[NiX2(NHC)2]complexes in the hydrosilylation of internal alkynes

Article abstract of DOI:10.1002/ejic.201100015

A number of nickel(II) dihalide complexes with small monodentate N-heterocyclic carbene ligands was synthesized and tested for their catalytic activity in the hydrosilylation of internal alkynes. The nickel(0) active species was obtained from the starting

Full text of DOI:10.1002/ejic.201100015

FULL PAPER
DOI: 10.1002/ejic.201100015
[NiX2(NHC)2] Complexes in the Hydrosilylation of Internal Alkynes
Joris Berding,[a] John A. van Paridon,[a] Vincent H. S. van Rixel,[a] and
Elisabeth Bouwman*[a]
Keywords: Hydrosilylation / Nickel / Carbene ligands / NHC complexes / Alkynes / Homogeneous catalysis
A number of nickel(II) dihalide complexes with small mono- of the symmetric alkyne 3-hexyne in 60 min at 50 °C, with
dentate N-heterocyclic carbene ligands was synthesized and
tested for their catalytic activity in the hydrosilylation of in-
ternal alkynes. The nickel(0) active species was obtained
from the starting nickel(II) complex by reduction with dieth-
ylzinc. In all cases the catalytic reaction yielded the syn prod-
uct selectively. The fastest catalysts reached full conversion
5 mol-% catalyst loading. The asymmetrically substituted in-
ternal alkyne 1-phenyl-1-propyne gave full conversion
within 30 min. The major product (83%) was shown to be the
expected (E)-1-phenyl-1-(triethylsilyl)propene. The active
catalyst was demonstrated to be a homogeneous species.
been reported to catalyze this reaction. In general, the
hydrosilylation of internal alkynes is less explored than the
hydrosilylation of terminal alkynes.[15]
Introduction
N-Heterocyclic carbenes (NHCs) have been shown to be
versatile ligands in organometallic chemistry and cataly-
sis.[1] The bonding properties of these ligands are often
compared to those of well-known trialkylphosphanes, and
they may even be better σ-donors than these phosphanes.
This makes them good candidates for the stabilization of
transition-metal catalysts in various oxidation states during
the catalytic cycle. In addition, the shape and size of NHCs
can easily be modified by the introduction of various sub-
stituents on the heterocycle.
The hydrosilylation of C–C double and triple bonds is
one of the most important methods for the formation of
C–Si bonds and can be used to functionalize organic mole-
cules.[2] Vinylsilanes, obtained from the reaction between a
silane and an alkyne, are useful reagents in organic synthe-
sis.[3] A large issue in the direct hydrosilylation of alkynes is
the problem of stereoselectivity and regioselectivity, as the
use of a terminal alkyne can result in three isomeric vinyl-
silanes:[4] the α-silyl product and the β-(E)- and the β-(Z)-
stereoisomers (Scheme 1). Internal alkynes can give rise to
regioisomers and (E)- and (Z)-isomers. The selectivity
towards any of these products depends upon several factors,
such as the substituents on the alkyne and the silane, the
catalyst, and the reaction conditions. The hydrosilylation of
alkynes may be catalyzed by a number of different metal
complexes, including rhodium,[5–7] iridium,[6,8] ruthe-
nium[9,10] and platinum.[11–13] Complexes of the less pre-
Scheme 1. Products in the hydrosilylation of terminal alkynes.
Recently, Chaulagain et al. reported that a catalyst de-
rived from Ni(COD)2 (COD = cycloocta-1,5-diene), a bulky
1,3-diarylimidazolium salt and KOtBu is effective in the
hydrosilylation of alkynes.[17] The active catalyst is assumed
to form in situ by deprotonation of the imidazolium salt
and formation of the Ni0 NHC complex. In contrast, a mix-
ture of Ni(COD)2 and tributylphosphane was inactive un-
der the same conditions. In order to further investigate the
nickel-NHC complex catalyzed reaction we decided to pre-
pare a larger range of ligands, and to develop a protocol
starting from preformed and fully characterized nickel(II)-
NHC complexes.
In this report the preparation is described of a variety of
nickel(II) complexes bearing two monodentate NHC li-
gands and two halide anions, based on literature synthe-
ses.[18,19] These complexes are used in the catalytic hydro-
silylation of internal alkynes.
cious metals nickel,[14] cobalt,[15] and titanium[16] have also Results and Discussion
Ligand Synthesis
[a] Leiden Institute of Chemistry, Leiden University,
An overview of ligand precursors used in this study is
shown in Figure 1. The (benz)imidazolium salts were pre-
pared by the direct alkylation of a number of N-substituted
P. O. Box 9502, 2300 RA Leiden, The Netherlands
Fax: +31-71-5274761
E-mail: bouwman@chem.leidenuniv.nl
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[Nickel Complexes in the Hydrosilylation of Internal Alkynes
imidazoles and benzimidazoles. These quaternization reac-
tions are commonly performed in refluxing THF or aceto-
nitrile. However, to reduce reaction times the higher boiling
1,4-dioxane was used in a number of cases. The salts were
obtained in good yields as white to off-white solids and
most were found to be hygroscopic. These carbene precur-
1
sors were characterized by H and 13C NMR and IR spec-
troscopy, elemental analysis and mass spectrometry (ESI-
MS). The NMR spectra of the (benz)imidazolium salts in
deuterated DMSO showed a downfield shifted signal at
around 10 and 140 ppm, characteristic of the imidazolium
NCHN proton and carbon, respectively.[20]
Figure 2. Nickel complexes prepared in this study. R1 and R2 are
as given in Figure 1 for 1 and 2, respectively.
4d and 4e could not be observed, probably due to peak
broadening. Due to its poor solubility in common solvents,
a
13C NMR spectrum could not be recorded for complex
4e. The NMR spectra of 3b, 3e, 4b, 4c and 4d showed split-
ting of a number of resonances, indicating the presence of
two rotamers in solution, due to restricted rotation about
the Ni–C bond. This behaviour has been reported before
for complex 3d.[19] The NMR spectra of 4b and 4c, which
only differ in halide, are quite similar, as expected. Interest-
ingly, however, the isopropyl-CH and the NCH3 resonances
are shifted more downfield in the case of the bromide com-
plex 4b. The isopropyl-CH resonances of 4b are obscured
by resonances of the aromatic protons, but could be as-
Figure 1. Overview of ligand precursors used in this study.
1
signed with the aid of a H COSY NMR spectrum. The
Synthesis of Nickel(II) Complexes Bearing Two
Monodentate NHC Ligands
isopropyl-CH resonances of the iodide complex 4e are also
shifted less downfield than those reported for the bromide
analogue,[21] indicating a trend in the electronic properties
of the complexes depending on the halide anion. This
phenomenon has also been observed for the corresponding
PdII compounds.[24]
Following literature procedures,[19] the (benz)imid-
azolium salts, with the exception of 1f, were reacted at high
temperatures with Ni(OAc)2 to yield the [NiX2(NHC)2]
complexes depicted in Figure 2. In the cases in which the
melting point of the imidazolium salt was too high for the
reaction to occur successfully an additional low-melting
salt, tetrabutylammonium halide, was added as a solvent,
in an adaptation of the original procedure reported by
Huynh et al.[18] After aqueous work-up and purification,
Catalytic Studies: General Remarks
Complexes 3a–f and 4a–f were tested for their catalytic
the [NiX2(NHC)2] complexes were obtained as stable, activity in the hydrosilylation of internal alkynes with tri-
orange-red to purple powders. Complexes 3c, 3d, 4a and ethylsilane. As a benchmark, the symmetric 3-hexyne was
4f have been synthesized before, following these literature chosen as the internal alkyne. The results of the catalytic
procedures.[18,19,21] Complex 3a has been synthesized before, studies are given in Table 1 and Table 2. The activity is re-
by an analogous reaction in nitromethane.[22] Complex 3f ported as the time needed to consume all alkyne substrate
was obtained by transmetalation of the ligand from the sil- (t99), i.e. the time after which the alkyne could no longer be
ver(I) complex to NiBr2(PPh3)2. The silver(I) complex was detected by GC. As full conversion is reached asymptoti-
prepared in situ from ligand precursor 1f and Ag2O in cally, exact determination of t99 is difficult, and therefore
dichloromethane.[23]
the time at which 50% of the alkyne was consumed (t50) is
1
The H NMR spectra of all nickel complexes lacked the reported as well. A typical example of the evolution of the
characteristic imidazolium NCHN resonance at around substrate and the product in time for an active catalyst is
10 ppm, indicating successful carbene generation. The other shown in Figure 3, for a nickel compound that is preacti-
peaks present in the NMR spectra of the starting imid- vated with diethylzinc (Table 2, entry 12; see next section).
azolium salts could successfully be identified, albeit shifted
Two different isomers of the product may be obtained
slightly from their original position. The carbene carbon from the hydrosilylation of 3-hexyne (Figure 4), i.e. the (E)-
atom of the novel complexes 3e, 3f and 4b is observed in and the (Z)-isomer. In the current experiments, however,
their 13C NMR spectra at 174.7, 169.0 and 188.0 ppm, only one isomer was observed. Isolation and characteriza-
respectively. The carbene carbon atoms of complexes 3b, tion revealed it to be the (E)-isomer, in which the H and
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J. Berding, J. A. van Paridon, V. H. S. van Rixel, E. Bouwman
FULL PAPER
Table 1. The use of different compounds for the activation of com-
plex 3a in the hydrosilylation of 3-hexyne.[a]
Entry
Activator[b]
Amount[c] t50 [min][d] t99 [min][e]
1
2
3
4
5
6
7
8
none[f]
–
2
2
4
10
8
2
–
–
BuLi (0 °C)[g]
BuLi (–78 °C)
PhMgCl[f]
PhMgCl
10–50
40
–
200
–
70
40
30–250
140
–
n.d.
–
240
140
[f]
AlEt3
ZnEt2
ZnEt2
4
[a] Reagents and conditions: 0.05 mmol 3a, 1.0 mmol 3-hexyne, ac-
tivator, 2.0 mmol triethylsilane, 5 mL of THF, 50 °C. All catalytic
runs were performed in duplicate. The (E)-product is obtained se-
lectively. [b] Activator was added at room temp. unless noted other-
wise. [c] Equivalents of activator relative to nickel. [d] Time needed
to reach 50% conversion of 3-hexyne. [e] Time needed to reach full
conversion. [f] No conversion was observed after 5 h. [g] The ex-
periment was difficult to reproduce, and was performed 4 times. In
one run no activation occurred.
Figure 3. Typical example of the consumption of the substrate 3-
hexyne (᭡) and the evolution of the product (᭹) in time using
complex 4f (Table 2, entry 12). Solid line: regular run; dashed line:
with 100 equiv. Hg added after 30 min.
Table 2. Nickel-catalyzed hydrosilylation of 3-hexyne using com-
plexes 3 and 4.[a]
Figure 4. Possible isomers of the product of the hydrosilylation of
3-hexyne.
Entry Catalyst
Standard procedure[b]
With pre-activation[c]
t50 [min][d] t99 [min][e] t50 [min][d] t99 [min][e]
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
40
40
40
60
40
25
40
20
60
50
300
25
–
140
120
100
150
120
70
160
60
130
140
n.d.
80
40
45
35
40
14
22
45
25
25
25
160
150
120
140
60
80
180
80
plexes may be activated by reaction with an appropriate
silane, however, this process requires high temperatures.[26]
As a reducing agent butyllithium was considered, although
it was unclear whether this reagent could be used to activate
nickel(II) complexes, rather than nickel(II) salts.[27–29] In ad-
dition, triethylaluminium, diethylzinc and phenylmagne-
sium chloride were evaluated. Trialkylaluminium com-
pounds are known for the activation of Ni(acac)2 in Ziegler-
type catalysts, the zinc reagent is anticipated to have similar
alkylating properties,[30] and the Grignard reagent is used
for the activation of nickel dihalide complexes in the Kum-
ada coupling reaction.[31–34] The results of the catalytic
hydrosilylation of 3-hexyne with triethylsilane using com-
plex 3a in combination with these activators are summa-
rized in Table 1.
4a
4b
4c
4d
4e
4f
9
80
120
10
11
12
13
200 (45)[f] n.d. (150)[f]
20
60
[g]
–
–
[a] Reagents and conditions: 0.05 mmol Ni, 5 mL of THF,
0.2 mmol ZnEt2, 1.0 mmol 3-hexyne, 2.0 mmol triethylsilane,
50 °C. All catalytic runs were performed in duplicate. The (E)-prod-
uct is obtained selectively. [b] All reagents were mixed at room
temp. before heating to 50 °C. [c] Reaction mixture was stirred
10 min at 50 °C, before the alkyne and the silane were added. [d]
Time needed to reach 50% conversion. [e] Time needed to reach
Initially, in the butyllithium-activated experiments, the
full conversion. [f] Reaction mixture was stirred 30 min at 50 °C, activator was added to the nickel(II) complex at 0 °C (entry
before the alkyne and the silane were added. [g] No nickel complex
2), which caused a rapid color change of the solution from
was added. No conversion was observed after 5 h in the presence
red to yellow. As the catalytic results were difficult to repro-
of 0.2 mmol ZnEt2.
duce, it was suspected that this activation method was too
vigorous. It was decided to moderate the activation, by add-
the SiEt3-group are located on the same face of the double
ing the butyllithium at –78 °C, and allowing the reaction
bond, in agreement with the results of other nickel cata-
mixture to slowly warm up to room temperature (entry 3).
lysts.[12] In contrast, the Lewis acid-catalyzed hydro-
This modified procedure greatly improved the reproduc-
ibility of the catalytic experiments. In the case of the acti-
vation using AlEt3, PhMgCl or ZnEt2, no rapid color
silylation of alkynes has been reported to yield the (Z)-iso-
mer selectively.[25]
change was observed during the addition of the activator
to the solution of the nickel(II) complex at room tempera-
ture, and cooling was not employed in these experiments.
Catalyst Activation Procedures
The starting point of the catalytic cycle of the hydro- Unfortunately, no catalytic activity was observed after the
silylation of alkynes is a nickel(0) species. The nickel(II)- addition of AlEt3 (entry 6), and even an excess of PhMgCl
NHC compounds indeed are not active in the hydro- (entry 5) gave only partial activation.
silylation reaction under the conditions used in our study
In the catalytic experiments in which butyllithium was
(Table 1, entry 1). It has been shown that nickel(II) com- employed as activating agent, small amounts of two side-
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[Nickel Complexes in the Hydrosilylation of Internal Alkynes
products were observed. These were identified as butyltri- in a number of cases an induction time of 5 to 10 min is
ethylsilane and ethoxytriethylsilane (Figure 5). The first observed before full catalytic activity starts and with com-
may be formed by a reaction between butyllithium and tri- plex 4e no conversion is observed in the first 45 min.
ethylsilane, while the latter is most likely derived from
To eliminate the influence of the slow activation on the
butyllithium, triethylsilane and ethanol used in the work- overall activity, all catalytic runs were repeated with an al-
up procedure. In addition, it was observed that the use of ternative procedure in which the nickel(II) complex is acti-
a larger excess of butyllithium resulted in larger amounts of vated prior to the addition of the two reagents. Preacti-
these side products. As both side products are derived from vation is accomplished by stirring the mixture of the nickel
the silane reagent, which is used in excess, they are not complex with diethylzinc at 50 °C for 10 min, before the
taken into consideration for the calculation of the selectivity alkyne and the silane are added. The results obtained with
of the catalyst, which is based on conversion of the alkyne. this alternative procedure are included in Table 2.
In a number of cases the preactivation of the nickel(II)
complex significantly decreases the time needed to bring the
reaction to full conversion. For instance, using complex 3e
with pre-activation the time needed to reach full conversion
Figure 5. Side products observed in the butyllithium-activated cata- is halved compared to the regular procedure. Nickel iodide
lytic experiments.
complexes with isopropyl- and phenyl-substituted ligands
appear to benefit the most from the preactivation pro-
The catalyst activation with butyllithium at low tempera-
cedure. In some runs the catalytic activity appears to have
ture and the activation with 4 equiv. diethylzinc yielded a
decreased slightly, possibly due to catalyst decomposition
catalyst of equal activity. Because of the formation of side
in the absence of substrate, although this may be within
products and the laborious activation (cooling to –78 °C)
experimental error. However, as expected, the halide of the
when using butyllithium, and the insufficient activating
nickel(II) starting complex no longer has an influence on
properties of phenylmagnesium chloride and triethylalu-
the catalytic activity, as it is now removed at the activation
minium, it was decided to use 4 equiv. of diethylzinc as acti-
vator for the remainder of catalytic experiments.
step before the catalytic reaction starts (Table 2, entries 8
and 9). Complex 4e is highly insoluble and even after preac-
tivation for 30 min, some undissolved starting complex is
still present in the reaction mixture. Therefore, the results
shown for this complex are an underestimation of the true
Catalytic Activity: Effect of Different NHC Ligands
The results of the diethylzinc-activated, nickel-catalyzed catalytic activity of the nickel(0) species.
hydrosilylation of 3-hexyne using complexes 3a–f and 4a–f
are summarized in Table 2. In all cases the catalytic reaction
was performed using the following procedure: the nickel
Comparison of the Catalysts
complex is dissolved/suspended in THF and 3-hexyne is
The catalytic activity of the various nickel complexes is
added, followed by diethylzinc. Next, the silane is added dependent on the ligand substituents. For instance, an in-
and the reaction vessel is immediately placed in a preheated crease in the length of the alkyl chain from methyl to propyl
oil bath. All nickel complexes bearing monodentate N-het- in complexes 3a–3c, leads to a decrease in the time needed
erocyclic carbenes tested in this study are active hydro- to reach full conversion, although the more bulky isopropyl
silylation catalysts after activation with diethylzinc.
substituent leads to a less active catalyst in the case of the
The t50 and t99 values given in Table 2 are an indication imidazole-based carbene ligands. In the case of the benz-
the activity of the catalyst. However, if the activation is a imidazole-based carbene ligands, an increase in bulk
slow process, the nickel complex is not activated immedi- around the metal center clearly leads to enhanced catalytic
ately after the starting point of the catalytic run and the activity, although this is not apparent for complex 4e, which
true activity of the catalyst may be underestimated. There- was only partly activated.
fore, the overall activity of the catalyst is dependent on the
The most active catalysts found in this study are the N-
rate of the activation of the initial nickel(II) complex. The methyl-NЈ-phenyl-substituted imidazole-based complex 3e
activation of the catalyst may be influenced by (a) the halide and N,NЈ-dibenzyl-substituted benzimidazole-based com-
ion and (b) the solubility of the starting complex.
plex 4f, which both reach full conversion within 60 min.
It is often assumed that the activity of a complex is inde- The highest turnover frequency was observed with complex
pendent on the halide of the starting compound, as this 3e, as it reached 50% conversion in 14 min, which is equal
halide is removed during the activation process. However, it to 43 mol(mol cat)–1 h–1. In the case that no catalyst preac-
is clear (Table 2, entries 8 and 9) that the halide does have tivation is performed, the bromide complexes (3f, 4b and
an effect on the overall activity, as the substrate is con- 4f) are the most active.
sumed faster when starting from the bromide complex 4b,
To confirm that the nickel complex is the source of the
than from the iodide complex 4c. This indicates that in the catalytic activity, one experiment was performed without
case of the iodide complexes the activation is rather slug- addition of the nickel complex. As expected, after 5 h of
gish. Complexes 4a, 4c, and especially 4e are poorly soluble stirring at 50 °C no product could be observed (Table 2, en-
in THF, which may cause slow catalyst activation. Indeed, try 13).
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J. Berding, J. A. van Paridon, V. H. S. van Rixel, E. Bouwman
One method for excluding the observed catalytic activity
FULL PAPER
The hydrosilylation of 3-hexyne with triethylsilane has
been reported a number of times in the literature. For in- to be due to nickel nanoparticles is the so called “mercury
stance, 0.5 mol-% of a cationic rhodium complex in an test”. During the catalytic run a drop of metallic mercury
aqueous micellar system yielded 53% of the (E)-product in is added to the reaction mixture, which, in the case of nickel
3 h at room temperature.[35] A cobalt(I) complex was shown nanoparticles, should quench the reaction due to the forma-
to yield 71% of the (E)-product in 10 h, using 5 mol-% cata- tion of an HgNi3 or HgNi4 amalgam.[38] If, on the other
lyst loading at 40 °C.[15] However, using heterogeneous hand, the reaction continues, it is generally accepted that
platinum on carbon, the same product could be obtained the catalyst must be a homogeneous species. In the present
in 95% yield in 3 h using only 0.02 mol-% Pt at 80 °C.[12] study, 100 equiv. of mercury on nickel were added after
Although the latter catalyst is clearly the most efficient, the 30 min of reaction time. To ensure a large contact surface,
nickel catalyst under study may be an attractive alternative the reaction mixtures were stirred vigorously to create small
as it does not make use of the rather scarce and expensive mercury droplets. To exclude the possibility of an unin-
platinum or rhodium.
tended active heterogeneous catalyst being present, the mer-
In addition to the benchmark substrate 3-hexyne, the re- cury test was performed with all catalysts. In all cases the
activity and selectivity of a small series of other internal addition of mercury had no noticeable effect on the out-
alkynes was investigated as well. The results of this survey come of the catalytic reaction, in terms of rate or selectivity.
using complex 3a under the standard conditions are shown A comparative example of the evolution of 3-hexyne and
in Table 3. For example, the more sterically hindered di- the product in time during the mercury test is included in
phenylacetylene was fully converted into 1,2-diphenyl-1-tri- Figure 3.
ethylsilyl-cis-ethene (5) in 150 min (entry 1). In contrast, no
Although the mercury test is a fast and easy-to-perform
activity was observed using the even larger bis(triethylsilyl)- method for distinguishing homogeneous and heterogeneous
substituted acetylene (entry 3). Within 2 h the substrate 1,4- catalysts, it is not always conclusive.[39] For instance, poi-
dimethoxy-2-butyne was fully and cleanly converted into soning may be incomplete if the contact between the mer-
(E)-1,4-dimethoxy-2-triethylsilyl-2-butene (6). The asym- cury drop and the catalyst solution is not sufficient, the
metrically substituted internal alkyne 1-phenyl-1-propyne mercury may cause side reactions, or there simply may not
gave full conversion within 30 min. However, two products be enough mercury present to bind all nanoparticles.[40]
could be detected by GC. The major product was shown to Therefore, it was decided to perform a kinetic study on se-
be the expected (E)-1-phenyl-1-(triethylsilyl)propene (7a). lected catalytic systems. In theory, in the case of a homo-
NMR measurements indicate that the minor product is the geneous catalyst an increase in catalyst concentration
regioisomer (E)-1-phenyl-2-(triethylsilyl)propene.
should lead to a proportional increase in the overall rate of
the reaction, while for heterogeneous catalysts this is not
necessarily the case, due to the formation of different sizes
of metal clusters. The results of this kinetic study using
complex 3e are shown in Figure 6. Using different catalyst
concentrations, the product yield was determined after 15,
30 and 60 min. Clearly, an increase in catalyst concentration
gives a linear increase in the conversion within experimental
error and thus it may be concluded that the ZnEt2-activated
catalyst is truly homogeneous. Comparable results were ob-
tained using complex 3a.
Table 3. Catalytic hydrosilylation of various internal alkynes using
complex 3a.[a]
Entry Alkyne R–CϵC–RЈ
Product (yield)[b] t50 [min][c] t99 [min][d]
R
RЈ
1
2
3
4
Ph
Ph
5 (quant.)
50
40
–
150
120
–
CH3OCH2 CH3OCH2 6 (quant.)
(Et)3Si
Ph
[e]
(Et)3Si
Me
–
7a (83%)[f]
10
30
[a] Reagents and conditions: 0.05 mmol 3a, 5 mL of THF,
0.2 mmol ZnEt2, 1.0 mmol alkyne, 2.0 mmol triethylsilane, 50 °C.
All catalytic runs were performed in duplicate. All reagents were
mixed at room temp. before heating to 50 °C. [b] GC yield; isolated
yields are given in the experimental section. [c] Time needed to
reach 50% conversion. [d] Time needed to reach full conversion.
[e] No conversion was observed after 5 h. [f] 17% minor product
(7b).
Homogeneous vs. Heterogeneous Catalysis
A continuing discussion in homogeneous catalysis in-
volving zero-valent transition-metal intermediate species is
the question whether the active catalyst is indeed the homo-
geneous zero-valent metal complex, or that heterogeneous
metal nanoparticles or clusters are catalytically active. This
is of importance, especially since nanoparticles have been
shown to be catalytically active in a number of related
Figure 6. Product yield as a function of catalyst concentration after
a fixed time: (᭿) 60 min; (♦) 30 min; (᭡) 15 min.
As a final check for homogeneity it was attempted to
nickel-catalyzed reactions, such as hydrogenation[36] and the deliberately prepare a heterogeneous catalyst and to test its
Heck reaction.[37]
activity in the hydrosilylation of internal alkynes. The
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[Nickel Complexes in the Hydrosilylation of Internal Alkynes
heterogeneous nickel-catalyzed hydrosilylation of terminal resulting in the 1:1 adduct. Although there is no direct ex-
olefins has been reported.[41] However, to the best of our perimental evidence for this hypothesis, it is supported by
knowledge, no internal alkynes have been used in these reports of the isolation and characterization of nickel com-
studies, except one: Lappert et al. reported that Ni(acac)2, plexes with one NHC ligand and two coordinated alkenes
reduced by AlEt3, was inactive for the hydrosilylation of 2- as well as nickel complexes with two NHC ligands and only
hexyne and 4-octyne,[42] although with terminal alkynes a one coordinated alkene or alkyne.[44,45]
2:1 adduct could be obtained (Scheme 2).[43] Indeed, using
Ni(acac)2 and AlEt3 under the conditions of the present
Conclusions
study did not result in any conversion of 3-hexyne. There-
fore, it is concluded that the activity observed with com-
plexes 3a–f and 4a–f must arise from a homogeneous cata-
lyst.
In conclusion, the synthesis and characterization of
twelve monodentate N-heterocyclic carbene complexes of
nickel(II) and their activity in the catalytic hydrosilylation
of internal alkynes is reported. Four activating agents were
evaluated, from which diethylzinc was selected as being the
most efficient. Using a procedure in which the nickel cata-
lyst is preactivated, N-methyl-NЈ-phenyl-substituted com-
plex 3e and N,NЈ-dibenzyl-substituted complex 4f show the
highest activity in the hydrosilylation of 3-hexyne with tri-
ethylsilane, giving the desired (E)-product in quantitative
yield within 60 min at 5 mol-% catalyst loading.
Scheme 2. Hydrosilylation and dimerization of terminal alkynes
with a Ziegler-type catalyst.[42]
Mechanistic Considerations
Using the mercury test and kinetic studies, it was unam-
biguously shown that the active catalyst is a homogeneous
species.
Chaulagain et al.[17] reported that a mixture of 10 mol-
% Ni(COD)2, 10 mol-% of a bulky N,NЈ-diarylimidazolium
salt, such as N,NЈ-dimesitylimidazolium chloride, and
10 mol-% KOtBu in THF is active in the hydrosilylation
of internal alkynes, when using 2 equiv. of silane at room
temperature. However, slow addition of the alkyne was re-
quired to obtain the 1:1 adduct in good yield. Fast addition
of the alkyne led to the formation of a 2:1 adduct
(Scheme 3). In contrast, our catalytic system with two carb-
ene ligands is less active, and requires elevated tempera-
tures in order to proceed at an appreciable rate. In this case,
only the (E)-alkene product is observed and may be isolated
quantitatively.
Experimental Section
General Considerations: All experiments were performed under air
and moisture-free conditions under an argon atmosphere, unless
indicated otherwise. All chemicals were obtained from commercial
sources and used as received. THF and 1,4-dioxane were distilled
under an argon atmosphere from CaH2 and stored on molecular
sieves. Dry Ni(OAc)2 was obtained by heating the hydrate at 165 °C
under a stream of argon. Triethylsilane and 3-hexyne were degassed
and stored under argon on activated molecular sieves. N-Iso-
propylimidazole,[20] N-phenylimidazole,[46] N-isopropylbenzimid-
azole,[20] N-phenylbenzimidazole,[47] 1a,[48] 1b,[49] 1f,[23] 2e,[24] 3c,[19]
3d,[19] 4a,[18] and 4f[21] were synthesized according to literature pro-
cedures.
1H and 13C NMR spectra were recorded on a Bruker DPX300.
Chemical shifts are reported as referenced against residual solvent
signals and quoted in ppm relative to tetramethylsilane. IR spectra
were recorded with a Perkin–Elmer FT-IR Paragon 1000 spectro-
photometer equipped with a golden-gate ATR device, using the
reflectance technique. C, H, N determinations were performed on
a Perkin–Elmer 2400 Series II analyzer. Electrospray mass spectra
were recorded on a Finnigan TSQ-quantum instrument using an
electrospray ionization technique (ESI-MS). GC measurements
were performed on a Varian CP3800 with a 25 m WCOT fused
silica column and an autosampler, calibration lines were made
using heptane as internal standard. Peaks were identified by com-
parison with the pure compound (3-hexyne, Et3SiH, 3-triethylsilyl-
cis-hex-3-ene, butyltriethylsilane,[50] ethoxytriethylsilane,[50] 1,2-di-
phenyl-1-triethylsilyl-cis-ethene) and by GC-MS analysis (butyltri-
ethylsilane, ethoxytriethylsilane). Diethylzinc was added as a 1.0 m
solution in hexanes, n-butyllithium was added as a 1.6 m solution
in hexanes, triethylaluminium was added as a 0.6 m solution in hep-
tane and phenylmagnesium chloride was added as a 25 wt.-% solu-
tion in THF.
Scheme 3. Hydrosilylation of an internal alkyne using Ni(COD)2
and a bulky imidazolium salt (NHC·HCl is N,NЈ-dimesitylimid-
azolium chloride).[17]
The differences between the two systems may be ex-
plained by the different ligand-to-metal ratio applied. In the
case of the 1:1 ligand-to-metal ratio, the nickel center is
highly coordinatively unsaturated, which enables the coor-
dination of two alkynes and one silane substrate, leading to
the 2:1 adduct. The lack of steric bulk around the metal
center may also account for the high reactivity of this sys-
tem. In contrast, the use of a 2:1 ligand-to-metal ratio – as
in the current paper – leads to a catalyst to which only
General Reaction for the Synthesis of (Benz)imidazolium Salts: A
one alkyne and one silane may be bound simultaneously, solution of N-substituted (benz)imidazole and about 1.1 equiv. of
Eur. J. Inorg. Chem. 2011, 2450–2458
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2455

J. Berding, J. A. van Paridon, V. H. S. van Rixel, E. Bouwman
FULL PAPER
haloalkane in dry THF or 1,4-dioxane was placed under an argon
atmosphere and stirred at 80 or 100 °C, respectively, for 24 h. When
1080 (m), 828 (m), 785 (m), 747 (s), 697 (s), 600 (s), 484 (m) cm–1.
C14H13IN2 (336.18): calcd. C 50.02, H 3.90, N 8.33; found C 50.11,
using iodomethane as alkylating agent, the reaction mixture was H 3.92, N 8.39. MS (ESI): m/z (%) = 209 (100) [[M – I]+].
stirred at room temperature. The off-white precipitate was collected
General Procedure for the Synthesis of Nickel Complexes: A mixture
by filtration and recrystallized from methanol/diethyl ether to yield
of (benz)imidazolium halide, 0.5 equiv. anhydrous NiII acetate and
a white solid, which was dried in vacuo.
about 50% of the combined weight of the two reagents of the corre-
sponding tetrabutylammonium halide was dried in vacuo at 60 °C
for 1 h. The temperature was then raised to 130 °C (bromide salts)
or 155 °C (iodide salts) and kept at this temperature in vacuo for
several hours. After cooling, water was added and the mixture was
triturated thoroughly. After isolation of the crude product by fil-
tration, the pure compound was obtained either by repeated wash-
ing of a dichloromethane solution of the crude product with water,
evaporation of the solvent and precipitation with diethyl ether
(compounds 3a,b,e and 4b), or by recrystallization from hot DMF
(compounds 4c,d,e), and were isolated as red-orange to purple sol-
ids.
N-Methyl-NЈ-phenylimidazolium Iodide (1e): The synthesis was per-
formed according to the general procedure, starting from 2.16 g of
N-phenylimidazole (15 mmol) and 2.42 g of iodomethane
(17 mmol) in 20 mL of THF; yield 4.21 g (98%). 1H NMR
(300 MHz, 300 K, [D6]DMSO): δ = 9.80 (s, 1 H, NCHN), 8.31 (s,
1 H, NCH), 7.98 (s, 1 H, NCH), 7.79 (m, 2 H, Ar-H), 7.68–7.58
(m, 3 H, Ar–H), 3.96 (s, 3 H, NCH3) ppm. 13C NMR (75 MHz,
300 K, [D6]DMSO): δ = 137.2 (NCHN), 136.0 (Cq), 131.5 (CAr),
131.0 (CAr), 125.7 (NCH), 123.1 (CAr), 122.2 (NCH), 37.6 (NCH3)
ppm. IR (neat): ν = 3447 (m), 3371 (m), 3094 (m), 3029 (m), 1599
˜
(w), 1576 (m), 1553 (m), 1496 (m), 1423 (w), 1222 (m), 1068 (m),
813 (w), 758 (s), 682 (s), 611 (s) cm–1. C10H11IN2·0.5H2O (295.12):
calcd. C 40.70, H 4.10, N 9.49; found C 40.69, H 4.01, N 9.48. MS
(ESI): m/z (%) = 159 (100) [[M – I]+].
Bis(N,NЈ-dimethylimidazol-2-ylidene)diiodidonickel(II) (3a): Follow-
ing the general procedure, the complex was obtained from 1.12 g of
imidazolium iodide 1a (5.0 mmol) and 0.44 g of nickel(II) acetate
(2.5 mmol), without the addition of tetrabutylammonium iodide,
at 155 °C; yield 0.69 g (55%). NMR spectra are identical to those
reported in the literature.[22]
N-Methyl-NЈ-isopropylbenzimidazolium Bromide (2b): Following
the general procedure, the compound was obtained from 1.06 g of
N-methylbenzimidazole (8.0 mmol) and 1.23 g of 2-bromopropane
(10.0 mmol) in 30 mL of 1,4-dioxane and isolated as a white solid;
yield 1.41 g (69). 1H NMR (300 MHz, 300 K, [D6]DMSO): δ = 9.93
(s, 1 H, NCHN), 8.12 (m, 1 H, Ar–H), 8.03 (m, 1 H, Ar–H), 7.69
(m, 2 H, Ar–H), 5.06 (sept, J = 7 Hz, 1 H, NCH), 4.07 (s, 3 H,
NCH3), 1.60 (d, J = 7 Hz, 6 H, CH3) ppm. 13C NMR (75 MHz,
300 K, [D6]DMSO): δ = 141.1 (NCHN), 132.0 (Cq), 130.3 (Cq),
126.4 (CBim), 126.3 (CBim), 113.8 (CBim), 113.6 (CBim), 50.3 (NCH3),
Bis(N-ethyl-NЈ-methylimidazol-2-ylidene)diiodidonickel(II)
(3b):
Following the general procedure, 1.19 g of imidazolium iodide 1b
(5.0 mmol) and 0.44 g of nickel(II) acetate (2.5 mmol) were reacted
in 0.8 g of tetrabutylammonium iodide at 155 °C; yield 0.47 g
(35%). 1H NMR (300 MHz, 300 K, CDCl3): δ = 6.77 (m, 4 H,
NCH), 4.84 (2ϫq, J = 7 Hz, 4 H, NCH2), 4.26 (s, 6 H, NCH3),
1.69 (2ϫt, J = 7 Hz, 6 H, CH3) ppm. 13C NMR (75 MHz, 300 K,
CDCl3): δ = 123.2 (NCH), 120.5 (NCH), 45.6 (NCH2), 37.9
33.2 (NCH), 21.6 (CH ) ppm. IR (neat): ν = 3080 (w), 2978 (w),
˜
3
1564 (m), 1456 (m), 1436 (m), 1349 (m), 1262 (m), 1216 (m), 1100
(m), 1016 (w), 830 (w), 759 (s), 615 (m), 552 (m) cm–1. C11H15BrN2
(255.16): calcd. C 51.78, H 5.93, N 10.98; found C 51.98, H 6.15,
N 10.85. MS (ESI): m/z (%) = 175 (100) [[M – Br]+], 133 [[M –
Br – C3H7 + H]+].
(NCH ), 15.2 (CH ) ppm. IR (neat): ν = 3098 (w), 2972 (w), 1558
˜
3
3
(w), 1455 (m), 1401 (m), 1256 (m), 1219 (s), 1085 (m), 954 (m), 796
(m), 732 (s), 697 (s) cm–1. C12H20I2N4Ni (532.84): calcd. C 27.05,
H 3.78, N 10.52; found C 27.27, H 3.49, N 10.44. MS (ESI): m/z
(%) = 446 (100) [[M – I + MeCN]+].
N-Methyl-NЈ-isopropylbenzimidazolium Iodide (2c): According to
the general synthesis, 2.64 g of N-methylbenzimidazole (20 mmol)
was treated with 4.25 g of 2-iodopropane (25 mmol) in 25 mL of
THF. The compound was obtained as a white solid; yield 3.87 g
Bis(N-methyl-NЈ-phenylimidazol-2-ylidene)diiodidonickel(II) (3e):
Following the general procedure, the compounds was obtained
starting from 2.28 g of imidazolium salt 1e (8.0 mmol) and 0.71 g
of nickel(II) acetate (4.0 mmol) in 1.5 g of tetrabutylammonium
1
1
(64%). H NMR (300 MHz, 300 K, [D6]DMSO): δ = 9.81 (s, 1 H,
iodide at 155 °C; yield 0.73 g (29%). H NMR (300 MHz, 300 K,
NCHN), 8.12 (m, 1 H, Ar–H), 8.02 (m, 1 H, Ar–H), 7.69 (m, 2 H,
Ar–H), 5.05 (sept, J = 7 Hz, 1 H, NCH), 4.06 (s, 3 H, NCH3), 1.60
(d, J = 7 Hz, 6 H, CH3) ppm. 13C NMR (75 MHz, 300 K, [D6]-
DMSO): δ = 141.2 (NCHN), 131.9 (Cq), 130.3 (Cq), 126.4 (CBim),
126.3 (CBim), 113.7 (CBim), 113.6 (CBim), 50.3 (NCH3), 33.3 (NCH),
CDCl3): δ = 8.28 (d, J = 8 Hz, 4 H, Ar-H), 7.61 (t, J = 8 Hz, 4 H,
Ar-H), 7.49 (t, J = 8 Hz, 2 H, Ar-H), 7.00 (d, J = 2 Hz, 2 H, NCH),
6.84 (d, J = 2 Hz, 2 H, NCH), 4.01 (s, 6 H, NCH3) ppm. 13C NMR
(75 MHz, 300 K, CDCl3): δ = 174.7 (Ni-C), 140.8 (Cq), 128.8 (CAr),
128.1 (CAr), 126.4 (CAr), 123.6 (NCH), 122.6 (NCH), 38.2 (NCH3)
21.6 (CH ) ppm. IR (neat): ν = 3022 (w), 2979 (w), 1610 (w), 1567
˜
ppm. IR (neat): ν = 3129 (w), 1598 (w), 1497 (s), 1444 (m), 1403
˜
3
(m), 1456 (m), 1431 (m), 1352 (w), 1260 (m), 1215 (m), 1135 (m),
760 (s), 618 (m), 603 (m), 425 (m) cm–1. C11H15IN2 (302.16): calcd.
C 43.73, H 5.00, N 9.27; found C 43.88, H 5.39, N 9.37. MS (ESI):
m/z (%) = 175 (100) [[M – I]+], 133 [[M – I – C3H7 + H]+].
(w), 1229 (m), 1069 (m), 915 (m), 759 (m), 724 (m), 690 (s), 623
(m), 547 (m) cm–1. C20H20I2N4Ni (628.93): calcd. C 38.20, H 3.21,
N 8.91; found C 38.23, H 3.00, N 8.86. MS (ESI): m/z (%) = 542
[[M – I + MeCN]+], 501 (100) [[M – I]+], 228 [[M – 2I +
MeCN]2+].
N-Methyl-NЈ-phenylbenzimidazolium Iodide (2d): Following the ge-
neral procedure, the compound was obtained as a white solid from
1.17 g of N-phenylbenzimidazole (6.0 mmol) and 0.85 g of iodo-
methane (7.0 mmol) in 15 mL of THF; yield 1.67 g (83%). 1H
Bis(N,NЈ-dibenzylimidazol-2-ylidene)dibromidonickel(II) (3f):
A
mixture of 0.23 g of ligand precursor 1f (1.0 mmol) and 0.14 g of
silver(I) oxide (0.5 mmol) in 10 mL of dichloromethane was stirred
NMR (300 MHz, 300 K, [D6]DMSO): δ = 10.12 (s, 1 H, NCHN), in the absence of light at room temperature for 2 h. The resulting
8.15 (m, 1 H, Ar-H), 7.87–7.68 (m, 8 H, Ar–H), 4.17 (s, 3 H, solution was filtered with the aid of a membrane filter and added
NCH3) ppm. 13C NMR (75 MHz, 300 K, [D6]DMSO): δ = 143.1 to a solution of 0.34 g of Ni(PPh3)2Br2 (0.5 mmol) in dichlorometh-
(NCHN), 133.1 (Cq), 131.8 (Cq), 130.8 (Cq), 130.4 (2ϫCPh), 127.4 ane. A rapid color change from green to red occurred and stirring
(CBim), 126.9 (CBim), 125.1 (CPh), 113.9 (CBim), 113.3 (CBim), 33.5
was continued for 30 min. The off-white precipitate that had
(NCH ) ppm. IR (neat): ν = 3021 (w), 1563 (m), 1557 (s), 1487 formed was removed by filtration to give an orange solution, which
˜
3
(m), 1424 (w), 1308 (w), 1263 (m), 1239 (m), 1161 (m), 1133 (m),
was concentrated in vacuo until a red solid started to precipitate.
2456
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2450–2458

[Nickel Complexes in the Hydrosilylation of Internal Alkynes
After filtration and washing with hexane the red product was
recrystallized from DMF/diethyl ether; yield 0.15 g (42%). 1H
ppm. Due to the poor solubility, no 13C NMR spectrum was re-
corded. IR (neat): ν = 2976 (w), 1472 (m), 1417 (m), 1385 (m),
˜
NMR (300 MHz, [D6]DMSO, 300 K): δ = 7.55 (d, J = 4 Hz, 4 H, 1343 (m), 1306 (m), 1139 (m), 1095 (m), 745 (s), 551 (m) cm–1.
Ar-H), 7.29 (m, 16 H, Ar-H), 6.94 (s, 4 H, NCH), 6.09 (s, 8 H,
C26H36I2N4Ni·2H2O (753.15): calcd. C 41.46, H 5.35, N 7.44;
NCH2) ppm. 13C NMR (75 MHz, [D6]DMSO, 300 K): δ = 169.0 found C 41.45, H 5.01, N 7.55. ESI (MS): m/z (%) = 630 (100)
(Ni-C), 136.9 (Cq), 128.5 (CAr), 128.4 (CAr), 127.7 (CAr), 122.0 [[M – I + MeCN]+].
(NCH), 53.3 (NCH ) ppm. IR (neat): ν = 3134 (w), 2930 (w), 1494
˜
2
Typical Procedure for the Nickel-Catalyzed Hydrosilylation of In-
ternal Alkynes: In a Schlenk tube at room temperature, 0.050 mmol
nickel complex 3a (25.2 mg) was dissolved/suspended in 5.0 mL of
dry THF and 1.0 mmol 3-hexyne (82.1 mg) was added. Then,
0.20 mmol diethylzinc (1.0 m in hexanes, 0.20 mL) was added with
stirring, followed by 2.0 mmol triethylsilane (232.5 mg). The tube
was then placed in an oil bath, preheated to 50 °C and the mixture
was kept at this temperature. The reaction was followed by taking
samples at regular intervals, which were quenched by the addition
of ethanol and which were analyzed by GC. In the initial studies
heptane was added to the reaction mixture as an internal standard;
as it appeared that only one product was obtained in all cases, for
monitoring the conversion in time in the screening studies the rela-
tive intensities of the substrate and product were used.
(w), 1452 (m), 1417 (m), 1342 (w), 1228 (m), 1150 (w), 1028 (w),
769 (w), 728 (m), 692 (s), 594 (m) cm–1. C34H32Br2N4Ni·0.25DMF
(733.45): calcd. C 56.91, H 4.64, N 8.12; found C 57.01, H 5.06,
N, 8.07. MS (ESI): m/z (%) = 676 (100) [[M – Br + MeCN]+].
Bis(N-methyl-NЈ-isopropylbenzimidazol-2-ylidene)dibromidonickel-
(II) (4b): Following the general synthesis, the complex was obtained
from 1.28 g of benzimidazolium bromide 2b (5.0 mmol) and 0.44 g
of nickel(II) acetate (2.50 mmol) in 0.8 g of tetrabutylammonium
bromide at 130 °C; yield 0.57 g (40%). 1H NMR (300 MHz, 300 K,
CDCl3): δ = 7.50 (m, 2 H, Ar–H), 7.33–7.14 (m, 8 H, Ar–H +
NCH), 4.72 (2ϫs, 6 H, NCH3), 1.95 (2ϫd, J = 7 Hz, 12 H, CH3)
ppm. 13C NMR (75 MHz, 300 K, CDCl3): δ = 183 (Ni-C), 136.7
(Cq), 132.7 (Cq), 122.0 (CAr), 121.8 (CAr), 111.7 (CAr), 109.7 (CAr),
54.0 (NCH ), 34.2 (NCH), 21.2 (CH ) ppm. IR (neat): ν = 2974
˜
3
3
To isolate the product after full conversion was reached, water was
added to the reaction mixture and the product was extracted using
ethyl acetate. After drying of the organic layer with magnesium
sulfate, and removal of the solvents in vacuo, the residue was puri-
fied by column chromatography (silica gel, hexanes) to give the
product, which analyzed as 3-triethylsilyl-cis-hex-3-ene, as a color-
less oil;[12] yield 186 mg (94%).
(w), 1484 (m), 1435 (m), 1386 (m), 1344 (m), 1293 (m), 1135 (m),
1092 (m), 780 (w), 744 (s), 564 (m) cm–1. C22H28Br2N4Ni (567.01):
calcd. C 46.60, H 4.98, N 9.88; found C 46.55, H 4.97, N 9.79. MS
(ESI): m/z (%) = 528 (100) [[M – Br + MeCN]+].
Bis(N-methyl-NЈ-isopropylbenzimidazol-2-ylidene)diiodidonickel(II)
(4c): This complex was obtained following the general synthesis,
starting from 0.50 g of benzimidazolium salt 2c (1.65 mmol) and
0.14 g of nickel(II) acetate (0.82 mmol) in 0.4 g of tetrabutylammo-
nium iodide at 155 °C; yield 0.25 g (46%). 1H NMR (300 MHz,
300 K, CDCl3): δ = 7.48 (m, 2 H, Ar–H), 7.31 (m, 2 H, Ar–H),
7.18 (m, 4 H, Ar–H), 6.90 (m, 2 H, NCH), 4.47 (2ϫs, 6 H, NCH3),
1.91 (d, J = 7 Hz, 12 H, CH3) ppm. 13C NMR (75 MHz, 300 K,
CDCl3): δ = 188.0 (Ni-C), 137.4 (Cq), 133.1 (Cq), 121.8 (CAr), 121.7
(CAr), 111.7 (CAr), 109.5 (CAr), 54.0 (NCH3), 34.4 (NCH), 20.4
The mercury test was performed by the addition of 5.0 mmol mer-
cury (1.0 g) to the reaction mixture, 30 min after the tube was trans-
ferred to the oil bath.
1,2-Diphenyl-1-triethylsilyl-cis-ethene (5): Following the procedure
described above, the product was obtained from 178.2 mg of di-
phenylacetylene (1.0 mmol) and 232.5 mg of triethylsilane
(2.0 mmol). The product was purified by column chromatography
(silica gel, petroleum ether), and obtained as a colorless oil, which
analyzed as 1,2-diphenyl-1-triethylsilyl-cis-ethene;[12] yield 283 mg
(96%).
(CH ) ppm. IR (neat): ν = 2975 (w), 1483 (w), 1436 (w), 1392 (m),
˜
3
1351 (m), 1294 (m), 1137 (w), 1090 (m), 744 (s), 563 (m), 427 (m)
cm–1. C22H28I2N4Ni (661.01): calcd. C 39.98, H 4.27, N 8.48; found
C 39.85, H 4.36, N 8.26. MS (ESI): m/z (%) = 574 (100) [[M – I +
MeCN]+].
(E)-1,4-Dimethoxy-2-triethylsilyl-2-butene (6): Following the pro-
cedure described above, the product was obtained starting from
114.1 mg of 1,4-dimethoxy-2-butyne (1.0 mmol) and 232.5 mg of
triethylsilane (2.0 mmol). The product was purified by chromatog-
raphy (silica gel, petroleum ether), and obtained as a colorless oil,
which analyzed as (E)-1,4-dimethoxy-2-triethylsilyl-2-butene;[15]
yield 219 mg (95%).
Bis(N-methyl-NЈ-phenylbenzimidazol-2-ylidene)diiodidonickel(II)
(4d): This complex was obtained following the general complex
synthesis, starting from 0.20 g of benzimidazolium salt 2d
(0.6 mmol) and 53 mg of nickel(II) acetate (0.3 mmol) in 0.2 g of
tetrabutylammonium iodide at 155 °C; yield 0.14 g (63%). 1H
NMR (300 MHz, 300 K, CDCl3): δ = 8.17 (m, 4 H, CHPh), 7.75
(m, 4 H, CHPh), 7.67 (m, 2 H, CHPh), 7.21 (m, 4 H, CHBim), 7.13
(m, 4 H, CHBim), 4.06 (s, 6 H, NCH3) ppm. 13C NMR (75 MHz,
300 K, CDCl3): δ = 138.2 (Cq), 136.8 (Cq), 136.7 (Cq), 129.1 (CPh),
128.8 (CPh), 127.9 (CPh), 122.5 (CBim), 122.4 (CBim), 110.2 (CBim),
(E)-1-Phenyl-1-(triethylsilyl)propene (7a): Following the procedure
described above, the product was obtained starting from 116.2 mg
of 1-phenyl-1-propyne (1.0 mmol) and 232.5 mg of triethylsilane
(2.0 mmol). The product was purified by chromatography (silica
gel, petroleum ether), and obtained as a colorless oil, which ana-
lyzed as (E)-1-phenyl-1-(triethylsilyl)propene;[17] yield 167 mg
(72%).
109.1 (CBim), 34.5 (NCH ) ppm. IR (neat): ν = 3054 (w), 1597 (w),
˜
3
1500 (m), 1435 (m), 1389 (m), 1339 (m), 1231 (m), 1090 (m), 917
(w), 750 (s), 695 (s), 613 (m), 554 (m), 490 (m) cm–1. C28H24I2N4Ni
(729.05): calcd. C 46.13, H 3.32, N 7.69; found C 46.07, H 3.46, N
7.72. ESI (MS): m/z (%) = 642 (100) [[M – I + MeCN]+].
Acknowledgments
Bis(N,NЈ-diisopropylbenzimidazol-2-ylidene)diiodidonickel(II) (4e):
The compound was prepared following the general procedure, from
3.90 g of N,NЈ-diisopropylbenzimidazolium iodide (11.8 mmol)
and 1.04 g of nickel(II) acetate (5.9 mmol) in 2.0 g of tetrabutylam-
monium iodide at 155 °C; yield 1.26 g (30%). 1H NMR (300 MHz,
We thank Prof. J. Reedijk for continuous support of our work and
valuable discussions. This work has been supported financially by
the National Graduate Research School Combination NRSC-Ca-
talysis, a joint activity of the graduate research schools “Neder-
300 K, CDCl3): δ = 7.48 (m, 4 H, Ar–H), 7.12 (m, 4 H, Ar–H), lands Instituut voor Onderzoek in de Katalyse” (NIOK), “Holland
6.93 (sept, J = 7 Hz, 4 H, NCH), 1.88 (d, J = 7 Hz, 24 H, CH3) Research School of Molecular Sciences” (HRSMC), and the
Eur. J. Inorg. Chem. 2011, 2450–2458
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Berding, J. A. van Paridon, V. H. S. van Rixel, E. Bouwman
FULL PAPER
[25] N. Asao, T. Sudo, Y. Yamamoto, J. Org. Chem. 1996, 61, 7654–
7655.
[26] Y. Kiso, M. Kumada, K. Maeda, K. Sumitani, K. Tamao, J.
Organomet. Chem. 1973, 50, 311–318.
“National Dutch graduate school of Polymer Science & Technol-
ogy” (PTN).
[1] W. A. Herrmann, Angew. Chem. 2002, 114, 1342; Angew.
Chem. Int. Ed. 2002, 41, 1290–1309.
[27] S. B. Blakey, D. W. C. MacMillan, J. Am. Chem. Soc. 2003, 125,
6046–6047.
[2] B. Marciniec, Silicon Chem. 2002, 1, 155–175.
[3] E. Langkopf, D. Schinzer, Chem. Rev. 1995, 95, 1375–1408.
[4] B. M. Trost, Z. T. Ball, Synthesis 2005, 853–887.
[5] A. Mori, E. Takahisa, Y. Yamamura, T. Kato, A. P. Mudalige,
H. Kajiro, K. Hirabayashi, Y. Nishihara, T. Hiyama, Organo-
metallics 2004, 23, 1755–1765.
[28] Y. Sato, R. Sawaki, M. Mori, Organometallics 2001, 20, 5510–
5512.
[29] T. N. Tekavec, G. Zuo, K. Simon, J. Louie, J. Org. Chem. 2006,
71, 5834–5836.
[30] S. Benson, B. Payne, R. M. Waymouth, J. Polym. Sci. Polym.
Chem. Ed. 2007, 45, 3637–3647.
[6] E. Mas-Marza, M. Poyatos, M. Sanau, E. Peris, Inorg. Chem.
2004, 43, 2213–2219.
[31] J. P. Corriu, J. P. Masse, J. Chem. Soc., Chem. Commun. 1972,
144.
[7] M. V. Jimenez, J. J. Perez-Torrente, M. I. Bartolome, V. Gierz,
F. J. Lahoz, L. A. Oro, Organometallics 2008, 27, 224–234.
[8] V. S. Sridevi, W. Y. Fan, W. K. Leong, Organometallics 2007,
26, 1157–1160.
[32] K. Tamao, K. Sumitani, M. Kumada, J. Am. Chem. Soc. 1972,
94, 4374–4376.
[33] J. Berding, M. Lutz, A. L. Spek, E. Bouwman, Organometallics
2009, 28, 1845–1854.
[34] J. Berding, T. F. van Dijkman, M. Lutz, A. L. Spek, E. Bouw-
man, Dalton Trans. 2009, 6948–6955.
[35] A. Sato, H. Kinoshita, H. Shinokubo, K. Oshima, Org. Lett.
2004, 6, 2217–2220.
[9] Y. Kawanami, Y. Sonoda, T. Mori, K. Yamamoto, Org. Lett.
2002, 4, 2825–2827.
[10] B. M. Trost, Z. T. Ball, J. Am. Chem. Soc. 2005, 127, 17644–
17655.
[11] C. A. Tsipis, J. Organomet. Chem. 1980, 188, 53–61.
[12] M. Chauhan, B. J. Hauck, L. P. Keller, P. Boudjouk, J. Or-
ganomet. Chem. 2002, 645, 1–13.
[13] G. De Bo, G. Berthon-Gelloz, B. Tinant, I. E. Marko, Organo-
metallics 2006, 25, 1881–1890.
[14] A. Tillack, S. Pulst, W. Baumann, H. Baudisch, K. Kortus, U.
Rosenthal, J. Organomet. Chem. 1997, 532, 117–123.
[15] L. Yong, K. Kirleis, H. Butenschon, Adv. Synth. Catal. 2006,
348, 833–836.
[16] T. Takahashi, F. Y. Bao, G. H. Gao, M. Ogasawara, Org. Lett.
2003, 5, 3479–3481.
[17] M. R. Chaulagain, G. M. Mahandru, J. Montgomery, Tetrahe-
dron 2006, 62, 7560–7566.
[18] H. V. Huynh, C. Holtgrewe, T. Pape, L. L. Koh, E. Hahn, Or-
ganometallics 2006, 25, 245–249.
[19] D. S. McGuinness, W. Mueller, P. Wasserscheid, K. J. Cavell,
[36] F. Alonso, I. Osante, M. Yus, Tetrahedron 2007, 63, 93–102.
[37] A. Houdayer, R. Schneider, D. Billaud, J. Ghanbaja, J. Lam-
bert, Synth. Met. 2005, 151, 165–174.
[38] G. M. Whitesides, M. Hackett, R. L. Brainard, J. Lavalleye,
A. F. Sowinski, A. N. Izumi, S. S. Moore, D. W. Brown, E. M.
Staudt, Organometallics 1985, 4, 1819–1830.
[39] J. A. Widegren, R. G. Finke, J. Mol. Catal. A 2003, 198, 317–
341.
[40] K. S. Weddle, J. D. Aiken, R. G. Finke, J. Am. Chem. Soc. 1998,
120, 5653–5666.
[41] P. Boudjouk, B. H. Han, J. R. Jacobsen, B. J. Hauck, J. Chem.
Soc., Chem. Commun. 1991, 1424–1425.
[42] M. F. Lappert, S. Takahashi, J. Chem. Soc., Chem. Commun.
1972, 1272.
[43] M. F. Lappert, T. A. Nile, S. Takahashi, J. Organomet. Chem.
1974, 72, 425–439.
B. W. Skelton, A. H. White, U. Englert, Organometallics 2002, [44] N. D. Clement, K. J. Cavell, L. L. Ooi, Organometallics 2006,
21, 175–181.
25, 4155–4165.
[20] O. V. Starikova, G. V. Dolgushin, L. I. Larina, P. E. Ushakov,
T. N. Komarova, V. A. Lopyrev, Russ. J. Org. Chem. 2003, 39,
1467–1470.
[45] T. Schaub, U. Radius, Chem. Eur. J. 2005, 11, 5024–5030.
[46] Y. Z. Huang, H. Miao, Q. H. Zhang, C. Chen, J. Xu, Catal.
Lett. 2008, 122, 344–348.
[47] M. A. Phillips, J. Chem. Soc. 1929, 131, 2820–2828.
[48] Y. Kondo, S. Izawa, S. Kusabayashi, J. Chem. Soc. Perkin
Trans. 2 1988, 1925–1928.
[49] N. L. Lancaster, P. A. Salter, T. Welton, G. B. Young, J. Org.
Chem. 2002, 67, 8855–8861.
[50] R. N. Meals, J. Am. Chem. Soc. 1946, 68, 1880–1881.
[21] H. V. Huynh, L. R. Wong, P. S. Ng, Organometallics 2008, 27,
2231–2237.
[22] W. A. Herrmann, G. Gerstberger, M. Spiegler, Organometallics
1997, 16, 2209–2212.
[23] J. Berding, H. Kooijman, A. L. Spek, E. Bouwman, J. Or-
ganomet. Chem. 2009, 694, 2217–2221.
[24] Y. Han, H. V. Huynh, L. L. Koh, J. Organomet. Chem. 2007,
692, 3606–3613.
Received: January 6, 2011
Published Online: April 7, 2011
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