Transfer hydrogenation of ketones catalysed by half-sandwich (η6-p-cymene) ruthenium(II) complexes incorporating benzoylhydrazone ligands

Article abstract of DOI:10.1002/aoc.3648

Neutral half-sandwich η⁶-p-cymene ruthenium(II) complexes of general formula [Ru(η⁶-p-cymene)Cl(L)] (HL?=?monobasic O, N bidendate benzoylhydrazone ligand) have been synthesized from the reaction of [Ru(η⁶-p-cymene)(μ-Cl)Cl]₂ with acetophenone benzoylhydrazone ligands. All the complexes have been characterized using analytical and spectroscopic (Fourier transform infrared, UV–visible, ¹H NMR, ¹³C NMR) techniques. The molecular structures of three of the complexes have been determined using single-crystal X-ray diffraction, indicating a pseudo-octahedral geometry around the ruthenium(II) ion. All the ruthenium(II) arene complexes were explored as catalysts for transfer hydrogenation of a wide range of aromatic, cyclic and aliphatic ketones with 2-propanol using 0.1?mol% catalyst loading, and conversions of up to 100% were obtained. Further, the influence of other variables on the transfer hydrogenation reaction, such as base, temperature, catalyst loading and substrate scope, was also investigated.

Full text of DOI:10.1002/aoc.3648

Received: 26 July 2016
DOI 10.1002/aoc.3648
Revised: 2 September 2016
Accepted: 2 September 2016
F U L L PA P E R
Transfer hydrogenation of ketones catalysed by half‐sandwich
6
(
η ‐p‐cymene) ruthenium(II) complexes incorporating
benzoylhydrazone ligands
Nanjan Mohan | Rengan Ramesh
Centre for Organometallic Chemistry, School of
6
Neutral half‐sandwich η ‐p‐cymene ruthenium(II) complexes of general formula [Ru
Chemistry, Bharathidasan University,
Tiruchirappalli, Tamil Nadu, India
6
(η ‐p‐cymene)Cl(L)] (HL = monobasic O, N bidendate benzoylhydrazone ligand)
have been synthesized from the reaction of [Ru(η ‐p‐cymene)(μ‐Cl)Cl]2 with
acetophenone benzoylhydrazone ligands. All the complexes have been characterized
using analytical and spectroscopic (Fourier transform infrared, UV–visible, H
NMR, C NMR) techniques. The molecular structures of three of the complexes
have been determined using single‐crystal X‐ray diffraction, indicating a pseudo‐
octahedral geometry around the ruthenium(II) ion. All the ruthenium(II) arene
complexes were explored as catalysts for transfer hydrogenation of a wide range of
aromatic, cyclic and aliphatic ketones with 2‐propanol using 0.1 mol% catalyst load-
ing, and conversions of up to 100% were obtained. Further, the influence of other
variables on the transfer hydrogenation reaction, such as base, temperature, catalyst
loading and substrate scope, was also investigated.
6
Correspondence
Rengan Ramesh, Centre for Organometallic
Chemistry, School of Chemistry, Bharathidasan
University, Tiruchirappalli 620 024, Tamil Nadu,
India.
1
1
3
Email: ramesh_bdu@yahoo.com
KEYWORDS
6
benzoylhydrazone, crystal structure, transfer hydrogenation, η ‐p‐cymene ruthenium
(II) complex
[
12]
[13]
1
| INTRODUCTION
(II)
and Ir(III) complexes,
which have been used and
proved to be effective catalysts. As ligands of the catalysts
6
[14]
In recent years, (η ‐arene) ruthenium(II) derivatives have
attracted considerable interest because of their potential roles
as catalysts for a number of organic reactions, such as hydro-
for transfer hydrogenation, arenes such as p‐cymene
and
[
15]
hexamethylbenzene
for ruthenium(II) complexes have
been the most popular. Furthermore, Ru, Rh and Ir complexes
bearing N, P, O, S, C element‐based ligands with various
forms (such as metal–N‐heterocyclic carbenes, half sandwich,
multidentate metal complexes and their combinations) are
perhaps the most classic and popular catalysts for transfer
hydrogenations.
Aroylhydrazones, R─CO─NH─N═CH─R′, are an
important class of Schiff base ligands, coordinating through
protonated/deprotonated amide oxygen and the imine nitro-
gen of hydrazone moiety; very often an additional donor site
[
1]
[2]
gen transfer, ring‐closing metathesis, stoichiometric C─C
couplings, catalytic oxidative Heck reactions, allyl alco-
hol isomerization to alkyne hydration, asymmetric catalysis
in Diels–Alder reactions, asymmetric transfer hydrogena-
[
3]
[4]
[5]
[6]
[
7]
[8]
tion of ketones, hydration of nitriles and 1,3‐dipolar
[
9]
cycloaddition reactions of nitrones with methacrolein. In
6
addition, the [Ru(η ‐arene)(chelating‐ligand)Cl]‐type com-
plexes reveal the characteristic ‘piano stool’ structure, with
the unreactive arene as a ‘spectator ligand’ in the metal coor-
dination sphere and the chloride as a suitable ‘leaving
(usually N or O) is provided by the aldehyde or ketone
[
10]
[16–18]
group’.
These structural features appear favourable for
forming the hydrazone Schiff base.
Moreover, metal
providing sequential reactions involved in catalysis. Today
most transfer hydrogenations are performed using transition
complexes of aroylhydrazones continue to be an attractive
area of research in view of the simplicity of their synthesis
with main group elements, transition metals, and lanthanides,
[
11]
metal complexes, predominantly based on Rh(I),
Ru
Appl Organometal Chem 2016; 1–9
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
MOHAN AND RAMESH
which exhibit a variety of molecular architectures as well as
very interesting chemical, electrochemical, spectroscopic,
[19–23]
magnetic and biological properties.
The catalytic transfer hydrogenation of carbonyl groups
to their corresponding alcohols as key intermediates spans
from fine chemicals to pharmaceutical synthesis. In addi-
tion, catalytic transfer hydrogenation is considered as a
promising alternative to the more classic reduction methods
[
24,25]
using molecular hydrogen or metal hydrides.
Among
[
24,26]
[27]
various possible hydrogen sources, alcohols,
water,
have found
[
28]
[29]
6
SCHEME 1 Synthesis of η ‐p‐cymene ruthenium(II) benzoylhydrazone
formic acid
and alkylammonium formats
complexes
a wide range of applications. In this reaction, 2‐propanol is
a preferred hydrogen source having favourable properties:
it is stable, easy to handle, non‐toxic and inexpensive.
[
7]
2.1 | Spectral characterization
In most previous studies of transfer hydrogenation in 2‐
propanol, however, a strong base such as KOH, NaOH or
KOtBu is essential to promote the reduction smoothly.
Noyori and co‐workers and Morris and co‐workers reported
useful chiral Ru complexes as catalysts for asymmetric
The Fourier transform infrared (IR) spectra of the ligands
exhibit a medium to strong band in the region 3180–
3196 cm− which is characteristic of the ν(N─H) functional
groups. The free ligand also displays ν(C═N) at 1636–
1648 cm− and ν(C═O) absorptions in the region 1610–
1
1
[30]
−1
hydrogenation under base‐free conditions.
Thus far, the
1653 cm . In addition, the bands due to N─H and C═O
transfer hydrogenation reaction has been extensively studied
and continues to generate a high degree of interest, given the
need to develop environmentally friendly and simple
stretching vibrations are not observed in the spectra of the
complexes indicating that the ligand undergoes
tautomerization and subsequent coordination of the imidolate
oxygen to the ruthenium(II) ion. This is further supported by
the appearance of new bands in the region 1596–1621 cm
which may be attributed to the C═N─N═C fragments and
[
31]
processes.
−
1
Continuing with these studies, we focus our attention
6
mainly on the synthesis and characterization of the new η ‐
p‐cymene ruthenium(II) complexes bearing acetophenone
benzoylhydrazone ligands. The structures of three representa-
tive compounds have been established in the solid state using
single‐crystal X‐ray diffraction. The complexes were
assessed for transfer hydrogenation of a wide range of
ketones using 2‐propanol as hydrogen source with low cata-
lyst loading.
disappearance of ν(C═O) and appearance of ν(N═C─O) and
−
1
ν
(C─O) in the region 1482–1486 and 1369–1376 cm ,
[32]
respectively,
in the spectra of all the metal(II) complexes
therefore indicating the coordination mode of the
benzoylhydrazone ligand to ruthenium(II) ion via the
azomethine nitrogen and the imidolate oxygen.
6
The electronic absorption spectra of the η ‐p‐cymene
ruthenium(II) complexes were recorded in dry chloroform
solution in the range 200–800 nm. All the complexes display
three intense absorptions in the region 230–400 nm. The
2
| RESULTS AND DISCUSSION
6
absorption spectra of the η ‐p‐cymene ruthenium(II)
The acetophenone benzoylhydrazone ligand derivatives
benzoylhydrazone complexes exhibit very intense band
around 266–288 nm and 242–246 nm assigned to ligand‐
centred π–π* and n–π* transitions, respectively. The lowest
energy absorption bands in the electronic spectra of the com-
plexes in the visible region, 319–328 nm, are ascribed to
metal‐to‐ligand charge transfer transitions. Based on the pat-
tern of the electronic spectra of all the complexes an octahe-
dral environment around the ruthenium(II) ion is proposed
similar to that of other octahedral ruthenium(II) complexes.
Further, the binding of the ligand to ruthenium was con-
HL1–HL4 were prepared by modification of a reported pro-
[
20]
cedure,
by Schiff base condensation reaction of
acetophenone with substituted benzhydrazides (R = H, Cl,
Br, OCH ) in an equimolar ratio in refluxing methanol. The
3
6
interaction of 1 equiv. of the appropriate [Ru(η ‐p‐cymene)
(
μ‐Cl)Cl] dimer with 2 equiv. of benzoylhydrazone ligand
2
in benzene with triethylamine afforded the new complexes
6
of the general formula [Ru(η ‐p‐cymene)Cl(L)] (Scheme 1).
The addition of triethylamine to the reaction mixture was
used to remove a proton from the imidol oxygen and to
facilitate the coordination of the imidolate oxygen to the
ruthenium(II) ion. All the complexes are obtained as air‐
stable yellow solids, which are soluble in solvents such as
benzene, toluene, chloroform, dichloromethane, acetonitrile,
dimethylformamide and dimethylsulfoxide. The analytical
1
firmed from H NMR spectra of all the complexes, as shown
in Figures S1‐S4 (supporting information). The multiplets
observed in the region around 6.7–8.0 ppm for all the com-
plexes are assigned to the aromatic protons of the
benzoylhydrazone ligand. The singlet in the region 2.7–
2.8 ppm is due to methyl protons nearer to coordinated
azomethine group. In addition, peaks around 8.9–9.2 ppm
for ─NH disappear for all the complexes due to enolization
6
data for all the η ‐p‐cymene ruthenium(II) complexes are in
good agreement with the molecular structures proposed.

MOHAN AND RAMESH
3
followed by deprotonation on imidole oxygen indicating that
the ruthenium is coordinated through imidolate oxygen. The
four aromatic hydrogens of the p‐cymene ligand appear as
four sets of doublets in the range 3.4–5.3 ppm and methoxy
protons are observed as a singlet for complex 4 at 3.7 ppm.
The isopropyl CH proton appears as a septet in the range
2.4–2.5 ppm and methyl protons appear as a singlet at 2.0–
2
.1 ppm. Additionally, the protons of isopropyl group of the
6
η ‐p‐cymene ligand appear as two sets of doublets in the
range 1.0–1.1 ppm.
2.2 | X‐ray crystallographic studies
The molecular structures of three of the complexes, i.e. 1, 3
and 4, have been determined using single‐crystal X‐ray crys-
tallography. Single crystals of the neutral arene Ru(II)
acetophenone benzoylhydrazone complexes suitable for X‐
ray diffraction analyses were obtained by slow evaporation
of their concentrated solutions in CH Cl –petroleum ether.
FIGURE 2 ORTEP diagram of the complex [Ru(η
showing 30% probability level
6
‐p‐cymene)Cl(L3)] (3),
2
2
The ORTEP views of 1, 3 and 4 are shown in Figures 1, 2
and 3, respectively. A summary of crystal data, data collection
and refinement parameters is given in Table 1. Moreover, the
bond parameters associated with the metal centre are listed in
Table 2. The coordination geometry of the ruthenium atoms is
the classic three‐legged piano‐stool arrangement with a
pseudo‐octahedral geometry and is essentially identical in all
three complexes and display features similar to those of struc-
turally related ruthenium complexes. The coordination sites
about the metal centre in these complexes are occupied by
benzoylhydrazone ligand, a chloro group and p‐cymene ring
6
in η ‐manner. Complex 1 crystallizes in the monoclinic crys-
tal system in the P2 /n space group, while an orthorhombic
1
crystal system in the Pbcn space group is found for complex
6
FIGURE 3 ORTEP diagram of the complex [Ru(η ‐p‐cymene)Cl(L4)] (4),
showing 30% probability level
3
and triclinic crystal system in the P‐1 space group is found
for complex 4. In all complexes Ru─O and Ru─N bond dis-
tances are 2.050(2)–2.057(3) and 2.110(3)–2.117(2) Å,
respectively. The Ru─Cl bond lengths are very similar in all
three complexes and are comparable to the average bond
length of 2.411 Å as in the other Ru(II) complexes. The
Ru─C distances for the coordinated p‐cymene are 2.15–
2
.23, 2.16–2.22 and 2.13–2.21 Å. The distortion of the octa-
hedral geometry is shown by the values of the O(1)─Ru
1)─N(1), O(1)─Ru(1)─Cl(1) and N(1)─Ru(1)─Cl(1) angles,
which are in the range 76.62(10)–86.06(7)°.
(
2
.3 | Catalytic transfer hydrogenation of ketones
6
The synthesized complexes were explored for their catalytic
efficiency in transfer hydrogenation of ketones. For this
FIGURE 1 ORTEP diagram of the complex [Ru(η ‐p‐cymene)Cl(L1)] (1),
showing 30% probability level

4
MOHAN AND RAMESH
TABLE 1 Crystal data and structure refinement for complexes 1, 3 and 4
1
3
4
Empirical formula
Formula weight
Temperature
C
25
H
27ClN
2
ORu
C
25
H
26BrClN
2
ORu
C
26
H
29ClN
2
O
2
Ru
508.01
586.91
538.03
296(2) K
296(2) K
296(2) K
0.71073 Å
Triclinic
P‐1
Wavelength
0.71073 Å
0.71073 Å
Crystal system
Space group
Monoclinic
P2 /n
Orthorhombic
Pbcn
1
Unit cell dimensions
a = 9.6667(4) Å
α = 90°
a = 18.2461(6) Å
α = 90°
a = 9.3997(12) Å
α = 89.873(3)°
b = 18.1338(9) Å
β = 107.121(2)°
c = 13.4109(5) Å
γ = 90°
b = 8.3239(3) Å
β = 90°
c = 30.6984(12) Å
γ = 90°
b = 10.0756(10) Å
β = 77.456(3)°
c = 13.6762(13) Å
γ = 74.156(3)°
3
3
3
Volume
2246.67(17) Å
4662.4(3) Å
1214.0(2) Å
3
8, 1.672 Mg m−3
2.521 mm−1
2, 1.472 Mg m−3
0.781 mm−1
Z, calculated density
Absorption coefficient
F(000)
4, 1.502 Mg m−
0.836 mm−
1
1040
2352
552
Crystal size
0.60 × 0.40 × 0.20 mm
2.47 to 28.44°
−9 ≤ h ≤ 12,
0.50 × 0.35 × 0.30 mm
1.33 to 28.30°
0.60 × 0.40 × 0.20 mm
1.53 to 28.39°
Theta range for data collection
Limiting indices
−24 ≤ h ≤ 22,
−7 ≤ k ≤ 11,
−40 ≤ l ≤ 40
−12 ≤ h ≤ 12,
−13 ≤ k ≤ 13,
−16 ≤ l ≤ 18
−
22 ≤ k ≤ 24,
16 ≤ l ≤ 17
−
Reflections collected / unique
18 588 / 5528
R(int) = 0.0320]
29 403 / 5735
[R(int) = 0.0418]
9833 / 5881
[R(int) = 0.0264]
[
Completeness to theta =28.44
Absorption correction
97.7%
98.9%
96.5%
Semi‐empirical from equivalents
Semi‐empirical from equivalents
0.5184 and 0.3654
Semi‐empirical from equivalents
0.8595 and 0.6516
Max. and min. Transmission
Refinement method
0.8507 and 0.6340
2
2
2
Full‐matrix least‐squares on F
Full‐matrix least‐squares on F
Full‐matrix least‐squares on F
Data / restraints / parameters
5528 / 0 / 271
1.116
5735 / 0 / 284
1.182
5881 / 0 / 289
1.057
2
Goodness‐of‐fit on F
Final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0391,
wR2 = 0.0895
R1 = 0.0430,
wR2 = 0.0956
R1 = 0.0432,
wR2 = 0.1097
R1 = 0.0576,
wR2 = 0.0975
R1 = 0.0854,
wR2 = 0.1208
R1 = 0.0615,
wR2 = 0.1340
0.495 and − 0.525 e Å−
3
0.781 and − 0.552 e Å−3
0.975 and − 0.738 e Å−3
Largest diff. Peak and hole
TABLE 2 Selected bond lengths (Å) and angles (°) for complexes 1, 3 and 4
1
3
4
Bond lengths
N(1)–N(2)
1.411(3)
2.117(2)
2.050(2)
2.412(8)
1.287(3)
1.304(3)
1.299(4)
N(1)–N(2)
N(1)–Ru(1)
O(1)–Ru(1)
Cl(1)–Ru(1)
C(9)–O(1)
C(9)–N(2)
C(7)–N(1)
1.405(4)
2.113(3)
2.057(3)
2.410(13)
1.286(5)
1.299(5)
1.296(5)
N(1)–N(2)
1.402(4)
2.110(3)
2.054(2)
2.413(11)
1.300(4)
1.303(5)
1.297(4)
N(1)–Ru(1)
N(2)–Ru(1)
O(1)–Ru(1)
Cl(1)–Ru(1)
C(18)–O(1)
C(18)–N(1)
C(20)–N(2)
O(1)–Ru(1)
Cl(1)–Ru(1)
C(17)–O(1)
C(17)–N(2)
C(18)–N(1)
Bond angles
N(2)–N(1)–Ru(1)
C(17)–N(2)–N(1)
C(17)–O(1)–Ru(1)
O(1)–Ru(1)–N(1)
O(1)–Ru(1)–Cl(1)
N(1)–Ru(1)–Cl(1)
113.23(16)
111.10(2)
112.84(17)
76.65(8)
N(2)–N(1)–Ru(1)
C(9)–N(2)–N(1)
C(9)–O(1)–Ru(1)
O(1)–Ru(1)–N(1)
O(1)–Ru(1)–Cl(1)
N(1)–Ru(1)–Cl(1)
112.70(2)
N(1)–N(2)–Ru(1)
C(18)–N(1)–N(2)
C(18)–O(1)–Ru(1)
O(1)–Ru(1)–N(2)
O(1)–Ru(1)–Cl(1)
N(2)–Ru(1)–Cl(1)
113.50(2)
111.90(3)
112.90(2)
76.62(10)
85.01(8)
85.78(9)
112.10(3)
112.40(2)
76.91(11)
84.72(10)
85.74(10)
86.06(7)
84.03(7)

MOHAN AND RAMESH
5
a
study, the reduction of acetophenone to 1‐phenylethanol was
taken as a model reaction which was carried out at 82 °C with
acetophenone (0.36 mmol), complex 2 (1 mol%) and base
TABLE 4 Catalytic activity of the complexes
(0.007 mmol) in 2‐propanol. First, the corresponding blank
and control experiments were carried out for this reaction.
Without the addition of the base, no conversion is obtained.
Also, using the base without the catalyst gives no appreciable
conversions (Table 3).
b
Entry
Complex
Conversion (%)
1
2
3
4
1
2
3
4
97
100
98
In order to optimize the reaction conditions, first of all,
the influence of base was investigated. We initially carried
out the reaction of acetophenone using complex 2 as a test
catalyst in the presence of 2‐propanol with various bases.
Strong inorganic bases like NaOH or KOH give higher con-
versions of 100 and 98%, respectively. Whereas weak inor-
ganic bases such as NaHCO and Na CO give moderate
96
a
Reaction conditions: acetophenone (0.36 mmol), catalyst 1–4 (1 mol%), base
0.007 mmol) in 2‐propanol (5 ml) at 82 °C for 5 h.
(
b
Conversion determined by GC–MS analysis using m‐xylene as internal standard.
2). The catalytic activity reveals that introduction of electron‐
withdrawing substituents onto the coordinative
3
2
3
conversions of 25 and 41%, respectively. For organic bases
such as triethylamine we observed only traces amount of
alcohol formed. The conversions are summarized in Table 3
benzoylhydrazone ligand enhances the catalytic activity in
comparison with electron‐donating substituents. It is
observed that the presence of electron‐withdrawing group
on the aromatic ring possibly decreases the electron density
on the metal centre and hence the rate of transfer hydrogena-
tion increases.
In order to optimize the effect of catalyst loading, differ-
ent catalyst‐to‐substrate(C:S) ratios were tested in the transfer
hydrogenation of acetophenone using complex 2 as a catalyst
in 2‐propanol–NaOH. When increasing the C:S ratio to
1:100, 1:500 and 1:1000 the reaction proceeds with good
conversions. The reaction proceeds with high conversion of
acetophenone to 1‐phenylethanol when a C:S ratio of
1:1000 (0.1 mol%) is used. The results are summarized in
Table 5. Thus, it is concluded that a C:S ratio of 1:1000 is
the best compromise between optimum reaction rate and C:
S ratio. Thus, the results of the optimization studies reveal
that the best conversions are obtained using NaOH as
the base in 2‐propanol solvent at 82 °C with 0.1 mol% of
catalyst 2.
.
Therefore, the activity trend is found to be in the order
NaOH > KOH > Na CO > NaHCO > CH COONa > Et N,
2
3
3
3
3
consistent with the relative strength of the bases. The slight
t
differences in the activities of NaOH, BuOK and KOH high-
light their similar strengths in deprotonating 2‐propanol as
[33]
previously observed by Aydemir et al.
We screened the catalytic activity of the four complexes
in the transfer hydrogenation using acetophenone as a repre-
sentative substrate with 2‐propanol and NaOH as a base at
82 °C. The catalytic reactions were performed using
0.36 mmol of acetophenone, 1 mol% of complexes 1–4 and
NaOH (0.007 mmol) in 2‐propanol reflux for 5 h at 82 °C.
All the complexes efficiently catalyse the transfer hydrogena-
tion of acetophenone with maximum conversions within 5 h.
The results in Table 4 show the influence of the nature of R
on catalyst activity. Complex 2 shows maximum conversion
in the transfer hydrogenation of acetophenone (Table 4, entry
Due to the highest catalytic activity of complex 2, it was
applied as a catalyst to explore the substrate scope in the
transfer hydrogenation of ketones (Table 6). All reactions
were carried out under identical conditions to allow compar-
ison of results. The products were identified by GC–MS after
doing the required workup. The complex catalyses the trans-
fer hydrogenation of ketones to their corresponding alcohols
TABLE 3 Effect of base and temperature in transfer hydrogenation of
acetophenone
a
b
Entry
Base
NaOH
Temp. (°C)
Time (h)
Conversion (%)
1
2
3
4
5
6
7
8
9
82
82
50
82
82
82
82
82
82
5
5
100
98
a
TABLE 5 Effect of catalyst loading
KOH
NaOH
5
52
—
c
24
5
0
NaOH
NaHCO
Na CO
CH COONa
Et
22
b
c
Entry
Ru (mol%)
Conversion (%)
TON
3
8
25
2
3
8
41
1
2
3
1
100
94
100
470
920
3
12
12
18
0.2
0.1
3
N
Trace
92
a
a
Reaction conditions: acetophenone(0.36 mmol), catalyst 2 (1 mol%), base
0.007 mmol) in 2‐propanol (5 ml).
Reaction conditions: acetophenone(0.36 mmol), catalyst 2 (1.0 ‐ 0.1 mol%), base
(
b
(0.007 mmol) in 2‐propanol (5 ml) at 82 °C for 5 h.
b
Conversion determined by GC–MS analysis using m‐xylene as internal standard.
Conversion determined by GC–MS analysis using m‐xylene as internal standard.
c
c
Reaction carried out in absence of catalyst.
Turnover number = ratio of moles of product formed to moles of catalyst used.

6
MOHAN AND RAMESH
a
TABLE 6 Transfer hydrogenation of ketones using complex 2
TABLE 6 (Continued)
Conversion
Conversion
b
TONc
b
c
Entry
Substrate
Product
(%)
Entry
Substrate
Product
(%)
TON
1
92
920
13
98
980
750
710
1
1
4
5
75
71
2
97
970
3
4
99
990
a
Reaction conditions: ketone (3.6 mmol), catalyst
2 (0.1 mol%), NaOH
(
b
0.007 mmol) in 2‐propanol (5 ml) at 82 °C for 5 h.
Conversion determined by GC–MS analysis using m‐xylene as internal standard.
c
Turnover number = ratio of moles of product formed to moles of catalyst used.
100
1000
with good to excellent conversions in all cases.
Acetophenone with substituents of varying electronic proper-
ties are efficiently reduced to the corresponding secondary
alcohols in excellent conversions (entries 1–6). The presence
of electron‐withdrawing or electron‐donating substituents on
the aromatic ring has a significant effect in the conversion.
In general, electron‐withdrawing substituents (4′‐chloro, 4′‐
bromo, 4′‐nitro) on the aryl ring give higher conversions
5
90
900
6
7
100
90
1000
900
(entries 2–4) when compared to that of acetophenone (entry
1
) whereas electron‐donating substituents (4′‐methyl) on the
ring give lower conversions (entry 5) than that of
acetophenone. Substrates with ortho‐substituted electron‐
withdrawing group such as fluoro play a significant role
and are catalysed with 100% conversions (entry 6). The com-
plex catalyses the conversion of acetophenone derivatives
with electron‐withdrawing substituents to the corresponding
alcohols almost quantitatively (100% conversion) in 5 h. As
the steric bulk of the acyl chain increases, the conversion
markedly decreases. Sterically hindered ketones such as
benzophenone (entry 7) undergo hydrogenation to give the
corresponding secondary alcohols in moderate conversions
of 90%. Cyclic ketones containing five‐ to eight‐membered
rings (entries 8–12) are reduced effectively to give the corre-
sponding alcohols with conversions of up to 100%. The
complex also efficiently catalyses the reduction of aliphatic
ketones such as 2‐pentanone, 3‐pentanone and 4‐methyl‐2‐
pentanone (entries 13–15) to their corresponding secondary
alcohols with high conversions. At this point, a comparison
in terms of catalytic efficiency and scope of application can
be established between our complex 2 and other structurally
related arene ruthenium catalysts. Albrecht and co‐workers
have reported triazolylidene arene ruthenium(II) complexes
for the transfer hydrogenation of acetophenone of high con-
8
9
100
100
1000
1000
1
1
0
1
100
88
1000
880
1
2
56
560
(
Continues)
[
34]
versions with 0.5 mol% catalyst loading.
Ruthenium(II)
(
arene) complexes of bispyrazolylazines act as catalysts in
[
35]
hydrogen transfer of ketones with low turnover numbers.

MOHAN AND RAMESH
7
6
Further, neutral η ‐arene ruthenium(II) complexes with
monodentate P‐donor ligands show excellent yields in
transfer hydrogenation reaction with high catalyst load-
4.2 | Physical measurements
IR spectra were recorded with a JASCO FT/IR 400 Plus
instrument. Elemental analysis (C, H, N) was carried out
with a Vario EL III CHNS elemental analyser at the Sophis-
ticated Test and Instrumentation Centre (STIC), Cochin Uni-
versity, Kochi. Electronic spectra in chloroform solution
were obtained with a CARY 300 Bio UV–visible spectrom-
[36]
ing.
Nieto et al. have reported transfer hydrogenation
of acetophenone by arene ruthenium(II) bipyridyl com-
plexes with maximum conversions in longer reaction
[
37]
time.
Recently, arene ruthenium(II) Schiff base
complexes have been shown to give moderate to excellent
conversions in transfer hydrogenation of carbonyl
compounds with high loading of catalyst in long reaction
1
eter (Varian). All H NMR spectra were recorded with a
Bruker 400 MHz instrument using tetramethylsilane as an
internal reference. Melting points were recorded with a
Boetius micro‐heating table and are corrected. Organic com-
pounds in catalysis were identified using GC–MS with a
Shimadzu (Japan) QP2010 SE spectrometer using an
RTX‐5 MS detector equipped with a capillary column
[
38]
time.
Further, the workup process is very simple for this
catalytic system, as the catalyst is stable in all organic
solvents and it can be recovered easily. Assuming that these
catalytic transformations follow the classic pathway in
which the ketone coordinates to ruthenium hydride inter-
(30 m × 0.25 mm × 0.25 μm) and high‐purity helium as car-
[
7,39–41]
mediates as proposed by several research groups,
rier gas.
the observations seem to indicate that hydride transfer from
the metal centre to the coordinated ketone is the rate‐limit-
ing step (rather than ketone complexation) in the catalytic
4
.3 | Preparation of acetophenone benzoylhydrazone
[
41]
ligand
cycle.
A mixture of 4‐substituted benzhydrazide (0.01 mmol) and
acetophenone (0.01 mmol) in methanol (30 ml) was refluxed
for 30 min. The separated solid was filtered and dried in air.
Yield: 87–91%.
3
| CONCLUSIONS
6
Four new η ‐p‐cymene ruthenium(II) complexes of formula
6
6
[Ru(η ‐p‐cymene)Cl(L)] (HL
=
monobasic bidendate
4.4 | Synthesis of η ‐p‐cymene ruthenium(II)
benzoylhydrazone ligand) have been synthesized by treating
benzoylhydrazone complexes
6
[Ru(η ‐p‐cymene)(μ‐Cl)Cl] with acetophenone benzoyl-
2
The complexes were prepared using a general procedure in
hydrazone ligands. The composition of the complexes was
6
which [Ru(η ‐p‐cymene)(μ‐Cl)Cl] (25 mg, 0.04 mmol) was
2
determined using elemental analysis, and IR, UV–visible,
added to benzoylhydrazone ligand (19.4–25.8 mg,
1
13
H NMR and C NMR spectroscopic methods. X‐ray dif-
0
.08 mmol) in the presence of Et N (0.5 ml) in benzene
3
fraction analysis of three of the complexes confirms the O
and N coordination of the ligands and indicates the presence
of pseudo‐octahedral geometry around ruthenium ion.
Further, the ruthenium(II) arene benzhydrzone complexes
were developed as efficient catalysts in transfer hydrogena-
tion of a diverse range of ketones giving excellent conver-
sions in the presence of NaOH–2‐propanol. The catalysts
work well at a loading of 0.1 mol% and exhibit appreciable
turnover number in relatively short time.
(
30 ml). The resulting solution was refluxed for 5 h and the
progress of the reaction was monitored by TLC. At the end
of the reaction, the solution was concentrated to about 3 ml
and petroleum ether was added whereby solid separated
out. The obtained yellow solid was recrystallized from
CH Cl –petroleum ether at room temperature. Yield:
2
2
7
5–86%.
6
4
.4.1
|
[Ru(η ‐p‐cymene)(Cl)(L1)] (1)
Yellow; yield 86%; m.p. 194 °C (with decomposition). Anal.
Calcd for C H ClN ORu (%): C, 59.10; H, 5.35; N, 5.51.
2
5
27
2
−
1
4
4
| EXPERIMENTAL
Found (%): C, 59.20; H, 5.27; N, 5.55. IR (KBr, cm ):
621 ν(C═N─N═C), 1486 ν , 1373 ν(C─O). UV–visible
1
(
(
(N═C─O)
3
CH CN, λ /nm; ε/dm mol−1 cm ): 323 (720), 270
−1
.1 | Reagents and materials
3 max
1
1196), 243 (1429). H NMR (400 MHz, CDCl , δ, ppm):
3
RuCl ⋅3H O is commercially available and was used as sup-
7.1–8.0 (m, 10H, aromatic), 5.3 (d, J = 6 Hz, 1H, cymene
Ar‐H), 5.0 (d, J = 6 Hz, 1H, cymene Ar‐H), 4.7 (d,
J = 5.6 Hz, 1H, cymene Ar‐H), 3.4 (d, J = 6 Hz, 1H, cymene
3
2
plied from Loba Chemie Pvt Ltd. Benzoylhydrazide deriva-
tives and ketones were purchased from Sigma‐Aldrich and
used as received. All other chemicals were obtained from
commercial sources and used as received. All the solvents
were dried and distilled prior to use using standard proce-
Ar‐H), 2.7 (s, 3H, CH of ligand), 2.4 (m, 1H, CH of p‐
3
cymene), 2.0 (s, 3H, CH3 of p‐cymene), 1.0 (dd,
13
J = 12.8 Hz, J = 6.8 Hz, 6H, 2CH of p‐cymene).
C
3
[
42]
6
dures.
The precursor [Ru(η ‐p‐cymene)(μ‐Cl)Cl] dimer
NMR (100 MHz, CDCl , δ, ppm): 17.8 (CH of p‐cymene),
2
3
3
[43]
was prepared according to a literature procedure.
21.0, 21.5 (2CH of p‐cymene), 22.9 (CH of ligand), 29.7
3
3

8
MOHAN AND RAMESH
(
CH of p‐cymene), 78.5–85.1 (aromatic carbons of p‐
J = 5.6 Hz, 1H, cymene Ar‐H), 3.4 (d, J = 5.6 Hz, 1H,
cymene Ar‐H), 3.7 (s, 3H, OCH ), 2.7 (s, 3H, CH of ligand),
cymene), 98.0 and 100.5 (quaternary carbons of p‐cymene),
3
3
126.6, 127.0, 127.8, 127.9, 129.1, 131.6 (CHarom and Carom),
2.4 (m, 1H, CH of p‐cymene), 2.0 (s, 3H, CH of p‐cymene),
3
1
42.4 (C═N), 167.4 (C─O).
1.0 (dd, J = 12.8 Hz, J = 6.8 Hz, 6H, 2CH of p‐cymene).
3
13
C NMR (100 MHz, CDCl , δ, ppm): 18.8 (CH of p‐
3
3
6
cymene), 22.1, 22.4 (2CH of p‐cymene), 23.9 (CH of
4
.4.2
|
[Ru(η ‐p‐cymene)(Cl)(L2)] (2)
3
3
ligand), 30.7 (CH of p‐cymene), 80.3–86.1 (aromatic carbons
of p‐cymene), 98.9 and 101.5 (quaternary carbons of p‐
Yellow; yield 82%; m.p. 180 °C (with decomposition). Anal.
Calcd for C H Cl N ORu (%): C, 55.35; H, 4.83; N, 5.16.
Found (%): C, 55.31; H, 4.77; N, 5.20. IR (KBr, cm ): 1596
ν(C═N─N═C), 1483 ν , 1374 ν(C─O). UV–visible
25
26
2 2
−
1
cymene), 55.2 (OCH ) 112.9, 125.3, 128.0, 128.8, 130.5,
3
1
61.2 (CHarom and Carom), 143.5 (C═N), 167.6 (C─O).
(N═C─O)
3
−1
−1
(CH CN, λ /nm; ε/dm mol cm ): 319 (616), 268
3 max
1
(924), 242 (1087). H NMR (400 MHz, CDCl , δ, ppm):
4.5 | X‐ray crystallographic study
3
7
.1–8.0 (m, 9H, aromatic), 5.3 (d, J = 6 Hz, 1H, cymene
Crystals were obtained after slow evaporation of CH Cl –
2
2
Ar‐H), 5.0 (d, J = 6.4 Hz, 1H, cymene Ar‐H), 4.7 (d,
J = 6 Hz, 1H, cymene Ar‐H), 3.4 (d, J = 5.6 Hz, 1H, cymene
Ar‐H), 2.7 (s, 3H, CH of ligand), 2.4 (m, 1H, CH of p‐
cymene), 2.0 (s, 3H, CH3 of p‐cymene), 1.0 (dd,
J = 12.4 Hz, J = 6.8 Hz, 6H, 2CH of p‐cymene).
6
petroleum ether solutions of [Ru(η ‐p‐cymene)Cl(L)] 1, 3
and 4 at room temperature. The single crystals were mounted
on a glass fibre and transferred to a Bruker AXS Kappa
APEX II single‐crystal X‐ray diffractometer with graphite‐
monochromated Mo Kα radiation (λ = 0.071073 nm). Data
were collected at 296(2) K. Intensities were corrected for
Lorentz and polarization effects and empirical absorption.
The structures were solved by direct methods and refined
3
13
C
3
NMR (100 MHz, CDCl , δ, ppm): 17.8 (CH of p‐cymene),
2
3
3
1.0, 21.4 (2CH of p‐cymene), 22.9 (CH of ligand), 29.7
3
3
(CH of p‐cymene), 78.5–85.0 (aromatic carbons of p‐
cymene), 98.1 and 100.6 (quaternary carbons of p‐cymene),
1
1
2
by full‐matrix least‐squares on F . All non‐hydrogen atoms
26.8, 127.1, 128.0, 129.2, 130.1, 135.1 (CHarom and Carom),
42.3 (C═N), 167.8 (C─O).
were refined with anisotropic displacement parameters. All
hydrogen atoms were geometrically placed and their coordi-
nates were refined riding on their parent atoms. CCDC refer-
ence numbers are 1449542, 1046405 and 1449623.
6
4
.4.3
|
[Ru(η ‐p‐cymene)(Cl)(L3)] (3)
Yellow; yield 79%; m.p. 172 °C (with decomposition). Anal.
Calcd for C H BrClN ORu (%): C, 51.16; H, 4.46; N,
25
26
2
4.6 | Procedure for catalytic transfer hydrogenation of
ketones
4
.77. Found (%): C, 51.11; H, 4.54; N, 4.85. IR (KBr, cm
−
1
)
: 1610 ν(C═N─N═C), 1482 ν
, 1376 ν(C─O). UV–vis-
(N═C─O)
−1
3
−1
A mixture of ketone (3.6 mmol), ruthenium catalyst (0.1 mol)
and base (0.007 mmol) was heated to reflux in 5 ml of 2‐
propanol for an appropriate period of time as mentioned.
The catalyst was removed as a precipitate from the reaction
mixture by the addition of diethyl ether followed by filtration
and subsequent neutralization with 5 ml of 1 N HCl, washed
with water and dried over anhydrous Na SO . Then the ether
ible (CH CN, λ /nm; ε/dm mol cm ): 328 (467), 288
3
max
1
(714), 244 (816). H NMR (400 MHz, CDCl , δ, ppm):
3
7.2–8.0 (m, 9H, aromatic), 5.3 (d, J = 6.4 Hz, 1H, cymene
Ar‐H), 5.1 (d, J = 6 Hz, 1H, cymene Ar‐H), 4.7 (d,
J = 5.6 Hz, 1H, cymene Ar‐H), 3.5 (d, J = 5.6 Hz, 1H,
cymene Ar‐H), 2.8 (s, 3H, CH of ligand), 2.5 (m, 1H, CH
3
2
4
of p‐cymene), 2.1 (s, 3H, CH3 of p‐cymene), 1.1 (dd,
13
layer was passed through a short path of silica gel and the fil-
trate was subjected to GC–MS analysis. The conversions
obtained are related to the residual unreacted ketone and are
averages of two runs in the case of all catalytic reactions.
J = 12.4 Hz, J = 6.8 Hz, 6H, 2CH of p‐cymene).
C
3
NMR (100 MHz, CDCl , δ, ppm): 17.8 (CH of p‐cymene),
3
3
2
1.0, 21.4 (2CH of p‐cymene), 22.9 (CH of ligand), 29.7
3
3
(CH of p‐cymene), 78.5–85.0 (aromatic carbons of p‐
cymene), 98.1 and 100.6 (quaternary carbons of p‐cymene),
ACKNOWLEDGMENTS
1
1
23.6, 127.1, 128.0, 129.4, 129.8, 130.6 (CHarom and Carom),
42.3 (C═N), 167.9 (C─O).
One of the authors (N.M.) thanks the University Grants Com-
mission (UGC), New Delhi, for the award of UGC‐RFSMS.
We express sincere thanks to DST‐FIST, India for the use
of Bruker 400 MHz spectrometers at the School of Chemis-
try, Bharathidasan University, Tiruchirappalli. We thank
CEFIPRA, India for the GC–MS analysis through research
project (ref. no. IFC/5005‐1).
6
4
.4.4
|
[Ru(η ‐p‐cymene)(Cl)(L4)] (4)
Yellow; yield 75%; m.p. 165 °C (with decomposition). Anal.
Calcd for C H ClN O Ru (%): C, 58.04; H, 5.43; N, 5.20.
2
6
29
2 2
−
1
Found (%): C, 58.10; H, 5.46; N, 5.12. IR (KBr, cm ): 1605
ν(C═N─N═C), 1487 ν , 1369 ν(C─O). UV–visible
(N═C─O)
3
−1
−1
(
(
CH CN, λ /nm; ε/dm mol cm ): 322 (688), 270
3 max
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