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L.. Tusek-Bozic, R. Trojko / Thermochimica Acta 339 (1999) 41±45
44
17% for the chloride and 27% for the bromide com-
plex corresponds well with this assumption. It was
shown that decomposition of these complexes started
with their dehalogenation at 225±2308C, and is indi-
as are bromo complexes compared to their chloro
analogues, most probably due to the higher capacity
of bromide ligands to establish a ꢁ-bond with the
metal by retrodonation. Chelate complex is dehy-
drated and exhibits a broad dehydration step over
the range of 34±1608C, indicating that two water
molecules are lattice-held [18]. Decomposition of
the complex includes deesteri®cation and other ligand
degradation processes. The ®nal decomposition pro-
duct of all the complexes is a mixture of metallic Pd
and P2O5, similarly as was obtained for palladium(II)
halide complexes of various aminophosphonic acid
derivatives [8,11]. This was con®rmed by infrared
spectroscopic and X-ray diffraction analysis. Differ-
ences obtained between the calculated and found
pyrolytic residue could be ascribed to partial sublima-
tion of P2O5 which takes place above 3008C [19].
The thermal data of palladium(II) halide complexes
of 2-mqmp were compared with those obtained for the
complexes of 2-dqmp [8], which are included in Table
1 for comparison. The diester ligand, similarly as its
monoester analogue, forms N-bonded palladium diha-
lide complexes Pd(2-dqmp)2X2 (X = Cl, Br), while in
the acidic media the quinolinium salts of tetrahalo-
cated by an intensity decrease of the Pd±X stretching
1
vibration which appeared at 358 and 326 cm
,
respectively [13]. On the other hand the loss of the
ethyl ester group is accompanied by a decrease of the
absorption bands arising from various modes of the P±
O±Et vibrations in the region of 1160±1040 cm 1 and
1
C±C ethyl vibrations between 980 and 930 cm as
well as by an increase of the ꢀ(P=O) band around
1
1230 cm [8,15]. It was shown that dehalogenation
and deesteri®cation processes were completed at ca.
3008C and were accompanied by an endothermic
effect in DTA curves at 2708C in both complexes.
In the ionic quinolinium trihalopalladium complexes
dehalogenation starts at lower temperatures and
occurs in two steps separated by an in¯ection in the
TG curve but without a distinct plateau. The ®rst step
between 1658C and 1858C for the chloride and
between 1908C and 2108C for the bromide complex
corresponds approximately to the loss of one halide
ion forming dihalide complexes, similarly as was
obtained by heating these complexes in methanol
and some other solvents. Palladium as a soft metal
has a strong preference for N-donor ligands and there-
fore complexes with O-phosphonate-bonding were
easily transformed into those with N-quinoline-bond-
ing. In addition, it has been shown that some ion-pair
palladium salt complexes of various amine, pyridine
and quinoline also give neutral palladium(II) dihalide
complexes by solid-phase thermal transformation
[8,16,17]. In the second step the complete dehalogena-
tion along with deesteri®cation ends at ca. 3008C,
similarly as was observed in dihalide adducts. In the
DTA curves these processes are visible as two
endothermic peaks at 2108C and 2858C for the chlor-
ide and at 2258C and 2908C for the bromide complex.
These decomposition steps are followed by a contin-
uous weight loss with several unclear peaks in the
DTA curves. On the bases of the initial decomposition
temperatures, which correspond to the beginning of
the dehalogenation process, it could be concluded that
the thermal stability of the complexes depends on both
the type of the complex and the halide ligand bonded
to palladium. In general, dihalide adducts are more
stable than ionic trihalopalladium complexes as well
palladate anions [2-Hdqmp]2 [PdX4]2 Á2H2O (X =
+
Cl, Br) as dihydrate compounds are formed. From
the results obtained it could be concluded that com-
plexes of the monoester are more stable. Thus in the
case of dihalide complexes, dehalogenation in com-
plexes of 2-mqmp begins at higher temperatures (30±
608C) than for the corresponding 2-dqmp complexes.
This observation is interesting in view of the impor-
tance of the Pd±halogen bond strength for the binding
of the complexes to DNA strands, which in turn is
related to their antitumor activity. It has been sug-
gested that the requirements for a potential antitumor
complex compound are a pair of strongly covalently
bonded ligands and a pair of moderately labile ligands
such as halides [20]. A smaller antitumor effect might
be expected when the leaving ability of the labile
ligands is lower. Our results are in agreement with this
assumption. It was found that complexes of the diester
are in general more effective than the corresponding
complexes of the monoester [3,14]. It should be
pointed out, that although the Pd±X stretching fre-
quency was found to increase in monoester complexes
with respect to the corresponding diester complexes,
these differences are rather small suggesting the simi-
Tusek-Bozic, Lj.
