Startseite Template synthesis, characterization, and antimicrobial activity of a new lead (II) Schiff base complex
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Template synthesis, characterization, and antimicrobial activity of a new lead (II) Schiff base complex

  • Nesrin Beynek EMAIL logo , Nurcihan Tan und Hayrettin Beynek
Veröffentlicht/Copyright: 6. November 2015
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Main Group Metal Chemistry
Aus der Zeitschrift Main Group Metal Chemistry Band 38 Heft 5-6

Abstract

A novel lead (II) complex of Schiff base containing three bipyridine units was synthesized by the template reaction between a new amine compound, 6-methyl-6′-(2-aminophenoxymethyl)-2,2′-bipyridine, and 2,2′-bipyridine-6,6′-dicarboxaldehyde in the presence of a Pb(II) perchlorate ion. The new amino compound and Schiff base complex, [PbL](ClO4)2·2H2O, were characterized by using matrix-assisted laser desorption/ionization with time-of-flight mass spectroscopy, FAR FT-IR, UV/visible spectroscopy, 1H NMR, elemental analyses (CHN), and molar conductivity measurements. Then the synthesized complex was screened in vitro for its antibacterial and antifungal activities against three Gram-negative and two Gram-positive bacteria and also against Candida albicans as a fungus by the microdilution broth method as a minimal inhibitory concentration. [PbL](ClO4)2·2H2O exhibits moderate activity on Staphylococcus aureus and Listeria monocytogenes when compared with standard drug and also shows moderate antifungal activity on the fungus Candida albicans.

Keywords: antimicrobial activity; lead (II); Schiff base; template reaction

Introduction

The chemistry of metal Schiff base complexes is a fascinating area of intense study for inorganic chemists. The ease of synthesis, stability under a variety of oxidative and reductive conditions, and structural versatility make Schiff bases very important chelating ligands in main group metal and transition metal coordination chemistry. In particular, mixed donor macrocyclic and acyclic Schiff base ligands have been widely studied as complexing agents that may be used for the selective extraction of heavy metals, an area of great interest in environmental and biological chemistry (Izatt and Christensen, 1987; Lindoy, 1989; Bashall et al., 1994; Dietrich et al., 1995; Gerbeleu et al., 1999; Uluçam et al., 2008; Gomez et al., 2010; Mewis and Archibald, 2010; Martinez et al., 2013; Rezaeivala and Keypour, 2014). The coordination chemistry of Pb(II) with N- and O-donor ligands has been investigated in the past decade and frequently discussed in regard of the stereochemical activity of the lone pair of electrons (Davidovich et al., 2009). Recently, we synthesized and characterized some heavy metal ions of mixed donor acyclic and macrocyclic Schiff base ligands and studied their some cytotoxic activities (Adatia et al., 1995; Beynek et al., 1998; Uluçam et al., 2008). Here, we describe a new complex of N8O2 donor ligands, which were obtained from the reaction of Schiff base, resulting from the condensation of 6-methyl-6′-(2-aminophenoxymethyl)-2,2′-bipyridine and 2,2′-bipyridine-6,6′-dicarboxaldehyde with Pb(II) perchlorate in methanol.

Results and discussion

The starting material to amine, 6-bromo-2-methylpyridine (2), was obtained by treating 6-amino-2-methylpyridine (1) in concentrated aqueous hydrobromic acid with bromine and then with sodium nitrite. Then, 6,6′-dimethyl-2,2′-bipyridine (3) was prepared using the coupling of 2 with the nickel catalyst prepared from NiCl2(PPh3)2 and zinc in the presence of Et4NI in THF (Iyoda et al., 1990). Bromination of 3 with N-bromosuccinimide in reflux CCl4 gave 6-methyl-6′-bromomethyl-2,2′-bipyridine (4) (30% yield). 6,6′-Bis(bromomethyl)-2,2′-bipyridine was also isolated, the 1H NMR spectrum of which shows a peak at 4.63 for the methylene hydrogens. Then, the nitro compound, 6-methyl-6′-(2-nitrophenoxymethyl)-2,2′-bipyridine (5), was prepared using Williamson ether synthesis from the same equivalents of 2-nitrophenol and 4 in DMF and nitro group was reduced to amine using metallic iron (Fenton et al., 1987) (Scheme 1). The dialdehyde compound, 2,2′-bipyridine-6,6′-dicarboxaldehyde (7), was synthesized according to our previous report (Uluçam et al., 2008).

Scheme 1: Synthetic route for 6-methyl-6′-(2-aminophenoxymethyl)-2,2′-bipyridine (6). (i) 47% HBr, Br2, NaNO2 then NaOH; (ii) NiCl2(PPh3)2, Zn, Et4NI, THF, under Ar; (iii) NBS, CCl4, benzoyl peroxide; (iv) 2-nitrophenol, DMF, K2CO3; (v) Fe powder, glasial, CH3COOH.
Scheme 1:

Synthetic route for 6-methyl-6′-(2-aminophenoxymethyl)-2,2′-bipyridine (6). (i) 47% HBr, Br2, NaNO2 then NaOH; (ii) NiCl2(PPh3)2, Zn, Et4NI, THF, under Ar; (iii) NBS, CCl4, benzoyl peroxide; (iv) 2-nitrophenol, DMF, K2CO3; (v) Fe powder, glasial, CH3COOH.

A new lead (II) complex was prepared according to the template procedure in which the Schiff base acyclic ligand, (N,N′E,N,N′E)-N,N′-(2,2′-bipyridine-6,6′-diyl-bis(methan-1-yl-1-ylidene))-bis(2-((6′-methyl-2,2′-bipyridin-6-yl)methoxy)benzenamine), L, was condensed from 6-methyl-6′-(2-aminophenoxymethyl)-2,2′-bipyridine (6) and 7 in the presence of the Pb(II) perchlorate ion (Scheme 2).

Scheme 2: Synthetic route for the complex, [PbL](ClO4)2·2H2O.
Scheme 2:

Synthetic route for the complex, [PbL](ClO4)2·2H2O.

The infrared spectra of [PbL](ClO4)2·2H2O in the region of 400–4000 cm-1 show the absorption band 1618 cm-1, which is assigned to the ν(C=N) stretching vibration, indicating the formation of Schiff base products. Furthermore, the absence of the ν(C=O) and ν(N-H) stretching vibrations in the spectra of the complex, as compared to the aldehyde and diamine, respectively, further indicates the formation of the Schiff base. The bands at 1585 and 1487 cm-1 are associated with the ν(C=N)py and ν(C=C) vibrations from the pyridine and phenyl rings (Gill et al., 1961). For the [PbL](ClO4)2·2H2O complex, absorptions at 1085 and 620 cm-1 (very strong broad and strong sharp, respectively) were assigned to the ν3 and ν4 stretching modes of the tetrahedral ionic perchlorate (Nakamoto, 1997). The unsplit nature of these bands indicates that the anions are not coordinated. These results suggest that the perchlorate anions are not coordinated to the metal ion in solution, reflecting the weak coordination ability of this anion. In the spectra of the complex, [PbL](ClO4)2·2H2O, broad band at 3535 cm-1 is considered to arise from vibration due to the O-H stretchings of a water molecule and is consistent with the formulation proposed from elemental analysis.

[PbL](ClO4)2·2H2O was dissolved in acetone and the molar conductivity of 10-3m of its solution at 26.0°C was measured. It is concluded from the result that the complex is considered 2:1 electrolytes with the molar conductance value at 155 Ω-1 mol-1 cm2, indicating the ionic nature of the complex (Geary, 1971). Thus, the complex may be formulated in solution as [PbL](ClO4)2·2H2O. This result is supported by the IR spectra of the complex.

The matrix-assisted laser desorption/ionization with time-of-flight (MALDI-TOF) mass spectrum of [PbL](ClO4)2·2H2O is structurally enlightening since it displays a series of breakdown species. The sequential loss of an anionic perchlorate ion from the neutral parent molecule generates [Pb-208(L-H)(ClO4)], which is the major peak observed in the mass spectra. The loss of the second perchlorate anion occurs to generate [(Pb-208(L-H)]+. The final peaks of interest arise at m/z 756.880 and 782.815 in the spectra from the separation of the metal ion [L-H]- and with sodium [L+Na]+, respectively (Figure 1 and Table 1). The complex [PbL](ClO4)2·2H2O had a 1:2:1 (aldehyde:amine:metal ion) stoichiometry. The formula is in agreement with the mass spectral data. Microanalytical data for the complex also denote a metal-to-ligand ratio of 1:1. In addition, the complex has two moles of water solvate associated with its formulation.

Figure 1: MALDI TOF mass spectrum of [PbL](ClO4)2·2H2O.
Figure 1:

MALDI TOF mass spectrum of [PbL](ClO4)2·2H2O.

Table 1

Fragmentation of [PbL](ClO4)2·2H2O.

Ions from fragmentation
m/zIntensity (%)Assignment
1064.68096.7[Pb-208(L-H)(ClO4)]
964.89020.3[Pb-208(L-H)]+
782.81568.5[L+Na]+
756.88017.2[L-H]-

The proton NMR spectrum of the soluble complex [PbL](ClO4)2·2H2O in acetone-d6 has resonance signals integrating for 38 protons, which is consistent with the given structure (see Supplementary Figure 1). The imine protons appear as the characteristic most downfield singlet (δ 9.40). The aldehyde carbonyl proton (CH=O) appearance of the most downfield singlet at δ 10.18 ppm in the 1H NMR spectrum of dialdehyde compound 7 (Uluçam et al., 2008) and attributable to aldehydic function is noticeably absent. Additionally, in the IR spectra the absence of any carbonyl (ca. 1715–1695 cm-1) peak and the presence of an absorbance at 1618 cm-1 confirmed the formation of an imine-containing complex [PbL](ClO4)2·2H2O. The methyl protons were assigned to the most upfield signal (3.78 ppm) as a singlet, and methylene protons appeared at 5.80 ppm and aromatic protons at δ 6.93 to δ 8.93 in the spectra.

The electronic spectra of [PbL](ClO4)2·2H2O exhibit an intense absorption band in the 293 nm region, which may be assignable to the intra-ligand (π→π*) transitions of the conjugated aromatic rings. The complex has also broad band around the 310 nm region, which may be caused by the nitrogen→metal charge transfer (L→M charge transfer) (Pretsch et al., 1989).

Antimicrobial activity

The complex [PbL](ClO4)2·2H2O was tested using the microbroth dilution method for its inhibitory effects on the growth of bacteria: Escherichia coli, Listeria monocytogenes, and Salmonella thphimurium as Gram-negative; Staphylococcus aureus and Bacillus cereus as Gram-positive; and standard yeast strain Candida albicans because such organisms can achieve resistance to antibiotics through biochemical and morphological modification. The results are compared with ampicillin for bacteria and antifungal activity is referenced against Amphotericin B. The antimicrobial activity results are presented in Table 2. The complex [PbL](ClO4)2·2H2O exhibits moderate activity on S. aureus and L. monocytogenes [minimum inhibitory concentration (MIC) ranged from 32.0 to 64.0 μg mL-1] and also shows antifungal activity on the fungus, C. albicans (MIC value 64.0 μg mL-1).

Table 2

In vitro antimicrobial activity (MIC, μg mL-1) of the Schiff base complex [PbL](ClO4)2·2H2O and standard reagents.

Test compoundsGram-positiveGram-negativefungi
S. aureusB. cereusE. coliL. monocytogenesS. thphimuriumC. albicans
[PbL](ClO4)2·2H2O32128>12864>12864
DMSO (1%)>128>128>128>128>128>128
Ampicillin<0.5>1281624–
Amphotericin B–––––<0.5

Conclusions

In the present work, we have reported the synthesis of a novel lead (II) complex of the acyclic Schiff base containing three bipyridine units. The spectroscopic data from the mass, FT-IR, and 1H NMR spectroscopy together with elemental analysis provide useful information about their formation and structural characterization. The coordination chemistry of the toxic heavy metal ions is of crucial importance to the design of complexing agents which may serve as sensors or extractants of these metals for environmental or recycling purposes. Its antimicrobial properties have also been discussed. The complex [PbL](ClO4)2·2H2O exhibits moderate activity on S. aureus and L. monocytogenes (MIC ranged from 32.0 to 64.0 μg mL-1) and also shows antifungal activity on the fungus, C. albicans (MIC value 64.0 μg mL-1). Therefore, it can be said that this complex appears to be a promising candidate for environmental and biological applications.

Experimental details

Materials and equipment

6-Methyl-6′-(bromomethyl)-2,2′-bipyridine (4) (Beynek et al., 2008) and 2,2′-bipyridine-6,6′-dicarboxaldehyde (7) (Uluçam et al., 2008) were synthesized according to the published procedures. All other chemicals were purchased from Merck or Sigma-Aldrich Chemical Company and were used without further purification.

Melting points were determined using a Gallenkamp MPD350.BM2.5 digital melting point apparatus and were uncorrected. The compounds were checked for purity by TLC on silica gel 60 F254 (Merck). Fourier transform infrared (FT-IR) spectra were recorded on a Perkin Elmer Frontier FAR-FT-IR spectrometer, fitted with a GladiATR platform, at 4000–400 cm-1. 1H NMR spectra were recorded with a Varian spectrometer (300 MHz) in CDCl3 or acetone-d6 as solvent. 13C NMR spectra were recorded with a Varian spectrometer at 75.5 MHz in CDCl3 as solvent. Chemical shifts are reported as δ values in parts per million (ppm). Splitting patterns were designated as follows: s=singlet, d=doublet, t=triplet, m=multiplet, br=broad; Matrix-assisted laser desorption/ionization timeof-flight mass spectrometry (MALDI-TOF-MS) measurements were performed on a Bruker Microflex LT MALDI-TOF. This instrument has a maximum resolution of 4000. Elemental analyses were performed on a CHNS-O Thermo Finnigan Flash EA1112 elemental analyzer. Electronic spectra were recorded at room temperature on a Schimadzu UV-1700 Pharma spectrophotometer using 1 cm quartz cuvettes and DMSO as solvent. Molar conductance was measured using a WTW Inolab Thermal 720P conductivitymeter where the cell constant was calibrated with 0.001 m KCl solution, and acetone was used as solvent.

Syntheses

6-Methyl-6′-(2-nitrophenoxymethyl)-2,2′-bipyridine (5):

The new precursor nitro compound 5 was prepared using a method adapted from that described by Cannon et al. (1967) for synthesis of 1,2-bis(2-nitrophenoxy)ethane. A solution of 2-nitrophenol (1.392 g, 10 mmol) in hot anhydrous DMF (40 mL) was treated with K2CO3 (0.690 g, 5 mmol) and gently boiled. To this solution 4 (2630 g, 10 mmol) in 20 mL anh. DMF was added for 30 min. Gently, reflux was maintained for 4 h and a half of the solvent was distiled from the mixture, which was then poured into H2O (400 mL). Yellow product was filtered off and washed with dilute aq. NaOH solution and H2O, and then dried. Yield: 2.90 g (90.2%). M.p. 118–120°C. 1H-NMR (300 MHz; CDCl3): δ 2.63 (3H, s, CH3), 5.43 (2H, s, CH2), 7.07 (1H, d, J 7.3 Hz), 7.20 (1H, d, J 7.5 Hz), 7.53 (1H, d, J 6.7 Hz), 7.67–7.89 (3H, m), 7.84–7.92 (2H, m), 8.16 (1H, d, J 7.2 Hz) 8.33 (1H, d, J 7.3 Hz), 13C-NMR (75 MHz; CDCl3): δ 24.93 (CH3), 71.88 (CH2), 115.1, 118.41, 120.62, 120.98, 121.17, 123.65, 126.11, 134.56, 137.34, 138.26, 140.13 (C), 152.00 (C), 155.55 (C), 156.21 (C), 157.08 (C), 158.35 (C).

6-Methyl-6′-(2-aminophenoxymethyl)-2,2′-bipyridine (6)

The reduction of 6-methyl-6′-(2-nitrophenoxymethyl)-2,2′-bipyridine (5) to the amine compound was effected by modification of a previously reported method (Fenton et al., 1987). 6-Methyl-6′-(2-nitrophenoxymethyl)-2,2′-bipyridine (5) (1.6286 g, 5 mmol) was slurried in EtOH-H2O (1:1, 80 mL) containing iron powder (1.3963 g, 25 mmol) and brought to reflux. Glacial acetic acid (1.2 mL) was added dropwise and reflux was maintained for a further 18 h after which the mixture was filtered hot (using well-packed Celite aid). The cooled filtrate was extracted with chloroform and oily product was obtained by evaporation of organic phase under vacuum. Crystallization from EtOH:H2O (1:1, 20 mL) gave the pale brown powder product in 55% (0.88 g) yield. M.p. 118–120°C. IR: νmax/cm-1; 3375 m (NH2), 3290 m δ(NH2), 1614 (aromatic ring). 1H-NMR (300 MHz; CDCl3): δ 2.57 (3H, s, CH3), 3.87 (2H, s, (br) NH2), 5.24 (2H, s, CH2), 6.60–6.84 (4H, m), 7.17 (1H, d, J 7.32 Hz), 7.47 (1H, t, J 7.3 Hz), 7.7 (1H, t, J 7.6 Hz), 7.81 (1H, t, J 7.7 Hz), 8.74 (1H, d, J 7.63 Hz), 8.32 (1H, d, J 7.32 Hz. 13C-NMR (75 MHz; CDCl3): δ 24.93 (CH3), 71.56 (CH2), 112.61, 115.59, 118.49, 118.76, 120.31, 121.17, 121.91, 123. 6, 136.69, 137.34, 137.87 (C), 146.40 (C), 155.70 (C), 156.23 (C), 157.07 (C), 58.23 (C).

Macrocyclic Schiff base complex [PbL](ClO4)2·2H2O:

2,2′-Bipyridine-6,6′-dicarboxaldehyde (7) (0.221 g, 1.0 mmol) was dissolved in MeOH (20 mL) containing Pb(ClO4)2·H2O (0.424 g, 1.0 mmol). The solution was stirred vigorously and heated while a methanolic solution of 6-methyl-6′-(2-aminophenoxymethyl)-2,2′-bipyridine (6) (0.60 g, 2.0 mmol) was added slowly. The resultant solution was refluxed for 4–5 h and filtered hot. The solvent of the reaction mixture was reduced to half its original volume, and then the mixture was placed in a refrigerator to induce precipitation. The yellow powder product was filtered and dried. Yield 0.65 g, 55%. νmax/cm-1; 3535 br (OH), 1618 m (C=N), 1585 m, 1487 m, 1439 m, 1370 w, 1291 w, 1260 m, 1232 w, 1180 w, 1154 w, 1085 vs. (ClO4-), 1035 m, 999 m, 907 w, 784 s, 761 m, 669 w, 620 vs. (ClO4-), 592w. 1H NMR (300 MHz; acetone-d6): δ 2.83 (6H, s, CH3), 5.80 (4H, s, CH2), 6.93 (2H, d, J 7.9 Hz), 7.16 (2H, t, J 7.1 Hz), 7.28–7.49 (10H, m), 7.68 (2H, t, J 7.8 Hz), 7.81 (2H, d, J 7.9 Hz), 7.87 (2H, d, J 7.5 Hz), 8.36 (2H, d, J 7.6 Hz), 8.68 (2H, t, J 7.9 Hz), 8.93 (2H, d, J 8.3 Hz), 9.40 (2H, s, HC=N). ΛM (Ω-1 mol-1 cm2) (acetone, 10-3m) 26.0°C: 155 (2:1). μeff (BM), 24°C: 1.03. Electronic spectrum (DMSO). λmax: 348 nm π→π* (C=N) , 293 nm π→π* (aromatic ring). MALDI-TOF (m/z): found: 1064.68, calc.: 1064.24 for [Pb-208(L-H)(ClO4)]; found: 964.890 calc: 965.281 for [Pb-208(L-H)]+; found: 782.815, calc: 782.302 for [L+Na]+; found: 756.880, calc: 757.304 for [L-H]- (Table 1). Anal. calc. for C48H42N8O12 Cl2Pb; C 48.00; H 3.52; N 9.33; found: C 48.22; H 3.72; N 9.24.

Antibacterial and antifungal assays:

The microdilution broth method was used in antimicrobial activity studies according to CLSI (Clinical Laboratory Standards Institute) standards (National Committee for Clinical Laboratory Standards, 1997, 2002). Bacterial strains used in this study were E. coli ATCC 25922, L. monocytogenes ATCCF19115, and S. thphimurium ATCC 14028 as Gram-negative; S. aureus ATCC 25923 and B. cereus ATCC 11778 as Gram-positive bacteria; and standard yeast strain was C. albicans ATCC 10231. Different concentrations of the complex [PbL](ClO4)2·2H2O (0.5; 1.0; 2.0; 4.0; 8.0; 16.0; 32.0; 64.0; 128.0 μg mL-1) in DMSO (1%) solution of culture medium were prepared in 96-well flat-bottomed microculture plates by the double dilution method. After micro-plates were prepared and inoculated, they were incubated for bacteria at 37°C for 24 h. The inoculum density of each bacterial and fungus isolate was standardized with 0.5 McFarland turbidity standards. The suspension had a final inoculum of 5×107 cfu mL-1. The MIC were defined as the lowest concentration of the compound giving complete inhibition of visible growth. Antimicrobial effects of [PbL](ClO4)2·2H2O were investigated against test microorganisms. The solvent, DMSO (1%), used for the preparation of compounds did not show inhibition against the tested organisms. The serial dilutions of an antibacterial agent without microorganism was used as negative control, whereas media with ampicillin (standard antibiotic) and amphotericin B (standard antifungal drug) were used as the positive controls and media with microorganism was used as growth control.

Corresponding author: Nesrin Beynek, Faculty of Science, Department of Chemistry, Trakya University, Edirne 22030, Turkey, e-mail:

Acknowledgments

The authors thank the Trakya University for financial support under the research grant no. TUBAP-2013/29.

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The online version of this article (DOI: 10.1515/mgmc-2015-0018) offers supplementary material, available to authorized users.

Received: 2015-5-12
Accepted: 2015-10-6
Published Online: 2015-11-6
Published in Print: 2015-12-1

©2015 by De Gruyter

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antimicrobial activity; lead (II); Schiff base; template reaction
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  1. Frontmatter
  2. Research Articles
  3. Two structurally related, hydrogen-bonded polymorphs of the zwitterionic complex trichlorido- ((dimethylphosphoryl)methanaminium-κO)zinc(II)
  4. CuO/polypyrrole nanocomposites as a marker of toxic lead ions for ecological remediation in contrast with CuO and polypyrrole
  5. Template synthesis, characterization, and antimicrobial activity of a new lead (II) Schiff base complex
  6. Synthesis of novel penta and hexa coordinated monobutyltin(IV) derivatives based on oximes and N-protected amino acids
  7. Synthesis, structural characterization, and antibacterial activity of tricyclohexyltin aryloxyacetates
  8. Short Communications
  9. Synthesis and crystal structures of extremely bulky phosphinoamido and phosphinoamino germanium(II) chloride complexes
  10. [4-tBu-2,6-{P(O)(OiPr)2}2C6H2Sn(PPh3)Cr(CO)5]ClO4 – a salt containing a cationic triphenylphosphane-stabilized organostannylene transition metal complex
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Heruntergeladen am 21.9.2025 von https://www.degruyterbrill.com/document/doi/10.1515/mgmc-2015-0018/html
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