Spectrothermal studies on Co(II), Ni(II), Cu(II) and Zn(II) salicylato (1,10-phenanthroline) complexes

Article abstract of DOI:10.1023/B:JTAN.0000032264.74742.27

The cobalt, nickel, copper and zinc atoms in bis(1, 10-phenanthroline) bis(salicylato-O)metal(II) monomeric octahedral complexes [M(Hsal) ₂(phen)₂]nH₂O, (M: Co(II), n=1; Cu(II), n=1.5 and Ni(II), Zn(II), n=2) are coordinated by the salicylato monoanion (Hsal) through the carboxyl oxygen in a monodentate fashion and by the 1,10-phenanthroline (phen) molecule through the two amine nitrogen atoms in a bidentate chelating manner. On the basis of the DTG_max, the thermal stability of the hydrated complexes follows order: Ni(II) (149°C)>Co(II) (134°C)>Zn(II) (132°C)>Cu(II) (68°C) in static air atmosphere. In the second stage, the pyrolysis of the anhydrous complexes takes place. The third stage of decomposition is associated with a strong exothermic oxidation process (DTA curves: 410, 453, 500 and 450°C for the Co(II), Ni(II), Cu(II) and Zn(II) complexes, respectively). The final decomposition products, namely CoO, NiO, CuO and ZnO, were identified by IR spectroscopy.

Full text of DOI:10.1023/B:JTAN.0000032264.74742.27

Journal of Thermal Analysis and Calorimetry, Vol. 76 (2004) 793–800
SPECTROTHERMAL STUDIES ON Co(II), Ni(II), Cu(II)
AND Zn(II) SALICYLATO (1,10-PHENANTHROLINE)
COMPLEXES
H. Olmez, F. Arslan and H. Icbudak*
Department of Chemistry, Faculty of Arts and Sciences, Ondokuz Mayis University,
55139 Kurupelit-Samsun, Turkey
(Received July 21, 2003; in revised form December 16, 2003)
Abstract
The cobalt, nickel, copper and zinc atoms in bis(1,10-phenanthroline)bis(salicylato-O)metal(II)
monomeric octahedral complexes [M(Hsal)2(phen)2]×nH2O, (M: Co(II), n=1; Cu(II), n=1.5 and Ni(II),
Zn(II), n=2) are coordinated by the salicylato monoanion (Hsal) through the carboxyl oxygen in a
monodentate fashion and by the 1,10-phenanthroline (phen) molecule through the two amine nitrogen
atoms in a bidentate chelating manner. On the basis of the DTGmax, the thermal stability of the
hydrated complexes follows order: Ni(II) (149°C)>Co(II) (134°C)>Zn(II) (132°C)>Cu(II) (68°C) in
static air atmosphere. In the second stage, the pyrolysis of the anhydrous complexes takes place. The
third stage of decomposition is associated with a strong exothermic oxidation process (DTA curves:
410, 453, 500 and 450°C for the Co(II), Ni(II), Cu(II) and Zn(II) complexes, respectively). The final
decomposition products, namely CoO, NiO, CuO and ZnO, were identified by IR spectroscopy.
Keywords: salicylato-phenanthroline complexes, spectrothermal analysis
Introduction
Salicylic acid and metal salicylates have been used for many years as anti-in-
flammatory, antipyretic and analgesic drugs in medicine [1–4]. Salicylic acid (H2sal)
is a diprotic acid which forms in aqueous solutions hydrogen salicylate (Hsal–) and
salicylate (sal2–) ions. From a coordination standpoint, salicylate is a versatile ligand,
displaying a variety of bonding modes. For example, Hsal– anion is known to bond to
metals as a unidentate carboxylate, as a bidentate chelating ligand using one
carboxylate oxygen and hydroxyl oxygen and as a bidentate bridging carboxylate
ligand. Both the hydroxyl and carboxyl protons can be lost from the parent acid to
generate sal2– which seems to be invariably chelating through the hydroxyl and one
of the carboxyl oxygens [5].
*
Author for correspondence: E-mail: icbudak@omu.edu.tr
1388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
© 2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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Mononuclear metal (II) complexes of salicylate derivatives with basic ligands
containing nitrogen donors have been reported and few X-ray structures of them are
known [2–6]. It has been found that by increasing the acidity of the alkylcarboxylate
ligands or by increasing the basicity of the basic ligands, the tendency towards for-
mation of monomeric complexes increases [2]. The X-ray crystal structure of poly-
meric [Mn(Hsal)2(bipy)]×H2O has also been reported [7].
Experimental
Materials and instrumentation
All chemicals used were analytical reagent products. Elemental analyses for C, H and N
were carried out at the TUBITAK Marmara Research Centre in Turkey. The
conductivity data were recorded using a Crison conductimeter, model 522. The
concentrations of the solutions were approximately 10–3 M. Magnetic susceptibility
measurements at room temperatures were performed using a Sherwood Scientific MXI
model Gouy magnetic balance. UV-VIS spectra were obtained from the ethanol
solutions of the complexes with a Unicam UV2 spectrometer in the range of
900–190 nm. IR spectra were recorded in the 4000–200 cm–1 region with a Mattson 1000
FT-IR spectrometer using KBr pellets. A Rigaku TG8110 thermal analyzer was used to
record simultaneous TG, DTG and DTA curves in a static air atmosphere, at a heating
rate of 10 K min–1 in the temperature range 20–1000°C using platinum crucibles. Highly
sintered a-Al2O3 was used as a reference and the DTG sensitivity was 0.05 mg s–1.
Preparation of complexes
[M(Hsal)2(H2O)4]×4H2O, (M: Co(II), Ni(II), Cu(II), Zn(II))
A solution of sodium salicylate, NaC7H5O3, (1.60 g, 10 mmol) in water (25 mL) was
added dropwisely with stirring at 50°C to the solutions of MSO4 (5 mmol) in water
(25 mL). The mixture was stirred for 4 h at 50°C in a temperature-controlled bath and
the clear solution was then cooled to room temperature. The crystals formed were fil-
tered and washed with 10 mL of cold water and acetone and dried in vacuo. Yield:
84, 88, 82, 76% respectively.
[M(Hsal)2(phen)2]×nH2O, (M:Co(II), n=1; Cu(II), n=1.5 and Ni(II), Zn(II), n=2)
A solution of 1,10-phenanthroline (0.748 g, 4 mmol) in ethanol (15 mL) was added
dropwisely upon stirring to a solution of [M(Hsal)2(H2O)4]×4H2O (2 mmol) in water
(15 mL). The solution was heated to 50°C and stirred for 12 h in a tempera-
ture-controlled bath. The reaction mixture was then cooled to room temperature. The
crystals formed were filtered and washed with a 10 mL of cold water and acetone and
dried in vacuo. Yield: 89, 92, 72, 78%; found: C, 63.69, 62.34, 63.14, 62.21; H, 3.99,
4.02, 3.93, 3.91; N, 7.41, 7.48, 7.50, 7.62; calc.: C, 64.14, 62.58, 62.94, 62.01;
H, 3.97, 4.15, 4.03, 4.11; N, 7.87, 7.68, 7.73, 7.61 for C38H28N4CoO7, C38H30N4NiO8,
C38H29N4CuO7,5, C38H30N4ZnO8, respectively.
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Results and discussion
The molar conductivity values of the complexes (L=11, 17, 19 and 14 S cm2 mol–1
for the Co(II), Ni(II), Cu(II) and Zn(II) complexes, respectively) indicate that these
complexes are non electrolyte in DMF. The Co(II), Ni(II) and Cu(II) complexes ex-
hibit an experimental magnetic moment value of 3.38, 2.32 and 1.36 mB, respectively,
which are consistent with a weak field octahedral geometry as expected. However,
these magnetic moment values are lower than the spin only values for three, two and
one unpaired electrons, indicative of some antiferromagnetic behavior [4]. The
Zn(II) complex is diamagnetic.
UV-VIS spectra
For the pale-pink complex [Co(Hsal)2(phen)2]×H2O displays the metal-to-ligand charge
transfer bands in the 220–273 nm region (e=4032–3890 dm3 mol–1 cm–1) and a shoulder
at 348 nm (e=1616 dm3 mol–1 cm–1) tailing into the visible region. The occurrence of two
d–d bands at 482 nm (e=35 dm3 mol–1 cm–1) as a shoulder and 860 nm
(e=2 dm3 mol–1 cm–1) for the Co(II) complex is consistent with an octahedral structure.
The electronic spectrum for the light blue complex [Ni(Hsal)2(phen)2]×2H2O exhibits two
weak d–d absorption transitions centered at 559 nm (e=8 dm3 mol–1 cm–1) and 879 nm
(e=6 dm3 mol–1 cm–1) which support octahedral geometry. The spectrum of Ni(II)
complex also exhibits a charge transfer band at 317 nm (e=2850 dm3 mol–1 cm–1) and a
shoulder at 345 nm (e=940 dm3 mol–1 cm–1) tailing into the visible region. The UV
spectrum for the turquoise complex [Cu(Hsal)2(phen)2]×1.5H2O exhibits a very broad
d–d absorption transition centered at 683 nm (e=98 dm3 mol–1 cm–1). This value was
assigned to the 2Eg®2T2g d–d transition, thereby suggesting octahedral geometry around
Cu(II). The spectrum also exhibits the bands at ca. 246–309 nm
(e=4523–3560 dm3 mol–1 cm–1) tailing into the visible region and it is assigned to charge
transfer. The colorless complex [Zn(Hsal)2(phen)2]×2H2O exhibits no d–d absorption
band as expected and displays the metal-to-ligand charge transfer bands in the
240–305 nm region (e=3916–3233 dm3 mol–1 cm–1). A similar band between 400 and
450 nm previously reported for mononuclear copper(II) salicylate compounds was
ascribed to a ligand-to-metal charge transfer transition [3, 4, 8].
IR spectra
The bands due to the asymmetric and symmetric stretching n(COO) of the solid sali-
cylic acid are positioned at 1657 and 1393 cm–1, respectively. The n(C=O) band
at 1657 cm–1 is shifted to a lower frequency in the all complexes [e.g. 1595 cm–1 for
Co(II); 1585 cm–1 for Ni(II); 1588 cm–1 for Cu(II); 1596 cm–1 for Zn(II)] showing that
the coordination takes place through the carboxyl group [9, 10]. The difference be-
tween the wavenumbers of nasym and nsym bands (D) is the identifying parameter of the
structure of metal-carboxylate anion bond. The observed D values for the ligand
(H2sal) and its Co(II), Ni(II), Cu(II) and Zn(II) complexes were 264, 205, 196, 185
and 207 cm–1, respectively. The observed D values indicate that the coordination is
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monodentate [3, 11]. The phenolic d(OH) bending peak for the salicylic acid re-
mained almost in the same position at 1250 cm–1 in all complexes suggesting that the
phenol group retains the proton. All the complexes show a broad band of strong in-
tensity in the range of 3403–3434 cm–1 responsible from the n(OH) of crystallization
water. The sharp absorption bands in the range of 1434–1427 and 858–848 cm–1 in
complexes are due to the n(C-N) and d(C–H) vibrations, respectively. The stretching
band for the n(C=N) of the solid phenanthroline observed at 1643 cm–1 [12] was
shifted to lower frequencies in the spectrum of the complexes [e.g. 1634 cm–1 for
Co(II); 1634 cm–1 for Ni(II); 1629 cm–1 for Cu(II); 1634 cm–1 for Zn(II)]. The low in-
tensity bands in the region of 424–391 cm–1 are attributed to M–N and M–O vibra-
tions [13]. On the basis of IR spectra, it may be said that the structures of the studied
metal complexes are somewhat resemblant.
Thermal analysis
The thermal analysis curves (TG, DTA and DTG) of the studied complexes are shown in
Figs 1–4. The thermoanalytical results are summarized in Table 1. The experimental
results of TG, DTG, DTA and IR spectra show that the first stages in the decomposition
process of all the complexes is related to dehydration. According to the DTG and DTA
curves, the dehydration process takes place in a single step which corresponds exactly to
the loss of the crystal waters. The dehydration temperature of the complexes lies in the
range of 54–189°C. On the basis of the DTGmax, the thermal stability of the hydrated
complexes follows order: Ni(II) (149°C)>Co(II) (134°C)>Zn(II) (132°C)>Cu(II) (68°C).
These hydrated complexes show similar sequence in the nicotinamide-acetylsalicylato
mixed-ligand complexes with the identical metal ions [14].
In the second stage, the pyrolysis of the anhydrous complexes takes place. The
Co(II) and Ni(II) complexes decompose in a similar manner. The second mass loss
stages of these complexes were observed in the range of 196–309°C for the Co(II)
complex and 193–286°C for the Ni(II) complex. However, the decomposition process
Fig. 1 TG, DTA and DTG curves of [Co(Hsal)2(phen)2]×H2O
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Fig. 2 TG, DTA and DTG curves of [Ni(Hsal)2(phen)2]×2H2O
Fig. 3 TG, DTA and DTG curves of [Cu(Hsal)2(phen)2]×1.5H2O
Fig. 4 TG, DTA and DTG curves of [Zn(Hsal)2(phen)2]×2H2O
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799
is initially endothermic, an exothermic contribution is also observed in the DTA curve
of the Co(II) complex. The similar but opposite contribution is also observed for the
Ni(II) complex. These stages of the complexes are related to the partial decomposition
of phenanthroline ligand and the release of CO2 from the salicylato ligand. The
exothermic contribution is likely due to the release of CO2 [15]. Early reported data on
thermal analysis and mass spectrometric studies indicate that the salicylate-metal
complexes decompose by releasing of CO2 [16–18]. A decrease in the CO2 stretching
intensity of the intermediate products was observed in the vibrational spectra of the
Co(II) and Ni(II) complexes. In the Co(II) and Ni(II) complexes, the pyrolysis occurred
in two successive steps resulting stable intermediate products. Since both ligands
decomposed partially in the thermal process, the intermediate products were not
identified. The anhydrous Cu(II) and Zn(II) complexes begin to decompose with
melting at 222 and 217°C, respectively, according to the DTA curves.
Similar to the other salicylato and benzoate derivative complexes [9, 19–21], the
third stage of decomposition is associated with a strong exothermic oxidation process
(DTA curves: 410, 453, 500 and 450°C for the Co(II), Ni(II), Cu(II) and Zn(II) com-
plexes, respectively). In the Co(II), Ni(II) and Zn(II) complexes, the strong exother-
mic mass loss processes occur in two steps, a single DTG peak was observed for the
Cu(II) complex. During this stage, the remaining organic part is abruptly burnt in all
complexes. In the Co(II) complex, the endothermic peak at 915°C is related to the
conversion of Co3O4 to CoO. The final decomposition products, namely CoO, NiO,
CuO and ZnO, were identified by IR spectroscopy with corresponding spectra ob-
tained under the same conditions as the pure oxides. The values for the final products
given in Table 1 show good agreement with the corresponding oxides.
Fig. 5 Suggested structures of the complexes
The structures of the synthesized complexes which are presented in Fig. 5, are
consistent with the chemical, thermal and spectroscopic properties which were
determined.
* * *
We thank to Ondokuz Mayis University Research Found in Turkey for the financial support to this work.
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