Journal of Thermal Analysis and Calorimetry

, Volume 104, Issue 3, pp 817–821 | Cite as

Thermal properties of solid complexes with biologically important heterocyclic ligands

Part III. Thermal decomposition and infrared spectra of thiocyanato Mg(II) complexes with 2-hydroxypyridine, quinoline, and quinoxaline
  • E. Jóna
  • L’. Lajdová
  • L’. Kvasnicová
  • S. Lendvayová
  • M. Pajtášová
  • D. Ondrušová
  • P. Lizák
  • S. C. Mojumdar
Article

Abstract

The thermal decomposition of the complexes Mg(SCN)2(2-OHpy)4·H2O(I), Mg(SCN)2(quin)4·2H2O(II) and Mg(SCN)(quinox)4·5H2O(III) (2-OHpy–2-hydroxypyridine, quin–quinoline, quinox–quinoxaline) has been investigated in static air atmosphere at 20–1000 °C by means of thermogravimetry (TG), differential thermal analysis (DTA), and infrared (IR) spectroscopy. The composition of the complexes had been identified by means of elemental analysis and complexometric titration. The possible scheme of destruction of the complexes is suggested. The final product of the thermal decomposition was MgS. IR data suggest that heterocyclic ligands were coordinated to Mg(II) through the nitrogen atom of their heterocyclic ring. Thiocyanate group is also coordinated through the nitrogen atom.

Keywords

Thermal decomposition Mg(II) complexes 2-hydroxypyridine Quinoline Quinoxaline TG DTA IR 

Introduction

The relationship between the thermolyses and structures of metal complexes and the influence of the natures of the ligands on the process of thermal decomposition are very important. Therefore, many authors have studied thermal properties of many coordination compounds with heterocyclic ligands [1, 2] since these compounds play an important role in many biological systems. The authors have studied the thermal behavior of Mg(II) complexes with pyridine and quinoline derivates to improve the understanding of drug-metal ion interactions. This study is a continuation of previous studies on the stereochemistry [3] and thermal properties [4, 5, 6, 7, 8, 9, 10, 11] of Mg(II) complexes with N-donor ligands. This article deals with the preparation of thiocyanato Mg(II) complexes with 2-hydroxypyridine, quinoline and quinoxaline (Fig. 1) together with the thermal and IR spectral analyses of these complexes.
Fig. 1

Structures of 2-hydroxypyridine (a), Quinoline (b), and Quinoxaline (c)

Experimental

Synthesis of Mg(II) complexes

Compounds I and III were prepared by dissolving 20.30 g (0.1 mol) MgCl2·6H2O in ethanol and gradually adding 19.40 g (0.2 mol) KSCN. KCl was filtered off from the solution and then, 32.22 g (0.4 mol) 2-hydroxypyridine or 52.06 g (0.4 mol) quinoxaline was added, respectively, to filtrate. The resulting solutions were reduced in volume at room temperature and the complexes which formed, were filtered off, washed with ether and dried at room temperature.

The complex II was prepared by treating Mg(SCN)2·5H2O 2.305 g (0.01 mol) in ethanol with quinoline (2.46 mL, 0.02 mol). The solution was left to stand at room temperature. The fine microcrystals thus precipitated were filtered off, washed with cold ethanol and finally dried at room temperature.

Measurements

Elemental analyses (C, H, and N) were carried out on a Carlo Erba 1106 Analyser and the content of Mg(II) was determined by complexometric titration.

Thermal decomposition was studied on a Derivatograph OD 102 (MOM Budapest) in air atmosphere using a ceramic crucible with a sample mass of 100 mg from room temperature to 900 °C. A heating rate of 10 °C min−1 was chosen for all measurements.

The infrared (IR) spectra were obtained on a Philips analytical PU 9800 FTIR spectrometer using KBr pallet in the range 400–4000 cm−1.

Results and discussion

Analysis of compound

The content of carbon, hydrogen, and nitrogen was determined by elemental analysis and the content of magnesium was estimated by complexometric titration. The analysis confirms (Table 1) the theoretical expectation of the studied complexes I–III.
Table 1

Elemental analysis and complexometric titration data of the complexes I–III

Complex

Theoretical/%

Experimental/%

C

H

N

Mg

C

H

N

Mg

Mg(SCN)2(2-OHpy)4·H2O (I)

49.03

4.11

15.59

4.51

50.30

4.02

15.74

4.55

Mg(SCN)2(quin)2·2H2O (II)

55.20

4.17

12.88

5.59

52.71

4.32

16.80

5.57

Mg(SCN)2(quinox)4·5H2O (III)

54.37

4.56

18.65

3.23

55.45

4.53

17.71

3.19

Thermal behavior of the compounds

The thermal decomposition data of the compounds I–III are summarized in Table 2. The complexes I–III are relatively thermally stable. Thermal decomposition of these compounds started with the dehydration process and followed by the elimination of the ligands. The final solid product was always identified as MgS.
Table 2

Thermal decomposition data

Complex

DTA

TG

Tpeaks/°C

Trange/°C

Mass loss/% found/calc.

Lost component

Residue

I

120

108–205

3.00/3.34

H2O

 
 

286

205–430

53.00/52.94

3(2-OHpy)

 
 

484

430–575

18.00/17.64

1(2-OHpy)

 
 

682

575–795

16.00/15.61

SCN,CN

MgS

II

132

107–170

9.00/8.29

2H2O

 
 

209

170–344

60.00/59.41

2(quin)

 
 

419

344–460

12.00/13.36

SCN

 
 

501

460

6.00/5.98

CN

MgS

III

128

105–177

12.00/11.99

5H2O

 
 

211

177–301

70.00/69.31

4(quinox)

 
 

413

301–549

8.00/7.73

SCN

 
 

590

549–730

4.00/3.46

CN

MgS

The TG and DTA curves for Mg(SCN)2 (2-OHpy)4 H2O (I) are shown in Fig. 2. The TG and DTA curves of that complex indicate that it is thermally stable up to 108 °C, where the dehydration process commences. This is followed by two mass loss steps between 205–430 °C and 430–575 °C. Based on the mass loss values (Table 2), these two steps were attributed to the formation of two intermediate decomposition products, i.e., Mg(NCS)2(2-OHpy) and Mg(NCS)2, while the final solid product is concluded to be MgS. The thermal decomposition scheme is:
$$ {\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \left( {{\text{2-OHpy}}} \right)_{ 4} \cdot {\text{H}}_{ 2} {\text{O}}\mathop{\longrightarrow}\limits^{{108{-}205^\circ {\text{C}}}}{\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \left( {{\text{2-OHpy}}} \right)_{ 4} $$
(1)
$$ {\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \left( {{\text{2-OHpy}}} \right)_{ 4} \mathop{\longrightarrow}\limits^{{205{-}430^\circ {\text{C}}}}{\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \left( {{\text{2-OHpy}}} \right) $$
(2)
$$ {\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \left( { {\text{2-OHpy}}} \right)\mathop{\longrightarrow}\limits^{{430{-}575^\circ {\text{C}}}}{\text{Mg}}\left( {\text{SCN}} \right)_{ 2} $$
(3)
$$ {\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \mathop{\longrightarrow}\limits^{{575{-}795^\circ {\text{C}}}}{\text{MgS}} $$
(4)
Fig. 2

TG and DTA curves of Mg(SCN)2(2-OHpy)4·H2O (I)

The DTA curve of complex I (Fig. 2) displays three endothermic peaks maximized at 120, 286, and 484 °C corresponding to the loss of 1 mol H2O, 3 mol 2-OHpy, and 1 mol 2-OHpy, respectively, and one exothermic peak maximalized at 682 °C corresponding to the decomposition reaction of Mg(SCN)2 with simultaneous formation of MgS.

As summarized in Table 2, the TG curve for the complex Mg(SCN)2(quin)2·2H2O(II) indicate that mass loss starts observable at ~107 °C (in the temperature range 50–900 °C) and four mass loss steps were observed. The first step between 107 and 170 °C is accompanied by 9% mass loss and corresponds to the release of 2 molecules H2O. The second step took place between 170 and 344 °C is accompanied by 60% mass loss and corresponds to the release of 2 molecules quin. The third step between 344 and 460 °C, and fourth step between 460 and 695 °C correspond to the decomposition of Mg(SCN)2 and Mg·SCN to MgS as final decomposition product. The most probable thermal decomposition scheme is given below:
$$ {\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \left( {\text{quin}} \right)_{ 2} . 2 {\text{ H}}_{ 2} {\text{O}}\mathop{\longrightarrow}\limits^{{107{-}170^\circ {\text{C}}}}{\text{ Mg}}\left( {\text{SCN}} \right)_{ 2} \left( {\text{quin}} \right)_{ 2} $$
(5)
$$ {\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \left( {\text{quin}} \right)_{ 2} \mathop{\longrightarrow}\limits^{{170{-}344^\circ {\text{C}}}}{\text{ Mg}}\left( {\text{SCN}} \right)_{ 2} $$
(6)
$$ {\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \mathop{\longrightarrow}\limits^{{344{-}460^\circ {\text{C}}}}{\text{ Mg}}\cdot {\text{SCN}} $$
(7)
$$ {\text{Mg}}\cdot {\text{SCN}}\mathop{\longrightarrow}\limits^{{460{-}695^\circ {\text{C}}}}{\text{ MgS}} $$
(8)
The DTA curve for the complex II (Fig. 3) shows two endothermic peaks at 132 and 209 °C corresponding to the loss of 2 molecules H2O and 2 molecules quin, respectively, and two exothermic peaks with maxima at 419 and 501 °C, corresponding to decomposition reactions of Mg(SCN)2 and Mg·SCN with simultaneous formation of MgS.
Fig. 3

TG and DTA curves of Mg(SCN)2(quin)2·2H2O (II)

The TG and DTA curves of Mg(SCN)2(quinox)4·5H2O(III) are shown in Fig. 4. The TG curve of that complex indicates that it is thermally stable up to 105 °C, where the dehydration process commences. This is followed by three mass loss steps between 105–177, 177–301, and 301–549 °C. Based on the mass loss values (Table 2), these three steps were attributed to the formation of three decomposition products, i.e., Mg(SCN)2(quinox)4, Mg(SCN)2, and Mg·SCN, while the final solid product is concluded to be MgS. The most probable thermal decomposition scheme is:
$$ {\text{Mg}}\left( {\text{SCN}} \right)\left( {\text{quinox}} \right)_{ 4} . 5 {\text{H}}_{ 2} {\text{O}}\mathop{\longrightarrow}\limits^{{105{-}177^\circ {\text{C}}}}{\text{ Mg}}\left( {\text{SCN}} \right)_{ 2} \left( {\text{quinox}} \right)_{ 4} $$
(9)
$$ {\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \left( {\text{quinox}} \right)_{ 4} \mathop{\longrightarrow}\limits^{{177{-}301^\circ {\text{C}}}}{\text{ Mg}}\left( {\text{SCN}} \right)_{ 2} $$
(10)
$$ {\text{Mg}}\left( {\text{SCN}} \right)_{ 2} \mathop{\longrightarrow}\limits^{{301{-}549^\circ {\text{C}}}}{\text{ Mg}}.{\text{SCN}} $$
(11)
$$ {\text{Mg}}.{\text{SCN}}\mathop{\longrightarrow}\limits^{{549{-}730^\circ {\text{C}}}}{\text{ MgS}} $$
(12)
Fig. 4

TG and DTA curves of Mg(SCN)2(quinox)4·5H2O (III)

The DTA curve of complex III (Fig. 4) displays two endothermic peaks maximized at 128 and 211 °C corresponding to the loss of 5 molecules H2O and 4 molecules quinox, respectively, and two exothermic peaks maximized at 413 and 590 °C, corresponding to the decomposition reactions of Mg(SCN)2 and Mg·SCN with simultaneous formation of MgS.

IR spectra

The most important infrared spectral data of studied complexes are reported in Table 3. The absorption bands which occur in the range 3500–3200 cm−1 (symmetric and antisymmetric OH stretching) and 1630–1590 cm−1 (HOH bending) confirm the presence of water in the complexes I–III [4, 10].
Table 3

Some IR spectral data (450–2600 cm−1) of complexes I–III

Assignment

Complex

I

II

III

ν(CN) NCS

2081

2083

2083

ν(CS) NCS

767

764

766

ν(CN) py ring

1615

1612

1614

ν(CC)

951

959

959

ν(CH) ring

858

864

866

γ(CCC)

767

764

766

ν (OH)

3258

3244

3234

δ (HOH)

1633

1632

1630

The stretching vibration ν(C–H) in the pyridine ring appeared at 1590 cm−1. Upon complex formation the peak shifts to higher frequencies. The shifts in complexes I–III (in the range 1600–1620 cm−1) may suggest that the bond formation of the metal with the N of pyridine ring increases the dipolar contribution of C=N+ in the heterocyclic ring [11].

The thiocyanate groups as a ligand can principally occur as a monofunctional, bonded through nitrogen or sulfur, as a bifunctional or trifunctional ligand in the function of a bridge. At coordination of the SCN group, the changes of the C–N and C–S stretching vibrations are studied in the first place. According to the literature data [12], the main criterion of the M-NCS or M-SCN bondings is the band position corresponding to the C–S stretching vibration: the region from 770 to 860 cm−1 is assigned to the M-SCN bonding, from 690 to 730 cm−1 to the M-NCS bonding. The C–N stretching vibrations, observed in the region 2080–2110 cm−1 indicate M-NCS bonding, in the region 2110–2130 cm−1 M-SCN bonding. The ν(C–S) and ν(C–N) vibrations for studied complexes (Table 3) confirm the coordination of SCN group to Mg(II) through nitrogen atom.

Conclusions

All of the studied complexes I–III are hydrated, stable in air and soluble in water, ethanol, methanol, and dimethylsulfoxide. In complex I, loss of N-heterocyclic ligands occurs (on the TG curve) in the two steps and in complexes II and III in one step. The thermal stability of the complexes can be ordered in the sequence I > II > III (but the differences are minimum). The results reveal that MgS is left as residue at the end of the thermal degradation experiments of the compounds I–III. The preliminary study has shown that the complexes do have biological activities.

Notes

Acknowledgements

The authors wish to thank the Slovak Grant Agency (project VEGA 1/330/09) for the financial support.

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Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2011

Authors and Affiliations

  • E. Jóna
    • 1
  • L’. Lajdová
    • 1
  • L’. Kvasnicová
    • 1
  • S. Lendvayová
    • 1
  • M. Pajtášová
    • 1
  • D. Ondrušová
    • 1
  • P. Lizák
    • 2
  • S. C. Mojumdar
    • 1
    • 3
    • 4
  1. 1.Department of Chemical Technologies and EnvironmentFaculty of Industrial Technologies, Trencin University of A. DubcekPúchovSlovakia
  2. 2.Department of Industrial DesignFaculty of Industrial Technologies, Trencin University of A. DubcekTrencinSlovakia
  3. 3.University of New BrunswickFrederictonCanada
  4. 4.Deptartment of Chemical and Biochemical EngineeringThe University of Western OntarioLondonCanada

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