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Photoluminescent metal-organic framework with hex topology constructed from infinite rod-shaped secondary building units and single e,e-trans-1,4-cyclohexanedicarboxylic dianion
Inorganica Chimica Acta 360 (2007) 3108–3112 www.elsevier.com/locate/ica
Note
Photoluminescent metal-organic framework with hex topology constructed from infinite rod-shaped secondary building units and single e,e-trans-1,4-cyclohexanedicarboxylic dianion Miao Yu, Linhua Xie, Shuxia Liu *, Chunling Wang, Haiyan Cheng, Yuanhang Ren, Zhongmin Su Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun City, JiLin 130024, PR China Received 23 December 2006; received in revised form 23 February 2007; accepted 3 March 2007 Available online 20 March 2007
Abstract Hydrothermal reaction produced a three-dimensional zinc 1,4-cyclohexanedicarboxylate formulated as Zn5(l3-OH)2(trans-chdc)4 (chdc = 1,4-cyclohexanedicarboxylic dianion) in high purity and good yield, which is constructed from infinite rod-shaped Zn–O–C secondary building units interconnected by –C6H12-cyclohexane rings of the ligands. The topology of the framework can be regarded as hex type. Though it is synthesized from 1,4-cyclohexanedicarboxylic acid with mixed conformations (trans and cis), interestingly, the ligands in the compound Zn5(l3-OH)2(trans-chdc)4 are uniformly in e,e-trans conformation. This may be related to its synthetic conditions. Photoluminescence measurement reveals that the compound exhibits intense violet-blue fluorescent emission at room temperature. Origin of the emission can be assigned to intraligand transitions by comparison of the fluorescent emission bands for the free ligand chdcH2 and the compound Zn5(l3-OH)2(trans-chdc)4. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Metal-organic framework; Hex topology; Carboxylate ligands; Rod-shaped SBU; Fluorescence
1. Introduction In recent years, metal-organic frameworks (MOFs) have been investigated extensively due to their potential as functionalized materials as well as the variety of their intriguing topologies [1–11]. For the richness of transition metals with different d electron configurations and numerous available organic ligands, varied functional materials can be obtained. The extensive investigation in this area not only aims to seek the structure–property relationships but also to explore the correlations between synthetic conditions and resultant structures. Compared to the rigid ligand 1,4-benzenedicaryboxylic acid that has been turned out to
*
Corresponding author. Fax: +86 431 85099328. E-mail address: [email protected] (S. Liu).
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.03.020
be versatile for the design of porous crystalline material [12,13], 1,4-cyclohexanedicarboxylic acid (chdcH2) is a flexible ligand always existing with three conformations in common conditions (Scheme 1). For 1,3-diaxial hindrance a,a-trans form is least thermodynamically stable and seldom separated into solid-state compounds [14]. The e,e-trans form is most stable due to its two equatorial substituents. However, the dynamic equilibrium between e,e-trans and e,a-cis forms results in the difficult anticipation that which form would be crystallized into final products. Jung and Kim have separated 2D and 3D La1,4-cyclohexanedicarboxylates coordination networks in hydrothermal conditions with the chdc conformations controlled by the solution pH value [15]. More recently, Cao and co-workers have reported two cadmium coordination polymers with single e,e-trans and e,a-cis chdc, respectively, obtained in hydrothermal conditions with different pH
M. Yu et al. / Inorganica Chimica Acta 360 (2007) 3108–3112
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Scheme 1. The three conformations of the ligand chdcH2.
values and reaction temperatures [16]. Many other works reveal that the tendency that which form of chdc will be separated is also influenced by some other synthetic conditions, such as the nature of metal ions and assistant ligands [17–22]. These factors reflect the mutual selectivity between the coordination geometries of metal centers and the conformations of the ligand. Herein, we report a zinc 1,4-cyclohexanedicarboxylate Zn5(l3-OH)2(trans-chdc)4 (1) (chdc = 1,4-cyclohexanedicarboxylic dianion) synthesized in hydrothermal conditions in high purity and good yield. It is a three-dimensional metal-organic framework constructed from infinite rodshaped Zn–O–C secondary building units (SBUs) [2] interconnected by totally deprotonated chdcH2 ligands. It’s topology can be regarded as hex type according to the nomenclature defined by Yaghi and Keeffe [23]. It is interesting that the chdc ligands are in single e,e-trans conformation within compound 1 although started from chdcH2 with mixed conformations. The causation for that may be correlated to the synthetic condition. Photoluminescence measurements reveal that the compound exhibits intense violet-blue fluorescent emission at room temperature. Origin of the emission is discussed. 2. Experimental 2.1. Materials All chemicals were obtained from commercial sources and used without further purification. Elemental analyses were performed on a PLASMA-SPEC(I) ICP atomic emission spectrometer and a Perkin–Elmer 240C elemental analyzer. IR spectrum was recorded in the range 400– 4000 cm1 on an Alpha Centaurt FT/IR spectrophotometer using KBr pellets. TG analysis was performed on a Perkin–Elmer TGA7 instrument in flowing N2 with a heating rate of 10 °C/min. Powder X-ray diffraction measurement was performed on a Rigaku D/MAX-3 instrument with Cu Ka radiation in the angular range 2h = 3–90° at 293 K. Photoluminescence measurements were carried out on a Hitachi F-4500 Fluorescence Spectrophotometer. 2.1.1. Preparation of 1 A mixture of Zn(NO3)2 Æ 6H2O (0.8 mmol, 0.211 g), chdcH2 (mixture of cis and trans 99%) (0.8 mmol, 0.137 g), NaOH (1.6 mmol, 0.064 g) and water (10 mL) was sealed in Teflon-lined autoclaves and heated at
180 °C for 6 days, followed by slow cooling to room temperature. Compound 1 was collected as colorless block crystals with a yield about 0.142 g (85% based on Zn). Elemental Anal. Calc. for C32H42O18Zn5: C, 36.90; H, 4.06; Zn, 31.40. Found: C, 36.11; H, 4.68; Zn, 31.80%. IR (KBr, cm1): 3380 (m), 2945 (m), 2862 (m), 2361 (w), 2342 (w), 1542 (s), 1428 (s), 1364 (m), 1332 (m), 1284 (m), 1213 (m), 1047 (w), 929 (w), 890 (w), 798 (m), 687 (m), 571 (w). 2.1.2. X-ray crystallography Diffraction intensities for compound 1 was collected on a Siemens Smart CCD diffractometer with Mo Ka mono˚ ) at 293 K. The linear chromated radiation (k = 0.71073 A absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were taken from International Tables for X-ray Crystallography. Empirical absorption corrections were applied. The structures were solved by the direct method and refined by the full-matrix least-squares method on F2 using the SHELXTL crystallographic software package. Anisotropic thermal parameters were used to refine all non-hydrogen atoms. Partial H Table 1 Crystal data and structure refinement parameters for 1 Empirical formula C32H42O18Zn5 M 1041.5 Crystal system triclinic Space group P 1 2h Range (°) 1.95–26.37 ˚) a (A 8.6515(4) ˚) b (A 10.6723(6) ˚) c (A 11.8128(6) a (°) 113.855(1) b (°) 96.305(1) c (°) 106.316(1) ˚ 3) V (A 925.80(8) Z 1 F(0 0 0) 528 Dcalc (g cm3) 1.868 l(Mo Ka) (mm1) 3.271 Total data collected 5278 Unique data 3716 Observed data [I > 2r(I)] 3120 Rint 0.0146 Goodness-of-fit 1.034 R indexes [I > 2r(I)] R1 = 0.0292,a wR2 = 0.0615b R indexes (all data) R1 = 0.0379,a wR2 = 0.0662b P P a R1 ¼ iFoj jFci/ jFoj. P P b wR2 ¼ ½ wðF 2o F 2c Þ2 = wðF 2o Þ2 1=2 ; w = 1/[r2(Fo)2 + (aP)2 + bP], 2 where P ¼ ðMaxðF o ; 0Þ þ 2F 2c Þ=3, a = 0.0257, b = 0.71.
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atoms on the ligands are placed geometrically and the others (for the hydroxide ion and the 1,4-carbons of cyclohexane ring of the ligands) were located from the difference Fourier maps and refined isotropically. Details of the crystal data and final structure refinements of compound 1 are summarized in Table 1. The powder X-ray diffraction pattern for bulk sample of 1 agrees well with that simulated from the single-crystal data. The diffraction peaks on both experimental and simulated patterns match well in positions, indicating its phase purity (see Supplementary material Fig. 1S). 3. Results and discussion 3.1. Synthesis and structure We noted that the yield of the product was sensitive to the ratio of the original materials. The moderate ratio of chdcH2 to NaOH (about 1:2) is crucial for the good yield of 1. Larger or smaller ratios would produce worse results and even cannot yield compound 1. The phenomenon may result from the fact that a larger ratio cannot make all the chdcH2 deprotonated for further reaction and a smaller ratio would result in too much oxide of zinc with inert coordination reactivity. Single crystal X-ray diffraction study reveals that 1 is a three-dimensional framework built from infinite rodshaped secondary building blocks, which are interconnected by –C6H12– cyclohexane rings of the chdc ligands. The infinite SBU that can be formulated as [–Zn5–(OH)2– (COO)4–]1 consists of zinc atoms locked by hydroxide ions and carboxyl groups of chdc. Within these units, there are three types of zinc atoms (Zn1, Zn2, Zn3), one type of hydroxide ion (O1) and four types of carboxyl groups. (Fig. 1) Zn1, located at the inverse center position, coordinates with six O atoms, two hydroxide O atoms and four carboxyl O atoms, with distorted octahedron geometry. Zn2 and Zn3 are five coordinated with distorted squarepyramid and trigonal-bipyramid geometries, respectively, which are completed by five O atoms, one from hydroxide ion and the others from carboxyl groups. The selected Zn– O bond lengths and Zn–O–Zn angles are listed in Table 2. The hydroxide ion (O1) bridging the three zinc atoms can be distinctly identified for that its proton can be found in the difference Fourier maps reasonably. The four types of carboxyl groups from four chdc are at least chelated or/ and bridged to two zinc atoms, two of which are mode A and the other two are mode B and C, respectively (Fig. 2a, Scheme 2). With such complex connectivity, these distorted octahedra, square-pyramids and trigonal-dipyramids form the infinite rod-shaped SBU by corner-sharing and edge-sharing (Fig. 2b). The rods are stacked in parallel along [1 0 0] direction and interconnected by the –C6H12– parts of chdc ligands (Fig. 2c). Each rod is connected to six neighboring rods by eight chdc connectors. Because there are four of the eight chdc connectors linking two sets of rods, the vertex of the framework of 1 should be 8-con-
Fig. 1. A segment of the framework of 1 represented by 30% thermal ellipsoids. All H atoms have not been shown for clarity. Zn: teal; O: red; C: black. Symmetry code: (A) 1 x, 2 y, 2z. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 2 ˚ ] and angles [°] for 1 Selected bond lengths [A Zn(1)–O(1)[a] Zn(1)–O(1) Zn(1)–O(9)[a] Zn(2)–O(5) Zn(2)–O(1) Zn(2)–O(9) Zn(3)–O(4) Zn(3)–O(2)
2.057(2) 2.057(2) 2.1196(18) 1.922(2) 1.994(2) 2.019(2) 2.149(2) 2.213(2)
Zn(1)–O(9) Zn(1)–O(7)[a] Zn(1)–O(7) Zn(2)–O(6) Zn(2)–O(8)[b] Zn(3)–O(3) Zn(3)–O(8) Zn(3)–O(1)
2.1196(18) 2.131(2) 2.131(2) 2.023(2) 2.337(2) 1.933(2) 1.971(2) 1.974(2)
O(1)[a]–Zn(1)–O(1) O(1)[a]–Zn(1)–O(9)[a] O(1)–Zn(1)–O(9)[a] O(1)[a]–Zn(1)–O(9) O(1)–Zn(1)–O(9) O(9)[a]–Zn(1)–O(9) O(1)[a]–Zn(1)–O(7)[a] O(7)[a]–Zn(1)–O(7) O(9)–Zn(2)–O(6) O(5)–Zn(2)–O(8)[b] O(1)–Zn(2)–O(8)[b] O(9)–Zn(2)–O(8)[b] O(6)–Zn(2)–O(8)[b] O(1)–Zn(3)–O(4) O(3)–Zn(3)–O(2) O(8)–Zn(3)–O(2) O(1)–Zn(3)–O(2)
180.000(1) 80.51(8) 99.49(8) 99.49(8) 80.51(8) 180.000(1) 87.78(8) 180.000(1) 104.37(8) 98.75(9) 85.90(8) 145.49(8) 58.85(8) 95.02(8) 92.42(10) 84.37(8) 89.18(8)
O(1)–Zn(1)–O(7)[a] O(9)[a]–Zn(1)–O(7)[a] O(9)–Zn(1)–O(7)[a] O(1)[a]–Zn(1)–O(7) O(1)–Zn(1)–O(7) O(9)[a]–Zn(1)–O(7) O(9)–Zn(1)–O(7) O(5)–Zn(2)–O(1) O(5)–Zn(2)–O(9) O(1)–Zn(2)–O(9) O(5)–Zn(2)–O(6) O(1)–Zn(2)–O(6) O(3)–Zn(3)–O(8) O(3)–Zn(3)–O(1) O(8)–Zn(3)–O(1) O(3)–Zn(3)–O(4) O(8)–Zn(3)–O(4)
92.22(8) 89.30(8) 90.70(8) 92.22(8) 87.78(8) 90.70(8) 89.30(8) 110.94(9) 115.64(9) 84.54(8) 108.85(10) 129.89(9) 118.86(9) 114.67(9) 126.26(9) 93.81(9) 86.04(8)
[a]
Symmetry transformations used to generate equivalent atoms: x + 1, y + 2, z + 2. [b] Symmetry transformations used to generate equivalent atoms: x + 2, y + 2, z + 2.
nected rather than 10-connected. Consequently, the topology of 1 can be regarded as hex type (coordination number is 8) according to the nomenclature defined by Yaghi and
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Fig. 2. (a) Ball-and-stick and (b) polyhedral representation of the infinite [–Zn5–(OH)2–(COO)4–]1 rod-shaped SBU. (c) Perspective view of the framework of 1 down [1 0 0] direction. All H atoms have not been shown for clarity. Zn: teal; O: red; C: black. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Scheme 2. Schematic representation of the three coordination modes for the carboxyl groups in 1.
Keeffe [23]. However, the corresponding complex connectivity results in the lower symmetry of the framework of 1 ðP 1Þ compared to an ideal hex framework (P6/mmm). Although one of the four chdc per formula in 1 is statistically disordered and located at two positions (see Supplementary material, Fig. 4S), we note that the chdc ligands in 1 are totally e,e-trans conformation. As e,e-trans conformation is the one of most thermodynamic stable in the three ones, the formation of 1 with single e,e-trans conformation chdc may be correlated to the reaction conditions with high temperature, long reaction time and moderate high pH value. Its high yield indicates the high selectivity of the zinc ions and the e,e-trans form chdc at the relevant synthetic conditions. 3.2. Photoluminescence property Solid state sample of compound 1 exhibits intense photoluminescence with max emission peak at k = 413 nm upon excitation at 263 nm at room temperature (Fig. 3). To understand the nature of the emission band, photoluminescence measurement of the raw material of the ligand chdcH2 was carried out, the result of which reveals that it is also photoluminescent with max emission peak at 425 nm upon excitation peak 264 nm (Fig. 3). The emission
Fig. 3. Photoluminescence spectra for compound 1 (blue) and free ligand chdcH2 (black) with excitation at 263 nm and 264 nm, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
band of the free ligand is attributed to the intraligand p* ! n transitions [24,25]. The profile of the emission band for 1 resemble that for the free ligand except (1) obvious intensity enhancement at low energy region but slight weakening at high energy region; (2) a shift towards shorter wavelength. These observations indicate that the fluorescent emission of 1 can be assigned to intraligand transitions in nature, but not MLCT (metal-to-ligand charge transfer) or LMCT (ligand-to-metal charge transfer) [26–28]. The hypsochromic shift must be related to that n orbitals (highest occupied molecule orbitals, HOMO) located on the carboxyl O atoms are perturbed by the zinc cations
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after their coordination [24]. The intensity enhancement around 413 nm must be for that the coordination of ligand to metal center effectively increase the rigidity of the ligand and reduce the loss of energy by nonradiation decay [29,30]. What cause the weakening of the emission band round 324 nm is not clear now. We presume there may be some kind of absorption for the compound 1 at this wavelength region, which result in the weakening of the emission band. 3.3. Thermogravimetric analysis The thermogravimetric analysis (TGA) curve recorded at 20–800 °C reveals that there are two step weight losses for compound 1 (see Supplementary material Fig. 3S). The first stage occurred between 70 and 200 °C is corresponded to the detachment of one chdc per formula (weight loss: calc., 14.8%; found, 12.5%). The second step from 400 to 530 °C results from the loss of the other three chdc (weight loss: calc., 44.4%; found, 45.2%). Final species is corresponded to ZnO at 800 °C (weight: calc., 39.1%; found, 40.1%). 4. Conclusions We have successfully prepared a three-dimensional zinc 1,4-cyclohexanedicarboxylate built from rod-shaped SBUs and single e,e-trans chdc. This work provides a new example capable of separating e,e-trans conformation chdcH2 into solid-state solid from the mixed conformation in good yield and high purity. Compound 1 may also be served as advanced material for violet-blue light emitting device. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 20571014) and the Scientific Research Foundation for Returned Overseas Chinese Scholars, the Ministry of Education. We also thank Prof. Chunshan Shi for his discussion on the photoluminescence property of the compound. Appendix A. Supplementary material CCDC 602852 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223336-033; or e-mail: [email protected]. Supplemen-
tary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2007.03.020. References [1] S.L. James, Chem. Soc. Rev. 32 (2003) 276. [2] O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Nature 423 (2003) 705. [3] S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem., Int. Ed. 43 (2004) 2334. [4] C.N.R. Rao, S. Natarajan, R. Vaidhyanathan, Angew. Chem., Int. Ed. 43 (2004) 1466. [5] J.L.C. Rowsell, O.M. Yaghi, Micropor. Mesopor. Mater. 73 (2004) 3. [6] M.L. Tong, X.M. Chen, S.R. Batten, J. Am. Chem. Soc. 125 (2003) 16170. [7] H. Chun, D. Kim, D.N. Dybtsev, K. Kim, Angew. Chem., Int. Ed. 43 (2004) 971. [8] D.N. Dybtsev, H. Chun, K. Kim, Chem. Commun. (2004) 1594. [9] L.H. Xie, S.X. Liu, B. Gao, C.D. Zhang, C.Y. Sun, D.H. Li, Z.M. Su, Chem. Commun. (2005) 2402. [10] S.C. Xiang, X.T. Wu, J.J. Zhang, R.B. Fu, S.M. Hu, X.D. Zhang, J. Am. Chem. Soc. 127 (2005) 16352. [11] X.C. Huang, Y.Y. Lin, J.P. Zhang, X.M. Chen, Angew. Chem., Int. Ed. 45 (2006) 1557. [12] H. Li, M. Eddaoudi, M. O’Keeffe, O.M. Yaghi, Nature 402 (1999) 276. [13] G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surble, I. Margiolaki, Science 309 (2005) 2040. [14] F.A. Cotton, J.P. Donahue, C. Lin, C.A. Murillo, Inorg. Chem. 40 (2001) 1234. [15] Y. Kim, D.Y. Jung, Chem. Commun. (2002) 908. [16] W.H. Bi, R. Cao, D.F. Sun, D.Q. Yuan, X. Li, Y.Q. Wang, X.J. Li, M.C. Hong, Chem. Commun. (2004) 2104. [17] Y.J. Qi, Y.H. Wang, C.W. Hu, M.H. Cao, L. Mao, E.B. Wang, Inorg. Chem. 42 (2003) 8519. [18] M. Kurmoo, H. Kumagai, S.M. Hughes, C.J. Kepert, Inorg. Chem. 42 (2003) 6709. [19] M. Kurmoo, H. Kumagai, M. Akita-Tanaka, K. Inoue, S. Takagi, Inorg. Chem. 45 (2006) 1627. [20] M. Du, H. Cai, X.J. Zhao, Inorg. Chim. Acta 358 (2005) 4034. [21] Y. Gong, C.W. Hu, H. Li, K.L. Huang, W. Tang, J. Solid State Chem. 178 (2005) 3152. [22] A. Thirumurugan, M.B. Avinash, C.N.R. Rao, Dalton Trans. (2006) 221. [23] N.L. Rosi, J. Kim, M. Eddaoudi, B.L. Chen, M. O’Keeffe, O.M. Yaghi, J. Am. Chem. Soc. 127 (2005) 1504. [24] W. Chen, J.Y. Wang, C. Chen, Q. Yue, H.M. Yuan, J.S. Chen, S.N. Wang, Inorg. Chem. 42 (2003) 944. [25] Q.R. Fang, G.S. Zhu, M. Xue, J.Y. Sun, G. Tian, G. Wu, S.L. Qiu, Dalton Trans. (2004) 2202. [26] C. Kutal, Coord. Chem. Rev. 99 (1990) 213. [27] M.H. Zeng, X.L. Feng, X.M. Chen, Dalton Trans. (2004) 2217. [28] H.T. Zhang, Y.Z. Li, T.W. Wang, E.N. Nfor, H.Q. Wang, X.Z. You, Eur. J. Inorg. Chem. (2006) 3532. [29] X.L. Wang, Q. Chao, E.B. Wang, X. Lin, Z.M. Su, C.W. Hu, Angew. Chem., Int. Ed. 43 (2004) 5036. [30] X.L. Wang, C. Qin, E.B. Wang, Z.M. Su, Chem. Eur. J. 12 (2006) 2680.
Note
Photoluminescent metal-organic framework with hex topology constructed from infinite rod-shaped secondary building units and single e,e-trans-1,4-cyclohexanedicarboxylic dianion Miao Yu, Linhua Xie, Shuxia Liu *, Chunling Wang, Haiyan Cheng, Yuanhang Ren, Zhongmin Su Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal University, Changchun City, JiLin 130024, PR China Received 23 December 2006; received in revised form 23 February 2007; accepted 3 March 2007 Available online 20 March 2007
Abstract Hydrothermal reaction produced a three-dimensional zinc 1,4-cyclohexanedicarboxylate formulated as Zn5(l3-OH)2(trans-chdc)4 (chdc = 1,4-cyclohexanedicarboxylic dianion) in high purity and good yield, which is constructed from infinite rod-shaped Zn–O–C secondary building units interconnected by –C6H12-cyclohexane rings of the ligands. The topology of the framework can be regarded as hex type. Though it is synthesized from 1,4-cyclohexanedicarboxylic acid with mixed conformations (trans and cis), interestingly, the ligands in the compound Zn5(l3-OH)2(trans-chdc)4 are uniformly in e,e-trans conformation. This may be related to its synthetic conditions. Photoluminescence measurement reveals that the compound exhibits intense violet-blue fluorescent emission at room temperature. Origin of the emission can be assigned to intraligand transitions by comparison of the fluorescent emission bands for the free ligand chdcH2 and the compound Zn5(l3-OH)2(trans-chdc)4. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Metal-organic framework; Hex topology; Carboxylate ligands; Rod-shaped SBU; Fluorescence
1. Introduction In recent years, metal-organic frameworks (MOFs) have been investigated extensively due to their potential as functionalized materials as well as the variety of their intriguing topologies [1–11]. For the richness of transition metals with different d electron configurations and numerous available organic ligands, varied functional materials can be obtained. The extensive investigation in this area not only aims to seek the structure–property relationships but also to explore the correlations between synthetic conditions and resultant structures. Compared to the rigid ligand 1,4-benzenedicaryboxylic acid that has been turned out to
*
Corresponding author. Fax: +86 431 85099328. E-mail address: [email protected] (S. Liu).
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.03.020
be versatile for the design of porous crystalline material [12,13], 1,4-cyclohexanedicarboxylic acid (chdcH2) is a flexible ligand always existing with three conformations in common conditions (Scheme 1). For 1,3-diaxial hindrance a,a-trans form is least thermodynamically stable and seldom separated into solid-state compounds [14]. The e,e-trans form is most stable due to its two equatorial substituents. However, the dynamic equilibrium between e,e-trans and e,a-cis forms results in the difficult anticipation that which form would be crystallized into final products. Jung and Kim have separated 2D and 3D La1,4-cyclohexanedicarboxylates coordination networks in hydrothermal conditions with the chdc conformations controlled by the solution pH value [15]. More recently, Cao and co-workers have reported two cadmium coordination polymers with single e,e-trans and e,a-cis chdc, respectively, obtained in hydrothermal conditions with different pH
M. Yu et al. / Inorganica Chimica Acta 360 (2007) 3108–3112
3109
Scheme 1. The three conformations of the ligand chdcH2.
values and reaction temperatures [16]. Many other works reveal that the tendency that which form of chdc will be separated is also influenced by some other synthetic conditions, such as the nature of metal ions and assistant ligands [17–22]. These factors reflect the mutual selectivity between the coordination geometries of metal centers and the conformations of the ligand. Herein, we report a zinc 1,4-cyclohexanedicarboxylate Zn5(l3-OH)2(trans-chdc)4 (1) (chdc = 1,4-cyclohexanedicarboxylic dianion) synthesized in hydrothermal conditions in high purity and good yield. It is a three-dimensional metal-organic framework constructed from infinite rodshaped Zn–O–C secondary building units (SBUs) [2] interconnected by totally deprotonated chdcH2 ligands. It’s topology can be regarded as hex type according to the nomenclature defined by Yaghi and Keeffe [23]. It is interesting that the chdc ligands are in single e,e-trans conformation within compound 1 although started from chdcH2 with mixed conformations. The causation for that may be correlated to the synthetic condition. Photoluminescence measurements reveal that the compound exhibits intense violet-blue fluorescent emission at room temperature. Origin of the emission is discussed. 2. Experimental 2.1. Materials All chemicals were obtained from commercial sources and used without further purification. Elemental analyses were performed on a PLASMA-SPEC(I) ICP atomic emission spectrometer and a Perkin–Elmer 240C elemental analyzer. IR spectrum was recorded in the range 400– 4000 cm1 on an Alpha Centaurt FT/IR spectrophotometer using KBr pellets. TG analysis was performed on a Perkin–Elmer TGA7 instrument in flowing N2 with a heating rate of 10 °C/min. Powder X-ray diffraction measurement was performed on a Rigaku D/MAX-3 instrument with Cu Ka radiation in the angular range 2h = 3–90° at 293 K. Photoluminescence measurements were carried out on a Hitachi F-4500 Fluorescence Spectrophotometer. 2.1.1. Preparation of 1 A mixture of Zn(NO3)2 Æ 6H2O (0.8 mmol, 0.211 g), chdcH2 (mixture of cis and trans 99%) (0.8 mmol, 0.137 g), NaOH (1.6 mmol, 0.064 g) and water (10 mL) was sealed in Teflon-lined autoclaves and heated at
180 °C for 6 days, followed by slow cooling to room temperature. Compound 1 was collected as colorless block crystals with a yield about 0.142 g (85% based on Zn). Elemental Anal. Calc. for C32H42O18Zn5: C, 36.90; H, 4.06; Zn, 31.40. Found: C, 36.11; H, 4.68; Zn, 31.80%. IR (KBr, cm1): 3380 (m), 2945 (m), 2862 (m), 2361 (w), 2342 (w), 1542 (s), 1428 (s), 1364 (m), 1332 (m), 1284 (m), 1213 (m), 1047 (w), 929 (w), 890 (w), 798 (m), 687 (m), 571 (w). 2.1.2. X-ray crystallography Diffraction intensities for compound 1 was collected on a Siemens Smart CCD diffractometer with Mo Ka mono˚ ) at 293 K. The linear chromated radiation (k = 0.71073 A absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were taken from International Tables for X-ray Crystallography. Empirical absorption corrections were applied. The structures were solved by the direct method and refined by the full-matrix least-squares method on F2 using the SHELXTL crystallographic software package. Anisotropic thermal parameters were used to refine all non-hydrogen atoms. Partial H Table 1 Crystal data and structure refinement parameters for 1 Empirical formula C32H42O18Zn5 M 1041.5 Crystal system triclinic Space group P 1 2h Range (°) 1.95–26.37 ˚) a (A 8.6515(4) ˚) b (A 10.6723(6) ˚) c (A 11.8128(6) a (°) 113.855(1) b (°) 96.305(1) c (°) 106.316(1) ˚ 3) V (A 925.80(8) Z 1 F(0 0 0) 528 Dcalc (g cm3) 1.868 l(Mo Ka) (mm1) 3.271 Total data collected 5278 Unique data 3716 Observed data [I > 2r(I)] 3120 Rint 0.0146 Goodness-of-fit 1.034 R indexes [I > 2r(I)] R1 = 0.0292,a wR2 = 0.0615b R indexes (all data) R1 = 0.0379,a wR2 = 0.0662b P P a R1 ¼ iFoj jFci/ jFoj. P P b wR2 ¼ ½ wðF 2o F 2c Þ2 = wðF 2o Þ2 1=2 ; w = 1/[r2(Fo)2 + (aP)2 + bP], 2 where P ¼ ðMaxðF o ; 0Þ þ 2F 2c Þ=3, a = 0.0257, b = 0.71.
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atoms on the ligands are placed geometrically and the others (for the hydroxide ion and the 1,4-carbons of cyclohexane ring of the ligands) were located from the difference Fourier maps and refined isotropically. Details of the crystal data and final structure refinements of compound 1 are summarized in Table 1. The powder X-ray diffraction pattern for bulk sample of 1 agrees well with that simulated from the single-crystal data. The diffraction peaks on both experimental and simulated patterns match well in positions, indicating its phase purity (see Supplementary material Fig. 1S). 3. Results and discussion 3.1. Synthesis and structure We noted that the yield of the product was sensitive to the ratio of the original materials. The moderate ratio of chdcH2 to NaOH (about 1:2) is crucial for the good yield of 1. Larger or smaller ratios would produce worse results and even cannot yield compound 1. The phenomenon may result from the fact that a larger ratio cannot make all the chdcH2 deprotonated for further reaction and a smaller ratio would result in too much oxide of zinc with inert coordination reactivity. Single crystal X-ray diffraction study reveals that 1 is a three-dimensional framework built from infinite rodshaped secondary building blocks, which are interconnected by –C6H12– cyclohexane rings of the chdc ligands. The infinite SBU that can be formulated as [–Zn5–(OH)2– (COO)4–]1 consists of zinc atoms locked by hydroxide ions and carboxyl groups of chdc. Within these units, there are three types of zinc atoms (Zn1, Zn2, Zn3), one type of hydroxide ion (O1) and four types of carboxyl groups. (Fig. 1) Zn1, located at the inverse center position, coordinates with six O atoms, two hydroxide O atoms and four carboxyl O atoms, with distorted octahedron geometry. Zn2 and Zn3 are five coordinated with distorted squarepyramid and trigonal-bipyramid geometries, respectively, which are completed by five O atoms, one from hydroxide ion and the others from carboxyl groups. The selected Zn– O bond lengths and Zn–O–Zn angles are listed in Table 2. The hydroxide ion (O1) bridging the three zinc atoms can be distinctly identified for that its proton can be found in the difference Fourier maps reasonably. The four types of carboxyl groups from four chdc are at least chelated or/ and bridged to two zinc atoms, two of which are mode A and the other two are mode B and C, respectively (Fig. 2a, Scheme 2). With such complex connectivity, these distorted octahedra, square-pyramids and trigonal-dipyramids form the infinite rod-shaped SBU by corner-sharing and edge-sharing (Fig. 2b). The rods are stacked in parallel along [1 0 0] direction and interconnected by the –C6H12– parts of chdc ligands (Fig. 2c). Each rod is connected to six neighboring rods by eight chdc connectors. Because there are four of the eight chdc connectors linking two sets of rods, the vertex of the framework of 1 should be 8-con-
Fig. 1. A segment of the framework of 1 represented by 30% thermal ellipsoids. All H atoms have not been shown for clarity. Zn: teal; O: red; C: black. Symmetry code: (A) 1 x, 2 y, 2z. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 2 ˚ ] and angles [°] for 1 Selected bond lengths [A Zn(1)–O(1)[a] Zn(1)–O(1) Zn(1)–O(9)[a] Zn(2)–O(5) Zn(2)–O(1) Zn(2)–O(9) Zn(3)–O(4) Zn(3)–O(2)
2.057(2) 2.057(2) 2.1196(18) 1.922(2) 1.994(2) 2.019(2) 2.149(2) 2.213(2)
Zn(1)–O(9) Zn(1)–O(7)[a] Zn(1)–O(7) Zn(2)–O(6) Zn(2)–O(8)[b] Zn(3)–O(3) Zn(3)–O(8) Zn(3)–O(1)
2.1196(18) 2.131(2) 2.131(2) 2.023(2) 2.337(2) 1.933(2) 1.971(2) 1.974(2)
O(1)[a]–Zn(1)–O(1) O(1)[a]–Zn(1)–O(9)[a] O(1)–Zn(1)–O(9)[a] O(1)[a]–Zn(1)–O(9) O(1)–Zn(1)–O(9) O(9)[a]–Zn(1)–O(9) O(1)[a]–Zn(1)–O(7)[a] O(7)[a]–Zn(1)–O(7) O(9)–Zn(2)–O(6) O(5)–Zn(2)–O(8)[b] O(1)–Zn(2)–O(8)[b] O(9)–Zn(2)–O(8)[b] O(6)–Zn(2)–O(8)[b] O(1)–Zn(3)–O(4) O(3)–Zn(3)–O(2) O(8)–Zn(3)–O(2) O(1)–Zn(3)–O(2)
180.000(1) 80.51(8) 99.49(8) 99.49(8) 80.51(8) 180.000(1) 87.78(8) 180.000(1) 104.37(8) 98.75(9) 85.90(8) 145.49(8) 58.85(8) 95.02(8) 92.42(10) 84.37(8) 89.18(8)
O(1)–Zn(1)–O(7)[a] O(9)[a]–Zn(1)–O(7)[a] O(9)–Zn(1)–O(7)[a] O(1)[a]–Zn(1)–O(7) O(1)–Zn(1)–O(7) O(9)[a]–Zn(1)–O(7) O(9)–Zn(1)–O(7) O(5)–Zn(2)–O(1) O(5)–Zn(2)–O(9) O(1)–Zn(2)–O(9) O(5)–Zn(2)–O(6) O(1)–Zn(2)–O(6) O(3)–Zn(3)–O(8) O(3)–Zn(3)–O(1) O(8)–Zn(3)–O(1) O(3)–Zn(3)–O(4) O(8)–Zn(3)–O(4)
92.22(8) 89.30(8) 90.70(8) 92.22(8) 87.78(8) 90.70(8) 89.30(8) 110.94(9) 115.64(9) 84.54(8) 108.85(10) 129.89(9) 118.86(9) 114.67(9) 126.26(9) 93.81(9) 86.04(8)
[a]
Symmetry transformations used to generate equivalent atoms: x + 1, y + 2, z + 2. [b] Symmetry transformations used to generate equivalent atoms: x + 2, y + 2, z + 2.
nected rather than 10-connected. Consequently, the topology of 1 can be regarded as hex type (coordination number is 8) according to the nomenclature defined by Yaghi and
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Fig. 2. (a) Ball-and-stick and (b) polyhedral representation of the infinite [–Zn5–(OH)2–(COO)4–]1 rod-shaped SBU. (c) Perspective view of the framework of 1 down [1 0 0] direction. All H atoms have not been shown for clarity. Zn: teal; O: red; C: black. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Scheme 2. Schematic representation of the three coordination modes for the carboxyl groups in 1.
Keeffe [23]. However, the corresponding complex connectivity results in the lower symmetry of the framework of 1 ðP 1Þ compared to an ideal hex framework (P6/mmm). Although one of the four chdc per formula in 1 is statistically disordered and located at two positions (see Supplementary material, Fig. 4S), we note that the chdc ligands in 1 are totally e,e-trans conformation. As e,e-trans conformation is the one of most thermodynamic stable in the three ones, the formation of 1 with single e,e-trans conformation chdc may be correlated to the reaction conditions with high temperature, long reaction time and moderate high pH value. Its high yield indicates the high selectivity of the zinc ions and the e,e-trans form chdc at the relevant synthetic conditions. 3.2. Photoluminescence property Solid state sample of compound 1 exhibits intense photoluminescence with max emission peak at k = 413 nm upon excitation at 263 nm at room temperature (Fig. 3). To understand the nature of the emission band, photoluminescence measurement of the raw material of the ligand chdcH2 was carried out, the result of which reveals that it is also photoluminescent with max emission peak at 425 nm upon excitation peak 264 nm (Fig. 3). The emission
Fig. 3. Photoluminescence spectra for compound 1 (blue) and free ligand chdcH2 (black) with excitation at 263 nm and 264 nm, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
band of the free ligand is attributed to the intraligand p* ! n transitions [24,25]. The profile of the emission band for 1 resemble that for the free ligand except (1) obvious intensity enhancement at low energy region but slight weakening at high energy region; (2) a shift towards shorter wavelength. These observations indicate that the fluorescent emission of 1 can be assigned to intraligand transitions in nature, but not MLCT (metal-to-ligand charge transfer) or LMCT (ligand-to-metal charge transfer) [26–28]. The hypsochromic shift must be related to that n orbitals (highest occupied molecule orbitals, HOMO) located on the carboxyl O atoms are perturbed by the zinc cations
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after their coordination [24]. The intensity enhancement around 413 nm must be for that the coordination of ligand to metal center effectively increase the rigidity of the ligand and reduce the loss of energy by nonradiation decay [29,30]. What cause the weakening of the emission band round 324 nm is not clear now. We presume there may be some kind of absorption for the compound 1 at this wavelength region, which result in the weakening of the emission band. 3.3. Thermogravimetric analysis The thermogravimetric analysis (TGA) curve recorded at 20–800 °C reveals that there are two step weight losses for compound 1 (see Supplementary material Fig. 3S). The first stage occurred between 70 and 200 °C is corresponded to the detachment of one chdc per formula (weight loss: calc., 14.8%; found, 12.5%). The second step from 400 to 530 °C results from the loss of the other three chdc (weight loss: calc., 44.4%; found, 45.2%). Final species is corresponded to ZnO at 800 °C (weight: calc., 39.1%; found, 40.1%). 4. Conclusions We have successfully prepared a three-dimensional zinc 1,4-cyclohexanedicarboxylate built from rod-shaped SBUs and single e,e-trans chdc. This work provides a new example capable of separating e,e-trans conformation chdcH2 into solid-state solid from the mixed conformation in good yield and high purity. Compound 1 may also be served as advanced material for violet-blue light emitting device. Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 20571014) and the Scientific Research Foundation for Returned Overseas Chinese Scholars, the Ministry of Education. We also thank Prof. Chunshan Shi for his discussion on the photoluminescence property of the compound. Appendix A. Supplementary material CCDC 602852 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223336-033; or e-mail: [email protected]. Supplemen-
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