Synthesis, structural, and antibacterial studies of some transition metal complexes derived from thiosemicarbazone and semicarbazone

Article abstract of DOI:10.1080/10426500802274450

A series of metal complexes of Pd(II), Pt(II), Rh(III), Ir(III), and Ru(III) with thiosemicarbazone and semicarbazone of 2-acetyl thiophene have been synthesized. Their structures were determined on the basis of elemental analyses; molar conductance; magnetic susceptibility measurements; and IR, 1H NMR, and electronic spectral studies. On the basis of molar conductance, the complexes may be formulated as [M(L)2]Cl2 and [M'(L)2Cl2]Cl [where M = Pd(II), Pt(II) and M' = Rh(III), Ir(III), and Ru(III)] due to their 1:2 and 1:1 electrolytic nature. On the basis of IR, 1H NMR, and electronic spectral studies, an octahedral geometry has been assigned for Rh(III), Ir(III), and Ru(III), while square planar geometry has been assigned for the Pd(II) and Pt(II) complexes. The synthesized ligands and their complexes have been screened for bactericidal activity against several bacterial species (i.e., B. macerans, A. aureus, E. coli), and it is shown that the metal complexes act as more active antimicrobial agents than the uncomplexed ligands from which they derive.

Full text of DOI:10.1080/10426500802274450

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Synthesis, Structural, and
Antibacterial Studies of
Some Transition Metal
Complexes Derived from
Thiosemicarbazone and
Semicarbazone
Sulekh Chandra a & Monika Tyagi a
a Department of Chemistry, Zakir Husain College,
New Delhi, India
Published online: 19 Feb 2009.
To cite this article: Sulekh Chandra & Monika Tyagi (2009): Synthesis, Structural,
and Antibacterial Studies of Some Transition Metal Complexes Derived from
Thiosemicarbazone and Semicarbazone, Phosphorus, Sulfur, and Silicon and the
Related Elements, 184:3, 778-789
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Phosphorus, Sulfur, and Silicon, 184:778–789, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802274450
Synthesis, Structural, and Antibacterial Studies of Some
Transition Metal Complexes Derived from
Thiosemicarbazone and Semicarbazone
Sulekh Chandra and Monika Tyagi
Department of Chemistry, Zakir Husain College, New Delhi, India
A series of metal complexes of Pd(II), Pt(II), Rh(III), Ir(III), and Ru(III) with
thiosemicarbazone and semicarbazone of 2-acetyl thiophene have been synthesized.
Their structures were determined on the basis of elemental analyses; molar con-
1
ductance; magnetic susceptibility measurements; and IR, H NMR, and electronic
spectral studies. On the basis of molar conductance, the complexes may be formu-
lated as [M(L)2]Cl2 and [Mꢀ(L)2Cl2]Cl [where M = Pd(II), Pt(II) and Mꢀ= Rh(III),
Ir(III), and Ru(III)] due to their 1:2 and 1:1 electrolytic nature. On the basis of IR,
1H NMR, and electronic spectral studies, an octahedral geometry has been assigned
for Rh(III), Ir(III), and Ru(III), while square planar geometry has been assigned
for the Pd(II) and Pt(II) complexes. The synthesized ligands and their complexes
have been screened for bactericidal activity against several bacterial species (i.e.,
B. macerans, A. aureus, E. coli), and it is shown that the metal complexes act as
more active antimicrobial agents than the uncomplexed ligands from which they
derive.
Keywords Biological screening; semicarbazone; thiosemicarbazone; transition metal
complexes
INTRODUCTION
Thiosemicarbazones and their metal complexes are widely known as
having a large range of biological applications such as antiviral, anti-
fungal, antibacterial, antimalarial, and anticancer agents.1−7 The ac-
tivity of these compounds is strongly dependent upon the nature of the
heteroaromatic ring and the position of attachment to the ring, as well
as the form of the thiosemicarbazone moiety.8,9 Thiosemicarbazones
and semicarbazones usually react with metallic cations, giving com-
plexes in which the ligand behaves as a chelating agent bonding coor-
dination through the hydrazine nitrogen and sulfur/oxygen atoms.10
Received 24 March 2008; accepted 16 June 2008.
The authors are thankful to the UGC New Delhi for financial support.
Address correspondence to Sulekh Chandra, Department of Chemistry, Zakir Husain
College, J.L.N. Marg, New Delhi 110 002, India. E-mail: schandra 00@yahoo.com
778

Some Transition Metal Complexes
779
FIGURE 1 Suggested structure of the ligands.
After the discovery of the chemotherapeutically active platin com-
plexes of thiosemicarbazide derivatives,11 many thiosemicarbazone
compounds showing biological activity were synthesized. There
are only a few reports of palladium and platinum complexes of
thiosemicarbazones.12−14 It is well known that several metal ions
enhance the biological activities of thiosemicarbarbazones; the new
metals to add to such a list are ruthenium and rhodium.15,16 Much
attention has been drawn towards the chemistry of ruthenium and
rhodium in different coordination spheres.17,18 Due to the different
oxidation states of ruthenium and rhodium, their reactivity depends
upon the stability of the oxidation states. Due to the growing inter-
est of thiosemicarbazones of palladium, platinum, ruthenium, and
rhodium metal ions, here we report the synthesis, characterization,
and biological activities of Pd(II), Pt(II), Rh(III), Ir(III), and Ru(III)
complexes of 2-acetyl thiophene thiosemicarbazone and semicabazone
(Figure 1).
RESULTS AND DISCUSSION
A systematic study of the reactions of metal chlorides with ligands in a
1: 2 molar ratio has been carried out. The reactions can be represented
by the following reactions:
Reflux
L2 + MCl2 −→ ML2Cl2
Reflux
L2 + M’Cl3 −→ M’L2Cl3

780
S. Chandra and M. Tyagi
where L L1, L2, M Pd(II), Pt(II), and M’
and Ru(III).
Rh(III), Ir(III)
On the basis of elemental analysis, the complexes were assigned to
possess the composition as shown in Table I. The Pd(II) and Pt(II)
complexes are 1:2 electrolytes with conductance values of 202–212
ꢀ
−1cm2mol−1. However, the molar conductance data of the Rh(III),
Ir(III), and Ru(III) complexes lies in the range 92–102 ꢀ−1cm2mol−1
indicating a 1:1 electrolytic nature. Thus these complexes may be for-
mulated as [M(L)2]Cl2 and [M’(L)2Cl2]Cl [where L L1and L2, M =
Pd(II), Pt(II) and Mꢀ Rh(III), Ir(III), and Ru(III)].
1H NMR Spectra
The 1H NMR spectra of the free ligands show three singlets at δ
3.40–3.46, 11.91–11.95, and 8.01–8.09 ppm due to -NH2 proton, -
NH proton, and azomethane proton (CH N-), respectively. The -NH2
proton and -NH proton gives the same singlet in the complexes,
which indicates that these groups are involved in coordination. The
azomethane proton signal shows a change as a downfield shift in
the complexes, indicating involvement of nitrogen in coordination
(Table II).
IR Spectra
The IR spectra of the free ligands display two sharp bands at ca
3432 and 3357 cm−1, assignable to asymmetric and symmetric NH2
groups, respectively, which remain at almost the same positions in
the metal complexes, suggesting that amino group is not involved in
the coordination. The bands at 1592 and 1579 cm−1 in ligands L1
and L2, respectively, due to ν(C N) are shifted to lower wave num-
bers in the metal complexes, suggesting coordination through the
azomethane nitrogen.19,20 The strong bands at 756 cm−1 and 1670
cm−1 are due to ν(C S) and ν(C O) groups, respectively. Upon com-
plex formation, the position of these bands is shifted towards the
lower side, indicating coordination of the sulfur and oxygen atom
to the central metal atoms.21,22 Bands at ca 454–475, 308–318, and
411–420 cm−1 can be assigned to ν(M N), ν(M S), and ν(M O), re-
spectively. The appearance of these bands further supports the bond-
ing of the ligands to the metal through the nitrogen, sulfur, and
oxygen atoms of ν(C N), ν(C S), and ν(C O) groups respectively
(Table III).

781

782
S. Chandra and M. Tyagi
TABLE II 1H NMR Spectral Data (δ ppm) of the Ligands and Their
Metal Complexes
Compounds
-NH2
-NH
HC
N
L1
3.46
3.44
3.45
3.46
3.47
3.40
3.41
3.41
3.42
3.40
11.91
11.90
11.92
11.92
11.91
11.95
11.94
11.93
11.95
11.96
8.09
[Pd(L1)Cl2
[Pt(L1)Cl2
[Rh(L1)2Cl2]Cl
[Ir(L1)2Cl2]Cl
L2
8.28
8.32
8.29
8.30
8.01
8.22
8.25
8.20
8.23
[Pd(L2)Cl2
[Pt(L2)Cl2
[Rh(L2)2Cl2]Cl
[Ir(L2)2Cl2]Cl
Magnetic Moments and Electronic Spectra
Pd(II) and Pt(II) Complexes
All the complexes are diamagnetic as expected for square planar
d8 metal complexes. The electronic spectra of Pd(II) and Pt(II) com-
plexes show three d-d spin-allowed transitions. These correspond to
the transitions from the three lower lying “d” levels to the empty d2x–2y
1
orbital. The ground state is A1g and excited states corresponding to
1
1
1
the A2g, B1g, and Eg transitions in order of increasing energy.23,24
Two d-d transitions are observed in the range of 18760–21276 cm−1
and 20811–25310 cm−1. These bands are attributed to A1g → A2g(ν1)
1
1
1
1
and A1g → B1g(ν2) transitions, respectively. The electronic spectra of
these complexes indicate the square planar geometry around the Pd(II)
and Pt(II) ion.
By assuming a value of F2 = 10F4 = 600 for Slater-Condon interelec-
tronic repulsion parameters for both Pd and Pt , it is possible to calcu-
late the value of ꢁ1 (Table IV) from the first spin-allowed d-d transition.
The splitting parameter increases in the expected order Pt>Pd.
Rhodium(III) and Iridium(III) Complexes
All of the formed complexes have been found to be +3 oxidation state.
All the complexes are diamagnetic. Rhodium(III) complexes exhibit in
the electron spectra with transitions at 17550–17600 cm−1, 20210–
20280 cm−1, and 27170–27400 cm−1. These bands resemble those of
reported transitions for other hexacoordinated rhodium complexes25
3
1
1
and may be assigned 1A1g → T1g, 1A1g → T1g, and 1A1g → T2g, respec-
tively, which agree with an octahedral geometry. The B and Dq values

Some Transition Metal Complexes
783
TABLE III Infrared Absorption Frequencies (cm−1) of Ligands and
Their Metal Complexes
Assignments
Compound
ν (C N)
ν(C S)
ν(C O)
ν(M S)
ν(M O)
ν(M N)
L1
1592
1578
1582
1570
1576
1580
1579
1567
1560
1554
1562
1550
756
732
740
738
729
745
—
—
—
—
—
—
—
—
—
—
—
308
312
318
315
310
—
—
—
—
—
—
—
—
—
—
—
[Pd(L1)2]Cl2
[Pt(L1)2]Cl2
[Rh(L1)2Cl2]Cl
[Ir(L1)2Cl2]Cl
[Ru(L1)2Cl2]Cl
L2
460
472
465
468
458
—
475
454
462
470
465
—
—
—
1670
1659
1650
1647
1645
1664
[Pd(L2)2]Cl2
[Pt(L2)2]Cl2
[Rh(L2)2Cl2]Cl
[Ir(L2)2Cl2]Cl
[Ru(L2)2Cl2]Cl
-411
418
404
416
420
—
—
were calculated from the positions of their electronic bands using the
following equations:
86(B)2
10 Dq
2(B)2
10 Dq
ν1 = 10 Dq − 4B +
,
.
ν2 = 10 Dq + 12B +
The B values are 55–62% of the free ion value. The decrease in B values
from the free ion value suggests that there is a considerable orbital
overlap with strong covalency in the metal ligand σ bond.26 Jorgensen
has demonstrated that the decreasing the value of β is associated with
a reduction in the positive charge of the cation and with the tendency
to be reduced to the next lower oxidation state.
The electronic spectra of the iridium(II) complexes under study dis-
play bands in the range of 30800 and 34000–34250 cm−1, which may
1
1
1
1
be assigned to A1g → T1g(ν1) and A1g → T2g(ν2) transitions in order
of increasing energy. The two transitions ν1 and ν2 have been used to
evaluate the ligand field parameters (Table IV). The values of these
ligand field parameters are compared with those reported for other
iridium(III) complexes involving similar donors.27
Ruthenium(III) Complexes
The ground state of ruthenium(III) is 2T2g and the first excited dou-
2
2
blet levels in order of increasing energy are A2g and T2g, which

784
S. Chandra and M. Tyagi
TABLE IV Electronic Spectral Bands (cm−1) and Ligand Field
Parameters
Dq
B
C
ꢁ
1
(cm−1
)
Complex
λ
max
(cm−1
)
(cm−1
)
(cm−1
)
(cm−1
)
β
υ /υ
2
1
[Pd(L1)2]Cl2
[Pt(L1)2]Cl2
18639, 20811
21276, 25310
–
–
–
–
435
200
470
–
–
–
–
–
–
–
20439
23376
–
–
[Rh(L1)2Cl2]Cl 17600, 20210, 27170 2195
1740 0.60 1.15
800 0.33 1.10
2882 0.75 1.28
–
–
1780 0.62 1.15
850 0.32 1.11
2705 0.76 1.27
[Ir(L1)2Cl2]Cl
30800, 34000
3160
[Ru(L1)2Cl2]Cl 13500, 17260, 23550 2738
–
[Pd(L2)2]Cl2
[Pt(L2)2]Cl2
18760, 22565
20080, 24815
–
–
–
–
–
–
20560
22180
–
–
–
–
[Rh(L2)2Cl2]Cl 17550, 20280, 27400 2206
445
212
475
[Ir(L2)2Cl2]Cl
[Ru(L2)2Cl2]Cl 14000, 17850, 23600 2725
30800, 34250
3085
are known to arise from T4 e1g configuration.28 The ruthenium(III)
complexes display electronic2sgpectra with transitions at 13500–14000
cm−1, 17260–17850 cm−1, and 23550–23600 cm−1. These bands may
2
4
2
4
2
2
2
be assigned due to T2g → T1g, T2g → T2g, and T2g → A2g, T1g in
TABLE V Antibacterial Activity Data of Ligands and Their Metal
Complexes
Diameter of Inhibition Zone (mm)(conc.in µgml−1
B. macerans A. aureus E.coli
250 500 250
11
)
Compounds
250
500
500
L1
9
10
14
12
11
10
11
7
13
23
17
15
12
16
8
12
16
15
13
14
14
9
14
27
24
18
20
22
12
20
18
16
13
14
34
[Pd(L1)2]Cl2
[Pt(L1)2]Cl2
[Rh(L1)2Cl2]Cl
[Ir(L1)2Cl2]Cl
[Ru(L1)2Cl2]Cl
L2
13
12
11
9
10
6
21
19
14
13
19
9
[Pd(L2)2]Cl2
[Pt(L2)2]Cl2
[Rh(L2)2Cl2]Cl
[Ir(L2)2Cl2]Cl
[Ru(L2)2Cl2]Cl
Streptomycin (standard)
11
9
7
8
9
17
14
12
14
15
24
12
11
9
7
8
19
16
15
11
13
25
13
12
10
9
11
19
14
16

Some Transition Metal Complexes
785
increasing order of energy. The B, C, and 10 Dq parameters were cal-
culated using the following equations29:
2T2g(t5) = 0,
4T1g(t4 e) = 10 Dq − 5B 4C,
4T2g(t4 e) = 10 Dq + 5B − 4C,
2A2g,2 T1g(t4 e) = 10 Dq − 2B − C.
The values of these ligand field parameters are comparable to those
reported for other trivalent ruthenium complexes involving nitrogen
and sulfur donor molecules. The considerable decrease in Racah in-
terelectronic repulsion parameter, B, suggests the presence of strong
covalent bonding between the donor and metal ions. The overall effect
will be an increase in the observed Dq value; high Dq values are usually
associated with considerable electron delocalization.30
On the basis of spectral studies, the following structure may be sug-
gested for the complexes (Figure 2).
EXPERIMENTAL
All the chemicals used were of Anala R grade and procured from Sigma-
Aldrich and Fluka. Metal salts were purchased from E. Merck and
were used as received. All solvents used were of standard/spectroscopic
grade. C, H, and N were analyzed on a Carlo-Erba 1106 elemental an-
alyzer. Molar conductance was measured on the ELICO (CM82T) con-
ductivity bridge. Magnetic susceptibility was measured at room tem-
perature on a Gouy balance using CuSO4.5H2O as a calibrant. 1H NMR
spectra were recorded at room temperature on a Bruker Avance DPX-
300 spectrometer using DMSO-d6 as a solvent. IR spectra (KBr) were
recorded on a FTIR spectrum BX-II spectrophotometer. The electronic
spectra were recorded in DMSO on a Shimadzu UV mini-1240 spec-
trophotometer.
The Synthesis of Ligands
Ligands L1 & L2 were prepared, and the methods of their preparation
are given below.
Ligand L1
A hot ethanolic (20 mL) solution of thiosemicarbazide (0.91 g, 0.01
mol) and an ethanolic (20 mL) solution of 2-acetyl thiophene (1.26 g,

786
S. Chandra and M. Tyagi
FIGURE 2 Suggested structure of the complexes.
0.01 mol) were mixed slowly with constant stirring. This mixture was
refluxed at 70–80◦C for 2–3 h. Upon cooling, a white-colored compound
precipitated. It was filtered, washed with cold EtOH, and dried un-
der vacuum over P4O10. Yield: 70%, m. 159–161◦C. Elemental analysis
found % C 42.20, H 4.59, N 21.15 calculated for C7H9N3S2(molecular
mass 199 amu) % C 42.21, H 4.52, N 21.10.
Ligand L2
An aqueous (20 mL) solution of semicarbazide hydrochloride( 1.11
g, 0.01 mol) and an ethanolic (20 mL) solution of 2-acetyl thiophene
(1.26 g, 0.01 mol) were mixed in the presence in sodium acetate (1.36g,

Some Transition Metal Complexes
787
0.01 mol). The reaction mixture was stirred vigorously for 1 h. Upon
cooling, a white compound precipitated. It was filtered, washed with
cold EtOH, and dried under vacuum over P4O10. Yield 73%, mp 195–
197◦C. Elemental analysis found % C 46.96, H 4.87, N 22.89 calculated
for C7H9N3OS (molecular mass 183 amu) % C 45.90, H 4.91, N 22.95.
The Synthesis of Complexes
Hot ethanolic solutions (20 mL) of the corresponding metal salts (0.01
mol) were mixed with a hot ethanolic solution (20 mL) of the respective
ligands (0.02 mol). The reaction mixture was refluxed on a water bath
for about 4–5 h. Upon cooling, a colored complex precipated in each
case. The complex was filtered, washed with cold ethanol, and dried
under vacuum over P4O10.
ANTIBACTERIAL SCREENING
The antibacterial activitity of the ligand and its metal complexes were
tested by using the paper disc diffusion method31−33 against B. mac-
erans, A. aureus, and E. coli. Nutrient agar medium was prepared by
using peptone, beef extract, NaCl, agar–agar, and distilled water. The
test compounds in measured quantities were dissolved in DMF to give
concentrations of 250 and 500 ppm of compounds. 25 mL nutrient agar
media (NA) was poured into each Petri plate. After solidification, 0.1 mL
of test bacteria was spread over the medium. The disc of Whatmann
no. 1 filter paper of diameter 5.00 mm each containing 1.5 mg cm−1
of the compounds was placed at 4 equidistant places at a distance of
2 cm from the center in the inoculated Petri plates. A filter paper disc
treated with DMF served as a control, and streptomycin was used as
a standard drug. All determinations were made in duplicate for each
of the compounds. An average of two independent readings for each
compound was recorded. These Petri plates were kept in a refrigerator
for 24 h for prediffusion. Finally the Petri plates were incubated for
26–30 h 28 2◦C. The zone of inhibition was calculated in mm.
The antimicrobial screening data show that metal chelates exhibit
more inhibitory effects than the parent ligands (Table V). The increased
activity of the metal chelates can be explained on the basis of chelation
theory.34 It is known that chelation tends to make the ligand act as
more powerful and potent bactericidal agents, thus killing more of the
bacteria than the ligand. It is observed that in the complex, the positive
charge of the metal is partially shared with the donor atoms present
in the ligands, and there is π-electron delocalization over the whole

788
S. Chandra and M. Tyagi
chelating structure. This increases the lipophilic character of the metal
chelate and favors its permeation through the lipid layer of the bacte-
rial membranes. There are other factors that also increase the activity
including solubility, conductivity, and bond length between the metal
and ligand.
CONCLUSION
The present study confirmed an octahedral geometry around the
Rh(III), Ir(III), and Ru(III), whereas we confirmed square planar geom-
etry for the Pd(II) and Pt(II) complexes with bidentate ligands coordi-
nationg through the N and O/S donors. It is also interesting to note that
the sulfur-bonded ligands and their complexes are more active than the
oxygen-bonded ligands and their complexes.
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Some Transition Metal Complexes
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