# Synthesis and Structural Studies of a New Complex of Di[hexabromobismuthate (III)] 2,5-Propylaminepyrazinium [C10H28N4]Bi2Br10

Authors: Mohamed El Mehdi Touati; Habib Boughzala
Journal: Advances in Chemistry
 (2015)

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## 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.


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## Body

## 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 formulaR+a  (MbX3b+a)-a (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 F2 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 Table1.Table 1
Experimental data for C10H28N4Bi2Br10.
Crystal data
Data collection
Empirical formula
C10H28N4Bi2Br10
Diffractometer
Enraf-Nonius CAD-4
Formula weight
1417.42 (g·mol−1)
Wavelength:λ
0.71073 Å
Crystal system
Monoclinic
θ range
2.55° ≤θ ≤ 26.97°
Space group
P21/c
Temperature
298 (2) K
Unit celldimensions
a = 11.410 (4) Åb = 11.284 (4) Åc = 12.599 (3) Åβ = 115.93 (2)°
Limiting indicesh, k, and l
−14 ≤h ≤ 130 ≤ k ≤ 140 ≤ l ≤ 16
Crystal habit
Yellow prism
Absorption correction
Psi scan
Volume
V = 1458.8 (8) Å3
Standard reflection
2 every 120 min
Z
Z = 4
Measured reflections
3697
Absorptioncoefficient
µ = 25.75 mm−1
Independent reflections
3177
F(000)
1264
Observed reflectionsI > 2σ(I)
2352
Crystal size (mm3)
(0.3× 0.3 × 0.1)
R
int
⁡
0.062
Densitycalc
D
calc = 3.236
T
min
⁡/Tmax⁡
0.001/0.074
Refinement
Refinement
Least square onF2
Δ
ρ
max
⁡
4.95 eÅ−3
R
[
F
2 > 2σ(F2)]
0.066
Δ
ρ
min
⁡
−7,09 eÅ−3
S
1.06
wR
0.169


### 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 Figure1. It reveals the presence of the chemical elements identified by the single crystal X-ray diffraction.Figure 1
Qualitative analysis by EDX of C10H28N4Bi2Br10.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 (Figure2). This result allows identifying the crystallographic axis and shows the absence of preferential orientation of crystallites.Observed and calculated morphologies of C10H28N4Bi2Br10 crystals.
(a)
(b)


### 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.ORTEP of the inorganic part [Bi2Br10]4− in (a) and the organic moiety [C10H28N4]4+ in (b) with 50% of probability level.
(a)
(b)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.Table 2
Selected interatomic distances (Å) and angles (°) in the structure of C10H28N4Bi2Br10.
(a)
Octahedra BiBr6
Bi–Br4
2.744 (2)
Br5–Bi–Br2
169.15 (6)
Bi–Br3
2.748 (3)
Br4–Bi–Br1
98.30 (6)
Bi–Br5
2.793 (2)
Br3–Bi–Br1
170.84 (6)
Bi–Br2
2.894 (2)
Br5–Bi–Br1
84.48 (7)
Bi–Br1
2.981 (4)
Br2–Bi–Br1
91.96 (6)
Bi–Br1i
3.079 (2)
Br4–Bi–Br1i
170.99 (5)
Br1–Bii
3.079 (2)
Br3–Bi–Br1i
82.72 (6)
Br4–Bi–Br3
90.61 (7)
Br5–Bi–Br1i
90.14 (6)
Br4–Bi–Br5
84.30 (6)
Br2–Bi–Br1i
100.00 (6)
Br3–Bi–Br5
94.36 (7)
Br1–Bi–Br1i
88.19 (5)
Br4–Bi–Br2
86.09 (6)
Bi–Br1–Bii
91.81 (5)
Br3–Bi–Br2
90.75 (7)
(b)
Organic group
C1–N2
1.490 (2)
N2–C1–C2
111.30 (1)
C1–C2
1.500 (2)
C1–C2–C3
109.00 (1)
C2–C3
1.500 (2)
C2–C3–N1
113.30 (1)
C3–N1
1.501 (2)
N1–C4–C5ii
113.60 (1)
C4–N1
1.454 (2)
N1–C5–C4ii
112.50 (1)
C4–C5ii
1.520 (2)
C4–N1–C3
115.10 (1)
C5–N1
1.510 (2)
C4–N1–C5
110.00 (1)
C5–C4ii
1.520 (2)
C3–N1–C5
109.50 (1)
Symmetry codes: (i) –x + 1, -y, -z + 2; (ii) –x + 1, -y, z + 1.The BiBr6 octahedra are somewhat distorted. As described by Shannon [13], the distortion index of this polyhedra (ID(Bi–Br) = 1.88 10−3) indicates a significant dissymmetry in the dinuclear entity [Bi2Br10]4−(1)WithIDBi–Br=16∑i=16BiBri-BiBrmBiBrm2,
where BiBrm 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.Table 3
Hydrogen bonds for C10H28N4Bi2Br10 (D: donor; A: acceptor).
D–H⋯A
D–H (Å)
H⋯A (Å)
D⋯A (Å)
D–H⋯A (°)
N1–H1⋯Br5
0.909
2.779 (3)
3.534 (1)
141.27 (7)
N2–HA⋯Br3
0.890
2.703 (2)
3.537 (2)
156.38 (1)
N2–HB⋯Br2
0.890
2.792 (2)
3.521 (2)
149.03 (1)
N2–HB⋯Br4
0.890
2.829 (2)
3.487 (2)
131.92 (1)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 (Table3). 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].Table 4
Infrared bands observed and assigned to vibration modes for C10H28N4Bi2Br10.
Observed frequencies (cm−1)
Attributions
3470
Stretching (N–H)
2925
Stretching (C–H)
1640
Bending (N–H)
1389
Bending (C–H) and stretching (C–C)
1114
Stretching (C–N)Figure 4
Observed IR spectrum of the compound C10H28N4Bi2Br10.After an optimization of the molecular configuration, the calculated spectrum, presented in Figure5, 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.Figure 5
Calculated IR spectrum of the compound C10H28N4Bi2Br10.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 Figure6 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 (ΔH) 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.Differential scanning calorimetry curves of C10H28N4Bi2Br10.
(a)
(b)
(c)


### 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 R factors:
(2)Rp=∑Yi·obs-1/cYi(cal)∑Yi·obs,Rwp=∑wiYi·obs-1/cYi·(cal)2wiYi·obs2,GOF=∑wiYi·obs-1/cYi·(cal)2N-p,
where Yiobs and Yi(cal) are the observed and calculated intensities at the ith step in the pattern, respectively. wi is the reciprocal of the variance of each observation; the summation is carried out over all the observations and “c” 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.Table 5
Unit cell parameters and details of Rietveld refinement of C10H28N4Bi2Br10.
Formula
C10H28N4Bi2Br10
System
Monoclinic
Space group
P21/c
Z
4
Unit cell
a/Å
11.387 (1)
b/Å
11.260 (1)
c/Å
12.577 (1)
β/(°)
115.925 (3)
Volume/Å3
1450.45 (2)
Density
3.245 (5)
Zero point 2θ/(°)
0.0182 (8)
Reliability factors (%)
Rp
3.85
Rwp
5.89
RB
4.917
RF
2.22Figure7 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%.Figure 7
Experimental and calculated X-ray diffraction patterns and their difference for C10H28N4Bi2Br10.


### 3.7. The HOMO-LUMO Gap

Crystalline materials can be classified according to their band gap. The C10H28N4Bi2Br10 exhibits absorption bands around (-1,287)-(-9,179)=7,891 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.Figure 8
Calculated frontier molecular orbital of the title compound.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].


## 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 F2 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 Table1.Table 1
Experimental data for C10H28N4Bi2Br10.
Crystal data
Data collection
Empirical formula
C10H28N4Bi2Br10
Diffractometer
Enraf-Nonius CAD-4
Formula weight
1417.42 (g·mol−1)
Wavelength:λ
0.71073 Å
Crystal system
Monoclinic
θ range
2.55° ≤θ ≤ 26.97°
Space group
P21/c
Temperature
298 (2) K
Unit celldimensions
a = 11.410 (4) Åb = 11.284 (4) Åc = 12.599 (3) Åβ = 115.93 (2)°
Limiting indicesh, k, and l
−14 ≤h ≤ 130 ≤ k ≤ 140 ≤ l ≤ 16
Crystal habit
Yellow prism
Absorption correction
Psi scan
Volume
V = 1458.8 (8) Å3
Standard reflection
2 every 120 min
Z
Z = 4
Measured reflections
3697
Absorptioncoefficient
µ = 25.75 mm−1
Independent reflections
3177
F(000)
1264
Observed reflectionsI > 2σ(I)
2352
Crystal size (mm3)
(0.3× 0.3 × 0.1)
R
int
⁡
0.062
Densitycalc
D
calc = 3.236
T
min
⁡/Tmax⁡
0.001/0.074
Refinement
Refinement
Least square onF2
Δ
ρ
max
⁡
4.95 eÅ−3
R
[
F
2 > 2σ(F2)]
0.066
Δ
ρ
min
⁡
−7,09 eÅ−3
S
1.06
wR
0.169


## 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 Figure1. It reveals the presence of the chemical elements identified by the single crystal X-ray diffraction.Figure 1
Qualitative analysis by EDX of C10H28N4Bi2Br10.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 (Figure2). This result allows identifying the crystallographic axis and shows the absence of preferential orientation of crystallites.Observed and calculated morphologies of C10H28N4Bi2Br10 crystals.
(a)
(b)


## 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.ORTEP of the inorganic part [Bi2Br10]4− in (a) and the organic moiety [C10H28N4]4+ in (b) with 50% of probability level.
(a)
(b)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.Table 2
Selected interatomic distances (Å) and angles (°) in the structure of C10H28N4Bi2Br10.
(a)
Octahedra BiBr6
Bi–Br4
2.744 (2)
Br5–Bi–Br2
169.15 (6)
Bi–Br3
2.748 (3)
Br4–Bi–Br1
98.30 (6)
Bi–Br5
2.793 (2)
Br3–Bi–Br1
170.84 (6)
Bi–Br2
2.894 (2)
Br5–Bi–Br1
84.48 (7)
Bi–Br1
2.981 (4)
Br2–Bi–Br1
91.96 (6)
Bi–Br1i
3.079 (2)
Br4–Bi–Br1i
170.99 (5)
Br1–Bii
3.079 (2)
Br3–Bi–Br1i
82.72 (6)
Br4–Bi–Br3
90.61 (7)
Br5–Bi–Br1i
90.14 (6)
Br4–Bi–Br5
84.30 (6)
Br2–Bi–Br1i
100.00 (6)
Br3–Bi–Br5
94.36 (7)
Br1–Bi–Br1i
88.19 (5)
Br4–Bi–Br2
86.09 (6)
Bi–Br1–Bii
91.81 (5)
Br3–Bi–Br2
90.75 (7)
(b)
Organic group
C1–N2
1.490 (2)
N2–C1–C2
111.30 (1)
C1–C2
1.500 (2)
C1–C2–C3
109.00 (1)
C2–C3
1.500 (2)
C2–C3–N1
113.30 (1)
C3–N1
1.501 (2)
N1–C4–C5ii
113.60 (1)
C4–N1
1.454 (2)
N1–C5–C4ii
112.50 (1)
C4–C5ii
1.520 (2)
C4–N1–C3
115.10 (1)
C5–N1
1.510 (2)
C4–N1–C5
110.00 (1)
C5–C4ii
1.520 (2)
C3–N1–C5
109.50 (1)
Symmetry codes: (i) –x + 1, -y, -z + 2; (ii) –x + 1, -y, z + 1.The BiBr6 octahedra are somewhat distorted. As described by Shannon [13], the distortion index of this polyhedra (ID(Bi–Br) = 1.88 10−3) indicates a significant dissymmetry in the dinuclear entity [Bi2Br10]4−(1)WithIDBi–Br=16∑i=16BiBri-BiBrmBiBrm2,
where BiBrm 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.Table 3
Hydrogen bonds for C10H28N4Bi2Br10 (D: donor; A: acceptor).
D–H⋯A
D–H (Å)
H⋯A (Å)
D⋯A (Å)
D–H⋯A (°)
N1–H1⋯Br5
0.909
2.779 (3)
3.534 (1)
141.27 (7)
N2–HA⋯Br3
0.890
2.703 (2)
3.537 (2)
156.38 (1)
N2–HB⋯Br2
0.890
2.792 (2)
3.521 (2)
149.03 (1)
N2–HB⋯Br4
0.890
2.829 (2)
3.487 (2)
131.92 (1)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 (Table3). 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].Table 4
Infrared bands observed and assigned to vibration modes for C10H28N4Bi2Br10.
Observed frequencies (cm−1)
Attributions
3470
Stretching (N–H)
2925
Stretching (C–H)
1640
Bending (N–H)
1389
Bending (C–H) and stretching (C–C)
1114
Stretching (C–N)Figure 4
Observed IR spectrum of the compound C10H28N4Bi2Br10.After an optimization of the molecular configuration, the calculated spectrum, presented in Figure5, 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.Figure 5
Calculated IR spectrum of the compound C10H28N4Bi2Br10.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 Figure6 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 (ΔH) 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.Differential scanning calorimetry curves of C10H28N4Bi2Br10.
(a)
(b)
(c)


## 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 R factors:
(2)Rp=∑Yi·obs-1/cYi(cal)∑Yi·obs,Rwp=∑wiYi·obs-1/cYi·(cal)2wiYi·obs2,GOF=∑wiYi·obs-1/cYi·(cal)2N-p,
where Yiobs and Yi(cal) are the observed and calculated intensities at the ith step in the pattern, respectively. wi is the reciprocal of the variance of each observation; the summation is carried out over all the observations and “c” 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.Table 5
Unit cell parameters and details of Rietveld refinement of C10H28N4Bi2Br10.
Formula
C10H28N4Bi2Br10
System
Monoclinic
Space group
P21/c
Z
4
Unit cell
a/Å
11.387 (1)
b/Å
11.260 (1)
c/Å
12.577 (1)
β/(°)
115.925 (3)
Volume/Å3
1450.45 (2)
Density
3.245 (5)
Zero point 2θ/(°)
0.0182 (8)
Reliability factors (%)
Rp
3.85
Rwp
5.89
RB
4.917
RF
2.22Figure7 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%.Figure 7
Experimental and calculated X-ray diffraction patterns and their difference for C10H28N4Bi2Br10.


## 3.7. The HOMO-LUMO Gap

Crystalline materials can be classified according to their band gap. The C10H28N4Bi2Br10 exhibits absorption bands around (-1,287)-(-9,179)=7,891 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.Figure 8
Calculated frontier molecular orbital of the title compound.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.



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