Novel dinuclear platinum(II) Complexes containing mixed nitrogen-sulfur donor ligands

Article abstract of DOI:10.1021/ic2018902

A series of novel dinuclear platinum(II) complexes were synthesized containing a mixed nitrogen-sulfur donor bidentate chelate system in which the two platinum centers are connected by an aliphatic chain of variable length. The bidentate chelating ligands were selected to stabilize the complex toward decomposition. The pK_a values and reactivity of the four synthesized complexes, namely, [Pt₂(S¹,S₄-bis(2- pyridylmethyl)-1,4-butanedithioether)(OH₂)₄] ⁴⁺(4NSpy), [Pt₂(S₁,S₆-bis(2- pyridylmethyl)-1,6-hexanedithioether)(OH₂)₄]⁴⁺ (6NSpy), [Pt₁(S¹,S⁸-bis(2-pyridylmethyl)-1,8- octanedithioether)(OH₂)₄]⁴⁺ (8NSpy), and [Pt₂(S ¹,S ₁₀-bis(2-pyridylmethyl)-1,10- decanedithioether)-(OH₂)₄]⁴⁺ (10NSpy), were investigated. This system is of special interest because only little is known about the substitution behavior of dinuclear platinum complexes that contain a bidentate chelate that forms part of the aliphatic bridging ligand. Moreover, the ligands as well as the dinuclear complexes were examined in terms of their cytotoxic activity, and the 10NSpy complex was found to be active. Spectrophotometric acid-base titrations were performed to determine the pK _a values of all the coordinated water molecules. The substitution of coordinated water by thiourea was studied under pseudo-first-order conditions as a function of nucleophile concentration, temperature, and pressure, using stopped-flow techniques and UV-vis spectroscopy. The results for the dinuclear complexes were compared to those for the corresponding mononuclear reference complex [Pt(methylthiomethylpyridine)(OH₂)₂]²⁺ (Pt(mtp)), by which the effect of the increasing aliphatic chain length of the bridged complexes could be investigated. The results indicate that there is a clear interaction between the two platinum centers, which becomes weaker as the chain length between the metal centers increases. Furthermore, differences and similarities of the N,S-system were compared to the corresponding dinuclear N,N-system studied previously in our group. In addition, quantum chemical calculations were performed to support the interpretation and discussion of the experimental data.

Full text of DOI:10.1021/ic2018902

Article
pubs.acs.org/IC
Novel Dinuclear Platinum(II) Complexes Containing Mixed Nitrogen−
Sulfur Donor Ligands
Stephanie Hochreuther,† Ralph Puchta,†,‡ and Rudi van Eldik*,†
†Inorganic Chemistry, Department of Chemistry and Pharmacy, University of Erlangen-Nurnberg, Egerlandstrasse 1, 91058 Erlangen,
̈
Germany
‡Computer Chemistry Center, Department of Chemistry and Pharmacy, University of Erlangen-Nurnberg, Nagelsbachstrasse 25,
̈
̈
91052 Erlangen, Germany
S
* Supporting Information
ABSTRACT: A series of novel dinuclear platinum(II) complexes
were synthesized containing a mixed nitrogen−sulfur donor
bidentate chelate system in which the two platinum centers are
connected by an aliphatic chain of variable length. The bidentate
chelating ligands were selected to stabilize the complex toward
decomposition. The pKa values and reactivity of the four
synthesized complexes, namely, [Pt2(S1,S4-bis(2-pyridylmethyl)-
1,4-butanedithioether)(OH2)4]4+ (4NSpy), [Pt2(S1,S6-bis(2-pyri-
dylmethyl)-1,6-hexanedithioether)(OH2)4]4+ (6NSpy), [Pt2(S1,S8-
bis(2-pyridylmethyl)-1,8-octanedithioether)(OH2)4]4+ (8NSpy),
and [Pt2(S1,S10-bis(2-pyridylmethyl)-1,10-decanedithioether)-
(OH2)4]4+ (10NSpy), were investigated. This system is of special
interest because only little is known about the substitution behavior
of dinuclear platinum complexes that contain a bidentate chelate that forms part of the aliphatic bridging ligand. Moreover, the
ligands as well as the dinuclear complexes were examined in terms of their cytotoxic activity, and the 10NSpy complex was found
to be active. Spectrophotometric acid−base titrations were performed to determine the pKa values of all the coordinated water
molecules. The substitution of coordinated water by thiourea was studied under pseudo-first-order conditions as a function
of nucleophile concentration, temperature, and pressure, using stopped-flow techniques and UV−vis spectroscopy. The results
for the dinuclear complexes were compared to those for the corresponding mononuclear reference complex
[Pt(methylthiomethylpyridine)(OH2)2]2+ (Pt(mtp)), by which the effect of the increasing aliphatic chain length of the bridged
complexes could be investigated. The results indicate that there is a clear interaction between the two platinum centers, which
becomes weaker as the chain length between the metal centers increases. Furthermore, differences and similarities of the
N,S-system were compared to the corresponding dinuclear N,N-system studied previously in our group. In addition, quantum
chemical calculations were performed to support the interpretation and discussion of the experimental data.
consequence in a next step the spectator ligands were changed,
which resulted in the development of numerous different
compounds, for example, oxaliplatin (a third generation
drug).11,12 In oxaliplatin the two ammine ligands were replaced
by a single bidentate ligand, namely, (1R,2R)-cyclohexane-1,2-
diamine.13 Oxaliplatin shows no cross-resistance to cisplatin
and carboplatin because of a differently formed DNA adduct.14
By synthesizing different platinum compounds, investigating
their cytostatic activity and comparing the results, the so-called
structure−activity relationship was summarized.15 This postu-
lation was helpful for further synthetic work to be able to predict
cytostatic activity. Naturally, research always continues and
scientists like Farrell and co-workers developed the so-called
nonclassical platinum drugs,16 which are multinuclear and
therefore offer a completely different mechanism of binding to
INTRODUCTION
■
During the past decades, numerous Pt-containing compounds
have been synthesized, and their cytostatic activity has been
analyzed.1 The main goal was to create new possibilities for
anticancer therapy. Since the discovery of the cytostatic activity
of cisplatin, the first platinum containing anticancer drug, in
1969,2,3 much more has been learned in terms of cytostatic
properties of platinum drugs. Mononuclear Pt(II) as well as
Pt(VI) compounds have been developed and improved step by
step (Figure 1). Cisplatin was part of the first generation of
Pt-containing drugs. Efforts to develop cisplatin analogues with
reduced toxicity and a broader range of therapeutic activity
led to the discovery of carboplatin (a second generation drug).
By replacing chloride with 1,1-cyclobutanedicarboxylate, as new
leaving group, carboplatin aquates slower compared to cisplatin
and therefore is significantly less toxic,4,5 but displays similar
activity when used to treat ovarian cancer.6−10 Another problem
was the cross resistance of carboplatin to cisplatin, and as a
Received: August 29, 2011
Published: November 15, 2011
© 2011 American Chemical Society
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Figure 1. Selection of Pt(II)-based anticancer drugs, which have
gained marketing approval for human use (upper row) or which have
entered phase II clinical trials (BBR3464).
Figure 2. Structures of the studied dinuclear N,S−Pt(II) complexes
(NSpy system) and their mononuclear analogue Pt(mtp).
more flexible and long ranged, with a high degree of interstrand
cross-links.35 Furthermore, it is interesting to study the
combination of a strong σ-donor and a π-acceptor within the
same complex. Surprising results may show up as observed in the
work of Hofmann et al.,36 who combined pyridyl (π-acceptor)
with benzyl moieties (σ-donor) within the same complex.
In the present work, we focused on the nature of the
substitution reactions with thiourea and their dependence on
the chain length of the complexes. We used thiourea as a strong
S-containing nucleophile because of its good water solubility,
high nucleophilicity, and neutral character. The high affinity of
Pt(II) for sulfur is also of interest in terms of chemo-protective
ability. Thiourea or methionine were especially used against
nephrotoxicity, to suppress the coordination of platinum to
S-containing proteins, which is responsible for several side
effects.37 We report thermodynamic (pKa values) and kinetic
data (rate and activation parameters) for the complexes studied,
and used quantum chemical calculations to support the inter-
pretation of the experimental results. The corresponding
Pt(mtp) reference complex32 and the previously obtained
information on its kinetic behavior is useful for comparison
with the here studied dinuclear system. In addition, cytostatic
cell tests of all ligands and dinuclear complexes were carried out
using the HeLa S3 cell line.
DNA, although some violate the classical structure−activity
relationship.17−21 BBR3464 is a trinuclear Pt(II) drug chosen as
the lead agent from a family of di- and trinuclear complexes. It
displays higher and faster uptake into cisplatin sensitive and
resistant cells22 and faster DNA binding.23 However, the crucial
point is the formation of a range of unique DNA adducts that
are thought to be the mechanism by which it derives its potent
cytotoxicity.
In the past almost all medicinal used platinum drugs con-
tained exclusively nitrogenous donor ligands. Not until
the past 10 years have S-containing donor ligands and their
corresponding platinum complexes been synthesized. The
group of Sadler for instance developed a sulfur analogue of
the well-studied Pt(H2NCH2CH2NH2)Cl2 complex, namely,
̌ ́
Pt(CH3SCH2CH2SCH3)Cl2, and Bugarcic et al. found the
complex to be cytostatic active against MCF7 (breast cancer)
cell lines.24 Besides exclusively sulfur donor containing com-
pounds, complexes with mixed spectator ligands have been
developed.25−27 For example the complex dichloro(2-
methylthiomethylpyridine)platinum(II), Pt(mtp)Cl2, is a com-
plex with nitrogen as well as sulfur donor ligands. In the
literature different synthetic pathways are described,28−30 and
the bidentate mixed N,S-complex was found to be a promising
cytostatic agent.31 Furthermore, the corresponding diaqua
complex was recently studied in detail in our group.32 Hardly
any literature is presently available on di- or polynuclear Pt(II)
complexes with S-containing donor ligands.
EXPERIMENTAL SECTION
■
Chemicals. The starting materials 2-picolyl chloride hydro-
chloride, the different alkyl dithiols, 1,10-decanedibromide, the
nucleophile thiourea, trifluoromethanesulfonic (triflic) acid, and silver
triflate were purchased from Acros Organics. Potassium
tetrachloroplatinate(II) was obtained from Strem Chemicals. The
deuterated chemicals, such as D2O, deuterated triflic acid (CF3SO3D)
and NaOD, were obtained from Deutero GmbH. All other used
chemicals were of the highest purity commercially available and were
used without further purification. For all preparations of aqueous
solutions, ultra pure water was used.
The present study is a continuation of earlier work performed
in our group, where dinuclear nitrogen donor containing Pt(II)
complexes were investigated.33 We now switched from
exclusively N-containing to mixed N- and S-containing dinuclear
Pt(II) complexes. Figure 2 shows the novel complexes used in
this study as well as their mononuclear analogue, which was also
investigated in our group.32 The complexes consist of a pyridyl
unit (N-donor) and a thiomethylether unit (S-donor). The two
Pt(II) centers are connected by an aliphatic chain of variable
length. The dinuclear complexes are abbreviated according to
their aliphatic chain length, the employed N,S-donor atoms, and
the presence of a pyridine ring, namely, 4NSpy, 6NSpy, 8NSpy,
10NSpy, and Pt(mtp) for the mononuclear reference complex,
respectively.
Preparation of the (N,S)-Ligand System. S1,S4-bis(2-pyridyl-
methyl)-1,4-butanedithioether (1), S1,S6-bis(2-pyridylmethyl)-1,6-
hexanedithioether (2), S1,S8-bis(2-pyridylmethyl)-1,8-octanedi-
thioether (3), and S1,S10-bis(2-pyridylmethyl)-1,10-decanedithioether
(4), were prepared according to a well-known procedure:38 A sodium
ethoxide solution was prepared by addition of 0.92 g (40 mmol) of
sodium metal to 20 mL of dry ethanol at 0 °C followed by addition of
the corresponding dithiol (5 mmol) and an ethanolic solution of
3.28 g (20 mmol) of 2-picolyl chloride hydrochloride, while keeping
the temperature at 0−5 °C. The resulting suspension was allowed to
reach room temperature, stirred for 30 min, followed by stirring for 2 h
Usually, complexes with bidentate coordination spheres show
higher cytostatic activity than their analogue with only one
vacant coordination site.34 In addition, it is known from multi-
nuclear complexes that the DNA adducts of such complexes are
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Article
under reflux. The mixture was cooled to room temperature, and 50 mL
of water was added to destroy remaining sodium ethoxide, and finally
the ethanol was removed under reduced pressure. The product was
extracted with dichloromethane (4 × 20 mL), the combined organic
layers were dried over Na2SO4, and the product was obtained by
removing the solvent under reduced pressure. The 4-(N,S)- and
6-(N,S)-ligands were obtained as orange oils, while the ligands with
longer chains such as the 8-(N,S)- and 10-(N,S)-ligands were found to
be lightly yellow solids. All ligands were characterized and identified by
NMR spectroscopy and used for complex-formation without further
purification.
5: Yield: 337 mg (0.40 mmol, M = 836.4 g/mol, 84%). Anal. Calcd
for C16H20Cl4N2Pt2S2: C, 22.97; H, 2.41; N, 3.35; S, 7.67%. Found: C,
22.90; H, 2.40; N, 3.25; S, 7.92%. 1H NMR (dmso-d6, 298.2 K): δ 9.38
(d, 3JHH = 4.9 Hz, 2H), δ 8.16 (t, 3JHH = 7.7 Hz, 2H), δ 7.80 (d, 3JHH
=
2
7.7 Hz, 2H), δ 7.54 (m, 2H), δ 4.76 (d, AB-spin system, JHH = 17.0
Hz, 2H), δ 4.48 (d, AB-spin system, 2JHH = 17.0 Hz, 2H), δ 2.88−2.67
(m, 4H), δ 1.91−1.76 (m, 4H). MS-FAB (NBA, 70 eV) 801 [M−Cl]+.
6: Yield: 278 mg (0.32 mmol, M = 864.5 g/mol, 67%). Anal. Calcd
for C18H24Cl4N2Pt2S2: C, 25.01; H, 2.80; N, 3.24; S, 7.42%. Found: C,
24.83; H, 2.77; N, 2.96; S, 7.61%. 1H NMR (dmso-d6, 298.2 K): δ 9.41
(d, 3JHH = 5.5 Hz, 2H), δ 8.17 (t, 3JHH = 7.7 Hz, 2H), δ 7.82 (d, 3JHH
=
2
The bridging reagent 1,10-decanedithiol (4a) is commercially not
available and was therefore prepared following the reported procedure
with slight modifications:39 To a solution of 6.0 g (20 mmol) of
1,10-decanedibromide in 50 mL of ethanol, 3.2 g (42 mmol) of solid
thiourea were added within 10 min and stirred under reflux overnight.
The reaction mixture was then cooled to room temperature, treated
dropwise with 90 mL of 10% NaOH solution and again refluxed
(110 °C) for 3 h. After cooling to room temperature, the mixture was
diluted with 100 mL of a 1 M HCl solution until pH 7 was reached.
The product was extracted with diethyl ether (2 × 200 mL) and
obtained as colorless oil after evaporation of the solvent. The crude
product was identified by NMR spectroscopy and used for the next
step without further purification.
7.7 Hz, 2H), δ 7.56 (m, 2H), δ 4.76 (d, AB-spin system, JHH = 17.0
Hz, 2H), δ 4.48 (d, AB-spin system, 2JHH = 17.0 Hz, 2H), δ 2.89−2.65
(m, 4H), δ 1.71−1.56 (m, 4H), δ 1.44−1.34 (m, 4H). MS-FAB (NBA,
70 eV) 830 [M−Cl]+.
7: Yield: 300 mg (0.34 mmol, M = 892.6 g/mol, 70%). Anal. Calcd
for C20H28Cl4N2Pt2S2: C, 26.91; H, 3.16; N, 3.14; S, 7.14%. Found: C,
27.02; H, 3.19; N, 2.88; S, 7.43%. 1H NMR (dmso-d6, 298.2 K): δ 9.40
(d, 3JHH = 5.4 Hz, 2H), δ 8.16 (t, 3JHH = 7.8 Hz, 2H), δ 7.81 (d, 3JHH
=
2
7.8 Hz, 2H), δ 7.55 (m, 2H), δ 4.75 (d, AB-spin system, JHH = 17.0
Hz, 2H), δ 4.48 (d, AB-spin system, 2JHH = 17.0 Hz, 2H), δ 2.86−2.64
(m, 4H), δ 1.75−1.59 (m, 4H), δ 1.40−1.31 (m, 4H), δ 1.22−1.15 (m,
4H). MS-FAB (NBA, 70 eV) 857 [M−Cl]+.
8: Yield: 358 mg (0.39 mmol, M = 920.6 g/mol, 81%). Anal. Calcd
for C22H32Cl4N2Pt2S2: C, 28.70; H, 3.50; N, 3.04; S, 6.97%. Found: C,
28.44; H, 3.39; N, 2.82; S, 6.97%. 1H NMR (dmso-d6, 298.2 K): δ 9.40
1
1: C16H20N2S2 (M = 304.5 g/mol). H NMR (CDCl3, 298.2 K): δ
8.43 (d, 3JHH = 4.8 Hz, 2H), δ 7.56 (dt, 3JHH = 7.7 Hz, 4JHH = 1.7 Hz,
2H), δ 7.27 (d, 3JHH = 7.7 Hz, 2H), δ 7.06 (dd, 3JHH = 4.8 Hz, 4JHH
=
(d, 3JHH = 5.4 Hz, 2H), δ 8.16 (t, 3JHH = 7.8 Hz, 2H), δ 7.81 (d, 3JHH
=
7.7 Hz, 2H), δ 3.73 (s, 4H), δ 2.38 (t, 3JHH = 6.5 Hz, 4H), δ 1.59−1.52
(m, 4H). 13C NMR (CDCl3, 298.2 K): δ 158.7, 149.0, 136.5, 122.8,
121.6, 37.9, 30.9, 28.0.
7.8 Hz, 2H), δ 7.55 (m, 2H), δ 4.76 (d, AB-spin system, JHH = 17.0
Hz, 2H), δ 4.49 (d, AB-spin system, 2JHH = 17.0 Hz, 2H), δ 2.86−2.64
(m, 4H), δ 1.75−1.59 (m, 4H), δ 1.40−1.31 (m, 4H), δ 1.22−1.15 (m,
4H). MS-FAB (NBA, 70 eV) 885 [M−Cl]+.
2
1
2: C18H24N2S2 (M = 332.5 g/mol). H NMR (CDCl3, 298.2 K): δ
8.43 (d, 3JHH = 4.8 Hz, 2H), δ 7.62 (dt, 3JHH = 8.5 Hz, 4JHH = 1.7 Hz,
Preparation of the Complex Solutions.41 Solutions of the aqua
complexes were prepared by suspending a defined amount of the
corresponding chloro complexes 5−8 in 0.001 M triflic acid and
adding a stoichiometric excess of silver triflate (4.1 equiv) with respect
to the chloro ligands. The mixture was then stirred in the dark for at
least 2 days at 45 °C, which led to the formation of silver chloride. The
precipitated silver chloride was filtered off, and the pH of the resulting
solution was increased to 11 by addition of small amounts of 0.1 M
NaOH. This led to the formation of brown silver oxide (from the
excess silver triflate), which was then removed with a Millipore filter,
and the pH of the remaining solution was adjusted to pH 2 with triflic
acid to give the desired complex concentration of 0.2 mM. For all
kinetic investigations the pH of the solution was kept at 2, and the
ionic strength was adjusted to I = 0.01 M (triflic acid). After aquation,
the complex solutions were characterized using NMR spectroscopy
and mass spectrometry (see further Discussion).
Cytostatic Tests. Cell tests for all four dinuclear Pt(II) tetra-
chloro complexes (5−8) and for all ligands (1−4) were performed.
The cytotoxicity of the complexes was studied using the AlamarBlue
assay in the human cervix carcinoma cell line HeLa S3. The AlamarBlue
assay is characterized by its linearity and its high sensitivity, while its
handling is less error prone in contrast to the MTT assay. Living cells
reduce the sodium salt of the dark blue, nonfluorescent dye resazurin
(7-hydroxy-3H-phenoxazin-3-one-10-oxide) to the pink, highly fluo-
rescent resurufin (7-hydroxy-3H-phenoxazin-3-one). In the commonly
used MTT assay, the formed formazan is insoluble in the cell-culture
medium and requires dimethyl sulfoxide (dmso, DMSO) as an
additional solubilizer. The handling of the AlamarBlue assay is thus
much simpler, readout by fluorescent detection is directly achieved
without additional steps.42 Cells were cultivated at 37 °C and 5% CO2
atmosphere in DMEM medium (Gibco) with 10% FCS (Biochrome
AG) and 1% Pen/Strep (GIBCO) added. Cells were split twice per
week. For the assay, cells were seeded in 96-well plates (4 000 cells/
well) and allowed to attach for 24 h. Complexes to be tested were
dissolved in a suitable amount of DMSO. Different concentrations were
prepared by serial dilution with medium to give final concentrations
with a maximum DMSO content of 1%. The cells were then incubated
for 48 h with 100 μL each of above dilution series. After removal of the
medium, AlamarBlue solution (10 μL (BioSource Europe) diluted with
2H), δ 7.34 (d, 3JHH = 7.8 Hz, 2H), δ 7.12 (dd, 3JHH = 4.8 Hz, 4JHH
=
7.8 Hz, 2H), δ 3.79 (s, 4H), δ 2.44 (t, 3JHH = 7.4 Hz, 4H), δ 1.55−1.46
(m, 4H), δ 1.35−1.26 (m, 4H). 13C NMR (CDCl3, 298.2 K): δ 158.9,
149.0, 136.5, 122.9, 121.7, 38.1, 31.5, 28.9, 28.2.
1
3: C20H28N2S2 (M = 360.6 g/mol). H NMR (CDCl3, 298.2 K): δ
8.43 (d, 3JHH = 4.8 Hz, 2H), δ 7.56 (dt, 3JHH = 8.5 Hz, 4JHH = 1.6 Hz,
2H), δ 7.29 (d, 3JHH = 7.8 Hz, 2H), δ 7.06 (dd, 3JHH = 4.8 Hz, 4JHH
=
7.8 Hz, 2H), δ 3.75 (s, 4H), δ 2.39 (t, 3JHH = 7.3 Hz, 4H), δ 1.50−1.41
(m, 4H), δ 1.28−1.14 (m, 8H). 13C NMR (CDCl3, 298.2 K): δ 158.9,
149.0, 136.4, 122.8, 121.6, 38.1, 31.5, 29.0, 28.8, 28.5.
1
4a: C10H24S2 (M = 206.4 g/mol). H NMR (CDCl3, 298.2 K): δ
2
2.46 (t, JHH = 7.2 Hz, 4H), δ 1.60−1.50 (m, 4H), δ 1.35−1.20 (m,
12H). 13C NMR (CDCl3, 298.2 K): δ 34.01, 29.41, 29.02, 28.34,
24.63.
1
4: C22H32N2S2 (M = 388.6 g/mol). H NMR (CDCl3, 298.2 K): δ
8.44 (d, 3JHH = 4.8 Hz, 2H), δ 7.57 (dt, 3JHH = 8.5 Hz, 4JHH = 1.7 Hz,
2H), δ 7.30 (d, 3JHH = 7.8 Hz, 2H), δ 7.07 (dd, 3JHH = 4.8 Hz, 4JHH
=
7.8 Hz, 2H), δ 3.75 (s, 4H), δ 2.40 (t, 3JHH = 7.3 Hz, 4H), δ 1.52−1.42
(m, 8H), δ 1.19−1.12 (m, 8H). 13C NMR (CDCl3, 298.2 K): δ 159.0,
149.1, 136.5, 122.9, 121.7, 38.1, 31.6, 29.3, 29.1, 29.0, 28.7.
Synthesis of the (N,S)-Complex System. [Pt2(S1,S4-bis(2-
pyridylmethyl)-1,4-butanedithioether)Cl4] (5), [Pt2(S1,S6-bis(2-pyri-
dylmethyl)-1,6-hexanedithioether)Cl4] (6), [Pt2(S1,S8-bis(2-pyridyl-
methyl)-1,8-octanedithioether)Cl4] (7), and [Pt2(S1,S10-bis(2-pyridyl-
methyl)-1,10-decanedithioether)Cl4] (8) were synthesized according
to a literature method.40 To 0.48 mmol of the corresponding ligands
(1−4), a solution of 400 mg (0.96 mmol) of K2PtCl4 in water was
slowly added using a dropping funnel, under vigorous stirring at room
temperature. The reaction mixture was further stirred until the
characteristic red color of dissolved K2PtCl4 disappeared (3−6 h). The
resulting yellow precipitate was filtered off, washed with water,
ethanol, and diethyl ether, and dried under vacuum. During com-
plexation the SR2−sulfur donor atom becomes a stereogenic center
and three configurations, namely, R,R; S,S and R,S are possible. We
were not able to isolate single diastereomers because of rapid
(S)-inversion at the sulfur atom (for further discussion, see kinetic
measurements), and consequently used a mixture of the generated
species in all measurements.
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90 μL DMEM medium) was directly added, and the cells were
incubated for 90 min. After excitation at 530 nm, fluorescence at
590 nm was measured using a Synergy HT Microplate Reader (Bio-
TEK). Cell viability is expressed in percent with respect to a control
containing only pure medium and 1% DMSO incubated under identical
conditions. All experiments were repeated two times with each
experiment done in four replicates. The resulting curves were fitted
using Sigma plot 10.0 (Systat Software, Inc. 2006).
Instrumentation and Measurements. NMR spectroscopy
(Bruker Avance DPX 300) and a Carlo Erba Elemental Analyzer
1106 were used for ligand and chloro complex characterization and
chemical analysis, respectively. 195Pt-NMR measurements were
performed on a Bruker Avance DRX 400WB with K2PtCl4 (in D2O/
CF3SO3D) as reference (δ = −1620 ppm). Mass spectrometric
measurements were performed on an UHR-TOF Bruker Daltonik
(Bremen, Germany) maXis, an ESI-TOF mass spectrometer capable of
a resolution of at least 40000 fwhm used by the group of Prof. Ivana
Ivanovic-Burmazovic at the University of Erlangen-Nurnberg. A Varian
́
́
̈
Cary 1G spectrophotometer equipped with a thermostatted cell holder
was used to record UV−vis spectra for the determination of the pKa
values of the aqua complexes. Kinetic measurements on fast reactions
were carried out on an Applied Photophysics SX 18MV stopped-flow
instrument and for the study of slow reactions a Shimadzu UV-2010PC
spectrophotometer with a thermo-electrically controlled cell holder was
used. Experiments at elevated pressure were performed on a laboratory-
made high-pressure stopped-flow instrument for fast reactions.43 The
temperature of the instruments was controlled throughout all kinetic
experiments to an accuracy of ±0.1 °C. The values of the reported
pseudo-first-order rate constants are the average of at least four kinetic
measurements.
1
Figure 3. H NMR spectrum of the 6NSpy complex at pH 2 (D2O/
CF3SO3D).
Computational Details. We performed B3LYP/LANL2DZp
hybrid density functional calculations, that is, with pseudopotentials
on the heavy elements and the valence basis set augmented with
polarization functions.44−50 During the optimization of the structures
no constraints other than symmetry were applied. In addition, the
resulting structures were characterized as minima by computation of
vibrational frequencies. The Gaussian 03 suite of programs was used
throughout.51
Figure 4. 195Pt-NMR spectrum of the 6NSpy complex at pH 2 (D2O/
CF3SO3D).
RESULTS AND DISCUSSION
■
Characterization of the Tetraaqua Complexes. The
chloro complexes of the NSpy system were fully characterized
as reported in the Experimental Section. However, a closer look
at the resulting solution has to be taken following aquation and
removal of chloride ions. Aquation of the chloro complexes
involves an increase in pH up to 11. This high pH leads to
deprotonation of the water ligands and probably to the
formation of μ-OH dimers. After slow addition of concentrated
triflic acid, the pH is decreased to 2 and the dimers are expected
to decompose. To clarify the composition of the solution after
removing the chloride ions, we performed NMR spectroscopic
and mass spectrometric measurements.
group “5” between the pyridine ring and bridge (see drawing in
Figure 3), and demonstrates the purity of the aqua complex
solution. The concentration of the solution is quite low, which
can be seen in the large D2O solvent peak at 4.79 ppm.
Dimeric as well as OH-containing species can be analyzed
using 195Pt-NMR. In general, bridged μ-OH dimers show a
clear shift to lower fields because of deshielding of the Pt(II)
center.52 Furthermore, different species present would lead to
more than one signal in the spectra. The Pt(mpt) chloro
complex, [Pt(2-methylthiomethylpyridine)Cl2], as mononu-
clear analogue was used as reference to locate the ppm region
for the subsequent measurements of the dinuclear system.
Pt(mpt) was measured in dmso-d6 because of the poor
solubility of the neutral chloro complex in D2O and exhibits
one peak at −2780.5 ppm and its reference-spectrum is
reported in Figure S7 of the Supporting Information. The 195Pt-
NMR measurements were performed in two ppm ranges from
−2000 to −3000 ppm, where the signals of the dinuclear
system are supposed to occur, and also at lower field from
−1000 to −2000 ppm to check for the presence of dimeric
species. Because of the symmetry of the dinuclear system, the
two Pt(II) centers are equivalent and exhibit the same chemical
shift. According to this, exactly one peak was observed in the
195Pt-NMR spectra, which can be seen in Figure 4 for the
For NMR measurements the aquation was carried out as
mentioned in the Experimental Section using D2O, deuterated
triflic acid (CF3SO3D), and NaOD instead of H2O, triflic acid,
and NaOH. After aquation the pH* (without correction) of the
samples was adjusted using drops of concentrated deuterated
triflic acid to obtain a pH* of 2. We performed H- and 195Pt-
1
NMR measurements to detect the species present in solution
and to check the purity of the solution. By way of example,
1
Figures 3 and 4 show the H- and 195Pt-NMR spectra of
6NSpy. The spectra of the three remaining NSpy complexes
are shown in Figures S1−S6 of the Supporting Information.
1
The H NMR spectrum clearly shows evidence for complex-
formation because of the presence of two doublets at 4.45 and
4.19 ppm, which represent the AB-spin system of the CH2
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Figure 5. Mass spectrum of the 10NSpy complex at pH 2 (left) and the isotopic pattern as well as the simulated pattern for the main peak at m/z =
406 (right).
−
6NSpy complex. We note that because of the low solubility of
the 10NSpy complex, the poor signal-to-noise ratio of the 195Pt
spectrum hampers the analysis. However, only one signal is
recognizable and furthermore the 1H spectrum also clearly
shows the presence of only one species. No signals were observed
in the low field range. The chemical shifts of the dinuclear
Pt(II) complexes are independent of the chain length and
appear in the same region. We found values of −2324.0 ppm
(4NSpy), −2323.4 ppm (6NSpy), −2324.3 ppm (8NSpy), and
−2323.0 ppm (10NSpy), which are in good agreement for
platinum complexes in the oxidation state 2+ and bidentate
coordination spheres.52
In addition, mass spectrometric measurements were
performed and in general confirm the information obtained
from the NMR measurements. As an example, Figure 5 depicts
the mass spectrum in the m/z range of 200−1500 for the
10NSpy complex. The main peaks can be found at m/z = 406
(2+), 961 (1+), and 1039 (1+), which consist of [(C22H32-
N2Pt2S2)(OH)2], [(C22H32N2Pt2S2)(OH)2(CF3SO3)], and
[(C22H32N2Pt2S2)(OH)(CF3SO3)2] and represent fragments
of the 10NSpy aqua complex. Furthermore, Figure 5 depicts the
isotopic pattern at m/z = 406 and its corresponding simulated
pattern (also Figures S8−S13, Supporting Information).
The mass spectra of all aqua complexes show similar
fragments for the entire dinuclear system, that is, no dimers
or polymers were observed. Such species would exhibit a
different isotopic pattern and would have been detected during
simulation of each peak. Intramolecular μ-OH species can not
be distinguished from normal OH-coordinated species in the
mass spectrometric measurements, since they exhibit the same
isotopic pattern, but can be excluded on the basis of the NMR
spectra. The appearance of hydroxo ligands on the complex
observed in the mass spectra is due to the ionization of
coordinated water. Beside the complex peaks, a few single peaks
can be found that belong to organic fragments generated during
the evaporation procedure (180 °C).
treated with different kinds of counteranions, such as ClO4 ,
−
−
BF4 , PF6 , and NTf22−, but no crystals were obtained.
pKa Determination of the Tetraaqua Complexes. For
further mechanistic clarification, it is important to obtain
information on the acidity of the coordinated water ligands and
therefore on the reactivity of the aqua complexes. For this
reason spectrophotometric pH titrations with NaOH as base in
the pH range of 1−10 were performed. The spectrophoto-
metric pKa determination was done using a Hellma optical cell
for flow-through measurements attached to a peristaltic pump.
Thereby, it was necessary to keep the concentration of the
complex solution constant, to avoid absorbance corrections
because of dilution. This can be achieved by using a large
volume of 100 mL complex solution during the titration.
Starting at pH 1, the pH was increased by addition of small
portions of solid NaOH to the solution until a pH of 3 was
reached. For further increase in pH, a series of NaOH solutions
of different concentrations were used. The consecutive pH
changes were obtained by dipping a needle into these NaOH
solutions and then in the complex solution. After each addition
of base, samples of 500 μL were taken from the complex
solution, and the pH was measured at 25 °C using an InLab
Semi-Micro pH electrode. This electrode was calibrated using
standard buffer solutions at pH 4, 7, and 10, purchased from
Fisher Scientific. Afterward the samples were discarded because
of a possible chloride ion contamination coming from the pH
electrode. Each pKa titration was performed at least two times
and an average of both values was taken. The pH dependence of
the aqua complex was monitored by UV−vis spectroscopy. The
so-obtained data were analyzed using the Specfit Global Analysis
software. Specfit is a multivariant data analysis program for
modeling and fitting of chemical kinetics and a variety of
equilibrium titrations that are obtained from multiwavelength
spectrophotometric measurements. An important advantage of
this program is the inclusion of all the data measured as a
function of wavelength for the calculated pKa values.
On the basis of the NMR and mass spectrometric
measurements, we conclude that after aquation and increasing
the pH to 11, the possible generated hydroxo and μ-OH
bridged species decompose during acidification of the solution.
Consequently, the tetraaqua complexes are predominantly
present in solution at pH 2, which could also be verified by
performing a pH speciation for the complete dinuclear NSpy
system (see further discussion).
We also tried to isolate crystals for the aqua and chloro
complexes, but unfortunately were not successful. The chloro
complexes are neutral compounds and poorly soluble in any
common solvent. Furthermore, the aqua complexes were
The pKa values were determined by probing different models
for the systems studied. The model selected was that which
gave the best statistical fit. A very low complex concentration of
0.05 mM was used during the pH titration to prevent dimer or
polymer formation, since it is known from the literature that
μ-OH bridged dimers, trimers, or tetramers can accumulate in
solution at higher pH.53−55 Because of the dinuclear system, a
local high concentration can lead to intramolecular ring-closure
through μ-OH bridges during the pH titration. Trials were
made to fit the spectrophotometric data assuming the
formation of a ring-closed dimer at higher pH, but this resulted
in a very poor fit of the data.
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Figure 6 (and Figures S14−S16, Supporting Information)
shows a typical example for 8NSpy of a titration with its
Figure 6. UV−vis spectra recorded for 0.05 mM 8NSpy in the pH
range 1−10 at 25 °C.
spectral changes recorded as a function of pH. The obtained
pKa values for the coordinated water ligands are summarized in
Table 1. The fit-report for each complex is summarized in
Tables S1−S4 of the Supporting Information.
Figure 7. Plots of absorbance versus pH at specific wavelengths for the
NSpy complex system.
methylene chain with an inductive effect. This of course has an
influence on the pKa values and leads to a decrease in pKa1
because of a higher positive charge of the tetra-aqua complex,
and to an increase in pKa3 because of the electron donating
effect of the aliphatic chain trans to the second deprotonated
water molecule.
In general, we found four pKa values for the platinum bound
water molecules for 4NSpy, 6NSpy, and 8NSpy. The 10NSpy
complex with the longest methyl chain shows only two
pKa values. Plots of absorbance versus pH illustrate this fact.
Figure 7 depicts the absorbance traces at one specific
wavelength for all four complexes. Although this wavelength
exhibits only small spectral changes, it was selected on purpose
because in this region the absorbance spectra reflected best the
change from one species into another. Figures S17−S20
(Supporting Information) report the data for three other
wavelengths by way of further examples. We note that not every
deprotonation step is visible at each wavelength.
A similar behavior is already known from the work of Farrell
et al.56 and was also observed in our group.33,57 The
determination of pKa values for polynuclear Pt(II) complexes,
where the relevant Pt centers are far away from each other, show
only one pKa value for two symmetrically equivalent water
molecules. Thus, we assume for the complexes studied in this
work, that on reaching a chain length of 10 methylene groups
(distance between Pt−Pt = 18.40 Å (B3LYP/LANL2DZp)), it
is not possible to differentiate between the two metal centers in
terms of their acidity anymore. The dinuclear 10NSpy complex
behaves like two mononuclear complexes and shows only two
pKa values for a total of four water molecules. For better
comparison with the other dinuclear complexes, we entitled the
deprotonation of the first two water molecules trans to the
pyridine nitrogen as pKa1 and the second deprotonation of the
last two water molecules trans to the sulfur as pKa3 in Table 1. It
is noticeable that the two pKa values for 10NSpy and Pt(mtp)
differ in the first as well as in the second value (see Table 1).
The 10NSpy complex behaves like the mononuclear complex,
but it is and remains dinuclear with an overall charge of 4+ and a
Since the complex system includes two different type of
donors, namely, an aromatic pyridine unit with π-accepting
ability and a sp3-hybridized thioether unit (SR2) with strong
σ-donating ability, it is possible to distinguish which water
molecule is deprotonated first. The sulfur donor with its strong
σ-donor effect labilizes the water molecule in the trans position,
such that the Pt−O bond trans to sulfur is elongated, which
leads to a higher pKa value. Moreover, it is known from
previous work that through the presence of an in-plane pyridine
unit in the chelate system, the electron density on the Pt(II)
center and the hydroxo ligand is stabilized better by the
electron withdrawing ability of the π-accepting pyridine ring.58
Consequently, the water molecule trans to the pyridine
nitrogen donor is deprotonated first and the resulting hydroxo
ligand with its negative charge can perfectly be stabilized trans
to the pyridine donor. Scheme 1 shows the proposed stepwise
deprotonation steps for the pH dependence of the dinuclear
system.
As mentioned before, the results shown in Table 1 indicate a
correlation between the pKa values and the chain length of
the complexes. We found lower pKa1 values for the dinuclear
N,S-system compared to the Pt(mtp) complex. This can be
explained by the higher charge of 4+ compared to 2+ and
was also found for the corresponding dinuclear N,N-system
studied in our group before.33 The pKa1 value is very similar for
all dinuclear complexes with a slight increase on going from
4NSpy to 10NSpy. This effect was also observed for the
Table 1. Summary of pKa Values Obtained for the Stepwise Deprotonation of Platinum Bound Water
4NSpy
6NSpy
8NSpy
10NSpy
Pt(mtp)32
pKa1
pKa2
pKa3
pKa4
2.16 ± 0.01
5.22 ± 0.02
7.14 ± 0.03
9.33 ± 0.03
2.38 ± 0.03
5.09 ± 0.06
7.44 ± 0.06
9.21 ± 0.03
2.32 ± 0.01
4.86 ± 0.09
7.86 ± 0.03
9.23 ± 0.03
2.46 ± 0.07
pKa1
pKa2
3.15 ± 0.03
8.45 ± 0.07
6.84 ± 0.07
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increasing chain length the difference between pKa1 and pKa2
becomes smaller.
Scheme 1. Proposed Stepwise Deprotonation for the pH
Dependence of the Dinuclear System
At this stage both water ligands trans to pyridine are
deprotonated, and the overall charge of the diaqua-dihydroxo
species is 2+. The third deprotonation step now occurs trans to
the sulfur donor. Because of the decreased overall charge, the
pKa3 values are higher compared to those for the first two steps.
As seen in Table 1, the pKa3 values are drastically influenced by
the aliphatic chain length. The labilizing effect of the sulfur
donor is accompanied by the inductive effect of the methyl
chain. With increasing chain length the SR2 unit receives more
electron density and therefore exhibits a stronger σ-donor effect
on the trans localized water molecule. Consequently, the water
molecule becomes more labilized and the pKa3 values increase
in the order 4NSpy < 6NSpy < 8NSpy < 10NSpy. Although
the 10NSpy complex offers only two pKa values, it fits perfectly
into the series and shows the highest pKa3 value.
Naturally, the final deprotonation step shows the highest pKa4
values. We found very similar values for the three complexes
4NSpy, 6NSpy, and 8NSpy, and assume that the presence of
three hydroxo ligands in the complex overrules the inductive
effect of the aliphatic chain. Therefore, the pKa4 values are
independent of the chain length. We want to note that the dif-
ference between pKa3 and pKa4 becomes smaller with increasing
chain length as seen in Table 1. This again is a charge effect as
mentioned above.
Finally, we want to discuss the relatively low pKa1 values for
the dinuclear NSpy complexes as compared to the correspond-
ing dinuclear NNpy system (for more information concerning
the NNpy system see ref 33). Such low pKa values for platinum
bound water molecules are known in the literature. For
corresponding N,N-complex system,33 and we suggest that the
slight increase arises from a decrease in communication
between the two Pt(II) centers accompanied by a decrease in
the localized charge. We want to note that this small decrease in
acidity is a result of charge effects and not of σ-donation by the
aliphatic chains. The second deprotonation step occurs trans to
the pyridine unit at the second Pt(II) center, and the pKa2
values are significantly higher compared to pKa1. This effect can
be ascribed to the change in overall charge from 4+ to 3+ and
therefore to less electrophilic metal centers. As mentioned
above, 10NSpy shows no second deprotonation step in this pH
area since it is not possible to distinguish between the two
water molecules in each Pt(II) center. In addition, it is
noticeable that the differences between pKa1 and pKa2 become
smaller with increasing chain length (about 0.2 units for each
additional two CH2 groups). This again corroborates earlier
work performed in our group.33,57 As mentioned above, the
effect of charge addition decreases as the distance between the
Pt(II) centers becomes longer. This effect explains why with
́
example, Soldatovic et al. determined a pKa1 value for a mono-
nuclear N,S−Pt(II) complex of 3.49,59 but offered no explana-
tion for the relatively low value. To find similarities and
differences in the NSpy and NNpy systems, we took a closer
look at the geometry using quantum chemical calculations. We
simulated the first two deprotonation steps to end at the 2+
charged diaqua-dihydroxo species for all dinuclear complexes.
From a comparison of the tetraaqua species with the diaqua-
dihydroxo species of the 4NSpy with the 4NNpy complexes
as shown in Table 2, and also for the other complexes in
Tables S5−S7 (Supporting Information), the lengths of the
Pt−O bond trans to the pyridine nitrogen are essentially the
Table 2. Calculated Bond Lengths and Angles (B3LYP/LANL2DZp) for the 4NSpy and 4NNpy Complexes and for the
Corresponding Deprotonated Species
4NSpy [Pt2(OH2)4]4+
4NNpy [Pt2(OH2)4]4+
4NSpy [Pt2(OH2)2(OH)2]2+
4NNpy [Pt2(OH2)2(OH)2]2+
α (deg)
92.3
85.4
96.3
85.8
2.31
2.01
2.16
2.13
94.4
97.7
95.6
82.2
2.07
2.00
2.13
2.15
100.2
76.3
97.4
86.1
2.28
2.07
2.15
1.99
102.5
77.7
97.4
82.3
2.05
2.06
2.12
2.00
β (deg)
γ (deg)
δ (deg)
d Pt−X (Å)
d Pt−Npy (Å)
d Pt−Otrans X (Å)
d Pt−Otrans py (Å)
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same for both complex systems. The differences in the Pt−O
bond length trans to X, where X is either NH or S (see drawing
in Table 2), are mainly due to the stronger σ-labilization in case
of NSpy, but have no influence on the first pKa1 value. Further
differences can be seen in terms of the angles, where the major
difference is related to angle β, the angle between the two
platinum bound water ligands (see drawing in Table 2).
During deprotonation trans to the pyridine nitrogen donors,
the angle between the two water molecules decreases more in
the case of the NNpy system. Consequently, the necessary
rearrangement in geometry after deprotonation is more favored
in the NSpy system, where only small changes in geometry
occur. Therefore, we assume that the resulting diaqua-dihydroxo
NSpy species is more stable and consequently leads to lower
pKa1 values compared to its N,N-analogues.
Kinetic Measurements with Thiourea. The kinetics of
the substitution of coordinated water was investigated
spectrophotometrically by following the change in absorbance
at suitable wavelengths as a function of time. A pH of 2 was
selected to guarantee the presence of tetra-aqua species in
solution. The investigations act as preliminary tests to study the
behavior of the aqua complexes and to compare the results for
the series complexes with different chain length. By way of
example, the absorbance changes observed during the reaction
of 0.1 mM 10NSpy with 5 mM thiourea are presented in
Figure 8, and Figures S21−S23 (Supporting Information) show
chain is flexible and can easily undergo rotation and flipping. We,
therefore, assume that the evidence of (S)-inversion is of significant
importance in our case because it explains why in solution a mixture
of all possible configurations was always observed and that it is
impossible to isolate one of them in a pure form. Because the
nucleophilic attack occurs above or below the square planar
coordination sphere of the Pt(II) complexes, the configuration of
the sulfur donor atom has no effect on the substitution rate.
In the kinetic measurements at least a 50-fold excess of
thiourea relative to the complex concentration was used to
guarantee pseudo-first-order conditions for the consecutive
substitution reactions. Since it is known that triflate anions do
not coordinate to Pt(II) metal centers,61 0.01 M triflic acid
(pH 2) was used as solvent for all measurements, which leads to
an ionic strength of I = 0.01 M. Furthermore, the pH of 2 was
selected to guarantee the evidence of the tetra-aqua complex
and to prevent the formation of hydroxo species, which are
known to be inert against further substitution.62,63 The time
dependent spectra observed for the reactions of 4NSpy, 6NSpy,
and 8NSpy can be analyzed using a double-exponential fit,
which indicates that there are two consecutive reactions observed
at 310 nm (see Figures S24−S26, Supporting Information). The
10NSpy complex is an exception in this respect and shows a
different behavior compared to its shorter-bridged analogues in
that it exhibits only one reaction step at 310 nm, which can be
fitted by a single exponential function as illustrated in Figure S27,
Supporting Information. This different behavior in kinetic
reactivity fits perfectly to the information based on the pKa
values for 10NSpy (see further discussion). At 290 nm, the data
for the slowest reaction clearly shows a single exponential
behavior for all four studied complexes because at this selected
wavelength, the first rapid steps were not to be taken into
account (see Figures S28−S31, Supporting Information). On
using thiourea as a neutral nucleophile, the charge on the
complex remains constant during the substitution steps, and
because of the Ci-symmetry of the complexes, we expect two
water ligands to be equivalent and substituted simultaneously.
In recent work, we performed a detailed study on the
mononuclear NSpy complex (Pt(mtp), see Figure 1).32 This
complex now acts as a reference for the here studied dinuclear
system. In general, we found similar and well-established
reactivity with thiourea as entering nucleophile. Especially the
10NSpy complex with the longest methyl chain shows very
similar behavior compared to Pt(mtp), whereas the shorter-
bridged complexes differ in reactivity. For the further discussion
and for clarity concerning the introduced rate constants, we
suggest the successive substitution steps outlined in Scheme 2
by way of a first approach.
Figure 8. UV−vis spectra recorded during the reaction of 0.1 mM
10NSpy with 5 mM thiourea at 25 °C and pH 2 (I = 0.01 M, triflic
acid). Inset: absorbance spectrum of 0.1 mM 10NSpy in the absence
of thiourea.
the plots for the other three complexes. The spectra illustrate a
large change in absorbance that results in an isosbestic point
around 310 nm, which represents the first fast substitution step,
followed by a steady increase in absorbance until the end of the
reaction is reached. On the basis of the information obtained
from these spectra, the wavelength 310 nm was selected to
follow the first fast reaction and 290 nm was selected to observe
the second slower substitution step. In addition, it should be
pointed out that the N,S-system includes a stereogenic center at
each sulfur donor atom SR2. Yates et al. studied the behavior
of the mononuclear NSpy complex [Pt(methylthio-
methylpyridine)Cl2] in solution using variable-temperature
NMR spectroscopy.60 Their experiments showed that the
dichloro complex undergoes fluxional processes in solution at
ambient temperature, which was confirmed by density func-
tional theory (DFT) calculations to be mainly (S)-pyramidal
inversion at the sulfur center. For the dinuclear system there is
a long aliphatic chain instead of a methyl group. However, the
Scheme 2. Suggested Substitution Mechanism for the
Dinuclear Complex System with Thiourea (tu)
Because of the strong trans labilizing effect of the sulfur
donor, the displacement of the water ligand trans to sulfur
occurs in the first reaction step. For this step the different chain
lengths have an influence on the rate of the substitution
reaction. As the aliphatic chain becomes longer, more electron
density is pushed into the metal center because of the inductive
effect of the methylene groups, with the consequence that the
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Table 3. Summary of the Rate Constants and Activation Parameters for the Displacement of Coordinated Water by Thiourea at
25 °C and pH 2
Pt(mtp)32
4NSpy
252 ± 3
6NSpy
221 ± 5
8NSpy
211 ± 2
10NSpy
203 ± 5
k1 (M−1 s−1
k2 (M−1 s−1
)
)
634 ± 10
18.8 ± 0.3
0.031 ± 0.004
16.7 ± 0.1
0.034 ± 0.001
17.7 ± 0.2
0.033 ± 0.001
k−2 (s−1
)
k3K2 (M−2 s−1
k4K3 (M−2 s−1
)
)
73.3 ± 0.8
23.1 ± 0.2
24.9 ± 0.4
34 ± 1
−84 ± 3
48 ± 1
−59 ± 4
65 ± 4
−54 ± 13
22.1 ± 0.6
37.2 ± 0.3
−71 ± 1
47 ± 2
−63 ± 6
64 ± 2
21.8 ± 0.2
39 ± 1
−66 ± 4
47 ± 2
−64 ± 5
64 ± 5
−57 ± 16
ΔH# in kJ mol−1
30 ± 1
−90 ± 3
41.3 ± 0.4
−61 ± 1
1
ΔS# (J mol−1 K−1
)
1
ΔH# (kJ mol−1
)
2
ΔS# (J mol−1 K−1
)
2
ΔH# (kJ mol−1
)
−2
ΔS# (J mol−1 K−1
)
)
)
−57 ± 7
−2
ΔH# (kJ mol−1
)
12.5 ± 0.5
−169 ± 2
18.0 ± 0.3
−158 ± 1
2,3
ΔS# (J mol−1 K−1
2,3
ΔH# (kJ mol−1
)
18.8 ± 0.6
−155 ± 2
−7.5 ± 0.2
−4.8 ± 0.2
−10.9 ± 0.5
18.7 ± 0.8
−155 ± 3
−6.96 ± 0.03
−4.6 ± 0.2
−8.7 ± 0.2
18.2 ± 0.6
−157 ± 2
−5.6 ± 0.2
−4.9 ± 0.3
−7.7 ± 0.2
3,4
ΔS# (J mol−1 K−1
3,4
ΔV# (cm3 mol−1
)
−5.5 ± 0.1
−4.73 ± 0.02
1
ΔV# (cm3 mol−1
)
2
ΔV# (cm3)
−2
Pt(II) center becomes less electrophilic and the nucleophilic
attack occurs slower. The described effect leads to a clear
decrease in reactivity in the order 4NSpy (252 M−1 s−1) >
6NSpy (221 M−1 s−1) > 8NSpy (211 M−1 s−1) > 10NSpy (203
M−1 s−1) at 25 °C (also Table 3). The first substitution step for
the Pt(mtp) complex is almost three times faster (634 M−1
s−1), which is in good agreement with the obtained pKa values
and the reduced charge of 2+. The observed rate constants for
the dinuclear systems and, for comparison, the rate constants
for the mononuclear complex are summarized in Table 3,
where eq 1 presents the rate law for the first substitution step.
σ-donor ligands at the Pt(II) center (viz. SR2, first and
second thiourea ligands), which leads to a labilization of the
second entered thiourea ligand and displacement by water.
Furthermore, the rate constants k−2 for the back reaction are
the same within the experimental error limits (0.033 s−1).
No second substitution reaction on this time scale could
be found for the 10NSpy complex at 310 nm as mentioned
above.
A final reaction step could be observed for all four dinuclear
complexes at 290 nm. It is the second observed reaction step,
k
obs3, for 10NSpy and the third observed reaction step, kobs4, for
4NSpy, 6NSpy, and 8NSpy. All four complexes show nearly
the same observed rate constants, kobs3,4, for this final step
(approximately 23 M−2 s−1 in terms of a third-order rate
constant), from which we suggest that the final step involves
dechelation of the pyridyl rings. The rate for dechelation
does not depend on the length of the aliphatic chain because
two thiourea ligands have already entered the Pt(II)
coordination sphere and overrule the earlier effect of the
(1)
The determined pseudo-first-order rate constants, kobs1, were
plotted against the thiourea concentration, and resulted in a linear
dependence with no meaningful intercept as seen in Figure 9 (see
also Table S8, Supporting Information). This indicates that the
reverse reaction with water is too slow and can be neglected.
The subsequent reaction occurs trans to the pyridine unit and
involves the displacement of the second water ligand at each
Pt(II) center. This substitution step is about ten times slower
compared to the first one, since the first thiourea ligand has
already entered the coordination sphere, thereby increases the
electron density on the Pt(II) center with its σ-donating ability,
which results in a slower nucleophilic attack. Furthermore, the
second step is not influenced by the bridging element. In
consequence, we found nearly the same rate constants k2 for the
complexes 4NSpy (18.8 M−1 s−1), 6NSpy (16.7 M−1 s−1), and
8NSpy (17.7 M−1 s−1 at 25 °C) within the standard error limits
(see Table 3). Figure 10 shows plots of kobs2 against the
nucleophile concentration, which result in a linear dependence
with a small but clear intercept (see also Table S9, Supporting
Information). The corresponding rate law is given in eq 2 and
the values of k2 and k−2 are summarized in Table 3.
aliphatic chain. Plots of the obtained rate constants, kobs3
,
against the thiourea concentration lead to a square
concentration dependence that is shown exemplarily in
Figure 11 (see also Table S10, Supporting Information) for
the 10NSpy complex, and Figures S32−S34 (Supporting
Information) show plots of kobs4 against the thiourea
concentration for the 4NSpy, 6NSpy, and 8NSpy complexes,
which also result in square concentration dependences (see
also Table S11, Supporting Information).
In summary, we observed a different reaction behavior for
the 10NSpy complex than for the shorter-bridged NSpy
complexes, and they are therefore discussed separately. As
known from the pKa determination, the 4NSpy, 6NSpy, and
8NSpy complexes involve platinum centers that can
communicate among each other. Therefore, changes in charge
during substitution reactions could play a crucial role. The first
and the second reaction steps, k1 and k2, were found to be the
displacement of coordinated water by neutral thiourea ligands
and consequently the overall charge of 4+ remained constant.
As seen in Scheme 3, the dechelation of one pyridine ring and
the immediate protonation of the pyridine nitrogen atom lead
(2)
The back reaction (k−2) for the second substitution step
can be accounted for in terms of the competing three sulfur
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Figure 9. Plots of kobs1 vs thiourea concentration for the reaction with 0.1 mM complex in 0.01 M triflic acid (pH 2, I = 0.01 M) at 25 °C.
to an increase in charge from 4+ to 5+. Because of the
interaction of the two Pt(II) centers, two dechelation steps are
expected, where the first dechelation, k3, occurs as a pre-
equilibrium, K3, followed by a fast second dechelation step, k4.
This behavior leads to a square concentration dependence, and
the rate law for the third-order rate constant, k4K3, is given by
eq 3.
(3)
The substitution of the second water molecule leads to three
sulfur donor atoms at each Pt center. The remaining Npy−Pt
bond is strongly labilized by the trans effect of the thiourea
nucleophile. We, therefore, suggest that the third stepthe
dechelation of the NSpy ligandhappens fast. Moreover, fast
dechelation is enforced by the immediate protonation of the
free pyridine nitrogen atom. Fischer et al.64 found a pKa value
of about 4.50 for such substituted pyridine derivatives. This
means that at the studied pH of 2 the nitrogen atom of the
pyridine ring is protonated and avoids rechelation, and
therefore forces the reaction in one direction (Scheme 3).
It is known from the literature65 that at a specific chain
length the two metal centers within a dinuclear complex do not
interact with each other anymore. The required chain length to
reach two independent Pt(II) centers is different for each
complex system. Therefore, we suggest that the 10NSpy
complex with the longest aliphatic chain behaves like a
mononuclear complex and consequently has similar mecha-
nistic properties to the Pt(mtp) reference complex as
mentioned before. The observed behavior during the pH
titration confirms this assumption. In a recent study,32 we
postulated for the Pt(mtp) complex that the displacement of
the last water molecule and the dechelation of the pyridyl unit
occur as a pre-equilibrium followed by a fast second step, which
can not be separated from each other. This behavior results in a
Figure 10. Plots of kobs2 vs thiourea concentration for the reaction with
0.1 mM of the 4NSpy, 6NSpy, and 8NSpy complexes in 0.01 M triflic
acid (pH 2, I = 0.01 M) at 25 °C.
Figure 11. Plots of kobs3 vs thiourea concentration for the reaction with
0.1 mM 10NSpy in 0.01 M triflic acid (pH 2, I = 0.01 M) at 25 °C.
Inset: Plot of kobs3 vs [thiourea]2 for the 10NSpy complex.
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Scheme 3. Proposed Reaction Mechanism for the Reactions of 4NSpy, 6NSpy, and 8NSpy with Thiourea (n = 4, 6, and 8)
third-order rate constant with a square nucleophile concen-
tration dependence.32 Since the 10NSpy complex behaves like
the mononuclear reference complex, we suggest the same
steady state process with a fast equilibrium (K2 = k2/k−2),
followed by an irreversible reaction with the rate constant k3,
which can be seen in Scheme 4. The last reaction is the
Tables S15−S21, Supporting Information). Figure 12 shows
the Eyring plots for the three determined reaction steps
Scheme 4. Proposed Reaction Pathway Exclusively for the
Reaction of 10NSpy Complex with Thiourea
Figure 12. Eyring plots for the determination of the thermal activation
parameters for the successive reaction steps of 0.1 mM 6NSpy with
thiourea at pH 2 (I = 0.01 M triflic acid).
dechelation of the pyridyl rings. Because the Pt(II) centers do
not interact with each other anymore, the change in charge has
no influence on the substitution process, and dechelation
occurs simultaneously for both pyridyl rings. The so-defined
rate law is given in eq 4 and the obtained third-order rate
constant, k3K2, for 10NSpy is summarized in Table 3.
exemplarily for the 6NSpy complex. The data were calculated
using the Eyring equation for which the observed first-order
rate constants kobs1 were converted to second-order rate
constants (k = kobs/[tu]) based on eq 1 and converted to the
third-order rate constants (k = kobs/[tu]2) based on eq 3, and
the results are summarized in Table 3.
As expected, the activation enthalpy ΔH#1 and entropy ΔS#
1
of the first reaction step increase with elongation of the
aliphatic chain. This collaborates perfectly with the obtained
rate constants. A slower reaction is accompanied by an
unfavorably high transition state and therefore higher activation
enthalpies and less negative activation entropies, thus a higher
activation barrier. Only for the shorter-bridged complexes
4NSpy, 6NSpy, and 8NSpy the displacement of the second
water molecule at each Pt(II) center could be followed. In that
case, the values of k2 in general decrease compared to the rapid
first step, which is also reflected in the activation parameters.
The activation enthalpy as well as the activation entropy show
lower values than for the first step. Moreover, nearly the same
values for ΔH#2 (47 kJ mol−1) and for ΔS#2 (−62 J mol−1 K−1)
can be found. Since the substitution reaction occurs trans to the
pyridine nitrogen donor, the rate and activation parameters are
not influenced by the chain length. The back reaction k−2 also
shows similar activation parameters (ΔH#−2 ≈ 64 kJ mol−1 and
(4)
Finally, we note that after coordination of a third thiourea
ligand to each Pt(II) complex, the strong trans-labilizing effect
of thiourea will also affect the thioether donor, which is the
only connection in the dinuclear system. The excess of thiourea
is large enough to displace the last donor in the ligand system
to produce [Pt(tu)4]2+. However, this decomposition is very
slow and was not studied in the present work.
Activation Parameters for the Reaction with
Thiourea. In addition, temperature and pressure dependence
studies were performed to gain more insight into the nature of
the substitution mechanism from the thermal and pressure
activation parameters. The thermal parameters ΔH# and ΔS#
were determined by measuring the rate constants of the first
and last substitution step of each complex at a fixed thiourea
concentration as a function of temperature (Figures S35−S42
and Tables S12−S14, Supporting Information). Complete
concentration dependence studies at different temperatures
were performed for the second reaction step since it includes an
intercept, namely, the back reaction (Figures S43−S51 and
ΔS# ≈ −57 J mol−1 K−1) for the three complexes, which
−2
means that the displacement of thiourea by water is also not
influenced by the chain length. The same trend can be seen
for the last step, k3,4, for the shorter-bridged NSpy complexes.
We found very similar values for ΔH#3,4 (18.5 kJ mol−1) and for
ΔS#3,4 (−155 J mol−1 K−1), which again indicates that the third
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reaction step belongs to dechelation of the pyridine unit, which
is independent of the bridging element. The stepwise
dechelation of the first and second pyridine ring cannot be
observed separately, and consequently the activation parame-
ters ΔH#3,4 and ΔS#3,4 are composed of both steps. In addition,
we note that the activation parameters for the displacement of
the last water molecule and the simultaneous dechelation of
both pyridine rings shows similar values for ΔH# and ΔS#
reason it is not surprising that we found very similar ΔV#
2
values for 4NSpy, 6NSpy, and 8NSpy as seen in Table 3. Since
the second substitution reaction includes a back reaction, the
activation volumes ΔV#
for the back reaction were
−2
determined and found to be significantly negative. We found
a clear tendency for the activation volumes of the back reaction
in the order 4NSpy < 6NSpy < 8NSpy. However, in this case
the change is not caused by electron donation, but in fact by
charge effects. With elongation of the methylene chain, the
overall charge at each Pt(II) center decreases. On using water
as a weak nucleophile (compared to thiourea), we suggest that
the slight change in charge has an influence on the activation
2,3
2,3
in the case of 10NSpy.
In addition, pressure dependence studies were performed for
the first and second substitution steps by measuring the rate
constants kobs1 for each complex at a fixed thiourea con-
centration (25 mM) as a function of pressure for the first step
(Figures S52−S55 and Table S22, Supporting Information) and
by performing a concentration dependence at different pres-
sures for the second substitution step k2 (Figures S56−S64 and
Tables S23−S28, Supporting Information). The volumes of
activation ΔV#, calculated from the slope of plots of ln(k)
versus pressure, are summarized in Table 3, and by way of
example Figure 13 depicts plots of ln(kobs) versus pressure for
the successive reaction steps of the 8NSpy complex.
volume ΔV# such that the back reaction has a stronger
−2
associative character for the shorter aliphatic chain and
therefore a slightly more electrophilic metal center.
Cytostatic Activity. Dr. Thomas Huhn from the University
of Konstanz and his team performed cell tests for all four
dinuclear Pt(II) tetra-chloro complexes. The cytotoxicity of the
chloro complexes 5−8 (see Figure 2) was studied using the
AlamarBlue assay in the human cervix carcinoma cell line HeLa
S3. Of all new complexes tested only 8, the complex with the
longest methylene chain, showed cytostatic activity. We found
an IC50 value of 21 ± 9 μM for complex 8, which describes the
concentration at which 50% of the cells remained viable with
respect to the control (see Table S29, Supporting Information).
The resulting dose−response curve for the HeLa S3 cell line is
shown in Figure 14. Cisplatin was also tested as a reference
Figure 13. Plots of ln(kobs) vs pressure for the first and the second
reaction steps of 0.1 mM 8NSpy with 25 mM thiourea at 25 °C and
pH 2 (I = 0.01 M triflic acid).
Throughout the series of complexes the substitution
reactions were accelerated with increasing pressure. The so-
obtained activation volumes are very typical for associative
substitution reactions on square-planar complexes,66−68 where
the volume decrease results from bond formation on going
from the square-planar reactant state to the trigonal
bipyramidal transition state. This mechanistic assignment is
furthermore in good agreement with the negative activation
entropies found for the two reaction steps for all dinuclear
complexes (see Table 3). The ΔV#1 values increase in the order
4NSpy (−7.5 cm3 mol−1) < 6NSpy (−6.96 cm3 mol−1) <
8NSpy (−5.6 cm3 mol−1) < 10NSpy (−4.73 cm3 mol−1) at
25 °C. Since the first substitution step takes place trans to the
sulfur donor, the chain length has an influence on the activation
Figure 14. Dose−response curves for the HeLa S3 cell line for
complex 8.
compound and exhibited an IC50 value of 1.2 ± 0.4 μM. We
note that the free ligand itself shows some cytotoxic activity
(67 μM, not shown), but the IC50 values clearly show that
complex 8 results in a much higher activity than the ligand.
The chloro complexes instead of the corresponding aqua
species were used for the cell tests because of their biological
and medical relevance. In general, a platinum drug such as
cisplatin is injected into the body as the corresponding chloro
species. On its way to the cell, the relatively high chloride
concentration in blood and other tissues suppress the
hydrolysis reactions. After entering the cell (influx), where a
much lower chloride concentration is present, the platinum
drug undergoes hydrolysis. It is nowadays well-accepted that
the active species for cisplatin is [cis-Pt(NH3)2(OH2)Cl]+,
where one labile water molecule can easily be displaced.69
Determination of the pKa values is therefore of special interest,
volume ΔV# . With elongation of the methylene bridge, more
1
electron density is pushed into the metal center, which leads to
a decrease in electrophilicity of the Pt(II) center. In
consequence, the entering nucleophile is less attracted, which
leads to a less associative transition state and therefore to
smaller negative activation volumes. For the displacement of
the second water ligand at each metal center the activation
volume ΔV#2 could only be determined for the shorter-bridged
complexes. This substitution step occurs trans to the pyridine
unit and is therefore independent of the chain length. For this
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Table 4. Summary of the Chemical Quantum Calculations for the Mononuclear and Dinuclear Complexes
Pt(mtp)
4NSpy
6NSpy
8NSpy
10NSpy
Φ (S−C−H2− Cpy−Npy) (deg)
d Pt−S (Å)
d Pt−Otrans to S (Å)
d Pt−Npy (Å)
d Pt−Otrans to py (Å)
d Pt−Pt (Å)
d S−(CH2n−S) (Å)
29.4
2.30
2.17
2.00
2.14
27.3
2.31
2.16
2.01
2.13
10.81
7.04
27.6
2.31
2.17
2.01
2.13
13.33
9.57
27.7
2.31
2.17
2.01
2.13
15.88
12.10
27.9
2.31
2.18
2.01
2.13
18.40
14.66
since if the water molecules at the metal center are too acidic,
hydroxo species will be generated under physiologically
relevant pH conditions and such complexes are known to be
inert to further substitution reactions. On the basis of the pKa
values for the cytostatic active 10NSpy complex, we know that
at physiological pH the diaqua-dihydroxo species is present in
solution, where one water and one hydroxo ligand are
coordinated at each Pt(II) center.
The complexes with shorter aliphatic chains show no
cytostatic activity. The mononuclear reference complex Pt-
(mtp) was found to be active.31 We assume that the cytostatic
activity derives from the same Pt(II) moiety, namely, the
pyridine donor linked to a thioether donor. Furthermore, it was
demonstrated from the pKa determination that at a bridge
length of 10 methylene groups the two Pt(II) metals do not
interact with each other and behave like mononuclear
compounds. However, the complex is dinuclear, and it is
known from literature that polynuclear complexes exhibit
different DNA adducts because of the ability to form long-
ranged interstrand cross-links.70 Widespread studies in cell-free
systems indicate that the formation of interstrand cross-links is
a major event for di- or polynuclear complexes.19 Thus,
dinuclear cytostatic active complexes may have the advantage of
a different binding-mechanism to DNA. As mentioned above,
they form long-ranged interstrand DNA-adducts and the so
caused DNA-damage cannot be sufficiently repaired.71
Consequently, polynuclear complexes may help in terms of
overcoming Pt-drug resistance.
DFT Calculations. Table 4 summarizes the results of the
quantum chemical calculations. The data show in general
shorter Pt−N bonds than the Pt−O bonds, and of course the
Pt−Npy bond is shorter than the Pt−S bond for each complex.
Furthermore, the Pt−O bond trans to sulfur is longer than the
Pt−O bond trans to Npy for each dinuclear complex. This
nicely depicts the trans labilizing effect of the sulfur donor. The
calculated dihedral angles (Npy−Pt···Pt−Npy) indicate a twist in
the ring arrangement. An analysis of the calculated Pt−Pt
distances show that two additional CH2 groups add 2.52 Å to
Figure 15. Calculated structure (B3LYP/LANL2DZp) (Ci) of the
4NSpy complex.
CONCLUSIONS
■
In this study, a series of novel dinuclear platinum(II) complexes
were synthesized containing a mixed nitrogen and sulfur donor
bidentate chelate system. The two platinum centers are
connected by an aliphatic chain of variable length by which
the effect of elongating the methylene chain was studied.
Furthermore, it was interesting to study the influence of
combining a strong σ-donor and a π-acceptor within one
complex system. We performed spectrophotometric acid−base
titrations to determine the pKa values for the coordinated water
ligands. In general, we found four pKa values for 4NSpy,
6NSpy, and 8NSpy. The 10NSpy complex with the longest
methylene chain shows only two pKa values. This can be
ascribed to a clear correlation between chain length and acidity
of platinum bound water. With elongation of the chain, the
communication between the two Pt(II) centers becomes
weaker and finally ends as seen for the 10NSpy complex,
which behaves like two mononuclear complexes with two
equivalent water molecules at each metal center. Furthermore,
we established a deprotonation pattern, where the first
deprotonation occurs trans to the pyridine nitrogen donors
and the last deprotonation occurs trans to the sulfur donors.
Thereby, the first step is not influenced by the aliphatic chain
length, whereas the last step shows increased pKa values with
increasing chain length, which can be accounted for by the
increasing electron donation effect of the chain. On comparing
the acidity of the N,S-complexes with their corresponding N,N-
analogues, two differences are noted. The pKa values of the
NSpy system are approximately one pK unit lower, which can
be explained by a more favored geometric arrangement
following deprotonation. In contrast to 10NSpy, the two
Pt(II) centers can interact with each other in the case of the
10NNpy. Kinetic measurements were performed with thiourea
as a strong nucleophile under pseudo-first-order conditions as a
function of nucleophile concentration, temperature, and
pressure, using stopped-flow techniques and UV−vis spectros-
copy. The different behavior of the shorter-bridged complexes
and 10NSpy during the pH titrations is also reflected in their
the Pt−Pt distance, which is very similar to the results of Erturk
̈
et al.57 and also perfectly comparable to the results of our
recent work.33 Because of the larger sulfur donor atoms, the
Pt−Pt distances for all dinuclear N,S-complexes increases by
0.8 Å compared to the corresponding N,N-analogues (for more
information concerning the NNpy system see ref 33).
By way of example, Figure 15 shows the calculated structure
of the 4NSpy complex (for the other structures see Figures
S65−S68, Supporting Information). It is noticeable that the
CH2-group between the pyridine ring and the sulfur donor
extremely sticks out of the square coordination sphere, and the
bridging methylene chain behaves like a staggered zigzag chain.
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kinetic reactivity. We found three successive reaction steps for
the shorter-bridged 4NSpy, 6NSpy, and 8NSpy complexes,
which are ascribed to the stepwise displacement of water by
thiourea, followed by a successive dechelation of the pyridine
rings because of the favored protonation of the nitrogen atom
under the selected experimental conditions. The reactivity of
10NSpy is perfectly comparable to the mononuclear reference
complex studied before.32 Two reaction steps are observed that
belong to the displacement of water by thiourea and finally to a
simultaneous dechelation of both pyridine rings because of
the lack of communication between the metal centers. Since the
first substitution step occurs trans to the σ-donating sulfur,
the results for k1 indicate that there is a clear dependence on
the aliphatic chain length. With elongation of the chain, more
electron density is pushed onto the metal center, which leads to
slower substitution reactions in the order 4NSpy > 6NSpy >
8NSpy > 10NSpy. Although the dechelation of one donor
atom occurs relatively fast, we found the remaining
[Pt2(H2NpyS)(tu)6]6+ species to be stable for at least 12 h.
The final decomposition pattern of the complete system was
not studied in this work.
of Chemistry and Konstanz Research School Chemical Biology,
University of Konstanz, Germany) for help with the biological
assays and the kind cooperation. Finally, the authors kindly
thank Prof. Dr. Ivana Ivanovic-
́
Burmazovic
́
and Oliver
Troppner for their support with the mass spectrometric
̈
measurements and Dr. Achim Zahl for help with the 195Pt-
NMR measurements.
REFERENCES
■
(1) Wheate, N. J.; Walker, S.; Craig, G. E.; Oun, R. Dalton Trans.
2010, 39, 8113−8127.
(2) Rosenberg, B.; VanCamp, L.; Krigas, T. Nature 1965, 205, 698−
699.
(3) Rosenberg, B.; VanCamp, L.; Trosko, J. E.; Mansour, V. H.
Nature 1969, 222, 385−386.
(4) Neidle, S.; Ismail, I. M.; Sadler, P. J. J. Inorg. Biochem. 1980, 13,
205−212.
(5) Frey, U.; Ranford, J. D.; Sadler, P. J. Inorg. Chem. 1993, 32,
1333−1340.
(6) Wilkinson, R.; Cox, P. J.; Jones, M.; Harrap, K. R. Biochimie
1978, 60, 851−857.
(7) Cleare, M. J.; Hydes, P. C.; Malerbi, B. W.; Watkins, D. M.
Biochimie 1978, 60, 835−850.
It is known from the literature34 that complexes with
bidentate coordination spheres show higher cytostatic activity
compared to their monodentate analogues with only one vacant
coordination site. In addition, it is well-known from multi-
nuclear complexes that the DNA adducts of such complexes are
more flexible and long ranged, with a high degree of interstrand
cross-links, and consequently are promising compounds to
overcome cisplatin resistance.35 Therefore, cytostatic cell tests
for all dinuclear complexes were carried out using the HeLa
S3 cell line, and the tetrachloro complex 8 with the longest
chain was found to be cytostatic active with an IC50 value of
21 ± 9 μM.
(8) Calvert, A. H.; Harland, S. J.; Newell, D. R.; Siddik, Z. H.; Jones,
A. C.; McElwain, T. J.; Raju, S.; Wiltshaw, E.; Smith, I. E.; Baker, J. M.;
Peckham, M. J.; Harrap, K. R. Cancer Chemother. Pharmacol. 1982, 9,
140−147.
(9) Evans, B. D.; Raju, K. S.; Calvert, A. H.; Harland, S. J.; Wiltshaw,
E. Cancer Treat. Rep. 1983, 67, 997−1000.
(10) Barnard, C. F. J.; Cleare, M. J.; Hydes, P. C. Chem. Brit. 1986,
22, 1001−1004.
(11) Armand, J.-P.; Boige, V.; Raymond, E. Semin. Oncol. 2000, 27,
96−104.
(12) Extra, J. M.; Marty, M.; Brienza, S. Semin. Oncol. 1998, 25, 13−
22.
(13) Kelland, L. Nat. Rev. Cancer 2007, 7, 573−584.
(14) Boulikas, T.; Vougiouka, M. Oncol. Rep. 2003, 10, 1663−1682.
(15) (a) Thompson, A. J.; Williams, R. J. P.; Reslova, S. Struct.
Bonding (Berlin) 1972, 11, 1−46. (b) Sherman, S. E.; Lippard, S. J.
Chem. Rev. 1987, 87, 1153−1181.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures S1−S7 show NMR spectra and Figures S87−S13 mass
spectra of the dinuclear complexes. Figures S14−S16
summarize UV−vis spectra for the complexes 4NSpy, 6NSpy,
and 10NSpy in the pH range of 1−10, and Figures S17−S20
depict absorbance traces as a function of pH for all dinuclear
complexes. Figures S21−S64 illustrate different UV−vis
spectral changes, kinetic traces, concentration, temperature,
and pressure dependent studies for all dinuclear complexes.
Figures S65−S68 report the calculated structures of the
dinuclear complexes. Tables S1−S4 summarize the fit reports
for the pKa determination, Tables S5−S8 depict the calculated
bond lengths and angles, and Tables S8−S28 summarize all
values of kobs determined for all reactions as a function of
nucleophile concentration, temperature, and pressure for all
dinuclear complexes. Table S29 summarizes the experimental
results for the cytostatic cell tests. This material is available free
of charge via the Internet at http://pubs.acs.org.
(16) Farrell, N.; de Almeida, S. G.; Skov, K. A. J. Am. Chem. Soc.
1988, 110, 5018−5019.
(17) Costa, L. A. S.; Rocha, W. R.; de Almeida, W. B.; Dos Santos, H.
F. J. Chem. Phys. 2003, 118, 10584−10592.
(18) Farrell, N. Comments Inorg. Chem. 1995, 16, 373−389.
(19) Brabec, V.; Kasp
Qu, Y.; Farrell, N. Biochemistry 1999, 38, 6781−6790.
̌ ́ ́ ́ ́ ́
arkova, J.; Vrana, O.; Novakova, O.; Cox, J. W.;
(20) Qu, Y.; Farrell, N. J. Am. Chem. Soc. 1991, 113, 4851−4857.
(21) (a) Farrell, N.; Qu, Y. Inorg. Chem. 1989, 28, 3416−3420.
(b) Farrell, N.; Qu, Y.; Hacker, M. P. J. Med. Chem. 1990, 33, 2179−
2184. (c) Roberts, J. D.; Van Houten, B.; Qu, Y.; Farrell, N. P. Nucleic
Acids Res. 1989, 17, 9719−9733. (d) Kraker, A.; Elliott, W.; Van
Houten, B.; Farrell, N.; Hoeschele, J.; Roberts, J. J. Inorg. Biochem.
1989, 36, 160.
(22) Roberts, J. D.; Beggiolin, G.; Manzotti, C.; Piazzoni, L.; Farrell,
N. J. Inorg. Biochem. 1999, 77, 47−50.
(23) Roberts, J. D.; Peroutka, J.; Farrell, N. J. Inorg. Biochem. 1999,
77, 51−57.
(24) Bogdanovic,
́
G.; Kojic,
́
V.; Srdic,
́
T.; Jakimov, D.; Djuran, M.;
AUTHOR INFORMATION
̌
■
Burgarci
33−43.
̌
c,
́
Z. D.; Baltic, M.; Baldic,
́
́
V. V. Metal-Based Drugs 2002, 9,
Corresponding Author
*E-mail: vaneldik@chemie.uni-erlangen.de.
(25) Mandal, S.; Castineiras, A.; Mondal, T. K.; Mondal, A.;
̃
Chattopadhyay, D.; Goswami, S. Dalton Trans. 2010, 39, 9514−9522.
(26) Pitteri, B.; Bortoluzzi, M.; Marangoni, G. Trans. Met. Chem.
2005, 30, 1008−1013.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge continued financial support
from the Deutsche Forschungsgemeinschaft. The authors thank
Dipl. Chem. Malin Bein and Dr. Thomas Huhn (Department
̌
́
̌ ́ ́ ́
(27) (a) Burgarcic, Z. D.; Soldatovic, T.; Jelic, R.; Alguerο, B.;
Grandas, A. Dalton Trans. 2004, 22, 3869−3877. (b) Annibale, G.;
Brandolisio, M.; Pitteri, B. Polyhedron 1995, 14, 451−453.
12760
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Article
(28) Baldo, A.; Chessa, G.; Marangoni, G.; Pitteri, B. Polyhedron
1985, 4, 1429−1431.
(29) Jones, R. C.; Madden, R. L.; Skelton, B. W.; Tolhurst, V.-A.;
White, A. H.; Williams, A. M.; Wilson, A. J.; Yates, B. F. Eur. J. Inorg.
Chem. 2005, 6, 1048−1055.
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision B.03; Gaussian Inc.: Wallingford, CT, 2004.
(52) (a) Qu, Y.; Farrell, N. J. Inorg. Biochem. 1990, 40, 255−264.
(b) Khokhar, A. R.; Deng, Y.; Al-Baker, S.; Yoshida, M.; Siddik, Z. H.
J. Inorg. Biochem. 1993, 51, 677−687. (c) Shen, W.-Z.; Schneckbeck,
R.-D.; Freisinger, E.; Lippert, B. Dalton Trans. 2008, 30, 4044−4049.
(d) Pizarro, A. M.; Munk, V. P.; Navarro-Ranninger, C.; Sadler, P. J.
Angew. Chem., Int. Ed. 2003, 42, 5339−5342.
(30) Jones, R. C.; Skelton, B. W.; Tolhurst, V.-A.; White, A. H.;
Wilson, A. J.; Canty, A. J. Polyhedron 2007, 26, 708−718.
(31) Du Preez, J. G. H. Platinum(II) complexes, preparation and use.
Document number: CA Patent 2547275, 2005; p 38.
(32) Hochreuther, S.; Nandibewoor, S. T.; Puchta, R.; van Eldik, R.
Dalton Trans. 2011, in press.
(53) Lee, K. W.; Martin, D. S. Inorg. Chim. Acta 1976, 17, 105−110.
(54) Lim, M. C.; Martin, R. B. J. Inorg. Nucl. Chem. 1976, 38, 1911−
1914.
(33) Hochreuther, S.; Puchta, R.; van Eldik, R. Inorg. Chem. 2011, 50,
8984−8996.
(34) Qu, Y.; Farrell, N. J. Am. Chem. Soc. 1991, 113, 4851−4857.
(35) (a) Kloster, M. B. G.; Hannis, J. C.; Mudiman, D. C.; Farrell, N.
(55) Faggiani, R.; Lippert, B.; Lock, C. J. L.; Rosenberg, B. J. Am.
Chem. Soc. 1977, 99, 777−781.
Biochemistry 1999, 38, 14731−14737. (b) Brabec, V.; Kasp
Vrana, O.; Novakova, O.; Cox, J. W.; Farrell, N. Biochemistry 1999, 38,
6781−6790. (c) Kasparkova, J.; Zehnulova, J.; Farrell, N.; Brabec, V. J.
̌ ́ ́
arkova, J.;
(56) Davies, M. S.; Cox, J. W.; Berners-Price, S. J.; Barklage, W.; Qu,
Y.; Farrell, N. Inorg. Chem. 2009, 39, 1710−1715.
́
́
́
̌
́
́
(57) Erturk, H.; Hofmann, A.; Puchta, R.; van Eldik, R. Dalton Trans.
̈
Biol. Chem. 2002, 277, 48076−48086. (d) Qu, Y.; Scarsdale, N. J.;
Tran, M.-C.; Farrell, N. P. J. Biol. Inorg. Chem. 2003, 8, 19−28. (e) Qu,
Y.; Scarsdale, N. J.; Tran, M.-C.; Farrell, N. J. Inorg. Biochem. 2004, 98,
1585−1590. (f) Hegmans, A.; Berners-Price, S. J.; Davies, M. S.;
Thomas, D. S.; Humphreys, A. S.; Farrell, N. J. Am. Chem. Soc. 2004,
126, 2166−2180. (g) Qu, Y.; Tran, M.-C.; Farrell, N. P. J. Biol. Inorg.
Chem. 2009, 14, 969−977.
2007, 22, 2295−2301.
(58) Hofmann, A.; Jaganyi, D.; Munro, O. Q.; Liehr, G.; van Eldik, R.
Inorg. Chem. 2003, 42, 1688−1700.
̌
́ ̌ ́
(59) Soldatovic, T.; Bugarcic, Z. D.; van Eldik, R. Dalton Trans. 2009,
23, 4526−4531.
(60) Jones, R. C.; Madden, R. L.; Skelton, B. W.; Tolhurst, V.-A.;
White, A. H.; Williams, A. M.; Wilson, A. J.; Yates, B. F. Eur. J. Inorg.
Chem. 2005, 6, 1048−1055.
(36) Hofmann, A.; Dahlenburg, L.; van Eldik, R. Inorg. Chem. 2003,
42, 6528−6538.
(61) Fallis, S.; Anderson, G. K.; Rath, N. P. Organometallics 1991, 10,
3180−3184.
(37) (a) Reedijk, J. Chem. Commun. 1996, 801−806. (b) Burchenal,
J. H.; Kalaher, K.; Dew, K.; Lokys, L.; Gale, G. Biochimie 1978, 60,
961−965.
(62) Mahal, G.; van Eldik, R. Inorg. Chem. 1985, 24, 4165−4170.
(63) Mahal, G.; van Eldik, R. Inorg. Chem. 1987, 127, 203−208.
(64) Fischer, A.; King, M. J.; Robinson, F. P. Can. J. Chem. 1978, 56,
3059−3067.
(38) Sarkar, S.; Patra, A.; Drew, M. G. B.; Zangrando, E.;
Chattopadhyay, P. Polyhedron 2009, 28, 1−6.
(39) (a) Horie, M.; Sakano, T.; Osakada, K. J. Organomet. Chem.
2006, 691, 5935−5945. (b) Bierer, D. E.; Dener, J. F.; Dubenko, L. G.;
Gerber, R. E.; Litvak, J.; Peterli, S.; Peterli-Roth, P.; Truong, T. V.;
Mao, G.; Bauer, B. E. J. Med. Chem. 1995, 38, 2628−2648. (c) Gerber,
R. E.; Hasbun, C.; Dubenko, L. G.; King, M. F.; Bierer, D. E. Org.
Synth. Coll. X 2004, 234−239, 186−196.
(65) Davies, M. S.; Cox, J. W.; Berners-Price, S. J.; Barklage, W.; Qu,
Y.; Farrell, N. Inorg. Chem. 2000, 39, 1710−1715.
(66) Stochel, G.; van Eldik, R. Coord. Chem. Rev. 1999, 187, 329−
374.
(67) Helm, L.; Elding, L. I.; Merbach, A. E. Helv. Chim. Acta 1984,
67, 1453−1460.
(40) Djuran, M. I.; Milinkovic, S. U.; Habtemariam, A.; Parsons, S.;
Sadler, P. J. J. Inorg. Biochem. 2002, 88, 268−273.
(68) Helm, L.; Elding, L. I.; Merbach, A. E. Helv. Chim. Acta 1985,
24, 1719−1721.
(41) Hofmann, A.; van Eldik, R. Dalton Trans. 2003, 15, 2979−2985.
(42) Hamid, R.; Rotshteyn, Y.; Rabadi, L.; Parikh, R.; Bullock, P.
Toxicol. In Vitro 2004, 18, 703−710.
(69) Berners-Price, S. J.; Appleton, T. G. Platinum-based Drugs
in Cancer Therapy; Kelland, L. R., Farrell, N. P., Eds.; Humana Press
Inc.: Totowa, N.J., 2000.
(43) van Eldik, R.; Gaede, W.; Wieland, S.; Kraft, J.; Spitzer, M.;
Palmer, D. A. Rev. Sci. Instrum. 1993, 64, 1355−1357.
(70) (a) Cox, J. W.; Berners-Price, S. J.; Davies, M. S.; Qu, Y.; Farrell,
N. J. Am. Chem. Soc. 2001, 123, 1316−1326. (b) Brabec, V.;
Kasparova, J. Drug Resist. Updates 2005, 8, 131−146.
(71) (a) Kloster, M. B. G.; Hannis, J. C.; Mudiman, D. C.; Farrell, N.
(44) Becke, A. D. J. Chem. Phys. 1993, 98, 564−5652.
(45) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623−11627.
̌ ́ ́
Biochemistry 1999, 38, 14731−14737. (b) Kasparkova, J.; Zehnulova,
(46) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter
Mater. Phys. 1988, 37, 785−789.
J.; Farrell, N.; Brabec, V. J. Biol. Chem. 2002, 277, 48076−48086.
(c) Qu, Y.; Scarsdale, N. J.; Tran, M.-C.; Farrell, N. P. J. Biol. Inorg.
Chem. 2003, 8, 19−28. (d) Qu, Y.; Scarsdale, N. J.; Tran, M.-C.;
Farrell, N. J. Inorg. Biochem. 2004, 98, 1585−1590. (e) Hegmans, A.;
Berners-Price, S. J.; Davies, M. S.; Thomas, D. S.; Humphreys, A. S.;
Farrell, N. J. Am. Chem. Soc. 2004, 126, 2166−2180. (f) Qu, Y.; Tran,
M.-C.; Farrell, N. P. J. Biol. Inorg. Chem. 2009, 14, 969−977.
(47) Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry;
Plenum: New York, 1976; Vol. 3, pp 1−28.
(48) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284−298.
(49) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310.
(50) Huzinaga, S., Ed.; Gaussian Basis Sets for Molecular Calculations;
Elsevier: Amsterdam, The Netherlands, 1984.
(51) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery Jr., J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, J.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A., G.;
12761
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