The Effect of the Aminic Substituent on the Thermal Decomposition of Cyclic Dithiocarbamates

Estudos termogravimétricos e calorimétricos diferenciais para ditiocarbamatos de NH4 , Na, Zn, Cd e Pb, derivados de aminas cíclicas contendo nitrogênio como heteroátomos, foram realizados em atmosferas de ar e nitrogênio, para avaliar a influência da tensão angular dos anéis na decomposição térmica destes compostos, em relação à formação de tiocianatos metálicos como via de decomposição. Os intemediários formados foram caracterizados por difração de raios-X, tendo sido encontrados oxissulfatos de Zn, Cd e Pb, sob atmosfera de ar, o que sugere a decomposição térmica nestas condições como via sintética para estes compostos. Os produtos de decomposição final obtidos foram sulfetos metálicos sob nitrogênio e óxidos dos metais de transição e sulfato de sódio sob ar. Entalpias de fusão são também descritas, com base nos resultados de DSC.

In those reviews is possible to note that the knowledge of DTC thermal behaviour and information about their stability and decomposition residues, are important data and can show synthetic routes for unusual compounds.
In three reviews about the thermal behaviour of DTC compounds 1,2,4 , most of references are related with the aliphatic derivatives in which a decomposition mechanism involving metallic thiocyanate as the main intermediate.However, few studies has been presented in relation to the cyclic DTC, such as pyrrolidinedithiocarbamate (Pyr) and piperidinedithiocarbamate (Pip), in which the formation of the thiocyanate must be related with cleavage of C-N bond in the ring.
Pyr complexes are widely used in solvent extraction and other analytical procedures, because of their resistance to acidic media 10 .In applications such as in atomic absorption spectrophotometry, the thermal decomposition data can help in understanding more about atomization mechanisms of those complexes 11 .
The Pip complexes of Zn and Cd are largely applied as curing agents in rubber processing 12 and in photographic films 13,14 were the thermal behaviour is an important feature.
Recently Fernandez-Alba et al. 15 , reported studies on thermal decomposition of NH 4 Pyr and other DTC, with agricultural applications.Vibrational Spectroscopy was used to propose an structure to the complexes.
Diaz et al. 16 studied decomposition fragments in mass spectroscopy for Bi, Cd, Co, Cr, Cu, Fe, Ni, Pb, Sb, Zn and ammonium in Pyr compounds.
Gomicek et al. 17 , presented X-ray diffraction studies for the characterisation of decomposition products of several Pyr complexes with divalent cations.The decomposition products were obtained at 80, 150, 250, 300, 400 and 600 °C, under oxygen and argon atmospheres.The compounds were obtained in an oven without use of the TG to follow up the thermal decomposition pathways.
The same authors 18 , investigated the thermal behaviour of Cd, Co, Cu, Fe, Ni, Pb and V -Pyr complexes in graphite tubes for atomic absorption.
Sceney et al. 19 , studied the thermal decomposition of seven copper(II) dithiocarbamates by thermogravimetry, under air and nitrogen atmospheres.Complete TG and DTA studies are reported to copper(II)-diethyldithiocarbamate complex under air, vacuum and nitrogen.
It is also important to note that few DSC results, about thermal DTC behaviour has been previously reported.The most thermal differential data are from DTA results.
In the present work the thermal behaviour of ammonium, sodium, zinc, cadmium and lead complexes with Pyr and Pip under nitrogen and air atmospheres was investigated with the purpose of evaluating the influence of the different angular tension of the 5 and 6 members aminic rings on the decomposition and identify the intermediates formed.

Syntesis of the ligands and complexes
Ammonium and sodium salts of the ligands, were prepared by direct reaction between the piperidine or pyrrolidine and carbon disulphide in basic media and recrystallised from acetone (NH 4 Pip) or ethanol-water 1:1 (v/v) (NaPip, NaPyr and NH 4 Pyr) and then dried in a vacuum oven at 50 °C, for 8 h.
The metallic complexes were obtained by direct reaction of the DTC sodium salt and a soluble salt of the desired cation.The resulting precipitates were filtered, washed with acetone and dried as above.

Equipment
Characterazation of the compounds was performed using Vibrational Spectroscopy (KBr pellets) in a Nicolet 5SXC spectrophotometer; NMR-1 H in a Brücker AC-200 spectrometer and Flame Atomic Absorption Spectroscopy in an Intralab AA12/1475 (Gemini) spectrophotometer and Elemental Analysis using a Fisons EA 1108 CNHS-O instrument.
TG curves were recorded in a DuPont 9900 thermoanalyser coupled with a TGA 951 Thermogravimetric Module under a gas flow of 200 mL min -1 (N 2 or air), in a platinum crucible, at 10 °C min -1 heating rate and using samples of about 7 mg of each compound at atmospheric pressure.For the ammonium salts (Pyr and Pip), the TG experiments were carried out only in nitrogen.The DSC curves were recorded in a DuPont 9900 thermoanalyser coupled with a DSC 910 Calorimetric Module under a gas flow of 200 mL min -1 (N 2 or air), in aluminium hermetic pans, at 10 °C min -1 heating rate and using samples of about 5 mg of each compound at atmospheric pressure.
X-ray diffraction patterns of intermediates and final products, were obtained in an HZ-Karl Zeiss or a D5.000 Siemens diffractometers.The melting points were also determined in a dry furnace Electrothermal digital melting point apparatus.
Fusion enthalpies were determined by DSC after calibration of the equipment with indium metal.The area of the melting peak for this metal was stored in the system and compared with the area of sample melting peaks in a standard procedure.Temperatures are also measured in relation to the In fusion, calibration.

Results and Discussion
The structural formula of the ligands are: The proposition of the earlier works for thiocyanate 1 as the decomposition intermediate implies that the decomposition of the DTC complexes occurs with rupture of C-N-C bonds.For a divalent cation-Pyr complex we can propose: (1) The differences in the angular tension between the 5 and 6 membered rings of pyrrolidine and piperidine respec- However the results obtained here suggest the decomposition in a direct way (except to CdPyr 2 ), without the thiocyanates as the main decomposition pathway, in agreement with some observations of Fernandez-Alba et al. 15 The spectrometric characterisation results are shown in Table 1 and the elemental analysis data for all the compounds are shown is in Table 2.The TG-DTG curves for Pyrrolidinedithicarbamates are shown in Fig. 1 under nitrogen and air atmospheres.The DSC curves under both atmospheres are in Fig. 2.
Mass losses, temperature ranges and residues for the decomposition of all Pyr compounds are described in Ta-ble 3. The melting points and enthalpies are shown in Table 4.

NH4Pyr
The TG/DTG curves of ammonium salt showed a single decomposition step between 70 and 175 °C ( In order to attribute the peaks a sample was heated in a glass tube (20 x 2 cm) immerse in a glycerine bath.Around 140 °C the gas produced by heating the sample was bubbled into a phenolphtalein solution turning it to red and showed a characteristic smell of ammonia 26 .Near 150 °C melting Figure 1.TG (solid line) and DTG (dashed line) curves for the Pyrrolidinedithiocarbamates under N2 atmosphere: a) NH4 + ; b) Na + ; c) Zn 2+ ; d) Cd 2+ ; e) Pb 2+ ; and under air: f) Na + ; g) Zn 2+ ; h) Cd 2+ and i) Pb 2+ . of sample was observed producing a brown viscous liquid, which IR spectra suggests that it is the HPyr (Table 1).The gas at 165 °C when bubbled in a Sb 3+ solution produced an orange solid and showed a characteristic smell of H 2 S. The colour is characteristic for Sb 2 S 3 26 .
According to these observations it was shown that the NH 4 Pyr decomposes by the following steps: NH 4 Pyr → NH 3 + Hpyr (2) Table 1.Molecular absorption spectrometry (UV), infrared spectrometry (IR) and nuclear magnetic resonance 1 H (NMR) data obtained for the compounds.

NaPyr
Sodium salt showed the decomposition in two steps, relative to the loss of 2H 2 O, with sodium polissulphide and a carbonaceous residue under nitrogen atmosphere (340-394 °C) according to Figure 1.b and sodium sulphate air (Fig. 1.f), and it was the most stable compound in the series here studied.The presence of the polisuphide is in agree-ment with the observed in earlier works 27,28 , and were characterised by X-ray diffraction.
The DSC curves for sodium salt evidenced peaks attributed to a loss of water and decomposition under N 2 (Fig. 2.b).The experiment carried out in air (Fig. 2.g) showed the loss of water followed by exothermic processes of decomposition and oxidation to Na 2 SO 4 , respectively.

ZnPyr2
TG/DTG curves of zinc compound showed decomposition in a single step resulting mainly in ZnS and a small amount of Zn ° in the residue under nitrogen (Fig. 1.c).This mixture is slowly reduced up to 811 °C producing Zn°.Under air (Fig. 1.g) three steps were observed.The first was the decomposition and formation of ZnS, followed by Figure 2. DSC curves for the Pyrrolidinedithiocarbamates under N2 atmosphere: a) NH4 + ; b) Na + ; c) Zn 2+ ; d) Cd 2+ ; e) Pb 2+ ; and under air: f) NH4 + g) Na + ; h) Zn 2+ ; i) Cd 2+ and j) Pb 2+ .production of ZnSO 3 and ZnS and pure ZnO.All these data were confirmed by X-ray diffraction data.
The DSC curves under N 2 atmosphere (Fig. 2.c), showed a endothermic peak at 269 °C (∆H = 8.32 kJ mol -1 ), attributed to a crystalline phase change followed by a melting process, coincident with the observation in a capillary tube.The endothermic peak at 331 °C, was attributed to decomposition of the compound.Under air atmosphere (Fig. 2.h) endothermic peaks at 268 °C (∆H = 8.40 kJ mol -1 ), and at 294 °C, were attributed to crystalline phase change and melting respectively, in agreement with the data under nitrogen atmosphere.The decomposition of the complex is represented by endothermic peaks at 325 and exothermic at 346 °C.ZnPyr 2 , is the unique compound that showed crystalline phase transition in the present series.

CdPyr2
The TG curves in N 2 atmosphere (Fig. 1.d) showed the decomposition with generation of CdS and a amorphous material according the X-ray diffraction data.The black amorphous material was insoluble in HCl and was suspect to carbonaceous residue.
The DTG curve suggests that the first step is split in two consecutive reactions.This could be related with the formation of thiocyanide considering that the Cd 2+ is soft acid with affinity to soft base represented by that anion, according to Pearson's concept 29 and the weakening of the C-N ring bonds by angular tension.Above 566 °C volatilisation of the CdS is observed, represented by a yellow substance impregnating the exhausting furnace tube, with black residue of 3.6% at the crucible, probably carbon.
Under air atmosphere (Fig. 1.h) the residues showed the presence of CdS at 400 °C and pure Cd 3 OSO 4 at 700 °C.The final residue is CdO.All these results were confirmed by X-ray diffraction data.
The DSC curves do not evidenced the occurrence of melting in agreement with capillary tube observation.Under N 2 atmosphere (Fig. 2.d), is possible to note the decomposition, related with the endothermic peaks at 320 and 345 °C.Under air atmosphere (Fig. 2.i) decomposition is represented by the peaks at 309, 336 °C and 346 °C in agreement with TG data.The exothermic processes at 446 °C, was attributed to carbon burning.

PbPyr2
Under nitrogen atmosphere (Fig. 1.e) the PbPyr 2 TG curves showed two steps.The first one resulting in a black residue of PbS, confirmed by X-ray diffraction measurements and by smell of H 2 S in presence of hydrochloric acid.The second step was related to volatilization of the PbS; resulting in a small amount of intermetallic Pb-Pt at crucible, at 1030 °C, according the X-ray diffraction data.
Under air atmosphere (Fig. 1.i) the decomposition occurred with formation of a mixture of PbS and PbSO 4 , followed by oxidation to PbSO 4 and a small amount lead oxysulphate, Pb 3 O 2 SO 4 , according the X-ray diffraction results.
The DSC curve under N 2 atmosphere (Fig. 2.e), showed a sharp endothermic peak, attributed to the melting, followed by an endothermic decomposition peak at 321 °C.The curve in air (Fig. 2.j) also suggests a melting process.In the sequence occurred the decomposition of compound with peaks at 320 °C and at 334 °C, followed by the oxidation process represented by the exothermic peak at 453 °C.
The TG-DTG curves for Piperidinedithicarbamates are shown in Fig. 3 under nitrogen and air atmospheres.The DSC curves under both atmospheres are in Fig. 4. Mass losses, temperature ranges and residues for the decomposition of all Pyr compounds are described in Table 5.The melting points and enthalpies are shown in Table 4.

NH4Pip
The TG/DTG curves of this salt showed a single mass loss (85-180 °C), without residues in the crucible (Fig. 3.a).The DSC curves showed a sharp endothermic peak, at 172 °C and 175 °C under N 2 (Fig. 2.a) and air (Fig. 2.f) atmospheres respectively, without evidence of melting.
Experiment in an open glass tube showed that the compound sublimates without decomposition since the IR spectra of the sublimate product is identical to the original salt.

NaPip
This salt showed a very similar behaviour with the NaPyr.The decomposition occurred in two steps under N 2 (Fig. 3.b).The first relative to the loss of 2H 2 O, in agreement with Kudela et al. report 23 .The second step is related to decomposition and formation of sodium polisulphide as residue under nitrogen 27,28 .Under air (Fig. 3.f) the TG curve showed the water loss was and formation of sodium sulphate in the range.All these residues were confirmed by X-ray diffraction, being the most stable piperidine derivative in the series here studied.
The DSC curves for sodium salt showed three endothermic peaks under N 2 atmosphere (Fig. 4.b).The first at 142 °C related to the water loss, the second a sharp peak at 302 °C, attributed to a melting process.The last one at 338 °C, was attributed to the decomposition.Under air (Fig. 4.g) showed the loss of water (endothermic peaks at 148 and 165 °C), followed by complex exothermic processes of decomposition represented by the peaks at 291 and 320 °C and oxidation to Na 2 SO 4 at 417 °C.The melting is probably masked by the strong exothermic decomposition of the compound under oxidative atmosphere.Under air the peaks at 321 and 335 °C were attributed to the decomposition, and the exothermic peak at 448 °C was related with the production of Cd 3 OSO 4 .

PbPip2
The TG/DTG curves under nitrogen (Fig. 3.e) for the lead compound showed a decomposition, resulting in a black residue of PbS confirmed by X-ray diffraction and test with HCl.Above 700 °C, volatilization of the PbS occurred, similarly to the PbPyr 2 .
Under air atmosphere (Fig. 3.i) a decomposition occurred, with formation of a mixture of PbS and traces of lead oxysulphate Pb 2 OSO 4 .Then the mixture is converted to Pb 2 OSO 4 , and finally PbSO 4 .The residues were identified by X-ray diffraction.
The DSC curve under N 2 atmosphere (Fig. 4.e), showed a sharp endothermic peak at 244 °C attributed to melting, followed by a decomposition peak at 323 °C.The curve under air atmosphere (Fig. 4.j) showed the melting peak.At the sequence the decomposition of the compound is observed with peaks at 316 °C, 333 °C, and 416 °C.

Final Comments
It was expected that the angular tension in the fivemembered pyrrolidine ring, should lead to a weakening of the C-N ring bonds favouring its rupture generating the metallic thiocyanate but this is not true.
According to the results obtained here, both cyclic DTC, decomposed preferentially in a direct way, without the formation of a thiocyanate intermediate, in a different .TG (solid line) and DTG (dashed line) curves for the Piperidinedithiocarbamates under N2 atmosphere : a) NH4 + ; b) Na + ; c) Zn 2+ ; d) Cd 2+ ; e) Pb 2+ ; and under air: f) Na + ; g) Zn 2+ ; h) Cd 2+ and i) Pb 2+ .behaviour described to non cyclic derivatives 1 .The CdPyr suggested the unique exception.
The most remarkable difference in the decomposition as a function of the aminic group was observed for the ammonium salts.Some oxysulphates were obtained as products, suggesting that the thermal decomposition of the dithiocarbamate complexes may be an useful synthetic route to obtain such compounds.DSC curves for the Piperidinedithiocarbamates under N2 atmosphere: a) NH4 + ; b) Na + ; c) Zn 2+ ; d) Cd 2+ ; e) Pb 2+ ; and under air: f) NH4 + g) Na + ; h) Zn 2+ ; i) Cd 2+ and j) Pb 2+ .
Pyr derivatives, a more tense 5 member ring, are supposed to decompose preferentially by the thiocyanate.

4 )
HPyr (l) → H 2 S + other volatiles (As the DSC experiments were carried out in an hermetic crucible the temperatures are higher than the observed in the open tube.These observations are not described in the earlier reports.When the DSC experiment is carried out in an open crucible, a single peak at 155 °C is observed, under both atmospheres.In the TG experiments these features are not observed probably because the crucible is open.

Table 4 .
Melting points and energies, determined by dsc and capillary tubes.
* base line not clear for integration procedure; ** masked by the decomposition process.

Table 3 .
Mass loses and temperature range corresponding to the decomposition of the pyr compounds.
-not calculated, mixture; a: relative to a mass loss; b: relative to the residue.

Table 5 .
Mass loses and temperature range corresponding to the decomposition of the pip compounds.
-not calculated, mixture; a: relative to a mass loss; b: relative to the residue.