Two New Metal−Organic Frameworks with Mixed Ligands of

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DOI: 10.1021/cg100599z

Two New Metal-Organic Frameworks with Mixed Ligands of Carboxylate and Bipyridine: Synthesis, Crystal Structure, and Sensing for Methanol

2010, Vol. 10 5020–5023

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Li-Fang Song,†,‡ Chun-Hong Jiang,†,‡ Cheng-Li Jiao,†,‡ Jian Zhang,† Li-Xian Sun,*,† Fen Xu,*,§ Wan-Sheng You,§ Zhong-Gang Wang, and Ji-Jun Zhao^ †

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Materials and Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, ‡Graduate School of the Chinese Academy of Sciences, Beijing 100049, China, §College of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, China, Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China, and ^School of Physics and Optoelectronic Technology and College of Advanced Science and Technology, Dalian University of Technology, Dalian 116024, China Received May 6, 2010; Revised Manuscript Received October 13, 2010

ABSTRACT: Two new mixed ligand metal-organic frameworks, [Cu2(OH)(2,20 -bipy)2(BTC) 3 2H2O]n (1) and [Co(4,40 -bipy)(m-BDC)]n (2), have been synthesized by hydrothermal reaction of Cu(COO)2 3 H2O with 1,3,5-benzenetricarboxylic acid (H3BTC) and 2,20 -bipyridine and of Co(NO3)2 3 6H2O with 1,3-benzenedicarboxylic acid (m-H2BDC) and 4,40 -bipyridine, respectively. They were characterized by single-crystal X-ray diffraction analysis, powder XRD, elemental analysis, TGA, and IR spectroscopy. The adsorption characteristics of compounds 1 and 2 for methanol were investigated in situ with a quartz crystal microbalance (QCM), which indicated that they were highly sensitive to methanol at room temperature. Introduction During the past decades, the preparation of metal organic frameworks (MOFs) has attracted a great deal of interest for their diverse topology frameworks as well as their potential applications in the fields of gas storage,1 ion exchange,2 catalysis,3 chemical sensors,4 and separation.5 This series of materials are constructed of metal ions as connected centers and polyfunctional organic ligands as linkers usually. In principle, the extended coordination framework solids with desired structural features and physicochemical properties greatly depend on the nature of the organic ligands and metal ions. Among the numerous organic ligands, the aromatic carboxylic acids and the bipyridine ligands are favored for their strong coordinating ability, which stabilizes the framework architecture, including that of honeycomb, grid, T-shaped, ladder, diamondoid, and octahedral structures. More recently, the organic aromatic polycarboxylate ligands, especially, 1,3-benzenedicarboxylic acid (m-H2BDC) and 1,3,5benzenetricarboxylic acid (H3BTC), have been extensively applied in the construction of a rich variety of MOFs because of their diverse coordination modes and high structural stability.6 In the present paper, we report two new metal-organic frameworks based on polycarboxylate ligands and bipyridine ligands, [Cu2(OH)(2,20 -bipy)2(BTC) 3 2H2O]n (1) and [Co(4,40 -bipy)(m-BDC)]n (2). We reveal that these two metal-organic frameworks with hydrophilic structure could only allow comparatively small and polar solvent molecules to access the channel or interact with the surface of the material. The adsorption and desorption of methanol molecules on the frameworks have been investigated with an in situ quartz crystal microbalance (QCM).7

Experimental Section Materials and Physical Measurements. All chemicals and reagents of analytical grade were commercially available and used as received. The IR spectra were recorded with a Nicolet Avatar 380 FT-IR spectrometer using the KBr pellet technique. *Fax: þ86-411-84379213. Telephone: þ86-411-84379213. E-mail: lxsun@ dicp.ac.cn (L.-X.S.); [email protected] (F.X.). pubs.acs.org/crystal

Published on Web 10/25/2010

Thermogravimetric analysis (TGA) was carried out on a Cahn Thermax 500 instrument with a heating rate of 10 C/min under flowing air. Adsorption properties were characterized on a research quartz crystal microbalance (Maxtek, USA). Powder XRD experiments were carried out on a PANalytical X-ray diffractometer (X’Pert MPD PRO, Cu KR, 40 kV, 40 mA). The program Mercury 1.4.2 was used for calculation of X-ray crystallographic powder patterns of compounds 1 and 2. The diffuse reflectance UV-visible spectra were recorded at room temperature on a Cintra (GBC) apparatus with BaSO4 as a reference.

Preparation. Synthesis of [Cu2(OH)(2,20 -bipy)2(BTC) 3 2H2O]n (1). A mixture of Cu(CH3COO)2 3 H2O (0.199 g, 1 mmol),

H3BTC (0.141 g, 0.67 mmol), 2,20 -bipyridine (0.156 g, 1 mmol), NaOH (0.080 g, 2 mmol), and H2O (15 mL) was sealed in a 40 mL Teflon-lined stainless steel autoclave, heated at 120 C for 24 h, and then cooled to room temperature naturally. After filtration, the product was washed with distilled water and then dried at 50 C under vacuum overnight. Dark blue block crystals suitable for X-ray diffraction analysis were obtained in ca. 52% yield based on Cu(II). Elemental analysis results for C29H24N4O9Cu2. Calcd: C, 49.79; H, 3.46; N, 8.01 (%). Found: C, 50.67; H, 2.99; N, 8.10 (%). IR data (KBr pellet, ν [cm-1]): 3420(s), 2920(w), 1610(vs), 1560(s), 1440(s), 1360(vs), 771(s), and 721(s). Synthesis of [Co(4,40 -bipy)(m-BDC)] (2). A mixture of Co(NO3)2 3 6H2O (0.291 g, 1 mmol), m-H2BDC (0.125 g, 0.75 mmol), 4,40 -bipyridine (0.039 g, 0.25 mmol), and H2O (6 mL) was sealed in a 40 mL Teflon-lined stainless steel autoclave, heated at 120 C for 90 h, and then cooled to room temperature naturally. After filtration, the product was washed with distilled water and then dried at 50 C under vacuum overnight. Dark purple-red block crystals suitable for X-ray diffraction analysis were obtained in ca. 60% yield based on Co(II). Elemental analysis results for C18H12N2O4Co. Calcd: C, 57.01; H, 3.19; N, 7.39 (%). Found: C, 55.07; H, 3.03; N, 7.58. IR data (KBr pellet, ν [cm -1]): 3450(w), 3060(m), 1610(vs), 1550(s), 1450(s), 1410(vs), 810(s), 720(s), and 633(s). Crystal Structure Determinations. Suitable single crystals of two compounds were mounted on glass fibers for X-ray r 2010 American Chemical Society

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measurement. Crystallographic data for 1 and 2 were collected on a Bruker SMART APEX II-CCD single-crystal X-ray diffractometer equipped with a fine-focus sealed tube with a graphite-monochromated Mo KR (λ = 0.71073 A˚) radiation source at room temperature. All absorption corrections were performed using the SADABS program. Crystal structures were solved by the direct method. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were fixed at calculated positions with isotropic thermal parameters. All calculations were performed using the SHELX-97 program.8 Crystal Data for 1. C29H24N4O9Cu2, Mr = 699.60, crystal dimensions 0.12  0.10  0.10 mm3, triclinic, space group P1, T=293 K, a = 8.7523(15) A˚, b = 9.9715(17) A˚, c = 17.610(3) A˚, R=92.857(2), β=104.123(2), γ=107.665(2), V=1407.3(4) A˚3, Z = 2, μ(Mo KR)=1.575 mm-1, Dcalc = 1.651 g cm-3. Final residuals were R1 = 0.0456 for 4890 reflections with I>2σ(I), wR2 = 0.1024, GOF = 1.040, largest diff. peak and hole were 0.590 and -0.543 e 3 A-3. Crystal data for 2: C18H12N2O4Co, Mr=379.23, crystal dimensions 0.100.10  0.08 mm3, triclinic, space group P1, T = 273 K, a = 9.222(6) A˚, b = 10.094(7) A˚, c=10.107(7) A˚, R = 77.489(9), β = 72.452(9), γ = 79.864(8), V = 869.5 (10) A˚3, Z = 2, μ(Mo KR) = 1.011 mm-1, Dcalc = 1.448 g cm-3. Final residuals were R1 = 0.1016 for 2932 reflections with I > 2σ(I), wR2 = 0.2548, GOF = 1.047, largest diff. peak and hole were 3.792 and -0.693 e 3 A-3. Selected bond lengths and bond angles are listed in Table S1 (Supporting Information). Full crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited in the Cambridge Crystallographic Data Center with CCDC No. 697530 for 1 and 670354 for 2. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax: 44-1223 336033 or e-mail: [email protected]).

Results and Discussion 0

Structural Analysis and Discussion. [Cu2(OH)(2,2 -bipy)2(BTC) 3 2H2O]n (1). The single-crystal diffraction analysis of

compound 1 reveals a two-dimensional (2D) coordination framework. The asymmetric unit and metal coordination of compound 1 are shown in Figure 1a. There are two crystallographically unique CuII centers in the crystal structure. Each CuII center is located in a slightly distorted tetragonal pyramidal geometry and coordinated by two carboxyl group O atoms from the different BTC ligands, one O atom from water, and two chelated N atoms from bipyridine. The two CuII atoms are linked by one bridging bidentate carboxyl group and one O atom from coordinated water, as a consequence forming a sixmembered ring. The Cu 3 3 3 Cu contact distance across the dinuclear unit is 3.319 A˚. The distances of Cu-OBTC (from benzene tricarboxylate acid) and Cu-Owt (from coordinated water) are in the ranges 1.915(3)-1.920(3) A˚ and 1.933(3)2.331(3) A˚, respectively, which are close to those usually reported for copper-oxygen donor compounds.9 The distances of Cu-N (from 2,20 -bipyridine) are in the range of 2.016(3)-2.042(3) A˚. The O-Cu-O bond angles vary from 90.62(11) to 96.60(11), the O-Cu-N bond angles vary from 91.16(12) to 170.50(14), and the N-Cu-N bond angles are 79.52(14) and 79.98(14); thus, the CuII ion has a distorted tetragonal pyramid coordination geometry. Each BTC ligand acts as a μ4-bridge to link four CuII ions as shown in Figure 1b, in which one carboxyl group ligates two metal ions in dimonodentate fashion, and the other two carboxyl groups ligate one metal ion in monodentate mode, respectively. The BTC ligands bridge the three adjacent metal ions into a layer. The layers are further assembled together through intermolecular C-H 3 3 3 O, O-H 3 3 3 O, and N-H 3 3 3 O hydrogenbonding interactions and π-π interaction between the aromatic and the bipyridine rings, resulting in a 3D supramolecular

Figure 1. (a) Coordination geometry of Cu atoms in 1 (thermal ellipsoids are at the 30% probability level; hydrogen atoms are omitted for clarity); (b) perspective view of the 2D layered framework of 1.

architecture. The solvent-accessible volume was calculated by SOLV analysis within PLATON as 2.2% of the crystal volume (30.8 out of the 1407.3 A˚3 unit cell volume). [Co(4,40 -bipy)(m-BDC)]n (2). Compound 2 displays a 2D coordination framework. The asymmetric unit and metal coordination of compound 2 are shown in Figure 2a. Single-crystal X-ray diffraction analysis shows that the CoII center is located in a six-coordinated slightly distorted octahedron and surrounded by four carboxylate oxygen atoms from two m-BDC ligands and two pyridyl nitrogen atoms from two different 4,40 -bipyridine ligands. CoII is connected to the three nearest bridged m-BDC ligands through their oxygen atoms. The O-Co-O bond angles range from 60.02(18) to 152.37(19), the O-Co-N bond angles range from 86.7(2) to 95.6(2), the N-Co-N bond angle is 177.52(19), and thus the CoII ion has a distorted octahedral coordination geometry. As a result of the distortion of the coordination octahedral around CoII atom, the bond lengths of the four Co-O bonds vary from 2.019 to 2.248 A˚. The Co-N distance is 2.154 A˚. The interatomic distances are close to those usually reported for cobalt solids.10 As illustrated in Figure 2a, one carboxylate is chelating bidentate whereas the other adopts a syn-anti bridging mode. As a consequence, each CoII ion forms a 8- or 16-membered chelated ring with m-BDC, and the adjacent CoII centers are connected by m-BDC components with a separation of 4.177(18) A˚ to furnish a polymeric chain. The chains are connected together by 4,40 -bipyridine into layers, and then the layers are assembled together through π-π interaction between adjacent m-BDC, resulting in a 3D supramolecular architecture. The solvent-accessible volume was calculated by SOLV analysis within PLATON as

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Figure 3. Frequency shift for the dehydrated 1 coated QCM response to methanol of 40 μL, 60 μL, 80 μL, and 100 μL; the corresponding concentrations of methanol in the chamber are 64 ppm, 96 ppm, 128 ppm, and 160 ppm.

Figure 2. (a) Coordination geometry of the Co atom in 2 (thermal ellipsoids are at the 30% probability level; hydrogen atoms are omitted for clarity); (b) perspective view of the 2D layered framework of 2.

12.5% of the crystal volume (108.5 out of the 869.5 A˚3 unit cell volume). The phase purity is confirmed through powder XRD, the experimental and simulated XRD patterns of the two compounds are matched well, respectively, and their peak positions are in good agreement with each other (Supporting Information, Figure.S1 and Figure.S2). Thermal Stability. To examine the thermal stability of these two frameworks, thermogravimetric analysis (TGA) was carried out. The thermal stability of compound 1 has been discussed in our previous work.11 TGA (Supporting Information, Figure S3) of 1 displays a four-stage mass loss in the temperature range 25-600 C. The two coordinated water lost in the first step starts at 67 C with a weight loss of 4.8% (calculated 5.2%). The powder XRD patterns and IR spectra of dehydrated and rehydrated compound 1 (Supporting Information, Figure.S4 and Figure.S5) indicate the structure of dehydrated compound 1 can go back to the original structure through rehydration. Further, three stages of decomposition occur in the region 117-461 C in a continuous way according to the degradation of the BTC and 2,20 -bipyridine ligands. The sample was decomposed to CuO until the end of the temperature. TGA of compound 2 (Supporting Information, Figure S6) reveals a three-stage weight loss when heated between room temperature and 600 C. The first two stages start at about 170 C, and mass loss is about 10.1%, which is due to the decomposition of part of the 4,40 -bipyridine ligands. Further decomposition occurs in the region 430-480 C according to the degradation of the 1,3-BDC and 4,40 -bipyridine ligands, which means the degradation of the frameworks. The overall weight

loss of the compound is about 77.5% (calculated 80.2%), which indicates that compound 2 was probably decomposed to CoO. This decomposition process is very similar to our previous work.12 Solid State Electronic Absorption Spectra. The diffuse reflectance UV-vis spectra of 1 and 2 (Supporting Information, Figure.S7 and Figure.S8) show strong adsorption bands in the UV region, which correspond to intraligand n-π* and π-π* transitions. In the visible region, the spectrum of 1 exhibits a broad band at 660 nm which is attributed to the d-d transition typical for CuII, and 2 exhibits a broad absorption band centered at 520 nm owing to the spin-forbidden d-d transitions for CoII ions, which are in good agreement with the literature values of relevant complexes.13 Chemical Sensing Properties of Solvent Molecules. To our knowledge, although there are various mixed ligand metalorganic frameworks reported, the sensor properties of those are still rarely discussed. In the present article, a quartz crystal microbalance (QCM) (see the Supporting Information) was used to study the sensor performance of compounds 1 and 2 for potential analysis of small solvents molecules such as methanol in direct methanol fuel cells. The increase in film mass is signaled by the decrease in the crystal’s resonance frequency. When compound 1 coated QCM is exposed to different injections of methanol from 40 to 100 μL in a 0.5 L chamber, the variety of the frequency shift is shown in Figure 3. The QCM curve displays an illustrative set of results for compound 1 coated QCM frequency changes. The frequency shift gets a positive change with the increase in the content of methanol in the chamber; after 1-2 min the mass-uptake slows down and eventually saturates. In addition, compound 1 has a reversible response to methanol. When the compound 2 coated QCM is exposed to different injections of methanol vapor varying from just 10 to 1 μL in the same chamber, the variety of the frequency shift is shown in Figure 4, indicating that compound 2 has a highly sensitive reversible response to methanol compared with compound 1. To understand the adsorption mechanism between methanol and the compound, we compared the IR spectra of methanol adsorbed and desorbed compounds. Single crystals of compounds 1 and 2 were exposed to methanol vapor at room temperature for 24 h respectively; both of them get moistened without dissolution. Then the methanol adsorbed single crystals were heated at 100 C for 24 h to remove methanol completely. The color of methanol adsorbed and desorbed crystals remained the same. The IR spectra (Supporting Information, Figure.S9

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of Explosion Science and Technology, Beijing Institute of Technology (Grant No. KFJJ10-1Z). Supporting Information Available: Details of crystallographic data in CIF format, FT-IR spectra, powder XRD pattern, TGA, UV-vis spectra, and quartz crystal microbalance test (PDF). This material is available free of charge via the Internet at http://pubs. acs.org.

References

Figure 4. Frequency shift for the dehydrated 2 coated QCM response to methanol of 10 μL, 9 μL, 8 μL, 7 μL, 4 μL, and 1 μL; the corresponding concentrations of methanol in the chamber are 16 ppm, 14 ppm, 13 ppm, 11 ppm, 6 ppm, and 2 ppm.

and Figure.S10) remained overall similar, except the strong and broad ν(O-H) stretching bands centered at 3400 cm-1 become a little stronger when exposed in methanol vapor. Supriya et al. reported that reversible single crystal to single crystal transformations driven by a gas-solid reaction between crystal and methanol occurred.14 The water coordinated compound 1 has a response to methanol probably caused by the reaction with methanol. Compound 2 possesses a higher resonance frequency to methanol in the QCM test, while 2 has no coordinated water. We think that the higher solvent accessible volume and the hydrophilic structure play a major role in the amount of methanol uptake.

Conclusions In summary, we have reported two new metal-organic frameworks generated from aromatic polycarboxylate acids and a dipyridyl-type ligand system under hydrothermal conditions. The hydrophilic framework of 2 is highly sensitive to polar molecule methanol, which indicates the metal organic frameworks have a potential use as chemical sensors for direct methanol fuel cells, gas separation and storage, etc. Further investigations based on these two metal organic frameworks are still in progress. Acknowledgment. The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (No. 20833009, 20873148, 20903095, 50901070, 51071146, 51071081, and U0734005), the National Basic Research Program (973 program) of China (2010CB631303), IUPAC (Project No. 2008-006-3-100), Dalian Science and Technology Foundation (2009A11GX052), and the State Key Laboratory

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