Ruthenium indenylidene complexes bearing N-alkyl/N-mesityl-substituted N-heterocyclic carbene ligands

Article abstract of DOI:10.1039/c5dt00967g

We report on the synthesis and characterization of second generation ruthenium indenylidene catalysts bearing unsymmetrical N-heterocyclic carbene (NHC) ligands denoted as RuCl₂(3-phenyl-1-indenylidene)(1-mesityl-3-R-4,5-dihydroimidazol-2-ylidene)(PCy₃), in which R is methyl 8a, octyl 8b or cyclohexyl 8c. The characterization of 8a-c was performed by NMR spectroscopy, elemental analysis, IR, HRMS and single-crystal X-ray diffraction analysis. In addition, the catalytic activity of the obtained initiators was evaluated in various representative metathesis reactions. The results reveal that the complexes 8a-c, bearing an N-alkyl side on the NHC, show a faster catalytic initiation than the reference complex 2. Complex 8a, which performs the best among the investigated indenylidene complexes, exhibits slower initiation but better overall efficiency than its benzylidene analogue 1c, especially in a low catalyst loading.

Full text of DOI:10.1039/c5dt00967g

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Journal Name
RSCPublishing
ARTICLE
Ruthenium Indenylidene Complexes Bearing N-
Alkyl/N-Mesityl-Substituted N-Heterocyclic Carbene
Ligands
Cite this: DOI: 10.1039/x0xx00000x
d
c
e
d
,a,b,d
Baoyi Yu, Fatma B. Hamad, Bert Sels , Kristof Van Hecke, and Francis Verpoort*
Received 00th January 2012,
Accepted 00th January 2012
We report on the synthesis and characterization of second generation ruthenium indenylidene
catalysts bearing unsymmetrical ꢀheterocyclic carbene (NHC) ligands denoted as RuCl (3ꢀ
N
2
phenylꢀ1ꢀindenylidene)(1ꢀmesitylꢀ3ꢀRꢀ4,5ꢀdihydroimidazolꢀ2ꢀylidene)(PCy ), in which R is
DOI: 10.1039/x0xx00000x
3
methyl 8a, octyl 8b or cyclohexyl 8c. The characterization of 8a-c was performed by NMR
spectroscopy, elemental analysis, IR, HRMS and singleꢀcrystal Xꢀray diffraction analysis.
Additionally, the catalytic activity of the obtained initiators was evaluated in various
www.rsc.org/
representative metathesis reactions The results reveal that the complexes 8a-c, bearing an
alkyl side on the NHC, show a faster catalytic initiation than reference complex . Complex
, which performs best among the investigated indenylidene complexes, exhibits slower
Nꢀ
2
8
a
initiation but better overall efficiency than its benzylidene analogue 1c, especially in a low
catalyst loading.
1
0b,12
NHCꢀbearing ruthenium catalyst has been explored.
Among these investigated catalysts, 1c bearing an ꢀmethyl
group exhibited outstanding metathesis performance compared
N
Introduction
NꢀHeterocyclic carbenes (NHCs) are widely employed as ligands in to that of its analogues and had activity comparable to the
1
organometallic complexes. The strong σꢀdonating and πꢀback parent catalyst 1a. Recently, Grubbs’ group reported a library
1
3
donating properties enable NHC ligands to be strongly bonded to the of catalysts (
3 ) bearing a bidentate unsymmetrical NHC
2
14
metal center, thereby, stabilizing the formed complexes. The ligand, affording high
successful example in olefin metathesis is the substitution of a PCy3
Zꢀselectivity.
ligand from bisꢀPCy coordinated “first generation” rutheniumꢀbased
3
catalysts with an NHC ligand, resulting in the formation of more
stable “secondꢀgeneration” catalysts with better catalytic
3
performance (Fig. 1; 1a, 2). The ease of electronic and steric
modification on the scaffold of NHC allows a fineꢀtuning of the
catalytic performance of the secondꢀgeneration catalysts by testing
4
various substituents on an NHC ligand. These modifications include
5
6
7
steric addition, steric reduction, ionizedꢀgroup addition, electronic
8
9
tuning and solid support anchoring.
Of particular interest is the unsymmetrical modification that
has resulted in catalysts with great selectivity in different
metathesis reactions, such as E:Z selectivity in cross metathesis
(
CM), selectivity in diastereo ringꢀclosing metathesis (RCM) and cis
4c
selectivity in ringꢀopening metathesis (ROMP). Several reports
have shown that replacement of one of the side substituents from a
ruthenium coordinate SIMes ligand by an aliphatic group could
strengthen the σꢀdonation of NHC to the ruthenium center. The
enhanced σꢀdonation property is expected to be transferred to the
ruthenium core and thereby enhancing the catalytic efficiency.
However, in most cases, the activity of these metathesis catalysts is
1
0
Fig. 1 Selected examples of second-generation ruthenium-based catalysts.
Ruthenium indenylidene catalysts (e.g. 2) constitute a
influenced by steric rather than electronic properties. For example, famous family of rutheniumꢀbased olefin metathesis catalysts,
in 2003, Mol’s group reported an adamantylꢀsubstituted along with other families of ruthenium benzylidene and
1
1
15
unsymmetrical NHC ruthenium benzylidene catalyst 1b
to the large size of the adamantyl group, which contributes to indenylidene catalysts have attracted great interest because of
the extra obstruction for substrates,
low metathesis their ease of preparation, comparative catalytic activities in
performance of 1b was demonstrated. Furthermore, a variant of metathesis reactions and relatively greater stability.
stericꢀreduced ꢀalkyl/
ꢀmesityl substituted unsymmetrical Interestingly, the exploration of the indenylidene family of
.
Due ruthenium benzylidene etherꢀchelating catalysts. Ruthenium
a
1
6
N
N
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J
/
o
C
u
5DrnT0
a
0
l
9
N
6
a7Gme
3
1
1
ruthenium metathesis initiators is similar to that of benzylidene
The isolated complexes were characterized by P{ H} (Figs. S3,
1
13
1
initiators in many aspects. For example, after the successful S6, and S9), Hꢀ (Figs. S4, S7, and S10) and C{ H} (Figs. S5, S8,
development of the4 unsymmetrical NHC on ruthenium and S11) NMR spectroscopy. The proton and carbon resonances
c
benzylidene catalysts, unsymmetrical modifications of NHCs were further assigned using additional 2D spectroscopy as follows:
17
1
1
bearing ruthenium indenylidene complexes were also reported.
Grela’s group reported a series of catalysts bearing various
homoꢀnuclear H{ H} COSY, TOCSY, and NOESY; heteroꢀnuclear
1
13
3g
31
H{ C} HSQC and HMBC NMR spectroscopy. Each PꢀNMR
The catalytic tests spectrum of complexes 8a-c consists of two singlet peaks to which
1
7a,17b
substituents on an Nꢀbenzyl group (4a-g).
revealed that the dissociation of the phosphine ligand was two rotamers could be attributed: 8a, the major peak (δ = 32.2 ppm)
accelerated by the repulsion between the Nꢀbenzyl arms and the and the minor peak (δ = 14.7 ppm) in a 100:5 ratio; 8b, the major
PCy3 ligand. However, for the complexes containing sulfurꢀ peak (δ = 29.2 ppm) and the minor peak (δ = 15.0 ppm) in a ratio of
substituted (4e-f) or pentafluoroꢀsubstituted (4g) Nꢀbenzyl groups, a 100:2.5 ratio; and 8c, the major peak (δ = 22.7 ppm) and the minor
3
1
latent property or reduced catalytic activity was observed. The peak (δ = 16.0 ppm) in a 100:2.5 ratio. Consistent with the PꢀNMR
1
authors suggested that the heteroꢀatom may coordinate to the spectra, each HꢀNMR spectrum of 8a-c shows two sets of proton
1
9
ruthenium center diminishing the amount of active species, thereby resonances to which the rotamers can be attributed. These
decreasing the catalytic performance in the metathesis process. major/minor rotamers were also observed for unsymmetrical NHC
17c
Moreover, Mauduit’s group reported indenylidene complexes (5a-b) bearing ruthenium indenylidene complexes 5a-d. However, in the
bearing Nꢀcyclopentyl and Nꢀcyclododecyl side groups on case of the Nꢀalkyl/Nꢀmesityl mixed NHCꢀcontaining rutheniumꢀ
unsymmetrical NHC ligands and their congeners featuring an benzylidene complexes (1b and 1c), only one conformer existed in
1
7c
10b,11ꢀ12
unsaturated backbone 5c-d.
Catalytic tests showed that both the formed complex.
cyclopentylꢀcontaining complexes 5a and 5c afforded better catalytic
performance compared to 5b and 5d.
With respect to these resonances of the major rotamers, the
NOESY spectra of 8a-c (Fig. S1 for 8a) show several NOE
On the basis of the previous reports, we investigated rutheniumꢀ correlations between methyl groups (from the mesityl groups) and
based indenylidene catalysts containing unsymmetrical NHCs with indenylidene fragments, indicating their relative position.
different sized amino side groups (Nꢀmethyl, Nꢀoctyl and Nꢀ Considering the Nꢀalkyl groups on the NHC ligands in 8a-c the
cyclohexyl). Although the NHCs bearing electronꢀdonating Nꢀalkyl NOESY spectra reveal no NOE correlation between the Nꢀalkyl
substituents may increase the catalyst initiation and efficiency, groups and indenylidene moieties. Thus, for the major rotamers of
especially for stable ruthenium indenylidene catalysts, the decreasing 8a-c, the indenylidene ligand is closer to the mesityl group than to
size of side Nꢀalkyl groups on the NHC group might exhibit less the Nꢀalkyl group. Furthermore, elemental analysis, mass spectra and
hindrance for the substrates hence resulting in greater catalytic single crystal Xꢀray analysis of the complexes confirm their purities
performance.
and configurations.
Single-crystal X-ray diffraction analysis
Results and Discussion
Synthesis of the complexes
Single crystals suitable for Xꢀray diffraction of 8a-c were
grown by slow evaporation of the complexes in
hexane/EtOAc/CH Cl solutions. Complexes 8a-b crystallized
2
2
The unsymmetrical NHC precursors—specifically, 1ꢀmesitylꢀ3ꢀ
methylꢀ4,5ꢀdihydroꢀimidazolium chloride (Scheme 1, 6a), 1ꢀ
mesitylꢀ3ꢀoctylꢀ4,5ꢀdihydroꢀimidazolium chloride (6b), and 1ꢀ
mesitylꢀ3ꢀcyclohexylꢀ4,5ꢀdihydroꢀimidazolium chloride (6c) —
were prepared according to previously reported procedures.
The synthesis of complexes 8a-c was performed through the
in the triclinic centroꢀsymmetric space group
Pꢀ1 and complex
8
c
crystallized in the monoclinic centroꢀsymmetric space group
C2/c. The asymmetric unit of the structure of 8a consists of one
complex molecule and the 3ꢀphenylꢀ1ꢀindenylidene ligand is
completely disordered over two positions, are rotated over
1
2
1
6, 18 approximately 175° with respect to each other. In the case of
reaction of firstꢀgeneration ruthenium indenylidene complex 7
8b, the asymmetric unit contains two complex molecules; in the
case of 8c, the asymmetric unit accommodates one complex
with the unsymmetrical free Nꢀheterocyclic carbenes. The carbenes
were generated from the deprotonation of NHC precursors 6a-c with
potassium hexamethyldisilazide (KHMDS) in toluene at room
temperature. The courses of reactions were monitored by TLC. The
products were purified by silica gel chromatography and
subsequently washed with methanol and pentane to give airꢀstable
reddishꢀbrown solids in moderate yields (55ꢀ63%).
molecule and half an additional CH Cl molecule.
2
2
R
N
N
Mes
Cl
Cl
Ph
(
1). KHMDS / Toluene, 0.5 h, r.t.
N
N
R
Ru
(
2). 7 / Toluene, r.t.
Cl
PCy3
6a-c
8a-c
Cy3P
Cl
Cl
Ph
a: R = Methyl
b: R = Octyl
c: R = Cyclohexyl
Ru
PCy3
7
Scheme 1 General applied synthesis strategy for the synthesis of complexes 8a-c
.
2
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Fig. 2 ORTEP representations of the structures of 8a-c (thermal ellipsoids are
drawn at the 30% probability level), showing the atom labeling scheme of the
heteroatoms. The hydrogen atoms are omitted for clarity. For 8a, the disorder
of the 3-phenyl-1-indenylidene moiety is omitted. For 8b, one of complex
2 2
molecules is omitted. For 8c, the CH Cl solvent molecule is omitted.
Table 1 Selected bond lengths (Å) and bond angles (°) for complexes 8a-c
and 1c
8
a
8b
8c
1c
CNHC-Ru-P
158.6(1)
160.7(2)/
165.5(1)
160.6(1)
1
61.1(2)
C
NHC-Ru=CInd
Cl-Ru-Cl
Ru=CInd
Ru-CNHC
Ru-P
101.0(2)/
99.7(2)/
99.6(2)
160.03(5)/
102.1(2)
158.96(4)
1.856(4)
2.064(4)
2.469(1)
2.404(1)
2.410(1)
100.7(1)
164.29(3)
1.840(3)
2.064(4)
2.4243(9)
2.4061(8)
2.3793(8)
1
03.3(7)
160.31(4)
1
60.18(5)
1.871(5)/
1.876(5)/
1.877(7)
2.072(5)/
1
.92(2)
2.067(4)
2.4378(9)
2.399(1)
2
.072(6)
2.467(1)/
.456(1)
2.402(2)/
.405(1)
2.401(1)/
.401(1)
2
Fig. 3 Stability tests of complexes 8a-c together with complex
solution at 20 C in open air. The lines are intended as a visual aid.
2 in a CDCl
3
Ru-Cl
°
2
2
.402(2)
2
Stability studies of the complexes
The overall arrangement of the ligands surrounding the Before the catalytic testing, complexes 8a-c were subjected to
ruthenium center are similar for all the new complexes (Fig. 2), stability tests. Degradation studies of the catalysts were performed in
with the mesityl group side of the NHC pointing toward the a nonꢀpretreated polar solvent, CDCl (0.6 mL), at 20 °C in open air,
indenylidene moiety, similar to previously reported complexes of and the course of the decomposition processes was monitored by Hꢀ
3
1
9
a,12,17,20
this type.
All atoms coordinate with the ruthenium center in NMR using bis(3,5ꢀdimethoxyphenyl)methanone as an internal
a typical distorted squareꢀpyramidal geometry, and the indenylidene indicator. In general, the reference complex 2 exhibited greater
moieties occupy an apical position. Indeed, for most of the stability than species 8a-c under the test conditions (Fig. 3). After 11
ruthenium indenylidene (including 8a-c) and benzylidene days, 25% of the initial complex 2 remained and 5ꢀ11% of
complexes bearing monoꢀunsymmetrical NHC (1ꢀmesitylꢀ3ꢀ complexes 8a-c persisted. During the course of the decomposition
alkylꢀ4,5ꢀdihydroimidazolꢀ2ꢀylidene) groups, the mesityl group tests, complexes 8a-c exhibited the similar stability and readily
was observed in parallel over the indenylidene plane or phenyl decomposed irrespective of the remaining residual complex
ring to form an intramolecular πꢀπ interaction. This arrangement is concentrations. Nevertheless, the reference complex 2 exhibited a
considered to enhance the stabilization of the formed complexes. In concentration dependent decomposition rate, and this rate decreased
contrast, the failure in isolation of ruthenium benzylidene complexes with lower concentrations. However, with respect to the time
bearing a symmetrical NHC substituted with alkyl groups has been required for a catalytic run, none of these compounds would
12
reported as a consequence of their instability.
decompose to any appreciable extent, even in presence of oxygen.
Some selected bond lengths and angles in the complexes 8a-c
are listed in Table 1, along with the similar data for 1c (for the
structural representation of 1c, see Fig. S2, ESI). Remarkably, the
bent angles of the CNHCꢀRuꢀP follow a trend of the bulkiness of Nꢀ
alkyl substituents on 8a-c, which are 158.6(1), 160.7(2)/161.1(2) and
COOEt
COOEt
COOEt
COOEt
1
mol% [Ru]
CH Cl , 30 oC
2
2
1
65.5(1), respectively. The larger bent angles indicate that, because
9
10
Eq. 1
of the steric congestion, the bulkier Nꢀalkyl substituents on the NHC
ligand induce a stronger repulsive interaction with a PCy group.
3
The CNHCꢀRu=CInd bent angles for 8a-c also show obvious
differences from each other, with the angles of 101.0(2)/103.3(7) for
8
a, 99.7(2)/99.6(2) for 8b and 102.1(2) for 8c. 8a-b exhibit similar
ClꢀRuꢀCl angles (160°), which are comparable with the
corresponding angle in 8c (159°). The RuꢀCInd and RuꢀCNHC bond
lengths for 8a-c can be summarized as 1.9 and 2.1 Å, respectively.
For all complexes 8a-c, the bond lengths of RuꢀP range from
2
2
.4378(9) to 2.467(1) Å and the bond lengths of RuꢀCl are between
.399(1) and 2.410(1) Å.
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DOI: 10.1039/C 096
Jou5DrnT0al Na7Gme
Fig. 4 RCM of
The lines are intended as a visual aid.
9
with complexes
2
and 8a-c (1 mol%) at 30 °C in CH
2
Cl
2
(0.1 M).
3
Fig. 5 ROMP of COD with complexes 2 and 8a-c (0.05 mol%) at 40 °C in CDCl (0.6
mL). The lines are intended as a visual aid.
The ROMP of COD proceeded with
2 and 8a-c with a
Catalytic activity of complexes 8a-c
22
catalyst loading of 0.05 mol% and at 40 °C in CDCl3. Under
and 8a-c reached full
The catalytic ability of complexes 8a-c related to that of complex 2 these conditions, all catalysts
2
in metathesis reactions was investigated in RCM of sterically conversion within two hours; however, different kinetic
hindered diethyl 2ꢀallylꢀ2ꢀ(2ꢀmethylallyl)malonate (9) (Eq. 1) under performance profiles were observed (Fig. 5). 8a exhibited a fast
2
1
standard conditions established by Grubbs’ group. Interestingly, all reaction rate and reached full conversion of COD after only 20
the unsymmetrical NHCꢀcontaining complexes 8a-c showed faster minutes, followed by 8b, which achieved full consumption of
catalytic initiation than the reference complex 2 (Fig. 4). Among COD after approximately 80 minutes. The reference catalyst
2
complexes 8a-c, 8a (bearing the smallestꢀsized NHC with an amino also fully converted the COD in the same time as 8b, but a
side methyl group) performed better in both catalytic initiation and longer induction period in the kinetic profile of the reaction was
efficiency, followed by complex 8b and then complex 8c.
observed. Complex 8a showed a faster increase of reaction rate
The good performance exhibited by the new complexes 8a- (slope) than the other three complexes, this probably was due to
prompted us to further examine their efficiency and the faster initiation of complex 8a and the presence of a smaller
c
selectivity in additional benchmark metathesis reactions, as sized methyl group makes it easier for the approach of COD to
follows: ROMP of cis
cisꢀcyclooctaꢀ1,5ꢀdiene (COD) (Eq. 2); the ruthenium center. Interestingly, 8c significantly
ringꢀclosing enyne metathesis (RCEYM) of (1ꢀ(allyloxy)propꢀ outperformed with respect to the initiation rate, but 8c
ꢀyneꢀ1,1ꢀdiyl)dibenzene (11) (Eq. 3); and CM between exhibited inferior catalytic performance in the propagation
,
2
2
allylbenzene and cisꢀ1,4ꢀdiacetoxyꢀ2ꢀbutene (Eq. 4).
stage, therefore requiring the longest time (approximately 120
minutes) to afford a full conversion of COD.
0
.05 mol% [Ru]
CDCl3, 40 o
Ph
Ph
(
)
Ph
O
Ph
C
n
2 mol% [Ru]
O
Toluene, 50 o
C
11
12
COD
Eq. 2
Eq. 3
4
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DOI: 10.1039/C5DT00967G
ligand. While in case of ruthenium indenylidene complexes, the
associative or interchange pathways were suggested, where the
release of the phosphine ligand happens after or simultaneously
during the attachment of the olefin to the ruthenium core. The
involvement of different pathways is due to the bulky indenylidene
ligand that is not flexible enough to reach a kind of low energy state
of the 14ꢀelectron species, which the smaller sized benzylidene
ligand easily can obtain. In this study, the tested initiation rate of 8a
is identical to its benzylidene analogue 1c and all complexes 8a-c
show significant faster initiation rates than complex 2. This suggests
that all the unsymmetrical NHC bearing complexes follow a
different initiation pathway then complex 2.
A comparison of the different performances among 8a-c
revealed that the size of the Nꢀalkyl group dominated the catalytic
activity. The catalyst bearing the smallest Nꢀalkyl group performed
better in both initiation and propagation stages. The smallest NHC
remaining on an active species may allow the substrates to more
easily approach the ruthenium center to finalize a catalytic cycle than
26
the complex bearing a bulkier NHC ligand.
Fig. 6 RCEYM of 11 with complexes
2
M). The lines are intended as a visual aid.
and 8a-c (2 mol%) at 50 °C in toluene (0.1
The introduction of one aliphatic group instead of one mesityl
group from the original symmetrical NHCꢀbearing complexes 1a
10b,11
failed to improve catalytic efficiency.
Among the investigated
In addition, the synthesized complexes were examined in complexes, even the bestꢀperforming catalyst 1c only exhibited
1
2
the RCEYM of the substrate 11 (Eq. 3) using a catalyst loading catalytic activity comparable to this of the original complex 1a.
A
of 2 mol% at 50 °C in toluene. The kinetic profiles achieved by similar conclusion has been reported with respect to an investigation
2
and 8a-c (Fig. 6) were similar to those obtained from the on benzylidene etherꢀchelating complexes (GrubbsꢀHoveyda
27
RCM of
9
(Fig. 4). Quantitative consumption of 11 required catalysts). In the present study, however, the same NHC ligand
whereas incorporated on the scaffold of indenylidene significantly improved
and 8b required approximately 220 minutes to the efficiency of the resulting catalyst relative to the indenylidene
achieve full conversion. Similar to 8a and 8b
8c exhibited a reference complex 2, which may be a consequence of the electronic
faster catalytic initiation than
, but a full conversion of the and steric modification of NHC on a stable indenylidene complex
substrate in the case of 8c required additional time (5 hours) exerting greater influence than the benzylidene complexes on the
full conversion is not shown in Fig. 6). During the course of catalytic activity.
approximately 90 minutes with complex 8a
complexes
,
2
,
2
(
RCEYM of 11, the differences among the reaction rates of the
complexes are less significant in comparison with
aforementioned RCM and ROMP reactions. This is probably
because of the RCEYM reaction was performed at a relatively
higher temperature and therefor, the catalyst initiation for all
the complexes was promoted.
OEt
OEt
[
Ru] 2.5 mol%
+
+
CH Cl , 35 oC
2
2
OEt
Ph
Ph
The steric and electronic effects of the substituents on the NHC
ligand could significantly influence the catalyst performance in
13
Eq. 4
4
metathesis reactions. Bulky NHC ligands are well known to
stimulate the initiation and improve the efficiency of a catalyst by
Table 2 CM of allylbenzene with 2 eq. of cisꢀ1,4ꢀdiacetoxyꢀ2ꢀbutene
2 2
catalyzed by ruthenium complexes 2 and 8a-c (2.5 mol%) at 35 °C in CH Cl
quickly releasing the PCy group, which stimulates the first catalytic
3
5,17a,17b,23
cycle.
In this study, although complex 2 harbored a bulkier
NHC compared to those in 8a-c, it exhibited slower catalytic
initiation than all of the newly obtained unsymmetrical NHCꢀbearing
complexes under the tested conditions (extra information on
initiation studies see ESI), which disagrees with the previously
established conclusion related to the steric aspect. Therefore, the
faster initiation of 8a-c over 2 maybe due to the stronger σꢀdonating
Entry
Catalyst
2
Time
Conversion to
E/Z
13
75
1
3h
3.0/1
2
3
4
5
6
7
8
3d
3h
3d
3h
3d
3h
3d
78
79
78
77
78
75
75
10.7/1
5.8/1
6.1/1
4.5/1
5.5/1
3.6/1
4.5/1
8a
8b
8c
1
0
property of Nꢀalkyl/mesityl mixed NHC ligand. This hypothesis is
consistent with the results reported by Plenio’s group, that for the
Grubbs second generation type catalysts, which bearing an electronꢀ
donating group substituted NHC ligand, showed faster initiation than
2
4
the one with an electronꢀwithdrawing group.
In addition, carefully analyzing the initiation profiles of 1c, 2
and 8a-c reveals that the catalyst initiation rates are in an order of 1c
≥
8a > 8b ≥ 8c >> 2. Studies of the initiation mechanism show that
In addition to the observed efficiency in the RCM, ROMP
selectivity in CM may also be
ꢀolefins widely exist in natural
ruthenium benzylidene complexes (e.g. 1a-c) and ruthenium and RCEYM reactions,
indenylidene complexes (e.g. 2) follow different pathways . In case important because
E/Z
25
Z
2
8
of ruthenium benzylidene complexes, the initiation is consistent with compounds. Different from the RCM and ROMP reactions,
a dissociation pathway, where the approaching of the olefin to the the CM reaction often lacks driven forces, which make it a
29
ruthenium center occurs after the dissociation of the phosphine challenging type of olefin metathesis reaction.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5

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ARTICLE
DOI: 10.1039/C 096
Jou5DrnT0al Na7Gme
The CM reaction was examined between allylbenzene and 2 eq.
of cisꢀ1,4ꢀdicacetoxyꢀ2ꢀbutene using a catalyst loading of 2.5 mol%
2 2
Fig. 7 RCM of 14 with complexes 1c and 8a (0.125-0.5 mol%) at 30 °C in CH Cl
(
0.1 M) . The lines are intended as a visual aid.
at 35 °C in CH Cl . Under these reaction conditions, all complexes
2
2
achieved a conversion to product 13 within a range of 75ꢀ79%
within three hours (Table 2). The lowest E/Z ratio of 3.0/1 (Entry 1) verified in the RCM of diethyl 2,2ꢀdiallylmalonate (14) (Eq. 5) using
was achieved with catalyst 2, whereas complexes 8a-c afforded 5.8/1, various catalyst loadings (0.125ꢀ0.5 mol%) at 30 °C in CH Cl . In
.5/1 and 3.6/1 E/Z ratios (Entries 3, 5 and 7), respectively. When general, with all the used catalyst loadings, benzylidene complex 1c
the reaction was extended to three days, a much higher E/Z ratio of exhibited faster initiation than indenylidene complex 8a, whereas
A comparison of the performance between 1c and 8a was
2
2
4
1
8
0.7/1 (Entry 2) was observed for 2, whereas the ratios in cases of complex 8a outperformed 1c in overall catalyst efficiency (Fig. 7).
a-c slightly increase to 6.1/1 for 8a, 5.5/1 for 8b and 4.5/1 for 8c When the catalyst loading was decreased, the decrease in catalytic
(Entries 4, 6 and 8), respectively.
efficiency was more pronounced with 1c than with 8a, which led to a
In the initial conversion, a moderate amount of
Z
ꢀisomer clear discrepancy between the reaction profiles of the two catalysts.
often exists in the product mixtures, whereas the secondary The difference between the conversions achieved with 8a and 1c was
metathesis favors the formation of the thermodynamic
only 2% at a catalyst loading of 0.5 mol%, and this difference
isomer. The faster initiation of a catalyst allows a rapid increased to 15% at a catalyst loading of 0.125 mol%.
conversion of substrates. At the same time, concomitant
This observation is consistent with the general conclusion
formation of stable
ꢀisomer in the secondary metathesis also that ruthenium indenylidene complexes are more stable than
E
ꢀ
3
0
E
3
0g
16b,31
promoted. Thus, higher
for
were observed during first three hours. When the reaction catalyst precursor releases a higher concentration of active
time was extended from three hours to three days, the observed species to the reaction solution. On one hand, this higher
smaller increase of the
ratios for complexes 8a-c relative to concentration of active species will cause a higher rate of
that for
might have been due to the less stable property of the catalytic performance. On the other hand, the higher
E
/
Z
ratios for 8a-c compared to that their benzylidene counterparts.
A faster initiation of a
2
E/Z
2
unsymmetrical NHCꢀbearing complexes (see the section on concentration of active species are responsible for the faster
stability testing), because the isomerization process for the extinction of their catalytic ability, since the active species of
formation of
occurs after the disappearance of active species
Eꢀisomer from original formed Z
ꢀisomer no longer the NHC coordinated methylidene 14ꢀelectron ruthenium
3
0e
32
.
complex could decompose via a bimolecular routine and the
3
3
decomposition rate is proportional to its concentration.
Although, there will be an equal total number of active species
that could be generated from 8a and 1c, the average life time
for the active species that are derived from 8a could be slightly
longer than 1c, because of the slightly slower initiation rate of
Comparison of complex 8a with its benzylidene analogue 1c
in metathesis reaction
As demonstrated in aforementioned metathesis reactions, the
new complexes 8a-b exhibited better catalytic activity relative
to the classical SIMesꢀbearing ruthenium indenylidene complex
8
a
. The longer average lifetime of the active species that are
derived from 8a causes a better overall performance of 8a than
1c.
2
. Thus, we compared the performance of the most active
complex 8a (in current studying) to that of the previously
1
0b,12
explored benzylidene analogue 1c
.
Conclusions
In summary, three new airꢀstable ruthenium indenylidene
complexes containing
Nꢀalkyl/Nꢀmesityl mixed NHC ligands
COOEt
COOEt
COOEt
were successfully synthesized and isolated in moderate yields.
Singleꢀcrystal Xꢀray analyses revealed the configuration of the
obtained complexes and showed that the repulsion interaction
between the NHC and PCy3 groups are correlated to the
0
.125-0.5 mol% [Ru]
, 30 o
2
C
CH Cl
2
COOEt
14
15
Eq. 5
bulkiness of the
catalysts 8a-c and the reference catalyst
replacement of one ꢀmesityl group by different
N
ꢀalkyl groups. A comparison between the
revealed that
ꢀalkyl groups
2
N
N
could significantly improve the initiation of the resulting
complexes. This faster initiation of the new catalysts might
stem from the relatively stronger
unsymmetrical NHC ligands. The best catalytic efficiency was
achieved with complex 8a, which had the smallestꢀsized
σꢀdonating properties of the
Nꢀ
alkyl group on the NHC ligand among the investigated
catalysts. The greater activity of 8a was attributed to it faster
initiation and the smaller obstruction of the NHC for the
substrates during the metathesis process. Complex 8a not only
significantly
outperformed
the
reference
ruthenium
indenylidene complex
2
with respect to initiation and efficiency
under the tested conditions, but it also provided higher overall
conversion of diethyl 2,2ꢀdiallylmalonate compared to its
benzylidene analogue 1c, especially at low catalyst loadings.
This result suggests that the optimization of both steric and
electronic properties is critical in designing metathesis catalysts
because the same ligand can be more beneficial in one family
of catalysts than in another one. Moreover, tuning of the steric
properties also influences the initiation pathway of complex.
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

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DOI: 10.1039/C5DT009 G
ART LE
67IC
2
247; (d) J. Huang, E. D. Stevens, S. P. Nolan and J. L. Petersen, J. Am.
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Acknowledgements
F.V. would like to express his deep appreciation to the State
Key Lab of Advanced Technology for Materials Synthesis and
Processing for financial support (Wuhan University of
Technology). F.V. acknowledges the Chinese Central
Government for an Expert of the State Position in the Program
of Thousand Talents and the support of the Natural Science
Foundation of China (No. 21172027). K.V.H. would like to
thank the Hercules Foundation (project AUGE/11/029 "3Dꢀ
SPACE: 3D Structural Platform Aiming for Chemical
Excellence") and the Research Fund – Flanders (FWO) for
funding. B.Y., B.S. and F.V. appreciate the Research Fund ꢀ
Flanders (FWO) (project No. 3G022912) for funding.
4
(a) C. Samojłowicz, M. Bieniek and K. Grela, Chem. Rev., 2009, 109,
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1
708; (b) G. C. Vougioukalakis and R. H. Grubbs, Chem. Rev., 2009,
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Chem. Rev., 2013, 257, 2274; (d) N. Ledoux, B. Allaert, A. Linden, P.
Van der Voort, F. Verpoort, Organometallics., 2007, 26, 1052.
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2
013, 49, 3188.
Notes
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8
K. Skowerski, C. Wierzbicka, G. Szczepaniak, Ł. Gułajski, M. Bieniek
and K. Grela, Green Chem., 2012, 14, 3264.
(a) T. Ritter, M. W. Day and R. H. Grubbs, J. Am. Chem. Soc., 2006,
a
Laboratory of Organometallics, Catalysis and Ordered Materials, State
Key Laboratory of Advanced Technology for Materials Synthesis and
Processing; School of Chemistry, Chemical Engineering and Life
1
28, 11768; (b) N. Ledoux, R. Drozdzak, B. Allaert, A. Linden, P. Van
Der Voort, F. Verpoort, Dalton Trans., 2007, 44, 5201.
Sciences, Wuhan University of Technology, Wuhan 430070, P.R. China.
9
(a) S. Prühs, C. W. Lehmann and A. Fürstner, Organometallics, 2004,
23, 280; (b) M. Mayr, M. R. Buchmeiser and K. Wurst, Adv. Synth.
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Xie, Y. Lu, F. Ding, Y. Sun and F. Verpoort, Curr. Org. Chem., 2013,
b
,
National Research Tomsk Polytechnic University. Tomsk, Russian
Federation.
c
Dar es Salaam University College of Education, Faculty of Science,
Department of Chemistry, P.O.Box 2329, Dar es Salaam, Tanzania.
1
7, 2592; (e) B. Allaert, N. Dieltiens, N. Ledoux, C. Vercaemst, P. Van
der Voort, C.V. Stevens, A. Linden, F. Verpoort, J. Mol. Cat. AꢀChem.,
006, 260, 221.
d
Department of Inorganic and Physical Chemistry, Ghent University,
2
Krijgslaan 281ꢀS3, 9000, Ghent, Belgium. Fax: +32ꢀ9ꢀ264ꢀ4183. Eꢀmail:
1
0
(a) J. Huang, L. Jafarpour, A. C. Hillier, E. D. Stevens and S. P. Nolan,
Organometallics, 2001, 20, 2878; (b) K. Vehlow, S. Maechling and S.
Blechert, Organometallics, 2006, 25, 25.
M. B. Dinger, P. Nieczypor and J. C. Mol, Organometallics, 2003, 22,
5
N. Ledoux, B. Allaert, S. Pattyn, H. V. Mierde, C. Vercaemst and F.
Verpoort, Chem. Eur. J., 2006, 12, 4654.
K. Endo and R. H. Grubbs, J. Am. Chem. Soc., 2011, 133, 8525.
V. M. Marx, L. E. Rosebrugh, M. B. Herbert and R. H. Grubbs,
Ruthenium in Catalysis, Topics in Organometallic Chemistry, ed. P. H.
Dixneuf and C. Bruneau, Springer Verlag, Germany, 2014, vol. 48, pp.
Francis.Verpoort@UGent.be
e
Centre for Surface Chemistry and Catalysis, Leuven University, 3001,
1
1
1
2
Leuven, Belgium
291.
†
CCDCꢀ1049430ꢀ1049431ꢀ1049433ꢀ1049495
contain
the
supplementary crystallographic data for this paper and can also be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12, Union Road,
1
1
3
4
Cambridge
deposit@ccdc.cam.ac.uk).
Electronic Supplementary Information (ESI) available: [Detailed
synthesis procedure of complexes 8a-c; Initiation studies of complexes
CB2
1EZ,
UK;
fax:
+44ꢀ1223ꢀ336033;
or
1
ꢀ17.
1
5
(a) F. Boeda, H. Clavier and S. P. Nolan, Chem. Commun., 2008, 2726;
(b) A. M. LozanoꢀVila, S. Monsaert, A. Bajek and F. Verpoort, Chem.
Rev., 2010, 110, 4865.
‡
1
13
31
16 (a) K. Harlow, A. Hill and J. E. WiltonꢀEly, J. Chem. Soc., Dalton
Trans., 1999, 285; (b) A. Fürstner, O. Guth, A. Düffels, G. Seidel, M.
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Kędziorek, M. Malińska, K. Woźniak and K. Grela, Organometallics,
1c, 2 and 8a-c; Xꢀray figure of complex 1c; Hꢀ, Cꢀ and PꢀNMR
spectra of obtained compounds are included here]. See
DOI: 10.1039/b000000x/
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Jou5DrnT0al Na7Gme
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| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5DT00967G
Three ruthenium indenylidene complexes containing
N-alkyl/N-mesityl mixed N-heterocyclic carbene
ligands show significant improvement in their catalytic initiation rate. The smaller sized NHC
contributes to a better catalytic performance.