SYNTHESIS, CHARACTERIZATION AND BIOLOGICAL STUDIES OF NI (II) COMPLEXES WITH SCHIFF BASE CO-LIGAND DERIVED FROM 5,6-DIAMINO-1,10-PHENANTHROLINE AND BENZENE-1,4-DICARBALDEHYDE

The Schiff base (PDB) was synthesized by refluxing ethanolic solution of 5,6-diamino-1,10-phenanthroline with Benzene-1,4-dicarbaldehyde for 3 hours. Similarly the complexes were synthesized by refluxing equimolar solution of PDB with [Ni(phen)2Cl2] and [Ni(bpy)2Cl2] for 5 hours each and characterized by FTIR, UV-visible, Mass and Elemental Analysis. The absorption band in IR spectrum of PDB at 1593 cm was assigned to C=N stretching frequency which was shifted to 1550 cm and 1549 cm in the spectra of [Ni(phen)2PDB](PF6)2 and [Ni(bpy)2PDB](PF6)2 respectively. The new absorption bands at 721 cm and 754 cm in the spectra of both complexes were due to M-N coordinate bond. The UVvisible absorption band at 390 nm was due to * transitions for azomethine. The low energy band at 450 nm for both complexes is due to MLCT Ni(d) PDB(*) transition. The molecular ion peak (M) in the mass spectra for PDB and the complexes were observed at 443 m/z, 984.75 m/z and 940.95 m/z respectively. The compounds were evaluated for antimicrobial activities, by well diffusion method against bacteria using agar nutrient as the medium and fungi using potato dextrose agar as medium. The free Schiff base has low activity compared to the complexes with inhibition zone between 4.0 mm to 6.6 mm whereas the complexes ranges from 7.5 mm to 11.2 mm.


INTRODUCTION
Schiff bases are compounds containing imine or azomethine (-C=N-) functional group. Which are produced through the condensation reaction of primary amines with carbonyl-containing compounds (Fig.1) and they were firstly reported by Hugo Schiff (Khudheyer and Murad, 2018). Schiff bases are considered as privileged ligands, because of their ability to form complexes with a wide range of metal ions resulting in the formation of stable complexes (Ajlouni et al, 2018). Schiff bases bearing aryl or heterocyclic groups have also been shown to exhibit a broad range of biological activities, including, antibacterial and antifungal properties (Lashanizadegan and Jamshidbeigi, 2011).
Transition metals have initiated the development of metal based drugs with promising pharmacological applications and offer unique therapeutic opportunities. Research has shown significant progress in utilization of transition metal complexes as drugs to treat several human diseases like carcinomas, lymphomas, infections control, anti-inflammatory and neurological disorders. The recognition of Schiff base complexes as models for biologically active compounds has brought rapid advancement within the field of coordination and bio-inorganic chemistry and spawned extensive research on their synthesis and applications (Chohan and Sheazi, 1999). Metal complexes containing diimine ligands such as 1,10phenanthroline and its derivatives have gained importance because of their versatile roles as building blocks for the synthesis of metallo-dendrimers and as molecular scaffolding for supramolecular assemblies, and in analytical chemistry, catalysis, electrochemistry, ring-opening metathesis polymerization and biochemistry, (Mesut et al., 2013) Mesut et al., 2013, reported the synthesis and characterization of new Co(II), Ni(II) and Cu(II) complexes with 1,10-Phenanthroline imidazole derivative, which is 2-p-tolyl-1Himidazo[4,5-f][1,10] phenanthroline (L). Those compounds were screened for antibacterial activity against these bacterial strains; A. hydrophila, S. aureus, K. pneumoniae, P. aeruginosa, S. marcescens, E. aerogenes, B. subtilis, E. coli and E. faecalis. This research is aimed at the synthesis and characterization of Nickel (II) complexes derived from [Ni(phen)2Cl2]. 2H2O and [Ni(bpy)2Cl2].2H2O and to evaluate their antibacterial and antifungal activities. The proposed targets may result in the development of drugs with increased cytotoxicity compared to commercially available drugs.

EXPERIMENTAL MATERIALS AND METHODS
All chemicals were obtained from Sigma-Aldrich and used without purification. Tetrabutyl ammonium chloride (TBACl) and palladium on activated charcoal 10%Pd/C were purchased from E. Merck (India). All the reactions were monitored by checking TLC of the reaction mixture. The complexes were purified by column chromatography. The ligand and the complexes were characterized by standard analytical techniques (FT-IR, Mass, UV-visible spectroscopy and Elemental analysis).

Preparation of the starting materials
The following precursor molecules that are necessary for the synthesis of new Schiff base used in this study have been prepared by adopting the published procedures.

Preparation of Precursor Complexes
The following precursor complexes that are necessary for the synthesis of new complexes in this study have been prepared by adopting published procedures.

Molar Conductivity Measurements
The molar conductance of the complexes was measured at room temperature in DMF using 0.001 molL -1 solution of both complexes.

Antimicrobial activity
The ligand PDB and its Ni(II) complexes were evaluated for antimicrobial activity by the well diffusion method against the bacteria Salmonella typhi, Pseudomonas aeruginosa, Escherichia coli and Staphylococcus aureus using agar nutrient as the medium and antifungal activities against the fungi Aspergillus niger, Aspergillus flavus and Rhizoctonia bataicola cultured on potato dextrose agar as medium. The stock solution (10 -3 molL -1 ) was prepared by dissolving the compounds in DMSO. The antimicrobial activities were performed in triplicate and the average was taken as the final reading. The well was made on the agar medium inoculated with microorganisms and filled with the test solution. The plate was incubated for 24 hours for bacteria and 72 hours for fungi at 35 °C. During this period, the test solution was diffused and the growths of the inoculated microorganisms were affected. The inhibition zone was developed and it was measured in mm. Zone of inhibition of the investigated compounds against the bacteria and fungi are summarized in Table 4 and 5. Streptomycin and Ketoconazole were used as standard reference compounds for antibacterial and antifungal studies respectively (Sheikh et al., 2004). The Schiff base has a percentage yield of 78%, whereas the two red and orange crystalline powdered complexes were obtained in appreciable yield of 71% and 69% respectively. Both two complexes [Ni(phen)2PDB]Cl2 and [Ni(bpy)2PDB]Cl2 are electrolytic in nature having molar conductance values of 97 Ω -1 cm 2 mol -1 and 89 Ω -1 cm 2 mol -1 respectively, which are in good agreement with similar complexes reported in literature.   . 3, September, 2020, pp 132 -141 attached to the phenanthroline moiety. The high-energy bands (PDB: 246 nm and 276 nm) are attributed to the * transitions corresponding to the phenanthroline moiety of the ligand. The low energy band at 450 nm for both complexes is due to MLCT Ni(d) PDB(*) transition. The band centered at 280 nm and 275 nm for [Ni(phen)2(PDB)](PF6)2 and [Ni(bpy)2(PDB)](PF6)2 are attributed to intra-ligand* transitions. All the electronic transitions were found to be similar to those reported by Suma et al., (2012).

Mass Spectra for Schiff base and Ni(II) Complexes
The mass spectrum of the Schiff base showed its molecular ion peak at 443 m/z also a base-peak at 211 m/z is observed due to C12H6N4(M -C14H10O2). In the case of corresponding mixedligand Ni(II) complexes the molecular ion peak for the complex([Ni(phen)2(PDB)]PF6) + were seen at 984.75 m/z (M) + . Similarly the molecular ion peak for the complex ([Ni(bpy)2(PDB)]PF6) + was obtained at 940.95 m/z (M) + which are in good agreement with the complexes obtained and reported by Suma et al., (2012).