Homobimetallic ruthenium-N-heterocyclic carbene complexes: Synthesis, characterization, and catalytic applications

Article abstract of DOI:10.1002/adsc.200600515

Two new homobimetallic ruthenium-arene complexes [(p-cymene)Ru(μ-Cl) ₃RuCl(η²-C₂H₄)(L)], where L = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (3a) or 1,3-bis(2,4,6- trimethylphenyl)-4,5-dichloroimidazolin-2-ylidene (3b), were isolated in high yields upon heating a toluene solution of [RuCl₂ (p-cymene)] ₂ with 1 equivalent of carbene ligand under an ethylene atmosphere. They were characterized by NMR and TGA. Their catalytic activity was investigated in the atom transfer radical polymerization of vinyl monomers. In the polymerization of methyl methacrylate, complex 3a displayed faster reaction rates than 3b and the related phosphine-based complex 2a (L = tricyclohexylphosphine), although control was more effective with the latter catalyst. When n-butyl acrylate or styrene served as monomer, a major shift of reactivity was observed between complex 2a that promoted controlled radical polymerization, and complexes 3a or 3b that favored metathetical coupling. Further homocoupling experiments with various styrene derivatives confirmed the outstanding aptitude of complex 3a (and to a lesser extent of 3b) to catalyze olefin metathesis reactions. Contrary to monometallic ruthenium-arene complexes of the [RuCl₂(p-cymene)(L)] type, the new homobimetallic species did not require the addition of a diazo compound or visible light illumination to initiate the ring-opening metathesis of norbornene or cyclooctene. When α,ω-dienes were exposed to 3a or 3b, a mixture of cycloisomerization and ring-closing metathesis products was obtained in a non-selective way. Addition of a terminal alkyne co-catalyst enhanced the metathetical activity while completely repressing the cycloisomerization process. Thus, quantitative conversions of diethyl 2,2-diallylmalonate and N,N-diallyltosylamide were achieved within 2 h at room temperature using 2 mol% of catalyst precursor 3 a and 6 mol % of phenylacetylene.

Full text of DOI:10.1002/adsc.200600515

FULL PAPERS
DOI: 10.1002/adsc.200600515
Homobimetallic Ruthenium–N-Heterocyclic Carbene Complexes:
Synthesis, Characterization, and Catalytic Applications
Xavier Sauvage,a Yannick Borguet,a Alfred F. Noels,a Lionel Delaude,a,*
and Albert Demonceaua,*
a
Center for Education and Research on Macromolecules (CERM), Institut de Chimie (B6a), UniversitØ de Li›ge,
Sart-Tilman par 4000 Li›ge, Belgium
Fax : (+32)-4-366-3497; e-mail: l.delaude@ulg.ac.be or a.demonceau@ulg.ac.be
Received: August 18, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Two new homobimetallic ruthenium-arene outstanding aptitude of complex 3a (and to a lesser
complexes [(p-cymene)Ru
where L=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2- Contrary to monometallic ruthenium-arene com-
ylidene (3a) or 1,3-bis(2,4,6-trimethylphenyl)-4,5-di- plexes of the [RuCl2(p-cymene)(L)] type, the new
A
N
AHCTREUNG
chloroimidazolin-2-ylidene (3b), were isolated in homobimetallic species did not require the addition
high yields upon heating a toluene solution of [RuCl2 of a diazo compound or visible light illumination to
A
under an ethylene atmosphere. They were character- cyclooctene. When a,w-dienes were exposed to 3a or
ized by NMR and TGA. Their catalytic activity was 3b, a mixture of cycloisomerization and ring-closing
investigated in the atom transfer radical polymeri- metathesis products was obtained in a non-selective
zation of vinyl monomers. In the polymerization of way. Addition of a terminal alkyne co-catalyst en-
methyl methacrylate, complex 3a displayed faster re- hanced the metathetical activity while completely re-
action rates than 3b and the related phosphine-based pressing the cycloisomerization process. Thus, quanti-
complex 2a (L=tricyclohexylphosphine), although tative conversions of diethyl 2,2-diallylmalonate and
control was more effective with the latter catalyst. N,N-diallyltosylamide were achieved within 2 h at
When n-butyl acrylate or styrene served as mono- room temperature using 2 mol% of catalyst precur-
mer, a major shift of reactivity was observed be- sor 3a and 6 mol% of phenylacetylene.
tween complex 2a that promoted controlled radical
polymerization, and complexes 3a or 3b that favored Keywords: arene ligands; homogeneous catalysis;
metathetical coupling. Further homocoupling experi- metathesis; polymerization; radical reactions; ruthe-
ments with various styrene derivatives confirmed the nium
Introduction
mental reality of stable nucleophilic N-heterocyclic
carbenes (NHCs), which were first isolated and char-
Ruthenium-arene complexes[1] are versatile and effi- acterized by Arduengo and co-workers in 1991.[7]
cient catalyst precursors for various important organic These divalent carbon species are neutral, two-elec-
transformations.[2] This is due in part to the lability of tron ligands with only little p-back-bonding tenden-
the h6-arene ligand that can be easily removed upon cy.[8] They behave as phosphine mimics, yet they are
thermal[3] or photochemical[4] activation to release
highly active, coordinatively unsaturated species.
During the 1990s, we demonstrated that [RuCl
A
cymene)(PR3)] complexes bearing basic and bulky
ACHTREUNG
phosphine ligands, such as tricyclohexylphosphine
(PCy3, see structure 1a), were highly effective preca-
talysts for ring-opening metathesis polymerization
(ROMP)[5] and atom transfer radical polymerization
(ATRP).[6] The past decade also witnessed the experi-
Adv. Synth. Catal. 2007, 349, 255 – 265
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255

Xavier Sauvage et al.
FULL PAPERS
better s-donors and they form stronger bonds to
metal centers than most phosphines. Their electronic
and steric properties are liable to ample modification
simply by varying the substituents on the heterocyclic
ring. Therefore, NHCs have become ubiquitous li-
gands in organometallic chemistry and catalysis.[9]
They have also acquired a place on their own as re-
agents and catalysts in organic synthesis, since they
behave as powerful nucleophilic agents.[10] In order to
further expand the scope of ruthenium-arene catalyst
precursors, we have adopted this class of ancillary li-
gands instead of phosphines to generate [RuCl
A
cymene)(NHC)] species, either preformed or in
ACHTREUNG
situ.[11] Thus, complex 1b bearing the 1,3-dimesitylimi-
dazolin-2-ylidene ligand (nicknamed IMes) displayed
a remarkable activity for initiating ROMP of cyclooc-
tene under visible light illumination,[12] whereas com-
plex 1c sporting a modified NHC ligand (4,5-dichloro-
1,3-dimesitylimidazolin-2-ylidene, IMesCl2) was an at-
tractive challenger for promoting atom transfer radi-
Scheme 1. Synthesis of homobimetallic ruthenium-NHC
complexes 3a and 3b.
cal addition (ATRA)[13] and polymerization[14] pro- talline orange powders in high yields (70% for 3a,
cesses. 80% for 3b) by simple filtration and washing. They
In 2005, Severin and co-workers investigated the re- could be conveniently handled in air for short periods
action of [RuCl2(p-cymene)]2 with 1 equivalent of of time, although prolonged exposure to oxygen and
AHCTREUNG
PCy3 under an ethylene atmosphere. Under these moisture led to progressive color changes indicative
conditions, the ruthenium dimer afforded a new type of degradation. Under normal atmosphere, complex
of molecular scaffold, in which a RuCl
A
ACHTREUNG
bridges to a ruthenium-(p-cymene) moiety. Complex position. Thus, for long-term storage, they were kept
2a displayed an outstanding catalytic activity in in Schlenk flasks under an inert atmosphere at
ATRA reactions.[15] By varying the nature of the À188C. Thermogravimetric analyses also pointed to
arene ligand, the same group also synthesized the re- an enhanced stability of complex 3b compared to 3a,
lated homobimetallic complex 2b and successfully em- since the decomposition onset occurred at 2148C for
ployed it as catalyst precursor for the ATRP of meth- 3b and 1208C for 3a. For the sake of comparison,
acrylate monomers at 358C.[16]
TGA of 2a was also carried out. Despite its remarka-
In view of the enhancements brought by the re- ble stability in open air at room temperature, this
placement of phosphines by NHCs in monometallic complex began loosing weight at 828C.
ruthenium-arene catalyst precursors of type 1, we de-
Complexes 3a and 3b readily dissolved in dichloro-
cided to investigate the effect of similar modifications methane at room temperature or in toluene upon
on the catalytic activity of complexes of type 2. In this heating. Since the alkene ligand was rather labile in
contribution, we disclose the synthesis, characteriza- solution, all NMR spectra were recorded in CD2Cl2
tion, and catalytic applications of two new homobime- saturated with ethylene. Both complexes displayed
1
tallic ruthenium-arene complexes 3a and 3b bearing, similar H NMR features. Integration of the various
respectively, the IMes and IMesCl2 ligands.
signals confirmed the presence of the NHC, ethylene,
and arene ligands in 1:1:1 proportions. The h2-C2H4
fragment gave rise to two multiplets centered at
about 3.3 and 3.7 ppm, respectively. Only two aromat-
ic protons from the p-cymene ligand were visible.
They resonated as two distinct multiplets forming an
AB system between 5.0 and 5.1 ppm. The two other
p-cymene ring protons were masked by the more in-
Results and Discussion
Synthesis and Characterization of Complexes 3a and
3b
The preparation of complexes 3a and 3b was rather tense absorptions of CH2Cl2 and free C2H4 at ca. 5.2
straightforward and could be achieved in a single step and 5.3 ppm, respectively. Yet, the non-equivalence of
by heating the commercially available [RuCl2(p- the four aromatic protons on the arene ligand was
G
cymene)]2 dimer with 1 equivalent of IMes[7b,c] or confirmed by 13C{1H} NMR spectroscopy that showed
[17]
IMesCl2 in toluene under an ethylene atmosphere four distinct lines for the carbon atoms bearing these
(Scheme 1). Both products were isolated as microcrys- protons. Further evidence for the asymmetric nature
256
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Homobimetallic Ruthenium–N-Heterocyclic Carbene Complexes
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Table 1. Metathesis and atom transfer radical polymerization of methyl methacrylate (MMA), n-butyl acrylate (nBA), and
styrene catalyzed by complexes 2a, 3a, and 3b at 858C or 1108C.
[a]
Monomer
MMA[c]
Catalyst
Metathesis Yield [%]
ATRP Yield [%]
Mn [kgmolÀ1][a]
Mw/Mn
f[b]
kapp [10À5 sÀ1]
2a
3a
3b
2a
3a
3b
2a
3a
3b
0
78
81
47
16
27
2
62
0
0
36.5
40
19
5
24
-
24
-
-
1.05
1.25
1.10
1.25
1.85
-
1.35
-
-
0.85
0.8
1.0
1.25
0.45
-
1.0
-
-
2.11
2.81
0.93
0.23
<1
<1
1
16
41
0
nBA[d]
[f]
-
-
Styrene[e]
1.63
-
-
100
100
[a]
Determined by size-exclusion chromatography in THF with PMMA or polystyrene calibration.
Initiation efficiency f=Mn,theor./Mn,exp. with Mn,theor. =([monomer]/[initiator])0 ” Mw ” conversion.
[b]
[c]
[d]
[e]
[f]
Initiator: ethyl 2-bromo-2-methylpropionate, [MMA]0:[initiator]0:
Initiator: ethyl 2-bromopropionate, [nBA]0:[initiator]0:[complex]0 =600:2:1, 16 h at 858C.
[complex]0 =750:2:1, 16 h at 1108C.
[complex]0 =800:2:1, 16 h at 858C.
AHCTREUNG
Initiator: (1-bromoethyl)benzene, [styrene]0:[initiator]0:
Uncontrolled polymerization.
ACHTREUNG
of type 3 complexes came from the observation of
five different signals for the methyl groups on the
arene and NHC ligands.
The most striking difference between the NMR
spectra of complexes 3a and 3b concerned the reso-
nance of the imidazole C4 and C5 carbons. The re-
placement of hydrogen atoms by chloro substituents
on these positions led to a significant decrease of
their chemical shift (from 125.6 ppm in 3a to
117.9 ppm in 3b). The carbene center was also affect-
ed by the modification of electron density on the het-
erocyclic ring, although to a lesser extent (167.8 ppm
for C2 in 3a, 172.3 ppm in 3b). These variations are in
line with NMR data previously reported for rutheni-
um-benzylidene metathesis catalysts [RuCl
(PCy3)
IMes.[18]
G
Figure 1. . Dependence of PMMA molecular weight Mn on
&
U
A
Table 1 for reaction conditions.
Crystals of 3a were grown by slowly cooling a satu-
rated toluene solution of the complex under an ethyl-
ene atmosphere. X-ray diffraction analysis at 120 K methylpropionate as initiator under standard experi-
showed that each unit cell was made of two non- mental conditions.[6] The reactions were carried out in
equivalent molecules of 3a, one of them being disor- toluene under the exclusion of oxygen for 16 h at
dered, and one molecule of solvent. It confirmed un- 858C with an initial monomer/initiator/catalyst molar
ambiguously the validity of the structure depicted in ratio of 800:2:1. With both complexes 3a and 3b, the
Scheme 1 for 3a. Unfortunately, all our attempts to molecular weights increased linearly with conversion
refine the structure remained unsuccessful as it was (Figure 1). The semilogarithmic plot of ln([M]0/[M]t)
not possible to come up with an R parameter lower versus time also followed
a linear relationship
than 0.16 even when data sets were collected at low (Figure 2). These results strongly suggest that radical
temperature on freshly synthesized crystals.
polymerization took place in a controlled fashion with
both catalysts.[19] The nature of the ancillary ligand
significantly influenced the rate of reaction. IMes af-
forded a much more active catalyst than its dichloro
derivative IMesCl2. Indeed, the pseudo-first order
Catalytic Investigations
We have investigated the catalytic activity of homobi- rate constant (kapp) computed for complex 3a was
metallic ruthenium complexes in the ATRP of vinyl three times larger than for complex 3b. In terms of ki-
monomers (Table 1). First, methyl methacrylate netics, complex 3a also outperformed the tricyclohex-
(MMA) was polymerized using ethyl 2-bromo-2- ylphosphine-based bimetallic species 2a (see Table 1
Adv. Synth. Catal. 2007, 349, 255 – 265
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257

Xavier Sauvage et al.
FULL PAPERS
duction of 1 mL of neat substrate in the reaction
medium led to an initial monomer/initiator/catalyst
molar ratio of 600:2:1. Acrylates are much more chal-
lenging monomers for ATRP than methacrylates and
only a few ruthenium catalytic systems allow a good
control of their radical polymerization.[21] Complex 2a
deserves a honorable mention for this task, despite
the fact that only modest yields were achieved after
16 h. Complex 3a was more active but required an in-
duction period and did not afford any control. With
complexes 3a and 3b, the most salient feature was the
formation of significant amounts of di(n-butyl) fuma-
G
rate and maleate in a 98:2 ratio. These products result
from the homocoupling of the unsaturated ester via
olefin metathesis. A similar reaction had also occur-
red with MMA. In this case, however, it remained es-
sentially negligible compared to ATRP and only
traces of (E)- and (Z)-dimethyl 2,3-dimethylbut-2-
enedioate were detected by gas chromatography
(GC).
Figure 2. Time dependence of ln([M]0/[M]t) for the ATRP of
&
*
MMA at 858C with catalysts 2a ( ), 3a ( ), and 3b (&). See
Table 1 for reaction conditions.
and Figure 2). When molecular weight distributions
In a third series of experiments, we investigated the
were examined, however, complex 2a took prece- ATRP of styrene at 1108C with an initial monomer/
dence over its NHC-containing congeners. Although initiator/catalyst molar ratio of 750:2:1 (Table 1).
the number-average molecular weight (Mn) reached Complex 2a proved once again its efficiency as a radi-
with this catalyst at 858C exceeded the value calculat- cal initiator, as it led to a well-behaved polymeri-
ed from the initial monomer-to-initiator ratio (initia- zation process with a satisfactory control over the mo-
tion efficiency f=0.85), polydispersity remained as lecular weight distribution and no stilbene formation.
narrow as Mw/Mn =1.05, whereas complexes 3a and It should be pointed out, however, that a small peak
3b led to PDIs of 1.25 and 1.10, respectively, under due to low molecular weight polystyrene remained
the same experimental conditions. Moreover, the for- visible in the GPC traces during the whole duration
mation of small amounts of low molecular weight of the reaction (see Supporting Information). A simi-
polymers that did not disappear over the course of lar phenomenon already occurred in the polymeri-
the reaction was observed with the latter catalysts zation of MMA with complexes 3a and 3b, although
(see Supporting Information). Although minor (less to a lesser extent. It was assigned to a persistent radi-
than 1% of the total polymer mass), this phenomenon cal effect (vide supra). Most interestingly, replace-
was not detected with catalyst 2a. It is attributed to ment of tricyclohexylphosphine by a NHC ligand led
the coupling or dismutation of oligomeric chains to a complete change of selectivity. Thus, when sty-
during the early stages of the polymerization process rene was reacted with complexes 3a and 3b under the
(“persistent radical effect”).[20]
same experimental conditions that were employed for
When the polymerization of MMA was carried out 2a, ATRP was totally suppressed and replaced by
at 358C instead of 858C with complex 3a, the semilo- olefin metathesis. Despite the presence of (1-bromo-
garithmic plot of ln([M]0/[M]t) versus time remained ethyl)benzene as initiator, no polystyrene was isolated
linear, but high molecular weight polymers were and a quantitative yield of stilbene [almost exclusively
formed from the very beginning of the reaction and the (E)-isomer] was obtained within 30 min.
Mn as well as Mw/Mn remained almost constant
To further evaluate the metathetical activity of ho-
(ꢀ76 kgmolÀ1 and 1.60, respectively) throughout the mobimetallic ruthenium-(p-cymene) complexes, we
entire run. This behavior sharply contrasts with the have performed the homocoupling of styrene deriva-
results previously reported for complex 2b at 358C tives bearing various electron-withdrawing or -donat-
that met all the criteria of controlled polymeri- ing substituents on their aromatic rings (Table 2). The
zation.[16] With the NHC-based complex 3a, the de- reactions were carried out in toluene at 858C using
crease of temperature most likely induced a switch of 0.2 mol% of catalyst. Conversions were monitored by
mechanism, from controlled ATRP to a redox-initiat- GC at regular time intervals using an internal stan-
ed free-radical process,[14] although further investiga- dard. To speed up the screening, yields were not sys-
tions are needed to fully clarify this point.
tematically determined. However, whenever stilbene
Next, we probed the reactivity of n-butyl acrylate products were separated from the reaction mixtures
(nBA) at 85 8C (Table 1). The experimental procedure and purified by column chromatography, isolated
used with MMA was kept unchanged and the intro- yields closely matched the conversions recorded,
258
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Homobimetallic Ruthenium–N-Heterocyclic Carbene Complexes
FULL PAPERS
Table 2. Metathesis of styrene derivatives catalyzed by com-
plexes 2a, 3a, and 3b at 858C.[a]
action. trans-Anethole [1-methoxy-4-(1-propenyl)ben-
zene] reacted at about the same rate as styrene and
was therefore more reactive than 4-methoxystyrene
(entry 9). It is also an inexpensive natural product,
whereas 4-methoxystyrene is not widely available.
Thus, trans-anethole is a prime starting material to
access the important class of polyhydroxystilbenes via
olefin metathesis.[22,23] The chemical synthesis of natu-
rally occurring stilbenoid compounds, including re-
sveratrol (3,4’,5-trihydroxystilbene), has attracted a
lot of attention recently, because of the remarkable
physiological properties and potential therapeutic
values of these phytoalexins.[24] Hence, we were
pleased to note that the presence of a terminal
methyl group on the C=C double bond of trans-ane-
thole did not hinder the metathesis process. On the
other hand, attempts to homocouple a-methylstyrene
with complex 3a were completely unsuccessful. This
failure might be attributed to the greater steric con-
gestion imposed to a metal-alkylidene or a metallacy-
clobutane intermediate by a 1,1-disubstituted double
bond compared to its 1,2-disubstituted isomer or a
terminal olefin. Another styrene derivative that resist-
ed metathesis under our experimental conditions was
Entry Substrate
Catalyst Conversion
[%][b]
1
2
3
4
5
6
7
8
9
Styrene
Styrene
Styrene
4-Acetoxystyrene
4-Chlorostyrene
4-Methylstyrene
4-Methoxystyrene
4-Trifluoromethylstyrene
trans-Anethole
2a
3a
3b
3a
3a
3a
3a
3a
3a
0
93
70
100
100
89
91
70
90
[a]
Experimental conditions: Ru cat. (0.004 mmol), substrate
(2 mL of 1M solution in toluene, 2 mmol), 2 h at 858C.
Determined by GC using dodecane as internal standard.
[b]
thereby confirming the validity of the analytical 2-methoxystyrene. In this case, the lack of reactivity
method employed. was most likely due to the formation of a stabilized
With styrene itself, results obtained with complexes ruthenium-benzylidene species possessing a chelated
2a, 3a, and 3b corroborated the trends already ob- methoxy ligand.[23,25]
served under ATRP conditions (cf. Table 1). Thus,
To expand the scope of homobimetallic ruthenium
complex 2a was completely devoid of metathetical ac- catalysis in olefin metathesis, we have examined the
tivity (Table 2, entry 1), while complex 3a was highly ROMP of two representative cycloalkenes in the pres-
efficient at transforming styrene into (E)-stilbene and ence of complexes 2a, 3a, and 3b (Table 3). Polymeri-
ethylene. A 93% conversion was achieved after 2 h zations of cyclooctene were carried out in chloroben-
(entry 2) and full consumption of the substrate occur- zene at 608C for 2 h using a monomer-to-catalyst
red within 5 h at 858C. Complex 3b displayed a lesser ratio of 250. In order to impede gelification and for-
activity and afforded only a 70% conversion after 2 h mation of insoluble cross-linked polymers, norbor-
(entry 3). The intervention of a photochemical activa- nene was reacted in more dilute solutions with 0.1
tion step was ruled out by carrying out reactions in mol% of catalyst. Due to its high strain, ring-opening
the dark. Results paralleled those obtained under metathesis of this bicyclic monomer is particularly
normal lighting conditions. Upon irradiation of the re- easy and occurs almost under any circumstances, pro-
action mixtures with a strong visible light source, vided that enough time is allowed to the reaction.[26]
rapid darkening of the catalyst solutions ensued and It was therefore not surprising to observe the full con-
conversions dropped significantly.
version of norbornene into polynorbornene with cata-
Introduction of slightly deactivating acetoxy or lyst 2a, along with 3a and 3b. Yet, recourse to the
chloro substituents on the para-position of the aro- phosphine-based complex afforded a much broader
matic ring speeded up the reaction that became com- molecular weight distribution and a lower proportion
plete after 2 h with complex 3a (entries 4 and 5). Con- of cis double bonds in the polymer backbone, thereby
versely, the presence of 4-methyl or 4-methoxy groups underlining the influence of the ancillary ligand on
slightly reduced the reaction rate. This effect was best the polymerization outcome.
evidenced after 1 h and started to level off after 2 h
Compared to norbornene, cyclooctene is signifi-
(entries 6 and 7). Eventually, almost quantitative cantly more difficult to ring-open.[27] Hence, forma-
yields of (E)-4,4’-dimethylstilbene or (E)-4,4’-dime- tion of polyoctenamer occurs only at a reasonable
thoxystilbene were obtained after 5 h. With the rate with highly active catalytic systems. With this mo-
strongly deactivated 4-trifluoromethylstyrene, the re- nomer, a clear-cut distinction could be established be-
action rate further dropped. A decent 70% conver- tween complex 2a that did not afford any reaction
sion was reached after 2 h (entry 8) but did not signif- after 2 h at 608C and the NHC-containing ruthenium-
icantly increase within the next 3 h allowed to the re- arene species 3a and 3b, which led to complete con-
Adv. Synth. Catal. 2007, 349, 255 – 265
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259

Xavier Sauvage et al.
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Table 3. Ring-opening metathesis polymerization of norbornene and cyclooctene catalyzed by complexes 2a, 3a, and 3b at
608C.
[b]
[c]
Monomer
Catalyst
Conversion [%][a]
Yield [%]
Mn [kgmolÀ1][b]
Mw/Mn
scis
Norbornene[d]
2a
3a
3b
2a
3a
3b
>99
>99
>99
0
>99
>99
88
91
87
0
93
95
115
480
150
-
250
230
5.1
1.5
1.8
-
1.8
1.6
0.18
0.43
0.39
-
0.26
0.34
Cyclooctene[e]
[a]
Determined by GC using internal standards.
[b]
[c]
[d]
[e]
Determined by size-exclusion chromatography in THF with polystyrene calibration.
Fraction of cis double bonds in the polymer, determined by 13C NMR spectroscopy.
Experimental conditions: Ru cat. (0.015 mmol), PhCl (12 mL), norbornene (15 mmol), 2 h at 608C.
Experimental conditions: Ru cat. (0.03 mmol), PhCl (5 mL), cyclooctene (7.5 mmol), 2 h at 608C.
versions under the same experimental conditions follow the mechanism commonly accepted for
(Table 3). Although the polymerizations were not ROMP[30] and lead to chain growth (Scheme 2).
controlled, almost quantitative yields of high molecu-
In a final series of experiments, we probed the cata-
lar weight polymers were obtained without the need lytic activity of complexes 2a, 3a, and 3b in the ring-
to add a diazo compound, nor to photochemically ac- closing metathesis (RCM) of two benchmark sub-
tivate the catalyst precursor, as previously required strates, viz. diethyl 2,2-diallylmalonate (4) and N,N-di-
with monometallic ruthenium-arene complexes bear- allyltosylamide (5). Preliminary tests were carried out
ing phosphine[5] or NHC[12] ligands, respectively. The in toluene at 858C using 2 mol% of ruthenium cata-
intimate details of the mechanism remain, however, lyst. Under these conditions, no reaction occurred
elusive. Only a small amount of active species are with the phosphine-based bimetallic species 2a.
generated from the bulk of catalyst precursor and all Indeed, GC or NMR analyses performed to monitor
our attempts to acquire spectroscopic evidence for the transformation of substrates 4 and 5, respectively,
the transformation of ruthenium-arene complexes did not show any sign of conversion (Table 4, entries 1
into the actual ruthenium-alkylidene centers required and 4). With the NHC-containing catalyst precursor
to initiate olefin metathesis have remained unsuccess- 3a, full consumption of the starting materials occurred
ful so far. While thermal or photochemical arene de- within 2 h. The expected RCM cycloadduct A was,
coordination was postulated to be the first step with however, accompanied by a higher molecular weight
monometallic complexes of type 1, results obtained product B formed in almost equimolar proportion
here suggest that homobimetallic ruthenium com- (entries 2 and 5). Structural analysis of this product
plexes 3a and 3b could be activated via an alternative by NMR and GC-MS revealed the presence of adja-
pathway involving exchange of the labile ethylene cent methyl and exo-methylene groups on a five-
ligand for a cycloolefin monomer. Formation of a re- membered ring. Thus, ruthenium complex 3a acted as
active carbene moiety could then occur through direct catalyst precursor for both RCM and cycloisomeriza-
(albeit unfavorable) equilibration, as proposed by tion of the acyclic a,w-dienes. A similar twofold reac-
Mühlebach and co-workers,[28] although a more com- tion was also observed with complex 3b (entries 3 and
plex sequence involving the intermediacy of rutheni- 6). Yet, conversions were slightly lower and cycloiso-
um-hydride species cannot be ruled out.[29] Subse- merization took precedence over RCM, in line with
quent steps of metallacyclization and cycloreversion the reduced metathetical activity of 3b compared to
3a. Reports from the groups of Kurosawa[31] and Dix-
neuf[32] had already established that selected monome-
tallic ruthenium-arene complexes displayed such a
dual activity. A mechanism involving oxidative cou-
pling of the diene to a ruthenium(II) center followed
by b-elimination to generate a hydrido-ruthenium(IV)
intermediate and reductive elimination was proposed
to account for the cycloisomerization process
(Scheme 3).[32b]
The Japanese and French teams were able to tune
their catalytic systems to promote exclusively the
Scheme 2. Possible mechanism for the ROMP of cycloole-
fins.
transformation of a,w-dienes into metathesis prod-
260
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Adv. Synth. Catal. 2007, 349, 255 – 265

Homobimetallic Ruthenium–N-Heterocyclic Carbene Complexes
FULL PAPERS
Table 4. Ring-closing metathesis and cycloisomerization of
a,w-dienes catalyzed by complexes 2a, 3a and 3b at 858C.
Entry
Substrate
Catalyst
Conversion [%]
A:B
1
2
3
4
5
6
4[a]
4[a]
4[a]
5[b]
5[b]
5[b]
2a
3a
3b
2a
3a
3b
0
-
99
98
0
98
96
53:47
40:60
-
49:51
41:59
Scheme 4. Catalytic cyclic for the RCM of a,w-dienes in the
presence of a terminal alkyne.
[a]
Experimental conditions: Ru cat. (0.004 mmol), substrate
(2 mL of 0.1M solution in toluene, 0.2 mmol), 2 h at
858C. Conversions and product distributions were deter-
mined by GC using n-dodecane as internal standard.
Experimental conditions: Ru cat. (0.002 mmol), substrate
(1 mL of 0.1M solution in toluene-d8, 0.1 mmol), 2 h at
858C. Conversions and product distributions were deter-
and 5 were reacted with catalyst precursors 2a, 3a,
and 3b (2 mol%) in the presence of phenylacetylene
(6 mol%). The results are listed in Table 5. To our
great satisfaction, the alkyne co-catalyst was highly ef-
fective at suppressing the cycloisomerization process
altogether. At the same time, it significantly increased
the rate of metathesis. Hence, with catalyst 3a addi-
[b]
1
mined by H NMR spectroscopy.
Table 5. Ring-closing metathesis of a,w-dienes catalyzed by
complexes 2a, 3a and 3b at 25 or 858C in the presence of
phenylacetylene.
Entry Substrate Catalyst Temperature
Conversion
[%]
Scheme 3. Catalytic cycle for the cycloisomerization of a,w-
dienes.
[8C]
1
2
3
4
5
6
4[a]
4[a]
4[a]
5[b]
5[b]
5[b]
2a
3a
3b
2a
3a
3b
85
25
25
85
25
25
81
98
87
62
99
71
ucts, while completely inhibiting the cycloisomeriza-
tion process, by adding a small amount of terminal
alkyne to the reaction media.[31,32] The alkyne co-cata-
lyst probably reacts with coordinatively unsaturated
ruthenium centers to form vinylidene species. Al-
though less active than their alkylidene counterparts,
complexes featuring an Ru=C=CHR moiety are well-
known initiators for various types of olefin metathesis,
including ROMP and RCM (Scheme 4).[33]
[a]
Experimental conditions: Ru cat. (0.004 mmol), phenyl-
acetylene (0.012 mmol), substrate (2 mL of 0.1M solution
in toluene, 0.2 mmol), 2 h. Conversions and product dis-
tributions were determined by GC using n-dodecane as
internal standard.
[b]
Experimental conditions: Ru cat. (0.002 mmol), phenyl-
acetylene (0.006 mmol), substrate (1 mL of 0.1M solution
in toluene-d8, 0.1 mmol), 2 h. Conversions and product
We have applied this strategy to alter the course of
the reaction between a,w-dienes and our homobime-
tallic ruthenium-arene complexes. Thus, substrates 4
1
distributions were determined by H NMR spectroscopy.
Adv. Synth. Catal. 2007, 349, 255 – 265
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www.asc.wiley-vch.de
261

Xavier Sauvage et al.
FULL PAPERS
tion of phenylacetylene allowed us to lower the reac- bornene or cyclooctene. When a,w-dienes were ex-
tion temperature from 858C to 258C while maintain- posed to 3a or 3b, a mixture of cycloisomerization
ing a quantitative conversion within 2 h (entries 2 and and RCM products was obtained in a non-selective
5). Replacement of the IMes ligand with its 4,5-di- way. Addition of a terminal alkyne co-catalyst provid-
chloro derivative in complex 3b once again reduced ed a convenient method to enhance the metathetical
the catalytic efficiency toward metathesis, although activity while completely repressing the cycloisomeri-
more than satisfactory conversions were achieved al- zation process. Thus, quantitative conversions of sub-
ready at 258C in the presence of the alkyne co-cata- strates 4 and 5 were achieved within 2 h at room tem-
lyst (entries 3 and 6). Complex 2a that was completely perature using 2 mol% of catalyst precursor 3a and 6
inactive on its own (cf. Table 4) also benefited from mol% of phenylacetylene.
the adjuvant and became moderately active at pro-
To sum up, our results clearly demonstrate that an-
moting RCM. Heating was, however, still required to cillary ligands exert a critical influence on the catalyt-
achieve satisfactory conversions (entries 1 and 4). In ic activity of homobimetallic ruthenium-arene com-
all cases, small amounts of 2-phenylbutadiene were plexes. Whereas phosphine-containing species are
detected in the reaction media (up to 6 mol% com- active only for controlled radical reactions, the new
pared to the main cycloadducts). Formation of this compounds 3a and 3b bearing NHC ligands are highly
byproduct formally results from an enyne metathesis efficient catalyst precursors for various types of olefin
between phenylacetylene and ethylene. A control ex- metathesis. Thus, they significantly broaden the appli-
periment was performed by stirring a 0.1M phenyl- cation field of their predecessors 2a and 2b, and they
acetylene solution in toluene under 0.5 bar of ethyl- provide a valuable alternative to preformed rutheni-
ene for 1 h at 258C in the presence of complex 3a (2 um-alkylidene catalysts for initiating metathetical
mol%). After solvent evaporation and fractional dis- transformations in polymer chemistry and fine organ-
tillation under reduced pressure, 2-phenylbutadiene ic synthesis.
was isolated in 98% yield [Eq. (1)].
Experimental Section
General Remarks
All syntheses were carried out under an inert atmosphere
using standard Schlenk techniques. Solvents and monomers
were distilled from appropriate drying agents and deoxygen-
Conclusions
ated prior to use. The [RuCl2(p-cymene)]2 dimer was pur-
A
chased from Strem. 1,3-Bis(2,4,6-trimethylphenyl)imidazo-
lin-2-ylidene (IMes)[7b,c] and 1,3-bis(2,4,6-trimethylphenyl)-
4,5-dichloroimidazolin-2-ylidene (IMesCl2)[17] were synthe-
By heating a toluene solution of the commercially
available [RuCl2(p-cymene)]2 dimer and 1 equivalent
ACHTREUNG
1
sized according to published procedures. H and 13C NMR
of IMes or IMesCl2 under an ethylene atmosphere,
we were able to isolate two new homobimetallic
ruthenium-arene complexes 3a and 3b in high yields.
The catalytic activity of these new compounds bearing
NHC ligands was first investigated in the ATRP of
vinyl monomers. In the polymerization of MMA,
complex 3a displayed faster reaction rates than 3b
and the related phosphine-based complex 2a, al-
though control was more effective with the latter cat-
alyst. When n-butyl acrylate or styrene served as mo-
nomer, a major shift of reactivity was observed be-
tween complex 2a that promoted controlled radical
polymerization, and complexes 3a or 3b that favored
metathetical coupling. Further homocoupling experi-
ments with various styrene derivatives confirmed the
outstanding aptitude of complex 3a (and to a lesser
extent of 3b) to catalyze olefin metathesis reactions.
Contrary to monometallic ruthenium-arene com-
plexes of type 1 that were previously investigated in
our group, the new homobimetallic species of type 3
did not require the addition of a diazo compound or
visible light illumination to initiate the ROMP of nor-
spectra were recorded at 298 K on a Bruker DRX 400 spec-
trometer operating at 400.13 and 100.62 MHz, respectively.
Chemical shifts are listed in parts per million downfield
from TMS and are referenced from the solvent peaks or
TMS. Thermogravimetric analyses were performed on a TA
Q500 instrument. Gas chromatography was carried out on a
Varian 3900 instrument equipped with a flame ionization de-
tector and a WCOT fused silica column (stationary phase:
CP-Sil 5CB; column length: 15 m; inside diameter:
0.25 mm; outside diameter: 0.39 mm; film thickness:
0.25 mm). Gel permeation chromatography was performed
in THF at 458C on a SFD S5200 autosampler liquid chroma-
tograph equipped with a SFD 2000 refractive index detector
and a battery of 4 PL gel columns fitted in series (particle
size: 5 mm; pore sizes: 105, 104, 103, and 102 Š; flow rate:
1 mLminÀ1). The molecular weights (not corrected) are re-
ported versus monodisperse polystyrene or PMMA stand-
ards used to calibrate the instrument. Elemental analyses
were carried out in the Laboratory of Pharmaceutical
Chemistry at the University of Li›ge.
262
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2007, 349, 255 – 265

Homobimetallic Ruthenium–N-Heterocyclic Carbene Complexes
FULL PAPERS
was used (see Table 1 for further information on the initia-
tor, temperature, and reaction time).
Synthesis of Complexes 3a and 3b
A suspension of [RuCl2(p-cymene)]2 (0.4 g, 0.65 mmol) and
A
IMes or IMesCl2 (0.65 mmol) in dry toluene (20 mL) was
heated at 708C for 24 h under an ethylene atmosphere.
After cooling to room temperature, the reaction mixture
was allowed to settle down and the supernatant solution was
removed with a cannula. The solid product was rinsed with
small portions of n-pentane and dried under high vacuum.
Complex 3a was isolated as an orange powder; yield:
0.37 g (70%). 1H NMR (400 MHz, CD2Cl2 +C2H4, 258C):
Self-Metathesis of Styrene
A ruthenium complex (0.004 mmol) was placed in a 15-mL
Schlenk tube containing a magnetic stirring bar and capped
with a septum. Air was expelled by three vacuum-argon
cycles before 2 mL of a styrene solution (1M in toluene,
2 mmol) was added with a dried syringe under argon. The
reaction mixture was heated for 2 h in an oil bath at 858C
under an inert atmosphere. The conversion was monitored
by gas chromatography using n-dodecane as internal stan-
dard. After cooling to room temperature, the crude mixture
was purified by column chromatography on silica gel using a
10:1 v/v mixture of petroleum ether (b.p. 40–608C) and
ethyl acetate as eluent to afford pure (E)-stilbene.
d=1.19 [d, 3JH,H =6.4 Hz, 6H, CH
CH3 Mes+CH3 p-cym), 2.06 (s, 6H, o-CH3 Mes), 2.29 (s,
(CH3)2], 2.03 (s, 9H, o-
3
6H, p-CH3 Mes), 2.68 [sept, JH,H =6.4 Hz, 1H, CH(CH3)2],
A
3
3.23 (m, 2H, C2H4), 3.60 (m, 2H, C2H4), 4.97 (d, JH,H
=
3
5.6 Hz, 1H, CHar p-cym), 5.05 (d, JH,H =5.6 Hz, 1H, CHar p-
cym), 5.24 (2 H CHar p-cym+free C2H4), 6.56 (s, 2H, CH
Im), 6.83 (d, J=6.0 Hz, 4H, CHar Mes); 13C NMR
(100 MHz, CD2Cl2 +C2H4, 258C): d=17.9 (CH3), 18.4
(CH3), 18.5 (CH3), 20.5 (CH3), 20.8 (CH3), 22.0 (CH3), 30.6
Other styrene derivatives were reacted using the same ex-
perimental procedure (see Table 2 for their list).
[CH(CH3)2], 60.0 (C2H4), 77.6 (CHar p-cym), 77.9 (CHar p-
E
cym), 78.2 (CHar p-cym), 78.7 (CHar p-cym), 95.1 (Car p-
cym), 100.4 (Car p-cym), 125.6 (CH Im), 127.8 (CHar Mes),
136.5 (Car Mes), 136.7 (Car Mes), 137.6 (Car Mes), 167.8
(CRu Im); analysis (%) calcd. for C33H42Cl4N2Ru2 (810.65):
C 48.89, H 5.22, N 3.46; found C 50.36, H 5.32, N 3.06.
Complex 3b was isolated as an orange powder; yield:
0.46 g (80%). 1H NMR (400 MHz, CD2Cl2 +C2H4, 258C):
ROMP of Cycloolefins
A ruthenium complex (0.03 mmol) was placed in a 25-mL
round-bottom flask containing a magnetic stirring bar and
capped with a three-way stopcock. Air was expelled by
three vacuum-argon cycles before dry chlorobenzene (5 mL)
and cyclooctene (1 mL, 7.5 mmol) were added. All liquids
were handled with dried syringes under argon. The reaction
mixture was stirred for 2 h in an oil bath at 608C under an
inert atmosphere. The conversion was monitored by gas
chromatography using the cyclooctane impurity of cyclooc-
tene as an internal standard. After cooling, the resulting gel
was diluted with CHCl3 (20 mL) and slowly poured into
methanol (500 mL) under vigorous stirring. The precipitated
polyoctenamer was filtered with suction, dried under dy-
namic vacuum, and characterized by GPC and NMR spec-
troscopy.
d=1.27 [d, 3JH,H =6.4 Hz, 6H, CH
CH3 Mes), 2.11 (s, 3H, CH3 p-cym), 2.13 (s, 6H, o-CH3
A
Mes), 2.75 [sept, 3JH,H =6.4 Hz, 1H, CH
C
2H, C2H4), 3.66 (m, 2H, C2H4), 5.05 (d, JH,H =4.0 Hz, 1H,
3
CHar p-cym), 5.14 (d, JH,H =4.0 Hz, 1H, CHar p-cym), 5.40
(2 H CHar p-cym+free C2H4), 6.96 (d, J=4.8 Hz, 4H, CHar
Mes); 13C NMR (100 MHz, CD2Cl2 +C2H4, 258C): d=17.9
(CH3), 18.4 (CH3), 20.6 (CH3), 21.4 (CH3), 21.9 (CH3), 30.5
[CH(CH3)2], 61.0 (C2H4), 77.5 (CHar p-cym), 77.8 (CHar p-
R
cym), 78.2 (CHar p-cym), 78.8 (CHar p-cym), 95.3 (Car p-
cym), 100.4 (Car p-cym), 117.9 (CCl Im), 128.0 (CHar Mes),
134.1 (Car Mes), 137.6 (Car Mes), 137.8 (Car Mes), 138.7 (Car
Mes), 172.3 (CRu Im); analysis (%) calcd. for
C33H40Cl6N2Ru2 (879.54): C 45.06, H 4.58, N 3.18; found C
44.12, H 4.88, N 3.07.
Norbornene was polymerized following a similar proce-
dure (see Table 3 for further information on the relative
proportions of catalyst, monomer, and solvent).
RCM of Diethyl 2,2-Diallylmalonate
A ruthenium complex (0.004 mmol) was placed in a 15-mL
Schlenk tube containing a magnetic stirring bar and capped
with a septum. Air was expelled by three vacuum-argon
cycles before 2 mL of a diethyl 2,2-diallylmalonate solution
(0.1M in toluene, 0.2 mmol) possibly containing 6 mol% of
phenylacetylene was added with a dried syringe under
argon. The reaction mixture was stirred for 2 h at 25 or
858C. Conversions and product distributions were moni-
tored by GC using n-dodecane as internal standard.
ATRP of Vinyl Monomers
A ruthenium complex (0.0117 mmol) was placed in a glass
tube containing a magnetic stirring bar and capped with a
three-way stopcock. Air was expelled by three vacuum-ni-
trogen cycles before methyl methacrylate (1 mL, 9.35 mmol)
and ethyl 2-bromo-2-methylpropionate (0.1M in toluene,
0.25 mL) were added. All liquids were handled with dried
syringes under nitrogen. The reaction mixture was heated
for 16 h in an oil bath at 858C under an inert atmosphere.
After cooling to room temperature, it was diluted with THF
(5 mL) and poured in n-heptane (600 mL) under vigorous
stirring. The precipitated polymer was filtered with suction,
dried overnight under dynamic vacuum, and characterized
by GPC.
RCM of N,N-Diallyltosylamide
A ruthenium complex (0.002 mmol) was placed in a NMR
tube equipped with a J. Young valve. Air was expelled by
three vacuum-argon cycles before 1 mL of a N,N-diallyltosyl-
amide solution (0.1M in toluene-d8, 0.1 mmol) possibly con-
taining 6 mol% of phenylacetylene was added with a dried
syringe under argon. The reaction was monitored by
1H NMR spectroscopy for 2 h at 25 or 858C.
n-Butyl acrylate and styrene were polymerized according
to similar procedures. Poly(n-butyl acrylate) was isolated by
evaporating the reaction mixture. Polystyrene was precipi-
tated from methanol. In all experiments, 1 mL of monomer
Adv. Synth. Catal. 2007, 349, 255 – 265
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
263

Xavier Sauvage et al.
FULL PAPERS
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Acknowledgements
We thank the “Fonds National de la Recherche Scientifique”
(FNRS), Brussels, for the purchase of major instrumentation.
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