Backbone tuning in indenylidene-ruthenium complexes bearing an unsaturated N-heterocyclic carbene

Article abstract of DOI:10.3762/bjoc.6.128

The steric and electronic influence of backbone substitution in IMes-based (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2- ylidene) N-heterocyclic carbenes (NHC) was probed by synthesizing the [RhCl(CO)₂(NHC)] series of complexes to quantify

Full text of DOI:10.3762/bjoc.6.128

Backbone tuning in indenylidene–ruthenium
complexes bearing an unsaturated
N-heterocyclic carbene
César A. Urbina-Blanco1, Xavier Bantreil1, Hervé Clavier1,2,
Alexandra M. Z. Slawin1 and Steven P. Nolan*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, 1120–1126.
1EaStCHEM School of Chemistry, University of St Andrews, St
Andrews KY16 9ST, United Kingdom and 2present address: Institut
des Sciences Moléculaires de Marseille Université Aix-Marseille,
UMR CNRS 6263, France
doi:10.3762/bjoc.6.128
Received: 17 September 2010
Accepted: 03 November 2010
Published: 23 November 2010
Email:
César A. Urbina-Blanco - caub@st-andrews.ac.uk; Steven P. Nolan* -
snolan@st-andrews.ac.uk
Guest Editor: K. Grela
© 2010 Urbina-Blanco et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
N-heterocyclic carbene; olefin metathesis; percent buried volume;
ruthenium–indenylidene; Tolman electronic parameter
Abstract
The steric and electronic influence of backbone substitution in IMes-based (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
ylidene) N-heterocyclic carbenes (NHC) was probed by synthesizing the [RhCl(CO)2(NHC)] series of complexes to quantify exper-
imentally the Tolman electronic parameter (electronic) and the percent buried volume (%Vbur, steric) parameters. The corres-
ponding ruthenium–indenylidene complexes were also synthesized and tested in benchmark metathesis transformations to establish
possible correlations between reactivity and NHC electronic and steric parameters.
Introduction
The use of N-heterocyclic carbenes (NHC) as spectator ligands featured in a number of catalysts that display excellent activity
in ruthenium-mediated olefin metathesis represents one of the in olefin metathesis. NHCs have become the ligand par excel-
most important breakthroughs in this field [1-8]. Mixed lence in olefin metathesis (Figure 1) [7,8].
complexes bearing both a phosphane and a NHC ligand,
so-called 2nd generation catalysts, typically display better In order to improve catalytic activity, the possibility of fine-
thermal stability and activities compared to 1st generation cata- tuning of NHC steric and electronic properties has been
lysts [9,10]. Key to the success and research activity involving exploited. Bulkier and more electron-donating NHCs allow
2nd generation catalysts has been the wide selection of NHCs faster initiation with usually a concurrent increase in reaction
available [11,12]. These highly basic ligands have now been rate when the olefin substrate is of low steric hindrance [13-17].
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Beilstein J. Org. Chem. 2010, 6, 1120–1126.
Figure 1: Representative olefin metathesis catalysts.
Results and Discussion
Evaluation of the ligand electronic and steric
properties
Less sterically demanding NHCs are typically used for the syn-
thesis of highly encumbered olefins [18]. Recent studies have
shown that backbone substitution in saturated NHCs greatly
improves catalyst stability by restricting rotation around the
N–Caryl bond (Figure 2); this presumably slows catalyst decom-
position via an observed C–H activation route [19].
Previous studies have shown that the electronic parameter of
NHC (and other) ligands can be quantified employing the
stretching frequency of CO (νCO) in various transition
metal–carbonyl complexes [26-32]. This method was initially
developed by Tolman [33], using the average infrared
frequency of CO in [Ni(CO)3L] complexes. This electronic
parameter has become known as the Tolman electronic para-
meter (TEP) and has been used to quantify the electron donor
ability of phosphanes, and more recently, has been used to study
the electronic properties of NHCs [34].
However, the high toxicity of [Ni(CO)4] encouraged the search
for analogous systems using different metals to determine the
TEP. One of the most popular and suitable alternatives to nickel
is a rhodium carbonyl system, since it is easily synthesised and
handled [34]. In this work, a series of [RhCl(CO)2(NHC)]
complexes were synthesized in order to evaluate the electronic
donor ability of the NHCs.
Figure 2: Highly active olefin metathesis catalysts bearing NHC with
backbone substitution.
These results encouraged us to explore the electronic influence
of backbone substitution in unsaturated NHCs with
ruthenium–indenylidene complexes. Indenylidene catalysts are
rapidly becoming quite popular [20,21], due to the availability
of ruthenium precursors [22] and their straightforward syn-
thesis. The higher steric hindrance and improved electronic
donor ability of the indenylidene moiety also contribute to the
observed increased stability compared to benzylidene
congeners. This family of complexes displayed interesting
stability even when forcing reaction conditions are required
[13,23-25].
The free carbenes were prepared according to literature pro-
cedures. Free IMes (4b) [35] and IMesMe (4a) [36] were
synthesized from the corresponding tetrafluoroborate salts; free
IMesBr (4c) [37] and IMesCl (4d) [38] were synthesized in situ
prior to complex synthesis by reacting free IMes with CBr4 and
CCl4, respectively (Scheme 1).
The complexes 5a–d were prepared by reacting [Rh(CO)2Cl]2
Herein, we present the synthesis and characterization of three with the corresponding free carbene in THF (Scheme 2). After
new ruthenium–indenylidene catalysts and their performance in stirring for 4 h at room temperature, removal of the solvents and
benchmark metathesis transformations. In order to quantify the washing of the residue with pentane, the corresponding
Tolman electronic parameter associated with IMes-type (IMes = complexes were obtained as yellow microcrystalline solids, in
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) ligands good yields (71–80%).
possessing variable backbone substitution patterns, the corres-
ponding series of [RhCl(CO)2(NHC)] complexes was synthe- Infrared spectra were recorded in DCM for 5a–d and the car-
sized. X-ray diffraction studies permit the determination of the bonyl stretching frequencies (νCOav) were used to provide the
percent buried volume (%Vbur) of these NHCs ligands and TEP (Table 1). As expected, the backbone substitution pattern
quantify their respective steric parameter.
has a profound effect on the electronic donor capacity of the
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Beilstein J. Org. Chem. 2010, 6, 1120–1126.
Scheme 1: Synthesis of the free NHCs.
Scheme 2: Synthesis of [RhCl(CO)2(NHC)] complexes.
NHC, and a linear correlation between the electronegativity of Analysis of the crystal structures of 5a–d, in conjunction with
the backbone substituent (measured as the Hammett parameter, the aforementioned computational tool, allow us to conclude
σp) and the average carbonyl stretching frequency (νCOav) in that a hydrogen–methyl or hydrogen–halogen exchange in the
[RhCl(CO)2(NHC)] complexes is observed (R2 = 0.98).
backbone creates small steric variation in the NHC evidenced
by the very close values obtained for the %Vbur [40]. However,
the %Vbur for the ligands correlates very well with the size of
the substituent: IMesCl ≈ IMesBr > IMes ≈ IMesMe.
The electron donating nature of the NHC decreases along the
series: IMesMe > IMes > IMesBr > IMesCl. As an internal
check of the data, it is worth noting that the calculated TEP for
IMes (2050.3 cm−1) agrees well with the experimentally
obtained value in the nickel system (2051.5 cm−1) [34].
Synthesis of ruthenium–indenylidene cata-
lysts and their performance in olefin
metathesis
The ruthenium–indenylidene complexes were synthesized in
order to establish how strongly the electronic and steric parame-
ters of the NHC influence catalytic activity in olefin metathesis.
As reported for 6b [41], precatalysts 6a, 6c and 6d were synthe-
sized by exchange between PCy3 and the corresponding free
carbene in [RuCl2(PCy3)2(Ind)] (Scheme 3). The new
complexes proved challenging to purify by recrystallization,
Given their steric and geometric variability, evaluating the steric
parameters of NHCs poses a more challenging task. One of the
more recent methodologies defines a percent buried volume
(%Vbur), which quantifies the volume of a sphere centred
around the metal (with a specific radius distance) occupied by
the ligand. The more sterically demanding ligands will corres-
pond to larger %Vbur values [36,39].
Table 1: Electronic and steric parameters of NHCs in [RhCl(CO)2(NHC)] complexes.
Complex
νCOav (cm−1)
TEPa (cm−1)
σp
%Vbur
[RhCl(CO)2(IMesMe)]
[RhCl(CO)2(IMes)]
2034.8
2037.6
2041.3
2042.5
2048.0
2050.3
2053.3
2054.2
−0.170
0.000
0.227
0.232
31.7 ± 0.1b
31.8 ± 0.5b
32.6
[RhCl(CO)2(IMesBr)]
[RhCl(CO)2(IMesCl)]
32.7
aTEP calculated using equation TEP = 0.8001 νCOav + 420.0 cm−1. bAverage of the independent structures.
1122

Beilstein J. Org. Chem. 2010, 6, 1120–1126.
Scheme 3: Synthesis of [RuCl2(NHC)(PCy3)(Ind)] complexes.
however flash column chromatography on silica gel afforded showed little decomposition even after 24 h in dichloro-
highly pure compounds in moderate yields (52–79%). The use methane-d2 at 40 °C. Traces of degradation could be observed
of this purification technique also attests to the robustness of the after 1 h in toluene at 80 °C with complete decomposition after
novel complexes.
24 h.
Complexes 6a, 6c and 6d are stable in the solid state under Complexes 6a–d were then tested in benchmark metathesis
aerobic conditions and exhibit remarkable stability in solution transformations with substrates featuring different steric prop-
under inert atmosphere. 1H NMR analysis of their solutions erties (Table 2). The catalysts were found to perform very
Table 2: Catalytic evaluation of 6a–d in benchmark metathesis transformations.a
Loading
(mol %)
Convb (%)
(yield (%))
Substrate
Product
Catalyst
T (°C)
Time (h)
6a
6b
6c
6d
22
49
9
1
1
rtc
24
2
3
6d
80
<99 (95)
6a
6b
6c
6d
33
39
65
33
rtc
24
2
6d
80
<99 (97)
6d
1
1
80
80
2
2
<99 (98)
6d
6d
<99 (85)
<99 (96)
1
1
80
80
2
5
b: 69 (55)
E/Z > 20:1
c: 9
6d
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Beilstein J. Org. Chem. 2010, 6, 1120–1126.
Table 2: Catalytic evaluation of 6a–d in benchmark metathesis transformations.a (continued)
6a
6b
6c
6d
62
37
69
5
5
80
80
5
2
78 (72)
6a
6b
6c
6d
31
36
18
43
6a
6b
6c
6d
58
86
98
2
2
80
80
3
3
98 (95)
6a
6b
6c
6d
90
97
99
99 (99)
aReaction conditions: substrate (0.5 mmol), toluene (0.1 M), N2, 80 °C. bConversions determined by 1H NMR. cDCM (0.1 M).
modestly in the synthesis of poorly hindered substrates 7b and substituents on the NHC lead to the following ligand electronic
8b at room temperature, but their performance improves signifi- donor scale: IMesMe > IMes > IMesBr > IMesCl. The size of
cantly upon thermal activation. Thus, 6d achieves full conver- the substituent also affects the steric hindrance of the ligands,
sion within 2 h at 80 °C. Similar results were achieved with and the percent buried volume of the NHCs decreases in the
substrates 9a–12a. Interestingly transformations at room following order: IMesCl ≈ IMesBr > IMes ≈ IMesMe. A
temperature exhibit no correlation between the electronic prop- modest trend between the electronic properties of the carbene
erties of the carbene and the catalytic outcome. However, more and the catalytic outcome was found in the synthesis of tetra-
challenging substrates that effect the formation of tetrasubsti- substituted olefin. This was attributed to improved stability of
tuted double bonds do present a trend. Even if catalysts the catalyst derived from lower electron-donating properties of
performed similarly, the highest conversions were constantly the NHC.
reached with the catalyst bearing the least electron-donating
carbene, namely IMesCl (6d). These results can be rationalized
Supporting Information
in terms of the mechanism of the reaction. Although a more
electron-donating NHC should better stabilize the 14-electron
Detailed experimental procedures for the synthesis of
active species and allow better catalytic activity, the faster initi-
complexes 5a–d, 6a, 6c and 6d, and procedures for the
ation is also related to faster catalyst decomposition; at 80 °C,
catalysis are available in the supporting information. The
this deactivation contributes considerably to the catalytic
CIF files of crystal structures 5a–d have been deposited
outcome. In conclusion, we suggest that 6d represents the most
with the CCDC, No. 793640–793643, respectively. Copies
advantageous catalyst owing to its improved stability, which is
of the data can be obtained free of charge on applications to
attributed to reduced initiation from poorer electron-donating
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, fax:
ability of the NHC ligand.
+44 1223 336 033; http://www.ccdc.cam.ac.uk; e-mail:
deposit@ccdc.cam.ac.uk.
Conclusion
Supporting Information File 1
Supporting Information.
The effects of modulating the nature of substituents on the
backbone (C4 and C5) positions of the IMes ligand have
permitted a quantification of the electronic and steric parame-
ters associated with these synthetic variations. Using a rhodium
carbonyl system, the electronic variations brought about by
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-128-S1.pdf]
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Beilstein J. Org. Chem. 2010, 6, 1120–1126.
24.Bieniek, M.; Michrowska, A.; Usanov, D. L.; Grela, K. Chem.–Eur. J.
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Acknowledgements
We are grateful to the EC for funding through the seventh
framework program (CP-FP 211468-2-EUMET). SPN is a
Royal Society-Wolfson Research Merit Award holder.
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