Synthesis, characterization, and transfer hydrogenation of Ru(II)-N-heterocyclic carbene complexes

Article abstract of DOI:10.1080/00958972.2014.911291

A new series of ruthenium(II) N-heterocyclic carbene complexes [RuL ^1,2,3(p-cymene)Cl₂] (3a-c) (where L is a N-heterocyclic carbene), have been synthesized via transmetalation. The new ruthenium(II)-NHC complexes were applied to transfer hydrogenation of acetophenone derivatives and aldehydes using 2-propanol as a hydrogen source and KOH as a co-catalyst. The results show that the corresponding alcohols could be obtained in good yield with high catalyst activity (up to 100%) under mild conditions. [RuL ¹(p-cymene)Cl₂] (3a) is much more active than the other complexes in transfer hydrogenation. Reactions, catalyzed by 3a-c, showed the highest reaction rates and yields of alcohol when the substrates bear more electron-withdrawing substituents. All new compounds were characterized by IR, elemental analysis, LC-MS (ESI), and NMR spectroscopy.

Full text of DOI:10.1080/00958972.2014.911291

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Synthesis, characterization, and
transfer hydrogenation of Ru(II)-N-
heterocyclic carbene complexes
S. Yaşara, S. Çekirdeka & İ. Özdemirb
a Department of Chemistry, Gaziosmanpaşa University, Tokat,
Turkey
b Department of Chemistry, İnönü University, Malatya, Turkey
Accepted author version posted online: 04 Apr 2014.Published
online: 29 Apr 2014.
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To cite this article: S. Yaşar, S. Çekirdek & İ. Özdemir (2014) Synthesis, characterization, and
transfer hydrogenation of Ru(II)-N-heterocyclic carbene complexes, Journal of Coordination
Chemistry, 67:7, 1236-1248, DOI: 10.1080/00958972.2014.911291
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 7, 1236–1248, http://dx.doi.org/10.1080/00958972.2014.911291
Synthesis, characterization, and transfer hydrogenation of
Ru(II)-N-heterocyclic carbene complexes
S. YAŞAR*†, S. ÇEKIRDEK† and İ. ÖZDEMIR‡
†Department of Chemistry, Gaziosmanpaşa University, Tokat, Turkey
‡Department of Chemistry, İnönü University, Malatya, Turkey
(Received 3 July 2013; accepted 26 February 2014)
A new series of ruthenium(II) N-heterocyclic carbene complexes [RuL1,2,3(p-cymene)Cl2] (3a–c)
(where L is a N-heterocyclic carbene), have been synthesized via transmetalation. The new ruthe-
nium(II)-NHC complexes were applied to transfer hydrogenation of acetophenone derivatives and
aldehydes using 2-propanol as a hydrogen source and KOH as a co-catalyst. The results show that
the corresponding alcohols could be obtained in good yield with high catalyst activity (up to 100%)
under mild conditions. [RuL1(p-cymene)Cl2] (3a) is much more active than the other complexes in
transfer hydrogenation. Reactions, catalyzed by 3a–c, showed the highest reaction rates and yields
of alcohol when the substrates bear more electron-withdrawing substituents. All new compounds
were characterized by IR, elemental analysis, LC–MS (ESI), and NMR spectroscopy.
Keywords: Ruthenium; Silver; N-Heterocyclic carbine; Transfer hydrogenation; Transmetalation
1. Introduction
Ligand effects are extremely important in homogeneous catalysis by metal complexes.
Carbenes are both reactive intermediates and ligands in catalysis. After the first stable
N-heterocyclic carbene ligands were reported by Arduengo et al., N-heterocyclic carbenes
*Corresponding author. Email: sedat.yasar@gop.edu.tr
© 2014 Taylor & Francis

Ru(II)-N-heterocyclic carbene complexes
1237
(NHCs) have been indispensable ligands for transition metals and homogeneous catalysis
[1–3]. Due to their topological and electronic versatility, N-heterocyclic carbene-based
ligands have widespread applications not only in organometallic chemistry but also in
industrial applications of homogeneous catalysis. Strong σ-donation from NHCs increases
the stability of the complexes and gives complexes with advantageous properties in catalytic
reactions [1–15]. The first catalytic applications of NHC complexes were reported by
Herrmann in 1995, together with the recognition that NHCs are excellent ligands for many
homogeneous catalysts [2]. Different NHC complexes and their catalytic applications have
been reported by many research groups [4–15]. Continued research has focused on synthe-
sis of new functional NHC complexes. Functionality of the complexes depends on the steric
and electronic effect of the ligands on the metal center. Ru–NHC complexes can be func-
tionalized by N-substituted NHC ligands. In this context, our research group has focused on
synthesis, characterization, and catalytic activity of functional N-heterocyclic carbene
ligands and their metal complexes [16–19].
Synthesis of different alcohols is an important application for transition metal-catalyzed
reactions for economical and environmental reasons [20]. Various Ir, Rh, and Ru complexes
have catalyzed different types of ketone to alcohol conversions under mild reaction condi-
tions [21, 22]. Ligand choice has a large effect on metal center selectivity and activity. So
far, phosphine, N-heterocyclic carbene, and amine-based metal complexes have been cata-
lysts for transfer hydrogenation reactions.
Based on our experience in the synthesis of N-heterocyclic carbene ligands and their
applications in homogeneous catalysis, we explore the activity of new Ru–NHC (3a–c)
complexes in transfer hydrogenation of ketones and aldehydes. All proposed structures are
supported by NMR and IR spectra.
Figure 1. Synthesis of imidazolium salts (1a–c).

1238
S. Yaşar et al.
2. Results and discussion
1
The hygroscopic imidazolium salts (1a–c) (figure 1) were characterized by H and 13C
1
NMR mass, IR spectroscopy, and elemental analyses. The H NMR spectra of the imidazo-
lium salts supported the assigned structures; the resonances for acidic C(2)–H were
observed as sharp singlets at 10.59, 10.48, and 10.28 ppm for 1a–c, respectively. There was
no direct effect of the R groups on the chemical shifts of the protons of 1a–c. 13C NMR
chemical shifts were consistent with the proposed structure; the imino carbon appeared as a
1
typical singlet in the H-decoupled mode at 137.2, 137.9, and 137.7 ppm for imidazolium
salts (1a–c), respectively. The NMR values are similar to those found for other imidazolium
salts [22].
Silver complexes of N-heterocyclic carbenes were prepared by the addition of Ag2O to
imidazolium salts solutions (1a–c) in CH2Cl2 (figure 2). The resulting suspension was stir-
red at room temperature overnight, filtered through Celite, and crystallized with CH2Cl2.
White crystals (in 70–80% yield) were washed and dried in vacuum. When the white crys-
tals were left in air, they began to slowly turn black. Low field signals attributed to acidic
1
NCHN protons were not observed in the H NMR spectra of the new Ag–NHC complexes
(2a–c). This observation confirmed that the imidazolium salts were deprotonated. However,
coordination of N-heterocyclic carbene ligands to the metal center resulted in low field reso-
nances for carbene carbon at δ 180–181.2 ppm.
Ag–NHC complexes are widely used as transfer compounds. To show the feasibility of
this approach for 2a–c, a series of Ru(II)-NHC complexes were synthesized in good yield
as stable dark red solids. 3a–c were obtained by reaction of [RuCl2(p-cymene)]2 with a
solution of 2a–c in the dark (figure 2). The low field resonance signals of Ag–Ccarb shifted
to higher field when NHC bonded to ruthenium. This observation confirms that Ru–NHC
complexes were formed by transmetalation. The air and moisture-stable ruthenium N-het-
erocyclic carbene complexes (3a–c) were soluble in polar and halogenated solvents.
Transition metal-catalyzed transfer hydrogenation of ketones in 2-propanol has become
an efficient method for synthesis of alcohols [22–29].
In order to show the usefulness of the catalysts in a more general manner, we employed
3a–c in the transfer hydrogenation of acetophenone derivatives and aldehydes (figure 3).
2-Propanol and KOH were used as a hydrogen donor and promoter, respectively.
Figure 2. Synthesis of 2a–c and 3a–c NHC complexes.

Ru(II)-N-heterocyclic carbene complexes
1239
The transfer hydrogenation reactions are very susceptible to the type of the base and its
concentration. In order to confirm this, a blank reaction was carried out. As expected, no
product was observed in the absence of base (table 1, entry 7). We screened the effect of
base type and base ratio in the catalytic transfer hydrogenation. Different inorganic bases
such as NaOH, Cs2CO3, t-BuOK, KOH, and K2CO3 were tested on this catalytic system;
we obtained the best conversion rates with KOH. High concentrations of KOH promoted
conversions but there was no conversion difference between 4 and 2 mM% of KOH (Con-
version means conversion to the desired product; see table 1 footnote.). However, the lower
base ratio (2 mM% KOH) was preferred for atom economy (table 1).
Electronically different acetophenone and aldehyde derivatives were selected as sub-
strates for hydrogen transfer reactions. According to the literature, no decomposition or side
reactions of these substrates have been detected in transfer hydrogenation [19].
As shown in table 2, these ketones and aldehydes were reduced to the corresponding sec-
ondary alcohols under mild conditions. Reaction time was determined as the time required
for 90% conversion (determined by GC). Several factors play important roles in catalyst
activity. Catalyst performance depends on the nature of substrates, the behavior of the
ligands in the complexes, and concentration of the catalyst. Electrophilic effect of the sub-
strate, nature of the catalysts, donor function, and flexibility of the backbone in the NHC
ligands can be responsible for activity of the complexes. Steric and electronic properties of
the substrates also affect conversions. Substrates bearing electron-withdrawing groups were
reduced to secondary alcohols in better yield than substrates bearing electron-donating
Figure 3. Transfer hydrogenation of ketones and aldehydes catalyzed by 3a–c.
Table 1. Influence of the nature of the base co-catalyst with 3a on
the reduction of p-chloroacetophenone by transfer hydrogenation.a
Entry
Base
Base (mM)
Conversion (%)b
1
2
3
5
4
5
6
7
Cs2CO3
tBuOK
K2CO3
NaOH
KOH
KOH
KOH
–
4
4
4
4
4
2
1
–
83
100
23
90
100
100
85
0
aReaction conditions: p-chloroacetophenone (1 mM), 3a (0.75 M%), 80 °C,
30 min.
bDetermined by GC.

1240
S. Yaşar et al.
Table 2. Transfer hydrogenation of ketones and aldehydes catalyzed by 3a–c (0.75 M%).
Cat: 0.75 mol%
Cl
Cl
N
N
Ru
1 mmol
R
O
O
OH
H
OH
3a, 3b, 3c
2 mmol KOH
2-propanol (5 mL), 80 o
X
X
or
or
C
R1
OH
X= CH3,
H
R1
R1=Cl,H,CH3
Entry
Catalyst
Substrate
Conversiona
Product
1
2
3
3a
3b
3c
100
97
92
4
5
6
3a
3b
3c
91
88
87
7
8
9
3a
3b
3c
93
96
85
10
11
12
3a
3b
3c
100
100
100
13
14
15
3a
3b
3c
100
100
100
16
17
18
3a
3b
3c
100
100
100
19
20
21
22
23
3a
3b
3c
3c
3b
93c
97c
90c
14d
19d
24
25
26
27
28
3a
3b
3c
3c
–
1e
3e
1e
65b
20f
(Continued)

Ru(II)-N-heterocyclic carbene complexes
1241
Table 2. (Continued).
Entry
29
Catalyst
Substrate
Conversiona
15f
Product
–
30
31
–
30f
3a
0g
32
33
34
35
36
37
3a
3b
3c
3a
3b
3c
95h
94h
89h
65h
56i
51i
aReaction conditions: The reactions were conducted at a substrate/catalyst/base (S/C/base) molar ratio of 1 : 0.0075 : 2, i-PrOH (5
mL), KOH (4 mM), 3a–c (0.75 M%), 80 °C, 30 min. Purity of compounds is checked by GC and GC–MS (three independent
catalytic experiments) and conversions are based on ketones. Conversions were determined by GC (three independent catalytic exper-
iments).
bAt normal atmospheric condition with undried 2-propanol.
c0.375 M% catalyst concentration.
dAt 50 °C.
eAt room temperature.
fNo 3a–c catalyst.
gNo base.
hCatalyst concentration 0.1 M%.
iCatalyst concentration 0.025 M%.
groups (table 2, entry 1–3, 4–6, and 13–15). Considering yield and reaction time, 3a and
3b showed better performance than 3c with acetophenone derivatives, presumably due to
increased steric bulk around the metal or lower electron donation provided by the carbene
ligands. When the bulkiness of R increased, catalyst activity did not change much (table 2,
entry 1, 2, 4, 7, 8).
2-Propanol is used as hydrogen donor in transfer hydrogenation reactions due to its
stability, low toxicity, and moderate boiling point. With 2-propanol, our results were
promising when compared with literature as well as with our previous work [16–19,
30–36].
There are some applications for hydrogen transfer reactions with metal-free catalytic sys-
tems. However, metal-free catalytic systems cannot compete in terms of efficiency and time
with metal-based systems [37–40]. Therefore, a blank reaction was tested in the absence of
the 3a–c complexes and results agreed with the literature (table 2, entry 28–30).
The stability of metal carbene complexes against moisture and oxygen were reported pre-
viously by Schultz et al. [14]. Addition of water to the catalyst system (10 M%) decreased
conversions and yields [14]. Observations made on the basis of our experiments are in
agreement (table 2, entry 27). The performance of 3a–c was negatively affected by moisture
in 2-propanol with lower yields observed when 2-propanol was not stirred over CaH2 over-
night before distillation. However, 3c gave 65% conversion (table 2, entry 27). Temperature
has a potent effect on the catalyst performance for transfer hydrogenation [41]; catalyst
performance of 3a–c was very low at low temperatures (table 2, entry 22–26).

1242
S. Yaşar et al.
To compare catalyst performance, 3a–c were tested under the same conditions. 4-Chloro-
acetophenone was reduced to almost 100% of the corresponding alcohol in 30 min by 0.75
M% 3a–c. At lower 3a–c concentration (0.375, 0.1, and 0.025 M%) with 4-chloroacetophe-
none, very high conversions were obtained (table 2, entry 19–21, 32–37). Although 3a–c
gave excellent performance with all substrates, there were slight differences in performance
due to the N-heterocyclic carbene ligands at the metal center.
A plot of conversion versus time for 3a–c with 4-chloroacetophenone is shown in figure
4. The reaction was monitored by taking small portions from the reaction mixture under a
nitrogen atmosphere at set intervals and the percentage conversion was determined. It is
clear that this sampling of the reaction medium and reduction in the concentration of cata-
lyst negatively affected the catalyst performance (figure 4).
When the catalytic reactions were performed without interruption (table 2, entry 1–3), the
results were clearly better than the interrupted reactions.
Figure 4. Time dependence of the catalytic transfer hydrogenation of 4-chloroacetophenone catalyzed by 3a–c
(0.75 M%) in 2-propanol at 80 °C with KOH as base. Every 5 min reactions were stopped and small portions taken
for analyses.
Figure 5. Lifetime study for 3b (0.75 M%) with 4-chloroacetophenone. Catalytic run was initiated using standard
conditions but after 30 min operating time, when >90% conversion had been obtained, an additional 1 mM of sub-
strate was added to the solution and the reaction monitored for a further 30 min (total conversion (99 + 93%)), after
which time a third aliquot of substrate was added and the reaction monitored for further 30 min (total conversion
(99 + 93 + 87%)), after which time a fourth aliquot of substrate was added and the reaction again monitored (total
conversion (99 + 93 + 87 + 56%)).

Ru(II)-N-heterocyclic carbene complexes
1243
Stability, activity, and continuity of 3b for hydrogen transfer were tested with 4-chloroac-
etophenone under normal operating conditions. After 30 min, an additional 1 mM of 4-chlo-
roacetophenone was added to the reaction mixture and reaction monitored, and after
90 min, a third aliquot of substrate was loaded. Although catalyst concentration was
reduced, catalyst performance of 3b hardly decreased with time (figure 5).
The catalytic activities of 3a–c are comparable with previously reported catalyst systems
and our previous results [20, 30–36]. In our previous studies [17], concentrations of cata-
lysts and KOH were higher and reaction times were longer than in our current study. With
this catalysis system, we developed reaction parameters such as reaction time, conversion,
and catalyst quantity. Catalyst systems 3a–c showed excellent activity with different sub-
strates and 3b performed with very high catalyst stability.
In summary, a new series of N-heterocyclic carbene ligands (1a–c), silver-NHC complexes
(2a–c), and ruthenium-NHC complexes (3a–c) have been synthesized. To demonstrate the
efficiency of 3a–c as catalyst precursors, transfer hydrogenations of ketones and aldehydes
were performed. Excellent activity and good catalyst stability were observed under mild reac-
tion conditions. Structural differences in 3a–c did not affect yields significantly. The
Ru–NHC catalysts described here gave one of the highest conversion rates to date for transfer
hydrogenation reaction catalyzed by Ru–NHC complexes with low base loading.
3. Experimental setup
3.1. Materials and methods
All procedures were carried out under an inert atmosphere using standard Schlenk line tech-
niques. Chemicals and solvents were purchased from Sigma Aldrich Co. (Dorset, UK). The
solvents used were purified by distillation over the drying agents indicated and were trans-
ferred under argon: Et2O (Na/K alloy), CH2Cl2 (CaH2), hexane, and toluene (Na). Elemen-
tal analyses were performed by the Turkish Research Council (Ankara, Turkey) Microlab.
NMR spectra and mass spectrometry (ESI) were recorded at Gaziosmanpasa University. IR
spectra were recorded at Gaziosmanpasa University on a Perkin Elmer Paragon 1000 FT-IR
Spectrometer employing a KBr disk. All reactions involving silver compounds were per-
formed with the exclusion of light.
3.2. Melting point determination
Melting points were measured in open capillary tubes with an Electrothermal-9200 melting
point apparatus and are uncorrected.
3.3. NMR spectroscopy
1H NMR and 13C NMR spectra were recorded using a Varian AS 400 Merkur spectrometer
operating at 400 MHz (1H), 100 MHz (13C) in CDCl3 with tetramethylsilane as an internal
reference. The NMR studies were carried out in high-quality 5 mm NMR tubes. Signals are
quoted in parts per million as δ downfield from tetramethylsilane (δ 0.00). Coupling con-
stants (J values) are given in Hz.

1244
S. Yaşar et al.
3.4. Gas chromatography
GC analyses of reactions mixtures have been done using a Shimadzu GC 2010-Plus GC-
FID system. Column: TeknokromaTRB-5 capillary column, 30 m × 0.32 mm × 0.25 μm.
Initial temperature at 50 °C, held for 1 min, ramp 2 °C/min next 90 °C, held for 3 min, ramp
40 °C/min next 240 °C held for 10 min. The temperature of the injector and detector were
held at 240 °C.
3.5. General preparation of 1,3-dialkylimidazolium salts
New unsymmetrical 1,3-dialkylimidazolium salts (1a–c) were prepared according to known
methods [23]. The unsymmetrical NHC precursors were prepared according to the general
reaction pathway shown in figure 1. To a solution of 1-methyl imidazole (0.78 g, 10.0 mM)
in DMF (5 mL) was added slowly the corresponding alkyl or aryl halogen (12.0 mM) com-
pounds, and the resulting mixture was stirred at 80 °C for 24 h. Diethyl ether (10 mL) was
added to obtain a creamy hydroscopic solid. The creamy solids were washed with diethyl
ether (3 × 10 mL) and the corresponding imidazolium chlorides obtained in (80–85%) yields
after drying under vacuum.
3.5.1. 1-Methyl-3-(2-methylbenzyl)imidazolium chloride, 1a. Yield: 1.77 g (80%). ν(CN)
1
= 1475 cm−1. H NMR (399.9 MHz, CDCl3) δ (ppm) = 2.26 [s, 3H, CH2C6H4(CH3)-2];
4.04 [s, 3H, NCH3]; 5.51 [s, 2H, CH2C6H4(CH3)-2]; 7.11–7.23 [m, 5H, NCHCHN and
CH2C6H4(CH3)-2]; 7.69 [s, 1H, NCHCHN]; 10.59 [s, 1H, NCHN]. 13C NMR (100 MHz,
CDCl3) δ (ppm) = 18.5 [CH2C6H4(CH3)-2]; 35.9 [NCH3]; 50.8 [CH2C6H4(CH3)-2]; 120.8,
123.2; 126.2, 129.0, 129.1 and 130.1 [CH2C6(CH3)-2]; 130.5 and 136.2 [NCHCHN]; 137.2
[NCHN]. LC–MS (ESI): m/z (%) 187.2 (100) [M–Cl]+. Anal. Calcd for C12H15N2Cl (%):
C, 64.71; H, 6.79; N, 12.58. Found: C, 64.78; H, 6.85; N, 12.68.
3.5.2. 1-Methyl-3-(3,4,5-trimethoxybenzyl)imidazolium chloride, 1b. Yield: 2.5 g (85%).
ν(CN)
= δ (ppm) = 3.79 [s, 3H,
1432 cm−1. 1H NMR (399.9 MHz, CDCl3)
CH2C6H2(OCH3)-4]; 3.86 [s, 6H, CH2C6H2(OCH3)-3,5]; 4.02 [s, 3H, NCH3]; 5.45 [s, 2H,
CH2C6H2(OCH3)3-3,4,5]; 6.83 [s, 2H, CH2C6H2(OCH3)3-3,4,5]; 7.39 and 7.46 [s, 2H,
NCHCHN]; 10.48 [s, 1H, NCHN]. 13C NMR (100 MHz, CDCl3) δ (ppm) = 35.9 [NCH3];
52.8 [CH2C6H2(OCH3)-4]; 55.8 [CH2C6H2(OCH3)-3,5]; 60.1 CH2C6H2(OCH3)3-3,4,5];
[105.8, 121.2, 122.5; 153.1, [CH2C6(OCH3)3-3,4,5]; 128.1 and 138.0 [NCHCHN]; 137.1
[NCHN]. Anal. Calcd for C14H19N2ClO3 (%): C, 56.28; H, 6.41; N, 9.38. Found: C, 56.36;
H, 6.45; N, 9.38.
3.5.3. 1-Methyl-3-(2-metoxhyethyl)imidazolium chloride, 1c [23]. Yield: 1.4 g (82%).
1
ν(CN) = 1432 cm−1. H NMR (399.9 MHz, CDCl3) δ (ppm) = 3.25 [s, 3H, CH2CH2OCH3];
3.36 [t, J = 4 Hz, 2H, CH2CH2OCH3]; 4.00 [s, 3H, NCH3]; 4.49 [t, J = 4 Hz, 2H,
CH2CH2OCH3]; 7.54 [s, 1H, NCHCHN]; 7.58 [s, 1H, NCHCHN]; 10.28 [s, 1H, NCHN].
13C NMR (100 MHz, CDCl3) δ (ppm) = 36.4 [CH2CH2OCH3]; 49.6 [CH2CH2OCH3]; 59.0
[NCH3]; 70.1 [CH2CH2OCH3]; 123.2 [NCHCHN]; 137.7 [NCHN]. LC–MS (ESI): m/z (%)
141.22 (100) [M–Cl]+. Anal. Calcd for C7H13N2OCl (%): C, 47.60; H, 7.42; N, 15.86.
Found: C, 47.54; H, 7.35; N, 15.80.

Ru(II)-N-heterocyclic carbene complexes
1245
3.6. General procedure for the preparation of the silver-NHC complexes (2a–c)
The silver-NHC complexes were prepared via reaction of Ag2O (5 mM) with dialkyl imi-
dazolium salts (10 mM) (1a–c) in CH2Cl2 (20 mL) and stirred overnight at room tempera-
ture (figure 2). Overnight stirred Ag–NHC solutions were filtered through Celite and diethyl
ether added to get white crystals. The white crystals slowly turn black in 3–4 days when
standing in light. The molecular structure of Ag–NHC complexes was determined by spec-
troscopic techniques.
3.6.1. Chloro-[1-methyl-3-(2-methylbenzyl)imidazole-2-ylidene]silver(I), 2a. Yield: 2.5 g
1
(76%), m.p.: 146–147 °C. ν(CN) = 1604 cm−1. H NMR (399.9 MHz, CDCl3) δ (ppm) =
2.28 [s, 3H, CH2C6H4(CH3)-2]; 3.86 [s, 3H, NCH3]; 5.27 [s, 2H, CH2C6H4(CH3)-2]; 7.11–
7.23 [m, 6H, NCHCHN and CH2C6H4(CH3)-2]. 13C NMR (100 MHz, CDCl3) δ (ppm) =
19.6 [CH2C6H4(CH3)-2]; 39.0 [NCH3]; 54.0 [CH2C6H4(CH3)-2]; 121.0, 122.1; 126.9,
128.6, 129.1 and 131.3 [CH2C6H4(CH3)-2];133.1 and 136.6 [NCHCHN]; 180.7 [Ag–Ccarb].
LC–MS(ESI): m/z = 480.2 [(L2Ag)AgCl2-AgCl2]. Anal. Calcd for AgC12H14N2Cl (%): C,
43.73; H, 4.28; N, 8.50. Found: C, 43.70; H, 4.23; N, 8.45%.
3.6.2. Chloro-[1-methyl-3-(3,4,5-trimethoxybenzyl)imidazole-2-ylidene]silver(I), 2b. Yield:
1
3 g (75%), m.p.: 158–159 °C. ν(CN) = 1602 cm−1. H NMR (399.9 MHz, CDCl3) δ (ppm) =
3.84 [m, 12H, NCH3 and CH2C6H2(OCH3)3-3,4,5]; 3.86 [s, 6H, CH2C6H2(OCH3)-3,5];
5.17 [s, 2H, CH2C6H2(OCH3)3-3,4,5]; 6.52 [s, 2H, CH2C6H2(OCH3)3-3,4,5]; 7.01 [s, 2H,
NCHCHN]. 13C NMR (100 MHz, CDCl3) δ (ppm) = 38.9 [NCH3]; 56.1 [CH2C6H2(OCH3)-
4]; 56.4 [CH2C6H2(OCH3)-3,5]; 60.8 CH2C6H2(OCH3)3-3,4,5]; [105.3, 121.1, 122.6;
153.6, [CH2C6(OCH3)3-3,4,5]; 131.1 [NCHCHN]; 180.2 [Ag–Ccarb]. LC–MS (ESI): m/z =
631.2 [(L2Ag)AgCl2-AgCl2]. Anal. Calcd for AgC14H18N2ClO3 (%): C, 41.45; H, 4.47; N,
6.91. Found: C, 41.39; H, 4.43; N, 6.88.
3.6.3. Chloro-[1-methyl-3-(2-metoxhyethyl)imidazole-2-ylidene]silver(I), 2c. Yield: 2 g
1
(76%), m.p.: 74 °C. ν(CN) = 1608 cm−1. H NMR (399.9 MHz, CDCl3) δ (ppm) = 3.22 [s,
3H, CH2CH2OCH3]; 3.38 [t, J = 4 Hz, 2H, CH2CH2OCH3]; 4.08 [s, 3H, NCH3]; 4.55 [t, J
= 4 Hz, 2H, CH2CH2OCH3]; 7.54–7.58 [m, 2H, NCHCHN]. 13C NMR (100 MHz, CDCl3)
δ
(ppm)
=
36.9 [CH2CH2OCH3]; 50.0 [CH2CH2OCH3]; 59.6 [NCH3]; 72.1
[CH2CH2OCH3]; 123.6 [NCHCHN]; 181.2 [Ag–Ccarb]. LC–MS (ESI): m/z = 388.2 [(L2Ag)
AgCl2-AgCl2]. Anal. Calcd for AgC7H12N2OCl (%): C, 29.66; H, 4.27; N, 9.88. Found: C,
29.62; H, 4.22; N, 9.81.
3.7. General method for the preparation of the ruthenium-NHC complexes (3a–c)
The ruthenium-NHC complexes (3a–c) were synthesized through transmetalation via silver
NHC complexes (2a–c) with the method reported by Wang and Lin [36]. We reacted Ag–
NHC complexes (5 mM) with [RuCl2(p-cymene]2 (10 mM) in the dark and the mixture was
stirred for 24 h at room temperature. The solution was filtered through Celite and crystal-
lized from dichloromethane:diethyl ether (1 : 2) at room temperature (figure 2).

1246
S. Yaşar et al.
3.7.1. Dichloro-[1-methyl-3-(2-methylbenzyl)imidazole-2-ylidene](p-cymene)ruthenium
1
(II), 3a. Yield: 0.3 g (60%), m.p.: 207–208 °C. ν(CN) = 1612 cm−1. H NMR (399.9 MHz,
CDCl3) δ (ppm) = 1.25 [d, J = 6.9 Hz, 6H, p-CH3C6H4CH(CH3)2]; 2.03 [s, 3H, p-
CH3C6H4CH(CH3)2]; 2.40 [s, 3H, CH2C6H4(CH3)-2]; 2.88 [h, J = 6.9 Hz, 1H, p-
CH3C6H4CH(CH3)2]; 4.07 [s, 3H, NCH3]; 4.92 [s, 2H, CH2C6H4(CH3)-2]; 5.30 and 6.12
[s, 4H, p-CH3C6H4CH(CH3)2]; 6.91 and 7.06 [d, J = 2 Hz, 2H, NCHCHN]; 7.14–7.25 [m,
4H, CH2C6H4(CH3)-2]. 13C NMR (100 MHz, CDCl3) δ (ppm) = 18.6 [p-CH3C6H4CH
(CH3)2]; 19.4 [p-CH3C6H4CH(CH3)2]; 30.7 [p-CH3C6H4CH(CH3)2]; 39.8 [CH2C6H4(CH3)-
2], 52.7 [NCH3]; 76.6 [CH2C6H4(CH3)-2]; 98.6, 107.8, 123.5, 123.9 [NCHCHN and p-
CH3C6H4CH(CH3)2]; 125.9, 126.3, 127.7, 130.6, 135.7 and 136.6 [CH2C6H4(CH3)-2];
174.6 (Ru–Ccarb). LC–MS (ESI): m/z (%) 457.1 (100) [M–Cl]+. Anal. Calcd for
RuC22H28N2Cl2 (%): C, 53.66; H, 5.73; N, 5.65. Found: C, 53.70; H, 5.78; N, 5.85.
3.7.2. Dichloro-[1-methyl-3-(3,4,5-trimethoxybenzyl)imidazole-2-ylidene](p-cymene)-
1
ruthenium(II), 3b. Yield: 0.4 g (71%), m.p.: 196–197 °C. ν(CN) = 1614 cm−1. H NMR
(399.9 MHz, CDCl3) δ (ppm) = 1.25 [d, J = 6.9 Hz, 6H, p-CH3C6H4CH(CH3)2]; 2.09 [s,
3H, p-CH3C6H4CH(CH3)2]; 2.97 [h, J = 6.9 Hz, 1H, p-CH3C6H4CH(CH3)2]; 3.84 [s, 6H,
CH2C6H2(OCH3)3-3,5]; 3.85 [s, 3H, CH2C6H2(OCH3)3-4]; 4.05 [s, 3H, NCH3]; 5.10 [s,
2H, CH2C6H2(OCH3)3-3,4,5]; 5.40 and 5.98 [s, 4H, p-CH3C6H4CH(CH3)2]; 6.65 [s, 2H,
CH2C6H2(OCH3)3-3,4,5]; 6.89 and 6.99 [s, 2H, NCHCHN]; 13C NMR (100 MHz, CDCl3)
δ (ppm) = 18.8 [p CH3C6H4CH(CH3)2]; 30.9 [p-CH3C6H4CH(CH3)2]; 39.8 [p-CH3C6H4CH
(CH3)2]; 55.0 [CH2C6H2(OCH3)3-4]; 56.4 [CH2C6H2(OCH3)3-3,5]; 60.9 [NCH3]; 76.6
[CH2C6H2(OCH3)3-3,4,5]; 99.2103.2, 105.6, 108.9 [p-CH3C6H4CH(CH3)2]; 122.5 123.8
[NCHCHN]; 132.5, 137.7, 153.5 [CH2C6H2(OCH3)3-3,4,5]; 174.4 (Ru–Ccarb). LC–MS
(ESI): m/z (%) 533.2 (100) [M–Cl]+. Anal. Calcd for RuC24H32N2Cl2O3 (%): C, 50.70; H,
5.67; N, 4.93. Found: C, 50.68; H, 5.60; N, 4.89.
3.7.3. Dichloro-[1-methyl-3-(methoxyethyl)imidazole-2-ylidene](p-cymene)ruthenium(II),
3c. Yield: 0.33 g (75%), m.p.: 146–147 °C. ν(CN) = 1621 cm−1. 1H NMR (399.9 MHz,
CDCl3) δ (ppm) = 1.26 [d, J = 6.9 Hz, 6H, p-CH3C6H4CH(CH3)2]; 2.10 [s, 3H, p-
CH3C6H4CH(CH3)2]; 2.93 [h, J = 6.9 Hz, 1H, p-CH3C6H4CH(CH3)2]; 3.33 [s, 3H,
CH2CH2(OCH3)]; 3.71 [m, 2H, CH2CH2(OCH3)]; 3.99 [s, 3H, NCH3]; 4.26 and 4.76 [m,
2H, CH2CH2(OCH3)]; 5.15 and 5.42 [s, 4H, p-CH3C6H4CH(CH3)2]; 6.97 [d, J = 2 Hz, 1H,
NCHCHN]; 7.32 [d, J = 2 Hz, 1H, NCHCHN]. 13C NMR (100 MHz, CDCl3) δ (ppm) =
18.5 [p-CH3C6H4CH(CH3)2]; 30.7 [p-CH3C6H4CH(CH3)2]; 39.5 [p-CH3C6H4CH(CH3)2];
51.5 [CH2CH2(OCH3)]; 53.48 [CH2CH2(OCH3)]; 58.8 [CH2CH2(OCH3)]; 73.2 [NCH3];
82.6, 85.1, 99.5, 108.6 [p-CH3C6H4CH(CH3)2]; 122.8 and 123.6 [NCHCHN]; 173.7 (Ru–
Ccarb). LC–MS (ESI): m/z (%) 411.1 (100)[M–Cl]+. Anal. Calcd for RuC17H26N2Cl2O
(%): C, 45.74; H, 5.87; N, 6.28. Found: C, 45.69; H, 5.78; N, 6.22.
3.8. General method for transfer hydrogenation of ketones and aldehydes
Ketones or aldehydes (1 mM) were added to a mixture of the ruthenium catalyst, 3a–c
(0.75 M%) and KOH (2 mM) in i-PrOH (3 mL), and the mixture was heated to 80 °C for
30 min (figure 3). Solution was cooled to room temperature and passed through silica gel.
Then volatiles were removed under reduced pressure and conversion distribution was
1
determined by H NMR spectroscopy and GC.

Ru(II)-N-heterocyclic carbene complexes
1247
Acknowledgement
This work was financially supported by Gaziosmanpasa University Research Fund (GOP. B.A.P:
2010/84, 2011/51 and 2011/105).
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