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SYNTHESIS AND CHARACTERIZATION OF M2+ (M = Mn, Cu, Cd and Zn) COMPLEXES WITH A NEW SCHIFF BASE LIGAND CONTAINING PARA- FLOUROBENZEN AND THEIR ANTIMICROBIAL STUDIES.

Byline: TaiebehTamoradi, Hamid Goudarziafshar, ArashGhorbaniChoghamarani, FatemehKatouzian and FiroozehChalabian

ABSTRACT

The template synthesis and characterization of several metal complexes (metal ions: Mn2+, Cu2+, Cd2+ and Zn2+) of macroacyclic Schiff-base ligand (L) are reported. L has also been prepared from the condensation reaction of 1,4-di-(4- fluoro-2aminophenoxy)butane (DFAB) and Salicylaldehyde in ethanol and characterized by IR, 1H-NMR and 13C-NMR spectroscopy and mass spectrometry. We are also especially interested in the antimicrobial activity of these new complexes. The in vitro antibacterial activity of the metal ion, free ligand and their complexes were tested against the gram -positive bacteria; Bacillus anthracis RTCC 1036, Staphylococcus epidermidis PTCC 1114, Staphylococcus aureus RTCC 1885, Enterococcus faecalis RTCC 2121, Bacillus subtilis PTCC 1715 and gram-negative bacteria; Listeria monocytogenes RTCC 1885, Pseudomonas aeruginosa RTCC 1547, Klebsiella pneumonia. RTCC 1247, Enterobacteraerogenes PTCC 1221, Eschericha coli.

RTCC1330 by paper disc diffusion and millidilution broth methods.The results showed that in some cases the antimicrobial activity of the complexes exceeded the Tetracycline and Gentamycine used as a standard.

Keywords: Antimicrobial activity, Schiff-base, Template Synthesis, Metal Complexes, Macrocyclic

INTRODUCTION

Schiff bases and their complexes are of substantial importance to chemists and biologists becauseof their industrial applications and biological activity, for example antibacterial, antimalarial, antifungal, antiviral, and anticancer, which is related to their chemical structure. They are used as pigments and dyes, catalysts, intermediates in organic synthesis, and as polymer stabilizers [1, 2]. The Schiff bases ligands are readily available, versatile and, depending on the nature of the starting materials (primary amines and carbonyl precursors), they exhibit various denticities and functionalities. All these advantages make Schiff bases very good candidates in the effort to synthesize metal complexes of interest in bioinorganic chemistry, catalysis, encapsulation, transport and separation processes, magneto chemistry [3].

There is a continuing interest in transition metal complexes of Schiff bases because of the presence of both nitrogen and oxygen donor atoms in the backbones of these ligands, some of these complexes have been exhibit interesting physical and chemical properties and potentially useful biological activities [4]. The applications of such complexes depend to a large extent on their molecular structure. Schiff base ligands are able to coordinate many different metals, and to stabilize them in various oxidation states. During the past two decades, considerable attention has been paid to the chemistry of the metal complexes of Schiff bases containing nitrogen and other donors. This may be attributed to their stability, biological activity and potential applications in many fields such as oxidation catalysis, and electrochemistry. Schiff base ligands have an affinity for transition metals such as Cu, Ni and Fe.

Some of these complexes have been studied in great deal for their various structures, steric effects and their coordination chemistry [5-7]. MacroacyclicSchiff-base complexes based on transition metals and multidentate ligands are an interesting chemical field and have been the subject of extensive research due to their potential applications in fields of magnetochemistry, material science, catalysis, separation and ncapsulation processes, hydrometallurgy, formation of compounds with unusual properties and metalmetal interactions [8-25].

EXPERIMENTAL

2.1. General considerations

All chemicals except 1,4-di-(4-fluoro-2- aminophenoxy)butane were purchased from Merck and used without further purification. IR spectra were recorded (KBr) on a BrukerVERTEX 70 spectrometer. 1H NMR and 13C NMR spectra were recorded on a Bruker- AV400MHz. Mass spectra were recorded on a 5973 Tecnology Agilent (HP) spectrometer (EI = 70 eV).

2.2. Synthesis

2.2.1. Synthesis of 1,4-di-(4-fluoro-2-nitrophenoxy)butane To a solution of 6.28 g 4-fluoro-2-nitrophenol (40mmol) in 100 cm3 ethanol was added 1.6 g NaOH (40 mmol) in 10 cm3 water under stirring at room temperature. After the color of solution changed from yellow to red, a solution of 0.432 g1,4-dibromo butane (.5mmol) in 20 cm3 ethanol was added dropwise and was refluxed for 48 haccording to eq. (1). After completion of the reaction, the cream solid crude was filtered and washed with ethanol.Yield: 2.93 g (80%), mp: 121 C. IR (KBr, , cm-1): 3091(CHaromatic), 2957(CHaliphatic),1518 as(NO), 1475 (C=Caromatic), 1342 s(NO), 1259 (CN), 1194 (CO), 1140, 1039, 971, 937, 803, 763, 727, 675.

2.2.2. Synthesis of 1,4-di-(4-fluoro-2- aminophenoxy)butane (DFAB) 0.74 g 1,4-di-(4-fluoro-2-nitrophenoxy)butane (2mmol) was dissolved in 100 cm3HCl (5 N) at temperature of 40-50C. Then 4.8 g SnCl2.H2O (23mmol) was gradually added to the mixture of reaction. After the addition of SnCl2, a turbid solution was formed. The mixture of reaction was refluxed till the solution become colorless and transparent (24 h) according to eq. (2). After completion of the reaction, the white solid crude was filtered and washed with cold ethanol Yield: 73 %; mp 132-133 C, IR (KBr, , cm-1): 3450s(N H),3050(CHaromatic), 2942(CHaliphatic),1579, 1504 (C=Caromatic), 1304 s(CN), 1279 (CF), 1223 (C O),1155, 803 (CH), 715, 504, 450.

2.2.3. Synthesis of macroacyclic Schiff-base Ligand (L) To a solution of 0.224 gsalicylaldehyde (1 mmol) in 20 cm3 ethanol was added dropwise a solution of 0.39 g diamine salt (DFAB) in 20 cm3 water and 50 cm3 ethanol over 30 minutes. Then the solution of NaOH (5M) was added drop by drop until pH of the mixture of reaction reached 7. The reaction was stirred at room temperature for 24 h to be completed according to eq. (3). After completion of the reaction, the whity precipitate was filtered and washed with acetonitrile and cold methanol.Yield: (71)%; mp 150-151C, IR (KBr, , cm-1): 2955(CHaromatic), 2879(CHaliphatic), 2853s(C Himinic), 1618s(C=N),1616 (C=C), 1471, 1157 (CF), 1115 (CO). m/z = 518 [M++1], 517 [M+].1H NMR (d, DMSO-d6, MHz): 1.93 (4H; Ha), 4.14 (4H; Hb), 6.94-7.59 (14H; Hd, He, Hg, Hk, Hl, Hm, Hn), 8.99 (2H; Hi), 13.7 (2H; Hphenolic). 13C NMR (d, DMSO-d6, MHz): 25.88 (4C; Ca), 68.85 (2C; Cb), 106.42-155.76 (25C; Cc, Cd, Ce, Cf, Cg, Ch, Cj, Ck, Cl, Cm, Cn, CO), 167.15 (2C; Ci).

2.2.4. Template Synthesis of Metal complexes of L with M2+ (M = Mn,Cu, Cd and Zn) To a solution of 0.224 gsalicylaldehyde (2 mmol) in 20 cm3 ethanol was added appropriate amount of M 1 mmol) and it was stirred at room temperature for 2 h. After that dropwise a solution of 0.39 g diamine salt (DFAB) (1mmol) in 20 cm3 water and 50 cm3 ethanol over 30 minutes. Then the solution of NaOH (5M) was added drop by drop until pH of the mixture of reaction reached 7. The mixture of reaction was refluxed for 36 h. After completion of the reaction, the whity precipitate was filtered and washed with acetonitrile and cold methanol.

[Mn(L)]Cl2(1). Yield 78 %; mp220-222 C; FAB MS (positive FAB in nitrobenzyl alcohol): m/z 573 (MH+, [C30H24F2MnN2O4]+), m/z 516 (MH+ , [C30H26F2N2O4]+). IR (KBr, , cm-1): 2935(CHaromatic), 2873(CHaliphatic), 2853s(CHiminic), 1665 (C=C), 1620 (C=N), 1217 (C-N), 1145 (C-F).

[Cu(L)](NO3)2(4). Yield 82 %;mp201-203 C; FAB MS (positive FAB in nitrobenzyl alcohol): m/z 577 (MH+, [C30H24F2CuN2O4]+), m/z 518 (MH+, [C30H26F2N2O4]+), m/z 414 (MH+, [C23H22F2N2O3]+). IR (KBr, , cm-1): 1611 (C=N), 1599 (C=C), 1253 (C-N), 1208 (C-O), 1127 (C- F).

[Zn(L)](NO3)2(5). Yield 91 %; mp 249-250 C; FAB MS (positive FAB in nitrobenzyl alcohol): m/z 578 (MH+, [C30H24F2ZnN2O4]+), m/z 516 (MH+, [C30H26F2N2O4]+), m/z 412 (MH+, [C23H22F2N2O3]+), m/z 303 (MH+, [C16H14F2N2O2]+), m/z 411 (MH+, [C23H20F2N2O3]+), m/z 305 (MH+, [C16H14F2N2O2]+). IR (KBr, , cm-1): 2954(C Haromatic), 2933(CHaliphatic), 2875 s(CHiminic), 1619 (C=N), 1594 (C=C), 1288 (C-N), 1228 (C-O), 1201 (C-F). 1H NMR (d, DMSO-d6, MHz): 1.89 (4H; Ha), 3.96 (4H; Hb), 5 (4H; He, Hg) 6.21-6.74 (10H; Hd, Hk, Hl, Hm, Hn), 8.99 (2H; Hi), 13.7 (2H; Hphenolic). 13C NMR (d, DMSO-d6, MHz): 26.09 (4C; Ca), 68.58 (2C; Cb), 100.57-156.36 (24C; Cc, Cd, Ce, Cf, Cg, Ch, Cj, Ck, Cl, Cm, Cn, CO), 167.4 (2C; Ci). [Cd(L)](NO3)2(6). Yield 75 %; mp 190-192 C; FAB MS (positive FAB in nitrobenzyl alcohol): m/z 627 (MH+, [C30H24F2CdN2O4]+), m/z 522 (MH+, [C23H19F2CdN2O3]+), m/z 517 (MH+, [C30H26F2N2O4]+), m/z 412 (MH+, [C23H22F2N2O3]+), m/z 306 (MH+, [C16H18F2N2O2]+.

IR (KBr, , cm-1): 2956(CHaromatic), 2932(CHaliphatic), 2879 s(CHiminic), 1619 (C=N), 1595 (C=C), 1263 (C-N), 1227 (C-O), 1151 (C-F). 1H NMR (d, DMSO-d6, MHz): 1.92 (4H; Ha), 4.09 (4H; Hb), 6.94-7.59 (14H; Hd, He, Hg, Hk, Hl, Hm, Hn), 8.99 (2H; Hi), 13.69 (2H; Hphenolic). 13C NMR (d, DMSO-d6, MHz): 25.88 (4C; Ca), 68.86 (2C; Cb), 106.33- 156.36 (25C; Cc, Cd, Ce, Cf, Cg, Ch, Cj, Ck, Cl, Cm, Cn, CO), 163.86 (2C; Ci).

Antimicrobialactivity

The in vitro antibacterial activity of the metal ion, free ligand and their complexes were tested against the gram-positive bacteria; Bacillus anthracis RTCC 1036, Staphylococcus epidermidis PTCC 1114, Staphylococcus aureus RTCC 1885, Enterococcus faecalis RTCC 2121, Bacillus subtilis PTCC 1715 and gram-negative bacteria; Listeria monocytogenes RTCC 1885, Pseudomonas aeruginosaRTCC 1547, Klebsiella pneumonia. RTCC 1247, Enterobacteraerogenes PTCC 1221, Eschericha coli.RTCC1330by paper disc diffusion and millidilution broth methods.

Bacteria cultures were obtained from Tehran Islamic Azaduniversity Hospital, Microbiology Department. Microbial strains were cultured overnight at 310 K in Nutrient Broth. During the survey, these stock cultures were stored in the dark at 277 K.Theinocula of microorganisms were prepared from 12 h broth cultures and suspensions were adjusted to 0.5 McFarland standard turbidity. The synthesized compounds were dissolved in methanol (%20 methanol) to a final concentration of 2.0 mg mL-1and sterilized by filtration by 0.45 lm Millipore filters. Antimicrobial tests were then carried out by the disc diffusion method using 100 mL of suspension containing 108 CFU mL-1bacteria spread on a nutrient agar (NA) medium. The discs were impregnated with 50 mL of each compound (100 mg/disc) at the concentration of 2.0 mg mL-1and placed on the inoculated agar. Methanol impregnated discs were used as negative control.

Tetracycline and Gentamycine (100 mg/disk) was used as positive reference sta ndard to determine the sensitivityof one strain/isolate in each microbial species tested. Antimicrobial activity in the disc diffusion assay was evaluated by measuring the zone of inhibition against the test organisms.

Each assay in this experiment wasrepeated twice. For investigation of antimicrobial activity of as preparedmetal ion, free ligand and their complexes. The inhibitory effect of complexes on the growth of microbes were studied. The results can be seen in Table 1.

It is apparent the metal complexes have greater antimicrobial activity but related free ligands and metal ions have not antimicrobial activity.

Tweedy's chelation theory is a good clarification for this phenomenon.

Micro Dilution Assays

All tests were performed in a nutrient broth supplemented with methanol to a final concentration of 20 % (v/v) to enhance their solubility. Test strains were suspended in thenutrient broth by adjusting to the 0.5 McFarland standards.The compounds to be tested were dissolved in methanol to give the highest concentration (2,000 mg mL-1), and serial dilutions thereafter in sterile 10 mL test tubes containing nutrient broth to give sample concentrations of the range 7.811,000 mg mL-1. The minimum inhibitory concentration (MIC) values of compounds were determined using a modification of the micro-well dilution assay method.

100 mL from test compounds initially prepared at 1,000 mg mL-1 concentration was added into the first wells. Then, 100 mL from the serial dilutions was transferred into eight consecutive wells. The contents of the wells were mixed and the microplates were incubated at 37 C for 24 h. The compounds were tested against each microorganism twice.

The values obtained are average of the two results. The MIC values were determined from visual examinations as the lowest concentration of the extracts in the wells with no bacterial grow

RESULTS AND DISCUSSION

A potentially macroacyclic Schiff-base ligand (L) was prepared by condensation reaction of 1,4-di-(4-fluoro-2- aminophenoxy)butane (DFAB) and salicylaldehyde in ethanol. DFAB was prepared through reduction of NO2 groups of 1,4-di-(4-fluoro-2-nitrophenoxy)butane (DFNB) using Cannon reaction. DFNB was also synthesized by nucleophilic substitution reaction (SN2) of 4-fluoro-2- nitrophenole and 1,4-dibromo butane. The complexation of L

Table 1: Antimicrobial activity of M2+ (M = Mn, Cu, Cd and Zn) Complexes Gentamycineand Tetracycline (as a standards compound)

###Disk diffusion

###TE###GM

###MIC

###30mcg/ml###10mcg/ml

###Zn###Mn###Cd###Cu

###Tetracycline###Gentamycie

###Gram-positive

###Bacillus anthracis###30mm###--------###25mm###10mm###20mm

###25mm

###RTCC1036###6.25mg/ml###12.5mg/ml###100mg/ml

###Staphylococcus###40mm###20mm###44mm###40mm

###25mm###25mm

###epidermidisPTCC1114###1.56 mg/ml###25 mg/ml###1.56 mg/ml###1.56 mg/ml

###Staphylococcus###40mm###20mm###40mm###30mm

###25mm###30mm

###aureusRTCC1885###1.56###25mg/ml###1.56mg/ml###6.25mg/ml

###Enterococcus faecalis###20mm###--------###---------------###15mm

###30mm###15mm

###RTCC2121###25mg/ml###-------###--------###100mg/ml

###Bacillus subtilisPTCC1715###30mm###15mm###35mm###25mm

###20mm###20mm

###6.25mg/ml###100mg/ml###3.12mg/ml###12.5mg/ml

###TE###GM

###Gram-negative###Zn###Mn###Cd###Cu

###30mcg/ml###10mcg/ml

###ListeriaMonocytogenes###36mm###----------###42mm###30mm

###20mm###25mm

###RTCC1885###6.25mg/ml###----------###1.56mg/ml###6.25 mg/ml

###Pseudomonas aeruginosa###20mm###------------###-----------###15mm

###-------###15mm

###RTCC 1547###25mg/ml###-------###-------###100mg/ml

###50mm###15mm###25mm###20mm

###Klebsiella pneumoniaRTCC###20mm###25mm

###0.75mg/ml###100mg/ml###12.5mg/ml###25mg/ml

###1247

###Enterobacteraerogenes PTCC###30mm###------------###25mm###15mm

###20mm###15mm

###1221###6.25mg/ml###12.5mg/ml###100mg/ml

###30mm###25mm###10mm

###Eschericha coliRTCC1330###18mm###10mm

###6.25mg/ml###12.5mg/ml###100mg/ml

Was carried out toward M2+ (M = Mn,Cu, Cd, and Zn) using one-pot template reactions. The resulted complexes were investigated by IR and mass spectrometry.

IR, NMR and mass data gives useful information on the structure of DFAB, DFNB, Schiff-base ligand (L) and its metal complexes. The spectrum of the free ligand was compared with the spectrum of the metal complexes. The structurally significant IR, NMR and mass spectral data of free

ligand and its metal complexes have been reported in the experimental section.

The vibration bands that appeared in the IR spectrum of DFNB at 1342 and 1518 cm-1 are assigned to symmetric and asymmetric stretching vibrations of the NO2 groups in the molecule, respectively. The disappearance of vibration band at 3500 cm-1 related to stretching vibration of phenolic OH groups confirmed the formation of DFNB. The band at 3450 cm-1 in the IR spectrum of DFAB is ascribed to the phenolic N-H stretching vibration. The vibration band at 1223 cm-1 indicates that etheric bond cleavage has not occurred through reduction of NO2 groups. The vibration band related to stretching vibrations of the NO2 groups is absent showing the reduction of NO2 groups has been completed. The strong absorption band at approximately 1618 cm-1in the IR spectrum of L is ascribed to the stretching vibration of iminic C=N bonds.

The 1H NMR spectrum of the ligand L showed signals at 1.93 (4H); 4.14 (4H); 8.99 (2H) and 13.7 (2H) ppm which are attributed to methylene Ha; methylene Hb, iminic and phenolic hydrogens, respectively. The 13C NMR spectrum of L showed signals at 25.88 (4C); 68.85 (2C), 106.42-155.76 (25C) and 167.151 (2C) ppm which are attributed to methylene Ca; methylene Cb, aromatic and iminic carbons, respectively.

The IR spectra of Mn (II), Cu (II) and Zn (II) complexes exhibited vibration bands at 1620 cm-1, 1611 cm-1 and 1619 cm-1 respectively which can be assigned to the C=N stretching vibration of L. The frequency of this vibration for Cd (II) complexe is the same as Zn (II) complex. Also, vibration bands at 1145, 1208 and 1228 cm-1 confirmed the maintenance of C-O bonds of L for Mn (II), Cu (II) and Zn (II) complexes respectively. These results in addition to mass results clearly indicate the formation and coordination of macroacyclic ligand (L) to metal ions through one-pot template reaction.

Because single crystals of these complexes could not be isolated from any solvents, no definitive crystal structures could be assigned. However, on the basis of characterization results, the molecular ratio of the ligand to metal ions could be confirmed as 1:1.

CONCLUSION

A new macraocyclic Schiff-base ligand derived from condensation of 1,4-di-(4-fluoro-2-aminophenoxy)butane and salicylaldehyde has been synthesized and it's complexation capacity towards Mn2+, Cu2+, Cd2+, and Zn2+ has been studied by adopting one-pot template method. The structures of the complexes were confirmed by 1H-NMR, 13C-NMR, IR spectroscopy and mass spectrometry. We are also especially interested in the antimicrobialactivity of these new complexes.

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