The amidosulfonic acid or sulfamic acid (NH₂SO₃H) has molar mass 97.10 g mol^-1. When dry it is stable, but in solution, it is easily hydrolyzed, forming ammonium bisulfate. It is relatively soluble in water, moderately soluble in alcohol, poorly soluble in acetone, insoluble in ether and very soluble in nitrogenous bases, liquid ammonia, pyridine, formamide and dimethylformamide. It is classified as a strong acid (pH = 1.2 in 1% aqueous solution and 25 ^oC). It is used as standard in alkalimetry, in steel cleaning, in removal of nitrites and stabilization of chlorine in pool water. It is a toxic compound, used as poison for rats. Handling requires careful care as it easily irritates the skin and mucous membrane. It forms orthorhombic crystals and has melting point near 205 ^oC ¹ . In recent years sulfamic acid has been used as an efficient heterogeneous catalyst in a series of organic reactions, such as acetylation, esterification, condensation, transesterification, among others ² . It is a ZWITTERION, in other words, a dipolar ion having opposite charges on different atoms. In the formation of metal complexes, both amine and sulfonate groups participate in the coordination with the metal ion.

Few studies report the chemical and thermal properties of metal salts containing sulfonic acid derivatives, although the preparation of these salts is easy to perform ^3-10 . Maksin and Standritchuk ³ studied the water solubility of Ni(II) and Co(II) sulfamates. Budurov et al. ^4,5 studied by DSC the phase transformations that occur in the heating of some amidosulfonates, determining the energy involved in the processes represented by endothermic peaks. Also, by DSC, Thege ⁶ investigated the thermal behavior of (NH₄)2SO₄, NH₄HSO₄ and NH₄NH₂SO₃. The crystalline structure and growth of Li[NH₂SO₃] monocrystals was studied by Stade, Held and Bohaty ⁷ , and it was possible to establish the spatial group, cell parameters and refractive index for the crystals obtained. The elastic properties of sulfamic acid and various sulfamates were studied by Haussül and Haussül and reported phase transformations ⁸ . Shimizau et al. ⁹ determined the crystallographic properties of silver amidosulfonate and described a lamellar structure with potential chemical applications. Squattrito and coworkers studied the layered structures of metal salts of sulfonic acid derivatives ^10-12 . Jaishree et al. ¹³ studied the optical and thermal properties of a monocrystal of amidosulfonic acid and reported the absorption bands in the infrared region. The main absorptions were attributed to the vibrational modes of the -SO₃- and -NH₃+ group. Brahmaji et al. also identified the functional groups by FTIR and reported the changes observed in the pure crystals and doped with Tb3+ 14. Wickleder ¹⁵ studied the synthesis, crystal structure, and thermal behavior of some rare earth amidosulfonates. Luiz, Nunes and Matos ¹⁶ studied the thermal behavior of all the amidosulfonates of the rare earth series and observed a mass gain between 250 ^oC and 350 ^oC, attributed to an oxidative process SO₃2- → SO₄2-, which is more evident in a lower heating rate. Brahmaji et al. ¹⁴ also observed mass gain in this temperature range.

All chemicals used in this study were of analytical grade. Metallic chlorides were obtained from Sigma Aldrich, while the sodium hydrogen carbonate and silver nitrate were obtained from Merck and were used without further purification.

The elemental analysis results are presented in Tab. 1 and are in agreement with the proposed general formula M(NH₂SO₃)₂.xH₂O, where M = Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ and x the number of water molecules ranging from 1 to 4, where x = 4 for Mn²⁺, 3 for Co²⁺ and Zn²⁺, 2 for Ni²⁺ and 1 for Cu²⁺.

Table 1. Table 1. Elemental analysis of the solid compounds.

The main infrared absorption bands were associated with the N-H stretching, namely, a NH₃ ⁺ broad band at 3400-3300 cm^-1, the N-H stretching as a weak band at 2873 cm^-1 and the absorptions observed at 1533 and 1433 cm^-1 due to symmetric stretching mode of NH₃ ⁺ while at 1567 cm^-1 is due to asymmetric mode. The SO₃ ^- stretching vibration was observed in a 1065 cm^-1 and at 685 cm^-1, the N-S stretching mode can be seen, all these absorptions agree with the literature ^4-14 . The absence of that weak band of N-H stretching is an evidence that de coordination of the metal ions occurred by NH₃ ⁺ group. Another fact is the increase observed in the wavenumber for the N-S stretching, suggesting that the N-S bond becomes strongest. The vibrational spectra in the infrared region of amidosulfonic acid and metal amidosulfonates are show in the Fig. 1.

The X-ray powder pattern of the metal amidosulfonates are shown in the Fig. 2. The X-ray diffractograms indicate that the compounds were obtained with a certain crystallinity degree, showing no evidence of isomorphism. The compounds showed low relative intensity (Io) reflections: Mn (2θ = 19.0; I_o; = 845); Co (2θ = 19.3; I_o = 3621); Ni (2θ = 26.3; I_o = 6534); Cu (2θ = 25.4; I_o = 4010); Zn (2θ = 19.4; I_o = 4146).

By the TG/DTA curves it was possible to establish the stoichiometry of the compounds, such as: ML2.xH2O, where M represents the metallic ions M = Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺; L represents the anion NH2SO3- and x the number of water molecules ranging from 1 to 4, where x = 4 for Mn²⁺, 3 for Co²⁺ and Zn²⁺, 2 for Ni²⁺ and 1 for Cu²⁺. In a synthetic air atmosphere, the events associated with the thermal decomposition of the ligand occur at slightly lower temperatures than in a nitrogen atmosphere. Tables 2 and 3 show the results extracted from the TG and DTA curves in the atmosphere of synthetic air and nitrogen. Figures 3 and 4 show the TG and DTA curves in the atmosphere of synthetic air and nitrogen, respectively.

Figure 1. Figure 1. Vibrational spectra in the infrared region of amidosulfonic acid and metal amidosulfonates.

Figure 2. Figure 2. X-ray diffraction pattern of the metal amidosulfonates.

Figure 3. Figure 3. TG and DTA curves of the obtained compounds, in a synthetic air atmosphere. Sample mass: Mn = 9.680 mg, Co = 2.980 mg, Ni = 10.194 mg, Cu = 10.314 mg and Zn = 10.064 mg.

Figure 4. Figure 4. TG and DTA curves of the obtained compounds, in a nitrogen atmosphere. Sample mass: Mn = 10.383 mg, Co = 3,794 mg, Ni = 2,984 mg, Cu = 10.414 mg and Zn = 10.193 mg.

Table 2. Table 2. Results extracted from the TG curves.

Table 3. Table 3. Results extracted from the DTA curves.

The main absorptions in the infrared: vO-H (3600 - 2700 cm^-1) in the salts; vN-H (3400-3300 cm^-1) in amidosulfonic acid; vsS-O (1260-1140 cm^-1) and vasS-O (1040-1020 cm^-1) emphasize the differences between the spectra of the salts and of the amidosulfonic acid, evidence the bond to the metal suggesting that the coordination occurs by the sulphonic grouping.

The X-ray diffractograms indicate that the compounds were obtained with a certain crystallinity degree, showing no evidence of isomorphism. The compounds showed relatively low intensity reflections.

The thermoanalytical results (TG/DTA) allowed to establish the stoichiometry of the compounds as: ML₂.xH₂O, where M represents the metallic ions Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺; L represents the anion NH₂SO^3-; x represents the number of water molecules, where x = 4 for Mn²⁺, x = 3 for Co²⁺; and Zn²⁺, x = 2 for Ni²⁺ and x = 1 for Cu²⁺. No significant differences were observed between the analysis carried out in synthetic air or nitrogen atmosphere, only the thermal decomposition of the ligand in synthetic air atmosphere occur at slightly lower temperatures that in a nitrogen atmosphere. For all compounds, the MSO4 stable intermediate was observed, which later decomposes to the respective oxide, that is: Mn₃O₄, CoO, NiO, CuO and ZnO. In some samples the residual oxide is produced at temperatures above 950 ^oC.