Thermal behaviour of platinum(II) complexes of diethyl and monoethyl 2-quinolylmethylphosphonates

Article abstract of DOI:10.1007/s10973-005-0761-z

Thermal study of the molecular and ionic platinum(II) complexes of diethyl (2-dqmp) and monoethyl (2-Hmqmp) ester of 2-quinolylmethylphosphonic acid: dihalide adducts trans-PtL₂X₂ (L=2-dqmp, X=Cl, Br; L=2-Hmqmp, X=Cl), methylquinolin

Full text of DOI:10.1007/s10973-005-0761-z

Journal of Thermal Analysis and Calorimetry, Vol. 81 (2005) 153–157
THERMAL BEHAVIOUR OF PLATINUM(II) COMPLEXES OF DIETHYL
AND MONOETHYL 2-QUINOLYLMETHYLPHOSPHONATES
Lj. Tušek-Boꢀiº* and R. Trojko
Ru»er Boškoviº Institute, Bijeni¹ka 54, 10002 Zagreb, Croatia
Thermal study of the molecular and ionic platinum(II) complexes of diethyl (2-dqmp) and monoethyl (2-Hmqmp) ester of
2-quinolylmethylphosphonic acid: dihalide adducts trans-PtL2X2 (L=2-dqmp, X=Cl, Br; L=2-Hmqmp, X=Cl), methylquinolinium
tetrahaloplatinates [LH+ ]2[PtX24– ] (L=2-dqmp, X=Cl, Br; L=2-Hmqmp, X=Cl, Br), methylquinolinium hexahalodiplatinates
[LH+ ]2[Pt2X62– ](L=2-Hmqmp, X=Cl, Br) and chelate complex Pt(2-mqmp)2⋅2H2O, has been carried out using TG-DTA techniques
and the infrared spectroscopic study. There are great differences in the thermal behaviour between various types of complexes, es-
pecially between the molecular and the ion-pair salt complexes.
Keywords: DTA, phosphonate complex, platinum antitumor agent, platinum(II) complex, quinoline complex, thermal decomposition
Introduction
previously reported for the palladium(II) complexes of
the same phosphonate ligands [9, 10] and discussed
Studies of platinum(II) and palladium(II) complexes
with nitrogen donor ligands are of current interest not
only because of their wide-range application in organic
synthesis and catalysis [1, 2], but also owing to their
biological and pharmacological importance [3, 4]. Fol-
lowing the discovery of the antitumor activity of
cisplatin and its analogues, a great deal of effort was
expanded in developing of new platinum-based com-
plexes with intention of improving the cytotoxicity and
therapeutic properties of this class of compounds. Our
studies in this field are directed to palladium(II) and
platinum(II) complexes of quinoline and aniline-based
alkyl phosphonates, which might be of interest due to
their biological activity [5–8]. A different type of mo-
lecular and ionic metal complexes, such as molecular
dihalide adducts with trans and cis configuration,
mononuclear and binuclear metallocyclic and ion-pair
salt complexes with antitumor activity, have been pre-
pared and investigated.
with respect to their structure-stability relationship.
Experimental
The platinum(II) complexes of 2-dqmp and 2-Hmqmp
were prepared and characterized according to the
published methods [7].
The thermogravimetric analyses (TG) were car-
ried out on a Cahn RG electromicrobalance in air at-
mosphere at a heating rate of 4 K min–1 up to 850°C.
Differential thermal analyses (DTA) were performed
with a Netzsch 406 differential thermal analyzer ap-
plying a heating rate of 5 K min–1 in static air atmo-
sphere. The reference substance was pure alumina.
The samples were diluted with the reference sub-
stance in the ratio 1:1 by mass.
FTIR spectra were recorded on an ABB Bomem
MB102 spectrophotometer using KBr (4000–250 cm–1)
and polyethylene (400–200 cm–1) pellets.
We recently reported the synthesis of platinum(II)
complexes with diethyl (2-dqmp) and monoethyl
(2-Hmqmp) 2-quinolylmethylphosphonates as well as
their spectroscopic analysis and antitumor activity [7].
Most of these compounds have been found cytostatic
in vitro against human and animal tumor cell lines. As
an extension of these investigations, in the present pa-
per we describe the decomposition behaviour of these
biologically interesting metal complexes investigated
by thermogravimetry (TG) and differential thermal
analysis (DTA) accompanied by the IR spectroscopic
studies. The results obtained were compared with those
The X-ray powder diffraction patterns were
taken with a Philips counter diffractometer (mono-
chromatised CuK radiation).
α
Results and discussion
Investigations of the interaction of diethyl ester and
monoethyl ester of 2-quinolylmethylphosphonic acid
with PtX24– (X=Cl, Br) have shown that either the mo-
lecular or the ionic complexes could be formed de-
*
Author for correspondence: tusek@rudjer.irb.hr
1388–6150/$20.00
© 2005 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands

TUžEK-BO¦I‡, TROJKO
pending on the acidity of the alcoholic/aqueous reac-
broad, dehydration step between 42–112°C with the
mass loss close to the calculated value for evolution of
two molecules of water. In the IR spectra the water loss
is indicated by a disappearance of the rather intense ab-
tion solution (Fig. 1) [7]. While in the neutral medium
dihaloplatinum complexes, PtL2X2, containing N-
bonded ligand in a trans square planar fashion were ob-
tained, from the HX acidic medium at pH<3 the salt
complexes, [LH+ ]2[PtX 42– ], were isolated. The ligand
is protonated forming methylquinolinium ion, while
platinum remains in the form of square planar mono-
meric tetrahaloplatinum complexes. In addition, the
monoethyl ester at pH~3 forms dimeric hexahalodi-
platinum complexes [LH+ ]2[Pt 2X62– ], while the che-
late complex PtL2⋅2H2O with N,O bonded ligand
through the quinoline nitrogen and the phosphonic
acid oxygen was obtained at pH>6. Decomposition be-
haviour of the complexes was investigated by thermo-
gravimetry and differential thermal analysis, and ex-
amples of the appropriate TG-DTA plots are presented
in Figs 2–4. Thermal decomposition of the complexes
is not simple; it takes place through a multistep pro-
cess. Although no stable intermediate products, except
the corresponding anhydrous compounds in the case of
the hydrated complexes, were found owing to the com-
plexity and the overlap of the degradation processes,
thermal analysis subsequently followed by the infrared
spectroscopic study provides valuable information
about decomposition properties of the complexes. IR
spectra were recorded every 50°C and are compared
with the corresponding spectra at room temperature [7].
The hydrated complexes exhibit a single, somewhat
sorptions
in
the
ν(OH)
region
between
3460–3210 cm–1. Degradation of the halide complexes
begins by a dehalogenation process overlapped with
deesterification of the organophosphorus ligand, and is
strongly influenced by the type of complex. In dihalide
adducts of 2-dqmp this step is rather sharp and covers
approximately the range of 150°C (Fig. 2). The mass
loss of about 22% for the chloro and 40% for the
bromo complex corresponds approximatively to evolu-
tion of two halide ions and four ethyl ester groups
(calc. values 19.5 and 37.2%, respectively). The de-
composition of the complexes started with their
dehalogenation, which is indicated by an intensity de-
crease of the Pt–Cl/Br stretching vibrations in the far
infrared region between 400–200 cm–1. On the other
hand, the loss of the ethyl ester groups is accompanied
by a decrease of the absorption bands arisen from vari-
ous modes of the P–O–Et vibrations between
1150–1020 cm–1 and C–C ethyl vibrations in the re-
gion of 990–960 cm–1. The dehalogenation and
deesterification processes are completed at ca. 300°C
and are accompanied by broad and poorly resolved en-
dothermic peaks in DTA curves. The decomposition of
the complexes continues with a progressive mass loss
to a mixture of Pt and P2O5. The platinum metal was
Fig. 1 Platinum(II) complexes of 2-dqmp and 2-Hmqmp
154
J. Therm. Anal. Cal., 81, 2005

PLATINUM(II) COMPLEXES OF DIETHYL AND MONOETHYL 2-QUINOLYLMETHYLPHOSPHONATES
Fig. 2 TG-DTA curves for complexes: — – Pt(2-dqmp)2Br2
---
Fig. 3 TG-DTA curves for complexes:
— – Pt(2-Hmqmp)2Cl2⋅2H2O,
and
– [2-Hdqmp]2[PtBr4]⋅2H2O
---
– [2-H2mqmp]2[PtCl4]⋅2H2O and
– [2-H2mqmp]2[Pt2Cl6]
⋅⋅⋅
identified as the pyrolytic residue in a number of plati-
num complexes [11, 12]. Formation of P2O5 in the IR
spectra is followed by the intensity decrease of the
phosphonate ester bands and by the intensity increase
of the P=O stretching vibrations around 1260 cm–1
[13–15]. Dichloro adduct of monoester 2-Hmqmp is
more stable than the corresponding dichloro adduct of
diester 2-dqmp. Its dehalogenation and deesterification
process occurs at higher temperatures between
190–390°C (Fig. 3). This decomposition step is contin-
ued by the pyrolytic decomposition of ligand visible in
the DTA curve as a few exothermic peaks.
calc. 26.9, 30.5, 18.4 and 24.5% for
[2-Hdqmp]2[PtCl4]⋅2H2O, [2-Hdqmp]2[PtBr4]⋅2H2O,
[2-H2mqmp]2[PtCl4]⋅2H2O and [2-H2mqmp]2[PtBr4]
complex, respectively). The mass loss in the second
step corresponds approximately to the release of two
remaining halogens (found mass loss: 8.4, 15.8, 9.1
and 17.1%; calc. 7.6, 14.4, 8.1 and 15.7%).
Dehalogenation in hexahalodiplatinum com-
plexes of 2-Hmqmp also occurs in two steps. The first
relatively broad step starts at about 70°C covering ap-
proximately the range up to 220°C. The mass loss of
ca. 13% (calc. 14.5%) for the chloro and of ca. 17%
(calc. 18.2%) for the bromo complex corresponds ap-
proximately to two halide ions and two ethyl ester
groups. The second rather sharp step ranged up to
ca. 320–330°C could be attributed to complete de-
halogenation of the complexes what is confirmed by
the absence of the ν(Pt–Cl/Br) absorptions in the IR
spectra. It is overlapped by the pyrolytic decomposi-
tion of the ligand, which continues with a progressive
mass loss and a strong exothermic peak around 360°C.
Chelate complex exhibits a broad dehydration step
between 50–120°C (Fig. 4), indicating that two water
In the ionic tetrahaloplatinate complexes after
the dehydration step begins dehalogenation process
between 120–130°C, at lower temperatures than in
the corresponding dihaloplatinate adducts. It occurs
in two steps, as revealed by the very distinct inflec-
tions in the TG curves, although well-defined TG pla-
teaus are not reached (Figs 2 and 3). In the DTA
curves these processes are visible as slight endother-
mic effects. First step, ranged up to 260–270°C for
the 2-dqmp complexes and up to 220–240°C for the
2-Hmqmp complexes, corresponds to the loss of two
halogens accompanied by the ligand deesterification
(found mass loss: 27.6, 28.7, 17.9 and 22.8%;
J. Therm. Anal. Cal., 81, 2005
155

TUžEK-BO¦I‡, TROJKO
to the beginning of the dehalogenation process in the
halide complexes, it was shown that dihalide adducts
of the monoethyl ester are more stable compounds
with respect to those of the diethyl ester, as well as are
chloro complexes compared to their bromo ana-
logues. It may be presumed that this arises from the
steric effects that increase from monoester to diester
and from chloro to bromo derivatives. In the ion-pair
salt complexes with the protonated phosphonate
ligand as cation and the tetrahaloplatinate or
hexahalodiplatinate complex as anion, these differ-
ences are less pronounced. Comparing thermal prop-
erties of the platinum and palladium quinolylmethyl-
phosphonate complexes, it could be shown that there
are small differences in thermal stability between
their ionic complexes and N,O-chelates [9, 10]. On
the other hand, palladium dihalide adducts are more
stable than their platinum analogues. Their de-
halogenation process begins at 20–40°C higher tem-
peratures. It is interesting to note that most of plati-
num complexes of 2-Hmqmp show smaller antitumor
activity than their palladium analogues [6, 7] while
activity of 2-dqmp complexes of both metals are more
or less comparable [5, 7]. In addition, complexes of
diester are more active than those of monoester.
These findings indicate that besides the breaking abil-
ity of the Pt/Pd-halogen bonds which promotes bind-
ing of the metal complexes to DNA strands, there are
a lot of other factors that influence biological activity
of the complexes such as their solubility, lipophilicity
as well as their reactions prior to DNA binding [19].
It was shown that dehalogenation of the com-
plexes and ligand deesterification are the main pro-
cesses in the course of decomposition of the plati-
num(II) and palladium(II) halide complexes of both
phosphonate ligands. These results are in agreement
with those obtained by studying decomposition be-
haviour of a number of halide complexes with various
alkyl aminophosphonates by FAB-mass spectros-
copy, in which the main fragment ions are those ob-
tained by the sequentially losses of halide ions and
those corresponding to loss of the phosphonate ester
group or its fragments [20].
Fig. 4 TG-DTA curves for — – 2-Hmqmp and --- – its com-
plex Pt(2-mqmp)2⋅2H2O
molecules are lattice-held [16]. Decomposition of the
complex includes deesterification and other ligand deg-
radation processes in the 200–500°C region visible in
the DTA curve as a broad exothermic peak centered
around 450°C. A continuous thermal degradation could
be seen also in the free monoester (Fig. 4). Its TG curve
shows a rather strong mass loss of ca. 45% between 170
and 290°C arisen from its partial pyrolytic decomposi-
tion, which is continued in the second broad step
over 290°C giving the pyrolytic residue P2O5. This is
confirmed by the IR and X-ray data. Difference ob-
tained between the calculated and found residue value
could be ascribed to partial sublimation of P2O5, which
takes place at higher temperatures [14, 17]. Decomposi-
tion of 2-mqmp is not followed by clear peaks in the
DTA curve. Only is visible one endothermic effect
at 150°C, which is not accompanied by the mass loss in
the TG curve, and corresponds approximately to the
melting point of this monoester [18].
Acknowledgements
The financial support of the Croatian Ministry of Science, Ed-
ucation and Sports (grants 0098035 and 0098065) is gratefully
acknowledged.
It could be concluded that stability of complexes
greatly varies depending on the type of complex as
well as on the halide and organophosphorus ligand
bonded to platinum. In general, molecular complexes
are more stable than the ionic complexes. From the
initial decomposition temperatures, which correspond
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