Crystalline and Molecular Structure of Hydrated 2 , 6-Xylidinium Tetrachlorozincate ( II )

A new zinc complex compound of formula [2,6-(CH3)2C6H3NH3]2ZnCl4. 2H2O has been prepared and characterized by X-ray crystallography, thermal analysis and IR spectroscopy. The complex crystallizes in the monoclinic space group P21/a with a minimal tetrahedral distortion of the ZnCl4 2ion, a = 7.448(1) Å, b = 38.486(3) Å, c = 8.298(1) Å, β = 112.52(1) °, V = 2197.4(4) Å and Z = 4. The crystal structure was solved and refined to R = 0.047 with 3270 independent reflections. It can be described by inorganic layers of [ZnCl4.(H2O)2] 2parallel to (010) plane, between which the 2,6-xylidinium cations are located. The grouping is maintained by different types of interactions (electrostatic, H-bonds, Van der Walls and π-π stacking). A characterization of this compound by thermal analysis and IR spectroscopy are also reported.


Introduction
Hybrid materials with organic and inorganic components are an attractive field of research due to their ability to combine the properties of organic and inorganic compounds within one single molecular scale, so as to exhibit some interesting crystal structure and some special properties in several areas, such as nonlinear optical (NLO), magnetism, luminescence, etc [1][2][3][4][5][6][7][8] .It is therefore vital to design and synthesize novel salts with inorganic anions and organic cations to explore their various properties.A recent advance in this system is to design the coordination frameworks of metal halides by the incorporation of various organic structuredirecting agents and generating new supramolecular entities [9][10][11][12][13] .In the present investigation we report the synthesis and crystal structure of a new organic-inorganic hybrid zinc complex, [2,6-(CH 3 ) 2 -C 6 H 3 NH 3 ] 2 ZnCl 4 .2H 2 O (Figure 1).Its characterization by thermal analysis and IR spectroscopy are also reported.

Synthesis
Crystals of Zn(II) complex with 2,6-xylidine, (C 8 H 12 N) 2 ZnCl 4 .2H 2 O, were prepared by slow evaporation of an aqueous solution of 2,6-xylidine, HCl and ZnCl 2 (2:2:1) molar.Crystals are stable for a long-time in normal conditions of temperature and humidity.The chemical formula was determined when resolving the crystal structure by X-ray diffraction spectroscopy.

Investigation techniques X-ray diffraction
A single crystal of the complex was mounted on a MACH 3 Enraf Nonius diffractometer operating at 296 K with a monochromated Mo radiation source (λ=0.7107Å).The structure was solved by direct methods using the SIR92 14 program and refined by full matrix leastsquares techniques based on F using teXsan 15 .The hydrogen atoms were not refined, and all non-hydrogen atoms were refined anisotropically.Further details of structural analyses are summarized in Table 1.The final atomic coordinates of the non-hydrogen atoms and their equivalent isotropic displacement parameters, Beq, are given in Table 2.The drawings were made with Diamond 16 .Crystallographic data (CIF) for the structure reported in this paper have been deposited in the Cambridge Crystallographic Data centre as supplementary materials No 682367.Copies of the data can be obtained, free of charge, on application to the CCDC, 12 Union Road, Cambridge CB12EZ, UK

IR Spectroscopy
IR spectrum was recorded in the range 4000-400 cm -1 with a "Perkin-Elmer Spectrum 1000" spectrophotometer using a sample dispersed in spectroscopically pure KBr pellet.

Thermal analysis
Thermal analysis was performed using the "multimodule 92 Setaram analyzer" operating from room temperature up to 723 K at an average heating rate of 5 K.min -1 .

Structure description
The main geometrical features of different entities are reported in Table 3. Figure 1 shows the ORTEP 17  A view of the structure projected along the axis is given in Figure 2.This projection shows that the most striking feature of this architecture is the assembly of the (ZnCl 4 ) 2-anions and the water molecules into two-dimensional network, parallel to ac planes at y=(2n+1)/8, with the hydrogen-bonded cations protruding on both sides of the network.Adjacent twodimensional networks interpenetrate each other and pack through π-π stacking interactions between the xylidinium cations.There are two different π-π interactions between identical antiparallel cations, with face to face distances of 3.649 and 3.735 Å, less than 3.8, maximum value accepeted for π-π interactions 18 .As shown in Figure 3, in the crystal structure of the title complex, the inorganic and organic species interact by mean of a network of four O-H…Cl, two N-H…O and four N-H…Cl hydrogen bonds (Table 4), giving rise to a porous layer.It is notable that the distances H…Cl varies between 1.83 and 2.58 Å.These values are smaller than the sum of the radii of Van Der Waals of the chlorine and hydrogen atoms (r Cl + r H ≤ 2.81 Å).Consequently, these values correspond well to strong bonds.All these hydrogen bonds, as well as coulombic attractions, Van der Walls forces and π-π interactions, act as an important and predictable structural organizing tool and increase so the structure stability of the title hybrid material.As expected, in the ZnCl 4 2-anion, the Zn atom has a tetrahedral coordination surrounded by four Cl atoms.In the [ZnCl 4 ] tetrahedra, the mean values of Zn-Cl lengths and Cl-Zn-Cl angles are 2.267 Å and 109.5°,respectively, which are in agreement with those found in anilinium tetrachlorozincate [19][20][21] .The slightly deviation from the perfect tetrahedron around Z(II) can be explained by the involving of the chlorine ions in the hydrogen bonding.However, the Zn-Cl bond lengths do not differ appreciably from the shortest and longest bond lengths of 2.245(1) and 2.289(1) Å associated to Zn-Cl1 and Zn-Cl3, respectively.In addition, the Cl-Zn-Cl bond angles are also comparable, varying between 105.15(5) and 114.71(6)°.All these data indicate a slight deviation from perfect tetrahedral environment around the zinc cation, according to literature results 22 and in comparaison to the terachlorocuprate salt 23 .Indeed, in this compound, the Cu-Cl bond leghts and Cl-Cu-Cl bond angles vary from 2.216(7) to 2.283(6) Å and from 94.75(2) to 138.95(3°), respectively, indicating a large deviation from perfect tetrahedral environment around Cu(II).An examination of the organic moiety geometrical features shows that, the carbon atoms building the two phenyl rings of the title compound have a good coplanarity and they form a conjugated planes with average deviation of 0.025 and 0.037 Å, and they form between them a dihedral angle of 7.32°.The two crystallography independent 2,6-xilidinium cations involved in this crystal packing, exhibit C-C and C-N distances and C-C-C and C-C-N angles in the range usually found for this molecule [24][25] .

Thermal analysis
Curves corresponding to DTA and TGA analysis are reported in Figure 5.The DTA curve shows a succession of endothermic peaks.The first one (89°C) corresponds to an elimination of the two crystallization water molecules, well confirmed by the weight loss observed on TGA curve (% water experimental 7.30, calculated 7.38).After this dehydratation, the anhydrous compound melts towards 197 °C, confirmed by a separate melting point determination of the title complex.By heating further, the important weight loss observed in the large temperature range, 250-500 °C, corresponds to a decomposition of the obtained phase.

Conclusion
Good quality single crystals of 2,6-xylidinium tetrachlorozincate were grown by slow evaporation technique at room temperature.The structure of the title material is stabilized by different types of interactions and particularly an interesting H-bond network.When heated, this complex loses the crystallisation water molecules and continues to decompose in a large temperature range.

Table 1 .
Crystal and experimental data.