3D Chiral Microporous (10,3)-a Topology Metal–Organic Framework

ARTICLE pubs.acs.org/crystal

3D Chiral Microporous (10,3)-a Topology Metal
Organic Framework Containing Large Helical Channels Mei-Na Li, Dong-Ying Du, Guang-Sheng Yang, Shun-Li Li, Ya-Qian Lan,* Kui-Zhan Shao, Jun-Sheng Qin, and Zhong-Min Su* Institute of Functional Material Chemistry, Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China

bS Supporting Information ABSTRACT: A new 3D chiral metal
organic framework [Co2(BTT)(OH)(H2O)5] 3 5CH3OH 3 6DMA (1) has been synthesized using an achiral tritopic bridging ligand, 1,3,5tris(2H-tetrazol-5-yl)benzene (H3BTT). 1 possesses a (10,3)a net containing large helical channels of 17.43 Å and exhibits a high solvent-accessible volume (calculated 79%). The N2 adsorption and magnetic properties for 1 have been examined.

’ INTRODUCTION Metal
organic frameworks (MOFs) have recently attracted considerable interest, owing to their versatile framework topologies as well as their potential applications as functional materials in molecular magnetism, catalysis, gas sorption, fluorescent sensing, and optoelectronic devices.1,2 Especially, MOFs with chiral frameworks have aroused a great deal of attention, owing to not only their intriguing helical topologies but also their potential applications, such as enantioselective synthesis, asymmetric catalysis, and optical materials.3
7 Among these, the synthesis of chiral MOFs has sparked keen interest in developing microporous MOFs as multifunction materials. A helix is one of the most attractive and evocative expressions of chirality as well as a good model to transmit chiral information.8 Recently, facile and versatile triangular building blocks, such as carbonates, borates, and 1,3,5-benzenetricarboxylate (H3BTC),9
11 have been used to obtain networks such as (12,3)-, (10,3)-, and (8,3)-nets with intrinsic helical channels.12 The chiral (10,3)-a network is a classical chiral topology net in the field of porous MOFs; this network can be predictably synthesized from three-connected organic bridging ligands and inorganic nodes.13 Thus, the synthesis of chiral MOFs based on 3-connected ligand is a feasible route. However, difficulties must be solved when targeting such MOFs: (a) expanded connections generally produce brittle architectures; (b) the large channel size within the crystal framework makes it more often susceptible to interpenetrate, preventing high porosity.14 It is key to select or design a suitable organic ligand to construct chiral porous MOFs possessing a desired topology. So we selected rigid chelate ligand in order to avoid interpenetration and collapse to build chiral porous MOFs. Considering that multitopic tetrazolate-based organic ligands may have great r 2011 American Chemical Society

potential for generating MOF materials with novel topologies, we selected 1,3,5-tris(2H-tetrazol-5-yl)benzene (H3BTT) as the assembly ligand, because twelve N atoms of the ligand can exhibit diverse coordination fashions and C
C single bonds between benzene rings and tetrazole groups can turn around discretionarily. As three tetrazoles in the H3BTT ligand all act in Mode III (μ2tetrazolate mode),15 the ligand may form a trigonal-planar coordination geometry to construct a 3-connected topology. The MOFs based on Co ions have wide application in magnetic material.16 For constructing the multifunctional MOFs, we choose cobalt ions to combine with H3BTT ligand. Fortunately, we synthesized a new 3D chiral metal
organic framework [Co2(BTT)(OH)(H2O)5] 3 5CH3OH 3 6DMA (1) (DMA = N, N-dimethylacetamide) based on a (10,3)-a net using H3BTT and metal cobalt atoms. It is noteworthy that 1 possesses a high solvent-accessible volume (calculated 79%). In addition, the N2 adsorption and magnetic properties of compound 1 have been investigated in detail.

’ EXPERIMENTAL SECTION General Procedures. All reagents were purchased commercially and used without further purification. H3BTT was synthesized readily by the procedure reported in the literature.17 Synthesis of [Co2(BTT)(OH)(H2O)5] 3 5CH3OH 3 6DMA (1). A mixture of Co(Ac)2 3 4H2O (0.075 g, 0.30 mmol), H3BTT (0.028 g, 0.10 mmol), and DMA/methanol (1:1) was stirred for 10 min in air at room temperature. Then the mixture was transferred and sealed in a Teflon Received: March 2, 2011 Revised: April 11, 2011 Published: April 27, 2011 2510

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Table 1. Crystal Data and Structure Refinements for Compound 1 compound

1

formula

C9H14N12O6Co2

formula weight crystal system

493.09 cubic

space group

P4132

T/K

293(2)

λ

0.71069

a/Å

18.6400(18)

R/deg

90

V/Å3

6476.5(11)

Z Flack factor

4
0.01(4)

μ/mm
1

0.528

Dcalc/g 3 cm
3

0.506

F(000)

972

reflections collected/unique

33031/1929 [Rint = 0.0892]

final R indices [I > 2σ(I)]

R1 = 0.0417, wR2 = 0.0969

R indices (all data)

R1 = 0.0891, wR2 = 0.1112

goodness-of-fit

0.894

reactor (18 mL) and heated at 85 °C for 3 days. After the mixture was cooled to room temperature at 5 °C/h, red crystals of 1 were obtained (40.2% yield based on H3BTT). Elemental Anal. Calcd for C38H88N18O17Co2: C, 38.45; H, 7.42; N, 21.25. Found: C, 38.57; H, 7.39; N, 21.34%. IR data (KBr, cm
1): 3426 (s), 2940 (w), 1624 (s), 1508 (w), 1404 (m), 1262 (w), 1191 (w), 1065 (w), 1020 (m), 904 (w), 791 (s), 596 (m), 475 (s). Physical Measurements. Elemental analyses of carbon, hydrogen, and nitrogen were carried out with a Carlo Erba 1106 elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the range 4000
400 cm
1 on a Mattson Alpha-Centauri spectrometer. X-ray photoelectron spectroscopy (XPS) analyses were performed on a thermo ECSALAB 250 spectrometer with an Al KR (1486.6 eV) achromatic X-ray source running at 15 kV. The XPS binding-energy (BE) was internally referenced to the aliphatic C(1s) peak (BE, 284.6 eV). Thermal gravimetric analysis (TGA) was performed on a PerkinElmer TG-7 analyzer heated from room temperature to 800 °C under nitrogen. PXRD patterns were recorded on a Siemens D5005 diffractometer with Cu KR (λ = 1.5418 Å) radiation in the range 5
40°. X-ray Crystallography. Data collection of compound 1 was performed on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo Ka radiation (λ = 0.71069 Å) at room temperature. All absorption corrections were performed by using the SADABS program. The crystal structure was solved by direct methods and refined with full-matrix least-squares (SHELXTL-97) with atomic coordinates and anisotropic thermal parameters for some non-hydrogen atoms. The hydrogen atoms of the aromatic rings were included in the structure factor calculation at idealized positions by using a riding model. Disordered, independent solvent molecules inside the frameworks were eliminated in the refinement by PLATON18/SQUEEZE, and the results were attached to the CIF file. The detailed crystallographic data and structure refinement parameters are summarized in Table 1 and Table S2 (Supporting Information). Further details of the crystal structure determination have been deposited with the Cambridge Crystallographic Data Centre as a supplementary publication. CCDC 796005 for 1 contains the supplementary crystallographic data for this paper. Structure Description of 1. Single-crystal X-ray diffraction measurements reveal that complex 1 crystallizes in the chiral cubic

Figure 1. (a) Coordination environment of Co(II) atoms in 1; hydrogen atoms were omitted for clarity. (Symmetry transformations used to generate equivalent atoms: (A)
x þ 1/2,
y, z
1/2; (B)
x, y þ 1/2,
z þ 1/2.) (b) Co2-based SBUs [Co2N6O6].

Figure 2. (a) Left-handed and right-handed helical chains generated by the SBU in 1 (left); four left-handed helical chains surrounding the righthanded helical chains (right). (b) Space-filling representation of 1 (Co, orange; N, blue; C, gray; O, red; H, green). P4132 space group. The enantiomer of complex 1 in the P4132 space group has been measured randomly (Table S2, Supporting Information). The crystal structure of 1 shows that three tetrazole rings are nonplanar and the dihedral angles between the phenyl ring and each of the tetrazole planes are all 16.5°. Complex 1 displays a threedimensional (3D) structure, consisting of one type of ligand and one type of Co(II) atom. Each cobalt center of 1 is coordinated by three nitrogen atoms (Co1
N1 2.119(2) Å) from different BTT ligands, and each of two cobalt atoms combines with six oxygen atoms from one OH group and five water molecules (Figure 1a). Each ligand coordinates to six Co(II) atoms through six N atoms and each tetrazole group in a bidentate fashion (Figure S1, Supporting Information). Two crystallographically equivalent Co(II) ions are bridged together by three tetrazole groups of BTT ligands bound in the bidentate fashion to give a [Co2N6] fragment with the Co 3 3 3 Co distance of 3.760 Å. Then, six oxygen atoms of one OH group and five water molecules bound to two Co atoms, giving Co2-based secondary building units (SBUs) [Co2N6O6] (Figure 1b). The XPS analysis, the bond valence sum calculations, and acid
base titration and back-titration in the Supporting Information could prove the formula of 1. The two type single infinite helical chains are made up of Co2-based SBUs and ligands BTT. Each type of helices is homochiral, which is obtained by 2511

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Figure 3. (a) Ball-and-stick and polyhedral representations of the 3-connected H3BTT ligand and 3-connected Co2-based SBU [Co2N6O6]. (b) Chiral (10,3)-a net of 1. the tetrazolate groups linking Co(II) through its 2- and 3-position N atoms coordination, giving rise to infinite left- and right-handed helical chains (Co2-BTT)n along the c axis (Figure 2a). Every left-handed helical chain is surrounded by four right-handed helical chains, and likewise each right-handed helical chain is enclosed by four left-handed helical chains (Figure 2a). All two adjacent helical chains are bridged by sharing partial BTT ligands (one tetrazolate group and one benzene ring) (Figure S2, Supporting Information). Interweaving of two types of single-helical chains leads to an unprecedented three-dimensional (3D) homochiral network with large helical channels of 17.43 Å (Figure 2). Calculations with PLATON show that the effective volume for the inclusion is 5135.8 Å3 per unit cell, which is 79.3% of the crystal volume. Every [Co2N6O6] cluster is surrounded by three BTT ligands; thus, the [Co2N6O6] cluster can be considered as a three-connected node (Figure 3a). Due to three tetrazoles in the H3BTT ligand, all acting in Mode III, the BTT ligand in the structure can correspond with the trigonal-planar coordination geometry, and each ligand linking three [Co2N6O6] units can be considered as a three-connected node. The overall structure of complex 1 is an extended neutral 3D noninterpenetrating (10,3)-a network in the well-known topology of the Si net of the SrSi2-type structure19 (Figure 3b). Although the (10,3)-a networks based on oxygen and nitrogen donors have been widely investigated, most (10,3)-a networks are constructed by tridentate carboxylate ligands and its derivatives. The chiral (10,3)-a net is a common structural type for MOFs containing planar BTC, but there are no such examples reported which are constructed based on binuclear metal unit SBUs and nonplanar ligand H3BTT.

’ PXRD, THERMAL ANALYSIS, AND N2 ABSORPTION The powder X-ray diffraction (PXRD) pattern of bulk crystals is very similar to that simulated from single crystal X-ray data of 1 (Figure S3, Supporting Information). Upon putting the as-synthesized material in vacuum at room temperature for 2 h, the peak of the powder pattern (Figure S3c) becomes slightly diverse from that of the pristine solids (Figure S3b), which indicates that the open structure is maintained even after removal of solvent molecules. A further adsorption study was carried out on the evacuated solid after reimmersing it in DMA/methanol (1:1) for 2 h. The PXRD pattern (Figure S3d) shows that the positions and intensities of the peak are coincident to the as-synthesized solids, which confirms the reversibility of the inclusion process. To study the thermal stability of complex 1, thermogravimetric analyses were performed on polycrystalline samples under a nitrogen atmosphere with a heating rate of 10 °C min
1 (Figure S4, Supporting Information). Thermogravimetric analysis (TGA) of 1 shows a gradual weight loss of about 57.32% below 275 °C corresponding to the loss of all guest methanol and DMA (calcd 57.53%). And then the compound begins to decompose, with loss of coordination water molecules. The N2 sorption isotherm of 1 at 77 K reveals type I behavior characteristic of a microporous material (Figure 4). The average diameter of the large helical channels of compound 1 is 17.43 Å. The uptake amount of N2 increases abruptly at the start of the experiment, and the maximum N2 uptake of 53.9 cm3/g (at standard temperature and pressure, 2512

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Figure S5, Supporting Information) for 1 could be fit with the Curie
Weiss equation from 50 to 300 K, giving C = 3.71 cm3 K mol
1 and θ =
51.50 K, with the small negative Weiss constant indicating the occurrence of an antiferromagnetic interaction between the nearest neighbor cobalt(II) atoms.22

’ CONCLUSION Herein we have successfully constructed a 3D chiral microporous (10,3)-a topology metal
organic framework based on binuclear cobalt units and H3BTT ligands. Noteworthily, complex 1 exhibits large helical channels of 17.43 Å and a high solvent-accessible volume (calculated 79%). In addition, the gas adsorption experiments of N2 and magnetic property of 1 have been investigated. Further studies for synthesis and applications of porous chiral MOFs are in progress in our laboratory. Figure 4. Gas sorption isotherms of 1 for N2 at 77 K.

’ ASSOCIATED CONTENT

bS

Supporting Information. X-ray crystallographic files (CIF), diagrams of the structure, selected bond distances and angles, PXRD, TGA, IR, and XPS. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: (þ86)-0431-85099108. E-mail address: lanyq176@ nenu.edu.cn; [email protected].

Figure 5. Plots of χm and χmT vs T for a polycrystalline sample of complex 1.

STP) was reached at 1 atm, which corresponds to apparent Brunauer
Emmett
Teller (BET) areas of 202 m2/g and Langmuir surface areas of 235 m2/g, respectively. The nitrogen adsorption and desorption are reversible.

’ MAGNETIC PROPERTIES The temperature dependence of the molar magnetic susceptibility (χm) was investigated on polycrystalline samples of 1 to probe magnetic superexchange between the S = 3/2 Co(II) ions in the temperature range 2
300 K with an applied field of 1000 Oe. As shown in Figure 5, upon cooling of the sample, the χm value of complex 1 increases, reaching a maximum of 0.21 cm3 mol
1 at 2.0 K. The χmT value observed at 300 K (3.20 cm3 mol
1 K) is larger than the value for a spin-only S = 3/2 system (1.875 cm3 mol
1 K). This is as expected for octahedral highspin cobalt(II) ions, which have a large first-order orbital contribution to the magnetic moment.20 As the temperature decreases, the χmT value decreases and reaches a value of 0.85 cm3 mol
1 K at 2 K. This decrease is indicative of the single-ion effects of the cobalt(II) centers.21 The 1/χm versus T plot (see

’ ACKNOWLEDGMENT We are thankful for financial support from the Program for Changjiang Scholars and Innovative Research Team in University, the National Natural Science Foundation of China (No. 20901014 and 21001020), the Science Foundation for Young of Jilin Scientific Development Project (No. 20090125 and 20090129), the Fundamental Research Funds for the Central Universities (No. 20090407 and NENU-STC08019), the Ph.D. Station Foundation of Ministry of Education for New Teachers (No. 20090043120004), the Postdoctoral Foundation of Northeast Normal University, and the Postdoctoral Foundation of China (No. 20090461029). ’ REFERENCES (1) (a) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed. 1999, 38, 2638. (b) Khlobystov, A. N.; Blake, A. J.; Champness, N. R.; Lemenovskii., D. A.; Majouga, A. G.; Zyk, N. V.; Schr€oder, M. Coord. Chem. Rev. 2001, 222, 155. (c) Robson, R. J. Chem. Soc., Dalton Trans. 2000, 3735. (d) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629. (e) Zaworotko, M. J. Chem. Commun. 2001, 1. (f) Ockwig, N. W.; Delgado-Friedrichs, O.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2005, 38, 176. (g) Jiang, H.-L.; Tatsu, Y.; Lu, Z.-H.; Xu, Q. J. Am. Chem. Soc. 2010, 132, 5586. (2) (a) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc. 1994, 116, 1151. (b) Fletcher, A. J.; Cussen, E. J.; Prior, T. J.; Rosseinsky, M. J.; Kepert, C. J.; Thomas, K. M. J. Am. Chem. Soc. 2001, 123, 10001. (c) Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511. (d) Zou, R.-Q.; Sakurai, H.; Song, H.; Zhong, R.-Q.; Xu, Q. J. Am. Chem. Soc. 2007, 129, 8402. (e) Kitagawa, S.; Kitaura, R.; Noro, S. I. Angew. Chem., Int. Ed. 2004, 43, 2334. (f) Wang, X.-L.; Qin, C.; Wang, E.-B.; Su, Z.-M. Chem.
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