Preparation of a series of Ru(p-cymene) complexes with different N-heterocyclic carbene ligands for the catalytic β-alkylation of secondary alcohols and dimerization of phenylacetylene

Article abstract of DOI:10.1021/om800377m

A series of five different (p-cymene)Ru(NHC) complexes (NHC = imidazolin-2-ylidene, imidazolin-4-ylidene, and pyrazolin-3-ylidene) have been obtained and fully characterized. The crystal structure of two of the new complexes has been determined by X-ray diffraction methods. All five complexes have been tested in the catalytic β-alkylation of secondary alcohols with primary alcohols and the dimerization of phenylacetylene, showing an excellent activity in both processes. A clear improvement on die catalytic activity of the complexes is observed when the more basic NHC ligands are used. The pyrazolylidene-Ru complex lies among the best catalysts for the β-alkylation of secondary alcohols reported to date.

Full text of DOI:10.1021/om800377m

4254
Organometallics 2008, 27, 4254–4259
Preparation of a Series of “Ru(p-cymene)” Complexes with Different
N-Heterocyclic Carbene Ligands for the Catalytic ꢀ-Alkylation of
Secondary Alcohols and Dimerization of Phenylacetylene
Amparo Prades,† Mo´nica Viciano,† Mercedes Sanau´,‡ and Eduardo Peris*,†
Departamento de Qu´ımica Inorga´nica y Orga´nica, UniVersitat Jaume I, AVenida Vicente Sos Baynat s/n,
Castello´n, E-12071, Spain, and Departamento de Qu´ımica Inorga´nica, UniVersitat de Vale`ncia, C/Dr.
Moliner s/n, Burjassot-Valencia, E-46100, Spain
ReceiVed April 29, 2008
A series of five different “(p-cymene)Ru(NHC)” complexes (NHC ) imidazolin-2-ylidene, imidazolin-
4-ylidene, and pyrazolin-3-ylidene) have been obtained and fully characterized. The crystal structure of
two of the new complexes has been determined by X-ray diffraction methods. All five complexes have
been tested in the catalytic ꢀ-alkylation of secondary alcohols with primary alcohols and the dimerization
of phenylacetylene, showing an excellent activity in both processes. A clear improvement on the catalytic
activity of the complexes is observed when the more basic NHC ligands are used. The pyrazolylidene-
Ru complex lies among the best catalysts for the ꢀ-alkylation of secondary alcohols reported to date.
have also been described, and the effects of a full set of NHC
ligands with different topologies have been studied.5
Introduction
The use of N-heterocyclic carbene ligands (NHCs) for the
preparation of homogeneous catalysts has now become one of
the most productive fields in organometallic chemistry. Since
1995, when Herrmann reported the first use of a NHC ligand
in the preparation of a homogeneous catalyst,1 there has been
an enourmous development of NHC chemistry, which has been
triggered by the search for enhanced or even new catalytic
processes.2 Probably, one of the main benefits of NHCs is that
their preparation is simple and that the coordination methodolo-
gies that are now available allow the preparation of NHC
complexes of almost any transition metal in a variety of
oxidation states, affording a wide set of potential catalytic
applications.
With the rapid development of Ru-NHC chemistry, it is
strange that some of the simplest NHCs have not been tested
in Ru-catalyzed reactions. For example, since the description
of the abnormal coordination of NHCs (aNHCs) by Crabtree
and co-workers,6 there has been an increasing number of
complexes with aNHCs,7,8 but this type of carbene has not been
used yet in the development of a Grubbs-type catalyst. In fact,
the only Ru(aNHC) complexes known so far are those recently
reported by Whittlesey and co-workers in a Ru3 cluster,9 with
the interesting feature that one of them is a doubly bound
abnormal-NHC that is bridging two Ru atoms.
Although the coordination of pyrazole-based carbenes (pyra-
zolylidenes) has been known since 1997,10 only a few examples
regarding Rh-,10 Cr-,11 and Pd-pyrazolylidenes12 have been
published, probably because the lower C-H acidity of the
pyrazolium ion makes its deprotonation more difficult than in
other azolium ions. The search for cheap, simple, and accessible
methods for the preparation of homogeneous catalysts with
improved efficiencies is a continuous demand in chemistry, and
pyrazolylidenes are known to be stronger σ-donors than
When talking about Ru-NHC chemistry, it is inevitable to
mention the enourmous applications that these compounds have
achieved in the development of highly effective catalysts for
the metathesis of olefins,3,4 probably the most important
achievement of any of the M-NHC complexes known to date.
Other interesting catalytic applications of Ru-NHC complexes
* Corresponding author. E-mail: eperis@qio.uji.es.
† Universitat Jaume I.
(5) Dragutan, V.; Dragutan, I.; Delaude, L.; Demonceau, A. Coord.
Chem. ReV. 2007, 251, 765.
(6) Grundemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree,
R. H. Chem. Commun. 2001, 2274.
‡ Universitat de Vale`ncia.
(1) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J.
Angew. Chem., Int. Ed. Engl. 1995, 34, 2371.
(2) (a) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed. Engl. 1997,
36, 2163. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1291. (c)
Mata, J. A.; Poyatos, M.; Peris, E. Coord. Chem. ReV. 2007, 251, 841. (d)
Pugh, D.; Danopoulos, A. A. Coord. Chem. ReV. 2007, 251, 610. (e) Peris,
E.; Crabtree, R. H. C. R. Chim. 2003, 6, 33. (f) Peris, E.; Crabtree, R. H.
Coord. Chem. ReV. 2004, 248, 2239. (g) Gade, L. H.; Bellemin-Laponnaz,
S. Coord. Chem. ReV. 2007, 251, 718. (h) Cesar, V.; Bellemin-Laponnaz,
S.; Gade, L. H. Chem. Soc. ReV. 2004, 33, 619. (i) Perry, M. C.; Burgess,
K. Tetrahedron: Asymmetry 2003, 14, 951.
(3) (a) Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45, 3760. (b) Trnka,
T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (c) Grubbs, R. H.
Tetrahedron 2004, 60, 7117. (d) Hoveyda, A. H.; Zhugralin, A. R. Nature
2007, 450, 243.
(7) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.; Crabtree,
R. H. Organometallics 2004, 23, 2461.
(8) (a) Appelhans, L. N.; Zuccaccia, D.; Kovacevic, A.; Chianese, A. R.;
Miecznikowski, J. R.; Macchioni, A.; Clot, E.; Eisenstein, O.; Crabtree,
R. H. J. Am. Chem. Soc. 2005, 127, 16299. (b) Arnold, P. L.; Pearson, S.
Coord. Chem. ReV. 2007, 251, 596. (c) Song, G. Y.; Zhang, Y.; Li, X. W.
Organometallics 2008, 27, 1936. (d) Heckenroth, M.; Kluser, E.; Neels,
A.; Albrecht, M. Angew. Chem., Int. Ed. 2007, 46, 6293.
(9) Ellul, C. E.; Mahon, M. F.; Saker, O.; Whittlesey, M. K. Angew.
Chem., Int. Ed. 2007, 46, 6343.
(10) Kocher, C.; Herrmann, W. A. J. Organomet. Chem. 1997, 532,
261.
(11) Tafipolsky, M.; Scherer, W.; Ofele, K.; Artus, G.; Pedersen, B.;
Herrmann, W. A.; McGrady, G. S. J. Am. Chem. Soc. 2002, 124, 5865.
(12) (a) Han, Y.; Huynh, H. V. Chem. Commun. 2007, 1089. (b) Han,
Y.; Huynh, H. V.; Tan, G. K. Organometallics 2007, 26, 6581.
(4) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.; Scholl,
M.; Choi, T. L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc.
2003, 125, 2546.
10.1021/om800377m CCC: $40.75
2008 American Chemical Society
Publication on Web 07/29/2008

“Ru(p-cymene)” Complexes with Different NHC Ligands
Organometallics, Vol. 27, No. 16, 2008 4255
Scheme 1
Scheme 2
Scheme 3
imidazolylidenes, thus making them a good choice for the
preparation of C-H activation catalysts.
We now report the preparation of a series of “(p-cyme-
ne)Ru(NHC)” complexes (NHC ) imidazolin-2-ylidene, imi-
dazolin-4-ylidene, and pyrazolin-3-ylidene; 1-4, Scheme 1),
not only aiming to fulfill the gap in the chemistry of aNHCs
and pyrazolylidenes of ruthenium but also trying to have a wide
set of topologically similar ligands with different electron-donor
properties and study their effect in catalysis. For this purpose
we have studied the catalytic activity of our new compounds
in the ꢀ-alkylation of secondary alcohols with primary alcohols
and in the dimerization of phenylacetylene.
corresponding to the proton at the backbone of the abnormal
coordinated azole, at δ 6.6 (2a) and 6.9 (2b). The 13C NMR
spectra show the characteristic signals due to the abnormal
coordinated carbene carbons at 152.2 (2a) and 154.3 (2b) ppm.
The 1H NMR spectrum of the pyrazolylidene complex 3
shows two representative doublets due to the protons of the azole
ring at δ 7.2 and 6.5 (3JH-H ) 2.2 Hz). The signals due to the
protons of the N-Me groups appear at different chemical shifts
as a consequence of the loss of symmetry of the ligand upon
coordination (4.0 and 3.8 ppm). The 13C NMR spectrum shows
a signal at 180.3 ppm due to the metalated carbon. The 1H NMR
spectrum of 4 is qualitatively similar to that shown by 3,
although the comparison of the integrals of the signals assigned
to the pyrazolylidene and the p-cymene ligands allowed us to
determine the number of azoles bound to the metal. The 13C
NMR spectrum shows a signal at 175.1 ppm due to the Ccarbene
atom.
Results and Discussion
To selectively prepare aNHC complexes, one of the most
convenient methods is to block the imidazolium ligand precur-
sors by substitution at C2 with alkyl or aryl groups.7,13 The
coordination of 1,2,3-trimethylimidazolium iodide to [RuCl2(p-
cymene)]2 was performed by transmetalation of the previously
obtained (not isolated) silver-carbene in CH2Cl2, as shown in
Scheme 2. The reaction product, 2a, was obtained in moderate
yield (40%). Compound 2b (X ) Ph) was obtained by a similar
procedure using 1,3-dimethyl-2-phenylimidazolium iodide (yield
65%). Under the reaction conditions used, we did not see the
formation of cationic biscarbene complexes, not even using an
excess of the imidazolium salt.
The transmetalation from a preformed silver carbene was also
effective for the coordination of the pyrazolylidene ligand to
[RuCl2(p-cymene)]2. The reaction of 1,2-dimethylpyrazolium
iodide with Ag2O in CH2Cl2 afforded the correponding Ag-
NHC complex, which was used in situ for the transmetalation
of the carbene to [RuCl2(p-cymene)]2. Depending on the Ru/
pyrazolium molar ratio used, the monocarbene compound 3
(60% yield) or the cationic bis-carbene complex 4 (35% yield)
was obtained, as shown in Scheme 3.
The molecular structures of 2b and 3 were unequivocally
determined by means of X-ray diffraction studies. Figure 1 and
2 show the ORTEP diagrams of 2b and 3, respectively.
Both structures can be regarded as three-legged piano stools.
Together with the azolylidene ligand, two chloro ligands and
the p-cymene ring complete the coordination sphere about the
Compounds 2a, 2b, 3, and 4 were characterized by spectro-
scopic techniques and elemental analysis. The most significant
1H NMR signal for the aNHC complexes 2a and 2b is the one
Figure 1. Molecular diagram of compound 2b. Hydrogen atoms
have been omitted for clarity. Ellipsoids are at 30% probability.
Selected bond distances (Å) and angles (deg): Ru(1)-C(01)
2.084(7), Ru(1)-Cl(1) 2.4212(16), Ru(1)-Cl(2) 2.4465(14),
Ru(1)-Ccentroid 1.688, C(01)-Ru(1)-Cl(1) 85.14(19), C(01)-Ru(1)-
Cl(2) 85.73(19), Cl(1)-Ru(1)-Cl(2) 89.55(5).
(13) (a) Chianese, A. R.; Zeglis, B. M.; Crabtree, R. H. Chem. Commun.
2004, 2176. (b) Bacciu, D.; Cavell, K. J.; Fallis, I. A.; Ooi, L. L. Angew.
Chem., Int. Ed. 2005, 44, 5282. (c) Alcarazo, M.; Roseblade, S. J.; Cowley,
A. R.; Fernandez, R.; Brown, J. M.; Lassaletta, J. M. J. Am. Chem. Soc.
2005, 127, 3290.

4256 Organometallics, Vol. 27, No. 16, 2008
Prades et al.
Table 1. ꢀ-Alkylation of Secondary Alcohols with Primary Alcoholsa
entry cat.
R1
Ph
Ph
Ph
Ph
Ph
R2
t (h) yield (%) alcohol:ketone
1
2
3
4
5
1
Pr
Pr
Pr
Pr
Pr
22
22
22
13
10
60
>95
86
>95
95
78:22
90:10
91:9
90:10
90:10
2a
2b
3
4
Figure 2. Molecular diagram of compound 3. Hydrogen atoms have
been omitted for clarity. Ellipsoids are at 30% probability. Selected
bond distances (Å) and angles (deg): Ru(1)-C(1) 2.044(5),
Ru(1)-Cl(1) 2.4295(13), Ru(1)-Cl(2) 2.4201(14), Ru(1)-Ccentroid
1.695, C(1)-Ru(1)-Cl(1) 88.54(13), C(1)-Ru(1)-Cl(2) 86.47
(14), Cl(2)-Ru(1)-Cl(1) 87.31(5).
6
7
8
9
1
Ph
Ph
Ph
Ph
Ph
3-Cl(C6H4)
3-Cl(C6H4)
3-Cl(C6H4)
3-Cl(C6H4)
3-Cl(C6H4)
24
24
24
10
8
95
94
86
>95
>95
92:8
81:19
100:0
100:0
85:15
2a
2b
3
10
4
11
12
13
14
15
1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
24
24
8
8
8
57
>95
>95
>95
>95
100:0
88:12
93:7
97:3
88:12
metal atoms. The molecular structure of compound 2b confirms
that the carbene ligand is bound in an abnormal coordination
mode. The Ru-Ccarbene distance is 2.084 Å, in the range of other
(p-cymene)Ru(NHC) complexes.14,15 The Ru-Ccarbene distance
in compound 3 is 2.044 Å, very similar to the analogous distance
found for 2b. All other distances and angles are unexceptional.
2a
2b
3
4
16
17
18
19
20
1
Ph
Ph
Ph
Ph
Ph
4-Cl(C6H4)
4-Cl(C6H4)
4-Cl(C6H4)
4-Cl(C6H4)
4-Cl(C6H4)
24
8
24
8
87
>95
90
>95
>95
92:8
2a
2b
3
90:10
83:17
77:23
96:4
Compounds 2a, 2b, 3, and 4 and the previously reported
complex 115 contain three different types of N-heterocyclic
carbenes with similar topological frameworks but different
electron-donor power. Pyrazolin-3-ylidenes11 and aNHCs7 are
known to be better σ-donors than imidazolin-2-ylidenes, so the
use of 1-4 in different catalytic reactions under the same
reaction conditions can provide useful information in the design
of future catalysts.
4
8
21
22
23
24
25
1
C5H11 Ph
C5H11 Ph
C5H11 Ph
C5H11 Ph
C5H11 Ph
20
14
20
10
8
>95
>95
>95
>95
>95
100:0
100:0
100:0
100:0
100:0
2a
2b
3
4
a Reaction conditions:
1 mmol of primary alcohol, 1 mmol of
We have recently studied the catalytic ꢀ-alkylation of
secondary alcohols with primary alcohols.16,17 Despite the
important benefits of this process, apart from our two works,
we found only a few pioneering examples of this catalytic
reaction described in the literature, referring to Ru18,19 and to
Ir20 complexes. This reaction is believed to involve oxidation
of both alcohols to form a ketone and an aldehyde, which
undergo an aldol condensation, giving an R,ꢀ-unsaturated
ketone, which is further reduced to give the saturated alcohol.20,21
Taking this into account, we thought that complexes 1-4 could
be good candidates for this reaction, because Ru-NHC com-
plexes have proven to be excellent catalysts for the transfer
hydrogenation reactions between alcohols and ketones.22 NHC
secondary alcohol, 1 mmol of KOH, and 0.01 mmol of catalyst in 0.3
mL of toluene at 110 °C. Yields and ratios were determinad by 1H
NMR.
ligands have also been shown to be efficient catalysts toward
C-H activation processes.4,23
The reactions were carried out under atom-economic condi-
tions, using an equimolecular amount of the primary and
secondary alcohols. A fixed catalyst loading of 1 mol % was
used in the presence of KOH in toluene at a temperature of
110 °C. The reaction was performed using 2-phenylethanol and
2-heptanol as secondary alcohols and four different primary
alcohols (n-butanol, benzyl alcohol, 3-chlorobenzyl alcohol, and
4-chlorobenzyl alcohol). Table 1 shows the catalytic results for
this process, and the reaction times correspond to the maximum
conversions achieved by each catalyst. All catalysts 1-4 show
good catalytic activities in this reaction, although significant
differences are observed between them. The cationic compound
4 invariably shows the best catalytic activity, achieving full
conversions in short reaction times (8-10 h). The monopyra-
zolylidene complex 3 also shows an excellent activity in all
the reactions tested, although slightly longer reaction times are
needed for completion (8-13 h). The normal-NHC complex 1
shows lower activity than the rest of the catalysts, providing
(14) (a) Poyatos, M.; Mas-Marza, E.; Sanau, M.; Peris, E. Inorg. Chem.
2004, 43, 1793. (b) Cariou, R.; Fischmeister, C.; Toupet, L.; Dixneuf, P. H.
Organometallics 2006, 25, 2126. (c) Ozdemir, I.; Demir, S.; Cetinkaya,
B.; Toupet, L.; Castarlenas, R.; Fischmeister, C.; Dixneuf, P. H. Eur.
J. Inorg. Chem. 2007, 2862.
(15) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J.
Chem.-Eur. J. 1996, 2, 772.
(16) da Costa, A. P.; Viciano, M.; Sanau, M.; Merino, S.; Tejeda, J.;
Peris, E.; Royo, B. Organometallics 2008, 27, 1305.
(17) Viciano, M.; Sanau, M.; Peris, E. Organometallics 2007, 26, 6050.
(18) (a) Cho, C. S.; Kim, B. T.; Kim, H. S.; Kim, T. J.; Shim, S. C.
Organometallics 2003, 22, 3608. (b) Guillena, G.; Ramon, D. J.; Yus, M.
Angew. Chem., Int. Ed. 2007, 46, 2358.
(19) (a) Martinez, R.; Ramon, D. J.; Yus, M. Tetrahedron 2006, 62,
8988. (b) Martinez, R.; Ramon, D. J.; Yus, M. Tetrahedron 2006, 62, 8982.
(20) Fujita, K.; Asai, C.; Yamaguchi, T.; Hanasaka, F.; Yamaguchi, R.
Org. Lett. 2005, 7, 4017.
(23) (a) Scott, N. M.; Pons, V.; Stevens, E. D.; Heinekey, D. M.; Nolan,
S. P. Angew Chem., Int. Ed. 2005, 44, 2512. (b) Dorta, R.; Stevens, E. D.;
Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5054. (c) Huang, J. K.; Stevens,
E. D.; Nolan, S. P. Organometallics 2000, 19, 1194. (d) Prinz, M.; Grosche,
M.; Herdtweck, E.; Herrmann, W. A. Organometallics 2000, 19, 1692. (e)
Danopoulos, A. A.; Winston, S.; Hursthouse, M. B. J. Chem. Soc., Dalton
Trans. 2002, 3090. (f) Corberan, R.; Sanau, M.; Peris, E. Organometallics
2006, 25, 4002. (g) Corberan, R.; Sanau, M.; Peris, E. J. Am. Chem. Soc.
2006, 128, 3974.
(21) Hamid, M.; Slatford, P. A.; Williams, J. M. J. AdV. Synth. Catal.
2007, 349, 1555.
(22) (a) Poyatos, M.; Mata, J. A.; Falomir, E.; Crabtree, R. H.; Peris,
E. Organometallics 2003, 22, 1110. (b) Danopoulos, A. A.; Winston, S.;
Motherwell, W. B. Chem. Commun. 2002, 1376.

“Ru(p-cymene)” Complexes with Different NHC Ligands
Organometallics, Vol. 27, No. 16, 2008 4257
Table 2. Dimerization and Cyclotrimerization Reaction of
Phenylacetylenea
zolylidene) are providing the best activities, and among these,
the pyrazole-based NHC seems to be the most active one. Since
there are no studies yet comparing the σ-donating powers of
aNHCs and pyrazolylidenes, we cannot fully attribute the
differences in activity to the electronic differences between the
ligands, but it seems clear that the pyrazolylidenes should be
considered as a good choice for catalyst design.
Compounds 1-4 were also tested in the catalytic dimerization
of phenylacetylene to provide the corresponding enynes. The
direct coupling of two terminal alkynes is an interesting process
because enynes may serve as building blocks for the synthesis
of natural products. One of the main challenges in this type of
reaction is the preparation of highly effective catalysts capable
of affording good selectivities in the formation of the E-dimers,
for which commercially available metal precursors do not seem
to give the desired results, and the preparation of more
sophisticated organometallic species is needed. Several reports
have appeared in which ruthenium complexes have provided
good E-selectivities in this reaction,24,25 but still the reactions
fail to give high yields and long reaction times are needed.
The reactions were carried out with phenylacetylene and a
fixed amount of catalyst (5 mol %) in acetonitrile-d3 at 70 °C,
in the presence of NEt3. The dimer [RuCl2(p-cymene)]2 was
also tested for comparison. Table 2 shows the catalytic results
for the process. As observed, high conversions are achieved in
all cases, although the dimerization competes with the trimer-
ization of the acetylene, generating two possible trisubstituted
benzenes. Catalysts 1-4 provide yields in the range 20-42%
in the dimerization products with a moderate E-selectivity, while
[RuCl2(p-cymene)]2 affords only the cyclotrimerization com-
pounds under the same reaction conditions. The results obtained
for compounds 1-4 compare well with the activities shown by
other ruthenium catalysts recently reported.24,26 For this reaction
we did not find any substantial differences in the catalytic
activities of 1-4, although it becames clear that the introduction
of the basic NHC ligand provides an inversion of the catalytic
behavior compared to [RuCl2(p-cymene)]2.
entry
catalyst
conversion (%)
ratio a:b:c
1
2
3
4
5
6
1
97
88
79
95
81
56
33:9:58
14:5:81
26:10:64
21:8:71
28:9:63
0:0:100
2a
2b
3
4
[RuCl2(p-cym)]2
a Reaction conditions: 0.3 mmol of phenylacetylene, 0.075 mmol of
NEt3, and 0.015 mmol of catalyst in 0.3 mL of CD3CN at 70 °C for
8 h. Yields and ratios were determinad by 1H NMR.
Table 3. Crystallographic Data
2b
3
empirical formula
fw
C21H26Cl2N2Ru
478.41
0.71073
273(2)
orthorhombic
Pna2 (1)
14.5949(5)
10.8530(4)
26.1994(9)
90
C15H22Cl2N2Ru
402.32
0.71073
273(2)
monoclinic
P2(1)/c
14.2961(17)
7.0647(9)
16.3109(19)
90
92.052(3)
90
1646.3 (3)
4
1.623
1.040
wavelength (Å)
temperature (K)
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
ꢀ (deg)
90
90
γ (deg)
V (Å)3
4149.9 (3)
8
1.531
1.020
32 497
1.006
R1 ) 0.0495
wR2 ) 0.1030
Z
density (calcd) (Mg/m3)
absorp coeff (mm-1
)
Conclusions
no. of reflns collected
goodness-of-fit on F2
R indices (all data)
10 295
1.002
R1 ) 0.0448
wR2 ) 0.0973
We have prepared a set of simple “(p-cymene)Ru(NHC)”
complexes, with the NHC ligands being of the type normal-
NHC, abnormal-NHC, and pyrazolin-3-ylidene. All complexes
were obtained by the transmetalation from the corresponding
silver-carbene preformed complexes. While we found that there
is only one publication describing Ru-aNHC complexes in the
literature, our Ru-pyrazolylidene complex is the first one to
coordinate such a ligand to ruthenium and one of the few
examples known for transition metal compounds,10–12,26 a fact
that is even more remarkable if we take into account that three
of the five examples reported to date refer to remote pyra-
zolylidenes (pyrazolin-4-ylidenes).12,27
only moderate yields in rather long reaction times (20-24 h).
The abnormally-bound NHC complexes 2a and 2b show good
activity in terms of conversions achieved, but longer reaction
times are needed than those for 3 and 4. The phenyl-substituted
NHC complex (2b) shows lower efficiency than 2a, probably
because of the higher steric crowding around the metal center.
For most of the reactions, the process is very selective in the
production of the alkylated alcohols, although in some cases
small amounts of the alkylated ketones were obtained as
secondary products.
It is worth mentioning that compound 4 lies among the most
efficient catalysts for this type of reaction, taking into account
the short reaction times needed and the low catalyst loadings
used. To our knowledge, only an iridium compound reported
by us displays a similar activity for this type of reaction,16 but
among ruthenium complexes 4 seems to be the most active
reported to date.
The “(p-cymene)Ru(NHC)” complexes were tested in two
catalytic reactions, namely, the ꢀ-alkylation of secondary
alcohols with primary alcohols and the dimerization of pheny-
lacetylene, showing good activities in both of them. The results
(24) (a) Bassetti, M.; Pasquini, C.; Raneri, A.; Rosato, D. J. Org. Chem.
2007, 72, 4558. (b) Gao, Y.; Puddephatt, R. J. Inorg. Chim. Acta 2003,
350, 101.
(25) (a) Bassetti, M.; Marini, S.; Diaz, J.; Gamasa, M. P.; Gimeno, J.;
Rodriguez-Alvarez, Y.; Garcia-Granda, S. Organometallics 2002, 21, 4815.
(b) Slugovc, C.; Mereiter, K.; Zobetz, E.; Schmid, R.; Kirchner, K.
Organometallics 1996, 15, 5275.
(26) Daniels, M.; Kirss, R. U. J. Organomet. Chem. 2007, 692, 1716.
(27) Schutz, J.; Herdtweck, E.; Herrmann, W. A. Organometallics 2004,
23, 6084.
An interesting feature about the comparison of the results
that we present in Table 1 is that the different types of NHC
ligands are providing different activities in a quite rational way.
It seems that the more σ-donating NHCs (aNHCs and pyra-

4258 Organometallics, Vol. 27, No. 16, 2008
Prades et al.
Synthesis of 2b. A suspension of 1,3-dimethyl-2-phenylimida-
zolium iodide (176 mg, 0.59 mmol) and silver oxide (204 mg, 0.88
mmol) in CH2Cl2 was stirred at room temperature for 2 h. The
mixture was filtered through Celite, and then [RuCl2(p-cymene)]2
(150 mg, 0.25 mmol) was added. The mixture was refluxed for
3 h, and the white precipitate formed (silver halide) was separated
by filtration through Celite. The solvent was evaporated, and the
crude solid was purified by column chromatography using silica
gel. Elution with CH2Cl2/acetone (4:1) afforded the separation of
an orange band that contained 2b. Complex 2b was obtained as a
brown solid by precipitation from CH2Cl2/Et2O solution (yield: 155
mg, 65%). 1H NMR (CDCl3, 300 MHz): δ 7.61-7.58 (m, 3H,
CHPh), 7.38-7.35 (m, 2H, CHPh), 6.91 (s, 1H, CHimid), 5.30 (d,
obtained for the ꢀ-alkylation of secondary alcohols lie among
the best reported to date and allowed a clear comparison of the
activities provided by the different ligands, the pyrazolylidene
being the best one. In the dimerization of phenylacetylene, we
have observed that the introduction of the NHC ligand affords
a clear improvement to the formation of the dimerization
products compared to the results provided by [RuCl2(p-
cymene)]2 under the same reaction conditions.
Our results prove that pyrazolylidene ligands are an excellent
choice for the design of simple and highly effective catalysts.
Studies on the modification of the topologies of this type of
ligands and their coordination to other potential catalytically
active metal fragments are underway.
3
3JH-H ) 5.70 Hz, 2H, CHpcym), 5.17 (d, JH-H ) 5.70 Hz, 2H,
CHpcym), 3.81 (s, 3H, NCH3), 3.51 (s, 3H, NCH3), 2.86-2.74 (m,
1H, CHisop pcym), 2.15 (s, 3H, CH3pcym), 1.23 (d, 3JH-H ) 6.90 Hz,
6H, CH3isop pcym). 13C{1H} NMR (CDCl3, 125 MHz): δ 154.3 (C-
Ru), 143.6 (Cqimid), 131.5 (CHimid), 130.2, 129.8, 126.4 (CHPh),
124.6 (CqPh), 102.8, 99.3 (Cqpcym), 84.1, 84.0 (CHpcym), 34.8 (CHisop
pcym), 31.1, 30.9 (NCH3), 22.6 (CH3isop pcym), 18.6 (CH3pcym).
Electrospray MS (20 V, m/z): 443.1 [M - Cl]+. Anal. Calcd for
C21N2RuCl2H26 (mol wt 478.42): C, 52.72; H, 5.48; N, 5.86. Found:
C, 52.78; H, 5.60; N, 5.71.
Experimental Section
NMR spectra were recorded on Varian Innova 300 and 500 MHz
spectrometers, using CDCl3 and DMSO-d6 as solvents. Elemental
analyses were carried out in an EA 1108 CHNS-O Carlo Erba
analyzer. Electrospray mass spectra (ESI-MS) were recorded on a
Micromass Quatro LC instrument, and nitrogen was employed as
drying and nebulizing gas. 1,2,3-Trimethylimidazolium iodide,28
1,2-dimethylpyrazolium iodide,10 [RuCl2(p-cymene)]2,29 and 115
were prepared according to literature procedures. All other reagents
are commercially available and were used as received.
Synthesis of 1,3-Trimethyl-2-phenylimidazolium Iodide. To
a round-bottomed flask were added 2-phenylimidazolium (1 g, 6.9
mmol), NaOH (416 mg, 10.4 mmol), TBABr (50 mg, 0.15 mmol),
and a few drops of water. The mixture was stirred at room
temperature for 1 h, and then iodomethane (650 µL, 10.40 mmol)
was added. After being stirred for 48 h at room temperature, the
reaction mixture was extracted with CH2Cl2/H2O and the organic
extracts were collected and dried over Na2SO4. Evaporation of the
solvent under vacuum gave an oil. The oil was redissolved in
CH3CN (10 mL), and iodomethane (650 µL, 10.40 mmol) was
added. The mixture was refluxed overnight. The volatile components
were removed under vacuum, and the product was washed with
CH2Cl2 to give the desired product. Yield: 80%. 1H NMR (500
MHz, DMSO-d6): 7.88 (s, 2H, CHimid), 7.77-7.69 (m, 5H, CHPh),
3.69 (s, 6H, NCH3). 13C{1H} NMR (125 MHz, DMSO-d6): 144.8
(Cq), 133.0 (CqPh), 131.3 (CHimid), 130.1, 123.8, 121.8 (CHPh), 36.5
(NCH3). Electrospray MS: m/z 173.3 [M+].
Synthesis of 3. Silver oxide (174 mg, 0.75 mmol) was added to
a solution of 1,2,-dimethylpyrazolium iodide (110 mg, 0.50 mmol)
in CH2Cl2, and the mixture was stirred at room temperature for
2 h. Then [RuCl2(p-cymene)]2 (150 mg, 0.25 mmol) was added.
The mixture was refluxed for 3 h. The suspension was filtered
through Celite, and the solvent was evaporated under reduced
pressure. The crude solid was purified by column chromatography.
Elution with a mixture of CH2Cl2/MeOH (9:1) afforded a yellow
band that contained compound 3. The pure compound was
precipitated from a mixture of CH2Cl2/hexanes (yield: 120 mg,
1
3
60%). H NMR (CDCl3, 500 MHz): δ 7.25 (d, JH-H ) 2.50 Hz,
1H, CHpyrazole), 6.51 (d, 3JH-H ) 2.00 Hz, 1H, CHpyrazole), 5.19 (d,
3JH-H ) 5.99 Hz, 2H, CHpcym), 5.03 (d, JH-H ) 5.99 Hz, 2H,
3
CHpcym), 4.01 (s, 3H, NCH3), 3.78 (s, 3H, NCH3), 2.70-2.65 (m,
1H, CHisop pcym), 2.02 (s, 3H, CH3pcym), 1.14 (d, 3JH-H ) 6.50 Hz,
6H, CH3 isop pcym). 13C{1H} NMR (CDCl3, 75 MHz): δ 180.3 (C-
Ru), 133.0, 117.1 (CHpyrazole), 104.8, 99.3 (Cqpcym), 84.5, 84.1
(CHpcym), 31.8 (NCH3), 30.7 (CHisop pcym), 22.8 (CH3isop pcym), 14.3
(CH3pcym). Electrospray MS (15 V, m/z): 367.0 [M - Cl]+. Anal.
Calcd for C15N2RuCl2H22 (mol wt 402.3): C, 44.78; H, 5.51; N,
6.96. Found: C, 44.65; H, 5.62; N, 6.88.
Synthesis of 2a. A suspension of 1,2,3-trimethylimidazolium
iodide (93 mg, 0.39 mmol) and silver oxide (136 mg, 0.59 mmol)
in CH2Cl2 was stirred at room temperature for 2 h. The product
mixture was filtered through Celite, and then [RuCl2(p-cymene)]2
(100 mg, 0.16 mmol) was added to the solution. The mixture was
refluxed for 3 h and then was filtered through Celite. The solvent
was evaporated and the crude solid purified by column chroma-
tography using silica gel. Elution with CH2Cl2/MeOH (9:1) afforded
the separation of an orange band that contained 2a. Complex 2a
was obtained as a brown solid by precipitation from CH2Cl2/Et2O
solution. Yield: 53 mg, 40%. 1H NMR (CDCl3, 300 MHz): δ 6.59,
Synthesis of 4. In an analogous manner to the preparation of 3,
the transmetalation was carried out in dichloromethane with 1,2,-
dimethylpyrazolium iodide (172 mg, 0.78 mmol), silver oxide (274
mg, 1.18 mmol), and [RuCl2(p-cymene)]2 (120 mg, 0.20 mmol).
Elution with 20 mL of CH2Cl2/acetone (1:1) with 30 mg of KPF6
afforded the separation of a yellow band that contained compound
4 and residual KPF6, which was filtered off. Complex 4 was
obtained as a green solid by precipitation from a CH2Cl2/Et2O
solution (yield: 85 mg, 35%). 1H NMR (CDCl3, 300 MHz): δ 7.44
3
3
3
(s, 1H, CHimid), 5.22 (d, JH-H ) 5.70 Hz, 2H, CHpcym), 5.08 (d,
(d, JH-H ) 3.00 Hz, 2H, CHpyrazole), 6.55 (d, JH-H ) 2.70 Hz,
3JH-H ) 5.70 Hz, 2H, CHpcym), 3.81 (s, 3H, NCH3), 3.55 (s, 3H,
NCH3), 2.75-2.66 (m, 1H, CHisop pcym), 2.47 (s, 3H, CCH3), 2.09
3
2H, CHpyrazole), 5.44 (d, JH-H ) 6.00 Hz, 2H, CHpcym), 5.16 (d,
3JH-H ) 6.00 Hz, 2H, CHpcym), 3.90 (s, 6H, NCH3), 3.53 (s, 6H,
NCH3), 2.70-2.60 (m, 1H, CHisop pcym), 1.88 (s, 3H, CH3pcym), 1.16
(d, 3JH-H ) 6.90 Hz, 6H, CH3isop pcym). 13C{1H} NMR (CDCl3, 75
MHz): δ 175.1 (C-Ru), 133.6, 118.4 (CHpyrazole), 112.2, 100.9
(Cqpcym), 92.0, 88.8 (CHpcym), 37.2, 36.8 (NCH3), 30.6 (CHisop pcym),
22.6 (CH3isop pcym), 18.5 (CH3pcym). Electrospray MS (15 V, m/z):
463.1 [M]+. Anal. Calcd for C20N4RuClH30PF6 (mol wt 607.97):
C, 39.51; H, 4.97; N, 9.22. Found: C, 39.35; H, 5.27; N, 5.90.
3
(s, 3H, CH3pcym), 1.18 (d, JH-H) 6.90 Hz, 6H, CH3isop pcym).
13C{1H} NMR (CDCl3, 75 MHz): δ 152.2 (C-Ru), 141.0 (Cqimid),
125.1 (CHimid), 103.1, 99.1 (Cqpcym), 83.9, 83.8 (CHpcym), 37.1, 34.3
(NCH3), 30.8 (CHisop pcym), 22.6 (CH3isop pcym), 18.6 (CH3pcym), 10.7
(CCH3). Electrospray MS (25 V, m/z): 381.3 [M - Cl]+. Anal.
Calcd for C16N2RuCl2H24 (mol wt 416.04): C, 46.16; H, 5.81; N,
5.73. Found: C, 45.96; H, 5.95; N, 5.75.
ꢀ-Alkylation of Secondary Alcohols with Primary Alcohols.
Standard Procedure. The reaction was carried out with secondary
alcohol (1 mmol), primary alcohol (1 mmol), 1 mol % of catalyst,
and base, KOH (1 mmol), in toluene (0.3 mL) at 110 °C. The
(28) Ricciardi, F.; Romanchick, W. A.; Joullie, M. M. J. Polym. Sci.
Polym. Chem. 1983, 21, 1475.
(29) Bennett, M. A.; Smith, A. K. J. Chem. Soc., Dalton Trans. 1974,
233.

“Ru(p-cymene)” Complexes with Different NHC Ligands
Organometallics, Vol. 27, No. 16, 2008 4259
reaction was monitored by 1H NMR spectroscopy, by introducing
aliquots of the reacting solution inside an NMR tube with 0.5 mL
of CDCl3. The evolution was determined by integration. The signals
due to reagents and products were taken from the literature.19
Dimerization of Phenylacetylene. Standard Procedure. The
reaction was carried out with phenylacetylene (0.3 mmol), 5 mol
% of catalyst, and base, Et3N (0.075 mmol), in acetonitrile-d3 (0.3
temperature on a Siemens Smart CCD diffractometer using graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å). The diffraction
frames were integrated using the SAINT package.30
Space group assignment was based on systematic absences, E
statistics, and successful refinement of the structures. The structure
was solved by direct methods with the aid of successive difference
Fourier maps and refined using the SHELXTL 6.1 software
package.31 All non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were assigned to ideal positions and refined
using a riding model.
1
mL) at 70 °C. The reaction was monitored by H NMR spectros-
copy, by introducing aliquots of the reacting solution inside an NMR
tube with 0.5 mL of CDCl3. The evolution was determined by
integration, using ferrocene as internal standard. The signals due
to reagents and products were taken from the literature.26
X-ray Diffraction Studies. Crystals for X-ray diffraction of 2b
and 3 were obtained by slow diffusion of pentane in a concentrated
solution of the compound in dichloromethane. Crystal data are
summarized in Table 3. Data collection was performed at room
Acknowledgment. We gratefully acknowledge financial
support from the MEC of Spain (CTQ2005-05187), Bancaixa
(P1.1B2007-04).
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determinations of 2b and 3. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
(30) SAINT, Version 5.0; Bruker Analytical X-ray System: Madison, WI,
1998.
(31) Sheldrick, G. M. SHELXTL, Version 6.1; Bruker AXS, Inc.:
Madison, WI, 2000.
OM800377M