Antioxidant, electrochemical, thermal, antimicrobial and alkane oxidation properties of tridentate Schiff base ligands and their metal complexes

Article abstract of DOI:10.1016/j.saa.2011.05.106

In this study, two Schiff base ligands (HL¹ and HL²) and their Cu(II), Co(II), Ni(II), Pd(II) and Ru(III) metal complexes were synthesized and characterized by the analytical and spectroscopic methods. Alkane oxidation activities of the metal complexes were studied on cyclohexane as substrate. The ligands and their metal complexes were evaluated for their antimicrobial activity against Corynebacterium xerosis, Bacillus brevis, Bacillus megaterium, Bacillus cereus, Mycobacterium smegmatis, Staphylococcus aureus, Micrococcus luteus and Enterococcus faecalis (as Gram-positive bacteria) and Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Yersinia enterocolitica, Klebsiella fragilis, Saccharomyces cerevisiae, and Candida albicans (as Gram-negative bacteria). The antioxidant properties of the Schiff base ligands were evaluated in a series of in vitro tests: 1,1-diphenyl-2- picrylhydrazyl (DPPH) free radical scavenging and reducing power activity of superoxide anion radical generated non-enzymatic systems. Electrochemical and thermal properties of the compounds were investigated.

Full text of DOI:10.1016/j.saa.2011.05.106

Spectrochimica Acta Part A 81 (2011) 184–198
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Antioxidant, electrochemical, thermal, antimicrobial and alkane oxidation
properties of tridentate Schiff base ligands and their metal complexes
a
b,∗
c
d
d
˙
Gökhan Ceyhan , Cumali C¸ elik , Serhan Uru s¸ , Ibrahim Demirta s¸ , Mahfuz Elmasta s¸ ,
Mehmet Tümera
Kahramanmara s¸ Süt c¸ ü Imam University, Chemistry Department, 46100 K.Maras, Turkey
Yalova University, Polymer Engineering, 77100 Yalova, Turkey
K.Mara s¸ Süt c¸ ü Imam University, University-Industry-Public Collaboration, Research-Development-Application Centre, 46100 K.Mara s¸ , Turkey
a
b
c
d
Gaziosmanpasa University, Department of Chemistry, 60240 Tokat, Turkey
a r t i c l e i n f o
a b s t r a c t
In this study, two Schiff base ligands (HL1 and HL ) and their Cu(II), Co(II), Ni(II), Pd(II) and Ru(III) metal
complexes were synthesized and characterized by the analytical and spectroscopic methods. Alkane oxi-
dation activities of the metal complexes were studied on cyclohexane as substrate. The ligands and their
metal complexes were evaluated for their antimicrobial activity against Corynebacterium xerosis, Bacillus
brevis, Bacillus megaterium, Bacillus cereus, Mycobacterium smegmatis, Staphylococcus aureus, Micrococcus
luteus and Enterococcus faecalis (as Gram-positive bacteria) and Pseudomonas aeruginosa, Klebsiella pneu-
moniae, Escherichia coli, Yersinia enterocolitica, Klebsiella fragilis, Saccharomyces cerevisiae, and Candida
albicans (as Gram-negative bacteria). The antioxidant properties of the Schiff base ligands were eval-
2
Article history:
Received 22 December 2010
Received in revised form 26 May 2011
Accepted 30 May 2011
Keywords:
Schiff bases
Antimicrobial
Antioxidant
Catalysis
•
uated in a series of in vitro tests: 1,1-diphenyl-2-picrylhydrazyl (DPPH ) free radical scavenging and
Electrochemical
Alkane-oxidation
reducing power activity of superoxide anion radical generated non-enzymatic systems. Electrochemical
and thermal properties of the compounds were investigated.
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
orrhagic shock, emphysema, cirrhosis, diabetes mellitus, hepatitis,
cancer, atherosclerosis as well as other ailments [4–7].
Free radicals contain one or more unpaired electrons, produced
in normal or pathological cell metabolism. Reactive oxygen species
ROS) react easily with these free radicals to become radicals them-
Schiff bases form an important class of organic compounds with
a wide variety of biological properties [8]. Development of a new
chemotherapeutic Schiff bases is now attracting the attention of
medicinal chemist [9]. Many studies have been reported regarding
the biological activities of Schiff bases, including their anticancer
[10], antibacterial [11], antifungal, and herbicidal activities. Schiff
bases derived from various amine and carbonyl derivatives were
reported to possess genotoxicity [12,13], antimicrobial [14], and
antifungal activities [15]. A number of Schiff bases [16] have been
tested for antibacterial [17], antifungal [18], anticancer [19], and
herbicidal [20] activities. Although many studies have investigated
the antioxidant properties of resveratrol [21], there have been
only a few reports of antioxidant and antiproliferative effects of
hydroxyl-substituted Schiff bases.
In this study, two Schiff base ligands and their metal com-
plexes were synthesized and characterized by the analytical and
spectroscopic methods. Antioxidant properties of the hydroxyl-
substituted Schiff base ligands were investigated. The antimicrobial
activities of the ligands and their metal complexes were stud-
ied using the bacteria and yeast. The redox properties of the
compounds were investigated by the cyclic voltammetry. The cat-
alytic properties of the metal complexes were investigated on
(
selves. ROS are various forms of activated oxygen, which include
−
free radicals such as superoxide anion radicals (O2 ) and hydroxyl
•
radicals (OH ), as well as non-free radical species (H O ) and the
2
2
singled oxygen (O ) [1,2]. They are formed in living organisms in
2
different ways, including normal aerobic respiration, stimulated
polymorphonuclear leukocytes and macrophages, and peroxi-
somes. They are natural by-products of our body’s metabolism.
They are dangerous; however, when present in excess, they can
attack biological molecules such as lipids, proteins, enzymes, DNA
and RNA, leading to cell or tissue injury [3]. The cells experience
an oxidative stress which contributes in a various clinical disorders
such as cancer, heart diseases, neurogenerative diseases like mul-
tiple sclerosis, Parkinson’s disease, autoimmune disease, stroke,
arthritis, ischemia, reperfusion injury, acute hypertension, haem-
∗ Corresponding author. Tel.: +90 344 219 10 93; fax: +90 344 219 10 42.
E-mail address: mtumer@ksu.edu.tr (C. C¸ elik).
1
386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.05.106

G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
185
–HC(CH ) ). 13C NMR (DMSO-d6 as solvent, ı in ppm): ı 169.0
cyclohexane as substrate. Thermal properties of the metal com-
plexes were investigated in the 20–1000 C temperature range.
3
2
◦
(CH N), 115.0–145.0 (Ar–C), 24,09 (CH), 40,75 (t-CH ), 40.54 (qua-
3
ternary C), 34.6 (–CH), 21.5 (–CH ). Mass spectrum (LC/MS APCI):
3
m/z 599 [M]+ (10%), 466 [M−144] (100%), 430 [M+2] (30%).
+
+2
2
. Experimental
UV–vis (ꢀmax, nm, DMF as solvent): 213, 277, 381, 386, 392, 394,
97. FT-IR (KBr, cm ): 3430 ꢁ(OH), 2998, 2892 ꢁ(CH2)alph, 1624
−
1
3
2
.1. General
ꢁ
(CH N), 1274 ꢁ(C–OH).
HL : Elemental analyses, found (calcd. %): C, 78.15 (78.39), H
.74 (9.21), N 4.90 (4.78). HNMR (CDCl , ( ppm): 9.80–10.10 (4, 3H,
2
The metal salts CuCl ·2H O, CoCl ·6H O, NiCl ·6H O,
2
2
2
2
2
2
1
9
3
RuCl ·3H O, Pd(AcO) and organic substances were purchased
3
2
2
OH), 8.60 (s, 2H, CH N), 7.40–8.20 (m, 6H, Ar–H), 1.13–1.64 (s, 27H,
–
from commercial sources and used as received, unless noted
otherwise. The Bu NBF compound was obtained from Fluka. Ele-
13
C(CH ) ). C NMR (CDCl , ı ppm) 169.00 (CH N), 115.00–145.00
3 3 3
4
4
(Ar–C), 40,77 (t-CH ), 40.56 (quaternary C). Mass spectrum (LC/MS
3
+
mental analyses (C, H and N) were performed using a LECO CHNS
9
metal complexes were obtained using KBr discs (4000–400 cm
with a Perkin Elmer spectrum 400 FT-IR spectrophotometer. FAR
spectra of the complexes were recorded using a Perkin Elmer
spectrum 400 FT-IR/FT-FAR instrument. The electronic spectra
were obtained in the 200–900 nm range by a Perkin Elmer Lambda
+
APCI): m/z 613 [M] (15%), 412 [M−201] (100%). UV–vis (ꢀmax,
32 elemental analyser. Infrared spectra of the ligands and their
−
1
nm, DMF as solvent): 201, 203, 217, 221, 402. FT-IR (KBr, cm ):
−
1
)
3
1
450 ꢁ(OH), 2975–2900 ꢁ(C–H)alph, 2875 ꢁ(C–H), 1643 ꢁ(C N),
290 ꢁ(C–OH), 750 ꢁ(C–H).
2
.3. Preparation of the Cu(II), Co(II), Ni(II) and Ru(III) complexes
4
5 spectrophotometer. Magnetic measurements were carried
Copper(II), cobalt(II), nickel(II) and ruthenium(III) complexes
out by the Gouy method using Hg[Co(SCN) ] as a calibrant. Mass
4
were obtained according to a general procedure: CuCl ·2H O
2
2
spectra of the ligands were recorded on a LC/MS APCI AGILENT
(
2 mmol, 0.241 g) was dissolved in ethanol and stirred under reflux
100 MSD spectrophotometer. 1H and
13
C NMR spectra were
1
for 30 min, followed by the addition of the Schiff base (1 mmol;
recorded on a Bruker 400 MHz instrument. TMS was used as
internal standard and CDCl3 as solvent. The thermal analyses
studies of the complexes were performed on a Perkin Elmer Pyris
Diamond DTA/TG Thermal System under nitrogen atmosphere at
1
2
0
.599 g for HL and 0.613 g for HL ) in ethanol and the reaction mix-
ture stirred upon heating for 24 h. The obtained complexes were
filtered, washed with methanol and dried in a vacuum desiccator
over CaCl2.
◦
a heating rate of 10 C/min.
Analytical and spectroscopic data of the metal complexes are
Cyclic voltammograms were recorded on an Iviumstat Electro-
chemical workstation equipped with a low current module (BAS
PA-1) recorder. The electrochemical cell was equipped with a BAS
glassy carbon working electrode (area 4.6 mm ), a platinum coil
auxiliary electrode and an Ag /AgCl reference electrode filled with
given below:
1
Co (L )(Cl) ·4H O: Elemental analyses, found (calcd. %): C 56.47
2
3
2
(
56.98), H 6.40 (6.50), N 2.41 (2.20). UV–vis (ꢀmax, nm, DMF as
solvent): 281, 327, 381, 410, 576. FT-IR (KBr, cm ): 3475 ꢁ(OH),
2
−
1
+
2
ꢁ
950–2800 ꢁ(C–H), 2850 ꢁ(C–H), 1650 ꢁ(C N), 1365 ꢁ(C–O), 950
(C–H), 540 ꢁ(M–O), 475 ꢁ(M–N). ꢂeff B.M. (298 K): 4.28 (for per
tetrabutylammonium tetrafluoroborate (0.1 M) in CH CN and DMF
3
solvents and adjusted to 0.00 V vs. SCE. Cyclic voltammetric mea-
surements were carried out at room temperature in an undivided
cell (BAS model C-3 cell stand) with a platinum counter elec-
trode and an Ag /AgCl reference electrode (BAS). All potentials
are reported with respect to Ag /AgCl. The solutions were deoxy-
metal).
Cu (L )(Cl) ·5H O: Elemental analyses, found (calcd. %): C 56.36
1
2
3
2
(
56.35), H 6.04 (6.43), N 3.29 (3.37). UV–vis (ꢀmax, nm, DMF as sol-
+
−
1
vent): 217, 236, 278, 280, 313, 410. FT-IR (KBr, cm ): 3452 ꢁ(OH),
+
2
8
960–2930 ꢁ(C–H)alph, 2870 ꢁ(C–H), 1629 ꢁ(C N), 1300 ꢁ(C–OH),
00 ꢁ(C–H), 575 ꢁ(M–O), 478 ꢁ(M–N). ꢂeff B.M. (298 K): 1.88 (for
genated by passing dry nitrogen through the solution for 30 min
prior to the experiments during the experiments.
per metal).
The microwave experiments were carried out using a Berghof
MWS3+ equipped with pressure and temperature control.
Microwave experiments were done in closed DAP 60 vessels. The
reaction products were characterized and analyzed by Perkin Elmer
Clarus 600 GC equipped with MS dedector fitted with Elite-5 MS
and FID detectors with BPX5 capillary columns.
1
Ni (L )(Cl) ·3H O: Elemental analyses, found (calcd. %): C 56.47
2
3
2
(
57.01), H 6.49 (6.50), N 3.33 (3.41). UV–vis (ꢀmax, nm, DMF as sol-
vent): 214, 220, 237, 265, 268, 401, 494, 598. FT-IR (KBr, cm ):
−
1
3
ꢁ
431 ꢁ(OH), 2966 ꢁ(C–H)alph, 2877 ꢁ(C–H), 1649 ꢁ(C N), 1350
(C–OH), 887 ꢁ(C–H), 574 ꢁ(M–O), 472 ꢁ(M–N). ꢂeff B.M. (298 K):
diamagnetic.
1
Ru(L )(Cl) ·6H O: Elemental analyses, found (calcd. %): C 59.00
2
2
2
.2. Preparation of the ligands (HL1 and HL2)
(59.06), H 6.75 (6.78), N 3.60 (3.62). UV–vis (ꢀmax, nm, DMF as sol-
vent): 201, 212, 217, 206, 209, 214, 399. FT-IR (KBr, cm ): 3452
−
1
The amine compound, 2,6-di-tert-butyl-4-hydroxy aniline, and
ꢁ(OH), 2966 ꢁ(C–H)alph, 2870 ꢁ(C–H), 1643 ꢁ(C N), 1300 ꢁ(C–O),
two carbonyl compounds (2,6-di-formyl-4-isopropyl phenol (A)
and 2,6-di-formyl-4-tert-butyl phenol (B)) were synthesized to
the reported methods [22,23]. The carbonyl compounds (1 mmol,
846 ꢁ(C–H), 532 ꢁ(M–O), 472 ꢁ(M–N). ꢂ B.M. (298 K): 1.74.
eff
1
Pd (L )(AcO) ·4H O: Elemental analyses, found (calcd. %): C
2
3
2
51.05 (50.99), H 6.60 (6.66), N 2.70 (2.64). UV–vis (ꢀmax, nm, DMF as
−
1
0.192 g for A and 0.206 g for B) in ethanol (20 mL, absolute) and
solvent): 214, 218, 220, 226, 232, 236, 252, 418. FT-IR (KBr, cm ):
3439 ꢁ(OH), 2945 ꢁ(C–H)alph, 2870 ꢁ(C–H), 1622 ꢁ(C N), 1350
ꢁ(C–O), 812 ꢁ(C–H), 552 ꢁ(M–O), 455 ꢁ(M–N). ꢂeff B.M. (298 K):
diamagnetic.
amine compound (2 mmol, 0.443 g) in ethanol (20 mL) were mixed
◦
and refluxed for about 4 h at 80 C. The color of the solution changed
to light yellow. After cooling the solution, the resulting precipi-
tate was filtered, and washed with cold ethanol. The ligands were
recrystallized from ethanol.
2
Co (L )(Cl) ·4H O: Elemental analyses, found (calcd. %): C 56.64
2
3
2
(57.46), H 6.62 (6.63), N 3.30 (3.35). UV–vis (ꢀmax, nm, DMF as sol-
−
1
Analytical and spectroscopic data of the Schiff base ligands are
given below:
vent): 201, 218, 220, 226, 228, 233, 268, 390, 400. FT-IR (KBr, cm ):
3441 ꢁ(OH), 2972 ꢁ(C–H)alph, 2875 ꢁ(C–H), 1647 ꢁ(C N), 1400
ꢁ(C–O), 827 ꢁ(C–H), 540 ꢁ(M–O), 467 ꢁ(M–N). ꢂeff B.M. (298 K):
4.19 (for per metal).
1
HL : Elemental analyses, found (calcd. %): C, 78.76 (78.22); H,
9
9
2
.29 (9.19); N, 4.33 (4.68). 1H NMR (CDCl3 as solvent, ı in ppm):
.90–10.50 (s, 3H, OH), 8.60 (s, 2H, CH N), 7.0–8.0 (m, 6H, Ar–H),
.40 (m, 1H, –CH), 1.10–1.70 (m, 18H, –C(CH ) ), 1.60 (d, 6H,
2
Cu (L )(Cl) ·4H O: Elemental analyses, found (calcd. %): C 56.05
2
3
2
(56.83), H 6.90 (6.54), N 3.39 (3.31). UV–vis (ꢀmax, nm, DMF as
3
3

1
86
G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
R
R
N
OH
N
N
O
N
nH O
2
HO
OH
M
M
HO
OH
Cl
Cl
Cl
M: Cu(II), Co(II), Ni(II), n: 3, 4 or 5
R
N
O
N
nH O
2
Ru
Cl
HO
OH
Cl
OH2
n: 5
1
2
R: 4-iso-propyl (HL ), 4-tert-butyl(HL )
Fig. 1. Proposed structures of the ligands and their metal complexes.
solvent): 218, 221, 231, 234, 237, 268, 373, 405. FT-IR (KBr, cm−1):
oxidation experiments. After immediately closing the vessels with
their captures or caps, the vessels were placed inside the Berghof
MWS3+ microwave oven and applied the microwave program with
the power of 300 W for 30 min. The temperature was controlled
automatically by the microwave instrument at about 100 C. The
pressure increased to 30–35 bar due to the evaporation of solvent
3
ꢁ
1
444 ꢁ(OH), 2966 ꢁ(C–H)alph, 2870 ꢁ(C–H), 1629 ꢁ(C N), 1350
(C–O), 839 ꢁ(C–H), 532 ꢁ(M–O), 481 ꢁ(M–N). ꢂeff B.M. (298 K):
.90 (for per metal).
2
◦
Ni (L )(Cl) ·5H O: Elemental analyses, found (calcd. %): C 57.06
2
3
2
(
57.49), H 6.37 (6.63), N 3.48 (3.35). UV–vis (ꢀmax, nm, DMF as sol-
−
1
vent): 218, 225, 230, 235, 264, 397, 409. FT-IR (KBr, cm ): 3439
and substrate. In order to stop the oxidation process, H O (1 mL)
2
ꢁ
8
(OH), 2958 ꢁ(C–H)alph, 2877 ꢁ(C–H), 1649 ꢁ(C N), 1350 ꢁ(C–O),
46 ꢁ(C–H), 566 ꢁ(M–O), 478 ꢁ(M–N). ꢂeff B.M. (298 K): diamag-
was added to the vessels. The oxidized organic products, except
the organic acids, were extracted with 10 mL CH Cl2 and injected
2
netic.
to GC and GC–MS to be analyzed and characterized. CyH, Cy–OH
and Cy O amounts were calculated from the external calibration
curves which were prepared before analyses.
2
Ru(L )(Cl) ·6H O: Elemental analyses, found (calcd. %): C 59.89
2
2
(
59.91), H 7.06 (7.16), N 3.55 (3.49). UV–vis (ꢀmax, nm): 217, 221,
−1
2
2
4
26, 229, 387, 397. FTIR (KBr, cm ): 3452 ꢁ(OH), 2958 ꢁ(C–H)alph,
877 ꢁ(C–H), 1629 ꢁ(C N), 1300 ꢁ(C–O), 839 (C–H), 550 ꢁ(M–O),
55 ꢁ(M–N). ꢂ B.M. (298 K): 1.68.
2.5. Free radical scavenging activity
eff
2
Pd (L )(AcO) ·5H O: Elemental analyses, found (calcd. %): C
2
3
2
1
2
The free radical scavenging activities of HL and HL were mea-
5
1.50 (51.45), H 6.82 (6.76), N 2.67 (2.61). UV–vis (ꢀmax, nm, DMF as
•
sured by the 1,1-diphenyl-2-picryl-hydrazil (DPPH ) compound
using the method of Blois [24]. According to this method, 0.1 mM
solution of the DPPH radical in ethanol was prepared, and 1 mL of
−
1
solvent): 217, 220, 228, 231, 235, 410. FTIR (KBr, cm ): 3439 ꢁ(OH),
2
ꢁ
966 ꢁ(C–H)alph, 2862 ꢁ(C–H)alph, 1622 ꢁ(C N), 1300 ꢁ(C–O), 852
(C–H), 557 ꢁ(M–O), 458 ꢁ(M–N). ꢂeff B.M. (298 K): diamagnetic.
Some physical properties of the Schiff base ligands and their
•
this solution was added to the solutions (3 mL) of the Schiff base
ligands in ethanol at different concentrations (10, 40, 80, 125 and
metal complexes were given in Supplementary material. In addi-
tion to, the preparing method of the palladium(II) complexes was
given in Supplementary material.
2
00 ␮g/mL). The mixture was shaken vigorously and allowed to
stand at room temperature for 30 min. Then, the absorbance at
17 nm was measured by a spectrophotometer. Lowering of the
5
absorbance of the reaction mixture indicated the higher of the free
radical scavenging activity.
The capability to scavenge the DPPH radical was calculated using
the following equation:
2
.4. Cyclohexane oxidation under microwave irradiation
The catalytic oxidation of cycloclohexane under microwave
ꢀ
ꢁ
irradiation was performed as follows: catalyst (0.02 mmol),
A − A
•
0
1
DPPH scavenging effect (%) =
× 100
cyclohexane (2 mmol) and H O2 (4 mmol) were microwaved
2
A
0
for 25 min at 300 W (30% of maximum output power). The
catalyst:substrate:oxidant ratio is 1:100:200, respectively. The
complexes were dissolved in 5 mL acetonitrile and then cyclo-
where A0 is the absorbance of the control solution, and A1 is the
absorbance in the presence of sample solutions or standards for
positive control.
hexane and H O2 were added to the microwave vessels for each
2

G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
187
1
2
Fig. 2. The FAR spectra of the [Cu(L )(Cl)3]·5H2O (a) and [Co(L )(Cl)3]·4H2O (b) complexes.
2.6. Reducing power activity
2.7. Preparation of microbial culture
1
2
The reducing power of the ligands HL and HL was determined
according to the method of Oyaizu [25]. Different concentrations
10, 20, 50, 100 ␮g/mL) in ethanol solution (1 mL) of the ligands
The Schiff base ligands and their metal complexes were
evaluated for their in vitro antibacterial activity against the
Corynebacterium xerosis, Bacillus brevis, Bacillus megaterium, Bacillus
cereus, Mycobacterium smegmatis, Staphylococcus aureus, Micrococ-
cus luteus and Enterococcus faecalis (as Gram-positive bacteria)
and Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli,
Yersinia enterocolitica, Klebsiella fragilis, Saccharomyces cerevisiae
and Candida albicans (as Gram-negative bacteria) by agar-well
diffusion. The antimicrobial activity studies were done according
to the literature method [26]. The compounds were dissolved in
(
1
2
HL and HL were mixed with a phosphate buffer (2.5 mL, 0.2 M,
pH 6.6) and potassium ferricyanide [K Fe(CN) ] (2.5 mL, 1%). The
mixture was incubated at 50 C for 20 min. A portion (2.5 mL) of
trichloroacetic acid (10%) was added to this mixture, and then
centrifuged for 10 min at 3.000 rpm. The upper layer of solution
3
6
◦
(
2.5 mL) was mixed with distilled water (2.5 mL) and FeCl (0.5 mL,
.1%), and the absorbance of the solution was measured at 700 nm
3
0
◦
with a UV–vis spectrophotometer. Higher absorbance of the reac-
tion mixture indicated greater reducing power of the Schiff base
ligands. Same procedure was also applied for ␣-tocopherol (vita-
min E), butylated hydroxy toluene (BHT) and butylated hydroxy
anisole (BHA) analysis as positive control.
DMSO (0.2 mL). The petri dishes were left at 4 C for 2 h and then
◦
the plats were incubated at 30 C for bacterium (18–24 h) and at
◦
25 C for fungi (72 h). At the end of the period, inhibition zones
formed on the medium were evaluated as millimeters (mm). The
control samples were only absorbed in DMSO.

1
88
G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
Fig. 3. The mass spectrum of the Schiff base ligand HL2.
3
. Results and discussion
In this study, we prepared the Schiff base ligands 4,4 -{(2-
atoms of the tert-butyl groups were shown at 40.54 and 40.56 ppm.
In the spectrum of the ligand HL , the signals at 21.5 and 34.6 ppm
1
ꢀ
can be attributed to the carbon atoms of the CH and CH groups of
3
hydroxy-5-isopropylbenzene-1,3-diyl) bis [methylidenenitrilo ] } -
the isopropyl group, respectively.
1
ꢀ
bis(2,6-di-tert-butylphenol)
(HL ),
4,4 -{(2-hydroxy-5-tert-
The FT-IR spectral data of the ligands and their metal complexes
−
1
butylbenzene-1,3-diyl) bis [methylidenenitrilo ] } bis (2,6-di -tert -
are given in Section 2. The broad bands at the 3450 and 3430 cm
can be attributed to the ꢁ(OH) groups. The bands observed at 1643
2
butylphenol) (HL ) and their transition metal complexes [Cu(II),
−
1
Co(II), Ni(II), Pd(II) and Ru(III)] (Fig. 1). In order to prepare the
ligands, we obtained the dicarbonyl compounds from the reaction
of the formaldehyde with 4-isopropyl- and 4-tert-butylphenol
in the alkali media. After the 3,5-di-tert-butyl-4-hydroxy ani-
line compound was synthesized, it was used immediately to
synthesize the Schiff base ligands. The condensation reaction of
aldehydes with 3,5-di-tert-butyl-4-hydroxy aniline readily forms
the corresponding imines. The yield of the ligand HL2 was very
low compared to the yields of other ligand and metal complexes.
The analytical data in Section 2 for the ligands and their metal
and 1624 cm can be assigned to the ꢁ(CH N) groups. The pheno-
−
1
lic (C–OH) groups are seen at 1290 and 1274 cm . In the Pd(II) and
−
1
Ru(III) complexes, the broad bands in the 3452–3439 cm range
can be attributed to the coordinated water molecules. In the spec-
tra of the complexes, the azomethine vibrations of the ligands were
shifted to the lower or higher regions which confirmed that the
nitrogen atom of the azomethine group coordinated to the metal
−1
ions [28]. The bands in the 575–532 and 478–455 cm range can
be attributed to the ꢁ(M–O) and ꢁ(M–N) vibrations, respectively
[29].
n
complexes agree with the general formula [M(L )(Z)m(H O)]·xH O
In order to investigate the substituent effect upon both the ␦-
and ␲-contribution to the total metal-nitrogen and metal-oxygen
bond strength, the FT-FAR method were used. Furthermore, the
effect of the substituents on the metal–halogen bond of the Schiff
base metal complexes was also examined. The infrared spectra in
2
2
−
(
n: 1 or 3, Z: Cl for Cu(II), Co(II), Ni(II), and Ru(III), AcO for Pd(II), m:
2
or 3, x: 3, 4, 5). All the compounds are stable at room temperature
in the solid state without decomposition. However, the ligands
and their metal complexes are very hygroscopic. These properties
were obtained from the results of the thermal and analytical
studies. The ligands are more soluble in common polar organic
−
1
the region 350–30 cm were measured for the complexes, and the
maximum absorption frequencies are listed in Table 1. The spec-
1
2
solvents, such as CHCl , EtOH, and THF, but partially soluble in
tra of the [Cu(L )(Cl) ]·5H O and [Co(L )(Cl) ]·4H O complexes are
3
3
2
3
2
non-polar organic solvents, such as hexane, heptane, toluene. The
complexes soluble only in the polar organic solvents, such as DMF
and DMSO.
given in Fig. 2. The FAR spectra of Cu(II), Co(II), Ni(II) and Ru(III)
complexes of the Schiff base complexes have similar features,
they are characterized by two strong metal–chlorine stretching
bands and two medium metal–nitrogen (ligand) stretching bands
in this region. Therefore, it can be reasonably assumed that the
substituents (isopropyl and tert-butyl) on the para-position of the
formyl ring do not change the stereo-configuration of the metal
complexes. On the whole, analogous to the case of the copper(II)
The H(13C) NMR spectra of the ligands were recorded using
CDCl3 as the solvent and obtained data are given in Section 2.
In the 1H NMR spectra of the ligands, the broad signals in the
1
9
.90–10.10 ppm range can be attributed to the protons of the phe-
nolic OH groups. The signal belonging to the OH proton disappears
in D O solution. The signal of the hydrogen atoms of the azome-
2
thine groups was seen at 8.60 ppm. The multiplets of the hydrogen
atoms of the aromatic rings were determined in the 7.00–8.20 ppm
range. The singlets of the hydrogen atoms of the tert-butyl groups
were observed in the 1.10–1.70 ppm range. In the spectrum of the
Table 1
The metal–halogen and metal–nitrogen stretching frequencies of the complexes of
the Schiff base ligands.
Complexes
ꢁ(M–Cl)
ꢁ(M–Cl)
ꢁ(M–N)
ꢁ(M–N)
1
ligand HL , the signals at 1.6 and 2.4 ppm can be attributed to the
1
[
[
[
[
Cu2(L )(Cl)3]·5H2O
339
330
327
338
335
327
328
335
311
302
298
300
304
301
302
295
265
247
245
230
274
260
246
228
102
94
93
105
110
100
97
hydrogen atoms of the CH and CH groups of the isopropyl group,
3
1
Co2(L )(Cl)3]·4H2O
respectively.
1
Ni2(L )(Cl)3]·3H2O
In the 13C NMR spectra of the ligands, the signal at 169.00 ppm
can be assigned to the azomethine carbon atom [27]. Aro-
matic ring carbon atoms of the ligands were determined in the
1
Ru(L )(Cl)2(H2O)]·5H2O
2
[Cu2(L )(Cl)3]·4H2O
2
[
[
[
Co2(L )(Cl)3]·4H2O
2
Ni2(L )(Cl)3]·5H2O
1
15.00–145.00 ppm range. The signals at 40.75 and 40.77 ppm can
2
Ru(L )(Cl)2(H2O)]·5H2O
102
be attributed to the tert-butyl carbon atoms. The quaternary C

G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
189
HL1
HL2
α-Tocopherol
BHT
0
0
0
0
0
0
,6
,5
,4
,3
,2
,1
0
0
20
40
60
80
100
Concentration (µg/mL)
Fig. 4. Free radical scavenging activity of different concentrations of HL1,
HL , BHT and ␣-tocopherol by 1,1-diphenyl-2-picrylhydrazyl radicals.
1
Fig. 5. Comparison of reducing power of different concentrations of HL ,
2
HL , BHT and ␣-tocopherol by spectrophotometric detection of the Fe+3-Fe+2
2
1
ꢀ
(
HL :
4,4 -{(2-hydroxy-5-isopropylbenzene-1,3-diyl)bis[methylidenenitrilo]}
1
ꢀ
transformation at 700 nm. (HL : 4,4 -{(2-hydroxy-5-isopropylbenzene-1,3-
2
ꢀ
bis(2,6-di-tert-butylphenol) and HL : 4,4 -{(2-hydroxy-5-tert-butylbenzene-
2
ꢀ
diyl)bis[methylidenenitrilo]}bis(2,6-di-tert-butyl phenol), HL : 4,4 -{(2-hydroxy-
-tert-butylbenzene-1,3-diyl)bis[methylidenenitrilo]}bis(2,6-di-tert-butylphenol)
and BHT: butylated hydroxytoluene).
1
,3-diyl)bis[methylidenenitrilo]}bis(2,6-di-tert-butylphenol).
BHT,
butylated
5
hydroxytoluene.
complexes, the metal–chlorine stretching (metal: Co(II), Ni(II) and
Ru(III)) frequencies due to the effects of both electron-releasing and
electron-attracting substituents. This phenomenon is apparently
due to the delocalized nature of the coordination bonds in the com-
plexes. In the complexes, the ꢁ(M–Cl) stretching frequency was
2
complexes, are diamagnetic, indicating low-spin (dsp ) square-
8
planar geometries as expected for the d system. In addition to the
other transitions, the spectra are characterized by strong absorp-
tion bands within the 397 and 401 nm which can be attributed
to intramolecular charge transfer (CT) interactions and to ␲–␲*
internal ligand transitions. The magnetic moment results clearly
indicate that the square-planar geometry for the Cu(II) systems,
which should contain one unpaired electron, where ꢂeff values
would be at 1.88 and 1.90 B.M. The palladium(II) complexes have
diamagnetic and square-planar geometry.
−1
seen in the 339–295 cm range. The ꢁ(M–N) stretching frequen-
−1
cies were shown in the 274–93 cm range.
Electronic spectra of the ligands and their complexes were
recorded in DMF in the 200–900 nm range and obtained data
are given in Section 2. In the ligands, the absorption bands in
the 277–201, 386–277 and 402–392 nm ranges can be attributed
to the ␦–␦*, ␲–␲* and n–␲* transitions, respectively [28,29].
In the complexes, the bands at the 576 and 598 nm may be
assigned to d-d transitions. In the complexes, charge transfer
bands (MLCT) were shown in the 494–400 nm range. The n–␲*,
1
2
The mass spectra of the ligands HL and HL were investigated
and obtained data are given in Section 2. The spectrum of the ligand
HL is given in Fig. 3. The molecular ion peak [M ] of the ligand
2
+
1
+
2
HL was seen at m/z 599. The [M+1 ] peak of the ligand HL was
1
obtained at m/z 613. The highest intensity peaks of the ligands HL
␲
–␲* and ␦–␦* transitions were determined in the 397–201 nm
and HL2 were seen at m/z 466 and 412 (100%) due to the loosing
range [28,29].
• •
C8H19O ] and [C10H34NO2 ] groups, respectively.
[
The magnetic moment values of the complexes at room tem-
perature (298 K) are given in Section 2. Tetrahedral cobalt(II)
complexes have magnetic moments at the 4.19 and 4.28 B.M. [28],
higher than that of the spin-only value. The magnetic moment
data for the Ru(III) complexes are at the 1.68 and 1.74 B.M., corre-
3.1. Antimicrobial activity studies
The biological activities of the Schiff base ligands (HL1 and
HL ) and their metal complexes were tested against the bacteria
sponding to one unpaired electron with low spin t52 configuration
2
g
for Ru(III) in an octahedral environment. The room temperature
magnetic moment measurements show that the nickel–Schiff base
and yeast because bacteria can achieve resistance to antibiotics
through biochemical and morphological modifications [30]. The
Table 2
Antimicrobial activity data of the Schiff base ligands and their metal complexes.
a
Microorganisms
CFU /mL inoculum
1
2
3
4
5
6
7
8
9
10
11
12
Control
Gram-positive bacteria
C. xerosis
B. brevis
B. megaterium
B. cereus
M. smegmatis
S. aureus
8
17b
15
–
12
9
13
–
10
6.9 × 10
13
22
15
18
–
9
–
–
20
7
18
17
9
10
14
16
25
28
20
29
27
30
24
30
17
15
10
12
9
7
17
15
20
25
21
16
22
19
16
14
15
12
16
18
9
12
10
9
10
17
9
12
17
–
23
17
20
22
18
15
17
12
7
11
20
16
19
25
27
30
18
22
15
18
20
17
13
16
23
19
21
27
16
18
12
15
–
–
–
–
–
–
–
–
8
7.7 × 10
8
7.3 × 10
8
6.9 × 10
8
9.9 × 10
8
8.5 × 10
8
M. luteus
E. faecalis
5.5 × 10
10
8
8
7.2 × 10
Gram-negative bacteria
P. aeruginosa
K. pneumoniae
E. coli
Y. enterocolitica
K. fragilis
8
5.9 × 10
21
23
19
17
–
15
17
–
12
14
–
15
13
16
10
9
8
11
12
16
9
20
14
13
21
24
16
14
28
28
31
29
32
35
30
18
16
30
13
17
–
24
27
26
28
30
25
24
19
20
–
–
17
–
17
23
19
15
16
18
18
30
18
27
20
25
24
26
–
23
24
18
25
27
24
–
–
–
–
–
–
–
8
7.8 × 10
8
6.0 × 10
8
5.5 × 10
8
7.5 × 10
8
S. cerevisiae
C. albicans
8.7 × 10
–
14
6
8
13
10
8
6.9 × 10
–
16
20
a
Number of colony forming units.
b
1
2
1
1
1
1
Inhibition zone (mm). Blanks mean not investigated. 1: HL , 2: HL , 3: [Co2(L )(Cl)3]·4H2O, 4: [Cu2(L )(Cl)3]·5H2O, 5: [Ni2(L )(Cl)3]·3H2O, 6: [Ru(L )(Cl)2(H2O)]·5H2O, 7:
1
2
2
2
2
2
[
Pd2(L )(AcO)3(H2O)]·3H2O, 8: [Co2(L )(Cl)3]·4H2O, 9: [Cu2(L )(Cl)3]·4H2O, 10: [Ni2(L )(Cl)3]·5H2O, 11: [Ru(L )(Cl)2(H2O)]·5H2O, 12: [Pd2(L )(AcO)3(H2O)]·4H2O.

1
90
G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
1
1
1
Fig. 6. (a–c) The thermal curves of the [Ni2(L )(Cl)3]·3H2O (a), [Pd2(L )(AcO)3(H2O)]·3H2O (b) and [Ru(L )(Cl)2(H2O)]·5H2O (c) complexes.
organisms used in the present study are C. xerosis, B. brevis, B.
megaterium, B. cereus, M. smegmatis, S. aureus, M. luteus and E. fae-
calis (as Gram-positive bacteria) and P. aeruginosa, K. pneumoniae,
E. coli, Y. enterocolitica, K. fragilis, S. cerevisiae, and C. albicans (as
Gram-negative bacteria). The diffusion agar technique was used to
evaluate the antibacterial activity of the synthesized complexes.
The results of the bactericidal screening of the synthesized com-
pounds are given in Table 2. While the Schiff base ligand HL has not
1
any activity B. megaterium, M. luteus, S. cerevisiae, and K. fragilis, it
shows activity against to the other microorganisms. The ligand HL2

G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
191
O
OOH
OH
CO H
catalyst
H O , MeCN
2
+
+
+
CO H
2
2
2
CyH
Cy-OH
Cy=O
Cy-OOH
Adipic acid
Fig. 7. Catalytic oxidation of cyclohexane under microwave irradiation.
does not show any activity against M. smegmatis, M. luteus, E. fae-
calis, E. coli, S. cerevisiae, and C. albicans, but, it has moderate activity
against the other microorganisms. The remarkable activity of the
Schiff base ligands may be arise from the hydroxyl groups, which
may play an important role in the antibacterial activity, as well as
the presence of two imine groups which imports in elucidating the
mechanism of transformation reaction in biological systems. The
obtained results indicate that the complexes have higher activity
than the ligands against the same microorganisms under identi-
cal experimental conditions. This suggests that the chelation could
facilitate the ability of a complex to cross a cell membrane and
can be explained by Tweedy’s chelation theory [31]. Chelation con-
siderably reduces the polarity of the metal ion because of partial
sharing of its positive charge with donor groups and possible elec-
tron delocalization over the whole chelate ring. Such a chelation
could enhance the lipophilic character of the central metal atom,
which subsequently favors its permeation through the lipid layer
of the cell membrane. The variation in the effectiveness of differ-
ent compounds against different organisms depends either on the
impermeability of the cells of the microbes or on differences in
ribosome of microbial cells. The activity of the Schiff base ligands
and their metal complexes increases as the concentration increases
because it is a well known fact that concentration plays a vital role in
increasing the degree of inhibition. Almost all compounds (except
Gram-negative bacteria is based on trans envelope effux driven by
a resistance or nodulation. Nickel toxicity is comparable to that of
cobalt (Table 2), but its toxic property is higher than the cobalt(II)
complexes. Pd(II) complexes have higher biological activity than
both Cu(II) and Ni(II) transition metal complexes for Gram-negative
bacteria. While Pd(II) complexes contain the acetate ligands, other
complexes have the chloride ions as the ligand. The high activity
of palladium(II) complexes may be due to the effect of the acetate
ions.
3.2. Free radical scavenging activity
•
The model of scavenging the stable DPPH radical is a widely
used method to evaluate antioxidant activities in a relatively short
time compared with other methods. The maximum absorption of a
•
stable DPPH radical in ethanol was at 517 nm. When the reaction
•
between antioxidant molecules and DPPH radical, which results
in the scavenging of the radical by hydrogen donation, the antiox-
•
idants cause to a decrease of the absorbance of DPPH radical.
This is visually noticeable as a discoloration from purple to yellow.
•
Hence, DPPH is usually used as a substrate to evaluate antiox-
idant activity of antioxidants [33]. The effect of antioxidants on
•
DPPH radical scavenging was thought to be due to their hydro-
•
gen donating ability. DPPH is a stable free radical and accepts
2
1
the complexes [Cu (L )(Cl) ]·4H O, [Pd (L )(AcO) (H O)]·3H O,
an electron or hydrogen radical to become a stable diamagnetic
molecule [34]. Fig. 4 illustrates a significant (P < 0.05) decrease the
2
3
2
2
3
2
2
2
2
[
Co (L )(Cl) ]·4H O and [Pd (L )(AcO) (H O)]·4H O have activity
2
3
2
2
3
2
2
2
•
against the bacteria and yeasts. The [Cu (L )(Cl) ]·4H O com-
concentration of DPPH radical due to the scavenging ability of
2
3
2
the sterically hindered Schiff base ligands (HL1 or HL ), and stan-
2
plex has not any activity against E. coli, Y. enterocolitica and
1
1
S. cerevisiae yeast. The complexes [Pd (L )(AcO) (H O)]·3H O,
dards. Solution of the sterically hindered Schiff base ligands (HL
2
3
2
2
2
2
2
•
[
Co (L )(Cl) ]·4H O and [Pd (L )(AcO) (H O)]·4H O do not have
and HL ) showed DPPH scavenging activity. Free radical scaveng-
2
3
2
2
3
2
2
1
any activity against S. cerevisiae, P. aeruginosa and S. aureus,
ing activity of the sterically hindered Schiff base ligands (HL and
HL ) also increased with increasing concentration. The scaveng-
1
2
respectively. The complex [Ru(L )(Cl) (H O)]·5H O has the high-
2
2
2
1
2
est activity against the E. coli, Y. enterocolitica, S. cerevisiae, and
C. albicans (yeast). The copper(II) complexes have higher activity
against Gram-positive bacteria than Gram-negative bacteria. Cop-
per may easily interact with radicals, especially with molecular
oxygen. Its radical character makes copper very toxic, and many
organisms are more sensitive to copper [32] than E. coli. The Co(II)
complexes are of medium toxicity (Table 2). Resistance to cobalt in
ing effect of the Schiff base ligands (HL and HL ) and standards at
•
200 ␮g/mL on the DPPH radical scavenging in the order of BHT > ␣-
1
tocopherol > ethanol extract of the ligand HL > water extract of
the ligand HL2 and were 94%, 91%, 83%, and 69%, respectively, at
the same concentration (200 ␮g/mL). These results indicate that
the Schiff base ligands have a noticeable effect on the scaveng-
ing free radical. Based on the data obtained from this study, the
Table 3
Catalytic oxidation of cyclohexane (CyH) with H2O2 under microwave irradiation.
Catalysta
CyH conversion (mol%)
Desired products (mol%)
ol:one
By products (mol%)
Blankb
0.86
98.27
98.03
99.98
99.10
99.56
100
99.92
100
99.74
99.98
Cy–OH (0.54); Cy O (0.32)
Cy–OH (41.94); Cy O (21.03)
Cy–OH (41.86); Cy O (23.23)
Cy–OH (40.27); Cy O (20.01)
Cy–OH (42.03); Cy O (19.73)
Cy–OH (3.80); Cy O (5.38)
Cy–OH (8.67); Cy O (13.14)
Cy–OH (20.29); Cy O (20.57)
Cy–OH (19.85); Cy O (22.12)
Cy–OH (1.39); Cy O (1.38)
Cy–OH (1.05); Cy O (0.91)
1.72
1.99
1.80
2.01
2.13
0.71
0.66
0.99
0.90
1.01
1.15
–
1
c
c
[
[
[
[
[
[
[
[
[
[
Cu2(L )(Cl)3]·5H2O
35.30
32.94
39.70
37.34
90.38
78.19
59.06
58.03
96.97
98.02
2
Cu2(L )(Cl)3]·4H2O
1
c
c
Co2(L )(Cl)3]·4H2O
2
Co2(L )(Cl)3]·4H2O
1
c
Ni2(L )(Cl)3]·3H2O
2
c
Ni2(L )(Cl)3]·5H2O
1
c
c
Pd2(L )(AcO)3(H2O)]·3H2O
2
Pd2(L )(AcO)3(H2O)]·4H2O
1
c
c
Ru(L )(Cl)2(H2O)]·5H2O
2
Ru(L )(Cl)2(H2O)]·5H2O
a
b
c
◦
3
2
0
00 W power was applied for 30 min. The reaction temperature and pressure were held at around 100 C and 30 bar in closed DAP60 vessels.
mmol cyclohexane:4 mmol hydrogen7 peroxide and 5 mL acetonitrile were used in experiment without catalyst.
.02 mmol catalyst:2 mmol cyclohexane:4 mmol hydrogen peroxide (1:100:200) and 5 mL acetonitrile were used for each reaction.

1
92
G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
Fig. 8. Influence of the complexes in cyclohexane oxidation under microwave irradiation. 0.02 mmol catalyst:2 mmol cyclohexane:4 mmol hydrogen peroxide (1:100:200)
◦
and 5 mL acetonitrile were used for each reaction. 300 W power was applied for 30 min. The reaction temperature and pressure were held at around 100 C and 30 bar in
closed DAP60 vessels.

G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
193
1
2
sterically hindered Schiff base ligands (HL and HL ) are free radical
inhibitors or scavengers, as well as a primary antioxidant that reacts
with free radicals, which may limit free radical damage occurring
in the human body.
3.3. Reducing power activity
Fig. 5 shows the reductive capabilities of the sterically hin-
dered Schiff base ligands (HL1 and HL ) compared to BHT, and
2
␣
-tocopherol. For the measurements of the reductive ability, we
+3
+2
investigated the Fe to Fe transformations in the presence of
the Schiff base ligand (HL1 and HL ) samples using the method of
Oyaizu [25]. The reducing capacity of a compound may serve as a
significant indicator for its potential antioxidant activity [35,36].
The antioxidant activity of putative antioxidants were attributed
to various mechanisms, among which are prevention of chain ini-
tiation, binding of transition metal ion catalysts, decomposition of
peroxides, prevention of continued hydrogen abstraction, reduc-
tive capacity, and radical scavenging [37]. Reducing power of the
Schiff base ligands (HL1 and HL ) and standard compounds fol-
2
2
2
1
lowed the order: BHT > ␣-tocopherol > HL > HL . Reduction power
1
2
activities of the Schiff base ligands (HL and HL ) had lower activ-
ities than the standards (BHT and ␣-tocopherol). But, the reducing
2
power activity of the ligand HL was higher than the Schiff base
1
ligand HL .
3.4. Thermal analysis studies
Thermal studies of all the complexes were performed
under nitrogen atmosphere in the temperature range of
◦
1
2
(
(
0–1000 C. The thermal curves of the [Ni (L )(Cl) ]·3H O
2
3
2
1
1
a), [Pd (L )(AcO) (H O)]·3H O (b) and [Ru(L )(Cl) (H O)]·5H O
2
3
2
2
2
2
2
c) complexes are given in Fig. 6a–c. All complexes have the
adsorbed and/or hydrated water molecules. As seen from Fig.
6
a–c and the figures of the other complexes, the hydrated water
◦
molecules were eliminated at the first step in the 40–85 C tem-
perature range. The intermediates are stable within the interval
of 150–170 C. The loss of the hydrated water molecules was seen
◦
as endothermic peaks in the DTA curves. While the palladium(II)
and ruthenium(III) complexes contain the coordinated water
molecules, chloride and acetate ions, the copper(II), cobalt(II)
and nickel(II) complexes contain only coordinated chloride ions.
Coordinated water molecules in the Pd(II) and Ru(III) complexes
◦
eliminate within the temperature range of 150–170 C. In the
Pd(II) complexes, the acetate ions loss from the complexes at
◦
the 195 and 200 C temperatures. The coordinated chloride
ions in the complexes, except the Pd(II) complexes, loss from
◦
the molecule in the 250–320 C temperature range [38]. This
process is connected with the elimination of two and or three
n
coordinated chloride ions and the formation of [M(L )] species
which is thermally stable up to the higher temperatures. Main
weight losses were at the higher temperatures. Then, the weights
◦
of the samples decreased slightly up to 1000 C. From the cal-
culations, it follows that the final decomposition product can
be MO/M O /M O (MO is for the Cu(II), Ni(II) and Pd(II) com-
2
3
3
4
plexes; M O is for the Ru(III) complexes; M O4 is for the Co(II)
2
3
3
complexes).
3.5. Cyclohexane oxidation under microwave irradiation
The optimum conditions were obtained as cata-
lyst:substrate:oxidant ratio of 1:100:200 in acetonitrile under
3
00 W microwave power for 30 min. The temperature and pres-
◦
sure were controlled at about 100 C and 30 bar by the instrument.
A blank has been run under the same conditions without any cat-
alyst. While the copper(II), cobalt(II), nickel(II) and palladium(II)

1
94
G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
Fig. 9. (a and b) Cyclic voltammograms of the Schiff base ligands (HL and HL ) in the presence of 0.1 M Bu4NBF4 in DMF solution and at different scan rates. (a) HL1 (DMF,
1
2
× 10 M), (b) HL2 (DMF, 1 × 10 M).
−
4
−4
1
complexes have the binuclear nature, the ruthenium(III) com-
plexes are the mononuclear. According to the possible oxidation
mechanism, the first and slow step is the oxidation of CyH to
Cy–OH and then Cy O and other further oxidized products. If the
first step is controlled, the ratio of desired products Cy–OH and
Cy O selectivity can be increased. The possible reaction products
are shown in Fig. 7. It has been shown that the microwave power
and the novel catalysts could affect the selective oxidation of
CyH to Cy–OH and Cy O. The binuclear Cu(II) complexes showed
well catalytic activity than the Ni(II), Pd(II) and Ru(III) complexes.
The Co(II) complexes have been outlined as a good catalyst in
many literatures [39,40]. In our study, the Co(II) complex showed
high catalytic activity on cyclohexane. This situation may be due
to the structure of the ligand. In the complexes, the ␲–␲ elec-
tron delocalization phenomena occur throughout the ligand for
cyclohexane. This situation may effect the oxidation reaction. The
best selectivities of the desired products Cy–OH and Cy O were
obtained in the Cu(II) complex (Table 3 and Fig. 8). The catalytic
performance of the binuclear Cu(II) complexes are comparable to
the activity exhibited by the earlier reported complexes [39,40].
The key point in the conversion of cyclohexane to the oxidized
products is the reduction of Cu(II)–L to Cu(I)–L (L:ligand). This
reduction to Cu(I)–L is facilitated by the ligands available around
the metal cation. The formation of the oxidation products Cy–OH
and Cy O show the preferential attack of the activated bonds.
Tetrahedral geometry of the binuclear Co(II) complexes may be the
reason of having high activity on cyclohexane. In the formation of
the oxidation product Cy O, binuclear Pd(II) complexes have also

G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
195
Table 5
The ꢃEp and E1/2 data of the Schiff base ligands and their metal complexes at the different scan rates and in the 1 × 10 M and 1 × 10−4 M concentrations.
−3
1 × 10− M (1 × 10−4 M)
3
1 × 10−3 M (1 × 10−4 M)
aE1/2 (E1/2)
Compound
aꢃEp (ꢃEp)
bꢃEp (ꢃEp)
cꢃEp (ꢃEp)
bE1/2 (E1/2)
cE1/2 (E1/2)
HL1
−0.45 (−0.12)
0.20 (−)
−0.57 (−0.05)
0.28 (0.20)
0.20 (0.21)
0.12 (0.03)
−0.18 (0.15)
0.15 (0.14)
−0.01 (−0.12)
−0.13 (0.15)
−0.16 (0.14)
0.25 (0.16)
0.20 (0.05)
0.21 (0.17)
−0.48 (−0.12)
1.53 (0.13)
0.16 (0.21)
0.15 (0.03)
0.24 (0.12)
0.14 (0.09)
−0.08 (−0.16)
0.15 (0.13)
0.23 (0.28)
0.22 (0.12)
−0.17 (0.14)
0.24 (0.13)
0.08 (1.23)
– (–)
– (–)
0.10 (–)
– (–)
– (–)
−0.07 (0.14)
−1.04 (−)
1.14 (–)
−0.75 (–)
– (0.10)
– (–)
0.03 (0.73)
– (–)
– (–)
– (0.76)
– (–)
– (–)
– (–)
– (–)
– (–)
– (0.72)
– (–)
– (–)
– (–)
– (–)
– (–)
1
[
[
[
[
[
Cu2(L )(Cl)3]·5H2O
1
Co2(L )(Cl)3]·4H2O
0.16 (0.29)
0.17 (0.17)
−0.16 (0.15)
0.18 (0.22)
−0.07 (−0.13)
−0.06 (−0.18)
−0.02 (0.16)
0.07 (0.23)
0.16 (0.16)
0.24 (0.17)
1
Ni2(L )(Cl)3]·3H2O
1
Pd2(L )(AcO)3(H2O)]·3H2O
1
Ru(L )(Cl)2(H2O)]·5H2O
2
HL
0.002 (−)
2
[
[
[
[
[
Cu2(L )(Cl)3]·4H2O
− (−)
2
Co2(L )(Cl)3]·4H2O
− (−)
2
Ni2(L )(Cl)3]·5H2O
− (−)
– (–)
– (0.10)
– (–)
2
Pd2(L )(AcO)3(H2O)]·4H2O
– (−0.75)
– (–)
2
Ru(L )(Cl)2(H2O)]·5H2O
+
All the potentials are referenced to Ag /AgCl; where Epa and Epc are anodic and cathodic potentials, respectively. ꢃEp = Epa − Epc. E1/2 = 0.5 × (Epa + Epc).
a
b
c
−1
−1
−1
2
2
5
00 mV s
50 mV s
00 mV s
.
.
.
−
4
high activity. The ruthenium(III) complexes have mononuclear
nature and therefore, their catalytic activities are very low than
other binuclear metal complexes.
rates in DMF solution (1 × 10 M) are given in Fig. 9a and b. The
1
−3
M
HL shows the irreversible process (I :I # 1.0) in the 1 × 10
pa pc
−
1
DMF solutions at the 200 and 500 mV s scan rates. Their potential
ranges change from −0.60 V to 1.32 V (Epc) and −1.08 V to 0.70 V
−
1
scan rate, the ligand HL1 shows the
3
.6. Electrochemical properties of the ligands and their metal
(Epa). At the 250 mV s
∼
complexes
reversible process (Ipa:Ipc = 1.0) at the 1.19 V (Epc) and −1.12 V
(
Epa). At this potential, the Schiff base ligand converts to the sec-
Cyclic voltammograms of the ligands and their complexes
were run in DMF–0.1 M Bu NBF4 and CH CN–0.1 M Bu NBF as
supporting electrolyte at 293 K. All potentials quoted refer to
amine compound [41]. This process is shown in Fig. 10. Similarly,
in the 1 × 10 M DMF solution, the HL2 ligand at the 200, 250
−
3
4
3
4
4
−
1
and 500 mV s scan rates shows the reversible processes in the
−0.75 + 1.56 V (cathodic peak potentials) and −0.76 + 1.51 V (anodic
peak potentials) range. The HL2 ligand shows the both imine to
sec-amine and imine to quinoid forms (Fig. 10) [42]. The quinoid
process would involve self-protonation reactions where the
measurements run at a scan rates (v) of 100, 150, 200, 250 and
−
1
5
00 mV s
and against an internal ferrocene–ferrocenium stan-
dard, unless otherwise stated. In order to investigate the effect of
the ligand concentration, the electrochemical studies were done in
−
3
−4
the 1 × 10 and 1 × 10 M solutions. The electrochemical data of
phenolic
hydroxyl
groups
act
as
proton
donors.
the ligands and their complexes are summarised in Tables 4 and 5.
The CV curves of the ligands at the 200, 250 and 500 mV s scan
Oxidation–reduction peaks of the ligands at the different scan
rates shifted to the lower or higher potentials.
−
1
R
R
N
OH
N
+
-
NH OH HN
2
H + 2e
+
-
HO
OH
-2H - 2e
HO
OH
imine
sec-amine
+
-
+
-
-
2H - 2e 2H + 2e
R
N
OH
N
O
O
quinone
R: tert-butyl, sec-propyl
Fig. 10. Reversible reduction–oxidation processes of the Schiff base ligands in DMF solution.

1
96
G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
1
electron reduction. The Pd(II) complex of the Schiff base ligand HL1
The electrochemical curves of the [Co (L )(Cl) ]·4H O,
2
3
2
and
1
2
[
[
Pd (L )(AcO) (H O)]·3H O,
[Cu (L )(Cl) ]·4H O
shows the reversible process at the −0.99 V (Epc) and 0.93 V (Epa)
2
3
2
2
2
3
2
2
−1
−3
Pd (L )(AcO) (H O)]·4H O complexes at 100, 150, 200, 250
potentials at the 250 mV s scan rate in the 1 × 10 M solution.
2
3
2
2
−1
2
and 500 mV s scan rates are shown in Fig. 11c–f. In addition to
the ligand-based processes, all the complexes show the reversible,
irreversible or quasi-reversible redox processes with one or two-
The [Pd (L )(AcO) (H O)]·4H O has reversible process at the 1.07,
2
3
2
2
1.06 and −0.72 V (Epc) and 0.98 and 0.96 V (Epa) potentials at the
−
1
−4
200, 250 and 500 mV s scan rates in the 1 × 10 M solution. In
Fig. 11. (c–f) Cyclic voltammograms of the metal complexes in DMF solutions at the different scan rates.

G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
197
Fig. 11. (Continued).
−
4
−1
scan rate, the Cu(II),
the 1 × 10 M solution, at the 200 mV s
The electrochemical results indicate that the electron transfer
process may occur as follows for all complexes:
For the irreversible reactions:
2
Co(II) and Ni(II) complexes of the ligand HL show the reversible
process in the −1.19 to (+)1.08 V potential range. The Ru(III)
complexes are the irreversible at the all scan rates and in the
× 10 and 1 × 10 M concentrations.
−
3
−4
−
1
M(II) + e → M(I)

1
98
G. Ceyhan et al. / Spectrochimica Acta Part A 81 (2011) 184–198
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[
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(
HL and HL ) had lower activities than the standards (BHT and ␣-
[
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ligand HL was higher than the Schiff base ligand HL . As the ligands
have the sterically hindered properties, they are good stabilizator.
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catalytic activity than the other complexes on cyclohexane.
[
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