Synthesis and physico-chemical studies of a novel bis [3,5-diamino-4H- 1,2,4-triazol-1-ium] dichloride monohydrate

The title new compound, (C2H6N5 + )2, 2Cl − .H2O, contains two 3,5-diamino-4H-1,2,4-triazol-1-ium cations, two chloride anions and one water molecule. The crystal structure is stabilized by O H···Cl, N H···Cl, N H···O and N H···N hydrogen bonds, one of them being a three-center interaction. Strong π π stacking interactions between neighboring triazolium rings are present, with a centroid centroid distance of 3.338 (7) Å. The exocyclic N atoms are sp 2 hybridized, as evidenced by bond lengths and angles, in agreement with an enamine-imine tautomerism. A dielectric spectroscopic study of the title compound was performed. The 13 C CP-MAS NMR spectrum is in agreement with crystallographic data. The infrared spectrum has been recorded at ambient temperature and interpreted on the basis of literature data. The temperature dependence of the imaginary part of the permittivity constant was analyzed with the Cole Cole formalism in the temperature range 325 375 K.


INTRODUCTION
Chemists and physists of the solid state have shown an increasing interest in the study of halide anion salts containing organic cations owing to their numerous practical and potential applications in various fields such as supramolecular chemistry [1] and biochemical processes [2]. Among all halide anions, the chloride one has been successfully used to assemble double -helical motifs of various molecules [3]. We report here the synthesis and the crystal structure of a new member of this family, (C2H6N5 + )2, 2Cl − .H2O (I), which was isolated during our studies of the preparation of new organic hydrochloride compounds [4,5].

Chemical preparation
An aqueous 1M HCl solution and 3,5-diamino-1,2,4-triazole in a 1:1 molar ratio were mixed and dissolved in sufficient ethanol. Crystals of (I) grew as the ethanol evaporated at 293 K over the course of a few days (yield = 65 %).

Investigation techniques
The characterization of the title compound was carried out by X-ray diffraction, solid state NMR, IR and dielectric spectroscopies and DFT calculations.

X-ray single crystal structural analysis
A suitable crystal was selected and mounted on a Gemini kappa-geometry diffractometer (Agilent Technologies UK Ltd) equipped with an Atlas CCD detector and using Mo Kα radiation ( = 0.71073 Å). Intensities were collected at 100 K by means of the CrysalisPro software [6]. Reflection indexing, unit-cell parameters refinement, Lorentz-polarization correction, peak integration and background determination were carried out with the CrysalisPro software [6]. Analytical absorption correction was applied using the modeled faces of the crystal [7].The structure were solved by direct methods with SIR97 [8] and the least-square refinement on F 2 was achieved with the CRYSTALS software [9]. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were located in a difference map, but those attached to carbon atoms were repositioned geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry (C---H in the range 0.93 -0.98, N---H in the range 0.86 -0.89 and O---H = 0.82 Å) and Uiso(H) (in the range 1.2 -1.5 times Ueq of the parent atom), after which the positions were refined with riding constraints. The drawings were made with Diamond [10]. Crystal data and experimental parameters used for the intensity data collection are summarized in Table 1.

Physical measurements
The 13 C NMR spectrum of the title compound was recorded on a solid-state high-resolution Bruker DSX-300 spectrometer operating at 75.49 MHz with a classical 4 mm probehead allowing spinning rates up to 10 kHz. The chemical shifts are given relative to tetramethylsilane (precision 0.5 ppm). The spectra was recorded by use of cross-polarization (CP) from protons (contact time 5 ms) and MAS. Before recording the spectrum it was checked that there was a sufficient delay between the scans allowing a full relaxation of the protons. The IR spectrum was recorded in the range 4000 -400 cm -1 with a "Perkin-Elmer FTIR" spectrophotometer 1000 using a sample dispersed in spectroscopically pure KBr pressed into a pellet. A 7280 DSP Lock-in Amplifier was used in the frequency range 100 Hz -2 MHz for dielectric measurements. The finely grain samples were pressed into pellets of 8 mm diameter and 1.2 mm thickness using a hydraulic press at 3000 Kg/cm 2 . Contacts were made by coating the surfaces of the sample with silver paste. Measurements were made in the temperature range 325 -375 K. The temperature regulation was provided by a programmable DC power supply (HP E3632A), a programmable Keithley multimeter (Model 2000) and an oven developed in our laboratory. The temperature was controlled to within ± 0.1 °C. Before performing the measurement, a sufficient time was allowed for thermal stabilization.

DFT calculations
The infrared spectrum was calculated with the Gaussian 09 software by using different starting molecules: only one organic cation, the organic cation and the chlorine atom linked to it and all the atoms of the asymmetric unit. The positions of the protons were optimized in all cases by using the B3LYP/6-31++G** method and finally the infrared spectra was calculated. In all cases at least one imaginary frequency (close to zero) was obtained and finally a calculation was made with only one organic cation and a full optimization before determination of the frequencies. In that case no imaginary frequency was obtained and a comparison of the frequencies at wavenumbers higher than 400 cm -1 showed that quite the same results (in terms of position, intensity and nature of vibration) were obtained than with the other systems for the vibrations having a sufficient intensity (naturally in the case of the system containg all the atoms of the asymmetric unit the bands were dedoubled). Finally the spectrum was drawn by broadening the peaks with a lorentzian. J u l y 2 6 , 2 0 1 4

Structure description
The main geometrical features of the different chemical entities of the coordination compound (C2H6N5 + )2, 2Cl − .H2O are reported in Tables 2 and 3.

NMR spectroscopy
The 13 C CP-MAS NMR spectrum of (C2H6N5 + )2, 2Cl − .H2O is shown in Fig. 4. It exhibits only one broad and quite asymmetric resonance signal at ca. 150 ppm corresponding to the four crystallographically independent aromatic carbon atoms. The peaks of these carbon atoms are overlapping probably due to their relatively similar chemical environments. J u l y 2 6 , 2 0 1 4

IR absorption spectroscopy
FTIR spectroscopy was used to verify the functional groups present in the crystal and their vibrational behavior in the solid state. Hydrogen bonding interactions are able to affect both frequency as well as intensity and width of the vibrations. The IR spectrum of the title crystalline compound is shown in Fig. 5. The characteristic vibrational modes of these compounds can be compared to those of similar materials [13 -15]. In the high frequency region, the observed bands between 3600 and 3100 cm -1 can be assigned to the OH stretching mode and to the asymmetric and symmetric stretching modes of the NH2 group, respectively [16]. The peaks situated between 2918 and 2670 cm -1 correspond to nonfundamental NH stretching modes and to the CH stretching modes [17]. The bands observed at 1700, 1510 and 1460 cm -1 are assigned to the (OH), to the asymmetric as(NH2) and symmetric s(NH2) bending modes, respectively, and to C-C, C-N and N-N stretching vibrations [18,19]. The bands between 1000 and 600 cm -1 are assigned to the out of plane bending modes γ(Cary-H), γ(Cary-C) and γ(N-H) [20].

DFT calculations
The DFT calculations show that all observed vibrations are due to the organic cation and to the water molecule. However the vibrations of water lead to intensities much smaller than those of the organic cation as shown when performing the calculations on a system containing all atoms of the asymmetric unit. Even if the calculation led to some slightly imaginary frequencies (ca. -50 cm -1 which can be explained by a small variation, within the experimental errors, of the position of one atom) it is this calculation which was taken into account for the comparison The corresponding spectrum is shown on Figure 6 while the comparison between experimental and theoretical values is shown on Figure 7. Clearly there is a good agreement between the experimental and theoretical values if the scale factor is taken into account. The attributions which can then be deduced from these calculations are the same than those given above.

Dielectric spectroscopy
The frequency dependent complex permittivity of the studied compound (        i ) ( * ) was described analytically by the generalized Cole -Cole model [21]. For the first relaxation process one can write:  is the high frequency permittivity, s  is the low frequency permittivity,  and  are the relaxation times and the distribution parameters ( 1 0    ), respectively. For a single Debye type relaxation, the distribution parameter  is equal to zero.
The frequency dependence of the imaginary part of the dielectric permittivity is shown on Figure. 8. A considerable deviation from the Debye-type behavior at low frequencies is observed which can be attributed to the DC conductivity.

Conclusions
The asymmetric unit of the crystal structure of the title compound contains two 3,5-diamino-4H-1,2,4-triazol-1-ium cations, two chloride anions and one water molecule. The organic cations containing the N(4), N(5), N(7), N(9) and N(10) nitrogen atoms are interconnected via N(7) -H(71)···N(4) hydrogen bonds to form organic chains. The organic cations containing the N(11), N(13), N (14), N (16) and N(17) nitrogen atoms are associated by N(11) -H(111)···N(13) hydrogen bonds leading to the formation of dimers characterized by strong ππ stacking interactions between neighboring triazolium rings. These dimers, the Cl − anions and the water molecules are located between the organic chains through a set of hydrogen bonds. The 13 C CP-MAS NMR spectrum agrees with the structural data. The vibrational properties were investigated by infrared spectroscopy. Dielectric measurements show a deviation from the Debye-type behavior at low frequencies which can be attributed to the DC conductivity. J u l y 2 6 , 2 0 1 4