Transition Metal Schiff base Complexes with N, S and O donors – Synthesis, Characterisation and Antimicrobial Studies.
P. Muthuselvan1, S. Theodore David1* and M. Sivasankaran Nair2
1Department of Chemistry, St. John’s College, Tirunelveli-627 002, India.
2Manonmaniam Sundaranar University, Tirunelveli-627 012, India.
*Corresponding Author E-mail: stheodore_david@yahoo.co.in
ABSTRACT:
The Schiff base complexes of Thiophene-2-carboxaldehyde and L-histidine with Co(II), Ni(II), Cu(II) and Zn(II) were synthesized and characterized by elemental analysis, molar conductance, IR, UV-Vis, magnetic moment, CV, and thermal analyses (TG and DTA). All the complexes were found to be non-electrolytic in nature. FTIR spectra show that the Schiff base ligand is bound to the metal ions in a tridentate manner involving thiophene sulphur, azomethine nitrogen and carboxylato oxygen atoms. The fourth position in the coordination sphere is occupied by the chloride ion. The geometrical structures of Co(II),Ni(II) and Zn(II) complexes are found to be tetrahedral whereas Cu(II) complexes are square planar. The antimicrobial results show that the metal complexes were found to be more active than the ligand. The complexes showed increased nuclease activity in the presence of an oxidant when compared to the ligand.
KEYWORDS: Schiff base; Transition metal complexes; Spectral Thermal analysis; Biological activities.
1. INTRODUCTION:
The Schiff bases and their metal complexes are well known for their multifaceted applications1-3. The metal complexes of Schiff bases containing sulphur donors gain special interest due to their unusual configuration, structural lability and biological activity4,5. Literature survey shows that there are some recent reports on the metal complexes of Schiff bases derived from thiophene-2-carboxaldehyde6-8. However, reports of the Schiff base complexes derived from thiophenal and amino acids are few9. This paper describes the synthesis and characterization of a novel Schiff base ligand viz. thiophene-2-carboxaldehyde-L-histidine (thiophenal-his) and their complexes with Co(II), Ni(II), Cu(II) and Zn(II). The in vitro antimicrobial studies have also been reported.
L-histidine was purchased from Sigma, thiophene-2-carboxaldehyde from Himedia and the Co(II), Ni(II), Cu(II) and Zn(II) chlorides were Merck samples. All the auxiliary reagents and solvents were of pure quality.
2.2. Synthesis of Schiff base ligand:
A solution of L-histidine (2 mmol) and potassium hydroxide pellets (2 mmol) in 50 ml of methanol is kept under continuous stirring in a 100 ml RB flask. A solution of thiophene-2-carboxaldehyde (2 mmol) in 50 ml methanol is then added slowly to the flask. The reaction mixture was vigorously stirred at room temperature for 15 minutes and then refluxed at 70–80oC for 8 hrs. The yellow solution is reduced to 1/3 of its original volume by rotary evaporation. The concentrated filtrate thus obtained is poured in to diethyl ether. A brownish yellow precipitate was formed, which is collected by vacuum filtration and washed several times with anhydrous ether and then dried in vacuum over anhydrous calcium chloride. The yield of the isolated ligand is found to be 70-75 %.
2.3. Synthesis of Schiff base complexes:
A solution of thiophenal-his (L) (2 mmol) in methanol (20 ml) was added to a solution of metal(II) chloride (2 mmol) in 20 ml of aqueous methanol, and the reaction mixture was stirred and then refluxed for 2 hrs. The resulting solution was cooled to room temperature and the volume was reduced to half of the initial volume under reduced pressure. The precipitate was filtered off, washed several times with cold ethanol, ether and then dried in vacuum over anhydrous calcium chloride.
Elemental analysis was done using a Perkin-Elmer elemental analyzer. Molar conductance of the complexes was measured in methanol (10-3 M) solutions using a coronation digital conductivity meter. IR spectra were recorded in KBr disc on a JASCO FT/IR-410 spectrometer in the 4000-400 cm-1 region. The electronic spectra were recorded on a Perkin Elmer Lambda-25 UV-VIS spectrometer. Room temperature magnetic measurements were performed on a Guoy balance by making diamagnetic corrections using Pascal’s constant. Cyclic voltametric measurements were carried out in a Bio-Analytical system (BAS) model CV-50W electrochemical analyzer. The three electrode cell comprised of a reference Ag/AgCl, auxiliary platinum and working glass electrodes. Tetrabutylammonium perchlorate was used as supporting electrolyte. Thermal studies were effected upon a Perkin-Elmer 7 series thermal analyzer, with a heating rate of 20oC/min in nitrogen atmosphere.
2.5. In vitro antimicrobial studies:
The in vitro biological screening effects of the synthesized complexes were tested against the bacterial species, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumonia, Proteus vulgaries and Staphylococcus aureus; fungal species, Aspergillus niger and Candida albicans by the disc diffusion method10. Chloroamphenicol and Nystatin were used as the standard antibacterial and antifungal agents. The test organisms were grown on nutrient agar medium in petri plates. The compound was dissolved in DMF solution and soaked in filter paper disc of 5 mm diameter and 1 mm thickness. The discs were placed on the previously seeded plates and incubated at 37 oC and the diameter of inhibition zone around each disc was measured after 24 hrs for bacterial and 72 hrs for fungal species.
2.6. DNA cleavage studies:
Cleavage reactions were run between the metal complexes and pUC19 DNA.The prepared solutions were diluted with loading dye using 1% agarose gel. Then 3 mL of ethidium bromide (0.5 mg/mL) was added to the above solution and mixed well. The warmed agarose was poured and clamped immediately with comb to form sample wells. The gel was mounted into electrophoretic tank; enough electrophoretic buffers were added to cover the gel to a depth of about 1 mm. The DNA sample (30 mM), metal complex (50 mM) and H2O2 (500 mM) in 50 mM Tris-HCl buffer (pH=7.1) were mixed with loading dye and loaded into the well of the submerged gel using a micropipette. The electric current (50 mA) was passed into running buffer. After 1-2 hrs, the gel was taken out from the buffer. After electrophoresis, the gel was photographed under UV transluminator (280 nm) and documented.
3. RESULT AND DISCUSSION:
The analytical and physical properties of thiophenal-his (L) are listed in Table 1. The Schiff base ligands are air sensitive in nature and soluble in all common organic solvents. The CoLCl complex is air sensitive, hygroscopic in nature and soluble in water, DMF and DMSO, while other complexes are stable towards air and moisture and soluble in acetonitrile, DMF and DMSO. The molar conductance values (Table 1) of metal complexes measured in DMSO (10-3 M) reveal that they are non-electrolytic nature11.
3.1 IR spectra:
The IR spectra of the complexes studied are shown in Fig.1 and the spectral data of the Schiff base ligand and their complexes are given in Table 2. IR spectrum of the ligand (L) show strong bands respectively at 1629 cm-1 which can be attributed to azomethine (-C=N-) stretching frequency. During complexation, this band is shifted to lower frequency indicating the coordination of azomethine nitrogen to the metal ion. The strong band present at 1589 cm-1 for L can be assigned to asymmetric stretching frequency of carboxylato group. The Schiff base ligand also display bands at 1409 cm-1 respectively due to symmetric stretching vibration of carboxylato group20. On complexation, the asymmetric and symmetric stretching bands are shifted to lower frequency for all the complexes, which reveals the formation of a linkage between the metal ion and carboxylato oxygen atom. Moreover, the large difference (~200) between the asymmetric and symmetric stretching modes indicates the monodentate binding of the carboxylato group in the complexes12,13. The IR spectra of the free Schiff base ligand exhibits a sharp band at 832 cm-1, due to the ν(C-S). On complexation this band has been shifted to a lower frequency in the 825-826 cm-1 range indicating the coordination of the thiophene sulphur atom to the metal ion.
The spectrum of all the metal complexes show new bands in the 530-560 cm-1 and 475-448cm-1 regions, which can be attributed to the formation of M-O and M-N bonds, respectively14,15.
Table 1. Analytical and physical properties of the Schiff base ligands and their complexes
|
Complexes |
Empirical formula |
Colour |
Calculated (Found) (%) |
ΛM Ω-1 cm2 mol-1 |
|||||
|
C |
H |
N |
O |
S |
M |
||||
|
L |
C11H10SN3O2K |
Yellow |
45.97 (45.95) |
45.97 (45.95) |
3.51 (3.47) |
14.62 (14.59) |
11.16 (11.15) |
-- |
-- |
|
[CoLCl] |
C11H10N3SO2ClCo |
violet |
38.56 (38.67) |
2.94 (3.01) |
12.26 (12.31) |
9.34 (9.43) |
9.36 (9.28) |
17.20 (17.09) |
6.0 |
|
[NiLCl] |
C11H10N3SO2ClNi |
green |
38.58 (38.71) |
2.94 (3.02) |
12.27 (12.37) |
9.34 (9.44) |
9.36 (9.26) |
17.14 (17.08) |
11.0 |
|
[CuLCl] |
C11H10N3SO2ClCu |
brown |
38.04 (38.13) |
2.90 (3.01) |
12.04 (12.16) |
9.21 (9.33) |
9.23 (9.14) |
18.30 (18.16) |
7.5 |
|
[ZnLCl] |
C11H10N3SO2ClZn |
yellow |
37.84 (37.95) |
2.89 (2.99) |
12.04 (12.19) |
9.17 (9.29) |
9.18 (9.04) |
18.73 (18.63) |
14.0 |
Table 2. IR Spectral data (cm-1)
|
Complexes |
νC=N |
νasym (coo-) |
νsym (coo-) |
νC-S |
νM-O |
νM-N |
|
L |
1629 |
1589 |
1409 |
832 |
-- |
-- |
|
[CoLCl] |
1622 |
1574 |
1382 |
825 |
537 |
461 |
|
[NiLCl] |
1620 |
1581 |
1382 |
825 |
551 |
461 |
|
[CuLCl] |
1616 |
1581 |
1375 |
826 |
564 |
454 |
|
[ZnLCl] |
1619 |
1581 |
1382 |
826 |
562 |
461 |
Fig.1 IR spectrum of Schiff base complexes
(a) CoLCl (b) NiLCl (c) CuLCl and (d) ZnLCl
3.2 Electronic spectra:
The electronic spectral data of the compounds are given in Table 3. The electronic spectra of L exhibit a broad band at 275 nm, respectively, which can be assigned to π-π* transition of the azomethine(>C=N) chromophore. On complexation this band was shifted to lower wavelength region suggesting the coordination of azomethine nitrogen to the central metal ion.
The electronic spectrum of tetrahedral Co(II) complexes is reported to have only one absorption band in the visible region due to 4A2(F) → 4T1(P) transition16. The spectrum of the present CoLCl complex has only one band in the visible region at 600, which indicates tetrahedral geometry for the complex. The electronic spectrum of the NiLCl complexes show an intense absorption band at 625 nm, which may be due to the 3T1(F) → 3T1(P) transition corresponding to tetrahedral geometry on Ni(II) [16]. The electronic spectrum of CuLCl complex exhibit a broad band with λmax at 630 nm. This may be due to the 2B1g → 2A1g transition of the square planar environment around Cu(II)16.
Table 3. Electronic spectral data of the complexes
|
Complexes |
Absorption (nm) |
Transition |
Geometry |
|
[CoLCl] |
600 |
4A2(F)→4T1(P) |
Tetrahedral |
|
[NiLCl] |
625 |
3T1(F)→3T1(P) |
Tetrahedral |
|
[CuLCl] |
630 |
2B1g→2A1g |
Square planar |
|
[ZnLCl] |
240, 265 |
п-п*, n-п* |
Tetrahedral |
3.3 Magnetic measurements:
The magnetic measurement data is given in Table 4. The magnetic susceptibility value of 4.58BM for CoLCl complex is indicative of the tetrahedral geometry for the complex17,18. The square planar Ni(II) complexes are diamagnetic while tetrahedral complexes will have moments in the range 3.2-4.1BM. The NiLCl complex reported herein has a room temperature magnetic moment value of 3.62, which corresponds to the tetrahedral structure of Ni(II) complex17,18. The magnetic moment value of 2.01BM for CuLCl, falls within the range normally observed for monomeric Cu(II) square planar complexes17,18.
Table 4 Magnetic data of the Schiff base metal complexes
|
Complexes |
χM х 106 c.g.s. unit |
χD х 106 c.g.s. unit |
χ’M х 106 c.g.s. unit |
μeff BM |
Geometry |
|
[CoLCl] |
8474 |
170 |
8644 |
4.58 |
Tetrahedral |
|
[NiLCl] |
5230 |
170 |
5400 |
3.62 |
Tetrahedral |
|
[CuLCl] |
1495 |
170 |
1665 |
2.01 |
Square planar |
|
[ZnLCl] |
- |
- |
- |
- |
Tetrahedral |
χM = molar susceptibility; χD = diamagnetic correction; χ’M = corrected molar susceptibility
3.4 Electrochemical studies:
The electrochemical data obtained for the M(II)LCl complexes by cyclic voltammetry are given in Table 5. The cyclic voltammogram of the CoLCl complex shows a well defined redox process corresponding to the formation of the quasi-reversible Co(II)/Co(I) couple. The cathodic peak at −0.872 V versus Ag/AgCl and the associated anodic peak at −0.640 V corresponds to the Co(II)/Co(I) couple. The peak to peak separation (ΔEp = 0.232 V) indicates a quasi-reversible one electron transfer process. The redox property of the NiLCl complex displayed an anodic and associated cathodic peaks at -0.898 and -0.746 V respectively, corresponding to the formation of the quasi-reversible (ΔEp = 0.152 V) one electron reduction Ni(II)/Ni(I) couple.
The CuLCl complex displayed a cathodic peak at -0.866 V versus Ag/AgCl with the corresponding anodic wave at -0.656 V on the reverse scan. The peak separation value (ΔEp = 0.210 V) indicates a totally quasi-reversible character for the one electron transfer reaction of metal-based Cu(II)/Cu(I) couple. The Zn(II) with d10 configuration is expected to be electrochemically inactive.
Table 5 Electrochemical data of the Schiff base metal complexes
|
Comp lexes |
Couple/Peak |
Cathodic, Epc (V) |
Anodic, Epa (V) |
ΔEp (V) |
|
[CoLCl] |
Co(II)/Co(I) |
-0.872 |
-0.640 |
0.232 |
|
[NiLCl] |
Ni(II)/Ni(I) |
-0.746 |
-0.898 |
0.152 |
|
[CuLCl] |
Cu(II)/Cu(I) |
-0.866 |
-0.656 |
0.210 |
|
[ZnLCl] |
- |
- |
- |
- |
3.5 Thermal analysis:
Thermal decomposition curves (Table 6, Fig.2) of all the complexes show a similar sequence of two decomposition steps. The thermograms showed no weight loss upto 300 oC, indicating the absence of water molecule in the complexes. The first stage of decomposition take place at 300oC corresponding to 50-60% weight loss. The weight loss may be due to the loss of chloride ion and partial decomposition of coordinated ligand, indicating that the chloride ion is present within the coordination sphere. This step is accompanied by an endothermic peak at 300-320oC range. The next exothermic step of the thermal degradation that occurs between 360–615oC (20-25% weight loss) corresponds to the removal of remaining organic ligand moiety leaving a metal oxide residue.
From the above results, we can conclude that the Schiff base ligand (thiophenal-his), binds the M(II) through its imino nitrogen, thiophene sulphur and carboxylato oxygen atoms. The molar conductance, thermal and cyclic voltammetry data indicates that the fourth position in the coordination sphere is occupied by the chloride ion.
Fig.2 TGA DTA curves: (a) [CoLCl]; (b) [NiLCl]; (c) [CuLCl]; (d) [ZnLCl] complexes
3.6 Antimicrobial activity:
The in vitro antifungal and antibacterial activity of the ligands and their complexes have been carried out against the bacteria species, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumonia, Proteus vulgaries and Staphylococcus aureus; fungal species, Aspergillus niger and Candida albicans. The results of the antimicrobial activities are summarized in Table 7. The antimicrobial activity of the ligands and their metal complexes indicate that the complexes acquire higher growth compared to those of ligands as seen in earlier reports19,20. The high activity of the thiophenal-his (L) complexes against most of the microbes, may be due to the presence of heterocyclic imidazole ring there in. In the thiophenal-his (L) complexes, the ligand have uncoordinated imidazole nitrogen atom, which enhance the activity of the complexes by bonding with trace elements present in microorganism. This may combine with the uncoordinated site and inhibit the growth of microorganism. The bioactivity of the thiophenal-his (L) and its complexes are found to be in the order: Co(II)>Cu(II)>Ni(II)>ligand.
Antimicrobial activity of all the complexes at low concentration towards the microbes is very low. The activity of the Schiff base ligands and their complexes increases with the increases concentration because the concentration plays a vital role in increasing the degree of inhibition. The mode of action of the complexes may involve the formation of the hydrogen bond through the azomethine group (>C=N) with the active centers of the cell constituents resulting in the interference with normal cell process.
3.7 DNA cleavage studies:
Gel electrophoresis experiments using pUC19 DNA were performed with metal(II) Schiff base complexes in the presence of H2O2 as oxidant. The nuclease activity is greatly enhanced by the incorporation of metal ion in the respective Schiff base ligand. However, the ligand and ZnLCl complex exhibits no significant cleavage activity in the presence of H2O2. The results show that the complexes cleave DNA more efficiently in the presence of oxidant, which may be due to the formation of hydroxyl free radicals. The production of hydroxyl free radical is due to the reaction between the metal complex and oxidant. These hydroxyl free radicals participate in the oxidation of the deoxyribose moiety, followed by the hydrolytic cleavage of the sugar phosphate backbone21. The more pronounced nuclease activity in the metal complexes in the presence of H2O2 as oxidant may be due to the increased production of hydroxyl radicals.
The cleavage efficiency was measured by determining the ability of the complex to convert the Supercoiled DNA to open circular form or nicked form. There is a considerable increase in the intensity of bands for open circular form in the case of metal complexes.
Table 6 Thermo analytical results of M(II)-L complexes
|
Complexes |
Temperature range t (oC) |
% weight loss Obs.(calculated) |
DTA peak t (oC) |
Process
|
|
[CoLCl] |
310-475, 550-740 |
~60 ~20 |
410 (endo) 670 (exo) |
loss of chloride ions and decomposition of ligand moiety |
|
[NiLCl] |
315-435, 480-730 |
~60 ~20 |
412 (endo) 720 (exo) |
loss of chloride ions and decomposition of ligand moiety |
|
[CuLCl] |
320-445, 455-735 |
~60 ~20 |
407 (endo) 735 (exo) |
loss of chloride ions and decomposition of ligand moiety |
|
[ZnLCl] |
330-475, 475-810 |
~60 ~20 |
420 (endo) 670 (exo) |
loss of chloride ions and decomposition of ligand moiety |
Table 7 The in- vitro antimicrobial data of the complexes
|
Complexes |
Zone of inhibition (mm) |
||||||
|
Bacterial species |
Fungal species |
||||||
|
E.coli |
K.pneumoniae |
P.aeruginosa |
P.vulgaris |
S aureus |
A niger |
C.albi cans |
|
|
L |
11.3 |
11.9 |
10.2 |
12.3 |
12.7 |
11.6 |
11.1 |
|
[CoLCl] |
12.5 |
11.5 |
12.5 |
12.5 |
14.5 |
13.5 |
15.5 |
|
[NiLCl] |
13.5 |
11.5 |
11.5 |
12.5 |
15.0 |
10.5 |
15.5 |
|
[CuLCl] |
15.0 |
11.5 |
13.5 |
12.5 |
13.5 |
12.5 |
15.5 |
|
[ZnLCl] |
10.5 |
11.5 |
11.5 |
12.5 |
14.5 |
14.5 |
15.5 |
4. CONCLUSION:
The Schiff base ligand, thiophenal-his (L) and its Co(II), Ni(II), Cu(II) and Zn(II) complexes were prepared and characterized using various spectral techniques. The results indicate that the complexes belong to the tetrahedral geometry for Co(II) and Ni(II) complexes whereas Cu(II) complexes have square planar geometry. The IR spectra show that the Schiff base ligand, thiophenal-his, coordinates to the metal ion through the thiophene sulphur, imino nitrogen and carboxylato oxygen atoms. The fourth position of coordination is occupied by the chloride ion. The complexes show very good antimicrobial and DNA cleavage activities.
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Received on 02.06.2011 Modified on 15.06.2011
Accepted on 23.06.2011 © AJRC All right reserved
Asian J. Research Chem. 4(8): August, 2011; Page 1305-1310