Synthesis, Spectral Characterization and Biological Studies of Lanthanum(III) Complexes with 3-Substituted-4-Amino-5-Mercapo-1,4-Triazoles Schiff Bases

Some lanthanum(III) complexes have been synthesized by reacting lanthanum(III) metal salt with Schiff bases derived from 3-substituted-4-amino-5-mercapto-1,2,4-triazole and glyoxal/ biacetyl/ benzyl. All these complexes are not soluble in common organic solvents. However sparingly soluble in DMF and DMSO. The chemical analysis of the complexes confirmed to the stoichiometry of the type La(III)LNO 3 ·H 2 O. La(III)LCl·H 2 O and La(III)LNCS·H 2 O respectively. The chelation of the complexes has been proposed in the light of analytical, spectral studies. The measured molar conductance values indicate that, the complexes are non-electrolytes. The Schiff bases and their complexes have been screened for their antibacterial and antifungal activities. The results of these studies show the metal complexes to be more antibacterial and antifungal as compared to the uncomplexed coumarins.

In our pursuit of new ligands for metal complexes, we have synthesized a new series of Schiff bases derived by the condensation of 3-substituted-4-amino-5-mercapto-1,2,4-triazole and gyyoxal/biacetyl/benzyl. These ligands have donor sites with the ONNO sequence and varied coordination abilities. The nature of ligands has attributed our attention and aroused our interest in elucidating the structures of La(III) complexes with the Schiff bases as there is scant information on these complexes of the ligands. Hence, in this paper we have report the synthesis, spectral and biological activities of La(III) complexes with the following synthesized ligands (Fig. 1

EXPERIMENTAL
All chemicals used were of reagent grade. Substituted salicylaldehydes were prepared as described in the literature 15 .

Synthesis of Lanthanum(III) Complexes
A solution of lanthanum(III) nitrate / lanthanum(III) chloride / lanthanum(III) thiocyanate (0.01 mol) was treated with (0.01 mol) Schiff bases in super dry alcohol. The reaction mixture was refluxed for about 2-3 hrs. After cooling pH of the reaction mixture was adjusted to about ca 7 by adding dilute ammonia with constant stirring. The light yellow precipitate of complex was obtained. Then the precipitated complex was filtered, washed thoroughly with dry ethanol and ether and finally dried over fused calcium chloride in vacuum.

3. Analysis and Physical Measurements
The lanthanum in the complex was determined by volumetric method using EDTA solution 18 . The nitrogen was determined by Dumas method. The results of chemical analysis and molar conductance values are listed in Table 1.
The IR spectra of the ligands and their lanthanum(III) complexes were recorded on a HITACHI-270 IR spectrophotometer in the 4000-250 cm -1 region in KBr disc. The electronic spectra of the complexes were recorded on a VARIAN CARY 50-BIO UVspectrophotometer in the region of 200-1100 nm.

RESULTS AND DISCUSSION
The Lanthanum(III) complexes are colored, stable and non-hygroscopic in nature. The elemental analyses shows that, all the Lanthanum(III) complexes have 1:1 stoichiometry of the type La(III)LNO 3  conductance values are too low to account for any dissociation of the complexes in DMF, indicating the non-electrolytic nature of the complexes (Table 1).
In order to establish whether water molecule present in the complexes coordinated to the metal ion, weighed complexes (1) and (2) were dried over P 2 O 5 in a vacuum for ca 1h and then weighed again. No loss in weight was observed.
This was confirmed by heating the complexes for ca. 2 h at 105 °C and no weight loss was observed. These observations suggest the water molecule in the complexes is coordinated to the metal ion.
The complexes of LaNCS·H 2 O exhibit the N-bonded mode of coordination of thiocyante group which is confirmed by the appearance of the bands around 2105,785 and 475cm -1 , which are assigned to ν(C=N) of NCS and ν(C=S) of NCS and NCS bending vibrations respectively. This is further supported by an additional band due to ν(La-N) vibration which has been found to appear around 285 cm -1 . This is further supported by an additional band due to ν(La-N) vibration which has been found to appear around 285 cm -1 .
The complexes of the type LaLCl·H 2 O show the band due to ν(La-Cl) at 250 cm -1 which is assigned in view of previous reports 23 . Metal ligand vibrations are difficult to assign on the empirical bases since their frequencies are sensitive to the metal ligand. A comparison of IR spectra between lagand and its metal complexes fail to give clear cut assignment because some ligand vibrations activated by complex formation which may appear in the same region as metal ligand vibrations. In the far IR spectra of the complexes of the several bands observed, some of them have been assigned tentatively to ν(M-N) and ν(M-S) vibrations 24,25 . All the complexes show medium intensity bands in the region 430-460 cm -1 has been considered to be due to ν(M-N) and the high intensity bands found in the 280-230 cm -1 region may be regarded due to ν(M-S) vibration.

2. NMR Spectra
The NMR spectra of lanthanum(III) complexes of the ligands, the signal due to NH proton disappears suggesting that, the ligands react with the metal ion in thiol form via deprotonation. This observation supports IR inferences. The another characteristic signal due to azomethine proton in the complex of ligand I appears at 8.4 ppm indicates a downfield shift with respect to ligand I (8.2 ppm). This observation suggests the coordination of the azomethine moiety through the nitrogen to the metal ion. A further support for coordination of C=N nitrogen to the downfield shift of the aromatic proton signals appear in the range of 7.2-8.3 ppm with respect to ligand No. IX (6.9-7.9 ppm). A signal due to water proton in the complexes appears at 5.4 and 5.6 ppm respectively. The signal due to CH=N of triazole ring of the complex 9 ppm appeared at 8.44 ppm. All these observations provide support for the IR inferences.