Synthesis of rhodium N-heterocyclic carbene complexes and their catalytic activity in the hydrosilylation of alkenes in ionic liquid medium

Article abstract of DOI:10.1016/j.jorganchem.2010.11.017

Rhodium complexes bearing N-heterocyclic carbene (NHC) ligands were prepared from bis(η⁴-1,5-cyclooctadiene) dichlorodirhodium and 1-alkyl-3-methylimidazolium-2-carboxylate, and the catalytic properties of rhodium complexes prepared in the hydrosilylation of alkenes in ionic liquid media were investigated. It was found that both the catalytic activity and selectivity of the rhodium complexes bearing NHC ligands were influenced by the attached substituents of the imidazolium cation. Additionally, rhodium complexes bearing NHC ligands in ionic liquid BMimPF₆ could be reused without noticeable loss of catalytic activity and selectivity.

Full text of DOI:10.1016/j.jorganchem.2010.11.017

Journal of Organometallic Chemistry 696 (2011) 2116e2121
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of rhodium N-heterocyclic carbene complexes and their catalytic
activity in the hydrosilylation of alkenes in ionic liquid medium
,
,
a,
Jiayun Li a b, Jiajian Peng b, Ying Bai b, Guoqiao Lai b , Xiaonian Li
*
*
a Resources & Environment Catalysis Institute of Zhejiang University of Technology, State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Hangzhou 310032,
People’s Republic of China
b Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 310012, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Rhodium complexes bearing N-heterocyclic carbene (NHC) ligands were prepared from bis(h
4-1,5-
Article history:
Received 14 September 2010
Received in revised form
9 November 2010
cyclooctadiene) dichlorodirhodium and 1-alkyl-3-methylimidazolium-2-carboxylate, and the catalytic
properties of rhodium complexes prepared in the hydrosilylation of alkenes in ionic liquid media were
investigated. It was found that both the catalytic activity and selectivity of the rhodium complexes
bearing NHC ligands were influenced by the attached substituents of the imidazolium cation. Addi-
tionally, rhodium complexes bearing NHC ligands in ionic liquid BMimPF6 could be reused without
noticeable loss of catalytic activity and selectivity.
Accepted 12 November 2010
Keywords:
Rhodium N-heterocyclic carbene complexe
1,3-Dialkylimidazolium-2-carboxylate
Hydrosilylation
Ó 2010 Elsevier B.V. All rights reserved.
Alkene
Ionic liquid
1. Introduction
organosilicon monomers containing functional groups have been
synthesized via this reaction [9,10]. Apart from platinum complexes,
Transition metal complexes with N-heterocyclic carbenes (NHCs)
have attracted considerable interest over the last two decades [1,2].
Many transition metal complexes with NHCs have been synthesized
and provided the powerful homogeneous catalysts [3,4]. The NHCs
most commonly encountered are based on the imidazole ring
system. This electron-rich heterocycle provides a suitable frame-
work that stabilizes the carbene center located between two
rhodium complexes have been widely applied as highly effective
catalysts in hydrosilylation processes [11,12]. Rhodium complexes
with phosphine ligands such as Rh(PPh3)3Cl are often used. Though
the catalytic hydrosilylation process is well established, on the basic
of economic criteria concern, further improvement of the catalytic
activity and selectivity still remains a big challenge and continues to
be the focus of intense research. N-heterocyclic carbenes (NHCs)
have become universal ligands in organometallic and inorganic
coordination chemistry [13e17]. The use of NHCs as replacements
for phosphines has provided some significant improvements over
traditional phosphine-based ligands [6]. Rhodium complexes such
as [RhCl(cod)NHC], [RhCl(CO)NHC], [RhCl(PPh3)2NHC], etc. as
effective catalysts in the hydrosilylation of alkenes and alkynes with
silanes have been demonstrated [18e21]. Very recently, a new
hydrosilylation process in a scCO2/IL system with a rhodium
complex as the catalyst had been reported by our group [22]. During
this process, rhodium complexes of NHC were formed by direct
carboxylation of 1,3-dialkylimidazolium hexafluorophosphate with
CO2 in situ. Herein, we described the synthetic strategy for the
preparation of rhodium complexes bearing NHC ligands by using
1-alkyl-3-methylimidazolium-2-carboxylate as NHC transfer agent,
and the catalytic activity and selectivity of rhodium complexes
prepared in the hydrosilylation of olefins with triethoxysilane in
ionic liquid were investigated (Scheme 1).
nitrogen atoms [5], and in fact behaves like typical s-donor ligand
[6]. In general, NHCs can be synthesized from a precursor imidazo-
lium salts by using a strong base, such as potassium tert-butoxide or
sodium hydride as deprotonation reagent. Lin et al. [7] reported that
AgeNHC complexes can be synthesized from the addition of imi-
dazolium salt and Ag2O, and the AgeNHC complexes formed can
readily transfer the NHCs to palladium or rhodium to form PdeNHC
or rhodiumeNHC complexes. Crabtree et al. [8] reported that N, N0-
dimethylimidazolium-2-carboxylate can be used as an NHC transfer
agent to transition metals.
On the other hand, hydrosilylation is one of the most significant
SieC bond formation reactions in organosilicon chemistry. Many
* Corresponding authors. Fax: þ86 571 28865135.
E-mail addresses: gqlai@hznu.edu.cn (G. Lai), xnli@zjut.edu.cn (X. Li).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.11.017

J. Li et al. / Journal of Organometallic Chemistry 696 (2011) 2116e2121
Si(R1)3
2117
R1
N
R1
α-adduct
O
O
N
R
R
neat
MeOH
+
Si(R1)3
Si(R1)3
Rh Complex
O
O
N
N
O
1a-10a
R1
HSi(R1)3
R
β-adduct
+
alkane
R1
N
R
N
dehydroganative
silylation product
MeCN
O
O
R
[Rh(cod)Cl]2
CO2
+
N
Rh
R1: OEt, Et
Cl
N
Scheme 1. Hydrosilylation of alkenes with triethoxysilane catalysed by rhodium
complex.
1a-10a
1b-10b
O
O
N
N
neat
2. Experimental sections
+
EtOH
O
O
N
N
O
2.1. General information
2a
Styrene was washed with 5% NaOH and dried with Na2SO4. After
filtration the styrene was distilled under reduced pressure. All
other substances were purchased from Aldrich and were used as
received.
1-Methyl-3-butylimidazolium hexafluorophosphate (BMimPF6)
was prepared according to literature procedures [23].
Compound
Rhodium NHC
R1
methyl
ethyl
1a
2a
3a
4a
5a
6a
7a
8a
9a
10a
1b
2b
3b
4b
5b
6b
7b
8b
9b
10b
butyl
Gas Chromatography: Trace DSQ GC Column
¼
DB-5
hexyl
30 m ꢀ 2.5 mm ꢀ 0.25
m
m, split ¼ 50:1, flow ¼ 1 mL minꢁ1 constant
flow, inlet temperature ¼ 260 ꢂC, column temperature ¼ 50 ꢂC
(hold 1 min) then 15 ꢂC minꢁ1 up to 260 ꢂC (hold 10 min).
1H and 13C spectra were measured using a Bruker AV400MHz
spectrometer operating at 400.13 and 100.62 MHz, respectively.
Elemental analyses were performed on a VARIO EL-3 elemental
analyzer.
2-propenyl
phenyl
benzyl
4-methylphenyl
2, 6-diisopropylphenyl
2, 4, 6-trimethylphenyl
2.2. Synthesis of 1,3-dialkylimidazolium-2-carboxylates
A
screw-top pressure tube was charged with dimethyl
Scheme 2. Synthesis of imidazolium-2-carboxylates and rhodium N-heterocyclic
carbonate (3.0 mL), 1-alkylimidazole (2.0 mL) and a stir bar. It was
sealed and heated for 30e70 h at 80e90 ꢂC to give a brown oil,
which was washed with diethyl ether (3 ꢀ 10 mL). It was purified by
recrystallization in acetonitrile to give a yellow solid, which was
dried in vacuo to give the product (Scheme 2).
carbene complexes.
2.2.4. 1-Hexyl-3-methylimidazolium-2-carboxylate (4a)
Synthesized by the reaction of 1- hexylimidazole and dimethyl
carbonate (Yield: 83%).1H NMR (400 MHz, CDCl3)
d (ppm): 0.76
2.2.1. 1,3-Dimethylimidazolium-2-carboxylate (1a)
(t, J ¼ 8 Hz, 3H, CH3), 1.05e1.80 (m, 8H, CH2), 3.87 (s, 3H, NeCH3),
4.08 (m, 2H, NeCH2), 7.32 (brs, 1H, imidazol), 7.52 (brs, 1H, imi-
dazol). Analysis calculated for (C11H18N2O2): C 62.83%; H 8.63%; N
13.32%; O 15.22%. Found: C 62.85%; H 8.58%; N 13.33%; O 15.24%.
Synthesized by the reaction of 1-methylimidazole and dimethyl
carbonate (Yield: 92%). 1H NMR (400 MHz, D2O)
d (ppm): 4.00
(s, 6H, NeCH3), 7.39 (s, 2H, imidazol). Analysis calculated for
(C6H8N2O2): C 51.42%; H 5.75%; N 19.99%; O 22.83%. Found: C
51.44%; H 5.76%; N 19.98%; O 22.81%.
2.2.5. 1-(2-Propenyl)-3-methylimidazolium-2-carboxylate (5a)
Synthesized by the reaction of 1-(2-Propenyl) imidazole and
2.2.2. 1-Ethyl-3-methylimidazolium-2-carboxylate (2a)
dimethyl carbonate (Yield: 87%).1H NMR (400 MHz, CDCl3)
d (ppm):
Synthesized by the reaction of 1-ethylimidazole and dimethyl
carbonate (Yield: 88%) or synthesized by the reaction of 1-meth-
ylimidazole and diethyl carbonate (Yield: 45%). 1H NMR (400 MHz,
3.88 (s, 3H, NeCH3), 4.93 (m, 2H, NeCH2), 5.25e5.32 (m, 2H, CH]
CH2), 6.05 (m, 1H, CH]CH2), 7.34 (brs, 1H, imidazol), 7.56 (brs, 1H,
imidazol). Analysis calculated for (C8H10N2O2): C 57.82%; H 6.07%; N
16.86%; O 19.26%. Found: C 57.83%; H 6.05%; N 16.87%; O 19.24%.
CDCl3)
d
(ppm): 0.84 (t, J ¼ 8 Hz, 3H, CH3), 3.84 (s, 3H, NeCH3), 4.62
(m, 2H, NeCH2), 7.39 (brs, 1H, imidazol), 7.42 (brs, 1H, imidazol).
Analysis calculated for (C7H10N2O2): C 54.54%; H 6.54%; N 18.17%; O
20.76%. Found: C 54.51%; H 6.55%; N 18.16%; O 20.78%.
2.2.6. 1-Phenyl-3-methylimidazolium-2-carboxylate (6a)
Synthesized by the reaction of 1-phenylimidazole and dimethyl
carbonate(Yield:66%).1HNMR (400 MHz,CDCl3)
d(ppm):3.84(s, 3H,
2.2.3. 1-Butyl-3-methylimidazolium-2-carboxylate (3a)
NeCH3), 7.01e7.52 (m, 5H, Ph), 7.37 (brs, 1H, imidazol), 7.39 (brs, 1H,
imidazol). Analysis calculated for (C11H10N2O2): C 65.34%; H 4.98%;
N 13.85%; O 15.82%. Found: C 65.31%; H 4.98%; N 13.86%; O 15.85%.
Synthesized by the reaction of 1-butylimidazole and dimethyl
carbonate (Yield: 86%).1H NMR (400 MHz, CDCl3)
d (ppm): 0.74
(t, J ¼ 8 Hz, 3H, CH3), 1.19e1.70 (m, 4H, CH2), 3.95 (s, 3H, NeCH3),
4.14 (m, 2H, NeCH2), 7.39 (brs, 1H, imidazol), 7.54 (brs, 1H, imida-
zol). Analysis calculated for (C9H14N2O2): C 59.32%; H 7.74%; N
15.37%; O 17.56%. Found: C 59.34%; H 7.70%; N 15.38%; O 17.58%.
2.2.7. 1-Benzyl-3-methylimidazolium-2-carboxylate (7a)
Synthesized by the reaction of 1-benzylimidazole and dimethyl
carbonate (Yield: 71%). 1H NMR (400 MHz, CDCl3)
d (ppm): 3.84

2118
J. Li et al. / Journal of Organometallic Chemistry 696 (2011) 2116e2121
(s, 3H, NeCH3), 6.08 (m, 2H, NeCH2), 7.32e7.52 (m, 5H, Ph), 7.32
(brs, 1H, imidazol), 7.45 (brs, 1H, imidazol).Analysis calculated for
(C12H12N2O2): C 66.65%; H 5.59%; N 12.96%; O 14.80%. Found: C
66.61%; H 5.58%; N 12.98%; O 14.83%.
(C14H22RhN2Cl): C 47.14%; H 6.22%; N 7.85%. Found: C 47.11%; H
6.22%; N 7.88% (Yield: 88%).
2.3.3. Chloro(h
4-1,5-cyclooctadiene)(1-butyl-3-methylimidazole-
2-ylidene)rhodium(I) (3b)
2.2.8. 1-(4-Methylphenyl)-3-methylimidazolium-2-carboxylate (8a)
Synthesized by the reaction of 1-(4-methylphenyl)imidazole
and dimethyl carbonate (Yield: 67%). 1H NMR (400 MHz, CDCl3)
1H NMR (400 MHz, CDCl3)
d
(ppm): 6.81 (br d, J ¼ 4 Hz, 2H,
imidazol), 5.01 (m, 2H, COD CH), 4.49 (m, 2H, NeCH2), 4.07 (m,
3H, NeCH3), 3.34 (m, 1H, COD CH), 3.25 (m, 1H, COD CH), 2.38 (m,
4H, COD CH2), 1.98 (brm, 6H, 4HCOD CH2 and 2H Bu CH2), 1.29
(m, 2H, Bu CH2), 0.92 (t, J ¼ 8 Hz, 3H, CH3); 13C NMR (100 MHz,
d
(ppm): 2.25(s, 3H, PhCH3), 3.94 (s, 3H, NeCH3), 7.15e7.28 (m, 4H,
Ph), 7.32 (brs, 1H, imidazol), 7.35 (brs, 1H, imidazol). Analysis
calculated for (C12H12N2O2): C 66.65%; H 5.59%; N 12.96%; O 14.80%.
Found: C 66.63%; H 5.59%; N 12.98%; O 14.80%.
CDCl3)
d
: 13.8, 18.4, 28.6, 32.6, 33.0, 37.7(JNeC ¼ 7 Hz), 50.5
(JNeC ¼ 10 Hz), 67.2 (d, JRheC ¼ 14 Hz), 98.3 (d, JRheC ¼ 7 Hz), 120.1,
122.0, 181.8 (d, JCeRh 50 Hz). Analysis calculated for
¼
2.2.9. 1-(2,6-Diisopropylphenyl)-3-methylimidazolium-
2-carboxylate (9a)
(C16H26RhN2Cl): C 49.95%; H 6.81%; N 7.28%. Found: C 49.89%; H
6.90%; N 7.26% (Yield: 88%).
Synthesized by the reaction of 1-(2,6-diisopropylphenyl)imid-
azole and dimethyl carbonate (Yield: 62%). 1H NMR (400 MHz,
2.3.4. Chloro(h
4-1,5-cyclooctadiene)(1-hexyl-3-methylimidazole-
CDCl3)
d
(ppm): 1.21(d, J ¼ 8 Hz, 12H, CH(CH3)2), 2.50(m, 2H, CH
2-ylidene)rhodium(I) (4b)
(CH3)2), 3.92 (s, 3H, NeCH3), 7.32e7.52 (m, 3H, Ph), 7.32 (brs, 1H,
imidazol), 7.38 (brs, 1H, imidazol). Analysis calculated for
(C17H22N2O2): C 71.30%; H 7.74%; N 9.78%; O 11.17%. Found: C
71.28%; H 7.74%; N 9.79%; O 11.19%.
1H NMR (400 MHz, CDCl3)
d
(ppm): 6.81 (br d, J ¼ 4 Hz, 2H,
imidazol), 4.92 (m, 2H, COD CH), 4.46 (m, 2H, NeCH2), 4.07 (m, 3H,
NeCH3), 3.35 (m, 1H, COD CH), 3.24 (m, 1H, COD CH), 2.40 (m, 4H,
COD CH2), 1.29e2.00 (brm, 12H, 4HCOD CH2 and 8H CH2), 0.92
(t, J ¼ 8 Hz, 3H, CH3); 13C NMR (100 MHz, CDCl3)
d: 13.9, 11.3, 28.7,
2.2.10. 1-(2,4,6-Trimethylphenyl)-3-methylimidazolium-
2-carboxylate (10a)
29.8,30.3, 31.1, 32.1, 33.0, 37.8 (JNeC ¼ 7 Hz), 50.2 (JNeC ¼ 10 Hz), 67.6
(d, JRheC ¼ 14 Hz), 98.5 (d, JRheC ¼ 7 Hz), 120.3, 121.9, 182.1
(d, JCeRh ¼ 50 Hz). Analysis calculated for (C18H30RhN2Cl): C 52.37%;
H 7.33%; N 6.79%. Found: C 52.36%; H 7.31%; N 6.81% (Yield: 84%).
Synthesized by the reaction of 1-(2,4,6-trimethylphenyl)imid-
azole and dimethyl carbonate (Yield: 64%). 1H NMR (400 MHz,
CDCl3)
d (ppm):2.09 (s, 6H, CH3), 2.33 (s, 3H, CH3), 3.88 (s, 3H,
NeCH3), 7.08 (s, 2H, Ph), 7.35 (brs, 1H, imidazol), 7.41 (brs, 1H,
imidazol). Analysis calculated for (C14H16N2O2): C 68.83%; H 6.60%;
N 11.47%; O 13.10%. Found: C 68.81%; H 6.59%; N 11.49%; O 13.11%.
These data were consistent with those reported in the literature [24].
2.3.5. Chloro(
dazole-2-ylidene]rhodium(I) (5b)
1H NMR (400 MHz, CDCl3)
h
4-1,5-cyclooctadiene)[1-(2-propenyl)-3-methylimi-
d
(ppm): 6.82 (br d, J ¼ 4 Hz, 1H,
imidazol), 6.62 (br d, J ¼ 4 Hz, 1H, imidazol), 4.80 (m, 2H, COD CH),
4.15 (m, 2H, 1H NeCH2 and1H NCH2CH]CH2), 3.95 (m, 3H,
NeCH3), 3.67(d, J ¼ 12 Hz, 1H, NeCH2), 3.32 (m, 1H, COD CH), 3.24
(m, 1H, COD CH), 2.23 (m, 4H, COD CH2), 2.18e1.87 (brm, 6H,
2.3. Synthesis of rhodium N-heterocyclic carbene complexes
4HCOD CH2 and 2H NCH2CH]CH2); 13C NMR (100 MHz, CDCl3)
d:
These compound were prepared by stirring a mixture of
28.4, 32.1, 35.6, 38.1(JNeC ¼ 7 Hz), 47.2, 52.4(JNeC ¼ 10 Hz), 66.5 (d,
JRheC ¼ 14 Hz), 97.35(d, JRheC ¼ 7 Hz), 119.6, 123.2, 182.3 (d,
JCeRh ¼ 50 Hz). Analysis calculated for (C15H22RhN2Cl): C 48.86%; H
6.01%; N 7.60%. Found: C 48.84%; H 6.00%; N 7.62% (Yield: 93%).
500 mg (1.01 mmol) of bis(h
4-1,5-cyclooctadiene) dichloro-
dirhodium and 2.2 mmol of 1-alkyl-3–methyl imidazolium-2-
carboxylate in 10 mL of acetonitrile in a Schlenk flask for 40 min
at room temperature. The reaction mixture was evaporated
under reduced pressure and washed with diethyl ether
(3 ꢀ 10 mL). The yellow solid was dissolved in methylene chlo-
ride (2 mL) and purified by repeated recrystallization from
diethyl ether.
2.3.6. Chloro(h
4-1,5-cyclooctadiene)(1-phenyl-3-methylimidazole-
2-ylidene)rhodium(I) (6b)
1H NMR (400 MHz, CDCl3)
d (ppm): 8.03(m, 2H, Ph), 7.56e7.46
(m, 3H, Ph), 6.97 (br d, J ¼ 4 Hz, 1H, imidazol), 6.82 (br d, J ¼ 4 Hz,
1H, imidazol), 4.60 (m, 2H, COD CH), 4.03 (m, 3H, NeCH3), 3.28 (m,
1H, COD CH), 3.21 (m, 1H, COD CH), 2.34 (m, 4H, COD CH2), 1.97
2.3.1. Chloro(h
4-1,5-cyclooctadiene)(1-methyl-3-methylimidazole-
2-ylidene)rhodium(I) (1b)
(brm, 4H, 4HCOD CH2); 13C NMR (100 MHz, CDCl3)
d: 28.5, 32.5, 37.8
1H NMR (400 MHz, CDCl3)
d
(ppm): 6.72 (s, 2H, imidazol), 4.95
(JNeC ¼ 7 Hz), 67.1 (d, JRheC ¼ 14 Hz), 98.3 (d, JRheC ¼ 7 Hz), 120.1,
(m, 2H, COD CH), 4.00 (m, 6H, NeCH3), 3.33 (m, 2H, COD CH), 2.33
122.5, 125.8, 128.8, 129.0, 138.9, 181.9 (d, JCeRh ¼ 50 Hz), due to
(m, 4H, COD CH2), 1.89 (m, 4H, COD CH2); 13C NMR (100 MHz,
overlapping of signals, the carbon resonances from d 125 to 139
CDCl3)
d
: 28.9, 33.0, 37.6(JNeC ¼ 7 Hz), 67.6 (JRheC ¼ 15 Hz), 98.5
were listed. Analysis calculated for (C18H22RhN2Cl): C 53.42%; H
5.48%; N 6.92%. Found: C 53.40%; H 5.48%; N 6.93% (Yield: 89%).
(JRheC ¼ 7 Hz), 121.9, 182.8 (JCeRh ¼ 50 Hz). Analysis calculated for
(C13H20RhN2Cl): C 45.57%; H 5.88%; N 8.18%. Found: C 45.59%; H
5.86%; N 8.19% (Yield: 92%).
2.3.7. Chloro(h
4-1,5-cyclooctadiene)(1-benzyl-3-methylimidazole-
2-ylidene)rhodium(I) (7b)
2.3.2. Chloro(
h
4-1,5-cyclooctadiene)(1-ethyl-3-methylimidazole-
1H NMR (400 MHz, CDCl3)
d (ppm): 7.56e7.26 (m, 5H, Ph), 6.87
2-ylidene)rhodium(I) (2b)
(br d, J ¼ 4 Hz, 1H, imidazol), 6.60 (br d, J ¼ 4 Hz, 1H, imidazol), 5.58
(br s, 2H, CH2Ph), 4.87 (m, 2H, COD CH), 4.23 (m, 3H, NeCH3), 3.29
(m, 1H, COD CH), 3.25 (m, 1H, COD CH), 2.38 (m, 4H, COD CH2), 1.95
1H NMR (400 MHz, CDCl3)
d
(ppm): 6.82 (br d, J ¼ 4 Hz, 2H,
imidazol), 5.00 (m, 2H, COD CH), 4.47 (m, 2H, NeCH2), 3.72 (m, 3H,
NeCH3), 3.29 (m, 1H, COD CH), 3.25 (m, 1H, COD CH), 2.37 (m, 4H,
(brm, 4H, 4HCOD CH2); 13C NMR (100 MHz, CDCl3)
d: 28.8, 32.1, 37.8
COD CH2), 1.98 (brm, 4H, 4HCOD CH2), 1.22 (t, J ¼ 8 Hz, 3H, CH3); 13
C
(JNeC ¼ 7 Hz), 54.6(JNeC ¼ 11 Hz), 67.3 (d, JRheC ¼ 14 Hz), 98.7 (d,
JRheC ¼ 7 Hz), 119.9, 121.3, 128.1, 128.3, 129.0, 135.6181.9 (d,
JCeRh ¼ 50 Hz). Analysis calculated for (C19H24RhN2Cl): C 54.49%; H
5.78%; N 6.69%. Found: C 54.47%; H 5.77%; N 6.69% (Yield: 89%).
NMR (100 MHz, CDCl3)
d
: 15.8, 28.5, 32.5, 37.8(JNeC ¼ 7 Hz), 46.6
(JNeC ¼ 10 Hz), 67.1 (d, JRheC ¼ 14 Hz), 98.3 (d, JRheC ¼ 7 Hz), 120.3,
122.2, 181.9 (d, JCeRh 50 Hz). Analysis calculated for
¼

J. Li et al. / Journal of Organometallic Chemistry 696 (2011) 2116e2121
2119
2.3.8. Chloro(
methylimidazole-2-ylidene]rhodium(I) (8b)
1H NMR (400 MHz, CDCl3)
(ppm): 7.26e7.92(m, 4H, Ph), 6.99
h
4-1,5-cyclooctadiene)[1-(4-methylphenyl)-3-
N
d
N
Rh
Cl
(br d, J ¼ 4 Hz, 1H, imidazol), 6.88 (br d, J ¼ 4 Hz, 1H, imidazol),
4.64 (m, 2H, COD CH), 4.13 (m, 3H, NeCH3), 3.18 (m, 1H, COD CH),
3.23 (m, 1H, COD CH), 2.33(s, 3H, PhCH3), 2.01 (m, 4H, COD CH2),
1.87 (brm, 4H, 4HCOD CH2); 13C NMR (100 MHz, CDCl3)
d: 22.1,
5b
27.9, 32.8, 38.8(JNeC ¼ 7 Hz), 67.6 (d, JRheC ¼ 14 Hz), 97.7
(d, JRheC ¼ 7 Hz), 121.1, 122.7, 127.7, 131.6, 133.3, 138.9183.9
(d, JCeRh ¼ 50 Hz), due to overlapping of signals, the carbon
Scheme 3. Complex 5b.
resonances from
d 125 to 139 were listed. Analysis calculated for
CH3), 3.65 (q, J ¼ 8 Hz, 1H, SieCH), 3.76 (q, J ¼ 8 Hz, 6H, OeCH2),
7.12e7.19 (m, 5H, Ph).
(C19H24RhN2Cl): C 54.49%; H 5.78%; N 6.69%. Found: C 54.47%;
H 5.78%; N 6.70% (Yield: 89%).
2.4.1.3. Ethylbenzene. 1H NMR (CDCl3, 400 MHz)
J ¼ 8 Hz, 3H, CH3), 2.64 (q, J ¼ 8 Hz, 2H, CH2), 7.11e7.25 (m, 5H, Ph).
d (ppm): 1.15 (t,
2.3.9. Chloro(
3-methylimidazole-2-ylidene]rhodium(I) (9b)
1H NMR (400 MHz, CDCl3)
(ppm): 7.17e7.35(m, 3H, Ph), 6.99
h
4-1,5-cyclooctadiene)[1-(2,6-diisopropylphenyl)-
d
2.4.2. Hydrosilylation reaction of styrene with triethylsilane
(br d, J ¼ 4 Hz, 1H, imidazol), 6.83 (br d, J ¼ 4 Hz, 1H, imidazol), 4.66
(m, 2H, COD CH), 4.13 (m, 3H, NeCH3), 3.26 (m, 1H, COD CH), 3.14
(m, 1H, COD CH), 2.93 (m, 2H, CH(CH3)2), 2.13 (m, 4H, COD CH2),
1.97 (brm, 4H, 4HCOD CH2), 1.23(d, J ¼ 8 Hz, 6H, CH(CH3)2); 13C
2.4.2.1.
400 MHz)
b
-Adduct [triethyl(phenethyl)silane]. 1H NMR (CDCl3,
d
(ppm): 0.66 (q, J ¼ 8 Hz, 6H, SieCH2), 092 (t, J ¼ 8 Hz,
9H, CH3), 0.98 (t, J ¼ 8 Hz, 2H, SieCH2), 2.71 (t, J ¼ 8 Hz, 2H, CH2),
7.11e7.27 (m, 5H, Ph).
NMR (100 MHz, CDCl3)
d
: 22.6, 26.0, 28.7, 33.4, 37.8(JNeC ¼ 7 Hz),
68.3 (d, JRheC ¼ 14 Hz), 98.1 (d, JRheC ¼ 7 Hz), 121.7, 122.8, 123.7,
2.4.2.2.
400 MHz)
a
-Adduct [triethyl(1-phenylethyl)silane]. 1H NMR (CDCl3,
125.9, 130.0, 134.2181.9 (d, JCeRh ¼ 50 Hz), due to overlapping of
d
(ppm): 0.69 (q, J ¼ 8 Hz, 6H, SieCH2), 097 (t, J ¼ 8 Hz,
signals, the carbon resonances from
d 125 to 139 were listed.
9H, CH3), 1.21 (d, J ¼ 9 Hz, 3H, CH3), 3.58 (q, J ¼ 8 Hz, 6H, OeCH2),
Analysis calculated for (C24H34RhN2Cl): C 58.96%; H 7.01%; N 5.73%.
Found: C 58.97%; H 7.00%; N 5.72% (Yield: 84%).
7.15e7.23 (m, 5H, Ph).
2.4.2.3. Dehydrogenative silylation product [triethyl(styryl)silane]. 1H
2.3.10. Chloro(
3-methylimidazole-2-ylidene]rhodium(I) (10b)
1H NMR (400 MHz, CDCl3)
(ppm): 7.06(s, 2H, Ph), 6.99 (br d,
J ¼ 4 Hz, 1H, imidazol), 6.83 (br d, J ¼ 4 Hz, 1H, imidazol), 4.46 (m,
2H, COD CH), 4.23 (m, 3H, NeCH3), 3.30 (m, 1H, COD CH), 3.26 (m,
1H, COD CH), 2.32 (s, 3H, CH3), 2.28 (s, 3H, CH3), 2.18 (s, 3H, CH3),
2.02 (m, 4H, COD CH2), 1.61 (brm, 4H, 4HCOD CH2); 13C NMR
h
4-1,5-cyclooctadiene)[(2,4,6-trimethylphenyl)-
NMR (CDCl3, 400 MHz)
J ¼ 8 Hz, 6H, SieCH2), 6.47 (d, J ¼ 12 Hz, 1H, SiCH), 6.98(d, J ¼ 12 Hz,
1H, CHPh), 7.16e7.34 (m, 5H, Ph).
d
(ppm): 0.93 (t, J ¼ 9 Hz, 9H, CH3), 1.33 (q,
d
2.4.3. Hydrosilylation of 1-hexene with triethoxysilane
2.4.3.1.
b
-Adduct (hexyltriethoxysilane). 1H NMR (CDCl3, 400 MHz)
d
(ppm): 0.64 (t, J ¼ 6 Hz, 2H, SieCH2), 0.89 (t, J ¼ 8 Hz, 3H, CH3),
(100 MHz, CDCl3)
d
: 18.5, 20.0, 21.4, 28.5, 32.5, 37.8(JNeC ¼ 7 Hz),
1.22e1.42 (m, 17H, CH2 CH3), 3.81 (q, J ¼ 8 Hz, 6H, OeCH2).
These data were consistent with those reported in the works
[12,22].
68.7 (d, JRheC ¼ 14 Hz), 96.3 (d, JRheC ¼ 7 Hz), 122.1, 124.5, 125.3,
128.8, 137.0, 138.4, 183.5 (d, JCeRh ¼ 50 Hz), due to overlapping of
signals, the carbon resonances from
d 125 to 139 were listed.
Analysis calculated for (C21H28RhN2Cl): C 56.45%; H 6.32%; N 6.27%.
Found: C 56.44%; H 6.31%; N 6.29% (Yield: 87%).
These data were consistent with those reported in the literature
[24].
2.4.4. Chloro(h
4-1,5-cyclooctadiene){1-[3-(triethoxysilyl)propyl]-3-
methylimidazole-2-ylidene}rhodium(I) (11b)
At the end of the reaction, the catalyst phase (5b/BMimPF6) was
separated from the product by decantation and the structure was
determined by 1H NMR (Scheme 4).
2.4. Catalytic hydrosilylation of alkene with triethoxysilane
1H NMR (400 MHz, CDCl3)
d
(ppm): 7.02 (br d, J ¼ 4 Hz, 1H,
imidazol), 6.81 (br d, J ¼ 4 Hz, 1H, imidazol), 4.82 (m, 2H, COD CH),
4.24(m, 2H, NeCH2), 3.93 (m, 3H, NeCH3), 3.82 (q, J ¼ 8 Hz, 6H,
OeCH2), 3.31 (m, 1H, COD CH), 3.24 (m, 1H, COD CH), 2.21 (m, 4H,
COD CH2), 2.18e1.87 (brm, 4H, 4HCOD CH2), 1.64e1.12 (m, 11H, CH2
CH3), 0.71(t, J ¼ 6 Hz, 2H, SieCH2)/BMimPF6: 1H NMR (400 MHz,
Typical hydrosilylation reaction procedures were as follows: A
given amount of catalyst and ionic liquid were added to a 10 mL
round bottomed flask equipped with a magnetic stirrer and the
alkene and silane were then added. This mixture was heated to the
appropriate temperature and the hydrosilylation reaction was
allowed to proceed with constant stirring for 5 h. At the end of the
reaction, the product phase was separated from the catalyst by
decantation and the conversion of alkene and the selectivity were
determined by GC. The catalyst was recharged with fresh alkene
and silane for the next catalytic run.
CDCl3)
d (ppm): 8.49 (s, 1H, imidazol), 7.33e7.29 (m, 2H, imidazol),
4.17 (t, J ¼ 8 Hz, 2H, NeCH2), 3.95 (s, 3H, NeCH3), 1.87e1.22(m, 4H,
CH2), 0.95 (t, J ¼ 8 Hz, 3H, CH3).
N
N
(EtO)3Si
(EtO)3SiH
2.4.1. Hydrosilylation of styrene with triethoxysilane
2.4.1.1.
400 MHz)
b
-Adduct [triethoxy(phenethyl)silane]. 1H NMR (CDCl3,
N
Rh
Cl
N
Rh
Cl
d
(ppm): 1.00 (t, J ¼ 8 Hz, 2H, SieCH2),1.24 (t, J ¼ 9 Hz, 9H,
BMimPF6
CH3), 2.74 (t, J ¼ 8 Hz, 2H, CH2), 3.84 (q, J ¼ 8 Hz, 6H, OeCH2),
7.16e7.27 (m, 5H, Ph).
5b
11b
2.4.1.2.
400 MHz)
a
-Adduct [triethoxy(1-phenylethyl)silane]. 1H NMR (CDCl3,
(ppm): 1.18 (t, J ¼ 8 Hz, 9H, CH3), 1.33 (d, J ¼ 8 Hz, 3H,
d
Scheme 4. Complex 11b.

2120
J. Li et al. / Journal of Organometallic Chemistry 696 (2011) 2116e2121
Table 1
3. Results and discussion
Synthesis of imidazolium-2-carboxylates and rhodium N-heterocyclic carbene
complexes.
3.1. Synthesis and characterization of 1-alkyl-3-
methylimidazolium-2-carboxylate
Entry
a
Material
Yielda (%)
b
Yielda (%)
1
2
3
4
5
1a
2a
3a
4a
5a
1-Methylimidazole
1-Ethylimidazole
1-Butylimidazole
1-Hexylimidazole
1-(2-Propenyl)
Dimethyl 92
carbonate 88
1b
2b
3b
4b
5b
92
88
88
84
93
There are only a few literatures concerning imidazolium-
2-carboxylates. In 1974, Schlösser and Regitz [25] first isolated 1,
3-diphenylimidazolinium-2-carboxylate by hydrolysis of more
complex zwitterionic carbamate derivatives. Kuhn et al. [26] used
CO2 to trap 1, 3-diisopropyl-4, 5-dimethylimidazol-2-ylidene as
stable adduct in 1999. In 2005, Crabtree et al. [8,24] showed that N,
N0-dimethylimidazolium-2-carboxylate can be an NHC transfer
agent to transition metals. And the syntheses of N, N0-dimethyli-
midazolium-2-carboxylate usually use base and in some cases a high
pressure of CO2. But the experimental procedure did not easily
operate and hard to isolate product. In 2003, Tkatchenko and co-
workers [27] reported the formation of 1,3-dimethylimidazolium-
2-carboxylate from 1-methylimidazole and dimethyl carbonate. In
2005, Tommasi and Crabtree [8,24] subsequently extended the
procedure to the preparation of 1-butyl-3-methylimidazolium-
2-carboxylate. But this method that was limited to the preparation
of 1,3-dimethylimidazolium-2-carboxylate and 1-butyl-3-methyl-
imidazolium-2-carboxylate was only reported in a few literatures.
We have extended this procedure to the preparation of 1-alkyl-3-
methylimidazolium-2-carboxylate. A series of 1-alkyl-3-methyl-
imidazolium-2-carboxylate (2ae10a) were synthesized by the
reaction of 1-alkylimidazole and dimethyl carbonate. At the same
time, we find that 1-ethyl-3-methylimidazolium-2-carboxylate
could also be synthesized by the reaction of 1-methylimidazole and
diethyl carbonate. These ten products are characterized by NMR
spectroscopy and elemental analysis. And the NMR spectroscopy
and elemental analysis data of the prepared compounds are in
agreement with the assigned structures.
86
83
87
imidazole
6
7
8
6a
7a
8a
1-Phenylimidazole
1-Benzylimidazole
1-(4-Methylphenyl)
imidazole
1-(2,6-Diisopropyl
phenyl) imidazole
66
71
67
6b
7b
8b
89
89
89
9
9a
62
64
9b
84
10
11
10a 1-(2,4,6-Tri
methylphenyl)imidazole
1-Methylimidazole
10b 87
2b 88
2a
Diethyl
45
carbonate
a
Isolated yields.
3.3. Hydrosilylation catalysed by rhodium N-heterocyclic carbene
complexes in the ionic liquid medium
Recently, our group [22] reports a new hydrosilylation process in
a scCO2/IL system with a rhodium complex as the catalyst. During
this process, Rh complexes of NHCs exhibited excellent catalytic
activity and selectivity was synthesized by direct carboxylation of
1,3-dialkylimidazolium hexafluorophosphate with CO2 in situ. In
order to research effect of Rhodium complexes bearing NHC ligands,
we had synthesized a series of rhodium complexes bearing NHC
ligands. The results listed in Table 1 indicate that Rh(PPh3)3Cl and
Table 2
3.2. Synthesis and characterization of rhodium N-heterocyclic
carbene complexes
Effect of the catalyst/IL system on the hydrosilylation reaction of styrene with
triethoxysilane.
Entry Catalyst
BMimPF6 Conv. (%) Select. (%)
Ton
1-Allyl-3-methylimidazolium-2-carboxylate, air- and moisture-
stable species, can transfer NHCs to [Rh(cod)Cl]2 with release of
(substrate mol %) (mL)
b
a
Byproduct
CO2. For example, chloro(h
4-1,5-cyclooctadiene)(1,3-dimethylimi-
1
2
3
Rh(PPh3)3Cl 0.1
[Rh(cod)Cl]2 0.05
1b 0.1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
4
6
2
2
2
2
2
78.1
45.6
91.3
88.8
86.4
84.5
89.1
88.2
93.5
94.9
95.6
96.8
81.2 16.5 2.3a
781
59.1 39.8 1.1a
456b
913
888
864
845
911
882
935
949
956
968
200
491
1582
970
969
938
936
934
931
924
dazole-2-ylidene)rhodium(I) (1b) was synthesized by the 1,3-
54.1 45.9
59.4 40.6
64.7 35.3
70.3 29.7
86.8 13.2
84.5 15.5
82.1 17.9
87.5 12.5
91.5 8.5
90.2 9.8
89.7 10.3
90.0 10.0
90.7 9.3
90.2 9.8
90.3 9.7
90.1 9.9
90.4 9.6
90.3 9.7
90.4 9.6
90.6 9.4
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
dimethylimidazolium-2-carboxylate reacts and [Rh(cod)Cl]2; chloro
4
2b 0.1
(h
4-1,5-cyclooctadiene)[1-phenyl-3-methylimidazole-2-ylidene]
5
3b 0.1
rhodium(I) (6b) was synthesized by the 1-phenyl-3-methyl-
imidazolium-2-carboxylate reacts and [Rh(cod)Cl]2.1bwas obtained
in 92% isolated yield and 6b was obtained in 89% isolated yield.
And a series of rhodium complexes bearing NHC ligands were
synthesized by this way and characterized by NMR spectroscopy
and elemental analysis. The NMR spectroscopy (1H NMR and 13C
NMR) and elemental analysis data of the prepared compounds
were in agreement with the assigned structures.
6
4b 0.1
7
5b 0.1
8
6b 0.1
9
7b 0.1
10
11
12
13
14
15
16
17
18c
19d
20e
21f
22g
8b 0.1
9b 0.1
10b 0.1
10b 0.5
10b 0.2
10b 0.05
10b 0.1
10b 0.1
10b 0.1
10b 0.1
10b 0.1
10b 0.1
10b 0.1
100
98.2
79.1
97.0
96.9
93.8
93.6
93.4
93.1
92.4
The chloro(h
4-1,5-cyclooctadiene)[1-(2-propenyl)-3-methyl-
imidazole-2-ylidene]rhodium(I) (5b) was synthesized by the
reaction of the 1-(2-propenyl)-3-methylimidazolium-2-carboxy-
late and [Rh(cod)Cl]2 for the first time. We find h
2-coordination
mode of 5b (Scheme 3) in CD3Cl is visible in the NMR spectroscopy
(1H NMR). The signals for the protons of the allylic double bond in
5b are significantly shifted to higher field compared to compound
Reaction conditions: styrene 30 mmol; triethoxysilane 36 mmol; 70 ꢂC; 2 h.
Byproduct: ethylbenzene and unsaturated-adduct.
5a. The allylic CH and CH2 resonances appear at
d
¼ 6.05 and
a
d
¼ 5.25e5.32 ppm for 5a and at
d
¼ 4.15 and
d
¼ 1.87e2.18 ppm
Ethylbenzene.
Based on Rh.
Second run.
Third run.
Fourth run.
Fifth run.
Sixth run.
b
for 5b, respectively. Upon coordination of the allylic double bond,
the NeCH2 protons of this group become enatiotopic and give rise
c
d
to two signals at
compound 5a exhibits only one resonance for allylic NeCH2
protons
¼ 4.93 ppm.
d
¼ 4.15 and 3.67 ppm, while the spectrum of
e
f
g
d

J. Li et al. / Journal of Organometallic Chemistry 696 (2011) 2116e2121
2121
Table 3
Rhodium complexes bearing NHC ligands (1be4b, 6be10b)
showed a pronounced solubility in BMimPF6. They were specially
designed to be used in ionic liquid biphasic systems and had been
employed to circumvent catalyst problems. 10b in BMimPF6 could
be reused without noticeable loss of catalytic activity and selec-
tivity (Table 2, entries 18e22).
Results of the hydrosilylation reaction of other alkenes with triethoxysilane.
Entry Alkene
Silane
Conv. (%) Selectivity (%)
Ton
b
a
Byproduct
1
2
3
1-Hexene
Triethoxysilane
100
100
～100
100
100
100
e
e
e
e
e
e
e
e
e
5000
1-Heptene
1-Octene
1-Dodecene
2-Methyl-
styrene
4-Methyl-
styrene
4-Methoxy-
styrene
4-Fluoro-
styrene
4-Chloro-
styrene
5000
～5000
4995
4
99.9 100
4. Conclusion
5a
94.2
97.4
98.1
97.1
96.9
99.1
92.3 7.7
89.7 10.3
87.1 12.9
90.7 9.3
91.4 8.6
1796
6a
e
e
e
e
1874
1916
1882
1872
991
In summary,
carboxylate was synthesized by the reaction of 1-alkylimidazole
and dimethyl carbonate. 1-Alkyl-3-methylimidazolium-2-
a series of 1-alkyl-3-methylimidazolium-2-
7a
8a
carboxylate, air- and moisture-stable species, could transfer NHCs
to [Rh(cod)Cl]2 with release of CO2, and a series of rhodium
complexes bearing NHC ligands are synthesized. The catalytic
properties of rhodium complexes prepared in the hydrosilylation of
alkenes in ionic liquid media were investigated. It was found that
both the catalytic activity and selectivity of the rhodium complexes
bearing NHC ligands were influenced by the attached substituents
of the imidazolium cation. Additionally, rhodium complexes
bearing NHC ligands in ionic liquid BMimPF6 could be reused
without noticeable loss of catalytic activity and selectivity.
9a
10b
Styrene
Triethylsilane
71.7 4.2 24.1
Reaction conditions: alkene 30 mmol; triethoxysilane 36 mmol; 10b 0.02 mol% of
styrene; BMimPF6 2 mL; 70 ꢂC; 2 h.
a
Reaction conditions: styrene 30 mmol; triethoxysilane 36 mmol; 10b 0.1 mol%
of styrene; BMimPF6 2 mL; 70 ꢂC; 2 h.
b
Reaction conditions: styrene 30 mmol; triethylsilane 36 mmol; 10b 0.1 mol% of
styrene; BMimPF6 2 mL; 70 ꢂC; 2 h. Byproduct: dehydrogenative silylation product;
no alkane.
Acknowledgements
[Rh(cod)Cl]2 had low catalytic activity and selectivity in BMimPF6,
respectively. In contrast to Rh(PPh3)3Cl and [Rh(cod)Cl]2, no by-
We are grateful to the Fund of Zhejiang province (Y4100248)
and Technologies R&D Program of Hangzhou city (20100331T16)
for financial support.
product was detected when chloro(
3-methylimidazole-2-ylidene)rhodium(I) was used as catalyst. The
1be4b had higher catalytic activity and lower selectivity of the
h
4-1,5-cyclooctadiene)(1-alkyl-
b-
adduct in BMimPF6, respectively. And the catalytic activities of
rhodium complexes bearing NHC ligands slightly decreased with
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