Cationic versus Anionic Palladium Species in the Suzuki–Miyaura Cross-Coupling

Article abstract of DOI:10.1002/ejic.201701000

The cationic complex [(pym-Im-Me)₂PdCl][PF₆] (pym = pyrimidyl; Im = imidazolin-2-ylidene) has been identified as a highly active and reliable catalyst in the Suzuki–Miyaura cross-coupling reaction in ethanol for a wide range of substrates without the need for an excess of boronic acid. The catalytic activity of the cationic complex is retained at very low catalyst loadings, whereas quick decomposition occurs when using the anionic complex [Bu₄N][Pd(DMSO)Cl₃] (DMSO = dimethyl sulfoxide). Further investigations of the induction period, the effect of catalyst poisons, and SEM analysis of the particles formed under catalytic conditions show that in both cases the catalytically active species is – at least partially – heterogeneous.

Full text of DOI:10.1002/ejic.201701000

DOI: 10.1002/ejic.201701000
Full Paper
Palladium Cross-Coupling Catalysts
Cationic versus Anionic Palladium Species in the
Suzuki–Miyaura Cross-Coupling
Felix Schroeter[a] and Thomas Strassner*[a]
Abstract: The cationic complex [(pym-Im-Me)2PdCl][PF6] using the anionic complex [Bu4N][Pd(DMSO)Cl3] (DMSO = di-
(pym = pyrimidyl; Im = imidazolin-2-ylidene) has been identi-
fied as a highly active and reliable catalyst in the Suzuki–
Miyaura cross-coupling reaction in ethanol for a wide range of
substrates without the need for an excess of boronic acid. The
catalytic activity of the cationic complex is retained at very low
catalyst loadings, whereas quick decomposition occurs when
methyl sulfoxide). Further investigations of the induction pe-
riod, the effect of catalyst poisons, and SEM analysis of the par-
ticles formed under catalytic conditions show that in both cases
the catalytically active species is – at least partially – heteroge-
neous.
Introduction
the cation of 1 have been employed by Chen,[19] as well as
complexes similar to the anion of 1 by Trzeciak,[20] Dyson,[21]
and Nolan.[22] Complexes 1–3 have been characterized by solid-
state structures.[17,18] Thus, we chose to study the performance
of complex 1 in the Suzuki–Miyaura cross-coupling in compari-
son to complexes 2–4, each containing only one palladium
species,[17,18] to identify which of the palladium species in 1
provides the ideal precatalyst for the Suzuki–Miyaura cross-cou-
pling reaction.
The Suzuki–Miyaura cross-coupling[1] is one of the most power-
ful C–C bond-forming processes[2] and as such has been
awarded the 2010 Nobel Prize in chemistry.[3] The palladium-
catalyzed synthesis of unsymmetric biaryls is of great value
both for laboratory-[4] and industrial-scale processes.[5] To en-
hance the catalytic activity, many types of ligands,[6] for exam-
ple phosphines,[7] as well as ligand-free catalytic systems[8] have
been investigated. Since the isolation of the first free N-hetero-
cyclic carbene (NHC) by Arduengo,[9] many palladium–NHC
complexes have been employed in the Suzuki–Miyaura cross-
coupling,[10] most frequently with monodentate NHC ligands.[11]
On the other hand, chelating ligands generally increase the
stability of a complex, which may also improve its performance
in catalytic reactions.[12] For example, bis(NHC) complexes have
been successfully employed in the Suzuki–Miyaura cross-cou-
pling.[13] Furthermore, (C^N)-chelating NHC ligands for palla-
dium have proven their effectiveness in cross-coupling reac-
tions,[14] either by directly stabilizing the catalyst or by their
potential hemilability, allowing them to stabilize labile and oth-
erwise low-coordinate intermediates.[15]
Scheme 1. Palladium complexes for the Suzuki–Miyaura cross-coupling used
in the present study.
Among these, palladium complexes with chelating pyrimidyl-
imidazolin-2-ylidene ligands have been successfully employed
in various catalytic reactions.[16] In 2009, we reported complex
1, containing both a palladium cation and a palladium anion
(Scheme 1).[17] Compound 1 was an active precatalyst in the
Mizoroki–Heck reaction, as well as the oxidation of methane.
Further studies assigned the high catalytic activity in the
Mizoroki–Heck reaction to the palladium anion.[18] In the
Suzuki–Miyaura cross-coupling reaction, compounds similar to
Results and Discussion
Catalysis
The reaction of 4-bromotoluene with phenylboronic acid has
been chosen as a model reaction for the Suzuki–Miyaura cross-
coupling (Scheme 2). The reaction conditions were preopti-
mized by using complex 1 (Table S1, see Supporting Informa-
tion). It should be noted that the catalyst load is given in mol-
% of palladium to take into account that complex 1 contains
two potentially catalytically active palladium species. Compari-
son of the different precatalysts 1–4 shows moderate to high
yields at higher catalyst loadings (1 and 0.5 mol-%) for all em-
[a] Fachrichtung Chemie und Lebensmittelchemie, TU Dresden,
01062 Dresden, Germany
E-mail: thomas.strassner@chemie.tu-dresden.de
http://www.chm.tu-dresden.de/oc3/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201701000.
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ployed compounds (Figure 1). However, only cationic complex tion (Figure 2). The cationic complex 2 shows a comparatively
2 maintains its catalytic activity at lower catalyst loadings down long induction period of 15 min and a subsequent quick onset,
to 0.1 mol-% (Figure 1). In general, anionic complexes 3 and 4 reaching a yield of 70 % after 60 min. The anionic complexes 3
are less efficient compared to the NHC-containing complexes 1 and 4 show no observable induction period, but after 30 min,
and 2.
the reaction proceeds very slowly. As already mentioned, we
observed a quick formation of palladium black for anionic com-
plexes 3 and 4. Rapidly formed palladium nanoparticles, which
aggregate quickly, might explain the high initial reaction rate
and the incomplete conversion.
Scheme 2. Model reaction for the Suzuki–Miyaura cross-coupling used in the
present study.
Figure 2. Initial yield of 4-methylbiphenyl for the precatalysts 1–4. Reaction
conditions: 4-bromotoluene (1 mmol), phenylboronic acid (1.5 mmol), K3PO4
(2 mmol), toluene (5 mL), [Pd] (0.5 mol-%), 60 °C. The yield was determined
by GC with use of dodecane as the internal standard and is given as an
average of two runs.
Figure 1. Yield of 4-methylbiphenyl for the precatalysts 1–4 at different cata-
lyst loads and reaction times. Reaction conditions: 4-bromotoluene (1 mmol),
phenylboronic acid (1.5 mmol), K3PO4 (2 mmol), toluene (5 mL), 60 °C. The
yield was determined by GC with use of dodecane as the internal standard
and is given as an average of two runs.
The mixed complex 1 indeed behaves as a mixture of the
anionic and cationic species, as it also showed no induction
period but retained its catalytic activity over a longer period of
time.
Prolonging the reaction time to 24 h with 0.1 mol-% catalyst
loading leads to full conversion only for complex 2 (Figure 1).
With catalysts 1, 3, and 4, containing the anionic palladium
species, lower yields were obtained, accompanied by an almost
instantly visible formation of palladium black, indicating de-
composition of the anionic species. Similar observations have
been reported in the literature.[20a] For complex 2, formation
of palladium black is only observed after completion of the
reaction.
Generally, all precatalysts show an unusual correlation be-
tween catalyst load and product yield. The highest yields are
obtained at 0.5 mol-% catalyst load, while increasing the cata-
lyst load to 1 mol-% slightly lowers the yield (Figure 1). Similar
observations have been made for other cross-coupling reac-
tions.[23] Additionally, complexes 3 and 4 show a relatively high
deviation of their catalytic activity, even in identical catalytic
runs. Quick formation of palladium black can be observed and
indicates the decomposition of the anionic complex to palla-
dium nanoparticles. This might explain the deviation, since the
exact conditions of the decomposition are difficult to repro-
duce. To maintain their high catalytic activity, nanoparticles
generally need a stabilizer,[8a] which is not present in the reac-
tion. Thus, only complex 2 retains its performance at catalyst
loadings as low as 0.01 mol-%.
Poisoning Experiments
To gain more insight into the catalytically active species, we
conducted the reaction in the presence of CS2, which acts as a
catalyst poison for homogeneous and heterogeneous catalysts
at temperatures below 50 °C (Table 1).[24] Since only the surface
atoms of a heterogeneous catalyst are catalytically active, a sub-
stoichiometric amount of a catalyst poison should be able to
completely shut down the catalytic activity.[25] Indeed, the
addition of a substoichiometric amount of CS2 led to complete
inhibition of catalytic activity in every case and indicates the
involvement of a heterogeneous catalytic reaction.
Table 1. Results of the Suzuki–Miyaura cross-coupling in the presence of cata-
lyst poisons.[a]
Entry
Catalyst
Yield
[%]
Yield with CS2
[%][b]
Yield with Hg
[%][c]
1
2
3
4
1
2
3
4
90[d]
78[e]
40[e]
80[d]
0
20
44
23
23
0[e]
0[e]
0
[a] Reaction conditions: 4-bromotoluene (1 mmol), phenylboronic acid
(1.5 mmol), K3PO4 (2 mmol), [Pd] (0.5 mol-%), toluene (5 mL), 2 h. The yield
was determined by GC with use of dodecane as the internal standard and is
given as an average of two runs. [b] In the presence of CS2 (0.15 mol-%), at
50 °C. [c] One drop of Hg was added with a syringe, at 60 °C. [d] At 50 °C.
[e] At 40 °C.
Identification of the Catalytically Active Species
To account for the reactivity differences of the complexes, we
In addition, mercury-poisoning tests were performed
investigated the induction period and initial phase of the reac- (Table 1). Mercury adsorbs onto metal surfaces and/or
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amalgamates metals, especially palladium, while homogeneous
complexes remain unaffected.[25] The catalysis in the presence
of mercury was not completely inhibited. This indicates that a
homogeneous catalytically active species is also formed.[26] In
this experiment, precatalyst 2 significantly outperforms the
other precatalysts, indicating that the formation of a heteroge-
neous catalyst is retarded, and instead, a highly active homoge-
neous catalyst is formed first, albeit in small quantities, since
substoichiometric quantities of CS2 completely shut down the
catalytic activity.
SEM Investigation
All experiments performed invoke the participation of nanopar-
ticles in the catalytic reaction.[27] Thus, we isolated the palla-
dium metal formed during the catalytic run and analyzed it by
scanning electron microscopy (SEM). For the cationic precata-
lyst 2, small uniform nanoparticles were formed, with an esti-
mated size of 7 2 nm (Figure 3). For the anionic precatalyst 3,
larger nanoparticles were formed (estimated size 11 8 nm),
with a significantly higher variation in size (Figure 4). These
nanoparticles appear to be aggregated to larger spherical struc-
tures. The anionic precatalyst 4 yielded nanoparticles similar to
complex 3 (Figure S1, see Supporting Information), while for
the mixed complex 1, different regions were visible in the SEM
images. One region resembled the shape and size distribution
of the anionic precatalysts (Figure S2, see Supporting Informa-
tion). The other one resembled shape and size distribution of
the cationic complex 2 (Figure S3, see Supporting Information).
Figure 4. SEM image of isolated nanoparticles formed in the Suzuki–Miyaura
cross-coupling with precatalyst 3. Particle size 11 8 nm.
Further Optimization
All performed experiments show that both the cation and the
anion contribute to the observed catalytic activity in complex
1. Due to the higher activity and stability of the active species
formed from complex 2, it was chosen as the precatalyst for
further optimization (Table 2).
Table 2. Optimization of reaction conditions with use of complex 2.[a]
Entry
Base (mmol)
Solvent
Time [h]
Yield [%]
1[b]
2[b]
3
4
5
K3PO4 (2)
K3PO4 (2)
K3PO4 (2)
K2CO3 (2)
Cs2CO3 (2)
K3PO4 (2)
K3PO4 (2)
K3PO4 (1.5)
K3PO4 (2)
toluene
toluene
toluene
toluene
toluene
EtOH
EtOH
EtOH
EtOH
2
5
5
5
5
5
5
5
5
83
100
100
100
0
6
100 (37)[c]
77 (0)[e]
90[f]
7[d]
8
9
0[g]
[a] Reaction conditions: 4-bromotoluene (1 mmol), phenylboronic acid
(1 mmol), K3PO4 (2 mmol), 2 (0.1 mol-%), solvent (5 mL), 60 °C. The yield was
determined by GC with use of dodecane as the internal standard and is given
as an average of two runs. [b] Phenylboronic acid (1.5 mmol, instead of
1 mmol). [c] In parentheses: one drop of Hg was added with a syringe. [d] At
50 °C. [e] In parentheses: in the presence of CS2 (0.03 mol-%). [f] After 5 and
24 h. [g] Blank run without catalyst 2.
Prolonging the reaction time from 2 to 5 h leads to full con-
version (Table 2, Entries 1 and 2). At this point, the amount
of phenylboronic acid could be reduced from 1.5 equiv. to an
equimolar amount without a decrease in yield (Table 2, Entry 3).
Different bases were tested, with potassium carbonate being
equally suited, while the use of cesium carbonate did not result
Figure 3. SEM image of isolated nanoparticles formed in the Suzuki–Miyaura
cross-coupling with precatalyst 2. Particle size 7 2 nm.
From these experiments it can be concluded, that the NHC- in product formation (Table 2, Entries 4 and 5). Changing the
containing precatalyst 2 forms the smallest and most efficiently solvent to ethanol also led to complete conversion after 5 h
stabilized nanoparticles, thus outperforming anionic complexes (Table 2, Entry 6) and encouraged us to use ethanol for further
3 and 4. Indeed, NHC-stabilized nanoparticles of several metals studies, since it is less toxic and can be used without prior distil-
were reported as catalysts.[28] In the latter compounds, larger lation or drying. Performing the reaction at 50 °C led to a de-
and thus less active nanoparticles are formed, which aggregate crease in yield, but it allowed us to perform another poisoning
quickly because of the absence of stabilizing agents.[8a]
experiment with CS2 (Table 2, Entry 7), which, together with the
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mercury poisoning experiment (Table 2, Entry 6), confirmed the of the anionic complex 3 leads to quick catalyst decomposition,
heterogeneous nature of the catalytic reaction even at lower as indicated by rapid formation of palladium black during the
catalyst loads of complex 2. However, the reduction of the reaction, which in turn prevents full conversion. This is in con-
amount of base led to incomplete conversion, even after a pro- trast to the Mizoroki–Heck reaction, where the anion was re-
longed reaction time (Table 2, Entry 8).
sponsible for the high activity.[18] The presence of an induction
period, the observed catalyst poisoning in the presence of CS2
and mercury, and the SEM images indicate the participation of
a heterogeneous catalyst for each of the compounds 1–4.
Substrate Scope
Under the optimized conditions, various aryl bromides and
boronic acids can be coupled (Scheme 3). Aryl bromides with
electron-withdrawing and electron-donating functional groups
are tolerated (Scheme 3, conditions A). However, for aryl
bromides with coordinating functional groups or steric hin-
drance the catalyst amount has to be increased to enable prod-
uct formation (Scheme 3, conditions B). On the other hand,
various arylboronic acids are tolerated regardless of their elec-
tronic and steric situation. The coupling of dibromobenzene re-
quires an increased amount of base, but also proceeds in high
yields (Scheme 3, conditions C). Aryl chlorides were unreactive,
even under more forcing reaction conditions.
Experimental Section
General Considerations: All reactions were performed under ar-
gon. Solvents of at least 99.0 % purity were used in all reactions in
this study. Toluene and N,N-dimethylacetamide were distilled from
calcium hydride and stored over molecular sieves (4 Å). Dioxane
and pyridine were stored over molecular sieves (4 Å). Dry aceto-
nitrile and CH2Cl2 were obtained from an MBraun solvent purifica-
tion system. Ethanol was used as received. All other chemicals were
obtained from common suppliers and used without further purifica-
tion. 1H and 13C NMR spectra were recorded with Bruker Avance
300 and Bruker DRX 500 NMR spectrometers. 1H and 13C NMR spec-
tra were referenced internally using the solvent resonances (1H: δ =
7.26 ppm, 13C: δ = 77.0 ppm for CDCl3). Chemical shifts are given
in ppm, coupling constants J in Hz. Elemental analyses were per-
formed by the Microanalytical Laboratory of our institute with a
Hekatech EA 3000 Euro Vector elemental analyzer. GC–MS measure-
ments were carried out with an Agilent 7890A Gas Chromatograph
and an Agilent 5975 Series MSD. SEM micrographs were performed
with a Zeiss DSM-982 GEMINI with an emission voltage of 7 kV.
General Catalytic Procedure: Into a 10 mL crimp vial all solid rea-
gents were added, and the vial was capped by a butyl rubber sep-
tum. The vial was evacuated and subsequently filled with argon
three times. Then, the solvent and all liquid reagents were added
with a syringe. The vials were placed in a preheated aluminum
block and stirred for the indicated time. For determination of the
GC yield, dodecane (175 mg) was added as an internal standard,
and aliquots (100 μL) of the reaction mixture were placed onto a
plug of silica and rinsed with CH2Cl2 (5 mL). The obtained solution
was analysed by GC–MS. The yield was determined using a calibra-
tion curve. All yields given are an average of two separate runs. For
the determination of the isolated yields, the reaction mixture was
filtered through a plug of silica and rinsed with CH2Cl2 (2 × 20 mL).
The solvent was evaporated under reduced pressure and the prod-
uct dried in vacuo. For reactions without full conversion, the raw
product was purified by column chromatography.
SEM Measurements: The reaction mixture was washed with water
(5 mL), and ethanol (5 mL) was added to the organic phase. The
nanoparticles were separated from the solution by centrifugation
and washed with water and ethanol (5 mL each). During workup,
aggregation of the dispersed metal powder to bulk metal occurred.
The palladium metal was dried in vacuo at 50 °C and analyzed
by SEM. From the SEM images, the average nanoparticle size was
estimated by measuring 55 randomly chosen visible particles for
each catalyst. The confidence interval is given as 2σ.
Scheme 3. Substrate scope. General reaction conditions: aryl bromide
(1 mmol), boronic acid (1 mmol), ethanol (5 mL), 60 °C. Conditions A: 2
(0.1 mol-%), K3PO4 (2 mmol), 5 h. Conditions B: 2 (0.5 mol-%), K3PO4 (2 mmol),
24 h. Conditions C: 2 (0.1 mol-%), phenylboronic acid (2 mmol), K3PO4
(4 mmol), 5 h.
Conclusions
Supporting Information (see footnote on the first page of this
article): Additional tables and figures experimental procedures,
product characterization data, and NMR spectra.
Both the anion and cation of the mixed compound 1 have been
shown to be catalytically active in the Suzuki–Miyaura cross-
coupling by comparison of the precatalysts 2–4. Cationic com-
plex 2 has been identified as an active precatalyst for the cou-
pling of various aryl bromides and boronic acids in ethanol as
Acknowledgments
an inexpensive and nontoxic solvent. On the contrary, the use We are grateful to S. Goldberg for the SEM measurements.
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Keywords: NHC complexes · Palladium · Suzuki–Miyaura
reaction · Cross-coupling · Nanoparticles
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Received: August 21, 2017
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