(p-cymene)RuLCl2 (L = 1,3-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and related complexes as ring closing metathesis catalysts

Article abstract of DOI:10.1021/om990357f

Complexes of (η⁶-arene)ruthenium bearing the carbene ligand 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) Ru(IMes)(Cl)₂(η⁶-arene), Ru-(IPr)(Cl)₂

Full text of DOI:10.1021/om990357f

3760
Organometallics 1999, 18, 3760-3763
Notes
(p-cym en e)Ru LCl2 (L )
1,3-Bis(2,4,6-tr im eth ylp h en yl)im id a zol-2-ylid en e a n d
1,3-Bis(2,6-d iisop r op ylp h en yl)im id a zol-2-ylid en e) a n d
Rela ted Com p lexes a s Rin g Closin g Meta th esis Ca ta lysts
Laleh J afarpour, J inkun Huang, Edwin D. Stevens, and Steven P. Nolan*,†
Department of Chemistry, University of New Orleans, New Orleans, Louisiana, 70148
Received May 12, 1999
Summary: Complexes of (η6-arene)ruthenium bearing
the carbene ligand 1,3-bis(2,4,6-trimethylphenyl)imida-
zol-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene (IPr) Ru(IMes)(Cl)2(η6-arene), Ru-
(IPr)(Cl)2(η6-arene), and [RudCdCdCPh2(IMes)(Cl)(η6-
arene)]PF6 were prepared and found to be efficient
catalyst precursors for ring closing olefin metathesis.
metathesis (RCM) reactions.4a,b,5 In every example
mentioned above the use of sterically demanding and
electron-donating phosphines is required to stabilize
reactive intermediates. We have previously shown that
the nucleophilic carbene ligand, 1,3-bis(2,4,6-trimeth-
ylphenyl)imidazol-2-ylidene (IMes) (4), can act as a
tertiary phosphine analogue and is capable of support-
ing metathesis activity.6 The increased catalytic activity
in ring closing metathesis reactions was illustrated in
the case of the IMes analogue of the Grubbs’ catalyst,
RuCl2(dC(H)Ph)(PCy3)(IMes) (5). The role of the IMes
ligand is twofold: IMes is a better donor than PCy3 and
it is more sterically demanding, which helps prevent (or
slow) bimolecular carbene decomposition. Recently, we
have been able to synthesize 1,3-bis(2,6-diisopropyl-
phenyl)imidazol-2-ylidene (IPr) (6). To study the effect
of replacing a phosphine donor with the IMes or IPr
ligand, we now describe the synthesis and characteriza-
tion of (p-cymene)RuCl2(IMes) (7), (p-cymene)RuCl2(IPr)
(8), and [(p-cymene)RuCl(IMes)(dCdCdCPh2)]PF6 (9).
Furthermore, the catalytic activities of 7, 8, and 9 in
RCM reactions has been investigated and are compared
to those of the phosphine-containing complexes (p-
cymene)RuCl2(PCy3) (2) and (p-cymene)RuCl2(Pi-Pr3)
(10).
In tr od u ction
Recent developments in the area of olefin metathesis
have had a tremendous impact on this field.1 The use
of the ruthenium carbene complex, RuCl2(dC(H)Ph)-
(PCy3)2 (1), developed by Grubbs et al. in organic and
polymer chemistry is now widespread.1a,b,2 Although 1
is a highly efficient and tolerant catalyst precursor for
numerous metathetical reactions,3 recently, alternative
metathesis catalyst precursors that are more accessible
and have comparable activity have been introduced.4
For example, it has been shown that (p-cymene)RuCl2-
(PCy3) (2), and its cationic, 18-electron allenylidene
derivative, [(p-cymene)RuCl(PCy3)(dCdCdCPh2)]PF6
(3), are active catalyst precursors for various ring closing
† E-mail: snolan@uno.edu.
(1) (a) Nguyen, S. T.; J ohnson, L. K.; Grubbs, R. H.; Ziller, J . W. J .
Am. Chem. Soc. 1992, 114, 3974-3975. (b) Nguyen, S. T.; Grubbs, R.
H.; Ziller, J . W. J . Am. Chem. Soc. 1993, 115, 9858-9859. (c) Schwab,
P.; France, M. B.; Grubbs, R. H.; Ziller, J . W. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2039-2041. (d) Belderrain, T. R.; Wilhelm, T. A.;
Grubbs, R. H. Organometallics 1997, 16, 4001-4003. (e) Wilhelm, T.
A.; Belderrain, T. R.; Brown, S. N.; Grubbs, R. H. Organometallics
1997, 16, 3867-3869. (f) Wolfe, J .; Stu¨er, W.; Gru¨nwal, C. H.; Werner,
H.; Schwab, P.; Schulz, M. Angew. Chem., Int. Ed. Engl. 1998, 37,
1124-1126. (g) Katayama, H.; Ozawa, F. Organometallics 1998, 17,
5190-5196. (h) Ivin, K. J .; Mol, J . C. Olefin Metathesis and Metatthesis
Polymerization; Academic Press: New York, 1997. (i) Grubbs, R. H.;
Miller, S. J .; Fu, G. C. Acc. Chem. Res. 1995, 28, 446-452. (j) Schuster,
M.; Blechert, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 2036-2065.
(k) Fu¨rstner, A. Top. Catal. 1997, 4, 285-299.
Resu lts a n d Discu ssion
Reaction of the commercially available [(p-cymene)-
RuCl2]2 (11) with IMes or IPr in THF at room temper-
ature resulted in the formation of orange microcrystal-
line solid (p-cymene)RuCl2(IMes) (7) and brown micro-
crystalline solid (p-cymene)RuCl2(IPr) (8) in 90% yields,
respectively. Complexes were characterized by 1H NMR
spectroscopy and elemental analysis (see Experimental
Section). When 7 is reacted with 1,1-diphenylprop-2-
ynyl alcohol in the presence of NaPF6, [(p-cymene)RuCl-
(2) Schwab, P.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem. Soc. 1996,
118, 100-110.
(3) (a) Tindall, D.; Pawlow, J . H.; Wagner, K. B. Top. Organomet.
Chem. 1998, 1, 180. (b) Kiessling, L. L.; Strong, L. E. Top. Organomet.
Chem. 1998, 1, 196, and references therein.
(4) (a) Fu¨rstner, A.; Picquet, M.; Bruneau, C.; Dixneuf, P. H. Chem.
Commun. 1998, 1315-1316. (b) Fu¨rstner, A.; Ackermann, L. Chem.
Commun. 1999, 95-96. (c) Harlow, K. J .; Hill, A. F.; Wilton-Ely, J . D.
E. T. J . Chem. Soc., Dalton, Trans. 1999, 285-291. (d) Fu¨rstner, A.;
Hill, A. F.; Lieble, M.; Wilton-Ely, J . D. E. T. Chem. Commun. 1999,
601-602.
(5) Picquet, M.; Bruneau, C.; Dixneuf, P. H. Chem. Commun. 1998,
2249-2250.
(6) (a) Huang, J .; Stevens, E. D.; Nolan, S. P.; Petersen, J . L. J . Am.
Chem. Soc. 1999, 121, 2674-2678. (b) Weskamp, T.; Schattenmann,
W. C.; Spiegler, M.; Hermann, W. A. Angew. Chem., Int. Ed. Engl.
1998, 37, 2490-2493. Weskamp, T.; Schattenmann, W. C.; Spiegler,
M.; Hermann, W. A. Angew. Chem., Int. Ed. Engl. 1999, 38, 262. (c)
Scholl, M.; Trnka, T. M.; Morgan, J . P.; Grubbs, R. H. Tetrahedron
Lett. 1999, 40, 2247-2250.
10.1021/om990357f CCC: $18.00 © 1999 American Chemical Society
Publication on Web 08/06/1999

Notes
Organometallics, Vol. 18, No. 18, 1999 3761
Ta ble 1. Cr ysta llogr a p h ic Da ta for Com p lex 9
formula
fw
color
C53.56H61.25ClF6N2O2PRu
1046.54
brown
space group
P2(1)/c
a, Å
b, Å
c, Å
18.8203(7)
17.4709(6)
15.9621(6)
90
104.5250(10)
90
5080.7(3), 4
1.368
0.0479
R, deg
â, deg
γ, deg
volume (Å3), Z
density (calcd) (g/cm3)
R
Figu r e 1. ORTEP of (p-cymene)RuCl(IMes)(dCdCdCPh2)-
PF6 (9) with ellipsoids drawn with 50% probability. Hy-
drogen atoms and PF6 counterion have been omitted for
clarity.
Rw
0.1313
754
57962
8945
no. refined params
no. data collected
no. unique data, I > 3σ
goodness of fit, F2
0.941
Sch em e 1
Ta ble 2. Selected Bon d Dista n ces (Å) a n d Bon d
An gles (d eg) for 9
Ru(1)-C(22)
Ru(1)-C(10)
Ru(1)-C(39)
Ru(1)-C(40)
Ru(1)-C(38)
Ru(1)-C(37)
Ru(1)-Ct*a
Ru(1)-Cl(1)
Ru(1)-C(42)
Ru(1)-C(41)
1.890(4)
2.077(4)
2.224(5)
2.249(4)
2.356(4)
2.383(4)
1.804(4)
2.3903(11)
2.246(4)
2.258(4)
C(22)-Ru(1)-Cl(1)
C(22)-Ru(1)-C(10)
C(10)-Ru(1)-Cl(1)
C(22)-C(23)-C(24)
Ct-Ru-C(34)
84.95(12)
92.62(15)
85.38(11)
171.8(4)
131.8
Ct-Ru-C(59)
122.9
125.4
Ct*-Ru-Cl(1)
a
*Ct is the center of the arene ring.
Ta ble 3. Rin g Closin g Meta th esis of
Dia llylm a lon a te Usin g Ca ta lyst P r ecu r sor s
2, 7, 8, 9, a n d 10
catalyst
precursor
temp
(°C)
time
(h)
conversion
(%)b
entry
solvent
1
2
3
4
5
6
7
8
9
2
10
9
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
CD2Cl2
toluene-d8
toluene-d8
toluene-d8
40
40
40
40
40
40
80
80
80
27
27
27
27
27
27
2
47
48
85
78
40
7
8
8a
7
40
100
100
100
7a
8
2
2
a
The experiment was performed in the absence of light.
Calculated from NMR spectra.
b
(IMes)(dCdCdCPh2)]PF6 (9) is produced as brown
crystals in 91% yield (Scheme 1).
Complex 9 is characterized by H NMR spectroscopy
to those of (p-cymene)RuCl2(PR3) (R ) Cy (2) and i-Pr
(10)). Results are presented in Table 3.
1
and elemental analysis. The X-ray crystal structure of
9 has been determined, and an ORTEP of 9 is shown in
Figure 1. The coordination geometry around the Ru
center can be considered a three-legged piano stool.
p-Cymene is bound to ruthenium in a η6 fashion; the
isopropyl groups on the arene are distorted away from
the metal center presumably due to unfavorable steric
factors. The two mesityl groups on the IMes ligand are
bent toward the ruthenium center with the dihedral
angles of 78.2(2)° and 89.9(2)°, providing steric crowd-
ing, which appears beneficial in RCM reactions.2c,6 The
allenylidene group is not linear but rather bent at the
middle carbon (C22-C23-C24 ) 171.8°). The Ru-C22
(1.890(4) Å) bond distance is considerably shorter than
the Ru-C10 single bond (2.077(4) Å). Selected bond
lengths and bond angles are reported in Table 2.
The catalytic activities of 7, 8, and 9 have been tested
by using the standard ring closing metathesis (RCM)
substrate, diethyldiallylmalonate (eq 1), and compared
When the reactions were carried out in CD2Cl2 and
heated to 40 °C, 9 catalyzed reaction 1 with a conversion
of 85%, whereas 7 showed a conversion of 78%. The use
of 2 and 10 as catalyst precursors led to the yields of
48% and 47%, respectively, after 27 h (Table 3, entries
1-4). The catalytic activity of 8 at this temperature
(40% yield, Table 3, entry 5) was in the range of those
of the phosphine-containing complexes 2 and 10. To
investigate the role of solvent, temperature, and light,
RCM was performed with 7 and 8 as the catalyst
precursors and toluene-d8 as the solvent. Upon heating
the reaction mixtures to 80 °C, a 100% conversion to

3762 Organometallics, Vol. 18, No. 18, 1999
Notes
from Strem Chemicals, Aldrich, or Organometallics, Inc., and
were used as received. Methylene chloride was distilled from
CaH2, and toluene was dried over Na/K and vacuum trans-
ferred to flame-dried glassware. NMR spectra were recorded
using a Oxford 400 MHz spectrometer.
The compound [RuCl2(p-cymene)]2 (11)10 and carbene ligands
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) (4)11
and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) (6)8
were synthesized according to the literature procedure. The
synthesis of other organoruthenium complexes, (p-cymene)-
RuCl2(P(i-Pr)3) (10) and (p-cymene)RuCl2(PCy3) (2), have
previously been reported.12 Experimental synthetic procedures,
leading to isolation of previously unreported complexes, are
described below.
Gen er a l P r oced u r e for Syn th esis of (p-cym en e)Ru Cl2-
(IMes) (7) a n d (p-cym en e)Ru Cl2(IP r ) (8). A 100 mL flask
was charged with 0.970 g (1.58 mmol) of [RuCl2(p-cymene)]2,
3.186 mmol of IMes (4) or IPr (5), and 30 mL of THF. The
clear orange solution was stirred at room temperature for 60
min, after which the solvent was removed under vacuum. The
residue was washed with hexane (2 × 10 mL), filtered, and
dried under vacuum, which afforded the tan product (yield:
90%).
F or (7): 1H NMR (400 MHz, THF-d8) δ 1.08 (d, J ) 7.2 Hz,
6H, CH(CH3)2), 1.51 (s, 3H, CH3), 2.22 (s, 12H, Mes-2,6-CH3),
2.32 (s, 6H, Mes-4-CH3), 2.43 (m, 1H, CH(CH3)2), 4.55 (d, J )
6 Hz, 2H, C6H4), 5.00 (d, J ) 6 Hz, 2H, C6H4), 6.90 (d, 4H,
Mes-3, 5-H), 7.05 (s, 2H, NCHCHN). Calcd for C31H38N2Cl2-
Ru: C, 60.98; H, 6.27; N, 4.59. Found: C, 60.64; H, 6.20; N,
4.31.
F or (8): 1H NMR (400 MHz, C6D6) δ 1.00 (d, J ) 6.4 Hz,
12H, IPr CH(CH3)2), 1.05 (d, J ) 7.0 Hz, 3H, p-cymene CH-
(CH3)2), 1.15 (d, J ) 7.0 Hz, 3H, p-cymene CH(CH3)2), 1.46 (d,
J ) 6.4 Hz, 12H, IPr CH(CH3)2), 1.75 (s, 3H, p-cymene CH3),
2.72 (m, 1H, p-cymene CH(CH3)2), 3.24 (m, 4H, IPr CH(CH3)2),
4.48 (d, J ) 6 Hz, 2H, p-cymene C6H4), 4.85 (d, J ) 6 Hz, 2H,
p-cymene C6H4), 6.47 (s, 2H, NCHCHN), 7.03 (t, J ) 7 Hz,
1H, IPr C6H4), 7.10 (d, J ) 9 Hz, 4H, IPr C6H4), 7.22 (t, J ) 7
Hz, 1H, IPr C6H4). Calcd for C37H50N2Cl2Ru: C, 63.96; H, 7.25;
N, 4.03. Found: C, 64.20; H, 7.30; N, 4.10.
(p-cym en e)Ru Cl(IMes)(dCdCdCP h 2)P F 6 (9). A 50 mL
flask was charged with 118 mg (0.568 mmol) of 1,1-diphenyl-
prop-2-ynyl alcohol, 231.2 mg (0.3786 mmol) of (p-cymene)-
RuCl2(IMes), 200 mg (760 mmol) of NaPF6, and 20 mL of
MeOH. The reaction mixture was stirred at room temperature
overnight. The solvent was then evaporated in vacuo, and the
residue was extracted with CH2Cl2 (20 mL). Filtration and
removal of the volatiles yielded a brown solid, which was
washed with hexanes (2 × 5 mL) and then dried under vacuum
to afford 467 mg of the red-brown product (yield: 91%). The
crude product can be purified by recrystalization from CH2-
Cl2-hexane to obtain the dark brown crystals: 1H NMR (400
MHz, THF-d8) δ 1.08, 1.17 (d, J ) 6.8 Hz, 6H, CH(CH3)2), 1.99
(s, 3 H, CH3), 2.04, 2.15, 2.18, 2.53 (s, 18 H, Mes-2, 4, 6-CH3),
3.22 (m, 1H, CH(CH3)2), 5.17, 5.54, 5.68, 5.85 (d, 4 H, C6H4,
product was observed after only 2 h in both cases (Table
3, entries 7-9). Performing the reactions in the dark
did not change the outcome and yields of the reactions
(Table 3, entries 6 and 8), which would indicate that
the catalytic reactions are not photoinduced. This is in
contrast with the complexes of the type M(p-cymene)-
Cl2(PR3) (M ) Ru, Os; R ) Cy, i-Pr), which have been
reported to become active ROMP catalysts only when
activated by UV irradiation.7 It has also been reported
that RCM in the presence of [(p-cymene)(PCy3)ClRud
CdCdCPh2]PF6 and Ru(p-cymene)Cl2(PCy3) is acceler-
ated by exposure to UV or neon light.4b,5b No such effect
is observed for our system. Examination of data gath-
ered in Table 3 shows that the IMes-containing com-
plexes 7 and 9 are the best catalyst precursors found
in this study, whereas the ruthenium complex incorpo-
rating the IPr ligand, 8, showed similar reactivity to
those of 2 and 10. Solution calorimetric investigations
indicate that the Ru-L bond strength decreases in the
following order: Ru-IMes (15.6 kcal/mol) > Ru-IPr
(11.2 kcal/mol) > Ru-PCy3 (10.5 kcal/mol) > Ru-Pi-
Pr3 (9.4 kcal/mol).6a,8 The IMes ligand proved to be a
stronger binder than the IPr ligand, whose relative
enthalpy is comparable to that of the PCy3 ligand. The
initial step in the ring closing metathesis mechanism
using the (p-cymene)RuLCl2 complexes must involve the
formation of a ruthenium-carbene complex,2c,9 and in
the case of ruthenium-arene complexes the carbene
moiety can presumably be formed by the change in the
hapticity of the arene ring, leading to vacant sites on
Ru. The more electron-donating ligand (IMes) can
facilitate this process more easily than either IPr or the
phosphines, and this reason is proved to be the origin
of the higher catalytic activity of 7 compared to those
of 8, 2, and 10 at 40 °C. When the temperature is raised
to 80 °C, both 7 and 8 show the same activity. It could
be argued that at higher temperatures the activation
barrier for the change in arene hapticity has already
been overcome and under these conditions the electronic
differences between the ligands are not very important.
We have shown that the ruthenium complexes incor-
porating the nucleophilic carbene ligands 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene (IMes) (4) and 1.3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) (6) are
more active catalyst precursors in RCM reactions than
those containing phosphine ligands. These RCM reac-
tions are not light induced and proceed to completion
at higher temperatures (27 h at 40 °C vs 2 h at 80 °C).
The novel complexes display high stability at elevated
temperatures.
J
) 6 Hz), 7.12-7.86 (m, 16 H, all the other Ar-H and
NCHCHN). Calcd. for C31H38N2Cl2Ru: C, 60.69; H, 5.31; N,
3.08. Found: C, 60.64; H, 5.21; N, 3.31.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations involving
organoruthenium complexes were performed under inert
atmospheres of argon or nitrogen using standard high-vacuum
or Schlenk tube techniques or in an M Braun glovebox
containing less than 1 ppm oxygen and water.
Rin g Closin g Meta th esis P r oced u r e. In the drybox
catalyst precursor (5 mol %) was accurately weighed in a
Wiland screw-capped NMR tube and dissolved in CD2Cl2 or
toluene-d8 (0.4 mL). Diethyldiallyl malonate (0.02 g, 0.1 mmol)
was added to the solution, and the sealed NMR tube was
heated to 40 °C in the case of CD2Cl2 and to 80 °C when
Sta r tin g Ma ter ia ls. The phosphine ligands, 1,1-diphenyl-
prop-2-ynyl alcohol and diethyl allyllmalonate, were purchased
(10) Bennett, M. A.; Huang, T. N.; Matheson, T. W.; Smith, A. K.
Inorg. Synth. 1982, 21, 74-79.
(7) Hafner, A.; Mu¨hlebach, A.; van der Schaaf, P. A. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2121-2124.
(11) Arduengo, A. J ., III; Dias, H. V. R.; Calabrese, J . C.; Davidson,
F. J . Am. Chem. Soc. 1992, 114, 9724-9725.
(8) J afarpour, L.; Stevens, E. D.; Nolan, S. P. Manuscript in
preparation.
(12) (a) Demonceau, A.; Noels, A. F.; Saive, E.; Hubert, A. J . J . Mol.
Catal. 1992, 76, 123-132. (b) Serron, S. A.; Nolan, S. A. Organome-
tallics 1995, 14, 4611-4616.
(9) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J . Am. Chem. Soc. 1997,
119, 3887-3897.

Notes
Organometallics, Vol. 18, No. 18, 1999 3763
toluene-d8 was used as the solvent. Product formation and
diene disappearance were monitored by integrating the allylic
methylene peaks. Product formation was confirmed by com-
parison with literature NMR data.9 The reactions were carried
out under an ordinary neon lamp. For the reactions that were
performed in the absence of light, the NMR tubes were
wrapped in aluminum foil.
X-r a y Diffr a ction Mea su r em en ts. A single crystal of 9
was coated with paratone oil and then sealed in a glass
capillary tube. The X-ray data were collected at low temper-
ature using graphite-monochromated Mo KR radiation on a
Siemens P4 automated X-ray diffractometer. The structure
was solved using direct methods (SHELXS-86) and refined by
full matrix least-squares techniques. Initial fractional coordi-
nates for the Ru atom were determined by heavy-atom
methods, and the remaining non-hydrogen atoms were located
by successive difference Fourier calculations, which were
performed with algorithms provided by SHELXTL IRIS op-
erating on a Silicon Graphics IRIS Indigo workstation. Crys-
tallographic data can be found in Table 1, and selected bond
distances and bond angles are presented in Table 2.
Ack n ow led gm en t. S.P.N. acknowledges the Na-
tional Science Foundation for partial support of this
work.
Su p p or tin g In for m a tion Ava ila ble: Details of crystal
structure determinations for 9 (PDF). This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
OM990357F