A bidentate NHC-alkenyl ruthenium(II) complex via vinyl C-H bond activation

Article abstract of DOI:10.1021/om060087i

A bidentate NHC - alkenyl ruthenium(II) complex has been obtained on deprotonation of imidazolium salts with a N-CH₂CH₂X branch (X = OMe, Cl) followed by reaction with [RuCl₂(p-cymene)] ₂. Low-temperature NMR studies and intermediate trapping with BH ₃ indicate the formation of a CH₂= CH - NHC carbene followed by the intramolecular activation of a vinyl sp² C - H bond on coordination to a ruthenium(II) center.

Full text of DOI:10.1021/om060087i

2126
Organometallics 2006, 25, 2126-2128
Communications
A Bidentate NHC-Alkenyl Ruthenium(II) Complex via Vinyl C-H
Bond Activation
Renan Cariou,† Ce´dric Fischmeister,† Lo¨ıc Toupet,‡ and Pierre H. Dixneuf*,†
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-UniVersite´ de Rennes, and Groupe Matie`re
Condense´e, UMR 6626 CNRS- UniVersite´ de Rennes, Campus de Beaulieu, AVenue du ge´ne´ral Leclerc,
35042 Rennes, France
ReceiVed January 27, 2006
Summary: A bidentate NHC-alkenyl ruthenium(II) complex has
been obtained on deprotonation of imidazolium salts with a
N-CH2CH2X branch (X ) OMe, Cl) followed by reaction with
[RuCl2(p-cymene)]2. Low-temperature NMR studies and inter-
mediate trapping with BH3 indicate the formation of a CH2d
CH-NHC carbene followed by the intramolecular actiVation
of a Vinyl sp2 C-H bond on coordination to a ruthenium(II)
center.
precursors [RuCl(PCy3)(indenylidene)(arene)]X6 by replacing
the phosphine PCy3 with a more electron releasing NHC ligand
with a lateral chelating arm. Our attempt to introduce a NHC
ligand containing a N-CH2CH2OMe chain to [RuCl2(p-
cymene)]2 (1) led us to discover an unexpected chelating ligand
in a ruthenium(II) complex.
We now wish to report that the deprotonation of the
imidazolium salts 2 (X ) OMe (a), Cl (b)) and addition of the
expected resulting NHC ligand to [RuCl2(p-cymene)]2 (1) does
not lead to the expected RuCl2(p-cymene)(NHC) complex but
to the new chelating mixed carbene alkenyl complex 3 (eq 1).
The evidence for intramolecular vinyl sp2 CH bond activation
is presented.
Nitrogen-containing heterocyclic carbene (NHC) metal com-
plexes1 constitute crucial keys for the development of new
catalytic reactions or the creation of more efficient catalysts for
the formation of carbon-carbon bonds,2 alkene metathesis,3 or
enantioselective catalysis.4 Consequently, a strong motivation
is leading to the design of functional NHC carbenes, especially
chelating mixed carbenes and bis(carbenes), for the tuning of
catalyst activity.5 Of special interest are functional carbenes
containing a weakly coordinating lateral group allowing its
reversible coordination to the metal center, to temporarily
stabilize a catalyst precursor and at the same time to allow the
substrate coordination and activation.
For the design of new alkene metathesis ruthenium catalysts,
an objective was to modify the olefin metathesis catalyst
* To whom correspondence should be addressed. Tel: + 33 2 23 23 62
80. Fax: + 33 2 23 23 69 39. E-mail: pierre.dixneuf@univ-rennes1.fr.
† Institut des Sciences Chimiques de Rennes.
‡ Groupe Matie`re Condense´e.
We have shown recently that the allenylidene-rutheni-
um complex [RudCdCdCPh2(Cl)(PCy3)(p-cymene)]OTf (5),
readily obtained from RuCl2(PCy3)(p-cymene) (4), is active in
alkene metathesis7 and proceeds via a rearrangement leading
to the corresponding indenylidene-ruthenium complex 6 either
by heating8 or more readily and quantitatively on addition of
acid.6
(1) For reviews on N-heterocyclic carbenes see: (a) Crudden, C. M.;
Allen, D. P. Coord. Chem. ReV. 2004, 248, 2247. (b) Herrmann, W. A.
Angew. Chem., Int. Ed. 2002, 41, 1290. (c) Jafarpour, L.; Nolan, S. P. AdV.
Organomet. Chem. 2000, 46, 181. (d) Bourissou, D.; Guerret, O.; Gabbai,
F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39. (e) Arduengo, A. J. Acc.
Chem. Res. 1999, 32, 913.
(2) (a) Herrmann, W. A.; O¨ fele, K.; Preysing, D. V.; Schneider, S. K. J.
Organomet. Chem. 2003, 687, 229 and references therein. (b) McGuinness,
D. S.; Green, M. J.; Cavell, K. J.; Skelton, B. W.; White, A. H. J.
Organomet. Chem. 1998, 565, 165. (c) Navarro, O.; Kelly, R. A.; Nolan,
S. P. J. Am. Chem. Soc. 2003, 125, 16194 and references therein. (d) Hadei,
N.; Kantchev, E.-A. B.; O’Brien, C. J.; Organ, M. G. Org. Lett. 2005, 7,
3805.
(3) (a) Connon, S. J.; Blechert, S. In Topics in Organometallic Chemistry;
Dixneuf, P. H., Bruneau, C., Eds.; Springer-Verlag: Berlin, 2004; Vol. 11
(Ruthenium Catalysts and Fine Chemistry), p 93. (b) Astruc, D. New J.
Chem. 2005, 29, 42. (c) Grubbs, R. H. Tetrahedron 2004, 60, 7117. (d)
Connon, S. J.; Blechert, S. Angew. Chem., Int. Ed. 2003, 42, 1900. (e)
Se´meril, D.; Bruneau, C.; Dixneuf, P. H. AdV. Synth. Catal. 2002, 344, 1.
(f) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (g) Fu¨rstner,
A. Angew. Chem., Int. Ed. 2000, 39, 3012. (h) Maier, M. E. Angew. Chem.,
Int. Ed. 2000, 39, 2073.
(5) (a) Cetinkaya, B.; Demir, S.; Ozdemir, I.; Toupet, L.; Se´meril, D.;
Bruneau, C.; Dixneuf, P. H. Chem. Eur. J. 2003, 9, 2323. (b) Peris, E.;
Crabtree, R. H. Coord. Chem. ReV. 2004, 248, 2239. (c) Poyatos, M.; Mas-
Marza`, E.; Sanau´, M.; Peris, E. Inorg. Chem. 2004, 43, 1793. (d) Arnold,
P. L.; Scarisbrick, A. C.; Blake, A. J.; Wilson, C. Chem. Commun. 2001,
2340. (e) Hermann, W. A.; Schwarz, J.; Gardiner, M. G. Organometallics
1999, 18, 4082. (f) McManus, H. A.; Guery, P. J. Chem. ReV. 2004, 104,
4151. (g) Schneider, N.; Ce´sar. V.; Bellemin-Laponnaz, S.; Gade, L. H.
Organometallics 2005, 24, 4886.
(6) (a) Castarlenas, R.; Dixneuf, P. Angew. Chem., Int. Ed. 2003, 42,
4524. (b) Castarlenas, R.; Vovard, C.; Fischmeister, C.; Dixneuf, P. H. J.
Am. Chem. Soc., in press.
(7) (a) Fu¨rstner, A.; Liebl, M.; Lehmann, C. W.; Picquet, M.; Kunz, R.;
Bruneau, C.; Touchard, D.; Dixneuf, P. H. Chem. Eur. J. 2000, 6, 1847.
(b) Castarlenas, R.; Fischmeister, C.; Bruneau, C.; Dixneuf, P. H. J. Mol.
Catal. A: Chem. 2004, 213, 31.
(4) (a) Ce´sar, V.; Bellemin-Laponnaz, S.; Gade, L. Chem. Soc. ReV. 2004,
33, 619. (b) Enders, D.; Balensiefer, T. Acc. Chem. Res. 2004, 37, 534. (c)
Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003, 14, 951.
10.1021/om060087i CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/25/2006

Communications
Organometallics, Vol. 25, No. 9, 2006 2127
Whereas the indenylidene complex 6 is very active at room
temperature,6 its stability in solution at higher temperature is
moderate, as the catalytic species is highly coordinatively
unsaturated by loss of the p-cymene ligand. To improve the
stability of the system, an attempt was made to replace the PCy3
ligand of the precursor 4 by the electron-donating, bulky, and
chelating NHC ligand with a N-CH2CH2OMe branch arising
from the deprotonation of the imidazolium salt 2a (eq 2). Thus,
Figure 1. Molecular structure of complex 3 (50% probability
ellipsoids). H atoms and the CH2Cl2 molecule of crystallization
are omitted for clarity. Selected bond lengths (Å): Ru1-C15 )
2.028(4), Ru1-C11 ) 2.071(5), C11-C12 ) 1.350(7), N1-C12
) 1.424(6), N2-C16 ) 1.456(6), C13-C14 ) 1.350(7).
Scheme 1
the imidazolium salt 2a was first prepared by reaction of
mesitylimidazole in ClCH2CH2OMe and obtained in 75% yield.
The imidazolium salt 2a was dissolved in THF and deprotonated
at -20 °C by n-butyllithium. The reaction mixture was then
added to [RuCl2(p-cymene)]2 (1) in THF at room temperature.
The expected product [RuCl2(NHC)(p-cymene)] (7) was not
formed. The 1H NMR analysis of the resulting complex showed
the absence of the CH2CH2OMe branch and the presence of
the p-cymene ligand, with four different aromatic protons,
suggesting its coordination to a chiral ruthenium center. An
alkenyl group was observed with deshielded protons (δ 8.65
(d), 7.06 (d), 3JHH ) 5.6 Hz). The 13C NMR spectrum showed
a signal at 188.92 ppm characteristic of the coordinated NHC
carbon atom. Another low-field signal at 154.71 ppm was
attributed to the vinylic carbon atom bonded to the ruthenium.
These NMR data were in favor of the complex 3 (eq 2). The
same complex 3 was also obtained by starting from the
chlorinated salt 2b instead of 2a in a slightly better yield (37%
vs 30% from 2a).
for a ruthenium complex containing a chelating (NHC) carbene
arene ligand, [RuCl2{(η1-CN[CH2(η6-C6H2Me3-2,4,6)])CHCHN-
(CH2(η6-C6H2Me3-2,4,6)}], with a Ru-C bond length of 2.04
Å.5a
The formation of complex 3 has been investigated by means
of NMR experiments. Two reasonable pathways have initially
been considered. One route involves the double deprotonation
of the imidazolium salt leading to the vinylcarbene [A] followed
by elimination of HCl on coordination to the ruthenium(II)
center. The second possibility is the formation of the carbene
[B], leading to complex 7, followed by the elimination of XH
and HCl (Scheme 1).
The structure of 3 was confirmed by X-ray diffraction analysis
(Figure 1).
The molecular structure shows the presence of the p-cymene
ligand and only one chloride ligand. The most surprising feature
is the bidentate mixed carbene-vinyl ligand, forming a met-
allabicyclic unit. The C11-C12 and C13-C14 bond lengths
of 1.350 and 1.349 Å, respectively, are consistent with CdC
double bonds. It is noteworthy that the NHC carbene ligand
and the five-membered ruthenacycle Ru1-C11-C12-N1-C15
are coplanar, as the sum of the angles (C13N1C15), (C15N1C12),
and (C12N1C13) reaches 360.0° and the torsion angle
(C12N1C15N2) equals 179.45°. The Ru-C15 bond length (2.03
Å) is slightly shorter than typical ruthenium-carbon bonds of
various (arene)ruthenium-monodentate NHC complexes (typi-
cally 2.06-2.09 Å).5a,9 This feature was previously observed
1
The deprotonation of 2b by nBuli has been followed by H
NMR at low temperature by the observation of the imidazole
NCHdCHN protons. When the deprotonation was performed
at -20 °C, the total disappearance of the more acidic NCHN
proton of the imidazolium salt was observed. The analysis of
the region between 4.7 and 6.5 ppm showed two important
features. First, we note the presence of four NCHdCHN vinylic
protons at 5.61 and 6.21 ppm and at 5.71 and 6.13 ppm, showing
the presence of two carbene species.10-12 The second informa-
tion comes from the signals at about 5 and 6.5 ppm characteristic
(8) Bassetti, F.; Centola, F.; Se´meril, D.; Bruneau, C.; Dixneuf, P. H.
Organometallics 2003, 22, 4459.
(9) Ozdemir, I.; Yigit, B.; Cetinkaya, B.; Ulku, D.; Tahir, M. N.; Arici,
C. J. Organomet. Chem. 2001, 633, 27.
(10) Arduengo, A. J.; Krafczyk, R.; Schmutzler, R. Tetrahedron 1999,
55, 14523.

2128 Organometallics, Vol. 25, No. 9, 2006
Communications
of a CH2dCH- vinylic system. These observations support the
formation of the intermediate vinyl carbene [A]. The structure
of the second species is likely to be of the [B] type, although it
was not unambiguously identified by NMR.
The formation of 3 constitutes a new example of the capability
of ruthenium complexes to activate usually inert C-H bonds.15
Whereas the C-H bond activation by ruthenium(0) is now well
established,15 the reaction presented here reveals an unprec-
edented example of C-H bond activation of an NHC vinyl
group by a ruthenium(II) species. Previous C-H bond activa-
tions by ruthenium(II) complexes were discovered in the ortho
metalation of phosphites16 and arylamines.17 The ethene C-H
bond activation with rhodium(I) and iridium(I) complexes is a
well-known process,18 whereas vinyl CH bond activation of
vinylpyridine has been performed with osmium polyhydrides19
and the mixed pyridine-N-heterocyclic carbene on reaction with
A second experiment was carried out to confirm these
preliminary observations. It is known that in situ generated NHC
carbenes can be trapped by BH3 or BF3.13 Thus, 2a was
deprotonated at - 20 °C and then the formed species were
trapped by BH3. When 1 equiv of nBuli was used at -20 °C,
the 1H NMR spectrum revealed the absence of the NCHN proton
and the presence of two boron adducts, one of these containing
a vinyl group. This observation is consistent with the presence
of derivative 8 (eq 3). The second species was not clearly
20
IrH5(PPh3)2 is known to “abnormally” bind to iridium via
imidazolium (NCHdCHN)C-H bond activation. These metal
polyhydride complexes generate a coordinatively unsaturated
metal moiety on loss of hydrogen before C-H bond activa-
tion.19,20 The hypothesis is made here that, for both ortho
metalation and the present reaction, the coordination of the
substrate (arylamine, aryl phosphite, or vinyl NHC) takes place
first, followed by C-H bond activation by the 18-electron
ruthenium(II) center.
The above results show the formation of a new chelating
mixed carbene-vinyl ruthenium complex. The X-ray structure
shows the planarity of the NHC ligand with the five-membered
ruthenacycle. Low-temperature NMR observations are consistent
with the initial formation of an N-vinyl N-mesityl imida-
zolylidene carbene, and thus, the formation of the complex
implies an intramolecular activation of an usually inert vinyl
sp2 C-H bond.
identified, but NMR data support derivative 9 (eq 3). However,
when 2 equiv of nBuli was added to 2a, as in the formation of
3, only species 8 was observed. Its 1H NMR spectrum shows a
vinyl group linked to a heteroatom, as three doublets of doublets
were observed (1H NMR (200 MHz): δ 4.41 (Hb), 4.66 (Hc),
7.82 (Ha) 2JHbHc ) 1.6 Hz; 3JHaHb ) 9.0 Hz, 3JHaHc ) 16.0 Hz).
Acknowledgment. We are grateful to the CNRS, the
University of Rennes, and the European Network IDECAT for
financial support.
These observations support the in situ formation of both
intermediates [A] and [B] by deprotonation of salt 2. However
in the formation of 3, 2 equiv of nBuLi is used to deprotonate
the salt 2 before addition to complex 1, and under these
conditions only the vinyl-containing species is observed by
NMR. It is thus likely that species [A] is predominantly formed
from 2 and that its coordination to the ruthenium atom of 1
favors the intramolecular activation of a vinyl sp2 C-H bond
with formal elimination of HCl leading to complex 3. The
released HCl is expected to decompose the carbene [A]; thus,
the yield cannot exceed 50%. However, the addition of NEt3 to
the reaction mixture did not increase the yield of 3. The addition
of HBF4 to 3 led to its decomposition rather than to the
formation of a CHdCH2 group or the formation of an alkylidene
ligand, as observed by Werner14 on protonation of a vinyl
derivative.
Supporting Information Available: Text and figures giving
experimental details and crystallographic data for complex 3. This
material is available free of charge via the Internet at http://
pubs.acs.org. CCDC reference number 276178 also contains the
supplementary crystallographic data for this paper. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html.
OM060087I
(15) (a) Kakiuchi, F.; Murai, S. Activation of unreactive bonds and
organic synthesis. In Topics in Organometallic Chemistry; Springer-
Verlag: Berlin, 1999; Vol. 3, p 47. (b) Kakiuchi, F.; Murai, S. Acc. Chem.
Res. 2002, 35, 826. (c) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002,
102, 1731.
(16) Lewis, L. N.; Smith, J. F. J. Am. Chem. Soc. 1986, 108, 2728.
(17) (a) Ritleng, V.; Sutter, J. P.; Pfeffer, M.; Sirlin, C. Chem. Commun.
2000, 129. (b) Ritleng, V.; Pfeffer, M.; Sirlin, C. Organometallics 2003,
22, 347.
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S.; Trujillo, M.; Carmona, E. Organometallics 1999, 18, 4304. (b) Paneque,
M.; Poveda, M. L.; Salazar, V.; Taboada, S.; Carmona, E. Organometallics
1999, 18, 139.
(19) Recently Esteruelas et al. have reported the activation of an olefinic
sp2 CH bond of vinylpyridine promoted by the osmium polyhydride
complexes OsH3(SnPh2Cl)(PiPr3)219a and OsH6(PiPr3)2:19b (a) Eguillor, B.;
Esteruelas, M. E.; Olivan, M.; On˜ate, E. Organometallics 2005, 24, 1428.
(b) Barrio, P.; Esteruelas, M. E.; On˜ate, E. Organometallics 2004, 23, 3627.
(20) (a) Gru¨ndemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.;
Crabtree, R. H. Chem. Commun. 2001, 2274. (b) Chianese, A. R.;
Kovacevic, A.; Zeglis, B. M.; Faller, J. W.; Crabtree, R. H. Organometallics
2004, 23, 2461.
(11) Although the resolution of the low-temperature NMR experiments
was not very good, the ratio of the two carbene species could be estimated
as 1/1.
(12) The involvement of an electron-rich olefin as the carbene source
(Wanzlick equilibrium) was discarded, since experiments designed to prepare
and use this type of intermediate failed to produce any complex. The
experimental conditions used were those reported in ref 5a.
(13) Ramnial, T.; Jong, H.; McKenzie, I. D.; Jennings, M.; Clyburne, J.
A. C. Chem. Commun. 2003, 1722.
(14) Jung, S.; Brandt, C. D.; Wolf, J.; Werner, H. Dalton Trans. 2004,
375.