Synthesis, crystal structure, infrared spectroscopy, thermal analysis and Hirshfeld surface analysis of a new hemihydrate of [Zn (H2O) 6][{(CH2)6N4}ZnCl3]2·0.5H2O
Graphical abstract
Introduction
In the last decade, hybrid materials have attracted more attention because their properties combine the characteristics of both organic and inorganic components. When the interactions between the inorganic and organic components are significantly weaker than covalent or ionic bonds, they are sometimes called Class I hybrids. Some of these compounds are interesting due to the possibility of possessing new properties and multifunctional character. Potential applications have been described for such diverse fields as catalysis, optics, electronics, medicine, conductivity and biology [[1], [2], [3], [4], [5], [6], [7], [8]].
Zinc is well suited for the construction of a diversity of coordination compounds, as its d10 electronic configuration allows a variety of coordination numbers and geometries which are not dependent on ligand-field stabilization but rather on ligand charge and size. The zinc atom can accommodate different coordination geometries including tetrahedral (four-coordinate) [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]], square-pyramidal (five coordinate) [20] and octahedral geometry (six-coordinate) [21,22]. Zinc is involved in catalytic and structural functions of proteins; it is an essential element for all living organisms and it is the second most abundant metal in the human body [[23], [24], [25], [26], [27]].
Hexamethylenetetramine (HMTA; IUPAC name: 1,3,5,7-tetraazatricyclo[3.3.1.1]decane) is an important product obtained by condensation of formaldehyde and ammonia at room temperature and atmospheric pressure in aqueous acidic conditions. It has been known for over 130 years. It was the first organic crystal structure determined and it was found to have tetrahedral symmetry. It is a four-ring heterocycle and may act as an acceptor of two, three or four hydrogen bonds. HMTA is a tertiary amine, a colourless solid and a versatile reagent in organic synthesis [[28], [29], [30], [31], [32]].
Indeed, complexation of HMTA with zinc and numerous other metals was observed in early coordination chemistry studies [33]. Powder diffraction studies as early as 1954 yielded tentative cell constants for some complexes [34], but clear structural analyses were not forthcoming until much later. In that regard, Mak et al. reported an accurate structure analysis of the hexahydrate [Zn(H2O)6][{(CH2)6N4}ZnCl3]·6H2O [35].
In the present paper, we present the synthesis and characterization, including a single crystal structural study, of a new organic-inorganic Class I hybrid compound which is also a new, diminished hydrate of [Zn(H2O)6][{(CH2)6N4}ZnCl3], namely the hemihydrate. Its infrared spectroscopic properties, thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are discussed. Its Hirshfeld surfaces are also analyzed to clarify the nature of the intermolecular interactions. The new hybrid material has some structural similarities to the hexahydrate, but the two structures are clearly distinct.
Section snippets
Synthesis
The hemihydrate [Zn(H2O)6][{(CH2)6N4}ZnCl3]2·0.5H2O was prepared by evaporation of an aqueous solution at room temperature. This solution was prepared from a mixture of hexamethylenetetramine (0.25 g), ZnSO4·7H2O (0.51 g) and a few drops of concentrated HCl (37%) in water. The molar ratio is Zn: Cl: HMTA: H2O = 1:1:1:50. After a few days of slow evaporation at room temperature, colourless single crystals were obtained.
Physical measurements
The infrared (IR) spectra were collected on a Perkin-Elmer Spectrum 100 FT-IR
Results and discussion
A note on nomenclature: In order to make the text as simple as possible, we use additive nomenclature for the discussion of the compounds under consideration. As detailed in the IUPAC Red Book [39], this nomenclature emphasizes a central atom and the ligands attached to it. Thus, we indicate ligated water as the aqua ligand, as distinct from unligated water, for which we retain the more generic term 'hydrate.' Of the other two principal alternative systems given in the IUPAC Red Book, one of
Conclusion
A novel Class I organic-inorganic hybrid compound of Zn(II) has been synthesized. Its structure was determined using single crystal X-ray diffraction and it was characterized by FTIR, DTA and TGA studies. The structure of the title compound can be described as consisting of stacks parallel to the crystallographic c axis: Firstly, cationic Zn1(O1WH2)6 octahedra and disordered unligated water molecules line up along the three-fold symmetry axis at x = 0, y = 0; secondly, the anionic complexes are
Funding
Support was provided by the Ministerio de Economía y Competitividad, Spain, under Grant MAT2015-68200-C2-1-P and with European FEDER funds. Funding from the Diputación General de Aragón (ProjectM4, E16) is gratefully acknowledged.
Funding from the Ministry of Higher Education and Scientific Research of Tunisia is gratefully acknowledged.
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