一種三維四元酸銅配合物的合成、結(jié)構(gòu)及其性質(zhì)

邵彩云1，劉孟巖1，練晨1，王力冊(cè)1，許珍煥1，郭旭1，龍銀雙1，路云霞2，楊立榮1*

(1. 河南大學(xué) 化學(xué)化工學(xué)院，河南省多酸化學(xué)重點(diǎn)實(shí)驗(yàn)室,分子與晶體工程研究所，河南 開封 475004；

2. 河南大學(xué)圖書館，河南 開封 475001)

摘要：通過(guò)水熱法合成了一種具有三維孔狀結(jié)構(gòu)的金屬有機(jī)框架物{[Cu(BPTC)0.5·(bpy)]·H2O}∞ (BPTC = 3,3′,4,4′-苯甲酮-四甲酸和 bpy = 4,4′-聯(lián)吡啶). 運(yùn)用元素分析、紅外光譜、X射線單晶衍射以及X射線粉末衍射等對(duì)其進(jìn)行了結(jié)構(gòu)表征, 利用熱重分析評(píng)價(jià)了其熱穩(wěn)定性. X射線單晶衍射分析表明標(biāo)題配合物通過(guò)一維右手螺旋鏈聯(lián)接成為二維層狀結(jié)構(gòu), 進(jìn)而經(jīng)由4,4′-聯(lián)吡啶聯(lián)接成為三維孔狀結(jié)構(gòu). 當(dāng)該孔狀結(jié)構(gòu)中包容的水分子被移除后，該金屬有機(jī)框架物依然保持三維孔狀結(jié)構(gòu). 此外，實(shí)驗(yàn)還研究了標(biāo)題配合物的磁學(xué)和固態(tài)熒光性質(zhì).

關(guān)鍵詞：有機(jī)-金屬框架物;水熱合成;磁性;熒光性質(zhì)

Received date： 2015-05-28.

Foundation item： The Natural Science Foundation of Henan Province (13A15006 and 15NB005).

Biography： SHAO Caiyun (1969-), female, senior experimentalist, main direction is coordination chemistry.*Corresponding author, E-mail: lirongyang@henu.edu.cn.

Studies on metal-organic frameworks (MOFs) are of considerable interest due to their fascinating network topologies and potential applications in molecular sieves, sensors, gas storage, ion exchange, size-selective separation, molecular magnetism, photoluminescence and heterogeneous catalysis[1-6]. A large number of MOFs with plentiful structures and unique properties have been designed and synthesized. The construction of MOFs is highly influenced by some certain factors，such as the coordination tendencies of metal ions, the structural characters of organic ligands, the properties of solvent and pH value, and so on[7-12].

Among various organic ligands, multidentate organic ligands like multicarboxylate acids are often used as the linkers of metal ions to construct diversified MOFs. Certainly, 3,3′,4,4′ -benzophenone-tetracarboxylate was selected as multifunctional organic ligands based on several intriguing characters: i) quadruple carboxylic dentate arms are benefit to forming helical structures; ii) eight potential donor oxygen atoms contribute to construct multi-dimensional frameworks; iii) the noncovalent interactions, such as hydrogen bonds and π…π stacking exist in the benzene rings; iv) the pH values may exert an effect on the deprotonated degree of the ligands. What′s more, auxiliary ligands (for example, 4,4′-bipyridine) may lead to abundant network topology as linkers[13-18].

Here in, we report the hydrothermal synthesis, characteristics, thermal analysis and the crystal structure of the 3D porous coordination complex of Cu(II) center with H4BPTC and 4,4′-bipyridine ligand, namely, {[Cu(BPTC)0.5·(bpy)]·H2O}∞. The typical coordination mode of H4BPTC is summarized in Fig. 1. Additionally, we investigated the thermal analysis, FT-IR spectroscopy and X-ray powder diffraction of the coordination complex, and found that the 3D porous framework remains intact after the guest water molecules are removed.

Fig.1　The coordination mode of the BPTC ligand in the complex

1Experimental

1.1　Reagent and instrument

All reagents were analysis grade. Elemental analysis (C, H and N) was performed by a Perkin-Elmer 2400-II CHNS/O analyzer. Infrared (IR) spectrum was recorded on a Bruker VERTEX 70 IR spectrometer in the range of 4 000-400 cm-1(using KBr in pellets). XRD data were recorded on a DX-2700 instrument with Cu Kαradiation (λ= 0.154 056 nm) in the angular range 2θ= 545oat 293 K. Magnetic susceptibility measurements were carried out by using a Quantum Design MPMS-5 magnetometer in the temperature range of 2-300 K. TG analysis was measured on a Perkin-Elmer TGA7 instrument with a heating rate of 10 ℃/min from 25 to 1 000 ℃ in N2flow. The photoluminescence property was performed on a HITACHI F-7000 fluorescence spectrophotometer in the solid state at the room temperature. Single crystals for X-ray structure analysis was performed on Bruker CCD Apex-II diffractometer with Mo Kαradiation (λ= 0.071 073nm) at 296 K. The structure of the complex was solved by direct methods and further refined by full-matrix least-squares refinements onF2using the SHELXL-97 software and an absorption correction was performed by the SADABS program[19].

1.2　Synthesis of the title complex

The title complex was synthesized from the reaction mixture of copper acetate (0.25 mmol, 49.9 mg), 3,3′,4,4′-benzophenone-tetracarboxylate (0.25 mmol, 89.6 mg), 4,4′-bipyridine (0.25 mmol, 39.0 mg ) in 10 mL distilled water and was adjusted to pH = 5 with 1 mol/L NaOH solution. The resultant mixture was homogenized under stirring for 30 min at ambient temperature, transferred into 25 mL Teflon-lined stainless steel autoclave under autogenous pressure at 160 ℃ for 4 d, and then cooled to room temperature at a rate of 5 ℃/h. After filtration, the product was washed with distilled water and then dried, and yellowish transparent block crystals suitable for X-ray diffraction analysis were obtained. Anal. Calc. for C19H10CuN2O5(409.83, %): C 55.68, H 2.46, N 6.84; Found(%): C 54.75, H 2.98, N 6.90. Selected IR (cm-1): 3 563(w), 3 420(w), 3 146(w), 2 946 (w), 1 654 (w), 1 629(s), 1 612 (s), 1 528(m), 1 414(s), 1 385(s), 1 368 (s), 1 296 (w), 1 277(m), 1 244 (m) 1 244(m), 1 083(w), 1 071(m), 1 012(w), 912(w), 849(w), 833(m), 803(w), 765(m), 754(w), 726(w), 704(w), 666(w), 643(m), 604(w), 515(m), 441(w), 418(w).

2Results and Discussion

2.1　Structural description of the complex

The crystal data and structure renement detail for the complex are summarized in Table 1. Single crystal X-ray diffraction reveals that the complex crystallizes in the orthorhombic space groupF-ddd. Each Cu(II) atom is coordinated by two oxygen atoms from two BPTC ligands (Cu(1)-O(1) 0.194 3(6) nm, Cu(1)-O(4) 0.195 8(6) nm, respectively) and two nitrogen atoms from two bpy ligands (Cu(1)-N(1) 0.203 8(7) nm and Cu(1)-N(2) 0.203 0(8) nm), as shown in Fig. 2a. Weak interaction occur between the carboxylic oxygen atoms and central Cu(II) ion (Cu(1)-O(2) 0.270 5(7) nm and Cu(1)-O(5) 0.277 7(7) nm) owing to the Jahn-Teller effect. The bond length data (see Table 2) in the present work are consistent with those in previous work covering Cu(II) ions coordination polymers[20-21].

Table 1　Crystal data and structure refinement parameters for complex

Table 2　Selected bond lengths and bond angles for complex

It is noteworthy that each BPTC anion coordinates to four Cu(II) cations to generate a 2D chiral layer alongadirection to form rhombic windows with dimensions of 1.145 1 nm×2.000 9 nm (Fig. 2b). Within the layer, right-handed helical chains formedviacarbonyl groups of BPTC anions with the pitches of 1.145 2 nm are observed. At the same time, bpy pillars connect Cu(II) ions to generate a 1D chain [-bpy-Cu(1)-bpy-Cu(1)-]∞(Fig. 2c). Furthermore, the adjacent Cu-BPTC 2D layers are linked by bpy linkers to form a cavities-containing 3D framework, as illustrated in Fig. 2d, in which the lattice water molecules are encapsulated in the cavities.

Fig.2　a) Coordination environment of the complex, the asymmetric unit and the related coordination atoms are labeled, lattice water and hydrogen atoms are omitted for clarity; b) the 1D ribbon of [bpy-Cu-bpy-Cu] ∞ in the complex; c) the 2D layer of Cu-BPTC. insert: left- and right-Cu-phthalate helical chain; d) the 3D framework of the complex

2.2　FT-IR spectroscopy and X-ray powder diffraction

The title complex are insoluble in common solvents such as CH3COCH3, CH3CH2OH, CH3CN and THF, but slight soluble in DMF. The structure of the complex is identied by satisfactory elemental analysis as well as FT-IR and single crystal X-ray diffraction. The strong vibrations appeared at about 1 606 and 1 395 cm-1in the complex are ascribed to the coordinated carboxylates. The values of Δ[νas-νs] is 211 cm-1, which indicates that the carboxyl groups are coordinated with the metal ionsvia monodentate mode[22-23]. The sharp peaks ofδO-C-Ovibration in plane emerge in the range of 660-760 cm-1. The absence of the characteristic bands ranging from 1 725 to 1 785 cm-1indicates that the H4BPTC ligands are completely deprotonated in the form of BPTC4-anions upon reaction with the metal ions[24-25]. The results are in good agreement with the single-crystal X-ray structural analysis and well match to the literature reports.

Fig.3　The simulated and experimental (25 ℃ and 250 ℃) powder XRD patterns of the complex

To confirm the purity of the complex and further investigate whether the 3D porous framework would collapse upon removal of the guest water molecules, the original as-synthesized complex and the processed sample were characterized by X-ray powder diffraction (XRD) at the same conditions, the corresponding XRD patterns are shown in Fig. 3. The guest water molecules can be removed by heating at 250 ℃ for 24 h. By comparison, the corresponding positions and intensities of peaks in the as-synthesized patterns of the complex were unchanged when it was heated over 250 ℃, which suggests that the 3D porous framework remains intact after the guest water molecules are removed. This result may also be evidenced by the IR spectra. The corresponding characteristic peaks of the as-synthesized complex do not shift after the guest water molecules are removed by heating to 250 ℃ (see Fig.4).

Fig.4　IR spectrum of the complex

2.3　Photoluminescent properties

The solid-state photoluminescence spectrum of the complex was investigated at room temperature intersecting with incidence at an angle of 45° (see Fig.5). The complex exhibits two strong emission bands centered at 483 and 590 nm upon excitation at 374 nm. From the band position and shape, it is reasonable to speculate that the emissions of the complex is tentatively assigned to the mixture effects of intraligand charge transfer and ligand-to-metal-charge-transfer (LMCT) transitions[26-27].

Fig.5　Emission and excitation spectrum for the complex at room temperature

2.4　Thermal analysis

The thermal analysis of the complex has been measured from room temperature to 1 000 ℃ with a heating rate of 10 ℃·min-1in N2flow (see Fig. 6), which indicates that the complex decomposes in two steps, giving a total weight loss of 81.80%. The first weight loss of 3.11% between room temperature and 247 ℃, which is attributed to the loss of the lattice water (calc. 4.40%). The second stage weight loss (78.69%) of 250-1 000 ℃ conform to the segmental decomposition of 4,4′-bipyridine and 3,3′,4,4′-benzophenone-tetracarboxylate ligands. The remnant of the complex is 19.20%, which suggests that CuO is the final product (calc. 19.40%).

Fig.6　Thermogravimetric analysis of the complex from 25 to 1 000 ℃

2.5　Magnetic Properties

Variable-temperature magnetic susceptibility of the complex is measured in the 2.0-300 K. The variation of the ratio of the magnetic,χM-1andχMTare shown in Fig. 7. The thermal evolution ofχM-1obeys the Curie-Weiss law,χM=C/(T-θ) in the range of 2-300 K with a Weiss constantθ=-12.668 K and a Curie constantC= 0.390 cm3·K·mol-1, respectively. At 300 K, theχMTvalue is 0.069 cm3·mol-1·K (0.744μB), which is lower than the expected value (0.375 cm3·K·mol-1, 1.731μB) for magnetically isolated high-spin Cu(II) (SCu= 1/2,g=2.0). With the temperature decreasing, theχMTvalue changes smoothly to a minimum of 0.011 cm3·K·mol-1at 2 K. The negativeθvalue and theχMTvsTcurve reveal typical antiferromagnetic interactions between the Cu(II) centers. The shortest Cu…Cu distance across the (O2C-C-C-CO2)2bridge (0.523 0(1) nm) is shorter than the bpy bridge (1.115 8(3) nm), suggesting that the observed antiferromagnetic interaction should arise from the magnetic super exchange through the (O2C-C-C-CO2)2bridges.

Fig.7　Thermal variation of χ M and χ MT for the complex. Insert: Plot of thermal variation of χ M -1

3Conclusions

In summary, we report here a novel 3D porous metal-organic framework generated from 3,3′,4,4′-benzophenone-tetracarboxylate in the presence of auxiliary 4,4′-bipyridine ligand, and the 3D framework remains intact after the guest water molecules encapsulated in the cavities are removed. The 2D layer in the complex constitutes of right-handed helical chains. Moreover, the complex presents antiferromagnetic behavior and photoluminescence property, the result suggest that the as-synthesized complex may be potential multifunctional materials in photoluminescence and magnetism.

CCDC-995728 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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