Studies of Distorted Octahedral Complexes  of Cobalt, Nickel and Copper and Their Antibacterial Properties

Introduction

The thiosemicorbazide and derivatives has broad spectrum of applications in various fields. In biological field they may be used as fungicides, insecticides, algaecides, plant growth regulators, enzymatic aldolisation catalyst, antiviral antibiotic agents etc.^1-6 One of the major development in the field of bio-organic chemistry is the finding that tin compounds of thiosemicorbazides can play an important role in anticortiogenesis as Tondon et.al⁷ have recently screen the di-n-butyltin complexes of Schiff base derived from S-substituted dithiocarbazate and fluoroaniline for their antitumour activity in lymphocyte leukaemia tumour system. The Schiff’s base of S-methyldithiocarbazate and its transition metal complexes have been studied for their cytotoxic action on potent antifungal activities.^8-9 Terepthalic acid bis (4-phenylthiosemicarbazone) has been found tetradentete chelating agent coordinating through the thiol-S and azomethine-N. Semicarbazones and thiosemicarbazones are important class of organic compounds, which have remarkable biological properties.^10-12 Due to their pharmaceutical properties and chelating nature, their complexes have extensively been studied during the recent years.^13-16

These significances of thiosemicarbazone complexes, have steered us to undertake the present study that involves the synthesis and characterisation of some complexes of Co(II), Ni(II) and Cu(II) with thiosemicarbazone of 3-hydroxy-4-methoxybenzaldhyde and their antibacterial evaluation.

Material and Methods

All the reagents used were of Anal. R. Grade. 3-Hydroxy-4-methoxybenzaldhyde was procured from Aldrich while thiosemicarbazide hydrochloride was procured from BDH.

A Schiff’s base ligand, 3-hydroxy-4-methoxybenzalde hydethiosemicarbazone was prepared by the condensation of 3-hydroxy-4-methoxybenzaldehyde and thiosemicarbazide and was used for complexation with Co(II), Ni(II) and Cu(II). The metal salts (MX₂.6H₂O, where X = Cl^– , NO^–3, CH₃ COO^–) and ligand were taken in 1 : 2 molar ratio in ethanolic medium and solution was refluxed for 3 to 4 hours. On cooling the solution, coloured precipitate was obtained, which was filtered and washed with alcohol and dried in decicator on anhydrous CaCl₂. The complexes were micro analysed for C, H, N and S and their m.p. was noted down.

Scheme 1 Click here to View scheme

 

The molar conductivity of complexes was determined in DMF solution of 10^–3 M concentration on conductivity meter, C.G. 857 Schoolgrate GmbH having Pt-electrode. The magnetic susceptibility of complexes has been determined on Guoy balance at room temperature. The I.R. spectra of complexes were recorded Shimadzu FTIR-8400S spectrophotometer using KBr disc method. Electronic spectra of complexes were recorded on Shimadzu UV-160 spectrophotometer in DMF solution.

Table 1: Physical and Micro analytical Data of Ligand and Complexes

Compounds	Elemental analysis (%): Calcd. (Found)						%yield	m.p. (ºC)	Molarconductivity (λ) (ohm–1 cm2mol–1)	Mag. Moment (µeff) (B.M.)
	Metal	C	H	N	S	Cl
1. Ligand(HMBTS):[C9H11N3O2S)	–	48.00(48.35)	4.89(4.70)	18.67(18.85)	14.23(14.18)	–	85	312	11.50	4.70
2. [Co(HMBTS)2Cl2]	10.16(09.86)	37.25(37.78)	03.80(03.71)	14.48(14.42)	10.16(09.86)	12.24(12.11)	78	314	15.00	4.69
3. [Co(HMBTS)2(NO3)2]	09.31(09.19)	34.13(34.35)	03.48(03.42)	17.70(17.61)	10.11(10.06	–	83	315	12.35	4.72
4.[Co(HMBTS)2(CH3COO)2]	09.40(09.32)	42.11(42.34)	04.47(04.35)	13.40(13.35)	10.21(10.11)	–	74	318	17.00	3.25
5. [Ni(HMBTS)2Cl2]	10.11(10.25)	37.27(37.39)	03.80(03.39)	14.49(17.63)	11.04(10.00)	12.25	75	317	18.00	3.20
6. [Ni(HMBTS)2(NO3)2]	09.26(09.14)	34.14(34.32)	03.48(03.39)	17.70(17.63)	10.12	–	77	316	14.00	3.30
7. Ni(HMBTS)2(CH3COO)2]	09.35(09.22)	42.13(42.36)	04.47(04.40)	13.40(13.34)	10.21(10.12)	–	81	330	15.32	1.95
8. [Cu(HMBTS)2Cl2]	10.86(10.77)	36.95(37.24)	03.76(03.71)	14.37(14.30)	10.95(10.88)	12.15(12.08)	86	329	15.50	2.04
9. [Cu(HMBTS)2(NO3)2]	09.96(09.87)	33.90(34.12)	03.45(03.33)	17.56(17.48)	10.13(09.92)	–	83	332	15.48	2.00
10. [Cu(HMBTS)2(CH3COO)2]	10.06(09.96)	41.80(41.92)	04.43(04.36)	13.30(13.18)	10.13(10.02)	–	85	312	11.50	4.70

 

Result and Discussion

The percentage composition of the ligand and complexes has been given in table 1. On the basis of their molar conductivity and percentage composition the general formula of (ML₂X₂) has been assigned for the complexes.

Where, L = 3-Hydroxy-4-methoxybenzaldehydethiosemicarbazone and   X = Cl^– , NO^–3, and CH₃ COO^–

The low value of molar conductivity of the complexes shows their non-electrolytic nature.

IR. Spectra

The sharp bond appearing at 3520 cm^–1 is assigned v_O-H to phenolic group^17-21 in the spectra of free ligand. This bond does not undergo any change in the complexes, which shows its non-participation in coordination. The sharp band appearing at 1640 cm^–1 in the spectra of the free ligand and is assigned to v_CH=N, which undergoes red shift and appears at 1615-1610 cm^–1 in the I.R. spectra of complexes, which is indicative of co-ordination through azomethine nitrogen of the ligand.^22-23 A doublet observed at 1270 cm^–1 and 1260 cm^–1 is farely assigned to v_CH3O group of the ligand.^24,25 These bands remain intact in I.R. spectra of complexes. A sharp band appearing at 1200 cm^–1 in the I.R. spectra of free ligand is assigned to v_C=S. This band gets shifted to 1170-1160 cm^–1 in the I.R. spectra of complexes confirming the coordination through thione sulphur of the ligand. The band appearing at 3470, 3290 and 1360 Cm^–1 due to v_NH2 (assy.), v_NH2 (Symm.) and v_NH^31–34 respectively in the I.R. spectra of the free ligand do not show appreciable change in their frequency in complexes. Thus it is obvious that these groups are not envolved in coordination. The appearance of new bands at 1460–1445 cm^–1, 1320–1310 cm^–1 and 1000–900 cm^–1 in the I. R. spectra of complexes number 3, 6 and 9 show the presence of monodentately coordinated  in these complexes.^35-38 The new bands appearing at 1630–1620 cm^–1 in the spectra of complexes numbers 4, 7 and 10 clearly indicates the monodentately coordination of CH₃COO^– ion to the metal ions in these complexes^39–41. The bands appearing in the far infrared region at 500–490, 420–400 and 370–365 cm^–1 are assigned to v_M-N, v_M-S, and v_M-Cl stratching vibrations respectively. Thus the ligand behaves as neutral bidentate coordinating through thione sulphur and azomethine nitrogen forming a very stable five membered ring.

Magnetic Moment and Electronic Spectra

The magnetic moment of Co(II) complexes has been found to be 4.69 – 4.72 B.M., which is greater than µ_S corresponding to three unpaired electrons

in high spin octahedral complexes of Co(II). This may be attributed to the lowering of symmetry from cubic one due to presence of different axial ligands. Thus µ_eff. values predict distorted octahedral symmetry around Co(II) in these complexes. In perfect octahedral symmetry of Co(II) complexes three bands are expected to appear in their electronic spectra due to three allowed transitions i.e., ⁴T_1g → ⁴T_2g , ⁴T_1g → ⁴T_2g  and ⁴T_1g → ⁴T_2g (P). But here in the present case complexes display four bands which show the further splitting of ⁴T_1g (F), ⁴T_2g and ⁴T_1g(P).⁴⁶ The band have been given in table-2.

Table 2 (cm^–1)

Complexes	Band-1	Band-2	Band-3	Band-4
[Co(HMBTS)2Cl2]	7,840	10,200	19,980	20,800
[Co(HMBTS)2(NO3)2]	8,200	8,560	16,280	20,500
[Co(HMBTS)2(CH3COO)2]	7,280	10,230	19,490	21,000

 

The first three bands assigned to the transitions as v₂ = ⁴A_2g →⁴E_g (b), v₃ = ⁴A_2g →⁴B_2g and v₄ = ⁴A_2g →⁴B_1g. The fourth band is broad and is not well resolved, hence it is assumed to consist of two transitions i.e., ⁴A_2g →⁴E_g (c) and ⁴A_2g →⁴A_2g (c). The bands corresponding to v₁ = ⁴A_2g →⁴E_2g (a) is not observed in the spectra.

On the basis of energy levels of different crystal field terms the various crystal field parameters and ν₁ have been derived and the values have been displayed in table-3.

Table 3 (cm^–1)

Complexes	Dq(xy)	Dq(z)	Ds	Dt	ν1
1. [Co(HMBTS)2Cl2]	978	1278.30	–2,335.4	–171.60	3,368
2. [Co(HMBTS)2(NO3)2]	770	818.00	–2,110.0	–27.60	1,974
3. [Co(HMBTS)2(CH3COO)2]	826	1201.50	–2,289	–214.60	2,297

 

The values are in good agreement with that of tetragonally distorted octahedral complexes of Co(II).^47,48

The magnetic moment of Ni(II) complexes has been found 3.20–3.30 B.M., which are in good agreement with the values reported for six coordinated Ni(II) complexes.^49,50 The electronic spectra of Ni(II) complexes display four bands which have been given in table-4.

Table 4: (cm^–1)

Complexes	ν1	ν2	ν3	ν4
1. [Ni(HMBTS)2Cl2]	8,600	17,750	15,200	19,400
2. [Ni(HMBTS)2(NO3)2]	8,500	14,200	15,800	19,600
3. [Ni(HMBTS)2(CH3COO)2]	8,750	14,600	15,400	19,350

 

The assignment of these bands with their energy are given below :-

ν₂ is free from D_t and D_s and thus it is a measure of 10D_q in plane, i.e., 10D_qxy. The various crystal field parameters have been derived from the energy associated with different transitions and the values have been displayed in table-5.

Table 5: (cm^-1)

Complexes	Dq(xy)	Dq(z)	Ds	Dt
1. [Ni(HMBTS)2Cl2]	1475.00	245.00	846.40	702.80
2. [Ni(HMBTS)2(NO3)2]	1420.00	280.00	769.00	651.40
3. [Ni(HMBTS)2(CH3COO)2]	1460.00	290.00	797.60	668.57

 

The values support a tetragonal distortion in the octahedral symmetry of Ni(II) complexes. From the value of D_t and D_s, it is clear that the distortion capacity is maximum for Cl^– and minimum for NO^–₃ ligand.

Cu(II) complexes have shown magnetic moment 1.95–2.04 B.M., which corresponds the presence of one unpaired electron indicating mononuclear nature of complexes with dilute para magnetism.^53-56

These complexes display three bands in their electronic spectra, which has been given in table-6

Table 6: (cm^–1)

Complexes	ν1	ν2	ν3
1. [Cu(HMBTS)2Cl2]	12,100	14,100	17,100
2. [Cu(HMBTS)2(NO3)2]	9,250	11,250	13,250
3. [Cu(HMBTS)2(CH3COO)2]	11,000	13,400	15,900

 

These bands has been assigned to the following spin allowed transition with their energy.⁵⁷

v₁ = ²B_1g →²A_1g = –6D_q + 2D_s + 6D_t + 2D_s–D_t = 4D_s + 5D_t

v₂ = ²B_1g →²B_2g = 4D_q – 2D_s + P_t + 6D_q + 2D_s–D_t = 10D_q.

and

v₃ = ²B_1g →²E_g = 4D_q + D_s – 4D_t + 6D_q – 2D_s – D_t = 10D_q + 3D_s– 5D_t.

The values of different parameter have been derived using above equations and shown in table-7.

Table 7: (Cm^–1)

Complexes	Dq(xy)	Dq(z)	Ds	Dt
1. [Cu(HMBTS)2Cl2]	1410	1503	3775	1665
2. [Cu(HMBTS)2(NO3)2]	1125	1128	2812.5	1287.5
3. [Cu(HMBTS)2(CH3COO)2]	1340	1328.75	3375	1525.00

 

The values are in good agreement with those reported for tetragonally distorted Cu(II) complexes.^58-61

Bioactivity Measurements

The ligands as well as all the complexes were screened for the invitro antibacterial activities against Escheraichia Coli, citrobactor gillenii using the agar disk diffusion method.⁶² The antibiotic, Chloramphenicol has been taken as standard for comparison. Autoclave Nutrient agar medium was poured into sterine peri-dish after dipping the test compound in DMSO solution. The width of the growth inhibition zone around the disc was measured after 24 hours incubation at 35°C temperature. Since DSMO was used as solvent, it was also screened against whole organisms and no activity was found. The antibacterial data have been given in table-8.

Table 8: Antibactarial activity data of ligand and complexes Inhibition Zone in mm.

		E-coli			Citrobactor gillenii
		Concentration			Concentration
S.No.	Compounds	500 µg/mL	250 µg/mL	100 µg/ mL	500 µg/ mL	250 µg/ mL	100 µg/ mL
1	3-hydroxy-4-methoxybenzaldehyde thiosemicarbazone(HMBTS)	7	6	4	8	8	7
2	[Co(HMBTS)2Cl2]	14	14	8	13	12	8
3	[Co(HMBTS)2(NO3)2]	15	14	9	14	12	8
4	[Co(HMBTS)2(CH3COO)2]	13	12	8	14	13	9
5	[Ni(HMBTS)2Cl2]	15	13	10	14	11	8
6	[Ni(HMBTS)2(NO3)2]	14	13	9	15	13	9
7	Ni(HMBTS)2(CH3COO)2	13	13	10	13	13	9
8	[Cu(HMBTS)2Cl2]	18	17	11	16	15	10
9	[Cu(HMBTS)2(NO3)2]	19	17	10	17	15	10
10	[Cu(HMBTS)2(CH3COO)2]	16	15	10	16	14	9
11	Standard (Chloramphenicol)	40	39	33	35	34	30

 

It is obvious from the data that all the complexes exhibit enhanced biological activity than the ligand, but lesser activity than the standard at whole the concentration. It is also clear that at 100 µg/ml concentration the inhibition is very low compare to higher concentration. The complex of Cu(II) are found more active than other complexes at all the concentration. This may be due to stronger biological activity of Cu (II) than Co(II)  and Ni(II). The enhancement of biological activity of the ligand after complexation may be attributed by chelate formation which fascinates the complex across the cell memebrance.^63-65

Scheme 3 Click here to View scheme

 

Conclusion

The ligand 3-Hydroxy-4-methoxybenzaldehyde thiosemicarbazone acts as neutral bidentate ligand coordinating through thione sulphur and azomethine nitrogen forming five membered ring with the metal ions. Due to the presence of different axial ligands all these complexes are appreciably tetragonally distorted. It is also revealed that the distortion capacity of chloride is maximum while that of nitrate is minimum, which may be attribute to the greater coordinating capacity of NO-in respect of Cl– and CH₃COO^–. Cl^– having weakest coordinating capability remain at lower distance from metal ions along z-axis and hence their is greater tetragonal distortion in octahedral symmetry of complexes. The tentative structure may be given as below.

Bioactivity measurement reveals that all the complexes are biologically more active against both E- coli and c-gillenii than the free ligand but less active than standard Chloramphenicol.

Acknowledgement

The authors are thankful to the Dr Sunil Kumar Singh, Professor and Head, P.G. Department of Zoology, M.U. Bodh Gaya for providing lab. Facility to carry out the biological evaluation of ligand and newly synthesized coordination compounds.

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