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Crystal structure, spectral, thermal and dielectric studies of a new zinc benzoate single crystal
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 1002–1006
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Crystal structure, spectral, thermal and dielectric studies of a new zinc benzoate single crystal B.R. Bijini a, S. Prasanna a, M. Deepa b, C.M.K. Nair a, K. Rajendra Babu a,⇑ a b
Deparetment of Physics, M.G. College, Thiruvananthapuram 695004, India Department of Physics, All Saints’ College, Thiruvananthapuram 695037, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Metal–organic frameworks (MOFs)
" "
" "
find potential applications in chemical engineering, chemistry, and materials science. Single crystals of zinc complex of benzoic acid by gel method. The crystal shows a linear polymeric structure along b-axis which has not been reported so far. FT-IR to determine the functional groups. Thermal decomposition behavior studied by TGA/DTA.
a r t i c l e
i n f o
Article history: Received 27 April 2012 Received in revised form 21 July 2012 Accepted 26 July 2012 Available online 4 August 2012 Keywords: Zinc benzoate Crystal structure Sodium metasilicate gel Crystal growth Dielectric property
a b s t r a c t Single crystals of zinc benzoate with a novel structure were grown in gel media. Sodium metasilicate of gel density 1.04 g/cc at pH 6 was employed to yield transparent single crystals. The crystal structure of the compound was ascertained by single crystal X-ray diffractometry. It was noted that the crystal belongs to monoclinic system with space group P21/c with unit cell parameters a = 10.669(1) Å, b = 12.995(5) Å, c = 19.119(3) Å, and b = 94.926(3)°. The crystal was seen to possess a linear polymeric structure along b-axis; with no presence of coordinated or lattice water. CHN analysis established the stoichiometric composition of the crystal. The existence of functional groups present in the single crystal system was confirmed by FT-IR studies. The thermal characteristic of the sample was analysed by TGA– DTA techniques, and the sample was found to be thermally stable up to 280 °C. The kinetic and thermodynamic parameters were also determined. UV–Vis spectroscopy corroborated the transparency of the crystal and revealed the optical band gap to be 4 eV. Dielectric studies showed decrease in the dielectric constant of the sample with increase in frequency. Ó 2012 Elsevier B.V. All rights reserved.
Introduction Metal–organic frameworks (MOFs), as a new class of porous materials, possess a wide array of potential applications in chemical engineering, chemistry and materials science, including gas storage, gas separation, and catalysis [1–3]. The metal–carboxylate bond formation is reversible, facilitating the formation of well-ordered ⇑ Corresponding author. Tel.: +91 944 7963076. E-mail address: [email protected] (K. Rajendra Babu). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.07.107
crystalline MOFs. The bridging bidentate coordination ability of carboxylate groups favors the high degree of framework connectivity and strong metal–ligand bonds necessary to maintain MOF architecture under the conditions required to evacuate the solvent from the pores. The replacement of the organic linker in MOF materials with some of the more efficient aromatics enhance the H2 storage properties of mixed inorganic–organic materials [4]. One of the areas in which MOFs started to appear recently is biomedical applications [5]. The unique physical and chemical characteristics of MOFs make them promising candidates for drug storage and drug delivery,
B.R. Bijini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 1002–1006
imaging and sensing [6,7]. One of the most important challenges in drug delivery research is the efficient delivery of drugs in the body using nontoxic nanocarriers. Toxicity and biocompatibility are the two other important criteria related to the material which is considered as a potential novel drug carrier. However these areas have not been explored as broadly as gas storage and gas separation. Benzoic acid, C7H6O2 (or C6H5COOH), is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, which was for a long time the only source for benzoic acid. Benzoic acid is also a common food preservative. Benzoic acid helps to prevent infection caused by bacteria. Resistance to the currently accessible antibiotics has motivated the search for new agents with antibacterial activity. Among such agents, metal complexes of biologically active ligands are attractive as metal ions can interact with different steps of pathogenic life cycles [8]. Zinc is an essential element in human growth. This element is known to regulate activity in over 300 metalloenzymes and as a component of ‘‘zinc fingers’’ participates in the reliable transfer of genetic information [9]. In addition to physiological functions, zinc and its compounds have important roles in clinical medicine. Certain zinc salts are biologically active ingredients useful to counter bacterial attachment. Zinc and its compounds have anti-bacterial and anti-viral activity and the wound-healing effect of zinc containing ointments has been known for several centuries. Among many methods available for crystal growth, gel technique is commonly adopted due to its simplicity and ability to suppress nucleation centers. The present study mainly focuses on the growth of zinc benzoate crystals by incorporating benzoic acid in the gel. The crystal structure of tetra nuclear zinc benzoate (Zn4O(C6H5CO2)6) has already been reported [10]. In the present paper, a new single crystal of zinc complex of benzoic acid – C28H20O8Zn2 – grown by conventional gel technique and its characterization has been discussed. The crystal structure was determined from single crystal XRD and was characterized by FT-IR, UV–Vis and elemental studies. TGA–DTA studies have been carried out to investigate the thermal properties of the grown crystals.
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0.9 cm1 resolution, DTGS detector), and TGA/DTA studies [Perkin Elmer Diamond, sample heated between ambient (30 °C) and 800 °C]. The carbon, hydrogen and nitrogen contents in the grown crystals were determined using Elementar Vario EL III CHNS analyzer. The electronic spectrum of the grown crystal was measured using a Varian Cary 5000 UV–Vis–NIR Spectrometer. The dielectric studies were conducted in the frequency region 500 Hz–4 MHz using LCR meter (Hioki 3532-50, LCR Hitester, Nagano, Japan). Results and discussion Crystal growth Tiny crystals were formed at the gel-solution interface 6 days after the incorporation of the top solution. At pH 5–5.5, poly crystals were found both at the gel–solution interface as well as inside the gel and, at pH 6 prismatic single crystals were obtained at the gel–solution interface. Crystals of size 8 mm 6 mm 3 mm were formed at pH 6 for a gel density of 1.04 g/cc and is shown in Fig. S1 (suppl. data). Single crystal X-ray diffraction studies A well formed single crystal was subjected to SXRD studies. The crystallographic data and processing parameters are given in Table 1. Anisotropic displacement parameters were applied to non hydrogen atoms in full matrix least square refinement based on F2. The hydrogen atoms were assigned common isotropic displacement factors and included in the refinement cycles by the use of geometrical restraints. The programs APEX 2/SAINT, SAINT/XPERP, SHELXL-97, and SIR 92 were used for computation. The IUCR software MERCURY was used for molecular graphics. The co-ordination environment of the complex with atom numbering scheme is shown in Fig. 1 and the packing of the crystal viewed along the a-axis is shown in Fig. S2 (suppl. data). In the present structure each zinc atom is in a distorted tetrahedral environment, coordinated by four oxygen atoms from four
Experimental procedure Crystallization method The apparatus used for crystallization of single crystals by gel technique consists of borosilicate glass tubes of length 20 cm and diameter 2.5 cm. Analytical grade (E.Merck & CDH) chemicals and reagents were used for the study. Sodium metasilicate solution was prepared by dissolving it in double distilled water and 10 ml of this solution was taken in the glass tube and 5 ml of 1 M benzoic acid in ethanol was added slowly with continuous stirring to avoid any local ion concentration, which would otherwise cause premature local gelling and make the final solution inhomogeneous. The pH of the final solution was adjusted within the range 5–6 by adding acetic acid. The experiment was set for different values of specific gravity of sodium metasilicate solution varying from 1.04 to 1.06 g/cc. Test tubes were sealed with sheet of plastic to avoid evaporation of the solution and contamination by impurities. Over the set gel, an aqueous solution of zinc chloride (1 M) was poured carefully along the walls of the test tube so as to avoid any gel breakage. Characterisation The crystal and molecular structure of the grown crystals were determined from single crystal XRD data obtained from Bruker Kappa Apex II CCDC X-ray diffractometer. The grown crystals were subjected to FT-IR analysis (Thermo Nicolet, Avatar 370 model,
Table 1 Crystal data and structure refinement parameters. Formula CCDC deposit No. Chemical formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions (Å) a = 10.669(1) Å c = 19.119(3) Å Volume Z Calculated density Absorption coefficient F(0 0 0) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 28.29 Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices[I > 2r(I)] R indices (all data) Largest diff. peak and hole
C28H20O8Zn2 826,000 615.22 296(2) 0.71073 Monoclinic P21/c b = 12.995(5) Å b = 94.926(3)° 2641.1(3) Å3 4 1.547 g/cm3 1.865 mm1 1248 0.35 0.30 0.25 mm3 2.48–28.37° 14 h 13, 14 k 17, 25 l 25 24,296 6553 [R(int) = 0.0724] 99.1% Semi-empirical from equivalents 0.6528 and 0.5614 Full-matrix least-squares on F2 6553/0/343 1.007 R1 = 0.0408, wR2 = 0.1014 R1 = 0.0830, wR2 = 0.1230 0.364 and 0.398 e Å3
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B.R. Bijini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 1002–1006
Fig. 1. Co-ordination environment of C28H20O8Zn2 with atom numbering. Hydrogen atoms omitted for clarity.
different benzoate ligands. All the benzoate ligands are coordinated to zinc atoms in bidentate mode. One oxygen atom of the carboxylate anion is coordinated to one zinc atom and the other oxygen atom to another zinc atom. The Zn–O distances vary from 1.917 to 1.940 Å. The bonds Zn(1)–O(8), Zn(2)–O(6) and Zn(2)– O(7) are polymeric and builds a linear polymeric structure along b-axis. O–Zn–O angles vary from 97.31° to 119.09°. The deviation from regular tetrahedral geometry may be due to aromatic groupings. The bond distances, bond angles and torsion angles are shown in Table 2. The reported structure of tetranuclear zinc benzoate (Zn4O(C4H5CO2)6) belongs to cubic system with each zinc atom in a slightly distorted tetrahedral environment with Zn–O dis-
Table 2 Selected bond lengths (Å), bond angles and torsion angles (°) for the complex. Zn(1)–O(1) Zn(1)–O(2) Zn(2)–O(3) Zn(2)–O(6) C(1)–O(1) C(1)–O(7)#1 C(15)–O(3) C(16)–O(4) O(8)–Zn(1)–O(5) O(5)–Zn(1)–O(1) O(5)–Zn(1)–O(2) O(4)–Zn(2)–O(6) O(6)–Zn(2)–O(7) O(6)–Zn(2)–O(3) O(2)–C(8)–C(9)–C(10) O(6)–C(8)–C(9)–C(10) O(1)–C(1)–C(2)–C(3) O(7)–C(1)–C(2)–C(3) O(3)–C(15)–C(17)– C(18) O(8)–C(15)–C(17)– C(18) O(4)–C(16)–C(23)– C(24) O(5)–C(16)–C(23)– C(24) O(3)–C(15)–O(8)–Zn(1) O(7)–C(1)–O(1)–Zn(1) O(1)–C(1)–O(7)–Zn(2) O(6)–C(8)–O(2)–Zn(1) O(2)–C(8)–O(6)–Zn(2)
1.923(2) 1.933(2) 1.940(2) 1.922(2) 1.259(4) 1.253(4) 1.245(4) 1.241(3) 99.40(9) 106.36(1) 109.46(1) 109.56(1) 112.89(1) 113.98(1) 0.2(5) 179.9(3) 176.9(3) 2.5(5) 9.0(5)
Zn(1)–O(5) Zn(1)–O(8) Zn(2)–O(4) Zn(2)–O(7)00 C(8)–O(2) C(8)–O(6)#1 C(15)–O(8)#2 C(16)–O(5) O(8)–Zn(1)–O(1) O(8)–Zn(1)–O(2) O(1)–Zn(1)–O(2) O(4)–Zn(2)–O(7) O(4)–Zn(2)–O(3) O(7)–Zn(2)–O(3) O(2)–C(8)–C(9)–C(14) O(6)–C(8)–C(9)–C(14) O(1)–C(1)–C(2)–C(7) O(7)–C(1)–C(2)–C(7) O(3)–C(15)–C(17)–C(22)
1.9179(2) 1.918(2) 1.9202(2) 1.935(2) 1.264(4) 1.246(4) 1.249(3) 1.258(3) 115.00(1) 119.09(1) 106.60(1) 112.41(1) 97.31(9) 109.76(1) 175.8(3) 4.4(4) 1.5(5) 179.1(3) 171.1(3)
171.8(3)
O(8)–C(15)–C(17)–C(22)
8.1(5)
158.2(3)
O(4)–C(16)–C(23)–C(28)
17.0(4)
16.6(5)
O(5)–C(16)–C(23)–C(28)
168.2(3)
9.4(5) 3.9 2.7(5) 9.3(5) 10.7(5)
176.7(2) 177.9(2) 170.9(2) 169.1(2) 175.1(2)
O(8)–C(15)–O(3)–Zn(2)
4.1(5)
O(3)–C(15)–O(8)–Zn(1)
9.4(5)
O(5)–C(16)–O(4)–Zn(2)
2.4(4)
C(2)–C(1)–O(1)–Zn(1) C(2)–C(1)–O(7)–Zn(2) C(9)–C(8)–O(2)–Zn(1) C(9)–C(8)–O(6)–Zn(2) C (17)–C(15)–O(3)– Zn(2) C (17)–C(15)–O(8)– Zn(1) C (23)–C(16)–O(4)– Zn(2) C (23)–C(16)–O(5)– Zn(1)
O(4)–C(16)–O(5)–Zn(1)
125.5(3)
tances varying from 1.930 to 1.950 Å and O–Zn–O angles varying from 104.6° to 113.0° [10]. In this structure Zn–Zn distance is 3.15 Å. The Zn–Zn distance in the present structure is 3.252 Å, indicating the absence of direct metal–metal interaction. The observed C–O distances of the coordinated carboxylate group range from 1.241 to 1.264 Å. These values are comparable with other reported bidentate bridging benzoates [10–12]. But in the reported structure of (1,3-dimethyl-4,5-dimethylimidazol-2-ylidene) Ag(I) (benzoate) the C–O bond length of coordinated carboxylate oxygen of benzoate coordinated to Ag(I) is 1.273 Å [13]. Also the O–C–O angle of carboxyl group of benzoate coordinated to Ga in the structure of C25H26Cl2GaN3O2 is 117.6(4)° and that of benzoate coordinated to In in the structure of C32H31ClInN3O4 are 119.6° and 120.2° [14]. But in the present structure the O–C–O angle of carboxyl group of benzoate ligand namely O(8)–C(15)–O(3), O(4)–C(16)–O(5), O(7)–C(1)–O(1), O(6)–C(8)–O(2) are 124.9°, 121.8°, 124.4° and 124.7° respectively, greater than 120° expected for a sp2 hybridized RCO2 moiety. The change in bond angle may be due to the strain on this group resulting from interaction of benzoate ligands in the equivalent position nearer to a particular carboxyl group. This leads to difference in electro negativity causing deviations in electron density. The change in bond angle also raises the probability of O(8) O(3), O(1) O(7), O(2) O(6) interactions within the carboxyl group and O(8) O(5) interaction between carboxyl oxygen of adjacent benzoate ligands. The strain may also arise due to change in torsion angles shown in Table 2. The data obtained further ascertains that the benzoate ligands attached to the Zn cation is completely interlaced among each other indicating the existence of relatively strong inter molecular interaction. The packing analysis of the complex also reveals that crystal lattice is stabilized by p–p interaction between different layers of aromatic rings. Elemental analysis Results obtained from CHN analysis, C = 54.5%, and H = 3.0% were comparable with the theoretical values of C = 54.66% and H = 3.2%. This confirms that the composition of the grown crystal is C28H20O8Zn2 as obtained from single crystal X-ray diffraction pattern. Fourier transform infrared (FTIR) spectra The FT-IR spectrum of the grown crystal is shown in Fig. 2. The peak at 3060 cm1 is due to C–H stretching vibrations [15]. The asymmetric and symmetric stretching vibration bands of COO–
169.8(2) 172.3(2) 59.9(5)
Fig. 2. FT-IR spectrum of the grown crystals.
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B.R. Bijini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 1002–1006 Table 3 Kinetic and thermodynamic parameters of dehydration (stage I) and decomposition (stage II) of C28H20O8Zn2. Kinetic parameters
Thermodynamic parameters
Stage
n
E (KJ/mol)
log A (S1)
S (J/Kmol)
DH (KJ/mol)
DG (KJ/mol)
I II
1.2 0.9
90.48 232.68
3.90 8.213
177.46 98.013
78.6 215.56
205.4 316.45
group emerge at 1576 and 1417 cm1 respectively [10,16]. The Dt (tasy tsy) separation 159 cm1 indicates the bridging bidentate coordination of carboxylate group to the central metal ion as evident from single crystal x-ray diffraction studies. The bands at 1596 and 1494 cm1 are due to C–C stretching [17]. The band at 717 cm1 is attributed to the C–H out of plane bending [18]. The band at 495 cm1 corresponds to Zn–O bond [19]. Thermal study The TGA/DTA of the sample was conducted in nitrogen atmosphere at a heating rate of 10 °C/min. The results obtained from thermal studies are shown in Fig. S3 (suppl. data). The compound is thermally stable up to 280 °C and above this temperature the compound melts and decomposes. The DTA curve shows two sharp endothermic peaks at 320.15 °C, 441.52 °C and one broad peak at 756 °C. The first sharp endothermic peak may be the melting point of the complex [20]. The melting is followed by decomposition of the complex releasing two molecules of (C6H5)2CO (experimental mass loss 56.2%, theoretical mass loss 59.23%) [19]. Next step of the thermal decomposition in the temperature range 700–890 °C is the release of two CO2 molecules(experimental mass loss 14.42%, theoretical mass loss 14.31%) and this decomposition corresponds to the endothermic peak 756.39 °C on the DTA curve [15,16,21]. The remaining weight of 29.21% corresponds to the ZnO (calculated 28.2%) which is left as residue. The Coats and Redfern method has been used to determine the kinetic parameters of decomposition i.e., order of reaction, activation energy (E), and pre-exponential term log A [22]. The other thermodynamic parameters such as the standard enthalpy and standard Gibb’s free energy (DG) were also calculated [23]. The calculated values of kinetic parameters are given in Table 3. UV–Vis–NIR spectroscopy The UV–Vis–NIR spectrum of C28H20O8Zn2 taken in the wavelength range 200–2000 nm is shown in Fig. 3. It is seen from the spectra that the maximum absorption occurs around 285 nm. There is no considerable absorption of light in the visible range of electromagnetic spectrum. A graph is drawn between photon energy (ht) verses (aht) where a is the absorption coefficient. The band gap is estimated as 4 eV by extrapolating the linear portion of the curve to zero absorption [Fig. S4 (suppl. data)]. The polarisability is highly sensitive pffiffiffiffi to band gap and hence the empirEg 24 ical relationship a ¼ ½1 4:06 M cm3 , where M is q 0:396 10 the molecular mass and q is the density, can be used to calculate the electronic polarisability [24]. Dielectric studies The variation of dielectric constant and dielectric loss of the grown crystal as a function of frequency is shown in Fig. S5 (suppl. data). From the graph, the dielectric constant seems to decrease with increase in frequency. The large value of dielectric constant at low frequency is due to the presence of space charge polarization [25]. The inverse relation of dielectric constant with frequency is due to the fact that the frequency of electric charge carriers can-
Fig. 3. UV–Vis spectrum of the grown crystal.
Table 4 Plasma energy and polarisability of C28H20O8Zn2 crystal. Parameters
Values
Plasma energy (eV) Penn gap (eV) Fermi gap (eV) Polarizability (cm3) (a) Penn analysis (b) Clausius–Mossotti Equation (c) From band gap
28.8 9.22 15.458 8.83 1023 9.43 1023 7.99 1023
not follow the alternation of the applied ac electric field beyond a certain critical frequency [26]. The very low value of dielectric constant at higher frequencies is important for the fabrication of materials for ferroelectric, photonic and electro–optic devices. The value of dielectric constant at higher frequencies can be used to calculate Penn gap, Fermi energy and polarisability of the grown crystals [24]. The calculated values are depicted in Table 4. The calculated values are in agreement with those calculated from the ClausiusMossotti relation.
Conclusions Single crystals of C28H20O8Zn2 with a new structure have been successfully grown by gel diffusion method. Sodium metasilicate of gel density 1.04 g/cc and pH 6 produced good quality crystals. Single crystal X-ray diffraction study confirms that the grown crystals belong to monoclinic system (C2) with unit cell parameters a = 10.669(1) Å, b = 12.995(5) Å, c = 19.119(3) Å, b = 94.926(3)°. The compound is polymeric and builds a linear polymeric structure along b-axis. The elemental analysis is consistent with the formula C28H20O8Zn2. Kinetic and thermodynamic parameters were calculated from TGA/DTA studies. Fermi gap, Penn gap, and Plasma energy of the grown crystals were calculated from unit cell
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parameters and dielectric studies. Energy gap is determined from UV–Vis spectroscopy. The value of polarisability determined from UV–Vis–NIR and dielectric studies agrees well with that calculated using Clausius–Mossotti relation. Zinc benzoate structure with its unique physical and chemical features find applications in areas such as drug storage, drug delivery, imaging and sensing. As a metal complex of biologically active ligand, this structure is also feasible in biological applications since metal ions interact effectively with various stages of pathogenic life cycles. The high porosity of the compound lends itself to widespread use in gas storage, separation and adsorption. The potential of the complex as an efficient drug carrier and its biocompatibility may have to be explored. Acknowledgements One of the authors Bijini B.R. is grateful to the UGC for awarding teacher fellowship under FDP. The authors also express their gratitude to STIC, Kochi for providing facilities for characterization studies. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.07.107. References [1] A. Corma, H. García, F.X.L.I. Xamenaa, Chem. Rev. 110 (2010) 4606–4655. [2] S. Turner, O.I. Lebedev, F. Schroder, R.A. Fischer, G.V. Tendeloo, Mater. Sci. 2 (2008) 275–276.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Crystal structure, spectral, thermal and dielectric studies of a new zinc benzoate single crystal B.R. Bijini a, S. Prasanna a, M. Deepa b, C.M.K. Nair a, K. Rajendra Babu a,⇑ a b
Deparetment of Physics, M.G. College, Thiruvananthapuram 695004, India Department of Physics, All Saints’ College, Thiruvananthapuram 695037, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Metal–organic frameworks (MOFs)
" "
" "
find potential applications in chemical engineering, chemistry, and materials science. Single crystals of zinc complex of benzoic acid by gel method. The crystal shows a linear polymeric structure along b-axis which has not been reported so far. FT-IR to determine the functional groups. Thermal decomposition behavior studied by TGA/DTA.
a r t i c l e
i n f o
Article history: Received 27 April 2012 Received in revised form 21 July 2012 Accepted 26 July 2012 Available online 4 August 2012 Keywords: Zinc benzoate Crystal structure Sodium metasilicate gel Crystal growth Dielectric property
a b s t r a c t Single crystals of zinc benzoate with a novel structure were grown in gel media. Sodium metasilicate of gel density 1.04 g/cc at pH 6 was employed to yield transparent single crystals. The crystal structure of the compound was ascertained by single crystal X-ray diffractometry. It was noted that the crystal belongs to monoclinic system with space group P21/c with unit cell parameters a = 10.669(1) Å, b = 12.995(5) Å, c = 19.119(3) Å, and b = 94.926(3)°. The crystal was seen to possess a linear polymeric structure along b-axis; with no presence of coordinated or lattice water. CHN analysis established the stoichiometric composition of the crystal. The existence of functional groups present in the single crystal system was confirmed by FT-IR studies. The thermal characteristic of the sample was analysed by TGA– DTA techniques, and the sample was found to be thermally stable up to 280 °C. The kinetic and thermodynamic parameters were also determined. UV–Vis spectroscopy corroborated the transparency of the crystal and revealed the optical band gap to be 4 eV. Dielectric studies showed decrease in the dielectric constant of the sample with increase in frequency. Ó 2012 Elsevier B.V. All rights reserved.
Introduction Metal–organic frameworks (MOFs), as a new class of porous materials, possess a wide array of potential applications in chemical engineering, chemistry and materials science, including gas storage, gas separation, and catalysis [1–3]. The metal–carboxylate bond formation is reversible, facilitating the formation of well-ordered ⇑ Corresponding author. Tel.: +91 944 7963076. E-mail address: [email protected] (K. Rajendra Babu). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.07.107
crystalline MOFs. The bridging bidentate coordination ability of carboxylate groups favors the high degree of framework connectivity and strong metal–ligand bonds necessary to maintain MOF architecture under the conditions required to evacuate the solvent from the pores. The replacement of the organic linker in MOF materials with some of the more efficient aromatics enhance the H2 storage properties of mixed inorganic–organic materials [4]. One of the areas in which MOFs started to appear recently is biomedical applications [5]. The unique physical and chemical characteristics of MOFs make them promising candidates for drug storage and drug delivery,
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imaging and sensing [6,7]. One of the most important challenges in drug delivery research is the efficient delivery of drugs in the body using nontoxic nanocarriers. Toxicity and biocompatibility are the two other important criteria related to the material which is considered as a potential novel drug carrier. However these areas have not been explored as broadly as gas storage and gas separation. Benzoic acid, C7H6O2 (or C6H5COOH), is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, which was for a long time the only source for benzoic acid. Benzoic acid is also a common food preservative. Benzoic acid helps to prevent infection caused by bacteria. Resistance to the currently accessible antibiotics has motivated the search for new agents with antibacterial activity. Among such agents, metal complexes of biologically active ligands are attractive as metal ions can interact with different steps of pathogenic life cycles [8]. Zinc is an essential element in human growth. This element is known to regulate activity in over 300 metalloenzymes and as a component of ‘‘zinc fingers’’ participates in the reliable transfer of genetic information [9]. In addition to physiological functions, zinc and its compounds have important roles in clinical medicine. Certain zinc salts are biologically active ingredients useful to counter bacterial attachment. Zinc and its compounds have anti-bacterial and anti-viral activity and the wound-healing effect of zinc containing ointments has been known for several centuries. Among many methods available for crystal growth, gel technique is commonly adopted due to its simplicity and ability to suppress nucleation centers. The present study mainly focuses on the growth of zinc benzoate crystals by incorporating benzoic acid in the gel. The crystal structure of tetra nuclear zinc benzoate (Zn4O(C6H5CO2)6) has already been reported [10]. In the present paper, a new single crystal of zinc complex of benzoic acid – C28H20O8Zn2 – grown by conventional gel technique and its characterization has been discussed. The crystal structure was determined from single crystal XRD and was characterized by FT-IR, UV–Vis and elemental studies. TGA–DTA studies have been carried out to investigate the thermal properties of the grown crystals.
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0.9 cm1 resolution, DTGS detector), and TGA/DTA studies [Perkin Elmer Diamond, sample heated between ambient (30 °C) and 800 °C]. The carbon, hydrogen and nitrogen contents in the grown crystals were determined using Elementar Vario EL III CHNS analyzer. The electronic spectrum of the grown crystal was measured using a Varian Cary 5000 UV–Vis–NIR Spectrometer. The dielectric studies were conducted in the frequency region 500 Hz–4 MHz using LCR meter (Hioki 3532-50, LCR Hitester, Nagano, Japan). Results and discussion Crystal growth Tiny crystals were formed at the gel-solution interface 6 days after the incorporation of the top solution. At pH 5–5.5, poly crystals were found both at the gel–solution interface as well as inside the gel and, at pH 6 prismatic single crystals were obtained at the gel–solution interface. Crystals of size 8 mm 6 mm 3 mm were formed at pH 6 for a gel density of 1.04 g/cc and is shown in Fig. S1 (suppl. data). Single crystal X-ray diffraction studies A well formed single crystal was subjected to SXRD studies. The crystallographic data and processing parameters are given in Table 1. Anisotropic displacement parameters were applied to non hydrogen atoms in full matrix least square refinement based on F2. The hydrogen atoms were assigned common isotropic displacement factors and included in the refinement cycles by the use of geometrical restraints. The programs APEX 2/SAINT, SAINT/XPERP, SHELXL-97, and SIR 92 were used for computation. The IUCR software MERCURY was used for molecular graphics. The co-ordination environment of the complex with atom numbering scheme is shown in Fig. 1 and the packing of the crystal viewed along the a-axis is shown in Fig. S2 (suppl. data). In the present structure each zinc atom is in a distorted tetrahedral environment, coordinated by four oxygen atoms from four
Experimental procedure Crystallization method The apparatus used for crystallization of single crystals by gel technique consists of borosilicate glass tubes of length 20 cm and diameter 2.5 cm. Analytical grade (E.Merck & CDH) chemicals and reagents were used for the study. Sodium metasilicate solution was prepared by dissolving it in double distilled water and 10 ml of this solution was taken in the glass tube and 5 ml of 1 M benzoic acid in ethanol was added slowly with continuous stirring to avoid any local ion concentration, which would otherwise cause premature local gelling and make the final solution inhomogeneous. The pH of the final solution was adjusted within the range 5–6 by adding acetic acid. The experiment was set for different values of specific gravity of sodium metasilicate solution varying from 1.04 to 1.06 g/cc. Test tubes were sealed with sheet of plastic to avoid evaporation of the solution and contamination by impurities. Over the set gel, an aqueous solution of zinc chloride (1 M) was poured carefully along the walls of the test tube so as to avoid any gel breakage. Characterisation The crystal and molecular structure of the grown crystals were determined from single crystal XRD data obtained from Bruker Kappa Apex II CCDC X-ray diffractometer. The grown crystals were subjected to FT-IR analysis (Thermo Nicolet, Avatar 370 model,
Table 1 Crystal data and structure refinement parameters. Formula CCDC deposit No. Chemical formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions (Å) a = 10.669(1) Å c = 19.119(3) Å Volume Z Calculated density Absorption coefficient F(0 0 0) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 28.29 Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices[I > 2r(I)] R indices (all data) Largest diff. peak and hole
C28H20O8Zn2 826,000 615.22 296(2) 0.71073 Monoclinic P21/c b = 12.995(5) Å b = 94.926(3)° 2641.1(3) Å3 4 1.547 g/cm3 1.865 mm1 1248 0.35 0.30 0.25 mm3 2.48–28.37° 14 h 13, 14 k 17, 25 l 25 24,296 6553 [R(int) = 0.0724] 99.1% Semi-empirical from equivalents 0.6528 and 0.5614 Full-matrix least-squares on F2 6553/0/343 1.007 R1 = 0.0408, wR2 = 0.1014 R1 = 0.0830, wR2 = 0.1230 0.364 and 0.398 e Å3
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Fig. 1. Co-ordination environment of C28H20O8Zn2 with atom numbering. Hydrogen atoms omitted for clarity.
different benzoate ligands. All the benzoate ligands are coordinated to zinc atoms in bidentate mode. One oxygen atom of the carboxylate anion is coordinated to one zinc atom and the other oxygen atom to another zinc atom. The Zn–O distances vary from 1.917 to 1.940 Å. The bonds Zn(1)–O(8), Zn(2)–O(6) and Zn(2)– O(7) are polymeric and builds a linear polymeric structure along b-axis. O–Zn–O angles vary from 97.31° to 119.09°. The deviation from regular tetrahedral geometry may be due to aromatic groupings. The bond distances, bond angles and torsion angles are shown in Table 2. The reported structure of tetranuclear zinc benzoate (Zn4O(C4H5CO2)6) belongs to cubic system with each zinc atom in a slightly distorted tetrahedral environment with Zn–O dis-
Table 2 Selected bond lengths (Å), bond angles and torsion angles (°) for the complex. Zn(1)–O(1) Zn(1)–O(2) Zn(2)–O(3) Zn(2)–O(6) C(1)–O(1) C(1)–O(7)#1 C(15)–O(3) C(16)–O(4) O(8)–Zn(1)–O(5) O(5)–Zn(1)–O(1) O(5)–Zn(1)–O(2) O(4)–Zn(2)–O(6) O(6)–Zn(2)–O(7) O(6)–Zn(2)–O(3) O(2)–C(8)–C(9)–C(10) O(6)–C(8)–C(9)–C(10) O(1)–C(1)–C(2)–C(3) O(7)–C(1)–C(2)–C(3) O(3)–C(15)–C(17)– C(18) O(8)–C(15)–C(17)– C(18) O(4)–C(16)–C(23)– C(24) O(5)–C(16)–C(23)– C(24) O(3)–C(15)–O(8)–Zn(1) O(7)–C(1)–O(1)–Zn(1) O(1)–C(1)–O(7)–Zn(2) O(6)–C(8)–O(2)–Zn(1) O(2)–C(8)–O(6)–Zn(2)
1.923(2) 1.933(2) 1.940(2) 1.922(2) 1.259(4) 1.253(4) 1.245(4) 1.241(3) 99.40(9) 106.36(1) 109.46(1) 109.56(1) 112.89(1) 113.98(1) 0.2(5) 179.9(3) 176.9(3) 2.5(5) 9.0(5)
Zn(1)–O(5) Zn(1)–O(8) Zn(2)–O(4) Zn(2)–O(7)00 C(8)–O(2) C(8)–O(6)#1 C(15)–O(8)#2 C(16)–O(5) O(8)–Zn(1)–O(1) O(8)–Zn(1)–O(2) O(1)–Zn(1)–O(2) O(4)–Zn(2)–O(7) O(4)–Zn(2)–O(3) O(7)–Zn(2)–O(3) O(2)–C(8)–C(9)–C(14) O(6)–C(8)–C(9)–C(14) O(1)–C(1)–C(2)–C(7) O(7)–C(1)–C(2)–C(7) O(3)–C(15)–C(17)–C(22)
1.9179(2) 1.918(2) 1.9202(2) 1.935(2) 1.264(4) 1.246(4) 1.249(3) 1.258(3) 115.00(1) 119.09(1) 106.60(1) 112.41(1) 97.31(9) 109.76(1) 175.8(3) 4.4(4) 1.5(5) 179.1(3) 171.1(3)
171.8(3)
O(8)–C(15)–C(17)–C(22)
8.1(5)
158.2(3)
O(4)–C(16)–C(23)–C(28)
17.0(4)
16.6(5)
O(5)–C(16)–C(23)–C(28)
168.2(3)
9.4(5) 3.9 2.7(5) 9.3(5) 10.7(5)
176.7(2) 177.9(2) 170.9(2) 169.1(2) 175.1(2)
O(8)–C(15)–O(3)–Zn(2)
4.1(5)
O(3)–C(15)–O(8)–Zn(1)
9.4(5)
O(5)–C(16)–O(4)–Zn(2)
2.4(4)
C(2)–C(1)–O(1)–Zn(1) C(2)–C(1)–O(7)–Zn(2) C(9)–C(8)–O(2)–Zn(1) C(9)–C(8)–O(6)–Zn(2) C (17)–C(15)–O(3)– Zn(2) C (17)–C(15)–O(8)– Zn(1) C (23)–C(16)–O(4)– Zn(2) C (23)–C(16)–O(5)– Zn(1)
O(4)–C(16)–O(5)–Zn(1)
125.5(3)
tances varying from 1.930 to 1.950 Å and O–Zn–O angles varying from 104.6° to 113.0° [10]. In this structure Zn–Zn distance is 3.15 Å. The Zn–Zn distance in the present structure is 3.252 Å, indicating the absence of direct metal–metal interaction. The observed C–O distances of the coordinated carboxylate group range from 1.241 to 1.264 Å. These values are comparable with other reported bidentate bridging benzoates [10–12]. But in the reported structure of (1,3-dimethyl-4,5-dimethylimidazol-2-ylidene) Ag(I) (benzoate) the C–O bond length of coordinated carboxylate oxygen of benzoate coordinated to Ag(I) is 1.273 Å [13]. Also the O–C–O angle of carboxyl group of benzoate coordinated to Ga in the structure of C25H26Cl2GaN3O2 is 117.6(4)° and that of benzoate coordinated to In in the structure of C32H31ClInN3O4 are 119.6° and 120.2° [14]. But in the present structure the O–C–O angle of carboxyl group of benzoate ligand namely O(8)–C(15)–O(3), O(4)–C(16)–O(5), O(7)–C(1)–O(1), O(6)–C(8)–O(2) are 124.9°, 121.8°, 124.4° and 124.7° respectively, greater than 120° expected for a sp2 hybridized RCO2 moiety. The change in bond angle may be due to the strain on this group resulting from interaction of benzoate ligands in the equivalent position nearer to a particular carboxyl group. This leads to difference in electro negativity causing deviations in electron density. The change in bond angle also raises the probability of O(8) O(3), O(1) O(7), O(2) O(6) interactions within the carboxyl group and O(8) O(5) interaction between carboxyl oxygen of adjacent benzoate ligands. The strain may also arise due to change in torsion angles shown in Table 2. The data obtained further ascertains that the benzoate ligands attached to the Zn cation is completely interlaced among each other indicating the existence of relatively strong inter molecular interaction. The packing analysis of the complex also reveals that crystal lattice is stabilized by p–p interaction between different layers of aromatic rings. Elemental analysis Results obtained from CHN analysis, C = 54.5%, and H = 3.0% were comparable with the theoretical values of C = 54.66% and H = 3.2%. This confirms that the composition of the grown crystal is C28H20O8Zn2 as obtained from single crystal X-ray diffraction pattern. Fourier transform infrared (FTIR) spectra The FT-IR spectrum of the grown crystal is shown in Fig. 2. The peak at 3060 cm1 is due to C–H stretching vibrations [15]. The asymmetric and symmetric stretching vibration bands of COO–
169.8(2) 172.3(2) 59.9(5)
Fig. 2. FT-IR spectrum of the grown crystals.
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B.R. Bijini et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 1002–1006 Table 3 Kinetic and thermodynamic parameters of dehydration (stage I) and decomposition (stage II) of C28H20O8Zn2. Kinetic parameters
Thermodynamic parameters
Stage
n
E (KJ/mol)
log A (S1)
S (J/Kmol)
DH (KJ/mol)
DG (KJ/mol)
I II
1.2 0.9
90.48 232.68
3.90 8.213
177.46 98.013
78.6 215.56
205.4 316.45
group emerge at 1576 and 1417 cm1 respectively [10,16]. The Dt (tasy tsy) separation 159 cm1 indicates the bridging bidentate coordination of carboxylate group to the central metal ion as evident from single crystal x-ray diffraction studies. The bands at 1596 and 1494 cm1 are due to C–C stretching [17]. The band at 717 cm1 is attributed to the C–H out of plane bending [18]. The band at 495 cm1 corresponds to Zn–O bond [19]. Thermal study The TGA/DTA of the sample was conducted in nitrogen atmosphere at a heating rate of 10 °C/min. The results obtained from thermal studies are shown in Fig. S3 (suppl. data). The compound is thermally stable up to 280 °C and above this temperature the compound melts and decomposes. The DTA curve shows two sharp endothermic peaks at 320.15 °C, 441.52 °C and one broad peak at 756 °C. The first sharp endothermic peak may be the melting point of the complex [20]. The melting is followed by decomposition of the complex releasing two molecules of (C6H5)2CO (experimental mass loss 56.2%, theoretical mass loss 59.23%) [19]. Next step of the thermal decomposition in the temperature range 700–890 °C is the release of two CO2 molecules(experimental mass loss 14.42%, theoretical mass loss 14.31%) and this decomposition corresponds to the endothermic peak 756.39 °C on the DTA curve [15,16,21]. The remaining weight of 29.21% corresponds to the ZnO (calculated 28.2%) which is left as residue. The Coats and Redfern method has been used to determine the kinetic parameters of decomposition i.e., order of reaction, activation energy (E), and pre-exponential term log A [22]. The other thermodynamic parameters such as the standard enthalpy and standard Gibb’s free energy (DG) were also calculated [23]. The calculated values of kinetic parameters are given in Table 3. UV–Vis–NIR spectroscopy The UV–Vis–NIR spectrum of C28H20O8Zn2 taken in the wavelength range 200–2000 nm is shown in Fig. 3. It is seen from the spectra that the maximum absorption occurs around 285 nm. There is no considerable absorption of light in the visible range of electromagnetic spectrum. A graph is drawn between photon energy (ht) verses (aht) where a is the absorption coefficient. The band gap is estimated as 4 eV by extrapolating the linear portion of the curve to zero absorption [Fig. S4 (suppl. data)]. The polarisability is highly sensitive pffiffiffiffi to band gap and hence the empirEg 24 ical relationship a ¼ ½1 4:06 M cm3 , where M is q 0:396 10 the molecular mass and q is the density, can be used to calculate the electronic polarisability [24]. Dielectric studies The variation of dielectric constant and dielectric loss of the grown crystal as a function of frequency is shown in Fig. S5 (suppl. data). From the graph, the dielectric constant seems to decrease with increase in frequency. The large value of dielectric constant at low frequency is due to the presence of space charge polarization [25]. The inverse relation of dielectric constant with frequency is due to the fact that the frequency of electric charge carriers can-
Fig. 3. UV–Vis spectrum of the grown crystal.
Table 4 Plasma energy and polarisability of C28H20O8Zn2 crystal. Parameters
Values
Plasma energy (eV) Penn gap (eV) Fermi gap (eV) Polarizability (cm3) (a) Penn analysis (b) Clausius–Mossotti Equation (c) From band gap
28.8 9.22 15.458 8.83 1023 9.43 1023 7.99 1023
not follow the alternation of the applied ac electric field beyond a certain critical frequency [26]. The very low value of dielectric constant at higher frequencies is important for the fabrication of materials for ferroelectric, photonic and electro–optic devices. The value of dielectric constant at higher frequencies can be used to calculate Penn gap, Fermi energy and polarisability of the grown crystals [24]. The calculated values are depicted in Table 4. The calculated values are in agreement with those calculated from the ClausiusMossotti relation.
Conclusions Single crystals of C28H20O8Zn2 with a new structure have been successfully grown by gel diffusion method. Sodium metasilicate of gel density 1.04 g/cc and pH 6 produced good quality crystals. Single crystal X-ray diffraction study confirms that the grown crystals belong to monoclinic system (C2) with unit cell parameters a = 10.669(1) Å, b = 12.995(5) Å, c = 19.119(3) Å, b = 94.926(3)°. The compound is polymeric and builds a linear polymeric structure along b-axis. The elemental analysis is consistent with the formula C28H20O8Zn2. Kinetic and thermodynamic parameters were calculated from TGA/DTA studies. Fermi gap, Penn gap, and Plasma energy of the grown crystals were calculated from unit cell
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parameters and dielectric studies. Energy gap is determined from UV–Vis spectroscopy. The value of polarisability determined from UV–Vis–NIR and dielectric studies agrees well with that calculated using Clausius–Mossotti relation. Zinc benzoate structure with its unique physical and chemical features find applications in areas such as drug storage, drug delivery, imaging and sensing. As a metal complex of biologically active ligand, this structure is also feasible in biological applications since metal ions interact effectively with various stages of pathogenic life cycles. The high porosity of the compound lends itself to widespread use in gas storage, separation and adsorption. The potential of the complex as an efficient drug carrier and its biocompatibility may have to be explored. Acknowledgements One of the authors Bijini B.R. is grateful to the UGC for awarding teacher fellowship under FDP. The authors also express their gratitude to STIC, Kochi for providing facilities for characterization studies. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.07.107. References [1] A. Corma, H. García, F.X.L.I. Xamenaa, Chem. Rev. 110 (2010) 4606–4655. [2] S. Turner, O.I. Lebedev, F. Schroder, R.A. Fischer, G.V. Tendeloo, Mater. Sci. 2 (2008) 275–276.
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