Indian Journal of Chemistry Vol. 4 1 A, May 2002, pp. 942-949

Synthesis, spectral, redox and biological studies of some Schiff base copper(II), nickel(II), cobalt(II), manganese(II), zinc(II) and oxovanadium(II) complexes '

derived from I-phenyl-2,3-dimethyl-4( 4-iminopentan-2-one)­pyrazol-5-one and 2-aminophenol/2-aminothiophenol

N Raman*, A Ku landaisamy & K Jeyasubraman ian l

Department of Chemistry, VHNSN Co llege, Virudhunagar 626 00 I, Tamilnadu, India

E-mai l: drnJaman @yahoo.eo.in

Received 6 Allfillst 200 1; revised 17 Jallll{l/Y 2002

Neu tral tetradentate complexes of Cu( II ). Ni(II), Co(l l). Mn(II). Zn(ll) and VO(II) have been synthesised us ing new Sch iff bases derived from 2-am inophenol/2-aminothiophenol and l-phenyl-2,3-dimethyl-4(4-iminopentan-2-one)-pyrazol -5-one in ethanol and characterized by microanalytical data, IR , UV -Vi s .. I H I-IMR and ESR spectra. Non-electrolytic behaviour and monomeri c type of the chelates have been assessed from their 10Vl conductance and magnetic usceptibility va lues respectively . The IR and UV- Vis. spectra suggest that all the complexes have square planar g ometry except vanady l and manganese complexes whi ch show square pyramidal and octahedral geometry respectively. The redox behaviour of copper and vanady l complexes has been studied by cycl ic voltammetry and the ESR spectra of copper and vanady l complexes are recorded and discussed. Antimicrobial act ivity of the Schiff bases and their complexes have been ex tensively studied on some microorganisms like Staphylococclls allreus. Bacillus subtilis, Klebsiella pnellmoniae. Sallllonelia (vphi. Pselldolllonolls aemginosa. Shigella j/exneri. Aspergillus niger and Trichoderma viridi. Most of the complexes have hi gher activi ty than the free li gand.

The Schiff base derivatives of 4 -aminoantipyrine (1-pheny l-2,3-d imethyl-4-amino pyrazo l-5-one) received much attention due to their app licatio ns including biological , c lini cal, ana lytical and pharmacological areas. Eventhough 4-aminoantipyrine itself exhibits ant imi crobia l ac ti vity, it has been en hanced due to the condensation with aldehydes , ketones, thiosemicarba­zides, carbazides etc. In contrast to the considerable growth o f literature on the Schiff base derivatives of 4-aminoantipyrine complexes of inner transi tion metals, the chemistry of transiti on metal(ll) complex­es, their biologica l and redox behaviours have not been much probed. As a part of our continuing efforts to study the sy nthesis and characterization of transition metal(ll) co mplexes and to evaluate the ir biological sc reening potential and electrochem ical behav io ur using Schiff base deri vatives of 4-amino­antipyrine ligands I, we report herein the synthes is of new tetradentate ligand sys tems by condensing J­pheny 1-2,3-d i methy 1-4( 4-i mi nopentan-2-o ne )-py razol-5-one wi th 2-aminopheno ll 2-aminothi opheno l. The sy nth sised material has N20 i N20S donor type and its coordi nati on behaviour with different metal ions are stud ied . The enolic structure of the Schiff bases are:

tDepartment of Chemistry, MEPCO SCHLENK Engineering College, Sivakasi 626005, Tamilnadu, India.

HzLI (X = 0);

Materials and Methods All the reagents used were of Merck products. In

the vo ltammetric experiments, Me.jNCl0 4 (TMAP) was used as supporting e lectrolyte and was purchased from Sigma. Spectroscopic grade so lvents were used for spectral and cyclic vo ltammetri ~ measurments. The carbon, hydrogen and nitrogen contents in each sample were performed at RSrC, CDRI Lucknow. I H NMR spectra of the samples were measured in DMSO-d(i at IICT, Hyd rabad. The IR spectra were recorded in KBr pellets using a Perkin-Elmer 783 spectrophotometer. The UV -Vis spectra of the complexes were recorded on a Shimadzu UV-1601 spectrophotometer. The X-band ES R spectra of copper and vanady l complexes were recorded in DMSO at 300 K and 77 K on a Varian ESR spectrometer using dipheny lp icrylhyrazyl (DPPH) as

RAMAN e/ al.: STUDY OF SOME SCHIFF BASE COMPLEXES 943

internal standard at RSIC , TIT, Chennai . Magnetic measurements of the complexes were carried out using Guoy balance. Copper sulphate was used as the calibrant. Cyclic voltammogram of copper and vanadyl complexes were recorded in aceton itrile ( I mmol) solution at 300 K on a BAS CY 50 electrochemical analyser. The three electrode cell comprised a reference Agi AgCl, auxi lI ary Pt and the working glassy carbon (0.07 cm2 dia) electrodes. The molar conductivity was measured on a systronic conductivity bridge with a dip type cell , using lOA M solution of complexes in ethanol.

Synthesis of Schiff" bases l-phen yl-2, 3 -dilllethyl-4( 4-illlinopelltan-2-one)­pyrazol-5-iminophellol (H2L' )

I-pheny 1-2,3-d i methy 1-4( 4-i mi nopentan-2-one )-py­razol-5-one (2.85 g, 10 mmol) in 50ml of ethanol was refluxed with 2-aminophenol (1.09 g, 10 mmol) for about 12 h. On cooling, the orange solid separated (H2LI) was filtered and recrystallised from ethanol.

f -pheny l-2,3 -dilllethyl-4( 4 -iminopentan -2 -one) ­pyrazol-5-iminothiophenol (H2L2)

l-phenyl-2, 3-dimethyl-4( 4-i mi nopentan-2-one)-py­razol-5-one (2.85 g, 10 mmol ) in 50 ml of ethanol was refluxed with 2-aminothiophenol (1.25 g, 10 mmol) for about 12 h. The volume of the solution was reduced to one third on a water path and 10 ml of pet. ether (60-80°C) was added with constant stirring. The yellow solid separated was filtered and recrystallised from ethanol.

Synthesis of complexes To a solution of MCI2 ( 10 mmol) in 20 ml ethanol,

H2LI/H2L2 ( 10 mmol) was added and the contents were heated on a water path under reflux for 5-6 h. The resulting solution was concentrated to 5 ml on a water bath and the product was separated by adding 10 ml of pet. ether (60-80°C) with stirring. The solid product formed was separated by filtration and washed thoroughly with hot ethanol and then dried in vacuo. Oxovanadium(lI) complexes were synthesised by the same procedure but in the presence of 5% aq . sod ium acetate solution. All the complexes were recrystallised in acetonitri le. The yield varied from 55-65%.

Antibacterial activity The in vitro biological scre ening effects of the

investigated compounds were tested against two Gram-positive bacteria, Staphylococcus aureus and Bacillus subtilis and four Gram-negative bacteria,

Klebsiella pneumoniae, Salmonella /yphi, Pseudomonas aeruginosa and Shigella jlexileri by the disc diffusion method using agar nutrient as medi um and utilising silver nitrate as control. The stock solutions were prepared by dissolving the compounds in acetonitrile and all the blank discs were moistened with the solvent. For di sc assays, paper (6 mm) containing the compounds was placed on the surface of the nutrient agar pl ates previously spread with 0.1 ml of overnight cultures of microorganisms. After 36 h of incubation at 3rC, the diameter of the inhibition zones was measured and is li sted in Table 5.

Ant(fungal activity The antifungal activity of the compounds was

evaluated by the well diffusion method against the fungi Aspergillus niger and Rhizoctollia hataicola cu ltured on potato dextrose agar as med ium. Acetonitrile was used as the solvent and the drug amphotericin as control. In a typical procedure, a well was made on the agar medium inoculated with microorganisms. The well was filled with the test solutions using a mi cropipette and the plates were incubated at 35°C for 72 h. During this period, the test solution was diffused and affected the growth of the inoculated microorganisms. A zone was developed on the plate and the inhibition zones were measured and are reported in Table 5.

Results and Discussion The analytical data along with some physical

properties of the new compounds are summarised in Table I. The analytical data of the complexes correspond well with the general formul a ML, whil e the manganese complexes have MnL.2H20 where M = Cu(Il), Ni(I1), Co(II), Zn(1I) and YO(lI) , L = LI/L2(C22H22N402/Cn Hn N40S). The proposed molecular structure of the new complexes are:

M = Cu(II), Ni(rI ), Co(ll) and Zn(rI) L' (X = 0); L2 (X = S)

Table !--Characterization data of the complexes

Compound M. wt. Colour Melt/ Found (Calcd, %)

decamp. M c H temperatu re °C

H2L1 374 Orange 146 69.86 (70.2 1) 6.32 (6.38)

[CuL 1] 436 Brown 172 13.87 (14.52) 57.8 1 (58.05) 4.96 (5.02)

[NiL1l 431 Brown 23 1 13.26 (1 3.56) 60.83 (6 1.01) 4.92 (5 .08)

[CoL 1] 431 Red 212 13.17 (13.61) 60. 23 (60.98) 4.99 (5.08)

[MnL 12Hz0] 463 Brown 236 I 0.93 (11.82) 56.3 1 (56.78) 5.56 (5.59)

[ZnL 1] 437 Colour less 233 15.17 (14.88) 59.64 (60.08) 4.93 (5.01)

[VOL 1] 439 Green 192 10.98 (11.05) 57.15 (57 .27) 4.64 (4.77)

H2L2 390 Yellow 97 67.32 (67.35) 5.95 (6.12)

[CuL2] 452 Brown 158 13.94 (14.01) 58.17 (58.2 1) 4.69 (4.85)

[NiL2] 447 Brown 217 12.78 (13.08) 57.91 (58.84) 4.76 (4 .90)

[CoL2j 447 Red 188 12.83 (13 .13) 58. 17 (58.8 1) 4.81 (4.90)

[MnL22HzO] 449 Brown 223 11.72 (13.43) 54.46 (54.89) 4.34 (4.57)

[znel 453 Brown 197 14.18 (14.36) 57.39 (57.97) 4.68 (4.83)

[VOL2] 455 Pale Green 152 10.97 (11.15) 57.46 (57.78) 4.57 (4.8 1)

(AM)Molar N conductance

mho cm2

mol" 1 X 10'3

14.73 (14.89)

12.65 ( 12.80) 1.40

12.71 (12.94) 1.80

12.56 (12.94) 1.40

12.18 (12.05) 2.20

12.37 (12.75) 1.90

12.12 (12.15) 1.17

14.17 (14.29)

12.24 (12.35) 1.48

12.29 ( 12.48) 2.37

12.32 ( 12.47) 1.70

11.53 (11.64) 2.30

12.26 (12.30) 1.92

12.85 (12 .26) 1.28

Magnetic moment

J..lerr (B. M)

1.79

3.68

5.97

1.78

1.89

3.92

6.0 1

1.80

\0 -+>--+>-

z 0 > z ._ (j :r: tTl 3:: . en tTl (j

? 3:: > -< N

8 N

RAMAN et al.: STUDY OF SOME SCHIFF BASE COMPLEXES 945

The magnetic susceptibility values of the complexes at room temperature are consistent with that of square planar geometry around the central metal ion except for Mn(II) and VO(II) complexes which show octahedral geometry and square pyramidal geometry respectively. Thermal analysis shows that manganese complexes lose two water molecules at about 170°C which suggests the presence of two water molecules coordinated to the central metal ion which is further confirmed from their characteristic IR spectrum. The low conductance values of the chelates support the non-electrolytic nature of the metal complexes.

In order to study the binding mode of the Schiff bases to metal in the complexes, IR spectra of the free ligands were compared with the spectra of the metal complexes. The Schiff base H2L1 shows a weak broad band in the region 2950-3300 cm- 1 assignable to intra­molecular hydrogen bonding between enolisable -C=O of pentan-2,4-dione and phenolic group. Absence of this band in complexes indicates the deprotonation of the intra-molecular hydrogen bonded enolic and phenolic groups upon complexation. The IR spectrum of H2L2 ligand shows a strong broad band in the region 3100-3400 cm· 1 and a weak band at 2580 cm-1 which are assigned to enolisble -C=O group of pentan-2,4-dione and -SH group respectively. The disappearance of above two bands in complexes indicates the deprotonation of the enolic and thiol groups during complexation. Coupled with this observation, absence of a band at ca. 1660-1700 cm- 1

in both the ligands (characteristics for the free -C=O groups) suggests that carbonyl group present in the pentan-2,4-dione moiety is in enolic form. Both the ligands show their characteristic -C=N bands in the region ca. 1620-1600 cm-1 which are shifted to lower frequencies in complexes (1590-1560 crn- 1

) indicating the involvement of azomethine nitrogen atom on coordination to the metal ion . The metal chelates show some new bands in the region 480-450 cm- 1 and 400-350 cm·1 which are due to the formation of M-0 and M-N bonds respectively2

. The manganese complexes show a broad band at 3500-3100 cm-1

which suggests that water molecule is coordinated to the central metal ion3

. In addition to the other bands, vanadyl complexes show an additional band at 950-930 cm· 1 attributed to Yv=o frequency4

•

The electronic absorption spectra of the Schiff bases, Cu(II), N i(II), Co(ll) and VO(II) complexes were recorded at 300 K. The solvent, absorption region, assignment and the proposed geometry of the

complexes are given 111 Table 2. These values are comparable with that of the other reported complexes5

.

Further, the coordination of the Schiff base ligands was confirmed from their 1H NMR spectra. The free ligand H2L1 in DMSO-d6 solution shows the following signals: C6H5 multiplet at 6.6-7.8 8 range, =C-CH3 at 1.9 8, -N-CH3 at 2.5 8, -CH3 at 2.8 -3 .2 8 and =C-CH at 4.8 8. The peaks at 9.0 8 and I 0.2 8 are attributable to the phenolic -OH group present in 2-aminophenol and enolic -C=O group present in pentan-2,4-dione moiety. The absence of these two peaks, in the zinc complex indicates the deprotonation of the phenolic group and enolic -C=O group of pentan-2,4-dione moiety of the Schiff base on chelation. Slight downfield shift is observed in all other signals of the zinc complex.

The cyclic voltammograms of copper and vanadyl complexes were recorded in dry acetonitrile solution at room temperature in the absence of molecular oxygen. The cyclic voltammetric data of the complexes are given in Table 3. A noteworthy feature has been observed in the cyclic votammogram of [CuL 1] complex which is shown in Fig. I a. It shows two peaks in the cathodic side which correspond to the reduction of Cu(ll) __, Cu(I) and Cu(l) __, Cu(O) respectively. After an initial scan, if the potential is reversed towards the anodic direction starting from -1.20 V, a stripping peak was observed and is attributable to the oxidation of deposited metal6 to Cu(II). The two-electron nature of the process is established by the comparison of Ipc and Ipa values. The [CuL2

] complex in acetonitrile solution (Fig. 1b) shows two quasi-reversible peaks for the redox couple Cu(II)/Cu(l) and Cu(I)/Cu(O) .

Cyclic voltammetric studies of [VOL 1] and [VOL2

]

complexes in acetonitrile solution show two well defined one electron transfer redox processes corresponding to the formation of the couples VO(II)IVO(III) and VO(II)IVO(l). In both the vanadyl complexes, VO(II)IVO(III) couple is reversible on the basis of peak height ratio data (Table 3) . Very similar electochemical behaviour of vanadyl complexes has been reported by Murray et aC and Wieghardt et a/. 8

. The peak current functions of both waves in both the complexes are different which indicate that two different species are electroactive in solution corresponding to VO(III) and VO(I).

The ESR spectra of Cu(Il) complexes were recorded in DMSO at 300 K and 77 K. The spectra of

946 INDI AN J CHEM. SEC A, MA Y 2002

the copper complexes at 300 K show one intense absorption band in the high field and is isotropic due to the tumbling motion of the molecules. However, these complexes in the frozen state show four well­reso lved peaks with low intensities in the low field region and one intense peak in the hi gh field region9

.

No band corresponding to ms = ± 2 transition was observed in the spectra, ruling out any Cu-Cu interaction. The spin Hamilton ian parameters of the complexes are given in Table 4.

The g tensor values of copper(l J) complex can be used to deri ve the ground state. In square pl anar

Table 2 - Electronic absorpti on specl ral data of the compounds

Compound Absorpti on reg ion (cm' l)

H2L I 280 11 , 42016

[CUl l] 28985, 43860 17090

[Nill] 30770, 4587 1 19600 21980

[COl i] 289 10, 43860 19500

(VOLI) 28730, 43860 13140 19040

H2L2 28985, 45870

[CUL2) 248 14, 40486 18657

[ il2) 46948, 34965 24096 16233

[Col 2) 32680, 48076 17094

[VOl2) 29850, 48309 22675 12180

INCT-Intraligand charge transfer band

E cn,-I mol·1 1" 1 Band ass ignment

65

40 28

45

68 42

30

40 65

28

85 40

INCT

INCT IAlg~IA2g IA l g~I B l g

IN CT I B2~2E

I B 2~2AI

INCT

INCT IA lg~ I A2g IAlg~ IBl g

INCT IB2~2E

IB2~2AI

Geomelry

Sqaure planar

Square planar

Square planar

Square pyramidal

Sqaure planar

Square planar

Square planar

Square pyramidal

Table 3 - Cyc lic voltammetri c data of Cu(lI) and VO(l I) complexes in acetonitrile containing 0.1 M TMAP Scan rate 100 mVS·1

Complex Couple Epc(V) Ep,(V) Ipc(/lA ) Ipa(/lA)

(CuLl] Cu(II)/Cu(l ) 0.22 21.6 Cu(l )/Cu(O) -0.82 11.12 Cu(O)/Cu(ll ) 0.48 -32.8

Cu(Il )/Cu(I) 0.14 0.26 13.8 -11.4 Cu(I)/Cu(O) -0.66 -0.37 5.70 -5.42

[VOLI) VO(J I)/VO(III ) 0.53 0.58 9.98 -10.06 VO(II )/VO(J) -0.98 -0.33 9.20 -8 .40

VO( Il )/VO(JIl ) 0 .56 0.62 15,26 - 15.53 VO( Il )/VO(l) -1.04 -0.58 6.73 -7.42

"

RAMAN el 01.: STUDY OF SOME SCHI FF BASE COM PLEXES 947

complexes, the unpaired electron lies in the d/_/ orbitals giving 2B I8 as the ground state with

811>8.1>2.00 whi le the unpaired electron lies in the d/ orbital giving 2A I8 as the ground state with

8.1>811>2.00. From the observed values, it is c lear that

811>8.1>2.00 which sugges ts that the complex is present in square planar geo metry. Further, it is also supported from the fact that the unpaired electron lies

d . I ' h d 2 2 b' l iD pre oml11ant y 111 t e x _y or Ita . The molecular orbital coeffi cients viz. in-plane n­

bonding ( ~2) and in-p lane a-bonding (a2) were calcu lated using the following express ions:

a? = -(A II/0.036) + (8 11-2 .0023) + 3/7 (8r 2.0023) + 0.04

~2 = (gIl2 .0023) E /- 8Aa2

1 3pA

« ~

r-

z

a:

a: I i . 1.00 - 1.20

OJ

w

ISf!A

a

I

-1 .20

E {VI ver sus Ag / AgC l

Fig. I - Cyclic vo ltammogram of [CULl] (a) and [CuL2] (b) at 300 K in (0.1 M TMAP) acetonitrile solution, Scan rate 100 mVS·1

where A = 828 cm-I for free ion and E is the elec tron ic

transition energy of 2B lg~2AI g . From the table, a 2 and ~2 values indicate that there is substanti al interact ion

in the in-plane a-bonding whereas the in-plane n­bonding is almost ionic . The results are anti cipated because there are no appropriate ligand orbitals to combine with dxyorbital of copper(II) ion.

The ESR spectra of vanadyl complexes were recorded in DMSO solution at 300 K and 77 K. T he typical ESR spectra of [YOe ] are shown in Fig. 2 . The room temperature spectra of the complexes are typical eight-line pattern which shows that a s ing le van adium is present in the molecule, i.e., it is monomer. In frozen state, the spectra show two types of resonance components, one set due to the parallel features and the other set due to the perpendi cular features, whi ch indicates ax ially symmetri c ani sotropy with well-resolved sixteen-line hyperfi ne

3200G

!

t--<

100 G

b

>--< 100 G

Df'PH

Fig. 2 - ESR spectrum of [VOL2j at 300 K (a) and 77 K (b) in DMSO solution

Table 4 - The spin Hamiltoni an parameters of Cu(lI ) and VO(l I) complexes in DMSO solution

Complex All Al. A ;so )

~2 gil gl. g js(I u-H:l l2erfineconstant x 104 em-I

CuLl 143 42 76 2.3 1 2.06 2. 14 0.59 0.86

CuL2 164 47 75 2.27 2.05 2. 16 0.8 1 0.93

VOL I 196 76 115 1.95 1.97 1.96 0.62 0.96

VOL2 194 73 108 1.96 1.97 1.94 0.52 0.99

948 INDIAN j CHEM, SEC A, MA Y 2002

splitting, characteri s tic of inte rac tion between the e lectron and the vanadium nuclear spins. The various parameters calculated from the spectra are given in Table 4. The o bserved parameters ind icate that the mo lecule ex ists in a square pyramidal geometry which is characteri sti c of oxovanad ium(ll) che lates II.

The mo lecul ar orbital coeffici ents, a 2 and ~2 were also ca lcu lated for the co mplex using the fo llowing

. II equations :

a 2 = (2.0023-g ll ) £ 1 8AB2

~2 = 7/6 (-AII /P + Al /P + gll _5114 gl -911 4 ge)

where P = 128x I 0 -4 cm-I, A = 135 cm-I and £ is the

e lectronic trans ition energy of 28 2 -1 2£. The

calcu lated in-plane n-bonding coeffic ient va lue (~2) does not deviate much from unity. For most of the

complexes, the in-plane n-bondi ng coeffici ent remain s constant and spans the reg io n from 0.94 to 1.00. Th is is consistent with Ki velson's conclusion l2

which suggests that the dXJ orbital is essentia ll y no n­

bonding while ~2 remains constant. Essenti a ll y, the de localisati on o f e lec tron into the ligand can be

gauged from the in-pl ane a-bonding coeffici ent (a2)

values. This fo llows the a-donor strength of the li gand and it usually decreases as the covalent bonding increases. From Table 4, it is c lear that the

in-p lane a-bonding (a2) is signi ficant. The M.O.

coeffi c ients a lso show that the meta l io n has some cova lent character in the ligand env ironment.

Table S- Antimicrob ial acti viti es of transition metal cO '~lp l exes (Zone formation in mm)

Compound S. Ihypi S. al/rel/s K. p" elllo"iae B. sl/blillis S. jlexneri P. af'lOg illo.m

Can trol* 20 II 18 14 37 36

H~L ' 8 II 13 7 16 I I

[Cull] 16 13 IS 13 2 1 22

INiL '] 17 16 13 16 27 19

[CoL'] 13 17 2 1 19 28 25 [MnL '2H2O] 19 2 1 25 22 17 28

[ZnL'] IS 26 26 23 25 26

[VOL'] 14 14 2 1 29 12 29

HlL2 II 18 10 IS 8 14

[CuL"] 13 14 27 12 13 25 [NiL2] IS 22 29 18 2 1 16 [CaLl] 12 26 IS 2 1 IS 19

IMn L22H2O] 18 17 18 IS 27 22 lZnL?] 24 2 1 23 28 18 3 1 [VOL2] 25 22 19 17 36 2 1

A. lliger R. bataicola

100 200 300 ( ~l g) 100 200 300 ( ~l g)

Cantrol * 8 14 22 10 IS 23 H2L' 6 10 17 8 13 17

[CuL ' 1 9 12 16 I I IS 19

INiL '] I I IS 19 12 17 26

[CoL'1 12 IS 2 1 IS 23 31

[MnL '2H2O] 8 II 17 13 18 34

[ZnL '] I I 17 28 9 13 21

[VOL' ] 7 12 18 10 17 26

H2Ll 8 14 17 12 19 29

[CuLl] 12 17 23 14 17 ,, -. ~ ..,

[NiL2] 14 18 26 7 11 13

[CoL2] 11 17 2 1 9 14 17

[MnL22H2O] 10 15 23 12 16 2 1

[Zn L2J 9 i4 25 13 18 26 IVOL~] 8 12 9 IS 23 * Sil ver nitrate for bacteria; *Amphotericin far fungi

RAMAN et 01.: STUDY OF SOME SCHIFF BASE COMPLEXES 949

Biological screening study The ill vitro biological screening effects of the

inves tigated compounds were tested against six bacteria: Staphylococcus aureus, Bacillus subtilis, Klebsiella pneul71oniae, Salmonella typhi, Pseudomonas aerugillosa, Shigella flexneri and two fungi Aspergillus niger and Rhizoctonia bataicola. The measured zone of inhibition against the growth of various microorganisms is li sted in Table 5.

A comparative study of the ligands and their complexes indicates that most of the metal chelates exhibit higher antimicrobial activity than that of the free li gand and the control. The increase in the antifungal activity of metal chelates with increase in concentration is due to the effect of metal ion on the normal cell process. Such increased activity of the metal chelates can be explained on the basis of Overtone 's concept l3 and Chelation theoryl 4.

According to Overtone's concept of cell permeability, the lipid me mbrane that surrounds the cell favours the passage of only lipid soluble materi als due to which liposolubi~ity has important factor which controls the antimicrobial activity. On chelation, the po larity of the metal ion will be reduced to a greater ex tent due to the overlap of the ligand orbital and partial sharing of positive charge of metal ion with donor groups . Further, it increases the delocali sation of 7t-electrons over the whol e chelate ring and enhances the lipophilicity of the complexes . Thi s increased lipophilicity enhances the penetration of the complexes into lipid membrane and blocking the metal binding sites on enzymes of microorganism. These complexes also disturb the respiration process of the cell and thus block the synthesis of proteins which restrict the further growth of the organism.

Furthermore, the mode of action of the compounds may involve formation of hydrogen bond through azomethine group with the active centres of cell constitutents, resulting in interference with the normal cell process 15.

Acknowledgement The authors express the ir sincere thanks to the

UGC, New Delhi, for financial support and Prof P R Athappan, School of Chemistry, M adurai Kamaraj

University for CY facilities . The authors also thank the Managing Board, Principal and Head of the Department of Chemistry of Yirudhunagar Hindu Nadars ' Senthikumara Nadar College for providing research facilities. K J thanks the Management and Principal of MEPCO SCHLENK Engineering College, for their support. One of the authors (A K) thanks CSIR, New Delhi, for the award of a seni or research fellowship.

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