N-Heterocyclic carbene ligands bearing hydrophilic and/or hydrophobic chains: Rh(i) and Pd(ii) complexes and their catalytic activity

Article abstract of DOI:10.1039/b715412g

A series of novel imidazolium salts bearing hydrophilic tetraethylene glycol (TEG) and/or hydrophobic long-chain alkyl (n-C₁₂) functionalities, which are precursors for desired N-heterocyclic carbene (NHC) ligands, were synthesized and characterized. Rh(i)-NHC complexes were prepared in good yields by the silver carbene transfer method with NHC-Ag species derived from the imidazolium salts. The molecular structure of the Rh(i)-NHC complex having n-C₁₂ chains has been determined by a single-crystal X-ray diffraction study and the complex is found to possess extended alkyl chains with anti conformations in the solid state. Hydrosilylation with the Rh(i) complexes and Suzuki-Miyaura coupling reactions with the Pd(ii) complexes with these NHC ligands were carried out. In the latter case, the TEG moiety enhances catalytic activity considerably. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715412g

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An international journal of inorganic chemistry
www.rsc.org/dalton
Number 3 | 21 January 2008 | Pages 305–412
ISSN 1477-9226
PAPER
Yasushi Tsuji et al.
PERSPECTIVE
N-Heterocyclic carbene ligands
bearing hydrophilic and/or
hydrophobic chains: Rh(I) and Pd(II)
Vincent Artero et al.
Modelling NiFe hydrogenases:
nickel-based electrocatalysts for
complexes and their catalytic activity hydrogen production
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PAPER
www.rsc.org/dalton | Dalton Transactions
N-Heterocyclic carbene ligands bearing hydrophilic and/or hydrophobic
chains: Rh(I) and Pd(II) complexes and their catalytic activity†‡
Hidetoshi Ohta,a,b Tetsuaki Fujiharaa and Yasushi Tsuji*a
Received 8th October 2007, Accepted 2nd November 2007
First published as an Advance Article on the web 22nd November 2007
DOI: 10.1039/b715412g
A series of novel imidazolium salts bearing hydrophilic tetraethylene glycol (TEG) and/or hydrophobic
long-chain alkyl (n-C12) functionalities, which are precursors for desired N-heterocyclic carbene (NHC)
ligands, were synthesized and characterized. Rh(I)–NHC complexes were prepared in good yields by
the silver carbene transfer method with NHC–Ag species derived from the imidazolium salts. The
molecular structure of the Rh(I)–NHC complex having n-C12 chains has been determined by a
single-crystal X-ray diffraction study and the complex is found to possess extended alkyl chains with
anti conformations in the solid state. Hydrosilylation with the Rh(I) complexes and Suzuki–Miyaura
coupling reactions with the Pd(II) complexes with these NHC ligands were carried out. In the latter
case, the TEG moiety enhances catalytic activity considerably.
Introduction
N-Heterocyclic carbene (NHC) ligands have attracted consider-
able attention in both organometallic chemistry and homoge-
neous catalysis.1 Their stronger bonds to metals, as compared
with conventional ligands such as phosphines, diminish ligand
dissociation. In addition, the NHC complexes are favored by facile
preparation methods, better stability toward air and moisture,
and high efficiency as catalysts. Therefore, much effort has been
devoted to the synthesis of a wide variety of NHC ligands and
their transition-metal complexes.2–5
Polyether substituents such as polyethylene glycol (PEG)6 may
Fig. 1 Structure of ligands used in this work.
add new functionality to transition-metal catalysts.7,8 Recently,
Grubbs and co-workers reported highly active water-soluble
Ru catalysts bearing PEG-attached NHC ligands for olefin
metathesis.8a,b In the present work, we designed a new type of
Suzuki–Miyaura coupling reaction with the Pd(II) complexes as
a catalyst, the TEG functionality of A enhanced catalytic activity
significantly.
polyether-functionalized NHC ligands 1a and 1b (Fig. 1). The
Results and discussion
NHC ligand 1a possesses the moiety A which has three hydrophilic
tetraethylene glycol (TEG) chains on both sides of the NHC ring.
On the other hand, 1b possesses the moiety B which has three
hydrophobic n-C12 chains in addition to the moiety A. The TEG
chains on these ligands (1a and 1b) can come close to the carbene
center and may bring about a unique catalytic environment in some
catalytic reactions. Furthermore, the ligand 1c bearing only the
moieties B and the ligand 1d without such long-chain substituents
were prepared for comparison. Here, we report the synthesis of
these novel NHC ligand precursors and the catalytic activities of
the Rh(I) and Pd(II) complexes with these NHC ligands. In the
Synthesis of imidazolium derivatives
The imidazolium salts 2a and 2c, which are precursors for 1a
and 1c, were synthesized by the reaction of imidazole with the
corresponding benzyl chlorides R–Cl (3a9: R = A or 3b10: R = B) as
shown in Scheme 1. The imidazolium salt 2b bearing both TEG (A)
and n-C12 chains (B) was synthesized in two steps via N-substituted
imidazole (411) bearing n-C12 chains (B). The imidazolium salt 2d
aDepartment of Energy and Hydrocarbon Chemistry, Graduate School of
Engineering, Kyoto University, Kyoto, 615-8510, Japan. E-mail: ytsuji@
scl.kyoto-u.ac.jp; Fax: +81-75-383-2514; Tel: +81-75-383-2515
bDivision of Chemistry, Graduate School of Science, Hokkaido University,
Sapporo, 060-0810, Japan
† CCDC reference numbers 662977 and 662978. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b715412g
‡ Electronic supplementary information (ESI) available: Spectroscopic
3
data for [PdCl(g -C3H5)(1a–d)] and computational details. See DOI:
10.1039/b715412g
Scheme 1 Synthesis of imidazolium salts 2a–d.
Dalton Trans., 2008, 379–385 | 379
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◦
˚
without such long-chain substituents was also synthesized by the
reaction of imidazole with 3c. These new compounds were fully
characterized by elemental analysis, MALDI-TOF mass spectra
and NMR measurements.
Table 1 Selected bond lengths (A) and angles ( ) for 5c and 5d
5c
5d
Rh(1)–C(1)
Rh(1)–Cl(1)
Rh(1)–C(4)
Rh(1)–C(5)
Rh(1)–C(6)
Rh(1)–C(7)
C(2)–C(3)
2.025(8)
2.4021(14)
2.121(6)
2.107(6)
2.189(10)
2.194(8)
1.334(11)
2.026(3)
2.3946(7)
2.119(3)
2.102(3)
2.202(3)
2.211(3)
1.340(4)
Synthesis and structural characterization of Rh(I)–NHC complexes
Rhodium(I) complexes 5a–d bearing the NHC ligands 1a–d were
synthesized by the carbene-transfer method12 with NHC–Ag
species derived from 2a–d (Scheme 2). The complexes (5a–d) were
isolated in their pure forms and fully characterized. MALDI-
TOF mass spectra of 5a–d showed fragment ion peaks [M −
Cl]+ at m/z = 1696, 1630, 1564, and 639, respectively. In the
C(1)–Rh(1)–Cl(1)
C(1)–Rh(1)–C(4)
C(1)–Rh(1)–C(5)
C(1)–Rh(1)–C(6)
C(1)–Rh(1)–C(7)
N(1)–C(1)–N(2)
N(1)–C(1)–Rh(1)
N(2)–C(1)–Rh(1)
88.28(17)
96.4(3)
90.2(3)
165.6(2)
157.9(3)
103.8(6)
130.5(6)
125.3(4)
88.85(8)
93.88(12)
88.96(12)
162.82(12)
160.86(13)
104.3(2)
1
13C{ H} NMR spectra of 5a–c having TEG and/or n-C12 chains,
the carbene carbons on the Rh center appeared as sharp doublets
at 183.0 ppm (JRh-C = 50.5 Hz), 183.0 ppm (JRh-C = 51.5 Hz), and
183.0 ppm (JRh-C = 50.5 Hz), respectively. The carbene carbon
resonance of 5d appeared at 183.1 ppm with a comparable JRh-C
value (52.5 Hz), indicating that the TEG and n-C12 substituents
do not affect the carbene carbon resonance.
130.7(2)
125.0(2)
in Fig. 2a and Fig. 2c, 5c possesses long straight alkyl chains which
extend in an anti conformation and these n-C12 chains are stacked
on top of each other in the solid state.14
For 5a and 5b, bearing TEG chains, all the trials to obtain single
crystals suitable for crystallographic analysis were unsuccessful.
Thus, the molecular structure of 5a was optimized by ONIOM cal-
culation (B3LYP/LANL2DZ:UFF).15 At first, the structure was
optimized with the TEG chains keeping their anti conformations,
as observed in the structure of 5c (Fig. 2a). The resulting optimized
structure (local minimum) is shown in Fig. 3a. However, during
the conformation analysis of 5a, many more stable conformers
were obtained. Among them, it was found that the significantly
folded structure shown in Fig. 3b is close to a global minimum: the
calculated energy difference between the structures in Fig. 3a and
Fig. 3b is 28.4 kcal mol−1. These calculation suggest that the TEG
moieties of 5a are quite flexible and would easily take the folded
structure shown in Fig. 3b due to C–C and C–O bond rotation.
As indicated by Fig. 3b, the TEG chains of 5a are located in close
proximity of the metal center. Therefore, the TEG moieties of 1a
Scheme 2 Synthesis of rhodium complexes 5a–d.
The molecular structures of 5c and 5d have been successfully
determined by X-ray structural analysis (Fig. 2). Selected bond
lengths and angles are listed in Table 1. In the structures,
each rhodium atom has a distorted square-planar coordination
geometry. The average Rh(1)–C(1) bond lengths of 5c and 5d are
˚
2.025(8) and 2.026(3) A, respectively. These bond lengths are usual
and similar to those of other Rh(I)–NHC complexes.13 As shown
Fig. 2 ORTEP drawings of (a) 5c and (b) 5d with thermal ellipsoids drawn at the 50% probability level. (c) Projection of the crystal structure of 5c; H
atoms are omitted for clarity.
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reaction was carried out in benzene, the yields of the product
were considerably decreased (entries 9–12). Although some solvent
effect was observed, the catalytic activity of 5a–d was not
substantially affected by introducing the long-chain substituents
onto the NHC ligands.
Suzuki–Miyaura coupling
Next, we studied Suzuki–Miyaura coupling reaction using a
palladium catalyst bearing 1a–d as a ligand. Suzuki–Miyaura
coupling is one of the most important and versatile methods for the
construction of carbon–carbon bonds.17 At first, we compared the
effectiveness of the NHC ligands 1a–d in the reaction of 2-bromo-
1,3-dimethylbenzene with phenylboronic acid. The palladium(II)–
3
NHC catalysts [PdCl(g -C3H5)(1a–d)] were generated in situ by
3
the reaction of [PdCl(g -C3H5)]2, 2a–d, and KOtBu.18 The reaction
was carried out in THF at 65 ◦C with K3PO4·H2O as a base
(Table 3). As a result, the palladium catalysts bearing 1a and 1b
afforded the product in 96% and 92% yields, respectively (entries 1
and 2). On the other hand, the NHC ligands 1c and 1d gave the
product only in 18% and 29% yields, respectively (entries 3 and
4). Thus, the NHC ligands bearing the TEG moieties (1a and
1b) were apparently more effective than the ligands without the
TEG moiety (1c and 1d). In toluene, 1a and 1b also afforded the
product in good yields (entries 6 and 7), while 1c and 1d were not
effective, providing the product in much lower yields (entries 8 and
9). The effectiveness of the NHC ligands bearing the TEG chains
(1a and 1b) was also observable in the reaction of various aryl
bromides and arylboronic acids in the presence of KF as a base
in THF at 65 ◦C (Table 4). Unfortunately, the Pd catalyst systems
with 1a–d gave almost no coupling products in the reaction of
4-chlorotoluene with phenylboronic acid.
Fig. 3 Optimized structures of 5a [(a) extended and (b) folded structures]
calculated by ONIOM (B3LYP/LANL2DZ:UFF).
and 1b may bring about a unique catalytic environment in some
catalytic reactions.
Hydrosilylation
We recently reported Rh(I)–NHC complexes with Fre´chet-type
flexible dendritic frameworks, and found that their unique catalytic
environment affected hydrosilylation of ketones.16 To explore
how the long-chain substituents attached to the NHC ligands
work in catalytic reactions, we investigated hydrosilylation of
cyclohexanone using the rhodium complexes 5a–d as catalysts
(Table 2). The reaction was carried out in the presence of a catalytic
amount (0.5 mol%) of 5a–d and diphenylsilane (1.2 equiv.) at room
temperature for 3 h. In THF, the rhodium–NHC complexes 5a–
c bearing TEG and/or n-C12 substituents afforded cyclohexanol
in 53%, 61%, and 65% yields, respectively (entries 1–3). On the
other hand, 5d without such long-chain substituents provided
the product in 49% yield (entry 4). These complexes 5a–d also
showed similar activity in CH2Cl2, providing the product in 61%,
64%, 57%, and 51% yields, respectively (entries 5–8). When the
In this manner, the TEG chains on the NHC ligands enhance
the catalytic activity considerably, even the reaction carried
out in THF. To shed light on a role of TEG chains, control
experiments in the presence of tetraethylene glycol dimethyl
ether (CH3O(CH2CH2O)4CH3: TEGDME) were carried out. As
a result, the addition of TEGDME to the catalyst system with 1d
Table 3 Suzuki–Miyaura coupling of 2-bromo-1,3-dimethylbenzenea
Table 2 Hydrosilylation of cyclohexanone with H2SiPh2 catalyzed by 5.a
Entry
Solvent
THF
Ligand (1)
Yieldb (%)
Entry
Solvent
THF
Catalyst
Yieldb (%)
1
2
3
1a
1b
1c
1d
1d
1a
1b
1c
1d
1d
96 (92c)
92 (90c)
18
29
27
66
68
37
18
1
2
3
4
5
6
7
8
9
5a
5b
5c
5d
5a
5b
5c
5d
5a
5b
5c
5d
53
61
65
49
61
64
57
51
22
19
20
14
4
5d
6
CH2Cl2
Toluene
7
8
9
10d
29
Benzene
10
11
12
a Reaction conditions: [PdCl(g -C3H5)]2 (1 mol%), 2 (2 mol%), KOtBu
(2 mol%), 2-bromo-1,3-dimethylbenzene (1.0 mmol), phen◦ylboronic acid
(1.5 mmol), K3PO4·H2O (2.0 mmol), solvent (1.0 cm3), 65 C, 15 h. b GC
yields. c Isolated yields. d Tetraethylene glycol dimethyl ether (12 mol%)
was added.
3
a Reaction conditions: cyclohexanone (1.0 mmol), diphenylsilane
(1.2 mmol), 5 (0.5 mol%), solvent (2.0 cm3), rt, 3 h. b GC yields.
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Table 4 Suzuki–Miyaura coupling of various aryl bromides and aryl-
with 1a–d, the TEG moieties on the NHC ligands enhance the
catalytic activity considerably. Further catalytic applications of
the novel NHC ligands bearing TEG moieties are currently being
investigated.
boronic acidsa
Experimental
Entry
ArBr
ArꢀB(OH)2
Ligand (1)
Yieldb (%)
General procedures and materials
1
2
3
4
1a
1b
1c
1d
90 (85c)
70
43
All manipulations were performed under an argon atmosphere us-
ing standard Schlenk-type glassware on a dual-manifold Schlenk
line. Reagents and solvents were dried and purified before use by
69
the usual procedures.21 1H NMR and 13C{ H} NMR spectra were
1
1
measured with a JEOL ECX-400 spectrometer. The H NMR
5
6
7
8
1a
1b
1c
1d
88 (85c)
89 (85c)
35
chemical shifts are reported relative to tetramethylsilane (TMS,
0.00 ppm) or residual protiated solvent (7.26 ppm) in CDCl3.
The 13C NMR chemical shifts are reported relative to CDCl3
(77.0 ppm). MALDI-TOF mass spectra were recorded on a Bruker
Autoflex. Elemental analysis was carried out at Center for Organic
Elemental Microanalysis, Graduate School of Pharmaceutical
Science, Kyoto University. TLC analysis was performed on Merck
silica gel 60 F254. Column chromatography was carried out on silica
gel, Wakogel C-200 or Kanto N60 (spherical, neutral, 63–210 lm).
Preparative recycling gel permeation chromatography (GPC) was
performed with a JAI LC9104. GC analysis was carried out using
a Shimadzu GC-17A equipped with an integrator (C-R8A) with
a capillary column (CBP-1 or CBP-5, 0.25 mm i.d. × 25 m). 3a9
and 3b10 (Scheme 1) were prepared by reported methods. 3,4,5-
Trimethoxybenzyl chloride (3c) was purchased from Wako Pure
Chemical.
9
9
10
11
12
1a
1b
1c
1d
65 (53c)
52
33
40
13
14
15
16
1a
1b
1c
1d
66 (60c)
58
21
28
17
18
19
20
1a
1b
1c
1d
84 (79c)
67
41
Synthesis of 2a
69
3
a Reaction conditions: [PdCl(g -C3H5)]2 (1 mol%), 2 (2 mol%), KOtBu
(2 mol%), aryl bromide (1.0 mmol), arylboronic acid (1.5 mmol), KF
(2.0 mmol), THF (1.0 cm3), 65 ◦C, 15 h. b GC yields. c Isolated yields.
A suspension of imidazole (0.136 g, 2.00 mmol), 3a (1.49 g,
2.00 mmol) and CsF–Celite (0.200 g)22 in CH3CN (20 cm3) was
refluxed under Ar for 36 h. After removal of the solvent, CH2Cl2
was added to the residue. The resulting suspension was filtered
through Celite, and the filtrate was concentrated to dryness. After
purification by silica gel column chromatography (10% MeOH–
CHCl3, v/v), the product was further purified by preparative
recycling GPC. Removal of volatiles under vacuum gave 2a
(0.891 g, 0.585 mmol, 59%) as a colorless oil. 1H NMR (400 MHz,
CDCl3, TMS): d 11.17 (s, 1H, Im), 7.26 (s, 2H, Im), 6.83 (s, 4H,
Ar), 5.38 (s, 4H, CH2Im), 4.20 (t, J = 4.5 Hz, 8H, OCH2), 4.14
(t, J = 5.0 Hz, 4H, OCH2), 3.83 (t, J = 4.5 Hz, 8H, OCH2), 3.76
(t, J = 5.0 Hz, 4H, OCH2), 3.71–3.61 (m, 60H, OCH2), 3.55–
3.52 (m, 12H, OCH2), 3.37 (s, 6H, OCH3), 3.36 (s, 12H, OCH3);
(TEGDME : Pd = 6 : 1) did not improve the yields substantially
(Table 3, entries 5 vs. 4 and 10 vs. 9). These results strongly suggest
that the TEG moieties introduced onto the NHC ligands are
important and operate to increase catalytic activity. In the Suzuki–
Miyaura coupling reaction, the transmetallation step would be
crucial.17b,c,19 The TEG moieties can be folded and located in
close proximity to the Pd center in a similar way to that shown
in Fig. 3b.20 Such a catalytic environment may facilitate the
transmetallation step via coordination of the oxygen atoms of
the TEG moieties onto K+ and/or arylboronic acids.
1
13C { H} NMR (100 MHz, CDCl3): d 153.1, 139.1, 137.7, 128.2,
121.5, 109.0, 72.2, 71.8, 70.51, 70.48, 70.45, 70.43, 70.41, 70.35,
70.3, 69.6, 69.0, 58.9, 53.5; MALDI-TOF MS (DIT): m/z 1486
[M − Cl]+; Anal. Calcd for C71H125ClN2O30·2H2O: C, 54.73; H,
8.34. Found: C, 54.42; H, 8.23%.
Conclusions
In summary, we have synthesized and characterized a series of
novel imidazolium salts bearing TEG and/or n-C12 function-
alities (2a–c). Rh(I) complexes with the NHC ligands (1a–d),
[RhCl(COD)(1a–d)] (5a–d), were prepared and the molecular
structure of 5c and 5d were successfully determined by X-ray
structural analysis. In hydrosilylation catalyzed by 5a–d, the
long-chain substituents (TEG and n-C12) did not substantially
change the catalytic activity. On the other hand, in the Suzuki–
Miyaura coupling reaction catalyzed by the Pd catalyst systems
Synthesis of 2c
A suspension of imidazole (34.0 mg, 0.500 mmol), 3b (0.680 g,
1.00 mmol) and CsF–Celite (0.100 g)22 in CH3CN (5.0 cm3)
was refluxed under Ar for 24 h. After removal of the solvent,
CH2Cl2 was added to the residue. The resulting suspension
was filtered through Celite, and the filtrate was concentrated to
382 | Dalton Trans., 2008, 379–385
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1
dryness. After purification by silica gel column chromatography
(10% MeOH–CHCl3, v/v), the product was further purified by
preparative recycling GPC. Removal of volatiles under vacuum
gave 2c (0.450 g, 0.323 mmol, 65%) as a colorless solid. 1H NMR
(400 MHz, CDCl3, TMS): d 11.20 (s, 1H, Im), 7.02 (s, 2H, Im),
6.64 (s, 4H, Ar), 5.40 (s, 4H, CH2Im), 3.97–3.91 (m, 12H, OCH2),
1.81–1.71 (m, 12H, CH2), 1.50–1.22 (m, 108H, CH2), 0.88 (t, J =
2.02 mmol, 81%) as a colorless semisolid. H NMR (400 MHz,
CDCl3, TMS): d 11.32 (s, 1H, Im), 7.16 (s, 1H, Im), 7.02 (s, 1H,
Im), 6.82 (s, 2H, Ar), 6.64 (s, 2H, Ar), 5.41 (s, 2H, CH2Im), 5.36
(s, 2H, CH2Im), 4.19 (t, J = 4.8 Hz, 4H, OCH2), 4.14 (t, J =
5.0 Hz, 2H, OCH2), 3.98–3.92 (m, 6H, OCH2C11H23), 3.82 (t, J =
4.8 Hz, 4H, OCH2), 3.76 (t, J = 5.0 Hz, 2H, OCH2), 3.70–3.61
(m, 30H, OCH2), 3.55–3.52 (m, 6H, OCH2), 3.37 (s, 3H, OCH3),
3.35 (s, 6H, OCH3), 1.82–1.69 (m, 6H, CH2), 1.50–1.43 (m, 6H,
CH2), 1.39–1.21 (m, 48H, CH2), 0.88 (t, J = 6.8 Hz, 9H, CH3);
1
6.8 Hz, 18H, CH3); 13C { H} NMR (100 MHz, CDCl3): d 153.8,
139.1, 138.1, 127.3, 120.8, 107.7, 73.4, 69.5, 54.0, 31.9, 30.3, 29.72,
29.70, 29.68, 29.66, 29.63, 29.57, 29.43, 29.38, 29.36, 29.3, 26.10,
26.07, 22.7, 14.1; MALDI-TOF MS (DIT): m/z 1354 [M − Cl]+;
Anal. Calcd for C89H161ClN2O6·H2O: C, 75.88; H, 11.66. Found:
C, 76.08; H, 11.77%.
13C { H} NMR (100 MHz, CDCl3): d 153.8, 153.1, 139.3, 139.0,
1
137.9, 128.1, 127.2, 121.3, 120.9, 109.1, 107.7, 73.4, 72.2, 71.83,
71.81, 70.6, 70.52, 70.50, 70.47, 70.45, 70.43, 70.40, 70.37, 69.6,
69.4, 69.1, 58.92, 58.89, 53.9, 53.6, 31.9, 30.3, 29.67, 29.65, 29.64,
29.62, 29.58, 29.5, 29.4, 29.33, 29.31, 29.29, 26.1, 26.0, 22.6, 14.0;
MALDI-TOF MS (DIT): m/z 1420 [M − Cl]+; Anal. Calcd for
C80H143ClN2O18: C, 65.97; H, 9.90. Found: C, 65.67; H, 9.92%.
Synthesis of 2d
A
suspension of imidazole (1.70 g, 25.0 mmol), 3,4,5-
trimethoxybenzyl chloride (3c: 10.8 g, 50.0 mmol) and CsF–
Celite (5.00 g)22 in CH3CN (250 cm3) was refluxed under Ar
for 24 h. After removal of the solvent, CH2Cl2 was added to
the residue. The resulting suspension was filtered through Celite,
and the filtrate was concentrated to dryness. After purification by
silica gel column chromatography (10% MeOH–CHCl3, v/v), the
product was recrystallized from CH3CN. The resulting crystals
were collected, washed with Et2O, and dried under vacuum to
afford 2d (7.21 g, 15.5 mmol, 62%) as colorless crystals. 1H NMR
(400 MHz, CDCl3, TMS): d 11.04 (s, 1H, Im), 7.28 (d, J =
1.4 Hz, 2H, Im), 6.80 (s, 4H, Ar), 5.44 (s, 4H, CH2Im), 3.87
Synthesis of [RhCl(COD)(1a)] (5a)
A suspension of 2a (0.944 g, 0.620 mmol) and silver oxide
(71.8 mg, 0.310 mmol) in 1,2-dichloroethane (62 cm3) was stirred
◦
at 50 C for 12 h under Ar. After cooling to room temperature,
[RhCl(COD)]2 (153 mg, 0.310 mmol) was added to the reaction
mixture. The resulting mixture was stirred at room temperature
for 24 h under Ar. After removal of the solvent, CH2Cl2 was
added to the residue. The resulting suspension was filtered through
Celite, and the filtrate was concentrated to dryness. The residue
was purified by silica gel column chromatography. The first band
eluting with CH2Cl2 contained [RhCl(COD)]2 and was discarded.
The second yellow band eluting with 5% MeOH–CH2Cl2 (v/v)
was collected, evaporated, and dried under vacuum to afford 5a
(0.902 g, 0.521 mmol, 84%) as a yellow oil. 1H NMR (400 MHz,
CDCl3): d 6.68 (s, 4H, Ar), 6.66 (s, 2H, NHC), 5.95 (d, J = 14.5 Hz,
2H, CH2-NHC), 5.40 (d, J = 14.5 Hz, 2H, CH2-NHC), 5.03 (br,
2H, COD), 4.18–4.08 (m, 12H, OCH2), 3.81 (t, J = 5.0 Hz, 8H,
OCH2), 3.76 (t, J = 5.2 Hz, 4H, OCH2), 3.71–3.67 (m, 12H,
OCH2), 3.66–3.61 (m, 48H, OCH2), 3.54–3.51 (m, 12H, OCH2),
3.36 (s, 6H, OCH3), 3.35 (s, 12H, OCH3), 3.31 (br, 2H, COD), 2.34
1
(s, 12H, CH3), 3.82 (s, 6H, CH3); 13C { H} NMR (100 MHz,
CDCl3): d 153.8, 138.7, 137.6, 128.3, 121.4, 106.4, 60.7, 56.5, 53.7;
MALDI-TOF MS (DIT): m/z 429 [M − Cl]+; Anal. Calcd for
C23H29ClN2O6·H2O: C, 57.20; H, 6.47. Found: C, 56.91; H, 6.18%.
Synthesis of 4
A mixture of imidazole (1.43 g, 21.0 mmol), 3b (14.3 g, 21.0 mmol),
K2CO3 (2.90 g, 21.0 mmol), KOH (1.18 g, 21.0 mmol) and
tetra(n-butyl)ammonium bromide (339 mg, 1.05 mmol) in toluene
(210 cm3) was refluxed under Ar for 24 h. After cooling to room
temperature, the suspension was filtered through Celite and the
filtrate was concentrated to dryness. The residue was purified by
silica gel column chromatography (3% MeOH–CH2Cl2, v/v) to
(br, 4H, COD), 1.90 (br, 4H, COD); 13C { H} NMR (100 MHz,
1
CDCl3): d 183.0 (d, JRh-C = 50.5 Hz, NCN), 152.8, 138.2, 131.8,
120.8, 107.9, 98.8 (d, JRh-C = 6.7 Hz, COD), 72.2, 71.9, 70.7,
70.59, 70.57, 70.53, 70.51, 70.45, 70.41, 69.7, 69.0, 68.8 (d, JRh-C
=
1
give 4 (11.4 g, 16.1 mmol, 77%) as a colorless solid. H NMR
14.3 Hz, COD), 59.0, 54.6 (CH2-NHC), 32.9 (COD), 28.8 (COD);
MALDI-TOF MS (DIT): m/z 1696 [M − Cl]+; Anal. Calcd for
C79H136ClN2O30Rh: C, 54.77; H, 7.91. Found: C, 54.99; H, 8.15%.
(400 MHz, CDCl3, TMS): d 7.53 (s, 1H, Im), 7.08 (t, J = 1.2 Hz,
1H, Im), 6.90 (t, J = 1.2 Hz, 1H, Im), 6.31 (s, 2H, Ar), 5.00 (s,
2H, CH2Im), 3.94–3.87 (m, 6H, OCH2), 1.80–1.69 (m, 6H, CH2),
1.50–1.41 (m, 6H, CH2), 1.40–1.24 (m, 48H, CH2), 0.88 (t, J =
1
6.8 Hz, 9H, CH3); 13C { H} NMR (100 MHz, CDCl3): d 153.5,
Synthesis of [RhCl(COD)(1b)] (5b)
138.1, 137.4, 131.0, 129.7, 119.2, 105.8, 73.4, 69.2, 51.0, 31.93,
31.91, 30.3, 29.73, 29.71, 29.68, 29.65, 29.62, 29.58, 29.4, 29.3,
26.10, 26.05, 22.7, 14.1; MALDI-TOF MS (DIT): m/z 712 [M +
H]+; Anal. Calcd for C46H82N2O3: C, 77.69; H, 11.62. Found: C,
77.95; H, 11.53%.
The complex was synthesized with 2b (0.874 g, 0.600 mmol) by a
similar method to that used for 5a. After purification by silica gel
column chromatography, removal of the volatiles under vacuum
1
gave 5b (0.978 g, 0.587 mmol, 98%) as a yellow oil. H NMR
(400 MHz, CDCl3): d 6.69 (s, 2H, Ar), 6.68 (d, J = 2.0 Hz, 1H,
NHC), 6.65 (d, J = 2.0 Hz, 1H, NHC), 6.62 (s, 2H, Ar), 6.03 (d, J =
14.5 Hz, 1H, CH2-NHC), 5.87 (d, J = 14.5 Hz, 1H, CH2-NHC),
5.50 (d, J = 14.5 Hz, 1H, CH2-NHC), 5.34 (d, J = 14.5 Hz, 1H,
CH2-NHC), 5.04 (br, 2H, COD), 4.19–4.09 (m, 6H, OCH2), 3.99–
3.90 (m, 6H, OCH2C11H23), 3.81 (t, J = 5.0 Hz, 4H, OCH2), 3.77
(t, J = 5.2 Hz, 2H, OCH2), 3.71–3.68 (m, 6H, OCH2), 3.66–3.62
Synthesis of 2b
A solution of 4 (1.78 g, 2.50 mmol) and 3a (1.86 g, 2.50 mmol)
in CH3CN (25 cm3) was refluxed under Ar for 57 h. The solvent
was removed in vacuo, and the residue was purified by preparative
recycling GPC. Removal of volatiles under vacuum gave 2b (2.94 g,
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Dalton Trans., 2008, 379–385 | 383
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(m, 24H, OCH2), 3.55–3.52 (m, 6H, OCH2), 3.364 (s, 3H, OCH3),
3.359 (s, 6H, OCH3), 3.32 (br, 2H, COD), 2.35 (br, 4H, COD),
1.90 (br, 4H, COD), 1.79–1.69 (m, 6H, CH2), 1.48–1.41 (m, 6H,
CH2), 1.34–1.22 (m, 48H, CH2), 0.87 (t, J = 6.8 Hz, 9H, CH3);
General procedure for hydrosilylation
A rhodium catalyst 5a–d (0.005 mmol) was placed in a 20 cm3
Schlenk tube under an Ar atomosphere. Anhydrous THF (2.0 cm3)
and cyclohexanone (1.0 mmol) were added by syringe under
an Ar flow. After the reaction mixture was stirred for 5 min,
diphenylsilane (1.2 mmol) was added via a syringe. The reaction
mixture was stirred at room temperature for 3 h. The desilylation
was performed with HCl–MeOH (2 M, 1.0 cm3) and the mixture
stirred for 1 h. After tridecane (0.25 mmol) as an internal standard
was added, the reaction mixture was diluted with Et2O (10 cm3).
The yield of the product was determined by GC analysis relative
to an internal standard.
1
13C { H} NMR (100 MHz, CDCl3): d 183.0 (d, JRh-C = 51.5 Hz,
NCN), 153.4, 152.9, 138.2, 138.0, 131.9, 131.4, 120.9, 120.6, 108.0,
107.0, 98.7 (brm, COD), 73.4, 72.3, 71.9, 70.7, 70.62, 70.60, 70.56,
70.54, 70.48, 70.4, 69.7, 69.3, 69.0, 68.28 (d, JRh-C = 14.3 Hz,
COD), 68.25 (d, JRh-C = 14.3 Hz, COD), 59.0, 54.8 (CH2-NHC),
54.6 (CH2-NHC), 32.91 (COD), 32.86 (COD), 31.91, 31.89, 30.3,
29.73, 29.71, 29.69, 29.66, 29.64, 29.60, 29.5, 29.42, 29.36, 29.3,
28.8 (br, COD), 26.14, 26.11, 22.7, 14.1; MALDI-TOF MS (DIT):
m/z 1630 [M − Cl]+; Anal. Calcd for C88H154ClN2O18Rh: C, 63.42;
H, 9.31. Found: C, 63.53; H, 9.62%.
General procedure for Suzuki–Miyaura coupling
3
A mixture of 2a–d (0.02 mmol), KOtBu (0.02 mmol) and [PdCl(g -
Synthesis of [RhCl(COD)(1c)] (5c)
C3H5)]2 (0.01 mmol) in anhydrous THF (1.0 cm3) was stirred
at room temperature for 30 min under an Ar atomosphere. The
reaction mixture was concentrated to dryness. Almost quantitative
A suspension of 2c (0.695 g, 0.500 mmol) and silver oxide (57◦.9 mg,
0.250 mmol) in 1,2-dichloroethane (50 cm3) was stirred at 50 C for
12 h under Ar. After cooling to room temperature, [RhCl(COD)]2
(123 mg, 0.250 mmol) was added to the reaction mixture. The
resulting mixture was stirred at room temperature for 24 h under
Ar. After removal of the solvent, CH2Cl2 was added to the residue.
The resulting suspension was filtered through Celite, and the
filtrate was concentrated to dryness. The residue was purified
by silica gel column chromatography (eluting with CH2Cl2). The
first band containing [RhCl(COD)]2 was discarded. The second
yellow band was collected, evaporated, and dried under vacuum
3
formation of the palladium complex [PdCl(g -C3H5)(1a–d)] was
confirmed by 1H NMR analysis (see ESI‡). Arylboronic acid
(1.5 mmol) and KF (2.0 mmol) were added to the residue under
Ar flow. Anhydrous THF (1.0 cm3) and aryl bromide (1.0 mmol)
was added via a syringe. The resultant suspension was stirred at
room temperature for 10 min. Then, the reaction was carried out
at 65 ◦C for 15 h. After cooling to room temperature, tridecan
(0.205 mmol) as an internal standard was added. The mixture was
diluted with ethyl acetate (15 cm3) and filtered through a pad of
Celite. The yield of the product was determined by GC analysis
relative to an internal standard.
1
to afford 5c (0.677 g, 0.423 mmol, 85%) as a yellow solid. H
NMR (400 MHz, CDCl3): d 6.67 (s, 2H, NHC), 6.64 (s, 4H, Ar),
5.99 (d, J = 14.3 Hz, 2H, CH2-NHC), 5.40 (d, J = 14.3 Hz, 2H,
CH2-NHC), 5.06 (br, 2H, COD), 4.00–3.90 (m, 12H, OCH2), 3.33
(br, 2H, COD), 2.36 (br, 4H, COD), 1.91 (br, 4H, COD), 1.79–
1.70 (m, 12H, CH2), 1.50–1.40 (m, 12H, CH2), 1.38–1.20 (m, 96H,
X-Ray diffraction study
Single crystals of [RhCl(COD)(1c)] (5c) and [RhCl(COD)(1d)]
(5d) suitable for X-ray diffraction study were obtained by diffusion
of methanol into 5c or 5d in CH2Cl2. Data were collected
on a Rigaku/Saturn70 CCD diffractometer using graphite-
CH2), 0.88 (t, J = 6.6 Hz, 18H, CH3); 13C { H} NMR (100 MHz,
1
CDCl3): d 183.0 (d, JRh-C = 50.5 Hz, NCN), 153.4, 138.0, 131.4,
120.7, 107.0, 98.7 (d, JRh-C = 6.7 Hz, COD), 73.4, 69.3, 68.2 (d,
JRh-C = 14.3 Hz, COD), 54.8 (CH2-NHC), 32.9 (COD), 31.9, 30.3,
29.74, 29.72, 29.70, 29.67, 29.65, 29.61, 29.5, 29.44, 29.38, 29.36,
28.8 (COD), 26.2, 26.1, 22.7, 14.1; MALDI-TOF MS (DIT): m/z
1564 [M − Cl]+; Anal. Calcd for C97H172ClN2O6Rh: C, 72.78; H,
10.83. Found: C, 72.44; H, 10.92%.
˚
monochromated Mo-Ka radiation (k = 0.71070 A) at 153 K, and
processed using CrystalClear23 (Rigaku). The structures of 5c and
5d were solved by direct methods (SIR200224 and SHELX97,25
respectively) and refined by full-matrix least-squares refinement
on F2. The non-hydrogen atoms were refined anisotropically. All
hydrogen atoms were located at calculated positions and not re-
fined. All calculations were performed using the CrystalStructure26
crystallographic software package.
Synthesis of [RhCl(COD)(1d)] (5d)
The complex was synthesized with 2d (0.465 g, 1.00 mmol) by
a similar method to that used for 5a. After purification by silica
gel column chromatography, removal of volatiles under vacuum
Crystal data for 5c. C97H172N2O6ClRh, M = 1600.79, T =
¯
˚
◦
153 K, triclinic, space group P1 (No. 2), a = 10.179(10) A, b =
1
◦
gave 5d (0.595 g, 0.881 mmol, 88%) as a yellow solid. H NMR
˚
˚
12.521(16) A, c = 41.57(16) A, a = 84.3(9) , b = 88.4(9) , c =
◦
3
−1
(400 MHz, CDCl3): d 6.75 (s, 4H, Ar), 6.69 (s, 2H, NHC), 6.26
(d, J = 14.5 Hz, 2H, CH2-NHC), 5.25 (d, J = 14.5 Hz, 2H, CH2-
NHC), 5.07 (br, 2H, COD), 3.84 (s, 12H, CH3), 3.82 (s, 6H, CH3),
˚
61.4(3) , U = 4629(19) A , Z = 2, l(Mo-Ka) = 2.641 cm , unique
reflections 21826 (Rint = 0.077), observed reflections 8854 (I >
3r(I)), R1, wR2 = 0.0645, 0.1684. GOF = 0.939.
3.36 (br, 2H, COD), 2.39 (br, 4H, COD), 1.95 (br, 4H, COD); 13
C
1
{ H} NMR (100 MHz, CDCl3): d 183.1 (d, JRh-C = 52.5 Hz, NCN),
153.5, 137.8, 132.0, 120.8, 105.7, 99.0 (d, JRh-C = 7.6 Hz, COD),
68.3 (d, JRh-C = 14.3 Hz, COD), 60.8, 56.4, 54.7 (CH2-NHC), 32.9
(COD), 28.8 (COD): MALDI-TOF MS (DIT): m/z 639 [M −
Cl]+; Anal. Calcd for C31H40ClN2O6Rh: C, 55.16; H, 5.97. Found:
C, 55.05; H, 5.86%.
Crystaldata for 5d. C31H40N2O6ClRh, M = 675.03, T = 153 K,
˚
monoclinic, space group P21/c (No. 14), a = 9.9534(4) A, b =
◦
3
˚
˚
˚
22.3490(9) A, c = 13.4724(6) A, b = 96.826(3) , U = 2975.7(2) A ,
Z = 4, l(Mo-Ka) = 7.088 cm−1, unique reflections 6767 (Rint
=
0.048), observed reflections 6767 (all data), R1, wR2 = 0.0425,
0.1150. GOF = 1.002.
384 | Dalton Trans., 2008, 379–385
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The Royal Society of Chemistry 2008
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Acknowledgements
This work was supported by CREST, Japan Science and Techno-
logy Agency.
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©