Synthesis, Characterization and Thermal Analysis of an Organic-Inorganic Hybrid Salt Involving Trans -Diaquabis(oxalato- κ 2 O 1 , O 2 )chromate(III) Complex Anion with Piperidinium as Counter Cation

A new organic-inorganic hybrid salt pipéridinium trans-diaquabis(oxalato)-chromate(III) tetrahydrate, (C 5 H 10 NH 2 )[Cr(C 2 O 4 ) 2 (H 2 O) 2 ]·4H 2 O (1), has been synthesized in water and characterized by FTIR and UV-Vis spectroscopies, elemental and thermal analyses and by single-crystal X-ray diffraction. 1 crystallizes in the orthorhombic non-centrosymmetric space group Cmc2 1 with the unit cell parameters a = 7.4329(3), b = 9.9356(5), c = 23.6756(11) Å, α = β = γ = 90˚, V = 1748.45(14) Å 3 and Z = 4. The structure of 1 consists of [Cr(C 2 O 4 ) 2 (H 2 O) 2 ] − mononuclear anions, piperidinium cations and uncoor-dinated compatible with an anionic chromium(III) complex in an octahedral environment. Thermal analysis shows a three-step decomposition of 1, leading to formation of a metal oxide residue.

[Cr(C 2 O 4 ) 2 (H 2 O) 2 ] − mononuclear anions, piperidinium cations and uncoordinated water molecules. The Cr III ion in the complex [Cr(C 2 O 4 ) 2 (H 2 O) 2 ] − is coordinated in a slightly distorted octahedral environment by four O atoms from two chelating oxalate dianions in the equatorial plane, and two O atoms from trans-coordinated water molecules occupying the apical positions. In the crystal, N-H⋯O and O-H⋯O hydrogen bond interactions connect the components into a 3-D framework. The IR spectrum of 1 is consistent with the presence of the various molecular building constituents, namely oxalato and aqua ligands, piperidinium cations and solvent water molecules. The UV-Vis spectrum shows two absorption bands around 564 and 416 nm which are

Introduction
A great interest has been paid to the synthesis of novel organic-inorganic hybrid salt due to their structural diversity [1] and potential applications in gas storage [2], ion exchange and catalysis [3], photoluminescence [4] [5] and magnetism [6]. In this respect, the diaquabis(oxalato)metalate(III) complex anions, [M III (C 2 O 4 ) 2 (H 2 O) 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 [7] [8] [9] [10]. 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 [7] [9] [11]- [16]. Furthermore, complexes involving piperidinium cations or their derivatives are known to be useful in the fields of medicine, bioinorganic chemistry and catalysis [17] [18] [19]. 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 aspects of focal relevance are associated with 1: a) its framework is non-centrosymmetric; 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.

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 Mett-ler-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).

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 Su-perNova 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 [20]. The crystal structure was solved by direct method of SHELXT-2014 [21] and refined by full-matrix leastsquare techniques on F 2 using the SHELXL-2018 program package [22]. All nonhydrogen 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 [23] 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.

Infrared Spectrum of 1
The FTIR spectrum of 1 ( Figure 1)

UV-Vis Spectrum of 1
The electronic absorption spectrum of 1 ( Figure 2)

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

Crystal Structure of 1
The asymmetric unit of the title compound is shown in Figure 4.  Table 2). The two     [14]. This fact of matter reinforces the crystal packing framework through extended hydrogen bridgings.

Conclusion
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 Cmc2 1 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 [33] [34] [35]. It could be also worth studying the magnetic properties of 1 as well as its antibacterial activities as with related compounds [10] [13]. Work in this direction is in progress in our group.

Supplementary Material
Detailed crystallographic data in CIF format for this paper were deposited with the Cambridge Crystallographic Data Centre (CCDC-1988875