Synthesis, Crystal Structure, and Characterization of the first Organic Cation Hexaoxoperiodate

The title compound is an organic-inorganic hybrid material. The single crystal X-ray diffraction investigation reveals that the studied compound crystallizes in the monoclinic system, space group P21 /c with the following lattice parameters: a = 7.551 (2) Å, b = 6.694 (3) Å, c = 14.783 (2) Å, β = 97.61 (2)° and Z = 2. The crystal lattice is composed of a discrete (H4IO6)− anions surrounded by piperazinium cations and water molecules. Complex hydrogen bonding interactions between the different chemical spices form a three-dimensional network. Room temperature IR, 13 C NMR spectroscopy, thermogravimetric analysis and optical absorption of the title compound were recorded and analyzed.


INTRODUCTION
The chemistry of periodic acid is rather complex as a great number of periodate oxo-anions HXIO6 (5-x)have been elaborated and structurally investigated [1][2]. IO4 anion is called metaperiodate, H4IO6 -, H3IO6 2-, H2IO6 3-, HIO6 4and IO6 5-anions are called orthoperiodates. H4IO6 anion is known only with a limited number of cations. Sebert characterized some compounds containing anions by IR and Raman spectroscopy, but no detailed structural analysis can be found due to twining of the crystals. Recently, Sr(H4IO6)2.3H2O [1] and Be(H4IO6)2.4H2O [2] are structurally characterized on single crystals. In literature, there are some organic cations metaperiodates [3]. Up to now, there is no organic cation orthoperiodate structurally well characterised. At our acknowledgement the title compound, (C4H12N2)(H4IO6)2.2H2O, is the first one of this family. We report here its synthesis and crystal structure. Piperazine and some derivates are biologically active across a number of different therapeutic areas [4] such as antifungal, antibacterial, antimalarial, antipsychotic, antidepressant and antitumour activity against colon, prostate, breast, lung and leukemia tumors [5].

Material Preparation
Crystals of piperazinium bis-tetrahydrogen-hexaoxoperiodate dihydrate were obtained by mixing, in stoichiometric ratio, a solution of periodic acid (1 M) with an aqueous solution of piperazine (0,4g ). The mixture was stirred for 5 hours, and then the precipitated was filtered. The obtained solution was slowly evaporated at room temperature. After 2 weeks of evaporation at 278 K, colorless prism shaped crystals appear in the solution.

Physical measurement
X-ray diffraction measurements were collected using CAD4 diffractometer MACH III, (graphite-monochromated Ag. Kα radiation λ = 0.56087 Å). All calculations were performed using SHELXL97 [6] for structure determination and refinement. The positional parameters of the iodine atoms were obtained from the three-dimensional Patterson map, whereas the remaining atoms were found from successive refinement and difference-Fourier synthesis. After introducing anisotropic thermal factors for the non hydrogen atoms and isotropic ones for H atoms [O−H bond distances and distances between two atoms from each water molecule were restrained to be 0.85(2) and 1.37(2) Å, with Uiso(H)= 1.5 Ueq (O)], the final refinement cycles led to R1 = 0.062 and wR2 = 0.156.
The infrared spectrum of the (C4H12N2)(H4IO6)2.2H2O was recorded on a Nicolet IR200 FT-IR Spectrometer at ambient temperature.
The thermogravimetric measurements were performed with the use of the multimodule 92 Setaram analyzer apparatus in the 25-500 °C temperature range in the argon flow. The rate of heating was 5°C / min; the mass of the sample was 13.83 mg.
The UV adsorption and optical diffuse reflectance spectra were measured at room temperature with a Perkin Elmer Lambda 11 UV/Vis spectrophotometer in the range of 200-800 nm.
The 13 C NMR spectra was measured with a Bruker ULTRASHIELD PLUS 500 spectrometer

X-ray diffraction analysis
Crystal data summary of intensity data collection and structure refinement are reported in Table 1.
The piperazinium bis-tetrahydrogen-hexaoxoperiodate dihydrate (I) has an asymmetric unit built of one H4IO6 anion, one half of piperazinium cation and one water molecule ( Figure 1). H4IO6 anions are coupled into pairs through H-bonds O2-H7…O3. The formed dimmers are connected by O4-H4…O1 to built layers parallel to (b,c) plane at x = 0. These layers are interconnected with the water molecules and piperazinium dications via Ow-H…O, N-H…O and C-H…O hydrogen bonds (Figure 2). In this hydrogen bonding, the water molecules play a double role, acceptor in O5-H5…O7 and donor in O7-H15…O5 and O7-H14…O3. In the IO6 octahedra, there are two groups of distances (I-O) short distances ranging from 1.807 (4)    Hydrogen bonds originating from the H4IO6 anions are expected to be much stronger than those originating from the water molecules. Relatively short distances (2.657 and 2.623Å) are found between the oxygen atoms O2 and O3 and O4 and O1 respectively, of the periodate ions (Table 3). Similar bonding are observed in inorganic compounds Sr(H4IO6)2.3H2O [2]. So, in this case we expect strong hydrogen bonds which link periodate anions to built layers parallel to (b, c) planes (Figure 3). In these latter the tetrahydrogenperiodate anions are associated so as to create R1 6 (18) and R1 2 (8) graph-set motif.  The organic piperazinium dication lies at an inversion centre and adopts a typical chair geometry, as evidenced by the mean deviation (± 0.038) from the least square plane, with normal valence bond lengths and angles [7], as observed in the structures of piperazinediium tetrachloridozincate [8] and piperazinediium tetrachloridozincate monohydrate [9]. A p r i l 1 8 , 2 0 1 4 Table 3

Infrared Spectroscopy
The infrared absorption spectrum of crystalline iodate is shown in Figure 4. To assign the IR peaks to vibrational modes, we examined the modes and frequencies observed in similar compounds [1]. The bands between 3758 cm -1 and 2843 cm

UV Absorption and Diffuse Reflectance
The UV absorption spectrum revealed one little intensity band at 274 nm. These band indicates the n→π* transition of the periodate IO6 anions. Optical diffuse reflectance spectrum ( Figure 5) indicates an optical band gap of 4neV, hence, Wavelength (cm -1 ) A p r i l 1 8 , 2 0 1 4 (C4H12N2)(H4IO6)2.2H2O is a wide-band-gap of dielectric material. This value suggests that this compound could be used for optical applications in far UV region [12].

Thermogravimetric analysis
The simulations TG-DTA analysis curves ( Figure 6) of the title compound show that this material is thermally stable up to 364K where it begins to dehydrate. In fact, the series of endothermic peaks, observed in the temperature range [363-403 K] in DTA curve correspond to the dehydratation of the periodate to give a new phase of diperiodate to give a new phaseof diperiodate to diperiodate of piperazinium. This reaction is confirmed by the first weight loss (12%) in the TG curve. This weight loss is very close to the percentage of four water molecules in the title compound   Figure 7 shows the 13 C MAS-NMR Spectrum of crystalline orthoperiodate (C4H12N2)(H4IO6)2.2H2O. This Spectrum is in good agreement with the X-ray structure. Indeed, it exhibits tow resonnance peaks at 43.57 and 41.05 ppm, with their corresponding satellite spinning side bands. These NMR components are related to the tow cristallographicallly inequivalent carbon which exist in the assymetric unit

Conclusion
The first organic cation periodate, (C4H12N2)(H4IO6)2.2H2O, has been prepared and structurally characterized. The atomic arrangement of this periodate consists of a three-dimensional network of tetrahydrogenperiodate anions, water molecules and piperazinium dication connected by Ow-H…O, N−H … O and C−H … O hydrogen bonding interactions. Upon heating, this salt lost the crystallization water molecules then begins to degrade. The UV cut of wavelength indicates an energy gap 4 eV, a value indicating that this material could be used for optical applications in far UV region.