Structural Studies of Divalent Transition Metal Cobalt with bis-( 2 , 6-diacetyl pyridine monoxime ) 1 , 2-di-iminobenzene

The complexes of divalent Cobalt with bis(2,6-diacetyl pyridine monoxime)1,2-diiminobenzene have been prepared separately in presence of respective salts. The general molecular formula of the complexes have been found to be [Co(L)]X2 where L=Bis(2,6-diacetyl pyridine monoxime)1,2-di-iminobenzene, X=chloride, bromide, sulphate, perchlorate, nitrate, acetate ions, On the characterisation of the complexes by usual physico-chemical method, all the complexes have been found to be monomeric and octahedral in geometry.


INTRODUCTION
The co-ordination chemistry of Schiff base ligand is interesting because of their ability to form complexes with transition metal ions.In general transition metal ions and its complexes with Schiff base ligand have been studied widely in world due to their vast scope in the field of agriculture 1 and medicine 2,3 .There are numerous ligand containing nitrogen and oxygen donor atoms, out of which Schiff base of drug chemical is widely used as the ligand which contains oxygen donor atom in addition to the nitrogen atom of the azomethine (>c=o) group to form the stable complexes with the transition metals.
In this work the preparation of the Schiff base ligand, bis(2,6-diacetyl pyridine monoxime)1,2-di-iminobenzene was done by only two steps reaction and both the steps are condensation process.A number of complexes of Co(II) with ligands having nitrogen and oxygen as donor atoms are reported in which ligands behave as hexadented ligands.
In conclusion, we have developed and synthesized very useful, absolutely clean and very high yielding, eco friendly Co(II) complexes.

EXPERIMENTAL
Preparation of the Schiff base ligand 12 The Schiff base ligand bis-(2,6-diacetyl pyridine monoxime) 1,2-di-iminobenzene has been prepared in two steps.In first step, 3.5 grams (0.05 mole) of concentrated aqueous solutiom of hydroxylamine hydrochloride was added to an alcoholic solution of 60.5 grams (0.04) 2,6-diacetyl pyridine and cooled upto -5 0 C. To this reaction mixture further added 20% aqueous solution of 12 grams (0.03 mole) NaOH dropwise with rapid stirring and temperature of the resultant mixture was maintained below 0 o c.The colour of the solution became pink.After 90 minutes it was diluted with water and just acidified with glacial acetic acid.Allowed the mixture to stand for 30 minutes and then filtered through succession.The residue was recrystallised from aqueous alcohol (60 volume % alcohol).A white needle type crystal were obtained which was washed with water and dried.The melting point of this monoxime was found to be 218 o C.The chemical reaction taking place during condensation process is given in figure-1.
In second steps, 1.08 grams (0.01 mole) 1,2-diaminobenzene was mixed with 2,6-diacetyl monoxime (0.02 mole) with the help of agate and mortar.They went into homogeneus viscous liquid after triturating them together for nearly two hours.The mortar was kept in refrigerator for overnight.The compound was dried under vaccum and recrystallised from a little of alcohol.The melting point was found to be 184 0 C. The compound was further analysed and found to contain C=67.11%,H=5.43% and N=19.26% respectively, Which corresponds to the molecular formula (C 24 H 24 N 6 O 2 ).The chemical reaction taking place during condensation process is given in figure-2.

Preparation of the complexes
An alcoholic solution of 0.43 grams (0.001 mole) of the Schiff base ligand bis-(2,6-diacetyl pyridine monoxime) 1,2-di-iminobenzene was gradually added to an alcoholic solution of 0.24 grams (0.001 mole) of Cobalt (II) chloride hexahydrate.The molor ratio of metal and the ligand was always kept in 1:1 ratio respectively.The resulting solution was shaken well and refluxed on water bath about 45 minutes.On cooling the solution under tap water, dark brown precipitate was obtained which was separated by filtration.The precipitate was washed with alcohol, recrystallised and dried over KOH pellets placed in desiccators and found to contain Cobalt=10.40%,Carbon= 51.36%, Hydrogen=4.32%,Nitrogen= 15.20%, and Chlorine=20.76%respectively, which corresponds to the molecular formula [Co(C 24 H 24 N 6 O 2 )]Cl 2 .
The complexes with the ligand was prepared separately keeping the metal-ligand molar ratio as 1:1 in each case in presence of respective metal salts.

RESULT AND DISCUSSION
The structure of the complexes are proposed by the interpretation of the result of elemental analysis, magnetic moment, electrical conductance, electronic spectra behaviour of the complexes and I.R. spectral nature of the ligand

Magnetic properties of complexes
The magnetic moment of the complexes were measured by Gouy´s method using the [ Hg(NCS) 4 ] as calibrant.The value of magnetic moment of Co(II) complexes with the ligand obtained in the range of 4.84 to 5.02 B.M 13 , which indicate octahedral arrangement of the donor atoms of the ligand molecule around the central metal cation.

Electrical conductivity
The electrical conductivity of solution of complexes were measured by conductivity meter, bridge in DMF at 25 0 c.Pure DMF and conductivity water were used as solvent .The value of electrical conductance of Cobalt(II) complexes with ligand obtained in the range of 152 to 164 ohm -1 cm 2 mole - 1 indicate electrolytic nature of the complexes.

Electronic spectra
The electronic absorption spectra of the complexes were recorded with Hitachi-320 spectrophotometer.
In our present investigation two bands are obtained for the Cobalt(II) complexes.The first band obtained in the range of 14400-15200 cm -1 is broad and unsymmetrical due to J.T.distortion.The first transition takes place between 4 t 1g (F)→ 4 T 2g (F) ( 4 E g + 4 B 2g ).The second band obtained in the range of 22250 to 23150 cm -1 is sharp and symmetrical due to 4T 1g (F)→4T 1g (P) which is spin as well as symmetry allowed transition.The electronic transition spectra indicates octahedral nature of Cobalt(II) complexes.The appearance of spectrum of three more bands in the spectrum of ligand in the vicinity of (1570-1580), (600-610) & (400-415) is an evidence in support of the presence of pyridine ring [15][16][17][18][19][20][21] .These bands can be assigned to pyridine ring deformation, in plane deformation and out of plane ring deformation. .The upward shifting further indicates that nitrogen atom of pyridine ring has taken part in co-ordination with metal ion for the complex formation.The highest energy pyridine band 1570-1580 cm -1 corresponds to í C =N of the heterocyclic ring.A shift on high frequency side has indicated that there has been increase in the double bond character as a consequence of co-ordination with N-donor atom.
By the comparison of I.R. Spectral behaviour of the ligand and complexes, it is concluded that the ligand behaves as hexadentate molecule.
A sharp and medium band obtained in the range of 410 to 425 cm -1 in the complexes due to the vibration of (M-N) bond confirms the coordination of nitrogen atoms of different environment in the bond formation with the metal cations.Thus two aldimino nitrogen atom, two nitrogen atoms of pyridine ring and two oximino nitrogen atoms are the bonding sites of the ligand molecule.
The complexes of Co(II) cations with the ligand bis-(2,6-diacetyl pyridine monoxime) 1,2-diiminobenzene are cationic complex their charges has been satisfied by the anions of respective metal salts used for the complex formation.
A pair of band obtained in the range of 2910 cm -1 to 2924 cm -1 and 1370 cm -1 to 1380 cm - indicates the presence of C-H and -CH 3 group in the ligand as well as in the complexes.There are no band around 1740 cm -1 for >C=O group.The absence of this band in the complexes makes it clear that the carbonyl group of 2,6-diacetyl pyridine was completely replaced by >C=N-when allowed to react with O-phenylene diaminobenzene.
Thus on the basis of elemental analysis, value of electrical conductivity and magnetic moments and electronic spectra and interpretation of the I.R.Spectra of the ligand and the complexes, the probable structure of [Co(L)]X 2 complexes are suggested to be tetragonally distorted octahedral in nature as shown in figure-3.