A great interest has been paid to the synthesis of novel organic-inorganic hybrid salt due to their structural diversity  and potential applications in gas storage , ion exchange and catalysis , photoluminescence   and magnetism . In this respect, the diaquabis(oxalato)metalate(III) complex anions, [MIII(C2O4)2(H2O)2]−, have been extensively used as building blocks for the generation of multifunctional materials formed by two distinct molecular networks, each furnishing a particular structural and physical property    . Among the organic cations involved in these hybrid salts, the use of pyridinium cations and their derivatives to build supramolecular architectures has become an extremely active field of investigation around the world    - . Furthermore, complexes involving piperidinium cations or their derivatives are known to be useful in the fields of medicine, bioinorganic chemistry and catalysis   . These cations obtained via protonation of the imine group of such organic molecules have the ability to self-assemble through the charge-assisted hydrogen bonds.
In continuation of the systematic search for other members of this family of materials, herein we report the synthesis, characterization and thermal analysis of a new organic-inorganic hybrid salt, piperidinium trans-diaquabis(oxalato-κ2O1,O2)chromate(III) tetrahydrate, (C5H10NH2)[Cr(C2O4)2(H2O)2]∙4H2O (1). Two aspects of focal relevance are associated with 1: a) its framework is non-centro-symmetric; b) the number of solvent water molecules per formula unit seems to be the highest obtained so far for this family of bis(oxalato)metalate(III) salts.
2.1. Materials and Physical Measurements
Reagents were obtained from Prolabo (oxalic acid and piperidine) and Riedel-de Haën (chromium(III) chloride hexahydrate) and used as such without further purification. Elemental analyses were performed using a Thermo Scientific FLASH 2000 Analyzer. The FTIR spectrum was perfomed with an Alpha-P spectrophotometer in the range 4000 - 400 cm−1 using KBr pallets. The UV-Vis spectrum was recorded on an Aqualytic spectrophotometer in water solution in the range 200 - 800 nm. Thermogravimetric (TG) analysis was investigated on a Mettler-Toledo TGA/DSC Thermogravimetric Analyser with a heating rate of 10˚C/min in flowing air. Single-crystal X-ray measurements were performed using the Agilent SuperNova diffractometer (λ = 0.71073 Å).
2.2. Synthesis of (C5H10NH2)[Cr(C2O4)2(H2O)2]·4H2O (1)
Chromium(III) trichloride hexahydrate, CrCl3·6H2O (6 mmol ; 1.6 g) was dissolved in 40 mL of water : solution A. Commercial piperidine C5H11N (6 mmol ; 0.43 g) and oxalic acid H2C2O4·2H2O (12 mmol ; 1.52 g) were dissolved in 30 mL of water ( solution B) and added dropwise in solution A. The mixture was stirred at room temperature for 5 h, then filtered and the filtrate was left to stand in the hood at room temperature. After two weeks, violet crystals suitable for X-ray structure determination were harvested. Yield: 71% based on CrCl3·6H2O. Anal. Calcd. for C9H24NCrO14 (422.29 g∙mol−1): C, 25.60; H, 5.73; N, 3.32%. Found: C, 25.23; H, 5.68; N, 3.32%. FTIR (KBr disk, cm−1): 3502, 3393, 3135, 2960, 1705, 1400, 1261, 903, 622, 482. UV-Vis (H2O solution, nm): 416, 564.
2.3. Crystal Structure Determination and Refinement
A suitable single crystal of the material was selected and mounted on a glass fiber. Diffraction data were obtained at 100 K on a Rigaku Oxford Diffraction SuperNova diffractometer with Mo-Kα radiation (λ = 0.71073 Å). The X-ray intensities were corrected using numerical absorption correction based on Gaussian integration over a multifaceted crystal model . The crystal structure was solved by direct method of SHELXT-2014  and refined by full-matrix least-square techniques on F2 using the SHELXL-2018 program package . All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were added in idealized geometrical positions for the organic cations. The positions of hydrogen atoms from the water molecules were assigned from the electron density map generated by Fourier difference and they were refined freely apart from the hydrogen atoms of O5W for which a restraint (O-H bond length of (0.9 ± 0.020) Å was used. They were included as riding atoms with isotopic displacement parameters ADPs ( ). DIAMOND program  was used to deal with the processed crystallographic data and artwork representations. Details of the structure determination and final refinements are summarized in Table 1 and selected bond lengths (Å) and angles (˚) around the central chromium (III) ion are listed in Table 2.
3. Results and Discussion
3.1. Formation of (C5H10NH2)[Cr(C2O4)2(H2O)2]·4H2O (1)
The combination of piperidine (C5H10NH) with oxalic acid in aqueous medium generates in situ (C5H10NH2)2C2O4 which reacts with an aqueous solution of CrCl3·6H2O. This reaction affords, by slow evaporation of the resulting solution at room temperature after two weeks, the title compound (C5H10NH2)[Cr(C2O4)2(H2O)2]·4H2O (1) as violet prismatic crystals. 1 is thermally stable up to 90˚C.
Table 1. Crystal data and structure refinement details for 1.
Table 2. Selected bond lengths (Å) and bond angles (˚) of 1.
Symmetry code: (i) −x + 1, y, z.
3.2. Infrared Spectrum of 1
The FTIR spectrum of 1 (Figure 1) exhibits a weak absorption band centered at 3502 cm−1 attributed to νN−H of piperidinium cations. The sharp bands observed at 3393 cm−1 and 3135 cm−1 can be assigned to the well-known νO-H vibrations of the H2O molecules of crystallization involved in hydrogen bonding and the H2O ligands that are coordinated to the CrIII sites respectively  . The stretching vibration band of C-H (νC-H) is situated at 2960 cm−1. The strong band appearing at 1705 cm−1 corresponds to νC=O  and the medium-size band at 1400 cm−1 can be assigned to the symmetric stretching absorption of the carboxylate groups of the oxalato ligand . Strong to medium well-resolved bands appear at 1261 cm−1 (νC−N), 1181 cm−1 (νC−O), 1081 cm−1 (νC−C)  . The pattern of the δO-C=O vibrations range of 944 - 875 cm−1 supports the presence of chelating oxalate dianions in the structure of 1. Medium to weak bands observed in the region 622 - 482 cm−1 may be attributed to vibrations νCr−O within the coordination spheres around the metallic centers ions. These results are consistent with the presence of cation, [Cr(C2O4)2(H2O)2]− complex anion, and H2O molecules of crystallization in 1.
3.3. UV-Vis Spectrum of 1
The electronic absorption spectrum of 1 (Figure 2) reveals two absorption bands at 416 nm (24038 cm–1) and 564 nm (17730 cm–1) corresponding respectively to 4A2g → 4T1g(F) and 4A2g → 4T2g (d−d) transitions within the octahedral [Cr(C2O4)2(H2O)2]− anionic complex of 1   . Obviously, the present electronic absorption spectrum is virtually superimposable with that reported since the spectral information thus obtained solely relates to [Cr(C2O4)2(H2O)2]− species.
3.4. Thermal Analysis of 1
The thermogravimetric (TG) and differential scanning calorimetry (DSC) curves of 1 depicted in Figure 3 evidence three distinct weight losses in the temperature range 90˚C - 389˚C with endothermic processes. The possible decomposition reactions, the experimental and calculated percentage weight losses are summarized in
Figure 1. Infrared absorption spectrum of 1.
Figure 2. Electronic absorption spectrum of 1.
Figure 3. TG curve (blue) and DSC curve (orange) of 1.
Table 3. In the temperature range 90˚C - 130˚C, 1 suffers a first weight loss of 18.48% (calc. 18.47%) corresponding to the release of the four water molecules of crystallization and three molecules of hydrogen from a partial degradation of organic moieties. A second weight loss of 8.56% (calc. 8.57%) occurs between 156˚C - 196˚C, corresponding to the partial decomposition of the [Cr(C2O4)2(H2O)2]− complex anion framework with release of the coordinating water molecules. The third weight loss of 55.83% (calc. 54.94%) in the temperature range 250˚C - 389˚C, attributed to the total decomposition of 1, leads to a final Cr2O3 residue  .
3.5. Crystal Structure of 1
The asymmetric unit of the title compound is shown in Figure 4. The molecular structure is formed by the piperidinium (C5H10NH2)+ cation in chair conformation, the complex anion, [Cr(C2O4)2(H2O)2]– in trans-geometry and four water molecules of crystallization. In the [Cr(C2O4)2(H2O)2]– complex anion, the CrIII ion adopts a slightly distorted octahedral coordination environment defined by two chelating bidentate oxalate ligands in the equatorial plane and by two water O atoms in the axial sites. The expected ideal values 90˚ and 180˚ in bond angles O-Cr-O in the above complex anion, vary within a range from 82.32 (5)˚ to 97.56 (3)˚ and from 179.87 (4)˚ to 179.99 (7)˚ respectively (see Table 2). The two
Table 3. Thermal decomposition data of 1.
Figure 4. Molecular structure of 1 with the atom numbering scheme. (i = x + 1, y, z).
Figure 5. Packing diagram of 1, viewed along the b axis, showing pillars of alternating complex anions and piperidinium cations and highlighting the interconnection of the components into a tridimensional network by hydrogen bonds (dashed lines).
Table 4. Hydrogen bond lengths (Å) and bond angles (˚) of 1.
Symmetry codes: (i) x, −y + 1, z−1/2; (ii) x−1/2, y−1/2, z; (iii) x + 1/2, y−1/2, z; (iv) x−1/2, y+1/2, z; (v) –x + 1/2, y+1/2, z; (vi) –x + 1, −y + 1, z−1/2; (vii) –x + 1, y, z.
pairs of equatorial Cr-O(ox) distances 1.9754 (8) and 1.9701 (8) Å and the two axial Cr-Ow distances 1.9589 (13) and 1.9653 (14) (Å) are comparable to those reported in similar compounds        and in agreement with the Cr-O(ox) (2.189 ± 0.293) Å and Cr-Ow (2.226 ± 0.411) Å mean values found in the CSD . By contrast, the Cr-Ow bond lengths are slightly shorter than the Cr-O(ox) ones. This situation was not previously observed in homologous salts involving [Cr(C2O4)2(H2O)2]– complex anion  - . Packing diagram of 1, viewed along the b axis, showing its layered structure formed of pillars of alternating [Cr(C2O4)2(H2O)2]– complex anions and piperidinium (C5H10NH2)+ cations plus H2O molecules of crystallization (Figure 5). In fact, the layered framework is exclusively the result of 3D interconnectivity between CrIII ions, oxalato and aqua ligands. The framework formed by [Cr(C2O4)2(H2O)2]− carries an overall excess negative charge and delineates unoccupied spaces parallel to the a axis, encapsulating water molecules of hydration and charge-balancing piperidinium cations, (C5H10NH2)+. The components are linked by N-H∙∙∙O [2.24 (2) to 2.32 (2) Å] and O-H∙∙∙O [1.76 (3) to 2.12 (2) Å] hydrogen bonds (dashed lines), generating a non-centrosymmetric 3D framework. The values of hydrogen bond lengths (Å) and angles (˚) are summarized in Table 4. To the best of our knowledge, 1 and 4-aminopyridinium trans-diaquabis(oxalato-κ2O1,O2)chromate(III) monohydrate  seem to be the only compounds having a non-centrosymmetric structure built with the [Cr(C2O4)2(H2O)2]− complex anions so far. In the other hand, it is worth noting that the number of solvent water molecules per formula unit in 1 is higher, compared to the homologous bis(oxalato)metalate(III) salts known hitherto    . This fact of matter reinforces the crystal packing framework through extended hydrogen bridgings.
In summary, compound 1, a novel organic-inorganic hybrid salt comprising the trans-diaquabis(oxalato)chromate(III) complex anion and piperidinium cation
has been synthesized and characterized spectroscopically, thermally and structurally. Thermal studies revealed that compound 1 is stable to heat up to 90˚C. It crystallizes in the chiral orthorhombic Cmc21 space group. This work confirms the great flexibility of synthetic manoeuvres for the self-assembly of bis(oxalato)chromate(III) complex anions with various organic cations. One could consider with serenity the fabrication of a compound in which the organic cation compensating the charge of the anionic complex would be solely replaced by small charged species such as hydronium ions. Such a system with protons balancing the negative charge of the anionic framework could be a good candidate for the exploration of the concept of one-dimensional proton conducting solids   . It could be also worth studying the magnetic properties of 1 as well as its antibacterial activities as with related compounds  . Work in this direction is in progress in our group.
Detailed crystallographic data in CIF format for this paper were deposited with the Cambridge Crystallographic Data Centre (CCDC-1988875). The data can be obtained free of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html [or from Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 IEZ, UK; fax: +44 (0) 1223-336033; e-mail: firstname.lastname@example.org].
We thank Prof. Elisa Barea, Department of Inorganic Chemistry, University of Granada (Spain) for assistance with the thermal measurements and elemental analysis. We are grateful to Prof. Justin Nenwa, Department of Inorganic Chemistry, University of Yaounde 1, Yaounde (Cameroon), for fruitful discussions. The authors also thank the International Union of Crystallography (IUCr) and the Cambridge Crystallographic Data Center (CCDC) for the promotion of Crystallography in Cameroon.
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