The synthesis of the N-(3-pyridyl)-isonicotinamide (3-pina) ligand was carried out following the procedure described by Gardner and colleagues [26] and later modified by Encarnação Amorim [25] . Reagents including 3.0090 g of 3-aminopyridine and 5.6500 g of Isonicotinoyl chloride hydrochloride were prepared. These were dissolved in 60.0 mL of pyridine in a 250.0 mL volumetric flask. The mixture was then sealed and stirred magnetically for 4 days. After this period, a biphasic separation occurred, resulting in the formation of a white precipitate. This precipitate was vacuum filtered through filter paper, transferred to a 250.0 mL beaker, and dissolved in 50.0 mL of distilled water. The mixture was then subjected to ultrasonic treatment.

Shortly thereafter, a white solid precipitated and the pH was adjusted to approximately 7 using a spatula tip amount of NaHCO₃. Following another filtration step with distilled water, 5.4310 g of a fine, shiny white powder was obtained and dried in a desiccator for 4 days. Once removed and reweighed, its mass was 2.5720 g. The sample underwent Fourier-transform infrared spectroscopy (FTIR), showing characteristic peaks corresponding to those expected. However, further drying in an oven at 90˚C - 100˚C for 24 hours was necessary to remove any residual moisture. Post-oven treatment, 1.2150 g of an opaque white solid was obtained and analyzed again using FTIR.

The purification process was carried out as described in the literature by Encarnação Amorim [27] with some modifications to enhance yield. A total of 0.499 g of the white, amorphous powder was dissolved in hot distilled water (40.0 mL at 70˚C) and stirred on a hot plate for 30 minutes. After reducing the volume to 30.0 mL, the solution was filtered to remove impurities and then cooled in an ice bath to precipitate a translucent crystalline solid resembling small needles. This solid was filtered, dried in a desiccator for 48 hours, weighed, and stored.

This synthesis was prepared from adaptations of the synthesis reported by Gerisch et al. [28] . A solution of 0.3000 g (1.22 mmol) of 4,4'-dimethyl-2,2'-bipyridine in 20.0 mL of ethanol was refluxed, and 0.1500 g (0.56 mmol) of rhodium chloride was added. The mixture was kept under reflux for 4 hours, yielding a light-yellow precipitate. The solvent was then evaporated under reduced pressure until one-third of its original volume remained. The solution was cooled in a refrigerator for 24 hours, and the precipitated solid was collected by filtration, washed with ethanol and ether, and dried under vacuum to yield a light-yellow product.

In this assay, 70.0 mg (0.12 mmol) of the initial compound [RhCl₂(4,4'-Met-2,2'-Bipy)₂]Cl∙nH₂O was dissolved in 2.0 mL of distilled water in a 50 mL round-bottom flask and treated in an ultrasonic bath with the addition of 5 drops of ethyl alcohol to enhance solubility. Next, 44 mg (0.36 mmol) of Isonicotinamide was added. The reaction mixture was semi-sealed and stirred magnetically for 2 hours, turning yellow. After cooling for 24 hours, a solid precipitated but was found to be soluble at room temperature. The solution was filtered and washed with ether, yielding a yellow solid.

The purification process followed was described in the literature by Santos [29] . In a beaker, 5 g of Isonicotinamide was dissolved in 15.0 mL of distilled water on a hot plate. Subsequently, 0.5 g of activated charcoal was added, and the mixture was hot filtered. The filtrate was passed through filter paper and/or absorbent cotton in a separating funnel. Upon cooling in an ice bath, a white solid in the form of fine needles precipitated, was filtered again, washed with ether, and stored.

20.0 mg (0.035 mmol) of the starting compound was dissolved in 10 mL of distilled water with 3 drops of ethyl alcohol in a 50.0 mL round-bottom flask to improve the solubility of the ligand. Following the addition of 7.7 mg (0.038 mmol) of the purified reagent 3-pina (1:1), the system was sealed and magnetically stirred at room temperature for 2 hours. The solution was then cooled in an ice bath, but no precipitate formed due to the solubility of the solid in the medium. Consequently, the solvent was evaporated, and after 24 hours, a yellow solid was obtained. This solid was vacuum filtered, washed with ether, and left in a desiccator for 24 hours.

The analysis of variance test followed by Tukey’s test was used for statistical analysis in the Bioestat 5.0 software, adopting a significance level of p < 0.05.

The experimental procedures were conducted at the Laboratory for the Study of Materials (LEMat). Spectroscopic analysis in the infrared region and thermoanalytical analyses (TG/DTG and DSC) were performed at the Multiuser Research Center. Infrared spectroscopy was conducted using a FTIR, specifically the Perkin Elmer Spectrometer 100, featuring a resolution of 4 cm⁻¹ in the range of 4000 - 500 cm⁻¹. This analysis utilized an accessory for the attenuated total reflectance technique with a germanium crystal. The thermoanalytical techniques were detailed through TG/DTG and TG-DSC curve infographics, obtained using the Mettler Toledo TGA/DSC equipment. The system was calibrated according to the manufacturer’s specifications. The curves were recorded using an α-Al₂O₃ crucible (70.0 μL) with a sample mass of approximately 5.0 mg, under a heating rate of 20˚C∙min⁻¹, dry air atmosphere with a flow rate of 60.0 mL∙min⁻¹ and a temperature range of 30˚C - 1000˚C.

For the C1 complex, a broad band was observed at 3374 cm⁻¹, indicating the presence of a dipole moment due to hydrogen bonding (O-H) interactions [35] [36] . This suggests the presence of water of hydration in the material, a finding that is corroborated by the thermogram and evidenced in the broad absorption band shown in Figure 1 . In the absorption region between 3060 - 3018 cm⁻¹, low-intensity overlapping peaks are characteristic of the stretching (ν) of the C-H bonds in sp2 hybridized carbons, which are typical of aromatic groups. The ligand 4,4-dimethyl-2,2-bipyridine contains primary carbons within its structure; this is evidenced by an absorption band at 2920 cm⁻¹, with the corresponding absorption range for aliphatic C-H groups being 2960 - 2850 cm⁻¹ [35] . Vibrations attributed to C=C and C=N bonds, which are characteristic of aromatic rings, were identified between 1617 and 1556 cm⁻¹ [37] . Lastly, in the range of 1416 - 1446 cm⁻¹, the harmonic bands due to angular deformations of the methyl group (−CH₃) can possibly be seen [38] .

In a second analysis, the molecular structure of the Isonicotinamide ligand, a primary amide, exhibits two strong bands in the region corresponding to the 3361 cm⁻¹ to 3179 cm⁻¹ stretches of the ν (N-H) group [30] , indicating its presence in this region. However, the compound’s hydration water, as observed in the thermogram, leads to an overlap of the peaks corresponding to these stretches. Meanwhile, a band indicative of the carbonyl group ν (C=O) stretching shifts to a lower energy region, presenting an intense peak ( Figure 2 ), characteristic of the dipole moment from molecular interactions, at 1683 cm⁻¹, consistent with the L2 ligand peak [35] . The presence of C=C and C=N type vibrations, pertaining to aromatic rings, is evident between 1618 and 1556 cm⁻¹ [31] [32] . Additionally, angular deformations linked to the compound’s methyl group occur in the range between 1416 and 1446 cm⁻¹ [37] .

In an attempt to synthesize the third compound, the infrared spectrum was obtained and did not show significant differences between the synthesized compounds, as identified by the vibration bands in Table 1 . This observation was consistent both in infrared spectroscopy and in thermogravimetric analysis. The material presented identical behavior to the initial compound (C1), exhibiting the same peaks in their respective analysis regions ( Figure 3 ). Consequently, it is imperative to reexamine the factors that may have interfered with the synthesis, including solubility, temperature, activation energy and chemical stability of the ligand in relation to the metal center, since the reaction did not occur spontaneously as expected.

*No bands; ** Evidence of overlapping bands; Ʋ: stretching; ẟ: angular deformation.

The complex [Rh(Met-byp)₂Cl₂]Cl∙5/2H₂O demonstrated in this analysis, between 60˚C and 150˚C, a probable loss of hydration water from the synthesized material, as indicated by peaks in the DTG and DSC ( Figure 4 ). This evidence suggests an energy absorption at 100˚C, consisting of an endothermic process [36] . The second thermal event occurred at 300˚C, resulting in a loss of stability and initiating the decomposition of the material, with a maximum rate of loss observed at 475˚C. During this thermal interval, a single exothermic event was recorded in the DSC ( Figure 5 ), with the maximum peak likely due to sublimation and/or thermal decomposition of the organic compound in the formed complex [39] . Finally, the last stage demonstrated an oxidative event at 503˚C that stabilized at 730˚C, as shown by the TG and DTG curves [40] . This stability was attributed to the presence of an O₂-rich atmosphere, which facilitated the formation of an oxide residue pertaining to the metal in the complex, possibly Rh₂O₃. The significant values in percentages are found in Table 2 .

For the modified compound [Rh(Met-byp)₂(Iso)₂]Cl₃·1/2H₂O, with the addition of the isonicotinamide ligand, the thermogravimetric curves identified in the TG-DTG are depicted in Figure 6 , and the thermal behavior as per the DSC is presented in Figure 7 . During the analysis, it was observed that there were at least three stages of mass loss and one stage of mass gain throughout the thermal analysis process, as indicated in Table 3 . Initially, between 70˚C - 150˚C, a likely loss of the material’s water of hydration was detected, manifesting as a low-intensity peak in both the DTG and DSC, indicative of a possible endothermic process at 100˚C. Subsequent stability was observed up to 173˚C. This stage was succeeded by the second mass loss of the analyte, peaking at 263˚C as evidenced in the DTG, accompanied by the first exothermic reaction of the compound revealed in the DSC. Following this, the material underwent continued thermal decomposition of the organic compounds coordinated to the metal, culminating in a maximal loss at 505˚C. The DSC illustrates a second peak characterized by high energy absorption intensity, thus corroborating the second exothermic reaction of the newly formed compound [36] [41] . In the final stage, a mass gain was recorded, which stabilized at 760˚C [40] . This stabilization is attributed to an O₂-rich atmosphere, thereby facilitating the formation of an oxide under atmospheric conditions and potentially leading to the formation of RhO₂ residue.

*Oxidative event.

For the compound synthesized with the second ligand (3-pina), the following thermogravimetric analyses were conducted: TG-DTG ( Figure 8 ) and TG-DSC ( Figure 9 ). The initial stage of the analysis identified the loss of volatile compounds and water of hydration. During the second phase, at 297˚C, the material exhibited the initial stage of thermal decomposition of its coordination sphere. The maximum rate of decomposition was observed at 530˚C. At the conclusion of the analysis, an oxidative stage of the material was identified [40] , resulting in an oxide residue that stabilizes at 600˚C. Despite these findings, it was noted that the formation of a new coordinated complex did not occur. This observation was based on the similarities in the consecutive peaks observed in the TG-DTG and the thermal processes in the DSC ( Figure 9 ), when compared to the initial synthesis compound depicted in Figure 4 [41] .