PEPPSI-type palladium complexes containing basic 1,2,3-triazolylidene ligands and their role in Suzuki-Miyaura catalysis

Article abstract of DOI:10.1002/chem.201103719

A series of PEPPSI-type palladium(II) complexes was synthesized that contain 3-chloropyridine as an easily removable ligand and a triazolylidene as a strongly donating mesoionic spectator ligand. Catalytic tests in Suzuki-Miyaura cross-coupling reactions revealed the activity of these complexes towards aryl bromides and aryl chlorides at moderate temperatures (50 °C). However, the impact of steric shielding was the inverse of that observed with related normal Nheterocyclic carbenes (imidazol-2-ylidenes) and sterically congested mesityl substituents induced lower activity than small alkyl groups. Mechanistic investigations, including mercury poisoning experiments, TEM analyses, and ESI mass spectrometry, provide evidence for ligand dissociation and the formation of nanoparticles as a catalyst resting state. These heterogeneous particles provide a reservoir for soluble palladium atoms or clusters as operationally homogeneous catalysts for the arylation of aryl halides. Clearly, the substitution of a normal N-heterocyclic carbene for a more basic triazolylidene ligand in the precatalyst has a profound impact on the mode of action of the catalytic system. Copyright

Full text of DOI:10.1002/chem.201103719

FULL PAPER
DOI: 10.1002/chem.201103719
PEPPSI-Type Palladium Complexes Containing Basic 1,2,3-Triazolylidene
Ligands and Their Role in Suzuki–Miyaura Catalysis
Daniel Canseco-Gonzalez,[a] Andrzej Gniewek,[b] Michal Szulmanowicz,[b]
Helge Mꢀller-Bunz,[a] Anna M. Trzeciak,*[b] and Martin Albrecht*[a]
Abstract: A series of PEPPSI-type pal-
ladium(II) complexes was synthesized
that contain 3-chloropyridine as an
easily removable ligand and a triazolyli-
dene as a strongly donating mesoionic
spectator ligand. Catalytic tests in
Suzuki–Miyaura cross-coupling re-
G
for ligand dissociation and the forma-
tion of nanoparticles as a catalyst rest-
ing state. These heterogeneous parti-
cles provide a reservoir for soluble pal-
ladium atoms or clusters as operation-
ally homogeneous catalysts for the ary-
lation of aryl halides. Clearly, the
substitution of a normal N-heterocyclic
carbene for a more basic triazolylidene
ligand in the precatalyst has a profound
impact on the mode of action of the
catalytic system.
ACHTUNGTRENNUNGactions revealed the activity of these
complexes towards aryl bromides and
aryl chlorides at moderate tempera-
tures (508C). However, the impact of
steric shielding was the inverse of that
observed with related normal N-
Introduction
steric demand have been developed over the last few years
that have indeed boosted the catalytic activity of the palladi-
Palladium complexes have found widespread academic and
industrial application as catalyst precursors for carbon–
um center, including P
(biaryl) (Cy=cyclohexyl,[6] and 1,3-dimesitylimidazol-2-yli-
(o-tol)3 (tol=tolyl),[4] (tBu)3,[5] PCy2-
ACHTUNGTRENNUNG PACHTUNGTRENNUNG
AHCTUNGTRENNUNG
dene (IMes).[7] In particular, the combination of IMes, as
a strongly electron-donating and bulky N-heterocyclic car-
bene (NHC), and 3-chloropyridine, as an easily removable
ligand, with PdCl2 gave a highly active catalyst precursor for
cross-coupling reactions (A, Figure 1).[8] Because of the ben-
eficial role of the pyridine ligand, the term PEPPSI
À
À
carbon cross-coupling and for C N and C O bond-forming
reactions.[1] Typically, a C X bond (X=halide or pseudo-
À
A
stantial progress has also recently been made in the direct
[2]
À
functionalization of C H bonds. Cross-coupling of cheap
and readily available chlorocarbons (X=Cl) generally in-
À
volves oxidative addition of the C Cl bond to a low-valent
(pyridineACTHNGUTERNNeUG nhanced precatalyst preparation, stabilization, and
initiation) was coined.
and coordinatively unsaturated palladium species as the
rate-limiting step.[3] Appropriate balancing of the stability of
the critical palladium(0) species is thus an essential aspect in
designing better catalysts. This stability should be high
enough to prevent decomposition, yet sufficiently low to
Recent work has shown that the electron-donor proper-
ties of N-heterocyclic carbene ligands[9] substantially in-
crease when the heteroatoms of the ring are positioned at
remote positions.[10] For example, mesoionic[11] 1,2,3-triazol-
5-ylidenes (B, Figure 1) bear only one nitrogen adjacent to
À
favor oxidative C Cl addition. Following these generic
guidelines, ligands with enhanced basicity and increased
[a] D. Canseco-Gonzalez, Dr. H. Mꢀller-Bunz, Prof. Dr. M. Albrecht
School of Chemistry and Chemical Biology
University College Dublin
Belfield, Dublin 4 (Ireland)
Fax : (+353)1716-2501
E-mail: martin.albrecht@ucd.ie
[b] Dr. A. Gniewek, M. Szulmanowicz, Prof. Dr. A. M. Trzeciak
Faculty of Chemistry, University of Wroclaw
14 F. Joliot-Curie St., 50-383 Wroclaw (Poland)
E-mail: anna.trzeciak@chem.uni.wroc.pl
Figure 1. PEPPSI-type IMes palladium(II) complex A and its triazolyli-
dene congener B (PEPPSI=pyridine-enhanced precatalyst preparation,
stabilization, and initiation).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103719.
Chem. Eur. J. 2012, 18, 6055 – 6062
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1
the carbene bonding site and are more basic than classical
imidazol-2-ylidene NHCs.[12] This type of triazolylidene[13]
has recently been applied in the development of a variety of
catalytic systems.[14] The triazolium ligand precursor is readi-
ly available through versatile copper-catalyzed [2+3] cyclo-
addition (click chemistry)[15] and subsequent N-alkylation.
This synthetic flexibility paired with the stronger electron-
donor properties suggests that these ligands will facilitate
the H NMR spectrum, and by the presence of stoichiomet-
ric quantities of the diagnostic set of four resonances due to
the 3-chloropyridine ligand. In addition, the resonance of
the N3-bound CH3 group is shifted upfield by 0.3–0.4 ppm,
and the a-protons of the N1-bound alkyl group become
more shielded (complexes 3a–c). In contrast, the 13C NMR
resonances are barely affected by the palladation reaction.
The chemical shift of the C5 nucleus moves by between 0.2
and 8 ppm upon changing its substituent from hydrogen (in
1) to palladium (in 3), and the C4 frequency did not vary by
more than 1 ppm. The absolute chemical shift of the C5 nu-
cleus does not correlate with the electronic impact of the
ortho substituents at N1 and C4,[18] suggesting that the reso-
nance frequency is governed by a combination of steric and
electronic factors.
À
C Cl oxidative addition in cross-coupling chemistry. Based
on these considerations, we became interested in synthesiz-
ing a set of PEPPSI-type palladium complexes B with differ-
ent triazolylidene ligands, as well as in evaluating their cata-
lytic activity. Remarkably, the modification of the heterocy-
clic carbene ligand has a profound impact on the working
mode of the catalyst, which may be rationalized by a de-
À
creased Pd Ccarbene bond stability in the triazolylidene
Complexes 3b, 3c, and 3d were analyzed by single-crystal
X-ray diffraction (Figure 2). In all three complexes, the li-
gands around the square-planar palladium center adopt the
system.[16]
À
expected trans arrangement. The Pd Ctrz (trz=triazolyli-
dene) bond length in all complexes is 1.96(1) ꢂ, which is in
Results and Discussion
line with those of related triazolylidene–palladium com-
À
Synthesis of the complexes: The triazolium salts 1a–e were
conveniently accessible through conventional and versatile
Cu-catalyzed click cycloaddition of the appropriate azide
and alkyne, followed by chemoselective methylation of the
N3 position by using MeI (Scheme 1).[17] Metalation of the
plexes (Table 1).[14e,f,19] The Pd Npyr (pyr=pyridine) bond
Table 1. Selected bond lengths [ꢂ] and angles [8] in complexes 3b, 3c,
and 3d.
3b
3c
3d
molecule 1 molecule 2[a]
À
Pd C(1)
1.9601(16)
2.1105(13)
2.3028(4)
2.3051(4)
178.87(6)
88.63(5)
90.45(4)
88.68(5)
92.25(4)
88.68(5)
1.9650(13)
2.1192(11)
2.3103(3)
2.3054(3)
177.70(4)
89.27(4)
91.83(3)
88.67(4)
90.36(3)
88.67(4)
1.957(3)
2.129(3)
2.3115(8)
2.3046(9)
175.81(11)
88.18(10)
92.12(8)
88.76(10)
90.75(8)
88.76(10)
1.967(3)
2.126(3)
2.2961(9)
2.2969(9)
178.71(12)
89.63(9)
89.76(9)
90.23(9)
90.48(9)
90.23(9)
174.89(4)
À
Pd N(4)
À
Pd Cl(1)
À
Pd Cl(2)
C(1)-Pd-N(4)
C(1)-Pd-Cl(1)
N(4)-Pd-Cl(1)
C(1)-Pd-Cl(2)
N(4)-Pd-Cl(2)
C(4)-Pd-Cl(2)
Cl(1)-Pd-Cl(2) 176.732(16) 175.849(12) 176.04(3)
[a] Second independent molecule in the unit cell, atom labeling adapted.
length is weakly affected by the substitution pattern on the
triazolylidene ligand and increases slightly in the series 3b<
3c<3d. The effect is very small and may indicate enhanced
steric congestion in this series. The most pronounced distinc-
tion between the three complexes pertains to the different
twist angle of the heterocycles out of the palladium coordi-
nation plane. The dihedral angle of the two heterocycles is
nearly identical in 3b and 3c, 30.6(2)8 and 27.57(16)8, re-
spectively, and yet it is significantly larger in 3d, 68.2(3)8.
This almost perpendicular arrangement is probably a direct
consequence of the shielding properties of the ortho-methyl
groups of the mesityl substituent in 3d. This shielding factor
may also explain the fact that the torsion angle of the pyri-
dine ligand out of the metal plane is smallest for 3d
(29.3(3)8), and increases for 3c (36.54(11)8) and 3b
(45.55(13)8). In all three complexes, the dihedral angle be-
tween the palladium square plane and the triazolylidene
Scheme 1. Synthesis of complexes 3.
triazolium salts with Ag2O and subsequent transpalladation
in 3-chloropyridine as the solvent afforded the PEPPSI-type
palladium complexes 3a–e as air- and moisture-stable solids.
Yields of the pure materials ranged between only 20 and
60%, partially because of the formation of an insoluble
pale-yellow side product, and partially because product frac-
tions were washed away during purification. The complexes
are highly soluble in chlorinated solvents, and also dissolve
substantially in Et2O and similar low-polarity solvents that
are typically used for precipitation of organometallic com-
pounds.
Successful palladation was indicated spectroscopically by
the absence of the low-field triazolium proton resonance in
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PEPPSI-Type Palladium Complexes
FULL PAPER
The role of the pyridine
ligand as an activator was inves-
tigated by comparing the activi-
ty of 3b to related complexes 4
and 5. Complex 4 is a dimeric
version of 3b that may split
into two coordinatively unsatu-
rated [PdACHTUNRGTNE(UGN trz)X2] fragments
akin to the intermediate expect-
ed upon pyridine dissociation
from complex 3b. Under the
optimized reaction conditions,
complex 4 exhibits a lower cata-
lytic activity and reaches only
60% conversion.[17] Complex 5,
comprising two triazolylidene
ligands, gave an appreciable
75% conversion, only marginal-
ly lower than the conversions
achieved with complex 3b.[20]
The high activity of complex 5
is in agreement with earlier
studies on an analogous com-
plex derived from 1e[14b] and
may be related to that observed
for a mixed C2-/C4-bound bis-
AHCTUNGTRENNUNG
(imidazolylidene) system,[21] in
which the C4-bound imidazoly-
lidene is supposed to be less
strongly bound to the palladium
Figure 2. ORTEP diagrams of complex a) 3b, b) 3c, and c) one of the two crystallographically independent
molecules of complex 3d (50% probability level; co-crystallized solvent molecules and hydrogen atoms omit-
ted for clarity).
ligand is significantly larger (68–798) than the 29–468 be-
tween the palladium coordination plane and the pyridine
heterocycle.
Catalytic application: The PEPPSI-type complexes were
tested as catalyst precursors in Suzuki–Miyaura cross-cou-
Table 2. Conversion [%] of 3-bromoanisole in the Suzuki–Miyaura cross-
coupling catalyzed by 3b.[a]
pling reactions by using meta-bromoanisole and phenylbor-
onic acid as model substrates. A first set of experiments was
aimed at identifying the most suitable reaction conditions to
achieve this coupling at mild reaction temperatures (508C).
The activity of complex 3b was evaluated by determining
the conversion after 2 h. Initial variations included the sol-
vent (dimethylacetamide (DMA), dioxane, and iPrOH) and
the base (K2CO3, NaOAc, tBuOK, and Bu4NF; Table 2).
These experiments indicated that the best performance oc-
curred with Bu4NF in iPrOH, and further catalytic evalua-
tions were thus carried out by using this combination. The
reaction is sensitive to thermal variations. At 508C, the cata-
lytic solution remained transparent over extended periods of
time. When heated to 808C, however, the catalytic reaction
mixture became cloudy within the first 10 min, and a black
precipitate gradually started to appear. In contrast, reducing
the temperature to 308C preserved the transparent appear-
ance, yet significantly compromised the conversion (33%
after 2 h, 58% after 4 h).
Base
Solvent
K2CO3
NaOAc
tBuOK
Bu4NF
DMA
dioxane
iPrOH
30
8
64
9
2
10
4
2
14
3
4
80
[a] General reaction conditions: bromoanisole (1.0 mmol), PhB(OH)2
(1.2 mmol), base (1.5 mmol), and complex 3b (5 mmol, 0.5 mol%) in the
appropriate solvent (3.0 mL) at 508C for 2 h.
center than the C2-bound analogue.[22] In complex 5, one tri-
azolylidene ligand may thus mimic the activating role of 3-
chloropyridine in 3b, thus also generating a [PdACTHNUGTRENUNG(trz)X2] spe-
cies.
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6057

M. Albrecht, A. M. Trzeciak et al.
Variation of the substituents at the triazolylidene ligand
revealed moderate dependence on steric factors
(Figure 3). The smallest substituents (3a and 3b) induced
homogeneous catalysis.[24] However, the delicate impact of
temperature, as well as the trend observed in the steric
demand of the triazolylidene ligand, which is inverse to the
trend observed for normal NHC–palladium complexes,[8]
suggest a catalytic mechanism that differs from classical
PEPPSI-type systems. To further elucidate the mode of
action, a set of analyses was performed, including microsco-
py, mass spectrometry, and mercury-poisoning experi-
ments.[25]
a
In a representative experiment by using complex 3b
under the standard conditions (bromoanisole as the sub-
strate, see Figure 3), a large excess of elemental mercury
(about 7 g, 35 mmol, ca. 7000 molequiv vs. palladium) was
added after 5 min (14% conversion). This addition efficient-
ly stalled the catalytic activity, and the same 14% conver-
sion was determined after 120 min. In contrast, a parallel
mercury-free reaction reached 80% over the same period.
The outcome was qualitatively identical when the substrate
was replaced with chlorobenzaldehyde. In this case, addition
of mercury after 5 min (4% conversion) inhibited further
catalytic coupling and no biaryl product was formed during
the subsequent 120 min (compare with the 47% yield in
a parallel unpoisoned reaction run over the same time
span). These results indicate that the catalytic working
mode is not fundamentally different when changing the sub-
strate from an aryl bromide to an aryl chloride. Moreover,
the well-known capability of mercury to poison heterogene-
ous, but not homogeneous catalysts[26] is in apparent contra-
diction with the assumption that palladium-catalyzed cross-
coupling of aryl chlorides under mild conditions requires
molecular palladium.[1,24]
Figure 3. Time–conversion profile for the coupling of bromoanisole
(1.0 mmol) and phenylboronic acid (1.2 mmol) catalyzed by complex 3
(0.5 mol%) with Bu4NF (1.5 mmol) in iPrOH (3 mL) at 508C. The inset
shows approximate turnover frequencies at 50% conversion (TOF50).
the highest activity, whereas the di
ACHTUNGTNER(NUNG mesityl)-substituted tri-
ACHTUNGTRENNUNGazolylidene palladium complex 3e displayed comparably
slow conversion. For example, 35% conversion was reached
with complex 3a after 15 min, whereas 60 min were required
with 3e to accomplish the same conversion. The low perfor-
mance of 3e is remarkable when considering the excellent
catalytic activity of its imidazol-2-ylidene analogue (com-
pare with A, Figure 1).[8,23] Complexes 3c and 3d, comprised
of moderately demanding triazolylidene substituents, display
activities that are between those of 3a and 3e. These results
suggest that the bulkiness around the palladium center mat-
ters, either for catalyst activation, or for substrate conver-
sion.
Analysis of catalytic reaction mixtures by transmission
electron microscopy (TEM) revealed further details. A mi-
crograph of a sample taken from the cross-coupling of bro-
moanisole by using 3b after 35 min (Figure 4a) is represen-
The most active complexes 3a and 3b were subsequently
tested for the conversion of more demanding aryl chlorides.
With para-chlorobenzaldehyde as the substrate, the activity
of 3a, containing a small Me substituent at the triazolyli-
dene ligand, was distinctly higher than that of 3b. At 508C
and a 2 mol% loading of palladium complex (vs. aryl chlo-
ride), 3a gave appreciable catalytic activity with a turnover
frequency at 50% conversion (TOF50) of 53 hÀ1. In contrast,
complex 3b, comprising an ethyl ortho substituent at the tri-
azolylidene, was markedly less active (TOF50 =17 hÀ1). With
both complexes, conversion ceased at about 60%, which
suggests a limited lifetime of the catalytically active species.
No improvement in conversion was achieved when increas-
ing the loading of the catalyst precursor from 2 to 5 mol%.
Figure 4. TEM micrographs of post-reaction mixtures after the Suzuki–
Miyaura reaction with a) 3b and bromoanisole and b) 3a and chloroben-
zaldehyde.
tative of all of the complexes (3a–e) and indicates the for-
mation of palladium nanoparticles with an average size of
2–3 nm. Slightly larger particles were formed in the conver-
sion of chlorobenzaldehyde (3–5 nm when using 3a, Fig-
ure 4b). Nanoparticle formation was also observed when
either of the two coupling partners was omitted. In the ab-
sence of phenylboronic acid, the particles were consistently
larger (4–8 nm). In addition, some redispersion was noted
Mechanistic insights: Aryl chloride conversion under rela-
tively mild conditions, as applied herein, typically suggests
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PEPPSI-Type Palladium Complexes
FULL PAPER
by the presence of larger crystallites, suggesting Ostwald rip-
ening under these conditions. In the absence of the aryl
halide, the particle size does not differ significantly from
that of the original catalytic mixture, which suggests that the
phenylboronic acid may play a role in stabilizing the nano-
particles and in controlling their size. The observed Ostwald
ripening may provide an explanation for the catalytic activi-
ty of the system because the particle size redistribution re-
quires partial dissolution of particles, thus generating a dis-
solved palladium(0) species[27] that either undergoes oxida-
tive addition with an aryl halide and thus enters the catalytic
cycle, or aggregates with a nanoparticle to contribute to the
Ostwald ripening process.[28]
Electron-spray ionization mass spectrometry (ESI-MS)
measurements were performed to validate the mechanistic
hypothesis. In agreement with the TEM data and with the
formation of colloidal palladium(0), ESI-MS spectra of the
catalytic reaction mixture did not show any signals in the
low-molecular range that would indicate the presence of de-
fined molecular species. Irrespective of the sampling time
(after 10, 20, and 35 min), only signals in the m/z 900–1600
range were detected. The spectra were almost identical for
all complexes (3a–e) and do not indicate a correlation to
the observed catalytic conversions. For example, when using
complex 3b, signals at m/z 1020, 1443, 1490, and 1534 were
measured. Their isotopic distribution pattern indicates the
presence of up to four palladium atoms per species. More-
over, the m/z 1443 and 1490 signals were observed when
using different triazolylidene palladium complexes as cata-
lyst precursors, suggesting that these species do not contain
a triazolylidene ligand.
decrease in concentration was observed upon prolonged
heating.
The interplay between NHC metal complexes and nano-
particles has received considerable attention recently. Poten-
tially in relation to our observations, the degradation of
NHC ruthenium complexes into nanoparticles in the pres-
ence of N-heterocyclic carbenes has been reported.[30] More-
over, it was shown that solid metals can be used as a precur-
sor to generate a variety of molecular carbene–metal com-
plexes.[31] Whilst imidazolium and ammonium salts effective-
ly stabilize palladium nanoparticles and prevent their aggre-
gation,[32] ammonium salts have also been shown to facilitate
the formation of soluble palladium species from palladi-
um(0) nanoparticles in the presence of aryl halides.[33]
Taking these precedents into account, it seems plausible that
colloidal palladium is the resting state of the catalyst derived
from complex 3,[34] although the catalytically active species
results from dissociation of a palladium cluster or of single
palladium atoms, either as a solvated palladium(0) species,
or as triazolylidene adducts.
Heterogenization of the molecular precatalyst to palladi-
um nanoparticles and subsequent dissolution of palladium
atoms from these nanoparticles constitutes a model that ac-
counts for all of the observations detailed above. Such
a model has been suggested, although only rarely underpin-
ned with experimental evidence.[35] The dissociated “naked”
or triazolylidene-bound palladium atoms are expected to be
sufficiently activated to oxidatively add both aryl bromides
and aryl chlorides under mild conditions.[36] Elemental mer-
cury poisons the reaction by preventing dissociation of a pal-
ladium atom from the colloidal reservoir and thus shuts
down the activation step from the catalyst resting state. In
addition, iPrOH may be the solvent of choice because of its
potentially reducing character,[37] thus providing smooth
access to low-valent palladium. Formation of colloidal palla-
dium may further explain the delicate role of the tempera-
ture, since elevated temperatures accelerate colloid forma-
tion and eventually lead to aggregation of the colloids into
large particles that cannot easily expel a palladium atom. In
contrast, too low a temperature appears to compromise the
decomposition of the precursor complex 3, leading to low
activity. Likewise, the substituents on the triazolylidene
system may fulfill a role that is different from the steric pro-
tection of the metal center, as in PEPPSI-type imidazol-2-yl-
idene palladium catalysts A.[8] In the catalytic system de-
scribed herein, a low steric impact of the carbene ligand fa-
cilitates palladium dissociation, and little steric congestion
also provides triazolium salts that may assist in stabilizing
the palladium colloids. In addition to this effect on the cata-
lytic reaction rate,[26e] this type of stabilization is supposed to
be essential to keep the nanoparticles small and to enhance
the propensity for a palladium atom to dissociate from the
particle and hence enter the catalytic cycle.
In contrast, ESI-MS measurements on solutions, in
iPrOH, containing only the complex and neither cross-cou-
pling substrates nor Bu4NF, revealed signals for various low-
molecular weight species resulting from loss of chloride, and
from triazolylidene ligand transfer. For example, a solution
of 3b gave signals at m/z 479.1, attributed to [Pd
ACHUTGTNRNEUG(N trz)ACHTUNGTNER(NUGN py-
Cl)Cl]+ and at m/z 517.1, assigned to [Pd(trz)2Cl]+. The
AHCTUNGTRENNUNG
latter species again underlines the activating role of the
chloropyridine ligand. Transfer of the triazolylidene ligand
may be a consequence of the sample ionization in the mass
spectrometer, since no indications of ligand transfer were
obtained from NMR measurements in solution. Similar ef-
fects have been observed previously with imidazolylidene–
palladium complexes.[29] Warming of a solution of 3b in
iPrOH to 508C, that is, to the catalytic reaction temperature,
resulted in substantially more signals in the ESI-MS spectra,
including dimeric species, such as [Pd
(trz)3Cl3]+ (m/z 880.1)
2ACHTUNGTRENNUNG
and [Pd2A(trz)3A
C
CHTUNGTRENNUNG
creased number of species and the presence of dimetallic
systems that were not detected when keeping the solution at
room temperature suggest that the complexes are thermally
1
unstable. Indeed, H NMR spectrum monitoring of the sta-
bility of 3b in [D8]-iPrOD at 508C by using hexamethylben-
zene as the internal standard revealed a drop in the concen-
tration of 3b to 57(Æ1)% after 2 h. In addition, a yellow
precipitate was formed, but no palladium black. No further
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M. Albrecht, A. M. Trzeciak et al.
with pentane (3ꢃ25 mL) and dried in vacuo. Analytical data for the new
triazolium salts are compiled in the Supporting Information.
Conclusion
General procedure for the synthesis of the carbene–palladium complexes
3: The carbene–silver complex 2 and PdCl2 were suspended in 3-chloro-
pyridine (7 mL) and stirred at 1008C for 16 h. The reaction mixture was
cooled to RT, diluted with CH2Cl2 (7 mL) and passed through a short
column of SiO2 covered with a pad of Celite. After product elution with
CH2Cl2 was complete (TLC), all volatiles were evaporated under reduced
pressure. The product was precipitated from pentane, collected by filtra-
tion, and dried in vacuo. The residue was dissolved in a minimum
amount of CH2Cl2 and poured into Et2O (50 mL). The precipitate was re-
moved by filtration and the filtrate was evaporated to dryness, affording
pure complex 3.
We have developed a straightforward synthetic methodology
to obtain a series of C5-bound 1,2,3-triazolylidene palladium
complexes containing a 3-chloropyridine ligand as an easily
cleavable ligand. The activity of these complexes in Suzuki–
Miyaura cross-coupling can be tailored by varying the sub-
stituents on the triazolylidene ring. Steric effects govern the
catalytic activity and less bulky substituents induce better
catalytic activity than bulkier IMes-type analogues compris-
ing two mesityl groups in the ortho positions, which is in
contrast to the trends observed with the imidazol-2-ylidene
congeners. Mechanistic work has demonstrated that this op-
posite trend originates from a fundamentally different mode
of action of the triazolylidene complexes. In contrast to the
homogeneous catalysis observed with the original PEPPSI
system, all experimental evidence indicates that the triazoly-
lidene complexes undergo a heterogenization process that
generates palladium nanoparticles as the resting state of the
catalyst. Leaching of palladium atoms from these nanoparti-
cles provides a molecular, catalytically active species that is
able to convert aryl chlorides under relatively mild condi-
tions. This type of mechanism may be relevant for a range
of catalyst precursors, and care should be taken when using
aryl chloride conversion as a probe because spectator li-
gands may not be preserved, but may instead be involved in
a heterogenization–homogenization pathway, and could also
assist in reconstituting a dissolved active species from a col-
loidal reservoir. Moreover, this work highlights substantial
differences between abnormal triazolylidenes and classical
NHCs, such as imidazol-2-ylidenes, in metal bonding.
Complex 3a: The reaction of 2a (500 mg, 0.61 mmol) and PdCl2 (217 mg,
1.23 mmol) according to the general procedure afforded 3a as a yellow
4
solid (200 mg, 35%). 1H NMR (CDCl3 500 MHz): d=9.00 (d, JHH
=
2.3 Hz, 1H; HPy), 8.89 (dd, 3JHH =5.5, 4JHH =1.2 Hz, 1H; HPy), 7.93 (dd,
3JHH =8.1, 4JHH =1.4 Hz, 2H; HAr), 7.72 (ddd, 3JHH =8.2, 4JHH =2.3,
5
1.2 Hz, 1H; HPy), 7.56 (m, 3H; HAr), 7.28 (ddd, 3JHH =8.2, 5.5, JHH
=
0.4 Hz, 1H;
HPy), 4.59 (s, 3H; NCH3), 3.98 ppm (s, 3H; NCH3);
13C{1H} NMR (CDCl3, 125 MHz): d=150.6 (CPy), 149.5 (CPy), 144.2
(CtrzPh), 139.0 (CPy), 138.0 (CtrzPd), 132.7 (CPy), 131.3 (m-CAr), 130.9 (p-
CAr), 129.8 (o-CAr), 128.3 (i-CAr), 126.6 (CPy), 41.8 (NCH3), 37.5 ppm
(NCH3); elemental analysis calcd for C15H17Cl3N4Pd (466.10): C 38.65, H
3.68, N 12.02; found: C 38.16, H 3.49, N 11.64.
Complex 3b: The reaction of 2b (500 mg, 0.59 mmol) and PdCl2 (217 mg,
1.23 mmol) according to the general procedure afforded 3b as a yellow
4
solid (226 mg, 40%). 1H NMR (CDCl3 500 MHz): d=9.00 (d, JHH
=
2.2 Hz, 1H; HPy), 8.89 (dd, 3JHH =5.5, 4JHH =1.2 Hz, 1H; HPy), 7.93 (dd,
3JHH =8.1, 4JHH =1.4 Hz, 2H; HAr), 7.72 (ddd, 3JHH =8.2, 4JHH =2.2,
5
1.2 Hz, 1H; HPy), 7.56 (m, 3H; HAr), 7.28 (ddd, 3JHH =8.2, 5.5, JHH
=
0.4 Hz, 1H; HPy), 5.03 (q, 3JHH =7.3 Hz, 2H; NCH2Me), 3.99 (s, 3H;
NCH3), 1.86 ppm (t, 3JHH =7.3 Hz, 3H; NCH2CH3); 13C{1H} NMR
(CDCl3, 125 MHz): d=150.5 (CPy), 149.5 (CPy), 143.8 (CtrzPh), 138.0
(CPy), 136.6 (CtrzPd), 133.0 (CPy), 131.3 (m-CAr), 130.9 (p-CAr), 129.1 (o-
CAr), 126.7 (i-CAr), 124.7 (CPy), 50.6 (NCH2Me), 37.6 (NCH3), 15.4 ppm
(NCH2CH3); elemental analysis calcd for C16H19Cl3N4Pd (480.13):
C
40.03, H 3.99, N 11.67, found: C 40.25, H 3.91, N 11.54.
Complex 3c: The reaction of 2c (670 mg, 0.78 mmol) and PdCl2 (276 mg,
1.56 mmol) according to the general procedure afforded 3c as a yellow
Experimental Section
4
powder (400 mg, 53%). 1H NMR (CDCl3 500 MHz): d=9.10 (d, JHH
=
2.0 Hz, 1H; HPy), 8.98 (dd, 3JHH =5.4, 4JHH =1.2 Hz, 1H; HPy), 7.75 (ddd,
General: 1-Methyl-4-phenyl-1,2,3-triazole, 1-mesityl-4-phenyl-1,2,3-tria-
zole, the triazolium salt 1c, and the silver carbenes 2b and 2e have been
described previously.[12,14b,38] All other reagents are commercially avail-
able and were used as received. Microwave reactions were carried out by
using a Biotage Initiator 2.5, operating at 100 W irradiation power.
Unless otherwise specified, NMR spectra were recorded at 258C on
Varian Innova spectrometers operating at 300, 400, or 500 MHz
(1H NMR) and 75, 100, or 125 MHz (13C{1H} NMR). Chemical shifts (d
in ppm and coupling constants J in Hz) were referenced to residual sol-
vent resonances. Assignments are based on homo- and heteronuclear
shift correlation spectroscopy. Elemental analyses were performed by the
Microanalytical Laboratory at University College Dublin, Ireland, by
using an Exeter Analytical CE-440 Elemental Analyzer.
5
3JHH =8.1, 4JHH =2.0, 1.2 Hz, 1H; HPy), 7.31 (ddd, 3JHH =8.1, 5.4, JHH
=
0.4 Hz, 1H; HPy), 4.82 (m, 2H; NCH2), 3.95 (s, 3H; NCH3), 3.04 (t,
3JHH =7.8 Hz, 2H; CtrzCH2), 2.27 (quint, 3JHH =7.5 Hz, 2H; NCH2CH2),
3
2.00 (quint, JHH =7.8 Hz, 2H; CtrzCH2CH2), 1.50 (m, 4H; NCH2CH2CH2,
C
trzCH2CH2CH2),
1.03,
1.02 ppm
(2ꢃt,
3JHH =7.3 Hz,
3H;
(CDCl3,
NCH2CH2CH2CH3,
CtrzCH2CH2CH2CH3);
13C{1H} NMR
125 MHz): d=150.6 (CPy), 149.5 (CPy), 144.2 (CtrzBu), 138.0, 134.9, 132.8
(3ꢃCPy), 129.2 (CtrzPd), 54.6 (NCH2), 36.3 (NCH3), 32.0 (NCH2CH2), 31.4
(CtrzCH2),
25.0
(CtrzCH2CH2),
22.8,
20.1
(CtrzCH2CH2CH2,
NCH2CH2CH2),
14.0,
13.8 ppm
(CtrzCH2CH2CH2CH3,
NCH2CH2CH2CH3); elemental analysis calcd for C16H25Cl3N4Pd (480.13):
C 39.53, H 5.18, N 11.52; found: C 39.46, H 5.09, N 11.36.
Complex 3d: The reaction of complex 2d (975 mg, 0.95 mmol) and PdCl2
(338 mg, 1.90 mmol) according to the general procedure afforded 3d as
a light yellow powder (184 mg, 18%). 1H NMR (CDCl3 500 MHz): d=
8.80 (d, 4JHH =2.2 Hz, 1H; HPy), 8.68 (dd, 3JHH =5.5, 1.1 Hz, 1H; HPy),
8.11 (d, 3JHH =7.0 Hz, 2H; HAr), 7.59 (m, 3H; HAr), 7.56 (m, 1H; HPy),
7.11 (m, 1H; HPy), 7.06 (m, 2H; HMes), 4.13 (s, 3H; NCH3), 2.39 (s, 3H;
ArCH3), 2.30 ppm (s, 6H; ArCH3); 13C{1H} NMR (CDCl3, 125 MHz): d=
150.6 (CPy), 149.6 (CPy), 144.3 (CtrzMes), 140.6 (CAr), 140.4 (CAr), 137.7
(CPy), 136.1 (CPy), 135.3 (CAr), 132.3 (CtrzPd), 130.8 (CAr), 130.4 (CAr),
129.5 (CAr), 129.1 (CAr), 127.1 (CPy), 124.6 (CAr), 37.9 (NCH3), 21.5
(ArCH3), 19.0 ppm (ArCH3); elemental analysis calcd for
C23H23Cl3N4Pd·H2O (568.23): C 47.12, H 4.30, N 9.56, found: C 46.96, H
4.21, N 9.10.
General procedure for the synthesis of the triazolium iodides 1: MeI was
added to a solution of triazole in MeCN and the mixture was stirred
under microwave irradiation at 908C for 5 h. All volatiles were then re-
moved in vacuo. The residue was washed several times with copious
amounts of Et2O and dried in vacuo to afford the crude triazolium salt 1.
Microanalytically pure samples were obtained by recrystallization from
hot acetone. Analytical data for the new triazolium salts are compiled in
the Supporting Information.
General procedure for the synthesis of the carbene–silver complexes 2:
Ag2O (0.5 equiv) was added to
a solution of the triazolium salt
1 (1.0 equiv) in CH2Cl2. The mixture was stirred in the absence of light at
room temperature for 2 h and then filtered through Celite. The solvent
was removed in vacuo at room temperature and the residue was washed
6060
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 6055 – 6062

PEPPSI-Type Palladium Complexes
FULL PAPER
Complex 3e: The reaction of complex 2e (200 mg, 0.18 mmol) and PdCl2
(64 mg, 0.36 mmol) according to the general procedure afforded 3e as
a yellow powder (125 mg, 57%). H NMR (CDCl3 500 MHz): d=8.75 (d,
ylation Methods (Ed.: L. Ackermann), Wiley-VCH, Weinheim,
2009; c) Palladacycles: Synthesis, Characterization and Applications
(Eds.: J. Dupont, M. Pfeffer), Wiley-VCH, Weinheim, 2010; d) A.
Suzuki, Angew. Chem. 2011, 123, 6854–6869; Angew. Chem. Int. Ed.
2011, 50, 6722–6737; e) E. Negishi, Angew. Chem. 2011, 123, 6870–
6897; Angew. Chem. Int. Ed. 2011, 50, 6738–6764; f) I. P. Beletskaya,
A. V. Cheprakov, Chem. Rev. 2000, 100, 3009–3066; g) J. F. Hartwig,
Nature 2008, 455, 314–319; h) J.-P. Corbet, G. Mignani, Chem. Rev.
2006, 106, 2651–2710; i) J. Magano, J. R. Dunetz, Chem. Rev. 2011,
111, 2177–2250.
1
4JHH =2.2 Hz, 1H; HPy), 8.65 (dd, 3JHH =5.5, 4JHH =1.1 Hz, 1H; HPy), 7.56
(ddd, 3JHH =8.1, 4JHH =2.2, 1.1 Hz, 1H; HPy), 7.08 (m, 1H; HPy), 7.07 (s,
2H; HMes), 7.05 (s, 2H; HMes), 3.82 (s, 3H; NCH3), 2.39, 2.38 (2ꢃs, 3H;
ArCH3), 2.32 ppm (s, 12H; ArCH3); 13C{1H} NMR (CDCl3, 125 MHz):
d=150.7 (CPy), 149.8 (CPy), 143.6 (CtrzMes), 140.8 (CAr), 140.6 (CAr), 140.5
(CAr), 139.5 (CAr), 137.5 (CPy), 136.1 (CPy), 135.6 (CAr), 132.1 (CtrzPd),
129.5 (CAr), 129.1 (CAr), 124.4 (CPy), 122.9 (CAr), 36.6 (NCH3), 21.6
(ArCH3), 21.5 (ArCH3), 21.2 (ArCH3), 19.0 ppm (ArCH3); elemental
analysis calcd for C26H29Cl3N4Pd·H2O (610.30): C 49.70, H 4.97, N 8.92,
found: C 50.12, H 4.61, N 8.85.
[2] For examples, see: a) D. Alberico, M. E. Scott, M. Lautens, Chem.
Rev. 2007, 107, 174–238; b) T. W. Lyons, M. S. Sanford, Chem. Rev.
2010, 110, 1147–1169; c) X. Chen, K. M. Engle, D.-H. Wang, J.-Q.
Yu, Angew. Chem. 2009, 121, 5196–5217; Angew. Chem. Int. Ed.
2009, 48, 5094–5115.
[3] a) N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457–2483; b) M.
Portnoy, D. Milstein, Organometallics 1993, 12, 1665–1673.
[4] M. Beller, H. Fischer, W. A. Herrmann, K. ꢄfele, C. Brossmer,
Angew. Chem. 1995, 107, 1992–1993; Angew. Chem. Int. Ed. Engl.
1995, 34, 1848–1849.
[5] A. F. Littke, G. C. Fu, Angew. Chem. 1998, 110, 3586–3587; Angew.
Chem. Int. Ed. 1998, 37, 3387–3388.
[6] D. W. Old, J. P. Wolfe, S. L. Buchwald, J. Am. Chem. Soc. 1998, 120,
9722–9723.
[7] C. Zhang, J. Huang, M. L. Trudell, S. P. Nolan, J. Org. Chem. 1999,
64, 3804–3805.
[8] a) M. G. Organ, S. Avola, I. Dubovyk, N. Hadei, E. A. B. Kantchev,
C. J. OꢅBrien, C. Valente, Chem. Eur. J. 2006, 12, 4749–4755; b) C. J.
OꢅBrien, E. A. B. Kantchev, C. Valente, N. Hadei, G. A. Chass, A.
Lough, A. C. Hopkinson, M. G. Organ, Chem. Eur. J. 2006, 12,
4743–4748; c) E. A. B. Kantchev, C. J. OꢅBrien, M. G. Organ,
Angew. Chem. 2007, 119, 2824–2870; Angew. Chem. Int. Ed. 2007,
46, 2768–2813; for a related concept, see: d) O. Navarro, N. Marion,
J. Mei, S. P. Nolan, Chem. Eur. J. 2006, 12, 5142–5148; e) M.-T.
Chen, D. A. Vicic, M. L. Turner, O. Navarro, Organometallics 2011,
30, 5052–5056.
[9] a) N-Heterocyclic Carbenes in Synthesis (Ed.: S. P. Nolan), Wiley-
VCH, Weinheim, 2006; b) Catalysis by Complexes, Vol. 32, Heterocy-
clic Carbenes in Transition Metal Catalysis and Organocatalysis
(Ed.: C. S. J. Cazin), Springer, Berlin, 2010; c) N-Heterocyclic Car-
benes: From Laboratory Curiosities to Efficient Synthetic Tools (Ed.:
S. Diez-Gonzalez), RSC, Cambridge, 2010; d) D. Bourissou, O.
Guerret, F. P. Gabbai, G. Bertrand, Chem. Rev. 2000, 100, 39–91;
e) W. A. Herrmann, C. Kçcher, Angew. Chem. 2002, 114, 1342–
1363; Angew. Chem. Int. Ed. 2002, 41, 1290–1309; f) F. E. Hahn,
M. C. Jahnke, Angew. Chem. 2008, 120, 3166–3216; Angew. Chem.
Int. Ed. 2008, 47, 3122–3172; g) special issue: A. J. Arduengo, G.
Bertrand, Chem. Rev. 2009, 109, 3209–3210.
General procedures for Suzuki–Miyaura Cross-Coupling: Phenylboronic
acid (147 mg, 1.2 mmol), base (1.5 mmol), 3-bromoanisole (127 mL,
1.0 mmol), and 2-propanol (3.0 mL) were added to a Schlenk tube con-
taining complex 3 (0.5 mol% or 1.0 mol%) and a magnetic stirring bar.
The Schlenk tube was placed in a preheated (508C) oil bath and stirred
for the time indicated. The mixture was quenched with water, extracted
with CH2Cl2, dried over MgSO4, and filtered through a pad of Celite. The
solvent was removed under reduced pressure and the residue was ana-
lyzed by 1H NMR spectroscopy. Chemical shifts of the starting materials
and products were compared with data reported previously.[39]
ESI-MS analyses: Complex 3 (1 mg) was dissolved in iPrOH (3 mL) at
508C and phenylboronic acid (7 mg), Bu4NF (50 mL), and 3-bromoanisole
(7 mg) were added. MS spectra were recorded after 5, 20, and 35 min for
samples (10 mL) diluted with iPrOH (1.5 mL) on an Apex-Qe Ultra 7T
instrument (Bruker Daltonics, Germany) equipped with an ESI source.
The potential between the spray needle and the orifice was set to 4.5 kV,
the source accumulation time was 0.5 s and the ion accumulation time
was 0.5 s.
Transmission electron microscopy: Complex 3 (1 mg), phenylboronic acid
(7 mg) and 3-bromoanisole (7 mg) were placed in a test tube and subse-
quently iPrOH (3 mL) and Bu4NF (50 mL) were added. The tube was
sealed with a plastic stopper and heated at 508C for 35 min with magnet-
ic stirring. A droplet of the reaction mixture was placed on a carbon-
coated microscope grid and dried for 40 min. TEM measurements were
carried out by using a FEI Tecnai G2 20 X-TWIN electron microscope
(TEM) operating at 200 kV.
Crystallographic details: Crystal data for complexes 3b, 3c, and 3d were
collected by using an Agilent Technologies SuperNova A diffractometer
fitted with an Atlas detector that uses monochromated MoKa radiation
(0.71073 ꢂ). A complete dataset was collected, assuming that the Friedel
pairs are not equivalent. An analytical numeric absorption correction
was performed.[40] The structures were solved by direct methods using
SHELXS-97 and refined by full-matrix least-squares fitting on F2 for all
data using SHELXL-97.[41] Hydrogen atoms were added at calculated po-
sitions and refined by using a riding model. Anisotropic thermal displace-
ment parameters were used for all nonhydrogen atoms. Further crystallo-
graphic details are compiled in the Supporting Information (Tables S1–
S3). CCDC-855567 (3d), CCDC-855568 (3b), and CCDC-855569 (3c)
contains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[10] a) P. L. Arnold, J. Pearson, Coord. Chem. Rev. 2007, 251, 596–609;
b) M. Albrecht, Chem. Commun. 2008, 3601–3610; c) O. Schuster,
L. Yang, M. Albrecht, H. G. Raubenheimer, Chem. Rev. 2009, 109,
3445–3478; d) M. Melaimi, M. Soleilhavoup, G. Bertrand, Angew.
Chem. 2010, 122, 8992–9032; Angew. Chem. Int. Ed. 2010, 49, 8810–
8849; e) D. G. Gusev, Organometallics 2009, 28, 6458–6461; f) Y.
Han, H. V. Huynh, Dalton Trans. 2011, 40, 2141–2147; g) M. Igle-
sias, M. Albrecht, Dalton Trans. 2010, 39, 5213–5215.
[11] a) A. Lawson, C. E. Searle, J. Chem. Soc. 1957, 1556–1561; b) A. D.
McNaught, A. Wilkinson in IUPAC Compendium of Chemical Ter-
minology (the “Gold Book”) 2nd ed., Blackwell Scientific Publica-
tions, Oxford, 1997.
[12] P. Mathew, A. Neels, M. Albrecht, J. Am. Chem. Soc. 2008, 130,
13534–13535.
[13] G. Guisado-Barrios, J. Bouffard, B. Donnadieu, G. Bertrand, Angew.
Chem. 2010, 122, 4869–4872; Angew. Chem. Int. Ed. 2010, 49, 4759–
4762.
Acknowledgements
´
We thank Dr. Ewa Mieczynska and Mr. Marek Jon (Faculty of Chemis-
try, University of Wrocław) for experimental assistance. Financial support
from Science Foundation Ireland, the European Research Council (ERC
Starting Grant), the Polish Ministry of Science and Higher Education
(N164/COST/2008), and COST D40 is gratefully acknowledged.
[14] a) R. Lalrempuia, N. D. McDaniel, H. Mꢀller-Bunz, S. Bernhard, M.
Albrecht, Angew. Chem. 2010, 122, 9959–9962; Angew. Chem. Int.
Ed. 2010, 49, 9765–9768; b) T. Nakamura, K. Ogata, S. Fukuzawa,
Chem. Lett. 2010, 39, 920–922; c) K. J. Kilpin, U. S. D. Paul, A.-L.
[1] a) Metal-Catalyzed Cross-Coupling Reactions, 2nd ed. (Eds.: A. de
Meijere, F. Diederich), Wiley-VCH, Weinheim, 2004; b) Modern Ar-
Chem. Eur. J. 2012, 18, 6055 – 6062
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6061

M. Albrecht, A. M. Trzeciak et al.
Lee, J. D. Crowley, Chem. Commun. 2011, 47, 328–330; d) T. Naka-
mura, T. Terashima, K. Ogata, S. Fukuzawa, Org. Lett. 2011, 13,
620–623; e) A. Poulain, D. Canseco-Gonzalez, R. Hynes-Roche, H.
Mꢀller-Bunz, O. Schuster, H. Stoeckli-Evans, A. Neels, M. Albrecht,
Organometallics 2011, 30, 1021–1029; f) A. Prades, E. Peris, M. Al-
brecht, Organometallics 2011, 30, 1162–1167; g) R. Saravanakumar,
V. Ramkumar, S. Sankararaman, Organometallics 2011, 30, 1689–
1694; h) J. Bouffard, B. K. Keitz, R. Tonner, G. Guisado-Barrios, G.
Frenking, R. H. Grubbs, G. Bertrand, Organometallics 2011, 30,
2617–2627; i) L. Bernet, R. Lalrempuia, W. Ghattas, H. Mueller-
Bunz, L. Vigara, A. Llobet, M. Albrecht, Chem. Commun. 2011, 47,
8058–8060; j) J. D. Crowley, A.-L. Lee, K. J. Kilpin, Aust. J. Chem.
2011, 64, 1118–1132.
[30] P. Lara, O. Rivada-Wheelaghan, S. Conejero, R. Poteau, K. Philip-
pot, B. Chaudret, Angew. Chem. 2011, 123, 12286–12290; Angew.
Chem. Int. Ed. 2011, 50, 12080–12084.
[31] B. Liu, Q. Xia, W. Chen, Angew. Chem. 2009, 121, 5621–5624;
Angew. Chem. Int. Ed. 2009, 48, 5513–5516.
[32] C. C. Cassol, A. P. Umpierre, G. Machado, S. I. Wolke, J. Dupont, J.
Am. Chem. Soc. 2005, 127, 3298–3299.
[33] A. Gniewek, A. M. Trzeciak, J. J. Ziolkowski, L. Kepinski, J.
Wrzyszcz, W. Tylus, J. Catal. 2005, 229, 332–343.
[34] Heterogenization has been report to occur, although, generally, the
catalytic activity has been attributed to the nanoparticles themselves
and not to redissolved molecular palladium species. See: a) M. T.
Reetz, J. G. de Vries, Chem. Commun. 2004, 1559–1563; b) J. G.
de Vries, Dalton Trans. 2006, 421–429; c) D. Astruc, Inorg. Chem.
2007, 46, 1884–1894; d) Y. Tsuji, T. Fujihara, Inorg. Chem. 2007, 46,
1895–1902; e) A. Gniewek, J. J. Ziolkowski, A. M. Trzeciak, M. Za-
wadzki, H. Grabowska, J. Wrzyszcz, J. Catal. 2008, 254, 121–130;
f) Y.-H. Chen, H.-H. Hung, M. H. Huang, J. Am. Chem. Soc. 2009,
131, 9114–9121; g) P. J. Ellis, I. J. S. Fairlamb, S. F. J. Hackett, K.
Wilson, A. F. Lee, Angew. Chem. 2010, 122, 1864–1868; Angew.
Chem. Int. Ed. 2010, 49, 1820–1824; h) A. N. Kashin, I. P. Belet-
skaya, Russ. J. Org. Chem. 2011, 47, 475–479; for critical reviews,
see also: i) I. P. Beletskaya, A. V. Cheprakov, J. Organomet. Chem.
2004, 689, 4055–4082; j) L. Yin, J. Liebscher, Chem. Rev. 2007, 107,
133–173.
[35] Metal leaching from solid supports has been demonstrated for the
Heck reaction at high temperatures: a) S. S. Prçckl, W. Kleist, M. A.
Gruber, K. Kçhler, Angew. Chem. 2004, 116, 1917–1918; Angew.
Chem. Int. Ed. 2004, 43, 1881–1882; b) S. S. Prçckl, W. Kleist, K.
Kçhler, Tetrahedron 2005, 61, 9855–9859; c) J. M. Richardson, C. W.
Jones, Adv. Synth. Catal. 2006, 348, 1207–1216; d) I. Pryjomska-Ray,
A. Gniewek, A. M. Trzeciak, J. J. Ziolkowski, W. Tylus, Top. Catal.
2006, 40, 173–184; e) S. MacQuarrie, J. H. Horton, J. Barnes, K.
McEleney, H.-P. Loock, C. M. Crudden, Angew. Chem. 2008, 120,
3324–3328; Angew. Chem. Int. Ed. 2008, 47, 3279–3282; f) P.-P.
Fang, A. Jutand, Z.-Q. Tian, C. Amatore, Angew. Chem. Int. Ed.
2011, 50, 12184; for a relevant example in palladium-catalyzed addi-
tion reactions, see: g) V. P. Ananikov, N. V. Orlov, I. P. Beletskaya,
V. N. Khrustalev, M. Y. Antipin, T. V. Timofeeva, J. Am. Chem. Soc.
2007, 129, 7252–7253; for an excellent review, see: h) V. P. Anani-
kov, I. P. Beletskaya, Organometallics 2012, 31, 1595–1604.
[36] a) M. Weck, C. W. Jones, Inorg. Chem. 2007, 46, 1865–1875; b) K.
Kçhler, W. Kleist, S. S. Prçckl, Inorg. Chem. 2007, 46, 1876–1883.
[37] a) G. Zassinovich, G. Mestroni, S. Gladiali, Chem. Rev. 1992, 92,
1051–1069; b) D. Klomp, U. Hanefeld, J. A. Peters, in Handbook of
Homogeneous Hydrogenation, Vol. 2 (Eds.: J. G. de Vries, C. J.
Elsevier), Wiley-VCH, Weinheim, 2007, 585–630; c) S. Gladiali, E.
Alberico, Chem. Soc. Rev. 2006, 35, 226–236; d) J. S. M. Samec, J.-E.
Bꢆckvall, P. G. Andersson, P. Brandt, Chem. Soc. Rev. 2006, 35, 237–
248.
[15] a) R. Huisgen, Angew. Chem. 1963, 75, 604–637; Angew. Chem. Int.
Ed. Engl. 1963, 2, 565–598; b) H. Kolb, M. G. Finn, K. B. Sharpless,
Angew. Chem. 2001, 113, 2056–2075; Angew. Chem. Int. Ed. 2001,
40, 2004–2021; c) M. Meldal, C. W. Tornoe, Chem. Rev. 2008, 108,
2952–3015; d) special issue: M. G. Finn, V. V. Fokin, Chem. Soc.
Rev. 2010, 39, 1231–1232.
[16] Similar observations have recently been reported in ruthenium-cata-
lyzed olefin metathesis: B. K. Keitz, J. Bouffard, G. Bertrand, R. H.
Grubbs, J. Am. Chem. Soc. 2011, 133, 8498–8501.
[17] See the Supporting Information for details.
13
À
[18] The pertinent C NMR chemical shifts for C Pd are 138.0 (3a),
136.6 (3b), 129.2 (3c), 132.3 (3d), and 132.1 ppm (3e).
[19] A. Poulain, M. Iglesias, M. Albrecht, Curr. Org. Chem. 2011, 15,
3325–3336.
[20] Similar to 3b, complexes 4 and 5 exhibit poor catalytic performance
when DMA or dioxane were used as the solvent. See the Supporting
Information for a time–conversion profile of 4 and 5.
[21] H. Lebel, M. K. Janes, A. B. Charette, S. P. Nolan, J. Am. Chem.
Soc. 2004, 126, 5046–5047.
[22] a) D. Bacciu, K. J. Cavell, I. A. Fallis, L.-I. Ooi, Angew. Chem. 2005,
117, 5416–5418; Angew. Chem. Int. Ed. 2005, 44, 5282–5284; b) M.
Heckenroth, A. Neels, M. G. Garnier, P. Aebi, A. W. Ehlers, M. Al-
brecht, Chem. Eur. J. 2009, 15, 9375–9386.
[23] J. Nasielski, N. Hadei, G. Achonduh, E. A. Kantchev, C. J. OꢅBrien,
M. G. Organ, Chem. Eur. J. 2010, 16, 10844–10853.
[24] a) A. F. Littke, G. C. Fu, Angew. Chem. 2002, 114, 4350–4386;
Angew. Chem. Int. Ed. 2002, 41, 4176–4211; b) R. B. Bedford,
C. S. J. Cazin, D. Holder, Coord. Chem. Rev. 2004, 248, 2283–2321;
nanoparticles have been reported to catalyze aryl chloride conver-
sion: c) R. B. Bedford, M. E. Blake, C. P. Butts, D. Holder, Chem.
Commun. 2003, 466–467; d) N. E. Leadbeater, M. Marco, Org. Lett.
2002, 4, 2973–2976.
[25] I. Blaszczyk, A. Gniewek, A. M. Trzeciak, J. Organomet. Chem.
2011, 696, 3601–3607.
[26] a) D. R. Anton, R. H. Crabtree, Organometallics 1983, 2, 855–859;
b) G. M. Whitesides, M. Hackett, R. L. Brainard, J. P. P. M. Laval-
leye, A. F. Sowinski, A. N. Izumi, S. S. Moore, D. W. Brown, E. M.
Staudt, Organometallics 1985, 4, 1819–1830; c) J. A. Widegren,
R. G. Finke, J. Mol. Catal. A: Chem. 2003, 198, 317–341; d) R. H.
Crabtree, Chem. Rev. 2012, 112, 1536–1554; mercury poisoning in
palladium-catalyzed cross-coupling is not unambiguous, see:
e) N. T. S. Phan, M. van der Sluys, C. W. Jones, Adv. Synth. Catal.
2006, 348, 609–679.
[38] a) P. Appukkuttana, W. Dehaena, V. Fokin, M. Van der Eycken, Org
Lett. 2004, 6, 4223–4225; b) K. Barral D. A. Moorhouse, J. E.
Moses, Org Lett. 2007, 9, 1809–1811; c) D. Kumar, V. B. Reddy, Syn-
thesis 2010, 1687–1691.
[39] J. Jin, M. M. Cai, J. X. Li, Synlett 2009, 2534–2538.
[40] CrysalisPro, Version 1.171.34.36, Oxford Diffraction Limited, 2010.
[41] G. M. Sheldrick, Acta Crystallogr. A 2008, 64, 112–122.
[27] R. D. Glover, J. M. Miller, J. E. Hutchision, ACS Nano 2011, 5,
8950–8957.
[28] I. P. Beletskaya, A. N. Kashin, I. A. Khotina, A. R. Khokhlov, Syn-
lett 2008, 1547–1552.
[29] W. Zawartka, A. Gniewek, A. M. Trzeciak, J. J. Ziolkowski, J.
Pernak, J. Mol. Catal. A: Chem. 2009, 304, 8–15.
Received: November 25, 2011
Revised: February 9, 2012
Published online: March 29, 2012
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