Phosphinic platinum complexes with 8-thiotheophylline derivatives: Synthesis, characterization, and antiproliferative activity

Article abstract of DOI:10.1021/ic062133c

The platinum mixed-phosphine complexes (SP-4,2)-[PtCl(8-MTT)(PPh ₃)(PTA)] (2) and cis-[Pt(8-MTT)₂(PPh₃)(PTA)] (3) (MTTH₂ = 8-(methylthio)theophylline, PTA = 1,3,5-triaza-7- phosphaadamantane) have been prepared from the precursor cis-[PtCl ₂(PPh₃)(PTA)] (1), which has been fully characterized by X-ray diffraction determination. Antiproliferative activity tests indicated that the presence of one lipophilic PPh₃ and one hydrophilic PTA makes 1-3 more active than the analogues bearing two PPh₃ or two PTA. The reactivity of cis-[PtCl₂(PPh₃)₂], cis-[PtCl₂-(PTA)₂], and cis-[PtCl₂(PPh ₃)(PTA)] with the bis(thiopurines) bis(S-8-thiotheophylline)methane (MBTTH₂), 1,2-bis-(S-8-thiotheophylline)ethane (EBTTH₂), and 1,3-bis(S-8-thiotheophylline)propane (PBTTH₂) has also been investigated. New binuclear complexes have been prepared and identified by spectroscopic techniques and their antiproliferative activities on T2 and SKOV3 cell lines evaluated.

Full text of DOI:10.1021/ic062133c

Inorg. Chem. 2007, 46, 4267−4276
Phosphinic Platinum Complexes with 8-Thiotheophylline Derivatives:
Synthesis, Characterization, and Antiproliferative Activity
P. Bergamini,*,† V. Bertolasi,† L. Marvelli,† A. Canella,‡ R. Gavioli,‡ N. Mantovani,† S. Man˜as,§ and
A. Romerosa*,§
Dipartimento di Chimica dell’UniVersita´ di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy,
Dipartimento di Biochimica e Biologia Molecolare dell’UniVersita` di Ferrara, Via L. Borsari 46,
44100 Ferrara, Italy, and AÄ rea de Qu´ımica Inorga´nica, Facultad de Ciencias Experimentales,
UniVersidad de Almer´ıa, 04071 Almer´ıa, Spain
Received November 10, 2006
The platinum mixed-phosphine complexes (SP-4,2)-[PtCl(8-MTT)(PPh3)(PTA)] (2) and cis-[Pt(8-MTT)2(PPh3)(PTA)]
(3) (MTTH2 8-(methylthio)theophylline, PTA 1,3,5-triaza-7-phosphaadamantane) have been prepared from
)
)
the precursor cis-[PtCl2(PPh3)(PTA)] (1), which has been fully characterized by X-ray diffraction determination.
Antiproliferative activity tests indicated that the presence of one lipophilic PPh3 and one hydrophilic PTA makes
1−3 more active than the analogues bearing two PPh3 or two PTA. The reactivity of cis-[PtCl2(PPh3)2], cis-[PtCl2-
(PTA)2], and cis-[PtCl2(PPh3)(PTA)] with the bis(thiopurines) bis(S-8-thiotheophylline)methane (MBTTH2), 1,2-bis-
(S-8-thiotheophylline)ethane (EBTTH2), and 1,3-bis(S-8-thiotheophylline)propane (PBTTH2) has also been investigated.
New binuclear complexes have been prepared and identified by spectroscopic techniques and their antiproliferative
activities on T2 and SKOV3 cell lines evaluated.
Introduction
hamper the cell replication mechanism, particularly in fast
replicating populations such as cancer cells.3 While the ideal
model structure for reaching the nucleic acids inside the cell
nucleus seems to be a cisplatin-resembling pattern, i.e., a
neutral square planar Pt(II) complex bearing two neutral and
two anionic ligands, the complex distribution and concentra-
tion in various body areas are presumably influenced by some
properties of the ligands like polarity, hydrophilicity, steric
requirements, etc.
In this perspective, we report here a group of new Pt-
phosphine complexes bearing 8-thiotheophylline derivatives4
(Chart 1) as anionic ligands.
Although some Pt-phosphine complexes have been tested
unsuccessfully in the past as anticancer compounds,5 we
believe it is worthwhile to reconsider the issue by associating
phosphines with other bioactive ligands or by using phos-
phines with peculiar characteristics like the protonable
The design and synthesis of Pt(II) complexes capable of
interacting with biomolecules producing a pharmacological
action is still a lively topic of research, spanning from the
production of new analogues of the classical Pt-containing
anticancer drugs1 to the more recent attempts to develop
complexes with an anti-HIV activity.2
It is now well-known that the biological activities of
platinum complexes originate from their interactions with
intranuclear molecules, in particular from the formation of
specific DNA-platinum adducts. They are responsible of
deformations of the DNA double strand structure which
* To whom correspondence should be addressed. E-mail: p.bergamini@
unife.it (P.B.); romerosa@ual.es (A.R.).
† Dipartimento di Chimica dell’Universita` di Ferrara.
‡ Dipartimento di Biochimica e Biologia Molecolare dell’Universita` di
Ferrara.
§ Universidad de Almer´ıa.
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10.1021/ic062133c CCC: $37.00
Published on Web 04/20/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 10, 2007 4267

Bergamini et al.
Chart 1
one, or on the contrary, the two sites could display an
independent reactivity and, therefore, it should be possible
to obtain species where a single N7 is bonded to platinum
and to synthesize heterobimetallic complexes with purinic
sites bonded to different metals. At last, cooperative or
synergic effects of the two metal nuclei, likely to amplify
the antiproliferative activity, could be observed.
Experimental Section
amino-phosphine PTA (1,3,5-triaza-7-phosphaadamantane),
which is at present attracting great interest in the field of
metal-based drugs.6 In a recent paper, we showed that Pt
complexes containing PPh3 and the N7-coordinated anionic
ligand 8-MTT- show on some cancer cell lines a remarkable
antiproliferative activity, which decreases when PPh3 is
replaced by the more hydrophilic PTA.7 On both cisplatin-
sensitive T2 and cisplatin-resistant SKOV3 cell lines, the
complex cis-[PtCl(8-MTT)(PPh3)2] (8-MTT- ) 8-(meth-
ylthio)theophyllinate) showed an activity comparable with
cisplatin while the analogue complex cis-[PtCl(8-MTT)-
(PTA)2], containing PTA ligands instead of PPh3, displayed
only a residual activity.
In this paper we expand the series of Pt-phosphine-
thiopurine complexes with the aim of finding a favorable
combination of ligands enhancing both the antiproliferative
activity and solubility in water. Following the hypothesis that
complexes with one PPh3 and one PTA could represent the
right balance between lipophilicity and hydrophilicity, we
have prepared the 8-MTTH derivatives (SP-42)-[PtCl(8-
MTT)(PPh3)(PTA)] (2) and cis-[Pt(8-MTT)2(PPh3)(PTA)] (3)
from cis-[PtCl2(PPh3)(PTA)] (1).
The comparative study has also been extended to new
binuclear complexes bearing PPh3, PTA, and the bis-
(thiopurines): bis(S-8-thiotheophylline)methane (MBTTH2)
(Chart 1, b); 1,2-bis(S-8-thiotheophylline)ethane (EBTTH2)
(Chart 1, c); 1,3-bis(S-8-thiotheophylline)propane (PBTTH2)
(Chart 1, d). These ligands, containing two purines linked
by a dithiopolymethylenic chain of variable length, are able
to bind two platinum nuclei via the N7 imidazolic atom of
the two linked purines. Such bis(purines) bear two coordina-
tion sites at a variable distance depending on the length of
the spacing group. The study of the interaction of such
ligands with metals could provide new information about
the interaction between platinum and oligonucleotides. In
fact the coordination of a Pt to a site of the ligand could
favor the coordination of the second nucleus on the other
Ligands 8-(methylthio)theophylline (8-MTTH2), bis(S-8-thio-
theophylline)methane (MBTTH2), 1,2-bis(S-8-thiotheophylline)-
ethane (EBTTH2), and 1,3-bis(S-8-thiotheophylline)propane (PBTTH2)
have been prepared by following the procedure described by
literature methods.4,8 The ligand PTA was prepared as reported9 as
well as Pt complexes cis-[PtCl2(PPh3)2] and cis-[PtCl2(PTA)2].10
All the other chemicals and solvents were used as purchased
(reagent grade). Elemental analyses (C, H, N, S) were performed
using a Carlo Erba instrument model EA1110. FT-IR spectra were
recorded on a Nicolet 510P FT-IR instrument (4000-200 cm-1
using CsI. NMR spectra were recorded on a Bruker AM spectrom-
eter operating at 200 MHz (1H) and 81.15 MHz (31P) and a Bruker
DRX300 spectrometer operating at 300.13 MHz (1H), 75.47 MHz
(13C), and 121.49 MHz (31P). A Varian Gemini 300 spectrometer
(121.42 MHz) was also used for variable-temperature studies. Peak
positions relative to tetramethylsilane, were calibrated against the
residual solvent resonance (1H) or the deuterated solvent multiplet
(13C), and were measured relative to external 85% H3PO4 with
downfield values taken as positive (31P). All the NMR spectra were
acquired at 293 K, unless differently specified.
)
Synthesis of cis-[PtCl2(PTA)(PPh3)] (1). Solid PTA (0.09 g,
0.632 mmol) was added to a clear solution obtained by dissolving
cis-[PtCl2(PPh3)2] (0.5 g, 0.632 mmol) in 16 mL of CH2Cl2. The
reaction mixture was kept under stirring for 1 h, and then the solvent
was removed under vacuum. The remaining oil was treated with
diethyl ether giving a white suspension, which was filtered out and
dried over P2O5.
Crystals for X-ray determination of 1 were obtained by recrys-
tallization of the crude product from CHCl3 and Et2O.
Yield: 0.412 g, 95%. Anal. Found: C, 41.94; H, 4.26; N, 5.76.
Calcd for C24H27Cl2N3P2Pt: C, 42.06; H, 3.97; N, 6.13. IR (CsI,
cm-1): 3049 (CH arom), 2944 ν(CH PTA); 1610, 1500, 1436
(ν(CC) Ph), 1010, 975, 950 (PTA), 280 and 258 ν(PtCl). 1H NMR
(CDCl3), δ (ppm): 3.8 NCH2N, 6H; 4.5 NCH2P, 6H; 7-8 Ph, 15H.
31P{1H} NMR (CDCl3), δ (ppm): 15.1 (d, PPh3, 1JPtP ) 3750 Hz);
1
2
-63.3 (d, PTA, JPtP ) 3195 Hz), JPP ) 18 Hz.
Synthesis of (SP-42)-[PtCl(8-MTT)(PPh3)(PTA)] (2). A solu-
tion containing 0.04 g of 8-MTTH (0.20 nmol) in 2 mL of 1 N
NaOH was added to a suspension of cis-[PtCl2(PPh3)(PTA)] (0.13
g, 0.20 mmol) in CH2Cl2 (14 mL). The biphasic reaction mixture
was kept under stirring at room temperature for 1 h, and then the
two phases were separated. The water solution was extracted three
times with CH2Cl2, and the washings were added to the organic
(6) (a) Ang, W. H.; Daldini, E.; Scolaro, C.; Scopelliti, R.; Juillerat-
Jeannerat, L.; Dyson, P. J. Inorg. Chem. 2006, 45, 9006-9013. (b)
Dorcier, A.; Ang, W. H.; Bolano, S.; Gonsalvi, L.; Juillerat-Jeannerat,
L.; Laurenczy, G.; Peruzzini, M.; Phillips, A. D.; Zanobini, F.; Dyson,
P. J. Organometallics 2006, 25, 4090-4096. (c) Romerosa, A.;
Campos-Malpartida, T.; Lidrissi, C.; Saoud, M.; Serrano-Ruiz, M.;
Peruzzini, M.; Garrido-Cardenas, J. A.; Garcia-Maroto, F. Inorg. Chem.
2006, 45, 1289-1298. (d) Scolaro, C.; Geldbach, T. J.; Rochat, S.;
Dorcier, A.; Gossens, C.; Bergamo, A.; Cocchietto, M.; Tavernelli,
I.; Sava, G.; Rothlisberger, U.; Dyson, P. J. Organometallics 2006,
25, 756-765. (e) Scolaro, C.; Bergamo, A.; Brescacin, L.; Delfino,
R.; Cocchietto, M.; Laurenczy, G.; Geldbach, T. J.; Sava, G.; Dyson,
P. J. J. Med. Chem. 2005, 48, 4161-4171.
(8) (a) Romerosa, A.; Lo´pez-Magan˜a, C.; Saoud, M.; Colacio, E.; Sua´rez-
Varela, J. Inorg. Chim. Acta 2000, 307, 125-130. (b) Romerosa, A.;
Lo´pez-Magan˜a, C.; Saoud, M.; Man˜as, S. Eur. J. Inorg. Chem. 2003,
348-355.
(9) Daigle, D. J.; Decuir, T. J.; Robertson, J. B.; Darensbourg, D. J. Inorg.
Synth. 1998, 32, 40-43.
(10) (a) Bailar, J. C.; Itatani, H., Jr. Inorg. Chem. 1965, 11, 1618-1620.
(b) Darensbourg, D. J.; Decuir, T. J.; Stafford, N. W.; Robertson, J.
B.; Draper, J. D.; Reibenspies, J. H. Inorg. Chem. 1997, 36, 4218-
4226.
(7) Romerosa, A.; Bergamini, P.; Bertolasi, V.; Canella, A.; Cattabriga,
M.; Gavioli, R.; Man˜as, S.; Mantovani, N.; Pellicani, L. Inorg. Chem.
2004, 43, 905-913.
4268 Inorganic Chemistry, Vol. 46, No. 10, 2007

Phosphinic Platinum Complexes
1
phase. The solution was dried over Na2SO4, and then the solvent
was removed under vacuum. The yellow solid residue was dried
over P2O5.
Yield: 0.06 g, 54%. Anal. Found: C, 43.82; H, 3.75; N, 10.98;
S, 4.20. Calcd for C32H36ClN7O2P2PtS: C, 43.88; H, 4.11; N, 11.20;
S, 3.66. IR (CsI, cm-1) (selected data): 1650 and 1684 ν(CO),
1527 ν(CdC) + ν(CdN). 1H NMR (CD2Cl2), δ (ppm): 2.5 (SMe),
Hz, JPtP ) 3830 Hz). 31P{1H} NMR (DMSO-d6), δ (ppm): Pa +
Pa′ 8.05 and 8.2 (2 d, PPh3 trans to N7-purine, 2JPP ) 19 Hz, 1JPtP
) 3218 Hz); Pb + Pb′ 13.9 and 14.1 (2 d, PPh3 trans to Cl, 2JPP
)
19 Hz, 1JPtP ) 3827 Hz). 31P{1H} NMR in DMSO-d6 was checked
every 10 °C from 25 to 100 °C.
Synthesis of cis-[{PtCl(PPh3)2}2(µ-N,N-PBTT)] (6). The syn-
thesis of complex 6 was achieved by the same way as 4, using
PBTTH2 (0.029 g, 0.063 mmol) dissolved in 1.3 mL of 0.1 M
NaOH and cis-[PtCl2(PPh3)2] (0.1 g, 0.13 mmol) in 10 mL of
CH2Cl2.
3.2 (N1Me) and 3.3 ppm (N3Me), 3.7-4.4 (PTA) and 7-8 ppm
1
(PPh3). 31P{1H} NMR (CDCl3), δ (ppm): 11.06 (d, PPh3, JPtP
)
1
2
3817 Hz), -73.8 (d, PTA, JPtP ) 2942 Hz), JPP ) 21.4 Hz.
Synthesis of cis-[Pt(8-MTT)2(PTA)(PPh3)] (3). A solution
containing 0.129 g of 8-MTTH (0.57 mmol) in 0.1 N NaOH (5.7
mL) was added to a suspension of cis-[PtCl2(PPh3)(PTA)] (0.196
g, 0.29 mmol) in 20 mL of CH2Cl2. The biphasic reaction mixture
was treated as above-reported for 2.
Yield: 0.122 g, 99%. Anal. Found: C, 54.37; H, 4.03; N, 5.31;
S, 3.74. Calcd for C89H78Cl2N8O4P4Pt2S2: C, 54.14; H, 3.95; N,
5.68; S, 3.24. IR (CsI, cm-1) (selected data): 1688 + 1649 ν(CO);
1527 ν(CdC) + ν(CdN). 1H NMR (CDCl3, 300 MHz), δ (ppm):
2.24 (qt, SCH2CH2CH2S, 2H), 3.31, 3.32 (2s, N1-CH3, 6H), 3.40
(s, N3-CH3, 6H), 3.41 (SCH2CH2CH2S, 4H), 7.0-8.0 (m, Ph, 60H).
31P{1H} NMR (CDCl3, 121.42 MHz), δ (ppm): Pa + Pa’ 8.35 and
8.38 (d, PPh3 trans to N7-purine, JPP ) 19.2 Hz, JPtP ) 3240
Hz), Pb + Pb′ 14.01 and 14.03 (d, PPh3 trans to Cl, 2JPP ) 19.2 Hz,
1JPtP ) 3831 Hz).
Reactivity in 1:1:1 and 1:2:1 Molar Ratio of cis-[PtCl2-
(PPh3)2], NaOH, and Respectively MBTTH2, EBTTH2, and
PBTTH2. In a 1:1:1 ratio experiment, MBTTH2 (0.028 g, 0.063
mmol) was dissolved in 0.65 mL of aqueous 0.1 M NaOH (0.065
mmol) and the resulting solution added to cis-[PtCl2(PPh3)2] (0.05
g, 0.063 mmol) in 10 mL of CH2Cl2. The reaction mixture was
treated as above. From the organic phase 4 was recovered (0.060
g, 48% based on the ligand).
Similar experiments were carried out using EBTTH2 and
PBTTH2: EBTTH2 gave 5 (42%); PBTTH2 gave 6 (43%).
In the same way three experiments in 1:2:1 molar ratio were
performed and gave respectively 4 (48%), 5 (47%), and 6 (49%).
Synthesis of cis-[{Pt(PTA)2}2(µ-Cl)(µ-N,N-MBTT)]Cl (7). The
ligand MBTTH2 (0.050 g, 0.114 mmol) was suspended in 10 mL
of water, and 2.3 mL of aqueous NaOH (0.1 M) was added leading
to a colorless solution. Addition of solid cis-[PtCl2(PTA)2] (0.134
g, 0.23 mmol) gave a pale yellow suspension. After 18 h, a yellow
solid was obtained by evaporation of the solvent under reduced
pressure. The solid was extracted with CH2Cl2 (3 × 3 mL) and the
solution filtered and taken to dryness, leaving the product as a
yellow solid.
Yield: 0.192 g, 62%. Anal. Found: C, 44.7; H, 3.97; N, 13.98;
S, 6.80. Calcd for C40H45N11O4P2PtS2: C, 45.00; H, 4.23; N, 14.47;
S, 6.01. IR (CsI, cm-1) (selected data): 1653 and 1684 ν(CO),
2
1
1
1529 ν(CdC) + ν(CdN). H NMR (CD2Cl2), δ (ppm): 2.2 and
2.45 (2 SMe), 3.2 and 3.3 (2 N1Me), 3.45 and 3.5 (2 N3Me), 3.6-
4.3 (PTA), 7.3-8.5 (PPh3). 31P{1H} NMR (CDCl3), δ (ppm): 1.6
(d, PPh3, 1JPtP ) 3403 Hz), -76.0 (d, PTA, 1JPtP ) 3111 Hz), 2JPP
)24 Hz.
Reactivity of Complex 1 with 8-MTTH2. A biphasic reaction
similar to the previously described synthesis of 3 was studied by
extraction and 31P{1H} NMR checking of 0.3 mL aliquots every
10 min using a capillary containing C6D6. The fully consumption
of 1 was observed after ca. 15 min to give 2 which finally evolved
to 3 after 1 h.
Synthesis of cis-[{PtCl(PPh3)2}2(µ-N,N-MBTT)] (4). The ligand
MBTTH2 (0.028 g, 0.063 mmol) was dissolved in 1.3 mL of
aqueous NaOH (0.1 M) and then added dropwise to a solution of
cis-[PtCl2(PPh3)2] (0.1 g, 0.13 mmol) in 10 mL of CH2Cl2. The
reaction mixture was kept under stirring at room temperature for 4
h. In this time the organic phase became yellow, and it was then
separated and dried over Na2SO4. The solvent was then removed
under vacuum and the yellow solid residue dried over P2O5.
Yield: 0.116 g, 95%. Anal. Found: C, 53.48; H, 3.86; N, 5.50;
S, 3.55. Calcd for C87H74Cl2N8O4P4Pt2S2: C, 53.69; H, 3.80; N,
5.76; S, 3.30. IR (CsI, cm-1) (selected data): 1688 + 1650 ν(CO);
1
1526 ν(CdC) + ν(CdN). H NMR (CDCl3), δ (ppm): 3.1 (s,
N1CH3, 3H), 3.4 (s, N3CH3, 9H), 5.0 (m, SCH2S, 2H), 7.0-8.0
(m, Ph, 60H). 31P{1H} NMR (CDCl3), δ (ppm): Pa + Pa′ 7.7 and
8.0 (2 d, PPh3 trans to N7-purine, 2JPP ) 20 Hz, 1JPtP ) 3259 Hz);
Yield: 0.133 g, 77%. Anal. Found: C, 30.47; H, 4.35; N, 18.37;
S, 3.99. Calcd for C39H62Cl2N20O4P4Pt2S2: C, 30.73; H, 4.07; N,
18.38; S, 4.20. IR (CsI, cm-1) (selected data): 1685 + 1639 ν(CO);
1527 ν(CdC) + ν(CdN). 1H NMR (CDCl3, 300 MHz), δ (ppm):
3.32-3.55 (8 s, N1-CH3 + N3-CH3, 4 × 3 H); 3.88-4.5 (m,
Pb + Pb′ 13.5 and 13.9 (2d, PPh3 trans to Cl, 2JPP ) 20 Hz, 1JPtP
)
3811 Hz). 31P{1H} NMR (DMSO-d6), δ (ppm): Pa + Pa′ 8.01 and
2
1
PTA, 48 H); 4.21 (s; SCH2S, 2 H). 31P{1H} NMR (CDCl3, 121.49
8.17 (2 d, PPh3 trans to N7-purine, JPP ) 18 Hz, JPtP ) 3222
Hz); Pb + Pb′ 14.08 and 14.26 (2 d, PPh3 trans to Cl, 2JPP ) 18 Hz,
1JPtP ) 3808 Hz). 31P{1H} NMR in DMSO-d6 was checked every
10 °C from 25 to 170 °C.
1
MHz), δ (ppm): Pa -68.5 (m, PTA trans to N7(purine), JPtP
)
2950 Hz), Pb -57.7 (m, PTA trans to Cl, JPtP ) 3378 Hz). 31P-
{1H} NMR in CDCl3 was checked every 10 °C from 50 to -60
°C. 13C{1H} NMR (CDCl3, 75.47 MHz), δ (ppm): 27.99 (s, N1-
CH3); 30.01 (s, N3-CH3); 35.85 (SCH2); 52.3 (NCH2 PTA); 56.05
1
Synthesis of cis-[{PtCl(PPh3)2}2(µ-N,N-EBTT)] (5). The com-
plex 5 was prepared in the same way as 4, mixing a solution of
EBTTH2 (0.029 g, 0.065 mmol) in 1.3 mL of 0.1 M NaOH with a
solution of cis-[PtCl2(PPh3)2] (0.1 g, 0.13 mmol) in 10 mL of
CH2Cl2.
1
(PCH2 PTA, JPC ) 50.9 Hz); 114.60 (s, C5); 149.093 (s, C4);
150.52 (s, C8); 151.78 (s, C6); 155.85 (s, C2)
Anion Exchange of cis-[{Pt(PTA)2}2(µ-Cl)(µ-N,N-MBTT)]Cl
(7) with NaBPh4. Synthesis of cis-[{Pt(PTA)2}2(µ-Cl)(µ-N,N-
MBTT)](BPh4) (7BPh4). A solution of 7 (0.05 g, 0.033 mmol) in
2 mL of MeOH was added to a solution (1 mL) of NaBPh4 (0.012
g, 0.035 mmol) in the same solvent. As soon as the solutions were
mixed, a white solid began to precipitate. After 1 h the precipitate
was filtered out, washed with Et2O (2 × 1 mL), and air-dried. The
Yield: 0.117 g, 99%. Anal. Found: C, 54.11; H, 4.10; N, 5.67;
S, 3.19. Calcd for C88H76Cl2N8O4P4Pt2S2: C, 53.98; H, 3.88; N,
5.72; S, 3.27. IR (CsI, cm-1) (selected data): 1690 + 1654 ν(CO);
1527 ν(CdC) + ν(CdN). 1H NMR (CDCl3, 300 MHz), δ (ppm):
3.2, 3.4 (2s, N1CH3 + N3CH3, 12H), 3.6 (m, S2CH2CH2S, 4H),
7.0-8.0 (m, Ph, 60H). 31P{1H} NMR (CDCl3), δ (ppm): Pa + Pa′
2
1
-
8.0 and 8.1 (2 d, PPh3 trans to N7-purine, JPP ) 20 Hz, JPtP
)
31P NMR in CDCl3 allowed us to identify the product as the BPh4
salt of cis-[{Pt(PTA)2}2(µ-Cl)(µ-N,N-MBTT)]+.
3221 Hz); Pb + Pb′ 13.5 and 13.7 (2 d, PPh3 trans to Cl, 2JPP ) 20
Inorganic Chemistry, Vol. 46, No. 10, 2007 4269

Bergamini et al.
Yield: 97%. 31P{1H} NMR (CDCl3, 121.49 MHz), δ (ppm): Pa
Table 1. Crystallographic Data for 1
1
-67.8 (m, PTA trans to N7(purine), JPtP ) 2895 Hz), Pb -56.8
formula
Mr
C24H27Cl2N3P2Pt
685.42
1
(m, PTA trans to Cl, JPtP ) 3415 Hz).
space group
cryst system
a/Å
b/Å
c/Å
P1h
Synthesis of cis-[{Pt(PTA)2}2(µ-Cl)(µ-N,N-EBTT)]Cl (8). Com-
plex 8 was prepared by reaction of EDTTH2 (0.050 g, 0.110 mmol)
dissolved in 2.2 mL of 0.1 M NaOH with cis-[PtCl2(PTA)2] (0.129
g, 0.22 mmol) in 10 mL of water. After 18 h under stirring, the
solvent was eliminated by reduced pressure. The yellow solid
residue was extracted with CH2Cl2 (10 mL), and the obtained yellow
solution was then reduced to 1.0 mL. The addition of 5 mL of
Et2O induced the precipitation of a yellow solid, which was filtered
out, washed with Et2O (2 × 2 mL), and air-dried.
triclinic
9.8553(1)
10.5419(2)
13.0706(3)
84.685(1)
72.933(1)
70.871(1)
1226.46(4)
2
R/deg
â/deg
γ/deg
V/Å3
Z
Dc/g cm-3
F(000)
1.856
668
60.87
19 814
7031
0.068
6355
3.2-30.0
-13, 13; -13, 14; -17, 18
0.0330
0.0825
290
1.051
-1.70; 1.57
Yield: 0.06 g, 36%. Anal. Found: C, 31.17; H, 4.17; N, 18.24;
S, 4.01. Calcd for C40H64Cl2N20O4P4Pt2S2: C, 31.23; H, 4.16; N,
18.22; S, 4.16. IR (CsI, cm-1) (selected data): 1685 + 1644 ν(CO);
1526 ν(CdC) + ν(CdN). 1H NMR (CD2Cl2, 300.13 MHz), δ
(ppm): 1.92 (s, SCH2CH2S, 4 H), 3.36 (s, N1-CH3, 6 H); 3.48,
3.54 (2 s, N3-CH3, 6 H); 3.65-4.58 (m, PTA, 48 H). 31P{1H}
NMR (CD2Cl2, 121.49 MHz), δ (ppm): Pa + Pa′ -69.1 (m, PTA
µ(Mo KR)/cm-1
measd reflcns
unique reflcns
Rint
obsd reflcns [I g 2σ(I)]
θmin-θmax/deg
hkl ranges
R(F2) (obsd reflcns)
wR(F2) (all reflcns)
no. of variables
goodness of fit
1
trans to N7(purine), JPtP ) 2847 Hz), Pb + Pb′ -57.9 (m, PTA
trans to Cl, 1JPtP ) 3460). 13C{1H} NMR (CD2Cl2, 75.47 MHz), δ
(ppm): 27.99 (s, N1-CH3); 28.04 (s, N3-CH3); 34.22 (s, SCH2-
CH2); 35.86 (CH2CH2S); 52.3 (NCH2 PTA); 56.0 (PCH2 PTA, 1JPC
) 50.9 Hz); 114.65 (s, C5); 149.09 (s, C8); 150.53 (s, C6); 151.76
(s, C4); 155.84 (s, C2)
F
min, Fmax/e Å-3
cis-[PtCl2(PPh3)(PTA)] (0.16 g, 0.228 mmol) in 10 mL of CH2Cl2.
After 18 h, the 31P{1H} NMR inspection of aliquots of the organic
phase of each reaction showed the formation of a mixture of
isomeric platinum complexes.
31P{1H} NMR (CDCl3, 121.49 MHz), δ (ppm) data for detectable
species:
(i) Reaction with MBTTH2: 10a, [{Pt(PPh3)(PTA)Cl}2(µ-N,N-
MBTT)] (66%) Pa + Pa′ -73.6 and -73.9 (2d, PTA trans N7-
(purine), 1JPtP ) 2975 Hz), Pb + Pb′ 10.7 and 10.9 (2d, PPh3 trans
Reactivity of EBTTH2 with NaOH and cis-[PtCl2(PTA)2].
Observation of [{PtCl(PTA)2}2(µ-N,N-EBTT)] (8a). The reaction
of EBTTH2 (0.050 g, 0.110 mmol) with 2.2 mL of 0.1 M NaOH
and cis-[PtCl2(PTA)2] (0.129 g, 0.22 mmol) in 10 mL of water
produced a yellow solution. After overnight stirring it was taken
to dryness leaving a yellow residue. The NMR analysis of the solid
dissolved in CD2Cl2 showed the presence of two products: 8 (80%)
and [{PtCl(PTA)2}2(µ-N,N-EBTT)] (8a) (20%). 31P{1H} NMR
(CD2Cl2, 121.49 MHz), δ (ppm): 8, see above for data; cis-[{PtCl-
(PTA)2}2(µ-N,N-EBTT)], 8a, Pa + Pa′ -71.6 (bt, PTA trans to N7-
1
2
Cl, JPtP ) 3790 Hz), JPP ) 20 Hz; 10b, [{Pt(PPh3)(PTA)Cl}-
[{Pt(PTA)(PPh3)Cl}(µ-N,N-MBTT)] (ca. 20%) Pa -64.7 (2d, PTA
1
trans to Cl, JPtP ) 3424 Hz) and Pa′ -76.0 (2d, PTA trans to N,
1
1
(purine), JPtP ) 2940 Hz), Pb + Pb’ -57.8 (bt, PTA trans to Cl,
1JPtP ) 2900 Hz), Pb 7.0 (2d, PPh3 trans to N, JPtP ) 3314 Hz),
1JPtP ) 3329 Hz). 31P{1H} NMR in CD2Cl2 was checked every 10
°C from 50 to -60 °C.
2JPP ) 20 Hz.
(ii) Reaction with EBTTH2: 11a, [{Pt(PPh3)(PTA)Cl}2(µ-N,N-
EBTT)] (64%) Pa + Pa′ -73.5 and -74.0 (2d, PTA trans to N7-
(purine), 1JPtP ca. 2950 Hz), Pb + Pb′ 10.7 and 10.9 (2d, PPh3 trans
Synthesis of cis-[{Pt(PTA)2}2(µ-Cl)(µ-N,N-PBTT)]Cl (9). Com-
plex 9 was prepared as above-described for 7, using PDTTH2 (0.040
g, 0.086 mmol) dissolved in 1.72 mL of aqueous NaOH (0.1 M)
and cis-[PtCl2(PTA)2] (0.100 g, 0.172 mmol) in 10 mL of water.
Yield: 0.165 g, 0.106 mmol, 62%. Anal. Found: C, 31.80; H,
4.33; N, 18.21; S, 4.52. Calcd for C41H66Cl2N20O4P4Pt2S2: C, 31.75;
H, 4.25; N, 18.05; S, 4.13. IR (CsI, cm-1) (selected data): 1685 +
1
2
to Cl, JPtP ) ca. 3810 Hz), JPP ) 22 Hz; 11b, [{Pt(PPh3)(PTA)-
Cl}[{Pt(PTA)(PPh3)Cl}(µ-N,N-EBTT)] (16%) Pa -64.7 (2d, PTA
1
trans to Cl, JPtP ) 3408 Hz) and Pa′ -76.7 (2d, PTA trans to N,
1JPtP ) 2933 Hz), Pb 7.0 (2d, PPh3 trans to N, JPtP ) 3244 Hz),
1
2JPP ) 20 Hz.
1
(iii) Reaction with PBTTH2: 12a, [{Pt(PPh3)(PTA)Cl}2(µ-N,N-
PBTT)] (ca. 70%) Pa + Pa′ -73.7 (m, PTA trans to N7, JPtP
2961 Hz) -76.7 (m, PTA trans to N7, JPtP ) 2924 Hz), Pb + Pb′
10.8 (m, PPh3 trans to Cl, JPtP ) 3798 Hz), together with other
1644 ν(CO); 1526 ν(CdC) + ν(CdN). H NMR (CDCl3, 300.13
1
)
MHz), δ (ppm): 2.034 (bs, SCH2CH2, 2H); 3.39 (bm, SCH2CH2,
4H), 3.32 (s, N1-CH3, 6 H); 3.54 (s, N3-CH3, 6 H); 3.87-4.56
(m, PTA, 48 H). 31P{1H} NMR (CDCl3, 121.49 MHz), δ (ppm):
Pa + Pa’ -68.72 (1JPtP ) 3016 Hz) and -71.4 (1JPtP ) 2990 Hz)
(2m, PTA trans to N7(purine), Pb + Pb′ -57.5 (1JPtP ) 3387 Hz)
and -58.1 (1JPtP ) 3396 Hz) (2m, PTA trans to Cl). 31P{1H} NMR
in CDCl3 was checked every 10 °C from 50 to -60 °C. 13C{1H}
NMR (CD2Cl2, 75.47 MHz), δ (ppm): 28.06 (bs, N1-CH3); 30.10
(bs, N3-CH3); 32.00 (bs, SCH2CH2-); 32.20 (bs, SCH2CH2-);
52.70 (m, NCH2 PTA); 56.0 (d, PCH2 PTA, 1JPC ) 50.6 Hz); 112.20
(bs, C5); 150.78 (bs, C4); 151.86 (bs, C8); 152.03 (bs, C6); 155.90
(bs, C2).
Reactivity of cis-[PtCl2(PPh3)(PTA)] with NaOH and Re-
spectively MBTTH2, EBTTH2, and PBTTH2. In three parallel
experiments, MBTTH2 (0.050 g, 0.114 mmol), EBTTH2 (0.050 g,
0.110 mmol), and PBTTH2 (0.050 g, 0.110 mmol) dissolved in 2.3
mL of 0.1 M NaOH (0.23 mmol) were mixed with a solution of
1
1
unidentified species.
X-ray Crystal Structure Determination. X-ray diffraction data
for compound 1 (Table 1) were collected on a Nonius Kappa CCD
diffractometer, at room temperature (T ) 295 K), with graphite-
monochromated Mo KR radiation (λ ) 0.7107 Å) and corrected
for Lorentz, polarization, and absorption (SORTAV)11 effects. The
structure was solved by direct methods (SIR97)12 and refined
(SHELXL-97)13 by full-matrix least squares with anisotropic non-H
and hydrogen atoms on calculated positions, riding on their carrier
atoms.
(11) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33.
(12) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
4270 Inorganic Chemistry, Vol. 46, No. 10, 2007

Phosphinic Platinum Complexes
Chart 2
Growth Inhibition Assays. Cell growth inhibition assays were
carried out using the cisplatin-sensitive T2 human cell line and the
cisplatin-resistant SKOV3 cell line. T2 is a cell hybrid obtained
by the fusion of the human lynphoblastoid line 174 (B lynphocyte
transformed by the Epstein-Barr virus) with the CEM human
cancer line (leukaemia T) while SKOV3 is derived from a human
ovarian tumor. The cells were seeded in triplicate in 96-well trays
at a density of 50 × 103 in 50 µL of AIM-V medium for T2 and
25 × 103 in 50 µL of AIM-V medium for SKOV3. Stock solutions
(10 mM) of the Pt(II) complexes were made in DMSO and diluted
in AIM-V medium to give final concentration of 2, 10, and 50
µM. Cisplatin was employed as a control for the cisplatin-sensitive
T2 cell line and for the cisplatin-resistant SKOV3. Untreated cells
were placed in every plate as a negative control. The cells were
exposed to the compounds for 72 h, and then 25 µL of a (4,5-
dimethylthiozol-2-yl)-2,5-diphenyltetrazolium bromide solution (12
mM) was added. After 2 h of incubation, 100 µL of lysing buffer
(50% DMF + 20% SDS, pH 4.7) was added to convert (4,5-
dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide into a brown
formazane. After an additional 18 h the solution absorbance,
proportional to the number of live cells, was measured by
spectrophotometer and converted in % of growth inhibition.14
Trying to find a convenient compromise between the
lipophilicity which seems to be associated with a higher
activity and the hydrophilicity generally welcomed for a
convenient drug formulation and administration,15 we rea-
soned that the mixed analogues with one PPh3 and one PTA
could represent the right balance between the two types and
therefore we report here the preparation of (SP-42)-[PtCl-
(8-MTT)(PPh3)(PTA)] (2) and cis-[Pt(8-MTT)2(PPh3)(PTA)]
(3) from their precursor, the new complex cis-[PtCl2(PPh3)-
(PTA)] (1) (Chart 2).
We found that the most convenient way to obtain the new
precursor 1 is the treatment of cis-[PtCl2(PPh3)2] with 1 equiv
of PTA in dichloromethane. The more nucleophilic PTA
replaced one PPh3 molecule giving 1 that was isolated by
quantitative precipitation with ether and was characterized
spectroscopically and through the determination of its X-ray
crystal structure. The 31P{1H} NMR spectrum of 1 in CDCl3
shows two coupled doublets (2JPP ) 18 Hz), the first due to
coordinated PPh3 at 15.1 ppm (1JPtP ) 3750 Hz) and the
second to PTA at -63.3 (1JPtP ) 3195 Hz). It is interesting
to point out that the 1JPtP for coordinated PTA is ca. 500 Hz
smaller than that for PPh3 although both phosphines are trans
to chloride. This significant difference could be the conse-
quence of the smaller size of PTA cone angle (102°) with
respect to PPh3 (145°).16 The 1H NMR spectrum of 1 displays
few broad signals for PTA aliphatic protons at 3.8 ppm
(NCH2N) and 4.5 ppm (NCH2P) and a 5:4 relative integrals
ratio for aromatic versus aliphatic protons. The IR shows
Results and Discussion
Complexes Containing 8-MTTH. We have recently
reported7 the lipophilic thiopurinic complexes cis-[PtCl(8-
MTT)(PPh3)2] (I) and cis-[Pt(8-MTT)2(PPh3)2] (II), which
display antiproliferative activity either on cisplatin-sensitive
T2 or on cisplatin-resistant SKOV3 cell lines, while the more
hydrophilic analogues cis-[PtCl(8-MTT)(PTA)2] (III) and
cis-[Pt(8-MTT)2(PTA)2] (IV) are less active on the same cell
lines.
(15) Henderson, W.; McCaffrey, L. J.; Nicholson, B. K. J. Chem. Soc.,
Dalton Trans. 2000, 2753-2760.
(16) Assefa, Z.; McBurnett, B. G.; Staples, R. J.; Fackler, J. P.; Assmann,
B., Jr.; Angermaier, K.; Schmidbaur, H. Inorg. Chem. 1995, 34, 75-
83.
(13) Sheldrick, G. M. SHELXL-97, Program for Crystal Structures Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(14) Hansen, M. B.; Nielsen, S. E.; Berg, K. J. Immunol. Methods 1989,
119, 203.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4271

Bergamini et al.
Chart 3
two characteristic PtCl stretching signals at 280 and 258
cm-1, which are consistent with a cis geometry.17
The thiopurinic complexes 2 and 3 were prepared from 1
by reaction respectively with 1 and 2 equiv of 8-MTTH and
NaOH in a double phase mixture of dichloromethane and
water (Chart 2). In their 1H NMR spectrum, the signals due
to coordinated 8-MTT- are present: for 2 three singlets at
2.5 (SMe), 3.2 (N1Me), and 3.3 (N3Me) ppm are observed,
while for 3 two close sets of signals are visible belonging to
the two inequivalent thiopurines (trans to PPh3 and trans to
PTA): there are two singlets at 2.2 and 2.45 ppm (SMe),
two singlets at 3.2 and 3.3 ppm (N1Me), and two singlets at
3.45 and 3.5 ppm (N3Me).
The synthesis of 3 by reaction of 1 with 2 equiv of
8-MTTH was followed by 31P{1H} NMR. A two step process
was observed: the first one, complete in about 15 min, led
to 2 through the substitution of the chloride trans to PTA by
MTT-, as indicated by the 10 ppm upfield shift of the PTA
signal (from -63.3 ppm in 1 to -73.8 ppm in 2). The second
step provided 3 in a further 60 min by substitution of the
residual chloride by the remaining equiv of deprotonated
ligand MTT-.
Figure 1. 31P{1H} NMR of 5 in DMSO-d6 at 25 °C (lower trace) and 80
°C (upper trace).
room temperature. Variable-temperature NMR experiments
showed that the inequivalence survives in solution up to 110
°C in DMSO-d6 due to a high conformational barrier. Finally,
the X-ray crystal structure for MBTTH2 gave evidence for
a stacking interaction between the two thiotheophyllines. To
confirm the hypothesis of a rotational origin also for the
inequivalence of the two sides observed for 4, a variable-
temperature 31P{1H} NMR experiment was carried out for a
solution of 4 in DMSO-d6 by running the spectrum every
10 °C from 25 up to 170 °C. The complete coalescence of
the two sets of signals was not obtained, but it was only
partial at 170 °C.
The identity of 4 is supported also by the IR spectrum
that shows absorptions for the thiopurine and PPh3 but no
bands for ν(NH). The 1H NMR displays signals for MBTTH2
and phosphines with an integral ratio in agreement with the
proposed molecular composition. The SCH2S group signal
arises at 5.00 ppm, very close to the corresponding signal
of the free ligand (5.12 ppm).4
Reactivity of Bis(thiopurines) MBTTH2, EBTTH2, and
PBTTH2 with Pt Phosphine Complexes. (a) Reactions
with cis-[PtCl2(PPh3)2]: Synthesis of 4-6. The reactions
of MBTTH2, EBTTH2, or PBTTH2 in CH2Cl2/H2O with 2
equiv of NaOH and 2 equiv of cis-[PtCl2(PPh3)2] gave the
binuclear complexes 4-6, respectively (Chart 3).
The longer central chain in EBTTH2 and PBTTH2 allows
a major conformational freedom, and therefore, for the free
ligands the inequivalence of the two extremities of the bis-
1
(purine) molecule was not observed by H NMR at room
temperature.4 On the contrary, both complexes 5 and 6 show
in 31P{1H} NMR a double pattern of signals resembling the
one just mentioned for 4, as a consequence of the loss of
conformational freedom upon the coordination of two
hindered [PtCl(PPh3)2]+ units. The two sets of signals in the
31P{1H} NMR (CDCl3) of 5 are closer in chemical shift than
the corresponding ones in 4 (the differences (∆(δ)) in ppm
between the same signal in the two sets are the following:
for 5, ∆(δ)PPh3-Pt-Cl ) 0.1 and ∆(δ)PPh3-Pt-N7 ) 0.2;
for 4, ∆(δ)PPh3-Pt-Cl ) 0.3 and ∆(δ)PPh3-Pt-N7 ) 0.4).
For 5 the coalescence of the two sets was observed in
DMSO-d6 at about 80 °C (Figure 1).
The 31P{1H} NMR for complex 4 displays two partially
overlapped sets of signals, each made up by two doublets
2
with satellites at 7.7, 8.0 (PPh3 trans to N7-purine, JPP
)
1
20.0, JPtP ) 3259 Hz) and 13.5, 13.9 ppm (PPh3 trans to
2
1
Cl, JPP ) 20.0 Hz, JPtP ) 3811 Hz).
The assignment of signals to P trans to Cl (downfield shift,
larger 1JPtP coupling constant) and P trans to N (upfield shift,
smaller 1JPtP coupling constant) is based on the comparison
with the data of the previously reported mononuclear
complexes cis-[PtCl2(PPh3)2], cis-[PtCl(8-MTT)(PPh3)2], and
cis-[Pt(8-MTT)2(PPh3)2].7 The two sets of signals show very
close chemical shifts and undistinguishable values for
coupling constants. We believe this feature is due to the
inequivalence of the corresponding phosphorus nuclei bonded
to different platinum centers (Pa * Pa′, Pb * Pb′).
Similarly, complex 6 shows a single 31P{1H} NMR pattern
at 81.15 MHz, but at 121.42 MHz the signals show some
splitting indicating the presence of a second almost coincident
pattern (6: ∆(δ)PPh3-Pt-Cl ) 0.02 and ∆(δ)PPh3-Pt-
N7 ) 0.03).
It is known4 that, in the ligand MBTTH2 itself, the two
thiotheophyllinic moieties are magnetically inequivalent at
We have performed a series of experiments trying to obtain
mononuclear complexes to be subsequently used as starting
materials for heterobimetallic complexes. The choice of
(17) (a) Adams, D. M.; Chatt, J.; Gerrat, J.; Westland, A. D. J. Chem. Soc.
1964, 734-739. (b) Gillard, R. D.; Pilbrow, M. F. J. Chem. Soc.,
Dalton Trans. 1974, 2320-2325.
4272 Inorganic Chemistry, Vol. 46, No. 10, 2007

Phosphinic Platinum Complexes
Chart 4
comparison with chemical shifts in 2 and 3 (PTA trans to
chloride downfield with respect to PTA trans to N7).
Variable-temperature NMR experiments from 40 to -60 °C
showed further splitting of the signals indicating the existence
of an electronic relation between the two {(PTA)2Pt}
moieties. This hypothesis was confirmed by a 31P-31P{1H}
COSY experiment which showed a clear coupling (4JPa/Pa′
) ca. 8 Hz) between phosphorus nuclei bonded to different
Pt centers.
The cationic structure cis-[{Pt(PTA)2}2(µ-Cl)(µ-N,N-
MBTT)]+ (7 in Chart 4), in which two platinum moieties
are bridged by a chloride that electronically connects the two
metal nuclei, is consistent with the above-reported NMR
observations, allowing coupling through a four-bonds dis-
tance. The cationic nature of the complex was confirmed
also by mixing a solution of the chloride of 7 with NaBPh4
in MeOH, producing the precipitation of the insoluble
MBTTH2 is particularly convenient because the mono- and
bimetallic species should be easily distinguishable by 31P-
{1H} NMR. In fact, while the above-described binuclear
complex 4 shows two close but clearly separated sets of
signals, the expected mononuclear complex should be
characterized by a single pattern. When we tried to obtain
the mononuclear 8-membered chelate, the stoichiometric ratio
of the reaction among MBTTH2, cis-[PtCl2(PPh3)2], and
NaOH was at first set at 1:1:2, but also under these
conditions, the above-described binuclear complex 4 was
obtained as the only Pt-containing product in less than 50%
yield. Analogue experiments with EBTTH2 and PBTTH2
gave the respective products in less than 50% yields.
complex
cis-[{Pt(PTA)2}2(µ-Cl)(µ-N,N-MBTT)](BPh4)
(7BPh4).
For compound 8, a 31P{1H} NMR pattern similar to that
of 7 was observed and therefore a similar structure was
proposed (Chart 4). Nevertheless the NMR observation of
the crude reaction mixture of EBTTH2 with NaOH and cis-
[PtCl2(PTA)2] showed the formation of an additional species
(8a) together with 8. The NMR features of 8a are similar to
those found for above-described bis-{(PPh3)2Pt} complexes
(4-6) indicating a noncyclic neutral structure with non-
equivalent extremities: the 31P{1H} NMR of 8a is constituted
by two pseudotriplets in the chemical shift range character-
istic for coordinated PTA trans to Cl (ca. -57.8, 1JPtP ) 3329
Hz) and trans to N7 (ca. -71.6, 1JPtP ) 2940 Hz). The signals
become two doublets of doublets at -40 °C. Considering
that 8 and 8a are formed in a constant ratio, which does not
change with the temperature, and the solution of pure 8 does
not show any sign of formation of 8a in 10 days, we can
conclude that 8 and 8a are not in equilibrium but originate
from different formation pathways.
A further confirmation of the tendency of these ligands
to give binuclear complexes was provided by the reaction
of L/cis-[PtCl2(PPh3)2]/NaOH in 1:1:1 ratio (L ) MBTTH2,
EBTTH2, or PBTTH2), which gave for each ligand 0.5 equiv
of the respective binuclear platinum complex. Therefore, it
is possible to conclude that the two imidazolic N7 atoms of
the two sides in MBTTH2, EBTTH2, and PBTTH2 do not
have independent reactivity with cis-[PtCl2(PPh3)2], but the
coordination of the first Pt seems to favor the entrance of
the second, the two steps occurring nearly simultaneously.
As a consequence, it was not possible to obtain monoplati-
num or heterobimetallic complexes in this way. It is worth
noticing that we have previously reported4 that the reaction
of MBTTH2 with cis-[PdCl2(PPh3)2] and NaOH under similar
reaction conditions produced exclusively the dipalladium
complex trans-[{PdCl(PPh3)2}2(µ-N,N-MBTT)]. This sort
of “cooperative effect” between two metal nuclei may
be involved in multiple-platinum coordination by oligo-
nucleotides or nucleic acids interacting with platinum-based
drugs.
Similarly, complex 9 was isolated from the reaction of
PBTTH2 with NaOH and cis-[PtCl2(PTA)2]. It displays a 31P-
{1H} NMR constituted by two multiplets of PTA trans to
N7 at -68.7 (1JPtP ) 3016) and -71.4 (1JPtP ) 2990) (Pa +
Pa′) and two multiplets of PTA trans to Cl at -57.5 (1JPtP
)
3387 Hz) and -58.1 (1JPtP ) 3396 Hz) (Pb + Pb′). This
feature is due to two partially overlapped sets of signals with
satellites. Variable-temperature NMR experiments in CDCl3
did not show signals of coalescence over a temperature range
from +50 to -60 °C. A 2D-31P{1H} COSY experiment in
CDCl3 showed also in this case cross-peaks between the
signals at -68.72 and -71.4 and between the signals at
-57.5 and -58.1 ppm, showing a through-bond electronic
connexion between the two coordination spheres. This result
allows us to confirm the cationic chloride-bridged structure
also for 9.
(b) Reactions with cis-[PtCl2(PTA)2]: Synthesis of 7-9.
Reactions of MBTTH2, EBTTH2, and PBTTH2 in H2O with
2 equiv of NaOH and cis-[PtCl2(PTA)2] allowed us to prepare
the cationic platinum/PTA complexes 7-9, in which two
metal nuclei are bridged by a chloride atom and a bis-
(thiopurinic) dianion through its imidazolic N7 atoms (Chart
4).
Compound 7, obtained from MBTTH2, displays a 31P{1H}
NMR characterized by two groups of very close signals, one
at -57.7 ppm (1JPtP ) 3378 Hz), assigned to PTA trans to
chloride, and the second one at -68.5 ppm (1JPtP ) 2950
Hz), attributed to PTA trans to N7, on the basis of a
(c) Reactions with cis-[PtCl2(PPh3)PTA]. In principle,
the geometrical isomers obtainable from the reaction of bis-
(thiopurine) with cis-[PtCl2(PPh3)PTA] (1) are three for the
open form (a-c) and three for the cationic cyclic form (d-
f) (a and d, both Ph3P trans to chloride; b and e, one PPh3
Inorganic Chemistry, Vol. 46, No. 10, 2007 4273

Bergamini et al.
Chart 5
trans to chloride and the other one trans to nitrogen; c and
f, both PPh3 trans to N) (Chart 5).
of doublets with satellites. Two of these signals (same
intensity) are in the range of coordinated PTA (-76 and
-64.7 ppm) with 1JPtP indicating that the first one is trans to
nitrogen (2900 Hz) while the second one is trans to chloride
(3424 Hz). The third signal is located in the PPh3 zone (7.0
The reaction of MBTT2- with 2 equiv of cis-[PtCl2(PPh3)-
PTA] (1) gave rise to a mixture of isomeric forms, and we
did not succeed in separating them. The major species
amounted to ca. 66% of the mixture, on the basis of the
integrated signals in 31P{1H} NMR. Although it was not
possible to separate and purify this complex, its structure
could be assigned from its NMR features. A characteristic
two partially overlapped doublets at 10.7 and 10.9 ppm was
observed in its 31P{1H} NMR, both due to Pt coordinated
PPh3 with 1JPtP of ca. 3790 Hz, indicating that this phosphine
is trans to chloride, and two other very close doublets at
-73.6 and -73.9 ppm (1JPtP of ca. 2975 Hz), for the
coordinated PTA trans to nitrogen. It is reasonable to suppose
that all the signals belong to the open form 10a (Chart 6),
and also in this case the double pattern is due to the two
inequivalent Pt coordination spheres at the two sides of the
complex.
1
ppm, JPtP ) 3314 Hz, trans to nitrogen). A further signal
(PPh3 trans to Cl) could be coincident with the corresponding
of the main species. These data support the hypothesis that
the minor species is the open isomer 10b (Chart 7).
Chart 7
Chart 6
The same distribution of products can be observed for the
mixture obtained from the reaction of 1 with EBTT2-,
containing 11a as the main species (yield 64% based on its
31P{1H} NMR: Pa + Pa′ -73.5 and -74.0, 2d, PTA trans
1
to N7(purine), JPtP ) ca. 2950 Hz; Pb + Pb′ 10.7 and 10.9,
2d, PPh3 trans to Cl, 1JPtP ) ca. 3810 Hz) and 11b as minor
species (Pa -64.7, 2d, PTA trans to Cl, 1JPtP ) 3408 Hz; Pa′
1
-76.7, 2d, PTA trans to N, JPtP ) 2933 Hz; Pb 7.0, 2d,
1
PPh3 trans to N, JPtP ) 3244, Pb′ trans to Cl could be
coincident with the corresponding of 11a).
Finally the reaction of PBTT2- with 2 equiv of cis-[PtCl2-
(PPh3)PTA] (1) gave a main pattern that tentatively we
A small amount of a minor species (ca. 20% by 13P NMR)
is also observed in the spectrum, giving rise to three doublets
4274 Inorganic Chemistry, Vol. 46, No. 10, 2007

Phosphinic Platinum Complexes
Table 3. Estimated IC50 (µM) on Cisplatin-Sensitive T2 Cell Line and
on Cisplatin-Resistant SKOV3
complex
T2
SKOV 3
1, cis-[PtCl2(PTA)(PPh3)]
2, cis-[PtCl(8-MTT)(PPh3)(PTA)]
3, cis-[Pt(8-MTT)2(PTA)(PPh3)]
I, cis-[PtCl(8-MTT)(PPh3)2]
II, cis-[Pt(8-MTT)2(PPh3)2]
III, cis-[PtCl(8-MTT)(PTA)2]
IV, cis-[Pt(8-MTT)2(PTA)2]
cis-[PtCl2(PTA)2]
ca. 10
2-10
2-10
ca. 2
10-50
>50
10-50
10-50
10
ca. 50
>50
>50
>50
>50
>50
>50
>50
>50
>50
cis-[PtCl2(PPh3)2]
Na(8-MTT)
cis-[PtCl2(NH3)2]
ca. 50
>50
<2
H2A‚‚‚Cl1 distances of 2.84 and 2.94 Å and C1-
H1B‚‚‚Cl2 and C2-H2A‚‚‚Cl1 angles of 159 and 140°,
respectively.
Cell Growth Inhibition of Complexes 1-3. In Table 3
are reported the estimated concentrations reducing to 50%
the cell growth (IC50) of T2 (cisplatin-sensitive) and SKOV3
(cisplatin-resistant) cell lines for the complexes 1-3, in
comparison with the previously reported data7 for cis-[PtCl-
(8-MTT)(PPh3)2] (I), cis-[Pt(8-MTT)2(PPh3)2] (II), cis-[PtCl-
(8-MTT)(PTA)2] (III), cis-[Pt(8-MTT)2(PTA)2] (IV), and
their dichloride precursors. The data obtained for cisplatin
in the same set of experiments are also given.
The comparison points out that on the T2 cell line the
compounds 2 and 3 are about as active as the triphenylphos-
phine analogues I and II and more active than PTA
complexes III and IV, while on SKOV3 (cisplatin-resistant)
only the mixed phosphine complexes 1-3 show appreciable
activity.
Figure 2. Crystal structure of 1.
Table 2. Selected Bond Distances (Å) and Angles (deg)
Pt1-Cl1
Pt1-Cl2
2.337(1)
2.369(1)
Pt1-P1
Pt1-P2
2.250(1)
2.255(1)
Cl1-Pt1-Cl2
Cl1-Pt1-P1
Cl1-Pt-P2
86.76(4)
175.09(4)
90.75(4)
Cl2-Pt1-P1
Cl2-Pt1-P2
P1-Pt1-P2
88.33(4)
175.76(4)
94.16(4)
attribute to isomer 12a (estimated ca. 90% on its 31P{1H}
NMR), characterized by two multiplets one for two PTA
both trans to N7(purine) at -73.7 (1JPtP ) 2961 Hz) and
-76.7 (1JPtP ) 2924 Hz) and the other multiplet (or two
overlapped multiplets) at 10.8 ppm (1JPtP ) 3798 Hz) due
to PPh3 trans to Cl. Also in this case a group of minor signals
is present due to PTA trans to Cl and an unresolved multiplet
due to PPh3 probably trans to nitrogen.
Crystal Structure of 1. An ORTEP18 drawing of 1 is
shown in Figure 2. Selected bond distances and angles are
given in Table 2. The coordination geometry is distorted
square planar with angles ranging from 86.76(4) to 94.16-
(4)° around the Pt1 atom. The Cl-Pt-P angle observed for
PTA (Cl2-Pt1-P1 ) 88.33(4)°) and PPh3 (Cl1-Pt-P2 )
90.75(4)°) is in agreement with their respective ligand cone
angles (PTA ) 102°; PPh3 ) 145°);16 the biggest volume
of the PPh3 leads to a reduced Cl-Pt-PTA angle. The largest
deviations from the least-squares plane through Pt1, Cl1, Cl2,
P1, and P2 are 0.076(1) and 0.060(1) Å for Cl2 and P2,
respectively.
Both Pt-Cl distances of 2.337(1) and 2.369(1) Å, with
Cl atoms in trans positions to tertiary phosphines, are
significantly longer than the average Pt-Cl distance of 2.29-
(1) Å calculated for square-planar Pt complexes having Cl
atoms in mutual trans positions.19 These data reflect the
strong trans influence exerted by phosphine groups.20
In the crystal packing the molecules of complex are linked
by a centrosymmetric dimers by means of C1-H1B‚‚‚Cl2
and C2-H2A‚‚‚Cl1 short interactions with H1B‚‚‚Cl2 and
It is reasonable to suppose that the {Pt(PTA)(PPh3)} group
could confer to 1-3 the ability either to dissolve in water-
based biological fluids and to cross the lipophilic membranes
of the cell and of the nucleus, reaching their target, the
nucleic acids.
The activities of the dichloride precursors 1, cis-[PtCl2-
(PPh3)2], and cis-[PtCl2(PTA)2] and of the salt Na(8-MTT)
are reported for comparison in Table 3. The activity of the
dichlorides on both cell lines is lower than that found for
the corresponding thiopurinic derivatives but shows the same
trend: 1 is the most active dichloride, followed by the PPh3
complex, and then by the nearly inactive cis-[PtCl2(PTA)2].
The free ligand salt Na(8-MTT) does not show appreciable
activity on its own, but the presence of 8-MTT- as ligand
in the platinum complexes in most cases improves their
antiproliferative activity as it is evident from the comparison
of the Pt dichloride precursors with the corresponding Pt-
MTT complexes.
Cancer Cell Growth Inhibition of Binuclear Com-
plexes. The results of the cell growth inhibition obtained
with the binuclear complexes 4-9 (together with their
dichloride precursors and the ligands salts) for cisplatin-
(19) Palenik, G. J.; Giordano, T. J. J. Chem. Soc., Dalton Trans. 1987,
1175-1179.
(20) (a) Oberhauser, W.; Bachman, C.; Stampfl, T.; Haid, R.; Langes C.;
Rieder, A.; Bru¨ggeller, P. Inorg. Chim. Acta 1998, 274, 143-154.
(b) Johansson, M. H.; Malmstro¨m, T.; Wendt, O. F. Inorg. Chim. Acta
2001, 316, 149-152. (c) Song, H. B.; Zhang, Z. Z.; Mak, T. C. W.
Polyhedron 2002, 21, 1043-1050.
(18) Burnett, M. N.; Johnson, C. K. ORTEPIII; Report ORNL-6895; Oak
Ridge National Laboratory: Oak Ridge, TN, 1996.
Inorganic Chemistry, Vol. 46, No. 10, 2007 4275

Bergamini et al.
Table 4. Estimated IC50 (µM) on Cisplatin-Sensitive T2 Cell Line and
on Cisplatin-Resistant SKOV3 for Binuclear Complexes
Conclusion
The new complex cis-[PtCl2(PPh3)(PTA)] (1), containing
two different phosphines, one of them being water soluble
(PTA), has been prepared and structurally characterized. It
has been shown that its derivatives with 8-MTT- (2, 3) in
most cases display an antiproliferative activity on model
tumoral cell lines (T2 and SKOV3) better than their
analogues (I-IV) containing a single phosphine type. This
improvement is likely to be related with a more favorable
balance between lipophilicity and hydrophilicity due to the
presence of two different phosphines bonded to platinum.
The reactions of cis-[PtCl2(PPh3)2], cis-[PtCl2(PTA)2], and
cis-[PtCl2(PPh3)(PTA)] with bis(S-8-thiotheophylline)methane
(MBTTH2), 1,2-bis(S-8-thiotheophylline)ethane (EBTTH2)
and 1,3-bis(S-8-thiotheophylline)propane (PBTTH2) in vari-
ous stoichiometric ratios gave invariably binuclear products,
suggesting a sort of cooperative effect between the two sites
of the ligand with regard to coordination.
The antiproliferative activity of triphenylphosphine bi-
nuclear complexes was found lower than those for corre-
sponding mononuclear complexes, indicating that biological
activity of the two platinum centers is not addictive, while
among all the examined PTA complexes only the binuclear
7-9 showed some activity, probably because a more
convenient hydrophilicity/lipophilicity balance is reached in
this case.
complex
T2
SKOV 3
4
5
6
7
8
9
10-50
10-50
10-50
2-10
2-10
2-10
ca. 50
>50
>50
>50
ca. 50
10-50
10-50
10-50
>50
cis-[PtCl2(PPh3)2]
cis-[PtCl2(PTA)2]
Na2(MBTT)
Na2(EBTT)
Na2(PBTT)
>50
>50
>50
>50
>50
>50
>50
cis-[PtCl2(NH3)2]
<2
>50
sensitive T2 cell line and for cisplatin-resistant SKOV3 cell
line are reported in Table 4.
It is necessary to underline that, for all the binuclear
platinum complexes, the platinum concentration is effectively
double that of the complex concentration. In spite of this,
on both cancer lines the triphenylphosphine derivatives 4-6
present an activity lower than that observed for corresponding
mononuclear analogues I and II, showing the lack of synergic
effect between the two platinum centers. On the contrary,
the bimetallic PTA complexes 7-9 are much more active
than the mononuclear complexes III and IV. This result
could be again related to the hydrophilicity/lipophilicity
balance: in fact, among all tested PTA complexes activity
was found only in binuclear complexes where lipophilic bis-
(thiopurinic) ligands are introduced beside the hydrophilic
{(PTA)2Pt} group.
In a comparison of the activity of the binuclear complexes
4-9 with their dichloride precursors cis-[PtCl2(PPh3)2] and
cis-[PtCl2(PTA)2], a clear conclusion is pulled: a thiopurinic
ligand on platinum enhances the antiproliferative activity of
the complex which seems to be independent of the length
of the bridging group in binuclear complexes. We also tested
the disodium salts of MBTTH2, EBTTH2, and PBTTH2 as
comparison, and we found that they do not show any
appreciable activity on their own.
Acknowledgment. We thank the PAI (Junta de Andalu-
c´ıa, FQM-317; Project AM56/04), the MCYT (Spain) for
the Projects PPQ2003-01339 and CTQ2006-06552/BQU, and
Consorzio Interuniversitario di Ricerca in Chimica dei
Metalli nei Sistemi Biologici. This work was supported by
the EC through the MCRTN program AQUACHEM (MRTN-
CT-2003-503864) and the COST Actions D17 and D29.
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determination of compound 1 and
antiproliferative activity tables including 10-12 as isomeric
mixtures. This material is available free of charge via the Internet
at http://pubs.acs.org. Complete crystallographic data (excluding
structural factors) for the structure in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication number CCDC 643227. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K. [fax +44(0)-1223-336033; e-mail
deposit@ccdc.cam.ac.uk or via www.ccdc.cam.ac.uk/conts/
retrieving.html].
Unfortunately, the reactions of MBTTH2, EBTTH2, and
PBTTH2 with the mixed phosphine precursor 1 invariably
gave only mixtures of several isomeric platinum complexes
which we did not succeeded in separating in pure form. For
this reason it was not possible to draw any definitive
conclusion from the tests of the antiproliferative activity of
the group of binuclear products bearing the {Pt(PTA)(PPh3)}
moiety (see Supporting Information).
IC062133C
4276 Inorganic Chemistry, Vol. 46, No. 10, 2007