Abstract
A novel coordination polymer, poly[(μ-diacetato)(tetraphthalato)pentabarium(II)], with acetate bridged moiety has been successfully grown by gel diffusion technique. Single crystal X-ray diffraction analysis showed that the compound crystallizes in triclinic space group P1̅. The grown crystals were further characterized by elemental analysis, FT-IR, UV-visible spectroscopic analysis, and thermogravimetric and powder X-ray diffraction studies. The photoluminescent properties of the complex and the ligand were also investigated.
Introduction
The relevance of coordination polymers in research area is due to their catalytic activity, magnetic property, high thermal stability and conductivity (Lehn, 1995; Zhao et al., 2012). Coordination polymers of aromatic carboxylates exhibit variety of coordination modes and have considerable applications in gas storage, gas separation and molecular recognition (Williams et al., 2008; Natarajan et al., 2012). The aromatic polycarboxylic acids are of increasing interest due to their structural diversity. Benzene 1,2-dicarboxylic acid (phthalic acid) shows applications in synthesizing resin, medicine, fiber and analytical reagent (Cingi et al., 1978; Williams et al., 1994). The variety of coordination modes of the ligand shows a tendency to form supramolecular coordination compounds and makes it possible to prepare metal phthalates with various structural moieties (Baca et al., 2004; Zhang et al., 2011).
Many reports of metal complexes of benzene 1,3-dicarboxylic acid and benzene 1,4-dicarboxylic acid are known (Reineke et al., 1999; Fang et al., 2004; Banerjee et al., 2005). The alkaline earth metal carboxylates are widely used in material science as oxide precursors by soft chemistry route (Yaghi and Li, 1995). Phthalic acid commonly adopts 1,6-bridging mode to the metal atom and forms the polymeric structure as found in Cu(II), Co(II) and Zn(II) metal phthalates with [M(amine)2]2+. Moreover, 1,6-bridging mode is present, and the other carboxylate oxygen atoms of phthalate ligand coordinate with additional metal atoms and form the complicated polymeric structure that are rarely reported (Baca et al., 2003, 2004). Considering all these structural and functional versatilities, we have selected phthalic acid for the preparation of the title compound.
Detailed literature study revealed some reports of phthalate-based metal complexes. One of the new inorganic-organic hybrid framework, [Ba5(OBDC)4(H2O)2(NO3)2]n (H2OBDC=phthalic acid), containing a two-dimensional (2D) Ba–O–Ba inorganic layer with the OBDC ligands and nitrate ions residing on both sides of the inorganic layer have been reported by solvothermal method (Zhang et al., 2012). The chelating behavior of o-phthalate anion is explained in the crystal structure of barium diaquadi(o-phthalato)cuprate(II) dihydrate, and the crystal structure of [BaCu(H2O)4(OOC-C6H4-COOH)2]n has also been reported (Cingi et al., 1978; Balegroune et al., 2009).
In the present study, we report a new coordination polymer of Ba(II) complex, poly[(μ-diacetato)(tetraphthalato)pentabarium(II)] (PDTP). The crystals were obtained by single gel diffusion technique (Dhanya et al., 2014) at room temperature. A remarkable feature of the title complex is that there is a bridging acetate molecule coordinating to one of the metal ion (Kariuki and Jones, 1993; Wan et al., 2002; Shyu et al., 2009; Saravanan et al., 2015). This type of coordination of bridging acetate has not yet been reported for metal phthalates, and this is the first report of the title compound. The grown crystals were characterized by elemental analysis, FT-IR, UV-visible, thermogravimetric, powder X-ray diffraction and single crystal X-ray diffraction studies. The luminescent properties of the complex and the ligand are also discussed.
Results and discussion
Crystal growth
Single crystals of PDTP suitable for X-ray diffraction analysis were grown in gel medium within 3 days. The optimum condition for crystal growth is, the density of sodium metasilicate =1.04 g cm−3, pH=5.5, concentration of phthalic acid=0.5 m and concentration of barium chloride=0.5 m. The crystals were separated and washed with doubly distilled water and dried. The photograph of the grown crystals of PDTP taken in a test tube is shown in Figure 1.
Photograph of the crystalline PDTP.
FT-IR spectroscopic studies
The FT-IR spectrum of the title complex exhibits several bands corresponding to various functional groups present in the complex. The IR spectrum of the phthalic acid is compared with that of the title complex. On the basis of some reported IR spectra of metal complexes of phthalic acid, the important bands in the complex were interpreted (Wang et al., 2003; Tang et al., 2004; Nicola et al., 2014). The carbonyl absorption, observed at 1684 cm−1 in the FT-IR spectrum of ligand, is absent in the complex indicating that the ligand coordinates to the metal ion through the carboxylate group after deprotonation. The asymmetric and symmetric stretching vibrations of carboxylate groups observed at 1535 and 1423 cm−1, respectively, support this observation. The scissoring vibration of COO− group appears at 852 cm−1. Ba–O stretching vibrations are observed at 691 cm−1. The FT-IR spectrum is shown in Figure S1 (see online supplementary data).
Single crystal X-ray diffraction studies
Single crystal X-ray diffraction analysis reveals that the title compound crystallizes in triclinic space group P1̅. The crystal data and structure refinement parameters are summarized in Table 1. Selected bond lengths and bond angles are shown in Table S1 (supplementary data).
Crystal data and structure refinement parameters of [Ba5(CH3COO)2(C8H4O4)4].
| Parameters | [Ba5(CH3COO)2(C8H4O4)4] |
|---|---|
| Empirical formula | C36 H22 Ba5 O20 |
| Formula weight | 1461.23 |
| Temperature | 296(2) K |
| Wavelength | 0.71073 Å |
| Crystal system, space group | Triclinic, P1̅ |
| Unit cell dimensions | a=7.3156(6) Å α=109.213(3)° |
| b=10.5778(8) Å β=92.871(4) | |
| c=14.0701(11) Å γ=104.989(3)° | |
| Volume | 982.19(14) Å3 |
| Z, Calculated density | 1, 2.470 Mg/m3 |
| Absorption coefficient | 5.022 mm−1 |
| F(000) | 678 |
| Crystal size | 0.350×0.300×0.300 mm |
| Theta range for data collection | 2.915 to 28.395° |
| Limiting indices | −9 ≤ h ≤ 9, −13 ≤ k ≤ 14, −18 ≤ l ≤ 18 |
| Reflections collected/unique | 11,221/4836 [R(int)=0.0205] |
| Completeness to theta | =25.242 99.6% |
| Absorption correction | Semi-empirical from equivalents |
| Maximum and minimum transmission | 0.3121 and 0.2702 |
| Refinement method | Full-matrix least-squares on F2 |
| Data/restraints/parameters | 4836/0/278 |
| Goodness-of-fit on F2 | 0.956 |
| Final R indices [I>2sigma(I)] | R1=0.0174, wR2=0.0446 |
| R indices (all data) | R1=0.0188, wR2=0.0454 |
| Largest diff. peak and hole | 0.510 and −0.890 e.A−3 |
wR2=[Σw(Fo2-Fc2)2]1/2 R1=Σ//Fo/−/Fc///Σ/Fo/.
The asymmetric unit of the molecule along with atom numbering pattern is shown in Figure 2. Taking the asymmetric unit, one of the acetate groups is chelated to Ba3, and Ba1 is surrounded by four carboxylate oxygen atoms of two phthalate ligand. At the same time O1 is bridged to Ba1 and Ba2. The three different barium atoms, Ba1, Ba2 and Ba3 are 12, 9 and 8 coordinated, respectively.
Asymmetric unit of the compound with atom numbering scheme.
The coordination geometry of Ba1 is best described as cuboctahedron BaO12, in which Ba1 is coordinated to 12 chelating phthalate carboxylate oxygen atoms (O1, O9, O5, O6, O7 and O8) from six different phthalate groups. Ba2 is surrounded by nine carboxylate oxygen atoms, of which seven are from different phthalate groups and a bridged carboxylate oxygen atoms (O2 and O10) from a phthalate ligand and it can be defined by a tricapped trigonal prism BaO9. The coordination geometry of Ba3 can be best explained as a square antiprism BaO8, in which four carboxylate oxygen atoms are from different phthalate groups, two oxygen atom donors from a phthalate group and a bridged acetate carboxylate group coordinated to Ba3. The coordination geometry of barium atoms is shown in Figure 3, and the coordination polyhedrons of barium atoms are shown (Figure S2A-C, supplementary data). As reported by Zhang et al. (2012), in the compound [Ba5(OBDC)4(H2O)2(NO3)2]n, there exist three different barium centers in the crystal structure, but the coordination geometry is different from that of PDTP. All the barium atoms Ba1, Ba2 and Ba3 are eight coordinated and exhibit slightly distorted bicapped octahedral geometry in Ba1 and slightly distorted bicapped triangular prism in both Ba2 and Ba3. The crystal structure of [Sr5-(C8H4O4)4(NO3)2(H2O)2] exhibits three distinct SrII cations with coordination numbers, 12 (Sr1), 10 (Sr2) and 8 (Sr3) (Stein and Ruschewitz, 2005). The Ba–O bond lengths range from 2.706(2) to 3.091(3) Å, and O–Ba–O bond angles range from 44.29(5)° to 180° and are comparable to those found in other Ba(II) carboxylates (Lo et al., 1998; Djehni et al., 2007; Wu et al., 2015). The barium centers Ba(1)…Ba(2), Ba(2)…Ba(3) and Ba(1)…Ba(3) are at a distance of 4.3668(3), 4.3932(3) and 4.1893(4) Å, respectively. The atom with highest coordination number shows increase in bond length and decrease in bond angle (Casas et al., 2000). Ba1 shows the highest coordination, and so the bond lengths and bond angles relative to Ba1 show the above trend. The highest bond length of Ba(1)–O(7), 3.0696 (18) Å, is shown by Ba1. The tridendate chelating bridging oxygens, Ba(1)–O(9) and Ba(1)–O(7) having bond lengths 3.0188(17) and 3.0696 (18) Å, are greater than that of bidendate bridging Ba(1)–O(1) and Ba(1)–O(8) distances. The bond angles of tridendate chelating bridging oxygens Ba(3)–O(9)–Ba(1), Ba(2)–O(9)–Ba(1), Ba(3)–O(7)–Ba(1) and Ba(2)–O(7)–Ba(1) are 92.89(5)°, 94.26(5)°, 90.49(5)° and 94.28(5)°, respectively, which is lower than that of bidendate bridging Ba(2)–O(1)–Ba(1) [104.02(6)°] and Ba(2)–O(8)–Ba(1) [101.83(6)°]. The highest bond angle 180° is observed in O(1)–Ba(1)–O(1), O(5)–Ba(1)–O(5), O(6)–Ba(1)–O(6), O(7)–Ba(1)–O(7), O(8)–Ba(1)–O(8) and O(9)–Ba(1)–O(9).
View of the coordination environments of barium atoms.
All the phthalate units are deprotonated in the compound, and this is evident from the C–O distances ranging from 1.242(3) to 1.271(3) Å which further reveals the delocalization of π electron density over the carboxylate groups. The C–O and C–C distances range from 1.242(3) to 1.271(3) Å and 1.377(4) to 1.502(3) Å, respectively. The single bond character of C–COO distances range from 1.494(3) to 1.504(3) Å. In each phthalate unit, two carboxylate ends of phthalate group adopting a bridging μ2-η2η3 mode (one oxygen atom connects two metal ions, other connects three metal ions) (O6 of one of the carboxylate group is coordinated to Ba1 and Ba3, another oxygen O5 is coordinated to Ba1, Ba2 and Ba2) is shown in Figure 4. Similarly, O7 of another carboxylate group is coordinated to Ba1, Ba2, Ba3 and O8 is coordinated to Ba1 and Ba2). O5 and O7 of carboxyl group is tridendately attached to Ba1, Ba2 and Ba3, while O6 and O8 are bidendately attached to Ba1, Ba3 and Ba1, Ba2, respectively.
Phthalate coordination geometry of PDTP.
In contrast to the title compound, the barium based phthalate complex [Ba5(OBDC)4(H2O)2(NO3)2]n, (Zhang et al., 2012), exhibited two different coordination modes of phthalate ligand [μ5-(η2-O′,O″),O,O′,O″,O″′ and μ6-(η2-O′,O″),O,O,O″,O″′,O″′]. In the previous reports of most of the metal phthalates, phthalate adopts different coordination mode unlike PDTP (Wan et al., 2002; Pizon et al., 2010). The special and interesting behavior of carboxylate group in phthalate ligand is that one of the oxygen atoms is linked to three different barium centers and the other is linked to two different barium centers on either side; such type of coordination has never been reported so far. Only a rare example of phthalate ligand exhibits one of the carboxylate oxygen coordinations of three different metal centers (Wang et al., 2014). The bonds Ba2–O5 and Ba1–O7 with bond lengths of 2.9093 and 3.0696 Å correspond to the tridendate chelating bridging oxygen. This bond length is greater than that of bidendate bridging Ba–O bond lengths (Ba3–O6, 2.7354 Å and Ba2–O8, 2.7384 Å). The bond angles of O5–Ba1–O6 (45.38°) and O8–Ba1–O7 (44.25°) are smaller than that of bidendate chelating bridging O–Ba–O bond angles.
The acetate coordination geometry adopts bridging μ2-η1,η2 (one oxygen atom connects one metal ion, the other connects two metal ions; O3 connected to Ba3 and O4 connected to Ba2 and Ba3) (Figure S3). The oxygen atoms connect adjacent barium centers and form different types of four-membered Ba2O2 rings. O2 oxygen atom connects adjacent Ba2 atoms; O5 connects adjacent Ba3 atoms and forms two different Ba2O2 rings. Ba2 and Ba3 connect O2 and O7 to form one Ba2O2 ring. Ba1 and Ba2 connect O5, O8 and O9, and Ba1 and Ba3 connect O7, O6 and O9 to generate four Ba2O2 rings. The different types of Ba2O2 rings are shown in Figure 5.
The Ba2O2 rings present in PDTP.
The seven-membered and four-membered chelating rings formed around barium centers enhance the thermodynamic stability of the complex. Unlike PDTP, three- and four-membered rings are formed in [Ba5(OBDC)4(H2O)2(NO3)2]n (H2OBDC=phthalic acid) (Zhang et al., 2012). In the title compound, the four-membered chelating ring is formed by carboxylate oxygen atoms of three phthalate units in Ba1. In Ba2 and Ba3, the seven-membered chelating rings are formed by carboxylate oxygen atoms of phthalate units. The four-membered chelating ring is formed by oxygen atoms of the acetate group in Ba3. The chelating rings formed around three barium centers are depicted in Figure S4A–C (supplementary data). The phthalate and acetate groups linked by three barium atoms are extended to form a 2D polymeric structure. The 2D structure with porous architecture is used for gas storage, gas separation and catalysis. The packing diagram viewed along the ‘c’ axis shows the 2D layered architecture of the title compound (Figure S5, supplementary data). The packing diagram showing the polymeric structure viewed along the ‘b’ axis and of 2D inorganic layer of polymeric structure viewed along the ‘c’ axis are shown in Figure 6. The packing diagram of PDTP viewed along the ‘a’ axis shows phthalate and acetate connecting three barium environments (Figure S6), and the benzene ring of the phthalate groups are pointing upward and downward the layers.
(A) The packing diagram of PDTP viewed along the ‘b’axis. (B) 2D inorganic layer viewed along the ‘c’ axis.
Powder X-ray diffraction studies
The crystalline nature of the title compound is revealed from the well-defined Braggs peak at specific 2θ angles. In order to check the bulk purity of the compound, powder X-ray diffraction pattern was experimentally recorded and compared with X-ray diffraction pattern obtained from mercury software using single crystal XRD data. These two results are in good agreement which indicate that the compound possesses bulk purity. The powder X-ray diffraction pattern of PDTP is shown in Figure 7.
The powder X-ray diffraction patterns of PDTP.
Thermogravimetric analysis
The thermal stability of the title compound is studied by using thermogravimetry. The compound is stable up to 508°C. This indicates the absence of lattice and coordinated water. One strikingly clean weight loss step occurred at 569°C with a mass loss percentage of 23.00% (calc. 22.45%) corresponding to the removal of two phthalate molecules. Beyond 610°C, the decomposition was apparent. Thermogravimetric/Derivative thermogravimetric (TG/DTG) curve of PDTP is shown in Figure 8.
TG/DTG curve of PDTP.
UV-Visible spectroscopic studies
The solid state absorption spectrum of PDTP is shown in Figure 9. The absorbance at 281 nm indicates the intraligand charge transfer (Srinivasan and Rane, 2009). The compound is highly transparent to all the visible radiations (400–800 nm).
UV-Vis spectrum of PDTP.
Photoluminescent studies
The solid state photoluminescent spectra of the title compound and the ligand were recorded at room temperature, and the spectral pattern is shown in Figure 10. The free ligand shows broad emission at 352 nm upon excitation at 290 nm. The complex shows broad emission with a maximum wavelength of 422 nm (λexc=290 nm). It reveals that the absorption is red shifted in the complex. The spectral pattern is due to ligand-centered charge transfer (n-π* or π-π*) (Yang et al., 2013; Yang et al., 2014). The shift of emission band observed in the compound is probably due to the deprotonation of the ligand followed by coordination to Ba2+ ion, which increases the ligand confirmation rigidity and thereby reduces the energy loss by radiationless decay (Che et al., 2001). This observation suggests that the compound may be an excellent candidate for potential photoactive material. The average luminescent lifetime of a solid compound is 3.36 ns at 422 nm (Figure 11).
The solid state photoluminescent spectra of phthalic acid and complex.
The solid state luminescent lifetime of the complex.
Conclusion
Single crystals of a novel 2D coordination polymer, PDTP, was grown by single gel diffusion technique at room temperature. Single crystal X-ray diffraction analysis reveals that the compound crystallizes in triclinic space group P1̅. The interesting aspect of the crystal structure is that the compound possesses a 2D polymeric structure with acetate bridges. The chelate rings formed in barium centers enhance the thermodynamic stability of the complex. The functional groups present in the crystals were identified by FT-IR analysis. The chemical composition was confirmed by Carbon Hydrogen Nitrogen (CHN) analysis. The transparent nature of the grown crystal is evident from the UV-visible spectroscopic studies, and the wide transparency of the compound in the entire visible region makes it suitable for optoelectronic applications. The thermal decomposition behavior of the complex was identified by TGA analysis. The crystalline nature of the compound was confirmed by powder X-ray diffraction studies. The photoluminescent property suggests that the compound can be used as potential photoactive material.
Experimental
Crystal growth procedure
Single crystals of the title complex were prepared by gel diffusion techniques. Analytical Reagent sodium metasilicate was used for gel preparation. All the chemicals and reagents are obtained from Central Drug House (New Delhi, India). The crystals were grown in simple test tubes of length 20 cm and 2.5 cm of diameter. The gel was prepared by dissolving the phthalic acid ligand solution of molarity (0.5 m) in ethanol to sodium metasilicate solution of density 1.04 g cm−3. The pH of the gel was adjusted by using glacial acetic acid in the pH range of 5 to 7.5. The gel solution was transferred to several test tubes and covered with transparent plastic sheet to avoid evaporation of solution. After setting the gel the barium chloride solution of molarity 0.5 m is added as top solution along the sides of the test tube. Transparent good crystals were grown in the gel medium within 3 days.
Elemental analysis
The stoichiometry of the grown crystal was determined by elemental analysis. The carbon and hydrogen present in the compound were theoretically calculated and compared with experimentally found values. Anal. Calc. for [Ba5(CH3COO)2(C8H4O4)4]: C, 29.56; H, 1.51. Found: C, 29.44; H, 1.07%. The elemental analysis data obtained of the grown crystal corresponds to the composition [Ba5(CH3COO)2(C8H4O4)4].
Characterization
The FT-IR Spectra were recorded in a range 4000–400 cm−1 using Thermo Nicolet, Avatar 370 Spectrometer in KBr pellets (Thermo Elecron Corporation, Madison, WI, USA). The composition of carbon and hydrogen in the compound was determined by using Elementar Vario-EL 111 CHN analyzer (Elementar AnalysensystemeGmbH, Hanau, Germany). The UV-Visible spectroscopic studies were carried out using Varian Cary 5000 UV-Vis-NIR spectrometer (Agilent Technologies, Santa Clara, CA, USA) in the range 200–1200 nm. The powder X-ray diffraction studies were conducted using a Bruker AXS D8 advance XRD (Bruker AXS GmbH, Karlsruhe, Germany) with Cu Kα radiation (λ=1.54056 Å). Single crystal X-ray diffraction studies of the complex were collected using Bruker AXS Kappa Apex2 CCD diffractometer (Bruker, AXS GmbH, Karlsruhe, Germany) with graphite monochromated Mo Kα radiation (λ=0.71073 Å). The unit cell dimensions and intensity data were recorded at 296 K. The programme SAINT/XPREP was used for data reduction and APEXT2/SAINT for cell refinement (Bruker, 2004). The structures were solved by direct methods using SIR92, and refinement was carried out by full-matrix least squares on F2 using SHELXL-97 (Sheldrick, 1997a,b). All the non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were refined isotropically. The structures were plotted by DIAMOND software version 3.1f (Brandenburg, 2008) and IUCr software Mercury (Version 3) (Mercury 1.4.1, 2001/2005). The thermal analysis of the crystal 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 photoluminescence and the luminescent lifetime of solid compound are recorded using Fluoromax-3 spectroflurometer (JY Horiba PL meter at room temperature) consisting of 150 W Xenon arc lamp, monochromator and a detector.
Supplementary crystallographic data
CCDC 1416725 contains the supplementary crystallographic data for the compound PDTP. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2, 1EZ, UK; Fax: (+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk. Additional information is available as electronic supplementary material in the online version.
Acknowledgments
One of the authors, U.S.S.M., is thankful to the UGC for providing the financial assistance in the form of Junior Research Fellowship. The authors are thankful to SAIF, CUSAT, Kochi, India for analytical facilities and Dr. A. Santhosh Kumar, School of Pure and Applied Physics, M. G. University, Kottayam, India, for providing photoluminescent and lifetime measurements. We are also thankful to Dr. Shibu M. Eapen, SAIF, CUSAT, Kochi, India, for single crystal X-ray diffraction measurements.
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