New ruthenium metathesis catalysts with chelating indenylidene ligands: Synthesis, characterization and reactivity

Article abstract of DOI:10.1039/c2dt12271e

Six new ruthenium complexes bearing a bidentate (κ₂O,C)- isopropoxy-indenylidene and PPh₃ or PCy₃ ligands have been synthesized and characterized by ¹H, ¹³C NMR spectroscopy and X-ray crystallography.

Full text of DOI:10.1039/c2dt12271e

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PAPER
New ruthenium metathesis catalysts with chelating indenylidene ligands:
synthesis, characterization and reactivity†‡
Anzhelika Kabro,a Ghazi Ghattas,a Thierry Roisnel,b Cédric Fischmeister*a and Christian Bruneau*a
Received 25th November 2011, Accepted 4th January 2012
DOI: 10.1039/c2dt12271e
Six new ruthenium complexes bearing a bidentate (κ2O,C)-isopropoxy-indenylidene and PPh3 or PCy3
ligands have been synthesized and characterized by 1H, 13C NMR spectroscopy and X-ray
crystallography. Some of these complexes were synthesized in dimethyl carbonate, a green solvent that
was recently shown to be suitable for several catalytic transformations including olefin metathesis. The
thermal stability and catalytic efficiency of the PCy3-containing complexes have been evaluated in a
series of test reactions.
We are now reporting on three new congeners of this family,
where the steric and electronic demands on the bidentate indeny-
lidene ligand have been modified. The influence of these vari-
Introduction
Olefin metathesis1 is well established as an efficient and accessi-
ble synthetic tool for the modification and creation of simple and
complex organic molecules2 as well as for the production of
polymers3 and the transformation of renewable resources4 by
self-5 and cross-metathesis with ethylene, functional alkenes6
and alkynes.7 This tremendous impact of olefin metathesis on
molecular synthesis is in a large part the result of the continuous
development of more efficient and stable transition metal cata-
lysts, especially ruthenium-based complexes. Since the first well-
defined complex described by Grubbs in 1992,8 more than 350
catalysts sharing the same architecture, i.e. RuX2(L)2(vCR1R2)
have been reported.9 In particular, the Grubbs,10 Hoveyda11 and
indenylidene12 catalysts are the three main families of neutral
complexes whereas several cationic and neutral complexes13
have also been described. In 2010, we have reported the first
member of a new family of ruthenium catalysts featuring a che-
lating indenylidene ligand 1 (Fig. 1).14,15 This catalyst displayed
an extremely high thermal stability associated with a high cata-
lyst activity, in some cases higher than the related Hoveyda or
classical indenylidene catalysts. In fact, complex 1 displayed
some features characterizing latent olefin metathesis catalysts.16
ations has been studied in terms of thermal stability as well as
catalytic activity. We also show that in some cases dichloro-
methane can be advantageously replaced by the greener dimethyl
carbonate (DMC) in organometallic syntheses.
Results and discussion
Synthesis and characterization
The syntheses of the new complexes 2, 3, and 4 based on the
structure of the recently reported complex 1 have been per-
formed in order to evaluate the influence of steric and electronic
effects on the stability and catalytic efficiency of these com-
plexes (Fig. 1). Dichloromethane and toluene are the preferred
solvents in olefin metathesis both for catalytic transformations
and catalyst synthesis. Recently, we have shown that the greener
aSciences Chimiques de Rennes. Catalyse et Organométalliques,
UMR 6226-CNRS-Université de Rennes 1, Campus de Beaulieu, 263,
avenue du général Leclerc, 35042 Rennes, France. E-mail:
cedric.fischmeister@univ-rennes1.fr, christian.bruneau@univ-rennes1.fr;
Fax: (+33) 223236939
bCentre de diffractométrie X, UMR 6226-CNRS-Université de Rennes 1,
Campus de Beaulieu, 263, avenue du général Leclerc, 35042 Rennes,
France
†Dedicated to Professor David Cole-Hamilton on the occasion of his
retirement and for his outstanding contribution to transition metal
catalysis.
‡Electronic supplementary information (ESI) available: Full experimen-
tal details and cif files. CCDC reference numbers 856296–856300. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt12271e
Fig. 1 Complexes 1, 2, 3 and 4.
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DMC17 was perfectly suitable for olefin metathesis transform-
ations such as ring-closing metathesis (RCM), cross-metathesis
(CM) and ethenolysis of fatty ester derivatives.18
During our research aimed at the synthesis of new olefin
metathesis catalysts we have evaluated the potential of DMC in
organometallic synthesis for the preparation of complexes such
as 6 and 1 (Scheme 1). Complex 6 was originally prepared in
THF using a large excess of CuCl (8 equiv.) in order to trap the
released triphenylphosphine. For practical reasons several
solvent changes (THF, CH2Cl2, toluene) were necessary in order
to ensure an efficient separation of the formed copper–phosphine
compound yielding 6 in 67%.14
Scheme 3 Propargylic alcohols 7, 8 and 9, precursors of complexes
10, 11, 12 and 2, 3, 4.
treated with 2 equiv. of PCy3 in CH2Cl2 yielding 2 in a 38%
yield after purification by column chromatography (16% overall
yield for the 2 steps). Alternately, 2 could also be prepared in
DMC in one step without isolating the intermediate complex 10.
In this case 2 was isolated in a slightly higher 24% yield
(2 steps).
Scheme 1 Synthesis of complex 6.
The synthesis of 6 was then attempted in DMC and performed
well using only 2.5 equiv. of CuCl yielding 6 in 73%. In particu-
lar, the work-up procedure was drastically simplified as the
copper–phosphine complex precipitated in DMC.19 Encouraged
by this result, the one-pot synthesis of 1 was carried out in DMC
without isolating the intermediate complex 6 (Scheme 2). With
this procedure, 1 was isolated in a reasonable and competitive
40% yield for the two step synthesis.
Scheme 4 Synthesis of complex 2.
The synthesis of complex 3 was performed in 2 steps and
started by the initial reaction of the propargylic alcohol 8 with
RuCl2(PPh3)3 in DMC at 70 °C. In that case the reaction was
attempted without CuCl. This procedure furnished complex 11
in a 61% isolated yield. The same reaction carried out in THF
with CuCl as the phosphine scavenger provided 11 in a 56%
yield. 11 was further reacted with 2 equiv. of PCy3 in CH2Cl2 to
give 3 in a 60% yield (Scheme 5). X-ray structures of complexes
11 and 3 were recorded from crystals obtained by slow diffusion
of hexane in CH2Cl2 and THF, respectively.
Scheme 2 One-pot synthesis of 1.
Having improved the protocol for the preparation of complex
1, the synthesis of complexes 2, 3 and 4 was then investigated
and started by the preliminary synthesis of the propargylic alco-
hols 7, 8 and 9 (Scheme 3). These syntheses were realized
according to the previously reported synthesis of 5.14 However,
it was necessary to adapt this protocol to the specificity of each
compound (see ESI).‡
Synthesis and characterization of complexes 2, 3, 4, 10, 11
and 12
Scheme 5 Synthesis of complex 3.
Finally, complex 4 was prepared in a single step in DMC
without isolating the intermediate complex 12, which was never-
theless identified by its characteristic 31P NMR shift at
63.0 ppm. 12 was then reacted with PCy3 at room temperature to
furnish 4 in an overall yield of 50% for the two steps
(Scheme 6). Unfortunately it was not possible to obtain crystals
of 4 suitable for X-ray characterization.
Having demonstrated its beneficial impact, DMC was used for
the synthesis of complexes 2, 3 and 4. RuCl2(PPh3)3 and 7 were
thus reacted at 70 °C in DMC in the presence of 2.5 equiv. of
CuCl. A white powder of [CuClPPh3]4 precipitated upon cooling
the reaction to room temperature and complex 10 was isolated in
38% yield (Scheme 4). Crystals suitable for X-ray analysis were
obtained by slow diffusion of hexane in CH2Cl2. 10 was then
3696 | Dalton Trans., 2012, 41, 3695–3700
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Scheme 6 Synthesis of complex 4.
Molecular structure analysis
The molecular structures of complexes 10, 11 and 3 were ana-
lysed and compared to the previously reported molecular struc-
tures of complexes 6 and 1, respectively, with the aim of
establishing structure–stability and structure–activity relation-
ships. Significant structural data of complexes 6, 10 and 11 are
collected in Table 1. These data showed that the torsion angle
[C2–C3–C4–C5] constituted the single significant difference
between these complexes. In fact this torsion angle was almost
similar in 6 and 10 hence showing that the meta-substitution by
the bulky –OiPr in complex 6 has no impact on this torsion
angle (Fig. 2). By contrast, as one could anticipate, the introduc-
tion of an o-methyl substituent in 11 induced an almost perpen-
dicular orientation of the 2,6-dimethyphenyl fragment (Fig. 3).
Fig. 3 Molecular structure of complex 11 represented at 50% ellipsoid
probability. H atoms are omitted for clarity.
Table 1 Structural data of complexes 6, 10 and 11
Structural data
6
10
11
Ru–C1[a]
1.854(2)
2.2304(5)
2.424(2)
2.3177(6)
2.3058(7)
1.470(3)
147.11(2)
178.38(4)
50.8(3)
1.849(5)
2.223(1)
2.429(3)
2.302(1)
2.312(1)
1.490(1)
142.60(5)
176.76(8)
46.7(1)
1.854(2)
2.2285(6)
2.474(2)
2.2995(6)
2.3054(6)
1.494(3)
139.72(2)
175.01(4)
83.3(3)
Ru–P[a]
Ru–O[a]
Ru–Cl1[a]
Ru–Cl2 [a]
C3–C4
Fig. 4 Molecular structure of complex 3 represented at 50% ellipsoid
probability. H atoms are omitted for clarity.
Cl1–Ru–Cl2[b]
P–Ru–O[b]
C2–C3–C4–C5
hindrance of the 2,6-dimethylphenyl substituent cannot account
for the smaller Cl1–Ru–Cl2 angle as all the structural data other
than the torsion angle [C2–C3–C4–C5] are very similar in 6, 10
and 11. The variations in the Cl1–Ru–Cl2 are hence likely due
to an electronic effect on the conjugated indenyl-backbone.
During this work the synthesis of a non-substituted, i.e. C(3)–H,
complex was attempted. Although the PPh3 complex was
obtained, its stability and synthesis repeatability were poor.
However an X-ray structure could be obtained and showed a
Cl1–Ru–Cl2 angle of 144.16°, intermediate between 6 and 10.21
From the three new complexes with the PCy3 ligand, only 3
furnished crystals suitable for X-ray analysis (Fig. 4). This struc-
ture was compared to that of the parent PPh3 complex 11 and
that of the previously reported PCy3 complex 1. The substitution
by a more donating and bulky phosphine had no major impact
on the molecular structure of 3. In fact, all the characteristic
bond lengths and angles remained almost unchanged as com-
pared to 11 except the Cl1–Ru–Cl2 angle which increased from
139.72 to 145.00 in order to accommodate one bulky cyclohexyl
ring from the phosphine ligand (Ru–C1: 1.854(2) Å; Ru–P:
2.2595(6) Å; Ru–O: 2.489(2) Å; Ru–Cl1: 2.3136(7) Å; Ru–Cl2:
2.3077(7) Å; P–Ru–O: 176.20(4)°; C2–C3–C4–C5: 88.0(3)°.
Fig. 2 Molecular structure of complex 10 represented at 50% ellipsoid
probability. H atoms are omitted for clarity.20
In addition, it was observed that the Cl1–Ru–Cl2 angle was
sensitive to the nature of the C3-substituent. In fact, the steric
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Scheme 7 RCM of diethyl diallylmalonate.
Scheme 8 RCM of diethyl allyl(2-methylallyl)malonate.
Fig. 5 Thermal stability at 110 °C in toluene-d8 of complexes 1, 2, 3
1
and 4 and Hoveyda first generation complex (HI) measured by H NMR
vs. trimethoxybenzene as the internal standard.
The molecular structure of 3 was then compared to that of
complex 1 bearing a m,m-diisopropyloxyphenyl C(3)-substitu-
ent. As observed when comparing 6 to 11, the main structural
difference between 3 and 1 concerned the torsion angle [C2–
C3–C4–C5] that varied from 25.87° for 1 to almost 90° for 3
(87.97). With all these structural data collected that highlighted
some structural differences, the thermal stability and catalytic
activity of complexes 2–4 were then investigated in detail.
Fig. 6 RCM conversion of DEDAM with complexes Ru–Ind (■),
HI (▲) and 1 (●) at 30 °C.
Thermal stability and catalytic activity
Having previously shown the latent character of complex 1, the
thermal stability in solution of the new complexes 2, 3 and 4
was evaluated and compared to that of the previously reported
complex 1 and to the parent Hoveyda first generation complex.
As depicted in Fig. 5, all the new complexes were less thermally
stable than 1. In particular complex 2 was found to be the less
stable of all the complexes including the first generation
Hoveyda catalyst. By contrast complexes 3 and 4 featuring
ortho-substitution on the pending aryl fragment were more stable
than 2 and Hoveyda first generation catalyst but yet less stable
than 1. Without structural data on complexes 4 and 12 it would
be risky to draw any conclusion on the influence of the C(3)-aryl
substitution pattern. However, the superior stability of 1 shows
that strong electron donating substituents on the C(3)-pending
phenyl substituent is a prerequisite for thermal stability. Further-
more, if the C3–C4 bond length can be used as a probe of the
conjugation between the indenyl fragment and the pending aryl
substituent, the nearly constant value of the C3–C4 bond length
in 6, 10 (Table 1) and 1 (1.464(6) Å) shows that conjugation
effects are not determinant to account for the stability variations
observed with the different complexes (conjugation in 11 and 3
should be negligible due to the nearly perpendicular arrangement
of the indenyl and phenyl substituent).
Fig. 7 RCM conversion of DEDAM with complexes 1–4 at 30 °C in
dichloromethane.
We have previously reported the comparison between
complex 1 and the structurally related Ru–indenylidene and
Hoveyda first generation complexes, and we have shown that all
complexes performed well but with different reaction profiles. In
particular 1 showed a sigmoidal curve resulting from a slow
initiation step (Fig. 6).
When the new complexes 2, 3 and 4 were engaged in the
RCM of DEDAM under the same conditions, a similar trend
was observed with, however, an extended induction period for
all three new complexes (Fig. 7). Of note, the more thermally
stable complex 1 initiated faster than its less stable congeners 2,
The catalytic efficiency of the three new complexes was then
evaluated in the RCM reactions of diethyl diallylmalonate
(DEDAM) and diethyl allyl(2-methylallyl)malonate using a set
of well defined experimental conditions allowing for comparison
with other catalysts in dichloromethane22 but it must be men-
tioned that they perform with the same efficiency in greener
DMC18 (Schemes 7 and 8).
3698 | Dalton Trans., 2012, 41, 3695–3700
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associated with the longest initiation period. In this regards, this
complex might be very interesting as a latent catalyst for
polymerization reactions such as the polymerization of dicyclo-
pentadiene. These results clearly show that simple chemical
modifications on the pending C(3)-aryl substituent should allow
for fine tuning of the catalyst initiation and overall efficacy in
RCM and polymerization reactions.
Acknowledgements
The authors are grateful to the French Agence Nationale de la
Recherche for financial support and post-doctoral grant to
A. K. (ANR-08-EFC-03-01 BIOTANDEM).
Fig. 8 RCM conversion of diethyl allyl(2-methylallyl)malonate with
complexes 1–4, HI and Ru–Ind at 30 °C in dichloromethane.
Notes and references
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3 and 4. With little knowledge on the initiation mechanism of
such complexes it is difficult at the moment to explain this cata-
lyst hierarchy.23 However, the extended induction period
observed with 3 in particular, is very interesting regarding latent
catalysts and shows that electronic and steric variations at the
pending C(3)-aryl substituent can induce modifications of the
catalytic properties.
Complexes 2, 3 and 4 were then tested in the more difficult
RCM reaction of diethyl allyl(2-methylallyl)malonate
(Scheme 8). As observed with 1, the new complexes showed
improved performances as compared to other first generation
complexes as they reached higher conversions in the same
amount of time (Fig. 8).
As observed for the RCM of DEDAM (Fig. 7) 4 displays the
closest reaction profile to 1 and provided almost full conversion
(97%) of substrate upon an extended reaction time (600 min).
The same results were obtained at 700 min with 2 (92%) and 3
(93%), when the first generation Hoveyda and Ru–indenylidene
catalysts failed to reach 80 and 70% conversion, respectively.
Recently, we have shown that slow addition of catalysts into the
reaction media led to enhanced performances in metathesis trans-
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to a low concentration of that species.6b In the present case we
believe that the very slow activation of complexes 1–4 may have
the same effect by gradually releasing the active catalyst into the
reaction media.
Conclusions
We have synthesized and characterized 6 new complexes of the
recently disclosed family of (κ2O,C)-ruthenium–indenylidene
complexes and we have shown that these organometallic synth-
eses could be performed in DMC, a much greener solvent than
the usual tetrahydrofuran, dichloromethane and toluene most
often used in such syntheses. Of the three new (PCy3) complexes
prepared, complex 4 bearing an o-F substituents on the C(3)-
pending aryl group was found to be the closest to 1 in terms
of catalytic activity and thermal stability. Complex 3 bearing
an o-CH3 substituent also displayed a good thermal stability
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simultaneously
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3700 | Dalton Trans., 2012, 41, 3695–3700
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