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International Journal of Inorganic Chemistry
Volume 2013 (2013), Article ID 614628, 10 pages
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
1Department 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
3Department of Chemistry, M. S. Ramaiah Institute of Technology, Bangalore 560 054, India
4Department of Chemistry, Inje University, Kimhae 621749, Republic of Korea
5Department of Chemistry, J. S. S. Academy of Technical Education, Bangalore 560060, India
Received 17 December 2012; Revised 10 February 2013; Accepted 26 February 2013
Academic Editor: Alfonso Castiñeiras
Copyright © 2013 N. Shashidhar Reddy et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Six new complexes of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) with substituted benzofuran derivatives have been synthesized and characterized by elemental analysis, magnetic moments, conductance measurements, spectral characterization, and so forth. Elemental data coincide with the general formula , where L = (E)-7-Methoxy-N1-(2,4,5-trimethoxy benzylidene) benzofuran-2-carbohydrazide (L1) or (E)-N1-(2,6-dichloro benzylidene)-7-methoxy benzofuran-2-carbohydrazide (L2), of the complexes. The ligands coordinate to the metal ions through the oxygen of the carbonyl group and the nitrogen of the hydrazine group. Electronic spectral data of the complexes suggests the probable geometry is octahedral in nature. All the complexes and ligands were screened for their antibacterial activity. Among them, Co, Ni, and Cu complexes of L2 showed good activity against all microbes.
Schiff’s bases are the most widely studied chelating ligands in coordination chemistry . They are useful in catalysis in organic synthesis and in medicine as antibiotics, antiallergic, and antitumor agents . 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 [3–7]. Among them, benzofuran based fused heterocycles have been of great interest as they are abundant in nature and have wide pharmacological activities . Benzofuran based compounds have been reported to show activities as anti-infective agents, like antifungal [9–12], antiprotozoal, and antitubercular [13–15], and also in the treatment of antiarrhythmic  and cardiovascular  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  or 2,3-disubstituted benzofurans derivatives . 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 . 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 .
Therefore we thought it is worthwhile to synthesis and characterize the benzofuran based ligands and their transition metal complexes and investigate their antimicrobial properties.
As a part of our ongoing study on benzofuran derivatives [6, 7, 22–24], we report herein the synthesis, characterization, and antibacterial activity of (E)-7-Methoxy-N1-(2,4,5-trimethoxy benzylidene) benzofuran-2-carbohydrazide (L1) and (E)-N1-(2,6-dichloro benzylidene)-7-methoxy benzofuran-2-carbohydrazide (L2) and their metal complexes such as Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) as shown in Scheme 1.
2,4,5-Trimethoxybenzaldehyde, 2,6-dichlorobenzaldehyde, and 2-hydroxy-3-methoxybenzaldehyde were of reagent grade purchased from Sigma-Aldrich. All other chemicals were of AR grade and used as supplied. The solvents were dried and distilled before use. The starting material, 7-methoxy-1-benzofuran-2-carbohydrazide, was prepared according to the literature method .
2.1. Preparation of (E)-7-Methoxy-N1-(2,4,5-trimethoxybenzylidene)benzofuran-2-carbohydrazide (L1)
To a solution (25 mL) of 7-methoxy-1-benzofuran-2-carbohydrazide (2.5 g, 0.01 mol) dry ethanol (25 mL), a solution of 2,4,5-trimethoxybenzaldehyde (2.378 g, 0.01 mol) in dry ethanol (10 mL) was added. The reaction mixture was refluxed on a water bath. A light yellow crystalline product formed during 7-8 hrs; the reaction mass was cooled and collected by filtration. The solid was washed several times with hot water (10 mL × 5) and then with diethyl ether (10 mL × 3) and finally dried in vacuo. The obtained ligand (L1) was recrystallized from dry ethanol. Mol. Formula, C20H20N2O6: Mol. Wt. 383.378, M.P. 252°C, Yield: 75%.
2.2. Preparation of (E)-N1-(2,6-Dichlorobenzylidene)-7-methoxybenzofuran-2-carbohydrazide (L2)
To an ethanolic solution (25 mL) of 7-methoxy-1-benzofuran-2-carbohydrazide (2.5 g, 0.01 mol), 2,6-dichlorobenzaldehyde (2.122 g, 0.01 mol) in dry ethanol (10 mL) was added. The reaction mixture was refluxed on a water bath for 7-8 hrs. A light yellow crystalline solid formed. The reaction mixture was cooled and the solid was collected by filtration. This solid was washed with hot water ( mL) and then with diethyl ether and then dried in vacuo. A crystalline solid was obtained by recrystallization from ethanol. Mol. Formula, C17H12N2O3: Mol. Wt. 363.198, M.P. 130°C, Yield: 80%.
2.3. 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 CaCl2. The formation of the complexes may be represented by the following equations:
3. 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 X-band 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. 1H NMR spectra were recorded in DMSO-d6 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.
4. 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 Gram-negative 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 μg/μL. Pure DMSO was taken as the control. 100 μL 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 μL 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.
5. Results and Discussion
5.1. 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 due to (NH) and (NH) stretching vibrations, respectively, of secondary amide. Strong bands observed at 1670 and 1702 cm−1 in free ligands are assigned to ν(C=O) of -CONH group which matches with the literature reports . Medium to strong intense bands at 1606 and 1596 cm−1 in L1 and L2 were assigned to ν(C=N) of the azomethine group . The medium bands at 942 and 958 cm−1 were assigned to ν(N–N) stretching vibrations of hydrazine and ν(C–O–C) stretching vibrations of furan was observed in the region of 1029–1125 cm−1 [29, 30]. 1H NMR spectra of ligands L1 and L2 were recorded in DMSO-d6. The singlet at δ, 12.00–12.10 ppm was assigned to amide proton (–CONH–). The singlet at δ, 8.30–8.65 ppm were assigned to azomethine (-HC=N-) proton in both ligands. The aromatic ring protons were appeared as multiplet at δ, 7.0–7.7 ppm in the 1H NMR spectra. The 1H 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.
5.2. Characterization of the Metal Complexes
The analytical data (Table 1) of the complexes, 1–6 indicated that all the complexes were of stoichiometry, [M(L1/L2)Cln], where M = Co(II) , Ni(II) , Cu(II) , Zn(II) , Cd(II) , and Hg(II) . The molar conductivity values of the complexes in DMF (10-3 M) are in the range 13.8–20.3 Ω−1 cm2 mol−1 suggested non-electrolytic nature of the complexes . All the complexes are partially or almost insoluble in common organic solvents, but soluble in DMF, DMSO, and pyridine. Room temperature magnetic susceptibility measurements indicated paramagnetic nature for Co, Cu, and Ni complexes. The six-coordinate Co(II) complexes exhibit magnetic moments of 4.68 and 4.87 B.M., suggesting octahedral geometry for Co(II) . Ni(II) complexes showed magnetic moment values of 2.85 and 2.92 B.M., slightly higher than the spin only (2.83 B.M.) value, indicating an octahedral environment around Ni(II) ion . The observed magnetic moments for Cu(II) complexes are 1.74 and 1.79 B.M., suggesting a distorted octahedral geometry around Cu(II). FT-IR spectra of ligands and their metal complexes are presented n Table 2. The FT-IR stretching frequencies of free ligands were compared with that of their metal complexes. The FT-IR spectrum of ligands showing a strong band at 3354 cm−1 and 3185 cm−1, respectively, is due to (NH) and (NH) stretching vibrations of secondary amide. According to Halli et al.  coordination of NH results in splitting or shifting of bands to lower frequency side or decrease in intensity. In the present study no such changes were observed, instead ν(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 ν(C=O) band to lower region in complexes was observed indicating participation of the carbonyl oxygen in coordination . In free ligands at ν(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 . The medium bands in ligands due to ν(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 ν(C–O–C) of furan remains unaltered in the metal complexes, indicating non-participation 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 ν(M–O), ν(M–N), and ν(M–Cl) stretching vibrations, respectively . 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.
In electronic spectra of Co(II) complexes appeared two absorption bands at 16628, 21154 cm−1 due to L1 and 15902, 20752 cm−1 due to L2 respectively. These bands are assigned to and transitions, respectively, in an octahedral environment. The band could not be observed; however, would be calculated using band fitting procedure . The octahedral geometry is further supported by the values of ligand field parameters such as Racah inter-electronic repulsion parameter (), ligand field splitting energy (10), covalency factor (), and ligand field stabilization energy (LFSE).
The six coordinate Ni(II) complexes exhibit bands at 15342, 25810 cm−1 and 15530, 25637 cm−1 assignable to and 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 , , , %, ratio, and LFSE value. The 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 .
For the Co(II) complex the Racah inter-electronic repulsion parameter () is calculated using the following equation: while for Ni(II) complexes and are calculated from The covalency factor () was obtained in the following manner:
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, , , and , 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 . 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)Cl2] is given in Figure 4. Neiman and Kivelson  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 suggests that the unpaired electron lies predominantly in the orbital  characteristic of square planar or octahedral geometry in copper(II) complexes . The value for these complexes is greater than 2 indicating the presence of covalent character . 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 . Values from 165 to 171 cm−1 for the present complexes suggest a moderate to considerable distortion in the geometry.
6. 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)Cl2], [Ni(L2)Cl2], and [Co(L2)Cl2] complexes showed very good activity against all organisms (MIC = 25 μg/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.
Based on stoichiometries and analytical data of the ligands, (E)-7-Methoxy-N1-(2,4,5-trimethoxy benzylidene) benzofuran-2-carbohydrazide (L1) or (E)-N1-(2,6-dichloro benzylidene)-7-methoxy benzofuran-2-carbohydrazide (L2) are neutral, bidentate ligands coordinating through the “O” and “N” of the amide and azomethine group, respectively.
All the complexes possess 1 : 1 (M : L) stoichiometry based on analytical and spectral data to chloride bridged polymeric octahedral structures of Cu(II), Co(II), and Ni(II) complexes (Figure 5(a)) and monomer/dimeric tetrahedral structures to Zn(II), Cd(II), and Hg(II) complexes (Figure 5(b)). Benzofuran ligands are more active against almost all microbes. Hg complexes are less active; Zn and Cd complexes are moderately active Cu, Co, and Ni complexes showed very good activity against all bacteria.
Sincere thanks go to Principals of SDM College of Engineering and Technology, Dharwad, and M. S. Ramaiah Institute of Technology, Bangalore, for providing necessary facilities to carry out research work and Visvesvaraya Technological University, Belgaum, for financial assistance in the form of a research grant. Authors are thankful to TEQIP Phase-II at SDMCET-Dharwad, for providing the financial support.
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