versão On-line ISSN 1996-840X
S.Afr.j.chem. (Online) vol.67 Durban Jan. 2014
Synthesis, characterization and antimicrobial activity of copper(II) complexes of some ortho-substituted aniline Schiff bases; crystal structure of bis(2-methoxy-6-imino)methylphenol copper(II) complex
Abdullahi Owolabi SobolaI, II, *; Gareth Mostyn WatkinsII; Bernadus Van BrechtIII
IDepartment of Chemistry, Lagos State University, Ojo, Nigeria
IIDepartment of Chemistry, Rhodes University, Grahamstown, 6140 South Africa
IIIDepartment of chemistry, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa
This study presents the synthesis, characterization and antimicrobial activity of copper(II) complexes of some ortho-substituted aniline Schiff bases (L1-L8). The Schiff bases and their respective copper(II) complexes were characterized by a combination of elemental analysis, infrared and UV/Visible studies. The structures of the ligands were also confirmed from 1H- and 13C-NMR spectral data. The infrared and electronic transition studies showed that the ligands are bidentate coordinating via the imine nitrogen and the phenolic oxygen atoms in a planar configuration. Introduction of aqueous ammonia to the ethanolic solution of L3/L4 and Cu(OAc)2.H2O aliquot yielded an ammonia-based complex due to the hydrolysis of the imine bond. The crystal structure of the resulting complex indicated a square planar geometry. The copper(II) ion crystallizes in the monoclinic system with a space group of P21/c having a = 10.9309, b = 4.85600, c = 17.7030, α = γ = 90 ° and β = 119.658 °. The geometry is slightly distorted from planarity with O1-Cu1-N1 bond angles of 92.19 ° and 87.8 °. The Schiff base ligands and their respective copper(II) complexes were screened for their in vitro antibacterial and antifungal activity against Escherichia coli ATCC® 8739™*, Staphylococcus aureus subsp. aureus ATCC® 6538™*, Bacillus subtilis subsp. spizizenii ATCC® 6633™* and Candida albicans ATCC® 2091™*. The o-vanillin-based ligands exhibited higher activity than the salicylaldehyde derivatives which were virtually non-active against the tested organisms.
Keywords: o-Vanillin, substituted-aniline, Schiff bases, Cu(II) complexes, antimicrobial activity.
The chemistry of Schiff base compounds has been studied extensively. They have various applications as coordinating ligands,1-6 as catalysts,7-9 in electrochemistry,10-11 and medicinal values. Schiff bases possess antimicrobial 12-19 and anti-inflammatory activity.20 It has been discovered that the biological activity of some Schiff base ligands became enhanced upon chelation with metal ions.21-22 In some cases, however, the free ligands do possess higher activity than the corresponding metal com-plexes,17 an indication that activity does not depend solely on the presence of metal ions but rather a synergistic effect of many factors. Many Schiff base metal complexes have been prepared and screened for their antimicrobial activity.
Ortho-hydroxysalicylaldimines possess N2O2 donor atoms and consequently, form stable chelates with metal ions.15,21,23-24 The configuration of the chelate group in the four coordinate complexes may be square-planar, tetrahedral, distorted tetrahedral or distorted trigonal pyramidal with the metal atom at the apex. These configurations depend primarily on the nature of the metal atom and also on the magnitude and symmetry of the ligand field.25 Many bis-(salicylaldimine)Cu(II) complexes Cu(Sal.R)2, have been prepared and most of them are considered to be square-planar.25,26 For (Sal.R)2, R being alkyl or aryl groups; the square-planar configuration is stable, but when there is considerable steric hindrance, the distortion from the planarity may occur.
This study presents the synthesis, characterization and antimicrobial activity of copper(II) complexes of some Schiff base ligands (L1-L8) derived from 2-hydroxy-3-methoxybenzalde-hyde (o-vanillin)/2-hydroxybenzaldehyde (salicylaldehyde) and ortho-substituted anilines (R = Cl, Br, CH3 and OCH3). In addition, the crystal structure of bis-(2-methoxy-6-imino) methylphenol copper(II) complex, obtained from the hydrolysis of the methyl and methoxyl derivatives of o-vanillin Schiff bases in the presence of aqueous ammonia, is also discussed.
2. Materials and Methods
All the chemicals and reagents were of analytical grade and used without further purification. The 1H- and 13C-NMR spectra were recorded in CDCl3 with SiMe4 as internal standard on a Bruker Avance NMR operating at 400 MHz. The mid-infrared absorption frequencies (4000-700 cm-1) were recorded on a PerkinElmer Spectrum 100 FT-IR equipped with universal attenuated total reflectance (ATR) accessory while the far-infrared (700-30 cm-1) spectra were recorded in nujol mull on a PerkinElmer Spectrum 400 FT-IR. The UV/Visible spectra were obtained from PerkinElmer Lambda 25 spectrophotometer. The elemental analysis, CHN, was done on Vario MICRO V1.6.2 elemental analysen systeme GmbH while the percentage metal content was determined on a PerkinElmer A Analyst atomic absorption spectrometer. Molar conductivity measurement for the complexes was done in DMF, using Az® 86555 pH/mV/ Cond./TDS/Temp. The melting points (uncorrected) of the compounds were determined using the Galenkemp melting point apparatus. The microorganisms were purchased from Micro-biologics (Cape Town, South Africa).
2.1. Synthesis of the Schiff Base Ligands
The Schiff base ligands were synthesized according to the general procedure in the literature27 by condensing o-vanillin and salicylaldehyde with 2-chloroaniline, 2-bromoaniline, 2-methylaniline and 2-methoxyaniline, respectively, as presented in Scheme 1. The synthesized ligands are designated as ovan-2-cla, ovan-2-bra, ovan-2-tol, ovan-2-nis, sal-2-cla, sal-2-bra, sal-2-tol and sal-2-nis, corresponding to ligands L1-L8.
2.1.1. Ligand L1 (ovan-2-cla)
1.522 g (10.00 mmol) of o-vanillin was refluxed with 0.91 mL (10.00 mmol) of o-chloroaniline in ethanol for 2 h to obtain an orange solution. The solution was concentrated under reduced pressure, and an orange precipitate was obtained. The precipitate was filtered under suction, washed with ethanol and recrystallized from ethanol. It was dried over silica gel in a desiccator. Yield: 1.387 g (53 %); δH (400 MHz, CDCl3) 13.65(1H, s, -OH); 8.68(1H, s, HC=N); 7.52(1H, d, J 7.6 Hz); 7.36(2H, m); 7.25(1H, t, J 8.4 Hz); 7.06(2H, t, J 8.7 Hz); 6.93(1H, t, J 8.0 Hz) and 3.98(3H, s, -OCH3). δC (400 MHz, CDCl3): 163.51(HC=N); 152.09(Ar-OCH3); 149.06(Ar-OH); 145.49(Ar-N = C); 130.68, 128.23, 128.11, 124.32, 119.04, 115.57(Ar-C) and 56.62(-OCH3); (Found: C, 63.82; H, 4.55; N, 5.10 %. Cal. for C14H12NO2Cl: C, 64.27; H, 4.62; N, 5.35 %).
2.1.2. Ligand L2 (ovan-2-bra)
The procedure was the same as for ligand L1 using o-vanillin and o-bromoaniline. An orange precipitate was obtained. Yield: 0.890 g (29 %); δH (400 MHz, CDCl3): 13.73(1H, s, -OH); 8.68(1H, s, HC=N); 7.52(1H, d, J 7.5 Hz); 7.34(2H, m); 7.25(1H, t, J 8.4 Hz); 7.06(2H, t, J 8.5 Hz); 6.93(1H, t, J 7.8 Hz) and 3.98(3H, s, -OCH3). δC (400 MHz, CDCl3): 163.24(HC=N); 152.02 (Ar-OCH3); 148.81(Ar-N=C); 145.27(Ar-OH); 130.68(Ar-Br); 128.27, 128.14, 124.30,119.00,115.44(Ar-C) and 56.51(-OCH3); (Found: C, 54.84; H, 3.94; N, 4.47 %; Cal. for C14H12NO2Br: C, 54.87; H, 3.92; N, 4.57 %).
2.1.3. Ligand L3 (ovan-2-tol)
The procedure was the same as for ligand L1 using o-vanillin and 2-methylaniline (o-toluidine). A yellow precipitate was obtained. Yield: 1.148 g (48 %); dH (400 MHz, CDCl3): 13.85(1H, s,-OH); 8.64(1H, s, HC=N); 7.31(2H, t, J 8.7 Hz); 7.23(1H, d, J 8.2 Hz); 7.18(1H, d, J 7.6 Hz); 7.05(2H, m); 6.93(1H, t, J 7.5 Hz ); 3.99(3H, s, -OCH3) and 2.44(3H, s, -CH3). δC (400 MHz, CDCl3): 162.36(HC = N); 152.12(Ar-OCH3); 148.96(Ar-N = C); 147.37(Ar-OH); 133.01(Ar-Br); 128.27, 128.14, 124.30, 119.00, 115.44(Ar-C), 56.57(-OCH3) and 18.66(-CH3); (Found: C, 74.11; H, 6.26; N, 5.68 %; Cal. for C15H15NO2: C, 74.66; H, 6.27; N, 5.80 %).
2.1.4. Ligand L4 (ovan-2-nis)
The procedure was the same as for ligand L1 using o-vanillin and 2-methoxyaniline (o-anisidine). A red precipitate was obtained. Yield: 2.30 g (89 %); δH (400 MHz, CDCl3): 14.42(1H, s,-OH); 8.74(1H, s, HC=N); 7.27(2H, t, J 7.3 Hz); 7.20(1H, d, J 8.5 Hz); 7.15(1H, d, J 8.2 Hz); 7.05(2H, m); 6.93(1H, t, J 8.1 Hz); 3.97(3H, s,-OCH3); and 3.93(3H, s,-OCH3). δC (400 MHz, CDCl3): 161.77(HC=N); 153.49, 153.08(Ar-OCH3); 136.79 (Ar-N=C); 149.19(Ar-OH); 128.42, 123.95, 119.99, 118.40, 112.42, 110.90(Ar-C); 56.57, 56.28(-OCH3). Found: C, 69.35; H, 5.98; N, 5.30 %; Cal. for C15H15NO3: C, 70.02; H, 5.88; N, 5.44 %).
2.1.5. Ligand L5 (sal-2-cla)
The procedure was the same as for ligand L1 using salicylaldehyde and 2-chloroaniline. A yellow precipitate was obtained. Yield: 1.05 g, 45 %; δH (400 MHz, CDCl3): 13.21(1H, s,-OH); 8.66(1H, s, HC=N); 7.75(1H, d, J 8.5 Hz ); 7.50(2H, t, J 8.2 Hz); 7.42(1H, d, J 8.0 Hz); 7.30(1H, d, J 7.4 Hz); 7.22(1H, t, J 7.8 Hz); 7.11(1H, d, J 8. 5 Hz); 7.00(1H, t, J 8.0 Hz). δC (400 MHz, CDCl3): 163.74(Ar-OH); 161.81(HC = N); 145.86(Ar-N = C); 130.65(Ar-Cl); 134.11,132.90,129.95,128.16,128.12,119.65,119.54, 117.93(Ar-C); (Found: C, 66.97; H, 4.31; N, 6.00 %; Cal. for C13H10NOCl: C, 67.39; H, 4.35; N, 6.05 %).
2.1.6. Ligand L6 (sal-2-bra)
The procedure was the same as for ligand L1 using salicylaldehyde and 2-bromoaniline. A yellow precipitate was obtained. Yield: 2.15 g (78 %); δH (400 MHz, CDCl3): 13.13(1H,s,-OH); 8.64(1H, s, HC=N); 7.71(1H, d, J 8.2 Hz); 7.45(2H, t, J 7.4 Hz); 7.39(1H, d, J 7. 8 Hz); 7.27(1H, d, J 8.1 Hz); 7.18(1H, t, J 7. 7 Hz); 7.09(1H, d, J 8.8 Hz); 6.99(1H, t, J 8.4 Hz). δC (400 MHz, CDCl3): 163.64(Ar-OH); 161.73(HC = N); 147.26(Ar-C=N); 134.13, 132.94, 128.90, 128.39, 120.37, 119.57, 119.46,117.94(Ar-C); (Found: C, 56.43; H, 3.56; N, 5.04 %; Cal. for C13H10NOBr: C, 56.49; H, 3.62; N, 5.79 %).
2.1.7. Ligand L7 (sal-2-tol)
The procedure was the same as for ligand L1 using salicylaldehyde and o-methylaniline (o-toluidine). Yield: 1.55 g (73 %); δH (400 MHz, CDCl3): 13.48(1H, s, -OH); 8.62(1H, s, HC=N); 7.44 (2H, m); 7.32(2H, t, J 8.6 Hz); 7.24(1H, d, J 8.8 Hz); 7.15(1H, d, J 7.6 Hz); 7.08(1H, d, J 8. 38 Hz); 6.99(1H, t, J 7. 8 Hz); 2.44(3H, s, -CH3). δC (400 MHz, CDCl3): 162.69 (Ar-OH); 161.65 (Ar-OCH3; Ar-N=C); 147.93(Ar-OH); 133.56(Ar-CH3); 132.77, 132.64,131.15,127.47,127.30,119.79,119.48,118.19,117.66(Ar-C) and 18.68(-CH3). (Found: C, 78.84; H, 6.19; N, 6.6 %); Cal. for C14H13NO: C, 79.52; H, 6.15; N, 6.63 %.
2.1.8. Ligand L8 (sal-2-nis)
The procedure was the same as for ligand L1 using salicylaldehyde and o-methoxyaniline (o-anisidine). Yield: 1.395 g (61 %); δH (400 MHz, CDCl3): 13.41(1H, s, -OH); 8.64(1H, s,HC=N); 7.38(2H,m); 7.29(2H,t,J7.80Hz); 7.20(1H,d,J8.2Hz); 7.10(1H, d, J 8.2 Hz); 7.05(1H, d, J 8.4 Hz); 6.90(1H, t, J 8.0 Hz); 3.88(3H, s, -OCH3). δC (400 MHz, CDCl3): 162.54(Ar-OH); 162.02(HC=N); 153.40(Ar-OCH3); 137.55 (Ar-N=C); 133.32, 132.39,128.30,121.39,120.09,119.95,119.17,117.82,112.33(Ar-C); and 56.29(-OCH3). Found: C, 73.12; H, 5.89; N, 6.12 %. Cal. for C14H13NO2: C, 73.99; H, 5.77; N, 6.16 %
2.2. Synthesis of the Complexes
0.114 g (0.573 mmol) of copper acetate monohydrate, Cu(OAc)2.H2O, was dissolved in 10 mL ethanol and added dropwisely to a vigorously stirring 0.30 g (1.15 mmol) ethanolic solution of ligand L1. The resulting solution was refluxed for 6 h to obtain a brown precipitate. It was filtered under suction, washed thoroughly with ethanol and dried over silica gel in a desiccator. The same procedure was repeated for all the complexes. However, the addition of aqueous ammonia to L3/ Cu(OAc)2.H2O aliquot resulted in the hydrolysis of the imine bond and consequent formation of an ammonia-based complex as shown in Schemes 2 and 3. The same product was obtained with ligand L4. The analytical and the spectral data for the complexes are presented in Tables 1 and 2.
3. X-ray Crystallography Data
Single crystals of the [Cu(ovan-NH)2].H2O complex were obtained via slow evaporation of the saturated DMF solution of the complex. A suitable single crystal of the complex was diffracted using a Bruker KAPPA APEX II single crystal x-ray diffrac-tometer, with a 4-circle Kappa goniometer. The crystallographic data was collected at 200 K and 0.7107 nm (lll) on a sensitive CCD detector with graphite-monochromated MoK α radiation, a total of 1978 reflections was collected of which 1836 were considered observed [I > 2σ(I)]. The structure was solved by direct method using the program SHELXL-9728 and refined anisotropically by full matrix least-squares on F2 using SHELXL-97. The details of crystallographic parameters, data collection and refinements are listed in Table 3 while selected bond lengths and angles are presented in Table 4.
4. Results and Discussion
4.1. Elemental Analysis and Conductivity Measurement
The elemental analysis results for the complexes suggest that the complexes are of the form CuL2, with the Cu2+ ion coordinating to the Schiff base ligands in a ratio 1:2. The low conductivity values for the complexes imply the Schiff base ligands coordinated as neutral species29via the deprotonated phenolic oxygen-O, and the imine nitrogen-N, atoms.
4.2. 1H- and 13C- NMR
The assignments of the main NMR signals are given in the experimental section. The azomethine proton (HC=N) absorbed downfield in all the Schiff base ligands as one proton singlet at 8.64-8.68 ppm while the broad signal at 13.8213.13 ppm is assigned to the phenolic hydroxyl proton. The position of the hydroxyl signal is attributable to a strong intramolecular hydrogen bonding through the imine -N and the phenolic -O. The signal at 163.51-160.88 ppm is due to the imine functional group (C=N), and further confirms the formation of the Schiff base ligands. The purity of the ligands was, however, indicated by the disappearance of the amino and aldehyde protons at 3-4 ppm and 9-10 ppm, respectively. The methoxy substituent of the o-vanillin analogues was observed as a strong singlet at 3.99-3.97 ppm. In addition, the signals at 2.44 ppm and 3.93-3.88 ppm correspond to the methyl and methoxy substitu-ents of the toluidine and anisidine-based ligands, respectively.
4.3. Infrared Study
The imine (C=N) functional group of the free ligands was observed as a strong band at 1615-1612 cm-1. As expected, the band red-shifted in the spectra of the complexes, except [Cu(ovan-NH)2].H2O, indicating coordination of the Schiff base ligands via the azomethine N-atom.24,26,30-31 In addition, the strong band at 1280-1273 cm-1 corresponds to the phenolic C-O stretching vibration ofthe free ligands. This band blue-shifted to 1336-1325 cm-1 upon complexation,5,31-32 suggesting the involvement of the phenolic oxygen atom in the coordination sphere. All the infrared spectra of the free ligands were characterized by a broad band at 3220-2284 cm-1 corresponding to the O-H stretching vibration of the ligands. The broadness was due to a strong intramolecular hydrogen bonding between the imine nitrogen and the hydroxyl group.33-34 The hydroxyl band, however, disappeared in the spectra of the complexes due to deprotonation and involvement of the oxygen atom in the coordination sphere. The mode of coordination of the Schiff base ligand was further substantiatedby the appearance of two new bands in the far infrared spectra of the complexesat 580-541 cm-1 and 482-449 cm-1. These bands are assigned to the VCu-O and vCu-N stretches, respectively.4,5,33
Furthermore,the infrared spectrum ofCu(ovan-NH)2has two additional strong bands at 3413 cm-1 and 3294 cm-1. The bands are attributed to the N-H stretching vibrations of ligand ovan-NH that was obtained upon the addition of aqueous ammonia to the aliquot of L3/L4 and Cu(OAc)2H2O as shown in Scheme 3.
4.4. UV/Visible Study
The electronic transition study of the compounds was carried out in methanol. Three absorption bands were observed in the spectra ofthe Schiffbase ligands at229-211 nm, 292-268nmand 394-319nm. The bandsathigherenergyvalues(229-211 nm)are attributed to the π→π* transition of the benzene ring3 while the other two bands are assigned to the π→π*and n →π* transi- tions of the azomethine chromophore, respectively.3,12 These transitions are observed at a lower energy values in the spectra of the complexes, indicating coordination via the azomethine functional group (HC=N).
The d → d transition of the copper(II) ion was observed as a broad band at 683-654 nm, indicating a square planar geometry. Bhunora et al.observedsimilarbandat662-628nmforaseries of square planar salicyaldiminecopper(II) complexes.26 The Schiff base ligands showed an additional absorption band at above 400 nm (470-442 nm) in methanol, and this new band has been previously assigned to the keto-imine form of ortho-hydroxy-salicylaldiminesinpolarandnon-polarsolvents.3The tautomer-ism is thought to occur via an intramolecular (or intermolecular in protonic solvents) hydrogen transfer to the imine nitrogen.
4.5. Molecular Structure of [Cu(ovan-NH)2].H2O
The labelled ORTEP diagram of [Cu(ovan-NH)2].H2O is presented in Fig. 1 while the bond lengths and angles are presented in Table 4. The crystal structure consists oftwo units ofthe Schiff base ligands chelatingwiththe copper(II)ion in a planar milieu. The copper(II) ion crystallizes in the monoclinic system with a space group of P21/chavinga=10.9309,b=4.85600, c-= 17.7030, α= γ = 90 °and β = 119.658 °. The bidentate Schiff base ligand formed a four coordinate chelate with the copper(II) ion in a trans configuration through the imine nitrogen and the phenolic oxygen atoms. The geometry is slightly distorted from planarity with O1-Cu1-N1 bond angles of 92.19 ° and 87.8 °. The bond lengths for the Cu-O (1.9447Å) and Cu-N (1.915 Å) fall within the range of reported values for similar structures in the literature (1.999 and 1.869; 1.8959 and 1.9769; 1.901 and 1.9769)Å.26 The structures reported by Bhunora et al.,26 equally have the copper(II) ion in a planar geometry within the trans configuration of the N2O2 donor atoms. In addition, there exist intra-molecular hydrogen bonding with average bond length (O-H) 0.76Å(Fig. 2),occasionedbythe presence of a water molecule in the crystal lattice of the complex.
5. Biological Study
The Schiff base ligands and their respective copper(II) complexes were screened for their in vitro antibacterial and antifungal activity against Escherichia coli ATCC® 8739™*, Staphylococcus aureus subsp. aureus ATCC® 6538™*, Bacillus subtilis subsp. spizizenii ATCC® 6633™* and Candida albicans ATCC® 2091™*.
5.1. Disc Diffusion Technique
The qualitative antimicrobial susceptibility testing of the compounds was evaluated using the disc diffusion technique.34 Each test organism was inoculated onto a nutrient agar plate and incubated at 37 °C for 24 h to obtain the primaryculture. Several discrete colonies were picked from the culture to make a bacterial suspension(10mL) inatesttube usingsaline water. The turbidity ofthe suspensionwascomparedwith0.5 McFarlandstandardto obtain 106-108 CFUs. The bacterial suspension (0.1 mL) was inoculated onto Mueller Hinton plate and the sterile discs that have been impregnated with the test compounds were firmly placed on it. The assay was inoculated at 37 °C for 16 h and the zone of inhibition was measured as millimetres diameter. Ampicillinwas usedas the standardantibacterialdrugwhile the control solvent was dimethylformamide (DMF). The test was repeated two more times for those compounds that showed activity of more than 6.5 mm, and their activity was recorded as average zone of inhibition in Table 5. Similar procedure was repeated for the antifungal susceptibility testing of the com- pounds using the potato disc assay instead of Mueller Hinton agar. The standard antifungal drug used for the study was ketoconazole.
5.2. Minimum Inhibitory Concentration (MIC)
The quantitative antimicrobial activity of the test compounds was evaluated using micro dilution broth method according to Clinical and Laboratory Standard Institute (formally NCCLS).34 Two-fold serial dilutions of the compounds were prepared in 96 microwell plates using sterile nutrient broth as the diluent. The plates were inoculated with 5µL bacterial suspensions containing 106-108 CFUs and incubated at 37 °C for 16-18 h. The MIC value was defined as the lowest concentration of the compounds giving complete inhibition of visible growth. The MIC values for the compounds varied from 6.250-0.1953 mg/mL and the result is presented in Table 6.
The diameters of the zone of inhibition and the minimum inhibitory concentrations (MIC) of the compounds are presented in Tables 5 and 6, respectively. The compounds were not as active as penicillin, but showed significant antifungal activity against the tested organisms.
The o-vanillin-based ligands exhibited higher activity than the salicylaldehyde-based compounds which were virtually non-active against the tested organisms. This could be due to the presence of the methoxy group at the ortho position of the aldehyde moiety of the Schiff base ligands. Likewise, ligands L3 and L4 (toluidine and anisidine derivatives) showed significant potency against the microorganisms (Fig. 3); thus relating the activity of the Schiff bases to the electronic effects of the substitu-ents, since the chloro- and the bromo- analogues exhibited only slight activity. Ansari et al.35 has, however, reported high antimicrobial activity for the o-chloro, o-methyl, o-methoxyl and p-amino analogues in a series of substituted benzimidazole Schiff base ligands.
On chelation, however, the presence of the Cu(II) ions did not enhance the antimicrobial activity of the free ligands. The copper complexes exhibited lower activity than the Schiff base ligands. Though, it is believed that chelation increases the lipophilicity of the metal ion and consequently increases the antimicrobial activity of metal complexes relative to the free ligands. Conversely, it has been observed that the free ligands do possess higher activity in some cases; an indication that activity does not depend solely on the presence of metal ions but rather a synergistic effect of many factors. Thus, understanding the mechanisms of action of potential antimicrobial agent is considered a critical aspect of our study for the future.
The ortho-substituted Schiff bases coordinated as monobasic bidentate ligands via the imine nitrogen and phenolic oxygen atoms. The crystal structure of bis(3-methoxyl-2-methylimine) copper(II) complex, indicated a planar configuration for the complexes. The antimicrobial study of the compounds revealed higher activity for the o-vanillin derivatives than the salicylaldehyde-based ligands.
7. Supplementary Data
X-ray crystallography data of bis(3-methoxy((imine)methyl-phenol)Cu(II) complex has been deposited with the Cambridge Crystallographic Data Centre (CCDC) and can be obtained free of charge on request at http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033; e-mail: firstname.lastname@example.org, quoting the CCDC number, 888472.
We acknowledge the effort of Dr Sheriff Adewuyi of Federal University of Agriculture (FUNAAB), Abeokuta, Nigeria, for perusing the manuscript.
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Received 18 May 2013
Revised 28 October 2013
Accepted 3 March 2014