Transition Metal Ion Complexes of Schiff-bases. Synthesis, Characterization and Antibacterial Properties

Some novel transition metal [Co(II), Cu(II), Ni(II) and Zn(II)] complexes of substituted pyridine Schiff-bases have been prepared and characterized by physical, spectral and analytical data. The synthesized Schiff-bases act as deprotonated tridentate for the complexation reaction with Co(II), Ni(II) and Zn(II) ions. The new compounds, possessing the general formula [M(L)2] where [M=Co(II), Cu(II), Ni(II) and Zn(II) and HL=HL1, HL2, HL3 and HL4] show an octahedral geometry. In order to evaluate the effect of metal ions upon chelation, the Schiff bases and their complexes have been screened for antibacterial activity against the strains such as Escherichia coli,Staphylococcus aureus, and Pseudomonas aeruginosa. The complexed Schiff bases have shown to be more antibacterial against one more bacterial species as compared to uncomplexed Schiff-bases.


INTRODUCTION
Much attention '8 has been devoted by bioinorganic as well as by medicinal chemists to the relationship between the metal ions and their complexes as antitumour 9and antibacterial 35 agents. In vivo studies have indicated that some biologically active compounds may become more carcinostatic and bacteriostatic upon chelationS622. Such interactions of transition metal ions with amino acids, peptides and nucleic acids, are of immense biological importance23"25. Several reviews 26"29 showed that the metallo-organic chemistry of $30-33 such compounds greatly influence their biological action hghlghtng the catalytic function metal in many biological processes 34"37 Several studies have revealed 38"39 that by condensation of salicylaldehyde with different heterocyclic compounds, derivatives with potent antibacterial and antifungal activity are obtained. Osman et al 4 prepared thiadiazole derived compounds of salicylaldehyde which were found to be highly potent antibacterial against Bacilhts cereus and antifungal, against Aspergillus niger. Several compounds 4 incorporating piperazinyl guanidine, when condensed with sa[icy[aldehyde were found to exhibit cardiovascular and vasodepressive activity. Studies of Shah et a[ 42 also showed thiazolidinone-derived salicylaldehydes to possess good antimicrobial activity. Keeping in view the significance of metals in biology, we have previously reported 4347 several series of biologically active compounds and have evaluated the role of metal ions on their biological activity. In continuation to the same research topic, we report here some novel substituted pyridine Schiff bases obtained from salicylaldehyde. We have also studied the effect of substituents as well as metals ions on the biological activity of these derivatives.

Material and Methods
All chemicals and solvents used were of Analar grade. All metal(II) salts were used as chlorides IR spectra were recorded on a Philips Analytical PU 9800 FTIR spectrophotometer. UV (1). All other metal complexes were prepared respectively following the same method.
Antibacterial studies The synthesized metal complexes, in comparison to the uncomplexed Schiff-bases were screened for their antibacterial activity against pathogenic bacterial strains, Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa. The paper disc diffusion method 4s49 reported elsewhere was adopted for the determination of antibacterial activity.

Physical properties
The Schiff bases (HLI-HLa) ( Fig. 1) were prepared by refluxing an appropriate amount of 5-hydroxy-, nitro-, methoxyor bromo-substituted 2-aminopyridine and salicylaldehyde in hot ethanol in l:l molar ratio respectively. The structures of these Schiff bases were established with the help of their IR, NMR, and microanalytical data (Tables and 2) These Schiff bases were then used for the complexation with Co(II), Cu(II), Ni(II) and Zn(II) ions. All of the synthesized metal complexes [(1)-(16)] (Table 3) were air and moisture stable. These were prepared by the stoichiometric reaction of the corresponding metal salts (as chlorides) and the Schiff base in molar ratios M:L of 1:2. The complexes are intensely colored, amorphous solids, which decompose above 200 C. They are insoluble in common organic solvents such as ethanol, methanol, chloroform or acetone, but soluble in DMSO and DMF. Molar conductance values of the soluble complexes in DMF showed low values (12)(13)(14)(15)(16)(17)(18)(19) ohmcmZmol) indicating s them to be non-electrolytic. Infrared spectra IR spectra of the Schiff bases showed the absence of bands at 1735 and 3420 cm "l due to cat;bonyl v(C=O) and v(NH2) stretching vibrations and, instead, appearance of a strong new band at-1635 cm" assigned 5 to the azomethine, v(HC=N) linkage. It suggested that amino and aldehyde moieties of the starting reagents are absent and have been converted into the azomethine moiety (Fig.l). The comparison of the IR spectra of the Schiff bases and their metal chelates (Table 4) indicated that the Schiff bases were principally coordinated to the metal atom in three ways, representing thus the ligands acting in a tridentate manner.  d) The band appearing at 1635 cm due to the azomethine was shifted to lower fi'equency by -10-15 cm " Indicating participation of the azomethine nitrogen in the complexation.
The band at 1620 assigned to pyridine ring v(C=N) nitrogen also shifted to lower frequency by -15-25 cm " which was indicative of the involvement of ring nitrogen of pyridine in chelation.
A band appearing at 3425 cm assigned to v(OH) in the Schiff base compounds was not found in the spectra of their metal complexes indicating deprotonation and coordination of the hydroxyl oxygen to the metal atom. Further conclusive evidence of the coordination of these Schiff base compounds with the metals, was shown by the appearance of weak low frequency new bands at 525-530 and 455-460 cm . These were assigned 5 to the metal-nitrogen v(M-N) and metal-oxygen v(M-O) respectively. These new bands were observable only in the spectra of the metal complexes and not in the spectra of its uncomplexed Schiff base compounds thus confirming participation of these hetero groups (O or N) in the coordination.

NMR spectra
The IH NMR spectra of the Schiff bases and of their Zn(II) complexes taken in DMSO-do are listed in Table   2. The Schiff bases exhibited signals due to all the expected protons in their expected region and have been identified from the integration curve found to be equivalent to the total number of protons deduced from their proposed structures. These were compared with the reported 54 signals of the known identical compounds and give further support for the compositions of the new ligands as well as their complexes suggested by their IR and elemental analyses data. Comparison of the chemical shifts of the uncomplexed Schiff bases with those of the corresponding complex show that some of the resonance signals experience shifts upon the complexation. In each case, the protons assigned due to heteroaromatic (HC=N), azomethine (HC=N), Transition Metalion Complexes ofSchfff-Bases. Synthesis, Characterization and Antibacterial Properties hydroxyl group (OH) and substituted aromatic were found at around 8.8, 7.3, 9.9 and 6.8-7.7 ppm in the spectra of the ligands. The protons due to heteroaromatic, azomethine and substituted aromatic undergo shift towards downfield by 0.9-1.0 ppm in the complexes indicating coordination of these groups with the metal atom. Also, protons due to hydroxyl group (OH) were found absent in the spectra of the complexes. The absence of these signals suggested 55 the deprotontion of the hydroxyl group and the involvement of the oxygen atom in complexation. Magnetic moments and UV-visible spectra The room temperature magnetic moment of the solid cobalt (II) complexes was found to lie in the range (4. 30210-30575 cm which may be assigned to T, --+ T,(F), T,g -+ A,g(F) and T,g -+ Tg(P) transitions 55859 respectively and are suggestive of the octahedral geometry around the cobalt ions.
The Cu(ll) complexes showed three absorption bands between 10 Dq band for a distorted octahedral geometry corresponding 6'6 to the transitions 2Eg -+2T28. The bands at 22152-22355 and 30550-30645 cm may be due to intra-ligand charge transfer transitions.
The Ni(II) complexes exhibited three spin-allowegt bands at 815-10145,.5945-16250 and 28540-2910 cm" signable 6263respectively, to the transitions aA,g(F) 'T2g(F)(v), "Ag(F)'T(F)(v2) and "Ag(F) ---> T3(P)(v3) which were characteristic of their octah-edral geometry (Fig 2). Table 4. IR and UV-visible.. spectral data of the metal(II) chelates R tcm "), Escherichia coli (a), Staphylococcus aureus (b) and Pseudomonas aeruginosa (c). The compounds were tested at a concentration of 30 lag/0.01 mL in DMF solution using the paper disc diffusion method. The susceptibility zones were measured in diameter (mm) and the results are reproduced in Table 5. The susceptibility zones measured were the clear zones around the discs killing the bacteria. All the Schiff bases and their complexes individually exhibited varying degrees of inhibitory effects on the growth of the tested bacterial species. The antibacterial results evidently show that the activity of the Schiff base compounds became more pronounced when coordinated to the metal ions. All metal ions have varying antibacterial influence on bacterial species. The Co(II) complex of HLI was more antibacterial against one species and less against the other as compared to the Co(II) complex of the other Schiff bases. Same results were found for other metal complexes. It is however, not possible to make out exactly which metal ion is playing more antibacterial role against one or the other bacterial species but, it is definitive that metal ions do play a significant role in enhancing the antibacterial activity of antibacterial agents on chelation. It is suggested that in the chelated complex, the positive charge of the metal ion is partially shared with the donor atoms and there is g-electron delocalization over the whole chelate ring. This increases the lipophilic character of the metal chelate and favors its permeation through lipoid layers of the bacterial membranes. It is Characterization and Antibacterial Properties also suspected that factors such as solubility, dipole moment and cell permeability mechanisms are also influenced by the presence of the metal ions, which are responsible in enhancing this role of metals as bactericidal. Our in vitro studies are in progress, which would help us in determining further the actual mechanism involved in enhancing this activity.