Arene diruthenium(II)-mediated synthesis of imines from alcohols and amines under aerobic condition

Article abstract of DOI:10.1002/aoc.6122

The utility and selectivity of the newly synthesized dinuclear arene Ru(II) complex were demonstrated towards the synthesis of imines from coupling of alcohols and amines in the aerobic condition. Analytical and various spectral methods have been used to establish the unprecedented formation of the new thiolato-bridged dinuclear ruthenium complex. The molecular structure of the titled complex was evidenced with aid of X-ray crystallographic technique. A wide range of imines were obtained in good-to-excellent yields up to 98% and water as the by-product through an acceptorless dehydrogenative coupling of alcohols with amines. The catalytic reaction operated a concise atom economical without any oxidant with 1 mol% of the catalyst load. Further, the role of base, solvent and catalyst loading of the coupling reaction has been investigated. A plausible mechanism has been described and was found to proceed via the formation of an aldehyde intermediate. Short synthesis of antibacterial drug N-(salicylidene)-2-hydroxyaniline illustrated the utility of the present protocol.

Full text of DOI:10.1002/aoc.6122

Received: 28 October 2020
DOI: 10.1002/aoc.6122
Revised: 27 November 2020
Accepted: 28 November 2020
F U L L P A P E R
Arene diruthenium(II)-mediated synthesis of imines from
alcohols and amines under aerobic condition
Veerappan Tamilthendral1 | Rengan Ramesh1
| Jan Grzegorz Malecki2
1Centre for Organometallic Chemistry,
The utility and selectivity of the newly synthesized dinuclear arene Ru(II)
complex were demonstrated towards the synthesis of imines from coupling of
alcohols and amines in the aerobic condition. Analytical and various spectral
methods have been used to establish the unprecedented formation of the
new thiolato-bridged dinuclear ruthenium complex. The molecular structure
of the titled complex was evidenced with aid of X-ray crystallographic tech-
nique. A wide range of imines were obtained in good-to-excellent yields up to
98% and water as the by-product through an acceptorless dehydrogenative
coupling of alcohols with amines. The catalytic reaction operated a concise
atom economical without any oxidant with 1 mol% of the catalyst load. Fur-
ther, the role of base, solvent and catalyst loading of the coupling reaction has
been investigated. A plausible mechanism has been described and was found
to proceed via the formation of an aldehyde intermediate. Short synthesis of
antibacterial drug N-(salicylidene)-2-hydroxyaniline illustrated the utility of
the present protocol.
School of Chemistry, Bharathidasan
University, Tiruchirappalli, India
2Department of Crystallography, Institute
of Chemistry, University of Silesia,
Katowice, Poland
Correspondence
Rengan Ramesh, Centre for
Organometallic Chemistry, School of
Chemistry, Bharathidasan University,
Tiruchirappalli, 620 024 Tamilnadu, India.
Email: rramesh@bdu.ac.in
Funding information
Science and Engineering Research Board,
Grant/Award Number: EMR/2016/004952
K E Y W O R D S
aerobic oxidation, imine synthesis, thiolato Ru(II) catalyst, thiourea
1 | INTRODUCTION
higher reaction time are required in many situations.[5]
Imines have been also synthesized in different circum-
Imines are profound important class of nitrogen com-
pounds owing to their high reactivity.[1] They are ubiqui-
tous intermediates in many organic reactions such as
cyclization, cycloaddition, multicomponent reactions and
condensation.[2] They are adaptable nitrogen sources that
find applications in pharmaceuticals, industrial and
agriculture.[3] Further, many nitrogen-containing bioac-
tive compounds such as amines, amides and pyrrolines
can be constructed from imine functional group
(Figure 1).[4] Hence, synthesis and applications of imines
are essentially ever-appealing topics in synthetic organic
chemistry.
stances includes Schmidt reaction, Aza-Wittig reaction[6]
and oxidation of secondary amines using oxidizing agents
(Scheme 1).[7]
Although a number of methods are known for imine
synthesis in the literature, they largely suffer from draw-
backs like use of toxic reagents, poor atom economy,
harsh synthetic process and low level of selectivity.[8]
To overcome the aforementioned limitations, the
metal-catalyzed direct synthesis of imines from alcohols
with amines through acceptorless dehydrogenation cou-
pling mechanism is an alternative approach. The strategy
consists of two steps: (i) aerobic oxidation of alcohol in
The conventional approach for imine synthesis
involves the direct coupling of amines with aldehydes or
ketones with Lewis acid or dehydrating agents and
the presence of a transition metal catalyst and
(ii) generation of imine. More advantageously, the accept-
orless dehydrogenative methodology is a greener protocol
Appl Organomet Chem. 2020;e6122.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6122

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TAMILTHENDRAL ET AL.
FIGURE 1 Examples for bioactive imine
analogues
SCHEME 1 Imine formation via
traditional method and new approaches by
aerobic oxidation of alcohols and amines (a, b
and c)
for the coupling of alcohol and amine to desired imine
with the water as the by-product.
system used as a catalyst in the presence of additives.[18]
Maggi et al. demonstrated the catalytic performance of
Ru–NHC complex (5 mol%) in imine synthesis using
DABCO ligand in the presence of molecular sieves for
24 h.[10b] The catalytic activity of Co(II)–NNN pincer
complex has been explored for imine synthesis, and the
reaction was carried out with n-octane as a solvent at
high temperature[19] (Scheme 2).
Overall, a large number of metal complexes with dif-
ferent ligand systems have been explored as catalysts for
this reaction. In particular, metal-based catalysts for the
synthesis of imines with phosphine labile ligands have
been well explored. However, the catalytic condition
showed some drawbacks such as higher temperature,
higher catalyst load and inert atmosphere. To overcome
the above issues, we are interested to execute the imine
synthesize protocol using metal complexes with
phosphine-free ligands. Generally, metal complex con-
taining phosphorus-free ligands has salient features like
ease of synthesis and air stability, to avoid tedious separa-
tion and catalyst recovery. In the present art of research,
we have described the synthesis and characterization of
Milstein and co-workers reported Ru–PNP-type pin-
cer complex that promoted synthesis of imines from alco-
hols and amines under nitrogen atmosphere.[9] This
significant breakthrough methodology has garnered
much attention from the researchers towards imine syn-
thesis. Several wide transition metal complexes such as
Ru, Os, Pd, Pt and Au have been reported as catalysts for
imine synthesis under high temperature, inert atmo-
sphere, special condition and long reaction time.[10–14]
Shiraishi and co-workers employed a Pt/TiO2 heteroge-
neous catalyst for the imine synthesis with UV radiation
and used nitrogen atmosphere protection.[13] Soule and
co-workers have reported synthesis of imines by gold/pal-
ladium alloy nanoparticles (1.5 mol%) in the presence of
oxidant.[15] Donthiri et al. have described synthesis of
imines by using NaOH (10 mol%) as a catalyst at high
temperature.[16] The Tian research group explored imine
formation by employing CuI/bipyridine/TEMPO under
neat conditions.[17] Later, the Zhang group reported mild
one-pot synthesis of imines using as Fe(NO3)3/TEMPO

TAMILTHENDRAL ET AL.
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catalysts could be achieved by careful design of two metal
active sites (Figure 2).
2 | RESULTS AND DISCUSSION
1-(5-Methylthiazole-2-yl)-3-phenylthiourea
ligand
(HL) was prepared from phenyl isothiocyanate with
5-methylthiazole-2-amine in the equimolar ratio in the
presence of dimethylformamide (DMF) medium.[21]
The synthesis cationic arene Ru(II) thiourea complex
can be accomplished in good yield from complexation
of ruthenium starting precursor [(η6-p-cymene)RuCl2]2
(1.0 mmol) with thiourea ligand in 1:2 molar respec-
tively in benzene under the open-air condition. The
complex was yellow in colour and air stable. It was
easily soluble in solvents like CH2Cl2, CHCl3, CH3CN,
dimethyl sulfoxide (DMSO), and tetrahydrofuran
(THF). The resulting complex was crystallized from the
mixture of solvents dichloromethane and methanol
(1:1) (Scheme 3).
SCHEME 2 Synthetic strategies of imine reaction
new binuclear Ru(II) complex of thiourea ligand and
used as a catalyst for imine synthesis under the aerobic
catalytic condition. A catalyst featuring two closely asso-
ciated metal active sites is one of the emerging areas in
homogeneous catalysis. This bimetallic catalytic system
complements the traditional focus on parameters in order
to optimize catalytic behaviour in a better way. Change
of the steric and electronic properties of the ligands can
fine-tune the performance of the bimetallic system. Such
catalysts introduce new optimization parameters such as
catalyst nuclearity and synergistic cooperation between
the two metal active sites and the bridging ligands.[20]
Hence, controlling selectivity and activity of the catalytic
transformations will be offered by the suitable design of
bimetallic catalysts. Exquisite levels of activities of these
In the IR spectrum, thiazole N–H and phenyl group
N–H in the ligand showed bands in the regions 3,366 and
3,162 cm−1, respectively. Also, free ligand displayed the
thiocarbonyl (νC S) stretching frequencies at 1,254 cm−1
.
On complexation, thiazole-attached N–H stretching
vibration was not observed in the complex, indicating
that the ligand underwent enolization and decrease in
νC (1,150 cm−1). The shift in these bands revealed the
S
coordination of ligand to the metal via thiazole nitrogen
and thiocarbonyl sulfur.
FIGURE 2 Structure–function
relationship available in bimetallic catalysis

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TAMILTHENDRAL ET AL.
SCHEME 3 Synthetic route to [Ru
(η6-p-cymene)(HL)]2Cl2 complex
1
In the H NMR spectrum, the free ligand showed sig-
nals at 12.30 and 10.23 ppm due to N–H protons. Upon
complexation, the thiazole-connected –NH proton dis-
appeared from the complex, further supporting
enolization and coordination through thiocarbonyl sulfur
to the Ru(II) ion. All aromatic protons of the complex
appeared as multiplet in the region of 7.30–7.60 ppm. The
arene protons of the complex were observed at
5.30–5.56 ppm. The methyl protons of isopropyl group in
p-cymene moiety exhibited as singlet in the region
1.21–1.26 ppm. A septet appeared in the range of 2.80 ppm
due to a methine proton of the isopropyl group. Further,
signals due to the methyl protons of the p-cymene were
observed at 2.43 ppm as singlet (Figures S1, S2).
diffraction studies confirmed the structure proposed with
the aid of other spectroscopic techniques.
With the novel dinuclear arene Ru(II) thiourea
complex in hand, we wish to study the catalytic utility
in the synthesis of imines from coupling of alcohols
and amines at open atmospheric conditions. For that,
we initiated with test reaction between the equimolar
amounts of 4-methylbenzyl alcohol and aniline with
complex (1 mol%) as a catalyst with various solvents
and KOH as a base to optimize the reaction condition
(Table 1). When toluene was used as solvent, the
corresponding imine product 3a was obtained 83%
yield in 12 h (Table 1, entry 1). Switching the solvent
to xylene and benzene is also effective, furnishing
imines up to 80% and 72% yield, respectively (Table 1,
entries 2 and 3). Moderate yields of imines were
obtained when the reaction was performed in various
polar solvents like dioxane, THF, acetonitrile, DMF
The solid state structure of the complex [Ru(η6-
p-cymene)(HL)]2Cl2 has been studied by X-ray crystallo-
graphic technique. Crystals of suitable size were obtained
from mixed solvents of dichloromethane and methanol
(1:1). The Oak Ridge thermal ellipsoid plot (ORTEP) view
of the complex is shown in Figure 3, and the crystallo-
graphic data and selected bond distances and bond angles
are shown in the supporting information (Table S1, S2).
The crystal belongs to the monoclinic space group ‘C 2/c’
with Z = 4. The thiourea chelates to Ru(II) ion through
the two thiolato sulfur ions and thiazole nitrogen, and
the remaining position is occupied by arene moiety for-
ming a pseudo octahedral geometry. A four-membered
Ru–S–Ru–S ring system is formed owing to the bridging
position of sulfur atoms between the two Ru ions. The
unprecedented formation of bridging system is due to
pushing of electron density by the thiazole group through
the amino nitrogen atom. This enabled the sulfur atom to
make the new Ru–S bond, resulting in dimer formation.
The observed dimeric structure is similar to the related
compound containing a [Rh–N–C–S]2 sulfur-bridged din-
uclear unit.[22] The Ru2S2 core is essentially planar,
which indicated that the cymene ligands adopted cis
arrangement in the complex, similar to the arrangement
FIGURE 3 Oak Ridge thermal ellipsoid plot (ORTEP)
representation of complex [Ru(η6-p-cymene)(HL)]2Cl2 with 50%
probability level. All hydrogen atoms were omitted for clarity.
Selected bond lengths [Å] and angles [ꢀ]: Ru(1)–S [2] = 2.3765 [15],
Ru(2)–S [2] = 2.4204 [16], Ru(1)–N [1] = 2.120 [5], Ru(1)–C
[12] = 2.261 [6], Ru(1)–C [13] = 2.189 [6]; Ru(1)–S(2)–Ru
[2] = 99.25 [6], S(2)–Ru(1)–S [1] = 80.75 [6], N(1)–Ru(1)–S
[2] = 86.99 [14], N(1)–Ru(1)–C [12] = 122.3 [2], N(1)–C(1)–S
[1] = 112.2 [5], C(5)–S(2)–Ru [1] = 103.4 [2], C(12)–Ru(1)–S
[2] = 97.35 [17]
observed
in
[(η6-C6H3Me3)Ru{SCMe2CH–(CO2H)
NH2}2]2.[23] All of the Ru–S distances of complex are basi-
cally of equal length [range 2.3765(15)–2.4204(16) Å],
indicating symmetrical sulfur atoms. It has been observed
that the Ru–S–Ru bond angle [99.25 (6)ꢀ] is slightly larger
than the corresponding chloride bridging Ru–Cl–Ru
[98.22ꢀ] bond angle.[24] Hence, the single-crystal X-ray

TAMILTHENDRAL ET AL.
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TABLE 1 Screening of solvents,
bases and temperaturesa
Entry
1
Solvent
Toluene
Xylene
Base
Temp. (ꢀC)
110
140
80
Yield(%)b
83
KOH
2
KOH
80
3
Benzene
1,4-Dioxane
THF
KOH
72
4
KOH
100
66
65
5
KOH
78
6
Acetonitrile
DMF
KOH
82
60
7
KOH
150
65
52
8
9c
Methanol
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
KOH
70
—
r.t
NR
NR
10
10c
11d
12
13
14
15
16
17
18
19e
—
80
t-BuOK
NaOH
NaOMe
K2CO3
CS2CO3
t-BuOK
t-BuOK
t-BuOK
t-BuOK
110
110
110
110
110
110
80
82
85
79
80
88
90
60
98
r.t
70
Abbreviations: DMF, dimethylformamide; THF, tetrahydrofuran.
The bold data in the table 1 indicates the best optimized reaction condition.
aConditions: 4-methyl benzyl alcohol (1 mmol), aniline (1 mmol), catalyst, (1.0 mol%) and base (0.5 mmol)
in the presence of solvent (5 ml) at 60ꢀC for 12 h.
bIsolated yield.
cAbsence of base.
dAbsence of catalyst.
eTime for 24 h.
and methanol (Table 1, entries 4–8). These results indi-
cated that nonpolar solvents outperformed polar sol-
vents in the test reaction. No further reaction
proceeded in the absence of a base or a catalyst
(Table 1, entries 9–11). Furthermore, good product
yields are observed in the presence of NaOH and
NaOMe (Table 1, entries 12 and 13). In addition, up to
80% of imines were noted when K2CO3 and Cs2CO3
are present (Table 1, entries 14 and 15). Further, it
has been observed that t-BuOK outperformed other
bases, which afforded 3a in 90% yield of imine
(Table 1, entries 16 and 17). Notably, the cationic din-
uclear ruthenium complex catalyzed effectively the
coupling of alcohol and amine and yielded 98% of
selective imine under the optimized condition of tolu-
ene/t-BuOK at 60ꢀC (Table 1, entry 18).
Further, the effectiveness of our catalyst was exam-
ined with different catalyst loadings for the test reaction
(Table 2). Upon reducing the catalyst loading from 1 mol
% to 0.25 mol%, there was a substantial decrease in yields
(Table 2, entries 1–4). Therefore, 1 mol% catalyst loading
is the best choice for optimization.
The substrate scope of the reaction with respect to
various types of alcohols and amines under the optimized
catalytic conditions is displayed in Table 3. Fabulously,
electron-rich functionalities of benzyl alcohols (–CH3, –
OCH3) are efficiently reacted with aniline to yield the
respective imines 3a–3c in 83–95% of isolated yields

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TAMILTHENDRAL ET AL.
TABLE 2 The screening of the
catalyst loadingsa
Entry
Catalyst (mol %)
Yield (%)
1
2
3
4
1.0
0.5
0.3
0.1
97
80
61
39
The bold data in the table 2 indicates the best optimized condition.
aConditions: 4-methylbenzyl alcohol (1 mmol), aniline (1 mmol) and base (0.5 mmol) in the presence of
solvent (5 ml) at 60ꢀC for 12 h.
(Figures S3–S8). Further, electron-withdrawing substitu-
ents (–Cl, –F) on benzyl alcohols were tolerated well with
aniline that acquired desired imines 3d and 3e in the
yields of 88–78% (Figures S9–S12). In addition, the
coupling reactions between different benzyl alcohols
and 4-ethoxy and 4-methoxy anilines afforded the
corresponding imines 3f and 3g in 80–83% of isolated
yields (Figures S13–S16). More interestingly, the complex
catalysed well in the coupling of sterically hindered
2-bromobenzyl alcohol with 4-methoxy aniline to afford
the respective imine 3h in 75% of yield (Figures S17,
S18). However, the electron-withdrawing substituent of
4-chlorobenzyl alcohol with 4-methoxy aniline showed a
better result in the formation of respective imine 3i in
79% of yield (Figures S19, S20). The high yield of 82% for
3j was obtained by the reaction of 4-methoxybenzyl alco-
hol and 4-methoxy aniline (Figures S21, S22). Coupling
of benzyl alcohol-bearing electron-donating and with-
drawing substituents (4-methyl and 4-chloro) with
4-bromoaniline gave respective imines 3k and 3l in 98%
and 90% of yields (Figures S23–S26). Importantly,
piperonyl-based imine moieties were found to be effective
in pharmaceutically active ingredients. But the synthesis
of piperonyl-derived imines is less covered in previous lit-
erature.[25] Hence, we are interested in coupling the
piperonyl alcohol with various amines. More signifi-
cantly, we attained the piperonyl-derived imines 3m–3o
up to 94% of yields (Figures S27–S32). Gratifyingly, the
catalytic efficiency of the present complex proved the
synthesis of bis-imine product 3p with the appreciable
yield of 75% under the optimized condition (Figures S33,
S34). Deliberately, a chiral imine 3q was achieved from
the coupling of 4-methylbenzyl alcohol with (R)-
(Figures S35, S36). Notably, the complex efficiently pro-
moted the synthesis of imines from heterocyclic alcohols,
and amines resulted in good yields of imine products 3r
and 3s (Figures S37–S40). The attempt taken for coupling
of alcohol and aliphatic amine to provide the expected
product 3t was successful (Figures S41, S42). Further, the
catalytic condition was found to be ineffective for the
coupling of aliphatic alcohols with amines.
It is crucial at this point to compare the catalytic effi-
ciency and scope of our catalytic system with other
reported ruthenium(II) catalysts. Maggi et al. demon-
strated the catalytic performance of Ru–NHC complex
(5 mol%) in imine synthesis using DABCO ligand in the
presence of molecular sieves for 24 h.[10b] Musa et al.
have reported the catalytic activity of bifunctional
Ru(II) PCP pincer complexes towards synthesis of imine
from alcohols and amines in p-xylene medium with
2 mol% catalyst loading for 24 h under argon atmo-
sphere.[26] The binuclear Ru catalyst was documented to
catalyze an imine formation reaction with 5 mol%
DABCO ligand, and molecular sieves for 24 h were
reported.[27] In addition, Higuchi and co-workers
reported the ruthenium complex catalyzed imination
reaction with Zn (OCOCF3)2 (1 mol%) and KOtBu
(20 mol%) as a base in dioxane medium.[28] The arene
Ru(II) complex has considerable benefits over other
reported catalysts. In contrast, the salient features of
titled catalysts are insensitive towards air and simple,
convenient catalytic method for the synthesis of imines.
Further, the bimetallic catalytic system, with a coopera-
tive effect between the two metal centres, enhances the
strong metal–metal interaction, which interact with
the substrates, increasing the rate of the reaction than
the monometallic system. We speculated that the
(+)-α-methylbenzylamine
gave
an
84%
yield

TAMILTHENDRAL ET AL.
7 of 12
TABLE 3 Synthesis of imines from alcohols and aminea
Entry
1
2
3
Yieldb %
1
(3a)
(3b)
(3c)
95
80
83
2
3
4
5
(3d)
(3e)
88
78
6
7
(3f)
80
83
(3g)
8
(3h)
75
(Continues)

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TAMILTHENDRAL ET AL.
TABLE 3 (Continued)
Entry
1
2
3
Yieldb %
9
(3i)
79
82
10
(3j)
11
12
(3 k)
98
90
(3 l)
13
14
(3m)
(3n)
70
94
15
(3o)
82

TAMILTHENDRAL ET AL.
9 of 12
TABLE 3 (Continued)
Entry
1
2
3
Yieldb %
16c
(3p)
75
c
17
18
(3q)
(3r)
84
70
19
20
(3s)
(3t)
87
60
aReaction conditions: 1a (1 mmol), 2a (1 mmol), catalyst (1 mol%), t-BuOK (0.5 mmol) and toluene (5 ml) stirred for 12 h in open air.
bIsolated yields.
cReaction for 24 h.
SCHEME 4 Preparation of N-(salicylidene)-
2-hydroxyaniline using our protocol

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TAMILTHENDRAL ET AL.
3 | CONTROL EXPERIMENTS FOR
THE MECHANISTIC
INVESTIGATIONS[A
Control experiments were performed under standard
conditions in order to examine the mechanism of the
imination (Scheme 5). Initially, oxidation of alcohol leads
to the formation of aldehyde. Further, a mixture of prod-
ucts aldehyde and imine was obtained when the reaction
was conducted in the presence of amine for 8 h. Com-
plete imine product was obtained only after 12 h of the
reaction. Hence, the formation of aldehyde clearly indi-
cates that the reaction proceeds via oxidation of alcohol
as an initial step (Figures S45–S50).
SCHEME 5 Control experiments
A plausible mechanism has been proposed based on
the results from the control experiments and on the previ-
ously reported literature (Scheme 6). The reaction involves
the formation of ruthenium alkoxide species from the cat-
alyst through deprotonation of the alcohol followed by
β-hydride elimination to form aldehyde. This aldehyde
intermediate further reacts with amines to produce
imines, and water is eliminated as a by-product. Further,
the ruthenium hydride[10b,30] complex reacts with alcohol
to form the next catalytic cycle with the release of two mol-
ecules of water. The detailed studies on the mechanism for
imine synthesis are under investigation.
catalytic performance may be from two active metal cen-
tres of the complex working independently, or only an
active metal centre under the electronic influence of the
second one. Hence, the catalyst loading is 1 mol% suffi-
cient to catalyse the reaction with good-to-excellent
yields.[29]
It is worth to note that one of the antibacterial drugs,
namely, N-(salicylidene)-2-hydroxyaniline, was synthe-
sized from 2-hydroxybenzyl alcohol and 2-amino phenol
using our present protocol (Scheme 4), and an excellent
yield of 95% was obtained (Figures S43, S44).
SCHEME 6 Plausible mechanism
for imine formation

TAMILTHENDRAL ET AL.
11 of 12
4 | CONCLUSIONS
for 12 h under open-air atmosphere in 5 ml of toluene,
and the reaction was monitored by thin-layer chromatog-
raphy (TLC) until completion. Then, the reaction mixture
was cooled and diluted with ethyl acetate (10 ml). For
calculation of isolated yield, the layers were formed upon
addition of water (5 ml), and organic layer was separated.
The organic phase was dried over Na2SO4 and concen-
trated in vacuo. The resulting residue was purified by col-
umn chromatography using EtOAc:hexane to afford
imine products.
Summing up, we have presented the first example of
thiolato-bridged dinuclear arene Ru(II) arene complex
that promoted green synthesis of highly desirable imines
obtained from readily available alcohols and amines in
open air as an eco-friendly oxidant. To our knowledge,
this is a convenient and straightforward method for one-
pot synthesis of imines from alcohols and amines. The
catalyst system provides selective imination reactions of
substituted alcohols with various amines with good toler-
ance to reducible functional groups. The complex was
demonstrated as an efficient catalyst with 1 mol% loading
under optimized conditions to afford up to 98% yield.
ACKNOWLEDGMENTS
The authors are thankful to the Science and Engineering
Research Board (SERB) (Scheme No. EMR/2016/004952)
for providing financial support and for Junior Research
Fellow to V. T. We are also thankful to the DST-India
(FIST Programme) for the use of instrumental facilities
at the School of Chemistry, Bharathidasan University,
India.
5 | EXPERIMENTAL SECTION
5.1 | General method for the synthesis of
binuclear p-cymene ruthenium(II)
complex
AUTHOR CONTRIBUTIONS
[RuCl2(η6-p-cymene)]2 (1 equiv) and 1-(5-methylthiazole-
2-yl)-3-phenylthiourea(HL) (2 equiv) were dissolved in
25 ml of benzene and stirred for 6 h. The solution was
reduced to 2 ml, and addition of petroleum ether
(60–80ꢀC) in excess gave a clear yellow solid.
Veerappan Tamilthendral: Investigation; methodol-
ogy; writing-original draft. Rengan Ramesh: Conceptu-
alization; supervision; validation; writing-review and
editing. Jan Grzegorz Malecki: Software.
CONFLICT OF INTEREST
The authors declare no competing financial interest.
5.1.1 | [Ru(η6-p-cymene)(HL)]2Cl2
Yellow solid. Yield: 92%: Anal. Calcd. For
C42H48Cl2N6Ru2S4: C, 48.59; H, 4.66; N, 8.09. Found: C,
48.62; H, 4.60; N, 8.01. H NMR (400 MHz, CDCl3): δ
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are avail-
able in the supplementary material of this article.
1
(ppm) = 11.55 (s, 2H, N–H(phenyl)), 7.58–7.60 (m, 4H,
ArH(ligand)), 7.43–7.47 (m, 4H, ArH(ligand)), 7.36–7.38 (m,
4H, ArH(ligand)), 5.43–5.57 (m, 2H, CH(p-cymene)),
5.30–5.38 (m, 6H, CH(p-cymene)), 2.80 (sept, 2H, CH (CH3)2
(p-cymene)), 2.43 (s, 6H, CH3(p-cymene)), 2.08 (s, 6H, CH3
(ligand)), 1.21–1.26 (m, 12H, CH (CH3)2(p-cymene)). 13C{1H}
NMR (100 MHz, CDCl3): δ (ppm) = 175.35 (C–S), 158.22
(C N), 140.52, 136.32, 129.19, 128.95, 127.89, 125.28
(Ar carbons of ligand), 106.66 and 100.06 (quaternary car-
bons of p-cymene), 86.29, 84.89, 84.73, 84.24 (Ar carbons
of p-cymene), 30.78 (CH of p-cymene), 22.52, 22.17
(2CH3, p-cymene), 18.54 (CH3, p-cymene), 12.53 (CH3 of
ligand). Fourier transform infrared (FT-IR) (cm−1): 2,925
(N–H), 1,642 (C N), 1,594 (C C), 1,261 (N–C S), 1,149
and 910 (C S). UV–vis (CHCl3): λmax (nm) 280, 340, 463.
ORCID
Rengan Ramesh https://orcid.org/0000-0001-8358-9583
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
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How to cite this article: Tamilthendral V,
Ramesh R, Malecki JG. Arene diruthenium(II)-
mediated synthesis of imines from alcohols and
amines under aerobic condition. Appl Organomet
Chem. 2020;e6122. https://doi.org/10.1002/
aoc.6122