Good-quality single crystals of polydistrontiumdimalatepentahydrate, a two-dimensional porous metal-organic framework, were successively grown by gel diffusion technique. Sodium metasilicate was used for gel preparation. Single crystal X-ray diffraction analyses showed that the compound crystallizes in triclinic space group P-1. The functional groups present in the crystals were identified by Fourier transform-infrared analysis. Elemental, powder X-ray diffraction and UV-visible spectral analyses of the compound were also performed. Thermal stability of the grown crystals was analyzed by thermogravimetry. Dielectric analyses of the title compound were conducted at room temperature in the frequency range of 400 Hz–4 MHz.
Metal-organic frameworks (MOFs) are an important class of porous crystalline materials composed of infinite networks of metal centers or inorganic clusters bridged by simple organic linkers through metal-ligand coordination bonds (Cheetham et al., 2006). MOFs are promising materials with potential applications in the fields of catalysis, magnetism, gas storage and adsorption, photoluminescence and so on (Kuppler et al., 2009; Lee et al., 2009). Nowadays, dicarboxylate ligands are extensively used to bridge metal centers leading to MOFs with interesting structure and topologies (Rao et al., 2004).
Maleic acid (cis-butenedioic acid) is an unsaturated organic dibasic acid, and it was selected for the present study because it shows reactions typical of both olefins and carboxylic acids. Malic acid (2-hydroxysuccinic acid) is a common metabolite of plants and animals. It also acts as an intermediate in the tricarboxylic acid Krebs cycle (Berger, 1981; Voet and Voet, 2004). The crystal structural, spectral and thermal properties of strontium malate [Sr(C4H4O5)]·3H2O (Jini et al., 2006) and the crystal structure of [Sr L-(C4H4O5)·3H2O]·H2O (Ghosh and Rao, 2008) have been reported. In the present work, we report the crystal structure of a two-dimensional (2D) metal-organic framework polydistrontiumdimalatepentahydrate [Sr2(C4H4O5)2(H2O)5] (DSDMP). We started the experiment with maleic acid and strontium nitrate, and to our surprise, we got crystals whose structure is similar to that reported by Fleck et al. (2001) obtained by the hydrothermal reaction of DL-malic acid and strontium carbonate followed by slow evaporation. To the best of our knowledge, this is a novel crystal growth method for the metal-malate system, and no documented example of maleate ligand of such a phenomenon has been reported. We report the crystal structure of DSDMP by the gel diffusion technique and provide more insight into its crystal structure. The gel diffusion technique is one of the simplest and most cost-effective techniques for obtaining good-quality crystals at room temperature (Dhanya et al., 2011). We also performed Fourier transform infrared-spectral (FT-IR), elemental, UV-visible absorption, thermogravimetric/derivative thermogravimetric (TG/DTG), dielectric and powder X-ray diffraction analyses.
High-quality crystals essential for the single crystal X-ray diffraction analysis appeared at the gel interface after 3 days. The growth procedure was completed within 2 weeks. The crystals were then separated from the gel interface, washed with distilled water and dried. Better results were obtained with a gel density of 1.05 g cm-3 at pH 6.5. Figure 1 shows the photograph of the crystalline DSDMP.
The chemical composition of the synthesized material was determined by elemental analysis. DSDMP contains 18.06% carbon (18.10%) and 3.22% hydrogen (3.40%) (the theoretical percentage is represented by figures in parentheses). The theoretical values were calculated using the formula [Sr2(C4H4O5)2(H2O)5]. The experimental and calculated values of carbon and hydrogen agree well with the composition of the complex.
The FT-IR spectrum is shown in Figure S1 (supplementary data). The spectrum shows peaks characteristic of coordinated malate ligand. The strong band at 1700 cm-1 characteristic of ν(C=O) of the carboxylic acid group is absent from the spectrum of the complex, indicating coordination of the ligand to the metal through the carboxylate group. The absorption for the asymmetric stretching vibration of carboxylate group gives a strong peak at 1573 cm-1, and the corresponding symmetric stretching vibration appears at 1417 cm-1. The peak at 1120 cm-1, which corresponds to ν(OH) vibration of the CH-OH group, is shifted to a lower frequency at 1106 cm-1 in the spectrum of the complex, indicating the involvement of the OH group of the ligand on coordination (Suciu et al., 2006). The broad band at 3380 cm-1 corresponds to ν(OH) of the coordinated water molecule. Two weak peaks at 2958 and 2919 cm-1 correspond to ν(CH) and ν(CH2) vibrations, respectively (Beghidja et al., 2008). Deformation modes of CH2 appear at 1332 and 1292 cm-1. The peaks at 1193 and 935 cm-1 may be assigned to C-C asymmetric and symmetric stretching vibrations, respectively. Rocking and wagging modes of the water molecule appear at 899 and 809 cm-1, respectively (Thomas et al., 2013). Peaks at 674, 557 and 477 cm-1 may be attributed to the bending, wagging and rocking vibrations of the COO- group, respectively.
To test the crystalline quality of the title compound, powder X-ray diffraction analyses were performed. The crystalline nature of the title compound is confirmed from the well-defined peaks displayed in the spectrum. The phase purity of the bulk sample was determined by comparing the powder XRD pattern with the simulated one obtained from single crystal X-ray diffraction data using the Mercury software. As shown in Figure 2, the peak positions of the experimental pattern of the complex match well with the simulated one, confirming that the complex had been obtained as pure crystalline phases.
The single crystal X-ray diffraction analysis indicates that DSDMP is a 2D metal-organic framework. More insight into its crystal structure was provided. The asymmetric unit of [Sr2(C4H4O5)2(H2O)5] consists of two strontium atoms, two malate ligands and five coordinated water molecules. The repeating unit of DSDMP along with atom-numbering scheme is shown in Figure 3.
The coordination environments of the two strontium atoms are different in the complex owing to the presence of coordinated water molecules. The crystallographic data along with details of structure solution refinements are given in Table 1. Some selected bond lengths and bond angles are summarized in Table S1 (supplementary data). Sr1 is nona-coordinated by six oxygen atoms (O1, O2, O3, O4, O9 and O10) from different malate ligands and the remaining three (O1w, O2w and O3w) by coordinated water molecules. Six oxygen atoms (O4, O5, O6, O7, O8 and O10) from four different malate moieties and two water molecules (O4w and O5w) are coordinated to form a slightly distorted square antiprism polyhedron configuration around Sr2 (Figure S2, supplementary data). In Sr1 and Sr2, a five-membered chelate ring is formed by carboxylate oxygen and oxygen atom of the alcoholic group, which further enhances the thermodynamic stability of the complex. The Sr1-O and Sr2-O bond distances are in the range of 2.596(2)–2.858(3) Å and 2.468(2)–2.723(3) Å, respectively. Sr1 and Sr2 polyhedra are edge shared through O4 and O10, which connects them to a dinuclear segment with Sr1-Sr2 non-bonding distance of 4.3944 Å.
|Empirical formula||C8 H18 O15 Sr2|
|Formula weight (M)||529.46|
|Temperature (T) (K)||293|
|Wavelength (Mo Kα) (Å)||0.71073|
|Unit cell dimensions||a (Å)=6.3885(2), α=81.127(2)°|
|b (Å)=10.1288(3), β=78.601(1)°|
|c (Å)=13.5147(4), γ=79.646(2)°|
|Volume V (Å3)||836.88(4)|
|Z, Dcalc (ρ) (Mg/m3)||2, 2.101|
|Absorption coefficietnt, μ (mm-1)||6.450|
|θ Range for data collection (°)||1.55–27.49|
|Independent reflections (Rint)||0.0326|
|Limiting indices||-8≤ h≤8, -13≤k≤10, -17≤l≤ 15|
|Completeness to θ||97.4%|
|Absorption correction||Semi-empirical from equivalents|
|Max. and min. transmission||0.3586 and 0.2478|
|Refinement method||Full-matrix least squares on F2|
|Goodness-of-fit on F2||1.047|
|Final R indices [(I>2σ(I)]||R1=0.0328, wR2=0.0792|
|R indices (all data)||R1=0.0472, wR2=0.0861|
|Largest difference peak and hole (e Å-3)||0.584 and -0.946|
In this structure, the malate molecule coordinates to the metal atom as a tridentate ligand bonding through the alcoholic and carboxylic oxygen atoms on one side of the molecule and to coordinate bidentately through the carboxyl oxygen atoms on the other end of the molecule. Apart from coordinating bidentately to the metal atom, O2 of the carboxyl group coordinates to the neighboring metal atom of another dimeric unit, which extends them into a polymeric structure (Figure 4) similar to the case of O7 oxygen of the carboxyl group. Moreover, the carboxylate groups of the malate ligand exhibit two kinds of coordination mode in the complex (i) μ2-η1, η1, that is, each oxygen atom of the carboxyl group connects one metal atom and (ii) μ2-η1, η2, that is, one oxygen atom of the carboxyl group connects one metal atom while the other one connects two metal atoms. In the packing diagram, Sr1 and Sr2 polyhedra are interconnected through malate ligand, which assembles them to a complicated 2D structure. The packing diagram of DSDMP viewed along the ‘b’ axis is shown in Figure 4. The Sr1 and Sr2 polyhedra are connected by O4 and O10 to form a one-dimensional chain. This chain is further connected by malate ligand to form the 2D polymeric structure. The packing diagram shows that DSDMP has a porous architecture. These pores can be clearly seen in Figure 5. The absence of water molecules indicates the hydrophobic nature of these pores. Porous metal-organic frameworks have potential applications in gas storage and adsorption (Duren et al., 2004; Li et al., 2009; Ma, 2009).
There are extensive networks of hydrogen-bonding interactions in the crystal structure of DSDMP. The hydroxide and water moieties are responsible for inter- and intramolecular hydrogen bonding. The hydrogen bonds are listed in Table S2 (supplementary data). Of these, three are intramolecular hydrogen bonds: O1w-H11···O1 (symmetry code -x+1,-y+1,-z+2), O3w-H31···O1 (symmetry code x+1,y,z) and O4w–H42···06 (symmetry code x+1,y+1,z-1). The alcoholic group is hydrogen bonded to coordinated water moieties of the adjacent molecule (O3-H3···O5w, O8-H8···O2w). Another type of intramolecular hydrogen bonding exists between coordinated water molecules, (O2w-H22···O1w, O5w-H52···O4w). The presence of all these hydrogen bonds could be an important factor for the overall stability of the compound DSDMP. Hydrogen-bonding interactions are shown in Figure 6.
The UV-visible absorption analysis was performed to determine the absorption range of the grown crystals, the spectrum of which is shown in Figure 7. The grown crystals are highly transparent in the entire visible region (400–800 nm) as expected for Sr (II) complexes. The cut-off wavelength is observed at 270 nm. A weak absorption peak at about 204 nm may be attributed to the n-π* transition of the carboxyl group (Guo et al., 2004).
Figure 8 shows the TG/DTG curve of the sample DSDMP. The compound decomposes in three stages. The first stage (80–160°C) corresponds to the loss of two coordinated water molecules. The observed and calculated mass losses are 6.79% and 6.8%, respectively. The second weight loss of 10.58% observed in the temperature range of 170–260°C corresponds to the loss of the remaining water molecules, the theoretical mass loss being 10.95%. The third weight loss in the temperature range of 290–580°C corresponds to the decomposition of malate ligand to obtain finally a mixture of SrCO3 and a residue of carbon (observed 27.24%, theoretical 27.35%).
Dielectric properties are correlated with the electro-optic property of the crystals, particularly when they are non-conducting materials (Boomadevi and Dhanasekaran, 2004). The dielectric constant of the sample is calculated from the capacitance and dissipation factor measurement results using an H10K1 3532 LCR HITESTER in the frequency range of 400 Hz–4 MHz. The powdered sample was pressed into pellets 11 mm in diameter and 2 mm in thickness by applying pressure. The opposite faces of these pellets placed between two copper electrodes were then uniformly coated with high-grade air-drying silver paste to form a parallel plate capacitor. The capacitance of the sample was then measured by varying the frequency at room temperature. The measured capacitance can then be used to calculate dielectric constant εr using the equation
where C is the capacitance, t is the thickness of the sample, ε0 is the permittivity of free space and A is the area of cross-section. The variation of dielectric constant and dielectric loss vs. log frequency is shown in Figures 9 and S3 (supplementary data), respectively. From the plot, it is clear that the dielectric constant has a higher value in the low frequency region, which decreases with increasing frequency and becomes constant at higher frequency. This is because all the four polarizations, namely, space charge, electronic, dipolar and ionic polarization responsible for the dielectric constant of the materials might be active at low frequency. The lower value of dielectric constant at high frequency is due to the fact that beyond a certain frequency of the electric field, the dipole does not follow the alternating current (Hill et al., 1969; Batra et al., 2005). The low value of dielectric loss at high frequency implies that the crystals possess enhanced optical quality with lesser defects (Balarew and Duhlew, 1984).
Good-quality single crystals of [Sr2(C4H4O5)2(H2O)5] were successfully grown by the gel diffusion technique at room temperature and characterized by single crystal X-ray diffraction analysis. The single crystal X-ray diffraction analysis revealed that the compound is a porous metal-organic framework that crystallizes in triclinic space group P-1 with two crystallographically unique Sr (II) centers. The elemental analysis established the composition of the grown crystal. The FT-IR data suggest that the ligand is coordinated to the metal through the carboxylate and OH groups. The powder X-ray diffraction analyses revealed that the grown crystals are of good crystallinity. The optical absorption analysis indicates minimum absorption in the entire visible region. The thermal studies showed that the sample decomposes in three stages, and a mixture of SrCO3 and a residue of C were obtained as the final decomposition products. The dielectric analyses revealed that the crystals possess low dielectric constant and dielectric loss at high frequency and its enhanced optical quality with lesser defects.
Silica gel made up of sodium metasilicate (CDH) was used for preparing single crystals because of its easy availability and better performance. Sodium metasilicate solution with a density of 1.04–1.05 g cm-3 was prepared by dissolving sodium metasilicate in doubly distilled water. The pH ranging from 5 to 7 was adjusted by adding maleic acid (CDH) (1 m) with continuous stirring. The solution was then transferred to several boiling tubes 2.5 cm in diameter and 20 cm in length. After the gel was set, strontium nitrate solution (CDH) (1 m) was added carefully along the sides of the boiling tube as the upper reactant without disturbing the gel. The boiling tubes were then covered with sheets of plastic to prevent contamination and evaporation of the solution.
The elemental analysis of the grown crystals was performed using Elementer Vario-EL 111 CHNS analyzer (Elementar Analysensysteme GmbH, Hanau, Germany). The FT-IR spectrum was recorded using potassium bromide pellets on a Thermo Nicolet, Avatar 370 spectrometer (Thermo Electron Corporation, Madison, WI, USA) in the range of 4000–400 cm-1. The powder X-ray diffraction analyses were conducted using a Bruker AXS D8 advance XRD (Bruker AXS GmbH, Karlsruhe, Germany) with Cu Kα radiation (λ=1.54056 Å). The single crystal X-ray diffraction analysis of the grown crystal DSDMP was recorded using a Bruker AXS Kappa Apex2 CCD diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) at room temperature with graphite monochromated Mo Kα (λ=0.71073 Å) radiation. The unit cell dimensions and intensity data were recorded at 293 K. The program SAINT/XPREP was used for data reduction and APEX2/SAINT for cell refinement (Bruker, 2004). The structure was solved by direct methods and was refined by full-matrix least squares on F2 using SHELXS-97 and SHELXL-97 computer programs (Sheldrick, 1997a,b). All non-hydrogen atoms were refined with anisotropic thermal parameters. Carbon-bound H-atoms were placed in calculated positions and included in the refinement in the riding model approximation. The hydroxy and water H-atoms were located in a difference Fourier map, and refined isotropically. DIAMOND software version 3.1f was employed for structure plotting (Brandenburg, 2008). The optical absorption (Agilent Technologies, Santa Clara, CA) spectrum was recorded using Varian Cary 5000 UV-vis-NIR spectrometer in the range of 200–1200 nm. The thermal analysis of the crystals was carried out using a Perkin Elmer Diamond TG/DTG analyzer (PerkinElmer Inc., Wellesley, USA) instrument with a heating rate of 10°C/min in nitrogen atmosphere. The dielectric analyses were conducted at room temperature using H10K1 3532 LCR HITESTER (HIOKI E.E. Corporation, Nagano, Japan) in the frequency range of 400 Hz–4 MHz.
CCDC 935149 contains the supplementary crystallographic data for the compound [Sr2(C4H4O5)2(H2O)5]. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2, 1EZ, UK; Fax: (+44) 1223-336-033, or e-mail: email@example.com.
The authors are grateful to the authorities of Sophisticated Analytical Instrumental Facilities (SAIF), Cochin University of Science and Technology, Kochi and NIIST, Thiruvananthapuram for providing the instrumental facilities. We are also thankful to Dr. Shibu M. Eapen, SAIF, CUSAT, Kochi, India for single crystal X-ray diffraction measurements. R.M.N. is grateful to the University of Kerala for the award of a fellowship.
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