Rotamers of palladium complexes bearing IR active N-heterocyclic carbene ligands: Synthesis, structural characterization and catalytic activities

Article abstract of DOI:10.1016/j.jorganchem.2008.10.044

The preparation and properties of mono- versus bis(carbene) Pd(II) complexes bearing unsymmetrical cyano- and ester-functionalized NHC ligands as potential IR probes were studied in detail. Direct reaction of Pd(OAc)₂ with functionalized imidazolium salts afforded either bis(carbene) (3a, c) or monocarbene complexes (5, 6) with a N-coordinated imidazole co-ligand. The latter were exclusively obtained with N-ethylene substituted salts, which were found to undergo N-C cleavage reaction. The milder Ag-carbene transfer reaction on the other hand was tolerable to the length of the substituents and the nature of the functional groups. All bis(carbene) complexes (3a-c, 4a-c) were obtained as a inseparable mixture of square-planar trans-anti and trans-syn rotamers. The identity, ratio and dynamic equilibrium of these rotamers have been investigated and the relatively high rotational barrier for rotamers of 3a was estimated to be about 74 kJ mol^-1 at 380 K. All eight complexes were fully characterized by NMR and IR spectroscopies, ESI mass spectrometry and X-ray single crystal and powder diffraction. A preliminary catalytic study showed that ester-functionalized complexes 4a and 4b gave rise to highly active catalyst in the double Mizoroki-Heck coupling of aryl dibromides, while the in situ ester-hydrolyzed complexes were also active in the coupling of activated aryl chlorides.

Full text of DOI:10.1016/j.jorganchem.2008.10.044

Journal of Organometallic Chemistry 694 (2009) 323–331
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Rotamers of palladium complexes bearing IR active N-heterocyclic carbene ligands:
Synthesis, structural characterization and catalytic activities
*
Han Vinh Huynh , Jiang Wu
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation and properties of mono- versus bis(carbene) Pd(II) complexes bearing unsymmetrical
cyano- and ester-functionalized NHC ligands as potential IR probes were studied in detail. Direct reaction
of Pd(OAc)2 with functionalized imidazolium salts afforded either bis(carbene) (3a, c) or monocarbene
complexes (5, 6) with a N-coordinated imidazole co-ligand. The latter were exclusively obtained with
N-ethylene substituted salts, which were found to undergo N–C cleavage reaction. The milder Ag-carbene
transfer reaction on the other hand was tolerable to the length of the substituents and the nature of the
functional groups. All bis(carbene) complexes (3a–c, 4a–c) were obtained as a inseparable mixture of
square-planar trans-anti and trans-syn rotamers. The identity, ratio and dynamic equilibrium of these
rotamers have been investigated and the relatively high rotational barrier for rotamers of 3a was esti-
mated to be about 74 kJ molꢀ1 at 380 K. All eight complexes were fully characterized by NMR and IR spec-
troscopies, ESI mass spectrometry and X-ray single crystal and powder diffraction. A preliminary catalytic
study showed that ester-functionalized complexes 4a and 4b gave rise to highly active catalyst in the
double Mizoroki–Heck coupling of aryl dibromides, while the in situ ester-hydrolyzed complexes were
also active in the coupling of activated aryl chlorides.
Received 23 September 2008
Received in revised form 27 October 2008
Accepted 28 October 2008
Available online 5 November 2008
Keywords:
N-heterocyclic carbene
Palladium
Rotamers
C–C coupling
Homogeneous catalysis
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Result and discussion
In recent years, N-heterocyclic carbenes (NHCs) have attracted
increasing attention in organometallic chemistry and catalysis
mainly due to their strong donor-abilities surpassing those of clas-
sical phosphines and the ease of their preparation [1]. Moreover,
additional functionalities can be introduced at the nitrogen atoms
of the N-heterocyclic ring in a straightforward manner, and accord-
ingly, various donor-functionalized NHCs and their complexes
have been explored [2], some of which show hemilabile behaviour
[3]. In addition to their extensive use in catalysis, some NHC com-
plexes of Ag, Au and Pd have also been investigated for potential
biomedical applications [4–8]. Furthermore, application of NHCs
in material science [9] and supramolecular chemistry [10] are just
beginning to unfold. In view of these wide applications, we became
interested in the introduction of IR active functions in NHC com-
plexes as an additional spectroscopic probe, which may be useful
in the characterization of materials and for sensing purposes. Here-
in, we report the design of six unsymmetrical carbene precursors
functionalized with IR active cyano- and ester-groups and an initial
study on their coordination chemistry on palladium(II). Further-
more, we describe properties, molecular structures and some cata-
lytic activities of the resulting complexes.
2.1. Ligand precursors
The cyano- and ester-functionalized imidazolium bromide salts
(1a–c and 2a–c), were prepared by direct alkylation of N-mesityl-
imidazole with alkyl bromides bearing the desired functional
groups as shown in Scheme 1. The alkylating agents methyl-3-
bromopropionate and methyl-4-bromobutyrate were prepared by
esterification of the respective bromo-substituted carboxylic acids
in methanol with catalytic amount of concentrated sulfuric acid.
Formation of imidazolium salts was indicated by the presence
of a downfield chemical shift at ꢁ9.50–10.50 ppm in their 1H
NMR spectra for the proton on the C2 carbon of the heterocyclic
ring. The yield for most salts was generally quite high. The rela-
tively lower yields for salts 1b and 2b may be due to subsequent
base-catalyzed N-alkyl cleavage of the ß-cyanoethyl and ß-ester-
ethyl group from the heterocycle (vide infra) [11]. Ligand precur-
sors 1 and 2 are all solids due to the N-mesityl group. Other
known cyano- [12] or ester-functionalized [13] imidazolium salts
bearing purely N-alkyl substituents are ionic liquids at room tem-
perature. Increasing the chain length of the N-alkyl substituent
leads to more hygroscopic carbene precursors. In general, imidazo-
lium salts with ester functional groups tend to be more hygro-
scopic than their cyano counterparts, which we tentatively
ascribe to stronger interaction of the salts with water molecules
via hydrogen bonds to the oxygen of the ester function.
* Corresponding author. Tel.: +65 6516 2670; fax: +65 6779 1691.
E-mail address: chmhhv@nus.edu.sg (H.V. Huynh).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.10.044

324
H.V. Huynh, J. Wu / Journal of Organometallic Chemistry 694 (2009) 323–331
R
1a: n = 1, R = CN, 97%
n
1b: n = 2, R = CN, 67%
N
N
N
N
Toluene
1c: n = 3, R = CN, 85%
Br
n Br
+
R
2a: n = 1, R = CO2Me, 92%
2b: n = 2, R = CO2Me, 75%
2c: n = 3, R = CO2Me, 94%
70-90°C
Scheme 1. Preparation of cyano- and ester-functionalized imidazolium salts.
2.2. Synthesis of Pd–NHC complexes
for the synthesis of another two ester-functionalized Pd–NHC com-
plexes [PdBr2(meCO2MeNHC)2] (4a) and [PdBr2(propCO2MeNHC)2] (4c)
to avoid possible hydrolysis of the ester group on the functional-
ized imidazolium salt.
To study the coordination chemistry of proligands 1a–c and
2a–c, we opted for palladium(II) as a common and catalytically ac-
tive metal center in NHC chemistry. The general method for the
preparation of dihalo-bis(carbene) Pd(II) complexes by reacting
Pd(OAc)2 with 2 equiv. of imidazolium salt works well for
cyanomethyl and cyanopropyl derivatives 1a and 1c yielding 85%
and 94% of the desired products [PdBr2(meCNNHC)2] (3a) and
[PdBr2(propCNNHC)2] (3c), respectively (Method A, Scheme 2).
However, reaction of the cyanoethyl substituted imidazolium
salt 1b under similar reaction conditions gave the monocarbene
complex [PdBr2(etCNNHC)(imd)] (5) with a N-coordinated mesityl-
imidazole ligand (imd), which presumably resulted from N–C
cleavage of the ß-cyanoethyl group under the basic and relatively
harsh reaction condition [6]. Similar cleavage was observed in
the analogous reaction of the esterethyl substituted imidazolium
salt 2b resulting in complex [PdBr2(etCO2MeNHC)(imd)] (6) (Scheme
3b). The relatively high yields of these two complexes indicate that
only 1 equiv. of the precursor salts was successfully deprotonated
to give the carbene ligand, whereas the second equivalent under-
went base-catalyzed N-alkyl cleavage under elimination of HBr
and an alkene (Scheme 3a). To prevent this cleavage, the relatively
milder Ag-carbene-transfer method was explored (Method B,
Scheme 2). Indeed, this method gave the desired bis(carbene) com-
plexes, [PdBr2(etCNNHC)2] (3b) and [PdBr2(etCO2MeNHC)2] (4b), as
major products in yields of 67 and 75% with only minor amounts
of the monocarbene complexes. This method has also been applied
2.3. Characterization of Pd–NHC complexes
All Pd–NHC complexes (3–6), were fully characterized by elec-
trospray ionization (ESI) mass spectrometry, IR, 1H and 13C NMR
spectroscopies. In the ESI mass spectra of the bis(carbene) com-
plexes 3 and 4, m/z values corresponding to both [MꢀBr]+ and
[M+Na]+ ([M+NH4]+ for 4b) fragments were observed, with the lat-
ter as the base peak. The spectra for the monocarbene complexes 5
and 6, on the other hand, showed only [MꢀBr]+ fragments. The iso-
topic envelopes match with the calculated pattern as illustrated in
Fig. 1 using the zoom scan spectra of complex 3c as an example.
The cyano stretching frequencies of complexes 3 and 5 and car-
bonyl stretching frequencies of 4 and 6 measured by IR spectros-
copy are summarized in Table 1. The intensity of the C@O
stretching band is generally strong, while that of C„N is relatively
weak. In complex 3a, the C„N stretching is so weak that it could
not be distinguished from the baseline. This might be due to the
small difference in bond polarity when the C„N group is in close
proximity to the N-atom of the heterocycle. As a consequence, a
potential application of cyanomethylene-derivatives for IR sensing
purposes can be excluded.
In both 1H NMR and 13C NMR spectra for the bis(carbene)
complexes 3 and 4, two sets of signals were observed. These are
no shielding
R
R
R
R
n
n
n
n
Method A
N
N
(3a,c)
N
N
N
Br N
N
N
N
Br
Pd
Br
+
Pd
or
N
Br
Br
Method B
(3b,4a-c)
n
R
shielding
trans-anti
trans-syn
3a: n = 1, R = CN, 85%,
3b: n = 2, R = CN, 65%,
3c: n = 3, R = CN, 94%,
anti:syn = 1:4
anti:syn = 2:1
anti:syn = 3:1
4a: n = 1, R = CO2Me, 93%, anti:syn = 1:4
4b: n = 2, R = CO2Me, 89%, anti:syn = 1:1.5
4c: n = 3, R = CO2Me, 97%, anti:syn = 1.3:1
Scheme 2. Preparation of Pd(II) complexes. Method A (3a,c): 0.5 equiv. Pd(OAc)2/DMSO/90 °C. Method B (3b, 4a–c): (i) 0.5 equiv. Ag2O/CH2Cl2, (ii) 0.5 equiv. [PdBr2(MeCN)2].
All anti-syn ratios determined by 1H NMR in CDCl3 solutions.

H.V. Huynh, J. Wu / Journal of Organometallic Chemistry 694 (2009) 323–331
325
R
R
a)
b)
N
N
N
N
N
N
Br
Pd
Base, Δ
-HBr
Pd(OAc)2
N
N
Br
DMSO
90°C
Br
-
R
5: R = CN, 78%
6: R = CO2Me, 78%
Scheme 3. Base-catalyzed N-alkyl cleavage reaction (a) and preparation of Pd(II) monocarbene complexes (b).
Fig. 1. Experimental isotopic pattern for complex fragments [3cꢀBr]+ (a) and [3c+Na]+ (c) obtained by ESI-MS; simulated isotopic pattern for complex fragments [3cꢀBr]+ (b)
and [3c+Na]+ (d).
assigned to trans-anti and trans-syn isomers, which are due to a
Table 1
hindered rotation of the unsymmetrical ligand around the car-
bene–palladium bond. Notably, the isomeric 1H NMR resonances
for the NCH2 protons are well separated, which allows an easy
assignment and determination of their relative ratios for all rota-
mers of 3 and 4 (Fig. 2). In the trans-anti rotamers, the NCH2 groups
are situated in the shielding region of the aromatic mesityl substi-
tuent giving rise to distinctively more highfield shifts as compared
to those in the syn rotamers, which do not experience shielding by
the ring current (Scheme 2). Correspondingly, all bis(carbene)
complexes also show two carbene resonances ranging from 168
C„N or C@O stretching frequencies of complexes 3–6 measured as KBr pellets.
Complex
C„N stretch (cmꢀ1
)
C@O stretch (cmꢀ1
)
3a
3b
3c
4a
4b
4c
5
Too weak to detect
2253 (medium)
2246 (medium)
–
–
–
–
–
–
1757 (strong)
1725 (strong)
1734 (strong)
–
2252 (medium)
–
6
1733 (strong)

326
H.V. Huynh, J. Wu / Journal of Organometallic Chemistry 694 (2009) 323–331
Fig. 2. Variable-temperature 1H NMR spectral plot of complex 3a in d6-DMSO.
to 172 ppm, which is indicative of trans configured bis(imidazolin-
2-ylidene) complexes. The difference between these two carbene
signals is less than 0.2 ppm corroborating the presence of rotamer-
ic pairs. A separation of these rotamers was unsuccessful, which
leads to the assumption of a dynamic equilibrium between them
in solution. cis-Complexes, on the other hand, would usually give
rise to more highfield shifts of ꢁ160–165 ppm that are absent in
all spectra. The observed preference for trans isomers is most likely
due to the bulky N-mesityl substituent in all NHC ligands.
obtained by slow evaporation of a chloroform/toluene (3a), ace-
tone (3b, 4a/b, 6), acetonitrile (5), and methanol solutions (4b) or
by cooling of a hot saturated dichloromethane solution (3c). The
molecular structures of trans-anti-3c, trans-syn-4a and 5 along
with their numbering scheme and selected bond lengths and
angles are depicted in Figs. 3–5 as representatives.
All bis(carbene) complexes crystallized in a trans configuration
with two carbene and two bromo ligands coordinated to the metal
center in a nearly perfect square-planar geometry. In complexes
3a, 4a, and 4b, the two carbene ligands are orientated in a trans-
syn manner, while in complexes, 3b, 3c and 4c, a trans-anti config-
uration is adopted. In all cases, the configuration found in solid
state also represents the major isomer found by 1H NMR in solu-
tion. For all 6 bis(carbene) complexes 3 and 4, the Pd–Ccarbene bond
distances range from 2.015–2.036 Å. The monocarbene complexes
5 and 6, on the other hand, have slightly shorter Pd–Ccarbene bond
lengths ranging from 1.960–1.976 Å, which is due to the weaker
trans influence of the N-coordinated mesitylimidazole ligand as
compared to that of a NHC ligand.
In order to provide concrete evidence that the single crystals
used for crystal diffraction, and thus the determined solid-state
conformation, is representative of the crystalline bulk, X-ray pow-
der diffraction analysis was performed. Crystalline materials of 3c
were ground into fine powders, which were subjected to X-ray
powder diffraction. The experimental powder pattern agrees well
with the calculated pattern for trans-anti-3c (Fig. 6) suggesting
that only one isomeric form crystallized and isomerization occurs
upon dissolution (vide supra). To find further support for this
statement, a solution of initially pure trans-anti-3b was evapo-
rated under vacuum and the residue thus obtained was analyzed
by X-ray powder diffraction. A comparison with the calculated
data based on trans-anti-3b revealed an additional set of peaks
in the experimental data attributable to its rotamer trans-syn-3b,
which apparently has formed upon dissolution of trans-anti-3b
(Fig. 7).
In order to study this equilibrium, 1H NMR experiments and
X-ray powder diffraction (vide infra) were conducted. It was found
that pure single crystals of trans-anti-3c (vide infra) undergo isom-
erization upon dissolution in CDCl3 to give a stable anti:syn rot-
amer ratio of 3:1. Similarly, we observed that 3a gives rise to
different rotamer-ratios in different deuterated solvents. For exam-
ple, the anti:syn rotamer ratio amounts to 1:4 in CDCl3, while the
ratio is 1:12 in d6-DMSO. Apparently, the polarity of 3a rotamers
is so different that chloroform and DMSO stabilize them differ-
ently. Furthermore, variable-temperature (VT) 1H NMR spectra of
3a were recorded in d6-DMSO from 300–380 K (Fig. 2). The ratio
of the two different isomers was observed to change slowly with
increasing temperatures, which suggests that the activation barrier
for this isomerization process is relatively high. At >340 K, the sig-
nals broaden and coalescence for the NCH2 protons occurs at 380 K
allowing for an estimate of the rotational barrier as about
74 kJ molꢀ1 [14]. In general, the trans-anti configuration is fa-
voured in complexes with longer substituents. The monocarbene
complexes 5 and 6 show as expected one set of signals with equiv-
alent o-CH3 group of the N-mesityl substituents, which points to a
trans arrangement of the NHC and the imidazole ligands.
2.4. Molecular structures
Single crystals of complexes 3a ꢂ 0.5(C6H5CH3), 3b, 3c,
4a ꢂ (CH3)2CO, 4b ꢂ CH3OH, 4c,
5
and 6 ꢂ 0.25(CH3)2CO, were

H.V. Huynh, J. Wu / Journal of Organometallic Chemistry 694 (2009) 323–331
327
Fig. 5. Molecular structure of 5 showing 50% probability ellipsoids. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Pd1–C1
1.969(5), Pd1–Br1 2.4178(7), Pd1–Br2 2.4361(8), N1–C1 1.358(7), N2–C1 1.395(7),
N4–C16 1.335(7), N5–C16 1.340(7), N3–C15 1.122(8); C1–Pd1–Br1 89.59(16), C1–
Pd1–Br2 90.75(16), N4–Pd1–Br1 90.49(13), N4–Pd1–Br2 89.07(13), N1–C1–N2
104.9(5), N4–C16–N5 110.7(5).
Fig. 3. Molecular structure of trans-anti-3c showing 50% probability ellipsoids.
Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]:
Pd1–C1 2.019 (4), Pd1–Br1 2.4365(4), N1–C1 1.362(5), N2–C1 1.369(5), N3–C16
1.141(6); C1–Pd1–Br1 91.08(11), C1–Pd1–Br1A 88.92(11), N1–C1–N2 103.7(4).
Fig. 4. Molecular structure of trans-syn-4a ꢂ (CH3)2CO showing 50% probability
ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity. Selected
bond lengths [Å] and angles [°]: Pd1–C1 2.036(9), Pd1–C16 2.027(8), Pd1–Br1
2.4339(12), Pd1–Br2 2.4516(11), N1–C1 1.356(10), N2–C1 1.351(11), N3–C16
1.340(11), N4–C16 1.367(11), O2–C14 1.182(13), O4–C29 1.195(11); C1–Pd1–Br1
88.9(2), C1–Pd1–Br2 91.1(2), C16–Pd1–Br1 90.4(2), C16–Pd1–Br2 89.6(2), N1–C1–
N2 104.5(8), N3–C16–N4 104.2(7).
Fig. 6. Experimental (a) and calculated (b) X-ray powder diffraction pattern for
trans-anti-3c.
2.5. Mizoroki–Heck catalysis
Complexes of N-heterocyclic carbene ligands have been applied
in both homogeneous and heterogeneous catalysis. In particular,
Pd–NHC complexes have been widely used in the making of C–C
bonds often employing the Mizoroki–Heck and Suzuki–Miyaura
coupling reactions. A preliminary study was carried out in this work
to investigate the catalytic activities of the ester-functionalized

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H.V. Huynh, J. Wu / Journal of Organometallic Chemistry 694 (2009) 323–331
Table 2
Mizoroki–Heck Coupling reactions catalyzed by complexes 4a and 4b.a
O
O
O
Z
Z
1 mol% [Pd]
R
O
+
+
X
R
O
O
DMF, NaOAc
-HX
O
O
Br
Z
Br
1 mol% [Pd]
Z
O
O
DMF, NaOAc
-HX
Z = C, N; X = Br, Cl; R = CHO, H
Entry
Catalyst
Aryl halide
Yield (%)b
1
2
3
4
5
6
7
8
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4a
4b
4-Bromobenzaldehyde
4-Bromobenzaldehyde
1,3-Dibromobenzene
1,3-Dibromobenzene
2,6-Dibromopyridine
2,6-Dibromopyridine
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
2-Chloropyridine
>99
>99
90c
c
9
91c
93c
51c
58c
5c
9
10
11
12
13
14
2-Chloropyridine
3c
4-Chlorobenzaldehyde
4-Chlorobenzaldehyde
2-Chloropyridine
71c,d
86c,d
9c,d
8c,d
2-Chloropyridine
a
Reaction conditions generally not optimized.
b
Yields were determined by 1H NMR spectroscopy and calculated based on
average of two independent runs.
c
With addition of 1.5 equiv. of [N(n-C4H9)4]Br.
With addition of 0.01 mmol of NaOH.
d
hyde could thus be efficiently improved from 58% to 86% in the
case for 4b (Entry 8 versus Entry 12). The yield improvement for
complex 4a on the other hand was less substantial, which may
be due to the short methylene-spacer restricting an efficient coor-
dination of the carboxylato group (Entry 7 versus Entry 11). The
coupling of 2-chloropyridine, on the other hand, could not be im-
proved. Overall, the imidazolium-based NHC complexes 4a and
4b are less active than some benzimidazolium-derived NHC Pd
complexes reported by us earlier [17,18].
Fig. 7. Experimental X-ray powder diffraction pattern for precipitated 3b (a) and
calculated pattern for trans-anti-3b (b).
complexes 4a and 4b in the Mizoroki–Heck reaction. The coupling
of aryl bromides and chlorides with tert-butyl acrylate at 120 °C
in DMF with 1 mol% catalyst loading and a reaction time of 18 h
was chosen as a standard test reaction.
3. Conclusion
The results summarized in Table 2 showed that both complexes
are highly efficient in the coupling of the activated aryl bromides
achieving >90% yield (Entries 1–6). Double Mizoroki–Heck reac-
tions have also been investigated with our catalysts, as shown in
Entries 3–6. Notably, all double Mizoroki–Heck reactions pro-
ceeded smoothly even though the introduction of a vinylic substi-
tuent would deactivate the substrate for the second coupling step.
A similar coupling reaction of 1,3–dibromobenzene with tert-butyl
acrylate to give (E,E0)-di-tert-butyl-3,30-(1,3-phenylene)diacrylate
has been reported with 2 mol% Pd(OAc)2/tri-o-tolylphosphine and
higher temperature giving a lower yield of 80% [15]. In our work,
very high yields of >90% were obtained with 1 mol% of our pre-cat-
alysts as shown in Entries 3 and 4. The extension of this double
coupling protocol to the heterocyclic substrate 2,6-dibromopyri-
dine, which has been rarely studied, also led to a high yields for
the doubly-coupled product (E,E0)-di-tert-butyl-3,30-(pyridine-2,6-
diyl)diacrylate of 91% and 93%, respectively, which to the best of
our knowledge has not been reported so far. The coupling of aryl
chlorides was less successful and afforded only moderate yields
for 4-chlorobenzaldehyde (Entries 7/8) and even lower yields for
2-chloropyridine (Entries 9/10). Recently, we reported that mixed
dicarboxylato-bis(carbene)palladium(II) complexes could activate
aryl chlorides readily [16]. In an attempt to generate chelating
mixed carbene-carboxylato ligands in situ for catalysis, 1 mol% of
NaOH was added. The yield for the coupling of 4-chlorobenzalde-
A series of palladium(II) mono- and bis(carbene) complexes
bearing IR active cyano (3a–c, 5) or ester-groups (4a–c, 6) have
been prepared. It was found that N-ethylene substituents undergo
C–N bond cleavage reactions under basic/harsh conditions giving
rise to formation of monocarbene complexes. Apart from 3a, all
complexes show medium to strong IR bands, which render them
potentially useful for IR sensing purposes. Furthermore, the
trans-anti and trans-syn rotamers of all bis(carbene) complexes
have been studied in detail. A preliminary study showed that com-
plexes 4a and 4b have good catalytic activities in double Mizoroki–
Heck coupling reactions with aryl dibromides yielding diacrylates.
Further studies are underway to explore the potential of these
complexes as IR sensing probes in bio-related areas and for the elu-
cidation of reaction mechanism.
4. Experimental
4.1. General considerations
Unless otherwise noted all operations were performed without
taking precautions to exclude air and moisture. All solvents and
chemicals were used as received without any further treatment if
not noted otherwise. 1-Mesitylimidazole was prepared according

H.V. Huynh, J. Wu / Journal of Organometallic Chemistry 694 (2009) 323–331
329
to a previously reported method [19]. 1H and 13C NMR spectra were
recorded on Bruker ACF 300 spectrometer or AMX 500
4.6. Synthesis of 1-mesityl-3-(3-methylpropionate)imidazolium
bromide (2b)
a
spectrophotometer and the chemical shifts (d) were internally ref-
erenced by the residual solvent signals relative to tetramethylsilane
(1H, 13C). ESI Mass spectra were measured using a Finnigan MAT
LCQ spectrometer. Infrared spectra were measured with a Varian
3100 FT-IR spectrophotometer as KBr pellets. Elemental analyses
were performed on a Perkin–Elmer PE 2400 elemental analyzer at
the Department of Chemistry, National University of Singapore.
A solution of mesitylimidazole (373 mg, 2.0 mmol) and methyl-
3-bromopropionate (0.218 mL, 2.0 mmol) was heated in THF
(20 mL) at 75° C for 36 h. The solvent was removed under vacuum
and the residue was dissolved in dichloromethane and extracted
with dilute NaHCO3 solution. The aqueous layer was collected and
evaporated to dryness. The white solid obtained was then dissolved
in chloroform and filtered over celite. The filtrate was dried under
vacuum to obtain the desired product (529 mg, 1.50 mmol, 75%).
1H NMR (300 MHz, CDCl3): d 10.05 (s, 1H, NCHN), 8.15 (s, 1H,
CHimid), 7.13 (s, 1H, CHimid), 6.99 (s, 2H, Ar-H), 5.00 (t, 3J(H,H) = 5.8 Hz,
2H, NCH2), 3.67 (s, 3H, OCH3), 3.20 (t, 3J(H,H) = 5.7 Hz, 2H, CH2C@O),
2.32 (s, 3H, p-CH3), 2.05 (s, 6H, o-CH3). 13C{1H} NMR (125.8 MHz,
CDCl3): d 172.1 (C@O), 142.0 (NCN), 138.9, 134.9, 131.3, 130.5,
124.9, 123.4 (Ar-C), 52.9 (OCH3), 46.7 (NCH2), 35.4 (CH2C@O), 21.7
(p-CH3), 18.2 (o-CH3). ESI-MS: m/z: 273 [MꢀBr]+.
4.2. Synthesis of 1-mesityl-3-cyanomethylimidazolium bromide (1a)
A mixture of mesitylimidazole (373 mg, 2.0 mmol) and bromo-
acetonitrile (0.139 mL, 2.0 mmol) was dissolved in toluene (5 mL)
and stirred at 90 °C for 36 h. The off-white precipitate formed on
heating was then collected via filtration and washed with diethyl
ether (597 mg, 1.95 mmol, 97%). 1H NMR (500 MHz, CDCl3): d
10.22 (s, 1H, NCHN), 8.43 (s, 1H, CHimid), 7.27 (s, 1H, CHimid), 7.00
(s, 2H, Ar-H), 6.47 (s, 2H, NCH2), 2.34 (s, 3H, p-CH3), 2.09 (s, 6H,
o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d 142.3 (NCN), 138.9,
134.9, 131.1, 130.6, 124.8, 124.3 (Ar-C), 114.3 (CN), 39.3 (NCH2),
21.8 (p-CH3), 18.5 (o-CH3). ESI-MS: m/z 226 [MꢀBr]+.
4.7. Synthesis of 1-mesityl-3-(4-methylbutyrate)imidazolium
bromide (2c)
A solution of mesitylimidazole (373 mg, 2.0 mmol) and methyl-
4-bromobutyrate (0.252 mL, 2.0 mmol) was heated in THF (20 mL)
at 75 °C for 36 h. The solvent was removed under vacuum and the
residue was stirred in diethyl ether for 4 h. The diethyl ether was
decanted and the insoluble product was dried under vacuum to af-
ford a hygroscopic solid (682 mg, 1.87 mmol, 94%). 1H NMR
(500 MHz, CDCl3): d 10.17 (s, 1H, NCHN), 8.07 (s, 1H, CHimid),
7.21 (s, 1H, CHimid), 6.91 (s, 2H, Ar-H), 4.73 (t, 3J(H,H) = 5.8 Hz,
2H, NCH2), 3,57 (s, 3H, OCH3), 2.45 (t, 3J(H,H) = 5.7 Hz, 2H,
CH2C@O), 2.27 (m, 2H, CH2CH2CH2), 2.26 (s, 3H, p-CH3), 1.99 (s,
6H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d 173.2 (C@O), 141.7
(NCN), 138.2, 134.7, 131.2, 130.3, 124.2, 123.9 (Ar-C), 52.4
(OCH3), 49.7 (NCH2), 30.9 (CH2C@O), 26.4 (CH2CH2CH2), 21.6 (p-
CH3), 18.1 (o-CH3). ESI-MS: m/z: 287 [MꢀBr]+.
4.3. Synthesis of 1-mesityl-3-(2-cyanoethyl)imidazolium bromide (1b)
A mixture of mesitylimidazole (373 mg, 2.0 mmol) and 3-bro-
mopropionitrile (0.167 mL, 2.0 mmol) was heated in toluene
(5 mL) at 75 °C for 6 h. The off-white product was collected via fil-
tration and then purified by column chromatography (429 mg,
1.34 mmol, 67%). 1H NMR (300 MHz, CDCl3): d 10.05 (s, 1H, NCHN),
8.36 (s, 1H, CHimid), 7.21 (s, 1H, CHimid), 7.01 (s, 2H, Ar-H), 5.14
(t, 3J(H,H) = 6.2 Hz, 2H, NCH2), 3.48 (t, 3J(H,H) = 6.2 Hz, 2H, CH2CN),
2.36 (s, 3H, p-CH3), 2.06 (s, 6H, o-CH3). 13C{1H} NMR (75.5 MHz,
CDCl3): 142.3 (NCN), 138.6, 135.0, 131.2, 130.6, 124.8, 124.1
(Ar-C), 117.4 (CN), 46.7 (NCH2), 21.8 (CH2CN), 21.2 (p-CH3), 18.3
(o-CH3). ESI-MS: m/z 240 [MꢀBr]+.
4.4. Synthesis of 1-mesityl-3-(3-cyanopropyl)imidazolium bromide
(1c)
4.8. Method A: synthesis of bis(carbene) complexes 3a, 3c and mono-
carbene complexes 5 and 6
A mixture of mesitylimidazole (373 mg, 2.0 mmol) and 4-bro-
mobutyronitrile (0.200 mL, 2.0 mmol) was heated in toluene
(5 mL) at 90 °C for 48 h. After removing toluene under vacuum,
the sticky oil remained was stirred in THF overnight to give a hygro-
scopic white powder (569 mg, 1.7 mmol, 85%). 1H NMR (300 MHz,
CDCl3): d 10.19 (s, 1H, NCHN), 8.17 (s, 1H, CHimid), 7.18 (s, 1H,
CHimid), 6.99 (s, 2H, Ar-H), 4.93 (t, 3J(H,H) = 7.0 Hz, 2H, NCH2), 2.75
(t, 3J(H,H) = 6.8 Hz, 2H, CH2CN), 2.48 (m, 2H, CH2CH2CH2), 2.32
(s, 3H, p-CH3), 2.07 (s, 6H, o-CH3). 13C{1H} NMR (75.5 MHz, CDCl3):
d 141.7 (NCN), 138.1, 134.7, 131.1, 130.4, 124.7, 123.9 (Ar-C), 119.3
(CN), 49.1 (NCH2), 27.0 (CH2CN), 21.6 (CH2CH2CH2), 18.2 (o-CH3),
14.9 (p-CH3). ESI-MS: m/z 254 [MꢀBr]+.
A mixture of Pd(OAc)2 (68 mg, 0.3 mmol) and appropriate
amount of imidazolium salt (0.6 mmol) were heated at 90 °C for
12 h in DMSO (5 mL). The reaction mixture was filtered over celite
and the DMSO of the filtrate was removed via vacuum distillation.
The residue was then dissolved in dichloromethane (25 mL) and
extracted with water (3 ꢃ 25 mL). The organic layer was collected,
dried over Na2SO4 and the solvent was removed under vacuum to
give a pale yellow solid.
4.9. Method B: synthesis of bis(carbene) complexes 3b and 4a–c
Appropriate amount of imidazolium salt (0.6 mmol) and silver
oxide (70 mg, 0.3 mmol) were suspended in dichloromethane
and stirred at ambient temperature for 12 h shielded from light.
The mixture was then filtered into an acetonitrile solution of
[PdBr2(CH3CN)2]. After stirring at ambient temperature for 12 h,
the reaction mixture was filtered over celite and the solvent was
removed under vacuum to give pale yellow powders.
4.5. Synthesis of 1-mesityl-3-(methylacetyl)imidazolium bromide (2a)
A mixture of mesitylimidazole (373 mg, 2.0 mmol) and methyl-
bromoacetate (0.139 mL, 2.0 mmol) were dissolved in toluene
(5 mL) and stirred at 90° C for 18 h. The off-white precipitate
formed was filtered, washed with diethyl ether and dried under
vacuum (0.627 g, 1.85 mmol, 92%). 1H NMR (500 MHz, CDCl3): d
9.99 (s, 1H, NCHN), 8.06 (s, 1H, CHimid), 7.17 (s, 1H, CHimid), 7.00
(s, 2H, Ar-H), 5.94 (s, 2H, NCH2), 3.82 (s, 3H, OCH3), 2.33 (s, 3H,
p-CH3), 2.09 (s, 6H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d
167.7 (C@O), 142.1 (NCN) 139.7, 135.0, 131.3, 130.5, 125.5, 123.1
(Ar-C), 54.1 (NCH2), 51.4 (OCH3), 21.8 (p-CH3), 18.2 (o-CH3).
ESI-MS: m/z 259 [MꢀBr]+.
4.10. trans-Dibromo-bis(1-mesityl-3-cyanomethylimidazolin-2-
ylidene)palladium(II) (3a)
Yield for both rotamers: 183 mg (0.255 mmol, 85%). trans-syn-
3a: 1H NMR (500 MHz, CDCl3): d 7.20 (d, 3J(H,H) = 1.9 Hz, 2H,
CHimid), 6.87 (s, 4H, Ar-H), 6.82 (d, 3J(H,H) = 1.9 Hz, 2H, CHimid),
5.71 (s, 4H, NCH2), 2.47 (s, 6H, p-CH3), 1.92 (s, 12H, o-CH3).

330
H.V. Huynh, J. Wu / Journal of Organometallic Chemistry 694 (2009) 323–331
13C{1H} NMR (126 MHz, CDCl3): d 172.1 (NCN), 138.9, 136.4, 135.4,
129.8, 125.1, 121.5 (Ar-C), 115.1 (CN), 40.2 (NCH2), 21.9 (p-CH3),
19.9 (o-CH3). trans-anti-3a: 1H NMR (500 MHz, CDCl3): d 7.22
(d, 3J(H,H) = 1.9 Hz, 2H, CHimid), 7.03 (s, 4H, Ar-H), 6.90
(d, 3J(H,H) = 1.9 Hz, 2H, CHimid), 5.18 (s, 4H, NCH2), 2.39 (s, 6H,
p-CH3), 2.23 (s, 12H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d
172.2 (NCN), 140.1, 136.9, 135.6, 129.8, 125.3, 121.0 (Ar-C), 114.4
(CN), 39.4 (NCH2), 21.8 (p-CH3), 20.3 (o-CH3). Anal. Calc. for
C28H30Br2N6Pd: C, 46.92; H, 4.22; N, 11.72. Found: C, 47.20; H,
4.27; N, 11.73%. ESI-MS: m/z = 637 [MꢀBr]+, 739 [M+Na]+.
(OCH3), 53.1 (NCH2), 21.9 (p-CH3), 19.9 (o-CH3). trans-anti-4a: 1H
NMR (500 MHz, CDCl3): d 7.07 (d, 3J(H,H) = 1.9 Hz, 2H, CHimid),
6.98 (s, 4H, Ar-H), 6.80 (d, 3J(H,H) = 1.9 Hz, 2H, CHimid), 5.07
(s, 4H, NCH2), 3.73 (s, 6H, OCH3), 2.37 (s, 6H, p-CH3), 2.24
(s, 12H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d 171.9 (C@O),
169.1 (NCN), 139.2, 137.0, 136.1, 129.5, 124.1, 122.8 (Ar-C), 53.2
(OCH3), 52.5 (NCH2), 21.8 (p-CH3), 20.4 (o-CH3). Anal. Calc. for
C30H36Br2N4O4Pd: C, 46.03; H, 4.64; N, 7.16. Found: C, 45.75; H,
4.71; N, 6.99%. ESI-MS: m/z = 703 [MꢀBr]+, 805 [M+Na]+. FT-IR
ꢀ1
~
m
(KBr pellet):
(C@O) 1757 cm (s).
4.11. trans-Dibromo-bis[1-mesityl-3-(2-cyanoethyl)imidazolin-2-
4.14. trans-Dibromo-bis[1-mesityl-3-(3-methylpropionate) imida-
ylidene]palladium(II) (3b)
zolin-2-ylidene]palladium(II) (4b)
Yield for both rotamers: 146 mg (0.196 mmol, 65%). trans-anti-
3b: 1H NMR (500 MHz, CDCl3): d 7.09 (d, 3J(H,H) = 1.9 Hz, 2H,
CHimid), 7.01 (s, 4H, Ar-H), 6.81 (d, 3J(H,H) = 1.9 Hz, 2H, CHimid),
4.47 (t, 3J(H,H) = 7.0 Hz, 4H, NCH2), 2.90 (t, 3J(H,H) = 7.0 Hz, 4H,
CH2CN), 2.37 (s, 6H, p-CH3), 2.21 (s, 12H, o-CH3). 13C{1H} NMR
(126 MHz, CDCl3): d 171.3 (NCN), 139.8, 137.1, 136.0, 129.7,
124.2, 122.4 (Ar-C) 118.0 (CN), 47.4 (NCH2), 21.8 (p-CH3), 20.4
(CH2CN), 19.8 (o-CH3). trans-syn-3b: 1H NMR (500 MHz, CDCl3): d
7.15 (d, 3J(H,H) = 1.9 Hz, 2H, CHimid), 6.83 (s, 4H, Ar-H), 6.74
(d, 3J(H,H) = 1.9 Hz, 2H, CHimid), 4.92 (t, 3J(H,H) = 7.0 Hz, 4H,
NCH2), 3.44 (t, 3J(H,H) = 7.0 Hz, 4H, CH2CN), 2.47 (s, 6H, p-CH3),
1.92 (s, 12H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d 171.1
(NCN), 138.7, 136.4, 135.5, 129.8, 124.7, 122.0 (Ar-C), 118.3 (CN),
47.6 (NCH2), 22.0 (p-CH3), 20.6 (CH2CN), 20.0 (o-CH3). Anal. Calc.
for C32H40Br2N4O4Pd: C, 47.40; H, 4.97; N, 6.91. Found: C, 45.80;
H, 5.07; N, 6.85% (the unsuccessful attempts to obtain better
values were attributed to the general lower thermal stability of
N-ethylene substituents.). ESI-MS: m/z = 665 [MꢀBr]+, 767
Yield for both rotamers: 217 mg (0.268 mmol, 89%). trans-syn-
4b: 1H NMR (500 MHz, CDCl3): d 7.09 (d, 3J(H,H) = 1.9 Hz, 2H,
CHimid), 6.82 (s, 4H, Ar-H), 6.65 (d, 3J(H,H) = 1.9 Hz, 2H, CHimid),
4.93 (t, 3J(H,H) = 7.0 Hz, 4H, NCH2), 3.72 (s, 6H, OCH3), 3.33
(t, 3J(H,H) = 7.0 Hz, 4H, CH2C@O), 2.45 (s, 6H, p-CH3), 1.92 (s,
12H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d 172.8 (C@O),
170.7 (NCN) 138.3, 136.6, 136.0, 129.6, 123.7, 122.4 (Ar-C), 52.6
(OCH3), 47.4 (NCH2), 36.2 (CH2C@O), 21.9 (p-CH3), 20.0 (o-CH3).
trans-anti-4b: 1H NMR (500 MHz, CDCl3): d 7.09 (d, 3J(H,H) =
1.9 Hz, 2H, CHimid), 6.96 (s, 4H, Ar-H), 6.70 (d, 3J(H,H) = 1.9 Hz,
2H, CHimid), 4.50 (t, 3J(H,H) = 7.0 Hz, 4H, NCH2), 3.69 (s, 6H,
OCH3), 2.86 (t, 3J(H,H) = 7.0 Hz, 4H, CH2C@O), 2.34 (s, 6H, p-CH3),
2.22 (s, 12H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d 173.1
(C@O), 170.7 (NCN), 139.2, 137.2, 136.4, 129.4, 123.2, 123.0
(Ar-C), 52.4 (OCH3), 47.0 (NCH2), 35.6 (CH2C@O), 21.7 (p-CH3),
20.4 (o-CH3). Anal. Calc. for C30H34Br2N6Pd: C, 48.38; H, 4.60; N,
11.28. Found: C, 49.14; H, 5.31; N, 11.42% (the unsuccessful
attempts to obtain better values were attributed to the general
lower thermal stability of N-ethylene substituents.). ESI-MS:
~
+
ꢀ1
~
m (CN) 2253 cm (m).
[M+Na] . FT-IR (KBr pellet):
m/z = 731 [MꢀBr]+, 828 [M+NH4]+. FT-IR (KBr pellet):
m (C@O)
4.12. trans-Dibromo-bis[1-mesityl-3-(3-cyanopropyl)imidazolin-2-
1725 cmꢀ1 (s).
ylidene]palladium(II) (3c)
4.15. trans-Dibromo-bis[1-mesityl-3-(4-methylbutyrate)imidazolin-
Yield for both rotamers: 217 mg (0.280 mmol, 94%). trans-anti-
2-ylidene]palladium(II) (4c)
3c: 1H NMR (500 MHz, CDCl3):
d 7.00 (s, 4H, Ar-H), 6.99
(d, 3J(H,H) = 2.5 Hz, 2H, CHimid), 6.79 (d, 3J(H,H) = 1.9 Hz, 2H,
CHimid), 4.36 (t, 3J(H,H) = 7.0 Hz, 4H, NCH2), 2.39 (s, 6H, p-CH3),
2.22 (s, 12H, o-CH3), 2.22 (m, 4H, CH2CH2CH2), 2.08 (t, 3J(H,H) =
7.0 Hz, 4H, CH2CN). 13C{1H} NMR (126 MHz, CDCl3): d 170.4
(NCN), 139.6, 137.4, 136.4, 129.5, 124.0, 122.5 (Ar-C), 119.8 (CN),
50.0 (NCH2), 26.6 (CH2CN), 21.9 (p-CH3), 20.4 (o-CH3), 15.2
(CH2CH2CH2). trans-syn-3c: 1H NMR (500 MHz, CDCl3): 7.02
(d, 3J(H,H) = 2.5 Hz, 2H, CHimid), 6.83 (s, 4H, Ar-C), 6.72
(d, 3J(H,H) = 1.9 Hz, 2H, CHimid), 4.77 (t, 3J(H,H) = 7.0 Hz, 4H,
NCH2), 2.62 (m, 4H, CH2CH2CH2), 2.50 (t, 3J(H,H) = 7.0 Hz, 4H,
CH2CN), 2.47 (s, 6H, p-CH3), 1.90 (s, 12H, o-CH3). 13C{1H} NMR
(126 MHz, CDCl3): d 170.2 (NCN), 138.4, 137.4, 136.0, 129.8,
124.5, 121.9 (Ar-C), 119.8 (CN), 50.0 (NCH2), 26.9 (CH2CN), 21.8
(p-CH3), 19.9 (o-CH3), 15.0 (CH2CH2CH2). Anal. Calc. for
C32H38Br2N6Pd: C, 49.73; H, 4.96; N, 10.87. Found: C, 49.48; H,
4.97; N, 10.68%. ESI-MS: m/z = 693 [MꢀBr]+, 795 [M+Na]+. FT-IR
Yield for both rotamers: 217 mg (0.290 mmol, 97%). trans-anti-
4c: 1H NMR (500 MHz, CDCl3):
d 6.97 (s, 4H, Ar-H), 6.95
(d, 3J(H,H) = 1.9 Hz, 2H, CHimid), 6.74 (d, 3J(H,H) = 1.9 Hz, 2H,
CHimid), 4.27 (t, 3J(H,H) = 6.9 Hz, 4H, NCH2), 3.71 (s, 6H, OCH3),
2.49 (t, 3J(H,H) = 6.9 Hz, 4H, CH2CO), 2.35 (s, 6H, p-CH3), 2.23 (s,
12H, o-CH3), 2.15 (m, 4H, CH2CH2CH2). 13C{1H} NMR (126 MHz,
CDCl3): d 174.2 (C@O), 170.4 (NCN), 139.2, 137.3, 136.5, 129.5,
123.7, 121.8 (Ar-C), 52.3 (OCH3), 50.7 (NCH2), 31.4 (CH2C@O),
26.3 (CH2CH2CH2), 21.9 (p-CH3), 20.4 (o-CH3). trans-syn-4c: 1H
NMR (500 MHz, CDCl3): d 6.98 (d, 3J(H,H) = 1.9 Hz, 2H, CHimid),
6.83 (s, 4H, Ar-H), 6.68 (d, 3J(H,H) = 1.9 Hz, 2H, CHimid), 4.69
(t, 3J(H,H) = 6.9 Hz, 4H, NCH2), 3.71 (s, 6H, OCH3), 2.50
(t, 3J(H,H) = 6.9 Hz, 4H, CH2CO), 2.46 (s, 6H, p-CH3), 2.16 (m, 4H,
CH2CH2CH2), 1.92 (s, 12H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3):
d 174.1 (C@O), 170.3 (NCN), 138.2, 136.6, 136.1, 129.6, 123.9, 121.6
(Ar-C), 52.4 (OCH3), 50.9 (NCH2), 31.3 (CH2C@O), 26.5
(CH2CH2CH2), 21.7 (p-CH3), 20.0 (o-CH3). Anal. Calc. for
C34H44Br2N4O4Pd: C, 48.68; H, 5.29; N, 6.68. Found: C, 48.80; H,
5.07; N, 6.85%. ESI-MS: m/z = 759 [MꢀBr]+, 862 [M+Na]+. FT-IR
ꢀ1
~
m
(KBr pellet):
(CN) 2246 cm (m).
4.13. trans-Dibromo-bis[1-mesityl-3-(methylacetyl)imidazolin-2-
ylidene]palladium(II) (4a)
ꢀ1
~
m
(KBr pellet):
(C@O) 1734 cm (s).
Yield for both rotamers: 218 mg (0.278 mmol, 93%). trans-syn-
4a: 1H NMR (500 MHz, CDCl3): d 7.10 (d, 3J(H,H) = 1.9 Hz, 2H,
CHimid), 6.84 (s, 4H, Ar-H), 6.74 (d, 3J(H,H) = 1.9 Hz, 2H, CHimid),
5.45 (s, 4H, NCH2), 3.84 (s, 6H, OCH3), 2.45 (s, 6H, p-CH3), 1.93 (s,
12H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d 171.8 (C@O),
169.2 (NCN), 138.4, 136.6, 135.8, 129.6, 124.1, 122.9 (Ar-C), 53.4
4.16. trans-Dibromo-(mesitylimidazole)(1-mesityl-3-cyano-
ethylimidazolin-2-ylidene)palladium(II) (5)
Yield: 0.162 mg (0.234 mmol, 78%). 1H NMR (500 MHz, CDCl3):
d 8.02 (s, 1H, CHimid), 7.66 (s, 1H, CHimid), 7.33 (s, 1H, CHimid), 6.99
(s, 2H, Ar-H), 6.92 (s, 2H, Ar-H), 6.90 (s, 1H, CHimid), 6.70 (s, 1H,

H.V. Huynh, J. Wu / Journal of Organometallic Chemistry 694 (2009) 323–331
331
CHimid), 5.06 (t, 3J(H,H) = 7.0 Hz, 2H, NCH2), 3.46 (t, 3J(H,H) = 7.0 Hz,
Acknowledgements
2H, CH2CN), 2.34 (s, 3H, p-CH3), 2.30 (s, 3H, p-CH3), 2.28 (s, 6H,
o-CH3), 1.94 (s, 6H, o-CH3). 13C{1H} NMR (126 MHz, CDCl3): d
153.1 (NCN), 141.2, 140.0, 139.9, 136.9, 135.6, 135.2, 133.2,
130.7, 130.0, 129.8, 125.8, 123.0, 120.2 (Ar-C) 118.4 (CN), 47.9
(NCH2), 21.8 (p-CH3), 21.6 (p-CH3), 20.5 (o-CH3), 20.3 (CH2CN),
18.2 (o-CH3). Anal. Calc. for C27H31Br2N5Pd: C, 46.88; H, 4.52; N,
10.12. Found: C, 46.93; H, 4.62; N, 10.34%. ESI-MS: m/z = 612
The authors would like to thank the National University of Sin-
gapore for financial support (Grant No. R 143-000-327-133) and
especially Ms. Geok Kheng Tan, Ms Su Fen Woo and Prof. Lip Lin
Koh for determining the X-ray molecular structures.
Appendix A. Supplementary material
+
ꢀ1
~
[MꢀBr] . FT-IR (KBr pellet):
m (CN) 2252 cm (m).
CCDC 699197, 699198, 699199, 699200, 699201, 699202,
699203 and 699204 contain the supplementary crystallographic
data for 5, 6 ꢂ 0.25(CH3)2CO, 3b, 3c, 4c, 3a ꢂ 0.5(C6H5CH3),
4a ꢂ (CH3)2CO, and 4b ꢂ CH3OH. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
(Synthesis and analytical data of bromo-esters; analytical data
for double Mizoroki-Heck coupling products; ORTEP plots with se-
lected bond lengths and angles of complexes trans-syn-3a, trans-
anti-3b, trans-syn-4b, trans-anti-4c and 6; selected crystallographic
data for all complexes) associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2008.10.044.
4.17. trans-Dibromo-(mesitylimidazole)[1-mesityl-3-(3-methyl-
propionate)imidazolin-2-ylidene]palladium(II) (6)
Yield: 0.162 mg (0.234 mmol, 78%). 1H NMR (500 MHz, CDCl3):
d 8.04 (s, 1H, CHimid), 7.68 (s, 1H, CHimid), 7.29 (s, 1H, CHimid), 6.98
(s, 2H, Ar-H), 6.91 (s, 2H, Ar-H), 6.81 (s, 1H, CHimid), 6.68 (s, 1H,
CHimid), 5.06 (t, 3J(H,H) = 7.0 Hz, 2H, NCH2), 3.73 (s, 3H, OCH3),
3.37 (t, 3J(H,H) = 7.0 Hz, 2H, CH2C@O), 2.33 (s, 3H, p-CH3), 2.30 (s,
3H, p-CH3a), 2.27 (s, 6H, o-CH3), 1.95 (s, 6H, o-CH3). 13C{1H} NMR
(126 MHz, CDCl3): d 173.0 (C@O), 151.7 (NCN), 141.3, 140.0,
139.6, 137.1, 135.7, 133.4, 130.8, 130.6, 130.0, 129.8, 125.1,
123.5, 120.1 (Ar-C), 52.6 (OCH3), 47.7 (NCH2), 35.7 (CH2C@O),
21.8 (p-CH3), 21.7 (p-CH3), 20.6 (o-CH3), 18.3 (o-CH3). Anal. Calc.
for C28H34Br2N4O2Pd: C, 46.40; H, 4.73; N, 7.73. Found: C, 45.98;
H, 4.69; N, 7.54%. ESI-MS: m/z = 645 [MꢀBr]+. FT-IR (KBr pellet):
References
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ꢀ1
~
m
(C@O) 1733 cm (s).
4.18. General procedure for the Mizoroki–Heck coupling
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In a typical run, a reaction tube was charged with a mixture of
aryl halide (1.0 mmol for monohalides, 0.5 mmol for dihalides),
anhydrous sodium acetate (1.5 mmol), tert-butyl acrylate
(1.4 mmol), catalyst (0.01 mmol) and DMF (3 mL). The reaction
was stirred and heated at 120 °C for 18 h. After the mixture was
cooled to the ambient temperature, dichloromethane (10 mL)
was added. The organic layer was then extracted with water
(6 ꢃ 8 mL) and dried over NaSO4. The solvent was allowed to evap-
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4.19. X-ray diffraction studies
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[14] The following approximate equation was used for the calculation of
D
Gà:
Diffraction data for complexes 3–6 were collected with a Bruker
AXS APEX CCD diffractometer equipped with a rotation anode at
D
Gà = RTc(22.96 + ln (Tc/d
m
)) (J molꢀ1) with R = 8.314 J molꢀ1 Kꢀ1, Tc = 380 K
and d
m = 265 Hz, according to: H. Günther, NMR Spectroscopy, Georg Thieme
223(2) K using graphite monochromated Mo
Ka radiation
Verlag, Stuttgart, Germany, 1973.
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(k = 0.71073 Å). Data were collected over the full sphere and were
corrected for absorption. Structure solutions were found by the
Patterson method. Structure refinement was carried out by full-
matrix least squares on F2 using SHELXL-97 [20] with first isotropic
and later anisotropic displacement parameters for all non-hydro-
gen atoms. A summary of the most important crystallographic data
is given in the Supplementary material.
[20] G.M. Sheldrick, SHELXL-97, Universität Göttingen, Germany, 1997.