Co (II) and Zn (II) Metal Complexes of Heterocyclic Schiff Bases: A Synthesis, Spectral and Antimicrobial Study

INTRODUCTION

Metal complexes of Schiff bases are extensively studied due to synthetic flexibility, selectivity and sensitivity towards a variety of metal atoms¹. Schiff bases are most widely used as chelating ligands in coordination chemistry². They are also useful in catalysis and in medicine as antibiotics, antiallergic and antitumor agents³. The metal complexes of Schiff bases derived from heterocyclic compounds have been the centre of attraction for many workers in recent years^4-8. A study through the literature revealed that no work has been realized on the preparation and characterization of metal complexes of mixed heterocyclic Schiff bases derived by the reaction of Co (II) and Ni (II) Benzoinoxime primary ligand with heterocyclic compounds such as 2-aminothiazole and 8-hydroxyquinoline to form the secondary ligand, an attempt has been made to synthesize and characterize some novel metal complexes of these Schiff base ligands. The prepared ligands as well as their metal complexes were also investigated for their in vitro activity against some bacterial and fungal species. The purpose of the work was to understand the coordination as well as the biological chemistry of these novel synthesized compounds.

MATERIAL AND METHOD

Synthesis of Benzoinoxime

Benzoinoxime was prepared by standard procedure⁹ in which 10 gm (0.047 mol) of benzoin and 20 gm (25 ml) of rectified spirit together with an aqueous solution of 8.0 gm (0.087 mol) of hydroxylamine was taken together in a 250 ml round bottom flask. Before using hydroxylamine hydrochloride was neutralized with 4.4 gm (0.091 mol) of sodium hydroxide. The mixture was refluxed for 60 min. Then water was added to precipitate benzoinoxime. It was cooled in ice bath. The solid was filtered with solution at pump and it was washed with water finally the product was recrystallised by using ethanol.

The purity of the product was checked by TLC and Melting point. Melting point of Benzoin Oxime is 151⁰C.

 

Scheme 1  Click here to View Scheme

 

Preparation of simple complexes of Benzoinoxime

The following complexes of CO-Benzoinoxime and Zn- Benzoinoxime have been synthesized as follows^10-14.

The above metal complexes were prepared by mixing the solution of the primary ligand  [(PL₂ Benzoinoxime) ( 0.06 mole)] in 80 ml of boiling ethanol with 0.03 mole of  metal(II) chloride solution in 20 ml of distilled water. The mixture was magnetically stirred for about one hour. Then coloured precipitate (except Zn) were formed which was filtered out and washed with hot water or methanol then dried over vacuum. The purity was checked by using TLC.   

Preparation of mixed ligand complexes of Co (II), Zn (II) Benzoinoxime with S-L₁ and S-L₃

Co (II), Zn (II) Benzoinoxime complex solution (0.01 mol) in ethanol and solution of S-L₁ and S-L₃ were dissolved in 25 ml ethanol or 25 ml CH₂Cl₂ were mixed separately in 250 ml round bottom flask. The reaction mixture was refluxed on a water bath for 2-4 hrs on cooling the coloured precipitate separate that was filtered, washed with cold hexane solvent and dried under vacuum over night.

Table-I: Analytical data of Co (II), Zn (II) B.O. + S-L₁, S-L₃

Complex of ligand	Molecular formula	Colour	M.P.0C	%  element    Found  (Calculated)						Mol.Wt
				C	H	O	N	S	M
B.O.Co(II)	[C28H24O4N2(CO)]	Faint pink	267	65.88 (65.75)	4.70 (4.50)	12.54 (12.45)	5.49 (5.42)	—	11.37	510
B.O.Zn(II)	[C28H24O4N2(Zn)]	Colourless	242	64.99 (64.90)	4.64 (4.52)	12.37 (12.21)	5.41 (5.22)	—	12.57	517
Co (II) B.O. + 2-amino thiazole	[C34H30O4N6S2Ni]	Reddish pink	271	57.59 (57.60)	4.23 (4.26)	9.02 (9.03)	11.84 (11.84)	9.02 (9.07)	8.32 (8.30)	709
Co(II) B.O. + 8-hydroxy quinoline	[C37H28 O5N3Ni]	Brown	276	67.99 (68.01)	4.28 (4.30)	12.25 (12.23)	6.43 (6.44)	—	9.03 (9.01)	653
Zn(II) B.O. + 2-amino thiazole	[C34H30 O4N6S2Zn]	Faint yellow	246	57.08 (57.10)	4.19 (4.23)	8.95 (8.98)	11.74 (11.77)	8.95 (8.98)	9.09 (9.02)	715
Zn(II) B.O.+ 8-hydroxy quinoline	[C37H28 O5N3Zn]	White	252	67.37 (67.40)	4.24 (4.27)	12.13 (12.19)	6.37 (6.43)	—	9.86 (9.88)	659

 

RESULT AND DISCUSSION

Elemental Analysis and Magnetic Moment

Elemental analysis data reveals that the observed values of percentage of C, H, N, halogen and metal ion are in good agreement with values predicted and calculated for Co(II) complexes with 1:2 metal to ligand ratio for simple complexes and 1:2:2 for metal complex with oxime as a primary ligand and SL₁ as a secondary ligand while it is 1:2:1 for metal complex with oxime as a primary ligand and SL₃ as a secondary ligand. The elemental analysis indicates that Zn(II) complexes of mixed ligand exhibit the stoichiometry 1:2:2. The Zn (II) complexes undergo decomposition at high temperature.

The magnetic moment values are in the range of 2.74 to 3.25 B.M. of mixed ligand complexes in present work indicates octahedral geometry¹⁵. Due to completely filled‘d’ shell as expected for Zn(II) ion it exhibits diamagnetic  nature. The present complexes are found to be diamagnetic in nature for the square plannear¹⁶ geometry.

The electronic spectra of Co (II) complexes exhibit absorption bands at 10661-10870 cm^-1(D₁), 16694-18149 cm^-1 (D₂) and 20661-25189(D₃) from lower energy level to higher energy level, which also clearly indicate the square planer and octahedral stereochemistry of Co (II) complexes¹⁵.

The ligand field parameter values i.e. LFSE observed in the range 30.383 to 30.979 Kcal/mole for mixed ligand complexes and 42.986 to 43.445 kcal/mole for simple complexes are in good agreement with octahedral CO (II) complexes and square planner complexes respectively. In present work the electronic absorption spectrum of Zn (II) complexes shows only one absorption band in UV-visible spectrum in the range 357 nm (2800 cm^-1 ) to 372 nm ( 26882 cm^-1 ). This is due to absence of d-d transition. This observed peak is because of metal to ligand charge transfer spectra. The values are in agreement with octahedral geometry of these complexes^{17, 18, 19}.

IR Spectroscopy

The azomethine C=N stretching is found at 1650 cm^-1 for benzoinoxime²⁰. For Co(II) and Zn(II) Benzoinoxime the same band shifted to lower frequency and appears at 1623 cm^-1, 1647 cm^-1, for their adducts it is again to lower frequency and appear in the range of  1576-1580 cm^-1 and 1571-1578 cm^-1. This indicates that the azomethine group of the ligand is coordinated to metal ion via nitrogen atom²¹.

The ligand benzoinoxime exhibit four peaks in the range of 1430 cm^-1 to 1575 cm^-1 attributed to aromatic C=C stretch for two aromatic ring while its Co(II) and Zn(II) complexes exhibits the same stretch at 1415-1600 cm^–1 and at 1446-1601 cm-1and the mixed ligand complexes exhibitit in the range of 1414-1600 cm^-1, 1418-1600 cm^-1 .

In the case of mixed ligand complex of 2-aminothiazole and 8-hydroxyquinoline frequency of OH is shifted to 3389-3398 cm^-1 showing weakening of intramolecular hydrogen bonding.

The ligand band at 1403 cm^-1 is due to the O-H deformation mode of the oximino hydroxyl group it identified at 1640-1645 cm^-1 for Co(II) and Zn(II) adducts ( mixed ligand complexes) indicating that the oximino hydroxyl group remains intact²² in the complex of Co(II) with all heterocylic ligands used in present work. However the O-H deformation band of the phenolic oxygen appears at the 1315 cm^-1 in the ligands disappears in the complexes showing co-ordination through phenolic oxygen.

Co (II), Zn (II) of Benzoine oxime complexes shows at 486 cm^-1 due to metal-nitrogen stretching and at 492 cm^-1, 539 cm^-1 due to metal-oxygen stretching. The metal oxygen bond shifted to higher value in case of mixed ligand complex of Co(II), Zn(II) of Benzoineoxime and appears in the range 504-511cm^-1, 544-547 cm^-1.

A metal nitrogen weak band appears for all mixed ligand complexes in the range 491-495 cm^-1 this is due to formation of a bond between nitrogen atom of secondary ligand and central metal cobalt ion. In case of mixed ligand complex of Co (II), Zn (II) Benzoineoxime and 8-hydroxyquinoline one more band is observed at 430 cm^-1, 429 cm^-1. This clearly indicates that the phenolic oxygen bonded to Co (II).

In mixed ligand complexes of 2-aminothiazole and 2-aminopyridine the bands appears at 3450 cm^-1 and 3248 cm^-1. This is due to free N-H- stretch proves that the –NH₂group do not participate in complex formation.

Table-II: Selected IR bands (cm^-1) of Co (II) and Zn (II) BO with SL_1, SL₃ (mixed ligand) complexes

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