Preparation of a series of Ru(ii) complexes with N-heterocyclic carbene ligands for the catalytic transfer hydrogenation of aromatic ketones

Article abstract of DOI:10.1039/c1dt11203a

The reaction of [RuCl₂(p-cymene]₂ with Ag-N-heterocyclic carbene (NHC) complexes yields a series of [(p-cymene)Ru(NHC)] complexes (2a-f). All synthesised compounds were characterized by elemental analysis, NMR spectroscopy and the mo

Full text of DOI:10.1039/c1dt11203a

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PAPER
Preparation of a series of Ru(II) complexes with N-heterocyclic carbene
ligands for the catalytic transfer hydrogenation of aromatic ketones†
a
a
a
b
c
¨
¨
˙
¨
Nevin Gu¨rbu¨z, Emine Ozge Ozcan, Ismail Ozdemir,*‡ Bekir C¸ etinkaya, Onur S¸ahin and
Orhan Bu¨yu¨kgu¨ngo¨rc
Received 24th June 2011, Accepted 3rd November 2011
DOI: 10.1039/c1dt11203a
The reaction of [RuCl2(p-cymene]2 with Ag–N-heterocyclic carbene (NHC) complexes yields a series of
[(p-cymene)Ru(NHC)] complexes (2a–f). All synthesised compounds were characterized by elemental
analysis, NMR spectroscopy and the molecular structure of 2a was determined by X-ray
crystallography. All complexes have been tested as catalysts for the transfer hydrogenation of aromatic
ketones, showing excellent activity in this reaction.
activities.14 The first application of NHC complexes for the transfer
Introduction
hydrogenation reaction was reported by Nolan in 2001.15 With
regard to transfer hydrogenations different carbene or carbene-
phosphane systems containing Rh,16 Ir16,17 Ru18 and Ni19 have
been reported.
N-heterocyclic carbenes (NHC) have emerged as a useful class
of ligands in the development of homogeneous organic and
organometallic catalysts. They have proven an alternative to
tertiary phosphines in homogeneous catalysis, because of the
strong s-donating and negligible p-accepting character, NHCs
can form stable bonds with various metals from main group to
transition metals in different oxidation states and stabilize catalyt-
ically active intermediates.1,2 The last few decades have witnessed
an extensive exploration of metal-NHC complexes as catalysts
for a wide variety of organic transformations including olefin
metathesis,3 hydrosilylation,4 hydroformylation,5 hydrogenation,6
copolymerization,7 furan synthesis,8 C–C9 and C–N10 coupling
reactions and asymmetric transformations.11
Transfer hydrogenation is the addition of hydrogen to an
unsaturated molecule by a reagent other than H2. Typically a
sacrificial reagent (hydrogen donor) such as 2-propanol together
with a strong base and a Ru, Rh or Ir catalyst are used. Transfer hy-
drogenation generates few by-products, avoids hazardous reagents,
and uses readily available and benign starting materials such
as carbonyl compounds, imines, alkenes and alkynes.12 Transfer
hydrogenation is preferred for large-scale industrial use in the hope
of developing a greener process by reducing waste production and
energy use and lowering toxicity.13 Recently, research has also been
devoted to the synthesis of functionalized ligands containing NHC
moieties, in order to modify the ligand properties and catalytic
Recently, our research group has focused on N-heterocyclic
carbene derivative ligands and their metal complexes as syn-
thesis, characterization and catalytic activity.20 We have also
reported the application of imidazolidin-2-ylidene ruthenium
complexes and benzimidazolidin-2-ylidene ruthenium complexes
in transfer hydrogenation of aromatic ketones.21 In this ar-
ticle, the new imidazolidinium salts that are precursors to
these NHC ligands are readily synthesized and the subsequent
formation of complexes of Ag–NHC has proved facile. The
reaction of the Ag–NHC complexes with [RuCl2(p-cymene)]2
in dichloromethane afforded the Ru-NHC complexes (2a–
2f). All synthesized compounds were characterized by 1H
NMR, 13C NMR, IR and elemental analysis techniques which
support the proposed structures. The molecular and crys-
tal structure of dichloro-[1,3-bis(2-methylbenzyl)imidazolidin-2-
ylidene](p-cymene)ruthenium(II) complex was determined by sin-
gle crystal X-ray diffraction technique. Further, the synthesized
complexes have been effectively used as catalyst in transfer
hydrogenation of ketones in the presence of isopropanol and KOH
as base.
Results and discussion
a
˙
Ino¨nu¨ University, Faculty of Science, Department of Chemistry,
44280, Malatya, Turkey. E-mail: ismail.ozdemir@inonu.edu.tr;
Fax: +904223410212
Synthesis of imidazolidinium salts
bEge University, Faculty of Science, Department of Chemistry, 35100,
A series of new symmetrical 1,3-dialkyl-imidazolidinium salts
(1) were prepared according to known methods22 and were
conventional NHC precursors. The symmetrical NHC precursors
were prepared according to general reaction pathway depicted in
Scheme 1. Treatment of ethylenediamine with 2 equivalents of
aromatic aldehyde in methanol at room temperature led to the
˙
Bornova-Izmir, Turkey
cOndokuz Mayıs University, Department of Physics, 55139, Samsun, Turkey
† CCDC reference numbers 828989. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11203a
‡ Dedicated to Prof. Dr Christian Bruneau on the occasion of his 60th
birthday.
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Scheme 1 Preparation of imidazolidinium salts.
formation of the corresponding diimine. Their reduction with
sodium borohydride in methanol, followed by treatment with
triethylorthoformate in the presence of ammonium chloride with
continuous elimination of ethanol led to the formation of the
expected imidazolidinium chlorides in excellent yields.
Ru.25 Several other procedures have also been developed in recent
years.26
The procedure involving Ag–NHC carbene, generated by treat-
ment of imidazolium salt with Ag2O is probably one of the most
general methods, because it generates an air stable intermediate
under mild reaction-conditions, thus allowing an easy access to
a wide range of transition metal complexes. It is often used
successfully when other methods fail.27 The use of Ag–NHC
complexes as carbene transfer reagents provides in many cases
a convenient way to overcome the difficulties arising from using
strong bases, inert atmospheres, and complicated workups. This
method was used for the preparation of complexes 2a–2f. Firstly,
we examined the formation of variously substituted NHC–Ag
complexes having two alkyls on the NHC moiety. All these NHC–
Ag compounds were obtained in high yields.28 The isolated Ag-
complex prepared from 1a–1f and Ag2O were converted into the
red brown monocarbene Ru–NHC complexes (2a–2f) in high
yields (Scheme 2). The air and moisture-stable ruthenium carbene
complexes (2a–f) were soluble in solvents such as dichloromethane,
chloroform, toluene, and tetrahydrofurane and insoluble in non-
polar solvents.
The complexes 2a–f, which are very stable in the solid state have
been characterized by analytical and spectroscopic techniques.
Ruthenium complexes exhibit a characteristic u(NCN) band typically
at 1495, 1515, 1497, 1492, 1498 and 1515 cm-1 respectively for
2a–2f.29 13C chemical shifts provide a useful diagnostic tool for
this type of metal carbene complex. The chemical shifts for the
carbon atom fall in the 206.6–209.7 ppm and are similar to those
found in other ruthenium–carbene complexes. These complexes
show typical spectroscopic signatures, which are in line with those
recently reported for other [RuCl2(NHC)(arene)] complexes.29
The imidazolidinium salts were isolated as colourless solids in
1
very good yields and fully characterized by H and 13C NMR,
and IR spectroscopies, elemental analyses, and their melting
points were determined (see experimental section). The 1H NMR
spectra of the imidazolidinium salts further supported the assigned
structures; the resonances for acidic C(2)-H were observed as
sharp singlet at the 9.06, 10.51, 10.39, 10.61, 8.32 and 10.65 ppm
respectively for 1a–f. The positions of the methyl groups on the
benzyl rings have also a strong influence on the chemical shift
of the proton. For the imidazoldinium salts 1a, there is shielding
due to the ortho methyl groups, so the chemical shift is observed
singlet at the 9.06 ppm. Whereas the 4-substitution pattern in 1b
leads to 10.51 ppm. 13C NMR chemical shifts were consistent with
the proposed structure; the imino carbon appeared as a typical
singlet in the 1H-decoupled mode in the 158.6, 158.5, 158.4, 158.6,
156.2 and 158.8 ppm respectively for imidazolidinium salts (1a–f).
The IR data for imidazolidinium salts 1a–f clearly indicate the
presence of the –C N– group with a n(C N) vibration at 1660,
1669, 1661, 1669, 1653 and 1646 cm-1 respectively for 1a–f. The
NMR values are similar to those found for other imidazolidinium
salts.20b,23 The salts are air- and moisture stable both in the solid
state and in solution.
Preparation of ruthenium–carbene complexes 2a–f
The complexation protocols in NHC chemistry are mainly based
on the following routes: i) the complexation of the free, preisolated
NHC with metal; ii) in situ deprotonation of the azolium salt
by base, either exogenous or embedded in the metal precursor,
and subsequent metal-complexation; iii) use of basic Ag2O to
generate a Ag–NHC complex, followed by the NHC transfer
to a late transition metal via transmetallation.24 Recently, CuI–
NHC complexes have been shown to transfer NHC to Au, Pd and
Catalytic transfer hydrogenation of ketones
Transition-metal catalyzed transfer hydrogenation using 2-
propanol as a hydrogen source has become an efficient method
in organic synthesis as illustrated by several useful applications
reported in recent years.12–19 The reaction conditions for this
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Scheme 2 General preparation of Ru–NHC complexes.
important process are economic, relatively mild and environmen-
tally friendly. The volatile acetone product can also be easily
removed to shift an unfavorable equilibrium. Owing to its effi-
ciency in the transfer hydrogenation of acetophenone derivatives,
ruthenium(II) complexes (2a–2f) were further investigated by
transfer hydrogenation of various methyl aryl ketones.
The ruthenium(II) complexes 2a–2f catalyzed the reduction of
ketones to the corresponding alcohols via hydrogen transfer from
2-propanol with KOH as the promoter. As the starting point,
the performance of the catalysts in the transfer hydrogenation
were screened by using acetophenone as a model substrate
(eqn(1)).
In a typical experiment the preformed, isolated crystalline
catalyst (0.01 mmol) was dissolved in 2-propanol. After the
catalysts had completely dissolved, acetophenone (1.00 mmol) and
a base (4 mmol) were added and the reaction was performed at
80 ◦C. The reactions were conducted at a substrate/catalyst/base
(S/C/base) molar ratio of 1 : 0.01 : 4. In the transfer hydrogenation
reaction, the base facilitates the formation of ruthenium alkoxide
by abstracting proton from the alcohol and subsequently alkoxide
undergoes b-elimination to give ruthenium hydride, which is
an active species in this reaction. Since the base facilitates the
formation of a ruthenium alkoxide by abstracting the proton
from isopropanol, different bases were used as promoters in the
transfer hydrogenation of ketones. Acetophenone was kept as a
test substrate and allowed it to react in isopropanol with catalytic
quantities of complex 2 in the presence of different bases like
Cs2CO3, K2CO3, NaOH, KOH, t-BuOK and NaOAc. It has
been observed that NaOH and KOH are shown to have good
conversions when compared to the Cs2CO3, K2CO3, t-BuOK and
NaOAc in the hydrogenation reactions. The stronger the base
the higher the conversion rate, NaOAc (51%) < K2CO3(58%) <
Cs2CO3(61%) < t-BuOK(82%) < NaOH (91%) < KOH(97%).
As in the previous study the best results were obtained with
KOH.21c Hence, it is decided that base KOH is the best compromise
between optimum reaction rate in isopropanol and reaching 97%
conversion for acetophenone within 1 h. In the absence of a
base no transfer hydrogenation of the ketones was observed.
Under the reaction conditions complex 2b proved to be the most
effective catalyst relative to 2a, 2c, 2d, 2e and 2f. The reduction of
acetophenone with 2b was completed within 1 h reaching 97%. In
contrast, acetophenone was reduced within 1 h using 2a, 2c, 2d, 2e
and 2f with 94, 96, 94, 96 and 90% conversion, respectively (Table
1, entries 1–6). Also, we tried this reaction at 30 min. However, the
(1)
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Table 1 Transfer hydrogenation of ketones catalyzed by 2a–f
Entry
1
Cat.
Substrate
Product
Yield (%)
94
2a
2
4
4
5
6
7
8
9
10
11
12
13
2b
2c
2d
2e
2f
97
96
94
96
90
2a
2b
2c
2d
2e
2f
75a
82a
72a
69a
76a
68a
92
2a
14
15
16
17
18
19
2b
2c
2d
2e
2f
94
92
90
91
90
94
2a
20
21
22
23
24
25
2b
2c
2d
2e
2f
98
95
94
92
95
92
2a
26
27
28
29
30
31
2b
2c
2d
2e
2f
90
91
89
90
91
63
2a
32
33
34
35
36
37
38
2b
2c
2d
2e
2f
2a
2b
68
61
67
68
60
—
—
39
2a
91
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Table 1 (Contd.)
Entry
40
Cat.
Substrate
Product
Yield (%)
95
2b
41
42
43
44
2c
2d
2e
2f
91
92
93
90
Reaction conditions: 1.0 mmol substrate, iPrOH (10 mL), KOH (4 mmol), Ru–NHC (0.01 mmol), 80 ◦C, 1 h. Purity of compounds is checked by GC and
GC-MS and yields are based on ketones. Yields were determined by GC.a Reaction time is 30 min.
yields lower than 1 h, for example the reduction of acetophenone
with 2b was completed within 30 min 82% (Table 1, entries 7–12).
A variety of ketones were transformed to the corresponding
secondary alcohols. Typical results are shown in Table 1. Under
Experimental section
All reactions for the preparation imidazolidinium salts (1) and
ruthenium-(NHC) complexes (2) were carried out under argon
in flame-dried glassware using standard Schlenk techniques. The
those conditions p-methoxyacetophenone, p-fluoroacetophenone
solvents used were purified by distillation over the drying agents
and 3,4,5-trimethoxyacetophenone react very cleanly and in
indicated and were transferred under Ar: Et2O (Na/K alloy),
good yields with 2-propanol (Table 1, entries 14, 20, and 25).
CH2Cl2 (P4O10), hexane, toluene (Na). All reagents were purchased
The presence of electron withdrawing (F) or electron donating
from Aldrich Chemical Co. Melting points were determined in
(OCH3) substituents on acetophenone (Table 1, entries 16, 20,
glass capillaries under air with an Electrothermal-9200 melting
and 26) has a significant effect on the reduction of ketones to
point apparatus. FT-IR spectra were recorded as KBr pellets in
their corresponding alcohols. The maximum conversion of 4-
the range 400–4000 cm-1 on a ATI UNICAM 1000 spectrometer.
fluoroacetophenone to corresponding alcohol was achieved over
1H NMR and 13C NMR spectra were recorded using a Varian AS
a period of 1 h (Table 1, entry 20).
400 Merkur spectrometer operating at 400 MHz (1H), 100 MHz
The conversion of ketones with bulky substituents on the
(13C) in CDCl3 and DMSO-d6 with tetramethylsilane as an internal
aromatic ring was not observed or slightly decreased. For example,
reference. All reactions were monitored on a Agilent 6890 N GC
when 2,4,6-trimethylbenzyl-methyl ketone was used conversion
system by GC-FID with a HP-5 column of 30 m length, 0.32 mm
decreased (Table 1, entries 31–36) and when ketone with pen-
diameter and 0.25 mm film thickness. Column chromatography was
performed using silica gel 60 (70–230 mesh). Elemental analyses
were performed by Turkish Research Council (Ankara, Turkey)
Microlab.
tamethyl on aromatic ring was used in the transfer hydrogenation,
conversion was not observed (Table 1, entries 37–38). The
ruthenium complexes also catalyzed the transfer hydrogenation
of benzophenone very effectively (Table 1, entries 39–44). Among
the tested complexes, the complex 2b is highly efficient in the
transfer hydrogenation of ketone to secondary alcohol. Also, in
our previous work, reaction time was chosen as 12 h and AgOTf
was used for the activation of Ru–(NHC) complexes. However
in this present work AgOTf is not used and the reaction time is
lowered to 1 h and higher conversion is obtained as compared with
our previous results.21a,b
General preparation of 1,3-dialkylimidazolidinium salts
The aromatic aldehyde (20 mmol) and the ethylenediamine
(10 mmol) were stirred overnight in methanol. The diimine
was collected as a white solid, filtrated and recrystallized in an
alcohol/ether mixture. The diimine (10 mmol) was subsequently
reduced by NaBH4 (30 mmol) in CH3OH (30 mL). The solution
was then treated by 1 N HCl, and the organic phase was
extracted with CH2Cl2 (3 ¥ 30 mL). After drying over MgSO4
and evaporation, the diamine was isolated as a solid. The diamine
was then treated in a large excess of triethyl orthoformate (50 mL)
Conclusions
From readily available starting materials, such as 1,3-
dialkylimidazoldinium salts, six [RuCl2(p-cymene)(1,3-dialkyl-
imidazolidin-2-ylidene)] complexes (2a–f) have been prepared by
transmetallation from Ag–NHC complexes. Complex 2a has been
characterized by single crystal X-ray diffraction studies. The
efficiency of complexes 2a–f as catalyst precursors for the transfer
hydrogenation of ketones has been established.
◦
in the presence of 10 mmol of NH4Cl at 110 C in a distillation
apparatus until the removal of ethanol ceased. Upon cooling to
RT a colourless solid precipitated, which was collected by filtration
and dried under vacuum. The crude product was recrystallized
from absolute ethanol to give colourless needles and the solid was
washed with diethyl ether (2 ¥ 10 mL) and dried under vacuum.
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1,3-Bis(2-methylbenzyl)imidazolidinium chloride, 1a
Calcd. for C25H37N4Cl: C, 67.99; H, 8.69; N, 13.06. Found: C,
67.89; H, 8.91; N, 13.05%.
Yield: 4.43 g; 90%; m.p.: 169–170 ◦C; n(CN): 1660 cm-1. 1H
NMR (d, CDCl3): 2.30 (s, 6H, CH2C6H4(CH3)-2), 3.73 (s,
4H, NCH2CH2N), 4.72 (s, 4H, CH2C6H4(CH3)-2), 7.23–7.33(m,
8H, CH2C6H4(CH3)-2), 9.06 (s, 1H, NCHN). 13C{H}NMR
(d, CDCl3): 19.3 (CH2C6H2(CH3)-2), 48.7 (NCH2CH2N), 49.7
(CH2C6H4(CH3)-2), 127.0, 129.3, 129.9, 131.4, 132.4 and 137.5
(CH2C6H4(CH3)-2), 158.6 (NCHN). Anal. Calcd. for C19H23N2Cl:
C,72.48; H, 7.36; N, 8.90. Found: C, 72.54; H, 7.47; N, 8.90%.
1,3-Bis(3,4-dimethoxybenzyl)imidazolidinium chloride, 1f
Yield: 4,95 g; 81%; m.p: 403–404 ◦C; n(CN)
:
1646 cm-1.
1H NMR (d, CDCl3): 3.72 (s, 4H, NCH2CH2N), 3.84 (s,
6H, CH2C6H3(OCH3)2-3), 3.91 (s, 6H, CH2C6H3(OCH3)2-
4), 4.77 (s, 4H, CH2C6H3(OCH3)2-3,4), 6.78–7.11(m, 6H,
CH2C6H3(OCH3)2-3,4), 10.65 (s, 1H, NCHN). 13C{H}NMR
(d, CDCl3): 47.3 (NCH2CH2N), 55.9 (CH2C6H3(OCH3)2-3),
56.5 (CH2C6H3(OCH3)2-4), 52.8 (CH2C6H3(OCH3)2-3,4), 111.1,
112.1, 121.5, 124.9, 129.6, 149.7 (CH2C6H3(OCH3)2-3,4), 158.8
(NCHN). Anal. Calcd. for C21H27N2ClO4: C,61.99; H, 6.69; N,
6.88. Found: C, 61.73; H, 6.64; N, 6.92%.
1,3-Bis(4-methylbenzyl)imidazolidinium chloride, 1b
Yield: 3,87 g; 82%; m.p: 227–228 ◦C; n(CN): 1669 cm-1. 1H
NMR (d, CDCl3): 2.30 (s, 6H, CH2C6H4(CH3)-4), 3.71 (s,
4H, NCH2CH2N), 4.79 (s, 4H, CH2C6H4(CH3)-4), 7.13 and
7.25 (d, J = 7.8 Hz, 8H, CH2C6H4(CH3)-4), 10.51 (s, 1H,
NCHN). 13C{H}NMR (d, CDCl3): 21.2 (CH2C6H2(CH3)-4), 47.5
(NCH2CH2N), 51.6 (CH2C6H4(CH3)-4), 128.8, 129.5, 129.9, 138.9
(CH2C6H4(CH3)-4), 158.5 (NCHN). Anal. Calcd. for C19H23N2Cl:
C,72.48; H, 7.36; N, 8.90. Found: C, 72.52; H, 7.40; N, 8.92%.
General procedure for the preparation of the ruthenium–NHC
complexes (2a–f)
The ruthenium complexes were prepared by means of Ag-
carbene transfer method developed by Wang and Lin.30 The silver
monocarbene complexes, which should subsequently serve as a
carbene-transfer agent, were synthesized by the reaction of Ag2O
with 2 equiv. of salts (1) in CH2Cl2 at ambient temperature. The
formulation of Ag–NHC was determined by elemental analyses
and spectroscopic techniques.28 We conveniently reacted Ag–NHC
with [RuCl2(p-cymene]2 in dark condition and the mixture was
allowed to stir for 24 h at room temperature. The solution was
filtered through Celite, and the solvent was removed under vacuum
to afford the product as a red-brown powder. The crude product
was recrystallized from dichloromethane : diethyl ether (1 : 2) at
room temperature.
1,3-Bis(4-ethylbenzyl)imidazolidinium chloride, 1c
Yield: 4,52 g; 88%; m.p: 243–244 ◦C; n(CN): 1661 cm-1. 1H
NMR (d, CDCl3): 1.12 (t, J = 7.5 Hz, 6H, CH2C6H4(CH2CH3)-
4), 2.56 (q, J = 7.5 Hz, 4H, CH2C6H4(CH2CH3)-4), 3.68
(s, 4H, NCH2CH2N), 4.73 (s, 4H, CH2C6H4(CH2CH3)-4),
7.07 and 7.22 (d, J = 7.8 Hz, 8H, CH2C6H4(CH2CH3)-
4), 10.39 (s, 1H, NCHN). 13C{H}NMR (d, CDCl3):
15.4 (CH2C6H2(CH2CH3)-4), 28.5 (CH2C6H2(CH2CH3)-4), 47.5
(NCH2CH2N), 51.9 (CH2C6H4(CH2CH3)-4), 128.7, 128.9, 129.7,
145.2 (CH2C6H4(CH2CH3)-4), 158.4 (NCHN). Anal. Calcd. for
C21H27N2Cl: C,73.56; H, 7.94; N, 8.17. Found: C, 73.61; H, 7.90;
N, 8.21%.
Dichloro-[1,3-bis(2-methylbenzyl)imidazolidin-2-ylidene](p-
cymene)ruthenium(II), 2a
1,3-Bis(4-i-propylbenzyl)imidazolidinium chloride, 1d
Yield: 0,33 g; 69%; m.p: 225–227 ◦C; u(CN) = 1495 cm-1. 1H NMR
(d, CDCl3): 1.21 (d, J = 7.2 Hz, 6H, p-CH3C6H4CH(CH3)2), 1.94 (s,
3H, p-CH3C6H4CH(CH3)2), 2.35 (s, 6H, CH2C6H4(CH3)-2), 2.56
(h, J = 7.2 Hz, 1H, p-CH3C6H4CH(CH3)2), 3.46–3.61 (m, 4H,
NCH2CH2N), 4.84 ve 5.59 (d, J = 15.6 Hz, 4H, CH2C6H4(CH3)-
2), 5.11 ve 5.33 (d, J = 6.3 Hz, 4H, p-CH3C6H4CH(CH3)2)),
7.23–7.41 (m, Hz, 8H, CH2C6H4(CH3)-2). 13C{H}NMR (d,
CDCl3): 17.9 (p-CH3C6H4CH(CH3)2), 19.3 (CH2C6H4(CH3)-2),
22.5 (p-CH3C6H4CH(CH3)2), 30.58 (p-CH3C6H4CH(CH3)2), 49.4
(NCH2CH2N), 52.7 (CH2C6H4(CH3)-2), 84.8, 85.2, 96.2, 106.3 (p-
CH3C6H4CH(CH3)2), 126.3, 127.1, 130.8, 135.8 (CH2C6H4(CH3)-
2), 209.7 (Ru–Ccarb). Anal. Calcd for RuC29H36N2Cl2: C, 59.58; H,
6.21; N, 4.79; found: C, 59.60; H, 6.21; N, 4.78%.
Yield: 4,34 g; % 78; m.p.: 246–247 ◦C; n(CN): 1669 cm-1. 1H NMR
(d, CDCl3): 1.90 (d, J = 6.9 Hz, 12H, CH2C6H4(CH(CH3)2)-
4), 2.89 (h, J = 6.9 Hz, 2H, CH2C6H4(CH(CH3)2)-4), 3.74
(s, 4H, NCH2CH2N), 4.73 (s, 4H, CH2C6H4(CH(CH3)2)-4),
7.18 and 7.29 (d, J = 5.1 Hz, 8H, CH2C6H4(CH(CH3)2)-
4), 10.61 (s, 1H, NCHN). 13C{H}NMR (d, CDCl3): 23.8
(CH2C6H2(CH(CH3)2)-4), 33.8 (CH2C6H2(CH(CH3)2)-4), 47.5
(NCH2CH2N), 52.0 (CH2C6H4(CH(CH3)2)-4), 127.3, 128.9,
129.9, 149.8 (CH2C6H4(CH3)-4), 158.6 (NCHN). Anal. Calcd. for
C23H31N2Cl: C,74.47; H, 8.42; N, 7.55. Found: C, 74.41; H, 8.70;
N, 7.61%.
1,3-Bis(4-diethylaminobenzyl)imidazolidinium chloride, 1e
Dichloro-[1,3-bis(4-methylbenzyl)imidazolidin-2-ylidene](p-
cymene)ruthenium(II), 2b
Yield: 3.40 g; 79%; m.p.: 113–115 ◦C; n(CN): 1653 cm-1. 1H NMR
(d, CDCl3): 1.15 (t, J = 7.05 Hz, 12H, CH2C6H4N(CH2CH3)2-
4), 3.36 (q, J = 6.9 Hz, 8H, CH2C6H4N(CH2CH3)2-4), 3.74
(s, 4H, NCH2CH2N), 4.52 (s, 4H, CH2C6H4N(CH2CH3)2-4),
6.62 and 7.11 (d, J = 8.7 Hz, 8H, CH2C6H4N(CH2CH3)2-
4), 8.32 (s, 1H, NCHN). 13C{H}NMR (d, CDCl3): 12.5
(CH2C6H2N(CH2CH3)2-4), 44.3 (CH2C6H2N(CH2CH3)2-4), 47.5
(NCH2CH2N), 52.0 (CH2C6H4N(CH2CH3)2-4), 111.8, 118.2,
130.3, 148.1 (CH2C6H4N(CH2CH3)2-4), 156.2 (NCHN). Anal.
Yield: 0,33 g; 69%; m.p: 230–231 ◦C; u(CN) = 1515 cm-1. 1H NMR
(d, CDCl3): 1.29 (d, J = 6.9 Hz, 6H, p-CH3C6H4CH(CH3)2),
2.17 (s, 3H, p-CH3C6H4CH(CH3)2), 2.91 (h, J = 6.9 Hz,
1H, p-CH3C6H4CH(CH3)2), 2.34 (s, 6H, CH2C6H4(CH3)-4),
3.38–3.53 (m, 4H, NCH2CH2N), 4.88 (d, J = 6.9 Hz, 2H,
CH2C6H4(CH3)-4) and 5.37 (m, 2H, CH2C6H4(CH3)-4), 5.17
and 5.43 (d, J = 6 Hz, 4H, p-CH3C6H4CH(CH3)2), 7.17 and
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7.31 (d, J = 8.1 Hz, 8H, CH2C6H4(CH3)-4). 13C{H}NMR (d,
CDCl3): 18.8 (p-CH3C6H4CH(CH3)2), 21.4 (CH2C6H2(CH3)3-4),
22.6 (p-CH3C6H4CH(CH3)2), 30.7 (p-CH3C6H4CH(CH3)2), 48.8
(NCH2CH2N), 55.6 (CH2C6H4(CH3)-4), 83.7, 85.7, 97.6, 108.2 (p-
CH3C6H4CH(CH3)2), 127.8, 129.4, 134.1, 137.3 (CH2C6H4(CH3)-
4), 208.1 (Ru–Ccarb). Anal. Calcd. for RuC29H36N2Cl2: C, 59.58; H,
6.21; N, 4.79; found: C, 59.62; H, 6.23; N, 4.75%.
12.5 (CH2C6H4N(CH2CH3)2-4), 18.8 (p-CH3C6H4CH(CH3)2),
22.7 (p-CH3C6H4CH(CH3)2), 30.71 (p-CH3C6H4CH(CH3)2),
44.35 (CH2C6H4N(CH2CH3)2-4), 48.59 (NCH2CH2N), 55.31
(CH2C6H4(CH3)-4), 83.5, 85.6, 86.6, 97.6 (p-CH3C6H4CH(CH3)2),
147.3, 129.1, 123.4, 111.8 (CH2C6H4(CH3)2-4). 206.6 (Ccarb). Anal.
Calc. For RuC29H36N2Cl2: C, 59.58; H, 6.21; N, 4.79; found: C,
59.60; H, 6.21; N, 4.78%.
Dichloro-[1,3-bis(4-ethylbenzyl)imidazolidin-2-ylidene](p-
cymene)ruthenium(II), 2c
Dichloro-[1,3-bis(3,4-dimethoxybenzyl)imidazolidin-2-ylidene](p-
cymene)ruthenium(II), 2f
Yield: 0.33 g; % 65; m.p: 206–207 ◦C; u(CN) = 1497 cm-1. 1H
NMR (d, CDCl3): 1.24 (t, J = 7.5 Hz, 6H, CH2C6H4(CH2CH3)-
Yield: 0.34 g;
% = 1515
63; m.p: 202–203 ◦C; u(CN)
cm-1. 1H NMR (d, CDCl3): 1.34 (d, J = 6.9 Hz, 6H, p-
CH3C6H4CH(CH3)2), 2.24 (s, 3H, p-CH3C6H4CH(CH3)2), 3.03
(h, J = 6.9 Hz, 1H, p-CH3C6H4CH(CH3)2), 3.54–3.91 (m,
4H, NCH2CH2N), 3.89 (s, 6H, CH2C6H3(OCH3)2-4), 3.90
(s, 6H, CH2C6H3(OCH3)2-4), 4.63 and 5.56 (d, J = 14.4 Hz,
4H, CH2C6H3(OCH3)2-3,4) 5.12 and 5.47 (d, J = 6 Hz, 4H,
p-CH3C6H4CH(CH3)2), 6.82–7.22 (m, 6H, CH2C6H4(OCH3)2-
3,4). 13C{H}NMR (d, CDCl3): 19.0 (p-CH3C6H4CH(CH3)2),
22.6 (p-CH3C6H4CH(CH3)2), 30.8 (p-CH3C6H4CH(CH3)2),
4), 1.28 (d,
J
= 6.9 Hz, 6H, p-CH3C6H4CH(CH3)2),
2.17 (s, 3H, p-CH3C6H4CH(CH3)2), 2.65 (q, J = 7.5 Hz,
4H, CH2C6H4(CH2CH3)-4), 2.90 (h, J = 6.9 Hz, 1H, p-
CH3C6H4CH(CH3)2), 3.55–3.97 (m, 4H, NCH2CH2N), 4.92 and
5.33 (d, J = 15 Hz, 4H, CH2C6H4(CH2CH3)-4) 5.17 and 5.44
(d, J = 6 Hz, 4H, p-CH3C6H4CH(CH3)2), 7.19 and 7.34 (d, J =
7.8 Hz, 8H, CH2C6H4(CH2CH3)-4). 13C{H}NMR (d, CDCl3):
15.6 (CH2C6H4(CH2CH3)-4), 18.8 (p-CH3C6H4CH(CH3)2),
22.6 (p-CH3C6H4CH(CH3)2), 30.7 (p-CH3C6H4CH(CH3)2), 48.8
(NCH2CH2N), 55.6 (CH2C6H4(CH2CH3)-4), 83.7, 85.7, 97.6,
108.0 (p-CH3C6H4CH(CH3)2), 127.8, 128.2, 134.3, 143.6
(CH2C6H4(CH2CH3)-4), 208.1 (Ru–Ccarb). Anal. Calcd. for
RuC31H40N2Cl2: C, 60.78; H, 6.58; N, 4.57; found: C, 60.81; H,
6.64; N, 4.58%.
48.8
(NCH2CH2N),
55.9
(CH2C6H3(OCH3)2-3),
56.1
(CH2C6H3(OCH3)2-4), 53.4 (CH2C6H3(OCH3)2-3,4), 82.3,
86.7, 98.2, 109.4 (p-CH3C6H4CH(CH3)2), 110.7, 111.9, 120.0,
129.4, 148.6, 149.3 (CH2C6H3(OCH3)2-3,4), 207.8 (Ru–Ccarb).
Anal. Calcd. for RuC31H40N2Cl2O4: C, 55.03; H, 5.96; N, 4.14;
found: C, 54.99; H, 5.98; N, 4.10%.
Dichloro-[1,3-bis(4-i-propylbenzyl)imidazolidin-2-ylidene](p-
cymene)ruthenium(II), 2d
General procedure for the transfer hydrogenation of ketones
Under an inert atmosphere a mixture containing ketones (1 mmol),
the ruthenium catalyst, 2a–2f, (0.01 mmol) and KOH (4 mmol)
was heated to reflux in 10 mL of i-PrOH for 1 h. The solvent was
then removed under reduced pressure and product distribution
was determined by 1H-NMR spectroscopy and GC and GC-MS.
Yield: 0,36 g; % 71; m.p: 226–227 ◦C; u(CN) = 1492 cm-1. 1H
NMR (d, CDCl3): 1.26 (t, J = 7.2 Hz, 12H, CH2C6H4(CH(CH3)2)-
4), 1.28 (d, J = 7.8 Hz, 6H, p-CH3C6H4CH(CH3)2), 2.17
(s, 3H, p-CH3C6H4CH(CH3)2), 2.95 (h, J = 6.9 Hz, 3H,
CH2C6H4(CH(CH3)2)-4 and p-CH3C6H4CH(CH3)2), 3.46 (m,
4H, NCH2CH2N), 4.97 and5.31 (m, 4H, CH2C6H4(CH(CH3)2)-
4) 5.17 and 5.44 (d, J = 6 Hz, 4H, p-CH3C6H4CH(CH3)2),
7.23 and 7.34 (d, J = 8.1 Hz, 8H, CH2C6H4(CH(CH3)2)-
4). 13C{H}NMR (d, CDCl3): 18.8 (p-CH3C6H4CH(CH3)2),
22.6 (p-CH3C6H4CH(CH3)2), 24.0 (CH2C6H2(CH(CH3)2)-4),
30.7 (p-CH3C6H4CH(CH3)2), 33.8 CH2C6H2(CH(CH3)2)-4), 48.8
(NCH2CH2N), 55.6 (CH2C6H4(CH3)-4), 83.7, 85.8, 97.6,
108.0 (p-CH3C6H4CH(CH3)2), 126.7, 127.7, 134.4, 148.2
(CH2C6H4(CH(CH3)2)-4), 208.2 (Ru–Ccarb). Anal. Calcd. for
RuC33H44N2Cl2: C, 61.86; H, 6.92; N, 4.37; found: C, 61.81; H,
6.95; N, 4.41%.
Structural characterization of 2a
Crystal of 2a suitable for X-ray analysis was obtained from a
dichloromethane solution layered with diethyl ether. The single-
crystal X-ray data were collected on a STOE diffractometer with
an IPDS II image plate detector Mo-Ka radiation l = 0.71069
˚
A. The structure was solved by direct-methods using SHELXS-
9731 and refined by full-matrix least-squares methods on F2 using
SHELXL-9732 from within the WINGX33,34 suite of software. The
crystal was very small and thin and so the reflection intensities were
too low. So, the observed/unique data ratio (23%) of data was not
as desired. The absorption correction applied is also not as good
as expected due to the smallness of the crystal and the absorption
coefficient of Ru is large. This causes also additional unsuitability
for the Rint and Goof parameters. Some C atoms show a large
displacement parameter and the direction of motion between these
atoms was restrained (DELU instruction in SHELXL97). The
parameters for data collection and structure refinement of complex
2a are listed in Table 2. All non-hydrogen atoms were refined with
anisotropic parameters. Hydrogen atoms bonded to carbon were
Dichloro-[1,3-bis(4-diethylaminobenzyl)imidazolid-2-ylidene](p-
cymene)ruthenium(II), 2e
Yield: 0,280 g; 70%; m.p: 199–201 ◦C; u(CN)
cm-1. 1H NMR (d, CDCl3): 1.17 (t, J = 7.2 Hz, 12H,
CH2C6H4N(CH2CH3)2-4), 1.31 (d, 7.2 Hz, 6H, p-
= 1498
J
=
CH3C6H4CH(CH3)2), 2.19 (s, 3H, p-CH3C6H4CH(CH3)2), 2.94
(h, J = 7.2 Hz, 1H, p-CH3C6H4CH(CH3)2), 3.36 (q, J = 7.2 Hz,
8H, CH2C6H4N(CH2CH3)2-4), 3.51–3.67 (m, 4H, NCH2CH2N),
4.77–5.16 (m, 2H, CH2C6H4(CH3)2-4), 5.19 ve 5.46 (d, J =
6.3 Hz, 4H, p-CH3C6H4CH(CH3)2), 6.66 ve 7.20 (d, J =
12.9 Hz, 8H, CH2C6H4(CH3)2-4). 13C{H}NMR (d, CDCl3):
˚
placed in calculated positions (C–H = 0.93–0.97 A) and treated
using a riding model with U = 1.2 times the U value of the parent
atom for CH, CH2 and CH3. Molecular diagrams were created
2336 | Dalton Trans., 2012, 41, 2330–2339
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Table 2 Parameters for data collection and structure refinement of 2a
Empirical formula
Formula weight
T/K
C62H82Cl4N4ORu2
1243.26
296
Monoclinic
P21/c
15.5879(8)
13.1132(5)
30.0461(16)
90
91.148(4)
90
6140.4(5)
4
1.345
0.71
2584
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
r
calcd/Mgm-3
m/mm-1
F(000)
Crystal size/mm
q Range for data collection/◦
Reflections collected
Independent reflection
Rint
Max./min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
R1 (I > 2s(I))
wR2 (I > 2s(I))
R1 (all data)
0.08 ¥ 0.05 ¥ 0.02
1.3–26.0
41355
11549
0.273
0.915 and 0.986
11549/10/649
0.66
0.060
0.071
0.279
0.123
0.33 and -0.55
Fig. 1 ORTEP drawing of complex 2a showing 30% probability
wR2 (all data)
˚
thermal ellipsoids. Selected bond lengths [A]:Ru1–C30 = 2.018(13),
-3
˚
Drmax./min./e A
Ru1–Cl1 = 2.417(3), Ru1–Cl2 = 2.424(4), C1–N3 = 1.346(14), C1–N4 =
1.350(14), C2–C3 = 1.507(17), Ru2–C1 = 2.026(14), Ru2–Cl13 = 2.416(3),
◦
˚
Ru2–Cl14
= 2.430(4), C30–N1 = 1.348(13), C30–N2 = 1.361(13),
Table 3 Hydrogen-bond parameters (A,
)
C31–C32 = 1.516(15); angles [◦]: C30–Ru1–Cl1 = 88.1(3), C30–Ru1–Cl2 =
89.3(4), N1–Ru1–C30 = 130.2(9), N2–C30–Ru1 = 127.1(9), N1–C30–N2 =
D–H ◊ ◊ ◊ A
D–H
H ◊ ◊ ◊ A
D ◊ ◊ ◊ A
D–H ◊ ◊ ◊ A
102.7(10), N1–C41–C42
= 113.0(10), Cl3–Ru1–Cl14 = 83.38(12),
C4–H4B ◊ ◊ ◊ Cl4
C6–H6 ◊ ◊ ◊ Cl2a
0.97
0.93
0.93
0.97
0.93
0.93
0.97
0.97
0.93
0.97
0.93
0.93
0.98
2.34
2.70
2.59
2.63
2.55
2.66
2.78
2.60
2.79
2.64
2.54
2.69
2.83
3.240 (14)
3.572 (17)
2.920 (19)
3.253 (13)
2.879 (16)
3.524 (12)
3.415 (13)
3.326 (12)
3.605 (12)
3.276 (13)
2.864 (19)
3.531 (12)
3.394 (16)
153
156
102
122
101
154
123
132
147
124
101
151
118
C54–C53–Ru1 = 68.6(7), C1–Ru2–Cl14 = 92.4(4), C1–Ru2–Cl13 = 89.6(4),
N4-Ru2–C1 = 125.9(10), N3–Ru2–C1 = 128.1(10), N3–C1–N4 = 105.7(12),
N3–C12–C13 = 116.4(11), Cl1–Ru1–Cl2 = 84.06(12), C22–C21–Ru2 =
76.2(9).
C6–H6 ◊ ◊ ◊ N4
C12–H12B ◊ ◊ ◊ Cl3
C14–H14 ◊ ◊ ◊ N3
C22–H22 ◊ ◊ ◊ Cl3b
C31–H31A ◊ ◊ ◊ Cl4
C33–H33B ◊ ◊ ◊ Cl2
C35–H35 ◊ ◊ ◊ Cl2
C41–H41A ◊ ◊ ◊ Cl1
C47–H47 ◊ ◊ ◊ N1
C53–H53 ◊ ◊ ◊ Cl1c
C56–H56 ◊ ◊ ◊ Cl2
Symmetry codes:a x, y - 1, z; b -x + 1, -y + 1, -z + 1; c -x, -y + 2, -z + 1.
using ORTEP-III.35 Geometric calculations were performed with
PLATON.36 Fig. 1 shows the molecular structure of 2a.
The molecules are linked into sheets by a combination of three
C–H ◊ ◊ ◊ Cl hydrogen bonds (Table 3). Atom C22 in the reference
molecule acts as a hydrogen-bond donor, via H22, to atom Cl3ii, so
forming a centrosymmetric R2 (8)37 ring centred at (1/2, 1/2, 1/2).
2
Similarly, atom C53 in the reference molecule acts as a hydrogen-
bond donor, via H53, to atom Cl1iii, so forming a centrosymmetric
2
R2 (8) ring centred at (0, 1, 1/2). The combination of C–H ◊ ◊ ◊ Cl
hydrogen bonds generate a chain of rings running parallel to the
ab plane (Fig. 2).
Fig. 2 Packing arrangement of the molecules in the unit cell.
Supplementary material
CCDC-828989 contains the supplementary crystallographic
data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.†
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The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 2330–2339 | 2337
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View Article Online
123, 8410–8411; (d) S. Lee and J. F. Hartwig, J. Org. Chem, 2001, 66,
Acknowledgements
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3402–3415; (e) I. Ozdemir, S. Demir, B. C¸ etinkaya, C. Gourlaouen, F.
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This work was financially supported by the Technological and
˙
Scientific Research Council of Turkey TUBITAK (107T098)],
˙ ¨
Ino¨nu¨ University Research Fund (I.U. B.A.P: 2011/Gu¨du¨mlu¨-4)
and the Faculty of Arts and Sciences, Ondokuz Mayıs University,
Turkey, for the use of the Stoe IPDS-II diffractometer (purchased
from grant no. F279 of the University Research Fund).
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