Synthesis and characterization of a free phenylene bis(N-heterocyclic carbene) and its di-Rh complex: Catalytic activity of the di-Rh and CCC-NHC Rh pincer complexes in intermolecular hydrosilylation of alkynes

Article abstract of DOI:10.1016/j.jorganchem.2005.07.088

1,3-Bis(3-butylimidazolium-1-yl)benzene diiodide (1) was reacted with Li(2,2,6,6-tetramethylpiperidine) yielding the free bis-carbene, 1,3-bis(3-butylimidazol-2-ylidene-1-yl)benzene (3), which has been spectroscopically characterized. Combining the free b

Full text of DOI:10.1016/j.jorganchem.2005.07.088

Journal of Organometallic Chemistry 690 (2005) 5938–5947
www.elsevier.com/locate/jorganchem
Synthesis and characterization of a free phenylene bis(N-heterocyclic
carbene) and its di-Rh complex: Catalytic activity of the di-Rh
and CCC–NHC Rh pincer complexes in intermolecular
hydrosilylation of alkynes
Gurusamy Thangavelu Senthil Andavan, Eike B. Bauer, Christopher S. Letko,
*
T. Keith Hollis , Fook S. Tham
Department of Chemistry, Physical Sciences 1, University of California, Riverside, CA 92521-0403, United States
Received 12 June 2005; received in revised form 22 July 2005; accepted 23 July 2005
Available online 8 September 2005
Abstract
1,3-Bis(3-butylimidazolium-1-yl)benzene diiodide (1) was reacted with Li(2,2,6,6-tetramethylpiperidine) yielding the free bis-
carbene, 1,3-bis(3-butylimidazol-2-ylidene-1-yl)benzene (3), which has been spectroscopically characterized. Combining the free
bis-carbene with [Rh(COD)Cl]2 yielded the corresponding di-Rh bis(N-heterocyclic carbene) complex (4) that was structurally char-
acterized. The di-Rh bis-carbene complex was found to exhibit complex solution 13C and 1H NMR spectra that have been assigned
as a mixture of diastereomers. The crystal structure of the di-Rh bis-carbene compound 4 was composed of a pair of enantiomeric
atropisomers. The diastereomeric atropisomers were assigned as the source of the spectral complexities. The di-Rh di-carbene com-
plex 4 and the CCC–NHC Rh pincer complex 2 were applied as catalysts in hydrosilylation reactions of terminal and internal alky-
nes. Both catalysts are highly active, regioselective, stereoselective, and chemoselective: terminal alkynes give predominantly the
b-(Z) isomer and internal alkynes afford the b-(E) isomer in chloroform or benzene. One of the strongest attributes of the catalyst
systems is that the results were achieved without exclusion of air and without purification of commercially available reagents.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Free bis-carbene; Pincer complex; Hydrosilylation; N-heterocyclic carbene; Rhodium complex; Carbene complex
1. Introduction
of the imidazolium nucleus in stabilizing a carbene cen-
ter at the C-2 position has been demonstrated by the iso-
lation and X-ray crystal structure characterization of the
free carbene [3,4], and by the preparation of their com-
plexes with transition metals [5]. The field of N-hetero-
cyclic carbene chemistry has exploded since that time
and many new synthetic methods have been developed,
and the applications of NHC ligands have multiplied [6].
We and several other groups have recently been en-
gaged in the replacement of phosphines with NHCÕs in
pincer complexes. Much new and interesting chemistry
has resulted. Pincer complexes, especially PCP pincers,
have been demonstrated to be useful catalysts for
The preparation of N-heterocyclic carbene complexes
of transition metals was first reported in 1968 [1]. The
synthetic difficulty of accessing the ligand motif ham-
pered development of the field for many years [2]. In
1991, Arduengo reported the deprotonation of imidazo-
lium salts to prepare the free carbene thus opening up a
synthetic route to the ligand [3]. The remarkable ability
*
Corresponding author. Fax: +1 951 827 4713.
E-mail address: keith.hollis@ucr.edu (T.K. Hollis).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.088

G.T.S. Andavan et al. / Journal of Organometallic Chemistry 690 (2005) 5938–5947
5939
dehydrogenation of alkanes [7]. Very recently, a PCP
pincer was designed to be even more electron rich, and
it was found to be capable of oxidative addition of the
N–H bond of ammonia [8]. The phosphines in pincer li-
gands have been replaced by NHCÕs to produce pincer
architectures incorporating the carbene as the r-donor.
We were the first to report the CCC–NHC pincer archi-
tecture illustrated in Scheme 2, where the NHC architec-
ture with a M–C bond at the central position has been
directly mapped atom for atom onto the PCP pincer
architecture [9]. Several different types of NHC pincer
complexes have been reported and been extensively
studied. These pincer NHC complexes have been dem-
onstrated to be catalytically active for transfer hydroge-
nation, C–C coupling reactions in Heck and related
chemistries, hydroformylation and the hydroamination
of alkynes [10]. The synthetic routes to the known types
of NHC pincer analogues illustrated in Scheme 1 are li-
gand and metal specific [11,12]. A generally applicable
route to CCC–NHC pincer complexes has not been
available until our recent report [9].
which was found to effectively transmetallate to late
transition metals like Rh yielding CCC–NHC pincer
complex 2 [9]. The ligand precursor 1 was also envi-
sioned to position two mono(NHC) coordinated metals
in proximity to one another for potential applications in
bimetallic cooperativity [14]. We report here the synthe-
sis and characterization of the free bis-carbene, its di-Rh
complex, and comparison to the di-Rh and pincer com-
plexes as catalysts for hydrosilylation reactions.
2. Results
2.1. Bimetallic rhodium N-heterocyclic carbene synthesis
We first envisaged the synthesis of a novel electron-
rich di-Rh bis-carbene complex based on the bis(imi-
dazolium) salt 1 (Scheme 4). The imidazolium salt 1
was obtained by alkylating 1,3-bis(imidazol-1-yl)ben-
zene according to the standard method employed in
our laboratory [13]. The synthesis of a di-Rh bis-carbene
complex from 1 would give a complex in which the two
Rh atoms are in close proximity perhaps giving rise to
enhanced reactivity due to cooperative effects. Thus,
we sought a method for deprotonating the bis(imidazo-
lium) salt 1 to form the free carbene and to subsequently
metallate it with an appropriate Rh source.
We have established the efficient one-pot synthesis of
1,3-bis(imidazol-1-yl)benzene followed by alkylation
yielding bis(imidazolium) salts 1 [13], and we have re-
cently reported an efficient metallation/transmetallation
strategy to tris-activate imidazolium salt 1 in the prepa-
ration of CCC–NHC pincer complexes as illustrated in
Scheme 3. Reaction with a Zr amido reagent activates
all three C–H bonds generating the pincer architecture,
Several standard bases that have been reported previ-
ously to produce NHCÕs were evaluated in THF at
ꢀ78 ꢁC by 13C NMR spectroscopy analysis. Potassium
hydride produced 0% conversion, and lithium hexam-
ethyldisilazine yielded only 60% conversion. It was
found that 1,3-bis(3-butylimidazol-1-yl)benzene diio-
dide 1 reacted with 2,2,6,6-(tetramethylpiperidine)lith-
ium (TMP-Li) in THF-d8 at ꢀ78 ꢁC quantitatively
A
N
N
M
Ln
N
N
N
N
N
M
1
yielding bis-carbene 3 as characterized by H and 13C
R
N
R
R
N
Ln
NMR spectroscopy at ꢀ50 ꢁC [5e,15]. The 1H NMR sig-
nals for the iminium protons of 1 were no longer ob-
served, and the 13C NMR spectrum revealed a new
peak at 201 ppm, which is characteristic of carbenes.
The bis-carbene was not stable above ꢀ30 ꢁC and
decomposed to numerous unidentified products.
A = C, N
R
Scheme 1. Recently reported NHC pincer architectures.
C
C
N
N
N
C
N
or
Bis-carbene 3 and [Rh(COD)Cl]2 were reacted at
ꢀ78 ꢁC in situ to yield the di-Rh bis-carbene complex
4 as a yellow powder in 61% isolated yield [16]. The sig-
nal for the carbene carbon of 4 was doubled with peaks
observed at d 182.0 (1JRh–C = 49 Hz) and d 181.9
D
D
M
M
M
N
R
N
R
N
R
N
R
Ln
Ln
Ln
Scheme 2. Atom mapping of pincer complexes onto a CCC–NHC
architecture.
1. 2.5 Zr(NMe2)4
2. [Rh(COD)Cl]2
N
N
N
N
I
I
N
N
n-Bu
N
N
Rh
I
n-Bu
n-Bu
I
n-Bu
Me2HN
1
2
Scheme 3. Preparation of a CCC–NHC Rh pincer complex.

5940
G.T.S. Andavan et al. / Journal of Organometallic Chemistry 690 (2005) 5938–5947
Rh(COD)I
TMP-Li
[Rh(COD)Cl]2
- 78 ˚C, THF
N
N
N
N
N
N
- 78 ˚C, THF
N
N
nBu
2 I
nBu
N
N
N
N
Rh(COD)I
nBu
nBu
nBu
nBu
quantitative by NMR
61%
1
3
4
Scheme 4. Preparation of the carbene 3 and the di-Rh complex 4.
(1JRh–C = 49 Hz), upfield relative to the free carbene and
Table 1
˚
Selected bond distances (A) and angles (ꢁ) for di-Rh bis-carbene
complex 4
with the characteristic doublet from coupling to Rh. The
1
13C and H NMR spectra of compound 4 had a dou-
˚
Distances (A)
bling of many of the observed peaks (vide infra). An ex-
act mass determination indicated the corresponding
parent ion m/z = 998 corroborating the formation of
di-Rh bis-carbene 4.
Rh1–C2
Rh1–C31
Rh1–C32
Rh1–C35
Rh1–C36
Rh1–I1
Rh10–C20
Rh10–C310
Rh10–C320
Rh10–C350
Rh10–C360
Rh10–I10
N1–C2
N1–C21
N1–C5
C2–N3
N3–C7
N3–C4
N10–C20
N10–C210
N10–C50
C20–N30
N30–C70
N30–C40
2.0201(11)
2.1912(11)
2.2392(12)
2.1105(12)
2.1324(11)
2.68340(11)
2.0150(11)
2.2143(12)
2.2445(12)
2.1025(11)
2.1304(11)
2.67326(11)
1.3477(14)
1.4649(15)
1.3913(16)
1.3713(14)
1.4268(14)
1.3954(14)
1.3547(14)
1.4644(15)
1.3888(15)
1.3665(14)
1.4278(13)
1.3916(14)
A X-ray quality single crystal of 4 was grown from a
saturated toluene solution. The molecular structure was
determined and0 is depicted in Fig. 1. The Rh(1)–C(2)
0
˚
and Rh(1 )–C(2 ) bond distances are 2.020 and 2.015 A.
The structure reveals that Rh centers are in a square-
planar environment. Selected geometric parameters are
collected in Table 1. Details of data collection and solu-
tion of the X-ray crystal structure are listed in Table 2.
2.2. Catalytic hydrosilylation
Hydrosilylation is a powerful tool in organic synthe-
sis. It is an atom-economical method of synthesis [17]
and allows access to versatile silanes [18], which are
important building blocks in organic chemistry [19].
However, the synthetic utility of addition of silanes to
terminal alkynes is limited. As shown in Eq. (1), three
isomeric vinylsilanes b-(E), b-(Z) and a may be formed
in addition to dehydrosilylation [20]. The hydrosilyla-
tion of internal alkynes still gives b-(E) and b-(Z) iso-
mers (Eq. (2)). Thus, a highly regio- and stereospecific
synthesis is highly desirable, and a plethora of transition
metal catalysts for this reaction have been developed.
Angles (ꢁ)
N1–C2–Rh1
N3–C2–Rh1
C2–N3–C7
128.42(8)
127.18(8)
125.42(9)
89.25(3)
155.61(4)
167.70(5)
91.06(5)
91.06(4)
36.38(5)
81.43(5)
82.23(5)
38.92(5)
128.76(8)
126.18(7)
124.17(9)
92.04(3)
158.56(5)
164.12(5)
87.53(5)
89.64(4)
35.98(5)
81.29(5)
81.54(5)
39.06(5)
C2–Rh1–I1
C2–Rh1–C31
C2–Rh1–C32
C2–Rh1–C35
C2–Rh1–C36
C31–Rh1–C32
C35–Rh1–C32
C36–Rh1–C31
C35–Rh1–C36
N10–C20–Rh10
N30–C20–Rh10
C20–N30–C70
C20–Rh10–C310
C20–Rh10–C310
C20–Rh10–C320
C20–Rh10–C350
C20–Rh10–C360
C310–Rh10–C320
C350–Rh10–C320
C360–Rh10–C310
C350–Rh10–C360
C24
C23
C32’
C35’
C31’
C21
I1
C5
N1
C22
C4
N3
I1’
C2
Rh1
C31
C7
Rh1’
C24’
C36’
C6
C35
C32
C8
C7’
C2’
C21’
C9
C36
N1’
C8’
N3’
C4’
C23’
C22’
C5’
Fig. 1. ORTEP diagram of 4 (50% probability, hydrogen atoms are
omitted for clarity).

G.T.S. Andavan et al. / Journal of Organometallic Chemistry 690 (2005) 5938–5947
5941
Table 2
Crystallographic data for 4
R
SiR'3
R
R
R
R
HSiR'3
+
SiR'3
β-(E)
ð2Þ
R
Empirical formula
Formula weight
Temperature (K)
C36H50I2N4Rh2
998.42
100(2)
0.71073
Orthorhombic
Pbca
β-(Z)
˚
Wavelength (A)
Hydrosilylation of multiple bonds catalyzed by metal
Crystal system
Space group
Unit cell dimensions
complexes typically involves oxidative addition of the si-
lane to the metal as the first step [27]. Therefore, we
envisaged our new electron-rich di-Rh complex 4 as a
potential hydrosilylation catalyst. Thus, complex 4 and
for comparison the pincer Rh complex 2 were employed
as catalysts for this reaction. Experiments were first con-
ducted under conditions similar to those applied by
Peris and co-workers [21e]. Dimethylphenylsilane
(Me2PhSiH), the alkyne, CDCl3 and the catalyst (2–
3.5 mol%) were combined in a screw-capped pressure
vial and maintained at different temperatures. Schlenk
techniques were not applied and neither was purification
of starting materials performed. Conversion of the alky-
nes was ascertained by NMR spectroscopy, and prod-
uct distributions were determined by comparison of
1H and 13C NMR data with literature values [20,23a,
24a,25b,28]. The coupling constants for the vinylic pro-
tons are very indicative and range from 13 to 15 Hz for
b-(Z), 18 to 20 Hz for b-(E) and ꢁ2 Hz for a isomers.
The results of the catalytic experiments are compiled
in Table 3. In general and unlike previous reports,
hydrosilylation of terminal and internal alkynes was
successful. Most of the alkynes were quantitatively con-
verted to the products in 2 h or less at 80 ꢁC indicating
the high activity of both catalysts. Switching the solvent
to benzene gave similar results (entries 1, 7, 21).
The product distribution for the hydrosilylation of
terminal alkynes shows in general a strong preference
for the b-(Z)-alkene for both catalysts. This finding is
consistent with earlier reports showing b-(Z)-selectivity
for neutral Rh complexes [23]. Only t-Bu-acetylene gives
mainly the b-(E) isomer. Internal alkynes are smoothly
hydrosilylated and give predominantly the b-(E)-prod-
ucts. The stereoselectivities are good to excellent, and
only small amounts of the a product were formed. In
general, the pincer complex 2 shows somewhat better
selectivity than the di-Rh complex 4. The activities are
comparable, as an experiment at room temperature re-
vealed. Phenylacetylene was reacted with catalysts 2 or
4 with a catalyst load of 2 mol% based on Rh under
˚
a (A)
14.0393(2)
16.9296(2)
31.2725(4)
90
90
90
7432.84(17)
8
1.784
2.579
3920
0.45 · 0.37 · 0.26
1.95–40.25
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
Volume (A )
Z
Density (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
Crystal size (mm)
h Range for data collection (ꢁ)
3
˚
)
Index ranges
ꢀ25 6 h 6 25, ꢀ30 6 k 6 30,
ꢀ56 6 l 6 56
Reflections collected
276835
Independent reflections [Rint
Completeness to h = 40.25ꢁ
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
]
23383 [0.0328]
99.9%
Semi-empirical from equivalents
0.5481 and 0.3892
Full-matrix least-squares on F2
23383/0/449
1.033
R1 = 0.0227, wR2 = 0.0543
Over the past decade significant progress has been made
in this area, and catalysts for regio- and stereocontrolled
syntheses of vinylsilanes are now available [21]. Plati-
num catalysts were among the first ones investigated
and typically lead to the b-(E) isomers [22]. Rhodium
complexes were intensively studied, and it has been
shown that neutral Rh complexes tend to form b-(Z)
isomers [23], whereas cationic Rh complexes such as
[Rh(COD)2]BF4 give the b-(E) compounds [18,24]. Ste-
reodivergent syntheses of b-(E) and b-(Z) isomers were
reported [25], and enantioselective hydrosilylation cata-
lysts are available [26].
However, the regioselectivity and stereoselectivity of
the hydrosilylation of alkynes still depends on several
factors such as temperature, reaction time, substrates,
additives and catalyst. In many cases, long reaction
times and high temperatures are necessary and the
exclusion of moisture and air is required. Thus, the
search for generally applicable catalyst systems in this
reaction is ongoing.
1
otherwise the same conditions described in Table 3. H
NMR spectra of the crude product showed ꢁ27% con-
version for both catalysts after three hours, and the
product distribution was similar to that reported in
Table 3.
To demonstrate functional group compatibility, some
experiments were conducted with alkyne-containing
alcohols (entries 21–26). 5-Hexyne-1-ol (entries 21 and
22) was readily hydrosilylated and showed a similar
product distribution as the other terminal alkynes.
R
R
SiR3
HSiR3
+
R
SiR3
β-(E)
β-(Z)
ð1Þ
R
SiR3
+
R3Si
R

5942
G.T.S. Andavan et al. / Journal of Organometallic Chemistry 690 (2005) 5938–5947
Table 3
Catalytic hydrosilylation resultsa
Entry
Alkyne
Catalyst
Time (h)
Temp. (ꢁC)
Conversionb (%)
b-(E)c,d
b-(Z)c,d
ac,d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
a
PhC„CHe
PhC„CH
PhC„CH
4
2
2
4
4
2
4
2
4
2
2
4
2
4
2
4
4
2
4
2
4
2
4
2
4
2
2
2
12
2
16
1
2
2
2
1
18
18
2
2
12
2
20
18
2
1.5
1.5
2
7
6.5
7
80
80
rt
80
rt
100
100
87
100
52
8
5
5
10
14
8
16
11
9
92
95
95
82
76
89
80
83
84
88
86
10
20
20
11
15
15
15
21
28
87
77
52h
71h
10
15
0
0
0
8
10
3
4
6
7
4
H3C(CH2)2C„CH
H3C(CH2)2C„CH
H3C(CH2)2C„CH
H3C(CH2)6C„CHe
H3C(CH2)6C„CH
H3C(CH2)7C„CH
H3C(CH2)7C„CH
H3C(CH2)7C„CH
t-BuC„CH
t-BuC„CH
PhC„CPh
PhC„CPh
EtC„CEt
80
80
80
80
80
80
60
80
80
60
80
80
80
80
80
80
80
80
80
45
80
100
100
100
100
100
100
100
57
100
100
100
100
100
100
100
100
100
100
100
100
100
8
9
5
80
69
80
89
85
85
85
79
72
8
15
–
–
69
73
10f
11f
–
–
–
–
–
EtC„CEt
EtC„CEt
PrC„CPr
PrC„CPr
–
–
g
CH2(OH)(CH2)3C„CHe
CH2(OH)(CH2)3C„CH
PrCH(OH)C„CH
PrCH(OH)C„CH
c-C6H10(C„CH)(OH)j
c-C6H10(C„CH)(OH)j
5
8
–
–
21i
12i
2
Catalyst (2–3.5 mol%) and silane (1.1 equiv.) were mixed in CDC13 10 min before addition of the alkyne (1 equiv).
Conversion of the alkyne. The consumption of internal alkynes was confirmed by 13C NMR.
Determined by 1H NMR of the crude reaction mixture.
The percentages of b-(Z), b-(E), and a are in relation to each other only.
In C6D6.
b
c
d
e
f
ꢁ40% hydrogenated product.
g
h
i
ꢁ15% hydrogenated product.
No b-(E) and a-products were observed, other side products.
ꢁ20% hydrogenated product.
j
1-Ethynyl-1-cyclohexanol.
Hydrosilylation of 1-hexyne-3-ol and 1-ethynyl-1-cyclo-
As reported before [25a], salt effects can dramatically
change the selectivity of catalyst systems. Thus, several
catalysis experiments were repeated with phenylacety-
lene under conditions as reported in Table 3 with the
addition of LiCl, water, and n-Bu4NBr. The results of
these experiments are compiled in Table 4. Both cata-
lysts, 2 and 4, were found to produce different b-(E)/
b-(Z) selectivity in the presence of additives. The most
dramatic effect was observed in the hydrosilylation of
phenylacetylene with added n-Bu4NBr (entry 4). Exclu-
sively the b-(E)-hydrosilylation product was observed,
but nearly 50% of the phenylacetylene was converted
to styrene. Addition of LiCl typically causes the catalysts
to switch from producing predominantly the b-(Z) iso-
mer with terminal alkynes (entries 1, 3, 5, and 7) to pro-
ducing more of or predominantly the b-(E) isomer
(compare to entries 2, 4, 6, and 8, respectively). Similar
patterns were observed for n-Bu4NBr. When water was
added the amount of a-product increased. There was
no significant salt effect observed when LiCl was added
hexanol (entries 23–26) required longer reaction times
for catalyst 4, and the product distribution differs from
the other terminal alkynes. No O-silylated products
were detected.
Further experiments were conducted to better under-
stand the selectivity of our Rh carbene catalysts. Hydro-
silylations were conducted at room temperature to
evaluate the temperature dependence of the catalytic
activity and selectivity. The reaction occurs at room
temperature also, but incomplete conversion was ob-
served (see entries 3 and 5) without significant change
in the product distribution. The stability of the product
ratio over time was evaluated and established to be the
result of kinetic control. Thus, we checked the E/Z ratio
of the hydrosilylation of 1-decyne and 3-hexyne after 1 h
and maintained the reaction mixture at 80 ꢁC for 18 h
(entries 10, 11, 16, and 17). The ratio did not change sig-
nificantly, establishing that isomerization to the thermo-
dynamically favored b-(E) product does not take place.

G.T.S. Andavan et al. / Journal of Organometallic Chemistry 690 (2005) 5938–5947
5943
Table 4
(3) took place. Thus, we conclude that the dehydrogena-
tion of the silane is a side reaction with our catalyst sys-
tem with substrates that hydrosilylate slowly.
Effect of additive on product selectivitya
Entry Alkyne
Catalyst
b-(E)b,c b-(Z)b,c ab,c
1
2
3
4
5
6
7
8
9
PhC„CHd
4
29
55
40
100
5
61
35
43
0
10
10e
17
0f
0
To examine the dehydrogenation of the silane, a con-
trol experiment was performed. Catalyst 4 and Me2Ph-
SiH were combined as illustrated in Eq. (4) without
substrate under the conditions described in Table 3 for
the catalytic experiments. 1H NMR and GC–MS analy-
sis showed that the silane was consumed. Silyl-contain-
ing byproducts similar to those of the reaction in Eq.
(3) were observed by GC–MS. Thus dehydrogenative
coupling of the silane took place, even without an
appropriate hydrogen acceptor. It is not clear what hap-
pens to all of the hydrogen produced, but cyclooctene
and cyclooctane were detected in the reaction mixture.
Thus, the cyclooctadiene (COD) serves as a hydrogen
acceptor.
PhC„CH
4 + LiCl
4 + LiCl/H2O
4 + NBu4Br
2
PhC„CHd
PhC„CH
PhC„CHd
PhC„CH
95
40
43
43
88
53
32
84
73
29
21
20
28
27
2 + LiCl
2 + LiCl/H2O
2 + NBu4Br
2
51
37
56
8
9
PhC„CHd
PhC„CH
20
3e
4
H3C(CH2)7C„CHd
10
11
12
13
14
15
16
17
18
a
H3C(CH2)7C„CH 2 + LiC1
34
33
9
13
35
7
H3C(CH2)7C„CH 2 + NH4BuBr
H3C(CH2)7C„CHd
4
H3C(CH2)7C„CH 4 + LiCl
16
44
79
80
72
73
11
26
–
–
–
H3C(CH2)7C„CH 4 + NH4BuBr
PrC„CPrd
PrC„CPr
PrC„CPrd
PrC„CPr
4
4 + LiCl
2
4, CDCl3
2 + LiCl
–
silyl side-
products
ð4Þ
Me2PhSiH
Catalyst (2–3.5 mol%) and silane (1.1 equiv.) were mixed in CDC13
80 ˚C, 1.5 h
10 min before addition to the alkyne (1 equiv.) and the corresponding
salt (1 equiv.) followed by heating at 80 ꢁC for 2.5 h.
Finally, the chemoselectivity of the hydrosilylation
catalysts was evaluated in a competition experiment
involving 1 equiv. of styrene and phenylacetylene and
1.1 equiv. of Me2PhSiH. Combination with catalyst 2
or 4 under standard conditions as shown in Eq. (5)
was observed to produce the corresponding vinylsilanes
with a product distribution similar to that reported in
Table 3. The styrene remained unreacted with catalyst
4, and ꢁ12% ethylbenzene had formed with catalyst 2.
Thus, hydrosilylation of alkynes in the presence of alk-
enes chemoselectively was achieved.
b
Determined by 1H NMR spectroscopy of the crude reaction mix-
ture. Average of two or three experiments.
c
The percentages of (Z), (E), and a are in relation to each other
only.
d
In CDC13 without additive.
e
ꢁ20% hydrogenated product.
f
ꢁ50% hydrogenated product.
to reactions of internal alkynes (entries 9–12). Added
salts have some influence on the selectivity of addition
to terminal alkynes with catalysts 2 and 4, therefore care
should be taken to exclude ionic impurities.
Ph
SiMe2Ph
20%
Ph
0%
Hydrosilylation of styrene was not successful under
the reaction conditions described in Table 3. However,
1.0 Ph
+
+
Ph
+
4, CDCl3
+
1
careful analysis of the H and 13C NMR spectra of the
SiMe2Ph
15%
1.0
Ph
80 ˚C, 1.5 h
Ph
reaction mixture revealed that ethylbenzene was formed;
the silane had completely converted and ꢁ50% of the
styrene remained unreacted. When two equivalents of si-
lane were employed, no more styrene starting material
was left over. Thus, a hydrogen transfer from the silane
to the alkene took place, giving rise to a process as
shown in Eq. (3). A GC–MS analysis of the crude prod-
uct mixture showed ethylbenzene and silane-containing
byproducts such as Me2PhSiSiMe2Ph, Me2PhSi–O–Si-
Me2Ph and Me2PhSiCl.
+
61%
SiMe2Ph
1.1 Me2PhSiH
Ph
14%
ð5Þ
3. Discussion
3.1. Bimetallic rhodium N-heterocyclic carbene synthesis
4, CDCl3
silyl side-
ð3Þ
+
+ 2 Me2PhSiH
The di-Rh bis-carbene system 4 that is reported is re-
lated to the 1,2-ethylene bridged analogue reported by
Herrmann et al. [5e] and the 2,6-pyridylene bridged ana-
logue reported by Peris and co-workers [21e]. The
Rh(1)–Rh(1 ) distance of 7.9 A precludes a M–M bond
and is similar to the di-Rh bis-carbene pyridyl bridged
structure reported by Peris and co-workers [21e]. An
analysis of Cambridge Crystallographic Database
including the May 2005 release data revealed 127
products
80 ˚C, 1.5 h
Ph
Ph
Hydrosilylation of carbonyls to give O-silylethers was
not successful under the reaction conditions described
above. Cyclohexanone or benzaldehyde was reacted un-
der the conditions described in Table 3. The carbonyl
starting materials were recovered, but the silane had
been completely consumed. Presumably, a dehydroge-
nation of the silane in a reaction similar to that in Eq.
0
˚

5944
G.T.S. Andavan et al. / Journal of Organometallic Chemistry 690 (2005) 5938–5947
examples of structurally characterized Rh–NHC bonds.
sponding silanes, and alkenes do not react in the
presence of alkynes as shown in Eq. (5). Thus, the re-
ported catalysts are selective for alkynes.
˚
The mean of the Rh–C bond distances was 2.023 A with
˚
a population standard deviation of 0.035 A. The maxi-
˚
mum value reported was 2.111 A, and the minimum
˚
was 1.914 A. Evaluation of this data and comparison
with our Rh–NHC bond distances of 2.0201(11) and
2.0150(11) reveal that the distances fall in the lower half
of the reported bond distances between the first quartile
4. Conclusions
In conclusion, we performed the synthesis of the di-
Rh bis-carbene complex 4 in a straightforward one-pot
synthesis from readily available starting materials. Cat-
alyst 4 and the analogous CCC–NHC pincer complex
2 were demonstrated to have high reactivity in hydrosi-
lylation reactions. Hydrosilylations are highly regio- and
stereospecific: terminal alkynes give b-(Z) isomers and
internal alkynes lead to b-(E) isomers (65–89%). There-
fore, the di-Rh complex 4 and pincer complex 2 are
promising candidates for other catalytic applications
requiring electron-rich metal centers. Further catalytic
studies of these new complexes are ongoing.
˚
value of 2.0115 A and the second quartile value of
˚
2.029 A.
Apparently, hindered rotation of the aryl-NHC bond
generates diastereomers at the axes of chirality in solu-
tion due to the steric bulk of the COD ligand, iodide,
and the presence of the ortho-hydrogen resulting in the
complex NMR spectra observed. Such an observation
has not been reported for the ethylene or pyridylene
bridged systems [5e,21e], thus the ortho-hydrogen and
the halide have a significant impact on the dynamics
of the system. The crystal structure contains the R,R
and S,S enantiomeric pair related through a center of
inversion. Quickly collecting a 13C NMR spectrum upon
dissolution of the crystals reveals a diastereomeric ratio
of ꢁ2:1, which is consistent with rapid equilibration.
The assignment of the major isomer in solution as rac
or meso has not been made, and the mixture is reported
in Section 5.
5. Experimental
5.1. General considerations
For the ligand and complex synthesis, manipulations
were carried out under an inert atmosphere of Ar with
the use of standard Schlenk, vacuum line, and glove
box techniques. Dry oxygen-free solvents were used.
The deuterated chlorinated solvents were filtered
through basic alumina before dissolving the samples.
[Rh(COD)Cl]2 was purchased from Strem Chemicals
and was used as received. NMR spectra were recorded
on Bruker Avance 300 (300.132 MHz) and Varian Inova
(500.140 and 300.053 MHz) instruments. Chemical
shifts reported are relative to tetramethylsilane, and
were referenced by assigning the residual solvent peak
(C6D5H to 7.16 ppm and CHCl3 to 7.28 ppm). Reagents
and solvents for the hydrosilylation reactions were used
as received and reactions were performed in the air.
3.2. Catalytic hydrosilylation
The activities of our carbene complexes 2 and 4 are
high. Rh-based catalyst systems recently published by
Peris and co-workers (24–72 h at 60 ꢁC; 33–80% b-(E)
selectivity for phenylacetylene) [21e] or Faller (2.5 h at
45 ꢁC; 81–99% b-(E) selectivity for phenylacetylene)
[18] show activities and selectivity which are comparable
to our system. However, complexes 2 and 4 can be han-
dled in air and no special treatment of solvents or sub-
strates is necessary. Furthermore, they selectively
hydrosilylate alkynes, making them an attractive alter-
native to established catalyst systems. The findings here-
in are consistent with the generally accepted mechanism
of the hydrosilylation reaction of terminal alkynes
involving Rh-containing catalysts proposed by Crabtree
[28a] and Ojima et al. [23a].
5.2. Synthesis of 1,3-bis(3-butylimidazolium-1-yl)benzene
diiodide (1)
The experiments with t-Bu-acetylene and the alkyne-
containing alcohols nicely show the influence of steric
effects. As described earlier, the bulky t-Bu substituent
directs the silyl group in the b-(E)-configuration and
some hydrogen transfer from the silane to the alkyne
to give an alkene product was observed. In 1-hexyn-3-
ol and 1-ethynyl-1-cyclohexanol (entries 23–26) the al-
kyne units are in a positions adjacent to secondary
and tertiary carbon atoms making the hydrosilylation
more difficult. The silyl group is directed to the b-(E) po-
sition and hydrogen transfer is promoted. Carbonyl
compounds and alcohols are not converted to the corre-
1,3-Bis(imidazol-1-yl)benzene (0.840 g, 4.00 mmol),
1-iodobutane (2.94 g, 16.0 mmol), and toluene (8 mL)
were reacted in a sealed tube at 150 ꢁC for 8 h. The sus-
pension was then allowed to reach room temperature,
and the contents were washed with THF (3 · 10 mL).
The residue was dried under high vacuum to obtain 1
as a yellow colored solid (1.78 g, 77%): 1H NMR
(CDCl3): d 11.23 (s, 2H), 9.01 (s, 1H), 8.72 (s,
J = 2.1 Hz, 2H), 8.24 (dd, J = 8.1 and 2.1 Hz, 2H),
7.77 (t, J = 8.1 Hz, 1H), 7.5 (s, 2H), 4.46 (t,
J = 8.0 Hz, 4H), 2.06 (pseudo-quintet, J = 8.0 Hz, 4H),
d
1.49 (pseudo-sextet, J = 7.5 Hz, 4H), 1.05 (t,

G.T.S. Andavan et al. / Journal of Organometallic Chemistry 690 (2005) 5938–5947
5945
J = 7.5 Hz, 6H); 13C{1H} (75 MHz, DMSO-d6): d 137.6,
133.7, 127.2, 125.4, 124.5, 122.9, 117.6, 51.2, 32.9, 20.7,
15.2.
121.2, 120.3, 116.5, 95.4 (d, J = 4.5 Hz), 71.9 (d,
J = 12.8 Hz), 51.6, 31.6, 29.4, 19.9, 13.6. LSIMS: 998
(M+), 896 (M+ ꢀ COD), 871(M+ ꢀ I), 763 (M+ ꢀ
COD ꢀ I), 425 (M+ ꢀ 2I ꢀ 2COD ꢀ Rh).
5.3. Synthesis of 1,3-bis(3-butylimidazol-2-ylidene-1-yl)-
benzene (3)
5.5. Hydrosilylation experiments
A mixture of 2,2,6,6-(tetramethylpiperidine)lithium
salt (0.088 g, 0.6 mmol) and 1,3-bis(3-butylimidazo-
lium-1-yl)benzene iodide (0.173 g, 0.3 mmol) was cooled
to ꢀ78 ꢁC and THF (3 mL) was added dropwise. The
colorless suspension was allowed to react at ꢀ78 ꢁC
for 1 h. The 13C{1H} and 1H NMR spectra of the result-
ing orange colored solution were recorded at ꢀ50 ꢁC
Me2PhSiH (0.012 g, 0.085 mmol) and the catalyst (2–
3.5 mol%) were dissolved in CDCl3 or benzene-d6
(0.75 mL) in a screw-capped pressure vial. After
10 min it was combined with the alkyne (0.077 mmol),
and the sealed vial was immersed in an oil bath pre-
heated to the desired temperature. The products were
characterized by NMR spectroscopy (see text).
1
and showed quantitative conversion: H NMR (THF-
d8): d 8.20 (broad s, 1H), 7.95 (broad s, 2H), 7.82 (broad
s, 2H), 7.56 (broad s, 3H), 4.32 (b, 4H), 1.85 (b, 4H),
1.63 (b, 4H), 0.96 (b, 6H); 13C{1H} (125 MHz, THF-
d8): d 200.5, 142.7, 130.9, 122.4, 120.6, 115.9, 49.7,
31.8, 18.9, 14.4 .
5.6. X-ray structure determination for compound 4
Measurements were carried out on a Bruker APEX 2
(version 1.0–22) [29] platform-CCD X-ray diffractome-
˚
ter system (Mo-radiation, k = 0.71073 A, 50 kV/40 mA
power) for Rh. Data were collected at low temperature
at T = 100(2) K. Frames were integrated with the aid
of the Bruker SAINT version 7.06A software [30] and
using a narrow-frame integration algorithm.
5.4. Synthesis of bis(1,5-cyclooctadiene)diiodo[1,3-bis(3-
butylimidazol-2-ylidene-1-yl)benzene]dirhodium(I) (4)
A mixture of 2,2,6,6-(tetramethylpiperidine)lithium
salt (0.088 g, 0.6 mmol) and 1,3-bis(3-butylimidazo-
lium-1-yl)benzene iodide (0.173 g, 0.3 mmol) was cooled
to ꢀ78 ꢁC and THF (3 mL) was added dropwise. The
colorless suspension was kept at ꢀ78 ꢁC for 1 h which
afforded an orange colored solution. [Rh(COD)Cl]2
(0.148 g, 0.297 mmol) dissolved in THF (7 mL) was
added slowly. The reaction was then allowed to reach
r.t. and stirred for another 3 h (with no change in the
color). The reaction was concentrated and the solid
was triturated with toluene and filtered to remove the
residual solid. Pentane was added to the filtrate produc-
ing a powder that was removed by filtration. The filtrate
was concentrated to obtain orange colored crystals
(0.183 g, 61%). Single crystals of 4 were grown from a
For 4 a total of 4800 frames were collected for a
hemisphere of reflections (with scan width of 0.3ꢁ in x
and / angles of 0ꢁ, 90ꢁ, 180ꢁ, and 270ꢁ for every 600
frames in starting 2h = ꢀ30ꢁ, and ꢀ60ꢁ, 10 s/frame
exposure time). Based on an orthorhombic crystal sys-
tem, the integrated frames yielded a total of 276835
˚
reflections at a maximum 2h angle of 80.50ꢁ (0.55 A res-
olution), of which 23383 were independent reflections
(Rint = 0.0328, Rsig = 0.0169, redundancy = 11.8, com-
pleteness = 99.9%) and 20037 (85.7%) reflections were
greater than 2r(I). The unit cell parameters were,
˚
˚
˚
˚
a = 14.0393(2) A, b = 16.9296(2) A, c = 31.2725(4) A,
3
a = b = c = 90ꢁ, V = 7432.84(17) A , Z = 8, calculated
density Dc = 1.784 g/cm3. Absorption corrections were
applied (absorption coefficient l = 2.579 mmꢀ1; max/
min transmission = 0.5481/0.3892) to the raw intensity
data using the SADABS program (version 2004/1) [31].
The Bruker SHELXTL software package (version 6.14)
[32] was used for phase determination and structure
refinement for 4. Direct methods of phase determination
followed by subsequent Fourier cycles of refinement led
to an electron density map from which most of the
non-hydrogen atoms were identified. With subsequent
isotropic refinement, all of the non-hydrogen atoms were
identified. Atomic coordinates, isotropic and anisotropic
displacement parameters of all the non-hydrogen atoms
were refined by means of a full-matrix least-squares pro-
cedure on F2. The H-atoms were included in the refine-
ment in calculated positions riding on the atoms to
which they were attached. The largest peak/hole in
1
saturated solution of toluene. H NMR (300.053 MHz,
CD2Cl2): Major isomer: d 9.02 (t, J = 2.1 Hz, 1H), 8.59
(dd, J = 2.1 and 8.1 Hz, 1H), 7.66 (d, J = 2.1 Hz, 1H),
7.14 (d, J = 8.1 Hz, 1H), 5.25–5.18 (m, 2H), 4.95–4.83
(m, 4H), 4.62–4.57 (m, 2H), 3.5–3.4 (m, 2H), 3.1–3.0
(m, 2H), 2.42–2.28 (m, 2H), 1.98–1.93 (m, 2H), 1.73–
1.53 (m, 2H), 1.07 (t, J = 7.5 Hz, 3H). Minor isomer: d
10.04 (t, J = 2.1 Hz, 1H), 8.77 (dd, J = 2.1 and 8.1 Hz,
1H), 7.60 (d, J = 2.1 Hz, 1H), 7.17 (d, J = 2.1 Hz, 1H),
5.20–5.10 (m, 2H), 4.90–4.71 (m, 4H), 4.58–4.54 (m,
2H), 3.6–3.5 (m, 2H), 3.05–2.85 (m, 2H), 2.36–2.20 (m,
2H), 1.98–1.88 (m, 2H), 1.73–1.53 (m, 2H), 1.07 (t,
J = 7.5 Hz, 3H). 13C {1H} (125 MHz, CDCl3): Major
isomer: d 182.0 (d, J = 49 Hz, NHC-Rh), 140.1, 128.8,
125.1, 121.8, 121.4, 120.8, 96.0 (d, J = 4.5 Hz), 72.7 (d,
J = 12.8 Hz), 52.1, 31.8, 29.9, 19.9, 13.6. Minor isomer:
d 181.9 (d, J = 49 Hz, NHC-Rh), 139.7, 128.1, 122.6,
3
˚
the final difference map was 1.456/ꢀ0.600 e/A , R1 =
0.0164, wR2 = 0.0405, with intensity, I > 2r(I).

5946
G.T.S. Andavan et al. / Journal of Organometallic Chemistry 690 (2005) 5938–5947
[6] For some recent reviews see: (a) D. Bourissou, O. Guerret, F.P.
Acknowledgment
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181;
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met. Chem. 48 (2001) 1;
Support of this work by the National Science Foun-
dation (CHE-0317089) is gratefully acknowledged.
(d) , For a recent example see:S. Conejero, Y. Canac, F.S. Tham,
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Appendix A. Supplementary data
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 279032 for compound 4. Fur-
ther details on the crystal structure investigation are
available free of charge at http:/www.ccdc.cam.ac.uk/
or upon request from the Director of the Cambridge
Crystallographic Data centre, 12 Union Road, GB-
Cambridge CB2 1EZ UK; fax: +44 1223 336 033, on
quoting the full journal citation. The 1H and 13C
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NMR spectra of 4, and the H NMR spectrum of 1
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