# Synthesis, Characterization, and In Vitro Cytotoxic Activities of Benzaldehyde Thiosemicarbazone Derivatives and Their Palladium (II) and Platinum (II) Complexes against Various Human Tumor Cell Lines

Authors: Wilfredo Hernándeza; Juan Paz; Abraham Vaisberg; Evgenia Spodine; Rainer Richter; Lothar Beyer
Journal: Bioinorganic Chemistry and Applications
 (2008)

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## Abstract

The palladium (II) bis-chelate Pd(L1−3)2 and platinum (II) tetranuclear Pt4(L4)4 complexes of benzaldehyde thiosemicarbazone derivatives have been synthesized, and characterized by elemental analysis and IR, FAB(+)-mass and NMR (1H,13C) spectroscopy. The complex Pd(L2)2 [HL2=m-CN-benzaldehyde thiosemicarbazone] shows a square-planar geometry with two deprotonated ligands (L) coordinated to PdII through the nitrogen and sulphur atoms in a transarrangement, while the complex Pt4(L4)4 [HL4=4-phenyl-1-benzaldehyde thiosemicarbazone] has a tetranuclear geometry with four tridentate ligands coordinated to four PtII ions through the carbon (aromatic ring), nitrogen, and sulphur atoms where the ligands are deprotonated at the NH group. The in vitro antitumor activity of the ligands and their complexes was determined against different human tumor cell lines, which revealed that the palladium (II) and platinum (II) complexes are more cytotoxic than their ligands with IC50 values at the range of 0.07–3.67 μM. The tetranuclear complex Pt4(L4)4, with the phenyl group in the terminal amine of the ligand, showed higher antiproliferative activity (CI50 = 0.07–0.12 μM) than the other tested palladium (II) complexes.


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## Body

## 1. Introduction

The synthesis of transition metal complexes with thiosemicarbazone ligands has been receiving considerable attention due to the  pharmacological properties of both ligands and complexes [1–3].
Thiosemicarbazone derivatives exhibit a great variety of biological activities,
such as antitumor [4], antifungal [5, 6], antibacterial [6, 7], and antiviral
[8] properties.The deprotonated
thiosemicarbazone ligands usually coordinate to
platinum, palladium, copper, ruthenium, and osmium through oxygen,
nitrogen, and sulphur donor atoms in their (N, S) bidentate form or  (N, N, S or O, N, S) tridentate form, to form metallic
complexes of different molecular geometry [9–11].The square
planar platinum (II) and palladium (II) complexes of M(HL)Cl2  and M(L)Cl
type with thiosemicarbazone ligands derived from phenylacetaldehyde and
2-formylpyridine showed high cytotoxicity in vitro against HL60 leukemia and P388 mouse leukemia cell
lines [12], while platinum(II) and palladium(II) binuclear complexes with p-isopropylbenzaldehyde
thiosemicarbazone ligands exhibit strong cytotoxic activities on mouse tumor
cell growth inhibition [4, 13].On the
other hand, C,N,S  thiosemicarbazone ligands can also be coordinated to  palladium (II) to form complexes with two fused
five-membered chelate rings containing a carbon-metalσ bond [14]. Although
there is little information about the antitumor activity of these tetranuclear
complexes, their pharmacological applications could be relevant.As
part of our continuing investigations about metal complexes with ligands
derived from thiourea [15–19] such as
thiosemicarbazones, we report here the synthesis, characterization, and
antitumor activity of palladium(II) bis-chelates Pd(L1–3)2  and platinum (II) tetranuclear complex Pt4(L4)4 with benzaldehyde
thiosemicarbazone and 4-phenyl-1-benzaldehyde thiosemicarbazone ligands,
R-PhCH=N-NH-C(=S)-NHR1, HL1(R, R1=H),  HL2(R=m-CN, R1=H), HL3(R=o-NO2, R1=H),
and HL4(R=H, R1=Ph).


## 2. Experimental

### 2.1. Materials and Measurements

Chemicals were reagent grade. Palladium (II)
bis(acetylacetonate), ammonium tetrachloroplatinate, thiosemicarbazide,
4-phenyl-thiosemicarbazide, benzaldehyde,m-CN-benzaldehyde,
and o-NO2-benzaldehyde
were purchased from Aldrich. Melting points were determined on a Büchi melting
point B-545 apparatus. Elemental
analyses
were determined on a Fisons-Carlo Erba Elemental Microanalyzer. The infrared
(IR) spectra were recorded in solid state (KBr pellets) on a Bruker FT-IR IFS
55 Equinox spectrophotometer in the 4000–400 cm−1 range.
The FAB(+) mass  spectra were recorded on a ZAB-HSQ (V.G. Analytical Ltd. Floats Roads, Wythenshawe,
Manchester, England) spectrometer, using 3-nitrobenzyl alcohol as the matrix.
The H1 (300 MHz)
and C13 (75.5 MHz) NMR spectra were recorded on a Bruker Advance DRX 300 spectrometer
at 300 K, using DMSO-d6 as solvent. The chemical
shifts (δ) in ppm were
measured relative to tetramethylsilane (TMS).


### 2.2. Synthesis of the Ligands

The
thiosemicarbazone derivatives (HL) were prepared according to the literature
[20] as shown in Scheme 1.Scheme 1
Synthesis of the benzaldehyde thiosemicarbazone and 4-phenyl-1-benzaldehyde
thiosemicarbazone ligands.General Method
To a hot solution of thiosemicarbazide (1.82 g, 20 mmol) or
4-phenylthiosemicarbazide (3.3 g, 20 mmol) in 160 mL, methanol was added dropwise a solution of the
corresponding benzaldehyde (20 mmol) in 70 mL methanol during 30 minutes. The
mixture was stirred and refluxed for 4 hours, it was filtered and the filtrate
was concentrated to half the volume under reduced pressure. After a slow evaporation of the
concentrate at room temperature, crystals were collected by filtration, washed
with cold ethanol, and dried in vacuo. For ligand HL4, the filtrate
was kept in the refrigerator and after several hours small rectangular crystals
were obtained. These crystals were suitable for structure analysis by
X-ray diffraction.


#### 2.2.1. Benzaldehyde Thiosemicarbazone (HL1)

Colorless
crystals. Yield 80%, m.p. 167–169°C. Anal. Calc. For C8H9N3S
(179,2 g/mol): C, 53.6%; H, 5.1%; N, 23.4%; S, 17.9%. Found: C, 53.5%; H, 5.3%;
N, 23.5%; S, 17.7%. FAB(+)-MS:
m/z 179  (M+, 70%); IR (KBr,
cm−1): ν(NH2) 3400, 3380; ν(NH) 3250; ν(C=N) 1600;
ν(C=S) 885.H1-NMR (DMSO-d6): δ  7.78 (d,
2Hortho, Ph, J=6.8 Hz),
7.39 (t, 2Hmeta, Ph,
J= 7.2 Hz), 7.40 (t, 1Hpara,
Ph, J= 7.2 Hz);  8.05 (s, 1H, HC=N);  8.19, 7.98 (d, 2H, NH2); 11.42 (s,
1H, =N–NH). C13-NMR (DMSO-d6): δ  128.65, 127.29, 129.83, 134.18 (Ph); 142.28
(HC=N);  178.0 (C=S).


#### 2.2.2. m-Cyanobenzaldehyde Thiosemicarbazone (HL2)

Colorless crystals. Yield
76%, m.p. 203-204°C.
Anal. Calc. For C9H8N4S (204,3 g/mol): C,
52.9%; H, 3.9%; N, 27.4%; S, 15.7%. Found: C, 52.8%; H, 3.7%; N,  27.6%; S, 15.5%.; FAB(+)-MS: m/z 205 (MH+, 100%); IR (KBr, cm−1): ν(NH2) 3410, 3397; ν(NH)  3236
ν(CN) 2233; ν(C=N)  1596;
ν(C=S)  880. H1-NMR (DMSO-d6): δ  7.81, 7.79 (s, d, 2Hortho, Ph, J =5.9 Hz); 7.58 (t, 1Hmeta, Ph, J = 7.7 Hz); 7.83
(t, 1Hpara, Ph,  J = 5.7 Hz);  8.03 (s, 1H, HC=N);  8.31, 8.26 (d, 2H, NH2); 11.60 (s,
1H, =N–NH).C13-NMR (DMSO-d6): δ  129.87, 118.6, 135.69, 132.32, 132.68 (Ph);
111.97 (CN); 139.66 (HC=N);  178.35
(C=S).135.69, 130.15.


#### 2.2.3. o-Nitrobenzaldehyde Thiosemicarbazone (HL3)

Yellow crystals. Yield 90%, m.p. 214-215°C. Anal. Calc. For C8H8O2N4S
(224.3 g/mol): C, 42.9%; H, 3.6%; N, 24.9%; S, 14.3%. Found: C, 42.6%; H, 3.5%;
N,  24.6%; S, 14.1%. FAB(+)-MS:  m/z 226
(MH+, 100%); IR (KBr, cm−1): ν(NH2)
3421, 3370; ν(NH) 3240; ν(C=N) 1602; ν(C=S)
890. H1-NMR (DMSO-d6): δ  7.58 (d,  1Hortho, Ph, J = 6.9 Hz); 8.02, 7.73 (d,d, 2Hmeta, Ph, J = 7.8 Hz); 7.61
(t, 1Hpara, Ph,  J =6.7 Hz);  8.12 (s, 1H, HC=N);  8.45, 8.41 (d, 2H, NH2); 11.73 (s,
1H, =N–NH).C13-NMR (DMSO-d6): δ 124.54, 137.24, 130.36,
133.36, 128.46, 128.33 (Ph); 148.29 (HC=N);
178.49 (C=S).


#### 2.2.4. 4-Phenyl-1-Benzaldehyde Thiosemicarbazone (HL4)

Yellow crystals. Yield 75%, m.p. 192–194°C. Anal. Calc. For C14H13N3S
(255.3 g/mol): C, 65.9%; H, 5.1%; N, 16.5%; S, 12.5%. Found: C, 65.4%; H, 5.3%;
N, 16.7%; S, 12.6%. FAB(+)-MS:
m/z 255  (M+, 48%); IR (KBr,
cm−1): ν(NH) 3245; ν(C=N) 1625; ν(C=S) 915. H1-NMR
(DMSO-d6): δ  7.42 (t, 1Hpara, Ph,  J = 6.8 Hz); 7.43
(t, 2Hmeta, Ph, J = 6.7 Hz);
7.91 (d, 2Hortho, Ph,  J = 5.3 Hz); 7.21 (t, 1Hpara, NHPh,
J = 6.2 Hz); 7.37 (t, 2Hmeta,
NHPh,  J = 6.4 Hz); 7.58 (d, 2Hortho,
NHPh, J = 5.6 Hz);  8.17 (s, 1H,
HC=N);  10.11 (s, 1H, NHPh); 11.83 (s,
1H, =N–NH). C13-NMR (DMSO-d6): δ  127.6, 128.6, 130.0, 134.0 (Ph); 125.3,
125.9, 128.0, 139.1 (NH-Ph); 142.9 (HC=N);
176.0 (C=S).


### 2.3. Synthesis of the Palladium (II) and Platinum (II) Complexes (see Scheme2)

A
solution of Pd(acac)2 (0.30 g, 1.0 mmol) in CH2Cl2/CH3OH
(30 mL, 2:1 v/v)  or a solution of (NH4)2PtCl4  (0.1865 g, 0.5 mmol) in water/ethanol (2:1, 15 mL) was added dropwise to a stirred solution of the corresponding
thiosemicarbazone (2.0 mmol) in 60 mL of
methanol. Sodium acetate (0.16 g, 2 mmol) in 3 mL of water was then added.  The solution was refluxed for 2 hours and
stirred for 24 hours at room temperature. The precipitate was collected by
filtration and dried in vacuo.Scheme 2
Synthesis of the palladium (II) bis-chelate complexes and the
platinum (II) tetranuclear complex.


#### 2.3.1. Palladium (II) Complex of Benzaldehyde Thiosemicarbazone, Pd(L1)2

Yellow solid. Yield 70%, m.p. 204-205°C. Anal. Calc. For C16H16N6S2Pd
(462.9 g/mol): C, 41.5%; H, 3.5%; N, 18.2%; S, 13.9%. Found: C, 40.9%; H, 3.6%; N, 18.6%; S,
13.5%. FAB(+)-MS: m/z  463 (M+,
60%); IR (KBr, cm−1): ν(NH2) 3390, 3367;  ν(C=N) 1582; ν(C=S) 805. H1-NMR
(DMSO-d6): δ  7.49, 7.45,
7.41, 7.39 (m, Ph);  8.13 (s, 2H,
HC=N);  8.29, 8.21 (d, 4H, NH2).


#### 2.3.2. Palladium (II) Complex of m-Cyanobenzaldehyde Thiosemicarbazone, Pd(L2)2

Crystals
suitable for X-ray structure determination were obtained by slowly evaporating
a methanol/dichloromethane (2:1) solution at room temperature.Orange
crystals. Yield 63%, m.p. >240°C (decomp.). Anal. Calc. for C18H14N8S2Pd·H2O
(530.9 g/mol): C, 40.7%; H, 3.0%; N, 21.1%; S, 12.1%;  Found: C, 42.0%; H, 2.7%; N, 21.6%; S, 12.3%.
FAB(+)-MS: m/z  513 (M+-H2O,
100%); IR (KBr, cm−1): ν(NH2) 3405, 3377;  ν(CN)
2230;  ν(C=N) 1570; ν(C=S) 815. H1-NMR
(DMSO-d6):
δ  7.75, 7.65, 7.55 (m, Ph);  8.06 (s, 2H, HC=N);  8.68 (s, 2H, NH2).


#### 2.3.3. Palladium (II) Complex of o-Nitrobenzaldehyde Thiosemicarbazone, Pd(L3)2

Yellow solid.
Yield 61%, m.p.   >260°C (decomp.). Anal. Calc. for
C16H14O4N8S2Pd (552.9 g/mol): C, 34.8%; H, 2.6%; N, 20.3%; S, 11.6%. Found: C,  34.6%; H, 2.5%; N,
20.6%; S, 11.3%.
FAB(+)-MS: m/z  553 (M+, 74%); IR (KBr, cm−1): ν(NH2) 3400, 3350; ν(C=N) 1585;
ν(C=S) 820. H1-NMR (DMSO-d6): δ  7.85, 7.80, 7.70 (m, Ph);  8.15 (d, 2H, HC=N);  8.44 (s, 2H, NH2).


#### 2.3.4. Platinum (II) Tetranuclear Complex, Pt4(L4)4

Crystals suitable
for structure determination by X-ray diffraction were obtained by slowly
evaporating an ethanol/chloroform (2:1) solution at room temperature.Red crystals.
Yield 57%, m.p. 188-189°C. Anal. Calc. For C56H44N12S4Pt4·2C2H5OH (1885.8 g/mol): C, 38.2%; H, 3.0%; N, 8.9%; S,
6.8%. Found: C, 38.0%; H, 3.0%; N, 8.6%; S, 6.5%; IR (KBr, cm−1):  ν(NHPh) 3200; ν(C=N) 1590; ν(C=S) 840. H1-NMR (DMSO-d6):  δ7.50–7.80 (m,
Ph); 7.0–7.4 (m, NHPh);  8.0 (d, 4H,
HC=N);  9.15, 9.68 (d, 4H, NHPh).


### 2.4. Crystal Structure Determinations

The data for the
crystal structure determinations were collected on a Siemens CCD smart diffractometer
(HL4, Pt4(L4)4) and a Stoe IPDS2 diffractometer
(Pd(L2)2) (MoKα  radiation, λ = 0.71073 Å, graphite monochromator).
The intensities were corrected for Lorentz and polarization effects and for
absorption using SADABS (Pt4(L4)4) and
X-RED/X-SHAPE (Pd(L2)2). The structures were solved by
direct methods, which revealed the positions of all nonhydrogen atoms and
refined on F2 by a full
matrix least-squares procedure using anisotropic displacement parameters with the
exception of the solvent molecules in Pt4(L4)4 which were refined isotropically. The hydrogen atoms for HL4 and
Pd(L2)2 were
located from different
Fourier syntheses and refined isotropically. For Pt4(L4)4,
the hydrogen atoms were included in calculated positions and refined in riding
mode. All calculations were carried out using the SHELXS-97 and SHELXL-97
programs [21, 22]. Crystal data collection and refinement details for the
ligand HL4, the palladium (II) complex Pd(L2)2,
and the platinum (II) tetranuclear complex Pt4(L4)4 are summarized in
Table 1.Table 1
Crystal
data and
refinement summary.
HL4Pd(L2)2⋅H2OPt4(L4)4⋅2C2H5OHEmpirical
formulaC14H13N3SC18H16N8OPdS2C60H56N12O2Pt4S4Formula weight (g/mol)255.33530.911885.77Crystal habit, colorYellow
platesOrange prismsRed prismsCrystal  systemTriclinicMonoclinicMonoclinicSpace groupP-1C2/cP21a (Å)5.988(1)20.016(4)12.708(1)b (Å)10.285(2)6.421(1)13.639(1)c (Å)11.410(2)18.511(4)17.031(1)α (°)68.040(2)β (°)82.514(3)120.78(3)94.054(1)γ (°)86.886(2)Volume (Å3)646.1(2)2044.0(7)2944.5(3)Z; F(000)2; 2684; 10642;
1784Densitycalc (g/cm3)1.3131.7252.127Crystal
size
(mm)0.44× 0.20 × 0.040.43× 0.39 × 0.070.16× 0.16 × 0.05μ(MoKα)(mm−1)0.2351.1419.6692θ range (°)3.8–50.06.8–56.23.8–59.0Temperature
(K)220213213Measured reflections3388947819123Rint0.02900.06350.0224Unique reflections2252246213693Observed reflections(I>2σ(I))1939201212253Refined parameters215170713wR2 (unique reflections)0.16930.09290.0774R1 (observed reflections)0.06180.03310.0338Largest difference peak  and hole (e/ Å3)0.38/−0.311.26/−0.881.71/−1.14


### 2.5. Biological Activity

#### 2.5.1. Cell Culture

The antitumor assays were
performed employing the following cell lines: H460 (human lung large cell
carcinoma), ME180 (human cervix epidermoid carcinoma), M-14 (human amelanotic
melanoma), DU145 (human prostate carcinoma), MCF-7 (human breast
adenocarcinoma), HT-29 (human colon adenocarcinoma), PC3 (human prostate
carcinoma), and K562 (human chronic myelogenous leukemia). Cells were maintained
in Dulbecco's Modified Eagle’s Medium (DMEM) supplemented with 10% fetal calf serum and 50μg/mL gentamycin, and grown at 37°C in a 5% CO2 humidified environment.


#### 2.5.2. Assessment of Cytotoxicity

Cells were inoculated into
96-well tissue culture plates at a density of 3000–5000 cells per
well and incubated at37°C with their corresponding growth medium for 24 hours to allow cells to attach. A
plate containing each of these cells was fixed in situ with trichloroacetic
acid (TCA) in order to obtain the cell values at zero time before adding the test
compounds. The rest of the plates containing the different cell lines received
serial dilutions of the ligands and palladium (II) complexes in DMSO to be
incubated at 37°C for 48 hours. The assay was terminated by the addition of
cold TCA. The cell numbers in each well was determined using the sulforhodamine
B (SRB) assay [23]. TCA-treated plates were incubated at 4°C for 1 hour and
then the cells were washed five times with tap water and dried completely at
room temperature. The cells were stained for 20 minutes with a solution of 0.4% sulforhodamine B in 1% acetic acid. At the end of the staining period,
unbound dye was removed by washing four times with 1% acetic acid until the
washing solution became colorless. After complete drying, bound dye was
solubilized with 10 mM Tris buffer (pH 10.5) and the absorbance reads on an automated plate reader at a wavelength
of 550 nm. The IC50 value was
defined as the concentration of test sample resulting in a 50% reduction of
absorbance as compared with untreated controls that received a serial dilution
of the solvent in which the test samples were dissolved, and was determined by
linear regression analysis.For K562 cells, which grow
in suspension, instead of fixing and staining with SRB, cells were counted
using a Coulter counter.


## 2.1. Materials and Measurements

Chemicals were reagent grade. Palladium (II)
bis(acetylacetonate), ammonium tetrachloroplatinate, thiosemicarbazide,
4-phenyl-thiosemicarbazide, benzaldehyde,m-CN-benzaldehyde,
and o-NO2-benzaldehyde
were purchased from Aldrich. Melting points were determined on a Büchi melting
point B-545 apparatus. Elemental
analyses
were determined on a Fisons-Carlo Erba Elemental Microanalyzer. The infrared
(IR) spectra were recorded in solid state (KBr pellets) on a Bruker FT-IR IFS
55 Equinox spectrophotometer in the 4000–400 cm−1 range.
The FAB(+) mass  spectra were recorded on a ZAB-HSQ (V.G. Analytical Ltd. Floats Roads, Wythenshawe,
Manchester, England) spectrometer, using 3-nitrobenzyl alcohol as the matrix.
The H1 (300 MHz)
and C13 (75.5 MHz) NMR spectra were recorded on a Bruker Advance DRX 300 spectrometer
at 300 K, using DMSO-d6 as solvent. The chemical
shifts (δ) in ppm were
measured relative to tetramethylsilane (TMS).


## 2.2. Synthesis of the Ligands

The
thiosemicarbazone derivatives (HL) were prepared according to the literature
[20] as shown in Scheme 1.Scheme 1
Synthesis of the benzaldehyde thiosemicarbazone and 4-phenyl-1-benzaldehyde
thiosemicarbazone ligands.General Method
To a hot solution of thiosemicarbazide (1.82 g, 20 mmol) or
4-phenylthiosemicarbazide (3.3 g, 20 mmol) in 160 mL, methanol was added dropwise a solution of the
corresponding benzaldehyde (20 mmol) in 70 mL methanol during 30 minutes. The
mixture was stirred and refluxed for 4 hours, it was filtered and the filtrate
was concentrated to half the volume under reduced pressure. After a slow evaporation of the
concentrate at room temperature, crystals were collected by filtration, washed
with cold ethanol, and dried in vacuo. For ligand HL4, the filtrate
was kept in the refrigerator and after several hours small rectangular crystals
were obtained. These crystals were suitable for structure analysis by
X-ray diffraction.


### 2.2.1. Benzaldehyde Thiosemicarbazone (HL1)

Colorless
crystals. Yield 80%, m.p. 167–169°C. Anal. Calc. For C8H9N3S
(179,2 g/mol): C, 53.6%; H, 5.1%; N, 23.4%; S, 17.9%. Found: C, 53.5%; H, 5.3%;
N, 23.5%; S, 17.7%. FAB(+)-MS:
m/z 179  (M+, 70%); IR (KBr,
cm−1): ν(NH2) 3400, 3380; ν(NH) 3250; ν(C=N) 1600;
ν(C=S) 885.H1-NMR (DMSO-d6): δ  7.78 (d,
2Hortho, Ph, J=6.8 Hz),
7.39 (t, 2Hmeta, Ph,
J= 7.2 Hz), 7.40 (t, 1Hpara,
Ph, J= 7.2 Hz);  8.05 (s, 1H, HC=N);  8.19, 7.98 (d, 2H, NH2); 11.42 (s,
1H, =N–NH). C13-NMR (DMSO-d6): δ  128.65, 127.29, 129.83, 134.18 (Ph); 142.28
(HC=N);  178.0 (C=S).


### 2.2.2. m-Cyanobenzaldehyde Thiosemicarbazone (HL2)

Colorless crystals. Yield
76%, m.p. 203-204°C.
Anal. Calc. For C9H8N4S (204,3 g/mol): C,
52.9%; H, 3.9%; N, 27.4%; S, 15.7%. Found: C, 52.8%; H, 3.7%; N,  27.6%; S, 15.5%.; FAB(+)-MS: m/z 205 (MH+, 100%); IR (KBr, cm−1): ν(NH2) 3410, 3397; ν(NH)  3236
ν(CN) 2233; ν(C=N)  1596;
ν(C=S)  880. H1-NMR (DMSO-d6): δ  7.81, 7.79 (s, d, 2Hortho, Ph, J =5.9 Hz); 7.58 (t, 1Hmeta, Ph, J = 7.7 Hz); 7.83
(t, 1Hpara, Ph,  J = 5.7 Hz);  8.03 (s, 1H, HC=N);  8.31, 8.26 (d, 2H, NH2); 11.60 (s,
1H, =N–NH).C13-NMR (DMSO-d6): δ  129.87, 118.6, 135.69, 132.32, 132.68 (Ph);
111.97 (CN); 139.66 (HC=N);  178.35
(C=S).135.69, 130.15.


### 2.2.3. o-Nitrobenzaldehyde Thiosemicarbazone (HL3)

Yellow crystals. Yield 90%, m.p. 214-215°C. Anal. Calc. For C8H8O2N4S
(224.3 g/mol): C, 42.9%; H, 3.6%; N, 24.9%; S, 14.3%. Found: C, 42.6%; H, 3.5%;
N,  24.6%; S, 14.1%. FAB(+)-MS:  m/z 226
(MH+, 100%); IR (KBr, cm−1): ν(NH2)
3421, 3370; ν(NH) 3240; ν(C=N) 1602; ν(C=S)
890. H1-NMR (DMSO-d6): δ  7.58 (d,  1Hortho, Ph, J = 6.9 Hz); 8.02, 7.73 (d,d, 2Hmeta, Ph, J = 7.8 Hz); 7.61
(t, 1Hpara, Ph,  J =6.7 Hz);  8.12 (s, 1H, HC=N);  8.45, 8.41 (d, 2H, NH2); 11.73 (s,
1H, =N–NH).C13-NMR (DMSO-d6): δ 124.54, 137.24, 130.36,
133.36, 128.46, 128.33 (Ph); 148.29 (HC=N);
178.49 (C=S).


### 2.2.4. 4-Phenyl-1-Benzaldehyde Thiosemicarbazone (HL4)

Yellow crystals. Yield 75%, m.p. 192–194°C. Anal. Calc. For C14H13N3S
(255.3 g/mol): C, 65.9%; H, 5.1%; N, 16.5%; S, 12.5%. Found: C, 65.4%; H, 5.3%;
N, 16.7%; S, 12.6%. FAB(+)-MS:
m/z 255  (M+, 48%); IR (KBr,
cm−1): ν(NH) 3245; ν(C=N) 1625; ν(C=S) 915. H1-NMR
(DMSO-d6): δ  7.42 (t, 1Hpara, Ph,  J = 6.8 Hz); 7.43
(t, 2Hmeta, Ph, J = 6.7 Hz);
7.91 (d, 2Hortho, Ph,  J = 5.3 Hz); 7.21 (t, 1Hpara, NHPh,
J = 6.2 Hz); 7.37 (t, 2Hmeta,
NHPh,  J = 6.4 Hz); 7.58 (d, 2Hortho,
NHPh, J = 5.6 Hz);  8.17 (s, 1H,
HC=N);  10.11 (s, 1H, NHPh); 11.83 (s,
1H, =N–NH). C13-NMR (DMSO-d6): δ  127.6, 128.6, 130.0, 134.0 (Ph); 125.3,
125.9, 128.0, 139.1 (NH-Ph); 142.9 (HC=N);
176.0 (C=S).


## 2.2.1. Benzaldehyde Thiosemicarbazone (HL1)

Colorless
crystals. Yield 80%, m.p. 167–169°C. Anal. Calc. For C8H9N3S
(179,2 g/mol): C, 53.6%; H, 5.1%; N, 23.4%; S, 17.9%. Found: C, 53.5%; H, 5.3%;
N, 23.5%; S, 17.7%. FAB(+)-MS:
m/z 179  (M+, 70%); IR (KBr,
cm−1): ν(NH2) 3400, 3380; ν(NH) 3250; ν(C=N) 1600;
ν(C=S) 885.H1-NMR (DMSO-d6): δ  7.78 (d,
2Hortho, Ph, J=6.8 Hz),
7.39 (t, 2Hmeta, Ph,
J= 7.2 Hz), 7.40 (t, 1Hpara,
Ph, J= 7.2 Hz);  8.05 (s, 1H, HC=N);  8.19, 7.98 (d, 2H, NH2); 11.42 (s,
1H, =N–NH). C13-NMR (DMSO-d6): δ  128.65, 127.29, 129.83, 134.18 (Ph); 142.28
(HC=N);  178.0 (C=S).


## 2.2.2. m-Cyanobenzaldehyde Thiosemicarbazone (HL2)

Colorless crystals. Yield
76%, m.p. 203-204°C.
Anal. Calc. For C9H8N4S (204,3 g/mol): C,
52.9%; H, 3.9%; N, 27.4%; S, 15.7%. Found: C, 52.8%; H, 3.7%; N,  27.6%; S, 15.5%.; FAB(+)-MS: m/z 205 (MH+, 100%); IR (KBr, cm−1): ν(NH2) 3410, 3397; ν(NH)  3236
ν(CN) 2233; ν(C=N)  1596;
ν(C=S)  880. H1-NMR (DMSO-d6): δ  7.81, 7.79 (s, d, 2Hortho, Ph, J =5.9 Hz); 7.58 (t, 1Hmeta, Ph, J = 7.7 Hz); 7.83
(t, 1Hpara, Ph,  J = 5.7 Hz);  8.03 (s, 1H, HC=N);  8.31, 8.26 (d, 2H, NH2); 11.60 (s,
1H, =N–NH).C13-NMR (DMSO-d6): δ  129.87, 118.6, 135.69, 132.32, 132.68 (Ph);
111.97 (CN); 139.66 (HC=N);  178.35
(C=S).135.69, 130.15.


## 2.2.3. o-Nitrobenzaldehyde Thiosemicarbazone (HL3)

Yellow crystals. Yield 90%, m.p. 214-215°C. Anal. Calc. For C8H8O2N4S
(224.3 g/mol): C, 42.9%; H, 3.6%; N, 24.9%; S, 14.3%. Found: C, 42.6%; H, 3.5%;
N,  24.6%; S, 14.1%. FAB(+)-MS:  m/z 226
(MH+, 100%); IR (KBr, cm−1): ν(NH2)
3421, 3370; ν(NH) 3240; ν(C=N) 1602; ν(C=S)
890. H1-NMR (DMSO-d6): δ  7.58 (d,  1Hortho, Ph, J = 6.9 Hz); 8.02, 7.73 (d,d, 2Hmeta, Ph, J = 7.8 Hz); 7.61
(t, 1Hpara, Ph,  J =6.7 Hz);  8.12 (s, 1H, HC=N);  8.45, 8.41 (d, 2H, NH2); 11.73 (s,
1H, =N–NH).C13-NMR (DMSO-d6): δ 124.54, 137.24, 130.36,
133.36, 128.46, 128.33 (Ph); 148.29 (HC=N);
178.49 (C=S).


## 2.2.4. 4-Phenyl-1-Benzaldehyde Thiosemicarbazone (HL4)

Yellow crystals. Yield 75%, m.p. 192–194°C. Anal. Calc. For C14H13N3S
(255.3 g/mol): C, 65.9%; H, 5.1%; N, 16.5%; S, 12.5%. Found: C, 65.4%; H, 5.3%;
N, 16.7%; S, 12.6%. FAB(+)-MS:
m/z 255  (M+, 48%); IR (KBr,
cm−1): ν(NH) 3245; ν(C=N) 1625; ν(C=S) 915. H1-NMR
(DMSO-d6): δ  7.42 (t, 1Hpara, Ph,  J = 6.8 Hz); 7.43
(t, 2Hmeta, Ph, J = 6.7 Hz);
7.91 (d, 2Hortho, Ph,  J = 5.3 Hz); 7.21 (t, 1Hpara, NHPh,
J = 6.2 Hz); 7.37 (t, 2Hmeta,
NHPh,  J = 6.4 Hz); 7.58 (d, 2Hortho,
NHPh, J = 5.6 Hz);  8.17 (s, 1H,
HC=N);  10.11 (s, 1H, NHPh); 11.83 (s,
1H, =N–NH). C13-NMR (DMSO-d6): δ  127.6, 128.6, 130.0, 134.0 (Ph); 125.3,
125.9, 128.0, 139.1 (NH-Ph); 142.9 (HC=N);
176.0 (C=S).


## 2.3. Synthesis of the Palladium (II) and Platinum (II) Complexes (see Scheme2)

A
solution of Pd(acac)2 (0.30 g, 1.0 mmol) in CH2Cl2/CH3OH
(30 mL, 2:1 v/v)  or a solution of (NH4)2PtCl4  (0.1865 g, 0.5 mmol) in water/ethanol (2:1, 15 mL) was added dropwise to a stirred solution of the corresponding
thiosemicarbazone (2.0 mmol) in 60 mL of
methanol. Sodium acetate (0.16 g, 2 mmol) in 3 mL of water was then added.  The solution was refluxed for 2 hours and
stirred for 24 hours at room temperature. The precipitate was collected by
filtration and dried in vacuo.Scheme 2
Synthesis of the palladium (II) bis-chelate complexes and the
platinum (II) tetranuclear complex.


### 2.3.1. Palladium (II) Complex of Benzaldehyde Thiosemicarbazone, Pd(L1)2

Yellow solid. Yield 70%, m.p. 204-205°C. Anal. Calc. For C16H16N6S2Pd
(462.9 g/mol): C, 41.5%; H, 3.5%; N, 18.2%; S, 13.9%. Found: C, 40.9%; H, 3.6%; N, 18.6%; S,
13.5%. FAB(+)-MS: m/z  463 (M+,
60%); IR (KBr, cm−1): ν(NH2) 3390, 3367;  ν(C=N) 1582; ν(C=S) 805. H1-NMR
(DMSO-d6): δ  7.49, 7.45,
7.41, 7.39 (m, Ph);  8.13 (s, 2H,
HC=N);  8.29, 8.21 (d, 4H, NH2).


### 2.3.2. Palladium (II) Complex of m-Cyanobenzaldehyde Thiosemicarbazone, Pd(L2)2

Crystals
suitable for X-ray structure determination were obtained by slowly evaporating
a methanol/dichloromethane (2:1) solution at room temperature.Orange
crystals. Yield 63%, m.p. >240°C (decomp.). Anal. Calc. for C18H14N8S2Pd·H2O
(530.9 g/mol): C, 40.7%; H, 3.0%; N, 21.1%; S, 12.1%;  Found: C, 42.0%; H, 2.7%; N, 21.6%; S, 12.3%.
FAB(+)-MS: m/z  513 (M+-H2O,
100%); IR (KBr, cm−1): ν(NH2) 3405, 3377;  ν(CN)
2230;  ν(C=N) 1570; ν(C=S) 815. H1-NMR
(DMSO-d6):
δ  7.75, 7.65, 7.55 (m, Ph);  8.06 (s, 2H, HC=N);  8.68 (s, 2H, NH2).


### 2.3.3. Palladium (II) Complex of o-Nitrobenzaldehyde Thiosemicarbazone, Pd(L3)2

Yellow solid.
Yield 61%, m.p.   >260°C (decomp.). Anal. Calc. for
C16H14O4N8S2Pd (552.9 g/mol): C, 34.8%; H, 2.6%; N, 20.3%; S, 11.6%. Found: C,  34.6%; H, 2.5%; N,
20.6%; S, 11.3%.
FAB(+)-MS: m/z  553 (M+, 74%); IR (KBr, cm−1): ν(NH2) 3400, 3350; ν(C=N) 1585;
ν(C=S) 820. H1-NMR (DMSO-d6): δ  7.85, 7.80, 7.70 (m, Ph);  8.15 (d, 2H, HC=N);  8.44 (s, 2H, NH2).


### 2.3.4. Platinum (II) Tetranuclear Complex, Pt4(L4)4

Crystals suitable
for structure determination by X-ray diffraction were obtained by slowly
evaporating an ethanol/chloroform (2:1) solution at room temperature.Red crystals.
Yield 57%, m.p. 188-189°C. Anal. Calc. For C56H44N12S4Pt4·2C2H5OH (1885.8 g/mol): C, 38.2%; H, 3.0%; N, 8.9%; S,
6.8%. Found: C, 38.0%; H, 3.0%; N, 8.6%; S, 6.5%; IR (KBr, cm−1):  ν(NHPh) 3200; ν(C=N) 1590; ν(C=S) 840. H1-NMR (DMSO-d6):  δ7.50–7.80 (m,
Ph); 7.0–7.4 (m, NHPh);  8.0 (d, 4H,
HC=N);  9.15, 9.68 (d, 4H, NHPh).


## 2.3.1. Palladium (II) Complex of Benzaldehyde Thiosemicarbazone, Pd(L1)2

Yellow solid. Yield 70%, m.p. 204-205°C. Anal. Calc. For C16H16N6S2Pd
(462.9 g/mol): C, 41.5%; H, 3.5%; N, 18.2%; S, 13.9%. Found: C, 40.9%; H, 3.6%; N, 18.6%; S,
13.5%. FAB(+)-MS: m/z  463 (M+,
60%); IR (KBr, cm−1): ν(NH2) 3390, 3367;  ν(C=N) 1582; ν(C=S) 805. H1-NMR
(DMSO-d6): δ  7.49, 7.45,
7.41, 7.39 (m, Ph);  8.13 (s, 2H,
HC=N);  8.29, 8.21 (d, 4H, NH2).


## 2.3.2. Palladium (II) Complex of m-Cyanobenzaldehyde Thiosemicarbazone, Pd(L2)2

Crystals
suitable for X-ray structure determination were obtained by slowly evaporating
a methanol/dichloromethane (2:1) solution at room temperature.Orange
crystals. Yield 63%, m.p. >240°C (decomp.). Anal. Calc. for C18H14N8S2Pd·H2O
(530.9 g/mol): C, 40.7%; H, 3.0%; N, 21.1%; S, 12.1%;  Found: C, 42.0%; H, 2.7%; N, 21.6%; S, 12.3%.
FAB(+)-MS: m/z  513 (M+-H2O,
100%); IR (KBr, cm−1): ν(NH2) 3405, 3377;  ν(CN)
2230;  ν(C=N) 1570; ν(C=S) 815. H1-NMR
(DMSO-d6):
δ  7.75, 7.65, 7.55 (m, Ph);  8.06 (s, 2H, HC=N);  8.68 (s, 2H, NH2).


## 2.3.3. Palladium (II) Complex of o-Nitrobenzaldehyde Thiosemicarbazone, Pd(L3)2

Yellow solid.
Yield 61%, m.p.   >260°C (decomp.). Anal. Calc. for
C16H14O4N8S2Pd (552.9 g/mol): C, 34.8%; H, 2.6%; N, 20.3%; S, 11.6%. Found: C,  34.6%; H, 2.5%; N,
20.6%; S, 11.3%.
FAB(+)-MS: m/z  553 (M+, 74%); IR (KBr, cm−1): ν(NH2) 3400, 3350; ν(C=N) 1585;
ν(C=S) 820. H1-NMR (DMSO-d6): δ  7.85, 7.80, 7.70 (m, Ph);  8.15 (d, 2H, HC=N);  8.44 (s, 2H, NH2).


## 2.3.4. Platinum (II) Tetranuclear Complex, Pt4(L4)4

Crystals suitable
for structure determination by X-ray diffraction were obtained by slowly
evaporating an ethanol/chloroform (2:1) solution at room temperature.Red crystals.
Yield 57%, m.p. 188-189°C. Anal. Calc. For C56H44N12S4Pt4·2C2H5OH (1885.8 g/mol): C, 38.2%; H, 3.0%; N, 8.9%; S,
6.8%. Found: C, 38.0%; H, 3.0%; N, 8.6%; S, 6.5%; IR (KBr, cm−1):  ν(NHPh) 3200; ν(C=N) 1590; ν(C=S) 840. H1-NMR (DMSO-d6):  δ7.50–7.80 (m,
Ph); 7.0–7.4 (m, NHPh);  8.0 (d, 4H,
HC=N);  9.15, 9.68 (d, 4H, NHPh).


## 2.4. Crystal Structure Determinations

The data for the
crystal structure determinations were collected on a Siemens CCD smart diffractometer
(HL4, Pt4(L4)4) and a Stoe IPDS2 diffractometer
(Pd(L2)2) (MoKα  radiation, λ = 0.71073 Å, graphite monochromator).
The intensities were corrected for Lorentz and polarization effects and for
absorption using SADABS (Pt4(L4)4) and
X-RED/X-SHAPE (Pd(L2)2). The structures were solved by
direct methods, which revealed the positions of all nonhydrogen atoms and
refined on F2 by a full
matrix least-squares procedure using anisotropic displacement parameters with the
exception of the solvent molecules in Pt4(L4)4 which were refined isotropically. The hydrogen atoms for HL4 and
Pd(L2)2 were
located from different
Fourier syntheses and refined isotropically. For Pt4(L4)4,
the hydrogen atoms were included in calculated positions and refined in riding
mode. All calculations were carried out using the SHELXS-97 and SHELXL-97
programs [21, 22]. Crystal data collection and refinement details for the
ligand HL4, the palladium (II) complex Pd(L2)2,
and the platinum (II) tetranuclear complex Pt4(L4)4 are summarized in
Table 1.Table 1
Crystal
data and
refinement summary.
HL4Pd(L2)2⋅H2OPt4(L4)4⋅2C2H5OHEmpirical
formulaC14H13N3SC18H16N8OPdS2C60H56N12O2Pt4S4Formula weight (g/mol)255.33530.911885.77Crystal habit, colorYellow
platesOrange prismsRed prismsCrystal  systemTriclinicMonoclinicMonoclinicSpace groupP-1C2/cP21a (Å)5.988(1)20.016(4)12.708(1)b (Å)10.285(2)6.421(1)13.639(1)c (Å)11.410(2)18.511(4)17.031(1)α (°)68.040(2)β (°)82.514(3)120.78(3)94.054(1)γ (°)86.886(2)Volume (Å3)646.1(2)2044.0(7)2944.5(3)Z; F(000)2; 2684; 10642;
1784Densitycalc (g/cm3)1.3131.7252.127Crystal
size
(mm)0.44× 0.20 × 0.040.43× 0.39 × 0.070.16× 0.16 × 0.05μ(MoKα)(mm−1)0.2351.1419.6692θ range (°)3.8–50.06.8–56.23.8–59.0Temperature
(K)220213213Measured reflections3388947819123Rint0.02900.06350.0224Unique reflections2252246213693Observed reflections(I>2σ(I))1939201212253Refined parameters215170713wR2 (unique reflections)0.16930.09290.0774R1 (observed reflections)0.06180.03310.0338Largest difference peak  and hole (e/ Å3)0.38/−0.311.26/−0.881.71/−1.14


## 2.5. Biological Activity

### 2.5.1. Cell Culture

The antitumor assays were
performed employing the following cell lines: H460 (human lung large cell
carcinoma), ME180 (human cervix epidermoid carcinoma), M-14 (human amelanotic
melanoma), DU145 (human prostate carcinoma), MCF-7 (human breast
adenocarcinoma), HT-29 (human colon adenocarcinoma), PC3 (human prostate
carcinoma), and K562 (human chronic myelogenous leukemia). Cells were maintained
in Dulbecco's Modified Eagle’s Medium (DMEM) supplemented with 10% fetal calf serum and 50μg/mL gentamycin, and grown at 37°C in a 5% CO2 humidified environment.


### 2.5.2. Assessment of Cytotoxicity

Cells were inoculated into
96-well tissue culture plates at a density of 3000–5000 cells per
well and incubated at37°C with their corresponding growth medium for 24 hours to allow cells to attach. A
plate containing each of these cells was fixed in situ with trichloroacetic
acid (TCA) in order to obtain the cell values at zero time before adding the test
compounds. The rest of the plates containing the different cell lines received
serial dilutions of the ligands and palladium (II) complexes in DMSO to be
incubated at 37°C for 48 hours. The assay was terminated by the addition of
cold TCA. The cell numbers in each well was determined using the sulforhodamine
B (SRB) assay [23]. TCA-treated plates were incubated at 4°C for 1 hour and
then the cells were washed five times with tap water and dried completely at
room temperature. The cells were stained for 20 minutes with a solution of 0.4% sulforhodamine B in 1% acetic acid. At the end of the staining period,
unbound dye was removed by washing four times with 1% acetic acid until the
washing solution became colorless. After complete drying, bound dye was
solubilized with 10 mM Tris buffer (pH 10.5) and the absorbance reads on an automated plate reader at a wavelength
of 550 nm. The IC50 value was
defined as the concentration of test sample resulting in a 50% reduction of
absorbance as compared with untreated controls that received a serial dilution
of the solvent in which the test samples were dissolved, and was determined by
linear regression analysis.For K562 cells, which grow
in suspension, instead of fixing and staining with SRB, cells were counted
using a Coulter counter.


## 2.5.1. Cell Culture

The antitumor assays were
performed employing the following cell lines: H460 (human lung large cell
carcinoma), ME180 (human cervix epidermoid carcinoma), M-14 (human amelanotic
melanoma), DU145 (human prostate carcinoma), MCF-7 (human breast
adenocarcinoma), HT-29 (human colon adenocarcinoma), PC3 (human prostate
carcinoma), and K562 (human chronic myelogenous leukemia). Cells were maintained
in Dulbecco's Modified Eagle’s Medium (DMEM) supplemented with 10% fetal calf serum and 50μg/mL gentamycin, and grown at 37°C in a 5% CO2 humidified environment.


## 2.5.2. Assessment of Cytotoxicity

Cells were inoculated into
96-well tissue culture plates at a density of 3000–5000 cells per
well and incubated at37°C with their corresponding growth medium for 24 hours to allow cells to attach. A
plate containing each of these cells was fixed in situ with trichloroacetic
acid (TCA) in order to obtain the cell values at zero time before adding the test
compounds. The rest of the plates containing the different cell lines received
serial dilutions of the ligands and palladium (II) complexes in DMSO to be
incubated at 37°C for 48 hours. The assay was terminated by the addition of
cold TCA. The cell numbers in each well was determined using the sulforhodamine
B (SRB) assay [23]. TCA-treated plates were incubated at 4°C for 1 hour and
then the cells were washed five times with tap water and dried completely at
room temperature. The cells were stained for 20 minutes with a solution of 0.4% sulforhodamine B in 1% acetic acid. At the end of the staining period,
unbound dye was removed by washing four times with 1% acetic acid until the
washing solution became colorless. After complete drying, bound dye was
solubilized with 10 mM Tris buffer (pH 10.5) and the absorbance reads on an automated plate reader at a wavelength
of 550 nm. The IC50 value was
defined as the concentration of test sample resulting in a 50% reduction of
absorbance as compared with untreated controls that received a serial dilution
of the solvent in which the test samples were dissolved, and was determined by
linear regression analysis.For K562 cells, which grow
in suspension, instead of fixing and staining with SRB, cells were counted
using a Coulter counter.


## 3. Results and Discussion

### 3.1. IR Spectra of the Ligands and Their Complexes

The
infrared absorption bands
become very useful for determining the mode of coordination of the
ligands to metal. In the IR spectra, the broad bands of the –NH group observed at 3236–3250cm−1   for the ligands disappear in the complexes spectra, which indicates the
deprotonation of the NH-CS group. The strong bands observed at 1596–1625 cm−1  range
in the free ligands have been assigned to ν(C=N) stretching vibrations [24]. On
complexation, these bands were observed to be shifted to lower frequencies (1570–1590 cm−1),
which are in agreement with the wave numbers for other bischelate complexes [6, 25, 26]. These results indicate that the imine nitrogen is coordinated to the
metal ion. All ligands showed medium bands in the 880–915 cm−1  range ascribed to ν(C=S) vibrations. These absorption bands shift 65–80 cm−1  to lower frequencies on the coordination of the thiocarbonyl sulfur to
palladium (II) or platinum (II) ion. These results are in agreement with other
thiosemicarbazone complexes [24, 27]. In addition, the vibrational frequencies
of the –NH2  groups remain unchanged for both the ligands and the
complexes. This evidence indicates the noncoordination of the –NH2 group to the Pd(II) center.


### 3.2. NMR Spectra of the Ligands and Their Complexes

In theH1-NMR spectra of the ligands, the signals of the =N–NH protons were
observed as singlets at δ 11.42–11.83. These
signals disappeared in the H1-NMR spectra of the palladium (II) and
platinum (II) complexes indicating the deprotonation of the =N–NH group. The
signals of the HC=N protons which appear as singlets at δ 8.03–8.17 in the ligands
show a shift to downfield  in δ 0.03–0.80 after
complexation. This shift indicates the coordination of the imine nitrogen to
the metal center [28]. The signals of the aromatic protons of the ligands
appeared at δ 7.21–7.91, and the
resonance lines found correspond to the calculated multiplicity. These signals
do not suffer relevant changes in the chemical shifts for the palladium (II)
and platinum (II) complexes. The NH2 signal in the ligands HL1,
HL2, and HL3appears as doublets at δ 7.98–8.45 due to the
nonequivalence of the amine protons. This evidence is attributed to the
restricted rotation around C–N bond (thiocarbonyl carbon and terminal amine
nitrogen) due to its partial double bond character [14, 29]. The
presence of the phenyl group on the terminal amine (NHPh) of the ligand HL4 produces a downfield chemical shift at δ 2.1 with respect
to the NH2 group of the ligand HL1. This reveals that HL4 is slightly less basic than HL1. The resonance signals of the –NH2 or NHPh groups in the palladium (II) and platinum (II) complexes do not change,
and this evidence indicates that the amine groups are not coordinated to the
metal ion [14].In theC13-NMR spectra, the carbon resonance signals of the HC=N group
appeared at δ 139.66–148.3. The
results are similar to the chemical shifts found for the ligands benzophenone
thiosemicarbazide and phenylpropenal thiosemicarbazone (both found at δ 141)
[30, 31]. The C=S signals observed at δ 178.5–176.0 are
characteristic for this group, while the aromatic carbons were observed at δ 139.1–124.5.


### 3.3. Structural Data

The
molecular structures of HL4, Pd(L2)2, and Pt4(L4)4 are shown in
Figures 1, 2, and
3, respectively, whereas their selected bond
lengths and bond angles are presented in
Tables 2 and 3.Table 2
Selected bond lengths (Å) for the HL4 ligand and the Pd(L2)2 and Pt4(L4)4 complexes.
HL4Pd(L2)2Pt4(L4)4S1–C11.686(3)1.746(2)1.812(9)N1–N21.378(4)1.378(2)1.389(10)N1–C81.284(4)1.293(11)N2–C11.354(4)1.296(11)N3–C11.342(4)1.348(3)1.354(11)N3–C21.422(4)1.427(12)C2–C31.472(3)C8–C91.457(4)1.441(13)N1–C11.316(3)N2–C21.306(3)Pd1–S12.297(1)Pd1–N22.049(2)Pt–S (intraligand)2.351(2)Pt–S′(bridging)2.298(2)Pt–N11.991(7)Pt–C102.015(8)Table 3
Selected bond angles (°) for the
HL4 ligand and the Pd(L2)2 and Pt4(L4)4 complexes.
HL4Pd(L2)2Pt4(L4)4N2–N1–C8115.0(3)118.5(7)N1–N2–C1119.8(3)113.4(7)C1–N3–C2127.7(3)127.6(8)S1–C1–N2118.7(2)124.8(7)S1–C1–N3126.1(2)117.4(2)114.4(7)N2–C1–N3115.2(3)120.7(8)N1–C8–C9121.8(3)115.0(8)N2–N1–C1113.5(2)N1–N2–C2116.5(2)S1–C1–N1125.3(2)N1–C1–N3117.3(2)N2–C2–C3131.6(2)S1–Pd1–N282.8(1)S–Pt–N183.1(2)N1–Pt–C1080.9(3)C10–Pt-S′94.8(3)S–Pt–S′101.5(1)Pt–S1–S193.6(3)Pt–N1–N2124.2(5)Pt–N1–C8117.2(6)Figure 1
Molecular structure of HL4.Figure 2
Molecular structure of Pd(L2)2.Figure 3
Molecular structure of Pt4(L4)4.


### 3.3.1. 4-Phenyl-1-Benzaldehyde Thiosemicarbazone HL4

The reaction
product of benzaldehyde and 4-phenyl thiosemicarbazide shows the expected bond
lengths, especially the N1–C8 double bond with a length of 1.284(4) Å (see
Figure1). The molecular fragment N3-C1(S1)-N2-N1-C8 is nearly planar. The C9–C14 phenyl
ring deviates only slightly from this mean plane and forms an angle of 56.4(1)° with the C2–C7 phenyl ring.There
are two hydrogen bonds: an intramolecular N3–H⋯ N1 [N3–H 0.85 Å, H ⋯ N1 2.17 Å,
N3 ⋯ N1 2.615 Å, N3–H ⋯ N1 112°]
hydrogen bond and an intermolecular
N2–H ⋯ S1 [N2–H 0.88 Å, H ⋯ S1 2.62 Å, N2 ⋯ S1 3.466 Å, N2–H ⋯ S1 162°]  hydrogen bond. The latter leads to the formation of pairs of molecules in the
crystal structure.


### 3.3.2. Bis(3-Cyanophenyl-1-Benzaldehyde Thiosemicarbazonato)palladium (II) Pd(L2)2

3-cyanophenyl-1-benzaldehyde
thiosemicarbazone reacts with palladium (II) acetylacetonate to form a
bis-chelate withCi  molecular symmetry (see
Figure 2). The
deprotonated ligand coordinates bidentately through S and N. The coordination
of the Pd atom is square planar with a transarrangement of the coordinating
atoms. It leads to a lengthening of the S1–C1 bond and a shortening of the N1–C1
bond. The Pd1S1N1N2C1 chelate ring is nearly planar and forms an angle of
11.5(1)° with the C3–C8 phenyl ring.In the
crystal structure, one molecule of water per formula unit of the chelate is
included.There
are three hydrogen bonds comprising the atom O1 of the water molecule and the
atom N4 of the cyano group: O1–H⋯ S1 [O1–H 0.79 Å, H ⋯ S1 2.65 Å, O1 ⋯ S1 3.418 Å,
O1–H ⋯ S1 166°], N3–H ⋯ O1 [N3–H 0.87 Å, H ⋯ O1 2.21 Å, N3 ⋯ O1 3.081 Å,
N3–H ⋯ O1 173°]  and N3–H′ ⋯ N4 [N3–H′ 0.85 Å, H′ ⋯ N4 2.22 Å, N3 ⋯ N4 3.054 Å, N3–H′ ⋯ N4 167°].


### 3.3.3. Tetrakis (4-Phenyl-1-Benzaldehyde Thiosemicarbazonato)tetraplatinum (II) Pt4(L4)4

4-phenyl-1-benzaldehyde
thiosemicarbazone reacts with ammonium tetrachloroplatinate (II) to form a
tetranuclear complex with slightly distorted square planar geometry (see
Figure3). The tridentate ligands are deprotonated at the NH group and
coordinated through S, N,
and C (aromatic ring). The fourth coordination site at each Pt atom is occupied
by a sulfur atom of a neighboring ligand. In this way, a puckered
eight-membered ring of alternating Pt and S atoms is formed as the core of the
molecule. Each of the four Pt atoms belongs to two fused five-membered chelate
rings: the C, N metallocycle and the N, S chelate moiety.The
Pt-Pt distances range from 3.43 Å to 3.84 Å. The Pt–S bonds form two distinct
groups with significantly differing lengths: Pt-Schelating 2.351(2) Å and Pt-Sbridging 2.298(2) Å (mean values). The coordination of the
ligand to the Pt atoms leads to a lengthening of the S–C1 bond (increased
single-bond character) and a shortening of the neighboring N2–C1 bond
(increased double-bond character) compared with the free ligand.In the
crystal structure, two molecules of ethanol per formula unit of the
tetranuclear complex are included, which stabilize the crystal structure by
hydrogen bonds. The O1 and O2 atoms of the ethanol molecules are bonded by
hydrogen bonds to N3 atoms: N3–H⋯ O1 [N3–H 0.86 Å, H ⋯ O1 2.07 Å, N3 ⋯ O1 2.898 Å,
N3–H ⋯ O1 162°]  and N3–H′ ⋯ O2 [N3–H′ 0.86 Å, H′ ⋯ O2 2.14 Å, N3 ⋯ O2 2.999 Å, N3–H′ ⋯ O2 173°].The formation of tetranuclear compounds was previously
observed for Pd complexes with similar thiosemicarbazone ligands [14].


### 3.4. Antitumor Evaluation

All ligands had a
50% inhibitory concentration (IC50)  >  40 μM against the used human tumor cell
lines. As shown in Table 4, the palladium (II) and platinum (II) complexes were
more cytotoxic (IC50 = 0.08–12.46 μM) than
their respective ligands. These results reveal that the cytotoxic activity
increases dramatically when ligands are coordinated to the metal ion [18, 19, 32].Table 4
IC50(μM)
values of the palladium complexes, Pd(1–3)2, and the
platinum tetranuclear complex, Pt4(L4)4,
against  the different human tumor cell
lines.
Human tumor
cell linesPd(L1)2Pd(L2)2Pd(L3)2Pt4(L4)4Lung large cell carcinoma (H460)8.733.535.820.09Cervix
epidermoid carcinoma (ME180)10.473.678.060.12Prostate
carcinoma (DU145)11.591.999.300.10Breast
adenocarcinoma (MCF-7)8.182.096.140.08Amelanotic
melanoma (M-14)12.461.878.740.11Colon
adenocarcinoma (HT-29)7.451.355.960.07Prostate
carcinoma (PC-3)10.201.567.360.08Chronic myelogenous leukemia (K562)6.150.453.040.08Cl50(a) corresponds to the concentration
required to inhibit a 50% of the cell growth when the cells are exposed to the
compounds during 48 hours. Each value is the average of two independent
experiments.The Pd(L2)2 complex  with a cyano group in the metaposition of the aromatic ring and
the Pt4(L4)4 tetranuclear complex with the
phenyl group in the terminal amine of the ligand showed to be more cytotoxic (IC50 =
0.45–3.67 and 0.07–0.12 μM, resp.)
than the other Pd(L1)2 and Pd(L3)2 complexes
against all tested human tumor cell lines. These results indicate that the
cytotoxic activity is enhanced when four ligands are coordinated to four
platinum atoms.Probably, the high cytotoxicity of the Pd(L2)2 and Pt4(L4)4 complexes may be related to the
intercalation of each metal complex between nitrogen bases of the DNA tumor
cells, causing greater conformational changes in the double helix of DNA and
then producing cell death [33, 34].On the other
hand, the Pt4(L4)4 tetranuclear complex was
more cytotoxic (IC50 = 0.08 μM)
compared with the cytotoxic activity shown by cisplatin (IC50 =
7.0 μM) assayed in (HL60) human leukemia cells [12]. With respect to the
cytotoxicity shown by the square planar copper (II) complexes with
carboxamidrazone ligands (IC50 =  3.0 μM), assayed in vitro against to the (MCF-7) human
breast adenocarcinoma cell line [35], the Pt4(L4)4 complex
resulted to be  more cytotoxic, while the Pd(L2)2 complex showed a similar inhibition concentration (IC50
= 2.09 μM). In relation to other palladium (II) and platinum (II) complexes of the
M(HL)Cl2 type with phenyl acetaldehyde thiosemicarbazone ligands (IC50 = 38
and 9 μM, resp.) assayed in the K562 human chronic
myelogenous leukemia cell line [12], the Pd(L2)2 and Pt4(L4)4 complexes
showed higher cytotoxic activity. In addition, the Pd(L2)2 complex resulted to be more cytotoxic (IC50 = 2.09 μM) than the
palladium (II) and copper (II) complexes of the M(L)Cl type (IC50 = 12.94
and 3.98, resp.) and the NiL2 complex (IC50 = 2.25 μM) with
1,2-naphthoquinone-1-thiosemicarbazone ligands, tested in vitro against to the MCF-7 human breast adenocarcinoma cell
line [34].In summary, we
have prepared the palladium (II) bis-chelate complexes and the platinum (II)
tetranuclear complex, Pd(L1–3)2, and Pt4(L4)4,
which were more cytotoxic on all the human tumor cell lines at low micromolar
concentrations with respect to the free ligands. The crystal structure of Pd(L2)2 shows that the palladium atom has a
square-planar geometry with two bidentate ligands having sulfur and
nitrogen as donor atoms in transpositions.
The complex Pt4(L4)4 has a tetranuclear
structure with four tridentate(C,N,S) ligands coordinated to four platinum
atoms.Supplementary Material
Further details
of the crystal structure determination are available on request from the Cambridge Crystallographic Data Center
(CCDC, 12 Union Road, Cambidge CB2 1EZ, UK;
fax: +44 1223 336033; email: deposit@ccdc.cam.ac.uk),
on quoting the depositing numbers CCDC 694972 for HL4, 694973 for
Pd(L2)2, and 694974 for Pt4(L4)4,
the names of the authors, and the journal citation.


## 3.1. IR Spectra of the Ligands and Their Complexes

The
infrared absorption bands
become very useful for determining the mode of coordination of the
ligands to metal. In the IR spectra, the broad bands of the –NH group observed at 3236–3250cm−1   for the ligands disappear in the complexes spectra, which indicates the
deprotonation of the NH-CS group. The strong bands observed at 1596–1625 cm−1  range
in the free ligands have been assigned to ν(C=N) stretching vibrations [24]. On
complexation, these bands were observed to be shifted to lower frequencies (1570–1590 cm−1),
which are in agreement with the wave numbers for other bischelate complexes [6, 25, 26]. These results indicate that the imine nitrogen is coordinated to the
metal ion. All ligands showed medium bands in the 880–915 cm−1  range ascribed to ν(C=S) vibrations. These absorption bands shift 65–80 cm−1  to lower frequencies on the coordination of the thiocarbonyl sulfur to
palladium (II) or platinum (II) ion. These results are in agreement with other
thiosemicarbazone complexes [24, 27]. In addition, the vibrational frequencies
of the –NH2  groups remain unchanged for both the ligands and the
complexes. This evidence indicates the noncoordination of the –NH2 group to the Pd(II) center.


## 3.2. NMR Spectra of the Ligands and Their Complexes

In theH1-NMR spectra of the ligands, the signals of the =N–NH protons were
observed as singlets at δ 11.42–11.83. These
signals disappeared in the H1-NMR spectra of the palladium (II) and
platinum (II) complexes indicating the deprotonation of the =N–NH group. The
signals of the HC=N protons which appear as singlets at δ 8.03–8.17 in the ligands
show a shift to downfield  in δ 0.03–0.80 after
complexation. This shift indicates the coordination of the imine nitrogen to
the metal center [28]. The signals of the aromatic protons of the ligands
appeared at δ 7.21–7.91, and the
resonance lines found correspond to the calculated multiplicity. These signals
do not suffer relevant changes in the chemical shifts for the palladium (II)
and platinum (II) complexes. The NH2 signal in the ligands HL1,
HL2, and HL3appears as doublets at δ 7.98–8.45 due to the
nonequivalence of the amine protons. This evidence is attributed to the
restricted rotation around C–N bond (thiocarbonyl carbon and terminal amine
nitrogen) due to its partial double bond character [14, 29]. The
presence of the phenyl group on the terminal amine (NHPh) of the ligand HL4 produces a downfield chemical shift at δ 2.1 with respect
to the NH2 group of the ligand HL1. This reveals that HL4 is slightly less basic than HL1. The resonance signals of the –NH2 or NHPh groups in the palladium (II) and platinum (II) complexes do not change,
and this evidence indicates that the amine groups are not coordinated to the
metal ion [14].In theC13-NMR spectra, the carbon resonance signals of the HC=N group
appeared at δ 139.66–148.3. The
results are similar to the chemical shifts found for the ligands benzophenone
thiosemicarbazide and phenylpropenal thiosemicarbazone (both found at δ 141)
[30, 31]. The C=S signals observed at δ 178.5–176.0 are
characteristic for this group, while the aromatic carbons were observed at δ 139.1–124.5.


## 3.3. Structural Data

The
molecular structures of HL4, Pd(L2)2, and Pt4(L4)4 are shown in
Figures 1, 2, and
3, respectively, whereas their selected bond
lengths and bond angles are presented in
Tables 2 and 3.Table 2
Selected bond lengths (Å) for the HL4 ligand and the Pd(L2)2 and Pt4(L4)4 complexes.
HL4Pd(L2)2Pt4(L4)4S1–C11.686(3)1.746(2)1.812(9)N1–N21.378(4)1.378(2)1.389(10)N1–C81.284(4)1.293(11)N2–C11.354(4)1.296(11)N3–C11.342(4)1.348(3)1.354(11)N3–C21.422(4)1.427(12)C2–C31.472(3)C8–C91.457(4)1.441(13)N1–C11.316(3)N2–C21.306(3)Pd1–S12.297(1)Pd1–N22.049(2)Pt–S (intraligand)2.351(2)Pt–S′(bridging)2.298(2)Pt–N11.991(7)Pt–C102.015(8)Table 3
Selected bond angles (°) for the
HL4 ligand and the Pd(L2)2 and Pt4(L4)4 complexes.
HL4Pd(L2)2Pt4(L4)4N2–N1–C8115.0(3)118.5(7)N1–N2–C1119.8(3)113.4(7)C1–N3–C2127.7(3)127.6(8)S1–C1–N2118.7(2)124.8(7)S1–C1–N3126.1(2)117.4(2)114.4(7)N2–C1–N3115.2(3)120.7(8)N1–C8–C9121.8(3)115.0(8)N2–N1–C1113.5(2)N1–N2–C2116.5(2)S1–C1–N1125.3(2)N1–C1–N3117.3(2)N2–C2–C3131.6(2)S1–Pd1–N282.8(1)S–Pt–N183.1(2)N1–Pt–C1080.9(3)C10–Pt-S′94.8(3)S–Pt–S′101.5(1)Pt–S1–S193.6(3)Pt–N1–N2124.2(5)Pt–N1–C8117.2(6)Figure 1
Molecular structure of HL4.Figure 2
Molecular structure of Pd(L2)2.Figure 3
Molecular structure of Pt4(L4)4.


## 3.3.1. 4-Phenyl-1-Benzaldehyde Thiosemicarbazone HL4

The reaction
product of benzaldehyde and 4-phenyl thiosemicarbazide shows the expected bond
lengths, especially the N1–C8 double bond with a length of 1.284(4) Å (see
Figure1). The molecular fragment N3-C1(S1)-N2-N1-C8 is nearly planar. The C9–C14 phenyl
ring deviates only slightly from this mean plane and forms an angle of 56.4(1)° with the C2–C7 phenyl ring.There
are two hydrogen bonds: an intramolecular N3–H⋯ N1 [N3–H 0.85 Å, H ⋯ N1 2.17 Å,
N3 ⋯ N1 2.615 Å, N3–H ⋯ N1 112°]
hydrogen bond and an intermolecular
N2–H ⋯ S1 [N2–H 0.88 Å, H ⋯ S1 2.62 Å, N2 ⋯ S1 3.466 Å, N2–H ⋯ S1 162°]  hydrogen bond. The latter leads to the formation of pairs of molecules in the
crystal structure.


## 3.3.2. Bis(3-Cyanophenyl-1-Benzaldehyde Thiosemicarbazonato)palladium (II) Pd(L2)2

3-cyanophenyl-1-benzaldehyde
thiosemicarbazone reacts with palladium (II) acetylacetonate to form a
bis-chelate withCi  molecular symmetry (see
Figure 2). The
deprotonated ligand coordinates bidentately through S and N. The coordination
of the Pd atom is square planar with a transarrangement of the coordinating
atoms. It leads to a lengthening of the S1–C1 bond and a shortening of the N1–C1
bond. The Pd1S1N1N2C1 chelate ring is nearly planar and forms an angle of
11.5(1)° with the C3–C8 phenyl ring.In the
crystal structure, one molecule of water per formula unit of the chelate is
included.There
are three hydrogen bonds comprising the atom O1 of the water molecule and the
atom N4 of the cyano group: O1–H⋯ S1 [O1–H 0.79 Å, H ⋯ S1 2.65 Å, O1 ⋯ S1 3.418 Å,
O1–H ⋯ S1 166°], N3–H ⋯ O1 [N3–H 0.87 Å, H ⋯ O1 2.21 Å, N3 ⋯ O1 3.081 Å,
N3–H ⋯ O1 173°]  and N3–H′ ⋯ N4 [N3–H′ 0.85 Å, H′ ⋯ N4 2.22 Å, N3 ⋯ N4 3.054 Å, N3–H′ ⋯ N4 167°].


## 3.3.3. Tetrakis (4-Phenyl-1-Benzaldehyde Thiosemicarbazonato)tetraplatinum (II) Pt4(L4)4

4-phenyl-1-benzaldehyde
thiosemicarbazone reacts with ammonium tetrachloroplatinate (II) to form a
tetranuclear complex with slightly distorted square planar geometry (see
Figure3). The tridentate ligands are deprotonated at the NH group and
coordinated through S, N,
and C (aromatic ring). The fourth coordination site at each Pt atom is occupied
by a sulfur atom of a neighboring ligand. In this way, a puckered
eight-membered ring of alternating Pt and S atoms is formed as the core of the
molecule. Each of the four Pt atoms belongs to two fused five-membered chelate
rings: the C, N metallocycle and the N, S chelate moiety.The
Pt-Pt distances range from 3.43 Å to 3.84 Å. The Pt–S bonds form two distinct
groups with significantly differing lengths: Pt-Schelating 2.351(2) Å and Pt-Sbridging 2.298(2) Å (mean values). The coordination of the
ligand to the Pt atoms leads to a lengthening of the S–C1 bond (increased
single-bond character) and a shortening of the neighboring N2–C1 bond
(increased double-bond character) compared with the free ligand.In the
crystal structure, two molecules of ethanol per formula unit of the
tetranuclear complex are included, which stabilize the crystal structure by
hydrogen bonds. The O1 and O2 atoms of the ethanol molecules are bonded by
hydrogen bonds to N3 atoms: N3–H⋯ O1 [N3–H 0.86 Å, H ⋯ O1 2.07 Å, N3 ⋯ O1 2.898 Å,
N3–H ⋯ O1 162°]  and N3–H′ ⋯ O2 [N3–H′ 0.86 Å, H′ ⋯ O2 2.14 Å, N3 ⋯ O2 2.999 Å, N3–H′ ⋯ O2 173°].The formation of tetranuclear compounds was previously
observed for Pd complexes with similar thiosemicarbazone ligands [14].


## 3.4. Antitumor Evaluation

All ligands had a
50% inhibitory concentration (IC50)  >  40 μM against the used human tumor cell
lines. As shown in Table 4, the palladium (II) and platinum (II) complexes were
more cytotoxic (IC50 = 0.08–12.46 μM) than
their respective ligands. These results reveal that the cytotoxic activity
increases dramatically when ligands are coordinated to the metal ion [18, 19, 32].Table 4
IC50(μM)
values of the palladium complexes, Pd(1–3)2, and the
platinum tetranuclear complex, Pt4(L4)4,
against  the different human tumor cell
lines.
Human tumor
cell linesPd(L1)2Pd(L2)2Pd(L3)2Pt4(L4)4Lung large cell carcinoma (H460)8.733.535.820.09Cervix
epidermoid carcinoma (ME180)10.473.678.060.12Prostate
carcinoma (DU145)11.591.999.300.10Breast
adenocarcinoma (MCF-7)8.182.096.140.08Amelanotic
melanoma (M-14)12.461.878.740.11Colon
adenocarcinoma (HT-29)7.451.355.960.07Prostate
carcinoma (PC-3)10.201.567.360.08Chronic myelogenous leukemia (K562)6.150.453.040.08Cl50(a) corresponds to the concentration
required to inhibit a 50% of the cell growth when the cells are exposed to the
compounds during 48 hours. Each value is the average of two independent
experiments.The Pd(L2)2 complex  with a cyano group in the metaposition of the aromatic ring and
the Pt4(L4)4 tetranuclear complex with the
phenyl group in the terminal amine of the ligand showed to be more cytotoxic (IC50 =
0.45–3.67 and 0.07–0.12 μM, resp.)
than the other Pd(L1)2 and Pd(L3)2 complexes
against all tested human tumor cell lines. These results indicate that the
cytotoxic activity is enhanced when four ligands are coordinated to four
platinum atoms.Probably, the high cytotoxicity of the Pd(L2)2 and Pt4(L4)4 complexes may be related to the
intercalation of each metal complex between nitrogen bases of the DNA tumor
cells, causing greater conformational changes in the double helix of DNA and
then producing cell death [33, 34].On the other
hand, the Pt4(L4)4 tetranuclear complex was
more cytotoxic (IC50 = 0.08 μM)
compared with the cytotoxic activity shown by cisplatin (IC50 =
7.0 μM) assayed in (HL60) human leukemia cells [12]. With respect to the
cytotoxicity shown by the square planar copper (II) complexes with
carboxamidrazone ligands (IC50 =  3.0 μM), assayed in vitro against to the (MCF-7) human
breast adenocarcinoma cell line [35], the Pt4(L4)4 complex
resulted to be  more cytotoxic, while the Pd(L2)2 complex showed a similar inhibition concentration (IC50
= 2.09 μM). In relation to other palladium (II) and platinum (II) complexes of the
M(HL)Cl2 type with phenyl acetaldehyde thiosemicarbazone ligands (IC50 = 38
and 9 μM, resp.) assayed in the K562 human chronic
myelogenous leukemia cell line [12], the Pd(L2)2 and Pt4(L4)4 complexes
showed higher cytotoxic activity. In addition, the Pd(L2)2 complex resulted to be more cytotoxic (IC50 = 2.09 μM) than the
palladium (II) and copper (II) complexes of the M(L)Cl type (IC50 = 12.94
and 3.98, resp.) and the NiL2 complex (IC50 = 2.25 μM) with
1,2-naphthoquinone-1-thiosemicarbazone ligands, tested in vitro against to the MCF-7 human breast adenocarcinoma cell
line [34].In summary, we
have prepared the palladium (II) bis-chelate complexes and the platinum (II)
tetranuclear complex, Pd(L1–3)2, and Pt4(L4)4,
which were more cytotoxic on all the human tumor cell lines at low micromolar
concentrations with respect to the free ligands. The crystal structure of Pd(L2)2 shows that the palladium atom has a
square-planar geometry with two bidentate ligands having sulfur and
nitrogen as donor atoms in transpositions.
The complex Pt4(L4)4 has a tetranuclear
structure with four tridentate(C,N,S) ligands coordinated to four platinum
atoms.Supplementary Material
Further details
of the crystal structure determination are available on request from the Cambridge Crystallographic Data Center
(CCDC, 12 Union Road, Cambidge CB2 1EZ, UK;
fax: +44 1223 336033; email: deposit@ccdc.cam.ac.uk),
on quoting the depositing numbers CCDC 694972 for HL4, 694973 for
Pd(L2)2, and 694974 for Pt4(L4)4,
the names of the authors, and the journal citation.



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*Source: 690952-2009-01-08.xml*