Green conversion of alcohols to carbonyls catalyzed by novel ruthenium-Schiff base-triphenylphosphine complexes

Article abstract of DOI:10.1016/j.inoche.2010.10.010

A series of new Ru^II-Schiff base complexes of containing Ph ₃P has been prepared and characterized. The complexes have been used as catalysts for the oxidation of alcohols in [EMIM]Cl ionic liquid-NaOCl system. Higher catalytic activity was found for RuL1. A plausible mechanism has been proposed.

Full text of DOI:10.1016/j.inoche.2010.10.010

Inorganic Chemistry Communications 14 (2011) 155–158
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Green conversion of alcohols to carbonyls catalyzed by novel ruthenium-Schiff
base-triphenylphosphine complexes
⁎
Dileep Ramakrishna, Badekai Ramachandra Bhat
Catalysis and Material Chemistry Division, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasanagar-575025, India
a r t i c l e i n f o
a b s t r a c t
A series of new RuII-Schiff base complexes of containing Ph3P has been prepared and characterized. The
complexes have been used as catalysts for the oxidation of alcohols in [EMIM]Cl ionic liquid–NaOCl system.
Higher catalytic activity was found for RuL1. A plausible mechanism has been proposed.
© 2010 Elsevier B.V. All rights reserved.
Article history:
Received 26 July 2010
Accepted 13 October 2010
Available online 20 October 2010
Keywords:
Ru complexes
Catalytic oxidation
Ionic liquid
NaOCl
Ru-based oxidation catalysis is a powerful and extremely versatile
synthetic tool to afford selectively oxygenated products both in
homogeneous and in heterogeneous conversions [1,2]. The ability of
ruthenium to assume a wide range of oxidation states and
coordination geometries provides unique opportunities for catalysis.
The oxidation of alcohols by ruthenium complexes along with
primary oxidants like iodobenzene [3], alkyl hydroperoxides [4],
NMO [5] and molecular oxygen [6] is well established. Issues such as
product separation from the catalyst and catalyst recovery remain
problematic. One of the alternative approaches for tackling these
problems is the use of ambient temperature ionic liquids (ILs) as
solvents. This is because of their solvating ability, negligible vapor
pressure, easy recyclability and reusability. Over the last several years,
many transition metals such as nickel [7], cobalt [8], ruthenium [9],
manganese [10], tungsten [11], rhenium [12], iron [13] and vanadium
[14] have been used as catalysts for alcohol oxidation with ILs as green
solvents [15]. The present work reports the synthesis of a series of
ruthenium triphenylphosphine complexes (RuL1–RuL5) containing
N-(2-pyridyl)-N'-(5-Sub-salicylidene) hydrazine (Scheme 1) and
their application as catalysts for the oxidation of alcohols to carbonyl
compounds in ethyl-methyl imidazolium chloride [EMIM] ionic liquid
media with NaOCl as co-oxidant.
RuL1–RuL5 were prepared by refluxing a benzene solution of [RuCl2
(PPh3)3] and ligand in a 1:1 molar ratio for 3 h [20].
All the complexes were greenish-black in colour and found to be
air stable at room temperature, and non-hygroscopic in nature. The
synthesized ruthenium (II) complexes are highly soluble in common
organic solvents.
The IR spectra of all the ligands exhibit significant bands around 1561–
1579 and 1268–1278 cm−1 which are tentatively assigned to ν–CH = N–
and phenolic ν–C–O stretching, respectively. On complexation ν–CH = N–
appears at 1590–1610 cm−1 and the red shift corroborates the N
(azomethine) coordination. In the spectra of complexes the coordination
through phenolic oxygen is confirmed by the increase in C–O at higher
frequencies in the region 1275–1296 cm−1. The positive shift is further
supported by the disappearance of the ν–OH band in the range 3430–
3454 cm−1 in all the complexes. Further, these complexes show a strong
band near 1440, 1100, and 690 cm−1 indicating the presence of
coordinated PPh3 ligands. Besides, all the complexes show strong bands
corresponding to ν–M–N and ν–M–O in the region 437–451 and 516–
525 cm−1, respectively [21–23]. All the complexes show multiplets at δ
6.75–8.60 ppm due to the coordinated PPh3 ligands. Among the expected
aromatic proton signals for the coordinated ligand, most have been clearly
observed while a few could not be detected due to their overlap with their
PPh3 signals. Additional methoxy signals are observed as singlet for
complex RuL5 at δ 2.24 ppm. A sharp singlet appeared for OH proton of all
ligands in the region δ 13.0 ppm is absent in all the complexes. A peak
observed at 8.5 ppm in the complexes has been assigned to azomethine
proton (−CH=N−). 31P NMR spectra exhibit a singlet at 33.5–34.6 ppm
suggesting that triphenylphosphine groups must be trans to each other
[24,25]. Electronic spectra of the complexes have been recorded in
dichloromethane solution. All the ruthenium complexes show four
intense absorptions in the visible and ultraviolet region 450–280 nm.
[RuCl2(PPh3)3] was prepared by refluxing RuCl3.3H2O and triphenyl-
phosphine in methanol for 4 h [16–18]. The Schiff bases were prepared in
70–80% yield by condensation reactions of 2-hydrazinopyridine with the
corresponding 5-substituted salicylaldehyde in methanol [19]. Complexes
⁎
Corresponding author. Tel.: +91 824 2474000x3204; fax: +91 824 2474033.
E-mail address: chandpoorna@yahoo.com (B.R. Bhat).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.10.010

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D. Ramakrishna, B.R. Bhat / Inorganic Chemistry Communications 14 (2011) 155–158
P
H
N
HO
R
N
Benzene
N
RuCl 2 (PPh3)3
N
N
N
H
R
Reflux, 4 hr
Ru
O
Cl
P
(RuL1: R=H, RuL2: R= Cl, RuL3: R=Br, RuL4: R=NO2, RuL5: R= OCH3).
Scheme 1. Synthesis of ruthenium (II) complexes.
The intense absorption in the visible region at 437–442 nm is probably
due to metal-to-ligand charge transfer transitions [26]. In addition, the
other high intensity bands in the 300–305 nm regions were characterized
by ligand-centered transitions taking place in the three coordinated
ligand. The pattern of the electronic spectra of all the complexes indicated
the presence of an octahedral environment around ruthenium (II) ion,
similar to that of other ruthenium complexes [27].
Benzylalcohol was selected as a model substrate for optimization
of the reaction conditions [28,29]. In order to study the effect of time
on the activity, the product analysis was done at regular intervals of
time under similar reaction conditions (Fig 1). It was found that the
total reaction time required was 30 min in EMIM-NaOCl. The
oxidation occurred only in poor yield by simply bubbling molecular
oxygen through reaction mixture under similar reaction conditions
(Table 1, entry 2). To evaluate the catalytic effect of RuL1, the
oxidation of benzylalcohol was carried out under similar reaction
conditions in the absence of catalyst and no conversion was observed
(Table 1, entry 3). The reaction was complete at 30 min at room
temperature (Table 1, entry 1). Further, when the oxidation of
benzylalcohol was carried out using less amount of RuL1, the yield
was low (Table 1, entry 5). Also the reaction was carried out using low
amount of oxidant under same conditions and was observed that the
yield was low (Table 1, entry 6).
The oxidation of other benzylic and allylic alcohols was then
examined using the optimized reaction conditions. All the synthesized
complexes were found to catalyze the oxidation of alcohols to
corresponding carbonyl compounds. RuL1 showed better results
than all the other complexes. This may be due to the fact that the
catalytic activity of complexes varies with the size of substituent. It
was observed that the activity decreases with increase of bulkiness of
the substituents. This may be due to steric hindrance caused by the
substituent, which can affect the planarity of the ligand in the
complexes [7]. The results for the oxidation of a variety of alcohols are
summarized in Table 2. Further oxidation to acids was ruled out. The
GC results showed no extra peak confirming the absence of carboxyl
acid formation. Inspite of IL being non-volatile and non-flammable, its
improper disposal would still lead to new pollution of environment
(because of minimal degradation in the environment). At the same
time ILs are also expensive reagents. Thus, attention needs to be paid
to recycling and re-use of ILs.
Fig. 2 shows the efficiency of the reaction in [EMIM]Cl till 7 runs
with benzylalcohol as substrate. It is clear from the figure that
recycling of ionic liquid slightly affects the conversion of the product.
Results of the present investigation suggest that the complexes are
able to react efficiently with NaOCl to yield high valent ruthenium-
Table 1
Oxidation of benzylalcohol to benzaldehydea in EMIM-NaOCl system.
Entry
Reaction time (min)
% conversion
1
30
360
360
30
92.1
b15
NRd
59.0
56.3
2b
3c
4e
5f
30
a
Unless otherwise indicated, all reactions were carried out with 1 mmol of
benzylalcohol with 0.01 mmol of RuL1 catalyst, 1 mmol of NaOCl.
b
The reaction was carried out under oxygen instead of NaOCl.
Reaction without catalyst.
NR: No reaction.
0.005 mmol of catalyst.
c
d
e
Fig 1. Effect of time with conversion. (1 mmol alcohol, 1 mmol NaOCl, 0.01 mmol RuL1,
0.1 mL EMIM, RT).
f
0.5 mmol of NaOCl.

D. Ramakrishna, B.R. Bhat / Inorganic Chemistry Communications 14 (2011) 155–158
157
Table 2
Oxidation of alcohols catalyzed by RuL1a in EMIM-NaOCl system.
Alcohols
Product
% conversion of carbonyl
compoundb
RuL1 RuL2 RuL3 RuL4 RuL5
O
90
89
87
87
83
OH
H
O
OH
H
88
89
88
88
85
87
85
87
82
82
H3C
H3C
O
OH
H
O
O
CH3
O
Fig 2. Effect of recycling on conversion of benzylalcohol.
OH
H
86
86
85
86
84
85
84
85
81
83
Cl
Cl
Cl
Cl
mass (obtained by evaporation of the resultant solution to dryness),
which shows a band at 820 cm−1, characteristic of oxoruthenium (IV)
species [30,31]. The proposed mechanism with NaOCl as co-oxidant is
shown in Scheme 2.
In conclusion, the ruthenium (II) complexes in ionic liquid
provides a greener route for the catalytic oxidation of alcohols to
carbonyls using NaOCl as a green oxidant. A smaller reaction time,
higher catalytic activity of the complexes, reasonable time and
recycling make this catalytic system a cheaper, easier and environ-
mental friendly process.
O
OH
H
NO2
NO2
O
OH
95
92
91
88
81
CH3
CH3
OH
O
90
86
81
84
89
85
90
84
88
84
88
83
87
84
87
80
80
80
80
81
O
OH
H
O
OH
OH
O
OH
OH
H
O
H3C
OH
78
79
78
77
78
78
77
78
78
72
74
75
70
72
70
H3C
H
H3C
OH
O
H3C
H3C
H
O
H3C
OH
H
O
70
72
70
72
69
71
69
71
68
68
OH
H3C
H3C
H3C
H
H
H3C
OH
O
a
1 mmol alcohol, 1 mmol NaOCl, 0.01 mmol RuL1, 0.1 mL EMIM, RT.
GC yield.
b
oxo species responsible for dehydrogenation of alcohols. This was
further supported by spectral changes that occur by addition of NaOCl
to an ionic liquid solution of the ruthenium (II) complexes. The
appearance of peak at 390 nm is attributed to the formation of
oxoruthenium (IV) species (Fig. 3), which is in conformity with other
oxo ruthenium (II) complexes [30–32]. Further support in favor of the
formation of such species is identified by the IR spectrum of the solid
Fig 3. UV–Vis spectra of (a) RuL1 (b) RuL1+NaOCl (spectra taken after 30 min of
mixing).

158
D. Ramakrishna, B.R. Bhat / Inorganic Chemistry Communications 14 (2011) 155–158
H2O
IIRuCl(L)(PPh3)2
NaOCl
H+
(PPh3)2(L)ClRuIIIH
(PPh3)2 (L)ClRuIV=O
+
NaCl
R1
C
R2
O
H+
+
R1 CH OH
R2
R1CH OH
R2
(PPh3)2 (L)ClRuIV=O
Scheme 2. Proposed mechanism for the oxidation of alcohols using Ru/NaOCl.
31P NMR (H3PO4, δ ppm): 34.2.UV λmax (nm) (ε (M−1 cm−1): 437 (6056), 349
(12566), 302 (18310).
References
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(0.16 mmol; 0.033–0.041 g) were taken in 20 ml benzene. The mixture was
refluxed for 3 h and was monitored by TLC. The solvent was partially evaporated
and light petroleum (600–800 C, 5 ml) was added to the reaction mixture to
separate the solid.
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separated and tested for its reusability.
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(Shimadzu 2014, Japan); the instrument has a 5% diphenyl and 95% dimethyl
siloxane Restek capillary column (30 m length and 0.25 mm diameter) and a
flame ionization detector (FID). The initial column temperature was increased
from 600 C to 1500 C at the rate of 100 C/min and then to 2200 C at the rate of
400 C/min Nitrogen gas was used as the carrier gas. The temperatures of the
injection port and FID were kept constant at 1500 C and 2500 C respectively
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65.87; H. 4.56; N, 4.79. FT-IR (KBr, cm−1): 3244, 1564, 1444, 1296, 1096, 612, 520,
489, 451. 1 H NMR (400 MHz, CDCl3, δ, ppm): 8.55 (s, -CH=N-), 6.75–8.15 (m, Ar-
H), 3.85 (s, -N-H). 31P NMR (H3PO4, δ ppm): 34.1.UV λmax (nm) (ε (M−1 cm−1):
442 (6126), 350 (12603), 301 (18250).
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63.47; H. 4.26; N, 4.57. FT-IR (KBr, cm−1): 3225, 1579, 1440, 1289, 1097, 611, 516,
491, 441. 1 H NMR (400 MHz, CDCl3, δ, ppm): 8.50 (s, -CH=N-), 6.75–8.15 (m, Ar-
H), 3.85 (s, -N-H). 31P NMR (H3PO4, δ ppm): 34.6.UV λmax (nm) (ε (M−1 cm−1):
439 (6084), 356 (12818), 305 (18493).
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H
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