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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.49 n.2 Concepción jun. 2004

http://dx.doi.org/10.4067/S0717-97072004000200012 

 

J. Chil. Chem. Soc., 49, N 2 (2004), pags.:169-172

STUDY OF STABILITY PHYSICAL CHEMICAL OF 3-[2´-THIAZOLYLAZO]-2,6-DIAMINOPYRIDINE AND THE FORMATION OF 3-[N,N-ETHYL-MET-AZO]-2,6-DIAMINOPYRIDINE AS NEW LIGAND CHROMOPHORE

César Soto SalazarI* and M. Inés ToralII

IDepartment of Analytical and Inorganic Chemistry, Faculty of Chemical Sciences, University of Concepción, Concepción, Chile. E-mail: cesarsoto@udec.cl.
IIDepartment of Chemistry, Faculty of Sciences, University of Chile, Santiago, Chile.


Abstract

The 3-[2´-thiazolylazo]-2,6-diaminopyridine or 2,6-TADAP have been used as chromophore reagent for the determination of metals of the platinum group. In this work a study of the stability of the 2,6-TADAP is included, a synergetic action of the temperature, the reaction time and the acidity of the medium was found. According to spectral behavior it can be postulated that 2,6-TADAP ligand suffers structural changes, having as consequence the formation of another azocompound. Besides, the mechanism of decomposition and characterization of the formed compound was also included. The characteristic chemical of new azocompound is appropriate for the development of analytic methods for the determination of metals belonging to the platinum group, using the new ligand called 3-[N,N-ethyl-met-azo]-2,6-diaminopyridine or 2,6-DAPEMA.

Key word: 3-[2´-thiazolylazo]-2,6-diaminopyridine, stability physical chemical, 3-[N,N-ethyl-met-azo]-2,6-diaminopyridine.


Introduction

The azocompounds are organic molecules highly conjugated with a high chromophore character. Among these compounds are the thiazolylazo diaminopyridine that have been used in the determination of transition metals such as palladium, platinum and cobalt (1-6 and 10). Some of the studies were oriented toward the acid-base equilibrium, determining the corresponding acidity constants values (7) and for analytical purposes (1-6, 9 and 10). Their capacities of forming complex with some metals of the platinum group were also studied. However, the study of the stability of these azocompound ligands has captured less attention

The preliminary assays of the stability for 2,6-TADAP, showed a great dependence to the temperature, acidity and reaction time, that which would be indicating a strong kinetic effect, or probable modifications in the molecular structure of this azocompound.

In this work, a study of the stability 3-[2´-thiazolylazo]-2,6-diaminopyridine or 2,6-TADAP is presented (Figure 1), in order to improve the knowledge about this ligand for analytical purpose. This ligand suffers structural changes, having as a consequence the formation of another azocompound, and the mechanism of decomposition and characterisation of the formed compound is also included.


 
Fig. 1: 3-[2´-thiazolylazo]-2,6-diaminopyridine or 2,6-TADAP.General procedures

The Table 1 describes the conditions, in which the study of the stability of the 2,6-TADAP was carried out, using 685.0 mg diluted to 25 ml with high-purity water.

The study of the formation of the complexes, in similar conditions was carried out using 15 mg of each metal. In all cases a reagent blank was prepared. All measures were carried out versus the correspond reagent blank.


Table 1: Procedure for the study of stability.

STUDY T° [°C] VHClO4 [ml] t [min]

Time Effect 90 6.0 10-60
Temperature Effect 15-90 6.0a, 7.5b 30c, 60d
VHClO4 Effect
(Equivalent to H0)
90 3.0-7.5 30c, 60d

a y c =ligand as reagent blank; b y d = complex.

Results and discussion

In this study, the univariable method was carried out. For a constant quantity of ligand 2,6-TADAP, the temperature, reaction time and the pherchloric acid concentration (acidity of the medium) were varied alternatively. The temperature was varied between 15° to 90°C, the reaction time between 10 min to 60 min and the concentration of pherchloric acid between 0.624 to 4.52 mol l-1. Beside this ligand was characterized by NMRH and FTIR (Table 2 and 3)

Table 2: NMRHI Results, for 2,6-TADAP.

N° of H d [ppm]# J[Hz] # d [ppm] J[Hz]

H5 0 6.20 9,0 6,13 8,8
H 7,42 5,0 7,41 5,0
H40 7,63 9,0 7,60 8,9
H 7,78 5,0 7,78 5,0

# Values cited in literature


Table3: FTIR Results, for 2,6-TADAP.

Groups

n [cm-1] #

n [cm-1]

-NH-(s)

3450 w; 3350 w, 3240 w 3433,7w, 3329w, 3220w
-NH2(aromatic) 1630 s 1633,6s
-N-N- 1450 m 1453,8m
N(aromatic) 1300 s 1293,2s
-CN- (aromatic) 1330 s 1333,2

# Values cited in literature

Study of stability of 2,6-TADAP

(i) Effect of the temperature:

The study of the temperature effect was carried out following the general procedure, but the temperature was varied between 15°C to 90°C. The Figure 2 (a, b), shows that with the increase of the temperature the 2,6-TADAP spectral band changes, taking place a displacement of the lmax approximately in 100 nm, to the blue region (UV), which indicates a decrease of the initial resonance, caused by changes at s and p orbital level of the ligand molecule. Beside a colour change was observed, the purple colour of ligand was changed to yellow.


 
Fig. 2: (a) Spectral bands of 2,6-TADAP to different temperatures values: (1) 15°C, (2) 30°C, (3) 40°C, (4) 60°C, (5) 70°C, (6) 80°C, (7) 90°C. (b) Effect of the temperature on the spectral bands of 2,6-TADAP

(ii) Effect of the reaction time:

The study of the reaction time was carried out following the general procedure but the time of reaction is varied between 10 to 80 min. In the Figure 3 (a, b) shows the effect of the time of reaction on the form of the spectral band. In this case a significant spectral changes was also found, which can be also attributed to structural changes.


 
Fig. 3: (a) Spectral bands of 2,6-TADAP to different values of time (min): (1) 10, (2) 20, (3) 30, (4) 40 and (5) 60. (b) Effect of the time on the spectral bands of 2,6-TADAP.

(iii) Effect of the acidity (H0):

The study of the acidity effect was carried out following the general procedure, but the acidity range used it was between of 2.1 to -6.4 (value of H0 x10-2). As can be seen in the Figure 4 (a, b), the band near 500 nm begins to disappear when increasing the acidity and further increase the spectral band to 400 nm, demonstrating the dependence of this reaction with the degree of acidity of the solution and agreeing with the results above in this study.


 
Fig. 4: (a) Spectral bands of 2,6-TADAP to different values of H0 x10-2: (1) 2.1; (2) ­1.6; (3) ­3.3; (4) ­4.5 and (5) ­5.6. (b) Effect of the acidity (H0) on the spectral bands of 2,6-TADAP, the absorbencies readings were made respectively to two values of lmax 503.2 and 404.1 nm

To evaluate if the change is due to a chemical reaction, the solution was cooled when the reaction was finished, however the initial colour is not recovered. In this context, it is possible to postulate that a chemical change in this condition was carried out. In order to elucidate, if the anion pherchlorate acts as oxidiser on the ligand, this was replaced for a similar quantity of hydrochloric acid and using the same procedure. This reaction gives similar results to those obtained with pherchloric acid, for this reason was possible to discard that the pherchloric acid acts as oxidiser.

Due to the characteristic of the reaction medium and to the sensitivity to react of the centres and since the nitrogen of the ring thiazolyc, this can receive a great quantity of electronic density (negative charge), conform with the protonation studies of this atom (7). Contrarily, in the sulphur atom it could receive positive charge due to their orbital disposition, (8 and 9) which are based on the evidence spectroscopic (Figure 2-4) (shift toward the UV, in 100 nm is indicative of a decrease in the resonance of the molecule original). In this context, it is possible to postulate two mechanisms by different ways (Figure 5), but both ways are catalysed for the temperature and the strongly acidic medium, where the ligand molecule suffers the protonation of all the basic centres corresponding to the nitrogen thiazolyc and aminic group (pKa1=-3.73; pKa2=1.16 and pKa3=4.19) (10).


 
Fig. 5: Mechanisms of Reaction M1 and M2, for the obtaining of L1 and L2, respectively

During the course of the experiment the presence of the H2S was observed, probably due to the elimination of this gas or of some thyolic compound (Figures 5: mechanism 1 "M1" and 2 "M2", respectively).

For the two ways mechanisms (Figure 5), it is possible to postulate that under conditions of extreme acidity and temperature, the ligand presents high lability of the bond S-C and the H2S is a good salient group, so that it is probable that L2 are not formed or this compound could be a precursor of L1.

The product of this reaction was analysed by NMRH and FTIR, for the first technique presents the characteristic signals of the protons pyridinic, besides here are presented three additional signals corresponding to, a methylic group (0.97 ppm), ethylic (3.5 ppm) and group CH (2.5 ppm), these two last present shifts (low fields) of its normal values, indicating that its chemical environment was altered (Table 4).


Table 4: NMRH1 Results, for 2,6-DAPEMA (L1)

N° de H d [ppm]# d [ppm]

CH3 0.97 0.97
CH2 2.5 3.4
CH2 2.5 2.7
NH 2.3 2.5

# Values cited in literature

The results of FTIR present characteristic signals (Table 5), corresponding to amines aliphatic (3449.5 cm-1), aromatic amines (1637.3 cm-1), group diazonic (1500 cm-1), aromatic nitrogen (1384.2 cm-1) and ethylic groups (2800 cm-1).


Table5: FTIR Results, for 2,6-DAPEMA.

Groups n [cm-1] # n [cm-1]

Amine Aliph. 3450 w; 3350 w, 3240 w 3449.5
-NH2(aromatic) 1630 s 1637.3s
-N-N- 1450 m 1500
N(aromatic) 1300 s 1294s
-CH(aliph) 2800-3000 2800
-CH3 1390 1384.2
-CH2 1490 1440
-CN- (alipha.) 1097 1121

# Values cited in literature

The results show that the ligand formed in this condition present the typical functional groups of the 3-[N,N-ethyl-met-azo]-2,6-diaminopyridine compound which was designed as 2,6-DAPEMA (L1). The formation of this azocompound correspond to mechanism 1 (M1) proposed.

For this reason it is necessary to maintain fixed the values of temperature, reaction time and acidity, to favour the formation of 2,6-DAPEMA, since this is the most important for analytic purposes, because under these conditions it no presents more structural changes.

In summary, according to the experimental conditions, it is possible to obtain 2,6-TADAP or 2,6-DAPEMA, as evidence of the interaction of these ligand with platinum, palladium and rhodium, respectively. As can be seen in the Figure 6, palladium forms complex with 2,6-TADAP and its bands are centred between 550 nm and 800 nm, according to Garcia M. (3), however when palladium reacts with 2,6-DAPEMA the formed complex is different to the complex Pd(IV)-2,6-TADAP, which also presents a very different spectral behaviour, presenting two bands centred to 411,6 nm and 502,6 nm. On the other hand the main band is higher in six times, for this reason the sensibility increases.


 
Fig. 6: Effect of the temperature on the formation of the complexes of Pd(II), in lMAX 445.0 and 662.5 nm, in different temperature values: (a) 15°C, and (b) 90°C.

For the case of Rh (III) (Figure 7), under the experimental conditions in that 2,6-TADAP is stable, practically there is not formation of complex and the analytical signal is uncertain, however under the conditions in which the formation of 2,6-DAPEMA is quantitative, this cation forms complex with 2,6-DAPEMA giving a very defined spectral signal, useful for analytical purposes. Similar behaviour for platinum was found.


 
Fig. 7: Effect of the temperature on the formation of the complex of Rh(III), in lMAX 452,8 nm, in different values of temperature: (a) 15°C and (b) 90°C.

On the base of the previous antecedents it is possible to determine that 2,6-TADAP can be used for analytical porpuses for determination, however variations of the work conditions can alter the accuracy of the results due to the transformation of this ligand.

The new ligand 2,6-DAPEMA, obtained starting from 2,6-TADAP, is a azocompound that can be used successfully for analytical purposes for the platinum, palladium and rhodium determination, because it forms complexes with very defined spectral bands and the structure stays constant under some specified conditions of acidity, temperature and reaction time. For these reason the use of the new ligand would allow the development of the analytical method with bigger sensibility and robustness.

 

Acknowledgements

The authors are grateful to the National Fund for Development of Sciences and Technology (FONDECYT), project 1020692 and Department of Postitle and Postgrade of University of Chile, Projects 1 and 14, for the financial support. Furthermore, and National Commission of Scientific and Technological Research (CONICYT), for the doctoral fellowships.

References

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* whom correspondence

Received: December 1, 2003 ­ Accepted: January 27, 2004

 

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