Synthesis, Crystal Structure and Thermal Behaviour of a New Three-dimensional Hybrid Fluoride Framework with Mixed Valence: (Fe2+/Fe3+)

Crystal structure of metal-organic frameworks (MOF’s) compound [Hdma]∙(FeFeF6(Hamtaz)2) was hydrothermally synthesized, eventually assisted via classical heating (Acid Digestion Autoclave). Crystalline structure determination is formed from single crystal X-ray diffraction data. The unit cell is orthorhombic space group Imma, with cell parameters a=15.9520(14) Å, b=9.4548(7) Å, c=9.7056(8) Å, V=1463.8(2) Å3 and Z=4. The structure [Hdma]∙(FeFeF6(Hamtaz)2) exhibits a three-dimensional inorganic network resulting from the association of FeFeF6N4 planes with Hamtaz molecules in the [010] direction, as well as uncoordinated dimethylammonium cations [Hdma] which are formed by the hydrolysis of DMF solvent. The thermal analysis (TG) of the title compound shows that the decomposition undergoes two steps between 200°C and 600°C and the total experimental mass loss 63.03% assuming that the hematite Fe2O3, is the final product of the decomposition. *Corresponding author: Mouna Smida, Faculté des Sciences et Techniques, Université du Maine, Avenue Olivier Messiaen, France, Tel: +0021695231854; E-mail: mouna.smida@yahoo.fr Received June 05, 2017; Accepted June 15, 2017; Published June 20, 2017 Citation: Smida M, Lhoste J, Dammak M, Ribaud AH, Leblan M, et al. (2017) Synthesis, Crystal Structure and Thermal Behaviour of a New Three-dimensional Hybrid Fluoride Framework with Mixed Valence: (Fe2+/Fe3+). Chem Sci J 8: 161. doi: 10.4172/2150-3494.1000161 Copyright: © 2017 Smida M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
The synthesis of compounds containing more than one ligand using hydrothermal methods is much more difficult to control, and experimental synthesis conditions play a fundamental role. The formation of hybrid compounds is strongly determined by several factors, such as the molar ratio of reagents, pH, reaction time, solvent, reaction temperature [1,2]. The hydrothermal synthesis of crystalline inorganic-organic hybrid compounds leads to two types of hybrid networks, according to the nature of bonding interactions [3]. In class I hybrids, the organic and inorganic parts are interlinked by weak interactions (van der Waals or hydrogen bonds) that contribute to the 3D structural stability. Whereas in the Class II hybrids, the metal atoms are strongly linked by covalent or iono-covalent bonds with the organic parts, to form metal-organic frameworks (MOF's). However, very few MOF's built up from fluorinated inorganic frameworks have been reported in the literature [4]. The interest for hybrid fluorinated materials is associated to the application in various domains ranging from gas storage (especially hydrogen) [5], catalysis [6], ion-exchange [7], magnetism [8], luminescence [9], biomedicine [10] etc. More recently, several crystalline forms have been proposed as components for secondary batteries, used as anode or cathode in rechargeable lithium batteries [11].
At the parallel, few hybrid fluoroferrate are listed. Most often, the metal fluoride species, which result from the condensation of FeA x B y units (A, B=N, O, F), are isolated polyanions or cluster such as FeF 6 , FeF 5 (H 2 O), FeF 2 (H 2 O) 4 , FeF 4 N 2 [12][13][14][15][16][17][18][19][20][21]. In fact, the research of inorganic 1D, 2D and 3D network structure is successful for class I and class II of hybrid fluoroferrate with triazole [22]. In this study, we have paid a great deal of attention to the hybrid iron fluoride compound as the object of our investigation. The aim of the present work is to study the structure determination and the thermal analysis of the second new three-dimensional network fluoroferrate with mixed valence Fe 2+ /Fe 3+ . We report here the results obtained by X-ray single-crystal diffraction and differential thermogravimetric analysis (TG).

X-ray crystallography
Single-crystal X-ray diffraction data was collected at room temperature on an APEX II Quazar diffractometer (4-circle Kappa goniometer, IμS microfocus source (M o , K α ), CCD detector). The structure was solved by direct methods which give the position of most of the atoms (iron, fluorine, nitrogen and carbon) developed by successive fourier maps and subsequent refinements using SHELXS-86 and SHELXL-97 [23][24][25] programs, where these last were included in WINGX package [26]. Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to carbon and nitrogen were placed in geometrically idealized positions and included as riding atoms. The structure graphics were created by the DIAMOND program [27]. Crystal data and structure refinement details for the title compound are summarized in Table 1. The final positions and equivalent isotropic thermal parameters for the new compound are shown in Tables 2 and 3, while the selected bond lengths in [Hdma]•(Fe 2+ Fe 3+ F 6 (Hamtaz) 2 ) is listed in Table 4. The Table  5 listed the hydrogen bond distances (Å) of the new three material [Hdma]•(Fe 2+ Fe 3+ F 6 (Hamtaz) 2 ). Finally, a selected bond angles are given in Table 6.

Thermal analysis
Thermogravimetric experiments was reported for the new threedimensional fluoroferrate with a thermoanalyzer NETZSCH STA 449 F3 under humid air atmosphere and a heating rate of 5°C.min -1 from 25°C up to 895°C.

Structure description of [Hdma]•(Fe 2+ Fe 3+ F 6 (Hamtaz) 2 )
Single crystal X-ray diffraction analyses, at room temperature indicate that the 3D [Hdma]•(Fe 2+ Fe 3+ F 6 (Hamtaz) 2 ) compounds crystallizes in the orthorhombic space group Imma. Final refinements of anisotropic displacement parameters (ADP) and secondary extinction converged to R=0.046% and W R =0.126% (1219 independent reflections and 91 parameters). Two iron atoms are located on special 4c position (Fe (2)) and general 4e position (Fe (1)). Successive Fourier maps and refinements allowed locating fluorine, nitrogen atoms and organic moieties. F(2), F(3) and F(4) atomic positions were statistically occupied with 0.75 site occupancy whereas for F(5) and F(6) atomic positions occupied with 0.25 site occupancy, moreover, one nitrogen atom and one carbonate atom were disordered on N(1)/C(1) sites for electronic neutrality and from distances and environment considerations. Furthemore, N(1) and C(1) atoms statistically distributed on a general position, as shown in Table 2. The fluoride atoms are randomly distributed over several positions. In fact, the occupation of the fluoride sites was done freely at their positions, which corresponds to six fluorine atoms per unit formula. Hydrogen atoms of Hamtaz neutral were placed with HFIX options (43 for CH, NH, and 93 for NH 2 ). The structure for the title compound is built up from Fe 2 F 6 (Hamtaz) 2 planes connected by Hamtaz ligands which separated by dimethylammonuim cations coming from the hydrolysis of DMF solvent, resulting a second three-dimensional hybrid fluoride with 3D inorganic connectivity (Figure 1a and 1b). It is crystallizes in the same system orthorhombic with identical Centro symmetric space group Imma, accompanying different cell parameters, comparable to the first 3D Fe 2 F 5 (Htaz) [22].
From the Figure 1, we note that the arrangement of the various octahedral and organic molecules forms a tunnels with square cross section where the [Hdma] cations are located, the side of the square is equal to 6.774 Å. Thus this structure of the new material involve anionic Fe 2 F 6 N 4 infinite trans-chains along (100) direction. The average <C-C> and <N-C> distances of Hamtaz molecules are varied between 1.32 Å and 1.42 Å, in fact there are the same geometry than that found in the succession of guanazolium fluoroaluminates [28,29]. Dimethylammonium cations are strongly disordered, in fact two different positions for N(4) and C(4) atoms were located with refined occupation being close to one quarter and one half, respectively, form one member of [Hdma] entity per unit formula. The formulation [Hdma]•(Fe 2 F 6 (Hamtaz) 2 ) was then obtained. A projection on the cb plane ( Figure 1) shows that this structure contains two type of octahedral forming a Fe III 2 Fe II F 6 N 4 trimers. As a consequence, the structure presents two oxidation states for iron atoms. Every anionic Fe III 2 Fe II F 6 N 4 trimer is connected to four neutral Hamtaz molecules by four symmetry nitrogen atoms N(2) along the (110)         state for Fe(1) and Fe(2) atoms are +III and +II, respectively, and the ligands established between Fe(2) and nitrogen atoms of Hamtaz molecules lead to deduce that this new fluoroferrate with mixed valence is a Class II hybrid (Figure 3).    distances between Fe atoms and terminal fluorine atoms F(2), F(3), F(4), F(5) and F (6), medium distances between Fe atoms and bridging F(1) atoms, and long distances between Fe atoms and nitrogen atoms N(2). The Fe III -F(1)-Fe II angle is equal to 170.64°, whereas the both F(1)-Fe III -F(1) and F(1)-Fe II -F(1) angle values counterpart to 179.94° and 180°, respectively (Table 6).  [12,13] (Table 5).

Thermogravimetric analysis
The TG thermal analysis in oxygen dynamic atmosphere of the title compound is given in Figure 5. During heating, it decomposes in different ways continuous at different temperature between 200°C and 600°C and it shows that the compound prepared is stable at room temperature. The weight loss is attributed to the decomposition of [Hdma]•(Fe 2+ Fe 3+ F 6 (Hamtaz) 2 ) to give the amine fluoride and HF gas. The first stage between 100°C and 350°C approximately, probably due to the release of 1.25 mol of Hamtaz molecule and eventually, 6 mol of hydrofluoric acid (HF), while the experimental mass loss equal to 52.12%. Moreover, the second step in the temperature range of 350°C and 550°C, approximately, is attributed to the elimination of the rest of organic moieties that corresponds to 10.91% of the experimental mass loss. In fact, the loss of HF and decomposition of Hamtaz molecules and the creation of FeF 2 , make up a total experimental mass loss of 63.03%. At high temperature, above 600°C the Fe 2 O 3 hematite compound is product [21][22][23]. The total experimental mass loss value 63.03% is in good agreement with the theoretical mass loss 63.03% assuming that the hematite Fe 2 O 3 , is the final product of the oxidation.

Conclusion
New three-dimensional mixed valence fluoroferrate obtained from the reaction of FeF 2 and FeF 3 with 3-amino-1,2,4-triazole and aqueous HF in DMF solvent. The structural properties and the thermal behavior for [Hdma]•(Fe 2+ Fe 3+ F 6 (Hamtaz) 2 ) are reported. The structure exhibit M II N 4 F 2 and M III F 6 octahedral units in which nitrogen atoms come from neutral amines, forming the infinitie layers of Fe 2 F 6 (Hamtaz) 2 , where the [Hdma] cations are located in the cavities. This structure while being regarded the notation of Cheetham [30] leading to 3-D dimensionality with respect to both organic connectivity between metal centers (O n ) and extended inorganic connectivity (I n ) therefore the notation of Cheetham is I 1 O 2 (I=inorganic and O=organic), note that the sum of the exponents gives the overall dimensionality of the structure. The results of thermal analysis suggest that the decomposition take place in two steps and at high temperature the Fe 2 O 3 hematite compounds is product.