Studies on metal complexes of 2-((8-hydroxyquinolin-5-yl)methylene)benzo[b]thiophen-3(2H)-one 1,1-dioxide

Studies on metal complexes of 2-((8-hydroxyquinolin-5-yl)methylene)benzo[b]thiophen-3(2H)-one 1,1-dioxide

Yogesh S. Patel

Received: 25 February 2014 / Accepted: 2 July 2014

� Springer Science+Business Media Dordrecht 2014

Abstract The novel Schiff base 2-((8-hydroxyquinolin-5-yl)methylene)benzo[b]

thiophen-3(2H)-one 1,1-dioxide (3) was synthesized by Remier–Tieman reaction of

benzo[b]thiophen-3(2H)-one 1,1-dioxide (1) with 8-hydroxyquinoline-5-carbalde-

hyde (2). Metal complexes (4a–f) of the Schiff base (3) were prepared from salts of

a variety of transition metal ions, viz., Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and

Zn(II). The Schiff base and its metal complexes were characterized by physico-

chemical, thermogravimetric, and spectroscopic analysis. Thermogravimetric ana-

lysis revealed the number of water molecules in the metal complexes. Electronic

spectral analysis and magnetic measurement studies were used to determine the

geometry of the complexes. The compounds were evaluated for antibacterial

activity against Gram-positive (Bacillus subtilis and Staphylococcus aureus) and

Gram-negative (Salmonella typhimurium and Escherichia coli) bacteria. Growth

inhibition was compared with that by the standard drug ciprofloxacin. Antifungal

activity was tested against four fungi (Penicillium expansum, Botryodiplodia

theobromae, Nigrospora sp., and Trichothesium sp.), with the antifungal drug

ketoconazole as positive control.

Keywords Metal complex � Spectral study � Thermal study � Antimicrobial

activity

Introduction

In the last few decades there has been increased interest in the synthesis and study of

metal complexes of 8-hydroxyquinolines (8HQs) [1–6]. By exploiting the reactivity

of this heterocycle and its derivatives, researchers have used different strategies to

Y. S. Patel (&)

Department of Chemistry, Government Science College, Gandhinagar 382015, Gujarat, India

e-mail: [email protected]

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Res Chem Intermed

DOI 10.1007/s11164-014-1764-9

synthesize metal complexes with different structures, or to modify these

structures [7–12]. 8HQ and its derivatives have been reported to have promising

bioactivity, including anticancer [13, 14], antibacterial [15, 16], antidyslipidemic

and antioxidative [17] activity, vasorelaxing properties [18], and antivirus and

antiplatelet [19–21] activity.

There has also been interest in the use of benzo[b]thiophene, a bioisostere of

indole, for synthesis of thioindigo dyes. Analogues of bioactive indole derivatives,

including indole alkaloids, were synthesized in the 1960s and 1970s, and synthesis

of several sulfur analogues of bioactive furanochromones and furanocoumarins has

also been reported in the literature. These analogues, with a benzo[b]thiophene core,

are usually obtained by use of suitable annulation reactions. This work is still in

progress, and its literature up to 1980 has been reviewed [22, 23].

We therefore decided to undertake a study of compounds containing a

combination of the benzo[b]thiophene and 8-HQ structures in single molecules.

Our prime intention was to synthesize metal complexes and to study their

antimicrobial activity. Our synthetic approach in the different phases of this work,

viz.:

1 synthesis of the Schiff base 2-((8-hydroxyquinolin-5-yl)methylene)benzo

[b]thiophen-3(2H)-one 1,1-dioxide;

2 synthesis of metal complexes by use of a variety of metal(II) acetates, e.g.

Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) metal ions is summarized in

Fig. 1 and details of the procedures and the results obtained are discussed

below.

Experimental

Materials and measurements

All common reagents and solvents were of analytical grade and were used without

further purification. Aluminium foil-backed pre-coated silica gel 60 F254 thin-layer

chromatography (TLC) plates purchased from E. Merck, Mumbai, India, were used to

check the purity of compounds and to study the progress of reactions. For visualization,

TLC plates were illuminated with ultraviolet light (254 nm), evaluated in I2 vapor, and

sprayed with Dragendorff’s reagent. Infrared spectra (FT-IR) in the range

4,000–400 cm-1 were obtained from KBr pellets by use of a Perkin Elmer Spectrum

GX spectrophotometer. 1H NMR and 13C NMR spectra were acquired at 400 MHz on a

Bruker NMR spectrometer, with DMSO-d6 as solvent and TMS as internal reference

standard. Microanalytical (C, N, H) data were obtained by use of a Perkin–Elmer 2400

CHN elemental analyzer. Diffuse electronic spectra of solids were recorded on a

Beckman DK-2A spectrophotometer with a solid reflectance attachment. MgO was

used as a reference. Magnetic moments [24] at room temperature were determined by

the Gouy method with mercury tetrathiocyanatocobaltate(II), HgCo(NCS)4, as calibrant

(Xg = 1,644 9 10-6 cgs units at 20 �C), by use of a Citizen balance. Molar

susceptibilities were corrected by use of Pascal’s constant [25]. Thermogravimetric

studies, in the temperature range 50–700 �C, in air, were performed with a Perkin–

Y. S. Patel

123

Elmer thermogravimetric analyzer; the heating rate was 10 � min-1. The metal content

of coordination compounds was determined by decomposing a weighed amount of each

compound with 1:1.5:2.5 HClO4–H2SO4–HNO3 followed by standard EDTA titra-

tion [26]. Melting points were determined by the standard open capillary method.

Synthesis of the Schiff base

Schiff base (3) was synthesized as reported elsewhere [27]. Piperidine (10 % mol

equiv) and benzo[b]thiophen-3(2H)one 1,1-dioxide (2 mmol) were added to a

solution of 8-hydroxyquinoline-5-carbaldehyde (2 mmol) in acetic acid (10 mL).

The reaction mixture was stirred under reflux for 30 min. After cooling, the

precipitate was isolated by filtration and crystallized from a mixture of acetic acid

and methanol to give 2-((8-hydroxyquinolin-5-yl)methylene)benzo[b]thiophen-

3(2H)-one 1,1-dioxide as yellow crystals. Yield 70 %; M.Wt. 337.35 g;

m.p. [250 �C; elemental analysis calculated for C18H11NO4S: C 64.09, H 3.29,

N 4.15, S 9.50 %; Found: C 64.12, H 3.26, N 4.17, S 9.52 %; 1H NMR (DMSO-d6,

d ppm): 5.87 (s, 1H, H11); 6.69 (dd, 1H, j1 = 7.5, j2 = 4.8, H16); 6.82 (d, 1H,

j = 7.3, H18); 6.96 (dd, 1H, j1 = 7.3, j2 = 4.8, H17); 7.12 (d, 1H, j = 7.5, H15);

7.26 (d, 1H, j = 8.4, H7); 7.45 (d, 1H, j = 8.4, H6); 7.60 (dd, 1H, j1 = 7.2,

j2 = 6.0, H3); 8.68 (d, 1H, j = 7.2, H4); 8.94 (d, 1H, j = 6.0, H2); 9.72 (s, 1H, –

OH); 13C NMR (DMSO-d6, d ppm): 111.23 (C7), 121.84, 124.28, 125.95, 127.12,

Fig. 1 Synthesis of the Schiff base and its metal complexes

Studies on metal complexes

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129.85 (C12), 132.4, 134.2, 136.91 (C9), 142.15 (C14), 144.72 (C11), 148.84 (C2),

153.90 (C8), 185.28 (C20); FT-IR (KBr, cm-1): 3339 m(Ar–OH), 3044 m(Ar–CH),

1694 m(–C=O), 1599 m(–C–N), 1504, 1570 m(–C=C–), 1344 m(–S=Oasym), 1179 m(–

S=Osym), 1227 d(–O–H), 1097 m(–C–O).

Synthesis of metal complexes

A hot solution of the metal(II) salt (2.5 mmol) in 50 % aqueous formic acid (5 mL)

was added dropwise with continuous stirring to a hot solution of the Schiff base

(5 mmol) in 20 % aqueous formic acid (20 mL). After addition of the metal salt

solution, the pH of the reaction mixture was adjusted (*5–6) with dilute ammonia

solution. The solid product which separated out as a suspension was digested on a

water bath for 2 h. The solid was isolated by filtration, washed with hot water then

ethanol, and then dried in air at room temperature. All the metal complexes were

characterized by determination of their physicochemical properties by use of

spectroscopic techniques and by thermogravimetric analysis. Electronic spectral

analysis and magnetic measurement studies were conducted to determine the

geometry of the metal complexes.

4a: Yield 55 %; M. Wt. 763.65 g; m.p. [250 �C; elemental analysis calculated

for C36H24N2O10S2Mn: C 56.62, H 3.17, N 3.67, S 8.40, Mn 7.19 %; Found: C

56.60, H 3.16, N 3.62, S 8.42, Mn 7.12 %; FT-IR (KBr, cm-1): 3439 m(–OH, H2O),

3034 m(Ar–CH), 1696 m(–C=O), 1580 m(–C–N), 1575, 1509 m(–C=C–), 1408

m(O–M), 1279 d(–OH, H2O), 1344 m(–S=Oasym), 1179 m(–S=Osym), 1117 m(C–O),

1092 m(C–O?M), 872 (H2Orocking), 730 m(M?N), 705 (H2Owagging), 539 m(M?N).

4b: Yield 55 %; M. Wt. 764.56 g; m.p. [250 �C; elemental analysis calculated

for C36H24N2O10S2Fe: C 56.55, H 3.16, N 3.66, S 8.39, Fe 7.30 %; Found: C 56.50,

H 3.13, N 3.69, S 8.34, Fe 7.33 %; FT-IR (KBr, cm-1): 3447 m(–OH, H2O), 3027

m(Ar–CH), 1695 m(–C=O), 1584 m(–C–N), 1571, 1503 m(–C=C–), 1404 m(O–M),

1349 m(–S=Oasym), 1276 d(–OH, H2O), 1182 m(–S=Osym), 1115 m(C–O), 1095

m(C–O?M), 879 (H2Orocking), 737 m(M?N), 709 (H2Owagging), 535 m(M?N).

4c: Yield 60 %; M. Wt. 767.65 g; m.p. [250 �C; elemental analysis calculated

for C36H24N2O10S2Co: C 56.33, H 3.15, N 3.65, S 8.35, Co 7.68 %; Found: C 56.30,

H 3.17, N 3.63, S 8.31, Co 7.70 %; FT-IR (KBr, cm-1): 3442 m(–OH, H2O), 3042




4d: Yield 58 %; M. Wt. 767.41 g; m.p. [250 �C; elemental analysis calculated

for C36H24N2O10S2Ni: C 56.34, H 3.15, N 3.65, S 8.36, Ni 7.65 %; Found: C 56.37,

H 3.15, N 3.62, S 8.33, Ni 7.60 %; FT-IR (KBr, cm-1): 3430 m(–OH, H2O), 3031




4e: Yield 65 %; M. Wt. 772.26 g; m.p. [250 �C; elemental analysis calculated

for C36H24N2O10S2Cu: C 55.99, H 3.13, N 3.63, S 8.30, Cu 8.23 %; Found: C 55.96,

H 3.10, N 3.66, S 8.32, Cu 8.26 %; FT-IR (KBr, cm-1): 3432 m(–OH, H2O), 3039


Y. S. Patel

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4f: Yield 55 %; M. Wt. 774.09 g; m.p. [250 �C; elemental analysis calculated

for C36H24N2O10S2Zn: C 55.86, H 3.13, N 3.62, S 8.28, Zn 8.45 %; Found: C 55.82,

H 3.10, N 3.63, S 8.25, Zn 8.48 %; FT-IR (KBr, cm-1): 3430 m(–OH, H2O), 3037



m(C–O?M), 879 (H2Orocking), 733 m(M?N), 702 (H2Owagging), 536 m(M?N). 1H

NMR (DMSO-d6, d ppm): 5.88 (s, 2H, H11); 6.67 (dd, 2H, j1 = 7.5, j2 = 4.8, H16);

6.82 (d, 2H, j = 7.3, H18); 7.09 (dd, 2H, j1 = 7.3, j2 = 4.8, H17); 7.21 (d, 2H,

j = 7.5, H15); 7.34 (d, 2H, j = 8.4, H7); 7.52 (d, 2H, j = 8.4, H6); 7.73 (dd, 2H,

j1 = 7.8, j2 = 6.4, H3); 8.87 (d, 2H, j = 7.8, H4); 9.08 (d, 2H, j = 6.4, H2).

Biological activity

Antibacterial activity (in vitro)

Compounds 3 and 4a–f were screened for in-vitro antibacterial activity against

Gram-positive (Bacillus subtilis (BS) and Staphylococcus aureus (SA)) and

Gram-negative (Salmonella typhimurium (ST) and Escherichia coli (EC) bacteria

by use of the agar diffusion assay [28, 29]. Wells were cut in the media by use

of a sterile metal borer and solutions of the test samples, at the recommended

concentration (1 mg/mL in DMSO), were placed in the wells. DMSO and the

reference antibacterial drug ciprofloxacin were placed in other wells as negative

and positive controls, respectively. The plates were immediately incubated at

37 �C for 24 h. Activity was determined by measuring the diameter (mm) of

zones indicative of complete inhibition. Growth inhibition was compared with

that for the standard drug. DMSO had no activity against any of the bacterial

strains.

Antifungal activity (in vitro)

Compounds 3 and 4a–f were also examined for activity against the fungi

Penicillium expansum (PE), Botryodiplodia theobromae (BT), Nigrospora sp.

(NS), and Trichothesium sp. (TS). The antifungal drug, ketoconazole was used as

positive control. The fungi were grown and maintained on potato dextrose agar

plates, and cultures of the fungi were purified by the single-spore isolation

technique. Solutions of compounds 3 and 4a–f and the positive control at the

recommended concentration were prepared in DMSO to test inhibition of

germination of the spores of each fungus. Fungal culture plates were inoculated

and incubated at 25 ± 2 �C for 48 h. The plates were then observed and diameters

(mm) of zones of inhibition were measured. Percentage inhibition after 5 days was

calculated by use of the formula:


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Percentage of inhibition ¼ 100 X � Yð ÞX

;

where X is the area of the colony on the control plate and Y is the area of the colony

on the test plate.

Results and discussion

We successfully synthesized compound 3 by reaction of benzo[b]thiophen-3(2H)-

one 1,1-dioxide (1) with 8-hydroxyquinoline-5-carbaldehyde (2). Complexes (4a–

f) of a variety of metal(II) salts with Schiff base (3) were also prepared. Compounds

3 and 4a–f were duly characterized.

Characterization of Schiff base 3

As far as we are aware, Schiff base 3 has not been reported previously.

Characterization of the reaction product furnished clear-cut proof of successful

synthesis of 2-((8-hydroxyquinolin-5-yl)methylene)benzo[b]thiophen-3(2H)-one

1,1-dioxide (3). The FTIR spectrum contained the most relevant peaks of the

quinoline and benzo[b]thiophen rings and typical absorption at 3339 cm-1 from the

hydroxyl group attached to the quinoline ring (Fig. 2). In the 1H NMR spectrum

(Fig. 3), the singlet at 9.72 d ppm was ascribed to protons of the –OH group, and

other relevant peaks were observed as expected; the structure was further confirmed

by 13C NMR data. The expected structure depicted in Fig. 1 was, thus, clearly

verified by the spectroscopic analysis, which also indicated the absence of any

detectable impurity, particularly of the two reagents used to prepare 3.

Characterization of metal complexes 4a–f

Metal complexes 4a–f were duly characterized by spectroscopic analysis. The

number of water molecules attached to the metal ion in the complex, and other

structural information, were determined by thermal analysis. The geometry of the

central metal ion was confirmed by study of electronic spectra and by magnetic

susceptibility measurements. Elemental analysis of all the metal complexes was in

good agreement with the predicted structures. From the results obtained it was

concluded that the metal to ligand stoichiometry of the complexes was 1:2. All the

data were consistent with the suggested structure of the metal complexes shown in

Fig. 4.

IR spectral bands of the Schiff base and its metal complexes suggest formation of

the desired metal complexes and support their structure. Spectral features provide

valuable information about the nature of the functional group attached to the metal

atom. To study the mode of bonding of the Schiff base to the metal complexes, the

IR spectrum of the free Schiff base (Fig. 2) was compared with those of the metal

complexes (Fig. 4). Substantial differences were expected. Characteristic absorption

bands observed for the free Schiff base were: mO–H (3,339 cm-1), mC–O

Y. S. Patel

123

Fig. 2 FT-IR spectrum of Schiff base 3

Fig. 3 NMR spectrum of Schiff base 3


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(1,694 cm-1), mC–N (1,599 cm-1), and dO–H (1,227 cm-1) [30]. Both of the two

characteristic absorption bands mO–H and dO–H disappeared after formation of the

coordinate complex, and the mC–O (1,097 cm-1) band shifted toward higher

frequency. According to Charles et al. [31], this may be because the transition metal

ion was coordinated with the ligand by formation of an M–O bond in which the

electronegativity of the metal ion was less than that of hydrogen whereas the

electronegativity of the oxygen was larger. In addition, the original absorption band

at mC–N (1,599 cm-1) was shifted toward lower frequency; this may be because the

transition metal ion was coordinated with the ligand by formation of an M–N bond.

From these results, we concluded that the transition metal ion was coordinated with

the hydroxyl oxygen atom and the hetero-nitrogen atom in the 8HQ. The bands

observed in the regions 3,430–3,442, 1,274–1,282, 872–882, and 702–709 cm-1 are

attributed to –OH stretching, bending, rocking, and wagging vibrations, respec-

tively, because of the presence of water molecules. The presence of the rocking

band indicates the nature of coordination of water molecules [32]. New bands, i.e.

bands which were not present in the spectrum of the Schiff base, appeared in the

spectra of the metal complexes. For example, sharp bands in the regions

532–540 cm-1 and 732–738 cm-1 can be assigned to mM–N [33], which indicated

involvement of the nitrogen atom in coordination. Medium-intensity bands for mM–

O [34] were observed at 625–640 cm-1 as a result of M–O coordination. The

appearance of mM–N and mM–O vibrations supports the involvement of N and O

atoms in complexation with the metal ions. These data suggest that the amide-N and

carboxylate-O groups are involved in coordination with the metal(II) ion in the

complexes. These features confirmed the proposed structure of the metal complexes

shown in Fig. 4.

Comparison of the 1H NMR spectra of the Schiff base and its complex with

Zn(II) revealed that a broad singlet at d 9.72 ppm in the former, attributed to the

OH proton [35], had disappeared from the spectrum of Zn(II) complex, suggesting

Fig. 4 FT-IR spectrum of metal complex 4e

Y. S. Patel

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this proton has been lost because of coordination of the oxygen atom with the

metal ion [36]. The H2 signal of the Zn(II) complex appeared at lower magnetic

field (d 9.08) than in the spectrum of the Schiff base (d 8.94), suggesting

involvement of the N atom in formation of complex. Absorption of all the

quinoline protons was shifted slightly downfield, except for that of H7 which was

shifted upfield [37]; this is indicative of coordination of the oxygen atom with the

metal ion.

Information about the geometry of the metal complexes was obtained from

electronic spectral data and the magnetic moments of the compounds (Table 1). The

diffuse electronic spectrum of [Cu(L)(H2O)2]n contains two broad bands at

approximately 15,622 and 25,757 cm-1, ascribed to the 2Eg ? 2T2g transition

and to charge transfer, respectively. This suggests distorted octahedral structure for

this complex, which was further confirmed by its leff value (1.87 B.M.). The

spectrum of [Ni(L)(H2O)2]n contains absorption bands at 9,891, 15,584, and

23,978 cm-1, ascribed to the 3A2g ? 3T2g(F), 3A2g ? 3T1g(F), and 3A2g ? 3T1g

transitions, respectively. The spectrum of [Co(L)(H2O)2]n contains absorption bands

at 22,041, 15,520, and 9,830 cm-1, corresponding to the 4T1g(F) ? 4T1g(P),

4T1g(F) ? 4A2g(F), and 4T1g(F) ? 4T2g(F) transitions, respectively. These results

are indicative of the octahedral configuration of the [Ni(L)(H2O)2]n and [Co(L)(H2-

O)2]n complexes [38]. This configuration was further confirmed by their leff values

(3.11 B.M. and 4.62 B.M.). The spectrum of [Fe(L)(H2O)2]n contains bands at

36,022 cm-1 and 19,038 cm-1 ascribed to the transitions 5T2g(F) ? 3T1g and

5T2g(F) ? 3Eg; these and the leff value of 5.02 B.M. again suggest octahedral

configuration. The spectrum of [Mn(L)(H2O)2]n contains weak bands at 16,486,

17,710, and 23,179 cm-1 ascribed to the transitions 6A1g ? 4T1g(4G),

Table 1 Electronic spectral data of the metal complexes

Metal

complex

Molecular formula leff (B.M.) Frequency

(cm-1)

Transition

4a [Mn(L)H2O)2] 5.44 16,486 6A1g ? 4T1g(4G)

C36H24N2O10S2Mn 17,710 6A1g ? 4T2g(4G)

23,179 6A1g ? 4A1g, 4Eg

4b [Fe(L)(H2O)2] 5.02 19,038 5T2g(F) ? 3Eg

C36H24N2O10S2Fe 36,022 5T2g(F) ? 3T1g

4c [Co(L)(H2O)2] 4.62 9,830 4T1g(F) ? 4T2g(F)

C36H24N2O10S2Co 15,520 4T1g(F) ? 4A2g(F)

22,041 4T1g(F) ? 4T1g(P)

4d [Ni(L)(H2O)2] 3.11 9,891 3A2g ? 3T2g

C36H24N2O10S2Ni 15,584 3A2g ? 3T1g(F) 3A2g ? 3T1g

(P)23,978

4e [Cu(L)(H2O)2] 1.87 15,622 2Eg ? 2T2g

C36H24N2O10S2Cu 25,757 Charge transfer

4f [Zn(L)(H2O)2] D

(diamagnetic)

– –

C36H24N2O10S2Zn


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6A1g ? 4T2g(4G), and 6A1g ? 4A1g,4Eg, respectively, again suggesting octahedral

structure for the [Mn(L)(H2O)2]n complex [39]. This configuration was further

confirmed by its leff value of 5.44 B.M. The spectrum of the [Zn(L)(H2O)2]n

complex was not easily interpreted, but its leff value shows the complex is

diamagnetic, as expected. Magnetic moments leff of all the other metal complexes

revealed they are paramagnetic.

The thermal behavior of complexes can provide much information. Decompo-

sition of all the metal complexes occurred in two steps (Table 2). The first step,

between 80 and 160 �C, might be attributed to loss of mass corresponding to water

molecules (Fig. 5). The weight loss observed for each complex was quite consistent

with the theoretical value for elimination of two water molecules, indicating two

water molecules were coordinated to the metal ion. The second step occurred

between 160 and 700 �C, and the mass loss corresponded to decomposition of the

ligand part of the complexes. Noticeable weight loss of the metal complexes

occurred between 350 and 550 �C, possibly because of acceleration by metal oxide

formed in situ. Each metal complex lost approximately 80 % of its weight when

heated to 700 �C. On the basis of the relative decomposition (% weight loss) and the

nature of the thermograms, the metal complexes may be arranged in order of

increasing stability as: Cu \ Fe \ Ni \ Co \ Zn \ Mn.

Biological activity

Antibacterial activity

Results from studies of antibacterial activity against Gram-positive and Gram-

negative bacteria indicated compounds 3 and 4a–f were active against both types of

bacteria, with zones of inhibition (ZOI) ranging from 22 to 38 mm (Table 3).

Schiff base 3 (ZOI[BS] = 24 mm, ZOI[SA] = 23 mm, ZOI[ST] = 22 mm,

ZOI[EC] = 24 mm) was less active than its metal complexes. Among compounds

4a–f, compound 4e (ZOI[BS] = 36 mm, ZOI[SA] = 33 mm, ZOI[ST] = 38 mm,

ZOI[EC] = 37 mm) was a potent antibacterial agent against all Gram-positive and

Gram-negative bacteria. Compound 4b (ZOI[BS] = 35 mm, ZOI[SA] = 32 mm,

ZOI[ST] = 37 mm, ZOI[EC] = 36 mm) also had good antibacterial activity against

Table 2 Thermogravimetric data of the metal complexes

Sample % weight left at different temperatures % wt. loss from

80–160 �C attributed to

coordinated H2O

100 �C 200 �C 300 �C 400 �C 500 �C 600 �C 700 �C Cald. Found

4a 96.6 93.3 89.9 79.9 44.0 23.9 19.5 4.71 4.8

4b 97.1 93.0 90.2 80.4 44.3 24.2 19.8 4.71 4.9

4c 97.3 93.3 90.7 80.0 44.4 24.6 20.1 4.69 4.6

4d 96.9 92.8 90.3 79.8 44.2 24.0 19.6 4.69 4.5

4e 97.5 93.1 90.8 80.3 44.7 24.2 20.0 4.66 4.7

4f 97.2 92.8 90.1 80.4 44.6 23.8 19.3 4.65 4.6

Y. S. Patel

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the bacteria. Compounds 4a, 4c–f had moderate antibacterial activity. Compounds 3and 4a–f had less antibacterial activity than the standard antibiotic drug,

ciprofloxacin (ZOI[BS] = 38 mm, ZOI[SA] = 36 mm, ZOI[ST] = 40 mm, ZOI[EC] =

39 mm).

As already stated, comparative study of the sizes of the growth-inhibition zones for

the Schiff base and its metal complexes indicated the metal complexes had greater

Fig. 5 Thermogram of metal complex 4e

Table 3 Antimicrobial activity of the Schiff base and its metal complexes

Compound Antibacterial activity Antifungal activity

Zone of inhibition Zone of inhibition

Gram ?ve Gram -ve PE BT NS TS

BS SA ST EC

3 24 23 22 24 28 24 25 22

4a 27 26 28 28 30 28 29 27

4b 35 32 37 36 36 33 37 34

4c 29 28 27 26 29 27 31 30

4d 30 29 31 29 27 28 30 27

4e 36 33 38 37 38 36 39 37

4f 26 28 29 28 32 31 30 28

Ciprofloxacin 38 36 40 39 – – – –

Ketoconazole – – – – 42 41 40 39

BS, Bacillus subtilis; SA, Staphylococcus aureus; ST, Salmonella typhi; EC, Escherichia coli;

PE, Penicillium expansum; BT, Botrydepladia thiobromine; NS, Nigrospora Sp.; TS, Trichothesium Sp.


123

antibacterial activity than the free Schiff base (Table 3). Such increased activity of the

metal complexes can be explained on the basis of Overtone’s concept and Tweedy’s

chelation theory [40]. According to Overtone’s concept of cell permeability, the lipid

membrane that surrounds the cell favors passage of lipid soluble materials only, and

liposolubility is regarded as an important aspect of antimicrobial activity. Chelation

reduces the polarity [41, 42] of the metal ion, mainly because of partial sharing of its

positive charge with the donor groups and, possibly, p-electron delocalization within

the whole chelate ring system as a result of coordination. This process of chelation

thus increases the lipophilic nature of the central metal atom, which in turn favors its

permeation through the lipoid layer of the membrane. This in turn is responsible for

increasing the hydrophobic character and liposolubility of the molecule, enabling it to

cross the cell membrane of the microorganism; this enhances the biological utilization

ratio and activity of the compound. The biological activity of compounds also

depends on the nature of the ligand, i.e. its concentration and lipophilicity, the nature

of the metal ion, the sites of coordination, and the geometry of the complex.

Antifungal activity

Results from screening of the antifungal activity of compounds 3 and 4a–f revealed

all the compounds were active against the four fungi (Table 3). Schiff base 3(ZOI[PE] = 28 mm, ZOI[BT] = 24 mm, ZOI[NS] = 25 mm, ZOI[TS] = 22 mm) was

less active than its metal complexes. Compound 4e (ZOI[PE] = 38 mm,

ZOI[BT] = 36 mm, ZOI[NS] = 39 mm, ZOI[TS] = 37 mm) was most active against

all the fungi. Compound 4b (ZOI[PE] = 36 mm, ZOI[BT] = 33 mm, ZOI[NS] = 37 -

mm, ZOI[TS] = 34 mm) also had good antifungal activity. Compounds 4a, 4c–f had

moderate antifungal activity. Compounds 3 and 4a–f were less active than the

standard antibiotic drug, ketoconazole (ZOI[PE] = 42 mm, ZOI[BT] = 41 mm,

ZOI[NS] = 40 mm, ZOI[TS] = 39 mm).

Conclusions

A new Schiff base and its metal complexes were prepared in good yield and duly

characterized. In the metal complexes, the Schiff base coordinates with one central

metal atom at four coordination sites, with two water molecules. Structures proposed

for the Schiff base and its metal complexes are consistent with results from elemental,

spectral, and thermal analysis. The geometry of the central metal ion was confirmed

by electronic spectra and magnetic susceptibility measurements. All the data provide

good evidence of complex formation. The complexes do not melt up to 400 �C. The

complexes have moderate thermal stability and good biological activity.

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