Ruthenium-based NHC-arene systems as ring-opening metathesis polymerisation catalysts

Article abstract of DOI:10.1002/ejic.200700665

The availability of starting materials, the complexity of the preparative routes and the cost to make the catalyst are key in the development of industrially relevant olefin metathesis catalysts. With this in mind, new synthetic routes, which lead to alte

Full text of DOI:10.1002/ejic.200700665

FULL PAPER
DOI: 10.1002/ejic.200700665
Ruthenium-Based NHC-Arene Systems as Ring-Opening Metathesis
Polymerisation Catalysts
Nele Ledoux,*[a] Bart Allaert,[a] and Francis Verpoort[a]
Keywords: Carbene ligands / Homogeneous catalysis / Olefin polymerisation / Ruthenium
The availability of starting materials, the complexity of the
preparative routes and the cost to make the catalyst are key
in the development of industrially relevant olefin metathesis
catalysts. With this in mind, new synthetic routes, which lead
ityl-4,5-dihydroimidazol-2-ylidene) was found to be unat-
tainable, bidentate NHC analogues were synthesized in-
stead. These are O-hydroxyaryl-substituted NHCs, capable
of binding with the metal centre through the oxygen atom as
to alternatives for the classic ruthenium benzylidene com- well as through the carbene carbon atom. Their chelating
plexes and circumvent the need for patented Grubbs inter-
mediates were explored. The research presented herein fo-
properties improved the stability of the corresponding ruthe-
nium arene complexes, which allowed more successful isola-
cuses on the coordination of “saturated” N-heterocyclic car- tion.
bene (NHC) ligands to Ru dimer [(p-cymene)RuCl2]2. As the
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
coordination of the standard NHC ligand H2IMes (1,3-dimes-
Germany, 2007)
of which catalyst 1 is the most popular, proceed through
bis(phosphane) benzylidene systems.[31,32] A more direct
pathway with the use of moderate reaction conditions and
readily available starting materials is quite desirable. This
also involves the need for alternative means to introduce
the alkylidene moiety, which avoid the quite cumbersome
preparative routes by diazo compounds.[33] In this context,
the Ru dimer [(p-cymene)RuCl2]2 (2) is air and moisture
stable, and it is an easy to handle precursor that is an ideal
starting material for the synthesis of allenylidene or vinyl-
idene catalysts to be used for metathesis.[34–44] In order to
find a resembling alternative for the classic Grubbs catalyst
1, it would be necessary to coordinate H2IMes (1,3-di-
mesityl-4,5-dihydroimidazol-2-ylidene) to Ru dimer 2.
Whereas several groups claim to have synthesized [(p-cy-
Introduction
Olefin metathesis is a fundamental catalytic process and
stands among the most prominent ways to build carbon–
carbon bonds.[1–6] The reaction consists of the cleavage and
reformation of alkene double bonds with a simultaneous
exchange of substituents. Cross metathesis (CM),[7,8] ring-
closing metathesis (RCM),[9] ring-opening metathesis poly-
merisation (ROMP)[10] and acyclic diene metathesis poly-
merisation (ADMET),[11] illustrate the versatility of the ole-
fin metathesis reaction. Thanks to the continuous improve-
ment of its initiators and a sharpened comprehension of its
reaction mechanism,[12–30] the olefin metathesis transforma-
tion has been upgraded to a very practical, efficient and
flexible synthetic methodology. Olefin metathesis now al-
lows cleaner, more efficient and less expensive industrial
production of polymers, fine chemicals, pesticides and phar-
maceutical intermediates. Despite the recent advances, the
search for commercially relevant catalyst systems remains
challenging. When thinking of commercial applications,
one should not only focus on catalyst activity, selectivity or
stability, as highly developed initiators are sometimes too
expensive for applications on an industrial scale. Other im-
portant aspects are the availability of the starting materials,
the complexity of the preparative routes and the cost to
make the catalyst. In addition, it is often relevant to search
for patent-free synthetic strategies. Up to now, most syn-
thetic pathways toward Grubbs 2nd generation complexes,
[a] Department of Inorganic and Physical Chemistry, Laboratory
of Organometallic Chemistry and Catalysis, Ghent University,
Krijgslaan 281 (S-3), 9000 Ghent, Belgium
Fax: +32-9-264-4983
E-mail: nele.ledoux@gmail.com
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Ru-Based NHC-Arene Systems as ROM Polymerisation Catalysts
mene)(H2IMes)RuCl2] in situ, it was never isolated, nor stereoisomers. Complex 7 shows high stability and can be
fully characterized.[45,46] Driven by the fascinating challenge handled in air without any sign of decomposition. To ex-
to find alternative synthetic strategies, we herein describe plore its thermal stability, a solution of 7 (16.5 mg,
the endeavour we undertook to contribute to this quest.
0.030 mmol) and coronene (internal standard, 9.0 mg,
0.030 mmol) in deuterated chlorobenzene (C6D5Cl,
0.50 mL) was followed over time at 100 °C by using 1H
NMR spectroscopy. After 48 h, no sign of decomposition
was observed. At 120 °C, the complex fully decomposed
within 4 h.
Results and Discussion
Whereas the coordination of IMes (1,3-dimesitylimid-
azol-2-ylidene) to Ru dimer 2 proceeds readily by standard
synthetic strategies (complex 4),[38,47] analogous binding of
H2IMes was found to be more problematic. Regardless of
the reaction conditions applied, a mixture of Ru complexes
was obtained (Scheme 1). Only when the chloroform adduct
H2IMes(H)(CCl3) was used as a carbene precursor could a
small rate of N-heterocyclic carbene (NHC) coordination
be evidenced through 13C NMR spectroscopy. A 13C NMR
resonance was found at δ = 202.5 ppm, which is characteris-
tic for the carbene carbon atom coordinated to the metal
centre. However, 1H NMR spectroscopic analysis of the re-
action products revealed that desired complex 5 was only
formed as a minor product together with undefined hyd-
ridic species. 1H NMR spectroscopic resonances were found
at δ = –3.64, –3.98, –5.09 and –5.62 ppm.
Scheme 2. Synthesis of NHC-arene complex 7.
Dark-red crystals of 7 suitable for X-ray structure analy-
sis were grown by slow evaporation of a toluene/CH2Cl2
solution, and its molecular structure is represented in Fig-
ure 1. The molecular species consists of a p-cymene ligand
showing η6-bonding, an anionic chloride ligand and a bi-
dentate (C,O) chelating NHC ligand. The isopropyl group
on the arene ligand is a little distorted away from the metal
centre as a result of steric factors.
Scheme 1. Synthesis of [(p-cymene)(H2IMes)RuCl2].
Figure 1. The molecular structure of 7; ellipsoids show 50% prob-
Because the failure in our efforts to isolate 5 was assigned
to a lack of stability of the complex, a bidentate analogue
of the NHC ligand was synthesized. The chelating proper-
ability.
The catalytic activity of 7 was evaluated in the ROMP of
ties of a bidentate NHC were expected to ameliorate the the highly strained monomer norbornene and compared to
complex stability through a “chelate effect”. The synthesis the activity of Ru dimer 2, complex 3 and 4 (Table 1). Our
of 1-(mesityl)-3-(2-hydroxyphenyl)-4,5-dihydroimidazolium results reveal that 7 displays very poor activity in metathesis
chloride salt (6) was straightforward following a procedure reactions; only 20% of norbornene polymerised during a
outlined by Grubbs et al.[48] In order to effectuate ligand reaction time of 3 h at 85 °C (Table 1, Entry 4). It has been
coordination, the NHC precursor was treated with 2 equiv. described in the literature that for this catalyst type decom-
of base; that is, 1 equiv. to liberate the free carbene and plexation of the arene ligand is the prior requirement for
1 equiv. to deprotonate the phenolic group. After 15 min of the generation of catalytic activity.[49–52] The complex must
reaction time, Ru dimer 2 was added to the reaction mix- be coordinatively unsaturated for a substrate to enter the
ture. As expected, the ligand bound to the metal centre in coordination sphere of the metal. In other words, for the
a chelating manner (Scheme 2). Because of the pseudotetra- catalyst to become active, a ligand has to dissociate to gen-
hedral arrangement of the ligands, a stereogenic centre was erate a vacant site. However, the high stability of complex 7
created at the Ru centre, leading to the existence of two indicates that arene ligand dissociation is greatly restrained.
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N. Ledoux, B. Allaert, F. Verpoort
FULL PAPER
Table 1. ROMP of 2-norbornene.[a]
NMR (C6D6) spectroscopic signal. Through breaking of
the chelate, the complex was expected to lose part of its
extreme stability and become more reactive. Entry 7 in
Table 1 shows that a loss of catalytic activity was observed
instead! Also, cationic counterpart 9 exhibited poor ROMP
activity (Table 1, Entry 8). As expected, losing the chelate
effect induced a significantly decreased stability of complex
8. The complex was found to decompose rapidly in solu-
tion. Even as a solid stored under inert atmosphere the
complex was unstable and slowly changed colour from
orange to green (time course of two weeks). NMR spectro-
scopic analysis of the decomposition product indicated that
both NHC and p-cymene ligand were still coordinated to
the Ru centre. Therefore, we suggest that 8 suffers from a
tendency to undergo ortho metallation with the 2-hy-
droxyphenyl amino side group.[46,56–58]
Entry Catalyst
Conversion Mn (*103)
PDI
cis
[%]
[%]
1
2
3
4
2
3
4
7
84
≈100
≈100
20
198
85
2.1
2.9
2.8
3.8
3.0
3.0
2.0
2.6
–
45
16
49
57
52
75
81
78
–
339
108
228
588
1954
1112
–
32
65
932
5
7 + phenylacetylene
33
6
7 + TMSD
21
7
8
17
8
9
16
9
10
4
10
11
12
10 + phenylacetylene
10 + TMSD
11
7
10
14
3.1
4.5
2.1
42
65
61
[a] Catalyst/norbornene, 1:2500, cat. conc. = 0.41 mm, 85 °C, sol-
vent = 0.5 mL of CH2Cl2 to dissolve the catalyst + 10 mL of tolu-
ene, reaction time = 3 h. A correction factor of 0.5*Mn was applied
for characterizing the PNBs by gel permeation chromatography.[53]
The cis fraction was determined by 1H NMR spectroscopy: δ =
5.36 CH=CH trans, 5.22 CH=CH cis.
With the aim to generate the vinylidene moiety in situ,
an excess amount of phenylacetylene (20 equiv.) was added
to the catalyst (Table 1, Entry 5). Only a minor activity en-
hancement was observed. Also, the addition of (trimethyl-
silyl)diazomethane (TMSD), which is known to activate [(p-
cymene)(PCy3)RuCl2] (3) with the in situ formation of
[RuCl2(=CHSiMe3)(PCy3)][49,54] did not significantly en-
hance the catalytic activity (Table 1, Entry 6). In a dissoci-
ative ligand substitution pathway, dissociation of the p-cy-
mene moiety is a prerequisite for the generation of an
empty coordination site where the alkylidene moiety can be
introduced. As a result of the exceptional stability of the
complex, which is accompanied by a low dissociation ten-
dency of the arene ligand, the addition of a terminal alkyne
or a diazo compound was found to have only a minor ef-
fect. Furthermore, we observed that in contrast to com-
plexes of type 3 or 4, which become more active towards
metathesis upon UV irradiation,[35,51,55] the ROMP-cata-
lyzed by 7 is not photoinduced, as no change in the out-
Scheme 3. Losing the chelate effect.
Anticipating that this ortho metallation might be avoided
come was seen when reactions were carried out in the dark by a small modification in the NHC framework, a methyl
or under constant irradiation with UV light (shortwave UV: unit was introduced at the carbon atom in the ortho posi-
254 nm, longwave UV: 365 nm).
tion of the 2-hydroxyphenyl group. Coordination of the new
The addition of an excess amount of ethereal HCl to 7 bidentate NHC afforded air- and moisture-stable complex
caused protonation of the phenolic oxygen atom, therefrom 10. Also, this complex is a poor initiator for metathesis and
affording complex 8 as a bright orange solid (Scheme 3). the addition of a terminal alkyne or TMSD group only
1
The OH proton was found at δ = 10.08 ppm as a broad H slightly increases the polymer yield (Table 1, Entries 9–11).
Scheme 4. Modified complexes 10 and 11.
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Ru-Based NHC-Arene Systems as ROM Polymerisation Catalysts
the air- and moisture-sensitive free carbene. To a dry Schlenk flask
charged with 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride
(0.490 g, 1.44 mmol) and dry toluene (5 mL) was added LiHMDS
(0.5 m in toluene, 2.88 mL, 1 equiv.). The resulting suspension was
stirred at room temperature for 15 min. Ru dimer 2 (0.43 g,
0.70 mmol) was then added as a solid, and the mixture was stirred
at room temperature for an additional 45 min. The solution was
filtered to remove residual salts, and the filtrate was concentrated
under vacuum. The residue was dissolved in a small amount of
CH2Cl2 (1 mL) and precipitated upon addition of hexane (25 mL).
The orange solid was filtered off and vacuum dried. Yield: 0.67 g,
78%. NMR spectroscopic data are similar to those reported by
Again, HCl was added to “break” the chelate to afford
complex 11. Thereupon the catalytic activity in the ROMP
of 2-norbornene was slightly enhanced (Table 1, Entry 12).
In spite of the small ligand modification (ortho methyl-
ation), also this complex was found to slowly decompose.
A solution of 11 in CH2Cl2 changed colour from orange to
dark green during a time course of 1 h. Unfortunately, a
complex mixture of products was obtained, and we were
not able to identify the decomposition products (Scheme 4).
1
Nolan et al.: H NMR (300 MHz, CDCl3): δ = 6.95 (s, 4 H, Mes-
Conclusions
3,5-H), 6.90 (s, 2 H, NCHCHN), 5.03 (d, J = 5.5 Hz, 2 H, p-cy-
mene aryl-H), 4.63 (d, J = 5.5 Hz, 2 H, p-cymene aryl-H), 2.51 (m,
1 H, p-cymene CH(CH3)2), 2.35 (s, 6 H, mesityl p-CH3), 2.23 (s, 6
From these results, it is clear that complexes of the type
[(p-cymene)(L)RuCl2] (with L = H2IMes or resembling
NHC) are not very reactive, and they too unstable for prac- H, mesityl o-CH3), 1.79 (s, 3 H, p-cymene CH3), 1.07 [d, J = 6.8 Hz,
6 H, p-cymene CH(CH3)2] ppm. 13C NMR (75 MHz, CDCl3): δ =
tical use. Likely due to stability problems, we were unable
6.95 172.0 (NCN), 139.0, 138.9, 130.1, 128.9, 125.4, 103.1, 96.0,
to isolate complex [(p-cymene)(H2IMes)RuCl2] (5). Similar
87.0, 85.9, 30.4 [p-cymene CH(CH3)2], 22.9, 22.7, 21.4, 19.3, 18.24,
complexes 8 and 11 were successfully isolated, but only
when an uncommon synthetic strategy was followed. The
18.0 ppm.
NHC-Arene Complex 7: The NHC precursor 1-mesityl-3-(2-hydroxy-
phenyl)-4,5-dihydroimidazolium chloride (0.765 g, 2.41 mmol) was
NHCs were first introduced as bidentate ligands, which
greatly enhanced the stability of the complex through the
chelate effect. Subsequent treatment with hydrochloric acid
caused a breaking of the chelate O–Ru bond and formation
of complexes [(p-cymene)(NHC)RuCl2] bearing mono-
dentate NHCs. Upon loss of the chelate effect, a dramatic
decrease in the stability of the complex was observed; as a
result, complexes 8 and 11 could not be kept for more than
a few days even when stored as a solid under an inert atmo-
treated with
a solution of potassium hexamethyldisilazide
(KHMDS; 0.5 m in toluene, 9.66 mL, 4.83 mmol, 2 equiv.) over
15 min at room temperature. A suspension of Ru dimer 2 (0.70 g,
1.14 mmol) in toluene was added, and the resulting mixture was
stirred for an additional 1.5 h at room temperature. The toluene
solution was filtered and washed with CH2Cl2 (2ϫ10 mL). After
evaporation of the filtrate, cold acetone was added while stirring.
Subsequent filtration allowed the isolation of the pure red-pink cat-
sphere. In solution, both complexes decomposed very alyst, which was thoroughly dried. Yield: 0.58 g, 46%. 1H NMR
(300 MHz, CDCl3): δ = 6.93–7.02 (m, 3 H, aryl CH), 6.79 (t, 3JH,H
rapidly, which drastically restricted their catalytic value. To
adequately compete with the Grubbs catalysts, it is unques-
3
= 7.3 Hz, 1 H, aryl CH), 6.71 (d, JH,H = 7.9 Hz, 1 H, aryl CH),
3
6.43 (t, JH,H = 7.3 Hz, 1 H, aryl CH), 5.52 (d, J = 4.6 Hz, 1 H, p-
tionable that better activity and stability levels have to be
cymene aryl-H), 5.26 (d, J = 4.6 Hz, 1 H, p-cymene aryl-H), 5.09
reached. Therefore, our attention has now been drawn to
(d, J = 3.4 Hz, 1 H, p-cymene aryl-H), 4.43 (pseudo q, Japp
=
other synthetic methods that first introduce an alkylidene
moiety and then, in a second reaction step, realize the coor-
dination of an NHC ligand. We hope to be able to report
on these results in the near future.
ca. 10.5 Hz, 1 H, NCH2CH2N), 4.18 (pseudo q, Japp = ca. 10.5 Hz,
1 H, NCH2CH2N), 4.02 (pseudo q, Japp = ca. 10.5 Hz, 1 H,
NCH2CH2N), 3.92 (pseudo q, Japp
= ca. 10.5 Hz, 1 H,
NCH2CH2N), 3.69 (d, J = 3.2 Hz, 1 H, p-cymene aryl-H), 2.50 (s,
3 H, mesityl CH3), 2.42 (s, 3 H, mesityl CH3), 2.41 [m, 1 H, p-
cymene CH(CH3)2], 2.33 (s, 3 H, mesityl CH3), 1.58 (s, 3 H, p-
3
Experimental Section
cymene CH3), 0.96 [d, JH,H = 6.7 Hz, 3 H, p-cymene CH(CH3)2],
0.83 [d, 3JH,H = 6.7 Hz, 3 H, p-cymene CH(CH3)2] ppm. 13C NMR
(75 MHz, CDCl3): δ = 202.7 (NCN), 157.4 (C-O), 139.7 (NHC
aryl-C), 139.0 (NHC aryl-C), 138.0 (NHC aryl-C), 136.2 (NHC
aryl-C), 130.9 (NHC aryl-C), 130.5 and 130.3 (NHC aryl-C), 128.9
and 128.8 (NHC aryl-C), 124.92 and 124.86 (NHC aryl-C), 121.3
and 121.2 (NHC aryl-C), 116.3 and 116.2 (NHC aryl-C), 113.9 and
113.8 (NHC aryl-C), 101.1 (p-cymene aryl-C), 95.2 (p-cymene aryl-
C), 94.7 (p-cymene aryl-C), 93.1 (p-cymene aryl-C), 83.8 (p-cymene
aryl-C), 79.4 (p-cymene aryl-C), 51.1 (CH2CH2), 48.7 (CH2CH2),
30.2 and 30.1 [p-cymene CH(CH3)2], 23.3 (CH3), 21.3 and 21.1
(CH3), 20.0 and 19.8 (CH3), 18.6 and 18.5 (CH3), 18.3 and 18.1
(CH3) ppm. Some carbon atoms have a double peak in the spectrum,
which indicates the presence of two diastereomers in roughly an equal
ratio. C28H33ClN2ORu (550.11): C 61.14, H 6.05, N 5.09; found C
61.39, H 6.08, N 5.12.
General: All reactions and manipulations involving organometallic
compounds were conducted in oven-dried glassware under an ar-
gon atmosphere by using standard Schlenk techniques. Solvents
were dried with appropriate drying agents and distilled prior to use.
1H and 13C NMR spectroscopic measurements were performed
with a Varian Unity-300 spectrometer. Ru dimer 2,[59] H2I-
Mes(H)(CCl3),[60] the bidentate NHC salts[47] and complex 3[35]
were prepared according to literature procedures. (Trimethylsilyl)-
diazomethane (TMSD) was purchased from Aldrich as a solution
in hexane. We were unable to do elemental analysis on complexes
9 and 11 because of the lack of stability of these compounds. For
these two complexes, we hope that characterisation through 1H
NMR and 13C NMR spectroscopy is sufficient.
Synthesis of the Complexed
[(p-cymene)RuCl2(IMes)] (4): The synthesis of complex 4 was pre-
viously reported by Nolan et al. to result from the reaction of Ru
dimer with free IMes carbene.[38] We herein describe a slightly al-
tered synthetic strategy, which avoids the isolation and handling of
NHC-Arene Complex 8: Complex 7 (0.125 g, 0.227 mmol) was dis-
solved in CH2Cl2 (5 mL). An excess amount of HCl (1 n in Et2O,
1 mL) was added, and the resulting mixture was stirred at room
temperature for 5 min. Solvents were evaporated and hexane
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N. Ledoux, B. Allaert, F. Verpoort
FULL PAPER
(25 mL) was added to precipitate complex 8 as a bright orange
123.8 (NHC aryl-C), 120.0 (NHC aryl-C), 114.9 (NHC aryl-C),
112.6 (NHC aryl-C), 99.5 (br., p-cymene aryl-C), 97.6 (p-cymene
solid. Yield: 0.12 g, 93%. 1H NMR (300 MHz, CDCl3): δ = 8.02
(d, J = 7.5 Hz, 1 H, aryl CH), 7.10 (s, 1 H, aryl CH), 7.03 (s, 1 H, aryl-C), 93.9 (br., p-cymene aryl-C), 91.9 (p-cymene aryl-C), 82.4
aryl CH), 6.96 (m, 1 H, aryl CH), 6.89 (m, 2 H, aryl CH), 5.70 (br. (br., p-cymene aryl-C), 78.0 (p-cymene aryl-C), 49.8 (NCH2CH2N),
s, 2 H, p-cymene aryl-H), 5.32 (br. s, 2 H, p-cymene aryl-H), 4.34
47.4 (NCH2CH2N), 28.9 [p-cymene CH(CH3)2], 22.1 (CH3), 20.0
(m, 2 H, NCH2CH2N), 4.10 (m, 1 H, NCH2CH2N), 3.98 (m, 1 H, (CH3), 19.8 (CH3), 18.6 (CH3), 17.3 (CH3), 16.9 (CH3) ppm.
NCH2CH2N), 2.86 [sept, J = 5.5 Hz, 1 H, p-cymene CH(CH3)2], C29H35ClN2ORu (564.14): calcd. C 61.74, H 6.25, N 4.97; found C
2.58 (s, 3 H, mesityl CH3), 2.38 (s, 3 H, mesityl CH3), 2.35 (s, 3 H,
mesityl CH3), 1.75 (s, 3 H, p-cymene CH3), 1.02 [m, 6 H, p-cymene
CH(CH3)2] ppm. Signals of the aromatic p-cymene protons appear
as broad signals due to internal rotation of the ligand. 1H NMR
(300 MHz, C6D6): δ = 10.08 (br. s, 1 H, OH) ppm. 13C NMR
(75 MHz, CDCl3): δ = 203.0 (NCN), 146.1 (C-OH), 139.8 (NHC
aryl-C), 138.8 (NHC aryl-C), 137.4 (NHC aryl-C), 135.5 (NHC
aryl-C), 130.4 (NHC aryl-C), 129.5 (NHC aryl-C), 129.2 (NHC
aryl-C), 125.1 (NHC aryl-C), 121.7 (NHC aryl-C), 118.9 (NHC
aryl-C), 117.7 (NHC aryl-C), 105.0 (br., p-cymene aryl-C), 97.3
(br., p-cymene aryl-C), 92.1 (br., p-cymene aryl-C), 82.3 (br., p-
cymene aryl-C), 52.0 (NCH2CH2N), 49.7 (NCH2CH2N), 30.4 [p-
cymene CH(CH3)2], 23.6 (CH3), 21.3 (CH3), 21.2 (CH3), 19.8
(CH3), 18.9 (CH3), 18.5 (CH3) ppm. C28H34Cl2N2ORu (586.57):
calcd. C 57.34, H 5.84, N 4.78; found C 56.62, H 5.79, N 4.54.
61.48, H 6.12, N 4.56.
NHC-Arene Complex 11: Analogous to 8, complex 11 was obtained
as an orange solid in good yield (83%). Characterisation through
1H NMR spectroscopy was troublesome as the aromatic p-cymene
protons and the backbone protons on the NHC appeared as very
broad signals due to internal rotation of the ligands. 1H NMR
(300 MHz, CDCl3): δ = 7.19 (m, 1 H, aryl CH), 7.07 (br. s, 1 H,
aryl CH), 7.03 (s, 1 H, aryl CH), 6.88 (m, 1 H, aryl CH), 6.58 (m,
2 H, aryl CH), 5.49–5.36 and 5.19 (br. signals, 4 H, p-cymene aryl-
H), 4.26–3.92 (br. m, 4 H, NCH2CH2N), 2.69–0.88 (several signals)
1
ppm. H NMR (300 MHz, C6D6): δ = 10.14 (br. s, 1 H, OH) ppm.
13C NMR (75 MHz, CDCl3): δ = 202.2 (NCN), 151.2 (C-OH),
138.2 (NHC aryl-C), 136.5 (NHC aryl-C), 134.9 (NHC aryl-C),
131.1 (NHC aryl-C), 129.4 (NHC aryl-C), 128.2 (NHC aryl-C),
124.3 (NHC aryl-C), 119.4 (NHC aryl-C), 118.1 (NHC aryl-C),
116.9 (NHC aryl-C), 98.1 (br., p-cymene aryl-C), 95.0 (br., p-cy-
mene aryl-C), 93.4 (br., p-cymene aryl-C), 80.9 (br., p-cymene aryl-
C), 53.8 (br., NCH2CH2N), 29.3 [p-cymene CH(CH3)2], 22.4
(CH3), 21.7 (CH3), 21.0 (CH3), 20.4 (CH3), 19.6 (CH3), 19.1 (CH3)
ppm.
Cationic NHC-arene Complex 9: Silver triflate (0.049 g, 0.19 mmol)
was added to a solution of 8 (0.109 g, 0.19 mmol) in CH2Cl2
(5 mL). The reaction mixture was stirred for 15 min at room tem-
perature. The solution was filtered to remove AgCl, and the solvent
was evaporated to almost complete dryness. The addition of hexane
(10 mL) led to precipitation of the desired cationic complex as a
bright orange solid, which was filtered off and dried in vacuo.
Catalytic Reactions
1
Yield: 0.13 g, 96%. H NMR (300 MHz, CDCl3): δ = 7.51 (d, J =
ROMP of 2-Norbornene: In a typical ROMP experiment (see
Table 1), the catalyst (0.004248 mmol) in CH2Cl2 (0.5 mL) was
transferred into a 15-mL vessel followed by the addition of 2-nor-
bornene (1 g, 2500 equiv.) and toluene (10 mL). The reaction was
then stirred at 85 °C for 3 h. To stop the polymerisation, a 2-ethyl
vinyl ether/BHT(2,6-di-tert-butyl-4-methylphenol) solution in
CHCl3 was added. The reaction mixture was then poured into
MeOH (50 mL) to precipitate the polymer. The polymer was iso-
lated upon filtration and analysed gravimetrically by 1H NMR
spectroscopy and GPC (gel permeation chromatography).
5.1 Hz, 1 H, aryl CH), 7.08 (s, 1 H, aryl CH), 7.02 (m, 4 H), 5.75
(br. s, 2 H, p-cymene aryl-H), 5.43 (br. s, 2 H, p-cymene aryl-H),
4.53 (m, 1 H, NCH2CH2N), 4.28 (m, 1 H, NCH2CH2N), 4.09 (m,
2 H, NCH2CH2N), 2.59 (s, 3 H, mesityl CH3), 2.47 [m, 1 H, p-
cymene CH(CH3)2], 2.36 (s, 3 H, mesityl CH3), 2.33 (s, 3 H, mesityl
CH3), 1.63 (s, 3 H, p-cymene CH3), 0.95 [m, 6 H, p-cymene
CH(CH3)2] ppm. 13C NMR (75 MHz, CDCl3): δ = 204.4 (NCN),
144.2 (C-OH), 139.9 (NHC aryl-C),138.8 (NHC aryl-C), 136.8
(NHC aryl-C), 135.5 (NHC aryl-C), 130.6 (NHC aryl-C), 129.4
(NHC aryl-C), 129.0 (NHC aryl-C), 125.7 (NHC aryl-C), 123.5
(NHC aryl-C), 118.4 (NHC aryl-C), 117.8 (NHC aryl-C), 91.8 (br.,
p-cymene aryl-C), 83.2 (br., p-cymene aryl-C), 52.1 (NCH2CH2N),
49.6 (NCH2CH2N), 30.6 [p-cymene CH(CH3)2], 23.0 (CH3), 21.2
(CH3), 21.1 (CH3), 19.9 (CH3), 18.7 (CH3), 18.2 (CH3) ppm.
CCDC-652147 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
NHC-Arene Complex 10: In an analogous procedure as for complex
7, 1-mesityl-3-(2-hydroxy-6-methylphenyl)-4,5-dihydroimidazolium
chloride (0.284 g, 0.86 mmol), KHMDS (0.5 m in toluene, 3.43 mL)
and Ru dimer 2 (0.25 g, 0.41 mmol) afforded complex 10 as an
orange-red solid. Yield: 0.15 g, 32%. 1H NMR (300 MHz, CDCl3):
δ = 7.02 (m, 2 H, aryl CH), 6.84 (t, J = 6.9 Hz, 1 H, aryl CH),
6.73 (d, J = 7.3 Hz, 1 H, aryl CH), 6.46 (m, 1 H, aryl CH), 5.56
(d, J = 6.3 Hz, 1 H, p-cymene aryl-H), 5.29 (br. s, 1 H, p-cymene
aryl-H), 5.11 (d, J = 5.0 Hz, 1 H, p-cymene aryl-H), 4.43 (pseudo
q, J = 10.5 Hz, 1 H, NCH2CH2N), 4.18 (pseudo q, J = 10.1 Hz, 1
H, NCH2CH2N), 4.04–3.92 (m, 2 H, NCH2CH2N), 3.71 (br. s, 1
H, p-cymene aryl-H), 2.53 (s, 3 H, mesityl CH3), 2.45 (s, 6 H, 2-
hydroxy-6-methylphenyl and mesityl CH3), 2.41 [m, 1 H, p-cymene
CH(CH3)2], 2.36 (s, 3 H, mesityl CH3), 1.61 (s, 3 H, p-cymene
CH3), 0.98 [d, J = 7.0 Hz, 3 H, p-cymene CH(CH3)2], 0.85 [d, J =
6.8 Hz, 3 H, p-cymene CH(CH3)2] ppm. 13C NMR (75 MHz,
CDCl3): δ = 201.5 (NCN), 156.1 (C-OH), 138.6 (NHC aryl-C),
137.8 (NHC aryl-C), 136.8 (NHC aryl-C), 134.9 (NHC aryl-C),
129.5 (NHC aryl-C), 129.2 (NHC aryl-C), 127.5 (NHC aryl-C),
Acknowledgments
The authors gratefully acknowledge the FWO-Flanders and the re-
search Fund of Ghent University for generous financial support
during the preparation of this manuscript. We wish to thank Oliv-
ier F. Grenelle and Dr. Marc G. Proot of Chevron Technology,
Ghent, for elemental analyses and dr determination and Anthony
Linden of the Institute of Organic Chemistry, University of Zürich,
for collecting the crystallographic data set.
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Received: June 27, 2007
Published Online: October 19, 2007
Eur. J. Inorg. Chem. 2007, 5578–5583
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5583