INTRODUCTION:

In the recent past a considerable attention was devoted to the preparation and properties of complexes of macrocyclic ligands by one pot synthesis due to their increased potential in basic applied chemistry and antimicrobial activity. The design and synthesis of these systems are very useful for the achievement of specific molecular synthesis and processes, it gives a knowledge of the unique physico-chemical properties of the self assembling aggregations for the preparation of quite sophisticated molecular devices. The macrocyclic complexes of the Schiff’s bases are synthesized by the template effect of the metal ion on the aldehydes (or) ketones and amines in bringing them to form a designed macrocycle in good yield and high purity which is otherwise impossible^1-5. Synthesis of the macrocyclic quadridantate (N₄) chelates complexes by the template effect of the metal ions Co(II), Ni(II) and Cu(II) on thiourea and acetyl acetone is carried out which are not formed in the absence of these metal ions. The imine group of the Schiff’s bases is responsible for the antimicrobial activity of these complexes. A similar study on these transition metal ions and their substituted derivatives is in progress.

MATERIALS AND METHODS:

All the reagents were of analar grade and the solvents were purified by standard methods. IR spectra (4000-450cm-1) were taken as KBr disc using a Perkin –Elmer spectrum ONE- NO17-1159 spectrophotometer. Microanalyses for carbon, hydrogen and nitrogen were obtained by using Elemental Analyzer^6,7. Magnetic moments were measured at room temperature on Guoy balance using Hg[Co(NCS)₄] as the calibrant .Conductance was determined on a Systronics conductivity meter type 304, in sterile water with a dip type cell having platinum electrode. The UV-visible spectra were run on a cary-5E spectrometer (200-900 nm) in nujol mull. TGA-DTA analyses were obtained by using NETZSCH STA-409C/CD thermal analyzer.

EXPERIMENTAL:

Preparation of [Co(C₁₂H₁₆N₄S₂)(H₂O)₂](NO₃)₂ and [Ni(C₁₂H₁₆N₄S₂)(H₂O)₂]Cl₂ complexes:

To a 25 ml of methanolic solution of acetyl acetone (4m mole, 41.1 ml in methanol), a solution of 25 ml of thiourea (4m mole, 0.3044 g in methanol) was added. To the above mixture a 25 ml solution of cobaltous nitrate (2m mole, 0.582 g in methanol) was added and the resultant mixture was refluxed well and kept aside. The product obtained was filtered off and washed thoroughly with a hot solution of methanol and dried. The yield was quantitative. The complex was recrystalized with a hot solution of water and methanol in 1:9 ratio. The purity of the product was checked by TLC. The same procedure was applied for the preparation of complex [Ni(C₁₂H₁₆N₄S₂)(H₂O)₂]Cl₂ using acetyl acetone, thiourea and nickel chloride.

Preparation of [Cu(C₁₂ H₁₆N₄S₂)Cl)₂] complex:

To a 25 ml methanolic solution of acetyl acetone (4m mole, 41.1 ml in methanol) a solution of 25 ml of thiourea (4 mmole, 0.3044 g in methanol) was added. To the above mixture a 25 ml solution of cupric acetate monohydrate (2 mmole 0.399 g in methanol,) was added as 2 ml of con. HCl was added drop wise and refluxed well. The precipitate obtained was filtered and washed with a hot solution of methanol and dried. The yield was quantitative. The complex was crystallized with a hot solution of water and methanol in 1:9 ratio. The purity of the product was checked by TLC.

RESULTS AND DISCUSSION:

All the complexes were soluble in water and common organic solvents. The molar conductance values of Co(II) and Ni(II) complexes show that they behave as 1:2 electrolytes, but the Cu(II) complex is a non-electrolyte. The analytical data shows (table-1) that the Schiff’s base behaves as a quadridendate ligand⁸.

Magnetic susceptibility:

The magnetic susceptibility measurements of the complexes were performed at room temperature. The magnetic moment value for Co(II), Ni(II) and Cu(II) complex were 4.95 BM, 2.89 BM and 1.98 BM respectively ^9,10. This shows that all of them are paramagnetic with the number of unpaired electrons 3, 2 and 1 respectively and the geometry of all these complexes is octahedral.

Electronic Spectra:

The electronic spectral data of Co(II), Ni(II) and Cu(II) complexes were shown in table (2). They have been studied with the view to obtain more information on stereochemistry of the complexes and to procure more support for the conclusion deduced with the help of magnetic data. The electronic absorption spectra of the Co(II) complex appears at 10066 cm^-1 , 21441 cm^-1 and 22406 cm^-1due to ⁴T_1g (F)→⁴T_2g(F), ⁴T_1g(F)→⁴A_2g(F)and ⁴T_1g(F)→⁴T_1g(P) transitions respectively ^11-14. The intense band around 29326 cm^-1 may be a charge transfer band. The ligand field parameters such as Dq, B’, β and β% have been calculated using band-fitting equation given by Underhill-Billing¹⁵, the crystal field splitting energy (Dq) value is 1137.0 cm^-1. These values are well within the range reported for most of the octahedral Co(II) complexes. The Racah parameter (B) is less than the free ion value (971 cm^-1) suggesting a considerable orbital overlap and delocalization of electrons on the metal ion. The nephelauxetic ratio (β) for the present Co(II) complex is 0.940. This is less than one, suggesting partial covalency in the metal-ligand bond. The values of Dq, β%, LFSE and υ₂/υ₁ suggest the octahedral geometry for Co(II) complex.

The electronic spectrum of Ni(II) complex shows two bands at 18,320 cm^-1and 28,850 cm^-1 assignable to ³A_2g→³T_1g(F) (υ₂) and ³A_2g→³T_1g(P) (υ₃) transitions respectively, as expected for an octahedral environment. The lowest band υ₁≈10 Dq was not observed and it is calculated as 11600 cm^-1 .The ligand field parameters show the presence of octahedral geometry. The Racah parameter of the complex (825.8 cm^-1) is less than the free ion (1030 cm^-1) value due to the covalent nature of metal-ligand bond and octahedral geometry around the Ni(II) ion. The electronic spectrum of Cu(II) complex exhibits a broad asymmetric band in the region 16411-12320 cm^-1with maxima at 14360 cm^-1 due to ²T_2g→²E_g transition indicating the presence of distorted octahedral geometry because of the John-Teller effect.

Infrared Spectra:

The significant IR band for the metal complex and their tentative assignment are compiled and represented in table-3. The infrared spectra of the Co(II), Ni(II) and Cu(II) complexes show absorption bands at 1584.35 cm^-1, 1612.50 cm^-1, and 1623.67 cm^-1 respectively. This shows the presence of methine moiety (-CH=N-). The existence of the bands respectively at 547.37 cm^-1, 544.36 cm^-1and 533.58 cm^-1 in these complexes confirm the coordination the methine nitrogen with these metal ions. It can be inferred that the Schiff’s base acts as a quadridentate ligand with four nitrogen atoms as donor sites^16,17. The presence of bands at 480.25 cm^-1 and 3428.58 cm^-1 in the Co(II) complex and the bands at 481.31 cm^-1, 3304.71 cm^-1 in the Ni(II) complex reveal coordination of the water molecules with these metal ions.

In the Cu(II) complex the aforesaid bands are absent which shows the absence of coordinated water molecules. The presence of a band at 472.98 cm^-1 confirms the existence of metal-chlorine bond. The presence of band in all these complexes in the range 1030-1100 cm^-1 indicates the presence of (-C=S) group.

Thermal Studies:

The TGA, DTA curves of the Co(II) and Ni(II) complexes show an endothermic band at about 100º C with a loss of mass corresponding to two coordinated water molecules in these complexes¹⁸. No endothermic band is found in the Cu(II) complex indicating the absence of coordinated water molecules.

Antimicrobial activity:

The in vitro antibacterial screening of the compounds was undertaken against the bacteria Escherichia coli (G-Ve) and Staphylococcus aureus (G+Ve) by cup-plate method using nutrient agar as medium. In a typical procedure, molten nutrient agar kept at 45º C was then poured into Petri dish and allowed to solidify. Then small wells (10 mm diameter) at 1 cm distance were punched carefully using a sterile cork and these were completely filled with test solutions of 0.04 mole (0.02 mg in 50 ml water). The plates were incubated for 24 hours at 37º C. The diameter of the zones of inhibition for all the test compounds was measured and the results were compared with standard drug of the same concentration.

Table-1

Complex	M.W.	calculated (found) %				μ_effBM	Λ_m Ohm^-1cm²mol^-1
Complex	M.W.	M%	C%	H%	N%	μ_effBM	Λ_m Ohm^-1cm²mol^-1
[Co(L)(H₂O)₂](NO₃)₂	499.39	11.80 (11.78)	28.86 (28.78)	4.04 (4.00)	16.83 (16.79)	4.95	164.8
[Ni(L)(H₂O)₂]Cl₂	446.06	13.16 (13.10)	32.31 (32.27)	4.52 (4.49)	12.56 (12.53)	2.89	128.1
[Cu(L)Cl₂]	414.87	15.32 (15.30)	34.74 (34.70)	3.89 (3.86)	13.50 (13.46)	1.98	8.3

Where L= C₁₂H₁₆N₄S₂

Table-2:

S. No	Complex	υ₁ cm^-1	υ₂ cm^-1	υ₃ cm^-1	Dq cm^-1	B cm^-1	β	β%	υ₂/υ₁	υ₃/υ₂	LFSE K cal mol^-1
1.	Co(II)	11415	21413	22421	1146.6	910.0	0.94	6.28	1.876	1.047	26.21
2.	Ni(II)	11655	18315	28902	1156.1	825.8	0.793	20.81	1.571	1.575	33.05
3.	Cu(II)	14367	---	---	1436.7	---	---	---	---	---	41.08

Table-3

S. No	Complex	υ _C=N	υ _C=S	υ _M-N	υ _M-O	υ_M-OH2	υ_M-Cl
1.	Co(II)	1584.35	1030.70	547.37	480.25	3428.38	---
2.	Ni(II)	1612.50	1087.36	544.36	481.31	3304.71	---
3.	Cu(II)	1623.67	1103.77	533.58	---	---	472.98

Table-4

S. No	Complex	Inhibition Zone (in mm)
S. No	Complex	Staphylococcus aureus (G+Ve)	Escherichia coli (G-Ve)	Aspergillus niger	Candida albicans
1.	Co(II)	6	-	-	-
2.	Ni(II)	7	5	-	-
3.	Cu(II)	15	8	9	8

The antifungal activity of the compounds was evaluated against the fungi Aspergillus niger and Candida albicans by cup-plate method and cultured on Sabouraud’s agar medium adapting similar procedure as described above. The plates were incubated for 24 hours at 37º C. The diameters of the zone of inhibition for all the test compounds were measured and the results were compared with that of the standards under identical conditions.

Since all the test compounds and standards were prepared in freshly distilled sterile water, its zone of inhibition was found to be very negligible and taken as zero mm. The antibacterial activity results revealed that the complexes show weak to good activity^19-21. The Co(II) and Ni(II) complexes show weak anti bacterial activity but the Cu (II) complex were found to be very effective antibacterial.

The antifungal activity results reveal that the complexes of Co(II) and Ni(II) inactive where as the Cu(II) complexes show considerable activity.

CONCLUSIONS:

The elemental analysis, magnetic susceptibility, electronic spectra, IR, TGA-DTA analysis reveal that the complexes are formed insitu by template effect. The Schiff’s base acts as a N₄ quadridentate chelate. All the complexes are paramagnetic, octahedral and they show considerable antimicrobial activity.

ACKNOWLEDGEMENTS:

The authors express their sincere thanks to the management, principal and staff members of PG and Research Department of Chemistry, C. Abdul Hakeem College, Melvisharam, for providing research facilities. We also thank SAIF, IIT MADRAS.

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Received on 05.04.2010 Modified on 30.04.2010

Asian J. Research Chem. 3(3): July- Sept. 2010; Page 732-734