Synthesis and characterization of para-pyridine linked NHC palladium complexes and their studies for the Heck-Mizoroki coupling reaction

Article abstract of DOI:10.1039/c2dt30520h

This paper describes the synthesis of 1-(pyridine-4-ylmethyl) NHC and their Pd(ii) and Ag(i) complexes, which are fully characterized. Interestingly, we have also synthesized a Pd complex 3a-CO₃ using a more direct treatment of K₂CO₃ with PdCl₂. 3a-CO ₃ represents the first reported solid structure of a Pd η²-carbonato complex stabilized by an NHC framework. 3a-CO ₃ can be easily converted to a PdCl₂ derivative by treating it with chloroform. We have found these palladium complexes mediate the Heck-Mizoroki coupling with a low catalyst loading. Furthermore, we also expand such catalytic manifold toward constructing fused polyaromatic substrates, a highly useful class of compounds in optoelectronic chemistry.

Full text of DOI:10.1039/c2dt30520h

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PAPER
Synthesis and characterization of para-pyridine linked NHC palladium
complexes and their studies for the Heck–Mizoroki coupling reaction†
Ya-Ming Liu,a,b Yi-Chun Lin,a Wen-Ching Chen,a Jen-Hao Cheng,a,c Yi-Lin Chen,a Glenn P. A. Yap,d
Shih-Sheng Sun*a and Tiow-Gan Ong*a
Received 4th March 2012, Accepted 13th April 2012
DOI: 10.1039/c2dt30520h
This paper describes the synthesis of 1-(pyridine-4-ylmethyl) NHC and their Pd(II) and Ag(I) complexes,
which are fully characterized. Interestingly, we have also synthesized a Pd complex 3a-CO3 using a
more direct treatment of K2CO3 with PdCl2. 3a-CO3 represents the first reported solid structure of a
Pd η2-carbonato complex stabilized by an NHC framework. 3a-CO3 can be easily converted to a PdCl2
derivative by treating it with chloroform. We have found these palladium complexes mediate the
Heck–Mizoroki coupling with a low catalyst loading. Furthermore, we also expand such catalytic
manifold toward constructing fused polyaromatic substrates, a highly useful class of compounds in
optoelectronic chemistry.
alkenyl substituents onto an aromatic molecule in a gentle
Introduction
manner. Since the pioneering work contributed by Heck and
Mizoroki in the early 1970s,5 a significant advance in this area
N-Heterocyclic carbenes (NHCs) are now ubiquitous in organo-
metallic chemistry due to their effective role as ligands in homo-
geneous transition metal catalysis. In the last decade, most
research efforts have been devoted to the development of hybrid
NHC ligand sets based on covalently attaching the ortho-pyridyl
pendant arm.1 The interest in these scaffolds is fuelled by the
chelating effect invoked by the N-donor site to enhance the stab-
ility of the resulting catalyst.1c,2 Amazingly, no such structural
NHC framework bearing a para-pyridyl pedant arm has been
reported in the literature, despite of many reports pertaining to
studies of C–C cross coupling reactions using ortho-pyridine
NHCs as supporting chelating ligands for metal complexes.3
Today, palladium-catalyzed cross coupling reactions are
among the most common and effective strategies employed by
chemists for constructing C–C bonds in molecular synthesis,4
which has been recognized with the Nobel Prize for Chemistry
in 2010. Heck–Mizoroki coupling represents one example of a
palladium-catalyzed reaction that permits a bond attachment of
has been achieved and applications have been found in the syn-
thesis of natural products, pharmaceuticals, and materials
science.6 Herein, we describe the development of the 1-(pyri-
dine-4-ylmethyl) NHC scaffold and the synthesis of their silver
and palladium complexes. These palladium complexes were
further tested and examined for their catalytic activity towards
the Heck–Mizoroki cross coupling reaction.
Results and discussion
Synthesis of 1-(4-pyridinylmethyl) imidazolium
The preliminary work was initiated with the synthesis of 1-(pyri-
dine-4-ylmethyl) imidazolium bearing a mesityl (1a) and isopro-
pyl (1b) side arm. The imidazolium compounds 1a and 1b were
prepared with a similar method to that described by Tolman
et al., as illustrated in Scheme 1.7 The quaternization of 1a and
1b was achieved through refluxing the suspension of 4-(chloro-
methyl)pyridine hydrochloride with NaHCO3 and mesityl–
imidazole in ethanol for a day. The identity of 1 is confirmed by
1H NMR spectroscopy with the signature peak of an N–CH–N
fragment at ∼10–11 ppm. Numerous attempts to obtain free
carbene species via base deprotonation of compound 1 were
unsuccessful and yielded intractable products.
aInstitute of Chemistry, Academia Sinica, Nangang, Taipei, Taiwan,
Republic of China. E-mail: tgong@chem.sinica.edu.tw
bDepartment of Chemistry, National Central University, Jhongli,
Taiwan, Republic of China
cHonour College, National Taiwan University of Science and
Technology, Taipei, Taiwan, Republic of China
dDepartment of Chemistry and Biochemistry, University of Delaware,
Newark, Delaware 19716, USA
†Electronic supplementary information (ESI) available: X-ray data col-
lection, solution, and refinement details for compounds 2a, 2b, 2b, 3a,
3b and 3a-CO3. Crystal data are also available as CIF files for these
compounds. CCDC reference numbers: 818961 (2a), 818962 (2b),
818963 (3a), 818964 (3a-CO3) and 818965 (3b). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2dt30520h
Synthesis and characterization of pyridine linked NHC silver
and palladium complexes
Considering that we are not able to obtain the free carbene
species via deprotonation of imidazolium 1, one alternate
synthetic method for obtaining NHC complexes is through
7382 | Dalton Trans., 2012, 41, 7382–7389
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Scheme 1 Preparation of the imidazolium compounds 1a and 1b, Ag and Pd complexes.
the measurement of steric constraints, indicated that mesityl is
obviously larger than isopropyl.10 This is counter-intuitive to the
anticipated structural behavior, where a metallic complex bearing
a ligand with a large steric protection should appear to disfavor
this dimerization or aggregation process like 2b. Yet, the mesityl
N substituent of 2b can rotate more freely than the isopropyl
group of 2a, permitting fluctuations in the steric interactions.11
Interestingly, the bond distance of Ag–carbene in 2a (Ag–C(10)
2.078(4) Å) has not been perturbed significantly by the change
of the pendant arm with respect to 2b. Finally, the crystal
packing in 2a and 2b demonstrated a lack of intermolecular
interaction between Ag and the pyridyl moiety, signifying that
the strong σ donation from the NHC has a saturated metal center
from accepting a higher electron density from the other ligand.
We then turned to the synthesis of Pd complexes by examin-
ing the transmetallation method through the corresponding
Ag–NHC complexes. Ag complex 2b, an isopropyl side arm
analogue, reacts readily with Pd(CH3CN)2Cl2 to produce a
Fig. 1 Molecular structures of 2b (left) and 2a (right) with thermal
ellipsoids drawn at the 30% probability level. All hydrogen atoms are
omitted for clarity.
transmetallation using silver NHC complexes, which are well
known for acting as carbene transfer agents.8 The reaction of 1
and Ag2O in dichloromethane at room temperature produces
compound 2 in a reasonable yield (2a (72%) and 2b (69%)). A
single-crystal X-ray diffraction experiment was undertaken to
determine the atomic connectivity in 2a and 2b, and structural
models are presented in Fig. 1, which are consistent with the
NMR spectroscopy and molecular mass analysis. The structure
of 2b consists of a linear arrangement of Ag coordinated to
NHC and chloride ligands with the Ag–C(4) distance being
2.081(2) Å, which is in agreement with the observed average for
five-membered carbene complexes.9
1
yellow solid 3b with a high yield. The representative H NMR
spectroscopic features of 3b display two singlets corresponding
to a total of 4 protons at 5.76 and 5.48 ppm for methylene moi-
eties and two doublets at 1.61 and 1.38 ppm corresponding to a
total of 12 protons for the methyl group for the isopropyl arms.
Nonetheless, the NMR spectra of 3b gave little in the way of
confirmation of the structure,12 as only minimal differences in
chemical shifts were observed relative to the Ag complex 2b. A
crystal of 3b suitable for single-crystal X-ray structural analysis
was obtained from a concentrated dichloromethane solution
ˉ
In contrast to 2b, the molecular structure of 2a consists of two
independent silver NHC dimer units bridged by chloride ligands.
Each unit seems to have an inversion point positioned at the
center of Ag–Cl–Ag bridging. The buried volume, a gauge for
and crystallized in the triclinic P1. The molecular structure of 3b
is illustrated in Fig. 2, and the crystal data and collection para-
meters are given in Table S5 (see the ESI†). The Pd center has a
square planar geometry with the two trans NHC ligands, whose
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Fig. 3 Molecular structure of 3a-CO3 with thermal ellipsoids drawn at
the 30% probability level. All hydrogen atoms are omitted for clarity.
Fig. 2 Molecular structure of 3b with thermal ellipsoids drawn at the
30% probability level. All hydrogen atoms are omitted for clarity.
nonbonded pyridyl wing tips are anti to each other. The NHC
rings are orthogonal to the coordination plane of the metal center
with a twist angle of 69.1°, presumably minimizing the
unfavourable steric interaction. The bond length between Pd and
Ccarbene in 3b is 2.0265(19) Å, which is in agreement with the
typical literature value.13
Regrettably, an NHC Pd complex similar to 3a cannot be pre-
pared via a silver transmetallation route in a clean fashion.
Therefore, we resorted to preparing the palladium complex based
on a similar method to that described by Lee et al.14 A direct
reaction between imidazolium precursors 1a, PdCl2, and K2CO3
in pyridine produces 3a-CO3 with a high yield (87%). The
proton NMR spectrum of 3a-CO3 has similar features to its
silver counterpart. In addition to the carbenic peak at 167 ppm,
the 13C NMR spectrum of 3a-CO3 appears to have a low field
resonance at 170 ppm, indicating the possibility of a carbonate
moiety in the complex. The identity of 3a-CO3 was unambigu-
ously confirmed by X-ray crystallography (Fig. 3), crystallizing
in the monoclinic P2(1)/n and displaying a non-ideal square
planar palladium(II) bound to two pyridine–NHCs in cis
fashion with a normal length of 1.973 and 1.980 Å and one η2-
coordinated carbonate ligand with a Pd–O bond length of 2.051
and 2.041 Å. The geometrical distortion in 3a-CO3 with a bite
angle of C10–Pd–O2 99.77(8)° and C28–Pd–O1 103.19(8)° is
due to the constraint of the four-membered metallacycle imposed
by the carbonate coordination. Few molecular structures of the
related carbonate palladium complexes supported by chelating
amine and phosphine ligands have been isolated and reported.15
To the best of our knowledge, 3a-CO3 represents the first
reported solid structure Pd η2-carbonate complex stabilized by an
NHC framework. More importantly, we are able to synthesize a
high yield of 3a-CO3 using a more direct treatment of K2CO3
with PdCl2, rather than through CO2 gas bubbling.
Fig. 4 Molecular structures of 3a with thermal ellipsoids drawn at the
30% probability level. All hydrogen atoms are omitted for clarity.
Table 1 Optimization of Heck–Mizoroki coupling in different
conditionsa
Entry Cat./mol% Base
Solvent Temp (°C) Time (h) Yieldb
1
2
3
4
5
6
7
8
3b/5
3b/5
3b/5
3b/5
3b/5
3b/1
3b/1
3b/1
K2CO3 DMF
KOtBu DMF
NaOtBu DMF
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
K2CO3 DMF
130
130
130
100
60
130
130
130
130
130
130
130
18
18
18
18
18
18
12
6
18
12
18
18
>95%
5%
16%
80%
6%
>95%
>95%
83%
>95%
79%
92%
83%
9
3b/0.1
3b/0.1
3a/0.1
10
11
12
3a-Co3/0.1 K2CO3 DMF
a The reactions were carried out using 10 (1.92 mmol) and 20
(1.28 mmol). b The isolated yield is based on 20 as the limiting reagent.
Finally, the synthesis of the pyridine–NHC palladium chloride
complex 3a is straightforward via 3a-CO3. Refluxing 3a-CO3 in
CDCl3 solution produced 3a with a quantitative yield. Like 3b,
3a exhibits a square planar geometry, warranting no further
comment (Fig. 4) with the exception that both chloride ligands
are cis to each other.
screening attempts were undertaken to determine the viability of
various bases with a 5 mol% loading of 3b pre-catalysts for the
C–C coupling of styrene (10) and bromobenzene (20) in a DMF
solution at 130 °C (entries 1–3, Table 1). As seen in entry 1 of
Table 1, the reaction produced a quantitative yield of the corre-
sponding product, stilbene (20aa) in the presence of K2CO3, a
milder base. However, lowering the reaction temperatures to 100
Catalytic Heck–Mizoroki coupling
The catalytic performance of complex 3 in the Heck–Mizoroki
coupling of aryl halides with alkenes was evaluated. Initial
7384 | Dalton Trans., 2012, 41, 7382–7389
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Table 2 Scope of the Heck–Mizoroki catalytic reaction with different
Table 3 Scope of the Heck–Mizoroki catalytic reaction with fused
aryl bromidesa
polyaromatic bromidea
Entry
Ar–Br
Cat. PdL2
0.1%
Time (h)
Yieldb
>95%
>95%
>95%
Entry
1
Ar–Br
Cat. PdL2
0.1%
Time (h)
18
Yieldb
86%
1
2
3
18
18
18
0.1%
0.1%
2
3
0.1%
0.1%
18
18
41%
4
5
6
0.1%
18
>95%
>95%
1%
0.1%
12
18
46%
45%
4
5
0.1%
0.1%
18
18
82%
80%
1%
0.1%
12
18
>95%
70%
7
1%
0.1%
12
18
82%
69%
6
0.1%
18
81%
8
1%
0.1%
12
18
67%
12%
9
0.1%
0.1%
18
88%
7
8
0.1%
0.1%
18
18
81%
90%
10
18
0%
a The reactions were carried out using 10 (1.92 mmol) and 2x
(1.28 mmol). b The isolated yield is based on aryl halide as the limiting
reagent.
a The reactions were carried out using 10 (1.92 mmol) and 3x
(1.28 mmol). b The isolated yield is based on aryl halide as the limiting
reagent.
and 60 °C caused the reaction to proceed slowly with 80% and
6% yields, respectively (entries 4 and 5, Table 1). Importantly,
the reactions could be performed using only 1.0 and 0.1 mol%
catalyst loading with little effect on the yield (entries 6 and 9,
Table 1). Next, screening of a similar catalytic reaction with the
other Pd complexes 3a and 3a-CO3 also underwent an effective
cross coupling of ∼90% yield (entries 11 and 12, Table 1).
With these results in hand, we further expanded the scope of
the cross coupling of styrene with different substituted aryl bro-
mides using catalyst 3b (see Table 2). In general, cross coupled
products can be obtained with high yield, and are insensitive to
the effect of electron donating or withdrawing substituted on aryl
bromide (entries 1–4, 7 and 9, Table 2). A moderate conversion
(67%) could also be achieved for the pyridine functionality com-
pound (entry 8, Table 2). Electron-rich substrates with para-
amino substituents produced a lower yield of 45% product 24aa
under identical conditions (entry 5, Table 2), but surprisingly,
meta-amino substituents delivered a quantitative conversion
(entry 6, Table 2). Regrettably, aryl chloride could not be con-
verted effectively (entry 10, Table 2).
2-bromonaphthalene (86%, entry 1, Table 3), 1-bromonaphtha-
lene (>95%, entry 13, Table 3) and 4-bromobiphenyl (82%,
entry 4, Table 3), yet diminishing yield was seen for 6-bromo-2-
naphthalenol (41%, entry 2, Table 3), suggesting the sensitivity
of the reaction to a hydroxy functional group. A fused aromatic
containing more than four membered rings, such as 9-bromoan-
thracene 34 (entry 5, Table 3), 9-bromo-10-phenylanthracene 35
(entry 6, Table 3) and pyrene 36 (entry 7, Table 3), also partici-
pated in the catalytic reaction in surprisingly high yields with
low catalyst loading.
We next examined whether 3b could also facilitate the reaction
with terpyridine, as such molecular motifs have now evoked
widespread attention in the area of supramolecular and coordi-
nation chemistry for materials with unique physical properties.21
The tridentate functionality of this substrate might partially
poison the catalysts. To our delight, product derived from 37 was
isolated with a 90% yield (entry 8, Table 3). Moreover, the pres-
ence of dangling pyridyl groups in catalyst 3b did not produce
any notable steric hindrance for large substrates approaching the
catalyst (entries 6–8, Table 3). Finally, Table 4 reveals that we
can also expand the scope of our catalytic reaction to butyl acry-
late with different aryl bromides as well as fused aromatic rings
in high yields.
Fused polyaromatic compounds have been widely employed
in many optoelectronic applications such as light-emitting
materials,16 organic thin-film transistors,17 nonlinear two-photon
absorption,18 photovoltaics,19 and bio-imaging materials.20 In
order to demonstrate the broader applicability of the method we
further set out to examine the coupling of styrene with fused
polyaromatic substrates in a similar reaction condition, as
listed in Table 3. Excellent conversion was observed for
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Table 4 Scope of Heck–Mizoroki catalytic reaction with butyl
using a more direct treatment of K2CO3 with PdCl2. 3a-CO3 can
be easily converted to a PdCl2 derivative by treating it with
chloroform. We have found that the palladium catalyst 3b
actively mediates Mirozoki–Heck coupling with a low catalyst
loading and a broad substrate scope. In particular, catalyst 3b
can be applied to substrates with electron donating groups
(Table 2, entries 3 and 6) and a catalyst poison site (Table 2,
entry 8 and Table 3, entry 8). Even a challenging substrate like
naphthol gave an encouraging yield (Table 3, entry 2). We can
also expand such a catalytic manifold toward other fused poly-
aromatic substrates. Ongoing work is focused on understanding
such cooperative effects exhibited by an uncoordinated pyridine
moiety and exploring the scope of other new organic reactions.
acrylatea
Entry
1
Ar–Br
Cat. PdL2
0.1%
Time (h)
18
Yieldb
>95%
2
3
0.1%
0.1%
18
18
>95%
>95%
4
0.1%
18
68%
Experimental
a The reactions were carried out using 11 (1.92 mmol) and aryl bromide
(1.28 mmol). b The isolated yield is based on aryl halide as the limiting
reagent.
General procedure
All air-sensitive manipulations were performed under an atmos-
phere of nitrogen using the Schlenk technique and/or a glovebox.
Toluene, hexane, THF and ether were purified by passage
through a column of activated alumina using a solvent purifi-
cation system purchased from Innovative Technology, Inc.
Deuterated benzene and toluene were dried by vacuum transfer
from activated molecular sieves. H and 13C NMR spectra were
1
recorded on Bruker 300 MHz, Bruker 400 MHz and Bruker
500 MHz spectrometers using the residual proton signal in the
deuterated solvent as a reference. PdCl2 was purchased from
Arcos Chemicals and used without further purification.
N-Mesityl-N′-(4-pyridylmethyl)imidazolium chloride (1a)
The compound 1a was prepared in a similar manner to that
reported by Tolman et al.7 Mesityl imidazole (21.95 mmol,
4.08 g) was added to the suspension of 4-(chloromethyl)pyridine
hydrochloride (18.24 mmol, 3.00 g) and sodium bicarbonate
(27.36 mmol, 2.10 g) in 50 mL ethanol. After being stirred and
refluxed for 18 h, volatiles were removed in vacuo to produce
the crude product. This was dissolved in dichloromethane
(100 mL), and diethyl ether (400 mL) was added slowly to pre-
cipitate the solid. Yield: 55% (3.15 g). 1H NMR (CDCl3,
Scheme 2 Heck–Mizoroki coupling reaction mediated by complex 4.
Several observations with these Pd(II) NHC catalysts
employed in the Mirozoki–Heck coupling reactions are worth
mentioning. Control experiments using complex 4,22 lacking a
pyridine moiety at the para position, led to lower yield of 89%
and 82% for 22aa and 23aa, respectively (Scheme 2). Second,
we also performed another catalytic reaction under similar con-
ditions with 0.3 mol% and 10 mol% of lithium chloride as an
additive to bind the pedant arm of the pyridine. Again, diminish-
ing yields of the product with 30% and 0%, respectively, were
obtained. All these findings have validated the importance of
the cooperative effect invoked by the pyridine moiety to assist
the efficiency of this catalytic process.
3
400 MHz, 25 °C): δ 10.91 (s, 1H, NCHN), 8.58 (d, JH–H = 6
3
Hz, 2H, Py), 7.97 (s, 1H, CH), 7.58 (d, JH–H = 6 Hz, 2H, Py),
7.12 (s, 1H, CH), 6.94 (s, 2H, C6H2), 6.09 (s, 2H, PyCH2), 2.29
(s, 3H, ArCH3), 1.99 (s, 6H, ArCH3); 13C NMR (CDCl3,
100 MHz, 25 °C): δ 150.8 (Py), 143.0 (Py), 141.5 (Ar), 138.7
(NCHN), 134.1 (Ar), 130.8 (Ar), 130.0 (Ar), 123.8 (CH), 123.6
(Py), 123.5 (CH), 51.6 (PyCH2), 21.2 (ArCH3), 17.7 (ArCH3);
HR-MS (FAB): m/z = 278.1658; calculated for C18H20N3
([M − Cl]+): 278.1658.
N-(i-Propyl)-N′-(4-pyridylmethyl)imidazolium chloride (1b)
Conclusions
The compound 1b was prepared in a similar manner to that
reported by Tolman et al.7 Isopropyl imidazole (29.30 mmol,
3.28 g) was added to the suspension of 4-(chloromethyl)pyridine
hydrochloride (24.40 mmol, 4.00 g) and sodium bicarbonate
(35.60 mmol, 3.075 g) in 50 mL ethanol. After being stirred and
In conclusion, this paper describes the synthesis of 1-(pyridine-
4-ylmethyl) NHC and their Pd(II) and Ag(I) complexes, which
are fully characterized including X-ray crystal structures. We are
also able to synthesize the Pd complex 3a-CO3 with a high yield
7386 | Dalton Trans., 2012, 41, 7382–7389
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refluxed for 18 h, volatiles were removed in vacuo to produce a
brown liquid residue, which was dissolved in 50 mL dichloro-
methane, followed by the addition of 400 mL diethyl ether to
in vacuo to produce the solid residue. The solid residue was
redissolved in 15 mL dichloromethane and washed twice with
water to remove the excess inorganic salt. The washed solution
was dried in vacuo to produce a yellow solid, which was further
purified by recrystallization from CH2Cl2–diethyl ether (3 : 1
volume) at 25 °C. Yield: 87% (0.20 g). 1H NMR (CDCl3,
1
precipitate the product. Yield: 70% (4.06 g); H NMR (CDCl3,
3
400 MHz, 25 °C): δ 11.09 (s, 1H, NCHN), 8.54 (d, JH–H
=
3
5.6 Hz, 2H, Py), 7.62 (s, 1H, CH), 7.47 (d, JH–H = 6 Hz, 2H,
Py), 7.41 (s, 1H, CH), 5.76 (s, 2H, PyCH2), 4.71 (m, 1H, CH),
1.56 (d, 6H, CH3); 13C NMR (CDCl3, 100 MHz, 25 °C): δ
150.2 (Py), 142.3 (Py), 136.0 (NCN), 123.1 (Py), 122.6 (CH),
120.4 (CH), 53.1 (PyCH2), 51.0 (CH), 22.7 (CH3); HR-MS
(FAB): m/z = 202.1348; calculated for C12H16N3 ([M − Cl]+):
202.1344.
3
400 MHz, 25 °C): δ 8.55 (d, JH–H = 6 Hz, 2H, P), 7.18 (d,
3JH–H = 5.6 Hz, 2H, Py), 7.13 (s, 1H, C6H2), 7.06 (s, 1H, C6H2),
2
6.82 (d, 1H, CH), 6.77 (d, 1H, CH), 5.54 (d, JH–H = 14.8 Hz,
2
1H, PyCH2), 3.83 (d, JH–H = 14.8 Hz, 1H, PyCH2), 2.42
(s, 3H, ArCH3), 2.18 (s, 3H, ArCH3), 1.61 (s, 3H, ArCH3); 13C
NMR (CDCl3, 100 MHz, 25 °C): δ 170.2 (CO3), 167.5
(NCHN), 150.9 (Ar), 143.8 (Ar), 140.2 (Ar), 139.0 (Ar), 135.3
(Ar), 133.5 (Ar), 130.9 (Ar), 129.1 (Ar), 124.4 (CH), 123.7 (Ar),
121.2 (CH) 52.8 (PyCH2), 21.3 (ArCH3), 19.0 (ArCH3), 17.7
(ArCH3); νmax/cm−1 1618s (CO); HR-MS (EI): m/z = 721.2121;
calculated for C37H39N6O3Pd ([M + H]+): 721.2118.
[N-Mesityl-N′-(4-pyridylmethyl)imidazol-2-ylidene] silver
chloride complex (2a)
Ag2O (0.48 mmol, 0.11 g) in 3 mL dichloromethane solution
was added to a solution of N-mesityl-N′-(4-pyridylmethyl)imida-
zolium chloride 1a (0.96 mmol, 0.30 g) at room temperature.
After stirring for 4 h, the solution was filtered and removed
in vacuo to produce the solid product, which was washed with
diethyl ether. Yield: 72% (0.291 g). 1H NMR (CDCl3,
[N-Mesityl-N′-(4-pyridylmethyl)imidazol-2-ylidene] palladium
(II) chloride complex (3a)
3
400 MHz, 25 °C): δ 8.60 (d, JH–H = 5.2 Hz, 2H, Py), 7.14
The 3 mL dichloromethane solution of NHC Pd complexes 4a
(0.27 mmol, 0.20 g) was stirred at 60 °C for 3 days. The cloudy
solution was filtered through Celite. The filtrate solution was
dried in vacuo to produce a pure color solid product. Yield: 95%
(s, 1H, CH), 7.08 (d, 3JH–H = 5.2 Hz, 2H, Py), 6.99 (s, 1H, CH),
6.94 (s, 2H, C6H2), 5.44 (s, 2H, PyCH2), 2.31 (s, 3H, ArCH3),
1.96 (s, 6H, ArCH3); 13C NMR (CDCl3, 100 MHz, 25 °C): δ
182.1 (NCHN), 150.4 (py), 144.8 (Ar), 139.6 (Ar), 135.1 (Ar),
134.5 (py), 129.4 (Ar), 123.6 (CH), 121.9 (CH), 121.8 (py),
54.1 (pyCH2), 21.0 (ArCH3), 17.6 (ArCH3); HR-MS (FAB): m/z
= 661.2218; calculated for C36H38N6Ag ([M − Cl]+): 661.2209;
anal. calculated (%) for C36H38N6ClAg: C 51.27, H 4.78, N
9.96; found: C 51.39. H 4.47, N 9.85.
(0.19 g). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 8.56 (d, 3JH–H
=
3
6 Hz, 2H, Py), 7.22 (s, 1H, C6H2), 7.20 (d, JH–H = 5.6 Hz, 2H,
Py), 7.15 (s, 1H, C6H2), 6.86 (d, 1H, CH), 6.31 (d, 1H, CH),
2
2
5.54 (d, JH–H = 15.2 Hz, 1H, PyCH2), 3.64 (d, JH–H = 15.2
Hz, 1H, PyCH2), 2.49 (s, 3H, ArCH3), 2.39 (s, 3H, ArCH3),
1.59 (s, 3H, ArCH3); 13C NMR (CDCl3, 100 MHz, 25 °C): δ
163.6 (NCHN), 150.3 (py), 143.9 (Ar), 140.1 (Ar), 139.5 (Ar),
135.2 (Ar), 133.9 (Ar), 131.1 (Ar), 129.0 (Ar), 124.2 (CH),
124.1 (Py), 121.2 (CH) 52.7 (PyCH2), 21.2 (ArCH3), 19.9
(ArCH3), 18.1 (ArCH3).
[N-(i-Propyl)-N′-(4-pyridylmethyl)imidazol-2-ylidene] silver
chloride complex (2b)
Ag2O (0.86 mmol, 0.20 g) in 3 mL dichloromethane solution
was added to a solution of N-isopropyl-N′-(4-pyridylmethyl)imi-
dazolium chloride 1b (1.68 mmol, 0.40 g) at room temperature.
After stirring for 4 h, the solution was filtered and removed
in vacuo to produce the solid product, which was washed with
diethyl ether. Yield: 69% (0.397 g). 1H NMR (CDCl3,
400 MHz, 25 °C): δ 8.55 (d, 3JH–H = 6 Hz, 2H, Py), 7.07 (s, 1H,
[N-(i-Propyl)-N′-(4-pyridylmethyl)imidazol-2-ylidene] palladium
(II) chloride complex (3b)
Pd(CH3CN)2Cl2 (0.87 mmol, 0.30 g) in dichloromethane was
added to the suspension of 2b (0.44 mmol, 0.11 g) at room
temperature and was stirred for 4 h. The cloudy solution was
filtered through Celite. The filtrate solution was dried in vacuo to
3
CH), 7.05 (d, JH–H = 5.6 Hz, 2H, Py), 6.95 (s, 1H, CH), 5.28
(s, 2H, PyCH2), 4.72 (m, 1H, CH), 1.45 (s, 6H, CH3); 13C NMR
(CDCl3, 100 MHz, 25 °C): δ 179.7 (NCNH), 150.7 (Py), 144.7
(Py), 122.2 (Py), 121.5 (CH), 118.5 (CH), 54.7 (PyCH2), 54.6
(CH), 24.0 (CH3). HR-MS (FAB): m/z = 308.0321; calculated
for C12H15N3Ag ([M − Cl]+): 308.0317.
1
produce a pure solid product. Yield: 83% (0.210 g); H NMR
(CDCl3, 300 MHz, 25 °C): δ 8.61 (br, 2H, Py), 8.46 (br, 2H,
3
3
Py), 7.38 (d, JH–H = 5.2 Hz, 2H, Py), 7.15 (d, JH–H = 4.8 Hz,
3
3
2H, Py), 6.94 (d, JH–H = 1.6 Hz, 1H, CH), 6.90 (d, JH–H = 1.6
3
3
Hz, 1H, CH), 6.73 (d, JH–H = 1.6 Hz, 1H, CH), 6.69 (d, JH–H
= 1.6 Hz, 1H, CH), 5.76 (s, 2H, PyCH2), 5.68 (m, 1H, CH),
5.50 (s, 2H, PyCH2), 5.35 (m, 1H, CH), 1.62 (d, 6H, CH3), 1.40
(d, 6H, CH3); 13C NMR (CDCl3, 75 MHz, 25 °C): 170.4
(NCN), 170.4 (NCN), 150.4 (Py), 150.3 (Py), 145.6 (Py), 145.4
(Py), 122.9 (Py), 122.6 (Py), 121.2 (CH), 121.10 (CH), 117.7
(CH), 53.2 (CH), 53.0 (CH), 52.5 (CH), 23.7 (CH3), 23.5 (CH3);
HR-MS (EI): m/z = 579.1023; calculated for C24H31N6Cl2Pd
([M + H]+): 579.1022.
[N-Mesityl-N′-(4-pyridylmethyl)imidazol-2-ylidene] palladium
(II) carbonate complex (3a-CO3)
The compound 4a was prepared in a similar manner as that
described by Lee et al.14 A suspension of 1a (0.63 mmol,
0.20 g), K2CO3 (2.56 mmol, 0.36 g), and PdCl2 (0.32 mmol,
0.06 g) in 3 mL pyridine was heated at 60 °C for 5 days. The
reaction mixture was allowed to cool down and was removed
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7382–7389 | 7387

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General procedure for the palladium-mediated Heck–Mirozoki
coupling of styrene with various aryl halides
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K2CO3 base was added to a DMF solution of 3b (0.1 mol%) and
styrene 10 or butyl acrylate 11 (1.92 mmol) and aryl halide
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purified by flash chromatography using hexane/ethyl acetate as
the eluent.
Single-crystal X-ray characterization
X-ray data collection, solution, and refinement details for 2a, 2b,
3a and 3b and 3a-CO3 are included in the ESI.† Crystals were
mounted using viscous oil or epoxy adhesive onto glass fibers
and cooled to the data collection temperature. Data were col-
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lected using a Nonius Kappa CCD diffractometer (MoKα
=
0.71073 Å). Multi-scan absorption corrections were employed.
The structures were solved by direct methods and refined using
the least-squares method on F2. The compound molecule 2a is
located with two symmetry independent molecules, each on an
inversion center. In 3a, the molecule is located on a two-fold
axis with cis ligands, while in 3b the trans molecule is located
on an inversion center. One molecule of methylene chloride
solvent was found co-crystallized per compound molecule in 3a-
CO3. All non-hydrogen atoms were refined with anisotropic dis-
placement parameters. Amine hydrogen atoms were located from
the difference map and refined with Uiso = 1.2Ueq of the attached
nitrogen atom. All other hydrogen atoms were treated as ideal-
ized contributions. The structure factors and anomalous dis-
persion coefficients are contained in the SHELXTL 6.12
program library.23 CCDC-818961 (2a), CCDC-818962 (2b),
CCDC-818963 (3a), CCDC-818964 (3a-CO3) and CCDC-
818965 (3b) contain the supplementary crystallographic data for
this paper.
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Acknowledgements
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We are grateful to the Taiwan National Science Council (NSC:
99-2113-M-001-011-MY3) and Academia Sinica for their
generous financial support.
12 The 1H NMR spectroscopic features of 3b display a mixture of two
isomers in solution with the ratio of 2/3. Two isomeric forms are thought
be anti–syn rotamer. Y. P. Huang, C. C. Tsai, W. C. Shih, Y. C. Chang,
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