Syntheses and Characterization of Some Tetradentate Schiff-Base Complexes and Their Heteroleptic Analogues

VO(IV), Ni(II) and Cu(II) complexes of the asymmetric Schiff base [(HOC6H3(OCH3)C(C6H5):N(CH2CH2)N:C(CH3)CH:C(C6H5)OH)], and their heteroleptic analogues with triphenyl phosphine and 2,2’-bipyridine have been synthesized and characterized by elemental analyses, conductance, magnetic, infrared and electronic spectral measurements. The ligand is tetradentate coordinating via the imine N and enolic O atoms. The Ni(II) and Cu(II) complexes adopt a four coordinate square planar geometry, the VO(IV) complex is five coordinate square-pyramidal and the heteroleptic complexes are 6-coordinate, octahedral. The assignment of geometry is collaborated by magnetic moments and electronic spectra measurements. The compounds are non-electrolyte in nitromethane and are magnetically dilute.

The elemental analyses C, H and N were determined by Microanalytical Laboratory of Dipartimento di Scienze Chimiche, Universita di Trieste, Italy.Nickel and copper were determined titrimetrically and oxovanadium(IV) was estimated gravimetrically 21 .The IR spectra of the compounds were recorded on a Perkin-Elmer 2000 FTIR spectrophotometer, as KBr discs in the range 4000 -400cm -1 while the solid reflectance spectra were measured on a Unicam Helios γ-spectrophotometer.Magnetic susceptibilities were measured on Johnson Matthey magnetic susceptibility balance and diamagnetic corrections were calculated using Pascal's constant 22 while conductivity measurements were measured using a MC-1, Mark V conductivity meter with a cell constant of 1.0.

Syntheses [HO(C 6 H 3 (OCH 3 )C(C 6 H 5 ):N(CH 2 ) 2 N:C(CH 3 )CH:C(C 6 H 5 )OH)], ( H 2 L)
The ligand was prepared by modification of a literature method 14 .A mixture of 0.1 mol (6.00 g; 6.70 mL) ethylenediamine, 0.1 mol (22.83 g) 2-hydroxy-4-methoxybenzophenone and 0.1 mol (16.22 g) of benzoyl acetone, in 100 mL of ethanol was refluxed for two hours.The reaction mixture was cooled in ice and the yellow product, which was first formed, was filtered and washed with ethanol and dried over anhydrous calcium chloride.

Preparation of the metal complexes
The various complexes were prepared by addition of 5 mmol of Ni(CH 3 COO) 2 .4H 2 O (1.24 g), Cu(CH 3 COO) 2 .H 2 O (1.00 g) or VOSO 4 .H 2 O (0.82 g) in bits to a stirring 5 mmol of the ligand (2.21g) in 60 mL of dried methanol at room temperature (the Oxovanadyl mixture was buffered with 10 mmol of triethyl amine (1.08 g, 1.49 mL ).After stirring for thirty minutes, the resulting homogeneous solution of the complex was refluxed for 2 h and the precipitated solids were filtered, washed with methanol and dried over anhydrous calcium chloride.

Preparation of 2, 2'-bipyridine and triphenyl phosphine heteroleptic complexes
Triphenyl phosphine (1.30 g, 5 mmol) or 2,2'-bipyridine (0.79 g, 5 mmol) dissolved in 10 mL of ethanol was added slowly to a solution of the Schiff-base chelates, (2.50-2.54g, 5 mmol) in 30 mL of ethanol while stirring.The mixture was refluxed for 1 h and the product formed was filtered, washed with ethanol several times and dried under vacuum.

Results and Discussion
The reactions of the various metal(II) Schiff bases and heteroleptic analogs can be represented by equations 1-3.The compounds are formed in moderate yields (30-80%).The general equation for the formation of the compounds is given below: The reactions of the metal(II) Schiff bases with triphenyl 2, 2'-bipyridine in ethanol to give the heteroleptic complexes is as shown below: ML + A ML (A) (3) (Where M =VO(IV), Ni(II), Cu(II) ; A = triphenyl phosphine/ 2, 2'-bipyridine).The C, H and N data, percentage yields and melting points are presented in Table 1.The molar conductances of the compounds that dissolved in nitromethane were below 15.00 ohm -1 cm 2 mol -1 indicating that they were non-electrolytes, since a value in the range 75-90 ohm -1 cm 2 mol -1 is expected for a 1:1 electrolyte.
Table 1 Analytical Data for Compounds.

Infrared Spectra
The strong band at 3300 cm -1 , which is conspicuously absent in the spectra of the complexes, is assigned as vOH stretching frequency in the ligand, which indicatives deprotonation and involvement of the enol O in chelation.The uncoordinated C=N and C=C stretching vibrations occur as coupled bands expectedly in the ligand at 1606-1542 cm -1 and are bathochromic shifted to 1600-1513 cm -1 on coordination to the metal ions [18][19] , indicative of the involvement of N and O donor atoms of C=N and enol groups respectively, in coordination to the metal ions.In heteroleptic complexes, these bands are hypsochromic shifted to 1608-1516 cm -1 which is indicative of a stronger M-P/M-N (base) bonds in them 19 .The v(V=O) bands appeared very strong between 940-980 cm -1 while those due to v(M-O) and v(M-N)/ v(M-P) were observed [14][15] at 494-444 and 599-525 cm -1 (Table 2).

Magnetic Moments
Oxovanadium(IV) complexes are expected to have a moment of 1.73B.M, regardless of the geometry, when the orbital contribution is completely quenched by low symmetry fields.However, in the compounds studied a moment in the range 1.74-1.76B.M is observed, indicative of monomeric compounds.In the Ni(II) compounds reported in this work, the complex is diamagnetic while the heteroleptic complexes have moment of 3.20 and 3.27 B.M, respectively.The former is indicative of a four coordinate, square planar geometry while the latter is suggestive of a six coordinate, octahedral geometry 22 .The magnetic moments of 1.98 -2.07 B.M. is observed for the copper(II) compounds, which indicate that they are magnetically dilute and are higher than the spin-only value of 1.73 B.M. as a result of orbital contribution and spin-orbit coupling 22 .

Conclusion
The asymmetric Schiff-base ligand coordinates to the VO(IV), Ni(II) and Cu(II) ions in a tetradentate manner using the azomethine N and enol O atoms.The assignment of a 4coordinate, square planar for Ni and Cu complexes, a 5-coordinate square-pyramidal geometry for the VO(IV) complex and 6-coordinate octahedral geometry (Figure 2) for their heteroleptic analogs is corroborated by magnetic, infrared and electronic spectral measurements.

Figure 1 .
Figure 1.The structural formula the ligand

Figure 2 .
Figure 2. The proposed coordination spheres of the Schiff-base Complexes (a) and their heteroleptic analogs (b and c).
→ a 1 (band III) transitions.The heteroleptic analogs, on the other hand display one to two bands at 14.05 and 18.35-19.00kK which is indicative of a six-coordinate tetragonal (octahedral) geometry 15.45, 17.55 and 26.50 kK respectively, which is consistent with a fivecoordinate, square-pyramidal geometry and is assigned to b 2 → e* π , (band I), b 2 → b 1 (band II) and

Table 2 .
Infrared and Electronic Spectral Data of the Complexes.