Cobalt complexes in [EMIM]Cl - A catalyst for oxidation of alcohols to carbonyls

Article abstract of DOI:10.1016/j.inoche.2009.11.014

A coordination complex system consisting of Cobalt (II)-Schiff bases with triphenylphosphine were synthesized and characterized. These catalysts were effective in the oxidation of primary and secondary alcohols. The oxidation reactions were carried out in ethyl-methyl-imidazolium ionic liquid in presence of NaOCl. Higher catalytic activity was observed for CoL1.

Full text of DOI:10.1016/j.inoche.2009.11.014

Inorganic Chemistry Communications 13 (2010) 195–198
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Cobalt complexes in [EMIM]Cl – A catalyst for oxidation of alcohols to carbonyls
*
Dileep Ramakrishna, Badekai Ramachandra Bhat
Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasanagar 575 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A coordination complex system consisting of Cobalt (II)-Schiff bases with triphenylphosphine were syn-
thesized and characterized. These catalysts were effective in the oxidation of primary and secondary alco-
hols. The oxidation reactions were carried out in ethyl-methyl-imidazolium ionic liquid in presence of
NaOCl. Higher catalytic activity was observed for CoL1.
Received 16 September 2009
Accepted 13 November 2009
Available online 18 November 2009
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Co(II) complexes
Schiff base
Catalytic oxidation
Green oxidation
The selective oxidation of primary and secondary alcohols to
the corresponding aldehydes and ketones plays a central role in
the synthetic organic chemistry. This represents an important
entry to essential functional groups, such as ketones, aldehydes,
and carboxylic acids [1]. Use of some homogeneous metal
complexes of Ru [2–7], Pd [8,9], Co [10,11], Os [12] and Cu
[13,14] has achieved high catalytic activity and selectivity. Hence
a wide variety of methods have been developed [15]. Stoichiome-
tric reactions include Swern oxidation [16], Dess–Martin oxidation
[17] and various metal oxidants [18], whereas catalytic reactions
involve transition metal complexes in combination with oxidants,
used as sacrificial reagents (N-oxides, S-oxides, NaIO4, NaOCl, etc.)
[19–22]. Thus, it is not surprising that transition metal complexes
have attracted a considerable interest in recent years. It is well-
known that the wide spread use of traditional organic solvents in
many chemical processes is an issue of great environmental con-
cern. Hence, the use of ionic liquids as reaction medium is a topic
of much current interest in the context of environmentally friendly
chemical reactions [23].
Cobalt species have been extensively studied as catalysts for a
wide range of oxidation reactions [24,25]. Cobalt-catalyzed oxida-
tion of alcohols has been studied by different groups. Sharma et al.
studied oxidation of secondary alcohols in acetonitrile catalyzed by
Cobalt (II)-Schiff base [26] and cobalt (II) acetylacetonate [27] with
O2 and N-bromosuccinimide as oxidants, respectively. Das and
Punniyamurthy used cobalt (II) complex to catalyze oxidation of
alcohols into carboxylic acids and ketones in acetonitrile with
H2O2 as oxidant [28]. N-bromosuccinimide, t-Butyl hydroperoxide
and H2O2 have also been used as oxidants in many cases for cobalt
catalyzed systems. However, the long reaction time reported in the
above systems makes it unfriendly for the industries. The green
solvent system for the catalytic oxidation of alcoholic to carbonyl
using the metal complexes has not been studied extensively.
We are reporting our attempts to synthesize a series of Cobalt
triphenylphosphine complexes (CoL1–CoL5) containing N-(2-pyri-
dyl)-N0-(salicylidene) hydrazine with its derivatives (Scheme 1)
and their application as catalysts for the oxidation of alcohols to
carbonyl compounds in ionic liquid.
The starting complex [CoCl2(PPh3)2] was prepared by the reac-
tion between CoCl2ꢀ6H2O and triphenylphosphine in glacial acetic
acid [29]. The Schiff bases were prepared in 70–80% yield by con-
densation reactions of 2-hydrazinopyridine with the correspond-
ing 5-substituted salicylaldehyde in methanol [30]. Complexes
CoL1–CoL5 were prepared by refluxing a dichloromethane solution
of [CoCl2(PPh3)2] and ligand in a 1:1 molar ratio for 3 h [31]. All the
complexes are dark green. They were found to be soluble in
CH3OH, CH3CN, C6H6, DMSO and DMF. The analytical data for these
complexes are in good agreement with the above molecular for-
mula. The process is on to get single crystals suitable for X-ray
diffraction.
The electronic spectra of the complexes showed many bands in
the region 250–490 nm. The bands appeared in the region
250–350 nm have been assigned to intraligand transitions. A less
intense band in range 390–490 nm corresponds to forbidden
d ? d transition. The IR spectra of the ligands exhibit a strong band
around 1610–1620 cmꢁ1, which is assigned to
m(C@N) vibration.
As a result of coordination, this band is altered in complexes. The
band in the region 1315–1330 cmꢁ1 which is assigned to phenolic
m(C–O) in the free ligand, is shifted to higher wave number in the
complexes suggesting the coordination of phenolic oxygen to me-
* Corresponding author. Tel.: +91 824 2474000x3204; fax: +91 824 2474033.
E-mail address: chandpoorna@yahoo.com (B. Ramachandra Bhat).
tal ion. The N–H stretching frequency occurs around 3100 cmꢁ1 in
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.11.014

196
D. Ramakrishna, B. Ramachandra Bhat / Inorganic Chemistry Communications 13 (2010) 195–198
P
R
O
N
P
NH
Cl
Co
P
N
Chloroform,
Δ
Co
+
Cl
N
Cl
OH
NH
(CoL1: R=H, CoL2: R= Cl, CoL3: R=Br, CoL4: R=NO2, CoL5: R= OCH3).
Scheme 1. Synthesis of cobalt (II) complexes.
ligands is unaltered in complexes. The pyridine vibrations around
610 cmꢁ1 (in-plane ring deformation) and 490 cmꢁ1 (out of plane
ring deformation) were also unchanged in complexes. These fac-
tors reveal the non participation of –NH group and pyridine-N in
coordination. The bands around 550 cmꢁ1 and 470 cmꢁ1 in the
90
80
70
60
50
40
30
complex is assigned to
m(M–O) and m(M–N), respectively. Bands
due to triphenylphosphine are also appeared in the expected re-
gion [32]. 1H NMR spectra of the complexes exhibit a multiplet
around 6.9–7.9 ppm which has been assigned to the protons of
phenyl groups present in Schiff base ligand and triphenylphos-
phine. A peak observed at 8.5 ppm in the complexes has been as-
signed to azomethine proton (–CH@N–). The absence of
a
resonance at 10.3 ppm due to phenolic hydrogen indicates the
deprotonation of the Schiff base [33]. In the 13C NMR spectra of
all the complexes, azomethine carbon resonances are observed in
the 151.12–154.49 ppm range. The resonances for C–N, C–O and
C–P are observed in the regions 149.32–149.99, 162.44–164.93
and 142.96–144.48 ppm, respectively. The 13C NMR spectra of
complexes revealed the presence of six different carbons (119.30,
129.46, 143.85, 143.98, 149.82 and 163.98 ppm). This indicates
that the three quaternary carbons arising from triphenylphosphine
aromatic units are in different magnetic environments. 31P NMR
spectra exhibits a singlet at 22.3–22.8 ppm suggesting the pres-
ence of one coordinated triphenylphosphine in the complexes. In
order to obtain further structural information, the magnetic mo-
ments of the complexes were measured. All cobalt (II) complexes
show magnetic moments in the 2.33–2.49 B.M. range indicating
that these complexes have a low spin square planar geometry [34].
The optimization of the reaction conditions was studied by tak-
ing benzyl alcohol as substrate with CoL1 in EMIM–NaOCl system
(Table 1). The benzaldehyde formed was quantified by GC [35]. In
order to study the effect of time on the activity, the product anal-
ysis was done at regular intervals of time under similar reaction
conditions (Fig. 1). It was observed that the total reaction time
was only 15 min even at room temperature. The experiments were
conducted at regular intervals of time beyond 30 min. The results
show that the conversion remains constant at about 90% after a
0
10
20
30
40
50
60
Time (min)
Fig. 1. Effect of time on conversion of benzyl alcohol to benzaldehyde.
reaction time of 15 min. This implies that CoL1 in EMIM–NaOCl
system showed good efficiency (Table 1, entries 3 and 9). The cat-
alytic activities of CoL2–CoL5 in EMIM–NaOCl were carried out
[36] (Table 2). The catalytic activity varies with the size of the sub-
stituent. It was observed that the activity decreased with increase
in the bulkiness of the substituents. This may probably be due to
steric hindrance caused by the substituent which can affect the
planarity of the ligand in the complexes.
The effect of the concentration of catalyst with respect to sub-
strate was carried out at different substrate to catalyst ratios. A
0.02 mmol of catalyst was sufficient for the effective transforma-
tion of benzyl alcohol to benzaldehyde (Table 1, entry 3). The reac-
tion was also studied in the absence of catalyst. The yield was
insignificant in this case (Table 1, entry 1). This observation reveals
the catalytic role of cobalt (II) complexes. The reaction was studied
at various substrate to oxidant ratios (Table 1). A minimum quan-
tity of 1 mmol of the oxidant was sufficient for the effective oxida-
tion of benzyl alcohol to benzaldehyde (Table 1, entry 9). All the
alcohols were oxidized in good to excellent yields. All the synthe-
sized cobalt (II) complexes were found to catalyze the oxidation of
alcohols (both aliphatic and aromatic) to corresponding carbonyl
compounds in 60–96% yield. Benzylic primary and secondary alco-
hols oxidize smoothly to give aldehydes and ketones respectively.
All the experiments are carried out in air atmosphere since there is
no change in conversion if reaction is carried out in inert atmo-
sphere. From this it is clear that air is not a significant co-oxidant
in the oxidation process and cobalt (II) complexes are air stable.
In conclusion, the series of experiments described represents a
useful method for the oxidation of alcohols to carbonyl compounds
at room temperature. The catalyst can be easily prepared and is
Table 1
Optimization of reaction conditions for oxidizing benzyl alcohol to benzaldehyde.a
Entry Amount of CoL1
Amount of NaOCl
(mmol)
Conversionb
(%)
(mmol)
1
2
3
4
5
6
7
8
0
1.0
1.0
1.0
1.0
1.0
1.0
0
0.5
1.0
1.5
2.0
1.2
0.01
0.02
0.03
0.04
0.05
0.02
0.02
0.02
0.02
0.02
60.2
89.2
89.3
89.8
89.7
27.6
68.6
89.2
89.6
89.4
9
10
11
a
1 mmol Benzyl alcohol, 0.2 mL EMIM, 15 min, room temperature.
Average of 3 trials.
b

D. Ramakrishna, B. Ramachandra Bhat / Inorganic Chemistry Communications 13 (2010) 195–198
197
Table 2
Oxidation of alcohols catalyzed by Co(II) complexes.a
Entry
Alcohols
Product
% Conversion of carbonyl compoundb
CoL1c
CoL2c
CoL3d
CoL4c
CoL5e
O
1
2
89.2
82.8
83.5
80.9
84.1
OH
H
O
90.2
87.3
90.6
91.1
89.2
81.4
79.5
76.0
81.6
OH
H
O
3
4
80.4
80.8
OH
H
O
O
O
87.6
87.5
90.2
77.5
87.0
85.2
79.0
71.3
OH
H
Cl
Cl
Cl
Cl
O
5
78.1
OH
H
NO2
NO2
OH
O
6
7
94.8
93.7
91.3
90.9
80.1
87.2
78.6
79.0
81.2
81.1
O
OH
O
8
9
90.5
94.8
91.7
91.5
86.0
86.3
73.4
73.4
76.4
78.6
OH
H
OH
OH
O
10
83.0
88.6
81.2
79.1
77.4
O
OH
H
OH
O
11
12
13
81.3
82.6
75.4
79.6
76.8
71.2
76.8
78.6
74.6
80.1
79.3
72.0
71.4
75.6
73.4
OH
H
O
OH
H
H
O
OH
14
15
76.7
72.6
74.7
69.1
73.4
67.1
75.2
66.5
75.9
64.3
OH
O
H
H
OH
O
a
b
c
1 mmol Alcohol, 1 mmol NaOCl, 0.2 mmol Co(II) complex, 0.1 mL EMIM, stirring at room temperature.
GC yield, average of 3 trials.
Reaction time, 15 min.
Reaction time, 20 min.
Reaction time, 25 min.
d
e
therefore extremely cost effective. The rapid reaction times for the
substrates means a large number of materials may be screened in
parallel over a short time period. EMIM ionic liquid as a solvent
will exhibit real advantages by providing a ‘green’ process with
safer operation, easier separation, high catalytic activity and with
no loss of solvent by oxidation.
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126.52, 124.23, 121.83, 121.35, 118.84, 114.76, 20.23. 31P NMR (H3PO4, d
ppm): 22.3. CHN found (calc.) for C30H24Cl2N3OPCo: C: 63.19(63.45), H:
4.14(4.26), N: 7.01(7.20); UV–Vis: kmax intraligand interactions: 256, 325,
369, d ? d forbidden transition: 443.
leff: 2.33.CoL3: Yield: 62%. IR (KBr,
cmꢁ1): 3109, 1585, 1439, 1340, 1093, 698, 615, 552, 495, 475. 1H NMR (CDCl3,
d ppm): 6.7–7.3 (m, 15H, Ar–H), 7.8–7.9 (m, 7H, Ar–H), 8.5 (d, 1H, –CH@N–);
13C NMR (ppm): 162.53, 153.46, 144.48, 144.33, 134.79, 134.66, 133.94,
131.84, 130.70, 130,67, 128.74, 128.66, 128.28, 128.14, 127.06, 123.07, 122.03,
119.90, 119.54, 114.95. 31P NMR (H3PO4, d ppm): 22.5. CHN found (calc.) for
C30H24BrClN3OPCo: C: 57.97(58.34), H: 3.89(3.95), N: 6.25(6.86); UV–Vis: kmax
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intraligand interactions: 253, 319, 369, d ? d forbidden transition: 449. leff:
2.44.CoL4: Yield: 72%. IR (KBr, cmꢁ1):, 1595, 1429, 1351, 1087, 695, 610, 549,
489, 479. 1H NMR (CDCl3, d ppm): 6.6–7.4 (m, 15H, Ar–H), 7.4–7.8 (m, 7H, Ar–
H), 8.6 (d, 1H, –CH@N–); 13C NMR (ppm): 162.83, 153.39, 144.42, 144.24,
136.46, 135.06, 134.79, 134.66, 130.72, 130.67, 128.73, 128.66, 128.29, 128.16,
127.09, 123.50, 122.05, 120.83, 114.97, 106.28. 31P NMR (H3PO4, d ppm): 22.8.
CHN found (calc.) for C30H24ClN4O3PCo: C: 62.13(62.29), H: 4.16(4.18), N:
9.50(9.69); UV–Vis: kmax intraligand interactions: 252, 320, 358, d ? d
forbidden transition: 440. l
eff: 2.49.CoL5: Yield: 55%. IR (KBr, cmꢁ1):, 1586,
1428, 1346, 1105, 695, 608, 550, 495, 476. 1H NMR (CDCl3, d ppm): 6.6–7.3 (m,
15H, Ar–H), 7.4–7.8 (m, 7H, Ar–H), 8.5 (d, 1H, –CH@N–); 13C NMR (ppm):
164.93, 151.12, 143.23, 142.96, 134.84, 134.63, 134.19, 130.64, 130.60, 129.70,
128.80, 128.32, 128.10, 127.58, 127.06, 126.03, 124.24, 122.77, 121.98, 118.94,
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[24] W. Partenheimer, Adv. Synth. Catal. 348 (2006) 559.
114.85, 110.29. 31P NMR (H3PO4,
d
ppm): 22.6. CHN found (calc.) for
C31H27ClN3O2PCo: C: 65.97(66.08), H: 4.36(4.83), N: 7.21(7.46); UV–Vis: kmax
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2.36.
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:
[35] The reaction product analysis was carried out using gas chromatography (GC)
(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 60 °C to 150 °C at the rate of 10 °C/min and then to 220 °C at the rate of
40 °C/min Nitrogen gas was used as the carrier gas. The temperatures of the
injection port and FID were kept constant at 150 °C and 250 °C, respectively
during product analysis. The retention times for different compounds were
determined by injecting commercially available compounds under identical
gas chromatography conditions. The oxidation products are commercially
available, and were identified by GC co-injection with authentic samples.
[36] A solution of cobalt (II) complex (0.02 mmol) in 0.1 mL ethyl-methyl-
imidazolium (EMIM) ionic liquid was added to the solution of substrate
(1 mmol) and NaOCl (1 mmol). The mixture was stirred at room temperature.
At the requisite times aliquots of the reaction mixture were removed and the
alcohol and aldehyde/ketone extracted with ether. The ether solution was then
analyzed by GC.
[30] A. Sarkar, S. Pal, Polyhedron 25 (2006) 1689.
[31] Procedure: To a solution of [CoCl2(PPh3)2] (0.2 mmol) in Chloroform (20 mL)
the corresponding Schiff base ligand (0.2 mmol) was added. The mixture was
refluxed for 3 h. After the completion of reaction, the solvent was removed
under vacuum. The residue was washed with diethyl ether and dried in vacuo
to form dark green color complexes.CoL1: Yield: 69%. IR (KBr, cmꢁ1): 3106,
1592, 1446, 1345, 1095, 697, 609, 550, 490, 470. 1H NMR (CDCl3, d ppm): 6.6–
7.3 (m, 15H, Ar–H), 7.4–7.7 (m, 8H, Ar–H), d 8.5 (d, 1H, CH@N). 13C NMR (ppm):
163.98, 154.51, 143.98, 143.85, 134.84, 134.72, 133.98, 133.00, 130.59, 130.57,
129.00, 128.62, 128.60, 128.21, 128.11, 126.70, 121.88, 121.63, 119.30, 115.44,
114.84. 31P NMR (H3PO4, d ppm): 22.4. CHN found (calc.) for C30H25ClN3OPCo:
C: 67.13(67.55), H: 4.65(4.72), N: 7.67(7.88); UV–Vis: kmax (nm) intraligand
interactions: 253, 322, 359, d ? d forbidden transition: 442.
leff: 2.41.CoL2:
Yield: 65%. IR (KBr, cmꢁ1): 3104, 1597, 1435, 1330, 1101, 690, 610, 545, 493,
472. 1H NMR (CDCl3, d ppm): 6.6–7.3 (m, 15H, Ar–H), 7.4–7.8 (m, 7H, Ar–H),
8.5 (d, 1H, –CH@N–); 13C NMR (ppm): 162.44, 154.25, 144.46, 144.32, 135.95,