Anomalous efficacy of bimetallic Au/Pd nanoclusters in CCl bond activation and formal metathesis-type CB bond activation at room temperature

Article abstract of DOI:10.1246/cl.2012.630

Au/Pd alloy nanoclusters stabilized by poly(N-vinylpyrrolidone) catalyze two different reactions of phenylboronic acid with 4-chlorobenzoic acid at room temperature in a single reaction cycle, cross coupling and metathesis-type homocoupling that is normally inaccessible through conventional catalysis.

Full text of DOI:10.1246/cl.2012.630

doi:10.1246/cl.2012.630
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Published on the web June 2, 2012
Anomalous Efficacy of Bimetallic Au/Pd Nanoclusters in C­Cl Bond Activation
and Formal Metathesis-type C­B Bond Activation at Room Temperature
1
1,2
Raghu Nath Dhital and Hidehiro Sakurai*
Department of Functional Molecular Science, School of Physical Sciences,
1
Graduate University for Advanced Studies (SOKENDAI), Myodaiji, Okazaki, Aichi 444-8787
2
Research Center for Molecular Scale Nanoscience, Institute for Molecular Science,
Myodaiji, Okazaki, Aichi 444-8787
(
Received March 19, 2012; CL-120235; E-mail: hsakurai@ims.ac.jp)
Au/Pd alloy nanoclusters stabilized by poly(N-vinylpyr-
rolidone) catalyze two different reactions of phenylboronic acid
with 4-chlorobenzoic acid at room temperature in a single
reaction cycle, cross coupling and metathesis-type homocou-
pling that is normally inaccessible through conventional
catalysis.
Bimetallic nanoclusters with an alloy structure have
attracted a great deal of interest during the past decade owing
to their enhanced catalytic activities, which are superior to those
of the corresponding monometallic counterparts.1 Among the
various bimetallic nanoclusters fabricated to date, Au/Pd alloy
clusters are particularly fascinating because of their excellent
,2
Scheme 1. Suzuki­Miyaura cross-coupling reaction (cycle A)
and aerobic (oxidative) homocoupling of arylboronic acid
(
cycle B).
3
catalytic activities for a diversity of chemical transformations.
Recent research reports have, however, dealt mainly with the
significant extension of catalytic reactivity and selectivity of
monometallic nanoclusters.3 Herein, we report the peculiar
catalytic activity of bimetallic Au/Pd alloyed nanoclusters in
coupling between 4-chlorobenzoic acid (CBA) and phenyl-
,4
boronic acid (PhB(OH) ) under aqueous conditions at room
2
temperature.
Among the organic halides including triflates, organic
chlorides are less reactive due to relatively high C­Cl bond
strength and their reluctance to undergo oxidative addition.
Recently significant progress has been made toward activation of
aryl chlorides by the use of highly electron-donative and bulky
5
ligands. These ligands make the metal more nucleophilic,
which facilitates oxidative addition by less active C­Cl bonds,
the initial step of cross-coupling reaction. We previously
reported that colloidal gold nanoclusters, such as poly(N-
vinylpyrrolidone)-protected gold (Au:PVP) or chitosan [¢-1,4-
linked poly(D-glucosamine)]-protected gold (Au:chit), are ex-
cellent catalysts for aerobic oxidation including homocoupling
reactions of arylboronic acids and their derivatives (cycle B in
Scheme 1) in water under ambient conditions.6 One character-
istic feature of colloidal nanogold is that a negatively charged
surface is generated due to the size effect of the clusters as well
as the donation effect from the matrices, realizing the facile
,7
Scheme 2. Coupling reactions between CBA and PhB(OH)2
catalyzed by (a) Au:PVP and (b) Pd:PVP, N/D: not detected.
8
adsorption of molecular oxygen on its surface. Such electroni-
of two metals (Au0.5Pd0.5:PVP) were prepared according to our
9
cally enriched cluster surface would also be expected to
accelerate the oxidative addition process in the Suzuki­Miyaura
cross coupling (cycle A in Scheme 1). Since Pd always favors
the cross coupling against homocoupling as compared with Au,
we started to develop PVP-stabilized bimetallic Au/Pd alloy
clusters for the activation of C­Cl bond under mild conditions in
the Suzuki­Miyaura reaction. The PVP-stabilized Au and Pd
monometallic and bimetallic alloy clusters containing 1:1 ratios
previous procedure.
First, the coupling reactions between CBA and PhB(OH)2
by monometallic clusters were investigated. Equimolar amounts
of CBA and PhB(OH)2 were used as reactants. The cross-
coupling product, biphenyl-4-carboxylic acid (1) was not
observed, but biphenyl (2) and phenol (3) were observed in
73% and 26% yields (Scheme 2a), respectively in water at room
temperature, which is in good agreement with our previous
Chem. Lett. 2012, 41, 630­632
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Figure 1. 11B NMR spectra recorded at pH 9.7 in 33% D2O.
Reaction conditions: CBA (0.15 mmol), PhBpin (0.15 mmol),
3
00 mol % K2CO3, 2 atom % Au0.5Pd0.5:PVP, 2 mL H2O, 1 mL
D2O, 27 °C, 24 h degassed.
PhB(OH) continued to occur in the absence of oxidant in faire
2
competition with cross-coupling product. The constant ratios
of 1 and 2 were observed in each time interval suggest that
both products are probably formed in a single catalytic cycle
Scheme 3. Coupling reactions between CBA and PhB(OH)2
catalyzed by (a) Au0.5Pd0.5:PVP and (b) Au:PVP + Pd:PVP.
1
6
(
Figure S5, Supporting Information ). This result indicates that
report that Au:PVP promotes only homocoupling (and the
the observed reaction is unlikely to follow the conventional
mechanism (Scheme 1) and is more likely to occur by a new
mechanism. To understand the reaction pathway of the homo-
7
a
oxygenation) and no activity towards the cross coupling. To
suppress the oxidative homocoupling and oxygenation, the
reaction was performed under carefully degassed conditions,
11
coupling, the reaction was monitored by B NMR spectroscopy
to follow the intermediates derived from PhB(OH)2. Pinacol
ester of phenylboronic acid (PhBpin) was used as a reactant
instead of PhB(OH)2 due to the expectation of these intermedi-
ates stabilized by the pinacol ester formation. We confirmed that
the same reaction also took place in the case of PhBpin to give
cross-coupling product and homocoupling product in 50% and
49% yield, respectively, after prolonged time (27 h). As shown
in Figure 1, bis(pinacolato)diborane (pin B ) was detected after
7
a,10
giving no reaction.
The results did not change when the
reaction was carried out at 90 °C. According to these results,
Au:PVP should not be able to catalyze the Suzuki­Miyaura
11
cross-coupling because Au is unable to activate C­Cl bonds.
Similar results were observed when Pd:PVP was used as a
catalyst although Pd:PVP showed inferior activity as compared
to Au:PVP towards the aerobic oxidations for yielding 2 and 3
(Scheme 2b). The lack of cross-coupling activity of Pd:PVP is
2
2
consistent with previous reports where Pd:PVP-catalyzed cou-
pling of aryl iodides or bromides as coupling partners were
described but not aryl chlorides.12
16 h. It can be concluded that a metathesis-type reaction of
PhB(OH)2 takes place under anaerobic conditions, giving
biphenyl (2) and tetrahydroxydiborane [(HO) B­B(OH) ]. To
2
2
To our delight, bimetallic clusters did exhibit a cross-
coupling activity and 1 was obtained in 14% yield, along with
the best of our knowledge, this is the first example of the
homocoupling reaction of organoboron compounds through
the formal metathesis reaction not by oxidative coupling. To
summarize, the total reaction occurs between two equivalents
of CBA and four equivalents of PhB(OH)2 to give two
equivalents of biphenyl-4-carboxylic acid (1), one equivalent
65% of 2 and 21% of 3 in the presence of Au0.5Pd0.5:PVP under
air (Scheme 3a). Although we knew that Au/Pd bimetallic
clusters are potentially active in the cross-coupling reaction, the
oxidative homocoupling and oxygenation side reactions could
not be suppressed unless the reactions were carried out under
of biphenyl (2), two equivalents of boric acid [B(OH) ], and
3
anaerobic conditions.7
a,10
We therefore examined the reactions
one equivalent of tetrahydroxydiborane in the presence of the
bimetallic catalyst in H2O at 27 °C as shown in Scheme 4.
Although the details of the reaction mechanism are still
unclear, mechanisms involving a single-metal (Au or Pd)
catalytic site can be ruled out for the following reasons. First,
it is impossible to express the overall reactions described in
Scheme 4 in terms of a single-site catalyst, at least if that single
site catalyst behaves in the same manner as in the conventional
under carefully degassed conditions and as a result 50% of 1 and
0% of 2 (yield based on PhB(OH) ; i.e., 2:1 molar ratios) were
5
2
obtained with Au0.5Pd0.5:PVP as the catalyst. To our surprise,
however, biphenyl was still obtained in 50% yield, in the
absence of oxidant although none of the oxygenated product 3
was produced. To confirm the significance of the bimetallic
cluster structure, the reaction was carried out in the presence of
a physical mixture of 1 atom % Au:PVP and 1 atom % Pd:PVP,
resulting in the trace amounts of 1 and 2 (Scheme 3b). These
results clearly showed that a synergistic effect originating from
the presence of an alloyed phase plays a crucial role in the
coupling reaction.
1
4
Suzuki­Miyaura coupling. Next, previous reports have sug-
gested that catalysis in palladium nanocluster-catalyzed cross-
coupling reactions might occur in solution involving atomic Pd
leached from clusters if oxidative addition occurs on a single
2+
palladium atom to afford a Pd intermediate. We examined the
leaching of Pd and Au by means of inductively coupled plasma
atomic-emission spectroscopy (ICP-AES) analysis after com-
pletion of the reaction, and the levels of Pd and Au ions were
found below the detection limits (<44 ppb of Pd and <50 ppb of
The oxidative homocoupling of phenylboronic acid
(Scheme 1, cycle B) could only occur in the presence of
stoichiometric amount of chemical oxidant or molecular oxygen
7
a,10,13
or air.
In contrast, we observed the homocoupling of
Chem. Lett. 2012, 41, 630­632
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H2O
reaction involves two processes of cross-coupling and meta-
thesis-type homocoupling in a single reaction cycle where both
Au and Pd are synergistically participating. We believe that the
modified electronic structure of the catalyst and/or the nature
of the active sites, resulting from a synergistic effect by the
presence of alloy phases, play a crucial role in generating the
unusual catalytic properties and open new possibilities for
reactions that are normally inaccessible through conventional
catalysis.
2
Cl
CO2H + 4 (HO) B
2
+ 2OH−
2
HO2C
+
+ (HO)2B B(OH)2 + 2B(OH)3
+
2Cl
−
Scheme 4. The overall reaction catalyzed by Au0.5Pd0.5:PVP.
References and Notes
1
2
3
4
a) R. Ferrando, J. Jellinek, R. L. Johnston, Chem. Rev. 2008, 108,
45. b) N. Toshima, T. Yonezawa, New J. Chem. 1998, 22, 1179.
J. R. Kitchin, J. K. Nørskov, M. A. Barteau, J. G. Chen, Phys. Rev.
Lett. 2004, 93, 156801.
a) G. J. Hutchings, Chem. Commun. 2008, 1148. b) M. Chen, D.
Kumar, C.-W. Yi, D. W. Goodman, Science 2005, 310, 291.
a) P. Landon, P. J. Collier, A. F. Carley, D. Chadwick, A. J.
Papworth, A. Burrows, C. J. Kiely, G. J. Hutchings, Phys. Chem.
Chem. Phys. 2003, 5, 1917. b) D. I. Enache, J. K. Edwards, P.
Landon, B. Solsona-Espriu, A. F. Carley, A. A. Herzing, M.
Watanabe, C. J. Kiely, D. W. Knight, G. J. Hutchings, Science 2006,
8
Scheme 5. Proposed model for synergistic involvement of
atoms of both metals in the oxidative addition processes.
Au). When the reaction was stopped after 8 h in the in situ
leaching test and samples of the filtrate were monitored
for further reaction, the conversion remained unchanged
311, 362. c) A. Murugadoss, H. Sakurai, J. Mol. Catal. A: Chem.
2011, 341, 1. d) H. Zhang, T. Watanabe, M. Okumura, M. Haruta, N.
16
(
Scheme S1, Supporting Information ). These experiments
Toshima, Nat. Mater. 2012, 11, 49.
strongly support the view that leaching of metal species (Pd or
Au) does not form part of the catalytic cycle. In addition, the
initial rates of formation of biphenyl-4-carboxylic acid (1) and
biphenyl (2) depend linearly on the concentration of the
5
a) A. F. Littke, G. C. Fu, Angew. Chem., Int. Ed. 2002, 41, 4176. b)
T. E. Barder, S. D. Walker, J. R. Martinelli, S. L. Buchwald, J. Am.
Chem. Soc. 2005, 127, 4685. c) A. Zapf, A. Ehrentraut, M. Beller,
Angew. Chem., Int. Ed. 2000, 39, 4153. d) G. Altenhoff, R. Goddard,
C. W. Lehmann, F. Glorius, J. Am. Chem. Soc. 2004, 126, 15195.
A review: T. Tsukuda, H. Tsunoyama, H. Sakurai, Chem.®Asian J.
16
Au0.5Pd0.5:PVP (Figure S7 ), providing strong evidence that
the oxidative addition occurs on the surface of bimetallic Au/
6
7
2011, 6, 736.
1
2a
Pd:PVP. Furthermore, the initial rate of formation of 1 and 2
was independent of the concentration of CBA and it showed
a) H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi, T. Tsukuda,
Langmuir 2004, 20, 11293. b) H. Sakurai, H. Tsunoyama, T.
Tsukuda, J. Organomet. Chem. 2007, 692, 368. c) R. N. Dhital, A.
Murugadoss, H. Sakurai, Chem.®Asian J. 2012, 7, 55.
16
zero-order kinetics with CBA (Figure S6 ), which indicated
that the oxidative addition of CBA is feasible on a bimetallic
surface. Since the presence of a bimetallic structure is essential
for the reaction to occur, as judged from the results shown in
Scheme 3b, the catalytic cycle of the reaction, particularly the
oxidative addition process, must involve multiple metals on the
surface of bimetallic clusters.
8
a) H. Tsunoyama, H. Sakurai, Y. Negishi, T. Tsukuda, J. Am. Chem.
Soc. 2005, 127, 9374. b) H. Tsunoyama, N. Ichikuni, H. Sakurai, T.
Tsukuda, J. Am. Chem. Soc. 2009, 131, 7086.
9
1
S. Yudha S., R. N. Dhital, H. Sakurai, Tetrahedron Lett. 2011, 52,
2633.
0 a) C. Adamo, C. Amatore, I. Ciofini, A. Jutand, H. Lakmini, J. Am.
Chem. Soc. 2006, 128, 6829. b) J. P. Parrish, Y. C. Jung, R. J. Floyd,
K. W. Jung, Tetrahedron Lett. 2002, 43, 7899.
On the basis of these results, we would like to propose a
model, shown in Scheme 5, for the oxidative addition at the
gold/palladium surface. In this model, more than two adjacent
atoms synergistically assist cleavage of the C­Cl bond (tran-
sition state A), so that an organic unit becomes attached to one
atom (probably Pd) and the chloride ion becomes attached to
other atoms, such as those on the Au surface, to form the stable
bridged intermediate (B). This process might prevent the
11 A recent paper has reported the Suzuki­Miyaura cross-coupling
reaction by poly(2-aminothiophenol)-protected nanogold (Au:PATP)
at high temperature: J. Han, Y. Liu, R. Guo, J. Am. Chem. Soc. 2009,
1
31, 2060.
2 a) Y. Li, X. M. Hong, D. M. Collard, M. A. El-Sayed, Org. Lett.
000, 2, 2385. b) Y. Li, M. A. El-Sayed, J. Phys. Chem. B 2001, 105,
1
2
8938. c) Y. Li, E. Boone, M. A. El-Sayed, Langmuir 2002, 18, 4921.
d) F. Durap, Ö. Metin, M. Aydemir, S. Özkar, Appl. Organomet.
Chem. 2009, 23, 498. e) P. J. Ellis, I. J. S. Fairlamb, S. F. J. Hackett,
K. Wilson, A. F. Lee, Angew. Chem., Int. Ed. 2010, 49, 1820.
3 a) N. Kirai, Y. Yamamoto, Eur. J. Org. Chem. 2009, 1864. b) G.
Cheng, M. Luo, Eur. J. Org. Chem. 2011, 2519. c) T. Vogler, A.
Studer, Adv. Synth. Catal. 2008, 350, 1963.
2+
formation of M intermediates through oxidative addition at
a single metal site, which would otherwise permit leaching to
occur and would also result in the dual-catalyst behavior in the
formal cross coupling and metathesis-type homocoupling
reactions. On the basis of DFT calculation, Corma and co-
workers have also proposed that a similar type of oxidative
addition process occurs during the reaction of iodobenzene with
1
14 a) A. V. Gaikwad, A. Holuigue, M. B. Thathagar, J. E. ten Elshof, G.
Rothenberg, Chem.®Eur. J. 2007, 13, 6908. b) M. B. Thathagar,
J. E. ten Elshof, G. Rothenberg, Angew. Chem., Int. Ed. 2006, 45,
2886. c) L. Strimbu, J. Liu, A. E. Kaifer, Langmuir 2003, 19, 483. d)
1
5
Au .
3
8
J.-S. Chen, A. N. Vasiliev, A. P. Panarello, J. G. Khinast, Appl.
Catal., A 2007, 325, 76. e) S. S. Soomro, F. L. Ansari, K.
Chatziapostolou, K. Köhler, J. Catal. 2010, 273, 138. f) P.-P. Fang,
A. Jutand, Z.-Q. Tian, C. Amatore, Angew. Chem., Int. Ed. 2011, 50,
In summary, we have demonstrated anomalous behavior
of bimetallic Au/Pd alloy systems in the coupling reaction
of PhB(OH) with CBA at room temperature under aqueous
2
12184.
conditions. The most important finding is that the reaction
mechanism is likely to be completely different from that of the
conventional Suzuki­Miyaura coupling, which can be described
in terms of the participation of a single metal (Pd). Instead, our
1
1
5 A. Corma, R. Juárez, M. Boronat, F. Sánchez, M. Iglesias, H. García,
Chem. Commun. 2011, 47, 1446.
6 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2012, 41, 630­632
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/