Synthesis, characterization and catalytic applications of 2,2-dimethylpropane-1,3-diaminopalladium(II) complex in Mizoroki-Heck reaction

Article abstract of DOI:10.14233/ajchem.2018.20900

New tetradentate Schiff base ligand (L_SB) and its palladium(II) complex, i.e., complexation of N,N'-bis(4-methoxysalicylidene)-2,2-dimethylpropane-1,3-diaminopalladium(II) (PdL_SB) has been successfully synthesized through condensation reaction between primary amine 2,2-dimethyl-1,3-propanediamine with 2-hydroxy-4-methoxybenzaldehyde. The palladium complex was then prepared by refluxing the corresponding L_SB with an equimolar amount of palladium(II) acetate to give C₂₁H₂₄N₂O₄Pd. The complex was characterized by using several techniques, such as the elemental chemical analysis CHN, FTIR, NMR spectroscopy and single crystal X-ray crystallography. This air/moisture stable complex was investigated to be a high class of homogeneous catalyst for Mizoroki-Heck reaction. The reaction was monitored by GC-FID at 12 h reaction period. The percentage conversion of 4-bromoacetophenone was 70 % indicated that PdL_SB can act as an ideal potential catalyst in the Mizoroki-Heck reaction.

Full text of DOI:10.14233/ajchem.2018.20900

Asian Journal of Chemistry; Vol. 30, No. 1 (2018), 81-84
ASIAN JOURNAL OF CHEMISTRY
https://doi.org/10.14233/ajchem.2018.20900
Synthesis, Characterization and Catalytic Applications of 2,2-Dimethylpropane-
1,3-Diaminopalladium(II) Complex in Mizoroki-Heck Reaction
2
SITI KAMILAH CHE SOH1,* and MUSTAFFA SHAMSUDDIN
1School of Marine and Environmental Sciences, Universiti Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia
2Faculty of Science, Universiti Teknologi Malaysia, 81030 Skudai, Johor Bahru, Johor, Malaysia
*Corresponding author: Fax: +60 9 6683193; Tel: +60 9 6683978; E-mail: sitikamilah@umt.edu.my
Received: 3 July 2017;
Accepted: 4 September 2017;
Published online: 30 November 2017;
AJC-18655
New tetradentate Schiff base ligand (LSB) and its palladium(II) complex, i.e., complexation of N,N'-bis(4-methoxysalicylidene)-2,2-
dimethylpropane-1,3-diaminopalladium(II) (PdLSB) has been successfully synthesized through condensation reaction between primary
amine 2,2-dimethyl-1,3-propanediamine with 2-hydroxy-4-methoxybenzaldehyde. The palladium complex was then prepared by refluxing
the corresponding LSB with an equimolar amount of palladium(II) acetate to give C21H24N2O4Pd. The complex was characterized by using
several techniques, such as the elemental chemical analysis CHN, FTIR, NMR spectroscopy and single crystal X-ray crystallography.
This air/moisture stable complex was investigated to be a high class of homogeneous catalyst for Mizoroki-Heck reaction. The reaction
was monitored by GC-FID at 12 h reaction period. The percentage conversion of 4-bromoacetophenone was 70 % indicated that PdLSB
can act as an ideal potential catalyst in the Mizoroki-Heck reaction.
Keywords: Tetradentate Schiff base, Homogeneous catalysis, Mizoroki-Heck.
catalysts in various reactions, since they have N,N,O,O donor
INTRODUCTION
set which possesses more benefits, such as facile approach,
The Mizoroki-Heck reaction is the palladium catalyzed
relative tolerance, readily adjusted ancillary ligands and tunable
carbon-carbon bond formation between aryl halides and olefins
steric and electronic coordination environments on the metal
is known to employ significant roles in modern synthetic chemistry
center [9,10]. Indeed, the unique properties of tetradentate
[1,2] and are used extensively for agrochemical industry, pharma-
Schiff base derivatives in numerous reported applications and
ceutical intermediates, conducting polymers, pesticides and
their tendency to form metal complexation with many metals
liquid crystals application [3-5]. Basically, Mizoroki-Heck
inspired researchers to design and synthesize 2,2-dimethyl-
reaction is carried out in an organic solvent, such as amine,
propane-1,3-diaminopalladium(II) complex. Thus, the perfor-
benzene, toluene, N,N-dimethylacetamide, N,N-dimethylform-
mance of synthesized complex was further investigated as a homo-
amide or tetrahydrofuran, which could give excellent perfor-
geneous catalyst in the catalytic studies of Mizoroki-Heck reaction.
mance and conversion to the catalytic product due to the solu-
EXPERIMENTAL
bility of most organic compound in N,N-dimethyl acetamide
[6,7]. Traditionally, Mizoroki-Heck reactions are extensively
catalyzed by phosphine palladium complexes. However, the
cost, air and moisture sensitive as well as less stable of phosphine
have led to the search for low-cost and easy accessible ligands
as synthons for a new palladium Mizoroki-Heck coupling
catalysts [8]. Therefore, the catalysis under phosphine-free
condition represents a challenge of high importantance. To
date, most of the efforts are emphasized in the search of more
promising, simple, mild, efficient and economical friendly
catalysts. Tetradentate ligands derived from Schiff base are
widely used in coordination chemistry and can coordinate with
a large number of transition metals. They can participate as
All the chemical reagents were purchased commercially
and used without further purification. Infrared spectra were
carried out on the Perkin Elmer Spectrum One FTIR Spectro-
meter in the range of 4000-400 cm-1 by using KBR disks. CHN
elemental analyses were recorded using a Thermo Finnigan
CE 125 CHN analyzer. 1H and 13C NMR were recorded on a
BrukerAvance 400 MHz spectrometer with CDCl3 as a solvent.
A single X-ray Crystallography was performed by using a Bruker
APEX DUO CCD area detector diffractometer. The gas chromato-
graphy analyses were carried out on a GC-Hewlett-Packard
5890 series II gas chromatograph equipped with a nominal
capillary column and flame ionization detector (FID).

82 Soh et al.
Asian J. Chem.
TABLE-1
Synthesis of N,N'-bis(4-methoxysalicylidene)-2,2-
FTIR DATA FOR LIGANDS (LSB) AND
PALLADIUM(II) COMPLEX (PdLSB)
dimethylpropane-1,3-diamine (LSB):A solution of 2,2-dimethyl-
1,3-propanediamine (5 mmol) in 10 mL absolute ethanol was
added to a solution of 2-hydroxy-4-methoxybenzaldehyde (10
mmol) in a three-necked round bottom flask. The resultant
yellow solution was refluxed under nitrogen for 5 h. The ligand
obtained as yellow microcrystals were filtered off, washed with
cold ethanol and dried in a vacuum desiccator over blue silica
gel [11]. Yield: 89 %. m.p: 126-127 ºC.
Synthesis of N,N'-bis-(4-methoxysalicylidene)-2,2-
dimethylpropane-1,3-diaminopalladium(II) complex (PdLSB):
The corresponding palladium(II) complex was synthesized by
mixing a solution of LSB (0.21 g; 0.62 mmol) in acetonitrile
(10 mL) with a solution of palladium(II) acetate in acetonitrile
(10 mL) in three necked round bottom flask. The resulting solution
was refluxed with stirring for 5 h under nitrogen atmosphere
(Scheme-I). The solid product of PdLSB was then recrystallized
from a mixture of dichloromethane/methanol (1:1 v/v). Slow
evaporation of the solvent at room temperature over several
days gave yellow crystals. Yield: 78 %. m.p: 323-324 ºC.
ν(C=C
aromatic)
Compd.
ν(O-H)
ν(C=N)
ν(C-O)
ν(C-N)
3436
–
1629
1604
1446
1118
1221
1337
1305
LSB
1440
PdLSB
signal in LSB appeared as a singlet peak at δ 8.17 ppm. The
signal was shifted up in the spectrum of PdLSB (δ 7.35 ppm),
suggesting that the shielding of the azomethine group due to
the π-back bonding from Pd(II). The absence of sharp singlet
resonance of the phenolic proton (δ 14.11 ppm) further app-
roved that (-OH) group had participated in chelation through
deprotonation. Meanwhile, 13C NMR showed the displacement
of (C=N) carbon to the lower field in PdLSB supported the
coordination of azomethine nitrogen and phenolic oxygen to
Pd metal [13]. Table-2 shows the summary data for 1H and 13C
NMR for LSB and PdLSB.
TABLE-2
1H AND 13C NMR SPECTRAL DATA FOR LSB AND PdLSB
H
H
Compd.
H
H
1H NMR δH (ppm)
13C NMR δC (ppm)
N
N
N
N
Pd(OAc)2
CH3CN
C-CH3 (24.19)
CH2-C-CH2 (36.23)
Ar-OCH3 (55.34)
N-CH2 (66.58)
Pd
C-CH3 (1.07)
N-CH2 (3.42)
Ar-OCH3 (3.83)
N=CH (8.17)
OH HO
O
O
H3CO
Scheme-I: Preparation of palladium(II) tetradentate Schiff base complex
(PdLSB
OCH3
H3CO
OCH3
LSB
C-H, aromatic (112.2-132.7)
C-OH (163.7)
)
C-H, aromatic (6.44-7.13)
C-OH (14.11)
C=N (164.7)
General procedure for homogeneous catalytic testing
in Mizoroki-Heck cross-coupling reaction: In a Radleys
carousel tube equipped with a magnetic stir bar, 4-bromo-
acetophenone (1 mmol), methyl acrylate (3 mmol), base (2.4
mmol), respective palladium(II) catalyst PdLSB (1 mmol%)
and N,N-dimethylacetamide (5 mL) were added whilst purged
with nitrogen and heated at 120 ºC for 12 h. The conversion of
the product was monitored by GC-FID. The synthetic approach
for the Mizoroki-Heck reaction is illustrated in Scheme-II.
C-CH3 (23.99)
CH2-C-CH2 (34.14)
Ar-OCH3 (55.27)
N-CH2 (71.62)
C-CH3 (1.06)
N=CH (7.35)
N-CH2 (3.34)
PdLSB
Ar-OCH3 (3.79)
C-H, aromatic (6.59-6.98)
C-H, aromatic (113.6-135.0)
C-OH (161.6)
C=N (165.6)
The elemental analysis of LSB and PdLSB was analyzed to
determine the agreement between the theoretical and experi-
mental values with molecular formula suggested in Table-3.
O
O
O
Pd catalyst
H2C
+
CH3
H3C
OCH3
TABLE-3
DMA, Base
OCH3
Br
ELEMENTALANALYSIS OF
LIGAND (LSB) AND COMPLEX (PdLSB)
O
Scheme-II: Mizoroki-Heck reaction of 4-bromoacetophenone with methyl
Elemental analysis (%): Found (calcd.)
Compd.
acrylate
C
H
N
68.20 (68.09)
53.25 (53.12)
7.31 (7.07)
5.26 (5.09)
7.53 (7.56)
6.20 (5.90)
LSB, C21H26N2O4
PdLSB, C21H24N2O4Pd
RESULTS AND DISCUSSION
The FTIR spectrum for LSB (Table-1) showed a strong band
at 3436 cm-1 which can be ascribed to the stretching of phenolic
ν(O-H) group. This band disappeared in the spectrum of the
complex, indicating the coordination of phenolic -OH with
palladium ion via deprotonation of phenolic hydrogen. The
IR spectrum of PdLSB exhibited a strong band at 1604 cm-1 which
corresponded to the stretching frequency of imine ν(C=N)
group. The contribution of C=N stretching mode was reduced
as the electron pair on the nitrogen atom is involved in bond
formation with metal ion [12].
The square planar of PdLSB was confirmed by single X-ray
crystallography analysis and found to be in agreement with the
literature [14].As shown in Fig. 1, Pd2+ ion is located in the inner
cis-N2O2 core of the ligand LSB and adopts a perfectly square
planar geometry where N(1A)-Pd(1)-N(1) and O(1A)-Pd(1)-
O(1) angles are 94.20(4)º and 85.03(3)º, respectively. The dihedral
angle between two benzene rings of the ligand was 79.21(4)º.
The Pd(1)-O(1) and Pd(1)-N(1) distances were 1.9932(6) Å and
2.0029(7) Å, respectively and all parameters were within normal
ranges (68) and comparable with the other related structures [15].
The selected bond lengths and angles are listed in Table-4.
In 1H NMR spectrum of ligand, methoxy proton (O-CH3)
was observed at δ 3.83 ppm. The azomethine proton (-HC=N)

Vol. 30, No. 1 (2018)
Synthesis and Catalytic Applications of 2,2-Dimethylpropane-1,3-Diaminopalladium(II) Complex 83
with 70 % conversion of starting material to the desired
Mizoroki-Heck product after 12 h reaction time probably due
to the presence of electron donating (-OCH3) group that can
accelerate the oxidative addition of aryl halide. However,
the conversion might not reach up 100 % and was found to be
time independent. To extend the research work scope, more
optimization conditions for catalytic study in Mizoroki-Heck
reaction should be considered by using a variety of reaction
parameters such as different bases, temperature, catalyst loading
and solvent in order to increase the percentage of C-C coupling
yield. It is worth mentioning here that these findings indicated
the synthesized palladium(II) complex (PdLSB) can act as a
potential homogeneous catalyst in the Mizoroki-Heck reaction.
The catalytic data (Table-5) showed that PdLSB efficiently
catalyzed the Mizoroki-Heck reaction of 4-bromoacetophenone
with methyl acrylate.
TABLE-5
PdLSB CATALYZED MIZOROKI-HECK COUPLING REACTION
OF 4-BROMOACETOPHENONE WITH METHYLACRYLATE
Catalyst
(PdLSB)
120
12
Reaction condition
Temperature (°C)
Control
120
24
Reaction time (h)
Fig. 1. Molecular structure of PdLSB, showing the atom-labelling system
Catalyst loading (mmol %)
% Conversion of 4-bromoacetophenone
0.00
17
1.00
70
and 50 % probability displacement ellipsoids
TABLE-4
Conclusion
SELECTED BOND DISTANCES (Å) AND ANGLES (°) FOR PdLSB
Bond
In this study, N,N'-bis(4-methoxysalicylidene)-2,2-dimethyl-
propane-1,3-diamine (LSB) and its palladium(II) complex
(PdLSB) were successfully synthesized with high percentage
of yields. Under the present experimental condition, the homog-
eneous catalyst was found to efficiently catalyze Mizoroki-Heck
reaction of 4-bromoacetophenone into the coupled product.
Assignment
Assignment
Angles (°)
distances (Å)
1.9932(6)
1.9932(6)
2.0029(7)
2.0029(7)
1.2951(10)
1.4695(10)
1.3092(9)
1.3637(10)
1.4366(11)
1.3835(11)
0.9500
Pd(1)-O(1)
Pd(1)-O(1A)
Pd(1)-N(1)
Pd(1)-N(1A)
N(1)-C(7)
N(1)-C(8)
O(1)-C(1)
O(2)-C(3)
O(2)-C(12)
C(2)-C(3)
C(2)-H2A
O(1A)-Pd(1)-O(1)
O(1A)-Pd(1)-N(1A)
O(1)-Pd(1)-N(1A)
O(1A)-Pd(1)-N(1)
O(1)-Pd(1)-N(1)
N(1A)-Pd(1)-N(1)
C(1)-O(1)-Pd(1)
C(3)-O(2)-C(12)
C(7)-N(1)-Pd(1)
C(8)-N(1)-Pd(1)
C(7)-N(1)-C(8)
85.03(3)
90.27(3)
174.10(3)
174.10(3)
90.27(3)
94.20(4)
119.13(5)
117.06(7)
121.25(5)
120.32(5)
118.40(7)
ACKNOWLEDGEMENTS
The authors thank the Ministry of Higher Education, Malaysia,
School of Marine and Environmental Sciences, Universiti Malaysia
Terengganu and Department of Chemistry, Universiti Teknologi
Malaysia for the research collaboration, facilities and publication
support.
Catalytic testing of homogeneous Mizoroki-Heck cross-
coupling reaction:After confirming the exact structure of the
synthesized complex PdLSB, the catalytic activity of organic
reaction was then tested by using 4-bromoacetophenone and
methyl acrylate in the presence of sodium acetate as base and
solvent at optimum reaction temperature (120 ºC) for 12 h. The
reaction was carried out in a Radleys 12-placed carousel reactor
vessel whilst continuously flushed with nitrogen gas. However,
the temperature must be carefully controlled to avoid the
palladium black formation that will inhibit the catalytic cycle
if the temperature was too high [16]. The catalyst loading was
kept to 1.0 mmol % so as to produce an expected turnover
number (TON) of 100 if 100 % conversion was achieved. The
reaction was monitored by using GC-FID.A controlled experiment
indicated that a small amount of the cross coupling product
was observed without any catalyst.
REFERENCES
1. S.B. Moosun, L.H. Blair, S.J. Coles, M.G. Bhowon and S. Jhaumeer-
Laulloo, J. Saudi Chem. Soc., 21, 441 (2017);
https://doi.org/10.1016/j.jscs.2015.10.003.
2. N. Khadir, G. Tavakoli,A.Assoud, M. Bagherzadeh and D.M. Boghaei,
Inorg. Chim. Acta, 440, 107 (2016);
https://doi.org/10.1016/j.ica.2015.10.035.
3. A. Suzuki, J. Organomet. Chem., 576, 147 (1999);
https://doi.org/10.1016/S0022-328X(98)01055-9.
4. S.P. Stanforth, Tetrahedron, 54, 263 (1998);
https://doi.org/10.1016/S0040-4020(97)10233-2.
5. S.R. Borhade and S.B. Waghmode, Tetrahedron Lett., 49, 3423 (2008);
https://doi.org/10.1016/j.tetlet.2008.03.109.
6. M. Bakherad, A. Keivanloo, B. Bahramian and S. Jajarmi, Appl. Catal. A.,
390, 135 (2010);
https://doi.org/10.1016/j.apcata.2010.10.003.
7. A. Battace, T. Zair, H. Doucet and M. Santelli, J. Organomet. Chem., 690,
3790 (2005);
As expected, the result demonstrated excellent catalytic
activity and efficiency of PdLSB as a homogeneous catalyst
https://doi.org/10.1016/j.jorganchem.2005.05.014.

84 Soh et al.
Asian J. Chem.
8. A.F. Littke and G.C. Fu, J. Org. Chem., 64, 10 (1999);
https://doi.org/10.1021/jo9820059.
13. G. Budige, M.R. Puchakayala, S.R. Kongara, A. Hu and R.Vadde, Chem.
Pharm. Bull. (Tokyo), 59, 166 (2011);
9. M. Wang, H. Zhu, K. Jin, D. Dai and L. Sun, J. Catal., 220, 392 (2003);
https://doi.org/10.1016/S0021-9517(03)00306-3.
10. F. Marchetti, C. Pettinari, R. Pettinari, A. Cingolani, D. Leonesi and
A. Lorenzotti, Polyhedron, 18, 3041 (1999);
https://doi.org/10.1248/cpb.59.166.
14. S.K.C. Soh, M. Shamsuddin, M.M. Rosli and H.K. Fun, Acta Crystallogr.,
68E, m514 (2012);
https://doi.org/10.1107/S1600536812013128.
https://doi.org/10.1016/S0277-5387(99)00230-2.
11. M. Montazerozohori, M.H. Habibi, A. Hojjati, R. Mokhtari, Y. Yamane
and T. Suzuki, Acta Crystallogr., 65E, o1662 (2009);
https://doi.org/10.1107/S1600536809022855.
15. L. Ding, Z. Chu, L. Chen, X. Lu, B. Yan, J. Song, D. Fan and F. Bao,
Inorg. Chem. Commun., 14, 573 (2011);
https://doi.org/10.1016/j.inoche.2011.01.028.
16. S.R. Borhade and S.B. Waghmode, Indian J. Chem., 47B, 1549 (2008).
12. A. Kumar, M. Agarwal, A.J. Singh and R.J. Butcher, Inorg. Chim. Acta,
362, 3208 (2009);
https://doi.org/10.1016/j.ica.2009.02.031.