Cobalt(III)-oxo cubane clusters as catalysts for oxidation of organic substrates

Article abstract of DOI:10.1007/s12039-011-0111-6

Transition metal coordination complexes play a vital role as catalysts in the oxidation of organic substrates including renewable chemicals in an economically viable and environmentally friendly way. Here we highlight the preparation, characterization and application of oxo-cubane complexes of cobalt(III) as oxidation catalysts using air and water as oxidants. Cobalt(III)-oxo complexes of the type Co₄O₄(O ₂CR)₄L₄ have been prepared by a general method and these have been characterized by analytical, spectroscopic, electrochemical and crystallographic methods. These soluble complexes have shown promising utility as catalysts in the aerobic oxidation of side chains of alkylaromatic hydrocarbon compounds. Oxidation of neat ethylbenzene has shown very high conversion and selectivity for acetophenone formation. On the other hand, oxidation of p-xylene has been found to yield both p-toluic acid and terephthalic acid. It is also possible to oxidize p-xylene in an aqueous medium under moderate applied O2 pressure. Selective epoxidation of α-pinene with air as the oxidant also takes place with the cobalt(III)-based homogeneous catalysts. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-011-0111-6

J. Chem. Sci. Vol. 123, No. 2, March 2011, pp. 163–173. ꢀc Indian Academy of Sciences.
Cobalt(III)-oxo cubane clusters as catalysts for oxidation of organic
substrates
BIRINCHI KUMAR DAS and RAJESH CHAKRABARTYb
a,∗
a
Department of Chemistry, Gauhati University, Guwahati 781 014, India
Present address: Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA
b
e-mail: Birinchi.Das@hotmail.com; rajeshch2001@yahoo.co.in
Abstract. Transition metal coordination complexes play a vital role as catalysts in the oxidation of organic
substrates including renewable chemicals in an economically viable and environmentally friendly way. Here
we highlight the preparation, characterization and application of oxo-cubane complexes of cobalt(III) as oxi-
dation catalysts using air and water as oxidants. Cobalt(III)-oxo complexes of the type Co4O4(O2CR)4L4 have
been prepared by a general method and these have been characterized by analytical, spectroscopic, electro-
chemical and crystallographic methods. These soluble complexes have shown promising utility as catalysts
in the aerobic oxidation of side chains of alkylaromatic hydrocarbon compounds. Oxidation of neat ethylben-
zene has shown very high conversion and selectivity for acetophenone formation. On the other hand, oxida-
tion of p-xylene has been found to yield both p-toluic acid and terephthalic acid. It is also possible to oxidize
p-xylene in an aqueous medium under moderate applied O2 pressure. Selective epoxidation of α-pinene with
air as the oxidant also takes place with the cobalt(III)-based homogeneous catalysts.
Keywords. Cobalt(III) cubane; aerobic oxidation; pinene epoxidation; alkylaromatic oxidation;
homogeneous catalysis.
1. Introduction
of transition metals have also been the subject of studies
because these complexes may help in understanding the
Transition metal coordination compounds have numer- redox and ligand substitution reactions in metal catal-
4
ous applications among which an important one is in ysed autoxidation processes. Clearly, metal cluster
the catalysis of various reactions. Catalytic use of these complexes of cobalt are of particular interest because
compounds is particularly pre-eminent in reactions of their catalytic activity towards autoxidation of vari-
involving the oxidation of organic compounds via C–H ous aromatic hydrocarbons. Trinuclear cobalt clusters,
activation. Along with other important metals such as [(py) Co O(OAc) OR][PF ] have been used as cata-
3
3
5
6
copper, iron, vanadium, osmium, etc., cobalt is active lyst precursors for the autoxidation of toluene in acetic
as a catalyst in the oxidation of substrates including acid containing LiBr with O as the oxidant to produce
2
1
5
alkylaromatics, alkenes and alcohols. Several indus- benzoic acid.
2
trial reactions make use of cobalt-based catalysts. Use
Since cobalt-oxo clusters of different nuclearities
of cobalt(II) salts and complexes is common in the may form under a given oxidative synthetic condition,
published literature. The resultant processes utilize the it appeared important for us to identify the complex
+
3 oxidation state of the metal in completing the cat- species that may play an active role in oxidation catal-
alytic cycle, but instances of either cobalt(III) salts or ysis. Presence of the oxide ion as a ligand in these sys-
complexes being used in organic oxidation are rather tems favours the formation of cobalt(III) complexes,
3
rare. Ratnasamy et al. have reported the selective oxi- but existence of the metal in the +4 oxidation state
dation of p-xylene to terephthalic acid by trinuclear also cannot be ruled out. Presence of nitrogen bases
oxo-bridged cluster complexes of cobalt(II/III) and as probable ligands in the oxidizing preparative condi-
manganese(II/III) including mixed-Co/Mn species in tion favours the formation of cobalt(III) complexes of
presence of dioxygen. Oxo-bridged cluster complexes reversible redox stability. Oxo-bridged transition metal
complexes with ancillary N-and O-donor ligands are
of considerable interest to synthetic inorganic chemists
because of their interesting structural motifs as well as
∗
For correspondence
163

164
Birinchi Kumar Das and Rajesh Chakrabarty
due to their magnetic, optical, and catalytic properties alcohols and alkenes with air, oxygen or tert-butyl
6
8,9,12,13
and biological relevance. The most common core units hydroperoxide (TBHP) as oxidants.
It is also
encountered in such tetranuclear clusters are [M (μ - possible to immobilize the complexes on mesoporous
4
3
O) ] (I), [M (μ -O) ] (II) and [M (μ -O) ] (III). Man- silica to obtain stable heterogeneous catalysts for the
2
4
3
3
4
3
4
1
0,11
ganese complexes of type III are well-studied because same reactions.
of their role as durable abiotic catalysts for the genera- In this paper, we discuss the synthesis and physico-
tion of oxygen and hydrogen from water by photolytic chemical characteristics of Co(III)-oxo cubane-like
7
water splitting.
clusters of the type Co O (O CR) L . The role of these
4 4 2 4 4
complexes in catalytic oxidation under homogeneous
conditions is also discussed briefly.
2
. Preparation of Co O (O CR) L
4 4 2 4 4
The synthetic strategy for the preparation of tetra-
Oxo-carboxylato cobalt(III) clusters having a nuclear Co(III)-oxo complexes of the type
cubane-like core (III) have been found to be important Co (O CR) , C , or a 4-
O
L
, with R = CH
H
6 5
4
4
2
4
4
3
in view of their promising activities in the catalytic substituted aryl group and L = pyridine or a 4-
oxidation of diverse organic substrates.8 The first substituted pyridine, involves the reaction of Co(NO
–13
)
2
·
3
example of cobalt(III) complexes of the above type was 6H
O with two equivalents of the sodium salt of a car-
boxylic acid, NaO CR and one equivalent of the chosen
The complex Co O (O CMe) (py) having the same N-donor ligand — pyridine (py), 4-cyanopyridine (4-
2
2+
ꢁ
14
[
Co O (O CMe) (bpy) ] (bpy = 2,2 -bipyridine).
2
4
4
2
2
4
4
4
4
2
4
4
+
[
Co O ] core was later isolated from a mixture CNpy), 4-methylpyridine (4-Mepy) or 4-ethylpyridine
of several other species formed by direct ozonol- (4-Etpy) — followed by H
ysis of cobalt(II) acetate in H O/AcOH/pyridine elevated temperature in commercial grade methanol.
followed by precipitation with NH PF . Our inter- Oxidation of the pink solution forming at the begin-
4
4
-oxidation of Co2 at an
+
O
2
2
9
2
15
4
6
ests on cobalt(III) cubane-like clusters of the type ning of the reaction with excess H
Co O (O CR) L , where R is methyl or an aryl group olive green solution from which dark olive green
O leads to an
2
2
4
4
2
4
4
and L is either pyridine or a substituted pyridine ligand, Co(III) complexes of the above type may be isolated.
originated from their relevance in oxidation catalysis. Scheme 1 summarizes the synthetic route followed by
A highly efficient heterogeneous catalyst prepared by us to obtain the cobalt(III)-oxo clusters in good to very
supporting a cobalt(III) species was earlier found to good yield. In most cases, particularly when R is an
show very high conversion and selectivity in the aerobic aryl group, the complexes are found to precipitate out
oxidation of alkylaromatic hydrocarbons.16 Detailed during the course of the reaction. However, isolated
studies have suggested that the metal complex is likely product yields are low in most cases. In view of this,
17
to belong to the type Co O (O CR) L . In the course we have developed an alternative work-up procedure
4
4
2
4
4
of our work several complexes of this class have been to optimize product yields. In this improved isolation
prepared by a convenient method and characterized by procedure the reaction mixtures obtained as described
a variety of techniques. More importantly, it has been in scheme 1 are concentrated via evaporation and
found that they act as homogeneous catalysts in the extracted with CH
oxidation of organic substrates such as alkylaromatics, be obtained from the Na
Cl
. The solid products may then
2
2
SO -dried CH Cl solutions
2
4
2
2
Scheme 1. General scheme for the synthesis of cubane-like cobalt(III)-oxo clusters.

Cobalt(III)-oxo clusters as catalysts for oxidation
165
by adding petroleum ether as the precipitating solvent. involving R = Me and L = py. Although single crys-
This procedure also ensures the isolation of the cubane tal X-ray diffraction data collected at room temper-
complexes without co-crystallizing sodium salts and ature was helpful in determining the identity of the
4
solvent molecules (vide infra).
complex species as Co O (O CMe) (py) (1), the least
4
4
2
4
18
The synthetic method developed by us is thus squares refinement was far from satisfactory. While
quite simple because in all cases we are able to use some of the carbon atoms belonging to the aromatic
Co(NO ) ·6H O in MeOH as the starting material and rings showed abnormal anisotropic thermal parameters,
3
2
2
H O oxidation of Co(II) in presence of the sodium salt the residuals obtained were also very high (R ∼ 0.22)
2
2
of the relevant carboxylic acid and the required pyri- because some electron density peaks could not be
dine or substituted pyridine for successful preparation included in structure refinement. Using intensity data
◦
of the cluster complexes. This makes our procedure obtained at −100 C it was possible to establish its com-
10
quite general for the synthesis of cubane-like tetrameric position as 1·0.5NaNO ·8H O. Presence of NaNO3
3
2
cobalt(III) complexes of the type Co O (O CR) L . as a co-crystallizing salt in the crystals also helped us
4
4
2
4
4
The significant aspect of our work is that, unlike in in rationalizing the observed electrical conductivity of
reported procedures,1
4,15
complexes of other nuclear- solutions of the crystals in water and methanol. The
ities are not found to occur in the isolated products. same tetrameric complex was also obtained as dark
Also, while using a dichloromethane extract of the olive olive-green crystals of composition 1·NaClO ·3.5H O
4
2
green products during work-up procedure for isolating from the concentrated reaction mixture to which aque-
9
the complexes in improved yields, the complexes were ous NaClO was added.
4
4
invariably found to be identical to the complex species
The complex Co O (O CMe) (py) (figure 1) con-
4
4
2
3
4
+
2−
present in the precipitated products obtained directly sists of four each of Co and O ions occurring
4+
from the reaction mixtures. The vacuum-dried samples at alternate corners of a cube to form a [Co O ]
obtained via CH Cl extraction correspond to the gen- core with acetato ligands bridging the Co ions along
4
4
3+
2
2
eral formula Co O (O CR) L as indicated by C, H, N four face diagonals of the Co O cube. On the other
4
4
2
4
4
4
4
and Co analyses.
hand, the four Co atoms form an approximate tetra-
The neutral tetrameric complexes of the type hedron with Co...Co separations of ∼ 2.75 Å. Pyri-
Co O (O CR) L are soluble in common organic sol- dine ligands occupy the most outlying sites on the
4
4
2
4
4
vents to varying degrees. While they readily dissolve octahedral Co(III) centres. Inspection of the structural
in CH Cl , their solubility is rather low in chloro- parameters leads to the conclusion that for the Co
2
2
form. They also have appreciable solubility in polar atoms, the Co–O(oxide) and Co–O(carboxylate) bond
solvents such as MeOH and MeCN. However, the lengths are in the fairly narrow ranges of 1.860(3)–
4
complex Co O (O CMe) (py) (1) dissolves in several 1.876(3) and 1.948(3)–1.962(3) Å, respectively. These
4
4
2
4
solvents of widely differing polarity including water,
DMF, DMSO, acetonitrile, ethanol, methanol acetone,
diethyl ether and dichloromethane. The complexes with
R = Me generally show solubility in water. The
integrity of the complexes in the dissolved state in vari-
ous solvents is evidenced by their UV-visible and NMR
spectra. These complexes are indefinitely stable in the
solid state at room temperature.
3
. Crystal structure analysis
The identity and structure of the olive green complexes
of the type Co O (O CR) L have been established by
4
4
2
4
4
single crystal X-ray diffraction. The crystal structure
analysis of the compounds has proven to be an impor-
tant part of our work due to several reasons. The first
compound of the series was isolated in the form of
Figure 1. Molecular structure of [Co4O4(O2CMe)4(py)4]
1) in 1·NaClO4·3.5H2O. Hydrogen atoms are omitted for
(
rod-like crystals from the preparative reaction mixture clarity.

166
Birinchi Kumar Das and Rajesh Chakrabarty
values are consistent with a low spin configuration of of co-crystallizing solvent molecules in crystals of the
Co(III). The average Co–N distance is of the order of cubane clusters has been noted by us in other com-
9
1
.96 Å. Two types Co...Co distances, close to (i) 2.82 Å pounds such as Co O (O CMe) (4-Mepy) ·3H O,
4
4
2
4
4
2
8
when Co...Co is bridged by two oxo-ligands only, and Co O (O CC H ) (py) .6MeOH·0.5H O and Co O
4
4
2
6
5
4
4
2
4
4
8
(
ii) 2.70 Å when bridged by two oxo-ligands and a (O CC H ) (4-Mepy) .3.5MeOH·5H O as well.
2
6
5
4
4
2
bidentate carboxylato ligand, are observed. The bond Crystal structure analysis of the last two compounds
4+
parameters for the cubane-like [Co (μ-O) ] core in show that the cubanes having aryl carboxylates
all compounds studied by X-ray diffraction method are also present metrically similar [Co (μ -O) ] cores
4
4
4+
4
3
4
similar.
(figure 3). It is also worth noting that despite the
presence of a number of hydrogen bonds forming via
+
In the crystal structure of 1·NaClO ·3.5H O the Na
4
2
ions are found to bridge the cubane-like complexes to the involvement of the solvent molecules, crystals of
form zigzag chains that run along the crystallographic Co O (O CC H ) (py) .6MeOH·0.5H O are not stable
4
4
2
6
5
4
4
2
a and b directions. The two crystallographically dis- at room temperature and as a result, the crystal had
tinct chains are related by 4 symmetry in the crystal to be inserted into a Lindemann capillary containing
lattice. Each sodium atom is in an approximately octa- mother liquor during intensity data collection. On the
hedral environment surrounded by six oxygen atoms, other hand, crystals obtained from less polar media
four from two carboxyl groups and the other two being were found to lose crystallinity in several cases.
contributed by two H O molecules of crystallization
It is seen from the above results that the cubane-like
2
9
4+
(
figure 2).
[Co O ] core present in the complexes prepared by
4
4
13
In the crystal structure of 1·0·5NaNO ·8H O
us is similar to those present in the related compounds
3
2
14,15
two molecules of the tetramer [Co (μ -O) (μ- which were reported earlier.
Important structural
4
3
4
O CMe) (py) ] along with one formula unit of NaNO
parameters of cubane tetramers of Co(III) are com-
2
4
4
3
and sixteen H O molecules are found to constitute pared in table 1. It is observed that the peripheral
2
the asymmetric unit of the triclinic unit cell. Thermo- ligands have only a minor influence on the geome-
gravimetric studies corroborate the number of water try of the tetrameric Co(III)-oxo core present in the
10
4+
molecules found from crystal structure analysis. The complexes. The [Co (μ -O) ] cuboid allows for two
4
3
4
observed weight loss of 11.4% matches very well with kinds of shortened Co...Co separations of ca. 2.8 Å
3+
3+
the calculated weight loss of 11.5% for the chemical and ca. 2.7 Å. Although these Co ...Co distances
formula given by crystal structure analysis. Presence are fairly long for direct Co–Co bond formation, the
Figure 2. Sodium-bridged zigzag supramolecular chain of 1 in the crystal structure of 1·NaClO4·3.5H2O. Hydrogen atoms
and uncoordinated water molecules are omitted for clarity.

Cobalt(III)-oxo clusters as catalysts for oxidation
167
compounds. A representative spectrum is shown as part
of supplementary materials.
(a)
1
The H NMR spectra of these complexes are char-
acterized by sharp signals due to their diamagnetic
6
nature. The cobalt(III) ions with a low-spin d electron
configuration are expected to be diamagnetic, particu-
20
larly in the presence of pyridine as a ligand. While
1
the H NMR spectrum of Co O (O CC H -4-OMe) (4-
4
4
2
6
4
4
1
3
Mepy) . is given as supplementary material, the
NMR spectrum of Co O (O CMe) (py) is shown in
C
4
4
4
4
2
4
figure 4. The simplicity of the spectra clearly indi-
cates the presence of only one type of environment for
pyridyl as well as carboxylato ligands and thus it is con-
sistent with a virtual T symmetry of the molecule in
d
solution. It suggests the retention of molecular integrity
on dissolution. For all other complexes in the series
we have obtained the proton NMR spectra to establish
that in all cases we isolated pure cobalt(III)-oxo cubane
clusters (selected spectral data are given as part of sup-
plementary materials, see www.ias.ac.in/chemsci). The
NMR spectra thus provide conclusive evidence for the
bulk purity of the product. At the same time, the spec-
tra also suggest that the oligonuclear complexes remain
intact in the solution phase as well.
(b)
UV-visible spectra of the tetranuclear cobalt(III)-
oxo clusters have been studied in the solution phase.
4
The spectral data for the complex Co O (O CMe) (py)
4
4
2
4
obtained using several common solvents are presented
in table 2. The electronic spectra for this compound in
Figure 3. (a) Molecular structure of the cubane
cluster and (b) Co · · · Co separations for the Co4
tetrahedron in the inner [Co4(μ3-O)4]4
+
core in
2
2
2
CH Cl and H O at three different concentrations are
Co4O4(O2CC6H5)4(py)4.6MeOH·0.5H2O.
shown in figure 5. As can be seen from the data, the
spectra are slightly solvent-as well as concentration-
dependent. The spectral bands found in MeOH occur at
−1
−1
6
5
17 nm (ε = 360 M cm ), 330 nm (ε =
900 M cm ) and 246 nm (ε = 21,900 M cm ).
existence of an approximately tetrahedral assembly
of four cobalt atoms in each compound is notable
−1
−1
−1
−1
figure 3(b)). Each O2− ligand may then be considered
The lowest energy band at 617 nm can be assigned
(
1
1
to the d–d transition involving either A → T or
1
1
to cap a triangular face of the Co tetrahedron.
4
1
1
6
A → T for the approximately octahedral low-spin d -
1
2
Co(III) centers. As judged from the observed intensi-
ties, the other two bands are attributable to ligand to
metal charge transfer transitions (LMCT). While the
4
. Physicochemical properties of Co O (O CR) L
4 4 2 4 4
It has been described above that a number of cubane- first LMCT band at 330 nm is likely to be due to a tran-
like complexes of general formula Co O (O CR) L
sition involving the μ -O-Co(III) moiety present in the
4
4
2
4
4
3
may be prepared by following the general proce complex, the latter occurring at 246 nm most probably
dures developed by us. The complexes belonging to involves a transition between a molecular orbital from
the series have been investigated by various spectro- the carboxylato ligands and a vacant cobalt(III) eg*
scopic and analytical techniques. Based on crystallo- orbital.
graphic results, it may be assumed that aided by short
As noted above, the spectral traces depend to some
III
Co −O(oxide) distances and Co · · · Co separations, extent on the solvent and concentration used. For
the tetranuclear core of the cubane complexes is quite example, the above complex shows bands in the vis-
robust. This may explain the preeminence of the molec- ible region in CH Cl and MeCN at 638 and 650 nm
2
2
ular ion peaks in the ESI-MS spectra obtained for the respectively indicating a significant shift in band

168
Birinchi Kumar Das and Rajesh Chakrabarty
◦
III
4+
Table 1. Average interatomic bond distances [Å] and angles [ ] for tetranuclear complexes having [(Co )4(μ3-O)4] core.
Compounda
Co–Narom. Co–(μ3–O) Co–Ocarbox. Co. . . Cob Co. . . Coc O–Co–Od Co–O–Cod Reference
[Co4O4(O2CC6H4-p-Me)2(bpy)4] 1.939
1.882
1.959
2.852
2.665
–
–
[14
]
(
ClO4)2·2MeCN
4
15
Co4O4(O2CMe)4(py) ·5CHCl3
1.973
1.960
1.962
1.865
1.887
1.865
1.962
1.945
1.953
2.818
2.78
2.815
2.683
–
2.702
85.9
83.9
85.0
93.6
95.8
94.6
[
[
[
]
1
9]
Co4(pg)4O4
Co4O4(O2CMe)4(py)4
10
]
·
0.5NaNO3·8H2O
Co4O4(O2CMe)4(py)
NaClO4·3.5H2O
4
9
1.968
1.870
1.956
2.823
2.709
84.4
95.4
[ ]
·
9
Co4O4(O2CMe)4(4-Mepy)4·3H2O 1.959
1.868
1.883
1.963
1.967
2.816
2.856
2.701
2.725
85.3
85.4
94.3
94.69
[ ]
8
Co4O4(O2CC6H5)4(py)4.6MeOH
0.5H2O
Co4O4(O2CC6H5)4(4-Mepy)4.
.5MeOH·5H2O
1.968
1.955
[ ]
·
8
1.866
1.951
2.826
2.708
84.21
95.15
[ ]
3
a
b
c
Hpg = N-(2-pyridylmethyl)glycine; bridged by two oxo ligands only; bridged by two oxo ligands and a bidentate
d
carboxylato ligand; only oxygen atoms of the Co4O4 core considered
13
4
Figure 4.
C NMR spectrum of Co4O4(O2CMe)4(py) in D2O.
4
Table 2. Electronic spectral data on Co4O4(O2CMe)4(py) in various solvents.
λmax, nm (ε, M 1cm
−
−1
)
Solvent
1
1
A1→ T1
LMCT (μ3-oxo-bridge)
LMCT
MeOH
CH2Cl2
MeCN
H2O
617 (360)
638 (sh)
650 (330)
618 (388)
330 (5,900)
362 (7,428)
355 (7,149)
354 (4,742)
246 (21,900)
227 (26,537)
–
257 (42,879)

Cobalt(III)-oxo clusters as catalysts for oxidation
169
(a)
(b)
4
Figure 5. UV-visible spectra of Co4O4(O2CMe)4(py) in (a) CH2Cl2 and (b) H2O obtained through measurements on
− −4 −5
3
1
0 , 10 and 10 M solutions.
−1
−1
positions. Again, the 362 nm (ε = 7,428 M cm ) presence of the triply bridging oxide ions in these com-
band in CH Cl becomes blue-shifted to 355 nm plexes is evidenced by the appearance of a moder-
2
−
2
1
−1
−1
(
ε = 7,149 M cm ) in MeCN. The characteristic vis- ate intensity band at ∼ 635 cm . A four-band pattern
−1
ible peak due to the d–d transition remains similar observed at ∼ 760, ∼ 695, ∼ 635 and ∼ 580 cm for
in shape, although the band position undergoes con- the cubane complexes (please see supplementary mate-
siderable shift from the values found in CH Cl and rials for examples) is a characteristic feature of the IR
2
2
MeCN. On the other hand, the visible band at 618 nm spectra of Co O (O CR) L .
4
4
2
4
4
−1
−1
(
ε = 388 M cm ) observed in H O occurs almost
The redox behaviour of the cubane clusters of
at the same energy with the band observed in MeOH cobalt(III) has been studied by cyclic voltammetry.
2
(617 nm), although the lower energy LMCT bands in
these two solvents appear at widely separated wave-
lengths (354 nm for H O and 330 nm for MeOH).
2
4+
From these results, it may be said that the [Co O ]
4
4
core of the complex undergoes deformation to varying
degrees in different solvents. At the same time, it is not
possible to rule out the possibility of H O and MeOH
2
entering into hydrogen bonding interactions with the
oxo-ligands so as to change the shape of the Co O
4
4
cube further. This could perhaps explain why the d–d
band positions in these two solvents appear at nearly
the same energies. The highest energy LMCT tran-
sition is not observed in MeCN for as yet unknown
reasons.
The above spectral results are typical of complexes
of the type Co O (O CR) L ; spectra of several other
4
4
2
4
4
complexes in the series have been recorded (please
see supplementary materials for spectral data on other
related complexes www.ias.ac.in/chemsci).
The mid-IR spectra recorded for the cubane-like
cobalt(III) complexes are also informative. It is possi- O CMe) (py) in MeCN containing 0.2 M TBAP and
−1
Figure 6. Cyclic voltammogram at 20 mVs on Pt elec-
trode (Ag/AgCl reference electrode) of Co4(μ3-O)4(μ-
2
4
4
3+
4+
ble to identify bands due to the pyridyl as well as car- ferrocene as an internal standard. E1 (Co /Co ) = 0.73 V,
/
2
boxylato ligands present in the complexes. Also, the ꢀEp = 66 mV.

170
Birinchi Kumar Das and Rajesh Chakrabarty
a
b
Table 3. CV and DPV data for a few Co4O4(O2CR)4(L)4 complexes in CH2Cl2–0.1M TBAP.
CV data
Complex
DPV data (V)
E1 (V) ꢀEp(mV)
/
2
Co4(μ3-O)4(μ-O2CMe)4(py)4
0.75
0.71
0.72
1.0
170
138
140
128
0.77
0.73
0.74
1.02
Co4(μ3-O)4(μ-O2CMe)4(4-Mepy)4
Co4(μ3-O)4(μ-O2CMe)4(4-Etpy)4
Co4(μ3-O)4(μ-O2CMe)4(4-CNpy)4
aScan rate, 20 mVs , bscan rate, 5 mVs , working electrode, Pt; reference electrode, Ag/AgCl
−1
−1
Scheme 2. Redox couple involved in the one electron oxidation of Co4O4(O2CMe)4L4.
Figure 6 shows the CV recorded for a 10−3 M solu-
tion of complex Co (μ -O) (μ-O CMe) (py) in MeCN Co O (O CR) L also have been studied by electro-
Several other cubane complexes of the type
4
3
4
2
4
4
4
4
2
4
4
containing 0.2 M TBAP as the supporting electrolyte chemical methods and in all cases we observe very
−
1
(
scan rate = 10 mVs ) and ferrocene as the internal similar electrochemical behaviour. The observed E
1
/
2
standard using platinum electrode. The electrochemical values may be correlated to Hammett constants for
studies reveal a reversible oxidation at 0.73 V for this substituents on aromatic rings.
8
particular complex. Other complexes also show similar
E
1
values, some of which are listed in table 3. How-
/
2
ever, change of solvent from MeCN to CH Cl makes
2
2
5. Catalytic behaviour in aerobic oxidation
the electrochemical process quasireversible. It is imper-
4
+
ative that the tetrameric [Co (μ -O) ] core undergoes A few of the cubane-like cobalt(III) complexes des-
4
3
4
5+
a one-electron oxidation to [Co (μ -O) ] to form a cribed above have been found to catalyse the oxidation
4
3
4
mixed valence tetranuclear complex that contains three of some commercially important organic substrates. It
III
IV
Co and one Co ions, as had been observed for is to be noted that the neutral tetrameric complexes dis-
a related complex. Such a favourable electrochemical play appreciable solubility in organic media at room
redox potential for the cubane complexes suggests their temperature and hence they may be expected to serve
suitability as redox catalysts in the oxidation of organic as homogeneous catalysts or catalyst precursors in the
substrates.
redox transformation of organic compounds.
4
The stoichiometry of electron transfer can be deter-
The complex Co O (O CMe) (py) has been found
4
4
2
4
mined from the peak current measurements using fer- to mediate the aerobic oxidation of neat ethylbenzene
rocene in equimolar concentration as an internal stan- to form acetophenone in ∼ 69% yield. No additional
dard. It is found that the ratio of observed peak cur- solvent is required in this conversion that produces 1-
rents for ferrocene and Co O (μ-O CMe) (py) is ∼ 4 phenylethanol (1%) as the only other minor product at
4
4
2
4
4
III
(
figure 6). This suggests that only one Co centre the end of 22 h (scheme 3). An excellent 99% selectiv-
per ‘(Co ) O ’ cluster undergoes oxidation. Thus the ity for acetophenone has been achieved. As indicated by
III
4
4
electron transfer process involves a one electron oxi- gas chromatographic studies, the conversion of ethyl-
III
4+
4
dation of the cubane core from [(Co ) (μ -O) ]
benzene proceeds more or less linearly with time at
4
3
III
IV
5+
◦
to [(Co ) Co (μ -O) ] (scheme 2). A literature the rate of ca. 5% per hour at 120 C for the first four
report on a structurally related complex [Co (μ - hours and thereafter the rate of oxidation slows down
O) (μ-O CMe) (bpy) ] in MeCN revealed a simi- a little, but the rise in conversion remains almost lin-
3
3
4
21
4
3
2+
4
2
2
2
lar oxidation process where the cubane core [Co (μ - ear up to 22 h after which the amount of acetophe-
4
3
IV
4+
III
III
O)4] containing 4Co was oxidized to a 3Co , Co
none levels off. Unlike in the cases of Co(II)-based pro-
cesses where the metal needs to undergo oxidation to
form.

Cobalt(III)-oxo clusters as catalysts for oxidation
171
attractive. Suitable modification of reaction conditions
as well as experimental set-up is expected to improve
yields and selectivities of products in these catalysed
oxidation processes. The survival of the cubane com-
plexes through the course of the oxidation reactions also
suggests that under appropriate conditions the reuse of
the catalysts may be possible.
Scheme 3. Aerobic oxidation of ethylbenzene catalysed by
Co4O4(O2CMe)4(py) .
4
Typically cobalt-catalysed autoxidation of p-xylene
in the liquid phase is carried out in acetic acid medium
at very high temperature (195–205 C) and pressure
Co(III) initially in a sluggish manner,22 no induction
period is observed in the use of the present Co(III)-
based reaction. Product selectivities are also much bet-
◦
(
∼ 30 bar) in the presence of halide ion promoter
that significantly changes the reaction kinetics of such
processes.
ter with our cubane complexes compared even to the
2,22
Co(III) complexes which are reported23 to be highly
Since the oxidation of Co(II) to Co(III)
is a slow process, use of promoters is necessary in
cobalt(II)-catalysed processes to prevent a possible
induction period during which no desired conversion
takes place. The avoidance of induction period and
polluting promoters, redundancy of solvents and the
usability of water as a solvent in alkyl aromatic oxida-
tion thus make these conversions quite friendly from an
environmental point of view.
active in the oxidation of ethylbenzene to acetophenone
at 150 C. The acetophenone productivity calculated for
◦
the present study is 2310 moles per mole of catalyst,
while the turnover frequency calculated for the first four
−1
hours of the reaction is 212 h . Our results obtained
under homogeneous conditions by making use of the
cobalt(III) cubane clusters also compare well with the
highly active heterogeneous catalyst, also based on a
Co(III)-oxo species.16
Oxidation of renewable feedstock to produce value-
The cubane complexes has also been found to be use- added chemicals constitutes an important area of
24
ful in the autoxidation of p-xylene with air under atmo- research. Transition metal based catalysts for such
25
spheric pressure and in absence of any co-catalysts or transformations are quite important. In the above con-
additives under solvent-free condition (scheme 4). Dur- text, oxidation of α-pinene, which occurs widely in the
ing autoxidation of p-xylene with compressed air for- plant kingdom, may be viewed as an important reaction
mation of solid product starts immediately. Due to the because oxidation products of α-pinene find use as the
build-up of a large quantity of solid product the reaction starting materials for fragrance, flavour and therapeu-
26
needs to be stopped at the end of 7 h. The isolated yields tic agents including taxol. The tetrameric cubane-like
of p-toluic acid and terephthalic acid are found to be complex [Co O (O CC H ) (4-CNpy) ] was examined
4
4
2
6
5
4
4
∼
25% and ∼ 1%. The compound Co O (O CMe) (4- as a catalyst for the homogeneous air oxidation of
4
4
2
4
CNpy) is also active under moderate O pressure in α-pinene under atmospheric pressure (scheme 5) and
4
2
aqueous medium. In this case also, p-toluic acid is the found to favour the preferential formation of the epox-
main product with only traces of terephthalic acid form- ide product.
ing as the other product.12 Our results thus indicate that
This reaction was carried out under homogeneous
the cobalt(III)-oxo clusters may be active as catalysts conditions at an elevated temperature using 1,4-dioxane
13
for p-xylene oxidation in highly polar (water) as well as the solvent. The effect of reaction temperature was
as non-polar (p-xylene) media. Further studies are how- studied by taking 25 mg (0.08 mol%) of the catalyst
◦
◦
ever necessary to make these findings commercially and allowing the reaction to take place at 60 C, 80 C
◦
or 100 C, respectively. By monitoring the progress of
the reaction by GC it was found that an increase in the
reaction temperature accelerates air oxidation of α-
pinene with the highest conversion of 66.8% being
◦
observed at 100 C after 24 h. At all the three temper-
atures air oxidation commenced quickly with a prefer-
ential formation of α-pinene oxide over other oxidation
products.
In order to evaluate the optimum catalyst require-
ment in the air oxidation of α-pinene, the catalyst con-
centration was varied between 0.01 mol% to 0.5 mol%.
Scheme 4. Aerobic oxidation of p-xylene catalysed by
Co4O4(O2CMe)4(py) .
4

172
Birinchi Kumar Das and Rajesh Chakrabarty
Scheme 5. Aerobic oxidation of α-pinene catalysed by Co4O4(O2CPh)4(4-CNpy)4.
GC analysis of products (figure 7) shows that the high- lead to effective yields of desirable products at high
est substrate conversion in the reactions is 81.4% with selectivity under moderate liquid phase conditions. In
a turnover frequency (TOF) of 105 after 24 h, while view of the solubility of the complexes in alkyl aro-
the selectivities for the epoxidation product, α-pinene matic substrates it is possible to carry out the oxidation
oxide, are in the range of 62–68%. It can be seen that reactions in the absence of added solvents. In one case
conversion and product yields are the highest when cat- it has been found that oxidation of p-xylene may also
alyst concentration is the lowest. This is in conformity be carried out using water as a solvent. The promis-
with an earlier report where higher concentration of ing catalytic behaviour shown by the complexes sug-
2
7
catalyst led to slower air oxidation of α-pinene.
gests that inclusion of more complexes in the studies
may lead to even better results. The environmentally
acceptable conditions used in the reactions indicate that
with further investigations it may be possible to develop
methods with potential for commercial exploitation.
6. Conclusions
It has been possible to develop a new general method
for the synthesis of cobalt(III)-oxo cubane clusters
of general formula Co O (O CR) L . Physicochemi-
cal behaviour of these soluble molecular complexes
4
4
2
4
4
Supplementary information
has been studied in detail. Molecular structure of the The electronic supporting information can be seen in
complexes has been analysed. A few complexes of www.ias.ac.in/chemsci.
the series have been evaluated as homogeneous cata-
lysts for the oxidation of ethylbenzene, p-xylene and
α-pinene using dioxygen as the oxidant. The reactions
Acknowledgements
Financial assistance from the Department of Sci-
ence and Technology (DST), Government of India is
gratefully acknowledged. RC thanks University Grants
Commission (UGC), India for a Research Fellowship.
The authors also thank Prof. JH Clark of the University
of York, UK for his help and suggestions in doing the
catalytic work.
References
1
. Sheldon R A and Kochi J A 1981 Metal-catalyzed oxi-
dations of organic compounds (New York: Academic
Press)
2
3
4
. Suresh A K, Sharma M M and Sridhar T 2000 Ind. Eng.
Chem. Res. 39 3958
. Chavan S A, Srinivas D and Ratnasamy P 2001 J. Catal.
204 409
. Sumner C E Jr and Steinmetz G R 1985 J. Am. Chem.
Soc. 107 6124
Figure 7. GC plots under optimized conditions for the cat-
alytic oxidation of α-pinene catalysed by a cubane complex
of cobalt(III) .
5. Sumner C E Jr and Steinmetz G R 1989 Inorg. Chem. 28
13
4290

Cobalt(III)-oxo clusters as catalysts for oxidation
173
6
7
. Winpenny R E P 2004 Comprehensive coordination 17. Chakrabarty R 2006 Ph.D. Thesis, Gauhati University,
chemistry (eds) J A McCleverty and T J Meyer (Oxford:
Pergamon Press: 7 125)
Assam
18. Chakrabarty R, Bora S J and Das B K 2004 Catal-
ysis in petroleum and petrochemicals industries (eds)
A K Talukdar and K G Bhattacharyya (New Delhi:
Narosa Publishing House) pp. 287–292
. Yagi M, Wolf K V, Baesjou P J, Bernasek S L and
Dismukes G C 2001 Angew. Chem. Int. Ed. Engl. 40
2925
8
. Chakrabarty R, Sarmah P, Saha B, Chakravorty S and 19. Ama T, Okamoto K, Yonemura T, Kawaguchi H,
Das B K 2009 Inorg. Chem. 48 6371 Takeuchi A and Yasui T 1997 Chem. Lett. 1189
. Chakrabarty R, Bora S J and Das B K 2007 Inorg. Chem. 20. Cotton F A and Willkinson G 1988 Advanced inorganic
6 9450 chemistry (New York: Wiley) 5th edn, pp. 735–738
0. Sarmah P, Chakrabarty R, Phukan P and Das B K 2007 21. Dimitrou K, Brown A D, Concolino T E, Rheingold A
J. Mol. Catal. A Chem. 268 36 L and Christou G 2001 Chem. Commun. 1284
1. Chakrabarty R, Das B K and Clark J H 2007 Green 22. Partenheimer W 1995 Catal. Today 23 69
9
4
1
1
1
1
1
1
1
Chem. 9 845
23. Qi J-Y, Ma H-X, Li X J, Zhou Z-Y, Choi M C K,
Chan, A S C and Yang O-Y 2003 Chem. Commun.
1294
2. Chakrabarty R, Kalita D and Das B K 2007 Polyhedron
26 1239
3. Chakrabarty R and Das B K 2004 J. Mol. Catal. A Chem. 24. Corma A, Iborra S and Velty A 2007 Chem. Rev. 107
23 39 2411
4. Dimitrou K, Folting K, Streib W E and Christou G 1993 25. Gallezot P 2007 Catal. Today 121 76
2
J. Am. Chem. Soc. 115 6432
5. Beattie J K, Hambley T W, Klepetko J A, Masters A F
and Turner P 1998 Polyhedron 17 1343
26. Maksimchuk N V, Melgunov M S, Chesalov Y A,
Mrowiec-Biało n´ J, Jarzêbski A B and Kholdeeva O A
2007 J. Catal. 246 241
6. Das B K and Clark J H 2000 Chem. Commun. 27. Lajunen M K, Maunula T and Koskinen A M P 2007
05 Tetrahedron 56 8167
6