Pd(II) complexes of Schiff bases and their application as catalysts in Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions

Article abstract of DOI:10.1002/aoc.3329

Schiff bases of 2-(phenylthio)aniline, (C₆H₅)SC₆H₄N=CR (R=(o-CH₃)(C₆H₅), (o-OCH₃)(C₆H₅) or (o-CF₃)(C₆H₅)), and their palladium complexes (PdLCl₂) were synthesized. The compounds were characterized using ¹H NMR and ¹³C NMR spectroscopy and micro analysis. Also, electrochemical properties of the ligands and Pd(II) complexes were investigated in dimethylformamide-LiClO₄ solution with cyclic and square wave voltammetry techniques. The Pd(II) complexes showed both reversible and quasi-reversible processes in the -1.5 to 0.3V potential range. The synthesized Pd(II) complexes were evaluated as catalysts in Mizoroki-Heck and Suzuki-Miyaura cross-coupling reactions.

Full text of DOI:10.1002/aoc.3329

Full paper
Received: 4 February 2015
Revised: 3 April 2015
Accepted: 10 April 2015
Published online in Wiley Online Library: 14 May 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3329
Pd(II) complexes of Schiff bases and their
application as catalysts in Mizoroki–Heck and
Suzuki–Miyaura cross-coupling reactions
Mustafa Keleş*, Hülya Keleş and Duygu Melis Emir
Schiff bases of 2-(phenylthio)aniline, (C6H5)SC6H4N¼CR (R = (o-CH3)(C6H5), (o-OCH3)(C6H5) or (o-CF3)(C6H5)), and their palladium
complexes (PdLCl2) were synthesized. The compounds were characterized using 1H NMR and 13C NMR spectroscopy and micro
analysis. Also, electrochemical properties of the ligands and Pd(II) complexes were investigated in dimethylformamide–LiClO4 so-
lution with cyclic and square wave voltammetry techniques. The Pd(II) complexes showed both reversible and quasi-reversible
processes in the ꢀ1.5 to 0.3 V potential range. The synthesized Pd(II) complexes were evaluated as catalysts in Mizoroki–Heck
and Suzuki–Miyaura cross-coupling reactions. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: Schiff base; palladium; Mizoroki–Heck coupling; Suzuki–Miyaura coupling; electrochemistry
2-methoxybenzaldehyde and 2-trifluoromethylbenzaldehyde
(Scheme 3). The Pd(II) complexes of these Schiff bases (1a–3a)
Introduction
The palladium-catalysed cross-coupling reactions of aryl halides
with arylboronic acids (Suzuki–Miyaura reaction)[1] and olefins
(Heck–Mizoroki reaction)[2] provide a powerful methodology for
constructing C(sp2)–C(sp2) bonds. The reason for the active research
is the fact that C–C coupling reactions have many areas of usage in
fields such as pharmacological agents, herbicides and the synthesis
of natural products.[3] Consequently, researchers have been working
on developing better catalysts and increasing yields in these areas
which are important for both industrial and scientific purposes.[4]
In Mizoroki–Heck and Suzuki–Miyaura reactions, phosphine com-
pounds are active due to their excellent donor capability. Neverthe-
less, they have some disadvantages like natural toxicity, sensitivity
to air, high cost, synthetic difficulties and limitations of use.[5] Within
this scope, nitrogen-based ligands are generally advantageous as
they are usually stable to air, inexpensive and easier to handle than
their phosphine counterparts.[6] Recently, Schiff bases and their palla-
dium complexes have been shown to have such catalytic activity in
Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions that they
can become possible alternatives to phosphine.[7] Researchers have
been developing these types of ligands and their palladium com-
plexes which provide more effective and easier oxidative addition.
In the study reported here, we prepared several new Schiff base
ligands containing hard nitrogen and soft sulfur donor atoms (1–3)
and their Pd(II) complexes (1a–3a). The Pd(II) complexes were used
in Mizoroki–Heck (Scheme 1) and Suzuki–Miyaura (Scheme 2) cross-
coupling reactions. The Schiff bases and their palladium complexes
were easily synthesized and the results demonstrate that these
compounds are also ideal catalysts and highly efficient complexes
for the C–C cross-coupling reaction.
were prepared under nitrogen atmosphere as shown in
Scheme 4. It is found that although the Pd(II) complexes are
soluble in dichloromethane, chloroform, ethanol, methanol and
acetone, they are insoluble in n-hexane and diethyl ether.
FT-IR analysis of Schiff bases and Pd(II) complexes
The FT-IR spectra of the synthesized ligands show a characteristic
peak in the region 1632–1640 cmꢀ1 indicating the formation of
Schiff bases. At the end of the reaction νN–H and νC¼ stretching
O
bands of amines and aldehydes disappear and new νC¼
N
stretching bands appear with medium intensity at 1632cmꢀ1 (1),
1635 cmꢀ1 (2) and 1640 cmꢀ1 (3). The FT-IR spectral bands of Pd
(II) complexes related to stretching vibrations shift towards lower
frequencies when compared to the free ligands: 1624 cmꢀ1 (1a),
1614 cmꢀ1 (2a) and 1619 (3a) cmꢀ1 [8] The slight shift to lower fre-
.
quencies after complexation discloses that the ligands are coordi-
nated to Pd(II) through the nitrogen atom.[9] Also, the aromatic
CH peaks for all compounds appear at 3060, 3050, 3047, 3027,
3043 and 3051 cmꢀ1, and aliphatic CH3 bands for 1 and 2 and their
Pd(II) complexes 1a and 2a are observed at 2967, 2982, 2939 and
2961 cmꢀ1, respectively.
NMR spectra
The structures of compounds were characterized using 1H NMR and
13C NMR spectroscopy. The 1H NMR signals of CH¼N proton are at
8.05 (1), 8.19 (2) and 8.32 (3) ppm, respectively. The CH¼N peaks of
*
Correspondence to: Mustafa Keleş, Chemistry, Osmaniye Korkut Ata University,
80000 Osmaniye, Turkey. E-mail: mkeles@osmaniye.edu.tr
Results and discussion
Ligands 1–3 were synthesized by treating 2-(phenylthio)ani-
line with appropriate aldehydes, i.e. 2-methylbenzaldehyde,
Osmaniye Korkut Ata University, Faculty of Arts and Sciences, Department of
Chemistry, 80000, Osmaniye, Turkey
Appl. Organometal. Chem. 2015, 29, 543–548
Copyright © 2015 John Wiley & Sons, Ltd.

M. Keleş et al.
acetonitrile. The values reveal that all the Pd(II) complexes behave
as non-electrolytes.[13]
Scheme 1. Mizoroki–Heck reaction.
Electronic spectra
The electronic spectra of the ligands and their Pd(II) complexes
were investigated in acetonitrile solvent (1× 10ꢀ4 M). The absorp-
tion spectra show two π–π* transitions attributed to phenyl rings
and azomethine chromophore in the range 286–290nm. After
complexation with Pd(II) the absorption bands are shifted to lower
energy (408 (1a), 404 (2a) and 394 (3a) nm).[14]
Scheme 2. Suzuki–Miyaura reaction.
Electrochemical studies of ligands and their Pd(II) complexes
Electrochemical properties of the Schiff base ligands (1, 2, 3) and
their Pd(II) complexes (1a, 2a, 3a) were studied using cyclic volt-
ammetry and square wave voltammetry in DMF with 0.1M LiClO4
as the supporting electrolyte within the potential range ꢀ1.5 to
0.3 V. The obtained electrochemical data are summarized in Table 1.
None of the studied Schiff base ligands 1, 2, 3 show discernible re-
sponses under identical conditions except in the potential range
ꢀ0.7 to ꢀ0.8V. The value observed at this potential range corre-
sponds to the intramolecular reduction/oxidation coupling of the
imine groups.
Scheme 3. Synthesis of Schiff bases 1–3.
Figure 1 shows the cyclic voltammetric responses of 1 and 1a. It
is seen that the Pd(II) complex 1a exhibits an irreversible redox
Table 1. Electrochemical parameters for Pd(II) complexes in DMF–
0.1 M LiClO4 solution
Scheme 4. Synthesis of Pd(II) complexes 1a–3a.
Compound Redox couple
Epc (V)
Epa (V) ΔEp (mV)a E1/2 (V)b
1a
2a
Pd+2/Pd+1
Pd+2/Pd+1
Pd+1/Pd0
Pd+2/Pd+1
Pd+1/Pd0
ꢀ0.475
—
—
257
74
—
1a–3a slightly shift (by 0.23–0.11 ppm) when compared to the free
ligands (1a, 8.28; 2a, 8.30; 3a, 8.45 ppm).[10] These results point to
π-electrons of azomethine group being attracted to d orbitals of
Pd(II). In the 13C NMR spectra, the azomethine carbon (C11 and
C11′) resonances appear in the region of 156.4–162.3 ppm: 159.5
(1), 159.5 (2), 156.4 (3), 162.3 (1a), 160.3 (2a) and 160.0 (3a) ppm.
These signals are assigned to CH¼N resonances. The shifting of
13C NMR spectra occurring after complexation proves that Pd(II) co-
ordinates to the ligands via nitrogen atoms.[11] The 1H NMR spectra
of ligands and their Pd(II) complexes 1, 2, 1a and 2a show signals at
2.50–3.86 ppm, corresponding to aliphatic CH3 protons. The peaks
of aliphatic CH3 carbons occur in the range 19.7–55.5 ppm in 13C
NMR spectra. The peaks observed in the region of 6.98–7.75ppm
ꢀ0.674 ꢀ0.417
ꢀ1.158 ꢀ1.084
ꢀ0.285 ꢀ0.322
ꢀ0.546
ꢀ1.121
ꢀ0.304
—
3a
37
ꢀ1.291
—
—
aΔEp = Epa ꢀ Epc
.
bE1/2 = (Epa + Epc)/2.
1
as multiplets in H NMR spectra belong to the aromatic protons
of the phenyl rings.[12] In the 13C NMR spectra of the ligands and
complexes, the phenyl carbon signals appear in the region of
111.0–156.4 ppm. No substantial change is noticed in the NMR
values of the aromatic protons and carbons when ligands are ex-
changed with complexes.
As a result of micro analysis, it is understood that all the ligands
and complexes have few impurities due to the organic solvent.
Micro analysis for C, H, N and S of ligands 1, 2, 3 and their Pd(II) metal
complexes 1a, 2a, 3a indicates that the metal-to-ligand ratio of com-
plexes is 1:1. The NMR, FT-IR and micro analysis results prove that Pd
(II) coordinates through nitrogen and sulfur atoms of the ligands.
Molar conductivity
Figure 1. Cyclic voltammograms of Schiff base ligand 1 (dotted curve) and
Pd(II) complex 1a (solid curve) in DMF–LiClO4 using glassy carbon electrode
with 0.1 V sꢀ1 scan rate (inset: square wave voltammogram of 1a: amplitude,
50 mV; frequency, 15 Hz; glassy carbon working electrode).
The molar conductivity of the complexes was investigated and
observed to be 2.3 (1a), 2.1 (2a) and 3.1 (3a) mS cm2 molꢀ1 in
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Appl. Organometal. Chem. 2015, 29, 543–548

Schiff base–Pd(II)-catalysed C–C coupling reactions
process during the negative scan. The cathodic reduction peak at
E
duction of Pd(II) to Pd(I) is clearly observed in the square wave volt-
ammogram of the inset.
Pd(II)/Pd(I) redox couple.[13,17] Pd(II)/Pd(I) redox process is clearly
observed in both the cyclic voltammogram and the square wave
voltammogram, whereas the process at ꢀ1.291 V in the cathodic
scan is irreversible and not observed in the square wave voltammo-
gram. This reductive response can be assigned as Pd(I) to Pd(0).[14]
According to Table 1, the cathodic peak potentials (Epc) of Pd(II)/
Pd(I) become less negative due to electron-withdrawing group –
CF3. In the case of compounds 1a and 2a, the presence of –CH3
and –OCH3 (electron donating) makes the potentials more negative
than that of 3a.[18]
pc = ꢀ0.475 V corresponds to the Pd(II)/Pd(I) redox pair.[14] The re-
Complex 2a shows a well-defined reversible electrochemical re-
sponse at Epa = ꢀ1.084 V and Epc = ꢀ1.158 V which correspond to
Pd(I)/Pd(0) redox pair (Fig. 2). This couple is found to be a reversible
process. This behaviour is similar to that reported previously in cy-
clic voltammetric studies of Pd(II) complexes.[15] A quasi-reversible
process is also observed in the cyclic voltammetric response of 2a
which is assigned to Pd(II)/Pd(I) couple at Epa = ꢀ0.417V and
Epc = ꢀ0.674 V.[16] Only a weak wave is observed in the anodic
square wave voltammogram of this redox couple (Fig. 2, inset).
The cyclic voltammograms of 1 × 10ꢀ3 M Schiff base ligand 3 and
Pd(II) complex 3a are presented in Fig. 3. Complex 3a shows an an-
odic peak at Epa = ꢀ0.322V. On the reverse scan, two cathodic
peaks are observed at Epc = ꢀ0.285 and ꢀ1.291 V, respectively.
The reversible process in the ꢀ0.3V potential range is assigned to
Mizoroki–Heck reaction
Palladium complexes are active catalysts for C–C coupling
reactions.[19,1] Because of this, complexes 1a–3a were tested in
Mizoroki–Heck and Suzuki–Miyaura reactions.
Firstly, the Mizoroki–Heck reaction of bromobenzene and sty-
rene was chosen as a model reaction, and the optimal conditions
were identified as shown in Scheme 1. Determination of the opti-
mum conditions is achieved by testing organic and inorganic bases
(NEt3, Na2CO3, NaOAc and K2CO3), various temperatures (80, 100,
120 and 140 °C) and various solvent systems (toluene, 1,4-dioxane,
DMF and N-methylpyrrolidone) in the presence of 1a (substrate/
catalyst: 250), with the samples taken at the end of the reaction be-
ing analysed using GC. In polar solvents, the conversions generally
appear to be higher than those obtained in non-polar solvents, and
the best conversion is found in 1,4-dioxane and K2CO3 at 140 °C.
The Mizoroki–Heck reaction was carried out with catalysts 1a–3a
between derivatives of aryl bromides and styrenes or acrylates
substituted in various positions (Tables 2 and 3) using the condi-
tions mentioned above.
Under the typical reaction conditions, non-activated neutral sub-
strates such as bromobenzene give moderate yields. Low conver-
sion is obtained with styrenes only in one reaction with catalyst
1a (Table 2, entry 1). High conversion is observed in the presence
of 3a (Table 2, entry 1). In the reaction between activated
electron-deficient substrates such as 4-bromobenzaldehyde with
styrenes, all three catalysts show high activity (Table 2, entries
5–7). In addition, the presence of electron-donating groups such
as methyl or methoxy moiety on the aryl bromide leads to lower
Figure 2. Cyclic voltammograms of Schiff base ligand 2 (dotted curve) and
Pd(II) complex 2a (solid curve) in DMF–LiClO4 using glassy carbon electrode
with 0.1 V sꢀ1 scan rate (inset: square wave voltammogram of 2a: amplitude,
50 mV; frequency, 15 Hz; glassy carbon working electrode).
Table 2. Results of reaction between olefins and aryl bromidesa
Conversion (%)b
Entry
R1
Aryl bromide
Phenyl bromide
1a
2a
3a
1
H
22
9
57
21
54
26
99
99
99
70
60
13
19
10
88
24
52
33
99
99
99
70
41
11
25
10
2
2-CH3
4-OCH3
4-Br
Phenyl bromide
3
Phenyl bromide
5
4
Phenyl bromide
20
99
99
99
67
30
3
5
H
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromobenzaldehyde
6-Methoxynaphthyl bromide
6-Methoxynaphthyl bromide
6-Methoxynaphthyl bromide
6-Methoxynaphthyl bromide
6
2-CH3
4-OCH3
4-Br
7
8
9
H
10
11
12
2-CH3
4-OCH3
4-Br
23
15
aReaction conditions: aryl bromide (0.1 mmol), olefin (0.12 mmol), K2CO3
(0.12 mmol), catalyst (4 × 10ꢀ4 mmol), 1,4-dioxane (3 ml), 140 °C, 6 h.
Figure 3. Cyclic voltammograms of Schiff base ligand 3 (dotted curve) and
Pd(II) complex 3a (solid curve) in DMF–LiClO4 using glassy carbon electrode
with 0.1 V sꢀ1 scan rate (inset: square wave voltammogram of 3a: amplitude,
50 mV; frequency, 15 Hz; glassy carbon working electrode).
bAnalyses were made using GC and in accordance to aryl bromide.
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Appl. Organometal. Chem. 2015, 29, 543–548

M. Keleş et al.
Table 3. Results of reaction between olefins and aryl bromidesa
Table 4. Results of reaction between aryl bromides and boronic acidsa
Conversion
Conversion (%)b
(%)b
Entry
R1
R2
1a
2a
3a
Entry
R1
CH3
Aryl bromide
Phenyl bromide
1a
2a
3a
1
2-C(O)CH3
4-C(O)H
H
H
H
H
99
92
94
89
87
93
≤99
74
89
88
99
65
94
89
≥99
90
86
94
99
30
78
89
99
40
95
91
≤99
92
95
98
96
31
89
86
99
66
1
2
3
4
5
6
7
8
9
3
7
37
36
53
37
84
99
10
90
30
37
59
83
70
99
99
12
69
20
2
CH2CH3
(CH2)3CH3
CH3
Phenyl bromide
3
2-C(O)H
Phenyl bromide
23
5
4
C4H3(m-OCH3)
2-C(O)CH3
4-C(O)H
4-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Bromobenzaldehyde
6-Methoxynaphthyl bromide
6-Methoxynaphthyl bromide
6-Methoxynaphthyl bromide
5
C4H4
CH2CH3
(CH2)3CH3
CH3
50
99
—
6
6
C4H4
7
2-C(O)H
C4H4
8
C4H3(m-OCH3)
2-C(O)CH3
4-C(O)H
C4H4
CH2CH3
(CH2)3CH3
9
2-CH3
2-CH3
2-CH3
2-CH3
99
10
11
12
2-C(O)H
aReaction conditions: aryl bromide(0.1 mmol), olefin (0.12 mmol), K2CO3
(0.12 mmol), catalyst (4 × 10ꢀ4 mmol), 1,4-dioxane (3 ml), 140 °C, 6 h.
bAnalyses were made using GC and in accordance to aryl bromide.
C4H3(m-OCH3)
aReaction conditions: aryl bromide (0.1 mmol), boronic acid (0.12 mmol),
K2CO3 (0.12 mmol), catalyst (4 × 10ꢀ4 mmol), 1,4-dioxane (3 ml)–H2O
(3 ml), 100 °C, 6 h.
bAnalyses were made using GC and in accordance to aryl bromide.
reaction rates and results in low conversions with styrenes. In the
reaction of 2-bromo-6-methoxynaphthalene with olefins, 1a–3a
show a lower activity compared to other aryl bromides. This is
probably because, unlike other aryl bromides, 2-bromo-6-
methoxynaphthalene is sterically hindered (Table 2, entries 9–12).
Carbon–carbon coupling reactions were also tested with
acrylates under the same reaction conditions as described above,
and higher conversions are obtained when compared with
styrenes. All catalysts show high activity in the reaction of
4-bromobenzaldehyde with butyl acrylate (Table 3, entry 6). In the
reaction between 4-bromobenzaldehyde and ethyl acrylate, only
two reactions show high activity (Table 3, entries 5 (3a) and 8
(2a)). Meanwhile, aryl bromides bearing electron-withdrawing
groups such as aldehyde (Table 3, entries 5 and 6) give excellent
conversions within 6 h. The reactions of 2-bromo-6-methoxy-
naphthalene with acrylates (Table 3, entries 8 (2a) and 9 (1a)) also
result in high conversions. Other conversions listed in Table 3 are
low and moderate because 2-bromo-6-methoxynaphthalene is ste-
rically hindered and catalysts cannot approach the active site when
compared to other aryl halides.
acid and 2-bromo-6-methoxynaphthalene being sterically bulky,
the catalysts cannot come close to these compounds, an effect that
has an influence on the reaction rates in these conditions. Therefore
the catalysts show less activity in these reactions than in those
reactions with other olefins and aryl halides. Whereas in the synthe-
sis of carbaldehyde derivative compounds the same catalysts give
high yields when coupling 2-bromo-6-methoxynaphthalene with
benzeneboronic acid (Table 4, entry 4).
Experimental
Materials and methods
All reactions were carried out under nitrogen atmosphere using
conventional Schlenk glassware. Solvents were dried using
established procedures and then immediately distilled under nitro-
gen atmosphere prior to use. 2-(Phenylthio)aniline and aldehydes
purchased from Sigma-Aldrich Chemie GmbH (Steinheim,
Germany) were used without further purification. Pd(cod)Cl2 was
prepared as described in the literature.[20]
Micro analysis (C, H, N and S) was performed using a LECO CHNS
932 instrument. Infrared spectra of synthesized compounds were
recorded with a PerkinElmer RX1 spectrophotometer in the range
650–4000 cmꢀ1. All 1H NMR (400.1MHz) data were obtained at
25°C with deuterated DMSO and CDCl3 using a Bruker NMR spec-
trometer. 13C NMR spectra were obtained with a Varian Mercury
100.6MHz NMR spectrometer. UV–visible spectra were recorded
with a PG T80+ spectrophotometer at room temperature. Conduc-
tivities were measured with a Hanna EC-215 conductivity meter.
The coupling products were analysed using a PerkinElmer Clarus
500 series gas chromatograph equipped with a flame ionization
detector and a 30 m × 0.25 mm × 0.25 μm film thickness β-Dex cap-
illary column. TLC was used for monitoring the reactions.
Suzuki–Miyaura reaction
The same approach was followed for the Suzuki–Miyaura reaction
as for the Mizoroki–Heck reaction. For this purpose, inorganic and
organic bases (NEt3, Na2CO3, NaOAc, NaOH and K2CO3), various
temperatures (80 and 100 °C) and various solvent systems
(toluene, 1,4-dioxane and DMF with water) were tested in the re-
action of aryl bromides and phenylboronic acids in the presence
of catalyst 1a, and samples taken at the end of the reaction were
analysed using GC (Scheme 2). The GC results confirm that the
conversion at 80 °C is low when compared to that at 100 °C
(Table 4).
Catalytic experiments were conducted with various aryl
halides and boronic acid derivatives at 100 °C using catalysts
1a–3a (K2CO3, 1,4-dioxane–water (3:3 ml)). The reactions of
3-bromobenzaldehyde with boronic acids were performed with
substrate-to-catalyst ratio of 250 at 100°C and they give excellent
yields (Table 4, entries 3, 7, 11). The lowest conversion is observed
in the reaction of 2-naphthaleneboronic acid and 2-bromo-6-
methoxynaphthalene (Table 4, entry 8). The reason for this low con-
version can be explained as follows. Due to 2-naphthaleneboronic
Cyclic voltammetry and square wave voltammetry data were ob-
tained with a CHI 6094D electrochemical analyser controlled by an
external PC. All electrochemical experiments were performed using
a
conventional three-electrode electrochemical cell system
consisting of a glassy carbon electrode (3.0mm in diameter) as
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Schiff base–Pd(II)-catalysed C–C coupling reactions
Preparation of [PdCl2(o-PPh2)C6H4CH¼N(o-CH3)(p-OH)C6H3] (1a)
working electrode, a platinum wire as auxiliary electrode and
Ag/AgCl electrode as reference.
Ligand 1 (155 mg, 0.51 mmol) was added to a solution of Pd(cod)Cl2
(145mg, 0.51 mmol) in dry CH2Cl2 (10 ml). The mixture was stirred
for 2 h at reflux. Then, addition of diethyl ether caused the forma-
tion of a dark yellow precipitate which was filtered off and dried
to afford the title compound 1a. Yield 0.22 mg (90%), m.p. 244 °C.
1H NMR (400.2 MHz, DMSO-d6, δ, ppm): 8.28 (s, 1H, HC¼N), 7.74
(d, 1H, J= 7.8 Hz, H13’), 7.54 (d, 1H, J = 6.9 Hz, H16’), 7.53–7.03 (m,
11H, H1’–3’,6’–9’,14’,15’), 2.50 (s, 3H, CH3). 13C NMR (100.6MHz, CDCl3,
δ, ppm): 162.3 (C11’, CH¼N), 149.9 (C10’), 146.4 (C4’), 136.9 (C5’),
136.6 (C8’), 132.5 (C17’), 132.3 (C16’), 131.7 (C1’), 131.0 (C14’), 130.7
(C2’), 129.8 (C12’), 128.9 (C15’), 127.6 (C6’), 126.3 (C7’), 125.4 (C3’),
116.9 (C13’), 115.0 (C9’), 30.8 (s, CH3). FT-IR (KBr, cmꢀ1): 3027 (CHar),
2939 (CH3), 1624 (C¼N). Anal. Calcd for C20H17Cl2NSPd (%): C,
49.97; H, 3.56; N, 2.91; S, 6.67. Found (%): C, 50.45; H, 3.86; N, 3.20;
S, 7.40.
Voltammetric experiments were performed in extra-pure DMF
(Merck) containing 0.1M LiClO4 as the supporting electrolyte.
High-purity argon was used for deoxygenating the solution at
least 15 min prior to each run and to maintain an argon blanket
during the measurements. In order to obtain a reproducible
active surface, the glassy carbon electrode was polished with alu-
mina suspension with a particle size of 0.05 μm. Square wave volt-
ammetry settings were: step potential, 4 mV; amplitude, 50 mV;
frequency, 15 Hz. The concentration of the Pd(II) complexes was
1 × 10ꢀ3 M during the voltammetric experiments. Scans were per-
formed in the potential range ꢀ1.5 to 0.3V starting from negative
potential. All electrochemical measurements were carried out at
room temperature.
Preparation of ligands and complexes
Preparation of Complexes 2a and 3a
Preparation of (C6H5)SC6H4N¼C–(o-CH3)(C6H5) (1)
Complexes 2a and 3a were prepared using a procedure similar to
that for complex 1a.
2-(Phenylthio)aniline (580 mg, 2.90 mmol) and 2-methylbenzal-
dehyde (350 mg, 2.90 mmol) were mixed and stirred for 1 h in eth-
anol (10 ml) at room temperature. The reaction was monitored
using TLC (hexane–ethyl acetate, 3:1). The yellow precipitate prod-
uct was washed with cold ethanol and dried under vacuum. The
yellow product was crystallized from methanol at ꢀ20 °C. Yield
Complex 2a. Orange solid; yield 0.20 g (78%); m.p. 214 °C. 1H
NMR (400.2MHz, DMSO-d6, δ, ppm): 8.30 (s, 1H, HC¼N), 7.52
(d, 1H, J= 7.5 Hz, H13’), 7.42 (d, 1H, J = 7.7 Hz, H16’), 7.73–7.03 (m,
11H, H1’–3’,6’–9’,14’,15’), 3.42 (s, 3H, OCH3). 13C NMR (100.6MHz, CDCl3,
δ, ppm): 160.3 (C11’), 146.6 (C10’), 136.9 (C4’), 136.6 (C5’), 132.8 (C8’),
132.3 (C17’), 131.7 (C16’), 131.4 (C1’), 131.0 (C14’), 130.7 (C2’), 129.9
(C12’), 129.0 (C15’), 127.6 (C6’), 127.4 (C7’), 126.3 (C3’), 125.4 (C13’),
116.8 (C9’), 54.8 (s, OCH3). FT-IR (KBr, cmꢀ1): 3043 (CHar), 2961
(OCH3), 1614 (C¼N). Anal. Calcd for C20H17Cl2ONSPd (%): C, 48.36;
H, 3.45; N, 2.82; S, 6.45. Found (%): C, 47.45; H, 3.96; N, 3.08; S, 7.40.
Complex 3a. Orange solid; yield 0.24 g (88%); m.p. 239 °C. 1H
NMR (400.2MHz, DMSO-d6, δ, ppm): 8.45 (s, 1H, HC¼N), 7.72 (d,
1H, J = 7.7 Hz, H13’), 7.42 (d, 1H, J = 6.2Hz, H16’), 7.75–7.10 (m, 11H,
1
0.70 g (81%), m.p. 71 °C. H NMR (400.2MHz, CDCl3, δ, ppm): 8.61
(s, H11, HC¼N), 7.44 (d, 1H, J = 6.9 Hz, H13), 7.28 (d, 1H, J = 7.7Hz,
H16), 7.18 (d, 2H, J = 7.5 Hz, H1), 7.11 (d, 1H, J= 7.4 Hz, H6), 7.20–
6.98 (m, 8H, H2,3,7–9,14,15), 2.55 (s, 3H, CH3). 13C NMR (100.6 MHz,
CDCl3, δ, ppm): 159.5 (s, C11, CH¼N), 150.8 (C10), 138.9 (C4), 134.2
(C5), 133.9 (C8), 133.1 (C16), 131.8 (C17), 131.1 (C1), 131.0 (C14),
129.3 (C2), 129.0 (C12), 128.8 (C15), 127.7 (C6), 127.1 (C7), 126.3 (C3),
126.2 (C13), 118.4 (C9), 19.7 (s, CH3). FT-IR (KBr, cmꢀ1): 3060 (CHar),
2967 (CH3), 1632 (C¼N). Anal. Calcd for C20H17NS (%): C, 79.17; H,
5.65; N, 4.62; S, 10.57. Found (%): C, 77.17; H, 5.69; N, 4.47; S, 10.52.
H1’–3’,6’–9’,14’,15’). 13C NMR (100.6 MHz, CDCl3, δ, ppm): 160.0 (C11’
,
CH¼N), 149.9 (C10’), 136.9 (C4’), 136.1 (C5’), 134.3 (C8’), 131.8 (C17’),
131.5 (C16’), 131.1 (C1’), 131.0 (C14’), 130.4 (C2’), 129.9 (C12’), 128.6
(C15’), 127.6 (C6’), 125.4 (C7’), 123.5 (C3’), 120.6 (C13’), 119.1 (C9’),
114.8 (s, CF3). FT-IR (KBr, cmꢀ1): 3051 (CHar), 1619 (C¼N). Anal. Calcd
for C20H14F3Cl2NSPd (%): C, 44.92; H, 2.64; N, 2.62; S, 6.00. Found
(%): C, 45.45; H, 2.96; N, 2.20; S, 6.40.
Preparation of Ligands 2 and 3
The Schiff bases 2 and 3 were prepared using a procedure similar to
that for ligand 1.
Ligand 2. Yellow solid; yield 0.67g (73%); m.p. 60.0°C. H NMR
1
(400.2 MHz, DMSO-d6, δ, ppm): 8.83 (s, 1H, HC¼N), 7.53 (d, 1H,
J = 7.7 Hz, H13), 7.43 (d, 1H, J = 7.0 Hz, H16), 7.31 (d, 2H, J = 7.1Hz,
H1), 7.10 (d, 1H, J= 7.3 Hz, H6), 7.33–7.17 (m, 8H, H2,3,7–9,14,15), 3.86
(s, 3H, CH3). 13C NMR (100.6MHz, CDCl3, δ, ppm): 159.5 (C11, CH¼N),
156.4 (C10), 150.5 (C4), 133.9 (C5), 133.4 (C8), 132.8 (C17), 132.4 (C16),
129.2 (C1), 128.5 (C14), 127.9 (C2), 127.7 (C12), 126.8 (C15), 125.9 (C6),
124.6 (C7), 120.8 (C3), 118.4 (C13), 111.0 (C9), 55.5 (s, OCH3). FT-IR (KBr,
cmꢀ1): 3050 (CHar), 2982 (OCH3), 1635 (C¼N). Anal. Calcd for
C20H17NOS (%): C, 75.20; H, 5.36; N, 4.39; S, 10.04. Found (%): C,
73.87; H, 5.19; N, 4.62; S, 9.76.
General procedure for Mizoroki–Heck coupling reaction
In a typical experiment, an oven-dried Schlenk tube was charged
with K2CO3 (1.2mmol) and organic solvent (3ml) under nitrogen at-
mosphere followed by addition of aryl halide (0.1mmol), olefin
(0.12 mmol) and Pd(II) catalyst (4× 10ꢀ4 mmol). The flask was
placed in an oil bath and the reaction mixture was stirred at appro-
priate temperatures for the required times. After completion of the
reaction, the mixture was cooled and extracted with ethyl acetate
(3× 20 ml). The extracts were collected and washed with brine
and dried over MgSO4, and then the solvent was evaporated. The
residue was purified by flash column chromatography on a silica
gel column.
1
Ligand 3. Yellow solid; yield 0.95 g (92%); m.p. 72 °C. H NMR
(400.2 MHz, DMSO-d6, δ, ppm): 8.73 (s, 1H, HC¼N), 7.61 (d, 1H,
J = 7.8 Hz, H13), 7.53 (d, 1H, J = 7.82Hz, H16), 7.44 (d, 2H, J = 7.1Hz,
H1), 7.32 (d, 1H, J = 7.4 Hz, H6), 7.28–7.02 (m, 8H, H2,3,7–9,14,15). 13C
NMR (100.6 MHz, CDCl3, δ, ppm): 156.4 (C11, CH¼N), 149.5 (C10),
134.0 (C4), 133.8 (C5), 133.1 (C8), 132.6 (C17), 132.2 (C16), 129.7 (C1),
129.5 (C14), 129.3 (C2), 129.1 (C12), 128.9 (C15), 127.8 (C6), 127.1
(C7), 125.7 (C3), 125.1 (C13), 123.3 (C9), 118.2 (s, CF3). FT-IR (KBr,
cmꢀ1): 3047 (CHar), 1640 (C¼N). Anal. Calcd for C20H14F3NS (%): C,
67.21; H, 3.95; N, 3.92; S, 8.97. Found (%): C, 66.27; H, 3.80; N, 4.07;
S, 9.31.
General procedure for Suzuki–Miyaura coupling reaction
An oven-dried Schlenk tube was charged with base (0.2mmol) and
organic solvent–H2O (3:3ml) under nitrogen atmosphere followed
by aryl halide (0.1 mmol), phenylboronic acid (0.12 mmol) and Pd
(II) catalyst (4× 10ꢀ4 mmol). The flask was placed in an oil bath
and then the reaction mixture was stirred at appropriate
Appl. Organometal. Chem. 2015, 29, 543–548
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

M. Keleş et al.
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The extracts were washed with brine and dried over MgSO4 and
the solvent was evaporated.
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New bidentate Schiff bases and their Pd(II) complexes were pre-
pared. The complexes were tested as catalysts for Mizoroki–Heck
and Suzuki–Miyaura reactions. High conversions were obtained in
the Suzuki–Miyaura reaction when carried out at 100 °C in K2CO3
media. These results indicate that all the examined catalysts were
effective for these reactions. Ligands with CH3, OCH3 and CF3 had
no influence on the catalytic activity because no significant differ-
ence was noticed in the resulting conversions when the catalysts
were compared with one another. Electrochemical properties of
the Pd(II) complexes investigated and E1/2 values of reversible redox
processes were determined. Ep values of irreversible processes have
also been reported.
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The authors thank Osmaniye Korkut Ata University for financial
support and the Scientific and Technological Research Council of
Turkey (project no. 113Z497) for a grant. The authors also thank Kirsi
Ekroos for technical support.
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wileyonlinelibrary.com/journal/aoc
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Appl. Organometal. Chem. 2015, 29, 543–548