Synthesis, Crystal Structure, and Characterization of a New Organic-Inorganic Hybrid Material: (H₃AEP)₂⋅(BiCl₆)⋅3Cl⋅2H₂O

Abstract

The title compound is an organic-inorganic hybrid material. The single crystal X-ray diffraction investigation reveals that the studied compound crystallizes in the orthorhombic system, space group Pbca with the following lattice parameters: 𝑎=12.108 (4) Å, 𝑏=19.403 (3) Å, 𝑐=24.933 (6) Å, and 𝑍=8. The crystal lattice is composed of a discrete (BiCl₆)³⁻ anion surrounded by piperazinium cations, chlorine anions, and water molecules. Complex hydrogen bonding interactions between (BiCl₆)³⁻, Cl⁻, organic cations, and water molecules form a three-dimensional network. Room temperature IR, Raman spectroscopy, and optical absorption of the title compound were recorded and analysed. The observed crystal morphology was compared to the simulated one using the Bravais-Friedel, Donnay-Harker model.

1. Introduction

Investigations of compounds, possessing organic cations and inorganic anions, evoke interesting properties because they provide original supramolecular networks. Heterocyclic cations and halobismuthates (III) strictly connected with the arrangement of hydrogen bond system generate interesting supramolecular assemblies [1]. The size of the organic cations, their symmetry, and ability to form the hydrogen bonds determine the physical properties in these materials (e.g., nonlinear optical properties, luminescence, conductivity etc.) [2–6]. The halobismuthate (III) family, in particular the chlorobismuthate, is composed of distorted (BiCl₆)³⁻ isolated octahedra or connected by corners, edges, or faces forming ribbons, plans, or layers. The cavities between inorganic moieties are filled by organic cations to anionic framework through hydrogen bonds and/or electrostatic interactions. To advance our understanding of the principals governing the structural properties, especially the role played by hydrogen bonds, we have replaced the alkyl ammonium cations used so far [7, 8] with those containing more nitrogen atoms in their asymmetric unit. In this case, we used the 1-(2-aminoethyl) piperazine (AEP), an electron donor, which is a derivative of piperazine and contains primary, secondary, and tertiary nitrogen atoms. We synthesized a new zero-dimensional organic-inorganic material (H₃AEP)₂⋅(BiCl₆)⋅3Cl⋅2H₂O. In this paper, we report the single crystal X-ray diffraction study, infrared spectroscopy (IR), Raman spectroscopy, and optical absorption measurements. The crystal shape morphology calculation using the Bravais-Friedel, Donnay-Harker (BFDH) model is also reported.

2. Experimental Details

2.1. Synthesis

Under ambient conditions, 2 mmol of BiCl₃ was dissolved in an aqueous solution of 1-(2-aminoethyl) piperazine with excess of HCl (to improve solubility). The mixture was stirred and kept at room temperature for several weeks. Bulks like orange crystals were obtained 6 months later.

2.2. X-Ray Data Collection

Data collection was performed with a CAD-4 Enraf-Nonius X-ray diffractometer [9] at 298 K with graphite monochromator using Mo Kα wavelength. An empirical psi-scan [10] absorption correction was applied. The structure was solved and refined by full-matrix least squares based on 𝐹² using SHELXS-97 and SHELXL-97 [11], respectively. The heavy atoms (Bi and Cl) were located first by Patterson method. The atoms of the organic part were positioned from Fourier difference maps after looking to the bond length calculations. The atomic positions were refined with anisotropic displacement parameters. Thus, abnormally large ellipsoids in C11 and N5 are observed. In by far most cases, this disorder affects small parts of molecules like organic side chains. Hydrogen coordinates were idealized using appropriate HFIX instructions and included in subsequent least-squares refinement cycles in riding-motion approximation. Molecular graphics were prepared using Diamond3 [12]. Crystal data and structure refinements details are reported in Table 1.

Table 1 

Crystal data and structure refinement details for (H₃AEP)₂·(BiCl₆)·3Cl·2H₂O.

2.3. Physicochemical Characterization Techniques

Raman scattering was exited at 488 nm and recorded at room temperature using a JOKIN-YVON (T64000) spectrometer.

Room temperature infrared absorption spectrum in 400–4000 cm⁻¹ frequency range was recorded on a Perkin Elmer Paragon 1000 Pc spectrometer by dispersing 2% of the studied compound in KBr discs.

Optical absorption spectrum of the (H₃AEP)₂  (BiCl₆)  3Cl2H₂O films was measured at room temperature using a UV-visible absorption spectrometer Perkin Elmer Lambda 45.

3. Results and Discussion

3.1. Structure Description

The crystal structure of the title compound is composed by structural units containing an isolated and distorted (BiCl₆)³⁻ octahedron, two independent organic cations, three unique chloride anions, and two independent water molecules (Figure 1) linked by hydrogen bonds to form a zero-dimensional organic-inorganic hybrid network (Figure 2). The bismuth (III) cation is in a distorted octahedral environment of chlorine anions with Cl-Bi-Cl angles ranging from 87.00 (6) to 95.77 (7)° for cis and 172.41 (7) to 176.60 (7)° for trans arrangements. The bond length for Bi--Cl in the (BiCl₆)³⁻ octahedron vary between 2.644 (2) and 2.781 (2) Å (Table 2). As already observed in the hexachlorobismuthates (III), the octahedral deformation of Bi (III) coordination is probably the partial effect of the stereo chemical activity of the Bi (III) 6s² lone electron pair (LEP) [16, 17].

Table 2 

Selected bond distances (Å) and angles (°) for (H₃AEP)₂·(BiCl₆)·3Cl·2H₂O.

Figure 1 

ORTEP of asymmetric unit of (H3AEP)2⋅(BiCl6)⋅3Cl⋅2H2O. Hydrogen atoms are omitted for clarity.

Figure 2 

Arrangement of cluster anions and cations in (H3AEP)2⋅(BiCl6)⋅3Cl⋅2H2O.

The charge of the inorganic moiety is balanced by triply protonated 1-(2-aminoethyl) piperazine (H₃AEP) molecules and chloride anions Cl⁻ located in the empty space.

In the H₃AEP molecule the piperazinium rings adopt the chair conformation. The ethylamine side chain presents two different conformations “A” and “B” (Figure 3(a)). In fact, they are arranged in pairs with two different orientations of the piperazinium rings and form alternate planes (Figure 3(b)).

(a)

(b)

Figure 3 

Configurations and arrangement of organic molecules. Hydrogen atoms are omitted for clarity.

Furthermore, the “A” conformation forms planes at 𝑧=0 and 𝑧=0.5, and the other “B” forms planes at 𝑧=0.25 and 0.75 (Figure 3(b)). These planes are connected through hydrogen bonds N--H⋯Cl via the free chlorine anions Cl7, Cl9, and Cl8, as shown in Figure 4.

Figure 4 

A view showing the layer propagated by water molecules and chlorine anions via O--H⋯N, O--H⋯Cl, and N--H⋯Cl hydrogen bonds (hydrogen bonds shown as dashed lines).

The observed distortion of the linear part in the “A” configuration is probably due to the presence of the water molecule (O2) near the terminal ammonium group. On the other hand, in the “B” conformation, the water molecule O1 is located near the cyclic part (Figure 4). Consequently, the dihedral angles between two planes containing N2--C5--C6 and N3--C5--C6 is 74.37° for “A” and 4.17° between N5--C11--C12 and C6--C12--C11 for “B.”

The water molecules insure the cohesion between the organic cations and the chlorine anions via strong N--H⋯O, O--HW⋯Cl hydrogen bonds. The (BiCl₆)³⁻ clusters are connected to the organic moiety through O--HW⋯Cl hydrogen bonds (via water molecules) and by N--H⋯Cl bonds (Table 3) giving up a three-dimensional network, as shown in Figure 5.

Table 3 

Hydrogen bonds for (H₃AEP)₂·(BiCl₆)·3Cl·2H₂O (Å, °).

Figure 5 

Hydrogen bonding schemes between oxygen water molecules, organic cations, and (BiCl6)3− octahedral.

3.2. Crystal Morphology

Bulk crystals of the title compound were grown from aqueous solution by a slow-evaporation at room temperature. Orange crystals of average size 0.4×0.4×0.5 mm were obtained. The crystal morphology prediction, obtained by BFDH (Bravais-Fridel and Donnay-Harker) [18–20] algorithm using Mercury (CSD 3.0.1) program [21], is very close to the experimental observation (Figure 6), predicting that the dominating faces are (002), (020) faces as well as (111), (102), and (021).

(a)  

(b)

Figure 6 

Good correlation between estimated morphology with the BFDH models (a) and the bulk single crystal (H3AEP)2⋅(BiCl6)⋅3Cl⋅2H2O(b).

3.3. Vibrational Study

The Infrared spectrum of the title compound is shown in Figure 7. Using previous works reported in the literature on similar compounds [22, 23], we propose in Table 4 an attempt of assignment of the main bands. The high frequencies domain in the spectrum is characterized by N--H and O--H stretching modes, combination bands, and harmonics, while the lower one corresponds to the bending and to the external modes. However, the band at 3437 and 3096 cm⁻¹ corresponds to the stretching asymmetric and symmetric vibrations of the N--H bond respectively. The bending vibration of N--H bond is located at 1571 cm⁻¹. The board band at 3005 cm⁻¹ corresponds to the O--H stretching vibration modes. The C--H stretching mode is superimposed in 2821 cm⁻¹ and 2703 cm⁻¹. The 1381, 1034, and 967 cm⁻¹ bands were associated with the asymmetric and symmetric stretching and the bending of C--C, group respectively. The absorption bands located at 1230 and 764 cm⁻¹ are assigned to the stretching and bending modes of C--N bonds. Finally, the absorption band at 596 cm⁻¹ is associated with the N--C--C--N stretching mode.

Table 4 

Wavenumbers (cm⁻¹) and relative assignments of the observed bands in the infrared and Raman spectrum of (H₃AEP)₂·(BiCl₆)·3Cl·2H₂O.

Figure 7 

Infrared spectrum of (H3AEP)2⋅(BiCl6)⋅3Cl⋅2H2O in the 500–4000 cm−1 range at room temperature.

The Raman spectrum of the title compound recorded at low frequencies between 0 and 500 cm⁻¹ is shown in Figure 8. The bands at 249 cm⁻¹ and 211 cm⁻¹ are assigned to the Bi--Cl external antisymmetric and symmetric stretching, respectively. Raman line at 101 cm⁻¹ most likely corresponds to the Bi--Cl deformation vibration. These assignments, presented in Table 4, are essentially based on comparison with the literature for numerous chlorobismuthates (III) compounds [24, 25].

Figure 8 

Low wavenumber range of (H3AEP)2⋅(BiCl6)⋅3Cl⋅2H2O at room temperature.

3.4. Optical Study

The optical absorption spectrum of the title compound films measured at room temperature and shown in Figure 9 exhibits two distinct absorption bands centred at 274 and 346 nm similar to those reported for the organic-inorganic hybrid type films [26, 27]. The strongest absorption is somewhat clipped, this is due to the saturation of the detector, producing an overflow. In the halogenobismuthates (III), the absorption spectrum exhibits several bands around 300–400 nm [14, 28]. Earlier, suggestions in the literature assigned these bands mostly to Metal-Centred (MC) transitions [29], but recent studies proved that some of these bands, especially those of higher energies, can rather be assigned partially to ligand to metal charge transfer (LMCT) transition [30, 31].

Figure 9 

Diffuse reflectance UV-visible absorption spectra for (H3AEP)2⋅(BiCl6)⋅3Cl⋅2H2O.

Figure 10(a) shows a schematic molecular orbital energy level diagram for the (BiX₆)³⁻ ions. On Bi(III), the Highest occupied molecular orbital (HOMO) is predominately 6s² and the lowest unoccupied molecular orbital (LUMO) is predominately 6p. Figure 10(b) illustrates the energy level correlation between Bi(III) atomic states and the 6s6p MC states for the (BiX₆)³⁻ ions in octahedral (O_h) symmetry. According to the literature, we note that replacing the halo ligands from chloride to bromide then iodide shifts the corresponding bands towards the longer wavelengths (Table 5).

Table 5 

Optic absorption bands of some halobismuthates (III) complexes.

(a)

(b)

Figure 10 

(a) Schematic molecular orbital energy level diagram. (b) Energy level correlation between atomic states and MC 6s to 6p transition for the (BiX6)3− ions assuming Oh symmetry.

A close examination of the absorption spectrum for the title compound shows that the first band at 274 nm corresponding to the higher energy band should be assigned to the (LMTC) transition from the np (t_1gt_2gt_1ut_2u) orbital of Cl to 6p (t_1u) orbital of Bi(III), whereas the second band at 346 nm corresponding to the lowest energy band ascribes to the 6s6p MC transition from the 6s² (a_1g) to the T_1u state which correlates with the ³P₁ atomic state in Bi(III).

4. Conclusion

The zero-dimensional organic/inorganic hybrid compound (H₃AEP)₂⋅(BiCl₆)⋅3Cl⋅2H₂O structure consists of isolated (BiCl₆)³⁻ entities surrounded by 1-(2-ammoniumethyl) piperazinium cations, chlorine anions, and crystallization water molecules. The cations are bonded to the water molecules by strong hydrogen bonds. The chlorobismuthates inorganic parts (BiCl₆)³⁻ are linked to organic cations by hydrogen bonds involving water molecules and Cl⁻ anions, forming a self-assembled zero-dimensional structure. The vibrational properties of the title compound were studied by Raman scattering, UV-visible, and infrared spectroscopy. Estimated morphology with the BFDH models is in agreement with the observed crystal shape.