N-donor complexes of palladium as catalysts for Suzuki cross-coupling reactions in ionic liquids

Article abstract of DOI:10.1016/j.molcata.2003.11.030

Palladium imidazole complexes have been used as catalyst precursors for the Suzuki cross-coupling reaction in 1-butyl-3-methylimidazolium-based ambient temperature ionic liquids. The system provides a stable, recyclable method for iodo- and bromoarenes. The preferred reaction conditions are explored and the effect of changing the ionic liquid components is investigated.

Full text of DOI:10.1016/j.molcata.2003.11.030

Journal of Molecular Catalysis A: Chemical 214 (2004) 27–32
N-donor complexes of palladium as catalysts for
Suzuki cross-coupling reactions in ionic liquids
Christopher J. Mathewsa, Paul J. Smithb, Tom Weltonb,∗
a
Syngenta, Jealotts International Research Centre, Bracknell RG42 6ET, UK
b
Department of Chemistry, Imperial College of Science Technology and Medicine, South Kensington, London SW7 2AY, UK
Received 31 July 2003; accepted 3 November 2003
Abstract
Palladium imidazole complexes have been used as catalyst precursors for the Suzuki cross-coupling reaction in 1-butyl-3-methylimida-
zolium-based ambient temperature ionic liquids. The system provides a stable, recyclable method for iodo- and bromoarenes. The preferred
reaction conditions are explored and the effect of changing the ionic liquid components is investigated.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Cross-coupling reaction; Ionic liquids; Palladium imidazole complexes
1. Introduction
the use of various palladium sources, either with phosphine
ligands or added phosphines. However, unexpected activity
observed during control experiments led us to investigate the
use of palladium complexes of N-donor ligands as catalysts
in ionic liquids. It is this work that we report here.
Palladium catalysed coupling and cross-coupling reac-
tions remain a method of choice for the formation of C–C
bonds. Recent developments have led to the activation of
aryl chlorides [1] and room temperature catalysis [2]. Sig-
nificant innovations have been made in the discoveries of
novel phosphine systems and in the modification of tra-
ditional phosphines. The replacement of the commonly
used triarylphosphine ligands with sterically-hindered and
electron-rich trialkylphosphines [3] phosphites [4] and
phospha-palladacycles (for a review see [5]) have all given
rise to highly active systems in palladium catalysed cou-
pling reactions.
Our interest has been in the use of ionic liquids as sol-
vents for the Suzuki reaction [6]. The application of ionic
liquids as solvents for transition metal catalysis is an area
of intense interest [7] and a wide range of processes are un-
der investigation. The first reported example of a palladium
catalysed coupling reaction in an ionic liquid was the Heck
reaction in phosphonium and ammonium based ionic liquids
[8]. Since then, palladium catalysed coupling reactions have
become one of the most active areas of catalyst research in
ionic liquids (some examples [9]). To date, we have reported
The use of N-coordinated ligands in palladium catalysed
coupling reactions has been an area of limited activity. Most
examples involve cyclometallated palladium complexes in-
corporating either the imine [10,11], amine [12], oxime
[13] or oxazoline [14] moieties. The remainder include
either chelating bi- or terdentate N-coordinating ligands
such as diazabutanes [15,16] bis(oxazolinyl)pyrrole [17]
or dipyridyl [18] ligands, respectively. A series of methyl
palladium(II) complexes have also been reported, incorpo-
rating N-coordinated bis- and tris-imidazole chelates that
were found to be catalytically active in the Heck reaction of
4-bromoacetophenone with n-butyl acrylate [19]. Recently,
an example of a mixed imidazolylidene-imidazole palla-
dium complex and its application in Sonogashira coupling
reactions has been reported [20]. We have reported the use
of palladium(II) imidazole based complexes in molecular
solvents as catalysts for the Suzuki reaction [21] (Scheme 1).
2. Results and discussion
∗
While searching for soluble sources of palladium for the
palladium/phosphine catalysed Suzuki reactions in ionic
Corresponding author. Tel.: +44-1715945763; fax: +44-1715945804.
E-mail address: t.welton@ic.ac.uk (T. Welton).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2003.11.030

28
C.J. Mathews et al. / Journal of Molecular Catalysis A: Chemical 214 (2004) 27–32
(CH3CN)PdCl2
N-donor ligand
Na2CO3 (aq)
and the ionic liquid itself, even after 4 weeks stored under
N2. However, upon addition of water the ESI MS revealed a
new peak at 307 Da, corresponding to [(mim)2PdCl]+. This
confirmed the formation of (mim)2PdCl2 in the ionic liquid
and that its formation is not due to 1-methylimidazole im-
purities in the ionic liquid. In our Suzuki reactions NaCO3
is added as an aqueous solution, so giving conditions for
the potential formation of (mim)2PdCl2. Hence, we chose
to investigate the use of palladium/imidazole catalysts for
the Suzuki reaction in ionic liquids.
Br
B(OH)2
+
Ionic Liquid
Scheme 1. The Suzuki coupling reaction of a bromobenzene with tolyl-
boronic acid.
Table 1
The Suzuki reaction of bromobenzene and tolylboronic acid in
[C4C1im][BF4] with a variety of palladium sources
b
Yielda (%)
TOF (h−1
)
Homoc (%)
Initial experiments, with 1-methylimidazole to determine
the reaction conditions to be used found that the addition
of (CH3CN)2PdCl2 and 1-methylimidazole directly to the
ionic liquid was the preferable method for the introduc-
tion of the catalyst. The solutions were then heated to
110 ◦C, during which time the yellow colour of the palla-
dium faded. Performing the Suzuki reaction without this
prior heating of the (CH3CN)2PdCl2/4mim (where mim
= 1-methylimidazole) in the [C4C1im][BF4] resulted in
decomposition of the catalyst during the reaction and a
decreased yield of 4-methylbiphenyl. Thus, as with the pal-
ladium/phosphine catalytic system that we have previously
reported [6], the initiation of the catalyst was fundamental
to the success of the ionic liquid mediated Suzuki reaction.
The resulting solution was cooled to room temperature, the
aqueous Na2CO3 solution and the starting materials added
and the reaction carried out at 110 ◦C for 20 min. In contrast
to the palladium/phosphine system, the catalyst preparation
in the ionic liquid could be conducted under air, without
any detrimental effect on the eventual Suzuki reaction.
It was found that, in order to prevent catalyst decom-
position, at least 4 equivalents of 1-methylimidazole were
required, relative to the palladium, whilst more than 4
equivalents inhibited the reaction. Using 4 equivalents of
1-methylimidazole the reaction had reached its maximum
yield after 1 h at 110 ◦C, achieving a 83.3% yield, with
only a marginal increase after 3 h (Table 2). The reaction
could also be performed at room temperature, achieving a
78.7% yield in 24 h. Unreacted bromobenzene was detected
at the end of all the reactions and less than a 4% yield of
4,4ꢀ-dimethylbiphenyl was generated from homo-coupling.
Palladium source
[C4C1im]2[PdCl4]
45.3d
65.8
68.2
113
165
171
3.7
2.9
2.1
[C4C1im]2[PdCl4]/4PPh3
6
Pd(PPh3)4
Conditions: 1.2 mol% catalyst, temperature = 110 ◦C, time = 20 min.
a
Isolated yields of 4-methylbiphenyl, based on bromobenzene.
b
TOF (h−1) = number of moles of desired product per mole of
catalyst used per hour.
c
Homo-coupled 4,4ꢀ-dimethylbiphenyl yield.
Extensive catalyst decomposition and leaching.
d
liquids the tetrachloropalladate salt, [C4C1im]2[PdCl4]1
was investigated. Its use in combination with triphenylphos-
phine in [C4C1im][BF4], gave an active system for the
reaction of bromobenzene and tolylboronic acid (Table 1),
with a 65.8% yield of 4-methylbiphenyl in 20 min at 110 ◦C
without any apparent catalytic decomposition. This was
typical of several of the palladium sources that we used in
this reaction. However, unlike the other palladium sources
used, which were inactive in the absence of PPh3, the use
of [C4C1im]2[PdCl4] alone gave rise to a 45.3% yield of
4-methylbiphenyl in 20 min (Table 1). Extensive catalyst
decomposition was evident. [C4C1im]2[PdCl4] has also
been reported to act as a catalyst for the Heck reaction in
[C4C1im][PF6] [22].
Palladium(II) salts have previously been reported to
catalyse coupling reactions without added ligands, although
long reaction times are generally required [23]. However,
[C4C1im]2[PdCl4] has been previously reported as a palla-
dium source for the hydrodimerisation of 1,3-butadiene in
both [C4C1im][BF4] and [C4C1im][PF6] [24]. Here it was
claimed that the active species was (mim)2PdCl2 generated
in situ from a reaction of the [C4C1im]2[PdCl4] with the
ionic liquids. This led us to test the possibility that a similar
chain of events was occurring in our Suzuki reactions, lead-
ing to palladium/imidazole complexes that act as catalysts,
or their immediate precursors, in these ionic liquids.
ESI mass spectra of a solution of [C4C1im]2[PdCl4]
in dry [C4C1im][BF4] revealed peaks attributable only
to the [C4C1im]2[PdCl4] {[(C4C1im)2PdCl3]+, 491 Da;
[(C4C1im)3PdCl4]+, 667 Da; [(C4C1im)4PdCl5]+, 842 Da}
Table 2
Investigation of reaction conditions (CH3CN)2PdCl2/4mim catalysed
Suzuki reactions
b
Time
Reaction
temperature (◦C)
Yielda (%)
TOF (h−1
)
Homoc (%)
20 min
1 h
3 h
110
110
110
25
42.4
83.3
86.1
78.7
106
69
24
3
3.3
2.0
3.8
2.9
24 h
Conditions: 1.2 mol% catalyst, L:catalyst = 4:1.
1
a
Note on nomenclature. The substituted imidazolium cations used in
Isolated yields of 4-methylbiphenyl product, based on bromobenzene.
this paper are abbreviated to [CnCꢀnim]+, where n and nꢀ are the chain
lengths of the respective alkyl chains. For example, [C4C1im]+ is the
1-butyl-3-methylimidazolium cation.
b
TOF (h−1) = number of moles of desired product per mole of
catalyst used per hour.
c
Homo-coupled 4,4ꢀ-dimethylbiphenyl product.

C.J. Mathews et al. / Journal of Molecular Catalysis A: Chemical 214 (2004) 27–32
29
Table 3
formed in solution with ESI-MS. When (CH3CN)2PdCl2
and 1-methylimidazole were added to the [C4C1im][BF4]
and heated at 110 ◦C for 1 h under N2 the ESI-MS revealed
signals at 387 {[(mim)3PdCl]+}, 371 {unidentified Pd com-
plex}, 346 {unidentified Pd complex}, 270 {[(mim)2Pd]+}
and 83 {[Hmim]+} m/z. Upon addition of aqueous sodium
carbonate and reheating of the solution a new signal
at 443 {[(mim)2Pd(C4C1imy)Cl]+} (where C4C1imy
= 1-butyl-3-methylimidazolylidene) appeared. This pro-
vides evidence that mixed imidazolylidene/1-methylimida-
zole palladium complexes could be generated in situ under
conditions similar to those used in catalytic reactions. How-
ever, it should be stressed that, unlike the phosphine system,
other palladium containing signals are present, some of
which are unidentified, that may result from other species
that could be catalysing the reaction. Attempts to indepen-
dently prepare the [(mim)2Pd(C4C1imy)Cl]+ complex were
unsuccessful.
The reaction of bromobenzene and tolylboronic acid in a variety of
solvents
b
Solvent
Yielda (%)
TOF (h−1
)
Homoc (%)
[C4C1im][BF4] run 1
[C4C1im][BF4] run 2
[C4C1im][BF4] run 3
[C4C1im][BF4] run 4
[C4C1im][BF4] run 5
42.4
45.4
43.2
40.7
42.9
106
114
108
102
107
3.3
3.8
2.5
3.2
2.7
Dioxane [21]
95.7d
89.8d
36.8d
92.8d
239
225
92
0
0
0
0
Toluene [21]
THF (60 ◦C) [21]
Water (100 ◦C) [21]
232
Conditions: 1.2 mol% catalyst, L:catalyst = 4:1, temperature = 110 ◦C,
time = 20 min.
a
Isolated yields of 4-methylbiphenyl product, based on bromobenzene.
b
TOF (h−1) = number of moles of desired product per mole of
catalyst used per hour.
c
Homo-coupled tolylboronic acid yield.
Yellow–orange solution afforded from the initiation process. Exten-
d
sive catalyst decomposition and leaching.
2.3. Variation of ionic liquid-cation effect
2.1. Comparison with molecular solvents
If the formation of palladium/imidazolylidene complexes
is important in the catalysis, it would be expected that there
would be a significant effect of changing the cation of the
ionic liquid on the reaction. Hence, a study of the effect
of different ionic liquids on the Suzuki reaction of bro-
mobenzene with tolylboronic acid was performed with the
(CH3CN)2PdCl2/4mim system (Table 4).
The first thing to notice is that the two ionic liq-
uids for which imidazolylidene formation is not possible
{[C4C1C1im][BF4] (where bmmim = 1-butyl-2,3-dimethy-
limidazolium) and [C4C1py][N(SO2CF3)2] (where bmpy
= 1-butyl-1-methylpyrolidimum)} give the highest yield-
ing reactions and extensive catalyst decomposition. These
ionic liquids generated a yellow–orange solution after the
initiation of the catalyst, rather than the colourless solution
formed by the lower yielding, but stable catalyst solutions.
We have reported the reaction of bromobenzene and tolyl-
boronic acid in molecular solvents previously (Table 3) [21].
It can be seen immediately that the reaction yields and
turnover numbers are greater in most of the molecular sol-
vents used. The only exception to this is THF, which due to
its boiling point operates at a significantly lower temperature
with an associated reduction in rate. It is also noticeable that
the reaction in molecular solvents leads to no homo-coupled
product being formed. However, in all of these solvents the
catalyst was seen to decompose and after extraction it was
not possible to repeat the reaction.
Only in the ionic liquid was it possible to recycle and reuse
the reaction system. The reaction in [C4C1im][BF4] was,
therefore, repeated. After washing with hexane to remove
the products the solution was further washed with aliquots
of water until a single phase was formed on addition of
the aliquot. The ionic liquid was then dried and fresh base
and starting materials added. It can be seen (Table 3) that
the ionic liquid provided a stable reaction system for the
reaction, with no significant loss in activity.
Table 4
Scope of the Suzuki reaction with different ionic liquid-cation effects
b
Solvent
Yielda (%)
TOF (h−1
)
Homoc (%)
Clearly there are two regimes, the first is highly active,
but unstable, the second is less active, but very stable.
We have previously invoked the formation of palladium/
imidazolylidene complexes in [C4C1im]+ ionic liquids to
explain the reactivity of palladium/phosphine mixtures in
ionic liquids [6]. It is possible that analogous mixed imi-
dazolylidene/imidazole catalysts are being formed in this
reaction.
[C2C1im][BF4]
[C4C1im][BF4]
[C6C1im][BF4]
[C4C4im][BF4]
18.8
42.4
32.4
55.5
47
100
81
1.7
2.7
2.0
3.2
139
[C4C1C1im][BF4]
[C4C1py][N(SO2CF3)2]
85.3d
83.4d
213
209
1.4
4.1
Conditions: 1.2 mol% catalyst, L:catalyst = 4:1, temperature = 110 ◦C,
time = 20 min.
a
Isolated yields of 4-methylbiphenyl product, based on bromobenzene.
b
TOF (h−1) = number of moles of desired product per mole of
2.2. The ESI-MS catalytic investigation in [C4C1im][BF4]
catalyst used per hour.
c
Homo-coupled tolylboronic acid yield.
Yellow–orange solution afforded from the initiation process. Exten-
d
In order to confirm the reason for the stability of the cat-
alyst system in the ionic liquids, we investigated the species
sive catalyst decomposition and leaching.

30
C.J. Mathews et al. / Journal of Molecular Catalysis A: Chemical 214 (2004) 27–32
Table 6
Analogy with the previously reported palladium/phosphine
systems [6] in ionic liquids suggests that the catalyst sta-
bility in the ionic liquids is generated by the ability of
the imidazolium ions to form imidazolylidene ligands with
the palladium. However, unlike the phosphine systems, the
ionic liquids in which palladium/imidazolylidene complexes
cannot be formed give more reactive catalysts, with reac-
tivities similar to that found in molecular solvents. There is
no obvious trend in the reactivities in the ionic liquids that
can form imidazolylidene ligands.
Suzuki reactions with different imidazoles in [C4C1im][BF4]
Yielda(%) TOF (h−1
)
Homoc (%)
b
Ligand
Imidazole
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2-Methylimidazole
2-Phenylimidazole
Benzimididazole
2-Methylbenzimidazole
1-Methylimidazole
1-Butylimidazole
1-Phenylimidazole
1-Methylbenzimidazole
42.4
40.2
76.0
88.2
100
101
190
221
2.7
3.6
2.5
3.7
2.4. Variation of ionic liquid-anion effect
1,2-Dimethylimidazole
1-Butyl-2-methylimidazole
1,2,4,5-Tetramethylimidazole 29.2
1-Methyl-2-phenylimidazole
1-Butyl-2-phenylimidazole
31.2
33.9
78
85
73
81
72
1.5
3.0
2.2
3.2
2.2
The effect of the anion in 1-butyl-3-methyl imidazolium
ionic liquids is less clear than that of the cation. Stable
catalyst systems were formed in all of these ionic liquids.
The highest yields were achieved in the [C4C1im][OSO2CF3]
and [C4C1im][PF6], (Table 5). [C4C1im][N(SO2CF3)2] and
[C4C1im][BF4] gave lesser yields with [C4C1im][OSO2CH3]
and [C4C1im]Cl giving no yield at all.
It has been shown that the basicity of the anions of the
ionic liquid can affect the electrophilicity of transition metal
centres [25]. The greater the basicity the stronger is the
interaction with the metal centre and the slower the reaction.
However, although this may be contributing to the observed
reactivities here it cannot be the complete explanation. While
it offers an explanation for the other ionic liquids, under
this argument the reactivities of the [N(SO2CF3)2]− and the
[OSO2CF3]− liquids would be expected to be reversed. We
are continuing to investigate this phenomenon.
32.4
28.9
Conditions: 1.2 mol% catalyst, L:catalyst = 4:1, temperature = 110 ◦C,
time = 20 min.
a
Isolated yields of 4-methylbiphenyl product, based on bromobenzene.
b
TOF (h−1) = number of moles of desired product per mole of
catalyst used per hour.
c
Homo-coupled tolylboronic acid yield.
(0–80%). Assuming other imidazoles exhibited similar ac-
tivities, these conditions allowed the widest window to ob-
serve any improvements or deteriorations. Furthermore, it
suggested some information about the reaction rates, as es-
timated by TOF (h−1), for the different catalysts due to the
presence of unreacted bromobenzene and absence of cata-
lyst decomposition at the end of the reaction period.
Imidazoles incorporating an N–H bond gave no Suzuki
cross-coupled product and unreacted bromobenzene was de-
tected (Table 6). The ‘ligandless’ {(CH3CN)2PdCl2} cat-
alytic system gave a 25.4% 4-methlybiphenyl yield and,
therefore, these imidazoles must be actively inhibiting the
Suzuki reaction.
2.5. Variation of imidazoles and other N-donor ligands
The Suzuki reaction of bromobenzene with tolylboronic
acid, in a 1:1 ratio, was investigated with (CH3CN)2PdCl2
and a small number of other potential N-donor ligands at
110 ◦C for 20 min (Tables 6 and 7). Under these condi-
tions a 42.4% 4-methylbiphenyl yield was obtained, when
1-methylimidazole was used as a ligand, which was ap-
proximately in the middle of the affordable yield range
Table 7
Scope of Suzuki reactions with other potential N-coordinated ligand
b
Ligand
Yielda (%) TOF (h−1
)
Homoc (%)
Table 5
Pyrazole
Tetrazole
0
0
0
0
0
0
Scope of the Suzuki reaction with different ionic liquid-anion effect
DBU
4-Methylthiazole
Pyridine
15.2
81.2d
75.4d
79.0d
92.3d
38
1.1
2.0
3.4
2.6
3.1
b
Solvent
Yielda (%)
TOF (h−1
)
Homoc (%)
203
189
200
231
[C4C1im][BF4]
[C4C1im][PF6]
[C4C1im][N(SO2CF3)2]
[C4C1im][OSO2CF3]
[C4C1im]Cl
42.4
59.5
36.4
62.4
0
100
150
90
156
0
2.7
2.3
3.3
1.8
0
Et3N
1,4-Bis-(diisopropylphenyl)-
1,3-diazabutadiene
2,2ꢀ-Bipyridyl
68.2d
171
1.9
[C4C1im]Br
[C4C1im][OSO2CH3]
24.9
0
63
0
1.3
0
Conditions: 1.2 mol% catalyst, L:catalyst = 4:1, temperature = 110 ◦C,
time = 20 min.
a
Conditions: 1.2 mol% catalyst, L:catalyst = 4:1, temperature = 110 ◦C,
time = 20 min.
Isolated yields of 4-methylbiphenyl product, based on bromobenzene.
b
TOF (h−1) = number of moles of desired product per mole of
a
Isolated yields of 4-methylbiphenyl product, based on bromobenzene.
catalyst used per hour.
b
c
TOF (h−1) = number of moles of desired product per mole of
Homo-coupled tolylboronic acid yield.
Yellow–orange solution afforded from the initiation process. Exten-
d
catalyst used per hour.
c
Homo-coupled tolylboronic acid yield.
sive catalyst decomposition observed during reaction.

C.J. Mathews et al. / Journal of Molecular Catalysis A: Chemical 214 (2004) 27–32
31
Different N-alkyl substituents had no effect on the yield,
4. Experimental
for example, 1-butylimidazole gave a comparable yield to
1-methylimidazole (Table 6). However, 1-phenylimidazole
gave a yield of 76.0% under these conditions and
1-methylbenzimidazole was found to be the most active
imidazole based systems tested (88.2% yield).
C-2 substitution of the imidazoles led to a reduction
in the yield in comparison to the parent imidazole, irre-
spective of the nature of the C-2 substituent. For example,
1-butyl-2-phenylimidazole afforded a comparable yield to
1,2-dimethylimidazole.
Other N-donor ligands were also investigated (Table 7).
The inhibiting nature of N–H bonds were also observed
for pyrazole and tetrazole ligands (Table 7). Other N-coor-
dinating ligands tested were shown to be highly active, al-
beit with extensive decomposition. This would suggest that
the nature of the N-donor ligand is important in determining
whether a palladium/imidazolylidene complex can form. Di-
azabicycloundecene (DBU) was found to produce a 15.2%
yield of 4-methylbiphenyl with no decomposition.
The N-bidentate ligands, 1,4-bis-(diisopropylphenyl)-1,3-
diazabutadiene and 2,2ꢀ-bipyridyl, investigated by Nolan
[15] were also examined in [C4C1im][BF4] (Table 7). These
catalytic systems performed significantly better in the ionic
liquid than in dioxane, in which 80 and 13% yields were
reported for the diazabutadiene and bipyridyl ligands, re-
spectively. However, catalyst decomposition was clearly
observed.
4.1. General
Unless otherwise stated, all materials were used
as-obtained without further purification. Acetonitrile,
CH2Cl2, dioxane and ethyl acetate were distilled from
CaH2, The ionic liquids were prepared using methods that
we have previously described [26]. The imidazoles were
freshly distilled from KOH under reduced pressure, as
were 3-methylthiazole, DBU, pyridine and triethylamine.
Pyrazole and tetrazole were recrystallised from CHCl3 and
hexane. NMR spectra were recorded on a JEOL GSX-270
FT spectrometer. ESI-MS were performed on a Waters
HPLC 600 with a micromass ZMD (capillary 3.64 kV;
cone = 31 V; extractor = 8 V; source temperature 100 ◦C
and desolvation temperature = 120 ◦C). GC analysis were
performed with a Hitachi 163 gas chromatograph using a
2 m × 3 mm carbowax column.
4.2. Preparation of bis-(acetonitrile)dichloropalladium(II),
(CH3CN)2PdCl2
A suspension of PdCl2 (2.00 g, 11.3 mmol) was heated
at reflux in CH3CN (100 cm3) with vigorous stirring for
18 h under N2. Hot filtration of the resultant wine-red
coloured solution through a celite pad into stirred petroleum
spirit (40–60 ◦C; 100 cm3) at ambient temperature gave
a yellow–orange solid. Recrystallisation from a CH3CN
(200 cm3), CH2Cl2 (300 cm3) and hexane (100 cm3) mix-
ture gave (CH3CN)2PdCl2 (1.78 g, 65.3%) as a bright
yellow powdery solid; m.p. 129–131 ◦C (decomp); Found:
C, 18.63; H, 2.27; N, 10.77; C4H6N2Cl2Pd requires C,
18.55; H, 2.33; N, 10.82%; νmax(KBr)/cm−1 3285, 2979,
2918, 2338, 1412, 1329, 1022, 959, 438 and 407; δH(270
MHz, d6-DMSO)/ppm 2.06 (6H, s, 2CH3); δC(68 MHz,
d6-DMSO)/ppm 118.0 (s, CH3CN) and 1.09 (s, CH3CN).
3. Conclusions
A number of substituted imidazoles, when used with pal-
ladium(II) sources in ionic liquids, provide a robust air-stable
system for Suzuki reactions. A successful Suzuki reaction
required the initiation of the catalytic system in the ionic
liquid prior to the reaction. This was achieved by heating
the palladium source with the imidazole in the ionic liquid
to generate a completely colourless catalytic solution. With-
out the initiation low yields were obtained with catalytic de-
composition. The catalytic activity and stability in the ionic
liquids was strongly dependent on the nature of the imida-
zole ligands and on the natures of the anion and cation of
the ionic liquid. The (CH3CN)2PdCl2/4mim catalytic system
was found to be highly active in [C4C1py][N(SO2CF3)2],
[C4C1C1im][BF4] and molecular solvents, although exten-
sive catalyst decomposition occurred.
The reactivities of the different solutions that were sta-
ble depend on both the cation and the anion of the ionic
liquid, the most active of those tested being [C4C1im][PF6]
and [C4C1im][OSO2CF3]. It has also been shown that it is
possible to use the imidazole ligand to improve reactivities,
with 1-phenylimidazole and 1-methylbenzimidazole giving
the most active catalysts in [C4C1im][BF4]. Hence there is
a great deal if synthetic flexibility that can be explored in to
improve upon and develop this new catalyst system.
4.3. A typical Suzuki reaction in ionic liquid
To a flask charged with [C4C1im][BF4] (1.6 cm3) was
injected (CH3CN)2PdCl2 (2.36 mg, 9.12 ␮mol, 1.2 mol%)
and 1-methylimidazole (3.00 mg, 36.5 ␮mol, 4.8 mol%) The
yellow solution was stirred at 110 ◦C for 1 h under N2,
during which time the colour disappeared. The flask was
then removed from the heat and allowed to cool to ambient
temperature. To the colourless solution Na2CO3 (161 mg,
1.52 mmol, 2 equivalents) in deoxygenated water (0.80 ml),
bromobenzene (119.3 mg (80.00 ␮l), 0.7597 mmol, 1 equiv-
alent) and the tolylboronic acid (114 mg, 0.837 mmol, 1.1
equivalent) was added. The solution was then heated to
110 ◦C for 20 min. At that point the flask was rapidly cooled
in a dry-ice/acetone bath (−78 ◦C), the reaction mixture
was diluted with water (2 ml) and extracted with hexane
(4×15 cm3). The extracts were washed with brine (3×40 ml)
and water (3 × 40 ml), filtered through a pad of silica and

32
C.J. Mathews et al. / Journal of Molecular Catalysis A: Chemical 214 (2004) 27–32
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(d) T. Fukuyama, M. Shinmen, S. Nishitani, M. Sato, I. Ryu, Org.
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evaporated to dryness to afford 4-methylbiphenyl (51.0 mg,
42.4%) as a colourless crystalline solid.
(e) R. Rajagopal, D.V. Jarikote, K.V. Srinivasan, Chem. Commun.
(2002) 616;
Acknowledgements
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We would like to thank the EPSRC and Syngenta for the
provision of a CASE award (PJS). We would also like to
thank the editors of this special edition for the invitation to
submit this paper.
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