Ruthenium(II) complexes bearing N-heterocyclic carbene ligands with wingtip groups and their catalytic activity in the transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.ica.2019.119199

A series of new silver(I) complexes bearing N-heterocyclic carbene (NHC) ligands with wingtip groups were synthesized by the reaction of 1,3-dialkylimidazolinium salts with silver(I) oxide in dichloromethane. The silver complexes were used as carbene-tran

Full text of DOI:10.1016/j.ica.2019.119199

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Research paper
Ruthenium(II) complexes bearing N-heterocyclic carbene ligands with wingtip
groups and their catalytic activity in the transfer hydrogenation of ketones
Beyhan Yiğit, Yılmaz Iş ık, Duygu Barut Celepci, Enes Evren, Murat Yiğit,
Nevin Gürbüz, İsmail Özdemir
PII:
S0020-1693(19)31202-2
https://doi.org/10.1016/j.ica.2019.119199
ICA 119199
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
14 August 2019
4 October 2019
5 October 2019
Please cite this article as: B. Yiğit, Y. Iş ık, D. Barut Celepci, E. Evren, M. Yiğit, N. Gürbüz, I. Özdemir,
Ruthenium(II) complexes bearing N-heterocyclic carbene ligands with wingtip groups and their catalytic activity
in the transfer hydrogenation of ketones, Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.
2019.119199
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Ruthenium(II) complexes bearing N-heterocyclic carbene ligands with wingtip groups
and their catalytic activity in the transfer hydrogenation of ketones
Beyhan Yiğit,1 Yılmaz Işık,1 Duygu Barut Celepci,2 Enes Evren,3 Murat Yiğit,1 Nevin
Gürbüz,3,4 İsmail Özdemir3,4
1Department of Chemistry, Faculty of Science and Art, Adıyaman University, 02040
Adıyaman, Turkey
2Department of Physics, Faculty of Science, Dokuz Eylül University, 35160-Buca, İzmir,
Turkey
3Catalysis Research and Application Center, İnönü University, 44280 Malatya, Turkey
4Department of Chemistry, Faculty of Science and Art, İnönü University, 44280 Malatya,
Turkey
Corresponding author e-mail address: byigit@adiyaman.edu.tr
Abstract
A series of new silver(I) complexes bearing N-heterocyclic carbene (NHC) ligands with
wingtip groups were synthesized by the reaction of 1,3-dialkylimidazolinium salts with
silver(I) oxide in dichloromethane. The silver complexes were used as carbene-transfer agents
to synthesize ruthenium(II) complexes with the general formula [RuCl2(NHC)(η6-p-cymene)].
All the synthesized complexes were characterized by elemental analysis, FT-IR, 1H NMR and
13C NMR spectroscopy, and the molecular and crystal structure of 3d was determined by
single-crystal X-ray diffraction. These ruthenium complexes were tested as catalysts in the
i
transfer hydrogenation of ketones using PrOH as a hydrogen source. All compounds tested
showed good catalytic activity in these reactions.
Keywords: N-Heterocyclic carbene; ruthenium complexes; silver complexes; transfer
hydrogenation.
1. Introduction
Functional group transformation is one of the most fundamental reactions in organic
synthesis, and considerable research efforts have been devoted to the development of new
methods in this area. Hydrogenation and transfer hydrogenation of unsaturated compounds
are the most commonly used organic transformation catalyzed by metal complexes [1], and a
broad variety of substrates include aldehydes, ketones, esters, nitriles, imines, alkenes,
alkynes, nitroarenes, azobenzene, and carbon dioxide can be reduced by these methods [2].

Transfer hydrogenation is the transfer of hydrogen from a reagent other than molecular
hydrogen to an unsaturated molecule via a metal catalyst. In this process, the metal catalyst is
able to abstract a hydride and a proton from the hydrogen donor and deliver them to the
i
hydrogen acceptor [3]. As a hydrogen donor, a wide number of alcohols such as PrOH, 2-
butanol, glycerol, ethanol or other hydrogen donors such as formic acid, aqueous sodium
formate, formic acid/triethylamine mixture are used for transfer hydrogenation [4]. Among
i
them, PrOH and formic acid have been widely used as a hydrogen donor. Transfer
hydrogenation is the operationally simple, safe and environmentally friendly method when
compared with the conventional hydrogenation reaction using the highly flammable and
potentially explosive hydrogen gas [5]. The most common application of this process is the
formation of secondary alcohols by transfer hydrogenation of ketones. Transfer hydrogenation
of ketones catalyzed by metal complexes is an environmentally benign procedure to produce
alcohols, and a broad range of alcohols are accessible by transfer hydrogenation of ketones
i
using PrOH under mild reaction conditions in the presence of various metal catalysts
including Ru, Ir, Rh and others [6]. Particularly, asymmetric transfer hydrogenation of
ketones provides an efficient method for the preparation of enantiopure chiral secondary
alcohols, which are useful intermediates for the synthesis of pharmaceuticals, agrochemicals
and fine chemicals [7]. To date, numerous catalytic systems using NN, PP, PN, CN, NNN,
NNC, CNC, SNS etc., type of ligands for transfer hydrogenation of ketones have been
developed [8]. A base is generally required for transfer hydrogenation. However, some
complexes are reported to catalyze the transfer hydrogenation under base-free conditions [9].
For example, Corberan and Peris have reported the catalytic transfer hydrogenation of
ketones, aldehydes and imines in iPrOH under base-free conditions at room temperature using
CpIrIII(NHC) complexes as the catalyst [9b]. Recently, N-heterocyclic carbenes (NHC) have
attracted much attention due to their numerous applications in catalysis [10], material [11] and
medicinal sciences [12]. NHCs are neutral, two-electron donor ligands, which have strong σ-
donating and weak π-accepting ability and they can form the more stable metal-ligand bond
with a range of transition metals than phosphine ligands [13]. This strong metal-carbon bond
prevents the decomposition of NHC complexes during catalytic reactions. The first examples
of N-heterocyclic carbenes as ligands for transition metals were independently reported by
Öfele [14] and Wanzlick in 1968 [15]. Since then, a variety of NHC complexes have been
successfully synthesized, and extensively used as catalysts in C-C bond formation and
functional group transformation reactions [16]. The first use of NHC complexes for the
transfer hydrogenation was reported by Nolan in 2001 [17]. After that, many metal-NHC

complexes have been used as the catalyst in transfer hydrogenation [18]. Among them, NHC
complexes of Ru, Ir, and Rh have demonstrated good activity for transfer hydrogenation
reactions [19]. Ruthenium(II) complexes are very efficient catalyst precursors in the transfer
hydrogenation of ketones [20] and offer a cost advantage over common used iridium and
rhodium for transfer hydrogenation. Various arene-Ru-NHC complexes have been reported as
effective catalysts for the transfer hydrogenation of ketones, aldehydes, imines, nitriles, and
alkenes [21].
In this study, the new [RuCl2(NHC)(p-cymene)] (NHC = imidazolidin-2-ylidene)
complexes were readily synthesized by treating the [RuCl2(p-cymene)]2 dimers directly with
the corresponding Ag(I)-NHC complexes, which were prepared by reaction of Ag2O with
symmetrical 1,3-dialkylimidazolinium salts in dichloromethane. The catalytic activity of
ruthenium complexes was evaluated in the reduction of ketones by transfer hydrogenation
using iPrOH as a hydrogen donor.
2. Experimental
2.1. Materials and methods
All reactions for the preparation of the ruthenium(II)-NHC complexes were carried out
under argon in flame-dried glassware using standard Schlenk techniques. The solvents used
were purified by distillation over the drying agents indicated and were transferred under Ar:
THF, Et2O (Na/K alloy), CH2Cl2 (P4O10), hexane, toluene (Na). All reagents were purchased
from Sigma-Aldrich, Merck and Fluka. [RuCl2(p-cymene)]2 was synthesized according to
1
13
published procedures [22]. H-NMR and C-NMR spectra were recorded with a Varian AS
400 Merkur spectrometer operating at 400 MHz (1H), 100 MHz (13C) in CDCl3 with
tetramethylsilane as an internal reference. Coupling constants (J values) are given in Hertz.
NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, sept =
septet, q = quartet and m = multiplet signal. FT-IR spectra were recorded on the ATR unit in
the range 400-4000 cm-1 on Perkin Elmer Spectrum 100. GC was measured by GC-FID on an
Agilent 6890N gas chromatograph equipped with an HP-5 column of 30 m length, 0.32 mm
diameter and 0.25 m film thickness. Melting points were measured in open capillary tubes
with Stuart SMP 40 melting point apparatus and uncorrected. Elemental analyses were
performed at the Inönü University research center.
2.2. Synthesis of imidazolinium salts, 1
A mixture of N,N’-dialkylethylenediamines (6.2 mmol), NH4Cl (6.2 mmol) and triethyl

orthoformate (10 mL) was heated for 12 h at 110 C. Upon cooling to room temperature,

colorless crystals were obtained. The crystals were filtered, washed with diethyl ether (3 × 15
mL) and dried under vacuum. The crude product was recrystallized from ethanol/diethyl
ether.
2.3. Synthesis of silver(I)-NHC complexes, 2
A solution of the appropriate imidazolinium chloride (1.12 mmol), Ag2O (0.56 mmol) and
activated molecular sieves 4Å in dichloromethane (25 mL) was stirred for 24 h at room
temperature in the dark conditions, covered with aluminum foil under argon. The reaction
mixture was filtered through celite, and the solvent was removed under vacuum to afford the
product. The crude product was recrystallized from dichloromethane: hexane (1:2) at room
temperature. The resulting white solid was isolated by filtration and dried in vacuum.
2.3.1. Chloro-[1,3-di(1-phenylethyl)imidazolidin-2-ylidene]silver(I), 2a
o
1
Yield: 0.38 g, 81%, m.p. 285-287 C. IR, υ: 1656 cm-1 (NCN). H NMR (CDCl3) δ: 1.63 (d,
6H, J = 4.0 Hz, CH(CH3)C6H5), 5.42 (q, 2H, J = 8.0 Hz, CH(CH3)C6H5), 3.20 and 3.48 (t, 4H,
J = 8.0 Hz, NCH2CH2N), 7.27-7.44 (m, 10H, Ar-H). 13C NMR (CDCl3) δ: 17.6
(CH(CH3)C6H5), 43.7 (NCH2CH2N), 59.2 (CH(CH3)C6H5), 126.5, 126.7, 126.8, 127.1, 128.2,
128.3, 128.7, 129.3, 138.7 (Ar-C). Anal. Calc. for C19H22N2AgCl: C, 54.09; H, 5.22; N, 6.64.
Found: C, 54.12; H, 5.23; N, 6.66%.
2.3.2. Chloro-[1,3-di(1-(4-methyphenyl)ethyl)imidazolidin-2-ylidene]silver(I), 2b
Yield: 0.40 g, 80%, m.p. 161-163 oC. IR, υ: 1663 cm-1 (NCN). 1H NMR (CDCl3) δ: 1.62 and
1.80 (d, 6H, J = 8.0 Hz, CH(CH3)C6H4-CH3), 5.27 and 5.38 (q, 2H, J = 8.0 Hz,
CH(CH3)C6H4-CH3), 2.34 and 2.35 (s, 6H, CH(CH3)C6H4-CH3), 3.08-3.71 (m, 4H,
13
NCH2CH2N), 7.10-7.33 (m, 8H, Ar-H). C NMR (CDCl3) δ: 17.7 and 19.0 (CH(CH3)C6H4-
CH3), 21.0 and 21.1 (CH(CH3)C6H4-CH3), 43.6 and 45.3 (NCH2CH2N), 57.5 and 58.9
(CH(CH3)C6H4-CH3), 126.7, 127.0, 129.5, 129.9, 134.4, 135.7, 138.0, 138.8 (Ar-C). Anal.
Calc. for C21H26N2AgCl: C, 56.06; H, 5.78; N, 6.23. Found: C, 56.09; H, 5.76; N, 6.21%.
2.3.3. Chloro-[1,3-di(1-(3,4-dimethylphenyl)ethyl)imidazolidin-2-ylidene]silver(I), 2c
o
1
Yield: 0.41 g, 77%, m.p. 148-150 C. IR, υ: 1632 cm-1 (NCN). H NMR (CDCl3) δ: 1.79 (d,
6H, J = 8.0 Hz, CH(CH3)C6H3-(CH3)-3,4), 5.23 (q, 2H, J = 4.0 Hz, CH(CH3)C6H3-(CH3)2-
3,4), 2.24 and 2.26 (s, 12H, CH(CH3)C6H3-(CH3)2-3,4), 3.53-3.73 (m, 4H, NCH2CH2N), 7.11-
13
7.16 (m, 6H, Ar-H). C NMR (CDCl3) δ: 19.1 (CH(CH3)C6H3-(CH3)2-3,4), 19.5 and 19.9
(CH(CH3)C6H3-(CH3)2-3,4), 45.4 (NCH2CH2N), 57.5 (CH(CH3)C6H3-(CH3)2-3,4), 124.4,
128.2, 128.3, 130.3, 130.4, 134.8, 134.9, 137.5, 137.6 (Ar-C). Anal. Calc. for C23H30N2AgCl:
C, 57.80; H, 6.28; N, 5.86. Found: C, 57.84; H, 6.26; N, 5.87%.

2.3.4. Chloro-[1,3-di(benzhydrylimidazolidin-2-ylidene)]silver(I), 2d
o
1
Yield: 0.53 g, 87%, m.p. 286-288 C. IR, υ: 1644 cm-1 (NCN). H NMR (CDCl3) δ: 4.06 (s,
4H, NCH2CH2N), 6.38 (s, 2H, CH-Ar), 7.12-7.40 (m, 20H, Ar-H). 13C NMR (CDCl3) δ: 48.8
(NCH2CH2N), 66.4 (CH-Ar), 126.8, 127.2, 128.3, 128.4, 128.5, 128.7, 128.9, 129.0, 129.3,
135.5 (Ar-C). Anal. Calc. for C29H26N2AgCl: C, 63.79; H, 4.77; N, 5.13. Found: C, 63,75; H,
4.78; N, 5.14%.
2.3.5.
Chloro-[1,3-di(1-phenyl-1-(4-methylphenyl)methyl)imidazolidin-2-
ylidene]silver(I), 2e
o
1
Yield: 0.56 g, 88%, m.p. 160-162 C. IR, υ: 1637 cm-1 (NCN). H NMR (CDCl3) δ: 4.04 (s,
4H, NCH2CH2N), 6.32 (s, 2H, CH-Ar), 7.03-7.37 (m, 18H, Ar-H), 2.32 (s, 6H, C6H4-CH3).
13C NMR (CDCl3) δ: 48.8 (NCH2CH2N), 66.2 (CH-Ar), 21.1 (C6H4-CH3), 127.1, 128.2,
128.3, 128.4, 128.7, 128.8, 128.9, 129.2, 129.9, 132.5, 135.7, 138.9 (Ar-C). Anal. Calc. for
C31H30N2AgCl: C, 64.86; H, 5.23; N, 4.88. Found: C, 64.89; H, 5.21; N, 4.89%.
2.4. Synthesis of ruthenium(II)-NHC complexes, 3
A solution of required silver(I)-NHC complex (0.33 mmol) and [RuCl2(p-cymene)]2 (0.16
mmol) in dichloromethane (20 mL) was stirred for 24 h at room temperature in the dark.
Afterwards, the resulting mixture was filtered through celite, and the solvent was removed
under vacuum to afford the product. The crude product was recrystallized from
dichloromethane: diethyl ether (1:2) at room temperature. The orange-brown crystals were
filtered off, washed with diethyl ether (3 x 10 mL) and dried under vacuum.
2.4.1. Dichloro-[1,3-di(1-phenylethyl)imidazolidin-2-ylidene](p-cymene)ruthenium(II),
3a
o
1
Yield: 0.15 g, 79%, m.p. 194-196 C. IR, υ: 1512 cm-1 (NCN). H NMR (CDCl3) δ: 1.74 (d,
6H, J = 4.0 Hz, CH(CH3)C6H5), 6.13 (q, 2H, J = 8.0 Hz, CH(CH3)C6H5), 3.66-3.77 (m, 4H,
NCH2CH2N), 7.36-7.55 (m, 10H, CH(CH3)C6H5), 1.03 (d, 6H, J = 8.0 Hz, p-
CH3C6H4CH(CH3)2), 1.70 (s, 3H, p-CH3C6H4CH(CH3)2), 2.31 (sept, 1H, J = 8.0 Hz, p-
13
CH3C6H4CH(CH3)2), 5.15 and 5.22 (d, 4H, J = 8.0 Hz, p-CH3C6H4CH(CH3)2). C NMR
(CDCl3) δ: 19.8 (CH(CH3)C6H5), 43.4 (NCH2CH2N), 57.5 (CH(CH3)C6H5), 125.6, 127.0,
129.0, 142.7 (CH(CH3)C6H5), 17.2 (p-CH3C6H4CH(CH3)2), 22.4 (p-CH3C6H4CH(CH3)2), 30.3
(p-CH3C6H4CH(CH3)2), 83.1, 87.2, 93.7, 105.4 (p-CH3C6H4CH(CH3)2), 209.2 (Ru-Ccarb).
Anal. Calc. for C29H36N2RuCl2: C, 59.58; H, 6.16; N, 4.79. Found: C, 59.55; H, 6.17; N,
4.80%.
2.4.2.
Dichloro-[1,3-di(1-(4-methylphenyl)ethyl)imidazolidin-2-ylidene](p-
cymene)ruthenium(II), 3b

Yield: 0.17 g, 85%, m.p. 165-167 oC. IR, υ: 1511 cm-1 (NCN). 1H NMR (CDCl3) δ: 1.62-1.68
and 1.76-1.82 (m, 6H, CH(CH3)C6H4-CH3), 6.05-6.26 (m, 2H, CH(CH3)C6H4-CH3), 2.30 and
2.39 (s, 6H, CH(CH3)C6H4-CH3), 3.28-3.67 (m, 4H, NCH2CH2N), 7.10-7.47 (m, 8H,
CH(CH3)C6H4-CH3), 1.14 and 1.27 (d, 6H, J = 4.0 Hz, p-CH3C6H4CH(CH3)2), 1.93 (s, 3H, p-
CH3C6H4CH(CH3)2), 2.64 (sept, 1H, J = 8.0 Hz, p-CH3C6H4CH(CH3)2), 5.13, 5.30, 5.34 and
13
5.45 (d, 4H, J = 4.0 Hz, p-CH3C6H4CH(CH3)2). C NMR (CDCl3) δ: 20.9 (CH(CH3)C6H4-
CH3), 22.8 (CH(CH3)C6H4-CH3), 43.6 and 43.9 (NCH2CH2N), 55.4 and 57.7 (CH(CH3)C6H4-
CH3), 125.1, 127.7, 128.6, 129.7, 136.6, 140.1 (CH(CH3)C6H4-CH3), 17.6 (p-
CH3C6H4CH(CH3)2), 22.6 (p-CH3C6H4CH(CH3)2), 30.6 (p-CH3C6H4CH(CH3)2), 83.2, 84.4,
86.2, 87.3, 94.2, 105.9 (p-CH3C6H4CH(CH3)2), 207.6 (Ru-Ccarb). Anal. Calc. for
C31H40N2RuCl2: C, 60.78; H, 6.53; N, 4.57. Found: C, 60.80; H, 6.52; N, 4.58%.
2.4.3.
Dichloro-[1,3-di(1-(3,4-dimethylphenyl)ethyl)imidazolidin-2-ylidene](p-
cymene)ruthenium(II), 3c
o
1
Yield: 0.17 g, 81%, m.p. 186-188 C. IR, υ: 1504 cm-1 (NCN). H NMR (CDCl3) δ: 1.71 (d,
6H, J = 4.0 Hz, CH(CH3)C6H3-(CH3)-3,4), 6.02 (q, 2H, J = 4.0 Hz, CH(CH3)C6H3-(CH3)2-
3,4), 2.34 and 2.41 (s, 12H, CH(CH3)C6H3-(CH3)2-3,4), 3.61-3.73 (m, 4H, NCH2CH2N), 7.17-
7.27 (m, 6H, CH(CH3)C6H3-(CH3)2-3,4), 1.04 (d, 6H, J = 4.0 Hz, p-CH3C6H4CH(CH3)2), 1.69
(s, 3H, p-CH3C6H4CH(CH3)2), 2.36-2.39 (m, 1H, p-CH3C6H4CH(CH3)2), 5.19 and 5.22 (d,
13
4H, J = 8.0 Hz, p-CH3C6H4CH(CH3)2). C NMR (CDCl3) δ: 19.3 (CH(CH3)C6H3-(CH3)2-
3,4), 19.8 and 20.3 (CH(CH3)C6H3-(CH3)2-3,4), 43.4 (NCH2CH2N), 57.2 (CH(CH3)C6H3-
(CH3)2-3,4), 122.9, 127.1, 130.0, 135.2, 137.1, 140.2 (CH(CH3)C6H3-(CH3)2-3,4), 17.2 (p-
CH3C6H4CH(CH3)2), 22.4 (p-CH3C6H4CH(CH3)2), 30.3 (p-CH3C6H4CH(CH3)2), 82.8, 87.4,
93.3, 105.6 (p-CH3C6H4CH(CH3)2), 208.6 (Ru-Ccarb). Anal. Calc. for C33H44N2RuCl2: C,
61.87; H, 6.87; N, 4.37. Found: C, 61.84; H, 6.89; N, 4.38%.
2.4.4. Dichloro-[1,3-dibenzhydrylimidazolidin-2-ylidene](p-cymene)ruthenium(II), 3d
Yield: 0.20 g, 87%, m.p. 227-229 oC. IR, υ: 1495 cm-1 (NCN). 1H NMR (CDCl3) δ: 2.95 and
3.82 (t, 4H, J = 12.0 Hz, NCH2CH2N), 7.75 (s, 2H, CH(C6H5)2), 7.20-7.66 (m, 20H,
CH(C6H5)2), 1.03 (d, 6H, J = 8.0 Hz, p-CH3C6H4CH(CH3)2), 1.59 (s, 3H, p-
CH3C6H4CH(CH3)2), 1.98 (sept, 1H, J = 8.0 Hz, p-CH3C6H4CH(CH3)2), 4.81 and 5.11 (d, 4H,
13
J = 8.0 Hz, p-CH3C6H4CH(CH3)2). C NMR (CDCl3) δ: 46.6 (NCH2CH2N), 64.1 (CH-Ar),
127.4, 127.5, 127.7, 127.9, 128.6, 130.9, 139.6, 141.8 (CH(C6H5)2), 17.2 (p-
CH3C6H4CH(CH3)2), 22.9 (p-CH3C6H4CH(CH3)2), 30.1 (p-CH3C6H4CH(CH3)2), 84.6, 85.6,
95.9, 104.1 (p-CH3C6H4CH(CH3)2), 208.5 (Ru-Ccarb). Anal. Calc. for C39H40N2RuCl2: C,
66.10; H, 5.65; N, 3.95. Found: C, 66.12; H, 5.64; N, 3.95%.

2.4.5.
Dichloro-[1,3-di(1-phenyl-1-(4-methylphenyl)methyl)imidazolidin-2-ylidene](p-
cymene)ruthenium(II), 3e
o
1
Yield: 0.20 g, 83%, m.p. 181-183 C. IR, υ: 1511 cm-1 (NCN). H NMR (CDCl3) δ: 3.80 (t,
4H, J = 8.0 Hz, NCH2CH2N), 7.00 and 7.02 (s, 2H, CH-Ar), 7.19-7.69 (m, 18H, C6H4-CH3,
C6H5), 2.27 and 2.46 (s, 6H, C6H4-CH3), 1.01 and 1.04 (d, 6H, J = 8.0 Hz, p-
CH3C6H4CH(CH3)2), 1.60 (s, 3H, p-CH3C6H4CH(CH3)2), 1.99 (sept, 1H, J = 8.0 Hz, p-
CH3C6H4CH(CH3)2), 4.79, 4.84, 5.07 and 5.14 (d, 4H, J = 8.0 Hz, p-CH3C6H4CH(CH3)2). 13C
NMR (CDCl3) δ: 46.6 (NCH2CH2N), 63.9 and 64.0 (CH-Ar), 22.7 and 23.1 (C6H4-CH3),
127.3, 127.4, 127.6, 127.9, 128.5, 128.6, 129.2, 130.6, 130.9, 136.6, 137.1, 137.3, 138.6,
139.8, 142.1 (CH(Ar-C)2), 17.2 (p-CH3C6H4CH(CH3)2), 21.1 (p-CH3C6H4CH(CH3)2), 30.0
(p-CH3C6H4CH(CH3)2), 84.3, 84.8, 85.3, 86.0, 95.8, 103.9 (p-CH3C6H4CH(CH3)2), 208.2
(Ru-Ccarb). Anal. Calc. for C41H44N2RuCl2: C, 66.84; H, 5.97; N, 3.80. Found: C, 66.87; H,
5.96; N, 3.81%.
2.5. General procedure for the transfer hydrogenation of ketones
Ketone (1 mmol) was added to a mixture of ruthenium catalyst 3a-e (0.01 mmol) and KOH (2
i
mmol) in PrOH (3 mL). The mixture was heated at 80 °C for 2-6 h. At the end of the
reaction, the mixture was cooled to room temperature and filtrated through to short silica gel
column. Afterwards, volatiles were removed under reduced pressure and product distribution
was determined by GC using dodecane used as an internal standard. Conversions are based on
ketone derivatives.
2.6. Mercury poisoning experiment
Ketone (1 mmol) was added to a mixture of ruthenium catalyst 3a/3d (0.01 mmol) and KOH
(2 mmol) in iPrOH (3 mL). One drop of Hg was added with a syringe to the reaction mixture.
The mixture was heated at 80 °C for 6 h. At the end of the reaction, the mixture was cooled to
room temperature and aliquots were withdrawn from the reaction vessel to filtrate through to
short silica gel column. Afterwards, volatiles were removed under reduced pressure and
product distribution was determined by GC using dodecane used as an internal standard.
Conversions are based on ketone derivatives.
2.7. X-ray crystallographic data
The crystallographic data collection of the complex 3d was conducted at 293(2) K on a Rigaku
Oxford Xcalibur diffractometer with an Eos-CCD detector using graphite-monochromated
MoKα radiation (λ = 0.71073 Å). Data collection, cell refinement and data reduction along
with absorption correction were performed using the CrysAlisPro software package [23]. The

crystal structure was solved with the SHELXT structure solution program using Intrinsic
Phasing method [24] embedded in the Olex2 [25]. The refinement of coordinates and
anisotropic thermal parameters of non-hydrogen atoms were carried out by the full-matrix
least-squares method in SHELXL [26]. All non-hydrogen atoms were refined anisotropically.
All hydrogen atoms of the complex were placed at idealized positions and refined using the
riding model. A summary of crystal data, experimental details, and refinement results for the
complex are given in Table 1.
3. Results and discussion
3.1. Synthesis of Ag(I)-NHC and Ru(II)-NHC complexes
In this study, a series of new Ag(I)-NHC and Ru(II)-NHC complexes were
synthesized and characterized (Scheme 1). The symmetrical imidazolinium chloride salts 1
used as the precursors of NHC ligands were synthesized by cyclization reactions of N,N’-
dialkylalkanediamines with triethyl orthoformate in the presence of ammonium chloride as
reported previously [27]. The Ag(I)-NHC complexes 2a-e were prepared via the in situ
deprotonation of imidazolinium salts by Ag2O according to the general method described by
Wang and Lin [28]. Treatment of the imidazolinium salts with Ag2O in dichloromethane at
room temperature in the dark afforded the expected silver complexes Ag(I)-NHC 2a-e
(Scheme 1). The silver-NHC complexes 2a-e was obtained in high yields as white solids,
soluble in halogenated solvents. These complexes are stable in air but are light sensitive. In
1
13
the H NMR and C NMR spectra of 2a-e, the disappearance of the downfield resonance
signals of imidazolinium (NCHN) proton and imidazolinium (NCN) carbon signal suggests
1
the formation of the Ag(I)-NHC complexes. In the H NMR spectrum of 2a-e, CH proton
signals bonded with the nitrogen of imidazolidine ring appear as a quartet at 5.42 ppm for 2a
and 5.23 ppm for 2c, two quartet at 5.27 and 5.38 ppm for 2b, a singlet 6.38 and 6.32 ppm
respectively for 2d and 2e. The methylene protons of imidazolidine ring give two triplet at
3.20 and 3.48 ppm for 2a and two multiplet in range of 3.20-3.73 ppm for 2b and 2c, whereas
the same signals were observed as a singlet at 4.06 and 4.04 ppm for 2d and 2e, respectively.
The aromatic protons of NHC ligand were observed as multiplet in range of 7.03-7.44 ppm in
1
the H NMR of 2a-e. In the 2a-e complexes, the resonances for carbene carbon were not
detected, which has also been mentioned in the literature and given as a reason for the
fluxional behavior of the NHC complexes [29]. Silver-NHC complexes 2a-e exhibit a
characteristic υ(C=N) bond vibrations typically at 1656, 1663, 1632, 1644 and 1637 cm-1,
respectively for 2a-e.

R
R
R'
R'
R
R
R'
R
R
R'
R'
Cl
Ru
Cl
N
N
N
N
N
Ag2O, CH2Cl2
RT, 24 h
[RuCl2(p-cymene)]2
+) Cl-
N
AgCl
CH2Cl2, RT, 24 h
R'
3
2
1
Cl
Cl
Ru
Cl
Cl
Ru
Cl
Cl
Ru
Cl
Cl
N
N
N
N
N
N
N
N
Ru
Cl
Ru
Cl
N
N
3c
3e
3a
3d
3b
Scheme 1. The synthesis of (p-cymene)-ruthenium(II)-NHC complexes.
The ruthenium(II)-NHC complexes 3a-e were prepared via transmetalation method
from the corresponding silver(I)-NHC complexes and [RuCl2(p-cymene)]2 (Scheme 1). The
reaction of [RuCl2(p-cymene)]2 with 2 equivalents of 2a-e in dichloromethane at room
temperature afforded the corresponding (p-cymene)-ruthenium(II)-NHC complexes 3a-e,
which were isolated as orange-brown crystals in moderate to good yields after purification. A
single crystal of complex 3d for X-ray diffraction analysis was obtained from slow vapor
diffusion of diethyl ether into a dichloromethane solution at room temperature. These
complexes are very stable against air and moisture in the solid-state. The new ruthenium
1
complexes were characterized using H NMR, 13C NMR, IR spectroscopy and elemental
analysis techniques, which support the proposed structures. The ruthenium complexes exhibit
characteristic υ(C=N) bands in the range of 1495-1512 cm-1 for 3a-e. NMR analyses of the
complexes showed that the N-heterocyclic carbene ligands had been transferred from the
silver complexes to ruthenium. In the 1H NMR spectrum of 3a-e, aromatic protons signals of
p-cymene ligands appear as two doublet in range of 4.79-5.22 ppm with the same coupling
constants for 3a and 3c-e, whereas aromatic protons of p-cymene ligand were observed as
1
four doublets in range of 5.13-5.45 ppm in the H NMR of 3b. The methyl signals of
isopropyl group on p-cymene ligands appear as two doublets in range of 1.01-1.27 ppm for 3b
and 3e, a singlet in range of 1.03-1.04 ppm for 3a, 3c, and 3d. The methyl signals of p-
cymene ligands were observed as a singlet in range of 1.59-1.93 ppm in the 1H NMR of 3a-e.

13
The C NMR chemical shifts provide a useful diagnostic tool for this type of metal carbene
complexes. The characteristic resonance of the carbene carbon was observed at 209.2, 207.6,
208.6, 208.5 and 208.2 ppm respectively for 3a-e. The resonances of aromatic carbons of
NHC ligands appear between 122.9 and 142.7 ppm, whereas resonances for aromatic carbons
of p-cymene ligands were found in the range 83.1-105.9 ppm for 3a-e, these values were
similar to those found for other (p-cymene)-ruthenium(II)-NHC complexes [19d, 21d]. The
elemental analysis data of the complexes 3a-e agrees closely with the theoretical requirements
of their structures.
3.2. Description of the crystal structure of complex 3d
The complex 3d crystallizes in the monoclinic space group P21/c; its molecular and
crystal structure is depicted in Fig. 1. In the complex, coordination geometry around the
Ru(II) atom can be described as a pseudo-octahedral piano-stool geometry, with three sites
occupied by the η6-p-cymene ligand and the remaining three by the two chlorides and an
NHC ligand. The NHC moiety makes coordination with two benzhydryl imidazolidin wingtip
substituents. The ruthenium atom is situated 1.7076(5) Å from the centroid of the p-cymene
ring, which is consistent with the values reported in similar complexes [30]. In the p–cymene
ligand, the arene ring (C5–C10) has a planar configuration with the rmsd at 0.026 Å. In the
crystal packing of 3d, intra- and intermolecular C–H···Cl type weak interactions play a role
for the stabilization of the complex. Fig. S1 displays the crystal packing fashion of the
complex with these intra- and intermolecular interactions.

Fig. 1. Crystal structure of [RuCl2(NHC)(η6-p-cymene)] complex (3d). Thermal ellipsoids are
depicted at the 30% probability level. Selected bond distances (Å) and angles (°): Ru1–Cl1
2.4336(9), Ru1–Cl2 2.4300(8), Ru1–C1 2.108(3), Ru1–C5 2.243(3), Ru1–C6 2.189(3), Ru1–
C7 2.154(3), Ru1–C8 2.263(3), Ru1–C9 2.229(3), Ru1–C10 2.186(3), C1–N1 1.356(3), C1–
N2 1.354(3); Cl1–Ru1–Cl2 82.21(3), C1–Ru1–Cl1 89.81(8), C1–Ru1–Cl2 92.80(8), Cl1–
Ru1–Carene 124.127(2), Cl2–Ru1–Carene 124.208(2), C1–Ru1–Carene 130.042(3), C1–N1–C14
127.6(3), C1–N2–C27 127.0(2). For the sake of clarity, hydrogen atoms are omitted.
3.3. Transfer hydrogenation of ketones
The catalytic activities of new ruthenium complexes 3a-e were investigated in the transfer
i
hydrogenation of ketones using PrOH as a hydrogen source. Initially, the reduction of
acetophenone to 1-phenyl ethanol was used as a model reaction to determine the optimal
catalytic conditions with 3a as a catalyst (Table 1). Although the the some reports under base-
free conditions have been known [9], the addition of base is frequently crucial to the
efficiency of transfer hydrogenation reaction. Seven different bases were tested and among
them, KOH displayed the highest reactivity (entry 5). It is noteworthy that, in the absence of
base, no conversion was detected and only 9% conversion was observed without catalyst
(entry 13). When the [RuCl2(p-cymene)]2 was used as catalyst, phenyl ethanol was obtained
in 59% conversion. Lower catalyst ratio was caused the by the drop of conversions (entries
10, 11). The catalytic system was used in transfer hydrogenation of acetophenone at room
temperature, the formation of phenyl ethanol was not appreciable. When this catalytic system
o
was used at 50 C for 6h, phenyl ethanol was observed only 15% conversion (entry 9),
whereas good conversion was observed at 80 °C for 6h.
Table 1. Screening of transfer hydrogenation reaction conditions
OH
H
O
O
OH
Cat
+
+
Entry
Cat.
Cat.
Base
Conversion
(% mol)
(%)
15
1
2
3
4
3a
3a
3a
3a
1
1
1
1
NEt3
Cs2CO3
K2CO3
NaOH
75
61
92

5
6
3a
1
1
KOH
t-BuOK
NaOAc
KOH
96
69
39
15b
65
42
59
9
3a
7
3a
1
9
3a
1
10
11
12
13
3a
3a
0.5
0.25
1
KOH
KOH
[RuCl2(p-cymene)]2
KOH
-
-
KOH
i
o
aReaction conditions: KOH (2 mmol), PrOH (10 mL), acetophenone (1.0 mmol), 80 C, 6 h.
The purity of compounds is checked by GC using dodecane as an internal standard and
b
i
conversions are based on ketones. KOH (2 mmol), PrOH (10 mL), acetophenone (1.0
mmol), 50 oC, 6 h.
Also, the time-dependent reaction profile at different catalyst loadings was followed with
complex 3a (Figure 2). When 1 mol% of complex 3a was used, the only trace amount of the
product was identified after 60 min period, but no induction period was observed for 0.5
mol% catalyst loading in the same time. The 1 mol% of complex 3a gave the 87% conversion
within 2h gradually and no remarkable change was observed in the conversion after 4h as
represented Figure 2. As demonstrated by TEP (Tolman Electronic Parameter) data, NHC
ligands with bulky substituents generally exhibit better donor properties [31]. Also, the
carbene ligand of the compound is seemingly bulky enough to slow down the approach of
substrates, while the compound with the sterically least demanding ligands on the NHC-N-
atoms might be unable to stabilize either the active species and/or the transition states. In here,
the high induction time might be the consequence of steric and electronic effects of the
wingtip groups in the ruthenium N-heterocyclic carbene complexes.

100
90
80
70
60
50
40
30
20
10
0
3a (1 mol%)
3a (0.5 mol%)
30
60
90
120
240
360
Time (minutes)
Fig. 2. Time dependence of the transfer hydrogenation. Reaction conditions: Catalyst (0.01
mmol or 0.005 mmol), substrate (1 mmol), iPrOH (3 mL), KOH (2 mmol), 80 oC.
The catalytic experiments were carried out using 1 mmol ketone, 0.01 mmol 3a-e, 2 mmol
KOH and 3 mL iPrOH at 80 oC for 2-6 h. Under these reaction conditions, complex 3d proved
to be the most effective catalyst relative to 3a, 3c, 3b, and 3e for transfer hydrogenation of
acetophenone. The reduction of acetophenone with 3d was completed within 6 h in 98%
(Table 2, entry 4). In contrast, acetophenone was reduced within 6h using 3a, 3b, 3c and 3e in
96, 94, 96 and 89% conversion, respectively (Table 2, entries 1-5).
We next extended our investigations to include transfer hydrogenation of substituted
acetophenone derivatives. A variety of ketones were transformed into the corresponding
secondary alcohols. The results are shown in Table 2. Under those conditions, p-
methylacetophenone, p-methoxyacetophenone, p-floroacetophenone, p-nitroacetophenone
react very cleanly and in good conversions with iPrOH (Table 2, entries 9, 14, 19 and 21). The
presence of electron withdrawing (NO2) or electron donating (CH3 or OCH3) substituent on
acetophenone (Table 2, entries 8, 14 and 21) has a significant effect on the reduction of
ketones to their corresponding alcohols. For example, when 3d was used as the catalyst, the p-
tolylethanol products was observed in 72% yield (Table 2, entry 9), whereas in the case of 3a,
3b, 3c and 3e p-methylacetophenone was reduced in 38, 33, 52 and 55% conversion,
respectively (Table 2, entries 6-10). p-Methoxyacetophenone was reduced using 3a-e in 33,
23, 43, 45 and 35% conversion, respectively (Table 2, entries 11-15). When p-

floroacetophenone was used as the substrate, yields were increased slightly (Table 2, entries
16-20). The maximum conversion of 4-nitroacetophenone to corresponding alcohol was
achieved over a period of 2h (Table 2, entries 21-25).
The ruthenium complexes also catalyzed the transfer hydrogenation of cyclohexanone
very effectively (Table 2 entries 26-30). The conversion of cyclohexanone to corresponding
alcohol was achieved over a period of 3h, the products were obtained with 98-99%
conversion. Among the tested complexes, the complex 3d is highly efficient in the transfer
hydrogenation of ketone to a secondary alcohol. When the obtained results compared with
similar previously Ru-NHC complexes, new Ru-NHC complexes (3a-e) have similar catalytic
activity [19c,d and 21].
Table 2. Hydrogen transfer from iPrOH to ketones in the presence of 3a-e and KOH.a
O
C
OH
O
OH
3a-3e (1 mol%)
+
+
H3C CH CH3
H3C C CH3
CH CH3
KOH, iPrOH, 80oC
CH3
R
R
Entry
Substrate
Product
Time
(h)
6
Cat.
Conversion
(%)
96
94
96
98
89
38
33
52
72
55
33
23
43
45
35
80
73
1
2
3a
3b
3c
3d
3e
3a
3b
3c
3d
3e
3a
3b
3c
3d
3e
3a
3b
6
OH
O
3
6
C CH3
C CH3
H
4
6
5
6
6
6
7
6
OH
O
H3C
CH CH3
8
6
H3C
C CH3
9
6
10
11
12
13
14
15
16
17
6
6
6
OH
O
H3CO
CH CH3
6
H3CO
C CH3
6
6
6
O
OH
F
C
CH3
F
CH CH3
6

18
19
6
6
3c
75
85
3d
20
21
22
23
24
25
6
2
2
2
2
2
3e
3a
3b
3c
3d
3e
74
98
96
94
95
96
O
C
OH
O2N
O2N
CH CH3
CH3
26
27
28
29
30
3
3
3
3
3
3a
3b
3c
3d
3e
99
98
98
99
98
O
OH
i
aReaction conditions: Cat. (0.01 mmol), substrate (1 mmol), PrOH (3 mL), KOH (2 mmol),
o
80 C. The purity of compounds is checked by GC using dodecane as an internal standard,
conversions are based on ketone derivatives.
The ability of Hg(0) to poison metal-particle heterogeneous catalysts, by amalgamating the
metal or adsorbing on the metal surface, has been known for more than 90 years and is a
widely used test [32]. This experiment is performed by adding Hg(0) to the reaction solution.
The suppression of catalysis by Hg(0) is evidence for a heterogeneous catalyst; if Hg(0) does
not suppress catalysis, that is evidence for a homogeneous catalyst. We also carried out the
mercury poisoning experiment to assess whether the reaction system is homogeneous or
heterogeneous. The Hg(0)-poisoning experiments were performed with catalysts 3a and 3d in
the presence of excess Hg. The results showed no significant inhibition of conversion to
products when the complex 3a was used as a catalyst at different times (Table 3, entries 1-3).
Thus, the present catalysis appears to be homogeneous in nature. We could mention that
influence of nanoparticles on the performance of the catalysts was excluded by such an
experiment and that this is not the reason for the long induction period.
Table 3. Mercury poisoning test of Ru catalysts in transfer hydrogenation of acetophenone.a
Entry
Cat.
Time
Conversion Conversion with Hg

(h)
1
(%)
2
(%)b
<2
1
2
3
3a
3a
3a
3
87
96
87
6
94
4
3d
6
98
97
i
aReaction conditions: Cat. (0.01 mmol), substrate (1 mmol), PrOH (3 mL), KOH (2 mmol),
o
80 C. The purity of compounds is checked by GC using dodecane as an internal standard,
conversions are based on ketone derivatives. bOne drop of Hg was added with a syringe.
4. Conclusion
From readily available starting materials, such as 1,3-dialkyl-imidazolidin-2-ylidene, five
[RuCl2(p-cymene)(1,3-dialkyl-imidazolidin-2-ylidene)] complexes 3a-e have been prepared
by transmetallation from Ag-NHC complexes. The silver(I) and ruthenium(II) NHC
complexes were characterized by elemental analysis and spectroscopic methods. Also, the
molecular and crystal structure of one of the [RuCl2(NHC)(η6-p-cymene)] complexes 3d, was
determined by the single-crystal X-ray diffraction technique. Structural studies indicate that
the half-sandwich Ru atom completes its coordination sphere with two chlorido ligands, the
η6-p-cymene ligand and a carbon atom of NHC, presenting a pseudo-octahedral piano-stool
geometry. The chlorido ligands are responsible for the stabilization of the crystal structure of
the complex, with C–H···Cl type intra- and intermolecular interactions. The ruthenium
complexes were applied to transfer hydrogenation of ketones. Transfer hydrogenation
i
reactions were carried out using PrOH as the hydrogen donor with 3a-e as the catalysts and
o
KOH as the base at 80 C for 2-6 h. High yields were obtained. These ruthenium complexes
demonstrated good catalytic activity in the transfer hydrogenation of ketones. The ruthenium
complex 3d containing an NHC ligand with benzhydryl substituents showed better catalytic
performance than the other complexes. Also, the fact that no catalyst poisoning was observed
in the presence of mercury indicates the participation of a homogeneous catalyst.
Acknowledgment
We thank the Adıyaman University Research Fund (FEFYL/2017-0004) for financial
support of this work. The authors acknowledge Dokuz Eylül University for the use of the
Rigaku Oxford Xcalibur Eos Diffractometer (purchased under University Research Grant No:
2010.KB.FEN.13).

Supplementary
Crystallographic data as .cif files for the structure reported in this paper have been
deposited at the Cambridge Crystallographic Data Center with CCDC 1890673 for the
complex 3d. Copies of the data can be obtained free of charge at
http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic
Data Center, 12, Union Road, Cambridge CB2 1EZ, UK. fax: (+44) 1223-336-033, email:
deposit@ccdc.cam.ac.uk.
Conflicts of interest
The authors declare that they have no conflict of interest.
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Highlights
*Silver and ruthenium-N-heterocyclic carbene complexes were synthesized and characterized.
*The silver complexes were used as carbene-transfer agents to synthesize ruthenium(II)
complexes.
*These ruthenium complexes were tested as catalysts in the transfer hydrogenation of ketones.
Conflicts of interest
The authors declare that they have no conflict of interest.

Ruthenium(II) complexes bearing N-heterocyclic carbene ligands with wingtip groups
and their catalytic activity in the transfer hydrogenation of ketones
Beyhan Yiğit, Yılmaz Işık, Duygu Barut Celepci, Enes Evren, Murat Yiğit, Nevin
Gürbüz, İsmail Özdemir
R
R
R'
R'
O
OH
CH
C CH3
Cl
Ru
Cl
N
N
CH3
R
+
R
OH
O
C
H3C CH CH3
H3C
CH3