Spectroscopic Characterization and In Vitro

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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry

ISSN: 1553-3174 (Print) 1553-3182 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt20

Spectroscopic Characterization and In Vitro Antibacterial Activity of Some Novel Metal Complexes With Schiff Base Ligands Derived From Thiosemicarbazide D. Sakthilatha, A. Deepa & R. Rajavel To cite this article: D. Sakthilatha, A. Deepa & R. Rajavel (2015) Spectroscopic Characterization and In Vitro Antibacterial Activity of Some Novel Metal Complexes With Schiff Base Ligands Derived From Thiosemicarbazide, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 45:2, 286-297, DOI: 10.1080/15533174.2013.831880 To link to this article: http://dx.doi.org/10.1080/15533174.2013.831880

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Date: 17 October 2016, At: 04:22

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry (2015) 45, 286–297 Copyright © Taylor & Francis Group, LLC ISSN: 1553-3174 print / 1553-3182 online DOI: 10.1080/15533174.2013.831880

Spectroscopic Characterization and In Vitro Antibacterial Activity of Some Novel Metal Complexes With Schiff Base Ligands Derived From Thiosemicarbazide D. SAKTHILATHA, A. DEEPA, and R. RAJAVEL Department of Chemistry, Periyar University, Salem, Tamil Nadu, India Received 1 May 2013; accepted 27 July 2013

Six novel mononuclear Schiff base metal complexes of the type [ML1L2]X and [ML1L2X], where M D Cu(II), Ni(II), Co(II), Mn(II), Zn(II), and VO(IV) have been synthesized by condensation of 1-(1-(pyridine-2-yl)ethylidene)thiosemicarbazide (L1) and 1-(1-(2,4dihydroxyphenyl)ethylidene)thiosemicarbazide (L2) in the presence of divalent metal salts. The synthesized ligands and complexes were structurally characterized on the basis of elemental analysis, IR, UV-visible, 1H NMR spectroscopy, molar conductance, magnetic susceptibility measurements, and electrochemical studies. The in vitro antibacterial study reveals that the metal complexes inhibit the growth of bacteria more than the free ligands. Keywords: Schiff base, thiosemicarbazide, electrochemical, antibacterial studies

Introduction Schiff bases continue to occupy an important position as ligands in metal coordination chemistry,[1] even almost a century since their discovery. Currently metal complexes of S-, N-, and O-chelating ligands have attracted considerable attention because of their interesting physicochemical properties, pronounced biological activities and their use as models for metalloenzyme active sites.[2] Mainly metal complexes with sulfur containing ligands are also of a great interest in inorganic and organometallic chemistry, especially due to their potential with novel electrical and magnetic properties.[3] Thiosemicarbazones and their derivatives are versatile ligands as they exhibit various binding modes with transition and some main group metals. They can act as mono- or bidentate ligands binding to metal ions via different donor atoms. The structure of the thiosemicarbazide moiety confers a good chelating capacity and this property can be increased in thiosemicarbazone by inserting suitable aldehyde or ketone possessing a further donor atom to render the ligand polydentate.[4] They show biological activities including antibacterial antifungal,[5] antidiabetic,[6] antitumor,[7,8] antiproliferative,[9] anticancer,[10] herbicidal,[11] anticorrosion, and antiinflammatory activities.[6] This type of Schiff bases represent

Address correspondence to R. Rajavel, Department of Chemistry, Periyar University, Salem-636 011, Tamil Nadu, India. E-mail:[email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsrt.

an important class of compounds because they are utilized as starting materials in the synthesis of industrial products.[12] For the most part herterocyclic thiosemicarbazones have aroused considerable interest in chemistry and biological activity.[13] The biological significance of this class of compounds can be judged from the data that they belong to the most important inhibitors of ribonucleotide reductase. On the other hand, between the activity of this enzyme and the rate of replication of cancer cells there is a positive correlation.[14,15] Moreover, the coordination chemistry of transition metal complexes with mixed ligands are of current interest because they can provide new materials with useful properties such as magnetic exchange,[16,17] electrical conductivity,[18] photoluminescence,[19] nonlinear optical property,[20] catalyst,[21,22] and antimicrobial activity.[23] The biological importance of mixed ligand complexes is that they are sometimes more effective than the free ligands.[24] Mixed-ligand complexes containing nitrogen and oxygen donors are important owing to their antifungal, antibacterial, and anticancer activities.[25] In the present article, we report the synthesis and spectral characterization of number of mixed ligand metal complexes of Cu(II), Ni(II), Co(II), Mn(II), Zn(II), and VO(IV) with the previously mentioned ligands. Their spectral and redox behavior and biological activities are discussed.

Experimental Material The entire chemicals used were of the analytical reagent grade, 2-acetylpyridine, 2,4-dihydroxyacetophenone, and

Spectroscopic Characterization and In Vitro Antibacterial Activity of Novel Metal Complexes S

H 3C S CH3 N

H2N

+

H N

C

C

NH2

287

H N

N

C

NH2

Reflux 5 h N

Thiosemicarbazide

O

1-(1-(pyridin-2-yl)ethylidene)thiosemicarbazide

2-acetylpyridine

Sch. 1. 1-(1-(pyridin-2-yl)ethylidene)thiosemicarbazide.

thiosemicarbazide procured from Acros Organics and Loba Chemie. Metal salts were purchased from Acros Organics, Aldrich, Loba Chemie, and Qualigens. Physical Measurements Melting points of all newly prepared compounds were determined in open capillaries and are uncorrected. The C, H, and N were analyzed on a Carlo Erba 1106 elemental analyzer. The metal content of the complexes were determined according to the literature methods.[26] The molar conductance of the Schiff base complexes was determined on a Systronic direct reading conductivity meter with a cell having cell constant of 1.1. The 1H NMR of ligands was recorded in DMSO-d6 on a BRUKER 500 MHz spectrometer at room temperature using TMS as an internal reference. The IR spectra of all complexes were recorded on an FT-IR spectrophotometer (Jasco FT-IR-410) in the range 4000–400 cm¡1, and potassium bromide disc method was employed for sample preparation. The UV-visible in the 200–900 nm range was recorded on UV/Vis Jasco 550 double beam spectrophotometer in DMSO solvent. The magnetic measurements were made by Gouy’s method at room temperature by using Hg [Co(SCN)6] as calibrant. Cyclic voltammograms were obtained using a CHI1 120A electrochemical analyzer in DMSO containing 0.1M n-Bu4NClO4 as the supporting electrolyte. A three-electrode system was employed with a carbon electrode working electrode, a Pt-wire as the auxiliary electrode, and an Ag/AgCl electrode as the reference electrode. Antibacterial Studies The synthesized Schiff bases and their corresponding Cu(II), Ni(II), Co(II), Mn(II), Zn(II), and VO(IV) complexes were screened for their biological activities by using four bacteria, namely Staphylococcus aureus, Bacillus subtilis, Pseudomonas

aeruginosa, and Salmonella typhi by the reported method.[27] The bacteria were subcultured in agar medium. The Petri dishes were incubated for 24 h at 37 C. The compounds (25, 50, and 100 mg/mL) to be tested were dissolved in DMSO. The standard antibacterial drug was also screened under similar conditions for comparison. The wells were dug in the agar media using sterile metallic borer. Activity was determined by measuring the diameter of the zone showing complete inhibition (mm). Growth inhibition was compared with standard drugs. In order to clarify any effect of solvent DMSO on the biological screening, separate studies were carried out with solvent DMSO only and it showed no activity against any microbial strains. Synthesis of 1-(1-(pyridin-2-yl)ethylidene)thiosemicarbazide Ethanolic solution (10 mL) of thiosemicarbazide (0.5 g, 5.48 mmol) was added drop wise to the ethanolic solution of 2-acetyl pyridine (0.61 mL, 5.48 mmol) and give pale yellow solution that was gently refluxed for about 5 h. Then a deep yellow solution was obtained which on slow evaporation under reduced pressure yielded shinny yellow crystals of the ligand. The product was collected by filtration, washed with cooled ethanol, diethyl ether and dried at room temperature (Scheme 1). Equimolar amount of 2,4-dihydroxyacetophenone (0.1 g, 0.65 mmol) and the thiosemicarbazide (0.06 g, 0.65 mmol) in absolute alcohol was refluxed for 5 h. The excess solvent was removed under reduced pressure. The resulting brown precipitate was filtered, vacuum dried, and recrystallized using absolute alcohol (Scheme 2). A mixture of 1-(1-(pyridin-2-yl)ethylidene)thiosemicarbazide (0.1g, 0.56 mmol) and 0.11 g (0.55 mmol) of 1-(1-(2,4dihydroxyphenyl)ethylidene) thiosemicarbazide in 10 mL ethanol was added to the metal salts (0.55 mmol) dissolved in the least amount of ethanol. The reaction mixture was stirred

CH3 C

O

OH

C

S +

HO

S

H3C

H2N

H N

N

H N

C

NH2

Reflux 5 h C

NH2

Thiosemicarbazide

2,4-dihydroxyacetophenone

Sch. 2. 1-(1-(2,4 -dihydroxyphenyl)ethylidene)thiosemicarbazide.

HO

OH

1-(1-(2,4 -dihydroxyphenyl)ethylidene)thiosemicarbazide

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Sakthilatha et al. S

S H3C

HN C

N

H3C

C

HN C

NH2

+

N

NH2

+

MX2.nH2O

M = Cu(II), Ni(II), Co(II), Mn(II), Zn(II), OV(IV) X = OAc-, ClO4-

N

C

OH

HO

n =1,4,5,6

1-(1-(pyridin-2-yl)ethylidene) thiosemicarbazide

1-(1-(2,4-dihydroxyphenyl)ethylidene) thiosemicarbazide

Metal salts

Stirrer 1 h, Reflux 5 h

Mononuclear complexes Sch. 3. Synthesis of mixed ligand metal complexes.

for 1 h and then refluxed for 5 h to ensure the complete formation of metal complexes. The precipitated colored solid complexes were filtered and washed several times with ethanol to remove any excess of the unreacted starting materials. Finally the complexes were washed with diethyl ether and dried in vacuum desiccators (Scheme 3).

Results and Discussion Schiff base ligands L1 and L2 and complexes are stable at room temperature and soluble in DMF and DMSO. Physical data and elemental analysis of ligands and their metal complexes are shown in Table 1. The results of elemental analysis are in good agreement with those required by the proposed formulae. Characterization of Ligands The Schiff base ligands (L1 and L2) were prepared by the condensation of 2-acetyl pyridine with thiosemicarbazide and

2,4-dihydroxyacetophenone with thiosemicarbazide stoichiometrically in the molar ratio 1:1. The structures of the ligands were elucidated by elemental analyses, IR, electronic, and 1H NMR, spectra. The infrared frequencies of the Schiff base ligands (L1 and L2) and their assignments are listed in Table 2. The absorption band of the CHO group of 2-acetyl pyridine and 2,4dihydroxyacetophenone disappeared in the infrared spectrum of the ligands, indicating that the condensation has occurred. This is supported by the appearance of the strong band at 1500–1629 cm¡1 in the spectrum of the ligands, which is assigned to the stretching mode of azomethine moiety, y(C D N).[28] The infrared spectra of the ligands show broad band at 3450–3100, corresponding to NH, NH2 stretching, and phenolic OH stretching.[29] Electronic spectral data of the Schiff base ligands (L1 and L2) were recorded in DMSO solution. In the electronic spectrum of the ligand, two absorption bands at 261–276 nm (38314–36231) cm¡1 and 319–323 nm (31347–30959 cm¡1) were observed. The first bands correspond to 1La ! 1A1 and

Table 1. Elemental analysis of the ligands and its metal complexes. Calcd. (Found)% Compound Ligand 1 Ligand 2 [CuL1L2]ClO4 [NiL1L2]ClO4 [CoL1L2(OAC)(H2O)] [MnL1L2(OAC)(H2O)] [ZnL1L2]Cl [VOL1L2]SO4

Molecular formula C8H1oN4S C19H11N3O2S [Cu(C17H20N7O6S2Cl)] [Ni(C17H20N7O6S2Cl)] [Co(C19H25N7O5S2)] [Mn(C19H25N7O5S2)] [Zn(C17H20N7O6S2Cl)] [V(C17H21N7O8S3)]

M.W

Color

194 Yellow 225 Brown 593.1 Dark Green 588.2 Dark Yellow 564.0 Brown 560.0 Brown 522.9 Pale Brown 596.1 Green

M.p. ( C)

C

H

N

M

>250 >250 >250 >250 >250 >250 >250 >250

49.48 (48.44) 48.00 (47.98) 34.39 (34.36) 34.67 (34.64) 40.42 (40.40) 40.71 (40.68) 39.00 (39.02) 34.22 (34.20)

5.15 (5.12) 4.88 (4.85) 3.37 (3.34) 3.39 (3.36) 4.43 (4.40) 4.46 (4.42) 3.82 (3.80) 3.52 (3.50)

28.86 (28.85) 18.66 (18.65) 16.52 (16.50) 16.65 (16.64) 17.37 (17.35) 17.49 (17.45) 18.73 (18.72) 16.44 (16.43)

— — 10.71 (10.68) 9.97 (9.95) 10.44 (10.42) 9.80 (9.79) 12.50 (12.48) 8.54 (8.53)

Spectroscopic Characterization and In Vitro Antibacterial Activity of Novel Metal Complexes

289

Table 2. Infrared spectra (cm¡1) of ligands and metal complexes. Compound Ligand 1 Ligand 2 [CuL1L2]ClO4 [NiL1L2]ClO4 [CoL1L2(OAC)(H2O)] [MnL1L2(OAC)(H2O)] [ZnL1L2]Cl [VOL1L2]SO4

y(OH)

y(NH2)

y(NH)

y(C D N)pyr

y(C D N)

y(C D S)

y(M-O)

y(M-N)

— 3388 3409 3422 3427 3447 3416 3421

3371 3388/3299 3310/3212 3328/3217 3291/3291 3380 3301/3227 3347/3248

3184 3173 3107 3128 3176 3110 3197 3169

1601 — 1622 1613 1602 1630 1614 1629

1500 1629 1562 1552 1533 1537 1556 1584

845 842 808 805 850 857 803 845/801

— — 462 462 515 459 438 489

— — 415 425 457 422 411 431

Table 3. 1H and 13C NMR d (ppm) spectra of ligands. Assignment Methyl proton Pyridine proton Aromatic proton NH2 proton NH proton –OH proton

Ligand 1

Ligand 2

2.09 7–8.57 — 8.58 10.32 —

2.26 — 6.2–7.7 8.03 9.6 10.36–12.61

Lb ! 1A1 transitions of the phenyl ring. The second band corresponds to the p!p* and n!p* transition of the azomethine group and intermolecular CT from phenyl ring.[30] The 1H NMR spectral data of the ligands to TMS (0 ppm) in DMSO-d6 are listed in Table 3 (Figures 1 and 2). The 1H NMR spectra of 1-(1-(pyridine-2-yl)ethylidene)

thiosemicarbazide (L1) showed two signals at 10.32 and 8.58 ppm for the protons of the NH-CHS and NH2-CHS groups respectively. A signal is observed at 2.265 ppm corresponding to the methyl group. The proton of pyridine ring moiety observed in the range of 7–8.5 ppm and the 1H NMR spectrum of 1-(1-(2,4-dihydroxyphenyl)ethylidene)thiosemicarbazide (L2) exhibits a single peak at 10.63 and 12.61 ppm is attributed to p- and o-position of phenolic –OH group[31]. The chemical shifts belonging to NH2 and NH protons at 8.03 and 9.67 ppm. The aromatic C H proton was observed at 6.2– 7.7 ppm and the methyl proton also observed at 2.26 ppm.[32]

1

Fig. 1. 1H NMR spectrum of Ligand 1.

Characterization of Metal Complexes Conductivity measurements The molar conductance values of the complexes in DMSO (10¡3 M solutions) were measured at room temperature

290

Sakthilatha et al.

Fig. 2. 1H NMR spectrum of Ligand 2.

and the results are listed in Table 4. The conductivity values (50–105 V¡1cm2mol¡1) showed that all (Cu(II), Ni(II), Zn (II), and VO(IV)) complexes, which are 1:1 electrolyte except Co(II) and Mn(II) complexes (11–15 V¡1cm2mol¡1) have nonelectrolytic nature.[33]

The free ligand exhibits a broad band at 3500–3000 cm¡1, which may be assigned to the phenolic y(O H), y(NH2asy/ sym) and y(NH) group. The ligand spectrum exhibits a y(CHN) band at 1601 cm¡1 assigned to the pyridine ring and band at 1500–1629 cm¡1 assigned to the vibration of the

IR spectral studies of complexes The data of the IR spectral data of the Schiff base ligands and its metal complexes are listed in Table 2 and Figure 3. The IR spectral data of the complexes is compared with those of the free ligands in order to determine the coordination sites that may be involved in chelation. Table 4. Molar conductivity data (ohm¡1cm2 mol¡1) of the complexes.

Compound [CuL1L2]ClO4 [NiL1L2]ClO4 [CoL1L2(OAc) (H2O)] [MnL1L2(OAc) (H2O)] [ZnL1L2]Cl [VOL1L2]SO4

Molar conductance Solvent Lm (ohm¡1cm2 mol¡1)

Magnetic moment meff B.M

DMSO DMSO DMSO

105.35 78.85 15.00

1.95 2.83 5.08

DMSO

11.20

6.38

DMSO DMSO

52.03 65.29

diamagnetic 1.68

Fig. 3. IR spectra of (a) L1, (b) L2, (C) [CuL1L2]ClO4, (d) [NiL1L2]ClO4, (e) [CoL1L2(OAc)(H2O)], (f) [MnL1L2(OAc) (H2O)], (g) [VOL1L2]SO4, (h) [ZnL1L2]Cl.

Spectroscopic Characterization and In Vitro Antibacterial Activity of Novel Metal Complexes

291

Table 5. Electronic spectral data (cm¡1) of the metal complexes. Compound

p! p*

n! p*

LMCT

d-d transition

Band Assignment

Geometry

Ligand 1 Ligand 2 [CuL1L2]ClO4

38,314 36,231 33,444

31,347 30,959 —

— — —

— — 24,399 15,948 13,210

— — Octahedral

[NiL1L2]ClO4

43,103

31,446



[CoL1L2(OAC)(H2O)]

37,174

32,154



[MnL1L2(OAC)(H2O)]

45,871

33,222



[ZnL1L2]Cl [VOL1L2]SO4

43,608 44052

31,446 31,446

25,252 24096

— — 2 B1g ! 2A1g 2 B1g ! 2B2g 2 B1g ! 2Eg 2 A2g ! 2T2g 3 A2g ! 3T1g 3 A2g ! 3T1g (P) 4 T1g ! 4T2g 4 T1g ! 4A2g 4 T1g ! 4T1g 6 A1g ! 4T1g 6 A1g ! 4A2g — 2 B2g !2E1g 2 B2g ! 2A1g

azomethine group. In the IR spectral data of the complexes a considerable Schiff base toward higher or lower frequencies is observed, indication that both the nitrogen atoms of azomethine and pyridine nitrogen are coordinated to the metal ion. The vibrations of the pyridinic ring of the complexes are observed in the range 1602–1630 cm¡1 and the azomethine group[34] in the range 1534–1590 cm¡1. Moreover, the fact that ligand is coordinated to the metal through the pyridinic nitrogen is confirmed. A strong band at 842–845 cm¡1 in ligands is mainly due to y(C D S) stretching vibration which shifted to lower frequencies and occurred at 803–808 cm¡1 in metal complexes, indicating the coordination of thione sulfur to metal atom.[35] But in oxovanadium complex, the bands appeared at 845 cm¡1 and 801 cm¡1, corresponds to L2 thio group only coordinated to metal ion. The y(C O) of the free ligands at 1226 cm¡1 was shifted to lower frequencies in the mononuclear complexes, suggested the participation of the deprotonated oxygen/phenolic –OH group in the chelation.[36] The infrared spectrum of acetate complexes of cobalt(II) and manganese(II) displays two bands at 1499–1455 cm¡1 and 1246–1260 cm¡1 assigned to y(COO)asym and y(COO)sym respectively. The separation of these peaks by 253– 195 cm¡1 indicates the monodentate coordination of acetate group.[37] Copper(II) and nickel(II) complexes showed two sharp peak at 1087–1108 cm¡1 and 625–628 cm¡1, which are assigned to perchlorate ions. These peak does not show any splitting, indicating the presence of an uncoordinated perchlorate ions.[38] Furthermore, the presence of coordinated water molecules as evidenced by broad band’s appeared at 3500–3400 cm¡1 in Co(II) and Mn(II) complexes may be attributed to y(O-H) stretching frequency. The characteristic band at 969 cm¡1 in VO(II) complex has been assigned to y(VHO) vibrations.[39] New bands at 459–515 and 410– 457 cm¡1 are tentatively assigned to y(M-O) and y(M-N) stretching bands.[40]

12,771 14,662 23,696 12,970 13,277 24.330 13157 21186 — 12,755 14,749

Octahedral

Octahedral

Octahedral Octahedral Distorted Octahedral

Electronic spectral and magnetic studies The electronic spectral data of the Schiff base ligands and its complexes are listed in Table 5 and Figure 4. The electronic spectrum of six coordinated copper(II) complex shows a broad band at 24390–13210 cm¡1. It is difficult to assign the transition of this band it is due to mixing of all transitions.[41] The magnetic moment of the copper complex is 1.95 B. M. falls within the range (1.80–2.10 B.M.) normally observed for octahedral arrangement.[42] The electronic spectra of the Ni(II) complexes show bands in the range 12970, 14662, and 23696 cm¡1 characteristic to an octahedral geometry[41] and may be assigned to the 3 A2g(F) ! 3T2g(F), 3A2g(F) ! 3T1g (F), and 3A2g(F) !

Fig. 4. UV spectra of (a) L1, (b) L2, (C) [CuL1L2]ClO4, (d) [NiL1L2]ClO4, (e) [CoL1L2(OAc)(H2O)], (f) [MnL1L2(OAc) (H2O)], (g) [VOL1L2]SO4, (h) [ZnL1L2]Cl.

292

Sakthilatha et al.

3

T1g(P) transitions, respectively.[43] The magnetic moment of the nickel(II) complex is 2.83 B.M., which agrees with the presence of Ni(II) ion in octahedral geometry.[44] The electronic spectrum of mononuclear Co(II) complex showed a strong bands in the range 12970, 13277, and 24330 cm¡1 characteristic to an octahedral geometry.[41,42] There bands may be assigned to 4T1g (F) ! 4T2g (F), 4T1g (F) ! 4A2g (F), and 4T1g (F) ! 4T1g (F), transitions, respectively. The meff value measured for the present complex is 5.08 B.M. The electronic spectrum of Mn(II) complexes exhibits multiple weak bands at 13157 and 21186 cm¡1 assignable to 6A1g ! 4T1g and 6A1g ! 4A2g respectively. The magnetic moment (6.38 B.M.) is commensurate with the presence of five unpaired electrons with orbital contribution calculated for the d5-system in a high-spin octahedral geometry.[45] Electronic spectra of oxovanadium(IV) shows two bands, observed at ranges of 12755 and 14749 cm¡1, which are assigned as 2B2g ! 2E1g and 2B2g ! 2A1g transitions, respectively.[46] Above transition have major contribution for distorted octahedral geometry oxovanadium(IV) complexes which is also supported by the reported literature.[47] The magnetic moment of oxovanadium complex is 1.68 B.M.[48] The Zn(II) complex is diamagnetic and it containing N,Odonor Schiff base and according to the empirical formula of these complexes, we proposed an octahedral geometry.

Cyclic voltammetry The redox potential is an important parameter as it characterizes the ability of the redox center to transfer electrons and also to act as a redox catalyst. In metal complexes, the redox behavior of metal center is also influenced by the nature of the bonded ligands to the metal. The cyclic voltammogram of complexes in the potential range 0 to –2 V and for scan rate of 100 mV/s, in DMSO solutions containing 0.1 M TBAP used as supporting electrolyte, are presented in Table 6 (Figures 5 and 6). The cyclic voltammogram of metal complexes shows an anodic peak at the potential, EPa D –0.56 to –0.90 V and a cathodic peak at the potential EPc D –0.93 to –1.46 V, the half wave potential located at E1/2 D –0.56 to –1.14 V, the difference between the peaks, DEp D 409–845 mV, suggests that it is a one electron, quasireversible process.[49] Table 6. Electrochemical data of Schiff base metal complexes (negative potential). Complex

Epc (V)

Epa (V)

E1/2 (V)

DEP (mV)

Ligand 1 Ligand 2 [CuL1L2]ClO4 [NiL1L2]ClO4 [CoL1L2(OAc)(H2O)] [MnL1L2(OAc)(H2O)] [ZnL1L2]Cl [VOL1L2]SO4

¡0.93 ¡1.26 ¡0.97 ¡1.37 ¡1.46 ¡1.36 ¡1.41 ¡1.29

¡0.19 ¡0.62 ¡0.56 ¡0.53 ¡0.81 ¡0.90 ¡0.79 —

¡0.56 ¡0.94 ¡0.77 ¡0.95 ¡1.14 ¡1.13 ¡1.10 —

733 638 409 845 644 468 620 —

Fig. 5. Cyclic voltammogram of (a) L1, (b) L2, (C) [CuL1L2] ClO4.

On the basis of the previous evidence and analyses, the possible structure of the complexes is given as in Scheme 4. Biological activities The in vitro antimicrobial activities of the Schiff base ligands and their metal complexes were tested by MIC methods against four microbes such as Bacillus subtilis, Staphylococcus aureus (Gram-positive bacteria), and Pseudomonas aeruginosa, Salmonella typhi (Gram-negative bacteria) using the well diffusion method.[50] The activity of the ligands and its complexes increases as the concentration increases because it is a well-known fact that concentration plays a vital role in increasing the zone of inhibition. Table 7 shows the results of antimicrobial studies for the Schiff base ligands, metal complexes, and standard (kanamycin). It has been suggested that the ligands with the N and O donor system might have inhibited enzyme production, as enzymes that require free hydroxyl groups for their activity appear to the especially susceptible to deactivation by the ions of the complexes. The complexes facilitate their diffusion through the lipid layer of spore membranes to the site of action ultimately killing them by combining with -OH group of certain cell enzymes. The variation in the effectiveness of different biocidal agents against organisms depends on the impermeability of the cell. Chelation reduces the polarity of the central metal atom, mainly because of partial sharing of its positive charge with the ligand; also, the normal cell process may be affected by the formation of hydrogen bond through the azomethine nitrogen atom with the active centers of cell constituents.[51] From the results, it is clear that all the complexes are higher antibacterial activity than Schiff base ligands. Zn(II) and Cu (II) complexes exhibit higher inhibition toward all the microorganisms and is given Figure 7. However, Ni(II) and VO(IV) complexes exhibit moderate inhibition and Co(II) and Mn(II) complexes less activity toward the microorganisms compared with other complexes. The ligands L1 and L2 are very lower activity than metal complexes. The higher antimicrobial

Spectroscopic Characterization and In Vitro Antibacterial Activity of Novel Metal Complexes

293

Fig. 6. Cyclic voltammogram of (a) [NiL1L2]ClO4, (b) [CoL1L2(OAc)(H2O)], (c) [MnL1L2(OAc)(H2O)], (d) [VOL1L2]SO4, (e) [ZnL1L2]Cl. Table 7. Antimicrobial activity of the Schiff base ligands and their complexes. Zone of inhibition (mm) Gram-positive Compound Ligand 1

Ligand 2

[CuL1L2]ClO4 [NiL1L2]ClO4 [CoL1L2(OAc)(H2O)] [MnL1L2(OAc)(H2O)] [ZnL1L2]Cl [VOL1L2]SO4 Kanamycin

Gram-negative

Conc. (mg/mL)

Staphylococcus aureus

Bacillus subtilis

Pseudomonas aeruginosa

Salmonella typhi

25 50 100 25 50 100 25 50 100 25 50 100 25 50 100 25 50 100 25 50 100 25 50 100 30

10 12 15 5 5 7 12 15 17 7 9 10 1 2 2 1 2 3 9 12 14 7 8 10 10

0 1 2 3 5 7 10 13 14 9 10 11 4 5 7 4 4 5 10 12 15 6 8 12 14

0 2 4 6 7 8 9 10 12 10 12 14 1 2 3 0 0 1 9 11 14 5 7 10 9

3 5 7 5 6 8 8 11 12 9 11 11 0 2 2 0 1 1 13 15 16 8 10 13 10

294

Sakthilatha et al. NH2 CH3 C

HN

C

N

S N

M HO

O

S

H2N

N

C

X

C

NH

CH3

M = Cu(II), Ni(II), Zn(II) X = ClO4-, ClH2N C CH3 C

S

NH OCOCH3

N

N

M HO

OH2

O

N

C

HN S

CH3

C NH2

M = Co(II), Mn(II) NH2 CH3 C

HN N

C O

S N

V HO

OH

H2N

Sch. 4. Structure of metal complexes.

S C

SO4 N

NH

C CH3

Spectroscopic Characterization and In Vitro Antibacterial Activity of Novel Metal Complexes

295

Fig. 7. Antibacterial activity of L1 D Ligand, 1 L2 D Ligand 2, M1 D [CuL1L2]ClO4, M2 D [NiL1L2]ClO4, M3 D [CoL1L2(OAc) (H2O)], M4 D [MnL1L2(OAc)(H2O)], M5 D [ZnL1L2]Cl, M6 D [VOL1L2]SO4.

activity of Zn(II) and Cu(II) complexes, compared with that of other complexes, is perhaps due to the change in structure due to coordination, and chelating tends to make metal complexes act as more powerful and potent bacteriostatic agents, thus inhibiting the growth of the microorganisms.

Conclusion In this study, we synthesized Schiff base metal(II) complexes by template method using the ligands L1 and L2 and metal salts. The mode of bonding and over all structure of the complexes were determined through physicochemical and spectroscopic methods. The IR spectra elucidate the Schiff base ligands coordinated to metal ions via, azomethine nitrogen, pyridine ring nitrogen and deprotonated oxygen/phenolic –OH group. Thus the octahedral structure was proposed for these complexes in accordance with electronic spectra and magnetic studies. The electrochemical data of the complexes suggest metal centered electro activity in the potential range. Zn(II) and Cu(II) complexes show an increased antibacterial activity when compared with standard Kanamycin.

Funding The authors are grateful to the University Grant Commission (UGC), New Delhi for financial support in the form of Major

Research Project [MRP-F.No 37-299/2009 (SR)] of this work.

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