Benzaldehyde thiosemicarbazone complexes of platinum: Syntheses, structures and cytotoxic properties

Article abstract of DOI:10.1016/j.poly.2012.07.037

The reaction of 4-R-benzaldehyde thiosemicarbazones (denoted in general as H₂L-R, where H₂ stands for the two dissociable protons and R (R = OCH₃, CH₃, H, Cl and NO₂) for the substituent on the phenyl ring) with [Pt(PPh₃)₂Cl ₂] in the presence of NEt₃ afforded a family of organometallic complexes of platinum(II) of the type [Pt(PPh₃)(L-R)]. Reaction of the same group of ligands with K₂[PtCl₄] in the presence of NEt₃ afforded complexes of the type [Pt(HL-R) ₂]. The crystal structure of [Pt(PPh₃)(L-CH₃)] and [Pt(HL-CH₃)₂] have been determined. In the [Pt(PPh₃)(L-R)] complexes, the benzaldehyde thiosemicarbazones are coordinated to platinum as dianionic tridentate C,N,S-donors. In the [Pt(HL-R)₂] complexes, the benzaldehyde thiosemicarbazones are coordinated to the metal center as bidentate N,S-donors, forming five-membered chelate rings, and with reference to the structure of the uncoordinated thiosemicarbazone ligand, this coordination mode is associated with a change in stereochemistry around the CN bond. All the [Pt(PPh₃)(L-R)] and [Pt(HL-R)₂] complexes display intense absorptions in the visible and ultraviolet regions. The cytotoxic effects of these complexes, examined on the human leukemia cell line HL-60 and human lymphoma cell line U-937, have shown that all the [Pt(PPh₃)(L-R)] and [Pt(HL-R)₂] complexes are cytotoxic in nature and their IC₅₀ values indicate their potential use as antitumor agents.

Full text of DOI:10.1016/j.poly.2012.07.037

Polyhedron 45 (2012) 177–184
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Benzaldehyde thiosemicarbazone complexes of platinum: Syntheses, structures
and cytotoxic properties
Sarmistha Halder a, Piyali Paul a, Shie-Ming Peng b, Gene-Hsiang Lee b, Asama Mukherjee c,
Sushanta Dutta c, Utpal Sanyal c, Samaresh Bhattacharya a,
⇑
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
b Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
c Department of Anticancer Drug Development, Chittaranjan National Cancer Institute, Kolkata 700 026, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of 4-R-benzaldehyde thiosemicarbazones (denoted in general as H2L-R, where H2 stands for
the two dissociable protons and R (R = OCH3, CH3, H, Cl and NO2) for the substituent on the phenyl ring)
with [Pt(PPh3)2Cl2] in the presence of NEt3 afforded a family of organometallic complexes of platinum(II)
of the type [Pt(PPh3)(L-R)]. Reaction of the same group of ligands with K2[PtCl4] in the presence of NEt3
afforded complexes of the type [Pt(HL-R)2]. The crystal structure of [Pt(PPh3)(L-CH3)] and [Pt(HL-CH3)2]
have been determined. In the [Pt(PPh3)(L-R)] complexes, the benzaldehyde thiosemicarbazones are coor-
dinated to platinum as dianionic tridentate C,N,S-donors. In the [Pt(HL-R)2] complexes, the benzaldehyde
thiosemicarbazones are coordinated to the metal center as bidentate N,S-donors, forming five-membered
chelate rings, and with reference to the structure of the uncoordinated thiosemicarbazone ligand, this
coordination mode is associated with a change in stereochemistry around the C@N bond. All the
[Pt(PPh3)(L-R)] and [Pt(HL-R)2] complexes display intense absorptions in the visible and ultraviolet
regions. The cytotoxic effects of these complexes, examined on the human leukemia cell line HL-60 and
human lymphoma cell line U-937, have shown that all the [Pt(PPh3)(L-R)] and [Pt(HL-R)2] complexes
are cytotoxic in nature and their IC50 values indicate their potential use as antitumor agents.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 12 May 2012
Accepted 1 July 2012
Available online 20 July 2012
Keywords:
Benzaldehyde thiosemicarbazones
Platinum complexes
Syntheses
Structures
Cytotoxic properties
1. Introduction
studies have shown that these selected ligands usually display
three modes of binding, viz. a N,S-mode of binding forming a
The chemistry of thiosemicarbazone complexes of transition
metal ions has been receiving significant current attention, pri-
marily because of the bioinorganic relevance of these complexes
[1–23]. A large majority of thiosemicarbazone complexes have
found wide medicinal applications owing to their potentially ben-
eficial biological (viz. antibacterial, antimalarial, antiviral and
antitumor) activities [24–41]. Systematic studies on the binding
of thiosemicarbazones of selected types to different transition
metal ions are of considerable importance in this respect. How-
ever, we have been exploring the chemistry of platinum metal
complexes of thiosemicarbazones [42–57], mainly because of
the variable binding mode displayed by these ligands in their
complexes and also to gain chemical control over the mode of
binding, and the present work has emerged out of this explora-
tion. For the present study, a group of 4-R-benzaldehyde thio-
semicarbazones (I) have been chosen, which, in the solid state,
are known to have the stereochemistry shown in I. Our earlier
four-membered chelate ring (II) [56,57], a N,S-mode of binding
forming a five-membered chelate ring (III), and a C,N,S-mode of
binding (IV). Among these three modes of binding, mode III is
associated with a stereochemical change across the C@N bond.
The primary objective of the present study has been to induce,
if possible, the C,N,S-mode of coordination (IV) of the 4-R-benzal-
dehyde thiosemicarbazones to platinum. The selected thiosemi-
carbazone ligands (I) are abbreviated in general as H2L-R, where
H2 stands for the two protons, viz. the hydrazinic proton and
one ortho proton of the phenyl ring, which need to be dissociated
to induce the C,N,S-mode of binding (IV), and R for the substitu-
ent in the phenyl ring. It may be relevant to mention here that
though the chemistry of platinum complexes of several thiosemi-
carbazones has received considerable attention [50,58–83], the
binding of platinum to the chosen ligands (I) has remained unex-
plored. Two different platinum compounds, viz. [Pt(PPh3)2Cl2] and
K2[PtCl4], have been utilized in this study as the source of plati-
num. Interactions of the chosen thiosemicarbazones (I) with these
two platinum compounds have afforded two sets of interesting
complexes, including a group of organoplatinum complexes. The
⇑
Corresponding author.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.037

178
S. Halder et al. / Polyhedron 45 (2012) 177–184
chemistry of all these complexes is described in this paper, with
special reference to their formation, structure and cytotoxic
properties.
7.00 (d, 1H, J = 8.2), 7.41–7.78 (azomethine + PPh3). IR (cmÀ1):
1622, 1598, 1579, 1553, 1501, 1462, 1453, 1431, 1217, 1181,
1133, 752, 695, 515.
2.2.1.2. [Pt(PPh3)(L-CH3)]. Yield: 59%. Anal. Calc. for C27H24N3PSPt:
C, 50.00; H, 3.70; N, 6.48. Found: C, 50.04; H, 3.71; N, 6.47%. 1H
NMR (300 MHz; CDCl3) d (ppm) (J (Hz)): 1.75 (CH3); 5.00 (NH2);
6.07 (s, 1H), 6.62 (d, 1H, J = 7.5), 6.94 (d, 1H, J = 7.5), 7.36–7.79
(azomethine + PPh3). IR (cmÀ1): 1624, 1597, 1577, 1551, 1500,
1462, 1450, 1433, 1215, 1180, 1130, 752, 694, 515.
R
R = OCH 3, CH3,
R
H, Cl, NO2
NH2
N
NH2
S
H
N
N
H
H
H
N
N
2.2.1.3. [Pt(PPh3)(L-H)]. Yield: 63%. Anal. Calc. for C26H22N3PSPt: C,
49.21; H, 3.47; N, 6.62. Found: C, 49.17; H, 3.46; N, 6.63%. 1H
NMR (300 MHz; CDCl3) d (ppm) (J (Hz)): 5.04 (NH2); 6.31 (d, 1H,
7.5), 6.49 (d, 1H, J = 7.0), 7.03 (d, 1H, J = 7.1), 7.39–7.81 (azome-
thine + PPh3). IR (cmÀ1): 1625, 1598, 1579, 1550, 1505, 1463,
1453, 1431, 1214, 1182, 1133, 750, 694, 516.
S
M
II
I
M
R
2.2.1.4. [Pt(PPh3)(L-Cl)]. Yield: 57%. Anal. Calc. for C26H21N3PSClPt:
C, 46.67; H, 3.14; N, 6.28. Found: C, 46.57; H, 3.13; N, 6.27%. 1H
NMR (300 MHz; CDCl3) d (ppm) (J (Hz)): 5.08 (NH2); 6.24 (s, 1H),
6.79 (d, 1H, J = 7.8), 6.96 (d, 1H, J = 7.8), 7.42–7.68 (azome-
thine + PPh3). IR (cmÀ1): 1624, 1596, 1577, 1552, 1501, 1460,
1450, 1433, 1211, 1184, 1130, 754, 696, 515.
N
S
M
NH2
N
S
H
N
R
NH2
III
IV
2.2.1.5. [Pt(PPh3)(L-NO2)]. Yield: 62%. Anal. Calc. for C26H21N4O
2PSPt: C, 45.95; H, 3.09; N, 8.25. Found: C, 45.89; H, 3.08; N,
8.26%. 1H NMR (300 MHz; CDCl3) d (ppm): 6.84 (NH2); 7.34–7.83
(azomethine + 3H + PPh3).1 IR (cmÀ1): 1626, 1599, 1579, 1556,
1502, 1461, 1456, 1435, 1213, 1188, 1126, 755, 694, 514.
2. Experimental
2.1. Materials
2.2.2. [Pt(HL-R)2] complexes
2.2.2.1. [Pt(HL-OCH3)2]. 4-Methoxybenzaldehyde thiosemicarba-
zone (101 mg, 0.48 mmol) was dissolved in methanol (30 mL)
and K2[PtCl4] (100 mg, 0.24 mmol) was added to this solution fol-
lowed by triethylamine (49 mg, 0.49 mmol). The solution was then
stirred for 24 h, whereby [Pt(HL-OCH3)2] separated out as a yellow
crystalline precipitate, which was collected by filtration, washed
with cold methanol, and dried in air. Yield: 70%. Anal. Calc. for
Choloroplatinic acid was obtained from Arora Matthey, Kolkata,
India. Potassium chloride and triphenylphosphine were purchased
from Loba, India, and Hydrazinium sulfate was purchased from
Merck, India. The [Pt(PPh3)2Cl2] and K2[PtCl4] complexes were pre-
pared by following reported procedures [84,85]. The thiosemicar-
bazide and para-substituted benzaldehydes were purchased from
Merck, India. The 4-R-benzaldehyde thiosemicarbazones (H2L-R)
were prepared by reacting equimolar amounts of the thiosemicar-
bazide and the respective para-substituted benzaldehyde in a 1:1
ethanol-water mixture. All other chemicals and solvents were re-
agent grade commercial materials and were used as received.
C18H20N6O2S2Pt: C, 35.35; H, 3.27; N, 13.75. Found: C, 35.26; H,
3.28; N, 13.76%. 1H NMR (300 MHz; CDCl3) d (ppm) (J (Hz)): 3.85
(OCH3); 6.29 (NH2); 6.91 (d, 2H, J = 6.3), 7.59 (d, 2H, J = 8.8), 7.77
(s, 2H). IR (cmÀ1): 1605, 1524, 1368, 1292, 1181, 810.
2.2.2.2. [Pt(HL-CH3)2]. 4-Methylbenzaldehyde thiosemicarbazone
(95 mg, 0.49 mmol) was dissolved in methanol (30 mL) and
K2[PtCl4] (100 mg, 0.24 mmol) was added to this solution followed
by triethylamine (49 mg, 0.49 mmol). The solution was then stirred
for 24 h producing a yellow solution. Evaporation of this solution
gave a yellow solid, which was subjected to purification by thin layer
chromatography on a silica plate. With benzene–acetonitrile (10:1)
astheeluantayellowbandseparated, whichwasextractedwithace-
tonitrile and evaporation of this extract gave [Pt(HL-CH3)2] as a yel-
low crystalline solid. Yield: 68%. Anal. Calc. for C18H20N6S2Pt: C,
37.31; H, 3.45; N, 14.51. Found: C, 37.39; H, 3.46; N, 14.52%. 1H
NMR (300 MHz; CDCl3) d (ppm) (J (Hz)): 2.25 (CH3); 6.16 (NH2);
7.01 (d, 2H, J = 7.4), 7.40 (d, 2H, J = 8.0), 7.63 (s, 2H). IR (cmÀ1):
1605, 1524, 1367, 1292, 1180, 810.
2.2. Preparation of the complexes
2.2.1. [Pt(PPh3)(L-R)] complexes
The [Pt(PPh3)(L-R)] (R = OCH3, CH3, H, Cl and NO2) complexes
were prepared by following a general procedure. Specific details
are given below for a particular complex.
2.2.1.1. [Pt(PPh3)(L-OCH3)]. 4-Methoxybenzaldehyde thiosemicar-
bazone (27 mg, 0.13 mmol) was dissolved in ethanol (40 mL) and
to this solution was added triethylamine (13 mg, 0.13 mmol) fol-
lowed by [Pt(PPh3)2Cl2] (100 mg, 0.13 mmol). The mixture was
then heated to reflux for 6 h to yield a reddish-orange solution.
Evaporation of this solution gave a reddish-orange solid, which
was subjected to purification by thin layer chromatography on a
silica plate. With benzene–acetonitrile (10:1) as the eluant (CAU-
TION! Benzene is carcinogenic.) an orange band separated, which
was extracted with acetonitrile and evaporation of this extract
gave [Pt(PPh3)(L-OCH3)] as an orange crystalline solid. Yield: 55%.
Anal. Calc. for C27H24N3OPSPt: C, 48.79; H, 3.61; N, 6.33. Found:
C, 48.71; H, 3.62; N, 6.34%. 1H NMR (300 MHz; CDCl3) d (ppm) (J
(Hz)): 3.08 (OCH3); 4.98 (NH2); 5.91 (s, 1H), 6.33 (d, 1H, J = 9.9),
2.2.2.3. [Pt(HL-H)2]. The [Pt(HL-H)2] complex was prepared by fol-
lowing the same above procedure using H2L-H instead of H2L-
CH3. Yield: 71%. Anal. Calc. for C16H16N6S2Pt: C, 34.85; H, 2.90; N,
1
All aromatic and PPh3 proton signal are observed in the region 7.47–7.91 and
could not be seen distinctly due to their overlap in this region.

S. Halder et al. / Polyhedron 45 (2012) 177–184
179
15.25. Found: C, 34.76; H, 2.89; N, 15.24%. 1H NMR (300 MHz;
CDCl3) d (ppm) (J (Hz)): 6.34 (NH2); 7.35-7.50 (t1 + t2, 3H), 7.69
(d, 2H, J = 9.1), 8.04 (s, 2H). IR (cmÀ1): 1604, 1524, 1368, 1293,
1180, 810.
solution and refinement were done using the SHELXS-97 and SHELXL
-
97 packages [87]. The structures were solved by the direct methods.
In the crystal of the [Pt(HL-CH3)2] complex, disorder was identified
in both the coordinated ligands by an unrealistic geometry and poor
refinement. Hence a split atom model for the ligands was intro-
duced, which refined satisfactorily.
2.2.2.4. [Pt(HL-Cl)2]. The [Pt(HL-Cl)2] complex was prepared by fol-
lowing the same above procedure using H2L-Cl instead of H2L-H.
Yield: 67%. Anal. Calc. for C16H14N6S2Cl2Pt: C, 30.97; H, 2.26; N,
13.55. Found: C, 30.89; H, 2.27; N, 13.54%. 1H NMR (300 MHz;
CDCl3) d (ppm) (J (Hz)): 6.43 (NH2); 7.37 (d, 2H, J = 8.5), 7.58 (d,
2H, J = 8.4), 7.82 (s, 2H). IR (cmÀ1): 1604, 1524, 1368, 1292, 1181,
810.
2.5. Cytotoxicity assay
2.5.1. Cell lines and culture
HL-60 and U-937 (purchased from the National Center for Cell
Sciences, Pune, India) were maintained in complete sterile growth
medium RPMI-1640 with L-glutamine (GIBCO, Invitrogen Corpora-
tion, USA), containing 10% heat inactivated fetal bovine serum
(GIBCO, Invitrogen Corporation, USA) and 1% antibiotics
(10000 U penicillin/ml and 10000 lg streptomycin/ml, BioWhit-
taker, Cambrex Bio Science, USA). Cell cultures were maintained
2.2.2.5. [Pt(HL-NO2)2]. The [Pt(HL-NO2)2] complex was prepared by
following the same above procedure, used for the preparation of
[Pt(HL-OCH3)2], using H2L-NO2 instead of H2L-OCH3. Yield: 69%.
Anal. Calc. for C16H14N8O4S2Pt: C, 29.95; H, 2.18; N, 13.10. Found:
C, 30.08; H, 2.19; N, 13.09%. 1H NMR (300 MHz; CDCl3) d (ppm)
(J (Hz)): 7.34 (NH2); 8.18-8.22 (s + d, 4H), 8.44 (d, 2H, J = 8.7). IR
(cmÀ1): 1605, 1524, 1367, 1293, 1181, 811.
in the log phase using a subculture ratio of 1:5 twice a week.
2.5.2. Preparation of test material
Working stock solutions of all the compounds were prepared by
dissolving 20 mg of each compound in 1 ml of DMSO (Hybri-Max
grade, Sigma-Aldrich Co., USA) and filtered under sterile condi-
tions. These were serially diluted with complete RPMI-1640 med-
ium prior to use to obtain different drug concentrations.
2.3. Physical measurements
Microanalyses (C,H,N) were performed using a Heraeus Carlo
Erba 1108 elemental analyzer. IR spectra were obtained on a Shima-
dzu FTIR-8300 spectrometer with samples prepared as KBr pellets.
Electronic spectra were recorded on a JASCO V-570 spectrophotom-
eter. 1H NMR spectra were recorded in CDCl3 solution on a Bruker
Avance DPX 300 NMR spectrometer using TMS as the internal stan-
dard. Optimization of ground-state structures and energy calcula-
tions for all the complexes were carried out by the density
functional theory (DFT) method using the GAUSSIAN 03 (B3LYP/SDD-
6-31G) package [86].
2.5.3. MTT-based cytotoxicity test
One hundred microliters of a cell suspension from the stock
2 Â 105 HL-60 or 1 Â 105 U-937 cells/ml were added to 96-well cell
culture plates. Next 10 ll of drug solutions of different concentra-
tions were added to the respective wells followed by the addition
of the medium (90 l) in triplicate (total volume 200 l/well). All
l
l
vehicle controls contained the same concentration of DMSO. Plates
were incubated for 72 h at 37 °C, 5% CO2/95% air with humidity.
After removal of 100 ll of media from each well, 10 ll of a 5 mg/
2.4. Crystallography
ml solution of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoli-
um bromide (MTT, Sigma, USA) in Dulbecco’s PBS (GIBCO, BRL)
were added to each well and the plate was incubated for 4 h at
Single crystals of [Pt(PPh3)(L-CH3)] and [Pt(HL-CH3)2] were ob-
tained by slow evaporation of 1:1 dichloromethane–acetonitrile
solutions of the complexes. Selected crystal data and data collection
parameters are given in Table 1. Data were collected on a BRUKER
SMART Apex CCD area detector. X-ray data reduction, structure
37 °C. Subsequently 100 ll of acid isopropanol solution (0.04 N
HCl in isopropanol) was added to the wells to dissolve the formed
formazan crystals. The plate was read in a microplate reader (Bio-
Rad, USA, Model 550) at 540 nm. The percentage cell viability was
calculated by dividing the average absorbance of the cells treated
with compounds by that of the control; IC50 (drug concentration
needed for 50% inhibition of cells relative to the control) was deter-
mined through Curvefit software by plotting% inhibition versus
Table 1
Crystallographic data for the [Pt(PPh3)(L-CH3)] and [Pt(HL-CH3)2] complexes.
[Pt(PPh3)(L-CH3)]
27H24N3PSPt
[Pt(HL-CH3)2]
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
C18H20N6S2P2Pt
579
monoclinic
P21/n
8.6432(5)
13.4128(8)
17.8874(10)
90
90.927
90
2073.4(2)
4
drug concentration and the result is expressed in lM concentration.
648
triclinic
ꢀ
P1
3. Results and discussion
9.9753(5)
10.8823(6)
12.4992(7)
90.663(1)
107.612(1)
109.444(1)
1209.93(11)
2
b (Å)
c (Å)
3.1. Syntheses and structures
a
(°)
b (°)
Five 4-R-benzaldehyde thiosemicarbazones (I), with different
substituents in the para position of the phenyl ring, have been se-
lected for this study. The reaction of the 4-R-benzaldehyde thio-
semicarbazones with [Pt(PPh3)2Cl2] has been carried out in
refluxing ethanol in the presence of triethylamine to afford a group
of yellow complexes of the type [Pt(PPh3)(L-R)] in decent yields.
The observed elemental (C,H,N) analytical data of these complexes
are consistent with their compositions. In order to authenticate the
coordination mode of the thiosemicarbazone in these complexes,
the structure of a representative member of this family, viz.
[Pt(PPh3)(L-CH3)], has been determined by X-ray crystallography.
The structure is shown in Fig. 1 and some selected bond parameters
are presented in Table 2. The structure shows that the thiosemicar-
c
(°)
V (Å3)
Z
F(000)
632
1120
Crystal size (mm)
T (K)
0.20 Â 0.10 Â 0.04
0.25 Â 0.10 Â 0.08
298
298
l
R1
wR2
(mmÀ1
)
5.972
0.0329
0.0655
1.061
6.984
0.0277
0.0669
0.940
a
b
Goodness-of-fit (GOF)c
a
b
c
R1
wR2 = [
GOF = [
=
R
||Fo| À |Fc||/
R|Fo|.
R
[w(F2o À F2c )2]/
R .
[w(F2o)2]]1/2
R
[w(F2o À F2c )2]/(M À N)]1/2, where M is the number of reflections and N
is the number of parameters refined.

180
S. Halder et al. / Polyhedron 45 (2012) 177–184
Fig. 2. Packing diagram of the lattice of [Pt(PPh3)(L-CH3)].
Fig. 1. View of the [Pt(PPh3)(L-CH3)] complex.
molecules through these intermolecular hydrogen-bondings, and
these extended hydrogen-bonding interactions are responsible for
holding the crystal together. As all the [Pt(PPh3)(L-R)] complexes
have been synthesized similarly and they show similar properties
(vide infra), the other four [Pt(PPh3)(L-R)] (R – CH3) complexes are
assumed to have a similar structure to [Pt(PPh3)(L-CH3)].
bazone ligand is coordinated to platinum in the tridentate C,N,S-
fashion (IV) and one PPh3 ligand is also coordinated to the metal
center. In this complex, platinum is sitting in a CNSP coordination
sphere, which is distorted square planar in nature, as reflected in
the bond parameters around the metal center. The Pt–C, Pt–N, Pt–
P and Pt–S distances are quite normal, as observed in structurally
characterized complexes of platinum containing these bonds [58–
83]. The C–S, C–N and other bond distances within the coordinated
thiosemicarbazone are consistent with the bonding description
shown in IV. The absence of any solvent of crystallization in the
crystal lattice of [Pt(PPh3)(L-CH3)] indicates the possible existence
of non-covalent interaction(s) between the individual complex
molecules. A closer look at the packing pattern of the crystal reveals
The observed C,N,S-mode of coordination (IV) of the 4-R-benzal-
dehyde thiosemicarbazones in the [Pt(PPh3)(L-R)] complexes is
quite interesting, and it is believed to occur via the formation of an
intermediate, as illustrated in Scheme 1. In the initial step the thio-
semicarbazone seems to coordinate platinum as a bidentate N,S-do-
nor, via displacement of PPh3 and a chloride from [Pt(PPh3)2Cl2)] to
form a reactive intermediate (A). In A the remaining chloride is trans
to the sulfur donor atom, and hence is in close proximity to the ortho
C–H of the phenyl ring. In the following step activation of this ortho
C–H bond takes place, leading to the formation of a Pt–C bond via
eliminationof HCl, and thusaffordingthe[Pt(PPh3)(L-R)]complexes.
Achievement of the targeted C,N,S-mode of coordination (IV) of
the 4-R-benzaldehyde thiosemicarbazones in the [Pt(PPh3)(L-R)]
complexes, via a C–H bond activation (Scheme 1) has been quite
encouraging. The presence of triphenylphosphine in the platinum
starting material, viz. [Pt(PPh3)2Cl2], seems to have favored the for-
mation of these organoplatinum complexes. In order to see whether
the same C,N,S-mode of binding can also be induced by interaction
of the same thiosemicarbazones (I) with a different platinum start-
ing material that does not contain any coordinated PPh3 ligand,
reactions of the selected thiosemicarbazones (I) have been carried
out with K2[PtCl4]. These reactions proceed smoothly in methanol
to afford complexes of the type [Pt(HL-R)2] in good yields. Microan-
alytical data of these complexes agree well with their compositions.
The structure of one representative member of this family, viz.
[Pt(HL-CH3)2], has been determined by X-ray crystallography to
ascertain the binding mode of the thiosemicarbazone molecule is
shown in Fig. 3 and some relevant bond parameters are presented
in Table 2. The structure shows that the thiosemicarbazone ligands
are coordinated to platinum as bidentate N,S-donors, forming five-
membered chelate rings (V) with a bite angle of 83.1(2)°. It is inter-
esting to note that the phenyl ring of both the coordinated thio-
semicarbazones has moved away significantly from the plane of
the Pt(N–S) chelate ring, presumably to avoid a possible steric prob-
lem with the metal center. The structure also shows that the two
coordinated nitrogens are cis and so are the two sulfurs. Platinum
is thus nested in a N2S2 core, which is distorted significantly from
that non-covalent interactions of five types, viz. N–H---S, N–H---
p,
C–H---N, C–H---S and C–H--- interactions, are active in the lat-
p
tice. A selected view of the packing diagram is shown in Fig. 2. Each
complex molecule is thus linked with five surrounding complex
Table 2
Selected bond distances (Å) and bond angles (°) for [Pt(PPh3)(bztsc-CH3)] and
[Pt(Hbztsc-CH3)2].
[Pt(PPh3)(bztsc-CH3)]
Bond lengths (Å)
Pt–C(8)
Pt–N(3)
Pt–P(1)
Pt–S(1)
2.045(4)
2.032(4)
2.230(10)
2.340(13)
C(2)–N(3)
C(1)–N(2)
C(1)–N(1)
N(2)–N(3)
C(1)–S(1)
1.269(7)
1.287(7)
1.354(6)
1.381(6)
1.755(5)
Bond angles (°)
C(8)–Pt–S(1)
N(3)–Pt–P(1)
162.52(13)
176.87(12)
C(8)–Pt–N(3)
N(3)–Pt–S(1)
80.85(17)
81.70(13)
[Pt(Hbztsc-CH3)2]
Bond lengths (Å)
Pt–N(3)
Pt–N(6)
Pt–S(1)
2.027(6)
2.050(6)
2.236(6)
2.286(2)
1.361(10)
1.354(10)
1.294(11)
C(10)–N(5)
C(2)–N(3)
C(11)–N(6)
C(1)–S(1)
C(10)–S(2)
N(2)–N(3)
N(5)–N(6)
1.462(15)
1.291(10)
1.354(5)
1.744(10)
1.741(14)
1.405(9)
1.352(6)
Pt–S(2)
C(1)–N(1)
C(10)–N(4)
C(1)–N(2)
Bond angles (°)
N(3)–Pt–S(2)
N(6)–Pt–S(1)
176.00(2)
168.19(15)
N(3)–Pt–S(1)
N(6)–Pt–S(2)
83.10(2)
80.79(11)

S. Halder et al. / Polyhedron 45 (2012) 177–184
181
R
R
R
PPh3
PPh3
Cl
[Pt(PPh3)2Cl2]
PPh3, HCl
Pt
A
Pt
-
-
-
HCl
N
H
NH2
S
N
S
N
S
H
N
H
N
H
N
NH2
NH2
I
[Pt(PPh3)(L-R)]
Scheme 1.
species upon interaction of [PtCl4]2À with neutral ligands is well
documented in the literature [88–96]. In intermediate B, the two
coordinated chlorides are believed to be hydrogen-bonded with
two proximal SH fragments of the N-coordinated thiosemicarba-
zones. In the following step elimination of HCl takes place, facili-
tated by the presence of NEt3 in the reaction medium, leading to
coordination of the thiolate sulfur, thus generated, affording the
[Pt(HL-R)2] complex.
ideal square planar geometry, as reflected in the bond parameters
around the metal center. The Pt–N and Pt–S distances, as well as
other bond distances in the coordinated ligands, are quite usual
[58–83]. In view of the similarity in the method of preparation of
all the [Pt(HL-R)2] complexes and the similarity in their properties
(vide infra), the other four [Pt(HL-R)2] (R – CH3) complexes are as-
sumed to have the same structure as their R = CH3 analogue.
R
3.2. Spectral properties
Pt
1H NMR spectra of the [Pt(PPh3)(L-R)] complexes have been re-
corded in CDCl3 solutions and they show all the expected signals.
The PPh3 ligands show broad signals within the range 7.34–
7.83 ppm. From the coordinated thiosemicarbazone, the azome-
thine proton signal could not be detected due to its overlap with
the PPh3 signals, but the NH2 signal is observed within the range
4.98–6.84 ppm. Most of the expected signals from the phenyl ring
of the coordinated thiosemicarbazone are clearly observed in the
aromatic region, while a few could not be detected due to their
overlap with other signals. Signals for the OCH3 and CH3 substitu-
ents are observed, respectively, at 3.08 and 1.75 ppm. 1H NMR
spectra of the [Pt(HL-R)2] complexes, recorded in CDCl3 solutions,
show the azomethine proton signal within the range 8.01–
9.34 ppm, the NH2 proton signal within 6.29–7.34 ppm and the ex-
pected aromatic proton signals are observed in the 6.91–8.44 ppm
region. Signals for the OCH3 and CH3 substituents are observed at
3.85 and 2.25 ppm, respectively. The 1H NMR spectral data of the
[Pt(PPh3)(L-R)] and [Pt(HL-R)2] complexes are therefore in well
accordance with their compositions.
N
S
H
N
NH
2
V
The results obtained from the reactions between the selected thio-
semicarbazones (I) and K2[PtCl4] thus indicate that the absence of
PPh3 in the platinum starting material has failed to induce the
C,N,S-mode of coordination (IV), and instead has led to a relatively
less common N,S-mode of binding (V). Formation of the [Pt(HL-R)2]
complexes seems to have taken place via the steps shown in
Scheme 2. Initially the imine-nitrogens of two thiosemicarbazone
ligands are believed to displace two chlorides from the [PtCl4]2À
complex anion generating an intermediate B, where the remaining
two coordinated chlorides are cis. Formation of such cis-dichloro
The infrared spectra of all the [Pt(PPh3)(L-R)] complexes show
many vibrations of different intensities in the 1600–400 cmÀ1 re-
gion. Assignment of each individual band to a specific vibration
has not been attempted. However, three strong bands displayed
near 515, 694 and 752 cmÀ1 by these complexes, are attributed
to the coordinated PPh3 ligand. A comparison with the spectrum
of the [Pt(PPh3)2Cl2] complex shows the presence of several new
bands (e.g. near 1130, 1180, 1215, 1433, 1450, 1462, 1500, 1551,
1577, 1597 and 1624 cmÀ1) in the spectra of the [Pt(PPh3)(L-R)]
complexes, which must be due to presence of the coordinated thi-
osemicarbazone ligand. Except the absence of the bands due to the
PPh3 ligand and small shifts in the positions of the remaining
bands, the infrared spectra of the [Pt(HL-R)2] complexes are found
to be similar to those of the respective [Pt(PPh3)(L-R)] complexes.
The [Pt(PPh3)(L-R)] and [Pt(HL-R)2] complexes are soluble in
common organic solvents like methanol, ethanol, acetone, acetoni-
trile, dichloromethane, chloroform, etc., producing intense yellow-
ish-orange and yellow solutions, respectively.2 Electronic spectra
of both sets of complexes have been recorded in dichloromethane
solution, and the spectral data are presented in Table 3. Complexes
of both types show several absorptions within the 500–250 nm re-
gion. In comparison with the spectra of the uncoordinated thio-
semicarbazone ligands (I), which show an intense absorption
near 300 nm, the absorptions in the UV region for all the com-
Fig. 3. View of the [Pt(HL-CH3)2] complex.

182
S. Halder et al. / Polyhedron 45 (2012) 177–184
Cl
R
Cl
NH2
SH
N
S
NH2
HS
N
H2N
S
H2N
Pt
Pt
R
[PtCl4]2-
N
N
N
H
N
H
N
N
N
H
NH2
S
-
-
2 HCl
-
2 Cl
H
H
H
N
R
R
R
I
B
[Pt(HL-R)2]
Scheme 2.
Table 3
Electronic spectral data for [Pt(PPh3)(bztsc-CH3)] and [Pt(Hbztsc-CH3)2].
Complex
kmax/nm (e )
/dm3 molÀ1 cmÀ1
[Pt(PPh3)(L-
OCH3)]a
494(1500), 364(7600), 346(7500), 308(17200),
262(18200)
[Pt(PPh3)(L-CH3)]a 498(700), 361(6300), 340(7600), 306(13800),
256(17300)
[Pt(PPh3)(L-H)]a
495(700), 358(75000, 338(11000), 305(17800),
251(26000)
495(800), 361(8100), 340(11000), 308(20600),
253(30800)
[Pt(PPh3)(L-Cl)]a
[Pt(PPh3)(L-
NO2)]a
471(14300), 401(11900), 326(12900), 263(19100)
[Pt(HL-OCH3)2]a
[Pt(HL-CH3)2]a
[Pt(HL-H)2]a
[Pt(HL-Cl)2]a
[Pt(HL-NO2)2]b
400(4600), 316(18900), 257(7600)
380(3400), 317(15400), 254(6700)
400(3400), 316(21900), 246(12000)
422(3100), 321(57000)
500(6600), 395(17700), 278(15700)
a
In dichloromethane solution.
In N,N-dimethylformamide solution.
b
Table 4
Composition of selected molecular orbitals of the complexes.
Complex
Contributing fragments % contribution of fragments
to
HOMO
LUMO
[Pt(PPh3)(L-OCH3)] Pt
4.22
95.78
–
3.35
96.65
–
4.71
95.29
–
3.56
96.44
–
2.81
97.11
0.08
2.55
97.36
0.09
2.18
97.76
0.06
2.02
97.87
0.11
2.98
96.89
0.13
5.62
94.38
5.39
94.61
4.98
95.02
4.87
95.13
5.89
94.11
L-OCH3
PPh3
Pt
L-CH3
PPh3
Pt
L-H
PPh3
Pt
[Pt(PPh3)(L-CH3)]
[Pt(PPh3)(L-H)]
[Pt(PPh3)(L-Cl)]
[Pt(PPh3)(L-NO2)]
L-Cl
PPh3
Pt
4.01
95.99
–
L-NO2
PPh3
Pt
HL-OCH3
Pt
[Pt(HL-OCH3)2]
[Pt(HL-CH3)2]
[Pt(HL-H)2]
5.73
94.27
4.97
95.03
6.22
93.78
5.09
94.91
5.53
94.47
HL-CH3
Pt
HL-H
Pt
[Pt(HL-Cl)2]
HL-Cl
Pt
[Pt(HL-NO2)2]
HL-NO2
Fig. 4. Contour plots of the HOMO and LUMO of the [Pt(PPh3)(L-CH3)] complex.
plexes are attributable to transitions within the thiosemicarbazone
ligand orbitals. To have an understanding of the origin of the low-
est-energy absorption (near 500 nm in the [Pt(PPh3)(L-R)] com-
plexes and near 400 nm in the [Pt(HL-R)2] complexes), DFT
calculations have been performed on all the complexes. The com-
positions of the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) are given in Table

S. Halder et al. / Polyhedron 45 (2012) 177–184
183
Table 5
Cytotoxic activities (IC50
and Industrial Research, New Delhi, India [Grant No. 01(2384)/
10/EMR-II] is gratefully acknowledged. Piyali Paul thanks the
Council of Scientific and Industrial Research, New Delhi, for her fel-
lowship [Grant No. 09/096(0588)/2009-EMR-I].
,
l
M) for the complexes.
HL-60
Complex
U-937
[Pt(PPh3)(L-OCH3)]
[Pt(PPh3)(L-CH3)]
[Pt(PPh3)(L-H)]
[Pt(PPh3)(L-Cl)]
[Pt(PPh3)(L-NO2)]
[Pt(HL-OCH3)2]
[Pt(HL-CH3)2]
[Pt(HL-H)2]
[Pt(HL-Cl)2]
[Pt(HL-NO2)2]
Cisplatin
BCNU
5-FU
inactive
138.36
10.50
26.40
inactive
3.48
4.87
2.81
7.15
6.90
17.77
9.88
26.92
4.60
42.95
10.37
7.25
8.34
32.80
8.58
3.2
12.3
Appendix A. Supplementary data
CCDC 878660 and 878661 contains the supplementary crystal-
lographic data. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.07.037.
7.0
30.5
266.0
204.0
4.7
115.0
Hydroxyurea
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