Abstract
A new organic-inorganic hybrid material, [C10H28N4]Bi2Br10, has been synthesized and characterized. The compound crystallizes in monoclinic P21/c space group with a = 11.410(4) Å, b = 11.284(4) Å, c = 12.599(3) Å, β = 115.93(2)°, and V = 1458.8(8) Å3. The structure consists of discrete dinuclear [Bi2Br10]4− anions and [C10H28N4]4+ cations. It consists of a 0-D anion built up of edge-sharing bioctahedron. The crystal net contains N–H⋯Br hydrogen bonds. The differential scanning calorimetry (DSC) reveals an irreversible phase transition at −17°C. The frontier molecular orbital and the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) calculation allow the classification of the material as an insulator.
1. Introduction
The results of systematic structural investigations of halobismuthate (III) compounds reveal a great variety of different anionic frameworks. Most of these compounds are described by a general formula (where R is organic cations, M is Bi, and X is Cl, Br, and I) and have a tendency to constitute bi- or polynuclear anions in the crystalline state.
Generally, in these compounds, the coordination sphere of bismuth appears to be dominated by the tendency towards hexacoordination with polybismuthate species arising from corner, edges, or faces sharing BiX6 distorted octahedra.
The formation of the anionic sublattice is clearly determined by the counteractions, but the effects of their most evident properties such as charge, size, and shape are almost not predictable. The organic moiety can be used as physical and electronic barrier, contributing to original electrical and optical behaviour. In addition, since, in the crystal state, important contribution to the lattice stabilization is due to hydrogen bonding interactions, it should be possible to influence the bismuth coordination geometry acting on the number and orientation of the hydrogen bond donor sites of the cations [1–4].
2. Synthesis Experimental Protocol
The title compound was synthesized by dissolving stoichiometric amounts of bismuth (III) bromide in piperazine in a mixture of water and HBr. The resulting solution was stirred well and then kept at room temperature. Few weeks later, transparent crystals, as bright-yellow prism, were grown by slow evaporation. The purity of synthesized compound was improved by successive recrystallization process.
3. Results and Discussion
3.1. X-Ray Data Collection
The X-ray diffraction intensities from a single crystal of about (0.3 × 0.3 × 0.1) mm3 were collected with a CAD-4 (Enraf Nonius) diffractometer using the Mo Kα radiation (λ = 0.71073 Å). The crystal structure was solved by direct methods using SHELXS-97 [5]. Full-matrix least-squares refinement and subsequent Fourier synthesis procedures were performed using SHELXL-97 [6]. Molecular graphics were prepared using Diamond 3 [7]. CCDC-1008226 contains the supplementary crystallographic data for this compound. These data can be obtained free of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK.
Hydrogen atoms were located at their idealized positions using appropriate HFIX instructions in SHELXL-97 and included in subsequent least-squares refinement cycles in riding-motion approximation. Crystal data and parameters of the final refinement are reported in Table 1.
3.2. Crystal Morphology
Crystal morphology is a key element in many industrial processes and has an enormous impact in the materials processing stages. Thus, rationalization of the relationships between crystal morphology and the arrangement of atoms in the bulk crystal lattice is of great interest in many areas of science.
The crystal morphology prediction was obtained by BFDH (Bravais-Fridel and Donnay-Harker) [8–10] algorithm calculation using Mercury (CSD 3.0.1) [11].
The program uses the crystal lattice parameters and the symmetry space group to generate a list of possible growth faces and their relative growth rates.
The qualitative analysis result obtained by energy dispersive X-ray spectroscopy (EDX) is presented in Figure 1. It reveals the presence of the chemical elements identified by the single crystal X-ray diffraction.
The scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) high resolution images of the surface topography produces an image of the focused crystal. The view of the observed and calculated crystal morphologies reveals a similarity between the two shapes (Figure 2). This result allows identifying the crystallographic axis and shows the absence of preferential orientation of crystallites.
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3.3. Structure Description
At room temperature, the present compound crystallizes in the monoclinic P21/c space group. The asymmetric unit contains two bromobismuthate [Bi2Br10]4− anions and [C10H28N4]4+ cation, as shown in Figure 3.
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The organic and inorganic moieties are linked by N–H⋯Br hydrogen bonds ensuring the structure cohesion.
The 1 : 5 stoichiometry of anionic part [Bi2Br10]4− can be realized by different types of anionic sublattices.
In this compound, two BiBr6 octahedra share two bridging Br atoms and consequently form dinuclear [Bi2Br10]4− anion. One bismuth (III) ion is surrounded by six bromine anions; however, Bi–Br distances fall into two ranges: 2.793(2) to 2.894(2) Å for terminal Br and 2.981(4) to 3.079(2) Å for the bridging ones (Table 2). Lower than the sum of Van der Waals radii (4.35 Å according Pauling [12]), we deduce that the bismuth-bromine bonds have a dominant covalent character.
The BiBr6 octahedra are somewhat distorted. As described by Shannon [13], the distortion index of this polyhedra ( = 1.88 10−3) indicates a significant dissymmetry in the dinuclear entity [Bi2Br10]4− where is the average value of the Bi–Br bond length. The bond angles values (see Table 3) confirm the octahedral distortion since they are, in some cases, 10° less than the ideal values.
The intermolecular N–H⋯Br hydrogen bonds can be also the reason of geometrical distortion of [Bi2Br10]4− anion, due to possibility of shifting of the halogen atoms in the direction of the positive charge located on the cations.
The protonation of [C10H28N4] leads to [C10H28N4]4+. These cations are stacked between the planes containing the bioctahedrons [Bi2Br10]4−.
Each bioctahedron is sandwiched between two sheets of organic cations, setting out its 6 vertices Br− where three links to the higher plane and the others with the lower one by hydrogen bonds involve the terminal amino group of the organic cations.
Hydrogen bonds are linking the organic and inorganic moieties (Table 3). This fact can explain the observed fragility of the crystals. The hydrogen bonds ensure the crystal cohesion by connecting the alternating organic-inorganic layers and building a three-dimensional framework.
3.4. Infrared Spectroscopy
The IR spectrum of C10H28N4Bi2Br10 (Figure 4 and Table 4) was recorded at room temperature in the range of 400 to 4000 cm−1 using the VERTEX 80/80 v FT-IR research spectrometer, by dispersing 2% of the studied compound in KBr discs. We have calculated the vibrational spectrum by using semiempirical PM3 geometry optimization by “CAChe” program [14].
After an optimization of the molecular configuration, the calculated spectrum, presented in Figure 5, is very helpful for the attribution of the observed spectroscopic bands. On the other hand, the observed bands assignment becomes easier by comparing the observed frequencies and those calculated.
Based on the previous literature results and the theoretical simulation of the IR spectrum, the large band around 3470 cm−1 is attributed to the stretching modes of (N–H) in the amine group. The out of plane bending mode of this group is probably responsible for the band located at 1640 cm−1. The (C–H) stretching of methylene group is centered on 2925 cm−1.
The band around 1389 cm−1 is probably the result of the bending vibration of (C–H) and the stretching of (C–C). The stretching modes of (C–N) are probably observed around 1114 cm−1.
3.5. Thermal Properties
The differential scanning calorimetry (DSC) thermogram, shown in Figure 6 was performed with a DSC EVO131 instrument under nitrogen atmosphere. Firstly, no phase transition was observed when the sample was under cooling until −45°C; then, during heating from −45 to 20°C, we observed an energetic effect that reveals a phase transition in the temperature range of (−17/−12)°C accompanied by a significant enthalpy transition () evaluated at 2.732 Jg−1. Comparing the heating and the cooling curves, the observed phenomenon seems to be irreversible. Further treatments are scheduled to know the structural behaviour of the resulting compound after the DSC experiment.
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3.6. X-Ray Powder Diffraction
The X-ray powder diffraction technique (XRD) was used to control the crystalline phase purity. The diffraction pattern was obtained on a D8 ADVANCE Bruker diffractometer with a Lynxeye accelerator using Cu (Kα1/α2 = 1.54060/1.54439 Å) wavelength. The measurement was performed in spinning mode (60 tr·mn−1) in order to minimize the preferential orientation effect of the crystallites with step-scanning (Δ2θ = 0.02°) constant time interval of 0.1 s. The quantitative criteria of goodness of fits are the following agreement factors: where and are the observed and calculated intensities at the ith step in the pattern, respectively. is the reciprocal of the variance of each observation; the summation is carried out over all the observations and “” is a scale factor.
The refinements were carried out using TOPAS program [15].
The basic structural model for the C10H28N4Bi2Br10 was taken from Topa et al. output [16]. Details of the refinement are given in Table 5.
Figure 7 shows good agreement between the observed and calculated XRD patterns which confirms the crystalline purity of the prepared compound with an experimental error of 3% of mass. Furthermore, the XRPD raw diffraction is marked with the presence of a large hump centered around 2θ = 13°, signature of an amorphous part. The TOPAS degree of crystallinity calculation gives about 86%.
3.7. The HOMO-LUMO Gap
Crystalline materials can be classified according to their band gap. The C10H28N4Bi2Br10 exhibits absorption bands around eV calculated by the program “CAChe” using the semiempirical PM3 method and corresponding to the electronic transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied one (LUMO). The atomic orbital compositions of the frontier molecular orbital are sketched in Figure 8.
We can note the HOMO contribution coming from the halogen. In the LUMO, the percentage of contribution from the bromine atoms is dominant. These calculations show that the electronic properties are roughly imposed by the inorganic part. The Bi coordination geometry is the dominant structural factor influencing the electronic structure of studied compound [17].
4. Conclusion
The present paper has shown that the new organic-inorganic hybrid C10H28N4Bi2Br10 was synthesized by slow evaporation. Its structure is built up by dibutylpyrazinium dications and discrete (0-D) bromobismuthate anions. Several experimental techniques have been used to characterize the new compound.
The crystal structure was solved by single crystal X-ray diffraction. The vibrational properties were studied by Raman scattering and infrared spectroscopy; the crystal morphology was carries out using the Bravais-Freidel and Donnay-Harker model, and the X-ray powder diffraction measurement was carried out to check the title compound’s purity.
The crystal structure of C10H28N4Bi2Br10 consists of discrete [Bi2Br10]4− anions with dinuclear geometry of two BiBr6 octahedra sharing two bridging Br atoms and [C10H28N4]4+. The cohesion is assumed by hydrogen bonds. The title compound undergoes one low-temperature phase transitions at −17°C identified by differential scanning calorimetry.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.