English

• 1. INTRODUCTION

  The functional materials in coordination chemistry have been studied rather extensively^[1-3], but they are still a topic of interest to researchers because of the diverse nature of application in luminescence^[4], electrochemical^{[5, 6]}, magnetism^[7], gas storage^[8] and catalysis^[9]. Magnetic and proton conduction materials are becoming more and more important in the study of functional materials, and many excellent studies have been reported^[10-15]. The study on magnetic properties of complexes has been going on for many years. To date, many types of complex magnets have appeared, such as single-molecule magnets, single-chain magnets, and ion magnets. All of the above studies are designed to understand basic magnetic and magnetic structure correlations as well as the development of new molecule-based materials. Proton carriers H₃O⁺/H⁺ provided by acid or OH groups are needed in these materials. Besides, proton-conducting pathways comprised of H-bond network are also required for conducting proton. Generally, proton conductivities of organic and inorganic hybrid compounds such as metal oxide and nafion have been researched due to their applications in sensors and fuel cells. But in recent years, MOFs as an intriguing class of crystalline materials have attracted much attention in proton conductivity. However, the study of magnetic materials with proton conduction is relatively small, so it is very important to study the properties of multifunctional complex. We report herein our success construction of bifunctional copper complex [C₆₀H₅₄Cu₄O₃₉]_n with magnetic and gas storage based on a multiple acid ligands (H₃cpia).

  2. EXPERIMENTAL

  2.1 Materials and measurements

  All the starting materials for synthesis were commercially available and used as received. IR spectrum was recorded in the range of 400~4000 cm⁻¹ using a VECTOR-22 spectrometer with KBr discs. X-ray powder diffraction (PXRD) spectra were recorded on an Empyrean (PANalytical B.V.) diffractometer for a Cu-target tube equipped with a graphite-monochromator. Simulation of the PXRD spectra was performed by the single-crystal data and diffraction-crystal module of the Mercury (Hg) program available free of charge via the Internet at http://www.iucr.org. Comprehensive thermal analyzer (TG-DSC, STA449F3, Germany) was applied to investigate the thermal stability of the sample under nitrogen atmosphere at a heating rate of 10 ℃⋅min^-1. Magnetic data were collected by a Quantum Design SQUID-VSM magnetometer. Alternating current (AC) impedance measurements with sample pellets (6 mm in diameter) were performed by the conventional quasi-four-probe method on the Zennium electrochemical workstation employing silver paste and silver wires. The frequency range of the AC source is 1 ~ 1×10⁷ Hz, and the amplitude of AC voltage is 100 mV.

  2.2 Synthesis of complex [C₆₀H₅₄Cu₄O₃₉]_n (1)

  A mixture of H₃cpia (0.25 mmol), Cu(NO₃)₂·3H₂O (0.75 mmol), NaOH (0.75 mmol) and distilled H₂O (10 mL) was sealed in a 25 mL Teflon-lined autoclave and heated to 120 ℃ in 12 h. After maintaining for 72 h, the reaction vessel was cooled to room temperature in 24 h. Green crystals were collected with ca. 43% yield based on H₃cpia. FT-IR (KBr pellets, cm^-1): 3381, 2361, 1621, 1507, 1384, 1236, 1169, 859, 770, 729, 692, 623.

  2.3 X-ray crystallography

  Single-crystal X-ray diffraction measurement for 1 was collected on a Bruker SMART APEX-Ⅱ CCD diffractometer equipped with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293(2) K using an ω-φ scan mode. Absorption correction was applied by using the SADABS^[16]. The structure was solved by direct methods and refined by full-matrix least-squares techniques on F² with SHELX-97 package^[17]. All non-hydrogen atoms were refined anisotropically and hydrogen atoms isotropically by full-matrix least-squares refinement. The hydrogen atoms of water molecules in 1 were added automatically by SHELXL-2014 using OLEX2 as a graphical user interface. Complex 1 is of triclinic system, space group P$ \overline 1 $ with a = 10.7318(18), b = 12.267(2), c = 14.528(2) Å, α = 113.560(2)^o, β = 96.156(3)^o, γ = 103.552(3)º, V = 1660.5(5) ų, Z = 1, S = 1.057, F(000) = 842, R = 0.0517 and wR = 0.1426 (I > 2σ(I)). R = Σ||F_o| − |F_c||/Σ|F_o|; wR = [Σ[w(F_o² − F_c²)²]/Σw(F_o²)²]^1/2. The selected bond lengths and bond angles are given in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for Complex 1

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–O(8)	1.947(4)		Cu(1)–O(6)	1.960(3)		Cu(2)–O(14)#2	1.934(3)
  Cu(1)–O(7)	1.954(3)		Cu(1)–O(10)	2.125(4)		Cu(2)–O(5)#3	1.970(3)
  Cu(1)–O(9)#1	21.978(4)		Cu(2)–O(15)	1.929(3)		Cu(2)–O(16)	2.131(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(8)–Cu(1)–O(7)#1	86.94(15)		O(7)#1–Cu(1)–O(10)	93.92(16)		O(15)–Cu(2)–O(4)#4	90.05(16)
  O(8)–Cu(1)–O(6)	90.13(15)		O(6)–Cu(1)–O(10)	97.65(16)		O(14)#2–Cu(2)–O(4)#4	89.61(16)
  O(7)#1–Cu(1)–O(6)	168.29(15)		O(9)#1–Cu(1)–O(10)	95.93(17)		O(15)–Cu(2)–O(16)	99.09(15)
  O(8)–Cu(1)–O(9)#1	168.47(15)		O(15)–Cu(2)–O(14)#2	167.60(15)		O(5)#3–Cu(2)–O(16)	100.09(15)
  O(8)–Cu(1)–O(10)	95.60(17)		O(15)–Cu(2)–O(5)#3	89.74(16)		O(4)#4–Cu(2)–O(16)	91.78(14)

  3. RESULTS AND DISCUSSION

  3.1 Crystal structural description

  X-ray diffraction analysis revealed that complex 1 crystallizes in triclinic P$ \overline 1 $ space group and the asymmetric unit consists of two CuII ions, two Hcpia ligands, two coordinated water molecules and six lattice water molecules. As depicted in Fig. 1a, Cu(1) (or Cu(2)) is surrounded by four oxygen atoms from four different Hcpia ligands, and one oxygen atom from one water molecule. The coordination geometries of Cu(1) and Cu(2) are viewed as spherical square pyramid (SSP) calculated by SHAPE 2.0. Two adjacent Cu(Ⅱ) atoms are connected by four carboxylate groups with syn, syn mode to form a paddle-wheel-like dinuclear cluster. The coordination number of Cu(1) and Cu(2) is all five and the coordination of copper ion can be seen as a distorted tetragonal pyramid. The O–Cu–O angles are in the range of 86.9(3)º~168.5(4)º. The Cu–O bond lengths from 1.929(9) to 2.142(7) Å fall in the normal range. Adjacent ligands are linked by Cu to form a 2D network structure. The adjacent two copper atoms can be considered as a dinuclear Cu(2) unit. Simplifying the dinuclear Cu(2) unit as a four-connecting node generates a (4, 4) topologic structure for complex.

  Figure 1

  

  Figure 1.  Views of (a) the coordination environment of Cu(Ⅱ) ions and linkage of Hcpia ligands in 1 (symmetry codes: A: 1–x, 2–y, z; B: –x, 2–y, –1–z); (b) Two-dimensional structure based on paddle-wheel-like dinuclear clusters

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  3.2 Thermogravimetric and PXRD analysis

  Before magnetic properties test and proton conduction analysis, powder diffraction experiments and thermogravimetric experiments of complex 1 were carried out. Powder diffraction experiment result showed that the experimental and simulated values are well matched. Thermogravimetric analysis of complex 1 revealed three steps of decomposition with the 71% total weight loss during 63~730 ℃ as shown in Fig. 2(a). The first major weight loss of 9% occurred at the temperature between 68 and 157 ℃, corresponding to the removal of water molecules. There is no weight loss between 157 and 263 ℃. The second weight loss of 62% occurred from 263 to 730 ℃ due to the decomposition of complex skeleton. Further heating shows no noticeable weight loss between 730 and 800 ℃. Thermogravimetric analysis indicates the complex has good thermal stability.

  Figure 2

  

  Figure 2.  (a) TGA result of complex 1; (b) PXRD pattern of complex 1

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  3.3 Magnetic properties

  From magnetic point of view, complex 1 can be simplified as dinuclear Cu₂ complex and its measurements were carried out on the crystalline sample. The temperature dependence of χ_mT for 1 as a curve of χ_mT versus T (χ_m is the molar magnetic susceptibility for two Cu²⁺ ions) in the range of 2~300 K under an applied field of 1 kOe is shown in Fig. 3a. the value of χ_mT at 300 K is 0.67 emu·mol^-1·K, which is somewhat smaller than the expected value (S = 1/2, g = 2.0, C = 0.75 emu·mol^-1·K). On cooling, the χ_mT value decreases rapidly until about 75 K, suggesting the existence of strong antiferromagnetic interaction between Cu²⁺ ions in the Cu₂ cluster. At low temperature, the χ_mT value almost remains a constant. It can be explained that the short distance (2.625 Å) between adjacent Cu²⁺ ions leads to the appearance of metal-metal bond besides the syn-syn carboxylates transfer the antiferromagnetic interaction. The field-dependent magnetization per two Cu²⁺ ions of complex 1 at 1.8 K increases slowly and tends to 0.23 Nβ at 70 kOe (far from the saturation value) at 70 kOe, which is in agreement with the strong antiferromagnetic coupling (Fig. 3b). All in all, complex 1 displays antiferromagnetic behaviour.

  Figure 3

  

  Figure 3.  (a) Plot ofχ_mT vs. T under 1 kOe dc field of 1; (b) Plot of M vs. H at 1.8 K of complex 1

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  3.4 Proton conduction

  The protonated carboxyl groups as proton carriers and free water chains as proton-conducting pathways make 1 suitable for potential proton-conducting solid materials. Generally, proton-conducting property of MOFs is influenced by relative humidity (RH) and temperature^[18]. Thus, the proton conductivities of solid samples are investigated by AC impedance spectroscopy. For 1, we aimed to obtain conductivity in the form of Nyquist plots (Z' versus Z") at different temperature under relatively lower humidity conditions (75.79% RH) by employing a compacted pellet of the powder sample, as shown in Fig. 4. They display one semicircle and the conductivities of the sample were derived from the impedance values by employing the following equation^[19].

  $ {\rm{ \mathsf{ σ} }} = \frac{d}{{Z \cdot S}} $

  Figure 4

  

  Figure 4.  Nyquist diagrams of 1 at 60 ℃ (a) and 75 ℃ (b) (RH = 75.79%)

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  where σ is the conductivity (S·cm^-1), d the thickness (cm) of the measured sample, S the electrode area (cm²) and Z the impedance (Ω). d and S are 0.044 cm and 0.2827 cm² and thus the σ values are 1.04 × 10^-6 S·cm^-1 (60 ℃) and 1.23×10^-5 S·cm^-1 (75 ℃), respectively, which are comparable to the reported MOFs.

  4. CONCLUSION

  In summary, a new 2D Cu^Ⅱ-based MOF derived from paddle-wheel-like dinuclear clusters has been synthesized via hydrothermal reaction of Cu^Ⅱ salt and tricarboxy ligand. Complex 1 exhibits strong antiferromagnetic behavior and high proton conductivity with carboxyl as proton carriers and lattice water as proton-conducting pathways.

  
  
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• 

  Figure 1  Views of (a) the coordination environment of Cu(Ⅱ) ions and linkage of Hcpia ligands in 1 (symmetry codes: A: 1–x, 2–y, z; B: –x, 2–y, –1–z); (b) Two-dimensional structure based on paddle-wheel-like dinuclear clusters

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  Figure 2  (a) TGA result of complex 1; (b) PXRD pattern of complex 1

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  Figure 3  (a) Plot ofχ_mT vs. T under 1 kOe dc field of 1; (b) Plot of M vs. H at 1.8 K of complex 1

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  Figure 4  Nyquist diagrams of 1 at 60 ℃ (a) and 75 ℃ (b) (RH = 75.79%)

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for Complex 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–O(8)	1.947(4)		Cu(1)–O(6)	1.960(3)		Cu(2)–O(14)#2	1.934(3)
  Cu(1)–O(7)	1.954(3)		Cu(1)–O(10)	2.125(4)		Cu(2)–O(5)#3	1.970(3)
  Cu(1)–O(9)#1	21.978(4)		Cu(2)–O(15)	1.929(3)		Cu(2)–O(16)	2.131(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(8)–Cu(1)–O(7)#1	86.94(15)		O(7)#1–Cu(1)–O(10)	93.92(16)		O(15)–Cu(2)–O(4)#4	90.05(16)
  O(8)–Cu(1)–O(6)	90.13(15)		O(6)–Cu(1)–O(10)	97.65(16)		O(14)#2–Cu(2)–O(4)#4	89.61(16)
  O(7)#1–Cu(1)–O(6)	168.29(15)		O(9)#1–Cu(1)–O(10)	95.93(17)		O(15)–Cu(2)–O(16)	99.09(15)
  O(8)–Cu(1)–O(9)#1	168.47(15)		O(15)–Cu(2)–O(14)#2	167.60(15)		O(5)#3–Cu(2)–O(16)	100.09(15)
  O(8)–Cu(1)–O(10)	95.60(17)		O(15)–Cu(2)–O(5)#3	89.74(16)		O(4)#4–Cu(2)–O(16)	91.78(14)

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•