Synthesis and cytotoxic activity of gold(I) complexes containing phosphines and 3-benzyl-1,3-thiazolidine-2-thione or 5-phenyl-1,3,4-oxadiazole-2-thione as ligands

Article abstract of DOI:10.1016/j.ica.2014.01.042

Heterocyclic compounds and their metal complexes display a broad spectrum of pharmacological properties. This work reports the preparation and characterization of four novel gold(I) complexes containing tertiary phosphine and 3-benzyl-1,3-thiazolidine-2-thione, 5-phenyl-1,3,4-oxadiazole-2-thione as ligands. The reaction of chloro(triphenylphosphine)gold(I) and chloro(triethylphosphine)gold(I) with thioamides, 3-benzyl-1,3-thiazolidine-2- thione and 5-phenyl-1,3,4-oxadiazole-2-thione in dichloromethane or dichloromethane/acetone resulted in the formation of the gold(I) complexes of general formula: [SAuPR₃]Cl, S = 3-benzyl-1,3-thiazolidine-2-thione, R = Ph or Et and [SAuPR₃]_, S = 5-phenyl-1,3,4-oxadiazole- 2-thione, R = Ph or Et. Spectroscopic evidence suggested that gold is coordinated to the exocyclic sulfur atom in all cases and this was confirmed by X-ray crystallographic data obtained for complex (4). The cytotoxicity of the compounds has been evaluated in comparison to cisplatin in two different tumor cell lines, colon cancer (CT26WT) and metastatic skin melanoma (B16F10), and also in a kidney normal cell (BHK-21). The gold complexes showed a better activity than cisplatin and presented a high selectivity index.

Full text of DOI:10.1016/j.ica.2014.01.042

Inorganica Chimica Acta 414 (2014) 85–90
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Inorganica Chimica Acta
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Synthesis and cytotoxic activity of gold(I) complexes containing
phosphines and 3-benzyl-1,3-thiazolidine-2-thione or
5-phenyl-1,3,4-oxadiazole-2-thione as ligands
Joana Darc S. Chaves a, Fernanda Neumann a, Thiago Martins Francisco c, Charlane Cimini Corrêa a,
Miriam Teresa Paz Lopes b, Heveline Silva a, Ana Paula S. Fontes a, Mauro V. de Almeida a,
⇑
a Departamento de Química, ICE, Universidade Federal de Juiz de Fora, 36036-330 Juiz de Fora, MG, Brazil
b Departamento de Farmacologia, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil
c Departamento de Física, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 October 2013
Received in revised form 14 January 2014
Accepted 22 January 2014
Available online 5 February 2014
Heterocyclic compounds and their metal complexes display a broad spectrum of pharmacological proper-
ties. This work reports the preparation and characterization of four novel gold(I) complexes containing
tertiary phosphine and 3-benzyl-1,3-thiazolidine-2-thione, 5-phenyl-1,3,4-oxadiazole-2-thione as
ligands. The reaction of chloro(triphenylphosphine)gold(I) and chloro(triethylphosphine)gold(I) with
thioamides, 3-benzyl-1,3-thiazolidine-2-thione and 5-phenyl-1,3,4-oxadiazole-2-thione in dichlorometh-
ane or dichloromethane/acetone resulted in the formation of the gold(I) complexes of general formula:
[SAuPR3]Cl, S = 3-benzyl-1,3-thiazolidine-2-thione, R = Ph or Et and [SAuPR3], S = 5-phenyl-1,3,4-oxadiaz-
ole-2-thione, R = Ph or Et. Spectroscopic evidence suggested that gold is coordinated to the exocyclic sulfur
atom in all cases and this was confirmed by X-ray crystallographic data obtained for complex (4). The
cytotoxicity of the compounds has been evaluated in comparison to cisplatin in two different tumor cell
lines, colon cancer (CT26WT) and metastatic skin melanoma (B16F10), and also in a kidney normal cell
(BHK-21). The gold complexes showed a better activity than cisplatin and presented a high selectivity
index.
Keywords:
Gold(I) complexes
Tertiary phosphines
Thioamides
Cancer
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
tumor cells and to have potential for treating cisplatin-resistant
tumors [3].
The discovery of the antitumor properties of cisplatin
(cis-[PtCl2(NH3)2]) by Rosenberg represents a landmark in medici-
nal chemistry [1]. The use of cisplatin in chemotherapy resulted in
a reduction of about 80% of the death rate among men suffering
from testicular tumor in 1978 [2]. Since then, interest in Medicinal
Inorganic Chemistry has continued to grow with the emergence of
new targets and novel opportunities for intervention of Coordina-
tion Chemistry in Medicinal Chemistry. Gold(I) complexes have at-
tracted great attention as potential antitumor agents due to the
fact that many of them have been shown to inhibit the growth of
Gold compounds have been used in medicine for a long time.
However, it was only after Forestier described the anti-arthritic
properties of gold(I) compounds in 1935 that the scientific com-
munity began a more methodical investigation of the toxic and
beneficial effects of gold compounds in rheumatoid arthritis [4].
Nowadays gold compounds such as auranofin (triethylphosphine-
(2,3,4,6-tetra-O-acetyl-b-1-D-thiopyranosato-S)gold(I)), solganol
(gold(I) thioglucose), and myochrysin (sodium gold(I) thiomalate),
are used clinically in the treatment of rheumatoid arthritis.
The antineoplastic activity of auranofin has been evaluated on a
variety of animal and human tumor cell lines in vitro and in vivo
[5,6]. These studies have shown that this drug presents potent
cytotoxic activity against several cell lines, specially for P338
leukemia.
The mechanism of action of gold compounds is still not well
understood and has been continually investigated by the scientific
community. Some investigations have suggested that DNA is not
Abbreviations: NMR, nuclear magnetic resonance; DMSO, dimethysulfoxide;
HRMS(ESI), high-resolution mass spectra (eletrospray ionization); TMS, tetrameth-
ylsilane; MTT, (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide;
RPMI, Roswell Park Memorial Institute Medium; HEPES, 4-(2-hydroxyethyl)-1-
piperazineethanesulfonic.
⇑
Corresponding author. Fax: +55 32 21023310.
E-mail address: mauro.almeida@ufjf.edu.br (M.V. de Almeida).
http://dx.doi.org/10.1016/j.ica.2014.01.042
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

86
J.D.S. Chaves et al. / Inorganica Chimica Acta 414 (2014) 85–90
the primary target for these compounds although some gold
complexes such as Bis[1,2-bis(diphenylphosphino)ethane]gold(I)
chloride produce DNA protein cross-links and DNA strand breaks
in cells [7]. The biological effects of gold complexes could be med-
iated by an anti-mitochondrial mechanism. Studies indicate that
the mechanism of action of these compounds involves the enzyme
thioredoxin reductase, which is involved in the mechanism of pro-
liferation of tumor tissues [8,9]. Previous work on the chemical
reactivity of auranofin and other gold(I) complexes containing
phosphine also demonstrated that these compounds may react
with serum proteins, cellular proteins, glutathione and other small
molecular weight thiols [7].
Advances in chemotherapy of gold compounds have been
pursued by several research groups [10]. A variety of auranofin
analogues and phosphine gold(I) compounds containing S-donor
ligands has been developed and shown to possess potent cytotoxic
activities [11].
Considering that auranofin has shown similar or greater in vitro
activity than cisplatin and has also exhibited potent cytotoxic
activity against melanoma and leukemia cell lines and anti-tumor
activity against leukemia, several complexes of gold(I) have been
evaluated for cytotoxic and anti-tumor activity [8].
The use of nanotechnology in cancer treatment is also an attrac-
tive research area. Gold nanoparticles have been investigated as
drug delivery systems and also in phothermal therapy due to their
unique properties. The field referred to as nanomedicine still faces
many challenges but gold nanoparticles are one of the more prom-
ising areas of research [12,13].
2. Experimental
2.1. Materials and methods
All reagents and solvents were reagent grade and were used
without prior purification. The progress of all reactions was moni-
tored by thin-layer chromatography which was performed on
2.0 ꢀ 6.0 cm aluminium sheets precoated with silica gel 60 (HF-
254, Merck) to a thickness of 0.25 mm. Infrared (IR) spectra were re-
corded in a Bomem FTIR MB-102 spectrometer in the region 4000–
360 cmꢁ1 of the sample supported as a KBr pellet, with 4 cmꢁ1 of
spectral resolution, and an average of 64 scans. Only significant
peaks were recorded. 1H NMR (300 MHz) and 13C NMR (75 MHz)
spectra were recorded as solutions in CDCl3 and DMSO-d6 on a Bru-
ker spectrometer. The chemical shifts were expressed as d (in ppm)
with respect to a standard internal TMS reference (1H NMR). Raman
spectra were obtained using a Bruker RFS 100 FT-Raman instru-
ment equipped with a germanium detector refrigerated by liquid
nitrogen, with excitation at 1064 nm from a Nd:YAG laser, power
between 103 mW for sample in solid phase, in the range between
4000 and 50 cmꢁ1, and spectral resolution of 4 cmꢁ1, with an aver-
age of 500 scans. The high-resolution mass spectra were recorded
on a Micromass LCT spectrometer with electrospray ionization, at
the Institut de Chimie des Substances Naturelles, Gif-sur-Yvette,
France. Elemental analyses were performed at Central Analítica,
USP-Brazil. Diffraction data for single crystals of C14H20N2OPSAu
(4) were collected using a Oxford GEMINI A Ultra diffractometer
with Mo K
a (k = 0.71073 Å) and temperature of 120 K. Data
The chemical structure of the ligands present in gold(I) com-
plexes is an important parameter for biological activity. For in-
stance, Yeo et al. [14] have investigated the influence of the R
substituints (R = Me, Et and iPr) in triphenylphosphinegold(I) car-
bonimidothioates upon in vitro cytotoxicity against HT-29 colon
cancer cell line and have found that R = Me results in the most ac-
tive compound. Several gold(I) and gold(III) complexes derived
from 2-(20-pyridyl)benzimidazole including mononuclear and
binuclear species have shown relevant antiproliferative activities
in vitro against A2780 human ovarian carcinoma cells, resistant
or sensitive to cisplatin [15].
Previous structure–activity relationship studies of auranofin
and other gold(I) compounds have shown the importance of the
phosphine ligand for biological potency [16]. Gold compounds con-
taining tertiary phosphines with a linear S–Au–P arrangement
have been found to be more active than similar compounds with
no phosphinic substituents. The lipophilicity introduced in the
compounds due to coordination of tertiary phosphines seems to
be responsible for their enhanced cytotoxicity since it facilitates
transport across cell membranes [10,16,17].
3-Benzyl-1,3-thiazolidine-2-thione and 5-phenyl-1,3,4-oxaz-
adiazole were chosen as ligands since they are members of the het-
erocyclic class of compounds which exhibit interesting biological
properties such as anti-inflammatory and analgesic activities. This
class of compounds also exhibits insecticide, herbicide and fungicide
activities [18]. Compoundscontaining the thiazolidine ringare ableto
interact with a variety of biological targets and have been extensively
reportedinthe literature[19]. The thiazole skeleton is a constituent of
many biomolecules including b-lactam antibiotics such as the
penicillins and natural products such as thiamin [20]. Compounds
containing the 1,3,4-oxadiazole moiety exhibit antimicrobial, anti-
HIV [21], antitubercular [22], antimalarial [23], anti-inflammatory
[24], anticonvulsant [25], and anticancer [26] properties.
collection, reduction and cell refinement were carried out by CRY-
SALIS RED, Oxford diffraction Ltda – Version 1.171.32.38 software
[27]. The structures were solved and refined using SHELXL-97 [28].
An empirical isotropic extinction parameter x was refined accord-
ing to the method described by Larson [29]. A Multiscan absorption
correction was applied [30]. The structures were drawn by ORTEP-3
for Windows [31] and MERCURY softwares [32].
2.2. Synthesis of ligands
Ligand (A) was prepared from benzyl chloride according to the
experimental procedure described in ref [33] and ligand (B) was
prepared from benzoyl chloride according to the experimental pro-
cedure described in ref [34].
(A): as a white solid, m.p. 131 °C, lit [33] m.p. 132–133 °C. IR
mmax KBr (cmꢁ1): 3024; 2942; 1488; 1178; 983; 731; 398; Raman
mmax (cmꢁ1): 1157; 1001; 707; 1H NMR (300 MHz, CDCl3) d: 3.23
(t, 2H, H5, J5,4 = 7.8 Hz); 3.94 (t, 2H, H4, J4,5 = 7.8 Hz); 4.97 (s, 2H,
H6); 7.23–7.34 (m, 5H, H–Ar). 13C NMR (75 MHz, CDCl3) d: 27.2
(C5); 52.8 (C6); 56.0 (C4); 128.2 (C8, C12); 128.3 (C10); 129.1
(C9, C11); 135.1 (C7); 197.2 (C2).
(B): as a white solid, m.p. 217–223 °C, lit [34] m.p. 219 °C. IR
mmax KBr (cmꢁ1): 3142; 3099; 2952; 1501; 1487; 967; 696; 684;
Raman mmax (cmꢁ1): 3072; 1488; 1359; 970; 698; 1H NMR
(300 MHz, DMSO-d6) d: 7.53–7.61 (m, 4H, H8, H9, H10, NH); 7.86
(dd, 2H, J7,9 = J11,9 = 1.5 Hz, J7,8 = J 11,10 = 8.1 Hz, H7, H11); 13C
NMR (75 MHz, DMSO-d6) d: 122.4 (C6); 125.9 (C8, C10); 129.3
(C7, C11); 132.1 (C9); 160.4 (C-5); 177.5 (C2); HRMS(ESI): m/z calc.
for [C8H6N2OS] [MꢁH]ꢁ 177.0123, found 177.0128.
2.3. Synthesis of complexes (Scheme 1)
In this work we report the preparation, characterization and
cytotoxic activity against two tumor cell lines of four new gold(I)
complexes containing 3-benzyl-1,3-thiazolidine-2-thione and
5-phenyl-1,3,4-oxadiazol-2-thione and tertiary phosphine (PPh3
or PEt3) as ligands.
To a solution of Au(PPh3)Cl (0.198 g, 0.4 mmol) or Au(PEt3)Cl
(0.140 g, 0.4 mmol) in dichloromethane (3 mL), ligand (A)
(0.4 mmol) dissolved in dichloromethane (4 mL) or ligand (B)
(0.4 mmol) dissolved in acetone (3 mL) was slowly added during
3 h. After stirring for 9 h at room temperature in the dark, the

J.D.S. Chaves et al. / Inorganica Chimica Acta 414 (2014) 85–90
87
Scheme 1. Synthesis of complexes (1), (2), (3) and (4).
solvent was removed under reduced pressure to furnish a white
residue which was purified by preparative chromatography
(eluant:dichloromethane) to give the desired compounds (1), (2),
(3) and (4) in 46%, 54%, 62% and 68% yields, respectively.
7.45(m, 5H, HAr); 13C NMR (75 MHz, CDCl3) d: (9.2 CH3); 18.3 (d,
CH2, JCH2-P = 36.2 Hz); 27.2 (C5); 52.8 (C6); 56.0 (C4); 128.3 (C8,
C12); 128.5 (C10); 129.1 (C9, C11); 135.2 (C7); 197.4 (C2);
HRMS(ESI): m/z calc. for [C16H26NPS2Au]Cl [M]+ 524.0910, found
524.0905; [M+H]+ calc. 525.0988, found 525.0956.
Compound Au(PEt3)Cl is commercially available and Au(PPh3)Cl
was synthesized from K[AuCl4] according to the literature [35].
Anal. Calc. for [C16H26NS2PAu]Cl: C, 34.32; H, 4.68; N, 2.50.
Found: C, 34.49; H, 4.70; N, 2.53%.
(1): as a white solid, m.p. 125–128 °C. IR
2919; 1488, 1179; 999; 749; 384; Raman
max (cmꢁ1): 1155, 1002;
m
max KBr (cmꢁ1): 3032;
m
(3): as a white solid, m.p. 65–69 °C. IR mmax KBr (cmꢁ1): 3054;
2926; 1437; 996; 747; 708; 691; 380 Raman mmax (cmꢁ1): 3058;
1443; 958; 715; 378; 1H NMR (300 MHz, DMSO-d6) d: 7.53–7.65
(m, 18H, H8, H9, H10, Ar-PPh3); 7.81–7.85 (m, 2H, H7, H11); 13C
NMR (75 MHz, DMSO-d6) d: 123.7 (C6); 125.7 (C8, C10); 128,4 (d,
696; 391; 1H NMR (300 MHz, CDCl3) d: 3.24 (t, 2H, H5,
J5,4 = 7.8 Hz); 3.95 (t, 2H, H4, J4,5 = 7.8 Hz); 4.99 (s, 2H, H6); 7.24–
7.32 (m, 5H, H–Ar); 7.35–7.56 (m, 15H, Ar–PPh3). 13C NMR
(75 MHz, CDCl3) d: 27.2 (C5); 52.8 (C6); 55.9 (C4); 128.3 (C8,
C12); 128.4 (C10); 128.5 (C10); 129.1 (C9, C11); 129.4 (d, C30, C50,
C10, J1 -P = 58.4 Hz) 129.2 (C7, C11); 129.6 (d, C30, C50, J3 -P = J5 -
0
0
0
J3 -P = J5 -P = 11.7 Hz); 131.1 (d, C40, J4 -P = 2.3 Hz); 134.3 (d, C20,
P = 11.5 Hz); 131.2 (C9); 132.3 (C40); 133.8 (d, C20, C60 J2 -P = J6 -
P = 13.9 Hz); 163.8 (C5); 168.1 (C2); HRMS(ESI): m/z calc. for [C26-
H20N2OPSAu] [M+H]+ (637.0778), found 637.0784.
0
0
0
0
0
C60, J2 -P = J6 -P = 14.0 Hz); 135.2 (C7); 197.4 (C2); HRMS(ESI): m/z
calc. for [C28H26NPS2Au]Cl [M]+ 668.0910, found 668.0911;
[M+H]+ Calc. 669.0988, found 669.0962.
0
0
Anal. Calc. for C26H20N2OSPAu: C, 49.07; H, 3.17; N, 4.40. Found:
C, 49.32; H, 3.22; N, 4.39%.
Anal. Calc. for [C28H26NS2PAu]Cl: C, 47.77; H, 3.72; N, 1.99.
Found: C, 47.79; H, 3.74; N, 2.28%.
(4): as a white solid, m.p. 87–88 °C. IR mmax KBr (cmꢁ1): 3076;
2963; 1438; 995; 757; 709; 693; 391 Raman mmax (cmꢁ1): 3064;
1440; 957; 700; 366; 1H NMR (300 MHz, DMSO-d6) d: 1.10–1.21
(m, 9H, CH3); 1.92–2.04 (m, 6H, CH2) 7.54–7.56 (m, 3H, H8, H9,
H10); 7.85–7.88 (m, 2H, H7, H11); 13C NMR (75 MHz, DMSO-d6)
d: 9.6 (CH3); 17.5 (d, JCH2-P = 34.7 Hz); 124.3 (C6); 126.2 (C8);
(2): as a white solid, m.p. 62–63 °C. IR mmax KBr (cmꢁ1): 3025;
2964; 2924; 2872; 1489; 982; 774; 383; Raman mmax (cmꢁ1):
1155, 1003; 686; 392; 1H NMR (300 MHz, CDCl3) d: 1.12–1.24
(m, 9H, CH3); 1.78–1.89 (m, 6H, CH2); 3.23 (t, 2H, H5,
J5,4 = 7.8 Hz); 3.94 (t, 2H, H4, J5,4 = 7.8 Hz); 4.97 (s, 2H, H6); 7.32–

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J.D.S. Chaves et al. / Inorganica Chimica Acta 414 (2014) 85–90
126.4 (C10); 129.8 (C7); 129.9 (C11); 131.7 (C9); 164.2 (C5); 169.1
(C2); HRMS(ESI): m/z calc. for [C14H20N2OPSAu] [M+H]+
(493.0778), found 493.0769.
Anal. Calc. for C14H20N2OSPAu: C, 34.15; H, 4.09; N, 5.69. Found:
C, 34.08; H, 4.07; N, 5.70%.
Fig. 1. Thione-thiol tautomerization of 5-phenyl-1,3,4-oxadiazole-2-thione.
significant change was observed in thioamide group IV, indicating
that after the coordination, C@S presents some C–S character. The
displacement of the thioamide IV band suggests that the metal is
bound to the sulfur of the thione group.
2.4. Cytotoxicity assay
Cytotoxic activity was investigated against the following tumor
cell lines: CT26WT – colon cancer cells, B16-F10 – mouse metastatic
skin melanoma, (Ludwig Institute of Cancer Research - São – Paulo
Brazil) and non-tumor cell, BHK-21 – Baby Hamster Kidney.
All the cell lines were propagated in RPMI 1640 culture medium
pH 7.4, supplemented with 10% heat-inactivated Fetal Bovine
Serum (FBS), Hepes (4.0 mM), NaHCO3 (14.0 mM), ampicillin
(0.27 mM), and streptomycin (0.06 mM).
The presence of a new absorption band around 380 cmꢁ1
,
attributed to mAu–S, was observed in the IR spectra of complexes
(1), (2), (3), and (4). For these complexes significant shifts of thio-
amide bands were also observed (Fig. S1 and S2 and Table S1).
According to Rao and Venkataraghavan [37], the extreme varia-
tions in the assignment of the C@S stretching frequency in nitro-
gen-containing thiocarbonyl derivatives is undoubtedly due to
Cells were harvested by trypsinization and seeded in 96-well
tissue culture plates (100
l
L/well) at defined density (1 ꢀ 103 via-
vibrational coupling effects. In most of these systems, the
stretching vibration is not localized.
mC@S
ble cells/well) and incubated at 37 °C in a humidified atmosphere
containing 5% CO2 for 24 h. Stock solutions of the test substances
in DMSO were serially diluted in cell culture medium (<1% DMSO).
After drug exposure for 72 h at 37 °C and 5% CO2, cells were incu-
bated with MTT (0.01 M in water solution – 10
37 °C and 5% CO2. MTT is metabolized by viable cells resulting in
a violet complex product that, after being solubilized in 100 L of
The absorption corresponding to mSH was not observed in the
infrared and Raman spectra of ligand (B), complexes (3) and (4).
We can observe an absorption band in the spectrum of the free li-
gand corresponding to
oamide bands [38] indicating that it is present in the tautomeric
m
NH at ꢂ3140 cmꢁ1 and characteristic thi-
lL/well) for 4 h at
form (X).
l
The thione form that predominates for ligand (B) can coordinate
to the gold(I) through N3 or S2. However, the sulfur atom tends to
be the most likely binding site for AuPPh3 and AuPEt3 since gold
tends to bind soft, polarizable atoms such as sulfur.
The infrared spectra of the complexes (3) and (4) have shown
DMSO, can be quantified through colorimetric assay using a plate
reader (absorbance at 570 nm).
The negative control (100% value of viability) was obtained with
cells exposure in RPMI 1640 medium supplemented with 10% FBS.
Cisplatin was used as positive control against these cell lines.
The raw data were normalized to the untreated control cells
and compared to the metabolic activity of the viable treated cells.
IC50 values were calculated by four parametric nonlinear regres-
sions using GRAPHPAD Prism 5.0 software.
the absence of
mSH,
m
NH and thioamide I, II, III bands and the
CS indi-
appearance of a new band at 747–757 cmꢁ1 attributed to
m
cating that the ligand assumes the tautomeric form (Y) after the
formation of the complexes.
The assignments of the absorptions in the Raman spectra have
also shown the absence of the NH tautomer in the solid state for
the two complexes. The most relevant infrared and Raman spectra
absorptions are summarized in Table S1.
3. Results and discussions
The complexes (Scheme 1) were synthesized by adding the cor-
responding ligands, previously dissolved in dichloromethane or
dichloromethane/acetone 1:1, to a solution of Au(PPh3)Cl or
Au(PEt3)Cl in dichloromethane at room temperature, and were iso-
lated by preparative plate chromatography.
3.2. X-ray structural determination
A crystal of C14H20N2OPSAu complex (4) (Fig. 2) was obtained
by recrystallization from a mixture of dimethylsulfoxide and
The complexes were characterized by 1H NMR, 13C NMR, infra-
red, Raman, high-resolution mass spectra and complex (4) by X-ray
crystallography. The integrations of the signals in the NMR spectra
showed that the stoichiometric ratio for the complexes formed was
1:1 ligand:complex.
Table 1
Crystal data, data collection and structure refinement for C14H20N2OPSAu (4).
Empirical formula
C14H20AuN2OPS
Ligand (A) presents three bonding sites: the thiocarbonyl sulfur
atom, the nitrogen atom and the endocyclic sulfur atom. The
isolated pairs of electrons from the endocyclic sulfur atom and
nitrogen should be in resonance with the thiocarbonyl group, thus
contributing to greater delocalization of electrons and conse-
quently decreasing the ability of coordination.
Formula weight (g molꢁ1
T (K)
k (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
)
492.32
120(2)
0.71073
orthorhombic
P212121
7.3012 (1)
13.4183 (2)
17.1808 (2)
90.00
Ligand (B) can display two tautomeric forms (X) and (Y) as
shown in Fig. 1.
a
(°)
b (°)
90.00
c
(°)
90.00
1683.20 (4)
4
V (Å3)
3.1. Vibrational spectra
Z
Dcalc (g cmꢁ3
)
1.943
The infrared and Raman spectra of complexes (1), (2), (3) and
(4) were analyzed in comparison to the IR and Raman spectra of
3-benzyl-1,3-thiazolidine-2-thione and 5-phenyl-1,3,4-oxadiaz-
ole-2-thione. A shift of some absorptions representing the contri-
bution of the C@S vibration can be observed. They are produced
by vibrational coupling of NAC@S due to the mixed vibrations
[36]. For complexes (1) and (2) it has been found that the most
Crystal size (mm3)
Extinction coefficient
Collected reflections
Independent reflections
S
0.43 ꢀ 0.26 ꢀ 0.19
None
120111
4559
1.120
0.034
0.083
R [I > 2
wR
r(I)]

J.D.S. Chaves et al. / Inorganica Chimica Acta 414 (2014) 85–90
89
hydrogens of the PPh3 group. For complexes (2) and (4) two signals
were observed in the following regions: d 1.12–1.24 ppm (CH3);
1.78–1.89 ppm (CH2) and d 1.10–1.21 ppm (CH3); 1.92–2.04 ppm
(CH2) corresponding to hydrogens of the PEt3 group.
No changes were observed in the 1H NMR spectra of compounds
(1) and (2) after coordination. In the 13C NMR spectra, the signal
corresponding to the carbon atom of the thione group showed a
small chemical shift, on the order of d 0.2 ppm.
In the 13C NMR spectra of the 5-phenyl-1,3,4-oxadiazol-2-thi-
one ligand signals were observed at d 177.5 (C2) and d 160.4 (C5)
ppm, whereas in the spectra of complexes (3) and (4) the corre-
sponding signals appear at d 168.1 (C2), d 163.8 (C5) and d 169.1
(C2), d 164.2 (C5) ppm, respectively, showing a chemical shift to
lower frequency for carbon (C2) due to the presence of the metal
and a chemical shift to higher frequency for (C5).
Fig. 2. Perspective view of complex (4), ellipsoids are drawn at the 50% probability
level and hydrogen atoms are shown as spheres of arbitrary radii.
3.4. Biological tests
dichloromethane and isolated by filtration. The structure of (4) was
determined using single-crystal X-ray diffraction. Crystal data, data
collection and structure refinementdetails are summarized in
Table 1. Complex (4) crystallizes in the orthorhombic crystal sys-
tem and space group P212121. The central metal ion, gold(I), is
coordinated by the sulfur atom of the 5-phenyl-1,3,4-oxadiazole-
2-thiol ligand and with a tertiary phosphine (PEt3) in a linear
geometry, with the bond angle P1–Au1–S1 = 175.85° (8), as shown
in Table 2 which lists some other relevant geometrical features of
the complex. The coordination environment of Au(I) in complex (4)
can be seen in Fig. 2. The 5-phenyl-1,3,4-oxadiazole-2-thiol ligand
is twisted relative to the angle of 180° formed between P1–Au1–
S1. The overall twist measured as the dihedral angle between the
mean plane of the phenyl-thiol rings and the plane formed
between P1–Au1–S1 is ca. 68.38° (9). The crystal structure of com-
plex (4) clearly indicates the dominance of the thiol form of the li-
gand (B), as evidenced by the absence of the H atom at the N1 atom
and the C2–S1 distance (1.709 (7) Å) which is intermediate be-
tween a single and a double bond (1.820 and 1.600 Å, respectively).
One hydrogen atom from a CH3 group of the phosphine is involved
in an intermolecular C–Hꢃ ꢃ ꢃN hydrogen bond forming a two-
dimensional array along the bc plane. Fig. 3 shows the crystal pack-
ing of the complex.
The antitumor activity of the four gold(I) complexes and two
ligands was evaluated in comparison with cisplatin in two differ-
ent cell lines: colon cancer (CT26WT) and metastatic skin mela-
noma (B16F10). They were also examined for their cytotoxic
properties in a normal kidney cell (BHK-21). The tumor cells were
chosen to compose two different tumor types, namely, melanoma
and carcinoma and to also assess the activity in cell lines of differ-
ent embryonic origin such as epithelial and fibroblast. This was
done in an effort to overcome and control any differences. The
third cell line is a normal cell, which was used to evaluate the
selectivity index, allowing comparison of the cytotoxicity of the
compounds in tumor and normal cells.
IC50 values, calculated from the cell viability dose response
curves obtained after 72 h drug treatment in the MTT test, are shown
in Table 3. Complexes (1) and (3) were more cytotoxic in colon cells
(CT26WT) exhibiting smaller IC50 < 0.10
more cytotoxic in melanoma cells (B16F10) exhibiting IC50 = 0.12 -
M. All the compounds were more cytotoxic than cisplatin in both
lM while complex (3) was
l
tumor cell lines. Although the ligands in the free form showed signif-
icant activity, complexation favors the biological activity in all cases.
Complex (3) was about 82 and 54 times more cytotoxic than ligand
(B) in CT26WT and B16F10 cells respectively, while complex (1)
showed a cytotoxicity about 72 times greater than ligand (A) in co-
lon cells (CT26WT). Complex (1) displayed greater activity in colon
3.3. NMR studies
In the 1H NMR spectra of complexes (1) and (3) signals were ob-
served in the d 7.35–7.65 ppm region corresponding to aromatic
Table 2
Selected geometrical parameters (Å, °).
Au1–P1
Au1–S1
S1–C2
S1–Au1A
P1–C31
P1–C30
P1–C32
P1–Au1A
2.2607 (15)
2.3270 (15)
1.709 (7)
2.3356 (19)
1.800 (7)
1.816 (7)
1.827 (8)
2.2777 (19)
1.370 (8)
1.382 (9)
175.85 (8)
104.1 (2)
100.7 (2)
111.8 (2)
115.6 (2)
112.5 (3)
107.9 (2)
111.7 (2)
120.2 (3)
126.8 (6)
120.2 (5)
O5–C2
O5–C5
P1–Au1–S1
C2–S1–Au1
C2–S1–Au1A
C31–P1–Au1
C30–P1–Au1
C32–P1–Au1
C31–P1–Au1A
C30–P1–Au1A
C32–P1–Au1A
N1–C2–S1
O5–C2–S1
Fig. 3. Crystal packing of (4) as seen along bc plane. Color codes: C: gray, H: white,
N: blue, O: red, P: yellow, Au: orange. (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this article.)

90
J.D.S. Chaves et al. / Inorganica Chimica Acta 414 (2014) 85–90
Table 3
Cytotoxic activities against cell lines. IC50
(l
M
SD) and SI.
Compounds
Tumor cells IC50
B16F10
(lM
SD)a
Non-tumor cells IC50
SIb
(lM
SD)a
BHK-21
SIb
CT26WT
(A)ligand
(1)
(2)
(B)ligand
(3)
(4)
2.28 1.4
0.40 0.10
0.13 0.05
6.53 0.6
0.12 0.03
0.18 0.05
6.40 2.20
36.7
7.8
17.23 1.0
<0.10
0.20 0.03
8.28 0.4
<0.10
4.9
31.0
45.5
10.1
33.5
15.4
12.0
83.70 1.4
3.10 0.30
9.10 2.40
83.60 1.0
3.35 0.50
6.15 0.20
8.40 1.90
70.0
12.8
27.9
34.2
1.3
0.40 0.04
0.70 0.20
Cisplatin
a
SD – standard deviation of triplicate of two independent experiments.
SI – selectivity index.
b
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lM) than in B16F10 cells (IC50 = 0.40 -
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Complexes (2) and (4) were also much more cytotoxic than
their respective free ligands. The highest SI was found for complex
(2) that presents a combination of alkyl phosphine and thiazolidine
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In general, aryl phosphine complexes (1) and (3) were more toxic
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The same trend was not observed on melanoma cell line (B16F10).
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