Platinum(II)-bis-(N-heterocyclic carbene) complexes: Synthesis, structure and catalytic activity in the hydrosilylation of alkenes

Article abstract of DOI:10.1016/j.jorganchem.2011.04.030

Chelating biscarbene ligands increase the stability of metal-organic catalyst systems. The catalytic activities of seven structurally different platinum(II)-bis-NHC-complexes in the hydrosilylation of alkenes have been investigated and compared with the c

Full text of DOI:10.1016/j.jorganchem.2011.04.030

Journal of Organometallic Chemistry 696 (2011) 2918e2927
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Platinum(II)-bis-(N-heterocyclic carbene) complexes: synthesis, structure and
catalytic activity in the hydrosilylation of alkenes
1
*
Maria A. Taige, Sebastian Ahrens , Thomas Strassner
Physikalische Organische Chemie, Technische Universität Dresden, D-01062 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Chelating biscarbene ligands increase the stability of metal-organic catalyst systems. The catalytic
activities of seven structurally different platinum(II)-bis-NHC-complexes in the hydrosilylation of alkenes
have been investigated and compared with the catalytic activity of the Karstedt catalyst and of a highly
Received 14 February 2011
Received in revised form
26 April 2011
active platinum(0)-NHC-complex. It is shown that
a fine-tuning of the catalytic activity of the
Accepted 27 April 2011
platinum(II)-bis-NHC-complexes is possible. The synthesis of a platinum(II)-bis-NHC-complex with
similar activity, but additional advantages compared to the Karstedt catalyst, is reported. The solid
state structure of 1,1-[Bis(3,30-(4-methoxyphenyl)-1,10-1H-imidazolium-2,20-ylidene)methanediyl]plati-
num(II)-dichloride is presented.
Keywords:
Platinum(II)-NHC-complexes
Syntheses
Solid state structure
Catalysis
Ó 2011 Elsevier B.V. All rights reserved.
Hydrosilylation
Hydrosilation
1. Introduction
vinylsilanes (C), saturated derivatives (D), disilanes (E) and iso-
merized alkenes (F) are produced (Chart 2) [23].
Several good reviews have been published on the hydro-
silylation of alkenes, which is of particular importance for the
silicon chemistry [1e14]. Alkyl-, alkenyl- and arylsilanes also find
useful applications in organic syntheses, where they often replace
the corresponding boron and tin compounds as they are in general
less toxic, cheaper and more stable [15e17]. The hydrosilylation
reaction can be initiated in numerous ways [18]. Among these, the
transition metal catalyzed reaction, especially by platinum systems,
is the most commonly used method.
Another disadvantage of the Karstedt catalyst (1) is the forma-
tion of platinum black during the hydrosilylation reaction, which is
responsible for some of the side products and leads to a contami-
nation of the hydrosilylation product [24].
In addition to platinum [19e21,25e32] complexes, numerous
other metal complexes [33e50] are used as catalysts in the hydro-
silylation of alkenes and alkynes. In comparison to platinum
complexes, these usually exhibit
selectivity [51].
a
lower activity and/or
The two standard catalysts for industrial applications are the
Speier catalyst [19] (H2PtCl6 * 6 H2O in i-PrOH) and the Karstedt
catalyst [20,21] (1, Chart 1). A few years ago, also platinum(0)-
complexes with monodentate NHC-ligands (2, Chart 1) have been
introduced and successfully employed in the hydrosilylation e.g. of
1-octene with bis(trimethylsiloxy)methylsilane [22,23].
Over the years, the Speier catalyst has been replaced by the more
active and selective Karstedt catalyst. However, 1 not only catalyzes
the formation of the desired hydrosilylation products A and B, but
its use also results in several undesirable side reactions, in which
Due to their high stability towards heat, oxygen and moisture,
metal complexes of N-heterocyclic carbenes (NHC’s) are often
N
N
R
R
Si Si
O
Si
Si
Si
Si
Pt
O
Pt
O
Pt
Si
Si
O
1
2: R = e.g. 4-Methoxyphenyl
Chart 1. Karstedt catalyst [20,21] (1) and Markó’s NHC-catalysts [22,23] (2).
Chart 2. Hydrosilylation of alkenes with the Karstedt catalyst.
* Corresponding author. Tel.: þ49 (0) 351 463 38571; fax: þ49 (0) 351 463 39679.
E-mail address: thomas.strassner@chemie.tu-dresden.de (T. Strassner).
X-ray analysis.
1
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.04.030

M.A. Taige et al. / Journal of Organometallic Chemistry 696 (2011) 2918e2927
2919
considered as valuable alternatives to phosphine ligands in
homogeneous catalysis [52e57]. Especially chelated biscarbene
complexes have been shown to be stable even under harsh reac-
tion conditions and are expected to be more stable than mono-
dentate NHC’s, although only few examples of platinum
complexes with chelating biscarbene ligands have been published
[58e67].
isopropanol (5 mL) and dried in vacuo (yield: 584 mg, 54%). 1H
NMR (300.13 MHz, CDCl3):
d
¼ 7.38 (m, 6H, o-CH of Ar and NeCH of
Im), 6.79 (d, J ¼ 8.9 Hz, 4H, m-CH of Ar), 3.77 (s, 6H, O-CH3), 1.91 (dd,
3J ¼ 9.3 Hz, 2J ¼ 1.9 Hz, 2H, CH ¼ CH2 eq), 1.53e1.60 (m, 4H,
SieCH ¼ and ¼ CH2 ax), 0.20 (s, 6H, Si-CH3 eq), ꢀ0.60 (s, 6H, Si-CH3
ax) ppm. 13C NMR (75.475 MHz, CDCl3):
(p-C of Ar), 133.5 (i-C of Ar), 125.6 (o-CH of Ar), 123.4 (m-CH of Ar),
113.6 (NeCH), 55.4 (OeCH3), 40.3 (CH2 CHeSi), 32.3
(CH2 ¼ CHeSi), 1.6 (SieCH3 eq), ꢀ2.9 (SieCH3 ax) ppm.
d
¼ 180.7 (NeCeN), 158.4
¼
2. Experimental
2.1. General procedure
2.3. Synthesis of 1,1-[Bis(3,30-(isopropyl)-1,10-1H-imidazolium-2,20-
ylidene)methanediyl]-platinum(II)-dibromide (8)
Catalysis reactions were generally carried out under an atmo-
sphere of dry argon [68e71], although there have been some
reports in the literature that molecular oxygen can have an
important effect in hydrosilylation catalysis, especially when
colloids are involved [72e80].
1,4-Dioxane was dried over 4 Å molecular sieve prior to use.
GCeMS spectra were measured on a HewlettePackard GC G1800A
system. The conversion was measured against cyclooctadiene as
internal standard. The turnover number (TON) is defined as (mol
product)/(mol platinum), while the turnover frequency (TOF) is
defined as (mol product)/[(mol platinum) * (reaction time in h)].
The amount of catalyst is given relative to the amount of the
olefin. All catalytic experiments are repeated at least twice, most
of them three times. The conversion, yield, TON and TOF given are
the average values of the GC and NMR measurements,
respectively.
The synthesis follows a modified literature procedure [86]. 1,10-
Diisopropyl-3,30-methylene-bisimidazolium dibromide (394 mg,
1 mmol) and platinum(II)acetylacetonate (393 mg, 1 mmol) are
dissolved in 6 mL dimethylsulfoxide. The reaction mixture was
stirred for 3 h at 60 ꢁC, for 3 h at 80 ꢁC and for 3 h at 110 ꢁC. After
removal of the solvent, the light brown residue was washed three
times with ethanol (5 mL) and three times with tetrahydrofurane
(5 mL). The product was obtained as a colorless solid (yield: 410 mg,
70%), m. p.: 326 ꢁC (decomp.). 1H NMR (300.13 MHz, DMSO-d6):
d
¼ 7.61 (s, 2H, NCH), 7.60 (s, 2H, NCH), 6.12 (d, J ¼ 13.1 Hz, 1H,
NCH2N), 6.03 (d, J ¼ 13.1 Hz, 1H, NCH2N), 5.61 (sept, J ¼ 6.6 Hz, 2H,
NCH(CH3)2), 1.51 (d, J ¼ 6.7 Hz, 6 H, NCH(CH3)2), 1.26 (d, J ¼ 6.8 Hz,
6 H, NCH(CH3)2) ppm. 13C NMR (75.475 MHz, DMSO-d6):
d
¼ 144.8
(NCN), 121.2 (NCH), 117.5 (NCH), 61.9 (NCH2N), 51.4 (NCH(CH3)2),
23.3 (NCH(CH3)2), 21.5 (NCH(CH3)2) ppm. Anal. Calc. for
C13H20N4PtBr2: C, 26.57; H, 3.43; N, 9.54. Found: C, 26.43; H, 3.46;
N, 9.37.
1H and 13C NMR spectra were recorded with a Bruker AC 300 P
spectrometer. The spectra were referenced internally to the reso-
nances of the deuterated solvent (1H, 13C). Elemental analyses were
performed by the microanalytical laboratory of our institute using
an EuroVektor Euro EA-300 Elemental Analyzer. Melting and
decomposition points of the complexes were determined by
a melting point apparatus (Wagner and Munz PolyTherm A) and
are uncorrected.
2.4. Synthesis of 1,1-[Bis(3,30-(cyclohexyl)-1,10-1H-imidazolium-
2,20-ylidene)methanediyl]-platinum(II)-dibromide (9)
The synthesis follows a modified literature procedure [86]. 1,10-
Dicyclohexyl-3,30-methylene-bisimidazolium dibromide (237 mg,
0.5 mmol) and platinum(II)acetylacetonate (197 mg, 0.5 mmol)
were dissolved in 6 mL dimethylsulfoxide. The reaction mixture
was stirred for 3 h at 60 ꢁC, for 3 h at 80 ꢁC and for 3 h at 110 ꢁC.
After removal of the solvent, the light brown residue was washed
three times with ethanol (5 mL) and three times with tetrahy-
drofurane (5 mL). The product was obtained as a colorless solid
(yield: 143 mg, 43%), m. p.: 366 ꢁC (decomp.). 1H NMR (300.13 MHz,
1-N-substituted imidazoles (substituent: 2,4,6-trimethylphenyl,
4-methoxyphenyl, 4-bromophenyl, isopropyl, cyclohexyl) [59,81],
1,10-N-disubstituted-3,30-methylene-bisimidazolium
dichlorides
(substituent: 2,4,6-trimethylphenyl, 4-methoxyphenyl, methyl)
[82], 1,10-N-disubstituted-3,30-methylene-bisimidazolium dibro-
mides (substituent: 2,4,6-trimethyl-phenyl, 4-bromophenyl, iso-
propyl, cyclohexyl) [59,83], 1,3-bis(4-methoxyphenyl)imidazolium
chloride [84], 1,1-[Bis(3,30-(N-disubstituted)-1,10-1H-imidazolium-
2,20-ylidene)methanediyl]-platinum(II)-dichlorides (substituent:
2,4,6-trimethylphenyl (3), 4-methoxyphenyl (5), methyl (7)) [82]
and 1,1-[Bis(3,30-(N-disubstituted)-1,10-1H-imidazolium-2,20-yli-
dene)methanediyl]-platinum(II)-dibromides (substituent: 2,4,6-
trimethylphenyl (4), 4-bromophenyl (6)) [59] were prepared
according to the literature procedures. The Karstedt catalyst and
N-Methylimidazol were purchased from ABCR and used as
received.
DMSO-d6):
d
¼ 7.61 (s, 2H, NCH), 7.58 (s, 2H, NCH), 6.13 (d,
J ¼ 13.1 Hz, 1H, NCH2N), 6.04 (d, J ¼ 13.0 Hz, 1H, NCH2N), 5.21 (m,
2H, NeCH of Cy), 1.88-1.17 (m, 20 H, CH2 of Cy) ppm. 13C NMR
(75.475 MHz, DMSO-d6):
d
¼ 144.6 (NCN), 121.0 (NCH), 118.2 (NCH),
61.7 (NCH2N), 58.3 (NCH of Cy), 33.3 (CH2 of Cy), 31.9 (CH2 of Cy),
25.3 (CH2 of Cy), 24.6 (CH2 of Cy), 24.5 (CH2 of Cy) ppm. Anal. Calc.
for C19H28N4PtBr2 * 0.5 DMSO: C, 34.01; H, 4.42; N, 7.93; S, 2.27.
Found: C, 34.19; H, 4.40; N, 7.84; S, 2.18.
2.2. Synthesis of 1,3-Bis(4-methoxyphenyl)imidazoline-2-ylidene
platinum(0) divinyltetramethyl-siloxane (2)
2.5. General procedure for the hydrosilylation of styrene with
bis(trimethylsiloxy)methylsilane
The synthesis follows a modified literature protocol [85]. Under
an argon atmosphere a dry Schlenk tube was loaded with 1,3-Bis(4-
methoxyphenyl)imidazolium chloride (567 mg, 1.97 mmol),
a solution of the Karstedt catalyst (2.01 g, 1.63 mmol of platinum) in
5 mL dry toluene and tBuOK (336 mg, 3 mmol). The reaction
mixture was stirred for 2 h at room temperature. The heteroge-
neous mixture was filtered over Celite and the filtrate was obtained
as a clear, orange solution. The solvent was removed in vacuo and
the resulting beige solid was washed three times with ice-cold
A degassed 10 mL Schlenk flask equipped with a reflux
condenser was first charged with the noted amount of the cata-
lyst, bis(trimethylsiloxy)methylsilane (4.4 mmol) and styrene
(4 mmol) under an atmosphere of argon before it immediately
afterwards was put in a preheated oil bath. Temperatures given in
the tables correspond to the temperature of the oil bath. For
analysis, aliquots (0.2 mL) were removed from the reaction
mixture after a defined period of time, put in an NMR tube, diluted
by 0.5 mL deuterated chloroform and analyzed by NMR

2920
M.A. Taige et al. / Journal of Organometallic Chemistry 696 (2011) 2918e2927
Table 1
Table 2
Crystal data and crystallographic details for compound 5.
Selected bond length (Å) and angles (ꢁ) of compound 5.
5
5
Formula
C21H20Cl2N4O2Pt *C2H6OS
Pt1eC1
1.956(2)
Formula weight (g/mol)
Color/shape
Crystal system
Space group
a (Å)
704.53
Colorless/block
Triclinic
Pt1eC11
Pt1eCl1
Pt1eCl2
C1eN1
C1eN2
C11eN3
C11eN4
C1ePt1eC11
C1ePt1eCl1
C1ePt1eCl2
C11ePt1eCl1
C11ePt1eCl2
Cl1ePt1eCl2
N1eC1eN2
N3eC11eN4
C1eN1eC4eC19
C11eN4eC9eC15
C1eN1eC4eC23
C11eN4eC9eC10
1.957(2)
2.3653(6)
2.3561(6)
1.352(3)
1.351(3)
1.353(3)
1.353(3)
84.62(9)
92.43(7)
174.55(7)
175.09(7)
92.33(7)
90.30(2)
104.70(19)
104.9(2)
57.9(3)
P1 (No. 2)
9.7010(4)
9.9370(8)
14.1590(8)
102.755(6)
106.255(5)
94.455(5)
1263.75(15)
2
1.852
5.878
198
0.71073
b (Å)
c (Å)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
V (Å3)
Z
rcalc (g/cm3)
m
(mmꢀ1
)
Temperature (K)
l
(Å)
qmin/max (ꢁ)
4.054/21.835
33084
5169
ꢀ45.2(4)
ꢀ124.4(3)
134.7(3)
Reflections integrated
Independent reflections (all data)
Observed reflections [I > 2
s
(I)]
4798
Parameter refined
315
R1 (observed/all data)
wR2 (observed/all data)
0.0147/0.0180
0.0313/0.0320
1.025/1.025
ꢀ0.501/0.538
2.7. General procedure for testing the reusability
Goodness-of-fit (observed/all data)
Residual electron density (e/Å3)
A 10 mL Schlenk flask equipped with a reflux condenser was
degassed and put under an atmosphere of argon. It was charged
with the denoted amount of the catalyst, bis(trimethylsiloxy)
methylsilane (4.4 mmol) and styrene (4 mmol) and put in a pre-
heated oil bath. Aliquots (0.2 mL) were removed from the reaction
mixture after a defined period of time, poured into an NMR tube,
diluted by 0.5 mL deuterated chloroform and analyzed by NMR
spectroscopy. After complete conversion of the starting materials
the reaction mixture was cooled to room temperature. Then, the
Schlenk flask was again charged with bis(trimethylsiloxy)methyl-
silane (4.4 mmol) and styrene (4 mmol) and put back in the pre-
heated oil bath. Aliquots (0.2 mL) were again removed from the
reaction mixture after a defined period of time and analyzed as
described above.
spectroscopy. The estimated error of this method is ꢂ5%, the
average standard deviation is 3.6.
2.6. General procedure for the hydrosilylation of styrene with
dimethylphenylsilane
A degassed 10 mL Schlenk flask equipped with a reflux
condenser was charged with the noted amount of catalyst, 2 mmol
cyclododecane, 2 mL dioxane, 2.2 mmol dimethylphenylsilane and
2 mmol styrene under an atmosphere of argon. Immediately
afterwards the flask was put in a preheated oil bath. Temperatures
given in the tables correspond to the temperature of the oil bath.
For analysis, aliquots (0.2 mL) were removed from the reaction
mixture after a defined period of time and diluted by dichloro-
methane (15 mL). The solution was analyzed by gas chromatog-
raphy. The estimated error of this method is ꢂ2%, the average
standard deviation is 3.5.
2.8. Structure determination of compound 5
Crystal data and details on the structure determination are
presented in Table 1. Table 2 summarizes selected bond lengths and
bond angles. Suitable single crystals for the X-ray diffraction study
were grown by cooling a saturated, hot solution of 5 in DMSO (see
Fig. 1. ORTEP plot of 5 in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are plotted as spheres of arbitrary radius. The DMSO molecule is
omitted for clarity.

M.A. Taige et al. / Journal of Organometallic Chemistry 696 (2011) 2918e2927
2921
Fig. 1). A clear colorless block (0.27 ꢃ 0.19 ꢃ 0.09 mm) was stored
under perfluorinated ether, transferred on a glass capillary and
fixed. Preliminary examination and data collection were carried out
on an area detecting system (kappa-CCD; Nonius) at the window of
a sealed X-ray tube (Nonius, FR590) and graphite monochromated
nature of the substituents on the aromatic ring from electron-
donating to electron-withdrawing!
Platinum(II)-NHC-complexes have been previously used as cata-
lysts for this reaction [32]. We therefore investigated the use of
chelating bis-NHC-complexes with avariety of aromatic and aliphatic
substituents. Mesityl(2,4,6-trimethylphenyl)(3,4),4-methoxyphenyl
(5) and 4-bromophenyl (6) as well as the aliphatic substituents
methyl- (7), isopropyl- (8) and cyclohexyl- (9) were used in combi-
nation with chloride and bromide counterions since we had previ-
ously observed significant counterion effects in other catalytic
reactions (Chart 3).
Mo K
obtained by full-matrix least-squares refinement of 153 reflections.
Data collection was performed at 198 K within a -range of 4.10 <
< 26.40ꢁ. A total number of 33084 intensities were integrated.
a
radiation (
l
¼ 0.71073 Å). The unit cell parameters were
q
q
Raw data were corrected for Lorentz, polarization, decay and
absorption effects. After merging (Rint ¼ 0.035) a sum of 5169
independent reflections remained and were used for all calcula-
tions. The structure was solved by a combination of direct methods
and difference Fourier synthesis. All non-hydrogen atoms were
refined with anisotropic displacement parameters. All hydrogen
atoms were calculated in ideal positions riding on the parent
Complex 2 was synthesized by a modified literature procedure
described in the experimental section. Complexes 3e7 were
synthesized as previously published [59,82].
3.2. Solid state structure of 5
carbon atoms. Full-matrix least-square refinements with 315
P
parameters were carried out by minimizing
wðF02 ꢀ Fc2Þ2 with
Crystals of 5 suitable for X-ray diffraction were obtained by
cooling of a hot DMSO solution. Complex 5 crystallizes as a DMSO
solvate with one DMSO molecule per platinum (Fig. 1). The struc-
ture of complex 5 is similar to the corresponding platinum-bis-
NHC-complex with bromide counterions (10) [82] and the
palladium-bis-NHC-complex 11 [65]. All three complexes contain
a bowl-shaped NHC-ligand. The platinum-carbene-C distances are
the same within the margin of the error. Only the torsional angles of
the plane of the imidazole rings towards the plane of the aromatic
rings represented by the torsional angle C11-N4-C9-C10 in Fig. 1
(5: 134.7ꢁ, 10: 128.0ꢁ, 11: 128.2ꢁ) differ slightly.
SHELXL-97 weighting scheme and stopped at shift/err < 0.001.
Neutral atom scattering factors for all atoms and anomalous
dispersion corrections for the non-hydrogen atoms were taken
from the International Tables for Crystallography [87]. All calcula-
tions were performed with the SHELXL-97 [88] package and the
programs COLLECT [89], DIRAX [90], EVALCCD [91], SIR92 [92],
SADABS [93], PLATON [94] and ORTEP III [95].
3. Results and discussion
3.1. Preparation and structural properties of selected platinum-
NHC-complexes
3.3. Catalysis
We have investigated the catalytic activity of selected platinum-
NHC-complexes (Chart 3) in the hydrosilylation reaction and
compared their activity to that of the Karstedt catalyst (1) and the
Markó catalyst (2). Of special interest was the catalytic activity of
N-aryl substituted platinum(II)-bis-NHC-complexes in comparison
to the catalytic activity of different N-alkyl substituted NHC-
complexes. It should be possible to systematically fine-tune the
catalytic activity of the N-aryl NHC complexes by changing the
We studied thehydrosilylation ofstyrenewith bis(trimethylsiloxy)
methylsilane in a more detailed investigation, and the slightly less
reactive dimethylphenylsilane using two different catalytic systems
(Chart 4) [96].
We observed the formation of the branched hydrosilylation
product A, the linear hydrosilylation product B and the dehy-
drosilylation product C. No other side products could be identified.
O
Br
N
N
Pt
N
N
Cl
Br
Br
N
N
N
N
X
X
N
N
Pt
N
Pt
N
Cl
O
Br
3: X = Cl
4: X = Br
5
6
N
Pt
N
N
Pt
N
Br
Br
N
N
Br
Br
N
N
N
Cl
N
N
Pt
N
Cl
7
8
9
Chart 3. Platinum(II)-bis-NHC-complexes tested in the hydrosilylation of alkenes.

2922
M.A. Taige et al. / Journal of Organometallic Chemistry 696 (2011) 2918e2927
H
Si
+
R1
R2
R1
cat. [Pt]
R1
R2
R1
R1
R2 R1
Si
R2
R1
Si
R2
R1
Si
R1
Si
R1
+ H2
+
Si
R1
+
+
+
R1
R2
C
A
B
E
D
Chart 4. Hydrosilylation of styrene with bis(trimethylsiloxy)methylsilane (R1 ¼ OSiMe3, R2 ¼ Me) and dimethylphenylsilane (R1 ¼ Me, R2 ¼ Ph).
3.4. Hydrosilylation of styrene with bis(trimethylsiloxy)
methylsilane
Nearly complete conversion is also obtained with the
4-bromophenyl-substituted complex (6) and the methyl-substituted
complex (7) after a reaction time of 6 h (Table 3, entries 8/9), but the
reaction starts slower. The catalytically active species is proposed to
be a platinum(0) species [74] and the formation of the platinum(0)
species seems to be promoted by the electron-donating effect of the
methoxy-substituents in the para-positions of the phenyl rings. We
believethatthesilanereducesthecomplex,whichissupportedbythe
observation that the reaction starts more quickly if the silane is added
to the solution before the addition of the olefin.
The catalytic activity is influenced not only by the reduction of
the platinum center, but also from the steric demand of the
substituents, which leads to a significantly lower activity of
complexes 4 and 9. The cyclohexyl-substituted complex 9 shows
the lowest activity of all investigated complexes (Table 3, entry 11),
which is especially obvious during the early stage of the reaction
(Fig. 2). The steric effect reduces the activity and leads to a slightly
prolonged initiation time for complex 4 in comparison to complex 6
(Fig. 2). The formation of platinum black is only observed for the
methyl substituted complex 7 after a reaction time of 3 h, all other
complexes are stable under the reaction conditions. The selectivity
of all investigated complexes is similar, we observe about 20%
formation of the branched product A and about 80% of the linear
product B.
The results for a series of hydrosilylation reactions are shown in
Table 3. To compare the catalytic activity of the platinum(II)-bis-
NHC-complexes 3e9 with that of the Karstedt catalyst (1) in the
hydrosilylation of styrene with bis(trimethylsiloxy)methylsilane
(Table 3), we carried out the reactions at 140 ꢁC with 0.5 mol%
catalyst.
There is a significant influence of steric and electronic effects on
the catalytic activity of the platinum(II)-bis-NHC-complexes. The
4-methoxyphenyl-substituted complex (5) turned out to be the
most reactive of the platinum(II)-bis-NHC-complexes investigated.
With 5 a yield of 69% of the hydrosilylation products A and B was
obtained after a reaction time of only 30 s (Table 3, entry 5), while
all other platinum(II) complexes did not show any conversion after
30 s. The activity of 5 is comparable to the activity of the Karstedt
catalyst (1). One obtains 86% of the hydrosilylation products after
a reaction time of only 10 min with complex 5 (Table 3, entry 6) and
83% after the same reaction time with 1 (Table 3, entry 1). Complete
conversion is probably achieved at a different time for each catalyst,
but for comparison both reactions were run for a time of 6 h
(Table 3, entries 7 and 2). In comparison to 1, complex 5 has some
important advantages. The formation of platinum black is not
observed for complex 5 during the reaction, while it is generated
from 1 immediately after the addition of the silane.
3.5. Reusability
Table 3
An important factor for an industrial application of a catalyst is
the number of catalytic cycles it allows the reaction to be run before
it looses its activity. Therefore we tested the loss of activity in
several runs for the most active catalyst 5 and the Karstedt catalyst 1
under the same reaction conditions (Table 4). Additional results for
the measurements after 2 and 4 h are given in the supplementary
material.
Hydrosilylation of styrene with bis(trimethylsiloxy)methylsilane.
Me
O
Me3Si
O
Me
O
SiMe3
Si
Me3Si
0.5 mol%
cat. [Pt]
Si
H
O
O
O
+
Me3Si
Si
SiMe3
+
140 °C
SiMe3
Me
A
B
Entry Cat. Time Conversion
Conversion
Yield
Ratio A:B
[min] (siloxane) [%]a (styrene) [%]a (hydrosilylation
products) [%]a
1
2
3
4
5
6
7
8
1
1
3
4
5
5
5
6
7
8
9
10
97
100
85
69
69
88
99
94
94
84
29
96
100
96
78
69
82
99
95
96
79
21
83
91
82
67
69
82
97
94
89
79
21
16:84
15:85
13:87
13:87
22:78
14:86
15:85
17:83
16:84
17:83
19:81
360
360
360
½
10
360
360
360
360
360
9
10
11
Fig. 2. Yields of the hydrosilylation products A and B in dependence of the reaction
time for catalysts 4, 6, 7 and 9.
a
Determined by NMR analysis (error: ꢂ 5%).

M.A. Taige et al. / Journal of Organometallic Chemistry 696 (2011) 2918e2927
2923
Table 4
Reusability of complex 5 in comparison to the Karstedt catalyst 1.
Me
O
Me3Si
O
Me
O
SiMe3
Si
Me3Si
0.5 mol%
cat. [Pt]
Si
H
O
O
O
+
Me3Si
Si
SiMe3
+
140 °C
SiMe3
Me
A
B
Cycle
Cat.
Conversion
Conversion
Yield
Ratio A:B
TON
TOF [hꢀ1
]
(siloxane) [%]a
(styrene) [%]a
(hydrosilylation
products) [%]a,b
1.
1.
2.
2.
3.
3.
4.
4.
5.
5.
6.
6.
1
5
1
5
1
5
1
5
1
5
1
5
100
99
97
94
90
96
99
88
99
89
99
86
100
99
96
94
89
95
95
92
98
99
94
97
91
97
87
85
86
95
85
78
93
84
94
74
15:85
15:85
16:84
15:85
16:84
17:83
16:84
17:83
17:83
14:86
15:85
16:84
182
194
174
170
172
190
170
156
186
168
188
148
30
32
29
28
29
32
28
26
31
28
31
25
a
Determined by NMR analysis (error: ꢂ 5%).
b
Reaction time 6 h.
During the first catalytic cycle, the yields of the hydrosilylation
3.6. Concentration dependency
products A and B are comparable for both catalysts (Table 4), but
the Karstedt catalyst 1 obviously produces more side reactions.
Keeping in mind that the catalyst concentration is reduced with
every sample drawn from the reaction mixture, somewhat lower
yields observed during the following catalytic cycle can be
explained. The yields of the hydrosilylation products are compa-
rable for both catalysts with somewhat higher yields for the
Karstedt catalyst. But complex 5 has two important advantages in
comparison with the industrially used Karstedt catalyst 1. On one
hand, no decomposition of complex 5 to platinum black can be
observed even after several catalytic cycles. On the other hand,
complex 5 is at room temperature less soluble in starting materials
and hydrosilylation products which makes it easier to separate it
after the reaction.
We also looked at the catalyst concentration (between 1 mol%
and 0.01 mol%) for the 4-methoxyphenyl-substituted complex 5
(Table 5).
As can be seen from Table 5 the catalyst is highly active with
yields of 90% even at a low catalyst loading of 0.01% (entry 12), but
the lowest amount of side-products is formed at a catalyst
concentration of 0.5 mol% (entry 6). Increasing the amount of
catalyst to 1 mol% does not lead to a better conversion, only to more
side products (entry 3). At 0.01 mol% concentration a significantly
reduced reactivity at the beginning of the reaction (Table 5, entry
10) is observed. The reaction seems to start slower, which might
also explain the good selectivity. Complex 5 shows a remarkably
high TON of 9000 and a TOF of 1500 hꢀ1
.
Table 5
Concentration dependence of the conversion with catalyst 5.
Me
O
Me3Si
O
Me
O
SiMe3
Si
Me3Si
Si
H
O
cat. 5
O
O
+
Me3Si
Si
SiMe3
+
140 °C
SiMe3
Me
A
B
Entry
mol% cat.
t [min]
Conver-sion
Conver-sion
Yield
Ratio A:B
TON
TOF [hꢀ1
]
(siloxane) [%]a
(styrene) [%]a
(hydrosilylation
products) [%]a
1
2
3
4
5
6
7
8
1
1
1
½
89
98
99
69
99
99
61
82
94
0
86
97
99
69
99
99
56
85
95
0
78
90
90
69
97
97
51
70
75
0
21:79
17:83
17:83
22:78
15:85
15:85
22:78
17:83
15:85
e
78
90
90
137
194
194
510
700
750
e
e
240
360
½
240
360
½
240
360
½
240
360
23
15
0.5
0.5
0.5
0.1
0.1
0.1
0.01
0.01
0.01
e
49
32
e
175
125
e
9
10
11
12
89
93
93
97
82
90
16:84
17:83
8200
9000
2050
1500
a
Determined by NMR analysis (error: ꢂ 5%).

2924
M.A. Taige et al. / Journal of Organometallic Chemistry 696 (2011) 2918e2927
Table 6
Temperature dependence.
Me
O
Me3Si
O
Me
SiMe3
O
Si
Si
Me3Si
0.5 mol%
cat. [Pt]
H
O
O
O
SiMe3
+
Me3Si
Si
SiMe3
+
Me
A
B
Entry
Cat.
T [ꢁC]
Conversion
Conversion
Yield
Ratio A:B
TON
TOF [hꢀ1
]
(siloxane) [%]a
(styrene) [%]a
(hydrosilylation
products) [%]a,b
1
2
3
4
5
6
7
8
1
2
5
1
2
5
1
5
1
5
1
5
1
5
50
50
50
70
70
70
80
80
100
100
120
120
140
140
100
95
99
100
85
97
100
100
100
98
100
99
97
94
99
99
100
98
96
100
99
98
83
89
85
99
95
85
97
90
94
86
97
63
91
91
97
17:83
20:80
19:81
17:83
17:83
15:85
17:83
18:82
15:85
18:82
16:84
17:83
15:85
15:85
178
170
200
190
200
194
180
188
172
194
126
182
182
194
30
28
33
32
33
32
30
31
29
32
21
30
30
32
9
10
11
12
13
14
95
100
99
100
99
a
Determined by NMR analysis (error: ꢂ 5%).
b
Reaction time 6 h.
3.7. Reaction temperature
by-products are formed at a reaction temperature of 50 ꢁC, as e.g. the
thermal polymerization of styrene does not proceed below a reaction
temperature of 90 ꢁC in a noteworthy amount. With catalyst 5 full
conversion of the starting materials to the hydrosilylation products is
observed at 50 ꢁC after a reaction time of 6 h (Table 6, entry 3). Hence,
the catalytically active species, which is formed from complex 5, is
highly reactive and very selective in the hydrosilylation reaction of
alkenes. The Karstedt catalyst 1 on the other hand produces 10% side
products already at 50 ꢁC (Table 6, entry 1).
Another parameter with an important effect on the catalytic
activity is the reaction temperature, which was varied between
50 ꢁC and 140 ꢁC for catalysts 1 and 5. Additionally 2 was tested at
50 ꢁC and 70 ꢁC. We could not run the reaction lower than 50 ꢁC due
to the poor solubility of 5 at lower temperatures.
Table 6 shows only selected data points, the full Table 6 is given in
the Supplementary Material. Catalyst 5performs very well over the full
temperature range, the yields have always been higher than 90%, while
the Karstedt catalyst 1 has significant limitations between 80 ꢁC and
120 ꢁC, where the yields were as low as 63% (Table 6, entry 11). Catalyst
2 on the other hand performs significantly better at 70 ꢁC. 5 was
superior to both othercatalysts at 50 ꢁCwiththeaddedbenefitthatless
3.8. Hydrosilylation of styrene with dimethylphenylsilane
As described in Chart 4 different products are formed under the
reaction conditions of the hydrosilylation of styrene with dimethyl-
Table 7
Hydrosilylation of styrene with dimethylphenylsilane.
Entry
Cat.
Conversion
(silane) [%]a
Conversion
Yield
Ratio A:B
TON
TOF [hꢀ1
]
(styrene) [%]a
(hydrosilylation
products) [%]a
1
2
3
4
5
6c
7
8
1
3
4
5
6
7
8
9
100
17
60
58
29
49
13
26
60
24
41
59
33
68
8
52 þ 2b
15 þ 1b
8 þ 42b
48 þ 8b
14 þ 5b
33 þ 3b
0
4:96
2:98
2:98
19:81
7:93
6:94
e
108
31
16
96
28
66
e
18
5
3
16
5
11
e
43
0.4
0:100
0.8
0.1
a
Determined by GCeMS analysis (internal standard: cyclododecane).
Dehydrosilylation product.
Reaction time 24 h.
b
c

M.A. Taige et al. / Journal of Organometallic Chemistry 696 (2011) 2918e2927
2925
Table 8
Influence of the concentration of 5 on the conversion of the hydrosilylation reaction.
Me
Si
H
Me
Me
Me
Si
Si
cat. 5
dioxane,
reflux
Me
+
Me
+
A
B
Entry
mol% Cat.
[mol%]
t [h]
Conversion
(silane) [%]a
Conversion
Yield(products) [%]a
Ratio A:B
TON
45
50
82
96
260
360
1500
2400
10000
26000
TOF [hꢀ1
]
(styrene) [%]a
1
2
3
4
5
6
7
8
9
1
1
0.5
0.5
0.1
0.1
0.01
0.01
0.001
0.001
4
6
4
6
4
6
4
6
4
6
72
92
48
58
28
43
35
41
42
51
77
91
50
59
24
36
22
29
13
28
45 þ 8b
50 þ 12b
41 þ 5b
48þ 8b
26 þ 3b
36 þ 5b
15
24
10
26
4:96
4:96
15:85
19:81
2:98
3:97
3:97
8:92
5:95
3:97
11
8
21
16
65
60
375
400
3250
4333
10
a
Determined by GCeMS analysis (internal standard: cyclododecane).
Dehydrosilylation product.
b
phenylsilane: the branched hydrosilylation product (Chart 4, A), the
linearhydrosilylationproduct(Fig. 6, B), thedehydrosilylationproduct
(Chart 4, C) and the 1,1,2,2-tetramethyl-1,2-diphenyldisilane (Chart 4,
E). The hydrogen, which is a byproduct of the disilane formation,
partially reacts with styrene in a platinum catalyzed hydrogenation to
1-ethylbenzene (Chart 4, D).
aliphatic substituents (8 and 9), nearly no hydrosilylation product is
formed after a reaction time of 6 or 24 h, respectively (Table 7,
entries 7/8). The methyl-substituted complex 7 shows a surpris-
ingly high reactivity in this reaction (Table 7, entry 6), but decom-
poses to platinum black after a reaction time of only 3 h.
Also in this case, the most active catalyst for the hydrosilylation
reaction of styrene with dimethylphenylsilane is the 4-metho-
xyphenyl-substituted complex 5 (Table 7, entry 4). Complex 5 shows
a comparable catalytic activity as the Karstedt catalyst 1 (Table 7, entry
1), again with the least amount of side-products. In the case of 1 the
silane is completely consumed for 52% product, while 5 produces
almost the same yield of product from a lot less starting material.
Comparing entries 2 and 3 of Table 7 one could also discuss
a counterion effect as the mesityl system leads to significantly
different results depending on the counterion. The mesityl-
substituted complex with bromide counterions 4 additionally is
the most active catalyst for the dehydrosilylation reaction (Table 7,
entry 3). It is well known that platinum complexes catalyze the
dehydrosilylation reaction of substrates like styrene [30,97,98] and
that the formation of the dehydrosilylation product depends not
As dimethylphenylsilane is
a less reactive substrate than
bis(trimethylsiloxy)methylsilane, we were curious whether we
would also find a significant substituent effect on the catalytic
activity of the platinum-bis-NHC-complexes for the hydrosilylation
of styrene (Table 7).
The yields of the hydrosilylation reaction of styrene with
dimethylphenylsilane are significantly lower compared to the
reaction with bis(trimethylsiloxy)methylsilane (Tables 3 and 7),
due to the lower reactivity of dimethylphenylsilane and to the
dilution of the reaction mixture as the reaction is run in dioxane. It
is therefore an excellent reaction to study the substituent influence
on the catalytic activity of the platinum-bis-NHC-complexes.
The difference between the substituents, especially between the
aliphatic and aromatic substituents is obvious from Table 7. For the
Table 9
Influence of the reaction temperature on the conversion of the hydrosilylation reaction with catalyst 5.
Me
Si
H
Me
Me
Me
Si
0.5 mol%
Si
cat. 5
Me
+
Me
+
dioxane
A
B
Entry
T [ꢁC]
t [h]
Conversion
(silane) [%]a
Conversion
Yield
Ratio A:B
TON
TOF [hꢀ1
]
(styrene) [%]a
(hydrosilylation
products) [%]a
1
2
3
4
5
50
70
80
100
reflux
6
6
6
6
6
72
50
55
49
64
67
46
57
48
61
67
46
55
48
61
1:99
3:97
5:95
3:97
3:97
144
104
112
98
24
17
19
16
21
126
a
Determined by GCeMS analysis (internal standard: cyclododecane).

2926
M.A. Taige et al. / Journal of Organometallic Chemistry 696 (2011) 2918e2927
only on the nature of the catalyst, but also on the substrates, the
ratio of the substrates, the solvent and the reaction temperature. It
is therefore not surprising that all investigated complexes with the
exception of the very unreactive complexes 8 and 9 catalyze the
hydrosilylation as well as the dehydrosilylation reaction. Still, these
effects need further investigation, which will be the subject of
future studies. For the in general most active complex 5 we also
looked at the concentration- and temperature effects for the
dimethylphenylsilane (Tables 8 and 9). For all concentrations no
reaction was observed after 30 s.
ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: þ44 1223
336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Appendix. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at: doi:10.1016/j.jorganchem.2011.04.030.
References
3.9. Concentration dependence
[1] G. Berthon-Gelloz, I.E. Marko, N-Heterocyclic Carbenes in Synthesis. Wiley-
VCH Verlag GmbH & Co. KGaA, 2006, 119.
[2] G. Berthon-Gelloz, O. Buisine, J.-F. Briere, G. Michaud, S. Sterin, G. Mignani,
B. Tinant, J.-P. Declercq, D. Chapon, I.E. Marko, J. Organomet. Chem. 690 (2005)
6156.
The results are in line with those given in Table 5. Increasing the
amount of catalyst from 0.5 to 1.0 mol% does not lead to higher
yields of hydrosilylation product. Reducing the catalyst amount
from 0.5 to 0.1 mol% leads to a lower conversion, but even with only
0.001 mol% 5, a yield of 26% is obtained after a reaction time of only
6 h. The highest selectivity for the formation of the linear hydro-
silylation product (Table 8, entries 2, 6, 7 and 10) was found for
0.1 mol%. Dehydrosilylation was only observed for the higher
catalyst concentrations 0.1e1 mol% (Table 8, entries 2, 4 and 6).
In the course of this study we had learned that in the case of the
bis(trimethylsiloxy)methyl-silane the reaction could be run at
lower temperatures (Table 6) and we therefore also looked at the
influence of the reaction temperature on the conversion of the
reaction of styrene with dimethylphenylsilane (Table 9).
[3] S.E. Denmark, Z. Wang, Org. Synth. 81 (2005) 54.
[4] S. Diez-Gonzalez, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612.
[5] J.F. Harrod, Organosilicon Chemistry VI: From Molecules to Materials. Wiley-
VCH Verlag GmbH & Co. KGaA, 2005, 392.
[6] B. Marciniec, Coord. Chem. Rev. 249 (2005) 2374.
[7] P.A. Mayes, P. Perlmutter, Modern Reduction Methods. Wiley-VCH Verlag
GmbH & Co. KGaA, 2008, 87.
[8] A.T. Normand, K.J. Cavell, Eur. J. Inorg. Chem. (2008) 2781.
[9] S. Putzien, O. Nuyken, F.E. Kuehn, Prog. Polym. Sci. 35 (2010) 687.
[10] A.K. Roy, Adv. Organomet. Chem. 55 (2008) 1.
[11] M. Drees, T. Strassner, Inorg. Chem. 46 (2007) 10850.
[12] Y.-D. Wu, L.W. Chung, X.-H. Zhang, Computational Modeling for Homoge-
neous and Enzymatic Catalysis. Wiley-VCH Verlag GmbH & Co. KGaA, 2008,
285.
[13] K. Yamamoto, T. Hayashi, in: , second ed., Transition Metals for Organic
Synthesis, vol. 2 Wiley-VCH Verlag GmbH & Co. KGaA, 2004, p. 167.
[14] I. Ojima, in: , The Chemistry of Organic Silicon Compounds, vol. 2, Wiley-
Interscience, New York, 1989, p. 1479.
3.10. Temperature dependence
[15] J.A. Reichl, D.H. Berry, Adv. Organomet. Chem. 43 (1998) 197.
[16] B.M. Trost, Z.T. Ball, Synthesis (2005) 853.
[17] B.M. Trost, Science 254 (1991) 1471.
[18] D.A. Armitage, Organomet. Chem. 10 (1982) 86.
[19] J.L. Speier, Adv. Organomet. Chem. 17 (1979) 407.
[20] B.D. Karstedt, Ger. Offen. DE 1941411, (1970).
[21] B.D. Karstedt, Ger. Offen. DE 2307085, (1973).
[22] I.E. Marko, S. Sterin, O. Buisine, G. Mignani, P. Branlard, B. Tinant,
J.-P. Declercq, Science 298 (2002) 204.
[23] I.E. Marko, S. Sterin, O. Buisine, G. Berthon, G. Michaud, B. Tinant,
J.-P. Declercq, Adv. Synth. Catal. 346 (2004) 1429.
[24] J. Stein, L.N. Lewis, Y. Gao, R.A. Scott, J. Am. Chem. Soc. 121 (1999) 3693.
[25] O. Buisine, G. Berthon-Gelloz, J.-F. Briere, S. Sterin, G. Mignani, P. Branlard,
B. Tinant, J.-P. Declercq, I.E. Marko, Chem. Commun. (2005) 3856.
[26] J.W. Sprengers, M.J. Mars, M.A. Duin, K.J. Cavell, C.J. Elsevier, J. Organomet,
Chem 679 (2003) 149.
[27] S.E. Gibson, M. Rudd, Adv. Synth. Catal. 349 (2007) 781.
[28] W. Caseri, P.S. Pregosin, J. Organomet. Chem. 356 (1988) 259.
[29] W. Caseri, P.S. Pregosin, Organometallics 7 (1988) 1373.
[30] A.M. LaPointe, F.C. Rix, M. Brookhart, J. Am. Chem. Soc. 119 (1997) 906.
[31] X. Wang, H. Chakrapani, J.W. Madine, M.A. Keyerleber, R.A. Widenhoefer,
J. Org. Chem. 67 (2002) 2778.
Also in this case the reaction works best at low temperatures. It
can be run as low as 50 ꢁC, with perfect selectivity and 67% yield
(Table 7, entry 1). Higher temperatures reduce the yields as well as
the selectivity. The dimethylphenylsilane is less reactive compared
to the bis(trimethylsiloxy)methylsilane and we observed in general
lower yields.
4. Conclusion
We could show that platinum(II)-bis-NHC-complexes are
highly active and very stable catalysts for the hydrosilylation
of alkenes.
A fine-tuning of the catalytic activity of the
platinum(II)-bis-NHC-complexes is possible by variation of the
substituents on the imidazole ring. The most active catalyst of all
investigated platinum(II)-bis-NHC-complexes is 1,1-[Bis(3,30-(4-
methoxyphenyl)-1,10-1H-imidazolium-2,20-ylidene)methanediyl]
platinum(II)-di-chloride (5). The catalytic activity of complex 5 is
comparable to the catalytic activity of the Karstedt catalyst (1) and
higher than that of the corresponding platinum(0)-NHC-complex 2.
Complex 5 does not decompose to platinum black during the
reaction and therefore less by-products are formed, which is an
important advantage of catalyst 5 in comparison to the Karstedt
catalyst 1. The optimum conditions are 0.5 mol % catalyst 5 at
a reaction temperature of 50 ꢁC.
[32] M. Poyatos, A. Maisse-Francois, S. Bellemin-Laponnaz, L.H. Gade, Organome-
tallics 25 (2006) 2634.
[33] P.B. Glaser, T.D. Tilley, J. Am. Chem. Soc. 125 (2003) 13640.
[34] I. Ojima, T. Fuchikami, M. Yatabe, J. Organomet. Chem. 260 (1984) 335.
[35] B. Marciniec, C. Pietraszuk, Top. Organomet. Chem. 11 (2004) 197.
[36] C. Menozzi, P.I. Dalko, J. Cossy, Synlett (2005) 2449.
[37] K.D. Hesp, D. Wechsler, J. Cipot, A. Myers, R. McDonald, M.J. Ferguson,
G. Schatte, M. Stradiotto, Organometallics 26 (2007) 5430.
[38] R. Goikhman, D. Milstein, Chem. Eur. J. 11 (2005) 2983.
[39] A.M. Archer, M.W. Bouwkamp, M.-P. Cortez, E. Lobkovsky, P.J. Chirik, Organ-
ometallics 25 (2006) 4269.
[40] A.J. Chalk, J. Organometal. Chem. 21 (1970) 207.
[41] I. Hyder, M. Jimenez-Tenorio, M.C. Puerta, P. Valerga, Dalton Trans. (2007)
3000.
Acknowledgments
[42] S. Rendler, R. Froehlich, M. Keller, M. Oestreich, Eur. J. Org. Chem. (2008)
2582.
[43] C. Sui-Seng, L.F. Groux, D. Zargarian, Organometallics 25 (2006) 571.
[44] J.F. Jensen, B.Y. Svendsen, C.T.V. La, H.L. Pedersen, M. Johannsen, J. Am. Chem.
Soc. 124 (2002) 4558.
We are grateful to the “Fonds der Chemischen Industrie” and to
the “Konrad-Adenauer-Stiftung“ (S. A.) for their support of our
research.
[45] T. Hayashi, S. Hirate, K. Kitayama, H. Tsuji, A. Torii, Y. Uozumi, J. Org. Chem. 66
(2001) 1441.
[46] M. Rastaetter, A. Zulys, P.W. Roesky, Chem. Eur. J. 13 (2007) 3606.
[47] A.Z. Voskoboynikov, A.K. Shestakova, I.P. Beletskaya, Organometallics 20
(2001) 2794.
Appendix A. Supplementary material
CCDC 822864 contains the supplementary crystallographic data
for 5, which can be obtained free of charge via http://www.ccdc.cam.
[48] Y. Horino, T. Livinghouse, Organometallics 23 (2004) 12.

M.A. Taige et al. / Journal of Organometallic Chemistry 696 (2011) 2918e2927
2927
[49] A.K. Dash, I. Gourevich, J.Q. Wang, J. Wang, M. Kapon, M.S. Eisen, Organo-
[74] L.N. Lewis, R.J. Uriarte, N. Lewis, J. Catal. 127 (1991) 67.
[75] L.N. Lewis, Chem. Rev. 93 (1993) 2693.
metallics 20 (2001) 5084.
[50] A. Corma, C. Gonzalez-Arellano, M. Iglesias, F. Sanchez, Angew. Chem. Int. Ed.
46 (2007) 7820.
[51] T. Tuttle, D. Wang, W. Thiel, J. Köhler, M. Hofmann, J. Weis, J. Organomet.
Chem. 692 (2007) 2282.
[52] D. Bourissou, O. Guerret, F.P. Gabbaie, G. Bertrand, Chem. Rev. 100 (2000) 39.
[53] V. Cesar, S. Bellemin-Laponnaz, Actual. Chim. 326 (2009) 8.
[54] T.N. Tekavec, J. Louie, Top. Organomet. Chem. 21 (2007) 159.
[55] F.E. Hahn, M.C. Jahnke, Angew. Chem. Int. Ed. 47 (2008) 3122.
[56] A.J. Arduengo III, H.V.R. Dias, R.L. Harlow, M. Kline, J. Am. Chem. Soc. 114
(1992) 5530.
[57] W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290.
[58] C.A. Quezada, J.C. Garrison, C.A. Tessier, W.J. Youngs, J. Organomet. Chem. 671
(2003) 183.
[76] L.N. Lewis, K.G. Sy, G.L. Bryant Jr., P.E. Donahue, Organometallics 10 (1991) 3750.
[77] L.N. Lewis, R.J. Uriarte, Organometallics 9 (1990) 621.
[78] C.F. De, J.A. Osborn, G. Wilkinson, J. Chem. Soc. A (1968) 787.
[79] A. Onopchenko, E.T. Sabourin, J. Org. Chem. 52 (1987) 4118.
[80] A.J. Chalk, J.F. Harrod, J. Am. Chem. Soc. 87 (1965) 16.
[81] J. Liu, J. Chen, J. Zhao, Y. Zhao, L. Li, H. Zhang, Synthesis (2003) 2661.
[82] S. Ahrens, T. Strassner, Inorg. Chim. Acta 359 (2006) 4789.
[83] E. Mas-Marza, E. Peris, I. Castro-Rodriguez, K. Meyer, Organometallics 24
(2005) 3158.
[84] M.H. Voges, C. Romming, M. Tilset, Organometallics 18 (1999) 529.
[85] G. De Bo, G. Berthon-Gelloz, B. Tinant, I.E. Marko, Organometallics 25 (2006)
1881.
[86] T. Strassner, Y. Unger, A. Zeller, WO 2008000726 (2008).
[87] A.J.C. Wilson, International Tables for Crystallography. Kluwer Academic
Publisher, Dodrecht, The Netherlands, 1992.
[59] S. Ahrens, E. Herdtweck, S. Goutal, T. Strassner, Eur. J. Inorg. Chem. (2006) 1268.
[60] S. Ahrens, A. Zeller, M. Taige, T. Strassner, Organometallics 25 (2006) 5409.
[61] M. Muehlhofer, T. Strassner, E. Herdtweck, W.A. Herrmann, J. Organomet,
Chem 660 (2002) 121.
[88] G.M. Sheldrick, SHELXL-97, Program for the Refinement of Structures.
University of Goettingen, Goettingen, Germany, 1997.
[62] Y. Unger, A. Zeller, S. Ahrens, T. Strassner, Chem. Commun. (2008) 3263.
[63] Y. Unger, A. Zeller, M.A. Taige, T. Strassner, Dalton Trans. (2009) 4786.
[64] A. Meyer, M.A. Taige, T. Strassner, J. Organomet. Chem. 694 (2009) 1861.
[65] M.A. Taige, A. Zeller, S. Ahrens, S. Goutal, E. Herdtweck, T. Strassner,
J. Organomet. Chem. 692 (2007) 1519.
[89] Data Nonius, Collection Software for Nonius-Kappa CCD Devices. Delft, The
Netherlands, 1997e2000.
[90] A.J.M. Duisenberg, J. Appl. Cryst 25 (1992) 92.
[91] A.J.M. Duisenberg, R.W.W. Hooft, A.M.M. Schreurs, J. Kroon, J. Appl. Cryst. 33
(2000) 893.
[66] T. Strassner, M. Muehlhofer, A. Zeller, E. Herdtweck, W.A. Herrmann,
J. Organomet. Chem. 689 (2004) 1418.
[92] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M.C. Burla, G. Polidori,
M. Camalli, J. Appl. Cryst. 27 (1994) 435.
[67] M.Muehlhofer,T.Strassner,W.A.Herrmann,Angew.Chem.Int.Ed.41(2002)1745.
[68] J. Beckmann, D. Dakternieks, A. Duthie, S.L. Floate, R.C. Foitzik, C.H. Schiesser,
J. Organomet. Chem. 689 (2004) 909.
[69] D. Marchand, J. Martinez, F. Cavelier, Eur. J. Org. Chem. (2008) 3107.
[70] A. Hopf, K.H. Dotz, J. Mol. Catal. A: Chem. 164 (2000) 191.
[71] P. Steffanut, J.A. Osborn, A. DeCian, J. Fisher, Chem. Eur. J. 4 (1998) 2008.
[72] L.N. Lewis, J. Am. Chem. Soc. 112 (1990) 5998.
[93] G.M. Sheldrick, SADABS, Version 2.10. University of Goettingen, Goettingen,
Germany, 2003.
[94] A.L. Spek, PLATON. Utrecht University, Utrecht, The Netherlands, 2001.
[95] M.N. Burnett, K.J. Carroll, ORTEP-III. Oak Ridge National Laboratory, Tenn-
nessee, USA, 1996.
[96] N.S. Shaikh, K. Junge, M. Beller, Org. Lett. 9 (2007) 5429.
[97] M. Tanaka, Y. Uchimaru, H.J. Lautenschlager, Organometallics 10 (1991) 16.
[98] M. Tanaka, Y. Uchimaru, H.J. Lautenschlager, J. Organomet. Chem. 428 (1992) 1.
[73] L.N. Lewis, N. Lewis, J. Am. Chem. Soc. 108 (1986) 7228.