Four new series of Schiff bases derived from semicarbazide and thiosemicarbazide were synthesized by the reaction of 2-acetyl thiophene and thiophene-2-aldehyde in ethanol. Metal complexes of these Schiff bases has also been derived with Cu(II), Pb(II), Zn(II), Co(II), Ni(II) and Mg(II) metal salts. Such compounds and metal complexes were characterized by different physio- chemical techniques such as melting point, elemental analysis, multinuclear 1H-NMR, 13C-NMR and IR studies. All the complexes were found to have good agreement with the analytical data received. The possible geometries of the complexes were assigned on the basis of electronic and infrared spectral studies. These Schiff bases and metal complexes have wide variety of applications in many fields, such as biological, inorganic, coordination and analytical chemistry.
ABSTRACT
Four new series of Schiff bases derived from semicarbazones and thiosemicarbazones were synthesized by the reaction 0f2-acetyi thiophene and thiophene-2-aldehyde in ethanol. Metal complexes of these Schiff bases has also been derived with Cu(II), Pb(II), Zn(II), Co(II), Ni(II) and Mg(II) metai salts. Such compounds and metai complexes were characterized by different physic- chemical techniques like melting point, elemental analysis, muitinuciear NMR (1 Ķ13 c') and IR studies. All the complexes were found to have good agreement with the analytical data received. The possible geometries of the complexes were assigned on the basis of electronic and infrared spectral studies.
Keywords: Schiff bases; Semicarbazones; Thiosemicarbazones; metal complexes, IR,1 H NMR.
1. INTRODUCTION
Schiff bases are condensation products of primary amines with carbonyl compounds and they were first reported by Schiff in 18641. The common structural feature of these compounds is the azomethine group with a general formula RHC=N-R1, where R and R1 are alkyl, aryl, cyclo alkyl or heterocyclic groups. Several studies showed that the presence of a lone pair of electrons in a sp2 hybridized orbital of nitrogen atom of the azomethine group is of considerable chemical and biological importance [2-9]. Because of the relative ease of preparation, synthetic route and the special property of C=N bond, Schiff bases are generally excellent chelating agents, [7-13] with N-0 and N-S donor ligands so as to form ring structure with metal ion, which can serve as models for biologically important species.
Schiff bases derived have a wide variety of applications in many fields, such as biological, inorganic, coordination and analytical chemistry. These compounds are further used as ionophore in membrane in ion selective electrodes study [14-16]. A large number of different Schiff base ligands have been used as cation carriers in potentiometric sensors as they have shown excellent selectivity, sensitivity and stability for specific metal ions such as Ag(II], Co(II], Cu(II], Al(III), Hg(II], Ni(II], Zn (II] and Pb(II] [17-22].
Metal complexes of thiosemicarbazones with transition metals have received attention because of their biological and electrochemical activity including antitumor, antibacterial, fungicidal and anticarcinogenic properties, [23-28] including as ion selective electrode as well29. Thiosemicarbazones usually act as chelating ligands with transition metal ions, bonding through the s and N atoms [30-32]. The high affinity for the chelation of the Schiff bases towards the transition metal ions is utilized in preparing their solid complexes33.
This paper presents a series of new Schiff bases with the synthesis and spectroscopic characterization of their metal complexes [34-36] with ligands 2-acetyl thiophene thiosemicarbazone (LL1], thiophene-2- aldehyde thiosemicarbazone (LL2], 2-acetyl thiophene semicarbazone (LL3], thiophene-2-aldehyde semicarbazone (LL4] (Schemes 2.1 & 2.2].
2. MATERIALS AND METHODS
All the chemicals used were of analytical grade (AR] and of the highest purity. They included 2-acetyl
thiophene (CDH], thiophene-2-aldehyde (CDH], thiosemicarbazide (CDH] and semicarbazide (CDH], Metal salts were purchased from E. Merck and were used as received.
2.1 Preparation of ligands
All the ligands were prepared by the methods reported earlier37 by the coupling reactions.
2.1.1 Synthesis of 2-acetyl thiophene thiosemicarbazone Schiff Base (LL1 )
1.26 g of 2-acetyl thiophene(0.01mmol] was mixed with equivalent amount of thiosemicarbazide in hot ethanolic solutions (20 mL] with few drops of acetic acid were mixed with constant stirring .The mixture was refluxed for 2 h and the solid product formed was separated out by filtration, washed several times with 50% ethanol and then dried.
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Scheme 1: Synthesis of ligand (LL1)
2.1.2 Synthesis of thiophene-2-aldehyde
thiosemicarbazone Schiff Base (LL2)
1.12 g of thiophene-2-aldehyde (O.Olmmol] was mixed with equivalent amount of thiosemicarbazide in hot ethanolic solutions (20 mL] with few drops of acetic acid were mixed with constant stirring .The mixture was refluxed for 2 h and the solid product formed was separated out by filtration, washed several times with 50% ethanol and then dried.
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Scheme 2: Synthesis of ligand (LL2)
2.1.3 Synthesis of 2-acetyl thiophene semicarbazone Schiff Base (LL3)
1.26 g of 2-acetyl thiophene (O.Olmmol] was mixed with equivalent amount of semicarbazide in hot ethanolic solutions (20 mL] with equivalent amount of sodium acetate were mixed with constant stirring .The mixture was refluxed for 2 h and the solid product formed was separated out by filtration, washed several times with 50% ethanol and then dried.
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Scheme 3: Synthesis of ligand (LL3)
2.1.4 Synthesis of thiophene-2-aldehyde semicarbazone Schiff Base (LL4)
1.12 g of thiophene-2-aldehyde (O.Olmmol] was mixed with equivalent amount of semicarbazide in hot ethanolic solutions (20 mL] with equivalent amount of sodium acetate were mixed with constant stirring .The mixture was refluxed for 2 h and the solid product formed was separated out by filtration, washed several times with 50% ethanol and then dried.
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Scheme 4: Synthesis of ligand (LL4)
2.2 Preparation of complexes
2.2.1 Synthesis of metal complexes of Schiff base iminės [M(LL)2]C12
A general method has been adopted for the synthesis of metal complexes. Hot aqueous ethanolic solution (20 mL 1:1 v/v] of ligands (0.02m mol] and a hot aqueous solution (20 mL] of the hydrated metal salts (0.01m mol] were mixed with constant stirring. The mixture was refluxed for about 6 hrs. On cooling a precipitate was separated out. The same was filtered, washed with 50% ethanol and dried. The complexes were recrystallized.
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Scheme 5: Synthesis of metal complexes [M(LL)2]C12
2.2 Physical measurements
The C, H, N and s were analyzed on a Vario Micro Cube elemental analyzer, Model Vario-III.1 H and13 c NMR spectra were recorded on a Bruker, Model DPX-300 NMR spectrophotometer using DMSO as solvent. Chemical shift are given in ppm relative to tetra methylsilane. The IR spectra were recorded as KBr pellets on a Perkin Elmer FT-IR spectrophotometer, Model No. BX-2
3. RESULTS AND DISCUSSION
The Schiff bases 2-acetyl thiophene thiosemicarbazone (LL1], thiophene-2-aldehyde thiosemicarbazone (LL2] and their metal complexes were subjected to elemental analyses. The results of elemental analyses (C, H, N, S] with molecular formula and melting points are presented in Table (1]. The results obtained are in good agreement with those calculated for the molecular formula and the melting points are sharp, indicating the purity of the compounds prepared. The structures of the ligand and metal complexes are also confirmed by IR,13 c and1 H NMR spectra, which are discussed below:
3.1. Infrared spectra of the ligands
IR spectra in the region 4000 - 400 cm1 ־ have been recorded for the ligand and its complexes and the assignments of the observed frequencies have been made to specific group vibrations by comparison with the spectra of related complexes. Infrared spectra of the ligands showed a strong band in the region ~1597 and 1535 cm1 ־ was assigned to the symmetric or asymmetric [v(C=N]] vibrations of the azomethine group 38. The strong band observed at 1294 cm1 ־ & 838 cm1 ־ in the spectrum was due to the v(C=S] & 5(C=S] 39. The bands observed at 3407 cm1 ־ and 3146 cm1 ־ were assigned to v(NH2] and v(N-H] vibrations respectively. This further indicates that the ligand remained in the thione form.
The diagnostic IR spectral bands of the complexes (Fig. 5] are presented in Table (2], together with their tentative assignments. On complex formation the position of these bands shifted toward higher or lower side, indicating its involvement in coordination with metal ion. The v(C=S] stretching frequency was lowered in the spectra of the complexes, indicating the involvement of the thioketo sulphur in the coordination. These findings are further supported by the appearance of new bands at 470-528 cm1 ־ and 410439 cm1 ־, which are assigned to v(M-N] and v(M-S] vibrations, respectively.
Therefore, it can be concluded that ligands (LL1 & LL2] binds to the metal ions through azomethine N and the thioketo s atoms which shows that the thiosemicarbazones act as bidentate chelating agent40.
3.2. NMR Spectroscopy
The NMR spectra of the metal complexes are examined by comparing with those of the parant Schiff bases. The NMR spectra of the complex shows the absence of the signals due to (-NH-] in1 H NMR and (C=S] in13 c NMR spectrum which were observed in the free ligand indicating the thiolization of (C=S] followed by deprotonation and complexation with metal ion. It was also found that a new signal was observed after complexation with metal ion [41-43].
3.2.1.1 H NMR
1 H NMR spectra were recorded in DMSO solution, using TMS as internal standard for the synthesized compounds. The NMR spectra of the Schiff bases with the chemical shifts of the different types of protons are listed in Table (3]. The resonance of protons has been assigned on the basis of their integration and multiplicity pattern. The1 H NMR spectra of the Schiff bases in DMSO exhibits signals at 10.28, 10.35 and 11.33 ppm attributed to -CH=N- protons. The multisignals within the range of 6.91- 7.58 ppm are assigned to the aromatic protons of the rings. The singlet peaks at 3.39, 2.92, 3.0 and 3.42 ppm are due to the -N=NH- group of Schiff bases synthesised. The peaks at 8.3, 8.14, 9.13 and 8.05 ppm are due to the - NH2 protons43. The shifting of azomethine peaks in the observed spectra indicates the formation of Schiff bases.
Table 1: Physical and elemental analysis of ligands and their metal complexes.
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Table 2: IR spectral data [cm[1]־) of ligands (LL1, LL2] and their metal complexes.
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3.2.2.13 c NMR
The 13 c NMR spectra provided further support for the structural interpretation of Schiff bases, 13 c NMR spectral data of Schiff bases with peak assignment have been listed in Table (4]. The presence of number of signals found in 13 c NMR spectra corresponding to different types of nonequivalent carbon atoms present in the compounds corresponds. The signals were assigned by comparing with the literature values 42. The Schiff base ligand shows the signal at 521.42 ppm due to carbon atoms of methyl groups. On complexation no change has been observed. The spectra of the Schiff base ligand exhibits a strong band at δ 179.2 ppm due to c=s group. On complex formation the position of this band undergoes upfield shift to 5171.9 - 172.5 ppm. This indicates that sulphur is involved in coordination. The 13 c NMR spectral data of the Schiff bases are in accordence with the proposed structures.
Table 3: 1H NMR data (δ ppm) of compounds with peak assignment.
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Table 4: 13C NMR data (δ ppm) of compounds with peak assignment.
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Figure 1: Infra-Red spectrum of ligand (LL1)
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Figure 2: Infra-Red spectrum of ligand (LL2)
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Figure 3: 1H NMR spectrum of ligand (LL1) and (LL2)
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Figure 4: 13C NMR spectrum of ligand (LL1) and (LL2)
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Figure 5: 13C NMR spectrum of ligand (LL3) and (LL4)
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4. CONCLUSION
Schiff bases and their metal complexes of 2-acetyl thiophene and thiophene-2- aldehyde with thiosemicarbazide and semicarbazide were synthesized and characterized by analytical and spectral techniques. Analytical spectra shows the complexation of these Schiff bases with metal ions such as Pb2 +, Cu2 +, Zn2 +, Ni2 + and Co2 + etc.
5. Acknowledgements
The authors are thankful to the University Grant Commission, New Delhi, for financial assistance in the form of RGNF and the Director, MAIT, New Delhi, for providing research facilities. The authors thank the US1C, Delhi University, for their help in carrying out c, H, N and s elemental analysis, FT-1R spectral analysis and Chemistry Department, IIT Delhi, for providing1 H and13 c NMR spectroscopy facilities.
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- Quote paper
- Chandra Mohan et al. (Author), 2009, Synthesis and characterization of semicarbazide and thiosemicarbazide ligand based metal complexes, Munich, GRIN Verlag, https://www.hausarbeiten.de/document/436342
