Symmetrical Palladium (II) N,N,O,O-Schiff Base Complex: Efficient Catalyst for Heck and Suzuki Reactions

doi:10.4236/csta.2012.13005

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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

Email: [email protected]

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

W. N. W. IBRAHIM, M. SHAMSUDDIN

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).

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

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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