Notes
2800 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 9 Notes
http://dx.doi.org/10.5012/bkcs.2013.34.9.2800
Synthesis, Crystal Structure and Photoluminescent Property of Cadmium(II) Metal-organic Framework with Mixed Carboxylate and Bis(imidazole) Ligand
Yong Hong Zhou* and Yu Peng Tian†
School of Chemistry and Material Science, Huaibei Normal University, Huaibei 235000, People’s Republic of China
*E-mail: [email protected]
†Department of Chemistry and Chemical Technology, Anhui University, Hefei 230039, People’s Republic of China Received May 4, 2013, Accepted June 5, 2013
Key Words : Metal-organic framework, Pyridine-2,3-dicarboxylic acid, Crystal structure
Metal-organic frameworks (MOFs) have received great attention in the fields of functional materials and crystal engineering, not only because of their potential applications ranging from gas adsorption, molecular recognition, optics, catalysis, and magnetic materials, but also because of their intriguing framework topologies.1-3 The rational design and appropriate use of the characteristic ligands is critical in the construction of the desired MOFs.4-6 Among various organic ligands, multidentate N- or O-donor ligands such as pyridine or pyrazine-carboxylic acids, have drawn extensive attention in the construction of coordination polymers owing to their versatile coordination modes and structural stability.7 For example, pyridine-2,6-, 2,5-, or 3,4-dicarboxylic and pyra- zile-2,3-dicarboxylic acids have been employed as success- ful multifunctional ligands, and a variety of MOFs with diverse topologies and interesting properties have been obtained.8-12 However, compared with other pyridine-di- carboxylic acids, pyridine-2,3-dicarboxylic acid (H2PDA) (Scheme 1) is rarely employed as a linkage ligand. The main reason is that it often acts as chelating bidentate ligand through one nitrogen atom and one oxygen atom of the carboxylic group in ortho position, and the second carboxyl group remains idle.13 In an attempt to address this point, we have introduced an effective strategy, in which the pyridine- 2,3-dicarboxylic acid is used to construct a basic skeleton and another auxiliary ligand 1,4-bis(imidazol-1-yl)-butane (bib) (Scheme 1) to extend the framework. As an excellent derivative of imidazole, bib not only possesses the merits of imidazole, but also can adopt different conformations com- pared with the corresponding imidazole ligand on the basis
of the relative orientations of its -(CH2)4- groups. Herein, utilizing Cd2+ ions as nodes and the pyridine-2,3-dicarbox- ylic acid and bib ligands as spaces, we have obtained a novel three dimensional MOF with interesting topologies, namely, [Cd(PDA)(bib)]·CH3OH·2.5H2O (1) (PDA2− = pyridine-2,3- dicarboxylate anion).
The framework [Cd(PDA)(bib)]·CH3OH·2.5H2O (1) was synthesized by the reaction of Cd(CH3COO)2·2H2O with H2PDA and bib under hydrothermal conditions. X-ray study reveals that compound 1 is a 3D coordination polymer constructed from Cd2+ ions, PDA2− bridging ligands and bib linkers. The Cd2+ is five-coordinated by three nitrogen atoms from two bib ligands and one PDA2− with Cd-N distances of 2.212(5)-2.283(4) Å, and N-Cd-N angles of 105.72(17)- 125.41(18)º, two carboxylate oxygen atoms from two PDA2−
ligands with Cd-O distances of 2.246(5)-2.337(4) Å and O- Cd-O angles of 169.3(2)º, forming distorted trigonal bi- pyramidal geometry. In compound 1, all carboxylate groups of the H2PDA ligand are deprotonated, and two carboxylate groups of the PDA2− link two Cd2+ with µ-(k3N, O2: O3) coordination mode; that is, the ortho-position carboxylate oxygen atom and the pyridyl nitrogen atom chelate one Cd2+
and the meta-position carboxylate oxygen atom binds to another Cd2+ (Scheme 2). The bridging PDA2− ligands link adjacent Cd2+ to construct a 1D zigzag chain with the Cd···Cd separation of 8.416 Å (Figure 1(a)).
The bib ligand has been purposefully selected for the design and construction of intriguing polymers in crystal engineering, owing to the flexibility of the spacer.14,15 In compound 1, the Cd2+ belonging to the adjacent 1D chains are bridged by the bib ligands to form a 2D layer, in which the bib adopts trans conformation and connect two Cd2+
with Cd···Cd distance of 16.312 Å. The Z-sharped bib
Scheme 1. Molecular structure of the ligands used in this paper. Scheme 2. Coordination mode of PDA2− ligand.
Notes Bull. Korean Chem. Soc. 2013, Vol. 34, No. 9 2801
ligands protruding from both sides of the layer enlarge the space within the layer, resulting in the formation of pipe-link channels, which parallel to the [Cd(PDA)]n chains in the b direction (Figure 1(b)). Furthermore, each layer is catenated through the bib ligand with adjacent one having identical motifs. Adjacent layers are rotated about the normal line of the layer by an angle of about 180, affording a 3D network (Figure 1(c)).
It is worth mentioning the difference of bib before and after coordinated with Cd2+ ions. As a free molecule, bib molecule adopts trans conformation (Figure 2) as demon- strated by the torsion angle C6-N1-C4-C7 (-67.0(3)o). The two imidazole rings are parallel each other, and the donor N2-N2A separation is 10.442 Å. Although bib remains adopting trans conformation after coordinated with Cd2+, the torsion angle C10-N2-C11-C12 has changed to 74.5(9)o. The dihedral angle between two imidazole rings is 18o, and the two donor N5 and N5C atoms are 7.572 Å apart from each
other. These changes indicate that bib can bend or rotate according to the geometric needs of metal ions and causes the structural diversity.
During the past decade, the photluminescent properties of many d10 metal coordination polymers have been studied, and the results reveal that their luminescence behaviors are closely associated with the metal ions and organic ligands.16,17 In the present work, the photoluminescent properties of free H2PDA, bib and complex 1 have been investigated in the solid state at room temperature. The free bib ligand displays luminescent with emission maximum at 340 nm upon ex- citation at 300 nm. However, fluorescent emission of H2PDA ligand is very weak, and the strong electron withdrawing group, the carboxyl group, results in fluorescence quench- ing, so H2PDA ligands almost have no contribution to the fluorescent emission of as-synthesized polymers.18 In com- parison with that of the free ligands, 1 shows strong emission band centered at 425 nm (λex = 368 nm) (Figure 3).
The emissions arising from the free ligands are not observed for compound 1. The absence of ligand-based emission sug- gests energy transfer from the ligands to the Cd(II) centers during photoluminescence. Thus, the fluorescent emission of 1 may be proposed to originate from the coordination of bib to the Cd(II) ions (ligand-to-metal charge transition, LMCT), as reported for other Cd(II) complexes with N- donor ligands.19,20
Experimental
All reagents and solvents employed were commercially available and used as received without further purification.
Figure 1. (a) View of the 1D chain formed by PDA2− and Cd2+
ions in 1. (b) A pipe-like channel running along the b axis. (c) Perspective view of the 3D network of 1. Hydrogen atoms have been omitted for clarity.
Figure 2. The conformation of free bib ligands (Symmetry code:
(A) −x+1, −y+1, −z+1.).
Figure 3. The photoluminescent spectra of complex 1 in the solid state at room temperature.
2802 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 9 Notes
The C, H and N microanalyses were carried out with a Vario EL III elemental analyzer. The IR spectra were recorded on KBr discs on a Nicolet 170SX FT-IR spectrophotometer in the 4000-400 cm−1 region. Luminescence spectra for the solid samples were recorded with a RF-5301PC fluorescence spectrophotometer.
Synthesis of Bib. Ligand bib was synthesized as reported previously.20 The product was recrystallized from water and crystals of bib were obtained. Elemental analysis (%) calcd for C10H18N4O2: C, 53.03; H, 7.95; N, 24.75. Found: C, 53.09; H, 8.01; N, 24.78. IR (cm−1, KBr): 3407br, 3183w, 3112w, 1507s, 1464w, 1281w, 1230s, 1083vs, 826s, 732w, 663w, 629w.
Synthesis of [Cd(PDA)(bib)]·CH3OH·2.5H2O (1). The mixture of Cd(CH3COO)2·2H2O (0.133 g, 0.5 mmol), H2PDA (0.084 g, 0.5 mmol), bib (0.095 g, 0.5 mmol), methanol (5.0 mL) and H2O (10.0 mL) was placed in a 25 mL Parr Teflon- lined stainless steel vessel and the pH was adjusted to 6.0 by addition of triethylamine. Then the vessel was sealed and heated at 130 °C for 72 h. After the mixture was slowly cooled to room temperature, colorless crystals of 1 were obtained with a yield of 60%. Elemental analysis (%) calcd for C36H52Cd2N10O15: C, 39.64; H, 4.77; N, 12.85. Found: C, 39.72; H, 4.80; N, 12.80. IR (cm−1, KBr): 3425br, 3127w, 2951w, 1638s, 1580s, 1375vs, 1101vs, 826m, 710w, 616w, 525w.
Structure Determination and Refinement. Crystallo- graphic data of 1 and bib were collected at room temperature with a Bruker SMART APEX CCD diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.071073 nm). Semi-empirical absorption corrections were applied using the SADABS program.21 The structures were solved by direct methods and refined on F2 by full-matrix least squares using SHELXTL.22 All non-hydrogen atoms were treated anisotropically. Hydrogen atoms were added in geometrical positions. Crystallographic data for the structure reported here has been deposited with CCDC (Deposition No. CCDC- 907114 for 1 and 935090 for bib). These data can be obtained free of charge via http://www.ccdc.cam.ac.
uk/conts/retrieving.html or from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, E-mail: [email protected].
Supporting Information. Crystallographic data and structure refinement parameters for 1 and bib. Selected bond lengths and angles. And the publication cost of this paper was supported by the Korean Chemical Society.
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