Heterometallic platinum(ii) compounds with β-aminoethylferrocenes: Synthesis, electrochemical behaviour and anticancer activity

Article abstract of DOI:10.1039/c1dt11358e

A new family of heterometallic compounds 3-6 containing ferrocenyl and platinum(ii) centers has been synthesized by reaction of 1-β- aminoethylferrocene (1) and 1,1′-bis(β-aminoethyl)ferrocene (2) with Pt(ii) precursors. Using K₂[PtCl₄] as the Pt(ii) source, the cis-square-planar neutral compounds [Fe{η⁵-C ₅H₄(CH₂)₂NH₂} ₂PtCl₂] (3) and [{Fe(η⁵-C₅H ₄(CH₂)₂NH₂)(η⁵-C ₅H₅)}₂PtCl₂] (5) were obtained. Reaction of cis-[PtCl₂(dmso)₂] with 1 and 2 resulted in the displacement of dmso and chloride ligands from the platinum coordination sphere, affording the cationic and neutral compounds [Fe{η⁵- C₅H₄(CH₂)₂NH₂} ₂Pt(dmso)Cl]Cl (4) and [Fe(η⁵-C₅H ₄(CH₂)₂NH₂)(η⁵-C ₅H₅)Pt(dmso)Cl₂] (6). Compounds 3-6 were thoroughly characterized using multinuclear (¹H, ¹³C, ¹⁹⁵Pt) NMR, IR spectroscopy, ESI mass spectrometry and elemental analysis. Single-crystal X-ray analysis of heterometallic 6 confirmed the cis geometry of the molecule and revealed that the platinum atom is held in a perfect square-planar geometry. The electrochemical behaviour of the heterometallic compounds 3-6, which has been examined by cyclic (CV) and square wave (SWV) voltammetries in dichloromethane and dmso solution, is characterized by the reversible one-electron oxidation of the ferrocene moieties. The results of the biological activity studies revealed that the organometallic complex 5 is active against all cell lines with GI₅₀ values in the range 1.7-2.3 μM. When compared to the standard anticancer drug cisplatin, heterotrimetallic 5, possessing two aminoethylferrocenyl units coordinated to the Pt(ii) center, showed a greater activity profile in the colon cancer cell line. Cell cycle studies revealed that the new mixed compound exhibits a mechanism of action different to cisplatin.

Full text of DOI:10.1039/c1dt11358e

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PAPER
Heterometallic platinum(II) compounds with b-aminoethylferrocenes:
synthesis, electrochemical behaviour and anticancer activity†
Daniel Nieto,a Ana M. Gonza´lez-Vadillo,*a Sonia Brun˜a,a Ce´sar J. Pastor,b Carla R´ıos-Luci,c Leticia G. Leo´n,c
Jose´ M. Padro´n,c Carmen Navarro-Ranningera and Isabel Cuadrado*a
Received 19th July 2011, Accepted 14th September 2011
DOI: 10.1039/c1dt11358e
A new family of heterometallic compounds 3–6 containing ferrocenyl and platinum(II) centers has been
synthesized by reaction of 1-b-aminoethylferrocene (1) and 1,1¢-bis(b-aminoethyl)ferrocene (2) with
Pt(II) precursors. Using K2[PtCl4] as the Pt(II) source, the cis-square-planar neutral compounds
5
5
5
[Fe{h -C5H4(CH2)2NH2}2PtCl2] (3) and [{Fe(h -C5H4(CH2)2NH2)(h -C5H5)}2PtCl2] (5) were obtained.
Reaction of cis-[PtCl2(dmso)2] with 1 and 2 resulted in the displacement of dmso and chloride ligands
from the platinum coordination sphere, affording the cationic and neutral compounds
5
5
5
[Fe{h -C5H4(CH2)2NH2}2Pt(dmso)Cl]Cl (4) and [Fe(h -C5H4(CH2)2NH2)(h -C5H5)Pt(dmso)Cl2] (6).
Compounds 3–6 were thoroughly characterized using multinuclear (1H, 13C, 195Pt) NMR, IR
spectroscopy, ESI mass spectrometry and elemental analysis. Single-crystal X-ray analysis of
heterometallic 6 confirmed the cis geometry of the molecule and revealed that the platinum atom is held
in a perfect square-planar geometry. The electrochemical behaviour of the heterometallic compounds
3–6, which has been examined by cyclic (CV) and square wave (SWV) voltammetries in
dichloromethane and dmso solution, is characterized by the reversible one-electron oxidation of the
ferrocene moieties. The results of the biological activity studies revealed that the organometallic
complex 5 is active against all cell lines with GI50 values in the range 1.7–2.3 mM. When compared to
the standard anticancer drug cisplatin, heterotrimetallic 5, possessing two aminoethylferrocenyl units
coordinated to the Pt(II) center, showed a greater activity profile in the colon cancer cell line. Cell cycle
studies revealed that the new mixed compound exhibits a mechanism of action different to cisplatin.
rocenes have been investigated as electroactive sensors of anions4 in
Introduction
the modification of electrode surfaces,5 as redox mediators in elec-
trochemical biosensing of DNA and proteins,6 and as ancillary lig-
ands (with Pd and Pt centers) for olefin polymerizations.7 An inter-
esting recent report has shown that 1,1¢-bis(amino)ferrocenes are
also useful as starting compounds for gadolinium(III)-containing
macrocyclic DPTA systems employed as MRI blood-pool contrast
agents.8
Ferrocene derivatives possessing reactive functionalities tethered
to the cyclopentadienyl rings have been extensively used to
synthesize a broad variety of ferrocenyl-containing molecules
with interesting applications, ranging from homogeneous catalysis
to molecular recognition and materials science.1 Among these
metallocenes, amino-functionalized ferrocenes2,3 are an attractive
group of compounds that play a significant role in areas such
as asymmetric catalysis, NLO materials, and coupling with
biomolecules.1 Furthermore, several amino-functionalized fer-
Among aminoferrocenes, mono and bisfunctionalized b-
5
5
aminoethylferrocenes Fe{(h -C5H4(CH2)2NH2)(h -C5H5)} (1) and
5
Fe{h -C5H4(CH2)2NH2}2 (2) (Chart 1) are particularly interesting
compounds since they are able to undergo condensation reactions
typical of the primary amino functional group. Pioneering studies
in this field were carried out by Gonsalves and Rausch, who have
used 1,1¢-bis(b-aminoethyl)ferrocene (2) as starting monomer for
the preparation of remarkable examples of ferrocene-containing
polyamides and polyureas (see example in Chart 1) via interfacial
condensation polymerizations.9 Likewise, our group has also used
aminoethyl ferrocenes 1 and 2 as useful precursors of silicon-
containing ferrocenyl model compound polymers (Chart 1).10
The high reactivity of the primary amine group of monofunc-
tionalized 1-b-aminoethylferrocene (1) toward Si–Cl bonds also
aDepartamento de Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad
Auto´noma de Madrid, Cantoblanco, 29049, Madrid, Spain. E-mail: anam.
gonzalez@uam.es, isabel.cuadrado@uam.es; Fax: +34 91 497 4833; +34 91
497 4843; +34 91 497 4834
bLaboratorio de Difraccio´n de Rayos X, SIDI, Universidad Auto´noma de
Madrid, Spain
cBioLab, Instituto Universitario de Bio-Orga´nica “Antonio Gonza´lez”
(IUBO-AG). Universidad de La Laguna, 38206, La Laguna, Spain
† Electronic supplementary information (ESI) available: Additional spec-
troscopic data for 1–6; electrochemical details for 3–6 and crystallographic
data for 6. CCDC reference number 834095. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt11358e
432 | Dalton Trans., 2012, 41, 432–441
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Results and discussion
Synthesis and characterization of b-aminoethylferrocenes
The starting ferrocenyl derivatives required for the present study
were 1-b-aminoethylferrocene (1) and 1,1¢-bis(b-aminoethyl)-
ferrocene (2). These compounds in particular were selected
since their reactive NH2 functionalities are separated from the
cyclopentadienyl ring by a two methylene flexible spacer. This
fact is of importance because it minimizes steric and electronic
effects of the organometallic ferrocene moiety. The use of a b-
functionalized metallocene, in addition, avoids the instability of
a-functional ferrocene derivatives resulting from the stability of
the a-ferrocenyl carbonium ion.24 Both aminoethylferrocenes,
1 and 2, were synthesized several decades ago,25,26 but slightly
modified preparations were used here to improve yields and to
ease purification. Scheme 1 summarizes the sequence of multiple
reaction steps to give 1 and 2 (Scheme 1, top and bottom,
respectively).
Chart 1 b-Aminoethylferrocene precursors 1 and 2 and selected polymer
and dendrimer derivatives.
allowed facile organometallic functionalization of carbosilane
frameworks, resulting in dendrimers with 4 and 8 ferrocenyl
moieties attached to the surface of the dendritic structure through
Si–NH linkages (see example in Chart 1).11 These ferrocenyl
dendrimers are able to act as anion receptors, in solution and
immobilized onto electrode surfaces.12 Surprisingly, and although
b-aminoethylferrocenes 1 and 2 are also valuable building blocks
for coordination compounds of higher nuclearity, their coordina-
tion chemistry remains largely unexplored. To our knowledge, only
a few examples of cyclometallated Pd(II) and Pt(II) compounds
derived from 1 have been reported.13
Scheme
1
Reagents and conditions for the synthesis of
On the other hand, derivatives based on typical organometallic
moieties such as ferrocene, titanocene dichloride, and arene
ruthenium units have been extensively investigated in recent years
as a novel class of medicinal compounds, potentially with new
metal-specific modes of action.14,15 In particular, a number of
simple neutral ferrocene derivatives as well as some cationic
ferrocenium salts exhibit cytotoxic behaviour and inhibit the
development of tumors in vivo.15–16 In addition, the ferrocene unit
has also been linked to platinum,17 palladium or ruthenium centers
in order to achieve synergistic biological effects between the two
active metals.18,19
b-aminoethylferrocenes 1 and 2. (i) KCN, H2O, reflux; (ii) LiAlH4,
Et2O, reflux; (iii) NaOCl, 50 ◦C; (iv) CH3OH, H2SO4, 90 ◦C; (v) LiAlH4,
Et2O/toluene (1/1, v/v), reflux; (vi) Py, PCl3, KCN, THF/H2O, 40 ◦C;
(vii) LiAlH4, AlCl3, Et2O, reflux.
Monofunctionalized b-aminoethylferrocene (1) was prepared
in two steps (i and ii, in Scheme 1, top) from N,N,N-
trimethylferrocenylmethyl-ammonium iodide as starting material
5
5
by adapting the literature procedures. Firstly, [(h -C5H5)Fe{(h -
C5H4)CH2N(CH3)3}]I was reacted with potassium cyanide, thus
resulting in 1-cyanomethylferrrocene which was isolated as a
yellow crystalline solid in 63% yield. The cyanomethylferrocene
was converted into b-aminoethylferrocene (1) by reduction with
LiAlH4 followed by treatment with aqueous sodium hydroxide.
After aqueous workup and distillation in vacuum, primary amine
1 was obtained as an amber-brown oil in 66% yield.
As part of our longstanding interest in the chemistry of
ferrocenyl-containing compounds and dendrimers20,21 and in
the preparation and study of platinum-based derivatives with
antitumoral properties,22 we recently became interested in using
b-aminoethylferrocenes 1 and 2 for the synthesis of new se-
ries of potentially cytotoxic heterometallic ferrocene-containing
Pt(II) and Pt(IV) complexes. In this context, we have recently
found that the reaction of 1 with the platinum(II) benzonitrile
complex cis-[PtCl2(PhCN)2] affords an interesting compound,
trans-[PtCl2{NH C(Ph)(NH(CH2)2Fc)}2] having newly formed
ferrocenyl-amidine ligands, which is able to act as an electro-
For the synthesis of bifunctionalized ferrocenyl derivative 2
(Scheme 1, bottom), 1,1¢-ferrocenyldicarboxylic acid was firstly
prepared on a large scale by oxidation of 1,1¢-diacetylferrocene
with sodium hypochlorite. After purification, dicarboxyferrocene
was converted into the corresponding dimethyl ester by acid-
catalyzed esterification (Scheme 1, step iv). Subsequent reduction
of the methyl diester with LiAlH4 (Scheme 1, step v) afforded
1,1¢-bis(b-hydroxymethyl)ferrocene. This diol was converted into
the nitrile and its subsequent reduction with LiAlH4 produced the
desired amine 2 (steps vi and vii in Scheme 1).
chemical sensor for the H2PO4 anion.23
-
Herein, we report on the reactivity of b-aminoethylferrocenes
1 and 2 towards K2[PtCl4] and cis-[PtCl2(dmso)2] complexes as
platinum(II)-containing sources, which has led to the isolation
of new electroactive heterometallic platinum-based compounds.
Their complete structural characterization and redox behaviour
are described, along with their cytotoxic activity against a panel
of human solid tumor cell lines containing examples of HBL-100
(breast), HeLa (cervix), SW1573 (non-small cell lung), and WiDr
(colon).
As only a few structural characterization data of 1 and 2 have
been previously reported, a detailed characterization of both
aminoethylferrocenes was accomplished by NMR spectroscopy,
1
1
including { H–13C}HMQC and { H–13C}HMBC experiments,
IR spectroscopy, and mass spectrometry (see ESI). H and 13C
1
NMR spectra of 1 and 2 showed the shifts for the CH2 groups
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bonded to the nitrogen atom at a higher d (ppm) than those
for the other CH2 groups. While the H NMR spectrum of 2
shiny solid whose characterization data (see below and ESI†)
1
5
agreed with those expected for the dimetallic compound [Fe(h -
5
shows the typical patterns of disubstituted ferrocene derivatives,
two pseudo-triplets, the 1H NMR spectrum of 1 shows the
characteristic pattern for monosubstituted ferrocenes, two
pseudo-triplets and a singlet. The amine group protons of both
compounds appear at about 1.3 ppm, which was proved by the
disappearance of the signals when some D2O drops were added to
the NMR sample. The most relevant feature of the IR spectra of
ferrocenylamines 1 and 2 consists of the presence of bands, in the
range of 3292–3368 cm-1 due to the N–H stretching vibrations.
The ESI mass spectra show a peak corresponding to the M+ ion,
at m/z 229 (for 1) and at 272 (for 2), together with some peaks
assignable to reasonable fragmentation products.
C5H4(CH2)2NH2)(h -C5H5)Pt(dmso)Cl2] (6). Coordination of b-
aminoethylferrocenes to platinum centers resulted in the displace-
ment of one dmso and one chloride ligand from the coordination
sphere of platinum, leading to the cationic compound 4, or
one dmso ligand yielding neutral bimetallic 6. This reactivity
is different to that found by Lopez et al. in the reactions
of ferrocenylamine 1 with palladium(II) complexes, in which
cyclopalladation of the b-aminoethylferrocene 1 was observed, as
a result of the coordination of the nitrogen atom of 1 to palladium
followed by intramolecular electrophilic attack to the s C–H bond
of the cyclopentadienyl ring.13a
The structural identity of the novel molecules 3–6 was straight-
forwardly established on the basis of elemental analysis, IR, multi-
nuclear (1H, 13C, 195Pt) NMR spectroscopy and mass spectrometry,
when it was possible. Although the poor solubility of compound
3 in common organic solvents precluded any reasonable NMR
analyses (1H and 13C NMR), its structure was confirmed by ESI
mass spectrometry. The ion [M-Cl]+ was clearly observed at m/z
501.0 and its calculated and experimental isotopic distributions
are in total agreement. Furthermore, the IR spectrum of 3 (in
KBr) agrees well with the proposed structure, as it exhibits a band
at 487 cm-1 due to the Pt–N stretching vibration, and two bands
due to the stretching of the Pt–Cl bonds (at 330 and 321 cm-1)
indicative of the cis geometry.27
Reactivity of b-aminoethylferrocenes 1 and 2 with K2[PtCl4] and
cis-[PtCl2(dmso)2] complexes
Our initial attempts to prepare mixed platinum(II)-
aminoethylferrocene compounds involved the use of the
salt K2[PtCl4] as the platinum(II)-containing source. The
b-aminoethylferrocene precursors 1 and 2 were soluble in
dichloromethane, whereas this platinum starting material was not.
For this reason, the heterometallic complexes, cis-dichloro[1,1¢-
bis(b-aminoethyl)ferrocene]platinum(II) (3) and cis-dichlorobis[1-
b-aminoethylferrocene]platinum(II) (5) were prepared using a
biphasic system with the aminoethylferrocenes 1 and 2 in a CH2Cl2
solution and the platinum starting salt dissolved in an ethanol–
water mixture. The use of this synthetic route avoids the difficulties
related to the basicity of ferrocenylamines in aqueous solution that
could lead to the formation of ferrocenylamine salts. The reaction
mixture was stirred vigorously at room temperature to increase the
interface of reaction between the solvent layers. A phase-transfer
reaction was established between the layers. The heterometallic
products were found to be soluble in the dichloromethane phase.
After appropriate work-up, compounds 3 and 5 (Scheme 2) were
obtained in reasonable yields as yellow-orange crystalline solids.
On the other hand, when cis-[PtCl2(dmso)2] was reacted with 2
in dichloromethane solution, a yellow solid product was obtained
Compounds 4, 5 and 6 proved to be much more soluble in
common organic solvents than compound 3, so further charac-
terization could be achieved by NMR spectroscopy. While proton
NMR spectra of 5 (in dmso-d6) and 6 (in CD2Cl2) show the typical
patterns of monosubstituted ferrocene derivatives (a singlet for the
C5H5 protons, and two pseudo-triplets for the C5H4 moiety, which
1
in 6 are under the singlet signal), the H NMR spectrum of 4,
in (CD3)2CO, shows two signals at 4.19 and 4.12 ppm, typical of
disubstituted ferrocene derivatives. The expected signals for the
protons of the CH2 groups (4–6) appear at higher ppm than the
same protons of the precursors 1 and 2. In addition, an intense
singlet, at 3.34 (for 4) and at 3.44 (for 6), assigned to the protons of
the dmso ligands, can be observed. In particular, the proton NMR
spectra of 4 and 6 display 195Pt satellites for this last resonance with
3J(195Pt–1H) = 19.1 Hz (4) and 25.5 Hz (6). A remarkable 1H NMR
feature of 4–6 is the chemical shift of the NH groups of the amine
ligands, as they are considerably shifted downfield (at 4.50 for 4,
5
which was identified as [Fe{h -C5H4(CH2)2NH2}2Pt(dmso)Cl]Cl
(4) (Scheme 2). Analogous reaction of cis-[PtCl2(dmso)2] with
mono-functionalized ferrocene 1 in the same medium afforded,
after appropriate column chromatographic purification, a yellow
Scheme 2 Synthesis of heterometallic compounds 3–6. Reagents and conditions: (i) K2[PtCl4], H2O/EtOH (2/1, v/v), CH2Cl2; (ii) cis-[PtCl2(dmso)2],
CH2Cl2, r.t.
434 | Dalton Trans., 2012, 41, 432–441
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Table 1 Selected crystallographic data for compound 6
4.98 for 5, and 3.82 for 6) in comparison with the signals of the
free ligands of 1 (1.30 ppm) and 2 (1.24 ppm).
6
The 195Pt resonance is extremely sensitive to the coordination
environment, so the 195Pt{H} NMR spectra of compounds 4–
6 provide convincing evidence for the coordination sphere and
structure of the mixed ferrocene and platinum complexes. While
the signal detected in the 195Pt{H} NMR spectrum of compound
4 (in (CD3)2CO) appears at -3120 ppm and for 6 (in CD2Cl2) at
-3097 ppm, for compound 5 (in dmso-d6) the signal is shifted
downfield (-2210 ppm). These values are consistent with a
N2ClS,28 NCl2S29 and N2Cl2,22b set of donor atoms around the
platinum center, respectively.
Empirical formula
Fw
C14H21Cl2FeNOPtS
573.22
T [K]
100(2)
1.54178
˚
l [A]
cryst syst
Monoclinic
P21/c
13.1200(12)
11.3110(10)
11.8689(10)
90
97.208(5)
90
1747.4(3)
4
2.179
25.317
1096
0.20 ¥ 0.14 ¥ 0.08
3.40 to 68.26.
-14 £ h £ 15
-12 £ k £ 13
- 13 £ l £ 13
13 604
Space group
˚
a, A
˚
b, A
a, deg
b, deg
˚
c, A
g , deg
The IR spectra of 3 and 5 (KBr) show two bands due to the
stretching of the Pt–Cl bond, according to a cis structure, and
one band due to the Pt–N stretching vibration. For 4 and 6, in
addition to the latter signals (see experimental section) the typical
bands due to the presence of dmso are observed. Namely n(S O)
appears at 1124 cm-1 for 4 and at 1123 cm-1 for 6, and n(Pt–S) at
441 cm-1 for 4 and at 444 cm-1 for 6.
The mass spectrometric studies of compounds 4–6 have pro-
vided further support for the existence of the heterometallic
compounds. In the ESI-MS spectrum of trimetallic 5 a peak at
m/z 723.0 corresponding to the molecular ion [M]+ was observed.
In addition, the spectrum exhibits a peak corresponding to the
cationic fragment [M-Cl]+ (at 688.0), due to the loss of Cl-.
Their isotopic distributions are in excellent agreement with the
calculated ones. Mass spectral analysis (ESI) of 4 and 6 confirmed
the proposed structures, showing the corresponding molecular
ions [M]+ at m/z 580.0 (for 4), and [M+Na]+ at m/z 594.9 (for 6),
as the most intense signals, with an excellent agreement between
the calculated and the found isotopic patterns (see Fig. S13 and
S23, ESI†).
3
˚
V, A
Z
density (calcd), mgm-3
m, mm-1
F(000)
Crystal size, mm3
q, deg
Index ranges
no. of reflns collected
no. of indep reflns
completeness
3017 [R(int) = 0.0572]
94.2% (to q = 68.26◦)
absorp corr
semi-empirical from equivalents
full-matrix least-squares on F2
3017/0/180
refinement method
no. of data/restraints/params
goodness-of-fit on F2
Final R indices (I > 2s(I))
R indices (all data)
largest diff peak and hole/e A )
1.048
R1 = 0.0431, wR2 = 0.0984
R1 = 0.0486, wR2 = 0.1026
4.028 and -2.078
-3
˚
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for compound 6
6
In order to confirm the relative arrangement of the ferrocenyl,
chloride and dmso ligands in heterometallics 3–6, several crystalli-
zation procedures were used; however, most of the experiments
failed. Only when a hexane/CH2Cl2 solution of 6 was evaporated
at +4 ◦C suitable crystals were obtained. The molecular structure
of the platinum complex 6 together with the atomic numbering
scheme is shown in Fig. 1. Crystallographic data are shown in
Table 1 while selected bond lengths and angles are shown in Table
2.
Heterobimetallic 6 crystallizes in the monoclinic P21/c space
group with Z = 4. The coordination environment of the platinum
ion is square planar, with two cis chloride ligands, one nitrogen
atom of the ferrocenylamine and one sulfur atom of the dmso
unit. Consequently, cis bond angles around the Pt center are very
close to the expected 90◦, and S(1)–Pt(1)–Cl(1) and N(1)–Pt(1)–
Cl(2) angles have values of 175–176◦. Moreover, the compound
exhibits two hydrogen bonds, one of them intramolecular between
the N(1)–H(1A) proton of the amine and the oxygen atom O(1),
and the other intermolecular between the N(1)–H(1B) proton and
the chlorine atom Cl(2) of another molecule with bond distances
Pt(1)–Cl(1)
Pt(1)–Cl(2)
Pt(1)–N(1)
Pt(1)–S(1)
Pt(1)–Fe(1)
S(1)–O(1)
S(1)–C(1)
2.324(2)
2.294(19)
2.089(8)
2.199(2)
7.002
1.465(6)
1.764(9)
1.767(10)
1.443(13)
0.920
Cl(1)–Pt(1)–S(1)
N(1)–Pt(1)–Cl(2)
N(1)–Pt(1)–S(1)
Cl(1)–Pt(1)–Cl(2)
S(1)–Pt(1)–Cl(2)
N(1)–Pt(1)–Cl(1)
Pt(1)–S(1)–O(1)
Pt(1)–S(1)–C(1)
Pt(1)–S(1)–C(2)
C(1)–S(1)–C(2)
Pt(1)–N(1)–C(3)
C(3)–C(4)–C(5)
176.5(8)
175.1(2)
89.9(2)
90.8(8)
92.2(8)
87.2(2)
114.4(3)
112.7(3)
110.1(3)
101.5(4)
111.1(6)
112.6(9)
S(1)–C(2)
N(1)–C(3)
N(1)–H(1A)
N(2)–H(1B)
C(3)–C(4)
C(4)–C(5)
0.920
1.550(14)
1.491(14)
The S atom in the dmso ligand is in an approximately
tetrahedral environment. The Pt(1)–S(1)–O(1) angle (114.4(3)◦)
is slightly larger than the Pt(1)–S(1)–C(1) and Pt(1)–S(1)–C(2)
angles (112.7(3) and 110.1(3)◦, respectively) and the C(1)–S(1)–
C(2) angle (101.5(4)◦) is smaller than the other angles around the
S atom, since the multiple S–O bond occupies more space than the
single S–C bonds.
˚
of 2.24 and 2.86 A, respectively.
In the ferrocenyl moiety, the two cyclopentadienyl rings are
essentially parallel and they are arranged, approximately, in a
nearly eclipsed conformation. The distance between the two metals
˚
The Pt–S length (S(1)–Pt(1) 2.199(2) A) is similar to those
reported for other Pt–sulfoxide complexes with a chlorine in the
trans position.30,31 The Pt–Cl bond trans to the sulfoxide ligand is
˚
in the molecule (Fe ◊ ◊ ◊ Pt) is 7.002 A.
˚
˚
longer (2.324(2) A) than the one trans to the amine (2.294(19) A).
This elongation of the Pt–Cl(1) distance confirms the major trans
effect of the sulfur donor atom.
The molecular arrangement of 6 in the crystal structure shown
in Fig. 2 is of interest. An examination of the crystal packing
diagram along the b-axis shows that association of similar metallic
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vs. SCE, exhibit a single oxidation about +0.45 V, assigned to the
Fe2+/Fe3+ redox system. Representative CVs and SWVs are given
in Fig. 3 (for compounds 5 and 6) as well as in the ESI.† In all
cases, the linearity of i vs. n1/2 plots (see ESI†) demonstrates that the
main mass transport of these compounds to the electrode surface
is controlled by the diffusion step. Likewise, the voltammetric
features (ipc/ipa essentially equal to unity, DEpeak values about 60–
85 mV and Epeak independent of the scan rate) show that oxidation
of these mixed ferrocene–platinum compounds is chemically and
electrochemically reversible, resulting in the production of stable
and soluble species [3+][PF6 ], [4+][PF6 ] and [6+][PF6 ].
-
-
-
Fig. 1 Molecular structure of 6 with 50% probability ellipsoids. Hydrogen
atoms are omitted for clarity.
Fig. 3 CV (at different scan rates) and SWV responses of compounds 5
in dmso (a and b) and 6 in CH2Cl2 (c and d) (5 ¥ 10-4 M) on platinum
working electrode (nominal area 0.020 cm2).
The electrochemical behaviour of heterotrimetallic 5, contain-
ing two ferrocenyl moieties and one platinum(II) center, in dmso
solution, is reversible and shows a single oxidation peak at
E1/2 = 0.45, vs. SCE (see Fig. 3a) suggesting the formation of
the stable and soluble cationic species [52+][PF6 ]2. Likewise, in the
-
Fig. 2 Crystal-packing diagram of trimetallic 6 along the b-axis, showing
the iron-rich and platinum-rich pseudolayers.
square wave voltammogram (Fig. 3b) only one oxidation wave is
observed. The fact that only a single redox process is observed
implies simultaneous two-electron transfers at the same potential
as the ferrocenyl units in 5, indicating the lack of electronic
communication between the two ferrocenyl centers.32
moieties, a type of self-assembly, is apparently driving the packing
in the crystal. The intermolecular ferrocene distances to the next
˚
neighboring molecules range between 5.967 and 6.245 A, placing
the metallocene moieties in a favorable proximity for cluster-
forming intermolecular solid-state reactions. Thus, one can clearly
observe repeating platinum-rich and ferrocenyl-rich layers lying in
the ac-plane.
Antiproliferative activity
As a model for anticancer activity we used the human solid tumor
cell lines HBL-100 (breast), HeLa (cervix), SW1573 (non-small
cell lung), and WiDr (colon). The in vitro antiproliferative activity
of complexes 4–6 was evaluated after 48 h of drug exposure using
the sulforhodamine B (SRB) assay.33 Compound 3 was not soluble
enough in dmso (40 mM) to proceed with biological experiments.
The results expressed as GI50 (50% growth inhibition)34 values are
shown in Table 3.
For comparison, we have included the antiproliferative data
of the standard anticancer drug cisplatin (CDDP) against the
same panel of solid tumor cell lines. The data shows that the
organometallic complex 5, with two aminoethylferrocenyl units
coordinated to the Pt(II) center, was the most potent compound
Electrochemical studies
The anodic electrochemistry of the heterometallic compounds 3–
6 was examined by cyclic voltammetry (CV) and square wave
voltammetry (SWV), using dichloromethane or dmso as solvents,
and tetra-n-butylammonium hexafluorophosphate (n-Bu4NPF6)
as the supporting electrolyte. The low solubility of 3 did not allow
us to carry out electrochemical studies in the non-nucleophilic
solvent dichloromethane.
The cyclic voltammograms of heterometallic compounds 3, 4
and 6 in dmso, in the anodic potential region between 0 and +1 V
436 | Dalton Trans., 2012, 41, 432–441
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Table 3 In vitro antiproliferative activity against human solid tumor cellsa
Cell line (type)
Compound
HBL-100 (breast)
HeLa (cervix)
SW1573 (lung)
WiDr (colon)
4
>100
>100
>100
>100
5
2.0 ( 0.49)
32 ( 0.74)
1.9 ( 0.16)
1.7 ( 0.19)
27 ( 7.4)
2.0 ( 0.3)
2.0 ( 0.71)
30 ( 3.4)
3.4 ( 0.7)
2.3 ( 0.52)
41 ( 13)
26 ( 5.6)
6
CDDP
a Values expressed as GI50 are given in mM and are means of two to six experiments, standard deviation is given in parentheses.
of the series with GI50 values in the range 1.7–2.3 mM. In contrast,
compound 6 showed reduced activities while complex 4 was
inactive. When compared to cisplatin, compound 5 showed a
superior activity profile in the more drug resistant35 cell line
WiDr. Compound 5 was selected as a lead for further biological
evaluation.
doses. The results indicate that the mechanism of antiproliferative
activity of the new compound is not affecting the cell cycle.
The combination of the ferrocene and the diamino-platinum
scaffolds indicates a possible role of the ferrocene moiety in
the modulation of the antiproliferative activity of the diamino-
platinum framework. Ongoing studies beyond the scope of this
work will help to establish the precise role of both active functions
and any possible synergistic interaction.
Cell cycle studies
To investigate the effects that the mixed ferrocene–platinum com-
plex 5 produces on tumor cells and to highlight possible differences
in its mode of action, cell cycle analysis36 by flow cytometry was
performed on the cancer cell lines HeLa, SW1573, and WiDr
after 24 h exposure to the drugs. The standard anticancer drug
cisplatin was used as a reference drug to compare the results. Cells
were exposed to each agent at two drug concentrations that were
chosen based on two premises:37 the GI50 values of the compounds
and the sensitivity of the cell line to drug treatment, since at higher
drug doses cell death prevents examination of the cell cycle phase
distribution. Thus, all cell lines were initially exposed for 24 h to
compound 5 at 2 and 6 mM drug doses and to cisplatin at 5 and
10 mM. The obtained results showed that the cell cycle in all cell
lines was little affected at the low dose of 2 mM. No clear cell
cycle arrest could be observed even at the higher dose of 6 mM
(Fig. 4). However, only HeLa cells showed significant cell death
(12%), similar to cisplatin (13% at 5 mM). Increasing the dose to
18 mM only produced significant cell death in SW1573 cells (12%),
while changes in the cell cycle of HBL-100 cells were irrelevant. In
contrast, cisplatin produces a clear S-phase arrest, even at higher
Experimental
Materials and equipment
All reactions and compound manipulations were performed in an
oxygen- and moisture-free atmosphere (N2 or Ar) using standard
Schlenk techniques. Solvents were dried by standard procedures
over the appropriate drying agents and distilled immediately
prior to use. The platinum precursor cis-[PtCl2(dmso)2] was pre-
pared by literature methods.38 1,1¢-Diacetylferrocene, methanol,
sodium hypochlorite solution 5% (Aldrich), potassium cyanide,
aluminum chloride (Fluka), potassium tetrachloroplatinate(II),
N,N,N-trimethylferrocenylmethyl-ammonium iodide and lithium
aluminum hydride (Alfa Aesar) were used without further pu-
rification. Phosphorus trichloride and pyridine (Aldrich) were
distilled prior to use. Silica gel 60 silanized (Merck) was used
for column chromatographic purifications. Infrared spectra were
recorded on Bomem MB-100 FT-IR and on Perkin Elmer 100 FT-
IR spectrometers. H, 13C, 195Pt NMR spectra, were recorded on
Bruker-AMX-300 and Bruker-DRX-500 spectrometers. Chemical
1
shifts were reported in parts per million (d) with reference to
1
1
residual solvent resonances for H and 13C NMR (CDCl3: H,
d 7.27 ppm, 13C, d 77.0 ppm; (CD3)2CO: 1H, d 2.09 ppm;
13C, d 205.9 and 30.6 ppm; dmso-d6: 1H, d 2.54 ppm, 13C, d
40.45 ppm; and CD2Cl2: 1H, d 5.33 ppm, 13C, d 53.8 ppm).
Electrospray ionization (ESI) mass spectra were recorded with
a QSTAR (Applied Biosystems) spectrometer, using methanol as
the ionizing phase, while FAB-mass spectra were obtained using
a VG AutoSpec (Waters) mass spectrometer with m-NBA as the
matrix. Samples were prepared in CH2Cl2 or dmso. Elemental
analyses were performed by the Microanalytical Laboratory, SIDI,
Universidad Auto´noma de Madrid, Spain.
Electrochemical measurements
Cyclic voltammetric and square wave voltammetric experiments
were recorded on a BAS-CV-50 W potentiostat. CH2Cl2 (dried
over CaH2) and dmso (spectrograde) for electrochemical mea-
surements were freshly distilled under argon. The supporting
Fig. 4 Cell cycle phase distribution of untreated cells (C) and cells treated
(drug dose in mM) with complex 5 and cisplatin (CDDP) for 24 h.
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electrolyte used was tetra-n-butylammonium hexafluorophos-
phate (Fluka), which was purified twice by recrystallization from
ethanol and dried in vacuum at 60 ◦C. The supporting electrolyte
concentration was typically 0.1 M. A conventional three-electrode
cell connected to an atmosphere of prepurified nitrogen was used.
All cyclic voltammetric experiments were performed using either
a platinum-disk working electrode (A = 0.020 cm2) or a glassy
carbon-disk working electrode (A = 0.070 cm2) (both Bioanalytical
Systems), each of which were polished on a Buehler polishing cloth
with Metadi II diamond paste, rinsed thoroughly with purified
water and acetone and dried. All potentials were referenced to
the saturated calomel electrode (SCE). Under our conditions, the
decamethylferrocene redox couple [FeCp*2]0/+ is -0.06 V vs. SCE
in CH2Cl2. A coiled platinum wire was used as a counter electrode.
Solutions were typically 5 ¥ 10-4 M in the redox active species.
The solutions for the electrochemical experiments were purged
with nitrogen and kept under an inert atmosphere throughout the
measurements. Square wave voltammetry (SWV) was performed
using frequencies of 10 Hz.
fetal calf serum (FCS) was from Gibco (Grand Island, NY),
trichloroacetic acid (TCA) and glutamine were from Merck
(Darmstadt, Germany), and penicillin G, streptomycin, dmso and
sulforhodamine B (SRB) were from Sigma (St Louis, MO).
Cells, culture and plating
The human solid tumor cell lines HBL-100, HeLa, SW1573, and
WiDr were used in this study. These cell lines were a kind gift
from Prof. G. J. Peters (VU Medical Center, Amsterdam, The
Netherlands). Cells were maintained in 25 cm2 culture flasks in
RPMI 1640 supplemented with 5% heat inactivated fetal calf
serum and 2 mM L-glutamine in a 37 ◦C, 5% CO2, 95% humidified
air incubator. Exponentially growing cells were trypsinized and re-
suspended in antibiotic containing medium (100 units penicillin
G and 0.1 mg of streptomycin per mL). Single cell suspensions
displaying >97% viability by trypan blue dye exclusion were
subsequently counted. After counting, dilutions were made to
give the appropriate cell densities for inoculation onto 96-well
microtiter plates. Cells were inoculated in a volume of 100 mL per
well at densities of 10 000 (SW1573 and HBL-100), 15 000 (HeLa),
and 20 000 (WiDr) cells per well, based on their doubling times.
X-ray crystal structure determination
Compound 6 was structurally characterized by single-crystal X-
ray diffraction. A suitable yellow crystal of 6, of dimensions
0.20 ¥ 0.14 ¥ 0.08 mm, was located and mounted on a glass
fiber with “magic oil”. The sample was transferred to a Bruker
SMART 6 K CCD area-detector three-circle diffractometer with
Chemosensitivity testing
Compounds were initially dissolved in dmso at 400 times the
desired final maximum test concentration. Control cells were
exposed to an equivalent concentration of dmso (0.25% v/v,
negative control). Each agent was tested in triplicate at different
dilutions in the range of 1–100 mM. The drug treatment was
started on day 1 after plating. Drug incubation times were 48 h,
after which time cells were precipitated with 25 mL ice-cold TCA
(50% w/v) and fixed for 60 min at 4 ◦C. Then the SRB assay was
performed.33 The optical density (OD) of each well was measured
at 492 nm, using BioTek’s PowerWave XS Absorbance Microplate
Reader. Values were corrected for background OD from wells only
containing medium.
˚
a MacScience rotating anode (Cu-Ka radiation, l = 1.54178 A)
generator equipped with Goebel mirrors at settings of 50 kV
and 100 mA. A total of 3017 independent reflections (Rint
=
0.0572) were colleted in the range 3.40◦ < q < 68.26◦. X-ray
data were collected at 100 K, with a combination of six runs
at different j and 2q angles, 3600 frames. Data were collected
using 0.3◦ wide w scans with a crystal-to-detector distance of
4.0 cm. The substantial redundancy in data allows empirical
absorption corrections (SADABS)39 to be applied using multiple
measurements of symmetry-equivalent reflections. Raw intensity
data frames were integrated with the SAINT program,40 which
also applied corrections for Lorentz and polarization effects. The
software package SHELXTL version 6.10 was used for space
group determination, structure solution and refinement.41 The
space group determination was based on a check of the Laue
symmetry and systematic absences and was confirmed using the
structure solution. The structure was solved by direct methods
(SHELXS-97), completed with difference Fourier syntheses, and
refined with full-matrix least-squares using SHELXL-97 minimiz-
Cell cycle analysis
Cells were seeded in six well plates at a density of 2.5–5 ¥ 105
cells well-1. After 24 h the products were added to the respective
well and incubated for an additional period of 24 h. Cells were
trypsinized, harvested, transferred to test tubes (12 ¥ 75 mm)
and centrifuged at 1500 rpm for 10 min. The supernatant was
discarded and the cell pellets were re-suspended in 200 mL of cold
PBS and fixed by the addition of 1 mL ice-cold 70% EtOH. Fixed
cells were incubated overnight at -20 ◦C, after which time they
were centrifuged at 1500 rpm for 10 min. The cell pellets were
resuspended in 500 mL of PBS and 5 mL of DNAse-free RNAse
solution (10 mg mL-1) was added. The mixture was incubated at 37
◦C for 30 min. Finally, 5 mL of PI (0.5 mg mL-1) was added. Flow
cytometric determination of DNA content (20 000 cells sample-1)
was analyzed on a LRSII Flow Cytometer (Becton Dickinson, San
Jose´, CA, USA). The fractions of the cells in the G0/G1, S, and
G2/M phases were analyzed using FACS Diva 6.0 (BD Software,
San Jose´, CA, USA) software.
ing w(Fo2–Fc )2.42,43 Weighted R factors (Rw) and all goodness
2
of fit S are based on F2 conventional R factors (R) are based
on F. All non-hydrogen atoms were refined with anisotropic
displacement parameters. The hydrogen atom positions were
calculated geometrically and were allowed to ride on their parent
carbon atoms with fixed isotropic U. All scattering factors and
anomalous dispersion factors are contained in the SHELXTL
6.10 program library.
Biology
All starting materials were commercially available research-grade
chemicals and used without further purification. RPMI 1640
medium was purchased from Flow Laboratories (Irvine, UK),
Synthesis of Fe{[(g5-C5H4)(CH2)2NH2](g5-C5H5)} (1). Mono-
functional 1 was synthesized in two steps from N,N,N-
trimethylferrocenylmethyl-ammonium iodide, by adapting the
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literature procedures.9a 1-Cyanomethylferrocene44 (5.55 g, 24.4
mmol) in dry diethyl ether (90 mL) was added dropwise to a stirred
suspension of LiAlH4 (1.63 g, 43 mmol) in 100 mL of dry diethyl
ether and heated at 35 ◦C. After being stirred for 3 h under reflux,
the reaction mixture was externally cooled with an ice bath, and
cold water was slowly added dropwise until H2 release stopped.
Once decanted, the solution was acidified by addition of H2SO4
6 M. The solid formed was separated by filtration and treated
with an 6 N NaOH aqueous solution to adjust the pH to ~11.
The dark orange mixture was extracted with diethyl ether, the
organic phases were combined, dried (over Na2CO3), filtered and
the solvent was removed under vacuum. A viscous brown oil was
obtained which was dissolved in n-hexane and filtered to remove
any solid impurities. Solvent removal afforded compound 1 as a
viscous oily dark amber-brown product, which can be purified by
distillation in vacuum. Yield: 3.7 g (66%). 1H NMR (CDCl3, 300
n(N–H) 3437, 3197 cm-1, n(C–H) 3080, 2918 cm-1, d(N–H) 1578
cm-1, d(C–H) 806 cm-1, r(Fe–ring) and n(Pt–N) 487 cm-1, n(Pt–Cl)
330, 321 cm-1. MS (ESI+): m/z 501.0 [M–Cl]+.
Synthesis of cis-chloro(dimethylsulfoxide)[1,1¢-bis(b-amino-
ethyl)ferrocene] platinum(II) (4). A solution of 2 (200 mg,
0.74 mmol) in 40 mL of dry CH2Cl2 was added, via cannula,
to a solution of cis-[PtCl2(dmso)2] (207.3 mg, 0.491 mmol) in
80 mL of the same solvent. The mixture was stirred at room
temperature for 72 h and the resulting solution was concentrated
under vacuum until the appearance of an orange solid, which
was filtered, washed with CH2Cl2 and acetone and dried under
vacuum to afford the desired compound 4. Yield: 115 mg (40%).
Elemental analysis (%) calcd. for C16H26ClFeN2PtSO: C 33.10,
H 4.52, N 4.83, S 5.51; found: C 32.98, H 4.61, N 4.68, S 5.62.
3
1H NMR ((CD3)2CO, 300 MHz): d 2.87 (t, J = 7.5 Hz, 4H,
3
3
Fc–CH2), 3.12 (q, J = 7.5 Hz, 4H, CH2–NH2), 3.34 (s, JPt-H
= 19.1 Hz, 6H, CH3), 4.19, 4.12 (m, 8H, C5H4), 4.50 (br, 4H,
3
MHz): d 1.30 (br, 2H, NH2), 2.50 (t, J = 7.0 Hz, 2H, Fc–CH2),
2.80 (t, 3J = 7.0 Hz, 2H, CH2–NH2), 4.07, 4.08 (m, 4H, C5H4), 4.10
NH2). 13C{ H} NMR ((CD3)2CO, 75 MHz): d 29.7 (Fc–CH2),
1
1
(s, 5H, C5H5). 13C{ H} NMR (CDCl3, 75 MHz): d 34.1 (Fc–CH2),
42.8 (CH3–S), 46.0 (CH2–NH2), 68.2, 68.7 (C5H4), 85.2 (Cipso).
195Pt NMR ((CD3)2CO, 64 MHz) d -3120. IR (KBr): n(N–H)
3409, 3181 cm-1, n(C–H) 3074, 2912 cm-1, d(N–H) 1583 cm-1,
n(S O) 1124 cm-1, d(C–H) 805 cm-1, r(Fe–ring) and n(Pt–N)
487 cm-1, n(Pt–S) 441 cm-1, n(Pt–Cl) 333 cm-1. MS (ESI+): m/z
580.0 [M+].
43.6 (CH2–NH2), 67.4, 68.4, 86.3 (C5H4), 68.6 (C5H5). IR (CsI):
n(N–H) 3368, 3292 cm-1, n(C–H) 3091, 2927, 2853 cm-1, d(N–H)
1587 cm-1, d(C–H) 818 cm-1, r(Fe–ring) 483 cm-1. MS (FAB+):
m/z 229.1 [M+].
Synthesis of Fe{g5-C5H4(CH2)2NH2}2 (2). Bifunctionalized
2 was synthesized in several steps starting from commercially
available 1,1¢-diacetylferrocene, by adapting the literature proce-
dures. To a room-temperature solution of LiAlH4 (0.82 g, 21.6
mmol) in 34 mL of dry diethyl ether was added dropwise another
solution of AlCl3 (2.82 g, 21.2 mmol) in 40 mL of the same solvent.
The mixture was treated, dropwise, with a third solution of 1,1¢-
bis(cyanomethyl)ferrocene9a (2.30 g, 8.72 mmol) in 128 mL of
diethyl ether. The reaction was refluxed and stirred for 2.5 h.
The resulting homogeneous solution was cooled with an external
bath and, at the same time, cool water was added dropwise to
remove the excess LiAlH4. When no more H2 was released the
addition of water was stopped and a 6 M solution of H2SO4 was
added to protonate the amine group. The aqueous yellow phase
was separated and was brought to pH ~ 11 with a solution of
KOH. The aqueous phase was extracted with diethyl ether and
the combined organic layers were dried with Na2CO3. Filtration
and solvent removal afforded compound 2 as a brown oil that was
dried in vacuum. Yield: 2.04 g (86%).1H NMR (CDCl3, 300 MHz):
d 1.24 (s, 4H, NH2), 2.50 (t, 3J = 6.9 Hz, 4H, Fc–CH2), 2.80 (t, 3J =
Synthesis of cis-dichlorobis[1-b-aminoethylferrocene] plati-
num(II) (5). A solution of 1 (500.5 mg, 2.18 mmol) in dry CH2Cl2
(17 mL) was added, via cannula, to another solution of K2[PtCl4]
(410.0 mg, 0.993 mmol) in a mixture of 17 mL of distilled water and
9 mL of ethanol. The mixture was vigorously stirred at 40 ◦C for
24 h. After the organic phase was extracted, CH2Cl2 was added and
the solution was kept at -30 ◦C for 2 h. The orange solid isolated
was separated by filtration, washed with CH2Cl2 and dried under
vacuum to give compound 5. Yield: 380 mg (53%). Elemental
analysis (%) calcd. for: C24H30Cl2Fe2N2Pt: C 39.83, H 4.18, N 3.87;
found: C 40.04, H 4.11, N 3.68. 1H NMR (dmso-d6, 300 MHz): d
2.63 (m, 4H, Fc–CH2), 2.82 (m, 4H, CH2–NH2), 4.08, 4.13 (m, 8H,
1
C5H4), 4.16 (s, 10H, C5H5), 4.98 (br, 4H, NH2). 13C{ H} NMR
(dmso-d6, 75 MHz): d 30.4 (Fc–CH2), 47.2 (CH2–NH2), 67.6, 68.0
(C5H4), 68.8 (C5H5), 85.5 (Cipso). 195Pt NMR (dmso-d6, 64 MHz):
d -2210. IR (KBr): n(N–H) 3458, 3221 cm-1, n(C–H) 3136, 2914
cm-1, d(N–H) 1581 cm-1, d(C–H) 811 cm-1, r(Fe–ring) and n(Pt–
N) 485 cm-1, n(Pt–Cl) 327, 317 cm-1. MS (ESI+): m/z 723.0 [M+],
688.0 [M–Cl]+.
1
6.9 Hz, 4H, CH2–NH2), 4.01, 4.02 (m, 8H, C5H4). 13C{ H} NMR
(CDCl3, 75 MHz): d 33.6 (Fc–CH2), 43.3 (CH2–NH2), 68.2, 69.1,
86.2 (C5H4). IR (CsI): n(N–H) 3358, 3292, cm-1, n(C–H) 3085,
2922, 2864 cm-1, d(N–H) 1663 cm-1, d(C–H) 818 cm-1, r(Fe–ring)
489 cm-1. MS (FAB+): m/z 272.1 [M+].
Synthesis of cis-dichloro(dimethylsulfoxide)(1-b-aminoethyl
ferrocene)platinum(II) (6). A solution of 1 (500 mg, 2.18 mmol)
in 68 mL of dry CH2Cl2 was added, via cannula, to a second
solution of cis-[PtCl2(dmso)2] (459.4 mg, 1.09 mmol) in 110 mL
of CH2Cl2. The mixture was stirred at room temperature for two
days and the resulting solution was concentrated to ca. 90 mL and
cooled to -30 ◦C. The orange precipitate formed was filtered off,
washed with CH2Cl2 and dried under vacuum. The compound
was purified by column chromatography on silica gel 60 silanized
using AcOEt as eluent to afford compound 6. Yield: 320 mg
(51%). Elemental analysis (%) calcd. for: C14H21Cl2FeNPtSO: C
29.37, H 3.70, N 2.45, S 5.59. Found: C 29.54, H 3.75, N 2.68,
Synthesis of cis-dichloro[1,1¢-bis(b-aminoethyl)ferrocene] plati-
num(II) (3). A solution of 2 (288 mg, 1.06 mmol) in 30 mL of dry
dichloromethane was added under argon to another solution of
K2[PtCl4] (200 mg, 0.482 mmol) in a mixture of 14 mL of distilled
water and 7 mL of ethanol. The mixture was stirred at room
temperature under argon for 48 h. The solution was concentrated
and the solid formed was filtered, washed with cold CH2Cl2, H2O
and EtOH and dried under vacuum to give compound 3. Yield:
105 mg (38%). Elemental analysis (%) calcd. for C14H20Cl2FeN2Pt:
C 31.28, H 3.75, N 5.22; found: C 31.57, H 3.71, N 5.59. IR (KBr):
1
3
S 5.38. H NMR (CD2Cl2, 300 MHz): d 2.69 (t, J = 7.0 Hz,
2H, Fc–CH2), 3.14 (m, 2H, CH2–NH2), 3.44 (s, 3JPt-H = 25.5 Hz,
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6H, CH3), 3.82 (br, 2H, NH2), 4.13 (m, 4H, C5H4), 4.14 (s, 5H,
8 H.-K. Kim, J.-A. Park, K. M. Kim, Sk. Md. Nasiruzzaman, D.-S. Kang,
J. Lee, Y. Chang and T.-J. Kim, Chem. Commun., 2010, 46, 8442–8444.
9 (a) K. E Gonsalves, R. Lenz and M. D. Rausch, Appl. Organomet.
Chem., 1987, 1, 81–93; (b) K. E Gonsalves and M. D. Rausch, J. Polym.
Sci., Part A: Polym. Chem., 1988, 26, 2769–2775; (c) K. Gonsalves, L.
Zhan-Ru and M. D. Rausch, J. Am. Chem. Soc., 1984, 106, 3862–3863.
10 C. M. Casado, M. Mora´n, J. Losada and I. Cuadrado, Inorg. Chem.,
1995, 34, 1668–1680.
11 (a) I. Cuadrado, M. Mora´n, J. Losada, C. M. Casado, C. Pascual, B.
Alonso and F. Lobete, In Advances in Dendritic Macromolecules, G.
R. Newkome Ed.; JAI Press, Greenwich, Connecticut, 1996; Vol 3, pp.
151–195; (b) I. Cuadrado, M. Mora´n, C. M. Casado, B. Alonso and J.
Losada, Coord. Chem. Rev., 1999, 193–195, 395–445.
1
C5H5). 13C{ H} NMR (CD2Cl2, 125 MHz): d 31.3 (Fc–CH2),
44.3 (CH3–S), 48.4 (CH2–NH2), 68.0, 68.2 (C5H4), 68.7 (C5H5),
83.2 (Cipso). 195Pt NMR (CD2Cl2, 107 MHz) d -3097. IR (KBr):
n(N–H) 3448, 3208 cm-1, n(C–H) 3092, 2914 cm-1, d(N–H) 1566
cm-1, n(S O) 1123 cm-1, d(C–H) 817 cm-1, r(Fe–ring) and
n(Pt–N) 483 cm-1, n(Pt–S) 444 cm-1, n(Pt–Cl) 341, 317 cm-1. MS
(ESI+): m/z 594.9 [M+Na]+.
Conclusions
12 C. M. Casado, I. Cuadrado, B. Alonso, M. Mora´n and J. Losada, J.
Electroanal. Chem., 1999, 463, 87–92.
In summary, a family of new electroactive heterometallic
compounds containing b-aminoethylferrocenyl moieties and
platinum(II) centers has been successfully prepared and charac-
terized. Electrochemical data revealed that all mixed ferrocene–
platinum compounds 3–6 undergo one-electron (for 3, 4 and 6) or
two-electron (for 5) reversible oxidations, and that in trimetallic 5
the two ferrocenyl redox units are electrochemically independent.
In vitro, heterometallic monoferrocenyl compounds 4 and 6 do
not show considerable cytotoxicity against representative human
cancer cell lines. In contrast, trimetallic 5, bearing two redox-active
ethylferrocenyl units linked to the platinum center through NH2
linkages, is active against all cell lines tested, even in the more
resistant colon cancer cell line. Cell cycle studies demonstrate
that the new compounds show a different mechanism of action
to that of the standard anticancer drug cisplatin. Although the
exact biological target remains unknown, ongoing work will shed
light on the specific mechanism of action of the new trimetallic
compound and its scope as an anticancer drug.
13 (a) R. Bosque and C. Lo´pez, Polyhedron, 1999, 18, 135–143; (b) P.
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Acknowledgements
We gratefully acknowledge financial support from the
Spanish Ministerio de Ciencia
e
Innovacio´n, Projects
CTQ2009-09125/BQU, SAF2009-09431 and CTQ2008-06806-
C02-01/BQU, MSC (RTICC RD06/0020/1046), the Canary
Islands ACIISI (PI 2007/021) and FUNCIS (PI 43/09). S. B.
thanks the Spanish Ministerio de Educacio´n y Ciencia for a FPU
grant and L. G. L. thanks the MSC-FIS for an S. Borrell contract.
17 For recent interesting examples of heterometallic ferrocene-platinum
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