Thermal behaviour of copper(II) complexes of haloaspirinates

Article abstract of DOI:10.1023/A:1010134702443

Complexes of Cu(II) with substituted o-acetoxy benzoic acids (5-haloaspirines, X-asp) with and without pyridine (py), of composition [Cu₂(X-asp)₄] and [Cu(X-asp)₂(py)₂] have been synthesized and characterized. E

Full text of DOI:10.1023/A:1010134702443

Journal of Thermal Analysis and Calorimetry, Vol. 60 (2000) 505–515
THERMAL BEHAVIOUR OF COPPER(II) COMPLEXES
OF HALOASPIRINATES
S. H. Tarulli1, O. V. Quinzani1*, J. Dristas2 and E. J. Baran3
1Departamento de Química e Ingeniería Química, Universidad Nacional del Sur, Av. Alem 1253
8000-Bahía Blanca
2Departamento de Geología, Universidad Nacional del Sur, 8000-Bahía Blanca
3Centro de Química Inorgánica (CEQUINOR), Facultad de Ciencias Exactas, Universidad
Nacional de La Plata, 1900-La Plata, Argentina
(Received June 14, 1999)
Abstract
Complexes of Cu(II) with substituted o-acetoxy benzoic acids (5-haloaspirines, X-asp) with and with-
out pyridine (py), of composition [Cu2(X-asp)4] and [Cu(X-asp)2(py)2] have been synthesized and
characterized. Electronic and vibrational spectroscopic data of these complexes are reported. Its ther-
mal behaviour was investigated by thermogravimetry and differential thermal analysis. In all complexes,
the haloaspirinate ligands decompose in two or three steps, starting with the break up of the coordinated
acetoxy groups. CuO is obtained as the final pyrolysis residue in all cases.
Keywords: Cu(II), electronic spectra, haloaspirinate complexes, IR specra, thermal analysis
Introduction
Aspirin has been widely used as an analgesic agent but it has the disadvantage that it
can produce ulcerogenic lesions [1]. The copper(II) complex of aspirin has demon-
strated major anti-inflammatory and antiulcer activity with less injurious effects than
the free ligand [2–4]. Similar effects have also been obtained with some ternary
Cu(II) complexes containing aspirin and a second ligand [5–8].
In this paper we report the thermal behaviour of the complete series of Cu(II)
haloaspirinates, prepared by the procedure of Meier et al. [9] and of a new series of
ternary complexes containing pyridine as the second ligand.
Experimental
Synthesis of the complexes
The copper(II) aspirinate, [Cu2(asp)4], was obtained by adding an aqueous solution of
copper acetate monohydrate (C. Erba, 1.5 mmol) over a 50% aqueous ethanolic solu-
*
Author to whom all correspondence should be addressed.
1418–2874/2000/ $ 5.00
Akadémiai Kiadó, Budapest
© 2000 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

506
TARULLI et al.: HALOASPIRINATE COMPLEXES
tion of aspirine (Sigma, 3.0 mmol).The mixture was shortly heated at 40°C and the
precipitated blue solid was filtered off, washed with aqueous ethanol and dried over
CaCl2.
The 5-halogen-substituted acetylsalicylic acids were prepared by reacting the ap-
propriate substituted salicylic acid (Aldrich) with acetic anhydride [10]. The copper(II)
complexes were prepared by the general procedure described by Meier et al. [9]. The ter-
nary complexes derived from fluoro-, chloro- and bromo-aspirinate and pyridine, were
crystallized by addition of dried n-hexane to concentrated solutions of the respective cop-
per haloaspirinates in pyridine. The corresponding complex with iodo-aspirinate could
not be obtained due to the rapid hydrolysis of this halo-aspirinate. All these new com-
plexes have the same composition, [Cu(X-asp)2(py)2].
The results of the chemical analysis of all the prepared complexes are shown in
Table 1.
Spectroscopic measurements
UV-visible spectra of methanolic solutions and Nujol mulls of the complexes were
recorded on a Perkin Elmer Lambda-2 spectrophotometer. IR spectra were recorded
down 400 cm–1 with a Nicolet 520 FTIR instrument, using KBr pellets and Nujol
mulls.
Thermal analysis
Thermal decomposition studies were carried out on a Rigaku Denki Thermoflex in-
strument, in flowing air or nitrogen (30 ml min–1), up to a final temperature of 750°C.
Heating rates, ranged between 2 and 5°C min–1 and sample masses between 7 and
18 mg. Al2O3 was used as the DTA reference standard.
Results and discussion
Copper(II) complexes of haloaspirinates
The X-ray diffraction study of Manojlovic-Muir has demonstrated that Cu(II) aspi-
rinate crystallize in dimeric units, [Cu2(asp)4], interconnected by acetoxy oxygens
from neighbouring units [11]. The stereochemistry of the dimetallic units resembles
that found in copper(II) acetate and other copper carboxylate complexes [12, 13]. The
same dimetallic units are also present in the haloaspirinate complexes [9].
Selected IR bands of these complexes, related to vibrational modes of the carboxy-
late and acetoxy groups, are shown in Table 2. In the IR spectra of [Cu2(asp)4] and
[Cu2(F-asp)4] there are strong bands corresponding to the C=O stretching of two types of
acetoxy groups, one of them free of additional interactions, named (I) in Table 2, and the
other coordinated to a Cu(II) center of a neighbouring dimeric unit, named as (II). In the
chloro-, bromo- and iodo-aspirinates, the vacant sites on the Cu(II) ions are evidently oc-
cupied by water or methanol molecules, as becomes evident from the absence of type II
bands in the spectra of these complexes.
J. Therm. Anal. Cal., 60, 2000

Table 1 Analytical data of the prepared copper(II) haloaspirinate complexes
C/%
H/%
N/%
Cu/%
Compound
Formula
calc.
51.25
47.13
41.04
35.06
32.56
54.59
51.82
45.74
obs.
–
calc.
3.35
2.64
3.06
2.62
2.73
3.60
3.42
3.02
obs.
calc.
–
obs.
–
calc.
obs.
15.88
13.98
11.40
11.20
8.85
[Cu2(asp)4]
C36H28O16Cu2
15.06
13.85
12.06
10.30
8.61
[Cu2(F-asp)4]
Cu36H24F4O16Cu2
Cu36H32Cl4O20Cu2
Cu36H32Br4O20Cu2
47.03
40.17
35.53
31.72
54.62
51.79
44.96
2.49
2.69
2.22
2.04
3.96
3.37
2.53
–
–
[Cu2(Cl-asp)4] 4H2O
[Cu2(Br-asp)4] 4H2O
–
–
–
–
[Cu2(I-asp)4] 4CH3OH Cu40H40I4O20Cu2
–
–
[Cu(F-asp)2(py)2]
[Cu(Cl-asp)2(py)2]
[Cu(Br-asp)2(py)2]
Cu28H22F2N2O8Cu
Cu28H22Cl2N2O8Cu
Cu28H22Br2N2O8Cu
4.55
4.32
3.81
4.64
4.27
3.69
10.32
9.79
10.17
9.82
8.64
8.70

Table 2 Selected IR spectral data for the investigated copper(II) haloaspirinates (values in cm–1)
R-CO2
Acetoxy groups
Pyridine
Compound
ν(CO)
(I)
ν(CO)
(II)
νas(COC)
(II)
νas(COC)
(I)
νs(COC)
(I)
νs(COC)
(II)
ν(CC)
γ(CH)
νas
νs
δ
[Cu2(asp)4]
1620 vs 1400 vs
1629 vs 1389 vs
1635 vs 1378 vs
1620 vs 1370 m
1020 m
1760 s
1725 s
1238 vs 1195 vs 1100 s
1036 m
–
–
[Cu2(F-asp)4]
1014 w 1763 vs 1717 vs 1249 vs 1184 vs 1127 m
1027 w
–
–
[Cu2(Cl-asp)4] 4H2O
[Cu2(Br-asp)4] 4H2O
1017 m 1740 vs
1013 m 1740 vs
1017 m 1739 vs
1018 m 1758 vs
1018 m 1764 vs
1014 m 1765 vs
–
–
–
–
–
–
–
–
–
–
–
–
1236 vs 1100 s
1200 s 1098 s
–
–
–
–
–
–
–
–
–
–
[Cu2(I-asp)4] 4CH3OH 1639 vs 1374 vs
1206 vs 1103 s
1185 vs 1127 m
1198 vs 1108 s
1203 vs 1099 s
–
–
[Cu(F-asp)2(py)2]
[Cu(Cl-asp)2(py)2]
[Cu(Br-asp)2(py)2]
1608 vs 1368 vs
1605 vs 1360 s
1610 vs 1364 vs
1448 s
1448 s
699 vs
698 vs
1449 m 700 vs
(X-asp) – haloaspirinate, ν – stretching, δ – in plane deformation, γ – out-of-plane deformation, vs – very strong, s – strong, m: medium, w – weak

TARULLI et al.: HALOASPIRINATE COMPLEXES
509
The electronic absorption spectra of these complexes present the three band pat-
tern which is characteristic of dimeric Cu–Cu units [9], and which origin has been ex-
tensively discussed in the literature [9, 14, 15]. Some data are presented in Table 3.
Table 3 Selected UV-Visible spectral data for copper(II) haloaspirinates
Wavelength/nm
Compound
State
Band I
675
Band II
350
Bands III
306
Ref.
9
[Cu2(F-asp)4]
Nujol
Nujol
HCCl3
[Cu2(Cl-asp)4] 4H2O
[Cu(asp)2(py)2]
705
350
305
285
9
725
br
265
278
8
[Cu(F-asp)2(py)2]
[Cu(Cl-asp)2(py)2]
Nujol
792/783
br
342
t.w.
CH3OH
Nujol
747
322
310
276
270
t.w.
t.w.
818
br
CH3OH
Nujol
765
br
316
312
315
277
268
273
t.w.
t.w.
t.w.
[Cu(Br-asp)2(py)2]
806
br
CH3OH
763
br
asp – aspirinate, br – broad, t.w. – this work
The TG and DTA curves of the five dimeric aspirinate complexes present some
common characteristics and peculiarities. As typical examples for the measured TG
and DTA traces, those obtained for the fluoro- and chloro-aspirinates under an air
flow, are presented in Figs 1 and 2, respectively. The obtained thermal data for the
full series of complexes are shown in Tables 4 and 5. The complexes containing wa-
ter or methanol show an endothermic peak between 40 and 120°C, both under air and
nitrogen, corresponding to the removal of these ligands. The IR spectra of residues
collected after this first step clearly shows that the sites occupied by water or metha-
nol are immediately occupied by acetoxy groups, in a similar way as in the aspirinate
or fluoro-aspirinate compounds.
For all five complexes there is an endothermic process in the range between
145–270°C, named step II in Tables 4 and 5, corresponding to the partial degradation
of the aspirinate ligand. These processes appear with almost identical characteristics
in the TG and DTA curves obtained under both atmospheres, suggesting a bond
breaking without oxygen consumption. The initial decomposition temperature is
higher for the fluoro-aspirinate complex, indicating a greater thermal stability, in
agreement with some preliminary thermodynamic calculations for the free ligands.
The lost mass fraction in step II was almost the same in all cases (about
100 g mol–1 Cu(II)) and the IR spectra of solid residues collected at the end of this
J. Therm. Anal. Cal., 60, 2000

510
TARULLI et al.: HALOASPIRINATE COMPLEXES
Fig. 1 TG and DTA curves of [Cu2(F-asp)4] in air. Sample mass:10.5 mg; heating rate:
5oC min–1
Fig. 2 TG and DTA curves of [Cu2(Cl-asp)4]4H2O in air. Sample mass: 13.3 mg; heating
rate: 3oC min–1
step show important intensity diminution in bands related to vibrations of acetoxy
groups coordinated to copper. Besides, several new bands appear in the 1600 cm–1 re-
gion. On the other hand, the permanency of the uncoordinated acetoxy stretching
bands, indicate that these groups decompose at higher temperatures, within the steps
named III and IV in Tables 4 and 5.
All these data suggest that, the thermal degradation starts on the acetoxy groups
bonded to neighbouring Cu(II) ions, involving these groups and an important part of
the aromatic ring.
After step II the final degradation occurs in the temperature range between 260 and
550°C and involves two highly exothermic processes, except for the unsubstituted
aspirinate and the fluoro-aspirinate complexes, which decomposes in only one step. For
all complexes the final pyrolysis residue was CuO. Calculated and observed percentages
of mass loss for step I as well as for the final residues obtained in air, are summarised in
Table 6.
It is also interesting to mention that in nitrogen atmosphere the transformations
occur at slightly higher temperatures, and eventually with a different degradation
mechanism, as suggested by differences in the DTA traces in both atmospheres.
J. Therm. Anal. Cal., 60, 2000

TARULLI et al.: HALOASPIRINATE COMPLEXES
511
Table 4 Mass loss (in %) and decomposition temperatures from TG and DTA curves (air atmosphere)
TG
Compound
Step I
Step II
25.8a
Step III
Step IV
55.9c
258–300
57.4c
270–350
[Cu2(asp)4]
–
–
210–258b
25.1
225–270
[Cu2(F-asp)4]
6.8
60–120
19.4
145–255
41.8
255–310
16.7
310–500
[Cu2(Cl-asp)4] 4H2O
[Cu2(Br-asp)4] 4H2O
[Cu2(I-asp)4] 4CH3OH
[Cu(F-asp)2(py)2]
[Cu(Cl-asp)2(py)2]
[Cu(Br-asp)2(py)2]
5.5
40–75
16.5
174–263
25.5
263–360
40.5
360–566
6.7
45–98
13.3
190–266
23.8
266–320
44.4
320–490
25.6
65–210
18.7
210–270
42.8c
270–400
24.0
75–190
15.6
190–262
23.4
262–368
24.3
368–570
20.6
60–190
14.9
190–255
14.8
255–355
38.8
355–600
amass loss/% ; btemperature range/oC; conly one sharp and continuous step was observed
DTA
Step# II
Compound
[Cu2(asp)4]
Step# I
Step* III
260–295
(278)
Step* IV
225–263
(244/251)d
–
[Cu2(F-asp)4]
240–275
(260)
275–320
(274/296)
–
[Cu2(Cl-asp)4]4H2O
[Cu2(Br-asp)4]4H2O
[Cu2(I-asp)4]4CH3OH
[Cu(F-asp)2(py)2]
[Cu(Cl-asp)2(py)2]
[Cu(Br-asp)2(py)2]
70–125
(100)
180–237
(227)
255–315
(291)
345–413
(395)
40–75
(60)
175–255
(244)
260–368
(304/312/334)
368–520
(447)
52–95
(64)
213–266
(251)
266–350
(308)
360–500
(424)
85–155
(143)
155–230
(195)
230–275
(258)
275–370
(298)
98–158
(128–142)
158–218
(189)
240–375
(298–306)
375–570
(400–488)
e
50–143
(120)
143–220
(191)
220–274
(255)
274–398
(304)
398–600
(450)
#endothermic peak(s); *exothermic peak(s); dpeak minimum or maximum; every distorted by step III
J. Therm. Anal. Cal., 60, 2000

Table 5 Mass loss (in %) and decomposition temperatures from TG and DTA curves (nitrogen atmosphere)
TG
DTA
Step* III
Compound
Step I
–
Step II
Step III
Step IV
Step# I
–
Step# II
Step* IV
29.5a
53.3
277–400
213–286
(258)
286–427
(341–376)c
[Cu2(asp)4]
240–277b
[Cu2(F-asp)4]
26.7
225–270
53.3
270–400
243–278
(266)
278–420
(339–377)c
–
–
[Cu2(Cl-asp)4] 4H2O
[Cu2(Br-asp)4] 4H2O
6.4
65–131
15.5
180–248
34.6
248–355
28.3
355–510
70–130
(115)
214–248
(237)
248–371
(315)
371–500
(445)
5.6
45–75
16.8
145–260
29.5
260–408
35.1
408–600
45–70
(62)
215–257
(235)
257–355
(315)
415–565
(460)
amass loss/%; btemperature/oC; #endothermic peak(s); * exothermic peak(s); cvery broad

TARULLI et al.: HALOASPIRINATE COMPLEXES
513
Table 6 Residue data of complexes and mass loss percentages of step I (air atmosphere)
Step I
mass loss/%
Residue/%
CuO
Coumpound
Ligand
obs.
calc.
–
obs.
calc.
18.9
17.4
15.1
12.9
10.8
12.9
12.3
10.8
[Cu2(asp)4]
–
–
18.3
17.5
15.3
13.0
10.7
12.8
12.1
10.6
[Cu2(F-asp)4]
–
–
–
[Cu2(Cl-asp)4] 4H2O
[Cu2(Br-asp)4] 4H2O
[Cu2(I-asp)4] 4CH3OH
[Cu(F-asp)2(py)2]
[Cu(Cl-asp)2(py)2]
[Cu(Br-asp)2(py)2]
6.8
5.5
6.7
25.6
24.0
20.6
6.8
5.8
8.7
25.7
24.4
21.4
–4H2O
–4H2O
–4CH3OH
–2 pyridine
–2 pyridine
–2 pyridine
Copper(II) complexes of haloaspirinates and pyridine
An X-ray diffraction study of the complex [Cu(asp)2(py)2] showed that monomeric
units are present in the lattice and that the Cu(II) ion is coordinated by two pyridine
N-atoms and two carboxylate O-atoms in a trans square planar arrangement whereas
the second oxygen atoms from the carboxylate groups are weakly bound to axial po-
sitions of the metal center (8]. In other ternary aspirinate complexes with imidazole
[16] or benzimidazole [17] Cu(II) present also monomeric coordination, as deduced
from EPR and electronic spectroscopic data. Therefore, a similar situation may be ex-
pected for the present complex compounds.
As shown in Table 2, the IR spectra of the pyridine containing complexes only con-
tains bands corresponding to vibrations of type I acetoxy groups and the carboxylic
stretching bands appear at lower frequencies than in the parent dimeric compounds. This
spectroscopic behaviour is in agreement with the expected monomeric structure [17, 18].
Data of the UV-visible spectra of the complexes are shown in Table 3. The pres-
ence of the broad bands at ca 750 nm in methanolic solution or at ca 800 nm in the
solid mulls as well as the absence of the characteristic band II, confirms again a
monomeric tetragonal ligand arrangement around the Cu(II) centers [8, 14, 17].
For these complexes the thermal behaviour was investigated only under an air flow.
The TG and DTA curves of the three complexes are shown in Figs 3–5. The analysis of
data is included in Tables 4 and 6. The three complexes gives up the two pyridine mole-
cules in a two-step process between 60 and 210°C. After this release the behaviour of the
fluoro-aspirinate complex is different to that of the other two compounds. For the former
complex, the IR spectrum of residues collected at 210°C is identical to the spectrum of
[Cu2(F-asp)4]. That means, that after the loss of pyridine a structural rearrangement takes
place generating again the dinuclear structure characteristic of the parent complex.
The final decomposition of the chloro- and bromo-aspirinate complexes present
similar patterns and occur in similar temperature ranges as for the dimeric parent
compounds. In the DTA traces, the signal corresponding to step II appears highfy dis-
J. Therm. Anal. Cal., 60, 2000

514
TARULLI et al.: HALOASPIRINATE COMPLEXES
Fig. 3 TG and DTA curves of [Cu(F-asp)2(py)2] in air. Sample mass: 7.8 mg;
heating rate: 5oC min–1
Fig. 4 TG and DTA curves of [Cu(Cl-asp)2(py)2] in air. Sample mass: 13.2 mg;
heating rate: 5oC min–1
Fig. 5 TG and DTA curves of [Cu(Br-asp)2(py)2] in air. Sample mass: 16.1 mg;
heating rate: 5oC min–1
torted by the strong endothermic peak related to step III, especialty for the chloro-as-
pirinate. On the basis of these results, we consider that after the loss of pyridine, the
structures of these two complexes are probably not reorganized to the dimeric ar-
rangement but the structure transformatian involves the coordination of acetoxy
groups to the metal center, as suggested by the appearance of type II acetoxy vibra-
tion in the IR spectra of products collected after the elimination of pyridine.
* * *
J. Therm. Anal. Cal., 60, 2000

TARULLI et al.: HALOASPIRINATE COMPLEXES
515
We are grateful to Dr. Nora Rodríguez for the synthesis of the 5-haloaspirines and to Dr. Silvia Acebal
for the AA analysis of copper. This work was supported by the Consejo Nacional de Investigaciones
Científicas y Técnicas de la República Argentina and the Agencia Nacional de Promoción Científica y
Tecnológica (Préstamo BID 802/OC-AR-PICT Nr.119) and Secretaría de Ciencia y Técnica, Univer-
sidad Nacional del Sur (Subsidio Resolución CSU 170–98).
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