Studies on some nickel(II) and cobalt(II) mixed ligand complexes of arylsalicyl-aldimine and other ligands

Article abstract of DOI:10.1023/A:1016190522455

Several new mixed ligand complexes of Ni(II) and Co(II) of p-tolylsalicylaldimine (Sal-TH) or p-anisylsalicylaldimine (Sal-AH) with 8-hydroxyquinoline (HOx), benzohydroxamic acid (BH₂) or α-picoline (α-pic) have been synthesized. The complexes have the general formulae: [M(Sal-T)L], [M(Sal-A)L]·H₂O or [Ni(Sal-T)₂(α-pic)₂]·H₂O where M=Ni(II) or Co(II), Sal-T=p-tolylsalicyl-aldiminate, Sal-A=p-anisylsalicylaldiminate, L=8-hydroxyquinolinate or benzohydroxamate and α-pic=α-picoline. Analytical, conductivity and spectral data support a square planar structure for Co(II) complexes and octahedral structure for Ni(II) complexes. The thermal behaviour of the complexes has been studied by TG and DTG techniques. It was proposed that the decomposition of the complexed Schiff base takes place via a radical mechanism while for 8-hydroxyquinolinate or benzohydroxamate via proton transfer. Kinetic parameters of the thermal decomposition process have been computed by means of Coats-Redfern and Horowitz-Metzger methods.

Full text of DOI:10.1023/A:1016190522455

Journal of Thermal Analysis and Calorimetry, Vol. 68 (2002) 917–929
STUDIES ON SOME NICKEL(II) AND COBALT(II)
MIXED LIGAND COMPLEXES OF ARYLSALICYL-
ALDIMINE AND OTHER LIGANDS
A. I. El-Said*
Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt
(Received April 21, 2001)
Abstract
Several new mixed ligand complexes of Ni(II) and Co(II) of p-tolylsalicylaldimine (Sal-TH) or
p-anisylsalicylaldimine (Sal-AH) with 8-hydroxyquinoline (HOx), benzohydroxamic acid (BH2) or
α-picoline (α-pic) have been synthesized. The complexes have the general formulae: [M(Sal-T)L],
[M(Sal-A)L] H2O or [Ni(Sal-T)2(α-pic)2] H2O where M=Ni(II) or Co(II), Sal-T=p-tolylsalicyl-
aldiminate, Sal-A=p-anisylsalicylaldiminate, L=8-hydroxyquinolinate or benzohydroxamate and
α-pic=α-picoline. Analytical, conductivity and spectral data support a square planar structure for
Co(II) complexes and octahedral structure for Ni(II) complexes. The thermal behaviour of the com-
plexes has been studied by TG and DTG techniques. It was proposed that the decomposition of the
complexed Schiff base takes place via a radical mechanism while for 8-hydroxyquinolinate or
benzohydroxamate via proton transfer. Kinetic parameters of the thermal decomposition process
have been computed by means of Coats–Redfern and Horowitz–Metzger methods.
Keywords: arylsalicylaldimine, 8-hydroxyquinoline, metal benzohydroxamate
Introduction
Several metal complexes of Schiff bases containing N, O donors have been exten-
sively studied [1–5]. This may be due to the reported anticarcinogenic and antiviral
activity of these donor ligands and their complexes [6, 7]. The Schiff bases derived
from salicylaldehyde and different amines are considered to be suitable models for B6
vitamins [8], they form stable complexes with metal ions.
Hydroxamic acids have wide use as colorimetric and analytical reagents [9–11].
Furthermore, naturally occurring hydroxamic acids may function variously as antibi-
otics, antibiotic antagonists and tumour inhibitor [12, 13]. Thermal decomposition of
the metal complexes of 8-hydroxyquinoline, hydroxamic acid and Schiff bases has
been extensively studied in recent years [14–17]. In this paper we report the synthesis
and characterization of some Ni(II) and Co(II) mixed ligand complexes containing
*
E-mail: asmaa@aun.eun.eg
1418–2874/2002/ $ 5.00
Akadémiai Kiadó, Budapest
© 2002 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
p-tolylsalicylaldimine or p-anisylsalicylaldimine (Fig. 1) with 8-hydroxyquinoline,
benzohydroxamic acid or α-picoline. The thermal properties of the solid complexes
have been studied as an extension of our work on the thermal decomposition of mixed
ligand complexes [18–22].
Fig. 1 Structure of the used Schiff bases; R=CH3 (Sal-TH) or R=OCH3 (Sal-AH)
Experimental
All chemicals were of analytical grade. The Schiff base ligands were prepared by the
usual condensation method [1]. The IR spectra were recorded on a Shimadzu IR-470
spectrophotometer and electronic spectra were obtained using a Shimadzu UV-2101
PC spectrophotometer. Conductivity were made using a LF Digi 550 conductance
bridge. Thermal studies were carried out on 2000 DuPont thermal analyzer, with the
heating rate 10°C min–1. The mass spectra were obtained on a JEOLJMS600 apparatus.
Preparation of the complexes
The preparation of all mixed ligand complexes containing 8-hydroxyquinolinate or
benzohydroxamate followed the same procedure. The preparation of (p-anisylsalicyl-
aldiminato)-(benzohydroxamato) cobalt(II) is typical.
To a 20 mL methanolic solution of p-anisylsalicylaldimine (0.23 g, 1 mmol), a
hot solution of cobalt(II) acetate (0.25 g, 1 mmol) in 15 mL methanol was added
dropwise, followed by the addition of 10 mL of a methanolic solution of benzo-
hydroxamic acid (0.14 g, 1 mmol). The reaction mixture was refluxed for 3 h. The
product was precipitated by addition of sodium acetate solution (0.2 M). The com-
plex was filtered, washed with methanol (5 mL) and dried over P4O10.
Preparation of bis(p-tolylsalicylaldiminato)bis(α-picolinato)nickel(II) complex
A methanolic solution (20 mL) of p-tolylsalicylaldimine (0.21 g, 1 mmol) was mixed
with nickel acetate (0.49 g, 2 mmol), methanolic solution (20 mL). To the above mix-
ture α-picoline (0.18 g, 2 mmol) in 10 mL methanol was added and the reaction mix-
ture was refluxed for 1 h. The complex was precipitated by adding (20 mL, 0.2 M) so-
dium acetate solution. The product was filtered, washed with (5 mL) methanol and
dried over P4O10.
Results and discussion
The synthesis of the complexes may be represented as follows:
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EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
919
sod. acetate
M(OAc)2 4H2O+Sal-TH+HL
→ M(Sal-T)L+2HOAc+H2O
sod. acetate
M(OAc)2 4H2O+Sal-AH+HL
→ M(Sal-A)L H2O+2HOAc+H2O
sod. acetate
Ni(OAc)2 4H2O+2Sal-TH+2α-pic
→ Ni(Sal-T)2(α-pic)2 H2O+2HOAc+H2O
where M=Ni(II) or Co(II), Sal-TH=p-tolylsalicylaldimine, Sal-AH=p-anisylsalicyl-
aldimine, HL=8-hydroxyquinoline or benzohydroxamic acid and α-pic=α-picoline.
All of the isolated compounds are air-stable, non-hygroscopic and colored sol-
ids; insoluble in water, ethanol and methanol, soluble in DMF and DMSO. The ele-
mental analysis data (Table 1) are consistent with the formation of the complexes in
the molar ratio 1:1:1 of the metal salt, the Schiff base and 8-hydroxyquinoline or
benzohydroxamic acid while the last complex (Table 1) however, is formed in the
molar ratio 1:2:2 between nickel acetate, p-tolylsalicylaldimine and α-picoline. The
molar conductance of the complexes (2.00–10.00 ohm–1 cm2 mol–1) in DMF
( 10–3 M) indicates their non-electrolytic nature [23].
Table 1 Color, elemental analyses and melting point of the complexes
C/
H/
N/
Decomp.
m.p./°C
Color
Compound
% found (calc.)
66.92
(66.87)
4.35
(4.39)
6.83
(6.78)
light green
290
190
125
185
195
192
150
145
191
Ni(Sal-T)(Ox)
reddish
brown
66.79
(66.83)
4.42
(4.39)
6.75
(6.78)
Co(Sal-T)(Ox)
62.29
(62.26)
4.50
(4.48)
6.95
(6.92)
dark green
Ni(Sal-T)(BH)
reddish
brown
62.25
(62.23)
4.46
(4.48)
6.89
(6.91)
Co(Sal-T)(BH)
yellowish
green
61.80
(61.78)
4.50
(4.51)
6.24
(6.27)
Ni(Sal-A)(Ox) H2O
Co(Sal-A)(Ox) H2O
Ni(Sal-A)(BH) H2O
Co(Sal-A)(BH) H2O
Ni(Sal-T)2(α-pic)2 H2O
reddish
brown
61.77
(61.75)
4.48
(4.51)
6.24
(6.26)
57.41
(57.44)
4.61
(4.59)
6.39
(6.38)
light green
reddish
brown
57.38
(57.41)
4.57
(4.59)
6.36
(6.38)
yellowish
green
70.33
(70.29)
5.91
(5.90)
8.22
(8.20)
IR spectra
The infrared spectra of the complexes present the characteristics of both the Schiff
base and the other respective ligands. IR spectra (Table 2) show a strong band in the
region 1603–1620 cm–1 which indicates the coordination of the arylsalicylaldiminate
through the azomethine nitrogen [24]. Also a band in the region 1230–1265 cm–1 is
J. Therm. Anal. Cal., 68, 2002

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EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
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EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
921
attributable to ν(C–O); this band is shifted to higher frequency in the complexes indi-
cating coordination of the phenolic oxygen to the metal ion [25, 26]. For the oxinate
part, ν(C–O) band is found in the range 1105–1110 cm–1, typical of oxinate com-
plexes of metals [27], ν(C=N) occurs in the region 1570–1580 cm–1. Coordination of
benzohydroxamate is manifested by the appearance of a 1555–1565 cm–1 band due to
ν(CO), the shift of which to lower energy indicates coordination of benzohydr-
oxamate through the ketonic oxygen atom [28]. Benzohydroxamate exhibits further
two bands in the 1460–1480 and 1140–1150 cm–1 regions, apparently associated with
ν(CN) and ν(NO), respectively [29]. The presence of secondary coligand (α-pico-
line) in the complex Ni(Sal-T)2(α-pic)2 H2O is confirmed from its vibrations as a
strong band at 1400 cm–1 and a medium band at 530 cm–1 which are characteristic for
methylpyridine adducts [30, 31]. For p-anisylsalicylaldiminate Ni(II) and Co(II)
mixed ligand complexes, the absorption at 3400–3450 cm–1 is attributed to O–H
stretching vibration of lattice water molecules.
Electronic spectra
The electronic spectra of the complexes were studied in DMF solution and the char-
acteristic bands are summarized in Table 2. Ni(II) complexes show a band in the
16929–16129 cm–1 region corresponding to 3A2g→3T1g (F) d–d transition. The reddish
brown color of the Co(II) complexes and the weak broad band observed at
12048–11627 cm–1 region are characteristic of square planar structure [32]. A strong
and broad band at 18182–17241 cm–1 is also observed in analogy with other square
planar Co(II) complexes [33]. Other bands above 11111 cm–1 are not observed on our
instrument. A band observed in all complexes at 24390–20408 cm–1 region is sup-
posed to be charge transfer. Additional bands in the 41666–26666 cm–1 region are as-
signed to π→π* and n→π* transitions.
Mass spectra
Mass spectra of the complexes Ni(Sal-T)(BH), Co(Sal-A)(BH) H2O and
Ni(Sal-T)2(α-pic)2 H2O have been investigated. The molecular ions for all complexes
are not detected probably due to their thermal instability in the ionisation beam.
Ni(Sal-T)(BH) and Co(Sal-A)(BH) H2O show similar fragmentation pattern. A rep-
resentative fragmentation scheme for Co(Sal-A)(BH) H2O is suggested in Scheme I.
For the complex Ni(Sal-T)2(α-pic)2 H2O, the fragment Ni(Sal-T)+2 was detected
in the mass spectrum, this might be regarded as direct fragments of the molecular ion
by the elimination of the water and α-picoline molecules. Evidence for this arises
from the existence of the water and the free base as can be deduced from the peaks at
m/z=18 and m/z=93, respectively. This pathway can be simplified as follows in
Scheme II.
J. Therm. Anal. Cal., 68, 2002

922
EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
Existence of the ions [M(Sal-A)]+ or [M(Sal-T)]+ and [(2-cyanophenolato)cobalt
(III)+] (I) or [(2-cyanophenolato)nickel (II)+] (II) in the mass spectra of all complexes in-
dicates that fragmentation of the complexes proceeds almost with the same mechanism.
In conclusion, a square planar structure is assigned to Co(II) complexes while an
octahedral structure is proposed for Ni(II) complexes. The six-coordination geometry
may be attained through molecular association. The proposed structures of the com-
plexes are shown in Fig. 2.
Thermal analysis
Thermogravimetric analyses of some of the synthesized complexes were carried out
in air with a heating rate of 10°C min–1. The complexes are Co(Sal-T)(Ox),
Ni(Sal-T)(BH), Ni(Sal-A)(Ox) H2O, Co(Sal-A)(Ox) H2O, Ni(Sal-A)(BH) H2O and
Ni(Sal-T)2(α-pic)2 H2O. Thermoanalytical data are shown in Table 3. The simulta-
neous TG-DTG curves of the complexes Ni(Sal-A)(Ox) H2O and Co(Sal-A)(Ox)
J. Therm. Anal. Cal., 68, 2002

EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
923
Fig. 2 Suggested molecular structures of the complexes; R=CH3 or OCH3; n=0 for
R=CH3 and n=1 for R=OCH3; O N=8-hydroxyquinolinate or benzohydroxa-
mate and α-pic=α-picoline
H2O show three-step decomposition process (Fig. 3). For the two complexes, the first
step indicates the loss of a water molecule at 100 and 98°C, respectively. Such a de-
hydration at low-temperature confirms the presence of water of crystallization [34].
The second step involves the loss of 4-methoxyphenyl free radical (III) from the
Schiff base moiety at 235 and 231°C, respectively. The loss of a part from
arylsalicylaldimine as a free radical is recently reported [18]. The third step is a com-
Fig. 3 TG-DTG curves of Co(Sal-A)(Ox) H2O
J. Therm. Anal. Cal., 68, 2002

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EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
J. Therm. Anal. Cal., 68, 2002

EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
925
posite one (Fig. 3), involving the decomposition of the 8-hydroxyquinolinate moiety
and the remainder of the complexes at 375 and 404°C for Ni(Sal-A)(Ox) H2O and at
436 and 448°C for Co(Sal-A)(Ox) H2O with the formation of MO as the end product.
Fig. 4 TG-DTG curves of Ni(Sal-T)(BH)
Fig. 5 TG-DTG curves of Ni(Sal-T)2(α-pic)2 H2O
The complexes Co(Sal-T)(Ox), Ni(Sal-T)(BH) and Ni(Sal-A)(BH) H2O decom-
pose in three steps. The first step involves the loss of 4-methylphenyl free radical (IV)
or 4-methoxyphenyl free radical (III) for the first two complexes, respectively and
with a water molecule for the third complex (Fig. 4). The second step includes the
loss of the respective secondary ligand (8-hydroxyquinoline or benzohydroxamic
acid) via a proton transfer. This step occurs for the three complexes at 350, 337 and
361°C, respectively. It is proposed that a proton transfers during thermal decomposi-
tion from the azomethine group to the other coordinated ligand [35]. The final de-
composition occurs at 455, 464 and 442°C, respectively, to produce the correspond-
ing metal oxide. A suggested thermal decomposition mechanism can be explained as
follows:
J. Therm. Anal. Cal., 68, 2002

926
EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
Table 4 Computed kinetic parameters of the complexes
E*/
∆H*/
∆S*/
Step
Method
Compound
J mol–1 K–1
kJ mol–1
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
Horowitz–Metzger
58
62
55
56
240
221
68
–
47
50
134
–
–65
–
–88
–87
–69
–
1
2
3
Co(Sal-T)(Ox)
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
Horowitz–Metzger
64
63
148
–
192
190
41
–
44
–
135
–
–87
–97
–58
–
–39
–
1
2
3
Ni(Sal-T)(BH)
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
27
30
57
55
88
89
–
–44
–
–83
–
1
2
Ni(Sal-A)(Ox) H2O
Co(Sal-A)(Ox) H2O
Horowitz–Metzger
60
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
28
26
57
55
89
95
52
50
–67
–
–88
–91
1
2
Horowitz–Metzger
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
Horowitz–Metzger
55
56
147
149
188
192
51
47
171
–
182
148
–81
–86
–50
–
–
–95
1
2
3
Ni(Sal-A)(BH) H2O
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
Horowitz–Metzger
Coats–Redfern
Horowitz–Metzger
–
14
62
59
290
289
–
11
77
–
184
183
–
–61
–79
–88
–101
–121
1
2
3
Ni(Sal-T)2(α−pic)2 H2O
J. Therm. Anal. Cal., 68, 2002

EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
927
For the complex Ni(Sal-T)2(α-pic)2 the decomposition proceeds in three steps
(Fig. 5). In the first step one molecule of water is lost at 100°C. The second step in-
cludes the loss of 4-methylphenyl free radical accompanied by two α-picoline mole-
cules at 325°C. The remainder of the complex is decomposed in the third step at
421°C to produce NiO as the end product (Table 3).
A computer program [36] which enables the determination of kinetic and ther-
modynamic parameters from experimental TG/DTG data has been used. This pro-
gram allows the Coats–Redfern and Horowitz–Metzger methods [37, 38] to be per-
formed, the results are summarized in Table 4. From the results obtained we can con-
clude the following:
1. From the observation that the obtained activation energy values for the loss of
4-methylphenyl free radical and 4-methoxyphenyl free radical are slightly different
(Table 4), it is assumed that the attack in these compounds is not directly at the
metal-ligand bonds but frequently at a point in the Schiff base moiety and indicates
that the decomposition takes place through the same mechanism [39].
2. The loss of 8-hydroxyquinoline is much easier than the loss of benzohydroxamic
acid which confirms the high thermal stability of benzohydroxamato complexes.
Microbiological activities
The biological activities of some of the prepared complexes were tested against a
number of fungi and bacteria. The used fungi were A. niger and Penicillum while the
tested bacteria were P. aeruginosa and E. coli. The culture media were nutrient agar
(N.A.) supplemented with one gm yeast/liter. The antifungal and antibacterial activ-
ity of each compound were evaluated by the classical filter paper technique. For each
complex, a mass of 0.01 g was dissolved in 5 mL DMF. The complexes show variable
antibacterial and antifungal activities against the used bacteria and fungi (Table 5).
Table 5 Microbiological activities of the complexes
Fungi
Bacteria
P. aeruginosa
Complex
A. niger
Penicillum
E. coli
++
+
+
+
+
++
+
++
++
+
Ni(Sal-T)(BH)
++
Co(Sal-T)(Ox)
–
+
Co(Sal-A)(Ox) H2O
Ni(Sal-T)2(α-pic)2 H2O
–
+
++
The diameter (D) of inhibition zone (in nm) is expressed as ‘+’ and ‘–’; ‘++’: D=15–20; ‘+’: D=1–15;
‘–’: D=0
Conclusions
All the studied complexes undergo thermal decomposition firstly via the loss of
4-methylphenyl free radical or 4 methoxyphenyl free radical from the Schiff base
J. Therm. Anal. Cal., 68, 2002

928
EL-SAID: NICKEL(II) AND COBALT(II) MIXED LIGAND COMPLEXES
moiety, this is accompanied by the release of H2O molecule or two α-pic molecules
in the complexes Ni(Sal-A)(BH) H2O and Ni(Sal-T)2(α-pic)2 H2O, respectively. TG
curves show evolution of the secondary ligand (HOx or BH2) in a separate stage for
the complexes Co(Sal-T)(Ox), Ni(Sal-T)(BH) and Ni(Sal-A)(BH) H2O or with the
remainder of the complex for Ni(Sal-A)(Ox) H2O and Co(Sal-A)(Ox) H2O. This can
be occurred via proton transfers from the azomethine group to the other coordinated
ligand. All the complexes form the corresponding metal oxide as the end product.
Appendix
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