Active ruthenium(II)-NHC complexes for alkylation of amines with alcohols using solvent-free conditions

Article abstract of DOI:10.1016/j.poly.2019.114234

A series of new ruthenium(II) complexes bearing N-heterocyclic carbene ligands with benzylic groups were prepared by transmetallation reactions between silver(I) N-heterocyclic carbene complexes and [RuCl₂(p-cymene)]₂. All of the obtained complexes were characterized by FT-IR, ¹H NMR and ¹³C NMR spectroscopy, and the molecular structure of compound 3c was also determined by X-ray crystallography. These ruthenium complexes were tested for the alkylation of aromatic amines with a wide range of primary alcohols under solvent-free conditions using the hydrogen borrowing strategy. All of the compounds tested here showed excellent catalytic activity for these reactions and N-monoalkylated products were obtained selectively using 2.5 mol% of the ruthenium complexes.

Full text of DOI:10.1016/j.poly.2019.114234

Journal Pre-proofs
Active ruthenium(II)-NHC complexes for alkylation of amines with alcohols
using solvent-free conditions
Beyhan Yiğit, Emine Özge Karaca, Murat Yiğit, Nevin Gürbüz, Hakan Arslan,
İsmail Özdemir
PII:
S0277-5387(19)30679-5
DOI:
https://doi.org/10.1016/j.poly.2019.114234
POLY 114234
Reference:
Polyhedron
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Revised Date:
Accepted Date:
25 September 2019
22 October 2019
26 October 2019
Please cite this article as: B. Yiğit, E. Özge Karaca, M. Yiğit, N. Gürbüz, H. Arslan, I. Özdemir, Active ruthenium(II)-
NHC complexes for alkylation of amines with alcohols using solvent-free conditions, Polyhedron (2019), doi:
https://doi.org/10.1016/j.poly.2019.114234
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Active ruthenium(II)-NHC complexes for alkylation of amines with alcohols using
solvent-free conditions
Beyhan Yiğit,1 Emine Özge Karaca,2 Murat Yiğit,1 Nevin Gürbüz,2,3 Hakan Arslan,4 İsmail
Özdemir2,3*
1Department of Chemistry, Faculty of Science and Art, Adıyaman University, 02040 Adıyaman, Turkey
2Catalysis Research and Application Center, İnönü University, 44280 Malatya, Turkey
3Department of Chemistry, Faculty of Science and Art, İnönü University, 44280 Malatya, Turkey
4Department of Chemistry, Faculty of Arts and Science, Mersin University, Mersin, TR-33343, Turkey
Abstract
A series of new ruthenium(II) complexes bearing N-heterocyclic carbene ligands with
benzylic groups were prepared by transmetallation reactions between silver(I) N-heterocyclic
carbene complexes and [RuCl2(p-cymene)]2. All of the obtained complexes were
characterized by FT-IR, 1H NMR and 13C NMR spectroscopy, and the molecular structure of
compound 3c was also determined by X-ray crystallography. These ruthenium complexes
were tested for the alkylation of aromatic amines with a wide range of primary alcohols under
solvent-free conditions using the hydrogen borrowing strategy. All of the compounds tested
here showed excellent catalytic activity for these reactions and N-monoalkylated products
were obtained selectively using 2.5 mol % of the ruthenium complexes.
Keywords: Ru-N-Heterocyclic carbene complex, X-ray crystallography, Borrowing
hydrogenation reaction, Alkylation of amine.
1. Introduction
The borrowing hydrogen methodology, also called hydrogen auto-transfer, is an attractive
method for the formation of carbon-carbon and carbon-heteroatom bonds in the presence of
transition metal catalysts and has received much attention as greener process in recent years
[1]. This process involves three steps, but it is carried out in one pot. In the initial step,
hydrogen is temporarily removed from an alcohol with a transition metal catalyst to form a
carbonyl compound, then the carbonyl compound reacts with a nucleophile to form an
unsaturated intermediate. In the last step, the unsaturated intermediate is reduced by the metal

hydride complex to obtain the final product. This methodology has atom efficiency and a low
environmental impact, in addition to operational simplicity [2], and permits to the alkylation
of many different compounds, such as amines, ketones, secondary alcohols, thiols, amides and
nitriles, using alcohols [3]. Alcohols as alkylating agents are readily available, relatively
cheap and less toxic than the corresponding alkyl halide, and water is the only by-product in
the overall process. Instead of alcohols, amines can also be used as the source of electrophiles
in this process, but the use of amines is very limited [4]. The alkylation of amines with
alcohols using transition metal catalysts as an alternative to conventional alkylation
procedures is an environmentally benign procedure to produce alkylated amines, which are
widely used as biologically active compounds, agrochemicals, functionalized materials and
dyes in both the bulk and fine chemical industries [5]. Alkylated amines are usually prepared
by the alkylation of amines with alkyl halides [6] or by reductive amination of carbonyl
compounds with amines using stoichiometric reducing agents [7]. However, these reactions
have significant drawbacks, such as toxicity of the alkylating and reducing agents, the
formation of large amounts of waste, acidic reaction conditions and unwanted side products.
Numerous catalytic methods, such as Buchwald-Hartwig amination [8], hydroamination [9],
and hydroaminomethylation [10] and the borrowing hydrogen methodology [11], have been
developed for the purpose of overcoming these issues. In 1981, Grigg [12] and Watanabe [13]
independently reported the first examples of the alkylation of amines with alcohols using
homogeneous catalysts by the borrowing hydrogen method. In these reports, RhH(PPh3)4 and
RuCl2(PPh3)3 complexes were used as a catalyst for the alkylation of amines. Since then, a
number of homogeneous transition metal-based catalytic systems have been developed for
alkylation of amines with alcohols [14]. Heterogeneous catalysts have also been explored for
this type of transformation [15], but some of them suffer from drawbacks such as low
selectivity and activity for mono-alkylation, and the need for high temperature and additives
or co-catalysts. In addition, biocatalysts [16] and chiral catalysts [17] have been applied for
the amination of alcohols to prepare enantiopure amines, which are key intermediates for the
synthesis of organic compounds in the fine chemicals and pharmaceutical industries. In recent
years, studies have been focused on the development of more efficient catalysts for
performing N-alkylation reactions under mild conditions. Especially, catalytic systems based
on iridium provide mild conditions for the N-alkylation of amines. For example, Martin-
Matute and co-workers [18] reported the alkylation of amines with alcohols using
Cp*Ir(NHC) complexes under base-free conditions with short reaction times. High yields
were also obtained at low temperature, but long reaction times were required. Li and

Andersson [19] demonstrated that a bidentate Ir(NHC-phosphine) complex catalyzed the N-
monoalkylation of aromatic amines with primary alcohols at room temperature and under
solvent-free conditions. Fernandes and Royo [20] recently described the use of
IrCl2Cp*(NHC) complexes as a catalyst for the N-alkylation of amines with alcohols in water
under base-free conditions. In addition to iridium-NHC complexes [21], ruthenium complexes
[22] are also highly active for the N-alkylation of amines under mild conditions. Enyong and
Moasser [23] achieved the alkylation of primary and secondary amines with alcohols at low
or room temperature using [RuCl2(p-cymene)]2/amino amide ligand catalyst systems. Many
protocols for the N-alkylation of amines have been reported that require the use of organic
solvents and long reaction times. Herein, we wish to report the synthesis and characterization
of new p-cymene-ruthenium(II)-NHC complexes and their use as catalysts for the N-
alkylation reactions of aniline with arylmethyl alcohols under solvent-free conditions.
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 [24], perhydrobenzimidazolium
salts [25,26] and the Ag-NHC complexes [27,28] were synthesized according to published
1
procedures. H and 13C NMR spectra were recorded with a Bruker Avance III Merkur
spectrometer operating at 400 MHz (1H) and 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 and m = multiplet
signal. FT-IR spectra were recorded on ATR unit in the range 400-4000 cm-1 on a Perkin
Elmer Spectrum 100. GC was measured by a Shimadzu GC-2010 Plus 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 an Electrothermal-9200
melting point apparatus and are uncorrected.
The X-ray single crystal diffraction data were recorded on a Bruker APEX-II CCD
diffractometer. A suitable crystal was selected, coated with Paratone oil and mounted onto a
Nylon loop on a Bruker APEX-II CCD diffractometer. The crystal was kept at T = 296(2) K
during the data collection. The data were collected with MoKα (λ = 0.71073 Å) radiation for

complex 3c at a crystal-to-detector distance of 40 mm. Using Olex2 [29], the structure was
solved with the Superflip [30-32] structure solution program, using the Charge Flipping
solution method and refined by full-matrix least-squares techniques on F2 using ShelXL [33]
with refinement of F2 against all reflections. Hydrogen atoms were constrained by difference
maps and were refined isotropically and all non-hydrogen atoms were refined anisotropically.
The molecular structure plots were prepared using PLATON [34].
2.2. Synthesis of the perhydrobenzimidazolium salts, 1
The perhydrobenzimidazolium salts, 1, were synthesized according to the literature [28].
2.3. Synthesis of the silver(I)-NHC complexes, 2
The silver(I)-NHC complexes, 2, were synthesized according to literature [28].
2.4. Synthesis of the ruthenium(II)-NHC complexes, 3
A solution of the required silver(I)-NHC complex (0.86 mmol) and [RuCl2(p-cymene)]2
(0.43 mmol) in dichloromethane (20 mL) was stirred for 24 h at room temperature in the dark.
The resulting mixture was then 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 thus
obtained were filtered off, washed with diethyl ether (3 x 10 mL) and dried under vacuum.
2.4.1.Dichloro-[1,3-bis(4-methylbenzyl)perhydrobenzimidazol
-2-ylidene](p-cymene) ruthenium(II), 3a
1
Yield: 0.42 g, 76%, m.p.: 210 °C. IR, υ, cm-1: 1513 (NCN). H NMR (CDCI3) δ, ppm:
0.90-1.10
(m,
4H,
NCHCH2CH2CH2CH2CHN);
1.55-1.71
(m,
4H,
NCHCH2CH2CH2CH2CHN); 3.14-3.28 (m, 2H, NCHCH2CH2CH2CH2CHN); 2.23 and 2.36
(s, 6H, CH2C6H4CH3-4); 5.07, 5.17, 5.30 and 5.41 (d, 4H, J = 8 Hz, CH2Ar); 7.12, 7.19 and
7.28 (d, 8H, J = 8 Hz, CH2C6H4CH3-4); 1.19 and 1.24 (d, 6H, J = 4 Hz, p-
CH3C6H4CH(CH3)2); 2.06 (s, 3H, p-CH3C6H4CH(CH3)2); 2.80 (sept, 1H, J = 8 Hz, p-
CH3C6H4CH(CH3)2); 4.66, 4.74, 5.69 and 5.75 (d, 4H, J = 16 Hz, p-CH3C6H4CH(CH3)2). 13C
NMR (CDCI3) δ, ppm: 23.2, 24.1, 29.4, 29.7, 54.0 and 55.4 (NCH(CH2)4CHN); 21.1
(CH2C6H4CH3-4); 68.1 and 69.1 (CH2Ar); 126.2, 127.7, 128.8, 129.2, 135.1 and 136.6
(CH2C6H4CH3-4); 18.4 (p-CH3C6H4CH(CH3)2); 21.8 (p-CH3C6H4CH(CH3)2); 30.5 (p-
CH3C6H4CH(CH3)2); 83.4, 85.0, 85.6, 85.7, 96.9 and 107.9 (p-CH3C6H4CH(CH3)2); 214.8
(Ru-Ccarb).
2.4.2.Dichloro-[1,3-bis(2,4-dimethylbenzyl)perhydrobenzimidazol-2-ylidene](p-
cymene)ruthenium(II), 3b

Yield: 0.44 g, 80%, m.p.: 223-224 °C. IR, υ, cm-1: 1502 (NCN). 1H NMR (CDCI3) δ, ppm:
1.05-1.33 (m, 4H, NCHCH2CH2CH2CH2CHN); 1.60-1.71 (m, 4H,
NCHCH2CH2CH2CH2CHN); 3.20-3.50 (m, 2H, NCHCH2CH2CH2CH2CHN); 2.29, 2.30, 2.33
and 2.36 (s, 12H, CH2C6H3(CH3)2-2,4); 4.99, 5.20, 5.24 and 5.32 (d, 4H, J = 8 Hz, CH2Ar);
6.98 and 7.04 (s, 2H, CH2C6H3(CH3)2-2,4); 7.08, 7.13, 7.29 and 7.39 (d, 4H, J = 8 Hz,
CH2C6H3(CH3)2-2,4); 1.09 and 1.12 (d, 6H, J = 4 Hz, p-CH3C6H4CH(CH3)2); 1.86 (s, 3H, p-
CH3C6H4CH(CH3)2); 2.50-2.60 (m, 1H, p-CH3C6H4CH(CH3)2); 4.20-4.30 and 5.95-6.05 (m,
13
4H, p-CH3C6H4CH(CH3)2). C NMR (CDCI3) δ, ppm: 23.0, 24.2, 28.7, 29.5, 50.6 and 51.5
(NCH(CH2)4CHN); 19.1 and 19.3 (CH2C6H3(CH3)2-2,4); 67.7 and 68.9 (CH2C6H3(CH3)2-
2,4); 126.2, 126.8, 131.4, 132.9, 134.8 and 136.2 (CH2C6H3(CH3)2-2,4); 17.3 (p-
CH3C6H4CH(CH3)2); 21.8 (p-CH3C6H4CH(CH3)2); 30.4 (p-CH3C6H4CH(CH3)2); 83.9, 84.2,
85.4, 86.9, 95.8, 105.8 (p-CH3C6H4CH(CH3)2); 215.2 (Ru-Ccarb).
2.4.3.
Dichloro-[1,3-bis(4-ethylbenzyl)perhydrobenzimidazol-2-ylidene](p-cymene)
ruthenium(II), 3c
Yield: 0.44 g, 78%, m.p.: 204-205 °C. IR, υ, cm-1: 1513 (NCN). 1H NMR (CDCI3) δ, ppm:
0.97-1.11
(m,
4H,
NCHCH2CH2CH2CH2CHN);
1.55-1.70
(m,
4H,
NCHCH2CH2CH2CH2CHN); 3.19-3.29 (m, 2H, NCHCH2CH2CH2CH2CHN); 1.27 (t, 6H, J =
8 Hz, CH2C6H4(CH2CH3)-4); 2.65 (q, 4H, J = 4 Hz, CH2C6H4(CH2CH3)-4); 5.07, 5.17, 5.30
and 5.44 (d, 4H, J = 8 Hz, CH2Ar); 7.14, 7.21 and 7.30 (d, 8H, J = 8 Hz, CH2C6H4(CH2CH3)-
4); 1.18 and 1.21 (d, 6H, J = 8 Hz, p-CH3C6H4CH(CH3)2); 2.06 (s, 3H, p-
CH3C6H4CH(CH3)2); 2.78 (sept, 1H, J = 8 Hz, p-CH3C6H4CH(CH3)2); 4.66, 4.78, 5.67 and
13
5.77 (d, 4H, J = 16 Hz, p-CH3C6H4CH(CH3)2). C NMR (CDCI3) δ, ppm: 23.2, 24.1, 29.3,
29.6, 54.0 and 55.3 (NCH(CH2)4CHN); 15.5 (CH2C6H4(CH2CH3)-4); 28.4
(CH2C6H4(CH2CH3)-4); 68.2 and 69.0 (CH2Ar); 126.2, 127.6, 128.0, 135.3, 136.5, 143.0
(CH2C6H4(CH2CH3)-4); 18.4 (p-CH3C6H4CH(CH3)2); 21.8 (p-CH3C6H4CH(CH3)2); 30.5 (p-
CH3C6H4CH(CH3)2); 83.3, 84.9, 85.6, 85.8, 97.0 and 107.8 (p-CH3C6H4CH(CH3)2); 214.8
(Ru-Ccarb).
2.4.4.Dichloro-[1,3-bis(4-isopropylbenzyl)perhydrobenzimidazol-2-ylidene](p-
cymene)ruthenium(II), 3d
1
Yield: 0.43 g, 72%, m.p.: 214 °C. IR, υ, cm-1: 1512 (NCN). H NMR (CDCI3) δ, ppm:
1.01-1.30
(m,
4H,
NCHCH2CH2CH2CH2CHN);
1.56-1.73
(m,
4H,
NCHCH2CH2CH2CH2CHN); 3.16-3.29 (m, 2H, NCHCH2CH2CH2CH2CHN); 1.26 (t, 12H, J
= 8 Hz, CH2C6H4(CH(CH3)2)-4); 2.91 (sept, 2H, J = 8 Hz, CH2C6H4(CH(CH3)2)-4); 5.07,
5.16, 5.30 and 5.44 (d, 4H, J = 8 Hz, CH2Ar); 7.17, 7.24 and 7.30 (d, 8H, J = 8 Hz,

CH2C6H4(CH(CH3)2)-4); 1.17 and 1.22 (d, 6H, J = 8 Hz, p-CH3C6H4CH(CH3)2); 2.05 (s, 3H,
p-CH3C6H4CH(CH3)2); 2.75 (sept, 1H, J = 8 Hz, p-CH3C6H4CH(CH3)2); 4.66, 4.78, 5.67 and
13
5.77 (d, 4H, J = 16 Hz, p-CH3C6H4CH(CH3)2). C NMR (CDCI3) δ, ppm: 23.1, 24.2, 29.3,
29.6, 54.0 and 55.2 (NCH(CH2)4CHN); 24.0 (CH2C6H4(CH(CH3)2)-4); 33.7
(CH2C6H4(CH(CH3)2)-4); 68.2 and 68.9 (CH2Ar); 126.2, 126.5, 127.6, 135.4, 136.6 and 147.7
(CH2C6H4(CH(CH3)2)-4); 18.4 (p-CH3C6H4CH(CH3)2); 21.8 (p-CH3C6H4CH(CH3)2); 30.5 (p-
CH3C6H4CH(CH3)2); 83.3, 84.7, 85.7, 85.9, 97.0 and 107.6 (p-CH3C6H4CH(CH3)2); 214.8
(Ru-Ccarb).
2.4.5.Dichloro-[1,3-bis(4-tert-butylbenzyl) perhydrobenzimidazol-2-ylidene](p-cymene)
ruthenium(II), 3e
Yield: 0.51 g, 82%, m.p.: 227-228 °C. IR, υ, cm-1: 1513 (NCN). 1H NMR (CDCI3) δ, ppm:
1.01-1.22
(m,
4H,
NCHCH2CH2CH2CH2CHN);
1.57-1.73
(m,
4H,
NCHCH2CH2CH2CH2CHN); 3.17-3.31 (m, 2H, NCHCH2CH2CH2CH2CHN); 1.32 and 1.34
(s, 18H, CH2C6H4C(CH3)3-p); 5.08, 5.17, 5.31 and 5.44 (d, 4H, J = 4 Hz, CH2Ar); 7.28 and
7.40 (d, 8H, J = 8 Hz, CH2C6H4C(CH3)3-p); 1.16 and 1.20 (d, 6H, J = 8 Hz, p-
CH3C6H4CH(CH3)2); 2.04 (s, 3H, p-CH3C6H4CH(CH3)2); 2.72 (sept, 1H, J = 8 Hz, p-
CH3C6H4CH(CH3)2); 4.64, 4.88, 5.58 and 5.80 (d, 4H, J = 16 Hz, p-CH3C6H4CH(CH3)2). 13C
NMR (CDCI3) δ, ppm: 23.1, 24.1, 29.2, 29.6, 53.9 and 55.0 (NCH(CH2)4CHN); 31.4
(CH2C6H4C(CH3)3-p); 34.5 (CH2C6H4C(CH3)3-p); 68.2 and 68.8 (CH2Ar); 125.0, 125.9,
127.3, 135.0, 136.3 and 149.9 (CH2C6H4C(CH3)3-p); 18.4 (p-CH3C6H4CH(CH3)2); 21.8 (p-
CH3C6H4CH(CH3)2); 30.4 (p-CH3C6H4CH(CH3)2); 83.3, 84.8, 85.8, 85.9, 96.9 and 107.5 (p-
CH3C6H4CH(CH3)2); 214.8 (Ru-Ccarb).
2.5. General procedure for the N-alkylation of amines with alcohols
Under an inert atmosphere, KOBut (1 mmol), the aromatic amine (1 mmol), the alcohol
derivative (1.5 mmol) and the Ru-NHC complex (2.5 mol%) were added to a Schlenk tube.
The sealed Schlenk tube was stirred at 120 °C for 24 h. At the end of the reaction, the mixture
was cooled to room temperature, CH2Cl2 (2 mL) was added and the resulting solution was
filtered through a short pad of SiO2. The filtrate was analysed by GC-MS with the calibrations
based on dodecane.
3. Results and discussion
3.1. Synthesis of the Ru(II)-NHC complexes
The symmetrical 1,3-dialkylperhydrobenzimidazolium salts 1, as NHC precursors, were
obtained in high yields from the cyclization of the N,N’-dialkylcyclohexan-1,2-diamines with

triethyl orthoformate in the presence of ammonium chloride. The coordination of the
perhydrobenzimidazol-2-yldene ligands to ruthenium was achieved by the transmetallation
with silver N-heterocyclic carbene complexes. This procedure, involving Ag-NHC
complexes, is probably one of the most general methods for the preparation of NHC
complexes, which has successfully been applied to many transition metals, including
ruthenium, rhodium, iridium, gold and nickel [35]. Treatment of the
perhydrobenzimidazolium salts with Ag2O in dichloromethane at room temperature in the
dark afforded the corresponding silver NHC complexes 2. The addition of [RuCl2(p-
cymene)]2 to a solution of the silver NHC complexes in dichloromethane gave the desired (p-
cymene)-ruthenium(II)-NHC complexes 3a-e in moderate to good yields (Scheme 1). Instant
precipitation of AgCl indicated the successful carbene transfer onto ruthenium centre to form
the ruthenium(II)-NHC complexes. The ruthenium(II)-NHC complexes were purified by
crystallization. These complexes are very stable against air and moisture in the solid state.
1
The new ruthenium complexes were characterized by H NMR, 13C NMR and IR
spectroscopy, together with elemental analysis techniques, which support the proposed
structures. NMR analyses of the complexes showed that the N-heterocyclic carbene ligands
had been transferred from the silver complexes to the ruthenium ion. They exhibit
characteristic υ(C=N) bands in the range 1502-1517 cm-1 for 3a-e. In the 1H NMR spectra of
3a-e, the benzylic protons signals of the N-heterocyclic carbene ligands appear as four
doublets in range δ 4.99-5.44 ppm, with the same coupling constants, indicating that the
methylene protons of the two benzyl groups are diastereotopic. The signals of non-equivalent
aromatic protons of the p-cymene ligands were observed as four doublets in range δ 4.64-5.80
1
ppm in the H NMR spectra of 3a-e. The methyl signals of the isopropyl group on the p-
cymene ligands appear as two doublets in range δ 1.09-1.24 ppm, whereas the methyl signals
13
of the p-cymene ligands were observed as a singlet in range δ 1.86-2.06 ppm. The C-NMR
chemical shifts provide a useful diagnostic tool for this type of metal carbene complex. The
resonance of the carbene carbon atom of complexes 3a-e was observed at δ 214.8 ppm for 3a
and 3c-e, and at δ 215.2 ppm for 3b, these values are similar to those found for other (p-
cymene)-ruthenium(II)-NHC complexes. These new complexes show typical spectroscopic
signatures which are in line with those recently reported for other [RuCl2(NHC)(arene)]
complexes [36,37].

R
R
R
R
R
Cl
Ru
Cl
N
N
N
N
N
+)
N
Ag2O, CH2Cl2
RT, 24 h
[RuCl2(p-cymene)]2
Cl-
AgCl
CH2Cl2, 24 , RT
R
2
1
3a-e
Cl
Ru
Cl
Cl
Cl
N
Cl
Ru
Cl
Cl
Ru
Cl
N
N
N
N
N
N
N
N
Ru
Cl
Ru
N
Cl
3a
3d
3b
3c
3e
Scheme 1. The synthesis of the (p-cymene)-ruthenium(II)-NHC complexes.
3.2. Crystal structure description of complex 3c
The molecular structure of complex 3c was determined by the single-crystal X-ray
diffraction technique and the obtained results confirmed all the spectroscopic data. A single
crystal of complex 3c for X-ray diffraction analysis was obtained by slow vapor diffusion of
diethyl ether into a dichloromethane solution of 3c at room temperature. The complex
crystallized in the monoclinic C2 space group with Z = 4. Details of the crystallographic data
and structure refinement parameters are summarized in Table 1. Since the low temperature
apparatus of the Bruker APEX-II CCD diffractometer instrument was not in service, we
collected single crystal X-ray diffraction data for complex 3c at room temperature and we
obtained some disorder problems in the molecular structure. However, it is clear that the
obtained molecular structure for the complex 3c confirms our suggested molecular structure.
The atom numbering for complex 3c is given in Figure 1 with the relevant bond distances and
angles collected (see supporting information, Table S1).

Table 1. Crystal data and details of the structure refinement for complex 3c.
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C35H44Cl2N2Ru
664.69
296.0(2)
Monoclinic
C2
a (Å)
49.015(3)
b (Å)
6.5288(3)
c (Å)
10.2986(5)
β (°)
96.749(2)
3272.8(3)
Volume (Å3)
Z
4
ρcalc (g/cm3)
1.349
0.668
Μ (mm-1)
F(000)
1384.0
Crystal size (mm3)
Radiation
0.43 × 0.11 × 0.1
MoKα (λ = 0.71073)
6.032 to 50.088
-58 ≤ h ≤ 58, -7 ≤ k ≤ 7, -12 ≤ l ≤ 12
46193
5793 [Rint = 0.0819, Rsigma = 0.0448]
5793/1/367
1.078
2Θ range for data collection (°)
Index ranges
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I≥2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole (e Å-3)
Flack parameter
R1 = 0.0607, wR2 = 0.1472
R1 = 0.0722, wR2 = 0.1526
1.93/-0.79
0.082(17)
The ruthenium atom in the molecule has a classical pseudo-tetrahedral piano-stool
coordination environment, being ligated by two chloride ligands, one carbon atom of the N-
heterocyclic carbene ligand and a η6-p-cymene ligand.
The Ru-Ccarben [2.072(11) Å] and the average Ru-Ccymene [2.207(12) Å ] and the average
Ru-Cl [2.427(3) Å] bond lengths are slightly elongated in comparison with those observed
previously in related ruthenium complexes [38-41]. The Ru-Cl distances in the coordination
sphere are equal to within experimental error [Ru-Cl1 = 2.427(3) Å and Ru-Cl2 = 2.428(4)
Å]. The ruthenium atom is situated 1.708(5) Å from the ring centroid of the η6-p-cymene
ligand and the angles of the ring centroid of the p-cymene ligand–Ru–Cl are 127.1(2) and
127.4(2)° [38]. The bond angles Cl1-Ru-Cl2 [83.70(14)°] and Cl-Ru-Ccarben [89.8(3) and
89.0(3)°] fall within the normal range for known analogous complexes [39-42]. The small
steric demand of the carbene ligand is reflected in the 89.8(3) and 89.0(3)° C–Ru–C1 angles.
All the other bond lengths and angles fall within the expected range. A part of the
benzimidazole ring (C2-C3-C4-C5-C6) exhibits a puckered conformation, with puckering

parameters q2 = 0.161(16) Å, q3 = 0.581(16) Å, QT = 0.604(15) Å, θ = 15.6(15)° and φ =
316(6)° [43]. The largest deviations from the mean plane are 0.327(12) Å for C7 and
0.304(14) Å for C2. This ring puckering analysis shows that the C2-C7 ring has a chair
conformation with equatorial substitution at C2 for N1.
In the crystal structure, the molecules are linked by an intermolecular short contact: C26-
H26···Cl1i, with C-H 0.93 Å, H···Cl 2.64 Å, C-H···Cl 169° [Symmetry code: (i) x, -1+y, z]
and an intramolecular short contact: C17-H17B···Cl1, C-H 0.97 Å, H···Cl 2.55 Å, C-H···Cl
129°.
Figure 1. Molecular structure of the complex 3c.
3.3. Catalytic studies
The catalytic activities of the new ruthenium complexes 3a-e for the N-alkylation of
aromatic amines with arylmethyl alcohols were evaluated. Initially, the reaction of aniline
with benzyl alcohol was selected as a model reaction to determine the optimal catalytic
conditions, with 3e as a catalyst. The effect of the base, reaction time, solvent and catalyst
loading were examined. The results are summarized in Table 2. A base was crucial to the
efficiency of these reactions. Cs2CO3, K2CO3, KOH, and KOBut were tested as the base.
Among the tested bases, KOBut displayed the highest reactivity (entry 3). KOH was less
effective (entry 2), while the alkali metal carbonates Cs2CO3 and K2CO3 stopped the reaction
completely at the dehydrogenation step. It is noteworthy that in the absence of a base or
ruthenium complex, no reaction was observed (entry 11). Toluene and dioxan, often used in
these reactions, gave excellent or poor conversions (entries 2 and 5). However, in the absence
of a solvent, an excellent conversion was also observed. Reducing the catalyst loading to 1

mol% decreased the conversion (entry 9). Excellent conversion and selectivity were obtained
the use of 2.5 mol% of 3e (entry 10). A lower temperature (100 °C) or shorter time (15 h)
reduced the conversion (entries 2 and 1), whereas full conversion was observed at 120 °C for
24 h (entry 7). The catalytic experiments were carried out using 1 mmol aromatic amine, 1.5
mmol arylmethyl alcohol, 1.0 mmol KOBut, and 0.025 mmol 3a-e at 120 °C for 24 h under
argon without any solvent or additive. Under these reaction conditions, the ruthenium
catalyzed N-alkylation of aromatic amines (aniline, 2,4-dimethylaniline, and 2-aminopyridine)
with arylmethyl alcohols (benzyl alcohol, 4-methyl benzyl alcohol, 4-methoxybenzyl alcohol
and furfuryl alcohol) were examined. In all cases, only mono-alkylated amines and imines
were formed; no bis-alkylated products were detected. The reaction products were
characterized by NMR spectroscopy. The conversions and selectivities were screened by GC
and GC-MS analysis.
Table.2. Determination of reaction conditions for the alkylation of aniline with benzyl
alcohol.a-h
NH2
+
NH
N
+
OH
A
B
Entry
3e
Base
Time
(h)
15
Temp
(oC)
100
100
100
150
100
120
120
120
120
120
120
Conversion
A/B (%)
(mol%)
(%)
1
2
1
1
KOH
KOH
KOBut
KOH
KOH
KOBut
KOBut
KOBut
KOBut
KOBut
-
50
36/64
44/56
57/43
49/52
13/87b
78/22
89/11
52/48c
60/40d
88/12d
-/-
24
73
3
1
24
83
4
1
24
88
5
1
24
9
90
6
1
24
7
2.5
2.5
1
24
100
8
24
97
9
24
84
10
2.5
24
100
11
-
24
no reaction
aReaction conditions: Aniline (1 mmol), benzylalcohol (1.5 mmol), base (1 mmol), Ru-NHC (0.01 mmol),
b
c
toluene (3 mL), 15-24 h, 100 °C. Dioxane. Base (0.5 mmol). dSolvent free. Yields are determined by GC and
GC-MS.

With the determined optimal conditions in hand, we first investigated, in the presence of
complexes 3a-e as catalysts, the N-alkylation of aniline with benzyl alcohol, 4-methylbenzyl
alcohol, 4-methoxybenzyl alcohol and furfuryl alcohol. In all the reactions, alkylation of
aniline with high selectivities was achieved. The reaction of aniline with benzyl alcohol gave
N-benzylaniline with 93-100% conversions (Table 3, entry 1). The formation of N-
benzylaniline with high selectivity was obtained with complex 3d (Table 3, entry 1). The
treatment of aniline with 4-methylbenzyl alcohol and 4-methoxybenzyl alcohol using
complexes 3a-e as catalysts also gave the corresponding mono-alkylated amines, with
conversions of 96-99% and 86-94%, respectively, under these conditions. 4-Methoxybenzyl
alcohol afforded the corresponding mono-alkylated product N-(4-methoxybenzyl)aniline with
excellent selectivities for all the complexes 3a-e (Table 3, entry 3). The best selectivity was
achieved with catalyst 3e (Table 3, entry 3). These results show that the electron-donating
substituent (4-methyl and 4-methoxy) on the benzyl alcohol gave higher selectivity when
compared benzyl alcohol itself. When using furfuryl alcohol as an alkylating agent, N-(furan-
2-ylmethyl)aniline with 100% selectivity was obtained for all the complexes 3a-e, but the
conversions of reactions were slightly lower than the other alkylating agents (Table 3, entry
4).
Table 3. N-alkylation of anilines with benzyl alcohols.a
3a-e, KOBut
NH2
+
NH
N
+
OH
R
R'
R
R
R'
R'
B
A
Entry
Amine
Alcohol
[Ru]
Conversion (Yield A:B%)
3a
3b
3c
3d
3e
1
2
3
95 (80/20) 98(70/30)
95(57/43)
93(98/2)
100(88/12)
OH
NH2
96(100/0)
90(99/1)
98(86/14)
94(97/3)
96(70/30)
86(98/2)
99(80/20)
86(99/1)
97(100/0)
93(100/0)
OH
O
OH

4
5
6
7
8
94(100/0)
73(81/19)
86(89/11)
82(54/46)
84(100/0)
88(100/0)
69(51/49)
95(82/18)
81(80/20)
81(100/0)
87(100/0)
78(57/43)
88(100/0)
86(65/35)
85(100/0)
83(100/0)
81(94/6)
85(66/34)
71(94/6)
OH
O
OH
98(100/0)
100(97/3)
92(100/0)
84(50/50)
83(76/24)
79(66/34)
H3C
NH2
CH3
OH
O
OH
OH
O
aReaction conditions: Complexes 3a-e (0.025 mmol), arylmethyl alcohol (1.5 mmol), heterocyclic amine (1
mmol), KOBut (1 mmol), 120 °C, 24 h. The conversions and the selectivities were determined by GC and GC-
MS analysis with calibrations based on dodecane.
We next examined the reactions of 2,4-dimethylaniline with benzyl alcohol, 4-
methylbenzyl alcohol, 4-methoxybenzyl alcohol and furfuryl alcohol in the presence of
complexes 3a-e as catalysts under the same reaction conditions (Table 3, entries 5-8). Thus,
2,4-dimethylaniline was treated with benzyl alcohol for all the complexes 3a-e under the
optimized reaction conditions to obtain the corresponding N-benzyl-2,4-dimethylaniline in
69-81% conversions with 51-94% selectivities (Table 3, entry 5). The reaction of 2,4-
dimethylaniline with 4-methylbenzyl alcohol and 4-methoxybenzyl alcohol also afforded the
corresponding mono-alkylated amines with conversions of 84-95% and 81-100%, with 50-
100% and 54-97% selectivities, respectively. The above results show that electron-donating
substituents, such as Me and Ome, on both benzyl alcohol and aniline slightly increased the
selectivities of the mono-alkylated amine products under the same conditions (Table 3, entries
1-3 and Table 3, entries 5-7). Similar trends have been observed for the other Ru(II) systems
bearing NHC ligands [22]. When furfuryl alcohol was used as an alkylating agent, only the
corresponding mono-alkylated amine was obtained, with 100% selectivity for complexes 3a-d
(Table 3, entry 8); only 3e gave a slightly lower result than the other complexes. No imine or
bis-alkylated products were detected.
Finally, we also investigated the N-alkylation of 2-aminopyridine with the same alcohols
(benzyl alcohol, 4-methylbenzyl alcohol, 4-methoxybenzyl alcohol and furfuryl alcohol),
using the ruthenium(II)-NHC catalysts 3a-e in order to obtain N-alkylated amines under the
same reaction conditions. The 2-(N-alkylamino)pyridines were obtained in good to excellent
selectivities in the presence of 2.5 mol% of the catalysts. Moreover, the heteroaromatic

moiety in 2-aminopyridine was also well-tolerated under this catalytic system. In all the
reactions, only the nitrogen atom of the amino group of 2-aminopyridine was alkylated, the
products with N-alkyl pyridine were not detected. 2-Aminopyridine was efficiently arylated
with benzyl alcohol and furfuryl alcohol for all the complexes 3a-e with 100% selectivity
(Table 4, entries 1 and 4). The reaction of 2-aminopyridine with 4-methylbenzyl alcohol and
4-methoxybenzyl alcohol also gave the corresponding products with conversions of 96-100%
and 98-100%, with 99-100% and 97-100% selectivities, respectively.
Table 4. N-alkylation of 2-pyridyl amine with arylmethyl alcohols.a
3a-e, KOBut
+
+
OH
N
NH2
N
N
NH
N
R
R
R
A
B
[Ru]
Conversion (Yield A:B%)
Entry
Alcohol
3a
3b
3c
3d
3e
98(100/0)
100(100/0)
99(98/2)
98(100/0)
97(100/0)
96(100/0) 98(100/0)
98(100/0) 99(99/1)
100(97/3) 98(97/3)
95(100/0) 100(100/0)
1
2
3
4
OH
OH
OH
OH
97(100/0)
100(98/2)
92(100/0)
96(100/0)
100(97/3)
97(100/0)
O
95(100/0)
O
aReaction Conditions: Aniline (1 mmol), alcohol (1.5 mmol), Ru-NHC (0.025 mmol), KOBut (1 mmol), 120 °C,
24 h. The conversions and the selectivities were determined by GC and GC-MS analysis with calibrations based
on dodecane.
The ruthenium(II)-NHC complexes showed good catalytic activity for the N-alkylation of
amines in the presence of KOH base, and the general mechanism reveals that the in situ
generated ruthenium hydride species is the catalytically active species [44, 45]. A possible
catalytic cycle is proposed in Scheme 2 on the basis of the results obtained and available

literature on similar ruthenium catalyzed transformations [46-48]. This catalysis is considered
to proceed via the (benzyloxy)ruthenium intermediate I, which undergoes -hydrogen
elimination to give the ruthenium hydride II. Dehydrative condensation of the aldeyde with
an amine forms an imine (R1CH=NR2). Insertion of the imine into the Ru-H bond of II,
followed by alcoholysis of the resulting (amido)ruthenium species III, affords the N-
alkylation product and reproduces the (benzyloxy)ruthenium intermediate I, completing the
catalytic cycle.
Cl Ru Cl
N
N
R
R
KOBut
H
H
O-K+
H
H
-KCl
-ButOH
R
R1
OH
R1
H
H
Ru
O
H
R2
Cl
R1
N
N
N
H
R
R
H
I
H
H
O
R1
OH
H
R1
H
H
R1
R2NH2
H2O
N
Ru
Ru
Cl
Cl
H
R2
N
N
N
N
R
R
R
R
III
II
R2
N
H
R1
Scheme 2. A proposed reaction pathway for the ruthenium(II)-NHC catalyzed N-alkylation.
4. Conclusion
In summary, ruthenium(II)-NHC complexes 3a-e have been easily prepared by the reaction
of silver(I)-NHC complexes as a carbene transfer reagent with [RuCl2(p-cymene)]2 in

dichloromethane at room temperature in good yields and the molecular structure of compound
3c has also been determined by X-ray crystallography. The catalytic activity of these
complexes was investigated for the N-alkylation reactions of aniline, 2,4-dimethylaniline and
2-aminopyridine with arylmethyl alcohols, including benzyl alcohol, 4-methyl benzyl alcohol,
4-methoxybenzyl alcohol and furfuryl alcohol. The N-alkylation of the amines was performed
under solvent-free conditions over 24 h at 120 °C using 2.5 mol% of complexes 3a-e. All of
these complexes were found to be suitable for the N-alkylation of aromatic amines with
arylmethyl alcohols via hydrogen borrowing reactions. In this study, high selectivities were
obtained. In all cases only monoalkylated amines were formed and no bis-alkylated products
were detected.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This work was supported by the İnönü University Research Fund [Project No:
2016/76(Güd)].
Appendix A. Supplementary data
CCDC 1910177 contains the supplementary crystallographic data for 3c. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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Active ruthenium(II)-NHC complexes for alkylation of amines with alcohols using
solvent-free conditions
Beyhan Yiğit, Emine Özge Karaca, Murat Yiğit, Nevin Gürbüz, Hakan Arslan, İsmail Özdemir
Active ruthenium(II)-NHC complexes for alkylation of amines with alcohols using
solvent-free conditions
Beyhan Yiğit, Emine Özge Karaca, Murat Yiğit, Nevin Gürbüz, Hakan Arslan, İsmail Özdemir
A series of new ruthenium(II) complexes bearing N-heterocyclic carbene ligands with
benzylic groups were prepared by transmetallation reactions between silver(I) N-heterocyclic
carbene complexes and [RuCl2(p-cymene)]2. All of the obtained complexes were
characterized by FT-IR, 1H NMR and 13C NMR spectroscopy, and the molecular structure of
compound 3c was also determined by X-ray crystallography. These ruthenium complexes
were tested for the alkylation of aromatic amines with a wide range of primary alcohols under
solvent-free conditions using the hydrogen borrowing strategy. All of the compounds tested

here showed excellent catalytic activity for these reactions and N-monoalkylated products
were obtained selectively using 2.5 mol % of the ruthenium complexes.