The synthesis of the single crystal of C₆H₄(NH₃)₂∙Cl₂ (I) and β-[C₆H₁₀N₂]₂ZnCl₄ (II) compounds was carried out at home-built Teflon-lined stainless steel pressure bombs of 90 ml maximum capacity as follow: 1,2-benzen diammonium C₆H₈N₂ (sigma aldrich) was dissolved in a mixture of distilled water (10 mL) and Hydrochloric acid (0.5 mL) (sigma aldrich) for a few minutes with agitation successive to ensure complete dissolution. Second, such solutions were slowly combined in an autoclave and kept at 95˚C under auto-genous pressure for 24 h. After cooling these solutions for five days at room temperature, colourless single crystals C₆H₄(NH₃)₂∙Cl₂ (I) suitable for an X-ray structure.

-1, 2-benzen diammonium C₆H₈N₂ (0.0801 g; 0.622 mmoles) and Zinc chloride ZnCl₂ (0.5032 g; 1.88 mmoles) (sigma aldrich) were dissolved in aqueous solution of HCl (1 M; 37%) for a few minutes with agitation successive to ensure complete dissolution of the compound hydrochloric acid (pH ≈ 1.8). The mixture was placed in a Teflon-lined autoclave that was then sealed and heated to 95˚C for 24 h. It was then allowed to cool to room temperature in a cold water bath. Autoclaves were opened in the air, and products were recovered through filtration. White stick-shaped crystals β-[C₆H₁₀N₂]₂ZnCl₄ (II) with suitable dimensions for crystallographic study were recovered (Scheme 1).

Schematically the reaction is shown in the following equation.

­ XRD measurement

Phase purity and homogeneity of synthesized of both compounds were determined by x-ray powder diffraction recorded at room temperature in the angular range 5˚ < 2θ < 80˚ (Figure 1) with a step Δ (2θ) = (0.0170)˚ using a X’Pert PROMPD diffractometer with CuKα1 radiation (λKα1 = 1.540598 Å).

­ Single-crystal data collection and structure determination

Suitable crystals selected through an optical examination were mounted a intensity data were obtained on a D8 VENTURE Bruker AXS diffractometer equip- ped with a (CMOS) PHOTON 100 detector using Mo K_α radiation (λ = 0.71073 Å, multilayer monochromator) through the Bruker AXS APEX2 Software Suite [36] .

Frame integration and data reduction were carried out with the program SAINT [37] . The program SADABS [38] was then employed for multiscan-type absorption corrections. The crystal structure was solved in the monoclinic symmetry, space group C2/c; of two compounds, respectively; according to the automated search for space group available in Wingx [39] .

Zinc and chloride atoms were located using the direct methods with the program SIR-2014 [40] . C and N atoms from the amine were found from successive difference Fourier calculations using SHELXL-2014 [41] . Their positions were validated from geometrical considerations as well as from the examination of possible hydrogen bonds. H atoms were positioned geometrically and allowed to ride on their parent atoms. Structural drawings have been made by Diamond 2 [42] program. Crystallographic data are given in Table 1.

­ Spectroscopy measurements (IR and NMR-MAS)

The IR absorption spectrum of the crystallized powders in KBr was recorded on a Perkin-Elmer FT-IR 1000 spectrometer in the 400 - 4000 cm⁻¹ range. Cross polarization (CP) is to take advantage of the presence of spins abundant (1H) that we will excite to be able to cause a transfer of energy towards the spins scarce (¹³C). This transfer will be much more effective than just irradiation one could practice on the small nuclei. The technique cross-polarization (CP) is applied to types of acquisitions. All chemical shifts (d) are given with respect to tetramethylsilane for ¹³C. According to the IUPAC convention, shielding corresponds to negative values. Spectrum simulation was performed by using Bruker WINFIT software [43] .

The NMR-MAS experiments were performed at room temperature on a Bruker WB 300. Then, the powdered sample was packed in a 4 mm diameter rotor and set to rotate at a speed up to 8 kHz with 63.669 resonance frequency in a Doty MAS probehead. During the whole acquisition time, the spinning rate of the rotor was locked to the required value using the Bruker pneumatic unit which controls both bearing and drive inlet nitrogen pressures. The ¹³C spectrum was collected by a cross-polarization of proton with 5 ms contact time.

­ Hirshfeld surfaces

Hirshfeld surface analysis is a powerful tool for obtaining additional information on the intermolecular interaction of molecular crystals. The size and the Hirshfeld surface form allow (allow) qualitative and quantitative investigation and visualization intermolecular close contacts in molecular crystals [44] . The 2D fingerprint of the Hirshfeld surface represents a new method for summarizing the complex information contained in a molecular crystalline structure in a single plot single color, which provides a “fingerprint” of intermolecular interactions in the crystal.

The combination of de and di in the form of a 2D fingerprint provides a summary of the contacts intermolecular in the crystal and complement the Hirshfeld surfaces [45] . Such parcels provide information on intermolecular transport interactions in the immediate environment of each molecule in the asymmetric unit.

XRD description:

The room temperature powder X-ray patterns of the (I) and (II) compounds are shown in Figure 1. The formation of pure phase for the both compounds without secondary phases has been confirmed. The peaks are fine, indicating that the samples are well crystallized. All the reflection peaks were indexed in the monoclinic system with the space group. All the reflection peaks were indexed in the monoclinic system with the space group C2/c. The lattice parameters were refined using the least-squares subroutine of a standard computer program. These refined lattice parameters of C₆H₄(NH₃)₂∙Cl₂ (I) and β-[C₆H₁₀N₂]₂ZnCl₄ (II) compounds are: a = 7.267 (7) Å; b = 14.403 (1) Å, c = 7.941 (1) Å, β = 94.568 (2)˚, V = 828.592 (2) ų; a = 7.411 (7) Å; b = 25.585 (1) Å, c = 9.342 (1) Å, β = 94.022 (2)˚ and V = 1776.93 (2) ų, respectively. The unit cell parameters are in good agreement with the single crystal X-ray values.

Structure description of C₆H₄(NH₃)₂∙Cl₂ compound (I):

The structure of C₆H₄(NH₃)₂∙Cl₂ compound (I) occurs as monoclinic crystals in the space group C2/c with each molecule occupying a general position within the unit cell. The unit asymmetric of compound (I) (Figure 2) consists of two isolated chloride atoms and half of the organic cation [C₃H₅]⁺. A projection along the c-axis of the structure can be described by alternating [C₆H₁₀N₂]²⁺ cationic layers and Cl⁻ anionic layers (Figure 3) through extensive intermolecular N-H∙∙∙Cl interactions.

Indeed, the pyridine rings of the original 1,2-benzen diammonium have undergone bi-protonation (H⁺) during the synthesis process. The organic layer, observed at z/b = 1/4, is formed by the cations [C₆H₁₀N₂]²⁺, arranged parallel plane in the direction a and interconnected via a p-p interaction.

In the half cation the C-C/C-N bonds are almost equivalent: C2-C3 = 1.3867 (2) Å; C3-C4 = 1.3929 (2) Å and C2-N1 = 1.4607 (2) Å. The C-C-C and C-C-N angles vary within the range from 119.40 (1)˚ and 119.815 (1), respectively. The main inter atomic distances and angles in the cation are reported in Table 2. These values are in agreement with those observed in similar compounds [46] [47] [48] .

Indeed, the crystal packing is assured by three hydrogen bonds (N-H∙∙∙Cl), N1-H1A···Cl1, N1-H1B···Cl1ⁱⁱ, N1-H1C···Cl1ⁱⁱⁱ (for details and symmetry code, see Table 3).

Structure description of β-[C₆H₁₀N₂]₂ZnCl₄ compound (II):

The crystallographic analysis revealed that the complex (β-[C₆H₁₀N₂]₂ZnCl₄ (I)) crystallized in the monoclinic system, space group C2/c, involving the coordination of 1,2-benzen diammonium cation to the Zn²⁺ anions. It is a three-di- mensional (3-D) structure with an asymmetric unit of one [C₆H₁₀N₂]²⁺ cation and one [ZnCl₄]²⁻ (Figure 4). Their following unit cell dimensions are grouped in Table 1.

Codes de Symétrie: (ii) −x + 3/2, −y + 1/2, −z + 1; (ii) −x + 1, y, −z + 1/2.

Projections along the c-axis of the structure present tetrahedral layers, [ZnCl₄]²⁻, sandwiched between two layers of pyridinium cations [C₆H₁₀N₂]²⁺ (Figure 5(a)).

The organic part is formed by a single type of cation [C₆H₁₀N₂]⁺ (Figure 5(a) and Figure 5(b)). The main details of interatomic distances and angles are summarized in Table 4. Bond distances and angles in the cations are normal [49] [50] , with average of C-C = 1.389 (6)Å, C-N = 1.439 (5)Å, C-C-C = 119.998 (3)˚ and C-C-N = 120.035 (2)˚. The small angle is of the order of 117.67 (2) presents a small variant with that observed in benzene, this is due to the link N-H∙∙∙Cl. Protonation double in N influences the angles, which indicates flattening of the benzene.A similar behaviour was noticed earlier for quinine and cinchonine [51] [52] . Furthermore, an influence of the proton (H⁺), attached to the nitrogen atome (N1 or N2), on the surrounding of this atom may be estimated by a parameter called pyramidality [53] . The C-C distances equal to 1.383 Å and C-C-C angles equal to 120˚ [54] . This variation is due to the existence of intermolecular interactions. The protonation in the N site is well confirmed by the C-NH₃ bond length, in fact the distance C1-N2 is equal to 1.413 (2) Å, which is significantly longer than in the case of the C-NH2 molecule (CN = 1.335 (3)) [55] , the CCN angles range from 119.01 (2)˚ to 121.45 (2)˚.

Geometry and coordination of the anion [ZnCl₄]²⁻:

The inorganic layer is made up of [ZnCl₄]²⁻ tetrahedral, observed at z/c = 1/4 and z/c = 3/4 (Figure 5(a)). The four distances between Zn-Cl ranged from 2.2561 (4) to 2.2938 (4)Å. They are equivalent two to two. Six Cl-Zn-Cl angles are varied between 105.155 (16)˚ and 115.72 (3)˚. They are grouped by three, two and a wide angle of the order of 115.72 (3) (for details see Table 5). These values are in agreement with those observed in similar compounds [56] [57] [58] [59] .

From the geometric parameters, the [ZnCl₄]²⁻ anion has a slightly distorted tetrahedral stereochemistry. The Baur distortion indices [60] can be calculated using the expressions:

I D ( Zn-Cl ) = ∑ i = 1 n 2 | d i − d m | n 1 d m   ;   I D ( Cl-Zn-Cl ) = ∑ i = 1 n 2 | a i − a m | n 2 a m   ;

where d is the (Zn-Cl) distance, a is the (Cl-Zn-Cl) angle, m is the average value, n₁ = 4, and n₂ = 6. The values of these indices were 0.00206 and 0.00438, respectively.

Hydrogen bonding interactions

The assembly between inorganic and organic entities in the isostructural compounds is provided by hydrogen bonds of the N-H∙∙∙Cl type. Hence, these interactions have a crucial role in the formation of 3D supramolecular structures. The hydrogen bond parameters are given in Table 3 and Table 6.

For the compound (I), the N∙∙∙Cl distances are between 3.093 Ǻ and 3.105 Å and the H∙∙∙Cl distances are in the range of 2.266 Ǻ to 2.314 Ǻ. However the NH∙∙∙Cl angles vary between 151.34˚ and 168.24˚ (Table 3).

For the compound (II), the greatest N∙∙∙Cl distance is 3.712 (2) Å and the H∙∙∙Cl distances are in the range of 2.40 (3) Ǻ to 2.93 Ǻ. However the NH∙∙∙Cl angles vary between 113.4 (2)˚ and 161 (2)˚. Whereas the N-H angle∙∙∙N is in the range 123˚ and 153˚ (Table 6). It is deduced that the NH∙∙∙N hydrogen bonds are moderate [61] [62] [63] [64] [65] .

The comparison between the angles NH∙∙∙Cl, allowed us to note that those of the compound (II) are larger. These are due to the chlorine atoms bound to the Zinc atom, against the chlorine of the first compound is free.

Based on values of Table 3 and Table 5, it can be concluded that the hydrogen bonds are weak [66] . The cations are connected to each other by π-π stacking interactions are of fundamental importance for the further development of supramolecular components and prediction of their crystal structures.

Hirchfeld surfaces, which define the electron density weighing the boundary surfaces between molecules in a crystal, are useful for analyzing and displaying intermolecular contacts [67] .

The Hirshfeld surfaces of the both compounds are showed in Figure 6 and Figure 7. Hirchfeld molecular surface calculations were performed using the Crystal program Explorer [68] [69] , CIF files have been registered in this program. Hirchfeld surfaces are calculated using the following equation:

d n o r m = d i − r i v d w r i v d w + d e − r e v d w r e v d w

where, d_e is the distance between the Hirshfeld surface and the nearest atom outside the surface, d_i is the distance between the Hirshfeld surface and the nearest atom inside the surface, d_norm is expressed as a function of d_e and d_i and the radius of Van Der Waals atoms. The d_norm value is negative or positive when the intermolecular contacts are shorter or longer than the Van Der Waals (VDW) rays, respectively. The Hirshfeld surface uses a red-blue-white color scheme: the red regions represent the closest contacts and a negative reference value; the blue regions represent longer contacts and a positive d_norm value; and the white regions represent a contact distance exactly equal to the VDW separation with a d_norm value of zero [70] .

D fingerprints of the Hirshfeld surface of the study structure to highlight atoms involved in close contact [71] [72] .

Figure 8 illustrates the 2D fingerprint of all the contacts contributing to the Hirshfeld surface of C₆H₄(NH₂)₂∙HCl (I) compound. The graph in Figure 9 illustrates the 2D Fingerprint of all the contacts contributing to the Hirshfeld surface of β-[C₆H₁₀N₂]₂ZnCl₄ (II) compound, associated with the hydrogen atoms.

These graphs represents the H-Cl/Cl-H contacts between the hydrogen atoms located inside the Hirshfeld surface and the externally located in chlorine atoms and vice versa. The H-Cl/Cl-H contacts have the largest contribution to the total Hirshfeld surface (41.9%).

In the crystallization of these compounds, intermolecular interactions of the H∙∙∙ClCl∙∙∙H and H∙∙∙H type are the most abundant. It is evident that van der Waals forces exert an important influence on the stabilization of packing in the crystal structure. While other interconnects contribute less to Hirshfeld surfaces: H-C/C-H (7.6%) (Figure 8(c)). However, the contacts Cl-Cl (Figure 8(f)), C-Cl/ C-Cl (Figure 8(f)) and N-H/H-N (Figure 8(g)) have contributions of very small percentages of interactions (0.6%), (0.1%) and (0.2%), respectively.

In order to give some insight into the nature of forces stabilizing the discussed structures, the intermolecular interactions were analyzed with the help of Hirshfeld surface. Figure 9 present the 3D Hirshfeld surface and 2D fingerprint histogram of the title molecule of for the β-[C₆H₁₀N₂]₂ZnCl₄ (II). This state indicates presence of the C-H∙∙∙Cl type intermolecular hydrogen bond interactions in crystal structure. The C-H∙∙∙Zn type intermolecular hydrogen bond distances in crystal. Similarly, as seen from crystallographic analysis, the contact distances between Cl are obtained between and hydrogen atoms have considerable small values and the Cl-H/H-Cl interactions contribute to Hirshfeld surface with 34.2% value. The major contributions to Hirshfeld surface are H-H with 32.2% and C-H/H-C with 7.3%. The fingerprint of β-[C₆H₁₀N₂]₂ZnCl₄ (II) compound, suggest that the H∙∙∙H and C∙∙∙H contacts dominate the crystal interactions. However, these interactions present higher interatomic distance. Interactions with less interatomic distance (stronger interactions) are observed in fingerprint plots as pointy regions. The H···H contacts are present in higher percentages on the surface probably due to the close packing, resulting in the greater proximity between the molecules in the crystal arrangements. The contacts C···H present contributions with close values in the crystalline forms, smaller in polymorph II. The wings represent contacts C-H∙∙∙π, present in all cases.

Figure 10 and Figure 11 present the infrared spectra of C₆H₄(NH₃)₂∙Cl₂ (I) and β-[C₆H₁₀N₂]₂ZnCl₄ (II) at ambient temperature; respectively; to give more information on the crystal structure. A band at 3400 cm⁻¹, attributed to asymmetric, stretching vibration νas (NH₃), and a less intense band at 3370 cm⁻¹ relative to the symmetrical elongation vibration νs (NH₃). The modes of vibration (C-H) of the benzene nucleus appear at 3220 cm⁻¹ and 2800 cm⁻¹.

The band at 1497 cm⁻¹ is attributed to C = C elongation vibration and the two vibration bands (C-C) are located between 1500 cm⁻¹ and 1526 cm⁻¹. Two bands: one at 1253 cm⁻¹ relative to asymmetric valence vibration νas (C-N) and the other less intense at 1189 cm⁻¹ correspond to symmetric valence vibration νs (C-N). The more intense band at 778 cm⁻¹ attributed the off-plane aromatic (CH) bond, while the two moderately intense bands in the 479 - 515 cm⁻¹ frequency range corresponds respectively to δ (CCN) and torsion outside the plan of the group NH₃. We conclude that infrared spectroscopy confirms the presence of the [C₆H₄(NH₂)]²⁺ cationic part of our compound C₆H₄(NH₃)₂∙Cl₂ (I) [73] [74] [75] [76] [77] .

The IR spectrum has several bands that can be assigned as follows:

­ The bands observed at high frequency between 3550 and 3800 cm⁻¹ in IR is related to symmetrical and asymmetric stretching of an amine group (NH₂). In addition, the observed groups in 3100, 3300 cm⁻¹ in IR are attributed to symmetric and asymmetric NH stretching.

A wide band containing two signals, one around 2800 cm⁻¹ and the other at 2550 cm⁻¹, correspond to the elongation vibrations of (=C-H).

­ The bung, which is located at 1620 cm⁻¹ is probably due to the shear vibration of δ (NH₂) which is combined with (N-H) + of the symmetrical plane (sway) appearing at 1500 cm⁻¹.

­ The elongation vibration (C = C) is observed at 1498 cm⁻¹. Stretching vibrations (C-C) were observed at 1250 cm⁻¹. A stretching, vibration (C-N) appeared at 1200 cm⁻¹. The symmetrical off-plane vibration (C-H) (C-H wagging oop) created a band at 750 cm⁻¹.

­ The band observed at 738 cm⁻¹ is attributed to (C-C) torsional vibration. Finally, the intense peak that appeared at 450 cm⁻¹ is due to the band (C-N) in the plane of flexion.

­ The bands observed at 450 cm⁻¹ correspond to (C-C-C) in the plane.

In conclusion, the infrared study only confirms the presence of the organic group [C₆H₉N₂]²⁺.

The experimental spectrum (in blue) and the spectrum obtained by smoothing, using the β-DMFIT software (in red) are shown in Figure 12.

The isotropic band of ¹³C CP-MAS-NMR spectrum of the crystalline powder sample rotating at the magic angle with frequency 8 kHz illustrated in Figure 11. Spinning the sample at different frequencies allows us to identify the isotropic band. The spectrum is composed of three isotropic bands P1, P2 and P3. Simulation of the isotropic bands permits to identify twelve peaks using Gaussian-Lo- rentzian functions by means of a Dmfit program [43] (Table 7). The deconvolution of three signal shows confirm that the nitrogen-bonded atoms (heteroatom) with a higher electronegativity undergo the phenomenon of shielding consequently these carbon atoms move towards the high chemical shifts in ppm, correspond to the P1 band (to 268.83 at 135.74 ppm). The least shielded peaks observed towards the low ppm; P2 band (130.97 at 117.28 ppm) correspond to the ring carbon atoms not bound to the most unshielded carbon nitrogen atoms; P3 band (108.82 - 87.75 ppm) (Table 7). The chemical shift values were commonly observed in similar compounds [78] [79] .

This study shows that the compound consists of organic groups that have non-equivalent carbons. This result will be confirmed by the study of single- crystal X-rays.