Studies on the Cu (II) and Co (II) complexes with N,N’–bis (2-benzothiazolyl) -2,6 pyridine dithiocarboxamides.

Introduction  

The coordination chemistry of amide ligand is an important part of a number of chemical investigations. Sigel and Martin¹ reviewed the structure and stability of metal ion complexes of amide, oligopeptides and related ligands. In recent years, pyridine carboxamides, a burgeoning class of multidentate ligands have received much importance because these ligands have found use in asymmetric catalysis,^2,3 molecular receptors,^4,5 dendrimer synthesis⁶. Many derivatives of pyridine dithiocarboxamides show anti inflammatory, antipyretic and analgesic activities^7-8.

In this paper the synthesis of N,N’–bis (2-benzothiazolyl) -2,6–pyridine dithiocarboxamide(I) and isolation of their Cu(II) and Co(II) complexes are reported. The complexes were characterized using elemental analysis, magnetic moments, UV – Visible spectra and IR Spectral Studies.

Figure 1 Click here to View figure

 

Material  and Methods

The solubility of all complexes were tested in cold and hot water, in common organic solvents and all the complexes were found to be insoluble.

UV – Vis spectrum was recorded in solid state by diffuse reflectance method due to insoluble nature of complexes using Barium sulphate as inert material. IR spectra were obtained on Perkin Elmer 157 or 577 model in nujol mull or Potassium bromide/ Cesium iodide disc. The estimation of Cu and Co were carried out in our laboratory by using Vogel’s method⁹.

Synthesis of Ligand BPD

2,6 – Pyridine dithiocarboxylic acid (1.99 g, 10mmol) and thionyl chloride (20-25 ml) were refluxed under anhydrous conditions for 4-6 h. The excess thionyl chloride was removed under reduced pressure and the remaining mixture was cooled at 0⁰C and dry pyridine (30-35 ml) was added followed by 2-aminobenzothiazole (3.0 g, 20 mmol) with occasional stirring, until the evolution of HCl had ceased. The solid obtained was poured into ice cold water (200 ml), filtered off and washed with 5% NaHCO₃ solution, then with hot water and ethanol. The resulted black crude product was recrystallized from dioxane. The purity of compound was checked by TLC.

Synthesis of the Complexes

A solution of 5mmol of MCl₂ [where M = Cu (II) and Co (II) ] in ethanol (10ml) was added to the ligand (2.31g, 5mmol) suspended in nitrobenzene (10ml) and the mixture was refluxed for 12h. The obtained precipitate was filtered off, washed with water and ethanol and dried in air. All the isolated complexes were insoluble in most organic solvents, but soluble in DMF and DMSO.

The complexes decompose without melting when heated above 310⁰C.

Results and Discussion

Elemental analysis showed that formula of ligands was C₂₁H₁₃N₅S₄(H₂L₁), C₂₃H₁₇N₅S₄(H₂L₂) and the formula of complexes were ML₂  where M = Cu (II) and Co (II) ions. The complexes were insoluble in water, benzene and other common solvents but were soluble in  DMSO and DMF. The complexes were characterized by IR and UV absorption data.

IR Spectra

The infrared spectrum of the ligand showed a band around 3254 cm^-1 which was assigned to v_NH vibration. The N-H stretching appeared in all complexes as a very weak band. While ˃ C = S group of ligand absorbed around 1180 cm^-1 and exhibited medium strong bands and ˃C = S  group of complexes registered a lower shift of around 25 to 27 cm^-1  and appeared at 1155cm^-1 as medium band. This indicated thio enolisation of ˃ C = S group during the complex formation.

The benzothiazole absorption and pyridine ring absorption of ligand is also merged with the thio group and therefore a strong and medium absorption is displayed. The pyridine ring stretching of complexes registered a lower shift (except the first band) which indicated the involvement of N -atom in the ring coordination.

Electronic Spectra

The electronic spectral bands in ligands were obtained at around 273 nm and 359 nm which might have arose due to n —> π* and   π —> π*  transitions.

The electronic spectral bands in Cu (II) observed around 360 nm appeared due to ²B_1g —> ²B_2g mixed with internal ligand transition and the ²B_1g —>²E_g transition coupled with internal ligand transition and charge transfer transition occurred at 275 nm. The only d-d transition of metal ion observed around 724 nm appeared to arise due to ²B_1g  —> ²A_1g transition.

 

 Table – 2: Characteristics iR Band (in Cm^-1) of Ligand and Complexes

Ligand	N-H	˃C=S	Thiocarboxamide	Benzothiazole	Pyridine ring stretching
					I	II	III	IV
H2 (L1)	3252(m)	1178(s)	1396(m),1049(m),786(w)	1610(s)	1609(m)	1540(m)	1457(m)	1444(m)
H2 (L2)	3256(m)	1180(s)	1398(m),1052(m),789(w)	1612(s)	1615(m)	1545(m)	1460(m)	1449(m)
H2 (L3)	3253(m)	1182(s)	1402(m),1054(m),793(w)	1614(s)	1618(m)	1549(m)	1464(m)	1453(m)
Cu (L1)2	3250(w)	1153(m)	1340, 1005,760	1610(m)	1618(m)	1520(m)	1420(m)	1340(m)
Cu (L2)2	3250(w	1155(m)	1343,1008,764	1613(m)	1620(m)	1520(m)	1421(m)	1341(m)
Cu (L3)2	3250(w)	1157(m)	1349,1012,768	1616(m)	1622(m)	1522(m)	1422(m)	1343(m)
Co (L1)2	—	1154(m)	1343,1009,764	1613(m)	1618(m)	1537(m)	1442(m)	1358(m)
Co (L2)2	—	1158(m)	1353,1015,772	1615(m)	1619(m)	1541(m)	1444(m)	1364(m)
Co (L3)2	—	1160(m)	1345,1012,766	1610(m)	1617(m)	1539(m)	1440(m)	1360(m)

 

Table -3:- Electronic spectral bands of Cu(II)and Co(II)complexes

Complexs	2B1g ─˃2A1g	2B1g─˃2B2g	2B1g─˃2Eg	4T1g(F)─˃4A2g	4T1g(F)─˃4T1g(P)
Cu(L1)2	719nm	356nm	270nm
Cu(L2)2	724nm	360nm	275nm
Cu(L3)2	728nm	365nm	280nm
Co(L1)2				512nm	460nm
Co(L2)2				514nm	463nm
Co(L3)2				516nm	465nm

 

Figure 2  Click here to View figure

 

Conclusion

The tentative structure of the complex, on the basis of above discussion were proposed as

References

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