Reactions of unsymmetrically substituted N-heterocyclic carbenes with Ru3(CO)12

Article abstract of DOI:10.1021/om2000647

Reactions of a series of unsymmetrically substituted N-heterocyclic carbene ligands with Ru₃(CO)₁₂ afforded a variety of mononuclear (1, 2), trinuclear (3, 4, 5, and 5'), and tetranuclear (6, 7) ruthenium complexes via intramolecular ruthenium-mediated sp₂ and sp ₃ C-H bond activations. Complexes 6 and 7 include an octahedral cluster skeleton consisting of four ruthenium atoms and two metalated carbon atoms. The formation of this type of ruthenium clusters is proposed to involve the initial C-C double-bond migration and subsequent sp₂ C-H bond activations. All new complexes were fully characterized, and the molecular structures of 1, 2, 5, 6, and 7 were determined by X-ray diffraction analysis.

Full text of DOI:10.1021/om2000647

ARTICLE
pubs.acs.org/Organometallics
Reactions of Unsymmetrically Substituted N-Heterocyclic Carbenes
with Ru3(CO)12
Congying Zhang,† Bin Li,† Haibin Song,† Shansheng Xu,† and Baiquan Wang*,†,‡
†State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071,
People’s Republic of China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China
S Supporting Information
b
ABSTRACT: Reactions of a series of unsymmetrically substi-
tuted N-heterocyclic carbene ligands with Ru3(CO)12 afforded a
variety of mononuclear (1, 2), trinuclear (3, 4, 5, and 50), and
tetranuclear (6, 7) ruthenium complexes via intramolecular
ruthenium-mediated sp2 and sp3 CꢀH bond activations. Com-
plexes 6 and 7 include an octahedral cluster skeleton consisting
of four ruthenium atoms and two metalated carbon atoms. The
formation of this type of ruthenium clusters is proposed to
involve the initial CꢀC double-bond migration and subsequent
sp2 CꢀH bond activations. All new complexes were fully
characterized, and the molecular structures of 1, 2, 5, 6, and 7
were determined by X-ray diffraction analysis.
’ INTRODUCTION
(tBu2Im = 1,3-bis-tert-butylimidazol-2-ylidene) and Ad2Im(Ad2Im = 1,
3-bis-adamantylimidazol-2-ylidene) with Ru3(CO)12 gave the
abnormal NHC ruthenium complexes Ru3(tBu2Im)(CO)11 and
Ru3(Ad2Im)(CO)11, where the NHC ligands coordinate one of
the ruthenium metal centers at their C4 position probably
because of steric effects from bulky subsitituents.7a,b Thermal
treatment of these two NHC ruthenium complexes led to further
CꢀH bond activation at the C5 position, generating unprece-
dented complexes including “double abnormal” NHC ligands, in
which the NHC heterocycles interact with the Ru3 core through a
five-centerꢀfour-electron (5c-4e) manner. These reports clearly
indicate that both reaction conditions and substrates (e.g.,
N-substituents at NHC ligands) are equally important in deter-
mining what types of ruthenium carbonyl clusters are formed.
Very recently, we found that the reactions of indenyl-functio-
nalized NHC ligands with Ru3(CO)12 gave two ruthenium
clusters via unexpected intramolecular multiple CꢀH activation
processes (Scheme 1).10 Herein we have systematically investi-
gated the reactions of various unsymmetrically substituted NHC
ligands with Ru3(CO)12. What is most remarkable is that we
observed the formation of tetranuclear ruthenium clusters
with four ruthenium atoms and two metalated carbon atoms
(i.e., a CꢀC triple bond) constituting an octahedral geometry.
The formation of the cluster apparently involves the initial
During the past decades, there has been considerable interest
in N-heterocyclic carbene (NHC) ruthenium complexes owing
to their wide applications as efficient catalysts in a variety of
homogeneously catalytic systems such as CꢀC bond forma-
tion1,2 and olefin metathesis.2 The first NHC-coordinated ruthe-
nium carbonyl complex, Ru3(CO)11(IEt2H2), was synthesized
by Lappert and Pye in 1977 through the reaction of Ru3(CO)12
with the electron-rich carbene dimer 1,3-diethylimidazolin-2-
ylidene dimer, (IEt2H2)2.3 Note that the first stable free NHC
was isolated by Arduengo and co-workers in 1991.4 This
pioneering work has stimulated many cluster chemists to explore
the N-heterocyclic carbene chemistry of multinuclear transition
metal carbonyls.5ꢀ11 Since ruthenium is a transition metal facilely
forming polynuclear clusters, a wide range of polynuclear carbo-
nyl ruthenium complexes carrying NHC ligands have been
reported.5ꢀ10 Cabeza and co-workers reported a series of normal
NHC ruthenium carbonyl complexes Ru3(R2Im)(CO)11 (R2Im = 1,
3-dialkylimidazol-2-ylidene, R = Me, Et, Mes), in which the NHC
ligands coordinate one of the ruthenium metal centers at their
C2 position.5 Surprisingly, thermal treatment of Ru3(Me2Im)-
(CO)11 gave Ru3(μ-H)2(MeImCH)(CO)9 via the unusual
reversible cleavage of two CꢀH bonds of a N-methyl group.
Furthermore, two novel complexes containing bridging carbyne
and carbide ligands were also formed via the activation of
multiple CꢀX (X = H, N) bonds in Ru3(μ-H)2(MeImCH)-
(CO)9 under harsh reaction conditions.5e Whittlesey and
Received: January 22, 2011
Published: May 09, 2011
t
co-workers reported that reactions of bulky NHCs Bu2Im
r
2011 American Chemical Society
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Scheme 1
Scheme 2
CꢀC double-bond migration and subsequent sp2 CꢀH bond
activation.
Figure 1. ORTEP diagram of 1. Thermal ellipsoids are shown at the
30% level. All the hydrogen atoms are omitted for clarity. Selected
bond lengths [Å] and angles [deg]: Ru(1)ꢀC(12) 2.05(2), Ru(1)ꢀ
C(4) 2.12(2), Ru(1)ꢀI(1) 2.935(3), C(12)ꢀRu(1)ꢀC(4) 73.5(6),
C(12)ꢀRu(1)ꢀI(1) 80.4(4), C(4)ꢀRu(1)ꢀI(1) 92.7(4), C(3)ꢀRu-
(1)ꢀC(1) 96.5(7), C(2)ꢀRu(1)ꢀC(1) 80.8(7), C(2)ꢀRu(1)ꢀC(3)
88.1(7).
’ RESULTS AND DISCUSSION
Reaction of 1-Phenyl-3-methylimidazol-2-ylidene with
Ru3(CO)12. Deprotonation of 1-phenyl-3-methylimidazolium
iodide using the previously well-established method (nBuLi in
THF at 0 °C) in our group,12 followed by reaction with 1/3 equiv
of Ru3(CO)12 and 1 equiv of anhydrous NaI at room tempera-
ture, gave the mononuclear complex 1 in 76% yield (Scheme 2).
The absence of the resonance signal for the proton at the C2
for the formation of the byproducts is currently unclear. How-
ever, the employment of an excess amount of the nucleophile Iꢀ
apparently favors the formation of 1 and simultaneously inhibits
the formation of the byproducts.
1
position of the imidazole ring in the H NMR spectrum of 1
suggests that the NHC ligand coordinates the ruthenium center
at the C2 position. The integration of the resonance signals of
protons in the aromatic region shows only four protons instead
of five protons besides two protons attributed to the NHC ligand,
implying that sp2 CꢀH bond activation on the phenyl ring might
occur. The IR spectrum of 1 shows three terminal carbonyl
absorptions at 2096, 2048, and 2013 cmꢀ1, indicating no
bridging carbonyl groups exist in 1. Since complex 1 contains a
RuꢀI bond (confirmed by single-crystal X-ray diffraction ana-
lysis) in its molecule, we propose that the formation of 1 involves
the iodination of the RuꢀH intermediate (A), probably formed
via the oxidative addition of the CꢀH bond on the phenyl group
to the Ru(0) atom, by lithium iodide, as shown in Scheme 2.
It should be noted that without addition of anhydrous NaI the
yield of 1 decreased from 76% to 16%. In this case, in addition to
1, some byproducts were also isolated from the reaction mixture
by chromatography. Their 1H NMR spectra show the existence
of a resonance signal attributed to the C2ꢀH of the imidazole
ring. Unfortunately we could not fully characterize these bypro-
ducts. On the basis of the limited data and the previous literature
data,5b we tentatively propose that these unidentified com-
pounds might contain ruthenium cluster anions and a 1-phenyl-
3-methylimidazolium cation in their molecules. The mechanism
The molecular structure of 1 was determined by X-ray diffrac-
tion analysis (Figure 1). The NHC ligand normally coordinates
to the Ru atom at its C2 position. Complex 1 displays an octa-
hedral geometry with the Ru center surrounded by six ligands:
the NHC, the phenyl ring, three carbonyls, and one iodine atom.
The bond lengths of Ru(1)ꢀC(12) [2.05(2) Å], Ru(1)ꢀC(4)
[2.12(2) Å], and Ru(1)ꢀI(1) [2.935(3) Å] are longer than the
corresponding RuꢀCcarbene [2.007(8) Å], cyclometalated
RuꢀC [2.060(9) Å], and RuꢀI [2.7217(12) Å] distances found
in [(p-cymene)Ru(MeImPh)I].12 The C(4)ꢀRu(1)ꢀC(12)
bond angle is 73.5(6)°, slightly smaller than that of [(p-
cymene)Ru(MeImPh)I] [76.3(4)°].12
The five-membered ruthenacycle formed by Ru(1), C(4),
C(9), N(1), and C(12) adopts a coplanar conformation. The
ruthenacycle shares the same plane as both the imidazole ring
and the phenyl ring. The three carbonyl ligands coordinating to
the Ru atom are mutually perpendicular.
Reaction of 1-tert-Butyl-3-methylimidazol-2-ylidene with
Ru3(CO)12. Aromatic sp2 CꢀH bond activation has been
achieved in complex 1, and sp3 CꢀH activation might be very
likely under the same conditions because numerous examples
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ARTICLE
Scheme 3
involving sp3 CꢀH activation in the NHC ruthenium system are
known.5a,c,e,6f,10,12,13 When 1-tert-butyl-3-methylimidazolium io-
dide was used as the NHC precursor, the expected sp3 CꢀH
activated product 2 (an analogue of complex 1) was obtained
in 65% yield (Scheme 3). The molecular structure of 2 was also
confirmed by single-crystal diffraction analysis. Just like the
mechanism we propose for the formation of complex 1, the
formation of 2 might also involve the oxidative addition of
the CꢀH bond of the tert-butyl group to the Ru(0) atom and
the subsequent iodination of the RuꢀH intermediate (B) by
lithium iodide. Similarly, without addition of anhydrous NaI, the
yield of 2 decreased from 65% to 15%.
Figure 2. ORTEP diagram of 2. Thermal ellipsoids are shown at the
30% level. All the hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and angles [deg]: Ru(1)ꢀC(8) 2.094(13), Ru(1)ꢀC(4)
2.136(13), Ru(1)ꢀI(1) 2.7699(15), C(8)ꢀRu(1)ꢀC(4) 79.0(5), C-
(8)ꢀRu(1)ꢀI(1) 87.2(3), C(4)ꢀRu(1)ꢀI(1) 90.4(4), C(1)ꢀRu-
(1)ꢀC(3) 94.5(5), C(1)ꢀRu(1)ꢀC(2) 93.7(6), C(3)ꢀRu(1)ꢀC(2)
96.2(5).
1
In the H NMR spectrum of 2, one singlet for the tert-butyl
Scheme 4
group at the imidazolium ring is replaced by two singlets at 1.61
and 1.88 ppm, attributed to the two diastereotropic methyl
groups of the original tert-butyl group, and two doublets at
3.04 and 2.27 ppm (2JHꢀH = 11.1 Hz), attributed to the inequi-
valent geminal protons of the metalated CH2 group from the sp3
1
CꢀH bond activation of the original tert-butyl group. The H
NMR spectrum of 2 also shows a singlet at 6.91 ppm for the
protons at the C4 and C5 positions of the imidazole ring. This
reveals that the NHC ligand coordinates normally to the Ru
center at the C2 position, which is different from the case where
the bulky NHC carrying two tert-butyl groups coordinates to the
ruthenium center at the C4 position.7a The IR spectrum of 2
shows three terminal carbonyl absorptions at 2011, 1973, and
activation processes (Scheme 1).10 Herein we would like to
further explore the scope of the novel reaction by replacing the
indenyl group with a phenyl group in NHC ligands. The reaction
of phenylethyl N-heterocyclic carbene (1-phenylethyl-3-methy-
limidazol-2-ylidene), prepared in situ from the deprotonation of
the corresponding imidazolium salt with nBuLi and subsequent
filtration through a Celite column, with Ru3(CO)12 in toluene at
room temperature gave the NHC-coordinated trinuclear ruthe-
nium carbonyl complex Ru3[(Me)(PhCH2CH2)Im](CO)11 (3)
in 76% yield (Scheme 4).
1925 cmꢀ1
.
The molecular structure of 2 is illustrated in Figure 2. It clearly
shows that the NHC ligand coordinates to the Ru atom at the C2
position. One of the nine CꢀH bonds of the tert-butyl group is
activated to form a five-membered ruthenacycle. Like complex 1,
2 also has an octahedral molecular skeleton. The bond lengths of
Ru(1)ꢀC(8) [2.094(13) Å] and Ru(1)ꢀC(4) [2.136(13) Å]
are longer than those found in [(p-cymene)Ru(MeImtBu)I]
[2.023(3), 2.134(3) Å]12 and 1 [2.052(18), 2.115(17) Å]. The
bond length of Ru(1)ꢀI(1) [2.7699(15) Å] is also longer than
that of [(p-cymene)Ru(MeImtBu)I] [2.7423(4) Å]12 but shorter
than that of 1 [2.935(3) Å]. The five-membered ruthenacycle
formed by Ru(1), C(4), C(5), N(2), and C(8) adopts a coplanar
conformation. The imidazole ring shares the same plane as the
ruthenacycle. The chelate bite angle [C(8)ꢀRu(1)ꢀC(4)] is
79.0(5)°, slightly larger than those of [(p-cymene)Ru(MeImtBu)I]
[77.24(11)°]12 and 1 [73.5(6)°]. Three carbonyl ligands coordi-
nating to the Ru center are also mutually perpendicular.
In the 1H NMR spectrum of 3, the absence of the resonance
signal for the proton at the C2 position of the imidazole ring
suggests that the NHC ligand coordinates the ruthenium center
at the C2 position. The presence of signals for both N-methyl and
N-phenylethyl protons suggests no CꢀH bond activation takes
place. The IR spectrum of 3 shows multiple terminal carbonyl
absorptions at 2095, 2039, 2004, 1961, and 1949 cmꢀ1, suggest-
ing the possible existence of a trinuclear ruthenium cluster. On
the basis of the aforementioned data together with the elemental
analysis of 3, we propose that complex 3 has a molecular
structure with only one carbonyl group of Ru3(CO)12 substi-
tuted by the NHC ligand.
Thermal treatment of 3 at 70 °C in toluene gave three new
complexes together with Ru3(CO)12 (Scheme 5). The result
is consistent with that previously reported by Cabeza and co-
workers5a but completely different from our observation between
Reaction of 1-Phenylethyl-3-methylimidazol-2-ylidene
with Ru3(CO)12. We have reported that the reactions of inde-
nyl-functionalized NHC ligands with Ru3(CO)12 afforded two
ruthenium complexes via unexpected sp3 and sp2 multiple CꢀH
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Scheme 5
Figure 3. ORTEP diagram of 5. Thermal ellipsoids are shown at the 30% level. All the hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [deg]: Ru(1)ꢀRu(3) 2.8143(16), Ru(1)ꢀRu(2) 3.0421(17), Ru(2)ꢀRu(3) 2.8161(15), Ru(1)ꢀC(9) 2.125(7), Ru(1)ꢀC(24) 2.080(8),
Ru(2)ꢀC(10) 2.091(7), Ru(3)ꢀC(9) 2.147(8), Ru(3)ꢀRu(1)ꢀRu(2) 57.32(3), Ru(3)ꢀRu(2)ꢀRu(1) 57.27(4), Ru(1)ꢀRu(3)ꢀRu(2) 65.41(4),
Ru(1)ꢀC(9)ꢀRu(3) 82.4(2).
the reactions of indenyl-functionalized NHCs with Ru3-
(CO)12.10 In the present case only two sp3 CꢀH activations
were observed, while in the case of indenyl-functionalized NHCs
both sp3 and sp2 CꢀH activations took place.10 This strongly
supports that reaction substrates are of significant importance to
the formation of different products in the ruthenium NHC
system. Note that the formation of Ru3(CO)12 and complexes 5
and 50 possibly involves ligand exchange between 3 and 4
because we found that the reaction between an excess amount
of the NHC ligand and Ru3(CO)12 gave only complexes 5
and 50.
[2.089(5) Å].5a The bond lengths of Ru(1)ꢀC(9) [2.125(7) Å]
and Ru(3)ꢀC(9) [2.147(8) Å] are longer than those in [Ru3-
(μ-H)2(μ3-MeImCH)(CO)9] [2.121(6), 2.106(5) Å].5a The IR
spectrum of a mixture of 5 and 50 shows only terminal carbonyl
absorptions at 2061, 2023, 2001, and 1942 cmꢀ1
.
Reactions of Alkene-Functionalized NHCs with Ru3(CO)12.
It is well-known that the electron-rich alkenes can be used as π-
donors or directing groups for the chelation-assisted CꢀH bond
activation during their reactions with transition metals.14 There-
fore, we explore reactions of alkene-functionalized NHCs with
Ru3(CO)12 under different conditions. Alkene-functionalized
imidazolium salts, 1-(but-3-en-1-yl)-3-mesitylimidazolium bro-
mide and 1-(but-3-en-1-yl)-3-methylimidazolium bromide, were
readily synthesized by nucleophilic substitution of 4-bromobut-
1-ene with 1-mesitylimidazole and 1-methylimidazole, respec-
tively. The reactions of alkene-functionalized N-heterocyclic
carbenes, prepared in situ from the deprotonation of the above-
1
The H NMR spectrum of a mixture of complexes 5 and 50
indicates that they are geometric isomers in the ratio of 1:2. We
could not separate them by flash chromatography. Fortunately,
we obtained the pure single crystals of 5 by recrystallization of a
mixture of 5 and 50 from a CH2Cl2/hexane solution at room
temperature. The molecular structure of 5 was determined
by X-ray diffraction analysis (Figure 3). The angles of Ru(3)ꢀ
Ru(2)ꢀRu(1), Ru(3)ꢀRu(1)ꢀRu(2), and Ru(1)ꢀRu(3)ꢀ
Ru(2) are 57.27(4)°, 57.32(3)°, and 65.41(4)°, respectively. The
bond lengths of Ru(2)ꢀC(10) [2.091(7) Å] and Ru(1)ꢀ
C(24) [2.080(8) Å] are practically identical with the Ruꢀ
Ccarbene distances found in [Ru3(μ-H)2(μ3-MeImCH)(CO)9]
n
mentioned imidazolium salts with BuLi and subsequent filtra-
tion through a Celite column, with Ru3(CO)12 in refluxing
toluene gave the ruthenium carbonyl complexes 6 and 7 in
23% and 22% yields, respectively (Scheme 6). Such low yields for
6 and 7 are due to the decomposition of Ru3(CO)12 under such a
harsh refluxing condition.
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Scheme 6
Figure 5. ORTEP diagram of 7 showing one of four independent
molecules in the unit cell. Thermal ellipsoids are shown at the 30% level.
All the hydrogen atoms and solvent are omitted for clarity. Selected
bond lengths [Å] and angles [deg] (parameters are given for one
independent molecule, and there are no significant differences among
the four symmetry-independent molecules): Ru(1)ꢀRu(2) 2.7468(5),
Ru(1)ꢀRu(3) 2.7922(5), Ru(1)ꢀRu(4) 2.7740(5), Ru(2)ꢀRu(3)
2.7310(5), Ru(3)ꢀRu(4) 2.7420(5), Ru(1)ꢀC(13) 2.138(4), Ru(2)ꢀ
C(13) 2.220(4), Ru(2)ꢀC(14) 2.202(4), Ru(3)ꢀC(14) 2.150(4),
Ru(4)ꢀC(13) 2.248(4), Ru(4)ꢀC(14) 2.190(4), Ru(4)ꢀC(18)
2.026(4), C(13)ꢀC(14) 1.466(6), N(2)ꢀC(18)ꢀN(1) 105.2(4),
N(1)ꢀC(18)ꢀRu(4) 117.3(3), C(18)ꢀRu(4)ꢀC(14) 78.39(16), C-
(18)ꢀN(1)ꢀC(15) 121.2(4), N(1)ꢀC(15)ꢀC(14) 108.3(3), C-
(15)ꢀC(14)ꢀRu(4) 111.1(3), C(14)ꢀC(13)ꢀC(12) 124.5(4),
C(13)ꢀC(14)ꢀC(15) 124.4(4).
Figure 4. ORTEP diagram of 6 showing one of three independent
molecules in the unit cell. Thermal ellipsoids are shown at the 30% level.
All the hydrogen atoms are omitted for clarity. Selected bond lengths
[Å] and angles [deg] (parameters are given for one independent
molecule, and there are no significant differences among the three
symmetry-independent molecules): Ru(1)ꢀRu(2) 2.7344(9), Ru(1)ꢀ
Ru(4) 2.7533(7), Ru(2)ꢀRu(3) 2.7701(8), Ru(2)ꢀRu(4) 2.8014(7),
Ru(3)ꢀRu(4) 2.7710(8), Ru(1)ꢀC(17) 2.218(5), Ru(1)ꢀC(18)
2.198(5), Ru(2)ꢀC(17) 2.144(5), Ru(3)ꢀC(15) 2.012(5), Ru(3)ꢀC-
(17) 2.180(5), Ru(3)ꢀC(18) 2.242(5), Ru(4)ꢀC(18) 2.136(5),
C(17)ꢀC(18) 1.472(7), C(15)ꢀRu(3)ꢀC(17) 78.02(19), N(2)ꢀC-
(15)ꢀN(1) 105.3(4), C(15)ꢀN(2)ꢀC(16) 120.2(4), N(2)ꢀC-
(16)ꢀC(17) 108.1(4), C(16)ꢀC(17)ꢀRu(3) 112.4(3), C(18)ꢀ
C(17)ꢀC(16) 124.5(4), C(17)ꢀC(18)ꢀC(19) 124.9(4).
terminal carbonyls except for the Rucarbene atom center, which is
coordinated by two terminal carbonyls and one NHC ligand. The
bond lengths of RuꢀCcarbene [2.012(5) Å for 6, 2.026(4) Å for 7]
lie in the range of RuꢀCcarbene distances found in other
ruthenium NHC complexes.5ꢀ10 The CꢀC lengths of the two
metalated carbons [1.472(7) Å for 6, 1.466(6) Å for 7] are
between the typical CꢀC single bond and the CꢀC double
bond. The metalated carbon resonances in the 13C{1H} NMR
spectra of 6 and 7 are both at 67.6 ppm.
The structures of 6 and 7 are isostructural with the previous
reported butterfly cluster Ru4(μ4-PhC2Ph)(CO)12, which was
obtained by the reaction of diphenylacetylene with Ru3(CO)12.15
But in our cases the formation of 6 and 7 apparently involves the
initial migration of the terminal CꢀC double bond to the internal
double bond and subsequent transformation from the CꢀC
double bond to the CꢀC triple bond via ruthenium-mediated sp2
CꢀH bond activations. Although the detailed mechanistic process
and the fate of two hydrogen atoms are still unclear, the evolvement
of hydrogen gas under such harsh conditions is very likely.
The absence of the resonance signals for C2ꢀH of the original
imidazolium ring and the three protons of the homoallyl group in
the original imidazolium salts in the 1H NMR spectra of 6 and 7
indicates that intramolecular CꢀH activations take place in the
formation of both 6 and 7. The signals for the inequivalent
geminal protons in the NꢀCH2 group are split into two doublets at
5.03, 4.40 ppm (2JHꢀH = 13.1 Hz) and 5.13, 4.51 ppm (2JHꢀH =13.2)
for 6 and 7, respectively. Their IR spectra show only terminal
carbonyl absorptions at 2072 and 2008 (broad band) cmꢀ1 for 6
and 2072, 2031, 2011, 1968, and 1934 cmꢀ1 for 7.
We were very glad to obtain the single crystals of both 6 and 7
and confirm their molecular structures (Figures 4 and 5) by X-ray
diffraction analysis. Both complexes show an identical octahedral
molecular skeleton consisting of four Ru atoms and two meta-
lated carbon atoms. Each metalated carbon atom is connected
with three Ru atoms. The two metalated carbon atoms can also
be considered as the coordination of a CꢀC triple bond to the
four Ru atoms. Each Ru atom center is coordinated by three
’ CONCLUSION
In summary, we have investigated the reactions of a series of
unsymmetrically substituted NHCs with Ru3(CO)12 and suc-
cessfully synthesized a variety of mononuclear, trinuclear, and
tetranuclear ruthenium complexes. The formation of such com-
plexes involves interesting intramolecular sp2 and sp3 CꢀH bond
activations. This work not only highlights the importance and
abundance of ruthenium NHC cluster chemistry but also helps
understand complex reaction intermediates and mechanisms in
ruthenium-mediated catalytic systems.
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’ EXPERIMENTAL SECTION
The second, yellow band afforded 30 mg (33%) of 4 as yellow crystals.
The third, red band afforded unreacted 3 (5 mg). The fourth, yellow
band afforded 10 mg (9%) of a mixture of 5 and 50 as yellow crystals.
Complex 4: mp 114ꢀ116 °C. Anal. Calcd for C21H14N2O9Ru3: C,
General Considerations. Schlenk and vacuum line techniques
were employed for all manipulations. All solvents were distilled from
appropriate drying agents under argon prior to use. H NMR spectra
1
1
34.01; H, 1.90; N, 3.78. Found: C, 34.12; H, 1.71; N, 3.63. H NMR
were recorded on a Bruker AV400 or Varian AS-400 at room tempera-
ture with TMS as internal standard, while IR spectra were recorded as
KBr disks on a Bruker Tensor 27 or Nicolet 380 FT-IR spectrometer.
Elemental analyses were performed on a Perkin-Elmer 240C analyzer. All
(CDCl3): δ 7.32ꢀ7.23 (m, 3H, ArꢀH), 7.10 (d, J = 7.21 Hz, 2H,
ArꢀH), 6.89 (s, 1H, NCH), 6.80 (s, 1H, NCH), 6.13 (s, 1H, CH), 4.04
(t, J = 6.74 Hz, 2H, CH2), 2.96 (t, J = 7.34 Hz, 2H, CH2), ꢀ12.68 (s, 1H,
μ2-RuꢀH), ꢀ16.61 (s, 1H, μ2-RuꢀH). 13C{1H} NMR (100 MHz,
CDCl3): δ 199.0 (CO), 198.4 (CO), 169.6 (carbeneꢀC), 136.9
(ArꢀC), 128.8 (ArꢀC), 127.0 (ArꢀC), 119.8 (ArꢀC), 118.3 (NCH),
103.9 (NCH), 52.2 (CH2), 37.4 (CH2), 29.7 (CH). IR (νCO): 2098,
imidazoliums were prepared according to literature methods.9,12 All 13
C
spectra signals were assigned according to published literature.5a,7c,8ꢀ10
Reaction of 1-Phenyl-3-methylimidazol-2-ylidene with
Ru3(CO)12. A mixture of [(Me)(Ph)ImH]þIꢀ (0.286 g, 1 mmol)
and nBuLi (1 mmol) was stirred in THF (30 mL) for 1 h under 0 °C.
Ru3(CO)12 (0.213 g, 0.33 mmol) and anhydrous NaI (0.150 g 1 mmol)
were then added. The mixture was stirred at room temperature for 6 h.
The solution turned dark red. After removal of solvents under vacuum,
the residue was chromatographed on an alumina column. Elution with
petroleum ether/CH2Cl2 (3:1) gave a red band, which afforded 0.362 g
(76%) of 1 as purple-red crystals. Mp: 166ꢀ168 °C. Anal. Calcd for
C13H9IN2O3Ru: C, 33.28; H, 1.93; N, 5.97. Found: C, 33.42; H, 1.89; N,
5.82. 1H NMR (CDCl3): δ 7.83 (d, J = 6.79 Hz, 1H, ArꢀH), 7.57 (s, 1H,
NCH), 7.27 (d, J = 8.07 Hz, 1H, ArꢀH), 7.12 (m, 2H, ArꢀH), 7.05 (s,
1H, NCH), 3.96 (s, 3H, CH3). 13C{1H} NMR (100 MHz, CDCl3): δ
190.8 (CO), 188.7 (CO), 188.4 (CO), 174.7 (carbeneꢀC), 146.5
(ArꢀC), 146.0 (ArꢀC), 141.2 (ArꢀC), 126.5 (ArꢀC), 124.9
(ArꢀC), 123.3 (ArꢀC), 115.7 (NCH), 112.8 (NCH), 38.6 (CH3). IR
2062, 2035, 2011, 1939, 1977, 1966 cmꢀ1
.
Complexes 5 þ 50: mp 138ꢀ140 °C. Anal. Calcd for C32H28-
N4O8Ru3: C, 42.71; H, 3.14; N, 6.23. Found: C, 42.47; H, 3.36; N,
6.29. 1H NMR (CDCl3): δ 7.36ꢀ7.22 (m, 8H, ArꢀH), 7.16ꢀ7.12 (m,
2H, ArꢀH), 6.95 (s), 6.86 (s), 6.80 (s), 6.79 (s), 6.76 (s), 6.72 (s),
6.62ꢀ6.61 (m), 6.08 (s) (total 4H, NCH), 6.14 (s, 5), 5.97 (s, 50) (total
1H, CH), 4.62 (m), 4.37 (m), 4.16ꢀ4.08 (m), 4.02ꢀ3.93 (m) (total 4H,
CH2), 3.89 (s, 3H, NCH3), 3.22ꢀ3.15 (m), 3.11ꢀ3.03 (m), 3.00ꢀ2.94
(m) (total 4H, CH2), ꢀ11.90 (s, 5), ꢀ12.11 (s, 50) (total 1H, μ2-
RuꢀH), ꢀ15.98 (s, 5), ꢀ17.43 (s, 50) (total 1H, μ2-RuꢀH). 13C{1H}
NMR (100 MHz, CDCl3): δ 205.5 (CO), 205.3 (CO), 201.4 (CO),
201.2 (CO), 200.9 (CO), 200.1 (CO), 200.1 (CO), 199.9 (CO), 199.9
(CO), 199.3 (CO), 198.9 (CO), 198.9 (CO), 193.7 (CO), 193.5 (CO),
193.4 (CO), 190.0 (CO), 189.7 (CO), 181.5 (carbeneꢀC), 169.8
(carbeneꢀC), 169.0 (carbeneꢀC), 137.8 (ArꢀC), 137.6 (ArꢀC),
137.5 (ArꢀC), 137.4 (ArꢀC), 129.0 (ArꢀC), 128.9 (ArꢀC), 128.8
(ArꢀC), 128.8 (ArꢀC), 128.7 (ArꢀC), 128.6 (ArꢀC), 127.0 (ArꢀC),
126.8 (ArꢀC), 122.8 (NCH), 121.0 (NCH), 119.9 (NCH), 118.7
(NCH), 117.5 (NCH), 117.3 (NCH), 108.6 (NCH), 98.0 (NCH),
53.3 (CH2), 52.1 (CH2), 40.2 (CH2), 37.6 (CH2), 37.4 (CH2), 31.6
(νCO): 2096, 2048, 2013 cmꢀ1
.
Reaction of 1-tert-Butyl-3-methylimidazol-2-ylidene with
Ru3(CO)12. Using a procedure similar to that described for 1, reaction
of [(Me)(tBu)ImH]þIꢀ (0.266 g, 1 mmol) with nBuLi (1 mmol),
Ru3(CO)12 (0.213 g, 0.33 mmol), and anhydrous NaI (0.150 g 1 mmol)
gave 2 (0.300 g) as purple-red crystals in 65% yield. Mp: 123ꢀ125 °C.
Anal. Calcd for C11H13IN2O3Ru: C, 29.41; H, 2.92; N, 6.24. Found: C,
29.12; H, 3.04; N, 6.21. 1H NMR (CDCl3): δ 6.91 (s, 2H, NCH), 3.81
(s, 3H, NꢀCH3), 3.04 (d, J = 11.1 Hz, 1H, CH2), 2.27 (d, J = 11.1 Hz,
1H, CH2), 1.61 (s, 3H, CH3), 1.38 (s, 3H, CH3). 13C{1H} NMR (100
MHz, CDCl3): δ 193.5 (CO), 191.3 (CO), 187.7 (CO), 170.3
(carbeneꢀC), 123.5 (NCH), 117.6 (NCH), 66.4 (CH2), 38.6 (CH3),
(CH), 22.6 (CH), 14.1 (CH3). IR (νCO): 2061, 2023, 2001, 1942 cmꢀ1
.
Reaction of 3-Homoallyl-1-methylimidazol-2-ylidene
with Ru3(CO)12. To a solution of 0.109 g (0.5 mmol) of imidazolium
salt [(Me)(homoallyl)ImH]þBrꢀ in 30 mL of toluene at 0 °C was added
dropwise a nBuLi hexane solution (0.5 mmol), and the resulting mixture
was stirred for another 1 h. Then the solution was passed through a
Celite column to remove LiBr, to which 0.320 mg (0.5 mmol) of
Ru3(CO)12 was added. The resulting mixture was stirred under reflux for
12 h. After removal of solvents, the residue was chromatographed on an
alumina column using petroleum ether/CH2Cl2 as eluent. The red band
afforded 97 mg (23%) of 6 as a red solid. Mp: 151ꢀ152 °C. Anal. Calcd
for C19H10N2O11Ru4: C, 26.96; H, 1.19; N, 3.31. Found: C, 27.12; H,
1.29; N, 3.36. 1H NMR (400 MHz): δ 7.07 (d, J = 1.44 Hz, 1H, NCH),
6.98 (d, J = 1.43 Hz, 1H, NCH), 5.03 (d, J = 13.1 Hz, 1H, CH2), 4.40
(d, J = 13.1 Hz, 1H, CH2), 3.82 (s, 3H, NCH3), 3.09 (s, 3H, CH3).
13C{1H} NMR (100 MHz, CDCl3): δ 204.9 (CO), 200.6 (CO), 199.8
(CO), 194.3 (CO), 190.9 (CO), 188.3 (CO), 186.5 (CO), 177.4
(carbeneꢀC), 123.9 (NCH), 118.6 (NCH), 67.6 (RuCRu), 45.2
32.0 (CH3), 31.7 (CH3), 25.7 (CH3). IR (νCO): 2011, 1973, 1925 cmꢀ1
.
Reaction of 1-Phenylethyl-3-methylimidazol-2-ylidene
with Ru3(CO)12. To a solution of 0.266 g (1.0 mmol) of imidazolium
salt [(Me)(PhCH2CH2)ImH]þBrꢀ in 30 mL of toluene at 0 °C was
added dropwise a nBuLi hexane solution (1.0 mmol), and the resulting
mixture was stirred for another 1 h. Then the solution was passed
through a Celite column to remove LiBr, to which 0.639 mg (1.0 mmol)
of Ru3(CO)12 was added. The resulting mixture was stirred at room
temperature for 6 h. After removal of solvents the residue was chroma-
tographed on an alumina column using petroleum ether/CH2Cl2 as
eluent. The first, yellow band afforded 73 mg (11%) of unreacted
Ru3(CO)12. The second, red band afforded 0.54 g (76%) of 3 as a red
solid. Mp: 147 °C (dec). Anal. Calcd for C23H14N2O11Ru3: C, 34.64; H,
1.77; N, 3.51. Found: C, 34.52; H, 1.81; N, 3.54. 1H NMR (CDCl3): δ
7.36 (t, J = 7.30 Hz, 2H, ArꢀH), 7.30 (d, J = 7.03 Hz, 1H, ArꢀH), 7.25
(d, J = 6.49 Hz, 2H, ArꢀH), 6.99 (d, J = 1.21 Hz, 1H, NCH), 6.87 (d, J =
1.33 Hz, 1H, NCH), 4.36 (t, J = 7.65, 8.06 Hz, 2H, CH2), 3.81 (s, 3H,
CH3), 3.11 (t, J = 7.97, 7.71 Hz, 2H, CH2). 13C{1H} NMR (100 MHz,
CDCl3): δ 204.6 (CO), 170.2 (carbeneꢀC), 136.9 (ArꢀC), 128.9
(ArꢀC), 127.1 (ArꢀC), 123.8 (NCH), 121.7 (NCH), 53.3 (CH2), 40.2
(NCH2), 38.4 (NCH3), 29.7 (CH3). IR (νCO): 2072, 2008 (br) cmꢀ1
.
Reaction of 3-Homoallyl-1-mesitylimidazol-2-ylidene
with Ru3(CO)12. Using a procedure similar to that described for 6,
reaction of [(Mes)(homoallyl)ImH]þBrꢀ (0.160 g, 0.5 mmol) with
nBuLi (0.5 mmol) and Ru3(CO)12 (0.320 g, 0.5 mmol) gave 104 mg
(22%) of 7 as red crystals. Mp: 164 °C (dec). Anal. Calcd for
C27H18N2O11Ru4: C, 34.11; H, 1.91; N, 2.95. Found: C, 34.28; H,
1
2.03; N, 3.04. H NMR (CDCl3): δ 7.25 (s, 1H, NCH), 7.08 (s, 1H,
MesꢀH), 7.05 (s, 1H, MesꢀH), 6.92 (s, 1H, NCH), 5.13 (d, J = 13.3
Hz, 1H, CH2), 4.51 (d, J = 13.2 Hz, 1H, CH2), 3.19 (s, 3H, CH3), 2.38 (s,
3H, MesꢀCH3), 2.11 (s, 3H, MesꢀCH3), 2.01 (s, 3H, MesꢀCH3).
13C{1H} NMR (100 MHz, CDCl3): δ 202.1 (CO), 201.3 (CO), 199.8
(CO), 194.3 (CO), 191.4 (CO), 188.7 (CO), 186.5 (CO), 178.7
(carbeneꢀC), 140.5 (MesꢀC) 136.4 (MesꢀC), 134.7 (MesꢀC),
(CH2), 37.4 (CH3). IR (νCO): 2095, 2039, 2004, 1961, 1949 cmꢀ1
.
A solution of 0.100 g (0.125 mmol) of 3 in 30 mL of toluene was
stirred at 70 °C for 12 h. After removal of solvent the residue was chro-
matographed on an alumina column using petroleum ether/CH2Cl2 as
eluent. The first, yellow band afforded 13 mg (15%) of Ru3(CO)12.
3034
dx.doi.org/10.1021/om2000647 |Organometallics 2011, 30, 3029–3036

Organometallics
ARTICLE
Table 1. Crystal Data and Summary of X-ray Data Collection for 1, 2, and 5ꢀ7
1a
2
5
6
7 1.5CH2Cl2
3
formula
fw
C13H9IN2O3Ru
469.19
C11H13IN2O3Ru
449.20
C32H26N4O8Ru3
897.78
C19H10N2O11Ru4
846.57
C28.5H21Cl3N2O11Ru4
1078.10
T, K
113(2)
113(2)
116(2)
116(2)
116(2)
λ, Å
0.71070
monoclinic
P21/c
0.71073
0.71073
0.71073
0.71073
cryst syst
space group
a, Å
monoclinic
C2/c
triclinic
triclinic
monoclinic
P21/n
P1
P1
7.124(6)
10.143(9)
19.925(14)
90
14.133(3)
9.1977(18)
22.062(4)
90
8.791(5)
13.011(8)
15.004(8)
88.28(2)
75.691(15)
87.41(2)
1660.9(16)
2
9.072(2)
17.853(4)
23.241(5)
73.353(7)
85.268(7)
89.581(9)
3593.8(14)
6
19.5678(16)
34.657(2)
21.7690(17)
90
b, Å
c, Å
R, deg
β, deg
γ, deg
V, Å3
Z
93.94(3)
90
93.07(3)
90
110.1910(10)
90
1436(2)
4
2863.8(10)
8
13855.8(18)
16
Dcalc, g cmꢀ3
μ, mmꢀ1
2.170
2.084
1.795
2.347
2.067
3
3.248
3.252
1.402
2.535
2.001
F(000)
888
1712
888
2412
8336
cryst size, mm
θ range, deg
0.16 ꢁ 0.14 ꢁ 0.12
2.05ꢀ25.00
7964
0.24 ꢁ 0.20 ꢁ 0.18
1.85ꢀ25.01
7878
0.20 ꢁ 0.18 ꢁ 0.12
1.40ꢀ25.02
13 216
0.20 ꢁ 0.19 ꢁ 0.12
1.84ꢀ25.02
24 023
0.26 ꢁ 0.20 ꢁ 0.18
1.54ꢀ25.02
95 074
no. of reflns collected
no. of indep reflns/Rint
no. of params
GOF on F2
2483/0.2125
182
2497/0.0541
166
5820/0.1565
425
11 979/0.0341
979
24 407/0.0584
1762
0.718
1.195
1.003
1.015
1.054
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
largest diff peak and hole, e Åꢀ3
0.0851/0.2061
0.1688/0.2897
3.429, ꢀ2.542
0.0654/0.2603
0.0729/0.2670
4.038, ꢀ2.438
0.0684/0.1636
0.0840/0.1753
1.096, ꢀ2.021
0.0560/0.1351
0.0578/0.1379
2.355, ꢀ2.686
0.0452/0.1081
0.0529/0.1177
1.082, ꢀ2.747
3
a Due to poor quality of the single crystal of 1, some atoms’ thermal parameters (C8, C9, C10, C3, O3, N1, N2, C1, and C4) were restrained to be
isotropic.
129.8 (MesꢀC), 129.5 (MesꢀC), 124.1 (NCH), 119.1(NCH), 67.6
(RuCRu), 45.8 (NCH2), 21.2 (CH3), 17.6 (MesꢀCH3). IR (νCO):
Foundation of China (Nos. 20872066 and 21072101) for
financial support.
2072, 2031, 2011, 1968, 1934 cmꢀ1
.
Crystallographic Studies. Crystals of 1, 2, and 5ꢀ7 suitable for
X-ray analysis were obtained from CH2Cl2/hexane at room temperature.
Data collections were performed on a Rigaku Saturn 70 (for 1 and 2) or
Rigaku Saturn 724 (for 5ꢀ7) diffractometer using graphite-monochro-
mated Mo KR radiation (ωꢀ2θ scans). Semiempirical absorption correc-
tions were applied for all complexes. The structures were solved by direct
methods andrefinedbyfull-matrixleast-squares. Allcalculationswere done
using the SHELXL-97 program system. The crystal data and summary of
the X-ray data collection are presented in Table 1.
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’ AUTHOR INFORMATION
Corresponding Author
*Fax: 86-22-23504781. E-mail: bqwang@nankai.edu.cn.
’ ACKNOWLEDGMENT
We thank Dr. Yongqiang Zhang for improving the English
of this article. We also thank the National Natural Science
3035
dx.doi.org/10.1021/om2000647 |Organometallics 2011, 30, 3029–3036

Organometallics
ARTICLE
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’ NOTE ADDED AFTER ASAP PUBLICATION
In the version of this paper published on May 9, 2011, we
mistakenly stated that we observed a novel ruthenium structure.
We have since learned that the structure has been previously
observed. In the version of the paper published on May 16, 2011,
changes have been made throughout the paper to correct this
error, and a reference has been added to cite the previous
observation.
3036
dx.doi.org/10.1021/om2000647 |Organometallics 2011, 30, 3029–3036