Synthesis , Characterization and Thermal Analysis of a New Acetic Acid ( 2-Hydroxy-benzylidene )-hydrazide and its Complexes with Hg ( II ) and Pd ( II )

The new complexes have been synthesized by the reaction of Hg(II) and Pd(II) with acetic acid(2-hydroxy-benzylidene)hydrazide (L). These new complexes were characterized by elemental analysis, IR, H NMR spectroscopy and UV spectral techniques. The changes observed between the FT-IR, H NMR and UV-Vis spectra of the ligands and of the complexes allowed us to establish the coordination mode of the metal in complexes. Thermal properties, TG-DTA of these complexes were studied. TGDTA and other analytical methods have been applied to the investigation of the thermal behavior and structure of the compounds [M(L)2]Cl2 M= Hg, Pd. Thermal decomposition of these compounds is multi-stage processes.


Introduction
Schiff bases play an important role in inorganic chemistry as they easily form stable complexes with most transition metal ions.The development of the field of bioinorganic chemistry has increased the interest in Schiff base complexes, since it has been recognized that many of these complexes may serve as models for biologically important species [1][2][3][4][5] .

Experimental
All reagents were supplied by Merck and were used without further purification.Melting points were determined in an electrothermal 9200.The FTIR spectra were recorded in the range 400-4000 cm -1 by KBr pellets using a Brucker Tensor 27 M 420 FT-IR spectrophotometer. 1 H-NMR spectra in CDCl 3 were recorded on Bruker AMX-500 spectrometers.The UV-Vis spectra in CH 3 CN were recorded with a Wpa bio Wave S2 100 spectrophotometer.

Results and Discussion
The complexes [acetic acid (2-hydroxy-benzyliden)-hydrazide] Hg(Π) and [acetic acid (2-hydroxy-benzyliden)-hydrazide] Pd(Π) were prepared in good yield by stirring stoichiometric amounts of HgCl 2 and PdCl 2 with L (Scheme 1).Elemental analysis data are summarized in Table 1.The complexes were characterized by the usual methods: elemental analysis, FT-IR, H-NMR and absorption electronic spectroscopy.The complexes are stable in air and light, and are soluble in organic solvents such as DMSO and insoluble in and water and n-hexane.
The infrared spectra of the complexes taken in the region 400-4000 cm -1 were compared with this of the free ligand.As seen data the IR spectra of two meta(II) complexes are very similar to each other, except some slight shifts and intensity change of a few vibration bands caused by different metal(II) ions, which indicate that the complexes have similar structures (Figure 1).There are some significant changes between the metal(II) complexes and their free ligand for chelation as expected.The main stretching frequencies of the IR spectra of the ligand L and their complexes are tabulated in Table 2.An exhaustive comparison of the IR spectra of the ligand and complexes gave information about the mode of bonding of the ligand in metal complexes.The IR spectra of complexes, the ligand acts as a neutral bidentate through the azomethine and carbonyl groups 14 .
Transmittance, % The azomethine band is shifted to lower frequency in all metal complexes, suggesting that this group takes part in coordination.The coordination of nitrogen to the metal atom would be expected to reduce the electron density on the azomethine link and thus cause a shift in the C=N band.Moreover, in the spectra of the complexes, a considerable negative shift in ν(C=O) are observed indicating a decrease in the stretching force constant of C=O as a consequence of coordination through the carbonyl-oxygen atom of the free ligands 13 .The band observed at 1681 cm −1 in the spectrum of the ligand [17][18][19][20] , which attributed to ν(C=O) mode are shifted, in spectra of all the complexes to lower wave number and appears at 1679 and 1678 cm −1 region indicating the involvement of O-atom of the carbonyl group in coordination 21 .The small shift to higher frequency of the band due to υ(N-N) can be taken as additional evidence of the participation of the azomethine group in bonding.This result is confirmed by the presence of a new band at 551, 553 cm -1 and 483, 471 cm -1 ; these bands can be assigned to υ(M-O) and υ(M-N) vibrations, respectively [22][23][24] .
In NMR spectra of complexes we observed a shift of electron density from the ligand to the metal. 1 H-NMR spectra of the complexes (1), ( 2) show all the expected signals (Figure 2).The 1 H-NMR spectrum of the complexes exhibits two signals at 10.28 and 11.19 ppm are assigned to the OH protons, respectively.In all the spectra, a singlet corresponding to a single proton is observed in the range δ 9.25, 10.37 ppm, which is attributed to the azomethine proton (-HC=N) in ( 1), (2) complexes, respectively.The 1 H-NMR spectra of the [HgL 2 ]Cl 2 complexes show a negative shift of the signal due to the NH group.This signal is observed at δ 8.47, 8.18 ppm in (1), (2) complexes, suggesting that the coordination proceeds through the carbonyl oxygen or azomethine nitrogen groups.The downfield shifts of the methyl group signal at 2.21, 2.16 ppm for the (1), (2) complexes, support the coordination via the azomethine nitrogen.

Thermal analysis
The thermal properties of metal(II) complexes were investigated by thermograms (TG, DTG and DTA) and are shown in Figure 1, Figure 2. The TG curve of 1 and 2 complexes showed decomposed in two successive stages decomposed.The first stage occurred in the temperature range 116-164 and 175-239 °C with a corresponding weight loss 13 and 39% for 1 and 2 complexes, respectively.Which are accompanied by endothermic effect for complexes 1 and 2 in the DTA curve in the range 201, 202 °C and may be due to the loss of water molecules (one non-coordinated molecule, two coordinated molecules and one noncoordinated molecule, respectively).The second stages of decomposition were observed at 203-286 °C (66% wt loss) and 303-340 °C (36% wt loss) for 1 and 2 complexes, respectively.Meanwhile the DTA curve exhibits endothermic effect for complex 1 and exothermic effect for complex 2 in the range 276, 339 °C which are accompanied by weight loss confirming.

Figure 1 .Table 2 .
Figure 1.IR spectra of (a) C 18 H 20 O 4 N 4 HgCl 2 , (b) C 18 H 20 O 4 N 4 PdCl 2 Table 2. Selected characteristic IR bands (400-4000 cm −1 ) of ligands and their complexes Compound ν(C=O) ν(C=N) ν(N-N) ν(M-N) ν(M-O) L 1681 1621 1032 --(1) 1679 1619 1034 483 551 (2) 1678 1610 1036 471 553The azomethine band is shifted to lower frequency in all metal complexes, suggesting that this group takes part in coordination.The coordination of nitrogen to the metal atom would be expected to reduce the electron density on the azomethine link and thus cause a shift in the C=N band.Moreover, in the spectra of the complexes, a considerable negative shift in ν(C=O) are observed indicating a decrease in the stretching force constant of C=O as a consequence of coordination through the carbonyl-oxygen atom of the free ligands13 .The band observed at 1681 cm −1 in the spectrum of the ligand[17][18][19][20] , which attributed to ν(C=O) mode are shifted, in spectra of all the complexes to lower wave number and appears at 1679 and 1678 cm −1 region indicating the involvement of O-atom of the carbonyl group in coordination21 .The small shift to higher frequency of the band due to υ(N-N) can be taken as additional evidence of the participation of the azomethine group in bonding.This result is confirmed by the presence of a new band at 551, 553 cm -1 and 483, 471 cm -1 ; these bands can be assigned to υ(M-O) and υ(M-N) vibrations, respectively[22][23][24] .In NMR spectra of complexes we observed a shift of electron density from the ligand to the metal.1 H-NMR spectra of the complexes (1), (2) show all the expected signals (Figure2).The 1 H-NMR spectrum of the complexes exhibits two signals at 10.28 and 11.19 ppm are assigned to the OH protons, respectively.In all the spectra, a singlet corresponding to a single proton is observed in the range δ 9.25, 10.37 ppm, which is attributed to the azomethine proton (-HC=N) in (1), (2) complexes, respectively.The 1 H-NMR spectra of the [HgL 2 ]Cl 2 complexes show a negative shift of the signal due to the NH group.This signal is observed at δ 8.47, 8.18 ppm in (1), (2) complexes, suggesting that the coordination proceeds through the carbonyl oxygen or azomethine nitrogen groups.The downfield shifts of the methyl group signal at 2.21, 2.16 ppm for the (1), (2) complexes, support the coordination via the azomethine nitrogen.