Structure analysis and catalytic property of a microporous framework based on a flexible tripodal ligand with novel conformations

Journal of Molecular Structure 996 (2011) 110–114

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Structure analysis and catalytic property of a microporous framework based on a flexible tripodal ligand with novel conformations Xianju Shi a,b, Xinhai Zhang c, Xiaoxia Li a, Hongwei Hou a,⇑, Yaoting Fan a a

Department of Chemistry, Zhengzhou University, Henan 450052, PR China Puyang Vocational and Technical College, Henan 457000, PR China c Jiaozuo Teachers College, Henan 454000, PR China b

a r t i c l e

i n f o

Article history: Received 29 October 2010 Received in revised form 17 April 2011 Accepted 17 April 2011 Available online 22 April 2011 Keywords: Crystal structure Tripodal ligand Microporous complex Catalytic property

a b s t r a c t A microporous metal–organic framework {[Cu3Cl6(ttmb)4]6H2O}n (1) (ttmb = 1,3,5-tris(1,2,4-triazol-1ylmethyl)-benzene) with tetranodal (3,4)-connected topological type was prepared by the self-assembly reaction of the flexible tripodal ligand ttmb with CuCl22H2O. The ligand exhibits two infrequent coordination conformations in complex 1, and the framework has a large pore volume (remove the solvent molecules) of 1781.6 Å3 (36.3% of the total). The test of 1 as the catalyst in the oxidative coupling reaction of 2,6-dimethylphenol indicates that it is catalytically active by showing high conversion of DMP under the optimized reaction condition. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Microporous metal–organic frameworks (MOFs) have received considerable attention because of their fascinating topologies and promising applications as adsorption, separation, ion exchange, and catalysis materials [1]. Unlike conventional porous zeolite materials, the pores within MOFs can be systematically tuned by the judicious choice of metal centers with various coordination geometries and multifunctional organic bridging spacers [2]. However, crystal engineering of microporous MOFs is still confronted with great challenges in control of framework structures, because the final structures are frequently modulated by various factors. Until now, much effort has been devoted to the rational design and syntheses of novel microporous MOFs [3], and the tripodal ligand with three coordination groups was proved to be an excellent constructor to synthesize robust microporous MOFs [4]. The coordination groups of the flexible tripodal ligands can bend and rotate freely in response to the changes of central metal coordination geometry and counter-anion, and then, various MOFs with different pore volume could be constructed. According to the relative orientation of coordination groups and central group, the coordination conformations of the tripodal ligands in most of the reported MOFs could be classified as those which are listed in Scheme 1 [5]. And the cis,trans,trans and cis,cis,cis conformations are the prevalent adopted types. Confessedly, the spatial conformation of tripodal ligand plays an important role in influencing the structural ⇑ Corresponding author. Tel./fax: +86 371 67761744. E-mail address: [email protected] (H. Hou). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.04.027

frameworks of the MOFs. In order to investigate the relationship between the conformation of the ligands and the resulting topological structures, a flexible tripodal ligand 1,3,5-tris(1,2,4-triazol-1ylmethyl)-benzene (ttmb) was used as the functional ligand. And the self-assembly reaction of ttmb with CuCl22H2O led to the formation of a microporous complex {[Cu3Cl6(ttmb)4]6H2O}n (1) exhibiting tetranodal (3,4)-connected topological type. The ttmb ligand exhibits two infrequent coordination conformations in this complex, and the framework has a large pore volume (remove the solvent molecules) of 1781.6 Å3 (36.3% of the total), in view of which the catalytic activity of 1 in the oxidative coupling reaction of 2,6-dimethylphenol was investigated. 2. Experimental 2.1. Materials and physical measurements The ligands H2bfcs [6] and ttmb [7] were prepared according to the literature methods. The other starting reagents and solvents employed were purchased from commercial sources and were used without further purification. The FT-IR spectra were recorded from KBr pellets in the range of 400–4000 cm1 on a Bruker Tensor 27 spectrophotometer. Elemental analyses (C, H, and N) were carried out on a FLASH EA 1112 elemental analyzer. 2.2. Synthesis of complex 1 An methanol solution (4 mL) of ttmb (6.4 mg, 0.02 mmol) was added into the 4 mL methanol solution of CuCl22H2O (3.4 mg,

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0.02 mmol), then 2 mL distilled water was added dropwise to the above mixture solution, homogenized and filtered. The resulting solution was allowed to stand at ambient temperature for 4 weeks. Finally, blue block-shaped crystals of complex 1 were obtained in 44% yield (based on Cu). IR (cm1, KBr): 3419 s, 3118 m, 1614 m, 1530 s, 1432 m, 1284 m, 1129 s, 1021 m, 889 m, 753 m, 676 m. Anal. Calc. for C60H72Cl6Cu3N36O6 (%): C, 40.11; H, 4.04; N, 28.06. Found: C, 39.96; H, 4.08; N, 27.95. 2.3. X-ray crystallographic analysis The single-crystal data of 1 were collected on a Oxford Xcalibur Eos CCD detector diffractometer, with graphite monochromated Mo Ka radiation (k = 0.71073 Å) at a temperature of 20 ± 1 °C and corrected for Lorenz-polarization effects. The structures were solved by direct methods and expanded using Fourier techniques. The non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed at calculated positions. All calculations were performed using the SHELXL-97 crystallographic software

Table 1 Crystal data and structure refinement for complex 1.

a b

Complex

1

Formula Fw Temp (K) Wavelength (Å) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dc (g cm3) Absorption coefficient (mm1) F (0 0 0) GOF on F2 R1[I > 2r(I)]a wR2 (all data)b Largest diff. peak and hole (e Å3)

C60H72Cl6Cu3N36O6 1796.81 293 (2) 0.71073 Monoclinic P21/c 16.764(2) 13.2012(12) 24.587(4) 90 115.502(10) 90 4911.1(11) 2 1.207 0.865 1818 1.011 0.0811 0.1842 0.535 and 0.331

P P R1 = ||Fo|  |Fc||/ |Fo|. 2 wR2 ¼ ½R  wðF o  F 2c Þ2 =R  wðF 2o Þ2 1=2 .

package [8], and refined by full-matrix least squares methods based on F2 with isotropic thermal parameters. Crystallographic crystal data and structure processing parameters for 1 are summarized in Table 1. Selected bond lengths and bond angles of 1 are listed in Table 2. 2.4. Experiment procedure for the catalytic oxidative coupling of DMP One milli molar DMP (122 mg) was dissolved in 5 mL water containing 1 mmol NaOH (40 mg) and 0.1 mmol sodium n-dodecyl sulfate (SDS) (29 mg). 0.02 mmol complex 1 with appropriate size was added to the above solution, and the mixture was stirred under air at 50 °C. Then, 10 lL H2O2 (30% aqueous solution) was slowly added into the mixture using a microinjector every 15 min (two times in all). After 8 h, the reaction was stopped and 1.17 g NaCl was added. Then the mixture was transferred into a separatory funnel, the organic materials were extracted by CH2Cl2 for three times. The combined organic extracts were dried by anhydrous MgSO4 and the filtrate was evaporated in vacuo. The products were separated by preparative TLC performed on dry silica gel plates with ethylether–petroleum ether (1:3 v/v) as the developing solvents. PPE and DPQ were collected and dried in vacuo. PPE: 1H NMR (300 MHz, CDCl3) d: 2.12 (s, 6H), 6.47 (s, 2H). 13C NMR (300 MHz, CDCl3) d: 16.4, 114.4, 132.5, 145.6, 154.5. 3. Results and discussion 3.1. Description of crystal structure A single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the monoclinic space group P21/c, and each asymmetric unit of it contains one (Cu1) and a half (Cu2) crystallographically independent Cu(II) centers, three Cl anions, two ttmb ligands and three solvent water molecules. Metal centers Cu1 and Cu2 exhibit the similar coordination geometries except that the bond lengths and bond angles around them are different. So, only the coordination environment around Cu1 is illustrated in this paper. As can be seen from Fig. 1a, Cu1 is six-coordinated in a distorted octahedral geometry, defined by N1, N4, N7 and N10 from four ttmb ligands in the equatorial plane, and two counter-anion Cl1 and Cl2 occupying the axial position. Owing to the existence of three flexible methylene groups, ttmb ligands can bend and rotate freely. On coordinating to Cu(II) ions, one triazol ring lies out-

Table 2 Selected bond lengths (Å) and angles (°) for complex 1. Complex 1 Cu(1)–N(1) Cu(1)–N(7) Cu(1)–Cl(1) Cu(2)–N(13)#1 Cu(2)–N(16)#3 Cu(2)–Cl(3) N(1)–Cu(1)–N(7) N(1)–Cu(1)–N(10) N(7)–Cu(1)–N(10) N(4)–Cu(1)–Cl(1) N(10)–Cu(1)–Cl(1) N(4)–Cu(1)–Cl(2) N(10)–Cu(1)–Cl(2) N(13)#1–Cu(2)–N(13)#2 N(13)#2–Cu(2)–N(16)#3 N(13)#2–Cu(2)–N(16) N(13)#1–Cu(2)–Cl(3) N(16)#3–Cu(2)–Cl(3)

2.010(6) 2.016(6) 2.701(5) 2.031(7) 2.034(7) 2.789(4) 89.4(2) 89.1(2) 178.0(3) 84.2(2) 90.0(2) 91.72(19) 90.55(19) 180.0(3) 89.4(3) 90.6(3) 91.5(3) 88.2(3)

Cu(1)–N(4) Cu(1)–N(10) Cu(1)–Cl(2) Cu(2)–N(13)#2 Cu(2)–N(16) N(1)–Cu(1)–N(4) N(4)–Cu(1)–N(7) N(4)–Cu(1)–N(10) N(1)–Cu(1)–Cl(1) N(7)–Cu(1)–Cl(1) N(1)–Cu(1)–Cl(2) N(7)–Cu(1)–Cl(2) Cl(1)–Cu(1)–Cl(2) N(13)#1–Cu(2)–N(16)#3 N(13)#1–Cu(2)–N(16) N(16)#3–Cu(2)–N(16) N(13)#2–Cu(2)–Cl(3) N(16)–Cu(2)–Cl(3)

2.013(6) 2.025(6) 2.715(2) 2.031(7) 2.034(7) 171.2(3) 91.1(2) 90.1(2) 87.0(2) 88.7(2) 97.0(2) 90.88(18) 175.88(12) 90.6(3) 89.4(3) 180.000(3) 88.5(3) 91.8(3)

Symmetry transformations used to generate equivalent atoms in complex (1): #1 x + 2, y + 1, z + 2; #2 x, y + 1, z; #3 x + 2, y + 2, z + 2.

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Fig. 1. (a) The rhombic Cu2(ttmb)2 unit consisting of two Cu1 centers and two ttmb ligands, the local coordination environment around Cu1 center is also shown; (b) schematic description of the scorpion-like conformation of ttmb in 1.

Fig. 2. Topological representation of the 2D layer structure formed by Cu1 (above); and the 1D double-chain structure formed by Cu2 (bottom).

side the phenyl plane, but the centers of the other two triazol rings are linearly in the phenyl plane to form a capital ‘‘T’’. Up to now, most of the reported flexible tripodal ligand exhibited the W-form (cis,cis,cis) or Y-form (cis,trans,trans) conformations, such T-form conformation is very infrequent [9]. Further structure analysis indicates that the triazol ring is not absolutely outside the phenyl plane, and it is almost parallel to one triazol ring of the other two (dihedral angle: 14.516°), just like a scorpion (Fig. 1b). In such conformation, two ttmb ligands adopt a top-to-bottom orientation link two Cu1 centers to form a rhombic Cu2(ttmb)2 unit (Fig. 1a). The phenyl plane of the two ttmb is absolutely parallel, the distance of the phenyl centers is 4.383 Å, and that of Cu  Cu is 13.723 Å. Four Cu2(ttmb)2 units are joined together from beginning to end, forming a Cu8(ttmb)8 macrocyclic unit. The longest and shortest diameter of the ring is 23.786 and 12.687 Å, respectively. Repeating the Cu8(ttmb)8 units in space, a 2D plane is formed (Fig. 2). It is worthy to be noticed that, the metal centers in this plane are all Cu1 atoms, and there exists 1D double-chain structure formed by Cu2 in this complex, linking the 2D planes together to form a 3D microporous framework (Fig. 3). The ttmb ligand between Cu1 and Cu2 shows a different coordination conformation from

T-form. In this ttmb ligand, one triazol ring lies outside the phenyl plane, the other two are close to it, but the centers of them are not

Fig. 3. Topological structure of the 3D microporous framework of 1, highlighting the 1D tunnel along a axis.

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N

N

N

N

N

N

N N N

N

N

N

N

R

N

N

N

N

N

R

R

R

R

N

R

N

N

R

R

N

N

R

N

cis,cis,cis

cis,trans,trans

cis

N

N

R N N

N

N

R N

N

R

N

N N

R N

R N

N

R N

N

N

R

N

N

N

N

N

R N

R

trans

N

N

N

N

N

T-form

N

propeller

Scheme 1. Versatile coordination conformations of the flexible tripodal ligands.

Scheme 2. The oxidative coupling reaction process of 2,6-dimethylphenol in water.

coplanar with the center of phenyl ring, and these two triazol ring bend to the same orientation. To the best of our knowledge, this is a new type conformation that has not been reported before. Topological analysis performed by the OLEX program [10] indicates that the 3-connected ttmb ligand can be classified to two sorts, and they have the same short symbol (4  82) but different long symbols. The symbols of the 4-connected Cu1 and Cu2 are (4  84  10) and (42  82  102), respectively. Considering the stoichiometry, the whole net can be defined as a tetranodal (3,4)-connected (4  82)(4  82)(4  84  10)(42  82  102) topological type. Furthermore, the pore volume of a unit is estimated, using the PLATON program [11], to be 1296.7 Å3 (26.4% of the total). Removal of the solvent water molecules would increase those values to 1781.6 Å3 (36.3% of the total). 3.2. Catalytic oxidative coupling of DMP Considering that this Cu(II) complex contains large pore volume which facilitate the approach of other reactant to Cu(II) centers, we then investigated its catalytic activity in the oxidative coupling reaction of 2,6-dimethylphenol (DMP). The main product of this reaction, poly(1,4-phenylene ether) (PPE), is an useful engineering plastic due to its high mechanical intensity and chemical stability [12]. However, the conventional coupling reaction was always carried out in organic or aqueous-organic biphasic solvent [13], which is considered as the environmentally undesirable process. In recent years, a new kind of clean, safe, and inexpensive reaction solvent– water was applied in this catalytic reaction [14]. So, in this paper, we select a green oxidative coupling reaction system of DMP in aqueous solvent with clean oxidant H2O2 (Scheme 2) for the purpose of protecting environment. Under the optimized reaction conditions [15], complex 1 is catalytically active by showing high DMP

conversion of 95.6%, 79.1% selectivity ([PPE]  100)/([PPE] + [DPQ]), and the yield of PPE is 49.8%. 4. Conclusions In conclusion, a microporous tetranodal (3,4)-connected framework based on the flexible tripodal ligand ttmb was presented in this paper. The framework has a large pore volume (remove the solvent molecules) of 1781.6 Å3 (36.3% of the total), and ttmb exhibits two different coordination conformations which are very infrequent in comparison to the literature reports. And this complex is catalytically active by showing high conversion of DMP in the green oxidative coupling reaction of 2,6-dimethylphenol. Acknowledgments This work was financially supported by the National Natural Science Foundation (Nos. 20971110 and J0830412), Program for New Century Excellent Talents of Ministry of Education of China (NCET) and the Outstanding Talented Persons Foundation of Henan Province, The Ministry of Science and Technology of China for the International Science Linkages Program (2009DFA50620). Appendix A. Supplementary material CCDC 771687 contains the supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.molstruc.2011.04.027.

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