Synthesis and characterization of a novel photoluminescent three-dimensional metal–organic framework

Inorganica Chimica Acta 362 (2009) 2510–2514

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Synthesis and characterization of a novel photoluminescent three-dimensional metal–organic framework Kou-Lin Zhang a,*, Fang Zhou a, Li-Min Yuan b, Guo-Wang Diao a, Seik Weng Ng c a

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, PR China Test and Analysis Center, Yangzhou University, Yangzhou 225002, PR China c Department of Chemistry, University of Malaya, 50603, Kuala Lumpur, Malaysia b

a r t i c l e

i n f o

Article history: Received 27 August 2008 Received in revised form 8 October 2008 Accepted 10 October 2008 Available online 22 October 2008 Keywords: Synthesis Zn(II) complex Single-stranded helice Metal–organic framework Luminescence

a b s t r a c t A novel metal–organic framework containing one-dimensional channels of formula [Zn3(Aco)2(H2O)6]n (H3Aco = aconitic acid) has been synthesized and characterized by FT-IR spectroscopy, thermogravimetric analysis (TG), X-ray analysis, and solid state photoluminescence spectra. X-ray crystallographic studies reveal that there are two kinds of crystallographically independent Zn atoms in the title complex. The most interesting feature of the structure is an unprecedented 3D MOF containing infinite Zn(1) linear chains and heterochiral Zn(2) single-stranded helices. The linear chains and helices happen to be perpendicular to each other. Photoluminescence properties of the title compound have been examined in the solid state at room temperature. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Recently, the design and synthesis of metal–organic frameworks (MOFs) that provide new shapes, sizes, and chemical environments causes an increasing interest in supramolecular chemistry and coordination chemistry. This is not only due to their abundant structural diversity, but also they are fundamental steps to discover and construct various molecular-based functional materials or technologically useful materials [1–12]. The most feasible strategy used to design and construct MOFs is to select suitable polyfunctional ligands that enable the control on the structural motifs [13–15]. Aconitic acid (H3Aco), an open chain aliphatic carboxylic acid with three carboxylate groups, is an attractive candidate to design and synthesize novel metal–organic coordination compounds with high dimensional networks [16– 18]. Firstly, it is more flexible than aromatic carboxylic acids due to the open carbon skeleton; the nonlinear flexibility of the carboxylate groups endows it a peculiar characterization to form interesting architectures with fascinating topologies [16,19]. Secondly, it can act as a multi-dentate ligand for its rich coordination modes because the three carboxylate groups may be partially or completely deprotonated [20]. So, we have recently started a systematic study on the assembly reactions of metal–H3Aco system under different reaction conditions. * Corresponding author. Tel./fax: +86 514 87695171. E-mail addresses: [email protected], [email protected] (K.-L. Zhang). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.10.008

In the present paper, we report a novel 3D MOF [Zn3(Aco)2(H2O)6] based on the H3Aco ligand, which consists of two different kinds of 1D chains. To the best of our knowledge, the title complex represents a rare example of a 3D MOF constructed by perpendicular Zn(1) linear chains and heterochiral Zn(2) single-stranded helices. The solid state photoluminescence properties have been studied at room temperature.

2. Experimental 2.1. Materials and characterization All reagents were used as purchased. Elemental analyses (C, H and N) were carried out on a 240 C Elemental analyzer. Infrared spectrum (400–4000 cm 1) was recorded from KBr pellet in Magna 750 FT-IR spectrophotometer. The solid state emission spectra of the title compound and H3Aco were recorded using 48000DSCF fluorescence spectrometer. 2.2. Synthesis of [Zn3(Aco)2(H2O)6]n Hydrazinecarboxamide (0.0194 g, 0.2586 mmol) was dissolved in water (5 ml) and then a solution of Zn(NO3)2
6H2O (0.0256 g, 0.0862 mmol) in water (6 ml) was added. To this solution a mixture of H3Aco (0.0150 g, 0.0862 mmol) and NaOH (0.0102 g, 0.2586 mmol) in water (5 ml) was added and then the mixed solution was filtered. To the filtrate was added methanol (5 ml).

K.-L. Zhang et al. / Inorganica Chimica Acta 362 (2009) 2510–2514 Table 1 Crystal data and structure refinement for the title complex. Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (Mg/m3) Absorption coefficient (mm 1) F(0 0 0) h Range for data collection (°) Index ranges

Reflections collected/unique (Rint) Completeness to theta = 27.50 Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) Largest difference in peak and hole (e Å

3

)

C12H18O18Zn3 646.37 293(2) 0.71073 monoclinic C2/c 17.156(2) 9.5733(8) 14.049(2) 90 116.000(2) 90 2073.9(4) 4 2.070 3.532 1296 2.50–27.50 22 6 h 6 22, 12 6 k 6 12, 18 6 l 6 18 8633/2310 (0.0333) 97.1% 0.868 and 0.775 full-matrix least-squares on F2 2310/0/153 1.095 R1 = 0.0549, wR2 = 0.1912 R1 = 0.0635, wR2 = 0.1998 1.110 and 1.153

The yellow block crystals were formed several days later. They were filtered and washed with methanol and dried in vacuum (yield: 40.1% based on H3Aco). Anal. Calc. for C12H18O18Zn3: C, 22.30; H, 2.81. Found: C, 22.54; H, 2.72%. 2.3. X-ray single-crystal structure determination Crystallographic data for the title compound were collected at 293(2) K with a Siemens SMART CCD diffractormeter using graphite-monochromated (Mo Ka) radiation (k = 0.71073 Å), w and x scans mode. The structure was solved by direct methods and refined by full-matrix least-squares on F2 method using SHELXL-97 program [21]. Intensity data were corrected for Lorenz and polarization effects and an empirical absorption correction was performed. All non-hydrogen atoms were refined anisotropically. All the hydrogen atoms were added geometrically and allowed to ride on their respective parent atoms. The contribution of these hydrogen atoms was included in the structure factor calculations. Details of crystal data, collection and refinement are listed in Table 1.

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carboxamide have the same structure with the title complex. The reason that the presence of hydrazinecarboxamide leads to the formation of single-crystals may be attributed to the fact that the Zn(II) ion coordinates with the hydrazinecarboxamide at first and then the substitution reaction between the ligand Aco and the Zn(II)–hydrazinecarboxamide complex takes place more slowly compared with the direct reaction between the Zn(II) ion and Aco ligand. The single-crystals were thus formed in the presence of hydrazinecarboxamide. The solvent methanol was also important for the crystal growth of the title complex. The title complex is stable in the open air. 3.2. Structural description of the title complex X-ray diffraction analysis indicates that the title complex is a 3D MOF consisting of single metal centers. All carboxylate groups are completely deprotonated (Fig. 1). This result is in agreement with the FT-IR spectra in which no absorption bands around 1730– 1650 cm 1 attributed to –COOH group were observed. The coordination modes of three carboxylate groups in Aco ligand can be classified into two types: one carboxylate group adopts a bidentate-chelating mode, which binds to one Zn atom; while the other two carboxylate groups link three Zn atoms in a chelating-bridging fashion. Thus, one Aco ligand acts as a l4-bridge linking four Zn centers. Two crystallographically independent Zn atoms, Zn(1) and Zn(2), in 1:2 ratio, are observed in the symmetric unit and they are both six-coordinated. At first glance, Zn(1) atom looks like taking a tetragonal antiprismatic geometry and coordinates to eight carboxylate oxygen atoms in bichelating fashions from four individual Aco ligands. However, the distance between Zn(1) and O(2) is 2.698 Å, suggesting that this interaction is weak and could be negligible. Thus, the coordination environment of Zn(1) can be regarded to be in a seriously distorted octahedral geometry ligated to four chelating and two bridging carboxylate oxygen atoms from four independent Aco ligands. The distance between the Zn(1) and the bridging O(1) [2.325(7) Å] is slightly shorter than those of the other Zn(1)–O bonds [2.332(7)– 2.474(8) Å]. The angles of O–Zn(1)–O range from 54.3(2)° to 154.7(2)°. Selected bond distances and angles are listed in Table 2. The Zn(1) atoms are connected by Aco ligand with the nearest Zn(1)


Zn(1) distance of 7.025(1) Å, resulting in the formation of an infinite Zn(1) linear chain as shown in Fig. 2.

3. Results and discussion 3.1. Preparation The solvent used to synthesize the compound and the method to grow single crystal usually has great effect on the structure of the compound. Self-assembly of stoichiometric amounts H3Aco, hydrazinecarboxamide, Zn(NO3)2
6H2O and NaOH in the mixed water–methanol solvent and slow evaporation of solution in open air successfully provided the single-crystal suitable for the X-ray analysis of the title compound. It should be noted that the title complex could not be formed under hydrothermal condition. Although the specie hydrazinecarboxamide does not appear in the title complex, omission of the specie leads to the formation of polycrystals instead of single-crystals. The powder X-ray analysis reveals that the polycrystals prepared in absence of hydrazine-

Fig. 1. A view of the coordination environment of Zn centers. (Symmetry code: #1 x + 1, y + 2, z + 1; #2 x, y + 2, z 1/2; #3 x + 1, y, z + 1/2; #4 x + 1/2, y + 1/ 2, z + 1/2; #5 x + 1/2, y 1/2, z + 1/2).

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Table 2 Selected bond lengths (Å) and angles (°) for the title complex. Zn(1)–O(6)#1 Zn(1)–O(1)#3 Zn(1)–O(5)#1 Zn(2)–O(4)#4 Zn(2)–O(3) Zn(2)–O(2W) O(6)#1–Zn(1)–O(6)#2 O(6)#2–Zn(1)–O(1)#3 O(6)#2–Zn(1)–O(1) O(6)#1–Zn(1)–O(5)#1 O(1)#3–Zn(1)–O(5)#1 O(6)#1–Zn(1)–O(5)#2 O(1)#3–Zn(1)–O(5)#2 O(5)#1–Zn(1)–O(5)#2 O(4)#4–Zn(2)–O(3) O(4)#4–Zn(2)–O(2) O(3)–Zn(2)–O(2) O(3W)–Zn(2)–O(2W) O(2)–Zn(2)–O(2W) O(3W)–Zn(2)–O(1W) O(2)–Zn(2)–O(1W) C(1)–O(1)–Zn(1) C(5)–O(3)–Zn(2) C(6)–O(5)–Zn(1)#1

2.332(7) 2.325(7) 2.474(8) 2.263(7) 2.294(7) 2.356(8) 97.3(4) 97.9(3) 147.9(2) 54.3(2) 154.7(2) 86.7(3) 79.6(3) 121.9(4) 164.2(3) 95.9(3) 98.7(3) 93.4(3) 156.4(3) 167.9(3) 80.1(3) 102.1(6) 136.0(6) 89.3(6)

Zn(1)–O(6)#2 Zn(1)–O(1) Zn(1)–O(5)#2 Zn(2)–O(3W) Zn(2)–O(2) Zn(2)–O(1W)

2.332(7) 2.325(7) 2.474(8) 2.277(8) 2.313(7) 2.353(7)

O(6)#1–Zn(1)–O(1)#3 O(6)#1–Zn(1)–O(1) O(1)#3–Zn(1)–O(1) O(6)#2–Zn(1)–O(5)#1 O(1)–Zn(1)–O(5)#1 O(6)#2–Zn(1)–O(5)#2 O(1)–Zn(1)–O(5)#2 O(4)#4–Zn(2)–O(3W) O(3W)–Zn(2)–O(3) O(3W)–Zn(2)–O(2) O(4)#4–Zn(2)–O(2W) O(3)–Zn(2)–O(2W) O(4)#4–Zn(2)–O(1W) O(3)–Zn(2)–O(1W) O(2W)–Zn(2)–O(1W) C(1)–O(2)–Zn(2) C(5)–O(4)–Zn(2)#5 C(6)–O(6)–Zn(1)#1

147.9(2) 97.9(3) 83.6(4) 86.7(3) 79.6(3) 54.3(2) 154.7(2) 93.5(3) 87.2(3) 109.6(3) 87.9(3) 76.3(3) 92.6(3) 84.1(3) 76.4(3) 127.4(6) 110.4(6) 94.9(6)

Symmetry transformations used to generate equivalent atoms: #1 x + 1, y + 2, z + 1; #2 x, y + 2, z 1/2; #3 x + 1, y, z + 1/2; #4 x + 1/2, y + 1/2, z + 1/2; #5 x + 1/2, y 1/2, z + 1/2.

Zn(2) is also in the octahedral coordination environment involving two bridging carboxylate oxygen atoms from two different ligands, two aqua ligands lying almost in the equatorial plane, as well as one bridging carboxylate oxygen atom and one aqua ligand occupying the two apical sites. The O–Zn(2)–O angles lie in the range from 76.3(3)° to 167.9(3)°. The carboxylate groups link Zn(2) atoms into a 21 single-helical chain with the nearest Zn(2)


Zn(2) distance of 5.004(1) Å (Fig. 3). The formation of the helix in the structure may be attributed to the fact that the steric orientation of the carboxylate groups is remarkably flexible. The interconnection between the Zn(1) linear chains and the Zn(2) helices through carboxylate groups results in the formation of a 3D MOF (Fig. 4). It is of great interest that these 1D Zn(1) chains and Zn(2) helices happen to be perpendicular to each other. To the best of our knowledge, this kind of 3D MOF has not been reported. The adjacent helices are with different handedness and alternatively appear in 1:1 ratio. The heterochiral single-stranded helices not only exist in the same layers which are constructed by 1D Zn(1) chains, but also in adjacent ones. Because the Aco anion is achiral, the crystal which consists of equal amount of leftand right-handed helices may be formed through the ligand self-discrimination process and an internal racemate was thus produced [22–23]. It further confirmed that the structure of the title complex is a Zn-centered symmetric one. The intramolecular hydrogen bonds exist extensively in the title complex, which further stabilize the 3D MOF structure. The details of the hydrogen bonds are listed in Table 3.

Fig. 3. Ball-and-stick representation of the Zn(2) single-stranded helical chain where the green solid line represents the 21 helical axis. (For interpretation of the references to colour in figure legends, the reader is referred to the web version of this article.)

3.3. FT-IR spectrum The FT-IR spectra of the title compound show broad peaks in the range of 3600–3200 cm 1 due to the existence of coordinated water molecules. In the middle of the stretching vibrations, the IR results are commonly employed to distinguish the coordination modes of the carboxylate groups. The vibrations of mas(COO) and ms(COO) are at 1566 and 1398 cm 1, respectively, for the title complex. The absence of absorption bands at 1730–1650 cm 1 was indicative of the entire deprotonation of H3Aco upon its coordination to Zn ions [24]. Both the symmetric and asymmetric bands were shifted to lower wave numbers compared to the free H3Aco acid. In all these cases, the results of the X-ray analysis allowed

Fig. 2. A side view of the 1D Zn(1) chain.

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Fig. 4. View of the 3D MOF consisting of the infinite perpendicular 1D chains and helices. The hydrogen bonds and hydrgen atoms are not shown for clarity.

Table 3 The details of the hydrogen bonds for the title complex. d(D–H) d(H


A) d(D


A) \DHA

Symmetry of A

O1W–H1W1


O4 O1W–H1W2


O1 O2W–H2W1


O1 O2W–H2W2


O2 O2W–H2W2


O5 O3W–H3W1


O1W O3W–H3W1


O3 O3W–H3W2


O6 C4–H4


O6

0.8500 0.8500 0.8500 0.8500 0.8500 0.8500 0.8500 0.8500 0.9300

x,

1.9200 1.9900 2.0800 2.4100 2.2700 2.3700 2.4000 1.8500 2.4800

2.7628 2.7949 2.8851 3.1990 2.8322 3.1331 2.7571 2.6821 2.8293

168.00 157.00 158.00 154.00 123.00 149.00 106.00 168.00 102.00

y, 1/2 + z x, y, 1/2 z 1/2 + x, 1/2 y, 1/2 + z 1/2 x, 1/2 + y, 1/2 z 1/2 + x, 1/2 y, 1/2 + z 1/2 x, 1/2 + y, 1/2 z 1/2 x, 1/2 + y, 1/2 z 1/2 x, 1/2 y, z

to assign unambiguously the binding modes of the carboxylate groups.

Relative intensities / counts

140

D–H


A

120 100 80 60 40 20 0 300

3.4. Thermogravimetric analysis

400

450

500

550

600

Wavelength / nm

To study the thermal stability of the title compound, the thermogravimetric analysis (TG) was performed on polycrystalline samples under a nitrogen atmosphere in flowing N2 with a heating rate of 10 °C min 1 (Fig. 5). The thermogravimetric analysis (TGA) of the title complex revealed two thermal events. The first step between 124 and 270 °C is ascribed to the gradual loss of the coordinated water molecules, leading to a corresponding weight loss of 18.02% (calculated 16.71%). This dehydration step is subsequently

Fig. 6. Solid-state emission spectra of the complex and H3Aco at room temperature (red: H3Aco, kex = 296 nm; black: complex, kex = 296 nm).

followed by the decomposition step of the compound. The decomposition is achieved at about 355 °C, and no further thermal events are observed. 3.5. Photoluminescence properties It has been well known that metal–organic polymeric complexes with a d10 closed-shell electronic configuration have been found to exhibit fluorescent properties. The solid-state photoluminescence spectra of the title complex and H3Aco at room temperature are depicted in Fig. 6. As shown in Fig. 6, the emission spectrum of the title complex under the excitation wavelength 296 nm shows one intense maximum emission at 385 nm, which is just equal to that of free H3Aco acid. The excitation wavelength of the H3Aco ligand is also the same as that of the title complex (kex = 296 nm). Therefore, the strong emission band at 385 nm of the title complex may be mainly ascribed to p-p* transition of the free H3Aco acid. The weak sharp band centered at 550 nm for the title complex may arise from the Zn(II) to Aco ligand charge transfer. The fluorescent intensity of the title complex is weaker than that of the free H3Aco acid, which may be due to the existence of the coordinated water molecules in the title complex.

100 80

Weight / %

350

60 40 20 0 0

100

200

300

400

Temperature/ °c Fig. 5. TG curve of the complex.

500

600

4. Conclusion In conclusion, one novel 3D MOF [Zn3(Aco)2(H2O)6]n based on the H3Aco ligand has been successfully synthesized and character-

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ized. X-ray analysis reveals that the title complex is a rare example of 3D MOF constructed by infinite 1D Zn(1) chains and heterochiral Zn(2) single-stranded helices which are perpendicular to each other. The compound exhibits photoluminescence with the maximum emission located in UV region (kem = 385 nm). Acknowledgment We gratefully acknowledge the Foundation of Jiangsu Provincial Key Program of Physical Chemistry in Yangzhou University. Appendix A. Supplementary material CCDC 697772 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via 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.ica.2008.10.008. References [1] H.K. Chae, D.Y. Siberio-Perez, J. Kim, Y. Go, M. Eddaoudi, A.J. Matzger, M. O’Keeffe, O.M. Yaghi, Nature 427 (2004) 523. [2] M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe, O.M. Yaghi, Science 295 (2002) 469. [3] B. Chen, N.W. Ockwig, F.R. Fronczek, D.S. Contreras, O.M. Yaghi, Inorg. Chem. 44 (2005) 181. [4] M. Eddaoudi, D.B. Moler, H. Li, B. Chen, T.M. Reineke, M. O’Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319.

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