Crystal Structure, Spectroscopic and Thermal Investigations of a New 4-Ammonio-2,2,6,6-

The new complex of Zn(II) with 4-ammino-2,2,6,6tetramethylpiperidine, [C9H22N2]ZnCl4, has been prepared and characterized by various physicochemical studies. This compound crystallizes in the monoclinic system, with the space group P21/c and the following cell parameters: a = 9.8682(7), b = 7.5164(5), c = 21.019(2) Å, β = 99.003 (7)°, V = 1539.8(2) Å and Z = 4. The crystal structure was solved and refined to R = 0.024 using 3070 reflections. The atomic arrangement can be described by slightly distorted tetrahedral ZnCl4 2anions and 4-ammmonio-2,2,6,6tetramethylpiperidinium cations holding together by different interactions. The four chlorine atoms of the ZnCl4 2tetrahedron are acting as acceptors of hydrogen bonds. The crystal packing is influenced by cation-to-anion N-H...Cl and C-H...Cl hydrogen bonds leading to open framework architecture.


Introduction
Chemists and physicists of the solid state have shown an increasing interest in the study of the organic-inorganic hybrid materials in recent years owing to their potential fundamental and practical interest such as, fuel cell, liquid crystal-material development, second-order nonlinear optical (NLO) responses, magnetism, luminescence, photography and drug delivery, etc [1][2][3][4][5][6][7][8][9][10][11] .Moreover, in the organic inorganic hybrid metal halides, hydrogen bonds play an important role in stabilizing the frameworks of the organ ammonium cations and their counter inorganic species.The energetic of N-H…halide-metal hydrogen bonds and their possible role in supramolecular chemistry continue to attract much attention 12 .It is therefore important to design and synthesize novel hybrid compounds by reaction between the zinc chloride and an organic ligand containing N-H bonds.In the present investigation, we report the synthesis and the physicochemical characterization of [C 9 H 22 N 2 ]ZnCl 4 .

Experimental
Crystals of the title compound were prepared according to the following chemical reaction: ZnCl 4 By slow evaporation of an aqueous solution of 4-ammino-2,2,6,6-tetramethylpiperidine, hydrochloric acid and zinc chloride.The crystals are stable under normal conditions of temperature and humidity.The chemical formula was determined during the X-ray crystal structure analysis.

X-ray diffraction
The characterization of these salts was carried out using X-ray diffraction, Infrared spectroscopy, thermal analysis and impedance spectroscopy.A single crystal was carefully selected under polarizing microscope in order to perform its structural analysis by X-ray diffraction.The intensity data were collected on an Enraf-Nonius MACH3 automated fourcircle diffractometer using the graphite monochromated MoKα radiation at λ = 0.7107 Å.The structure was solved by direct methods using the SIR97 13 program and refined by full matrix least-squares techniques using CRYSTALS 14 .The drawings were made with Diamond 15 .All non-hydrogen atoms were refined anisotropically.The hydrogen atom positions were located by difference-Fourier synthesis and refined with riding restraints.Crystal data and experimental parameters used for the intensity data collection are summarized in Table 1.The main geometrical features of the different entities are given in Tables 2 and 3.

Infrared spectroscopy
The infrared spectrum was recorded in the range 4000-400 cm -1 with a "Perkin-Elmer FTIR-1000" spectrophotometer using a sample dispersed in spectroscopically pure KBr pellets.

Thermal analysis
Thermal analysis was performed using the "multimodule 92 Setaram" analyzer operating from room temperature up to 500 °C at an average heating rate of 5 °C.min - .

Impedance spectroscopy
Impedance spectroscopy was used to measure the electrical conductivity [16][17] .The impedance diagrams were recorded in the 5 Hz -13 MHz frequency range using a Hewlett Packard HP 4192A analyser.To ensure electrical contacts, the two parallel surfaces of the sample (having a cylindrical shape) were coated by a layer of silver paint.The platinum wires and the sample were held in contact by a weak mechanical pressure controlled by a screw/spring system and transmitted by an alumina rod.

Structure description
As shown in Figure 1, to ensure charge equilibrium, the organic species is doubly protonated at N1 and N2 nitrogen atoms.Thus, the structure associates to each 4-ammmoni-2,2,6,6tetramethylpiperidinium cation one tetrachlorozincte anion.Figure 2 shows that the atomic arrangement of the title compound can be described by inorganic ZnCl   The Zn(II) ion is in a tetrahedral coordination environment composed of four Cl -anions.Each ZnCl 4 2-group is connected to its four adjacent neighbour's organic cations by N-H…Cl and C-H…Cl interactions involving four chlorine atoms (Table 3).The (N)-H…Cl distances, varying between 2.37 and 2.56 Ǻ, are smaller than the sum od Van der Waals radii of the chlorine and hydrogen atoms [r(Cl) + r(H) = 2.81 Ǻ].Consequently, these values correspond well to strong hydrogen bonds.However, the C-H…Cl hydrogen bonds play a subordinative role in stabilizing the structure 18,19 .Moreover, the remarkable feature in the complexe salt is the N-H…Cl hydrogen bonding which form ( ( ( ( ) ) ) )  Previous studies of tetrachlorozincates have shown that the Zn-Cl bond lengths and Cl-Zn-Cl bond angles in the ZnCl 4 2-anion are not equal to one another but depend on the environment around the anion 21 .For example, the presence of hydrogen bonds or repulsions influences appreciably these geometrical parameters.Indeed, the fact that the Zn-Cl1 bond length is the longest meets the expectation derived from hydrogen bonding effects, because the atom Cl1 accepts three hydrogen bonds, while the remaining Cl2, Cl3 and Cl4 chlorine atoms are doubly acceptors (Table 3 ).However, the mean values of the Zn-Cl bond lengths and Cl-Zn-Cl bond angles are 2.266(5) Ǻ and 109.47(2)°,respectively, are in agreement with those found in 2,6-Xylidinium tetrachlorozincate dihydrate [ 2.267 Ǻ and 109.5°] 22.In the title hybrid material, the Cl-Zn-Cl angles range from 106.12(2) to 113.86(2)°.These values indicate that the geometry around the zinc atom is approximately tetrahedral, according to the Bis(quinolinium) tetrachlorozincate(II) where the corresponding limit angles are 106.33(3)and 112.34(4)° 23 .However, in the 4-(2-ammonioethyl)) morpholin-4ium tetrachlorozincate these angle values vary from 98.90(4) to 114.74(4)° indicating that the coordination geometry of Zn atom could be regarded as a distorted tetrahedron 24 .

Infrared spectroscopy
The IR spectrum of crystalline [C 9 H 22 N 2 ]ZnCl 4 is shown in Figure 4. To assign the IR bands to vibrational modes, we have examined the modes and frequencies observed in similar compounds [25][26][27] .

Impedance spectroscopy
Typical impedance spectra were carried out in the temperature range 383-423 K for electrolyte (Figure 6).The total conductivity of the samples has been obtained from the extrapolated intercept of the semicircle with the real axis at low frequency.This intercept combines volume and grain boundary components of the total resistance.Figure 7 shows the temperature dependence of the electrical conductivity σ of the title compound.The increase of the conductivity as a function of temperature proves that this material has a non-metal behaviour.Where E a is the activation energy, k and T are the Boltzmann constant and the absolute temperature, respectively.The A 0 constant includes the carrier concentration as well as other material dependent parameters 32,33 .The Arrhenius plot shows that the activation energy is practically constant in the chosen temperature range which can be explained by only one mechanism of conduction.The obtained value for the activation energy (0.15 eV) is relatively weak and is in favour of protonic conductivity mechanism.

Conclusion
The new complex [C 9 H 22 N 2 ]ZnCl 4 was prepared as single crystals at room temperature and characterized by various physicochemical studies.The crystal structure contains isolated slightly distorted tetrahedral ZnCl 4 2anions held to the 4-ammmonio-2,2,6,6tetramethylpiperidinium cations by multiple hydrogen bonds which lead to an open framework architecture.When heated, the title compound undergoes a decomposition of the organic entity in a large temperature range.The activation energy issued form the impedance spectra suggests a protonic conductivity mechanism for this materials.

Supplementary material
Crystallographic data for the structural analysis have been deposited at the Cambridge Crystallographic Data Centre, CCDC No CCDC 736280.Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 IEZ, UK(fax: +44-1226-336033; e-mail: deposit@ccdc.camor www:http:/www.ccdc.cam.ac.uk).

4 2 -
species isolated by the organic cations.These entities are held together by coulombic attraction and multiple hydrogen bonds to form a three dimensional infinite network.This figure shows columns, formed by two ZnCl 4 2-anions and two 4-ammmoni-2,2,6,6-tetramethylpiperidinium cations, running along the b-axis at x = ½ and z = n/2.

Figure 1 .
Figure 1.Asymmetric unit of [C 9 H 22 N 2 ]ZnCl 4 with atom labels and 40% probability displacement ellipsoids for non-H atoms

Figure 2 .
Figure 2. Projection of the structure of [C 9 H 22 N 2 ]ZnCl 4 along the b axis.ZnCl 4 is given in polyhedral representation.Hydrogen bonds are denoted by dotted linesThe Zn(II) ion is in a tetrahedral coordination environment composed of four Cl -anions.Each ZnCl 4 2-group is connected to its four adjacent neighbour's organic cations by N-H…Cl and C-H…Cl interactions involving four chlorine atoms (Table3).The (N)-H…Cl distances, varying between 2.37 and 2.56 Ǻ, are smaller than the sum od Van der Waals radii of the chlorine and hydrogen atoms [r(Cl) + r(H) = 2.81 Ǻ].Consequently, these values correspond well to strong hydrogen bonds.However, the C-H…Cl hydrogen bonds play a subordinative role in stabilizing the structure18,19 .Moreover, the remarkable feature in the complexe salt is the N-H…Cl hydrogen bonding which form ( ring type20 as shown in Figure3.

Figure 3 . 2 N
Figure 3. Hydrogen bonding which form representation of

Table 1 .
Crystal data, experimental parameters used for the intensity data collection, strategy and final results of the structure determination [C 9 H 22 N 2 ]ZnCl 4