Synthesis, structural characterization and catalytic activity of a palladium(II) complex bearing a new ditopic thiophene-N-heterocyclic carbene ligand

Article abstract of DOI:10.1016/j.ica.2009.02.035

A new thiophene-functionalized benzimidazolium salt (2) has been prepared by reacting N-methylbenzimidazole with 2-bromomethylthiophene (1), which in turn was obtained by bromination of 2-thiophenemethanol with PBr₃. Subsequent reaction of salt 2 with Pd(OAc)₂ afforded the cis-configured bis(carbene) Pd(II) complex (cis-3), which in solution exists as an inseparable mixture of cis-anti and cis-syn-rotamers in a 3.5:1 ratio. All new compounds have been fully characterized by spectroscopic and spectrometric methods. A preliminary catalytic study shows that cis-3 is highly active in the Suzuki-Miyaura coupling of aryl bromides with phenylboronic acid in/on water as environmentally benign reaction media.

Full text of DOI:10.1016/j.ica.2009.02.035

Inorganica Chimica Acta 363 (2010) 1979–1983
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Inorganica Chimica Acta
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Synthesis, structural characterization and catalytic activity of a palladium(II)
complex bearing a new ditopic thiophene-N-heterocyclic carbene ligand
*
Han Vinh Huynh , Ying Xia Chew
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 new thiophene-functionalized benzimidazolium salt (2) has been prepared by reacting N-methylbenz-
imidazole with 2-bromomethylthiophene (1), which in turn was obtained by bromination of 2-thioph-
enemethanol with PBr3. Subsequent reaction of salt 2 with Pd(OAc)2 afforded the cis-configured
bis(carbene) Pd(II) complex (cis-3), which in solution exists as an inseparable mixture of cis–anti and
cis–syn-rotamers in a 3.5:1 ratio. All new compounds have been fully characterized by spectroscopic
and spectrometric methods. A preliminary catalytic study shows that cis-3 is highly active in the
Suzuki–Miyaura coupling of aryl bromides with phenylboronic acid in/on water as environmentally
benign reaction media.
Received 21 January 2009
Received in revised form 24 February 2009
Accepted 25 February 2009
Available online 9 March 2009
Dedicated to Prof. R.J. Puddephatt
Keywords:
N-Heterocyclic carbenes
Palladium
Ó 2009 Elsevier B.V. All rights reserved.
Thiophene
Suzuki–Miyaura reaction
Homogeneous catalysis
1. Introduction
2. Results and discussion
N-heterocyclic carbenes (NHCs) are nowadays common ligands
in organometallic chemistry and catalysis mainly due to their
strong donor-abilities surpassing those of classical phosphines
and the ease of their preparation [1]. Moreover, additional func-
tionalities can be easily introduced at the nitrogen atoms of the
N-heterocyclic ring, and accordingly, various donor-functionalized
NHCs and their complexes have been explored, and their use in
catalysis has been recently reviewed [2]. To this point, N-func-
tionalization with N-, O- and P-donor groups is relatively com-
mon, whereas carbene ligands bearing softer sulfur-donors are
surprisingly rare, although some thiolate-NHCs [3–6], thioether-
NHCs [7–10] and thiophene-NHCs [11,12] have been reported.
Notably all these examples are based on an unsaturated imidaz-
ole or a saturated imidazoline N-heterocyclic scaffold. To the best
of our knowledge, sulfur-functionalized NHCs as potentially
hemilabile ligands based on benzimidazole are unknown,
although benzannulated carbenes [13,14] are becoming increas-
ingly popular. As part of our on-going research on complexes of
benzannulated carbenes [15–21], we herein report the synthesis,
structural characterization and catalytic activity of a palladium(II)
complex bearing a new thiophene-functionalized benzimidazolin-
2-ylidene ligand.
2.1. Ligand precursor
The two-step synthesis of the new thiophene-functionalized
benzimidazolium salt 2, which serves as ligand precursor, is sum-
marized in Scheme 1. The first step involves the bromination of
commercially available 2-thiophenemethanol to afford 2-bromom-
ethylthiophene (1). In contrast to a previous report [22], this can be
easily accomplished with PBr3, which furnishes 2-bromomethyl-
thiophene (1) as a yellow liquid in 89% yield. Upon standing at
ambient temperature in air, compound 1 slowly decomposes under
formation of a dark brown solid with the evolution of fumes, pos-
sibly due to acid catalyzed polymerization [22,23]. However, bro-
mide 1 can be stored under nitrogen and at low temperature in
the dark for a few days without decomposition. Heating of 1 with
N-methylbenzimidazole in toluene at 90 °C finally afforded the de-
sired thiophene-functionalized benzimidazolium bromide 2 as an
off-white powder in 78% yield.
A characteristic downfield resonance at 11.43 ppm correspond-
ing to the NCHN (C2) proton in the 1H NMR spectrum indicates the
successful formation of salt 2. The chemical shift of its C2 carbon at
142.9 ppm is in agreement with reported data for other benzimi-
dazolium salts [24]. The ESI mass spectrum of the carbene precur-
sor 2 is dominated by a base peak centered at m/z = 229 for the
cation [MꢀBr]+, which arises from loss of the bromide counter an-
ion. For comparison purposes, we have also obtained single crys-
* Corresponding author. Tel.: +65 65162670; fax: +65 67791691.
E-mail address: chmhhv@nus.edu.sg (H.V. Huynh).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.02.035

1980
H.V. Huynh, Y.X. Chew / Inorganica Chimica Acta 363 (2010) 1979–1983
N
N
2.2. Synthesis and characterization of Pd–NHC complex
OH
S
Br
S
S
N
N
Reaction of two equiv of salt 2 with Pd(OAc)2 under in situ
deprotonation afforded the desired bis(carbene) complex cis-
[PdBr2(NHC)2] (cis-3) (Scheme 2), which can be isolated as a light
brown solid after washing of the crude reaction mixture with
diethyl ether and THF. The formation of a carbene complex is indi-
cated by the disappearance of the acidic C2 proton characteristic of
2 in the 1H NMR spectrum of cis-3. Furthermore, two sets of closely
spaced signals are observed indicating the existence of an insepa-
rable isomeric mixture, the ratio of which was determined to be
3.5:1 through integration (vide infra).
PBr3
Et2O
Br
90 °C, Toluene
2
1
Scheme 1. Two-step synthesis of 1-(2-thienylmethyl)-3-methylbenzimidazolium
bromide.
Whereas the NCH2 resonances of precursor 2 were detected as
singlets, these protons become diastereotopic in the carbene com-
plex cis-3 giving rise to a doublet for each proton (AA0 spin system,
Fig. 2). This observation is in agreement with a cis-arrangement of
the two carbene ligands, in which a free rotation around the Pd–C
bond is restricted due to steric repulsion of the bulky N-thienylm-
ethyl substituents. As a consequence, the two isomers have been
assigned to cis–syn and cis–anti rotamers, in which the unsymmet-
rical carbene ligands differ in the orientation of their N-substitu-
ents. Due to steric reasons, the major set of signals is assigned to
the cis–anti rotamer. Correspondingly, two sets of signals are also
observed in the 13C NMR spectrum, and the assignment is further
corroborated by two closely spaced carbene resonances at 174.5
and 174.2 ppm, which are in the typical range observed for cis-
bis(benzimidazolin-2-ylidene)complexes of Pd(II). The correspond-
ing trans isomers would exhibit more downfield carbene shifts of
ꢂ180 ppm [25,26]. Furthermore, rotamers in trans-bis(carbene)
complexes are relatively common, and we have recently reported
a detailed study on their isomerization behavior [27]. On the other
hand, cis-bis(carbene) complexes bearing unsymmetrical ligands
usually adopt a cis–anti arrangement due to steric constraints,
and the existence of less favorable cis–syn rotamers have rarely
been observed.
Single crystals of the rotamer cis–anti-3 selectively crystallized
as acetone solvate cis–anti-3 ꢁ 1.5CH3COCH3 upon slow evaporation
of a saturated acetone solution of 3. The solid state molecular
structure determined by X-ray diffraction studies is shown in
Fig. 3. As anticipated for the major isomer in solution, the two car-
bene ligands are coordinating the metal center in the sterically
more favorable cis–anti fashion, as it reduces the repulsion be-
tween the adjacent carbene ligands. The essentially square planar
coordination sphere is completed by two bromo ligands. Due to a
crystallographic symmetry, the pendant thienylmethyl groups
are oriented anti to each other. The dihedral angle between the car-
bene ring planes and the coordination plane is 73.5°. Compared to
its benzimidazolium salt precursor 2, the Ccarbene–N1/2 bonds have
Fig. 1. Molecular structure of 2 ꢁ 1.5H2O showing 50% probability ellipsoids. Water
molecules and hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (°): C1–N1 1.317(6), C1–N2 1.329(5), N1–C2 1.391(5), N1–C13 1.461(5),
N2–C7 1.383(5), N2–C8 1.470(5), C2–C7 1.397(5); N1–C1–N2 111.0(4), C1–N1–C2
108.4(3), C1–N1–C13 126.4(4), C1–N2–C7 107.8(3), C1–N2–C8 125.2(4).
tals of the new thioether-functionalized benzimidazolium salt
from a concentrated CDCl3 solution as water solvate 2 ꢁ 1.5H2O,
which were subjected to X-ray diffraction analysis. The asymmet-
ric unit contains two independent molecules, of which only one is
shown in Fig. 1 along with the crystallographic numbering scheme.
S
N
N
N
N
S
N
N
Br
Br
Br
Br
90 °C
N
N
N
N
+
Pd(OAc)2
+
2
Pd
Pd
Br
DMSO
S
2
become elongated by
Dd = 0.03 Å. Upon coordination, this bond
S
S
elongation is further accompanied by a decrease in the N–C–N an-
gle from 111.0(4)° in 2 to 105.4(6)° in cis–anti-3. Other structural
parameters remain largely unchanged indicating that coordination
to the Pd-centre only affects the carbene carbon and the neighbor-
ing nitrogen atoms.
cis-anti-
3
cis-syn-3
Scheme 2. Preparation of Pd(II) bis(carbene) complex cis-3.
Fig. 2. Diastereotopic 1H NMR signals of NCH2 protons in cis–anti and cis–syn rotamers of cis-3.

H.V. Huynh, Y.X. Chew / Inorganica Chimica Acta 363 (2010) 1979–1983
1981
of the activated aryl bromides at ambient temperature achieving
yields of >85% (Entries 1/2). For deactivated aryl bromides addition
of [N(n-C4H9)4]Br, longer reaction times and a higher reaction tem-
perature are required. Under these conditions, coupling occurs
smoothly giving quantitative or near-quantitative yields (Entries
3/4). Disappointingly, the limitation of this catalytic system be-
comes apparent in the coupling of more challenging aryl chlorides
resulting in low yields of only 9% and 14%, respectively (Entries 5/
6). A comparison with previously reported Pd(II) catalysts bearing
only one benzimidazolin-2-ylidene ligand shows that the current
bis(carbene) system is less effective for coupling of aryl chlorides
[21,24].
3. Conclusion
In summary, we have reported the facile synthesis of a new pal-
ladium(II) complex bearing two unsymmetrical, thienylmethyl-
functionalized benzimidazolin-2-ylidene ligands. In solution, this
complex exists as a mixture of cis–anti and cis–syn rotamers in a
3.5:1 ratio, from which the sterically more favorable cis–anti iso-
mer crystallizes. A preliminary catalytic study shows that complex
cis-3 is highly active in the Suzuki–Miyaura coupling of activated
and deactivated aryl bromides in/on water as green reaction med-
ia. The coupling of more challenging aryl chlorides, however, gave
only low yields. Research in our lab is currently on-going to extend
the coordination chemistry of thiophene-functionalized NHCs to
other transition metals and to explore their potential applications
in catalysis.
Fig. 3. Molecular structure of cis–anti-3 ꢁ 1.5CH3COCH3 showing 50% probability
ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity. Selected
bond lengths (Å) and angles (°): Pd1–C1 1.972(7), Pd1–Br1 2.4726(10), C1–N1
1.352(9), C1–N2 1.353(9), N1–C2 1.381(10), N1–C8 1.454(10), N2–C7 1.414(10),
N2–C9 1.476(9), C2–C7 1.386(11); C1–Pd1–C1a 94.2(4), Br1–Pd1–Br1a 93.08(5),
C1–Pd1–Br1 86.4(2), C1a–Pd1–Br1 178.3(2), C1–Pd1–Br1a 178.3(2), C1a–Pd1– Br1a
86.4(2), N1–C1–N2 105.4(6).
It is worth noting, that the dissolution of these single crystals
led to the formation of a rotameric mixture of cis–anti-3 and cis–
syn-3 as evidenced by 1H NMR spectroscopy and confirming the
facile interconversion of these rotamers in solution.
2.3. Suzuki–Miyaura catalysis
4. Experimental
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 [25,27,28] and Suzu-
ki–Miyaura [29,30] coupling reactions. NHCs with additional do-
nor-functions can potentially exhibit hemilabile behavior and
thus offer additional stability for catalytically active intermediates.
Thus, a preliminary study was carried out in this work to investi-
gate the catalytic activities of the N-thienylmethyl-functionalized
NHC complex cis-3 in the Suzuki–Miyaura reaction. The coupling
of aryl bromides and chlorides with phenylboronic acid to give
the respective biaryls in/on water as environmentally benign med-
ia with 1 mol% catalyst loading was chosen as a standard test reac-
tion. The results of these heterogeneous reactions summarized in
Table 1 show that complex cis-3 is highly efficient in the coupling
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. 1H and 13C NMR spectra were recorded on a
Bruker ACF 300 spectrometer or AMX 500 spectrophotometer
and the chemical shifts (d) were internally referenced by the resid-
ual solvent signals relative to tetramethylsilane (1H, 13C). ESI Mass
spectra were measured using a Finnigan MAT LCQ spectrometer.
Elemental analyses were performed on a Perkin–Elmer PE 2400
elemental analyzer at the Department of Chemistry, National Uni-
versity of Singapore.
4.2. Synthesis of 2-bromomethylthiophene (1)
PBr3 (0.47 mL, 5.00 mmol) was added dropwise to a solution of
2-thiophenemethanol (0.95 mL, 10.0 mmol) in diethyl ether
(10 mL) at 0 °C. The solution was stirred for 1 h at ambient temper-
ature and methanol (1.5 mL) was then added. The mixture was di-
luted with deionised water (50 mL) and extracted with diethyl
ether (1 ꢃ 50 mL, 3 ꢃ 30 mL). The combined organic layers were
dried over Na2SO4 and filtered. Removal of the solvent in vacuo
afforded the product as a yellow liquid (1.576 g, 8.90 mmol, 89%).
1H NMR (300 MHz, CDCl3): d 7.37 (d, 3J(H,H) = 5.1 Hz, 1H, Ar-H),
7.16 (d, 3J(H,H) = 3.2 Hz, 1H, Ar-H), 6.99 (m, 1H, Ar-H), 4.80 (s,
2H, CH2).
Table 1
Suzuki–Miyaura cross-coupling reactions catalyzed by cis-3 in aqueous media.a
1 mol% 3
+
R
X
B(OH)2
R
H2O, K2CO3
X = Br, Cl
R = COCH3, CHO, OCH3, CH3
Entry
Aryl halide
t (h)
Temperature (°C)
Yield (%)b
1
2
3
4
5
6
4-Bromobenzaldehyde
4-Bromoacetophenone
4-Bromoanisole
4-Bromotoluene
4-Chlorobenzaldehyde
4-Chloroacetophenone
8
8
21
21
21
21
RT
RT
85
85
85
85
>99
85
97c
>99c
9c
4.3. Synthesis of 1-(2-thienylmethyl)-3-methylbenzimidazolium
bromide (2)
14c
a
Non-optimized reaction conditions: 1 mmol of aryl halide; 1.5 mmol of phen-
Compound 1 (1.120 g, 6.33 mmol) was added to a solution of 1-
methylbenzimidazole (0.905 g, 6.85 mmol) in toluene (5 mL). The
reaction mixture was stirred overnight at 90 °C and a white precip-
ylboronic acid; 3 ml of water; 2 equiv. of K2CO3; 1 mol% of catalyst.
b
Yields were determined by 1H NMR spectroscopy for an average of two runs.
c
With addition of 1.5 equiv. of [N(n-C4H9)4]Br.

1982
H.V. Huynh, Y.X. Chew / Inorganica Chimica Acta 363 (2010) 1979–1983
Table 2
itate was formed. The reaction mixture was filtered through a sin-
tered funnel and the white precipitate was washed with toluene.
The product was dried in vacuo to afford an off-white powder
(1.522 g, 4.92 mmol, 78%). 1H NMR (300 MHz, CDCl3): d 11.43 (s,
1H, NCHN), 7.77–7.75 (m, 1H, Ar-H), 7.70–7.61 (m, 3H, Ar-H),
7.51 (m, 1H, Ar-H), 7.31 (dd, 3J(H,H) = 5.1 Hz, 4J(H,H) = 1.2 Hz, 1H,
Ar-H), 7.03–7.00 (m, 1H, Ar-H), 6.10 (s, 2H, CH2), 4.26 (s, 3H,
NCH3). 13C{1H} NMR (75.47 MHz, CDCl3): d 142.9 (s, NCHN),
134.2, 132.0, 130.8, 129.9, 127.7, 127.3, 127.3, 113.5, 112.9 (s,
Ar-C), 45.8 (s, CH2), 33.9 (s, NCH3). MS (ESI): m/z = 229 [MꢀBr]+.
Anal. Calc. for C13H13BrN2S (M = 309.22): C, 50.49; H, 4.24; N,
9.06. Found: C, 50.23; H, 4.51; N, 8.90%.
Selected crystallographic data for salt 2 and complex cis–anti-3.
2 ꢁ 1.5H2O
cis–anti-
3 ꢁ 1.5CH3COCH3
C26H24Br2N4PdS2
1.5CH3COCH3
809.95
Empirical formula
C13H13BrN2S ꢁ 1.5H2O
ꢁ
Formula weight
Color, habit
Crystal size (mm)
Crystal system
Space group
a (Å)
336.25
colorless, block
0.40 ꢃ 0.16 ꢃ 0.08
monoclinic
P21/c
17.1632(14)
7.3363(6)
22.9362(17)
90
colorless, needle
0.18 ꢃ 0.04 ꢃ 0.04
monoclinic
C2/c
18.4228(13)
17.1467(14)
11.6450(7)
90
b (Å)
c (Å)
a
(°)
b (°)
90.368(2)
90
2887.9(4)
8
1.547
2.986
1.19–27.50
12165
0.7961, 0.3813
102.643(2)
90
3589.3(4)
4
1.499
2.889
1.64–25.00
10375
0.8932, 0.6244
4.4. Synthesis of palladium complex cis-3
c
(°)
V (Å3)
A mixture of salt 2 (0.343 g, 1.11 mmol) and Pd(OAc)2 (0.120 g,
0.53 mmol) was dissolved in DMSO (12 mL) and stirred overnight
at 90 °C. The yellow solution first turned green after a few hours
and darkened after reacting overnight. The reaction mixture was
filtered through a sintered funnel and the solvent of the filtrate
was removed by vacuum distillation. The resulting residue was
dissolved in CH2Cl2 (30 mL) and extracted with H2O (5 ꢃ 20 mL).
The organic phase was dried over Na2SO4 and the solvent was re-
moved in vacuo. The solid obtained was washed with diethyl ether
and THF. Removal of the solvent in vacuo yielded a light brown
powder (0.191 g, 0.26 mmol, 50%). Slow evaporation of a concen-
trated acetone solution afforded the cis–anti complex as crystals.
Cis–anti-3: 1H NMR (500 MHz, CD2Cl2): d 7.20–7.12 (m, 4H, Ar-H),
7.05–7.00 (m, 1H, Ar-H) 6.70–6.66 (m, 2H, Ar-H), 4.16 (s, 3H,
NCH3). 13C{1H} NMR (125.77 MHz, CD2Cl2): d 174.2 (s, NCN),
137.0, 135.7, 133.8, 127.3, 126.2, 125.7, 124.1, 124.0, 111.5, 110.9
(s, Ar-C), 48.6 (s, NCH2), 36.1 (s, NCH3). Cis–syn-3: 1H NMR
(500 MHz, CD2Cl2): d 7.40–7.22 (m, 5H, Ar-H), 6.82–6.75 (m, 2H,
Ar-H), 4.32 (s, 3H, NCH3). 13C{1H} NMR (125.77 MHz, CD2Cl2): d
174.4 (s, NCN), 136.9, 135.5, 133.9, 127.2, 126.1, 125.7, 124.1,
123.9, 112.0, 110.7 (s, Ar-C), 48.8 (s, NCH2), 36.2 (s, NCH3). MS
(ESI): m/z = 643 [MꢀBr]+. Anal. Calc. for C26H24Br2N4PdS2ꢁ-
CH3COCH3 (M = 780.93): C, 44.60; H, 3.87; N, 7.17. Found: C,
44.46; H, 4.07; N, 6.89%.
Z
Dcalcd (g cmꢀ3
)
l
(mmꢀ1
)
h range (°)
No. of unique data
Maximum, minimum
transmition
Final R indices [I > 2
r
(I)]
R1 = 0.0485,
wR2 = 0.1242
R1 = 0.0737,
wR2 = 0.1368
1.030
R1 = 0.0637,
wR2 = 0.1521
R1 = 0.0987,
wR2 = 0.1693
1.046
R indices (all data)
Goodness-of-fit (GOF) on F2
Peak/hole (e Åꢀ3
)
0.728/ꢀ0.542
0.723/ꢀ0.599
non-hydrogen atoms were generally given anisotropic displace-
ment parameters in the final model. A summary of the most impor-
tant crystallographic data is given in Table 2.
Acknowledgements
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
4.5. General procedure for the Suzuki–Miyaura coupling
CCDC 716610 and 716611 contain the supplementary crystallo-
graphic data for 2 ꢁ 1.5H2O and cis–anti-3 ꢁ 1.5CH3COCH3. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2009.02.035.
In a typical run, a test tube was charged with a mixture of aryl
halide (1.0 mmol), phenylboronic acid (1.2 mmol), potassium car-
bonate (2 mmol), precatalyst cis-3 (0.01 mmol) and [N(n-
C4H9)4]Br (1.5 mmol) (for entries 3–6 in the Table 1). To the mix-
ture was added H2O (3 mL). The reaction mixture was vigorously
stirred at the appropriate temperature (RT or 85 °C). After the de-
sired reaction time, the solution was allowed to cool. 10 ml of
dichloromethane was added to the reaction mixture and the organ-
ic phase was extracted with water (6 ꢃ 5 mL) and dried over
MgSO4. The solvent was removed by evaporation to give a crude
product, which was analyzed by 1H NMR spectroscopy.
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X-ray data were collected with a Bruker AXS SMART APEX dif-
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