Synthesis, characterization and antioxidant studies on 4-phenyl-1,3,5- triazine-2,6-diamine Schiff bases and their nickel(II), copper(II) and zinc(II) complexes

Article abstract of DOI:10.14233/ajchem.2013.13544

The synthesis, characterization, thermogravimetric and antioxidant activity of Schiff bases derived from 4-phenyl-1,3,5-triazine-2,6- diamine and 3,5-di-tert-butylsalicyladehyde (H2L) with their nickel(II), copper(II) and zinc(II) complexes are reported.

Full text of DOI:10.14233/ajchem.2013.13544

Asian Journal of Chemistry; Vol. 25, No. 6 (2013), 3105-3108
http://dx.doi.org/10.14233/ajchem.2013.13544
Synthesis, Characterization and Antioxidant Studies on 4-Phenyl-1,3,5-triazine-2,6-diamine
Schiff Bases and Their Nickel(II), Copper(II) and Zinc(II) Complexes
*
ABDULAZIZ ALI , NORBANI ABDULLAH and MOHD JAMIL MAAH
Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
*Corresponding author: E-mail: abomaz1389@yahoo.com
(Received: 2 February 2012;
Accepted: 12 December 2012)
AJC-12528
The synthesis, characterization, thermogravimetric and antioxidant activity of Schiff bases derived from 4-phenyl-1,3,5-triazine-2,6-
diamine and 3,5-di-tert-butylsalicyladehyde (H2L) with their nickel(II), copper(II) and zinc(II) complexes are reported. The structures of
the Schiff bases and the metal complexes wer characterized by elemental analyses (CHN), Fourier transform infrared spectroscopy (FT-
1
IR) and ultraviolet-visible spectroscopy (UV-VIS). Additionaly, H- and 13C-nuclear magnetic resonance spectroscopy (NMR) were
recorded for the Schiff bases, while thermogravimetric analysis was recorded for the complexes. Spectral studies reveal that the ligands
were acting as tetradentate chelating agents and coordinated to the metal center via deprotonated phenolate oxygen and azomethine
nitrogen atoms in a 1:1 ligand to metal ratio. The antioxidant activities of the ligand and the metal complexes were examined using the
DPPH radical scavenging method. The results show that the ligand exhibit higher radical scavenging ability than the metal complexes.
Key Words: Synthesis, Antioxidant, Ni(II), Cu(II), Zn(II), Complexes, Schiff base.
INTRODUCTION
Schiff bases are organic compounds which contain the
azomethine group (-C=N-). These compounds are synthesized
by the reaction of a primary amine and an active carbonyl
N
N
compound (aldehyde or ketone)1-3.
A great deal of information regarding the properties of
synthetic Schiff bases of potential biological interest has arisen
during the last few years4,5, different classes of organic comp-
ounds containing rich conjugated system can be observed when
these compounds contain hydroxyl groups attached to aromatic
rings. For example, compounds containing an azomethine
group (-CH=N-) known as Schiff bases are formed by the
condensation of a primary amine with carbonyl compounds.
Schiff bases obtained from aromatic aldehydes and aromatic
amines have an effective conjugation system and quite stable.
Schiff bases are important compounds owing to their wide
range of biological activities and industrial application. They
have been found to posses the pharmacological activities such
as anticancer6, antimicrobial7, antiinflammatory8 and antioxi-
dants9,10. Such activities may be related to the structural
arrangements of the ligands and to the nature of the substituent
groups11.
N
OH
N
N
(H3C)3C
HO
C(CH3)3
C(CH3)3
(H3C)3C
Fig. 1. Proposed structures of the ligand H2L
complexes. The novel compounds were characterized on the
bases of elemental analysis, IR, 1H and 13C NMR spectroscopy,
UV-VIS as well as thermogravimetric analysis (TGA) with a
view to further investigating the radical scavenging abilities
of these novel compounds using the DPPH method.
EXPERIMENTAL
4-Phenyl-1,3,5-triazine-2,6-diamine (C6H9N5, FW 187.21),
3,5-di-tert-butylsalicyl-aldehyde (C15H22O2, FW 234.34),
triethylamine, copper(II) acetate monohydrate (CuC4H6O4·H2O;
FW 199.65), nickel(II) acetate tetrahydrate (NiC4H6O4·4H2O;
FW 248.86), zinc acetate dihydrate (C4H6O4Zn·2H2O, FW
In this work, we are encouraged to synthesize triazine
Schiff bases containing hydroxyl groups attached to the
aromatic ring (Fig. 1), along with their nickel, copper and zinc

3106 Ali et al.
Asian J. Chem.
219.50) and DPPH. These chemicals and common organic
solvents were commercially available and used as received.
IR spectra were recorded with a Perkin-Elmer FT-IR
spectrophotometer model Spectrum 2000 using KBr pellets
as support in the range 4000-370 cm-1. 1H and 13C NMR spectra
were recorded at room temperature on a JEOL ECA-400
spectrometer, operating with a frequency of 400 MHz, using
DMSO-d6 as solvent. Electronic spectra, in DMSO solution,
were obtained using a Varian 50 conc UV-visible spectropho-
tometer over the wavelength range 200-800 nm. Thermogravi-
metric analysis was carried out on Perkin Elmer Precisely TGA
4000 thermogravimetric analyzer. The instrument was adjusted
at a heating rate of 20 ºC/min. The heating was performed
from 50-900 ºC.
bases and the metal complexes were dissolved in DMSO to
obtain concentration of 1 mg/mL. These stock solutions were
then diluted to 5, 10, 25, 50 and 100 µg/mL. Then, 200 µL of
each sample solution were combined with 50 µL of DPPH
(0.3 mmL) in triplicate in a 96-well microtitre plate. Final
concentrations of the Schiff bases were 4, 8, 20, 40 and 80 µg/
mL. The microtitre plate was incubated for 0.5 h at room
temperature. The plate was then read at 515 nm for 3 h with
20 min intervals to reach a steady state against DMSO as a
blank. The percentage DPPH quenched was calculated accor-
ding to the equation:
(Abs.blank − Abs.sample
)
DPPH quenched(%) =
×100
Abs.blank
The DPPH free radical scavenging assay was performed
in triplicate and the average value was obtained. All determi-
nations were made using the Infinite® 200 PRO plate reader
(TECAN, Männedorf, Switzerland).
the percentage free radical scavenging activity (inhibition %)
was plotted against concentration in µg/mL.
RESULTS AND DISCUSSION
Synthesis of the ligand
The physical properties of the ligands and its metal
complexes were listed in Table-1. Elemental analyses for the
complexes confirm 1:1 metal to ligand stoichiometry. The
compounds are very stable at room temperature in the solid
state. The ligands are soluble in ethanol, methanol, acetone
and high boiling point solvents like DMSO and DMF, whereas
the complexes dissolves only in DMSO and DMF and not
soluble in either ethanol or methanol.
H2L: A solution of 3,5-di-tert-butylsalicylaldehyde (2 g,
8.53 mmol) and 4 phenyl-1,3,5-triazine-2,6-diamine (0.79 g,
4.21 mmol) was mixed and stirred under reflux for 2 h. The
pale yellow powder formed was filtered and recrystallized from
ethanol. It was dried in an oven at 80 ºC for 0.5 h. Selected
FT-IR data (KBr, νmax, cm-1): 3329 (m, -OH), 2923 (m, C-H),
1612 (s, C = N), 1273 (s, C-O).
IR spectra: The main stretching frequencies of the IR
spectra of the ligand and complexes were shown in Table-2.
For the ligand, the spectra show the characteristics strong
peak due to C=N stretching at 1612 cm-1, indicating the forma-
tion of the Schiff base. Another strong peak at 1273 cm-1 is
assigned to C-O phenolic stretching, while a sharp weak peak
at 3329 cm-1 is characteristic of free -OH group1. Other peaks
bands in the region 1500-1000 cm-1 arise from benzene ring
skeletal vibrations. The results strongly support the formation
of the Schiff base13.
General method for synthesis of the metal complexes:
A solution of H2L (0.50 g, 0.81 mmol) in ethanol (40 cm3)
was added to equimolar quantity of the metal acetate in ethanol.
Few drops of triethylamine were then added. The mixture was
magnetically stirred and refluxed for 3 h. The product formed
was filtered and recrystallized from DMSO. NiL·2H2O,
selected FTIR data (KBr, νmax, cm-1): 3054 (C-H, m), 1606
(C=N, s), 1239 (C-O, s), 550 (Ni-O, w). CuL·H2O, selected
FTIR data (KBr, νmax, cm-1): 2345 (C-H, s), 1605 (C=N, s),
1241 (C-O, s), 546 (Zn-O, w). ZnL·2H2O, selected FTIR data
(KBr, νmax, cm-1): 2532 (C-H, m), 1608 (C=N, s), 1240 (C-O,
s), 548 (Zn-O, w).
The IR spectra of the nickel complexes differ from that
of the ligand. It is further noted that the -OH peak, observed
for H2L at 3329 cm-1, is now observed at 3401 cm-1 and is
assigned to coordinated H2O molecules in agreement with the
results from the elemental analyses. The peaks for C=N at
1612 cm-1 and C-O at 1273 cm-1 observed for H2L have shifted
Antioxidant activity of the ligand and metal complexes
by DPPH method: Free radical scavenging activity of the
test compounds were determined by the 1,1-diphenyl
picrylhydrazyl (DPPH) assay method12. Each of the Schiff
TABLE-1
PHYSICAL PROPERTIES OF THE LIGAND AND COMPLEXES
Found. (calcd.) (%)
Compound
H2L
Formula
Yield (%)
Colour
f.w.
C
H
N
C39H49N5O2
79
75
78
75
Yellow
Green
619.89
712.58
699.43
719.28
75.49 (76.35)
65.67 (66.57)
66.91 (66.13)
65.06 (66.14)
7.90 (7.21)
7.15 (6.46)
7.01 (6.34)
7.09 (6.78)
11.29 (11.87)
9.82 (10.15)
10.01 (10.79)
9.73 (10.03)
NiL·2H2O
CuL·H2O
ZnL·2H2O
NiC39H51N5O4
CuC39H49N5O3
ZnC39H51N5O4
Green
Yellow
TABLE-2
IR SPECTRAL BANDS (cm–1) OF LIGAND AND ITS METAL COMPLEXES
Compound
ν(O-H)
3329
ν(C-H) Aliphatic
ν(C=N)
1612
ν(C-O)
1273
1239
1241
1240
ν(M-O)
–
ν(M-N)
–
H2L
2923
3054
2345
2532
NiL·2H2O
CuL·H2O
ZnL·2H2O
3401(H2O)
3400 (H2O)
3422 (H2O)
1606
550
546
548
523
496
500
1605
1608

Vol. 25, No. 6 (2013)
Studies on 4-Phenyl-1,3,5-triazine-2,6-diamine Schiff Bases and Their Metal(II) Complexes 3107
to lower energy at 1606 and 1239 cm-1, respectively in NiL.
These suggest that the phenolic oxygens and imino nitrogens
are coordinated to Ni(II). Additionally, a new peak observed
at 550 cm-1 may be assigned to Ni-O bond1.
these bands are assigned to the transitions 3A2g → 3T2g, 3A2g →
3T1g(F), respectively17.
The peak at 410 nm (ε = 0.5 × 104 M-1 cm-1) is assigned to
metal-ligand charge transfer (MLCT). The spectrum is also
compared with that of H2L. It is noted that the π-π* band
observed for H2L (288 nm) remains shifted in the complex
(274 nm) (ε = 2.4 × 104 M-1 cm-1). However, the n-π* band
may be hidden under the strong MLCT band at 348 nm (ε =
0.7 × 104 M-1 cm-1). Thus, this band is significantly red-shifted
from ca. 300-400 nm as a result of complexation to the Ni(II).
These results are in agreement with literature result indicating
the formation of the complexes18.
The IR spectra for Cu(II) complexes shows the expected
functional groups as previously discussed for the correspon-
ding Ni(II) complex. The C=N, C-O and Cu-O peaks for
[CuL6]·H2O are at 1605, 1241 and 546 cm-1, respectively. These
are almost similar to those of the corresponding Ni(II)
complex, suggesting similar bond strength.
The IR spectra for Zn(II) complexes shows the presence
of all the expected functional groups. The wavenumbers of
C=N (1608 cm-1) and C-O (1240 cm-1) groups are almost the
same as for [CuL6(H2O)] (1605 and 1241 cm-1, respectively.
1H NMR spectra: The 1H NMR spectra for H2L is consistent
with the expected structural formula (Fig. 1). The singlet at
9.96 ppm is due to phenolic hydrogen; a singlet at 8.24 ppm is
due to imino hydrogen; and a multiplet in the range 6.70-7.61
ppm is due to the aromatic hydrogens. Furthermore a singlet
at 1.33 ppm is due to tertiary butyl protons. The integration
ratio for these hydrogens is 1:1:5.5, respectively (expected
ratio = 1:1:5.5) and supports the molecular symmetry for the
Schiff base14.
13C NMR spectra: Further evidence for the proposed
structure in Fig. 1 can be drawn from13C NMR spectra. The
spectra shows 15 peaks, assigned as follows: 29 ppm (tertiary
carbon atoms in the tert-butyl groups), 31.8 ppm for the six
methyl groups, 159.4 ppm C-OH, 167.91 ppm C=N, The aro-
matic carbons lie between 120.6-141.9 ppm. However, the ex-
pected number of peaks, after taking account of the symmetry
of the structure, is14.
The UV-visible spectra of the copper complexes shows a
broad d-d peak at 670 nm (εmax = 200 M-1 cm-1). Thus, [CuL
(H2O)] is a mononuclear square pyramidal complex. The
π-π* and MLCT bands are at 297 nm (ε = 2.5 × 104 M-1 cm-1)
and 402 nm (ε = 1.02 × 104 M-1 cm-1), respectively which are
almost the same as for the corresponding Ni(II) complex (274,
410 nm) and may be similarly explained.
The UV-visible spectra of the zinc complexes shows shows
that the MLCT and 0-π* peaks 390 nm (ε = 1.7 × 104 M-1 cm-1)
and 275 nm (ε = 2.5 × 104 M-1 cm-1) are at almost the same
energy as the corresponding peaks for [CuL(H2O)] (402, 297
nm. Thus, both metal ions have insignificant effect on the elec-
tronic transitions of the organic moiety. The MLCT peak is
normally observed from 348-323 nm for Zn(II) complexes,
involving electronic transitions from the full d orbitals of the
metal ion (3d10) to antibonding orbitals of the ligand19.
Thermal analysis (TGA): The thermal decomposition
process for Ni(II), Cu(II) and Zn(II) complexes were examined
and assessed in the temperature range 50-900 ºC. The obtained
themo-analytical data from the thermogravimetric curves were
summarized in Table-4.
UV-visible spectra: The UV-VIS spectral data of the
ligand and their complexes in DMSO are listed in Table-3.
The UV-visible spectra for H2L shows two moderately intense
absorption bands at 288 nm (ε = 2.1 × 104 M-1 cm-1) and 351
nm (ε = 2.2 × 104 M-1 cm-1). The bands are assigned to π-π*
transition of the aromatic ring and n-π* transition of the
azomethine chromophores, respectively. These values are in
TABLE-4
THERMAL ANALYSIS DATA FOR THE COMPLEXES
Decomposition
Tmax (ºC)
Eliminated
species
Compound
Step
First
Second
Third
First
130
300
875
140
310
650
150
340
850
2H2O
Ligand
NiL·2H2O
agreement with other Schiff bases reported in the literatures15,16
.
Residue (NiO)
H2O
CuL·H2O
TABLE-3
UV-VIS OF THE LIGAND AND COMPLEXES
Second
Third
First
Ligand
Residue (CuO)
2H2O
Compound
Tentative assignment
λmax (nm)
288
ε (M-1 cm-1)
2.1 × 104
2.2 × 104
550
π-π*
n-π*
3A2g → 3T2g
ZnL·2H2O
Second
Third
Ligand
H2L
Residue (ZnO)
351
902
3A2g → 3T1g(F)
The data obtained indicate that nickel complexes are
thermally stable up to 246 ºC. The first weight loss of 2.7 % at
130 ºC corresponds to the loss of coordinated H2O molecules
(expected, 5.3 %). The next step represents a total weight loss
of 87.4 % and is assigned to the decomposition of the ligand
(expected, 87 %). The amount of residue at 875 ºC is 9.6 %.
Assuming that the residue is NiO, the expected value is 10.4 %,
which is within the acceptable experimental error.
736
257
0.5 × 104
2.4 × 104
0. 7 × 104
1.02 × 104
2.5 × 104
1.7 × 104
2.5 × 104
CT
π-π*
n-π*
CT
NiL·2H2O
410
274
348
402
CuL·H2O
ZnL·2H2O
π-π*
297
390
CT
π-π*
275
The data obtained indicate that copper complexes are
thermally stable up to 250 ºC. Thus, it is as thermally stable as
[NiL(H2O)2] (246 ºC).
For nickel complexes the UV-VIS spectra exhibits weak
d-d bands 902 (εmax = 550 M-1 cm-1) and 736 nm (εmax = 257 M-1
cm-1). These are consistent with an octahedral configuration
at Ni(II) and using the Tanabe-Sugano diagram for d8 complex,
The first weight loss of 3.5 % at 140 ºC corresponds to
the loss of coordinated H2O molecule (expected, 2.5 %). The

3108 Ali et al.
Asian J. Chem.
next step represents a total weight loss of 83.3 % and is
assigned to the decomposition of the ligand (expected, 88.6 %).
The amount of residue at about 650 ºC is 13.2 %. Assuming
that the residue is CuO, the expected value is 11.1 %. Thus,
the thermal properties of [CuL(H2O)] is similar from that of
[NiL(H2O)2].
that the ligands are more effective antioxidants than their metal
complexes. They mainly act as a hydrogen atom transferring
antioxidants in an oxidative process. However, chelating to
the metal ions suppresses this property through coordination
to the metal centre, thus hampers hydrogen atom abstraction
through deprotonation.
The data obtained indicate that zinc complexes are
thermally stable up to 245 ºC. Thus, it is as themally stable as
[CuL(H2O)] (250 ºC).
ACKNOWLEDGEMENTS
The authors thankfully acknowledged the support from
University of Malaya, IPPP grant No. P5340/2009C.
The first weight loss of 3.7 % at 150 ºC corresponds to
the loss of coordinated H2O molecules (expected, 5.1 %). The
next step represents a total weight loss of 86.1 % and is
assigned to the decomposition of the ligand (expected, 86.2 %).
The amount of residue at 850 ºC is 10.2 %. Assuming that the
residue is ZnO, the expected value is 11.3 %, which is within
the acceptable experimental error.
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Schiff base ligand derived from 4-phenyl-1,3,5-triazine-
2,6-diamine and 3,5-di-tert-butylsalicylaldehyde with their
nickel, copper and zinc complexes have been synthesised and
characterized. The antioxidant studies of the compounds reveal
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