Preparation and characterization of bis-(N,N-disalicylalethylene-diamine)-μ-aquadicobalt(II)

Aim :- 

Preparation and characterization of bis-(N,N-disalicylalethylene-diamine)-μ-aquadicobalt(II)

Apparatus :- 

1. Round bottom flask
2. Reflux condenser
3. Beaker
4. Sunction pump
5. Filter paper
6. Thermometer, 

Chemical Requirements :- 

Salicylaldehydrate , Cobalt (II) chloride hexahydrate , Ethanol, Ethylene diamine , Acetic acid, Methan ol , Hydrochloric acid , Hydrogen peroxide, Sodium acetate trihydrate , Sodium hydroxide, Bis-(N, N’ disalicylidine ethylenedia).

Theory :- 

bis-(N,N-disalicylalethylene-diamine)  μaquadicobalt(II) consists of a ligand known as salen ligand. Salen refers to tetradentate C2 symmetric ligand synthesized from salicylaldehyde (sal) and ethylenediamine (en). It may also refer to a class of compounds, which are structurally related to the classical salen ligand , primarily bis-Schiff bases.Schiff bases are characterized by the presence of a C=N double bond (imine) which is bound to an aryl group. They are synthesized from an aromatic amine and carbonyl compound by nucleophilic addition forming an unstable hemiaminal or carbinolamine followed by dehydration to give an imine. 

The formation of a Schiff base by condensation reaction is influenced by pH of the solution, steric and electronic effects of the carbonyl compound and amine. The reaction for the formation in highly acidic solutions is unfavourable as the amine is protonated hindering the nucleophilic ability and in very basic conditions, there is unavailability of sufficient protons to catalyze the elimination of the hemiaminal (carbinolamine) hydroxyl group (dehydration).

Salen ligand has been noted to possess four co-ordinating sites It forms planar complexes with various transition metal ions and nitrogen atoms in the ligand have a higher tendency to co-ordinate with metal ions than the oxygen atoms, a consequence of increase d basicity of nitrogen atoms over the oxygen atoms. Salen form complexes with metal ions through nitrogen and oxygen donor atoms .the nitrogen basicity is influenced by the electron withdrawing properties of the benzene ring. The basicity of nitrogen atoms is also influenced by the presence of sp2 hybridization resident in both nitrogen atoms. The double bond decreases the likelihood of flexibility of Salen and makes it less adaptable to co-ordination to metal ions.

Oxygen atoms in the hydroxyl group are another co-ordination site in Salen and it is expected to be less acidic than the phenolic hydroxyl group. Ortho substitution influences the electron density on the hydroxyl bond making the oxygen in hydroxyl group to be preferred to that in the phenol group. Some authors noted that bis(salicylidene)ethylenediamine being a tetradentate chelating ligand gives metal complexes of fairly rigid structure. As a consequence, one metal ion co-ordinates through the four co-ordination sites due to the presence of two nitrogen and two oxygen atoms to form three chelate rings. The basicity of both nitrogen atoms which leads to increased stability of the metal complex is a consequence of the double bonds attached to the nitrogen atoms. 

Procedure :- 

A] Preparation of bis-(N,N-disalicylidene ethylenediamine) :- 

• Dissolve salicylaldehyde (3.05 g, 0.025 mol) in ethanol (25 ml) in a round bottom flask. 
• Heat the solution to ~70oC and add a solution of ethylenediamine (0.75 g, 0.0125 mol) dissolved in ethanol (3ml). 
• Heat the resulting yellow solution under reflux for 30 mins. 
• Reduce the volume of the solution to half by evaporating the solvent and after cooling, filter the yellow crystalline precipitate under suction and wash with ice-cold ethanol.
• Recrystallize the product from methanol and dry at room temperature under vacuum. 
• Determine the melting point and calculate the percent yield.

B] Preparation of bis(N, N' disalicylalethylene-diamine)-μ- aquadicobalt(II) :-

• Dissolve finely ground N,N'-disalicylidene ethylenediamine (1.34 g) in water (150 ml) and to this solution add sodium hydroxide (3.9 g) and sodium acetate trihydrate (5 g). 
• Continue stirring for 10-15 minutes. While stirring is continued add a solution of cobalt (II) chloride hexahydrate (1.23 g) dissolved in hot water (25 ml). 
• Stir the reaction mixture until it turns to a reddish-brown paste. 
• Allow the reaction mixture to stand for at least 15 minutes. 
• Centrifuge the reaction mixture until most of the mother liquor has been removed and a hard cake remains. 
• Wash the precipitate three times with water (each time with about 10 ml of water). Remove the cake from the centrifuge tube and dry them.

UV VIS SPECTRA OF (N,N’ disalicylalethylene-diamine) {SALEN LIGAND} :- 

Salen ligand has two absorption maxima ( around 260 and 320 nm ).The UV- visible absorption spectra was recorded using uv-vis spectrophotometer in the range of 200 to 450 nm at a temp of 25º C. To study the effect of solvents on absorption spectrum of salen , a series of diluted solution ( 1.00 × 10-4 - 1.00 × 10-5) in different solvents were used.

The IR spectra of ligand indicates the appearance of peaks between 1660-1623cm-1 . These peaks are assigned to v C=N stretching vibrations indicating the condensation resulting in the formation of corresponding azomethine compounds. 

But when the complex is formed the frequency is reduced to and we will get peaks in the range of 1610-1600cm-¹

In the free ligand the characteristic phenolic O-H absorptions are also observed between 3450-3300 cm-1 . The broad nature of these peaks suggests the presence of a weak hydrogen bond as a part of resonating ring system.

But in complex we will get multiple peaks in the range of 3750-3300

The other characteristic phenolic C-O stretching vibrations are observed between 1174-1142 cm-1. 

N,N’ disalicylalethylene-diamine{SALEN LIGAND}:- 

NMR spectra of salen shows a broad band around 13.73 ppm and 13.75 ppm corresponding to the phenolic protons, and also a singlet at 8.34 ppm characteristic of azomethine hydrogen CH=N group. The multiplets corresponding to the phenyl protons for salens appear in the range ( 6.98- 7.2 ) ppm . The presence of peaks around 3.60 ppm indicates presence of methylene .

USES OF SALEN LIGAND AND ITS COMPLEXES :- 

Salen based complexes are potenially been used in fields like catalysis , biochemistry , electrochemistry, sensors , molecular magnetism and material sciences . Salen metal complexes are still leading in the field of homogenous catalysis of various organic reactions