INTRODUCTION:
A large number of Schiff bases1 and their complexes2 have been investigated for their interesting and important properties, such as their ability to reversibly bind oxygen, catalytic activity in the hydrogenation of olefins, photo chromic properties and complexing ability towards some toxic metals. Furthermore, complexes of Schiff bases showed promising biological activity and biological modeling applications3. The Schiff base ligands with sulphur and nitrogen donor atoms in their structures act as good chelating agents for the transition and non-transition metal ions4. Coordination of such compounds with metal ions, such as copper, nickel and iron, often enhances their activities5, as has been reported for pathogenic fungi6. There is a continuing interest in metal complexes of Schiff bases because of the presence of both hard nitrogen or oxygen and soft sulphur donor atoms in the backbones of these ligands. They readily coordinate wide range of transition metal ions yielding stable and intensely coloured metal complexes, some of which have been shown to exhibit interesting physical and chemical properties and potentially useful biological activities.7, 8
Schiff bases are still regarded as one of the most potential group of chelators. The high affinity for the chelation of the Schiff bases towards the transition metal ions is utilized in preparing their solid complexes9, 10.
This paper discusses the synthesis, characterization and complexation of benzene-1,2-diyldimethaniminedibenzoic acid ligand and its Co(II), Ni(II), Cu(II) and Zn(II) metal ions.
MATERIALS AND METHODS:
Materials and reagents:
All chemicals used were of the analytical reagent grade (AR). They included o-phthaldehyde (Sigma); 2-aminobenzoic acid (Sigma); copper(II) chloride dihydrate (Prolabo); cobalt(II) and nickel(II) chlorides hexahydrates (BDH); zinc(II) chloride dihydrate (Ubichem), copper(II) chloride (Prolabo).Organic solvents used included absolute ethyl alcohol, diethylether, and dimethylformamide (DMF). These solvents were spectroscopic pure from BDH. Hydrogen peroxide, hydrochloric and nitric acids (MERCK) were used. De-ionized water collected from all glass equipments was usually used in all preparations.
Instruments: The molar conductance of solid complexes in DMF (10−3 M) was measured using Sybron–Barnstead conductometer (Meter-PM.6, E = 3406). Elemental microanalyses of the separated solid chelates for C, H, N and S were performed on a Coleman C20 automatic analyzer. The analyses were repeated twice to check the accuracy of the data. Infrared spectra were recorded on a Perkin–Elmer FT-IR type 1650 spectrophotometer in the region 4000–400 cm−1 as KBr discs. The 1HNMR spectra were recorded with a JEOL EX-270 MHz in d6-DMSO as solvent, where the chemical shifts were determined relative to the solvent peaks. The electronic spectra were measured on a Shimadzu 3101 pc spectrophotometer.
Synthesis of the Schiff base (benzene-1, 2-diyldimethaniminedibenzoic acid): Hot solution (60°C) of 2-aminobenzoic acid (2.74 g, 20 mmol) was mixed with hot solution (60°C) of o-phthaldehyde (1.34 g, 10 mmol) in 50 ml ethanol. The resulting mixture was left under reflux for 4 h and the solvent was evaporated till deep yellow oil product separated out. This oil was poured on ice cold dilute HCl, yellow crystalline product separated out. The formed solid product was separated by filtration, purified by crystallization from ethanol, washed with diethyl ether and dried in a vacuum over anhydrous calcium chloride. The yield was 86%. Equation of reaction is shown in Scheme 1.
Synthesis of metal complexes:
The metal complexes were prepared by the addition of hot solution (60 °C) of the appropriate metal chloride (1 mmol) in an ethanol–water mixture (1:1, 25 ml) to the hot solution (60 °C) of the Schiff base (0.372 g, 1 mmol) in the same solvent (25 ml). The resulting mixture was stirred under reflux for one hour whereupon the complexes precipitated. They were collected by filtration, washed with a 1:1 ethanol: water mixture and diethylether. The microanalysis data for C, H and N were repeated twice.
RESULTS AND DISCUSSION:
The results of elemental analyses and the melting points are presented in Table 1. The results obtained are in good agreement with those calculated for the suggested formula. The melting points are sharp indicating the purity of the prepared Schiff base.
Elemental Analysis: The result of the elemental analysis in Table 1 suggested the formulae [M(L)2] where M = Co(II), Ni(II), Cu(II) and Zn(II).
Molar conductivity measurements: The chelates were dissolved in DMF and the molar conductivities of 10−3 M of their solutions at 25°C were measured. Table 1 shows the molar conductance values of the complexes. The molar conductance values of the complexes fall in the range 6.74–15.65Ω−1 mol−1 cm2 indicating that these chelates are non-electrolytes.
IR spectral studies: The IR spectra data of the Schiff base and its complexes are listed in Table 2. The IR spectra of the complexes are compared with those of the free ligand in order to determine the coordination sites that may involve in chelation. The position and/or the intensities of these peaks are expected to be changed upon chelation.
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comparison, it was found that the azomethine υ(C
N) stretching
vibration is found in the free ligand at 1604 cm−1. This
band is shifted to lower wavenumbers (1589–1595 cm−1) in
the complexes indicating the participation of the azomethine nitrogen in
coordination. The υ(OH), υ(CO), υasym(COO)
and υsym(COO) stretching vibrations are observed at 3300,
1682, 1588 and 1395 cm−1 for H2L. The absence
of the υ(OH) stretching vibrations in the complexes suggested
deprotonated carboxylate O in bonding. The participation of the carboxylate-O
atom in the complexes formation was evidenced from the shift in position of
these bands from 1588 cm−1 in the ligand to 1502–1508 cm−1
in the complexes and from 1395 cm−1 in the ligand to
1401-1403 cm−1 in the metal complexes. New bands are
found in the spectra of the complexes in the regions 530–535 cm−1,
which are assigned to υ(M–O) stretching vibrations. The bands at
464–488 cm−1 have been assigned to υ(M–N)
mode. Therefore, from the IR spectra, it is concluded that H2L
ligand behaves as a bi-negative tetradentate ligand, coordinating to the metal
ions via the azomethine N and deprotonated carboxylate O.
1HNMR spectra : The chemical shifts of the different types of protons in the 1HNMR spectra of the ligand and metal complexes are listed in Table 3. Upon comparison, the COOH signal is found at 11.641 ppm in the spectrum of H2L ligand. This signal is completely absent in the complexes indicating the participation of the COOH group in chelation with proton displacement.
13CNMR spectra: The chemical shifts of the different types of carbons in the 13CNMR spectra of the ligand and metal complexes are listed in Table 4. The chemical shifts at 125.3 – 144.8 in the ligand and metal complexes are assigned as aromatic carbons. The chemical shift value between 170.4 -170.9 are assigned C=O in acids.
COOH)