Combinatorial Design of Copper-Based Mixed Nanoclusters: New Catalysts for Suzuki Cross-Coupling

Article abstract of DOI:10.1002/adsc.200303045

Quantum dots (2-5 nm) of copper and copper/palladium mixtures are found to be good catalysts for Suzuki cross-coupling. The catalysts are applicable to a wide range of iodo- and bromoaryl substrates, and give moderate yields using chloroaryl substrates. Cluster activity and stability is found to depend strongly on the preparation method and the reaction conditions. The mechanism of cluster deactivation and the sensitivity of the cluster-catalysed reaction to substituent effects are studied and discussed.

Full text of DOI:10.1002/adsc.200303045

FULL PAPERS
Combinatorial Design of Copper-Based Mixed Nanoclusters:
New Catalysts for Suzuki Cross-Coupling
Mehul B. Thathagar, Jurriaan Beckers, Gadi Rothenberg*
Chemical Engineering Department, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam,
The Netherlands
Fax: ( ‡ 31)-20-525-5604, e-mail: gadi@science.uva.nl
Received: February 20, 2003; Accepted: May 8, 2003
This paper is dedicated to the memory of Niraj B. Thathagar, who died at 23 years in a tragic accident in Mumbai,
India.
Abstract: Quantum dots (2 5 nm) of copper and tivation and the sensitivity of the cluster-catalysed
copper/palladium mixtures are found to be good reaction to substituent effects are studied and dis-
catalysts for Suzuki cross-coupling. The catalysts are cussed.
applicable to a wide range of iodo- and bromoaryl
substrates, and give moderate yields using chloroaryl
substrates. Cluster activity and stability is found to Keywords: aryl halides; biaryls; bimetallic catalysts;
depend strongly on the preparation method and the combinatorial catalysis; copper; cross-coupling; nano-
reaction conditions. The mechanism of cluster deac- clusters
Introduction
aggregates, the catalyst passes through an intermediate
phase, forming small clusters. Such nanoclusters are
Catalysis is traditionally divided into −heterogeneous×, themselves capable of catalysing Suzuki,[17] Heck,[12] and
−homogeneous× and −enzymatic× subsections, each with Ullmann-type coupling reactions[18] (see Figure 1). If
its own methods and applications. Metal nanoclusters in aggregation is prevented, catalytic activity comparable
the 2 5 nm range (also called −quantum dots×) are too with homogeneous systems can be achieved. Multi-
big to be −homogeneous catalysts×, and too small to be metallic cluster systems are important as incorporation
considered −bulk metals×.[1] These nanoclusters can of different metals can induce large changes in the
display unique catalytic properties that differ from their structure and enhance the catalytic properties of the
homogeneous and/or heterogeneous analogues. A sec- active component.[19 22]
ond important aspect of metal nanoclusters is their huge
The high activity reported for monometallic[23 27] and
surface area. In small clusters most atoms are at the bimetallic[12] nanoclusters encouraged us to investigate
surface, which can lead to very high catalytic activity.[2,3] further cluster catalysts for the Suzuki reaction. Copper-
Transition metal nanoclusters, especially those of noble
metals, have been reported as catalysts in olefin hydro-
genation,[4 8] hydrosilation,[9] and C C coupling reac-
À
tions.[10 12]
Formation of carbon-carbon bonds via cross-coupling
of aryl halides with arylboronic acids (the Suzuki
Miyaura reaction) is a key synthetic step in the
production of agrochemicals, polymers, and pharma-
ceuticals.[13] This reaction benefits from low reagent
toxicity and easy separation of by-products. Tradition-
À
ally, C C coupling is carried out using palladium(0), in
the presence of ligands,[14,15] that are often laborious to
synthesise.[16] During the reaction, and especially at high
conversions, the palladium complexes deactivate and
ultimately precipitate as micrometric aggregates (palla-
dium black) or coat the vessel with a mirror. However,
Figure 1. Cross-coupling of phenylboronic acid and haloben-
zene to give a biaryl derivative, showing clustering of the
homogeneous palladium(0) catalysts followed by catalyst
between the active Pd(0) atoms and the inactive bulk deactivation through aggregation.
Adv. Synth. Catal. 2003, 345, 979 985
DOI: 10.1002/adsc.200303045
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Mehul B. Thathagar et al.
FULL PAPERS
based alternatives are attractive because they are both
cheaper and ™greener∫ than any noble metals. More-
over, Cu(I) salts and Cu2O have recently been reported
as co-reagents[28 30] and co-catalysts[31] in C C and
C N coupling reactions. In a preliminary communi-
À
[32]
À
cation, we reported the first application of copper-based
mixed nanoclusters as Suzuki cross-coupling cata-
lysts.[33] Here we present a detailed investigation of the
reaction scope and try to gain some insight into its
mechanism.
Results and Discussion
Nanocluster Catalysts via NaBH4 Reduction
We first synthesised nanoclusters by using sodium
borohydride as reducing agent in the presence of
tetraalkylammonium bromide (TOAB) as stabiliser. In
a typical synthesis, 0.26 equivalents of aqueous borohy-
dride were added dropwise to a toluene solution of
palladium chloride containing 1.5 equivalents of tetra-n-
octylammonium bromide (TOAB). A black colloidal
suspension formed. This suspension, containing the Pd
(or Ru or Pd/Ru) clusters, was tested as is in the cross-
coupling of phenylboronic acid and iodobenzene to give
biphenyl (Equation 1).
Figure 2. Suzuki coupling of PhB(OH)2 with PhI catalysed by
palladium nanoclusters (prepared via NaBH4 reduction).
Reaction conditions: 3.00 mmol PhI, 6.00 mmol PhB(OH)2,
*
6.00 mmol K2CO3, 50 mL toluene, 1108C. ( ) 1 mol %
*
catalyst, added in one portion at t ˆ 0; ( ) 1 mol % catalyst
in total, added in three equal portions at intervals of 30 min.
The preparation of TOAB-protected Pd and Ru
nanoclusters by borohydride reduction was straightfor-
ward, but our attempts to prepare Cu, Co, and Ni
clusters by this method were unsuccessful. In all three
cases metal aggregates formed in the aqueous phase,
indicating that the interaction of these metals with the
TOAB was insufficient. To avoid this biphasic system we
switched to dimethylformamide (DMF), but here all
clusters aggregated (including Pd and Ru) and precipi-
A mixture of iodobenzene, 1.5 equivalents of phenyl- tated out of the solution. The reason for this may be the
boronic acid, 3 equivalents of K2CO3 and 1 mol % of strong reducing power of borohydride in the mono-
metal clusters was stirred in toluene at 1108C under N2 phasic system.
for 3 h. Reaction progress was monitored by GC. The
palladium clusters were most active, Ru gave 7%
conversion after 3 h and no synergistic effect was Nanocluster Catalysts via Formate Reduction
observed for the bimetallic Pd/Ru clusters. The palla-
dium clusters aggregated and precipitated after 90 min. To control the reduction rate we adopted the approach
We thought that the high initial concentration of Pd reported by Reetz and Maase, who used tetraalkylam-
clusters led to more collisions between the particles and monium carboxylates both as reducing and as stabilising
faster aggregation. To study the deactivation of Pd agents.[34] The Pd clusters prepared in DMF were more
clusters, the same reaction was carried out using active than those prepared in toluene, perhaps because
1 mol % Pd in three portions (3 Â 0.33 mol % Pd), the polar solvent itself acts as a stabiliser.[35] The results
added at intervals of 30 min. The initial activity of the below all pertain to clusters prepared in DMF. A
clusters in the first two portions is high, but they also mixture design[36] assuming no prior knowledge was
*
*
deactivate quickly (cf. data for and in Figure 2). The applied to test the singular and the combined catalytic
third portion deactivates even faster than the first two effects of copper with three noble metals (palladium,
and this may be due to aggregates already present in platinum and ruthenium, see Figure 3). A library of 15
reaction mixture. Thus, it is not the high initial concen- colloidal mixtures was prepared, by mixing the appro-
tration that causes catalyst deactivation. Rather, the priate homogeneous stock solutions of the metal
deactivation is more likely due to thermal instability chloride precursors, followed by reduction with tetraoc-
and/or contamination by borides.
tylammonium formate (TOAF) in DMF at 658C. All
980
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Combinatorial Design of Copper-Based Mixed Nanoclusters
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Figure 4. Time-resolved reaction profiles observed for Suzuki
coupling of PhB(OH)2 and PhI using various nanocluster
catalysts. Duplicate experiments with catalysts 2, 5, and 12
were within Æ 3%. Reaction conditions are as in Table 1.
Figure 3. Cartoon of a metal cluster stabilised by tetraoctyl-
ammonium ions and experimental design for preparing
cluster catalyst mixtures of copper, palladium, platinum and
ruthenium.
2 4). Aryl bromides containing electron-withdrawing
groups (entries 8 11) gave quantitative yields after 1
syntheses, reactions, and catalyst storage were per- 6 h. 4-Bromoanisole, on the other hand, took 24 h to
formed under nitrogen. Cluster size was measured using reach full conversion. The catalyst was also active
XR diffraction. In all cases, the size of the metal core towards aryl chlorides with electron-withdrawing
(excluding the stabilising tetraoctylammonium shell) groups, indicating that it is active enough to initiate
À
varied between 1.6 and 2.5 nm in diameter, with a size the oxidative addition step of a strong C Cl bond. In
distribution of Æ 0.1 nm.
several experiments ca. 10% biphenyl was also ob-
This library was then screened for activity in the cross- served. The amount of copper in these experiments is
coupling of phenylboronic acid with iodobenzene. The not enough to form biphenyl via the stoichiometric
reactions were carried out using 2 mol % metal relative Ullmann reaction,[37] and in any case no 4,4'-dimethyl-
to PhI. The results are shown in Table 1. As expected, biphenyl was observed when p-iodotoluene was used as
among the homometallic catalysts palladium 2 was most the substrate. It is more likely that some homocoupling
efficient, giving 100% yield after 4 h at 1108C. No of phenylboronic acid occurs in these cases when the
reaction was observed with the platinum clusters, but cross-coupling is slow.[38,39] With hindsight, we see that
ruthenium and, surprisingly, copper clusters were found the choice of model reaction was not ideal, as it is
to be active and stable (see Figure 4). Cu/Pd 5 was the difficult to distinguish between the cross-coupling of
most active of the bimetallic combinations, comparable iodobenzene and boronic acid and the homocoupling of
to pure Pd 2 (cf. Table 1, entries 4 and 2). Note that the the latter. Also, in contrast with earlier studies, no
amount of Pd in 5 is half of that in 2. Apparently, there is reduction of the nitro group was observed (Table 2
a synergistic effect between Cu and Pd. However, the entries 12 16). We thought that this discrepancy was
lower activity of the Pd/Pt catalyst 9 shows that no the result of solvent contamination by water but
simple extrapolation can be made from one combina- repeated control experiments using dried solvents in
tion to another.
the presence and absence of 1 mL water showed that
No reaction was observed with catalysts 6, 7 and 10. water does not effect the nitro group reduction. Further
Activity trends continued with 12 > 11 > 14 for the work to study the nitro group reduction is underway.
trimetallic clusters. 15 was less active but stable, reach-
ing 100% conversion after 24 h. Duplicate experiments
carried out after 4 weeks using same stock solution of Mechanistic Studies
nanoclusters, confirmed that the results were reprodu-
cible, and that the catalysts retain their activity.
The accepted catalytic cycle for the homogeneous
The bimetallic Cu/Pd clusters were active over a wide complex-catalysed Suzuki cross-coupling begins with
range of substrates (Table 2). In general, aryl iodides oxidative addition of the aryl halide, followed by
reacted faster then the corresponding bromides (entries transmetallation and reductive elimination of the prod-
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Table 1. Biphenyl yields and rate constants using various nanoclusters catalysts.[a]
Entry
Catalyst[b]
Yield[c] [%]
kobs [L/mol minÀ1]
r2 [5 obs.]
1
2
3
4
5
6
7
8
9
1 (Cu)
2 (Pd)
4 (Ru)
5 (Cu/Pd)
62
100
40
100
94
3.2 Â 10À3
5.9 Â 10À2 [d]
2.0 Â 10À3
6.1 Â 10À2 [d]
9.7 Â 10À3
2.9 Â 10À2
2.5 Â 10À2
3.8 Â 10À2 [d]
7.3 Â 10À3
2.8 Â 10À3
0.985
0.990
0.938
0.992
0.934
0.985
0.991
0.996
0.923
0.992
8 (Pd/Pt)
9 (Pd/Ru)
11 (Cu/Pd/Pt)
12 (Cu/Pd/Ru)
14 (Pd/Pt/Ru)
15 (Cu/Pd/Pt/Ru)
100
92
100
81
10
62[e]
[a]
Standard reaction conditions: 0.50 mmol iodobenzene, 0.75 mmol phenylboronic acid, 1.5 mmol K2CO3, 0.01 mmol catalyst
(2 mol % total metal relative to PhI), 12.5 mL DMF, N2 atmosphere, 1108C.
No conversion was observed with catalysts 3, 6, 7, 10, and 13.
GC yield after 6 h, corrected for the presence of internal standard.
Value is the average of two repeated experiments.
[b]
[c]
[d]
[e]
100% yield obtained after 24 h.
uct.[40] Does this mechanism also apply to nanocluster
catalysis? A partial answer to this question can be
obtained by examining the influence of substituents on
the reaction rate. For the substrates we studied, the
reaction profile fits well to a second-order rate equation
(see, e.g., the data for p-bromobenzotrifluoride in
Figure 5). One may assume that the rate-determining
step with nanocluster catalysts is, similar to the homo-
geneous reaction, the oxidative addition of ArX to the
catalytic site. Indeed, a Hammett plot of the relative
reactivity of various substituted bromobenzenes vs. sP
gave 1 ˆ1.48 (Figure 6), in close comparison with the
value obtained by Weissman et al. using homogeneous
palladium complexes.[41] The positive 1 value indicates
that the reaction is moresensitive to the electronic effects
of electron-withdrawing substituents, which are expected
to accelerate the oxidative addition step. It is likely that
the two reactions involve similar transition states.
In principle, this similarity can also result from partial
dissolution of the clusters in the reaction medium.[42, 43]
Metal atoms could −leach out× from the cluster and
initiate a homogeneous catalytic cycle. We cannot
disprove this hypothesis, but it seems to us less likely,
especially considering the catalysis by copper nano-
Figure 5. Suzuki coupling of PhB(OH)2 with 4-bromobenzo-
triflouride using a mixed Cu/Pd nanocluster catalyst. Inset
shows the fit of the initial reaction rate to a second-order rate
equation. Reaction conditions are as in Table 1.
clusters. If atom leaching was the cause ofthe activity, we on the stabilising shell. We anticipate that metal(0)
would expect Cu(0) complexes to be good cross- nanclusters could have a high impact in catalysis of
coupling catalysts.
carbon-carbon coupling reactions, and perhaps provide
new possibilities normally inaccessible with conven-
tional homogeneous and heterogeneous catalysts.
Conclusion
In the case of the Suzuki reaction, Cu nanoclusters are
good catalysts, unlike Cu complexes or micrometric Cu
particles. Furthermore, mixing two or more metal
precursors can lead in this case to synergistic effects.
The activity and stability of such clusters is strongly
Experimental Section
Materials and Instrumentation
1H and 13C NMR spectra were recorded in deuterated DMFon
dependent on the reaction conditions and particularly a 500 MHz Varian Inova instrument. GC analysis was per-
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Table 2. Nanocluster-catalysed Suzuki cross-coupling with various substrates.[a]
Entry
Substrate
Catalyst [composition]
Time [h]
Conversion [%][b]
1
2
1 (Cu)
5 (Cu/Pd)
8
2
100
100
3
5 (Cu/Pd)
2
100[c]
4
5
5 (Cu/Pd)
5 (Cu/Pd)
2.5
24
100
100[d]
6
7
12 (Cu/Pd/Ru)
5 (Cu/Pd)
24
24
62
100
8
9
5 (Cu/Pd)
5 (Cu/Pd)
5 (Cu/Pd)
5 (Cu/Pd)
5 (Cu/Pd)
5 (Cu/Pd)
24
24
4
100[c]
100[c]
100
10
11
12
13
1
100
0.5
6
100[c]
100[c]
14
5 (Cu/Pd)
4
100
15
16
5 (Cu/Pd)
5 (Cu/Pd)
4
6
100
100[c]
17
18
19
5 (Cu/Pd)
48
48
24
43
5 (Cu/Pd)
54
12 (Cu/Pd/Ru)
25[c]
[a]
Standard reaction conditions: 0.50 mmol substrate, 0.75 mmol phenylboronic acid, 1.5 mmol K2CO3, 0.01 mmol catalyst
(2 mol % total metal colloid relative to PhI), 12.5 mL DMF, N2 atmosphere, 1108C.
GC conversion, corrected for the presence of an internal standard. Only the cross-coupling product was observed, unless
otherwise noted.
[b]
[c]
[d]
8
10% biphenyl as by-product.
75% conversion to 1,4-diphenylbenzene (terphenyl).
formed using an Interscience Trace GC-8000 gas chromato-
graph with a 100% dimethylpolysiloxane capillary column
(DB-1, 30 m  0.325 mm). GC/MS analysis was performed
using a Hewlett-Packard 5890/5971 GC/MS equipped with a
ZB-5 (zebron) column (15 m  0.25 mm). All products are
known compounds and were identified by comparison of their
spectral properties to those of authentic samples. Samples for
GC were diluted with 1 mL DMF and filtered through an
alumina plug prior to injection. GC conditions: isotherm at
1058C (1 min); ramp at 208C minÀ1 to 2608C; isotherm at
2608C (5 min). All reactions were carried out under N2
atmosphere in Schlenk-type glassware that was oven dried
prior to reaction. Solutions were dispensed using a micro-
pipette. Unless noted otherwise, chemicals were purchased
from commercial firms and were used as received. Tetra-n-
octylammonium formate (TOAF) was synthesised using the
procedure reported by Maase.[44]
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Synthesis of Colloids by Reduction with NaBH4
Example Pd colloids: A Schlenk-type vessel equipped with a
rubber septum and a magnetic stirrer was evacuated and
refilled with N2. An aqueous solution of NaBH4 (2.0 mL,
26.0 mM) was added dropwise over 1 h to the reaction mixture
in the vessel containing PdCl2 (0.20 mmol, 34.0 mg), TOAB
(0.30 mmol, 164.0 mg) and toluene (50 mL) at 208C. The
mixture was stirred further for 2 h after which the toluene layer
was separated and used directly in the reaction.
Synthesis of Colloid Catalysts 1 15 using TOAF
Example Cu/Pd 5: A Schlenk-type vessel equipped with a
rubber septum and a magnetic stirrer was evacuated and
refilled with N2. The vessel was charged with CuCl and PdCl2
solutions in DMF (3 mL of each, 3.33 mM) using a syringe.
0.5 mL of a 0.2 M TOAF solution in DMF was added in one
portion to the solution at 658C, and the mixture was stirred for
24 h under a slight overpressure of N2. The colour of the
mixture changed from reddish brown to black. The resulting
colloidal suspension (particle size ꢀ 2.5 nm) was then stored
under N2 and was used as a stock solution in the cross-coupling
reactions. The other catalysts were prepared in a similar
manner using stock solutions of the corresponding metal
chlorides as starting materials. The size of the metal nano-
clusters was determined by XR diffraction using rotating Cu-
anode and a 2D image plate detector.
Figure 6. Plot of the relative rate constants vs. sp (Hammett
*
*
plot); ( ) results for cluster catalysts; ( ) results reported for
palladacycle catalyst in ref.[41]
468C (lit. 49 508C [45]); 1H NMR: d ˆ 7.688 (d, 2H, J ˆ 8.4 Hz),
7.602 (d, 2H, J ˆ 8.1 Hz), 7.478 (t, 2H, J ˆ 7.8 Hz), 7.365 (t, 1H,
J ˆ 7.248 Hz), 7.309 (d, 2H, J ˆ 8.1 Hz), 2.376 (s, 3H);
13C NMR: d ˆ 141.0, 138.1, 137.3, 129.8, 129.1, 127.4, 126.9,
126.7, 20.5. Good agreement was found with literature
values.[45]
Cross-Coupling of Aryl Halides with Phenylboronic
Acid
Sets of five reactions were performed in parallel, and in order
to minimise errors and to test the reproducibility of the results,
one reaction from each set was duplicated in the next set.
Example (1) 4-Nitrobiphenyl: A Schlenk-type glass vessel
equipped with a rubber septum and a magnetic stirrer was
evacuated and refilled with N2. The vessel was then charged
with a Cu/Pd 5 colloidal suspension prepared in DMF
(10.0 mL, 10.0 mM, 2.0 mol %), phenylboronic acid (0.914 g,
7.5 mmol), and K2CO3 (2.07 g, 15.0 mmol). 4-bromonitroben-
zene (1.01 g, 5.0 mmol) was added and the mixture was stirred
at 1108C for 4 h under a slight overpressure of N2. Reaction
progress was monitored by GC and GC/MS. After 4 h the
reaction mixture was poured into a separatory funnel, diluted
with water (20 mL) and extracted with dichloromethane (2 Â
20 mL). The organic layers were washed with 1M aqueous
NaOH (20 mL), dried over anhydrous MgSO4 and concen-
trated under vacuum at 408C to afford a yellow solid; yield:
1.0 g ( > 99% yield based on 4-bromonitrobenzene). The crude
product was recrystallised from hot ethanol to give light-yellow
needles (0.955 g, 95.5% based on 4-bromonitrobenzene), mp
Experimental Procedure for Kinetic Studies
Conditions and apparatus were similar to the cross-coupling
procedure given above. The typical second-order rate con-
stants observed for coupling of unsubstituted and various
substituted bromobenzene with phenyl boronic acid were i)
À3
bromobenzene ( kobs 9.70  10 MÀ1 minÀ1, r2 ˆ 0.980 for 5
ˆ
observations), ii) 4-bromoanisole (kobs 7.50 Â 10À3 MÀ1 minÀ1,
ˆ
r2 ˆ 0.991 for 5 observations), iii) 4-bromofluorobenzene
(kobs 1.18  10À2 MÀ1 minÀ1, r2 ˆ 0.992 for 5 observations), iv)
ˆ
4-bromonitrobenzene (kobs 2.60  10 MÀ1 minÀ1, r2 ˆ 0.981
À1
ˆ
À1
ˆ
for 5 observations), v) 4-bromobenzonitrile (kobs 1.04 Â 10
MÀ1 minÀ1, r2 ˆ 0.966 for 5 observations), vi) 4-bromobenzo-
trifluoride (kobs 4.19  10 MÀ1 minÀ1, r2 ˆ 0.988 for 5 obser-
À2
ˆ
vations).
Acknowledgements
1
1068C (lit. 103 1048C [14]); H NMR: d ˆ 8.371 (d, 2H, J ˆ
8.7 Hz), 8.034 (d, 2H, J ˆ 5.5 Hz), 7.854 (d, 2H, J ˆ 7.5 Hz),
We thank Prof. M. T. Reetz and Prof. H. Bˆnnemann for
discussions and preprints, Dr. E. Eiser for performing the XR
diffraction measurements, and Hans F. M. Boelens for advice on
experimental design.
7.549 (t, 3H, J 7.2 and 7.5 Hz); 13C NMR: d ˆ 147.48, 147.43,
ˆ
138.69, 129.57, 129.37, 128.30, 127.72, 124.417. Good agree-
ment was found with literature values.[14]
Example (2) 4-Phenyltoluene: Reaction and work-up were
performed as above, but using 4-bromotoluene (1.026 g,
6 mmol), to give 0.865 g (84.3%) of product. The crude material
was recrystallised from hot ethanol to give a white solid, mp
984
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Combinatorial Design of Copper-Based Mixed Nanoclusters
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