Palladium(II) complexes containing a bulky pyridinyl N-heterocyclic carbene ligand: Preparation and reactivity

Article abstract of DOI:10.1016/j.jorganchem.2007.06.007

Coordination chemistry of a pyridine imidazole-2-ylidene ligand (pyN ?C) with sterically hindered substituents toward palladium(II) metal ions has been investigated. The palladium carbene complex [(C-pyN ?C)Pd(η³-allyl)Cl] (3) is prepared via the transmetallation from the corresponding silver carbene complexes with [ClPd(η³-allyl)]₂. Upon the abstraction of chloride, coordination of pyridinyl-nitrogen becomes feasible to form [C,N-(pyN ?C)Pd(η³-allyl)](BF₄) (4). Ligand substitution reaction of 4 with triphenylphosphine results in the formation of [(C-pyN ?C)Pd(PPh₃)(η³-allyl)](BF₄)], which the pyridinyl-nitrogen donor is substituted by the phosphine. This palladium complex appears to be base sensitive. Treatment of 4 with t-butoxide causes the decomposition to yield the metal nano-particles. Furthermore, de-complexation of 4 takes place under hydrogen atmosphere to generate the carbene precursor, 1-(6-mesityl-2-picolyl)-3-mesitylimidazolium salt. Nevertheless, the palladium complex 4 shows good catalytic activity on the Suzuki-Miyaura and Mizoroki-Heck reactions.

Full text of DOI:10.1016/j.jorganchem.2007.06.007

Journal of Organometallic Chemistry 692 (2007) 3976–3983
www.elsevier.com/locate/jorganchem
Palladium(II) complexes containing a bulky pyridinyl N-heterocyclic
carbene ligand: Preparation and reactivity
*
Chao-Yu Wang, Yi-Hong Liu, Shei-Ming Peng, Jwu-Ting Chen, Shiuh-Tzung Liu
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, ROC
Received 11 April 2007; received in revised form 27 May 2007; accepted 5 June 2007
Available online 15 June 2007
Abstract
Coordination chemistry of a pyridine imidazole-2-ylidene ligand (pyN^C) with sterically hindered substituents toward palladium(II)
metal ions has been investigated. The palladium carbene complex [(C-pyN^C)Pd(g3-allyl)Cl] (3) is prepared via the transmetallation
from the corresponding silver carbene complexes with [ClPd(g3-allyl)]2. Upon the abstraction of chloride, coordination of pyridinyl-
nitrogen becomes feasible to form [C,N-(pyN^C)Pd(g3-allyl)](BF4) (4). Ligand substitution reaction of 4 with triphenylphosphine results
in the formation of [(C-pyN^C)Pd(PPh3)(g3-allyl)](BF4)], which the pyridinyl-nitrogen donor is substituted by the phosphine. This pal-
ladium complex appears to be base sensitive. Treatment of 4 with t-butoxide causes the decomposition to yield the metal nano-particles.
Furthermore, de-complexation of 4 takes place under hydrogen atmosphere to generate the carbene precursor, 1-(6-mesityl-2-picolyl)-3-
mesitylimidazolium salt. Nevertheless, the palladium complex 4 shows good catalytic activity on the Suzuki–Miyaura and Mizoroki–
Heck reactions.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Carbene; Palladium; Catalysis; Nano-particle; Coupling reaction
1. Introduction
we pursue to investigate the coordination of pyN^C toward
palladium ions and their activities.
The use of diaminocarbenes (NHC) as ligands on stabil-
ization of transition metal ions for catalysis hasbeenreceived
much attention [1–11]. These carbene donors are generally
considered as an important class of donors with strong basi-
city and good r-donating properties. Thus, transition metal
complexes containing such carbene moieties are thermally
more stable than those with phosphine species [2e].
Regarding the chelating carbene ligands, quite a few het-
ero-functionalized diaminocarbene ligands are known [6–
10]. In our previous works, we also reported the study of
a bidentate ligand with sterically hindered groups on both
pyridine and imidazole rings pyN^C[11]. The pyridinyl-
nitrogen donor of [(C,N-pyN^C)Rh(COD)]BF4 was found
to be labile and could be replaced by various donors such
as phosphine, azide and halides. Continuing this trend,
Mes
N
N
:C
N
Mes
Mes = 2, 4, 6-trimethylphenyl
pyN^C
2. Results and discussion
2.1. Synthesis of p-allylpalladium carbene complexes
The synthetic approach leading to the desired palladium
complex 4 is shown in Scheme 1. Conversion of the pyrid-
inyl-imidazolium salt 1 into the silver carbene complex
*
Corresponding author. Tel.: +886 2 2366 0352; fax: +886 2 2363 6359.
E-mail address: stliu@ntu.edu.tw (S.-T. Liu).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.06.007

C.-Y. Wang et al. / Journal of Organometallic Chemistry 692 (2007) 3976–3983
3977
Mes
Mes
Ag2O
N
N
N
N
[PdCl(π-allyl)]2
Ag..AgI2
AgBF4
Mes
N
Cl
N
BF4
Mes
N
N
Pd
N
Pd
Br
N
Mes
N
N
Mes
2
Mes
Mes
Mes = 2, 4, 6-trimethylphenyl
1
2
3
4
Scheme 1. Preparation of the palladium carbene complex.
[C-(pyN^C)2AgAgI2] (2) was achieved via the method pre-
viously reported [11]. Treatment of 2 with [PdCl(p-allyl)]2
caused the carbene moiety transfer from Ag to Pd to yield
the palladium carbene complex 3. Abstraction of chloride
from complex 3 via the addition of silver ion readily
assisted the coordination of pyridinyl-nitrogen to the
palladium center, resulting in the formation of [C,N-
(pyN^C)Pd(p-allyl)]BF4 (4).
The structures of palladium complexes were deter-
mined by both spectroscopic and elemental analyses and
the complex 4 was further confirmed by its single crystal
structural determination. 13C NMR data for the coordi-
nating carbene carbons appear at d 179.7 for 3 and
176.3 for 4, respectively, suggesting the formation of the
palladium carbon bond. These signals are all in the typi-
cal range for Pd–C(carbene) observed for the analogues
[12]. The 1H NMR shifts corresponding to the proton
of pyridine ring on 3 are similar to those of the silver
complexes (Table 1), indicating that the pyridinyl-nitro-
gen donor remains un-coordinated, but not for 4. A sig-
nificant coordination chemical shifts appear for those
hydrogen atoms of the pyridine ring on 4, suggesting
the chelation of pyN^C toward the palladium center.
The detail coordination sphere around the metal center
of 4 was confirmed by the X-ray crystal structural
analysis.
Fig. 1. ORTEP plot of cationic portion of 4 (30% probability ellipsoids).
and Table 2). Notable feature of this structure is the angle
of Pd(1)–C(4)–N(1) [136.1(3)ꢁ] away from 120 ꢁ, presum-
ably due to the constrain raised from the chelation of
PyN^C. The bisect angle of imidazole ring and rhodium
coordination plane is 33.5ꢁ, revealing the non-coplanar
nature. Due to the trans influence, bond length of Pd(1)–
˚
C(1) [2.222(5) A] is slightly longer than that of Pd(1)–
˚
˚
C(3) [2.095(4) A] by about 0.12 A.
Comparison of structural variation among a series of
[(p-allyl)Pd(NHC)] complexes is summarized in Table 2.
The general trends on bond lengths and bond angles of 4
are quite similar to those of known species as the listed
(Scheme 2) except the angle of X–Pd–C(carbene). Due to
the chelation effect, the bite angle of 4 [N(3)–Pd(1)–C(4)
88.8(1)ꢁ] is smaller than 90 ꢁ.
Fig. 1 displays the ORTEP plot of the cationic portion
of 4. Considering the allyl being a bidentate, the metal cen-
ter in 4 displays a square planar geometry. Bond length of
˚
Pd–C(carbene) [2.052(4) A] is in the normal range as com-
pared to those of the related palladium species (Scheme 2
Table 1
Selected spectral data for metal carbene complexes
Hb
N
Ha
Hc
Hd
He
Mes
Complex
1H NMR
Ha
13C NMR
Hb
Hc
Hd and He
C(carbene)
2a
3
7.45 (d, J = 8.0)
7.90 (d, J = 7.6)
8.20 (dd, J = 7.6, 1.2)
7.74 (dd, J = 8.0, 8.0)
7.79 (dd, J = 7.6, 7.6)
8.03 (dd, J = 7.6, 7.6)
7.16 (d, J= 8.0)
7.18 (d, J = 7.6)
7.34 (dd, J = 7.6, 1.2)
5.51
183.5
179.7
176.3
5.91 (d, J = 14.8) 5.75 (d, J = 14.8)
5.81 (d, J = 14.4) 5.60 (br)
4
a
Ref. [11].

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C.-Y. Wang et al. / Journal of Organometallic Chemistry 692 (2007) 3976–3983
i-Pr
BF4
Ph
Mes
N
N
N
i-Pr
MeCN
O
Cl
N
N
N
Pd
i-Pr
Pd
Pd
i-Pr
Mes
Mes
I
II
III
Scheme 2. Representative [(p-allyl)Pd(NHC)] species.
Table 2
Fig. 2. 31P NMR spectra of 5a and 5b under various temperature.
˚
Comparison of selected bond distances (A) and bond angles (ꢁ) of
palladium complexes
Complex
4 (X@N)
I(X@N)a
II (X@O)b
III (X@Cl)c
Table 3. The free energy of this interconversion is derived
to be 1.4 0.2 kJ/mol, showing a very small energy differ-
ence between two species.
Pd–C(carbene)
2.052(4)
2.066(2)
2.022(4)
2.029
Pd–X
Pd-allyl
2.193(3)
2.222(5)
2.152(4)
2.095(4)
2.086(2)
2.199(3)
2.150(2)
2.081(2)
2.062(3)
2.204(4)
2.133(5)
2.113(4)
2.380
2.204
2.131
2.113
Complex 5 is fairly stable under nitrogen atmosphere
without any change for a week, but it slowly decomposed
in the presence of air. By monitoring the sample in a CDCl3
solution, 5 was slowly transferred into 4 accompanied with
the formation of triphenylphosphine oxide (Eq. (1)). After
stirring for 72 h, this solution of 5 was completely con-
verted into 4. Apparently, the dissociated phosphine is
readily oxidized by the oxygen to yield its oxide, and the
dissociation of phosphine allows the pyN^C ligand to
regain the chelation.
X–Pd–C(carbene)
88.8(1)
100.05(7)
91.70(14)
95.55
a
Ref. [13].
Ref. [14].
Ref. [15].
b
c
2.2. Reactivity of p-allylpalladium carbene complexes
air
2.2.1. Triphenylphosphine
5a þ 5b ! 4 þ O@PPh3
ð1Þ
Treatment of 4 with triphenylphosphine in dichlorometh-
ane at ambient temperature resulted in the de-coordination
of pyridinyl-nitrogen to yield the phosphine-substituted
complex (Scheme 3), indicating that the chelation does not
enhance its donating ability. This phosphine complex exists
in two isomers, 5a and 5b, due to the stereo-orientation of
p-allyl group relative to the un-symmetrical carbene ligand.
2.2.2. Alkoxide
It has been studied by Nolan and coworkers that reac-
tion of a carbene allylpalladium complex with t-butoxide
in the presence of phosphine would yield a divalent palla-
dium species (Eq. (2)) [15]. It is clear that the nucleophilic
attack of alkoxide toward coordinating p-allyl ligand
resulted in the formation Pd(0) species.
Both 5a and 5b show significant differences in H and 31P
NMR shifts. However, we were not able to isolate either
one in pure form due to the rapid interconversion of each
other.
The ratio of 5a to 5b is temperature dependent and was
estimated by the 31P NMR peak integration (Fig. 2). It
appears that 5b is slightly more stable than 5a presumably
due to the steric repulsion between the p-allyl group and
the mesityl substituent of carbene moiety. The equilibrium
constants at various temperatures are summarized in
1
R
N
R
N
R
OK
N
PCy3
PCy3
C
Pd
Cl
C
Pd
C
Pd
N
R
N
R
N
R
O
V
IV
ð2Þ
Under the similar conditions, complex 3 reacted with
t-butoxide in the presence of tricyclohexylphosphine in
benzene to give a solution of palladium nano-particles,
instead of the production of palladium carbene phosphine
complexes. In this reaction, we did not observe any
formation of t-butyl allyl ether. 31P NMR spectrum of
this solution appears one signal at d 47.2, which is in
Mes
Mes
N
N
PP
3
PPh3
Pd
PPh3
Pd
4
Keq
N
N
N
N
BF4
BF4
Table 3
Equilibrium constants between 5a and 5ba
T (K)
Keq.
a
213
2.19
233
2.07
253
1.97
273
1.82
293
1.73
5a
5b
In D2Cl2.
Scheme 3.

C.-Y. Wang et al. / Journal of Organometallic Chemistry 692 (2007) 3976–3983
3979
agreement with the coordinating tricyclophosphine on the
2.3. Catalysis
palladium nano-particles [16]. With the observation of
TEM (Fig. 3a), the sizes of particles are ranging from 2
to 10 nm, indicating these particles not uniformly formed.
From EDX analysis showed that phosphorus, potassium
and chlorine elements are presented on the all area of
the sample. This palladium nano-particle solution
appeared to be stable under nitrogen atmosphere for
weeks. Apparently, the resulting nano-particles are stabi-
lized by the phosphines. Similarly, complex 4 also decom-
posed in the presence of t-butoxide to form the palladium
nano-particles with the size in 2–4 nm (Fig. 3b).
2.3.1. Suzuki–Miyaura coupling
The palladium carbene complex 4 was subjected to evalu-
ate its catalytic activity on the coupling reaction of phenylbo-
ronic acid with aryl halides. In a typical experiment for the
reaction, aryl halide, phenylboronic acid and base in a ratio
of 1:1.1:2 were placed in the flask, followed by the addition of
the solvent and the catalyst. The mixture was heated to
60 ꢁC for a certain period. The organic product was isolated
by extraction and then analyzed by both GC and 1H NMR
spectroscopy. Results are summarized in Table 4.
Apparently, complexes 3 and 4 behave quite different
from those related complexes such as IV. Unlike the forma-
tion of the carbene phosphine complex V (Eq. (2)), reaction
of 3 or 4 with t-butoxide readily caused the decomposition
to yield the palladium particles. This is presumably due to
the presence of benzylic protons in this structure. It has
been demonstrated by Cavell’s and other groups that
deprotonation of the imidazolium salt 1 with a strong base
did not produce the corresponding carbene due to the
interference of the deprotonation of benzylic methylene
protons [6].
The initial screen on solvents and various bases were
performed using 4 as the catalyst precursor (Table 4,
entries 1–6). It appeared that a mixed solvent of water/tol-
uene gave better results than any other organic solvents. In
case of bases, all carbonate salts and lithium hydroxide
provided excellent results. It is noticed that the reaction
needs to carry out under atmosphere of nitrogen. In other
words, the catalyst is quite sensitive to air.
We then found that the addition of triphenylphosphine
to the catalytic system readily increased the reaction yield
up to 90% (Table 4, entry 7). The phosphine presumably
underwent the ligand substitution toward metal center as
illustrated in the previous section. Indeed, the use of 5 as
the catalyst for this coupling does provide the same result.
The beneficial effect of phosphine on the catalysis was also
applicable to reduce the amount of catalysts. With the
molar ratio of [substrate]/[Pd] = 2000, the addition of
phosphine can improve the yield of the coupling product
(Table 4, entry 8).
Under the optimized reaction conditions, the catalytic
activities of the palladium complexes toward aryl bromide
appear to be excellent, but poor for a steric hindered aryl
bromide (entry 12). Similar to other catalytic systems, this
palladium complex shows less active toward aryl chlorides,
with less yields and longer reaction times (entries 16 and
17). Nevertheless, the palladium complexes prepared in this
work behave good catalytic activity on the Suzuki–Miya-
ura coupling reaction.
2.2.3. Hydrogen
Under atmospheric pressure of hydrogen gas, both 3
and 4 are stable at room temperature, without any
decomposition for 48 h. However, complex 4 slowly
decomposed at 50 ꢁC to yield the corresponding imidazo-
lium salt and palladium black (Eq. (3)). This is presum-
ably due to the oxidative addition of hydrogen to the
metal ion followed by the reductive elimination of pro-
pene and ligand. In fact, the addition of HBF4 to a solu-
tion of 3 or 4 in CDCl3 also caused the decomposition to
yield the imidazolium salt.
H2 50 o
C
N
Mes
N
N
+ Palladium black
N
BF4
N
Mes
Pd
or HBF4
BF4
N
Mes
Mes
2.3.2. Mizoroki–Heck coupling
ð3Þ
Palladium carbene complexes have been shown to be
effectively active on the catalysis in a wide range of cou-
pling reactions. On the basis of the palladium catalyzed
Suzuki–Miyaura coupling described above, we found it
appropriate to test them in Mizoroki–Heck reaction. This
coupling reaction is an efficient way to prepare styrene
derivatives, which are important chemicals for many appli-
cations. On the screen of solvents and bases, we established
that NMP and diisopropylamine are the best combination
for this coupling reaction with the use of 4 as the catalyst
(Table 5). Similar to the finding in the Suzuki–Miyaura
reaction, the addition of phosphine undoubtedly increases
the activity of catalyst. As shown from the data in Table
5, most aryl bromides as substrates give excellent yields
except the indole derivative.
Fig. 3. TEM photography of Pd nano-particles (a) from 3 and (b) from 4.

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C.-Y. Wang et al. / Journal of Organometallic Chemistry 692 (2007) 3976–3983
Table 4
Coupling of arylhalides with arylboronic acid catalyzed by complex 4a
Entry
Ar–X
ArB(OH)2
Catalyst (mol%)
Base
Solvent
Yieldb
TOFc
1
2
3
4
5
4-BrC6H4CHO
4-BrC6H4CHO
4-BrC6H4CHO
4-BrC6H4CHO
4-BrC6H4CHO
4-BrC6H4CHO
4-BrC6H4CHO
4-BrC6H4CHO
4-BrC6H4NMe2
4-BrC6H4OH
2-BrC6H4CHO
2-BrC6H4OMe
2-Bromopyridine
4-BrC6H4CHO
2-BrC6H4CHO
4-ClC6H4CHO
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
2,4,6-Me3C6H2B(OH)2
2,4,6-Me3C6H2B(OH)2
C6H5B(OH)2
C6H5B(OH)2
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.05
0.05
0.05
0.5
0.5
0.5
0.05
0.5
0.5
0.5
Na2CO3 (0.38 mol)
Na2CO3 (0.38 mol)
Na2CO3 (0.38 mol)
K2CO3 (0.46 mol)
LiOH (1 mmol)
NaOAc (0.46 mmol)
Na2CO3 (0.38 mol)
LiOH (1 mmol)
LiOH (1 mmol)
LiOH (2 mmol)
LiOH (1 mmol)
LiOH (1 mmol)
LiOH (1 mmol)
LiOH (1 mmol)
LiOH (1 mmol)
LiOH (1 mmol)
LiOH (1 mmol)
DMF
IPA
0
0
79
88
99
0
0
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
Toluene/H2O
26
29
33
14
30
333
283
333
333
6
6
41
90
7d
8d
>99
85
>99
>99
18
20
40
51
77
9d
10d
11d
12d
13d
14d
15d
16d,e
17d,e
7
133
17
6
58
5
O2N
N
Cl
a
Ar–X: 0.45 mmol; ArB(OH)2 0.45 mmol at 90 ꢁC for 6h.
GC yield.
TOF: turnover frequency: mol product/mol of catalyst h.
Additive: PPh3 (4.5 · 10ꢀ3 mmol).
Reaction time: 24 h.
b
c
d
e
2.3.3. Mechanistic pathway of catalysis
Concerning the reaction pathway, we studied the course
of the coupling reaction by monitoring the reaction mix-
cles might be responsible for this catalysis. It has been dem-
onstrated that palladium nano-particles can catalyze the
C–C bond coupling reaction [17]. Although we can not
completely exclude the homogeneous pathway, it is quite
likely that the palladium nano-particles or clusters are the
active species for the catalysis.
1
ture with H NMR and TEM. Under the catalytic condi-
tions, an aliquot from the reaction of the palladium
complex with phenylboronic acid in the presence of base
was examined by transmission electron microscopy, show-
ing the formation of palladium particles with the diameter
in the range of 5–20 nm. Upon the addition of p-bromo-
benzaldehyde to the above solution, the coupling reaction
underwent smoothly via checking the reaction mixture with
NMR. This result suggests that the palladium nano-parti-
3. Summary
With this study we have demonstrated the versatility of
pyridinyl-carbene ligand. By using carbene-transfer, palla-
dium carbene complex [C-(pyN^C)Pd(p-allyl)Cl] (3) was
Table 5
Results of Mizoroki–Heck reaction catalyzed by 4a
O
O
Ar-X
+
Ar
OBu
Additive
OBu
Entry
Ar–X
Base
Solvent
Time
Yieldb (%)
1
2
3
4
5
6
7
8
9
C6H5Br
C6H5Br
C6H5Br
C6H5Br
C6H5Br
C6H5Br
P-Me2NC6H4Br
5-Bromoindole
2,4,6-Me3C6H2Br
p-ClC6H4CHO
Morpholine
iPr2NH
iPr2NH
iPr2NH
iPr2NH
Et2NH
iPr2NH
iPr2NH
iPr2NH
iPr2NH
–
–
–
–
NMP
NMP
Xylene
DMF
NMP
NMP
NMP
NMP
NMP
NMP
12
12
12
12
6
12
12
12
12
22
56
77
0
0
PPh3 (4 · 10ꢀ3 mmol)
PPh3 (4 · 10ꢀ3 mmol)
PPh3 (4 · 10ꢀ3 mmol)
PPh3 (4 · 10ꢀ3 mmol)
PPh3 (4 · 10ꢀ3 mmol)
PPh3 (4 · 10ꢀ3 mmol)
99
99
83
37
83
19
10
a
Reaction conditions: ArX (0.4 mmol); butyl acrylate (0.48 mmol); Pd complex (2 · 10ꢀ3 mmol) at 140 ꢁC.
b
GC yield.

C.-Y. Wang et al. / Journal of Organometallic Chemistry 692 (2007) 3976–3983
3981
obtained via the reaction of the silver carbene complex 2
with [(g3-allyl)PdCl]2. Abstraction of chloride from 3
allowed to form the chelation of pyridinyl-carbene toward
the metal center. This carbene chelating complex 4 decom-
poses in the presence of t-butoxide or hydrogen, but the
palladium complexes obtained have shown good catalytic
activity on Suzuki–Miyaura and Mizoroki–Heck reactions.
for C30H34N3ClPd: C, 62.29; H, 5.92; N, 7.26. Found: C,
62.18; H, 5.90; N, 7.06%.
4.2.2. [(C,N-pyN^C)Pd(g3-allyl)]BF4 (4)
A sample of 3 (89 mg, 0.15 mmol) was dissolved in
10 mL of dichloromethane and AgBF4 (30 mg, 0.15 mmol)
was added. The mixture was stirred for 2 h at ambient tem-
perature. The reaction mixture was filtered through Celite
and the filtrate was added to a solution of hexane
(10 mL) to yield the white precipitates. Upon filtration,
the desired complex was obtained white powders (85 mg,
88%). Recrystallization from acetone/hexane gave complex
4 as a colorless crystalline solid. 1H NMR (CDCl3,
400 MHz, 50 ꢁC): d 8.20 (dd, 1H, J = 7.6 Hz, 1.2 Hz, Py),
8.03 (dd, 1H, J = 7.6 Hz, 7.6 Hz, Py), 7.99 (d, 1H,
J = 1.6 Hz, Im), 7.34 (dd, 1H, J = 7.6 Hz, 1.2 Hz, Py),
6.95 (s, br, 2H, Mes), 6.89 (s, br, 2H, Mes), 6.84 (d, 1H,
J = 1.6 Hz, Im), 5.81 (d, 1H, J = 14.4 Hz, CH2), 5.60 (br,
1H, CH2), 4.76 (br, 1H, allyl), 3.34 (d, 1H, J = 6.8 Hz,
allyl), 2.83 (s, br, 1H, allyl), 2.33 (s, 3H, Me), 2.29 (s, 3H,
Me), 1.97 (s, 3H, Me), 1.93 (s, 6H, Me), 1.88 (s, 3H,
Me), 1.82 (d, 1H, J = 12.0 Hz, allyl). 13C NMR (CDCl3,
100 MHz, 50 ꢁC): d 176.3 (Pd@C), 162.1, 155.1, 140.3,
139.6, 139.4, 138.6, 136.2, 135.8, 135.2, 135.0, 129.1,
128.4, 126.9, 126.8, 125.4, 124.0, 123.9, 122.6, 117.4, 55.6
(CH2), 21.0, 20.9, 20.7, 17.7, 17.6. Anal. Calc. for
C30H34BF4N3Pd: C, 57.21; H, 5.44; N, 6.67. Found: C,
57.15; H, 5.09; N, 6.24%.
4. Experimental
4.1. General
All reactions and manipulations were performed under a
dry nitrogen atmosphere unless otherwise noted. Tetrahy-
drofuran was distilled under nitrogen from sodium benzo-
phenone ketyl. Dichloromethane was dried over CaH2 and
distilled under nitrogen. Other solvents were degassed
before use. Chemicals were purchased from commercial
source and used without further purification. 1-(6-Mesi-
tyl-2-picolyl)-3-mesitylimidazolium bromide and its silver
carbene complex were prepared according to the method
reported easier [11].
Nuclear magnetic resonance spectra were recorded in
CDCl3 or acetone-d6 on either a Bruker AM-300 or
AVANCE 400 spectrometer. Chemical shifts are given in
1
parts per million relative to Me4Si for H and 13C{1H}
NMR, and relative to 85% H3PO4 for 31P NMR. Infrared
spectra were measured on a Nicolet Magna-IR 550 spec-
trometer (Series-II) as KBr pallets, unless otherwise noted.
4.2.3. (C-pyN^C)Pd(g3-allyl)(PPh3) (5)
4.2. Synthesis and characterization
A flame dried Schlenk flask was charged with [(C,N-
pyN^C)Pd(g3-allyl)]BF4 (17.7 mg, 0.028 mmol) in dichlo-
4.2.1. [(C-pyN^C)Pd(g3-allyl)Cl] (3)
romethane (2 mL).
A
solution of PPh3 (7.4 mg,
A sample of [(g3-allyl)PdCl]2 (100 mg, 0.273 mmol) was
dissolved in dichloromethane (15 mL) and silver carbene 2
(345 mg, 0.273 mmol) was added. The color of the solution
changed immediately from colorless to yellow. The solu-
tion was stirred for 3 h at ambient temperature. The reac-
tion mixture was filtered through Celite and the filtrate
was concentrated. The residue was dissolved in dichloro-
methane/hexane and the desired complex was crystallized
from the solution as a yellow solid (217 mg, 86%): 1H
NMR (400 MHz, CDCl3): d 7.90 (d, 1H, J = 7.6 Hz, Py),
7.79 (dd, 1H, J = 7.6 Hz, 7.6 Hz, Py), 7.61 (s, br, 1H,
Im), 7.18 (d, 1H, J = 7.6 Hz, Py), 6.92 (s, 1H, Mes), 6.90
(s, 3H, Mes), 6.83 (d, 1H, J = 2.0 Hz, Im), 5.91 (d, 1H,
J = 14.8 Hz, CH2), 5.75 (d, 1H, J = 14.8 Hz, CH2), 4.89–
4.79 (m, 1H, allyl), 3.87 (d, 1H, J = 6.8 Hz, allyl), 3.56 (s,
br, 1H, allyl), 2.94 (d, 1H, J = 6.8 Hz, allyl), 2.69 (d, 1H,
J = 14.0 Hz, allyl), 2.30 (s, 6H, Me), 2.10 (s, 3H, Me),
1.96 (s, 6H, Me), 1.95 (s, 3H, Me) 13C NMR (100 MHz,
CDCl3): d 179.7 (Pd@C), 160.0, 155.7, 138.8, 137.9,
137.7, 137.6, 136.1, 135.6, 135.5, 135.0, 129.0, 128.7,
128.3, 124.6, 122.8, 122.1, 115.3, 77.5, 73.0, 56.4 (CH2),
49.2, 21.8, 21.1, 19.0, 18.7. HR-FAB for [MꢀCl]+: Calc.
542.1788 (C30H34N3106Pd), found: 542.1779. Anal. Calc.
0.028 mmol) in chloromethane (2 mL) was added via can-
nula and stirred at room temperature. After ꢁ30 min, the
desired complex precipitated from the solution. The prod-
uct was isolated by filtration, washed with hexane
(1 mL · 2) and dried under vacuum at room temperature
for 2 h. A light yellow solid was collected (24.9 mg, 99%):
1H NMR (400 MHz, CDCl3): d 7.81 (br, 1 H), 7.65–6.88
(m, 22H), 6.76 (s, 1H), 5.12–5.08 (br, 1H), 4.86 (d, 1H,
J = 14.0 Hz), 4.81 (br, 1H), 3.90 (br, 1H), 3.56 (m, 1H),
2.50 (d, 1H, J = 13.2 Hz), 2.29 (s, 6H), 2.22–2.20 (m, 1H),
1.90 (s, 6H), 1.72 (s, 3H), 1.45 (s, 3H) (major); 7.81 (br,
1H), 7.65–6.88 (m, 22H), 6.63 (s, 1H), 5.12–5.08 (br, 1H),
4.81 (br, 1H), 4.66 (d, 1H, J = 14.0 Hz), 3.90 (br, 1H),
3.46 (d, 1H, J = 6.0 Hz), 2.63 (m, 1H), 2.31 (s, 6H), 2.17
(m, 1H), 1.90 (s, 6H), 1.78 (s, 3H), 1.36 (s, 3H) (minor).
31P NMR (121.4 MHz, CDCl3): d 23.9 (major); 24.7
(minor). 13C NMR (100 MHz, CDCl3): d 177.6 (d, JP–
C = 16.2 Hz), 158.9, 154.0, 71.3 (allyl), 68.3 (allyl), 67.6 (d,
J = 29.7 Hz, allyl), 55.9 (CH2), 21.5, 21.4, 20.5, 18.1, 17.9
(major); 177.6 (d, JP–C = 16.2 Hz), 158.9, 154.3, 71.3 (allyl),
70.2 (d, J = 27.4 Hz), 68.3 (allyl), 21.5, 21.4, 20.5, 18.3, 18.1
(minor). Anal. Calc. for C48H49N3BF4PPd: C, 64.62; H,
5.54; N, 4.71. Found: C, 64.89; H, 5.94; N, 4.44%.

3982
C.-Y. Wang et al. / Journal of Organometallic Chemistry 692 (2007) 3976–3983
4.2.4. Decomposition of (C-pyN^C)Pd(g3-allyl)Cl in the
presence of HBF4
Acknowledgement
To a flask was placed with 3 (5.8 mg, 1.0 · 10ꢀ3 mmol).
The flask was evacuated and filled with nitrogen gas.
CDCl3 (2 mL) and HBF4 were then added and the resulted
solution was stirred at 50 ꢁC for 10 h. The reaction mixture
was filtrated through Celite and the filtrate was concen-
trated to give 1-(6-mesityl-2-picolyl)-3-mesitylimidazolium
tetrafluoroborate (4.0 mg, 83%). 1H NMR (400 MHz,
CDCl3): d 8.91 (s, 1H, Im-H2), 7.82–7.78 (m, 2H), 7.62
(d, 1H, J = 7.6 Hz, Py), 7.17 (d, 1H, J = 7.6 Hz, Py),
7.09 (m, 1H), 6.93 (s, 2H, Mes), 6.87 (s, 2H, Mes), 5,78
(s, 2H, CH2), 2.29 (s, 6H, Me), 1.87 (s, 12H, Me). 13C
NMR (100 MHz, CDCl3): d 160.2, 152.1, 141.2, 138.0,
137.7, 137.1, 135.4, 134.5, 130.6, 129.7, 128.3, 124.9,
124.0, 122.8, 121.3, 54.2 (CH2), 21.0, 20.1, 16.9. Anal. Calc.
for C27H30N3BF4: C, 67.09; H, 6.26; N, 8.69. Found: C,
66.69; H, 6.26; N, 8.48%.
We thank the National Science Council, Taiwan, ROC
for the financial support (NSC95-2113-M-002-038).
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