Synthesis, Physical Characterization and Biological Evaluation of Schiff Base Cr(III), Mn(III) and Fe(III) Complexes

M(III) complexes with Cr, Mn and Fe with Schiff base derived from 2-amino-4-ethyl-5-hydroxy benzaldehyde and oxocarbohydrazide were synthesized and characterized by several techniques using elemental analysis (C, H, N), molar conductance measurements, magnetic measurements, electronic, mass and IR spectral studies. On the bases of these studies, a five coordinated square pyramidal geometry for all these complexes has been proposed and found to be potential antimicrobial agent. The Schiff base ligand and complexes were also tested for their antimicrobial activity (against the bacteria Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Bacillus megaterium and the fungi Kluyveromyces fragilis, Rhodotorula rubra, Candida albicans and Trichoderma reesei) to assess their inhibiting potential. An attempt is also made to correlate the antimicrobial activity with geometry of the complexes. All complexes were found to be less active against the pathogens E.coli, S.aureus and P.aeruginosa. Cr(III) complex showed the best antimicrobial activity, but ligand alone was found to be active against the fungus T.reesei.


Introduction
The field of Schiff base complexes was fast developing on account of the wide variety of possible structure for the ligands depending up on the aldehydes and amines.Many Schiff base and their complexes have been widely studied because of their industrial and biological applications 1,2 .Some Schiff base was tested for fungicidal activity which is related to their chemical structure 3 .Schiff base compound containing an amine group (-RC=N-) are usually formed by the condensation of a primary amine with an active carbonyl.The cross linking agents can also be derived from metal complexes with O-N-or -S ligands for example, the intra coordination salt such as salicylates, anthranilates and the aliphatic or aromatic amines can form strong five or six membered chelates rings which are able to produce the metal containing crosslinking agents with required properties [4][5][6][7] .Synthesis of oxovanadium(IV) complexes of the Schiff base derived by the condensation of 2aminobenzaldehyde with various diamines (1, 2-diaminoethane, 1, 2-diaminopropane, 1, 3-diaminopropane) and characterized by elemental analysis, spectral data and electrochemical studies 8 .The complexes of Co(II), Ni(II) and Cu(II) ions with the Schiff base derived from the condensation of salicylaldehyde and o-aminophenol or 2-aminobenzoic acid were synthesized and characterized by using different techniques, in particular the elemental analysis, molar conductance measurements, IR and electronic spectra [9][10] .
The aim of the present paper is to prepare, characterize and study the antimicrobial activity of the Schiff base complexes derived from oxocarbohydrazide and 2-amino-4-ethyl-5-hydroxy benzaldehyde.

Synthesis of Schiff base
The Schiff base (ligand) was prepared by mixing a warm dilute ethanol solution of oxocarbohydrazide (5 mmol) with 2-aminobenzaldehyde (10 mmol) under reflux for two hours.The precipitate was then removed from the reaction mixture by filtration 10,11 , washed with ethanol and dried in desiccator over anhydrous CaCl 2 .The colour of the solid product was yellow (yield 65%).

Synthesis of complexes
The Schiff base was taken in ethanol (50 cm 3 ) and stirred gently for one hour to give homogeneous solution and added trivalent chromium, manganese or iron chlorides salts (25 cm 3 ).The resulting solution was refluxed for 6-8 hours.The mixture was concentrated to half of its initial volume and kept in a desiccator for two days over anhydrous CaCl 2 .
The complexes were then filtered, washed with ethanol and dried.The complexes are soluble in DMF and DMSO but are insoluble in other common organic solvent and water.They were thermally stable up to 523-553K and then decompose.

Methods
The microanalysis of C, H and N were estimated by elemental analyzer (Perkin Elmer 2400), at SAIF, CDRI, Lucknow, India and the metal contents of Mn(III), Co(III) and Cu(III) was determined using atomic absorption spectrophotometer Perkin Elmer 5000.(See Table 1 containing also some physical properties of the prepared compounds).The conductivity was measured on digital conductivity meter (HPG system, G-3001) in DMSO at room temperature.The magnetic susceptibility measurements were carried out on vibrating sample magnetometer (Model PAR 155) at room temperature.Electronic spectra (in DMSO) were recorded on a Hitachi 330 spectrophotometer (5815-32573 cm -1 ).The IR spectra were performed using a FT-IR spectrophotometer Perkin Elmer 1650 in the range 4000-200 cm -1 using Nujol Mull (In Tables 1 and 2 molar conductivities, magnetic moments and spectral data are given).The FAB mass spectra (at room temperature) were recorded on VG-70-S mass spectrometer.The purity of the Schiff base and its complexes were confirmed also by TLC [microcrystalline cellulose (E.Merck) was used for the preparation of thin layer, complete separation was achieved with the solvent system acetone/conc.HCl/water (86:8:7).R F values increase in the following order of M(II) complexes: Mn, Co, Cu (0.20, 0.34, 0.60)] and HPLC.ol -1 cm -1) for HL (35800), Cr(III) complex (24500, 8200), Mn(III) complex (32400, 12600) and Fe(III) complex (18300, 8900).

Antimicrobial activity
The antibacterial and antifungal activities of the newly synthesized compounds were evaluated by agar well diffusion method 12 .All the microbial cultures were adjusted to 0.5 McFarland standards, which is visually comparable to a microbial suspension containing approximately 1.5x10 8 cfu/mL 13 .20 mL of agar media was poured into each Petri plate and plates were swabbed with 100 µL inocula of the test microorganisms and kept 15 min for adsorption.Using sterile cork borer of 8 mm diameter, wells were bored into the seeded agar plates and these were loaded with a 100 mL volume with concentration of 1.0 mg/mL of each compound dissolved in the dimethylsulphoxide (DMSO).All the plates were incubated at 300K for 24 h.Antimicrobial activity of all the synthesized compounds was evaluated by measuring the zone of growth inhibition against the test organisms with zone reader (Hi antibiotic zone scale).The medium with dimethylsulphoxide (DMSO) as solvent was used as a negative control whereas media with Ciprofloxacin (standard antibiotic) and Griseofulvin (standard antifungal drug) were used as positive control.The experiments were performed in triplicates.
The ligand HL and its Cr(III), Mn(III) and Fe(III) complexes were tested for their antibacterial and antifungal activity against the bacteria: Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Bacillus megaterium and the fungi: Kluyveromyces fragilis, Rhodotorula rubra, Candida albicans and Trichoderma reesei procured from Microbial Type Collection and Gene Bank, Institute of Microbial Technology (IMTECH), Chandigarh, India.

Results and Discussion
The elemental analysis (C, H, N, M) data of the metal chelates showed that they may be represented by the formula [M(C 19 H 24 N 6 O 3 )X]X 2, (M(III) = Cr, Mn and Fe; X=Cl).The purity of the Schiff base and its complexes was also confirmed by TLC and HPLC techniques.
The measurements of molar conductance in DMSO show that these chelates are 1:2 type of electrolytes (conductance values were 30-60 S cm 2 mol -1 , whereas the literature range was 30-180 Scm 2 mol -1 ) 14 .The tests for anions are positive before decomposing and after decomposing the chelates with concentrated HNO 3 showing their presence outside as well as inside the coordination sphere.Several attempts failed to obtain a single crystal suitable for x-ray crystallography.However, the analytical, spectroscopic and magnetic data enable us to predict the possible structure of the synthesized complexes.The low molar conductance of the complexes might arise due to large size of the cation coordination sphere, which might have low ionic mobility, the value of molar conductance's suggest that complexes are electrolytes and that chloride is present as the counter ion.
The IR spectra provide valuable information regarding the nature of functional group attached to the metal atom.The presence of a single medium band in the region 3250-3330 cm - 1 in the complexes may be assigned to N-H stretching vibrations 15,16 .It was noted that a pair of bands corresponding to υ (NH 2 ) at 3245 cm ¯1 and 3309 cm ¯1 are present in the spectra of the thiocarbohydrazide .The value of υ (C=N) stretching vibration is found lower (1520-1560 cm ¯1) than the expected value (1580 -1650 cm ¯1).This may be explained on the basis of a drift of lone pair density of azomethine nitrogen towards the metal atom 17,18 indicating that coordination takes place through nitrogen of (C=N) groups.The bands present in the range 3020-3040 cm ¯1 may be assigned due to (C-H) stretching vibrations of benzil and naphthalene rings 19 .The C-N stretching was in the range 1000-1300 cm ¯1.The band near 1700 cm ¯1 in oxocarbohydrazide may be assigned to free υ (C=O).The band at 3292-3438 cm ¯1 is due to the presence of -OH group in the complexes.This band is also present in the spectra of all the complexes, which indicate that oxygen is not coordinating to the metal atom 20,21 .The far IR spectra show bands in the region 420-450 cm ¯1 corresponding to υ (M-N) vibrations [22][23][24] .The presence of bands in all complexes in the region 420-450 cm ¯1 originates from the (M-N) azomethine vibrational modes and identifies coordination of azomethine nitrogen 25 .The bands present at 290-310 cm ¯1 may be assigned to υ (M-Cl) vibrations 26,27 .The FAB mass spectra of ligand, Cr(III), Mn(III) and Fe(III) complexes have been recorded.All the spectra exhibit parent peaks due to molecular ions (M + ).The proposed molecular formula of these complexes was confirmed by comparing their molecular formula weights with m/z values.The molecular ion (M + ) peaks obtained for various complexes are as follows: m/z 384 (ligand), m/z 542 (Cr(III) complex), m/z 545 (Mn(III) complex), m/z 546 (Fe(III) complex) These data are in good agreement with the proposed molecular formula for these complexes i.e. [M(C 19 H 24 N 6 O 3 )X]X 2 .Where M = Cr(III), Mn(III) and Fe(III) and X = Cl ¯.In addition to the peaks due to the molecular ions, the spectra exhibit peaks assignable to various fragments arising from the thermal cleavage of the complexes.The peak intensity suggested the stability of the fragments.
The electronic spectra of the Schiff base and its complexes were recorded in DMSO.The absorption spectra of free ligand consist of an intense bands centered at 370 nm (27027 cm -1 ) attributed to n → π* transition of the oxo group.Another intense band in higher energy region of the spectra of the free ligand was related to n → π* transition of benzene rings 21 .
Magnetic moments of Cr(III) complex was found 4.35 µ B at room temperature which is close to the predicted values for three unpaired electrons in the metal ion 28 .The electronic spectrum of Cr(III) complex showed bands in the range 8950-9310 cm ¯1.The spectral bands are consistent with that of five-coordinated Cr(III) complex, whose structure has been confirmed with the help of x-ray measurements 29 .On the basis of the analytical data, spectral studies and electrolytic nature of the complex, a square-pyramidal geometry may be assigned for this complex 30 .The spectral bands may be assigned as: 4 The magnetic moment of Mn(III) complex was 4.85 µ B which indicates the high spin d 4 system 28 .The electronic spectra of manganese complex show three d-d band which lays in the range 12250-12590 cm ¯1 and may be assigned to charge transfer transitions.The spectra resemble to those reported for five coordinate square pyramidal manganese(III) complexes 30 .This is further supported by the presence of the broad ligand field band at 20400 cm ¯1.The various bands may be assigned as follows: 5  , respectively, in order of increasing energy.However, the complexes do not have idealized C 4V symmetry.
The magnetic moment of Fe(III) complex was 5.78 µ B corresponding to the five unpaired electrons and is close to the predicted high spin values for this metal ion 28 .The electronic spectra of Fe(III) complex show various bands in the range 9820-9970 cm ¯1 and are consistent with the range of spectral bands reported for five coordinate square pyramidal Fe(III) complex 31 .The various bands can be assigned as: d xy →d xz , d yz and d xy →d z 2 .Any attempt to make accurate assignment is difficult due to interactions of the metal-ligand π-bond systems lifting the degeneracy of the d xz and d yz pair.
Based on various studies like elemental analysis, conductance measurements, magnetic susceptibilities, IR, electronic and mass spectral studies the structure shown in Scheme 2 may be proposed for all of the complexes.
The results showed (Table 3) that HL (ligand) exhibits moderate activity against all tested bacteria and Candida albicans fungus.HL showed high antifungal activity against Kluyveromyces fragilis and Trichoderma reesei but no activity against Rhodotorula rubra.Cr(III) complex showed high antibacterial and antifungal activity against Bacillus megaterium, Kluyveromyces fragilis, Rhodotorula rubra and Candida albicans, moderate activity against Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa, but no activity against the Trichoderma reesei.Mn(III) complex exhibited moderate activity against Pseudomonas aeruginosa and Bacillus megaterium, was highly effective against the Kluyveromyces fragilis, Candida albicans and Trichoderma reesei but showed no activity against the Escherichia coli, Staphylococcus aureus and Rhodotorula rubra.Fe(III) complex exhibited moderate activity against Staphylococcus aureus, Pseudomonas aeruginosa, Bacillus megaterium and Trichoderma reesei, higher effect against Kluyveromyces fragilis and Candida albicans, but no activity against Escherichia coli and Rhodotorula rubra microorganisms.
Antibacterial and antifungal activity of the ligand and its metal complexes is compared with the standard drug Ciprofloxacin and Griseofulvin.The variation in the activity of different metal complexes against different microorganisms depends on their impermeability of the cell or the differences in ribosomes in microbial cell 32 .The lipid membrane surrounding the cell favors the passage of any lipid soluble materials and it is known that liposolubility is an important factor controlling antimicrobial activity 33,34 .
In the present study low activity of the some metal complexes is may be due to their low lipophilicity, because of which penetration of the complex through the lipid membrane was decreased and hence, they could neither block nor inhibit the growth of the microorganism.

Conclusion
The analytical data show the presence of one metal ion per ligand molecule and suggest a mononuclear structure for the complexes [M (C 19 H 24 N 6 O 3 )X]X 2 .The electronic spectral data is in the favor of square pyramidal geometry of the complexes.The ligand and its Cr(III), Mn(III) and Fe(III) complexes were tested for antimicrobial activity against some pathogens.All the complexes were found to be less active against the bacteria E.coli, S.aureus and P.aeruginosa, whereas Cr(III) complex showed the best antimicrobial activity against the bacterium B.megaterium and the fungi K.fragilis and R.rubra.The free ligand (C 19 H 24 N 6 O 3 ) was found to be active against the fungus T.reesei.

Scheme 2 .
Scheme 2. Synthesis of the M(III) Schiff base complexes.

Table 1 .
Elemental analysis in %, molar conductance (Λ M in DMSO) in S cm 2 mol -1 and magnetic moment (µ eff ) in µ B of the Schiff base ligand and its metal complexes.
a HL = free Schiff base C 19 H 24 N 6 O 2 S.

Table 2 .
IR spectral bands position/cm ¯1 and electronic spectral data (in DMSO) cm ¯1 of the Schiff base ligand and its metal complexes.

Table 3 .
Antibacterial and antifungal activity of the standards, solvent, ligand and its metal complexes.HL = free Schiff base C 19 H 24 N 6 O 2 S. a