Structural, spectroscopic, and biological aspects of S∩N donor Schiff-base ligands and their chromium(III) complexes

Article abstract of DOI:10.1080/00958972.2010.489110

New chromium(III) complexes are synthesized by classical thermal and microwave (MW)-irradiated techniques. The Schiff bases 2-acetylfuran-S-benzyldithiocarbazate (L¹H), 2-acetylthiophene-S-benzyldithiocarbazate (L²H), 2-acetylpyridin

Full text of DOI:10.1080/00958972.2010.489110

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Structural, spectroscopic, and
biological aspects of S∩N donor Schiff-
base ligands and their chromium(III)
complexes
Sumit Shrivastava a , Nighat Fahmi a , D. Singh b & R.V. Singh a
a Department of Chemistry, University of Rajasthan, Jaipur
302055, India
b Faculty of Engineering and Technology, C.S. Azad University
of Agriculture and Technology Campus, Etawah, Uttar Pradesh
206001, India
Published online: 11 Jun 2010.
To cite this article: Sumit Shrivastava , Nighat Fahmi , D. Singh & R.V. Singh (2010)
Structural, spectroscopic, and biological aspects of S∩N donor Schiff-base ligands and
their chromium(III) complexes, Journal of Coordination Chemistry, 63:10, 1807-1819, DOI:
10.1080/00958972.2010.489110
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Journal of Coordination Chemistry
Vol. 63, No. 10, 20 May 2010, 1807–1819
Structural, spectroscopic, and biological aspects of S\N donor
Schiff-base ligands and their chromium(III) complexes
SUMIT SHRIVASTAVAy, NIGHAT FAHMIy, D. SINGHz and R.V. SINGH*y
yDepartment of Chemistry, University of Rajasthan, Jaipur 302055, India
zFaculty of Engineering and Technology, C.S. Azad University of Agriculture and
Technology Campus, Etawah, Uttar Pradesh 206001, India
(Received 13 April 2009; in final form 23 February 2010)
New chromium(III) complexes are synthesized by classical thermal and microwave
(MW)-irradiated techniques. The Schiff bases 2-acetylfuran-S-benzyldithiocarbazate (L1H),
2-acetylthiophene-S-benzyldithiocarbazate (L2H), 2-acetylpyridine-S-benzyldithiocarbazate
(L3H), and 2-acetylnaphthalene-S-benzyldithiocarbazate (L4H) were prepared by condensation
of -S-benzyldithiocarbazate in ethanol with the respective ketones by using MW as well as
conventional methods. The chromium(III) complexes have been prepared by mixing
CrCl3 ꢀ 6H2O in 1 : 1 and 1 : 2 molar ratios with monofunctional bidentate ketimines. The
structure of the ligands and their transition metal complexes were confirmed by elemental
analysis, melting point determinations, molecular weight determinations, infrared (IR),
electronic and electron paramagnetic resonance (EPR) spectral, and X-ray powder diffraction
studies. On the basis of these studies it is clear that the ligands coordinated to the metal atom in a
monobasic bidentate mode by S\N donors. Thus, an octahedral environment around the
chromium(III) has been proposed. The growth inhibiting potential of the ligands and complexes
has been assessed against a variety of fungal and bacterial strains.
Keywords: Chromium(III)
Antimicrobial activity
complexes;
S-benzyldithiocarbazate;
Spectral
studies;
1. Introduction
We are increasingly aware of the environmental impact of human activity and the need
to develop cleaner and more energy-efficient technologies in chemical synthesis.
Solvent-free reactions have played strategic roles in methodologies of organic as well as
inorganic synthesis [1, 2]. Among the most promising pathways, microwave
(MW)-assisted technique has been popularly used since 1986 for organic synthesis
[3, 4] and for inorganic synthesis since 1989 [5]. Use of MW ovens in chemical synthesis
and analysis has increasingly grown due to its ability to dramatically reduce reaction
times, improve yield, and simplify procedures [6].
Resistance resulting from indiscriminate use of antibacterial and antifungal drugs
both in humans and animals is a serious public health problem [7] and preparation of
new antimicrobials with activity in low doses is extremely important. Schiff bases and
*Corresponding author. Email: rvsjpr@hotmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.489110

1808
S. Shrivastava et al.
their structural analogues as ligating compounds containing acyclic and cyclic imine
C¼N bonds are of importance in modern coordination chemistry [8]. Schiff bases are an
important class of ligands due to their synthetic flexibility, selectivity, and sensitivity
toward the central metal, structural similarities with natural biological substances, and
also due to the presence of imine group (4C¼N–) which is significant in elucidating the
mechanism of transformation and racemization reactions in biological systems [9].
Schiff bases are easily prepared by condensation between aldehydes and imines.
Stereogenic centers can be introduced in the synthetic design. Schiff bases coordinate
with many different metal ions and stabilize various oxidation states and have
numerous applications, e.g., anticancer, antibacterial, antiviral, antifungal, and other
biological properties [10]. S-alkyl and S-benzyl esters of dithiocarbazic acids and their
Schiff bases provide interesting series of ligands and most of them and their complexes
are biologically active [11]. Dithiocarbazates exhibit significant antifungal, antiproto-
zoal, antibacterial, and anticancer activity [12]. Recently, an in vitro insulinomimetic
potential of these compounds has been established [13]. Carcinostatic activities have
been found for metal complexes of dithiocarbazic acid and the Schiff base derived
from S-methyl ester [14]. Complexes of these ligand systems exhibit interesting
metal–nitrogen and metal–sulfur bonding features with increased electron delocaliza-
tion which may lead to improved biological activity [15].
Metal ions play a significant role in metabolic activity in the human body. Chromium
has been determined to be an essential micronutrient for maintenance of normal glucose
tolerance in animals. Chromium(III) acts as an antidiabetic agent [16]. Chromium(III)
is safe and required in a proper dietary regimen of animals and humans [17].
We, therefore, synthesize and screen some new Cr(III) Schiff-base complexes of
biologically potent ligands against a variety of pathogenic fungal and bacterial strains.
2. Experimental
The CrCl3 ꢀ 6H2O was purchased from Alfa Aesar. All reagents were dried and distilled
before use. 2-Acetylfuran, 2-acetylthiophene, 2-acetylpyridine, and 2-acetylnaphthalene
were purchased and used as received. S-benzyldithiocarbazate was prepared as
described previously [18].
2.1. Preparation of the ligands
Two different routes were employed for synthesis of ligands.
(1) In MW-assisted synthesis of the ligands, S-benzyldithiocarbazate of 2-acetylfuran,
2-acetylthiophene, 2-acetylpyridine, and 2-acetylnaphthalene were prepared by
condensation of 2-acetylfuran (0.01 mol), 2-acetylthiophene (0.01 mol),
2-acetylpyridine
(0.01 mol),
and
2-acetylnaphthalene
(0.01 mol)
with
S-benzyldithiocarbazate (0.01 mol) in 1 : 1 molar ratio using a beaker in a
conventional MW oven by taking 2–5 mL alcohol as solvent. The reactions were
completed in 4–7 min. The solution was then concentrated under reduced pressure,
which on cooling gave white or brown precipitates, which were recrystallized twice
in alcohol.

Schiff–bases S\N donor
1809
S
C
S
C
H
N
H
N
O
S
C
N
S
CH2
CH2
S
C
N
CH
CH
3
3
1
2
L H=2-Acetylfuran-S-benzyldithiocarbazate
L H=2-Acetylthiophene-S-benzyldithiocarbazate
S
S
H
H
N
CH2
S
CH2
C
N
N
C
S
C
N
N
C
CH
CH
3
3
3
L H=2-Acetylpyridine-S-benzyldithiocarbazate
4
L H=2-Acetylnaphthelene-S-benzyldithiocarbazate
Figure 1. Structure of the ligands.
Table 1. Comparison between conventional and MW methods of synthesis.
Yield (%) Solvent (mL)
Thermal
Time
Compound
MW
Thermal
MW
Thermal (h)
MW (min)
L1H
83
82
78
86
73
73
74
77
69
70
69
70
85
88
84
93
81
82
89
86
82
85
80
82
100
100
100
100
45
40
50
50
40
4
3
5
4
2
3
3
2
3
2
3
2
4
4
5
6
4.5
8
8
5
5
5
3
L2H
L3H
3.5
3.5
17
18
12
13
15
13
14
17
L4H
[Cr(L1)Cl2(H2O)2]
[Cr(L1)2ClH2O]
[Cr(L2)Cl2(H2O)2]
[Cr(L2)2ClH2O]
[Cr(L3)Cl2(H2O)2]
[Cr(L3)2ClH2O]
[Cr(L4)Cl2(H2O)2]
[Cr(L4)2ClH2O]
50
40
50
6
7
4
(2) For comparison, the above ligands were also synthesized by a thermal method,
where the starting materials were dissolved in ꢁ100 mL of alcohol and the contents
were refluxed for 4–5 h. The solution was then concentrated under reduced pressure,
which on cooling gave brown crystalline precipitates, which were recrystallized
twice in alcohol. The structures of ligands are shown in figure 1. A comparison
between thermal method and MW method is given in table 1.
2.2. Preparation of the complexes
The complexes were also prepared by two different routes.
(1) In MW-assisted synthesis, the complexes were prepared by irradiating the reaction
mixture of CrCl3 ꢀ 6H2O (0.001 mol) and respective ligand (0.001 and 0.002 mol)

1810
S. Shrivastava et al.
in 1 : 1 and 1 : 2 molar ratios using NaOH in appropriate stoichiometric proportions
in methanol. The products were recovered from the MW oven and dissolved in
2–5 mL of dry methanol, where sodium chloride precipitate formed during the
course of the reaction was removed by filtration and the filtrate was concentrated
under reduced pressure. The resulting compounds were washed and recrystallized
with cyclohexane.
(2) These complexes were also synthesized by the thermal method and were completed
in 14–18 h; the yield of product was also less than that obtained by the MW-assisted
synthesis. In this method, the methanolic solution of CrCl3 ꢀ 6H2O (0.001 mol) was
added to the methanolic solution of ligands (0.001 and 0.002 mol) in 1 : 1 and 1 : 2
molar ratios using NaOH in appropriate stoichiometric proportions. The resulting
mixture was refluxed for 14–18 h; the sodium chloride precipitate formed during the
course of the reaction was removed by filtration and the solvent was removed under
reduced pressure. The product was dried in vacuum. The resulting compounds were
washed and recrystallized with cyclohexane.
2.3. Physical measurements and analytical method
Both sets of the ligands and complexes were subjected to various physicochemical
measurements. The molecular weights were determined by the Rast camphor
method [19]. The metal contents were analyzed gravimetrically. Sulfur and nitrogen
were determined by Messenger’s [20] and Kjeldahl’s methods [21], respectively. Carbon
and hydrogen analyses were performed at the Central Drug Research Institute (CDRI),
Lucknow. Infrared spectra were recorded on a Nicolet Magna FTIR-550 spectro-
photometer using KBr pellets. Electronic spectra were recorded on a Varian–Cary/5E
spectrophotometer at SAIF, IIT Madras, Chennai. Electron paramagnetic resonance
(EPR) spectra of the complexes were monitored on a Varian E-4X band spectrometer at
SAIF, IIT Madras, Chennai.
2.4. Antimicrobial studies
2.4.1. Antifungal studies. Bioefficacies of the compounds synthesized by thermal and
MW methods were checked in vitro. The in vitro antifungal activities of the ligands and
their complexes have been evaluated against three pathogenic fungi, Candida albicans,
Aspergillus niger, and Fusarium oxysporum by the agar plate technique [22]. The potato
dextrose agar (PDA) medium was prepared in the laboratory to maintain fungal
growth. For PDA preparation, 20 g potato was extracted with distilled water (100 mL)
at 100ꢂC for 1 h and filtered by cotton filter. The potato juice was then mixed with 2 g
dextrose and 1.5 g agar and finally the pH of the prepared PDA media was adjusted
to 7. Solutions of the test compounds in methanol at 100 and 200 ppm were prepared
and then mixed with the medium. The medium was then poured into Petri plates and
spores of fungi were placed on the medium with the help of inoculum’s needle. These
Petri plates were wrapped in polythene bags containing a few drops of alcohol and were
placed in an incubator at 25 ꢃ 2ꢂC. The activity was determined after 96 h of
incubation at room temperature (25ꢂC). Controls were also run and three replicates
were used in each case. The linear growth of fungus was obtained by measuring the

Schiff–bases S\N donor
1811
diameter of the fungal colony after 4 days and the percentage inhibition was calculated
as 100 ꢄ (CꢅT )/C, where C is the diameter of the fungus colony in the control plate
after 96 h and T the diameter of the fungal colony in the test plates after the same
period. The antifungal screening data of compounds were compared with the standard
(Flucanazone).
2.4.2. Antibacterial screening. In vitro antibacterial screening is generally performed
by disk diffusion method [23] for primary selection of the compounds as therapeutic
agents. The antibacterial activity of the ligands and their chromium complexes were
evaluated against four bacteria including Gram-positive (Bacillus subtilis and
Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas
aeruginosa). The nutrient agar medium having composition of peptone 5 g, beef extract
5 g, NaCl 5 g, agar-agar 20 g, and distilled water 1000 mL was pipetted into the Petri
dish. When it solidified, 5 mL of warm-seeded agar was applied. The seeded agar was
prepared by cooling the molten agar to 40ꢂC and then adding 10 mL of bacterial
suspension. The compounds were dissolved in methanol in 500 and 1000 ppm
concentrations. Paper disks of Whatman No. 1 filter paper measuring diameter of
5 mm were soaked in these solutions of varied concentrations. The disks were dried and
placed on the medium previously seeded with organisms in Petri plates at suitable
distance. The Petri plates were stored in an incubator at 28 ꢃ 2ꢂC for 24 h. The
diameters of the zone of inhibition produced by the compounds were compared with the
standard antibiotic (Streptomycin). The zone of inhibition thus formed around each
disk containing the test compounds was measured accurately in millimeter.
2.4.3. Determination of minimum inhibitory concentration. Minimum inhibitory con-
centration (MIC) is the lowest concentration of test agent that inhibited visible growth
of bacteria after 18 h incubation at 37ꢂC. The determination of the MIC involves a
semi-quantitative test procedure, which gives an approximation to the least concen-
tration of an antimicrobial needed to prevent microbial growth. The MIC was
determined by the liquid dilution method [24]. Stock solutions of Cr(III) complexes
with 10–50 mg mLꢅ1 concentrations were prepared with aqueous methanol. Inoculum
of the overnight culture was prepared. In a series of tubes, 1 mL each of Cr(III) complex
solutions with different concentrations were taken and 0.4 mL of the inoculum was
added to each tube. Further, 3.5 mL of sterile water was added to each of the test tubes.
These test tubes were incubated for 24 h and observed for the presence of turbidity. The
absorbance of the suspension of the inoculum was observed with spectrophotometer at
555 nm. The end result of the test was the minimum concentration of antimicrobial
(test materials), which gave a clear solution, for example, no visual growth [25, 26].
3. Results and discussion
Reactions of CrCl3 ꢀ 6H2O with the ligands and a stoichiometric amount of NaOH were
carried out in 1 : 1 and 1 : 2 molar ratios in methanol. The successive replacement of
chloride resulted in [CrCl2(L)(H2O)2] and [(CrCl(L)2(H2O)]. The overall reaction of 1 : 1

1812
S. Shrivastava et al.
and 1 : 2 complexes are as follows:
1 : 1
CrCl3 ꢀ 6H2O þ LH þ NaOH ꢅ! ½CrCl2ðLÞðH2OÞ2ꢆ þ NaCl þ 5H2O
ð1Þ
ð2Þ
MeOH
1 : 2
CrCl3 ꢀ 6H2O þ 2LH þ 2NaOH ꢅ! ½CrClðLÞ2ðH2OÞꢆ þ 2NaCl þ 7H2O
MeOH
The physical properties and analytical data of the ligands and their metal complexes,
synthesized by MW technique as well as conventional method, are listed in
‘‘Supplementary material’’ and table 2. As indicated in table 1, the yield of
MW-assisted synthesized complexes is more and the time utilized is less compared to
conventionally synthesized ligands and complexes. All the chromium complexes are
dark green, stable at ambient temperature, slightly soluble in ethanol and methanol,
and highly soluble in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO).
Molecular weight determinations indicate their monomeric nature.
3.1. UV spectra
The electronic spectra of the complexes were recorded in DMSO. Three transitions are
expected for Cr(III) complexes and are observed. Bands at 15,344–17,490,
4
4
4
4
21,110–23,385, and 30,050–33,980 cmꢅ1 are due to A2g ! T2g(ꢀ1), A2g ! T1g(ꢀ2),
4
4
and A2g ! T1g(P)(ꢀ3) transitions, respectively, suggesting an octahedral geometry
around Cr3þ [27]. Various ligand field parameters like Dq, B, and ꢁ have been
calculated and given in table 3. Energy of the first spin allowed transition
[4A2g(F) ! T2g(F)] directly gives the value of 10Dq. Electronic repulsion parameter
4
is expressed in terms of Racah parameter and ‘‘B’’ has been evaluated during these
studies. The nephelauxetic ratio ꢁ indicates that the complexes have appreciable
covalent character.
3.2. ESR spectra and magnetic moment
The electron spin resonance (ESR) spectra of 1 : 1 and 1 : 2 chromium(III) complexes
synthesized by different routes were recorded at room temperature. These consist of a
single broad peak in each case and from which the Lande splitting factor (‘‘g’’ values)
has been calculated and given in table 4. The g values lie in the range 1.9783–1.9831
with giso (2.04), which are characteristic of octahedral geometry [28]. The room
temperature magnetic moment for [Cr(L1)2ClH2O] is slightly less than required. The
observed magnetic moment value of ꢁ3.74 BM and the electronic spectra of
[Cr(L1)2ClH2O] support the octahedral structure of the complexes [29].
3.3. IR spectra
The significant IR bands of the ligands and their metal complexes along with their
tentative assignments, reported in ‘‘Supplemental material’’, are used to establish the
mode of coordination of bidentate ligands toward the metal. IR spectra of the ligands
display a strong band at 3250–3200 cmꢅ1 due to ꢀ(NH) vibrations, absent in spectra of

Schiff–bases S\N donor
1813

1814
S. Shrivastava et al.
Table 3. Electronic spectral data (cmꢅ1) of the chromium(III) complexes synthesized through thermal and
MW methods.
Spectral bands cmꢅ1
(nm)
Compound
Transitions
10Dq Bo ꢁ ¼ B/Bo " ¼ A/cl (L/mol Lꢅ1
)
4
[Cr(L1)Cl2(H2O)2] A2g(F) ! 4T2g(F)
17,100 (584)
22,990 (434)
33,000 (303)
16,510 (605)
22,368 (467)
32,250 (310)
16,970 (589)
22,488 (425)
32,600 (306)
15,344 (651)
22,952 (435)
33,980 (294)
17,226 (580)
21,885 (456)
32,970 (303)
15,856 (630)
21,110 (473)
30,050 (332)
16,000 (625)
22,580 (442)
32,660 (306)
17,490 (571)
23,385 (427)
33,846 (295)
1710 574
1651 566
1697 589
1534 840
1722 428
1585 499
1600 664
1749 563
0.62
0.61
0.64
0.91
0.46
0.54
0.72
0.61
5.999 ꢄ 103
4A2g(F) ! 4T1g(F)
5.872 ꢄ 103
2.677 ꢄ 103
5.987 ꢄ 103
5.892 ꢄ 103
3.614 ꢄ 103
2.932 ꢄ 103
1.956 ꢄ 103
1.938 ꢄ 103
944 ꢄ 103
4A2g(F) ! 4T1g(P)
[Cr(L1)2ClH2O]
4A2g(F) ! 4T2g(F)
4A2g(F) ! 4T1g(F)
4A2g(F) ! 4T1g(P)
4
[Cr(L2)Cl2(H2O)2] A2g(F) ! 4T2g(F)
4A2g(F) ! 4T1g(F)
4A2g(F) ! 4T1g(P)
[Cr(L2)2ClH2O]
4A2g(F) ! 4T2g(F)
4A2g(F) ! 4T1g(F)
878 ꢄ 103
4A2g(F) ! 4T1g(P)
441 ꢄ 103
4
[Cr(L3)Cl2(H2O)2] A2g(F) ! 4T2g(F)
5.997 ꢄ 103
5.131 ꢄ 103
1.517 ꢄ 103
5.980 ꢄ 103
5.716 ꢄ 103
2.921 ꢄ 103
5.879 ꢄ 103
5.611 ꢄ 103
1.472 ꢄ 103
5.996 ꢄ 103
4.422 ꢄ 103
2.303 ꢄ 103
4A2g(F) ! 4T1g(F)
4A2g(F) ! 4T1g(P)
[Cr(L3)2ClH2O]
4A2g(F) ! 4T2g(F)
4A2g(F) ! 4T1g(F)
4A2g(F) ! 4T1g(P)
4
[Cr(L4)Cl2(H2O)2] A2g(F) ! 4T2g(F)
4A2g(F) ! 4T1g(F)
4A2g(F) ! 4T1g(P)
[Cr(L4)2ClH2O]
4A2g(F) ! 4T2g(F)
4A2g(F) ! 4T1g(F)
4A2g(F) ! 4T1g(P)
B, complex and Bo, free ion (918).
Table 4. ESR spectral data of the chromium(III) complexes synthesized through thermal and
MW methods.
Compound
Medium wave frequency (GHz)
H0
gII value
T (ꢂC)
[Cr(L1)2ClH2O]
[Cr(L2)Cl2(H2O)2]
[Cr(L3)Cl2(H2O)2]
[Cr(L4)2ClH2O]
9.38
9.38
9.38
9.38
3379.01
3375.02
3388.81
3380.93
1.9831
1.9856
1.9783
1.9833
25
25
25
25
the complexes. A sharp and strong band at 1625–1615 cmꢅ1 is due to the azomethine of
the ligands. IR spectra of the complexes showed a shift to lower wave numbers of
15–25 cmꢅ1. Strong bands at 1050–1100 cmꢅ1 in all the ligands, attributed to ꢀ(C¼S)
moiety, shifted to lower frequency upon coordination. In spectra of Cr(III) complexes a
band is observed at 850–875 cmꢅ1 which may be attributed to coordinated water.
A broad band around 3430–3455 cmꢅ1 may be due to ꢀ(O–H) of water. These data, on
comparison with the spectra of the ligands, suggest that the azomethine nitrogen and
thiolic sulfur are involved in coordination with Cr(III). The far IR spectra of these
metal complexes exhibited new bands, which are not present in spectra of the ligands.
The single band observed from 310 to 320 cmꢅ1 is due to ꢀ(Cr–CI), suggesting thereby
that the complexes have a trans structure [30]. Bands at 415–460 and 330–356 cmꢅ1 are

Schiff–bases S\N donor
1815
Suggested structures for chromium complexes
Cl
OH
S
R„
2
C
N
Cr
Cl
N
C
OH
2
R
CH
3
Where R=
,
,
,
O
S
S
N
CH
2
R„=
Figure 2. 1 : 1 complex.
R
H C
3
C
OH
2
N
S
R„
N
C
Cr
C
N
R„
N
C
S
Cl
R
CH
3
Where R=
,
,
,
O
S
N
S
CH
2
R„=
Figure 3. 1 : 2 complex.
assigned to ꢀ(Cr N) and ꢀ(Cr–S) [31], thus supporting bonding through the
azomethine nitrogen and thiolic sulfur.
The above data suggest that the ligands are bidentate toward the metal ion. On the
basis of magnetic measurement, IR and electronic spectral data with an octahedral
environment around the metal atom has been proposed and structures are shown in
figures 2 and 3.
3.4. X-ray structure determination
The possible lattice dynamics of the finely powdered product, [CrCl2(L1)(H2O)2], has
been deduced on the basis of X-ray powder diffraction. The observed interplanar

1816
S. Shrivastava et al.

Schiff–bases S\N donor
1817
Table 6. Antifungal screening data for the ligands and their complexes synthesized through thermal and
MW methods.
Antifungal activity: percentage inhibition after 96 h (concentration in ppm)
C. albicans
A. niger
F. oxysporum
Compound
100
200
100
200
100
200
L1H
42.0 ꢃ 0.4
34.0 ꢃ 0.9
51.0 ꢃ 0.4
40.0 ꢃ 0.3
59.0 ꢃ 0.4
60.0 ꢃ 0.2
50.0 ꢃ 0.8
49.0 ꢃ 0.5
78.0 ꢃ 0.6
69.0 ꢃ 0.5
61.0 ꢃ 0.4
60.0 ꢃ 1.1
95.0 ꢃ 0.8
47.0 ꢃ 0.6
44.0 ꢃ 0.6
56.0 ꢃ 0.8
50.0 ꢃ 1.1
65.0 ꢃ 0.4
78.0 ꢃ 0.5
66.0 ꢃ 0.7
52.0 ꢃ 0.7
80.0 ꢃ 0.5
77.0 ꢃ 0.7
67.0 ꢃ 0.3
65.0 ꢃ 0.3
10.0 ꢃ0.9
45.0 ꢃ 0.6
38.0 ꢃ 0.6
32.0 ꢃ 0.6
41.0 ꢃ 0.3
55.0 ꢃ 1.5
50.0 ꢃ 0.6
49.0 ꢃ 0.5
59.0 ꢃ 0.3
72.0 ꢃ 0.5
63.0 ꢃ 0.2
50.0 ꢃ 0.2
54.0 ꢃ 0.3
98 ꢃ 1
48.0 ꢃ 0.2
42.0 ꢃ 0.1
40.0 ꢃ 0.1
50.0 ꢃ 0.4
61.0 ꢃ 1.4
64.0 ꢃ 0.1
55.0 ꢃ 1.4
67.0 ꢃ 0.6
75.0 ꢃ 0.5
68.0 ꢃ 0.4
57.0 ꢃ 0.2
59.0 ꢃ 0.5
10 ꢃ 1
39.0 ꢃ 0.2
32.0 ꢃ 0.2
52.0 ꢃ 0.2
36.0 ꢃ 0.2
50.0 ꢃ 0.4
53.0 ꢃ 0.2
44.0 ꢃ 0.7
55.0 ꢃ 0.4
79.0 ꢃ 0.3
84.0 ꢃ 0.6
71.0 ꢃ 0.2
69.0 ꢃ 0.6
10 ꢃ 1
43.0 ꢃ 0.4
40.0 ꢃ 0.2
57.0 ꢃ 0.2
45.0 ꢃ 0.2
65.0 ꢃ 0.2
63.0 ꢃ 0.3
50.0 ꢃ 0.4
60.0 ꢃ 0.6
83.0 ꢃ 0.3
89.0 ꢃ 0.7
77.0 ꢃ 0.5
73.0 ꢃ 0.4
10 ꢃ 1
L2H
L3H
L4H
[Cr(L1)Cl2(H2O)2]
[Cr(L1)2ClH2O]
[Cr(L2)Cl2(H2O)2]
[Cr(L2)2ClH2O]
[Cr(L3)Cl2(H2O)2]
[Cr(L3)2ClH2O]
[Cr(L4)Cl2(H2O)2]
[Cr(L4)2ClH2O]
Flucanazone
˚
spacing values (‘‘d ’’ in A) have been measured from the diffractogram of the compound
and the Miller indices h, k, and l have been assigned to each d value and 2-ꢃ angles are
reported. The results show that the compound belongs to ‘‘orthorhombic’’ crystal
system having unit cell parameters as a ¼ 9.05, b ¼ 17.15, c ¼ 21.15, maximum deviation
of 2ꢃ ¼ 0.046 and ꢄ ¼ 90, ꢁ ¼ 90, and ꢅ ¼ 90 at wavelength ¼ 1.93728. We have tried to
isolate single crystal of a chromium(III) complex suitable for X-ray diffraction study,
but could not succeed. However, Byun and Han [32] have recently reported the single
crystal structure of a six-coordinate chromium(III) complex in which the bidentate
ligand adopts the most stereochemically favorable octahedral orientation.
3.5. Antimicrobial assay
The ligands and their chromium complexes synthesized through thermal and MW
methods were evaluated for antimicrobial activity against four bacteria, E. coli,
P. aeruginosa, B. subtilis, and S. aureus and three fungi, C. albicans, A. niger, and
F. oxysporum. The results, summarized in tables 5 and 6, were compared with those of
the standard drug Streptomycin for bacteria and Flucanazole for fungi. All the ligands
and their Cr(III) complexes were sensitive against all the fungal and bacterial strains.
The antimicrobial screening data indicate that the metal complexes are more potent
antimicrobial agents than the free ligands.
3.6. Minimum inhibitory concentration
MIC values calculated for the ligands and their chromium(III) complexes as shown in
table 7 indicated that the ligands and their metal complexes were active in inhibiting the
growth of the tested organisms between 15 and 40 MIC (mg mLꢅ1) against selected
bacteria and fungi.

1818
S. Shrivastava et al.
Table 7. MIC (mg mLꢅ1) of the ligands and their complexes synthesized through thermal and MW
methods.
Compound
E. coli
P. aeruginosa B. subtilis S. aureus C. albicans A. niger F. oxysporum
L1H
L2H
L3H
L4H
25.0 ꢃ 0.1
21.0 ꢃ 0.2
24.0 ꢃ 0.2
37.0 ꢃ 0.2
26.0 ꢃ 0.2
32.0 ꢃ 0.2
33.0 ꢃ 0.2
32.0 ꢃ 0.2
16.0 ꢃ 0.1
16.0 ꢃ 0.1
24.0 ꢃ 0.2
17.0 ꢃ 0.1
17.0 ꢃ 0.2
25.0 ꢃ 0.2
24.0 ꢃ 0.2
14.0 ꢃ 0.3
34.0 ꢃ 0.1 28.0 ꢃ 0.2 24.0 ꢃ 0.6 32.0 ꢃ 0.1
25.0 ꢃ 0.2 24.0 ꢃ 0.2 36.0 ꢃ 0.4 40.0 ꢃ 0.1
23.0 ꢃ 0.1 31.0 ꢃ 0.2 34.0 ꢃ 0.1 27.0 ꢃ 0.4
35.0 ꢃ 0.1 34.0 ꢃ 0.1 36.0 ꢃ 0.2 31.0 ꢃ 0.2
20.0 ꢃ 0.2 21.0 ꢃ 0.1 15.0 ꢃ 0.1 17.0 ꢃ 0.1
21.0 ꢃ 0.1 18.0 ꢃ 0.3 12.0 ꢃ 0.1 16.0 ꢃ 0.1
19.0 ꢃ 0.2 19.0 ꢃ 0.1 12.0 ꢃ 0.2 15.0 ꢃ 0.2
10.0 ꢃ 0.0 16.0 ꢃ 0.1 16.0 ꢃ 0.1 21.0 ꢃ 0.1
23.0 ꢃ 0.1 15.0 ꢃ 0.2 15.0 ꢃ 0.2 15.0 ꢃ 0.2
17.0 ꢃ 0.2 31.0 ꢃ 0.3 15.0 ꢃ 0.2 16.0 ꢃ 0.2
20.0 ꢃ 0.2 22.0 ꢃ 0.1 15.0 ꢃ 0.2 15.0 ꢃ 0.1
24.0 ꢃ 0.3 16.0 ꢃ 0.1 18.0 ꢃ 0.1 23.0 ꢃ 0.4
39.0 ꢃ 0.2
30.0 ꢃ 0.2
25.0 ꢃ 0.2
30.0 ꢃ 0.2
26.0 ꢃ 0.2
20.0 ꢃ 0.2
22.0 ꢃ 0.2
15.0 ꢃ 0.2
19.0 ꢃ 0.1
18.0 ꢃ 0.2
21.0 ꢃ 0.2
14.0 ꢃ 0.2
[Cr(L1)Cl2(H2O)2] 18.0 ꢃ 0.2
[Cr(L1)2ClH2O]
[Cr(L2)Cl2(H2O)2] 19.0 ꢃ 0.1
[Cr(L2)2ClH2O]
16.0 ꢃ 0.1
[Cr(L3)Cl2(H2O)2] 13.0 ꢃ 0.2
[Cr(L3)2ClH2O]
16.0 ꢃ 0.2
[Cr(L4)Cl2(H2O)2] 15.0 ꢃ 0.2
[Cr(L4)2ClH2O]
30.0 ꢃ 0.6
19.0 ꢃ 0.2
The biological activity of the ligands exhibited a marked enhancement on
coordination with the metal ions against all the test bacterial/fungal strains, which
shows that metal chelates are more active than the ligands [33–35]. Increase in
concentration of the compounds increases the activity.
A review of the literature indicated that a large number of Schiff bases and their
metal complexes have been tested for antimicrobial activity against microorganisms.
In most tests, the complexes showed higher inhibitory activity than the ligands proving
their potential usefulness as broad-spectrum antimicrobial agents [36]. The antimicro-
bial activity of Schiff-base ligand and its chromium(III) complex was evaluated by
Badwaik and Aswar [37] indicating that the metal complex has higher activity than the
free ligand against the same organism under identical experimental conditions. The
increased antimicrobial activity of complexes than free ligands has been attributed to
chelation theory [38].
4. Conclusions
MW irradiation is an efficient and environmentally benign method to accomplish
inorganic synthesis to afford products in higher yields in shorter reaction periods. The
ligands and their chromium complexes synthesized through thermal as well as MW
methods were used for analyzing their physicochemical properties and antimicrobial
activity. The results indicate that all the ligands behave as monofunctional bidentate.
Chromium complexes synthesized in 1 : 1 and 1 : 2 molar ratios with monofunctional
bidentate ligands were six-coordinate. Antimicrobial activity of the complexes and
ligands showed that the Cr(III) complexes are more active than the parent ligands.
Supplementary material
The related data to ESR and X-ray powder diffraction will be provided from the
authors on request.

Schiff–bases S\N donor
1819
Acknowledgment
The authors are thankful to CSIR, New Delhi, India for financial assistance through
grant no. 01(2307)/09 EMR-II.
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