Synthesis, Physicochemical, Thermal, Electrical and Antibacterial Studies of Transition Metal Chelates of Bistridentate Schiff Base Derived From 2-4 Dyhydroxy 5- Acetyl Acetophenone And 2-Aminothiophenol

The chemistry of transition metal complexes including ordinary complexes, polymers and mixed ligand complexes has been extensively studied till date for their biological importance as well as a wide range of physicochemical properties [1]. A variety of ligands in complexes has enabled the making of the properties of the metal complexes originating due to ligands. Transition metal complexes are a class of compounds which can have properties that are not present in ordinary compounds [2, 3]. However, synthetic route leading to metal complexes with pre-established structures and properties has always remained a challenge for chemists. Schiff bases are often used as ligands in coordination chemistry to form metal complexes [4, 5] owing to their metal binding ability. Schiff base ligands such as N₂O₂, NNNN and ONO form stable complexes with metal ions due to the close proximity of the donor sites. Numerous transition metal complexes derived from Schiff bases have been studied for the interesting and important properties, such as biological activity, catalytic activity in hydrogenation of olefins, transfer of an amino group, photochromic properties and complexation ability towards some toxic metals [6-8] and in the synthesis of important drugs, such as antibiotic, antiallergic, anticancer and antitumor [9, 10]. The aim of our present study is to synthesize and to introduce the Schiff base which engaged in the complexation by involving Ni (II) Cu (II), and Zn metal ions. The characterization of Schiff base ligand and their complexes have done which prompted us to carry such type of work keeping in view of interesting streochemical and biological aspects. Thus in this present study synthesis and characterization of Ni (II) Cu (II), and Zn (II) complexes with substituted acetophenone and aminothiophenol are described.

Experimental

All the chemicals used as starting materials for the synthesis of ligand and its metal complexes were of analytical grades and used as received. All the required solvent were purified and dried before use according to the standard method [11].

Synthesis of 2-4 dihydroxy 5-acetyl 2-amino thiophenol (DHAATP) ligands:

A hot ethanolic solution of 2,4-dihydroxy-5-acetyl acetophenone (DHA) (3.88 gm, 0.02 mol) was mixed with an ethanolic solution of 2-aminothiophenol (2.18 am, 0.02 mol) and the reaction mixture was refluxed for about 8 h. The reaction mix was cooled at room temperature and the golden brown coloured product obtained was filtered, washed with ethanol, petroleum ether and dried in vacuo. Yield. 79%, m.p.253 ⁰C

NMR (CDCl₃ + DMSO) δ 12.15 (2H, s, phenolic OH), δ 11.10 (2H. s, imino N-H),δ 2.8 (6H, s, methyl), 6.47 and 7.92 ppm (2H,s,Phenyl),δ 3.90 (1H, s, thiophenolic),7.3 ppm (2h, m, Phenyl),7.6 And 7.1 ppm (2H, m, phenyl)

Synthesis of Ni (II), Cu (II) and Zn (II) Complexes

The metal complex of Ni (II), Cu (II) and Zn (II) were prepared by following general method. Equimolar quantities (0.01 mol) of metal salts and ligand were dissolved separately in hot ethanol and DMF respectively. These clear solutions were mixed in hot condition with constant stirring. The reaction mixture was refluxed on sand bath for 4-5h.The progress of the reaction was signaled by color change of the resulting solution. The solid products are precipitate, filtered, washed with ethanol and DMF and dried over anhydrous CaCl₂ in a desiccator.

Biological Study

The antibacterial activities of the ligand and the complexes have been carried out against the bacteria Staphalococcus aureus, Bacillus Subtilis, Salmonella typhimurium, P. aeruginosa and Escherichia coli using nutrient agar medium by the disc diffusion method. Solutions of 100, 200 and 300 ppm of the compounds in DMSO were used for the studies. These discs were placed on the already seeded plates and incubated at 35⁰C for 24h. The diameter (mm) of the inhibition zone around each disc was measured after 24h.

Physical measurements

 The estimation of carbon, hydrogen and nitrogen were obtained on a Carlo-Erba 1108 C-H-N-analyzer at micro analytical unit SAIF, CDRI, Luknow. The IR spectra were recorded in KBr pellets on a Perkin-Elmer-1600 FT-IR spectrophotometer. The reflectance spectra of the complexes were recorded on a Carry-2390 spectrophotometer using BaSO₄ as a dilutant and MgO as a reference in the range 200-1500 nm at SAIF, IIT Chennai. ¹H-NMR spectrum of the ligand was recorded in a mixed solvent (CDCl₃ + DMSO) on a bruker AC-200F, 300MHZ, NMR spectrometer using TMS as an internal standard at RSIC- Punjab university, Chandigarh. Magnetic measurements were carried out at room temperature using Gouy?s method using Hg [Co (SCN)₄] as calibrant and values were corrected for diamagnetism by using Pascal?s constant. Thermogravimetric analyses of the complexes were carried out using a TGS-2Perkin Elmer thermal analyzer in the temperature range 50-700 ^oC at a heating rate of 10 ^oC min^-1. Electrical conductivity of the complexes was measured in their compressed pellet form over a range of 313-413 K temperature using Zentech electrometer. Antibacterial and antifungal activities of the ligand and their complexes were carried out against the bacteria, E. coli, S. abony, S. aureus, P. aeruginosa and B. subtilis by disc diffusion method. SEM image measurements of complex were recorded at SAIF, Kochi, India.

RESULTS AND DISCUSSIONS

The reaction of 2,4-dihydroxy-5-acetyl acetophenone and 2-aminothiophenol gives the desired symmetrical tetradentate Schiff base ligand in high yields and purity. The physical characterization and micro analytical data of ligand and its coordination complexes are given in (table 1). All the complexes are coloured solids, air stable and are having line solubility in polar solvents DMF and DMSO. The elemental analysis shown in table 1 indicates that all these complexes have 1:1 metal: ligand stoichiometry.

Electronic spectra and magnetic studies

The electronic spectrum of Ni (II) complex consisted of three bands at 10319, 17543 cm^-1 and 24937 cm^-1  assignable to the  transitions ³A_2g ? ³T_2g, ³A_2g (F) ?³T_2g and ³A_2g ? ³T_1g, (P),  respectively corresponds to octahedral geometry around the metal ion [15]. The observed magnetic moment value of Ni (II) complex 2.85 B. M. supported high-spin octahedral geometry for Ni(II) complex. The magnetic moment for Cu (II) complex was found to be 1.86 B.M. at room temperature corresponding to one unpaired electron [15]. The electronic spectrum of Cu (II) complex displayed bands at 17211, 19074 and 20920 cm^-1 as signed for the ²B_1g ? ²A_1g, ²B_1g  ? ²B_2g and ²B_1g  ? ²E_g  transitions, respectively. The electronic transitions and magnetic moment value suggests pseudo octahedral geometry around Cu (II) ion [16, 17]. The Zn (II) complex does not show any d-d bands and are found to be diamagnetic, as expected for d¹⁰ system. On the basis of literature survey, that the Zn (II) complex have tetrahedral geometry [18, 19].

Infrared Spectra

The analysis of IR spectra of DHAATP and its metal complexes are summaries in table 2 which are found to be comparable with each other.  The Schiff base ligand exhibit band at 2980cm^-1 due to the intramolecular hydrogen bonding n(OH) group [20]. The absence of this band in the spectra of complexes indicates the deprotonation of the phenolic n(O-H) group and coordination of oxygen atom to the metal ion. IR spectra show strong band at 1620 cm^-1 due to n(C=N) azomethine mode, which shifted to lower range by 33-27 cm^-1 in the complexes indicating coordination through azomelhine nitrogen to the metal ion. The negative shift may be attributed to the formation of coordinate bond by donation of lone pair of electrons from nitrogen to the metal ion thereby decreasing electron density on nitrogen and it therefore, withdraw electrons from C=N (Pi bond). As a consequence, double bond character decreases and energy required to stretch the bond also decreases.  Therefore red shift is occurs.  The upward shift of n(C-O) (phenolic) band at 1275 cm^-1 of the ligand by 10-24 cm^-1 and the appearance of new band at 580-513 cm^-1 in the spectra of all the complexes is observed which further supports the involvement of phenolic oxygen in coordination. The ν(S-H) stretching vibration of amino thiophenol is not useful since it display very weak band in the ligand and complex spectra.  However the participation of the ν(S-H) group in chelation is ascertained from the shift of the n_asy(C-S) and n_asy(C-S) from 705-748 cm^-1 [21] to lower or higher wavelength in the spectra of the complexes.  New bands are found in the spectra of the complexes in the region 513-580 cm^-1 which are assignable to n(M-O) stretching vibration for metal complexes. The band at 418-431cm^-1 assigned to (M-S) thiophenol [22]. For metal complexes a strong new band is observed at 472-430 cm^-1 which is attributed to n(M-N) vibrations. The IR spectra of all complexes show bands for the presence of water molecules.  A broad and strong band centered in the region 3300-3500 cm^-1 and another sharp shoulder in the region 1650-1500 cm^-1 may be assigned to n(OH) and of (HOH) vibrations of water molecules respectively [23].  The additional strong band at 878-820 cm^-1 arising due to ν(OH) rocking vibration in the spectra of  the complexes further confirms the coordinated water molecules.

TGA Analysis

The analysis of thermograms of DHAATP and its metal complexes (fig.1) reveals a two stage decomposition pattern for all the synthesized metal complexes. The ligand DHAATP and the metal complexes of Ni (II), Cu (II), and Zn (II) complexes show no weight loss upto 130⁰C revealing the absence of lattice water molecule. The TG curve show weight-loss upto 235⁰C corresponding to loss of two coordinated water molecules in the complexes of Ni (II), Cu (II), and Zn (II) [% wt. loss, obs./calcd.; Ni(II) : 7.24/7.15 ;  Cu(II): 7.15/7.09   and Zn(II) : 7.11/7.06  respectively. The thermograms of Ni (II) and Cu (II) show gradual weight loss upto the range 285 ⁰C corresponding to the loss of one acetate group[% wt. loss, obs./calcd.; Ni(II) : 5.35/5.08 ;  Cu(II): 5.88/5.57 ; In complexes of Ni (II) weight-loss have been observed due to the presence of one methanol group supported by IR spectral data. In all complexes, gradual but continues weight-loss observed beyond 355 ⁰C corresponding to the thermal degradation of coordinated ligand molecules. The horizontal curve are obtained in the temperature range 610-680 ⁰C indicating the formation of metal oxide. The thermal stability of metal complexes was found to increase periodically with increase in atomic number of the metal and large value of charge/radius ratio [24]. The thermogravimetric analysis has proved to be useful analytical technique in evaluating thermodynamic activation parameters of the complexes upon thermal decomposition process, such as energy of activation (Ea), frequency factor (Z) and entropy change (-ΔS) by employing Coats-Redfern method [25] and values obtained are given in table.3. The half decomposition temperature of the compounds decreases in the order:   

DHAATP > Ni (II) > Cu (II) > Zn (II)

Electrical conductivity

The temperature dependence of electrical conductivity of DHAATP and its complexes reveals a linear trends fig. 2 and data is cited in table 1 from the result it is observed that the electrical conductivity of DHAATP and its complexes show a tendency to increase by raising the temperature up to 403K. The electrical conductivity of these complexes lies in the range 2.17 x 10^-8 to 4.60 x 10^-10 W-1cm-1 at 373K. The activation energy of electrical conduction of the complexes is found to be in the range 0.280 to 0.610 eV. The order of electrical conductivity of HMACHZ and their complexes at 373K is found to be Zn (II) > Cu (II) > Ni (II).

Biological study

The ligand DHAATP and its complexes are found to show inhibitory activity against all the gram positive and gram negative bacterial strains (fig. 3 and table 4). The complexes of Ni (II), Zn (II) and ligand DHAATP show moderate antibacterial activity. The Cu (II) show maximum inhibitory activity against S. aureus. Ni (II) show maximum activity against the S. typhi (gram positive bacteria), Ni(II) and Zn(II) show moderate antibacterial activity against S. Typhi. All the metal complexes show moderate antibacterial activity against the P. Aeruginosa (gram negative bacteria) and B. Subtilis (gram positive bacterial strain). It can be concluded that the ligand DHAATP and its complexes are bioactive against all the bacterial strains. The increase or decrease of antimicrobial activity of ligands upon coordination may be due to structural change in functional group responsible for antimicrobial action. The antimicrobial effect varies with metal ion as well as organism. Similar correlation and increase in antibacterial activity of ligands on complexation with reference to parent ligands has been well cited. Such increased activity of the metal chelates can be explained on the basis of overtone concept and Tweedy Chelation, theory [26]. According to overtone concept of cell permeability, the lipid membrane that surrounds the cell favors the passage of only lipid soluble materials due to which liposolubility is an important factor that controls antimicrobial activity. On chelation the polarity of metal ion is reduced to a greater extent due to the overlap of the ligand. Orbital and partial sharing of the positive charge of the metal ion with donar group. Further, it increases the delocalization of electrons over the whole chelates ring and enhances lipophilicity of the complex. The increased lipophilicitiy of complexes permit easy penetration into lipid membranes of organisms and facilitates as blockage of metal binding site in enzymes.

ACKNOWLEDGEMENT

The authors wish to thank SAIF Chandigarh and CDRI Lucknow for recording elemental analyses, IR, NMR and electronic spectra, SAIF, Kochi for SEM image measurements and Sant Gadge Baba Amravati University, Amravati for providing the laboratory facility.

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Figure 1: Thermogram of DHAATP and its complexes