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).

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.

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 [20]. The crystal structure was solved by direct method of SHELXT-2014 [21] and refined by full-matrix least-square techniques on F² using the SHELXL-2018 program package [22]. 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 ( U i s o H = 1.2 U e q C = 1.2 U e q N = 1.2 U e q O w ). 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.

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.

Symmetry code: (i) −x + 1, y, z.

The FTIR spectrum of 1 (Figure 1) exhibits a weak absorption band centered at 3502 cm⁻¹ attributed to ν_N−H of piperidinium cations. The sharp bands observed at 3393 cm⁻¹ and 3135 cm⁻¹ can be assigned to the well-known ν_O-H vibrations of the H₂O molecules of crystallization involved in hydrogen bonding and the H₂O ligands that are coordinated to the Cr^III sites respectively [24] [25]. The stretching vibration band of C-H (ν_C-H) is situated at 2960 cm⁻¹. The strong band appearing at 1705 cm⁻¹ corresponds to ν_C=O [26] 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 [26]. Strong to medium well-resolved bands appear at 1261 cm⁻¹ (ν_C−N), 1181 cm⁻¹ (ν_C−O), 1081 cm⁻¹ (ν_C−C) [27] [28]. The pattern of the δ_O-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 ν_Cr−O within the coordination spheres around the metallic centers ions. These results are consistent with the presence of C 5 H 1 0 NH 2 + cation, [Cr(C₂O₄)₂(H₂O)₂]⁻ complex anion, and H₂O molecules of crystallization in 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 ⁴A_2g → ⁴T_1g(F) and ⁴A_2g → ⁴T_2g (d−d) transitions within the octahedral [Cr(C₂O₄)₂(H₂O)₂]⁻ anionic complex of 1 [8] [24] [29]. 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.

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

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 [28] [30].

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 Cr^III 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

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 [8] [9] [10] [11] [12] [30] [31] 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 [32]. 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 [7] - [12]. 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 Cr^III 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 [14] 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 [7] [10] [11] [14]. This fact of matter reinforces the crystal packing framework through extended hydrogen bridgings.