Tailored Ru-NHC heterogeneous catalysts for alkene metathesis

Article abstract of DOI:10.1002/chem.200901752

The preparation of highly active and stable Ru-NHC (ruthenium-N-heterocyclic carbene ligands) alkene metathesis catalysts through surface organometallic chemistry on hybrid mesostructured materials was reported. Two types of mesoporous hybrid materials we

Full text of DOI:10.1002/chem.200901752

DOI: 10.1002/chem.200901752
Tailored Ru-NHC Heterogeneous Catalysts for Alkene Metathesis
Iyad Karamꢀ,[a] Malika Boualleg,[a] Jean-Michel Camus,[a] Tarun K. Maishal,[a]
Johan Alauzun,[b] Jean-Marie Basset,[a] Christophe Copꢀret,[a] Robert J. P. Corriu,[b]
Erwan Jeanneau,[c] Ahmad Mehdi,[b] Catherine Reyꢀ,[b] Laurent Veyre,[a] and
Chloꢀ Thieuleux*[a]
The introduction of N-heterocyclic carbene ligands
(NHC) has led to major breakthroughs in homogeneous cat-
alysis.[1,2] However, such homogeneous catalysts can still
suffer from deactivation and problems related to catalyst
cost and recovery, as well as metal separation from the or-
ganic substrates. In the case of the very challenging and
promising reaction of alkene metathesis,[3–6] these drawbacks
have probably been delaying the development of economi-
cal industrial processes. One possible solution would be the
development of an efficient heterogeneous catalysts that is
highly active (TON and TOF), stable (minimum recycling
and leaching) and tolerant to functional groups. Despite nu-
merous efforts in this area (involving permanent grafting of
Ru-NHC complexes on various supports[7–10] or other immo-
bilization strategies[11]), heterogeneous catalysts has not ful-
filled the aforementioned requirements. Recently, tailored
made organic–inorganic materials have proved to be an al-
ternative and advantageous route towards highly active and
well-defined heterogeneous catalysts.[12] In particular, fully
characterized well-defined Ir-NHC materials displayed cata-
lytic performances comparable to those of homogeneous ho-
mologues. This has been attributed to the careful control of
the catalyst preparation: synthesis of materials containing
regularly distributed NHC-moieties and subsequent selec-
tive functionalization into Ir-NHC species, leading to the
“single-site” nature of these catalysts.
Here, we describe the preparation of highly active and
stable Ru-NHC alkene metathesis catalysts through surface
organometallic chemistry[13] on hybrid mesostructured mate-
rials[14] (Scheme 1).
[a] Dr. I. Karamꢀ, M. Boualleg, Dr. J.-M. Camus, Dr. T. K. Maishal,
Dr. J.-M. Basset, Dr. C. Copꢀret, L. Veyre, Dr. C. Thieuleux
Universitꢀ de Lyon, Institut de Chimie de Lyon C2P2 UMR 5265
CNRS–Universitꢀ Lyon 1–CPE Lyon, ESCPE Lyon
43, Bd du 11 Novembre 1918 69616 Villeurbanne (France)
Fax : (+33)472431795
E-mail: thieuleux@cpe.fr
[b] Dr. J. Alauzun, Prof. Dr. R. J. P. Corriu, Prof. Dr. A. Mehdi,
Prof. Dr. C. Reyꢀ
Institut Charles Gerhardt
Chimie Molꢀculaire et Organisation du Solide
Universitꢀ Montpellier, cc 1701. Place Eugꢁne Bataillon
34095 Montpellier Cedex 5 (France)
Scheme 1. a) Preparation of M-Ru-Pr and M-Ru-Bn: i) 30 TEOS+
1XCH2TSi(OR)3 +HX/H2O (pH 1.5), Pluronic P123, 458C, 2 days; ii)
mesitylimidazole (10 equiv), toluene, reflux, 2 days followed by hydroly-
sis with HX/H2O (458C, 2 h) and then treatment with excess TMSBr,
[c] Dr. E. Jeanneau
Centre de Diffractomꢀtrie Henri Longchambon
Universitꢀ Lyon 1
43, Bd du 11 Novembre 1918, 69616 Villeurbanne (France)
Et3N, toluene, RT, 24 h; iii) KHMDS (1 equiv) followed by [Cl2Ru
CHPh)(PCy3)2] (5–10 equiv). b) Analogous silylated Ru complexes, RuPr
and RuBn.
ACHTUNGTRENNUNG(=
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901752.
ACHTUNGTRENNUNG
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COMMUNICATION
First, two types of mesoporous hybrid materials were pre-
pared, displaying the same texture, porous network structure
and concentration of organic functionalities; the only differ-
ence being the nature of the spacers: propylmesityl imidazo-
lium iodide (M-ImPr) versus benzylmesityl imidazolium
chloride (M-ImBn) (see the Supporting Information for de-
tailed procedures and characterization data, Figures S1–S8).
These materials were prepared using a recently developed
procedure: i) co-hydrolysis and co-polycondensation in
acidic conditions[15] of p-chlorobenzyltrimethoxysilane[12] or
3-iodopropyltriethoxysilane[16] (1 equiv) and 30 equivalents
of (EtO)4Si in the presence of Pluronic P123 as the struc-
ture-directing agent, ii) subsequent treatment with mesityl-
about 0.01 mol% Ru of M-RuPr (1.08 wt%), conversion of
ethyl oleate reaches 50% (thermodynamic equilibrium) at
408C under neat conditions in 5 h with an initial TOF of
65 minÀ1. The M-RuBn displays comparable performances
with a slightly lower TOF of 30 minÀ1. It is worth noting that
whatever catalysts used (M-RuPr or M-RuBn), the initial
rates (TOF) are independent of loadings (0.3–1.0 wt%).
This is consistent with the fact that all the Ru-sites exhibit
the same activity, indicating a “single site” behavior in such
mesoporous hybrid materials. With a lower M-RuBn catalyst
loading (0.003 mol%) thermodynamic equilibrium was still
reached in about 24 h (ꢀ17000 TON) with an initial TOF of
30 minÀ1. Considering the high catalytic performances of
this heterogeneous catalyst, we have investigated its recycla-
bility and leaching. Using 0.25 mol% of M-RuBn and neat
ethyl oleate, the equilibrium conversion was reached within
6 h at room temperature. The supernatant was filtered off
and analyzed by ICP, the solid was washed with toluene, and
this process was repeated seven times without significant
loss of activity, which shows that the active sites are pre-
served after recycling (see Supporting Information, Fig-
ure S16). Moreover, no trace of Ru (<50 ppm detection
limit) was detected in the liquid fractions, revealing the ab-
sence of Ru leaching from the material. This activity is far
ACHTUNGTRENNUNGimidazole to generate the corresponding imidazolium func-
tionalities in quantitative yields, and iii) passivation of all re-
sidual alkoxy/silanol groups by reaction with HI or HCl and
then Me3SiBr/NEt3.
Second, M-ImPr and M-ImBn were typically converted
into their corresponding Ru-NHC derivatives, M-RuPr and
M-RuBn, by reaction with potassium hexamethyldisilylazide
(KHMDS) (1.0 equiv) and then [Cl2RuACHTNUGTRENNUG(=CHPh)CAHTUNGTRNE(NUGN PCy3)2]
(Ph=phenyl, Cy=cyclohexyl) (5–10 equiv) (see the Sup-
porting Information, Figures S9–12). KHMDS was found to
be the more efficient base to deprotonate the imidazolium
groups. Other bases such as tBuOK, nBuLi, NaH, and solid
bases (e.g. Na2CO3 and Ag2O) were not very compatible
with the silica material, 4-(dimethylamino)pyridine
greater than that of [Cl2RuACHTNUGTRENN(UG =CHPh)ACHTUNGTRNEN(UGN PCy3)2] used for graft-
ing (ꢀTON of 4000 in our experimental conditions), which
suggests that the active species are very likely different.
The stereoselectivity at low conversions was then used as
a tool to characterize the active sites in metathesis.[18] In
alkene metathesis, the nature of the products and their
E/Z ratio depend on the approach of the alkene towards the
alkylidene ligand: syn/anti and head/tail (Scheme 2). This
mechanism leads to both an E/Z-isomerization of the reac-
tant and to the formation of two products with a given E/Z
ratio. The initial selectivities are a characteristic of the
active sites (metal, coordination sphere, stability of the met-
allacyclobutane), and therefore they were examined for var-
ious Ru-based homogeneous catalysts and compared to
those of M-RuBn and M-RuPr. To suppress the contribution
of isomerization through metathesis of reactants and prod-
ucts during the catalytic process, the E/Z ratio of the prod-
ucts versus the E/Z ratio of the reactant, namely EE/EO for
ethyl elaidate/ethyl oleate ratio (EE/EO=0 at 0% conv.)
were plotted; the initial E/Z ratio at low conversion are
summarized in Table 1.
Analysis of the E/Z ratio plot of the products, as a func-
tion of the E/Z ratio of the reactants, reveals that there is at
first a fast increase of the E/Z ratio of the products and then
a linear evolution. The deviation from the expected linearity
is a clear indication of a change in catalyst structure occur-
ring during initiation of the initial metallocarbene. Recent
studies have shown that the olefin can approach either cis or
trans with respect to the NHC ligand,[19–21] which could ex-
plain the observed deviation until a steady state is reached.
We have therefore used to characterize the catalysts
(Scheme 2a, L1 =PCy3 vs. NHC): 1) the E/Z ratio at low
conversions and 2) the extrapolated E/Z ratio of products at
(DMAP) and 1,8-diazabicyclo
were not reactive enough, and phosphazene bases interfered
with [Cl2Ru(=CHPh)(PCy3)2]. The alternative approach,
ACHTUNGERTN[NUNG 5.4.0]undec-7-ene (DBU)
A
ACHTUNGTRENNUNG
À
using soluble silver salts providing Ag NHC containing ma-
terial followed by transmetalation with the Ru complex,
failed (in contrast with the observations for Ir).[12] In fact,
the synthesis of the corresponding molecular silylated ana-
logues RuPr and RuBn using Ag salts gave rise to low
yields, whereas the same complexes were prepared in high
yields using KHMDS (Scheme 1b, see Supporting Informa-
tion for the synthesis and characterization details including
the X-ray structure for RuBn: Figures S13–S15).[17] The Ru
elemental analysis and particularly the Ru/N ratio (expected
0.5 for total grafting of Ru per imidazole ligand; found=
0.12) showed an approximate 20% grafting per imidazolium
functionalities for both M-RuPr and M-RuBn materials. No-
tably, the grafting of Ir complexes, which was quantitative
using soluble Ag salts,[12] also led to a 20% grafting using
KHMDS. The reason for partial grafting is not fully under-
stood yet, but cannot be attributed to the lack of reactivity
or accessibility of imidazolium groups and does not depend
on the organometallic starting complex. Further characteri-
1
zation of the material by H, 13C, and 29Si MAS NMR pro-
vided the expected signals for the NHC ligand and the
tether. However it was not possible to observe the Ru alky-
lidene and Ru-NHC carbene carbons, because of the low
Ru loading and the difficulty to observe the Ru-NHC car-
bene carbon for molecular complexes.
The catalytic activity of these materials (M-RuPr and M-
RuBn) was tested in the metathesis of ethyl oleate. Using
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C. Copꢀret, C. Thieuleux et al.
low EE/EO of the reagent. With both values, there is a clear
difference between the active sites of the first and the
second-generation ruthenium catalysts with initial stereose-
lectivities of about 3 versus 2 for L1 =PCy3 and L2 =NHC,
respectively. These ratios are not significantly modified by
either the substituents of the NHC unit (Pr, Bn, Ms) or its
nature (saturated vs. unsaturated). We have therefore trans-
posed this method to evaluate the nature of the active sites
in the Ru-containing materials, M-RuPr and M-RuBn
(Figure 1). These materials display an initial E/Z ratio of
Scheme 2. a) Initiation; b) metathesis: four (of the eight) possible ap-
proaches of Z- dissymmetric alkenes towards a Ru-alkylidene species
and the corresponding products obtained through metathesis.
&
Figure 1. E/Z of diester products vs. EE/OE for a) M-RuPr ( ) and RuPr
&
&
(&) and b) M-RuBn ( ) and RuBn ( ).
about 2, which shows that they behave like their NHC ho-
mogeneous homologues (RuPr vs. M-RuPr and RuBn vs.
M-Ru-Bn). For more rigid tether (Bn), the values are the
same for the homogeneous and heterogeneous catalysts, al-
though they slightly (but significantly) differ for those
having a more flexible tether (Pr). Such deviation towards a
more Z selective catalyst for the catalyst with the more flex-
ible tether could result from a closer vicinity of the metal
center from the surface of the material, which would favor
the reaction pathway where the substituents of the carbene
and of the cis olefin point away from the surface.[22]
Table 1. Intrinsic stereoselectivity of the active sites of Ru-based homo-
geneous catalysts.
Catalysts[a]
E/Z ratio[b]
9-octadecene
diester
Grubbs I[23]
Hoveyda I[24]
GI-Indenylidene[25,26]
Grubbs II[27]
Hoveyda II[28]
Nolan[29]
2.7 (3.6)
3.2 (3.5)
3.2 (3.5)
1.5 (2.5)
1.6 (2.3)
1.7 (2.6)
1.8 (2.2)
1.8 (2.0)
3.0 (3.4)
3.2 (3.5)
2.7 (3.4)
1.7 (2.7)
2.0 (2.5)
2.0 (2.6)
2.1 (2.2)
2.0 (2.2)
RuPr
RuBn
In conclusion, we have successfully prepared highly active
Ru-based alkene metathesis catalysts, using a novel ap-
proach based on SOMC on tailored hybrid organic–inorgan-
ic materials. In particular, these catalysts display high activi-
ty (TOF) and stability (TON vs. time, recycling and leach-
ing). Furthermore, from the overall catalytic performances
and stereochemical studies, we could demonstrate that the
active “single site” corresponds to a Ru-NHC species. The
versatility of such synthetic methodology and its transfer to
various metals and ligands (including sensitive complexes) is
a very promising approach towards a wide range of tailored
made well-defined heterogeneous catalysts.
[a] See below the corresponding structure of the Ru complexes. [b] E/Z
ratio at very low conversions; the values in parentheses correspond to ex-
trapolated E/Z ratio from the extrapolated value at the steady state.
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Chem. Eur. J. 2009, 15, 11820 – 11823

Ruthenium Heterogeneous Catalysis
COMMUNICATION
[14] F. Hoffmann, M. Cornelius, J. Morell, M. Froba, Angew. Chem.
2006, 118, 3290; Angew. Chem. Int. Ed. 2006, 45, 3216.
[15] R. J. P. Corriu, L. Datas, Y. Guari, A. Mehdi, C. Reye, C. Thieuleux,
Chem. Commun. 2001, 763.
Acknowledgements
This research was sponsored by ANR PNANO 2005 (ANR-05-NANO-
034).
[16] J. Alauzun, A. Mehdi, C. Reye, R. Corriu, New J. Chem. 2007, 31,
911.
[17] Crystal data for RuBn: RuSiPCl4N2O3C54H81, Mr =1108.19, triclinic,
Keywords: alkenes · direct synthesis · hybrid materials ·
metathesis · ruthenium
¯
P1, a=12.293(3), b=14.702(5), c=15.923(5) ꢄ, a=91.486(1), b=
92.745(3), g=96.965(3)8, V=2852(2) ꢄ3, Z=2, 1calcd =1.29 mg.mÀ3
,
,
T=293(2) K, MoKa =0.71073 ꢄ, absorption coefficient 0.553 mmÀ1
FACHTUNGTRENNUNG
R
RACHTUNGTRENNUNG
[1] W. A. Herrmann, T. Weskamp, V. P. W. Bohm, Adv. Organomet.
Chem. 2001, 48, 1.
[2] W. A. Herrmann, Angew. Chem. 2002, 114, 1342; Angew. Chem. Int.
Ed. 2002, 41, 1290.
0.070. CCDC 693458 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
[3] A. Fꢃrstner, Angew. Chem. 2000, 112, 3140; Angew. Chem. Int. Ed.
2000, 39, 3012.
[18] J. L. Bilhou, J. M. Basset, R. Mutin, W. F. Graydon, J. Am. Chem.
Soc. 1977, 99, 4083.
[4] T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18.
[5] L. Jafarpour, S. P. Nolan, J. Organomet. Chem. 2001, 617, 17.
[6] R. R. Schrock, Angew. Chem. 2006, 118, 3832; Angew. Chem. Int.
Ed. 2006, 45, 3748.
[19] A. Correa, L. Cavallo, J. Am. Chem. Soc. 2006, 128, 13352.
[20] P. E. Romero, W. E. Piers, J. Am. Chem. Soc. 2007, 129, 1698.
[21] D. R. Anderson, D. J. OꢅLeary, R. H. Grubbs, Chem. Eur. J. 2008,
14, 7536.
[7] a) M. Mayr, B. Mayr, M. R. Buchmeiser, Angew. Chem. 2001, 113,
3957; Angew. Chem. Int. Ed. 2001, 40, 3839; b) M. R. Buchmeiser,
New J. Chem. 2004, 28, 549; c) T. S. Halbach, S. Mix, D. Fischer, S.
Maechling, J. O. Krause, C. Sievers, S. Blechert, O. Nuyken, M. R.
Buchmeiser, J. Org. Chem. 2005, 70, 4687.
[8] C. Copꢀret, J. M. Basset, Adv. Synth. Catal. 2007, 349, 78.
[9] H. Clavier, K. Grela, A. Kirschning, M. Mauduit, S. P. Nolan,
Angew. Chem. 2007, 119, 6906; Angew. Chem. Int. Ed. 2007, 46,
6786.
[10] M. R. Buchmeiser, Chem. Rev. 2009, 109, 303.
[11] B. Van Berlo, K. Houthoofd, B. F. Sels, P. A. Jacobs, Adv. Synth.
Catal. 2008, 350, 1949.
[12] T. K. Maishal, J. Alauzun, J.-M. Basset, C. Copꢀret, R. J. P. Corriu,
E. Jeanneau, A. Mehdi, C. Reyꢀ, L. Veyre, C. Thieuleux, Angew.
Chem. 2008, 120, 8782; Angew. Chem. Int. Ed. 2008, 47, 8654.
[13] C. Copꢀret, M. Chabanas, R. Petroff Saint-Arroman, J.-M. Basset,
Angew. Chem. 2003, 115, 164; Angew. Chem. Int. Ed. 2003, 42, 156.
[22] J. M. Basset, J. L. Bilhou, R. Mutin, A. Theolier, J. Am. Chem. Soc.
1975, 97, 7376.
[23] S. T. Nguyen, R. H. Grubbs, J. W. Ziller, J. Am. Chem. Soc. 1993,
115, 9858.
[24] J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus, A. H. Hoveyda, J.
Am. Chem. Soc. 1999, 121, 791.
[25] A. Fꢃrstner, M. Picquet, C. Bruneau, P. H. Dixneuf, Chem.
Commun. 1998, 1315.
[26] L. Jafarpour, H. J. Schanz, E. D. Stevens, S. P. Nolan, Organometal-
lics 1999, 18, 5416.
[27] M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999, 1, 953.
[28] S. B. Garber, J. S. Kingsbury, B. L. Gray, A. H. Hoveyda, J. Am.
Chem. Soc. 2000, 122, 8168.
[29] J. K. Huang, E. D. Stevens, S. P. Nolan, J. L. Petersen, J. Am. Chem.
Soc. 1999, 121, 2674.
Received: June 24, 2009
Published online: October 15, 2009
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