Schiff bases (also known as imine or azomethine) and their complexes are synthesized from the condensation of an amino compound with carbonyl compounds under specific conditions. They exhibit a broad range of biological activities including antifungal, anti-bacterial, antimalarial, anti-proliferative, anti-inflammatory, antiviral, antipyretic properties and extensively used for industrial purposes  -  . A number of Schiff base complexes exhibit excellent catalytic activity in a wide range of chemical reactions both in homogeneous and heterogeneous catalysis  . Over the past few years, there have been many reports on their applications as catalyst in several reactions such as polymerization reaction, reduction of thionyl chloride, oxidation of organic compounds, reduction reaction of ketones, aldol reaction, Henry reaction, epoxidation of alkenes, hydrosilylation of ketones, synthesis of bis(indolyl) methanes and DielseAlder reaction  -  . Structurally, a Schiff base is a nitrogen analogue of an aldehyde or ketone in which the carbonyl group (CO) has been replaced by an imine or azo-methine group, as shown in Figure 1     .
Methionine[2-amino-4-(methylthio) butanoic acid] is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated -COO− form under biological conditions), and an S-methylthio ether side chain, classifying it as a nonpolar, aliphatic amino acid. Methionine is a bipodal ligand which has greater interest in coordination chemistry, as shown in Figure 2(a). Salicylaldehyde (2-hydroxybenzaldehyde) is the organic compound with the formula C6H4CHO-2-OH. Salicylaldehyde is a key precursor to a variety chelating agents, some of which are commercially important, as shown in Figure 2(b). Schiff base ligands can form very stable complexes compare to some other ligands, as shown in Figure 2(c)    .
Chemicals and Instruments
All chemicals used were of analytical grade (AR) and of the highest purity
Figure 1. General scheme for formation of Schiff bases.
Figure 2. Chemical structure of methionine (a) salicyldehyde (b) and Schiff base (c).
available. Salicylaldehyde and methionine (Sigma-Aldrich, Germany) were used to prepare the Schiff base that was mixed with (each) Mn(II) chloride tetrahydrate, La(III) chloride hexahydrate and Co(III) nitrate hexahydrate (Merck, Germany) in order to synthesize their complexes. The organic solvents used included methanol, ethanol, chloroform, acetone and DMSO. These solvents were either spectroscopically pure from Merck, Germany or purified by the recommended methods and tested for their spectral purity.
Percentage of carbon, hydrogen, sulfur and nitrogen present in the complex were recorded in a CHNS elemental analyzer (VARIO-MICRO V1.6.1, GmbH, Germany). Melting points of the studied compounds were taken in a melting points apparatus (SMP 11, Stuart, England), which have the capacity of recording the temperature up to 350˚C. Infrared spectra of the compounds were recorded in a FTIR spectrophotometer (IRPrestige-21, Shimadzu, Japan) in the range of 400 - 4000 cm−1 using KBr pellets. Ultraviolet-visible spectral analysis was carried out at room temperature in dimethyl sulfoxide (DMSO) in a double beam Shimadzu UV-visible spectrophotometer, model UV-1650 PC. Magnetic susceptibility of the complexes were determined using SHERWOOD SCIENTIFIC Magnetic Susceptibility Balance, Cambridge, England, and Model Magway MSB Mk1. Thermo gravimetric analysis of the complexes was performed in nitrogen atmosphere, using a TGA-50 analyzer, Shimadzu, Japan.
Synthesis of the Schiff base ligand (L)
Methionine-salicyldehyde Schiff base was synthesized from 10 mL water containing 0.1497 g (1 × 10−3 M) methionine and 20 mL ethanol that had been dissolved in advance 0.1209 g (1 × 10−3 M) salicylaldehyde. In order to maintain the pH value 9.0 of the mixture, 1 × 10−3 M NaOH solution was added and the mixture was refluxed for 2 hours. The reaction is clarified in Figure 3:
Synthesis of the metal complexes (ML)
The metal complexes were synthesized in the same manner by mixing 1 × 10−3 M ethanolic solution (20 mL) of methionine-salicylaldehyde Schiff base with 20 mL ethanol containing Co(NO3)3∙6H2O (1 × 10−3 M) or LaCl3∙6H2O (1 × 10−3 M) or MnCl2∙4H2O (1 × 10−3 M). Then the mixtures were refluxed for 4 hours. Finally, the volume of solutions was reduced and in all cases, an occurrence of colored precipitate was observed this being assigned to Co(III) complex or Mn(II) complex or La(III) complex respectively. It was filtered, washed with water and dried at room temperature. The reaction is clarified in Figure 4.
3. Results and Discussion
The Schiff base ligand was synthesized by refluxing the appropriate amount of methionine with salicylaldehyde in mixed medium (water-ethanol). The metal complexes of Schiff base ligand were prepared by the stoichiometric reaction of the corresponding metal(II) chloride, metal(III) nitrate and metal(III)chloride with the ligand in a molar ratio of 1:1. The complexes were obtained as air-stable solids, which are insoluble in water, partially soluble in acetone, methanol, ethanol and soluble in dimethyl sulfoxide (DMSO). The color, elemental analysis data along with some other physical properties of the synthesized complexes are listed in Table 1    .
Figure 3. Synthesis route of methionine-salicylaldehyde Schiff base.
Figure 4. Synthesis route of metal-Schiff base complex.
Table 1. Analytical and physical data of the metal complexes.
Here, L = C12H13SNO3.
Infra red Spectra
IR spectra for methionine, Schiff base ligand and metal complexes were recorded in wavenumber range between 400 - 4000 cm−1. IR spectrum of methionine showed absorption bands at 3000 - 2850, 2800 - 2550, 2106, 1350 - 1000 and 900 - 690 cm−1 corresponding to v(N-H)sym, v(O-H)carboxylic, v(N-H)asym, v(C-N)amines and v(C-H)bent stretching frequencies, respectively. A comparison of the IR spectra of methionine and Schiff base ligand provides proof of the formation of the Schiff base between methionine and salicylaldehyde. The principal band responsible for this is the new absorption band at 1636 cm−1 attributed to v(C=N) stretching vibration. The IR spectra of different metal complexes are given bellow Figure 5. The most important absorption bands and their assignments are listed in Table 2.
Infrared Spectrum of methionineInfrared Spectrum of [Co(C12H13SNO3)] complexInfrared Spectrum of [La(C12H13SNO3)] complexInfrared Spectrum of [Mn(C12H13SNO3)] complex
Figure 5. Infra red spectrum of methionine and metal complexes.
Table 2. Relevant IR data of the Schiff base and its metal complexes (cm−1).
Sh = sharp, m = medium, br = broad, w = weak, s = small.
The main bands in the IR spectrum of the free Schiff base are at 3450, 1636, 1375 and 1225 cm−1 attributed to v(OH),v(C=N) azomethine, v(C-N) amine and v(CO) phenolic, respectively     . The band at 3450 cm−1 in the ligand spectrum disappears from the complexes as an indicative of phenolic group deprotonation. Instead, the band at 1252 cm−1 assigned to v(CO) phenolic vibration is shifted left in comparison with the ligand, as a result of both deprotonation and coordination of phenolic oxygen. These shifts indicate the participation of the oxygen atom of the deprotonated hydroxyl group which is situated in a favorable position (orto) towards the azomethine group. The spectra of metal complexes exhibited a broad band around 3348 cm−1, 3401 cm−1 and 3050 cm−1 which is assigned to different v(N-H), associated with the complexes. IR spectra of the ligand presented a band at 1636 cm−1 attributed to v(C=N) azomethine group, which is shifted to a higher value in two complexes at 1640 and 1645 cm−1 suggesting that the ligand is coordinated to the metal ion through nitrogen atom from azomethine group. The new absorption bands v(M-N), v(M-O) and v(M-S) observed in spectra of complexes at 545, 766, 486 cm−1; 539, 759, 416 cm−1 and 656, 897, 448 cm−1 respectively (see inserted spectrum in Figure 5) shows the coordination of the ligand through nitrogen and oxygen   . The IR spectra of the complexes contain several absorption bands from the Schiff base and also new absorption bands, these being attributed to the coordination of the ligand to metal ions through phenolic oxygen atom, carboxylic oxygen atom, sulphur atom and imino nitrogen atom from azomethine bond.
Magnetic measurements and electronic spectra
The complex [CoL](NO3) showed 4.59 BM, indicated a tetrahedral geometry, which is related to three unpaired electrons. Furtthermore, the square planar geometry of the complex [MnL] showed 5.24 BM which is related to four unpaired electron. On the other hand, square planar complex of [LaL](H2O)Cl have one unpaired electron which showed the magnetic moment 1.47 BM, as shown in Table 3.
Electronic spectra of the complexes showed in the Figure 6. The electronic spectra of the cobalt (III) complex exibit one absorption band at 364 nm which could be assigned ligand to metal charge transfer transition and the other absorption band at 478 nm corresponds to intra ligand charge transfer transition. These transitions suggest tetrahedral and square planar structure respectively. On the other hand, the lanthanide (III) complex shows absorption band at 362
Table 3. Magnetic moments and electronic spectra of metal complexes.
Figure 6. Electronic spectra of the complexes.
nm and 480 nm respectively. The intense transition at 362 nm occur due to the ligand to metal charge transfer transition. The manganese (II) complex shows only one intense absorption band at 376 nm due to intra ligand charge transfer transition  . The electronic spectra of the complexes are agreed with magnetic values.
Thermal analysis (TGA)
Thermogravimetric analysis of the metal complexes are used to: i) get information about the thermal stability of these new complexes,(ii) decide whether the water molecules (if present) are inside or outside the inner coordination sphere of the central metal ion, and iii) suggest a general scheme for thermal decomposition of these complexes  . In the present investigation, heating rate were suitably controlled at 10˚C min−1 under nitrogen atmosphere, and the weight loss was measured from the ambient temperature up to 800˚C. The data are provided in Table 4.
The thermogram of the [Co(III)L](NO3) complex shows three decomposition steps within the temperature range 25˚C - 800˚C. The first step of decomposition within the temperature range 150˚C - 320˚C correspond to the loss of salicylaldehyde molecule with a mass loss of 33.27% (calc: 39.38%). The second step of decomposition within the temperature range 320˚C - 640˚C correspond to the loss of methionine molecule with a mass loss of 38.19% (calc: 39.88%). The subsequent step (upto 799˚C) correspond to the removal of the organic part of the ligand, leaving metal oxide as a residue. The remaining residue corresponding to a mass of 24.45% (calc: 23.70%).
The thermogram of the [La(III)L](H2O)Cl complex shows four decomposition steps within the temperature range 28˚C - 800˚C. The first step of decomposition within the temperature range 28˚C - 166˚C correspond to the loss of water
Table 4. Characteristics of the thermal degradation steps of Co(III), La(III) and Mn(II) complexes.
molecules of hydration with a mass loss of 9.14% (calc: 8.40%). The second step of decomposition within the temperature range 166˚C - 360˚C correspond to the loss of methionine molecule with a mass loss of 31.11% (calc: 34.90%). The third step of decomposition within the temperature range 360˚C - 625˚C correspond to the loss of salicyldehyde molecule with a mass loss of 22.67% (calcd: 29.93%). The subsequent step (upto 799˚C) correspond to the removal of the organic part of the ligand, leaving metal oxide as a residue. The remaining residue corresponding to a mass of 35.40% (cala: 36.23%).
The thermogram of the [Mn(II)L] complex shows three decomposition steps within the temperature range 31˚C - 800˚C. The first step of decomposition within the temperature range 166˚C - 408˚C correspond to the loss of methionine molecule with a mass loss of 45.96% (calc: 43.42%). The second step of decomposition within the temperature range 408˚C - 708˚C correspond to the loss of salicylaldehyde molecule with a mass loss of 24.66% (calc: 35.56%). The subsequent step (upto 799˚C) correspond to the removal of the organic part of the ligand, leaving metal oxide as a residue. The remaining residue corresponding to a mass of 23.44% (cala: 25.30%).
X-ray Powder Diffraction (XRD)
X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions. The analyzed material is finely ground, homogenized, and average bulk composition is determined. X-ray diffractometers consist of three basic elements: an X-ray tube, a sample holder, and an X-ray detector. Cu Kα or Mo Kα radiations are used in the x-ray powder diffractometer. For typical powder patterns, data is collected at 2θ from ~5˚ to 70˚, angles that are preset in the X-ray scan. X-ray powder diffraction is most widely used for the identification of unknown crystalline material.
The XRD pattern of different metal complexes is given in Figure 7.
The sharp line diffractogram indicates that Co(III) and Mn(II) complexes are crystalline in nature. The XRD pattern of lanthanum complex indicates that it is amorphous in nature.
From literature we know, for body centred crystal (h + k + l = 2n) or the ratio of Sin2θ must be 2:4:6:8:10:12:14:16:18. The XRD data in Table 5 fulfills the condition. So the structure of the complex [Mn(C12H13SNO3)] and [Co(C12H13SNO3)](NO3) is body centred.
From the result of various studies described and discussed here and the concept of essential criteria of complex formation that the associated ligands must fulfill the coordination number and the oxidation state of the metal ion, the most probable structure of the complexes are given in Figure 8.
The new Co(III), Mn(II) and La(III) complexes with the Schiff base derived from methionine and salicylaldehyde were synthesized and characterized. The
Figure 7. XRD pattern of Co(III), La(III) and Mn(II) complexes.
Table 5. XRD data of [Mn(C12H13SNO3)] and [Co(C12H13SNO3)](NO3) complexes.
Figure 8. Probable structure of [Mn(C12H13SNO3)] complex (a), [La(C12H13SNO3)](Cl)(H2O) complex (b) and [Co(C12H13SNO3)](NO3) complex (c).
data collected from IR spectra showed that the Schiff base behaves as a tetra-dentate ligand coordinated in the lanthanum, cobalt and manganese complexes. Electronic spectra and magnetic measurements indicated tetrahedral geometry for the cobalt complex and square planer geometry for lanthanum and manganese complexes. The thermal analysis presented the thermal degradation data of the three complexes. The XRD data revealed that the manganese and cobalt complexes were crystalline whereas the lanthanum complex was amorphous in nature.
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