Multinuclear NMR spectroscopy and antiproliferative activity in vitro of platinum(II) and palladium(II) complexes with 6-mercaptopurine

Article abstract of DOI:10.1016/j.molstruc.2004.07.027

A series of Pd(II) and Pt(II) complexes with 6-mercaptopurine (6-Hmp) of formulae Pd(6-Hmp)₂Cl₂ (1), Pd(6-mp)₂· 2H₂O (2), Pt(6-mp)₂·2H₂O (3), Pt(6-mp)(dmso)Cl (4) was synthesized and studied by IR, far-IR, ¹H, ¹³C, ¹⁵N NMR. (1) has an ionic character and consists of distinct [Pd(6-Hmp)₂]²⁺ cations and uncoordinated Cl ^- anions, whereas (2,3) are neutral species with central atoms bis-chelated by the deprotonated 6-mp^- ligands. NMR studies suggest that S and N(7) are the complexation sites, while far-IR spectra indicate the square-planar geometry of Pd(II) or Pt(II). In (4) the Pt(II) atom is coordinated by one chelating 6-mp^- anion, S-bonded dmso molecule and a terminal chloride. The antiproliferative activity in vitro of (2-4) was tested against human leukaemia HL-60 cells, being exhibited for (2) at the level ca. six times lower than in case of cisplatin.

Full text of DOI:10.1016/j.molstruc.2004.07.027

Journal of Molecular Structure 707 (2004) 241–247
www.elsevier.com/locate/molstruc
Multinuclear NMR spectroscopy and antiproliferative activity in vitro
of platinum(II) and palladium(II) complexes with 6-mercaptopurine
a,
Iwona Łakomska , Leszek Pazderski , Jerzy Sitkowski , Lech Kozerski ,
a
b,c
b
*
d
d
d
Marzena Pełczy n´ ska , Anna Nasulewicz , Adam Opolski , Edward Szłyk
a
a
Faculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87-100 Toru n´ , Poland
b
National Institute of Public Health, Chełmska 30/34, 00-725 Warsaw, Poland
c
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
d
Department of Experimental Oncology, Institute of Immunology and Experimental Therapy, Polish Academy of Sciences,
R. Weigla 12, 53-114 Wrocław, Poland
Received 30 April 2004; revised 16 July 2004; accepted 20 July 2004
Available online 2 September 2004
Abstract
A series of Pd(II) and Pt(II) complexes with 6-mercaptopurine (6-Hmp) of formulae Pd(6-Hmp)
2
Cl
2 2 2
(1), Pd(6-mp) $2H O (2),
1
Pt(6-mp) $2H O (3), Pt(6-mp)(dmso)Cl (4) was synthesized and studied by IR, far-IR, H, C, N NMR. (1) has an ionic character and
13
15
2 2
2
consists of distinct [Pd(6-Hmp) ] cations and uncoordinated Cl anions, whereas (2,3) are neutral species with central atoms bis-chelated
K
by the deprotonated 6-mp ligands. NMR studies suggest that S and N(7) are the complexation sites, while far-IR spectra indicate the square-
C
K
2
K
planar geometry of Pd(II) or Pt(II). In (4) the Pt(II) atom is coordinated by one chelating 6-mp anion, S-bonded dmso molecule and a
terminal chloride. The antiproliferative activity in vitro of (2–4) was tested against human leukaemia HL-60 cells, being exhibited for (2) at
the level ca. six times lower than in case of cisplatin.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Platinum complex; Palladium complex; 6-mercaptopurine; Multinuclear NMR; Antiproliferative activity in vitro
II
1
. Introduction
[M (6-Hmp) ]X (nZ2,3) compounds [3,7]. In the case
n
2
K
of deprotonated 6-mp
M(6-mp) $xH O type were obtained [8–10]. Both 6-Hmp
anions, the species of
6
-mercaptopurine, formally often drawn as 6-purinethiol,
in the solid state occurs as a thione species with the two
2
2
K
and 6-mp ligands were usually coordinated via S and
H1,H7
mobile protons located at 1, 7 positions (6-Hmp
) [1,2],
N(7) although the other coordination modes were also
H1,H9
II
reported: [Hg (6-Hmp-S,
II
while in the majority of solvents the equilibrium of H(1),
H(7) and H(1), H(9) tautomers was noted (Fig. 1) [3–5].
This polydentate heterocycle was proved to be either N- or
S-donor. It could bind transition metal ions with a large
variety of possible coordination modes, often simultaneously
) Cl ] [11] or {[Cd (m-6-
2 2
0
7 H9
mp-S,S ,N , )
]$2H
O}
[3].
2
2
n
The discussed heterocycle is often used as a drug in the
treatment of human leukemias [12,13]. The impact of metal
coordination on the biochemistry of 6-Hmp and its
analogues, such as 2-amino-6-mercaptopurine (6-thiogua-
nine, ampH) was described as well. The studies of
Pd (amp) Cl (ampH) and Pt(amp) $3H O in the L1210
C
K
2-
with protonation (6-H mp ), deprotonation (6-mp , 6-mp )
2
or intramolecular proton transfer processes [6–11].
2
C
3
4
2
2
2
The complexation of M cations by uncharged 6-Hmp
II
molecules resulted in the formation of [M (6-Hmp) X ] or
lymphoid leukemia test system in mice revealed significant
antitumor activity [14]. For other divalent transition metals
2
2
(e.g. Cu(II)), the increase of the effectiveness of 6-mp
binding to RNA was noted [15]. Hence, the studies of
*
Corresponding author. Tel.: C48-56-611-4317; fax: C48-56-654-
477.
E-mail address: dziubek@umk.pl (I. Łakomska).
K
-Hmp/6-mp interactions with metal ions may be useful
6
in the design of slow-release and long-acting drugs
2
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.07.027

242
I. Łakomska et al. / Journal of Molecular Structure 707 (2004) 241–247
Fig. 1. Thiol and thione forms of neutral 6-mercaptopurine (6Hmp).
and the synthesized new complexes can be potentially used
as new release pro-drugs.
During the present work we have focused at the
unambiguous assignment, were reported for the free ligand
[19]. For the latter species we have generally confirmed
the literature results, additionally determining the full set of
K
6
-Hmp/6-mp coordination compounds of Pd(II) and
JCH coupling constants, that was useful for determining the
actual electronic structure of 6-Hmp and the location of
both mobile imine protons within the molecule in the
1
solution. We have also performed the low-temperature H
Pt(II). Similar complexes were reported by Grinberg et al.
10], who synthesized the compounds of M(6-Hmp) Cl ,
M(6-Hmp) SO , M(6-Hmp)(6-mp)Cl and M(6-mp) type
[
2
2
2
4
2
(
Bariyanga et al. [16] reported Pt(6-Hmp) Cl $dmf and
MZPd, Pt), but he measured only their IR spectra.
NMR measurements of 6-Hmp, which exhibited the
separation of the two tautomeric forms.
2
2
1
characterized it with IR and H NMR spectroscopy.
The only X-ray structures were described by Heitner et al.
For the complexes (1–4) we have discussed the problem
of chemical shift changes in relation to the Pd(II) or Pt(II)
metallation and simultaneous deprotonation or intramole-
cular proton transfer processes. To our best knowledge, the
above class of compounds was never studied from this point
of view. We have also tested their antiproliferative activity
in vitro, proving the antitumour properties for the
species (2).
II
7 H1
0
17] for cis-[Pd (9-benzyl-6-mp-S,N , ) ]$(N,N -
2
[
dimethylacetamide) and by Dubler [18] for trans-
II
7 H9
[
of the above species were studied by multinuclear NMR
Pt (6-mp-S,N , ) ]$2H O. It is worth noting that none
2 2
1
3
15
spectroscopy, particularly no data on their C and
chemical shifts and assignments were available.
N
1
13
15
In this paper we report the H, C and N NMR
spectroscopic studies offour Pd(II) and Pt(II) complexes with
neutral and deprotonated 6-mercaptopurine ligands, having
the formulae of Pd(6-Hmp) Cl (1), Pd(6-mp) $2H O (2),
2. Experimental
2
2
2
2
Pt(6-mp) $2H O (3), Pt(6-mp)(dmso)Cl (4) (Fig. 2). We
2
2.1. Materials
2
1
have assigned the C and N NMR signals for both 6-Hmp
3
15
1
and the above coordination compounds using the H{ C}
13
PdCl2 (99.9%) was purchased from POCh Gliwice
(Poland); K PtCl (98%) and 6-mercaptopurine monohy-
drate (6-Hmp$H O, 99%) from Aldrich; K PdCl was
1
13
1
15
HSQC, H{ C} HMBC and H{ N} HMQC two-dimen-
sional heteronuclear correlation techniques; formerly
2
4
2
2
4
1
only one-dimensional C data, in fact not allowing for
3
prepared from PdCl2 and KCl; cis-[Pt(dmso) Cl ] as
2 2
Fig. 2. Schematic presentation of the structure types of metal complexes used in study.

I. Łakomska et al. / Journal of Molecular Structure 707 (2004) 241–247
243
reported [20]. Anhydrous 6-Hmp was obtained by 24 h
heating of 6-Hmp$H O at 200 8C.
thermogravimetric analyses (mass decrement below 370 K of
ca. 8 and 6.5%, respectively).
2
2
.2. Methods
2.4. Cells
The content of C, H, N was determined by elemental
HL-60 (human acute promyelocytic leukemia) cell line
was obtained from European Type Culture Collection by
courtesy of Prof. Spik and Dr Mazurier (Laboratory of
Biological Chemistry USTL, Lille, France) and maintained
in the Head Institute of Immunology and Experimental
Therapy, Wroclaw, Poland. The cells were cultured in
RPMI medium supplemented with glutamine, antibiotics
and 10% fetal bovine serum.
semi-microanalysis, Pd gravimetrically as palladium(II)
bis(dimethylglioximate), Pt spectrophotometrically as
H PtCl on Specord M40 Carl Zeiss Jena (lZ400 nm).
2
6
Thermogravimetric analyses were performed with a
SDT-2960 TA Instruments (samples 5 mg, heating range
3
spectra were measured with a Perkin–Elmer Spectrum-2000
00–1000 K, heating rate 4 deg/min, N atmosphere). IR
2
K1
FT-IR spectrometer, using KBr (400–4000 cm ) and
K1
polyethylene discs (100–400 cm ). The H{ C} HSQC,
1
13
2.5. Biological methods
1
13
H{ C} HMBC and H{ N} HMQC spectra were
1
15
performed with a Varian INOVA-500 NMR spectrometer
equipped with an inverse Nalorac Z-gradient shielded
probe. The solvent was dmf-d (1) or dmso-d (2–4), the
Cells were patted in 96-well sterile plates (Sarstedt,
4
Costar) at the density of 10 cells per well (in 100 ml of
culture medium) and incubated for 24 h. The tested
complexes (2–4) were added to the final concentrations
3
from 0.1 to 100 mg/cm and the incubation continued
additional 72 h. Then 20 ml of MTT solution (MTT: 3-(4,5-
dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide,
3
Sigma) in the concentration of 5 mg/cm was added to
7
6
concentration of saturated solutions being ca. 0.01–0.05 M,
the temperature being 303 K. The reference standard was
1
TMS for H and C; CH NO for N.
13
15
3
2
2
.3. Synthesis
1) was obtained by the reaction of 0.326 g (1 mmol) of
each well and incubated for next 4 h. The mitochondria of
viable cells converted a pale yellow MTT to a navy blue
formazan. When the incubation time was completed, 80 ml
of the lysing mixture was added to each well (lysing
(
K PdCl , dissolved in 25 cm of 2 M HCl, with 0.340 g
3
2
4
3
(2 mmol) of 6-Hmp$H O in 25 cm of methanol. The
2
reaction mixture was heated to 60 8C and stirred for 3 h,
0
3
mixture: 225 cm of dimethylformamide, 67.5 g of sodium
3
dodecyl sulfate (Sigma), 275 cm of water). After 24 h the
3
.5 cm of 1 M HCl being added each 5 min. The
precipitated orange solid was washed with 1 M HCl,
methanol and dried in vacuum.
formazan crystals were dissolved and the optical densities of
all samples measured with an Multiskan RC photometer
(Labsystems) at 570 nm wavelength. The ID50 values (i.e.
the complex concentration that inhibits proliferation rate of
tumor cells by 50%, comparing to the untreated control
cells) were calculated from curves constructed by plotting
cell survival (%) versus compound concentration (mM). All
samples were tested in triplicates and experiments were
repeated at least three times.
(
1 mmol) of K PdCl or 0.415 g (1 mmol) of K PtCl in
2) and (3) were synthesized by the reaction of 0.326 g
(
2
4
2
4
3
5 cm of water, with 0.340 g (2 mmol) of 6-Hmp$H O in
2
2
2
3
5 cm of methanol. The reaction mixtures were stirred at
room temperature for 2 h. The precipitated yellow solids
were washed by methanol, diethyl ether and dried in
vacuum.
(
cis-[Pt(dmso) Cl ], dissolved in 40 cm of methanol, with
4) was obtained by the reaction of 0.422 g (1 mmol) of
3
2
2
3
.170 g (1 mmol) of 6-Hmp$H O in 15 cm of the same
0
3. Results and discussion
2
solvent. The reaction mixture was stirred at room tempera-
ture for 4 h. The yellow precipitate was filtered, washed
with methanol, aceton, diethyl ether and dried in vacuum.
The elemental analysis of (1) corresponds to Pd(6-
Hmp) Cl formula (%; calc. for C H Cl N S Pd/found): Pd
3.1. NMR spectroscopy
(1–4) are insoluble in CDCl but sparingly soluble in
3
1
dmso-d or dmf-d . Their H NMR spectra exhibit the
2
2
10
8
2
8
2
6
7
(
(
22.1/21.7), C (24.9/25.3), H (1.7/2.0), N (23.3/23.1); that of
2) to Pd(6-mp) $2H O (%; calc. for C H N O S
2 2-
signals of aromatic hydrogens H(2) and H(8) at the chemical
shifts significantly different from those of free 6-Hmp. The
prepared complexCligand mixtures (1:1) reveal two sets of
2
2
10 10
8
Pd/found): Pd (23.9/24.2), C (27.0/27.2), H (2.3/2.2), N
25.2/24.9); that of (3) to Pt(6-mp) $2H O (%; calc. for
C H N O S Pt/found): Pt (36.6/36.4), C (22.5/22.3), H
1
(
H resonances, what proves that in the solution the
2
2
molecules of (1–4) can co-exist with those of 6-Hmp as
distinct species. It excludes the occurrence of fast ligand
exchange processes and indicates the stability of formed
coordination bonding. Moreover, (1–4) appear inert towards
1
0 10 8 2 2
II
(1.9/2.1), N (21.0/20.6); that of (4) to [Pt (6-mp)(dmso)Cl]
(%; calc. for C H ClN OS Pt/found): Pt (42.4/42.0), C
(
7
9
4
2
18.3/18.2), H (2.0/1.9), N (12.2/12.6) The content of two
1
solvated water molecules in (2) and (3) has been confirmed by
both dmso-d and dmf-d as their H NMR spectra do not
6
7

244
I. Łakomska et al. / Journal of Molecular Structure 707 (2004) 241–247
Table 1
1 13
The H and C NMR chemical shifts (d, ppm) of 6-Hmp and (1–4), in dmso-d
6
7
or dmf-d (coordination shifts, DcoordZdcomplexKdligand, are in parentheses)
Compound
H(2)
H(8)
C(2)
C(4)
C(5)
C(6)
C(8)
a
b
b
b
b
6
-Hmp (dmso-d
6
a
)
8.18
8.41
144.6
145.3
–
150.3
152.2
–
128.4
170.9
144.7
145.3
–
6
-Hmp (dmf-d
7
)
8.33
8.49
128.5
172.2
(
(
(
(
1) (dmf-d
7
)
8.64 (C0.31)
8.56 (C0.38)
8.61 (C0.43)
8.63 (C0.30)
9.41 (C0.92)
9.17 (C0.76)
9.40 (C0.99)
9.52 (C1.03)
–
–
2) (dmso-d
3) (dmso-d
6
)
)
154.6 (C10.0)
156.3 (C11.7)
156.5 (C11.2)
147.7 (K2.6)
146.9 (K3.4)
148.2 (K4.0)
134.7 (C6.3)
134.5 (C6.1)
135.7 (C7.2)
170.1 (K0.8)
172.1 (C1.2)
172.9 (C0.7)
143.9 (K0.8)
142.9 (K1.8)
144.1 (K1.2)
6
7
4) (dmf-d )
a
1
Literature data for 6-Hmp: H: H(2) 8.17-8.18; H(8) 8.36-8.37 (dmso-d
13
), [16,21] C: C(2) 144.85/145.62, C(4) 150.91/151.97, C(5) 128.76/129.78, C(6)
6
1
71.25/172.79, C(8) 145.08/145.60 (dmso-d
b
6
/dmf-d
7
) [19].
Accurate d values: C(2) 144.56G0.02, C(8) 144.65G0.02 (dmso-d
6
7
); C(2) 145.28G0.02, C(8) 145.33G0.02 (dmf-d ).
vary with time. The lack of decomposition symptoms
and dmf-d ), however, they can be easily distinguished by
7
1
13
1
15
has prompted us to perform the H{ C} and H{ N}
long-lasting two-dimensional heteronuclear correlation
experiments (HMBC, HSQC, HMQC), which increase the
HSQC due to their one-bond correlation with already
1
assigned H(2) and H(8). The respective JC(2)H(2) and
1
JC(8)H(8) coupling constants are 207.2 and 211.5 Hz. In the
HMBC spectrum the only carbon atom interacting in
a similar way, i.e. via three bonds with both aromatic
1
3
15
sensitivity of C and N detection and allow for signals
assignment.
1
The H and C NMR chemical shifts of free 6-Hmp and
13
3
hydrogens (via three bonds) is C(4) ( J
3
12.7 Hz,
JC(4)H(8) 11.0 Hz); C(5) correlates much stronger with H(8)
C(4)H(2)
(
1–4) are listed in Table 1.
1
Our assignment of H NMR signals has followed the
3
than H(2) ( J
3
C(6) - reversibly ( J
4
6.2 Hz,
J
1.4 Hz), whereas
1.3 Hz). The
C(5)H(8)
C(5)H(2)
4
report of Cini et al. [21], who proved, by selective
deuteration of aromatic hydrogens, that for 6-Hmp the
high-frequency resonance was H(8) and that at low-
frequency was H(2). We mention it because many earlier
literature reports were incorrect—their authors suggested
the reverse sequence of peaks, i.e. H(2) at higher and H(8) at
lower d values [22–24]. This mistake was caused by an
7.3 Hz, J
C(6)H(8)
C(6)H(2)
3
1
obtained sequence of C signals C(6)OC(4)OC(8)O
C(2)OC(5) (decreasing d values) is identical with that
suggested on the basis of one-dimensional NMR methods
3
by Fischer et al. [19] Much larger value of J
3
than
JC(6)H(2) confirms that in free 6-Hmp the N(1) atom is
C(4)H(2)
protonated, while N(3) is bare. Also a higher value of
3
1
erroneous and unauthorized analogy with the H NMR
3
JC(4)H(8) in comparison to JC(5)H(8) indicates that the
spectrum of unsubstituted purine, for which the chemical
shifts actually decreased in order H(6)OH(2)OH(8) [25].
Besides H(2) and H(8), for 6-Hmp we have detected a
broad singlet of the two averaged NH groups (in dmso-d6:
dominating tautomeric form in the dmso-d solution is that
6
of N(7)H, N(9). Hence the electronic structure of the main
tautomer of 6-Hmp is as in the middle of Fig. 1. The above
conclusions are based on the well known fact that in purine
ring systems the three-bond JCH coupling constants are ca.
dZ13.75 ppm, n1/2Z380 Hz; in dmf-d : dZ14.0 ppm,
7
n1/2Z95 Hz; both at 30 8C). It appears in the range typical
for aromatic imine-type hydrogens, its accurate d value
depending on the solvent and sample concentration. Its half-
line width increases with temperature, to 230 Hz at 60 8C
and 465 Hz at 90 8C (in dmf-d ). The solution cooling
7
results, as expected, in the narrowing of this signal to 68 Hz
at 20 8C and 41 Hz at 0 8C. Finally at K40 8C the two sharp
3
–5 Hz larger when the nitrogen atom, located between the
respective carbon and hydrogen nuclei, keeps the lone pair
of electrons, in contrast to the case while it is protonated.
1
13
Our assignment of the H and C NMR spectra of (1–4)
has followed the same way. The deshielding of both
aromatic hydrogens has been observed upon complexation,
this effect being more expressed for H(8) (ca. 0.75–
(
n1/2Z5–10 Hz), surprisingly separated (by ca. 5 Hz)
1
.05 ppm) than H(2) (ca. 0.3–0.45 ppm). It suggests that
resonances appear. An analogous separation, however,
concerns also H(2) and H(8) peaks, each of them consisting
of the two lines shifted by ca. 1–4 Hz. Hence one
can assume that at low temperatures the two distinct
forms of 6-Hmp are detected. In our opinion these may be
the tautomers of 6-Hmp, differing in the distribution of the
mobile NH protons within the heterocyclic ring system (e.g.
N(7)H/N(9) and N(7)/N(9)H), undistinguishable at room
temperature in the NMR timescale.
binding of both types of 6-mercaptopurine ligands occurs
rather in the imidazole than pyrimidine ring.
The H(8) high-frequency coordination shifts increase
with the mass of a platinide ion, being larger for the
complexes of Pt(II) ((3), (4): ca. 1.0–1.05 ppm) than Pd(II)
(
(1), (2): ca. 0.75–0.9 ppm). The simultaneous deprotonation
K
of 6-mercaptopurine ligand (from 6-Hmp in (1) to 6-mp in
(2)) makes the deshielding smaller by ca. 0.15 ppm.
1
3
13
We have measured the C NMR spectra for (2–4), that
We have assigned the C NMR signals of 6-Hmp using
1
13
the H{ C} HSQC and HMBC techniques. The protonated
C(2) and C(8) carbons reveal very close chemical shifts
of (1) being undetectable due to the low solubility of the
1
3
latter complex. The patterns of C coordination shifts
(
their d values differing max. 0.1 ppm in both dmso-d6
are similar, being independent on the metal ion (2,3) or

I. Łakomska et al. / Journal of Molecular Structure 707 (2004) 241–247
245
the presence of auxiliary ligands (4). It suggests the identical
3.2. IR spectroscopy
We have measured the IR spectra of (1–4), 6-Hmp$H O
2
K
coordination mode of 6-mp anions in all species (2–4).
C(2) and C(5) are always deshielded (ca. 10–12 and
6
–7 ppm, respectively), C(4) and C(8) slightly shielded
ca. 3–4 and 1–2 ppm, respectively), the changes at C(6)
being of variable sign and negligible magnitude (ca. G
and anhydrous 6-Hmp, the two latter being already reported
[27]. Their absorption patterns are generally similar
(
K1
within the 400–1700 cm
region, being for (2) and (3)
and
1
1
ppm). The latter carbon atoms reveal the d values of 170–
73 ppm, what indicates their thionic character in (2–4).
nearly identical with those reported for Pd(6-mp)
Pt(6-mp) [10].
The characteristic vibration of the overall 6-mercapto-
2
2
1
Recently we have described the H{ N} HMQC
15
1
spectrum of free 6-Hmp, assigning all N resonances: N(1)
5
purine ring skeleton, n6-Hmp, observed for 6-Hmp$H
O at
(for
2
K1
K1
K189.9, N(3) K144.2, N(7) K205.2, N(9) K159.3 ppm
26]. We have tried to measure analogous spectra for (1–4),
1614 cm
and anhydrous 6-Hmp at 1604 cm
K1
[
unsubstituted purine at 1618 cm ), [27,28] appears at
K1
1
5
K1
detecting, however, the full set of N signals only for the best
soluble complex (2): N(1) K109.6, N(3) K144.2, N(7)/N(9)
K204.8/K214.0 ppm. Their assignment has followed
that for 6-Hmp, i.e. N(1) and N(3) correlate via two
bonds with H(2), while N(7) and N(9) with H(8). The former
two nitrogens can be easily distinguished by the large
difference in their chemical shifts. Comparing to the
free ligand, N(1) is deshielded ca. 80 ppm what reveals
much higher wave numbers: 1647 cm
K1
(1), 1622 cm
(2), 1629 cm (3), 1631 cm (4). This shift is larger for
K1
2
C
K1
cation in (1) (ca. 40 cm ) than for
the [Pd(6-Hmp)2]
K1
neutral species (2–4) (ca. 20–30 cm ), what leads to the
conclusion that n6-Hmp increases due to metal coordination
but decreases upon simultaneous deprotonation. It is
consistent with the literature data for [Pt(6-Hmp)
]Cl
$dmf
mp Cl (1619 cm ) and 6-
2
2
K1
2
K1
dmf (1635 cm ), 6-H
C
K
K
K
C
K1
that the deprotonation of 6-Hmp to 6-mp , simultaneous
mp Na H O (1588 cm ) [16,27].
2
with Pd(II) coordination, occurs just at this site. The d
values of N(3) in 6-Hmp and (2) are identical, what proves
this nitrogen is inactive in the complexation process. N(7)
and N(9) are more difficult to assign as they correlate
with the same H(8) hydrogen and have comparable
chemical shifts. However, their large shielding constants
indicate that both are involved into any chemical bonding,
most likely N(7) with Pd and N(9) with H. Because in case
of free 6-Hmp the dominating tautomer was that of
The nCZS stretching vibration, observed for 6-Hmp$H O
2
K1
K1
and anhydrous 6-Hmp at 1222 cm , [27]
at 1226 cm
appears at 1244 cm (1), 1238 cm (2), 1238 cm (3),
K1
K1
K1
K1
1237 cm (4), confirming the thionic character of sulfur.
K1
The IR coordination shifts of ca. 15–20 cm
to higher
wave numbers are in agreement with our suggestion that S
atom is bonded to Pd(II) or Pt(II).
In the spectrum of (4) two additional, intensive absorption
K1
bands are detected at 1135 and 1024 cm . The latter is most
likely the rCH3 rocking vibration, already reported for neat
N(7)H, N(9) (in dmso-d ) [26], hence for (2) the Pd(II)
6
K1
K1
dmso (1012 cm ) and its vapour (1016, 1006 cm ) [29].
coordination of N(7) must be followed by the intramolecular
proton transfer to N(9).
K1
The former, overlapping with another mode at 1144 cm
derives from the n stretching mode that was found at
,
For (3) we have been able to record only N(1) at
K105.7 and N(3) at K144.2 ppm, the signals of N(7) and
N(9) being undetectable. The two former d values are very
close to those found in case of (2), what suggests the same
coordination/protonation pattern for both Pd(6-mp) $2H O
SaO
K1
K1
1055 cm for liquid dmso and 1102 cm for its vapour
[29]. The increase of nSaO energy, comparing to the free
ligand, proves that the dmso molecule is bonded with the
central atom via sulfur, as it was in trans-Pd(dmso) Cl
2
2
2
2
K1
K1
(
2) and Pt(6-mp) $2H O (3) complexes. As their stoichi-
2
(1116 cm ),
trans-Pt(dmso) Cl
2
(1130 cm
)
2
2
K
K1
and cis-Pt(dmso) Cl (1160, 1140 cm ) [30–32].
2 2
ometry suggests that the 6-mp anions act as bidentate
ligands, keeping for Pd(II) and Pt(II) the typical coordi-
nation number 4, it is evident that also another heteroatom
must participate in the complexation process. In our
opinion the only other available metallation site is the
sulfur atom, which probably completes the Pt/Pd–S–C(6)–
C(5)–N(7) five-membered chelate ring. Such a coordi-
nation mode, via N(7) and S, would not change the thionic
character of sulfur in either (2) or (3), what is reflected by
the respective d values of C(6) being 170.1 and 172.1 ppm,
respectively. It is consistent with the X-ray structural data
Similar wave numbers were reported for trans- and
cis-Pt(thiazole)(dmso)Cl2 complexes, also containing
K1
S-bonded dmso ligand (1150 and 1140 cm , respectively)
[33]. Recently we have detected this band, for a
series of trans-PtL(dmso)Cl2 species (LZ1,2,4-tria-
zolo[1,5-a]pyrimidine or its 5,7-disubstituted derivatives),
K1
at 1142–1147 cm [34]. Assuming an alternative mode of
dmso binding, i.e. via oxygen, it would appear within much
K1
lower 900–1000 cm region [35].
K1
In the range 2300–3100 cm
some other overlapped
II
7 H9
of the complex trans-[Pt (6-mp-S,N , ) ]$2H O, already
2
absorption bands are observed for all complexes (1–4). They
must derive from the nC–H and nN–H stretching vibrations
[36]. (2) and (3) reveal also an additional, broad peak at ca.
2
K
reported by Dubler [18]. The same type of 6-Hmp/6-mp
binding can be also proposed for (1) and (4), basing on the
1
13
K1
similarity of their H and C NMR spectra with those of
2) and (3).
3400 cm , that can be assigned to the nOH mode of the
solvated water molecules [35].
(

246
I. Łakomska et al. / Journal of Molecular Structure 707 (2004) 241–247
3
.3. Far-IR spectroscopy
Table 2
Antiproliferative activity in vitro of (2-4) against the cells of human
leukemia HL-60 line
K1
(
1) reveals nearly no absorption in the 300–400 cm
range, what confirms that the chloride anions are uncoordi-
nated; otherwise the n stretching vibrations should appear
Complex
ID50 [mM]
2
3
4
Cisplatin
Pd–Cl
4.20G2.7
O150
O150
0.71G0.16
2K
K1
(
also do not absorb in this part of far-IR region, whereas in the
e.g. for PdCl it was reported at 321 cm ) [35]. (2) and (3)
4
(ID ) values are listed in Table 2, together with analogous
50
K1
spectrum of (4) a strong nPt–Cl mode is observed at 334 cm
.
Such a wave number is typical for Pt(II) chloride complexes
data for the reference compound cisplatin. No noticeable
biological effects are observed for both platinum(II)
compounds (3,4) at concentrations as high as 150 mM,
while the respective ID50 value for cisplatin is 0.71 mM. In
contrast, the palladium(II) complex (2) displays relatively
low ID50 (4.20 mM) against the leukemia cell line and then it
seems to have a significant cytotoxic activity. However, the
ID50 of (2) is still ca. six times higher that those of cisplatin,
what indicates the former complex has weaker cytostatic
properties than the latter reference compound, at least for
the cell tumor line assayed. Similar activity in vitro was
already observed for some other palladium(II) complexes
K1
2-
4
K1
(
313 cm
for PtCl , 365 cm
K1
for trans-Pt(NH ) Cl ,
3 2 2
K1
3
45 and 331 cm for cis-Pt(NH ) Cl , 342 cm for trans-
3 2 2
K1
Pt(py) Cl , 343 and 329 cm for cis-Pt(py) Cl ) [35,37]. It is
2
2
2
2
also close to the values reported for trans-Pt(dmso) Cl
2
2
K1
K1
(
trans-Pt(thiazole)(dmso)Cl2 (347 cm
351 cm ) and cis-Pt(dmso) Cl (334, 322 cm ) [31],
2 2
K1
) and cis-
Pt(thiazole)(dmso)Cl (345, 314 cm ) [33] as well as a
series of trans-Pt(1,2,4-triazolo[1,5-a]pyrimidine)(dmso)Cl
K1
2
2
K1
species (338–347 cm ) [34].
The n
stretching vibration appears for (4) at
09 cm , being slightly shifted to lower energies
Pt-S(dmso)
K1
3
in comparison to trans-Pt(dmso)l (329 cm ) and
0
with mepirizole [38] and N,N -dialkyl-1,10-phenantroline-
2
K1
2
,9-dimathanamine derivatives [39].
K1
cis-Pt(dmso) Cl (315 cm ) [31]. The intensive band at
2
2
K1
84 cm , absent for (1–3), seems to be the gPtSO
K1
3
deformation, reported for trans-Pt(dmso) Cl at 376 cm
2
2
K1
4. Conclusions
and cis-Pt(dmso) Cl at 379 cm
[31]. Another bending
K1
vibration, dPtSO, found for trans-Pt(dmso) Cl at 419 cm
2
2
2
2
K1
The complex (1) has an ionic character and consists
2C
and cis-Pt(dmso) Cl at 430 cm [31], appears for (4) at
2
2
K1
K1
of distinct [Pd(6-Hmp)
2
]
cations and uncoordinated
ca. 430 cm but heavily overlaps with another 444 cm
mode.
The analysis of far-IR spectra within the 200–300 cm
chloride anions, while (2) and (3) are neutral species of
Pd(6-mp) $2H O (2), Pt(6-mp) $2H O (3) containing
K1
2
2
2
2
15
K
deprotonated 6-mp ligands. Their N NMR studies
suggest the S, N(7) bis-chelation of the heterocyclic rings.
The far-IR spectra of (1–3) indicate at square-planar
geometry of Pd(II) or Pt(II). The species (4) contains the
range is difficult due to the problem with distinguishing of
nM-S(6-mp) and nM–N stretching modes. For (1–3) the
presence of four bands of medium intensity at 301, 284,
K1
K1
2
49, 227 cm (1); 300, 293, 237, 230 cm (2); 270, 263,
K1
K
three types of ligands: one bis-chelated 6-mp anion, a
2
44, 234 cm
(3) seems to confirm the bis-chelated
S-bonded dmso molecule and a terminal chloride. The
compound (2) exhibits antiproliferative activity in vitro
against the cells of human leukaemia line HL-60 with ID50
of 4.20, i.e. ca. six times higher than obtained for cisplatin.
square-planar geometry of Pd(II) or Pt(II). However,
their unambiguous assignment is difficult and then it
is not possible to determine whether (1–3) are cis- or
trans- isomers, especially when consider the lack of
clear geometric preference for the concerned type of
complexes
II
(cis-[Pd (9-benzyl-6-mp-
metal
7
S,N , ) ]$(N,N -dimethylacetamide) versus trans-
H1
0
Acknowledgements
2
II
7 H9
[Pt (6-mp-S,N , ) ]$2H O) [17,18]. The latter problem is
2 2
of course connected to the question of identity of the
I.Ł. and L.P. wish to thank to Polish Committee for
Scientific Research (KBN) grant No 4 T09A 116 23 for
financial support.
complex 3 with the compound reported by Dubler, i.e.
II
trans-[Pt (6-mp-S,N , ) ]$2H O) [18], what, in our
7 H9
2
2
opinion, is very probable, but not proved. The spectrum of
(
K1
4) reveals only two medium bands at 284, 245 cm , what
K
must be caused by the presence of only one 6-mp ligand
within the Pt(II) coordination sphere.
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