Palladium nanoclusters in Sonogashira cross-coupling: A true catalytic species?

Article abstract of DOI:10.1002/adsc.200505229

The question of whether palladium nanoclusters are the actual catalysts in the so-called 'cluster-catalyzed Sonogashira cross-coupling' is investigated, using the coupling of phenylacetylene with 4-bromobenzonitrile as a model reaction. By combining a detailed kinetic analysis with transmission electron microscopy (TEM), we show that a soluble species must be present in the system when Pd nanoclusters are used as catalysts. Various Pd clusters show similar kinetic profiles to that of a homogeneous Pd(dba)2 complex. Most importantly, TEM analysis of samples taken before, during, and after the reaction shows that the cluster size decreases during the reaction. Based on these findings, we present a possible two-path mechanism for Sonogashira cross-coupling reactions in the presence of Pd nanoclusters.

Full text of DOI:10.1002/adsc.200505229

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DOI: 10.1002/adsc.200505229
Palladium Nanoclusters in Sonogashira Cross-Coupling: A True
Catalytic Species?
a
b
a
a
Mehul B. Thathagar, Patricia J. Kooyman, Romilda Boerleider, Eveline Jansen,
a
a,
Cornelis J. Elsevier, Gadi Rothenberg *
a
van ꢀt Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam,
The Netherlands
Fax: þ31 20 525 5604, e-mail: gadi@science.uva.nl
b
DelftChemTech, Julianalaan 136, 2628 BL Delft, The Netherlands
Received: June 1, 2005; Accepted: September 5, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de or from the author.
Abstract: The question of whether palladium nano-
clusters are the actual catalysts in the so-called ꢀclus-
ter–catalyzed Sonogashira cross-couplingꢁ is investi-
gated, using the coupling of phenylacetylene with 4-
bromobenzonitrile as a model reaction. By combin-
ing a detailed kinetic analysis with transmission elec-
tron microscopy (TEM), we show that a soluble spe-
cies must be present in the system when Pd nanoclus-
ters are used as catalysts. Various Pd clusters show
similar kinetic profiles to that of a homogeneous
and mixed metal clusters can catalyze the Sonogashira
[
12]
cross-coupling. In this communication, we try to pin-
point the actual active species in the catalytic cycle.
Does catalysis occur on the cluster surface or via leach-
ing of Pd atoms/ions into solution? The answer to this
question will help in future to design atom-economical
[
13]
and selective cluster catalysts.
To try and answer this fundamental question, we stud-
ied the mechanism of the Sonogashira cross-coupling re-
[
14]
[15,16]
action, catalyzed by Pd clusters at 508C.
We com-
Pd(dba) complex. Most importantly, TEM analysis
bined kinetic studies and transmission electron micros-
copy (TEM) analysis. The results indicate that a homo-
geneously catalyzed mechanism is likely.
2
of samples taken before, during, and after the reac-
tion shows that the cluster size decreases during
the reaction. Based on these findings, we present a
possible two-path mechanism for Sonogashira
cross-coupling reactions in the presence of Pd nano-
clusters.
Pd clusters were synthesized by reduction of Pd(NO )
3
2
[
17]
with tetraoctylammonium glycolate in DMF. In a typ-
ical reaction (Figure 1, top), phenylacetylene (1; 1.5
equivs.) was mixed with 4-bromobenzontrile (2; 1
equiv.), Pd clusters (0.01 equiv.) and tetrabutylammoni-
um acetate (TBAA, 1.5 equivs.) as base. Quantitative
yields of 1-cyano-4-phenylethynylbenzene (3) were ob-
tained in 2 h. Diphenyldiacetylene (3–5%) from the ho-
mocoupling of excess 1 was also observed (see Support-
ing Information for full experimental details).
Keywords: aryl halides; catalysis; kinetics; leaching;
palladium; TEM
Cluster samples were taken for TEM analysis before,
Nanometric palladium clusters can exhibit catalytic ac- during, and after the reaction (Figure 1, bottom). Prior
tivities comparable to those of traditional Pd complexes to reaction one can see separate Pd particles, with an
[
1,2]
in Heck, Suzuki and Sonogashira cross-coupling. Sev- average particle size of 14.5 nm (for 49 particles im-
eral reports discuss the mechanisms for heterogeneous aged). About 20% of the clusters are triangular, and
[3,4]
Pd catalysts, but little is known about how cluster cat- the rest show an irregular morphology. Surprisingly,
[
5–7]
alysts work in the liquid phase.
For palladium on a the samples taken during and after the reaction show
[
8]
solid support, the Maitlisꢁ filtration test and ICP anal- smaller particles, ca 8.0 nm, compared to the sample be-
ysis of the filtrate can tell us whether Pd is leaching or fore the reaction. In both cases, the particle size distribu-
[9]
not. With nanoclusters, however, testing is difficult tion was narrow and similar (Æ1.0 nm). This observa-
[
10]
due to the small cluster size. El-Sayed et al. and Brad- tion is contrary to the Oswald ripening process, where
[
11]
ley and co-workers studied the cluster size effect in atoms detach from (thermodynamically unstable)
[18,19]
Suzuki and Heck reactions, and concluded that the reac- smaller clusters and reattach to bigger clusters.
tion occurs at low coordination sites of the Pd cluster sur- Such ripening usually leads to growth of the bigger clus-
face. Recently, we showed that Pd clusters, Cu clusters, ters and eventual disappearance of the smaller ones. In
Adv. Synth. Catal. 2005, 347, 1965 – 1968
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1965

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Mehul B. Thathagar et al.
Figure 3. Time-resolved profiles showing the effect of mixing
either 4-bromobenzonitrile 1 or phenylacetylene 2 with the
Pd clusters prior to reaction. The ꢀ*ꢁ and ꢀ*ꢁ symbols denote
{
1þPd} with 2 added after 30 min and vice versa, respectively.
Figure 1. Sonogashira coupling of phenylacetylene with 4-
bromobenzonitrile using Pd clusters as catalyst and TEM im-
ages before (left) and after (right) the reaction.
We also studied the effect of various counteranions on
the catalytic activity, using clusters prepared from
Pd(NO ) , PdCl and Pd(OAc) . Cluster synthesis using
3
2
2
2
different precursors yielded similar average particle sizes
and size distributions (14–16 nmÆ1.0 nm in all cases).
À
À
The activity decreased in the order NO >Cl >
OAc (Figure 2). Finke and Özkar
3
À
[26,27]
showed that the
coordinatingabilityofanionstotheclustersurfaceaffects
the clusterꢁs stability. Acetate is a strong coordinating ion
[
28,29]
to Pd, while nitrate is a weak one.
This difference af-
fects the stability, rendering the clusters more (or less)
susceptible to leaching. Furthermore, clusters prepared
from Pd(NO ) showed similar activity to that of a homo-
3
2
0
geneous Pd (dba) complex. This may reflect the weaker
2
À
coordination of NO3 that increases the Pd leaching rate.
Figure 2. Sonogashira coupling of 1 with 2 catalyzed by ho-
There are very few reports on the copper-free version
of the Sonogashira reaction, and the mechanism is not
mogeneous Pd(dba) or by the Pd clusters prepared from var-
2
ious precursors. Reaction conditions: 1.5 mmol phenylacety-
lene, 1.5 mmol TBAA, 1 mol % catalyst (total Pd clusters rel-
ative to Br-C H -CN), 8 mL DMF, 508C, N atmosphere.
[
30–32]
clear.
Using initial rate methods, we found that
the reaction follows the rate law v¼k[Pd][2]. Note
that [Pd] represents the initial concentration of the Pd
nanoparticles and not the leached Pd during the reac-
tion. Moreover, we observed a decelerating effect of 1
6
4
2
our case, the decrease in the particle size during the re- on the oxidative addition of 2 (Figure 3). This may be
action strongly indicates that Pd atoms and/or ions are due to the unreactive complex formed by the coordina-
leaching out of the clusters. The leached, “naked” Pd tion of 1 with the leached Pd species, as observed by Am-
[
20]
[33]
species may be the real catalysts, instead of the defect atore et al.
sites on the clustersꢁ surface.
[
20–23]
When the substrate is
Based on our results, we propose a mechanism for the
consumed, these homogeneous species can re-cluster to Pd cluster-catalyzed Sonogashira cross-coupling. This
form new (smaller) particles with the stabilizing influ- mechanism includes two pathways (Figure 4): Path A in-
[
24]
0
ence of the TBAA. We also investigated the catalytic volves the leaching of Pd from defect sites on the clus-
activity of these smaller particles for the second cycle by ter, that is then stabilized by the coordinating anions
0
adding another equivalent of the reactants and base to and the solvent, forming a homogeneous Pd complex.
the reaction mixture of the first run. We observed a Subsequent oxidative addition of the aryl bromide 2
drop in reaction rate and catalytic activity compared gives a Pd complex that enters the conventional cata-
[
25]
II
to the first run. These results further indicate that catal- lytic cycle. Alternatively, in path B, 2 first attacks a de-
II
ysis does not occur on the surface, as one would expect fect site on the Pd cluster, releasing a Pd complex
an increase in activity with the decrease in particle size. into solution, that then enters the catalytic cycle, as sur-
1966
asc.wiley-vch.de
ꢂ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2005, 347, 1965 – 1968

Palladium Nanoclusters in Sonogashira Cross-Coupling
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Figure 4. Proposed mechanism for the Pd cluster-catalyzed Sonogashira cross-coupling involving two possible pathways.
[
21]
mised by Biffis et al. based on the gathered evidence The vessel was then charged with a Pd cluster suspension pre-
from the literature. The nucleophilic attack of 1 on the pared in DMF (1.0 mL, 10.0 mM, 1.0 mol %), phenylacetylene
II
ArylÀPd -Br(solvent)X complex takes place concomi- (2 mL, 750 mM, 1.5 mmol), TBAA (2 mL, 750 mM,
tant with the deprotonation of 1. Finally, the dissolved 1.50 mmol) and 3 mL of DMF. 4-Bromobenzonitrile (0.182 g,
Pd species can re-cluster, to form new (smaller) particles 1.00 mmol) was added and the mixture was stirred at 508C
after the reductive elimination step. This mechanism is for 6 h under a slight overpressure of N . Reaction samples
2
consistent with the results obtained from TEM analysis were taken at regular intervals and monitored by GC (pentade-
and kinetic studies, as well as with the one proposed by cane internal standard).
[
7]
Hu and Liu for the analogous Suzuki cross-coupling.
In summary, we show here that the cluster-catalyzed
Sonogashira reaction probably proceeds via a soluble
Pd species, and that the counteranions present in solu-
tion influence the leaching of the Pd ions/atoms. Further
investigations into this mechanism will be the subject of
future research in our laboratory.
References and Notes
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