Schiff base transition metal complexes for Suzuki–Miyaura cross-coupling reaction

Article abstract of DOI:10.1007/s12039-017-1347-6

Abstract: Schiff base ligand and its complex with iron (Fe), cobalt (Co), nickel (Ni) and copper (Cu) ions were synthesized using 4-aminoacetophenone and salicylaldehyde and characterized. FTIR spectrum shows that bidentate coordination of metal ions with ligand where O, N are electron donating sites of azomethine group. The geometry of the complexes was deduced from the calculated magnetic moment values and SCXRD analysis. All complexes were studied for their catalytic activity in Suzuki–Miyaura cross-coupling reaction with Cu–L complex showing excellent coupling yield among others. The catalytic activity data show promising results in coupling efficiency, employing cheap, abundant and green metals. Graphical Abstract: Synopsis The transition metal Schiff base complexes synthesized and characterized by SCXRD, Mass spectrometry, TGA, UV–Vis and FTIR analysis. Magnetic susceptibility measurements were also examined to know the plausible geometry of the complex. The synthesized complexes were systematically investigated for Suzuki–Miyaura cross-coupling reactions and optimized to enhance the yield of the Suzuki reaction. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-017-1347-6

J. Chem. Sci.
© Indian Academy of Sciences
DOI 10.1007/s12039-017-1347-6
REGULAR ARTICLE
Schiff base transition metal complexes for Suzuki–Miyaura
cross-coupling reaction
RASHEEDA M ANSARI and BADEKAI RAMACHANDRA BHAT∗
Catalysis and Material Chemistry Laboratory, Department of Chemistry, National Institute of Technology
Karnataka, Mangalore, Karnataka 575 025, India
E-mail: ram@nitk.edu.in
MS received 11 April 2017; revised 12 July 2017; accepted 13 July 2017
Abstract. Schiff base ligand and its complex with iron (Fe), cobalt (Co), nickel (Ni) and copper (Cu) ions
were synthesized using 4-aminoacetophenone and salicylaldehyde and characterized. FTIR spectrum shows that
bidentate coordination of metal ions with ligand where O, N are electron donating sites of azomethine group.
The geometry of the complexes was deduced from the calculated magnetic moment values and SCXRD analysis.
All complexes were studied for their catalytic activity in Suzuki–Miyaura cross-coupling reaction with Cu–L
complex showing excellent coupling yield among others. The catalytic activity data show promising results in
coupling efficiency, employing cheap, abundant and green metals.
Keywords. Suzuki–Miyaura cross-coupling; transition metal complex; schiff base; biphenyl.
1. Introduction
sized through coupling of an aryl halide or triflate to
phenylboronic acid in the presence of a catalyst.12 In
practice, palladium (Pd) complexes are common cata-
lysts for Suzuki coupling reactions;13–16 however, they
beingexpensiveandlessabundantitisrequiredtosearch
for more cost-effective and eco-friendly catalysts.
Coupling of various vinyl halides with a Grignard
reagent in the presence of iron catalysts has been
reported.17 Cross coupling of aromatic iodides with ter-
minal acetylenes using iron salts and CuI have been
done.18 A series of Fe(III) complexes supported by
chloro- and fluoro-functionalized amine-bis(phenolate)
ligands has been used for C–C coupling of aryl and
allyl Grignard reagents with alkyl halides.19 Coupling of
aryl halides with 2-Chloropyrazine having a wide vari-
ety of functional groups has been reported with cobalt
catalyst.20 Moreover, cobalt-catalyzed electrochemical
cross-coupling of functionalized phenyl halides with 4-
chloroquinoline derivatives have also been surveyed.21
However, cobaltcatalyzedC–Ccouplinghasbeeninves-
tigated which is applicable in the synthesis of important
pharmaceutical intermediates.22 Additionally, nickel
salt was used for direct reductive cross coupling of aryl
halides with phenylboronic acid.23 Also, copper cat-
alysts for cross-couplings of different organometallic
Schiff base is a unique class of ligands with different
donor atoms exhibiting fascinating coordination style
towards numerous metal ions.1,2 The compatibility for
the chelation of the Schiff bases towards the transition
metal ions is applied in preparing their metal complexes.
Schiff base metal complexes are well-known for their
easy synthesis, stability and wide application.3,4 Metal
complexes containing Schiff base ligand have been stud-
ied for their interesting and important properties such
as their ability to bind with oxygen, photochromism,
antibacterial and antifungal properties, catalytic activity
in olefins, hydrogenation and complexing proficieency
towards a few toxic metals.5,6
Transition metal-catalyzed cross-coupling reactions
are undoubtedly the most utilized means for forming
C–C,7 C–N,8 C–O,9 and C–S10 bonds in synthetic
organic chemistry. Researchers are trying to develop
simple, fast, cheap and more efficient methodologies
for different coupling reactions.11 The Suzuki cross-
coupling reaction or simply Suzuki reaction is one of the
most used cross-coupling reactions in modern organic
synthesis, wherein a biphenyl molecule is being synthe-
*
For correspondence
Electronic supplementary material: The online version of this article (doi:10.1007/s12039-017-1347-6) contains supplementary material,
which is available to authorized users.

R M ANSARI and B R BHAT
reagents with alkyl, aryl and heteroaryl halides have series elemental analyzer. FTIR results were obtained on a
been studied.24
Bruker-Alpha ECO-ATR FTIR spectrophotometer. The spec-
tra were obtained as KBr pellets. Electronic spectra of ligand
There are various examples of activity of Schiff base
and complexes were measured on Analytik Jena SPECORD
complexes in homogeneous as well as heterogeneous
catalysis.25,26 N, O donor atom Schiff base complexes
S600 UV–Vis spectrophotometer in the 200–800 nm range.
The magnetic susceptibilities of the complexes were recorded
were intensively studied for catalytic and biological
at room temperature on a Sherwood UK magnetic balance,
properties.27 Transition metal complexes (apart from
Hg[Co(SCN)4] was used as a calibrant. Molecular mass was
Pd) with Schiff base have been widely used in applica-
determined using a Waters Q-ToF micro mass spectrometer
tions such as in polymer industry, dye industry, medici-
1
with an ESI source. The H NMR spectrum of the ligand
nal chemistry, agrochemical and biological activities.28
was recorded in Bruker AV 400 instrument using TMS as
Promising catalytic activity shown by these transition
metal complexes have encouraged the researchers to
an internal standard. Thermogravimetric measurements were
performed on (EXSTAR-6000) using nitrogen as the carrier
develop cheaper catalysts for coupling reactions com- gas (flow rate: 50 mL/min). The heating rate was 10◦C/min.
The coupling reaction product analysis was carried out using
Gas Chromatography (GC) (Shimadzu 2014, Japan), siloxane
Restek capillary column (30 m length and 0.25 mm diame-
ter) and Flame Ionization Detector (FID). The initial column
temperature was increased from 60 to 150◦C at the rate of
10◦C/min and then to 220◦C at the rate of 40◦C/min. Nitro-
gen gas was used as the carrier gas.
pared to the ones with Pd. Exploratory studies on
employing the first-row transition metal complexes as
precatalysts in coupling activity is an ongoing research
topic because of the several benefits like low cost and
abundance of metal, ease of synthesis, facile scale-up,
etc.29 Contributing to this subject of research, we have
attempted in this work to synthesize a few Schiff base
complexes of Fe, Co, Ni and Cu ions. The scope of our
research study is to throw more light on the chelation
behavior of Schiff base ligand towards the chosen metal
ions and their catalytic activity in the coupling of various
substituted aryl halides with phenylboronic acid.
2.3 Synthesis of Schiff base ligand (L)
4-aminoacetophenone (0.135 g, 1 mmol) and salicylaldehyde
(0.122 g, 1 mmol) were dissolved in ethanol and heated to
60◦C in a 50 mL round bottom flask. The reaction mixture was
thenrefluxedfor3hfollowedbyfiltrationofobtainedproduct,
washing with diethyl ether, re-crystallization using ethanol
and drying. Yield: 80.40%; Color: Yellow; C15H13NO2: Anal.
Found: C, 75.94; H, 5.40; N, 5.05% Calc.: C, 75.90; H, 5.48;
2. Experimental
N, 5.85%. Mol. Wt.: 239.00: FTIR (KBr) ( /cm−1): 3333
ν
2.1 Materials and reagents
ν
ν
(O–H), 1411–1490 ( C=C aromatic), 1577 ( C=N), 1279
1
ν
ν
ν
( C–O), 1112–1166 ( C–N), 3046.22 ( C–H); H NMR (δ,
ppm in CDCl3): 12.87 (s, 1H); 8.63 (s, 1H); 8.04 (d, 2H); 8.02
(d, 2H); 7.46–6.94 (m, 4H); 2.60 (s, 3H) (Scheme 1).
All chemicals were of analytical reagent (AR) grade and
used without any further purification. 4-aminoacetophenone,
salicylaldehyde, nickel acetate tetrahydrate, copper acetate
monohydrate, cobalt acetate tetrahydrate and ferric chloride
hexahydrate, potassium carbonate, sodium carbonate were
purchased from Merck India and used as received. Ace-
tonitrile, 1,4-dioxane, toluene, ethyl alcohol, diethyl ether,
dichloromethane (DCM), potassium tertiary butoxide, cesium
carbonate and triethylamine used in the study were purchased
from Sigma-Aldrich.
2.4 Synthesis of metal complexes
2.4a Synthesis of complex C-1: The metal complex C-
1 was prepared by the addition of a hot solution of the ferric
chloride (0.270 g, 1 mmol) in 50% v/v mixture of ethanol in
water (5 mL) to the hot solution of the synthesized Schiff base
(0.239g, 2mmol)inthesamesolventsystem(5mL). Themix-
ture was refluxed with continuous stirring for 5 h. The reaction
mixture was filtered, washed with methanol and dried. Yield:
84.50%;Color:Darkpurple;C30H24ClFeN2O4:Anal. Found:
2.2 Instrumentation
The C, H and N contents of the ligand and metal complexes C, 59.92; H, 4.35; N, 4.45% Calc.: C, 59.83; H, 4.33; N,
were determined using microanalysis Thermoflash EA1112 4.63%. Mol. Wt.: 568.10: FTIR (KBr) ( /cm−1): 3351.84
ν
N
NH2
Ethanol
Reflux
+
O
OH
H
3COC
H
3COC
L
OH
Scheme 1. Synthesis of Schiff base ligand (L).

Schiff base transition metal complexes
COCH3
COCH3
COCH3
Cl
Fe
O
N
HC
CH
N
FeCl3.6H2O
N
+
O
Ethanol Water
/
OH
H
3COC
C-1
COCH3
Scheme 2. Synthesis of complex C-1.
O
.4H O
N
N
HC
Co(OCOCH3)2
2
CH
N
Co
+
Ethanol Water
/
OH
O
H
3COC
C-2
COCH3
Scheme 3. Synthesis of complex C-2.
O
.4H O
N
N
HC
Ni(OCOCH3)2
2
CH
N
Ni
+
Ethanol Water
/
OH
O
H
3COC
C-3
COCH3
Scheme 4. Synthesis of complex C-3.
ν
ν
(O–H), 1425–1475 ( C=C aromatic), 1596 ( C=N), 1279 system (5 mL). The mixture was refluxed with continuous
ν
ν
ν
( C–O), 1112–1166 ( C–N), 3061.86 ( C–H) (Scheme 2).
stirring for 5 h. The reaction mixture was filtered, washed
with diethyl ether and dried. Yield: 78.0; Color: Bright green;
C30H24N2NiO4: Anal. Found: C, 67.00; H, 4.81; N, 5.11%
Calc.: C, 67.−017; H, 4.88; N, 5.21%. Mol. Wt.: 535.85: FTIR
2.4b Synthesis of complex C-2: The metal complex
C-2 was prepared by the addition of a hot solution of the
cobalt acetate tetrahydrate (0.249 g, 1 mmol) in 50% v/v
mixture of ethanol in water (5 mL) to the hot solution of
the synthesized Schiff base (0.239 g, 2 mmol) in the same
solvent system (5 mL). The mixture was refluxed with con-
tinuous stirring for 5 h. The reaction mixture was filtered,
washed with methanol and dried. Yield: 68.60%; Color:
Orange; C30H24CoN2O4: Anal. Found: C, 64.31; H, 4.75;
N, 4.56% Calc.: C, 64.43; H, 4.90; N, 4.70%. Mol. Wt.:
ν
ν
(KBr) ( /cm ): 3366 (O–H), 1419–1445 ( C=C aromatic),
ν
ν
ν
1588 ( C=N), 1279 ( C–O), 1124–1178 ( C–N), 3056.73
ν
( C−H) (Scheme 4).
2.4d Synthesis of complex C-4: Copper complex C-4
was prepared by the addition of a hot solution of the copper
acetate monohydrate (0.199 g, 1 mmol) in 50% v/v mixture
of ethanol in DCM (5 mL) to the hot solution of the synthe-
sized Schiff base (0.239 g, 2 mmol) in ethanol (5 mL). The
mixture was refluxed with continuous stirring for 5 h. Formed
product was filtered and washed with 50% ethanol-water mix-
ture, diethyl ether and dried. Yield: 80.50%; Color: Brown;
535.90: FTIR (KBr) ( /cm−1): 3421.85 (O–H), 1356–1437
ν
ν
ν
ν
ν
( C=C aromatic), 1556.55 ( C=N), 1264 ( C–O), 3063 ( C–
H) (Scheme 3).
2.4c Synthesis of complex C-3: Nickel complex C-3 C30H24CuN2O4: Anal. Found: C, 66.68; H, 4.40; N, 5.17%
was prepared by the addition of a hot solution of the nickel Calc.: C, 66.−712; H, 4.48; N, 5.19%. Mol. Wt.: 539.00: FTIR
ν
ν
acetate tetrahydrate (0.248 g, 1 mmol) in 50% v/v mixture (KBr) ( /cm ): 3446 (O–H), 1442–1531 ( C=C aromatic),
ν
ν
ν
of ethanol in water (5 mL) to the hot solution of the syn- 1606 ( C=N), 1270 ( C–O), 1150–1181 ( C−N), 2993.81
ν
thesized Schiff base (0.239 g, 2 mmol) in the same solvent ( C−H) (Scheme 5).

R M ANSARI and B R BHAT
COCH3
O
N
HC
CH
N
Cu
.H O
DCM /
N
Cu(OCOCH3)2
Ethanol
2
+
OH
O
H
3COC
C-4
COCH3
Scheme 5. Synthesis of complex C-4.
2.5 X-ray crystallography
Ligand and complex (C-4) crystal were obtained by a slow
evaporation method using 50% DCM in ethanol as solvent.
The X-ray diffraction studies for these crystals were per-
formed on a Bruker APEX-II CCD diffractometer with Mo
α
K radiation ( = 0.71073 Å) at 296 K. The structure was
λ
o
solved using SHELXL-2007/2014 software and refined by
full matrix least square methods.
2.6 General procedures for the Suzuki reaction
Aryl halide (1.0 mmol) was added to a mixture of phenyl-
boronic acid (1.5 mmol), Schiff base complex (0.02 mmol)
and base (3.0 mmol) in 3 mL of solvent and heated to 80◦C
for 16 h. The mixture was then cooled to room temperature.
After cooling, the organic phase was analyzed by gas chro-
matography. Internal standard was used and calibrated against
each and every one of the cross-coupling products. Biphenyl
was obtained as a side product due to the homocoupling of
phenylboronic acid.
Figure 1. UV–Vis spectra of ligand and complexes in
ethanol (Concentration=10−3 M, Path length=1 cm).
which appear around 400–500 cm−1 in the spectra of the
complexes may be assigned to the coordination of metal
with a nitrogen atom.
3.3 Electronic absorption spectra
The electronic spectra of synthesized ligand and com-
plexes were recorded in ethanol solvent (10−3 M)
(Figure 1). Quartz cells with a 1 cm pathlength were
employed in the 200–700 nm spectral range. The peak in
the region 218 nm is assigned to the π−π* transitions of
aromatic rings. The peak at 310–350 nm involves n−π*
transition of the CH=N group.31 Iron complex shows
bands at 210, 322, 511 nm (449–563 nm). In cobalt com-
plex, peaks at 208, 263, 296, 354 nm (460–514 nm) were
observed. Similarly, nickel complex shows two peaks
at 215 and 321 nm which are due to π−π*, n − π*
transitions, respectively. The electronic spectrum of Cu
complex shows peaks at 213 and 315 nm, 402, 512 nm
due to π−π* and n − π* respectively.
3. Results and Discussion
3.1 1H NMR and mass spectra
1
The H NMR (400 MHz) spectrum of ligand (L) was
recorded in CDCl3 (Figure S1 in Supplementary Infor-
mation). The electron impact mass spectra of the Schiff
base and metal complexes were recorded and investi-
gated at 50 eV of electron energy. The important mass
fragmentations of the ligand and complexes are shown
in the spectra (Figures S2–S6 in Supplementary Infor-
mation).
3.2 FTIR spectra
The synthesized ligand and complex molecules were
examined by the FTIR analysis (Figures S7–S11 in Sup-
3.4 Magnetic susceptibility measurements
plementary Information). The new bands in the range The magnetic moment for iron complex C-1 was found
580–672 cm−1 in complexes, tentatively assign coordi- to be 1.80 BM, which indicates that the complex is para-
30
ν
nation of metal with oxygen atom (M–O). The bands magnetic with one unpaired electron present (Table 1).

Schiff base transition metal complexes
1. Magnetic 3.5 Thermogravimetric analysis
Table
moment of the complexes.
The thermal degradations of synthesized complex (C-1
and C-2) were studied using temperature range from 50
to 700◦C. The data from the thermogravimetric analysis
clearly indicates the decomposition of the ligand in the
temperature range of 180–450◦C. Removal of the lig-
and molecule proceeds in a single step; further complete
decomposition was observed at ≥500◦C (Figure 2).
Compound
μ eff. (BM)
C-1
C-2
C-3
C-4
1.80
1.73
0.33
1.65
3.6 Structure of L and complex C-4
SelectedbondlengthsandbondanglesarelistedinTable
S1 (Supplementary information). The molecular struc-
tures of L and C-4 are shown in (Figure 3(a) and (b)).
3.7 Catalytic activity studies of synthesized
complexes for Suzuki reaction
3.7a Effect of solvent: The catalytic efficiency of the
synthesizedcomplexesC-1toC-4wasstudiedinSuzuki
reaction for cross-coupling of 4-bromobenzonitrile and
phenylboronic acid (Base: K2CO3, Concentration of cat-
alyst: 0.02 mmol) in various solvent media (Figure 4).
Among the different solvents used, highest catalytic
activity was observed with acetonitrile. Moderate cat-
alyst activities were found in other solvents such as
ethanol, THF, 1,4-dioxane and toluene. Acetonitrile was
chosen as the optimum solvent for carrying out the reac-
tions. The reaction is found to work out best at its reflux
temperature.
Figure 2. TGA curve of complex C-1 and C-2.
The observed magnetic moment supports the iron com-
plex to be in low-spin Fe (III) state. Similarly, cobalt
complex C-2 shows the magnetic moment of 1.73 BM
with one unpaired electron which indicates that cobalt
complex possesses low spin Co (II) state. The almost
zero magnetic moment of nickel complex C-3 clearly
confirms the absence of any unpaired electrons. It also 3.7b Effect of base: Further, the effect of a base on the
affirms the diamagnetic nature of the complex with reaction performance was studied using different bases
a possible square planar geometry.32 The magnetic such as K2CO3, Na2CO3, Et3N, Cs2CO3 and KOtBu in
moment of 1.65 BM supports the copper complex C-4 acetonitrile solvent (Figure 5). As shown in Figure 5,
to be in high spin Cu (II) state with tetrahedral geometry. Et3N is the desirable base for the reaction with high
Figure 3. Single crystal structure of (a) ligand (L) and (b) copper Schiff base complex C-4 (displacement
ellipsoids are drawn at the 30% probability level).

R M ANSARI and B R BHAT
Figure 6. Effect of catalyst concentration.
Figure 4. Effect of solvent on Suzuki coupling.
Figure 7. Effect of catalyst concentration.
due to the oxidative addition step of the catalytic cycle
getting affected in the presence of increased concentra-
tion of catalyst. Thus, we preferred Et3N as the base,
acetonitrile as solvent, and 0.02 mmol of the catalyst as
the optimal conditions for the reaction.
Figure 5. Effect of base on Suzuki coupling.
product yield. In the absence of catalyst, coupling prod-
uct was formed with very less yield using triethylamine
base (15.5%) (Scheme 6).
3.7d Effect of time: Moreover, product yield strictly
depends on the reaction time. This was investigated by
analyzing the reaction mixture at regular intervals of
3.7c Effect of catalyst concentration on the reaction:
Since catalyst loading in a reaction significantly decides time. The product conversion was observed to increase
the catalytic efficiency, the amount of catalyst required with reaction time till the completion at 16 h, beyond
for effective conversion was another parameter selected which extent of conversion remained almost insignifi-
and studied. Figure 6 shows the effect of catalyst loading cant (Figure 7). This led us to choose optimal reaction
on the yield of reaction. The yield was found to increase time for coupling as 16 h.
with the increase in the amount of catalyst used. How-
Further, different substituted aryl halides were used to
ever, after the catalyst concentration of 0.02 mmol, the extend the cross-coupling reaction with phenylboronic
yield of the reaction showed a negative trend, possibly acid using the optimized reaction conditions. The results
(0.02 mmol)
Catalyst
Ph
CN
+
Br
CN
B(OH)2
CH3CN
Scheme 6. Suzuki coupling of 4-bromobenzonitrile and phenylboronic acid using synthesized catalyst.

Schiff base transition metal complexes
Table 2. Catalytic studies of synthesized complex for Suzuki–Miyaura cross-coupling reac-
tions of aryl halides with phenylboronic acid.
Entry
R
X
Yielda,b
C-1
C-2
C-3
C-4
1
2
3
4
5
6
7
8
9
10
OCH3
H
CN
Br
64
60
50
78
83
74
50
54
40
58
79
64
52
80
82
73
55
60
42
65
81
65
62
90
80
72
65
62
50
75
91
55
82
86
75
59
61
46
72
86
COCH3
NHCOCH3
OH
CH3
F
OH
CN
I
I
a Reaction conditions: Aryl halide (1 mmol), phenylboronic acid (1.5 mmol), Et3N (3.0 mmol),
catalyst (0.02 mmol), solvent (3 mL), 16 h.
bGC Yield.
for different substituents are summarized in Table 2. An S2 to S6) and FTIR spectra (Figures S7–S11) are given
internal standard was used and calibrated against each in the supporting information available at www.ias.ac.
and every one of the cross-coupling products. Biphenyls in/chemsci.
were obtained as a side product due to the homocoupling
of phenylboronic acid.
Acknowledgements
The authors convey their genuine thanks to the National
Institute of Technology, Surathkal (Karnataka) for character-
ization facilities and financial support.
4. Conclusions
In conclusion, an efficient, cheap, simple, convenient
and environmentally benign Schiff base transition metal
(Fe, Co, Ni and Cu) complex has been synthesized
and characterized. Suzuki–Miyaura cross-coupling of
various aryl halides and phenylboronic acids was cat-
alyzed using the synthesized complexes. Among all the
complexes, copper complex-catalyzed Suzuki reaction
showed good yield. The reaction proceeds efficiently
under a mild condition and the corresponding biphenyls
were obtained in excellent yields. This catalytic system
is more attractive because it is very cheap compared with
palladium. The absence of reducing agent and phos-
phine ligand in the catalytic system together are other
advantages of this system.
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and L and complex C-4 have been deposited with the
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