Neutral gold complexes with tridentate SNS thiosemicarbazide ligands

Article abstract of DOI:10.1021/ic201905t

Na[AuCl₄]?2H₂O reacts with tridentate thiosemicarbazide ligands, H₂L1, derived from N-[N′,N′- dialkylamino(thiocarbonyl)]benzimidoyl chloride and thiosemicarbazides under formation of air-stable, green [AuCl(L1)] complexes. The organic ligands coordinate in a planar SNS coordination mode. Small amounts of gold(I) complexes of the composition [AuCl(L3)] are formed as side-products, where L3 is an S-bonded 5-diethylamino-3-phenyl-1-thiocarbamoyl-1,2,4-triazole. The formation of the triazole L3 can be explained by the oxidation of H₂L1 to an intermediate thiatriazine L2 by Au³⁺, followed by a desulfurization reaction with ring contraction. The chloro ligands in the [AuCl(L1)] complexes can readily be replaced by other monoanionic ligands such as SCN^- or CN^- giving [Au(SCN)(L1)] or [Au(CN)(L1)] complexes. The complexes described in this paper represent the first examples of fully characterized neutral Gold(III) thiosemicarbazone complexes. All the [AuCl(L1)] compounds present a remarkable cell growth inhibition against human MCF-7 breast cancer cells. However, systematic variation of the alkyl groups in the N(4)-position of the thiosemicarbazone building blocks as well as the replacement of the chloride by thiocyanate ligands do not considerably influence the biological activity. On the other hand, the reduction of Au^III to Au^I leads to a considerable decrease of the cytotoxicity.

Full text of DOI:10.1021/ic201905t

Article
pubs.acs.org/IC
Neutral Gold Complexes with Tridentate SNS Thiosemicarbazide
Ligands∥
Pedro I. da S. Maia,† Hung Huy Nguyen,‡ Daniela Ponader,§ Adelheid Hagenbach,§ Silke Bergemann,⊥
Ronald Gust,⊥ Victor M. Deflon,† and Ulrich Abram*,§
†Instituto de Química de Sao Carlos, Universidade de Sao Paulo, CP 780, Sao Carlos-Sao Paulo, Brazil
̃
̃
̃
̃
‡Department of Chemistry, Hanoi University of Science, 19 Le Thanh Tong, Hanoi, Vietnam
§Institute of Chemistry and Biochemistry, Freie Universitat Berlin, Fabeckstr. 34-36, D-14195 Berlin, Germany
⊥Institute of Pharmacy, Freie Universitat Berlin, Konigin-Louise-Str. 2 and 4, D-14195 Berlin, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: Na[AuCl4]·2H2O reacts with tridentate thio-
semicarbazide ligands, H2L1, derived from N-[N′,N′-
dialkylamino(thiocarbonyl)]benzimidoyl chloride and thiose-
micarbazides under formation of air-stable, green [AuCl(L1)]
complexes. The organic ligands coordinate in a planar SNS
coordination mode. Small amounts of gold(I) complexes of
the composition [AuCl(L3)] are formed as side-products,
where L3 is an S-bonded 5-diethylamino-3-phenyl-1-thiocar-
bamoyl-1,2,4-triazole. The formation of the triazole L3 can be explained by the oxidation of H2L1 to an intermediate thiatriazine
L2 by Au3+, followed by a desulfurization reaction with ring contraction. The chloro ligands in the [AuCl(L1)] complexes can
readily be replaced by other monoanionic ligands such as SCN− or CN− giving [Au(SCN)(L1)] or [Au(CN)(L1)] complexes.
The complexes described in this paper represent the first examples of fully characterized neutral Gold(III) thiosemicarbazone
complexes. All the [AuCl(L1)] compounds present a remarkable cell growth inhibition against human MCF-7 breast cancer cells.
However, systematic variation of the alkyl groups in the N(4)-position of the thiosemicarbazone building blocks as well as the
replacement of the chloride by thiocyanate ligands do not considerably influence the biological activity. On the other hand, the
reduction of AuIII to AuI leads to a considerable decrease of the cytotoxicity.
A new class of tridentate thiocarbamoylbenzamidines
containing an additional thiosemicarbazone moiety and their
rhenium complexes have been reported recently together with
the biological activity of the organic compounds and some of
their oxorhenium(V) and nitridorhenium(V) complexes.10 The
novel chelators, which combine a thiosemicarbazide moiety
with a thiourea building block can readily be prepared from the
corresponding benzimide chloride and various thiosemicarba-
zides (Chart 1).10
Here, we report about the synthesis and characterization of a
series of new gold(III) complexes with the potentially
tridentate ligands H2L1, as well as a first evaluation of their
in vitro cytotoxic activity as the basis for their evaluation as
potential anticancer and antituberculosis drugs.
INTRODUCTION
■
Thiosemicarbazones are versatile ligands of considerable
significance with respect to their variable coordination behavior
and promising biological and pharmaceutical properties,1,2 and
gold complexes are classical inorganic pharmaceuticals for
treatment of arthritis and rheumatism.3 Gold complexes with
thiosemicarbazones are comparatively rare, and some of the
recently published papers deal with gold(I) compounds.4 Of
particular interest, however, should be gold(III) compounds
which are isoelectronic with Pt(II) and also prefer square-
planar complexes. However, the first gold(III) compound with
a thiosemicarbazone ligand was published only in 1998,5 and
afterward only few complexes of this type have been reported.
In most of the published papers, the Au(III) thiosemicarbazone
complexes without an organometallic framework, which is
provided by the precursor dichloro[2-(dimethylaminomethyl)-
phenyl-C1,N]-gold(III), [Au(damp-C1,N)Cl2], are reported to
be not stable, which leads to reduction to Au(I) or to the
formation of decomposition products.6 Indeed, only a few
gold(III) thiosemicarbazone complexes without the damp
ligand have been structurally characterized up to now,7−9 and
no neutral complex have been reported. A very recent work
evaluates first the potential of radioactive 198Au thiosemicarba-
zone complexes for nuclear therapeutic applications.9
EXPERIMENTAL SECTION
■
Materials. All reagents used in this study were reagent grade and
used without further purification. The solvents were dried and used
freshly distilled prior to use unless otherwise stated. Na[AuCl4]·2H2O
was prepared by a standard procedure.11 The synthesis of N-
[(diethylamino)(thiocarbonyl)]benzimidoyl chloride was performed
by the procedure of Beyer and Widera.12 The thiosemicarbazides were
Received: August 31, 2011
Published: January 10, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Article
Chart 1. Synthesis of the Ligands Used Throughout This Study
prepared by methods reported before.13 The compounds H2L1a,
H2L1b, H2L1d, and H2L1e were prepared as reported in our previous
papers.10,14
Synthesis of the [AuIIICl(L1)] Type Complexes. 0.1 mmol of
H2L1 was added to a solution of 0.1 mmol of Na[AuCl4]·2H2O in
MeOH (1 mL). The resulting solutions were stirred for 2 h at room
temperature. Green solids precipitated during this time. The
precipitates were filtered off, washed with cold MeOH, n-hexane,
and dried in air. Recrystallization from MeOH/CH2Cl2 mixtures gave
green crystals. By slow evaporation of the remaining mother solutions,
small amounts (approximately 1−20%) of colorless solids appeared,
which could only be separated in reasonable amounts for the H2L1d
derivative. It could be characterized as a gold(I) complex of a
cyclization product of the ligand, [AuCl(L3d)].
Physical Measurements. IR spectra were measured as CsI pellets
on a Shimadzu IR Prestige-21 spectrophotometer between 250 and
1
4000 cm−1. H and 13C NMR spectra were taken with a JEOL 400
MHz multinuclear spectrometer. ESI+ mass spectra were measured
with an Agilent 6210 ESI-TOF (Agilent Technologies). The elemental
analyses of carbon, hydrogen, nitrogen, and sulfur were determined
using a Heraeus vario EL elemental analyzer. The elemental analyses
of the gold compounds showed systematically too low values for
hydrogen and sometimes carbon (in some cases in a significant
extent). This might be caused by an incomplete combustion of the
metal compounds and/or hydride formation, and does not refer to
impure samples. Similar findings have been observed for analogous
rhenium complexes with the same type of ligands before.10 We left
these values uncorrected. Additional proof for the identity of the
products is given by high-resolution mass spectra for selected
representatives. The conductivities of some representative complexes
were measured in dimethylsulfoxide (DMSO) with an Orion Star
Series conductometer.
[AuIIICl(L1a)] (1). Yield: 51% (32 mg). Anal. Calcd for
C20H23AuClN5S2: C, 38.13; H, 3.69; N, 11.12; S, 10.18%. Found: C,
37.92; H, 3.23; N, 11.21; S, 10.02%. IR (νmax/cm−1): 1552 m (CN),
1489 vs (CC), 335 w (Au−Cl). 1H NMR (CDCl3, ppm): 1.16 (t, J
= 7.2 Hz, 3H, CH3), 1.24 (t, J = 7.2 Hz, 3H, CH3), 3.12 (s, 3H, N−
CH3), 3.66 (q, J = 7.2 Hz, 4H, methylene), 7.15−7.32 (m, 8H, Ph),
7.58−7.61 (m, 2H, o-Ph). 13C NMR (CDCl3, δ, ppm): 12.7, 13.0
(CH3), 43.5 (NCH3), 45.3, 47.5 (NCH2), 126.8, 127.0, 127.3, 129.1,
129.4, 137.9, 146.1 (Ph), 155.3, 158.3, 160.2 (CN). ESI+ MS (m/z,
assignment): 594 [M-Cl]+, 630 [M]+, 652 [M + Na]+, 668 [M+K]+,
991 [Au(L1a)(L2a)]+. High resolution MS of molecular ion [M+H]+
Calcd: 630.0827, Found: 630.0824.
Preparations of the Ligands. To a mixture containing the
desired thiosemicarbazide (2 mmol) and 12 mmol of Et3N in dry
tetrahydrofuran (THF, 10 mL), N-(N′,N′-diethylaminothiocarbonyl)-
benzimidoyl chloride (2 mmol) was added. The mixture was stirred for
4 h at room temperature. The colorless precipitate of NEt3·HCl was
filtered off, and the solvent of the filtrate was removed under reduced
pressure. The residue was dissolved in diethyl ether (10 mL) and
stored at −20 °C. The colorless solid of H2L1, which deposited from
this solution, was filtered off, washed with n-hexane, and dried under
vacuum.
[AuIIICl(L1b)] (2). Yield: 47% (28 mg). Anal. Calcd for
C17H23AuClN5S2: C, 34.38; H, 3.90; N, 11.79; S, 10.80%. Found: C,
33.98; H, 3.52; N, 11.75; S, 11.02%. IR (νmax/cm−1): 1560 m (CN),
1
1500 vs (CC), 341 w (Au−Cl). H NMR (CDCl3, ppm): 1.12−
1.18 (m, 6H, CH3), 1.80 (s, br, 4H, CH2), 3.15 (s, br, 4H, CH2), 3.63
(s, br, 4H, NCH2, pyrrolidine), 7.27−7.36 (m, 3H, Ph), 7.58 (dd, 3J =
4
6.7 Hz, J = 2.3 Hz, 2H, o-Ph). ESI+ MS (m/z, assignment): 558 [M-
Cl]+, 594 [M+H]+, 616 [M + Na]+, 632 [M+K]+, 919 [Au(L1b)-
(L2b)]+. High resolution MS of molecular ion [M+H]+ Calcd:
594.0827, Found: 594.0811.
The compounds obtained by this procedure were used for the
syntheses of the complexes without further purification.
[AuIIICl(L1c)] (3). Yield: 48% (29 mg). Anal. Calcd for
C17H23AuClN5OS2: C, 33.48; H, 3.80; N, 11.48; S, 10.51%. Found:
C, 33.14; H, 3.38; N, 11.51; S, 10.38%. IR (νmax/cm−1): 1556 m (C
N), 1500 vs (CC), 339 w (Au−Cl). 1H NMR (CDCl3, ppm): 1.14−
1.28 (m, 6H, CH3), 3.17 (t, J = 4.8 Hz, 4H, N−CH2), 3.55 (t, J = 4.8
Hz, 4H, O−CH2), 3.68 (q, J = 7.0 Hz, 4H, methylene), 7.28−7.30 (m,
3H, Ph), 7.51 (d, J = 7.8 Hz, 2H, o-Ph). 13C NMR (CDCl3, δ, ppm):
12.7, 13.0 (CH3), 45.3, 47.5 (NCH2, ethyl), 49.7 (NCH2, morpho-
line), 66.5 (OCH2), 127.3, 129.3, 129.4, 137.7 (Ph), 155.1 (CN),
158.0 (CN), 160.9 (CN). ESI+ MS (m/z, assignment): 574 [M-
Cl]+, 610 [M+H]+, 632 [M + Na]+, 648 [M+K]+, 951 [Au(L1c)-
(L2c)]+, 1241 [2M+Na]+. Molar conductivity (10−3 M, DMSO): 2.5 S
cm2 mol−1. High resolution MS of molecular ion [M+H]+ Calcd:
610.0776, Found: 610.0784.
H2L1c. Colorless needles. Yield: 71% (540 mg). Anal. Calcd for
C17H25N5OS2: C, 53.80; H, 6.64; N, 18.45; S, 16.89%. Found: C,
53.81; H, 6.46; N, 18.20; S, 16.59%. IR (νmax/cm−1): 3182 s (NH),
1
1636 vs (CN). H NMR (CDCl3, ppm): 1.04 (t, J = 7.1 Hz, 3H,
CH3), 1.21 (t, J = 7.0 Hz, 3H, CH3), 3.51 (q, J = 7.1 Hz, 2H, CH2),
3.68 (t, J = 4.8 Hz, 4H, N−CH2 morph), 3.82−3.89 (m, 6H, CH2 +
O−CH2 morph), 7.42−7.49 (m, 3H, Ph), 7.86 (d, J = 7.8 Hz, 2H, o-
Ph), 9.49 (s, 1H, NH), 10.11 (s, 1H, NH). 13C NMR (CDCl3, δ,
ppm): 12.4, 13.0 (CH3), 45.0, 46.4 (NCH2, ethyl), 48.5 (NCH2,
morpholine), 66.1 (OCH2), 127.7, 128.8, 131.6, 132.8 (Ph), 148.9
(CN), 179.7, 183.2 (CS). ESI+ MS (m/z, assignment): 402 (M +
Na)+, 418 (M + K)+.
H2L1f. Colorless powder. Yield: 75% (590 mg). Anal. Calcd for
C19H31N5S2: C, 57.98; H, 7.94; N, 17.79; S, 16.29%. Found: C, 58.05;
H, 7.91; N, 17.47; S, 16.43%. IR (νmax/cm−1): 3240 s (NH), 1632 vs
(CN). 1H NMR (CDCl3, ppm): 0.91 (t, J = 7.5 Hz, 6H,
−CH2CH2CH3), 0.99 (t, J = 7.1 Hz, 3H, −CH2CH3), 1.23 (t, J = 7.1
Hz, 3H, −CH2CH3), 1.62 (sex, J = 7.5, 4H, −CH2CH2CH3), 3.53 (m,
6H, −CH2CH2CH3 + −CH2CH3), 3.90 (q, J = 7.1 Hz, 2H, −
CH2CH3), 7.37−7.47 (m, 3H, Ph), 7.89 (d, J = 6.8 Hz, 2H, o-Ph), 9.67
(s, 1H, NH), 9.87 (s, 1H, NH). 13C NMR (CDCl3, δ, ppm): 11.3,
12.4, 12.9 (CH3), 20.4 (CH2, n-propyl), 44.6, 46.2 (NCH2), 127.7,
128.7, 131.4, 133.1 (Ph), 149.2 (CN), 178.5, 183.2 (CS). ESI+
MS (m/z, assignment): 416 [M + Na]+, 432 [M + K]+.
[AuIIICl(L1d)] (4). Yield: 56% (35 mg). Anal. Calcd for
C19H27AuClN5S2: C, 36.69; H, 4.38; N, 11.26; S, 10.31%. Found: C,
36.43; H, 3.68; N, 11.31; S, 10.71%. IR (νmax/cm−1): 1549 m (CN),
1497 vs (CC), 337 w (Au−Cl). 1H NMR (CDCl3, ppm): 1.14 (t, J
= 7.0 Hz, 3H, CH3), 1.21 (t, J = 8.8 Hz, 3H, CH3), 1.43 (s, br, 4H,
CH2, azepine), 1.50 (s, br, 4 H, CH2, azepine), 3.28 (t, J = 5.8 Hz, 4H,
NCH2 azepine), 3.63 (m, 4H, NCH2CH3), 7.27−7.25 (m, 3H, Ph),
7.51 (dd, J = 6.6 Hz, J = 2.5 Hz, 2H, o-Ph). 13C NMR (CDCl3, δ,
ppm): 12.9, 13.0 (CH3), 26.9, 28.4 (CH2, azepine), 45.1, 47.3 (NCH2,
ethyl), 52.7 (NCH2, azepine), 127.2, 128.8, 129.3, and 138.2 (Ph),
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Inorganic Chemistry
Article
Table 1. X-ray Structure Data Collection and Refinement Parameters for [AuIIICl(L1d)] (4), [AuIIICl(L1e)] (5), [AuICl(L3d)]
(4a), [AuIII(SCN)(L1c)] (8), [Au(CN)(L1c)] (9), and [Au(CN)(L1d))] (10)
4
5
4a
8
9
10
formula
C19H27AuClN5S2
621.99
C15H21AuClN5S2
567.90
C19H27AuClN5S
589.93
C18H23AuN6OS3
632.57
C18H23AuN6OS2
600.51
C20H27AuN6S2
612.56
Fw
crystal system
space group
a (Å)
orthorhombic
Pbca
monoclinic
P21/n
monoclinic
P21/n
monoclinic
P21/c
monoclinic
P21/c
orthorhombic
Pbca
13.7878(6)
9.7254(5)
33.280(2)
90
10.8864(7)
8.0631(6)
22.032(1)
90
8.0871(7)
23.774(2)
11.614(1)
90
11.833(1)
8.2302(6)
22.484(2)
90
11.884(1)
14.571(1)
13.368(1)
90
13.927(1)
9.6701(8)
33.253(3)
90
b (Å)
c (Å)
α (deg)
β (deg)
90
99.57(1)
90
106.44(1)
90
95.41(1)
90
114.85(1)
90
90
γ (deg)
V (Å3)
90
90
4462.6(4)
8
1907.0(2)
4
2141.7(3)
4
2179.9(3)
4
2100.4(3)
4
4478.4(7)
8
Z
ρcalcd (g·cm−3)
μ (mm−1)
1.852
1.978
1.830
1.927
1.899
1.817
6.915
8.081
7.105
7.059
7.224
6.775
reflections collected
reflections unique/Rint
data/restraints/param.
absorption correction
max/min transmission
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
GOF
45569
13168
16009
13047
13384
13768
6026/0.0957
6026/0/256
integration
0.5417/0.3142
0.0500
5115/0.0590
5115/0/222
integration
0.7692/0.3744
0.0465
5757/0.0588
5757/0/247
integration
0.4632/0.2311
0.0345
5836/0.0739
5836/0/265
integration
0.7088/0.2120
0.0559
5609/0.0794
5609/0/256
integration
0.6790/0.3397
0.0394
5927/0.0605
5927/0/265
integration
0.4418/0.1816
0.0631
0.1199
0.1046
0.0776
0.1336
0.0813
0.1628
0.883
1.002
1.089
0.984
0.868
1.027
153.6, 157.3, 159.9 (CN). ESI+ MS (m/z, assignment): 586 [M-
Cl]+, 622 [M+H]+, 644 [M + Na]+, 660 [M+K]+, 975 [Au(L1d)-
(L2d)]+. Molar conductivity (10−3 M, DMSO): 2.2 S cm2 mol−1. High
resolution MS of molecular ion [M+H]+ Calcd: 622.1140, Found:
622.1129.
[M + Na]+, 662 [M+K]+, 979 [Au(L1f)(L2f)]+, 1269 [2M+Na]+, 1285
[2M+K]+. Molar conductivity (10−3 M, DMSO): 2.9 S cm2 mol−1.
High resolution MS of molecular ion [M+H]+ Calcd: 624.1297,
Found: 624.1277.
Synthesis of the [AuIII(SCN)(L1)] Type Complexes. To a
solution containing the corresponding [AuCl(L1)] complex (0.05
mmol) in CH2Cl2 (1 mL), NH4(SCN) (0.1 mmol) was added
dissolved in MeOH (1 mL). The resulting solution was stirred for 1 h
at room temperature and left for slow evaporation of the solvent for 2
days. The green-brown crystalline precipitates formed were filtered off,
washed with cold MeOH, and dried under vacuum. Crystals suitable
for X-ray diffraction studies were obtained for [Au(SCN)(L1c)].
[AuIII(SCN)(L1a)] (7). Yield: 83% (27 mg). Anal. Calcd for
C21H23AuN6S3: C, 38.65; H, 3.55; N, 12.88; S, 14.74%. Found: C,
38.28; H, 2.98; N, 12.73; S, 14.90%. IR (νmax/cm−1): 2131 w (CN,
[AuICl(L3d)] (4a). Yield: 17% (10 mg). Anal. Calcd for
C19H27AuClN5S: C, 38.68; H, 4.61; N, 11.87; S, 5.44%. Found: C,
38.65; H, 4.21; N, 11.76; S, 5.37%. IR (νmax/cm−1): 1585 vs (CN),
1508 vs (CC), 332 w (Au−Cl). 1H NMR (CDCl3, ppm): 1.32 (t, J
= 7.1 Hz, 6H, CH3), 1.4−2.2 (m, br, 8H, CH2, azepine), 3.4−3.6 (m,
br, 4H, NCH2, azepine), 3.9−4.1 (m, br, 4H, NCH2CH3), 7.37−7.39
(m, 3H, Ph), 8.01 (dd, J = 4.5 Hz, J = 2.1 Hz, 2H, o-Ph). 13C NMR
(CDCl3, δ, ppm): 12.8 (CH3), 25.7, 26.0, 27.6, 27.8 (CH2, azepine),
44.7 (N−CH2), 55.3, 56.3 (N−CH2, azepine), 127.0, 128.4, 129.9,
130.0 (Ph), 159.6, 162.7 (CN), 180.6 (CS). ESI+ MS (m/z,
assignment): 358 [L3d + H]+, 380 [L3d + Na]+, 396 [L3d + K]+, 715
[2L3d + H]+, 737 [2L3d + Na]+, 753 [2L3d + K]+, 911 [Au(L3d)2]+.
[AuIIICl(L1e)] (5). Yield: 43% (25 mg). Anal. Calcd for
C15H21AuClN5S2: C, 31.72; H, 3.73; N, 12.33; S, 11.29%. Found: C,
30.97; H, 3.16; N, 12.03; S, 11.45%. IR (νmax/cm−1): 1558 m (CN),
1503 vs (CC), 343 w (Au−Cl). 1H NMR (CDCl3, ppm): 1.14 (t, J
= 7.1 Hz, 3H, CH3), 1.23 (t, J = 7.1 Hz, 3H, CH3), 2.83 (s, 6H, N−
CH3) 3.64 (q, J = 7.1 Hz, 4H, methylene), 7.26−7.29 (m, 3H, Ph),
7.55 (d, J = 6.7 Hz, 2H, o-Ph). 13C NMR (CDCl3, δ, ppm): 12.8, 13.0
(CH3), 41.9 (N−CH3), 45.2, 47.4 (N−CH2), 127.2, 129.0, 129.4, and
138.0 (Ph), 154.1 (CN), 157.5 (CN), 161.4 (CN). ESI+ MS
(m/z, assignment): 532 [M-Cl]+, 568 [M+H]+, 590 [M + Na]+, 606
[M+K]+, 867 [Au(L1e)(L2e)]+.
1
thiocyanate), 1555 m (CN), 1497 vs (CC). H NMR (CDCl3,
ppm): 1.18 (t, J = 7.1 Hz, 3H, CH3), 1.26 (t, J = 7.1 Hz, 3H, CH3),
3.14 (s, 3H, N−CH3), 3.69 (m, 4H, methylene), 7.16−7.32 (m, 8H,
Ph), 7.61−7.63 (m, 2H, o-Ph). 13C NMR (CDCl3, δ, ppm): 12.9, 13.0
(CH3), 43.8 (NCH3), 45.5, 47.5 (NCH2), 111.8 (SCN), 126.8, 127.2,
127.4, 129.2, 129.4, 129.6, 137.4, 146.0 (Ph), 155.6 (CN), 157.1
(CN), 159.5 (CN). ESI+ MS (m/z, assignment): 653 [M+H]+,
675 [M + Na]+, 691 [M+K]+, 991 [Au(L1a)(L2a)]+, 1343 [2 M +
K]+, 1979 [3 M + Na]+. Molar conductivity (10−3 M, DMSO): 2.6 S
cm2 mol−1. High resolution MS of molecular ion [M+K]+ Calcd:
691.0449, Found: 691.0315.
[AuIII(SCN)(L1c)] (8). Yield: 88% (28 mg). Found: C, 34.14; H,
3.75; N, 13.12; S, 15.62. Anal. Calcd for C18H23AuN6OS3: C, 34.18; H,
3.66; N, 13.29; S, 15.20. IR (νmax/cm−1): 2129 m (CN, S-bonded
[AuIIICl(L1f)] (6). Yield: 61% (38 mg). Anal. Calcd for
C19H29AuClN5S2: C, 36.57; H, 4.68; N, 11.22; S, 10.28%. Found: C,
36.19; H, 3.62; N, 10.81; S, 10.57%. IR (νmax/cm−1): 1552 m (CN),
1497 vs (CC), 347 w (Au−Cl). 1H NMR (CDCl3, ppm): 1H NMR
(CDCl3, ppm): 0.64 (t, J = 7.7 Hz, 6H, −CH2CH2CH3), 1.13 (t, J =
7.0 Hz, 3H, −CH2CH3), 1.23 (t, J = 7.0 Hz, 3H, −CH2CH3), 1.38
(sex, J = 7.7, 4H, −CH2CH2CH3), 3.03 (t, J = 7.7 Hz, 4H, −
CH2CH2CH3), 3.62 (m, 4H, −CH2CH3), 7.25−7.27 (m, 3H, Ph),
7.45 (d, J = 6.6 Hz, 2H, o-Ph). 13C NMR (CDCl3, δ, ppm): 11.0, 12.8,
12.9 (CH3), 21.3 (CH2, n-propyl), 45.1, 47.4 (NCH2, ethyl), 54.8
(NCH2, n-propyl) 127.3, 128.5, 128.8, 138.4 (Ph), 153.8, 157.3, 159.9
(CN). ESI+ MS (m/z, assignment): 588 [M-Cl]+, 624 [M+H]+, 646
1
thiocyanate), 1558 m (CN), 1508 vs (CC). H NMR (CDCl3,
ppm): 1.17 (t, J = 7.1 Hz, 3H, CH3), 1.26 (t, J = 7.1 Hz, 3H, CH3),
3.20 (t, J = 4.8 Hz, 4H, N−CH2), 3.56 (t, J = 4.8 Hz, 4H, O−CH2),
3.66−3.70 (m, 4H, methylene), 7.27−7.32 (m, 3H, Ph), 7.54 (d, J =
7.9 Hz, 2H, o-Ph). 13C NMR (CDCl3, δ, ppm): 12.9, 13.0 (CH3), 45.6,
47.6 (NCH2, ethyl), 49.9 (NCH2, morpholine), 66.5 (OCH2), 111.7
(SCN), 127.4, 129.5, 129.6, 137.2 (Ph), 155.5 (CN), 156.8 (C
N), 160.3 (CN). ESI+ MS (m/z, assignment): 574 [M-(SCN)]+,
633 [M+H]+, 655 [M + Na]+, 671 [M+K]+, 951 [Au(L1c)(L2c)]+,
1287 [2 M + K]+, 1919 [3 M + Na]+. High resolution MS of molecular
ion [M+H]+ Calcd: 633.0839, Found: 633.0830.
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Synthesis of the [Au(CN)(L1)] Type Complexes. To a solution
containing the corresponding [AuCl(L1)] type complex (0.05 mmol)
in CH2Cl2 (1 mL), were added 2 equiv of KCN (0.1 mmol) dissolved
in MeOH (2 mL). The resulting solution was stirred for 15 min at
room temperature. Slow evaporation of the solvent gave dark red
crystalline precipitates. The crystals formed were filtered off, washed
with cold MeOH, and dried under vacuum. Crystals suitable for X-ray
diffraction studies were obtained for [Au(CN)(L1c)] and [Au(CN)-
(L1d)].
contained the corresponding amount of DMF and DMSO,
respectively. The medium was removed after reaching the appropriate
incubation time. Subsequently, the cells were fixed with a solution of
1% (v/v) glutaric dialdehyde in phosphate buffered saline (PBS) and
stored under PBS at 4 °C. Cell biomass was determined by means of a
crystal violet staining technique as described earlier.17 The
effectiveness of the complexes is expressed as corrected T/Ccorr [%]
or τ[%] values according to the following equation:
[AuIII(CN)(L1c)] (9). Yield: 53% (16 mg). Anal. Calcd for
C18H23AuN6OS2: C, 36.00; H, 3.86; N, 13.99; S, 10.68%. Found: C,
35.74; H, 3.06; N, 13.60; S, 10.93. IR (νmax/cm−1): 2175 vw (C≡N),
1564 m (CN), 1504 vs (CC), 436 w (Au−C). 1H NMR (CDCl3,
ppm): 1.18 (t, J = 7.0 Hz, 3H, CH3), 1.26 (t, J = 7.0 Hz, 3H, CH3),
3.23 (t, J = 4.8 Hz, 4H, N−CH2), 3.57 (t, J = 4.8 Hz, 4H, O−CH2),
3.70 (m, 4H, methylene), 7.28−7.32 (m, 3H, Ph), 7.53 (d, J = 6.0 Hz,
2H, o-Ph). 13C NMR (CDCl3, δ, ppm): 12.8, 13.0 (CH3), 45.7, 47.5
(NCH2, ethyl), 50.0 (NCH2, morpholine), 66.5 (OCH2), 102.3
(C≡N), 127.3, 129.6, 129.6, 137.2 (Ph), 155.6 (CN), 157.4 (C
N), 161.4 (CN). ESI+ MS (m/z, assignment): 601 [M+H]+, 623 [M
+ Na]+, 639 [M+K]+, 1223 [2 M + Na]+, 1823 [3 M + Na]+.
[AuIII(CN)(L1d)] (10). Yield: 49% (15 mg). Anal. Calcd for
C20H27AuN6S2: C, 39.22; H, 4.44; N, 13.72; S, 10.47%. Found: C,
38.46; H, 2.80; N, 13.27; S, 11.08%. IR (νmax/cm−1): 2172, 2133 vw
(C≡N), 1556 m (CN), 1501 vs (CC), 444 w (Au−C). 1H NMR
(CDCl3, ppm): 1.16 (t, J = 6.7 Hz, 3H, CH3), 1.24 (t, J = 6.7 Hz, 3H,
CH3), 1.4−1.6 (m, 8H, CH2 azepine), 3.35 (t, J = 5.6 Hz, 4H, N−
CH2), 3.67 (t, J = 6.8 Hz, 4H, NCH2CH3), 7.26−7.29 (m, 3H, Ph),
7.53 (d, J = 7.0 Hz, 2H, o-Ph). 13C NMR (CDCl3, δ, ppm):12.9
(CH3), 26.9, 28.4 (CH2, azepine), 45.5, 47.3 (NCH2, ethyl), 52.9
(NCH2, azepine), 103.1 (C≡N), 127.2, 129.1, 129.4, 137.7 (Ph),
154.8, 156.1, 160.4 (CN). ESI+ MS (m/z, assignment): 613 [M
+H]+, 635 [M + Na]+, 651 [M+K]+.
cytostatic effect: T/C [%] = [(T − C )/(C − C )]×
corr
0
0
100
cytocidal effect: τ[%] = [(T − C )/C ]× 100
0
0
whereby T (test) and C (control) are the optical densities at 590 nm
of crystal violet extract of the cells in the wells (i.e., the chromatin-
bound crystal violet extracted with ethanol (70%) with C0 being the
density of the cell extract immediately before treatment. For the
automatic estimation of the optical density of the crystal violet extract
in the wells, a microplate autoreader (Flashscan S 12; Analytik Jena,
Germany) was used.
RESULTS AND DISCUSSION
■
Reactions of H2L1 with Na[AuCl4]·2H2O in MeOH at room
temperature afford analytically pure microcrystalline green
precipitates of the composition [AuCl(L1)] in good yields
(Chart 2). These products are soluble in CH2Cl2, CHCl3,
acetone and DMSO and almost insoluble methanol or ethanol.
Chart 2. Synthesis of the Gold(III) Complexes
X-ray Crystallography. The intensities for the X-ray determi-
nations were collected on a STOE IPDS 2T instrument with Mo Kα
radiation (λ = 0.71073 Å). Standard procedures were applied for data
reduction and absorption correction. Structure solution and refine-
ment were performed with SHELXS-97 and SHELXL-97.15 Hydrogen
atom positions were calculated for idealized positions and treated with
the “riding model” option of SHELXL. More details on data
collections and structure calculations are contained in Table 1.
Biochemicals and Biological Studies. Cell Culture Condi-
tions. The human MCF-7 breastcancer cell line was obtained from the
American type Culture Collection (ATCC). This cell line was
maintained as a monolayer culture in L-glutamine containing
Dulbeccos Modified Eagles Medium (DMEM) with 4.5 g/L glucose
(PAA Laboratories GmbH, Austria), supplement with 10% fetal calf
serum (FCS; Gibco, Germany) using 25 cm2 culture flasks in a
humidified atmosphere (5% CO2) at 37 °C. The cell lines were
passaged twice a week after previous treatment with trypsin (0.05%)/
ethylenediaminetetraacetic acid (0.02% EDTA; Boehringer, Ger-
many). Jurkat cells were purchased from the German Collection of
Microorganisms and Cell Culture (Deutsche Sammlung von
Mikroorganismen and Zellkulturen, Braunschweig), DSMZ No ACC
282, LOT 7. The cells were maintained in RPMI 1640 (PAA) medium
supplemented with 10% fetal calf serum (PAA), 37 °C, 5% CO2, and
maximum humidity.
In Vitro Chemosensitivity Assay. The in vitro testing of the
substances for antitumor activity in adherent growing cell lines was
carried out on exponentially dividing human cancer cells according to
a previously published microtiter assay.16 Exponential cell growth was
ensured during the whole time of incubation. Briefly, 100 μL of a cell
suspension was placed in each well of a 96-well microtiter plate at 7200
cells/mL of culture medium and incubated at 37 °C in a humidified
atmosphere (5% CO2) for 3 d. By removing the old medium and
adding 200 μL of fresh medium containing an adequate volume of a
stock solution of metal complex, the desired test concentration was
obtained. Cisplatin was dissolved in dimethylformamide (DMF) while
dimethylsulfoxide (DMSO) was used for all other compounds. Eight
wells were used for each test concentration and for the control, which
The IR spectra of H2L1 are characterized by strong broad
NH absorptions in the range 3140−3240 cm−1 and very strong
and sharp bands around 1630 cm−1, which can be assigned to
CN stretches. In the spectra of the gold(III) complexes the
NH absorptions are not observed, as expected for the double
deprotonation of the ligands during the reaction and the
formation of neutral compounds. The CN stretches are
found in the region between 1558 and 1547 cm−1, which
corresponds to a bathochromic shift of the νCN stretches of
about 80 cm−1 compared to those of the uncoordinated ligand
and indicates chelate formation with a large degree of π-
electron delocalization within the chelate rings. Weak
absorptions in the range between 335 and 347 cm−1 can be
assigned to the νAuCl stretching bands.
1
The H NMR spectra of the complexes show the signals in
the expected regions.10 The signals of the two NH protons,
which appear as very broad singlets in the region between 10.11
and 9.49 ppm for the free ligands are expectedly not observed
1
in the H NMR spectra of the complexes. This fact confirms
the double deprotonation of the ligands which was also
confirmed by the IR data.
The 13C NMR spectra show the expected signals in the
appropriate regions. For the uncoordinated thiosemicarbazides,
the CN and CS signals of thiourea residues appear in the
regions around 149 ppm and 183 ppm, while the CS
resonances of the thiosemicarbazide moiety appear around 179
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ppm, as discussed before.10 Upon coordination and formation
of the [AuCl(L1)] complexes, a downfield shift is observed for
the 13C NMR signals of the CN (around 5 ppm), while the
CS carbon atom signals are upfield shifted, appearing
between 158 and 160 ppm. This is consistent with the S,N,S-
coordination and thioenolization of the CS of thiourea and
thiosemicarbazone moieties.
In the case of the H2L1 ligands, however, this reaction pathway
plays an marginal role only, and just in the case of H2L1d a
significant amount of the reduction product could be isolated
by slow evaporation of the mother solution as colorless crystals
(Chart 3). In the IR spectra of these crystals a very strong band
in the CN region is observed at 1585 cm−1. In the 1H NMR
spectrum of the compound, the signals are found downfield
shifted with respect to those in the spectrum of the gold(III)
complex [AuIIICl(L1d)]. In contrast to the situation in the 13C
NMR spectra of the compounds H2L1 and the [AuCl(L1)]
complexes, the rotation around the C-NR1R2 bonds of the
thiosemicarbazide unit is restricted in the gold(I) complex
[AuICl(L3d)]. This is indicated by the appearance of 13C NMR
signals for magnetically nonequivalent carbon atoms of the
hexamethyleneimine substituent. The observation of a signal at
180.6 ppm is indicative for the coordination of the L3d ligand
in its thione form in the gold(I) complex.
Figure 1 illustrates the molecular structure of the [AuIIICl-
(L1d)] (4) as representative of the [AuIIICl(L1)] complexes.
The ESI MS spectrum of the colorless gold(I) compound
indicated a rearrangement of the organic ligands and the
formation of a heterocyclic species after the abstraction of a
sulfur atom. Finally, the nature of the side-product could be
resolved with an X-ray structural analysis. Figure 2 shows the
molecular structure of the product, which represents the
expected gold(I) complex with an unusual thiourea ligand
together with selected bond lengths and angles. The structure
shows that the reduction of the gold atom occurs with a
cyclization of the {L1d}2− ligand and sulfur abstraction. The
resulting N-(hexamethylene)-N′-1-(5-diethylamino-3-phenyl-
1,2,4-triazolyl)thiourea (L3) acts as a monodentate ligand in
the product. A linear coordination environment, which is
completed by a chloro ligand, has been found for the gold
atom. The triazole ring system is perfectly planar with a
maximum deviation of 0.006 Å for the N1 atom. The C−S
bond of 1.708(5) Å is slightly elongated from a double bond,
which is related with the coordination to gold.
Figure 1. Molecular structure of the complex [AuIIICl(L1d)] (4).
Hydrogen atoms omitted for clarity.
Selected bond lengths and angles for this gold(III) complex are
shown together with the values of compound 5 in Table 2. The
arrangement around the gold atom is best described as slightly
distorted square-planar. The thiosemicarbazonate binds the
gold atom as S,N,S-chelate with double-deprotonation of the
coordinating backbone. Thus, the square plane is defined by the
donor atoms of the tridentate ligand and a remaining chloro
ligand, with the maximum deviation from the mean least-
squares plane of 0.010 Å for N2. The observed Au−Cl and
Au−S bond lengths are similar to previously reported values for
comparable gold(III) thiosemicarbazonates.6a,7 A considerable
delocalization of π-electron density in the six- and five-
membered rings is evidenced by the observed bond lengths.
The C−S and C−N bonds inside the chelate rings are within
the ranges between carbon−sulfur and carbon−nitrogen single
and double bonds, respectively. The Au−S1 and Au−S2 bond
distances are quite similar; showing strong bond length
equalization, which also influences the planarity of the ligand,
and makes the bonding backbone almost planar.
Since the [AuIIICl(L1)] compounds are stable both in the
solid state and also in solvents like CH2Cl2, CHCl3, or DMSO
even under reflux conditions for several hours, the formation of
the gold(I) complexes occurs most probably parallel to that of
the gold(III) complexes. The minor amounts of the side-
products, which could be detected for all other H2L1
derivatives, except than H2L1d, did not allow the isolation of
the Au(I) compounds in pure form.
Nevertheless, there is a strong hint from the mass spectra of
all [AuCl(L1)] complexes, that under high-energy conditions
cyclization reactions occur. Besides the expected molecular
peaks for the gold(III) complexes [AuCl(L1)], there appear
The reduction of Au(III) by thiosemicarbazones is not
unexpected and has been reported in several examples before.
Table 2. Selected Bond Lengths (Å) and Angles (deg) in [AuIIICl(L1d)] (4) and [AuIIICl(L1e)] (5)
4
5
4
5
Au−Cl
Au−S1
Au−N2
Au−S2
C1−N1
C1−S1
S1−Au−S2
N2−Au−S1
N2−Au−S2
C1−S1−Au
C3−S2−Au
2.308(2)
2.282(2)
2.018(5)
2.287(2)
1.317(9)
1.734(8)
176.87(7)
97.12(7)
86.03(7)
106.2(3)
94.2(3)
2.291(2)
2.289(2)
2.015(6)
2.286(2)
1.31(1)
N1−C2
C2−N2
N2−N3
N3−C3
C3−S2
C1−N5
N2−Au−Cl
S1−Au−Cl
S2−Au−Cl
N3−N2−Au
C2−N2−Au
1.352(9)
1.295(9)
1.394(8)
1.286(0)
1.768(8)
1.349(0)
176.80(7)
86.03(7)
90.85(7)
117.3(4)
126.8(5)
1.34(1)
1.318(9)
1.384(9)
1.29(1)
1.747(8)
1.34(1)
1.733(9)
176.20(8)
99.0(2)
175.3 (2)
85.51(8)
90.69(8)
119.4(5)
124.2(5)
84.8(2)
105.1(3)
94.9(3)
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Chart 3. Formation of a AuI Side-Product during the Reaction with H2L1d
Chart 5. Formation of the [Au(X)(L1)] Complexes (X =
SCN or CN)
Figure 2. Structure of the complex [AuICl(L3d)] (4a). Hydrogen
atoms are omitted for clarity. Selected bond length (Å) and angles
(deg): Au−Cl 2.271(1), Au−S 2.261(1), S2−C3 1.708(5), C3−N3
1.404(5), Cl−Au−S2 176.80(5), Au−S2−C3 108.9(2).
peaks for species of the composition [Au(L1)(L2)]+, where L2
corresponds to the primary cyclization product of the
corresponding thiosemicarbazone (Chart 4). As the samples
were crystalline and analytically pure, these species were formed
under measurement conditions. The observation of the
coordinated species L2 in the ESI+ MS spectra of almost all
Au(III) complexes studied and the crystallographic character-
ization of the final decomposition product L3d in complex 4a
allow the formulation of a pathway for the cyclization of the
ligands under the influence of gold ions (Chart 4). It should be
mentioned, that the MS spectra of the compounds H2L1 do not
give any evidence for similar reactions.
complexes. Additional 13C NMR signals around 112 ppm can
be assigned to the coordinated SCN− ligands. The correspond-
ing signal of potassium thiocyanate in water appears at 133.3
ppm.20
Crystals suitable for structural analysis were obtained for the
complex [Au(SCN)(L1c)] (8). The crystals appear as orange-
brown plates. Figure 3 illustrates the molecular structure of 8 as
the representative compound of such complexes together with
selected bond lengths and angles. The bonding situation inside
the organic ligand is only a little different from those observed
for the chloro complexes. All C−S and C−N bonds inside the
chelate rings are practically in the same ranges as observed for 4
and 5, but a stronger bond-length equalization of the C−N
bonds is observed, which is also extended to the C1−N5 bond.
The thiocyanato ligand shows the typical bent coordination,
with an Au−S−C angle of 105.0(3)°, and the arrangement of
the SCN− ligand is coplanar with the coordination sphere of the
thiosemicarbazonato ligand in a way that the thiocyanate ligand
lies almost parallel to the Au−S2 bond.
The chloro ligands in the [AuIIICl(L1)] complexes are
readily replaced by SCN− or CN− ligands without any change in
the oxidation state of the metal. Such reactions proceed at
room temperature in mixtures of CH2Cl2 and MeOH, which
yield the products as crystalline solids in excellent yields (Chart
5). First evidence of the successful exchange reaction with
SCN− was the IR spectrum of the resulting solid, which
expectedly shows only marginal shift of the band, which belong
to the thiosemicarbazone, but an additional band at about 2130
cm−1, which can be assigned to the coordination of S-bonded
thiocyanate.18,19 1H and 13C NMR spectra of [Au(SCN)(L1)]
complexes are very similar to those of the analogous chloro
The ongoing discussion about the role of cyano species in
the metabolism of gold drugs and the fact that stable CN−
Chart 4. Proposed Pathway for the Cyclization of H2L1 Based on the Ionization of the Complexes
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Figure 4. Molecular structure of the complex [AuIII(CN)(L1c)] (9).
Hydrogen atoms omitted for clarity.
Figure 3. Molecular structure of the complex [AuIII(SCN)(L1c)] (8).
Hydrogen atoms omitted for clarity. Selected bond length (Å) and
angles (deg): Au−S1 2.290(3), Au−S2 2.280(2), Au−S3 2.323(3),
Au−N2 2.022(8), S1−C1 1.775(9), C1−N1 1.32(1), N1−C2 1.32(1),
C2−N2 1.34(1), N2−N3 1.388(9), N3−C31.27(1), C3−S2 1.77(1),
S3−C6 1.66(1), C6−N6 1.15(1), S1−Au−N2 98.3(2), S1−Au−S2
176.6(1), S1−Au−S3 81.6(3), N2−Au−S2 84.7(2), N2−Au−S3
179.7(3), S2−Au−S3 95.37(9), Au−S3−C6 105.0(3), S3−C6−N6
176.1(1).
has also included gold complexes with N-heterocyclic
carbenes,30,31 and a number of cationic complexes with a
variety of preferably N-donor ligands.32−34 The latter
compounds show good to moderate cytotoxicities in cancer
cell lines including cisplatin-resistant cells.
Thiosemicarbazones exhibit various biological activities and
have therefore attracted considerable pharmaceutical interest.
Several mechanisms of antitumor action were proposed and
compelled the interest of an SAR study. Furthermore, it is well-
known that such compounds are potent metal chelators and the
cytotoxic properties of the compounds are influenced by
chelation. In many cases, an increased activity of the metal
complexes is observed, which is assumed to be an effect of a
metal-assisted transport, while complex dissociation inside the
cell releases the thiosemicarbazones as the biologically active
species.35 The compounds presented here represent a new
series of familiar compounds, which combine ligands with
potential biological activity with metal ions with pharmaceutical
potential. To obtain an overview of the influence of the
individual residues in the molecular framework of the
thiosemicarbazones, we modified R1 and R2 (compounds
H2L1a to H2L1e) and prepared the corresponding gold(III)
complexes [AuCl(L1)]. The exchange of the chloro ligands by
SCN− and CN− may allow an insight into the mechanism of
potential activity and the role of the relatively labile Cl− ligand.
Thus, we investigated the antiproliferative effects of the
ligands in relation to their [AuCl(L1)] complexes in a time
response as well as in a concentration response assay. From the
first study the response of the cells to the compounds can be
estimated, while the latter allows the calculation of IC50 values.
Additionally, each two representatives of the SCN− and CN−
compounds have been included into this study, to obtain
information about the influence of the halide/pseudohalide
ligand.
compounds are known for Au(I) and Au(III) stimulated us to
include CN− ligands in the present study.3b,20 [Au(CN)(L1c)]
and [Au(CN)(L1d)] are formed during reactions of the
corresponding chloro complexes with KCN in MeOH/CH2Cl2
mixtures. The products can be isolated in crystalline forms after
slow evaporation of the reaction mixtures. In the IR spectra of
these complexes, bands appear in the ranges between 2130 and
2175 cm−1 and 436−444 cm−1, which are related to νCN and
νAuC stretching vibrations, respectively.18 The relatively high
frequencies of the νCN stretches are indicative of a strong σ-
donation in these gold(III) complexes since electrons are
removed from the 5σ orbital, which is weakly antibonding,
while π-backbonding tends to decrease the νCN because the
electrons enter into the antibonding 2pπ* orbital.16 The H
1
NMR spectra of the cyano complexes are similar to those of the
[Au(SCN)(L1)] type complexes and reflect a similar bonding
situation. In the 13C NMR spectra of the [Au(CN)(L1)]
complexes, signals are found around 102 ppm, which confirm
the presence of cyanide coordinated to gold(III) centers. The
chemical shift for free cyanide is at 163.6 ppm and for
[Au(CN)4]− a value of 103.5 ppm was found in CD3OD.21 ESI+
mass spectra of 9 and 10 exhibit the expected molecular ion
peaks and the peaks related to the [M+H]+. [Au(L1)(L2)]+
species are not observed here, which may indicate that the Au−
C bond is stronger than the Au−Cl and Au−SCN bonds and
hinders the cyclization.
Figure 4 illustrates the molecular structure of 9 as an example
of the compounds. Selected bond lengths and angles for the
complexes 9 and 10 are compared in Table 3. There are no
significant differences in the bonding situation of the
thiosemicarbazonato ligands between the chloro and the
cyano derivatives.
Many efforts in pharmaceutical gold chemistry have been
dedicated to the development of antitumor agents on the basis
of both Au(I) and Au(III) compounds.3b,22−29 Recent research
The thiosemicarbazones H2L1 cause a strong reduction of
the growth of human MCF-7 breast cancer cells. Maximum
activity was already detected after an incubation time of 48 h
(Figure 5). At the concentrations from 1.25 to 10 μM cytocidal
effects were observed. The rising recuperation of the cells at
lower concentrations is characteristic for a cytostatic effect. The
time response curve of cisplatin is quite different, with a
maximum cytotoxicity appearing not before an incubation time
of more than 100 h. All compounds H2L1 show this interesting
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Table 3. Selected Bond Lengths (Å) and Angles (deg) in [Au(CN)(L1c)] (9) and [Au(CN)(L1d)] (10)
9
10
9
10
Au−N2
Au−S1
Au−S2
Au−C4
C2−N1
C1−N1
C4−N6
N2−Au−S2
S2−Au−S1
S2−Au−C4
2.015(6)
2.298(2)
2.286(2)
2.003(8)
1.352(9)
1.315(9)
1.12(1)
2.017(6)
2.290(2)
2.287(2)
2.00(1)
1.33(1)
1.34(1)
1.08(2)
85.9(2)
176.82(7)
90.7(3)
C1−S1
S2−C3
N3−C3
N3−N2
N2−C2
N5−C1
1.739(8)
1.775(8)
1.28(1)
1.731(8)
1.781(8)
1.27(1)
1.410(9)
1.31(1)
1.33(1)
1.415(8)
1.302(9)
1.35(1)
85.9(2)
N2−Au−S1
N2−Au−C4
97.2(2)
97.3(2)
176.84(8)
90.9(3)
176.8(3)
176.2(3)
Figure 5. Cytotoxic effects of selected ligands and complexes.
Table 4. Cytotoxic Effects of Ligands H2L1 and their Gold Complexes against MCF-7 Cells
IC50 [μM]
R1
CH3
R2
Ligand
[AuIIICl(L1)]
[AuIII(SCN)(L1)]
[AuIII(CN)(L1)]
a
H2L1a
H2L1b
H2L1c
H2L1d
H2L1e
H2L1f
Phenyl
0.85
0.68
0.61
0.63
0.66
0.66
0.85
0.99
a
(CH2)4
morpholinyl
(CH2)6
2.19
0.74
0.35
0.025
0.045
a
2.43
a
CH3
CH3
0.23
n-propyl
n-propyl
9.04
a
Values taken from ref 10.
behavior and comparable time response curves, which allows
the comparison of IC50 values (after an incubation time of 48 h,
see Table 4). The degree of cytotoxicity can be influenced by
the variation of the peripheral substituents R1 and R2 as has
already been stated in ref 10.
compounds is hitherto unclear and the thiosemicarbazide
ligands themselves possess a considerable degree of activity.
On the other hand the reduction of gold(III) to gold(I) with
oxidative cyclization of the thiosemicarbazone, as in the case of
the complex [AuICl(L3d)], leads to the formation of a much
less active compound (IC50: 5.6 μM). After substitution of the
chloro ligands by thiocyanato or cyano ones, no considerable
changes in the cytotoxicity with respect to the chloro
compounds was observed for the SCN− complexes, while the
activity is significantly increased for the cyano complexes.
Hence, this may imply that the [AuCl(L1)] and [Au(SCN)-
(L1] complexes have the same target in the cell. The effect that
can be obtained with the CN− substitution will be subject of
further studies, also including other cyano complexes of gold. In
many cases, also with thiosemicarbazone ligands, a metal-
assisted transport is discussed as the reason for higher activities
of metal complexes when compared with their uncoordinated
organic ligands, while complex dissociation inside the cell
After coordination of the thiosemicarbazones to Au(III), for
most of the compounds a clear increase of the activity can be
stated, displaying IC50 values around 0.6 μM, while only for the
most active compound H2L1e is a slight decrease observed
upon complex formation. It is obvious that the formation of the
gold complexes levels their cytotoxic effects to low concen-
trations irrespective to their substituents R1 and R2, which
clearly implies different mechanisms of the activity for the
ligands and their [AuCl(L1)] complexes. With the observed
IC50 values, the complexes under study seem to be more active
by at least 1 order of magnitude than the previously reported
cationic complexes with nitrogen donor atoms.33,34 However, it
must be mentioned that the mode of action of the present
1611
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Inorganic Chemistry
Article
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■
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ACKNOWLEDGMENTS
We gratefully acknowledge financial support of DAAD, CAPES,
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■
DEDICATION
■
∥Dedicated to Professor Reinhard Kirmse on the occassion of
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