Green and chemoselective oxidation of alcohols with hydrogen peroxide: A comparative study on Co(II) and Co(III) activity toward oxidation of alcohols

Article abstract of DOI:10.1016/j.poly.2011.07.030

Two new cobalt (II) and cobalt (III) complexes of a terpyridine based ligand, (4′-(2-thienyl)-2,2′;6′,2″-terpyridine (L)), were synthesized. Each complex has two units of the tridentate ligand. The complexes were fully characterized by spectroscopic methods as well as CHN analysis. Moreover, their solid state structures were determined by single crystal X-ray diffraction. The cobaltous complex has the formula [Co(L) ₂](NO₃)₂·2CH₃OH·H ₂O (1), whereas the cobaltic complex shows the formula [Co(L) ₂](NO₃)₃·2CH₃OH (2). Both complexes were tested as homogenous catalysts for the oxidation of a variety of aliphatic and aromatic alcohols utilizing aqueous hydrogen peroxide in water media. The Co(II) complex showed more activity in comparison with its isostructural Co(III) species. The results show that the aromatic alcohols were oxidized with higher conversions and selectivity compared to the aliphatic substrates, possibly due to their conjugation systems which thermodynamically stabilized the carbonyl products.

Full text of DOI:10.1016/j.poly.2011.07.030

Polyhedron 30 (2011) 2768–2775
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Green and chemoselective oxidation of alcohols with hydrogen peroxide: A
comparative study on Co(II) and Co(III) activity toward oxidation of alcohols
⇑
Ali Nemati Kharat , Abolghasem Bakhoda, Bahareh Tamaddoni Jahromi
School of Chemistry, University College of Science, University of Tehran, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
0
0
0
00
Article history:
Two new cobalt (II) and cobalt (III) complexes of a terpyridine based ligand, (4 -(2-thienyl)-2,2 ;6 ,2 -ter-
pyridine (L)), were synthesized. Each complex has two units of the tridentate ligand. The complexes were
fully characterized by spectroscopic methods as well as CHN analysis. Moreover, their solid state struc-
tures were determined by single crystal X-ray diffraction. The cobaltous complex has the formula
Received 24 April 2011
Accepted 6 July 2011
Available online 31 August 2011
[
(
Co(L)
2
](NO
3
)
2
ꢀ2CH
3
OHꢀH
2
O (1), whereas the cobaltic complex shows the formula [Co(L)
2
](NO
3
)
3
ꢀ2CH
3
OH
Keywords:
2). Both complexes were tested as homogenous catalysts for the oxidation of a variety of aliphatic and
Oxidation of alcohols
Hydrogen peroxide
Cobalt catalysts
X-ray crystallography
Oxidation state
aromatic alcohols utilizing aqueous hydrogen peroxide in water media. The Co(II) complex showed more
activity in comparison with its isostructural Co(III) species. The results show that the aromatic alcohols
were oxidized with higher conversions and selectivity compared to the aliphatic substrates, possibly due
to their conjugation systems which thermodynamically stabilized the carbonyl products.
Ó 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
Aldehydes and ketones are among the most important chemi-
oxidation of aromatic and aliphatic alcohols to the corresponding
aldehydes and ketones with both cobaltous and cobaltic complexes
of a N-donor
. Results and discussion
.1. Synthesis and characterization
a-terpyridine derivative ligand.
cals, as fine chemicals or intermediates for syntheses, e.g. precur-
sors in the pharmaceutical industry [1–3]. Although they can be
synthesized by direct oxidation of alkanes [4] or alkenes [5], these
reactions are typically not selective [5,6]. The liquid-phase oxida-
tion of alcohols to the corresponding carbonyl compounds seems
to be the simplest synthetic method [7]. However, these reactions
are often limited to benzylic and allylic compounds [8], and need
high catalyst loadings [7–10], long reaction times [7] and require
additives such as bases, phase-transfer catalysts [11], expensive
oxidants [12,13] and toxic solvents [14,15]. Moreover, in the case
of using precious metals, the catalysts are usually expensive, not
easy to prepare and are not highly active for the oxidation of inac-
tive alcohols [8,9]. Hence, a search for other synthetic routes to car-
bonyl compounds which would overcome at least some of the
above confines is a matter of present interest. Various compounds
of cobalt (II) salts [16] and cobalt Schiff base complexes [17] are
widely used as homogeneous catalysts in the oxidation of alcohols
to the corresponding carbonyl compounds. A literature survey
shows a lack of remarkable studies on the comparison of Co(II)
and Co(III) activities in the catalytic oxidation of alcohols to
carbonyl compounds. So, we are interested in investigating the
2
2
0
0
0
00
The 4 -(2-thienyl)-2,2 ;6 ,2 -terpyridine ligand (L), was prepared
by a one pot condensation of 2-acetylpyridine, thiophene-2
carboxaldehyde and ammonia in the presence of t-BuOK, accord-
-
ing to our previously published method [18]. Consequently, two no-
vel cobaltous and cobaltic complexes were synthesized, as
represented in Scheme 1.
For the preparation of the cobalt (II) complex, hydrated cobalt
nitrate and (L) were mixed in a methanol–chloroform mixture to
obtain a deep red solution. Also, for the preparation of the cobalt
(
III) species, diluted nitric acid and a small amount of hydrogen per-
oxide were added to the reaction vessel and the mixture was heated
in an oil bath for 3 h. In order to investigate the purity of the pre-
pared compounds, spectroscopic characterization, such as IR and
CHN elemental analysis, was employed on both the synthesized
crystals. The IR spectra of complexes are rather similar and exhibit
common features due to the related chelating N-donor iminic li-
ꢁ
1
gands. The presence of broad (OH) bands at 3128 and 3151 cm
,
ꢁ1
as well as (CH) vibrations in the range 2961–2874 cm are as-
cribed to solvents, which are in agreement with the published
works [19]. The presence of free nitrate anions in both compounds
⇑
Corresponding author. Address: School of Chemistry, University College of
Science, University of Tehran, P.O. Box 13145-1357, Tehran, Iran. Tel.: +98 21
6
1112499; fax: +98 21 66495291.
ꢁ1
E-mail address: alnema@khayam.ut.ac.ir (A. Nemati Kharat).
was confirmed by sharp bands at 1384 and 1325 cm . In addition,
0
277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.07.030

A. Nemati Kharat et al. / Polyhedron 30 (2011) 2768–2775
2769
Scheme 1. Reagents and reaction conditions for the synthesis of the complexes.
Table 1
Analytical and physical data of the cobalt complexes.
Table 2
a
Crystal data and structure refinement of both complexes.
Complex Color (%
yield)
M.p. (°C)
C%
(Found/
Calc.)
H%
(Found/
Calc.)
N%
(Found/
Calc.)
Complex 1
Complex 2
9 11 2
34CoN O S
939.81
150(1)
0.71073
Formula
Formula weight
T (K)
Wavelength k (Å)
Crystal system
Space group
Crystal size (mm)
a (Å)
C
40
H
36CoN
8
O
9
S
2
C
40
H
895.82
150(1)
0.71073
triclinic
P1ꢀ
Co(II)
Brownish-
red (66.3%)
Yellow
>311(Dec.) (53.69/
53.63)
>368
(4.06/
4.05)
(3.67/
3.65)
(12.49/
12.51)
(13.38/
13.41)
Co(III)
(51.19/
51.12)
monoclinic
P2 /n
(37.6%)
(Dec.)
1
0.25 ꢃ 0.15 ꢃ 0.10 0.23 ꢃ 0.08 ꢃ 0.08
12.5271(9)
13.3036(10)
14.5261(9)
67.076(3)
79.697(4)
63.379(4)
1993.3(2)
2
13.8345(4)
14.8250(7)
20.0930(6)
90
104.985(2)
90
3980.9(3)
4
1.568
2.64–27.43
1936
0.612
ꢁ17 6 h 6 17
ꢁ19 6 k 6 19
ꢁ25 6 l 6 26
0.930 and 0.814
b (Å)
c (Å)
there are bands at 1000–1150and 1600–1607 cm 1 which are asso-
ciated with C@C and C@N stretches of the terpyridine moiety,
respectively. The bands observed in the 830–837 cm region for
both compounds were attributed to C–S stretching. Also, Co–N
bond vibrations are seen at 477 and 453 cm for the cobaltic and
cobaltous complexes, respectively [20]. The analytical data are
summarized in Table 1.
ꢁ
a
(°)
b (°)
(°)
ꢁ
1
c
3
V (Å )
ꢁ
1
Z
Dcalc (g cm 1
ꢁ
)
1.493
2.76–27.6
926
h Ranges for data collection
F(000)
Absorption coefficient (mmꢁ1)
Index ranges
0.603
ꢁ15 6 h 616
2
.2. Crystal structure description
ꢁ
ꢁ
17 6 k 6 17
18 6 l 6 18
Recrystallization of microcrystals of both complexes in a meth-
Maximum and minimum
transmission
Number of observed reflections
Data collected
Unique data (Rint
Parameters, restrains
0.956 and 0.791
anol:acetonitrile (1:1) mixture yielded brownish-red and yellow
crystals that correspond to the Co(II) (1) and Co(III) (2) nitrate
complexes, respectively. Crystallographic data are summarized in
Table 2. The structure of the cobaltous species contains the
4946
21730
5211
26664
)
9021 (0.0754)
531, 21
0.0740, 0.1835
0.1487, 0.2248
1.049
8996 (0.0710)
582, 18
0.0566, 0.1204
0.1212, 0.1472
1.01
2
+
a
[
Co(L)
2
]
cation and two nitrate anions, accompanied by two
Final R , wR2 (observed data)
1
a
methanol and one water solvated molecules (see Fig. 1). As is de-
picted in Fig. 1, one of the thiophene rings of complex 1 is rotation-
ally disordered over two sets of sites with refined occupancies of
1
Final R , wR
2
(all data)
2
Goodness-of-fit (GOF) on F (S)
Largest difference in peak and hole 1.136, ꢁ0.65
0.532, ꢁ0.44
ꢁ3
(
e Å
)
0
.686(3) and 0.314(3).
The cobalt (II) environment consists of two 4 -(2-thienyl)-
a
(w(F 2 ꢁ Fc2) )/
2
w(F 2) ]1/2.
2
0
R1
=
R
||F | ꢁ |F ||/
R
|F |, wR2 = [
R
R
o
c
o
o
o
0
0
00
2
,2 ;6 ,2 -terpyridine ligands, giving a distorted octahedral geome-
try. The Co–N average bond distance is 2.009(4) Å, which is a little
shorter than the regular range observed for similar cobalt com-
plexes (ꢂ2.1 Å) [21]. The unit cell packing of the complex is pre-
sented in Fig. 1. Different types of hydrogen bonds exist in the
structure (Table 3); one exists between a free nitrate ion and meth-
between the nitrate anion and the hydrogen atom of a water mol-
ecule (O1 –H1 –O6#1 178.7° and
ꢀ ꢀ ꢀO6 2.542(5) Å, O1
O1 –H1 –O3 168.1°). Symmetry transfor-
ꢀ ꢀ ꢀO3 3.182(6) Å, O1
W
W
W
W
W
W
mations are used to generate equivalent atoms: #1 x, y ꢁ 1, z + 1,
#2 x, y, z ꢁ 1. The Co–N bond distance to the central pyridyl ring
of the terpyridine moiety is a little shorter than the other two
anol molecules (O1
S
ꢀ ꢀ ꢀO1#2 2.779(7) Å, O1
S S
–H1 –O1 166.7° and
O2
S
ꢀ ꢀ ꢀO1#2 2.775(7) Å, O4–H4–O5 169.9°), and another one exists

2
770
A. Nemati Kharat et al. / Polyhedron 30 (2011) 2768–2775
Fig. 1. An ORTEP representation of the coordination environment around the Co(II) ion in the complex [Co(L)
coordination sphere. Thermal ellipsoids are drawn at the 50% probability level and solvent molecules are omitted.
2
](NO
)
3 2
ꢀ2CH
3
2
OHꢀH O, showing the atom numbering in the
Table 3
Hydrogen bonds (Å and °) for both complexes.
expectable because of the smaller radius of the cobaltic center.
Some selected bond lengths are presented in Table 4. It is notewor-
thy in this complex that both thiophene rings are rotationally dis-
ordered and the refined occupancies are 0.575(2) and 0.425(2) for
one ring and 0.832(3) and 0.168(3) for the other, although this is
not shown in the figure.
D–Hꢀ ꢀ ꢀA
D–H Hꢀ ꢀ ꢀA Dꢀ ꢀ ꢀA
\DHA Symmetry code
Cobaltous complex
O(1 W)–H(1 WA)ꢀ ꢀ ꢀO(6)#1 0.84 2.00 2.842(5) 178.7 #1: x, y ꢁ 1, z + 1
O(1W)–H(1WB)ꢀ ꢀ ꢀO(3)
O(1S)–H(1S)ꢀ ꢀ ꢀO(1)#2
O(2S)–H(2S)ꢀ ꢀ ꢀO(1)#2
0.84 2.36 3.182(6) 168.1
0.84 1.95 2.779(7) 166.7 #2: x, y, z ꢁ 1
0.84 1.94 2.775(7) 169.9 #2: x, y, z ꢁ 1
There are also some hydrogen bonds in the cobaltic complex,
which are between the nitrate ion and free methanol molecules
Cobaltic complex
O(2S)–H(2S)ꢀ ꢀ ꢀO(1S)
0.84 1.94 2.751(4) 161
0.84 1.89 2.710(4) 164.9
(
O2
oxygen of a methanol molecule and a hydrogen of another metha-
nol (O1 = 2.710(4) Å, O1 –H1 –O2 164.9°) (see Table 3).
ꢀ ꢀ ꢀO2
S
ꢀ ꢀ ꢀO1
S S S S
= 2.751(4) Å, O2 –H2 –O1 161°), and also between the
O(1S)–H(1S)ꢀ ꢀ ꢀO(2)
S
S
S
S
S
pyridyl rings, making it tighter than the others for substitution
with other probable ligands in the reaction media for oxidation
reactions.
2
.3. Catalytic oxidation of alcohols in water
In order to investigate the effect of oxidant quantities in the cat-
0
The cobaltic complex contains two tridentate chelating 4 -(2-
alytic system, the oxidation of benzyl alcohol was carried out using
various molar ratios of oxidant to model substrate (see Fig. 3). The
oxidation reactions with 3 equiv. of H
conversions for [Co(L)
amounts of H O , up to 5 equiv., led to a growth in the conversions
but decreased the selectivity to the corresponding aldehyde, with
an increase in carboxylic acid yield, in both catalytic systems.
2
The decrease in selectivity is higher for [Co(L) ] compared to
0
0
00
thienyl)-2,2 ;6 ,2 -terpyridine ligands (L). The molecular structure
of [Co(L) ](NO OH is shown in Fig. 2. The complex has a
2
3
)
3
ꢀ2CH
3
O
afforded 94% and 34%
, respectively. Higher
2
2
3+
distorted octahedral geometry, and the two rigid tridentate chelat-
ing ligands are the main factors accounting for this distortion. The
average Co–N distance for the complex is 1.913(3) Å, which is in
agreement with the values reported for similar structures of Co(III)
2+
2
]
and [Co(L)
2
]
2 2
[
22]. These shorter bond lengths in the cobaltic complex are
2+
Fig. 2. View of the geometry around Co(III) ion in [Co(L)
drawn at the 50% probability level and methanol molecules are omitted for clarity.
2
](NO
3
)
3
ꢀ2CH
3
OH, showing the atom numbering scheme in the coordination environment. Thermal ellipsoids are

A. Nemati Kharat et al. / Polyhedron 30 (2011) 2768–2775
2771
Table 4
Bond lengths (Å) and angles (°) for the Co(II) and Co(III) complexes.
Co(II) complex
Co(1)–N(2)
Co(1)–N(5)
Co(1)–N(3)
Co(1)–N(1)
Co(1)–N(6)
Co(1)–N(4)
S(1)–C(34)
S(1)–C(31)
S(2)–C(38)
S(2)–C(35)
1.872(3)
1.899(3)
2.034(4)
2.035(4)
2.100(4)
2.117(4)
1.652(5)
1.689(5)
1.701(5)
1.702(4)
N(2)–Co(1)–N(1)
N(5)–Co(1)–N(1)
N(3)–Co(1)–N(1)
N(2)–Co(1)–N(6)
N(5)–Co(1)–N(6)
N(3)–Co(1)–N(6)
N(1)–Co(1)–N(6)
N(2)–Co(1)–N(4)
N(5)–Co(1)–N(4)
N(3)–Co(1)–N(4)
80.67(15)
100.55(14)
161.58(14)
99.72(14)
79.56(14)
91.08(14)
92.27(14)
101.31(14)
79.39(14)
91.26(14)
N(1)–Co(1)–N(4)
N(6)–Co(1)–N(4)
C(34)–S(1)–C(31)
C(1)–N(1)–Co(1)
C(5)–N(1)–Co(1)
C(10)–N(2)–Co(1)
C(6)–N(2)–Co(1)
C(15)–N(3)–Co(1)
C(11)–N(3)–Co(1)
C(16)–N(4)–Co(1)
92.08(14)
158.95(13)
93.1(2)
128.9(3)
113.4(3)
119.9(3)
120.4(3)
128.6(3)
113.1(3)
128.4(3)
Co(III) complex
Co(1)–N(5)
Co(1)–N(2)
Co(1)–N(3)
Co(1)–N(6)
Co(1)–N(1)
Co(1)–N(4)
S(1)–C(34)
S(1)–C(31)
S(2)–C(38)
S(2)–C(35)
1.860(2)
1.860(2)
1.928(3)
1.940(3)
1.940(3)
1.950(3)
1.615(4)
1.695(4)
1.679(3)
1.722(3)
N(5)–Co(1)–N(2)
N(5)–Co(1)–N(3)
N(2)–Co(1)–N(3)
N(5)–Co(1)–N(6)
N(2)–Co(1)–N(6)
N(3)–Co(1)–N(6)
N(5)–Co(1)–N(1)
N(2)–Co(1)–N(1)
N(3)–Co(1)–N(1)
N(6)–Co(1)–N(1)
178.01(12)
98.86(10)
82.12(11)
82.54(11)
99.19(10)
91.18(11)
96.48(11)
82.57(10)
164.64(10)
89.92(11)
N(5)–Co(1)–N(4)
N(2)–Co(1)–N(4)
N(3)–Co(1)–N(4)
N(6)–Co(1)–N(4)
N(1)–Co(1)–N(4)
C(34)–S(1)–C(31)
C(10)–N(2)–Co(1)
C(6)–N(2)–Co(1)
C(15)–N(3)–Co(1)
C(11)–N(3)–Co(1)
82.20(11)
96.08(10)
90.17(11)
164.71(10)
92.79(11)
93.5(2)
119.39(19)
118.6(2)
126.9(2)
114.46(19)
[
Co(L)
2
]3+. The highest selectivity in aldehyde formation with rea-
the oxidation of alcohols. The oxidation of benzyl alcohol as a mod-
el substrate was carried out in various solvents, such as water, ace-
tone, methanol, ethanol and acetonitrile. The reactions were run
with a molar ratio of 1:20:60 for catalyst:substrate:oxidant as
the optimum conditions. Water was chosen as the selected solvent
sonably high conversion efficiency was obtained with 3 equiv. of
H O .
2 2
In order to find the optimum conditions for the oxidation reac-
tions, an attempt was made to investigate the effects of solvent on
Fig. 3. The effect of the amount of oxidant on the oxidation of benzyl alcohol by (A) the cobaltous and (B) the cobaltic complex.

2
772
A. Nemati Kharat et al. / Polyhedron 30 (2011) 2768–2775
Fig. 4. Effect of solvent types on the oxidation of benzyl alcohol catalyzed by the Co(II) and Co(III) species.
since the studied polar solvents showed similar mediation proper-
ties based on our data, shown in Fig. 4.
Both complexes were examined at different temperatures. At
ambient temperature, the oxidation of benzyl alcohol to the corre-
sponding aldehyde catalyzed by cobaltous species is 94%, as men-
tioned above. The catalytic productivity decreases with increasing
temperature. Also, the activity of the complexes broke down to half
as the temperature increased from 60 to 90 °C. The lowest activity
Table 5
a,b
Oxidation of a wide range of alcohols using the Co(II) and Co(III) complexes as catalyst in water at room temperature.
Entry Substrate
Co(II) catalyst
Conversion
Co(III) catalyst
Carbonyl compound:carboxylic acid/
hydroperoxide
Conversion
(%)
Carbonyl compound:carboxylic acid/
hydroperoxide
(
%)
1
64
89
76
7:1
9:1
6:1
40
45
33
5:1
7:1
7:1
OH
2
3
4
5
6
69
91
88
8:1
21
42
40
5:1
7:1
6:1
10:1
8:1
7
94
13:1
34
6:1
OH

A. Nemati Kharat et al. / Polyhedron 30 (2011) 2768–2775
2773
Table 5 (continued)
Entry Substrate
Co(II) catalyst
Conversion
Co(III) catalyst
Carbonyl compound:carboxylic acid/
hydroperoxide
Conversion
(%)
Carbonyl compound:carboxylic acid/
hydroperoxide
(
%)
8
95
15:1
37
12:1
9
73
9:1
24
8:1
HO
Cl
1
0
OH
91
31:1
30
22:1
11
12
13
89c
87c
93d
5:1
42
49
36
4:1
OH
O
8:1
5:1
OH
S
OH
19:1
17:1
1
1
4
5
OH
96d
94e
22:1
19:1
39
37
19:1
18:1
MeO
OMe
OH
F
a
b
c
Substrate:catalyst:oxidant (20:1:60) were well-stirred in water (2 ml) at room temperature for 8 h.
The conversion was calculated based on the starting substrate, by using GC and with a comparison of authentic samples.
The reaction time was 2 h.
The reaction time was 12 h.
The reaction time was 6 h.
d
e
was observed in the temperature range 90–100 °C. Different cobalt
salts, such as the nitrate or acetate, were investigated, though these
salts did not show notable catalyst activity in the oxidation process
under the studied conditions. The decrease in activity at higher tem-
peratures could be related to enhanced decomposition of the cobalt
complexes. Besides, the more active cobaltous complex at high tem-
perature and in the presence of oxidant could be rapidly oxidized to
the cobaltic complex, and then after this it could decompose. The lat-
ter suppositionwas studied by UV–Vis spectroscopy(see Supporting
information). Forcomplex 1, theexpectedweak d–dtransitionsof an
octahedral cobalt (II) complex could not be observed. The band at
337 nm refers to a charge transfer transition (LMCT). Unfortunately,
the expected weak d–d transition in the visible region for the com-
plex could not be detected even with a concentrated solution; this
may be lost in the low energy tail of the charge transfer transition.
After addition of hydrogen peroxide and warming the solution, the

2
774
A. Nemati Kharat et al. / Polyhedron 30 (2011) 2768–2775
band at 337 started to diminish gradually and a new band appeared
at 576 nm. This newabsorption could be related to the formation of a
new unknown cobaltic species.
In two separate blank experiments, no considerable oxidation
was observed in the absence of either Co(II) or Co(III) catalyst. To
examine the versatility of the method, a variety of both aliphatic
and aromatic alcohols were allowed to be oxidized using the above
described reaction conditions (see Table 5).
Noteworthy, the alcohols which have more solubility in aque-
ous media (Table 5, entries 2, 5–7 and 10–12) exhibit more reactiv-
ity towards oxidation by hydrogen peroxide and need shorter
reaction times in comparison with the other tested alcohols. In
other words, the alcohols with lower solubility in aqueous media
need longer reaction times to obtain a mono-phase reaction sys-
tem. It should be mentioned that among the various types of sec-
ondary alcohols, those including two aromatic rings directly
attached to the carbon bearing the hydroxyl group show high
selectivity and require shorter reaction times for their oxidation
corded at room temperature on a Bruker AVANCE 300 MHz. The
NMR spectra are referenced to Me Si as external standards.
4
Elemental analysis was performed using a Heraeus CHN–O Rapid
analyzer. The course of the reactions was monitored by gas
chromatography (Agilent Technologies 6890N Instrument),
equipped with a capillary column (19019J-413 HP-5, 5% phenyl
methyl siloxane, capillary 60.0 mm ꢃ 250 mm ꢃ 1.00 mm) and a
flame ionization detector. UV–Vis spectra were recorded on a
Shimadzu 2100 spectrometer using a 1 cm path length cell.
2 3 2
4.2. Preparation of [Co(L) ](NO ) ꢀ2MeOHꢀH
2
O
0
0
0
00
To a solution of 4 -(2-thienyl)-2,2 ;6 ,2 -terpyridine (L) (0.2 g,
0.64 mmol) in CHCl
3
(5 ml) was added a solution of Co(NO
3
)
2
ꢀ6H
2
O
(0.093 g, 0.32 mmol) in methanol (10 ml). The color of the reaction
mixture immediately turned deep red. After stirring for 30 min at
ambient temperature, the brownish-red mixture was left to evap-
orate slowly at room temperature. After 2 weeks, brownish-red
needles of the title compound were isolated (yield 0.19 g, 66.3%,
decomposition >311 °C).
(Table 5, entries 13–15). The results clearly show the widespread
applicability of the protocol for the oxidation of various types of
structurally diverse alcohols.
According to the presented results, the cobaltous species is
more effective than the cobalt (III) compound as a catalyst for oxi-
dation. The higher activity of the cobalt (II) complex can be ex-
plained by the lability of the complex towards the hydroperoxide
anion. This was confirmed by the longer Co–N bond distance,
which can be associated with the easier dissociation of the Co–N
4.3. Preparation of [Co(L)
2
](NO
3
)
3
ꢀ2MeOH
0
0
0
00
To a solution of 4 -(2-thienyl)-2,2 ;6 ,2 -terpyridine (L) (0.2 g,
0.64 mmol) in CHCl
3
(5 ml) was added a solution of Co(NO
3
)
2 2
ꢀ6H O
(0.093 g, 0.32 mmol) in methanol (10 ml). The color of the solution
turned deep red. Then, 1 M nitric acid and 30% hydrogen peroxide
(1 equiv.) was added to the reaction vessel and the reaction was re-
fluxed for 6 h until the reaction mixture turned yellow. The mix-
ture was stirred for 30 min at room temperature. All solvents and
volatiles were evaporated using a rotary evaporator and the resi-
due was recrystallized from methanol–acetonitrile 1:1 mixture.
After 2 weeks, yellow needles of the cobaltic complex were ob-
tained (yield 0.113 g, 37.6%, decomposition >368 °C).
3
+
bond and the formation of a new Co–OOH bond to form ‘‘Co
-
Oxo’’, an active form of the catalyst which catalyses the oxidation
reaction. The next step is oxygen transfer, then finally formation
of cobaltous species to continue the catalytic cycle. Based on our
knowledge, there are no remarkable reports in the literature to
compare the activity of similar Co(II) and Co(III) complexes for cat-
alytic purposes.
4.4. Typical procedure for the oxidation of alcohols
3
. Conclusion
In a typical experiment, a mixture of 1 mmol of alcohol, 3 mmol
An investigation of the homogeneous catalytic oxidation of
alcohols by H O in the presence of cobalt (II) and cobalt (III) com-
2 2
pounds is reported. Cobalt (II) compounds provide higher oxida-
tion activity than cobalt (III) compounds. The observed
experimental data can be consistently explained by the proposed
of hydrogen peroxide, 0.05 mmol of catalyst were added to a vessel
containing 2 ml deionized water at room temperature. After the
appropriate time, the reaction mixture was analyzed by gas chro-
matography (GC). Reaction products were identified by injection
of diethyl ether diluted samples into the GC. The yields were calcu-
lated from standard curves. Most of the reactions were run at least
twice and the found values were averaged.
2 2
catalytic cycles, including the activation of H O and its coordina-
tion to Co ions followed by an oxygen transfer step. This work pro-
vides important insight for the comparison of cobaltous and
cobaltic complexes for homogenous catalytic alcohol oxidation
by hydrogen peroxide, which is one of the most convenient, cheap-
est and greenest methods for the synthesis of various carbonyl
compounds.
4
.5. Crystal structure determination and refinement
The crystallographic data was collected with a Nonius Kappa
CCD diffractometer, using graphite-monochromated MoK
tion (0.71073 Å). For [Co(L) ](NO OHꢀH O, a brownish-red
ꢀ2CH
block with dimensions 0.25 ꢃ 0.15 ꢃ 0.10 mm and for [Co(L)
NO OH, yellow needle crystal with dimension
a radia-
2
)
3 2
3
2
4
. Experimental
2
]
(
3
)
3
ꢀ2CH
3
a
4
.1. Materials and instrumentation
0.23 ꢃ 0.08 ꢃ 0.08 mm was mounted. Cell constants and an
orientation matrix for the data collection were obtained by
least-squares refinement of diffraction data from 9021 unique
reflections for the Co(II) complex and 8996 for the Co(III) complex.
Data were collected at a temperature of 150(1) K to a maximum 2h
value of 55.2° and 54.86° for the Co(II) and Co(III) complexes,
The ligand was synthesized according to the previously pub-
lished procedures [18]. All solvents were obtained from commer-
cial sources, and if required the solvents were further purified by
standard methods. 2-Acetylpyridine, thiophene-2-carboxaldehyde,
2
8.0–30% aqueous ammonia and potassium tert-butoxide were
respectively. The numerical absorption coefficients,
MoKa radiation are 0.603 and 0.612 mm for the Co(II) and Co(III)
l
, for the
ꢁ1
purchased from Merck chemicals and were used as received. The
hydrated cobalt nitrate was obtained from Fluka. Melting points
are uncorrected and were obtained with Electrothermal 9200
complexes, respectively. The structures were solved by direct
methods [23] and subsequent differences Fourier map and then re-
ꢁ1
2
melting point apparatus. Infrared spectra from 250 to 4000 cm
fined on F by a full-matrix least-squares procedure using aniso-
of solid samples were taken as a 1% dispersion in CsI pellets using
tropic displacement parameters [23]. Hydrogen atoms were
placed in calculated positions and refined in a riding model
a Shimadzu-470 spectrometer. 1H and C NMR spectra were re-
13

A. Nemati Kharat et al. / Polyhedron 30 (2011) 2768–2775
2775
approximation using the SHELXL defaults [24]. Cell refinements were
performed using the Denzo-SMN crystallographic software pack-
age [25]. A summary of the crystal data, experimental details and
refinement results is given in Table 2.
[3] F. Ullmann, W. Gerhartz, Y. Stephen Yamamoto, R. Pfefferkorn, Ullmann’s
Encyclopedia of Industrial Chemistry, sixth ed., Wiley-VCH, Weinheim, 2002.
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(
We would like to acknowledge Dr. Alan J. Lough for crystallo-
graphic discussions. Financial support from University of Tehran
is also gratefully acknowledged.
2009) 6371.
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[
[
3
Appendix A. Supplementary data
[
[
[
CCDC 799943 and 799944 contain the supplementary crystallo-
graphic data for the Co2 and Co complexes. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
+
3+
[
[
[
[
3
36-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
[20] M. Cañadas, E. López-Torres, A. Martínez-Arias, M.A. Mendiola, M.T. Sevilla,
Polyhedron 19 (2000) 2059.
associated with this article can be found, in the online version, at
doi:10.1016/j.poly.2011.07.030.
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55 (2003) 432.
23] G.M. Sheldrick, SHELXTL-PC V6.1, Bruker Analytical X-ray Systems, Madison, WI,
001.
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