Low-temperature carbon-chlorine bond activation by bimetallic gold/palladium alloy nanoclusters: An application to Ullmann coupling

Article abstract of DOI:10.1021/ja309606k

This paper describes the unique catalytic activity of bimetallic Au/Pd alloy nanoclusters (NCs) for Ullmann coupling of chloroarenes in aqueous media at low temperature. The corresponding reaction cannot be achieved by monometallic Au and Pd NCs as well as their physical mixtures. On the basis of quantum chemical calculation, it was found that the crucial step to govern the unusual catalytic activity of Au/Pd is the dissociative chemisorption of ArCl, which is unlikely in the monometallic Au and Pd NCs.

Full text of DOI:10.1021/ja309606k

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Communication
Low-temperature Carbon–Chlorine Bond Activation by Bi-metallic Gold/
Palladium Alloy Nanoclusters: An Application to Ullmann Coupling
Raghu Nath Dhital, Choavarit Kamonsatikul, Ekasith Somsook, Karan
Bobuatong, Masahiro Ehara, Sangita Karanjit, and Hidehiro Sakurai
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja309606k • Publication Date (Web): 01 Dec 2012
Downloaded from http://pubs.acs.org on December 6, 2012
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Low-temperature Carbon–Chlorine Bond Activation by Bi-
metallic Gold/Palladium Alloy Nanoclusters: An Application to
Ullmann Coupling
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Raghu Nath Dhital, † Choavarit Kamonsatikul, § Ekasith Somsook, § Karan Bobuatong, || Masa-
hiro Ehara,*, || Sangita Karanjit† and Hidehiro Sakurai*, †, ‡
†Department of Functional Molecular Science, School of Physical Sciences, Graduate University for Advanced Stud-
ies (SOKENDAI), Myodaiji, Okazaki 444-8787, Japan
‡Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787,
Japan
||Research Center for Computational Science, Myodaiji, Okazaki 444-8585, Japan
§NANOCAST Laboratory, Center for Catalysis, Department of Chemistry and Center for Innovation in Chemistry,
Faculty of Science, Mahidol University, 272 Thung Phaya Thai, Rama VI Rd. Ratchathewi, Bangkok 10400, Thailand.
Supporting Information Placeholder
under mild conditions. Such reactions can be catalyzed by
ABSTRACT: This paper describes the unique catalytic activi-
many types of homogenous or heterogeneous transition-
metal catalysts, such as Ni, Pd or Au.6,7 However, to the best
ty of bimetallic Au/Pd alloy nanoclusters (NCs) for Ullmann
coupling of chloroarenes in aqueous media at low tempera-
of our knowledge, there are no reports of any successful ex-
ture. The corresponding reaction cannot be achieved by
amples of Ullmann coupling reactions of chloroarenes under
ambient conditions. It is because of relative difficulty in the
tures. Based on quantum chemical calculation it was found
activation a C-Cl bond, which needs to occur twice within a
monometallic Au and Pd NCs as well as their physical mix-
that the crucial step to govern the unusual catalytic activity
of Au/Pd is the dissociative chemisorption of ArCl, which is
unlikely in the monometallic Au and Pd NCs.
single catalytic cycle, and because all C-M intermediates
need to resist hydrogenation by external co-reductant.8
Our investigation started by monitoring the Ullmann cou-
pling of 4-chlorotoluene (1a) (0.1 mmol) in N,N-
dimethylformamide (DMF)/ H2O (1:2 ratios) in the presence
of 300 mol% K2CO3 at 45 °C under argon atmosphere using
In the past decade, nanoclusters (NCs) of bimetallic alloys
monometallic Au, Pd or bimetallic Au/Pd-alloy NCs as cata-
have attracted considerable research interest because of their
lysts. These NCs were stabilized by a hydrophilic polymer,
unique catalytic properties, which differ substantially from
poly(N-vinylpyrrolidone) (PVP).9–11 The results are shown in
those of single-phase monometallic counterparts.1 Among
Table 1 (entries 1-6).
the various bimetallic NCs that have been fabricated to date,
gold/palladium alloy NCs are particularly fascinating because
of their high catalytic activities.2
The Ullmann coupling product, 4,4’-dimethylbiphenyl
(2a), was not obtained when monometallic Au:PVP or
Pd:PVP was used as the catalyst (entries 1 and 2). To our de-
light, however, Au/Pd bimetallic alloy NCs did exhibit
Ullmann coupling activities (entries 3-5) with 1a at 45 °C.
When Au0.8Pd0.2:PVP (80% of Au and 20% of Pd) was used as
a catalyst, the yield of 2a was only 30% (entry 3). The activity
increased markedly for a bimetallic catalyst containing 50%
of Pd, giving 2a in 98% yield with 100% conversion (entry 4).
However, an increase in the Pd content to 80% drastically
reduced the yield of 2a to 20% (entry 5). The presence of
physical mixtures of 1 atom% Au and 1 atom% Pd did not
promote the reaction, indicating that the presence of an alloy
structure was essential to the coupling reaction (entry 6).
Since no reaction occurred in the absence of a base as shown
in entry 7, the effect of the base was screened by quenching
the reaction after 3 h. Inorganic bases such as carbonates or
hydroxides worked effectively and the reaction rate was in
Because of the relatively high dissociation energy of C-Cl
bond in comparison with C-Br or C-I, it is difficult to activate
chloroarenes by undergoing oxidative addition on Pd with-
out ligation by nucleophilic ligands.3 Here we demonstrate a
new method for activation of C–Cl bonds at low temperature
as a result of bimetallic Au/Pd synergy and we report the
successful examples of the Ullmann coupling of chloroarenes
under ambient conditions. Ullmann coupling, which was
first reported in 1901,4 is a conventional method for the syn-
thesis of symmetrical biaryls. Initially, aryl iodides were used
in coupling reactions that were promoted by excess amount
of copper at high temperatures.4,5 Recent developments in
the Ullmann coupling reaction has permitted the use of aryl
bromides or chlorides as reactants in the presence of a co-
reductant such as Zn powder, formic acid, and dihydrogen
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order Na2CO3 < K2CO3 < Cs2CO3 < NaOH < KOH (entries 8-
withdrawing groups, chloroarenes containing electron-
donating groups also reacted under the current reaction
conditions. 4-Chloroanisole (1j) and 5-chloro-2-
methoxyaniline (1k) were each coupled under mild condi-
tions, to give 2j and 2k in 92 and 93% yields, respectively
(entry 10-11). 2-Chloronaphthalene (1l) also successfully un-
derwent the coupling to afford 2,2'-binaphthyl (2l) in 92%
yield for 24 h (entry 12).
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12). The reaction was completed within 1 h in the presence of
300 mol% of KOH. The coupling reaction occurred smoothly
even when the reaction temperature was decreased to 35 °C
or to 27 °C (room temperature), giving 2a quantitatively for 6
h and 24 h, respectively (entries 13-14). To the best of our
knowledge, this is the first example of Ullmann coupling of
chloroarenes under ambient conditions.
9
Table 1. Ullmann coupling of 4-chlorotoluene cata-
lyzed by monometallic and bimetallic catalyst
Table 2. Catalytic Ullmann coupling of chloroa-
renes using bimetallic Au0.5Pd0.5:PVP catalysta
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Entry
Substrate
Product
t (h) Yieldb (%)
Entry
Catalyst
Base
T (°C)
t (h)
Yielda (%)
1
2
Au:PVP
Pd:PVP
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
—
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27
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3
0
0
3
Au0.8Pd0.2
Au0.5Pd0.5
Au0.2Pd0.8
Au:PVP+Pd:PVP
Au0.5Pd0.5
Au0.5Pd0.5
Au0.5Pd0.5
Au0.5Pd0.5
Au0.5Pd0.5
Au0.5Pd0.5
Au0.5Pd0.5
Au0.5Pd0.5
Au0.5Pd0.5
30
98
20
0
4
5
6c
7
0
8
Na2CO3
K2CO3
Cs2CO3
NaOH
KOH
66
73
85
95
98
98
98
97
9
3
10
11
12
13
14
15c
3
3
1
KOH
6
KOH
24
6
KOH
a Isolated yield. b1 atom% of Au:PVP and 1 atom% of Pd:PVP
was used. c150 mol% of KOH was used.
Having established the optimal reaction conditions, the
protocol was extended to other chloroarenes to verify the
scope of the method, and the results are listed in Table 2.
The coupling of chlorobenzene (1b) proceeded smoothly to
yield biphenyl (2b) in 97% (entry 1). Sterically less-congested
substrates coupled in excellent yields, and in the case of 3-
chlorotoluene (1c), the reaction was completed for 16 hours,
whereas sterically hindered 2-chlorotoluene (1d) required 24
h for the reaction to reach completion (entry 3-4). Chloroa-
renes with an electron-withdrawing group in the para-
position (1e-1i) also underwent coupling, although byprod-
ucts from hydrodechlorination were also obtained in some
cases (entries 5-8). For example, the reaction of 4-
chlorobenzonitrile (1g), gave the corresponding biaryl 2g in
92% yield together with 8% yield of benzonitrile (entry 7). In
the case of 1-chloro-4-nitrobenzene (1h), KOH failed to give
satisfactory results, but 2h was obtained in 88% yield after 24
hours when 300 mol% of Cs2CO3 was used (entry 8). Note
that two byproducts, nitrobenzene and N,N-dimethyl-4-
nitroaniline, were obtained from this reaction in 6% and 5%
yield, respectively. The former is the hydrogenation product
and the latter may be formed through amination with dime-
thylamine generated from DMF (see in the table caption).
The reaction of acetyl derivatives (1i) was somewhat slower
and was incomplete after 24 h, giving 2i in 86% yield, togeth-
er with 5% of acetophenone and a 10% recovery of 1i (entry
9). In addition to chloroarenes containing electron-
aReaction conditions: 2 atom% Au0.5Pd0.5:PVP, chloroarene
(0.1 mmol), KOH 150 mol%, DMF/H2O (1:2, 1 mL DMF, 2 mL
H2O), 35 °C, under argon. bYield of isolated product. cYield of
hydrodechlorination product. d300 mol% of Cs2CO3 was used.
eYield of 4-NO2C6H4NMe2. f10% of 1i was recovered.
gDMF/H2O 1:1 ratio (1.5 mL DMF and 1.5 mL H2O) was used.
However, to our surprise, the reaction of bromotoluene
(1m) was slower than that of 1a. The reaction was incomplete
even after 24 h, giving 2a in only 65% yield with the recovery
of 1m (entry 13). A comparison of the observed reaction
rates12 for the decay of 1a (k1a) and of 1m (k1m) (see SI, Figure
S5 and S6) showed that the value of k1a (5.6 × 10–2 h–1) was
nearly three times that of k1m (2.0 × 10–2 h–1). The significant
difference in the rates of activation of C-Cl and C-Br showed
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Journal of the American Chemical Society
the opposite trend in the catalytic activity of bimetallic
obtained in the case of Au20– system. These results clearly
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Au/Pd catalyst to that of usual transition-metal catalyst.
show that the inclusion of Pd has an effect of stabilizing the
dissociative adsorption and reducing activation energy,
which is not possible in pure Au cluster.
The role of DMF might be critical. We have previously re-
ported that DMF behaves as an excellent reductant in
Au:PVP-catalyzed intramolecular hydroalkoxylation of al-
kenes through donation of hydrogen from its formyl group.13
Indeed, it was confirmed that DMF can play the role of a
reductant in Ullmann coupling. The greater kinetic isotope
effect on the pseudo-first-order kinetics of decay of 1a in the
reaction of 1a to form 2a (kH/kD = 7.0) between DMF and
DMF-d7 (see SI, Figure S7) showed that DMF participates in
the rate-limiting step, which is probably a hydrogen-transfer
process. The reaction of 1l in DMF-d7/H2O proceeded signif-
icantly slower than that in DMF/H2O and, after 48 h it gave
2l in only 80% yield together with 6% of deuterated 3l
(Scheme 1). The results confirmed that the source of hydro-
gen for the hydrodechlorination side reaction was DMF. As a
result, no external reductant such as Zn or formic acid was
necessary under the reaction conditions.
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Scheme 1. Ullmann coupling of 1-l catalysed by bime-
tallic Au0.5Pd0.5:PVP in the presence of DMF-d7/H2O
(1:1 ratio)
Figure 1. Energy profile diagram of oxidative addition of
chlorobenzene on Au20– and Au10Pd10– NCs
–
The model cluster Au16Pd4 that simulates Au0.8Pd0.2 alloy
also show catalytic activity. The structure relaxation of Pd20
cluster, on the other hand, is very large in Int_A3 and this
intermediate is very stable, which prevents the further reac-
tion. These results show that the catalytic activity is high in
the Au/Pd alloy NCs while the activity is lost in the case of
pure Pd NCs. Details of these results are described in sup-
porting information. Our results indicated that involvement
of Au as a nearest heteroatom is crucial to initiate the reac-
tion in one hand and its composition up to 50% in bimetallic
catalyst enhances the catalytic activities in other hand.
Therefore in the current Au/Pd bimetallic system, both alloy
effects2d such as “ligand effect” and “ensemble effect” are
significant that enhances catalytic performance.
An essential step, which determines the characteristics
difference in the catalytic activities of Au, Au/Pd, and Pd NCs
is the oxidative addition of chloroarene followed by the spill
over of Cl,11a and we therefore intensively studied only this
–
step at present. Our previous report demonstrated that Au20
,
a negatively charged homogeneous Au NCs, is a suitable
model for simulating reaction on the surface of Au:PVP.14 We
therefore examined the model systems Au20–, Au16Pd4 ,
–
Au10Pd10–, and Pd20 to compare the reaction pathway for oxi-
dative additions on Au, Pd, and Au/Pd alloy clusters. Fun-
damental consideration of these models and the computa-
tional details are given in supporting information. The calcu-
lations were performed by the Gaussian09 suite of pro-
grams.15 The energy diagram of the oxidative addition of
chlorobenzene (1b) on Au20– and Au10Pd10– is shown in Figure
1 where the energies are shown by taking those of adsorption
complexes being the same for these clusters.
Scheme 2. Possible pathways for Ullmann coupling of
chloroarenes catalyzed by bimetallic Au0.5Pd0.5:PVP
Ar-Cl (1)
Ar-Ar (2)
Au/Pd
HCl
A
D
Me2NH, CO2
KCl, HCl
H
In the case of the Au20– system, 1b is adsorbed on the facet
site and a local minimum is obtained for the dissociative
chemisorption (Int_B1). However, the calculated activation
energy barrier for this (C-Cl) dissociative chemisorption is
Cl
Au/Pd
2G
Cl
DMF, KOH
Ar-Ar (2)
Ar
Cl
Ar
Ar
Cl
Pd Pd
Au
Ar
Cl
H
Au/Pd
Ar-Cl (1)
G
Au/Pd
1A
1C
Ar
E
Ar
–
Cl
Cl
very high at 137.9 kJ/mol. In Au10Pd10 bimetallic cluster
Au/Pd
2E
DMF, KOH
F
Ar-H
DMF, KOH
which simulates Au0.5Pd0.5 alloy, we found two types of in-
termediates for chemisorption. The adsorption was calculat-
ed to proceed with low activation energy barrier of 63.1
kJ/mol to give adsorption intermediate (Int_A2) in which
both the phenyl group and the Cl atom are attached to the
same Pd site. The other intermediate for dissociative chemi-
sorption (Int_B2) is also stable as the adsorbed complex
(Ads) or Int_A2. The calculated activation energy barrier
from Int_A2 is 56.0 kJ/mol. The reaction pathway, which
directly provides Int_B2 from the adsorption complex is also
possible and has a moderate energy barrier of 52.6 kJ/mol.
Note that an intermediate corresponding to Int_A2 was not
Me2NH, CO2
KCl
B
H
Ar
Au/Pd
1B
C
Me2NH, CO2
KCl
Ar-Cl (1)
Ar-H
Possible catalytic pathways are depicted in Scheme 2. The
inclusion of Au as a nearest heteroatom might promote the
spill over of Cl during the oxidative addition, forming an
intermediate 1A (process A).16 Possible routes for the in-
volvement of DMF to complete the redox cycle are displayed
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(6) For Au-catalyzed Ullmann coupling, see: Karimi, B.; Esfahani,
F. K. Chem. Commun. 2011, 47, 10452–10454.
in the mechanism. In the process B, DMF donates the hydro-
gen to form Cl free intermediate 1B, followed by the oxida-
tive addition of ArCl to form intermediate 1C. Subsequent
reductive elimination of Ar-Ar and HCl would lead to regen-
erate the catalyst. The oxidative addition of second molecule
of ArCl before the participation of DMF may also be possible,
leading to the intermediate 2E (process E). Reductive elimi-
nation of Ar-Ar from 2E would afford the intermediate 2G,
followed by the reduction by DMF to complete the redox
cycle. The preliminary theoretical calculations for these other
steps indicate that the present reaction proceeds smoothly
with low energy barriers after the dissociative chemisorption
of ArCl (Int_B).
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(7) (a) Fanta, P. E. Chem. Rev. 1964, 64, 613–632. (b) Fanta, P. E.
Synthesis 1974, 9–621. (c) Hassan, J.; Sévignon, M.; Gozzi, C.; Schulz,
E.; Lemaire, M. Chem. Rev. 2002, 102, 1359–1470. (d) Nelson, T. D.;
Crouch, R. D. Org. React. (Hoboken, NJ, U. S.) 2004, 63, 265–555.
(8) For the recent reports on Ullmann couplings of chloroarenes
under mild conditions, see (a) Li, J.-H.; Xie, Y.-X.; Yin, D.-L. J. Org.
Chem. 2001, 68, 9867–9869. (b) Gallon, B. J.; Kojima, R. W.; Kaner,
R. B.; Diaconescu, P. L. Angew. Chem. Int. Ed. 2007, 46, 7251–7254.
(c) Monopoli, A.; Calò, V.; Ciminale, F.; Cotugno, P.; Angelici, C.;
Cioffi, N.; Nacci, A. J. Org. Chem. 2010, 75, 3908–3911.
(9) For reports on Au:PVP-catalyzed carbon-carbon bond forming
reactions see: (a) Tsukuda, T.; Tsunoyama, H.; Sakurai, H. Chem.
Asian J. 2011, 6, 736–748. (b) Sakurai, H.; Tsunoyama, H.; Tsukuda,
T. J. Organomet. Chem. 2007, 692, 368–374. (c) Tsunoyama, H.; Sa-
kurai, H.; Ichikuni, N.; Negishi, Y.; Tsukuda, T. Langmuir 2004, 20,
11293–11296.
(10) For reports on Pd:PVP-catalyzed carbon–carbon bond-
forming reactions, see: (a) Li, Y.; Hong, X. M. D.; Collard, M.; El-
Sayed, M. A. Org. Lett. 2000, 2, 2385–2388. (b) Ellis, P. J.; Fairlamb, I.
J. S.; Hackett, S. F. J.; Wilson, K.; Lee, A. F. Angew. Chem. Int. Ed.
2010, 49, 1820–1824.
(11) For reports on Au/Pd:PVP-catalyzed reactions, see (a) Dhital,
R. N.; Sakurai, H. Chem. Lett. 2012, 41, 630–632. (b) Yudha S, S.;
Dhital, R. N.; Sakurai, H. Tetrahedron Lett. 2011, 52, 2633–2637. (c)
Hou, W.; Dehm, N. A.; Scott, R. W. J. J. Catal. 2008, 253, 22–27.
(12) The conversions (C) of 1a and 1m were estimated from their
decay as a function of the reaction time (t). The observed linear
relationship between –ln(1–C) and t showed that the reaction is
pseudo-first-order with respect to the substrate (see SI, S6 and S7).
(13) (a) Sakurai, H.; Kamiya, I.; Kitahara, H. Pure Appl. Chem.
2010, 82, 2005–2016. (b) Kamiya, I.; Tsunoyama, H.; Tsukuda, T.;
Sakurai, H. Chem. Lett. 2007, 36, 646–647.
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In summary, Ullmann coupling of chloroarene under am-
bient conditions was achieved by using bimetallic Au/Pd
alloy NCs as a catalyst and DMF as both a co-solvent and
reducing agent. We successfully demonstrated the unique
example of bimetallic effect where each monometallic NCs
and their physical mixture did not catalyze the reaction. The
observed synergistic effect of Au/Pd alloy is different from
the reported bimetallic effect where at least one metal is
mainly in charge of catalytic activity. In addition, the unusual
character of bimetallic Au/Pd alloy NCs, which display a
higher catalytic activity towards chloroarenes than for bro-
moarenes, is interesting. We hope our simplified approach
for activation of strong C-Cl bonds might create new oppor-
tunities for the development of designer multimetallic cata-
lyst for C-C coupling reactions.
ASSOCIATED CONTENT
Supporting Information
(14) Bobuatong, K.; Karanjit, S.; Fukuda, R.; Ehara, M.; Sakurai, H.
Phys. Chem. Chem. Phys. 2012, 14, 3103-3111.
(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E. et
al. Gaussian 09, Revision B.01, Gaussian Inc., Wallingford, CT, 2010.
(16) Corma and co-workers have also shown a similar type of in-
termediate in the oxidative addition of iodobenzene on Au38. See,
Corma, A.; Juárez, R.; Boronat, M.; Sánchez, F.; Iglesias, M.; García,
H. Chem. Commun. 2011, 47, 1446–1448.
Experimental details, characterization, kinetics data and oth-
er theoretical results. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
hsakurai@ims.ac.jp, ehara@ims.ac.jp
ACKNOWLEDGMENT
This work was supported by ACT-C (JST).
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