Syntheses and catalytic activities of pseudo-pincer and CSC pincer-type Pd(II) complexes derived from benzannulated N-heterocyclic carbenes

Article abstract of DOI:10.1039/b907887h

Three new dibenzimidazolium salts bearing thioether (B·2HBr, B·2HNO₃) and sulfoxide (C·2HBr) containing bridges have been synthesized as sulfur-functionalized dicarbene precursors. Palladation of B·2HBr and C·2HBr afforded two new pseudo-pincer

Full text of DOI:10.1039/b907887h

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N-Heterocyclic Carbenes
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Westfälische Wilhelms-Universität, Münster, Germany
Published in issue 35, 2009 of Dalton Transactions
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www.rsc.org/dalton | Dalton Transactions
Syntheses and catalytic activities of pseudo-pincer and CSC pincer-type
Pd(II) complexes derived from benzannulated N-heterocyclic carbenes†
Han Vinh Huynh,* Dan Yuan and Yuan Han
Received 20th April 2009, Accepted 22nd July 2009
First published as an Advance Article on the web 31st July 2009
DOI: 10.1039/b907887h
Three new dibenzimidazolium salts bearing thioether (B·2HBr, B·2HNO3) and sulfoxide (C·2HBr)
containing bridges have been synthesized as sulfur-functionalized dicarbene precursors. Palladation of
2
B·2HBr and C·2HBr afforded two new pseudo-pincer complexes cis-[PdBr2(B-k C)] (1) and
2
cis-[PdBr2(C-k C)] (2), in which the sulfur-donor remains pendant. Reaction of precursor B·2HNO3 in
the presence of 1 equiv of KBr, on the other hand, yields the first CSC-Pd(II) pincer complex
3
[PdBr(B-k CSC)]NO3 (3) bearing two carbene moieties. All three complexes have been fully
characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction
analysis. Their catalytic activities in the Mizoroki–Heck reaction have been evaluated as well.
Introduction
Results and discussion
N-heterocyclic carbenes (NHCs) are powerful s-donors that have
become standard ligands in organometallic chemistry and cataly-
sis due to their stabilizing properties and ease of preparation.1 An
additional advantage is that they can be modified in many ways,
which also allows for a straightforward donor-functionalization2
giving access to ditopic and pincer-type ligands, the complexes of
which have found widespread applications in catalytic processes.3
Heteroatom-NHC pincers have been generally prepared by com-
bining one or two NHC moieties with nitrogen, phosphorus
or oxygen donors, whereas sulfur-containing analogues are very
rare, possibly due to lack of suitable synthetic methodologies.4
To the best of our knowledge, only two examples of dithiolato5,6
SCS NHC-based pincer-ligands have been reported so far. CSC
pincer ligands based on NHCs are on the other hand unknown.
Furthermore, most known pincer systems contain a rather rigid
structure, which is intended to yield increased complex stability.
On the other hand, a more flexible ligand backbone would allow
a more subtle interplay between lability and stability, which
may be beneficial for certain types of catalytic applications. In
relation to our research on less explored sulfur-functionalized
NHCs, we have recently reported on complexes bearing ditopic
and hemilabile thioether-7 and thiophene-8 NHC ligands. As an
extension of our work in this area, we herein report on the
syntheses, structural characterization and catalytic activities of the
first CSC-pincer-type Pd(II) complex and two of its “quasi-pincer”
analogues, in which the flexible and soft sulfur moiety remains
pendant.
Sulfur-functionalized dibenzimidazolium salts
The preparation of thioether- and sulfoxide-bridged dibenz-
imidazolium salts as precursors to CSC NHC-based pincer
ligands is summarized in Scheme 1. The reaction of benzyl-
benzimidazole with neat dibromoethane afforded 1-benzyl-3-
bromoethylbenzimidazolium bromide A in a yield of 92%. Salt A is
soluble in dichloromethane, and thus can be easily separated from
small amounts of the ethylene bridged dibenzimidazolium salt
that has formed as a by-product. The formation of A is supported
by a base peak in the ESI mass spectrum at m/z = 316 for the
molecular cation [M - Br]+. Furthermore, its 1H NMR spectrum
shows a downfield shift at 10.12 ppm characteristic for the NCHN
proton in azolium salts and two triplets at 5.03 and 4.08 ppm
Department of Chemistry, 3 Science Drive 3, National University of
Singapore, Singapore 117543. E-mail: chmhhv@nus.edu.sg; Fax: +65
67791691; Tel: +65 65162670
† CCDC reference numbers 728535 (B·2HBr·0.8H2O), 728532 (1), 728533
(2·0.5DMF) and 728534 (3). For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b907887h
Scheme 1 Synthesis of benzimidazolium salts.
7262 | Dalton Trans., 2009, 7262–7268
This journal is
The Royal Society of Chemistry 2009
©

3
with vicinal coupling constant J(H-H) = 5.8 Hz assignable to
conversion and their chemical shifts show little changes. The
benzimidazolium salts A, B·2HBr, B·2HNO3 and C·2HBr are all
non-hygroscopic powders and can be easily handled in air.
the two inequivalent methylene groups of the bromoethylene
N-substituent. The benzylic protons on the other hand resonate
as a singlet at 5.85 ppm.
Palladium(II) complexes
Two equiv of A can subsequently undergo nucleophilic substitu-
tion with Na2S to form the thioether-bridged dibenzimidazolium
dibromide B·2HBr, which was isolated as an off-white powder in
The suitability of the proligands B·2HBr, C·2HBr and B·2HNO3
to form Pd(II)-complexes was probed by reacting them with
Pd(OAc)2 in DMSO at elevated temperatures. The reaction
sequences are summarized in Scheme 2. Palladation of B·2HBr
leads to the pseudo-pincer dicarbene Pd(II) complex cis-[PdBr2(B-
1
87% yield. Compared to precursor A, the H NMR signals do
not change significantly upon thioether-formation. Only a slight
upfield shift was observed for the methylene groups adjacent to
the sulfur atom. Stronger evidence for the identity of B·2HBr
was provided by a base peak in the ESI mass spectrum at
m/z = 252 for the molecular dication [M - 2Br]2+ and a less
intense peak at m/z = 585 for the mono-cation [M - Br]+. Single
crystals of B·2HBr·0.8H2O were obtained by slow evaporation of
a concentrated methanol solution and subjected to single-crystal
X-ray diffraction. Fig. 1 depicting its molecular structure along
with the crystallographic numbering scheme, shows the expected
connectivity and the presence of the desired thioether function
bridging the two cationic benzimidazolium fragments. All bond
parameters are in the expected range and do not require further
comments.
2
k C)] (1), in which the thioether function remains pendant. The
neutral complex was isolated as a stable yellow solid soluble in
CH2Cl2, CHCl3, DMSO, and DMF, but insoluble in less polar
solvents such as THF, diethyl ether and hexane. The formation
of a Pd(II) carbene complex is supported by positive mode ESI
mass spectrometry, which shows a base peak at m/z = 689 for the
[M - Br]+ complex fragment. Besides, the disappearance of the
NCHN signal in the 1H NMR spectrum characteristic for B·2HBr,
the metallation results in diastereotopy of the benzylic protons
with chemical shifts of 6.35 and 5.30 ppm, respectively, with
geminal coupling constants of 16.4 Hz. Likewise, the methylene
protons of the bridge adjacent to the nitrogen also give rise to two
signals at 5.84 and 4.85 ppm. The methylene groups adjacent to
the sulfur atom, on the other hand, resonate as one multiplet at
3.50 ppm, which indicates a certain degree of rotational freedom
in line with a pendant sulfur function. This is further supported
by the 13C NMR signal for carbene carbon found at 175.2 ppm.
This chemical shift corroborates a cis-arrangement of the two
benzimidazolin-2-ylidene donors disfavoring coordination of the
thioether bridge in a square planar complex.9
Fig. 1 Molecular structure of B·2HBr·0.8H2O showing 50% probability
ellipsoids; hydrogen atoms and solvent m◦olecules are omitted for clarity.
˚
Selected bond lengths (A) and angles ( ): N3–C24 1.505(9), N3–C17
1.333(10), N4–C17 1.343(9), N4–C31 1.489(9), C31–C32 1.525(10),
C32–S1 1.819(7), S1–C16 1.842(7), C16–C15 1.513(11), N2–C15 1.493(8),
N2–C1 1.339(8), C1–N1 1.333(8), N1–C8 1.489(8); N3–C17–N4 110.9(7),
N2–C1–N1 110.1(5).
Anion exchange occurs smoothly by reacting B·2HBr with two
equiv of AgNO3 affording B·2HNO3 with precipitation of AgBr.
Again, ESI MS proved useful in the characterization of the product
with a base peak at m/z = 252 assignable for the [M - 2NO3]2+
dication and a less intense peak at m/z = 566 for the mono-cation
[M - NO3]+. In addition, the sulfoxide-bridged dibenzimidazolium
salt C·2HBr was prepared by oxidation of the thioether B·2HBr
with three equiv of H2O2 in acetic acid. Successful oxidation was
indicated by a base peak at m/z = 260 for the dication [M - 2Br]2+
and a smaller one centered at m/z = 601 for the mono-cation
[M - Br]+. Upon formation of the sulfoxide function, all protons of
the bridge and their respective carbon atoms shift downfield, which
is in line with the increased -I-effect of the resulting sulfoxide
group. Furthermore, the a-protons also become diastereotopic
due to the reduced inversion of the oxidized and pyramidal
sulfur atom. All other groups remain largely unaffected by this
Scheme 2 Synthesis of pseudo-pincer complexes 1, 2 and CSC pincer
complex 3.
The identity of 1 as a pseudo-pincer was finally confirmed
by X-ray diffraction analysis on a single crystal obtained from
slow vapour diffusion of diethyl ether into a concentrated DMF
solution. Fig. 2 depicts the molecular structure, and selected
crystallographic data are listed in Table 3. As found in solution, the
dicarbene ligand B coordinates the palladium(II) center in a cis-
chelating fashion, which results in a ten-membered metallacycle.
The pendant sulfur atom is oriented “exo” with respect to the metal
center and points away from the coordination plane. A similar
arrangement has been observed for a related Pd(II) quasi-pincer
complex of an ether-bridged diimidazolin-2-ylidene ligand.10 The
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7262–7268 | 7263
©

Table 1 Mizoroki–Heck coupling reactionsa catalyzed by complexes 1-3
Entry Catalyst Aryl halide
t [h] Temp [◦C] Yield [%]b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
4-bromo-1-nitrobenzene 24
4-bromo-1-nitrobenzene 24
4-bromo-1-nitrobenzene 24
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
140
140
140
140
140
140
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
50
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzaldehyde
4-bromobenzonitrile
4-bromobenzonitrile
4-bromobenzonitrile
4-bromoacetophenone
4-bromoacetophenone
4-bromoacetophenone
2,6-dibromopyridine
2,6-dibromopyridine
2,6-dibromopyridine
24
24
24
24
24
24
24
24
24
24
24
24
Fig. 2 Molecular structure of pseudo-pincer complex 1 showing 50%
probability ellipsoids; hydrogen atoms are omitted for clarity. Se-
◦
˚
lected bond lengths (A) and angles ( ): Pd1–C1 1.951(7), Pd1–C15
1.984(6), Pd1–Br1 2.4684(10), Pd1–Br2 2.4792(9), N1–C8 1.451(11),
N1–C1 1.368(10), N2–C1 1.357(10), N2–C29 1.462(10), N3–C15 1.348(9),
N3–C22 1.474(10), N4–C32 1.452(10), N4–C15 1.349(9), S1–C30
1.799(8), S1–C31 1.812(8); C1–Pd1–Br1 84.7(2), C1–Pd1–C15 94.5(3),
C15–Pd1–Br2 87.8(2), Br1–Pd1–Br2 93.06(4), C15–Pd1–Br1 178.8(2),
C1–Pd1–Br2 176.8(2), N1–C1–N2 104.7(6), N3–C15–N4 105.5(6);
PdC2Br2/NHC dihedral angle 82◦ and 71◦; inter-NHC angle 80◦.
1-bromo-4-chlorobenzene 24
1-bromo-4-chlorobenzene 24
1-bromo-4-chlorobenzene 24
58
45
4-bromotoluene
4-bromotoluene
4-bromotoluene
4-bromoanisole
4-bromoanisole
4-bromoanisole
24
24
24
24
24
24
53c
˚
1.951(7) and 1.984(6) A as well as the Pd–Br bonds amounting
52c
˚
to 2.4684(10) and 2.4792(9) A are comparable to those observed
47c
41c
for the only other structurally characterized dibenzimidazolin-2-
ylidene- dibromo-palladium(II) complex.11
35c
51c
The reaction of precursor C·2HBr gave similar results and
2
a Reaction conditions:
1 mmol of aryl halide (0.5 mmol of 2,6-
an analogous pseudo-pincer complex cis-[PdBr2(C-k C)] (2) was
dibromopyridine); 1.4 mmol of tert-butyl acrylate; 3 ml of DMF; 1.5 mmol
of NaOAc; 1 mol% of [Pd]. 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.
obtained, which is indicated by a base peak at m/z = 705 for the
[M - Br]+ ion in the positive mode ESI mass spectrum. Complex 2
dissolves sparingly in acetone and CH3CN, but shows good solu-
bility in DMSO and DMF. Compared to the 1H NMR spectrum of
its ligand precursor C·2HBr, in which only the methylene protons
adjacent to the sulfoxide group are diastereotopic (vide supra), all
three methylene groups of the ligand C split into six separate
signals in accord with an AB coupling pattern upon complex
formation. Furthermore, the carbene donors of 2 resonate slightly
highfield from those in complex 1 at 173.8 ppm, again indicating
a cis-arrangement of the benzannulated carbene moieties. The
proposed structure was finally confirmed by an X-ray diffraction
study on crystals obtained by diffusion of diethyl ether into a
DMF solution of 2. The molecular structure depicted in Fig. 3 is
similar to that of complex 1 and features a cis-chelating dicarbene
and two bromo ligands coordinating the palladium center in a
square planar fashion. The resulting 10-membered metallacycle
contains the pendant sulfoxide group, which points away from the
coordination plane. Similar to 1, the two carbene planes adopt an
angle of 76◦ relative to each other. Their dihedral angles of 84◦ and
81◦ with respect to the PdC2Br2 coordination plane fall, in contrast
to those in 1, in a narrow range. All other bond parameters are
unexceptional and do not require further comments.
Table 2 Mizoroki–Heck coupling reactionsa catalyzed by complex 3
Entry
[Pd] [mol%]
t [h]
Temp [◦C]
Yield [%]b
TON
500
1
2
3
4
5
0.2
0.02
0.002
0.001
0.0002
24
24
24
48
72
120
120
120
120
120
>99
>99
>99
>99
36
5000
50 000
100 000
180 000
a Reaction conditions: 1 mmol of 4-bromobenzaldehyde; 1.4 mmol of tert-
butyl acrylate; 3 ml of DMF; 1.5 mmol of NaOAc. b Yields were determined
by 1H NMR spectroscopy for an average of two runs.
two remaining coordination sites in the square planar complex are
taken up by two bromo ligands. The two carbene planes adopt an
angle of 80◦ to each other. Surprisingly, the amount by which the
two carbene planes deviate from the PdC2Br2 coordination plane
is with values of 82◦ and 71◦ quite different. At this point this
difference can only be ascribed to crystal packing effects, since
other interactions could not be discerned. The Pd–C bonds of
In an attempt to overcome the formation of pseudo-pincer
complexes and to enforce a CSC-pincer-type coordination mode
of ligand B, we substituted the ligand precursor B·2HBr with
B·2HNO3. The latter only contains weakly coordinating counter-
anions, which should facilitate binding of the soft thioether
7264 | Dalton Trans., 2009, 7262–7268
This journal is
The Royal Society of Chemistry 2009
©

Table 3 Selected X-ray crystallographic data for the dibenzimidazolium salt B·2HBr·0.8H2O and the complexes 1, 2·0.5DMF and 3
B·2HBr·0.8H2O
1
2·0.5DMF
3
Formula
Formula weight
Color, habit
C32H32Br2N4S·0.8H2O
C32H30Br2N4PdS
768.88
colorless, block
0.24 ¥ 0.10 ¥ 0.10
223(2)
triclinic
¯
P1
11.8217(9)
11.8738(9)
12.7681(9)
69.492(2)
75.399(2)
64.829(1)
1507.98(19)
2
1.693
Mo Ka
3.363
1.72–25.00
15898
5301
0.0557
0.7297, 0.4991
C32H30Br2N4OPdS·0.5C3H7NO C32H30BrN5O3PdS
678.91
821.43
750.98
colorless, block
0.40 ¥ 0.20 ¥ 0.10
243(2)
triclinic
¯
P1
11.504(3)
11.595(3)
12.552(3)
96.355(5)
99.264(5)
98.275(5)
1619.7(7)
2
1.398
Mo Ka
2.597
1.66–24.99
16547
5687
0.0806
0.7812, 0.4231
colorless, block
0.18 ¥ 0.16 ¥ 0.16
223(2)
triclinic
¯
P1
yellow, block
0.18 ¥ 0.12 ¥ 0.08
223(2)
orthorhombic
Pbcn
Cryst size/mm
Temperature/K
Crystal system
Space group
˚
a/A
11.0762(5)
12.6526(6)
13.6890(7)
90.219(1)
106.606(1)
97.676(1)
1820.15(15)
2
31.2166(19)
11.1730(7)
17.026(1)
90
90
90
5938.3(6)
8
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Dc/g cm-3
1.499
1.680
Radiation used
Mo Ka
2.795
Mo Ka
2.085
m/mm-1
q range/◦
1.63–27.49
24120
8345
1.30–25.00
32991
5235
Reflection collected
Independent reflections
R(int)
0.0569
0.6633, 0.6331
0.1100
Max., min. transmission
0.8510, 0.7054
R1 = 0.0530, wR2 = 0.1173
R1 = 0.0977, wR2 = 0.1342
1.020
Final R indices [I > 2s(I)] R1 = 0.0821, wR2 = 0.2327 R1 = 0.0732, wR2 = 0.1630 R1 = 0.0562, wR2 = 0.1463
R indices (all data)
R1 = 0.1055, wR2 = 0.2451 R1 = 0.0890, wR2 = 0.1697 R1 = 0.0904, wR2 = 0.1614
Goodness-of-fit on F2
1.062
1.215
2.060/-0.902
1.044
-3
˚
Peak/hole/e A
2.224/-0.522
1.013/-0.471
1.176/-1.277
site, the reaction proceeded smoothly and the desired product
3
[PdBr(B-k CSC)]NO3 (3), could be isolated in a good yield of
68%. The solubility of complex 3 in common organic solvents is
similar to that of 2. Although the ESI mass spectrum shows a
cationic base peak at m/z = 689 for the [M - NO3]+ complex
cation, it does not provide sufficient evidence for the formation of
3, since the same base peak has been observed for the [M - Br]+
fragment of 1. On the other hand, NMR spectroscopy reveals
distinct differences. The coordination of the sulfur atom results in
diastereotopy of the adjacent methylene protons, which resonance
at 5.03 and 3.44 ppm as two broad pseudo-triplets. Furthermore,
the carbenoid 13C signals are observed downfield from those
in 1 at 176.6 ppm, indicating a trans arrangement of the two
benzimidazolin-2-ylidene moieties. However, this chemical shift
difference is not as pronounced as in direct cis-trans isomers9 as
a consequence of the different overall Lewis acidity of neutral 1
and cationic 3. Vapour diffusion of diethyl ether into a DMF
solution afforded single crystals that were subjected to X-ray
diffraction studies. Fig. 4 depicts the molecular structure of the
first CSC-pincer complex bearing NHC ligands. As found in
solution, pincer-type coordination forces the two carbene donors
into a trans-arrangement incorporating the Pd(II) center into
two six-membered rings. As anticipated, the fourth coordination
site trans to the sulfur donor is taken up by a bromo ligand
resulting in a square planar complex-cation, the charge of which is
Fig.
3
Molecular structure of pseudo-pincer complex 2·0.5DMF
showing 50% probability ellipsoids; hydrogen atoms and the solvent
molecule are omitted for clarity. Selected bond lengths (A) and angles
˚
(◦): Pd1–C1 1.991(6), Pd1–C17 1.987(5), Pd1–Br1 2.4765(7), Pd1–Br2
2.4649(7), N1–C1 1.340(7), N1–C8 1.472(7), N2–C1 1.360(6), N2–C15
1.457(7), N3–C17 1.355(7), N3–C24 1.450(7), N4–C17 1.365(7), N4–C31
1.467(7), S1–O1 1.490(5), S1–C16 1.790(6), S1–C32 1.817(7); C1–Pd1–Br1
84.28(15), C1–Pd1–C17 96.1(2), C17–Pd1–Br2 86.57(15), Br1–Pd1–Br2
93.03(3), C1–Pd1–Br2 177.24(15), C17–Pd1–Br1 179.25(17), N1–C1–N2
106.5(5), N3–C17–N4 105.7(5); PdC2Br2/NHC dihedral angle 84◦, 81◦;
inter-NHC angle 76◦.
-
compensated by one NO3 counter-anion. Pincer formation also
donor. Disappointingly, the equimolar reaction of B·2HNO3 with
Pd(OAc)2 in DMSO at 80 ◦C only led to substantial formation of
Pd black.
It was anticipated that the lack of stabilizing anionic ligands
was the cause for this decomposition. Indeed with the addition
of 1 equiv of KBr, which can take up the fourth coordination
leads to a substantial decrease of the inter-NHC angle to 15◦. The
dihedral angles between the carbene ring planes and the PdC2SBr-
coordination plane also decrease to 47◦ and 61◦, respectively. As a
consequence of the trans arrangement, the Pd-C bonds of 2.033(6)
˚
and 2.000(6) A have become elongated compared those in 1 and
2. They are also similar to those reported for related CNC pincer
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7262–7268 | 7265
©

10-3 mol% when the reaction time was extended to 48 h. This
result is promising and indicates that the sulfur function in the
bridge does not poison the active Pd(0) catalyst. However, further
decrease of catalyst loading to 2 ¥ 10-4 mol% and extension of
the reaction time to 78 h results in a dramatic drop of yield
to 36%, although a relatively high TON of 180 000 could be
achieved. Again, these results are superior than those reported
for complexes bearing COC-quasi-pincer10 or CNC-pincer-type15
carbene ligands derived from imidazole.
Conclusions
In summary, we have reported on the design of three new
dibenzimidazolium salt precursors bearing thioether (B·2HBr,
B·2HNO3) and sulfoxide (C·2HBr) groups. Reaction of the dibro-
mide salts with Pd(OAc)2 under standard conditions afforded the
Fig. 4 Molecular structure of CSC-pincer complex 3 showing 50%
-
probability ellipsoids; hydrogen atoms and the NO3 counte◦r-anion are
˚
omitted for clarity. Selected bond lengths (A) and angles ( ): Pd1–C1
2.033(6), Pd1–C17 2.000(6), Pd1–S1 2.3078(18), Pd1–Br1 2.4341(8),
N1–C1 1.361(8), N1–C8 1.468(7), N2–C1 1.356(7), N2–C15 1.472(8),
N3–C17 1.347(7), N3–C24 1.472(8), N4–C31 1.461(7), N4–C17 1.345(8),
S1–C16 1.800(7), S1–C32 1.825(7); C1–Pd1–Br1 93.59(18), C1–Pd1–S1
93.16(18), C17–Pd1–S1 84.46(17), C17–Pd1–Br1 88.66(17), C17–Pd1–C1
177.3(2), S1–Pd1–Br1 171.10(5), N1–C1–N2 105.2(5), N3–C17–N4
106.3(5); PdC2SBr/NHC dihedral angle 47◦, 61◦; inter-NHC angle 15◦.
2
pseudo-pincer complexes cis-[PdBr2(B-k C)] (1) and cis-[PdBr2(C-
2
k C)] (2), in which the sulfur donors remain uncoordinated,
but potentially hemilabile. Using an improved protocol involving
palladation of B·2HNO3 in the presence of 1 equiv of KBr, the first
3
CSC pincer complex [PdBr(B-k CSC)]NO3 (3) could be isolated
in a good yield. All new complexes have been fully characterized,
and a first catalytic investigation showed that all complexes are
highly active only in the Mizoroki–Heck coupling of activated
aryl bromides, whereas deactivated substrates gave moderate to
poor results. More importantly, these results demonstrate that the
sulfur function did not poison the active catalyst and research in
our lab is currently on-going to extend the promising coordination
chemistry of these new ligands to different transition metals in
search for other catalytic applications.
complexes containing benzannulated carbene donors.12,13 On the
other hand, the Pd–Br bond of 2.4341(8) is shorter due to the
smaller trans-influence of the sulfur donor. Finally, the Pd–S bond
is comparable to that of a thioether-functionzed NHC complex
reported earlier.7
Catalysis
The Mizoroki–Heck reaction of aryl bromides with tert-butyl
acrylate in DMF was selected to test and compare the initial
catalytic activities of the pseudo-pincer complexes 1 and 2
as well as the pincer-complex 3. The results summarized in
Table 1 demonstrate that all three complexes are suitable catalyst
precursors leading to quantitative yields for a range of activated
substrates (entries 1–13). The coupling of dibromo-pyridine is
also successful giving quantitatively the doubly-coupled product
(E,E¢)-di-tert-butyl-3,3¢-(pyridine-2,6-diyl)diacrylate (entries 13–
15). The latter result is encouraging and demonstrates superior
activities of benzimidazole-derived NHCs over a previously re-
ported imidazole-based bis(carbene) system.14 Surprisingly, the
reaction with 1-bromo-4-chlorobenzene turned out to be rather
sluggish resulting in only moderate yields (entries 16–18). The
limitations of all three complexes finally become evident in the
coupling of deactivated substrates, where poor to moderate yields
were obtained (entries 19–24). A better performance has been
reported for a similar lutidine-bridged CNC pincer.12 Notably, a
comparison did not reveal the superiority of any complex. It was
observed that any of the three complexes can give rise to the best
yield dependant on the substrate used (entries 17, 19, 24). However,
as these differences are rather small, it can be anticipated that all
three complexes decompose under the relatively harsh conditions
to palladium nanoparticles of very similar size that do the catalytic
work.
Experimental
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-Benzylbenzimidazole was prepared
according to a previously reported method.16 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 residual 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 University of Singapore.
Syntheses
1-Benzyl-3-(2-bromoethyl)-benzimidazolium bromide (A).
A
mixture of 1-benzylbenzimidazole (1.458 g, 7 mmol) and 1,2-
dibromoethane (7 ml) was heated at 85 ◦C overnight. After
removing the volatiles in vacuo, CH2Cl2 (50 ml) was added to
the residue and the resulting suspension was filtered over Celite.
The remaining solid was washed with CH2Cl2 (4 ¥ 20 ml) and
the solvent of the filtrate was removed in vacuo to give a white
solid (2.550 g, 6.44 mmol, 92%). H NMR (300 MHz, DMSO-
d6): d 10.12 (s, 1 H, NCHN), 8.19 (dd, 1 H, Ar-H), 8.00 (dd, 1
H, Ar-H), 7.69 (m, 2 H, Ar-H), 7.51 (dd, 2 H, Ar-H), 7.41 (m,
In a separate study, the influence of the catalyst loading on the
Mizoroki–Heck coupling reaction catalyzed by the pincer complex
3 was investigated as well (Table 2). Here it was observed, that
3 could still give quantitative yield with a loading of as low as
1
7266 | Dalton Trans., 2009, 7262–7268
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The Royal Society of Chemistry 2009
©

3
3 H, Ar-H), 5.85 (s, 2 H, NCH2Ph), 5.03 (t, J(H,H) = 5.8 Hz,
(120 mg, 0.13 mmol, 42%). 1H NMR (500 MHz, DMSO-d6): d 7.75
(d, 2 H, Ar-H), 7.26 (m, 4 H, Ar-H), 7.19 (m, 4 H, Ar-H), 7.12 (m,
6 H, Ar-H), 6.86 (d, 2 H, Ar-H), 6.35 (d, 2J(H,H) = 16.4 Hz, 2 H,
2 H, NCH2), 4.08 (t, J(H,H) = 5.8 Hz, 2 H, CH2Br). 13C{ H}
NMR (75.47 MHz, DMSO-d6): 143.0 (s, NCHN), 133.9, 131.0,
130.7, 129.0, 128.8, 128.2, 126.9, 114.1, 114.0 (s, Ar-C), 49.9 (s,
NCH2Ph), 48.1 (s, NCH2), 31.1 (s, CH2Br). MS (ESI): m/z = 316
[M - Br]+.
3
1
2
NCHHPh), 5.84 (m, 2 H, NCHH), 5.30 (d, J(H,H) = 16.4 Hz,
2 H, NCHHPh), 4.85 (d, 2J(H,H) = 13.9 Hz, 2 H, NCHH), 3.50
1
(m, 4 H, CH2S). 13C{ H} NMR (125.77 MHz, DMSO-d6): 175.2
(s, NCN), 135.2, 133.6, 133.2, 128.4, 127.8, 126.9, 123.3, 123.2,
111.9, 111.7 (s, Ar-H), 51.5 (s, NCH2Ph), 50.3 (s, NCH2), 33.0
(s, CH2S). Anal. Calc. for C32H30Br2N4PdS: C, 49.99; H, 3.93; N,
7.29. Found: C, 49.94; H, 3.92; N, 7.25%. MS (ESI): m/z = 689
[M - Br]+.
B·2HBr. A mixture of salt A (792 mg, 2 mmol) and Na2S·9H2O
(240 mg, 1 mmol) in CH3CN was stirred at ambient temperature
for 48 h. After removing the volatiles in vacuo, CH2Cl2 (3 ¥ 20 ml)
was added to the residue and the resulting suspension was filtered
over Celite. The remaining solid left in the flask and on the Celite
was dissolved in methanol. The product was crystallized from
methanol (0.579 g, 0.87 mmol, 87%). 1H NMR (300 MHz, DMSO-
d6): d 10.39 (s, 2 H, NCHN), 8.20 (dd, 2 H, Ar-H), 7.98 (dd, 2 H,
Ar-H), 7.65 (m, 4 H, Ar-H), 7.55 (d, 4 H, Ar-H), 7.37 (m, 6 H,
cis-[PdBr2(C-j2C)] (2). A mixture of salt C·2HBr (205 mg,
0.3 mmol) and Pd(OAc)2 (67 mg, 0.3 mmol) in DMSO (5 ml)
was stirred at 80 ◦C overnight. The resulting mixture was filtered
over Celite, and the solvent of the filtrate was removed by vacuum
distillation. The resulting residue was washed with H2O (3 ¥ 20 ml)
and redissolved in CH2Cl2. Slow evaporation of CH2Cl2 solution
afforded the product as a white solid (120 mg, 0.15 mmol, 51%).
1H NMR (500 MHz, DMSO-d6): d 7.93 (d, 2 H, Ar-H), 7.35 (m,
2 H, Ar-H), 7.24 (m, 2 H, Ar-H), 7.18 (m, 6 H, Ar-H), 7.07 (m, 4
H, Ar-H), 6.90 (d, 2 H, Ar-H), 6.26 (d, 2J(H,H) = 16.4 Hz, 2 H,
NCHHPh), 5.99 (m, 2 H, NCHH), 5.35 (d, 2J(H,H) = 16.4 Hz, 2
H, NCHHPh), 4.82 (m, 2 H, NCHH), 4.31 (m, 2 H, CHHS), 3.71
3
Ar-H), 5.87 (s, 4 H, NCH2Ph), 4.87 (t, J(H,H) = 6.3 Hz, 4 H,
NCH2), 3.29 (t, J(H,H) = 6.3 Hz, 4 H, CH2S). 13C{ H} NMR
(75.47 MHz, DMSO-d6): 142.9 (s, NCHN), 134.0, 131.1, 130.6,
128.9, 128.7, 128.3, 126.74, 126.71, 114.2, 113.9 (s, Ar-C), 49.8 (s,
NCH2Ph), 46.0 (s, NCH2), 29.4 (s, CH2S). MS (ESI): m/z = 585
[M - Br]+, 252 [M - 2Br]2+.
3
1
B·2HNO3. A mixture of salt B·2HBr (266 mg, 0.4 mmol)
and AgNO3 (136 mg, 0.8 mmol) in methanol (20 ml) was stirred
at ambient temperature overnight. The resulting suspension was
filtered from the precipitated AgBr over Celite and the solvent was
removed in vacuo to give the product as a white powder (207 mg,
0.329 mmol, 83%). H NMR (300 MHz, DMSO-d6): d 10.15 (s,
2 H, NCHN), 8.16 (dd, 2 H, Ar-H), 7.97 (dd, 2 H, Ar-H), 7.66
(m, 4 H, Ar-H), 7.52 (dd, 4 H, Ar-H), 7.38 (m, 6 H, Ar-H), 5.83
1
(m, 2 H, CHHS). 13C{ H} NMR (125.77 MHz, DMSO-d6): 173.8
(s, NCN), 134.8, 133.7, 133.5, 128.4, 127.8, 126.8, 123.8, 123.7,
112.1, 111.8 (s, Ar-H), 51.8 (s, NCH2Ph), 51.2 (s, NCH2), 43.5 (s,
CH2S). Anal. Calc. for C32H30Br2N4OPdS: C, 48.97; H, 3.85; N,
7.14. Found: C, 48.87; H, 4.05; N, 7.50%. MS (ESI): m/z = 705
[M - Br]+.
1
3
[PdBr(B-j3CSC)]NO3 (3).
A
mixture of salt B·2HNO3
(s, 4 H, NCH2Ph), 4.83 (t, J(H,H) = 6.5 Hz, 4 H, NCH2), 3.24
(t, J(H,H) = 6.5 Hz, 4 H, CH2S). 13C{ H} NMR (75.47 MHz,
DMSO-d6): 142.9 (s, NCHN), 134.0, 131.2, 130.7, 129.0, 128.7,
128.2, 126.8, 126.7, 114.1, 113.9 (s, Ar-C), 49.9 (s, NCH2Ph), 46.0
(s, NCH2), 29.4 (s, CH2S). MS (ESI): m/z = 566 [M - NO3]+, 252
[M - 2NO3]2+.
3
1
(198 mg, 0.32 mmol), KBr (37.5 mg, 0.32 mmol) and Pd(OAc)2
(71 mg, 0.32 mmol) in DMSO (5 ml) was stirred at 80 ◦C overnight.
The resulting mixture was filtered over Celite, and the solvent of the
filtrate was removed by vacuum distillation. The resulting residue
was washed with H2O (3 ¥ 20 ml), subsequently with CH2Cl2
(3 ¥ 20 ml) and dried under vacuum to afford the product as a
yellow powder (162 mg, 0.215 mmol, 68%).1H NMR (500 MHz,
DMSO-d6): d 8.03 (d, 2 H, Ar-H), 7.70(d, 2 H, Ar-H), 7.47 (m, 2
H, Ar-H), 7.42 (m, 2 H, Ar-H), 7.35 (m, 4 H, Ar-H), 7.20 (m, 6
H, Ar-H), 6.36 (d, 2J(H,H) = 15.8 Hz, 2 H, NCHHPh), 5.85 (d,
2J(H,H) = 15.8 Hz, 2 H, NCHHPh), 5.37 (d, 2J(H,H) = 13.9 Hz,
2 H, NCHH), 5.03 (ps t, 2J(H,H) = 12.6 Hz, 2 H, CHHS), 3.73 (d,
2J(H,H) = 13.9 Hz, 2 H, NCHH), 3.44 (ps t, 2J(H,H) = 12.6 Hz,
C·2HBr. A mixture of salt B·2HBr (200 mg, 0.3 mmol) and
35% H2O2 (78 mL, 0.9 mmol) in acetic acid (5 ml) was stirred at
ambient temperature overnight. After removing the volatiles in
vacuo, the residue was washed with THF (3 ¥ 20 ml) to give an
off-white powder (185 mg, 0.27 mmol, 91%). 1H NMR (300 MHz,
DMSO-d6): d 10.15 (s, 2 H, NCHN), 8.14 (dd, 2 H, Ar-H), 7.95
(dd, 2 H, Ar-H), 7.67 (m, 4 H, Ar-H), 7.51 (m, 4 H, Ar-H), 7.37
(m, 6 H, Ar-H), 5.82 (s, 4 H, NCH2Ph), 5.05 (br t, 4 H, NCH2),
1
2 H, CHHS). 13C{ H} NMR (75.47 MHz, DMSO-d6): 176.6 (s,
3.70 (dt, J(H,H) = 13.6 Hz,3J(H,H) = 6.0 Hz, 2 H, CHHS),
2
3.56 (dt, J(H,H) = 13.6 Hz,3J(H,H) = 6.0 Hz, 2 H, CHHS).
2
NCN), 136.4, 134.2, 133.6, 129.0, 128.2, 127.7, 124.6, 124.5, 112.5,
112.4 (s, Ar-H), 51.4 (s, NCH2Ph), 49.0 (s, NCH2), the signal for
CH2S overlaps with residual signals of DMSO-d6. Anal. Calc. for
C32H30BrN5O3PdS: C, 51.18; H, 4.03; N, 9.33. Found: C, 50.81; H,
4.30; N, 9.00%. MS (ESI): m/z = 689 [M - NO3]+.
13C{ H} NMR (75.47 MHz, DMSO-d6): 143.1 (s, NCHN), 133.8,
1
131.2, 130.6, 128.9, 128.6, 128.3, 128.0, 126.7, 114.0, 113.97 (s, Ar-
C), 49.9 (s, NCH2Ph), 49.2 (s, NCH2), 41.3 (s, CH2S). MS (ESI):
m/z = 602 [M - Br]+, 260 [M - 2Br]2+.
cis-[PdBr2(B-j2C)] (1). A mixture of salt B·2HBr (200 mg,
0.3 mmol) and Pd(OAc)2 (67 mg, 0.3 mmol) in DMSO (5 ml)
was stirred at 80 ◦C overnight. The resulting mixture was filtered
over Celite, and the solvent of the filtrate was removed by vacuum
distillation. The resulting residue was washed with H2O (3 ¥ 20 ml).
The residue was then dissolved in CH2Cl2 and filtered over Celite.
The filtrate was dried under vacuum and the remaining solid was
washed with THF to obtain the desired product as a yellow solid
Mizoroki–Heck catalysis
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), anhy-
drous 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 24 h. After the mixture was cooled to
the ambient temperature, dichloromethane (10 ml) was added. The
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7262–7268 | 7267
©

organic layer was then washed with water (6 ¥ 8 ml) and dried over
Na2SO4. The solvent was allowed to evaporate and the residue was
analyzed by 1H NMR spectroscopy.
2 For a recent review, see: A. T. Normand and K. J. Cavell, Eur. J. Inorg.
Chem., 2008, 2781.
3 (a) M. Albrecht and G. van Koten, Angew. Chem., Int. Ed., 2001, 40,
3751; (b) I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100,
3009; (c) D. Morales-Morales and C. M. Jensen, The Chemistry of
Pincer Compounds, Elsevier, Amsterdam, Netherlands, 2007.
4 For a recent review, see: D. Pugh and A. A. Danopoulos, Coord. Chem.
Rev., 2007, 251, 610.
5 D. Sellmann, C. Allmann, F. Heinemann, F. Koch and J. Sutter,
J. Organomet. Chem., 1997, 541, 291.
6 N. Matsumura, J. Kawano, N. Fukunishi, H. Inoue, M. Yasui and F.
Iwasaki, J. Am. Chem. Soc., 1995, 117, 3623.
7 H. V. Huynh, C. H. Yeo and G. K. Tan, Chem. Commun., 2006,
3833.
8 H. V. Huynh and Y. X. Chew, Inorg. Chim. Acta, 2009,
DOI: 10.1016/j.ica.2009.02.035, in press.
9 H. V. Huynh, J. H. H. Ho, T. C. Neo and L. L. Koh, J. Organomet.
Chem., 2005, 690, 3854.
10 D. J. Nielsen, K. J. Cavell, B. W. Skelton and A. H. White,
Organometallics, 2006, 25, 4850.
11 C. Marshall, M. F. Ward and W. T. A. Harrison, Tetrahedron Lett.,
2004, 45, 5703.
X-Ray diffraction studies
Diffraction data were collected with a Bruker AXS SMART
APEX diffractometer, using Mo Ka radiation at 223(2) (1-3)
or 243(2) K (B·2HBr), with the SMART suite of Programs.17
Data were processed and corrected for Lorentz and polarisation
effects with SAINT,18 and for absorption effect with SADABS.19
Structural solution and refinement were carried out with the
SHELXTL suite of programs.20 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 positions. 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
the Table 3. CCDC reference numbers 728535 (B·2HBr·0.8H2O),
728532 (1), 728533 (2·0.5DMF) and 728534 (3).†
12 F. E. Hahn, M. C. Jahnke, V. Gomez-Benitez, D. Morales-Morales and
T. Pape, Organometallics, 2005, 24, 6458.
13 D. H. Brown, G. L. Nealon, P. V. Simpson, B. W. Skelton and Z. Wang,
Organometallics, 2009, 28, 1965.
14 H. V. Huynh and J. Wu, J. Organomet. Chem., 2009, 694, 323.
15 D. J. Nielsen, K. J. Cavell, B. W. Skelton and A. H. White, Inorg. Chim.
Acta, 2002, 327, 116.
16 H. V. Huynh, L. R. Wong and P. S. Ng, Organometallics, 2008, 27,
2231.
Acknowledgements
We thank the National University of Singapore for financial
support (Grant No. R 143-000-327-133) and the CMMAC staff
of our department for technical assistance.
17 SMART version 5.628, Bruker AXS Inc., Madison, Wisconsin, USA,
2001.
18 SAINT+ version 6.22a, Bruker AXS Inc., Madison, Wisconsin, USA,
2001.
19 G. W. Sheldrick, SADABS version 2.10, University of Go¨ttingen, 2001.
20 SHELXTL version 6.14, Bruker AXS Inc., Madison, Wisconsin, USA,
2000.
Notes and references
1 For a recent review, see: F. E. Hahn and M. C. Jahnke, Angew. Chem.,
Int. Ed., 2008, 47, 3122.
7268 | Dalton Trans., 2009, 7262–7268
This journal is
The Royal Society of Chemistry 2009
©