Synthesis, thermal characterization, and antimicrobial activity of lanthanum, cerium, and thorium complexes of amino acid Schiff base ligand
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- Alghool, S., Abd El-Halim, H.F., Abd El-sadek, M.S. et al. J Therm Anal Calorim (2013) 112: 671. doi:10.1007/s10973-012-2628-4
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Metal complexes of La(III), Ce(IV), and Th(IV), with the amino Schiff base ligand, [N-(2-hydroxybenzyl)-l-methionine acid](H3L), were prepared in the presence of triethylamine as a deprotonating agent. All synthesized compounds were identified and confirmed by mass spectra, elemental analyses, molar conductivities, and spectral analyses (UV–Visible, IR, 1H NMR, and 13CNMR). Conductance measurements suggest the non-electrolytic nature and the complexes were isolated in 1:1 ratios. The thermal decomposition of the complexes was discussed in relation to structure. The data from thermogravimetric analysis clearly indicated that the decomposition of the complexes proceeds in four or five steps and the organic part of the complexes decomposed in one or two intermediates. The decomposition of all complexes ended with metal oxide and carbon residue. The Schiff bases and their complexes were screened for their antibacterial (Escherichia coli, Staphylococcus aureus) and antifungal (Aspergillus flavus and Candida Albicans) activities.
KeywordsAmino acidThermal studiesSchiff baseMetal complexesBacterial
The chemistry of metal complexes of Schiff base ligands having nitrogen, oxygen, and sulfur at their donor sites has been extensively studied [1–4].The coordination chemistry of amino acid Schiff base ligands is considered to constitute a new kind of potential antibacterial, anti-inflammation, and anticancer agent [5–7]. Transition metal complexes of salicylaldehyde–amino acid Schiff bases are non-enzymatic models for pyridoxal–amino acid systems that are of considerable importance as key intermediates in many metabolic reactions of amino acids catalyzed by enzymes that require pyridoxal as a cofactor [8–10]. Considerable effort has been devoted to the preparation and structural characterization of Schiff base complexes derived from salicylaldehyde and amino acids such as glycine [11–13]. Tridentate amino acid Schiff base complexes within which oxygen of a phenolate group and oxygen of amino acid, imine nitrogen, and a solvent are presumably coordinated to the oxovanadium center in the equatorial position . The chemical behavior of Schiff base complexes with lanthanide has become increasingly significant in the last few years due to the wide variety of applications of lanthanide complexes in photochemistry, medicine , and supramolecular . Schiff base complexes were prepared in the past decade among some lanthanide complexes showing catalytic activity for polymerization reaction . Recently, a novel amino acid Schiff base heteronuclear complex of Ln(III) was synthesized and results showed that it has a catalytic activity for polymerization . Also, lanthanides(III) show antitumor and antimicrobial activities [19, 20]. In this article, we synthesized and characterized La(III), Ce(IV), and Th(IV) complexes derived from the amino acid Schiff base ligand. The solid complexes were characterized by elemental analysis, mass spectra, thermal analyses, IR, molar conductance measurements, 1HNMR, and 13CNMR. Thermal decomposition of the complexes was also used to infer the structure.
Materials and measurements
La(NO3)3·6H2O, Ce(NO3)3·6H2O, and Th(NO3)4·5H2O from Aldrich Chem. Co. and salicylaldehyde, l-methionine acid, and NaBH4 all from Sigma Chem. Co. were used as received. The solvents methanol, ethanol, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) were reagent grade.
Elemental analysis (C, H, and N) was performed using a Perkin-Elmer CHN 2400 elemental analyzer. The content of metal ions was calculated gravimetrically as metal oxides. Molar conductance measurements of the ligand and its complexes with 1.0 × 10−3 mol/l in DMSO were carried out using a Jenway 4010 conductivity meter. 1H NMR and 13CNMR spectra were recorded on a Bruker 300 MHz NMR spectrometer and referenced to the residual solvent peaks. FAB-mass spectra were recorded on a JEOL SX 102/DA-6000 mass spectrometer/data system using argon/xenon (6 kV, 10 A) as the FAB gas. The UV–Visible Spectra were obtained in DMSO solution (1.0 × 10−3 M) for the ligand and its metal complexes with a Jenway 6405 Spectrophotometer using 1 cm quartz cell, in the range 200–800 nm. IR spectra (4,000–400 cm−1) were recorded as KBr pellets on a Bruker FT-IR spectrophotometer. Thermogravimetric analysis (TG/DTG) was carried out in the temperature range from 25 to 800 °C in a stream of nitrogen atmosphere using a Shimadzu TG 50H thermal analysis instrument. The experimental conditions were platinum crucible, nitrogen atmosphere with a 30 mL min−1 flow rate and a heating rate 10 °C min−1. The purity of the complexes was checked by thin layer chromatography using silica gel-G glass plates as the stationary phase and dichloromethylene and petroleum ether (4:1) as the mobile phase.
For these investigations, the filter paper disk method was applied according to Gupta et al. . The investigated isolates of bacteria and fungi were seeded in tubes with nutrient broth (NB). The seeded NB (1 cm3) was homogenized in the tubes with 9 cm3 of melted (45 °C) nutrient agar. The homogeneous suspensions were poured into Petri dishes. The disks of filter paper (diameter 4 mm) were ranged on the cool medium. After cooling on the formed solid medium, 2 × 10−5 dm3 of the investigated compounds were applied using a micropipette. After incubation for 24 h in a thermostat at 25–27 °C, the inhibition (sterile) zone diameters (including the disk) were measured and expressed in mm. An inhibition zone diameter of over 7 mm indicates that the tested compound is active against the bacteria under investigation. The antibacterial activities of the investigated compounds were tested against Escherichia coli as Gram negative and Staphylococcus aureus as Gram positive. The antifungal activities of the investigated compounds were tested against Aspergillus flavus and Candida albicans. The free ligand and its metal complexes were tested as well as the standard (Tetracycline as an antibacterial agent and Amphotericin B as an antifungal agent). The concentration of each solution was 1.0 × 10−3 mol dm3. Commercial DMSO was employed to dissolve the tested samples.
Synthesis of ligand and its metal complexes
Synthesis of N-(2-hydroxybenzyl)-l-methionine acid, (H3L)
IR (KBr): ν(OH) 3423; ν( NH) 3170, νas(COO) 1619, νs(COO) 1380, ν(phenolic, CO) 1280, 759.
1H NMR (DMSO-d6): δ = 7.28 (dd, 1H, 3J = 7 Hz, 4J = 1.5 Hz, H-2), 7.25 (dd, 1H, 3J = 7.5 Hz, 4J = 1.5 Hz, H-4), 6.88–6.85 (m, 2H, H-1, H-2), 4.21 (AB System, 2H, JAB = 8 Hz, H-5), 3.60 (t, 1H, 3J = 6.5 Hz, H-6), 2.63 (dddd, 2H, 3J = 6.5 Hz, 3J = 13 Hz, 3J = 20 Hz, 4J = 2 Hz, H-7) , 2.13 (m, 2H, H-8), 2.08 (s, 3H, H-9) ppm.
13C NMR (CD3OD): δ = 16.82 (–SCH3), 31.90 (–CH2), 32.54 (–CH2), 45.10 (–CH), 62.32 (benzylic), 115.91, 120.20, 123.74, 131.08, 152.32 and 162.41 (aromatic), 174.00 (–COOH).
The final peak at 255 amu (C12H17O3NS) (calculated atomic mass 255 amu) and other peaks at 222, 192, 150, 122, 106, 93, 79, and 45 amu.
Synthesis of the complexes
The complexes were synthesized by a general method. The metal salt (1 mmol) in a mixture of 10 mL methanol and 5 mL H2O was added dropwise to a solution of ligand H3L (255 mg, 1 mmol) and triethylamine (0.42 mL, 3 mmol) dissolved in 25 mL methanol. The resulting solutions were stirred at RT for 2 h. The precipitates of the complexes were filtered and washed with ethanol and diethyl ether, and finally dried under vacuum. The purity of the complexes was checked by thin layer chromatography. The complexes were isolated as powdered material. Mass spectra, elemental analysis (C, H, N), IR, UV–Visible, and TG analyses confirm the composition of the complexes.
Results and discussion
Elemental analyses and physical data of H3L ligand and its La(III), Ce(IV), and Th(IV) complexes
Compounds' empirical formula
Ω−1 cm2 mol−1
% C Calc./found
Molar conductance values of the complexes in DMSO solvent (10−3 mol dm−3) were in the range of (18–24) Ω−1 cm2 mol−1 and are presented in Table 1. The values adequately confirmed the non-electrolytic nature of the complexes. This fact elucidated that the nitrate ions are absent from outside the coordination sphere .
UV–Visible spectra of the ligand and its metal complexes were recorded in DMSO solution in the wavelength range 200–800 nm. The UV–Visible spectrum of the ligand shows three bands at 252 and 311 nm, which may be attributed to π–π and n–π* transitions. In the metal complexes, this band was slightly red shifted to 256–260 and 317–320 nm, which can be attributed to the binding of these coordination centers to the central metal ions. There is a weak band in the spectra of the complexes at ~735 nm because of weak f–f transition. A new absorption band at 472 nm appeared in the spectra of Ce(IV) complex and may be related to the metal-ligand charge transfer excitation .
Infrared spectral data of H3L ligand and its La(III), Ce(IV) ,and Th(IV) complexes cm−1
Mass spectra studies of La(III), Ce(IV), and Th(IV) complexes
1HNMR and 13CNMR for La(III) and Th(IV)
The 1H NMR spectrum of the ligand was recorded in dimethylsulfoxide (DMSO-d6).The 1H NMR spectra of the complexes were examined in comparison with the H3L ligand. Upon examinations, it was found that the aromatic-H signals appeared in the spectrum of H3L ligand in the range of 6.85–7.28 ppm, which moved to higher frequencies in the complexes as a result of coordinating to lanthanide ion. The methylene-H also shifted to the higher frequencies compared to the parent ligand as a result of high electron density of the lanthanide ions.
AB system appeared in the ligand (each proton of the CH2 group at carbon number 5 was coupled with methyl proton and the adjacent two protons), but disappeared in the 1HNMR of the complexes.
The 13CNMR spectra of the ligand and its metal complexes were recorded in CD3OD. Comparing the Carbon chemical shifts of ligand H3L with complexes reveals that the carbon signals that appeared in the complexes moved to lower frequencies compared to H3L ligand.
1HNMR and 13CNMR for La(III)
1H NMR (DMSO-d6): δ = 7.33–722 (m, 2H, H-3, H-4), 6.88–6.85 (m, 2H, H-1, H-2), 3.98 (s, 2H, H-5), 3.44 (t, 1H, 3J = 6.5 Hz, H-6), 2.53 (d, 2H, 3J = 7 Hz, H-7), 2.23 (m, 2H, H-8), 2.03 (s, 3H, H-9) ppm.
13C NMR (CD3OD): d 15.22 (–SCH3), 30.43 (–CH2), 32.00 (–CH2), 44.76 (–CH), 61.54 (benzylic), 117.66, 121.91, 124.38, 133.11, 153.97 and 165.82 (aromatic), 178.42 (–COOH).
1HNMR and 13CNMR for Th(IV)
1H NMR (DMSO-d6): δ = 7.33–7.01 (m, 4H, H-1, H-2, H-3, H-4), 3.66 (s, 2H, H-5), 3.35 (t, 1H, 3J = 6.5 Hz, H-6), 2.52 (d, 2H, 3J = 7 Hz, H-7), 2.12 (m, 2H, H-8), 2.00 (s, 3H, H-9) ppm.
13C NMR (CD3OD): d 14.31 (–SCH3), 29.25 (–CH2), 30.08 (–CH2), 45.81 (–CH), 60.71 (benzylic), 119.21, 120.73, 125.44, 135.41, 155.35 and 167.31 (aromatic), 177.09 (–COOH).
Thermal analysis data
Thermal data of La(III), Ce(IV), and Th(IV) complexes
TG mass loss % calc./found
C3H7 NO3/2 S
½ (La2O3) + 3C
CeO2 + 3C
NO3 + C6H10SN
ThO2 + 2C
Thermal properties of [La(L)2H2O]·2H2O complex
Thermal properties of [Ce(L)·2H2O]·2H2O complexes
The TG of [Ce(L)·2H2O]·2H2O complex has been divided into the following four stages of decomposition pattern. The first stage, within the temperature range of 25–140 °C with an estimated mass loss of 7.02 % (calculated mass loss 7.75 %), represents the loss of two hydration water molecules. The second stage, within the temperature range of 140–240 °C with an estimated mass loss of 8.11 % (calculated mass loss 7.75 %), represents the loss of two coordinated water molecules. The third stage, within the temperature range of 240–450 °C with an estimated mass loss of 22.01 % (calculated mass loss 22.60 %), is corresponding to the loss of organic part (C3H7NOS). The fourth stage, within the temperature range of 430–600 °C with an estimated mass loss of 16.53 % (calculated mass loss 17.00 %), could be assigned to the successive loss of organic part (C6H7).The final products, formed above 600 °C, consist of CeO2 and three carbon residue with an estimated mass loss of 45.73 % (calculated mass loss 44.80 %).
Thermal properties of [Th(L)·NO3·H2O]·3H2O complex
The thermal decomposition of [Th(L)·NO3·H2O]·3H2O gives four stages of decomposition pattern. The first stage, within the temperature range of 25–140 °C with an estimated mass loss of 8.33 % (calculated mass loss 8.73 %), represents the loss of three hydration water molecules. The second stage, within the temperature range of 140–240 °C with an estimated mass loss of 3.4 % (calculated mass loss 2.91 %), represents the loss of one coordinated water molecule. The third stage, within the temperature range of 230–450 °C with an estimated mass loss of 34.08 % (calculated mass loss 33.50 %), could be assigned to the successive loss of organic part (NO3 + C6H10ONS). The fourth stage, within the temperature range of 450–600 °C with an estimated mass loss of 11.77 % (calculated mass loss 12.28 %), could be assigned to the successive loss of organic part (C6H4).The final products, formed above 600 °C, consist of (ThO2) and five carbon residue with an estimated mass loss of 45.73 % (calculated mass loss 46.57 %).
Antibacterial activity data of H3L ligand and its La(III), Ce(IV), and Th(IV) complexes' inhibition zone/mm
Staphylococcus aureus G+
Escherichia coli G−
Amphotericin B (standard)