Syntheses and catalytic activities of Pd(II) dicarbene and hetero-dicarbene complexes

Article abstract of DOI:10.1016/j.jorganchem.2011.07.018

A series of palladium(II) complexes (1-6) bearing cis-chelating homo-dicarbene ligands with varying alkyl bridges (C1-C3) and N-heterocyclic backbones (imidazole and benzimidazole) have been synthesized by reaction of Pd(OAc)₂ with the respective diazolium bromides (A·2HBr - F·2HBr) in DMSO. A comparative catalytic study employing aryl chlorides in the Mizoroki-Heck reaction revealed the superiority of methylene- and propylene-bridged dibenzimidazolin-2-ylidenes over their imidazole-derived analogues. Based on these results, two new propylene-bridged hetero-dicarbene complexes (7 and 8) were designed containing a mixed benzimidazole/imidazole- derived NHC-donor set. Notably, both complexes outperformed their homo-dicarbene analogues, which may be due to the electronic asymmetry induced by hetero-dicarbene ligands. The molecular structures of complex 6 and 8 are also presented.

Full text of DOI:10.1016/j.jorganchem.2011.07.018

Journal of Organometallic Chemistry 696 (2011) 3369e3375
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Syntheses and catalytic activities of Pd(II) dicarbene
and hetero-dicarbene complexes
*
Han Vinh Huynh , Ramasamy Jothibasu
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:
A series of palladium(II) complexes (1e6) bearing cis-chelating homo-dicarbene ligands with varying
alkyl bridges (C1-C3) and N-heterocyclic backbones (imidazole and benzimidazole) have been synthe-
sized by reaction of Pd(OAc)2 with the respective diazolium bromides (A$2HBr e F$2HBr) in DMSO. A
comparative catalytic study employing aryl chlorides in the MizorokieHeck reaction revealed the
superiority of methylene- and propylene-bridged dibenzimidazolin-2-ylidenes over their imidazole-
derived analogues. Based on these results, two new propylene-bridged hetero-dicarbene complexes (7
and 8) were designed containing a mixed benzimidazole/imidazole-derived NHC-donor set. Notably,
both complexes outperformed their homo-dicarbene analogues, which may be due to the electronic
asymmetry induced by hetero-dicarbene ligands. The molecular structures of complex 6 and 8 are also
presented.
Received 18 May 2011
Received in revised form
6 July 2011
Accepted 15 July 2011
Keywords:
N-Heterocyclic carbene
Hetero-dicarbene
Palladium
MizorokieHeck coupling
Electronic asymmetry
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
the spacer-length and the nature of the carbene unit has not been
reported yet. Herein, we report the synthesis and characterization
N-heterocyclic carbenes (NHCs) have become ubiquitous ligands
in organometallic chemistry [1,2]. Among the few basic types,
NHCs derived from imidazole are dominating this area, although
benzimidazolin-2-ylidenes are becoming increasingly popular.
The latter are slightly weaker electron-donors due to the negative
inductive (-I) effect arising from benzannulation [3]. Nevertheless,
benzimidazole-derived NHCs offer the distinct advantage of a more
straightforward complexation to metals, which can be explained by
the presence of only one acidic proton in their benzimidazolium
precursors [4]. Imidazolium rings, on the other hand, have three
competing acidic protons at C2, C4 and C5, which can lead to
complicated product mixtures containing classical and mesoionic
(abnormal) NHCs [5], particularly when bulky N-substituents are
present. Palladium complexes of both types have also been repor-
ted to be highly active catalysts for the MizorokieHeck reaction
[6,7]. Similarly, chelating ligands are believed to provide additional
catalyst stability and therefore increasing the conversion under the
relatively harsh conditions usually applied in the Heck protocol.
Several studies have appeared that describe applications of bridged
dicarbene Pd(II) complexes in such CeC couplings [8e13], but to
the best of our knowledge, a detailed and combined comparison on
of Pd(II) complexes bearing cis-chelating homo- and hetero-
dicarbene ligands of C1-C3 spacers and a comparison of their
catalytic activities in the MizorokieHeck reaction of aryl bromides
and chlorides.
2. Results and discussion
2.1. Syntheses, characterizations and catalytic activities of homo-
dicarbene Pd(II) complexes
The Pd(II) complexes 1e6 bearing cis-chelating symmetrical
dicarbene ligands derived from imidazole and benzimidazole have
been routinely synthesized by heating the respective diazolium
bromides A$2HBr e F$2HBr with 1 equiv Pd(OAc)2 in wet DMSO
(Scheme 1). The methylene-bridged dicarbene complexes 1 and 4
[14] (85e90% yield) and their properties have been reported
previously. In general, the ethylene-bridged palladium complexes 2
and 5 were obtained at lower yield (30e50%) as compared to their
propylene-bridged analogues (80e90%) 3 and 6. The difficulty of
converting ethylene-bridged diazolium halides to the correspond-
ing chelating metaledicarbene complexes has been noted previ-
ously [8,15,16]. Propylene-bridged dicarbenes on the other hand
can coordinate to metal centres with ease as we have already
demonstrated with Ni(II) [16]. All Pd(II) complexes 1e6 have been
obtained as air- and moisture-stable yellow solids. Their formation
* Corresponding author. Tel.: þ65 6516 2670; fax: þ65 6779 1691.
E-mail address: chmhhv@nus.edu.sg (H.V. Huynh).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.07.018

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H.V. Huynh, R. Jothibasu / Journal of Organometallic Chemistry 696 (2011) 3369e3375
spacers [18,14] whereas that in complex 6 amounts to 76ꢀ. The
(CH2)n
Br
Br
(CH2)n
PdeC and PdeBr bonds of 1.986(6) Å and 2.4832(8) Å, respectively,
are non-exceptional and as expected slightly longer than those
observed for its direct Ni(II) analogue [16].
N
N
N
N
Pd(OAc)2
N
N
DMSO
n = 1-3
N
N
Pd
Br
Br
A·2HBr – F·2HBr
1-6
Complexes 1e6 have been comparatively studied for their
catalytic activities to discern any influences brought across by (i)
the length of the bridging unit and (ii) the nature of the NHC
moiety. For this purpose, the MizorokieHeck coupling reaction of
4-bromobenzaldehyde with t-butyl acrylate at 0.5 mol% catalyst
loading was initially chosen as a standard test reaction. However,
this substrate turned out to be unsuitable for this study, since all 6
complexes gave quantitative conversion. The use of the more
difficult substrate 4-chlorobenzaldehyde finally allowed for
a comparison of the complexes, and the results are summarized in
Table 1. Surprisingly imidazole- and benzimidazole-derived dicar-
bene complexes showed a different trend. For example, while
ethylene-bridged 2 performed best among the diimidazolin-2-
ylidene complexes (entry 2), its benzannulated analogue 5 gave
the poorest results (entry 5). On the other hand, the propylene-
bridged complex 3 was the poorest in the imidazole-derived
series (entry 3), although its dibenzimidazolin-2-ylidene
analogue turned out to be overall the best precatalyst (entry 6).
In most cases, benzimidazole-derived diNHC complexes were
superior. A possible explanation for this observation may be the
absence of any acidic protons on the benzimidazole heterocycle.
Imidazole-derived NHCs on the other hand contain acidic protons
at C4/5, which may interfere with bases or active intermediates
providing for additional catalyst decomposition pathways.
Furthermore, small amounts of Pd black were only observed for
ethylene-bridged complexes 2 and 5 pointing to their generally
lower stability under the harsh reaction conditions (Table 2).
N
N
N
N
N
N
N
Br
N
N
N
N
N
Pd
Pd
Pd
Br
Br
Br
Br
Br
[PdBr2(A)] (1)
[PdBr2(B)] (2)
[PdBr2(C)] (3)
N
N
N
N
N
N
N
N
N
N
N
N
Pd
Pd
Pd
Br
Br
Br
Br
Br
Br
[PdBr2(D)] (4)
[PdBr2(E)] (5)
[PdBr2(F)] (6)
Scheme 1. Synthesis of Pd(II)-diNHC complexes 1e6.
as carbene complexes is generally indicated by the absence of the
downfield signal (w9e10 ppm) typical for their precursors in the
1H NMR spectra. In addition, it was noted that all methylene
protons of the bridges become diastereotopic as a result of ligand
coordination. The observed broadening of these resonances has
been attributed to the fluxionality of the seven- (2 and 5) or eight-
membered (3 and 6) palladacycles. The 13C signal for the carbene
carbon in complex 2 was observed at 156.9 ppm, which is in the
range observed for other ethylene-bridged analogues [8]. Due to
the poor solubility of the other complexes (3, 5 and 6), either their
13C NMR spectra could not be obtained or the carbene signal could
not be resolved despite prolonged acquisition time.
Single crystals suitable for X-ray diffraction analyses of complex
6 were obtained by slow diffusion of diethyl ether into a saturated
DMSO solution. The asymmetric unit cell consists of two inde-
pendent half molecules of the complex and a diethyl ether lying on
the mirror plane. The solid state structure of only one independent
molecule of 6 is depicted in Fig. 1 as a representative, which shows
a square-planar Pd(II) centre coordinated by a cis-chelating dicar-
bene ligand and two terminal bromido ligands balancing the
charges. The bite angle of 85.7(3)ꢀ is almost the same as that
observed for methylene-bridged analogues indicating that the
length of alkyl bridges cannot be used to alter the bite angle [17]. On
the other hand, the bridging units affect the dihedral angle between
the coordination and the carbene ring plane to a great extent. For
example, small angles ranging from 39ꢀ to 54ꢀ were reported for
Pd(II) complexes of dibenzimidazolin-2-ylidenes with methylene
2.2. Syntheses, characterizations and catalytic activities of hetero-
dicarbene Pd(II) complexes
Having identified the propylene-bridged dibenzimidazolin-2-
ylidene complex 6 as the best homo-dicarbene Pd(II) precatalyst,
we turned our attention to hetero-dicarbenes with propylene
spacers as new ligand systems that contain two different NHC units.
An obvious choice would be to combine imidazole- and
benzimidazole-derived NHCs in one diNHC ligand. Previously, we
have already reported a new methodology for the preparation of
hetero-bis(carbene) Pd(II) complexes, which contain two different
monodentate NHCs [3]. These complexes have been used as probes
for the donor strength determination of various carbenes and are
also catalyst precursors in their own right [19]. In this work,
however, we focus on cis-chelating bidentate ligands containing
two different carbene units. The electronic asymmetry induced by
such hetero-dicarbenes through their two electronically different
donors may lead to complexes of interesting properties and cata-
lytic activities.
The preparation of the hetero-dicarbene complexes 7 and 8 and
their hetero-diazolium salt precursors is summarized in Scheme 1.
The reaction of methyl- or benzylbenzimidazole with neat dibro-
mopropane afforded the bromopropyl-benzimidazolium bromides
H and G in yields of 80 and 72%, respectively. Both salts are soluble
in dichloromethane, and thus can be easily separated from small
amounts of the respective propylene-bridged dibenzimidazolium
salts that may form as by-products. The formation of H and G is
supported by base peaks in their ESI mass spectra at m/z ¼ 254 and
330 for their molecular cations [MeBr]þ. Furthermore, their 1H
NMR spectra show downfield signals at 10.05 and 9.92 ppm char-
acteristic for the NCHN protons in azolium salts and the expected
multiplets assignable to the inequivalent methylene groups of the
bromopropylene N-substituents (Scheme 2).
Fig. 1. Molecular structure of complex 6∙0.5Et2O showing 50% probability ellipsoids.
Only one of two independent half molecules is shown. Hydrogen atoms and the
solvent molecule have been omitted for clarity. Selected bond lengths [Å] and angles
[deg]: Pd1-C8 1.986(6), Pd1-Br1 2.4832(8); C8-Pd1-C8A 85.7(3), Br1-Pd1-Br1A
96.38(2), C8-Pd1-Br1 88.96(16), C8-Pd1-Br1A 174.53(17).

H.V. Huynh, R. Jothibasu / Journal of Organometallic Chemistry 696 (2011) 3369e3375
3371
Table 1
MizorokieHeck Coupling reactions catalyzed by complexes 1e6.a
O
[Pd] (0.5 mol%)
NaOAc
t
Bu
O
O
t
OHC
Cl
Bu
1.5 [Bu4N]Br
DMF, 150 °C
O
OHC
Entry
Catalyst
t [h]
Yield [%]b
TON
1
2
3
4
5
6
1
2
3
4
5
6
18
18
18
18
18
18
39
46
33
77
25
90
78
92
66
154
50
180
a
Reaction conditions generally not optimized.
b
Yields were determined by 1H NMR spectroscopy for an average of two runs.
The desired hetero-diazolium salts I$2HBr and J$2HBr were
174.3 and 159.6 ppm (7) and 175.0 and 160.2 ppm (8), respectively.
The more downfield resonances are assigned to the benzimidazo-
lin-2-ylidene donor.
furnished by nucleophilic substitution reactions of H and G with
methylimidazole and benzylimidazole, respectively. As expected,
the solubilities of both hetero-diazolium bromides decrease further
compared to those of their precursors making them only soluble in
highly polar solvents like water, alcohols, DMF and DMSO. Evidence
for their formation can be found in their ESI mass spectra, which
shows strong peaks at m/z ¼ 335 for the [MeBr]þ monocation of
I$2HBr and at m/z ¼ 204 for the [Me2Br]2þ dication for J$2HBr.
Their NMR spectra also show the presence of both imidazolium and
benzimidazolium units in a 1:1 ratio with two very downfield
signals in the 1H NMR spectrum for the acidic NCHN protons. The
lowest field signal was assigned to the more acidic benzimidazo-
lium C2-proton.
Single crystals of 8 suitable for X-ray diffraction were ob-
tained as the solvate 8$0.5CH3CN by slow evaporation of a mixed
acetonitrile/ethyl acetate solution. The molecular structure
depicted in Fig. 2 confirms its identity as a square-planar
dibromido-hetero-dicarbene Pd(II) complex. The bite angle of
the cis-chelating hetero-dicarbene ligand is with 85.66(18)ꢀ
essentially the same as in the homo-dicarbene complex 6. The
dihedral angles between both imidazole- and benzimidazole-
derived carbene planes and the PdC2Br2 coordination planes
are with values of 77ꢀ and 79ꢀ likewise very similar to those
found in 6. The PdeCcarbene bond distances of 1.977(7) and
Palladation occurs straightforwardly with Pd(OAc)2 in DMSO at
elevated temperatures affording the first hetero-dicarbene Pd(II)
complexes 7 and 8 as pale yellow or off-white solids. Like their
homo-dicarbene analogues, they are only soluble in more polar
organic solvents such as DMF, DMSO and sparingly soluble in
CH3CN. Complex 8 is also sparingly soluble in chlorinated solvents.
ESI mass spectrometry supports their formation by isotopic
patterns at m/z ¼ 482 for [MeBr þ CH3CN]þ and m/z ¼ 593 for
[MeBr]þ for complex fragments of 7 and 8, respectively. The
absence of downfield signals in their 1H NMR spectra characteristic
for their ligand precursors I$2HBr and J$2HBr also indicates the
formation of carbene complexes. Furthermore, all methylene
protons become diastereotopic upon coordination indicating a rigid
structure with restricted movement of the propylene bridges. As
expected, two carbene signals are observed for each complex at
1.967(4) Å are indistinguishable within 3s. The PdeBr distances,
on the other hand, are significantly different. Notably, the PdeBr
bond trans to the imidazolin-2-ylidene is with 2.4855(9) Å
markedly longer than that trans to the benzannulated NHC
[2.4783(6) Å], perhaps indicating a stronger trans influence of
imidazole-derived NHCs.
With the two hetero-dicarbene complexes 7 and 8 in hand, their
catalytic activities have been tested and compared to those of the
homo-dicarbene complexes 1e6. Under identical reaction condi-
tions, it was found that both 7 and 8 gave rise to better precatalysts
resulting in quantitative yields of the MizorokieHeck product.
When the reaction time was shorten to 12 h there was no drop in
yield (entries 1 and 2). Further shortening of the time to 6 h finally
revealed the superiority of complex 7 over 8. The former still gave
near-quantitative yield (entry 3), while the latter afforded only 43%
Table 2
MizorokieHeck Coupling reactions catalyzed by complexes 7 and 8.a
O
[Pd] (0.5 mol%)
NaOAc
t
Bu
O
O
t
OHC
Cl
Bu
1.5 [Bu4N]Br
DMF, 150 °C
O
OHC
Entry
Catalyst
t (h)
Yield (%)b
TON
1
2
3
4
5
7
8
7
8
7
12
12
6
6
3
>99
>99
98
43
78
200
200
196
86
156
a
Reaction conditions generally not optimized.
b
Yields were determined by 1H NMR spectroscopy for an average of two runs.

3372
H.V. Huynh, R. Jothibasu / Journal of Organometallic Chemistry 696 (2011) 3369e3375
bridges have been synthesized. Comparison of their catalytic
Br
N
N
N
N
activities in the MizorokieHeck reaction revealed that C1 and C3
bridged-dicarbenes gave generally better Pd-catalysts. Further-
more, benzimidazolin-2-ylidenes were found to be superior to their
imidazolin-2-ylidenes possibly due to the absence of any inter-
fering acidic protons on the heterocycle. Based on these results, two
Pd(II) complexes (7 and 8) of new propylene-bridged hetero-
dicarbenes containing both imidazole- and benzimidazole-derived
NHCs have been synthesized. Their catalytic activities turned out
to be superior to that of homo-dicarbene complexes, which may be
traced back to an electronic asymmetry induced by the unusual
hetero-dicarbene ligands. Research is currently underway to
expand the scope of new hetero-dicarbenes as cis-chelating ligands
for other transition metal and to explore their use in catalysis.
Br(CH2)3Br
70 °C
Br
R
R
R = methyl or benzyl
G: R = methyl
H: R = benzyl
N
N
R
N
N
N
N
Pd(OAc)2
DMSO
N
R
N
R
N
R
N
R
Pd
2Br
Br
Br
[PdBr2(I)] (7: R = methyl)
[PdBr2(J)] (8: R = benzyl)
I·2HBr: R = methyl
J·2HBr: R = benzyl
4. Experimental section
4.1. General considerations
Scheme 2. Syntheses of Pd(II) hetero-dicarbene complexes 7 and 8.
All manipulations were carried out without taking precautions
to exclude air and moisture unless otherwise stated. All solvents
were used as received. The preparation of dicarbene salt precursors
A$2HBr [10], B$2HBr [8], C$2HBr [20], D$2HBr [10,14], E$2HBr [10],
F$2HBr [16] and the dicarbene complexes 1 [21] and 4 [14] have
been reported elsewhere. 1H and 13C NMR spectra were recorded on
a Bruker ACF 300 or AMX 500 spectrometer. The chemical shifts
were internally referenced to the residual protio solvent signals
relative to tetramethylsilane. ESI mass spectra were obtained using
a Finnigan MAT LCQ spectrometer. Elemental analyses were per-
formed on a PerkineElmer PE 2400 elemental analyzer at the
Department of Chemistry, National University of Singapore.
(entry 4). Even after 3 h, a good yield of 78% could be obtained with
complex 7 (entry 5). It is interesting to note that complex 8 with
bulkier N-benzyl groups is inferior to its analogue 7 with N-methyl
groups. Again, the presence of rather acidic benzylic protons may
lead to a non-innocent behaviour of benzyl-substituted ligands
causing catalyst decomposition particularly under the harsh reac-
tion conditions employed. A cleavage of the benzylic CeN bond is
also feasible.
To study the scope and limitations of the most active and stable
catalyst precursor 7, it was subjected to a range of substrates
(Table 3). 9-Bromoanthracene and 2,6-dibromopyridine could be
coupled quantitatively giving the desired mono- and disubstituted
products (entries 1 and 2). Activated aryl chlorides with nitro and
cyano substituents also gave near-quantitative yields (entries 3 and
4). Surprisingly, chloroacetophenone gave only a moderate yield of
47% (entry 5), which is comparable to that of the parent chloro-
benzene (entry 6). Finally, the lowest yield of 22% was obtained
with the difficult substrate chloropyridine revealing the limitation
of complex 7.
4.1.1. 1-(3-bromopropyl)-3-methylbenzimidazolium bromide (G)
1-methylbenzimidazole (0.581 g, 4.4 mmol) was added to 1,3-
dibromopropane (6 mL), and the resulting mixture was stirred for
12 h at 70 ꢀC. The reaction mixture was added dropwise into a flask
containing diethyl ether (50 mL) under stirring, and the resulting
suspension was filtered through a sintered funnel. The solid
product was washed with diethyl ether and dried under vacuum.
Compound G (1.17 g, 3.52 mmol, 80%) was isolated as a white
powder. 1H NMR (300 MHz, DMSO-d6):
d 9.81 (s, 1 H, NCHN), 8.10
(dd, 1 H, Ar-H), 8.04 (dd, 1 H, Ar-H), 7.71 (m, 2 H, Ar-H), 4.62 (t,
2 H,3J(H,H) ¼ 6.9 Hz, NCH2), 4.08 (s, 3 H, NCH3), 3.62 (t, 2 H,
3J(H,H) ¼ 6.7 Hz, CH2Br), 2.46 (m, 2 H, CH2). 13C{1H} NMR
(75.47 MHz, DMSO-d6): 143.0 (NCN), 131.9, 130.9, 126.5, 126.4,
113.6, 113.4 (Ar-C), 45.2 (NCH2), 33.2 (NCH3), 31.5 (CH2Br), 30.7
(CH2). MS (ESI): m/z ¼ 254 [MeBr]þ.
3. Conclusion
Six homo-dicarbene Pd(II) complexes bearing imidazole- (1e3)
and benzimidazole-derived carbenes (4e6) with C1-C3 alkyl
4.1.2. 1-(3-bromopropyl)-3-benzylbenzimidazolium bromide (H)
1,3-dibromopropane (5 mL) was added to benzylbenzimidazole
(0.625 g, 3 mmol) and stirred for 12 h at 70 ꢀC. After cooling to room
temperature, the reaction mixture was added dropwise into a flask
containing diethyl ether (50 mL) under stirring. The resulting
suspension was filtered through a sintered funnel, and the residue
was subsequently washed with ethyl acetate and dried under
reduced pressure to afford compound H (0.885 g, 2.16 mmol, 72%)
as a white powder. 1H NMR (500 MHz, DMSO-d6):
d 10.05 (s, 1 H,
NCHN), 8.13 (d, 1 H, Ar-H), 7.96 (d, 1 H, Ar-H), 7.67 (m, 2 H, Ar-H),
7.54 (d, 2 H, Ar-H), 7.41 (m, 3 H, Ar-H), 5.79 (s, 2 H, NCH2Ph), 4.66
(t, 2 H, 3J(H,H) ¼ 6.95 Hz, NCH2), 3.66 (t, 2 H, 3J(H,H) ¼ 6.3 Hz,
CH2Br), 2.51 (m, 2 H, CH2). 13C{1H} NMR (125.77 MHz, DMSO-d6):
142.8 (NCN), 133.9, 131.3, 130.9, 128.9, 128.7, 128.3, 126.7, 113.9,
113.7 (AreC), 49.9 (NCH2Ph), 45.5 (NCH2), 31.3 (CH2Br), 30.8 (CH2).
MS (ESI): m/z ¼ 330 [MeBr]þ.
Fig. 2. Molecular structure of complex 8∙0.5CH3CN showing 50% probability ellipsoids.
Hydrogen atoms and the solvent molecule have been omitted for clarity. Selected bond
lengths [Å] and angles [deg]: Pd1-C1 1.977(7), Pd1-C11 1.967(4), Pd1-Br1 2.4783(6),
Pd1-Br2 2.4855(9); C1-Pd1-C11 85.66(18), Br1-Pd1-Br2 94.4(2), C1-Pd1-Br1 91.44(13),
C11-Pd1-Br2 88.45(13), C1-Pd1-Br2 173.91(13), C11-Pd1-Br1 176.73(14).

H.V. Huynh, R. Jothibasu / Journal of Organometallic Chemistry 696 (2011) 3369e3375
3373
Table 3
MizorokieHeck Coupling reactions catalyzed by complex 7.a
7 (0.5 mol-%)
O
NaOAc
O
t
Bu
t
Ar
X
Bu
O
Ar
1.5 [Bu4N]Br
DMF, 150 °C
O
Entry
1
Aryl halide
t [h]
Yield [%]b
TON
Br
18
>99
200
200
2c
18
>99
Br
N
Br
3
4
5
6
18
18
18
48
97
95
47
41
194
190
94
NC
O2N
Cl
Cl
Cl
H3COC
82
Cl
Cl
N
7
48
22
44
a
Reaction conditions generally not optimized.
b
c
Yields were determined by 1H NMR spectroscopy for an average of two runs.
The yield refers to the doubly coupled product.
4.1.3. Hetero-diazolium salt (I$2HBr)
Compound (0.27 g, 0.81 mmol) was added to 1-
7.85 (t, 1 H, Ar-H), 7.68 (m, 2 H, Ar-H), 7.55 (m, 2 H, Ar-H), 7.44e7.40
(m, 8 H, Ar-H), 5.82 (s, 2 H, NbimiCH2Ph), 5.46 (s, 2 H, NimiCH2Ph),
G
methylimidazole (3 mL) and the resulting mixture was stirred at
90 ꢀC for 12 h. After cooling to room temperature, the reaction
mixture was added to a flask containing diethyl ether under stir-
ring, and the resulting suspension was filtered through a sintered
funnel. The residue was subsequently washed with diethyl ether.
Upon drying the residue under reduced pressure, compound
I$2HBr (0.32 g, 0.77 mmol, 95%) was isolated as an off-white
4.63 (t,
2
H, 3J(H,H)
¼
7.05 Hz, NbimiCH2), 4.41 (t, 2 H,
3J(H,H) ¼ 7.05 Hz, NimiCH2), 2.56 (m, 2 H, CH2). 13C{1H} NMR
(75.47 MHz, DMSO-d6): 142.6 (NCbimiN),136.4 (NCimiN),134.7,133.9,
131.3, 130.9, 129.0, 128.9, 128.7, 128.4, 126.7, 126.6, 122.8, 122.6,
114.0, 113.9 (Ar-C), 51.9 (NbimiCH2Ph), 49.9 (NimiCH2Ph), 46.2 (Nbi-
miCH2), 44.0 (NimiCH2), 28.7 (CH2). MS (ESI): m/z ¼ 204 [Me2Br]2þ
.
powder. 1H NMR (300 MHz, DMSO-d6):
d
9.92 (s, 1 H, NCHbimiN),
4.1.5. Dibromido-(1,10-dimethyl-3,30-ethylenediimidazolin-2,20-
9.26 (s, 1 H, NCHimiN), 8.12 (dd, 1 H, Ar-H), 8.05 (dd, 1 H, Ar-H), 7.84
(s, 1 H, Ar-H), 7.83e7.70 (m, 3 H, Ar-H), 4.60 (t, 2 H, NbimiCH2), 4.37
(t, 2 H, NimiCH2), 4.12 (s, 3 H, NbimiCH3), 3.85 (s, 3 H, NimiCH3), 2.52
(m, 2 H, CH2). 13C{1H} NMR (75.47 MHz, DMSO-d6): 142.9 (NCbimiN),
136.8 (NCimiN), 131.8, 130.8, 126.5, 126.4, 123.7, 122.1, 113.6, 113.5
(Ar-C), 45.8 (NbimiCH2), 43.6 (NimiCH2), 35.8 (NbimiCH3), 33.3
(NimiCH3), 28.8 (CH2). MS (ESI): m/z ¼ 335 [MeBr]þ.
diylidene)palladium(II) (2)
A mixture of B$2HBr (352 mg, 1.0 mmol) and Pd(OAc)2 (225 mg,
1.0 mmol) was stirred in DMSO (10 mL) at 70 ꢀC for 24 h. The
solution was then filtered through a small column of Celite, and the
solvent was removed completely by vacuum distillation. The
residue was washed with water, ethanol, hexane and diethyl ether.
Drying under vacuum gave a yellow solid (150 mg, 0.33 mmol, 33%).
1H NMR (300 MHz, DMSO-d6):
d 7.38 (s, 2H, CH), 7.34 (s, 2H, CH),
4.1.4. Hetero-diazolium salt (J$2HBr)
5.19 (m, 2H, CHH), 4.50 (m, 2H, CHH), 3.87 (s, 6H, CH3). 13C{1H} NMR
(75.47 MHz, DMSO-d6): 156.9 (NCN), 123.7 (CH), 122.5 (CH), 46.7
(CH2), 35.8 (CH3). MS (ESI): m/z ¼ 376 [MeBr]þ.
Compound H (0.41 g, 1 mmol) was added to a CH3CN solution of
benzylimidazole (0.316 g, 2 mmol) and stirred for 12 h at 80 ꢀC. The
solvent of the reaction mixture was removed under reduced pres-
sure and the resulting residue was washed with diethyl ether and
CH2Cl2. Upon drying the residue under vacuum, compound I$2HBr
(0.517 g, 0.9 mmol, 91%) was isolated as a white powder. 1H NMR
4.1.6. Dibromido-(1,10-dimethyl-3,30-propylenediimidazolin-2,20-
diylidene)palladium(II) (3)
Complex 3 was synthesized in analogy to compound 2 from
C$2HBr (183 mg, 0.5 mmol) and Pd(OAc)2 (113 mg, 0.5 mmol) and
isolated as a yellow solid (204 mg, 0.403 mmol, 86.8%). 1H NMR
(300 MHz, DMSO-d6):
d 10.14 (s, 1 H, NCHbimiN), 9.47 (s, 1 H,
NCHimiN), 8.14 (dd, 1 H, Ar-H), 7.98 (m, 1 H, Ar-H), 7.91 (t, 1 H, Ar-H),

3374
H.V. Huynh, R. Jothibasu / Journal of Organometallic Chemistry 696 (2011) 3369e3375
Table 4
(300 MHz, DMSO-d6): d 7.31 (s, 2H, CH), 7.27 (s, 2H, CH), 4.84 (m, 2H,
Selected X-ray crystallographic data for complexes 6 and 8.
NCHH), 4.34 (m, 2H, NCHH), 3.91 (s, 6H, CH3), 2.33 (m, 1H, CHH), 1.73
(m,1H, NCHH).13C{1H}NMR(75.47MHz,DMSO-d6): 123.0 (CH),122.8
(CH), 51.3 (NCH2), 37.5 (CH3), 30.6 (CH2). The carbene signal of 3 could
not be detected due to poor solubility. MS (ESI): m/z ¼ 390 [MeBr]þ.
6∙0.5Et2O
8∙0.5CH3CN
Formula
C21H25Br2N4O0.5Pd
C28H27.5Br2N4.5Pd
693.27
Formula weight
Colour
607.67
Yellow
Colourless
223(2)
0.54 ꢁ 0.12 ꢁ 0.06
Monoclinic
C2/c
32.294(2)
9.1189(6)
20.7260(14)
90
116.8280(10)
90
5446.5(6)
8
1.691
2.01 to 27.50
18981
Temperature (K)
Crystal size (mm)
Crystal system
Space group
a (Å)
223(2)
4.1.7. Dibromido-(1,10-dimethyl-3,30-ethylenedibenzimidazolin-
2,20-diylidene)palladium(II) (5)
Complex 5 was synthesized in analogy to compound 2 from
E$2HBr (226 mg, 0.5 mmol) and Pd(OAc)2 (113 mg, 0.5 mmol) and
isolated as a yellow solid (131 mg, 0.24 mmol, 47%). 1H NMR
0.40 ꢁ 0.30 ꢁ 0.06
Orthorhombic
Pmn21
17.2475(9)
8.2837(4)
15.1503(8)
90
b (Å)
c (Å)
(300 MHz, DMSO-d6): d 7.69 (m, 4H, Ar-H), 7.39 (m, 4H, Ar-H), 5.62
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
(br s, 2H, CHH), 5.07 (br s, 2H, CHH), 4.14 (s, 6H, CH3). The 13C NMR
spectrum of 5 could not be obtained due to poor solubility. MS
(ESI): m/z ¼ 476 [MeBr]þ.
90
90
V (Å3)
Z
2164.57(19)
4
1.865
1.79 to 27.49
14782
Dc (g cmꢂ3
q
)
4.1.8. Dibromido-(1,10-dimethyl-3,30-propylenedibenzimidazolin-
2,20-diylidene)palladium(II) (6)
Complex 6 was synthesized in analogy to compound 2 from
F$2HBr (233 mg, 0.5 mmol) and Pd(OAc)2 (113 mg, 0.5 mmol) and
isolated as a yellow solid (246 mg, 0.43 mmol, 86%). 1H NMR
range (ꢀ)
Unique data
Final R indices [I > 2
s
(I)]
R1 ¼ 0.0384,
R1 ¼ 0.0505,
wR2 ¼ 0.1029
wR2 ¼ 0.1123
(300 MHz, DMSO-d6): d 7.71 (m, 2H, Ar-H), 7.60 (m, 2H, Ar-H), 7.31 (m,
4H, Ar-H), 5.23 (m, 2H, NCHH), 4.90 (m, 2H, NCHH), 4.23 (s, 6H, CH3),
2.73(m,1H, CHH), 2.27(m,1H, CHH). The13CNMRspectrumof6could
not be obtained due to poor solubility. MS (ESI): m/z ¼ 491 [MeBr]þ.
3.0 mmol for dihalides), anhydrous sodium acetate (1.5 mmol),
catalyst (0.005 mmol), and [Bu4N]Br (1.5 mmol). To the mixture was
then added DMF (3 mL). The reaction mixture was vigorously stirred
at 150 ꢀC. After the desired reaction time, the solution was allowed
to cool. Dichloromethane (10 mL) was added to the reaction
mixture, and the organic phase was washed with water (6 ꢁ 5 mL)
and dried over MgSO4. The solvent was removed by evaporation to
give a crude product, which was analyzed by 1H NMR spectroscopy.
4.1.9. Pd(II) hetero-dicarbene complex (7)
Pd(OAc)2 (0.15 g, 0.67 mmol) was added to a DMSO solution of
compound I$2HBr (0.333 g, 0.8 mmol). The orange solution was
stirred for 12 h at 90 ꢀC. The reaction mixture was filtered over
Celite, and the solvent of the filtrate was removed by vacuum
distillation. The resulting residue was washed with water and
subsequently with ethanol. Upon drying the residue under vacuum,
complex 7 (0.275 g, 0.53 mmol, 79%) was isolated as a pale yellow
4.3. X-ray diffraction studies
Suitable single crystals were mounted on glass fibres. X-ray data
were collected with a Bruker AXS SMARTAPEX diffractometer, using
Mo Ka radiation, with the SMART suite of programs [22]. Data was
processed and corrected for Lorentz and polarization effects with
SAINT [23], and for absorption effect with SADABS [24]. Structural
solution and refinement were carried out with the SHELXTL suite of
programs [25]. The structure was solved by direct methods to locate
the heavy atoms, followed by difference maps for the light, non-
hydrogen atoms. All hydrogen atoms were put at calculated posi-
tions. All non-hydrogen atoms were generally given anisotropic
displacement parameters in the final model. A summary of the most
important crystallographic data is given in Table 4.
powder. 1H NMR (300 MHz, DMSO-d6):
d 7.74 (m, 1 H, Ar-H), 7.62
(m, 1 H, Ar-H), 7.37e7.25 (m, 4 H, Ar-H), 5.16 (m, 1 H, NCHH), 4.89
(m, 2 H, NCHH), 4.41 (m, 1 H, NCHH), 4.20 (s, 3 H, NbimiCH3), 3.94 (s,
3 H, NimiCH3), 2.44 (m, 1 H, CHH), 1.82 (m, 1 H, CHH). 13C{1H} NMR
(75.47 MHz, DMSO-d6): 174.3 (NCbimiN),159.6 (NCimiN),133.9,133.4,
123.4, 123.3, 123.1, 111.0, 110.4 (Ar-C), 51.6 (NbimiCH2), 48.4
(NimiCH2), 37.6 (NbimiCH3), 35.0 (NimiCH3), 30.2 (CH2). MS (ESI): m/z
482 [MeBr þ CH3CN]þ.
4.1.10. Pd(II) hetero-dicarbene complex (8)
A mixture of salt J$2HBr (0.341 g, 0.6 mmol) and Pd(OAc)2
(0.112 g, 0.5 mmol) in DMSO (8 mL) was stirred at 85 ꢀC for 12 h. The
reaction mixture was filtered over Celite and the solvent from the
filtrate was removed by vacuum distillation. The resulting residue
was washed with water and ethanol. Upon drying under reduced
pressure, complex 2 (0.289 g, 0.43 mmol, 86%) was obtained as an
Acknowledgements
The authors thank the National University of Singapore for
financial support (Grant No. R 143-000-407-112) and especially Ms
Geok Kheng Tan and Prof. Lip Lin Koh for determining the X-ray
molecular structures.
off-whitepowder.1HNMR (500MHz, DMSO-d6):
d 7.77(d,1 H, Ar-H),
7.37e7.30 (br m, 8 H, Ar-H), 7.18 (m, 5 H, Ar-H), 6.97 (d, 1 H, Ar-H),
6.91 (s, 1 H, Ar-H), 6.25 (d, 1 H, NCHHPh), 5.88 (d, 1 H, NCHHPh),
5.34 (t, 1 H, NCHH), 5.25 (d, 1 H, NCHHPh), 5.05 (t, 1 H, NCHH), 4.93
(dd,1 H, NCHH), 4.70 (d,1 H, NCHHPh), 4.48 (dd,1 H, NCH2), 2.54 (m,
1 H, CHH), 2.00 (m,1 H, CHH).13C{1H} NMR (125.77 MHz, DMSO-d6):
175.0 (NCbimiN),160.2 (NCimiN),136.0,135.1,133.9,132.8,128.5,128.4,
128.0,127.9,127.8,127.3,124.3,123.5,123.3,121.3,111.8,110.9 (Ar-C),
52.8 (NbimiCH2Ph), 51.8 (NbimiCH2), 51.6 (NimiCH2Ph), 48.6 (NimiCH2),
30.1 (CH2). MS (ESI): m/z ¼ 593 [MeBr]þ.
Appendix A. Supplementary material
CCDC 825978 (for 6∙0.5Et2O) and 825979 (for 8∙0.5CH3CN);
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
4.2. General procedure for the MizorokieHeck reaction
Supplementary data
In a typical run, a test tube was charged with a mixture of aryl
halide or dihalide (1.0 mmol), tert-butyl acrylate (1.5 mmol or
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.jorganchem.2011.07.018. These

H.V. Huynh, R. Jothibasu / Journal of Organometallic Chemistry 696 (2011) 3369e3375
3375
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