Synthesis, characterization, and thermal behavior of amidosulfonates of transition metals in air and nitrogen atmosphere

32 Eclética Química Journal, vol. 45, n. 4, 2020, 32-39 ISSN: 1678-4618 DOI: 10.26850/1678-4618eqj.v45.4.2020.p32-39 ABSTRACT: The amidosulfonates of Mn, Co, Ni, Cu and Zn were prepared by the direct reaction between the metal carbonate and the amidosulfonic acid with heating and stirring. The compounds were characterized by infrared absorption spectroscopy (IRFT), elemental analysis, thermal analysis (TG and DTA) and Xray diffraction by the powder method. The absorptions observed in IR spectra are associated with N-H and O-H stretching, as well as symmetrical and asymmetric S-O stretching in the sulfonic group. The compounds present Xray diffraction pattern with well-defined reflections, showing no evidence of isomorphism. The TG-DTA curves allowed to establish the stoichiometry of compounds as M(NH2SO3)2.xH2O, where M = Mn, Co, Ni, Cu and Zn and x ranging from 1 to 4. Dehydration leads to the formation of stable anhydrous. In all cases the respective sulfates are formed as an intermediate. After consecutive steps of decomposition, the respective oxides were obtained: Mn3O4, CoO, NiO, CuO and ZnO. The TG-DTA curves are characteristic for each sample, with thermal events related to dehydration and ligand decomposition. Synthesis, characterization, and thermal behavior of amidosulfonates of transition metals in air and nitrogen atmosphere


Introduction
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 o C). 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 o C 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][4][5][6][7][8][9][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 -SO3and -NH3 + group. Brahmaji et al. also identified the functional groups by FTIR and reported the changes observed in the pure crystals and doped with Tb 3+ 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 o C and 350 o C, attributed to an oxidative process SO3 2-→ SO4 2-, which is more evident in a lower heating rate. Brahmaji et al. 14 also observed mass gain in this temperature range.

Materials and Methods
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.

Metal Carbonates
Carbonates of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) were prepared by adding slowly, with continuous stirring, a saturated sodium hydrogen carbonate solution to aqueous solutions metal chloride, until total precipitation of the metal ions. The precipitates were washed with distilled water until the elimination of chloride ions (qualitative test with AgNO3/HNO3 solution for chloride) and was placed in vacuum desiccator until constant mass.

Metal Amidosulfonates
The compounds were prepared by the direct reaction between the aqueous suspension of the metal carbonates (Mn, Co, Ni, Cu and Zn) and the amidosulfonic acid, with heating at 80 o C and stirring. The acid in powder form was slowly added until a small amount of the metal carbonate remained. The carbonate in excess was removed by filtration and the aqueous solution was evaporated slowly near to dryness. Subsequently, the solution was kept in vacuum desiccator until constant mass.

Characterization of samples
The infrared spectroscopy for amidosulfonic acid and its metal amidosulfonates were run on a Perkin-Elmer Spectrum 100 ATR FTIR spectrophotometer using ATR accessory with germanium crystal. The FTIR spectra were recorded with 16 scans per spectrum a resolution of 4 cm -1 .
The X-ray powder patterns were obtained by using a BRUKER System D8 Advance Diffractometer, employing CuK radiation (λ = 1.541 Å), 25 mA, 40 kV, 10 rpm rotation, 0.02 step, 0.6 slit mm, time 0.3 s, in the range 2 from 10 to 70 degrees. Elemental analysis for H, N and S was performed using a Leco CHNS Analyzer.
The thermal behavior was evaluated by thermogravimetry (TG) and differential thermal analysis (DTA) in the simultaneous module TG-DTA 6200 Extar 6000 from Seiko SII, with sample mass of the order of 3 to 10 mg, heating ratio ß = 20 o C min -1 (30 to 900 o C), alumina crucible, dynamic atmosphere of synthetic air and nitrogen, with flow of 100 mL min -1 . As reference for DTA, previously calcined alumina was used.

Results and discussion
The elemental analysis results are presented in Tab. 1 and are in agreement with the proposed general formula M(NH2SO3)2.xH2O, where M = Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ and x the number of water molecules ranging from 1 to 4, where x = 4 for Mn 2+ , 3 for Co 2+ and Zn 2+ , 2 for Ni 2+ and 1 for Cu 2+ . 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 SO3stretching 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][5][6][7][8][9][10][11][12][13][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 Xray diffractograms indicate that the compounds were obtained with a certain crystallinity degree, showing no evidence of isomorphism. The compounds showed low relative intensity (Io 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 = Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ ; L represents the anion NH2SO3and x the number of water molecules ranging from 1 to 4, where x = 4 for Mn 2+ , 3 for Co 2+ and Zn 2+ , 2 for Ni 2+ and 1 for Cu 2+ . 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.

Ni(NH2SO3)2.2H2O
The dehydration of the nickel (II) amidosulfonate occurs in a two overlapping steps, with elimination of two water molecules between

Cu(NH2 SO3)2.1H2O
The dehydration of the copper (II) amidosulfonate occurs in a two overlapping steps, eliminating one water molecule between 73 o C and 199 o C (synthetic air) and between 65 o C and 203 o C (N2) with mass loss of 6.9 % in both atmospheres. At this stage, two endothermic peaks were observed at 89. . Under N2 atmosphere a new stage of mass loss appears, which begins at 840 o C and probably ends at temperatures above 900 o C. The DTA profile of the beginning of this stage has endothermic characteristics, suggesting a Cu 2+ → Cu + reduction process.

Zn(NH2SO3)2.3H2O
Dehydration Zn(NH2SO3)2.3H2O occurs two overlapping steps, between 64 °C and 195 °C (synthetic air and N2), equivalent to the consecutive release of one and two water molecules, respectively, with losses of the order of 17 %, Endothermic peaks are observed at 97 o C and 163 o C (synthetic air and N2). After formation of the anhydrous compounds, an exothermic peak was observed around 202 o C (synthetic air and N2).

Conclusions
The main absorptions in the infrared: O-H (3600 -2700 cm -1 ) in the salts; N-H (3400-3300 cm -1 ) in amidosulfonic acid; sS-O (1260-1140 cm -1 ) and asS-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 Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ and Zn 2+ ; L represents the anion NH2SO3 -; x represents the number of water molecules, where x = 4 for Mn 2+ , x = 3 for Co 2+ ; and Zn 2+ , x = 2 for Ni 2+ and x = 1 for Cu 2+  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 o C.