Crystal Structure Theory and Applications, 2012, 1, 25-29
http://dx.doi.org/10.4236/csta.2012.13005 Published Online December 2012 (http://www.SciRP.org/journal/csta)
Symmetrical Palladium (II) N,N,O,O-Schiff Base Complex:
Efficient Catalyst for Heck and Suzuki Reactions
Wan Nazihah Wan Ibrahim1, Mustaffa Shamsuddin2
1Faculty of Applied Science, Universiti Teknologi MARA, Shah Alam, Malaysia
2Faculty of Science, Universiti Teknologi Malaysia, Johor, Malaysia
Received September 13, 2012; revised October 15, 2012; accepted October 26, 2012
ABSTRACT
Palladium is arguably the most versatile and most widely applied catalytic metal in the field of fine chemicals due to its
high selectivity and activity. Palladium catalyst offers an abundance of possibilities of carbon-carbon bond formation in
organic synthesis. In this research, three different Schiff base ligands have been prepared by condensation reaction be-
tween appropriate aldehyde or ketone with amine namely 2,2-dimethyl-1,3-propanediamine in the molar ratio of 2:1.
The corresponding palladium (II) Schiff base complexes were prepared through the reaction between the Schiff base
ligand with palladium (II) acetate in a molar ratio 1:1. FTIR, 1H-NMR and 13C-NMR spectroscopic data revealed that
the ligands are N,N,O,O-tetradentate coordinated to the Pd atom through both the azomethine N atoms and phenolic O
atoms. From X-ray Crystallographic analysis, it showed that the complex exists as square planar geometry. The synthe-
sized palladium (II) Schiff base complexes were then subjected in catalytic Heck and Suzuki reaction of iodobenzene.
Keywords: Palladium (II) Schiff Base Complex; Heck Reaction; Suzuki Reaction
1. Introduction
Tetradentate Schiff base is one of the most extensively
studied ligands in coordination chemistry. They can co-
ordinate with a large number of transition metals, as well
as possessing numerous interesting properties to related
catalysis and electrochemistry. One type of tetradentate
Schiff base ligand is N,N,O,O-tetradentate donor set
which possesses many advantages such as facile ap-
proach, relative tolerance, readily adjusted ancillary li-
gands, and tuneable steric and electronic coordination
environments on the metal center [1,2]. Furthermore,
they can be synthesized both simply and cheaply in a
bulk amount which properties that become very impor-
tant when industrial applications are being sought. Based
on these unique properties, N,N,O,O-tetradentate Schiff
base ligands and their transition metal complexes, as
catalyst, have attracted significant attention being rele-
vant for their application in agrochemical and radio-
pharmaceutical industries for cancer targeting, as model
system for biological macromolecule [3]. In this research,
we had prepared palladium (II) complexes coordinated
with various types of Schiff base ligands. The synthe-
sized palladium (II) Schiff base complexes were then
subjected to Heck and Suzuki reactions in order to evalu-
ate the activity of the catalyst.
2. Experimental
2.1. Chemicals and Instrumentation
All glass wares were dried overnight in oven. Commer-
cial grade solvents were distilled according to normal
procedures and dried over molecular sieves (4 Å) before
used. All other chemicals were purchased from Aldrich,
Merck or Fluka and were used without further purifica-
tion and all reactions were conducted under inert condi-
tion. The melting point of the solid products was meas-
ured using Electrothermal Digital Melting Point Appara-
tus and was uncorrected. The CHN elemental analysis
was determined using a Thermo Finnigan CE 125 CHN
analyzer. Fourier Transform Infrared (FTIR) spectra of
the synthesized samples were recorded on Shimadzu
FTIR spectrometer in the range of 4000 - 400 cm–1 as
potassium bromide (KBr) disc. The proton 1H and carbon
13C Nuclear Magnetic Resonance (NMR) spectra were
recorded in CDCl3 or DMSO on a Bruker Avance 300
MHz and 400 MHz spectrometer. The chemical shifts are
reported in ppm relative to trimethylsilane (TMS). Sin-
gle-crystal X-ray Crystallography was performed by us-
ing a Bruker SMART APEX2 CCD area detector dif-
fractometer. Determination of products from the catalytic
testing was carried out using gas chromatographic (GC)
technique. Reaction mixture was analyzed by Agilent
C
opyright © 2012 SciRes. CSTA

W. N. W. IBRAHIM, M. SHAMSUDDIN
26
Technologies GC equipped with a 30 m × 250 μm × 0.25
μm nominal capillary column (ULTRA-1.0.05, 100% di-
methylpolysiloxane) using Flame Ionization Detection
(FID).
2.2. Catalyst Preparation and Characterization
The stoichiometric amount of appropriate aldehyde or
ketone derivatives (10 mmol) was added dropwise to a
2,2-dimethyl-1,3-propanediamine solution (0.51 g; ρ =
0.851; 5 mmol; 0.6 mL) in 10 mL dry ethanol. The mix-
ture was refluxed (78˚C) under nitrogen gas atmosphere
after which the solid product had formed. The solid
products of Schiff base ligands were then separated by
vacuum filtration, washed with cold ethanol and dried in
vacuum desiccator (25˚C) for overnight. In order of com-
plexation method, Schiff base ligand (5 mmol) was dis-
solved in a 10 mL of dry acetonitrile. Palladium (II) ace-
tate (1.10 g; 5 mmol) which was dissolved separately in a
10 mL of dry acetonitrile was then added dropwise into
the flask containing the ligand solution. The mixture was
stirred and refluxed (90˚C) under N2 gas atmosphere after
which the solid product had formed. The product was
then separated by vacuum filtration, washed with cold
acetonitrile and dried in a vacuum desiccator (25˚C).
2.3. Physical Properties Elemental Carbon,
Hydrogen and Nitrogen Analysis (CHN)
Schiff base Ligand 1: yellow needles solid (80%); m.p
111˚C - 112˚C; C (76.07) H (8.46) N (7.39); Schiff base
Ligand 2: yellow needles solid (85%); m.p 95˚C - 96˚C;
C (74.60) H (7.27) N (9.17); Schiff base Ligand 3:
yellow solid (86%); m.p 132˚C - 133˚C; C (47.92) H
(4.40) N (6.02); Complex 1: green solid (90%); m.p
341˚C - 343˚C; C (59.29) H (6.16) N (5.41); Complex 2:
orange solid (80%); m.p 335˚C - 336˚C; C (55.46) H
(5.02) N (6.18); Complex 3: orange solid (87%); m.p
305˚C - 306˚C; C (40.12) H (3.45) N (5.21).
2.4. Fourier Transform Infrared (FTIR)
Analysis of FTIR shows that some significant changes of
the important bands can be observed from FTIR spectra
of free Schiff base ligands and their complexes. The dis-
placement of C = N stretching frequencies from 1615 -
1631 cm–1 in the free Schiff base ligands to lower values
of 1606 - 1611 cm–1 in the complexes indicating the co-
ordination of azomethine nitrogen to the palladium metal.
This result shows that the contribution of C = N stretch-
ing mode has been reduced as the electron pairs on a ni-
trogen atom is involved in bond formation with the pal-
ladium ion [4]. Besides, the broad OH band at >3400
cm–1 in the free Schiff base ligands spectrum has totally
disappeared in complexes spectrum, suggesting the
strong participation of the OH group in chelate formation
to the palladium atom via the deprotonation of phenolic
hydrogen [5].
Schiff base ligand 1: 3434 (OH) 1615 (C = N) 1544,
1455 (C = C ar.) cm–1; Schiff base ligand 2: 3440 (OH)
1631 (C = N) 1579, 1428 (C = C ar.) cm–1; Schiff base
ligand 3: 3446 (OH) 1631 (C = N) 1570, 1478 (C = C ar.)
cm–1; Complex 1: 1606 (C = N) 1528, 1478 (C = C ar.)
cm–1; Complex 2: 1611 (C = N) 1539, 1428 (C = C ar.)
cm–1; Complex 3: 1608 (C = N) 1526, 1461 (C = C ar.)
cm–1.
2.5. Nuclear Magnetic Resonance (NMR)
From 1H-NMR data, the strong participation of the OH
group in chelation through the deprotonation of phenolic
hydrogen is indicated by the disappearance of the OH
singlet signal at very downfield chemical shift in the free
Schiff base ligand spectrum [5]. Meanwhile, based on
13C-NMR analysis, displacement of phenolic carbon
(C-OH) and azomethine carbon (C = N) from upfield in
non-coordinated ligands to the downfield in the com-
plexes suggest the deprotonation and coordination of
azomethine nitrogen atom and phenolic oxygen atom to
the palladium atom. Besides that, the signals from aro-
matic carbons in the complexes have shifted from upfield
to the downfield after complexation, which further sup-
ported the FTIR data in which the ligand is coordinated
to the palladium atom through the azomethine nitrogen
atom and the phenolic oxygen atom.
Schiff base ligand 1: 1H-NMR 12.30 (C-OH) 6.57 -
7.40 (C-H ar.) ppm 13C-NMR 171.83 (C = N) 164.31
(C-OH) ppm; Schiff base ligand 2: 1H-NMR 13.60
(C-OH) 6.88 - 7.37 (C-H ar.) ppm 13C-NMR 165.71 (C =
N) 161.29 (C-OH) ppm; Schiff base ligand 3: 1H-NMR
13.53 (C-OH) 6.87 - 7.43 (C-H ar.) ppm 13C-NMR
164.59 (C = N) 160.26 (C-OH) ppm; Complex 1:
1H-NMR 6.39 - 7.26 (C-H ar.) ppm 13C-NMR 218.15 (C =
N) 167.09 (C-OH) ppm; Complex 2: 1H-NMR 6.49 -
7.25 (C-H ar.) ppm 13C-NMR 162.96 (C = N) 165.34
(C-OH) ppm; Complex 3: 1H-NMR 6.72 - 7.48 (C-H ar.)
ppm 13C-NMR 163.61 (C = N) 204.45 (C-OH) ppm.
2.6. X-Ray Crystallography Analysis
Suitable crystal of Schiff base ligand 1 for X-ray analysis
was obtained by slow evaporation of the ligand solution
in dichloromethane and n-hexane (1:1) mixture at low
temperature (4˚C). From analysis, the bond lengths of
azomethine C8-N1, 1.295(2) Å and C15-N2, 1.286(2) Å
in the molecule are consistent with normal C = N bond
lengths as observed in other similar azomethine com-
pound [6]. Besides, there are two intramolecular strong
hydrogen bonding, N1-H1A and N2-H2A which contri-
bute to the stability of the molecule (Figure 1).
Suitable crystal of palladium (II) complex 2 for X-ray

W. N. W. IBRAHIM, M. SHAMSUDDIN 27
analysis was obtained by slow evaporation of a mixture
solution of chloroform and n-hexane (1:1) at low
temperature (4˚C). As shown in Figure 2, the PdII metal
centre has a cis-planar coordination by the two phenolic
oxygen atoms and two imine nitrogen atoms. The Pd-O
distances are in the range 1.979(3) - 2.008(4) Å with
Pd-N distances 1.981(3) - 2.014(3) Å, which are typical
of the square-planar PdII complex of Schiff base ligand
[7]. The bond angles around PdII ions suggested that the
complex has a distorted square-planar geometry as indi-
cated by the angles O-Pd-O in the range 79.66(11) -
80.54(16), O-Pd-N in the range 92.14(13) - 92.95(11)
and N-Pd-N in the range 94.92(12) - 94.95(15), deviating
substantially from that expected for a regular square-
planar geometry.
2.7. Catalytic Testing
2.7.1. Heck Reactio n
Palladium (II) complexes were tested in a Heck reaction
between iodobenzene and methyl acrylate to produce
methyl cinnamate. The general procedure is as follow:
iodobenzene (0.20 g; ρ = 1.830; 0.11 mL; 1 mmol), me-
thyl acrylate (0.17 g; ρ = 0.955; 0.18 mL; 2 mmol), base
(2.4 mmol), palladium (II) Schiff base complex (1.0 mmol
%; 0.01 mmol) and solvent N,N-dimethylacetamide,
DMA (5 mL) were mixed together in a Radley’s 12-
placed reaction carousel and was reflux for 24 hours whilst
being purged with nitrogen. (Bases: Et3N, NaHCO3, Na2CO3
and NaOAc; Temperature: 100˚C, 120˚C and 140˚C).
C7
Figure 1. ORTEP plot of Schiff-base ligand 1.
Figure 2. ORTEP plot of complex 2.
2.7.2. Suzuki Reaction
Palladium (II) complexes were tested in Suzuki reaction
between iodobenzene and phenylboronic acid to produce
biphenyl. The general procedure is as follow: iodo-
benzene (0.20 g; ρ = 1.830; 0.11 mL; 1 mmol),
phenylboronic acid (0.24 g; 2 mmol), base (2.4 mmol),
palladium (II) Schiff base complex (1.0 mmol%; 0.01
mmol) and solvent DMA (5 mL) were mixed together in
Radley’s 12-placed reaction carousel and was reflux for
24 hours whilst being purged with nitrogen. (Bases: Et3N,
KF, K2CO3 and K3PO4; Temperature: 100˚C, 120˚C and
140˚C).
3. Results and Discussion
3.1. Catalytic Heck Reaction Studies
The synthesized palladium (II) complexes were subjected
in the catalytic Heck reaction of iodobenzene and methyl
acrylate by using DMA solve + nt. Catalyst loading was
kept to 1.0 mmol%, so as to give an expected TON of
100 if 100% conversion of iodobenzene was achieved.
3.1.1. Effect of Bases
The function of base in Heck reaction is to neutralize the
acidic condition product by hydrogen halide in reductive
elimination step and regeneration of catalyst to continue
the catalytic cycles. In this study, four types of bases
have been used; Et3N, NaHCO3, Na2CO3 and NaOAc, in
order to study the effect of bases towards the percentage
conversion of iodobenzene. Some organic bases act as a
source of hydride and promote the hydrogenation of aro-
matic compounds. Due to this property or effect, there is
usually a competition between vinylation and hydroge-
nation in the Heck reaction and the selectivity pattern
will be influenced by the type of base used. As reported
by Kiviaho [8], selectivity for a vinylation product like
methyl cinnamate is high when Et3N is the base used in
the reactions which correspond to our studies in which
Et3N gives 100% conversion of iodobenzene with com-
plex 1 and more than 90% conversion with the other two
complexes. In contrast, the low conversion achieved for
NaHCO3 is probably due to the insolubility of NaHCO3
in the organic solvent used.
3.1.2. Effect of Temperature
Reaction temperature assists in the activation of the io-
dobenzene which usually occurs at reaction temperature
more than 100˚C. However, the reaction temperature
must be carefully controlled to avoid the formation of
palladium black which will inhibit the catalytic cycle if
the temperature is too high. In order to study the effect of
temperature on the conversion of iodobenzene, the cata-
lytic reaction temperatures were varied at 100˚C, 120˚C
and 140˚C using Et3N as base. From the results, it

W. N. W. IBRAHIM, M. SHAMSUDDIN
28
showed that the best reaction temperature was found to
be 140˚C, which all the complexes gave 100% conver-
sion. Among all of the three complexes, complex 1 still
maintained with the highest conversion even at lower
temperature than 120˚C. The higher activities of this
complex may be explained in terms of ligand effect. The
bulkier and more electron-rich ligand is thought to accel-
erate the oxidative addition of aryl halides and reductive
elimination steps, so that the catalyst regeneration in
catalytic cycles is faster [9]. However from our observa-
tion, the Heck reaction using temperature greater than
140˚C was avoided due to the formation of palladium
black, which therefore terminated the catalytic cycle.
3.2. Catalytic Suzuki Reaction Studies
The synthesized palladium (II) complexes were subjected
in the catalytic Suzuki reaction of iodobenzene and
phenylboronic acid by using DMA solvent. Catalyst
loading was kept to 1.0 mmol%, so as to give an expect-
ed TON of 100 if 100% conversion of iodobenzene was
achieved.
3.2.1. Effect of Bases
In this study, four types of bases have been used; Et3N,
KF, K2CO3 and K3PO4, in order to study the effect of
bases towards the percentage conversion of iodobenzene.
The type of bases was chosen based on their performance
in Suzuki reaction to form similar biphenyl products
[10,11]. In Suzuki reaction, the presence of base is need-
ed since the cross coupling via transmetallation step is
difficult due to the low nucleophilicity of organic groups
(R) on the boron atom. The role of the base is explained
by activation of palladium (II) complex or boranes. Ac-
cording to Kotha [12], the base is involved in the co-
ordination sphere of the palladium to activate the pal-
ladium catalyst by formation of intermediate (alkoxo)
palladium species which is known to accelerate the trans-
metallation step. Besides, the nucleophilicity of organic
groups is enhanced by quaternization of the boron with
bases which facilitates transmetallation. From our obser-
vation, the Et3N is the most favourable one compared to
other bases. Most probably, the Et3N mixed well with the
reaction mixture since it exists in liquid form and easily
interferes in coordination sphere of palladium complex.
3.2.2. Effect of Temperature
In order to study the effect of temperature on the conver-
sion of iodobenzene, the catalytic reaction temperatures
were varied at 100˚C, 120˚C and 140˚C with Et3N as
base. From the results, it showed that the best reaction
temperature was found to be 140˚C, where 100% conver-
sion was achieved after 24 hours reaction by using com-
plex 1. Among three complexes, the complex 2 was the
worst performer with only giving 5% conversion com-
pared to 46% conversion with complex 1 at 100˚C reac-
tion temperature. This can be explained by the presence
of the methyl group on the carbon of the azomethine
group of complex 1which dramatically increases the rate
of reaction. This may be due to the electronic effect of a
methyl group, which ultimately increases the electronic
environment around the palladium centre [13]. The elec-
tronic properties on palladium centre facilitate the inter-
ruption of palladium catalyst in aryl or vinyl halides
bonding in the oxidative addition steps on the catalytic
cycles and ultimately increasing the rate of reaction.
4. Conclusion
In this research, three palladium (II) Schiff base com-
plexes have been successfully synthesized and character-
ized. Based on the elemental CHN analysis, FTIR, 1H,
13C-NMR spectral studies and X-ray crystallographic
analysis, we suggest that the Schiff base ligands acted as
N,N,O,O-tetradentate ligand and have bonded to the pal-
ladium atom through the azomethine nitrogen atom (C =
N) and the phenolic oxygen atom. These complexes were
then subjected in catalytic Heck and Suzuki reaction of
iodobenzene. The results showed that the complex 1
gives conversion up to 100% using triethylamine as base
at temperature 120˚C - 140˚C for both Heck and Suzuki
reaction of iodobenzene. The higher activities of the
complex 1 maybe can be explained in term of ligand ef-
fect. The oxidative addition is well known as the rate
determining step in cross coupling reaction, thus, elec-
tron-rich ligands are usually needed to make the palla-
dium metal easily oxidized. The presence of the methyl
group on the carbon azomethine (C = N) increased the
electronic environment around the palladium centre
which ultimately, accelerates the oxidative addition of
aryl halides and reductive elimination steps.
5. Acknowledgements
The research is financed by Ministry of Science Tech-
nology and Innovation (MOSTI) for the Science Fund
03-01-06-SF0273).
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