Thermal properties of solid complexes with biologically important heterocyclic ligands
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.
KeywordsThermal decomposition Mg(II) complexes 2-hydroxypyridine Quinoline Quinoxaline TG DTA IR
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.
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
Elemental analysis and complexometric titration data of the complexes I–III
Thermal behavior of the compounds
Thermal decomposition data
Mass loss/% found/calc.
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.
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.
Some IR spectral data (450–2600 cm−1) of complexes I–III
ν(CN) py ring
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 .
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 , 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.
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.
The authors wish to thank the Slovak Grant Agency (project VEGA 1/330/09) for the financial support.
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