All chemicals and solvents used in the isolation were obtained from Sigma Aldrich and used without further purification.

The diffraction data was performed using an Enarf-Nonius FR590 four-circle kappa geometry diffractometer type scintillation detector with graphite monochromator using Mo Kα (λ = 0.71073 Å) radiation.

X-ray powder diffraction measurements were performed on a “PAN-analytical, X’pert PRO MPD” diffractometer using Kα1 (Cu) (λ = 1.5406 Å) radiation.

The infrared spectrum of the title compound, made on a pellet containing the crushed product dispersed in a KBr matrix (approximately 4% by mass of the sample to be analyzed) at room temperature, was recorded using Perkin Elmer Spectrum spectrophotometer in the range of 4000 - 400 cm⁻¹.

The UV–visible absorption spectrum was recorded on a 2802 UV/VIS spectrophotometer (UNICO).

The thermal analysis (TGA-DTA) was carried out under a helium atmosphere at a heating rate of 10˚C min⁻¹ in the temperature range [35˚C to 600˚C] using a SETARAM brand thermogravimetric analyzer type SETSYS 1750.

The complex (C₈H₉N₂)₃[Al(C₂O₄)₃]∙3H₂O was prepared by adding an amount of oxalic acid dihydrate H₂C₂O₄∙2H₂O (99.5% purity) (2 mmol, 252.2 mg) in an aqueous solution of aluminum nitrate nonahydrate Al(NO₃)₃∙9H₂O (98% purity) (1 mmol, 212.99 mg) and 5-methylbenzimidazole (C₈H₈N₂) (98% purity) (1 mmol, 132.66 mg), each one has dissolved in 10 cm³ of water under magnetic stirring at room temperature for 30 minutes. The resulting aqueous solution was filtered. After 2 weeks of slow evaporation at room temperature, the colorless prismatic crystals were obtained with a quality judged exploitable in X-ray diffraction.

A colorless prismatic crystal was selected under a polarizing microscope for single crystal X-ray diffraction analysis. The raw diffraction data were collected using the EXPRESS-CAD4 program [10] and processed through the WINGX software package [11].

The crystalline structure of this compound was solved using the SIR 2014 program [12], which allowed the different atoms to be localized in the cell other than the hydrogen atoms. At this stage, an empirical psi-scan [13] absorption correction was applied.

The hydrogen atoms of the water molecules were found in a difference Fourier map and refined, using the SHELXL-2014 program [14], with restraint: d(O−H) = 1.11(1) Å. H atoms of the 5-methylbenzimidazole cation attached to carbon and nitrogen were fixed using the AFIX 43 instruction authorized by the SHELXL-2014 program. Those attached to carbon atoms were placed in calculated positions with C–H distance of 0.96 Å, while those attached to nitrogen were restrained with d(N−H) = 0.86 Å. Their anisotropic thermal parameters were maintained Uiso(H) = 1.2Ueq(X) (X = C or N). After final refinement, a Fourier-Difference examination reveals no significant peak. All graphic representations have been developed via the Diamond version 3.2 program [15].

The crystallographic data, the experimental details of the data collection and the results of refinement of the crystal structure are summarized in Table 1.

The PXRD diagram was obtained at room temperature. The measurement was made with Bragg-Brentano geometry at 2θ, in the range of 5˚ to 70˚ with a pitch of 0.02˚.

The X-ray powder diffraction data of the (C₈H₉N₂)₃[Al(C₂O₄)₃]∙3H₂O sample was analyzed with the Rietveld technical refinement by means of GSAS-EXPGUI software [16] [17]. The final Rietveld plot is shown in Figure 1.

The peaks of the experimental diffractogram and those of the theoretical one are in a good agreement, which confirms the purity of that phase (C₈H₉N₂)₃[Al(C₂O₄)₃]∙3H₂O.

*CIF file containing complete information about the structure of (C₈H₉N₂)₃[Al (C₂O₄)₃]∙3H₂O was deposited with the Cambridge Crystallographic Data Center (CCDC 1965815). The file is freely available upon request from the following web site: http://www.ccdc.cam.ac.uk/data_request/cif.

The IR spectrum of the compound tris-(5-methylbenzimidazole) tris-(oxalato)-aluminate (III) trihydrateis is shown in Figure 2.

In the region of high frequencies, the bands at 3458 and 3100 cm⁻¹ are due to the stretching vibrations of the O-H and C-Haromatic bonds [18]. Harmonic bands of weak deformations are observed between 2000 and 1980 cm⁻¹ [19]. The band at 1686 cm⁻¹ is assigned to the C=O stretching mode of the oxalate ligands [20].

The absorption band at 1419 cm⁻¹ is attributed to the C=C and C-C stretching modes of the 5-methylbenzimidazole ring [21]. The weak bands at 1295 and 813 cm⁻¹ are attributed to the ν(C-N_aromatic) and δ(C-H) modes [22].

The absorption band located at 722 cm⁻¹ corresponds to the δ(N-H) mode [23]. The band at 467 cm⁻¹ is assigned to the ν(Al-O) mode [24].

The obtained UV-Vis Spectrum for the studied tris-(5-methylbenzimidazole) tris-(oxalato)-aluminate(III) trihydrate salt is depicted in Figure 3. The UV-Vis spectrum reveals two distinct absorption bands around 273 and 279 nm attributed, respectively, to the π-π and π-π* absorption bands of the 5-methylbenzimidazole cations [25]. Thus, the value of the energy of optical gaps is obtained by the Tauc relation [26] :

(αhν)ⁿ = B(hν − E_g)

α: the absorption coefficient, hv: the energy of the incident photon, B: a constant that depends on the energy, E_g: the optical gap energy, n: a constant that takes different values depending on the type of electronic transition. The experimental band-gap energy (E_g) estimated by extrapolation of the linear part is about 2.88 eV (Figure 4). This band-gap value shows that this compound exhibits semiconductor behavior (the gap energy is less than 4 eV).

The thermogram analysis (TGA-DTA) of (C₈H₉N₂)₃[Al(C₂O₄)₃]∙3H₂O (Figure 5) shows that the percentage of the experimental mass loss is in the order of 6.90%, which is calculated in the order of 7.25%. Indeed, the latter is equivalent to starting from three salvation molecules of water at a temperature between 50˚C to 85˚C. This step is characterized by two endothermic peaks in a temperature

range of 65˚C - 120˚C. At around 120˚C, there is a second loss which reflects the decomposition of the organic part and oxalate [% calculated, 59.49 (experimental%, 53.86)]. A last loss of mass phenomenon is observed at 350˚C, which corresponds to the conversion of Al₂(CO₃)₃ to Al₂O₃ with the departure of CO₂ [% calc, 6.01 (Experimental %, 7.95)]. This step is characterized by an endothermic peak at 375˚C.