425 s, 1330 s,1265 m,1190 s,1018 s,962 s,765 s,721 m,669 s,592 s, 518 w. Anal. Calcd. for C60H113N15O31Cu3: C,41.63; H,6.58; N, 12.14%. Found: C,41.27; H,6.12; N,12.12%.
Synthesis of {[Cu3(tci)2(tpt)2(H2O)2]·2DMAc·2H2O}n (2): A mixture of Cu(NO3)2 3H2O (0.070 mmol),H3tci (0.035 mmol) and tpt (0.035 mmol) in 3 mL 2:1 (v/v) mixture of DMAc/H2O was sealed in a 10 mL vial and heated at 95°C for 72 h. The reaction vessel was cooled to room temperature and green crystals were collected with ca 50% yield based on H3tci. FT-IR (KBr pellets, cm-1 ): 3411 m,2966 w,1695 s,1622 s,1575 s,1519 s,1462 w, 1373 s,1342 s,1313 m,1211 s,1060 s,1018 m,985 w,871 s,804 s, 763 s,653 s,516 w. Anal. Calcd. for C68H72N20O23Cu3: C,46.77; H, 4.27; N,16.04%. Found: C,47.26; H,4.20; N,16.21%. 2.3. X-ray crystallography
Single crystal X-ray diffraction measurement for 1 and 2were collected on a Rigaku Saturn 70 diffractometer at 113(2) K. The unit cell parameters and data collections were determined with Mo-Ka radiation (l = 0.71073 Å) and unit cell dimensions were obtained with least-squares refinements. Semi-empirical absorption corrections were applied using SADABS program. The structure was solved by direct methods using the SHELXS program of the SHELXTL package and refined with SHELXL [12]. The final refinement was carried out by full matrix least-squares methods with anisotropic thermal parameters for non-hydrogen atoms on F2 . For 1,a number of diffused scattered peaks with electron density were observed from the difference Fourier map,which can be attributed to the disordered DMAc and water molecules. The attempt to model these peaks was unsuccessful because the obtained residual electron density peaks were diffused. PLATON/ SQUEEZE [13] was used to further refine the structure. Crystal data and structure processing parameters are collected in Table 1 and some selected bond lengths and bond angles are listed in Table S1 in Supporting information. CCDC-1023890 (1) and 1023891 (2) contain the supplementary crystallographic data that can be obtained free of charge from The Cambridge Crystallographic Data Centerviawww.ccdc.cam.ac.uk/data_request/cif.
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Table 1 Crystallographic data and refinement details for 1 and 2. |
Crystal structure of 1. Single-crystal structural analysis reveals that 1 crystallizes in the monoclinic space group P21/c and possesses a 2D coordination network. As shown in Fig. 1a,1 contains three crystallographically independent Cu(II) ions,and each Cu(II) ion locates in the center of a square-pyramidal coordination geometry completed by four O atoms from four tci3- ligands and one axial O atom from DMAc molecule. There exist two kinds of Cu2(CO2)4 paddle-wheel units in 1 that both are bridged by four carboxylate groups from four different tci3- ligands,one consists of two Cu(II)1 ions and the other of one Cu(II)2 and one Cu(II)3 ions,both with a Cu···Cu distance ofca.2.65 Å. The Cu-O bond distances (1.952(17)-2.117(17) Å ) are all within the normal range [14].
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| Fig. 1. (a) View of the coordination environment of the Cu ions in1. Symmetry codes: A:-x,-y,1-z;B:x,1/2 -y,-1/2 +z;C:x,1/2 -y,1/2 + z;D:-x,1/2 + y,3/2 -z. (b) The 2D layer generated from the tci3- ligands and Cu2(COO)4units. (c) The 2D topological net with the short Schla¨fli symbol of (42·6)(42·63·8). (d) The 3D framework structure formed by C-H···O interactions (magenta) between the adjacent 2D sheets. | |
Furthermore,every tci3- ligand in 1connects three Cu2(CO2)4 paddle-wheel units to result in a 2D layer structure (Fig. 1b). Simplifying tci3- ligand as a three connecting spacer and each paddle-wheel unit as a four connecting node gives rise to a 2D (3,4)-connecting topological net (Fig. 1c) with a short Schläfli symbol of (42·6)(42·63·8). This 2D layer comprises two kinds of windows: a 6-membered (three tci3- ligands and three Cu2(CO2)4 units) and a 4-membered rings (two tci3- ligands and two Cu2(CO2)4units) (Fig. S1 in Supporting information). Adjacent 2D layers are further linked through intermolecular C-H···O interactions (Table S2 in Supporting information) to generate a 3D framework structure (Fig. 1d and Fig. S2 in Supporting information). After removing solvent molecules filling in the pore space, the accessible volume is 48% (3787.0 Å3) per unit cell (7888.0 Å3), as estimated using PLATON [13].
Crystal structure of 2. When rigid N-donor ligand tpt is introduced as auxiliary ligand,a 2D network with (44·62) topology is produced for 2 that crystallizes in triclinic space group P1. Polymer 2 comprises two kinds of crystallographically independent Cu(II) ions. As illustrated in Fig. 2a,Cu(II)1 ion locates in the center of a distorted square pyramidal geometry surrounded by four pseudo-coplanar O atoms from two tci3- ligands and one N atom from tpt ligand at the axial position.
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| Fig. 2. (a) View of the coordination environment of the Cu ions in 2. Symmetry codes: A:-x,-y,1-z;B:-x,1-y,2-z;C:-1+x,y,1+z;D:1-x,1-y,1-z. (b) The 2D layer viewed along thea-axis andc-axis. (c) The 3D framework structure formed by C-H···O interactions (yellow) and π···π interactions (green) between the adjacent 2D sheets. | |
Adjacent Cu(II)1 and Cu(II)1A ions are bridged by four carboxylates in bidentate bridging mode,giving a Cu2(CO2)4 paddle-wheel unit with a Cu···Cu distance of 2.638 Å. Cu(II)2 ion is six-coordinated by two O atoms from two tci3- ligands,two O atoms from two water molecules and two N atoms from two tpt ligands,leading to a slightly distorted octahedral geometry. The Cu(II)2 ions and Cu2(CO2)4 units are further bridged by tci3- and tpt ligands to form a 2D layer (Fig. 2b). The Cu-O bond distances (1.957(2)- 2.483(4) Å ) and Cu-N bond distances (2.038(3)-2.159(2) Å) of 2 are all within the normal range [14].
Careful inspection of the crystal structure indicates that there exist intermolecular C-H···O and π···π stacking interactions between the 2D layers when viewed along the c direction (Fig. 2c and Table S2 in Supporting information),which brings about the parallel packing of 2D layers to form a 3D supramolecular structure. Therefore,C-H···O interactions and π···π stacking interactions play the important roles in stabilizing the structure. Topological analysis reveals that both Cu2(CO2)4 unit and Cu(II)2 ion connect to four organic ligands,and therefore can be simplified as a 4-connected node. Thus,the overall network of 2 can be described as a binodal 4-connected framework with a short Schla¨fli vertex symbol of (44·62) (Fig. S3 in Supporting information). 3.2. Thermal behavior
Thermogravimetric analysis (TGA) of 1 (Fig. 3) reveals a notable weight loss during the temperature range from 50°C to 180°C, which can be attributed to the loss of dissociated DMAc and water molecules. The successive weight loss starting from 240°C corresponds to the decomposition of framework. The TG curve (Fig. 3) of 2 also displays two-step weight loss in the temperature range of 60-700°C. The first step from 60°C to 220°C can be assigned to the loss of dissociated DMAc and water molecules. The decomposition of residual framework starts from 250°C and completes at about 400°C.
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| Fig. 3. Thermogravimetric curves of polymers 1 and 2. | |
UV-vis reflectance spectra (Fig. 4) of 1 and 2 showed broadpeaks from 250 nm to 800 nm,and two bands with maxima at 502 and 508 nm were observed for 1 and 2,respectively. The photoluminescence spectra of the free ligand H3tci and polymers 1-2 in the solid state have been investigated at room temperature. The luminescences exhibit three intense emission bands at 473,483,494 nm for 1and an intense fluorescent emission at 473 nm for 2,respectively, upon excitation at 300 nm (Fig. 5). In comparison,the H3tci ligand displays emission peak at 488 nm under the same condition. The closeness in the emission wavelength with H3tci indicates that the photoluminescence of 1 and 2could be mainly attributed to H3L ligand centered electronic excitation perturbed by the metal ion and other components.
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| Fig. 4. Solid-state UV-vis reflectance spectra of polymers 1 and 2. | |
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| Fig. 5. Luminescent emission spectrum of polymers 1 and 2 in the solid state. | |
Solid-state magnetic susceptibility measurement for 1 was performed in the range of 2-300 K under the field of 1000 Oe. The xMTversusTplot andxMversus Tplot are shown in Fig. 6. At 300 K, the value ofxMTis 0.265 cm3 mol-1 K,which is smaller than the value of a non-interacting spin-only Cu(II) ion (0.375cm3 mol-1 K at room temperature). Lowering the temperature,the xMT decreases rapidly to 0.0215 cm3 mol-1 K at 50 K and gradually approaches 0.0155cm3 mol-1 K at 2 K,indicating the occurrence of strong antiferromagnetic interactions between Cu(II) ions. The xMversus Tcurve further confirms antiferromagnetic interactions. It is well known that syn-syn carboxylate bridge in dinuclear copper(II) polymers can mediate strong antiferromagnetic interactions [15, 16, 17, 18]. Based on the structure information of 1,the xMT versus T curve was least-squares-fitted using the Bleaney-Bowers equation derived from the isotropic spin Heisenberg Hamiltonian H=-JS1S2 with local spin S= 1/2 [19]:
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| Fig. 6. The plot ofxMTversusTfor the polycrystalline sample and the plot of xM versus T for polymer1. The red solid line is theoretical fits based on the Bleaney- Bowers equation. | |
Two new coordination polymers based on tris(2-carboxyethyl)isocyanuric acid have been prepared and characterized. Polymer 1 shows a (3,4)-connected 2D layer structure; while polymer 2 comprises dinuclear and mononuclear copper building units, which are further bridged by tci3- and tpt ligands to give a 2D 4-connected topological net. Different structure verifies the vital role of auxiliary ligand playing in the design and construction of target coordination polymers. In addition,the luminescent properties and the solid-state UV-vis spectra of 1 and 2 were explored. Furthermore,antiferromagnetic behaviors were observed for polymer 1.
AcknowledgmentsThis work was supported by the 973 Program of China (No. 2014CB845600) and the NSFC (Nos. 21421001 and 21290171),and MOE Innovation Team (No. IRT13022) of China.
Appendix A. Supplementary dataSupplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.01.019.
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