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Journal of Chemistry
Volume 2013, Article ID 980243, 7 pages
http://dx.doi.org/10.1155/2013/980243
Research Article

Synthesis, Crystal Structure, and Luminescence Properties of a New Calcium(II) Coordination Polymer Based on L-Malic Acid

1Department of Chemistry, Chung Yuan Christian University, 200 Chung Pei Road, Chung Li 32023, Taiwan
2Master Program in Nanotechnology, Chung Yuan Christian University, 200 Chung Pei Road, Chung Li 32023, Taiwan

Received 22 February 2013; Accepted 15 April 2013

Academic Editor: Jhy-Der Chen

Copyright © 2013 Duraisamy Senthil Raja et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A new calcium coordination polymer [Ca(HL-MA)]n (H3L-MA = L-malic acid) has been solvothermally synthesized. The structure of the newly synthesized complex has been determined by single-crystal X-ray diffraction analysis and further characterized by elemental analysis, reflectance UV-Vis & IR spectra, powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA). The single crystal structure analysis showed that the complex forms three-dimensional framework. The new Ca(II) complex has displayed very high thermal stability which was inferred from TGA and PXRD results. As far as the optical property of the new complex is concerned, the complex emitted its own characteristic sensitized luminescence.

1. Introduction

Multidimensional coordination polymers (CPs) or metal-organic frameworks (MOFs) which are derived from metal ions and organic ligands are emerged in the past two decades as an important family of porous materials with intriguing new structural topologies and potential application as functional materials [120]. In this regard, studies on the application aspects of MOFs such as ion exchange [1, 2], separation [3, 4], gas storage [5], catalysis [69], sensor [10, 11], magnetism [12, 13], photoluminescence [14, 15], drug delivery [16, 17], and proton conduction [1820] are slowly developing. So, the synthesis and characterization of infinite one-, two-, and three-dimensional (1D, 2D, and 3D) networks has been an area of rapid growth. Apart from numerous application-oriented studies of CPs, considerable attention has also been devoted to their structures that were shown to change broadly from simple discrete species to complicated supramolecular networks depending on the nature of metal ion, ligand substituents, stoichiometry, and with the presence of additional components [21, 22].

On the other hand, the use of carboxylates can generate a large number of coordination polymeric complexes composed of chains, sheets, and 3D networks with an enormous variety of intriguing structural topologies [2330]. Such systems have received considerable attention due to their fascinating properties as well as their potential applications in many fields. However, the hydroxyl polycarboxylates (HPCs) such as malate and tartrate have less been studied as building blocks in the construction of MOFs though they can display versatile coordination modes [29, 30]. In contrast to the aliphatic dicarboxylate compounds, the presence of the hydroxyl group in HPCs mostly allows the formation of five- and six-member rings which can stabilize the solid networks [31]. In particular, the naturally existing chiral L-malic acid (H3L-MA) ligand, besides two terminal carboxyl groups, contains a hydroxyl group which can potentially provide an additional coordination site. In most of the malate-bridged CPs, the oxygen atoms of the alkoxy or hydroxyl groups participate in coordination along with the α-carboxyl and/or β-carboxyl groups. In addition, being an important biological ligand, H3L-MA has been found to have versatile coordination behaviour. Structurally characterized metal complexes of H3L-MA include monomers [32], dimers [33], and polymeric chain complexes [34], as well as 3D coordination polymers [35].

Moreover, only a little attention has been drawn on the coordination chemistry of metal complexes with H3L-MA ligand recently. And also, to the best of our knowledge, no attempts were made to synthesize calcium coordination polymers of H3L-MA. With the previous background in mind, herein, we reported the synthesis, characterization, crystal structure, and photoluminescence properties of a new Ca(II) coordination polymer, [Ca(HL-MA)]n.

2. Material and Methods

2.1. General

All chemicals were obtained from commercial sources and were used without further purification. Elemental analyses are performed on a PE-2400 CHN Elementar analyzer instrument. Reflectance UV-Vis spectra were recorded by Jasco V-630 spectrophotometer. Emission spectra were measured by Jasco F-4500 spectrofluorometer. Thermogravimetric analyses (TGA) were carried out using a DuPont TA Q50 analyzer on powder samples under flowing nitrogen gas with a heating rate of 10°C/min. Infrared (IR) spectra were recorded in the range of 400–4000 cm−1 on a JASCO FT/IR-4200 spectrophotometer by using KBr disks. Powder X-ray diffraction (PXRD) patterns were recorded on a Panalytical PW3040/60 diffractometer. Mercury (version 2.4) software has been used for the simulation of theoretical peaks using single-crystal X-ray diffraction data for PXRD measurement.

2.2. Synthesis of [Ca(HL-MA)]n

H3L-MA (0.0804 g, 0.6 mmol), Ca(NO3)2·4H2O (0.2361 g, 1.0 mmol), EtOH (5.0 mL), and H2O (3.0 mL) were placed in a in 23 mL Teflon-lined digestion bombs and stirred for 20 min at room temperature. The reaction mixture turned into homogenous solution with the pH value of 2.32. Then, the solvothermal reaction was carried out by heating the reaction mixture to 180°C for 2 days under autogenous pressure followed by slow cooling at the rate of 6°C h−1 to room temperature. The colorless single crystals were collected in 79.38% (0.082 g) yield on the basis of H3L-MA. Elemental Analysis: Found (calculated) (%) for C4H4CaO5: C, 27.78 (27.91); H, 2.30 (2.34). Reflectance UV-Vis: (nm): 212, 299, 409. IR: (cm−1): 3604 (w), 3456 (w), 3212 (w), 2982 (w), 2907 (w), 2820 (w), 2704 (w), 1564 (s), 1438 (s), 1398 (s), 1340 (w), 1307 (w), 1201 (m), 1107 (m), 1046 (m), 964 (w), 934 (w), 897 (w), 809 (w), 681 (s), 620 (m), 531 (m), 484 (m), 406 (w).

2.3. Crystal Structure Determination

Single-crystal X-ray diffraction data for the complex was collected on Bruker AXS KAPPA APEX II diffractometer. All data were corrected for Lorentz and polarization effects, and the program SADABS in APEX2 was used for the absorption correction [36, 37]. While the hydrogen atoms bound to carbon were placed in idealized positions, the hydrogen atoms bound to other atoms were located from the difference Fourier map and were set riding on the parent atom with idealized distances. All nonhydrogen atoms were refined with anisotropic thermal parameters. All the structures were refined (weighted least squares refinement on ) to convergence [38]. Relevant data concerning data collection and details of the structure refinements are summarized in Table 1. Selected Ca–O bond lengths are listed in Table 2.

tab1
Table 1: Crystallographic data for the complex.
tab2
Table 2: Selected bond lengths (Å) for the complex.

3. Results and Discussion

3.1. Synthesis and Characterization

The complex has been synthesized as described in the experimental section using solvothermal reaction condition. A particular (3 : 5) H3L-MA and metal salt molar ratio has been used to prepare the complex. The complex is stable in air and insoluble in common organic solvents such as acetone, methanol, ethanol, dichloromethane, acetonitrile, chloroform, DMF, and DMSO. The elemental analysis results of the complex are in accorded with the theoretical values. The diffuse reflectance UV-Vis spectra at room temperature have been recorded for the ligand and the complex (Figure 1). On comparing the ligand spectra with the complex, the observed peaks for the complex are mainly due to the intraligand charge transfer transitions. The IR spectra of the ligand and the complex have been given in Figure 2. The IR peak shifts of the ligand in the complex gave an idea about its coordination behaviour. The exact polymeric structure of the new complex was finally confirmed by single crystal X-ray crystallographic studies.

fig1
Figure 1: UV-Vis absorption spectra for the ligand and the complex.
fig2
Figure 2: FT-IR spectra of H3L-MA and complex.
3.2. Structural Description of [Ca(HL-MA)]n

Single-crystal X-ray diffraction result shows that the structure of the complex possesses a 3D framework and crystallizes in the space group of P212121. The asymmetric unit of the complex consists of one independent Ca2+ ion and one HL-MA ligand unit. The independent Ca2+ ion center is seven-coordinated and exhibits a distorted pentagonal bipyramidal geometry; Ca2+ is coordinated by two α-carboxylate O atoms (O4 and O5) belonging to two HL-MA ligands, four β-carboxylate O atoms (O1, O1, O2, and O2) belonging to three HL-MA ligands, and one alkoxyl O atom (O3) belonging to a similar HL-MA ligand mentioned for one of the two α-carboxylate O atoms (Figure 3(a)). The ligand in the asymmetric unit has acted as dianionic heptadentate and coordinated to five Ca2+ ions (Figure 3(b)). The two of the coordinated β-carboxylate O atoms (O1 and O2) form bridge between two Ca centers. The typical Ca–O bond distances range from 2.2974 (13) to 2.6222 (12) Å (Table 2). The participation of O atom (O3) of hydroxyl group (as neutral donor) in coordination along with the one α-carboxyl group O atom (O4) of the ligand forms five-member chelation ring consisting of Ca1, O4, C4, C3, and O3 atoms in the complex. As shown in Figure 3(c), the CaO7 pentagonal bipyramidal motifs which are connected by coordinated β-carboxylate O atoms (O1 and O2) display a 1D zigzag chain in the complex. These 1D chains which are linked together by HL-MA ligands generate the 3D structure (Figure 3(d)).

fig3
Figure 3: (a) The coordination spheres of calcium atom in the complex. (b) The coordination mode of the ligand in the complex. (c) The edge-sharing 2D layer in the complex. (d) The 3D network viewed along the a-axis (H atoms were omitted for clarity).
3.3. Powder X-Ray Diffraction Analysis of the Complex

The purity and homogeneity of the bulk products of the complex has been determined by the comparison of simulated and experimental X-ray powder diffraction patterns. The peak positions of the experimental patterns for the complex nearly matched with the simulated one generated from single-crystal X-ray diffraction data, as depicted in Figure 4. The differences in intensity may be due to the preferred orientation of the powder samples.

fig4
Figure 4: Powder XRD patterns of the complex (measured, (a); calculated, (b)).
3.4. Thermal Stability

The thermal stability of the complex has been studied using thermogravimetry analysis (TGA), and the TGA curves are given in Figure 5. TGA curve of the complex showed that the weight loss is observed only after 410°C which clearly indicated that the complex is very much stable up to 410°C. The thermal stability of the complex has further been studied with aid of PXRD technique. The PXRD pattern of the complex at various temperatures confirmed once again the high thermal stability of the complex framework (Figure 6). That is, the PXRD pattern of the complex revealed that the complex is stable up to 350°C. So, it is a rare example that one coordination polymeric complex has retained its original structure up to very high temperature like 350°C. It is also to be noted that the TGA curve of the complex and the two weight losses (62.5%) observed after 410°C suggested that the coordinated ligand started to decompose step by step upon increasing the temperature to 800°C.

980243.fig.005
Figure 5: The TGA curve of the complex.
980243.fig.006
Figure 6: Powder XRD patterns of the complex at various temperatures.
3.5. Photoluminescence Properties

The solid-state excitation-emission spectra of the ligand and its alkaline earth metal complexes have been studied at room temperature, and their corresponding spectra are shown in Figure 7. The strongest emission peak for the free H3L-MA is at 416 nm with the excitation peak at 337 nm. It is attributed to the transitions [39]. Compared with the free ligand, the strongest emission peak for the complex is at 418 nm, and its excitation spectrum mainly showed strong peak at 342 nm. The emission spectrum of the new complex is much similar to that of the free ligand transitions and it may be ligand-centered electronic transition perturbed by the coordination to metal ions rather than to protons. These observations suggest that the new Ca(II) coordination polymer will be a candidate for potential photoactive material.

fig7
Figure 7: Photoluminescence emission spectra of the ligand and the complex.

4. Conclusion

The present investigation demonstrated the structure and properties of a new calcium coordination polymer based on chiral L-malic acid ligand via solvothermal reaction. It is interesting to note that the complex exhibits 3D framework. The TGA and PXRD results revealed that the complex can retain its original framework structure up to 350°C. It is also to be noted that the complex exhibited promising photoluminescence properties. The observed results are very much comparable with our previous report [40].

Conflict of Interests

The authors have declared that there is no conflict of interests.

Acknowledgments

Financial assistance received from the National Science Council, Taiwan (NSC101-2113-M-033-007-MY3 and NSC101-2811-M-033-018) is gratefully acknowledged. Acknowledgment is also made to Mrs. C.-W. Lu, Instrumentation Center, National Taiwan, University, Taiwan for her help in elemental analyses of the new complexes.

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