Cationic ruthenium(II)–NHC pincer complexes: Synthesis, characterisation and catalytic activity for transfer hydrogenation of ketones

Article abstract of DOI:10.1002/aoc.6287

Cationic ruthenium pincer complexes, [Ru(CNC)(CO)(PPh₃)Cl]X (CNC = 2,6-bis(1-methylimidazol-2-ylidene)-pyridine, X = Cl^? [1a], PF₆^? [1b]), [Ru(CNC)(PPh₃)₂Cl]X (X = Cl^? [2a], PF₆^? [2b]) and [Ru(CNC)(PPh₃)₂(H)]X (X = Cl^? [3a], PF₆^? [3b]) with triphenylphosphine, CO and halides as coligands have been synthesised and characterised by ¹H, ¹³C, ³¹P NMR, mass and single-crystal X-ray crystallography. The application of Ru complexes in the transfer hydrogenation of a wide range of ketones with 2-propanol as the hydrogen source is explored. The in situ transformations observed during the synthesis help understand and suggest a plausible mechanism via the hydride complex 3b. All complexes appear to be efficient catalyst precursors for transfer hydrogenation of ketones.

Full text of DOI:10.1002/aoc.6287

Received: 5 January 2021
DOI: 10.1002/aoc.6287
Revised: 19 April 2021
Accepted: 25 April 2021
F U L L P A P E R
Cationic ruthenium(II)–NHC pincer complexes: Synthesis,
characterisation and catalytic activity for transfer
hydrogenation of ketones
Dibya Yadav
| Shilpi Misra
| Dheeraj Kumar
| Suryabhan Singh
|
Amrendra K. Singh
Discipline of Chemistry, Indian Institute
of Technology Indore, Indore, 453552,
India
Cationic ruthenium pincer complexes, [Ru(CNC)(CO)(PPh )Cl]X (CNC = 2,6-bis
3
ꢀ
ꢀ
(
1-methylimidazol-2-ylidene)-pyridine, X = Cl [1a], PF6 [1b]), [Ru(CNC)
ꢀ
ꢀ
ꢀ
(
PPh ) Cl]X (X = Cl [2a], PF6 [2b]) and [Ru(CNC)(PPh ) (H)]X (X = Cl
3
2
3 2
Correspondence
6ꢀ
Amrendra K. Singh, Discipline of
Chemistry, Indian Institute of Technology
Indore, Indore 453552, India.
Email: aks@iiti.ac.in
[3a], PF
[3b]) with triphenylphosphine, CO and halides as coligands
1
13
31
have been synthesised and characterised by H, C, P NMR, mass and
single-crystal X-ray crystallography. The application of Ru complexes in the
transfer hydrogenation of a wide range of ketones with 2-propanol as the
hydrogen source is explored. The in situ transformations observed during
the synthesis help understand and suggest a plausible mechanism via the
hydride complex 3b. All complexes appear to be efficient catalyst precursors
for transfer hydrogenation of ketones.
Funding information
Science and Engineering Research Board,
Grant/Award Number: EMR/2016/004076
K E Y W O R D S
catalysis, N-heterocyclic carbene, pincer ligand, Ru complex, transfer hydrogenation
1
| INTRODUCTION
coordination geometries, namely, square pyramidal,
[
1–4,9]
trigonal-bipyramidal and octahedral.
pincer complexes are widely reported in the literature
Ru–CNC-type
Transition metal complexes with pincer type ligands have
been investigated widely and used in numerous catalytic
transformations.
with various coligands including halides, CO and
[1–9]
[24,25]
The robust pincer ligand platform
phosphines.
Presence of different coligands on
provides transition metal complexes with high thermal
stability which have been applied to various catalytic
reactions and small molecule activations, for example,
ruthenium centre can influence the electronic and steric
properties and also allows interesting coordination chem-
istry. An important structural aspect of the complexes
based on CNC pincer ligands is the type of wing-tip
substituents on the N-heterocycle which are used to
influence the steric environment around the central
metal atom. CNC pincer ligands with bulky aromatic
substituents on the N-heterocycles are the most common
whereas examples with smaller alkyl substituents are
somewhat less explored.
[10–13]
dinitrogen activation,
labilisation of the N-H bonds
[14,15]
[16,17]
in ammonia,
carbon dioxide reduction
and
[18–22]
water splitting.
Among the variety of pincer ligands,
pyridine–dicarbene pincer ligands with N-heterocyclic
carbenes (CNC pincer ligands) have become increasingly
popular ligands which increase the electron density at
the coordinated metal and enhance the reactivity of the
[23]
metal centre.
Hydrogenation/dehydrogenation reactions play a
There has been significant interest in ruthenium
pincer complexes as catalysts due to being readily avail-
able in different stable oxidation states and different
significant role in synthetic organic chemistry, and
such reactions involving oxygenated compounds are
particularly useful for manufacturing agrochemicals,
Appl Organomet Chem. 2021;e6287.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
1 of 11
https://doi.org/10.1002/aoc.6287

2
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YADAV ET AL.
[26,27]
pharmaceuticals, foods and fuels.
Traditionally,
2.2 | Synthesis of [Ru(CNC)(CO)(PPh3)
these reactions have been carried out using high hydrogen
pressure or a stoichiometric amount of hazardous
reagents, various additives, and cocatalysts, which often
Cl]Cl, 1a
An oven-dried Schlenk tube with the magnetic stirring
[27]
produce copious waste.
On the other hand, transfer
bar was charged with the ligand precursor CNCꢁ2HBr
ꢂ
hydrogenation (TH) and acceptorless dehydrogenation are
two of the most atom-efficient ways to access valuable
intermediates and various organic transformations.
Ruthenium-based complexes for TH of ketones and dehy-
(0.200 g, 0.5 mmol) and dried under vacuum at 100 C for
2 h. The Schlenk tube was cooled to room temperature
under N atmosphere. Dry methanol (10 ml) was added,
2
followed by Ag O (0.116 g, 0.5 mmol) and stirred at room
2
[
1–4,26,27]
drogenation of alcohols have been well studied.
temperature in the dark, covered with aluminium foil.
After 30 min, a white precipitate had formed, and
[RuHCl(CO)(PPh ) ] (0.477 g, 0.5 mmol) was added to
Whereas Ru(II) complexes bearing pincer ligands have
been well studied, complexes with CNC pincer ligands are
3
3
[24,25]
less explored for the TH of ketones.
With an aim to
the reaction mixture. The reaction mixture was heated at
ꢂ
utilise the robustness provided by the pyridine–dicarbene
CNC pincer ligands while also allowing the freedom to
have a variety of bulkier coligands, we have started
investigating synthesis and reactivities of complexes with
CNC pincer ligands having smaller alkyl substituents.
Herein, we report the synthesis, structure and
catalytic activity of Ru(II)–CNC complexes, namely,
60 C for 24 h, which results in a brown colour solution
with some residue. The reaction mixture was filtered
through celite, and the filtrate was reduced in volume
(2 ml) followed by the addition of diethyl ether (5 ml).
The compound precipitated out as yellow solid. The
X-ray quality crystals of 1a with bromide as the halide
ꢂ
ligand were obtained, at ꢀ18 C, by slow diffusion of
ꢀ
ꢀ
ꢀ
[
[
Ru(CNC)(CO)(PPh )Cl]X (X = Cl [1a], PF [1b]) and
diethyl ether in acetonitrile solution of the crude reaction
3
6
ꢀ
1
Ru(CNC)(PPh ) Cl]X (X = Cl [2a] and PF6 [2b]). A
mixture. Yield: 0.180 g (40%). H NMR (400 MHz,
3
2
possible intermediate 3a/3b was observed and also
synthesised separately, which was supported by spectro-
scopic data. Further, the reactivities of complexes 1b, 2b
and 3b have been investigated for the TH of ketones,
and the better performing catalyst precursor 1b was
used with a variety of ketones giving high conversions.
DMSO-d , δ in ppm): δ 8.44 (d, J = 2.2 Hz, 2H), 8.11
6
(t, J = 8.1 Hz, 1H), 7.77 (d, J = 8.1 Hz, 2H), 7.56
(d, J = 2.2 Hz, 2H), 7.39–7.31 (m, 3H), 7.25 (td, J = 7.7,
2.1 Hz, 6H), 6.94 (dd, J = 10.5, 7.8 Hz, 6H), 3.59 (s, 3H);
13
C NMR (DMSO-d , δ in ppm): 192.34, 149.78, 140.41,
6
132.99, 132.59, 131.98, 129.77, 128.34, 124.54, 118.41,
3
1
1
(
06.33, 38.01; P NMR (DMSO-d , δ in ppm): 42.94; IR
6
ꢀ
1
cm ): C O (1955.77); liquid chromatography–mass
+
2
2
| EXPERIMENTAL
spectrometry (LCMS): [M] 666.08, [M–Cl + H]—632.10,
ꢀ
LCMS: [PF ] —144.96, high-resolution mass spectrome-
6
+
.1 | General procedure
try (HRMS) for [M] [C H ClN OPRu] Calculated—
32
28
5
6
66.0763,
Found—666.0783;
Anal.
Calcd.
For
All reactions and manipulations were carried out under
an inert atmosphere using the standard Schlenk tech-
nique. Solvents were purchased from S. D. Fine-Chem
Limited and purified by distillation under an inert atmo-
[C H ClN OPRu]Cl: C 59.12, H 4.34, N 10.77 Found: C
32 28 5
59.47, H 4.76, N 11.06.
2.3 | Synthesis of [Ru(CNC)(CO)(PPh3)
[28]
[29]
sphere. [RuHCl(CO)(PPh ) ]
and [RuCl (PPh ) ]
2 3 3
3
3
Cl]PF , 1b
6
were prepared by following the literature procedure using
RuCl ꢁ3H O. Deuterated dimethyl sulphoxide was
To a solution of 1a (0.100 g, 0.13 mmol) in 2 ml of metha-
3
2
purchased either from EURISOtop or Aldrich Chemical
Co. NMR spectra were taken on Bruker Avance (III)
nol, add NH PF6 (0.22 g, 0.13 mmol) and stirred for
4
30 min at room temperature. A yellow precipitate of 1b
1
31
ꢂ
spectrometer operating at 400 ( H), 162 ( P) and
slowly comes out, and on cooling at 0 C, some more
13
1
1
00 MHz ( C). NMR chemical shifts are reported in ppm
and referenced to the solvent peaks for H (DMSO-d , δ
C (natural abundance of
DMSO-d , δ 40.45 ppm) NMR. P NMR chemical shifts
precipitation occurred. Yield: 0.035 g (31%). H NMR
1
(400 MHz, DMSO-d , δ in ppm): δ 8.37 (d, J = 2.4 Hz,
6
6
13
13
2
.54 ppm) and
C in
2H), 8.11 (t, J = 8.1 Hz, 1H), 7.72 (d, J = 8.2 Hz, 2H),
31
7.54 (d, J = 2.2 Hz, 2H), 7.36 (t, 3H), 7.25 (td, J = 7.7,
6
13
are referenced to an external 85% H PO standard as
2.2 Hz, 6H), 6.96 (dd, 6H), 3.61 (s, 3H); C NMR
3
4
0
ppm. The mass chromatograms were recorded on
(DMSO-d , δ in ppm): 192.06, 149.50, 140.49, 132.78,
6
Bruker-Daltonics-microTOF-QII mass spectrometer. Gas
chromatography (GC) samples were analysed in
Shimadzu QP2010 Ultra, without an internal standard.
132.39, 131.93, 129.55, 127.93, 124.52, 118.02, 106.14,
31
37.82; P NMR (DMSO-d , δ in ppm): 43.27, 143.87; IR
6
ꢀ
1
+
(cm ): C O (1954.21); LCMS: [M] 666.08, [M–Cl

YADAV ET AL.
3 of 11
ꢀ
+
1
+
H]—632.11, LCMS: [PF ] —144.96, HRMS for [M]
precipitation occurred. Yield: 0.034 g (29%). H NMR
6
1
[
6
C H ClN OPRu] Calculated—666.0763, Found—
66.0807; Anal. Calcd. For [C H ClN OPRu]PF :
C 47.39, H 3.48, N 8.63 Found: C 47.74, H 3.87, N 8.95.
(DMSO-d , δ in ppm): H NMR (500 MHz, DMSO-d )
δ 8.22 (d, J = 2.2 Hz, 2H), 7.46 (d, J = 2.4 Hz, 2H), 7.36
(t, J = 8.1 Hz, 1H), 7.29 (t, J = 7.5 Hz, 6H), 7.19–7.12
m, 12H), 7.06–6.99 (m, 12H), 6.98 (d, J = 4.00 Hz, 2H),
.59 (s, 3H); C NMR (DMSO-d , δ in ppm): 188.74,
52.09, 132.93, 132.13, 129.18, 127.55, 125.58, 117.75,
05.16, 48.29, 36.72; P NMR (DMSO-d , δ in ppm):
32
28
5
6
6
32
28
5
6
(
13
3
1
1
6
2
2
.4 | Synthesis of [Ru(CNC)(PPh ) Cl]Cl,
3 2
a and [Ru(CNC)(PPh ) H]cl, 3a
3 2
31
6
+
2+
Similar procedure was followed as with 1a except
RuCl (PPh ) ] (0.480 g, 0.5 mmol) was added in place of
RuHCl(CO)(PPh ) ]. The solvent was reduced in volume
2 ml) followed by the addition of diethyl ether (5 ml)
31.70, 26.57, ꢀ144.18. LCMS: [M] 900.18, [M–Cl]
ꢀ
+
[
[
(
432.60, LCMS: [M] —144.96, HRMS for [M]
2
3 3
[C H ClN P Ru]
Calculated—900.1730,
Found—
3
3
49 43
5 2
900.1739; Anal. Calcd. For [C H ClN P Ru]PF :
49
43
5
2
6
resulting in the precipitation of compound which was
filtered and dried under vacuum. Further, the crude solid
was purified by column chromatography using neutral
alumina with eluting solvent ((hexane/CH Cl )/CH OH)
C 56.30, H 4.15, N 6.70 Found: C 56.47, H 4.42, N 7.21.
2
.6 | Synthesis of [Ru(CNC)(PPh ) H]Cl,
a from 2a
3 2
2
2
3
3
(
(1:1):5) gives 3a and ((hexane/CH Cl )/CH OH) ((1:1):7)
2 2 3
affords 2a as light yellow solids. The X-ray quality
crystals of 2a were obtained by slow diffusion of diethyl
Complex 2a was added (0.214 mmol, 0.200 g) in a
Schlenk tube followed by K CO (0.214 mmol, 0.029 g),
2
3
ꢂ
ether in acetonitrile solution at ꢀ18 C.
and then, i-PrOH was injected via the syringe. The reac-
1
ꢂ
Compound 2a: Yield: 0.155 g (25%). H NMR
tion mixture was refluxed at 85 C for 15 h. The colour of
(
2
7
DMSO-d , 500 MHz, δ in ppm): δ 8.31 (d, J = 2.2 Hz,
H), 7.49 (d, J = 2.1 Hz, 2H), 7.36 (t, J = 7.5 Hz, 1H),
the reaction mixture was changed from greenish-brown
to brown orange. After the completion of the reaction,
the mixture was filtered, and the solvent was evaporated
under a reduced vacuum to afford brown solid. Solid was
washed with diethyl ether and dried under vacuum. The
complex was obtained with 77.8% yield.
6
.29 (t, J = 7.4 Hz, 6H), 7.18–7.12 (m, 12H), 7.06
13
(
d, J = 8.2 Hz, 2H), 7.04–7.00 (m, 12H), 3.59 (s, 3H);
C
NMR (DMSO-d , δ in ppm): 188.91, 152.10, 132.92,
6
1
3
32.13, 129.19, 127.69, 125.53, 117.83, 105.22, 48.28,
3
1
6.73; P NMR (DMSO-d , δ in ppm): 31.79, 26.60.
6
+
2+
ꢀ
LCMS: [M] 900.17, [M–Cl]
1
Calculated—900.1730, Found—900.1714; Anal. Calcd.
For [C H ClN P Ru]Cl: C 62.89, H 4.63, N 7.48 Found:
432.59, LCMS: [M] —
2
.7 | Synthesis of [Ru(CNC)(PPh ) H]
3 2
+
44.9636, HRMS for
[M]
[C H ClN P Ru]
49 43 5 2
PF , 3b
6
To a solution of 3a (0.11 mmol, 0.100 g) in methanol,
49
43
5 2
C 63.14, H 4.89, N 7.72.
NH PF6 was added and stirred for 30 min at room
4
1
Compound 3a: Yield: 0.058 g (10%). H NMR
temperature. A precipitate of 3b slowly comes out, and
ꢂ
(
400 MHz, DMSO-d ): δ 8.35 (d, J = 2.3 Hz, 2H), 7.84
on cooling at 0 C, some more precipitation of [Ru(CNC)
6
1
(
(
(
t, J = 8.0 Hz, 1H), 7.61 (d, J = 7.9 Hz, 2H), 7.28
t, J = 7.4 Hz, 6H), 7.25 (d, J = 2.2 Hz, 2H), 7.18
t, J = 7.6 Hz, 12H), 6.85–6.70 (m, 12H), 2.46 (s, 6H),
(PPh ) (H)]PF occurred. Yield: 0.060 g (54%). H NMR
3
2
6
(400 MHz, DMSO-d ): δ 8.31 (d, J = 4.0 Hz, 2H), 7.85
6
(t, J = 8.1 Hz, 1H), 7.58 (d, J = 8.5 Hz, 2H), 7.28
(t, J = 7.5 Hz, 6H), 7.24 (d, J = 4.00 Hz 2H), 7.18
1
3
ꢀ
8.88 (t, J = 27.0 Hz, 1H); C NMR (DMSO-d , δ in
6
ppm): 198.09, 149.75, 135.85, 131.66, 128.89, 127.71,
(t, J = 7.6 Hz, 12H), 6.90–6.64 (m, 12H), 2.47 (s, 2H),
3
1
13
1
23.92, 117.19, 104.11, 48.43, 35.82; P NMR (DMSO-d ,
ꢀ8.86 (t, J = 27.0 Hz, 1H); C NMR (DMSO-d , δ in
6
6
+
δ in ppm): 52.10. LCMS: 866.17 [M] . HRMS for
ppm): 201.28, 149.43, 135.54, 131.38, 128.55, 127.41,
123.56, 116.80, 103.74, 47.11, 35.50; P NMR (DMSO-d ,
+
31
[
8
M] [C H N P Ru] Calculated—866.2123, Found—
49
44
5
2
6
+
66.2125; Anal. Calcd. For [C H N P Ru]Cl: C 65.29, H
δ in ppm): 51.91, ꢀ144.15. LCMS: 866.20 [M] . HRMS
49
44 5 2
+
4
.92, N 7.77 Found: C 65.57, H 5.27, N 8.06.
for
[M]
[C H N P Ru]
Calculated—866.2123,
49
44 5 2
Found—866.2168; Anal. Calcd. For [C H N P Ru]PF :
C 58.22, H 4.39, N 6.93 Found: C 58.63, H 4.81, N 7.25.
49
44
5
2
6
2.5 | Synthesis of [Ru(CNC)(PPh ) Cl]
3 2
PF , 2b
6
2
.8 | X-ray data collection and structure
To a solution of 2a (0.100 g, 0.11 mmol) in 2 ml of metha-
nol, add NH PF6 (0.19 g, 0.11 mmol) and stirred for
refinement
4
3
0 min at room temperature. A yellow precipitate of 2b
Single-crystal X-ray data of compounds 1a and 2a
were collected on Rigaku Oxford Diffractometer using
ꢂ
slowly comes out, and on cooling at 0 C, some more

4
of 11
YADAV ET AL.
3
1
graphite-monochromated Mo Kα radiation (λ = 0.7107 Å).
The data collection was evaluated with the help of
CrysAlisPro CCD software. Data collections for all
complexes were carried out at room temperature. Final
refinement included atomic positions for all the atoms,
anisotropic thermal parameters for all the nonhydrogen
atoms and isotropic thermal parameters for all the
hydrogen atoms. Full-matrix least-squares refinement
against jF2j was carried out using the WinGx package of
(Figures S3–S8). Spectroscopic data ( P NMR and LCMS)
of the crude reaction mixture indicates that 2a and 3a are
formed as minor impurity during the synthesis of 1a. The
dissociation of CO ligand and subsequent coordination of
PPh ligand, present in the solution, may result in the
3
formation of complex 2a. The complex 2a, subsequently,
undergoes chloride ligand substitution by a methoxy
ligand generated from methanol solvent followed by
β-hydride elimination leading to the synthesis of the
hydride complex 3a. Compound 1b was precipitated out
by treating the crude reaction mixture of 1a with NH PF
in methanol (Figures S9–S14). The minor impurities from
1a and 1b were removed upon precipitation followed by
recrystallisation.
[
30]
programs. In 2a, the disordered lattice chloride ion was
refined by splitting them into two parts without fixing any
site occupancy factor (sof). The occupancies of the split
atoms were refined by means of a free variable. Details of
the structural parameters and final refinements for the
compounds are given in Table S1.
4
6
A similar reaction condition was used to synthesise
2
a from [RuCl (PPh ) ] as the ruthenium source (Ru2).
2 3 3
However, the synthesis of compound 2a is always
accompanied by in situ generation of 3a. Compound 2a
was attempted to purify in the form of 2b, by
precipitation using NH PF . However, we are unable to
2
.9 | General procedure for catalytic
hydrogen transfer reaction
4
6
Typically, the ketone (2 mmol) and catalyst (1 mol%)
were dissolved in i-PrOH (5 ml), under inert atmosphere
in two-neck 25-ml R.B. flask equipped with a reflux
condenser, followed by addition of Na (1 eq., 2 mmol) to
generate i-PrONa, in situ. After all the sodium metal had
dissolved, the reaction mixture was quickly heated to
reflux by lowering into a preheated oil bath. The
conversion of the corresponding product at 15-min time
intervals was determined by the relative peak area of the
substrate and the product in GC without an internal
standard (Figures S44–S73). After the reaction was com-
pleted, the solution was cooled quickly in an ice bath and
analysed by GC–mass spectrometry (MS). The product
was purified by silica gel column chromatography using
hexane/ethyl acetate (typically 8:2) as eluent to deter-
mine the isolated yield. NMR data for alcohol products
are given in Figures S74–S81.
separate compound 3b from 2b, which also precipitated
during the anion exchange. The mixture of complexes 2a
and 3a can be converted to 3a, cleanly, as shown in
Scheme 2 (vide infra). Spectroscopically pure 2a was
obtained by alumina-gel chromatography followed by
recrystallisation (Figures S15–S21). Further, anion
exchange of 2a and 3a by precipitation using NH PF
4
6
gives 2b (Figures S22, S23 and S25–S29) and 3b,
respectively.
All the complexes are characterised by IR, mass
and multinuclear NMR spectroscopic techniques. In the
1
H NMR spectrum (Figure S1), the ligand precursor
CNCꢁ2HBr exhibits a singlet at δ = 10.59 ppm due to
imidazolium proton, and disappearance of this peak
indicated the carbene generation during complex
formation.
ꢀ
1
The C O stretching frequency of 1956 and 1954 cm
in 1a and 1b (Figures S8 and S14), respectively, is signifi-
ꢀ
1
[25]
cantly larger than 1922 cm of Ru–CNC pincer
and
3
3
| RESULTS AND DISCUSSION
comparable with previously reported CNC complexes
ꢀ
1 [32]
1
952 and 1954 cm
.
Complex 1a showed signals for
+
.1 | Synthesis of CNC pincer ruthenium
ESI LCMS at m/z 632.12 and 666.08 assigned to
+
+
1
complexes
[1a–2Cl + H] and [1a–Cl] , respectively (Figure S6). H
NMR of 1a and 1b are almost identical with the pyridine
protons appearing as a doublet at δ = 8.44 and a triplet
δ = 8.11 ppm, whereas two doublets are observed at
δ = 7.77 and δ = 7.56 for the imidazol-2-ylidene protons
The imidazolium ligand precursor CNCꢁ2HBr was
[
31]
prepared by the reported procedure in literature
and
1
13
characterised by H and C (Figures S1 and S2). The
13
reaction of imidazolium precursor with Ag O, in
(Figures S3 and S9). In the C NMR spectra, the carbene
2
methanol, affords silver–carbene complex, which
undergoes transmetallation with Ru precursors in situ to
give Ru–CNC complexes (Scheme 1). When the silver–
carbene complex was treated with the [RuHCl(CO)
carbon signals of 1a and 1b appear at 192 ppm (Figures S5
31
and S11). P NMR spectrum of 1a and 1b showed peaks
at 40.59 and 43.27 ppm (Figures S4 and S10), respectively,
for PPh3 ligand, comparable with previously reported
[
33]
(
PPh ) ] precursor (Ru1) for 24 h, 1a was obtained
NNN pincer complexes.
3
3

YADAV ET AL.
5 of 11
SCHEME 1 Synthesis of CNC pincer ruthenium complexes 1–3
+
Compounds 2a and 2b show ESI LCMS signal at
This assumption is confirmed by mass analysis where
+
+
m/z 900.00, assigned to [2a/2b–Cl] (Figures S18 and
S26). In H NMR of 2a, one doublet and triplet appear at
ESI LCMS signal at m/z 432.59 is observed and assigned
1
2+
2+
to [2a–Cl] /[2b–Cl] (Figures S19 and S27).
δ = 8.31 and δ = 7.36 for pyridine protons, whereas
imidazol-2-ylidene protons were shown as two doublets
at δ = 7.49 and δ = 7.06 (Figure S15). In the case of 2b,
pyridine and imidazolium protons are slightly shifted to
downfield than 2a (Figure S22); however, methyl protons
appear at the same value δ = 3.59 ppm for both the
3.2 | Description of the crystal
structures
The molecular structures of complexes 1a and 2a are
confirmed by X-ray crystal diffraction analysis.
Complexes 1a (Figure 1) and 2a (Figure 2) crystallised in
an orthorhombic system with P2 2 2 space group and
31
complexes (Figures S15 and S22). Interestingly, P NMR
spectra of complexes 2a and 2b show two singlets at
3
1.79, 26.60, 31.70 and 26.57 ppm (Figures S16 and S23)
1
1 1
whereas no dissociation of PPh was observed. The signal
monoclinic system with P21/c space group, respectively.
The ruthenium metal centre in all the complexes displays
distorted octahedral geometry. Selected bond lengths and
angles of complexes 1a and 2a are listed in Table S2.
Complex 1a crystallised with bromide ions from
the crude reaction mixture whereas the mass data of
the purified samples indicated chloride as the halide
present in the coordination sphere. The molecular struc-
ture of 1a consists of a six-coordinate Ru(II) centre with
3
at 26.60 ppm was ruled out to be due to O PPh by
3
recording the NMR after addition of O PPh in the NMR
3
sample of 2a and 2b (Figure S24). These complexes are
31
expected to exhibit one singlet in the P NMR consider-
ing the same chemical environment for the two phospho-
31
rus atoms. These two singlets in the P NMR are
attributed to the generation of two species in solution
due to dissociation of the coordinated chloride ligand.

6
of 11
YADAV ET AL.
SCHEME 2 Plausible mechanism for transfer hydrogenation catalysis by 2b
ꢀ
Br and triphenylphosphine at the axial positions, CO
trans to the pyridine nitrogen atom and CNC pincer
ligand at the meridional site (Figure 1). Another bromide
ion is present in the lattice. The CNC pincer ligand
occupies three meridional sites with C1-Ru1-C10 angle
of 152.3(4), shorter than the previously reported
length of 1.114(13) Å (Table S2) is comparable with NNN
[
33]
pincer (C-O, 1.105(6) Å)
complex and slightly shorter
than the previously reported CNC complex (C-O, 1.152
[
25]
(6) Å).
Complex 2a also has distorted octahedral geometry in
which Ru(II) is surrounded by one CNC pincer ligand,
two triphenylphosphines and one chloride ion (Figure 2).
The two bulky triphenylphosphines are situated trans to
[32]
ꢂ
complexes.
similar to the complex reported by Poyatos et al.
bond distances of Ru1-C1 (2.051(9) Å) and Ru1-C10
2.085(9) Å) are comparable with the reported ruthenium
NHC carbene complexes 2.056(5) and 2.062(5) Å.
The bite angle (N3-Ru1-C10) of 76.8(4) is
[
25]
The
ꢂ
each other. The N3-Ru1-C10 bite angle is 77.5(3) and
[25]
(
comparable with the previously reported complexes.
[
25]
The
The bond distance of Ru1-C1 (2.052 (6) Å) (Table S2) is
CO molecule is present trans to the pyridine ring, and
Ru-C (CO) bond length of 1.875(13) Å is equivalent to
similar to that in the complex 1a and comparable with
[
25]
the reported complex
(2.056(5) Å) distance whereas
[25,32]
those reported in the literature.
The C-O bond
Ru1-C10 (2.094(7) Å) was slightly larger than the

YADAV ET AL.
7 of 11
FIGURE 1 Molecular structure of 1a with
thermal ellipsoids drawn at the 50% level. All
hydrogen atoms and a bromide counterion are
omitted for clarity. Selected bond lengths (Å)
ꢂ
and angles ( ): Ru1-N3, 2.077(8), Ru1-C1, 2.051
(
9); Ru1-C10, 2.085(9); Ru1-C14, 1.875(13);
Ru1-P1, 2.342(2); Ru1-Br1, 2.5727(13);
C1-Ru1-C10, 152.3(4); N3-Ru1-C10, 76.8(4);
N3-Ru1-P1, 90.7(2); C10-Ru1-P1, 94.8(3);
C1-Ru1-Br1, 85.6(2); N3-Ru1-Br1, 87.2(2);
C10-Ru1-Br1, 86.6(3); P1-Ru1-Br1, 177.20(6)
FIGURE 2 Molecular structure of 2a with
thermal ellipsoids drawn at the 50% level. All
hydrogen atoms and a chloride counterion are
omitted for clarity. Selected bond lengths (Å)
ꢂ
and angles ( ): Ru1-N3, 2.020(5), Ru1-C1, 2.052
(
6); Ru1-C10, 2.094(7); Ru1-P1, 2.370(1); Ru1-P2,
.359(1); Ru1-Cl1, 2.447(3); C1-Ru1-C10, 156.1
3); N3-Ru1-C10, 77.5(3); N3-Ru1-P1, 90.76(13);
2
(
C10-Ru1-P1, 89.68(15); C1-Ru1-Cl1, 97.80(19);
N3-Ru1-Cl1, 176.05(18); C10-Ru1-Br1, 106.1(2);
P1-Ru1-Cl1, 87.73(7); P2-Ru1-Cl1, 89.12(7)

8
of 11
YADAV ET AL.
complex 1a and the previously reported complex (2.062
Entries 5–10). The conversion of cyclohexanone to
corresponding alcohol was achieved in 79%, 74% and 78%
in 30 min (Table 1, Entries 6, 8 and 10) and 55%, 71% and
0% in 15 min (Table 1, Entries 5, 7 and 9) using the bases
[25]
(5) Å).
The Ru-P (Ru1-P1, 2.370(1)Å and Ru1-P2,
2
.359(1) Å) bonds in case of 2a are slightly longer than
the 1a (Ru1-P1, 2.342 (2) Å) and previously reported
[24]
t
complex (Ru1-P1, 2.318(2) Å).
NaOH, KOH and KO Bu, respectively, indicating there is
t
a significant induction period for catalysis with KO Bu.
As the results, ruthenium complex 1b (1 mol%) as
catalyst and i-PrONa as base in isopropanol under reflux
temperature were chosen as suitable reaction condition.
The scope of the catalyst 1b was then examined using
various ketone substrates to establish the generality of
the reaction. Several ketone derivatives with aliphatic
and aromatic substituents, as well as both the electron-
donating and electron-withdrawing substituents, were
investigated (Table 2). Aliphatic cyclic ketones (Table 2,
Entries 1 and 3) gave good to moderate conversions
in 30 min and 1 h, However, the reaction proceeds
comparatively slowly in case of aliphatic acyclic ketone
(Table 2, Entry 2). For aromatic ketones, the yield varies
from 58% to 99% in 1 h (Table 2, Entries 4–10). The
electron-withdrawing substituents like Br at the para
position (Table 2, Entry 5) showed comparable conver-
sion with acetophenone, but in case of chloro (Table 2,
Entry 5), reactivity of reaction was slightly decreased.
Subsequently, the electron-donating methyl substituents
3
.3 | Catalytic application in TH
Ruthenium complexes 1b, 2b and 3b were used as cata-
lyst for TH of ketones using 2-propanol, and the reaction
was monitored by GC without internal standard. Initially,
the TH of cyclohexanone in refluxing 2-propanol was
selected as a model reaction to evaluate the catalytic
activity of complexes. Using 2 mmol of ketone, 1 mol%
of catalyst and 1 equivalent of sodium isopropoxide
(
i-PrONa) as base complex 1b showed higher catalytic
activity than other complexes, namely, 2b and 3b,
resulting in >99% conversion of cyclohexanone in 30 min
(Table 1, Entry 2). Under similar conditions, complexes
2
b and 3b exhibited slightly lower catalytic activity with
6
1% and 72% conversion (Table 1, Entries 3 and 4).
Further, the effect of various bases, for example, NaOH,
KOH and KO Bu, in different time intervals (15 and
t
3
0 min) with complex 1b was also studied (Table 1,
TABLE 1 Optimisation table of
different catalysts
[
a]
b
ꢀ1
Entry
Catalyst
1b
Base
Time (min)
Conversion (%)
TON/TOF (h
85/340
99/198
61/122
72/144
55/220
79/158
71/284
74/148
-
)
1
2
3
4
5
6
7
8
9
i-PrONa
i-PrONa
i-PrONa
i-PrONa
NaOH
NaOH
KOH
15
30
30
30
15
30
15
30
15
30
15
30
85
>99
61
72
55
79
71
74
0
1b
2b
3b
1b
1b
1b
1b
KOH
t
1b
KO Bu
t
10
11
12
1b
KO Bu
78
78/156
96/384
102/204
c
1b
i-PrONa
i-PrONa
48
c
1b
51
Note: TON = (Number of moles of substrate converted)/(Number of moles of catalyst), at the end of the
reaction. TOF = [(TON)/h].
a
ꢂ
Reaction conditions: ketone (2.0 mmol), catalyst ([Ru] 1 mol%), i-PrONa (1 eq.), i-PrOH (5 ml), at 82 C
under a slow N flow.
2
b
Determined by gas chromatography without internal standard.
c
Catalyst ([Ru] 0.5 mol%).

YADAV ET AL.
9 of 11
TABLE 2 Transfer hydrogenation of various ketones with catalyst 1b
b
%
Conversion (isolated yield)
a
ꢀ1
Entry
Reactant
Product
0.5 h
1 h
TON/TOF (h
)
1
>99 (98)
-
98/196
30/30
2
3
24
65
37 (30)
80 (62)
62/62
4
5
6
7
80
60
58
32
>99 (95)
97 (75)
84 (80)
58 (45)
95/95
75/75
80/80
45/45
8
9
24
12
68 (53)
73 (67)
53/53
67/67
1
0
70
83 (64)
64/64
Note: TON = (Number of moles of substrate converted)/(Number of moles of catalyst), at the end of the reaction. TOF = [(TON)/h].
a
ꢂ
2
Reaction conditions: ketone (2.0 mmol), 1b ([Ru] 1 mol%), i-PrONa (1 eq.), i-PrOH (5 ml), at 82 C under a slow N flow for 1 h.
b
Determined by gas chromatography without internal standard.
at para and meta positions (Table 2, Entries 7 and 8)
benzophenone, we believe that after the dissociation
relatively decelerated the rate of TH. Benzophenone
of one PPh ligand, the steric environment around Ru
3
was reduced in 1 h with 73% conversion, whereas
centre is not so crowded to prevent its coordination to
the Ru. Further, the bulkiness of the product may be
helpful in the dissociation from the catalyst which can,
0
4
,4 -dichlorobenzophene gave 83% conversion (Table 2,
Entries 9 and 10). For the unexpected high reactivity of

10 of 11
YADAV ET AL.
SCHEME 3 Synthesis of CNC pincer ruthenium complexes 3a and 3b from 2a
then, start another catalytic cycle. The alcohols were
isolated in good to excellent yield after column chroma-
tography as reported in Table 2.
A plausible mechanism for transfer hydrogenation is
shown in Scheme 3, with complex 2b as the catalyst
precursor and 3b as the Ru-hydride intermediate. A simi-
lar mechanism may also be suggested to be operating
during the catalysis with 1b. It is worth mentioning that
the corresponding Ru-hydride species for 1b is observed
LCMS of 3a displayed signal at m/z 866.2 assigned to
+
[3a]
(Figure S33), matching with the catalytic
sample mass. Anion exchange of complex 3a with
NH PF was carried out to obtain the cleaner data of 3b
4
6
1
(Figures S36–S41). In H NMR of 3b, the hydrido signal
gives a triplet at δ = ꢀ8.86 ppm, which is indicative of
Ru–H complex with two phosphines (Figure S36).
Similarly, signals assignable to the pyridine protons and
imidazol-2-ylidene protons appeared at δ = 8.31 as a
doublet, δ = 7.85 ppm as triplet and two doublets at
δ = 7.58 ppm and δ = 7.24 ppm (Figure S36). All the
aromatic protons are slightly shifted to downfield in com-
parison to 2a, though methyl protons show significant
in the fragmentation pattern of 1a in LCMS as [1a–2Cl
+
+
H] ; however, attempts to synthesise or identify this
Ru-hydride intermediate under catalytic conditions have
been unsuccessful, probably due to its high reactivity.
Therefore, mechanistic studies were performed on cataly-
31
upfield shift at δ = 2.47 ppm. P NMR spectrum of 3b
31
sis with complexes 2b and 3b. P NMR of an NMR scale
experiment with 1 equivalent each of 2b, base and
showed peaks at δ = 51.96 for PPh and δ = 144.16 ppm
3
for PF , respectively (Figure S37).
6
2
-propanol indicates the presence of free PPh ligand
3
and the generation of the hydride complex 3b in the
catalytic reaction mixture (Figure S43). A sample of the
reaction mixture during catalysis was taken for LCMS
mass analysis. Analysis of the mass data of the reaction
mixture during catalysis indicates the presence of hydride
complex 3b supporting the proposed mechanism
4 | CONCLUSIONS
In summary, we have investigated the synthesis
and characterisation of ruthenium pincer complexes
ꢀ
ꢀ
[Ru(CNC)(CO)(PPh )Cl]X (X = Cl [1a], PF6 [1b]),
3
ꢀ
ꢀ
(Figure S42).
[Ru(CNC)(PPh ) Cl]X (X = Cl [2a], PF6 [2b]) and
3 2
3
1
ꢀ
ꢀ
Based on the P NMR and mass analyses of the
[Ru(CNC)(PPh ) (H)]X (X = Cl
[3a], PF6
[3b])
3
2
catalytic samples, it is proposed that in the presence of
containing a ‘pyridine–dicarbene’ pincer ligand. All the
ruthenium complexes were found catalytically active for
the transfer hydrogenation of ketones using propan-
2-ol. The in situ transformations of these complexes
during their synthesis were also observed, which helps in
understanding their behaviour during transfer hydroge-
nation catalysis. Complex 1b was found to be more active
for these transformations than 2b and 3b under the
optimised conditions. Subsequently, the substrate scope
for transfer hydrogenation catalysis with a range of
substituted ketones was studied with complex 1b as the
catalyst precursor. Further investigations with lower
catalyst loadings while increasing the reaction time and
applications of these complexes in related catalyses like
acceptorless alcohol oxidation and N-alkylation of amines
are currently undergoing.
i-PrONa, complex 2b generates the ruthenium alkoxide
0
species A. The Ru–H intermediate 3b is formed from
A via β-H elimination by releasing one molecule of
acetone or by dissociation of a PPh ligand if starting
3
0
from 3b. Addition of a ketone to the intermediate 3b
produces another ruthenium alkoxide intermediate B,
which releases the hydrogenated product upon proton-
ation from i-PrOH resulting in the formation of A again.
To further confirm our proposed mechanism, we have
synthesised 3b from 2a and performed a catalytic test run
starting from 3b. The reaction of complex 3a with K CO3
2
in refluxing i-PrOH for 15 h affords clean synthesis of
hydride intermediate complex 3a (Scheme 3).
The pure hydride complex 3a was characterised by
H, C, IR and MS (Figures S30–S32, S34 and S35). ESI
1
13
+

YADAV ET AL.
11 of 11
ACKNOWLEDGEMENTS
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Yoshizawa, Y. Nishibayashi, Eur. J. Inorg. Chem. 2016, 30,
A.K.S. acknowledges Science and Engineering Research
Board, Department of Science and Technology, Govern-
ment of India for funding this study (EMR/2016/004076).
The authors are grateful to the Sophisticated Instrumen-
tation Centre (SIC), IIT Indore, for single-crystal X-ray
diffraction and other characterisation facilities.
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AUTHOR CONTRIBUTIONS
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17] A. Genoni, D. N. Chirdon, M. Boniolo, A. Sartorel, S.
Dibya Yadav: Data curation; formal analysis; investiga-
tion; methodology. Shilpi Misra: Formal analysis; inves-
tigation; methodology. Dheeraj Kumar: Data curation;
formal analysis. Suryabhan Singh: Data curation;
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funding acquisition; project administration; supervision.
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CONFLICT OF INTEREST
There are no conflicts of interest to declare.
2
[
DATA AVAILABILITY STATEMENT
Additional data that support the findings of this study are
available in the supplementary material of this article.
[
[
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Dibya Yadav https://orcid.org/0000-0001-7310-6883
Shilpi Misra https://orcid.org/0000-0002-2408-841X
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Suryabhan Singh https://orcid.org/0000-0002-9078-
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