Antibacterial Activity of Macrocyclic Complexes of Cu ( II ) , Ni ( II ) and VO ( II ) Derived from o-Aminobenzaldehyde

Novel Cu(II), Ni(II) and VO(II) complexes are synthesized with N,N-bis(2-aminobenzylidene)benzene-1,4-diamine (L). Complexes were characterized by elemental analysis, molar conductance, IR, UV and EPR. Spectral studies reveals a square planner geomentry for Cu(II), Ni(II) complexes and square pyramidal for VO(II) complex. The ligand and its complexes were also evaluated against the growth of gram positive bacteria and gram negative bacteria.


Introduction
Metal complexes containing synthetic macrocyclic ligands have attracted a great deal of attention because they can be used as models for more intricate biological macrocyclic systems like metalloporphyrins (hemoglobin, myoglobin, cytochromes, chlorophylls), corrins (vitamin B12) and antibiotics (valinomycin, nonactin).So it attracted the attention of both inorganic and bioinorganic chemists 1 .These discoveries have created supramolecular chemistry and its enormous diversity.The formation of macrocyclic complexes mainly depends on size of the internal cavity and rigidity of the macrocycle formed 2 .Schiff bases having donor atoms such as N 2 O 2 and N 4 around the metal ion have received more attention, mainly because of their extensive applications as catalysts for carbonylation, hydrogenation hydroformylation, epoxidation, nucleicacid modification, electrochemical reduction, alkanehydroxylation, Diels-Alder transformations, carboxylic acid decarboxylation, amines oxidation and medicinal studies as models for mimicking the superoxide dismutase [3][4][5][6][7][8] .The chemistry of macrocyclic complexes is also important due to its use as dyes, pigments and NMR shift reagents 9 .
Metal condensation reaction with the ligand often provides selective routes towards product that are not obtainable in absence of metal ions.A rational control of the nuclearity of transition metal complexes is important to design systems with the desired properties, as some of these applications require the presence of more than one metal center in the particular complex.Indeed, binuclear complexes may have very different reactivity than mononuclear counterparts, thereby enabling transformations inaccessible to single metal ions 10 .For instance, nucleic acid hydrolysis is postulated to be facilitated by the cooperative action of two metal ions 11 .Furthermore, bi or oligo nuclear complexes containing transition metals may be formed by bridging ligands that can mediate magnetic interactions between paramagnetic metal ions 12 .

Experimental
Materials o-Phthalaldehyde and p-phenylenediamine were purchased from Aldrich.Ethanol, methanol, DMSO and DMF were used as solvents.The solvents metals and reagents with analytical grade were obtained commercially and used without further purification.

Physical measurement
The elemental analysis was performed using Carlo-Eraba 1106 instrument.Molar conductance of the complexes in DMF solution was measured with ELICO CM 185 Conductivity Bridge.The Infrared spectra were recorded on the Perkin Elmer FT-IR-8300 model spectrometer using KBr disc.Electronic absorption spectra in the UV-Visible range were recorded on Perkin Elmer Lambda-25 between 200-800 nm by using DMSO as the solvent.ESR spectra of the complexes were recorderd as powder samples at room temperature on an E4-EPR spectrometer using DPPH as the g-marks.Magnetic susceptibility data were collected on powdered sample of the compounds at the room temperature with PAR155 vibrating sample magnetometer.

Synthesis of Cu(II) complex
The Cu(II) complex was prepared by methanolic solution of Cu(ClO 4 ) 2 .6H 2 O salt.This was slowly added with stirred hot methanolic solution of binucleating Schiff base ligand.After half an hour phthalaldehyde was added drop wise to the same solution.The strring was continued for 1 h, after it undergoes reflux for 10-12 h at ≈70 °C.A dark blue colored precipitate formed was filtered, washed several times with methanol, diethylether and dried in vaccum 15

Synthesis of Ni(II) complex
The same procedure was adopted for Ni(ClO 4 ) 2 .6H

Synthesis of vanadyl complex
The same procedure was adopted for VOSO 4 .4H

Antibacterial studies
The antibacterial activity of the macrocyclic binuclear Cu(II), Ni(II) and VO(II) complexes were checked by the disc diffusion technique 16 .This was done on gram negative bacterial like Klebsiella pneumoniae, Escherichia coli and gram positive bacterial like Staphylococcus aureus at 37°C.The disc of Whatman no. 4 filter paper having the diameter 8.00 mm were soaked in the solution of compounds in DMSO (1.0 mg cm -1 ).After drying it was placed on nutrient agar plates.The inhibition areas were observed after 36 hours.DMSO was used as a control and Stremptomycin as a standard.

Results and Discussion
A novel macrocyclic binuclear Cu(II), Ni(II) and VO(II) Schiff base complexes have been synthesized by template condensation of binucleating schiff base ligand with metal salt and o-phthalaldehyde.All the complexes were crystalline in nature, dark colored solid, stable at room temperature and soluble in DMF or DMSO.The ligand can be confirmed by 1 H NMR. All the complexes gave satisfactory elemental analysis results with the proposed structure of the complexes.The formation and their geometry were further confirmed by IR, UV Vis, magnetic, EPR spectral studies.The ligand and complexes were also screened for antibacterial activity against bacteria species.

H NMR spectra of Schiff base ligand
The formation of Schiff base ligand was observed by peak ratios in the 1 H NMR spectra. 1 H-NMR spectra of the ligand was taken using DMSO d6 solvent (Figure 2).The aromatic region was a set of multiplets in the range of 6-7.5 ppm, while the azomethine proton was observed in the range 8.25 ppm.

Magnetic and electronic studies
The electronic spectra of the Schiff base ligand and its complexes were recorded in DMSO solution.The absorption spectrum of free ligand consist of an intense bands centered at 265 nm and 370 nm attributed to π-π* and n-π* transition within the Schiff base ligand.These transitions are also found in spectra of the complexes, but they shifted, confirming the coordination of ligand to metal ions 17 .Further, the d-d transition showed a strong band at 580 nm for Cu(II) complex, this is due to 2 B 1 g → 2 A 1 g.This transition shows the square plannar geometry around the metal 18 .In room temperature magnetic moment values for Cu(II) is 1.65 B.M. In general, the low magnetic moment values of binuclear copper complex are attributed to the antiferromagnetic interaction between two central Cu(II) metal ions 19 .The spectra of Ni(II) complex in the visible region at about 520 nm and 480 nm is assigned to 1 A 1 g → 1 A 2 g, 1 A 1 g → 1 B 1 g, transitions, suggesting an approximate square planar geometry of the ligand around the metal ions 20 .The magnetic moment data of VO(II) complex was 1.75 B.M., which confirms the binuclear structure of the complex.The intense charge transfer band at 531 nm in VO(II) complex assigned to 2 B 2 → 2 E transition.This is due to electron delocalization over whole molecule on complexation.Based on these data, a square pyramidal geometry has been assigned to the complex VO(II) 21,22

EPR spectral studies
The X-band EPR spectrum of the Cu(II) and VO(IV) complexes were recorded at room temperature.Cu(II) complex consists of an axial symmetrical signal with g ll = 2.12 and g ┴ = 2.05.The unpaired electron lies in the (dx2-y2) orbital giving 2 B1g as the ground state with the g ll > g⊥ > ge (2.0023 free spin value) 23 .The observed g ll value for square planner geometry Cu(II) complex is less than 2.3, which is in agreement with the covalent character of metal ligand bond 24 .G value less than 4 indicates a considerable metal -metal interaction in a complex 25 .Vanadyl complexes show peaks with g ll = 2.08 and g ┴ = 2.01 which indicates that the unpaired electron is present in the dxy orbital with square pyramidal geometry around the VO(II) chelates 26 .

Antibacterial activity
The Schiff base ligand and their macrocyclic binuclear Schiff base complexes were tested in vitro to assess their growth inhibitory activity against Gram negative bacterial like Klebsiella pneumoniae, Escherichia coli and gram positive bacterial like Staphylococcus aureus are given in Table 1.The antibacterial activity of complexes arises due presence of several azomethine groups which coordinate with metal ions.Cu(II) has more activity than ligand and other complexes.Difference in the effectiveness was depends on the impermeability of microbial cells or on the difference in ribosome of microbial cells 27 .The increase in antibacterial activity of metal chelates with increase in concentration is due to the effect of metal ion on normal cell processes.Such increases in activity of metal chelates can be explained on the basis of chelation theory 28 .On chelating, the polarity of the metal ion will be reduced to a greater extent due to the overlap of the ligand orbital and partial sharing of positive charge of metal ion with donor groups.Further, it increases the delocalization of πelectrons, enhances liphophilicity and penetration of the complexes which blocks metal binding sites on enzymes of microorganism.These complexes also disturb the respiration process of cell and thus block the synthesis of proteins, which restricts further growth of organism 29,30 .

Conclusion
A novel macrocyclic binuclear schiff base ligand and complexes have been synthesized by using freshly prepared o-aminobenzaldehyde with p-phenylenediamine, and o-phthalaldehyde with respective metal salts.The metal-ligand ratio of 1:1 has been arrived at by estimating the elemental contents.The presence of counter ions has been confirmed from the IR and high molar conductance values. 1 H NMR confirms the complete condensation of the ligand.ESR shows a covalent bonding nature of the complexes and also the geometry of the complexes.The antibacterial activity for Cu(II) complexes was more pronounced because of the chelation theory.
. Antibacterial activity of the ligand and complexes