Bridge functionalized bis-N-heterocyclic carbene rhodium(I) complexes and their application in catalytic hydrosilylation

Article abstract of DOI:10.1016/j.jorganchem.2010.09.049

A series of chelating bridge functionalized bis-N-heterocyclic carbenes (NHC) complexes of rhodium (I) were prepared by reacting the corresponding imidazolium salts with [Rh(COD)Cl]₂ in an in-situ reaction. For the N-methyl substituted complex

Full text of DOI:10.1016/j.jorganchem.2010.09.049

Journal of Organometallic Chemistry 696 (2011) 687e692
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Bridge functionalized bis-N-heterocyclic carbene rhodium(I) complexes and their
application in catalytic hydrosilylation
a
b
a
ꢀ
ꢁ b
Claudia S. Straubinger , Nadezda B. Jokic , Manuel P. Högerl , Eberhardt Herdtweck ,
Wolfgang A. Herrmann a , Fritz E. Kühn
,
a,b,
**
*
a Lehrstuhl für Anorganische Chemie, Catalysis Research Center, Technische Universität München, Lichtenbergstraße 4, 85747 Garching, Germany
b Molecular Catalysis, Catalysis Research Center, Technische Universität München, Lichtenbergstraße 4, 85747 Garching, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2010
Received in revised form
20 September 2010
Accepted 22 September 2010
Available online 1 October 2010
A series of chelating bridge functionalized bis-N-heterocyclic carbenes (NHC) complexes of rhodium (I)
were prepared by reacting the corresponding imidazolium salts with [Rh(COD)Cl]2 in an in-situ reaction.
For the N-methyl substituted complex with a PF6-anion an X-ray crystal structure was exemplary
obtained. All complexes were spectroscopically characterized and tested for the hydrosilylation of
acetophenone.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
N-Heterocyclic carbene
Bridge functionalization
Rhodium
Hydrosilylation (hydrosilation)
1. Introduction
from the product [9]. In order to achieve immobilization more
easily a functional group is attached to the complexes to allow
NHC metal complexes [1e4] have found widespread applica-
tions in a multitude of catalytic transformations such as Heck-,
Suzuki- and Sonogashira reactions, aryl amination or metathesis
[5], in carbon monoxide/ethylene copolymerization [6] and
hydrosilylation [7]. N-heterocyclic carbenes as ligands in organo-
a connection to the surface. Ligands with tethered hydroxy groups
have been successfully utilized for the immobilization of other
catalysts before [10], usually accounting for quite low leaching [11].
This concept was applied to synthesize novel hydroxy-functional-
ized bridged bis-carbene complexes.
metallic complexes act as strong s-donors and weak p-acceptors.
In many cases NHC complexes are air- and moisture stable and
show high thermal stability. Especially chelating bis-NHC
complexes of Pd and Pt exhibit extreme thermal and moisture
stable complexes, leading to remarkable catalytic properties [8].
In recent years the interest in “green” environmentally benign
chemistry has been continuously increasing. In this context, the
immobilization of NHC metal complexes on insoluble supports to
facilitate their reuse and recycling is emerging as a preferable
alternative for the application of homogeneous catalysis, which
usually requires energy intensive processes to separate the catalyst
2. Experimental
Unless otherwise stated, all reactions were performed under
a dry argon atmosphere using standard Schlenk techniques.
Solvents and substrates were dried by standard procedures,
distilled under an argon atmosphere and stored over molecular
sieves. 1,3-dibromo-propane-2-ol was purchased from Acros and
used without further purification. 1H, 13C{1H}, 19F and 31P-NMR
spectra were recorded on a JEOL-JMX-GX 400 MHz spectrometer at
25 ꢀC. 1H-NMR peaks are assigned as singlet (s), doublet (d), triplet
(t), multiplet (m). Mass spectrometric measurements were per-
formed on a Finnigan MAT 90 spectrometer using the FAB tech-
nique. Elemental analyses were performed in the Microanalytical
Laboratory at the TU München. Compounds 1ae3a were prepared
according to a modified literature procedure. The spectroscopic
date of compounds 1a and 3a identical to those reported in the
literature [12].
* Corresponding author.
** Corresponding author. Molecular Catalysis, Catalysis Research Center, Techni-
sche Universität München, Lichtenbergstraße 4, 85747 Garching, Germany. Fax:
þ49 89 289 13473.
E-mail addresses: wolfgangherrmann@ch.tum.de (W.A. Herrmann), fritz.kuehn@
ch.tum.de (F.E. Kühn).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.049

688
C.S. Straubinger et al. / Journal of Organometallic Chemistry 696 (2011) 687e692
2.1. General procedure for bridged bis-imidazoliumbromide salts
(1ae3a)
minutes a precipitate yields in the corresponding tetraphenylbo-
rate salt. The solid was filtered off and washed with water, dieth-
ylether and pentane and dried under vacuum to yield the
imidazolium salt 3c in 72%.
In an ACE pressure tube 25 mmol 1-R-imidazole (R ¼ methyl,
isopropyl, benzyl) and 10 mmol 1,3-dibromo-propane-2-ol are
dissolved in 5 ml THF at room temperature. After stirring the
solution over night, the reaction mixture is heated for 72 h at
100 ꢀC. After cooling to room temperature the reaction solution is
filtered off and the precipitate washed twice with 5 ml THF. Drying
under reduced pressure leaves compound 1e3 as off-white solids.
2.3.1. 1,10-(2-Hydroxy-1,3-propanediyl)bis[3-benzyl-1H-
imidazolium]di(tetraphenylborate) 3c
1H-NMR(400 MHz, 298 K, d6-Aceton):
d
¼ 8.77 (2H, s, NCHN),
7.57 (4H, s, NCH), 7.40 (20H, d, HAr), 6.89 (20H, m, HAr),6.75 (10H, m,
HAr), 5.39 (4H, s, NCH2-Ph), 4.42 (2H þ 1H, m, NCH2 þ CH), 4.20 (dd,
2H, NCH2). MS (FAB): m/z (%): 693 (15, [M-BPh4]þ), 373 (60, [M-2x
BPh4]þ). Anal. Calc for C23H26B2N4O*1/2 KBr: C 79.52; H 6.20; N
5.22. Found: C 79.62; H 6.13; N 4.98%.
2.1.1. 1,10-(2-Hydroxy-1,3-propanediyl)bis[3-isopropyl-1H-
imidazolium]dibromid 2a
Yield: 96%.1H-NMR(400 MHz, 298 K, d6-DMSO):
d
¼ 9.28 (2H, s,
3
NCHN), 7.94 (2H, S, NCH), 7.78 (2H, S, NCH), 5.95 (1H, d, JH-
2.4. General procedure for the chelating bis(NHC)-Rh(I) complexes
(4e7)
¼ 5.6 Hz, OH), 4.67 (2H, sept, CHiPr), 4.40 (2H, d, 3J ¼ 13.6 Hz,
OH
CH2), 4.25 (1H, m, CH), 4.12 (2H, dd, 2J ¼ 8 Hz,3J ¼ 13.6 Hz, CH2), 1.46
(12H, d, CH3). 13C{1H}-NMR(100 MHz, 298 K, d6-DMSO):
d
¼ 135.4
NaH and [Rh(COD)Cl]2 were each dissolved in ethanol and the
solutionswerecombinedandstirredfor30minatroomtemperature.
The clear, yellow solution was added via canula to the hexa-
fluorophosphate salts (complexes 4e6) respectively tetraphenylbo-
rate salt (complex 7) was added and the mixture was stirred for 16 h.
After reaction, ethanol was removed under vacuum and the
complexes were extracted with dichloromethane. Recrystallization
from a saturated DCM-solution with diethylether yields in the
rhodium complexes 4e7 as yellow solids. Crystals suitable for X-ray
diffraction studies of complex 4 could be obtained byslowly diffusion
of pentane into a dichloromethane solution of complex 4.
(NCHN), 123.1 (NCH), 120.3 (NCH), 67.5 (C-H), 52.2 (CHiPr), 51.9
(NCH2) 22.2 (CH3). MS (FAB): m/z (%): 356.9 (40, [M-Br]þ), 277.0 (34,
[M-2xBr]þ). Anal. Calc for C37H52Br2N4O: C 41.11; H 5.98; N 12.79.
Found: C 40.64; H 6.09; N 12.65%.
2.2. General procedure for the PF6-Salts (1be3b)
The corresponding bromine salts 1ae3a were dissolved in
a minimum amount of water and added to a saturated solution of
KPF6 in water. The precipitated imidazolium hexafluorophosphate
salts are filtered off, washed with water and diethylether and dried
under vacuum yielding the imidazolium salts 1be3b.
2.4.1. [Rh(Me-Im)2(COD)](PF6) 4
Yield: 30%. 1H -NMR(400 MHz, 298 K, d2-DCM):
NCH), 6.77 (2H, d, NCH), 4.97 (2H, d, NCH2), 4.54 (4H, m, CH2-COD),
4.44 (1H, m, CH), 4.30 (2H, dd, NCH2), 3.89 (6H, s, NCH3), 2.45 (4H, m,
CH2-COD), 2.25 (4H, m, CH2-COD).13C{1H}-NMR(100 MHz, 298 K, d2-
d
¼ 6.97 (2H, d,
2.2.1. 1,10-(2-Hydroxy-1,3-propanediyl)bis[3-methyl-1H-
imidazolium]di(hexafluorophosphate)1b
Yield: 51%. 1H-NMR(400 MHz, 298 K, d6-Aceton):
d
¼ 9.00 (2H, s,
NCHN), 7.72 (4H, d,NCH), 5.50(1H, d,OH), 4.71(2H, d, NCH2), 4.56(1H,
DCM):
d
¼ 183.2 CCarben, 125.5 (NCH), 121.6 (NCH), 90.3 (COD), 67.0
m, CH), 4.42 (2H, dd, CH2), 4.07 (6H, s, NCH3). 13C{1H}-NMR(100 MHz,
(C-H), 55.9 (NCH2), 35.1 (NCH3), 30.9 (COD). 31P{1H}-NMR(161 MHz,
298 K, d6-Aceton):
d
¼ 138.0 (NCHN), 124.6 (NCH), 124.0 (NCH), 69.2
298 K, d2-DCM):
d
¼ ꢁ 130.5 to ꢁ 158.0. MS (FAB): m/z (%): 431 (100,
(C-H), 53.1 (NCH2) 36.5 (NCH3). 31P{1H}-NMR(161 MHz, 298 K, d6-
[M-PF6]þ), 323 (70, [M-PF6-COD]þ). Anal. Calc for C19H28F6N4OPRh:
C 39.60; H 4.90; N 9.72. Found: C 39.18; H 4.70; N 9.30%.
Aceton):
d
¼ ꢁ 130.5 to ꢁ 158.0. MS (FAB): m/z (%): 367 (75, [M-PF6]þ),
221 (100, [M-2xPF6]þ). Anal. Calc for C11H18F12N4OP2*2KBr: C 17.61; H
2.42; N 7.47. Found: C 17.84; H 2.42; N 7.33%.
2.4.2. [Rh (iPr-Im)2(COD)](PF6) 5
Yield: 45%. 1H-NMR(400 MHz, 298 K, d2-DCM):
d
¼ 7.29 (2H, d,
2.2.2. 1,10-(2-Hydroxy-1,3-propanediyl)bis[3-isopropyl-1H-
NCH), 6.75 (2H, d, NCH), 5.11 (2H, d, NCH2 þ 1H CH), 4.84 (2H, sept.,
CHiPr), 4.55 (4H, m, CH2-COD), 4.13 (2H, dd, NCH2), 2.38 (4H, m, CH2-
COD), 2.21 (4H, m, CH2-COD), 1.37 (12H, d, CH3). 13C{1H}-NMR
imidazolium]di(hexafluorophosphate) 2b
Yield: 59%.1H-NMR(400 MHz, 298 K, d6-Aceton):
d
¼ 9.21 (2H, s,
NCHN), 7.89 (2H, d, NCH), 7.77 (2H, d, NCH), 5.91 (1H, s, OH), 4.67
(2H, sept, CHiPr), 4.48 (2H, d, NCH2), 4.26 (1H, m, CH), 4.17 (2H, dd,
CH2), 1.48 (12H, s, NCH3). 31P{1H}-NMR(161 MHz, 298 K, d6-Ace-
(100 MHz, 298 K, d2-DCM):
d
¼ 179.7 CCarben, 125.4 (NCH), 115.8
(NCH), 90.1 (COD), 65.0 (C-H), 59. 6 (C-H), 52.4 (NCH2), 30.9 (COD),
23.8 (CH3). MS (FAB): m/z (%): 487 (100, [M-PF6]), 379 (100, [M-PF6-
COD]). Anal. Calc for C23H36F6N4OPRh: C 43.68; H 5.74; N 8.86.
Found: C 43.12; H 6.12; N 8.72%.
ton):
d
¼ ꢁ 131.0 to ꢁ 157.0. Anal. Calc for C15H26F12N4OP2*1/2 KBr:
C 28.70; H 4.17; N 8.92. Found: C 28.79; H 4.32; N 8.93%.
2.2.3. 1,10-(2-Hydroxy-1,3-propanediyl)bis[3-benzyl-1H-
imidazolium]di(hexafluorophosphate) 3b
2.4.3. [Rh(Bn-Im)2(COD)](PF6) 6
Yield: 28%. 1H-NMR(400 MHz, 298 K, d2-DCM):
d
¼ 7.35 (6H, m,
Yield: 75%.1H-NMR(400 MHz, 298 K, d6-Aceton):
d
¼ 9.17 (2H, s,
HAr), 7.11 (2H, d, NCH), 6,83 (4H, m, HAr), 6.64 (4H, d, NCH), 5.64 (2H,
d, Ph-NCH2), 5.12 (2H, d, Ph-NCH2), 4.86 (1H, m, CH), 4.64 (4H, m,
CH2-COD þ NCH2), 4.48 (4H, m, CH2-COD þ NCH2), 2.47 (4H, m,
CH2-COD), 2.25 (4H, m, CH2-COD). MS (FAB): m/z (%): 582 (100, [M-
PF6]þ), 474 (81, [M-PF6-COD]þ). Anal. Calc for C31H36F6N4OPRh: C
51.11; H 4.91; N 7.46. Found: C 50.27; H 4.98; N 7.69%.
NCHN), 7.75 (2H, d, NCH), 7.48 (2H, d, NCH), 7.43 (10H, m, HAr), 5.58
(5H s, br, 1H-OH þ 4H-CH2-Ph), 4.69 (2H, d, NCH2), 4.53 (1H, m, CH),
4.41 (2H, dd, CH2).31P{1H}-NMR(161 MHz, 298 K, d6-Aceton):
d
¼ ꢁ 130.0 to ꢁ 158.0. MS (FAB): m/z (%): 518.5 (100, [M-PF6]þ), 373
(72, [M-2xPF6]þ). Anal. Calc for C23H26F12N4OP2: C 41.58; H 3.94; N
8.43. Found: C 41.52; H 3.85; N 8.49%.
2.4.4. [Rh(Bn-Im)2(COD)](BPh4) 7
2.3. Synthesis procedure for the BPh4-Salt 3c
Yield: 37%. 1H-NMR(400 MHz, 298 K, d6-Aceton):
m, HAr), 7.04 (2H, s, NCH), 7.05 (10H, m, HAr), 7.04 (2H, s, NCH), 5.58
(4H, s, NCH2-Ph), 4.77 (2H þ 1H, m, NCH2 þ CH), 4.57 (4H, m, CH2-
COD), 4.35 (dd, 2H, NCH2), 2.44 (4H, m, CH2-COD), 2.24 (4H, m,
d
¼ 7.37 (20H,
The corresponding bromine salt 3a was suspended in acetone
and a saturated solution of KBPh4 in acetone was added. After a few

C.S. Straubinger et al. / Journal of Organometallic Chemistry 696 (2011) 687e692
689
CH2-COD). MS (FAB): m/z (%): 583 (100, [M-BPh4]þ), 475 (90, [M-
Table 2
BPh4-COD]þ). Anal. Calc for C55H56BN4ORh*1/2 DCM: C 70.52; H
6.08; N 5.93. Found: C 69.94; H 6.39; N 5.15%.
Crystallographic data for compound 4.
4
Formula
Fw
C19H28F6N4OPRh
576.33
2.5. Catalysisehydrosilation of 4-fluoro-acetophenone
Color/habit
Yellow/fragment
0.20 ꢂ 0.25 ꢂ 0.46
Monoclinic
P21/c (no. 14)
14.2479(6)
12.6485(5)
13.6367(6)
114.119(2)
2242.99(17)
4
Crystal dimensions (mm3)
Crystal system
Space Group
4-fluoro-acetophenone (0.504 mmol) and the rhodium complex
(0.02 mmol) in 0.3 mL solvent (DCM, THF or DCE) were stirred for
10 min. Diphenylsilane (0.765 mmol) was added and the mixture
stirred at 25 ꢀC (60 ꢀC) and monitored by 19F-NMR spectroscopy.
ꢂ
a (A)
ꢂ
b (A)
ꢂ
c (A)
b
(ꢀ)
3
2.6. Single crystal X-ray structure determination of compound 4
ꢂ
V (A )
Z
T (K)
123
1.707
0.902
1168
Crystal data and details of the structure determination are
presented in Tables 1 and 2. Suitable single-crystals for the X-ray
diffraction study were grown with standard cooling techniques.
Crystals were stored under perfluorinated ether and fixed on the
top of a glass fiber. Preliminary examination and data collection
were carried out on an area detecting system (BRUKER AXS, APEX II,
Dcalcd (g cmꢁ3
)
m
(mmꢁ1
)
F(000)
q
Range (ꢀ)
1.57e25.31
Index ranges (h, k, l)
No. of rflns. collected
No. of indep. rflns./Rint
No. of obsd. rflns. (I > 2
ꢁ 16/þ 17, ꢃ 15, ꢃ 16
60597
4083/0.021
3957
4083/0/291
0.0488/0.1141
0.0504/0.1154
1.160
k
-CCD; FR591 rotating anode) and graphite-monochromated Mo-
s(I))
ꢂ
Ka radiation (
obtained by full-matrix least-squares refinements during the
scaling procedure. Data collection was performed at 123
l
¼ 0.71073 A). The unit cell parameters were
No. of data/restraints/params
R1/wR2 (I > 2s
(I))a
R1/wR2 (all data)a
K
GOF (on F2)a
(OXFORD CRYOSYSTEMS cooling device). The crystal was measured
with a couple of data sets (five runs; 3065 frames) in rotation scan
ꢂꢁ3
Largest diff. peak and hole (e A
)
þ 1.06/- 0.86
a
R1 ¼ S(kFoj-jFck)/SjFoj; wR2 ¼ {S[w(F2o-Fc2)2]/Sw(F2o)2]}1/2; GOF ¼ {S[w(Fo2-F2c)2]/
modus (D4
/
Du ¼ 0.50ꢀ; dx ¼ 35 mm). Intensities were integrated
(n-p)}1/2
.
and the raw data were corrected for Lorentz, polarization, and,
arising from the scaling procedure for latent decay and absorption
effects. The structure was solved by a combination of direct
methods and difference Fourier syntheses. All non-hydrogen atoms
were refined with anisotropic displacement parameters. Methyl
hydrogen atoms were calculated as a part of rigid rotating groups,
atoms were taken from International Tables for Crystallography. All
calculations were performed with the WINGX system, including
the programs PLATON, SHELXL-97, and SIR92 [13].
ꢂ
3. Results and discussion
with dCeH ¼ 0.98 A and Uiso(H) ¼ 1.5Ueq(C). All other hydrogen atoms
were placed in ideal positions and refined using a riding model,
ꢂ
Herein we describe a series of new bridged Rh(I)(bis-NHC)X
complexes with a hydroxy functionality in the bridge. As it can be
seen in the crystal structure, the hydroxy group does not coordinate
to the metal center, being free for reaction with a linker group on
a solid support, e.g. a polymer such as Wang-resin or inorganic
silica materials such as MCM-41 for use in heterogeneous catalysis.
In order to examine whether the newly introduced functional
group would affect the catalytic activity, all complexes were
examined as catalysts for homogeneous hydrosilylation reaction of
acetophenone with diphenylsilane.
with methylene, methine and aromatic dCeH distances of 0.99 A,
ꢂ
ꢂ
1.00 A and 0.95 A, respectively, and Uiso(H) ¼ 1.2Ueq(C). The hydrogen
atom of the hydroxyl group was calculated as a part of a rigid
ꢂ
rotating group, with dOeH ¼ 0.84 A and Uiso(H) ¼ 1.2Ueq(O). Full-
matrix least-squares refinements were carried out by minimizing
Sw(Fo2eF2c)2 with the SHELXL-97 weighting scheme and stopped at
shift/err < 0.001. The final residual electron density maps showed
no remarkable features. Neutral atom scattering factors for all
atoms and anomalous dispersion corrections for the non-hydrogen
Table 1
3.1. Synthesis and characterization
Selected bond distances (A) and angles (ꢀ) for
ꢂ
compound 4.
The hydroxy-functionalized imidazolium salts 1ae3a were
prepared by reaction of 1,3-dibromo-propane-2-ol with the corre-
sponding N-subsituted imidazoles in THF in a pressure tube
(Scheme 1) [12]. All bromine salts are soluble in polar solvents such
as methanol and dimethylsulfoxide. They are stable both under air
and in the presence of moisture, however compound 2a is very
hygroscopic and should be stored under an argon atmosphere.
The formation of compounds 1a-3a was verified by the
appearance of the NCHN peak at around 9 ppm in the 1H-NMR
spectra and the peak at around 137 ppm in the 13C-NMR spectra
which are in the typical range for the NCHN-proton of imidazolium
salts. An X-ray structure of 1a has been obtained [12].
ꢂ
Bond distance (A)
RhꢁC1
2.030(5)
2.042(6)
2.100
2.082
1.418(7)
RhꢁC10
RhꢁCg1a
RhꢁCg2a
C6ꢁO1
Bond angle (ꢀ)
C1ꢁRhꢁC10
C1ꢁRhꢁCg1a
C1ꢁRhꢁCg2a
C10ꢁRhꢁCg1a
C10ꢁRhꢁCg2a
Cg1ꢁRhꢁCg2a
N1ꢁC1ꢁN2
C5ꢁC6ꢁO1
C7ꢁC6ꢁO1
C5ꢁC6ꢁC7
N3ꢁC10ꢁN4
83.6(2)
176.9
96.0
93.4
178.8
87.0
104.2(4)
111.1(4)
107.7(4)
118.4(5)
104.9(5)
N
N
THF, 100 °C
pressure tube
Br
Br
N
N
1a R = Me
2a R = iPr
3a R = Bn
+ 2
OH
OH
N
R
N
R
2 Br-
R
a
Cg is defined as the midpoint of the double bonds
Cg1: C12]C13, and Cg2: C16]C17, resp.
Scheme 1. Synthesis of the bis-imidazolium salts 1ae3a.

690
C.S. Straubinger et al. / Journal of Organometallic Chemistry 696 (2011) 687e692
H2O, rt
- 2 KBr
N
N
N
N
N
1b R = Me
2b R = iPr
3b R = Bn
+ 2 KPF6
OH
OH
N
R
N
R
N
R
N
R
-
2 Br-
2 PF6
Acetone, rt
- 2 KBr
N
N
N
+ 2 KBPh4
OH
OH
N
R
N
R
N
R
N
R
3c R = Bn
-
2 Br-
2 BPh4
Scheme 2. Anion exchange reaction.
OH
1) EtOH, NaH
4 R = Me, X = PF6
5 R = iPr, X = PF6
6 R = Bn, X = PF6
7 R = Bn, X = BPh4
N
N
[Rh(COD)Cl]2
N
R
N
2)
N
R
N
Rh
X-
OH
2 X-
N
R
N
R
Scheme 3. Syntheses of the Rh(I) NHC complexes 4e7.
The anion exchange from bromine salts to the corresponding
hexafluorophosphate salts was carried out by mixing an aqueous
solution of the bromine salt with a saturated aqueous solution of
KPF6 (Scheme 2). The PF6-salts precipitate instantly from the
solution. The formation of the hexafluorophosphate ligand salts
was verified by FAB mass spectrometry, elemental analysis and the
septet signal around ꢁ 140 ppm in the 31P-NMR spectra. The
exchange of the bromide anions by tetraphenylborate anions was
achieved by reaction of the bromine salts with KBPh4 in acetone
(Scheme 2) yielding compound 3c. The formation of compound 3c
was verified by FAB-MS, elemental analysis and NMR spectroscopy.
The rhodium(I) complexes 4e6 were prepared starting from [Rh
(COD)Cl]2 and two equivalents of the corresponding PF6-imidazo-
lium salts 1be3b (Scheme 3) at room temperature under an argon
atmosphere. The in-situ synthesis of [Rh(COD)(OEt)]2 in the pres-
ence of NaH in ethanol resulted in a colour change from orange to
bright yellow. Afterwards, the PF6-salt was added to the suspension
and stirred over night. A yellow precipitate formed and the solution
was filtered off. After extraction with DCM and precipitation after
addition of pentane the Rh-bis-NHC complexes 4e6 were obtained
as yellow crystals. All complexes are air- and moisture stable and
soluble in DCM, THF, acetone and 1,2-dichloroethane. They are
stable in the presence of water, but not soluble. The synthesis of
complex 7 was carried out in the same way starting from tetra-
phenylborate salt 3c.
Fig. 1. ORTEP style plot of the cationic part of compound 4 in the solid state. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for
clarity except the hydroxyl hydrogen atom.
The formation of the carbene complexes was established by the
absence of the carbon peak at around 137 ppm and the peak for the
transformation, as protected alcohols can be produced directly
from carbonyl compounds. The newly synthesized rhodium
complexes were tested with a catalyst loading of 2 mol % in
homogeneous hydrosilylation of acetophenone with diphenylsilane
yielding silylether A and silylenolether B [15]. The substrate ace-
tophenone was replaced by 4-fluoroacetophenone in order to
conveniently monitor the reaction progress in-situ by 19F-NMR
spectroscopy, to avoid taking samples and the associated workup
and to circumvent the problem of overlapping integrals as observed
in the 1H-NMR spectra. The results are summarized in Table 3. For
a better comparison of the relative catalyst activities the time-
conversion curves are shown in Fig. 2.
2H-imidazolium proton in the 1H-NMR spectra
d
(1H) ¼ 9 ppm and
by the presence of a doublet peak at
d (
13C) w 180 ppm, which can
be assigned to the 2C-imidazol-2-ylidene (carbene) carbons of the
complexes 4e7 with a typical Rh-C coupling constant of ca. 50 Hz.
The structure of 4 was determined by X-ray crystallography and is
shown in Fig. 1. As for related bis-carbene complexes a boat
conformation could be established [14] with a non-coordinating
hydroxy group pointing away from the metal center.
To the best of our knowledge no other bis-carbene rhodium(I)
complex with a hydroxy functionality directly bonded on a bridging
atom has been characterized and described before.
i
With different wingtip groups Me - complex 4 (entry 1), Pr -
complex 5 (entry 3) and Bn - complex 6 (entry 5) a roughly 10%
3.2. Catalysis
higher selectivity can be seen for the sterically more demanding iPr
and Bn groups. A five to ten times higher TOF is achieved for Pr
i
The hydrosilylation of ketones or aldehydes leading to silyl
ethers is a useful and experimentally relatively straightforward
(entry 3). However, even after long reaction times of 15 h and 24 h,
Me and Bn groups to not go to full conversion under these reaction

C.S. Straubinger et al. / Journal of Organometallic Chemistry 696 (2011) 687e692
691
Table 3
Results for the hydrosilylation of 4-fluoro-acetophenone.
entry
[Rh]
R
X-
T [ꢀC]
solvent
time [h]
conv. [%]
A [%]
B [%]
TOF [hꢁ1
]
1
2
3
4
5
6
4
4
5
5
6
7
Me
Me
iPr
iPr
Bn
Bn
PF6
PF6
PF6
PF6
PF6
BPh4
25
60
25
25
25
25
DCM
DCE
DCM
THF
DCM
DCM
15
4
2
4
24
50 min
73
100
100
100
81
51
63
66
40
64
72
49
37
34
60
36
28
15
30
70
55
5
100
70
Reaction conditions: 2 mol % catalyst, 0.504 mmol of 4-fluoro-acetophenone, 0.756 mmol of diphenylsilane, 0.3 ml dichloromethane (entry 2: 1,2-dichloroethane DCE).
Conversions were determined by 19F-NMR spectroscopy. No spectroscopic evidence was found for defluorination, nor the formation of other products than A and B. TOFs have
been calculated from at the maximal slope of the time-conversion curve.
Related immobilized complexes of palladium have been success-
fully used in Suzuki-Miyaura cross coupling reactions. Investiga-
tions with respect to the synthesis and catalytic performance of
such immobilized compounds are currently under way in our labs.
Acknowledgements
We gratefully acknowledge Dr. Monica Carril and Serena L.M.
Goh for helpful discussions. This work was supported by Uni-
versität Bayern e. V. (Ph.D. fellowship for N.B.J.) and the Wacker
Insitut für Silicium Chemie-Wacker AG (Ph.D. fellowship for M.P.H).
We further thank the International Graduate School of Science and
Engineering (IGSSE) for financial support.
Fig. 2. Time-conversion curves of the hydrosilylation reaction.
conditions. Using complex 4 as catalyst at 25 ꢀC, a TOF of 15 hꢁ1 is
achieved and a conversion of 73% after 15 h (entry 1). A tempera-
ture increase from room temperature to 60 ꢀC increases the turn-
over frequencies up to 30 hꢁ1 and complete conversion is achieved
after 4 h reaction time (entry 2). A solvent dependent selectivity
can be noted. Using complex 5 in DCM as solvent, a TOF of 70 hꢁ1 is
achieved and a product selectivity of 66% for the silylated alcohol A
(entry 3) could be reached. Application of the same complex in THF
as solvent leads to a reverse product selectivity, TOF is 55 hꢁ1 (entry
4). For complex 6, with a PF6-anion and 7, with a BPh4 anion, a clear
influence of the anion can be observed. Complex 6 displays a TOF of
5 hꢁ1 and even after 24 h no complete conversion can be obtained
(entry 5). Using complex 7 under the same conditions leads to full
conversion after only 50 min with a TOF of 70 hꢁ1 (entry 6).
Considering these results, we found that besides reaction
temperature and solvent, both the wingtip groups and the anion
play a important role in this hydrosilylation reaction. The observed
turnover frequencies are within the range of previously described
catalysts for hydrosilylation reactions of acetophenone [16]. All
complexes show high activities under mild conditions and in future
work the immobilization of these compounds will be tested.
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