Introduction

Hybrid organic–inorganic materials with distinctive properties have a great importance from combining both organic and inorganic components into the material [1,2,3,4]. They are characterized by a large variety of structures framework dimensionality build from chains, layers, or three-dimensional connections [5, 6] as well as a correlation between the structural features and a vast diversity of physical properties from their potential applications such as magnetic, optical, luminescence and electric [7,8,9,10,11]. Hybrid halogen-based materials have demonstrated exciting properties from the renewal of zero-dimensional structures belonging to alums and Tutton’s salts [12,13,14,15]. In addition, Tellurium metalloid, as a strong Lewis acid, has an extensive coordination chemistry [16,17,18]. A large number of salts containing tellurium halides octahedra anions [TeX6]2− (X = Cl, Br) has been prepared and structurally characterized [19, 20]. Besides, hybrids based on tellurium have caught attention as promising materials due not only to their interesting structural topologies, but also to their physical properties such as luminescence, nonlinear optical activity, ferroelectricity and semi-conductivity [21,22,23]. The introduction of tellurium in metal–organic frameworks as clusters [24, 25] or tellurium-coordination polymers [26] allows a better correlation between structural features and physical characteristics [27,28,29,30,31]. In the organic part, the use of amines as template, generally protonated, has provided many types of hybrid materials, which could exhibit interesting optical properties [32, 33]. In almost all of these materials, the amine cations interact with the inorganic part, through weak hydrogen bonds, to give supramolecular network. However, it seems that the amine group may have an influence on physical property. We recently showed that synthesized bromotellurate salt of 4-(ethylammoniummethyl)pyridinium, in its solid state, showed a maximum UV–visible absorption and photoluminescence emission based on that of the organic part [34]. The aim of this study is to introduce the influence of 2-methylimidazole diamine that includes delocalization of electrons from aromaticity in this class of materials to further explore interesting optical properties. The peculiarity of the used 2-methylimidazole, C4H6N2, is that it can be described as heterocyclic amine soluble in water characterized by the presence of a methyl group linked to an aromatic five-heterocycle which contains two nitrogen atoms. In the present study, the crystal structure of (C4H7N2)2TeBr6 is described in detail. An interest for the significant effect of the aromatic diamine template is given. We performed Hirshfeld surface analysis to further investigate and explain intermolecular interactions in the structure.

Experimental

Synthesis

The ligand 2-methylimidazole (0.082 g, 2 mmol) was added to an aqueous solution (10 ml) of Te(OH)6 (0.23 g, 1 mmol) which were dissolved in a concentrated hydrobromic acid HBr (3 ml) as shown in the following reaction:

$$ {2}\left( {{\text{C}}_{{4}} {\text{H}}_{{6}} {\text{N}}_{{2}} } \right) \, + {\text{ Te}}\left( {{\text{OH}}} \right)_{{6}} + {\text{ 6HBr}} \mathop{\longrightarrow}\limits^{{{\text{H}}_{{2}} {\text{O}}}} \left( {{\text{C}}_{{4}} {\text{H}}_{{7}} {\text{N}}_{{2}} } \right)_{{2}} {\text{TeBr}}_{{6}} $$

The resulting orange solution was stirred for about 10 min and then left to slowly concentrate by solvent evaporation at room temperature for a period of 10 days. Well-formed orange parallelepiped crystals of the product appeared and were collected by filtration.

Single-crystal data collection and structure determination

A tiny single crystal was carefully chosen from the available crystals under a polarizing microscope to perform its structural analysis by X-ray diffraction. Diffraction data were collected on a Bruker APEX II CCD diffractometer, using graphite-monochromated MoKa radiation (0.71073 Å) at 296 K. Intensity measurements were made between angles, namely 2, 6 and 30°, giving rise to the following miller indices: − 24 ≤ h ≤ 25; − 12 ≤ k ≤ 12; − 19 ≤ l ≤ 17. The structure was solved by direct methods using the SHELXS-2014 program [35]. However, the positions of the tellurium atoms were determined through a three-dimensional Patterson synthesis. Bromine, carbon, nitrogen and hydrogen atoms were located by a three-dimensional Fourier function. Structure solution and refinement were carried out using SHELX programs [36] and refined by full-matrix least squares methods on F2 with all non-hydrogen atoms anisotropic SHELXL-2014 [37]. The non-hydrogen atoms were refined anisotropically. The hydrogen atoms were attributed to isotropic thermal factors close to those of the atoms to which they are linked. The hydrogen atoms of the CH, NH and NH2 groups were fixed geometrically by the appropriate instructions of the program (AFIX 43, AFIX 43 and AFIX 93, respectively). The H-atom of the N is located by difference Fourier synthesis. A final refinement on F2 with 2783 unique intensities and 89 parameters converged at wR(F2) = 0.084 (R(F) = 0.063) for 1369 observed reflections with I > 2σ (I). Calculations were performed using the WinGX crystallographic software package [38]. The crystal data collection and structure refinement results of the compound are given in Table 1. The structural graph was created with Diamond program [39].

Table 1 Crystal data and experimental parameters used for the intensity data collection strategy and final results of the structure determination
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X-ray powder diffraction

X-ray powder diffraction pattern of ground crystals of the compound (C4H7N2)2TeBr6 was recorded with a PANalytical θ/θ Bragg–Brentano Empyrean diffractometer (CuKα1+2 radiations) equipped with the PIXcel1D detector. Data were collected in the [5–100°] 2θ scattering angle range with a 0.013° step and 30 s per step for a total acquisition of 15 min.

Hirshfeld surface calculations

To define the space occupied by molecules that come into contact in the crystal, we have required the Hirshfeld surface were constructed from CIF file to the analysis of the crystal structures generated by the Crystal Explorer [40] program. It allows the visualization of the different types of intermolecular contacts in the crystal by focusing on close ones between atoms in neighboring molecules. The Hirshfeld surface was computed around the asymmetric unit using the normalized contact distance surface (dnorm). It can be generated into 2D fingerprint plots, which is a two-dimensional summary of intermolecular interactions that we take into account in the crystal. The dnorm is given by the following equation:

$$ d_{{{\text{norm}}}} = \frac{{d_{{\text{i}}} - r_{{\text{i}}}^{{v{\text{dw}}}} }}{{r_{{\text{i}}}^{{v{\text{dw}}}} }} + \frac{{d_{{\text{e}}} - r_{{\text{e}}}^{{v{\text{dw}}}} }}{{r_{{\text{e}}}^{{v{\text{dw}}}} }} $$

while di and de are the distances to the nearest atoms inside (di) and outside (de) the surface, while ridvW and redvW are the van der Waals radii of the appropriate atoms internal or external to the surface [41]. The normalized contact distance is displayed using a red-white-blue color scheme; the red color is used for highlighting shorter contacts, white for contacts around the vdW separation and blue for longer contacts [42]. Two additional colored properties (shape index and curvedness) based on the local curvature of the surface can also be specified [43]. As shown in this paper, the Hirshfeld surfaces are mapped with dnorm, shape-index, curvedness and 2D fingerprint plots (full and resolved) were performed by the Crystal-Explorer program [44].

Spectroscopic measurements

Fourier transform infrared (FT-IR) measurements were performed at room temperature on a Perkin-Elmer FT-IR Paragon 1000 PC spectrometer over the 4000–400 cm−1 region. Samples were prepared in KBr pellets. As for the Raman scattering spectrum, it was recorded using a T-64000 Raman spectrometer (ISA, JobinYvon) with standard attachments. The beam sources were argon krypton titan-sapphire and semi-conductor lasers. The spectrum was recorded in the range of 50 cm−1 to 4000 cm−1. Infrared and Raman spectra are recorded with the same resolution of 3 cm−1.

Scanning electronic microscopy (SEM)

SEM images of (C4H7N2)2TeBr6 were obtained using a JEOL microscope (JSM 6510 LV). Acceleration voltages varied between 20 and 30 kV as a function of the analyzed samples. Elementary quantitative microanalyses were performed using an energy-dispersive X-ray (EDX) OXFORD detector (Aztec software).

Results and discussion

Preliminary characterizations

Crystals taken from the stock solution, washed with ethanol and dried were studied by energy-dispersive X-ray (EDX). The data collected by this technique clearly reveal the organic/inorganic nature of the analyzed compound (Fig. 1). The deduced composition is only used as a guide to confirm the presence of tellurium, bromine and nitrogen. In addition to the DRX data of the single crystal, we can affirm that it is a hybrid material.

Fig. 1
figure 1

Analysis of a single crystal of (C4H7N2)2TeBr6 by EDX and morphology of the analyzed crystal

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The recording of the X-ray powder diffraction pattern of the compound was carried out. The comparison between experimental pattern and theoretical pattern generated from the crystal structure determined by DRX from single crystal at room temperature (Fig. 2) shows that the sample of the synthesized hybrid compound presents a single phase.

Fig. 2
figure 2

X-ray powder diffraction patterns of (C4H7N2)2TeBr6. Red: simulated from the structure determined by DRX of the single crystal. black: experimental (color figure online)

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Crystallographic study

The crystal structure of the title compound (C4H7N2)2TeBr6 has been solved and refined in the centrosymmetric monoclinic space group C2/c, with eight formula cell unit (Z = 8). The different parameters of the crystallographic unit cell have the values of: a = 18.577(2)Å, b = 9.1497(10)Å, c = 13.5355 (17)Å, β = 123.835(5)°, and V = 1911.1(4)Å3. Further details are reported in Table 1.

The asymmetric part of the unit cell of the title compound consists of one 2-methylimidazolium [C4H7N2]2+ cation and one hexabromotellurate (IV) anion [TeBr6]2− (Fig. 3). The Te atom is sixfold coordinated by bromide ions, forming an octahedral polyhedron. Thus, to ensure charge equilibrium, the structure associates each hexabromotellurate (IV) anions with one 2-methylimidazole cation.

Fig. 3
figure 3

Asymmetric unit of the (C4H7N2)2 TeBr6 crystal

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The atomic arrangement in the structure of the title material (Fig. 1S), according to the (a, c) plan, shows an alternation of organic cations and mineral anions along the crystallographic [101] direction. Octahedral inorganic [TeBr6]2− entities are arranged along the crystallographic c axis and connected with protonated amines through N–H…Br hydrogen bonds, while the organic cations [C4H7N2]2+ are interconnected in the same direction by the π stacking between their aromatic rings in such a way to neutralize the negative charge of the inorganic part. The perspective view of the atomic arrangement of (C4H7N2)2TeBr6 compound is shown in Fig. 4. Actually, the structure is composed of parallel inorganic parts alternated by organic chains. The organic cations trapped in the spacing between the inorganic parts and connection between all molecules are made by hydrogen bonds to from an infinite zero-dimensional network. The supramolecular crystal packing is built by N–H…Br hydrogen bonds varying between 3.490 and 3.682 Å (Table 2). The anionic polyhedron [TeBr6]2− exhibits a slightly distorted octahedral coordination environment with Te–Br band lengths ranging from 2.6361 (11) Å to 2.7476 (11) Å and Br−Te−Br bans angles fall in the range of 88.58°(2) and 180° (Table 1S). The small differences of the Te–Br bond lengths are not insignificant considering the standard deviations. There appears to be no stereochemical active lone pair of electrons at Te (IV) in the hexahalogeno complex ion [45,46,47,48,49].

Fig. 4
figure 4

Hydrogen bonds between chlorine and organic entities in the crystal structure of (C4H7N2)2 TeBr6 in projection along the a-axis. (H-bonds are represented by dashed lines)

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Table 2 Hydrogen-bonds geometry (Ǻ)
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The examination of the geometric features of the organic molecule exhibits a regular spatial configuration with C–C, C–N distances and C–C–N, C–N–C, N–C–C and N–C–N angles, quite like those found in other compounds [50, 51]. The C–N band lengths vary from 1.305(8) to 1.366(8)Å, and those of C–C band vary from 1.323(10) to 1.476(9)Å and C–C–N, C–N–C, N–C–C and N–C–N angles are between 106.3°(6) and 127.2°(7) (Table 1S). The distance between two aromatic nuclei (centroids) was 3.656 Å < 3.8 Å (Fig. 5) which confirms that the π–π interaction is present in this compound [52].

Fig. 5
figure 5

View of the p...p interactions between ring centroids

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Molecular Hirshfeld surface analysis

The Hirshfeld surfaces have been made to explore the type of interactions, such as (C…H, H…H, C…C etc.) or hydrogen bonds and to study the various intermolecular interactions that play an important role in a molecular crystal. The molecular Hirshfeld surface, 3D dnorm, shape index and curvedness for (C4H7N2)2TeBr6 are illustrated in Fig. 6, showing surfaces that have been napped over dnorm ranging from − 0.246 to 1.026 Å, shape index ranging from − 1.000 to 1000 Å, and curvedness ranging from − 4.000 to 0.400 Å, respectively. The dnorm mapping indicates that strong hydrogen bond interactions, such as N–H…Br hydrogen bonding between amino group and bromide, appear as the main interaction between the complexes and are seen as a bright red area in the Hirshfeld surfaces (Fig. 6a). The brightest and largest red spots observed in the dnorm refer to the shorted intermolecular contacts indicating the existence of hydrogen bonds N–H…Br in the crystal packing. The superseding H…Br interactions viewed in Hirshfeld surfaces by the bright red are in Fig. 6a, which appear as distinct spike in the 2D fingerprints plots (Fig. 2S).

Fig. 6
figure 6

Hirshfeld surfaces of (4-ethylaminomethyl pyridinum Telluric chlorid VI): a 3D dnorm surface, b surface index, c curvedness

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We can decompose 2D fingerprints plots to highlight close contacts. This decomposition enables the separation of each contribution of the different interactions from the full fingerprint. Visible complementary regions are visible in the fingerprint plot where one molecule acts as a donor (de > di) and the other as an acceptor (de < di). The 2D fingerprint plots of the title compound (Fig. 2S) indicate the strong intermolecular contacts, which are Br-H, H–H, C–H, Br–Br, N–Br, C–N, C–Br, N–H. The H–H…Br/Br…H contacts exhibit the characteristic shape of two “wings” at the top left and the bottom right of the fingerprint plots (Fig. 2Sa). In fact, the H…Br/Br…H constitutes the most important interactions in the crystal and their relative contribution extends to 73.6% (indicated as red areas) due to the existence of N–H…Br hydrogen bonds. In contrast, the H–H contacts are rather homogeneously extended over a large range of (di, de) pairs with an average contribution of 12.4% (Fig. 2Sb). The C…H/H…C contacts are also broadly distributed with accumulation comprising 2.8% (Fig. 2Sc). However, Br…Br and N…Br/Br…N contacts display relatively a sharp and acicular distribution with an average of 2.7 and 2.7%, respectively (Fig. 2Sd,e). The intermolecular C…Br/Br…C and C…N/N…C interactions appear as short blue-colored patches with the proportion of 1.7 and 1.3%, respectively (Fig. 2Sf,g). In addition, N…H/H…N interactions are negligible and exhibit only 1.1% (Fig. 2S,h).

The dnorm values were mapped onto the Hirshfeld surface (Fig. 6a) using red, blue and white schemes as follows: red regions representing closer contacts and a negative dnorm value; blue regions representing longer contacts and a positive dnorm value; and white regions representing the contacts distance that is exactly equal to the vdW separation with a dnorm value of zero. These normalized contact distances (dnorm) disclose the adjacent contacts of hydrogen bond donors and acceptors as well as evident other close contacts. Indeed, Fig. 6a shows that the huge circular depressions are indicative of hydrogen bonding contacts and the prevailing interactions are Br-H. Nonetheless, other noticeable spots are due to H–H contacts, based on both de and di. The shape index is extremely susceptible to very delicate changes in the surface shape. Indeed, shape index (S) is 0D dimensionless surface property described in terms of the two major surface curvatures and can be used to differentiate complementary hollows (S− 1.0; concave region; red colored) and bumps (S+ 1.0; convex region; blue colored) where two molecular Hirshfeld surfaces come into contact [53,54,55,56,57]. In particular, adjacent red/orange and blue triangle like patches on a shape index map (Fig. 6b) provide information about π–π stacking interactions [53, 56, 57]. However, in this compound, the π–π interactions were present as indicated in Fig. 6b; thus, the Hirshfeld surface confirms the presence of the π–π stacking interactions in organic cations. The curvedness is a measure of the shape of the molecule surface area. As can be seen in Fig. 6c, while the flat surface areas correspond to the curvedness values, the sharp curvature areas correspond to the high curvedness values and usually tend to divide the surface into patches, representing interactions between neighboring molecules. The big flat region that is described by a blue outline refers to the π–π staking interactions.

Infrared and Raman spectroscopy

In order to obtain more information on the crystal structure, we studied the vibrational properties using Raman scattering and infrared absorption. IR and Raman spectroscopy is a proficient method to confirm the functional groups present in the crystal and to study the structural consequences such as in-plane or out-of-plane vibrations [58]. At this level, we have discussed the vibrational analyses of the compound and tried to give the most precise assignment of the detected bands.

In fact, the Raman bands correspond to the translational, vibrational and external modes of the inorganic groups [TeBr6]2− in the low-frequency range 50–250 cm−1 (Fig. 7). As for the bands observed between 250 and 4000 cm−1 in the Raman spectra and those in the IR spectra (Fig. 8), they are assigned to the internal modes of the cation. The assignment of the internal and lattice modes of the organic cations and inorganic anion is based on the comparison with the well-documented spectra of the homologous compounds [59,60,61,62,63].

Fig. 7
figure 7

Experimental Infrared spectrum of the (C4H7N2)2 TeBr6

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Fig. 8
figure 8

Exprimental Raman spectrum of the (C4H7N2)2 TeBr6

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In the following, the IR assignments will be described in detail. The Raman spectrum of this crystal confirms also the presence of the majority of vibration mode function for the organic molecule (Fig. 7, Table 3). We note that the broadening of the band is caused by hydrogen bonding interactions which influence the band frequency as well as the intensity of the peak [64].

Table 3 Experimental frequencies (cm−1) of the (C4H7N2)2TeBr6 compound
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The IR spectrum of this compound (Fig. 8 and Table 3) (in Supporting information) shows the characteristic absorption peaks of the [C4H7N2]2+ cation [65, 66]. The peaks at 3456 cm−1 and 3436 cm−1 correspond to the (N–H) stretching symmetric and asymmetric vibrations. However, the scissoring asymmetric and symmetric vibrations δ(N–H) have been identified as broad and weak bands at 1578 and 1521 cm−1 in the IR spectrum, respectively. In aromatic compounds, the C–H stretching wave numbers appear in the range of 3000–3100 cm−1. Accordingly, in the present study, the aromatic =C–H stretching gives bands at 3081 cm−1 in the IR spectrum. The symmetric and asymmetric stretching vibrations of (C–H) are observed at 2827 and 2525 cm−1. The bands observed at 1475 and 1456 cm−1 in FT-IR spectra are attributed to the scissoring symmetric and asymmetric vibrations δ(C–H), respectively. Moreover, the N–H and C–H in plane bending in aromatic groups are located at the interval 1120–1106 cm−1.

The vibrations modes between 3176 and 3126 cm−1 are due to the (CH3) asymmetric and symmetric stretching vibrations. The deformation asymmetric and symmetric of (CH3) bond is observed at 1563 and 1503 cm−1 in IR spectrum. The C=N aromatic stretches are predicted at 1618 cm−1 in IR spectra [67]. In the present work, the C=C aromatic stretch is observed at 1603 cm−1 in the FT-IR. The aromatic ring carbon–nitrogen υ(C–N) stretching vibrations are detected at 1395 cm−1. The strong bands identified at 1408 cm−1 in the IR spectrum are assigned to υ(C–C) stretching vibrations in the ligand. The bands relating to the deformation mode symmetric and asymmetric of the C–N group appear in IR spectrum at 1138 and 943 cm−1. The deformation of (C–C) bond is observed at 733 cm−1. Besides, the absorption bonds located at 1485 and 1436 cm−1 in IR spectrum are assigned to the (C–N–H) symmetric and asymmetric scissoring vibration.

The bands relating to the in-plane deformation modes of the C–C–N, N–C–C, N–C–N and C–N–C groups appear in the spectral range between 1077 and 1037 cm−1. The bands corresponding to the δ(C–C–N) scissoring in-plane mode appear at 524 cm−1. The band detected at 619 cm−1 corresponds to the in-plane bending of C–N–C. The β(C–C–H) in-plane bending mode is located at 1006 cm−1. The peak located in IR spectrum at 854 cm−1 is generated by the out-of-plane bending mode of γ(C–C–H) groups.

Conclusion

The present research work reported the synthesis and characterization of the new centrosymmetric hybrid compound (C4H7N2)2TeBr6. Thus, single crystals of (C4H7N2)2TeBr6 were collected from an aqueous solution by a slow evaporation technique. The obtained material crystallizes in the space group C2/c with monoclinic system. The structure of this compound consists of isolated [TeBr6]2− octahedral anions and 2-methylimidazole cations. The crystal structure is stabilized by N–H…Br hydrogen bonds and \(\pi-\pi \) stacking interactions. Hirshfeld surface allowed us to investigate the stabilization of the crystal packing and quantify the propensity of the intermolecular interactions to form the supramolecular assembly. The study of the vibration spectroscopy by IR, Raman at room temperatures proved the homogeneity and purity of this synthesis.