Immobilization of an Aminobisphosphine–PdII Complex over Graphene Oxide: An Efficient and Reusable Catalyst for Suzuki–Miyaura, Ullmann Coupling and Cyanation Reactions

Article abstract of DOI:10.1002/ejic.201800291

The grafting of an aminobis(phosphine)–Pd^II complex (PNP–Pd^II) [PdCl₂{(Ph₂P)₂N(CH₂)₃Si(OMe)₃}] (2) on graphene oxide (GO) has been carried out by a condensation reaction between methoxysilane groups of 2 and hydroxyl groups of GO. The composite material was characterized by FTIR spectroscopy, solid-state ³¹P NMR spectroscopy, SEM, TEM, XPS and ICP-AES techniques. All these tools support the clean immobilization of compound 2 on GO. The composite material showed high catalytic activity in Suzuki–Miyaura, Ullmann coupling and cyanation reactions. The heterogeneity of the composite was confirmed by a hot filtration test. The immobilized PNP–Pd^II shows comparable activity to its homogeneous analogue 2. The recycling ability of the catalyst was examined for five consecutive runs, which showed little or no reduction in its catalytic efficiency.

Full text of DOI:10.1002/ejic.201800291

A Journal of
Accepted Article
Title: Immobilization of aminobisphosphine-PdII complex, over
graphene oxide: An efficient and reusable catalyst for Suzuki-
Miyaura, Ullmann coupling and cyanation reactions
Authors: Maravanji Shivaramaiah Balakrishna, Debasish Sengupta,
Madhusudan K Pandey, Dipanjan Mondal, and Latchupatula
Radhakrishna
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800291
Link to VoR: http://dx.doi.org/10.1002/ejic.201800291

10.1002/ejic.201800291
European Journal of Inorganic Chemistry
Immobilization of aminobisphosphine-PdII complex,
[PdCl2{(Ph2P)2N(CH2)3Si(OMe)3}] over graphene oxide: An efficient and
reusable catalyst for Suzuki-Miyaura, Ullmann coupling and cyanation
reactions
Debasish Sengupta, Madhusudan K. Pandey, Dipanjan Mondal, Latchupatula Radhakrishna,
and Maravanji S. Balakrishna*
This article describes the first example of grafting of a aminobis(phosphine)-PdII complex on
graphene oxide (GO) and its utility in Suzuki-Miyaura, Ullmann coupling and cyanation
reactions.
1
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
Immobilization of aminobisphosphine-PdII complex over graphene oxide:
An efficient and reusable catalyst for Suzuki-Miyaura, Ullmann coupling
and cyanation reactions
Debasish Sengupta, Madhusudan K. Pandey, Dipanjan Mondal, Latchupatula Radhakrishna,
and Maravanji S. Balakrishna*
Phosphorus Laboratory, Department of Chemistry, Indian Institute of Technology Bombay,
Powai, Mumbai 400 076, India
E-mail: krishna@chem.iitb.ac.in or msb_krishna@iitb.ac.in
Abstract: The grafting of
a
aminobis(phosphine)-PdII complex (PNP-PdII)
[PdCl2{(Ph2P)2N(CH2)3Si(OMe)3}] (2) on graphene oxide (GO) has been carried out via a
condensation reaction between methoxysilane groups of 2 and hydroxyl groups of GO. The
31
composite material was characterized by FTIR, solid state P NMR, SEM, TEM, XPS and
ICP-AES techniques. All these tools support the clean immobilization of compound 2 on GO.
The composite material showed high catalytic activity in Suzuki-Miyaura, Ullmann coupling
and cyanation reactions. The heterogeneity of the composite was confirmed by hot filtration
test. The immobilized PNP-PdII shows comparable activity as its homogeneous analogue 2.
The recycling ability of the catalyst has been examined for five consecutive runs which
showed little or no reduction in its catalytic efficiency.
2
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European Journal of Inorganic Chemistry
Introduction
Graphene is the thinnest carbon material made up of sp2 hybridised carbon atoms, arranged in
a honeycomb crystal structure.1 In recent years graphene has attracted enormous interest
owing to its physical, mechanical and thermal properties.2 Graphene oxide (GO) is an
oxidised form of graphene with layered structure containing a range of reactive oxygen
functional groups such as epoxide, hydroxyl and carboxyl groups located on basal planes and
edges. These are the active sites normally used during functionalization and also for catalysis.
Functionalization of graphene oxide has been a subject of numerous studies due to the
potential applications in material chemistry,3 medicinal chemistry4 and also in catalysis.5
The transition-metal phosphine complexes find extensive applications in synthetic organic
chemistry as efficient homogeneous catalysts.6 The immobilization of such systems also
attracted much attention since it includes numerous advantages like separation of the catalyst
from reaction mixture and reutilization of the material in several cycles. Over the past years a
wide range of solid materials like polystyrene, mesoporous silica, alumina, MgO, TiO2,
carbon material etc., have been used for the immobilization of different homogeneous
catalysts including phosphines.7 Although, different immobilization methods such as,
encapsulation, coordination and ion interaction are known, the method of covalent attachment
is found to be more effective compared to other methods. Different monophosphines and
chelated bisphosphines having alkoxysilane groups easily react with silanol groups of silica
during immobilization.8 On the other hand tripodal linker systems have been connected on
silica through three silicon atoms.9 The major drawback of these immobilized catalyst
systems is the detachment of active species and the most common way of detachment is the
hydrolysis of Si−O−E bond (E = Si, Al or Ti).8
Recently Khatri and co-workers have reported the grafting of oxo-vanadium and rhenium-
oxo moieties on Schiff base functionalized GO and used as heterogeneous catalysts for
3
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
alcohol and amine oxidations, respectively.10 Few salen complexes of CuII, CoII, FeIII and
oxovanadium(IV) have been successfully immobilized on amino-functionalized GO and
applied in aerobic epoxidation of styrene.11 Fan et al. have shown the immobilization of well
known Wilkinson catalyst over amine-functionalized GO through coordination interaction
and employed in the hydrogenation of cyclohexene.12 Chemical fixation of carbon dioxide to
dimethylformamide has been carried out by using GO grafted heteroleptic iridium complex.13
Few GO immobilized PdII complexes have been used for Suzuki-Miyaura cross-coupling
reaction.14 On the other hand, reduced graphene oxide (rGO) has been utilized for the
immobilization of N-heterocyclic carbene complexes of palladium and ruthenium by π-
stacking.15 However, the covalent attachment of any phosphine based metal complex over
GO is still unknown (Scheme 1). As a part of our continued interest in phosphorus-based
ligands and their coordination chemistry and catalytic applications,16 herein we report for the
first time, the covalent attachment of a PNP-PdII complex on graphene oxide and its catalytic
investigations in Suzuki-Miyaura, Ullmann coupling and cyanation reactions.
4
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
Scheme 1 Grafting of various metal complexes over GO.
Result and Discussion
To begin our study, we synthesized GO from graphite powder using Hummer’s method.17
The reaction of 3-aminopropyltrimethoxysilane with two equivalent of PPh2Cl in the
presence of triethylamine afforded a short-bite alkoxy functionalized diphosphine ligand
1
13
31
(MeO)3Si(CH2)3N(PPh2)2 (1) in good yield.7j Compound 1 was characterized by H, C, P
NMR and HRMS data. Reaction of 1 with one equivalent of Pd(COD)Cl2 in dichloromethane
at room temperature yielded [PdCl2{(PPh2)2N(CH2)3Si(OMe)3}] (2). The molecular structure
of compound 2 was confirmed by single crystal X-ray diffraction study (Fig. 1). For
o
immobilization, compound 2 was treated with GO in toluene at 110 C for 24 h under N2
atmosphere (Scheme 2). After completion of the reaction, the initial yellow colour solution
turned colourless. The colourless liquid on evaporation of solvent (toluene) under reduced
pressure did not give any residue indicating the complete immobilization of compound 2 on
5
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European Journal of Inorganic Chemistry
GO. The composite material was collected as black powder (GO@PNP-Pd) (here after
compound 2, i.e. PNP-Pd in GO composite is referred to as GO@PNP-Pd) and fully
characterized using various analytical and spectroscopic methods and employed in Suzuki-
Miyaura, Ullmann coupling and cyanation reactions.
Fig. 1 Molecular structure of compound 2. All hydrogen atoms were omitted for clarity.
Scheme 2 Immobilization of compound 2 on GO.
Characterization of GO@PNP-Pd
Initially the palladium content in the composite (GO@PNP-Pd) was calculated by ICP-AES
analysis. For this purpose 10 mg of the sample was digested with aqua regia (6 mL) and then
6
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European Journal of Inorganic Chemistry
diluted with water (10 mL), filtered in a 50 mL volumetric flask and used for analysis. The
loading of Pd was found to be 2.8 wt%. The BET surface area of the composite (GO@PNP-
Pd), calculated by nitrogen physisorption at 77 K was 17.7 m2 g-1.
The FT-IR spectrum of GO@PNP-Pd was recorded and compared with GO and compound 2
(Fig. 2). GO exhibits an intense and broad peak at 3402 cm-1, due to the νO−H bonds. Apart
from this, peaks observed at 1725, 1614, 1240 and 1055 cm-1 were assigned to stretching
modes of COOH, C=C, C−OH, and C−O−C bonds, respectively. Compound 2 also showed
some characteristic peaks in the region 2800-3000 cm-1, attributed to νC−H(sp2) and νC−H(sp3),
respectively, and νSi−O was observed at 1100 cm-1. Although, composite GO@PNP-Pd
showed almost all characteristic peaks of GO and compound 2, considerable decrease in the
intensity of O−H peaks was observed as anticipated due to the covalent attachment of PNP-
Pd(II) compound on GO via condensation reaction.
Fig. 2 FT-IR spectra of the GO, compound 2 and GO@PNP-Pd composite.
Solid-state NMR spectroscopy served as a useful analytical tool in the structural investigation
of immobilized systems. The solid state 31P NMR spectra of PNP-PdII complex 2 and
GO@PNP-Pd are shown in Fig. 3. Compound 2 showed a major peak at 24 ppm with some
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
side bands, whereas GO@PNP-Pd showed a resonance at 26.8 ppm along with typical side
bands. The downfield shift of 2.8 ppm may be due to the inductive effect caused by covalent
attachment of 2 on GO surface. These results have clearly indicated the successful
immobilization of PNP-PdII complex keeping the metal coordination sphere intact.
140
120
100
80
60
40
20
0
-20
-40
-60
-80 ppm
140
120
100
80
60
40
20
0
-20
-40
-60
-80
ppm
Fig. 3 Solid state 31P NMR of PNP-PdII complex 2 (black) and GO@PNP-Pd composite
(blue).
XPS has been used to determine the oxidation state of palladium in GO@PNP-Pd composite.
Full XPS spectra of compound 2 and GO@PNP-Pd contains signature peaks of different
elements (Fig. 4). Comparison of Pd 3d XPS spectra of compound 2 (336.2 and 341.6 eV)
and GO@PNP-Pd (336.2 and 341.5 eV) confirmed the +2 oxidation state of palladium in the
composite.
8
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European Journal of Inorganic Chemistry
Fig. 4 (a) Full XPS spectra of complex 2, (b) Pd 3d XPS spectra of complex 2, (c) Full XPS
spectra of GO@PNP-Pd composite, (d) Pd 3d XPS spectra of GO@PNP-Pd.
The information about the morphology of composite material was extracted from SEM
analysis. FEG-SEM images of GO and composite (GO@PNP-Pd) are presented in Fig. 5. GO
showed smooth planar surface (Fig. 5a), but composite showed crumpling features due to the
chemical modification. The elemental mapping by energy dispersive X-ray absorption
spectroscopy (EDS) of the composite revealed the homogeneous distribution of different
elements: Pd, Si, P and Cl on the surface (Fig. 6).
9
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European Journal of Inorganic Chemistry
Fig. 5 SEM images of (a) GO and (b) GO@PNP-Pd composite.
Fig. 6 SEM EDS mapping of GO@PNP-Pd showing elements, Pd, P, Si & Cl.
The TEM images of GO and GO@PNP-Pd composite in different magnification are shown in
Fig. 7. GO showed a layer structure (Fig. 7a), whereas the composite (GO@PNP-Pd)
displayed different layers decorated with metal stuffs (Fig. 7b & 7c). Energy dispersive X-ray
scattering (EDS) analysis (Fig. 7d) of the composite also confirmed the presence of all
possible elements such as Pd, P, Si, Cl, N, O and C. Although nitrogen was not identified in
SEM analysis, TEM EDS and XPS data confirmed the presence of nitrogen as well.
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European Journal of Inorganic Chemistry
Fig. 7 TEM images in different magnification of (a) GO; (b) & (c) GO@PNP-Pd composite;
(d) EDS pattern of the composite GO@PNP-Pd.
Catalytic activity of GO@PNP-Pd
Initially, the catalytic activity of the newly synthesized GO@PNP-Pd composite was
evaluated in Suzuki-Miyaura cross-coupling reaction due to its wide application in the
pharmaceutical and fine chemical industries.18 Various palladium catalysts with ancillary
phosphine ligands have been used for Suzuki-Miyaura reaction.19 Different reusable
heterogeneous palladium systems are also equally efficient for Suzuki-Miyaura reaction.20
Very recently several research groups have studied the Suzuki-Miyaura coupling by
21
developing some mono- and bimetallic heterogeneous systems.14,
In spite of their
potentiality, these systems suffered from various disadvantages such as prolonged reaction
time, high catalyst concentration and low substrate scope. Herein, we report a convenient
route to perform Suzuki-Miyaura coupling by using low catalyst concentration with broad
range of substrates including benzylic systems. We optimized the reaction conditions with 4-
bromoacetophenone and phenylboronic acid as coupling partner using the newly developed
11
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European Journal of Inorganic Chemistry
GO@PNP-Pd as an immobilized homogeneous catalyst and K2CO3 as a base (Table 1) at 80
oC. Solvent plays a vital role in Suzuki-Miyaura cross-coupling reaction.22 In DMF 1 mol%
catalyst (with respect to palladium) was required to achieve 100 % conversation after 10 h,
whereas in DMF:H2O (1:1) the reaction was completed within 1 h with 0.01 mol% of catalyst
(entries 1 & 2). In water, reaction needs more time for the completion (entry 3). However in
methanol the reaction was completed within 30 min using the same amount of catalyst (entry
4). Further the conversion has been decreased when the reaction was performed at lower
temperature (entry 5). Reactions without the catalyst or only with GO did not show any
conversion (entries 6 & 7). Therefore, the entry 4 in Table 1 represents the optimized reaction
condition.
Table 1 Optimization of Suzuki-Miyaura reaction using GO@PNP-Pd.a
Sl. No.
Catalyst
Pd content (mol%)
Solvent
Time (h) Conversion
(%)b
1
2
3
1
0.01
0.01
0.01
0.01
DMF
DMF:H2O (1:1)
H2O
10
1
2.5
0.5
0.5
3
100
100
100
100
59
4
MeOH
MeOH
MeOH
MeOH
5c
6
none
Graphene oxide
0
0
7d
3
a4-Bromo acetophenone (1 mmol), phenylboronic acid (1.2 mmol), K2CO3 (1.2 mmol) and
o
b
c
o
solvent (2.5 mL), 80 C under N2. GC-MS conversion. Reaction performed at 60 C.
dGraphene oxide (1 mg).
To explore the scope and limitations of the catalyst, a range of substituted bromoarenes were
employed with different boronic acids (Table 2, entries 1-7) which resulted in desired
products in excellent yields. Heteroaryl bromides were equally effective for the reaction and
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
afforded corresponding cross-coupling products in excellent yield (Table 2, entries 8 and 9).
Di-bromo compounds such as 9,10-Dibromoanthracene, 1,2-dibromobenzene and 1,3-
dibromobenzene efficiently reacted with boronic acid derivatives (Table 2, entries 10-12). On
the other hand, reaction of 4-nitrochlorobenzene with phenylboronic acid gave only 34% of
the cross-couple product (Table 2, entry 13). Since aryl chlorides are less reactive compared
to aryl bromides in cross-coupling reactions, the catalyst loading was increased to 1 mol%
that resulted in better conversions. Several aryl chlorides were reacted with various boronic
acids and the cross-coupling products were obtained in moderate to good yields (Table 2,
entries 13-16). Reactions between aliphatic halides and boronic acids have been carried out
by employing 1 mol% of the catalyst GO@PNP-Pd (with respect to palladium). Benzyl
bromide, benzyl chloride and 2-bromobenzyl bromide afforded corresponding cross-coupled
products in good to excellent yield (Table 2, entries 17-19). The catalytic efficiency of PNP-
Pd(II) compound before (2) and after immobilization (GO@PNP-Pd) were comparable
(Table 2, entries 1 & 20), however, GO@PNP-Pd showed better durability and did not show
any reduction in the catalytic efficiency even after five consecutive catalytic cycles (see Fig.
8) and also in scaled up reactions (Table 2, entry 20).
In order to prove the heterogeneous nature of the catalyst, hot filtration test has been
performed. Fig. 9 shows that the Suzuki-Miyaura reaction between 4-bromoacetophenone
and phenylboronic acid was very fast and completed within 30 min. Therefore, in order to
perform the hot filtration test, the catalyst was filtered off after 1.5 min and the remaining
o
liquid part (without catalyst) was heated again at 80 C for another 30 min. After separation
of the catalyst from the reaction, catalytic process was quenched (see hot filtration graph in
Fig. 9) suggesting that Pd was not leached out from GO surface during initial course of the
reaction and GO@PNP-Pd catalyst was of heterogeneous in nature.
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Table 2 Suzuki-Miyaura reaction of aryl halides with boronic acids using GO@PNP-Pd.a,b
GO@PNP-Pd
X
B(OH)2
80 oC, K2CO3
R1
R2
R1
R2
30 min, MeOH, N2
with bromoarenes
O2N
O
F
CH3
NC
entry 2, 100% (93%)
entry 1, 100% (96%)
entry 3, 100% (94%)
Cl
entry 4, 100%
OCH3 H3C
entry 6, 100% (93%)
OCH3
N
entry 8, 98% (94%)
entry 5, 100%
entry 7, 89%
CH3
S
CH3
entry 11c, 100%
entry 10c, 100% (97%)
entry 9, 100%
entry 12c, 100%
with chloroarenes
NO2
O2N
NC
OCH3
O2N
entry 16d, 63%
entry 13d, 34%a, 74%
entry 15d, 72%
entry 14d, 100%
with benzyl halides
O
OCH3
entry 17d, x = Br; 100%
OCH3
entry 19e, x = Br; 83% (81%)
entry 18d, x = Cl; 85%
entry 20, 100%f, 93%g
aReaction condition: ArBr (1 mmol), boronic acid (1.2 mmol), K2CO3 (1.2 mmol),
GO@PNP-Pd (0.6 mg, 0.01 mol% Pd), MeOH (2.5 mL), heating the reaction mixture at 80
b
oC under N2 for 30 min. GC-MS Conversion & yield in bracket represents isolated yield.
cArBr2 (0.5 mmol), boronic acid (1.2 mmol), K2CO3 (1.2 mmol), GO@PNP-Pd (0.6 mg, 0.02
o
d
mol% Pd), MeOH (2.5 mL), heating the reaction mixture at 80 C for 30 min. ArCl or
benzyl halide (0.25 mmol), boronic acid (0.3 mmol), K2CO3 (0.3 mmol), GO@PNP-Pd (9.5
o
e
mg, 1 mol% Pd) heating the reaction mixture at 80 C under N2 for 8 h. 2-Bromobenzyl
bromide (0.25 mmol), boronic acid (0.6 mmol), K2CO3 (0.6 mmol), GO@PNP-Pd (9.5 mg, 1
o
f
mol% Pd) heating the reaction mixture at 80 C under N2 for 8 h. Using 0.01 mol% of
g
compound 2. Scaled up reaction: 4-bromoacetophenone (995 mg, 5 mmol), phenylboronic
acid (731.6 mg, 6 mmol), potassium carbonate (829.3 mg, 6 mmol) and GO@PNP-Pd (3 mg,
0.01 mol% Pd), MeOH (7 mL), heating the reaction mixture at 80 oC under N2 for 1 h.
14
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Fig. 8 Recycling experiments of GO@PNP-Pd catalyst in Suzuki-Miyaura reaction between
4-bromoacetophenone and phenylboronic acid.
Fig. 9 Comparison of normal time profile verses hot filtration test.
Ullmann coupling is one of the cost-effective method for the synthesis of biaryls since it
eliminates the use of arylmetals, like arylboronic acids, arylstannanes, arylzinc, aryl Grignard
regents etc. On the other hand, use of chloro arenes in Ullmann coupling reaction is more
economical compared to bromo and iodo arenes as aryl chlorides are inexpensive and
environmentally benign. Limited number of heterogeneous palladium catalysts have been
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reported for the Ullmann coupling of aryl chlorides and the major drawbacks are high catalyst
loading, hazardous solvents and prolong reaction time.23 Herein, the newly developed catalyst
(GO@PNP-Pd) has been used as a heterogeneous catalyst for Ullmann coupling reaction of
o
chloro arenes with a high efficacy. The reaction has been performed in methanol at 80 C
using 1 mol% (with respect to palladium) of the catalyst, GO@PNP-Pd. However, the yield
of the product decreases with the lowering of temperature. Different chloro arenes having
electron donating and withdrawing groups such as –CH3, –OH, –NO2, and –CN undergo
Ullmann reaction efficiently and produce corresponding biaryl compounds in high yields
(Table 3, entries 1-6). The catalyst has been used for five consecutive cycles with appreciable
conversion (see ESI S1).
Table 3 GO@PNP-Pd catalyzed Ullmann coupling of aryl chlorides.a,b
GO@PNP-Pd
80 oC, K2CO3
Cl
R1
R1
R1
8 h, MeOH, N2
H3C
CH3
HO
OH
entry 1, 97% (74%)c
entry 4, 87%
entry 3, 88%
N
entry 2, 94%
O2N
NO2
NC
CN
N
entry 6, 82%
entry 5, 90%
aReaction condition: ArCl (0.25 mmol), K2CO3 (0.3 mmol), GO@PNP-Pd (9.5 mg, 1 mol%
o
b
Pd), MeOH (2 mL) heating the reaction mixture at 80 C under N2 for 8 h; Isolated Yield.
cReaction was performed at 60 oC.
After successful completion of Suzuki-Miyaura and Ullmann coupling reaction, the
GO@PNP-Pd catalyst has been employed in the cyanation reaction. Aryl nitriles represent an
16
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European Journal of Inorganic Chemistry
important class of organic compounds used as intermediates in the synthesis of many
pharmaceuticals, agrochemicals and dyes.24 In recent times, transition metal catalysed
cyanation reaction of aryl halides has become more attractive as it eliminates the use of
o
elevated temperature (150-200 C) and stoichiometric amount of toxic cyanating reagents.24b
Many homogeneous and heterogeneous palladium catalysts are known for cyanation
reactions but most of the methods suffer from the limitation such as high reaction
temperature, use of additives and poor reusability factors.25 Several phosphine ligands are
well-known for performing the cyanation reaction under mild condition.26 Herein, the GO
immobilized phosphine complex (GO@PNP-Pd) has been used as a reusable heterogeneous
catalyst for the cyanation reaction of aryl bromides. The reaction was performed with 1 mol%
o
of the catalyst (with respect to palladium) in DMF at an optimized temperature of 120 C
with K4[Fe(CN)6]·3H2O as a cyanating reagent. On decreasing the temperature below 120 oC,
the yield was lowered considerably. Aryl bromides having different substituents efficiently
react with K4[Fe(CN)6]·3H2O to afford corresponding nitrile derivatives (Table 4, entries 1-
7). Finally the recyclability of the catalyst GO@PNP-Pd has been examined for five
consecutive runs (see ESI S1) with an average yield 96% of terephthalonitrile.
To compare the morphological nature of the catalyst before and after the reaction, TEM
images of the catalyst have been taken after 1st and 2nd runs of Suzuki-Miyaura reaction (see
ESI S2). TEM images indicate the agglomeration of the palladium catalyst occurred over GO
surface during the reaction. On the other hand, Pd 3d line in XPS spectra (see ESI S3)
indicates the partial reduction of PdII species to Pd0, however, the catalytic activity remains
unaltered.
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Table 4 GO@PNP-Pd catalyzed cyanation of aryl bromides.a,b
aReaction condition: ArBr (0.25 mmol), K4[Fe(CN)6]·3H2O (0.045 mmol), Na2CO3 (0.37
mmol), GO@PNP-Pd (9.5 mg, 1 mol% Pd), DMF (2 mL), heating the reaction mixture at 120
b
c
o
d
oC under N2 for 12 h. Isolated yield. Reaction was performed at 100 C. ArBr2 (0.125
mmol), K4[Fe(CN)6]·3H2O (0.045 mmol), Na2CO3 (0.37 mmol), GO@PNP-Pd (9.5 mg, 2
e
mol% Pd). ArBr3 (0.125 mmol), K4[Fe(CN)6]·3H2O (0.068 mmol), Na2CO3 (0.55 mmol),
GO@PNP-Pd (14.25 mg, 3 mol% Pd).
Conclusions
In summary, we have demonstrated a facile and efficient method to immobilize PNP-Pd(II)
complex on graphene oxide. The supported PNP-Pd(II) material (GO@PNP-Pd) has been
well characterized by different spectroscopic and microscopic techniques and applied as a
heterogeneous catalyst in Suzuki-Miyaura, Ullmann coupling and cyanation reaction. Aryl
bromides and chlorides were efficiently reacted with boronic acids to produce the cross-
coupling products in good to excellent yield. Aliphatic bromides are equally effective for the
Suzuki-Miyaura reaction. On the other hand aryl chlorides have been utilized in Ullmann
coupling reaction for the synthesis of symmetrical biaryls. The heterogeneous nature of the
catalyst has been examined by hot filtration test. Further investigation on grafting of other
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metal systems (ruthenium, copper etc) and their catalytic application in other cross-coupling
reactions, hydrogenation and transfer-hydrogenation reactions is in progress.
Experimental Section
General Information
All phosphine based works were performed using standard vacuum-line and Schlenk
techniques under nitrogen atmosphere. All solvents were purified by conventional procedures
and distilled prior to use. Chemicals were purchased and used directly. [Pd(COD)Cl2] was
prepared according to the published procedure.27
Instrumental Methods
FTIR spectra of the samples were recorded using Perkin-Elmer, Spectrum One FTIR
instrument by KBr pellet method. The Field Emission Gun-Scanning Electron Microscope
(FEG-SEM) measurements of the samples were carried out using a JSM-7600F.
Transmission electron microscopy (TEM) of the samples were carried out with a Tecnai G2,
F30 (Make- FEI) operating at 300 kV and CM 200 (Make-PHLIPS) operating voltages: 20-
200 kv. The concentration of Pd in the composite has been estimated by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) using ARCOS, simultaneous ICP
Spectrometer, Germany. X-ray photoelectron spectroscopic (XPS) measurements were
recorded on AXIS Supra, Make: Kratos Analytical, UK (SHIMADZU group). The 1H and 31P
NMR (δ in ppm) spectra were obtained from either Bruker Avance-400 MHz or Bruker
Avance- 500 MHz spectrometer. The spectra were recorded in CDCl3 solutions with CDCl3
as an internal lock; TMS and 85% H3PO4 were used as internal and external standards for 1H
31
31
and P{1H} NMR, respectively. The solid state P NMR spectra were recorded in Bruker
Avance- 500 MHz spectrometer. All GC-MS analyses were carried out in Agilent 7890A GC
system connected with 5975C inert XL EI/CI MSD.
19
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
Preparation of GO
To an ice-cold concentrated sulfuric acid (46 mL) was slowly added sodium nitrate (100 mg)
and then graphite powder (2 g) with vigorous magnetic stirring. After that potassium
permanganate (6 g) was added slowly to the reaction mixture keeping the temperature within
0–5 oC to avoid any possible explosion. The mixture was allowed to stir at room temperature
for 6 h forming a thick paste. It was diluted with millipore water (92 mL) under stirred
o
condition. The temperature of the solution was raised to about 90 C and the mixture was
allowed to stir for 30 min. Finally, 280 mL millipore water was added followed by slow
addition of 3 mL H2O2 (30%). The colour of the solution changes from dark brown to
yellowish brown. The overall solution was centrifuged at 10000 rpm to collect the solid mass
at bottom and the supernatant liquid was thrown out. The solid mass was washed with 1 (N)
HCl followed by water to remove the acid and metal ions. Finally, the brown mass was
collected and dried at 60 oC under vacuum to obtain solid graphene oxide.
Synthesis of (Ph2P)2N(CH2)3Si(OMe)3 (1)
A solution of chlorodiphenylphosphine (2.46 g, 11.158 mmol) in toluene (10 mL) was added
dropwise to the mixture of 3-aminopropyltrimethoxysilane (1 g, 5.579 mmol) and NEt3 (1.7
mL, 12.27 mmol) in toluene (30 mL) at -60 oC. The white colour solution thus obtained was
stirred at room temperature for 24 h. The reaction mixture was filtered to separate the
ammonium salt and the filtrate was evaporated under reduced pressure to afford compound 1
(2.63 g, Yield: 86 %) as light yellow oil.
31P{1H} NMR (CDCl3, 202 MHz) : 62.2 (s); 1H NMR (CDCl3, 500 MHz) : 0.23 (t, J = 8.5
Hz, 2H, CH2−Si), 1.21-1.24 (m, 2H, −CH2−CH2Si), 3.24 (t, J = 8.5 Hz, 2H, −CH2N), 3.39 (s,
9H, OCH3), 7.31-7.41 (m, 20H, ArH); 13C NMR (CDCl3, 125 MHz) : 6.1, 24.3, 50.3, 55.7,
128.0, 128.7, 132.7 (t, J = 11.1 Hz), 139.6 (t, J = 6.3 Hz). C30H36NO3P2Si [M+H]+: 548.1940;
Found: 548.1637.
20
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
Synthesis of [PdCl2{(Ph2P)2N(CH2)3Si(OMe)3}] (2)
A solution of Pd(COD)Cl2 (141 mg, 0.494 mmol) in CH2Cl2 (20 mL) was added slowly to a
solution of 1 (270.4 mg, 0.494 mmol) in CH2Cl2 (15 mL) and stirred the mixture for 6 h. The
solvent was evaporated under reduced pressure and the residue was washed with petroleum
ether to afford analytically pure product of 2 as a yellow solid. Yield: 87 % (0.312 g). Mp:
250 oC. 31P{1H} NMR (CDCl3, 202 MHz) : 30.96; 1H NMR (CDCl3, 500 MHz) : 0.20 (t, J
= 8.0 Hz, 2H, −CH2−Si), 1.22-1.26 (m, 2H, −CH2−CH2Si), 2.99-3.05 (m, 2H, −CH2N), 3.33
(s, 9H, OCH3), 7.55 (t, J = 7.0 Hz, 8H, ArH), 7.66 (t, J = 7.0 Hz, 4H, ArH), 7.86-7.90 (m,
8H, ArH). Anal. Calcd. for C30H35Cl2NO3P2PdSi: C, 49.70; H, 4.87; N, 1.93 %. Found: C,
49.15; H, 4.73; N, 2.26 %.
The crystal of compound 2 suitable for the X-ray diffraction study was obtained by slow
diffusion of petroleum ether into a dichloromethane solution of 2.
Synthesis of GO@PNP-Pd composite
400 mg GO was dispersed in 50 mL toluene by sonication (30 min) under nitrogen
atmosphere. Then 100 mg of compound 2 in toluene (20 mL) was added slowly to the stirred
o
solution of GO and heated the reaction mixture at 110 C, for 24 h under N2. After
completion of reaction the mixture was cooled to room temperate and stands for 30 min. The
colourless supernatant liquid was decanted in another flask and the black residue was wash
with toluene (3 x 10 mL) and diethyl ether (3 x 10 mL). The resulting black powder was
dried under vacuum and used for analysis and reactions.
Sample preparation for ICP-AES analysis
o
10 mg of GO@PNP-Pd composite was heated in aqua regia (6 mL) at 150 C for 1 h. After
cooling, the mixture was diluted with water (10 mL), filtered into a 50 mL volumetric flask
and used for analysis. The loading of Pd was found to be 2.8 wt%.
21
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
Representative procedure for Suzuki-Miyaura reaction with GO@PNP-Pd composite
A mixture of 4-bromoacetophenone (199 mg, 1 mmol), phenylboronic acid (146.3 mg, 1.2
mmol), potassium carbonate (165.8 mg, 1.2 mmol) and GO@PNP-Pd (0.6 mg) in MeOH (2.5
mL) was taken in a 15 mL screw cap reaction tube, flashed with N2 and heated the reaction
mixture at 80 oC for 30 min under gentle magnetic starring. After completion of reaction the
mixture was allowed to cool and diluted with ethyl acetate (3 mL), stirred and then allowed to
stand for 15 min. The supernatant liquid was decanted into another flask and this process was
repeated three times more. The organic part was washed with water, dried over anhydrous
Na2SO4, concentrated under vacuum. The residue was purified by column chromatography
over a short column of silica gel and eluting with 3% ethyl acetate-light petroleum to obtain
1
13
the pure product (189 mg, Yield: 96%). The product was characterized by H and C NMR
data (See ESI S5 & S6).
Catalyst Recovery
After completion of the reaction, the reaction mixture was allowed to cool to room
temperature and diluted with ethyl acetate (3 mL) and stirred for 15 min. This process
was repeated three times and the supernatant liquid was separated by decantation. The
residue (composite GO@PNP-Pd) was further washed with methanol (3 x 3 mL)
followed by diethyl ether (3 x 3 mL) and dried under vacuum to give free‒flowing
black powder which was used in subsequent catalytic cycles.
Representative procedure for Ullmann coupling with GO@PNP-Pd composite
A mixture of 3-chlorotoluene (31.6 mg, 0.25 mmol), potassium carbonate (41.46 mg,
0.3 mmol) and GO@PNP-Pd (9.5 mg) in MeOH (2 mL) was taken in a 15 mL screw
o
cap reaction tube, flashed with N2 and heated the reaction mixture at 80 C for 8 h
under gentle magnetic starring. After completion of reaction the mixture was allowed
to cool and diluted with ethyl acetate (3 mL), stirred and then allowed to stand for 15
22
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
min. The supernatant liquid was decanted into another flask and this process was
repeated three times more. The organic part was washed with water, dried over
anhydrous Na2SO4, concentrated under vacuum. The residue was purified by column
chromatography over a short column of silica gel and eluting with light petroleum to
1
obtain the pure product (21.4 mg, Yield: 94%). The product was characterized by H
and 13C NMR data (See ESI S5 & S6).
Representative procedure for cynation reaction with GO@PNP-Pd composite
A mixture of 4-bromobenzonitrile (45.5 mg, 0.25 mmol), K4[Fe(CN)6]·3H2O (19 mg,
0.045 mmol), sodium carbonate (39.2 mg, 0.37 mmol) and GO@PNP-Pd (9.5 mg) in
DMF (2 mL) was taken in a 15 mL screw cap reaction tube, flashed with N2 and
o
heated the reaction mixture at 120 C for 12 h under gentle magnetic starring. After
completion of reaction the mixture was allowed to cool and diluted with ethyl acetate
(3 mL), stirred and then allowed to stand for 15 min. The supernatant liquid was
decanted into another flask and this process was repeated three times more. The
organic part was washed with water, dried over anhydrous Na2SO4, concentrated
under vacuum. The residue was purified by column chromatography over a short
column of silica gel and eluting with 7% ethyl acetate-light petroleum to obtain the
1
13
pure product (48.5 mg, Yield: 95%). The product was characterized by H and C
NMR data (See ESI S5 & S6).
X-ray Crystallography
Single crystal X-ray data collection was performed in Rigaku Saturn724
Diffractometer. Data collected at 150 K using graphite monochromated Mo-K
α
radiation source. The structure was solved by direct method and refined by full-matrix
least-squares procedures using the SHELXTL program package.28 Crystal data and
23
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
summary of data collection for compound
information S4.
2
was given in electronic supplementary
Electronic Supplementary Information
Crystallographic information, XPS, TEM images of GO@PNP-Pd after Suzuki-
Miyaura reaction, NMR spectral data and spectra for isolated compounds.
Accession Codes
CCDC 1563182 (Compound 2) contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif,
or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: krishna@chem.iitb.ac.in or msb_krishna@iitb.ac.in.
ORCID
Maravanji Balakrishna
0000-0003-3736-6148
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
DS thanks IIT Bombay for postdoctoral fellowship. MP, LR thanks UGC and DM
thanks DST, New Delhi for providing fellowships. MSB thanks the Science &
Engineering Research Board (SERB), New Delhi, India, for funding through grant
SB/S1/IC-108/2014. We thank SAIF & ESCA lab of IIT Bombay for providing the
instrumental facilities.
24
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10.1002/ejic.201800291
European Journal of Inorganic Chemistry
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