Impact of electronic modification of the chelating benzylidene ligand in cis-dichloro-configured second-generation olefin metathesis catalysts on their activity

Article abstract of DOI:10.1021/om500315t

A series of electronically modified second-generation cis-dichloro ruthenium ester chelating benzylidene complexes was prepared, characterized, and benchmarked in a typical ring-opening metathesis polymerization (ROMP) experiment. The electronic tuning of the parent chelating benzylidene ligand (2-ethyl ester benzylidene) was achieved by substitution at the 4- and 5-positions with electron-withdrawing nitro or electron-donating methoxy groups. The effect of the electronic tuning on the cis-trans isomerization process was studied experimentally and theoretically. Density functional theory calculations clearly revealed the influence of electronic modification on the relative stability between the cis and trans isomers, which is decisive for the activity of the studied compounds as initiators in ROMP.

Full text of DOI:10.1021/om500315t

Article
pubs.acs.org/Organometallics
Impact of Electronic Modification of the Chelating Benzylidene
Ligand in cis-Dichloro-Configured Second-Generation Olefin
Metathesis Catalysts on Their Activity
Eva Pump,† Albert Poater,‡,§ Michaela Zirngast,† Ana Torvisco,∥ Roland Fischer,∥ Luigi Cavallo,⊥
and Christian Slugovc*,†
†Institute for Chemistry and Technology of Materials, Graz University of Technology, Stremayrgasse 9, A 8010 Graz, Austria
‡Catalan Institute for Water Research (ICRA), H2O Building, Scientific and Technological Park of the University of Girona, Emili
Grahit 101, E-17003 Girona, Spain
§
́
́
Institut de Quımica Computacional i Catalisi, Departament de Quımica, Universitat de Girona, Campus de Montilivi, E-17071
̀
Girona, Spain
∥Institute of Inorganic Chemistry, Graz University of Technology, Stremayrgasse 9, A 8010 Graz, Austria
⊥Physical Sciences and Engineering, Kaust Catalysis Center, King Abdullah University of Science and Technology (KAUST), Thuwal
23955-6900, Saudi Arabia
S
* Supporting Information
ABSTRACT: A series of electronically modified second-generation cis-dichloro
ruthenium ester chelating benzylidene complexes was prepared, characterized, and
benchmarked in a typical ring-opening metathesis polymerization (ROMP)
experiment. The electronic tuning of the parent chelating benzylidene ligand (2-
ethyl ester benzylidene) was achieved by substitution at the 4- and 5-positions with
electron-withdrawing nitro or electron-donating methoxy groups. The effect of the
electronic tuning on the cis−trans isomerization process was studied experimentally
and theoretically. Density functional theory calculations clearly revealed the
influence of electronic modification on the relative stability between the cis and
trans isomers, which is decisive for the activity of the studied compounds as
initiators in ROMP.
manner, the higher thermodynamic stability of the cis isomer is
assumed to appear if the second neutral ligand, in addition to
the N-heterocyclic carbene, is neither a strongly σ nor a
strongly π donating ligand.10 The slow initiation and
pronounced thermal stability are perhaps the most important
reasons for the interest in such catalysts today. Such features are
beneficial when the metathesis reaction has to be performed at
elevated temperatures7c,d,13d,e or when a thermally triggered
(ring-opening metathesis) polymerization is intended.5b,c,11b In
many cases, first the trans isomer is obtained, which
subsequently isomerizes, releasing the thermodynamically
preferred cis-configured complex. The underlying isomerization
process was studied by experiment9,13e and by means of density
functional theory based investigations.10,13e,14 As the essence of
these studies, the mechanism of the isomerization can be
described by either a dissociative (i.e., the neutral donor ligand
dissociates before isomerization) or a concerted pathway (the
neutral donor stays in bonding distance to the ruthenium
center during the rearrangement of the halides). Furthermore,
it was shown that, for energetic reasons, cis-dichloro species (at
INTRODUCTION
■
Olefin metathesis is a unique carbon−carbon double bond
forming reaction catalyzed by transition-metal complexes.1 In
particular, ruthenium-based complexes have attracted consid-
erable interest because they tolerate a wide array of functional
groups as well as water and (with some restrictions) oxygen.2
Typical well-defined ruthenium olefin metathesis catalysts are
made up by a carbene and two neutral (phosphines, N-
heterocyclic carbene) and two anionic ligands (in most cases
chlorides). Already in one of the very early papers on well-
defined ruthenium-based catalysts the coexistence of trans- and
cis-dichloro isomers was disclosed.3 Nevertheless, cis-dichloro
isomers led a miserable existence after that, although some
isolated reports are available.4 In 2004 the first N-heterocyclic
carbene bearing ruthenium carbene complexes with a cis-
dichloro stereochemistry were disclosed,5 and since then
considerable attention has been paid to the investigation of
the scope and the limitation of catalysts bearing this particular
stereochemistry. cis-Dichloro ruthenium benzylidenes bearing
N-heterocyclic carbenes as coligands are characterized by a
slower initiation in comparison to their trans-dichloro counter-
parts6 and exist with O-,5a,b,7 CC-,8 S-,9 Se-,10 N-,5c,11 Br-,
I-12 or P-based10,13 neutral coligands. Expressed in a generalized
Received: March 25, 2014
© XXXX American Chemical Society
A
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least those bearing N-heterocyclic carbenes as coligands) are
considerably worse in catalyzing olefin metathesis reactions in
comparison to their isomerization products and it is postulated
that isomerization to the trans species has to take place in order
to gain catalytic activity.14c,d
In this work we report on our studies on the impact of
electronic modification of the chelating carbene ligand on the
ratio of cis and trans isomers, on the isomerization process, and
further on the resulting catalytic properties of the different
complexes. As the starting point we used a cis-dichloro
ruthenium complex bearing a chelating ester substituted
benzylidene ligand (cis-1)5b and prepared analogues in which
the benzylidene ligand is further modified with electron-
donating (−OMe) or electron-withdrawing groups (−NO2)
(see Chart 1). The impact of a similar electronic tuning on the
Information).17 Complexes cis-1−cis-5 were obtained from a
carbene-exchange reaction of M31 with 1.2−1.5 equiv of L1−
L5. The reaction was performed in CH2Cl2 at 25 °C and is
typically complete after 17 h. Extraction of the reaction mixture
with 5 vol % hydrochloric acid allowed for the separation of
most of the pyridine, and subsequent precipitation of the
products with n-pentane yielded the desired complexes cis-1−
cis-5 in crude form. In particular, complexes cis-1−cis-3 are
contaminated with major impurities (6−13%), which were
tentatively assigned to the cationic pyridine-containing species
cis-1+py−cis3+py, as shown in Scheme 1.18 A similar impurity
a
Scheme 1. Syntheses of cis-1−cis-5
Chart 1. Compounds under Investigation
a
Legend: (i) CH2Cl2, 25 °C, 17 h.
was not present in the crude product mixtures of cis-4 and cis-
5. Upon subsequent purification by column chromatography
analytically pure cis-1−cis-5 were obtained in moderate to
1
good yield. The complexes were characterized by H and 13C
NMR spectroscopy, elemental analysis, and single-crystal X-ray
crystallography. 1H NMR spectroscopy suggested a cis-dichloro
structure in all cases, as evidenced by a complete loss of
symmetry (distinct signals for all four aromatic mesityl protons
and all six mesityl methyl groups) and a diastereotopic splitting
of the methylene group of the ethyl ester moiety, indicating a
chiral ruthenium center. Variation of the electron density of the
catalytic activity of trans-dichloro-configured second-generation
Hoveyda-type catalysts has already been described experimen-
tally15 and theoretically.16 In simplified terms, the activity
increases in this system with the implementation of electron-
withdrawing substituents on the benzylidene moiety and is
lowered in case of the presence of electron-donating groups.
The effect of the electronic tuning on the activity was primarily
explained by the variation of the energy of the transition state
for the decoordination of the O-donor of the chelating
benzylidene ligand.15,16 Taking these studies as an inspiration,
we herein wish to disclose the effect of the electronic tuning of
the benzylidene ligand in cis-dichloro species.
1
benzylidene ligand is reflected by the H NMR shift of the
carbene’s proton: electron-rich derivatives cis-3 and cis-2
showed the corresponding singlet at 18.57 and 18.92 ppm and
the electron-poor systems cis-4 and cis-5 exhibited the
resonance at lower field, namely at 19.09 and 19.30 ppm,
respectively. The unsubstituted complex cis-1 showed the
corresponding signal at 18.96 ppm. Similarly, the 13C resonance
of the carbene was shifted according to the electronic
modification of the benzylidene ligand. While cis-1−cis-3
exhibited deshielded carbene resonances in the range of 282.9−
283.8 ppm, the nitro-group-bearing derivatives gave signals at
lower fields (278.0 ppm for cis-4 and 276.7 ppm for cis-5). The
same trend could be found for the CO resonance of the ester
group. In this case, however, the trend was less pronounced.
Unsubstituted cis-1 showed the carbonyl C peak at 177.0 ppm.
Methoxy derivatives cis-2 and cis-3 were characterized by the
corresponding resonances at 176.1 and 178.2 ppm, respectively,
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Aiming at the prepara-
tion of electronically modified ester-chelating benzylidene
complexes, a series of differently substituted 2-vinylbenzoic
acid ethyl ester derivatives was prepared. Carbene-precursors
L1−L5 featured no additional substituent (L1), an electron-
donating methoxy group at the position meta (L2) or para
(L3) to the vinyl group, or an electron-withdrawing nitro group
at a position meta (L4) or para (L5) to the vinyl group and
were prepared following known protocols (cf. the Supporting
B
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and in the nitro derivatives cis-4 and cis-5 the peak was shifted
to higher fields (176.1 ppm for cis-4 and 175.7 ppm for cis-5).
Comparable carbene proton shifts were obtained for electroni-
cally modified Hoveyda-type initiators,15 indicating that the
strongest impact on the carbene proton shifts was evoked by
the corresponding para-substituted benzylidene ligands. An
EDG functionalization (R2 = OiPr) shields the proton, leading
to an upfield-shifted carbene proton (as for cis-3), while a nitro
group exerts the opposite effect (cis-5).
to their trans counterparts before they become active in olefin
metathesis transformations. Herein we aim at studying the
energetic isomerization profiles for the series of electronically
modified compounds under investigation. Three plausible cis−
trans isomerization mechanisms were examined by DFT
calculations. Crystal structures were taken as models to obtain
optimized geometries of cis-1−cis-5. The three pathways that
were considered can be characterized as a chloride dissociative
pathway through transition state TS1, a concerted pathway,
through transition state TS2, and an oxygen dissociative
pathway, through transition state TS3, as schematically shown
for cis-2 in Figure 2. Results for all complexes are given in
Table 2. The first pathway, corresponding to chloride
dissociation, was considered because cis-2 exhibits a
pronounced lability of the chloride in a position trans to the
NHC ligand.7f,19 DFT calculations confirmed that dissociation
of either Cl1 or Cl2 leads to the same product cis-Cl+ bearing
the newly formed vacant coordination site trans to the NHC
Single-crystal X-ray diffraction analysis confirmed the
1
structural suggestion from the evaluation of H NMR data
(cf. Figure 1). All compounds under investigation exhibited a
ligand. This step is distinctly higher in energy (ΔEcis‑Cl+‑cis
=
33.4−38.3 kcal/mol) than the highest transition states observed
for the other two pathways. Furthermore, after chloride
dissociation another energy barrier of about 15 kcal/mol has
to be overcome to finally arrive at the desired trans product.
Overall the upper barrier window for this first dissociative
pathway ranges from 49.7 to 53.9 kcal/mol. These data suggest
that this isomerization pathway is strongly disfavored.20
However, calculations reveal a distinctly higher energy demand
for chloride dissociation in the nitro-substituted derivatives
(ΔEcis‑Cl+‑cis = 38.3 and 38.1 kcal/mol for cis-4 and cis-5), in
comparison to the unsubstituted or methoxy-substituted
congeners (33.4−35.6 kcal/mol). These findings are the key
Figure 1. Crystal structures of cis-4 and cis-5. All non-carbon atoms
are shown as 30% ellipsoids. Hydrogen atoms are removed for clarity.
distorted-square-pyramidal coordination mode of the ruthe-
nium atom bearing a cis-dichloro arrangement. The apex of the
pyramid was in all cases formed by the carbene carbon
(C(22)). Important bond lengths and angles are presented in
Table 1. The electronic tuning of the benzylidene ligand hardly
influenced the Ru−Cl(1) (2.352(1)−2.374(3) Å) and Ru−
Cl(2) bond lengths (2.344(2)−2.367(2) Å). The same
accounted for the Ru−C(22) and Ru−C(1) distances. Minor
deviations did not systematically follow the anticipated effect of
the substitution. Only the Ru−O(1) distance somehow
reassembled the expected tendency of shorter Ru−O bonds
for methoxy-substituted complexes and longer Ru−O bonds for
the nitro derivatives. However, the effects were not very
pronounced, as can be seen in Table 1. All in all, all solid-state
structures were very similar, except for one feature worth
mentioning: the ethyl ester residue in cis-4 was out of plane,
pointing toward the NHC ligand, while in all other complexes it
was in a coplanar arrangement with the benzylidene ligand.
Although this phenomenon might be best explained by packing
effects, it nevertheless played a role in the theoretical study
described below.
for understanding why the cationic pyridine complexes cis-4+
py
and cis-5+py were not observed as byproducts in the synthesis of
the nitro complexes. The second and third pathways, i.e. the
concerted (TS2) and the dissociative mechanisms (TS3cis),
show similar upper barriers in the range of 27.4−31.3 kcal/mol.
Complexes cis-2 and cis-5 might isomerize rather through a
concerted transition state14b to form the trans counterpart. On
the other hand, complexes cis-1, cis-3, and cis-4 may pursue an
oxygen dissociative pathway, first overcoming TS3cis21 and then
reaching the generally accepted olefin metathesis active trans
14e complex by overcoming TS3trans, which is about 10 kcal/
mol lower in energy than TS3cis. However, there is no clear
trend to conclude that one of the two pathways is clearly
favored, since the transition states TS2 and TS3cis differ in
energy by less than 1.5 kcal/mol. Further, in some cases TS2 is
Considerations of the Isomerization. As discussed in the
Introduction, it is generally believed that second-generation cis-
dichloro ruthenium benzylidene complexes have to isomerize
Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for Complexes cis-1−cis-5
cis-15b
cis-27f
cis-3
cis-4
cis-5
Ru−C(22)
Ru−C(1)
Ru−O(1)
Ru−Cl(1)
1.821(2)
2.024(2)
2.088(1)
2.368(1)
2.356(2)
98.68(7)
85.81(4)
89.86(7)
91.88(6)
106.15(6)
90.49(2)
1.824(3)
2.010(2)
2.076(1)
2.363(2)
2.344(2)
98.7(1)
1.833(9)
2.008(8)
2.072(6)
2.374(3)
2.354(3)
97.7(4)
1.813(3)
2.022(3)
2.093(2)
2.352(1)
2.367(2)
97.5(1)
1.805(5)
2.033(5)
2.083(3)
2.356(1)
2.351(1)
99.0(2)
Ru−Cl(2)
C(1)−Ru−C(22)
O(1)−Ru−Cl(2)
C(22)−Ru−O(1)
C(22)−Ru−Cl(1)
C(22)−Ru−Cl(2)
Cl(1)−Ru−Cl(2)
85.53(6)
89.9(1)
84.6(2)
85.63(6)
90.2(1)
86.1(1)
89.4(3)
90.1(2)
89.6(1)
92.4(3)
90.6(1)
92.4(2)
111.3(1)
91.21(3)
107.9(3)
92.08(9)
107.8(1)
91.69(3)
102.8(2)
91.92(5)
C
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Figure 2. Potential pathways for cis−trans isomerization (for cis-2): halide dissociative (TS1+), concerted (TS2), and oxygen dissociative
mechanisms (TS3) (solvation model PCM; solvent CH2Cl2).
a
Table 2. Energies (kcal/mol) of cis-1−cis-5 according to the Activation Mechanism
initiator
cis
cis-Cl+
TS1+
trans-Cl+
TS2
TS3cis
14e
TS3trans
trans
transexp
1
2
3
4
5
0
0
0
0
0
35.6
35.4
33.4
38.3
38.1
51.1
51.4
49.7
53.9
52.9
38.0
37.5
36.9
39.6
40.4
29.4
30.0
28.8
30.1
28.7
29.1
31.3
27.4
29.7
29.0
13.9
13.1
13.7
14.4
12.2
20.2
22.3
20.2
18.7
18.8
1.6
2.0
0.9
0.4
0.6
1.6 0.4
2.3 0.6
1.3 0.3
0.4 0.2
1.0 0.3
a
Values in boldface type highlight the more likely pathway (solvent CH2Cl2, solvation model PCM).
slightly lower in energy and in the other TS3cis is lower in
energy (see Table 2).
isomerization,22 by determining the trans content after 1 h (at
room temperature in CDCl3). Complex cis-3 (TS = 27.4 kcal/
mol) reached almost the equilibrium after that time, resulting in
Having established a mechanistic understanding of the
isomerization pathway, we now discuss the position of the
cis−trans equilibrium. A solution of pure cis-1−cis-5 (c = 6.3
μM) in CDCl3 was prepared and the isomerization progress at
9
0.5% trans-3 (which is 90 5% of the theoretical amount
of trans-3). Complex cis-1 gave 3 0.5% of trans-1 (50 8%
of the theoretical amount, TS = 29.1 kcal/mol), and cis-5 gave
1
room temperature was monitored via H NMR spectroscopy.
4
0.5% of trans-5 (25 3%, TS = 28.7 kcal/mol) in 1 h at
After 3 days the equilibrium was balanced for cis-1−cis-3 and
cis-5 (trans-1, 6%; trans-2, 2%; trans-3, 10%; trans-5, 16%),
whereas for cis-4 balancing was finished in 15 days (trans-4,
33%). From this data set, the free energies (ΔGtrans−cis) were
calculated to be between 0.4 0.2 and 2.3 0.6 kcal/mol in
favor of the cis isomer and were found to be in good agreement
with the corresponding values determined by DFT calculations
(ΔEtrans−cis) in CH2Cl2 (cf. Table 2). Experimental isomer-
ization studies in solvents other than CDCl3 were not
conducted because more apolar solvents (such as benzene
and toluene) provide insufficient solubility of the cis
compounds at room temperature and more polar solvents
(such as pyridine and protic solvents) favor the formation of
cationic species (cf. Scheme 1). As an alternative, DFT
calculations simulating solutions in benzene, toluene, THF,
CHCl3, and CCl4 were carried out, revealing increased
populations of the trans species in solvents with dielectric
constants lower than that of CH2Cl2 (cf. the Supporting
Information). Similar results were obtained in preceding studies
and are discussed in detail there.9g,f
room temperature. In contrast, no trans product could be
observed in the cases of cis-4 (TS = 29.7 kcal/mol) and cis-2
(TS = 30.0 kcal/mol) after 1 h isomerization time.
Polymerization. Separation and isolation of initiators cis-
1−cis-5 rendered a comparative activity study possible, and
ring-opening metathesis polymerization (ROMP) of dimethyl
bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate (8) was used as the
benchmark reaction. First, monomer 8 was polymerized at
room temperature with cis-1−cis-5 as the (pre)initiators and
1
the reaction was monitored by means of arrayed H NMR
spectroscopy. The obtained data are depicted in Figure 3 in a
time/conversion plot. The most active compound is cis-5 (t1/2
= 98 min), followed by cis-4 (t1/2 = 122 min), cis-3 (t1/2 = 161
min), and cis-1 (t1/2 = 372 min); cis-2 (t1/2 = 440 min) is the
least active initiator of all. The descending order of activity
should in theory depend on the activation energy for the
isomerization (at least if it is assumed that no transition state
during propagation is higher than the activation energy for the
isomerization). This holds true for the activity increase from
cis-2 to cis-1 and to cis-3, but the two most active initiators,
the nitro derivatives cis-4 and cis-5, show higher barriers for
the cis−trans isomerization process than cis-3. Nevertheless the
Additionally, the isomerization rate could be roughly
correlated to the energy of the highest transition state for
D
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Table 3. Characterization of poly8 Prepared with cis-1−cis-5
a
at Elevated Temperature
b
c
c
initiator
time (min)
Mn
PDI
M31
cis-1
cis-2
cis-3
cis-4
cis-5
10
30
35
15
12
12
2
4
4
3
2
2
54
242
281
176
116
144
1.1
1.7
1.9
1.6
1.5
1.5
a
Reaction conditions: [8]:[initiator] = 300:1; [8] = 0.1 mol L−1;
reaction temperature 80 °C; solvent toluene; inert atmosphere of N2.
b
Time until conversion is complete (checked by thin-layer
c
chromatography). Determined by SEC relative to poly(styrene)
standards in THF.
approximately 2 times lower than in case of the fast and fully
initiating compound M31. These observations, i.e. low activity
at room temperature and moderate initiation efficacy at higher
temperature, are typical for cis-dichloro initiators in general.
Electronic modification either decreases (cis-2) or increases the
initiation efficacy (cis-3−cis-5). Analyzing these data, a linear
correlation between the measured Mn values and the difference
between the calculated thermodynamic stabilities of the trans-
configured and the cis-configured isomers (ΔEtrans−cis) can be
found (see Figure 4). The meaning of this correlation is that
Figure 3. Time/conversion plot for the polymerization of 8 with
1
initiators cis-1−cis-5 relying on H NMR data. Reaction conditions:
[8]:[initiator] = 70:1; [8] = 0.13 mol L−1; reaction temperature 25 °C;
solvent CDCl3; inert atmosphere of N2.
trans species is less disfavored in these two cases, which might
explain their higher activity in comparison to cis-1−cis-3.
However, this assumption also bears an inconsistency,
because the stated difference between the thermodynamic
stabilities of the cis and the trans species in cis-4 are smaller
than those in cis-5. Accordingly cis-5 should be more active
than cis-4, which is not the case. Here the thermodynamic
stability of the actual initiator, the 14e species, seems to be
decisive, because most active cis-5 bears the thermodynamically
most favored 14e species (12.2 kcal/mol) and slightly less
active cis-4 leads to a less stable 14e species (14.4 kcal/mol).
Accordingly it is proposed that the stationary concentration of
the 14e species is an important factor for the activity of the
initiators at room temperature. To shed more light on the
structure−activity relationship discussed so far, another series
of polymerizations of monomer 8, now at elevated temperature
of 80 °C, was carried out. The aim here was to provide the
system enough energy to overcome the barrier for cis−-trans
isomerization and to assess the initiation efficacy of cis-1−cis-5
under these conditions. Therefore, a monomer to initiator ratio
of 300:1 was chosen and the polymerization reaction was
carried out until complete conversion of 8 was assured.
Thereupon the polymers were isolated and their number-
average molecular weights (Mn) were examined by size
exclusion chromatography (SEC). This approach allows for
an indirect relative assessment of the initiation efficacy, because
Mn is proportional to the ratio of the propagation rate (kp) and
the initiation rate constant (ki), provided that no secondary
metathesis occurs. Because kp is the same in all cases (in all
cases the same propagating species occurs), Mn is only
dependent on ki. As a benchmark initiator M31 was included
in the study. M31 is known for providing fast and complete
initiation (ki ≫ kp) and poly8 prepared with M31 at room
temperature is characterized by a Mn value of 62 kg/mol and a
PDI of <1.1 (relative to poly(styrene) standards; [8]:[M31] =
300:1).
Figure 4. Correlation between the number-average molecular weight
of poly8 prepared with different initiators and theoretically determined
thermodynamic cis−trans equilibrium constant (solvation model
PCM; solvent toluene).
the initiation efficacy is above all determined by the position of
the cis−trans equilibrium, assuming that this equilibrium is
reached quickly at 80 °C. Thus, electronic tuning of the
benzylidene ligand translates into altered balancing of the cis−
trans equilibrium, which is in turn responsible for the observed
initiation efficacy of the initiators.
This finding has several important implications: (a) at a
reaction temperature which is high enough to provide a fast
(relative to initiation) balancing of the cis−trans equilibrium,
the initiation efficacy is dependent on the relative thermody-
namic stability of the trans isomer in comparison to the cis
isomer, (b) this postulate should hold true no matter if a pure
cis isomer or a pure trans isomer is initially used, and
accordingly, (c) a trans-configured initiator might be
deactivated at elevated temperature if a thermodynamically
more stable cis isomer is accessible. To further evaluate the
general validity of this correlation, the ΔEtrans−cis value for two
reported initiators (6 and trans-7)7a,11b was calculated (see the
At 80 °C and with M31 as the initiator, poly8 with an Mn
value similar to that at room temperature was obtained. The
polymerization was finished after about 10 min. cis-Dichloro
initiators under investigation gave poly8 with higher Mn values,
decreasing from 281 (cis-2) to 116 (cis-4) (cf. Table 3),
meaning that the initiation efficacy is in the best case
E
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mL). 5-Methoxy-2-vinylbenzoic acid ethyl ester (L3; 62.9 mg, 0.305
mmol, 1.2 equiv) was added. The reaction mixture was stirred under
an argon atmosphere for 20 h, until the color changed from deep red
to deep green. Pyridine was removed by extraction with HClaq (5 vol
%). The solvent was reduced in vacuo to 1−2 mL, and the title
compound was precipitated with n-pentane. The dark green precipitate
was filtered and washed with n-pentane. For purification column
chromatography on silica gel (CH2Cl2/MeOH, 20/1 (v/v)) was used.
Yield: 95.2 mg (75%) of dark green crystals. TLC: Rf = 0.4 (CH2Cl2/
MeOH, 20/1 (v/v)). Anal. Calcd for C32H38Cl2N2O3Ru (670.64): C,
Supporting Information) and plotted against the corresponding
Mn values of poly8 (obtained with these initiators in the
literature). Indeed all three data points23 fit, indicating a more
general applicability of the postulate that the assessment of the
thermodynamic equilibrium position of the cis−trans isomer-
ization is suited to predict the initiation efficacy of not only cis-
but also trans-dichloro ruthenium benzylidene initiators at
elevated temperature. Another experiment further confirms this
fact, as similar Mn values for poly8 were observed when
employing pure cis-4 (Mn = 116 kg/mol, PDI = 1.5, t = 12
min) or a mixture of 4 (trans:cis = 2:1, Mn = 110, PDI = 1.3,
and t = 10 min).
1
57.31; H, 5.71; N, 4.18. Found: C, 57.28; H, 5.91; N, 4.01. H NMR
(20 °C, CDCl3, 300 MHz): δ 18.57 (s, 1H, RuCH), 7.54 (d, 1H,
4JHH = 2.5 Hz, ph3), 7.20 (d, 1H, 3JHH = 8.5 Hz, ph3), 7.18, 6.94, 6.02
3
4
(bs, 4H, mes), 7.08 (dd, 1H, JHH = 8.6 Hz, JHH = 2.5 Hz, ph5), 6.74
(d, 1H, JHH= 2.5 Hz, ph6), 4.62, 4.41 (m, 2H, CH2CH3), 4.36−3.59
4
CONCLUSIONS
■
(m, 4H, H2Im), 3.90 (s, 3H, OCH3), 2.69, 2.48, 2.39, 2.12, 1.35 (s,
18H, mes CH3), 1.47 (t, 3H, 3JHH = 7.1 Hz, CH2CH3). 13C NMR (20
°C, CDCl3, 125 MHz): δ 282.8 (1C, Ru=CH), 217.4 (1C, Cq, CNN),
176.7 (1C, Cq, COOEt), 158.8 (1C, Cq, ph4), 139.8, 138.2, 136.3,
134.8, 130.9, 128.5 (6C, Cq, mes C), 138.4 (1C, Cq, ph1), 135.5, 132.5
(1C, Cq, mes N), 130.2 (1C, ph5), 129.5 (4 mes H), 122.5 (1C, Cq,
ph2), 120.2 (1C, ph6), 115.2 (1C, ph3), 64.7 (1C, OCH2CH3), 55.8
(1C, OCH3), 51.0 (2C, H2Im), 21.2, 20.0, 18.3, 16.4 (6C, mes CH3),
14.1 (1C, OCH2CH3).
In summary, we disclosed the effect of electronic tuning of the
chelating benzylidene ligand in cis-dichloro-configured second-
generation olefin metathesis (pre)initiators/catalysts on their
catalytic performance. The activation of cis-dichloro species in
olefin metathesis occurs through isomerization to the trans
species. Hence, the mechanism of this process was studied
experimentally and theoretically. DFT calculations revealed that
isomerization proceeds through either an oxygen dissociative or
a concerted pathway. Both pathways display similarly high
transition energies; however, no clear trend allows a
determination of which is the favored one. Experimentally it
was shown that energy barriers to overcome the transition state
can already be reached at room temperature. The isomerization
rate is dependent on the relative energy stability of the
transition state and can be accelerated at elevated temperatures,
leading to a specific thermodynamic equilibrium for each
system in a given solvent. Experimental results were found to
be in good agreement with theoretically calculated thermody-
namic equilibrium constants. At elevated temperatures (about
80 °C) and in the presence of a substrate the cis−trans
isomerization equilibrium is reached quickly and is the key for
explaining the observed activity of the (pre)initiators in ROMP.
Hence, the thermodynamic equilibrium constant for the cis−
trans isomerization is above all responsible for the initiation
efficacy of not only cis- but also trans-dichloro ruthenium
initiators at elevated temperatures.
Preparation of cis-Dichloro(H2IMes)(2-ethyl ester-5-nitro-
benzylidene-κ2C,O)Ru (cis-4). In a Schlenk flask, M31 (64.0 mg,
0.0859 mmol, 1.0 equiv) was dissolved in degassed, dry CH2Cl2 (10
mL). The carbene precursor 4-nitro-2-vinylbenzoic acid ethyl ester
(L4; 22.8 mg, 0.103 mmol, 1.2 equiv) was added, and the reaction
mixture was stirred in a Schlenk flask under a nitrogen atmosphere for
17 h. The color changed from deep red to red brown. As described
above, the raw product was treated with HClaq (5 vol %) and
subsequently purified by column chromatography with CH2Cl2/
MeOH 20/1 (v/v), giving a brownish red solid (22.8 mg, 39%). TLC:
0.4 (CH2Cl2/MeOH, 20/1 (v/v)). Anal. Calcd for C31H35Cl2N3O4Ru
(685.60): C, 54.31; H, 5.15; N, 6.13. Found: C, 54.35; H, 5.17; N,
6.09. 1H NMR (20 °C, CDCl3, 300 MHz): δ 19.09 (s, 1H, RuCH),
3
8.51 (dd, 1H, 3JHH = 8.6 Hz, 4JHH = 1.9 Hz, ph4), 8.21 (d, 1H, JHH
=
8.7 Hz, ph3), 8.11 (d, 1H, 4JHH = 2.1 Hz, ph6), 7.19, 7.04, 5.96 (s, 4H,
mes), 4.71, 4.50 (m, 1H, CH2CH3), 4.38−3.65 (m, 4H, H2Im), 2.67,
3
2.50, 2.41, 1.99, 1.32 (s, 18H, mes CH3), 1.52 (t, JHH = 7.0 Hz,
CH2CH3). 13C NMR (20 °C, CDCl3, 75 MHz): δ 278.0 (1C,
Ru=CH), 215.3 (1C, Cq, CNN), 176.1 (1C, Cq, COOEt), 152.4 (1C,
Cq, ph5), n.d. (8C, Cq, mes C, mes N), 142.5 (1C, Cq, ph1), 132.5 (1C,
Cq, ph3), 131.2, 129.8, 128.2 (4C, mes), 124.0 (1C, Cq, ph2), 122.3
(1C, ph4), 121.2 (1C, ph6), 65.9 (1C, OCH2CH3), 51.1 (2C, H2Im),
18.3 (6C, mes CH3), 14.1 (1C, OCH2CH3).
EXPERIMENTAL SECTION
■
Preparation of cis-Dichloro(H2IMes)(2-ethyl ester-4-nitro-
benzylidene-κ2C,O)Ru (cis-5). In a Schlenk flask, M31 (150.1 mg,
0.200 mmol, 1.0 equiv) was dissolved in degassed, dry CH2Cl2 (25
mL). 5-Nitro-2-vinylbenzoic acid ethyl ester (L5; 66.8 mg, 0.300
mmol, 1.5 equiv) was added, and the reaction mixture was stirred in a
Schlenk flask under an argon atmosphere until the color changed from
deep red to olive yellowish black. After 20 h of conversion the product
was extracted with HClaq (5 vol %) to get rid of pyridine. Afterward
the complex was precipitated in CH2Cl2 (1−2 mL) with n-pentane,
giving a brown solid, which was filtered and dried in vacuo. The raw
product was isolated from side products by column chromatography
on silica gel (CH2Cl2/MeOH, 20/1, v/v). Yield: 88.9 mg of an ocher
solid (65%). TLC: Rf = 0.4 (CH2Cl2/MeOH, 20/1 (v/v)). Anal. Calcd
for C31H35Cl2N3O4Ru (685.60): C, 54.31; H, 5.15; N, 6.13. Found: C,
54.33; H, 5.20; N, 6.11. 1H NMR (20 °C, CDCl3, 300 MHz): δ 19.37
(s, 1H, RuCH), 8.85 (d, 1H, 4JHH = 2.1 Hz, ph3), 8.45 (dd, 1H, 3JHH
General Considerations. All reactions were carried out under a
nitrogen atmosphere. Solvents were dried by distillation over
appropriate drying agents. Ligands L1−L5 were prepared, starting
from commercially available compounds, as described in the
Supporting Information. cis-Dichloro(H2IMes)(2-ethyl esterbenzyli-
dene-κ2C,O)Ru (cis-1) and cis-dichloro(H2IMes)(2-ethyl ester-5-
methoxybenzylidene-κ2C,O)Ru (cis-2) were prepared according to
literature procedures.5b,7f All other chemicals were purchased from
commercial resources (Alfa Aesar, Sigma-Aldrich, Roth) and used as
received. NMR spectra were recorded on a Bruker Avance 300 MHz
or a Varian INOVA 500 MHz spectrometer. Chemical shifts (δ) are
given relative to TMS and coupling constants (J) in Hz. Gel
permeation chromatography (GPC) was used to determine molecular
weights and the polydispersity index. Measurements were carried out
in THF with the following arrangement: a Merck Hitachi L6000
pump, separation columns of Polymer Standards Service (5 μm grade
size), and a refractive-index detector from Wyatt Technology. For
calibration, polystyrene standards purchased from Polymer Standard
Service were used. X-ray measurements were performed on a Bruker
AXS Kappa APEX II diffractometer using Mo Kα radiation.
4
= 8.5 Hz, JHH = 2.3 Hz, ph5), 7.47 (d, 1H, 3JHH = 8.5 Hz, ph6), 7.21,
7.19, 6.96, 5.95 (s, 4H, mes), 4.70, 4.51 (m, 2H, CH2CH3), 4.41−3.68
(m, 4H, H2Im), 2.66, 2.52, 2.46, 2.41, 2.09, 1.37 (s, 18H, mes CH3),
3
1.54 (t, 3H, JHH = 7.1 Hz, CH2CH3). 13C NMR (20 °C, CDCl3, 125
MHz): δ 276.7 (1C, RuCH), 215.2 (1C, Cq, CNN), 175.7 (1C, Cq,
COOEt), 143.8, 143.6 (2C, Cq, ph1,5), 140.5, 139.6, 138.8, 138.2,
136.3, 135.6, 131.1, 128.3 (8C, Cq, mes C, mes N), 131.7 (1C, ph4),
Preparation of cis-Dichloro(H2IMes)(2-ethyl ester-4-methox-
ybenzylidene-κ2C,O)Ru (cis-3). In a Schlenk flask, M31 (148.8 mg,
0.221 mmol, 1.0 equiv) was dissolved in degassed, dry CH2Cl2 (25
F
dx.doi.org/10.1021/om500315t | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
128.8 (1C, ph6), 130.7, 129.9, 129.85, n.d. (4 mes), 126.8 (1C, ph3),
120.7 (1C, Cq, ph2), 65.9 (1C, OCH2CH3), 51.1 (2C, H2Im), 21.3,
20.0, 19.8, 18.2, 16.3 (6C, mes CH3), 14.1 (1C, OCH2CH3).
ACKNOWLEDGMENTS
■
E.P. gratefully acknowledges the receipt of the “Chemical
Monthly Fellowship” financed by Springer Verlag, the Austrian
Polymerization. Experiments were performed in Schlenk flask in
degassed solvents under N2 conditions. The appropriate amount of
catalyst cis-1−cis-5 (0.00159 mmol, 1 equiv) was rinsed with toluene
(3.8 mL) into a Schlenk flask. The reaction mixture was placed in an
oil bath at 80 °C. Then monomer 8 (100 mg, 0.476 mmol, 300 equiv),
dissolved in 1 mL of toluene, was added with stirring, giving a
concentration of 0.1 M with respect to the monomer. The reaction
progress was monitored by TLC on silica gel, using Cy/EtOAc 3/1
(v/v). After reaction completion, an excess of ethyl vinyl ether (200
μL) was added to quench the reaction. Subsequently the polymer was
precipitated in methanol and dried under vacuum.
̈
̈
Academy of Sciences (OAW), and the Gesellschaft Osterrei-
̈
chischer Chemiker (GOCH). A.P. thanks the Spanish
MICINN for a Ramon y Cajal contract (RYC-2009-05226)
́
and the European Commission for a Career Integration Grant
(CIG09-GA-2011-293900).
REFERENCES
■
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1
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, figures, and CIF and xyz files giving detailed
crystallographic data cis-3−cis-5, procedures and character-
ization data for the carbene precursors L1−L5, NMR spectra
for complexes cis-1−cis-5, protocols and detailed results for the
polymerization and isomerization experiments, output files of
the DFT calculations, and relative energies in gas and solvent
phases. This material is available free of charge via the Internet
at http://pubs.acs.org. Crystallographic data for cis-3−cis-5
have also been deposited with the CCDC, nos. 977278−
977280, and can be obtained free of charge from http://www.
ccdc.cam.ac.uk.
(9) (a) Ben-Asuly, A.; Tzur, E.; Diesendruck, C. E.; Sigalov, M.;
Goldberg, I.; Lemcoff, N. G. Organometallics 2008, 27, 811−813.
(b) Kost, T.; Sigalov, M.; Goldberg, I.; Ben-Asuly, A.; Lemcoff, N. G. J.
Organomet. Chem. 2008, 693, 2200−2203. (c) Diesendruck, C. E.;
Vidavsky, Y.; Ben-Asuly, A.; Lemcoff, N. G. J. Polym. Sci., Part A:
Polym. Chem. 2009, 47, 4209−4213. (d) Ben-Asuly, A.; Aharoni, A.;
Diesendruck, C. E.; Vidavsky, Y.; Goldberg, I.; Straub, B. F.; Lemcoff,
N. G. Organometallics 2009, 28, 4652−4655. (e) Vidavsky, Y.;
Lemcoff, N. G. Beilstein J. Org. Chem. 2010, 6, 1106−1119. (f) Aharoni,
A.; Vidavsky, Y.; Diesendruck, C. E.; Ben-Asuly, A.; Goldberg, I.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail for C.S.: slugovc@tugraz.at.
■
Notes
The authors declare no competing financial interest.
G
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(18) The byproducts were not isolated in pure form. NMR
spectroscopic data retrieved from the crude product mixtures
suggested the formation of cationic pyridine complexes on the basis
of high-field-shifted carbene proton resonances (17.41−17.93 ppm)
and the presence of signals assigned to a coordinated pyridine (e.g.,
8.50−8.55 ppm). A similar complex has been isolated and fully
characterized recently; see ref 7e.
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metathesis catalysts in general see: Falivene, L.; Poater, A.; Cazin, C. S.
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(20) This situation might change in the presence of donor ligands
work on this particularity is currently ongoing in our laboratories.
(21) The corresponding cis 14e compound was never observed in
any calculations.
(22) An exact experimental determination of the activation energy
was not possible, due to an unknown concomitant decomposition
reaction becoming important at higher temperatures (cf. the
Supporting Information).
(23) The higher Mn value of poly8 obtained with trans-6 (in
compared to that with cis-6) was attributed to different solubilities of
the initiators; cf. ref 7a.
(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
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B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
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N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
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H
dx.doi.org/10.1021/om500315t | Organometallics XXXX, XXX, XXX−XXX