# Synthesis, Crystal Structure, Hirshfeld Surface, and Physicochemical Characterization of New Salt Bis(2-ethyl-6-methylanilinium)tetrachloromercurate (II) [C9H14N]2HgCl4

Authors: Chaima Ayari; M. G. Althobaiti; Abdullah A. Alotaibi; Abdulhadi. H. Almarri; Valeria Ferretti; Cherif Ben Nasr; Mohamed Habib Mrad
Journal: Journal of Chemistry
 (2021)

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## Abstract

In this paper, a new organic salt bis(2-ethyl-6-methylanilinium)tetrachloromercurate (II)) has been synthesized, characterized, and studied theoretically using DFT. The synthesized compound was found to crystallize in the monoclinic system with space group P21/c with the following unit cell parameters a = 23.1696(2), b = 25.7951 (6), c = 7.5980 (4) Å, β = 96.5790 (11)°, and Z = 8. Its atomic arrangement can be described asmutually alternating organic and inorganic entities in the (ab) plane. The cohesion is achieved through N-H…Cl hydrogen bonds and π…π stacking interacting neighboring aromatic cations. The PXRD was carried out. The intermolecular interactions in the crystal packing were investigated using Hirshfeld surface analysis. Its associated 2D fingerprint plots revealed their contribution quantitatively. High-resolution images of the surface were obtained by the SEM technique, and the EDX spectrum assured the existence of all nonhydrogen atoms. The infrared spectrum was used to gain more information about vibrational modes. Analysis of thermal differential and gravimetric (TGA/DTA) shows two-phase transitions observed at 362 and 394 K. AC conductivity measurements were performed to confirm these two-phase transitions.


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## Body

## 1. Introduction

The concept of organic salts materials appeared very recently and has attracted attention in scientific research due to their different applications in chemistry. The synergy between the electrical, thermal, and optical properties of inorganic entities and the physicochemical properties of organic materials opens up a vast field of their applications. These materials, made by combining different components, aim to produce desirable properties and functionalities and at the same time reduce or eliminate undesirable effects [1–8].Mercury has very endearing properties [9–12]. Concerning coordination chemistry, Hg (II) does not have a specific coordination number because it can adopt several coordination geometry numbers alternating from two to six due to the d10 configuration and the lack of stabilization of the field effects of the ligand [13, 14]. Thus, some compounds indicate that the Hg2+ ion has unusual coordination geometries, such as the geometry of the pseudocoordinate structure [15] and the heptacoordinate structure [16]. Therefore, Hg-based coordination compounds can result in a wide range of supramolecular structures ranging from discrete coordination complexes to coordination polymers [17–20]. Nevertheless, the most frequent formula of this group is R2[MX4], where M represents the divalent metal, X can be a halogen atom (chlorine, bromine or an iodine atom), and R represents an organic group. When the R group is a protonated amine (cyclic or acyclic), a wide variety of those complexes are known [21, 22]. As for the organic group, we oriented our study towards aniline because of its technological uses in various industrial processes such as dyes and pigments. Several compounds of this family are also used as intermediates in the manufacture of herbicides, insecticides, pharmaceuticals for the design of bactericidal agents, agricultural chemicals, and several industrial and commercial purposes. It has also been used for the synthesis of molecules with significant potential nonlinear optical effects [23–27].Considering the fascinating properties of halogenomercurate (II) and its emerging possibilities widens the development of such organic salts. So, this work reports the synthesis of a new compound bis(2-ethyl-6-methylanilinium) tetrachloromercurate (II) [C9H14N]2HgCl4. Its X-ray crystal structure, PXRD, spectroscopic features (FT-IR, SEM, EDX), and AC conductivity have been exploited. TG/DTA coupling has also been executed and commented on. The characteristics mentioned above helped understand and delineate the intermolecular interactions of crystal structure and HS along with its fingerprint plots.


## 2. Experimental Procedures

### 2.1. Assembling [C9H14N]2HgCl4

The title compound bis(2-ethyl-6-methylanilinium)tetrachloromercurate (II) was synthesized by acid-base reaction between the amine 2-ethyl-6-methylaniline (C9H13N) and mercury (II) chloride.0.28 mL of C9H13N (2 mmol) was dissolved in ethanol, drooped to a mixture of 0.275 g of mercury chloride (1 mmol), dissolved in a small amount of water and 10 mL of concentrated HCl, maintained under stirring for 30 minutes, and then left to evaporate slowly at ambient temperature. The chemical reaction equation is as follows:(1)HgCl2+2C9H13N+2HCl⟶C9H14N2HgCl4+12H2,After three weeks, pale brown plate crystals were obtained with good quality for single X-ray diffraction analysis. The elemental analysis of the prepared compound yielded that it comprises N: 4.35%/4.55%; C: 34.94%/35.14; and H: 4.23%/4.55% (exp./theor.).


### 2.2. Characterizations

#### 2.2.1. Powdered X-ray Diffraction Studies (PXRD)

D8 Advance Bruker powder diffractometer has been employed using Cu-Kα radiation with a 2θ in the range between 3.5 and 50°. Recording the PXRD patterns has been accomplished using mercury program and the single crystal data [28]. PXRD data were captured using a Nonius Kappa CCD diffractometer, and the absorptions were corrected employing the DENZO-SMN package [29]. The direct method was used to solve the structure [30], which is then refined by exploding the SHELXL2014/7 program on F2 utilizing the full-matrix least-squares method [31]. All other calculations were executed with WINGX [32]. The C9 and C18 methyl-terminal groups were found to be disordered over two equivalent positions (occupancy factor 0.5). Experimental details are given in Table 1.Table 1
Summary of experimental details.
Crystal dataChemical formula[C9H14N]2HgCl4Mr614.81Crystal system, space groupMonoclinic, P21/cTemperature (K)295a, b, c (Å)23.1696 (2), 25.7951 (6), 7.5980 (4)β (°)96.5790 (11)V (Å3)4511.1 (3)Z8Radiation typeMo-Kαμ (mm−1)7.30Crystal size (mm3)″0.31 × 0.24 × 0.07Data collectionDiffractometerNonius Kappa CCDTmin, TmaxTmin = 3.0°, Tmax = 28.0°No. of measured, independent, and observed [I > 2σ(I)] reflections29996, 10674, and 8223Rint0.093(Sinθ/λ)max (Å−1)0.661RefinementR[F2 > 2σ(F2)], wR(F2), S0.051, 0.129, 1.04No. of reflections10674No. of parameters449No. of restraints5Δρmin, Δρmax (e Å−3)−1.17, 1.59JEOL-6610LV electron microscope operating at 30 kV coupled with an Oxford X-Max microanalysis system has been used to get the compound SEM/EDX patterns.The projections of the organic salt structure and the ORTEP view were drawn using the DIAMOND 2.0 program [33]. The Crystal Explorer 3.1 software was used for evaluating the intermolecular interactions and exploring the Hirshfeld surface [34].


#### 2.2.2. FT-IR Spectra

The FT-IR spectrum was recorded at ambient conditions using a Nicolet 5700 spectrophotometer from potassium bromide (KBr) with a scan range from 4000 to 400 cm−1.


#### 2.2.3. Thermal Analysis

PYRIS 1 TGA instrument was employed in recording TGA/DTA spectra by varying the temperature from 300 to 700 K with a heating rate of 10 K min−1 under inert nitrogen atmosphere.


#### 2.2.4. Conductivity Measurements

In order to measure the DC resistivity of the samples, the contacts were deposited using silver paste on the opposite faces of the samples, which is pressed into circular pellets of 6 mm × 2 mm dimension. DC resistivity measurement was performed using a two-probe resistivity set up at room temperature by applying a voltage up to 5 V using DC resistivity setup of marine India. The setup is interfaced with a computer through the RS232 cable and supported by automated software. DC resistivity measurement was performed and calculated according to the formula:(2)R=ρlA,where ρ is resistivity and A is the area of the sheet.


## 2.1. Assembling [C9H14N]2HgCl4

The title compound bis(2-ethyl-6-methylanilinium)tetrachloromercurate (II) was synthesized by acid-base reaction between the amine 2-ethyl-6-methylaniline (C9H13N) and mercury (II) chloride.0.28 mL of C9H13N (2 mmol) was dissolved in ethanol, drooped to a mixture of 0.275 g of mercury chloride (1 mmol), dissolved in a small amount of water and 10 mL of concentrated HCl, maintained under stirring for 30 minutes, and then left to evaporate slowly at ambient temperature. The chemical reaction equation is as follows:(1)HgCl2+2C9H13N+2HCl⟶C9H14N2HgCl4+12H2,After three weeks, pale brown plate crystals were obtained with good quality for single X-ray diffraction analysis. The elemental analysis of the prepared compound yielded that it comprises N: 4.35%/4.55%; C: 34.94%/35.14; and H: 4.23%/4.55% (exp./theor.).


## 2.2. Characterizations

### 2.2.1. Powdered X-ray Diffraction Studies (PXRD)

D8 Advance Bruker powder diffractometer has been employed using Cu-Kα radiation with a 2θ in the range between 3.5 and 50°. Recording the PXRD patterns has been accomplished using mercury program and the single crystal data [28]. PXRD data were captured using a Nonius Kappa CCD diffractometer, and the absorptions were corrected employing the DENZO-SMN package [29]. The direct method was used to solve the structure [30], which is then refined by exploding the SHELXL2014/7 program on F2 utilizing the full-matrix least-squares method [31]. All other calculations were executed with WINGX [32]. The C9 and C18 methyl-terminal groups were found to be disordered over two equivalent positions (occupancy factor 0.5). Experimental details are given in Table 1.Table 1
Summary of experimental details.
Crystal dataChemical formula[C9H14N]2HgCl4Mr614.81Crystal system, space groupMonoclinic, P21/cTemperature (K)295a, b, c (Å)23.1696 (2), 25.7951 (6), 7.5980 (4)β (°)96.5790 (11)V (Å3)4511.1 (3)Z8Radiation typeMo-Kαμ (mm−1)7.30Crystal size (mm3)″0.31 × 0.24 × 0.07Data collectionDiffractometerNonius Kappa CCDTmin, TmaxTmin = 3.0°, Tmax = 28.0°No. of measured, independent, and observed [I > 2σ(I)] reflections29996, 10674, and 8223Rint0.093(Sinθ/λ)max (Å−1)0.661RefinementR[F2 > 2σ(F2)], wR(F2), S0.051, 0.129, 1.04No. of reflections10674No. of parameters449No. of restraints5Δρmin, Δρmax (e Å−3)−1.17, 1.59JEOL-6610LV electron microscope operating at 30 kV coupled with an Oxford X-Max microanalysis system has been used to get the compound SEM/EDX patterns.The projections of the organic salt structure and the ORTEP view were drawn using the DIAMOND 2.0 program [33]. The Crystal Explorer 3.1 software was used for evaluating the intermolecular interactions and exploring the Hirshfeld surface [34].


### 2.2.2. FT-IR Spectra

The FT-IR spectrum was recorded at ambient conditions using a Nicolet 5700 spectrophotometer from potassium bromide (KBr) with a scan range from 4000 to 400 cm−1.


### 2.2.3. Thermal Analysis

PYRIS 1 TGA instrument was employed in recording TGA/DTA spectra by varying the temperature from 300 to 700 K with a heating rate of 10 K min−1 under inert nitrogen atmosphere.


### 2.2.4. Conductivity Measurements

In order to measure the DC resistivity of the samples, the contacts were deposited using silver paste on the opposite faces of the samples, which is pressed into circular pellets of 6 mm × 2 mm dimension. DC resistivity measurement was performed using a two-probe resistivity set up at room temperature by applying a voltage up to 5 V using DC resistivity setup of marine India. The setup is interfaced with a computer through the RS232 cable and supported by automated software. DC resistivity measurement was performed and calculated according to the formula:(2)R=ρlA,where ρ is resistivity and A is the area of the sheet.


## 2.2.1. Powdered X-ray Diffraction Studies (PXRD)

D8 Advance Bruker powder diffractometer has been employed using Cu-Kα radiation with a 2θ in the range between 3.5 and 50°. Recording the PXRD patterns has been accomplished using mercury program and the single crystal data [28]. PXRD data were captured using a Nonius Kappa CCD diffractometer, and the absorptions were corrected employing the DENZO-SMN package [29]. The direct method was used to solve the structure [30], which is then refined by exploding the SHELXL2014/7 program on F2 utilizing the full-matrix least-squares method [31]. All other calculations were executed with WINGX [32]. The C9 and C18 methyl-terminal groups were found to be disordered over two equivalent positions (occupancy factor 0.5). Experimental details are given in Table 1.Table 1
Summary of experimental details.
Crystal dataChemical formula[C9H14N]2HgCl4Mr614.81Crystal system, space groupMonoclinic, P21/cTemperature (K)295a, b, c (Å)23.1696 (2), 25.7951 (6), 7.5980 (4)β (°)96.5790 (11)V (Å3)4511.1 (3)Z8Radiation typeMo-Kαμ (mm−1)7.30Crystal size (mm3)″0.31 × 0.24 × 0.07Data collectionDiffractometerNonius Kappa CCDTmin, TmaxTmin = 3.0°, Tmax = 28.0°No. of measured, independent, and observed [I > 2σ(I)] reflections29996, 10674, and 8223Rint0.093(Sinθ/λ)max (Å−1)0.661RefinementR[F2 > 2σ(F2)], wR(F2), S0.051, 0.129, 1.04No. of reflections10674No. of parameters449No. of restraints5Δρmin, Δρmax (e Å−3)−1.17, 1.59JEOL-6610LV electron microscope operating at 30 kV coupled with an Oxford X-Max microanalysis system has been used to get the compound SEM/EDX patterns.The projections of the organic salt structure and the ORTEP view were drawn using the DIAMOND 2.0 program [33]. The Crystal Explorer 3.1 software was used for evaluating the intermolecular interactions and exploring the Hirshfeld surface [34].


## 2.2.2. FT-IR Spectra

The FT-IR spectrum was recorded at ambient conditions using a Nicolet 5700 spectrophotometer from potassium bromide (KBr) with a scan range from 4000 to 400 cm−1.


## 2.2.3. Thermal Analysis

PYRIS 1 TGA instrument was employed in recording TGA/DTA spectra by varying the temperature from 300 to 700 K with a heating rate of 10 K min−1 under inert nitrogen atmosphere.


## 2.2.4. Conductivity Measurements

In order to measure the DC resistivity of the samples, the contacts were deposited using silver paste on the opposite faces of the samples, which is pressed into circular pellets of 6 mm × 2 mm dimension. DC resistivity measurement was performed using a two-probe resistivity set up at room temperature by applying a voltage up to 5 V using DC resistivity setup of marine India. The setup is interfaced with a computer through the RS232 cable and supported by automated software. DC resistivity measurement was performed and calculated according to the formula:(2)R=ρlA,where ρ is resistivity and A is the area of the sheet.


## 3. Results and Discussion

### 3.1. X-Ray Powder Diffraction Analysis

The experimental and simulated PXRD patterns are shown in Figure1. It can be seen that the simulated X-ray powder diffraction pattern closely resembles the experimental pattern where most of the positions of the peaks coincide with each other. We conclude that the synthesized bulk and the crystal data used for diffraction are homogenous.Figure 1
Patterns of the simulated and experimental powder XRD for (C9H14N)2HgCl4.


### 3.2. X-Ray Microanalyses and Micrographs

The surface morphology of the grown crystal (C9H14N)2HgCl4, obtained using the scanning electron microscope, is given in Figures 2(a)–2(c). The surface morphology appears as an assembly of fragments with a flat surface and uniformly distributed, indicating good quality. Moreover, EDX, a valuable tool to identify the elements of higher atomic numbers in crystals, demonstrated the presence of the majority of the atoms (carbon, mercury, chloride, and nitrogen) (Figure 2(d)).Figure 2
(a–c) SEM images; (d) typical spectrum of (C9H14N)2HgCl4.
(a)(b)(c)(d)


### 3.3. Description of the Structure

Figure3 shows the ORTEP of (C9H14N)2HgCl4. The asymmetric unit of the prepared salt is formed by two tetrachloromercurate [HgCl4]2− anions and four crystallographic independent 2-ethyl-6-methylanilinium (C9H14N)+ organic cations. It is worth noting that the methyl-terminal groups C9 and C18 are disordered over two equivalent positions (occupancy factor: 0.5 : 0.5). In this compound, the cations are located among the inorganic anions, and the N-H…Cl hydrogen bonds provide the linkage between the cationic and anionic entities (Figure 4). The N…Cl distances vary from 3.060 (6) to 3.509 (6) Å (Table 2) so that hydrogen bonds can be classified into two halves, one of them as weak and the other as strong intermolecular contacts [34] (Figure 5). In addition, as the second type of interactions in the crystal packing, the π-π stacking interaction between aromatic rings adds more stability to the three-dimensional framework with the distance between benzene rings extending from 3.733 to 3.833 Å (Figure 6).Figure 3
ORTEP of the (C9H14N)2HgCl4 compound showing both atom label and thermal ellipsoid, plotted at 50% probability level.Figure 4
Projection along thec-axis arrangement of (C9H14N)2HgCl4.Table 2
H-bonds geometry (Å, °).
D―H···AD ―H (Å)H···A (Å)D···A (Å)D―H···A (°)N1―H1A···Cl2I0.892.483.267 (6)147N1―H1B···Cl3i0.892.353.161 (6)152N1―H1B···Cl10.892.303.094 (6)149N2―H2A···Cl50.892.853.360 (6)118N2―H2A···Cl8i0.892.563.096 (6)119N2―H2B···Cl7ii0.892.703.509 (6)151N2―H2C···Cl60.892.453.248 (8)149N3―H3A···Cl40.892.703.488 (8)148N3―H3B···Cl10.892.413.093 (6)134N3―H3C···Cl2iii0.892.693.215 (6)119N3―H3C···Cl3i0.892.683.289 (8)127N4―H4C···Cl5iv0.892.333.187 (6)161N4―H4B···Cl80.892.303.060 (6)14N4―H4C···Cl6v0.892.563.187 (6)128Symmetry Codes. i: x, y, z+1 ; ii: –x+1, –y, –z+2 ; iii: –x+2, –y, –z+1 ; iv: x, y, z–1 ; v: –x+1, –y, –z+1 .Figure 5
(a) Thec-axis atomic arrangement projection of (C9H14N)2HgCl4. (b) Different hydrogen bonds with dotted lines observed in title salt.Figure 6
The differentπ-π stacking interactions between the neighboring aromatic organic cations in (C9H14N)2HgCl4.Dimers formed by [HgCl4]2− anions were identified by projection of the atomic arrangements in the (ab) plane such that each mercury atom is coordinated by four chlorine atoms located at (0, 0, 0); (1/2, 0, 0); (0, 1/2, 0); and (1/2, 1/2, 0). The Cl-Hg-Cl bond angles vary from 94.50 (5)° to 151.12 (7)°, and the Cl-Hg bond lengths are in the range of 2.389 (2)–2.681 (2)Å. Those characteristics concerning bond lengths and angles listed in Table s1 agree well with those reported for similar compounds [35, 36]. In the structure of the protonated amine 2-ethyl-6-methylanilinium (C9H14N)+, the two groups -CH3 and -CH2-CH3 are arranged on the same plane as their carrier phenyl ring (in the ortho position) since the angles of torsion involving these substituents have the values of 0°.The bond lengths and angles are grouped in Table 3 and emphasized that the carbons of the phenyl rings are regular, agreeing well with those reported for other compounds with anilinium groups [37–39].Table 3
Selected bond distances and angles of the cationic entities (C9H14N)+.
Distances (Å)Angles (°)C1―C21.392 (9)C2―C1―C6123.8 (6)C1―C61.393 (10)C1―C2―C3115.9 (7)C2―C31.399 (10)C4―C3―C2121.8 (7)C3―C41.370 (12)C5―C3―C2120.3 (7)C4―C51.360 (12)C4―C5―C6121.6 (8)C5―C61.390 (9)C5―C6―C1116.7 (7)C10―C111.385 (9)C11―C10―C15124.1 (5)C10―C151.393 (9)C12―C11―C10116.7 (7)C11―C121.380 (9)C13―C12―C11121.2 (7)C12―C131.365 (11)C12―C11―C10116.7 (7)C13―C141.372 (11)C12―C13―C14120.7 (6)C14―C151.386 (9)C13―C14―C15121.1 (7)C19―C201.379 (9)C20―C19―C24116.2 (7)C19―C241.397 (10)C19―C20―C21121.0 (6)C20―C211.381 (10)C20―C21―C22118.2 (7)C21―C221.381 (11)C23―C24―C19120.9 (7)C22―C231.358 (11)C29―C28―C33120.8 (7)C23―C241.384 (11)C28―C29―C30119.7 (7)C28―C291.383 (11)C31―C30―C29121.1 (9)C28―C331.389 (11)C32―C31―C30120.6 (9)C29―C301.410 (13)C31―C32―C33121.3 (11)C30―C311.361 (17)C28―C33―C32117.4 (9)C31―C321.358 (17)C32―C331.390 (11)N1―C11.470 (7)N2―C101.477 (7)N3―C191.462 (8)N4―C281.475 (8)


### 3.4. Hirshfeld Surface Analysis

The Hirshfeld surfaces of the synthesized compound were plotted over adnorm. The shape index and curvedness are shown in Figure 7. The intensive hot red spots on the dnorm surface are attributed to the Cl and H atoms interactions (Figure 8). Both blue and red triangles appear on the shape index’s cartography, and the large flat green areas delimited by the blue outline in the cartography of curvedness indicate the existence of the π-π stacking interactions between the aromatics cycles of the organic cations. The enrichment ratio EXY for a pair of elements (X, Y) is calculated and reported in Table s2. Moreover, the full fingerprint plots of the main contacts involved in the title compound are illustrated in Figure 9(a). Globally, H…H contacts are the most plenty interactions with a contribution percentage reaching 49% (Figure 9(b)) and appear as a large region plot with a high concentration in the middle due to the abundance of the H atoms on the surface, and they are moderately enriched (EHH = 1.02). The H…Cl/Cl…H contacts ranked as the second most abundant interactions (34%) due to the plenty of hydrogen and chloride atoms on the molecular surface (SH = 69.3% and SCl = 20.85%) with an enrichment ratio EHCl = 1.17 which manifested as two sharp spikes in the 2D fingerprint plot (Figure 9(c)). Indeed, there are fourteen hydrogen bonds of type N-H…Cl (Table 3). 5.9% of the Hirshfeld surface resulting from C…H/H…C contact appears in Figure 9(d) as a symmetric pair of wings with an enrichment ratio ECH = 0.19. This low value revealed that these pairs tend to repel contact with each other. The C…C contacts, which refer to the π-π stacking interaction SC = 21.7%, contribute with 4.7% of the Hirshfeld surface (Figure 9(e)). The other intermolecular contacts are Hg…Cl: 3.5% (Figure 9(f)), Cl…Cl; 2.1% (Figure 9(g)), and Hg…H; 0.7% (Figure 9(h)) contributing lower to the surface.Figure 7
HS cartography withdnorm (a), shape index (b), and curvedness (c) for (C9H14N)2HgCl4.
(a)(b)(c)Figure 8
Observed hydrogen bonds in the salt bydnorm.Figure 9
The two-dimensional fingerprint plots for (C9H14N)2HgCl4.
(a)(b)(c)(d)(e)(f)(g)(h)These contact contributions are in agreement with those of [CuCl2(C14H10N2) (H2O)]H2O for which the H…H contacts are the most abundant 44.0% followed by H…Cl contact 23.1% and then the C…H contact 13.2% [40].


### 3.5. Vibrational Study

The FT-IR measurements were used to recognize the (C9H14N)2HgCl4 functional groups. Figure 10 displays the infrared spectrum governed by the cation’s vibration bands since the Hg-Cl internal vibrational bands manifest below 400 cm−1 [4–44].Figure 10
The experimental infrared absorption spectra of (C9H14N)2HgCl4.The two large bands in the high-frequency region, at 2973 and 2564 cm−1, may be imputed to υs,as(N-H)Ar and υs,as(C-H)Ar. The hydrogen bonds of type N-H…Cl affect the region’s absorption position, and the bands recorded at 1590, and 1559 cm−1 correspond to δ(NH) of the NH3 groups. However, the two intense peaks detected at 1618 and 1500 cm−1 are assigned to the valence vibration υ(C=C) of the aromatic rings. Besides, the vibration bands observed in the range 1450–1150 cm−1 are characteristic of δs,as(CH3), δs,as(NH3), and δs,as(C-N). While the low-intensity bands appearing between 1000 and 500 cm−1 correspond to the deformations δ(Caryl-H), δ(Caryl-Caryl), δ(Caryl-N), and γ(N-H) of the phenyl ring.


### 3.6. Thermal Analysis

Simultaneous TGA-DTA analyses were carried out to study the thermal stability and the changes of the newly synthesized compound (C9H14N)2HgCl4. The results are shown in Figure 11. A series of endothermic and exothermic peaks are observed on the DTA curve, accompanied by a weight loss on the TGA curve. The two endothermic peaks detected on the DTA curve observed at 362 K and 391 K without mass loss correspond to two-phase transitions phenomena. The three exothermic peaks observed a 441, 474, and 530 K with ΔH = −1279.880 J/g, ΔH = −903.219 J/g, and ΔH = −128.806 J/g, respectively, correspond to the loss of the two organic cations with a total experimental mass of 45% compared to the calculated weight loss of 46%. The two exothermic peaks in the DTA curve observed at 561 and 600 K, with ΔH = −22.134 J/g and ΔH = −1212.156 J/g, respectively, are assigned to the elimination of four chlorine atoms (exp/calc: 21.5%/22%). The compound completely melts since there is no endothermic peak after 600 K.Figure 11
TGA/DTA curves for (C9H14N)2HgCl4.


### 3.7. Electric Conductivity

To better understand the phenomenon of electrical conduction, we will use the equation of Arrhenius’ law [45]:(3)σ.T=Aexp−EaKβ.T,where Ea is the activation energy, T is the absolute temperature, Kβ is the Boltzmann constant, and A is a constant depending on the material. Figure 12 shows the curve of plotting ln(σ,T) versus 1000/T. This graph is formed by three regions (I), (II), and (III) and two breaks between 301 and 471 K. Each domain is presented by an affine straight line which is in accordance with Arrhenius’ law. The first break is observed at 393 K, with an activation energy of 0.59 eV, and (II), with an activation energy of 4.21 eV. The second break is observed at 368 K for which the activation energy of (I) is 0.60 eV. A slope change at this temperature was observed in the DTA curve due to two endothermic peaks at 362 and 391 K, respectively. In the studied temperature interval, the conductivity increases with temperature showing that this material behaves like a semiconductor [46].Figure 12
Variation of ln (σT) versus 1000/T for (C9H14N)2HgCl4.


## 3.1. X-Ray Powder Diffraction Analysis

The experimental and simulated PXRD patterns are shown in Figure1. It can be seen that the simulated X-ray powder diffraction pattern closely resembles the experimental pattern where most of the positions of the peaks coincide with each other. We conclude that the synthesized bulk and the crystal data used for diffraction are homogenous.Figure 1
Patterns of the simulated and experimental powder XRD for (C9H14N)2HgCl4.


## 3.2. X-Ray Microanalyses and Micrographs

The surface morphology of the grown crystal (C9H14N)2HgCl4, obtained using the scanning electron microscope, is given in Figures 2(a)–2(c). The surface morphology appears as an assembly of fragments with a flat surface and uniformly distributed, indicating good quality. Moreover, EDX, a valuable tool to identify the elements of higher atomic numbers in crystals, demonstrated the presence of the majority of the atoms (carbon, mercury, chloride, and nitrogen) (Figure 2(d)).Figure 2
(a–c) SEM images; (d) typical spectrum of (C9H14N)2HgCl4.
(a)(b)(c)(d)


## 3.3. Description of the Structure

Figure3 shows the ORTEP of (C9H14N)2HgCl4. The asymmetric unit of the prepared salt is formed by two tetrachloromercurate [HgCl4]2− anions and four crystallographic independent 2-ethyl-6-methylanilinium (C9H14N)+ organic cations. It is worth noting that the methyl-terminal groups C9 and C18 are disordered over two equivalent positions (occupancy factor: 0.5 : 0.5). In this compound, the cations are located among the inorganic anions, and the N-H…Cl hydrogen bonds provide the linkage between the cationic and anionic entities (Figure 4). The N…Cl distances vary from 3.060 (6) to 3.509 (6) Å (Table 2) so that hydrogen bonds can be classified into two halves, one of them as weak and the other as strong intermolecular contacts [34] (Figure 5). In addition, as the second type of interactions in the crystal packing, the π-π stacking interaction between aromatic rings adds more stability to the three-dimensional framework with the distance between benzene rings extending from 3.733 to 3.833 Å (Figure 6).Figure 3
ORTEP of the (C9H14N)2HgCl4 compound showing both atom label and thermal ellipsoid, plotted at 50% probability level.Figure 4
Projection along thec-axis arrangement of (C9H14N)2HgCl4.Table 2
H-bonds geometry (Å, °).
D―H···AD ―H (Å)H···A (Å)D···A (Å)D―H···A (°)N1―H1A···Cl2I0.892.483.267 (6)147N1―H1B···Cl3i0.892.353.161 (6)152N1―H1B···Cl10.892.303.094 (6)149N2―H2A···Cl50.892.853.360 (6)118N2―H2A···Cl8i0.892.563.096 (6)119N2―H2B···Cl7ii0.892.703.509 (6)151N2―H2C···Cl60.892.453.248 (8)149N3―H3A···Cl40.892.703.488 (8)148N3―H3B···Cl10.892.413.093 (6)134N3―H3C···Cl2iii0.892.693.215 (6)119N3―H3C···Cl3i0.892.683.289 (8)127N4―H4C···Cl5iv0.892.333.187 (6)161N4―H4B···Cl80.892.303.060 (6)14N4―H4C···Cl6v0.892.563.187 (6)128Symmetry Codes. i: x, y, z+1 ; ii: –x+1, –y, –z+2 ; iii: –x+2, –y, –z+1 ; iv: x, y, z–1 ; v: –x+1, –y, –z+1 .Figure 5
(a) Thec-axis atomic arrangement projection of (C9H14N)2HgCl4. (b) Different hydrogen bonds with dotted lines observed in title salt.Figure 6
The differentπ-π stacking interactions between the neighboring aromatic organic cations in (C9H14N)2HgCl4.Dimers formed by [HgCl4]2− anions were identified by projection of the atomic arrangements in the (ab) plane such that each mercury atom is coordinated by four chlorine atoms located at (0, 0, 0); (1/2, 0, 0); (0, 1/2, 0); and (1/2, 1/2, 0). The Cl-Hg-Cl bond angles vary from 94.50 (5)° to 151.12 (7)°, and the Cl-Hg bond lengths are in the range of 2.389 (2)–2.681 (2)Å. Those characteristics concerning bond lengths and angles listed in Table s1 agree well with those reported for similar compounds [35, 36]. In the structure of the protonated amine 2-ethyl-6-methylanilinium (C9H14N)+, the two groups -CH3 and -CH2-CH3 are arranged on the same plane as their carrier phenyl ring (in the ortho position) since the angles of torsion involving these substituents have the values of 0°.The bond lengths and angles are grouped in Table 3 and emphasized that the carbons of the phenyl rings are regular, agreeing well with those reported for other compounds with anilinium groups [37–39].Table 3
Selected bond distances and angles of the cationic entities (C9H14N)+.
Distances (Å)Angles (°)C1―C21.392 (9)C2―C1―C6123.8 (6)C1―C61.393 (10)C1―C2―C3115.9 (7)C2―C31.399 (10)C4―C3―C2121.8 (7)C3―C41.370 (12)C5―C3―C2120.3 (7)C4―C51.360 (12)C4―C5―C6121.6 (8)C5―C61.390 (9)C5―C6―C1116.7 (7)C10―C111.385 (9)C11―C10―C15124.1 (5)C10―C151.393 (9)C12―C11―C10116.7 (7)C11―C121.380 (9)C13―C12―C11121.2 (7)C12―C131.365 (11)C12―C11―C10116.7 (7)C13―C141.372 (11)C12―C13―C14120.7 (6)C14―C151.386 (9)C13―C14―C15121.1 (7)C19―C201.379 (9)C20―C19―C24116.2 (7)C19―C241.397 (10)C19―C20―C21121.0 (6)C20―C211.381 (10)C20―C21―C22118.2 (7)C21―C221.381 (11)C23―C24―C19120.9 (7)C22―C231.358 (11)C29―C28―C33120.8 (7)C23―C241.384 (11)C28―C29―C30119.7 (7)C28―C291.383 (11)C31―C30―C29121.1 (9)C28―C331.389 (11)C32―C31―C30120.6 (9)C29―C301.410 (13)C31―C32―C33121.3 (11)C30―C311.361 (17)C28―C33―C32117.4 (9)C31―C321.358 (17)C32―C331.390 (11)N1―C11.470 (7)N2―C101.477 (7)N3―C191.462 (8)N4―C281.475 (8)


## 3.4. Hirshfeld Surface Analysis

The Hirshfeld surfaces of the synthesized compound were plotted over adnorm. The shape index and curvedness are shown in Figure 7. The intensive hot red spots on the dnorm surface are attributed to the Cl and H atoms interactions (Figure 8). Both blue and red triangles appear on the shape index’s cartography, and the large flat green areas delimited by the blue outline in the cartography of curvedness indicate the existence of the π-π stacking interactions between the aromatics cycles of the organic cations. The enrichment ratio EXY for a pair of elements (X, Y) is calculated and reported in Table s2. Moreover, the full fingerprint plots of the main contacts involved in the title compound are illustrated in Figure 9(a). Globally, H…H contacts are the most plenty interactions with a contribution percentage reaching 49% (Figure 9(b)) and appear as a large region plot with a high concentration in the middle due to the abundance of the H atoms on the surface, and they are moderately enriched (EHH = 1.02). The H…Cl/Cl…H contacts ranked as the second most abundant interactions (34%) due to the plenty of hydrogen and chloride atoms on the molecular surface (SH = 69.3% and SCl = 20.85%) with an enrichment ratio EHCl = 1.17 which manifested as two sharp spikes in the 2D fingerprint plot (Figure 9(c)). Indeed, there are fourteen hydrogen bonds of type N-H…Cl (Table 3). 5.9% of the Hirshfeld surface resulting from C…H/H…C contact appears in Figure 9(d) as a symmetric pair of wings with an enrichment ratio ECH = 0.19. This low value revealed that these pairs tend to repel contact with each other. The C…C contacts, which refer to the π-π stacking interaction SC = 21.7%, contribute with 4.7% of the Hirshfeld surface (Figure 9(e)). The other intermolecular contacts are Hg…Cl: 3.5% (Figure 9(f)), Cl…Cl; 2.1% (Figure 9(g)), and Hg…H; 0.7% (Figure 9(h)) contributing lower to the surface.Figure 7
HS cartography withdnorm (a), shape index (b), and curvedness (c) for (C9H14N)2HgCl4.
(a)(b)(c)Figure 8
Observed hydrogen bonds in the salt bydnorm.Figure 9
The two-dimensional fingerprint plots for (C9H14N)2HgCl4.
(a)(b)(c)(d)(e)(f)(g)(h)These contact contributions are in agreement with those of [CuCl2(C14H10N2) (H2O)]H2O for which the H…H contacts are the most abundant 44.0% followed by H…Cl contact 23.1% and then the C…H contact 13.2% [40].


## 3.5. Vibrational Study

The FT-IR measurements were used to recognize the (C9H14N)2HgCl4 functional groups. Figure 10 displays the infrared spectrum governed by the cation’s vibration bands since the Hg-Cl internal vibrational bands manifest below 400 cm−1 [4–44].Figure 10
The experimental infrared absorption spectra of (C9H14N)2HgCl4.The two large bands in the high-frequency region, at 2973 and 2564 cm−1, may be imputed to υs,as(N-H)Ar and υs,as(C-H)Ar. The hydrogen bonds of type N-H…Cl affect the region’s absorption position, and the bands recorded at 1590, and 1559 cm−1 correspond to δ(NH) of the NH3 groups. However, the two intense peaks detected at 1618 and 1500 cm−1 are assigned to the valence vibration υ(C=C) of the aromatic rings. Besides, the vibration bands observed in the range 1450–1150 cm−1 are characteristic of δs,as(CH3), δs,as(NH3), and δs,as(C-N). While the low-intensity bands appearing between 1000 and 500 cm−1 correspond to the deformations δ(Caryl-H), δ(Caryl-Caryl), δ(Caryl-N), and γ(N-H) of the phenyl ring.


## 3.6. Thermal Analysis

Simultaneous TGA-DTA analyses were carried out to study the thermal stability and the changes of the newly synthesized compound (C9H14N)2HgCl4. The results are shown in Figure 11. A series of endothermic and exothermic peaks are observed on the DTA curve, accompanied by a weight loss on the TGA curve. The two endothermic peaks detected on the DTA curve observed at 362 K and 391 K without mass loss correspond to two-phase transitions phenomena. The three exothermic peaks observed a 441, 474, and 530 K with ΔH = −1279.880 J/g, ΔH = −903.219 J/g, and ΔH = −128.806 J/g, respectively, correspond to the loss of the two organic cations with a total experimental mass of 45% compared to the calculated weight loss of 46%. The two exothermic peaks in the DTA curve observed at 561 and 600 K, with ΔH = −22.134 J/g and ΔH = −1212.156 J/g, respectively, are assigned to the elimination of four chlorine atoms (exp/calc: 21.5%/22%). The compound completely melts since there is no endothermic peak after 600 K.Figure 11
TGA/DTA curves for (C9H14N)2HgCl4.


## 3.7. Electric Conductivity

To better understand the phenomenon of electrical conduction, we will use the equation of Arrhenius’ law [45]:(3)σ.T=Aexp−EaKβ.T,where Ea is the activation energy, T is the absolute temperature, Kβ is the Boltzmann constant, and A is a constant depending on the material. Figure 12 shows the curve of plotting ln(σ,T) versus 1000/T. This graph is formed by three regions (I), (II), and (III) and two breaks between 301 and 471 K. Each domain is presented by an affine straight line which is in accordance with Arrhenius’ law. The first break is observed at 393 K, with an activation energy of 0.59 eV, and (II), with an activation energy of 4.21 eV. The second break is observed at 368 K for which the activation energy of (I) is 0.60 eV. A slope change at this temperature was observed in the DTA curve due to two endothermic peaks at 362 and 391 K, respectively. In the studied temperature interval, the conductivity increases with temperature showing that this material behaves like a semiconductor [46].Figure 12
Variation of ln (σT) versus 1000/T for (C9H14N)2HgCl4.


## 4. Conclusion

This work is devoted to synthesizing a new organic compound of formula (C9H14N)2HgCl4. The X-ray diffraction analysis revealed that this compound crystallized in the monoclinic system with P21/c space group. The crystal structure showed an alteration of anionic and cationic entities. The structure cohesion was manifested by both strong and weak hydrogen bonds as well as π-π interactions. The HS analysis showed that H…H intermolecular interactions exemplify most of the total surface. The PXRD was proportionally homogenous with the single crystal. The functional groups of the crystal were assigned using FT-IR spectroscopy based on the observed bands. SEM/EDX was also realized to determine the morphology and constituents of the complex, respectively. The thermal analysis clarified that this compound is stable up to 370 K. The conductivity study reveals the presence of two transition phases already detected by thermal analysis using DTA-TGA measurements.



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