Half-sandwich ruthenium-carbene catalysts: Synthesis, characterization, and catalytic application in the N-alkylation of amines with alcohols

Article abstract of DOI:10.1016/j.ica.2019.119163

In this study, the synthesis and characterization of new half-sandwich ruthenium complexes containing oxygen functionalised N-aryl and N-alkyl benzimidazol-2-ylidene ligands have been reported. All ruthenium complexes were tested as catalysts for a wide range of substrates in the N-alkylation of secondary cyclic amines such as pyrrolidine and piperidine, and 4-methylaniline which was a primary aromatic amine with alcohols by hydrogen-borrowing process. The catalytic reactions were performed with 1 mol% catalyst loading at 120 °C, 16 h under solvent-free conditions. All ruthenium complexes showed excellent catalytic activity, and N-alkylated products were obtained selectively.

Full text of DOI:10.1016/j.ica.2019.119163

InorganicaChimicaActa498(2019)119163
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Research paper
Half-sandwich ruthenium-carbene catalysts: Synthesis, characterization, and
catalytic application in the N-alkylation of amines with alcohols
⁎
Murat Kaloğlua,b,
a İnönü University, Faculty of Science and Arts, Department of Chemistry, 44280 Malatya, Turkey
b İnönü University, Catalysis Research and Application Center, 44280 Malatya, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this study, the synthesis and characterization of new half-sandwich ruthenium complexes containing oxygen
functionalised N-aryl and N-alkyl benzimidazol-2-ylidene ligands have been reported. All ruthenium complexes
were tested as catalysts for a wide range of substrates in the N-alkylation of secondary cyclic amines such as
pyrrolidine and piperidine, and 4-methylaniline which was a primary aromatic amine with alcohols by hy-
drogen-borrowing process. The catalytic reactions were performed with 1 mol% catalyst loading at 120 °C, 16 h
under solvent-free conditions. All ruthenium complexes showed excellent catalytic activity, and N-alkylated
products were obtained selectively.
N-Heterocyclic carbene
Ruthenium
N-Alkylation of amines
Hydrogen-borrowing process
1. Introduction
process than the use of conventional alkylating agents.
The first examples of homogeneous metal-catalyzed N-alkylation of
The synthesis of higher-order amines such as secondary and tertiary
amines which are more commercially valuable by N-alkylation of pri-
mary amines is one of the most fundamental and important reactions in
organic synthesis [1,2]. The most preferred method for N-alkylation of
amines is the coupling of amines with alkyl halides in the presence of a
stoichiometric amount of base. However, this method can be highly
problematic because of the toxic nature of many alkylating agents and
the formation of undesirable salts as by-products. Therefore, the de-
velopment of improved synthetic methodologies for the preparation of
the alkylated amines continue to be of significant interest to organic
chemists. N-Alkylation of amines with readily available and cheap al-
cohols by via hydrogen auto-transfer, also known as hydrogen-bor-
rowing process is an alternative approach that minimizes the genera-
tion of wasteful products [3,4]. This process involves the conversion of
alcohols to aldehydes or ketones as temporary intermediates via an
oxidative hydrogen elimination. The in situ generated carbonyl groups
are highly reactive to nucleophiles such as amines. After reduction of
the resulting imine, the expected N-alkylated amines are obtained and
only water is generated as by-product. Nowadays, this approach has
significantly been using as an effective method for making NeC(sp3)
bonds, which are frequently found in biologically active compounds
and functional materials [5]. This approach environmentally more at-
tractive because of it is more atom economical and less hazardous
aliphatic and aromatic amines were reported independently by Grigg
[6] and Watanabe [7] using rhodium- and ruthenium-phosphine cata-
lysts in the early 1980s. Murahashi [8] also reported an alternative
ruthenium-phosphine catalyst for alkylation of aliphatic amines. Then,
great contributions have been made for this approach by Zhao [9],
Williams [10], Beller [11], Milstein [12] and Fujita [13]. In most of the
these reports, alkylation of amines with alcohols have been generally
catalyzed by ruthenium and iridium complexes [14], and these com-
plexes are shown to be highly effective for this reaction. Recently,
important progress in hydrogen-borrowing processes were also
achieved based on other noble metals such as gold [15], silver [16],
rhodium [17] and palladium [18] as well as non-noble metals such as
iron [19], copper [20], nickel [21] and etc. [22].
N-Heterocyclic carbenes (NHCs) represent a prominent class of li-
gands, widely used in organometallic and inorganic chemistry [23].
The strong σ-donating but poor π-accepting ability of NHC ligands lead
to the formation of many stable metal-NHC complexes. Due to activity
and stability of metal-NHC complexes, they have been widely used in
organometallic chemistry and catalysis as an ideal alternative to well-
known phosphine ligands for the synthesis of homogeneous catalysts.
Many types of N-alkyl, N-aryl and N-benzyl (benz)imidazol-2-ylidene
ligands have been used in transition-metal catalysis [24] and medical
applications [25], and many research groups have provided a large
⁎ Address: İnönü University, Faculty of Science and Arts, Department of Chemistry, 44280 Malatya, Turkey.
E-mail address: murat.kaloglu@inonu.edu.tr, https://www.inonu.edu.tr/.
URL:
https://doi.org/10.1016/j.ica.2019.119163
Received 23 August 2019; Received in revised form 19 September 2019; Accepted 19 September 2019
Availableonline20September2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

M. Kaloğlu
InorganicaChimicaActa498(2019)119163
were synthesized by the reaction of N-(alkyl)benzimidazole with dif-
ferent benzyl halides in chloroform at 60 °C for 24 h. 1a [33], 1c [25a]
and 1d [25b] NHC ligand precursors have been prepared according to
the literature. The new 1b and 1e ligand precursors were confirmed by
1H NMR, 13C NMR, FT-IR spectroscopies and elemental analysis tech-
niques. As shown in Table 1, the FT-IR data clearly indicated that the
new 1b and 1e benzimidazolium salts exhibit a characteristic ν(CN) band
typically at 1561 and 1557 cm−1, respectively. In the 1H NMR spectra,
C(2)-H proton down-field resonance of 1b and 1e were observed as
sharp singlets at δ = 11.72 and 11.18 ppm, respectively. In the 13C
NMR spectra, C(2)-carbon resonance of 1b and 1e salts were appeared
at δ = 143.9 and 143.2 ppm, respectively as single signals. These
spectroscopic values are in line with those found for other benzimida-
zolium salts of the literature [24c,25c,33].
[PF6]
R
R2
R1
R1
N
Ru
N
N
N
N
Ir
I
I
Me
Peris's iridium(Cp*)-NHC complex Valerga's ruthenium-(picolyl-NHC) complex
Mes
N
Ru
Cl
N
O
S
O
O
2.2. Preparation of ruthenium-carbene complexes
Bruneau's ruthenium-(sulfonate-NHC) complex
The new 2a-2e ruthenium-NHC complexes were prepared by
transmetallation of the corresponding silver-carbene adducts and
[RuCl2(p-cymene)]2 dimer. The silver-carbene complexes, which should
subsequently serve as a carbene-transfer agent, were synthesized by the
reaction of Ag2O with benzimidazolium salts (1a-1e) under exclusion of
light. The non-isolated silver-carbene complexes were converted into
the half-sandwich ruthenium-carbene complexes (2a-2e) in moderate
to good yields after purification. Ruthenium-carbene complexes are
very stable against air and moisture in the solid state. They are soluble
in most organic solvents, such as dichloromethane, chloroform, me-
thanol and acetonitrile, with the exception of non-polar ones, such as
pentane, hexane and diethyl ether. Formation of half-sandwich ruthe-
nium-carbene complexes is supported by NMR, FT-IR spectroscopies
and microanalysis techniques. In the 1H NMR spectra, the absence of
C(2)-H down-field signals at δ = 10–12 ppm characteristic for the
benzimidazolium salts indicates the formation of the expected ruthe-
nium-carbene complexes. Also, there are two characteristic AB doublet
signals at range of δ = 5–6 ppm corresponding to the methylene bridge
protons of benzyl substituent, which become diastereotopic after co-
ordination of the NHC ligands to the Ru atom. 13C NMR chemical shifts
provide a useful diagnostic tool for this type of metal-carbene complex.
In the 13C NMR spectra, the carbene signals of the ruthenium-carbene
complexes were observed at δ = 190.1, 190.0, 191.6, 191.8 and
187.9 ppm, respectively. The FT-IR data clearly indicated that, ruthe-
nium-carbene complexes exhibit a characteristic v(CN) stretching fre-
quency peaks typically between 1454 and 1483 cm−1. Due to the flow
of electrons from the NHC ligand to the ruthenium, the CeN bond is
weakened, and as a result, a decreasing in the v(CN) stretching frequency
is expected. Also, the microanalysis datas of the ruthenium complexes
agrees closely with the theoretical requirements of their structures.
Half-sandwich ruthenium complexes (2a-2e) show typical spectro-
scopic signatures, which are in line with those recently reported for
other similar type ruthenium-carbene complexes [24g,30,34].
Fig. 1. Well-known iridium- and ruthenium-catalysts for the alkylation of
amines.
number of NHC-based complexes. In recent years, ruthenium- and ir-
idium-NHC complexes have been reported to be active catalysts for the
alkylation of amines with alcohols. In particular, ruthenium(arene)-
and iridium(Cp*)-NHC complexes (Cp* = pentamethylcyclopenta-
dienyl) were used in these reactions. As an early example, iridium(Cp*)-
type NHC complexes have been reported for the N-alkylation of aniline
by Peris et al. [26]. Later, Crabtree and co-workers have described the
use of NHC complexes of iridium and ruthenium containing chelating
pyrimidine ligand as effective catalysts for the alkylation of amines and
secondary alcohols [27]. Valerga and co-workers reported the N-alky-
lation of both aromatic and non-aromatic amines catalysed by ruthe-
nium-picolyl-NHC complexes [28]. Bruneau and co-workers have
shown that the half-sandwich ruthenium complexes bearing a chelating
sulfonate-NHC ligand were efficient catalysts for the N-alkylation of
aniline and cyclic secondary amines with benzylic and aliphatic alco-
hols, and the regioselective C3-alkylation of N-protected cyclic sec-
ondary amines with aldehydes [29], (Fig. 1). More recently, ruthenium
(arene) complexes (arene = η6-p-cymene) containing NHC ligand have
been used as catalyst to obtain tertiary amines from cyclic secondary
amines [30].
Up to now, many examples of transiton-metal-catalyzed synthesis of
higher amines have been reported [10b,10d,29,31]. In these prominent
reports, starting from primary amines to higher-order amines such as
secondary and tertiary amines, cyclic tertiary amines by means of diols,
and N-heterocyclic compounds by employing functionalized amine
substrates were obtained by transition-metal catalysis. In a similar
manner, secondary amines and ammonia were applied for the same
purpose. However, there are still very few reports in the literature on
the ruthenium-carbene catalyzed N-alkylation of cyclic secondary
amines based on hydrogen-borrowing protocols [10b,13c,32]. There-
fore, the synthesis of five new half sandwich ruthenium-carbene com-
plexes of the general formula [RuCl2(η6-p-cymene)(NHC)] (NHC = 1,3-
dialkylbenzimidazol-2-ylidene), 2a-2e were performed in this study. All
ruthenium complexes were characterized by 1H NMR, 13C NMR, FT-IR
spectroscopy and elemental analysis techniques. The ruthenium com-
plexes were tested as catalysts in the N-alkylation of cyclic secondary
amines such as pyrrolidine and piperidine, and 4-methylaniline which
was a primary aromatic amine with aliphatic and benzylic alcohols by
hydrogen-borrowing process (Fig. 2).
All compounds were prepared according to general reaction
pathway depicted in Scheme 1. The some physical and spectroscopic
datas of all new compounds were summarized in the Table 1.
2.3. Determination of the optimum conditions for the N-alkylation of
amines
In order to determine the optimum conditions in the reaction, al-
kylation of pyrrolidine with benzyl alcohol was investigated as a model
reaction (Table 2). It is known in the literature that a stoichiometric
amount of base is needed for N-alkylation of primary aromatic amines
[31l,34]. But, due to the high basicity (pKa ∼ 11) and high nucleophilic
properties of secondary aliphatic amines compared to primary aromatic
amines, the N-alkylation of such amines does not require the use of a
stoichiometric amount of base [35].
2. Results and discussion
2.1. Preparation of benzimidazolium salts
The new 1b and 1e benzimidazolium salts as NHC ligand precursors
Initially, after a series of trials to determine the most active catalyst
2

M. Kaloğlu
InorganicaChimicaActa498(2019)119163
Fig. 2. N-Alkylation of primary and secondary amines with alcohols catalyzed by half-sandwich ruthenium complexes.
Table 1
Physical and spectroscopic properties of all new compounds.
Compound
Formula
Isolated yield[%]
M.p. [°C]
FT-IR v(CN)[cm−1
]
C(2)-H 1H NMR[ppm]
C(2) 13C NMR[ppm]
1b
1e
2a
2b
2c
2d
2e
C24H25ClN2O2
75
80
79
54
74
66
82
121–122
213–214
156–157
127–128
138–139
124–125
190–191
1561
1557
1471
1483
1468
1454
1465
11.72
143.9
143.2
190.1
190.0
191.6
191.8
187.9
C
29H34ClN2O
11.18
C28H33Cl3N2ORu
–
–
–
–
–
C
34H38Cl2N2O2Ru
35H40Cl2N2O3Ru
C
C37H44Cl2N2O5Ru
C39H48Cl2N2ORu
among the 2a-2e catalysts (Table 2, entries 1–5), it was determined that
the most active catalyst was the 2d catalyst. Therefore, all other entries
were performed in the presence of the 2d catalyst. Later, lower reaction
times were tested to investigate the effect of reaction time on conver-
sion. When the reaction was carried out at 16 h, 94% conversion was
observed (Table 2, entry 6), but when the reaction time was reduced to
8 h, the conversion was significantly reduced (Table 2, entry 7). No
significant difference was observed on the conversion between the 24 h
or the 16 h. Therefore, it was decided to carry out the reaction at 16 h as
the optimum time. When the reaction was carried out in presence of
1 mol% catalyst loading under solvent-free conditions at 16 h (Table 2,
entry 8), similar results were obtained with the reaction using toluene
as the solvent. Therefore, the further reactions were carried out under
solvent-free conditions. When the catalyst loading was reduced from
1 mol% to 0.5 mol%, the conversion dropped to 36%, but no noticeable
effect on the selectivity of 3a/3′a was observed (Table 2, entry 9).
In the reaction of pyrrolidine with benzyl alcohol, N- and C(3)-di-
benzylated pyrrolidine (3′a) can be observed as a side-product. In 2010,
Bruneau and co-workers obtained the N- and C(3)-dibenzylated pro-
ducts as the main product using D-(+)-camphorsulfonic acid (CSA) as
an additive [36]. In this context, CSA was also used as an additive to
determine the effect on N- and C(3)-dibenzylated product formation.
Scheme 1. Synthesis of NHC ligand precursors (1a-1e) and their half-sandwich ruthenium-carbene complexes (2a-2e).
3

M. Kaloğlu
InorganicaChimicaActa498(2019)119163
Table 2
Influence of the reaction conditions for ruthenium-carbene catalyzed alkylation of pyrrolidine.a
Entry
[Ru]
Additive
[mol-%]
Solvent
Time [h]
Conversionb [%]
Ratio of
3a/3′ac
1
2a
2b
2c
2d
2e
2d
2d
2d
2d
2d
2d
2d
None
Toluene
24
24
24
24
24
16
8
83
78
90
98
87
94
53
94
36
88
90
55
88/12
90/10
86/14
100/0
90/10
100/0
100/0
100/0
100/0
68/32
54/46
66/34
2
None
Toluene
3
None
Toluene
4
None
Toluene
5
None
Toluene
6
None
Toluene
7
None
Toluene
8
none
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
16
16
16
16
16
9d
10
11
12
None
CSA (20)
CSA (40)
CSA (80)
a
b
c
Reaction conditions: [Ru] (0.01 mmol, 1 mol%), pyrrolidine (1.0 mmol), alcohol (1.5 mmol), no base, 120 °C.
Conversions (%) were calculated according to pyrrolidine by GC analysis using dodecane as an internal standard.
Ratio of N-alkylated and N,C(3)-dialkylated products were calculated by GC analysis.
0.005 mmol, (0.5 mol%) 2d catalyst was used.
d
When 20 mol% CSA was used, 88% conversion was obtained in the
presence of 2d catalyst. When heteroaromatic 2-furfuryl alcohol was
used, only N-(furan-2-ylmethyl) pyrrolidine (3c) was obtained as pro-
duct with 2b and 2e catalysts (Table 3, entries 12,15), while N- and
C(3)-dialkylated product was observed with 22% selectivity in the
presence of 2d catalyst (Table 3, entry 14). Then the effect of aliphatic
primary and secondary alcohols was examined. In general, the reaction
was carried out at higher conversions with primary alcohols, while
slightly lower conversions were obtained with secondary alcohols. For
example, it is interesting to note that the less reactive secondary alcohol
1-phenylethanol reacted smoothly to give the N-(1-phenylethyl) pyr-
rolidine in 46–74% yield (Table 3, entries 16–20). The primary and
secondary aliphatic alcohols proved to be less reactive but the con-
densation was also selective leading to the N-alkylated products. When
1-hexyl alcohol was used, N-(1-hexyl) pyrrolidine (3e) was obtained in
51–74% yields (Table 3, entries 21–25), while 2-hexyl alcohol was
used, N-(2-hexyl) pyrrolidine (3f) was obtained in 47–68% yields
(Table 3, entries 26–30). Similar results were observed when 2-octyl
alcohol was used and moderate to high yields were obtained (Table 3,
entries 31–35). In all cases except entry 32, N-(2-octyl) pyrrolidine (3g)
was obtained with full selectivity. When Citronellol, which a natural
acyclic monoterpenoid compound, was used, high yields were observed
in all cases (Table 3, entries 36–40). The product N-(3,7-dimethyloct-6-
en-1-yl) pyrrolidine (3h) was obtained in the presence of 2d catalyst in
74% isolated yield.
reaction, and the N- and C(3)-dibenzylated product ratio increased to
32% (Table 2, entry 10). When the amount of CSA was doubled under
similar conditions, it was observed that the ratio of N- and C(3)-di-
benzylated product increased to 46% (Table 2, entry 11). Interestingly,
when the amount of CSA was increased to 80 mol%, both the conver-
sion and the dibenzylated product ratio decreased (Table 2, entry 12).
In contrast to Bruneau's study, the use of CSA in this study did not
significantly increase the formation of N- and C(3)-dibenzylated pro-
ducts. After these preliminary trials, it was concluded that the best
conditions for N-alkylation were achieved without the use of additives
under solvent-free conditions at 120 °C, 16 h.
2.4. N-Alkylation of amines with alcohols
N-Alkylation of cyclic secondary amines such as pyrrolidine and
piperidine, and primary aromatic amine such as 4-methylaniline with
aliphatic and benzylic alcohols was evaluated using the determined
optimum conditions (Table 3–5). In all cases, only N-monoalkylated
amines were obtained as the main products. Since CSA was not used as
an additive, N- and C(3)-dialkylated products were not observed in
some cases. Conversions, selectivities and yields were determined by
GC analysis. The main products of the reaction were purified by column
chromatography, and they were characterized by NMR spectroscopy.
Initially, the alkylation of pyrrolidine with aliphatic and benzylic al-
cohol derivatives was carried out in the presence of 2a-2e catalysts
under the determined optimum conditions (Table 3). In the studies
conducted, it was observed that the reaction occurred with high con-
version. In all cases, N-alkylated pyrrolidine derivatives as the main
product were obtained with high yields. All the results of the alkylation
of pyrrolidine are summarized in Table 3.
Next, the scope of the reaction was extended to the six-membered
piperidine substrate. Piperidine was N-alkylated by the benzylic alco-
hols and aliphatic primary and secondary alcohols with high conversion
in similar conditions (Table 4).
High conversions were obtained when the benzyl alcohol was used
with piperidine under solvent-free conditions (Table 4, entries 1–5). N-
benzyl piperidine (4a) was obtained with 80% isolated yield in the
presence of 2d catalyst. Surprisingly, the use of benzyl alcohol led to a
small amount of N- and C(3)-dialkylated product (about 27% ratio)
with piperidine in presence of 2d catalyst (Table 4, entry 4). Similar
results were obtained when 4-isopropylbenzyl alcohol was used. In this
case N-(4-isopropylbenzyl) piperidine (4b) was obtained in 61–80% GC
yield (Table 4, entries 6–10). The 2-octyl alcohol, which an aliphatic
The reaction of pyrrolidine with benzyl alcohol was carried out in
71–94% conversions (Table 3, entries 1–5). Only N-benzyl pyrrolidine
(3a) was obtained as product in the presence of 2d catalyst in 85% GC
yield (Table 3, entry 4). Similar results were obtained when 4-iso-
propylbenzyl alcohol was used (Table 3, entries 6–10). N-(4-iso-
propylbenzyl) pyrrolidine (3b) was obtained in 75% yield in the
4

M. Kaloğlu
InorganicaChimicaActa498(2019)119163
Table 3
Ruthenium-carbene catalyzed alkylation of pyrrolidine with alcohols.a
Entry
Alcohol
[Ru]
Main product
Conversionb [%]
Ratio of 3/3′c
Yield of 3d [%]
1
2
3
4
5
2a
2b
2c
2d
2e
78
71
85
94
83
90/10
93/7
60
56
89/11
100/0
94/6
72
85 (80)
74
6
2a
2b
2c
2d
2e
69
72
81
86
79
98/2
55
7
88/12
100/0
85/15
96/4
60
8
73
9
75 (70)
68
10
11
12
13
14
15
2a
2b
2c
2d
2e
81
76
86
90
94
95/5
70
100/0
95/5
62
73
78/22
80
100/0
85 (72)
16
17
18
19
20
2a
2b
2c
2d
2e
62
67
80
83
75
90/10
88/12
94/6
46
55
68
97/3
74 (70)
100/0
63
21
22
23
24
25
2a
2b
2c
2d
2e
66
72
88
84
78
97/3
51
100/0
89/11
83/17
100/0
61
74 (79)
70
66
26
27
28
29
30
2a
2b
2c
2d
2e
58
66
72
77
65
95/5
47
97/3
58
100/0
90/10
95/5
60
68 (65)
57
31
32
33
34
35
2a
2b
2c
2d
2e
60
68
70
77
64
100/0
52
96/4
55
100/0
100/0
100/0
55
72 (65)
50
36
37
38
39
40
2a
2b
2c
2d
2e
66
70
81
88
77
98/2
58
100/0
100/0
89/11
100/0
61
69
80 (74)
64
a
b
c
Reaction conditions: [Ru] (2a-2e) (0.01 mmol, 1 mol%), pyrrolidine (1.0 mmol), alcohol (1.5 mmol), no base, no solvent, 120 °C, 16 h.
Conversions (%) were calculated according to pyrrolidine by GC analysis using dodecane as an internal standard.
Ratio of N-alkylated and N,C(3)-dialkylated products were calculated by GC analysis.
d
Yields (%) were calculated by GC analysis and the number in parenthesis corresponds to the isolated yield after purification by column chromatography.
secondary alcohol, was also led to 59–92% conversion with perfect
selectivity (Table 4, entries 11–15). N-(2-octyl) piperidine (4c) was
abtained with 73% isolated yield in presence of 2e catalyst (Table 4,
entry 15). When Citronellol was used, high selectivity N-(3,7-di-
methyloct-6-en-1-yl) piperidine (4d) was formed in the presence of 2b
and 2e catalysts (Table 4, entries 17,20). This product was obtained in
80% isolated yield in the presence of 2d catalyst (Table 4, entry 19). As
previously observed with pyrrolidine, the primary and secondary ali-
phatic alcohols were less reactive and the corresponding tertiary amines
were isolated in lower yields.
It must be noted that in the presence of a deprotonating reagent
t
such as BuOK or NaH in refluxing toluene, [RuX2(arene)(NHC)] com-
plexes have led to the selective formation of amides from primary and
secondary amines [35]. But, the N-alkylation of primary and secondary
amines was provided with 20 mol% of the weak base such as NaHCO3
in presence of [RuCl(PPh3)(p-cymene)(NHC)] complexes at 130 °C
under solvent-free conditions [37]. In this catalytic system, the basic
cyclic secondary amines might play the role of deprotonating reagent
5

M. Kaloğlu
InorganicaChimicaActa498(2019)119163
Table 4
Ruthenium-carbene catalyzed alkylation of piperidine with alcohols.a
Entry
Alcohol
[Ru]
Main product
Conversionb[%]
Ratio of 4/4′c
Yield of 4d [%]
1
2
3
4
5
2a
2b
2c
2d
2e
80
76
90
97
91
97/3
72
95/5
56
85/15
83/27
95/5
72
85 (80)
83
6
2a
2b
2c
2d
2e
73
79
89
93
80
100/0
61
7
95/5
67
8
98/2
70
9
85/15
96/4
80 (77)
71
10
11
12
13
14
15
2a
2b
2c
2d
2e
76
59
82
92
90
96/4
64
96/4
45
100/0
88/12
100/0
70
78
80 (73)
16
17
18
19
20
2a
2b
2c
2d
2e
75
70
91
98
90
98/2
63
100/0
88/12
94/6
59
77
86 (80)
79
100/0
a
b
c
Reaction conditions: [Ru] (2a-2e) (0.01 mmol, 1 mol%), piperidine (1.0 mmol), alcohol (1.5 mmol), no base, no solvent, 120 °C, 16 h.
Conversions (%) were calculated according to piperidine by GC analysis using dodecane as an internal standard.
Ratio of N-alkylated and N,C(3)-dialkylated products were calculated by GC analysis.
d
Yields (%) were calculated by GC analysis and the number in parenthesis corresponds to the isolated yield after purification by column chromatography.
Table 5
Ruthenium-carbene catalyzed mono-alkylation of 4-methyl aniline with alcohols.a
Entry
Alcohol
[Ru]
Main product
Conversionb[%]
Ratio of 5/5′c
Yield of 5d [%]
1
2
3
4
5
2a
2b
2c
2d
2e
81
86
91
95
87
98/2
70
100/0
100/0
100/0
100/0
73
80
83 (79)
73
6
2a
2b
2c
2d
2e
73
59
79
90
86
90/10
78/22
100/0
100/0
98/2
61
7
47
8
70
9
80 (75)
10
75
11
12
13
14
15
2a
2b
2c
2d
2e
66
75
96
92
87
82/18
54
96/4
61
100/0
100/0
100/0
85 (81)
80
75
2a-2e
a
b
c
t
Reaction conditions: [Ru] (2a-2e) (0.01 mmol, 1 mol%), 4-methylaniline (1.0 mmol), alcohol (1.5 mmol), BuOK (1.5 mmol), no solvent, 120 °C, 20 h.
Conversions (%) were calculated according to 4-methylaniline by GC analysis using dodecane as an internal standard.
Ratio of amine and imine were calculated by GC analysis.
d
Yields (%) were calculated by GC analysis and the number in parenthesis corresponds to the isolated yield after purification by column chromatography.
6

M. Kaloğlu
InorganicaChimicaActa498(2019)119163
which favours the formation of tertiary N-alkylated amines facilitated
under solvent-free conditions. These results contrast with the N-alky-
lation of aromatic amines such as anilines by alcohols with similar
catalytic systems based on [RuX2(p-cymene)(NHC)] complexes, which
required the use of a stoichiometric amounts of base. Due to the low
basicity of primary aromatic amines such as 4-methylaniline, there is a
need for the use of a stoichiometric amount of base for the N-alkylation
of such amines [31l,34]. Therefore, the N-alkylation of 4-methylaniline
was tested in presence of a stoichiometric amount of base under the
4. Experimental
4.1. General procedure for the preparation of NHC ligand precursors (1a-
1e)
N-(Alkyl)-1H-benzimidazole derivative (5.0 mmol) was dissolved in
degassed CHCl3 (10 mL) and alkyl halide (5.0 mmol) was added under
argon atmosphere. The reaction mixture was stirred at 60 °C for 24 h.
Further, the volume of the reaction mixture was reduced up to 5 mL,
and Et2O (15 mL) was added to obtain a white crystalline solid, which
was filtered off. The solid precipitate was washed with Et2O
(3 × 10 mL) and dried under vacuum. The crude product was re-
crystallized from EtOH/Et2O mixture (1:4, v/v) and completely dried
under vacuum. The new NHC ligand precursors 1b and 1e were isolated
as air- and moisture-stable white solids with 75% and 80% yields, re-
spectively. The 1a [33], 1c [25a] and 1d [25b] NHC ligand precursors
have already been reported in the literature. All the new NHC ligand
precursors were characterized by 1H NMR, 13C NMR, FT-IR and ele-
mental analysis techniques. For the 1H NMR, 13C NMR and FT-IR
spectrums of the new 1b and 1e benzimidazolium salts see SI file, pages
S1–S4.
t
determined optimum conditions. In these trials, BuOK base, which is
widely used in similar studies in the literature, was used [34a]. It is
known in the literature that increasing the catalyst loading led to
complete conversion but with more of the N,N-dialkylated product
[32d]. It is clear that there is a correlation between the amines nu-
cleophilicity and the overall reactivity. Electron-donating groups on the
aniline make the aniline ring more reactive, while the electron-with-
drawing groups reduce the reactivity of the aniline ring. In addition, the
increased chain length and steric properties of the alcohol leads to the
reaction advancing in favor of N-monoalkylated product formation
[32d]. Therefore, N-monoalkylation of 4-methyl aniline occurred with
high selectivity. (Table 5).
The reaction of 4-methyl aniline with benzyl alcohol gave the N-
(benzyl)-4-methyl aniline (5a) with excellent selectivities in 81–95%
conversions (Table 5, entries 1–5). This product was obtained with 79%
isolated yield in the presence of 2d catalyst (Table 5, entry 4). 4-Iso-
proylbenzyl alcohol afforded the corresponding N-monoalkylated pro-
duct, N-(4-isopropylbenzyl)-4-methyl aniline (5b), for the all catalysts
(Table 5, entries 6–10). When the reaction was carried out with Ci-
tronellol, the N-alkylamine product, N-(3,7-dimethyloct-6-en-1-yl)-4-
methyl aniline (5c), gave in moderate to high GC yield (Table 5, entries
11–15). This product was obtained in 81% isolated yield in the presence
of 2c catalyst (Table 5, entry 13).
4.1.1. 1-(3-Methoxypropyl)-3-(3-phenoxybenzyl)benzimidazolium
chloride, (1b)
Yield: 1.533 g, 75% (white solid); 1H NMR (300 MHz, CDCl3, 25 °C,
TMS): δ (ppm) = 2.31 (p, J = 5.6 Hz, 2H, NCH2CH2CH2OCH3); 3.22 (s,
3H, NCH2CH2CH2OCH3); 3.44 (t, J = 5.3 Hz, 2H, NCH2CH2CH2OCH3);
4.71 (t, J = 6.7 Hz, 2H, NCH2CH2CH2OCH3); 5.89 (s, 2H,
NCH2C6H4(OC6H5)-3); 6.86–6.91 (m, 3H, arom CHs of NC6H4N); 6.99 (s,
1H, arom CH of NC6H4N); 7.07 (t, J = 7.4 Hz, 1H, arom CH of
NCH2C6H4(OC6H5)-3);
7.19–7.30
(m,
4H,
arom
CHs
of
NCH2C6H4(OC6H5)-3); 7.52 (d, J = 8.0 Hz, 2H, arom CHs of
NCH2C6H4(OC6H5)-3); 7.56–7.61 (m, 1H, arom CH of NCH2C6H4(OC6H5)-
3); 7.75 (d, J = 8.1 Hz, 1H, arom CH of NCH2C6H4(OC6H5)-3); 11.72 (s,
1H, NCHN). 13C NMR (75 MHz, CDCl3, 25 °C, TMS): δ(ppm)
In the present work, the use of the oxygen functionalised N-aryl and
N-alkyl benzimidazol-2-ylidene ligands was preferred. The chelating
nature of these type ligands promotes production of highly stable ru-
thenium 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. At the same time
the strong donor carbene moiety remains connected to the metal centre
[28–30]. Using of the CSA was not have a significant effect on the
formation of N- and C(3)-dialkylated products, contrary to similar
studies in the literature [30a,31f,36]. Therefore, CSA was not used in
catalytic studies and N-monoalkylated amine derivatives were obtained
as the main product. Due to the high basicity of cyclic secondary
amines, the use of a stoichiometric amount of base was not needed in all
trials using these amines.
=
29.6 (NCH2CH2CH2OCH3); 45.1 (NCH2CH2CH2OCH3); 51.0
(NCH2C6H4(OC6H5)-3);
58.8
(NCH2CH2CH2OCH3);
68.9
(NCH2CH2CH2OCH3); 113.1, 113.7, 118.0, 118.8, 119.2, 122.7, 123.8,
127.1, 129.9, 130.8, 131.0, 131.8, 134.9, 156.4, 158.0 (arom. Cs of
NC6H4N and NCH2C6H4(OC6H5)-3); 143.9 (NCHN). Elemental analysis
calcd. (%) for C24H25ClN2O2 (M.w. = 408.93 g.mol−1): C 70.49, H 6.16, N
6.85; found (%): C 70.66, H 6.28, N 6.99; (for the NMR and FT-IR spec-
trum of 1b see SI file, pages S1–S2).
4.1.2. 1-(4-Phenoxybutyl)-3-(2,3,4,5,6-pentamethylbenzyl)
benzimidazolium chloride, (1e)
Yield: 1.848 g, 80% (white solid); 1H NMR (500 MHz, CDCl3, 25 °C,
TMS): δ (ppm) = 1.97 (p, J = 5.9 Hz, 2H, NCH2CH2CH2CH2OC6H5);
2.24, 2.28 and 2.30 (s, 15H, NCH2C6(CH3)5–2,3,4,5,6);
2.26 (p, J = 7.4 Hz, 2H, NCH2CH2CH2CH2OC6H5); 4.04 (t, J = 5.8 Hz,
3. Conclusions
In summary, five new half-sandwich ruthenium-carbene complexes
were synthesized, and they were characterized by various spectroscopic
and analytical methods. The catalytic activities of the ruthenium-car-
bene complexes was tested for the N-alkylation of amines with alcohols
by hydrogen-borrowing process. The catalytic studies carried out under
solvent-free conditions were used various substrates such as aliphatic
and benzylic alcohols. Thus, the N-alkylation of both the cyclic sec-
ondary amines and the primary aromatic amines was successfully car-
ried out. The desired product was selectively obtained in moderate to
high yields. By means of the present catalytic system, various higher-
order cyclic and aromatic amines have been synthesized under mild and
harmless conditions without producing wastes, and only water has re-
leased as by-product. Therefore, ruthenium-catalyzed N-alkylation of
amines with alcohols through the borrowing hydrogen approach pro-
vides an economically and environmentally attractive procedure for the
preparation of higher amines.
2H,
NCH2CH2CH2CH2OC6H5);
4.85
(t,
J = 7.4 Hz,
2H,
NCH2CH2CH2CH2OC6H5); 5.84 (s, 2H, NCH2C6(CH3)5–2,3,4,5,6); 6.82
(d, J = 8.0 Hz, 2H, arom. CH, NCH2CH2CH2CH2OC6H4); 6.93 (t,
J = 7.3 Hz, 1H, arom. CH, NCH2CH2CH2CH2OC6H4); 7.25 (t, J = 7.9 Hz,
2H, arom. CH, NCH2CH2CH2CH2OC6H4); 7.30 (d, J = 8.5 Hz, 1H, arom.
CH, NC6H4N); 7.50 (t, J = 7.8 Hz, 1H, arom. CH, NC6H4N); 7.60 (t,
J = 7.8 Hz, 1H, arom. CH, NC6H4N); 7.74 (d, J = 8.3 Hz, 1H, arom. CH,
NC6H4N); 11.18 (s, 1H, NCHN). 13C NMR (125 MHz, CDCl3, 25 °C, TMS):
δ
(ppm) = 17.0, 17.2 and 17.4 (NCH2C6(CH3)5–2,3,4,5,6); 26.1
(NCH2CH2CH2CH2OC6H5); 26.5 (NCH2CH2CH2CH2OC6H5); 47.4
(NCH2CH2CH2CH2OC6H5); 48.2 (NCH2C6(CH3)5–2,3,4,5,6); 66.8
(NCH2CH2CH2CH2OC6H5); 113.1, 113.8, 114.4, 120.9, 124.9 127.0,
127.1, 129.5, 131.5, 131.6, 133.5, 134.0, 137.3, 158.5 (arom. Cs of
NC6H4N, NCH2C6(CH3)5–2,3,4,5,6 and NCH2CH2CH2CH2OC6H5); 143.2
(NCHN). Elemental analysis calcd. (%) for
C29H34ClN2O
(M.w. = 462.05 g.mol−1): C 75.38, H 7.42, N 6.06; found (%): C 75.53,
7

M. Kaloğlu
InorganicaChimicaActa498(2019)119163
H 7.57, N 6.14; (for the NMR and FT-IR spectrum of 1e see SI file, pages
(arom. Cs of p-cymene); 99.2, 108.8, 111.1, 111.5, 115.6, 117.5, 119.2,
120.7, 123.0, 123.1, 123.7, 129.8, 130.4, 135.0, 135.5, 139.7, 156.4,
158.0 (arom. Cs of NC6H4N and NCH2C6H4(OC6H5)-3); 190.0 (Ru-
S3–S4).
C
carbene). Elemental analysis calcd. (%) for C34H38Cl2N2O2Ru
4.2. General procedure for the preparation of the half-sandwich ruthenium-
(M.w. = 678.66 g.mol−1): C 60.17, H 5.64, N 4.13; found (%): C 60.45,
H 5.78, N 4.19; (for the NMR and FT-IR spectrum of 2b see SI file, pages
S7–S8).
carbene complexes (2a-2e)
For the preparation of the ruthenium-carbene complexes (2a-2e), a
solution of NHC ligand precursor (0.5 mmol), Ag2O (0.25 mmol), acti-
vated 4 Å molecular sieves and anhydrous dichloromethane (10 mL)
were added to a Schlenk tube under argon atmosphere. This solution
was stirred at room temperature for 24 h in the dark conditions, and
4.2.3. Dichloro-[1-(2-(2-ethoxyphenoxy)ethyl)-3-(3-methoxylbenzyl)
benz-imidazol-2-ylidene](p-cymene)ruthenium (II), (2c)
Yield: 0.262 g, 74% (red-brown solid); 1H NMR (400 MHz, CDCl3,
25 °C, TMS): δ (ppm) = 1.25 (d, J = 6.9 Hz, 6H, CH(CH3)2 of p-
cymene); 1.45 (t, J = 7.0 Hz, 3H, NCH2CH2OC6H4(OCH2CH3)-2);
2.04 (s, 3H, CH3 of p-cymene); 2.93 (hept, J = 6.7 Hz, 1H,
CH(CH3)2 of p-cymene); 3.72 (s, 3H, NCH2C6H4(OCH3)-3);
4.09 (q, J = 7.0 Hz, 2H, NCH2CH2OC6H4(OCH2CH3)-2); 4.55
(t, J = 7.0 Hz, 2H, NCH2CH2OC6H4(OCH2CH3)-2); 5.07 (m, 1H,
NCH2CH2OC6H4(OCH2CH3)-2); 5.21 (s, 2H, NCH2C6H4(OCH3)-3);
5.36–5.40 (m, 2H, NCH2CH2OC6H4(OCH2CH3)-2 and arom. CHs of p-
cymene); 5.60–5.62 (m, 3H, arom. CHs of p-cymene); 6.58 (d,
J = 8.2 Hz, 1H, arom. CHs of NCH2CH2OC6H4(OCH2CH3)-2); 6.68 (s,
1H, arom. CH of NCH2C6H4(OCH3)-3); 6.78–6.90 (m, 5H, arom. CHs of
NCH2CH2OC6H4(OCH2CH3)-2 and NCH2C6H4(OCH3)-3); 7.02–7.26 (m,
4H, arom. CHs, NC6H4N); 8.05 (d, J = 8.2 Hz, 1H, arom. CHs of
NCH2CH2OC6H4(OCH2CH3)-2). 13C NMR (101 MHz, CDCl3, 25 °C,
filtered through
a Celite. After that, [RuCl2(p-cymene)]2 dimer
(0.25 mmol) was added to this solution under argon atmosphere, and
this solution was stirred for 24 h at room temperature. End of the re-
action, the solution was filtered through Celite and the all solvent was
removed under reduced vacuum to afford the product as brown
powder. The crude product was recrystallized from dichloromethane/n-
pentane mixture (1:5, v/v). All new ruthenium-carbene complexes were
isolated as air-stable and red-brown solids with 54–82% yields. For the
1H NMR, 13C NMR and FT-IR spectrums of the new 2a-2e ruthenium-
carbene complexes see SI file, pages S5–S14.
4.2.1. Dichloro-[1-(3-methoxypropyl)-3-(4-chlorobenzyl)benzimidazole-2-
ylidene](p-cymene)ruthenium(II), (2a)
Yield: 0.245 g, 79% (red-brown solid); 1H NMR (400 MHz, CDCl3,
25 °C, TMS): δ (ppm) = 1.18 (d, J = 6.9 Hz, 6H, CH(CH3)2 of p-
cymene); 1.93 (s, 3H, CH3 of p-cymene); 2.09 (p, J = 5.3 Hz, 2H,
NCH2CH2CH2OCH3); 2.87 (hept, J = 6.7 Hz, 1H, CH(CH3)2 of p-
cymene); 3.37 (s, 3H, NCH2CH2CH2OCH3); 3.53 (t, J = 5.3 Hz, 2H,
NCH2CH2CH2OCH3); 4.94 (t, J = 6.6 Hz, 2H, NCH2CH2CH2OCH3); 5.31
(s, 2H, NCH2C6H4(Cl)-4); 5.40–6.13 (m, 4H, arom. CHs of p-cymene);
6.89 and 7.50 (d, J = 8.0 Hz, 2H, arom. CHs of NCH2C6H2(Cl)-4);
6.99–7.08 and 7.15–7.26 (m, 6H, arom. CHs of NC6H4N and
TMS):
δ (ppm) = 15.1 (NCH2CH2OC6H4(OCH2CH3)-2); 18.4 (CH3
of p-cymene); 21.7 and 23.4 (CH(CH3)2 of p-cymene); 30,7
(CH(CH3)2 of p-cymene); 50.4 (NCH2CH2OC6H4(OCH2CH3)-2);
52.6 (NCH2C6H4(OCH3)-3); 55.2 (NCH2C6H4(OCH3)-3); 63.9
(NCH2CH2OC6H4(OCH2CH3)-2); 69.3 (NCH2CH2OC6H4(OCH2CH3)-2);
83.5, 85.1, 85.2, 86.5 (arom. Cs of p-cymene); 98.2, 108.5, 111.2,
111.9, 112.5, 114.1, 118.0, 120.9, 121.6, 123.2, 130.0, 135.3,
135.6, 139.3, 147.8, 148.7, 160.1 (arom. Cs of NC6H4N,
NCH2CH2OC6H4(OCH2CH3)-2 and NCH2C6H4(OCH3)-3); 191.6 (Ru-
Ccarbene). Elemental analysis calcd. (%) for C35H40Cl2N2O3Ru
(M.w. = 708.69 g.mol−1): C 59.32, H 5.69, N 3.95; found (%): C 59.75,
H 5.87, N 3.80; (for the NMR and FT-IR spectrum of 2c see SI file, pages
S9–S10).
NCH2C6H2(Cl)-4). 13C NMR (101 MHz, CDCl3, 25 °C, TMS):
δ
(ppm) = 18.7 (CH3 of p-cymene); 22.3 and 23.2 (CH(CH3)2 of p-
cymene);
30.7
(NCH2CH2CH2OCH3);
30,9
(CH(CH3)2
of
p-cymene); 48.0 (NCH2CH2CH2OCH3); 52.5 (NCH2C6H4(Cl)-4); 58.9
(NCH2CH2CH2OCH3); 70.5 (NCH2CH2CH2OCH3); 83.0, 83.7, 86.0, 86.2
(arom. Cs of p-cymene); 99.4, 109.2, 111.2, 111.5, 123.1, 123.2, 127.9,
128.9, 133.3, 135.2, 135.4, 135.9 (arom. Cs of NC6H4N and
NCH2C6H4(Cl)-4); 190.1 (Ru-Ccarbene). Elemental analysis calcd. (%) for
4.2.4. Dichloro-[1-(2-(2-ethoxyphenoxy)ethyl)-3-(3,4,5-
trimethoxylbenzyl)- benzimidazol-2-ylidene](p-cymene) ruthenium(II),
(2d)
C
28H33Cl3N2ORu (M.w. = 621.01 g.mol−1): C 54.16, H 5.36, N 4.51;
Yield: 0.254 g, 66% (red-brown solid); 1H NMR (400 MHz,
CDCl3, 25 °C, TMS): δ (ppm) = 1.18 (d, J = 7.0 Hz, 6H, CH(CH3)2
of p-cymene); 1.35 (t, J = 7.0 Hz, 3H, NCH2CH2OC6H4(OCH2CH3)-2);
1.98 (s, 3H, CH3 of p-cymene); 2.88 (hept, J = 6.9 Hz, 1H,
CH(CH3)2 of p-cymene); 3.57 (s, 6H, NCH2C6H2(OCH3)3–3,4,5); 3.74 (s,
3H, NCH2C6H2(OCH3)3–3,4,5); 3.99 (q, J = 7.0 Hz, 2H, NCH2CH2
OC6H4(OCH2CH3)-2); 4.45–4.51 (m, 2H, NCH2CH2OC6H4(OCH2CH3)-
2); 4.98 (m, 1H, NCH2CH2OC6H4(OCH2CH3)-2); 5.19 (d, J = 5.9 Hz,
2H, NCH2C6H2(OCH3)3–3,4,5); 5.31–5.36 (m, 2H, NCH2CH2OC6H4
(OCH2CH3)-2 and arom. CHs of p-cymene); 5.56–5.71 (m, 2H, arom.
CHs of p-cymene); 6.22 (s, 1H, arom. CH of p-cymene); 6.28 (s, 2H,
arom. CHs of NCH2C6H2(OCH3)3–3,4,5); 6.73–6.84 (m, 4H, arom. CHs,
NC6H4N and NCH2CH2OC6H4(OCH2CH3)-2); 6.99 (d, J = 8.1 Hz, 1H,
arom. CH of NC6H4N); 7.07 (t, J = 7.6 Hz, 1H, arom. CH of NC6H4N);
7.18 (t, J = 7.7 Hz, 1H, arom. CH of NC6H4N); 7.94 (d, J = 8.2 Hz, 1H,
arom. CH of NCH2CH2OC6H4(OCH2CH3)-2). 13C NMR (101 MHz,
CDCl3, 25 °C, TMS): δ (ppm) = 15.1 (NCH2CH2OC6H4(OCH2CH3)-2);
18.4 (CH3 of p-cymene); 21.8 and 23.4 (CH(CH3)2 of p-cymene); 30,7
(CH(CH3)2 of p-cymene); 50.4 (NCH2CH2OC6H4(OCH2CH3)-2);
found (%): C 54.35, H 5.25, N 4.40; (for the NMR and FT-IR spectrum of
2a see SI file, pages S5–S6).
4.2.2. Dichloro-[1-(3-methoxypropyl)-3-(3-phenoxybenzyl)benzimidazole-
2-ylidene](p-cymene)ruthenium(II), (2b)
Yield: 0.188 g, 54% (red-brown solid); 1H NMR (400 MHz, CDCl3,
25 °C, TMS): δ (ppm) = 1.24 (d, J = 6.9 Hz, 6H, CH(CH3)2 of p-
cymene); 1.94 (s, 3H, CH3 of p-cymene); 2.25–2.43 (m, 2H,
NCH2CH2CH2OCH3); 2.91 (hept, J = 6.9 Hz, 1H, CH(CH3)2 of p-
cymene); 3.43 (s, 3H, NCH2CH2CH2OCH3); 3.58 (t, J = 5.0 Hz, 2H,
NCH2CH2CH2OCH3); 4.96 (t, J = 6.9 Hz, 2H, NCH2CH2CH2OCH3);
4.54–4.56, 5.06–5.08, 5.31–5.33 and 5.44–5.46 (m, 4H, arom. CHs of p-
cymene); 5.62–5.67 and 6.52–6.57 (m, 2H, NCH2C6H4(OC6H5)-3); 6.59
(s, 1H, arom. CH of NCH2C6H4(OC6H5)-3); 6.84–6.93 (m, 4H, arom.
CHs of NC6H4N and NCH2C6H4(OC6H5)-3); 7.02 (d, J = 8.1 Hz, 1H,
arom. CHs of NC6H4N and NCH2C6H4(OC6H5)-3); 7.09 (t, J = 7.4 Hz,
1H, arom. CH of NCH2C6H4(OC6H5)-3); 7.16 (t, J = 7.6 Hz, 1H, arom.
CH of NCH2C6H4(OC6H5)-3); 7.24–7.30 (m, 4H, arom. CHs of NC6H4N
and NCH2C6H4(OC6H5)-3); 7.55 (d, J = 8.1 Hz, 1H, arom. CH of
NC6H4N). 13C NMR (101 MHz, CDCl3, 25 °C, TMS): δ (ppm) = 18.5
(CH3 of p-cymene); 21.6 and 23.5 (CH(CH3)2 of p-cymene); 30.7
(NCH2CH2CH2OCH3); 30,8 (CH(CH3)2 of p-cymene); 47.9
53.1
(NCH2C6H2(OCH3)3–3,4,5);
56.2
and
60.9
(NCH2
C6H2(OCH3)3–3,4,5); 63.8 (NCH2CH2OC6H4(OCH2CH3)-2); 69.1
(NCH2CH2OC6H4(OCH2CH3)-2); 83.9, 84.7, 85.2, 86.9 (arom. Cs of p-
cymene); 98.1, 103.4, 108.8, 111.4, 112.5, 114.1, 120.9, 121.7, 123.1,
133.0, 135.2, 135.6, 137.2, 147.7, 148.7, 153.6 (arom. Cs of NC6H4N,
(NCH2CH2CH2OCH3);
52.4
(NCH2C6H(CH3)4–2,3,5,6);
58.9
(NCH2CH2CH2OCH3); 70.6 (NCH2CH2CH2OCH3); 82.3, 83.6, 85.7, 87.3
8

M. Kaloğlu
InorganicaChimicaActa498(2019)119163
NCH2CH2OC6H4(OCH2CH3)-2 and NCH2C6H2(OCH3)3–3,4,5); 191.8
(Ru-Ccarbene). Elemental analysis calcd. (%) for C37H44Cl2N2O5Ru
(M.w. = 768.74 g.mol−1): C 57.81, H 5.77, N 3.64; found (%): C 58.05,
H 5.93, N 3.56; (for the NMR and FT-IR spectrum of 2d see SI file, pages
S11–S12).
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2019.119163.
4.2.5. Dichloro-[1-(4-phenoxybutyl)-3-(2,3,4,5,6-pentamethylbenzyl)
benz-imidazole-2-ylidene](p-cymene)ruthenium (II), (2e)
Yield: 0.300 g, 82% (red-brown solid); 1H NMR (400 MHz, CDCl3,
References
25 °C, TMS):
δ (ppm) = 1.22 (d, J = 5.8 Hz, 6H, CH(CH3)2 of
p-cymene);
2.03–2.12
(m,
22H,
NCH2CH2CH2CH2OC6H5,
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Declaration of Competing Interest
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