Latent thermo-switchable olefin metathesis initiators bearing a pyridyl-functionalized chelating carbene: Influence of the leaving group's rigidity on the catalyst's performance

Article abstract of DOI:10.1021/om900857w

The synthesis and characterization of two ruthenium complexes bearing chelating carbene ligands is described. Carbene precursors, 2-(2-vinylphenyl) pyridine and 10-vinylbenzo[h]quinoline, are applied to prepare (SPY-5-31)-dichloro-(k² (C,N)-N-2-(2-vinylbenzylidene)pyridine(l,3- bis(2,4, 6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)ruthenium (VIII) and (SPY-5-31)-dichloro-(k²(C,N)-2-(benzo[/!]quinolin-10-yl)methylidene) (l,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)ruthenium (IX). Both catalysts/initiators are used to perform ring-closing metathesis (RCM) and ringopening metathesis polymerizations (ROMP). RCM experiments reveal significant thermal stability of the catalysts under forcing reaction conditions such as boiling toluene for 48 h. Even challenging substrates such as diethylallyl(2-methylallyl)malonate are completely transformed with low catalyst loadings (0.1 mol % at 110 °C). The high thermal stability, i.e., latency, might be explained by a slow generation of the catalytically active methylidene species. This feature leads to high molecular weight polymers and a thermal switchability in ROMP. Initiation of polymerizations of several norbornene derivatives occurs at about 48 ± 5 °C in the case of initiator VIII and at 110 ± 9 °C in the case of IX. A substantial increase of the switching temperature in the following cases could be supported with higher rigidity of the chelating carbene moiety in IX when compared to VIII.

Full text of DOI:10.1021/om900857w

Organometallics 2010, 29, 117–124 117
DOI: 10.1021/om900857w
Latent Thermo-Switchable Olefin Metathesis Initiators Bearing a
Pyridyl-Functionalized Chelating Carbene: Influence of the Leaving
Group’s Rigidity on the Catalyst’s Performance
Anna Szadkowska,† Xaver Gstrein,‡ Daniel Burtscher,‡ Katarzyna Jarzembska,§
§
Krzysztof Wozniak, Christian Slugovc,* and Karol Grela*
,‡
,†
ꢀ
†Institute of Organic Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warsaw,
Poland, ‡Institute for Chemistry and Technology of Materials (ICTM), Graz University of Technology,
Stremayrgasse 16 A 8010 Graz, Austria, and Department of Chemistry, Warsaw University, Pasteura 1,
§
02-093 Warsaw, Poland
Received October 2, 2009
The synthesis and characterization of two ruthenium complexes bearing chelating carbene ligands
is described. Carbene precursors, 2-(2-vinylphenyl)pyridine and 10-vinylbenzo[h]quinoline, are
applied to prepare (SPY-5-31)-dichloro-(κ2(C,N)-N-2-(2-vinylbenzylidene)pyridine(1,3-bis(2,4,
6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)ruthenium (VIII) and (SPY-5-31)-dichloro-(κ2(C,N)-
2-(benzo[h]quinolin-10-yl)methylidene)(1,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)-
ruthenium (IX). Both catalysts/initiators are used to perform ring-closing metathesis (RCM) and ring-
opening metathesis polymerizations (ROMP). RCM experiments reveal significant thermal stability of
the catalysts under forcing reaction conditions such as boiling toluene for 48 h. Even challenging
substrates such as diethylallyl(2-methylallyl)malonate are completely transformed with low catalyst
loadings (0.1 mol % at 110 °C). The high thermal stability, i.e., latency, might be explained by a slow
generation of the catalytically active methylidene species. This feature leads to high molecular weight
polymers and a thermal switchability in ROMP. Initiation of polymerizations of several norbornene
derivatives occurs at about 48 ( 5 °C in the case of initiator VIII and at 110 ( 9 °C in the case of IX.
A substantial increase of the switching temperature in the following cases could be supported with higher
rigidity of the chelating carbene moiety in IX when compared to VIII.
Introduction
tolerance. A lot of effort has been aimed at altering the alky-
lidene moieties connected to the ruthenium core and at using
diverse types of anionic and neutral ligands. Recently, phos-
phine-free NHC-bearing ruthenium initiators have caused
remarkable effects on catalytic activity and stability.1-4
Nowadays, aspirations in the area of olefin metathesis are
still directed toward reactivity improvements of more de-
manding substrates.
Systematic studies on the reactivity of phosphine-free
NHC-Ru complexes have shown that the influence of steric
hindrance is essential.2 However, if the catalyst is overactive
under the given conditions, it might be difficult or even
impossible to impose a reasonable control over molecular
properties in ROMP or to achieve complete conversions in
RCM. This is because of fast catalyst degradation.5-8 In
some industrial setups of ROMP, for example RIM, it is
required to store a mixture of a monomer and initiator before
the metathesis process occurs.7,8 The presence of a NfRu,
A considerable amount of research has been done recently
in the field of new catalysts/initiators for olefin metathesis
reactions. Some discoveries in the development of catalysts,
which commenced in the early 1990s, promoted applications
of metals such as molybdenum and ruthenium (Figure 1) in
olefin metathesis.1-4 Especially ruthenium-based systems
have found widespread applications in organic synthesis,
particularly because of their extensive functional group
*Corresponding authors. E-mail: klgrela@gmail.com (K.G.); slugovc@
tugraz.at (C.S.).
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2009 American Chemical Society
Published on Web 12/14/2009
pubs.acs.org/Organometallics

118 Organometallics, Vol. 29, No. 1, 2010
Szadkowska et al.
Figure 1. Selected catalysts for olefin metathesis.
OfRu, or SfRu chelate can favor the resting state of the
precatalyst over its metathesis active form (Figure 2).9-12
After the initiation event (controlled by temperature,
light, or a chemical agent) and the first catalytic turnover, the
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room or slightly above room temperature are called “latent” or “dor-
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catalyst but also on monomer structure, solvent, and temperature, these
terms should be used with care.
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N- O-, and S-chelating ligands.
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loss of the donor group allows catalysis to proceed
quickly.10c,e-g This switch in initiator structure from che-
lated to nonchelated form makes the above-mentioned strat-
egy successful. It enables us not only to catalyze the
metathesis reactions but also to perform them in a well-
controlled manner.10e,11-17 Potential applications of such
catalysts are not limited to polymer chemistry only, but
could also be used in multitransformation or multicompo-
nent synthetic sequences involving a metathesis step at
higher temperatures.
The influence of the NfRu chelation on the catalytic activity
is still the subject of numerous investigations. In this contribu-
tion, we describe the effect of the rigidness of the N-chelating
ligand on the catalytic performance of the formed ruthenium
complex in RCM and ROMP. Our previous work described
some catalysts containing imine-based N-chelating ligands as
well as quinoline and quinoxaline ones. In general, these
compounds possess quite a good stability and can potentially
be applied at high temperatures. These results have encouraged
us to conduct yet another design leading to another class
of NfRu chelated carbene complexes.10f,13a,b We have
anticipated that using strongly chelating rigid ligands
(10-benzo[h]quinoline and 2-phenylpyridine) would lead to
initiators exhibiting favorable properties in the applications
mentioned above.
(14) Gulajski, L.; Michrowska, A.; Bujok, R.; Grela, K. J. Mol.
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Bieniek, M.; Grela, K. New Concepts in Designing Ruthenium based
Second Generation Olefin Metathesis Catalysts and their Application.
In Catalysis of Organic Reactions: Twenty-second Conference; Prunier,
M. L., Ed.; Chemical Industries Series, Vol. 123; CRC Press, 2008; p 568;
(b) Grela, K.; Szadkowska, A.; Barbasiewicz, M.; Kadyrow, R. (Degussa
AG) Neuartige schwefelhaltige Metathese Katalysatoren, DE Patent
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druck, C. E.; Sigalov, M.; Goldberg, I.; Lemcoff, N. G. Organometallics
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Article
Organometallics, Vol. 29, No. 1, 2010 119
Scheme 1. Synthesis of Compound VIII
Scheme 3. Synthesis of Ru-Initiator IX
Scheme 2. Alternative Synthesis of Ligand 7 and Complex VIII
of 5 mol % of dichloro(1,5-cyclooctadiene)ruthenium(II) to
yield two isomeric products: 6 and 7. Complexes 6 and 7 were
formed in a ratio of 80:20, according to GC analysis. This
isomeric mixture was not separated, but further isomerized,
to 7 in pure form. This was achieved by using Krompiec
(PPh3)2ClRu(H)(CO) catalyst (2.5 mol %) in dry toluene.19
This reaction proceeded smoothly to give 2-(2-prop-1-en-
yl)phenyl)pyridine (7) in 80% yield as the only product.
Synthesis of the catalyst VIII employed the metathesis-
ligand exchange reaction between Ind-II and 7 in the pre-
sence of CuCl, completed with the final 60% conversion into
VIII in pure form (after chromatographic workup).
Complex VIII was found to be perfectly air stable and was
fully characterized by spectral techniques and by elemental
1
analysis. The H NMR for VIII was consistent with the
expected Cs symmetric structure, and the benzylidene proton
resonance signal appeared at 18.85 ppm. The 13C NMR
spectrum showed the carbene carbon at 314.7 ppm, while the
NHC carbon gave a resonance at 216.5 ppm.
Results and Discussion
As the aim of the current work was to check whether
decreasing the flexibility of the chelating carbene moiety
might result in increased latency of the catalyst/initiator, the
catalyst IX was prepared. In IX, more pronounced rigidity
was introduced by using a more π-conjugated chelating
carbene ligand (Scheme 3). In the first step of the synthesis
of the ligand precursor, compound 9 was synthesized via
esterification of 10-hydoxybenzo[h]quinoline (8) with triflic
anhydride in pyridine.20 Further, in the Suzuki-Miyaura
cross-coupling reaction compound 10 was made,21 which
then was used to obtain the initiator IX via a carbene
exchange with Gr-III0 as the ruthenium source.
The preparation of the presented initiators was straight-
forward, based on well-known transformations. The synth-
esis of the designed precursor 4 consisted of a set of
Suzuki-Miyaura cross-couplings, using a 2,4,6-trivinylcy-
clotriboroxane-pyridyne complex to introduce the vinyl
group to the ligand precursor (Scheme 1). As shown in the
same scheme, catalyst 5 was obtained by the carbene ex-
change reaction of (H2IMes)(pyridine)2(Cl)2RudCHPh
(Gr-III0) with the vinyl derivative 4 described above. A
mixture of 1 equiv of Gr-III0 and 2 equiv of 4 was subjected
to overnight stirring in CH2Cl2 at 25 °C, leading to formation
of VIII as green microcrystals in 52% yield by precipitation
upon addition of Et2O.
1
In general, the H NMR spectrum of initiator IX was
similar to the one observed for the catalyst VIII (Cs
symmetry). The characteristic carbene proton appeared at
19.19 ppm. In the 13C NMR spectrum, the carbene carbon
Encouraged by the report on ortho-selective allylation of
2-pyridylarenes published by Inoue and co-workers, we tried
an alternative ligand synthesis (Scheme 2).18 In this case, the
synthesis of VIII starts from inexpensive 2-phenylpyri-
dine 5, which is allylated using allyl acetate, in the presence
(19) Krompiec, S.; Kuznik, N.; Bieg, T.; Adamus, B.; Majnusz, J.;
Grymel, M. Pol. J. Chem. 2000, 74, 1197.
(20) Stille, J. K.; Echavarren, A. M. J. Am. Chem. Soc. 1987, 109,
5478.
(18) Oi, S.; Tanaka, Y.; Inoue, Y. Organometallics 2006, 25, 4773.
(21) Kerins, F.; O Shea, D. F. J. Org. Chem. 2002, 67, 9681.

120 Organometallics, Vol. 29, No. 1, 2010
Szadkowska et al.
Figure 3. ORTEP drawing of two independent molecules of IX (the upper one disordered) represented by atomic displacement
parameters drawn at the 50% probability level. Hydrogen atom labels are excluded for clarity.
˚
Table 1. Selected Bond Lengths (A) and Angles (deg)
appeared at 290.4 ppm and the NHC carbon was observed at
215.5 ppm.
parameter
value
Having in mind that the trans-cis isomerization process
occurred in some previous N-chelating catalysts,12 we tried
to measure the kinetics of isomerization of the catalysts VIII
Ru(1)-C(22)
Ru(1)-C(1)
Ru(1)-N(3)
Ru(1)-Cl(2)
Ru(1)-Cl(1)
Ru(2)-C(62B)/Ru(2)-C(62A)
Ru(2)-C(41)
1.814(4)
2.039(4)
2.107(3)
2.339(1)
2.346(1)
1.661(18)/1.932(16)
2.039(4)
2.121(14)/2.117(16)
2.338(1)
2.358(1)
1
and IX to their cis-dichloro counterparts. Monitoring H
NMR spectra of VIII or IX at 23 °C in CD2Cl2 for two
months13a,b led us to conclude that no isomerization was
taking place. It is worth noting that also no sign of decom-
position of the compounds was observed. These results
correlate very well with the observed high stability of com-
plexes VIII and IX in the solid state.
To learn more about the structural peculiarities exhibited
by VIII and IX, we focused on obtaining single-crystal X-ray
crystallographic data for the complexes. Unfortunately,
despite many trials of crystallization of VIII, single crystals
suitable for X-ray measurement were not obtained. In the
case of IX, a single crystal of acceptable quality was obtained
by a slow diffusion of n-hexane into benzene solution. The
crystal structure was successfully solved and refined and is
shown in Figure 3. Other structural information for catalyst
IX is given in Table 1.
Compound IX crystallized in the tetragonal P42/n space
group with two independent molecules in the asymmetric
part of the unit cell, forming green needle-shaped crystals.
Both ruthenium complex molecules occupy general posi-
tions, and there is one disordered water molecule (in the
form of two halves) per two independent Ru moieties in the
crystal structure. These disordered water molecules are
Ru(2)-N(43B)/Ru(2)-N(43A)
Ru(2)-Cl(4)
Ru(2)-Cl(3)
C(22)-Ru(1)-C(1)
99.42(18)
C(1)-Ru(1)-N(3)
171.24(15)
88.82(17)
160.36(4)
C(22)-Ru(1)-N(3)
Cl(2)-Ru(1)-Cl(1)
C(62B)-Ru(2)-C(41)/C(62A)-Ru(2)-C(41)
104.0(7)/95.7(5)
C(62B)-Ru(2)-N(43A)/C(62B)-Ru(2)-N(43B) 77.4(7)/90.4(7)
C(62A)-Ru(2)-N(43A)/C(62A)-Ru(2)-N(43B) 85.8(6)/98.8(6)
C(41)-Ru(2)-N(43A)/C(41)-Ru(2)-N(43B)
Cl(4)-Ru(2)-Cl(3)
176.2(4)/165.5(4)
160.28(5)
located close to the 4 axes and thus form a regular, almost
planar, octagon (see red dots in Figure 4). Among these two
independent molecules of complex IX, one comprises atoms
of very well determined positions while the other one exhibits
a high degree of disorder in the ylidene group. In the case of
the disordered ylidene fragment, one benzene ring takes two
relatively well-defined positions in space, whereas the second
benzene ring apparently (large ADP values) is trying to
occupy two positions, although the resolution of the data
is not good enough to distinguish them. The benzoquinoline

Article
Organometallics, Vol. 29, No. 1, 2010 121
Figure 4. Projection of the unit cell contents along the z-axis. Water oxygen atoms are illustrated as red dots; red squares, 42 axis; black-
striped squares, 4 axis.
molecular fragment is also disordered and occupies two
alternative positions in the crystal lattice. However, the bond
lengths in the first molecule do not differ much from the
corresponding parameters in the second molecule (there is
only a slight difference in the length of the Ru-N bond:
(see Table 1), agrees well with related complexes studied in
recent years.12,13a However, the compound stands out by
being comparatively distorted in the chelating ring part. The
C(22)-Ru-N(3) angle is equal to 88.82(17)°, whereas all
other angles forming the chelating six-membered ring are
larger than 120°. Furthermore, the chelating six-membered
ring is highly distorted out of the plane. If one chooses a
plane passing by C(22), C(23), C(35), and C(34), to the N(3)
atoms, Ru lies slightly out of this plane, which corresponds
to a slight bending of the benzo[h]quinoline moiety with
respect to the Ru-N bond.
In order to establish a structure/reactivity relationship for
the two complexes, three model RCM reactions were used.
Catalysts VIII and IX were tested in ring-closing metathesis
of N,N-diallyltosylamine (S1), RCM of diethyldiallyl mal-
onate (S2), and cycloisomerization of enyne (S3) (Table 2).
The course of each reaction was monitored by GC with
n-dodecane or n-tetradecane as internal standard, measuring
the increase in the amount of product with time. In all
reactions tested, the initiator IX performed significantly
slower, as compared to the catalyst VIII under identical
conditions. Interestingly, the model reactions were far from
completion, even after 24 h. No byproduct formation was
detected, and it should be noted that after that time pure
initiator was observed in both reaction mixtures.
˚
2.107(3) and 2.121(14)/2.117(16) A in the ordered and dis-
ordered moiety, respectively). Consequently, both ruthe-
nium centers exhibit a deformed quadratic pyramid
geometry with the carbene carbon atom occupying the top
position and two chloro ligands in a trans arrangement
(SPY-5-31 isomer) with very similar bond lengths and va-
lence angles (Table 1). The base of the square pyramid is
formed by the two above-mentioned chloro ligands, Cl(1)
and Cl(2), the benzo[h]quinoline nitrogen N(3), and the C(1)
atom of the H2IMes ligand, while the carbene carbon atom,
C(22), forms the apex. Molecules of complex IX are arranged
in space in such a way that benzoquinoline ligands lie parallel
one above another (alternating between the two independent
molecules); see Figure 4. They are shifted relative to each
other to compromise interactions between steric hindrance
and stacking of the molecules. The ylidene ligands lie per-
pendicular to each other as a consequence of the presence of
the 4-fold axis with benzene rings forming perpendicular
planes to those defined by benzoquinoline ligands, as shown
below. There are two different kinds of cavities around the
4-fold axes. The cavities associated with the 4 axis contain
water molecules, as we have already mentioned, and the ones
around the 42 axis are relatively smaller and empty; however
they allow the ylidene fragments to pack most efficiently.
The basic layout of the two moieties (e.g., the Cl(1)-Cl(2)
vector being essentially perpendicular to the dihydroimida-
zole main plane), as well as the bond lengths and angles
The activity of complexes VIII and IX was then tested at
80 °C, using diethylallyl(2-methylallyl)malonate (S4) as the
more demanding model substrate. The 99% conversion
according to GC analysis was obtained after 24 h for both
of the catalysts tested (Scheme 4). This result proves that
elevated temperatures are necessary for ring-closing metath-
esis when both VIII or IX are used as initiators.

122 Organometallics, Vol. 29, No. 1, 2010
Szadkowska et al.
Table 2. Catalytic Activity of VIII and IX in Test Reactions at
Low Temperaturea
Figure 5. Monomers used in ROMP.
that in the case of the monomers M1-M3 different rates of
initiation and polymerization22 should be observed as a
consequence of different anchor groups.
As the first criterion, we monitored the performance of
complexes VIII and IX as initiators for the polymerization of
1
the monomers by H NMR spectroscopy in solution. One
equivalent of VIII or IX was added into a NMR tube and
dissolved in CDCl3 (0.5 mL) under an inert atmosphere
of nitrogen. Monomer M2 (20 equiv) dissolved in CDCl3
(0.2 mL) was added, and the NMR tube was tightly closed
with a rubber septum. 1H NMR spectra were recorded
immediately and, subsequently, every 2 days for 30 days,
maintaining the temperature of 22 °C. In the case of IX, no
polymerization was observed, whereas VIII promoted slow
polymerization of M2 under these conditions. After 20 days, a
conversion of 55% was achieved. At that time, the initiator was
consumed, as indicated by the disappearance of the character-
istic NMR signals of VIII, and no further conversion was
observed within another 8 days of monitoring. The observa-
tion might be explained by a moderate sensitivity presumably
toward oxygen of the propagating species, since both ruthe-
nium compounds are, even without exclusion of oxygen,
perfectly stable in CDCl3 solutions for more then 2 months.
Furthermore, polymerizations in solution were carried
out. For this purpose, toluene was used as the solvent. The
corresponding initiator (1 equiv) was dissolved in 3 mL of
toluene and added to 300 equiv of the monomer under an
inert atmosphere of argon. The reaction mixture was heated
to 110 °C, stirred at this temperature for 5 h, and allowed to
cool to room temperature. Then 50 μL of ethyl vinyl ether
was added. Finally, precipitation of the polymer was
achieved by slow addition of the reaction mixture to vigor-
ously stirred methanol. The resulting white precipitates were
sampled and dried in a vacuum, giving rise to yields of
27-96%. Also the remaining liquid was evaporated to
dryness and the residue was dried in vacuo in order to
quantify the amount of residual monomer.
Similarly, bulk polymerizations of M1-M3 were per-
formed using VIII and IX as the initiators. The correspond-
ing complex (1.0 equiv) was dissolved in 50 μL of CH2Cl2 and
added to the neat monomer (300 equiv) placed in a Schlenk
tube. The reaction vessel was put into a preheated (110 °C) oil
bath for 5 h and stirred continuously. After cooling the
reaction mixture to room temperature, the residue was
dissolved in CH2Cl2 and 50 μL of ethyl vinyl ether was
added. Further workup was done as in the case of the
solution polymerization. Yields and residual monomer con-
tents of these trials are presented in Table 3. All polymers
were characterized by 1H and 13C{1H} NMR and IR spectro-
scopies, SEC (size exclusion chromatography), and DSC
(differential scanning calorimetry). The spectroscopic meth-
ods (NMR and IR) revealed the occurrence of ROMP in all
a Conditions: 5 mol %, c[S1, S2, or S3] = 0.02 M, dichloromethane,
40 °C, 24 h; conversions according to GC.
Scheme 4. RCM of Diene S4a
a Conditions: c[S4] = 0.02 M, 5 mol % of VIII or IX, toluene, 80 °C,
24 h.
Having established the good application profile of VIII
and IX at elevated temperatures, we decided to verify how
the catalysts perform at lower loadings. It was found that
complexes VIII and IX applied in RCM of substrate S4 in
low amount (0.1 mol %) resulted in conversions of 54% and
47%, respectively, after 24 h at 110 °C in toluene. In order to
ascertain whether our latent catalysts were still active after
this time, we extended the time of the reaction of ring closing
of diethylallyl(2-methylallyl)malonate to 48 h. It appeared
that both complexes VIII and IX performed remarkably well
and reached a conversion of 99% after that time.
It should be noted that despite such extreme conditions
(boiling toluene, 24-48 h), reaction solutions remained
green, which is an indication that initiation of the catalyst
was rather small and the stability of VIII and IX is high even
in the presence of the substrate. Remarkable stability com-
bined with high efficiency at the elevated temperatures is
useful in metathesis of more demanding compounds, such as
tri- or tetrasubstituted dienes and, at the same time, deter-
mine VIII and IX as thermo-switchable latent initiators in
ROMP. To gain information on the latency of compounds
VIII and IX in ROMP, we conducted model polymeriza-
tions utilizing (()-endo,exo-5,6-bis(ethoxymethyl)bicyclo-
[2.2.1]hept-2-ene (M1), (()-endo,exo-bicyclo[2.2.1]hept-
5-ene-2,3-dicarboxylic acid diethyl ester (M2), and a mixture
of 78% endo- and 22% exo-bicyclo[2.2.1]hept-5-ene-2-car-
boxylic acid ethyl ester (M3) (Figure 5). It can be expected
(22) Slugovc, C.; Demel, S.; Riegler, S.; Hobisch, J.; Stelzer, F.
Macromol. Rapid Commun. 2004, 25, 475.

Article
Organometallics, Vol. 29, No. 1, 2010 123
Table 3. Yields of Polymers Obtained from Solution and Bulk
Polymerization
solutiona yield %
(residual monomer %)
bulkb yield %
(residual monomer %)
polymer
polyM1/VIII
91 (-)
95 (<3)c
Mn = 1 180 000;
PDI = 1.9
81 (15)
Mn = 1 230 000;
PDI = 2.2
96 (-)
Mn = 1 090 000;
PDI = 2.4
23 (68)
Mn = 1 280 000;
PDI = 2.3
27 (61)
Mn = 2140000;
PDI = 2.1
91 (-)
Mn = 2 080 000;
PDI = 2.6
87 (-)
Mn = 2 010 000;
PDI = 2.2
95 (<3)c
Mn = 231 000;
PDI = 2.1
72 (14)
polyM2/VIII
polyM3/VIII
polyM1/IX
polyM2/IX
polyM3/IX
Mn = 1 310 000;
PDI = 2.2
89 (<5)
Mn = 2 090 000;
PDI = 1.9
69 (12)
Figure 6. Course of polymerization of M1, M2, and M3 in-
itiated by VIII and IX (heating rate = 3 °C/min, initiator:mono-
mer ratio = 1:500).
Mn = 1 470 000;
PDI =2.4
Mn = 2 190 000;
PDI =2.3
a Reaction conditions: initiator:monomer = 1:300, solvent, toluene;
110 °C, 5 h. b Reaction conditions: initiator:monomer = 1:300, no
solvent; 110 °C, 5 h. c Insoluble polymer.
cases under these conditions. No differences in the stereo-
chemistry of the obtained polymers were observed while
using the different initiators. SEC of the polymeric samples
obtained from the solution polymerization in THF, relative
to polystyrene standards, revealed molecular weights (Mn’s)
of >1 000 000 g/mol in all cases. The Mn values for the
polymers obtained from the bulk polymerization were even
higher, exceeding 2 000 000 g/mol in all cases. Using an
initiator providing fast and complete initiation, Mn’s of
about 50 000 g/mol could be expected.23 Conclusively, in-
itiation is considerably small for all initiator-monomer
combinations under investigation and even lower when no
solvent is applied. There are no significant differences in the
Mn values of the polymers obtained with VIII and IX.
Polydispersities of the polymers are reported for complete-
ness; however, they do not add important information. They
ranged from 1.9 to 2.6. The results on yield of polymers
obtained from the solution and bulk polymerizations are
presented in Table 3. Mn and PDI for each polymerization
experiment can be found in the Supporting Material.
Further information on the course of the bulk polymer-
izations was obtained by means of monitoring the polymer-
ization by DSC.10f The initiator (1 equiv) and the monomer
(500 equiv) were filled into DSC pans, which were then
placed into the apparatus. A heating ramp of 3 °C/min
commenced at 20 °C. The reaction exotherm was read out
as a function of temperature, thus allowing an easy and
convenient characterization of the thermal switchability of
newly synthesized initiators. The “switching temperature”
for the initiators is characterized by the onset of the exother-
mic heat-flow. Figure 6 shows the corresponding graphs.
The polymerizations of M1-M3 initiated by VIII were
characterized by an onset of the exothermal heat flow in the
range 46 ( 1 to 52 ( 1 °C, while polymerizations with IX
commenced at distinctly higher temperatures. Onset tem-
peratures in the range 104 ( 2 to 117 ( 2 °C were determined.
This trend reassembles the results obtained from the catalytic
activity of these complexes in RCM presented before.
Figure 7. Proposed influence of the ligand rigidity on initiation
of precatalysts VIII and IX via ligand decoordination.
Polymerizations of monomers M2 and M3 commenced at
lower temperatures when compared to the experiments with
monomer M1. This trend was more pronounced in the case of
IX as the initiator. A similar observation was made before.13b
The results obtained in RCM and ROMP transformations
showed a significantly increased latency of complex IX
compared to VIII. The observed feature might be explained
by the increased rigidity of the chelating carbene ligand of
IX. For the generation of the 14-electron complex necessary
for initiation, the N-donor atom has to dissociate from the
ruthenium center.24 This is easier to achieve in the complex
VIII because of the possibility of free rotation of the aro-
matic rings in the 2-phenylpyridine moiety, whereas in IX the
halide ligands and mesityl substituents impede the departure
of the nitrogen atom from the vacant coordination site
(Figure 7). Additionally, the increased aromatic character
of the ruthenacycle might contribute to the pronounced
latency of IX.16c As we previously reported, the initiation
step of the Hoveyda-Grubbs-type catalysts remains depen-
dent on the topology of the ligand core. The presence of a
conjugated π-electron system within the chelating frame-
work tends to stabilize the catalyst resting state, thereby
retarding the initiation step.16c
Conclusions
In summary, we disclosed the syntheses of two new
ruthenium complexes bearing chelating carbene ligands.
Both complexes are stable at ambient conditions and exhibit
catalytic activity at elevated temperatures. Remarkably,
(23) Burtscher, D.; Lexer, C.; Mereiter, K.; Winde, R.; Karch.;
Slugovc, C. J. Polym Sci. Part A: Polym. Chem. 2008, 46, 4630.
(24) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 27, 6543.

124 Organometallics, Vol. 29, No. 1, 2010
Szadkowska et al.
0.1 mol % of either of the introduced catalysts accomplished
complete RCM of diethylallyl(2-methylallyl)malonate at
110 °C in less than 48 h. Although the reaction time is not
particularly impressive, the results are good indications of
the high temperature stability of the compounds under
investigation. The latency of complex IX, containing the
π-conjugated system, was more pronounced than that of the
more “flexible” counterpart VIII, as revealed by determina-
tion of the “switching temperatures” for initiators VIII and
IX in ROMP of several norbornene derivatives. Both dor-
mant catalysts/initiators could be useful in a range of high-
temperature applications.
Synthesis of Catalyst IX, (SPY-5-31)-Dichloro-(κ2(C,N)-
2-(benzo[h]quinolin-10-yl)methylidene)(1,3-bis(2,4,6-trimethyl-
phenyl)4,5-dihydroimidazol-2-ylidene)ruthenium . A Schlenk
tube equipped with a stirring bar was charged with (H2IMes)-
(PCy3)Cl2RudCHPh (Gru-III0) (622.0 mg, 0.733 mmol) and 10
(166.2 mg, 0.822 mmol). The tube was flushed with argon, and
anhydrous CH2Cl2 (25 mL) was added. The purple reaction
mixture was stirred at 25 °C for 48 h, whereupon the color
changed to dark green. The volume of the solvent was reduced to
about 5 mL, and upon addition of Et2O (50 mL) a green
precipitate formed, which was filtered off. The residue was
washed with Et2O (3 ꢀ 10 mL) and dried in vacuo. Yield:
282.1 mg (57%).
1H NMR (δ, 20 °C, 500 MHz, CDCl3): 19.19 (s, 1H,
3
3
RudCH), 8.25 (d, JHH = 5.0 Hz, 1H, bq2), 8.17 (d, JHH
=
Experimental Section25
7.5 Hz,1H, bq7), 8.07 (d, 3JHH = 8.0 Hz,1H, bq4), 7.90 (d, 1H,
bq5), 7.64 (m, 2H, bq6,8), 7.44 (dd, 3JHH = 8.0 Hz, 3JHH = 5.0
Hz, 1H, bq3), 7.17 (s, 4H, PhMes3,5), 7.10 (d, 3JHH = 7.5 Hz, 1H,
bq9), 4.20 (s, 4H, NCH2), 2.55 (bs, 12H, CH3), 2.50 (s, 6H, CH3).
13C NMR (δ, 20 °C, 125 MHz, CDCl3): 290.4 (1C, RudCH),
215.5 (1C, NCN), 147.7 (1C, bq2), 146.0 (1C, bq10b), 142.8, 139.4
(b), 138.8, 137.1, 136.1, 129.9, 129.8, 129.7, 128.6, 126.9, 124.9,
122.7, 121.6 (23C, bq3-10a, PhMes1-6), 51.7 (b, 2C, NCH2), 21.4
(2C, CH3), 19.5 (b, 4C, CH3). MS (FD/FI, m/z): calcd for [M]þ•
(C35 H35N335Cl2102Ru) 669.1252; found 669.1274. IR (KBr): ν
2951, 2916, 2855, 2737, 1739, 1622, 1607, 1581, 1516, 1481, 1431,
1419, 1402, 1379, 1336, 1314, 1292, 1261, 1213, 1185, 1170, 1135,
1034, 993, 976, 914, 882, 855, 839, 794, 759, 713, 643, 615, 578,
506, 458, 422, 411 cm-1. Anal. Calcd for C35H35N335Cl2102Ru:
C, 62.82; N, 6.27; H, 5.27; Cl, 10.58. Found: C, 63.00; N, 6.12; H,
5.43; Cl, 10.47.
General Syntheses of Catalysts. Synthesis of Catalyst VIII,
SPY-5-31)-Dichloro-(κ2(C,N)-N-2-(2-vinylbenzylidene)pyridine-
(1,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)-
ruthenium. To a solution of 4 (140 mg, 0.77 mmol) in CH2Cl2
(5 mL) was added (H2IMes)(pyridine)2Cl2RudCHPh (Gru-III0)
(280 mg, 0.33 mmol), and the reaction mixture was stirred at
25 °C for 17 h. Afterward the mixture was evaporated to dryness
and the residue was redissolved in CH2Cl2/Et2O. Upon addition
of n-heptane a green precipitate formed, which was separated by
filtration, washed with heptanes, and dried under vacuum.
Yield: 109 mg (52%).
1H NMR (500 MHz, CDCl3): δ 2.45-2.49 (m, 18H), 4.15 (m,
4H), 6.65 (d, 1H), 7.10-7.16 (m, 4H), 7.55 (t, 1H), 7.57 (t, 1H),
7.60 (m, 2H), 7.74 (t, 1H), 7.97 (d, 1H), 8.14 (d, 1H), 16.85 (s,
1H). 13C NMR (125 MHz, CDCl3): δ 19.4, 19.4, 21.0, 21.1, 26.0,
26.2, 26.7, 26.8, 31.4, 121.9, 122.1, 122.7, 126.3, 128.0, 128.2,
128.5, 128.7, 129.4, 130.0, 130.1, 130.2, 135.0, 136.7, 138.4,
140.4, 145.0, 148.2, 156.5, 216.5, 314.4. MS (FD/FI, m/z): calcd
for [M]þ• (C33H35N335Cl2102Ru) 646.1330; found 646.1346. IR
(KBr): ν 3502, 2950, 2914, 2857, 2735, 1955, 1730, 1630, 1600,
1556, 1479, 1438, 1419, 1380, 1307, 1262, 1159, 1121, 1099, 1033,
950, 851, 789, 754, 735, 677, 641, 630, 579, 503, 474, 451, 420
cm-1. Anal. Calcd for C33H35N335Cl2102Ru: C, 61.29; N, 6.49;
H, 5.61; Cl, 10.95. Found: C, 61.17; N, 6.42; H, 5.61; Cl, 11.06.
Alternative Synthesis of Catalyst VIII. A Schlenk tube
equipped with a stirring bar was charged with ruthenium
complex (Ind-II) (127 mg, 0.15 mmol) and CuCl (17.8 mg, 0.18
mmol). The tube was flushed with argon and charged with
anhydrous methylene chloride (3.5 mL). Next, a solution of 7
(32 mg, 0.18 mmol) in methylene chloride (4 mL) was added
under an argon atmosphere, and the resulting solution was
stirred at 40 °C for 0.5 h. After this time, TLC indicated
complete conversion of the substrate. The resulting mixture
was concentrated in vacuo, the residue was redissolved in
AcOEt, and the solution was passed through a Paster pipet
containing a small amount of cotton and evaporated to dryness.
The crude product was purified by column chromatography
(using eluents cyclohexane/ethyl acetate, 10:1 to 1:1 v/v). After
evaporation of the solvents, the resulting solid was collected and
washed a few times with AcOEt and with cold n-pentane.
Acknowledgment. This work was supported by grant
number N204157636, which was provided by the
Polish Ministry of Science and Higher Education. K.G.
and K.W. thank the Foundation for Polish Science for
the “Mistrz” professorships. Single-crystal X-ray mea-
surements were accomplished at the Structural Research
Lab (SRL) of the Chemistry Department, Warsaw Uni-
versity, Poland. SRL has been established with finan-
cial support from European Regional Development
Fund in the Sector Operational Program “Improvement
of the Competitiveness of Enterprises, years 2004-2006”
project no. WKP_ 1/1.4.3./1/2004 /72/72/165/2005/U.
Parts of this work were supported by the Polymer Com-
petence Center Leoben GmbH (PCCL, Austria) within
the framework of the Kplus-Program of the Austrian
Ministry of Traffic, Innovation and Technology
(project II 2.2). K.G. and C.S. thank the European
Community for generous support, which was provided
through the Seventh Framework Program (grant no. CP-
FP 211468-2 EUMET).
Supporting Information Available: Complete characterization
of all new compounds, catalytic procedures, X-ray crystallo-
graphic tables, and data in CIF format. This information is
available free of charge via the Internet at http://pubs.acs.org.
(25) For full experimental details and other information see the
Supporting Information.