Syntheses and structural studies of platinum(II) complexes of O-methylselenomethionine and related ligands

Article abstract of DOI:10.1016/j.ica.2006.03.008

The complexes dichloro[2-(phenylselanyl)ethanamine]platinum(II), dichloro[2-(benzylselanyl)ethanamine]platinum(II) and dichloro(O-methylselenomethionine)platinum(II) have been prepared and the structure of dichloro(O-methylselenomethionine)platinum(II) has been determined by single crystal X-ray diffraction. The Pt(II) is in a square planar environment and is coordinated by two cis chloride ligands and a chelating O-methylselenomethionine ligand. The cytotoxicities of the compounds have been assessed in the human cell lines HeLa and K562 and they are at least threefold less toxic than cisplatin in both cell lines.

Full text of DOI:10.1016/j.ica.2006.03.008

Inorganica Chimica Acta 359 (2006) 3252–3256
www.elsevier.com/locate/ica
Syntheses and structural studies of platinum(II) complexes of
O-methylselenomethionine and related ligands
a
a
b
b
Michael Carland , Brendan F. Abrahams , Teresa Rede , Jodie Stephenson ,
b
c
a,
*
Vincent Murray , William A. Denny , W. David McFadyen
a
School of Chemistry, The University of Melbourne, Melbourne, Vic. 3010, Australia
School of Biochemistry and Molecular Genetics, University of New South Wales, Sydney, NSW 2052, Australia
b
c
Auckland Cancer Society Research Centre, Faculty of Medicine and Health Science, The University of Auckland, Private Bag 92019, Auckland,
New Zealand
Received 13 January 2006; received in revised form 8 March 2006; accepted 10 March 2006
Available online 20 March 2006
Abstract
The complexes dichloro[2-(phenylselanyl)ethanamine]platinum(II), dichloro[2-(benzylselanyl)ethanamine]platinum(II) and dichloro(O-
methylselenomethionine)platinum(II) have been prepared and the structure of dichloro(O-methylselenomethionine)platinum(II) has
been determined by single crystal X-ray diffraction. The Pt(II) is in a square planar environment and is coordinated by two cis chloride
ligands and a chelating O-methylselenomethionine ligand. The cytotoxicities of the compounds have been assessed in the human cell lines
HeLa and K562 and they are at least threefold less toxic than cisplatin in both cell lines.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Selenium–nitrogen chelates; O-Methylselenomethionine; Platinum(II) complexes, Synthesis; Crystal structure
1. Introduction
reported by Faraglia and Fregona [5] who showed that
complexes of the form M(sem)X2 (M = Pd(II), X = ClÀ
or BrÀ; M = Pt(II), X = ClÀ) (for example 2) were readily
formed when MX2 reacted with selenomethionine. Struc-
turally related complexes derived from methionine, such
as 3, have also been described and the cytotoxic properties
and in vivo activity of this compound have been examined
[6]. In a recent report we have described our structural
studies on hydrogen-bonded networks formed by dimeriza-
tion of platinum(II) dichloro complexes formed from the
ligands 2-(phenylselanyl)ethanamine (4) and 2(benzylsela-
nyl)ethanamine (5) [7]. These complexes, like those of
composition M(sem)X2, also contain a bidentate ASe-
(CH2)nANH2 ligand system. In the present work, we
describe the preparation of these ligands and the corre-
sponding platinum (II) dichloro complexes 10 and 11. We
also describe the synthesis of O-methylselenomethionine
6, the corresponding platinum(II) complex 12 [dichloro-
(O-methylselenomethionine)platinum(II)] as well as the
crystal structure obtained for this complex.
For some time now there has been considerable interest
in the use of sulfur containing ligands as platinum rescue
agents and it has been shown, for instance, that L-MetH
may be used to reduce cisplatin toxicity [1–3]. Given the
chemical similarities between sulfur and selenium it is not
surprising that selenium containing compounds have also
been exploited in an effort to reduce the toxicity of plati-
num drugs and selenomethionine 1 has been shown to
reduce the renal toxicity of cisplatin in rats and mice [4].
It is with these considerations in mind that we have begun
to investigate the chemistry of platinum(II) with bidentate
ligands containing both amine and selenium donor groups.
The preparation of the palladium(II) and platinum(II)
dihalide complexes of selenomethionine 1 has been
*
Corresponding author. Tel.: +61 3 8344 4222; fax: +61 3 9347 5180.
E-mail addresses: wdmf@chemistry.unimelb.edu.au, wdmf@unimelb.
edu.au (W.D. McFadyen).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.03.008

M. Carland et al. / Inorganica Chimica Acta 359 (2006) 3252–3256
3253
2. Experimental
225.57; C14H21NO2Se requires: C, 53.50; H, 6,73; N,
4.46. Found: C, 53.54; H, 6.68; N, 4.39%.
2.1. General
2.4. Synthesis of dichloro[2-(phenylselanyl)ethanamine]-
platinum(II) (10)
NMR spectra were obtained on a Varian Unityplus 400
spectrometer at 400 MHz for 1H spectra, 100 MHz for 13
C
spectra, 85 MHz for 195Pt spectra and 76.2 MHz for 77Se
spectra. 195Pt spectra were externally referenced to potas-
sium tetrachloroplatinate at À1630 ppm. 77Se spectra were
externally referenced to diphenyldiselenide at 464 ppm.
195Pt spectra were recorded in N-methylpyrrolidinone
(NMP). ESI-MS spectra were recorded on a quadrupole
ion trap Finnigan-MAT LCQ mass spectrometer equipped
with electrospray ionisation (ESI). Melting points were
determined on a Gallenkamp melting point apparatus and
are uncorrected. Elemental analyses were performed by the
Campbell Microanalytical Laboratory, University of Otago,
Dunedin, New Zealand and by CMAS, Belmont, Vic.
2-(Phenylselanyl)ethanamine (4) (340 mg, 1.70 mmol)
was dissolved in water (20 mL) containing 1 drop of conc.
HCl and the pH was adjusted to 7.6 with dilute sodium
hydroxide solution. Potassium tetrachloroplatinate(II)
(776 mg, 1.87 mmol) dissolved in water (10 mL) was added
with immediate formation of a buff-colored solid. The mix-
ture was stirred for 16 h, then the solid was collected via
centrifugation and washed with acetone. The product was
recrystallised by diffusing acetone into a NMP solution of
the complex. This yielded the title complex as yellow crys-
tals, 515 mg (71%); 195Pt NMR (NMP) d À3088 (major),
À2967 (minor); C8H11Cl2NPtSe requires C, 20.61; H,
2.38; Cl, 15.21; N, 3.00. Found: C, 20.48; H, 2.25; Cl,
15.32; N, 2.70%.
2.2. Synthesis of O-methylselenomethionine (6)
This was prepared from L-(+)-selenomethionine in
quantitative yield in an analogous manner to that described
[9] for the preparation of O-methylmethionine and was
characterised as the hydrochloride salt 9. 1H NMR
(D2O) d 1.98 (s, 3H), 2.24 (m, 2H), 2.63 (t, 2H,
J = 4.2 Hz), 3.81 (s, 3H), 4.27 (t, 1H, J = 3.8 Hz) 13C
NMR (CDCl3) d 3.91, 21.01, 34.12, 52.01, 53.93, 175.88;
77Se NMR (D2O) d 78.50; MS m/z (relative intensity)
212.0 (M+H, 47), 195.0 (91) 151.9 (100); C6H14ClNO2Se
requires: C, 29.20; H, 5.47; N, 5.54. Found: C, 28.79; H,
5.38; N, 5.46%. HRMS (M+H) calculated: 212.0190.
Found: 212.0194.
2.5. Synthesis of dichloro[2-(benzylselanyl)ethanamine]-
platinum(II) (11)
2-[(Phenylmethyl)selanyl]-N-(boc)ethanamine (8) (361
mg, 1.15 mmol) was dissolved in methanol (10 mL) and
added dropwise to a solution of HCl in methanol/methy-
lacetate (40 mL) (prepared through dropwise addition of
acetyl chloride to methanol). The mixture was stirred at
room temperature for 3 h, at which time TLC indicated
the absence of starting material. The solvents were
removed under reduced pressure to afford a solid, which
was used immediately without further purification. The
solid was dissolved in water (50 mL) and the pH adjusted
to 7.6 with dilute NaOH solution at which time the solu-
tion became cloudy. Potassium tetrachloroplatinate(II)
(524 mg, 1.26 mmol) dissolved in water (10 mL) was added
to this stirred solution with prompt formation of a pink-
grey precipitate. Stirring was continued in the dark for
16 h, then the precipitate was collected and washed with
water and then acetone. The product was recrystallised
by diffusing acetone into a DMF solution of the complex.
This yielded the title complex as yellow prisms (monohy-
drate), 388 mg (68%); 195Pt NMR (NMP) d À3057 (major),
À2745 (minor); C9H15Cl2NOPtSe requires: C, 21.70; H,
3.03; N, 2.81; Cl, 14.23. Found: C, 21.41; H, 2.77; N,
2.94; Cl, 14.93%.
2.3. Synthesis of N-tert-butoxycarbonyl-2-
[(phenylmethyl)seleno]ethanamine (8)
Dibenzyldiselenide (1.00 g, 2.94 mmol) was suspended
in stirred ethanol (100 mL) and enough sodium borohy-
dride was added to discharge the intense yellow color, at
which time the cloudy mixture became pale yellow. 2-
Bromo-N-(boc)ethanamine (7) [8] (1.30 g, 5.70 mmol) dis-
solved in ethanol (10 mL) was added and the mixture
stirred under a nitrogen atmosphere at ambient tempera-
ture for 5 h. The reaction was quenched with 10% aqueous
NaHCO3 solution (50 mL), then most of the solvents
removed under reduced pressure. The residue was parti-
tioned between water and diethyl ether and the collected
solvent layers dried (MgSO4). The solvent was then
removed and the residue was then subjected to flash chro-
matography (1:2 ethyl acetate–hexane) followed by
removal of solvent to afford the title compound as a pale
yellow solid, 1.60 g (88%), m.p. 46–47 ꢁC; 1H NMR
(CDCl3) d 1.41 (s, 9H), 2.58 (t, 2H, J = 3.6 Hz), 3.33 (m,
2H), 3.78 (s, 2H), 4.84 (bs, 1H), 7.2–7.35 (m, 5H); 13C
NMR (CDCl3) d 24.07, 26.84, 28.35, 40.18, 64.40, 126.79,
128.53, 128.78, 138.98, 155.62; 77Se NMR (CDCl3) d
2.6. Synthesis of dichloro(O-methylselenomethionine)-
platinum(II) (12)
O-Methylselenomethionine hydrochloride (9) (190 mg,
0.770 mmol) was dissolved in water (10 mL) and the pH
adjusted to 8 with dilute NaOH solution, at which time
the stirred solution became cloudy. Potassium tetrachloro-
platinate(II) (351 mg, 0.846 mmol) dissolved in water
(10 mL) was added and a solid began to precipitate after

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M. Carland et al. / Inorganica Chimica Acta 359 (2006) 3252–3256
15 min. Stirring was continued overnight, then the solid
was collected and washed with water and acetone, then
recrystallised from NMP–acetone to afford the title com-
pound as colorless crystals, 209 mg (57%); 195Pt NMR
(NMP) d À2978, À2950; C6H13Cl2NO2PtSe requires C,
15.14; H, 2.75; N 2.94. Found: C, 15.07; H, 2.92; N, 2.93%.
a
Boc
Br
Boc
SeBn
N
N
H
H
8
7
SeMe
SeMe
SeMe
b
c
2.7. X-ray crystallography
H3N
CO2Me
H2N
CO2H
H2N
CO2Me
Cl
1
6
9
Single crystal X-ray data of a pale yellow crystal of 12
(0.04 · 0.06 · 0.40 mm) were collected on a Bruker CCD
Scheme 1. Reagents and conditions: (a) Bn2Se2/NaBH4, EtOH, 88%; (b)
MeOH, amberlyst-15, 68%; (c) HCl/MeOH, 2 min, 100%.
˚
diffractometer using Mo Ka radiation, k = 0.71073 A
(graphite monochromator) at 293(2) K. The crystal struc-
ture was solved using direct methods and refined using a
full-matrix least-squares refinement procedure based upon
F2 using all data [10].
thetic routes to the other ligands are given in Scheme 1. The
ligand 2-(benzylselanyl)ethanamine (5) was prepared from
reaction of 2-bromo-(N-boc)ethanamine (7) [8] with
sodium benzylselenolate (prepared through reduction of
dibenzyldiselenide with sodium borohydride), followed by
acidic deprotection. As both the free amine and the hydro-
chloride salt of 5 proved hygroscopic and difficult to handle
the ligand was characterized as its Boc protected precursor
8. The ligand 5 was generated in situ by acid deprotection
of 8 prior to its use in the synthesis of the platinum com-
plexes. O-Methylselenomethionine (6) was prepared via
acid-catalysed esterification of selenomethionine 1 with
methanol and amberlyst-15 [9]. This compound also
proved difficult to handle and as a result full characterisa-
tion was performed on its hydrochloride salt 9.
Crystal data and structure determination details for
12: Formula C6H13NO2Cl2SePt; M = 476.1; monoclinic,
˚
P21, a = 8.8109(15), b = 7.2134(13), c = 9.1222(16) A;
3
˚
b = 96.549(3)ꢁ; V = 575.99(17) A , Z = 2; Calc. den-
sity = 2.745 g cmÀ3; l = 15.77 mmÀ1; F(000) 436; Mo Ka-
radiation: 2hmax = 55ꢁ; 3635 reflections measured; 2344
independent reflections (all included in refinement); param-
eters = 118; Flack parameter = À0.006(14); goodness-of-
fit = 0.98; wR2 = 0.053; R1 = 0.026; maximum and minimum
˚
residual eÀle3ctron density = 1.01 (1.00 A from Pt(1)) and
˚
À0.72 e A
.
2.8. Cytotoxicity assays
Reaction of the ligands 4–6 with potassium tetrachloro-
platinate(II) in water at ambient temperature afforded the
dichloroplatinum(II) complexes 10–12. As neutral species,
none of the complexes was soluble in water and as such
were readily isolated and yielded satisfactory elemental
analyses. The complexes were of low solubility in common
solvents such as DMF (in which molecules like cisplatin are
soluble). It was found that NMP was the most effective
‘inert’ solvent, although heating and/or sonication was
needed in order to achieve full dissolution. Once dissolved,
the solutions could be diluted with DMF without the for-
mation of any precipitate. The complexes 10–12 could be
recrystallised through diffusion of acetone into the NMP–
DMF solutions of the complexes and in this way crystals
suitable for X-ray diffraction were obtained (see Fig. 1).
The structure determination of 12 indicates the expected
square planar Pt(II) centre (Fig. 2). The Pt centre is coor-
dinated by 2 cis chloride ions and the chelating O-methylse-
lenomethionine ligand, which binds to the metal centre
through the selenium and nitrogen atoms. The Pt centre
RPMI-1640 medium supplemented with 20 mM
HEPES, 10% (v/v) foetal calf serum and NaHCO3
(0.85 g/L) were used to culture the K562 and HeLa cell
lines in at 37 ꢁC with 5% CO2 tension in a humidified incu-
bator. The cells were passaged every 2–3 days.
Exponentially growing K562 cells and HeLa cells were
seeded at 3 · 104 cells per well and 0.5 · 104 cells per well,
respectively, into 96 well microtitre plates in 100 lL ali-
quots. The HeLa cells were incubated at 37 ꢁC overnight
to provide sufficient time to attach. The test compound
was initially dissolved in DMF; 2-fold serially diluted in
RPMI-1640 media; and a volume of 100 ll was added in
triplicate to each cell line. The cells were then incubated
for 3 days at 37 ꢁC with 5% CO2 tension in a humidified
incubator. The cell viability was then assessed via an ala-
mar blue assay (Trek Diagnostic Systems) using fluores-
cence with excitation at 544 nm and emission at 590 nm.
Control wells were treated as above except that 100 ll of
RPMI-1640 media replaced the test compound. The IC50
was defined as the concentration that resulted in 50% inhi-
bition of growth. An average IC50 was determined based
on at least two independent experiments [11].
˚
lies within 0.043(4) A of the mean plane defined by the four
donor atoms. The bite angle of the chelating ligand is
95.6(1)ꢁ. The chelate ring is somewhat puckered, with
atoms C(2), C(3) and C(4) lying 0.78(1), 0.26(1) and
˚
0.75(1) A respectively from the mean plane defined by the
3. Results and discussion
four coordinated atoms. The platinum to donor bond dis-
tances, (Pt(1)ASe(1) 2.3697(8), Pt(1)ACl(1) 2.294(2),
Pt(1)ACl(2) 2.322(2) and Pt(1)AN(1) 2.043(5) A) all fall
The ligand 2-(phenylselanyl)ethanamine (4) was pre-
pared according to a published procedure [12] and the syn-
˚

M. Carland et al. / Inorganica Chimica Acta 359 (2006) 3252–3256
3255
the signals. On the basis of the crystal structure we would
expect only a single resonance. We note however, that
the environment of the Se atom is pyramidal with the
methyl group lying above the plane of the donor atom
set (as do all the other non-hydrogen atoms of the com-
plex). We propose that in solution the methyl group on
the selenium atom occupies one of two energetically similar
positions; one above the plane and one below the plane
resulting in the existence of two diastereoisomers [5]. In
compounds 10 and 11 the terminal R group (R = phenyl
in 10; R = benzyl in 11) on the pyramidal Se centre may
lie above or below the plane of the donor atom set. The
two sides of this plane are intrinsically different owing to
the expected puckering of the 5-membered chelate ring
and thus two distinct diastereoisomers are possible in each
case (Fig. 3). As with compound 12, the 195Pt NMR spectra
of 10 and 11 also provide evidence of diastereomeric forms.
Compounds 10 and 11 each possess an intense signal
around À3000 ppm in addition to a smaller secondary peak
in each case at À2967 ppm for 10 and À2745 ppm for 11,
consistent with a second, less favorable diastereomeric
geometry. It is also possible that the presence of two peaks
in the 195Pt spectra of these compounds is due to NMP
entering the coordination sphere of platinum. This seems
unlikely as NMP is expected to be a poor ligand for
platinum.
SeMe
CO2H
MeO2C
H2N
HO2C
H2N
SMe
SeMe
Pt
Pt
Cl
Cl
H2N
Cl
Cl
1
2
3
SeMe
SePh
SeBn
H2N
H2N
4
H2N
CO2Me
5
6
MeO2C
H2N
Cl
Se
Cl
H2N
Se
Cl
H2N
SeMe
Ph
Bn
Pt
Pt
Pt
Cl
Cl
Cl
10
11
12
Fig. 1. Selenium containing ligands 4–6, derived complexes 10–12, and
related compounds 1–3.
The cytotoxicity of the selenium-containing compounds
11 and 12 was investigated using the two human cells lines,
HeLa and K562. The IC50 values (Table 1) indicate that the
selenium-containing complexes 11 and 12 are cytotoxic
although at least threefold less so than cisplatin in both
these cell lines. The sulfur analogue of 12, that is, complex
[Pt(O-methylmethionine)Cl2] 3 has been reported to show
moderate in vivo antitumour activity [6].
We also prepared the known complex dichloroseleno-
methionineplatinum(II) (2) [5], which we found to be effec-
Cl
Cl
Fig. 2. ORTEP diagram of 12. Ellipsoids are at the 50% probability level.
R
R
Cl
Pt
Se
Cl
Pt
Se
within expected values. The complex is essentially flat, with
no non-hydrogen atom further than 1.5 A from the mean
˚
H2N
H2N
plane defined by the four donor atoms. Interestingly, all
non-hydrogen atoms of the complex lie on the same side
of the plane defined by the metal atom donor set. The com-
plexes stack along a 21-axis (the b-axis) with the Pt centres
Fig. 3. The two diastereoisomeric forms of compounds 10 and 11.
˚
displaced by 0.70 A from the axis. Platinum centres belong-
Table 1
Cytotoxicity data in the human K562 and HeLa cell lines
˚
ing to adjacent complexes within the stack are 3.87 A
apart. Perhaps the most significant interaction between
complexes within the stack is between Se(1) and Cl(2) cen-
tres which are separated by 3.54 A.
Cisplatin (lM)
Complex 10
Complex 11 (lM)
Complex 12 (lM)
IC50 in K562 cells
9
nda
34
48
˚
The 195Pt NMR spectrum of compound 12 shows two
signals of comparable size within 30 ppm of each other at
d À2978 and À2950 ppm. The relatively small separation
between the signals suggests that the Pt centre is in a very
similar environment in each of the species represented by
IC50 in HeLa cells
16
nda
>65b
>65b
a
Not determined (due to the low solubility of this complex).
The solvent DMF is cytotoxic to HeLa and K652 cells at an equivalent
b
IC50 concentration present in 65 lM solutions of the complexes 11 and 12.

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M. Carland et al. / Inorganica Chimica Acta 359 (2006) 3252–3256
tively insoluble in DMF, NMP and other non-coordinating
solvents as well as in water, and this behaviour precluded
an examination of its cytotoxic properties.
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Copies of the data can be obtained, free of charge, on
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bridge CB2 1EZ, UK (fax: +44 1223/336 033 or e-mail:
deposit@ccdc.cam.ac.uk).
Acknowledgment
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Support of this work by The National Health and Med-
ical Research Council (W.D. Mc F., V.M., W.A.D.) is
gratefully acknowledged.