RETRACTED ARTICLE: Catalytic activity of Fe(II) and Cu(II) PNP pincer complexes for Suzuki coupling reaction

Article abstract of DOI:10.1002/aoc.4054

Iron and copper PNP pincer complexes of the type [Fe(L)SO₄] and [Cu(L)OCOCH₃] are reported and represented as C-1 and C-2 catalyst. Both the complexes were synthesized using bis(diphenylphosphino)pyridine-2,6-diamine [L], and salts of ‘Fe’ and ‘Cu’ by direct coordination method. The as synthesized complexes were characterized using FTIR, UV–Vis, mass analysis and TGA. The effect of reaction time, catalyst load, solvent and base on the reaction between phenylboronic acid and para substituted bromobenzenes in the presence of the catalysts were investigated for evaluating the catalytic efficiency of the complexes. The results obtained highlight the enhanced C-C coupling reactions with the use of 0.4?mol% of the catalyst C-1 in 14?h and 0.6?mol% of C-2 in 16?h respectively with Cs₂CO₃ base and ACN as solvent media. Of the two complexes reported, C-1 with iron as catalytically active metal is more stable and active towards coupling which is reflected in its better coupling yields in lesser reaction time compared to copper bearing C-2 complex.

Full text of DOI:10.1002/aoc.4054

Received: 24 May 2017
DOI: 10.1002/aoc.4054
Revised: 21 July 2017
Accepted: 23 July 2017
F U L L P A P E R
Catalytic activity of Fe(II) and Cu(II) PNP pincer complexes
for Suzuki coupling reaction
Lolakshi Mahesh Kumar
| Rasheeda M. Ansari | Badekai Ramachandra Bhat
Catalysis and Materials Laboratory,
Department of Chemistry, National
Institute of Technology Karnataka,
Surathkal, Srinivasnagar, ‐575025, India
Iron and copper PNP pincer complexes of the type [Fe(L)SO ] and [Cu(L)
4
OCOCH ] are reported and represented as C‐1 and C‐2 catalyst. Both the com-
3
plexes were synthesized using bis(diphenylphosphino)pyridine‐2,6‐diamine
[L], and salts of ‘Fe’ and ‘Cu’ by direct coordination method. The as synthesized
complexes were characterized using FTIR, UV–Vis, mass analysis and TGA.
The effect of reaction time, catalyst load, solvent and base on the reaction
between phenylboronic acid and para substituted bromobenzenes in the pres-
ence of the catalysts were investigated for evaluating the catalytic efficiency of
the complexes. The results obtained highlight the enhanced C‐C coupling reac-
tions with the use of 0.4 mol% of the catalyst C‐1 in 14 h and 0.6 mol% of C‐2 in
Correspondence
Badekai Ramachandra Bhat, Catalysis and
Materials Laboratory, Department of
Chemistry, National Institute of
Technology Karnataka, Surathkal,
Srinivasanagar‐575025, India.
Email: ram@nitk.edu.in
1
6 h respectively with Cs CO base and ACN as solvent media. Of the two com-
2 3
plexes reported, C‐1 with iron as catalytically active metal is more stable and
active towards coupling which is reflected in its better coupling yields in lesser
reaction time compared to copper bearing C‐2 complex.
KEYWORDS
pincer ligand, PNP, Suzuki‐Miyaura cross‐coupling
1
| INTRODUCTION
which despite having unsurpassed catalytic activity is less
[
13–15]
abundant and expensive. The metals like nickel
and
[
16,17]
Transition metals have displayed excellent catalytic activ-
ity enabling several unfeasible chemical processes both
viable as well as productive. Process viability depends
majorly on the overall cost and ease of preparation of
the pre‐catalysts which needs to be based on abundant
metals and inexpensive ligands. Complexes bearing pin-
cer ligands are among the organometallic complexes that
have been established and recognized as effective pre‐cat-
alysts for various reactions. Their unique metal bound
structures, high thermal stability, and process viability
makes them the most active catalysts for organic transfor-
ruthenium
are also employed which show good
activity, but deters the green motive of catalysis by being
environmentally less benevolent. In this context, the
study and development of catalytic reactions promoted
by inexpensive and environmentally benign metals is sig-
nificant and perpetual research activity in the field of
catalysis. In present work, we have considered the price
of metals and cost of ligand synthesis which led us to
focus our studies on the first‐row transition metal com-
plexes with a PNP‐pincer ligand. Among the first‐row
[1]
[
18–29]
[30–35]
transition metals, iron
and copper
are widely
[2]
mations involving the activation of bonds. Pincer com-
plexes are also well known to catalyze Suzuki‐Miyaura
cross‐coupling reaction with high catalytic activity and
studied for their catalytic potential. The importance of
these metals for developing catalysts is majorly due to
their low cost and toxicity. Iron (II) PNP pincer complexes
[3–6]
selectivity.
However, the most emphasized complexes
were synthesized and utilized for heterolytic cleavage of
[
3–12]
[27]
catalyzing this coupling reaction are with palladium,
dihydrogen via metal–ligand cooperation.
A new class
Appl Organometal Chem. 2017;e4054.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 8
https://doi.org/10.1002/aoc.4054

2
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KUMAR ET AL.
of iron hydride complexes bearing phosphinite‐based pin-
cer ligands was demonstrated as competent catalysts for
the hydrosilylation of aldehydes and ketones with differ-
temperature and refluxed overnight. The solution was
then filtered, washed with anhydrous hexane (2 x 10 ml),
and the solvent was removed under vacuum to afford
orange colored crude ligand. The crude compound was
purified by column chromatography and pure ligand (L)
was obtained as white solid (Figure 1).
[28]
ent functional groups.
Copper and copper/noble metal
combination nanoclusters were found to be active cata-
lysts in the Suzuki cross‐coupling of various aryl halides
[31]
with phenylboronic acid.
Iron(II) complexes bearing
Yield: 87.2%. Melting point: 152°C.
1
tridentate PNP pincer ligands were employed as catalysts
H NMR (400 MHz, CDCl ) δ 7.82–7.62 (m, 9H),
3
for the selective formation of 3‐hydroxyacrylates from aro-
7.61–7.54 (m, 3H), 7.53–7.40 (m, 8H), 7.40–7.27 (m, 3H),
3.78–3.49 (br s, 2H, NH) (Supplementary Information
Figure S1).
[36]
matic aldehydes and ethyldiazoacetate.
Iron(II) com-
plexes of the types [Fe(PNP)Br ] and [Fe(PNP)(CO)Br ]
2
2
31
1
with PNP pincer ligands based on triazine and pyridine
backbones were tested as catalysts for the alkylation of
P{ H}NMR (161.8 MHz, DMSO): δ (ppm) 25.46
(s, PN) (Supplementary Information Figure S2).
MS‐ESI: (m/z): 478.4 (Supplementary Information
Figure S3).
[37]
amines by alcohols.
Fe(II) complexes stabilized by a
PNP ligand based on the 2,6‐diaminopyridine scaffold
were reported to efficiently couple the alcohols and
Elemental analysis calculated for C H N P
25 3 2
29
[38]
amines.
well‐defined Cu(I) PNP pincer complex based on
,6‐diaminopyridine which actively catalyzed the cross‐
An air‐stable, thermally robust, and
(M = 477.1): C, 72.95; H, 5.28; N, 8.80. Found: C, 72.15;
r
H, 5.03; N, 8.56%.
2
couplings of a range of aryl and heteroaryl halides with
different organomagnesium reagents, alkynes, and aryl‐
amines giving excellent to good isolated yields was also
2
.3 | Synthesis of [Fe(L)SO ]and [Cu(L)
4
OCOCH ] pincer complexes (C‐1 and C‐2)
3
[39]
reported by the same group.
However, PNP pincer
The prepared ligand (0.5 mmol) was taken in dry THF
and refluxed with 0.5 mmol of FeSO .(H O) (0.5 mmol)
and Cu(OCOCH ) .H O (0.5 mmol) in separate round
bottom flasks for 4 h, respectively. The obtained precipi-
tates were decanted and washed with dry ether followed
by filtration and drying at 50 °C in a vacuum oven for
complex with iron or copper as active metal center have
not been reported for Suzuki coupling reactions. There-
fore, here we report the catalytic activity of iron and cop-
per based PNP pincer complexes in Suzuki coupling
reactions.
4
2
7
3
2
2
4
h to obtain complexes C‐1 and C‐2, respectively
(
Figure 2).
2
2
| EXPERIMENTAL
.1 | Materials
2
1
.3.1 | [Fe(SO ){C H N‐2,6‐(NHPPh ) }](C‐
)
4
5
3
2 2
Iron(II) sulphate heptahydrate and copper(II) acetate
monohydrate were purchased from Merck, India and
used
chlorodiphenylphosphine, phenylboronic acid, and aryl
halides were purchased from Sigma‐Aldrich and tetrahy-
Yield: 80.9%.
as
received.
2,6‐diaminopyridine,
+
MS‐ESI: (m/z): 630.3 [M] ((Supplementary Informa-
tion Figure S4).
drofuran (THF), triethylamine (Et N), acetonitrile
3
(ACN) were purchased from Alfa‐Aesar and used without
further purification.
2
6
2
.2 | Synthesis of (N ,N ‐
bis(diphenylphosphino)pyridine‐2,6‐
diamine) (ligand = L)
FIGURE 1 Synthesis of ligand, L
The ligand was synthesized as per the reported procedure
[40]
with certain modifications. In a typical synthesis pro-
cess, triethylamine (1.85 g, 18.3 mmol) was dissolved in
THF (20 ml) with 2,6‐diaminopyridine (1 g, 9.2 mmol).
The mixture was cooled to 0 °C followed by drop wise
addition of chlorodiphenylphosphine (4.04 g, 18.3 mmol)
under stirring. The solution was brought to room
FIGURE 2 Complexes C‐1 and C‐2

KUMAR ET AL.
3 of 8
Elemental analysis calculated for C H FeN O P S
M = 629.1): C, 55.34; H, 4.00; N, 6.68. Found: C, 55.15;
H, 3.92; N, 6.51%.
3 | RESULTS AND DISCUSSION
3.1 | Characterization studies
29
25
3 4 2
(
r
The following section describes the confirmation of the
prepared pincer complexes by using different analytical
techniques like FTIR, UV–Vis, magnetic susceptibility
and TGA. Figure 3 (a) represents the FTIR spectra of the
complexes C‐1 and C‐2 in comparison with the ligand
and 2,6‐diaminopyridine (precursor for ligand synthesis).
The significant decrease in the characteristic N‐H
stretching peak from the starting material to the ligand
indicates the formation of N‐P bond at the expense of
N‐H bond. It is further confirmed by the presence of dou-
2
.3.2 | [Cu(OCOCH ) {C H N‐2,6‐
3
2
5
3
(NHPPh ) }](C‐2)
2
2
Yield: 75.3%.
MS‐ESI: (m/z): 659.5 [M] (Supplementary Information
Figure S5).
Elemental analysis calculated for C H CuN O P
3 4 2
+
33
31
(M = 658.1): C, 60.13; H, 4.74; N, 6.38. Found: C, 59.85;
r
H, 4.47; N, 6.12%.
−
1
blet in the 690–740 cm region of spectra of ligand and
[
41]
complexes which is characteristic to the P‐N stretching
2.4 | General procedure for the Suzuki
[
42]
and P‐C stretching , respectively. Figure 3 (b) is the
reaction
−
1
fingerprint region of the spectra within 1600–400 cm
and shows the evidence of metal‐N bond formation.
,
The prepared pincer complex (C‐1/C‐2) were added to the
mixture of phenylboronic acid (1.3 mmol) and a base in a
dry solvent (5 ml) and allowed to stir for 30 min. Aryl
halide (1.0 mmol) was added slowly to the mixture and
refluxed for the reaction to complete. The progress of
the reaction was monitored by thin layer chromatography
and gas chromatography. The completed reaction was
cooled to room temperature and the organic phase was
decanted and washed with 1.5 N HCl and brine solution
followed by drying with sodium sulphate, and concen-
trated to obtain a crude product. The crude product
obtained was purified using column chromatography.
−
1
Stretching peak at 491 cm in C‐1 is due to the Fe‐N
bond stretching and at 479 cm in C‐2 is because of the
−
1
[
43]
Cu‐N bond stretching.
To further confirm the complex formation, UV–Vis
spectra was examined for the complexes, C‐1 and C‐2 in
comparison to the ligand (L) as shown in Figure 4. From
2.5 | Characterization methods
The C, H and N contents of the compounds were deter-
mined by Thermoflash EA1112 series elemental analyzer.
Magnetic susceptibility measurement was recorded on a
Sherwood Scientific magnetic susceptibility balance
(UK). TGA was carried out from room temperature to
7
00 °C at a heating rate of 10 °C/min on EXSTAR‐6000.
The UV–Vis spectrum of the complex was recorded using
Analytik Jena SPECORD S600. FT‐IR spectra were
recorded on a Bruker‐Alpha ECO‐ATR FTIR spectrome-
1
ter. H NMR (400 MHz) spectrum was recorded in Bruker
31
1
AV 400 using TMS as an internal standard. P{ H} NMR
161.8 MHz) spectrum was recorded in VARIAN using
H PO as an internal standard. The molecular mass of
(
3
4
the compounds was determined using a Waters Q‐TOF
micro mass spectrometer with an ESI source. The
coupling reaction product analysis was carried out using
Gas Chromatography (GC) (Shimadzu 2014, Mangalore,
Japan) with siloxane Restek capillary column (30 m
length and 0.25 mm diameter) and Flame Ionization
Detector. Nitrogen was used as the carrier gas.
FIGURE 3 FTIR spectra of ligand and complexes C‐1 and C‐2: (a)
at 4000–400 cm (b) 1600–400 cm
−
1
−1

4
of 8
KUMAR ET AL.
FIGURE 4 12UV–vis spectrum of ligand and complexes C‐1 and
C‐2
FIGURE 5 TGA curves of the complexes C‐1 and C‐2
the spectra, it is observed that the ligand has broad
absorption bands at 204 nm, 264 nm, and 289 nm. On
complexation, the spectra showed a red shift in absorption
peak from 204 nm to 213 nm in C‐1 and to 208 nm in C‐2.
Also a decrease in intensity of ligand peak at 264 nm is
observed for both C‐1 and C‐2 and the absorption at
two complexes, C‐1 (Fe pincer complex) is stable at
relatively lower temperature and likely to possess better
efficiency as a catalyst.
3.2 | Catalytic activity studies
2
89 nm in the ligand have intensified with red shifts in
The catalytic efficiency of the prepared complexes was
examined for the Suzuki coupling reaction. The study
included the reaction between aryl halides with phenyl
boronic acid in presence of a base, solvent and prepared
C‐1 and C‐2 pincer complex as catalyst. To achieve high
catalytic efficiency, yield and optimized reaction condi-
tions, various factors including solvent, catalyst loading,
alkalinity, temperature, and time were optimized. The
reaction between 4‐bromobenzonitrile and phenyl
boronic acid was selected as a model reaction to evaluate
the catalytic activity of the pincer complexes.
both the complexes. New peaks appear for the complexes,
C‐1 and C‐2 at 331 nm and 338 nm respectively.
Bathochromic shift observed from the free ligand to the
complexes indicates a charge transfer phenomenon and
it can be inferred that the transitions observed in the
spectra are majorly due to intra‐ligand charge transfer
transitions (ILCT) and ligand to metal charge transfer
transitions (LMCT). A small and broad peak at 655 nm
in C‐2 spectra signifies a possible d‐d transition for the
Cu complex.
The magnetic susceptibility measurement shows that
the complex C‐1 is paramagnetic at room temperature
with magnetic moment value of 2.97 BM supporting the
The solvent effect was examined for the reaction. The
most commonly used solvents in Suzuki coupling reac-
tions like ethanol, 1,4‐dioxane, toluene, and acetonitrile
were screened for the selection of optimized solvent. The
time taken for the completion of reaction with selected
solvents are represented in Figure 6 (a, b) for C‐1 and
C‐2 complexes respectively. From the figure, it is observed
that the pincer complex with ACN shows greater catalytic
efficiency and higher reaction yield than toluene and
ethanol, which can be attributed to the coordinating
nature of the ACN in the reaction process and easy
[44]
formation of Fe(II) complex.
Complex C‐2 also showed
paramagnetic behavior with the magnetic moment of 1.77
BM indicating a Cu(II) complex.
To determine the thermal behavior and stability of the
complexes, TGA analysis was performed from room
temperature to 700 °C at a heating rate of 10 °C/min.
The TGA results of C‐1 and C‐2 complex are represented
in Figure 5. Thermogram of C‐1 shows the thermal
decomposition around 500 °C in a single step process
which is attributed to the excellent thermal stability
expected of a pincer complex. Thermogram of C‐2 repre-
sents a two‐step dissociation, wherein the first dissocia-
tion begins at 130°C and continues up to 230°C showing
a cumulative weight loss of about 22%, constituting the
dissociating acetate ligands along with water of crystalli-
zation. The dissociating pattern infers that among the
[
45]
miscibility of reactants and the catalyst
. Even though
there is no significant difference observed in the reaction
yield and reaction time for ACN and dioxane as a solvent,
the lower reflux temperature of ACN makes it suitable
solvent for the catalyzed reaction.
The reaction was also optimized for efficient usage of
the base in the reaction. In this context, we investigated
the Suzuki coupling reaction with the use of Cs CO
2
3,

KUMAR ET AL.
5 of 8
catalyzed reaction yields 89.9%, whereas C‐2 yields 76.1%
of the product. Hence among the different bases
employed in the Suzuki reaction, Cs CO was considered
2
3
as a suitable base to achieve enhanced catalytic efficiency
and greater yield.
To study the effect of the concentration of catalyst, the
reaction was studied under optimized conditions of ACN
as solvent and Cs CO as a base, in the presence of differ-
2
3
ent amount of catalyst. Figure 7 shows the yield vs. cata-
lyst load study where the catalyst loads was varied from
0.1 mol% to 1.0 mol%. From the figure, it is observed that
the increase in catalyst loading linearly increases the cou-
pling yield, suggesting that the amount of catalyst plays a
crucial role in yield of the reaction. When the catalyst load
reaches 0.4 mol% and 0.6 mol% for C‐1 and C‐2 respec-
tively, the maximum conversion was observed. However,
with further raise in the catalyst load, there was no signif-
icant contribution to the product yield. Thus, from the
obtained results, we considered 0.4 mol% of the catalyst
C‐1 and 0.6 mol% of C‐2 to be the optimized catalyst load.
Effect of reaction time on the catalytic activity was
studied by analyzing the reaction aliquots at regular inter-
vals of time. Figure 8 shows GC yield vs. time taken to
complete the reaction in presence of C‐1 and C‐2 catalyst.
From the figure, it is observed that with optimized condi-
tions like catalyst concentration, alkalinity, and solvent,
the C‐1 shows maximum yield after 14 h as compared to
16 h for C‐2. However, further raise in the reaction time
does not contribute much to the increase in product yield
and the graph shows almost steady‐state yield. Thus, 14 h
and 16 h are the optimized reaction time for C‐1 and C‐2
catalysts respectively.
FIGURE 6 Effect of solvent on (a) C‐1, (b) C‐2
t
Et N, K CO Na CO and KO Bu in ACN solvent. The
3
2
3,
2
3
different bases produced different yields as represented in
Table 1. From the results, it is observed that Cs CO pro-
Further, all reactions between substituted aromatic
halides and phenylboronic acid were carried out with
the optimized conditions like Cs CO base, ACN solvent,
2
3
vide better yield than other bases used in the study which
can be attributed to the ability of the base to easily donate
its carbonate ion for replacement of the halide ion in the
2
3
0.4 mol% of C‐1 with a reaction time of 14 h and
[46]
oxidative addition step of the catalytic cycle
. The C‐1
TABLE 1 Study of effect of base on Suzuki coupling of 4‐
bromobenzonitrile and phenyl boronic acid using catalyst C‐1 and
C‐2
a
Yield (%)
Entry
Solvent
Base
Na CO
CO
C‐1
C‐2
1
ACN
2
3
74.8
65.1
2
3
4
5
K
2
3
81.8
89.9
66.7
82.4
71.2
76.1
58.0
71.7
Cs
2
CO
3
t
KO Bu
3
Et N
Reaction conditions: 4‐bromobenzonitrile (1.0 mmol), phenylboronic acid
1.3 mmol), base (2.0 mmol), catalyst (0.5 mol %), solvent (5.0 ml), 16 h.
(
a
GC yields
FIGURE 7 Effect of catalyst loading on reaction

6
of 8
KUMAR ET AL.
participation of the Fe in the rate determining oxidative
addition step can result in the formation of a more stable
intermediate compared to that in the case of Cu based C‐2
catalyst. This is because, during reduction of the pre‐
catalyst to its catalytically active form, C‐1 will lose its sul-
phate group, and in the oxidative addition step it further
acquires Fe(II) state which is a stable oxidation state of
iron. Thus, a stable intermediate is formed by the oxida-
tive addition process. However, Cu in C‐2 may lose a
single acetate ion to form Cu(I) state or both the acetate
[
47]
ions to form active Cu(0) species
. Considering this,
the formation of less stable Cu(III) intermediate in the
oxidative addition cycle along with the stable Cu(II) inter-
mediate state is possible. This largely explains the lower
coupling yield shown by the C‐2 complex in this study.
FIGURE 8 Effect of reaction time on C‐1 and C‐2
0
.6 mol% of C‐2 with a reaction time of 16 h respectively.
The coupling reaction results are summarized in Table 2.
From the tabulated results, it is observed that electron
donating substituents rather slowed the cross‐coupling
process, whereas the electron withdrawing substituents
were found to significantly increase the coupling
efficiency. Among the aryl halides, 4‐iodobenzonitrile
showed maximum GC conversion of 96.0% for C‐1 and
8
3.0% for C‐2. The result conveys that the leaving capacity
of iodide group is better among the halides.
Within the two catalysts employed in this study,
observed conversion rate was relatively high for C‐1
which is a Fe‐based complex. The enhanced and ready
TABLE 2 Catalytic activity study for complexes
a
Yield (%)
Entry
1
X
R
C‐1
C‐2
Br
H
64 (56.5)
51
2
3
4
5
6
7
8
9
CN
92 (84.4)
79 (69.3)
89 (76.3)
87 (80.2)
42 (34.0)
65 (56.4)
78 (70.0)
96 (89.6)
74 (66.8)
79
69
76
77
37
53
69
83
64
OCH
3
COCH
3
NHCOCH
F
3
CH
3
I
OH
CN
1
0
Cl
CHO
Reaction conditions: Aryl halide (1.0 mmol), Phenylboronic acid (1.3 mmol),
Cs CO (2.0 mmol), catalyst (C‐1 ‐ 0.4 mol%, 14 h / C‐2 ‐ 0.6 mol%, 16 h), sol-
FIGURE 9 Proposed mechanism for the coupling reaction with
2
3
vent (5.0 ml),
C‐1 and UV–vis spectra supporting the formation of active catalyst
(b)
a
GC yields, average of 3 trials (Isolated yield).

KUMAR ET AL.
7 of 8
3
.3 | Proposed mechanism
of C‐2 as catalyst. We further conclude that synthesized
transition metal PNP pincer complex can be a viable alter-
native to the more commonly used Pd catalysts in the
cross‐coupling reactions.
Inference of catalysis mechanism for metal catalyzed C–C
coupling reaction needs a thorough understanding of the
process. In Figure 9, we attempt to provide a mechanistic
insight of the catalysis cycle for complex C‐1 which is
showing enhanced coupling activity among the two com-
plexes reported. Catalytically active Fe(II) metal compo-
nent (a) complex gets reduced to Fe(0) in the presence
of a base and phenylboronic acid (b). This is followed by
the oxidative addition to aryl halide which leads to the
ACKNOWLEDGEMENTS
The author, Mrs. Lolakshi Mahesh Kumar acknowledges
National Institute of Technology Karnataka, Surathkal
for providing a research fellowship. We also thank
Dr. Reddy's Institute of Life Sciences, Hyderabad for
NMR and Mass analysis.
2
formation of R ‐Fe(II) intermediate (c). Further, in pres-
ence of the base Cs CO , the nucleophilic substitution of
2
3
the halide group and transmetallation takes place
2
between phenylboronic acid and R ‐Fe(L)‐CO (d) which
ORCID
3
1
2
would result in the formation of biaryl R ‐Fe(L)‐R (e) spe-
cies. Finally, there is the reductive elimination of biaryl as
product, and Fe(0) species is regenerated to continue the
catalytic cycle.
Lolakshi Mahesh Kumar http://orcid.org/0000-0002-9166-
2
223
Badekai Ramachandra Bhat
169-1055
http://orcid.org/0000-0002-
8
The conversion of pre‐catalyst (a) to the active catalyst
(b) via a reduction process in the reaction condition is fur-
ther confirmed by the UV–Vis spectroscopy. Absorption
spectra were obtained with the reaction mixture as a
reference. The spectra were recorded when the
catalyst was added to the reaction and at an intermediate
stage i.e.7 h of reaction. A sharp peak at 280 nm appears
on both the spectra is associated to Fe(II) state of pre‐
catalyst (a). Additionally, a small and broad peak is observed
in the spectrum of the intermediate stage at 352 nm which
corresponds to the formation of Fe(0) state (b) that is the
active catalyst in the coupling reaction.
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4
| CONCLUSIONS
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How to cite this article: Kumar LM, Ansari RM,
Bhat BR. Catalytic activity of Fe(II) and Cu(II) PNP
pincer complexes for Suzuki coupling reaction.
Appl Organometal Chem. 2017;e4054. https://doi.
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