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Thermal studies on mixed ligand oxo thiocyanato rhenium (V) complexes with 2-mercapto pyridine.

INTRODUCTION

Studies in the area of complex formation of transition metals with inorganic and organic Ligands have continued to receive the attention of researchers. There has also been a considerable growth in the study of one of such rare transition metals "Rhenium". Compounds of this metal with Organo ligating agents have been reported by Kotegov et al (1977), Kukushkin (1985), Aminjanov and Okoronkwo (1999).

The formation of colored complexes of rhenium (V) was the basis for proposing the spectrophotometric method of determining rhenium in its oxidation state of (V).

In recent times much attention is been given to the effect of heat on the complexes of most transition metal. Due to the catalytic property of transition metals, and their compounds, apparently the stability of these compounds will be of great significance to researchers and industrialists. Most co-ordination compounds are very stable under normal conditions but subjected to heat, then undergo both physical and chemical transformations. These transformations are predicted by the nature of the central metal and Ligands used in their synthesis. Studies of thermal stability of these complexes had been conducted in view of the temperature at which the Ligating compounds are eliminated, consequently determining the strength of the metal--Ligand bonds.

Interpretation of thermogram of these thermal studies is one of the most important methods of researching the chemistry of complex compounds in solid phase (Logvinenko, 1982). Studies of thermal transformation of complex compounds have helped in finding solutions to problems associated with catalysis, pure metal production and formulation of profitable technology base. The work presented here in has focused on the study of stability of solid phase oxo rhenium (V) complexes with mixed Ligands comprising of 2--mercapto pyridine, ammonia and thiocyanide.

EXPERIMENTAL PART

Materials

[H.sub.2][ReO[Cl.sub.5]] was synthesized using analytical grade reagents and by the methods earlier reported.(Ejovska et al, 1967 and Kotegov et al, 1977). This was consequently used for the synthesis of Oxo -rhenium (V) complex containing 2-mercapto pyridine.

The 2--mercapto pyridine and the acid, HCl were obtained from Aldrich chemical company and used without further purification.

Analytical Methods

Thermal studies on the complexes under review were conducted in Derivatograph Q1500D using the "Paulic--Paulic--Erdei" system at a temperature rising rate of 278K/min. Using the thermograms, it became possible to measure concurrently the loss in mass with temperature rise (TG), rate of change in mass or differential mass change (DTG), differential change in heat effects (DTA) and Linear temperature rise (T).

Micro- analysis for C, N, H, Cl, S and M were carried out in the micro analytical laboratory of the Department of Chemistry, Tajik State National University. Rhenium was determined by precipitation as nitron perrhenate (Hillebrand et al, 1953, Klimova, 1967). The original samples to be analyzed were treated with approximately 25ml of water, 10ml of 30% potassium hydroxide. The mixture was heated mildly until it became colorless, after which it was evaporated to about 10ml and then diluted. The evaporation and dilution repeated until the excess hydrogen peroxide had been destroyed. The sample finally was made up to 100ml and suitable aliquot parts were taken for analysis.

Infrared spectral analysis of the complex was conducted in the region of 400-4000[cm.sup.-1] and were recorded in "Specord--JR--75" as KBr pellet.

Preparation of the complexes:

The [[Re.sub.2][O.sub.3][L.sub.4][(SCN).sub.2][(N[H.sub.3]).sub.2]] [Cl.sub.2].3[H.sub.2]O and [ReO[L.sub.2][([SCNHN[H.sub.2]).sub.2] N[H.sub.2]] [H.sub.2]O complexes were respectively synthesized from their base components [ReO[L.sub.2][(SCN).sub.2]Cl]3[H.sub.2]O by treating 1g of this complex for 30 minutes with gaseous ammonia in a descicator and the second complex was synthesized by dissolving 2.0g of the base complex in 200ml aceton, then gaseous ammonia was passed through this solution for two hours. The resulting complexes were analyzed and characterized (Aminjanov and Okoronkwo, 1999).

RESULTS AND DISCUSSION

Thermogram of the complex [[Re.sub.2][O.sub.3][L.sub.4][(SCN).sub.2] [(N[H.sub.3]).sub.2]][Cl.sub.2].3[H.sub.2]O showed that there is an endothermic effect with a maximum on the DTA curve at 90[degrees]C, Figure 1. Between the temperature range of 40-140[degrees]C (TG curve), the complex looses about 5.0% of its mass. This mass loss corresponds to the elimination of the three molecules of water. Theoretically, this amounts to about 4.60% (Table 2.). Element analysis of this complex heated in isothermal condition and brought to constant mass at 140[degrees]C showed results corresponding to complex without water molecules as presented in table 1.

Infrared spectroscopic analysis of this complex heated at 140[degrees]C does not show bands corresponding to the Hydroxyl group, which naturally appeared in the original complex at 3400-3600[cm.sup.-1].

In the region of 140-180[degrees]C, the mass loss amounted to 2.5%. This is accompanied by a clear endothermic effect, which appears at 170[degrees]C on the DTA curve. At this stage, the complex is believed to have lost one molecule of ammonia in which case either the thiocyanate ion or one molecule of 2- mercapto pyridine becomes a bridge between the dimerized Rhenium. Elimination of ammonia molecules is associated with endothermic effect.

Between the temperature range of 180[degrees]C-260[degrees]C, the complex further looses 5.74% of its mass. This is accompanied with an exothermic effect having a peak on the DTA curve at 200[degrees]C. Presumably in this temperature range, the complex looses one molecule of HSCN consequently there is a re-orientation of the inner sphere of the complex with one molecule of the ligand acting as a bridge. Increasing the temperature from 260 to 280[degrees]C gave rise to another loss of the second ammonia molecule as mass loss within this temperature range is 1.9%. Though endothermic effects accompany the loss of ammonia molecule, the bonding strength of the two is not equal and so is not lost at the same time. Temperature range of 280-380[degrees] is characterized by a slow change in mass as indicated by the DTG curve, total mass loss of 7.2% correspond to further loss of one more molecule of HSCN or one molecule of HCl. At 390[degrees]C the complex begins intensive decomposition with strong sequential exothermic effects at 420 and 570[degrees]C. The complex completely decomposes at 700[degrees]C.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The thermogram of the complex [ReO[L.sub.2]/[(SCNHN[H.sub.2]).sub.2]N [H.sub.2]][H.sub.2]O presents a different picture (Figure 2). In the temperature range of 20-120[degrees]C, there is no mass loss and virtually no effect on the DTA curve. From the temperature of 120[degrees]C, there is a sharp reduction in mass and this process ends at 160[degrees]C. In this temperature range, the complex looses about 11.2% of its mass. This is accompanied by sharp, twin endothermic effects with peaks at 135[degrees]C and 140[degrees]C respectively. Analogical picture was not observed in the mixed thiocyano, 2- mercapto pyridine complexes above. This complex with deprotonated thiourea is believed to have dimerized with the massive loss of 6 molecules of ammonia and the molecule of water, which was not part of the inner complex. Schematic representation of the dimerization process with loss of 6 molecules of ammonia and one molecule of water is presented below:

[ReO[L.sub.2][(SCNHN[H.sub.2]).sub.2] N[H.sub.2]][H.sub.2]O [right arrow] [[Re.sub.2][O.sub.3][L.sub.4][(SCN).sub.4]] + 6N[H.sub.3] + [H.sub.2]O.

A confirmation of this proposed thermolysis scheme can be supported by infrared spectrum of the product of this reaction in which a band attributed to v(Re = 0) appears at 905[cm.sup.-1].This frequency is characteristic of dimerized oxygen bridged rhenium (V) complexes and different from the unheated complexes, the IR-Spectrum showed band with peaks around 2040[cm.sup.-1] and which are attributed to the presence of the thiocyanate groups.

Further increase in temperature led to decrease in mass with an intermediate speed. At the interval of 160-320[degrees]C there are two exothermic effects with maximum peaks appearing at 225[degrees]C and 245[degrees]C. Percentage mass loss amounted to 11.5%. It is presumed that at this stage of thermolysis two molecules of HSCN are lost. This complex begins to decompose at 360[degrees]C, at a high rate and with exothermic effects having peaks on the DTA curves at 405[degrees]C and 495[degrees]C. Complete decomposition of this complex appears to occur at 600[degrees]C.

CONCLUSION

From the studies conducted, the two complexes under investigation [[Re.sub.2][O.sub.3][L.sub.4][(SCN).sub.2][(N[H.sub.3]).sub.2]][Cl.sub.2].3 [H.sub.2]O and [ReO[L.sub.2][(SCNHN[H.sub.2]).sub.2]N[H.sub.2]][H.sub.2]O contain molecules of water in the outer coordination sphere. These water of crystallization molecules are lost at temperature of about 90[degrees]C and 135[degrees]C respectively. The ammonia components of the complex were also identified to have been lost in endothermic processes and in the temperature ranges of 140-170[degrees]C.

The thermo analysis of the complex synthesized in acetone medium indicated clearly that the medium played a major role in the formation of the complex. Formation of HSCN molecules from the deprotonated form of thiourea in the complex [ReO[L.sub.2] [(SCNH[N.sub.2]).sub.2]N[H.sub.2]][H.sub.2]O indicated a structural rearrangement of the said complex hence the subsequent dimerization. This study further showed that complexation had considerably increased the stability of the principal ligand 2-mercapto pyridine. Where as 2-mercapto pyridine melts at 121-124[degrees]C and practically decomposes at 140-280[degrees]C (Busiev, 1972), the intensive decomposition of rhenium (V) complexes start at 550-700[degrees]C.

REFERENCES

(1.) Aminjanov, A.A., and Okoronkwo, A.E. 1999. "Processes of Interaction of gaseous ammonia with thiocyanide pyridinethiol complexes of Rhenium (V)". Co-ordination Compounds and aspects of their usage,. Dushanbe, 3rd Edition .P.8-13

(2.) Busiev, A.I. 1972. "Synthesis of New Organic Reagents for Inorganic Analysis".Moscow Univ. Press.

(3.) Ejovka , T.B., Vaida , S. and Baluka , M. 1967. "Structure and Properties of Compounds of Technetium and Rhenium type [MOX5]2- . J. Struct. Chem. 8 (3): 519-523

(4.) Hillebrand ,W.F., Lundell, G.E.F., Bright, H.A. and Hoffman, J.I. 1953. "Applied Inorganic Analysis" John Wiley and Sons Inc. New York., P.321

(5.) Klimova, V.A. 1967. "Basic Methods of Analysis of Organic Compounds", Moscow, P.208

(6.) Kotegov, K.V., Aminjanov ,A.A. and Kukushkin, U.N. '"Study of Complex formation of rhenium oxobromide with 1-methyl -2-Mercapto imidazole,". J. Inorg Chem,Moscow. 22., (10): 2742

(7.) Kukushkin, U.N., Budanov, V.F. and Seedov, G.N. 1981. "Thermal Transformation of Coordination Compounds in Solid Phase", Leningrade, Leningrade Univ. Press, Russia.

(8.) Kukushkin, U.N. 1985. Chemistry of Co-ordination Compound . Moscow . 455.

(9.) Logvinenko, V.M. 1982. "Thermal analysis of co-ordination complexes" Science, Siberia. New Siberia.

A. E. Okoronkwo

Department of Chemistry, Federal University of Technology, Akure ,Nigeria Email--afamex@yahoo.com,
Table 1: Analytical data and selected i.r. wave numbers

Compound Found (calculated) %
 Re N S C

[[Re.sub.2][O.sub.3] 31.40 9.13 17.3 22.53
[L.sub.4][(SCN) (32.68) (9.83) (16.85) (23.17)
.sub.2][(N[H.sub.3])
.sub.2]][Cl.sub
.2].3[H.sub.2]O

[[Re.sb.2][O.sub.3] 35.03 11.2 18.72 23.86
[L.sub.4] [(SCN).sub.2] (34.31) (10.32) (17.69) (24.32)
[(N[H.sub.3]).sub.2]]
[Cl.sub.2].

[ReO[L.sub.2] 32.7 16.7 22.3 23.8
[(SCNHN[H.sub.2]). (33.54) (16.11) (21.05) (23.68)
sub.2]NH2][H.sub.2]O

[[Re.sub.2][O.sub.3] 34.6 10.5 22.8 25.6
[L.sub.4][(SCN).sub.4]] (34,0) (10.2) (23.3) (26.3)

 Wave numbers
 ([cm.sup.-1])
Compound Found calculated (%) numbers
 H Cl ([cm.sup.-1])

[[Re.sub.2][O.sub.3] 3.00 5.85 463s,694s, 907s, 14
[L.sub.4][(SCN) (2.81) (6.23) s, 907s,
.sub.2][(N[H.sub.3]) 1400s,
.sub.2]][Cl.sub 1604s
.2].3[H.sub.2]O 3125

[[Re.sb.2][O.sub.3] 2.2 7.35 463s,694s, 907s, 14
[L.sub.4] [(SCN).sub.2] (2.40) (6.54) s, 907s,
[(N[H.sub.3]).sub.2]] 1400s,
[Cl.sub.2]. 1604s,

[ReO[L.sub.2] 2.8 990s
[(SCNHN[H.sub.2]). (3.28)
sub.2]NH2][H.sub.2]O

[[Re.sub.2][O.sub.3] 2.0 905s, 2040s
[L.sub.4][(SCN).sub.4]] (1.8) 2040s

Table 2: Thermal data for the complexes

Compound Dec % weight loss DTA peak
 Interval Found calcd (C
 (C[degrees]) [degrees])

[[Re.sub.2][O.sub.3] 0 - 140 5.0 4.60 90 endo
[L.sub.4][(SCN).sub.2] 140 - 180 2.5 1.52 90 endo
[(N[H.sub.3]).sub.2]] 180 - 260 5.74 5.34 200 exo
[Cl.sub.2]x3[H.sub.2]O 260 - 280 1.90 1.63 270 endo
 280 - 380 7.2 5.74 310 exo

[ReO[L.sub.2][(SCNHN[H. 20 - 120 - - -
sub.2]).sub.2] 120 - 160 11.2 9.87 135 endo,
N[H.sub.2]][H.sub.2]O 140 exo
 160 - 320 11.5 10.76 225 exo,
 245 exo

Endo: decomposition endotherm; exo: decomposition exotherm.
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Author:Okoronkwo, A.E.
Publication:Bulletin of Pure & Applied Sciences-Chemistry
Date:Jul 1, 2006
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