Aryl appended neutral and cationic half-sandwich ruthenium(II)-NHC complexes: Synthesis, characterisation and catalytic applications

Article abstract of DOI:10.1039/c7nj02822a

Half-sandwich ruthenium(ii) complexes 1-6 bearing imidazolylidene and pyridyl-imidazolylidene ligands have been synthesised in good yields and were characterised on the basis of spectral and analytical evidence. In addition, the structures of the complexes 1-4 were unambiguously established through single crystal X-ray analysis. Transmetalation of the ligands followed by complexation with ruthenium precursors yielded the air and moisture stable complexes. The crystal structures of these complexes exhibited piano-stool geometries with η⁶-coordination of the p-cymene or hexamethylbenzene moieties. These complexes exhibited catalytic activity in the transfer hydrogenation of carbonyls in an alkaline medium using 2-propanol as the hydrogen source. The effect of variations in the catalyst structure on the transfer hydrogenation and stability was investigated in detail, and theoretical calculations were employed to understand the mechanism of the catalytic activity. The neutral ruthenium-NHC complexes 1 and 2 showed the efficiency of ca. 100% at a catalyst loading of ca. 2 mol% within 2 h of the reaction in 2-propanol, whereas quantitative yields were obtained in the presence of cationic ruthenium-NHC complexes 3-6 within 1 h at a low catalyst loading of ca. 0.5 mol%, thereby demonstrating their robustness for the transfer hydrogenation of the aromatic ketones.

Full text of DOI:10.1039/c7nj02822a

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: D. Ramaiah, M.
Viji, N. Tyagi and N. Naithani, New J. Chem., 2017, DOI: 10.1039/C7NJ02822A.
This is an Accepted Manuscript, which has been through the
Volume 40 Number
1 January 2016 Pages 1–846
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
Accepted Manuscripts are published online shortly after
New Journal of Chemistry
www.rsc.org/njc
A journal for new directions in chemistry
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Please do not adjust margins
New Journal of Chemistry
Page 1 of 11
View Article Online
DOI: 10.1039/C7NJ02822A
Journal Name
ARTICLE
Aryl Appended Neutral and Cationic Half-sandwich Ruthenium(II)-
NHC Complexes: Synthesis, Characterisation and Catalytic
Applications
Received 00th January 20xx,
Accepted 00th January 20xx
Mambattakkara Viji,a,b Nidhi Tyagi,a Neeraj Naithanic and Danaboyina Ramaiah*d
DOI: 10.1039/x0xx00000x
www.rsc.org/
Half-sandwich ruthenium(II) complexes 1-6 bearing imidazolylidene and pyridyl-imidazolylidene ligands have been
synthesised in good yields and were characterised on the basis of spectral and analytical evidence. In addition, the
structures of the complexes 1-4 were unambiguously established through single crystal X-ray analysis. Transmetalation of
the ligands followed by the complexation with ruthenium precursors yielded the air and moisture stable complexes. The
crystal structures of these complexes exhibited piano-stool geometries with ȵ6-coordination of the p-cymene or
hexamethylbenzene moieties. These complexes exhibited catalytic activity in the transfer hydrogenation of carbonyls in
alkaline medium using 2-propanol as the hydrogen source. The effect of variations in the catalyst structure on the transfer
hydrogenation and stability was investigated in detail as well as theoretical calculations were employed to understand the
mechanism of the catalytic activity. The neutral ruthenium-NHC complexes 1 and 2 showed the efficiency of ca. 100% at a
catalyst loading of ca. 2 mol% within 2 h of reaction in 2-propanol, whereas quantitative yields were obtained in presence
of the cationic ruthenium-NHC complexes 3-6 within 1 h at a low catalyst loading of ca. 0.5 mol%, thereby demonstrating
their robustness for the transfer hydrogenation of the aromatic ketones.
complexes bearing NHCs have exhibited high catalytic efficacy
for the reduction of ketones, aldehydes, imines, nitro
aromatics, alkenes, and carbon dioxide.7,8
Introduction
During the past decade, the development of ligands bearing
donors other than phosphorus has been an active area of
interest in homogeneous catalysis, and organic synthesis. Of
the various ligands reported, N-heterocyclic carbenes (NHCs)
Of the various reduction reactions, the transfer
hydrogenation is preferred in the large-scale industry due to
the advantages such as the reduced waste generation and
energy consumption.9 In this context, Noyori and Baratta and
co-workers have developed Ru(II) and Os(II) complexes as
efficient catalysts for the transfer hydrogenation of the
ketones and imines.10,11 Though, a good number of examples
of the metal-NHC-complexes have been reported for the
transfer hydrogenation reactions,12 the development of new
catalysts with high stability is quite challenging in recent years.
Herein, we report the synthesis and catalytic activity of a novel
have emerged as
a flexible class when compared to
phosphorous containing ligands.1 Furthermore, NHCs bearing
transition metal complexes have been well studied for their
use as catalysts in C=O, C=C and C=N reduction reactions and
also in olefin metathesis.2-4 Of the reported examples, the
metal complexes based on iridium, rhodium, and ruthenium
complexes having various ligands especially NHC ligands have
been found to be excellent catalysts for the reactions like
hydrogenation, dehydrogenation, transfer hydrogenation (TH),
and hydrosilylation.5,6 Among these examples, the ruthenium
series of neutral and cationic ruthenium(II)-NHC complexes 1-6
appended with anthracene and phenyl moieties. Uniquely,
these systems showed significantly increased air and moisture
stability due to the coordination of Ru(II) with NHC ligands.
These systems acted as efficient and better catalysts for the
reduction of the aromatic ketones, substituted with electron
donor and acceptor functional groups, when compared to the
literature known complexes under similar conditions.
a.Photosciences and Photonics, Chemical Sciences and Technology Division, CSIR-
National Institute for Interdisciplinary Science and Technology, Trivandrum
695 019, India.
b.
Academy of Scientific and Innovative Research (AcSIR), CSIR-NIIST Campus,
Trivandrum 695 019, India.
c. Analytical and Spectroscopy Division, Vikram Sarabhai Space Centre, Trivandrum
695 022, India
Results and discussion
d. CSIR-North East Institute of Science and Technology, Jorhat 785 006, India
E-mail: rama@neist.res.in or d.ramaiah@gmail.com
Synthesis and characterisation of the complexes
Electronic Supplementary Information (ESI) available: Figures S1-S24, Tables S1-S4
and Scheme S1 shows the characterization data and catalytic studies of the
complexes. See DOI: 10.1039/x0xx00000x
The ligands L1 and L2, substituted anthracene and phenyl
were prepared as per the reported procedure.13,14 The
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 2 of 11
View Article Online
DOI: 10.1039/C7NJ02822A
ARTICLE
Journal Name
subsequent reactions of L1 and L2 with Ag2O followed by
To understand the redox and thermal stability properties
, we have carried out cyclic voltammetry,
in ca. 39 square wave measurements and thermogravimetric analysis
and 40% yields, respectively (Scheme 1). The cationic (TGA) under different conditions. The oxidation potentials vs
complexes
, on the other hand, were synthesised in a three- Ag/Ag+ measured for all complexes, which showed one
complexation with [Ru(ȵ6-p-cymene)Cl2]2 resulted in the of the complexes
1-6
synthesis of dinuclear ruthenium complexes
1 and 2
3-6
step route starting from the imidazole derivative15 as shown in electron oxidation of the redox couple Ru(II)/Ru(III) as
Scheme 2. In the first step, the synthesis of the imidazolium reported in the literature for the similar systems.16 These
salts L3, L4 and L5 was achieved by the reaction of 2-(1H- complexes exhibited quasi-reversible electrochemical wave
imidazol-1-yl)pyridine with the corresponding aryl halide. and the potentials are found to be +1.13, +1.14, +1.48, +1.28,
Subsequently, the complexes
deprotonation of the ligand precursors with Ag2O followed by S13, ESI). In the thermogravimetric analysis of the complexes
complexation with the ruthenium precursors, such as [Ru(ȵ6-p- and
, we observed a two step decomposition pattern in the
3-6 were synthesised by the +1.47, and +1.49 V, respectively, for the complexes 1-6 (Fig.
1
2
cymene)Cl2]2 and [Ru(hexamethylbenzene)Cl2]2. These temperature range ca. 20-900 oC, and the weight loss was
complexes were isolated in moderate yields as their found to be ca. 66 and 59%, respectively (Fig. S14, ESI).
hexafluorophosphate salts after treatment with ammonium Similarly, the thermograms of the complexes 3-6 under similar
hexafluorophosphate.
conditions showed a weight loss of ca. 62, 72, 70, and 85%,
respectively. These observations can be attributed to the loss
of chloride ions first followed by the decomposition of the
aromatic moieties present in these complexes.
N
N
Br
CH3CN
Reflux
Br
CH3CN
Reflux
Single crystal X-ray analysis of the complexes 1-4
Br
Br
X-ray quality crystals of the complexes were grown in
triclinic and monoclinic fashion through slow evaporation of a
mixture (9:1) of chloroform and methanol. For example, in the
N
N
N
N
2Br
N
2Br
N
N
N
case of complexes
1 and 2, the coordination geometry around
L1
L2
both the Ru(II) centres is best described as pseudo-octahedral
with arene ring having three coordination sites in a η6-fashion,
carbene and two chloride ions occupying the remaining
coordination sites. Fig. 1 shows the ORTEP diagrams of the
Ag2O/DCM, RT, 8 h
Ag2O/DCM, RT, 8 h
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
N
N
Cl
Cl
N
Ru
Cl
complexes 1-4 along with the atom numbering scheme and
N
Cl
N
Ru
Cl
Cl
Cl
Ru
Cl
their selected crystallographic data, bond lengths and bond
angles are summarised in Tables S1-S2 (ESI).
Ru
N
N
N
1
2
Scheme 1. Synthesis of the neutral ruthenium-NHC complexes 1 and 2.
C)
A)
Cl1
N3
C1
Ru1
All the ligands and their metal complexes were
characterised on the basis of analytical and spectral evidence
(Figs. S1-S12, ESI). For example, the singlet at δ 8.87 ppm and
9.26 ppm corresponding to the imidazolium proton in L1 and L2
B)
was disappeared upon the formation of complexes 1 and 2,
respectively. In their 13C NMR spectrum, a new signal due to
D)
the C2 (carbene carbon) of the imidazol-2-ylidene units
Cl1
Ru1
C12
N4
appeared at δ 173.16 and 173.97 ppm, respectively for
1 and
2. Similarly, the ligands were characterised having downfield
signals for the strongly de-shielded C2 protons of the
imidazolium moiety, which appeared at δ 9.37, 10.32 and
10.08 ppm for L3, L4 and L5, respectively. The ruthenium
complexes 3-6 were furthermore characterised by the
disappearance of carbenic protons followed by the appearance
of downfield shift for the carbenic carbons at δ 186.14, 190.56,
186.22 and 185.88 ppm, respectively. In addition, the
Fig. 1. ORTEP diagrams of the complexes 1-4 at 50% probability level. A) 1: [Ru2(L1)(η6-
p-cymene)2Cl4], B) 2: [Ru2(L2)( η6-p-cymene)2Cl4], C) 3: [Ru(L3)(η6-p-cymene)Cl]+, and D)
4: [Ru(L3)(η6-hexamethylbenzene)Cl]+. (Hydrogen atoms, solvents and counter ions
were omitted for clarity.
Both the complexes
1 and 2 showed coordinatively
saturated (18e-) “three-legged piano-stool” geometry with the
π-bonded η6-arene ring forming the seat and a carbene donor
atom of L1/L2 and two Cl- ions constituting the legs of the stool.
structures of the representative complexes
1-4 have been
unambiguously established through single crystal X-ray
structure analysis (Fig 1).
2 | J. Name., 2013, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Page 3 of 11
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ02822A
Journal Name
ARTICLE
The Ru-C (carbene) distances of 2.074(7) Å in
1
and 2.056 Å in these complexes (
3
and
4), Ru-Cl and Ru-Npy bond lengths are
2
are comparable to those in the low-spin Ru(II)-carbene of approximately identical 2.4 and 2.1 Å, respectively and
complexes.17 The p-cymene ring in the both these complexes is analogous to the literature reports.20 The Ru–NHC bond
almost planar, and ruthenium is displaced by 1.719 Å (for lengths are 2.017 (for ) and 2.024 Å (for ), typical of Ru-NHC
and 1.693 Å (for ) from the centroid of p-cymene ring, which σ-bonds, indicating that the back-donation is negligible for
is similar to the reported Ru(II) arene complexes.18,19 In both these complexes.21 In particular, Ru-Ar distances are found to
these complexes, the coordination of two imidazolyl carbene be 1.714(3) (for ) and 1.740(2) (for ) from the centroid of the
1)
3
4
2
3
4
units to Ru(II) centres produces a twist for these moieties. The arene, which is comparable to reported literature values for
Ru(II) centres adopt a trans-position to each other in such a similar “three legged piano-stool” ruthenium complexes.22 In
way that each Ru(II) centre is 0.462 Å away from the both these complexes, the ligand (L3) binds to the ruthenium
anthracene plane in
1
, while in case of the complex
2
, we metal centre via NHC, Npy atom forming one five-membered
chelate ring having the bite angles of C(1)-Ru(1)-N(3) = 76.63°
and C(12)-Ru(1)-N(4) = 76.05°, for and , respectively.
observed Ru(II) centre is 3.297 Å away from the benzene ring.
3
4
Similarly, the ruthenium atom in the cationic complexes
3
Interestingly, the phenyl arm attached to imidazolyl nitrogen
and
and hexamethylbenzene in 4), NHC ligand (L3), and chloride
4
is surrounded by the η6-bonded arene (p-cymene ring in
showed a flip, when a plane is drawn through NHC ligand (L3),
3
towards the leg of the piano-stool geometry in the complex
In contrast, reverse to the leg of the piano-stool geometry was
observed in the case of the complex , which could be
3.
ion and thus attains a “three legged piano-stool” geometry;
which is archetypical of [(η6-arene)Ru(L3)Cl]+ half-sandwich
arene complexes. The arene ring constitutes a seat, while
chloride, pyridine nitrogen (Npy) and carbene carbon of the
NHC ligand (L3), constitutes three legs of the piano-stool. In
4
attributed due to the steric factors of the hexamethylbenzene
moiety in the latter case.
Scheme 2. Synthesis of the cationic ruthenium-NHC complexes 3-6.
Catalytic activity of the complexes
The reaction mixture was then refluxed at 80 °C for a specified
time period. The transfer hydrogenation products obtained
were analysed through GC-MS and NMR techniques (Figs. S15-
S24, ESI). Table 1, summarises transfer hydrogenation results
To understand potential use of the Ru-NHC complexes 1-6,
in transfer hydrogenation of carbonyl compounds, we
employed acetophenone as the model substrate (Scheme 3).
The catalytic reactions were carried out under different
conditions using 0.1 mmol of the substrate in presence of
different solvents, base promoters, 0.5-2 mol% of the catalysts
at 80 ± 2 °C. The catalysts in different solvents and bases were
pre-heated at 40 oC and then acetophenone was added slowly.
Scheme 3. Transfer hydrogenation of acetophenone in presence of the
complexes 1-6.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 4 of 11
View Article Online
DOI: 10.1039/C7NJ02822A
ARTICLE
Journal Name
using acetophenone as the substrate in presence of the Ru- (entries 7-8) and achieving a TOF (turn over frequency) of ca.
NHC complexes 1-6 under different conditions. For example, 50 h-1 at 50% formation of the product. The investigation of
the complexes 1-2 in t-butanol in the presence of both NaOH transfer hydrogenation reactions with the complexes 3-6 (0.5
and KOH as promoters showed negligible reaction for 2 h mol%) in presence of both NaOH and KOH in 2-propanol
(entries 1-2). In contrast, when we employed glycerol as the exhibited excellent catalytic efficiencies, affording ca. 99-100%
hydrogen source under similar reaction conditions and NaOH (entries 7-8) conversion for the substrate and a TOF of ca. 200
as the base promoter, we observed good substrate conversion h-1 at 50% formation of the product. When the catalyst loading
yields in the range ca. 48-65% (entry 3). We observed a non- was decreased by ca. 50%, the complexes
1 and 2 at 1 mol%,
negligible conversion of the substrate in presence of the showed lower efficiency for the conversion of the substrate
complexes 1-6, when we used Cs2CO3 (entry 4) and Na2CO3 (ca. 62 and 58%) (entry 9), respectively. Similarly reduced
(entry 5) as the base promoter. Upon changing the base to conversion yields of ca. 45-60% were observed with the
KOtBu, we obtained an increase in the reaction yields to the complexes 3-6 at a loading of 0.25 mol% (entry 9). These
tune of ca. 55-60% (entry 6) for the complexes
40-75% for 3-6. Interestingly, the complexes
1
and
2
, and ca. results demonstrate that we require a minimum of 0.5 and 2
1
and
2 in the mol% loading of the catalysts 3-6 and 1-2, respectively, to
presence of both NaOH and KOH have been proven to catalyse achieve ca. 100% transfer hydrogenation of acetophenone
the efficient reduction of acetophenone at a loading of 2 mol% under moderate reaction conditions.
in 2-propanol, affording 1-phenylethanol about ca. 95-100%
a
Table 1. Optimization of reaction conditions for transfer hydrogenation of acetophenone in presence of the complexes 1-6.
Conversion (%)
Entry
Solvent
Base
Complex 1
Complex 2
Complex 3
Complex 4
Complex 5
Complex 6
1
2
3
4
5
6
7
8
9
t-Butanol
t-Butanol
NaOH
KOH
nc
nc
nc
nc
nc
nc
nc
50
nc
48
nc
52
nc
60
nc
62
nc
65
Glycerol
NaOH
Cs2CO3
Na2CO3
KOtBu
KOH
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
<10
21
<10
15
10
15
10
12
6
3
15
18
60
55
40
46
64
75
>99
100
b62
95
>99
100
c45
>99
100
c48
100
100
c58
100
100
c60
NaOH
NaOH
>99
b58
a Average of three independent experiments and S.D ± 1% and experimental conditions: acetophenone (0.1 mmol), complexes 1-2 (2 mol%) and 3-6 (0.5 mol%), base (0.1 mmol),
solvent (2 mL), temperature (80 ± 2 °C), reaction time 2 h. Yields determined by GC-MS analyses. b Complexes 1-2 (1 mol%). c Complexes 3-6 (0.25 mol%). nc: no conversion.
To explore the substrate scope of the reaction, the neutral dimethoxyphenyl)-ethanone (entry 9), and 1-(3,4,5-
and cationic Ru(II)-NHC complexes 1-6 were tested as pre- trimethoxy-phenyl)ethanone (entry 10), we observed excellent
catalysts for the transfer hydrogenation of various aromatic conversion to their corresponding alcohols in the range ca. 90-
ketones (Tables 2 and 3). For example, the substrates bearing 99%, and NaOH used as the base promoter. Interestingly, upon
electronegative groups like 2,2-difluoro-1-phenylethanone changing the base promoter to KOH, we observed ca. 92-99%
(entry 2) exhibited ca. 100% conversion to the reduced conversion of the substrate to the product within 3 h of
product using the catalysts
1 and 2 in the presence of both reaction under identical conditions. The substrate scope of the
NaOH and KOH as the base promoters in 2-propanol. Similar reaction was investigated through employing aliphatic ketones
observations were also made with the chloro- and iodo- and aliphatic/aromatic aldehydes. Surprisingly, we observed
substituted acetophenone (entries 3-4), which have resulted in negligible conversion of these substrates to the reduced
ca. 95-100%. When we employed benzophenone (entry 5) and products in presence of both the complexes
(4-bromophenyl)(phenyl)methanone (entry 6) as the similar conditions (entries 11-12). These experiments confirm
substrates, we observed quantitative conversion (ca. 90-100%) that, both the neutral complexes and were found to be
1 and 2 under
1
2
under similar reaction conditions. Similarly, in the case of the quite efficient and selective only for transfer hydrogenation of
reactions with electron rich aromatic ketones such as the aromatic ketones, while the aliphatic ketones/aldehydes
tolylethanone (entry 7), di-tolylmethanone (entry 8), 1-(3,4- showed negligible reaction under identical conditions.
4 | J. Name., 2013, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 5 of 11
Journal Name
View Article Online
DOI: 10.1039/C7NJ02822A
ARTICLE
Table 2. Transfer Hydrogenation of the aromatic ketones catalyzed by the complexes 1 and 2.a
Entry
Substrate
Product
Complex 1
Complex 2
Conversion % (TOF, h-1)
NaOH
KOH
NaOH
KOH
100, 96c
(50.00)
>99, 94c
(49.75)
>99, 95c
(49.95)
95, 93c
(47.60)
1
2
100, 98c
(50.00)
100, 97c
(50.00)
100, 99c
(50.00)
100, 96c
(50.00)
>99, 97c
(49.98)
98, 95c
(49.00)
100, 97c
(50.00)
96, 92c
(48.00)
3
4
>99, 98c
(49.90)
99, 94c
(49.90)
>99,96c
(49.95)
95, 91c
(47.50)
97, 94c
(48.50)
96, 90c
(48.00)
100, 94c
(50.00)
>99, 96c
(49.55)
5
6
7
91, 89c
(45.50)
>99, 95c
(49.95)
95, 90c
(47.50)
98, 93c
(49.00)
>99, 92c
(49.60)
>99b,91c
(33.30)
98, 90c
(49.00)
>99b, 88c
(33.30)
98, 91c
(49.00)
95b, 89c
(31.00)
97, 92c
(48.50)
96b, 91c
(32.00)
8
9
93, 90c
(46.50)
97b, 93c
(32.30)
98 , 94c
(49.00)
96b, 91c
(32.00)
96, 92c
(48.00)
96b,90c
(32.00)
90, 85c
(45.00)
92b,88c
(30.00)
10
11
nc
nc
nc
nc
nc
nc
nc
nc
12
a
Average of three independent experiments and S.D ± 1% and experimental conditions: substrate (0.1 mmol), complexes 1 or 2 (2 mol%, 0.002 mmol), base (NaOH/KOH, 0.1
mmol), 2-propanol (2 mL), temperature (80 ± 2 °C), reaction time 2 h. b Reaction time 3 h. c Isolated yield. nc: negligible or no conversion. TON (turn over number) = (moles of
substrate/moles of catalyst); TOF (turn over frequency), h-1 = TON/time of reaction (h) at 50% product formation.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 6 of 11
View Article Online
DOI: 10.1039/C7NJ02822A
ARTICLE
Journal Name
Table 3. Transfer hydrogenation of the aromatic ketones catalyzed by the complexes 3- .
6 a
Entry
Substrate
Product
Conversion (%), (Time, h)
(TOF, h-1)
Complex 3
Complex 4
Complex 5
Complex 6
O
OH
CH3
> 99 (2)
(199)
99 (2)
(198)
100 (2)
(200)
100 (2)
(200)
CH3
1
2
OH
O
100 (1)
(400)
100 (1)
(400)
100 (1)
(400)
100 (1)
(400)
F
F
F
F
OH
CH3
O
100 (2)
(200)
100 (2)
(200)
100 (1)
(400)
100 (1)
(400)
CH3
3
4
5
6
Cl
Cl
Cl
Cl
OH
CH3
O
100 (2)
(200)
100 (2)
(200)
100 (2)
(200)
100 (1)
(200)
CH3
CH3
Cl
Cl
O
OH
CH3
100 (4)
(100)
> 99 (4)
(99)
97 (3)
(129)
100 (3)
(133)
H3C
H3C
O
OH
CH3
> 99 (5)
(80)
98 (5)
(78)
100 (3)
(133)
> 99 (3)
(132)
CH3
H3CO
H3CO
a
Average of three independent experiments and S.D ± 1% and experimental conditions: Substrate (0.1 mmol), complexes 3-6 (0.5 mol%, 0.0005 mmol), base (NaOH, 0.1 mmol),
2-propanol (2 mL) at 80 ± 2 °C. TON (turn over number) = (moles of substrate/moles of catalyst); TOF (turn over frequency), h-1 = TON/time of reaction (h) at 50% product
formation.
To understand the potential of the cationic complexes
3-
6,
To determine the specificity and efficacy of the complexes,
we have expanded the transfer hydrogenation reactions we have carried out the transfer hydrogenation by employing
towards some of the electron rich and deficient aromatic ligand precursors (L1-L5) and also compared with the literature
ketones as shown in Table 3. The reactions were performed by known ruthenium based catalysts under similar conditions
following the optimized conditions at different time scales. (Table S3, ESI). Interestingly, we observed negligible
With a catalyst loading of ca. 0.5 mol%, the complexes
3
-6
conversion of acetophenone to the desired product in
exhibited excellent catalytic efficiency for the transfer presence of the ligand precursors (entry 3). In contrast, the
hydrogenation of 2,2-difluoro-1-phenylethanone (entry 2) reactions with the commercially available ruthenium catalysts
within 1 h of reaction time, affording a highest TOF value of ca. such as [Ru(p-cymene)Cl2]2 (entry 4) and the second
400
h-1.
Likewise
the
reactions
with
1-(2,4- generation “Hoveyda-Grubbs” catalyst23 (entry 5) furnished
dichlorophenyl)ethanone (entry 3) and 1-(4-chlorophenyl- moderate yields of the product under identical conditions.
ethanone (entry 4) yielded quantitative conversion (ca. 100%) When compared to the catalytic activity of the complexes 3-6
of the substrates. To achieve the quantitative yields of ca. (TOF = ca. 200 h-1) with the literature reported Ru(II) pincer
100% conversion, we required nearly 2 h for the mononuclear complexes such as Ru(CNC)(CO)Br2,24 Ru(2,6-bis(arylimidazol-
complexes
3
and
4
, while only ca. 1 h is required for the 2-ylidene)-pyridine)-(PPh3)Cl2,25 and Ru(II)-NNC26, the latter
and
. Susequently, we have carried out examples showed the TOF values of ca. 117 h-1, 333 h-1 and
dinuclear complexes
5
6
the reactions using substrates such as 1-(p-tolyl)ethanone 245 h-1, respectively, employing acetophenone as the
(entry 5) and 1-(4-methoxyphenyl)ethanone (entry 6) under substrate. The high catalytic activity of the complexes can be
similar conditions, wherein we observed quantitative substrate attributed to the strong sigma binding affinity of N-
conversion (ca. 97-100%) at a catalyst loading of only 0.5 heterocyclic carbene (NHC) ligand, which facilitates the
mol%. The observed conversion efficiency and afforded TOF formation of strong metal-carbon bond and prevents the
values for these reactions clearly confirm the stability and decomposition of the catalysts under reaction conditions. In
recyclability of the catalysts.
comparison to the neutral ruthenium complexes 1 and 2, the
cationic ruthenium complexes 3-6 showed better catalytic
6 | J. Name., 2013, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 7 of 11
Journal Name
View Article Online
DOI: 10.1039/C7NJ02822A
ARTICLE
activity. This observation can be ascribed to the strong binding ketones and aldehydes showed negligible reaction under
of both NHC and pyridine substituents to the metal centre in identical conditions. Importantly, the neutral ruthenium-NHC
the latter cases.
complexes 1 and 2 at 2 mol% showed catalytic efficiency of ca.
100% within 2 h, whereas only ca. 0.5 mol% loading of catalyst
and 1 h required to achieve ca. 100% conversion with the
The mechanism of transfer hydrogenation of the aromatic
ketones to the corresponding aromatic alcohols using the
ruthenium complexes 1-6 is shown in Scheme S1 (ESI), based
on experimental evidence and literature reports.27 The loss of
chloride molecule followed by the addition of 2-propoxide
gives an active ruthenium catalyst species A, which can further
undergo β-hydride migration and loss of acetone molecule to
give the ruthenium hydride complex B. The subsequent
formation of the coordinated complex C can be rationalized
through the interaction of the intermediate complex B with
the substrate, which in turn can undergo hydride transfer to
form the intermediate D. The desired aromatic alcohol
formation can be postulated through the reaction of the
intermediate D with the solvent 2-propanol in the presence of
the base promoter and thereby generating the activated
ruthenium catalyst species A. The chelating capability of the
NHC ligands as well as availability of two cis labile chloride ions
provided the stability and reactivity to the ruthenium
complexes synthesised.
cationic ruthenium-NHC complexes 3-6
.
The detailed
investigations on the mechanistic aspects of the transfer
hydrogenation reactions through theoretical studies have
furthermore confirmed that the reaction to proceed through a
thermodynamically stable pathway. These complexes showed
superior activity and stability when compared to the reported
catalysts, thereby demonstrating their use as efficient catalysts
for the transfer hydrogenation of the aromatic ketones.
Experimental section
Materials and methods
General experiments and spectroscopic techniques for the
characterization of the compounds have been described in the
literature.14,31-32 Cyclic voltammetry measurements were
carried out using a CV-50W elctroanalyzer in dichloromethane
and acetonitrile solutions using platinum wire as auxiliary
electrode and glassy-carbon as working electrode. The
oxidation potentials of the complexes were referenced
Ag/AgCl electrode and ferrocene (0.45 V, E1/2) was used as an
external standard. Thermogravimetric analysis was performed
at a heating rate of 10 °C/min in N2 atmosphere using
The proposed mechanism was furthermore rationalized
through theoretical calculations employing density function
theory method using B3LYP level and double
ξ
basis sets.28-30
All calculations were done by Gaussion 09 software suite.
Solvent molecules (acetone, isopropanol), acetophenone, 1-
phenylethanol and the intermediate species A, B and D were
optimised at the same level of calculations (Table S4, ESI). In
the first step, conversion of the species A to B (Ru-H, 1.74 Å)
via β-hydride elimination, we observed a value of ca. 40.75
kcal/mol for the heat of reaction (energy required to break Ru-
O bond) with the release of acetone molecule. Furthermore,
the interaction of the intermediate B with acetophenone gives
the intermediate D for which we obtained the heat of reaction
value of ca. -40.71 kcal/mol (energy released during Ru-O
bond formation). Finally, for the hydrogen transfer step in the
transformation of the intermediates D to A, we observed a
value of ca. 0.28 kcal/mol. These studies have demonstrated
that the intermediates postulated were energetically
favorable, thereby confirming that this reaction proceeds
through a thermodynamically stable pathway.
Schimadzu,
DTG-60
equipment.
Anthracene,
Ag2O,
methylimidazole, benzyl bromide, [Ru(ȵ6-p-cymene)Cl2]2 and
[Ru(hexamethyl-benzene)Cl2]2 were purchased from S. D. Fine
Chemicals and Aldrich India, and used without further
purification. All the substrates used (aromatic/aliphatic
ketones and aldehydes) for the catalytic reactions were
purchased from Sigma and used without further purification.
All solvents were purified and dried prior to use.
X-ray crystallography
Single crystals of
from a CHCl3–CH3OH (9:1) mixture. The data sets for the
single-crystal X-ray studies for .3CHCl3 and .2CHCl3 were
collected with Mo Kα (λ = 0.71073 Å) radiation on a RIGAKU
diffractometer. The data set for the complexes and were
1.3CHCl3 and 2·2CHCl3 were obtained
1
2
3
4
collected with Mo Kα (λ = 0.71073 Å) on a Bruker APEX-II
diffractometer. All calculations were performed using SHELXTL.
Conclusions
DFT calculations
DFT calculations of the complexes
1 and 2 were carried out
In summary, we designed and synthesized a novel series of
mono- and dinuclear “piano-stool” structured ruthenium(II)-
NHC complexes 1-6 and evaluated their catalytic efficiency for
the transfer hydrogenation of the aromatic ketones under
different conditions. A high catalytic activity was observed
with all the complexes with a low catalyst loading. Our
investigations have revealed the important role of the N-
heterocyclic carbene (NHC) in the catalytic transfer
hydrogenation reactions. These complexes exhibited high
selectivity and specificity for the aromatic ketones bearing
both electron rich and deficient groups, whereas aliphatic
using Gaussian 09 program package.28 The Becke’s three
parameters hybrid exchange functional with the Lee–Yang–
Parr (LYP) non-local correlation functional was used
throughout the computational study.29,30 A LANL2DZ basis set
was used in the calculations.
Synthesis of the ligands and the complexes
Synthesis of the complex 1: To a solution of L1 (50 mg, 0.09
mmol) in dry CH2Cl2 (20 mL) was added Ag2O (22 mg, 0.09
mmol). The reaction mixture was stirred for 4 h at 25 °C
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 8 of 11
View Article Online
DOI: 10.1039/C7NJ02822A
ARTICLE
Journal Name
followed by the addition of p-cymene dimer precursor [Ru(ȵ6- Anal. Calcd for C25H27N3ClPF6Ru (%): C, 46.05; H, 4.33; N, 6.44.
p-cymene)Cl2]2 (55 mg, 0.09) and further stirred for 4 h. The Found: C, 46.26; H, 4.28; N, 6.46.
reaction mixture was then passed through celite column and
Synthesis of the complex 4: To a solution of L3 (50 mg, 0.15
the solvent was evaporated to dryness. Orange solid was mmol) in dry CH2Cl2 (15 mL) was added Ag2O (19 mg, 0.079
obtained by precipitation in diethyl ether, yielded the complex mmol). The reaction mixture was stirred for 4 h at 25 °C
1
in 39%. The single crystals were obtained by recrystallisation followed by addition of metal precursor [Ru(hexamethyl-
from a mixture (9:1) of CH3Cl and CH3OH. Mp > 300 °C; 1H NMR benzene)Cl2]2 (53 mg, 0.079 mmol) and further stirred for 4 h.
(500 MHz, CDCl3, TMS) δ 1.36-1.37 (d, 12H), 1.58 (s, 6H), 2.31 The reaction mixture was then passed through celite column
(s, 6H), 3.04-3.10 (quint, 2H), 4.10 (s, 4H), 5.43 (s, 4H), 5.63 (s, and the solvent was evaporated to dryness. The solid obtained
4H),6.13 (s, 2H), 6.74(s, 2H), 7.59 (m, 4H), 8.38 (m, 4H); 13C was again dissolved in water and 1.5 equivalent of ammonium
NMR (125 MHz, DMSO-d6); δ 20.9, 23.9, 30.6, 32.9, 47.1, hexafluorophosphate in water was added. The solid obtained
117.7, 123.9, 125.5, 126.0, 126.6, 127.9, 128.7, 131.2, 163.9, by precipitation was dried and further recrystallised from
173.1 ppm; Elemental Anal. Calcd for C44H50Cl4N4Ru2 (%): C, diethyl ether, to yield
4 in 60%. The single crystals were
53.88; H, 5.34; N, 5.71. Found: C, 53.65; H, 5.54; N, 5.88. obtained by recrystallisation from a mixture (9:1) of CH3Cl and
1
Synthesis of the complex 2: To a solution of L2 (50 mg, 0.11 CH3OH. Mp > 300 °C; H NMR (500 MHz, CD3CN, TMS) δ 2.19
mmol) in dry CH2Cl2 (20 mL) was added Ag2O (27 mg, 0.11 (s, 18H), 544-5.52 (q, 2H), 7.09-7.10 (d, 1H), 7.35-7.38 (m, 3H),
mmol). The reaction mixture was stirred for 4 h at 25 °C 7.46-7.50 (m, 3H), 7.71-7.73 (d, 1H), 7.80-7.81 (d, 1H), 8.06-
followed by the addition of p-cymene dimer precursor [Ru(ȵ6- 8.09 (m, 1H), 8.70-8.71 (m, 1H); 13C NMR (125 MHz, CD3CN) δ
p-cymene)Cl2]2 (67 mg, 0.11 mmol) and further stirred for 4 h. 16.5, 55.6, 99.9, 113.0, 124.1, 125.2, 130.3, 136.3, 141.9,
The reaction mixture was then passed through celite column 153.1, 153.7, 190.6 ppm; ESI-MS: m/z Calcd for C27H31ClN3Ru,
and the solvent was evaporated to dryness. Orange solid was 534.1250; Found, 534.1263 (M+); Elemental Anal. Calcd for
obtained by precipitation in diethyl ether, yielded the complex C27H31ClN3PF6Ru (%): C, 47.69; H, 4.74; N 6.18. Found: C, 47.65;
2
in 40%. The single crystals were obtained by recrystallisation H, 4.79; N, 6.14.
from a mixture (9:1) of CH3Cl and CH3OH. Mp > 300 °C ; 1H
Synthesis of the ligand L4: To a mixture of 2-(1H-imidazol-1-
NMR (500 MHz, CDCl3, TMS) δ 1.26-1.27 (d, 12H), 1.58 (s, 6H), yl)pyridine (200 mg, 1.37 mmol) in dry CH3CN (10 mL) was
2.07 (s, 6H), 2.91-2.97 (quint, 2H), 4.05 (s, 4H), 5.08 (s, 4H), added 1,4-bis(bromomethyl)benzene (182 mg, 0.69 mmol).
5.39 (s, 4H), 6.78 (s, 2H), 6.98 (s, 2H), 7.32 (s, 4H); 13C NMR The reaction mixture was refluxed for 24 h at 100 °C in a
(125 MHz, DMSO-d6) δ 21.4, 23.9, 30.3, 31.5, 117.2, 125.0, pressure tube and the product obtained was then filtered and
126.1, 127.4, 127.7, 128.4, 137.1, 165.4, 173.9 ppm; Elemental washed thoroughly with acetonitrile and dried under vacuum.
Anal. Calcd for C36H46Cl4N4Ru2 (%): C, 49.09; H, 5.49; N, 6.36. The product was further purified by recrystallisation from
1
acetonitrile to give precursor L4 in 80%. Mp > 200 °C; H NMR
Found: C, 48.99; H, 5.42; N, 6.30.
Synthesis of the complex 3: To a solution of L3 (50 mg, 0.15 (500 MHz, DMSO-d6, TMS) δ 5.58 (s, 4H), 7.62 (s, 4H), 7.64-
mmol) in dry CH2Cl2 (15 mL) was added Ag2O (19 mg, 0.079 7.67 (m, 2H), 8.03-8.05 (m, 4H), 8.20-8.24 (m, 2H), 8.55-8.56 (t,
mmol). The reaction mixture was stirred for 4 h at 25 °C 2H), 8.65-8.66 (m, 2H), 10.32 (s, 2H); 13C NMR (125 MHz,
followed by addition of metal precursor [Ru(ȵ6-p-cymene)Cl2]2 DMSO-d6) δ 52.0, 114.2, 119.7, 123.4, 125.2, 129.1, 135.1,
(48 mg, 0.079 mmol) and further stirred for next 4 h. The 140.5, 146.3, 149.1 ppm; ESI-MS: Calcd for C24H22N6, 394.1895;
reaction mixture was then passed through celite column and Found, 393.1836 (M+-1).
the solvent was evaporated to dryness. The solid obtained was
Synthesis of the complex 5: To a solution of L4 (50 mg, 0.09
again dissolved in water and 1.5 equivalent of ammonium mmol) in dry CH2Cl2 (20 mL) was added Ag2O (21 mg, 0.09
hexafluorophosphate in water was added. The solid obtained mmol). The reaction mixture was stirred for 4 h at 25 °C
by precipitation was dried and further recrystallised from followed by the addition of metal precursor [Ru(ȵ6-
diethyl ether, to give the complex
3 in 50%. The single crystals p-cymene)Cl2]2 (55 mg, 0.09 mmol) and further stirred for 4 h.
were obtained by recrystallisation from a mixture (9:1) of The reaction mixture was then passed through celite column
CH3Cl and CH3OH. Mp > 300 °C; 1H NMR (500 MHz, CD3CN, and the solvents were evaporated to dryness. The solid
TMS) δ 0.88-0.91 (d, 6H), 2.14 (s, 3H), 2.35-2.41 (quint, 1H), obtained was again dissolved in water and 2.5 equivalent
5.49-5.50 (dd, 1H), 5.54-5.57 (d, 1H), 5.72-5.75 (d, 1H), 5.86- of ammonium hexafluorophosphate in water was added.
5.88 (t, 1H), 5.97-5.98 (t, 1H), 6.09-6.11 (d, 1H), 7.25 (s, 1H), The solid obtained by precipitation was dried and further
7.26 (s, 1H), 7.45 (s, 5H), 7.78-7.80 (d, 1H), 7.88-7.89 (d, 1H), recrystallised from diethyl ether, yielded the complex 5 in 40%.
8.11-8.15 (m, 1H), 9.16-9.17 (t, 1H); 13C NMR (125 MHz, Mp > 300 °C; 1H NMR (500 MHz, CD3CN, TMS) δ 0.86-0.91
CD3CN) δ 19.2, 22.3, 24.3, 31.8, 55.8, 83.2, 87.9, 91.0, 92.1, (m, 12H), 2.25 (s, 6H), 2.3-2.4 (quint, 2H), 5.48-5.49 (d, 2H),
101.0, 106.3, 109.6, 113.6, 124.2, 125.5, 127.2, 129.3, 130.1, 5.55-5.58 (d, 2H), 5.74-5.77 (d, 2H), 5.85-5.88 (t, 2H), 5.99-6.02
136.5, 142.5, 152.7, 156.5, 186.1 ppm; ESI-MS: m/z Calcd for (m, 2H), 6.10-6.12 (m, 2H), 7.25-7.26 (d, 2H), 7.43-7.48
C25H27N3ClRu, 506.0937; Found, 506.0949 (M+); Elemental (m, 6H), 7.77-7.86 (m, 2H), 7.87-7.88 (t, 2H), 8.11-8.12 (d, 2H),
8 | J. Name., 2013, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 9 of 11
Journal Name
View Article Online
DOI: 10.1039/C7NJ02822A
ARTICLE
9.15-9.16 (d, 2H); 13C NMR (125 MHz, CD3CN) δ 19.2, 22.5,
24.3, 31.8, 55.4, 83.3, 88.0, 91.0, 92.1, 100.9, 106.4, 109.8,
113.6, 124.2, 125.4, 127.2, 130.0, 137.0, 142.5, 152.6, 156.5,
186.2 ppm; Elemental Anal. Calcd for C44H48Cl2N6P2F12Ru2 (%):
C, 43.11; H, 4.11; N, 6.86. Found: C, 43.09; H, 4.23; N, 6.79.
Synthesis of the ligand L5: To a solution of 2-(1H-imidazol-1-
yl)pyridine (200 mg, 1.37 mmol) in dry CH3CN (10 mL) was
added 9,10-dibromomethylanthracene (251 mg, 0.69 mmol).
The reaction mixture was refluxed for 24 h at 100 oC in a
pressure tube and the product obtained was then filtered and
washed thoroughly with acetonitrile and dried under vacuum.
The product was further purified by recrystallisation from
acetonitrile to give ligand L5 in 75%. Mp > 240 °C; 1H NMR (500
MHz, DMSO-d6, TMS) δ 6.72 (s, 4H), 7.59-7.60 (t, 2H), 7.63-
7.66 (m, 2H), 7.79-7.81 (m, 4H), 8.00-8.02 (d, 2H), 8.17-8.20
(m, 2H), 8.48-8.49 (d, 2H), 8.63-8.69 (d, 6H), 10.08 (s, 2H); 13C
NMR (125 MHz, DMSO-d6) δ 45.6, 114.5, 119.4, 123.2, 124.6,
125.3, 126.3, 127.7, 130.8, 134.6, 140.5, 146.2, 149.2 ppm; ESI-
MS: Calcd for C32H26N6, 494.2208; Found, 493.2213 (M+-1).
Synthesis of the complex 6: To a solution of L5 (50 mg, 0.076
mmol) in dry CH2Cl2 (20 mL) was added Ag2O (18 mg, 0.076
mmol). The reaction mixture was stirred for 4 h at 25 °C
followed by the addition of metal precursor [Ru(ȵ6-p-
cymene)Cl2]2 (47 mg, 0.076 mmol) and further stirred for 4 h.
The reaction mixture was then passed through celite column
and the solvent was evaporated to dryness. The solid obtained
was again dissolved in water and 2.5 equivalent of ammonium
hexafluorophosphate in water was added. The solid obtained
Acknowledgements
We acknowledge the financial support from Department of
Biotechnology (BMB/2015/53) and Council of Scientific and
Industrial Research. This is contribution No. PPG-367 from
CSIR-NIIST, Trivandrum. We are grateful to Dr. Sunil Varughese
(CSIR-NIIST, Trivandrum) and Mr. P. Alex Andrews (IISER,
Trivandrum) for single crystal X-ray structures analysis.
Notes and references
1
(a) S. Diez-Gonzalez and S. P. Nolan, Coord. Chem. Rev.,
2007, 251, 874-883; (b) A. G. Nair, R. T. McBurney, D. B.
Walker, M. J. Page, M. R. D. Gatus, M. Bhadbhade and B. A.
Messerle, Dalton Trans., 2016, 45, 14335-14342; (c) O.
Prakash, K. N. Sharma, H. Joshi, P. L. Gupta and A. K. Singh,
Dalton Trans., 2013, 42, 8736-8747; (d) J. M. Farrell, R. T.
Posaratnanathan and D. W. Stephan, Chem. Sci., 2015,
6,
2010-2015; (e) L. Agnes, D. C. Jean, L. J. Nicholas, W. J. P.
Andrew and P. Rinaldo, New J. Chem., 2011, 35, 2162-2168;
(f) A. K. Sharma, H. Joshi, K. N. Sharma, P. L. Gupta and A. K.
Singh, Organometallics, 2014, 33, 3629-3639; (g) J. A. Mataa,
M. Poyatos, and E. Peris, Coord. Chem. Rev., 2007, 251, 841-
859; (h) C. Cesari, A. Cingolani, C. Parise, S. Zacchini, V.
Zanotti, M. C. Cassani and R. Mazzoni, RSC Adv., 2015, 5,
94707- 94718.
2
(a) N. Marion, O. Navarro, J. Mei, E. D. Stevens, N. M. Scott
and S. P. Nolan, J. Am. Chem. Soc., 2006, 128, 4101-4111; (b)
S. Ganesamoorthy, P. Jerome, K. Shanmugasundaram and
R. Karvembu, RSC Adv., 2014, 4, 27955-27962; (c) A. Y.
Khalimon, E. M. Leitao, and W. E. Piers, Organometallics,
2012, 31, 5634-5637; (d) D. Wang, Q. Bi, G. Yin, W. Zhao, F.
Huang, X. Xieac and M. Jiang, Chem. Commun., 2016, 52
14226-14229.
by precipitation was dried and further recrystallised from
1
in 45%. Mp > 300 °C; H
,
diethyl ether, to yield the complex
6
NMR (500 MHz, CD3CN, TMS) δ 1.01-1.06 (m, 12H), 2.27 (s, 6H)
2.57-2.65 (q, 2H), 5.89-5.90 (d, 2H), 6.13-6.14 (d, 2H), 6.34-
6.35 (d, 2H), 6.44-6.45 (d, 2H), 6.61-6.68 (m, 4H), 6.77-6.81 (m,
2H), 7.48-7.51 (m, 2H), 7.68-7.69 (d, 2H), 7.73-7.76 (m, 8H),
8.12-8.16 (m, 2H), 8.66 (s, 2H), 9.25-9.27 (d, 2H); 13C NMR (125
MHz, CD3CN); δ 19.3, 22.5, 32.1, 49.3, 84.3, 87.7, 91.7, 92.8,
100.9, 107.1, 108.7, 113.6, 117.8, 124.3, 126.1, 128.7, 129.9,
132.1, 142.6, 152.5, 156.6, 185.8 ppm; Elemental Anal. Calcd
for C52H52Cl2N6P2F12Ru2 (%): C, 47.10; H, 4.10; N, 6.34. Found:
C, 47.32; H, 4.13; N, 6.28.
3
4
(a) G. Amenuvor, C. Obuah, E. Nordlander and J. Darkwa,
Dalton Trans., 2016, 45, 13514-13524; (b) Y. Baoyi, X. Yu,H.
B. Fatma, L. Karen, L. A. Alex, H. V. Kristof and V. Francis,
New J. Chem., 2015, 39, 1858-1867; (c) T. Jin, D. He, W. Li, C.
J. Stanton, S. A. Pantovich, G. F. Majetich, H. F. Schaefer, J.
Agarwal, D. Wang and G. Li, Chem. Commun., 2016, 52
14258-14261.
,
(a) D. Gülcemal, A. G. Gökçe, S. Gülcemal and B. Çetinkaya,
RSC Adv., 2014, , 26222-26230; (b) C. Ricardo, A. A. Inaam,
4
S. David, M. Bouchaib, G. Salaheddine and D. H. Pierre, New
J. Chem., 2003, 27, 6-8; (c) G. C. Vougioukalakis and R. H.
Grubbs, J. Am. Chem. Soc., 2008, 130, 2234-2245; (d) J. R.
Chen, C. F. Li, X. L. An, J. J. Zhang, X. Y. Zhu and W. J. Xiao,
Angew. Chem. Int. Ed., 2008, 47, 2489-2492; (e) O. Debleds,
and J.-M. Campagne, J. Am. Chem. Soc., 2008, 130, 1562-
1563.
General procedure for the transfer hydrogenation reaction:
Solvent (2 mL) was degassed by purging with N2. The desired
quantity of the catalyst was added and dissolved by sonicating
for 10 min at 25°C. The base (0.1 mmol) was added and the
mixture then preheated at 40 °C for 10 min. Then substrate
(0.1 mmol) was then added and the temperature was raised to
80 °C and allowed the reaction to continue for the specified
time. Aliquots were taken at different intervals and analyzed
by GC-MS. After the reaction time, the mixture was passed
through a pad of silica in hexane and the reaction mixture
collected was evaporated to dryness. The products were
further analyzed by various spectral and analytical techniques.
The catalytic reactions were carried out in triplicates to
confirm the reproducibility of the results.
5
(a) Y. H. Chang, W. J. Leu, A. Datta, H. C. Hsiao, C. H. Lin, J. H.
Guh and J. H. Huang, Dalton Trans., 2015, 44, 16107-16118;
(b) S. Sergio, B. Miriam and E. Peris, Organometallics, 2010,
29, 275-277; (c) O. Prakash, K. N. Sharma, H. Joshi, P. L.
Gupta and A. K. Singh, Organometallics, 2014, 33, 983-993;
(d) W. H. Wan, M. T. James, F. Etsuko and H. Yuichiro, New J.
Chem., 2013, 37, 1860-1866; (e) Z. Mingshuo, L. Le and B. H.
Seth, J. Am. Chem. Soc., 2014, 136, 7058-7067; (f) F. Saleem,
G. K. Rao, A. Kumar, G. Mukherjee and A. K. Singh,
Organometallics, 2014, 33, 2341-2351.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 10 of 11
View Article Online
DOI: 10.1039/C7NJ02822A
ARTICLE
Journal Name
6
(a) W. Baratta, D. R. Paolo, D. Z. Alessandro, S. Alessandra, Z. 18 E. M. Robert, E. A. Rhona, M. P. del Socorro, C. Haimei, C.
Ennio and R. Pierluigi, Angew. Chem. Int. Ed., 2004, 43, 3584-
3588; (b) O. Prakash, H. Joshi, K. N. Sharma, P. L. Gupta and
A. K. Singh, Organometallics, 2014, 33, 3804-3812; (c) C.
Jeff, D. H. Nathan, P. Simon, P. Andrew, B. Gary, I. J. Duncan
and J. S. Peter, J. Med. Chem., 2001, 44, 3616-3621.
19 G. Mani, L. Rangasamy, E. Suresh, R. Anvarbatcha, A. A.
Mohammad and P. Mallayan, Dalton Trans., 2014, 43, 1203-
1219.
Shujie, L. Guoping and C. Chun, New J. Chem., 2015, 39
,
5360-5365; (d) A. M. Alexandre, H. M. Robert, Inorg. Chem.,
2010, 49, 11039-11044; (e) O. Prakash, K. N. Sharma, H.
20 M. Zirngast, E. Pump, A. Leitgeb, J. H. Albering and C.
Slugovc, Chem. Commun., 2011, 47, 2261-2263.
Joshi, P. L. Gupta and A. K. Singh, Organometallics, 2014, 33
2535-2543.
,
21 D. Gnanamgari, E. L. O. Sauer, N. D. Schley, C. Butler, C. D.
Incarvito and R. H. Crabtree, Organometallics, 2009, 28, 321-
325.
7
8
9
(a) M. Yigit, B. Yigit, I. Ozdemir, E. Cetinkaya and B Cetinkaya,
Appl. Organomet. Chem., 2006, 20 322-327; (b) D.
,
Chatterjee, and P. Sarkar, J. Coord. Chem., 2016, 69, 650-
655; (c) F. Saleem, G. K. Rao, S. Kumar, M. P. Singh and A. K.
Singh, Dalton Trans., 2015, 44, 19141-19152; (d) S. Horn and
M. Albrecht, Chem. Commun., 2011, 47, 8802-8804; (e) M.
Poyatos, A. Maisse-Francois, S. Bellemin-Laponnaz, E. Peris
and L. H. Gade, J. Organomet. Chem., 2006, 691, 2713-2720.
22 D. Pandiarajan and R. Ramesh, J. Organomet. Chem., 2013,
723, 26-35.
23 J. Louie, C. W. Bielawski and R. H. Grubbs, J. Am. Chem. Soc.,
2001, 123, 11312-11313.
24 M. Poyatos, J. A. Mata, E. Falomir, R. H. Crabtree and E. Peris,
Organometallics, 2003, 22, 1110-1114.
(a) W. Chao-Yu, F. Ching-Feng, L. Yi-Hong, P. Shei-Ming and L.
Shiuh-Tzung, Inorg. Chem., 2007, 46, 5779-5786; (b) F.
Saleem, G. K. Rao, A. Kumar, G. Mukherjee and A. K. Singh,
Organometallics, 2013, 32, 3595-3603; (c) L. Yanyun, Y.
Shenluan, W. Xiaofeng, X. Jianliang, S. Weiyi, D. Zhenrong
and G. Jingxing, J. Am. Chem. Soc., 2014, 136, 4031-4039; (d)
25 A. A. Danopoulos, S. Winston and W. B. Motherwell, Chem.
Commun., 2002, 1376-1377.
26 F. Zeng and Z. Yu, Organometallics, 2008, 27, 6025-6028.
27 D. A. Alonso, P. Brandt, S. J. M. Nordin and P. G. Andersson,
J. Am. Chem. Soc., 1999, 121, 9580-9588.
S. Kumar, F. Saleem and A. K. Singh, Dalton Trans., 2016, 45
11445-11458.
,
28 M. J. Frisch, G. W. Trucks, H. B. Schlegel, et al., Gaussian 09,
Revision D.01, Gaussian, Inc., Wallingford CT, 2009.
(a) A. Arturo, A. M Jose, E. Peris, L. Frederic, M. Jean and C.
Evelina, Organometallics, 2012, 31 3911-3919; (b) W.
,
29 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785-
789.
Yawen, X. Dong, L. Qian, W. Chao and X. Jianliang, Green
Chem., 2013, 15, 629-634; (c) P. Om, N. S. Kamal, J. Hemant,
30 A. D. Becke, Phys. Rev. A, 1988, 38, 3098-3100.
L. G. Pancham and K. S. Ajai, Organometallics, 2014, 33
2535-2543.
,
31 (a) K. Bhattacharyya, D. Ramaiah, P. K. Das and M. V. George,
J. Phys. Chem., 1986, 90, 5984-5988; (b) J. Joseph, N. V.
Eldho and D. Ramaiah, J. Phys. Chem. B, 2003, 107, 4444-
4450; (c) M. Hariharan, S. C. Karunakaran and D. Ramaiah,
10 O. Takeshi, U. Noriyuki, T. Kunihiko, M. Kunihiko, S. Christian
and R. Noyori, J. Am. Chem. Soc., 2006, 128, 8724-8725.
11 W. Baratta, G. Chelucci, E. Herdtweck, S. Magnolia, K. Siega
and P. Rigo, Angew. Chem. Int. Ed., 2007, 46, 7651-7654.
Org. Lett., 2007, 9, 417; (d) E. Kuruvilla, P. C. Nandajan, G. B.
Schuster and D. Ramaiah, Org. Lett., 2008, 10, 4295-4298.
12 S. Dıez-Gonzalez, N. Marion and S. P. Nolan, Chem. Rev.,
2009, 109, 3612-3676.
32 (a) N. Tyagi, M. Viji, S. C. Karunakaran, S. Varughese, S.
Ganesan, S. Priya, P. S. Saneesh Babu, A. S. Nair and D.
Ramaiah, Dalton Trans., 2015, 44, 15591-15601; (b) D.
Ramaiah, I. Eckert, K. T. Arun, L. Weidenfeller and B. Epe,
Photochem. Photobiol., 2004, 79, 99-104; (c) V. S. Jisha, A. J.
Thomas and D. Ramaiah, J. Org. Chem., 2009, 74, 6667-6673.
13 K. Ganesan and Y. Alias, Int. J. Mol. Sci., 2008,
14 (a) M. Viji, A. K. Nair, P. C. Nandajan and D. Ramaiah, RSC
Adv., 2014, 47982-47986; (b) D. Ramaiah, P. P.
9, 1207-1213.
4
,
Neelakandan, A. K. Nair and R. R. Avirah, Chem. Soc. Rev.,
2010, 39, 4158-4168; (c) A. K. Nair, P. P. Neelakandan and D.
Ramaiah, Chem. Commun., 2009, 6352-6354.
15 (a) A. K. Verma, J. Singh, V. Kasi Sankar, R. Chaudhary and R.
Chandra, Tetrahedron Lett., 2007, 48, 4207-4210; (b) G. J.
Barbante, P. S. Francis, C. F. Hogan, P. R. Kheradmand, D. J.
D. Wilson and P. J. Barnard, Inorg. Chem., 2013, 52, 7448-
7459.
16 (a) R. Ramasamy, K. Christian and S. Fritz, Electrochem.
Commun., 2000, 2, 190-194; (b) K. Mahn-Jong, K. Rama, Y.
Hongwei, M. M. Frederick, P. Fausto, S. Scolastica, C.
Sebastiano, H. Tina, K. Gary and R. Krishnan, Inorg. Chem.,
2002, 41, 2471-2476.
17 (a) D. Joseph, K. Mukesh, Z. Matthias and T. P. Elizabeth,
Organometallics, 2013, 32, 966-979; (b) W. Julia, P.
Alexander, E. K. Fritz and W. Baratta, Organometallics, 2013,
32, 4042-4045.
10 | J. Name., 2013, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 11 of 11
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ02822A
TABLE OF CONTENTS
iPrOH, Base
H
O
O
R1
R1
R
R
Synthesized aryl appended half-sandwich Ru(II)-NHC complexes and
demonstrated their use as selective catalysts for transfer hydrogenation
of the aromatic ketones.