Schiff base ligands derived from 2-pyridinecarboxaldehyde and its complexes: characterization, thermal, electrochemical, and catalytic activity results

Article abstract of DOI:10.1007/s00706-014-1381-8

Abstract Two novel Schiff base ligands (E)-N,N′-bis(pyridin-2-ylmethylene)cyclohexane-1,4-diamine and (E)-N,N′-bis(pyridin-2-ylmethylene)benzene-1,4-diamine and their binuclear Cu(II), Ni(II), and Co(II) complexes have been synthesized from 2-pyridinecarboxaldehyde, 1,4-diaminobenzene, and trans-1,4-diaminocyclohexane. Characterizations of the synthesized compounds were carried out by thermal analysis, analytical and spectroscopic methods. Catalytic activity of the complex compounds showed some low and some high performances towards the oxidation reactions of cyclohexene and styrene, especially higher performances were obtained for the oxidation of styrene. Thermal examination of the title compounds revealed that complex compounds, apart from Cu(II) complex, were more resistant to temperature, which increases their chance to be used as catalysts towards chemical reactions carried out in high temperatures. Electrochemical features of both ligands and complexes have also been explained.

Full text of DOI:10.1007/s00706-014-1381-8

Monatsh Chem
DOI 10.1007/s00706-014-1381-8
ORIGINAL PAPER
Schiff base ligands derived from 2-pyridinecarboxaldehyde
and its complexes: characterization, thermal, electrochemical,
and catalytic activity results
•
Selma Bal Sedat S. Bal
Received: 2 July 2014 / Accepted: 8 December 2014
Ó Springer-Verlag Wien 2015
Abstract Two novel Schiff base ligands (E)-N,N0-
bis(pyridin-2-ylmethylene)cyclohexane-1,4-diamine and
(E)-N,N0-bis(pyridin-2-ylmethylene)benzene-1,4-diamine
and their binuclear Cu(II), Ni(II), and Co(II) complexes
have been synthesized from 2-pyridinecarboxaldehyde,
1,4-diaminobenzene, and trans-1,4-diaminocyclohexane.
Characterizations of the synthesized compounds were
carried out by thermal analysis, analytical and spectro-
scopic methods. Catalytic activity of the complex
compounds showed some low and some high performances
towards the oxidation reactions of cyclohexene and sty-
rene, especially higher performances were obtained for the
oxidation of styrene. Thermal examination of the title
compounds revealed that complex compounds, apart from
Cu(II) complex, were more resistant to temperature, which
increases their chance to be used as catalysts towards
chemical reactions carried out in high temperatures. Elec-
trochemical features of both ligands and complexes have
also been explained.
Introduction
Transition metal catalysing the transfer of oxygen atoms to
organic substrates is of interest in the study of bioinorganic
mechanisms. In particular, the catalysis of alkene oxidation
by soluble transition metal complexes is of great interest in
both biomimetic and synthetic chemistry [1–3]. Thermal
stability, in general, represents an important parameter
towards the use of new compounds as catalysts in various
chemical reactions that require high temperatures [4, 5] and
it is known that most of the oxidation reactions are carried
out in high temperatures. Therefore, it is vital that the
catalysts used in these kinds of reactions are stable to high
temperatures. So far various Schiff base complexes have
been employed to catalytic oxidation of olefins to epoxides
and aldehydes [6–16]. And much attention has been
focused on developing novel catalytic processes based on
efficient catalysts with easy recoverability and inexpensive
eco-friendly oxidants such as H2O2, O2, and air. As well as
their catalytic activities, various Schiff bases have also
been examined for their antimicrobial activity in many
previous studies [17–19]. During the last decade, transition
metal complexes of Schiff bases derived from 2-pyridine-
carboxaldehyde and different amines have received
considerable attention for their synthetic and catalytic
activities [20–25]. Pyridine 2-carboxaldehyde Schiff base
complexes have also been examined for their catalytic
properties for different chemical reactions like amination
of aryl halides [26], pinacol coupling of aromatic aldehydes
[27], primary aromatic oxidations [28] and olefin cyclo-
propanation [29]. Schiff base complexes derived from
pyridine 2-carboxaldehyde have been reported for their
catalytic activities towards olefin epoxidation [30, 31].
Catalytic epoxidation of carbon–carbon double bonds is of
pivotal importance in organic chemistry due to the wide
Keywords Schiff base ꢀ Complexes ꢀ Catalyst ꢀ
Thermal
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-014-1381-8) contains supplementary
material, which is available to authorized users.
S. Bal (&) ꢀ S. S. Bal
Chemistry Department, Faculty of Arts and Science,
Kahramanmaras Sutcu Imam University, Avsar Kampusu,
46100 Kahramanmaras, Turkey
e-mail: selmadagli9@hotmail.com
123

S. Bal, S. S. Bal
range applications of highly regio-selective ring openings
and other reactions of epoxides in the synthesis of a variety
of functionalized products. It is widely known that epox-
ides can easily be transformed into a large variety of
organic compounds that can be further used in the synthesis
of many different and versatile compounds, which can be
used in our everyday lives. For example, in addition for its
importance to be an intermediate for organic synthesis and
pharmaceutical industry, styrene oxide can be used as
ultraviolet absorbent and sweetener [32]. It is known that
2-phenylethanol synthesized from styrene oxide has a very
sweet smell which makes it to be used as sweetener in food
industry and in perfumery. With this study, in addition to
catalytic activity, the pyridine-2-carboxaldehyde Schiff
base ligands and their Cu(II), Ni(II), and Co(II) complexes
have also been examined for their thermal and electro-
chemical properties.
202–356 nm region. In the UV spectra of L2, p–p* and n–
p* transitions were at 225–368 nm. Compared to general
p–p* or n–p* transitions values, which are around 251 nm
for a non-substituted benzene, these transitions for L2 were
shifted to higher wavelengths due to increase in conjuga-
tion arising from the benzene ring and the pyridine ring
through the imine group. For all the complexes, d–d tran-
sitions were observed above 500 nm and the bands
observed in the 397–367 nm range can be attributed to the
charge-transfer bands from ligand to metal or from metal to
ligand centre [33–35]. All the infrared and UV results are
given in the experimental section.
The magnetic moment value of Co(II) complex of L2 was
recorded as 6.11 B.M., which suggests that this complex
might have a tetrahedral geometry. The B.M. value for the
Cu2L2(H2O)4 complex, however, is 1.88, which tells us that
this complex has square-planar geometry like the Ni(II)
complex [33, 35]. For the Co(II) complex of L1, however,
the magnetic moment value was recorded as 4.4 B.M.,
which suggests that this complex might have a tetrahedral
geometry. The B.M. value for the Cu(II) complex, however,
is 1.4, which tells us that this complex has square-planar
geometry like the Ni(II) complex [34, 35].
Results and discussion
NMR spectra of the ligands
Proton NMR spectrum of L1 reveals cyclohexane proton
signals at 3.38 ppm (2H-10) and at 1.87 ppm (8H-20) as
multiplets. The imine protons are observed at 8.46 ppm. In
its carbon NMR spectrum, the cyclohexane carbons are
seen at 68.8 ppm (C-10) and at 32.3 ppm (C-20). The imine
carbons are at 160.2 ppm. 1H NMR spectrum of ligand L2,
on the other hand, showed the aromatic protons between 7
and 9 ppm. In the pyridine ring, H-4 was at 7.39 ppm
underneath the big singlet at 7.39, which belonged to the
four protons in the 1,4-disubstituted aromatic ring. The rest
of the NMR data can be seen in the ‘‘Experimental’’ part.
Thermal properties of the synthesized compounds
TG traces of the synthesized compounds provided addi-
tional information regarding their thermal stabilities and
thermal degradation behaviour. We can conclude from the
examination of TG graphics that L1 is the most stable
ligand with a decomposition temperature starting at around
230 °C compared to L2. However, almost all the com-
plexes revealed similar decomposition patterns within their
groups, and apart from the H2O losses, showed more
resistance to the temperature compared to their ligands
with decomposition temperatures between 250 and 650 °C.
Only the copper(II) complex of both ligands revealed lower
decomposition temperatures compared to the others.
IR and UV spectra of the ligands and the complexes
For all the ligands and their complexes, the imine (C=N)
stretching frequencies were observed between 1,538 and
1,618 cm-1. Upon complexation, the imine stretching fre-
quencies of the ligands were red shifted, showing an increase
in C=N stretching frequencies. The pyridine ring C–H
stretchings seen at 1,494 and 1,583 cm-1 for the ligands
were also redshifted proving the chelation between the metal
atoms and the nitrogen atom in the pyridine ring. This che-
lation had also an effect on the stretching frequencies of Ar–
H in pyridine ring carrying them to higher or lower wave-
lengths. The O–H stretching frequencies in water molecules
assisting in coordination in all the complexes were seen as
broad singlets between 3,200 and 3,366 cm-1 [33].
Thermal studies performed on the title compounds
support the proposed structures both for the ligands and
their coordination complexes. TGA graphic of L1 shows
that our ligand decomposes in the temperature range 230
and 430 °C giving 99 % total mass loss corresponding two
units of C6H5N and one unit of C6H10N2 losses, respec-
tively. Copper complex of this ligand decomposes in three
successive overlapped and unresolved steps. These
decomposition steps occur in the temperature range
110–700 °C with a net mass loss of 52.54 %. The residual
part gives 17.27 % copper and 30.19 % undetermined
portion. According to the TG results (Table 1), if we dis-
regard the water losses, the most stable compound among
the complexes of L1 seems to be the nickel(II) complex
with a decomposing temperature of 260 °C and the least
The UV spectra were taken in ethanol in the range
between 200 nm and 700 nm. The n–p* and p–p* transi-
tions of the ligand L1 and its complexes were observed in
123

Schiff base ligands derived from 2-pyridinecarboxaldehyde and its complexes
Table 1 Thermal analysis of the ligands and its complexes
Compound
M.W.
T/°C
Mass loss/%
Found (calc.)
Assignment
Residual/%
Found (calc.)
L1
292.38
727.71
230–320
320–430
110–160
190–195
203–440
95–210
62.00 (61.87)
37.00 (37.12)
10.2 (9.89)
C6H5N
C6H10N2
H2O
1 (1.01)
[Cu2L1(H2O)4]ꢀ4AcO
30.19 (32.53)
25.34 (25.0)
17.00 (15.12)
9.80 (10.02)
24.15 (25.33)
14.6 (15.3)
C6H5N
C6H10N2
H2O
17.27 (17.46) (Cu)
[Co2L1(H2O)4]ꢀ4AcO
[Ni2L1(H2O)4]ꢀ4AcO
718.48
718
35.91 (32.94)
250–390
395–530
80–210
C6H5N
C6H10N2
H2O
15.54 (16.41) (Co)
10.12 (10.03)
25.77 (25.34)
15.83 (15.32)
62.72 (62.83)
26.56 (26.20)
7.41 (7.87)
32.78 (32.97)
260–360
360–620
110–400
C6H5N
C6H10N2
C6H5N
C6H4
15.5 (16.34) (Ni)
L2
286.33
721.66
9.78 (10.10) (N2)
0.94 (0.87)
[Cu2L2(H2O)4]ꢀ4AcO
50–200
H2O
16.38 (16.72) (CuO)
16.89 (15.01)
200–800
37.48 (37.61)
21.84 (22.79)
15.12 (15.24)
38.23 (38.45)
15.9 (15.86)
5.88 (5.76)
C6H5N
C6H4N2
H2O
[Co2L2(H2O)4]ꢀ4AcO
712.43
711.95
70–190
15.75 (15.81) (CoO)
9.12 (8.88)
285–410
410–520
520–550
50–250
C6H5N
C6H4
N2
[Ni2L2(H2O)4]ꢀ4AcO
15.13 (15.34)
38.25 (38.51)
21.85 (21.74)
H2O
15.69 (15.46) (NiO)
9.08 (8.95)
290–450
450–650
C6H5N
C6H4N2
All thermal analyses were done under nitrogen atmosphere between the temperature range 30 and 988 °C at a heating rate of 10 °C/min
Fig. 1 TG plots of L1 and its complexes recorded under nitrogen atmosphere between the temperature range 30 and 988 °C at a heating rate of
10 °C/min
stable one is the copper(II) complex compound. TG plots
of the title compounds and the complexes are given in
Figs. 1 and 2.
Examination of the TGA graphic of L2 revealed the
ligand decomposition between 110 and 400 °C with a total
mass loss of 89.28 % resulting from the loss of two units of
123

S. Bal, S. S. Bal
Fig. 2 TG plots of L2 and its complexes recorded under nitrogen atmosphere between the temperature range 3 and 988 °C at a heating rate of
10 °C/min
Fig. 3 Cyclic voltammograms of L1 in the presence of 0.1 M Bu4NBF4 in DMF solution at 1 9 10-3 M at 100 mV s-1 (a) and 250 mV s-1
(b) scan rates
C6H5N and one unit of C6H4. The TG plot of Co2L2(H2O)4
complex shows four decomposition steps belonging to four
units of water molecules, two units of C6H5N, one unit of
C6H4 and N2 corresponding to a total mass loss of 75.13 %
between the temperatures 70 and 550 °C. However, for this
temperature range, one could also argue that the loss of
C6H4N2 (21.84 %) might have happened in the temperature
range 410 and 550 °C in one step or it could happen in two
consecutive steps; loss of C6H4 (15.9 %) at 410–520 °C
and the loss of N2 (5.88 %) at 520–550 °C. The formation
of CoO in the residual part occurs over 550 °C.
quoted refer to measurements run at scan rates (m) of 100,
250, 500, 750, and 1,000 mV s-1 and against an internal
ferrocene–ferrocenium standard.
For the scan rate 100 mV s-1, our ligand L1 exhibits
potentials in the range from -2 to 1.5 V, two anodic peaks
1.18 and -0.57 V are due to oxidation processes of the
imine units in our ligand by losing electrons and thus
generating cation radicals or dications. On the reverse
scanning, the ligand exhibits one irreversible and one
quasi-reversible cathodic peak located at -1.10 V and at
0.89 V. The irreversible process at -1.10 (Epc) and 1.18
(Epa) might suit to two electron-transfer mechanisms. The
ligand also shows similar irreversible processes at -1.22
(Epc) and 1.09 (Epa) for scan rate 250 mV s-1 and at -1.37
(Epc), 1.21 (Epa) for scan rate 500 mV s-1. The rest of the
currents detected for all the scan rates give either pseudo-
reversible or irreversible processes. Electrochemical
graphics of the ligand L1 is given for 100 and 250 mV s-1
Electrochemical properties of the synthesized
compounds
Cyclic voltammogram studies of the title compounds were
run in DMF (1 9 10-3 M)–0.1 M Bu4NBF4 as supporting
electrolyte at 293 K. Unless otherwise stated, all potentials
123

Schiff base ligands derived from 2-pyridinecarboxaldehyde and its complexes
Fig. 4 Cyclic voltammograms of L2 in the presence of 0.1 M Bu4NBF4 in DMF solution at 1 9 10-3 M at 100 mV s-1 (a) and 250 mV s-1
(b) scan rates
(a)
(b)
Fig. 5 Reversible reduction–oxidation processes of the title compounds L1 (a) and L2 (b) in DMF (1 9 10-3 M) solution
scan rates in Fig. 3. Copper complex of L1 shows quasi-
reversible processes at scan rates 250 mV s-1 and
500 mV s-1, at 0.85 (Epc), 1.15 (Epa) and at 0.83 (Epc),
1.18 (Epa), respectively. Cathodic to anodic peak potential
ratios for these pairs are around 0.7 (Ipc:Ipa), similar to the
values reported in the literature for quasi-reversible pro-
cesses [19]. At scan rates 100 and 750 mV s-1, copper
complex exhibits both pseudo-reversible and irreversible
redox processes. Nickel(II) complex also presents irre-
versible one-electron redox processes Ni(II) $ Ni(I) at
scan rates 250 mV s-1 and 750 mV s-1, at -1.01 (Epc),
1.12 (Epa) and at -1.31 (Epc), 1.23 (Epa), respectively [18,
19]. At scan rates 500 and 750 mV s-1, it also shows
pseudo-reversible processes together with the irreversible
ones.
Our ligand L2 and its complexes exhibit potentials in the
range from -2 to 2 V. For the scan rate 100 mV s-1, the
Schiff base ligand showed two cathodic peaks in the for-
ward scan at -0.48 and 0.87 V, which presents a reversible
process with 0.73 V peak at the reverse scan showing the
cathodic and anodic ratio to be 1.1 (Ipc:Ipa) (Fig. 4a). The
peak at -0.48 V and the peaks at -0.63, -1.38 V present
irreversible reduction and oxidation processes for L2. For
the scan rates 250 mV s-1 (Fig. 4b) and 500 mV s-1, we
can say that our ligand reveals irreversible redox processes
at 1.02 (Epc), 0.71 (Epa) and 1.15 (Epc), 0.69 (Epa). The
other peaks assigned for these scan rates are also irre-
versible oxidation or reduction processes.
The coordination compounds of the ligand L2 revealed
irreversible processes for all the scan rates. These irrevers-
ible or quasi-reversible redox processes for all the
complexes can be defined as the formation of M(II) or
M(I) with a simple one-electron process [18, 19]. The pos-
sible redox process for the ligands can be shown in Fig. 5.
Catalytic activity
The optimum conditions for the oxidation reactions of
cyclohexene and styrene were obtained as catalyst:sub-
strate:oxidant ratio of 1:100:200 in acetonitrile under
400 W microwave power for 60 min. The temperature and
pressure were controlled at about 110 °C and 30 bar by the
instrument. The possible oxidation reactions are given in
Fig. 6. Unlike classical thermal oxidation techniques,
microwave power is easy to use and clean for organic and
inorganic synthesis. Classical thermal oxidation
123

S. Bal, S. S. Bal
(a)
(b)
Fig. 6 Various oxidation products of cyclohexene (a) and styrene (b). Recorded under 400 W microwave power for 60 min with
catalyst:substrate:oxidant ratio of 1:100:200 in acetonitrile
Table 2 Cyclohexene oxidation results
Table 3 Styrene oxidation results
Catalyst
Conversion/ Selectivity of products/%
%
Catalyst
Conversion/% Selectivity of products/%
Epoxy 1-ol 1-one 1-hydroperoxide
Styrene Benzaldehyde Benzoic
oxide
acid
2.13
Blanka
5.5
–
2.7
31.96 6.15 61.24
80.8 7.5 11.07
–
4.54
Blanka
4.32
–
1.57
75.7
Cu2L1(H2O)b4 61
Co2L1(H2O)b4 24
Ni2L1(H2O)b4 38.75
Cu2L2(H2O)b4 34.25
Co2L2(H2O)b4 29
Ni2L2(H2O)b4 26.7
0.65
0.63
0.52
0.72
0.52
0.37
Cu2L1(H2O)b4 58.65
Co2L1(H2O)b4 25.65
Ni2L1(H2O)b4 33.5
Cu2L2(H2O)b4 61.75
Co2L2(H2O)b4 72.9
Ni2L2(H2O)b4 33.75
5.28
4.3
10.14
9
81.28
61.79
57.98
72.00
56.47
47.22 6.32 45.94
56.00 5.36 37.92
68.28 6.72 24.48
68.68 5.28 25.67
5.07
6.39
2.87
2.94
15.37
11.49
7.2
23.38
For all the experiments 400 W power was applied for 60 min. The
reaction temperature and pressure were held at around 110 °C and
30 bar in closed DAP60 vessels
For all the experiments 400 W power was applied for 60 min. The
reaction temperature and pressure were held at around 110 °C and
30 bar in closed DAP60 vessels
a
2 mmol cyclohexene, 4 mmol hydrogen peroxide, and 5 cm3 ace-
tonitrile were used without catalyst
a
2 mmol styrene, 4 mmol hydrogen peroxide, and 5 cm3 acetonitrile
were used without catalyst
b
0.02 mmol catalyst, 2 mmol cyclohexene, 4 mmol hydrogen per-
oxide (1:100:200) and 5 cm3 acetonitrile were used
b
0.02 mmol catalyst, 2 mmol styrene, 4 mmol hydrogen peroxide
(1:100:200) and 5 cm3 acetonitrile were used
techniques, however, require longer times and the control
of reaction conditions is very difficult.
for cyclohexene oxide. Furthermore, the benzaldehyde
formations have the highest percentages for this oxida-
tion. Our complex compounds were proved to be effective
catalysts over the styrene oxidation, with hydrogen per-
oxide as the oxidant, giving considerably high results for
styrene oxide formation. This might be due to the binu-
clear nature of the complexes. In the literature, various
Schiff base complexes used for this kind of catalytic
activities show similar results with ours [36–38]. How-
ever, one should keep in mind that these experiments can
be carried out in vastly different reaction conditions,
using various catalysts, solvents, oxidants and methods.
The oxidation experiments without using the catalysts
under microwave irradiation, however, gave very poor or
no yields towards the oxidation products.
The results of cyclohexene and styrene oxidations
catalysed by bidentate Schiff base metal complexes under
microwave radiation using hydrogen peroxide are listed in
Tables 2 and 3. As can be seen, all of the bidentate Schiff
base metal complexes have catalytic activity over the
oxidation of both cyclohexene and styrene and in general
high selectivities have been obtained. For the cyclohexene
oxidation, among the oxidation products, 2-cyclohexene-
1-ol and 2-cyclohexene-1-hydroperoxide showed the
highest percentages; however, poor or moderate results
were obtained for both epoxycyclohexene and 2-cyclo-
hexenone. Styrene oxidation reactions, however, gave
higher values for the epoxide formation than those gained
123

Schiff base ligands derived from 2-pyridinecarboxaldehyde and its complexes
From the examination of catalytic activities of Co(II),
Cu(II), and Ni(II) complexes we can say that the most
effective complex towards the formation of epoxycyclo-
hexene is copper complex of L2 and the least effective one
is nickel complex of the same ligand. In the formation of
2-cyclohexene-1-ol, however, we get the highest percent-
ages from the use of these coordination compounds as
catalysts.
MSD spectrophotometer. Elemental analyses were per-
formed on a LECO CHNS 932 elemental analyzer and the
metal analyses were carried out on an Ati Unicam 929
Model AA spectrometer in solutions prepared by decom-
posing the compounds in aqua regia and subsequently
digesting them in conc. HCl. Thermal analyses of synthe-
sized ligands and their metal complexes were carried out on
a Perkin-Elmer Thermogravimetric Analyzer TG/DTA 6300
instrument under nitrogen atmosphere between the temper-
ature range 30 and 988 °C at a heating rate of 10 °C/min.
Cyclic voltammetry was performed in DMF (1 9 10-3 M)–
0.1 M Bu4NBF4 as supporting electrolyte and against an
internal ferrocene–ferrocenium standard using Iviumstat
Electro-chemical Workstation equipped with a low current
module (BAS PA-1) recorder.
The synthesized complex compounds showed the high-
est efficiency towards the oxidation of styrene. Especially,
for the styrene oxide formation we get moderately high
percentages from the catalytic use of copper and nickel
complexes of our ligands. Benzaldehyde, among the sty-
rene oxidation outcomes, is oxidatively most produced one.
The reaction products of the microwave experiments
were analysed using Perkin Elmer Clarus 600 GC equipped
with MS detector fitted with Elite 5 MS and FID detector
fitted with BPX5 capillary columns.
Conclusion
In summary, with this work highly efficient binuclear
catalysts have been synthesized, characterized, and used
for the oxidation reactions of styrene and cyclohexene. Our
catalytic activity reaction conditions gave high or moderate
results for both cyclohexene and styrene oxidations. Sty-
rene oxide formations were observed to be considerably
higher than cyclohexene oxide formations. Thermal anal-
yses revealed well-defined decomposition steps for most of
the synthesized compounds and supported the proposed
formulas. In addition, the coordination compounds showed
more resistance towards temperature than their title com-
pounds, which means that these compounds have potential
to be used as catalysts towards many chemical reactions
requiring elevated temperatures. Finally, the electronic
features of these compounds have also been reported.
(E)-N,N’-Bis(pyridin-2-ylmethylene)cyclohexane-1,4-
diamine (L1, C18H20N4)
Pyridine-2-carboxaldehyde (4 mmol, 0.429 g) was taken
into 100 cm3 round-bottom flask containing 10 cm3 etha-
nol and a magnetic stirrer. Over this solution, 0.228 g 1,4-
diaminocyclohexane (2 mmol) dissolved in 20 cm3 ethanol
was added dropwise (Fig. 7). Resulting solution was stirred
under reflux for 7 h and left at r.t. for overnight. The
resulting precipitate was crystallized from ethanol. Yield:
80 %; m.p.: 178.2 °C; UV-Vis (ethanol): kmax (e) = 202
(257,900), 236 (179,900), 240 (287,300), 274 (97,000) nm
(M-1 cm-1); FT-IR (KBr): m = 3,051, 3,004 (Ar–H),
ꢀ
2,930, 2,855 (CH2), 1,638 (C=N), 1,566, 1,468 (Py)
1
cm-1; H NMR (CDCl3): d = 1.87 (m, 8H-20), 3.38 (m,
2H-10), 7.30 (2H-4, ddd, J = 7.5, 4.9, 1.2 Hz), 7.74 (2H-3,
dt, J = 7.8, 1.4 Hz), 7.99 (2H-2, brd, J = 7.88 Hz), 8.46
(s, CH=N), 8.66 (2H-5, dd, J = 4.8, 1.6 Hz) ppm; 13C
NMR (CDCl3): d = 32.3 (C-20), 68.8 (C-10), 121.5 (C-2),
124.6 (C-4), 136.5 (C-3), 149.5 (C-5), 154.7 (C-1), 160.2
(CH=N) ppm; MS (LC/MS APCI): m/z = 315.1
([M ? Na]?), 279.1 ([M-12]?, loss of one carbon atom
from the parent ion), 215.29 ([M-77]?, H2C=N–C6H10–
Experimental
Pyridine-2-carboxaldehyde, 1,4-diaminobenzene, and trans-
1,4-diaminocyclohexane were purchased from Sigma
Aldrich. The acetate salts of copper(II), nickel(II) and
cobalt(II) were obtained from Fluka. Nuclear magnetic
resonance spectra of the synthesized ligands were recorded
on a Bruker AV 400-MHz spectrometer in the solvent
CDCl3. Infrared spectra were obtained using KBr discs on a
Shimadzu 8300 FT-IR spectrophotometer in the region of
400–4,000 cm-1. Ultraviolet spectra were run in ethanol on
a Shimadzu UV-160A spectrophotometer. Magnetic mea-
surements were carried out by the Gouy method using
Hg[Co(SCN)4] as a calibrant. Molar conductances of the
ligands and their transition metal complexes were deter-
mined in MeOH (*10-3) at room temperature using a
Jenway Model 4070 conductivity meter. Mass spectra of the
ligand were recorded on a LC/MS APCI AGILENT 1100
N=CH–C5H4N); KM = 1.7X-1 cm2 mol-1
.
(E)-N,N’-Bis(pyridin-2-ylmethylene)benzene-1,4-
diamine (L2, C18H14N4)
Synthesis of the second ligand was done according to the
procedure given above. Over 0.429 g pyridine-2-carboxalde-
hyde (4 mmol) in a 100-cm3 round-bottom flask containing
10 cm3 ethanol, 0.228 g 1,4-diaminobenzene (2 mmol) dis-
solved in 20 cm3 ethanol was added dropwise. Resulting
solution was stirred under reflux for 24 h and left at r.t. for the
next day. The resulting precipitate was recrystallized from
ethanol. Yield: 65 %; m.p.: 167.3 °C; UV-Vis (ethanol): kmax
123

S. Bal, S. S. Bal
Fig. 7 Synthesis scheme of the ligands and the proposed structures of the metal complexes. M: Cu(II), Ni(II), Co(II)
(e) = 202 (72,020), 242 (71,820), 272 (69,680), 281 (62,220)
filtered, washed with distilled water to get rid of the excess
salt, and dried in vacuum.
nm (M-1 cm-1); FT-IR (KBr): m = 3,048, 2,919 (Ar–H),
ꢀ
1,618 (C=N), 1,583, 1,494 (Py), 843, 877 (Ar–H bending)
cm-1; 1H NMR (CDCl3): d = 7.39 (s, 4H-20, H-4), 7.83 (dt,
J = 7.8, 1.6 Hz, H-3), 8.22 (brd, J = 7.9 Hz, H-2), 8.67 (s,
CH=N), 8.74 (dd, J = 4.8, 1.5 Hz, H-5) ppm; 13C NMR
(CDCl3): d = 121.9 (C-2), 123.2 (C-20), 125.2 (C-4), 136.7
(C-3), 149.5 (C-10), 149.8 (C-5), 159.4 (C-1), 160.2 (CH=N)
ppm; MS (LC/MS APCI): m/z = 309.1 ([M ? Na]?), 275.0
([M-12]?, loss of one carbon atom from the parent ion);
[(E)-N,N’-Bis(pyridin-2-ylmethylene)cyclohexane-1,4-
diamine]tetraaquadicopper(II) acetate
(C26H40Cu2N4O12)
Black coloured; yield: 72 %; m.p.: 180.5 °C; UV-Vis
(ethanol): kmax (e) = 230 (165,000), 278 (95,000), 356
(196,000), 384 (83,000), 575 (46,000) nm (M-1 cm-1);
FT-IR (KBr): mꢀ = 3,366 (O–H), 2,931 (CH2), 1,564 (C=N),
1,385 (Py), 672 (M–N) cm-1; KM = 63.1 X-1 cm2 mol-1
leff = 1.4 B.M.
;
KM = 2.5 X-1 cm2 mol-1
.
Synthesis of complex compounds
[(E)-N,N’-Bis(pyridin-2-ylmethylene)cyclohexane-1,4-
diamine]tetraaquadinickel(II) acetate
(C26H40N4Ni2O12)
The ratio of the metal salts and the ligands were taken as
2:1. A solution of the metal salt (2 mmol) in 25 cm3
absolute ethanol was added into the solution of the ligand
L1 or L2 (1 mmol) in 25 cm3 ethanol. The mixtures were
stirred under reflux for overnight. The precipitate was
Green coloured; yield: 68 %; m.p.: 178 °C; UV-Vis
(ethanol): kmax (e) = 251 (62,200), 270 (61,400), 350
(57,600), 397 (46,100), 586 (51,000) nm (M-1 cm-1); FT-
ꢀ
IR (KBr): m = 3,350 (O–H), 2,930 (CH2), 1,543 (C=N),
123

Schiff base ligands derived from 2-pyridinecarboxaldehyde and its complexes
1,406 (Py), 660 (M–N) cm-1; KM = 30.4 X-1 cm2 mol-1
leff: diamag.
;
Berghof MWS3? microwave oven. The temperature was
controlled automatically by the microwave instrument.
Following the completion of microwaving, 1 cm3 H2O was
added into the vessels and the extraction of oxidized pro-
ducts was carried out with 10 cm3 CH2Cl2. The extracts
were then analysed with GC and GC–MS. During the
oxidation process, the temperature and pressure were
controlled at about 110 °C and 30 bar throughout the
experiment.
[(E)-N,N’-Bis(pyridin-2-ylmethylene)cyclohexane-1,4-
diamine]tetraaquadicobalt(II) acetate
(C26H40Co2N4O12)
Brown coloured; yield: 72 %; m.p.: 181.5 °C; UV-Vis
(ethanol): kmax (e) = 235 (59,400), 250 (31,600), 297
(25,300), 367 (38,000), 610 (28,600) nm (M-1 cm-1); FT-
ꢀ
IR (KBr): m = 3,400 (O–H), 2,923 (CH2), 1,614 (C=N),
1,409 (Py), 650 (M–N) cm-1; KM = 70.5 X-1 cm2 mol-1
leff = 4.4 B.M.
;
Acknowledgments We would like to thank K. Maras Sutcu Imam
University Research Projects Coordination Unit for the financial
support.
[(E)-N,N’-Bis(pyridin-2-ylmethylene)benzene-1,4-
diamine]tetraaquadicopper(II) acetate
(C26H34Cu2N4O12)
References
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(34,110), 396 (2,912), 589 (23,610) nm (M-1 cm-1); FT-
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123