Accepted Manuscript Growth and characterisation of a new polymorph of barium maleate: A metal organic framework Lekshmi P. Nair, B.R. Bijini, S. Prasanna, S.M. Eapen, C.M.K. Nair, M. Deepa, K. RajendraBabu PII: DOI: Reference:
S1386-1425(14)01264-5 http://dx.doi.org/10.1016/j.saa.2014.08.073 SAA 12594
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
18 May 2014 6 August 2014 24 August 2014
Please cite this article as: L.P. Nair, B.R. Bijini, S. Prasanna, S.M. Eapen, C.M.K. Nair, M. Deepa, K. RajendraBabu, Growth and characterisation of a new polymorph of barium maleate: A metal organic framework, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.08.073
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
GROWTH AND CHARACTERISATION OF A NEW POLYMORPH OF BARIUM MALEATE: A METAL ORGANIC FRAMEWORK Lekshmi P.Nair1, B.R.Bijini4, S.Prasanna4, S.M.Eapen5, C.M.K.Nair1, M.Deepa3, K.RajendraBabu1*, 2. 1
Department of Physics, M.G. College, Thiruvananthapuram, India-695004
2
Heera College of Engineering & Technology, Thiruvananthapuram, India -695568
3
Department of Physics, All Saints’ College, Thiruvananthapuram, India-695037
4
Department of Physics, H.H.P.B.N.S.S. College, Thiruvananthapuram, India-695014
5
STIC, Cochin University of Science & Technology, Kochi, India-682022
*
Corresponding author Tel. No: +91 9447963076
E-mail address: [email protected] (K. RajendraBabu)
Abstract: A new polymorph of barium maleate (BM) with chemical formula C24 H14 O24 Ba5. 7H2O is grown by modified gel method. Transparent plate like crystals of dimensions 9 × 4 × 1 mm3 were obtained. Single crystal X-Ray Diffraction analysis was done to determine the structure and the crystal belongs to triclinic system, P-1 space group with cell dimensions a = 7.2929 (3) Å, b = 10.5454 (4) Å, c = 14.2837 (6) Å, α =102.0350(10)◦, β = 99.1580(10)◦, γ = 102.9170(10)◦. Hydrogen bonding stabilises the two dimensional polymeric crystal structure. Fourier Transform Infrared spectroscopic method was utilised for the analysis of various functional groups present in the complex. Elemental analysis confirmed the stoichiometry of the complex. Thermal properties of the crystal were studied by TGA/DTA. The material melts at 368◦C.The optical transparency of the crystal was studied using UV-Visible absorption spectra. The optical band gap is found to be 3.35eV. Key words: Metal Organic Frame work, barium maleate, gel method, neutral gel.
1. Introduction: Metal co-ordination complexes of dicarboxylic acids are of extensive research interests due to their wide range of potential applications in polymer industry, drug design, optoelectronic industry, gas storage etc. The co-ordination complexes of various transition and rare earth metals with interesting properties are reported [1-3]. Maleic acid (cis-butenedioic acid) is a dicarboxylic acid which is widely used in the production of synthetic resins and as an intermediate in the production of other chemicals. Chemistry of maleic acid and the availability of monomers in a cheaper rate help to synthesize maleic acid polymer easily. Biocompatibility, water solubility and generally well-defined structure of maleic copolymers suit them to be used in a variety of medical and pharmaceutical applications [4]. It is one of the biologically important dicarboxylic acids and its interaction with different metal ions opens new potentialities with targeted properties. Barium, an alkaline earth metallic element, is used in fireworks, glass making etc. Barium meal is medicinally used as X-ray contrast media. The electronic industries use barium based getters to remove traces of gases from cathode ray tubes, vacuum tubes and heat pipe solar collectors [5]. Barium salt of maleic acid is used in the catalysis process of hydrocarbon steam reforming [6]. Herein we report a new polymorph of barium maleate grown by modified gel method. The BM crystals grown by the conventional gel method provides the same structure as that of the reported one, while the introduction of neutral gel layer in our experiment provides a new polymorph of barium maleate belonging to the triclinic system [7].Single crystal XRD analysis, Fourier Transform (FT) infrared technique, UV-
Visible spectral studies, thermo-gravimetric analysis and differential thermal analysis were used for the crystal characterisation. 2. Experimental procedure: 2.1 Crystallisation method: The crystallisation of the barium complex of maleic acid was accomplished using modified gel diffusion technique. Good quality single crystals were obtained by controlled nucleation and convection less growth offered by gel technique. Crystals were grown in single glass tube of length 20 cm and diameter 2.5 cm. Silica gel of specific gravity 1.03 to 1.06 g/cc was prepared by dissolving sodium meta silicate (SMS) in double distilled water. Maleic acid of particular molarity (0.5 M – 1.5 M) was added drop by drop to the continuously stirred SMS. The gel was then acidified with 1M glacial acetic acid to get pH in the range 3 to 7. About 30 ml of above solution was taken in each test tube and kept undisturbed for setting. Over the set gel, a neutral gel layer (pH-7) was introduced without disturbing it. Aqueous solution of barium chloride (0.5 M –1.5 M) was added as top reagent over the set neutral gel without damaging the gel system. The open end of the test tubes was covered with transparent plastic sheets to avoid contamination of the solution. The experimental set up was kept undisturbed for crystallisation at ambient temperature. 2.2 Characterisation: The single crystal XRD analysis of the crystal was carried out using Bruker AXS Kappa Apex2 CCD diffractometer. FT-IR spectrum was recorded using KBr pellets on a Thermo Nicolet, Avatar 370 spectrometer with resolution of 0.9 cm-1, in the range 4000400 cm-1. Absorption spectrum of the crystals was studied using Varian Cary 5000 UV-
Vis-NIR spectrometer in the range 200-1200 nm. TGA/DTA experiments were carried out in SDT Q600 V8.3 Build 101 instrument with a heating rate of 10 ◦C/ min in nitrogen atmosphere. The carbon and hydrogen contents in the sample were determined using Elementor Vario-EL III CHNS Analyser. 3. Results and discussion: 3.1 Crystal growth: Crystals of BM were grown as per the crystal growth technique described in section 2.1. The experiment was first started with the conventional gel method with maleic acid impregnated in the gel with barium chloride as top solution, which provided crystals in the form of whiskers. To get perfect single crystals for structural elucidation, a neutral gel layer was added above the set gel. Top solution was gently poured over the set neutral gel layer. Tiny crystals were first sighted at the neutral gel region on the fourth week. Rectangular shaped single crystals were obtained in the neutral gel layer in the pH range 5-5.5.The addition of intermediate neutral gel slowed down the diffusion of barium chloride, reduced the nucleation sites, eliminated the formation of whisker like structures and produced transparent plate like single crystals. Growth process took about 3 months to complete. Good quality single crystals suitable for single crystal XRD studies were obtained in gel medium of pH 5.5, density 1.04g/cc, 1M maleic acid and 1M barium chloride. Crystals of size 9 × 4 × 1 mm3 were obtained. The characteristic shape of the crystal is shown in Fig.1. Fig1. Photograph of as grown crystals of BM. 3.2 Crystal structure:
The single crystal XRD data of a well formed BM crystal were collected using Bruker AXS Kappa Apex2 CCD diffractometer with graphite monochromated Mo Kα (λ= 0.71073Å) radiation. Data reduction was done using SAINT/XPREP program [8]. The program SIR92 was used for solving the crystal structure and the refinement was carried out by full-matrix least squares on F2 using SHELXL-97 [9, 10]. Anisotropic thermal parameters were applied to refine all the non-hydrogen atoms. The hydrogen atoms were located from the difference fourier maps and refined isotropically. The IUCR software Mercury (Version 3) was used to construct molecular graphics. Table1 provides the crystallographic data and processing parameters. Fractional atomic coordinates are given in table2. Table1. Crystal data and structure refinement for BM Table2. Atomic coordinates (x 104) and equivalent isotropic displacement parameters The whisker shaped crystals formed in this work by the conventional gel method have the same unit cell parameters as that of the reported structure (a= 9.3721Å, b= 20.5880 Å, c= 14.0744 Å and α =γ = 90◦, P 21/c) [7]. Here we are reporting a new polymorph of barium maleate crystallised using the modified gel method. The unit cell parameters and the crystal system of BM are entirely different from that of the reported one. Figure 2 denotes the co-ordination environment of BM with atom numbering scheme. Barium in this structure exhibits 3 different co-ordination environments. Barium atoms are surrounded by oxygen atoms of maleic acid units. Ba1, Ba2, Ba3 have the coordination numbers 10, 12, 8 respectively while Ba in the reported structure is seven coordinated. The Ba-O bond length ranges from 2.672(3) Å to 2.989(2) Å. Selected bond lengths and bond angles are given in table3 and table4 respectively. Taking the asymmetric unit, one of the carboxyl groups of maleate anion is chelated to Ba1through both oxygen atoms, O1 and O2, while the other end remains free. But in the polymorph
of BM, all the maleic acid units are deprotonated. The co-ordinated C1-O1 and C1-O2 bond distances are 1.263Å and 1.252Å while the free C4-O3 and C4-O4 bond lengths are1.233Å and1.299Å respectively. The longer C4-O4 bond length may be due to the presence of an intramolecular hydrogen bonding and is comparable with the corresponding C-O distance in the betaine complex of maleic acid where there exists an intramolecular hydrogen bonding between the oxygen atoms of the maleate anion. The intramolecular O4-O1 distance, 2.44Å is within the range of potentially symmetric hydrogen bonds as reported by Ilczyszyn et al., [11]. But the symmetry of the proton position in this intramolecular hydrogen bond could not be confirmed as the hydrogen atoms in BM are not accurately fixed although the hydrogen bonding in the structure of betaine complex of maleic acid is shown symmetric. The second maleate anion unit acts as monodendate ligand through oxygen atom O5 of one of the carboxyl group to Ba1 and the oxygen atoms, O7 & O8 of other carboxyl group is bidendately co-ordinated to Ba1, Ba2 and Ba2, Ba3 respectively. The third maleate anion unit acts as monodentate ligand through oxygen atom, O12 of one of the carboxyl group to Ba3 and O9 & O10 of the other carboxyl group is monodentately and tridentately attatched to Ba2 & Ba1,Ba2, Ba3 respectively. The bond Ba2-O10 with a bond length of 2.989 Å corresponding to the tridentate chelating- bridging oxygen is greater than that of monodentate and bidentate bridging Ba-O distances. This may be due to the fact that bond lengths involving same atoms increases with the increase in co-ordination number [12]. The bond angle O10Ba2-O9 with a value of 43.87◦ corresponding to the tridentate chelating bridging oxygen atoms is less than those of others due to the increase in co-ordination number [13].The OC-O angle of the carboxyl groups of maleate anions ranges from 120.09◦ to 123.15◦. As the Ba-Ba distance is greater than the sum of the vander Waals radii (4.36Å), there is no metal – metal interaction. In the reported structure, no water molecules are co-ordinated
to the asymmetric unit. In the present structure, a water molecule is co-ordinated to Ba3 with a bond Ba3-O13 of length 2.704Å. This co-ordinated water molecule forms intermolecular hydrogen bonds with the neighbouring carboxylic oxygen and the lattice water oxygen, as given in table5, thereby forming a 2D polymeric structure. This water mediated bridge between molecules actually stabilises this structure through hydrogen bonds. Thus the crystal structure is stabilised by intermolecular hydrogen bonds formed between the repeating units. The 2D porous polymeric structure of BM can be used for gas adsorption and storage [14]. The hydrogen bonding pattern through the water bridge is shown in the packing diagram of BM viewed along the b-axis as in Fig. 3. Perspective view of BM along a axis is shown in Fig. 4. Table3.Selected bond lengths in BM Table4.Selected bond angles in BM Table5. Hydrogen bonds for BM [A◦ and deg.] Fig. 2: Asymmetric unit of BM Fig. 3: View of packing along b axis in BM Fig. 4: Perspective view of packing along a axis in BM 3.3. FT-IR analysis: The FTIR spectrum of BM is shown in Fig. 5.The effect of the co-ordination of metals with the organic linkers can be observed as the changes in the molecular vibrations. The absorption bands of water of crystallisation in many carboxylic acids appear around 3400 cm-1[15]. The band observed at 3393cm-1 is assigned to the –OH vibration of the water of ligation. A strong absorption band at 1707cm-1 in the maleic
acid spectrum corresponding to the carboxylic acid stretching gets diminished to a weak band at 1705cm-1 in the BM spectra. This may be due to the presence of a –COOH group which is not co-ordinated with the metal atoms as evidenced from SXRD. Asymmetric stretch of carboxylate group in BM occurs at1628cm-1, 1538cm-1and symmetric stretch at 1416cm-1, 1360cm-1. The ∆ν of 268cm-1, 178cm-1, 212cm-1 may be due to unidendate, bidendate bridging and tridendate chelating bridging modes respectively of the carboxylate oxygen atoms in BM [16,17].Thus the different co-ordination modes in the crystal structure are reflected by the difference in the asymmetric and symmetric stretching frequencies of the –COO- groups. The band at 1263cm-1 attributed to the C-O stretching in maleic acid ligand is shifted to 1213 cm-1in the BM spectra. This frequency lowering is due to the co-ordination of carboxyl group with the barium atom [18]. The band at 976cm-1may be due to the O-H out of plane deformation. The band at 864 cm-1is assigned to the C-H out of plane deformation [19].The band at 641 cm-1corresponds to the carboxyl in-plane deformation mode of the carboxyl group involved in intramolecular hydrogen bonding which corroborates the existence of an intramolecular hydrogen bond described in the single crystal XRD structure [20]. Ba-O stretching is identified by the band at 548.97 cm-1. Fig. 5: FT-IR spectrum of BM 3.4. Elemental analysis: The elemental composition of BM crystals was determined both theoretically and experimentally. Both the values are in agreement with each other and the molecular formula is derived as C24 H14 O24 Ba5. 7H2O. Experimental: C- 19.19%, H- 1.45%; Calculated: C- 19.21%, H- 1.8%. 3.5. Thermal studies:
TGA and DTA studies of BM were carried out with a sample of initial mass 7.648mg using calcined alumina as reference material. The sample was heated at a rate of 10◦C/min in alumina cups. The results of TGA and DTA studies are given in Fig.6. The first stage of decomposition starts with the weight loss of 6% (obs-5.8%) which corresponds to the removal of five lattice water molecules. The endothermic peak found at 62◦C in the DTA curve maybe due to this dehydration. During the next stage of decomposition (170◦C-180◦C), two co-ordinated water molecules are liberated with a weight loss of 2.4% (obs- 2.52%). Thus the endothermic peak at 178◦ C corresponds to this loss. In the temperature range, 350◦ C-510◦C, one CO and seven C2 H2 molecules are evolved. The calculated mass loss corresponding to this proposal is 14.13% (obs14.75%). Then the complex decomposes by the emission of 4 CO2 to form BaCO3, as the final residue, giving an exothermic peak at 830◦C in the DTA curve with a mass loss of 65% (obs-63.8%). The kinetic parameters like activation energy (E), order of reaction (N), frequency factor (A) of decomposition were determined using the Coats and Redfern method [21]. The thermodynamic parameters such as standard enthalpy and standard Gibbs’s free energy were also calculated and tabulated in table6 [22]. Fig. 6: TGA/DTA curves of BM Table6. Kinetic and thermodynamic parameters of various stages of decomposition. 3.6. UV- Vis spectral studies: UV- Visible absorbance spectrum of BM crystals was carried out between 200 nm and 1200nm. Fig. 7 shows the absorbance spectrum of the complex. The crystal is found to be transparent in the entire visible region which enables it to be a good candidate for optoelectronic application [23]. A graph is drawn between photon energy
(hν) versus (αhν) (1/2), where α is the absorption coefficient and the electronic band gap is estimated as 3.35eV and is given in Fig.8. The Plasma energy and Fermi energy are calculated as 20.6eV and 16.4eV respectively. Fig. 7: Absorbance spectrum of BM Fig. 8: Plot of alpha energy versus photon energy 4. Conclusions: Single crystals of a new polymorph of barium maleate have been successfully grown by the gel diffusion method with the inclusion of intermediate neutral gel layer. Good quality single crystals belonging to the triclinic system with space group P-1are grown from the gel medium of pH 5.5and density 1.04g/cc. A 2D polymeric porous structure is revealed by the single crystal XRD data. Various functional groups present in BM are confirmed by the FTIR spectral analysis. Thermal decomposition behaviour of the complex in nitrogen atmosphere is provided by the TGA/DTA studies. Wide transparency of BM in the entire visible range makes it a suitable candidate for optoelectronic application. The porous structure of the complex makes it a potential metal organic frame work for gas adsorption and storage. SUPPLEMENTARY DETAILS CCDC No. 982996 contains the supplementary crystallographic data for the compound C24 H14 O24 Ba5. 7H2O. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 IEZ, UK; fax: (+44)1223-336-033; or e-mail: data request @ccdc.cam.ac.uk. ACKNOWLEDGEMENT
The authors are thankful to the STIC, Cochin University of Science and Technology and SCTIMST, Thiruvananthapuram for analytical facilities. We express our sincere gratitude to Dr. Shibu M. Eapen, STIC, Cochin University of Science and Technology, India for providing Single Crystal XRD data. References: [1]S. K. Arora, V. Patel, R.G. Patel, B. Amin, A. Kothari, J. Phys. Chem. Solids, 65(2004) 965-973. [2]T. Jini, K. V. Saban, G.Varghese, Cryst. Res. Technol., 41 (2006) 250-254. [3]T. Jini, K. V. Saban, G.Varghese, S. Naveen, M. A. Sridhar and J. S. Prasad, J.Alloys Compd.,433 (2007) 211-215. [4]I. Popescu, D. M. Suflet, I. M. Pelin, G. C. Chitanu, Rev Roum Chim, 56(3) (2011) 173-188. [5]A. Giedraitis, S. Tamulevi, R. Gudaitis, M. Andrulevicius, Mater. Sci.16 (1) (2010) ISSN 1392–1320, 12-23. [6]William G. Billings, N0 Drawing. Filed Jan. 2, 1968, Ser. No. 694,845, 7 Int. Cl. C01b 2/16; C10k 3/00; B01j 11/32 US. Cl. 48-214 7 Claims Patented Apr. 14, 1970 1C6 [7]V. Mahalakshmi, A. Lincy, J. Thomas, K.V. Saban, J. Phys. Chem. Solids 73 (2012) 584-588. [8]BRUKER, APEX2, SAINT, XPREP, Bruker AXS Inc., Madison, WI USA, 2004. [9]A. Altornare, G. Cascarano, C. Gracovazzo, A. Guagliardi , J. Appl. Cryst. 26 (1993) 343-350.
[10]G. M. Sheldrick, ActaCrystallogr. Sect. A 64 (2008) 112-122. [11]M. M. Ilczyszyn, T. Lis, H. Ratajczak, J. Mol. Struct. 372(1995) 9-27. [12]J. S. Casas, M. S. G. Tasende, J. Sordo,Coordin. Chem. Rev. 209 (2000) 197-261. [13]S. Wang, T. D. Westmoreland, Inorg. Chem. 19 48(2) (2009) 719-727. [14]B. Moulton, M. Zaworotko, J. Chem. Rev. 101(2001) 1629-1658. [15]J. C. Kim, A. J. Lough, J. Chem.Crystallogr. 35(7) (2005) 535-539. [16]K. N. Lazarou, A. K. Boudalis, S. P. Perleps, A. Terzis, C. P. Raptopoulou, Eur. J. Inorg. Chem. (2009) 4554-4563. [17]G. B. Deacon, R. J. Philips, Coord. Chem. Rev. 33(1980) 227-250. [18]A. U.Czaja, N. Trukhan, U. Muller, Chem. Soc. Rev.38 (2009) 1284-1293. [19]G. AnandhaBabu, P. Ramasamy, J. Cryst. Growth 311(2009)1185-1189. [20]N. E. Albert, Infrared absorption associated with strong hydrogen bonds, California Institute of technology (1961) 204. [21]A.W.Coats, J.P.Redfern, Nature 01 (1964) 68. [22]K.J. Laidler, Chemical Kinetics, Harper and Row, Newyork, 1987. [23]S. Singh, B.Lal, J.Cryst. Growth 312 (2010) 301-304.
Fig. 1: Photograph of grown BM.
Fig. 2: Asymmetric unit of BM
Fig. 3: View of Packing along b axis in BM
Fig. 4: Perspective view of packing diagram of BM along a axis
Fig. 5: FT-IR spectrum of BM
Fig. 6: TGA/DTA curves of BM
Fig. 7: Absorbance spectrum of BM
Fig. 8: Plot of alpha energy versus photon energy
Table1. Crystal data and structure refinement for BM Empirical formula
C24 H28 Ba5 O31
Formula weight
1499.09
Temperature
296(2) K
Wavelength
0.71073 Å
Crystal system
Triclinic
Space group
P-1
Unit cell dimensions
a = 7.2929 (3) Å
α =102.035 (10)◦.
b = 10.5454 (4) Å
β = 99.1580 (10)◦.
c = 14.2837(6) Å
γ = 102.9170 (10)◦.
Volume
1022.49 (7) Å 3
Z
1
Calculated density
2.415 g/cc
Absorption coefficient
4.845 mm-1
F(000)
688
Crystal size
0.35 x 0.30 x 0.25 mm3
Theta range for data collection
1.49 to 28.20 deg.
Limiting indices
-9
S1386-1425(14)01264-5 http://dx.doi.org/10.1016/j.saa.2014.08.073 SAA 12594
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
18 May 2014 6 August 2014 24 August 2014
Please cite this article as: L.P. Nair, B.R. Bijini, S. Prasanna, S.M. Eapen, C.M.K. Nair, M. Deepa, K. RajendraBabu, Growth and characterisation of a new polymorph of barium maleate: A metal organic framework, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.08.073
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
GROWTH AND CHARACTERISATION OF A NEW POLYMORPH OF BARIUM MALEATE: A METAL ORGANIC FRAMEWORK Lekshmi P.Nair1, B.R.Bijini4, S.Prasanna4, S.M.Eapen5, C.M.K.Nair1, M.Deepa3, K.RajendraBabu1*, 2. 1
Department of Physics, M.G. College, Thiruvananthapuram, India-695004
2
Heera College of Engineering & Technology, Thiruvananthapuram, India -695568
3
Department of Physics, All Saints’ College, Thiruvananthapuram, India-695037
4
Department of Physics, H.H.P.B.N.S.S. College, Thiruvananthapuram, India-695014
5
STIC, Cochin University of Science & Technology, Kochi, India-682022
*
Corresponding author Tel. No: +91 9447963076
E-mail address: [email protected] (K. RajendraBabu)
Abstract: A new polymorph of barium maleate (BM) with chemical formula C24 H14 O24 Ba5. 7H2O is grown by modified gel method. Transparent plate like crystals of dimensions 9 × 4 × 1 mm3 were obtained. Single crystal X-Ray Diffraction analysis was done to determine the structure and the crystal belongs to triclinic system, P-1 space group with cell dimensions a = 7.2929 (3) Å, b = 10.5454 (4) Å, c = 14.2837 (6) Å, α =102.0350(10)◦, β = 99.1580(10)◦, γ = 102.9170(10)◦. Hydrogen bonding stabilises the two dimensional polymeric crystal structure. Fourier Transform Infrared spectroscopic method was utilised for the analysis of various functional groups present in the complex. Elemental analysis confirmed the stoichiometry of the complex. Thermal properties of the crystal were studied by TGA/DTA. The material melts at 368◦C.The optical transparency of the crystal was studied using UV-Visible absorption spectra. The optical band gap is found to be 3.35eV. Key words: Metal Organic Frame work, barium maleate, gel method, neutral gel.
1. Introduction: Metal co-ordination complexes of dicarboxylic acids are of extensive research interests due to their wide range of potential applications in polymer industry, drug design, optoelectronic industry, gas storage etc. The co-ordination complexes of various transition and rare earth metals with interesting properties are reported [1-3]. Maleic acid (cis-butenedioic acid) is a dicarboxylic acid which is widely used in the production of synthetic resins and as an intermediate in the production of other chemicals. Chemistry of maleic acid and the availability of monomers in a cheaper rate help to synthesize maleic acid polymer easily. Biocompatibility, water solubility and generally well-defined structure of maleic copolymers suit them to be used in a variety of medical and pharmaceutical applications [4]. It is one of the biologically important dicarboxylic acids and its interaction with different metal ions opens new potentialities with targeted properties. Barium, an alkaline earth metallic element, is used in fireworks, glass making etc. Barium meal is medicinally used as X-ray contrast media. The electronic industries use barium based getters to remove traces of gases from cathode ray tubes, vacuum tubes and heat pipe solar collectors [5]. Barium salt of maleic acid is used in the catalysis process of hydrocarbon steam reforming [6]. Herein we report a new polymorph of barium maleate grown by modified gel method. The BM crystals grown by the conventional gel method provides the same structure as that of the reported one, while the introduction of neutral gel layer in our experiment provides a new polymorph of barium maleate belonging to the triclinic system [7].Single crystal XRD analysis, Fourier Transform (FT) infrared technique, UV-
Visible spectral studies, thermo-gravimetric analysis and differential thermal analysis were used for the crystal characterisation. 2. Experimental procedure: 2.1 Crystallisation method: The crystallisation of the barium complex of maleic acid was accomplished using modified gel diffusion technique. Good quality single crystals were obtained by controlled nucleation and convection less growth offered by gel technique. Crystals were grown in single glass tube of length 20 cm and diameter 2.5 cm. Silica gel of specific gravity 1.03 to 1.06 g/cc was prepared by dissolving sodium meta silicate (SMS) in double distilled water. Maleic acid of particular molarity (0.5 M – 1.5 M) was added drop by drop to the continuously stirred SMS. The gel was then acidified with 1M glacial acetic acid to get pH in the range 3 to 7. About 30 ml of above solution was taken in each test tube and kept undisturbed for setting. Over the set gel, a neutral gel layer (pH-7) was introduced without disturbing it. Aqueous solution of barium chloride (0.5 M –1.5 M) was added as top reagent over the set neutral gel without damaging the gel system. The open end of the test tubes was covered with transparent plastic sheets to avoid contamination of the solution. The experimental set up was kept undisturbed for crystallisation at ambient temperature. 2.2 Characterisation: The single crystal XRD analysis of the crystal was carried out using Bruker AXS Kappa Apex2 CCD diffractometer. FT-IR spectrum was recorded using KBr pellets on a Thermo Nicolet, Avatar 370 spectrometer with resolution of 0.9 cm-1, in the range 4000400 cm-1. Absorption spectrum of the crystals was studied using Varian Cary 5000 UV-
Vis-NIR spectrometer in the range 200-1200 nm. TGA/DTA experiments were carried out in SDT Q600 V8.3 Build 101 instrument with a heating rate of 10 ◦C/ min in nitrogen atmosphere. The carbon and hydrogen contents in the sample were determined using Elementor Vario-EL III CHNS Analyser. 3. Results and discussion: 3.1 Crystal growth: Crystals of BM were grown as per the crystal growth technique described in section 2.1. The experiment was first started with the conventional gel method with maleic acid impregnated in the gel with barium chloride as top solution, which provided crystals in the form of whiskers. To get perfect single crystals for structural elucidation, a neutral gel layer was added above the set gel. Top solution was gently poured over the set neutral gel layer. Tiny crystals were first sighted at the neutral gel region on the fourth week. Rectangular shaped single crystals were obtained in the neutral gel layer in the pH range 5-5.5.The addition of intermediate neutral gel slowed down the diffusion of barium chloride, reduced the nucleation sites, eliminated the formation of whisker like structures and produced transparent plate like single crystals. Growth process took about 3 months to complete. Good quality single crystals suitable for single crystal XRD studies were obtained in gel medium of pH 5.5, density 1.04g/cc, 1M maleic acid and 1M barium chloride. Crystals of size 9 × 4 × 1 mm3 were obtained. The characteristic shape of the crystal is shown in Fig.1. Fig1. Photograph of as grown crystals of BM. 3.2 Crystal structure:
The single crystal XRD data of a well formed BM crystal were collected using Bruker AXS Kappa Apex2 CCD diffractometer with graphite monochromated Mo Kα (λ= 0.71073Å) radiation. Data reduction was done using SAINT/XPREP program [8]. The program SIR92 was used for solving the crystal structure and the refinement was carried out by full-matrix least squares on F2 using SHELXL-97 [9, 10]. Anisotropic thermal parameters were applied to refine all the non-hydrogen atoms. The hydrogen atoms were located from the difference fourier maps and refined isotropically. The IUCR software Mercury (Version 3) was used to construct molecular graphics. Table1 provides the crystallographic data and processing parameters. Fractional atomic coordinates are given in table2. Table1. Crystal data and structure refinement for BM Table2. Atomic coordinates (x 104) and equivalent isotropic displacement parameters The whisker shaped crystals formed in this work by the conventional gel method have the same unit cell parameters as that of the reported structure (a= 9.3721Å, b= 20.5880 Å, c= 14.0744 Å and α =γ = 90◦, P 21/c) [7]. Here we are reporting a new polymorph of barium maleate crystallised using the modified gel method. The unit cell parameters and the crystal system of BM are entirely different from that of the reported one. Figure 2 denotes the co-ordination environment of BM with atom numbering scheme. Barium in this structure exhibits 3 different co-ordination environments. Barium atoms are surrounded by oxygen atoms of maleic acid units. Ba1, Ba2, Ba3 have the coordination numbers 10, 12, 8 respectively while Ba in the reported structure is seven coordinated. The Ba-O bond length ranges from 2.672(3) Å to 2.989(2) Å. Selected bond lengths and bond angles are given in table3 and table4 respectively. Taking the asymmetric unit, one of the carboxyl groups of maleate anion is chelated to Ba1through both oxygen atoms, O1 and O2, while the other end remains free. But in the polymorph
of BM, all the maleic acid units are deprotonated. The co-ordinated C1-O1 and C1-O2 bond distances are 1.263Å and 1.252Å while the free C4-O3 and C4-O4 bond lengths are1.233Å and1.299Å respectively. The longer C4-O4 bond length may be due to the presence of an intramolecular hydrogen bonding and is comparable with the corresponding C-O distance in the betaine complex of maleic acid where there exists an intramolecular hydrogen bonding between the oxygen atoms of the maleate anion. The intramolecular O4-O1 distance, 2.44Å is within the range of potentially symmetric hydrogen bonds as reported by Ilczyszyn et al., [11]. But the symmetry of the proton position in this intramolecular hydrogen bond could not be confirmed as the hydrogen atoms in BM are not accurately fixed although the hydrogen bonding in the structure of betaine complex of maleic acid is shown symmetric. The second maleate anion unit acts as monodendate ligand through oxygen atom O5 of one of the carboxyl group to Ba1 and the oxygen atoms, O7 & O8 of other carboxyl group is bidendately co-ordinated to Ba1, Ba2 and Ba2, Ba3 respectively. The third maleate anion unit acts as monodentate ligand through oxygen atom, O12 of one of the carboxyl group to Ba3 and O9 & O10 of the other carboxyl group is monodentately and tridentately attatched to Ba2 & Ba1,Ba2, Ba3 respectively. The bond Ba2-O10 with a bond length of 2.989 Å corresponding to the tridentate chelating- bridging oxygen is greater than that of monodentate and bidentate bridging Ba-O distances. This may be due to the fact that bond lengths involving same atoms increases with the increase in co-ordination number [12]. The bond angle O10Ba2-O9 with a value of 43.87◦ corresponding to the tridentate chelating bridging oxygen atoms is less than those of others due to the increase in co-ordination number [13].The OC-O angle of the carboxyl groups of maleate anions ranges from 120.09◦ to 123.15◦. As the Ba-Ba distance is greater than the sum of the vander Waals radii (4.36Å), there is no metal – metal interaction. In the reported structure, no water molecules are co-ordinated
to the asymmetric unit. In the present structure, a water molecule is co-ordinated to Ba3 with a bond Ba3-O13 of length 2.704Å. This co-ordinated water molecule forms intermolecular hydrogen bonds with the neighbouring carboxylic oxygen and the lattice water oxygen, as given in table5, thereby forming a 2D polymeric structure. This water mediated bridge between molecules actually stabilises this structure through hydrogen bonds. Thus the crystal structure is stabilised by intermolecular hydrogen bonds formed between the repeating units. The 2D porous polymeric structure of BM can be used for gas adsorption and storage [14]. The hydrogen bonding pattern through the water bridge is shown in the packing diagram of BM viewed along the b-axis as in Fig. 3. Perspective view of BM along a axis is shown in Fig. 4. Table3.Selected bond lengths in BM Table4.Selected bond angles in BM Table5. Hydrogen bonds for BM [A◦ and deg.] Fig. 2: Asymmetric unit of BM Fig. 3: View of packing along b axis in BM Fig. 4: Perspective view of packing along a axis in BM 3.3. FT-IR analysis: The FTIR spectrum of BM is shown in Fig. 5.The effect of the co-ordination of metals with the organic linkers can be observed as the changes in the molecular vibrations. The absorption bands of water of crystallisation in many carboxylic acids appear around 3400 cm-1[15]. The band observed at 3393cm-1 is assigned to the –OH vibration of the water of ligation. A strong absorption band at 1707cm-1 in the maleic
acid spectrum corresponding to the carboxylic acid stretching gets diminished to a weak band at 1705cm-1 in the BM spectra. This may be due to the presence of a –COOH group which is not co-ordinated with the metal atoms as evidenced from SXRD. Asymmetric stretch of carboxylate group in BM occurs at1628cm-1, 1538cm-1and symmetric stretch at 1416cm-1, 1360cm-1. The ∆ν of 268cm-1, 178cm-1, 212cm-1 may be due to unidendate, bidendate bridging and tridendate chelating bridging modes respectively of the carboxylate oxygen atoms in BM [16,17].Thus the different co-ordination modes in the crystal structure are reflected by the difference in the asymmetric and symmetric stretching frequencies of the –COO- groups. The band at 1263cm-1 attributed to the C-O stretching in maleic acid ligand is shifted to 1213 cm-1in the BM spectra. This frequency lowering is due to the co-ordination of carboxyl group with the barium atom [18]. The band at 976cm-1may be due to the O-H out of plane deformation. The band at 864 cm-1is assigned to the C-H out of plane deformation [19].The band at 641 cm-1corresponds to the carboxyl in-plane deformation mode of the carboxyl group involved in intramolecular hydrogen bonding which corroborates the existence of an intramolecular hydrogen bond described in the single crystal XRD structure [20]. Ba-O stretching is identified by the band at 548.97 cm-1. Fig. 5: FT-IR spectrum of BM 3.4. Elemental analysis: The elemental composition of BM crystals was determined both theoretically and experimentally. Both the values are in agreement with each other and the molecular formula is derived as C24 H14 O24 Ba5. 7H2O. Experimental: C- 19.19%, H- 1.45%; Calculated: C- 19.21%, H- 1.8%. 3.5. Thermal studies:
TGA and DTA studies of BM were carried out with a sample of initial mass 7.648mg using calcined alumina as reference material. The sample was heated at a rate of 10◦C/min in alumina cups. The results of TGA and DTA studies are given in Fig.6. The first stage of decomposition starts with the weight loss of 6% (obs-5.8%) which corresponds to the removal of five lattice water molecules. The endothermic peak found at 62◦C in the DTA curve maybe due to this dehydration. During the next stage of decomposition (170◦C-180◦C), two co-ordinated water molecules are liberated with a weight loss of 2.4% (obs- 2.52%). Thus the endothermic peak at 178◦ C corresponds to this loss. In the temperature range, 350◦ C-510◦C, one CO and seven C2 H2 molecules are evolved. The calculated mass loss corresponding to this proposal is 14.13% (obs14.75%). Then the complex decomposes by the emission of 4 CO2 to form BaCO3, as the final residue, giving an exothermic peak at 830◦C in the DTA curve with a mass loss of 65% (obs-63.8%). The kinetic parameters like activation energy (E), order of reaction (N), frequency factor (A) of decomposition were determined using the Coats and Redfern method [21]. The thermodynamic parameters such as standard enthalpy and standard Gibbs’s free energy were also calculated and tabulated in table6 [22]. Fig. 6: TGA/DTA curves of BM Table6. Kinetic and thermodynamic parameters of various stages of decomposition. 3.6. UV- Vis spectral studies: UV- Visible absorbance spectrum of BM crystals was carried out between 200 nm and 1200nm. Fig. 7 shows the absorbance spectrum of the complex. The crystal is found to be transparent in the entire visible region which enables it to be a good candidate for optoelectronic application [23]. A graph is drawn between photon energy
(hν) versus (αhν) (1/2), where α is the absorption coefficient and the electronic band gap is estimated as 3.35eV and is given in Fig.8. The Plasma energy and Fermi energy are calculated as 20.6eV and 16.4eV respectively. Fig. 7: Absorbance spectrum of BM Fig. 8: Plot of alpha energy versus photon energy 4. Conclusions: Single crystals of a new polymorph of barium maleate have been successfully grown by the gel diffusion method with the inclusion of intermediate neutral gel layer. Good quality single crystals belonging to the triclinic system with space group P-1are grown from the gel medium of pH 5.5and density 1.04g/cc. A 2D polymeric porous structure is revealed by the single crystal XRD data. Various functional groups present in BM are confirmed by the FTIR spectral analysis. Thermal decomposition behaviour of the complex in nitrogen atmosphere is provided by the TGA/DTA studies. Wide transparency of BM in the entire visible range makes it a suitable candidate for optoelectronic application. The porous structure of the complex makes it a potential metal organic frame work for gas adsorption and storage. SUPPLEMENTARY DETAILS CCDC No. 982996 contains the supplementary crystallographic data for the compound C24 H14 O24 Ba5. 7H2O. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 IEZ, UK; fax: (+44)1223-336-033; or e-mail: data request @ccdc.cam.ac.uk. ACKNOWLEDGEMENT
The authors are thankful to the STIC, Cochin University of Science and Technology and SCTIMST, Thiruvananthapuram for analytical facilities. We express our sincere gratitude to Dr. Shibu M. Eapen, STIC, Cochin University of Science and Technology, India for providing Single Crystal XRD data. References: [1]S. K. Arora, V. Patel, R.G. Patel, B. Amin, A. Kothari, J. Phys. Chem. Solids, 65(2004) 965-973. [2]T. Jini, K. V. Saban, G.Varghese, Cryst. Res. Technol., 41 (2006) 250-254. [3]T. Jini, K. V. Saban, G.Varghese, S. Naveen, M. A. Sridhar and J. S. Prasad, J.Alloys Compd.,433 (2007) 211-215. [4]I. Popescu, D. M. Suflet, I. M. Pelin, G. C. Chitanu, Rev Roum Chim, 56(3) (2011) 173-188. [5]A. Giedraitis, S. Tamulevi, R. Gudaitis, M. Andrulevicius, Mater. Sci.16 (1) (2010) ISSN 1392–1320, 12-23. [6]William G. Billings, N0 Drawing. Filed Jan. 2, 1968, Ser. No. 694,845, 7 Int. Cl. C01b 2/16; C10k 3/00; B01j 11/32 US. Cl. 48-214 7 Claims Patented Apr. 14, 1970 1C6 [7]V. Mahalakshmi, A. Lincy, J. Thomas, K.V. Saban, J. Phys. Chem. Solids 73 (2012) 584-588. [8]BRUKER, APEX2, SAINT, XPREP, Bruker AXS Inc., Madison, WI USA, 2004. [9]A. Altornare, G. Cascarano, C. Gracovazzo, A. Guagliardi , J. Appl. Cryst. 26 (1993) 343-350.
[10]G. M. Sheldrick, ActaCrystallogr. Sect. A 64 (2008) 112-122. [11]M. M. Ilczyszyn, T. Lis, H. Ratajczak, J. Mol. Struct. 372(1995) 9-27. [12]J. S. Casas, M. S. G. Tasende, J. Sordo,Coordin. Chem. Rev. 209 (2000) 197-261. [13]S. Wang, T. D. Westmoreland, Inorg. Chem. 19 48(2) (2009) 719-727. [14]B. Moulton, M. Zaworotko, J. Chem. Rev. 101(2001) 1629-1658. [15]J. C. Kim, A. J. Lough, J. Chem.Crystallogr. 35(7) (2005) 535-539. [16]K. N. Lazarou, A. K. Boudalis, S. P. Perleps, A. Terzis, C. P. Raptopoulou, Eur. J. Inorg. Chem. (2009) 4554-4563. [17]G. B. Deacon, R. J. Philips, Coord. Chem. Rev. 33(1980) 227-250. [18]A. U.Czaja, N. Trukhan, U. Muller, Chem. Soc. Rev.38 (2009) 1284-1293. [19]G. AnandhaBabu, P. Ramasamy, J. Cryst. Growth 311(2009)1185-1189. [20]N. E. Albert, Infrared absorption associated with strong hydrogen bonds, California Institute of technology (1961) 204. [21]A.W.Coats, J.P.Redfern, Nature 01 (1964) 68. [22]K.J. Laidler, Chemical Kinetics, Harper and Row, Newyork, 1987. [23]S. Singh, B.Lal, J.Cryst. Growth 312 (2010) 301-304.
Fig. 1: Photograph of grown BM.
Fig. 2: Asymmetric unit of BM
Fig. 3: View of Packing along b axis in BM
Fig. 4: Perspective view of packing diagram of BM along a axis
Fig. 5: FT-IR spectrum of BM
Fig. 6: TGA/DTA curves of BM
Fig. 7: Absorbance spectrum of BM
Fig. 8: Plot of alpha energy versus photon energy
Table1. Crystal data and structure refinement for BM Empirical formula
C24 H28 Ba5 O31
Formula weight
1499.09
Temperature
296(2) K
Wavelength
0.71073 Å
Crystal system
Triclinic
Space group
P-1
Unit cell dimensions
a = 7.2929 (3) Å
α =102.035 (10)◦.
b = 10.5454 (4) Å
β = 99.1580 (10)◦.
c = 14.2837(6) Å
γ = 102.9170 (10)◦.
Volume
1022.49 (7) Å 3
Z
1
Calculated density
2.415 g/cc
Absorption coefficient
4.845 mm-1
F(000)
688
Crystal size
0.35 x 0.30 x 0.25 mm3
Theta range for data collection
1.49 to 28.20 deg.
Limiting indices
-9
