Olefin-tethered organoruthenium carbene complexes: Synthesis, X-ray structure and catalytic insights on hydrogenation of esters

Article abstract of DOI:10.1016/j.ica.2018.10.017

A series of Ru(II) complexes encompassing imidazolylidene olefin arm have been designed. The newly synthesized ligands and complexes were fully characterized by ¹H, ¹³C NMR, and elemental analyses. Structural geometry for one of the envoy Ru(II) carbene complexes 3a was confirmed by single-crystal X-ray diffraction studies. The complexes acquired a distorted octahedral geometry. The highly active [Ru(II)-NHC] complex 3b, showed excellent catalytic performance for the hydrogenation of esters in 1,4-dioxane medium. The effects of solvent, base, wingtip substituents, time and catalyst loading were also investigated. The reported catalyst performed exceptionally well for a range of esters and furnishes very good yield of hydrogenated products.

Full text of DOI:10.1016/j.ica.2018.10.017

Accepted Manuscript
Research paper
Olefin-tethered organoruthenium carbene complexes: Synthesis, X-ray struc-
ture and catalytic insights on hydrogenation of esters
Muthukumaran Nirmala, Kaliyappan Murugan, Subbarayan Vijayapritha,
Periasamy Viswanathamurthi, Roberta Bertani, Jan Grzegorz Malecki
PII:
S0020-1693(18)30874-0
https://doi.org/10.1016/j.ica.2018.10.017
ICA 18563
DOI:
Reference:
Inorganica Chimica Acta
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6 June 2018
12 October 2018
12 October 2018
Please cite this article as: M. Nirmala, K. Murugan, S. Vijayapritha, P. Viswanathamurthi, R. Bertani, J.G. Malecki,
Olefin-tethered organoruthenium carbene complexes: Synthesis, X-ray structure and catalytic insights on
hydrogenation of esters, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.10.017
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Olefin-tethered organoruthenium carbene complexes: Synthesis, X-ray
structure and catalytic insights on hydrogenation of esters
Muthukumaran Nirmala,aKaliyappan Murugan,aSubbarayan Vijayapritha,aPeriasamy
Viswanathamurthi,*aRoberta Bertani,bJan Grzegorz Maleckic
aDepartment of Chemistry, Periyar University, Salem-636 011, India.
bDepartment of Industrial Engineering, University of Padova, Via Marzolo 9, 35131 Padova,
Italy.
cDepartment of Crystallography, Silesian University, Szkolna 9, 40-006 Katowice, Poland.
*To whom correspondence should be made, e-mail: viswanathamurthi@gmail.com;
Fax: +91 427 2345124
1

Abstract
A series of Ru(II) complexes encompassing imidazolylidene olefin arm have been designed. The
1
newly synthesized ligands and complexes were fully characterized by H, 13C NMR, and
elemental analyses. Structural geometry for one of the envoy Ru(II) carbene complexes 3a was
confirmed by single-crystal X-ray diffraction studies. The complexes acquired a distorted
octahedral geometry. The highly active [Ru(II)-NHC] complex 3b, showed excellent catalytic
performance for the hydrogenation of esters in 1,4-dioxane medium. The effects of solvent, base,
wingtip substituents, time and catalyst loading were also investigated. The reported catalyst
performed exceptionally well for a range of esters and furnishes very good yield of hydrogenated
products.
Key words:
imidazolylidene olefin arm
N-heterocyclic carbene
Ruthenium(II) complexes
X-ray diffraction
Hydrogenation of esters
2

1. Introduction
The chemistry of N-heterocyclic carbenes (NHCs) has experienced swift expansion over
the past decade [1], mainly after the report of the first stable NHC [2]. To date, a number of
NHCs with various backbones and topologies have been investigated. Besides, the N-functional
group in NHC provides the additional donor site like σ and π-bonding, which offers control over
the coordination environment of the metal as well as unusual behaviors. In recent years, the
olefin group attached N-heterocyclic carbenes have gained escalating attention due to their
essential role in homogeneous catalysis as a π-electron reservoir to stabilize the metal center in
the catalytic cycle [3-5]. In this circumstance, olefin tagged NHC ligands are fairly promising
architectures, due to their coordination and dissociation, which is a key feature in many catalytic
systems [6, 7]. When these two steps are in dynamic equilibrium, or when they operate in
tandem in a catalytic cycle, the hemilability effect [8, 9] may be exploited to improve catalytic
efficacy, even though the life span of some synchronized olefins may be too short for the system
to be detected or excluded [10]. The first olefin functionalized carbene was isolated in 2001, but
its potential in catalysis was not reported [3]. Afterwards, the iridium complexes of N-allyl-
substituted benzimidazol-2-ylidene compounds were synthesized and deliberated its catalytic
activity towards the hydrogenation of carbonyl compounds [11]. Followed by the above, use of
NHC–olefin ligands (NHC = N-heterocyclic carbene) in transition metal systems such as Pd(II),
Ru(II), Ir(I)) [4, 5,11, 18] and NSHC–olefin (NSHC = N,S-heterocyclic carbene) in Pd(II), Cu(I),
Au(III), Rh(I), Ir(I) [12, 13] have been investigated in copious catalytic reactions in which, the
olefin functionalized ruthenium carbene complexes have shown excellent catalytic efficiencies in
catalytic hydrogenation reactions [4]. The combinative use of a robust (carbene) and flimsy
(olefin) M–C bond is anticipated to facilitate the metal to be more flexible to coordinative
changes. Moreover, the ƞ2 σ-donor olefin side arm performs a vital role in the catalytic cycle to
offer the adequate stability to the catalytic transition state.
The reduction of esters to the corresponding alcohols is an important reaction in organic
synthesis as it is often used in natural product synthesis, for the preparation of organic building
blocks and in industry for the manufacture of pharmaceuticals, agrochemicals, flavors, and
fragrances [14a]. Usually, the classical procedures for the reduction of esters was carried out by
3

using stoichiometric amounts of metal hydride reagents (e.g. lithium aluminum hydride, dibutyl
aluminum hydride, or boron hydride), [14b-e] low atom economy and under anhydrous laborious
workup procedures etc. While transition metal based homogeneous systems capable of
hydrogenation of esters are very scarce [14b-e]. In these systems, large amounts of additives,
such as an organic base, [14b] inorganic acids, [14b] salts, [14c, d] zinc, [14e] and fluorinated
alcoholic solvents [14b] were needed to obtain high conversion of esters into alcohols.
Moreover, the catalytic hydrogenations might serve as a relatively green and environmentally
benign substitute since no side products are formed compared to the classic stoichiometric
reduction processes. Therefore, there is a growth of more active well defined homogeneous
catalysts for ester hydrogenation is noteworthy. Recently, ruthenium, iridium, and osmium
complexes have gained a reputed position in the homogeneous hydrogenation of esters under
milder reaction conditions. Among them, the ruthenium catalyzed hydrogenation of esters into
alcohols have been scarcely explored [15, 16].
Based on the above facts, and in continuance of our current research in the utility of
transition metal NHC complexes for diverse organic transformations [17], in this paper, we
account the synthesis and characterization of ruthenium(II) complexes bearing olefin N-
heterocyclic carbene (NHC) ligands with different wingtip substituents in the imidazole ring,
together with their catalytic properties with regard to hydrogenation of esters using KOtBu as the
base. The striking features of these reactions embrace the use of low toxicity organic materials,
admirable atom economy, water is the only by-product, and high selectivity towards the
products.
2. Experimental methods
2.1. General considerations
All experiments were performed using standard Schlenk technique or in a glovebox under
nitrogen atmosphere. All reagents were purchased from Aldrich chemical Co. and used as
received without further purification. The microanalysis of carbon, hydrogen and nitrogen were
carried out using a Vario EL III elemental analyzer. Infrared spectra of the ligands and the metal
complexes were recorded as KBr discs in the range of 4000-400 cm-1 using a Bruker FT-IR
13
spectrophotometer.1H (300.13 MHz) and C (75.47 MHz) NMR spectra were taken in DMSO-
4

d6 or CDCl3 at room temperature with a Bruker AV400 instrument with chemical shifts relative
to tetramethylsilane. Thin-layer chromatography (TLC) was performed on Merck 1.05554
aluminum sheets precoated with silica gel 60 F254, and the spots were visualized with UV light
at 254 nm or under iodine. Column chromatography purifications were performed by Merck
silica gel 60 (0.063-0.200 mm). Melting points were determined in open capillary tubes on a
Technico micro heating table and are uncorrected.
2.2. Synthesis of NHC ligands
The NHC ligands were prepared according to previous literature report [18]. To a
solution of 1-substituted imidazole (18.2 mmol) in 30 mL of acetonitrile, allyl bromide (2.2 g,
18.2 mmol) was added in a Schlenk tube. The mixture was stirred overnight, and then the solvent
was removed under vacuum to give brown oil. The product has been washed with diethyl ether
(2 X 3 mL) and dried under vacuum.
Synthesis of 1-methyl-3-allylimidazolium chloride (1a)
The synthetic procedure of this compound is same as that of above representative procedure,
using 1-methylimidazole to give brown oil 1a. Yield: 91%. Anal. Calcd. for C7H11N2Br (203.07):
1
C,41.40; H,5.46; N,13.79 %. Found: C,41.35; H,5.40; N, 13.84 %. H NMR (300.13 MHz,
CDCl3, δ): 9.32 (s, 1H, NCHN), 7.37 (d, 1H, Imi-H), 6.93 (d, 1H, Imi-H), 6.0-4.51 (allyl group
13
protons), 3.50 (s, 3H, NCH3). C NMR (75.47 MHz, CDCl3, δ): 137.1 (NCN), 131.8 (NCH2-
CH= CH2), 124.4 (NCHCHN), 122.7 (NCHCHN), 120.8 (NCH2-CH=CH2), 51.3 (NCH2-
CH=CH2), 36.2 (NCH3).
Synthesis of 1-ipropyl-3-allylimidazolium chloride (1b)
The synthetic procedure of this compound is same as that of above representative procedure,
using 1-i-propyl imidazole to give brown oil 1b. Yield: 87%. Anal. Calcd. for C9H15N2Br
(231.13): C, 46.77; H, 6.54; N, 12.12 %. Found: C, 46.69; H,6.62; N, 12.20 %. 1H NMR (300.13
MHZ, CDCl3, δ): 8.75 (s, 1H, NCHN), 7.11-7.10 (d,J = 1.5 Hz, 1H, Imi-H), 7.007-7.005 (d, J =
1.2 Hz, 1H, Imi-H), 5.65 (dd, J = 6.2 Hz, J = 2.4 Hz, 1H, -CH=CH2), 5.62 (dd, J = 6.7 Hz, J = 4
Hz, 1H, -CH=CHHcis), 5.58 (dd, J = 4 Hz, J = 6.1 Hz, 1H, -CH=CHHtrans), 4.99-4.98 (d, J = 4
Hz, 2H, CH2).13C NMR (75.47 MHz, CDCl3, δ): 137.1 (NCN), 132.3 (NCH2-CH=CH2), 124.2
5

(NCHCHN), 122.8 (NCHCHN), 120.6 (NCH2-CH=CH2), 51.1 (NCH2-CH=CH2), 58.9
CH(CH3)2), 28.1 CH(CH3)2).
Synthesis of 1-tbutyl-3-allylimidazolium chloride (1c)
The synthetic procedure of this compound is same as that of above representative
procedure, using 1-t-butyl imidazole to give brown oil 1c. Yield: 85%. Anal. Calcd. for
1
C10H17N2Br (245.15): C, 48.99; H, 6.99; N, 11.43 %. Found: C, 48.81; H, 6.65; N, 11.35 %. H
NMR (300.13 MHZ, CDCl3, δ): 9.56 (s, 1H, NCHN), 7.33 (d, 1H, Imi-H), 7.13(d, 1H, Imi-H),
13
5.5-4.2 (allyl group protons), 1.03 (s, 9H, C(CH3)3). C NMR (75.47 MHz, CDCl3, δ): 133.5
(NCN), 128.3 (NCH2-CH=CH2), 120.9 (NCHCHN), 119.5 (NCHCHN), 117.3 (NCH2-
CH=CH2), 51.1 (NCH2-CH=CH2), 50.2 N-C(CH3)3), 21.62 C(CH3)3).
2.3. Synthetic procedure for ruthenium(II) NHC complexes
1-substituted 3-allylimidazolium chloride (2 mmol) was dissolved in 25 mL of
dichloromethane and transferred into a Schlenk vessel. Excess Silver(I) oxide (1.5 mmol) was
added, and the mixture was stirred for 24 h at room temperature under argon atmosphere [19].
Then the unreacted Ag2O was filtered through a plug of Celite. The resulting solution was
filtered through celite, solvent was evaporated in vaccuo, washed with diethylether (2 ml x 2)
and dried in vacuo. [RuBr2(PPh3)3] (1 g, 1 mmol) was taken up in 5 mL of dichloromethane and
added to a solution of Ag complex in 10 mL of CH2Cl2. The mixture was stirred overnight at
room temperature. After filtration in air, the solvent was removed in vacuum to give the product.
The final compound is stable in air. Single crystals of the compound 3a were achieved by slow
evaporation of a concentrated solution in a mixture of dichloromethane and acetone.
Compound 3a (R = Me)
The synthetic procedure of this compound is same as that of above representative
procedure, using 1-methyl-3-allylimidazolium chloride to give complex 3a. Yield: 79%. M.pt.:
243-245 ⁰C. Anal. Calcd. for C14H20N4Br2Ru (505.21): C, 33.28; H, 3.99; N, 11.09 %. Found: C,
33.33; H 3.93; N, 11.02 %.1H NMR (300.13 MHZ, CDCl3, δ): 6.9 (d, 1H Imi-H); 6.8 (d, 1H Imi-
13
H); 5.8-3.9 (allyl group protons), 3.7 (s, 3H, NCH3). C NMR (75.47 MHz, CDCl3, δ): 198.04
6

(NCN), 124.18 (Imi-C), 122.95 (Imi-C), 108.98 (NCH2-CH=CH2), 102.59 (NCH2-CH=CH2),
48.25 (NCH2-CH=CH2), 36.08 (NCH3).
Compound 3b (R = ipropyl)
The synthetic procedure of this compound is same as that of above representative
procedure, using 1-(i-propyl-3-allylimidazolium chloride to give complex 3b. Yield: 82 %.
M.pt.: 237-240 ⁰C. Anal. Calcd. for C18H28N4Br2Ru (561.31): C, 38.52; H, 5.03; N, 9.98 %.
1
Found: C, 38.46; H, 5.10; N, 9.90 %. H NMR (300.13 MHZ, CDCl3, δ): 6.95 (d, 1H, Imi-H),
6.8 (d, 1H, Imi-H), 5.6-4.2 (allyl group protons), 3.7 (m, 1H, CH(CH3)2), 1.21 (d, 6H,
CH(CH3)2). 13C NMR (75.47 MHz, CDCl3, δ): 198.73 (NCN), 126.39 (NCHCHN), 119.21
(NCHCHN), 109.29 (NCH2-CH=CH2), 102.47 (NCH2-CH=CH2), 63.87 (NCH2-CH=CH2), 56.3
NCH(CH3)2), 26.2 CH(CH3)2).
Compound 3c (R = tbutyl)
The synthetic procedure of this compound is same as that of above representative
procedure, using 1-(t-butyl)-3-allylimidazolium chloride to give complex 3c. Yield: 81 %. M.pt.:
223-225 ⁰C. Anal. Calcd. for C20H32N4Br2Ru (589.37): C, 40.76; H, 5.47; N, 9.51 %. Found: C,
40.69; H, 5.40; N, 9.42 %.1H NMR (300.13 MHZ, CDCl3, δ): 6.9 (d, 1H, Imi-H), 6.8 (d, 1H,
Imi-H), 5.9-3.5 (allyl group protons), 1.14 (s, 9H, NC(CH3)3). 13C NMR (75.47 MHz, CDCl3, δ):
198.9 (NCN), 125.47 (NCHCHN), 124.37 (NCHCHN), 109.29 (NCH2-CH=CH2), 102.47
(NCH2-CH=CH2), 66.3 (NCH2-CH=CH2), 49.1 NC(CH3)3), 23.5 C(CH3)3).
2.4. Typical procedure for the catalytic hydrogenation
To a mixture of catalyst (0.01 mmol), KOtBu (10 mol %), and 1,4-dioxane (4.0 mL) in a
Parr high-pressure reactor was added the ester (1.0 mmol). The dark red solution was purged
with H2 and stirred under 400 psi of H2 at 105 °C for 8 h. Products isolation were performed via
column chromatography using silica gel as stationary phase and n-pentane/ethylacetate or n-
pentane/isopropanol mixture as eluent. The products were confirmed by 1H NMR.
2.5. X-ray crystallography
7

Crystal of complex 3a was mounted on glass fibers used for data collection. Crystal data
were collected at 295 K using a Gemini Ultra Oxford Diffraction automatic diffractometer.
Graphite monochromated Mo-Ka radiation (λ = 0.71073 Å) was used throughout. The absorption
corrections were performed by the multi-scan method. Corrections were made for Lorentz and
polarization effects. The structures were solved by direct methods using the program SHELXS
[20]. Refinement and all further calculations were carried out using SHELXL [20]. The H atoms
were included in calculated positions and treated as riding atoms using the SHELXL default
parameters. The non-hydrogen atoms were refined anisotropically using weighted full-matrix
least squares on F2. Atomic scattering factors were incorporated into the computer programs.
3. Results and discussion
3.1. Synthesis
Scheme 1. General preparation of olefin-tethered NHC ligands (1a-c)
Scheme 2. General synthesis of [Ru-NHC] complexes (3a-c)
Olefin-tethered NHC ligands (1a-c) were prepared by the reaction of allyl bromide with
various 1-substituted imidazoles in presence of acetonitrile medium under stirring at overnight
8

(Scheme 1). After evaporation of the solvent under vacuum, the oily products were washed with
diethyl ether and with dichloromethane to provide the target compounds as brown oils in very
good yield. The synthetic route for the preparation of Ag(I)–NHC complexes 2a-c illustrated in
this work is shown in Scheme 2. The Schlenk tube used for the synthesis of silver complexes
was draped with aluminum foil to prohibit the light. Silver complexes 2a-c were prepared by the
reaction of excess silver(I) oxide with the corresponding imidazolium salts 1a-c in
dichloromethane overnight [19]. The resultant black suspension was filtered through a pad of
Celite and the solvent was evaporated to dryness under reduced pressure. The residue was
washed with diethyl ether and afforded the corresponding silver complexes as thick yellow oils
in very good yields. The synthesis of ruthenium NHC complexes was attained by
transmetallation process. The corresponding Ag(I)–NHC complexes 2a-c were reacted with
dichloromethane solution of [RuBr2(PPh3)3] for 24 h. This reaction yielded a black AgBr
precipitate, which was filtered off using a pad of Celite, followed by the solvent was reduced
under vaccum. Diethyl ether was added to precipitate the complexes as pale yellow solids, which
were washed with an additional amount of diethyl ether and crystallized from
dichloromethane/acetone solvent mixture to render the complexes (3a-c) in good yields. They
are greatly soluble in CH2Cl2, CHCl3, THF and DMSO. Nonetheless, they are sparingly soluble
in acetonitrile, but insoluble in hexane, pentane and Et2O. All the complexes (3a-c) were
1
completely characterized by H, 13C NMR and elemental analyses. The CHN data of the
ruthenium-NHC complexes are in good concord with the proposed molecular formulae.
3.2. Spectroscopic description
The 1H NMR spectra of the ligands (1a-c) and complexes (3a-c) have shown the signals
in the expected region (Fig. S1-S5, ESI). As estimated the NHC hydrogen atom of the
imidazolium salts gives rise to a singlet at 9.5-8.7 ppm in the ligands. The generation of free
carbene and subsequent formation of the [Ru-NHC] complexes were unequivocally confirmed
1
by the nonappearance of the H NMR resonances of imidazolium (NCHN) protons. The allyl
protons of imidazolium ligands resonated at the region 6.0-4.0 ppm whereas in the ruthenium
complexes the allylic protons shifted to high field region. The significant high field shift of the
resonances of the allyl protons particularly for the CH=CH2 protons indicates the coordination of
the allylic double bond with the ruthenium metal. The shift in resonance is similar to an
9

observation previously described for N-allyl double bond coordination to metal centers [21, 11,
22]. In addition, a singlet appeared around 3.5-3.7 ppm for compounds 1a and 3a corresponding
to NCH3 group protons. Furthermore, the spectra of compounds 1b and 3b showed peaks in the
region ~1.2 ppm region for isopropyl group. Moreover, a clear singlet appeared around 1.1 ppm
for the compounds (1c & 3c) corresponding to tertiary butyl protons.
13
The C NMR spectra have shown the expected signals in the appropriate regions (Fig.
S6-S11, ESI). The spectra of complexes have shown the carbenic carbon signal in the region
198.04-198.90 ppm. For the ligand and complexes the imi-C appeared in the region 126.3-119.2
1
13
ppm. The disappearance of the H NMR signals of NCHC and the downfield shift of C NMR
signals of NCN carbene carbon at ca. 198.04-198.90 ppm in the 1H and 13C NMR spectra of [Ru-
NHC] complexes are indicative of the Ru-Ccarbene bond formation [23]. The downfield shift of the
resonances for the Ru-Ccarbene carbon is due to the coordination of the allylic C=C double bond
with metal as a σ donor/π acceptor, which weakens the ruthenium carbene interactions [21]. The
high field shift of resonances of allylic carbon of the complexes when compared to ligands
further supports the coordination of the allylic C=C bond with ruthenium metal. Moreover, the
signal for methyl, isopropyl and tertiary butyl groups are also appeared in the expected region in
the spectra of ligands and complexes.
3.3. X-ray crystal structure description of complex 3a
Even though the analytical and spectral data gives some idea about the molecular
formulae of the complexes, they do not indicate the exact coordination of bis(carbene) units in
them. To gain additional insight into the coordination chemistry and the structural parameters of
the complexes, single crystals of one of the complexes (3a) were grown and characterized by X-
ray diffraction analysis. Single crystals of 3a apt for X-ray diffraction studies were acquired by
slow evaporation of a concentrated solution in a mixture of dichloromethane and acetone. No
single crystals could be obtained for the other complexes, but the resemblance in their
spectroscopic uniqueness implies that 3a is a good structural replica for all three complexes.
They invariably have shown a common octahedral Ru(II) structure with the NHC-olefin ligand
with the anticipated bis-η2 coordinated olefin for 3a. In 3a, one carbene ligand was trans to
bromide (Ru(1)-C(8) = 2.045(4)). The Ru-CNHC bond lengths (2.033(4)- 2.045(4) Å) are
inconspicuous for ruthenium-carbene bonds [24, 25]. Chelation is inveterate for all donor groups,
10

including the η2-coordination mode of the olefin in 3a as construed from NMR spectroscopy.
The crystal data and structure refinement parameters for complex 3a were collected in Table 1
and the selected bond lengths and bond angels are depicted in table 2. The ORTEP view, packing
arrangement of atoms in unit cell and space fill model of the complex 3a are given in Fig. 1-3.
The average olefine–ruthenium bond distances of Ru(1)-C(12) and Ru(1)-C(13) is 2.308(5) Å
was slightly higher than Ru(1)- C(5) and Ru(1)- C(6) (average) (2.176 Å) bond distances and the
Ru(1)- Br(1) and Ru(1)- Br(2) bond distance are 2.5915(6) Å and 2.6404(6) Å respectively [4].
3.4. Catalytic hydrogenation of esters
For our initial catalytic exploration, we chose the reduction of methyl benzoate in the
presence of 1 mol % of catalyst 3a-3c at 30 bar H2 and 105°C (Table 3). Catalyst 3b was attested
to be the most efficient complex for the hydrogenation of methyl benzoate in terms of yield and
selectivity. To our pleasure, the preferred reaction was completed after 8 h giving a very high
yield (96%) of benzyl alcohol (Table 3, entry 8). Optimization of key reaction parameters such
as temperature, solvent, base and reaction time showed the better optimization condition for
hydrogenation of esters. In order to assess the crucial role of base in promoting the generation of
metal intermediates all along the catalytic cycle, various bases such as K2CO3, NaHCO3, KHCO3,
NaOH, KOH and KOtBu were screened. Among the bases employed, KOtBu was found to be
the best and led to higher yield for this reaction (Table 4, entry 6). At this point it is important to
note that for most active ruthenium catalysts a substantial amount of base is required to form the
catalytically active species.
It is well known that the solvent can have thoughtful effect on the hydrogenation
reaction. We are paying attention in exploring the solvent-dependent differences in the behavior
of catalysts on carrying out the model reaction in the most recurrently used solvents such as,
toluene, 1,4-dioxane, xylene, acetonitrile, DMF, DMSO, EtOH and H2O (Table 4). Aromatic
hydrocarbon solvents such as toluene and p-xylene (Table 4, entries 7, 12) were afford
moderately good yield than polar aprotic (DMF, DMSO; Table 4, entries 11, 13), whereas 1,4-
dioxane endow with excellent yield in 8 h (Table 4, entry 6), while water were unsuccessful
(Table 4, entries 14). Delightfully, decreasing the pressure from 30 bar H2 (Table 1, entry 8,
yield 96%) to10 bar H2 a good yield (79%) of the desired product (Table 1, entry 9) has been
achieved. The dihydrogen solubility of the solvents has been taken in to account [26]. However,
11

nature of solvents, their polarity and their ability to dissolve reactants and catalyst have also
played a vital role in the efficiency of the catalytic system and in some cases, which overcome
their solubility of dihydrogen. As expected, protic solvents except water, have shown to form a
good catalytic system. This may due to the poor solubility of catalyst in aqueous medium.
To prove the general applicability of the new catalyst system, diverse esters were reduced to the
corresponding alcohols using 1 mol % 3b in 4 mL 1,4-dioxane at 30 bar H2 (Table 5). Excellent
yield and conversion was obtained for ethyl benzoate, giving benzyl alcohol (Table 5, entry 1).
The model substrate with its fruity flavor was hydrogenated gives the benzyl alcohol with an
excellent yield of 96% (Table 5, entry 2). Short- chain, aliphatic linear compound ethyl acetate
reduced towards the corresponding alcohol with outstanding yield of 99 % (Table 5, entry 4).
The recent catalysts explored for the hydrogenation of benzyl benzoate (benzyl alcohol, isolated
yield 86 %) and propyl propionate (propan-1-ol, isolated yield 68 %) shows moderately lesser
yield when compared to our present catalytic system (benzyl alcohol, isolated yield 92 % and
propan-1-ol, isolated yield 77 %) [27]. However, the hydrogenation of butyl benzoate gave a
good yield of 89 % (Table 5, entry 3). Hydrogenation of benzyl benzoate led to high yield of
benzyl alcohol (Table 5, entry 5). The catalyst 3b hydrogenate bromo-substituted methyl
benzoate, affording the (4-bromophenyl)methanol in 96 % yield without dehalogenation (Table
5, entry 6). Even the γ-butyrolactone hydrogenated competitively offers the desired 1,4-diol was
obtained in 86 % yield (Table 5, entry 7). Interestingly, the isolated yield of butane-1,4-diol
under the present synthetic methodology is more convenient and most efficient catalytic route
(isolated 86 %) than the catalytic route demonstrated for the preparation of butane-1,4-diol
(isolated 73 %) by Osamu et. al. [16]. Methyl nicotinate was transformed into the corresponding
pyridin-3-ylmethanol in excellent yield (Table 5, entry 8). However, the methyl cyclohexane
carboxylate hydrogenated well and gives the cyclohexylmethanol in 93 % (Table 5, entry 9). The
aliphatic linear compound propyl propionate was reduced to the corresponding propan-1-ol in 77
% yield under optimized reaction condition (Table 5, entry 10). After the catalytic reaction, the
catalyst was recovered by the addition of CH2Cl2/diethylether mixture. Then, the catalyst was
thoroughly washed with diethyl ether and its NMR spectrum was recorded (Figure S12, see
supporting information). The NMR spectrum shows that, the resonance corresponding to the
coordinated olefin was disappeared in a very short period of time (within five minutes). On the
other hand, new peaks corresponding to –CH2 and –CH3 protons were appeared in the NMR
12

spectrum of recovered catalyst. Hence, complex 3b, comprising an olefin-tethered NHC ligand,
is not stable under hydrogenation conditions, and the olefin wingtip group is hydrogenated at an
early stage of the reaction. While this intramolecular olefin hydrogenation may generate
coordinatively unsaturated species as active catalyst, it also transforms the NHC into a
monodentate ligand. [4a]. The synthetic upshot of this finding and the detailed mechanism of the
reaction are being explored.
4. Conclusions
Efforts to provoke imidazolin-2-ylidenes and their ruthenium(II) complexes by
transmetallation method was demonstrated. The resultant complexes exhibit high catalytic
activity in the hydrogenation of esters. The complex 3a was characterized by single crystal X-ray
1
diffraction analysis, and all complexes were completely characterized by H and 13C NMR
spectroscopy. Under optimized conditions, diverse esters converted into their corresponding
alcohols in good to outstanding yields. All over, this study expounds that ruthenium complexes
of olefin-functionalized NHCs are active catalysts for the hydrogenation of ester reaction. The
dilatation to other donor functionalities, conspicuously olefin-tethered ruthenium(II) complexes
are a possible avenue for further catalyst optimization. Research efforts at such more decisive
ruthenium(II) NHC complexes are under evolution in our laboratory.
Acknowledgment
The authors express their sincere thanks to Science and Engineering Research Board
(SERB), New Delhi, India for financial support (Grant No. EMR/2015/001526). One of the
authors (KM) thanks SERB for the award of fellowship.
Appendix A. Supplementary data
1H and 13C NMR spectra of NHC ligands and their ruthenium complexes. CCDC reference
number 1469200 for complex 3a. This data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at http://
13

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17

Br2
Br1
N4
C13
C12
Ru1
C1
C6
C8
C5
N3
N1
N2
Figure 1. ORTEP representation of the X-ray crystal structure of [Ru-NHC] complex 3a.
Figure 2. Packing diagram of [Ru-NHC] complex 3a.
18

Figure 3. Space fill modeling of [Ru-NHC] complex 3a.
19

Table 1. Crystal data and structure refinement parameters of 3a
Parameters
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
3a
C14 H20 Br2 N4 Ru
505.23
295
Monoclinic
P 21/c
11.7563(4)
8.1471(3)
17.5324(7)
90
β/°
γ/°
90.671(4)
90
1679.12(4)
4
1.999
5.692
Volume (Å3)
Z
ρcalc/mg mm-3
Absorption coefficient (mm-1)
F(000)
984
Data collected
Unique data
Rint
8485
2959
0.0417
1.046
0.0358
0.0847
0.0470
0.0847
GOF on F2
R1 (I>2(I))
wR2 (I>2(I))
R1 values (all data)
wR2 values (all data)
20

Table 2. Selected structural parameters of 3a
Interatomic distances (Å)
Ru(1)- Br(1)
Ru(1)- Br(2)
Ru(1)- C(1)
Ru(1)- C(5)
Ru(1)-C(6)
Ru(1)-C(8)
Ru(1)-C(12)
Ru(1)-C(13)
2.5915(6)
2.6404(6)
2.033(4)
2.193(4)
2.159(4)
2.045(4)
2.305(5)
2.313(5)
Bond angles(ᵒ)
Br(1)-Ru(1)-Br(2)
C(1)-Ru(1)-Br(1)
C(1)-Ru(1)-Br(2)
C(1)-Ru(1)-C(5)
C(1)-Ru(1)-C(6)
C(1)-Ru(1)-C(8)
C(1)-Ru(1)-C(13)
C(5)-Ru(1)- Br(1)
C(5)-Ru(1)-Br(2)
C(5)-Ru(1)-C(12)
C(5)-Ru(1)-C(1)
C(6)-Ru(1)-Br(1)
C(6)-Ru(1)-Br(2)
C(6)-Ru(1)-C(5)
C(6)-Ru(1)-C(12)
C(6)-Ru(1)-C(13)
C(8)-Ru(1)-Br(1)
C(8)-Ru(1)-Br(2)
C(8)-Ru(1)-C(5)
C(8)-Ru(1)-C(6)
C(8)-Ru(1)-C(12)
C(8)-Ru(1)-C(13)
C(12)-Ru(1)-Br(1)
C(12)-Ru(1)-Br(2)
C(12)-Ru(1)-C(13)
C(13)-Ru(1)-Br(1)
C(13)-Ru(1)-Br(2)
N(3)-C(1)-Ru(1)
87.43(2)
98.52(12)
84.08(12)
78.08(17)
87.09(19)
93.18(17)
162.66(19)
158.12(13)
113.41(13)
84.05(19)
107.2(2)
164.73(16)
78.99(16)
37.01(19)
83.5(2)
87.9(2)
79.12(12)
165.74(12)
79.50(17)
114.9(2)
75.36(18)
103.97(19)
95.00(14)
110.86(13)
34.12(19)
82.52(15)
78.66(14)
117.3(3)
21

Table 3. Influence of wingtip substituent and catalyst loading on the catalytic activity of [Ru-
NHC] complexes
Amount
of
catalyst
p
[bar]
Entry Catalyst
Wingtip (R) Time(h)
Yield(%)a
(mol%)
1
2
3
4
5
3a
3b
3c
3a
3b
3c
3a
3b
3b
3c
-
0.5
0.5
0.5
1
1
1
1
1
1
1
-
Me
iPr
tBu
Me
iPr
8
8
8
8
8
8
8
8
8
8
24
50
50
50
50
50
50
30
30
10
30
30
75
86
79
83
98
88
82
96
79
86
n.r
6
tBu
Me
iPr
7
8c
9
10
11
iPr
tBu
-
a
Reaction conditions: catalyst (0.5-1 mol %), KOtBu (10 mol %), and 1,4-dioxane (4.0 mL),
ester (1.0 mmol)
b Isolated yield after column chromatography.
e
Better optimization condition.
22

Table 4. Evaluation of conditions for hydrogenation of ester reaction using 3ba
Entry
1
Base
K2CO3
NaHCO3
KHCO3
NaOH
KOH
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
-
Solvent
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
Toluene
Acetonitrile
1,4-dioxane
C2H5OH
DMF
p-xylene
Temp (˚C)
105
105
105
105
105
105
110
82
Time (h)
Yield (%)b
43
8
8
8
8
8
8
12
16
18
12
12
12
12
12
24
24
2
3
4
28
39
61
69
96
78
69
95
54
52
69
45
n.r
n.r
n.r
5
6e
7
8
9
101
78
10
11
12
13
14
15c
16d
150
138
140
100
105
105
DMSO
H2O
1,4-dioxane
1,4-dioxane
KOtBu
a Reaction conditions: catalyst (0.01 mmol), KOtBu (10 mol %), and solvent (4.0 mL) , ester (1.0
mmol)
b Isolated yield after column chromatography.
c The reaction was carried out without base
d The reaction was carried out without catalyst.
e
Better optimization condition.
23

Table 5. Catalytic hydrogenation of estersa
S. No
Reactant
Product
Yield (5)b %
1c
94
2c
96
3c
4
89
>99
92
-
-
5
6
7
8
-
-
-
96
86
95
9
-
-
93
77
10
a Reaction conditions: Substrate (1 mol %), catalyst (1 mol %), KOtBu (10 mol %), in
1,4-dioxane were heated to 105ºC for 8 h under 30 bar H2.
24

b Isolated yield after column chromatography.
c Yields of methanol and ethanol are not reported.
25

Graphical abstract
Olefin-tethered organoruthenium carbene complexes: Synthesis, X-ray
structure and catalytic insights on hydrogenation of esters
Muthukumaran Nirmala,a Kaliyappan Murugan,a Subbarayan Vijayapritha,a Periasamy
Viswanathamurthi,*a Roberta Bertani,b Jan Grzegorz Maleckic
aDepartment of Chemistry, Periyar University, Salem-636 011, India.
bDepartment of Industrial Engineering, University of Padova, Via Marzolo 9, 35131 Padova,
Italy.
cDepartment of Crystallography, Silesian University, Szkolna 9, 40-006 Katowice, Poland.
26

Research Highlights
 New olefin-tethered organoruthenium carbene complexes have been synthesized and
epitomized.
 X-ray studies confirmed an octahedral geometry of the new complexes.

The newly synthesized complexes catalyze the hydrogenation of esters efficiently.
27