Synthesis , Characterization , and Antibacterial Activity of Co ( II ) , Ni ( II ) , Cu ( II ) , Zn ( II ) , Cd ( II ) , and Hg ( II ) Complexes of Schiff ’ s Base Type Ligands Containing Benzofuran Moiety

1 Department of Chemistry, S. D. M. College of Engineering and Technology, Dharwad 580002, India 2Department of Chemistry, Sri Krishna Institute of Technology, Bangalore 560 090, India 3 Department of Chemistry, M. S. Ramaiah Institute of Technology, Bangalore 560 054, India 4Department of Chemistry, Inje University, Kimhae 621749, Republic of Korea 5 Department of Chemistry, J. S. S. Academy of Technical Education, Bangalore 560060, India


Introduction
Schiff 's bases are the most widely studied chelating ligands in coordination chemistry [1].They are useful in catalysis in organic synthesis and in medicine as antibiotics, antiallergic, and antitumor agents [2].Recently metal complexes of Schiff 's bases particularly derived from carbonyl compounds based on heterocyclic rings have been the centre of attraction in many areas [3][4][5][6][7].Among them, benzofuran based fused heterocycles have been of great interest as they are abundant in nature and have wide pharmacological activities [8].Benzofuran based compounds have been reported to show activities as anti-infective agents, like antifungal [9][10][11][12], antiprotozoal, and antitubercular [13][14][15], and also in the treatment of antiarrhythmic [16] and cardiovascular [17] diseases.According to the literature survey in the recent years, there has been an increased interest investigation of anti-microbacterial activities on benzofuran derivatives, especially 2-substituted [18] or 2,3-disubstituted benzofurans derivatives [19].It is due to the presence of benzofuran derivatives in natural compounds.For example, the seed oil of the Egonoki plant, which contains a benzofuran derivative called egonal (Figure 1(a)), is an effective synergist for rotenone and pyrethrum against house flies, mosquitoes, aphides, and many other insects [20].Similarly, Baker's yeast contains a benzofuran derivative (Figure 1(b)) that acts as an antioxidant preventing hemorrhagic liver necrosis in rats and hemolysis of red cells in Vitamin-E deficient rats [21].

Preparation of Complexes.
Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) complexes, were prepared by mixing calculated quantity of L1 and L2 in ethanol and an aqueous solution of the corresponding metal chlorides in 2 : 1 molar ratio.The reaction mixture was refluxed on a water bath for 3-4 hrs.The completion of the reaction was monitored by thin layer chromatography.Once the reaction was completed, the solvent was removed approximately 50%, from hot solution.Then the residue was cooled to room temperature.The solid complexes formed were filtered, washed with hot water (10 mL, 4-5 times) and ethyl alcohol (10 mL, 3-4 times), and finally dried in vacuum desiccators over anhydrous CaCl 2 .The formation of the complexes may be represented by the following equations: (1)

Physical Measurements
Elemental analyses were carried out using Vario EL CHNS analyzer.Magnetic susceptibility measurements were carried out on a magnetic susceptibility balance (Sherwood scientific, Cambridge, England).Molar conductance measurements were made on an Elico CM-82 conductivity bridge in DMF (10 −3 M) using a dip-type conductivity cell fitted with a platinum electrode having cell constant 0.1 Scm −1 .The ESR spectra of complexes were recorded on a Varian E-122 Xband spectrophotometer at liquid nitrogen temperature in DMSO.The FT-IR spectra of ligands and their complexes were recorded as KBr discs in the range 4000-350 cm −1 on Shimadzu FT-IR spectrophotometer. 1 H NMR spectra were recorded in DMSO-d 6 on a Bruker 300 MHz spectrophotometer using TMS as an internal standard.The electronic spectra were recorded on an Elico-SL-159 single beam UV-Vis.spectrophotometer in the range of 200-1100 nm in N, N-dimethyl formamide (DMF) (10 −3 M) solution.FAB mass spectra were recorded on a JEOL SX 102/DA-6000 mass spectrophotometer from CDRI Lucknow, 6 KV, and 10 mA using argon as the FAB gas and m-nitro benzyl alcohol as the matrix.

Antibacterial Activity
In vitro antibacterial activity of the newly synthesized compounds were investigated against five Gram-positive bacteria species Staphylococcus aureus, Staphylococcus citreus, Bacillus polymyx, Bacillus cereus, and Lactobacillus and five Gramnegative species Proteus mirabilis, Klebsiella pneumonia, E. coli, Salmonella typhi, and Pseudomonas aeruginosa by the agar well diffusion method [26,27].Nutrient agar (Hi Media, India) was used as the bacteriological medium.The extracts were dissolved in 10% aqueous dimethylsulfoxide (DMSO) to a final concentration of 50 g/L.Pure DMSO was taken as the control.100 L of inoculums was aseptically introduced on to the surface of sterile agar plates and sterilized cotton swabs were used for even distribution of the inoculums.Wells were prepared in the agar plates using a sterile cork borer of 6.0 mm diameter.50 L of test and control compound was introduced in the well.The same procedure was used for all the strains.The plates were incubated aerobically at 35 ∘ C and examined after 24 hours.The diameter of the zone of inhibition produced by each agent was measured with a ruler.

Characterization of Ligands L1 and L2. The ligands L1
and L2 are air stable crystalline solids.The analytical data are given in Table 1.The FT-IR spectrum of ligands L1 and L2 showed strong bands at 3354 and 3185 cm −1 due to  asy (NH) and ] sy (NH) stretching vibrations, respectively, of secondary amide.Strong bands observed at 1670 and 1702 cm −1 in free ligands are assigned to ](C=O) of -CONH group which matches with the literature reports [22].Medium to strong intense bands at 1606 and 1596 cm −1 in L1 and L2 were assigned to ](C=N) of the azomethine group [28].The medium bands at 942 and 958 cm −1 were assigned to ](N-N) stretching vibrations of hydrazine and ](C-O-C) stretching vibrations of furan was observed in the region of 1029-1125 cm −1 [29,30]. 1 H NMR spectra of ligands L1 and L2 were recorded in DMSO-d 6 .The singlet at , 12.00-12.10ppm was assigned to amide proton (-CONH-).The singlet at , 8.30-8.65 ppm were assigned to azomethine (-HC=N-) proton in both ligands.The aromatic ring protons were appeared as multiplet at , 7.0-7.7 ppm in the 1 H NMR spectra.The 1 H NMR spectrum of L1 is shown in Figure 2.
The mass spectrometric studies of L1 and L2 were performed.The mass spectra of the ligands showed molecular ion peaks (M + ) at 383 and 363, respectively, are the corresponding molecular weights of both L1 and L2. 1) of the complexes, 1-6 indicated that all the complexes were of stoichiometry, [M(L1/L2)Cl  ], where M = Co(II) [1], Ni(II) [2], Cu(II) [3], Zn(II) [4], Cd(II) [5], and Hg(II) [6].The molar conductivity values of the complexes in DMF (10 −3 M) are in the range 13.8-20.3Ω −1 cm 2 mol −1 suggested non-electrolytic nature of the complexes [31].All the complexes are partially or almost insoluble in common organic solvents, but soluble in DMF, DMSO, and pyridine.Halli et al. [23] coordination of NH results in splitting or shifting of bands to lower frequency side or decrease in intensity.

Characterization of the Metal Complexes. The analytical data (Table
In the present study no such changes were observed, instead ](NH) bands were observed at higher wave number indicating non-participation of the coordination of NH with metal ion.The shift of about 15-40 cm −1 of ](C=O) band to lower region in complexes was observed indicating participation of the carbonyl oxygen in coordination [34].In free ligands at ](C=N) of the azomethine group is observed at 1606 cm −1 and 1596 cm −1 , which was shifted to lower region by 20-52 cm −1 which indicated the involvement of the azomethine N in dative bonding with metal ions [35].The medium bands in ligands due to ](N-N) stretching vibrations are shifted to higher wave number region confirming the involvement of one of the nitrogens of -N-N-in bonding with metal ions.The stretching vibration due to ](C-O-C) of furan remains unaltered in the metal complexes, indicating nonparticipation of the O-furan ring in bonding with metal ions.Metal-ligand vibrations are generally observed in the far-IR region and usually give valuable information regarding the bonding of ligands to the metal ions.The weak non-ligand bands observed in the spectrum in the regions 515-575 cm −1 , 425-475 cm −1 , and 376-410 cm −1 are assigned to ](M-O), ](M-N), and ](M-Cl) stretching vibrations, respectively [36].The electronic spectra of the Cu(II), Co(II), and Ni(II) complexes were recorded in DMF (10 −3 M) solution at room temperature and spectral data are shown in Table 3.
The six coordinate Ni(II) complexes exhibit bands at 15342, 25810 cm −1 and 15530, 25637 cm −1 assignable to 3 A 2g → 3 T 1g (F)(] 2 ) and 3 A 2g → 3 T 1g (P)(] 3 ) transition, respectively.The high energy band in the region 29000-30000 cm −1 is assigned to charge transfer in the case of Co(II) and Ni(II) complexes.The octahedral geometry is further supported by ligand field parameters like ,  1 , , %, ] 2 /] 1 ratio, and LFSE value.The  1 values for the complexes were lower than the free ion values, which is an indication of the orbital overlap and delocalization of d-orbitals.The  values obtained were less than unity, suggesting a considerable amount of covalency for the metal-ligand bonds.The  value for the Ni(II) complex was less than that of the Co(II) complex, indicating the greater covalency of the M-L bond [24].
For the Co(II) complex the Racah inter-electronic repulsion parameter ( 1 ) is calculated using the following equation: while for Ni(II) complexes  and B are calculated from The covalency factor () was obtained in the following manner: , (where  is free ion value) .
The light green Cu(II) complexes exhibit a single broad asymmetric band in the region 18678-12345 cm −1 .The broadness of the band indicates the three transitions, 2 B 1g → 2 A 1g (] 1 ), 2 B 1g → 2 B 2g (] 2 ), and 2 B 1g → 2 E g (] 3 ), which are similar in energy and give rise to only one broad band.The high intensity band observed in the region 33,000-28,000 cm −1 was assigned to charge transfer [38].The broadness of the band may be due to dynamic John-Teller distortion.All of these data suggest a distorted octahedral geometry around Cu(II).Typical electronic spectrum of Cu(II) complex of L1 is shown in the Figure 3.
ESR spectra of the copper (II) complexes were recorded at liquid nitrogen temperature in DMSO.The  || and  ⊥ values were calculated from the spectrum using the tetracyanoethylene (TCNE) free radical as the "" marker.Typical ESR spectrum of [Cu(L1)Cl 2 ] is given in Figure 4. Neiman and Kivelson [39] have reported that  || is less than 2.3 for covalent character and greater than 2.3 for ionic character of the metal-ligand bond in complexes.As seen in Table 4,  || values for the present complexes are slightly higher than 2.3 suggesting a small amount of ionic character of the metal-ligand bond.The trend  || >  ⊥ > 2.0023 suggests that the unpaired electron lies predominantly in the   2 − 2 orbital [40] characteristic of square planar or octahedral geometry in copper(II) complexes [41].The  av value for these complexes is greater than 2 indicating the presence of covalent character [42].The axial symmetry parameter () values of the complexes (greater than 4) suggest that there are no interactions between the copper centres in DMSO medium.The empirical factor  (=  || / || ) cm −1 is a measure of deviation from idealized geometry [43].Values from 165 to 171 cm −1 for the present complexes suggest a moderate to considerable distortion in the geometry.
Based on the analytical data and spectral characterization of the compounds tentative structures of the complexes are shown in Figures 5(a

Antibacterial Activities
Antibacterial activity of substituted benzofuran derivatives and their metal complexes were tested in vitro against representative Gram-positive bacteria species like "Staphylococcus  aureus," "Staphylococcus citreus, " "Bacillus polymyx, " "Bacillus cereus, " and "Lactobacillus" and Gram-negative bacteria species such as "Proteus mirabilis", "Klebsiella pneumonia, " "E.coli, " "Salmonella typhi, " and "Pseudomonas aeruginosa" by agar well diffusion method.Compounds inhibiting growth of one or both microorganisms were further tested for their minimum inhibitory concentration (MIC) of the compound.Benzofuran ligands are more active against all most all microbes.Among all the complexes [Cu(L2)Cl 2 ], [Ni(L2)Cl 2 ], and [Co(L2)Cl 2 ] complexes showed very good activity against all organisms (MIC = 25 g/mL), whereas zinc and cadmium complexes are moderately active, but mercury complexes are least active.Least actitivity of mercury may be due to its toxic nature towards microbes.However, the synthesized compounds showed relatively higher or lower activ than the standard drug Streptomycin.It may be due the nature of the metal ion, the nature of the ligand, and orientation of the ligand around the metal ion.The results are summarized in Table 5.

Conclusion
Based on stoichiometries and analytical data of the ligands, (E)-

Figure 1 :
Figure 1: Structures of the benzofuran derivatives in (a) Egonoki plant and (b) Baker's yeast.

Figure 3 :Figure 4 :
Figure 3: A typical electronic spectrum of copper complex of L1.

Table 1 :
Analytical data of ligands and their metal complexes.

Table 2 :
Important IR spectral bands (cm −1 ) of ligands and their metal complexes.

Table 3 :
Electronic spectral bands and ligand field parameters of the Cu(II), Co(II), and Ni(II) complexes in DMF (10 −3 ) solution.

Table 5 :
Antibacterial activity of ligands and their metal complexes.