Bis-N-heterocyclic carbene ruthenium(II) carbonyl complexes: Synthesis, structural characterization and catalytic activities in transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.ica.2009.11.015

In this contribution, the synthesis and characterization of eight ruthenium(II) carbonyl complexes supported by chelating alkane-bridged bis-N-heterocyclic carbene ligands are reported. The products obtained are analyzed using infrared and NMR spectroscop

Full text of DOI:10.1016/j.ica.2009.11.015

Inorganica Chimica Acta 363 (2010) 430–437
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Bis-N-heterocyclic carbene ruthenium(II) carbonyl complexes:
Synthesis, structural characterization and catalytic activities
in transfer hydrogenation of ketones
c
Yong Cheng a,b, Xiang-Yong Lu a, Hui-Jun Xu a, Yi-Zhi Li a, Xue-Tai Chen a, , Zi-Ling Xue
*
a State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, People’s Republic of China
b School of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, People’s Republic of China
c Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2009
Received in revised form 16 September 2009
Accepted 10 November 2009
Available online 16 November 2009
In this contribution, the synthesis and characterization of eight ruthenium(II) carbonyl complexes sup-
ported by chelating alkane-bridged bis-N-heterocyclic carbene ligands are reported. The products
obtained are analyzed using infrared and NMR spectroscopies. The molecular structures of four metal
complexes were determined by X-ray crystallography, which exhibit the six-coordinate octahedral geom-
etry with two carbene carbon atoms from the bidentate Bi-NHCs, two carbonyl groups and two chlorine
atoms in the trans(Cl)–cis(CO) configuration. All these complexes show catalytic activities in transfer
hydrogenation of ketones.
Keywords:
N-heterocyclic carbene
Ruthenium(II)
Carbonyl
Ó 2009 Elsevier B.V. All rights reserved.
Catalyst
Transfer hydrogenation
1. Introduction
et al. [2c] have observed that the bis-NHCs coordinate to a Rh(I)
center in either a chelating or a bridging 2:1 (metal:bis-NHC) fash-
There have been considerable investigations on the syntheses of
transition-metal N-heterocyclic carbene (NHC) complexes and
their catalytic applications in organometallic chemistry. So far
many efficient NHC-based catalysts have been developed for a
wide range of transformation reactions [1]. The majority of NHC li-
gands used are monodentate. The chelating N-heterocyclic carb-
enes (NHCs) are likely to be useful in organometallic synthesis
and catalysis, which would provide a new dimension in the prep-
aration of new catalysts because it yields the metal complexes with
usually high thermal and air stabilities [1f,g].
ion, depending on the length of the linker between the two azole
groups and the steric size of N-substituents.
Despite the rich catalytic applications of ruthenium complexes,
the Ru bis-N-heterocyclic carbene complexes applied in catalysis is
restricted to those complexes with pyridine-bridged pincer ligands
[3]. They showed considerable catalytic activities in transfer
hydrogenations of ketone [3a], oxidation of olefins [3b], and
metathesis of olefins [3c]. There are few reports of ruthenium com-
plexes containing alkane-bridged bis-NHC ligands [4]. Poyatos
et al. [4a,b] have prepared several
g
6-arene ruthenium complexes
Many reports have emerged in recent years concerning biden-
tate alkane-bridged bis-NHC ligands [2]. Most of the metal com-
plexes containing bis-NHCs employ them as chelating ligands at
a single metal, although some have been reported in which the
bis-NHCs group bridges metal atoms [2]. For example, Jia et al.
[2a] have reported the synthesis of binuclear half-sandwich irid-
ium and rhodium carbene complexes containing 1,2-dichalcogeno-
lato carborane or carbonato ligands. Mata et al. [2b] and Leung
with chelating bis-NHC. Marshall et al. [4c] have synthesized
ruthenium(II) benzylidene complexes of the chiral chelating bis-
NHC. Although ruthenium(II) carbonyl complexes are well known
and have shown many applications [5], to the best of our knowl-
edge, no ruthenium(II) carbonyl complex containing a chelating
bis-NHC with an alkane bridge has been reported. In this paper,
we describe the preparation of the mononuclear bidentate ruthe-
nium(II) carbonyl complexes with the bis-NHCs that contain
(CH2)n linkers (n = 1–4). Influence of the different linker lengths
on the catalytic activities of these complexes in transfer hydroge-
nation of ketones has been studied.
* Corresponding author. Tel.: +86 25 83597147; fax: +86 25 83314502.
E-mail address: xtchen@netra.nju.edu.cn (X.-T. Chen).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.11.015

Y. Cheng et al. / Inorganica Chimica Acta 363 (2010) 430–437
431
All the compounds have been characterized by NMR, IR and ele-
mental analyses. The chelating character of the bis-NHC ligands
can be deduced from NMR spectroscopy by comparing with the
spectra of the well characterized metal complexes [2]. In com-
plexes 1a–4a and 1b–4b, only one set of 1H and 13C signals was ob-
served for the two half molecules, indicating that the two halves
are symmetry-related. The 1H NMR spectra of 1a–4a and 1b–4b
do not exhibit a signal at d 9–10, where the NCHN proton of the
imidazolium group was usually found, suggesting the coordination
of the carbene carbon in bis-NHC ligands to the ruthenium atoms.
The 13C{H} NMR spectra of these complexes show resonances for
the carbonyl carbon atoms at d 196–198, similar to those of
[Ru(L)Cl2(CO)2] (L = bpy, bipy, dmbpy, (PR3)2) [5]. The 13C{H}
NMR signals for the carbene carbon atoms (d 171–180) located in
the characteristic range reported for the carbene carbon atoms of
bis-NHCs metal complexes [5]. The IR spectra of complexes 1a–
2. Results and discussion
2.1. Synthesis of ruthenium complexes
The bis(imidazolium) salts were prepared in high yields by the
reactions of n-butylimidazole or n-benzylimidazole with the corre-
sponding dibromoalkanes (e.g., dibromomethane, 1,2-dibromoeth-
ane, 1,3-dibromopropane, and 1,4-dibromobutane) according to
the literature methods [2a,b,f]. The transmetallation route using
a silver N-heterocyclic carbene complex has proved to be a very
useful procedure in the preparation of metal–NHC complexes [6].
In the current work, this procedure was employed to prepare
ruthenium(II) carbonyl complexes 1a–4a and 1b–4b by a two-step
process (see Scheme 1). Reacting the bis(imidazolium) salts L1a
–
L4a, L1b–L4b with an excess of Ag2O in dichloromethane (for
R = nBu) or methanol (for R = Bn) afforded the corresponding silver
N-heterocyclic carbene complexes in situ, which were then treated
with [Ru(CO)2Cl2]n in dichloromethane to afford the desired com-
plexes 1a–4a and 1b–4b. The products were purified by column
chromatography (dichloromethane/methanol (40:1)). All these
complexes are very soluble in CH3OH and CH2Cl2, but insoluble
in diethyl ether and hydrocarbon solvents. These complexes are
stable in the solid state in the air.
It should be noted that only one type of mononuclear ruthe-
nium(II) complex with the chelating ligands was obtained with
the (–CH2)n linkers (n changing from 1 to 4). In contrast, the types
of the Rh complexes with those bis-NHCs in either a chelating or a
bridging 2:1 (metal:bis-NHC) fashion, have been obtained depend-
ing on the length of the linker [2b,c]. Attempts to prepare the ana-
4a and 1b–4b show two
m (CO) absorptions at 2030–2040 and
1960–1970 cmꢀ1, similar to those of [Ru(L)Cl2(CO)2] (L = bpy, bipy,
dmbpy, (PR3)2) [5].
2.2. Molecular structures of 1a, 3a, 4a and 3b
The molecular structures of 1a, 3a, 4a, and 3b are displayed in
Figs. 1–4, respectively. The crystallographic data for 1a, 3a, 4a
and 3b are given in Table 1, and selected bond lengths and angles
are given in Table 2.
The ORTEP drawings of 1a, 3a, 4a and 3b in Figs. 1–4 show that
the coordination geometry around the ruthenium atom is a slightly
distorted octahedron with carbon atoms from the chelating bis-
NHC, two chloride atoms occupying mutually trans positions, and
two CO groups locating trans to the carbene carbon, respectively.
The Ru–Ccarbene distances (Ru1–C1 and Ru1–C2) ranged from 2.10
t
logues complexes with the bulky Bu substituents in N-atom of
imidazole under identical reaction conditions failed to yield the de-
sired products.
Scheme 1. Synthesis of Ru(II) carbonyl complexes 1a–4a and 1b–4b.
Fig. 1. Molecular structure of complex 1a showing 30% probability ellipsoids. The hydrogen atoms are omitted for clarity.

432
Y. Cheng et al. / Inorganica Chimica Acta 363 (2010) 430–437
Fig. 2. Molecular structure of complex 3a showing 30% probability ellipsoids. The hydrogen atoms are omitted for clarity.
Fig. 3. Molecular structure of complex 4a showing 30% probability ellipsoids. The hydrogen atoms are omitted for clarity.
Fig. 4. Molecular structure of complex 3b showing 30% probability ellipsoids. The hydrogen atoms are omitted for clarity.

Y. Cheng et al. / Inorganica Chimica Acta 363 (2010) 430–437
433
Table 1
Summary of crystallographic data for 1a, 3a, 4a and 3b.
Compound
1a
3a
4a
3b
Formula
Formula weight
Crystal system
Space group
a (Å)
C17H24Cl2N4O2Ru
488.37
monoclinic
P2(1)/c
7.7934(9)
21.378(3)
13.4990(16)
109.549(8)
2119.4(5)
4
C19H28Cl2N4O2Ru
516.42
monoclinic
P2(1)/c
11.986(3)
14.657(4)
13.586(3)
106.611(4)
2287.2(10)
4
C20H30Cl2N4O2Ru
530.45
monoclinic
P2(1)/c
12.7794(9)
14.9518(10)
13.3881(9)
113.602(1)
2344.1(3)
4
C25H24Cl2N4O2Ru
584.45
monoclinic
P2(1)/c
8.2681(9)
20.236(2)
15.638(2)
102.313(1)
2556.3(5)
4
b (Å)
c (Å)
b (°)
V (Å3)
Z
T (K)
291(2)
291(2)
291(2)
291(2)
Radiation (Mo Ka)
0.71073
1.010
0.71073
0.940
0.71073
0.919
0.71073
0.852
Absorption coefficient (mmꢀ1
)
h Range for data collection (°)
Data/restraints/parameters
Reflections collected
1.86/26.00
4157/0/237
12 729
2.09/26.00
4481/0/255
12 063
1.70/26.00
4607/0/264
12 646
1.67/26.00
5019/0/ 307
13 657
Reflections unique
4157
4481
4607
5019
Rint
0.025
0.029
0.037
0.043
Maximum and minimum transmission
0.80 and 0.70
1.209
0.82 and 0.77
1.123
0.82 and 0.79
1.069
0.82 and 0.84
1.000
Goodness-of-fit (GOF) on F2
R1/wR2[I > 2
r
(I)]
0.0562/0.0688
0.0815/0.0718
0.872/ꢀ0.663
0.0376/0.0902
0.0451/0.0920
0.285/ꢀ0.733
0.0453/0.1004
0.0589/0.1030
0.337/ꢀ0.811
0.0451/0.0818
0.0628/0.0838
0.996/ꢀ0.967
R1/wR2 (all data)a
Largest peak and hole (e/Å)
P
P
P
P
a
R1
=
||Fo| ꢀ |Fc||/ |Fo|; wR2 = [ w(|Fo| ꢀ |Fc|)2/ w|Fo|2]1/2
.
Table 2
Selected bond lengths (Å) and angles (°) for complexes 1a, 3a, 4a and 3b.
[5]. The Ru–Cl (Ru1–Cl1 and Ru1–Cl2) bond lengths observed in
these compounds fall in the range 2.41–2.44 Å, similar to those
of [Ru(L)Cl2(CO)2] (L = bpy, bipy, dmbpy, (PR3)2) [5]. The C–O
(C3–O2 and C4–O1) bond lengths in these complexes are in the
normal range of 1.10–1.15 Å [5].
The bond angles Ru1–C3–O2 and Ru1–C4–O1 are in the range of
170–180°. The Ccarbene–Ru–Ccarbene (C1–Ru1–C2) bond angles of
complex 1a (81.99(16)°) is much smaller than those of the other
complexes 3a (95.88(12)°), 4a (94.26(14)°) and 3b (96.10(13)°),
which is due to the occurrence of six-membered chelating ring in
complex 1a in contrast with eight- or nine-membered chelating
rings in complexes 3a, 3b, and 4a.
1a
3a
4a
3b
Bond lengths
Ru1–C1
Ru1–C2
Ru1–Cl1
Ru1–Cl2
Ru1–C3
Ru1–C4
C3–O2
2.107(4)
2.138(4)
2.4069(12)
2.4161(12)
1.914(5)
1.919(5)
1.128(5)
1.156(5)
2.140(3)
2.157(3)
2.4485(9)
2.4244(9)
1.912(3)
1.908(3)
1.129(4)
1.100(4)
2.160(3)
2.152(4)
2.4189(8)
2.4272(8)
1.896(4)
1.883(4)
1.134(4)
1.140(4)
2.155(4)
2.151(3)
2.4170(9)
2.4282(9)
1.904(4)
1.904(4)
1.120(4)
1.129(4)
C4–O1
Bond angles
C3–Ru1–C1
C3–Ru1–C4
C4–Ru1–C1
C3–Ru1–C2
C4–Ru1–C2
C1–Ru1–C2
C3–Ru1–Cl1
C4–Ru1–Cl1
C1–Ru1–Cl1
C2–Ru1–Cl1
C3–Ru1–Cl2
C4–Ru1–Cl2
C1–Ru1–Cl2
C2–Ru1–Cl2
Cl1–Ru1–Cl2
Ru1–C3–O2
Ru1–C4–O1
175.35(17)
90.23(19)
93.49(18)
94.25(17)
175.46(18)
81.99(16)
86.60(14)
88.67(14)
90.70(11)
90.88(13)
92.98(14)
91.21(14)
89.72(11)
89.27(13)
179.57(4)
167.4(4)
172.89(13)
85.64(14)
87.63(12)
90.99(14)
175.13(13)
95.88(12)
87.87(10)
96.08(11)
90.60(8)
175.71(16)
85.86(16)
89.87(15)
90.01(15)
175.74(14)
94.26(14)
86.48(11)
92.54(11)
93.25(9)
175.16(14)
85.52(16)
90.06(15)
88.47(15)
172.21(15)
96.10(13)
95.87(11)
86.63(12)
85.82(9)
2.3. Catalytic transfer hydrogenation of ketones
Many ruthenium complexes have been found to be the active
catalysts for transfer hydrogenation reactions [7]. Only few ruthe-
87.30(8)
88.25(9)
89.06(9)
95
95.22(10)
87.33(11)
86.72(8)
89.48(8)
175.58(3)
172.9(3)
93.82(11)
87.19(11)
86.42(9)
92.04(9)
179.58(3)
175.8(4)
89.23(11)
95.64(12)
89.23(9)
89.23(9)
174.57(4)
175.0(4)
1a
1b
2a
2b
3a
3b
4a
4b
S
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
174.0(4)
172.2(3)
174.2(3)
171.1(4)
to 2.16 Å, typical for those found in Ru–NHC complexes [3,4]. It is
interesting to find that the Ru–Ccarbene bond become longer with
increasing of the (CH2)n linker length from 1a (2.107(4),
2.138(4) Å), 3a (2.140(3), 2.157(3) Å) to 4a (2.160(3), 2.152(4)
Å). Considering the same electronic donacity of the carbene carbon
atoms in 1a–4a, the lengthening of Ru–Ccarbene could be due to the
geometric constraints imposed by the growing chain-bridge on
chelating rings. The Ru–CO bond lengths (Ru1–C3 and Ru1–C4) be-
come increasingly shorter from 1a (1.914(5), 1.919(5) Å), 3a
(1.912(3), 1.908(3) Å) to 4a (1.896(4), 1.883(4) Å). The Ru–CO bond
lengths in these complex are in the range of 1.8–1.9 Å, similar to
those reported in the other ruthenium(II) carbonyl complexes
0
20
40
60
80
100
120
140
time (min)
Fig. 5. Conversion versus reaction time of the catalytic transfer hydrogenation of
acetophenone. Experimental conditions:
4 lmol of catalyst (1a–4a and 1b–4b),
0.2 mmol of KOH, 4 mmol of acetophenone, solvent iPrOH (10 mL), T = 355 K.
S = [Ru(CO)2Cl2]n.

434
Y. Cheng et al. / Inorganica Chimica Acta 363 (2010) 430–437
nium–NHC complexes, however, have been reported to be the cat-
alysts for this transformation [8]. The ruthenium(II) carbonyl com-
plexes 1a–4a and 1b–4b were thus studied as the catalysts for
transfer hydrogenation of ketone. In the meantime, the ruthenium
precursor [Ru(CO)2Cl2]n was examined under the same reaction
conditions in order to investigate the role of the bis-N-heterocyclic
carbene ligand.
The reduction of acetophenone to 1-phenylethanol by 2-propa-
nol was used as the model reaction to explore the catalytic behav-
ior for complexes 1a–4a and 1b–4b using 2-propanol as hydrogen
donor in the presence of base (Eq. (1)). The catalytic experiments
It was found that 3a is the most active catalyst among all of
these complexes. The length of –CH2 linker between two N-hetero-
cyclic carbene also influences the catalytic activity of the com-
plexes. The sequence of the activity is 3a > 4a > 2a > 1a > S
(S = [Ru(CO)2Cl2]n) and 3b > 4b > 2b > 1b > S. When the length of
–CH2 linker n = 3, the complex shows the best activity.
Since complex 3a was found to be the most efficient catalyst in
transfer hydrogenation of acetophenone, we decided to further ex-
plore its catalytic potentials in the reduction of other aryl and alkyl
ketones with the reaction condition similar to those used in the
transfer hydrogenation of acetophenone (Table 3). It was found
that 3a is efficient in transfer hydrogenation of cyclohexanone to
cyclohexanol (99.67% after 2 h, entry 6), 4-chloroacetophenone to
4-chloroacetophenol (98.00% conversion after 6 h, entry 1), 2-hep-
tanone to 2-heptanol (92.82% after 6 h, entry 7), moderate active in
the case of diphenylketone (72.84% after 6 h, entry 3), 4-methoxy-
acetophenone (84.41% conversion after 6 h, entry 5), but shows a
poor activity in reduction of 2,4,6-trimethylacetophenone
(27.21% after 6 h, entry 2). These different catalytic activities may
be attributed to the electronic and steric effects of the substituents
on the ketones.
Some Ru–NHC [8] and Ir–NHC complexes [9] have been demon-
strated to be the effective catalysts for the transfer hydrogenation
of ketones. It should be noted that these catalytic studies were
carried out under different reaction conditions and even with dif-
ferent substrates and bases. Compound 1a had achieved turnover
frequencies of 260 hꢀ1 for transfer hydrogenation of acetophenone
after 2 h, larger than the most closely related ruthenium(II)
carbonyl chlorides complexes with pyridine-functionalized N-het-
erocyclic carbenes trans(Cl)- and cis(Cl)-Ru(Py-NHC)Cl2(CO)2
were carried out using 4.0 mmol of substrate ketone, 4 lmol of
i
ruthenium complex catalyst, 0.2 mmol of KOH, 10 mL of PrOH,
with a catalyst/base/substrate (Cat/Base/S) ratio of 1:50:1000. A
i
i
base solution in PrOH was added to a PrOH solution containing
the catalyst and the substrate, which was kept at 82 °C. The con-
version of the product was monitored by GC and the time-depen-
dent conversions were followed (Fig. 5).
O
OH
O
OH
cat.
+
+
KOH, 82 o
C
ð1Þ
Fig. 5 shows that all the complexes except 1b are much more active
than ruthenium precursor [Ru(CO)2Cl2]n, suggesting that the pres-
ence of most of the chelating bis-N-heterocyclic carbene ligand is
beneficial for the transfer hydrogenation of ketone. The catalytic
activity of 1a–4a (R = nBu) is better than the respective complexes
1b–4b (R = Bn) with the order 1a > 1b; 2a > 2b; 3a > 3b; 4a > 4b.
Table 3
Catalytic transfer hydrogenation of ketones using complex 3a.a
Entry
1
Ketone
Alcohol
Conversion % (h)b
98.00 (6)
O
C
OH
CH
CH3
CH3
Cl
Cl
CH3
CH3
2
3
27.21 (6)
72.84 (6)
O
OH
CH
C
CH3
CH3
H3
C
CH3
H3
C
CH3
OH
CH
O
C
O
OH
CH
4
5
91.8 (40 min)
84.41 (6)
C
CH3
CH3
O
OH
CH
C
CH3
CH3
CH3O
OH
CH3O
O
6
7
99.67 (2)
92.82 (6)
O
OH
a
Experimental condition: reactions were carried out at 82 °C; acetophenone (4 mmol), complex 3a (4 lmol), KOH (0.2 mmol) in 2-propanol (10 mL); ketone/Ru/
KOH = 1000/1/50.
b
The conversion was determined by GC analysis.

Y. Cheng et al. / Inorganica Chimica Acta 363 (2010) 430–437
435
(Py-NHC = 3-n-butyl-1-picolylimidazol-2-ylidene) [8c]. However,
compound 1b (TOF = 25.59 hꢀ1) is in the same order of magnitude
found in trans(Cl)- and cis(Cl)-[Ru(Py-NHC)Cl2(CO)2] [Py-NHC = 3-
benzyl-1-picolylimidazol-2-ylidene] [8c]. The turnover frequencies
of 3a is 498 hꢀ1 for the hydrogenation of cyclohexanone after reac-
tion for 2 h, which is much lower than that observed for [(CO-
D)Ir(NHC)Br] (NHC = 1,3-dipropylbenzimidazol-2-ylidene, TOF =
6000 hꢀ1) reported by Hahn et al. [9a].
In order to determine the fate of carbonyl ligand in the catalytic
reaction, we have attempted to probe if the carbonyl ligand still ex-
isted in the coordination sphere of ruthenium for the catalytic
reduction of acetophenone with 3a. After the catalytic reaction,
the reaction mixture was evaporated to give a solid. The IR spectra
of the isolated solids showed the absence of carbonyl ligand
around the ruthenium atom, indicating the carbonyl group did
not survive under the reaction conditions. The active catalytic spe-
cies apparently do not contain the carbonyl group. It is thus rea-
sonable to assume that complexes 1a–4a and 1b–4b just acted as
precursors to the active catalysts.
treated with [RuCl2(CO)2]n (0.5 mmol) when R = nBu. For R = Bn, the
solvent MeOH was removed and replaced by 30 ml of CH2Cl2 and
treated with [RuCl2(CO)2]n (0.5 mmol). The mixture was stirred at
room temperature for 12 h, to give a precipitate. The suspension
was filtered through Celite to remove the silver halide, and the
solution was concentrated under reduced pressure. The product
was purified by column chromatography using silica gel. Elution
with dichloromethane/methanol (40:1) afforded the separation of
a yellow band that contained the desired product, which was ob-
tained as yellow powder after the volatiles were removed. Recrys-
tallisation from methanol gave pure product suitable for elemental
analysis and crystals suitable for X-ray diffraction (1a, 3a, 4a, and
3b).
4.2.1. Methylenebis(N-n-butylimidazol-2-
ylidene)dichlorodicarbonylruthenium (1a)
Yield: 0.18 g, 75%. Anal. Calc. for C17H24Cl2N4O2Ru: C, 41.81; H,
4.95; N, 11.47. Found: C, 41.63; H, 4.86; N, 11.51%. 1H NMR
3
(DMSO-d6, 500 MHz): d 7.63 (d, 2H, JH–H = 1.2 Hz, NCH), 7.58 (d,
3
2H, JH–H = 1.2 Hz, NCH), 6.48 (br, 2H, NCH2N), 4.32 (m, 4H,
NCH2CH2CH2CH3), 1.87 (m, 4H, NCH2CH2CH2CH3), 1.42 (m, 4H,
3. Conclusions
3
NCH2CH2CH2CH3), 0.96 (t, 6H, JH–H = 7.3 Hz, NCH2CH2CH2CH3).
13C{H} NMR (DMSO-d6, 125.7 MHz): d 196.0 (CO), 176.2 (NCN),
123.1 (NCH), 121.9 (NCH), 62.3 (NCH2N), 50.5 (NCH2CH2CH2CH3),
33.9 (NCH2CH2CH2CH3), 20.3 (NCH2CH2CH2CH3), 14.5 (NCH2-
The present work describes the synthesis and catalytic proper-
ties of a series of ruthenium(II) carbonyl complexes with bis-N-
heterocyclic carbenes, where the substituent group of the azole
ring are nBu and Bn. The molecular structures of the bis-carbene
complexes here indicate that the bis-NHC ligand is chelated to
the metal atom and these complexes exhibit the trans(Cl)–cis(CO)
configuration. These complexes are found to be efficient catalysts
in transfer hydrogenation of ketones.
CH2CH2CH3). IR (KBr, cmꢀ1):
m (CO) 2039 and 1977.
4.2.2. Ethylenebis(N-n-butylimidazol-2-
ylidene)dichlorodicarbonylruthenium (2a)
Yield: 0.20 g, 80%. Anal. Calc. for C18H26Cl2N4O2Ru: C, 43.03; H,
5.22; N, 11.15. Found: C, 42.93; H, 5.06; N, 11.09%. 1H NMR
(DMSO-d6, 500 MHz): d 7.74 (s, 2H, NCH), 7.63 (s, 2H, NCH), 5.06
(s, 4H, NCH2CH2N), 4.59 (m, 4H, NCH2CH2CH2CH3), 1.96 (m, 4H,
NCH2CH2CH2CH3), 1.55 (m, 4H, NCH2CH2CH2CH3), 1.11 (t, 6H,
3JH–H = 7.0 Hz, NCH2CH2CH2CH3). 13C{H} NMR (DMSO-d6, 125.7
MHz): d 198.0 (CO), 171.5 (NCN), 125.9 (NCH), 124.1 (NCH), 51.5
(NCH2CH2N), 50.7 (NCH2CH2CH2CH3), 34.3 (NCH2CH2CH2CH3),
4. Experimental
4.1. General comments
Unless otherwise noted, all reactions and manipulations were
performed under a dry nitrogen atmosphere using a standard
Schlenk technique. The solvents were purified using standard
methods and degassed before use. Methanol was dried over Mg/
I2, dichloromethane was dried over P2O5 and then distilled under
nitrogen. Other chemicals were purchased from commercial source
and used without further purification. The following starting
materials were prepared according to the literature methods:
[RuCl2(CO)2]n [5b], methylenebis(N-n-butylimidazolium) dibro-
20.0 (NCH2CH2CH2CH3), 14.4 (NCH2CH2CH2CH3). IR (KBr, cmꢀ1):
m
(CO) 2039 and 1975.
4.2.3. Trimethylenebis(N-n-butylimidazol-2-
ylidene)dichlorodicarbonylruthenium (3a)
Yield: 0.21 g, 80%. Anal. Calc. for C19H28Cl2N4O2Ru: C, 44.19; H,
5.46; N, 10.85. Found: C, 44.11; H, 5.49; N, 10.81%. 1H NMR
(DMSO-d6, 500 MHz): d 7.62 (s, 2H, NCH), 7.44 (s, 2H, NCH), 4.60
(m, 2H, NCHHCH2CHHN), 4.41 (m, 2H, NCHHCH2CHHN), 3.98 (m,
4H, NCH2CH2CH2CH3), 1.89 (m, 4H, NCH2CH2CH2CH3), 1.67 (m,
2H, NCH2CH2CH2N), 1.44 (m, 4H, NCH2CH2CH2CH3), 0.95 (t, 6H,
3JH–H = 7.3 Hz, NCH2CH2CH2CH3). 13C{H} NMR (DMSO-d6,
125.7 MHz): d 197.7 (CO), 176.4 (NCN), 123.4 (NCH), 123.3(NCH),
51.5 (NCH2CH2CH2N), 45.7 (NCH2CH2CH2CH3), 35.1 (NCH2CH2CH2N),
mide (L1a) [2f], ethylenebis(N-n-butylimidazolium) dibromide
(L2a
(L3a
)
)
[2a], trimethylenebis(N-n-butylimidazolium) dibromide
[2a], tetramethylenebis(N-n-butylimidazolium) dibromide
(L4a) [2a], methylenebis(N-benzylimidazolium) dibromide (L1b
[2f], ethylenebis(N-benzylimidazolium) dibromide (L2b) [2f], trim-
ethylenebis(N-benzylimidazolium) dibromide (L3b [2f], and
)
)
tetramethylenebis(N-benzylimidazolium) dibromide (L4b) [2f].
Elemental analyses were performed in an Elementar Vario ELIII
elemental analyzer. NMR measurements were obtained in CDCl3 or
DMSO-d6 on a Bruker AM-500 spectrometer. Chemical shifts are
given in parts per million for 1H and 13C NMR. The IR spectra
were recorded on a Bruker Vector 22 spectrophotometer with
KBr pellets in the 4000–400 cmꢀ1 region. The GC analyses of the
catalytic mixture were performed on a Shimadzu GC-2010.
34.0
(NCH2CH2CH2CH3),
20.2
(NCH2CH2CH2CH3),
(CO) 2033 and 1966.
14.6
(NCH2CH2CH2CH3). IR (KBr, cmꢀ1):
m
4.2.4. Tetramethylenebis(N-n-butylimidazole-2-ylidene)
dichlorodicarbonylruthenium (4a)
Yield: 0.22 g, 80%. Anal. Calc. for C20H30Cl2N4O2Ru: C, 45.28; H,
5.70; N, 10.56. Found: C, 44.98; H, 5.41; N, 10.38%. 1H NMR
(DMSO-d6, 500 MHz): d 7.65 (s, 2H, NCH), 7.55 (s, 2H, NCH), 4.64
(m, 2H, NCHHCH2CH2CHHN), 4.44 (m, 2H, NCHHCH2CH2CHHN),
4.37 (m, 2H, NCHHCH2CH2CH3), 3.63 (m, 2H, NCHHCH2CH2CH3),
1.95 (m, 2H, NCH2CHHCHHCH2N), 1.87 (m, 4H, NCH2CH2CH2CH3),
1.45 (m, 4H, NCH2CH2CH2CH3), 1.08 (m, 2H, NCH2CHHCHHCH2N),
4.2. General procedures
A suspension of the appropriate bis(imidazolium) dibromide
(0.5 mmol) and silver oxide (1.0 mmol) in 30 ml of dichlorometh-
ane (for R = nBu) or methanol (for R = Bn) was stirred at room tem-
perature for 2 h. Then, the mixture was filtered through Celite and
0.96 (t, 6H, JH–H = 7.4 Hz, NCH2CH2CH2CH3). 13C{H} NMR (DMSO-
d6, 125.7 MHz): d 197.5 (CO), 176.7 (NCN), 123.5 (NCH), 123.0
(NCH), 51.1 (NCH2CH2CH2CH2N), 46.9 (NCH2CH2CH2CH3),
3

436
Y. Cheng et al. / Inorganica Chimica Acta 363 (2010) 430–437
34.2(NCH2CH2CH2CH3),
22.6
(NCH2CH2CH2CH2N),
20.1
iPrOH, the reaction starts immediately. The reaction progress was
monitored by GC analysis.
(NCH2CH2CH2CH3), 14.6 (NCH2CH2CH2CH3). IR (KBr, cmꢀ1):
m
(CO) 2035 and 1970.
4.4. X-ray structure determination of complexes 1a, 3a, 4a and 3b
4.2.5. Methylenebis(benzylimidazol-2-ylidene)
dichlorodicarbonylruthenium (1b)
Crystal data and details of data collection and refinements of 1a,
3a, 4a and 3b are given in Table 1. Diffraction data were collected
on a Bruker SMART Apex II CCD diffractometer using graphite-
Yield: 0.10 g, 40%. Anal. Calc. for C23H20Cl2N4O2Ru: C, 49.65; H,
3.62; N, 10.07. Found: C, 49.23; H, 3.73; N, 9.79%. 1H NMR
monochromated Mo Ka (k = 0.71073 Å) radiation and collected
(DMSO-d6):
d 7.68 (d, JH–H = 1.5 Hz, 2H, NCH); 7.23 (d, JH–
for absorption using SADABS program [10]. The structures were
solved by direct methods and refined on F2 against all reflections
by full-matrix least-squares methods with SHELXTL (version 6.10)
program [11]. The hydrogen atoms in these compounds were posi-
tioned geometrically and refined in the riding-model approxima-
tion. All non-hydrogen atoms were refined with anisotropic
thermal parameters.
H = 1.5 Hz, 2H, NCH); 7.42 (m, 4H, 2,6-H of phenyl), 7.39 (m, 2H,
4-H of phenyl), 7.27 (m, 4H, 3,5-H of phenyl), 5.76 (br, 6H, NCH2N
and CH2Ph); 13C{H} NMR (DMSO-d6, 125.7 MHz): d 196.9 (CO);
177.2 (NCN); 137.3, 129.1, 128.3, 127.8 (C6H6), 123.2 (NCH),
122.1 (NCH); 62.1 (NCH2N), 53.4 (CH2Ph). IR (KBr, cmꢀ1):
m (CO)
2044 and 1986.
4.2.6. Ethylenebis(benzylimidazol-2-ylidene)
dichlorodicarbonylruthenium (2b)
Acknowledgments
Yield: 0.12 g, 42%. Anal. Calc. for C24H22Cl2N4O2Ru: C, 50.53; H,
3.89; N, 9.82. Found: C, 50.67; H, 3.68; N, 9.63%. 1H NMR (DMSO-
d6): d 7.49 (d, JH–H = 1.5 Hz, 2H, NCH); 7.14 (d, JH–H = 1.5 Hz, 2H,
NCH); 7.40 (m, 4H, 2,6-H of phenyl), 7.33 (m, 2H, 4-H of phenyl),
7.24 (m, 4H, 3,5-H of phenyl), 5.80 (s, 4H, CH2Ph); 5.02 (s, 4H,
NCH2CH2N). 13C{H} NMR (CDCl3, 125.7 MHz): d 196.2 (CO); 173.5
(NCH); 138.1, 129.0, 128.9, 128.1 (C6H6), 127.9 (NCH), 123.2
This work was supported by the National Natural Science Foun-
dation of China (NSAF No. 10676012), the National Basic Research
Program of China (Nos. 2006CB806104 and 2007CB925102) and
the US National Science Foundation (CHE-051692).
Appendix A. Supplementary material
(NCH); 54.7 (CH2Ph), 49.5 (NCH2CH2N). IR (KBr, cmꢀ1):
m (CO)
CCDC 722254, 722255, 722256 and 723250 contain the supple-
mentary crystallographic data for 1a, 3a, 4a and 3b. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.ica.2009.11.015.
2041 and 1986.
4.2.7. Trimethylenebis(benzylimidazol-2-ylidene)
dichlorodicarbonylruthenium (3b)
Yield: 0.15 g, 50%. Anal. Calc. for C25H24Cl2N4O2Ru: C, 51.38; H,
4.14; N, 9.59. Found: C, 51.11; H, 3.87; N, 9.70%. 1H NMR (DMSO-
d6): d 7.47 (s, 2H, NCH); 7.10 (s, 2H, NCH); 7.40 (m, 4H, 2,6-H of
phenyl), 7.35 (m, 2H, 4-H of phenyl), 7.35 (d, 4H, 3,5-H of phenyl);
6.14 (d, 2H, CHHPh), 5.58 (d, 2H, CHHPh); 4.17 (m, 2H,
NCHHCH2CHHN), 4.09 (m, 2H, NCHHCH2CHHN); 1.75 (s, 2H,
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