Au25 nanocluster-catalyzed Ullmann-type homocoupling reaction of aryl iodides

Article abstract of DOI:10.1039/c2cc34765b

The Au₂₅(SR)₁₈/CeO₂ nanocluster catalyst showed high activity in the homocoupling of aryl iodides (e.g. up to 99.8% yield with iodobenzene) and excellent recyclability. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2cc34765b

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COMMUNICATION
Au nanocluster-catalyzed Ullmann-type homocoupling reaction of
aryl iodidesw
25
a
a
b
a
Gao Li, Chao Liu, Yu Lei and Rongchao Jin*
Received 3rd July 2012, Accepted 22nd October 2012
DOI: 10.1039/c2cc34765b
The Au (SR) /CeO nanocluster catalyst showed high activity
2
sized nanoclusters for the Ullmann-type homocoupling reaction
of aryl iodides.
5
18
2
in the homocoupling of aryl iodides (e.g. up to 99.8% yield with
iodobenzene) and excellent recyclability.
The synthesis of Au25(SR)18 nanoclusters followed a literature
6
protocol. The crystal structure of Au25(SR)18 clusters consists of
7
0
a electron-rich Au13 core (i.e. Au ) and an exterior shell of 12 gold
The carbon–carbon coupling reactions are the most widely used
routes for the generation of biaryl compounds in organic synthesis
for the preparation of natural products, pharmaceuticals,
d+
atoms which bear positive charges (i.e. Au , 0 o d o 1). The
Au25(SR)18 cluster possesses quantized electronic states due to
7
1
strong quantum confinement of electrons in the metal core.
8
polymers, and so on. Ullmann-type reaction is one of the
These nanoclusters are thermally stable up to B190 1C and
can mediate electron transfer in organic reactions. The unique
important examples of such organic chemistry, and a prototypical
case is the homocoupling of aryl halides. In general, such
reactions are catalyzed by palladium, nickel and copper
9
10
core–shell geometric structure and charge distributions render
11
2
Au (SR) cluster quite interesting for catalysis. Theoretical
25 18
catalysts in the form of complexes or nanoparticles. Recently,
work indicates that aryl iodide molecules could be adsorbed on
the gold surface via a weak Iꢀ ꢀ ꢀAu bond and p–p interaction,
and subsequently the C–I bond could be activated on the gold
gold(I) and (III) complexes, and gold nanoparticles have been
3
reported to catalyze the carbon–carbon coupling reactions.
Palladium nanoparticles protected by functionalized thiol and
Au–Pd alloy nanoparticles were also used to catalyze the
1
2
surface.
Herein we chose several oxides (e.g. CeO
Al ) as catalytic supports for Au25(SR)18 nanoclusters.
4
2 2 2
, TiO , SiO ,
carbon–carbon coupling reactions.
2 3
O
Gold nanoparticles hold great promise for homogeneous
and heterogeneous catalysis in various organic reactions,
which have attracted considerable attention in recent years,
The catalyst was prepared by impregnation of oxide powders
in a dichloromethane solution of Au25(SR)18, followed by
centrifugation and drying in vacuum. The loading of
Au (SR) nanoclusters was approximately 1 wt%. The oxide
5
such as the oxidation and hydrogenation reactions. On the
other hand, recent advances in the synthesis of gold nano-
particles have permitted the preparation of ultrasmall particles
25
18
supported Au (SR) /TiO catalyst was characterized by scanning
18
25
2
transmission electron microscopy (STEM), as regular TEM
imaging is difficult to observe Au25(SR)18 clusters due to the
background of supports. As shown in Fig. 1, the measured
particle size (B1.3 nm) is comparable with the native size
(
o2 nm) with atomic precision, such as Au (SR) nanoclusters
25
18
(
where SR represents thiolates). These nanoclusters exhibit
non-metallic behavior (e.g. the disappearance of surface plasmon
resonance in the optical spectrum). The non-metallic behavior of
Au nanoclusters originates from the electron energy quantization
effect due to ultrasmall size. The distinct quantum effects may
largely affect the catalytic properties of such ultrasmall Au
nanoparticles. Herein, we report the catalytic properties of
oxide-supported Au25(SR)18 nanoclusters (hereafter R =
CH CH Ph) for the Ullmann-type homocoupling reaction of
7
of Au25(SR)18 (1.27 nm) determined by single crystal X-ray
crystallography. The as-prepared catalyst (without calcina-
tion) was then used in catalytic reactions.
We first optimize the solvent and base for the homocoupling
reaction using the Au (SR) /CeO catalyst (Table 1). The
25
18
2
CeO support was reported to be capable of promoting gold
2
2
2
catalyst’s activity in the coupling reaction compared to other
aryl iodides to yield biaryls. The catalyst gave rise to a very
high conversion of iodobenzene (up to 99.8% with the CeO2
support) and showed excellent recyclability. Moreover, the
catalyst is applicable for a range of aryl iodides. To the best of
our knowledge, this is the first report on the use of quantum
5h,13
supports. The reaction was performed at 130 1C for 2 days.
Initial studies on the solvent effect showed that non-polar
solvents, such as toluene and o-xylene, gave very low yields
(
oca.30%) with K
3 4 2 3
PO or K CO as the base in the homo-
coupling reaction of iodobenzene (Table 1, entries 1–4).
Interestingly, with dimethylformamide (DMF) as the solvent,
a
Department of Chemistry, Carnegie Mellon University, Pittsburgh,
PA 15213, USA. E-mail: rongchao@andrew.cmu.edu
Energy Systems Division, Argonne National Laboratory,
9700 South Cass Avenue, ES/362, Argonne, IL 60439, USA
3 4
the conversion was largely improved to 54.5% with K PO as
b
the base (Table 1, entry 5). The high conversion in the DMF
solvent may be attributed to the improved stability of metal
nanoclusters under the reaction conditions since DMF can
w Electronic supplementary information (ESI) available: Experimental
procedures and H NMR spectra. See DOI: 10.1039/c2cc34765b
1
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 12005–12007 12005

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Table 2 The catalytic performance of Au25(SR)18 nanoclusters
supported on various oxides for homocoupling of iodobenzene
a
Entry
Catalysts
Conversion (%)
1
2
3
4
5
6
Au25(SR)18/CeO
2
99.8
99.3
99.5
98.9
12.1
0
Au25(SR)18/TiO
Au25(SR)18/SiO
Au25(SR)18/Al
Unsupported Au25(SR)18
CeO
2
2
2 3
O
b
2
a
2 3
Reaction conditions: 0.2 mmol iodobenzene, 0.6 mmol K CO , 100 mg
Au25(SR)18/oxide (1 wt% loading), 1 mL DMF, 130 1C, reaction time:
2
Fig. 1 (left) STEM image of the Au25(SR)18/TiO
18ꢁ
7
mined by X-ray crystallography (unit cell dimensions: a = 16.156 A,
2
catalyst; (right)
b
days. The amount of unsupported Au25(SR)18 was 1 mg.
+
(counterion: N(C
total structure of Au25(SC
2
H
4
Ph)
8
H
17
)
4
deter-
˚
stably adsorbed on the support’s surface due to charge–
dipole interaction (note: the Au25 core bears ꢁ1 charge) and
van der Waals interactions. Thus, the supported catalysts are
more durable in the catalytic reaction. Control experiments
˚
b = 17.388 A, c = 18.641 A); Au25 diameter: 1.27 nm (Au atom edge-
˚
to-edge distance, we take the Au atom diameter to be 0.288 nm).
Table 1 Effects of solvents and bases on the homocoupling reaction
a
catalyzed by Au25(SR)18/CeO
2
(
using plain oxide powders) gave no conversion of iodobenzene
under identical reaction conditions, thus, the homocoupling
reaction of iodobenzene is catalyzed by gold nanoclusters.
The Au25(SR)18-catalyzed homocoupling reaction can also
be expanded to a series of substrates with various substituents.
As shown in Table 3, significant electronic effects of substrates
were observed. The electron-rich p-iodoanisole (Table 3, entry 2)
was homocoupled at a much higher conversion (99.5%) than the
electron-deficient 1-iodo-4-nitrobenzene (67.5%, Table 3, entry 3).
Meanwhile, when the ꢁCHO substituent was introduced into
phenyl iodide, the conversion was decreased to 78.2% (Table 3,
entry 4). Furthermore, it was observed that the Au (SR) /CeO
b
Conversion (%)
Entry
Solvent
Base
1
2
3
4
5
6
Toluene
Toluene
o-Xylene
o-Xylene
DMF
K
3
K
2
K
3
K
2
K
3
K
2
PO
CO
PO
CO
PO
CO
4
8.4
31.2
2.2
18.7
54.5
99.8
3
3
3
4
4
DMF
a
Reaction conditions: 0.2 mmol iodobenzene, 0.6 mmol base, 100 mg
(1 wt% loading), 1 mL solvent, 130 1C, reaction
time: 2 days. The conversion of iodobenzene was analyzed by
25
18
2
Au25(SR)18/CeO
2
b
catalyst can also act as an excellent catalyst for homocoupling
0
1
H NMR.
of 1-iodonaphthalene to yield 1,1 -binaphthyl with a 99.7%
conversion (Table 3, entry 5).
We further investigated the catalytic properties of the recycled
1
1b,14
protect nanoclusters as reported in the literature.
further improve the yield, we substitute K CO for K
To
PO
2 2
Au25(SR)18/CeO catalyst. In this test, the Au25(SR)18/CeO
2
3
3
4
catalyst was recovered by simple centrifugation after the reaction
was finished. Then, with the recycled catalyst, a fresh reaction
as the base and found that the conversion of iodobenzene was
significantly increased to 99.8% (Table 1, entry 6). The turnover
ꢁ
1
number (TON) is B16.4 h (calculated as the converted moles
of iodobenzene divided by two and further divided by the
reaction time and the moles of Au (SR) nanoclusters). In
Table 3 Homocoupling of aryl iodides using the Au25(SR)18/CeO2
catalyst
a
2
5
18
the following studies, we choose DMF as the solvent and
CO as the base for further investigation of the catalytic
K
2
3
properties of Au25(SR)18 nanoclusters. Of note, a base is
necessary in the Ullmann-type homocoupling reaction, as it
Entry
1
Substrates
Conversion (%)
99.8
0
could promote the process of the R ꢀ ꢀ ꢀAuꢀ ꢀ ꢀI intermediate to
0
0
the R ꢀ ꢀ ꢀAuꢀ ꢀ ꢀR intermediate, and finally to the R –R product.
To investigate the potential effect of the oxide support of the
Au25(SR)18 catalyst, we compared four different supports
2
3
4
5
a
99.5
67.5
78.2
99.7
(
Table 2). It is interesting to note that there is no obvious
difference in the catalytic performance of the catalysts with the
different supports (i.e. Au (SR) /CeO , Au (SR) /TiO ,
18
2
5
18
2
25
2
2 3 2
Au25(SR)18/Al O and Au25(SR)18/SiO ). In all cases, B99%
conversion of iodobenzene was obtained. On the other hand,
when unsupported Au25(SR)18 clusters were used as the
catalyst, only 12.1% conversion was obtained, which is caused
by gradual decomposition of the free (i.e. unsupported)
Au25(SR)18 clusters over the course of the 130 1C reaction.
In contrast, the supported clusters are much more stable
Reaction conditions: 0.2 mmol substrate, 0.6 mmol K
Au25(SR)18/CeO (1 wt% loading), 1 mL DMF, 130 1C, reaction time:
days.
2 3
CO , 100 mg
2
(at least can be recycled multiple times, see ESIw). Although
the ligands are present on the cluster surface, the clusters can be
2
1
2006 Chem. Commun., 2012, 48, 12005–12007
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Notes and references
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´
lez-
Scheme 1 Proposed mechanism for the Ullmann-type homocoupling
reaction of iodobenzene catalyzed by Au25(SR)18 nanoclusters (green:
surface Au atoms, yellow: core Au atoms, the thiol ligands are omitted
for clarity).
´
2
´
´
´
2
was performed with a fresh solvent and reactants under
identical conditions. We found that the conversion was only
slightly decreased (within 5%) after five cycles (see ESIw); thus,
131, 2060; (f) H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi
and T. Tsukuda, Langmuir, 2004, 20, 11293; (g) S. Carrettin,
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4, 2242; (h) B. Karimi and F. K. Esfahani, Chem. Commun.,
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2
the Au25(SR)18/CeO catalyst shows an excellent recyclability.
2
Soc., 2010, 132, 12859.
With respect to the catalytic mechanism, the reactant iodo-
benzene is thought to be first absorbed on the gold surface due
to the Iꢀ ꢀ ꢀAu interaction, and then the CꢁI bond becomes
activated on the nanocluster surface. In the case of gold(I)
complex catalysts, formation of the Iꢀ ꢀ ꢀAuꢀ ꢀ ꢀC intermediate is
supported by gas-phase studies of a combination of mass-
spectrometry-based experiments and density functional theory
4
5
(a) M. Cargnello, N. L. Wieder, P. Canton, T. Montini,
G. Giambastiani, A. Benedetti, R. J. Gorte and P. Fornasiero,
Chem. Mater., 2011, 23, 3961; (b) J. Xu, A. R. Wilson,
A. R. Rathmell, J. Howe, M. Chi and B. J. Wiley, ACS Nano,
2
011, 5, 6119.
(a) R. Jin, Nanotechnol. Rev., 2011, 1, 31, and references therein;
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e) Y. Liu, H. Tsunoyama, T. Akita and T. Tsukuda, Chem.
(
2
¨
3i,12
(DFT) calculations.
The above catalytic results demonstrate
(
the excellent catalytic performance of Au25(SR)18 nanoclusters in
the C–C homocoupling reaction of aryl iodides. The Au25(SR)18
Commun., 2010, 46, 550; (f) D. R. Kauffman, D. Alfonso,
C. Matranga, H. Qian and R. Jin, J. Am. Chem. Soc., 2012,
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Chem., Int. Ed., 2010, 49, 1295; (h) S. K. Beaumont, G. Kyriakou
and R. M. Lambert, J. Am. Chem. Soc., 2010, 132, 12246;
10
nanoclusters have a good capability of electron transfer.
We propose the following mechanism for the Ullmann-type
homocoupling reaction of iodobenzene catalyzed by Au (SR)
18
25
(
1
i) N. Thielecke, M. Aytemir and U. Drusse, Catal. Today, 2007,
21, 115.
nanoclusters (Scheme 1). First, the substrate is absorbed on the
0
surface of the Au (SR) /CeO catalyst to form [Au ](R I)
25
6 M. Zhu, E. Lanni, N. Garg, M. E. Bier and R. Jin, J. Am. Chem.
Soc., 2008, 130, 1138.
M. Zhu, C. M. Aikens, F. J. Hollander, G. C. Schatz and R. Jin,
J. Am. Chem. Soc., 2008, 130, 5883.
8 (a) Z. Wu and R. Jin, ACS Nano, 2009, 3, 2036; (b) Y. Zhu,
H. Qian and R. Jin, Chem.–Eur. J., 2010, 16, 11455.
M. Zhu, W. T. Eckenhoff, T. Pintauer and R. Jin, J. Phys. Chem.
C, 2008, 112, 14221.
0 M. Zhu, C. M. Aikens, M. P. Hendrich, R. Gupta,
2
5
18
2
2
0
(
R = Ph), and then the C–I bond activation should occur by
7
the exterior Au12 shell since the shell gold atoms appear positive
i.e. Aud+, 0 o d o 1), giving rise to [Au25
0
(
R
]I
2 2
intermediates.
0
The reductive elimination of [Au25
product biphenyl (Scheme 1).
R ]I generates the final
2 2
9
In summary, we have investigated the catalytic capability of the
Au25(SR)18/CeO catalyst for the carbon–carbon homocoupling
1
2
H. Qian, G. C. Schatz and R. Jin, J. Am. Chem. Soc., 2009,
31, 2490.
1
reaction of iodobenzene to the biphenyl product. No distinct effect
of the oxide supports was observed. We have demonstrated that the
catalyst can catalyze a series of aryl iodides with high conversion.
1
1 (a) Y. Zhu, Z. Wu, G. C. Gayathri, H. Qian, R. R. Gil and R. Jin,
J. Catal., 2010, 271, 155; (b) H. Yamamoto, H. Yano, H. Kouchi,
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R. A. J. O’Hair, Angew. Chem., Int. Ed., 2012, 51, 3878;
The catalyst can also be easily recovered. The positively charged
surface gold atoms (Aud+) of Au25(SR)18 nanoclusters are ratio-
nalized to be active sites for activating iodobenzene, whereas the
(
b) U. Mazurek and H. Schwarz, Chem. Commun., 2003, 1321.
10
electron-rich, redox-active Au13 kernel participates the electron
transfer process in the catalytic reaction. The gold nanoclusters hold
a great potential in catalysis and may be further explored for other
carbon–carbon coupling reactions.
13 (a) A. Goguet, M. Ace, Y. Saih, J. Sa, J. Kavanagh and
C. Hardacre, Chem. Commun., 2009, 4889; (b) C. Gonzalez-
´
Arellano, A. Abad, A. Corma, H. Garcıa, M. Iglesias and
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´
1
4 (a) X. Liu, C. Li, J. Xu, J. Lv, M. Zhu, Y. Guo, S. Cui, H. Liu,
S. Wang and Y. Li, J. Phys. Chem. C, 2008, 112, 10778;
This work is financially support by the Air Force Office of
Scientific Research under AFOSR Award No. FA9550-11-1-
(
b) H. Kawasaki, H. Yamamoto, H. Fujimori, R. Arakawa,
Y. Iwasaki and M. Inada, Langmuir, 2010, 26, 5926;
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Y. Obora, Chem. Commun., 2011, 47, 5750.
9
999 (FA9550-11-1-0147). NMR instrumentation at CMU is
(
partially supported by NSF (CHE-0130903).
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 12005–12007 12007