Half-sandwich Ru(ii) arene complexes bearing benzimidazole ligands for theN-alkylation reaction of aniline with alcohols in a solvent-free medium

Article abstract of DOI:10.1039/d1nj01539g

In this article, the directN-alkylation reactions of amines with alcohol derivatives using the borrowing hydrogen methodology have been investigated. For this purpose, a new series of half-sandwich ruthenium(ii) complexes bearing N-coordinated benzimidazole complexes have been synthesized and fully characterized by FT-IR,¹H NMR and¹³C NMR spectroscopies. Additionally, the structures of the complexes2a-2ehave been characterized by X-ray crystallography. All new complexes were investigated for their catalytic activities in the alkylation reaction of amines with alcohol derivatives. It was found that alkylation reactions in a solvent-free medium are efficient and selective.

Full text of DOI:10.1039/d1nj01539g

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Half-sandwich Ru(II) arene complexes bearing
benzimidazole ligands for the N-alkylation
reaction of aniline with alcohols in a solvent-free
Cite this: New J. Chem., 2021,
45, 11075
medium†‡
bc
Metin Çiçek,ab Nevin Gurbuz,bc Namık O¨ zdemir, d ˙Ismail O¨ zdemir
*
and
¨
¨
a
Esin ˙Ispir
*
In this article, the direct N-alkylation reactions of amines with alcohol derivatives using the borrowing
hydrogen methodology have been investigated. For this purpose, a new series of half-sandwich
ruthenium(II) complexes bearing N-coordinated benzimidazole complexes have been synthesized and
fully characterized by FT-IR, 1H NMR and 13C NMR spectroscopies. Additionally, the structures of the
complexes 2a–2e have been characterized by X-ray crystallography. All new complexes were
investigated for their catalytic activities in the alkylation reaction of amines with alcohol derivatives. It
was found that alkylation reactions in a solvent-free medium are efficient and selective.
Received 29th March 2021,
Accepted 11th May 2021
DOI: 10.1039/d1nj01539g
rsc.li/njc
Piribedil is an anti-Parkinson’s disease drug, Indobufen is a
reversible inhibitor of platelet cyclooxygenase (Cox) activity, and
Introduction
Amines, because of their various biological and pharmacological Naftifine is an antifungal drug characterized by a substituted
properties, are an important class of compounds that are benzylamine moiety among other interesting structural
commonly present in bioactive molecules, pharmaceuticals and features8–10 (Fig. 1).
natural products.1 In organic chemistry, they are among the most
Alkylated amines are usually prepared by the alkylation of
important building blocks because they are essential precursors amines with alkyl halides11 or by the reductive amination of
in the preparation of a wide range of pharmacologically carbonyl compounds with amines using stoichiometric reducing
significant therapeutic agents, natural products and agents.12,13 However, these reactions have significant drawbacks,
agrochemicals.2–4 There is growing interest in developing simple such as the toxicity of the alkylating and reducing agents, the
and effective methodologies for the synthesis of N-alkylated formation of large amounts of waste, acidic reaction
aliphatic or aromatic amines through the formation of C–N conditions and side products. Numerous catalytic methods, such
bonds, as they are used in the high-throughput synthesis of as the Buchwald–Hartwig amination,14 hydroamination,15,16
potential drug-like compound libraries and as significant inter- hydroaminomethylation17 and the borrowing hydrogen
mediates in the pharmaceutical industry.5–7 As selected examples methodology,18 have been developed to overcome these issues.
of substituted amines, Pyribenzamine is a drug that is used as an The borrowing hydrogen methodology is a new, effective and
antipruritic and first-generation antihistamine, Vascor is an alternative technique for alkylamine synthesis. To obtain the
amine calcium channel blocker once used to treat angina, alkylated amines, the use of alcohols instead of alkyl halides
is an attractive method since it does not create any dangerous
a
¨
¨
Department of Chemistry, Faculty of Science and Arts, Kahramanmaras- Sutçu
˙
Imam University, Kahramanmaras-, 46050-9, Turkey. E-mail: esinispir@gmail.com
b
c
¨ ¨
Catalysis Research and Application Center, Inonu University, 44280 Malatya,
Turkey. E-mail: ismail.ozdemir@inonu.edu.tr
¨ ¨
Department of Chemistry, Faculty of Science and Art, Inonu University,
44280 Malatya, Turkey
d Ondokuz Mayıs University, Faculty of Education, Department of Mathematics and
Science Education, 9055139 Samsun, Turkey
† Dedicated to Prof. Christian Bruneau for his outstanding contribution to
catalysis.
‡ Electronic supplementary information (ESI) available. CCDC 2061420–2061424.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1nj01539g
Fig. 1 Important biologically active N-alkylated amines.
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and/or wasteful by-products (only H2O as a by-product). Also, the synthesis and characterization of five novel ruthenium(II)
alcohols are in many cases more readily available than alkyl complexes of the general formula [RuCl2(N-alkylbenzimidazole)
halides and associated alkylating agents, and if the reaction (Z6-p-cymene)]. All of the synthesized compounds were character-
1
continues successfully with the use of equimolar quantities of ized by H and 13C NMR and IR techniques, which support the
starting materials, it is possible to realize an extremely high proposed structures. The molecular and crystal structures of all the
atomic economic system.19 In this process, first, a metal takes complexes were determined using single-crystal X-ray diffraction.
the hydrogen from the alcohol. The hydrogen in the alcohol is These complexes were tested as catalysts for the N-alkylation of
converted into an aldehyde by short-term removal and formation aniline with arylmethyl alcohols using the hydrogen borrowing
of a metal hydride. After removal of water, the aldehyde may react strategy in a solvent-free medium.
with an amine to give the imine, and finally, the transition metal
catalyst returns the borrowed hydrogen to the imine to form the
Experimental
secondary amine (or tertiary amine if a secondary amine is the
starting material).20,21
All manipulations were performed in Schlenk-type flasks under
In 1981, Grigg22 and Watanabe23 independently reported the
an argon atmosphere. Melting point measurements were
first examples of the alkylation of amines with alcohols using
determined in open capillary tubes using Electrothermal-9200
homogeneous catalysts by the borrowing hydrogen method.
melting point apparatus. The IR spectra were recorded using
In these reports, RhH(PPh3)4 and RuCl2(PPh3)3 complexes were
the Gladi ATR unit (Attenuated Total Reflection) in the range of
used as a catalyst for the alkylation of amines. Since then, many
450–4000 cmÀ1 with a PerkinElmer Spectrum 100 Fourier-
homogeneous transition metal-based catalytic systems have been
transform infrared spectrometer. Routine 1H NMR and 13C
developed for the alkylation of amines with alcohols.24,25 In
NMR spectra were recorded using a Bruker Ascendt 400 Avance
addition, heterogeneous catalysts,26 biocatalysts27,28 and chiral
III HD NMR spectrometer with sample solutions prepared in
catalysts29,30 have been applied for the amination of alcohols.
CDCl3. The chemical shifts (d) are reported in parts per million
Half-sandwich ruthenium(II) complexes are Z-arene ruthenium
(ppm) relative to tetramethylsilane (TMS) as an internal standard.
derivatives, where the ruthenium center adopts a half-sandwich
Coupling constants (J values) are given in hertz (Hz). NMR multi-
structure. Complexes with such a structure are widely known as
plicities are abbreviated as follows: s = singlet, d = doublet, t =
piano-stool complexes and are, without doubt, the most studied
triplet, hept = heptet, dd = doublet of doublets, m = multiplet.
complexes within the large family of Z6-arene ruthenium
1H NMR spectra are referenced to residual protiated solvents (d =
complexes. The coordinated aromatic ring introduces steric and
7.28 ppm for CDCl3), and 13C NMR chemical shifts are reported
electronic properties, for instance, it prevents the metal center
relative to deuterated solvents (d = 77.16 ppm for CDCl3).
from being rapidly oxidized to ruthenium(III). The other three
The catalytic solutions were analyzed using a Shimadzu GC
coordinating sites available in the ruthenium complexes are
2025 equipped with a GC-FID sensor and RX-5ms column of
opposite the arene ligands and can be used to introduce a wide
30 m length, 0.25 mm diameter and 0.25 mm film thickness.
variety of ligands with N-, O,- S- or P-donor atoms.31 It is popular
to use nitrogen-donating ruthenium(II) arene complexes as a
General procedure for the preparation of the N-
catalyst32–42 or an antimicrobial and anti-cancer agent.43–46 Coor-
alkylbenzimidazoles (1)
dination compounds of nitrogen donor ligands are interesting
because, due to their steric and electronic effects, they can easily
(10 mmol) in dry THF (30 mL), the mixture was stirred for
adjust the charge density on the metal center. Due to their ability
to easily change the groups around the ruthenium atoms, the
Benzimidazole (10 mmol) was added to a solution of NaH
1 h at room temperature and the corresponding alkyl halide
(10.1 mmol) was added dropwise and heated for 24 h at 60 1C.
adjustable nature of their complexes has led to many uses,
Then, the THF was removed under vacuum. Dichloromethane
especially in chemical technology and organic synthesis: olefin
metathesis,32 asymmetric transfer hydrogenation of ketones,47
(50 mL) was added to the solid. The mixture was filtered, and
oxidation of alcohols,48 hydrogenation of organonitriles,37
the obtained clear solution was concentrated under vacuum.
alkynes,49 and alkenes,38 Diels–Alder reactions50 and alpha alkyla-
Then the solution was distilled and N-alkylbenzimidazole (1a–e)
was obtained.
tion of ketones.51
Our research group has focused on N-coordinated azole
N-(2-Morpholinylethyl)]-5,6-dimethylbenzimidazole, 1a58
ligands and their metal complexes, and their application in
different catalytic systems.52,53 Owing to their use in many Yield: 87%, m.p.: 99–100 1C; FT-IR n(CN): 1494 cmÀ1; H NMR
1
applications of coordination, medicinal and supramolecular (400 MHz, CDCl3): d = 2.38 and 2.41 [s, 6H, C6H2-5,6-(CH3)2],
chemistry, benzimidazoles have gained a lot of interest. Metal 2.48 [t, 4H, J = 4.4 Hz, NCH2CH2O], 2.75 [t, 2H, J = 6.4 Hz,
complexes with benzimidazole ligands are significant due NCH2CH2N], 3.69 [t, 4H, J = 4.4 Hz, NCH2CH2O], 4.21 [t, 2H, J =
to their high thermal stability, good catalytic efficiency and 6.4 Hz, NCH2CH2N], 7.16 and 7.56 (s, 2H, NC6H2N], 7.88 [s, 1H,
superior optical properties.54,55 To the best of our knowledge, NCHN]. 13C NMR (100 MHz, CDCl3): d = 20.1 and 20.6 [C6H2-5,6-
studies on using ruthenium(II) complexes bearing N-coordinated (CH3)2], 42.4 [NCH2CH2], 53.8 [NCH2CH2O], 57.7 [NCH2CH2N],
benzimidazole in the alkylation of amines with alcohols are rare 66.9 [NCH2CH2O], 109.6, 120.4, 131.0, 132.0, 132.2 and 142.3
in the literature.56,57 So, in the present article, we now describe [NC6H2N], 142.6 [NCHN].
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N-(2,2-Dimethoxyethyl)-5,6-dimethylbenzimidazole, 1b59
NCH2CH2O], 4.04 [t, 2H, J = 6 Hz, NCH2CH2N], 5.37 and 5.53 [d,
4H, J = 5.6 Hz, (CH3)2CHC6H4CH3-p], 7.16 and 7.73 [s, 2H,
NC6H2N], 8.44 [s, 1H, NCHN]. 13C NMR (100 MHz, CDCl3): d =
18.5 [(CH3)2CHC6H4CH3-p], 20.5 and 20.7 [C6H2-5,6-(CH3)2],
22.4 [(CH3)2CHC6H4CH3-p], 30.7 [(CH3)2CHC6H4CH3-p], 42.3
[NCH2CH2], 53.4 [NCH2CH2O], 57.3 [NCH2CH2N], 66.9
[NCH2CH2O], 81.3, 82.3, 97.3 and 102.6 [(CH3)2CHC6H4CH3-p],
110.6, 120.3, 132.3, 132.5, 133.4 and 141.0 [NC6H2N], 144.2
[NCHN]. Elemental analysis: calcd for C25H35N3ORuCl2ÁH2O: C
51.46, H 6.39, N 7.20; found C 52.11, H 6.32, N 7.41%.
1
Yield: 85%, m.p.: 88–89 1C; FT-IR n(CN): 1488 cmÀ1; H NMR
(400 MHz, CDCl3): d = 3.38 [s, 6H, NCH2CH(OCH3)2], 4.26 [d, J =
5.2 Hz, 2H, NCH2CH(OCH3)2], 4.57 [t, J = 5.2 Hz, 1H,
NCH2CH(OCH3)2], 7.44 and 7.81 [d, 2H, J = 8 Hz, NC6H4N],
7.19–7.27 [m, 2H, NC6H4N], 7.88 [s, 1H, NCHN]. 13C NMR
(100 MHz, CDCl3):
d
=
47.3 [NCH2CH(OCH3)2], 55.2
[NCH2CH(OCH3)2], 102.7 [NCH2CH(OCH3)2], 109.5, 120.4,
122.1, 123.0 and 134.0 [NC6H4N], 143.0 [NCHN].
N-(2,2-Dimethoxyethyl)-5,6-dimethylbenzimidazole, 1c
Dichloro-[1-(2,2-dimethoxyethyl)benzimidazole](p-cymene)
ruthenium(II), 2b
Yield: 87%, m.p.: 235–236 1C; FT-IR n(CN): 1512 cmÀ1; 1H NMR
Yield: 75%, m.p.: 103–104 1C; FT-IR n(CN): 103–104 cmÀ1
;
1H NMR (400 MHz, CDCl3): d = 2.38 and 2.40 [s, 6H, C6H2-
5,6-(CH3)2], 3.37 [s, 6H, NCH2CH(OCH3)2], 4.20 [d, J = 5.6 Hz,
2H, NCH2CH(OCH3)2], 4.56 [t, J = 5.6 Hz, 1H, NCH2CH(OCH3)2],
7.19 and 7.56 [s, 2H, NC6H2N], 7.84 [s, 1H, NCHN]. 13C NMR (100
(400 MHz, CDCl3):
d
=
1.27 [d, 6H,
J
=
7.2 Hz,
(CH3)2CHC6H4CH3-p], 2.11 [s, 3H, (CH3)2CHC6H4CH3-p], 2.87
[hept, 1H, J = 6.8 Hz, (CH3)2CHC6H4CH3-p], 3.37 [s, 6H,
NCH2CH(OCH3)2], 4.01 [d, J = 4.4 Hz, 2H, NCH2CH(OCH3)2],
4.48 [t, J = 4.4 Hz, 1H, NCH2CH(OCH3)2], 5.37 and 5.53
[d, 4H, J = 6 Hz, (CH3)2CHC6H4CH3-p], 7.34 and 8.01 [m, 4H,
NC6H4N], 8.32 [s, 1H, NCHN]. 13C NMR (100 MHz, CDCl3):
d = 18.4 [(CH3)2CHC6H4CH3-p], 22.3 [(CH3)2CHC6H4CH3-p],
30.6 [(CH3)2CHC6H4CH3-p], 47.3 [NCH2CH(OCH3)2], 55.2
[NCH2CH(OCH3)2], 102.5 [NCH2CH(OCH3)2], 81.1, 82.9,
97.7 and 102.1 [(CH3)2CHC6H4CH3-p], 111.2, 120.3, 123.3,
124.2, 134.0 and 142.1 [NC6H4N], 145.8 [NCHN]. Elemental
analysis: calcd for C21H28N2O2RuCl2ÁH2O: C 47.55, H 5.70, N
5.28; found C 47.84, H 5.31, N 5.51%.
MHz, CDCl3):
d = 20.2 and 20.6 [C6H2-5,6-(CH3)2], 47.3
[NCH2CH(OCH3)2], 55.2 [NCH2CH(OCH3)2], 102.7 [NCH2CH
(OCH3)2], 109.7, 120.4, 131.1, 132.2, 132.5 and 142.1 [NC6H4N],
143.0 [NCHN].
N-(Diphenylmethyl)benzimidazole, 1d60
Yield: 73%, m.p.: 155–156 1C; FT-IR n(CN): 1448 cmÀ1; 1H NMR
(400 MHz, CDCl3): d = 6.75 [s, 1H, CH(C6H5)2], 7.14–7.36 [m,
13H, NC6H4N and CH(C6H5)2], 7.83[d, J = 8 Hz, Ar], 7.60 [s, 1H,
NCHN]. 13C NMR (100 MHz, CDCl3): d = 63.6 [CH(C6H5)2],
110.6, 120.4, 122.4, 123.0, 128.2, 128.5, 129.0, 134.1, 138.1
and 144.1 [NC6H4N and CH(C6H5)2], 142.3 [NCHN].
Dichloro-[1-(2,2-dimethoxyethyl)-5,6-dimethylbenzimidazole]
(p-cymene)ruthenium(II), 2c
N-(Diphenylmethyl)-5,6-dimethylbenzimidazole, 1e
Yield: 90%, m.p.: 248–249 1C; FT-IR n(CN): 1449 cmÀ1; 1H NMR
(400 MHz, CDCl3): d = 2.28 and 2.35 [s, 6H, C6H2-5,6-(CH3)2],
6.70 [s, 1H, CH(C6H5)2], 6.92 and 7.50 [s, 2H, NC6H2N], 7.14 and
7.35 [s, 10H, CH(C6H5)2], 7.58 [s, 1H, NCHN]. 13C NMR
(100 MHz, CDCl3): d = 20.3 and 20.6 [C6H2-5,6-(CH3)2], 63.4
[CH(C6H5)2], 110.7, 120.4, 128.2, 124.4, 129.0, 131.3, 132.1,
132.7, 138.4 and 141.9 [NC6H2N and CH(C6H5)2], 142.7 [NCHN].
Yield: 95%, m.p.: 237–238 1C; FT-IR n(CN): 1513 cmÀ1; 1H NMR
(400 MHz, CDCl3): d = 1.27 [d, 6H, J = 6.8 Hz, (CH3)2CHC6
H4CH3-p], 2.14 [s, 3H, (CH3)2CHC6H4CH3-p], 2.37 and 2.40 [s,
6H, C6H2-5,6-(CH3)2], 2.88 [hept, 1H, J = 6.8 Hz, (CH3)2CHC6
H4CH3-p], 3.38 [s, 6H, NCH2CH(OCH3)2], 4.09 [d, J = 4.4 Hz, 2H,
NCH2CH(OCH3)2], 4.55 [t, J = 4.4 Hz, 1H, NCH2CH(OCH3)2],
5.37 and 5.52 [d, 4H, J = 6 Hz, (CH3)2CHC6H4CH3-p], 7.16 and
7.74 [s, 2H, NC6H2N], 8.32 [s, 1H, NCHN]. 13C NMR (100 MHz,
CDCl3): d = 18.4 [(CH3)2CHC6H4CH3-p], 20.5 and 20.7 [C6H2-5,6-
(CH3)2], 22.3 [(CH3)2CHC6H4CH3-p], 30.6 [(CH3)2CHC6H4CH3-
p], 47.3 [NCH2CH(OCH3)2], 55.0 [NCH2CH(OCH3)2], 102.5
[NCH2CH(OCH3)2], 81.2, 82.9, 97.6 and 102.1 [(CH3)2CHC6
H4CH3-p], 110.7, 120.3, 132.5, 132.6, 133.6 and 140.9 [NC6H4N],
144.5 [NCHN]. Elemental analysis: calcd for C23H32N2O2RuCl2:
C 51.11, H 5.97, N 5.18; found C 51.31, H 5.74, N 5.481%.
General procedure for the preparation of the N-coordinated
benzimidazole ruthenium(II) complexes
A solution of N-alkylbenzimidazole (1.0 mmol) and [RuCl2
(p-cymene)]2 (0.5 mmol) in 10 mL toluene was heated under
reflux for 5 h. Upon cooling to room temperature, an orange
solid of 2a–e was obtained. The solid (2a–e) was filtered and
washed with diethyl ether (3 Â 10 mL) and dried under vacuum.
The crude product was crystallized in CH2Cl2/Et2O.
Dichloro-[1-(diphenylmethyl)benzimidazole](p-cymene)
Dichloro-[N-2-(morpholinyl)ethyl-5,6-dimethylbenzimidazole]
(p-cymene)ruthenium(II), 2a
Yield: 74%, m.p.: 246–247 1C; FT-IR n(CN): 1513 cmÀ1; 1H NMR (400 MHz, CDCl3): d = 1.12 [d, 6H, J = 6.8 Hz, (CH3)2CHC6
ruthenium(II), 2d
Yield: 80%, m.p.: 227–228 1C; FT-IR n(CN): 1501 cmÀ1; 1H NMR
(400 MHz, CDCl3):
d
=
1.28 [d, 6H,
J
=
6.8 Hz, H4CH3-p], 2.14 [s, 3H, (CH3)2CHC6H4CH3-p], 2.70 [hept, 1H, J =
(CH3)2CHC6H4CH3-p], 2.13 [s, 3H, (CH3)2CHC6H4CH3-p], 2.38 6.8 Hz, (CH3)2CHC6H4CH3-p], 5.31 and 5.41 [d, 4H, J = 6 Hz,
and 2.41 [s, 6H, C6H2-5,6-(CH3)2], 2.45 [t, 4H, J = 4.8 Hz, (CH3)2CHC6H4CH3-p], 6.77 [s, 1H, CH(C6H5)2], 7.09–7.38 [m,
NCH2CH2O], 2.69 [t, 2H, J = 6 Hz, NCH2CH2N], 2.92 [hept, 13H, NC6H4N and CH(C6H5)2], 8.13 [d, J = 8 Hz, Ar], 8.21 [s, 1H,
1H, J = 6.8 Hz, (CH3)2CHC6H4CH3-p], 3.68 [t, 4H, J = 4.8 Hz, NCHN]. 13C NMR (100 MHz, CDCl3): d = 18.5 [(CH3)2CHC6H4CH3-p],
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22.2 [(CH3)2CHC6H4CH3-p], 30.6 [(CH3)2CHC6H4CH3-p], 64.8 Results and discussion
[CH(C6H5)2], 81.7, 82.9, 97.9 and 102.3 [(CH3)2CHC6H4CH3-p],
Nitrogen-ligands play an important role in modern organometallic
chemistry and homogeneous catalysis. The properties and
reactivities of the many complexes reported in the literature
that contain such donors can be understood if one takes into
consideration the general rules of coordination chemistry.
Thus, in terms of their coordination behavior, it is fundamental
to recognize the most important differences between nitrogen-
and phosphorus-ligands: (i) the p back-bonding ability of
N-donors is insignificant, thus making them generally unsuited
for the stabilization of low oxidation states of the transition
metal center. (ii) The trans effect exerted by N-donors is
negligible when compared with that of most other ligands
encountered in organometallic chemistry, and thus their rates
of substitution reactions are usually high. (iii) The reactivity of,
for example, alkyl complexes of transition metals containing
N-donors are usually high.64
111.8, 120.9, 123.8, 124.1, 128.2, 129.0, 129.3, 133.9, 137.1 and
143.1 [NC6H4N and CH(C6H5)2], 144.5 [NCHN]. Elemental
analysis: calcd for C30H30N2RuCl2: C 61.02, H 5.12, N 4.74;
found C 60.55, H 4.71, N 4.85%.
Dichloro-[1-(diphenylmethyl)-5,6-dimethylbenzimidazole](p-cymene)
ruthenium(II), 2e
Yield: 92%, m.p.: 246 1C (decomp); FT-IR n(CN): 1494 cmÀ1
;
1H NMR (400 MHz, CDCl3): d = 1.12 [d, 6H, J = 6.8 Hz,
(CH3)2CHC6H4CH3-p], 2.14 [s, 3H, (CH3)2CHC6H4CH3-p], 2.25
and 2.38 [s, 6H, C6H2-5,6-(CH3)2], 2.70 [hept, 1H, J = 6.8 Hz,
(CH3)2CHC6H4CH3-p], 5.30 and 5.39 [d, 4H, J = 6.8 Hz,
(CH3)2CHC6H4CH3-p], 6.71 [s, 1H, CH(C6H5)2], 6.87 and 7.82
[s, 2H, NC6H2N], 7.13 and 7.35 [m, 10H, CH(C6H5)2], 8.09 [s, 1H,
NCHN]. 13C NMR (100 MHz, CDCl3): d = 18.5 [(CH3)2CHC6
H4CH3-p], 22.2 [(CH3)2CHC6H4CH3-p], 20.5 and 20.7 [C6H2-5,
6-(CH3)2], 30.5 [(CH3)2CHC6H4CH3-p], 64.6 [CH(C6H5)2], 81.3,
82.8, 97.8 and 102.3 [(CH3)2CHC6H4CH3-p], 111.6, 120.6, 128.1,
128.8, 129.2, 132.5, 132.9, 133.6, 137.4 and 141.7 [NC6H2N and
CH(C6H5)2], 144.0 [NCHN].
We are interested in the possibility of using benzimidazole
derivatives as ligands because they are structurally simple,
readily available, and inexpensive; they also allow for the
simplistic introduction of various substituents into their
structure. The N-substituted ligands (1) were prepared by the
reaction of benzimidazole and base with alkyl halides in a one-
step reaction (Scheme 1). The synthesis of ligands 1a, 1b and 1d
has been reported in the literature.58 All ligands were isolated
as solids which are colorless and air-stable. They were char-
Elemental analysis: calcd for C32H34N2RuCl2ÁH2O: C 60.37,
H 5.70, N 4.40; found C 60.80, H 5.70 N 4.48%.
General procedure for the N-alkylation of aniline with
alcohols
1
acterized by H NMR, 13C NMR, and IR, which confirmed the
proposed structures. The analytical data, which are in good
agreement with the compositions proposed for all the ligands,
are presented in Table 2.
Under the optimized reaction conditions, in a Schlenk tube,
a mixture of the aniline derivative (1.0 mmol), an excess
of the alcohol derivative (1.5 mmol), KOBut (1.0 mmol)
and the Ru catalyst (2a–e) (0.025 mmol, 2.5 mol%) was
stirred at 120 1C for 24 h under an argon atmosphere.
After cooling to room temperature, CH2Cl2 (2 ml) was added
to the crude mixture. The obtained mixture was passed
through a short pad of silica gel and the filtrate was
analyzed by GC.
Ruthenium(II) complexes bearing the benzimidazole ligand
were synthesized in good yields via the reaction of N-
substituted benzimidazole with [RuCl2(p-cymene)]2 in toluene
at 110 1C and under reflux for 5 h. Coordination was immediate,
as shown by a change of color from red to orange. All the
complexes were isolated as air-stable, non-hygroscopic solids;
they are soluble in dimethylformamide, dimethyl sulfoxide and
halogenated solvents such as chloroform and dichloromethane
but are insoluble in petroleum ether, diethyl ether, and n-hexane.
The complexes 2a–e were characterized by 1H NMR, 13C NMR, and
FT-IR spectroscopies. All of the complexes were studied using
X-ray crystallography. Synthesis of the N-coordinated benzimida-
X-ray analysis
X-ray diffraction data of the compounds were recorded using a
STOE IPDS II diffractometer at room temperature using
graphite-monochromated Mo Ka radiation by applying the
o-scan method. Data collection and cell refinement were zole ruthenium(II) complexes (2a–e) is summarized in Scheme 1.
carried out using X-AREA61 while data reduction was applied
The NMR spectra of all the complexes were analyzed in
using X-RED32.61 The structures were solved by a dual-space d-CDCl3. The 1H and 13C NMR spectra of 1a–1e showed that the
algorithm using SHELXT-201862 and refined by means of the NCHN resonances occurred between 7.58–8.22 ppm and 142.3–
full-matrix least-squares calculations on F2 using SHELXL- 143.8 ppm, respectively. The values of complexes 2a–2e also
2018.62 All the H atoms were inserted in idealized positions showed that the NCHN resonances occurred between 8.09–
and treated using a riding model, fixing the bond lengths at 8.44 ppm and 144.0–145.8 ppm, respectively. The 1H NMR
0.98, 0.93, 0.97 and 0.96 Å for methine CH, aromatic CH, CH2 values showed that the C2–H signals shifted to low field in
and CH3 atoms, respectively. The displacement parameters of the ruthenium complexes as compared with the corresponding
the H atoms were fixed at Uiso(H) = 1.2Ueq (1.5Ueq for CH3). The ligand due to an electron-withdrawing effect of the metal
crystallographic data and refinement parameters are summar- center. The presence of the –C–N– group in the complexes
ized in Table 1. Molecular graphics were created by using was confirmed by the (CQN) vibrations between 1494 and
OLEX2.63
1513 cmÀ1. These NMR and FT-IR values are in agreement with
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Scheme 1 Synthesis of the N-coordinated benzimidazole ligands and
ruthenium(II) complexes.
Fig. 2 View of 2a showing the atom-numbering scheme. Displacement
ellipsoids are drawn at the 20% probability level and H atoms have been
omitted for clarity.
Table
2
Selected analytical data for the ligand (1) and ruthenium
complexes (2)
1H NMR 13C NMR
from 1.6613(11) Å to 1.676(2) Å, while the Cl1–Ru1–Cg, Cl2–
Ru1–Cg and N1–Ru1–Cg angles are found in the range of
125.64(5)–126.43(6)1, 127.21(5)–128.45(9)1 and 128.70(9)–
130.52(7)1, respectively. The Cl1–Ru1–Cl2, Cl1–Ru1–N1 and
Cl2–Ru1–N1 angles are smaller than the ideal tetrahedral angle
(109.471), which is compensated for by extension of the Cg–Ru–L
(L is Cl1, Cl2 or N1) angles. The ruthenium atoms are p-bonded
to the benzene rings with a mean Ru–C bond distance of 2.19 Å,
whilst the Ru–Cl and Ru1–N bond distances change from
2.4112(7) to 2.4335(7) Å and from 2.129(2) to 2.141(2) Å,
respectively. These lengths are in line with Ru(II)–arene
complexes with the same coordination sphere.65,66
Yield
Compound (%) M.p. (1C)
n(CQN) NCHN
NCHN
(ppm)
cmÀ1
(ppm)
1a
1b
1c
1d
1e
2a
2b
2c
2d
2e
87
85
75
73
90
74
87
95
80
92
99–100
88–89
1494
1488
1468
1448
1449
1513
1512
1513
1501
7.88
7.88
7.84
7.60
7.58
8.44
8.32
8.32
8.21
8.09
142.6
143.8
143.0
142.3
142.7
144.2
145.8
144.5
144.5
144.0
103–104
155–156
248–249
246–247
235–236
237–238
227–228
246 (decomposition) 1494
Secondary amines can be synthesized by many different
reported data for N-coordinated benzimidazole ruthenium(II) methods. These methods generally require environmentally
complexes in the literature.52,53 See the ESI for 1H, 13C NMR harmful organic solvents, alkyl halides and/or stoichiometric
and FT-IR spectra. Also, the analytical data are in good agreement quantities of reducing agents. In recent decades, much attention
with the general molecular formulae proposed for all the has been focused on the use of inexpensive and low toxicity
complexes (Table 2).
alcohols as the alkylation reagent.67 N-Alkylation of amines with
The molecular structures of 2a–e with the adopted atom- alcohols presents a green method for the synthesis of substituted
labeling scheme are shown in Fig. 2–5, while important bond amines that have substantial importance in synthetic applications.
distances and angles are listed in Table 3. The complexes are The advantage of the procedure in comparison with N-alkylation
composed of a benzimidazole ligand with a RuII metal center, using alkyl halides is the desirable high selectivity.
one p-cymene ligand and two Cl ligands. The molecular
Here, we used the optimized conditions for the catalytic
variation in the complexes originates from the monodentate reaction, which were determined in our previous works.68 In a
benzimidazole ligand. The crystallization characteristics of the standard experiment, KOtBu (1 mmol), aromatic amine (1 mmol),
complexes are divided into two groups, with 2a–c crystallizing alcohol derivative (1.5 mmol), and the Ru-benzimidazole complex
%
in the trigonal space group R3 and 2d and e crystallizing in the 2a–2e (0.025 mmol) were added to a Schlenk tube under an inert
monoclinic space group P21/c.
atmosphere. The sealed Schlenk tube was stirred at 120 1C for
The ‘‘three-legged piano-stool’’ coordination geometry of the 24 h. The reaction mixture was cooled to room temperature at the
Ru atom is adopted in all five complexes. The Ru(II) centers end of the reaction before CH2Cl2 (2 mL) was added, before
have a pseudo-octahedral coordination geometry in which filtering through a short SiO2 pad. The filtrate was analyzed by
p-cymene occupies three facial coordination sites. However, GC. The yields were based on the corresponding aniline. The
the geometry around the metal atoms may be regarded as a reactions were performed at a molar ratio of 1:0.025:1 aniline/
tetrahedron with considerable trigonal distortion, considering precatalyst/base (S/C/base). It is worth noting that when an excess
the linkage to the hydrocarbon as a single bond. Defining Cg as of benzyl alcohol (5 mmol) is used, only the secondary amine is
the centroid of the aromatic ring, the Ru–Cg distances vary formed in the catalytic reaction.
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Table 3 Selected distances (Å) and angles (1) for 2a–e
Parameters
2a
2b
2c
2d
2.4112(7)
2.4335(7)
2.130(2)
2e
Bond distances
Ru1–Cl1
Ru1–Cl2
2.4228(12)
2.4297(12)
2.137(3)
2.4189(8)
2.4199(9)
2.136(2)
2.4213(7)
2.4293(8)
2.129(2)
2.4287(7)
2.4203(7)
2.141(2)
Ru1–N1
Ru1–Cg
1.676(2)
1.6725(17)
1.6759(15)
1.6613(11)
1.6673(11)
Bond angles
Cl1–Ru1–Cl2
Cl1–Ru1–N1
Cl2–Ru1–N1
Cl1–Ru1–Cg
Cl2–Ru1–Cg
N1–Ru1–Cg
88.02(5)
86.63(10)
84.59(10)
125.76(8)
128.45(9)
129.10(14)
87.52(3)
85.88(7)
84.73(7)
126.43(6)
128.07(6)
129.56(11)
88.70(3)
86.29(6)
84.72(6)
125.88(5)
128.40(6)
128.70(9)
88.76(3)
85.81(6)
85.18(6)
126.29(5)
127.46(4)
129.20(7)
88.30(3)
86.31(6)
84.59(6)
125.64(5)
127.21(5)
130.52(7)
Note: Cg is the centroid of the p-cymene ring.
Fig. 4 View of 2c showing the atom-numbering scheme. Displacement
ellipsoids are drawn at the 20% probability level and H atoms have been
omitted for clarity.
Fig. 3 View of 2b showing the atom-numbering scheme. Displacement
ellipsoids are drawn at the 20% probability level and H atoms have been
omitted for clarity.
The N-alkylation of aniline, 2,4-dimethylaniline and 3-(tri-
fluoromethyl)aniline, with the alcohols benzyl alcohol,
4-methybenzyl alcohol, 4-methoxybenzyl alcohol and furfuryl
alcohol was investigated using 2a–e precatalysts to obtain the
secondary amines under the reaction conditions. Table 4
summarizes the N-alkylation of these arylamines with
arylmethyl alcohols. When the reaction of aniline with benzyl
alcohol was performed using the complexes 2a–2e, N-
benzylaniline was obtained as the major product (Table 4, entry 1).
The selectivity was in a high ratio toward to the amine. For example,
when complex 2a was used as the precatalyst, 80% conversion with
an amine/imine ratio of 100/0 was observed. Alkylation reactions
with arylmethyl alcohols, bearing electron-donating methyl and
methoxy groups resulted in a high conversion where mainly the
Fig. 5 View of 2d and 2e showing the atom-numbering scheme.
Displacement ellipsoids are drawn at the 20% probability level and
H atoms have been omitted for clarity.
amine was obtained (Table 4, entries 2 and 3) and complex 2a was used as the alkylating agent, N-(4-methylbenzyl)aniline could be
the most efficient precatalyst. When 4-methylbenzyl alcohol was obtained in high conversion (Table 4, entry 2). When complex 2a
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Table 4 N-Alkylation of arylamine derivatives with different arylmethyl alcoholsa
[Ru] Conversionb (yield A : B%)
2a 2b
Entry
Ar–NH2
Alcohol
2c
2d
56(97/3)
2e
1
2
3
80(100/0)
66(99/1)
92(92/8)
58(98/1)
93(92/8)
55(97/3)
86(95/5)
91(83/17)
98(92/8)
85(91/9)
87(91/9)
73(87/13)
52(100/0)
72(88/12)
47(100/0)
68(88/12)
4
70(100/0)
42(100/0)
49(100/0)
5
6
7
79(80/20)
92(85/15)
33(4/96)
91(66/34)
73(66/34)
62(1/99)
75(56/44)
54(77/23)
40(3/97)
73(63/37)
69(82/18)
39(3/97)
61(64/36)
76(33/67)
27(7/93)
8
12(95/5)
22(98/2)
14(97/3)
19(100/0)
18(99/1)
9
98(100/0)
97(68/32)
99(99/1)
91(100/0)
88(1/99)
90(96/4)
93(100/0)
93(44/56)
89(94/6)
98(100/0)
92(86/14)
92(89/11)
97(100/0)
92(98/2)
94(92/8)
10
11
12
84(100/0)
82(100/0)
89(100/0)
88(100/0)
77(100/0)
13
14
15
100(100/0)
100(100/0)
100(97/3)
100(97/3)
100(100/0)
100(97/3)
100(100/0)
100(94/6)
100(94/6)
100(100/0)
100(96/4)
100(97/3)
100(100/0)
100(100/0)
100(94/6)
16
100(99/1)
100(96/4)
100(100/0)
100(97/3)
100(98/2)
a
Reaction conditions: complexes 2a–e (0.025 mmol, 2.5 mol%), arylmethyl alcohol (1.5 mmol), aromatic amine (1 mmol), KOtBu (1 mmol), 120 1C,
b
24 h. The conversions and the selectivities were determined by GC and dodecane was used as an internal standard. Refers to arylamine’s
conversion to the secondary amine and imine.
was used as the precatalyst, mono-alkylated N-alkylaniline was amine/imine ratio of 66/34 was observed (Table 4, entry 5).
formed selectively. All of the reactions using 4-methoxybenzyl alcohol 2,4-Dimethylaniline also reacted with 4-methylbenzyl alcohol to
resulted in the selective formation of N-(4-methoxybenzyl)aniline in give N-(4-methylbenzyl)-2,4-dimethylaniline in 54–92% yield
high yields (selectivity of amine, 83–91%) (Table 4, entry 3).
(Table 4, entry 6). Also, 2,4-dimethylaniline reacted with 4-
Examination of the table shows that conversions are relatively methoxybenzyl alcohol to give the imine derivatives in 27–62%
low in the case of sterically hindered 2,4-dimethylaniline yield (selectivity of the imine, 93–99%) (Table 4, entry 7). The
(Table 4, entries 5–9). When 2,4-dimethylaniline was used as results obtained show that steric hindrance at the ortho-position
the substrate, the reaction became more difficult and N-(benzyl)- of the aniline substrate partially inhibits the reaction.
2,4-dimethylaniline yields of only 61–79% could be obtained
The amine with an electron-withdrawing group like –CF3
with benzyl alcohol as the respective partner after 24 h (Table 4, was almost converted to an N-alkylated product (up to 100%)
entry 5). We obtained the corresponding N-benzyl-2,4- with arylmethyl alcohols using 2a–2e as the precatalyst (Table 4,
dimethylaniline with 56–80% selectivity. In this reaction when entries 9–12). The electron-donating substituents like Me and
complex 2b was used as the precatalyst, 91% conversion with an OMe on both benzyl alcohol and aniline significantly improved
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furfuryl alcohol and 3,4-dimethoxybenzyl alcohol also gave
excellent yields of the N-alkylated 2-aminopyridines.
The activity of ruthenium complexes may be attributed to the
presence of electron donating-oxygen in the alkyl group bound to
the nitrogen. Therefore, the presence of benzimidazole ligands
bearing a different donating group such as ether side chains on
the metal may increase the performance of the catalyst. The
chelating nature of these ligands promotes the production of
highly stable complexes. The hemilabile part of such ligands is
capable of reversible dissociation to produce vacant coordination
sites, allowing complexation of substrates during the
catalytic cycle.
A possible catalytic cycle is proposed in Scheme 2 on the
basis of the results obtained and available literature on similar
ruthenium catalyzed transformations.69–71 This catalysis is
considered to proceed via the (benzyloxy)ruthenium intermediate
A, which undergoes b-hydrogen elimination to give the ruthenium
hydride B. Dehydrative condensation of the aldehyde with an
amine forms the imine (R1CHQNR2). Insertion of the imine into
the Ru–H bond of B, followed by alcoholysis of the resulting
(amido)ruthenium species C, affords the N-alkylation product and
reproduces the (benzyloxy)ruthenium intermediate A and com-
pletes the catalytic cycle.
Scheme 2 Proposed mechanism for Ru(II)–azole-catalyzed N-alkylation.
Conclusion
the selectivity of the mono-alkylated amine products under the
same conditions. All of the reactions using benzyl alcohol
resulted in the selective formation of mono-alkylated aniline
in high conversions (Table 4, entry 9). When the reaction
was carried out in the presence of 2a as the precatalyst,
a high conversion was obtained with the mono-alkylated
N-alkylaniline formed selectively. The results are similar to
those of other aniline derivatives.
In summary, we prepared a library of five new well-defined half-
sandwich ruthenium(p-cymene) complexes. These complexes
were characterized using different spectroscopic and analytical
techniques. The structure of complexes 2a–2e was determined
using X-ray crystallography. The catalytic activities of all new
complexes were investigated on the alkylation reaction of
amines with alcohol derivatives. Ruthenium complexes were
used as efficient and selective catalysts in the synthesis of
N-alkylaromatic amines using the N-alkylation reaction between
the aromatic amines and alcohol derivatives under solvent-free
conditions. When the catalytic results were evaluated, all the
ruthenium complexes were found to be sufficient for the N-alky-
lation reaction of aromatic amines with alcohols via the
hydrogen borrowing strategy. Since the ruthenium complexes
are structurally similar, it is clear that there is no significant
difference between the complexes in terms of their catalytic
activity for the N-alkylation of aromatic amines with alcohols,
and the selectivity of the reaction depends strongly on the nature
of the amine and the alcohol.
When the furfuryl alcohol was used as an alkylating agent,
aniline and 4-trifluoromethylaniline gave the corresponding
mono-alkylated amine with 100% selectivity for all complexes
2a–2e (Table 4, entries 4 and 12). In all of the reactions the N-(2-
furfurylmethyl)aniline derivative was obtained as the major
product, and a very low imine formation ratio (selectivity of
imine o5%) was observed (Table 4, entries 4, 8 and 12).
In the presence of our optimized catalytic system based on
the catalysts 2a–2e, the primary heteroaromatic 2-aminopyridine
reacted readily with various alcohols to selectively give the
N-mono-alkylated secondary amines (Table 4). The N-alkylation
of 2-aminopyridine with the same alcohols (benzyl alcohol,
4-methybenzyl alcohol, 4-methoxybenzyl alcohol, furfuryl alcohol
and 3,4-dimethoxybenzyl alcohol) was also investigated using 2a–e
precatalysts to obtain N-alkylated amines under the same
conditions. 2-(N-Alkylamino)pyridines were obtained in good to
excellent selectivity in the presence of 2.5 mol% precatalysts. Also,
under these catalytic conditions, the heteroaromatic moiety in
2-aminopyridine was well tolerated. 2-Aminopyridine was
efficiently alkylated with benzyl alcohol for all complexes 2a–2e
Conflicts of interest
There are no conflicts to declare for this article.
Acknowledgements
with 100% selectivity (Table 4, entry 13). The reaction of This study was supported by Scientific Research Projects Unit of
2-aminopyridine with benzyl alcohol, 4-methoxybenzyl alcohol, Ondokuz Mayıs University (Project No. PYO.FEN.1906.19.001).
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