Synthesis, Crystal Structure and Thermal Behavior of a 3-D Barium(II)/Iron(III)-Oxalate Polymer ()
1. Introduction
During the last decades, the self-assembly via metal-ligand coordination bonding approach has been demonstrated to be a useful method for the design and construction of new advanced materials that are needed to foster emerging technologies [1] [2] . Compounds of interest in this context include optically active systems [3] [4] , polymeric microporous open frameworks [5] and, most importantly, molecular magnets [3] [4] [6] -[8] . In this respect, the tris(oxalato)metalate(III) anions are suitable for building multifunctional properties in a molecular lattice [9] -[11] .
It is well-known that the numerous donor sites of the flexible oxalato ligand grant it a significant versatility regarding the possible metal complexes it can form. Following the strategy of utilizing transition metals with versatile multidentate ligands like oxalate dianion, we recently synthesized the first 3-D polymeric salt, {Ba6(H2O)17[Cr(C2O4)3]4}∙7H2O [12] , with the ionic ratio of 3BaII vs. 2CrIII. This compound has proven to be a useful precursor for the synthesis of highly versatile complex salts with the unprecedented potential of accumulating within a single system a whole set of relevant functionalities, such as nanoscale structural features, extended hydrogen bonding, magnetic interactions [13] -[15] . Moreover, it has received great attention in the field of materials science among other reasons for its use as a convenient route for the preparation of technologically important metallic composite oxides [16] .
In order to extend this family of barium(II)/metal(III)-oxalate complex salts, we now describe the synthesis, crystal structure, and thermal behavior of the new salt, {Ba6(H2O)17[Fe(C2O4)3]4}∙7H2O, which turns out to be the iron(III) analog of the previously well-characterized barium(II)/chromium(III)-oxalate polymer [12] , where chromium(III) ions are replaced by iron(III) ions.
2. Experimental
2.1. Materials and Measurements
All the chemicals used were commercial grade materials. They were procured from Prolabo and Merck. The reagents were used without further purification. All reactions were carried out in distilled water as the solvent. Elemental analysis for carbon and hydrogen was performed on a Vario EL (Heraeus) CHNS analyzer. The infrared spectrum was recorded on a Perkin-Elmer (System 2000) FT-IR spectrometer with a pressed KBr pellet in the scan range 4000 - 400 cm−1. Thermal analyses (TGA and TDA) were performed with a Mettler Toledo TGA/SDTA 851 thermal analyzer. The powdered sample (ca. 15 mg) was heated from 25˚C to 600˚C with a rate of 10˚C/min in dry nitrogen gas flowing at 60 mL/min. The melting point was taken with an Electrothermal 9100 apparatus. Three capillary tubes were filled with a powder sample and lodged in a heating closure.
2.2. Synthesis of {Ba6(H2O)17[Fe(C2O4)3]4}∙7H2O
Fe(NO3)3∙9H2O (0.404 g, 1 mmol), H2C2O4∙2H2O (0.378 g, 3 mmol) and Ba(NO3)2 (0.392 g, 1.5 mmol) were dissolved in 20 mL, 10 mL, and 10 mL of water respectively. The solutions of oxalic acid and barium nitrate were added in successive small portions in the solution of iron(III) nitrate. The resulting green mixture was stirred at 333 K for 1 hour and then filtered off. The filtrate was allowed to evaporate in darkness at room temperature. After three weeks, yellow crystals suitable for X-ray diffraction were isolated by filtration, and dried in air. Yield: 0.967 g (84.7%). M. p.: > 240˚C. Anal. Calcd. for C24H48Ba6Fe4O72 (2536.06 g∙mol−1): C, 11.37; H, 1.91%. Found: C, 11.32; H, 1.79%. IR (KBr disk, cm−1): ν = 3420 (m), 1705 (m), 1644 (s), 1398 (s), 1271 (s), 897 (w), 793 (s), 531 (s), 477 (s).
2.3. X-Ray Crystallography
A yellow crystal with approximate dimensions 0.4 × 0.3 × 0.2 mm3 was selected under ambient conditions and attached to the tip of a nylon loop. It was mounted in a stream of cold nitrogen at 100(1) K and centered in the X-ray beam by using a video camera. The diffraction data were collected on a Bruker APEX-II CCD diffractometer and processed using the Bruker software [17] . The structure was solved by the direct method. The positions of the hydrogen atoms were calculated. The non-hydrogen atoms were refined with anisotropic displacement coefficients, and the hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients. Structure solution and refinement were performed using the SHELX-2013 program package [18] . Crystallographic data and refinement parameters are listed in Table 1. Selected bond distances and bond angles are given in Table 2.
3. Results and Discussion
3.1. Formation of {Ba6(H2O)17[Fe(C2O4)3]4}∙7H2O (1)
The title compound was prepared by the reaction in aqueous medium of Ba(NO3)2 with H2C2O4∙2H2O and Fe(NO3)3∙9H2O in the respective 1.5:3:1 molar ratio. Yellow good crystals were isolated from the filtrate with good yield. Elemental and full X-ray structure analyses converge to support the general formulation of 1 as {Ba6(H2O)17[Fe(C2O4)3]4}∙7H2O. This unambiguously demonstrates that the single crystal used for the X-ray
Table 1. Crystal data and structure refinement parameters for compound 1.
Table 2. Selected bond lengths [Å] and angles [˚] for 1.
structure determination is representative of the entire complex synthesized. Compound 1 is a light-sensitive material soluble in water. It does not melt up to 240˚C.
3.2. Energy Dispersive X-Ray Analysis
The EDX spectrum of 1 is shown in Figure 1. This spectrum clearly confirms the presence of chemical elements C, O, Ba and Fe in the crystal.
3.3. Thermal Analysis
The thermal behavior of 1 has been studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) in the temperature range of 25˚C to 600˚C at a heating rate of 10˚C per minute under nitrogen atmosphere. TGA and DTA curves are depicted in Figure 2. This compound shows two steps of weight loss with endothermic change between 60˚C and 200˚C. The first step occurs from 70˚C - 109˚C and this corresponds to the release of twelve H2O molecules (calc. 8.5%, exp.: 8.6%) and the second from 130˚C - 200˚C represents the release of 10 H2O molecules (calc.: 7.1%, exp.: 7.0%). Above 350˚C, the decomposition of the crystal network
Figure 1. EDX spectrum of compound 1 showing the nature of its different chemical elements.
Figure 2. TGA (curve a) and DTA (curve b) of compound 1.
commences with the major weight loss in the temperature range 350˚C - 394˚C, which can be attributed to the release of a combined mass of 8CO2 + 2H2O (calc. 15.3%, exp.: 15.1%) and formation of a mixture of 2(Fe2O3∙H2O) and BaC2O4. Further, a small loss of weight is finally observed between 395˚C - 500˚C, which may be due to the release of a combined mass of 4CO2 + 2H2O (calc. 8.4%, exp.: 8.5%) and formation of the residual compounds, Fe2O3 and BaCO3.
3.4. Crystal Structure
The molecular structure of 1 has been determined by X-ray study, which shows that it crystallizes in the triclinic space group
. The asymmetric unit is shown in Figure 3. Compound 1 is a 3-D polymeric structure with oxalate anions and water molecules bridging Ba2+ and Fe3+ ions. This compound is isotructural with its chromium analog {Ba6(H2O)17[Cr(C2O4)3]4}∙7H2O characterized earlier [12] . It seems relevant, therefore, to compare the structural features of 1 with those of the chromium analog. As to be expected from structural isomorphism, the crystal data of 1 compare with the reported values (in brackets) as follows: a = 10.9539(10) [10.941(1)] Å; b = 17.4492(15) [17.595(1)] Å; c = 18.700(2) [18.577(1)] Å; α = 88.868(5) [88.850(1)]˚; β = 86.160(7) [85.813(1)]˚; γ = 76.462(6) [76.546(1)]˚; V = 3467.2(6) [3469.0(3)] Å3; Z = 2 [2] ; Dx = 2.429 [2.413] g/mm3.
The constitutive ionic building blocks of the structure of 1 are the tris(oxalato)ferrate(III) anion, [Fe(C2O4)3]3−, and Ba(II) cations. The anionic building unit, [Fe(C2O4)3]3−, functions as a metalloligand or else as an “internetting bridge” [19] . Hence, this internetting bridge interconnects, across its twelve O atoms as numbered, the six independent metallic sites Ba1, Ba2, Ba3, Ba4, Ba5 and Ba6—differently coordinated both by H2O and oxalate ligands—into a three-dimensional polymeric network. Each of the four independent FeIII sites is pseudo-octahedrally chelated by three oxalate (
) ligands with helical orientation. Selected geometrical parameters are presented in Table 2. The Fe-Ooxalate bond lengths range from 1.986(5) Å to 2.063(5) Å and the bite angles, O-Fe-O, vary from 79.4(2) to 83.0(2)˚. These bond lengths and bond angles are in good agreement with those found in other tris(oxalato)metalate(III) complexes [12] [20] [21] .
The packing diagram for 1 showing hydrogen bonding interactions as viewed along the a-axis is shown in Figure 4. The bulk structure of 1 is consolidated by a three-dimensional network of hydrogen-bonding of the type O-H∙∙∙O, with O∙∙∙O distances ranging from 2.645(14) to 3.277(11) Å (Table 3).
4. Conclusion
A new complex salt, {Ba6(H2O)17[Fe(C2O4)3]4}∙7H2O (1), obtained from aqueous solution as yellow crystals,
Figure 3. The asymmetric unit of compound 1. All atoms are drawn as 50% thermal probability ellipsoids. All H atoms are omitted for clarity.
Figure 4. Packing diagram of compound 1 showing the H-bonding O−H∙∙∙O (dashed lines) as viewed along the a-axis.
Table 3. Hydrogen bond lengths [Å] and angles [˚] for 1.
Symmetry transformations used to generate equivalent atoms (D, donor; A, acceptor): 11 − X, 1 − Y, 1 − Z; 2+X, +Y, −1 + Z; 31 − X, −Y, 1 – Z.
has been characterized. It is the analog of the well-known barium(II)/chromuim(III)-oxalate polymer, {Ba6(H2O)17[Cr(C2O4)3]4}∙7H2O [12] , with Fe3+ in lieu of Cr3+ ions. The bulk structure of 1 is a 3-D metal-organic framework held together by intermetallic linkages across oxalate and aqua oxygen bridgings with extensive hydrogen-bonding interactions stabilizing the crystal packing. Thermal analysis of compound 1 reveals that it is stable up to ca. 80˚C. The synthesis of compound 1 provides, no doubt, a path for the preparation of further new metal(II)/meta(III)-oxalate frameworks, which may lead to the formation of a wide variety of topologies. Work is in progress in our laboratory on extending this interesting family of oxalate-based precursor materials.
Acknowledgements
The authors are grateful to Dr. Ilia A. Guzei, University of Wisconsin-Madison (USA), for the use of his group X-ray facilities and to Priv.-Doz. Dr. Boniface P. T. Fokwa, University of Aachen (Germany), for his help with EDX, thermal and elemental analyses.
Supplementary Material
Detailed crystallographic data in CIF format for this paper were deposited with the Cambridge Crystallographic Data Centre (CCDC-961313). The data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 IEZ, UK; fax: +44 (0) 1223-336033; e-mail: deposit@ccdc.cam.ac.uk].
NOTES
*Corresponding author.