Suzuki coupling reactions catalyzed by Schiff base supported palladium complexes bearing the vitamin B6 cofactor

Article abstract of DOI:10.1016/j.mcat.2021.111528

Novel Schiff bases were synthesized by condensing aromatic amines with pyridoxal-5’-phosphate and characterized by using FT-IR, ¹H NMR, and ¹³C NMR spectroscopic techniques. The resulting Schiff bases were utilized as bidentate ligands, coordinating via imine nitrogen and phenolate oxygen atoms, to stabilize palladium ions. Aryl substituents on imine nitrogen allowed for fine tuning of the stereoelectronic properties of the Schiff bases. The catalytic activity of the selected palladium complexes was investigated in the Suzuki cross-coupling reaction of a series of aryl halides with boronic acids in H₂O/EtOH (1:1). Out of four complexes investigated in the cross-coupling reactions, Pd(L₈)₂, bearing a methoxy substituent on aryl imine, showed the highest activity at low catalyst loading. The scope of the reaction was also investigated with 26 samples.

Full text of DOI:10.1016/j.mcat.2021.111528

Molecular Catalysis 505 (2021) 111528
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Suzuki coupling reactions catalyzed by Schiff base supported palladium
complexes bearing the vitamin B cofactor
6
a
,
a
,
b
a
¨
Abdollah Neshat *, Mohammad Gholinejad *, Hafize Ozcan , Faezeh Khosravi ,
a
b
¨
Ali Mousavizadeh Mobarakeh , Omer Zaim
a
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, 45137-66731, Iran
Department of Chemistry, Trakya University, Edirne, 22030, Turkey
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Novel Schiff bases were synthesized by condensing aromatic amines with pyridoxal-5’-phosphate and charac-
terized by using FT-IR, 1H NMR, and C NMR spectroscopic techniques. The resulting Schiff bases were utilized
as bidentate ligands, coordinating via imine nitrogen and phenolate oxygen atoms, to stabilize palladium ions.
Aryl substituents on imine nitrogen allowed for fine tuning of the stereoelectronic properties of the Schiff bases.
The catalytic activity of the selected palladium complexes was investigated in the Suzuki cross-coupling reaction
Schiff base
13
Pyridoxal-5-phosphate
Homogeneous palladium catalyst
Carbon-carbon cross coupling
of a series of aryl halides with boronic acids in H
coupling reactions, Pd(L , bearing a methoxy substituent on aryl imine, showed the highest activity at low
catalyst loading. The scope of the reaction was also investigated with 26 samples.
2
O/EtOH (1:1). Out of four complexes investigated in the cross-
8 2
)
1
. Introduction
The carbon nanospheres derived from areca nut kernel, indigenous to
Sirsi in India was used as a support for loading palladium metal. The
resulting composite material was used for the reduction of nitroarenes
and Suzuki–Miyaura coupling of bromoarenes with aryl boronic acids
Carbon-Carbon bond forming reactions are among the important
catalytic transformations in organic synthesis [1–14]. The
Suzuki-Miyaura reaction, a palladium catalyzed cross-coupling of an
aryl or an alkenyl halide (or sulfonate, triflate, etc.) with an aryl or
[19]. Highly efficient catalysts based on zirconium ferrite ((ZrO)
2 2 5
Fe O )
magnetic nanoparticles (MNPs) in C C cross-coupling reaction were
–
2
2
alkenyl boronic acid, forming a Csp –Csp bond, is an important strategy
in the synthesis of homo- and hetero-biaryls. Because of its tremendous
role in the advancement of synthetic organic strategies, the 2010 Nobel
Prize in Chemistry was shared among three pioneers in the field of cross
coupling reactions [15].
reported [20]. The influence of nitrogen donor bridged spacer ligands
have been investigated in the N-heterocyclic carbene (NHC) palladium
complex catalyzed Suzuki coupling reaction [21]. Other innovative
substrates designed for the stabilization of palladium complexes include
electrospinning technology that was used to prepare a novel type of
composite catalyst with ligands such as urea-Pd/PAN (polyacrylonitrile)
[22]. Scrophularia striata extract was used as a bidentate ligand in the
Palladium supported magnetic nanoparticle (Pd@Fe3O4/AMOCAA).
The resulting nanomaterial was used as effective heterogeneous catalyst
for Suzuki and Sonogashira cross-coupling reactions [23]. Novel mag-
netic nanoparticles as supports for loading L-Mthionine or 2-(amino-
methyl)phenols stabilized palladium ions were also reported. These
Heterogeneous catalysts have been highly investigated in the C C
–
cross coupling reactions. Recently, a procedure was introduced for the
preparation of a Cu(I) supported complex on Silica supported boehmite
nanoparticles [16]. The introduced catalyst showed high activity in the
C C coupling reactions. Palladium nanoparticles stabilized by trypto-
–
phan on boehmite was also reported by the same group. This catalytic
system was used as a thermally stable catalyst for Heck, Stille and Suzuki
cross-coupling reactions [17]. A novel catalyst based on Pd/me-
tal–organic framework (MOF) in the presence of dichlor-
oimidazolidinedione (DCID) was introduced for the Suzuki-type
cross-coupling of phenols with aryl boronic acids. This novel catalyst
afforded cross coupling products in high yields in tetrahydrofuran [18].
nanocatalysts were utilized successfully as green catalysts for C
coupling reactions [24,25].
–
C
Despite innovative catalyst designs, in order to reach high catalyst
efficiency, the majority of the heterogeneous catalytic systems intro-
duced above require refluxing temperatures of the solvents. A literature
*
Corresponding authors.
E-mail addresses: a.neshat@iasbs.ac.ir (A. Neshat), gholinejad@iasbs.ac.ir (M. Gholinejad).
https://doi.org/10.1016/j.mcat.2021.111528
Received 25 November 2020; Received in revised form 16 February 2021; Accepted 9 March 2021
Available online 22 March 2021
2
468-8231/© 2021 Elsevier B.V. All rights reserved.

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
survey in the Suzuki coupling reactions reveals that carbon-carbon cross
coupling reactions are not limited to palladium-based catalysts alone.
Furthermore, to carry out these reactions more efficiently, a multitude of
ligands derived from common ligand families such as phosphines, N-
heterocyclic carbenes and Schiff bases have been developed in recent
years [26–28]. Besides, a variety of catalyst recycling strategies have
also been implemented to achieve economically viable catalysts. In this
regard, there are numerous reports of catalysts anchored to an organic
polymer or silica-based support [29–36]. While these strategies help
achieve optimal catalyst recovery, they tend to reduce efficient
catalyst-substrate interactions, leading to an increased reaction times
and reduced selectivity.
spectrometer.
2.2. General procedure for conducting Suzuki–Miyaura coupling reaction
Aryl halide (0.5 mmol), arylboronic acid (0.75 mmol), K CO (0.75
2
3
mmol), and the Pd(L ) catalyst (0.65 mol%) were added to a 5 ml flask
8
2
containing a mixture of solvents (H O:EtOH with 1:1 ratio, 2 mL). The
2
reaction mixture was stirred at R.T. for aryl iodides and aryl bromides
◦
and 80 C for aryl chloride for 24 h. The progress of the coupling re-
action was monitored by GC. Then, the crude product was extracted
using ethyl acetate (3 × 5 mL). The products were purified using column
and plate chromatography on silica gel.
Rapid progress in catalyst heterogenation techniques has not hin-
dered chemists’ desire to develop catalyst recovery methods for homo-
geneous systems. Mastrorilli et al. have reported ionic liquids as a novel
reaction medium in cross coupling reaction of vinyltrifluoroborate and
arenediazonium salts [37]. Later, Roglans et al. developed a thorough
mechanistic investigation of cross-coupling between organo-
trifluoroborate and arenediazonium salts [38]. Along this line, San-
Martin et al. then introduced para-carboxyl substituted CNC-pincer
ligands and their palladium complexes for their use in coupling exper-
iments. The catalytic systems introduced by this group were proved to
be highly water soluble, stable and recoverable in coupling reactions
2.3. General procedure for recycling of Pd(L8)2 catalyst in
Suzuki–Miyaura coupling reaction of 4-bromobenzonitrile with
phenylboronic acid
4-bromobenzonitrile (0.5 mmol), phenylboronic acid (0.75 mmol),
K CO (0.75 mmol), Pd(L ) catalyst (0.65 mol%) were added to a 5 ml
2
3
8 2
flask containing a mixture of solvents (H O:EtOH with 1:1 ratio, 2 mL).
2
The reaction mixture was stirred at room temperature for 24 h. After
completing the reaction, the organic phase was extracted by ethyl ace-
tate and the aqueous phase was used in the next reaction. This recycling
was repeated for five consecutive runs with low decrease in the activity.
[
39].
Synthetic simplicity and high yielding procedures have made Schiff
bases an attractive target for the development of novel ligands in co-
ordination chemistry of transition metal ions. Jacobson’s manganese
and chromium-based oxidation catalysts as well as Noyori’s copper
complexes for asymmetric catalysis are among well-known examples of
Schiff base utility as ligand in catalysis [40,41].
2.4. Synthesis of ligands
All Schiff base compounds (L –L ) were synthesized using the one
step condensation method and were characterized by various spectro-
1
8
scopic techniques. A general synthesis procedure for L and a palladium
1
With regard to our experience in Schiff base synthesis and catalytic
coupling reactions, we set out to investigate the synthesis of novel Schiff
bases which a) are capable of inducing intermolecular hydrogen
bonding interactions, a collection of which in the product would
resemble organic polymeric entities and b) allow ease of stereoelectronic
tunability. To this end, we chose pyridoxal 5-phosphate, which contains
an aldehyde functional group and is suitable as condensation site with
amines. Besides, it was anticipated that the phosphate group on this
molecule would have the capacity to initiate hydrogen bonding in-
teractions in the resulting Schiff base. The presence of phosphate would
also help to solubilize the resulting Schiff base in an aqueous medium.
From sustainability standpoint, conducing catalytic reactions in a
medium of cost-friendly and low toxicity is highly attractive. In this
regard, water, being a non-flammable, low immiscible, and easy in
phase separation is considered an excellent solvent in organic reactions.
However, due to the low solubility of catalysts in aqueous media and the
resulting low reactivity, the design and synthesis of water-soluble li-
gands is very important. Along this line, using naturally occurring and
green ligand for the stabilization of metal catalysts in aqueous media is
highly desirable from the green chemistry’s standpoints.
complex, Pd(L2)2 is shown below. Other Schiff bases and complexes
were prepared by following this general procedure.
2.4.1. Synthesis of L1
Pyridoxal-5-phosphate (0.4 g, 1.6 mmol) was added to a solution of
aniline (0.15 g, 1.6 mmol) in methanol. The mixture was stirred at room
temperature for 24 h. The orange precipitate was removed by filtration
and washed with cold methanol and then n-hexane. Re-crystallization
◦
from ethanol solution (20 mL) at 0 C yielded analytically pure orange
solid.
2.4.2. Spectroscopic data
1
L : H NMR (DMSO-d , 400 MHz): δ 9.47 (s, 1 H), 7.95 (s, 1 H), 7.62
1
6
3
3
3
(d, 2H, J =8.0 Hz), 7.47 (t, 2H, J =8.0 Hz), 7.36 (t, 1H, J =8.0 Hz),
2
13
4.96, 4.95 (d, J =4.0 Hz), 2.44 (s, 3 H). C (100 MHz, DMSO-d ) δ
HH
6
161.7, 154.0, 149.5, 147.0, 139.2, 131.2, 130.0, 128.5, 122.5, 120.5,
61.5, 19.3. 31P (DMSO-d , 162 MHz) δ 0.11. FT-IR (KBr, cm ):
ꢀ 1
ν
(=C-H)
6
3064 (w),
ν(Cꢀ H) 2931 (w), ν(Cꢀ H) 2884 (w), ν(C=C) 1380 (m). Yield:
◦
0.46 g, 88 %, Mp: 212 C.
1
L2: H NMR (400 MHz, DMSO-d ) δ 9.39 (s, 1 H), 7.93 (s, 1 H), 7.61
6
3
3
Here, we report the synthesis and structural characterization of
(d, 2H, J =8.0 Hz), 7.44 (d, 2H, J =8.0 Hz), 4.94 (s, 2 H), 2.42 (s, 3 H).
1
3
Schiff bases derived from pyridoxal-5-phosphate, which is a vitamin B
6
C NMR (100 MHz, DMSO-d ) δ 162.1, 153.6, 149.5, 145.8, 139.2,
31
6
cofactor, to stabilize palladium ions and to investigate their utility as
green and efficient catalysts in the Suzuki cross-coupling reactions of
substituted aryl iodides, bromides and chlorides with arylboronic acids
in a mixture of aqueous and organic media.
6
132.8, 131.1, 129.8, 124.3, 120.4, 61.5, 19.2. P (DMSO-d , 162 MHz)
ꢀ 1
δ 0.21. FT-IR (KBr, cm ):
ν
(=C-H) 3045 (w),
ν
(-C-H) 2884 (w),
ν
(C =
◦
C) 1382 (m). Yield: 0.53 g, 93 %, Mp: 227 C.
1
L : H NMR (400 MHz, DMSO-d ) δ 9.43 (s, 1 H), 7.94 (s, 1 H), 7.70
3
6
(
s, 2 H), 7.49 (s, 1 H), 4.96 (s, 2 H), 2.42 (s, 3 H). 13C NMR (100 MHz,
DMSO-d
) δ 164.2, 153.4, 149.6, 139.3, 135.2, 134.6, 131.4, 127.5,
21.6, 120.1, 114.4, 112.1, 61.4, 19.3. 31P (DMSO-d
2
. Experimental
6
1
6
, 162 MHz) δ 0.29.
ν (C = C) 1384 (m). Yield:
2
.1. General procedures and materials
FT-IR:
ν
(=C-H) 3048 (w),
ν
(-C-H) 2886 (w),
◦
0
.62 g, 98 %, Mp: 199 C.
ꢀ 1
Reagents and solvents were used as received from commercial sup-
L
4
: FT-IR (KBr, cm ):
ν
(=C-H) 3050 (w),
ν
(-C-H) 2886 (w), (C = C)
ν
1
13
◦
pliers. H, C NMR spectra were recorded on a Brucker Avance DPX400
1382 (m). Yield: 0.56 g, 96 %, Mp: 196 C.
◦
1
1
MHz instrument in DMSO-d
6
at 25 C using standard pulse programs. H,
L
5
: H NMR (400 MHz, DMSO-d
6
) δ 8.99 (s, 1 H), 7.99 (s, 1 H), 7.15
3
1P and 13C shifts are quoted relative to the residual solvent signals.
3
3
2
(d, 2H, J =8.0 Hz), 7.07 (t, 1H, J =8.0 Hz), 4.87 (d, 2H, JHH =4.0 Hz),
Infrared spectra were measured on a Bruker Vertex 70 FT-IR
2.46 (s, 3 H), 2.21 (s, 6 H). 13C NMR (101 MHz, DMSO-d
6
) δ 166.50,
2

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
31P (DMSO-d
, 162 MHz) δ 0.51. FT-IR (KBr, cm ):
ꢀ 1
ν
(=C-H) 3079 (m),
6
ν
(=C-H) 3010 (m),
ν
(=C-H) 2931 (m),
ν
(=C-N) 1643,
ν
(C = C) 1548
◦
(
s),
ν
(C = C) 1498 (m). Yield: 0.35 g, 53 %, Mp: 170 C.
1
L
8
: H NMR (400 MHz, DMSO) δ 9.40 (s, 1 H), 7.92 (s, 1 H), 7.61 (d,
3 3 2
2
3
1
H, J =8.0 Hz), 6.98 (d, 2H, J =8.0 Hz), 4.95 (d, 2H, JHH =4.0 Hz),
.79 (s, 3 H), 2.42 (s, 3 H). 13C NMR (100 MHz, DMSO-d
6
) δ 159.7,
58.6, 153.6, 149.2, 139.8, 130.8, 123.9, 120.7, 115.0, 61.6, 55.8, 19.2.
3
1
ꢀ 1
P (DMSO-d
6
, 162 MHz) δ 0.18. FT-IR (KBr, cm ):
ν
(=C-H) 3046 (w),
◦
ν
(-C-H) 2883 (w),
ν
(C = C) 1382 (m). Yield: 0.52 g, 92 %, Mp: 231 C.
2
.4.3. Synthesis of palladium complexes
2
.4.3.1. Pd(L
2
)
2
. In a flask, L
2
(0.025 g, 70 mmol) was dissolved in 10
, 0.007 g, 70 mmol) was added.
(0.008 g, 35 mmol) was added to the flask and the
ml acetonitrile and triethylamine (NEt
Then, Pd(OAc)
3
2
mixture was stirred at room temperature for 24 h. The orange precipitate
dried under nitrogen and was collected by paper filtration.
8
Scheme 1. Synthetic route for Schiff bases L-L .
2
.4.4. Spectroscopic data
1
4
53.86, 149.60, 147.37, 139.22, 131.12, 128.66, 125.99, 120.30, 61.60,
ꢀ 1
2
.4.4.1. Pd(L
(C = C) 1575 (m),
C. Anal. Found (calcd for C28
2
)
2
. FT-IR (KBr, cm ):
(C = C) 1386 (m), Yield: 0.02 g, 69 %. Mp: 249
26Cl Pd): C, 41.12 (41.02); H,
ν
(=C-H) 3058 (w),
ν
(-C-H) 2890
3
1
ꢀ 1
9.05, 19.25, 18.65. P (DMSO-d
6
, 162 MHz) δ 0.26. FT-IR (KBr, cm ):
(
◦
w),
ν
ν
ν
(=C-H) 3050 (w),
ν
(-C-H) 2886 (w),
ν
(C = C)1382 (m). Yield: 0.55 g,
H
2 4 10 2
N O P
◦
9
8 %, Mp: 168 C.
3
.20 (3.11); N, 6.85 (6.83).
1
L
6
: H NMR (400 MHz, DMSO) δ 9.32 (s, 1 H), 7.91 (s, 1 H), 7.53 (d,
H, J =8.0 Hz), 7.11 (s, 1 H), 7.00 (d, 2H, J =8.0 Hz), 4.96 (d, 2H, JHH
3
3
2
1
2
.4.4.2. Pd(L
5
)
2
. Pd(OAc)
(-C-H) 2967 (w),
m), Anal. Found (calcd for C30
2
(0.015 g), L
5
(0.046 g); FT-IR (KBr, cm-1):
ν
=
4.0 Hz), 2.43 (s, 3 H), 2.32 (s, 3 H), 2.25 (s, 3 H). 13C NMR (100 MHz,
(
=C-H) 3073 (w),
ν
ν (-C = N) 1612 (m), ν (C = C) 1417
DMSO-d
6
) δ 159.4, 154.0, 149.4, 143.0, 138.9, 138.0, 133.0, 131.6,
(
H
32
N
4
O
10
P
2
Pd): C, 46.38 (46.32); H,
3
1
1
30.7, 128.1, 120.6, 118.3, 61.5, 21.0, 19.3, 18.4. P (DMSO-d
6
, 162
◦
4
.15 (4.10); N, 7.21 (7.15). Yield: 0.03 g, 56 %, Mp: 213 C.
ꢀ 1
MHz) δ 0.22. FT-IR (KBr, cm ):
ν
(=C-H) 3048 (w),
ν
(-C-H) 2886 (w),
ν
◦
(
C = C) 1380 (m). Yield: 0.50 g, 89 %, Mp: 212 C.
2
.4.4.3. Pd(L
6
)
2
. Pd(OAc)
(-C-H) 2898 (w),
(C = C) 1376 (m), Anal. Found (calcd for C30
2
(0.013 g), L
6
(0.070 g); FT-IR (KBr, cm-1):
(C = C) 1569
Pd): C,
ν
1
L
7
: H NMR (400 MHz, DMSO) δ 10.44 (s, 1 H), 9.88ꢀ 8.37 (br. 3 H),
(
=C-H) 3025 (w),
w),
ν
ν
(C = N) 1616 (w), ν
2 13
8
.06 (s, 1 H), 5.08 (d, 2H, JHH =4.0 Hz), 2.42 (s, 3 H). C NMR (100
(
ν
32 4 10 2
H N O P
MHz, DMSO-d
6
) δ 196.8, 154.1, 150.7, 137.2, 131.8, 122.4, 61.7, 19.1.
1
Fig. 1. H NMR spectrum of L
1
. The expanded aromatic region of the signal is also shown.
3

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
1
3
Fig. 2. C NMR spectrum of L1.
Fig. 3. DEPT-135 spectrum of L1.
4

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
Fig. 4. The general structure of the palladium complexes, Pd(L
n
)
2
(n = 2,5,6,8.
R: aromatic substituents on the imine nitrogen).
4
2
6.38 (46.30); H, 4.15 (4.10); N, 7.21 (7.15). Yield: 0.038 g, 47 %, Mp:
◦
24 C.
2
8 2 2 8
.4.4.4. Pd(L ) . Pd(OAc) (0.02 g), L (0.07 g); FT-IR: 3079 (w), 3012
2 2.
Fig. 5. Optimized structure of Pd(L )
(
(
(
w), 2942 (w), 1614 (m), 1573 (m), 1508 (m), 1384 (m), Anal. Found
calcd for C32
H
36
N
4
O
12
P
2
Pd): C, 45.92 (45.50); H, 4.34 (4.30); N, 6.69
◦
corresponding Schiff bases. It is worth mentioning that in all metalation
reactions, ligands were initially deprotonated by adding a few drops of
triethylamine prior to reaction with the metal salt. After a few minutes,
the initial clear orange solution turned cloudy and an orange powder
precipitated. Further details about metal complexation are provided in
the experimental section. All palladium complexes are slightly soluble in
the majority of organic solvents and it is speculated that at multiple
ligand sites the hydrogen bonding interactions are cause the palladium
complexes to become less soluble than the ligands in organic solvents. In
Fig. 4, a general structure is proposed for the resulting palladium com-
plexes and out of many possibilities, the hydrogen bonding interactions
through phosphate groups are shown. Despite several attempts, suitable
crystals for XRD experiments could not be obtained. However, all metal
complexes investigated in catalytic reactions were analyzed by IR
spectroscopy. The results of CHN analysis are also provided in the
experimental section. These results support the proposed metal complex
structures in Fig. 4.
6.50). Yield: 0.062 g, 77 %, Mp: 232 C.
3
. Results and discussion
1 8
Synthesis of Schiff bases, L -L , was carried out at room temperature
in a solution of aryl amines with pyridoxal 5-phosphate in methanol for
4 h; Scheme 1. Judicious choice of the aryl amine precursor allows for
2
fine tuning of the electronic and steric effects of ligands, influencing the
generic properties of the metal complexes. Novel ligands introduced in
this study were characterized by a combination of spectroscopic
1
13
methods such as H & C NMR and IR. The reaction leading to ligand
precursors is shown in Scheme 1.
FT-IR spectrum of Schiff bases, L
1 8
‒L , displayed characteristic sharp
bands related to aromatic and aliphatic C–H stretching vibrations be-
ꢀ 1
ꢀ 1
– –
– –
tween 2800 cm and 3100 cm as well as C N, C C, and C–O
ꢀ 1
ꢀ 1
stretching vibrations from 1300 cm to 1650 cm (Figs. S1, S3, S8,
S13, S14, S19, S24, S29).
In general, the disappearance of the O H stretching frequency
–
Due to the presence of multiple hydrogen bonding sites in ligand
structures, which lower their solubility in both polar and nonpolar
deuterated solvents, obtaining reliable NMR spectra of ligands proved
challenging. However, we noticed that the addition of a drop of an
organic base such as triethylamine to the NMR tube containing the
ligand samples improved their solubility, resulting in better quality
ꢀ 1
signal (usually observed at about 3200 cm ) and a shift in the vibra-
–
tional frequency of the imine C
–
N bond toward lower frequencies are
strong indication of metal coordination of a Schiff base ligand. For
ꢀ 1
example, the band at ~ 1614 cm associated with the stretching fre-
–
ꢀ 1
quency of the imine C
, indicating imine nitrogen coordination to the metal center.
The structure of the metal complexes utilized for the catalytic re-
–
N bond in L is shifted to ~ 1604 cm in Pd
1
2 2
(L )
NMR data. Among all ligands, L
shown in Fig. 1, methyl and methylene resonances in L
at 2.44 ppm and 4.95 ppm, respectively. Aromatic ring protons in L
4
was an exception to this procedure. As
1
were observed
actions was optimized by performing a geometry optimization calcula-
tion with Gaussian03 using the B3LYP level and LANL2DZ basis set [42].
1
appeared as triplet and doublet signals at 7.36 ppm, 7.47 ppm and 7.62
ppm. Two singlet signals at 7.95 ppm and 9.5 ppm were assigned to the
imine (N = C–H) and = C–H proton of the pyridine ring, respectively. It
is notable that due to fast proton exchanges, signals from the OH groups
were too broad.
The optimized structure of Pd(L
complexes adopt a square planar geometry. However, in Pd(L
two trans positioned oxides reveal five degrees of distortion out of
2
)
2
is shown in Fig. 5. All the metal
2 2
)
and Pd
8 2
(L )
the plane of the palladium and nitrogen atoms. The presence of het-
eroatoms (chloro and oxygen atoms of the methoxy) in the ligand
structures (the Z position, Scheme 1) and a plausible involvement in the
hydrogen bonding might have played a role in structural distortion,
however, this claim requires further experimental investigation. The
vibrational frequency calculation of the metal complexes was also ob-
tained using the same computational methods and the results are in
good accordance with the experimental infrared spectra, shown in
Table S1. For example, comparison of the calculated peak positions with
In the 13C NMR spectrum of L
1
, Fig. 2, the expected signals for ten
aromatic carbons were observed within the 120–162 ppm range. The
resonances for two aliphatic carbons were observed at 61.3 ppm and
9.6 ppm, assignable to methylene and methyl groups. In the 3 P NMR
1
1
spectrum of L
As shown in Fig. 3 and in order to differentiate CH, CH
the ligand structures, DEPT-135 experiments were conducted. Only five
signals for aromatic carbons in L were observed in its DEPT-135 spec-
1
(Fig. S2) a sharp signal was also observed at ¡ 0.1 ppm.
2
, and CH in
3
1
those observed for Pd(L
8
)
2
reveals that the two peaks observed above
belong to the symmetrical stretching frequencies of the
methyl and methoxy substituents. Multiple peaks observed between
trum, indicating the presence of five tertiary carbons in its structure.
Five positive signals were also observed in the aromatic region, indic-
ꢀ 1
3
000 cm
ative of five different = C–H, and one at 19.3 ppm, indicating CH
3
group.
ꢀ ꢀ 1
1
1
618 cm and 1551 cm , point to metal coordination via imine ni-
A negative signal at 65.5 ppm was assigned to the CH group.
2
–
–
–
N) vibrations
trogen owing to aromatic ring and imine (C
–
C and C
Because of better solubility of palladium acetate precursor in
acetonitrile and to avoid side reactions, all complexation reactions in
this study were performed in acetonitrile by mixing and allowing to
react one equivalent of palladium acetate with two equivalents of the
and their relative shifts compared to the free ligand. Finally, a medium
ꢀ 1
intensity peak observed at 1508 cm belongs to the symmetrical scis-
soring bending mode of the methyl substituents.
5

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
Table 1
3.1. Catalytic Suzuki cross coupling reactions
Optimization of the coupling reaction between 4-bromoanisole and phenyl-
boronic acid, for different catalyst types and quantities at different tempera-
tures. The optimized reaction condition is shown in bold.
3.1.1. Catalytic performance
The catalytic activity of selected palladium complexes was investi-
gated in the Suzuki-Miyaura coupling reactions of aryl halides with
arylboronic acids. To find out the optimal reaction conditions, the
coupling reaction of 4-bromoanisole with phenylboronic acid was
selected as the model reaction. Catalytic activities of 1.5 mol% of Pd
◦
(
L
2
)
2
, Pd(L
5
)
2
, Pd(L
6
)
2
and Pd(L
8
)
2
were observed at 80 C in a mixture
of H
2
O/EtOH in a 1:1 vol ratio. Potassium carbonate (K
2
CO
3
) was used
as base and the reaction time was set to 24 h (Table1, Entries 1–4). All
◦
Entry
Catalyst
Amount of catalyst (mol. %)
T ( C)
Yield (%)
these palladium complexes performed well under these reaction condi-
1
2
3
4
5
6
7
8
9
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
Pd(L
2
5
6
8
2
5
6
8
2
5
6
8
5
8
2
5
6
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
1.5
80
80
80
80
60
60
60
60
60
60
60
60
50
50
r.t
r.t
r.t
r.t
>99
>99
>99
>99
>99
>99
98
tions and gave excellent yields (>99 %). Then, the reaction temperature
1.5
◦
was lowered to 60 C, whereupon Pd(L
2
)
2
, Pd(L
5
)
2
and Pd(L
8
)
2
gave
1.5
1.5
yields of up to 99 % (Table 1, Entries 5–8). To continue the optimization,
Pd(L , Pd(L and Pd(L were selected and the same reactions were
1.5
)
)
8 2
)
2
2
5
2
1.5
◦
conducted at 60 C with the amounts of the catalysts reduced to a half
with respect to the previous reactions. Under these new reaction con-
1.5
1.5
>99
96
ditions, Pd(L
5
)
2
and Pd(L
8
)
2
gave higher yields than that observed with
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
◦
10
11
12
13
14
15
16
17
18
98
Pd(L
Pd(L
2
8
)
)
2
(Table 1, Entries 9–12). At a lower reaction temperature, 50 C,
performed better than Pd(L ) (Table 1, Entries 13 and 14).
5 2
95
2
98
Finally, all catalytic reactions were repeated at room temperatures, upon
98
8 2
which 0.75 mol% of Pd(L ) gave the highest yield (Table 1, Entries
98
73
15–18).
84
To ensure the correct choice of base and solvent, the effect of various
80
solvents such as EtOH, H
and THF were investigated using Pd(L
temperature. High reaction yields were obtained using a mixture of
2
O, MeOH, Toluene, PEG-200, DMF, CH
3
CN
Pd(L
)
8 2
90
8 2
)
in the model reaction at room
H
2
O/EtOH in a 1:1 vol ratio (Table 2, Entries 1–9).
Table 2
The effect of different bases such as NaOH, KOH, Et
3
N, DABCO, t-
CO (Table 3,
The influence of solvent type on the coupling of 4-bromoanisole and phenyl-
boronic acid. The ideal solvent is shown in bold.
BuOK was also tested and all gave lower yields than K
2
3
Entries 1–6).
After optimizing the reaction conditions, the Suzuki coupling re-
actions of structurally different aryl iodides, bromides and chlorides
were investigated with different arylboronic acids. The results exhibited
excellent yields obtained with iodobenzene as well as aryl iodides,
bearing electron donating methyl and methoxy substituents (Table 4,
Entries 1–5). Also, the desired biaryls, in 90–99 % yields, were obtained
by couplings of aryl iodides having electron withdrawing groups with
phenylboronic acid (Table 4, Entries 6–11). Moreover, the Suzuki re-
action of 2-iodothiophene and 2,5-diiodothiophene with phenylboronic
acid progressed quite well and yielded products up to 90 % and 85 %,
respectively (Entries 12 and 13). Reactions of 4-iodobenzonitrile and 4-
iodonitrobenzene with 1-naphthaleneboronic acid also gave high yields
of 95 % and 99 %, respectively. (Table 4, Entries 14 and 15).
Entry
Solvent
EtOH
Yield (%)
1
2
3
4
5
6
7
8
9
75
60
53
8
2
H O
MeOH
Toluene
PEG-200
DMF
70
20
10
6
CH
THF
O:EtOH
3
CN
In the next step, the Suzuki coupling reactions of different aryl bro-
mides with arylboronic acids were investigated under optimized reac-
tion conditions. Generally, excellent yields were achieved between aryl
bromides ds having both electron-donating and electron-withdrawing
H
2
90
Table 3
Optimization of different bases for the coupling between 4-bromoanisole and phenylboronic acid.
Entry
Base
Yield (%)
1
2
3
4
5
6
NaOH
KOH
84
70
68
60
75
90
3
Et N
DABCO
t-BuOK
2 3
K CO
6

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
Table 4
a
b
c
8 2
Suzuki coupling reactions of structurally different aryl iodides , bromides and chlorides using the Pd(L ) catalyst.
1
2
d
e
f
Entry
1
Ar
Ar BR
Product
Yield (%)
TON :TOF
g
>99
152 : 6.3
2
3
4
5
90
89
90
91
138 : 5.75
h
593:49
138 : 5.75
h
606:50
g
6
7
8
9
>99
152 : 6.3
152 : 6.3
141 : 5.9
138 : 5.75
g
>99
92
90
1
1
0
1
90
82
138 : 5.75
126 : 5.2
(
continued on next page)
7

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
Table 4 (continued)
1
2
d
e
f
Entry
Ar
Ar BR
Product
Yield (%)
TON :TOF
1
2
3
90
85
138 : 5.75
131 : 5.4
1
14
15
16
95
146 : 6
g
>99
152 : 6.3
113 : 4.7
85
1
7
86
114 : 4.7
g
18
19
20
21
22
23
>99
116 : 4.8
116 : 4.8
g
>99
g
h
>98
606:50
89
118 : 4.9
g
h
96
640:53
g
96
112 : 4.70
(
continued on next page)
8

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
Table 4 (continued)
1
2
d
e
f
Entry
Ar
Ar BR
Product
Yield (%)
TON :TOF
2
4
83
110 : 4.6
2
2
2
2
5
6
7
8
89
90
92
88
118 : 4.9
120 : 5
122 : 5.11
117 : 4.8
2
3
9
0
90
80
120 : 5
89 : 3.7
a
b
c
d
e
f
Reaction conditions: Aryl iodides (0.5 mmol), arylboronic acid (0.75 mmol), K
2
CO
3
(0.75 mmol), Pd(L
8
)
2
(0.65 mol %) and H
2
O:EtOH (1:1 mL), R.T.
Reaction conditions: Aryl bromides (0.5 mmol), arylboronic acid (0.75 mmol), K
2
CO
3
(0.75 mmol), Pd(L
(0.75 mmol), Pd(L
8
)
2
(0.75 mol %) and H
2
O:EtOH (1:1 mL), R.T.
◦
Reaction conditions: Aryl chloride (0.5 mmol), arylboronic acid (0.75 mmol), K
2
CO
3
8
)
2
(0.9 mol %) and H
2
O:EtOH (1:1 mL) at 80 C.
Isolated yields.
f
[
TON] values [(mol product/mol catalyst)], [TOF] values [(mol product/mol catalyst)/time of reaction (h)].
Yields determined by GC.
g
◦
Reaction conditions: Aryl halide (0.5 mmol), arylboronic acid (0.75 mmol), K
2
CO
3
(0.75 mmol), Pd(L
8
)
2
(0.15 mol %), H
2
O:EtOH (1:1 mL), 80 C, and during 12 h.
◦
groups, with phenylboronic acids (Table 4, Entries 16–24). Moreover,
we examined the effect of the Pd(L catalyst in the reaction of 4-bro-
utilizing 0.9 mol % of the Pd(L
8
)
2
catalyst at 80 C which produced the
8
)
2
desired biaryl product in 80 % yield, (Table 4, Entry 30). The calculation
of TON and TOF for Suzuki coupling reaction of aryl halides in Table 4
showed satisfactory results. However, in order to obtain higher TON and
TOF numbers, some of the reactions were carried out using lower
mobenzonitrile and 4-bromonitrobenzene with other boronic acids such
as 1-naphthaleneboronic acid, 4-methoxyboronic acid and benzo[b]
thien-3-ylboronic acid affording 88–92 % yields (Table 4, Entries
◦
2
5–28). The reaction of 5-bromopyrimidine, a heterocyclic aryl bro-
catalyst quantity under shorter reaction times at 80 C. Under these new
mide, with phenylboronic acid produced a coupling product in high
yield, indicating the high activity of the presented catalyst with respect
to rather challenging heterocyclic aryl halides (Table 4, Entry 29). We
also expanded the application scope of the present active catalyst for the
Suzuki coupling reaction of aryl chlorides. Initial results showed that a
conditions, better results were achieved (Table 4, entries 3, 5, 20, and
22).
The comparison of the activity of Pd(L
8 2
) with other catalysts in
Suzuki coupling reaction of 4-bromoanisole and phenylboronic acid
8 2
indicated higher catalytic performance of Pd(L ) in most cases
2
7 % yield obtained in the coupling reaction of 4-chlorobenzonitrile
(Table 5).
under optimized reaction conditions for aryl bromides at room tem-
perature. Hence, different reaction conditions were investigated
9

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
Table 5
8 2
3.1.2. Recycling of the Pd(L ) catalyst
Comparative catalytic activity of Pd(L
8
)
2
with other reported catalysts in Suzuki
We examined the recycling of the Pd(L ) catalyst for the coupling
8 2
coupling reaction of 4-bromoanisole and phenylboronic acid.
reaction of 4-bromobenzonitrile with phenylboronic acid under opti-
mized reaction conditions. After the completion of the reaction, the
organic phase was extracted using ethyl acetate, and the aqueous phase,
which contained the catalyst, was utilized in the next reaction. Results
revealed that the catalyst can be recycled and reused easily for five
sequential times, Fig. 6, with low decline in the catalytic activity.
A variety of catalytic systems, mainly based on the pallidum metal,
◦
Entry
Catalyst
T ( C),
Loading
(Pd mol
Yield
(%)
Refs.
time (h)
–
has been developed for the C C coupling reactions [55–60]. Despite
%
)
systematic differences in electronic and steric properties of the ligands
systems in these catalytic systems, the majority of the reports in regards
to the mechanistic studies show common major steps, an example of
which is shown in Scheme 2. The two major steps in these mechanisms
include the oxidative addition of the alkyl or aryl halide substrate and
the final reductive elimination step of the products from the interme-
diate palladium complex, which leads to the regeneration of the active
palladium species. In the current report, the variation in the electronic
and steric properties of the ligands has played a central role in the
optimization of catalytic coupling reactions. The comparison of the
1
2
3
GO–Pd
Pd-2QC
4
S
80, 6
0.5
0.55
1
73
64
70
[43]
[44]
[45]
80, 1.5
90, 24
Palladium(II) pyrazolated
thio/selenoethers complex
Phosphorus-Nitrogen ligands
stabilized with Pd NPs
4
60, 2
0.08
84
[46]
5
6
Pd (COD)Cl
2
complex
80, 1
1
8
53.8
83
[47]
[48]
Phosphorus-coordinated
palladium(II) complex IV
110, 24
7
Palladacycle [PdCl((C
HOC )CHNH(CH
H)]
6
H
5
)(2-
110, 25
0.001
84
[49]
6
H
4
) SePh
2 3
–
2 2 8 2
performance of Pd(L ) and Pd(L ) in the optimization table reveals
8
9
Palladium-Porphyin complex
Palladium(II)-(E,N,E) pincer
ligand (S) complex
100, 4
0.1
2
80
76
[50]
[51]
that the replacement of the chloro substituent in the aromatic para po-
sition with an electron donating methoxy substituent dramatically
changes catalyst efficacy in the coupling reactions. Even a comparison of
100, 12
1
1
1
0
1
2
Pd-SH-FSM
130, 20
50, 2
1
79
91
97
[52]
[53]
[54]
Palladated oxime resins
O-carboxymethyl chitosan
Schiff bases supported Pd(II)
catalyst
1
2 2 5 2 6 2
coupling reaction data for Pd(L ) in Table 1 with Pd(L ) and Pd(L ) ,
100, 48
0.04
which contains electron donor methyl substituents, reveals the elec-
tronic influence of these substituents on catalyst performance. An in-
crease in the electron donor capacity of the ligand increases the
likelihood of oxidative addition reaction and this observation might
explain higher efficiency of the catalysts with methoxy or methyl sub-
stituents than those with chloro substituents. Therefore, the following
mechanism for the catalyst developed in this study is presented.
Following oxidative addition, aryl boronic acid and base mediated
transmetalation reaction leads to the formation of alkyl-aryl palladium
intermediate. The C
–
C coupling product is released via reductive
elimination in the final step.
4
. Conclusion
In this work, we synthesized several Schiff bases derived from
6
pyridoxal-5-phosphate, a cofactor of vitamin B , and utilized them as
bidentate ligands to coordinate Pd(II) ions for the preparation of a new
series of palladium complexes. The complexes were characterized using
FT-IR, 1H NMR, and C NMR spectroscopic techniques and were
applied successfully as active catalysts in the Suzuki-Miyaura coupling
reactions of different aryl halides with arylboronic acids in alcoholic
13
Fig. 6. Catalyst recycling diagram.
8 2
aqueous media. Results showed that among complexes tested, Pd(L )
exhibited the highest activity compared to the other catalysts. Also, this
active catalyst was utilized successfully with a wide range of substrates
of aryl iodides and bromides including electron-donating and electron
withdrawing groups with different arylboronic acids. The reactions of
aryl iodides and bromides were conducted at room temperature and the
◦
optimal temperature for aryl chlorides was determined to be 80 C. This
catalyst was recycled and reused for five sequential times with low
decline in catalytic activity.
CRediT authorship contribution statement
Abdollah Neshat: Conceptualization, Supervision, Data Analysis,
Writing - original draft preparation. Mohammad Gholinejad: Catalytic
¨
reactions analysis, Manuscript editing. Hafize Ozcan: Schiff base ligand
preparation. Faezeh Khosravi: Catalytic reactions data collection. Ali
Mousavizadeh Mobarakeh: Metal complex preparation, DFT structure
¨
optimization. Omer Zaim: Reviewing and Editing.
Scheme 2. Proposed C C coupling reaction mechanism.
–
1
0

A. Neshat et al.
Molecular Catalysis 505 (2021) 111528
[27] A. Zapf, M. Beller, Top. Catal. 19 (2002) 101–109.
Declaration of Competing Interest
[
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30] S. Paul, J.H. Clark, Green Chem. 5 (2003) 635–638.
The authors report no declarations of interest.
31] C. Baleiz a˜ o, A. Corma, H. García, A. Leyva, Chem. Commun. (2003) 606–607.
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