Rhodium(iii) complexes with a bidentate N-heterocyclic carbene ligand bearing flexible dendritic frameworks

Article abstract of DOI:10.1039/b701104k

Rh(iii) complexes with a bidentate N-heterocyclic carbene ligand bearing flexible dendritic frameworks have been synthesized and fully characterized by X-ray crystallographic analysis, NMR measurements and theoretical calculations. The Royal Society of Chemistry.

Full text of DOI:10.1039/b701104k

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Rhodium(III) complexes with a bidentate N-heterocyclic carbene ligand
bearing flexible dendritic frameworks†‡
Tetsuaki Fujihara,a Yasushi Obora,b Makoto Tokunagab and Yasushi Tsuji*a,b
Received 24th January 2007, Accepted 6th March 2007
First published as an Advance Article on the web 20th March 2007
DOI: 10.1039/b701104k
Rh(III) complexes with a bidentate N-heterocyclic carbene
ligand bearing flexible dendritic frameworks have been syn-
thesized and fully characterized by X-ray crystallographic
analysis, NMR measurements and theoretical calculations.
dendrimer-bromides, Gn–Br (n = 0, 1 and 2) as shown in eqn
(1).11 Subsequently, the reaction of 1a–c with CH2I2 in toluene
afforded the corresponding imidazolium salts [(G0)(C3H3N2)-
CH2(C3H3N2)(G0)]I2 (2a),9a [(G1)(C3H3N2)CH2(C3H3N2)(G1)]I2
(2b), and [(G2)(C3H3N2)CH2(C3H3N2)(G2)]I2 (2c) (eqn (2)). The
new compounds (1b, 1c, 2b and 2c) were fully characterized by
elemental analysis, FD-MS spectra and NMR measurements.
Rh(III) complexes with the NHC ligand RhI2(OAc)[(G0)-
(C3H2N2)CH2(C3H2N2)(G0)] (3a), RhI2(OAc)[(G1)(C3H2N2)-
CH2(C3H2N2 )(G1 )] (3b), and RhI2(OAc)[(G2 )(C3H2N2 )CH2 -
(C3H2N2)(G2)] (3c) were synthesized by the reaction of 2a–2c
with [RhCl(COD)]2 in the presence of KI and KOAc (eqn (3)).10a
In addition, a methyl derivative RhI2(OAc)[Me(C3H2N2)CH2-
(C3H2N2)Me] (3d) was also synthesized with the same method
using [Me(C3H3N2)CH2(C3H3N2)Me]I2.12 The new complexes
Much attention has been paid to dendrimers with organometallic
centers since the well-defined hyperbranched frameworks will
bring about unique chemical properties including novel catalysis.1,2
We have reported various dendritic ligands with pyridine,3
phosphine4 and N-heterocyclic carbene (NHC)5,6 functionalities
at the core and used as transition-metal-catalyzed reactions.
Recently, we have synthesized a series of Rh(I) complexes with
a monodentate NHC ligand bearing Fre´chet-type7 flexible den-
dritic frameworks. In the hydrosilylation of ketones catalyzed by
them, yields of the corresponding alcohols increased with higher
dendrimer generation: a positive dendrimer effect was evident.5
Furthermore, Rh(I) complexes with a NHC ligand having a rigid
2,3,4,5-tetraphenylphenyl and its higher dendritic frameworks
were efficient catalysts in the hydrosilylation of a,b-unsaturated
ketones.6
The NHC ligands have been widely used in the field of
homogeneous catalysis and organometallic chemistry.8 Formation
of stronger bonds to the metals associated with NHC ligands
as compared with phosphines diminish their dissociation from
metal centers. Among them, bidentate NHC ligands are of special
interest since stability of their metal complexes is considerably en-
hanced by the chelating effect.9 However, examples of the bidentate
NHCligandsareverylimitedas comparedtothoseof monodentate
NHC cases. As for Rh(III) complexes,10 the complexes are efficient
catalysts in hydrosilylation10a and transfer hydrogenation.10b In
this study, novel Rh(III) complexes with a bidentate NHC ligand
having Fre´chet-type7 flexible polybenzylether dendrimers were
synthesized and fully characterized. Then, these complexes were
employed as catalysts in the hydrosilylation of 2-cyclohexen-1-one
and good 1,4-selectivity was observed.
1
(3a–3d) were also fully characterized. In 13C{ H} NMR spectra
of 3a–3d, the coordinated carbene carbons appeared as sharp
doublets at d = 151.5 ppm (JRh–C = 41.6 Hz), 155.0 ppm (41.6 Hz),
154.8 ppm (41.7 Hz), and 151.3 ppm (43.1 Hz), respectively. The
isolated complexes were highly stable both to air and moisture in
the solid state. The 1H NMR spectra of 3a–3d did not change over
a week, indicating these complexes are also considerably stable in
solution.
(1)
A series of imidazolium salts, which are precursors for the
desired bidentate NHC ligands, were synthesized in two steps.
At first, N-substituted imidazoles bearing the Fre´chet-type7
polybenzylether dendrimers (1a–c) were synthesized by the re-
action of imidazole with one equivalent of the corresponding
aDepartment of Energy and Hydrocarbon Chemistry, Graduate School of
Engineering, Kyoto University and CREST, Japan Science and Technology
Agency (JST), Kyoto, 615-8510, Japan. E-mail: ytsuji@scl.kyoto-u.ac.jp;
Fax: +81 75 383 2514
bCatalysis Research Center, Hokkaido University, Sapporo, 001-0021, Japan
† The HTML version of this article has been enhanced with additional
colour images.
(2)
‡ Electronic supplementary information (ESI) available: Synthetic method
and analytical data of 1, 2 and 3. See DOI: 10.1039/b701104k
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(3)
The molecular structures of 3a, 3b and 3d have been successfully
determined by X-ray diffraction study. Fig. 1 shows the ORTEP
drawing of 3a, 3b and 3d.§¶ In the structures, each rhodium atom
has distorted octahedral coordination geometry. The average Rh–
C bond lengths of 3a, 3b and 3d were 1.962(4), 1.969(4) and
˚
1.965(3) A, respectively. These bond lengths are similar to those of
Fig. 2 1H NMR spectra of (a) 3a, (b) 3b and (c) 3c in CDCl3.
other Rh(III)–NHC complexes.10 Fig. 1(b) showed that in the solid
state the benzylether moiety on the NHC ligand spatially spreads
out and covers the wide area from the Rh center.
and the NHC ring protons was observed, such up-field shifts
might be caused by the ring-current effect of aromatic rings of
the dendritic framework: the second generation dendritic moieties
would then come closer to the NHC center. Actually, ONIOM13
(B3LYP/LANL2DZ:UFF)-CAChe14/CONFLEX15/MM316 cal-
culations afforded the significantly folded structure shown in
Fig. 3. It is noteworthy that this structure is quite different from the
open structure in Fig. 1(b) determined by X-ray crystallography.
Fig. 1 ORTEP drawings of (a) 3a, (b) 3b and (c) 3d with thermal ellipsoids
at 50% probability levels.
Fig. 2 showed the 1H NMR spectra of 3a–3c in CDCl3. All the
1H and 13C NMR resonances were unambiguously assigned with
1
the aid of H–H COSY and HSQC spectroscopy. The H NMR
Fig.
3 An optimized structure of 3c calculated by ONIOM
spectrum of 3a exhibited the NHC ring proton resonances as two
doublet peaks at d = 6.98 and 6.82 ppm (J = 1.8 Hz; signals a
and b in Fig. 2(a)). A bridging methylene proton resonance was
observed at d = 6.05 ppm (signal c in Fig. 2(a)). Similarly, the 1H
NMR spectrum of 3b showed signals of two NHC ring protons
at d = 6.92 and 6.84 ppm, and the bridging methylene proton
(B3LYP/LANL2DZ:UFF)-CAChe/CONFLEX/MM3. The yellow part
indicates the RhI2(OAc)[(C3H2N2)CH2(C3H2N2)] core.
As a probe reaction to investigate the catalytic activity
of 3, hydrosilylation of 2-cyclohexen-1-one17 was carried out
(Scheme 1).* As catalysts, 3a, 3b and 3c showed high catalytic
activity to afford mixtures of cyclohexanone (1,4-adduct: 4) and
2-cyclohexen-1-ol (1,2-adduct: 5) in 91–99% yields with the 1,4-
adduct as a major product (79–81% selectivities). Although the
yield and 1,4-regioselectivity decreased, the corresponding methyl
derivative (3d) also showed moderate catalytic activity. Recently,
we reported that the 1,2-adduct (5) was obtained as the major
product (61–69% selectivities in 62–83% total yields) in the
same hydrosilylation of 2-cyclohexen-1-one catalyzed by Rh(I)
complexes having the monodentate NHC ligand: RhCl(COD)-
[(Gn)2(C3H2N2)] (Gn: n = 0–3).6 With the bidentate NHC ligand,
1
at d = 5.95 ppm (Fig. 2(b)). However, the H NMR spectrum
of the second generation (G2) complex 3c displayed the NHC
ring proton resonances at d = 6.68 and 6.62 ppm, and a bridging
methylene proton at d = 5.58 ppm (Fig. 2(c)). In the spectrum,
one of the NHC ring proton resonance (signal a) and the bridging
methylene proton resonance (signal c) showed considerable up-
field shifts. The latter peak (signal c) overlapped with the methylene
resonance of the benzyl moiety on the NHC ring (signal d in
Fig. 2(c)). Interestingly, these shifts were not observed in the
1H NMR spectrum of the corresponding imidazolium salt 2c.
Although no noe interaction between the aromatic ring protons
1568 | Dalton Trans., 2007, 1567–1569
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Scheme 1 Hydrosilylation of 2-cyclohexen-1-one catalyzed by 3. The
products were obtained after hydrolysis.
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the dendrimer moiety of higher generation must affect the catalytic
environment as shown in Fig. 2 and 3. However, almost no effect
of the dendrimer generation was observed in the hydrosilylation as
shown in Scheme 1. In the catalytic reaction, the robust chelating
structures of 3 are more important and successfully realized the
good 1,4-regioselectivity.
Notes and references
§ Crystaldata for RhI2(OAc)[(G0)(C3H2N2)CH2(C3H2N2)(G0)]·C4H8O
(3a·THF): C27H31I2N4O3Rh, M = 816.28, T = 113 K, monoclinic, space
˚
group P2◦1/c (No. 14), a = 12.126(5), b = 14.786(6), c = 15.987(6) A, b =
3
92.446(6) , U = 2863(2) A , Z = 4, l(Mo Ka) = 2.78 cm−1, observed
˚
reflections 5511 (I>2r(I)), R1, wR2 (all data) = 0.042, 0.098. For
RhI2(OAc)[(G1)(C3H2N2)CH2(C3H2N2)(G1)]·CH3CN (3b·CH3CN):
¯
C53H50I2N5O6Rh, M = 1209.72, T = 113 K, triclinic, space group P1
˚
(No. 2), a = 9.338(4), b = 13.972(6), c = 20.243(8) A, a = 20.243(8), b =
◦
3
−1
˚
85.57(1), c = 78.379(9) , U = 2461(2) A , Z = 2, l(Mo Ka) = 1.66 cm
,
observed reflections 8789 (I>2r(I)), R1, wR2 (all data) = 0.043, 0.102.
For RhI2(OAc)[Me(C3H2N2)CH2(C3H2N2)Me] (3d): C11H15I2N4O2Rh,
M = 591.98, T = 113 K, monoclinic, space group Cc (No. 5), a = 9.685(2),
◦
3
˚
˚
b = 13.321(3), c = 12.691(3) A, b = 94.340(2) , U = 1632.6(7) A , Z = 4,
l(Mo Ka) = 4.83 cm−1, Flack parameter = 0.222(16), observed reflections
2074 (I>2r(I)), R1, wR2 (all data) = 0.019, 0.059.
¶ CCDC reference numbers 633257, 633258 and 633259. For crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b701104k
* General procedure for hydrosilylation of 2-cyclohexen-1-one. A rhodium
catalyst 3a (3.7 mg, 0.005 mmol) was placed in a 10-cm3 Schlenk tube
under an Ar atmosphere. Anhydrous THF (1.0 cm3), 2-cyclohexen-1-one
(9.7 × 10−2 dm3, 1 mmol), and bibenzyl (45.6 mg, 0.25 mmol as an internal
standard) were added under an Ar flow and the solution was stirred for
5 min. Ph2SiH2 (0.22 dm3, 1.2 mmol) was added via a syringe and the
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reaction mixture was stirred at room temperature (22
1
◦C) for 20 h.
The desilylation was performed by adding K2CO3 (ca. 1 mg) and MeOH
(1 cm3), then the mixture was stirred for 30 min. A total yield (99%) and
a ratio (79/21) of cyclohexanone and 2-cyclohexen-1-ol were determined
by a GC (Shimadzu CPB10 column, 25 m length, 0.25 mm i.d.).
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This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 1567–1569 | 1569
©