Nickel(II) benzimidazolin-2-ylidene complexes with thioether-functionalized side chains as catalysts for Suzuki-Miyaura cross-coupling

Article abstract of DOI:10.1021/om500484q

Four bis(benzimidazolin-2-ylidene) nickel(II) complexes featuring thioether moieties in the side chain have been synthesized by reactions of the respective benzimidazolium salts with nickel(II) acetate in molten tetrabutylammonium bromide as an ionic liqu

Full text of DOI:10.1021/om500484q

Article
pubs.acs.org/Organometallics
Nickel(II) Benzimidazolin-2-ylidene Complexes with Thioether-
Functionalized Side Chains as Catalysts for Suzuki−Miyaura Cross-
Coupling
Jan C. Bernhammer and Han Vinh Huynh*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117513, Singapore
S
* Supporting Information
ABSTRACT: Four bis(benzimidazolin-2-ylidene) nickel(II)
complexes featuring thioether moieties in the side chain have
been synthesized by reactions of the respective benzimidazo-
lium salts with nickel(II) acetate in molten tetrabutylammo-
nium bromide as an ionic liquid. All complexes were obtained
as inseparable mixtures of trans-syn and trans-anti rotamers, as
evidenced by NMR spectroscopy. For one of the complexes,
X-ray diffraction confirmed the square-planar coordination
geometry. The catalytic activity of all complexes for Suzuki−Miyaura cross-coupling was examined. Under the optimized
conditions, both aryl bromides and aryl chlorides were successfully coupled in the presence of triphenylphosphine as additive.
Yields ranged from good to moderate for electron-deficient aryl halides, while electron-rich aryl halides were found to be
unreactive.
catalysts for the Kumada−Corriu cross-coupling,12 but they
INTRODUCTION
■
were also found to be active in other types of cross-
N-heterocyclic carbenes (NHC) have established themselves as
couplings,11,13 hydrosilylation reactions,14 the coupling and
an important class of ligands in transition-metal catalysis,
oligomerization of olefins,15 and numerous other trans-
rivaling the ubiquitous phosphine ligands.1 The reasons are the
robustness of their binding to the metal center,2 their strong
formations. A small number of sulfur-functionalized NHC
electron-donating abilities,3 their ease of synthesis,4 and their
complexes of nickel are known to date, but to the best of our
knowledge their catalytic activity has never been thoroughly
structural diversity, which allows for the fine tuning of steric
explored.16 For this reason, we decided to synthesize nickel(II)
and electronic properties independently from each other.5 A
complexes with recently successfully employed thioether-
dazzling variety of these ligands have been synthesized, and the
functionalized NHC ligands and explore their catalytic
quest for highly sterically demanding and unusually electron-
activities.17
rich NHC ligands is still ongoing.6 The incorporation of
additional donor functionalities in the side chains has been
RESULTS AND DISCUSSION
particularly fruitful for catalytic applications as well. Phospho-
rus-based donor functionalities enable the synthesis of chelating
complexes with a combination of the characteristic advantages
of both NHC and phosphine complexes,7 and hemilabile donor
functionalities based on nitrogen, oxygen, or sulfur allow for
efficient stabilization of the catalytically active species while
rapidly opening up free coordination sites for incoming
substrates.8,9 Numerous chelating and pincer-type ligands
based on NHCs have been reported and successfully applied
in catalysis.10
Among the various donor functionalities, sulfur-based donor
moieties have been comparatively less explored, despite
showing particularly promising hemilabile behavior in their
late-transition-metal complexes. Most of the research in this
field has focused on palladium(II) complexes, with less
attention being paid to the other group 10 metals.9d,e,h
However, nickel catalysis remains an attractive field of study,
due to the considerably lower cost associated with this metal,
while maintaining similar reactivity patterns.11 Nickel com-
plexes containing NHC ligands are well-known to be excellent
■
Synthesis of Complexes. Nickel(II) NHC complexes have
been prepared by the reaction of free carbenes with suitable
nickel precursors, by in situ deprotonation of the azolium
precursor salts using basic nickel species, and in some rare
instances by transmetalation.18,11−15 However, not all of these
methods are useful for the synthesis of homo(bis)-NHC
complexes with benzimidazolin-2-ylidene (bimy) ligands, since
these ligands are difficult to handle as free carbenes due to their
high tendency to dimerize. A convenient pathway for the
formation of these complexes is the reaction of Ni(OAc)2 with
benzimidazolium salts in a large excess of molten tetrabuty-
lammonium bromide (TBAB) under vacuum.19 Under these
conditions, acetic acid is constantly removed from the reaction
Special Issue: Catalytic and Organometallic Chemistry of Earth-
Abundant Metals
Received: May 7, 2014
© XXXX American Chemical Society
A
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mixture, driving the reaction toward completion. A high
vacuum was found to be absolutely crucial for this reaction to
proceed. Benzimidazolium salts 1−4 bearing alkyl thioether
side chains with ethylene or propylene linkers, and a benzyl or
isobutyl group as a second side chain, were reacted, and the
desired complexes were obtained in moderate to good yields
using this methodology (Scheme 1). TBAB could be
conveniently removed by triturating the solidified reaction
mixture with water, and the complexes were isolated by
subsequent filtration.
Figure 1. Dynamic equilibrium between syn and anti rotamers of
complex 5 and shielding effect of the phenyl rings.
Scheme 1. Synthesis of Homo(bis)-NHC Complexes of
Nickel(II)
through the magnetic anisotropy effect of the phenyl rings. The
major rotamer gives rise to a more downfield signal for the
benzylic methylene moiety and more upfield signals for the
SCH2 and SCH groups. In the trans-anti rotamer, the shielding
effect of the phenyl groups acts on the thioether side chain,
leading to an upfield shift of the affected resonances, while in
the trans-syn rotamer, the same effect influences the chemical
shift of the benzylic methylene group (Figure 1). Similar
shielding effects have been reported for rotamers of trans-
[MBr2(NHC)2] complexes (M = Pd, Ni), providing a useful
method for spectroscopic analysis.20
Thus, it can be concluded that the major rotamer of 5 and 7
is the trans-anti form. As mentioned above, no clear separation
of side chain signals was observed for complexes 6 and 8
bearing isobutyl-substituted NHC ligands. However, due to the
steric bulk of the isobutyl side chains, the trans-anti rotamer
might be slightly favored in 8, similarly to the benzyl-
substituted NHC complexes.
The complexes were obtained as deep red solids. However,
only complex 5 was isolated in a microcrystalline form, while
the other three complexes remained amorphous, waxy solids
despite our best efforts at crystallizing them from a variety of
different solvent systems. The long, flexible, yet sterically bulky
side chains might hamper efficient crystal packing. Trace
impurities could thus not be completely removed, making it
impossible to obtain matching elemental analyses for
compounds 7 and 8.
All complexes are readily soluble in most common organic
solvents, with the exception of very protic polar solvents such as
ethanol, methanol, and water. Purification by column
chromatography was possible, but in most runs, the product
was already obtained in sufficiently pure form after aqueous
washing.
Two sets of signals were observed in the 13C NMR spectra of
all complexes as well. The carbene carbon resonances have
chemical shifts in the narrow range from 183.9 to 185.5 ppm,
which is typical for trans-[NiBr2(bimy)2] complexes.14a,19,21
The assignment of the rotameric species was further
corroborated by means of computational chemistry. Using a
simplified model for complex 5 with ethyl side chains instead of
the conformationally more flexible 2-isopropylthioethyl side
chain, the geometries of the trans-anti and trans-syn rotamers
were optimized at the B3LYP/cc-pVDZ level of theory (Figure
2).22,23 The optimized geometry of the trans-anti rotamer is in
good agreement with the experimental molecular structure.
The trans-anti rotamer was found to be 1.85 kJ/mol lower in
energy than the trans-syn rotamer, confirming the results
obtained by NMR spectroscopy. This energy difference
translates to an equilibrium constant of 2.1, which is in
reasonably good agreement with the experimental value of
∼1.5.
The 1H NMR spectra of all complexes showed the
disappearance of the downfield resonance assigned to the
acidic proton in the precursor salts, indicating successful
formation of the desired complexes. Two distinct sets of signals
were observed for the complexes, corresponding to an
inseparable mixture of the trans-syn and trans-anti rotamers
(Figure 1).
No very clear preference for one rotameric form over the
other was observed. In complex 6, the observed ratio of
rotamers was ∼1:1, due to the close similarity in bulk between
the isobutyl groups and the thioether side chains. This changed
with the introduction of the benzyl group in complexes 5 and 7
or the longer side chain in complex 8, with a 3:2 ratio of
Molecular Structure. Single crystals of 5 suitable for X-ray
diffraction studies were obtained by slow evaporation of a
concentrated solution in a mixture of dichloromethane and
toluene. No single crystals could be obtained for the other
complexes, but the similarity in their spectroscopic character-
istics suggests that 5 is a good structural model for all four
complexes. As suggested by NMR spectroscopy, a square-planar
geometry around the nickel(II) center was found, with a trans
arrangement of the benzimidazolin-2-ylidene ligands (Figure
3). The Ni−C and Ni−Br bond length fall within the expected
range, with distances of 1.911(5) and 2.3124(8) Å,
respectively.19,21 The NHC ring planes are almost perpendic-
ular to the coordination plane with a dihedral angle of 75.5(5)°.
1
rotamers observed in H NMR. The correct assignment of the
NMR resonances to the respective rotamers is difficult for
complexes 6 and 8, which only contain aliphatic side chains.
However, the installation of the benzyl side chain in 5 and 7
allows for a clear spectroscopic differentiation between the
isomers. The 1H NMR resonances assignable to the side chains
in the trans-syn and trans-anti rotamers were well separated, and
the benzyl group acts as a “built-in spectroscopic probe”
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Figure 2. Optimized geometries of the trans-anti and trans-syn rotamers of the simplified model for complex 5.
catalytic activities in the presence of catalytic amounts of free
phosphine have been reported previously by Lee et al.,13a,28 and
this effect was also observed for complex 5 (Table 1, entries 3
and 8). Only trace amounts of biphenyl arising as a side
product from the homocoupling of 10 were observed.
a
Table 1. Optimization of Reaction Conditions
b
entry
base
K2CO3
temp (°C)
yield (%)
1
2
3
4
5
6
7
90
90
90
90
90
90
90
90
80
70
60
50
40
30
70
70
70
70
22
64
99
0
Cs2CO3
KOH
NaOAc
NaOAc·3H2O
KOAc
4
Figure 3. Molecular structure of trans-anti-5. Thermal ellipsoids are
shown at 50% probability; hydrogen atoms and disorder in the side
chain have been omitted for clarity. Selected bond lengths (Å) and
angles (deg): Ni1−C1 1.911(5), Ni1−Br1 2.3124(8); Br1−Ni1−C1
90.1(2)/89.9(2), Br1−Ni1−Br1′ 180.0(2), C1−Ni1−C1′ 180.0(2).
0
K3PO4·H2O
KOH
>99
32
>99
>99
>99
54
6
c
8
9
KOH
10
11
12
13
14
15
16
17
KOH
KOH
The unsymmetrically substituted bimy ligands adopt an anti
arrangement, in line with the spectroscopic data for the major
rotamer.
KOH
KOH
KOH
0
Suzuki−Miyaura Cross-Coupling. Among the various
cross-coupling reactions for the construction of C−C bonds,
Suzuki−Miyaura cross-coupling has gained the most popularity.
The reasons are its reliability, the exceptional functional group
tolerance, and the mild reaction conditions.24 The organoboron
reagents it requires are readily available and nontoxic and can
be handled in the presence of air and moisture. While
palladium compounds are still the catalysts of choice for this
reaction, nickel catalysts are increasingly being investigated for
economic reasons.11b−e,13a−d,25 Additionally, the smaller size of
nickel and the accessibility of both the Ni(I) and Ni(III)
oxidation states give it sometimes a distinct reactivity difference
from palladium, possibly allowing for the cross-coupling of
substrates that is otherwise difficult to achieve.11e,26,27
As a starting point for the study of the catalytic activity of 5−
8, the reaction between 4-bromoacetophenone (9) and
phenylboronic acid (10) was chosen (Scheme 2). Improved
KOH (2.0 equiv)
KOH (1.5 equiv)
KOH (1.3 equiv)
KOH (2.0 equiv)
>99
89
72
0
d
18
a
Reaction conditions: 5 (1 mol %), 4-bromoacetophenone (1.0
mmol), phenylboronic acid (1.3 equiv), PPh3 (2 mol %), base (2.6
equiv), anhydrous toluene (3 mL), 1 h. Yields determined by GC-MS
with decane as internal standard; average of two runs. Reaction run
without addition of PPh3. Reaction run without exclusion of air and
b
c
d
moisture.
In order to improve the reaction conditions, several strong
bases were tested (Table 1, entries 1−7). Among these, only
potassium hydroxide and potassium phosphate gave quantita-
tive yields, while a moderate yield was achieved when using
cesium carbonate. With other bases, only small amounts of
biaryl 11 were observed or the reaction did not proceed at all.
Since both hydroxide and phosphate anions are known to play
an active role in the catalytic cycle by activating the boronic
acid or acting as a strongly basic ligand, these findings are
understandable.29
Scheme 2. Suzuki−Miyaura Coupling of 4-
Bromoacetophenone and Phenylboronic Acid
Using potassium hydroxide as a base, the possibility of
achieving milder reaction conditions by lowering the temper-
ature was examined (Table 1, entries 9−14). Quantitative yields
C
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were obtained at reaction temperatures as low as 60 °C, with a
sharp decline below this threshold and no product formation at
30 °C. Taking into account that 4-bromoacetophenone is a
reactive substrate, further optimizations were carried out at 70
°C to stay well above the minimally required temperature.
Next, it was examined if the large excess of base could be
reduced. Indeed, a reduction from 2.6 equiv of base to 2.0 equiv
resulted in no noticeable change in yield (Table 1, entry 15).
However, lower yields were obtained when using 1.5 and 1.3
equiv of base.
Using the optimal conditions for catalyst 5, the substrate
scope was explored (Scheme 4). Electron-poor aryl bromides
Scheme 4. Substrate Scope for the Suzuki−Miyaura
Coupling of Aryl Halides
Finally, the reaction was run without precautions to exclude
air and moisture, since such conditions would be highly
preferable. However, since the reduction of nickel(II) by
phenylboronic acid to a sensitive, catalytically active nickel(0)
species occurs as the initiation step of the reaction, no product
was observed under these conditions.13b
With the improved conditions in hand, the catalytic activity
of complexes 5−8 was compared. To allow for a better
differentiation between the precatalysts, the more electron-rich
and less reactive 4-bromotoluene (12) was used as a substrate
instead of 4-bromoacetophenone (Scheme 3 and Table 2).
were coupled in good to quantitative yields. Notable exceptions
were 4-nitrobromobenzene, which failed to react completely,
and 3,5-difluorobenzene, which gave only a moderate yield
(Table 3, entries 7 and 8).
a
Table 3. Suzuki−Miyaura Coupling of Aryl Halides
b
yield (%)
entry
substrate
4-COCH3
aryl bromide
aryl chloride
c
1
100
100
75
100
c
2
4-CHO
75
3
3-CHO
Scheme 3. Catalyst Screening for the Coupling of 4-
Bromotoluene and Phenylboronic Acid
4
2-CHO
72
c
c
5
4-H
76
36
c
6
4-CN
100
0
100
7
4-NO2
0
c
8
3,5-difluoro
4-CH3
44
c
9
60
c
10
11
12
13
3-CH3
77
c
Table 2. Screening of Precatalysts and Further
2-CH3
72
a
Optimization
2-halopyridine
2-halothiophene
1,3-dihalobenzene
4-chlorobromobenzene
0
0
traces
b
entry
catalyst
yield (%)
d
e
14
38
1
2
3
4
5
0
d
f
15
56
5
6
7
8
5
5
5
5
23
19
16
22
27
33
35
43
a
Reaction conditions: precatalyst (1 mol %), aryl halide (1.0 mmol),
phenylboronic acid (1.3 equiv), PPh3 (2 mol %), K3PO4·H2O (2.0
b
equiv), anhydrous toluene (3 mL), 70 °C, 2 h. Yields determined by
c
GC-MS with decane as internal standard; average of two runs. 24 h.
cd
,
6
7
8
d
Phenylboronic acid (2.6 equiv), K3PO4·H2O (4.0 equiv), anhyd.
de
,
e
Toluene (6 mL) used instead. 1,3-terphenyl, trace amounts of biaryl.
df
,
f
biaryl, trace amounts of 1,4-terphenyl.
dg
,
9
a
Reaction conditions: precatalyst (1 mol %), 4-bromotoluene (1.0
Among the examined aryl chlorides, the electron-poor
substrates generally performed best as well, even though they
were found to react more sluggishly than their aryl bromide
counterparts. Electron-poor systems such as 4-chloroacetophe-
none and 4-chlorobenzonitrile gave quantitative yields, but to
achieve full conversion it was necessary to extend the reaction
time to 24 h instead of 2 h (Table 3, entries 1 and 6). This
points to a low turnover frequency of the catalyst, but catalyst
decomposition does not occur during this period in the absence
of oxygen. More electron rich aryl halides such as
bromotoluene and bromobenzene were cross-coupled in
moderate yields, and a low yield of biaryl was obtained when
using chlorobenzene (entry 5).
Steric hindrance has only a minor influence on product yield
in comparison to electronic factors. Bromobenzaldehydes with
the aldehyde moiety at the 4-, 3-, and 2-positions were coupled
in quantitative to excellent yields, with a smaller yield for the 3-
and 2-substituted aryl halides (Table 3, entries 2−4). In
contrast, the corresponding bromotoluenes gave moderate
yields and needed longer reaction times, and no correlation
mmol), phenylboronic acid (1.3 equiv), PPh3 (2 mol %), KOH (2.0
b
equiv), anhydrous toluene (3 mL), 70 °C, 1 h. Yields determined by
c
GC-MS with decane as internal standard; average of two runs. 2 h.
d
e
f
g
Single run. 3 h. 6 h. 2 h, K3PO4·H2O used instead of KOH.
While in the absence of any precatalyst no reaction occurred,
small amounts of biaryl 13 were formed in the presence of 1
mol % of the nickel complexes. Neither the nature of the
nonfunctionalized side chain nor the length of the thioether
side chain had a clearly discernible impact on the obtained
yields, which fluctuated in a narrow range from 16 to 23%.
However, since complex 5 gave a slightly better yield than its
congeners, it was used for all further reactions.
In an attempt to improve the paltry yields in the cross-
coupling of 12, the reaction time was expanded, yet the increase
in product formation was only moderate. By contrast, changing
the base from potassium hydroxide to potassium phosphate
almost doubled the yield with only a slightly expanded reaction
time (Table 2, entry 9).
D
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minor rotamer) 7.76−7.53 (m, 3 H, Ar-H), 7.53−7.28 (m, 9 H, Ar-H),
7.22−7.10 (m, 2 H, Ar-H), 7.08−6.87 (m, 4 H, Ar-H), 6.46 (s, 4 H,
NCH2Ph), 5.56−5.47 (m, 4 H, NCH2), 3.73−3.60 (m, 4 H, SCH2),
between yield and substitution pattern was observed at all
(entries 9−11).
Heterocycles are not good substrates for the reaction. No
cross-coupling was observed for either 2-bromopyridine or 2-
bromothiophene, and only a trace amount of biaryl was found
with 2-chloropyridine as a substrate.
1,3-Dibromobenzene gave almost exclusively the terphenyl
product. Only trace amounts of 3-bromobiphenyl were present
in the reaction mixture. In contrast, 4-chlorobromobenzene
gave almost pure 4-chlorobiphenyl, demonstrating that aryl
bromides react considerably faster than aryl chlorides.
While more efficient nickel(II) NHC catalysts have been
described, the overall activity of our system, representing the
first catalytic study on sulfur-functionalized NHC nickel
complexes, is nevertheless encouraging.11e,30
3
3
3.18 (sept, JH−H = 7 Hz, 2 H, SCH), 1.38 (d, JH−H = 7 Hz, 12 H,
CH3). 13C{1H} NMR (75 MHz, CDCl3): δ (trans-anti, major rotamer)
185.5 (Ccarbene), 136.1, 135.9, 135.5, 129.5, 128.8, 128.5, 123.2, 112.0,
110.4 (Ar-C), 54.2 (NCH2Ph), 49.2 (NCH2), 36.1 (SCH), 30.2
(SCH2), 24.2 (CH3); δ (trans-syn, minor rotamer) 185.5 (Ccarbene),
136.1, 135.9, 135.2, 129.3, 128.8, 128.5, 123.2, 111.6, 110.7 (Ar-C),
53.5 (NCH2Ph), 49.4 (NCH2), 36.3 (SCH), 30.5 (SCH2), 24.4
(CH3). Anal. Calcd for C38H44Br2N4NiS2: C, 54.37; H, 5.28; N, 6.67.
Found: C, 54.45; H, 4.79; N, 6.59. MS (ESI): m/z 759 [M − Br]+.
trans-Dibromidobis(1-isobutyl-3-(2-(isopropylthio)ethyl)-
benzimidazolin-2-ylidene)nickel(II) (6). 1-Isobutyl-3-(2-
(isopropylthio)ethyl)benzimidazolium bromide (357 mg, 1.00 mmol,
2.0 equiv), nickel(II) acetate (88 mg, 0.50 mmol, 1.0 equiv), and
TBAB (1.00 g) were dried under vacuum at 80 °C for 2 h. Then the
temperature was brought slowly to 130 °C and maintained constant
for 12 h. After this time the reaction mixture was cooled to ambient
temperature. Trituration with water and subsequent filtration gave the
product as a red solid (195 mg, 51%, mixture of trans-syn and trans-anti
CONCLUSION
■
A series of four nickel(II) homo(bis)-NHC complexes has been
synthesized. All complexes were obtained as a mixture of the
trans-syn and trans-anti rotamers and showed an uncommonly
high solubility in common organic solvents. The trans-anti
rotamer of one complex could be characterized by X-ray
crystallography, and all complexes were fully characterized by
NMR spectroscopy and mass spectrometry. They all showed
good to moderate activity for Suzuki−Miyaura cross-coupling
in the presence of triphenylphosphine, with little variance
between the individual complexes. Under optimized conditions,
electron-deficient aryl bromides were coupled in good to
excellent yields, and moderate yields were obtained for more
electron-rich aryl bromides. Electron-poor aryl chlorides
needed longer reaction times but could be coupled in good
to excellent yields, while more electron rich substrates showed
low reactivity. Overall, this study demonstrates that nickel
complexes of thioether-functionalized NHCs are active catalysts
for the Suzuki−Miyaura reaction. The extension to other sulfur
groups or incorporation of other donor functionalities,
especially tethered phosphines, is a possible avenue for further
catalyst optimization. A more comprehensive study could also
address the influence of the noncoordinating side chains.
Research efforts at such more efficient nickel(II) NHC
complexes are in progress in our laboratory.
1
rotamers). H NMR (500 MHz, CDCl3): δ (both rotamers) 7.49−
7.30 (m, 4 H, Ar-H), 7.28−7.15 (m, 4 H, Ar-H), 5.71−5.22 (m, 4 H,
NCH2), 5.05−4.88 (m, 4 H, NCH2), 3.71−3.57 (m, 2 H, SCH2),
3.57−3.45 (m, 2 H, SCH2), 3.41−3.21 (m, 2 H, SCH), 3.18−3.02 (m,
2 H, CH), 1.33 (d, 3JH−H = 6 Hz, 12 H, CH3), 1.26 (d, 3JH−H = 6 Hz, 6
3
H, CH3), 1.22 (d, JH−H = 6 Hz, 6 H, CH3). 13C{1H} NMR (125
MHz, CDCl3): δ (both rotamers) 184.5, 184.3 (Ccarbene), 136.5, 136.2,
135.6, 135.2, 122.9, 111.3, 111.2, 110.9, 110.5 (Ar-C), 56.3, 56.3, 49.4,
49.1 (NCH2), 36.5, 36.3 (SCH), 30.4, 30.1 (SCH2), 29.9, 29.6 (CH),
24.4, 24.3, 21.8, 21.7 (CH3). Anal. Calcd for C32H48Br2N4NiS2: C,
49.83; H, 6.27; N, 7.26. Found: C, 49.70; H, 6.23; N, 7.03. MS (ESI):
m/z 691 [M − Br]+.
trans-Dibromidobis(1-benzyl-3-(3-(isopropylthio)propyl)-
benzimidazolin-2-ylidene)nickel(II) (7). 1-Benzyl-3-(2-
(isopropylthio)propyl)benzimidazolium bromide (405 mg, 1.00
mmol, 2.0 equiv), nickel(II) acetate (88 mg, 0.50 mmol, 1.0 equiv),
and TBAB (1.00 g) were dried under vacuum at 80 °C for 2 h. Then
the temperature was brought slowly to 130 °C and maintained
constant for 12 h. After this time the reaction mixture was cooled to
ambient temperature. Trituration with water and subsequent filtration
gave the product as a red solid (249 mg, 57%, mixture of trans-syn and
trans-anti rotamers). 1H NMR (500 MHz, CDCl3): δ (trans-anti,
major rotamer) 7.69 (d, JH−H = 7 Hz, 2 H, Ar-H), 7.48−7.28 (m, 10 H,
Ar-H), 7.18−7.12 (m, 2 H, Ar-H), 7.05−6.94 (m, 4 H, Ar-H), 6.69 (s,
4 H, NCH2Ph), 5.33−5.26 (m, 4 H, NCH2), 2.81 (sept, 3JH−H = 7 Hz,
2 H, SCH), 2.71−2.62 (m, 4 H, SCH2), 2.59−2.54 (m, 4 H, CH2),
1.21 (d, 3JH−H = 7 Hz, 12 H, CH3); δ (trans-syn, minor rotamer) 7.69
(d, JH−H = 7 Hz, 2 H, Ar-H), 7.48−7.28 (m, 10 H, Ar-H), 7.18−7.12
(m, 2 H, Ar-H), 7.05−6.94 (m, 4 H, Ar-H), 6.46 (s, 4 H, NCH2Ph),
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out without
precautions to exclude air and moisture, unless stated otherwise. All
solvents and chemicals were used as received or dried using standard
procedures when appropriate. NMR chemical shifts (δ) were internally
referenced to the residual solvent signals relative to tetramethylsilane
(1H, 13C). Elemental analyses were performed at the Department of
Chemistry, National University of Singapore. The ligand precursors
1−4 were synthesized following a previously reported procedure.17a
trans-Dibromidobis(1-benzyl-3-(2-(isopropylthio)ethyl)-
benzimidazolin-2-ylidene)nickel(II) (5). 1-Benzyl-3-(2-
(isopropylthio)ethyl)benzimidazolium bromide (783 mg, 2.00 mmol,
2.0 equiv), nickel(II) acetate (177 mg, 1.00 mmol, 1.0 equiv), and
TBAB (2.00 g) were dried under vacuum at 80 °C for 2 h. Then the
temperature was brought slowly to 130 °C and maintained constant
for 12 h. After this time the reaction mixture was cooled to ambient
temperature. Trituration with water and subsequent filtration gave the
product as a red solid (631 mg, 75%, mixture of trans-syn and trans-anti
rotamers). 1H NMR (300 MHz, CDCl3): δ (trans-anti, major rotamer)
7.76−7.53 (m, 3 H, Ar-H), 7.53−7.28 (m, 9 H, Ar-H), 7.22−7.10 (m,
2 H, Ar-H), 7.08−6.87 (m, 4 H, Ar-H), 6.71 (s, 4 H, NCH2Ph), 5.47−
3
3
5.39 (t, JH−H = 7 Hz, 4 H, NCH2), 3.05 (sept, JH−H = 7 Hz, 2 H,
3
SCH), 2.93−2.83 (m, 8 H, SCH2, CH2), 1.35 (d, JH−H = 6.6 Hz, 12
H, CH3). 13C{1H} NMR (125 MHz, CDCl3): δ (trans-anti, major
rotamer) 185.3 (Ccarbene), 136.3, 136.1, 135.4, 129.6, 128.6, 128.4,
123.1, 123.0, 111.7, 110.5 (Ar-C), 54.0 (NCH2Ph), 47.9 (NCH2), 35.9
(SCH), 30.1 (SCH2), 28.6 (CH2), 24.1 (CH3); δ (trans-syn, minor
rotamer) 185.2 (Ccarbene), 136.2, 136.0, 135.6, 129.3, 128.7, 128.4,
123.1, 123.0, 111.4, 110.7 (Ar-C), 53.4 (NCH2Ph), 47.8 (NCH2), 36.0
(SCH), 30.4 (SCH2), 29.0 (CH2), 24.2 (CH3). MS (ESI): m/z 787
[M − Br]+. Despite our best efforts, no matching elemental analysis
could be obtained for this complex.
trans-Dibromidobis(1-isobutyl-3-(3-(isopropylthio)propyl)-
benzimidazolin-2-ylidene)nickel(II) (8). 1-Isobutyl-3-(2-
(isopropylthio)propyl)benzimidazolium bromide (371 mg, 1.00
mmol, 2.0 equiv), nickel(II) acetate (88 mg, 0.50 mmol, 1.0 equiv),
and TBAB (1.00 g) were dried under vacuum at 80 °C for 2 h. Then
the temperature was brought slowly to 130 °C and maintained
constant for 12 h. After this time the reaction mixture was cooled to
ambient temperature. Trituration with water and subsequent filtration
gave the product as a red solid (351 mg, 88%, mixture of trans-syn and
5.35 (m, 4 H, NCH2), 3.52−3.41 (m, 4 H, SCH2), 2.90 (sept, 3JH−H
=
3
7 Hz, 2 H, SCH), 1.15 (d, JH−H = 7 Hz, 12 H, CH3); δ (trans-syn,
E
dx.doi.org/10.1021/om500484q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
trans-anti rotamers). 1H NMR (500 MHz, CDCl3): δ (both rotamers)
7.54−7.45 (m, 2 H, Ar-H), 7.38−7.29 (m, 2 H, Ar-H), 7.24−7.15 (m,
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̈
4 H, Ar-H), 5.59−5.40 (m, 4 H, NCH2), 4.97 (d, JH−H = 8 Hz, 4 H,
̈
NCH2), 3.49−3.36 (m, 1 H, SCH), 3.30−3.19 (m, 1 H, SCH), 3.08−
2.95 (m, 2 H, CH), 2.89−2.73 (m, 8 H, SCH2, CH2), 1.34−1.29 (m,
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Despite our best efforts, no matching elemental analysis could be
obtained for this complex.
General Procedure for Suzuki−Miyaura Cross-Couplings. A
Schlenk tube was charged with precatalyst (10 μmol, 1.0 mol %),
triphenylphosphine (5.2 mg, 20 μmol, 2.0 mol %), phenylboronic acid
(158 mg, 1.30 mmol, 1.30 equiv), potassium phosphate hydrate (461
mg, 2.00 mmol, 2.00 equiv), and the respective bromo- or chloroarene
if it was a solid (1.00 mmol, 1.00 equiv) under an inert nitrogen
atmosphere. Anhydrous toluene (3 mL) was added, followed by
bromo- or chloroarene if it was a liquid (1.00 mmol, 1.00 equiv). The
Schlenk tube was immersed into an oil bath preheated to 70 °C, and
the mixture was allowed to react for 2 h. After this time, the Schlenk
tube was taken out of the oil bath, the solution was diluted with
dichloromethane (10 mL), and decane as an internal standard was
added. Samples were analyzed by GC-MS.
Computational Methods. All calculations were carried out using
hybrid density functional theory (DFT) employing the B3LYP
functional22 and the Gaussian 09 software.31 The nature of all
stationary points was confirmed by frequency analysis, and all
geometries were found to represent minima on the potential energy
surface. For the optimization of complex geometries and the
calculation of their Gibbs free energies, all elements were described
with the cc-pVDZ basis set.23 All basis sets were obtained from the
EMSL Basis Set Library.32
X-ray Diffraction Studies. X-ray data were collected with a
Bruker AXS SMART APEX diffractometer, using Mo Kα radiation at
100(2) K, with the SMART suite of programs.33 Data were processed
and corrected for Lorentz and polarization effects with SAINT34 and
for absorption effects with SADABS.35 Structure solution and
refinement were carried out with the SHELXTL suite of programs.36
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 placed at calculated positions. All non-hydrogen
atoms were generally given anisotropic displacement parameters in the
final model. A summary of crystallographic data is given in Figure 3
and the Supporting Information.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Figures giving 1H and 13C NMR spectra for all complexes, CIF
file and a table giving crystallographic data for 5, and tables
giving Cartesian coordinates for the calculated structures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
(12) (a) Bohm, V. P. W.; Weskamp, T.; Gstottmayr, C. W. K.;
̈
̈
■
Herrmann, W. A. Angew. Chem., Int. Ed. 2000, 39, 1602. (b) Matsubara,
K.; Ueno, K.; Shibata, Y. Organometallics 2006, 25, 3422. (c) Huynh,
H. V.; Jothibasu, R. Eur. J. Inorg. Chem. 2009, 1926. (d) Zhang, C.;
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Corresponding Author
*E-mail for H.V.H.: chmhhv@nus.edu.sg.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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(13) (a) Lee, C.-C.; Ke, W.-C.; Chan, K.-T.; Lai, C.-L.; Hu, C.-H.;
Lee, H. M. Chem. Eur. J. 2007, 13, 582. (b) Ritleng, V.; Oertel, A. M.;
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We thank the National University of Singapore and the
Singapore Ministry of Education for financial support (WBS R
143-000-483-112 and SINGA scholarship) and the CMMAC
staff of the department of chemistry for technical assistance.
F
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