PREPARATION, SPECTRAL STUDY AND ANTIMICROBIAL ACTIVITY OF BINARY Co(II) COMPLEXES DERIVED FROM 2’-HYDROXY CHALCONES

The present work comprises preparation, characterization, thermal behavior and growth inhibitory activity of some novel Co(II) complexes derived from substituted (E)-1-(1-hydroxy-4-iodonaphthalen-2-yl)-3phenylprop-2-en-1-one (L1) and (E)-1-(4-bromo-1-hydroxynaphthalen-2-yl)-3-phenylprop-2-en-1-one (L2-L6). Newly synthesized metal-ligand complexes were structurally confirmed with suitable spectroscopic technique such as FT-IR, EPR, NMR (both H and C). XRD analysis for complex C1 confirmed the crystal system; tetragonal and space group; P 42/n: 2 with unit cell dimensions a, b = 13.3516 Å, c = 10.8009 Å; α, β, γ = 90. The IR and EPR study demonstrated that interaction between metal ions and ligand occurs through carbonyl oxygen and hydroxyl oxygen. From the values of magnetic moment (μ) it was observed that synthesized complexes (C1-C6) are paramagnetic with three unpaired electrons contain one electron in t2g orbital and two electrons in eg orbitals. Further all these complexes have been evaluated in-vitro for their antimicrobial activity against the Gram positive bacteria Staphylococcus aureus, Gram negative bacteria Escherichia coli and the yeast Candida albicans. The complex C1 showed the significant antimicrobial activity, whereas the complexes C2, C4, C5 and C6 are moderately active against the tested pathogens. The antimicrobial data revealed that growth inhibitory activities of complexes were enhanced comparatively than its respective ligands. The enhanced antimicrobial activity is attributed to the presence of halogens (Br, Cl, I) and hydroxyl (OH) active substituents associated with the basic nucleus of complexes. Therefore, the present study helps to develop a new class of antimicrobial analogues.


INTRODUCTION
Chalcone occurring naturally in many plants or they can also be synthetically prepared in laboratory [1]. They are biogenic key precursors of flavonoids in higher plants [2,3]. Chalcones are important reactive intermediate to constitute a major class of oxygen containing heterocyclic compounds [4]. The chalcones and their related derivatives are well-known for wide range of biological activities such as antiviral [5], anti-inflammatory [6], antitumor [7], antimitotic [8], antimicrobial [9], antioxidant [10], anti-diabetic [11] and antimalarial [12] activities. The reason for biological activity may be due to the presence of reactive keto vinyl group which allows for interference with cell membrane of fungi and moulds and demonstrates static properties against pathogens [13].
1,3-Diaryl-2-propene-1-one compounds possess the reactive keto-ethylenic group which interconverts to its isomeric form using acid or alkali makes them important ligands [14,15]. Chalcones are effective metal ion chelators and can easily form metal-coordinated complex compounds. All types of chalcones possess three domains to react with metals such as functional groups present on aromatic ring, keto-enol moiety and olefinic moiety [16]. 2'-hydroxy chalcones, their related heterocyclic and naphthalene analogues have reported for binding ability to form coordination complexes [17]. In recent years, the metal ions play important role in living systems and various transition metal complexes have been used as medicinal compounds. Metal

Antimicrobial activity
In vitro antimicrobial activity of compounds was determined by the agar cup plate method. Minimum inhibitory concentrations of each compound were determined against the standard concentrations. The agar dilution method was employed for the determination. Different concentrations of sample and standard such as 1.0, 0.5, 0.25 and 0.12 mg/mL were prepared in dimethylsulfoxide by serial dilution. The volume of 100 µL was added to each well. Standard and blank control was maintained for each test. After incubation, visually the lowest concentration of test solution with no detectable bacterial growth was considered as minimum inhibitory concentration.

RESULTS AND DISCUSSION
The ligand and its metal complexes were synthesized by the general procedure mentioned above in the experimental section. All the metal complexes are brown coloured in nature and are stable towards air and moisture at room temperature. All these metal complexes are insoluble in most of the organic solvents except DMSO and DMF. Metal complexes are characterized by FTIR, X-ray diffraction, TGA, ESR and NMR analysis.

FT-IR spectra
FT-IR measurement analysis of all the complexes was performed by potassium bromide pellet technique. The appearance of medium vibrational stretching band for OH group in ligand at 3230-3425 cm -1 [27] whereas strong and broad band at around 3400 cm -1 in metal complexes, confirms the formation of Co(II) complexes. The coordinated water molecule in the complex was confirmed by the presence of strong absorption broad band at 3415-3450 cm -1 . The vibrational bands observed at 1578-1631 cm -1 , 1525-1597 cm -1 and 1241-1294 cm -1 demonstrated to corresponding functional groups C=O, C=C, C-O, respectively. Similarly, the characteristics band that appeared at 540-581 cm -1 demonstrates the Co-O (metal-ligand) bond stretching present in formed metal complexes.

Powder X-ray diffraction analysis
The X-ray powder diffraction analysis was performed on X-ray powder diffractometer with parameters scanning mode; 2Theta/Theta, scanning type; continuous, X-ray; 40 kV/20 mA, fixed monochromator with 2θ range 10 to 90 degree at step 0.01degree. To observe the novelty of synthesized complex comparison was made between the observed pattern and reported pattern with peak search method. The observed pattern is shown in Figure 1. The measurement showed the peaks present at different 2θ values (Table 1). From these values calculated the grain size, dislocation density, strain and unit cell parameters and the values are shown in Table 2.  Thermal gravimetric analysis Thermal investigation of synthesized complexes was performed to know the information about thermal stability, the water molecule is present or absent inside or outside the coordination sphere of central metal ion and their thermal decomposition. TGA analysis was performed in a nitrogen inert atmosphere. In the first initial step complex started decomposing gradually till it attained the temperature of 170 o C, which demonstrates the loss of coordinated water molecule. After 200 o C anhydrous complex started decomposing till the temperature around 950 o C which corresponds to the elimination of the ligand molecule. The total mass loss was around 69 to 74%. The final residue left was around 26-31% which corresponds to the formation of cobalt oxide. The overlaid thermogram pattern (Figure 2) of complexes C1, C2 and C5 is almost comparable; it indicates that studied complexes start to reduce their weight at a reasonably high temperature which signifies that the presence of water molecule is inside the coordination sphere of cobalt ion.

EPR analysis
The EPR analysis of synthesized complexes was performed at room temperature. From the representative spectrum Figure 3, the calculated value of gǁǁ and g┴ are 2.0153 and 2.0066, respectively. The trend gǁǁ > g┴ > ge observed for the complex, it designates that the observed complex has octahedral geometry and unpaired electron lies in dx 2 -y 2 orbital.

Magnetic moments
Co(II) complexes are paramagnetic and exhibit magnetic moments at room temperature in the solid state. The Co(II) complexes showed the magnetic moments in the range 4.42-4.87 B.M., suggests three unpaired electron in octahedral environment.

Antimicrobial activity
The in-vitro antimicrobial activity of complexes and their ligands are presented in Table 3. The complexes C1 and C6 exhibit significant antibacterial activity against the pathogen Staphylococcus aureus, depicted the largest zone of inhibition 23.62 mm and 18.36 mm, respectively, which were even higher than the zone of inhibition 18.14 mm of bactericidal drug Ampicilin. The complexes C1, C4 and C5 demonstrate the significant antifungal activity against the pathogen Candida albicans, depicted the largest zone of inhibition 19.25 mm, 16.43 mm and 16.58 mm, respectively, which were higher than zone of inhibition 16.24 mm of fungicidal drug Fluconazole. However, all the complexes showed moderate antibacterial activity against the pathogen Escherichia coli, depicted the lower zone of inhibition with respect to bactericidal drug Ampicilin.  Consequently, the complexes C1, C4, C5 and C6 showed the even higher activity index with respect to strain and standard drugs, indicates significant potency. Activity data showed that inhibition found enhanced in all complexes than its respective ligand. Structurally similar activity data were reported in our previous study for Cu (II) and Zn(II) complexes of 2'-hydroxy chalcones [30]. The reason is that synthesized complexes contain di-dentate coordinating sites, their ligands associated with multiple halogen or hydroxyl substituents. These substituent supports to boost the pharmacological activity. The activity index (A.I.) was calculated using average zone of inhibition by following formula.  Table 4. The complex C1 was most sensitive, being inhibited at MIC value 0.12 mg/mL against all pathogens with respect to the standard drug Ampicilin and Flucanozole inhibited at MIC values 0.25 mg/mL and 1.0 mg/mL, respectively. The complexes C2, C5 and C6 were most potent, being inhibited at MIC values 0.12 mg/mL against the pathogen Escherichia coli with respect to the standard drug Ampicilin inhibited at MIC values 0.25 mg/mL. The complexes C2, C4 and C5 were the most resistant, being inhibited at MIC values 0.12 mg/mL against the pathogen Candida albicans with respect to the standard drug Flucanozole inhibited at MIC values 1.0 mg/mL. All complexes showed the improved MIC values than the respective ligands. The increased potency is due to the presence of halogens or hydroxyl substitutes in the ligands.   [29].

CONCLUSION
A series of bi-coordinated Co(II) complexes with 1,3-diaryl-2-propene-1-ones derivatives have been synthesized. All synthesized complexes have been spectrally characterised by IR, NMR, TGA, XRD and EPR analysis. The data suggested the octahedral geometry of Co(II) and bidentate ligands complexes with 1:2 stoichiometry. The in-vitro antimicrobial activity of complexes evaluated against the gram positive bacteria Staphylococcus aureus, gram negative bacteria Escherichia coli and the yeast Candida albicans. All these complexes showed much improved antimicrobial activity than its ligands associated with halogen and hydroxyl moiety. The complex C1 showed significant activity against all tested pathogens. The complexes C2, C5 and C6 showed significant activity against the Escherichia coli and Candida albicans and the complexes C4 showed significant activity against the Staphylococcus aureus and Candida albicans. The complex C1 exhibited the MIC value of 0.12 mg/mL against all pathogens, the complex C2 and C5 showed the MIC value of 0.12 mg/mL against Escherichia coli and Candida albicans, whereas the complex C4 showed the MIC value of 0.12 mg/mL against Staphylococcus aureus and Candida albicans. Hence, this synthetic methodology and antimicrobial results might serve as preliminary screening for the development of new antimicrobial agents with structural modification.