Synthesis of a new bidentate NHC-Ag(I) complex and its unanticipated reaction with the Hoveyda-Grubbs first generation catalyst

Article abstract of DOI:10.1016/j.tet.2009.05.095

A new sterically demanding bidentate imidazolium bromide has been prepared and used as ligand precursor for the synthesis of the corresponding NHC-silver(I) complex. The X-ray analysis of the silver(I) complex revealed a rare Ag₄O₄ c

Full text of DOI:10.1016/j.tet.2009.05.095

Tetrahedron 65 (2009) 7186–7194
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Synthesis of a new bidentate NHC–Ag(I) complex and its unanticipated reaction
with the Hoveyda–Grubbs first generation catalyst
*
´
Giovanni Occhipinti, Vidar R. Jensen, Karl Wilhelm To¨rnroos, Nils Åge Frøystein, Hans-Rene Bjørsvik
Department of Chemistry, University of Bergen, Alle´gaten 41, N-5007 Bergen, Norway
a r t i c l e i n f o
a b s t r a c t
Article history:
A new sterically demanding bidentate imidazolium bromide has been prepared and used as ligand
precursor for the synthesis of the corresponding NHC–silver(I) complex. The X-ray analysis of the silver(I)
complex revealed a rare Ag4O4 core cubane cluster. The silver(I) complex reacts readily with the Grubbs
first generation catalyst providing a labile alkylidene complex. When the transmetallation was attempted
with Hoveyda–Grubbs first generation catalyst in the presence of THF as solvent, two very stable
phosphine free bis-bidentate N-heterocyclic carbene complexes, one green and one orange, were formed.
Notably, one of these complexes is the first observation of a metal alkylidene group substituted by a NHC
ligand, a surprising result since the new complex is formally derived from a nucleophile substitution of
a hydride by a NHC ligand on the alkylidene carbon. A proposal for the reaction mechanism is elaborated.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 14 March 2009
Received in revised form 7 May 2009
Accepted 28 May 2009
Available online 6 June 2009
1. Introduction
we wanted to investigate the electronic and steric features of the
sterically demanding 4-nitrobenzyloxy substituted N-heterocyclic
carbenes (1a–1d), Chart 1.
During the past decades N-heterocyclic carbenes (NHCs)1–4 have
emerged as a versatile class of dative ligands in metal coordination
chemistry, particularly in the field of homogeneous catalysis. The
particular electronic features5,6 of the these ligands allow formation
of stable complexes both with early7–9 and late transition metals10–12
both in low and high oxidation states as well with s,13,14 p,15 and f4,16
CH3
CH3
H3C
H3C
H3C
N
N
N
O
N
O
CH3
1b
CH3
CH3
CH3
NO2
NO2
NO2
block elements. Due to their excellent s-donor ability, which sur-
passes even that of the most basic phosphines,5,10 NHCs have been
extensively utilized as ligands in homogeneous catalysis. Sub-
stitution of phosphine ligands with NHC ligands has in many cases
provided both more robust and active catalysts.1,3,17–19 The ruthe-
nium based olefin metathesis catalysts where a phosphine ligand is
substituted with a NHC ligand provide an important modification
that in general affords more active and robust catalysts.19–21 N-het-
erocyclic carbenes possess a good structural flexibility that permits
functionalizations to operate as chelate or pincer ligands.1,8,9,14,22–27
Chelate NHC ligands have afforded particularly efficient and usable
catalysts for asymmetric catalysis purposes.23,24,28
1a
N
H3C
N
O
N
CH3
N
O
CH3
NO2
1c
1d
O2N
O2N
Chart 1.
Structurally similar ligands previously described by Zhang and
co-workers8,14,25,26 and Kawaguchi and co-workers,9 were used to
synthesize a series of Mg(II), Na(I), Li(I), Y(III), Yb(III), Fe(II), Ni(II),
Zr(IV), and Ti(IV) complexes.
A current project in our laboratories involves the design, syn-
thesis, and investigation of novel ruthenium alkylidene complexes
as catalysts for olefin metathesis. Monodentate ligands were ini-
tially investigated29 and the results directed us subsequently to the
design of bidentate and tridentate ligands30,31 with the ambition to
synthesize new asymmetrical ruthenium catalysts. In this context,
In a recent short communication32 we reported the first imida-
zolium-substituted metal alkylidene together with investigations of
theelectronicstructurebymeansofquantumchemistrycalculations.
Herein, a detailed account of the synthesis and a thoroughly
structural analysis of the precursor, a new rare bidentate NHC–Ag(I)-
complex is given. The reaction leading to the imidazolium-substituted
* Corresponding author. Tel.: þ47 55 58 34 52; fax: þ47 55 58 94 90.
E-mail address: hans.bjorsvik@kj.uib.no (H.-R. Bjørsvik).
ruthenium alkylidene is discussed in detail and
a reaction
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.05.095

G. Occhipinti et al. / Tetrahedron 65 (2009) 7186–7194
7187
mechanism is proposed. A detailed NMR-based structural analysis of
2.3. Synthesis of the bidentate NHC silver(I) complex
the imidazolium–alkylidene complex is presented along with new
olefin metathesis tests performed in ionic solvents.
In attempts to prepare a silver(I) complex with our novel
bidentate NHC ligand 1a, variations of reaction medium, reaction
time, reaction temperature, concentrations, and the molar ratios of
the imidazolium bromide 2a, and Ag2O were investigated. Initially,
we used a slightly modified procedure previously disclosed by Lin
and co-workers40 and by Hoveyda and co-workers.28 A protocol
providing good yield (z65%) of the bidentate NHC silver(I) com-
plex 4a is reported in Scheme 2. The silver(I) complex 4a was easily
isolated and purified, namely filtered through a pad of Celite after
which the solvent was removed under reduced pressure. Further
purification was conducted by dissolving the isolated solid in
methylene chloride followed by precipitation by the addition of n-
pentane.
2. Method and results
2.1. Synthesis of the new imidazolium salt
The synthesis of the novel imidazolium salt 1-(2-hydroxy-5-
nitrobenzyl)-3-(2,4,6-trimethylphenyl)-3H-imidazol-1-ium bromide
2a is outlined in Scheme 1. The intermediate product 1-(2,4,6-tri-
methylphenyl)-1H-imidazole 3 was prepared by means of a slightly
modified multi component reaction protocol previously disclosed
by Arduengo and co-workers:33 glacial acetic acid, aqueous form-
aldehyde and glyoxal were mixed and heated at 70 ꢀC, after which
a mixture of glacial acetic acid, ammonium acetate in water, and
2,4,6-trimethylphenylamine was added dropwise over a period of
30 min. During a reaction period of 18–20 h at 70 ꢀC, the in-
termediate product 3 was achieved in good yield (z70%). 1-(2,4,6-
O2N
Ag2O
NO2
(2.5 mmol)
Br
Trimethylphenyl)-1H-imidazole
3 and 2-hydroxy-5-nitrobenzyl
O
N
N
N
N
bromide was then reacted at refluxing temperature in toluene to
obtain the target imidazolium salt 1-(2-hydroxy-5-nitrobenzyl)-3-
(2,4,6-trimethylphenyl)-3H-imidazol-1-ium bromide 2a that was
easily isolated by filtration in an excellent yield (>98%). The X-ray
structure of the new imidazolium bromide 2a and the crystal data
of the structural refinement are given in Supplementary data.
Ag
HO
CH3
Reflux, 3h
THF (8.5 mL)
H3C
Benzene (8.5 mL)
CH3
H3C
4a
2a (1.2 mmol)
CH3
CH3
CH3
Scheme 2. The imidazolium bromide 2a reacts with silver (I) ions to produce bi-
dentate NHC silver(I) complex 4a.
O
NH2
CH3
CH3COOH
H2O
NH4
CH3
O
H3C
Initial structural analyses of the isolated product were con-
ducted using 1H NMR. As expected, the 1H NMR spectrum revealed
that the proton signals of both the phenolic group and the C2–H of
the imidazole ring were absent.
Dropwise added over
a period of 30 min.
H3C
An aliquot of the prepared silver(I) complex was further purified
by re-crystallization in THF. The solid was dissolved at 65–70 ꢀC,
followed by slowly cooling to room temperature. A suitable crystal
was selected from the crystallized product and investigated on an
X-ray diffractometer. The surprising result of that analysis is
presented in Figure 1, revealing the very first Ag4O4 core cubane
H
O
H
H
H
N
N
CH3
CH3
O
CH3COOH / H2O
70 °C, 18h
O
H3C
3 (70%)
H3C
Br
Br
N
N
OH
OH
H3C
Toluene
Reflux, 18h
O2N
O2N
2a (98.5%)
Scheme 1. Synthetic pathway to 1-(2-hydroxy-5-nitrobenzyl)-3-(2,4,6-trimethyl-
phenyl)-3H-imidazol-1-ium bromide 2a.
2.2. The synthetic plan
The synthetic plan to reach the novel ruthenium complexes
disclosed herein was initially attempted by reacting deprotonated
imidazolium bromide 2a with the commercially available Grubbs34
or Hoveyda–Grubbs35 first generation catalysts. This strategy im-
plies a direct substitution of a phosphine ligand and a chlorine
anion with the new bidentate ligand.36 However, attempts to
deprotonate 2a with potassium tert-butoxide resulted only in de-
composition of the ligand, a result that was in line with previous
reports12 using analogous conditions for other types of ligands
(methylene-linked imidazolium salts). This direct path for the
preparation of the Ru-complexes was thus abandoned in favor of
a strategy involving a transmetallation step using the corresponding
NHC–silver (I) complex of the bidentate ligand 1a. This approach has
also earlier been utilized successfully for the synthesis of NHC–
Ru(IV),24,37 NHC–Pd(II),18,38 and NHC–Ni(II)39 complexes.
Figure 1. Structure of one of the two enantiomers of the silver-oxygen cubane cluster
4a, where the ligand 1-(4-nitrobenzyloxy)-3-mesityl-imidazole-2-ylidene is co-
ordinated to Ag(I). The molecule has C2 symmetry, the symmetry related part (a),
ꢁxþ3/2, y, ꢁzþ1/2. Anisotropic displacement parameters are given at the 50% prob-
ability level. Hydrogen atoms are omitted for clarity.

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G. Occhipinti et al. / Tetrahedron 65 (2009) 7186–7194
cluster that also contains Ag–C bonds. Moreover, this cluster also
represents the first cubane cluster that contains a bidentate N-
heterocyclic carbene ligand. The only previous report demon-
strating the use of a monodentate NHC ligand in cubane clusters
was recently disclosed.41
only slightly different, namely 166.2ꢀ and 164.5ꢀ. Further details
regarding the crystal data and results of the structural refinement
for this silver(I)-oxygen cubane cluster are provided in Supple-
mentary data.
The tetrameric Ag4O4 structure disclosed herein, Figure 1, con-
sists of a dissymmetric silver(I)–oxygen cubane (Ag4O4) type cluster.
The silver complex 4a represents a rare oxygen–silver cubane
cluster. Even if the core Ag4O4 cubane is rather distorted, a C2
symmetry axis passes through the left hand, O1–Ag1a–O1a–Ag1
and the right hand face O2–Ag2a–O2a–Ag2. This implies that the
reported structure is chiral and both enantiomers are present in the
unit cell that contains eight THF molecules forming a solvent
channel along the b-axis. A figure is provided in Supplementary
data. The structure is composed of two different and non-planar
quadrilateral parts bonded into a sandwich structure through four
dative O–Ag bonds (two bonds of length 2.642 (2) Å for Ag1–O2, and
the other two of length 2.640(2) Å for Ag2–O1.). Each part forms the
face of a quadrangle having two equal covalent Ag–O bonds and two
equal dative Ag–O bonds. In the same face, each silver center is
bonded to the first ligand bya short dative NHC–Ag bond (2.075(3) Å
for Ag1–C8 and 2.069(2) Å for Ag2–C27). It is connected to the
second ligand by a covalent Ag–O bond (2.150(2) Å or 2.162(2) Å).
Finally, each silver atom is coordinated to the oxygen belonging to
the first ligand through a dative Ag–O bond. Half of the cubane
cluster contains the same Ag2O2 core complex of dimeric bidentate
NHC–silver(I) complexes as discovered by Hoveyda and co-
workers.28 However, the complexes found by those authors did not
allow further sandwich complexation to give a cubane cluster
similar to 4a. The reason for this is most probably the steric hin-
drance due to the more bulky bidentate ligands.
It is interesting to note that the covalent and dative bonds for Ag
or O belonging to various faces are rather different. However, when
comparing the sum of the Ag–O bond lengths in the cubane cluster,
it is evident that these are equivalent within the statistical experi-
mental significance. The sum of the three bond distances C8–Ag1–
O2–C20 and C1–O1–Ag2–C27 belonging to opposite faces is the
same (6.018 Å), see Figure 2, while the two intramolecular space
distances from the carbene carbon atom and the arene carbon atom
are significantly different, namely C8–C20¼4.053(3) Å and C1–
C27¼4.447(3) Å, differing by 0.394 Å, see Figure 2. The bonding
angle NHC–Ag–O where silver is covalently bonded to oxygen are
2.4. Reactivity of silver complex 4a with Grubbs first
generation catalyst
The reaction of the silver complex 4a with the Grubbs first
generation catalyst34 goes to completion in 20 min at a temperature
of 65 ꢀC, or in 2 h at room temperature, to provide a rather labile
alkylidene Ru complex (red colored) as the major product. The new
complex was identified by a sharp singlet 1H peak at
d 19.52 ppm,
which is a typical shift value for the ruthenium–alkylidene pro-
ton.17,20,23,35,42 Furthermore, the 1H NMR spectrum of the reaction
mixture43 suggests a mono substituted NHC ruthenium complex,
although we were not able to perform a complete characterization
of the structure due to the instability of the complex.
2.5. Reactivity of silver complex 4a with Hoveyda–Grubbs first
generation catalyst
The silver(I) cubane cluster 4a reacts with Hoveyda–Grubbs first
generation catalyst35,44 to provide two discrete phosphine free bis
coordinated ruthenium complexes 5 (green colored) and 6 (orange
colored) both of which proved to be very stable (Scheme 3). The
reaction is conducted at 65 ꢀC for at least 7 h in order to be brought
to completeness. If the reaction is conducted with tetrahydrofuran
as reaction medium, both complexes 5 and 6 were produced.
However, if toluene is used as reaction medium, only complex 5
was obtained, and a mixture of toluene and THF provided both
complex 5 and 6. It was observed that the presence of PCy3 results
in an immediate decomposition of the cubane cluster 4a. It was
thought that this could be used in order to promote the desired
reaction forming the complex 5 and 6, however, PCy3 suppresses
both the reaction pathways to 5 and 6 under the formation of other
alkylidene complexes.45
NO2
N
N
Mes
O
O2N
O
N
Ru
PCy3
Ru
Cl
N
O
Cl
Mes
O
5 (6%)
N
N Mes
O2N
THF, reflux, 7h
Mes
N
Ag
O
N
Cl
4
N
O
O2N
4a
Ru
O
Mes
O
N
O2N
6 (12%)
Scheme 3. Synthesis of the ruthenium complexes 5 and 6 using silver(I)-oxygen cu-
bane cluster 4a and Hoveyda-Grubbs first generation catalyst.
2.6. Molecular structures of the Ru-complexes 5 and 6
The Ru-complexes 5 and 6 were purified and isolated using
column chromatography on silica gel. Initial characterization was
conducted by means of 1H NMR, 13C NMR, mass spectrometry, high-
resolution mass measurements, and elemental analysis. The in-
terpretation of the MS and the NMR spectra were in agreement
with the structures of the two Ru-complexes both of which co-
ordinate two bidentate NHC ligands. The number of proton and
carbon resonances in 1H NMR and 13C NMR spectra reveal an
Figure 2. A simplified sketch of complex 4a. Selected bond and intra molecular dis-
tances [Å]: Ag1–O1a 2.162(2), Ag1–O1 2.664(2), Ag2–O2 2.149(2), Ag2–O2a 2.687(2),
Ag1–O2 2.642(2), Ag2–O1 2.640(2), Ag1–Ag1a 3.3893(4), Ag2–Ag2a¼3.3366(5), Ag1–
Ag2a 3.8403(3), Ag1–Ag2 3.7048(3). [Symmetry operator a; ꢁxþ3/2, y, ꢁzþ1/2].

G. Occhipinti et al. / Tetrahedron 65 (2009) 7186–7194
7189
asymmetrical structure for each of the two complexes. The Ru-
complex 5 appeared clearly as a ruthenium alkylidene complex
formally derived from the substitution of one tricyclohexyl-
phosphine ligand and two chloride ions by two bidentate NHC li-
gands. The alkylidene proton resonance (Ru]CH, 15.90 ppm in
CD2Cl2) appears shifted toward higher fields with respect to that of
the Hoveyda catalyst35 (Ru]CH, 17.44 ppm in CDCl3), while the
alkylidene carbon resonance (Ru]C, 297.9 ppm in CD2Cl2) is shif-
ted toward lower fields also with respect to the Hoveyda catalyst
(Ru]C, 280.6 ppm). Moreover the 1H NMR (5.78 ppm) and 13C
NMR (77.8 ppm) chemical shifts of the isopropyloxy group in
comparison with those of the Hoveyda–Grubbs first generation
catalyst, respectively, (5.28 ppm) and (75.5 ppm), strongly suggest
a structure having the isopropoxy group coordinated to the metal.
The NMR analysis suggests for complex 5 an octahedral ruthenium
alkylidene complex with the metal coordinated by the chelating
isopropoxy group and by two different NHC bidentate ligands.
MS spectra reveal that the Ru-complex 6 contains one chlorine
more and one hydrogen less than the Ru-complex 5.1H NMR spectra
reveal that the Ru-complex 6 does not contain any alkylidene pro-
ton, while the expected signal for the alkylidene carbon appears at
244 ppm, namely shifted 54 ppm toward high field compared to the
resonance observed in the Ru-complex 5. Moreover the 1H NMR
(4.55 ppm) and 13C NMR (70.0 ppm) chemical shifts of the iso-
propoxy group in comparison with the Ru-complex 5 and the
(244 ppm) and both hydrogens in the NHC backbone (6.97 and
7.25 ppm) and two cross-peaks between the carbene carbon of the
NHC ring (161.7 ppm) and two hydrogens belonging to the aryli-
dene group (6.5 ppm, JCH) and (6.3 ppm, JCH), see Figure 3 and
Supplementary data.
Taken together, the MS, NMR, and elemental analyses suggest
a penta-coordinated Ru-complex with one chlorine, two aryloxy
groups, a NHC ligand bonded to the Ru, and finally a NHC ligand
bonded to the Ru-alkylidene carbon, see Chart 2. Detailed assign-
ment of all carbon and proton resonances is given in Supplemen-
tary data.
4
5
Ligand II
40
34
35
36
38
33
23
20
24
43
37
39
44
48
N
N
45
4
42
30
29
47
Cl
31
28
O2N
3
25
26
46
1
41 O 2019
Ru
27
2
18
17
14
N
O
21
O
N
5
13
15 16
12
11
6
20
8
9
7
Ligand I
10
O2N
Hoveyda–Grubbs first generation catalyst
8 strongly suggest
Chart 2.
a structure having the isopropoxy group not coordinated to the
metal. Based on routine 1D 1H NMR,1D 13C NMR and MS analyses, it
was difficult to determine if the chlorine was coordinated to the
metal or to the alkylidene. Moreover the carbon resonance at
244 ppm could also be compatible with an alkylidyne carbon.46
Additional 2D NMR experiments were conducted using the
DQF-COSY,47 HSQC/HETCOR48 and HMBC49 techniques in order to
unveil more structural information for the Ru-complex 6.
By means of these techniques, we were able to correlate 1H NMR
and 13C NMR resonances together through homonuclear (JHH) and
heteronuclear (1JCH and nJCH) couplings and thereby assign most of
them to the correct ligand atoms. These experiments have enabled
us to assign the proton and carbon resonances of complex 6. From
HMBC analysis it is clear that the alkylidene carbon is substituted
with one NHC ligand. This is evident from several cross-peaks, in
particular two cross peaks (4JCH) between alkylidene carbon
Figure 3. Expanded region of a superposition of two 2D 1H–13C HMBC spectra of Ru-
complex 6 in dichloromethane-d2 at 298 K. The two HMBC experiments are optimized
for two different long range couplings (nJCH¼4.0 and 10.0 Hz). The corresponding re-
gions of the 1D 600.13 MHz 1H spectrum and the 1D 1H-decoupled 150.91 MHz 13C
spectrum are shown on the top and left edge of the contour plot, respectively. The
expanded region shows the most important cross peaks connecting the 13C resonance
belonging to C1 (Chart 2) to the ligand I (see Chart 2) 1H resonances.
Figure 4. ORTEP diagram of the complexes 5 and 6 with the ellipsoids drawn at 50%
probability level. The hydrogens have been removed for clarity. Color coding: Cddark
gray, Ndblue, Odred, Rudmagenta.

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G. Occhipinti et al. / Tetrahedron 65 (2009) 7186–7194
Table 2
2.7. Single crystal X-ray diffractometer experiments
Selected bond distances and bond angles for complex 6
Several experiments were conducted in order to produce suit-
able crystals for X-ray diffractometer investigations. Complex 6 was
crystallized by slow diffusion at room temperature of n-hexane into
a concentrated solution of 6 in dichloromethane. Similarly, crys-
tallization of complex 5 was conducted at room temperature by
diffusion of n-pentane into a concentrated solution of 5 in
dichloromethane. Crystals suitable for X-ray diffractometer in-
vestigations of both of the complexes 5 and 6 were selected and
successfully investigated.
Bond distances [pm]
Bond angles [deg.]
Ru1–C1
C1–C2
C1–C4
186
148
150
232
201
211
201
136
134
137
136
Cl1–Ru1–O2
Cl1–Ru1–C1
Cl1–Ru1–C3
N1–C2–N2
N3–C3–N4
C2–C1–C4
C3–Ru1–O1
C3–Ru1–C1
O1–Ru1–C1
O1–Ru1–O2
Ru1–C1–C4
148
100
93
106
103
114
169
97
Ru1–Cl1
Ru1–O2
Ru1–O1
Ru1–C3
C2–N1
C2–N2
C3–N3
C3–N4
94
87
124
ORTEP diagrams of 5 and 6 are provided in Figure 4. Detailed
crystal data and results of the structural refinement are provided in
Supplementary data.
2.8. Complex 5
N1 N2
C1
The X-ray diffractometer analysis of complex 5 confirms an
octahedral ruthenium alkylidene structures as previously proposed
on the basis of MS and NMR analysis. Complex 5 is a rather crowded
18-electron Ru complex having the metal directly connected to
three carbon atoms (two NHCs and one alkylidene group) and three
oxygen atoms (two covalent and one dative). Interestingly, oxygen-
based ligands and carbene ligands are clustered together avoiding
location in trans-positions to each other. Consequently, carbenes
and oxygens are situated in trans–positions to each other. The
analysis of their bond distances suggests the following order of
trans-influence: alkylidene >>NHC>4-NO2-phenoxide>ether. As
a consequence of the lower trans-influence and the steric conges-
tion present in complex 5, the three Ru–O bond distances are
particularly long, see Table 1. The largest deviation is seen for the
covalent Ru1–O1 bond distance that, situated in trans-position to
the alkylidene group, has a distance (224 pm) typical for a Ru–O
dative bond.
O2N
X
O
Y
Chart 3. Bidentate ligand framework: X¼(H, Ag, Ru, C); Y¼(H, Ag, Ru).
Table 3
Geometrical differences of imidazole ring moiety in the compounds 2a, 4a, 5, and 6
Structure
N1–C1 [pm]
C1–N2 [pm]
: N1–C1–N2 [ꢀ]
2a
133
135
135
136
139
134
136
134
135
136
138
136
137
137
108
104
104
103
103
106
103
4a (Bound to Ag1)
4a (Bound to Ag2)
5 (Equatorial)
5 (Axial)
6 (Ligand I)
6 (Ligand II)
Table 1
group32 reveled that the NHC ring, that is bonded to the alkylidene
carbon should be classified as an imidazolium salt.
Selected bond distances and bond angles for complex 5
Bond distances [pm]
Bond angles [deg.]
Ru1–C6
Ru1–C8
Ru1–C14
Ru1–O1
Ru1–O2
Ru1–O3
C6–N3
C6–N4
C8–N1
C8–N2
C4–C14
200
207
184
209
224
238
139
136
136
138
147
N1–C8–N2
N3–C6–N4
103
103
85
172
95
165
167
78
2.10. Reaction mechanism
O1–Ru1–C8
O1–Ru1–C14
O2–Ru1–C6
O2–Ru1–C8
O3–Ru1–C6
O3–Ru1–C14
O2–Ru1–O1
C6–Ru1–C14
C8–Ru1–C14
A proposal for a reaction mechanism leading to the Ru com-
plexes 5 and 6 is provided in Scheme 4. The first step involves
dissociation of the Ag–O cubane cluster 4a under the formation of
the reactive intermediate, the free NHC-carbene 7, which in the
following operates as the transmetallating species exchanging
silver with ruthenium under the production of the intermediate
Ru-complex 9 and equimolar quantities of AgCl, or (m2-chloro)
(tricyclohexylphosphine)silver as a side-product, a product we
previously have identified in similar substitution reactions.30 The
84
94
95
2.9. Complex 6
intermediate Ru-complex 9 dissociates into the 14-electron
complex 10 and the free dative phosphine ligand PCy3. The 14-
electron complex 10 can react in two distinct ways, namely (1) by
a nucleophilic attack of a second carbene reactive intermediate 7
on the Ru atom, or (2) by a nucleophilic attack on the alkylidene
carbon providing the intermediate complex 11. Pathway (1) then
proceeds in a similar manner as seen in the first transmetallation
step, but this time with the 14-electron complex 10 instead of the
Hoveyda–Grubbs first generation catalyst 8. The subsequent step
of pathway (2) involves a hydride loss from 9 and the co-
ordination of the aryloxy group to ruthenium providing the novel
complex 6.
X-ray analysis of complex 6 confirms the extraordinary structure
of an imidazolium alkylidene complex. Complex 6 is the first ob-
servation of a metal alkylidene group formally substituted by an N-
heterocyclic carbene32 and is structurally related to previously
described phosphonium-substituted metal alkylidenes50,51 and
metal-substituted ketenes.52
The distance between the metal and the alkylidene carbon is
1.855 Å, which is slightly longer than for a double bond, while the
distance between the alkylidene carbon and the NHC carbon is
1.479 Å, which is slightly shorter than a C(sp2)–C(sp2) single bond,
see Table 2. The geometrical features of the NHC ring are in be-
tween those of the imidazolium salt 2a and the corresponding N-
heterocyclic carbene coordinated to silver or ruthenium, see Chart
3 and Table 3. Previous theoretical investigations conducted by our
As mentioned above, the presence of tetrahydrofuran alone or in
mixture with other solvents appears indispensable in order to ob-
tain the Ru complex 6 (following pathway 2). An explanation for
this might be that the presence of a coordinating solvent such as

G. Occhipinti et al. / Tetrahedron 65 (2009) 7186–7194
7191
= PCy3
P
Cl
Ru
O
Cl
PCy3
Cl
8
Ru
N
N
Mes
Mes
O
N
N
iPr-O
N
N Mes
Ag
O
AgCl
Ag
O
4a
7
9
Mes
N
N
Ag
Cl
O
O
O
N
N
Mes
Mes
O
Ru
Ru
7
N
N
N
O
O
N
Mes
PCy3
Cl
Mes
10
iPr
AgCl
5
Ru
iPr-O
O
Ag
PCy3
Mes
N
N Mes
N
N
Cl
O
N
N
N
N
O
O
Ag
Cl
Mes
Cl
H
9
7
H
Ru
Ru
Ru
iPr-O
O
O
O
iPr-O
iPr-O
Mes
AgH
H
N
Mes
N
N
N
N
Mes
10
11
6
Scheme 4. Proposed reaction mechanism of reaction leading to Ru complexes 5 and 6.
tetrahydrofuran may obstruct a nucleophilic attack of the NHC 7 on
the Ru metal, and thus render a nucleophilic attack on the alkyli-
dene carbon more probable.
species of any olefin metathesis catalyst should be four-coordinated,
thus in order to fullfil this requirement the complex 5, that is
a hexacoordinated Ru-alkylidene need to dissociate concurrently
two donating groups in order to become an active catalyst.
The discovery that NHC can attack the alkylidene carbon con-
stitutes another ‘non-innocent’ behavior of NHCs53 and may have
important implications in olefin metathesis. For example this can
suggest new routes to deactivation or decomposition of olefin
metathesis catalysts coordinated by NHCs and may also explain the
low yields sometimes observed in the synthesis of ruthenium
alkylidene complexes coordinated by NHC ligands.20 Finally imi-
dazolium alkylidenes could find applications in novel olefin me-
tathesis catalysts, as found for the phosphonium alkylidenes.51
3. Conclusion
The present investigation has demonstrated the synthesis of
a novel NHC–Ag(I) complex 4a, and its atypical reaction with
Hoveyda–Grubbs first generation catalyst 8. The reaction of 4a with
8 can follow two distinct mechanistic pathways to provide the
complexes 5 and 6, respectively. The reaction leading to complex 6
represents an unprecedented reaction pathway.
2.11. Catalytic activity in RCM reactions
Finally complex 6 shows a very low but detectable olefin me-
tathesis activity, while complex 5 shows no such activity.
The complexes 5 and 6 were investigated as olefin metathesis
catalyst (1 mol %) in ring closing metathesis using the standard re-
agent diethyl diallylmalonate (DEDAM) as the substrate. The trials
were conducted using either toluene or the ionic solvent 1-ethyl-3-
methylimidazolium hexafluorophosphate as reaction medium. The
reactions were conducted over a period of 16 h at slightly elevated
temperatures (80 ꢀC and 65 ꢀC, respectively). The reaction products
were analyzed by means of 1H NMR and revealed that complex 5 is
inactive as catalyst, while complex 6 shows a very low activity
(providing 0.4% of RCM product) when using 1-ethyl-3-methyl-
imidazolium hexafluorophosphate as reaction medium. The in-
activity of the Ru-complex 5 may appear somewhat unexpected
since Hermann and co-workes54 previously have reported catalytic
activity for bis-NHC-Ru-alkylidenes. We believe that the absent
catalytic activity of complex 5 is mainly due to the chelating effect of
the two bi-dentate NHC ligands, which contribute to a substantially
lowering of the entropic effect associated to the dissociation of the
NHC donor. Moreover, Schrock’s empirical rule states that the active
4. Experimental section
4.1. General
4.1.1. 1-(2,4,6-Trimethylphenyl)-1H-imidazole (3)
Glacial acetic acid (10 mL), aqueous formaldehyde (3 mL), and
aqueous glyoxal (4.6 mL) were transferred to a round bottom flask
(50 mL) and heated at 70 ꢀC. A solution of glacial acetic acid
(10 mL), ammonium acetate in water (3.08 g in 2 mL), and mesi-
tylamine (5.6 mL) was then added dropwise over a period of
30 min. The solution was continuously stirred and heated at 70 ꢀC
for 18 h. The reaction mixture was then cooled to room tempera-
ture, and the reaction mixture was added dropwise to a stirred
solution of NaHCO3 (29.4 g) in water (300 mL) when the target
product precipitated. The precipitate was isolated on a filter frit,
washed with water (3ꢂ20 mL), and air dried to obtain a brown–
yellow solid (5.18 g). The crude product was re-crystallized by using

7192
G. Occhipinti et al. / Tetrahedron 65 (2009) 7186–7194
ethyl acetate, to obtain target product 1-(2,4,6-trimethylphenyl)-
(complex 5) as well the orange fraction (complex 6) were collected
and concentrated and afforded 43 mg (40.2%) and 3.2 mg (2.8%),
respectively.
1H-imidazole in a yield of 69.7%.
Mp¼112–114 ꢀC; 1H NMR (CDCl3, 400 MHz):
¼7.43 (m, 1H),
d
7.23 (m, 1H), 6.97 (m, 2H), 6.89 (m, 1H), 2.34 (s, 3H), 1.99 (s, 6H); 13C
Mp 190–192 ꢀC (dec); 1H NMR (CD2Cl2, 500 MHz):
d¼15.90 (s,
NMR55 (CDCl3, 400 MHz):
120.2, 21.1, 17.4; IR
[cmꢁ1]: 3114, 3094, 2970, 2954, 2921, 2860,
d¼138.9, 137.6, 135.6, 133.5, 129.7, 129.1,
1H), 8.21 (d, J¼3.0 Hz, 1H), 8.03 (dd, J¼9.2, 3.0 Hz, 1H), 7.97 (d,
J¼3.0 Hz, 1H), 7.71 (dd, J¼9.3, 3.0 Hz, 1H), 7.42–7.37 (m, 1H), 7.31 (d,
J¼13.6 Hz, 1H), 7.28 (d, J¼2.1 Hz, 1H), 7.21 (d, J¼1.9 Hz, 1H), 7.02 (s,
1H), 6.81–6.91 (m, 4H), 6.67 (d, J¼9.2 Hz, 1H), 6.60 (d, J¼2.1 Hz, 1H),
6.37 (d, J¼1.9 Hz,1H), 6.21 (s,1H), 6.08 (s,1H), 6.04 (d, J¼9.3 Hz,1H),
6.01 (d, J¼13.8 Hz, 1H), 5.78 (septet, J¼6.7 Hz, 1H), 4.50 (d,
J¼13.8 Hz, 1H), 4.40 (d, J¼13.6 Hz, 1H), 2.36 (s, 3H), 1.80 (s, 3H), 1.79
(s, 3H), 1.64 (d, J¼6.7 Hz, 3H), 1.56 (d, J¼6.7 Hz, 3H), 1.48 (s, 3H), 1.04
n
1642, 1594, 1499, 1465, 1446, 1379, 1314, 1284, 1237, 1112, 1097,
1068, 1035, 1017, 971, 937, 907, 871, 816, 783, 737, 671.
4.1.2. 3-(2-Hydroxy-5-nitrobenzyl)-1-(2,4,6-trimethylphenyl)
imidazolium bromide (2a)
1-(2,4,6-Trimethylphenyl)-1H-imidazole (0.61 g, 3.3 mmol) and
2-hydroxy-5-nitrobenzyl bromide (0.76 g, 3.3 mmol) was dissolved
in toluene (8 mL) and refluxed for 18 h. During the course of the
reaction, a pale yellow solid precipitated in the reaction mixture.
After the reaction mixture was cooled to room temperature, the
solid was isolated on a filter placed in a Bu¨chner funnel, washed
with methyl-tert-butyl ether (3ꢂ10 mL) and dried under vacuum to
obtain a yield of 98.5% (1.35 g). The imidazolium salt was succes-
sively crystallized by slow diffusion of diethyl ether into a concen-
trated solution of 2a in ethanol.
(s, 6H); 13C NMR (CD2Cl2, 125.8 MHz):
d¼297.9 (d, J¼9.8), 179.0,
177.7,176.4,173.1,154.5,146.0,140.0,138.6,136.9,136.3,135.3,135.2,
134.2, 133.6, 133.5, 130.1, 129.0, 128.9, 128.5, 127.7, 126.7, 126.5,
126.4, 125.9, 125.4, 125.2, 124.4, 123.6, 123.4, 121.9, 121.8, 120.4,
117.9, 77.8, 52.3, 51.2, 22.7, 21.9, 21.1, 20.9, 18.3, 17.5, 16.75, 16.66; IR
(film)3182, 3131, 2963, 2922, 2857, 1935, 1767, 1591, 1563, 1476,
1441, 1404, 1376, 1348, 1272, 1183, 1170, 1149, 1124, 1110, 1085, 1034,
1018, 965, 932, 836, 752, 710, 702, 686, 660 cmꢁ1. MS (EI) m/z: 922.
HRMS (ESIpos) m/z 923.270932 (MþH)þ; calcd for C48H49N6O7Ru:
923.272688.
Mp¼276–278 ꢀC (dec); 1H NMR ((CD3)2SO, 400 MHz):
¼11.83
d
(br, 1H), 9.59 (s, 1H), 8.39 (s, 1H), 8.21 (d, J¼8.6 Hz, 1H), 8.06 (s, 1H),
7.93 (s, 1H), 7.20–7.07 (m, 3H), 5.56 (s, 2H), 2.32 (s, 3H), 2.01 (s, 6H);
4.1.5. Complex 6
13C NMR ((CD3)2SO, 400 MHz):
d
¼162.4, 140.3, 139.4, 138.2, 134.3,
131.2, 129.2, 127.1, 126.9, 124.1, 123.3, 121.4, 115.9, 48.3, 20.6, 16.9; IR
[cmꢁ1]: 3161, 3067, 2943, 1617, 1593, 1567, 1549, 1524, 1495, 1437,
Hoveyda first generation catalyst (135 mg, 2.25ꢂ10ꢁ4 mol) and
silver complex 4a (200 mg, 4.45ꢂ10ꢁ4 mol) were transferred to
a 25 mL Schlenk flask. The flask was evacuated and back filled with
argon. Then 6 mL of dry THF were added to the Schlenk flask under
argon. The mixture was stirred at reflux for 7 h, the mixture cooled
to room temperature and the solvent removed under reduced
pressure. The residue, a dark-brown solid was passed through
a silica gel column using diethyl ether/hexanes (7:3) as eluent. The
green fraction (complex 5) as well as the orange fraction (complex 6)
were collected and concentrated and afforded, respectively,13.2 mg
(6.3%) and 25.5 mg (11.9%).
n
1388, 1340, 1281, 1197, 1154, 1133, 1118, 1092, 1068, 935, 925, 897,
856, 843, 831, 820, 790, 770, 753, 747, 732, 700, 664; MS (ESI): m/z:
338 [417ꢁBr]þ. Anal. Calcd for C19H20N3O3Br: C, 54.56; H, 4.82; N,
10.05; Found: C, 54.84; H, 4.76; N, 9.88.
4.1.3. Synthesis of the silver complex (4a)
3-(2-Hydroxy-5-nitrobenzyl)-1-(2,4,6-trimethylphenyl) imida-
zolium bromide (500 mg, 1.195ꢂ10ꢁ3 mol) and silver (I) oxide
(569.4 mg, 2.457ꢂ10ꢁ3 mol) were added to a round bottom flask
Mp 193–195 ꢀC (dec); 1H NMR (CD2Cl2, 500 MHz):
d¼8.05–8.00
equipped with
a
cooler. Then, dried THF and benzene
(m, 2H), 7.83 (d, J¼2.9 Hz, 1H), 7.79 (dd, J¼9.1, 2.9 Hz, 1H), 7.25 (ddd,
J¼8.5, 7.3, 1.8 Hz, 1H), 7.20 (d, J¼2.0 Hz, 1H), 6.99 (br s, 1H), 6.93 (d,
J¼2.0 Hz, 1H), 6.88 (br s, 1H), 6.75–6.69 (m, 2H), 6.55–6.59 (m, 2H),
6.47 (dd, J¼7.6, 1.8 Hz, 1H), 6.38 (d, J¼2.0 Hz, 1H), 6.34 (d, J¼9.1 Hz,
1H), 6.27 (td, J¼7.3, 0.8 Hz, 1H) 6.11 (d, J¼14.1 Hz, 1H), 5.56 (d,
J¼14.1 Hz, 1H), 4.55 (septet, J¼6.0 Hz, 1H), 4.48 (d, J¼14.1 Hz, 1H),
3.80 (d, J¼14.1 Hz, 1H), 2.35 (s, 3H), 2.16 (s, 6H), 1.85 (s, 3H), 1.59 (s,
3H), 1.47 (s, 3H), 1.37 (d, J¼6.0 Hz, 3H), 1.22 (d, J¼6.0 Hz, 3H); 13C
(8.5 mLþ8.5 mL), and molecular sieves 4 Å (1.115 g) were added. The
reaction mixture was stirred at reflux for 3 h, cooled to room tem-
perature and diluted with CH2Cl2 (80 mL). The mixture was then fil-
tered through a pad of Celite (2/5 cm, w/l), that was thenwashed with
CH2Cl2 (3ꢂ20 mL). The solvent was removed under reduced pressure
to provide a yellow solid, the silver complex in a yield of z65%
(342.5 mg).
Mp 178–180 ꢀC (dec); 1H NMR (CDCl3, 600 MHz):
d
¼8.14 (s, 1H),
NMR (CD2Cl2, 125.8 MHz):
d¼244.3, 179.9, 172.8, 171.5, 161.6, 149.1,
7.76 (d, J¼7.9 Hz, 1H), 7.36 (s, 1H), 7.00 (s, 2H), 6.96 (s, 1H), 5.80 (d,
145.6, 140.2, 138.7, 138.3, 138.2, 137.7, 135.0, 134.5, 134.3, 133.9,
130.7, 130.5, 129.7, 129.2, 129.1, 128.9, 127.9, 127.0, 126.2, 126.1,
125.6,125.0,124.9,123.1,123.0,121.8,121.7,120.8,120.6,120.5,111.3,
70.0, 52.2, 49.4, 22.8, 22.1, 21.1, 20.9, 19.0, 18.6, 18.3, 16.8; IR
(film)3165, 3126, 3098, 3064, 2963, 2923, 2845, 1593, 1564, 1472,
1439, 1404, 1382, 1278, 1258, 1172, 1153, 1121, 1087, 1016, 957, 932,
903, 867, 837, 792, 751, 732, 710, 697, 666 cmꢁ1. MS (EI) m/z: 956.
J¼7.9 Hz, 1H), 5.23 (s, 2H), 2.39 (s, 3H), 1.93 (s, 6H). 13C NMR (CDCl3,
500 MHz)56
124.4, 122.0, 121.7, 121.3, 50.7, 21.3, 18.0; IR
:
d
¼174.6, 139.7, 136.4, 135.2, 134.6, 129.3, 127.5, 127.0,
n n: 3127,
[cmꢁ1]: IR
2920,1589,1566,1469,1437,1408,1278,1253,1237,1188,1165,1151,
1085, 1031, 927, 854, 835, 820, 790, 768, 735, 706, 679, 652; Anal.
Calcd for C76H72O12N12Ag4: C, 51.37; H, 4.08; N, 9.46. Found: C,
50.49; H, 3.81; N, 8.92.
HRMS
(ESIpos)
m/z
957.231375
(MþH)þ;
calcd
for
C48H48ClN6O7Ru: 957.232952.
4.1.4. Complex 5
70 mg (1.17ꢂ10ꢁ4 mol) of Hoveyda first generation catalyst and
58 mg (1.31ꢂ10ꢁ4 mol) of silver complex 4a were added to a 25 mL
Schlenk flask. The flask was evacuated and back filled with argon.
Then 2.5 mL of dry toluene and 2.5 mL of dry THF were added to the
Schlenk flask under argon. The mixture was stirred at 55 ꢀC for
2.5 h, a further 46 mg of silver complex (1.03ꢂ10ꢁ4 mol) then
added and the reaction was continued at the same temperature for
another 2.5 h, after which the mixture was cooled to room tem-
perature and the solvent removed under reduced pressure. The
residue, a dark-brown solid was passed through a silica gel column
using diethyl ether/hexanes (9:1) as eluent. The green fraction
4.1.6. NMR Spectroscopy
Sample preparation: A sample of the Ru-complex 6 (z5 mg) was
transferred to a NMR tube (o.d. 5 mm, Wilmad, model 528-PP-7) and
dissolved in dichloromethane-d2.
High-resolution one-dimensional and two-dimensional 1H
(600.13 MHz) and 13C (150.91 MHz) NMR spectra were acquired on
a Bruker Biospin AVANCE AV600 spectrometer equipped with
a narrow bore UltraShieldÔ Plus magnet. A 5 mm inverse triple
resonance (1H, 13C, 15N) CryoProbe with a z-gradient coil and cryo-
genic preamplifier cooling for both the 1H and 13C channels was
used. 1D 1H NMR: The spectral width was 7184 Hz, the pulse width

G. Occhipinti et al. / Tetrahedron 65 (2009) 7186–7194
7193
M.; Oldham, W. J.; Scott, B. L.; Schake, A. R.; Costa, D. A.; Smith, W. H. Abstr. Am.
Chem. Soc. 2001, 221, U667; Oldham, W. J.; Oldham, S. M.; Scott, B. L.; Abney, K.
D.; Smith, W. H.; Costa, D. A. Chem. Commun. 2001, 1348.
3.2
points were acquired and 16 transients were averaged.1D 13C NMR:
The spectral width was typically 39,370 Hz, the pulse width 7.4
ms (45 degree flip angle), the recycling delay 2 s, 64 complex
ms
17. Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed. 1997, 36, 2163; Stauffer, S. R.;
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(45 degree flip angle), the recycling delay 2 s, 128 k complex points
were acquired and 256 transients were averaged. Broadband 1H
composite pulse decoupling was used during the acquisition.
Standard 2D gradient-selected (gs) DQF-COSY,57 1H–13C gs-HSQC,58
1H–13C gs-HMBC59 and 13C–1H HETCOR60 experiments were per-
formed. All 2D experiments were performed in the pure-absorption
mode except for HMBC and HETCOR for which magnitude mode
spectra were recorded. The HSQC and HETCOR experiments were
optimized for a 145 Hz one-bond coupling (1JCH). Two HMBC spectra
were recorded, one was optimized for a 10 Hz long-range coupling
(nJCH), the other for a 4 Hz coupling. The sample temperature was
kept at 298.0 K for all the experiments.
The NMR data were processed and displayed using both Bruker’s
TopSpin software suite version 1.3 and the general processing
program iNMR.61 The residual 1H signal of the deuterated solvent
and 13C solvent signal were used as secondary chemical shift ref-
erences for 1H (5.32 ppm) and 13C (53.8 ppm), respectively, and the
chemical shifts are thus referenced to internal solvent resonances
and reported in parts per million relative to TMS.
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Acknowledgements
25. Wang, Z. G.; Sun, H. M.; Yao, H. S.; Shen, Q.; Zhang, Y. Organometallics 2006,
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The Norwegian Research Council is acknowledged for financial
support through the KOSK program (Grant No. 160072/V30), the
NANOMAT program (Grant No. 158538/431), as well as for CPU
resources granted through the NOTUR supercomputing program.
The University of Bergen is acknowledged for financial support
through the Nanoscience program. Prof. George W. Francis is ac-
knowledged for linguistic assistance.
Supplementary data
29. Occhipinti, G.; Bjørsvik, H.-R.; Jensen, V. R. J. Am. Chem. Soc. 2006, 128, 6952.
30. Occhipinti, G.; Jensen, V. R.; Bjørsvik, H.-R. J. Org. Chem. 2007, 72, 3561.
31. Occhipinti, G.; Bjørsvik, H.-R.; To¨rnroos, K. W.; Jensen, V. R. Organometallics
2007, 26, 5803.
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2009.05.095.
32. Occhipinti, G.; Bjørsvik, H.-R.; To¨rnroos, K. W.; Fu¨ rstner, A.; Jensen, V. R.
Organometallics 2007, 26, 4383.
33. Arduengo, A.J. III; Gentry, F.P. Jr.; Taverkere, P.K.; Simmons, H.E., (E.I. Du Pont de
Nemours & Co., USA). US Patent 6,177,575, 2001.
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