Magnetic Property of a Three-dimensional Copper Metal-organic Framework①

GAO Yu-Jie TIAN Chong-Bin TANG Jing-Xiao CUI Mei-Yan ZHOU Chuang-Yu FENG Mei-Ling HUANG Xiao-Ying

?

Magnetic Property of a Three-dimensional Copper Metal-organic Framework①

GAO Yu-Jiea,bTIAN Chong-BinbTANG Jing-Xiaoa,bCUI Mei-YanbZHOU Chuang-YucFENG Mei-Lingb②HUANG Xiao-Yingb②

a(College of Chemistry, Fuzhou University, Fuzhou 350002, China)b(State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)c(College of Chemistry, Fujian Agriculture and Forestry University, Fuzhou 350002, China)

A copper metal-organic framework (MOF) compound based on 2,5-thiophenedi- carboxylic acid (H2TDC) ligand, namely Cu(TDC)(H2O)?DMA (1, DMA =,?-dimethylace- tamide), has been synthesized in gram-scale via a one-pot solvothermal route in a high yield of 81.3%.Single-crystal X-ray analysis reveals that the structure of 1 features a three-dimensional (3D) open framework constructed by TDC interconnecting 1D chains of [-Cu(COO)(H2O)Cu-].Thermal property was investigated by TG-MS.The magnetic measurements indicate the existence of weak antiferromagnetic interactions between the Cu2+centers in 1.

metal-organic framework, copper, solvothermal synthesis, structure, magnetic property;

1. INTRODUCTION

Metal-organic frameworks (MOFs), as a promi- sing class of crystalline organic-inorganic hybrid materials with properties of gas adsorption and separation, catalysis, luminescence and so on, have received great attention in the past three decades[1-4].As the targeted applications of MOFs become more prevalent, it is more and more important to control and predict the structures of these materials.Signi- ficant results have originated from the “reticular synthesis” approach by Yaghi et al.that defines the secondary building unit (SBU) as a rigid center and the organic linker as a rigid spoke connecting two or more centers together[5,6].While Ferey et al.presents “controlled SBU technique” in which the individual SBU is synthesized prior to MOF synthesis and used as a “seed” for crystal growth, effectively directing MOFs with various types of frameworks[7].Com- pared to the limited number of metallic elements as a single nod, the diverse SBUs are more effective in constructing MOFs with novel structures and excellent properties[8].For instance, compounds with one-dimensional (1D) spin-chain are often found to exhibit various fascinating magnetic properties[9, 10].

The ligand 2,5-thiophenedicarboxylic acid (H2TDC) is a representative of the heterocyclic dicarboxylic acid family.The H2TDC ligand is an aromatic rigid molecule and the sulphur atom of its thiophene ring contains a lone pair of electrons which can be easily delocalized within the ring.Thus the networks constructed from the H2TDC ligand often exhibitgood stability and unique physical and chemical properties[11].Up to now, the H2TDC ligand has been used to construct MOFs with all kinds of metal ions[8,12-16].For instance, the Cu2+ion having diverse coordination modes could form a variety of compounds such as 0D-[Cu(TDC)(nnen)2]·H2O(nnen=,?-dimethylethylenediamine), 1D-{[Cu2(l-TDC)2(ampy)2]2DMF}(ampy = 2-amino- methylpyridine), 2D-[Cu2(TDC)2(NH3)4], 2D-{[Cu2(TDC)2(MeOH)2]4}, and 3D-[Cu(TDC)-(bpy)(H2O)](bpy) (bpy = 4,4?-bipyridine)[11,17-26].Herein, we report the synthesis, characterization, and magnetic property of a Cu-TDC compound, namely Cu2(TDC)2(H2O)2?2DMA (1).Although itssingle crystal structure has been reported recently, the magnetic property of 1 has not been studied in detail[27].Moreover, as the different synthetic method for 1 from that in literature[27]was applied here, the gram-scale synthesis with high yield (81.31%) has been realized.The 3D framework of 1 is constructed fromthe 1D [-Cu(COO)(H2O)Cu-]chains as SBUs interconnected by TDC ligands.The magnetic property has been investigated deeply revealing the existence of weak antiferromagnetic interactions between the Cu2+centers in 1.

2 EXPERIMENTAL

Materials and synthesis CuCl2·2H2O (99.9%, Shanghai Qingong Chemical Co.Ltd.), 2,5-thiophenedicarboxylic acid (99%, Wuhan Chifei Chemical Co.Ltd.),,?-dimethylacetamide (DMA) solvent (99%, Sinopharm Chemical Reagent Co.Ltd.) and tap water were used without further purification.

Synthesis of 1 A mixture of CuCl2·2H2O (0.1 mmol, 0.0170 g) and H2TDC (0.1 mmol, 0.0172 g) in 2 mL,?-dimethylacetamide (DMA) and 1 mL H2O was stirred under ambient conditions until homogeneous.The resulting mixture was sealed in a 23 mL Teflon-lined stainless-steel reactor, heated at 90 °C for 3 days and then cooled to room tempera- ture.Blue block-like crystals were obtained by filtration without any impurities.The crystalline pro- ducts were air-dried.Anal.Calcd.for C20H26N2O12S2Cu2: C, 35.45; H, 3.87; N, 4.13%.Found: C, 35.37; H, 3.92; N, 4.12%.

Gram-scale synthesis of 1 A mixture of CuCl2·2H2O (4 mmol, 0.6819 g), H2TDC (4 mmol, 0.6881 g) in 20 mL DMA and 10 mL H2O was stirred under ambient conditions until homogeneous.The resulting mixture was sealed in a 100 mL Teflon- lined stainless-steel reactor, heated at 90 °C for 3 days and then cooled to room temperature.Blue block-like crystals were obtained without any impurity.The crystalline products were air-dried (Yield: 1.102 g, 81.31% based on Cu).

Physical measurements Elemental analyses (EA) of C, H, and N were measured by a German Elementary Vario EL III instrument.Thermogra- vimetric analysis-mass spectroscopy (TG-MS) was carried out with a STA449C-QMS403C thermal analysis-quadrupole mass spectrometer at a heating rate of 10oC/min under a nitrogen atmosphere.Magnetic susceptibility was measured under an applied field of 1000 Oe from 300 to 2 K by a commercial Magnetic Property Measurement System (MPMS).Magnetization was measured at 2 K in an applied field from 0 to 8 T by the Quantum Design Physical Property Measurement System (PPMS).Powder X-ray diffraction (PXRD) patterns were performed with a Miniflex II diffractometer at 30 kV and 15 mA using Cu(1.54178 ?) in the angular range of 2= 5～55° at room temperature.The simulated PXRD pattern through single-crystal X-ray data was generated by using the Mercury program.The single-crystal X-ray diffraction intensity data for 1 were collected using a XCaliburE diffractometer with graphite-monochromatic Mo(0.71073 ?) at 295(2) K.The structure was solved by direct methods and refined by full-matrix least-squares on2by using the program SHELX-2016.

3 RESULTS AND DISCUSSION

3.1 Gram-scale synthesis and thermal stability

A recent report indicated that compound1couldbe solvothermally synthesized by employing CuCl2·2H2O and H2TDC ligand as reactants in the single solvent of DMA, while a mixed-solvent solvothermal method was adopted in our synthesis, that is, H2O functioned as the auxiliary solvent, which could not only reduce the cost of synthesis but also help in increasing the yield of 1 greatly.Moreover, in our work, the gram-scale synthesis of 1 could be realized through a facile, one-pot and economical route, which could easily produce more than 1.0 g pure crystalline samples (Fig.1a).Remarkably, PXRD confirms the phase purity of the air-dried products without washing or manual selection (Fig.1b).Moreover, the yield with our synthetic method could reach 81.31% (based on Cu) which is much higher than that in the reported synthetic method (yield: 37%, based on Cu)[27].TGA-MS analysis showed that the molecular-ion mass peaks of DMA (/= 87) and H2O (/= 18) could be detected at the weight-loss step until 380oC (Fig.2).Thus the sharp weight loss of 1 that began at 100oC and finished at 380oC could be attributed to the removal of DMA and H2O molecules and the pyrolysis of the framework at the same time.After 380oC, the weight loss continued to 800oC.

Fig.1. (a) Photo of the crystalline product of 1 via agram-scale synthesis without washing or manual selection.(b) PXRD patten of 1 and the simulated calculated from single-crystal X-ray data of 1

Fig.2. TG-MS curves of 1

3.2 Description of the crystal structure of compound 1

Although thesingle-crystal structure of this com- pound has been reported recently[27],some details have not been described such as the secondary bon- ding, hydrogen bonding interactions and topology analysis.In this section, we thus further investigate the structure of 1.The structure of 1 features a 3D open framework fabricated by TDC2-ligands, H2O molecules and Cu2+nodes, with DMA molecules located in the channels.Its asymmetric unit contains one formula unit including two halves of Cu2+ions (Cu(1) and Cu(2)), one TDC2-ligand, one coordina- ted H2O molecule and one free DMA molecule (Fig.3a).The two Cu2+ions adopt different coordination geometries.Cu(1) is occupied by four oxygen atoms from four TDC2-ligands in the equatorial plane and coordinated by two H2O mole- cules in the apical positions through secondary bonding (Cu(1)–O(5), 2.542(2) ?; Fig.3b and S1a), while Cu(2) is coordinated by four oxygen atoms from two TDC2–ligands and two H2O molecules in the equatorial plane and coordinated by two oxygen atoms from two TDC2–ligands in the apical positions through weak secondary bonding (Cu(2)–O(3), 2.703(2) ?; Fig.3b and S1b).Each TDC2-ligand links to four Cu2+ions with its three carboxylic oxygen atoms (O(1), O(2), O(3)) through three coordination bonds and one secondary bond, leaving the fourth oxygen atom O(4) being uncoordinated but forming hydrogen bond with the coordinated water molecule O(5) (Fig.S1c).The O(5)–H(5A)?O(4)i(i:–1/2, –+1/2,+1/2) hydrogen bond distance andangle are 2.549(3)? and 164(4)o,respectively (Table S3).Along theaxis, the carboxylic groups of TDC2-ligands together with coordinated H2O molecules interconnect the Cu(1) and Cu(2) ions to form a [Cu1-Cu2-Cu1-Cu2]array characteristic of the SBU of [-Cu1-(COO)(H2O)-Cu2-](Fig.3b).Each chain links to other four same chains via TDC2-ligands along theandaxes.As a result, a 3D framework is formed containing large rhombohedral channels along theaxis with window size of 10.21 × 10.21 ?2in which the DMA mole- cules are located (Fig.3c).Note that the DMA molecules interact with the frameworkO–H···O and C–H···S hydrogen bonds; the O(5)–H(5B)···O(6) and C(10)–H(10C)···S(1)ii(ii.–+3/2,+1/2,–+3/2) hydrogen bond distances andangles are 2.623(4) and 3.534(10) ?, and 167(4) and 122.0o,respectively (Table S3).From the topological point of view, the 3D framework of 1 is a gwg net (Fig.3d).

3.3 Magnetic property

As compounds with one-dimensional (1D) spin- chain or based on copper are often found to have interesting magnetic properties[10, 28-29], the tempera- ture-dependent magnetic susceptibility of 1 was measured under a magnetic field of 1000 Oe in the temperature range of 2～300 K.The magnetic property of 1 is shown in the form ofmversusplot (Fig.4a), wheremis the molar magnetic susceptibility for one Cu2+ion.At room temperature, the value ofmis 0.43 cm3?K?mol-1, which is somewhat higher than the spin-only value (0.38 cm3?K?mol-1for= 1/2 and= 2.0) expected for an uncoupled Cu2+ion.With the decrease of temperature from 300 to 2 K, themvalues decreased smoothly and reached a value of 0.017 cm3?K?mol-1at 2 K.This phenomenon is indicative of antiferromagnetic interaction in 1, as suggested by the negative Weiss constant= ?29.38 K, obtained from the data of 1/mversusin the temperature range of 50～300 K by the Curie-Weiss law (Fig.S2).The magnetization increases linearly with increasing the magnetic field, and reaches a value of 0.116 Nat 8 T, which is lower than the saturation value of one Cu2+ion (1 Nfor= 2.0), suggesting again the antiferromagnetic interaction in 1 (Fig.4b).To confirm the presence of long range of antiferro- magnetic ordering, the heat capacity data were collected from 2 to 30 K.Fig.S3 shows the absence of-like peak, indicating a short-range antiferromag- netic ordering.

To estimate the exchange coupling constant between the adjacent Cu2+ions,mdata were fitted by using the infinite-chain mode of classical spins derived by Fisher with= ?ΣS?S+1(stands for the exchange constant between the adjacent Cu2+ions, andSis the classical spin vector).The results can be fitted using the numerical expression for< 0 and the corresponding analytical expression is as follows[30]:

Fig.3. (a) Structural fragment of 1.Symmetry codes: i:+1,+1; ii: –+1/2,+1/2, –+1/2; iii:–1/2, –+1/2,+1/2; iv: –+1, –+1, –+1; v:+1/2, –+1/2,+1/2.(b) [-Cu1-(COO)(H2O)-Cu2-]chain in 1.The secondary bonds are shown as dotted lines.(c) View of the 3D open-framework along the-axis in 1.The hydrogen bonds are shown as dotted yellow lines.(d) Topological net of gwg in 1

Fig.4. (a) Experimental and calculated temperature dependence ofmfor 1.(b) Plot of the reduced magnetizationversus the applied fieldat 2 K for 1

with= –/

The best fit in the whole temperature range gives= 2.18,= –25.34 cm?1, and= 5.25 × 10?6.The fitting result also reveals weak antiferromagnetic interaction between the Cu2+centers.The shortest distance of Cu???Cu is 3.3066(1) ? and the magnetic interactions among the spin centers were very weak.In comparison, thevalue (?29.38 K) andvalue (?25.34 cm?1) of 1 are significantly larger than the corresponding values (= ?1.31 K and= ?1.68 cm?1) of a reported compound [Cu(TDC)(dpa)](dpa = 4,4?-dipyridylamine)[26].

4 CONCLUSION

Using a facile solvothermal method, a copper based metal organic material has been synthesized.Remarkably, the ultrahigh yield (81.31%)of the compound has been reached in a one-pot and economical route.The 3D framework of the compound is constructed by TDC ligands intercon- necting 1D [-Cu1-(COO)(H2O)-Cu2-]chains.The magnetic property has been investigated deeply, which reveals weak antiferromagnetic interaction between the Cu2+centers in 1.

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3 January 2018;

28 February 2018 (CCDC 1569063)

① This project was supported by the NNSFC (No.21771183), and Chunmiao project of Haixi Institute of Chinese Academy of Sciences (CMZX-2014-001)

E-mail: Feng Mei-Ling, female, professor, research field: hybrid materials.Fax: +86-591-63173146; E-mail: fml@fjirsm.ac.cn; Huang Xiao-Ying, male, professor, research field: structure chemistry. Fax: +86-591-63173145, E-mail: xyhuang@fjirsm.ac.cn

10.14102/j.cnki.0254-5861.2011-1944