A gold-carbon nanoparticle composite as an efficient catalyst for homocoupling reaction

Article abstract of DOI:10.1039/c3cc43726d

Carbon nanoparticle supported Au nanoparticles could be synthesized starting with a Au nanoparticle-chitosan composite. The composite nanoparticles efficiently converted phenylboronic acid to biphenyl through homocoupling.

Full text of DOI:10.1039/c3cc43726d

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Gold-Carbon Nanoparticle Composite as Efficient Catalyst for
Homocoupling Reaction
Md Palashuddin Sk,a Chandan K. Jana*a and Arun Chattopadhyay*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
the catalyst was required for high conversion and selectivity. It
Carbon nanoparticle supported Au nanoparticle could be
synthesized starting with Au nanoparticle-chitosan composite.
50 has also been demonstrated ꢀ although only in two instances ꢀ that
Au NPs are efficient in catalyzing oxidative coupling of aryl
boronic acid. For example, PVPꢀstabilized Au NP catalyzes
homocoupling reaction under aerial condition.8c Even though
high conversion of starting material was achieved, poor
55 selectivity towards desired biaryl was observed. On the other
hand, MgꢀAl mixed oxide supported Au NPs require high
pressure of oxygen and higher temperature for higher conversion
and better selectivity.8b Therefore, developing novel catalyst, in
order to bridge the gap between ambient reaction conditions and
60 reactivityꢀselectivity, is desirable so that the application potential
of the reaction could be expanded further.
The
composite
nanoparticle
efficiently
converted
phehylboronic acid to biphenyl through homocoupling.
10 Progress in catalytic chemical reactions – the mainstay of
chemistry of new molecules or pathways of reactions – depends
on development of efficient, environmentally friendly and costꢀ
effective catalysts.1a Although the tremendous growth that has
taken place in this regard for the last one hundred years or so
15 seems to overwhelm any new attempt in this regard, there is still
plenty of opportunity for finding new catalysts, relook at old
catalysts in a modified form or for developing combination of
catalysts in order to have the best of all worlds.
For example, charcoalꢀsupported Pd or Pt metal catalysts
20 have contributed greatly in catalytic organic transformations.1
Recent reports suggest that carbon nanoparticles (CNPs) could
provide better option, owing to their stability, sizes, ease of
dispersion in liquid medium and facile formation of composites –
especially with catalytic metal nanoparticles (NPs).2 In addition,
25 carbon NP supported metal NPs have been used for
photocatalysis2d and surface enhanced Raman spectroscopy
(SERS).3 Further, porous carbon supported metal NPs have been
used extensively for reduction of nitroaromarotics.4
We report herein an easy and bioꢀfriendly method of
preparation of composite of Au NPs and CNPs, which was used
for carrying out facile homocoupling reaction of phenylboronic
65 acid to provide excellent conversion with high selectivity under
aerial reaction conditions. In the composite, the size of Au NPs
was 4.9±1.4 nm, while that of CNPs varied from 50 nm to 350
nm. In addition, a mechanistic proposal relying on the role of
AuꢀNP as the active site is described, which rationalizes the
70 solvent dependent reactivity and selectivity of the reaction.
Chitosanꢀstabilized Au NPs were synthesized by reduction of
HAuCl4 in the presence of NaBH4 and chitosan at room
0
temperature. This was followed by heating the mixture at 200 C
Among the metal NPs, catalytic activity of Au NP, supported
for 2 h which led to formation of a paste. The mixture was heated
75 additionally for 1 min, during which carbonization took place.
The details of the procedure are included in ESI†. The final dried
powder was black in colour, which could easily be dispersed in
water by mild sonication. Photographs of the powder and the
dispersion are shown in Fig. 1 (A and B). UVꢀvis spectrum of the
80 dispersion (Fig. 1C) consisted of three peaks at 267 nm, 345 nm
and at 522 nm. The peak at 267 nm and 345 nm are characteristic
of CNPs (imputed to the π–π* transition), whereas that at 522 nm
is due to Au NPs.2aꢀc On the other hand, as prepared chitosan –Au
NP composite exhibited a single peak at 510 nm, indicating the
85 presence of Au NPs only. Further, photoluminescence property of
the composite resembled that of carbon dots. For example, as
shown in Fig. S1, (ESI†), the excitationꢀtuneable emission peaks
in the range of 325 nm to 500 nm of the composite resembled
those of CNPs. However, the quantum yield of luminescence was
90 higher for the composite (0.54%) in comparison to that of the
CNPs (0.36%). Transmission electron microscopy (TEM)
measurements indicated the presence of distinct particles of
30 on the surfaces of various organic and inorganic substrates, has
8aꢀc
made Au an alternative choice.5,
For example, carbon
supported Au NPs have been used for performing reduction and
oxidation reactions.6 However, it is also important to develop
newer methods for pursuing coupling reactions on supported Au
35 NPs. A case could be the production of biaryls, which are an
important class of compounds being present as active moiety in
pharmaceutically important molecules, natural products and in
advanced functional materials.7 Transition metal catalysed
oxidative homocoupling of the arylboronic acid is a major
40 pathway to achieve the homomeric biaryls synthetically.8ꢀ10 In
this context, palladium based catalysis has dominated the field.10
However, several other metal catalysts as well as the reaction
conditions were varied to minimize the limitation and increase
the efficiency of the parent palladium catalysed coupling reaction.
45 In one such case, cationic Au either supported on metal oxides
and polysaccharides or complexed with organic ligands is shown
to serve as efficient catalyst for selfꢀcoupling of arylboronic
acid.8a, d, 11 However, higher loading of 2.25 to 30 mole percent of
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35 G bands of sp2 carbon, signifying the formation of CNPs (Fig. S6,
ESI†).
The above results indicated the formation of smaller Au NPs
embedded in the larger amorphous CNPs. Also, the absence of
any oxidation states other than Au(0) makes the AuꢀCNP
40 composite ideal for studying the efficacy of Au NP catalyzed
coupling reactions. It may be mentioned here that the smallness
of the sizes of the CNPs (in the composite), as compared to
traditional charcoal support makes, the composite an ideal
platform for microheterogeneous reactions. It is important to
45 mention here that catalytic reactions in microheterogeneous
medium is inherently advantageous over its heterogeneous
counterpart, owing to the ease of product diffusion, in addition to
availability of large surface area for reactions.
Table 1. AuꢀCNP catalysed homocoupling of phenylboronic acida
70oC
AuꢀCNP
Fig. 1 Photographs of (A) dry composite and (B) that of the composite
dispersed in water. (C) Absorption spectra of AuꢀCNP composite and Au
NPꢀchitosan. (D) TEM image of AuꢀCNP composite with HRTEM image
in the inset. (E) Size distribution of Au NP present in the composite as
calculated from TEM images.
OH
B(OH)2
+
H2O/Tolune
Ph-Phb Ph-OHb Conversion
Entry
Solvent
Catalyst Oxygen
1c
2
3
4
5
6
7
8
9
Toluene/H2O AuꢀCNP
Toluene/H2O ꢀꢀꢀꢀꢀꢀꢀꢀꢀ
Toluene/H2O Chitosan
atm.
atm.
86%
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
68%
12%
98%
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
86.3%
5
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀꢀꢀꢀ
18.3%
ꢀꢀꢀꢀꢀꢀꢀꢀ
26%
atm.
average size 4.9 ± 1.4 nm, embedded in larger particles of sizes in
the range of 50 nm to 350 nm (Fig. 1D, 1E and Fig. S3, S5;
ESI†). Also, a large number of Au NPs were embedded in each of
Toluene/H2O
Toluene
Toluene
H2O
CNP
atm.
AuꢀCNP
AuꢀCNP
AuꢀCNP
atm.
balloon
atm.
10 the larger particles, as is evident from TEM measurements. That
the smaller particles were due to Au was confirmed from selected
area electron diffraction (SAED) measurement (Fig. S4, ESI†).
Further, high resolution TEM (HRTEM) measurements indicated
the presence of planes owing to lattice spacing of 0.224 nm,
15 which is due to (111) planes of metallic Au. On the other hand,
the larger particles could be CNPs. Xꢀray diffraction (XRD)
measurements indicated the presence of four planes of Au,
namely, (111), (200), (220) and (311) leading to diffractions at
38.30, 44.60, 64.80, and 77.8o (JCPDS 4ꢀ0783) ,respectively (Fig.
Toluene/H2O AuꢀCNP
Toluene/H2O AuꢀCNP
free
ꢀꢀꢀꢀꢀꢀꢀꢀ
19% (24%)
balloon
74%
100%
50 aReaction Condition: 0.3 mmol phenylboronic acid, 1mL toluene, 0.5 mL water, 10
mg catalyst (Au loading 0.66 mol%), 70 0C in open flask and 7h. bIsolated yields.
cGasꢀchromatographic analysis showed the yield to be 88% (ESI).
In order to investigate the catalytic activity of the composite,
the wellꢀknown CꢀC bond forming homocoupling reaction of
55 phenylboronic acid was pursued. Thus, phenylboronic acid in a
binary solvent mixture (2:1 V/V ratio of toluene and water) was
allowed to react in the presence of catalytic amount of the
o
20 S4, ESI†). A broad peak at 24.8 is assigned to amorphous
CNPs.
0
composite, under the aerial conditions at 70 C, which was found
to be the best reaction temperature (ESI, Table S2). A conversion
60 of 98% of the phenylboronic acid in 7 h, in the presence of only
0.66 mol% of the gold leading to the TOF (turn over frequency)
of 19 was observed. Biphenyl was isolated with yield of 86%,
along with 12% of phenol, a byꢀproduct of oxidation of
phenylboronic acid, presumably by the H2O2 generated in the
65 reaction. Control experiments (Table 1) carried out in absence of
catalyst and in the presence of CNP or chitosan did not result in
the product formation. Thus Au NPs in the composite played key
role in the catalytic conversion of phenylboronic acid to biphenyl.
Further, reaction involving chitosanꢀstabilized Au NPs lacked
70 selectivity and had lower conversion efficiency (ESI, Table S1,
entry 3). Interestingly, when the reaction was carried out in
toluene only medium there was no product formation (entry 5),
even when gaseous O2 was passed through the reaction mixture.
On the other hand, in the water (only) medium 86% conversion –
75 although with reduced selectivity – was observed. Interestingly,
the extent of conversion was reduced when deaerated water was
used instead. Additionally, externally added oxygen facilitated
the reaction (entry 9), where the reaction was completed in 3.5 h
with a TOF of 32. However, more amount of phenol was formed
Fig. 2 Xꢀray photoelectron spectra of AuꢀCNP composite. (A) is for Au
4f and (B) is for C 1s.
Xꢀray photoelectron spectroscopy analysis of the composite
25 evidenced the presence of peaks at 84.1 eV and 87.8 eV (Fig. 2),
where the binding energies correspond to the 4f7/2 and 4f5/2 states
of metallic gold i.e. Au0. Thus the presence of Au+ or Au3+
species could be ruled out. Further, analysis of peaks due to
carbon species indicated the presence of CꢀC (284.9 ev), CꢀOH or
30 CꢀOꢀC (286.5 ev) and C=O (287.9 ev) functional groups. On the
other hand, inductive coupled plasma (ICP) spectroscopy
indicated 1.95% of Au loading in the composite. Additionally,
Raman spectroscopy measurements indicated the presence of
broad peaks at 1341 cmꢀ1 and 1581 cmꢀ1, corresponding to D and
2
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reducing the biphenyl to phenol selectivity to ~3:1 from 7:1
(entry 1).
Based on these experimental findings and the literature reports,
a plausible mechanism of AuꢀCNP catalyzed homocoupling
reaction is depicted in scheme 1.8aꢀc, 11 The surface of the AuꢀNP
is oxidized, by dissolved molecular oxygen,8bꢀc,12 to positively
successfully recycled twice without compromising the conversion
50 (98%) of the starting material as well as the yield (89%) of the
desired product.
In summary, we have developed a simple method to synthesize
AuꢀCNP composite, which was used for homocoupling reaction
of phenylboronic acid. Importantly, selectivity of the reaction in
5
charged species being associated with oxygenated ligands (e.g. 55 the presence of H2O and dissolved oxygen indicated the
ꢀ
O22ꢀ, O2 or OHꢀ), in the first step of the catalytic cycle. The
exceptional role played by Au NPs being supported on CNP. That
the catalyst was stable and could be recycled is important for its
practical application. This reaction may open new doors of
metalꢀCNP composite towards coupling reaction and production
electron rich nature of CNP probably facilitates the aerial
10 oxidation of Au NP, which is reflected in the enhanced catalytic
activity of AuꢀCNP compared to Auꢀchitosan.13 The baseꢀ
mediated activation of phenylboronic acid occurred, prior to the 60 of fine chemicals and pharmaceuticals of industrial importance.
double transmetallation on the charged Au surface. However, the
ligand (coordinated to Au) induced transmetallation could also be
15 possible. In parallel, H2O2 generated in situ from reduced oxygen
(super oxide and/or peroxide) could have oxidized phenyl boronic
We thank Department of Biotechnology, India, for fund. Helps
from CIF, IIT Guwahati, Dr. C.V. Sastri, Anil Kumar, P. Barman,
S. Das and S. K. Sailapu are acknowledged. C.K.J is recipient of
DAE Young Scientist Research Award. M.P.S thanks CSIR for a
acid to phenol.12b Finally, the reductive elimination with 65 fellowship (09/731(0095)/2010ꢀEMRꢀI).
consequent product dissociation provided desired biphenyl,
aDeparment of Chemistry, Indian Institute of Technology Guwahati,
returning the Auꢀnanoparticle in to the catalytic cycle for the next
20 turn over.
Guwahati-39, Assam, India. Fax: +91 361 258 2349; Tel: +91 361 258
2304/28; E-mail: arun@iitg.ernet.in, ckjana@iitg.ernet.in
70 bCentre for Nanotechnology, Indian Institute of Technology Guwahati,
Guwahati-39, Assam, India.
† Electronic Supplementary Information (ESI) available: Experimental
section and Fig. S1ꢀS11. See DOI: 10.1039/b000000x/
75 Notes and references
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Scheme 1. Proposed mechanism of aerobic homocoupling reaction of
phenylboronic acid catalyzed by AuꢀCNP composite.
85 3. P. Luo, C. Li, G. Shi, Phys. Chem. Chem. Phys., 2012, 14, 7360.
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The increase in the percentage of the phenol in the reaction in
25 only water compared to that in tolueneꢀwater mixture can be
rationalized with the help of proposed mechanism, by considering
the comparative rate of transmetallation and oxidation of starting
material. In contrast to the reaction in water, for the case of
tolueneꢀwater mixture, biphenyl being hydrophobic diffuses
30 immediately after its formation to toluene and thus the catalytic
sites become ready for another cycle. However, due to higher
amount of dissolved O2 in only water or in tolueneꢀwater medium
with externally added oxygen, the formation of higher amount of
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35 formation. In contrast to the mechanism proposed by Carrettin et
8a
al. using AuꢀCeO2,
our mechanistic proposal relies on the
positively charged Au NP as the active catalyst for the
homocoupling reaction of phenylboronic acid. Furthermore,
solvent dependent reactivity and the selectivity observed
40 experimentally are in accordance with the proposed mechanism.
The stability and the ability to recycle the catalyst were also
investigated. Firstly, the supernatant of the reaction mixture
showed no significant absorbance above 300 nm (Fig. S7, ESI†),
thus excluding the presence of ionic Au species in the medium
45 and at the same time indicating the stability of the NPs.
Moreover, TEM and XRD studies of the recovered catalyst
indicated the stability of the Au NPs in terms of their shape and
size (Fig. S8 & S9 ESI†). Accordingly, the catalyst was
13. a) V. F. Lapko, I. P. Gerasimyuk, V. S. Kuts, Y. A. Tarasenko, Russ.
J. Phys. Chem. A, 2010, 84, 934. b) X. Dou, Z. Lin, H. Chen, Y. Zheng,
C. Lua, J. Lin, Chem. Commun., DOI: 10.1039/c3cc41145a.
This journal is © The Royal Society of Chemistry [year]
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Text: Gold in(en) carbon as efficient catalyst
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