Binuclear cobalt complex with Schiff base ligand: Synthesis, characterization and catalytic properties in partial oxidation of cyclohexane

Article abstract of DOI:10.1016/j.ica.2012.06.017

A new binuclear cobalt complex with Schiff base ligand and amino acid was synthesized and characterized using XRD, elemental analysis, IR spectroscopy, XPS and electrochemical measurements. The presence of cobalt in the oxidation state 3+ in this complex was proved. The catalytic activity of this complex was investigated in the reaction of partial cyclohexane oxidation with air, and high activity and selectivity for cyclohexanol and cyclohexanone formation were demonstrated. The possibility of complex reuse in cyclohexane oxidation was studied and it was shown that the activity slightly decreases in a second oxidation cycle along with changes in main product distribution due to partial transfer of Co³⁺ to Co²⁺.

Full text of DOI:10.1016/j.ica.2012.06.017

Inorganica Chimica Acta 392 (2012) 221–228
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Binuclear cobalt complex with Schiff base ligand: Synthesis, characterization
and catalytic properties in partial oxidation of cyclohexane
a
a
b
Liudmila I. Rodionova a, , Andrey V. Smirnov , Natalya E. Borisova , Victor N. Khrustalev ,
⇑
Anna A. Moiseeva a, Wolfgang Grünert c
a Department of Chemistry, Lomonosov Moscow State University, Lenin Hills 1/3, 119991 Moscow, Russian Federation
b A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov Street 28, 119991 Moscow, Russian Federation
c Ruhr-Universität Bochum, P.O. Box 10 21 48, D-44801 Bochum, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A new binuclear cobalt complex with Schiff base ligand and amino acid was synthesized and character-
ized using XRD, elemental analysis, IR spectroscopy, XPS and electrochemical measurements. The pres-
ence of cobalt in the oxidation state 3+ in this complex was proved. The catalytic activity of this
complex was investigated in the reaction of partial cyclohexane oxidation with air, and high activity
and selectivity for cyclohexanol and cyclohexanone formation were demonstrated. The possibility of
complex reuse in cyclohexane oxidation was studied and it was shown that the activity slightly decreases
in a second oxidation cycle along with changes in main product distribution due to partial transfer of Co3+
Received 16 March 2012
Received in revised form 31 May 2012
Accepted 1 June 2012
Available online 19 June 2012
Keywords:
Cyclohexane oxidation
Binuclear cobalt complex
Schiff bases
to Co2+
.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
mic not only structural but also catalytic properties of metalloen-
zymes [12–15]. Based on such biological processes as methane
oxidation on monooxygenase [12,13,15] or steroids oxidation on
cytochrome P450 [14], new complexes for catalytic oxidation of al-
kanes, olefins, alcohols and aldehydes are being developed.
The detailed study of structure, state of metal and catalytic
activity of such complexes can result the in elucidation of the
mechanisms of biocatalyzed oxidation processes.
One of the most promising directions of petroleum hydrocarbon
feedstock conversion is its partial oxidation with air to valuable
products [1]. Among these processes, the catalytic oxidation of
cyclohexane to a mixture of cyclohexanol and cyclohexanone be-
comes increasingly important due to great need of these products
as precursors in caprolactam and Nylon-6 synthesis [2]. One of the
first industrial alkane oxidation processes – the Du Pont adipic acid
process [3], which involves homogeneous cyclohexane oxidation
with air on cobalt salts, such as naphthenate or acetate, initiated
an intense research activity devoted to catalytic cyclohexane oxi-
dation over heterogeneous and homogeneous cobalt systems [4–
10] using different oxidants such as air [5,6], pure oxygen [7],
hydrogen peroxide [8,9] and organic hydroperoxides [10].
As cyclohexanol and cyclohexanone are more easily oxidized
than the initial cyclohexane, conversion is kept at low levels (about
4–5%) to avoid the formation of side products by deeper oxidation
of the cyclohexanol–cyclohexanone mixture [3]. In this connection,
it is of prime importance to find new catalysts which selectively
oxidize only cyclohexane.
In this article a binuclear cobalt complex CoL with Schiff base
ligand and L-valine will be described. The composition and struc-
ture of this complex were characterized by IR spectroscopy, ele-
mental analysis, XRD, XPS and electrochemical measurements.
The complex was tested in cyclohexane oxidation with air and
showed high activity and selectivity to the cyclohexanol–cyclohex-
anone mixture.
2. Experimental
2.1. Synthesis
2.1.1. Synthesis of sodium salt of the ligand (see Scheme 2)
Mono- and binuclear complexes of transitional metals with
Schiff base ligands attract attention since they are structural and
composition analogs of some biological active compounds [11].
Many researchers are focused on producing systems which can mi-
To the solution of 115 mg (5 mmol) of Na in anhydrous metha-
nol (30 ml) 590 mg (5 mmol) of solid L-valine was added and the
mixture was stirred until complete dissolution. After the addition
of the equimolar amount of 5-chloro-2-hydroxy-3-nitrobenzalde-
hyde (5 mmol) the mixture was stirred for 3 h at room tempera-
ture and a dark crimson solution was formed. After evaporation
of this solution, a dark red product was formed.
⇑
Corresponding author. Tel.: +7 (495) 939 2054; fax: +7 (495) 939 3570.
E-mail address: ludmilarodionova@mail.ru (L.I. Rodionova).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.06.017

222
L.I. Rodionova et al. / Inorganica Chimica Acta 392 (2012) 221–228
Cl
H2O
Co
NO2
O
H2O
Cl
O
OH2
OH2
O
OH
+
N
O
.
H2N
+
NO2
Co(OAc) 4H O
2 2
2
2
2
O
Co
O
N
O
H2O
O
OH
O2N
Cl
Scheme 1.
Anal. Calc. for C12H12ClNaN2O5: C, 44.67; N, 8.68; H, 3.75; Na,
determined as Co3O4. FTIR spectra were recorded on a Nicolet Pro-
tege 460 spectrometer in KBr pellets.
7.22. Found: C, 44.5; N, 8.92; H, 3.63; Na, 6.94%. IR (KBr, cmꢀ1):
3427, 2965, 2929, 2862, 1648, 1609, 1534, 1510, 1398, 1340,
1280, 1221. 1H NMR (400 MHz, methanol-d4) d ppm 1.02 (t,
J = 6.85 Hz, 6 H) 2.40 (tq, J = 6.60, 5.26 Hz, 1 H) 3.98 (br, s, 1 H)
7.67 (d, J = 2.93 Hz, 1 H) 7.90 (s, 1 H) 8.11 (d, J = 2.69 Hz, 1 H)
8.48 (s, 1 H).
Electrochemical studies (cyclic voltammetry) were carried out
on a IPC-Pro M potentiostat. A glassy-carbon disk with diameter
of 2 mm was used as working electrode; a 0.1 M Bu4NClO4 (TBP)
solution in DMF (high-purity grade) served as the supporting elec-
trolyte; Ag/AgCl/KCl(satd.) was used as reference electrode and a
platinum electrode as counter electrode. The sample concentration
was 10ꢀ3 mol/l. All measurements were carried out under argon;
the samples were dissolved in the deaerated solvent. The scan rate
was 200 mV sꢀ1. All potentials are given taking into account IR-
compensation.
2.1.2. Synthesis of complex [16] (Scheme 1)
5-Chloro-2-hydroxy-3-nitrobenzaldehyde 1 g (5 mmol) and
590 mg (5 mmol) of L-valine were dissolved in 50 ml of an etha-
nol–water (3/1) mixture and heated for 2 h. After this period,
1.254 g (5 mmol) of solid cobalt(II) acetate tetrahydrate was added
and the mixture was diluted with 50 ml of water and sonicated for
10 min to avoid precipitation of unreacted cobalt salt. The reaction
mixture was filtered, washed with water and diethyl ether. The
collected solid was air dried overnight and then over NaOH in a
desiccator, whereupon dark brown crystals were formed.
The substance thus formed was soluble in methanol and insol-
uble in diethyl ether and water. The structure of complex, denoted
as CoL, was confirmed by IR spectroscopy and X-ray diffraction.
Anal. Calc. for C24H32Cl2Co1.5N4O15: C, 37.12; N, 7.22; H, 4.12;
Co, 11.4; Found: C, 36.75; N, 6.98; H, 4.04; Co, 11.7. IR (KBr,
cmꢀ1): 3376, 2967, 2929, 2862, 1647, 1597, 1533, 1515, 1438,
1383, 1354, 1315, 1287, 1228, 1197, 1135.
2.2.1. X-ray crystal structure determination
A
red prismatic crystal (0.1 ꢁ 0.1 ꢁ 0.3 mm3) of CoL
(C24H27N4O12.5Cl2Co1.5ꢂ2.5H2O, M = 775.83) is trigonal, space group
P3121, at T = 100 K: a = 15.7626(9) Å, c = 24.8669(14) Å, V =
5350.7(5) Å3, Z = 6, dcalc = 1.445 g/cm3, F(000) = 2391,
l =
0.923 mmꢀ1. A total of 51478 reflections (7047 unique reflections,
Rint = 0.065) were measured on a Bruker SMART APEX II CCD dif-
fractometer (k(Mo Ka)-radiation, graphite monochromator, x
and scan mode, 2hmax = 52°) and corrected for absorption using
u
the SADABS program [17]. The structure was determined by direct
methods and refined by a full-matrix least squares technique on F2
with anisotropic displacement parameters for non-hydrogen
atoms. The absolute structure of CoL was objectively determined
by the refinement of Flack parameter, which has become equal
to 0.03(3). The hydrogen atoms of the water molecules were local-
ized in the difference-Fourier map and included in the refinement
with fixed positional and isotropic displacement parameters. The
other hydrogen atoms were placed in the calculated positions
and refined within a riding model with fixed isotropic displace-
ment parameters (Uiso(H) = 1.5Ueq(C) for the CH3-groups and
Uiso(H) = 1.2Ueq(C) for the other groups). The final divergence
factors were R1 = 0.080 for 5218 independent reflections with
2.2. Physical measurements
The elemental analysis (carbon, hydrogen and nitrogen) of the
complex was obtained by a Thermo Flash EA 1112 series analyzer.
XPS (X-ray photoelectron spectroscopy) data were recorded with
the Leyboldt LH 10/100, using Magnesium
Ka excitation
(1253.6 eV). The residual pressure in the spectrometer during the
runs was about 10ꢀ8 Pa. The sample was vacuumized at liquid
nitrogen temperature. The spectrometer energy scale was cali-
brated using Au 4f7/2 photoelectron line at 84.0 eV, the correction
for surface charging was made by setting the C 1s signal of aro-
matic and aliphatic C to 284.5 eV. The metal content was measured
using thermogravimetric analysis (TA Instruments, SDT Q600) in
air flow by complex decomposition to constant mass, which was
I > 2r(I) and wR2 = 0.206 for all independent reflections, S = 1.007.
All calculations were carried out using the SHELXTL program [18].
2.3. Cyclohexane oxidation
Cyclohexane (CyH) oxidation was carried out in a 100 ml stain-
less steel batch reactor equipped with a magnetic stirrer. In a typ-
ical experiment 0.36 mol of cyclohexane, 0.09 mol of toluene as
internal standard and 6.5 ꢁ 10ꢀ6 mol of catalyst were added to
the autoclave. The system was pressurized with air and then
heated under permanent stirring. The operating temperature, mea-
sured by a thermocouple inside the reactor, was 140 °C. The oper-
ating pressure after heating was maintained at 25 atm. The main
products, such as cyclohexanol (CyOH) and cyclohexanone (CyO),
were analyzed by gas–liquid chromatography, cyclohexyl hydro-
peroxide (CyOOH) was detected by iodometric titration with
Cl
Cl
O
OH
NO2
H2N
+
NO2
+ Na
O
N
O
OH
Na
OH
O
Scheme 2.

L.I. Rodionova et al. / Inorganica Chimica Acta 392 (2012) 221–228
223
sodium thiosulfate, acids were analyzed by titration with NaOH, as
describe in literature [19].
in Figs. 1 and 2 along with the atomic numbering scheme, and se-
lected bond lengths and angles are listed in Table 1. The cationic
[Co(H2O)5]2+ fragment of CoL is disordered over two sites relative
to the twofold axis. The crystal of CoL contains four water solvate
molecules per complex. Three water solvate molecules are statisti-
cally disordered with the total positional occupancies of 0.5, one of
which is also disordered over two sites in the ratio of 0.3:0.2.
The crystal structure of CoL is very similar to the related
[Co(H2O)6][Co(C9H7NO3)2]2ꢂ2H2O (CoL0) [33] and [Co(H2O)6]
[Co(C9H7NO3)2]2ꢂH2O (CoL00) [34]. The main difference between
these structures is that CoL is one whole complex, whereas CoL0
and CoL00 are salts with separated cationic and anionic parts.
The Co(III) cobalt atom is six coordinate with a slightly distorted
octahedron. The six coordination sites are occupied by four oxygen
and two nitrogen atoms from two Schiff-base (salicylideneglycine)
ligands. The Co1 cobalt atom lies within the O1–O3–O6–O8 basal
plane and the N1 and N3 nitrogen atoms occupy the axial positions
forming a N1–Co1–N3 angle of 170.9(2)°. Comparing the N–Co–N
angles in CoL, CoL0 (173.6(3)°) and CoL00 (173.5(1)°), compound
CoL is more distorted than the compounds CoL0 and CoL00. The sal-
icylideneglycine ligands are not planar. The six-membered metal-
locycles adopt a sofa conformation with the deviation of the Co1
atom from the plane through the other atoms of the cycles
(0.817 and 0.656 Å, respectively), and the five-membered metallo-
cycles adopt an envelope conformation with the deviation of the
nitrogen atom from the plane through the other atoms of the cy-
cles (0.310 and 0.339 Å, respectively). Thus, the each ligand can
be described by two planes which are defined by their 5- and 6-
membered rings. The intersection angles of these planes with each
other (42.9° and 37.6°, respectively) indicate the buckled nature of
the ligands.
To determine the possibility to reuse of the CoL complex in
cyclohexane oxidation, CoL was precipitated from products with
water after the catalytic experiment, washed with water and ace-
tone and tested in the catalytic oxidation of cyclohexane in a sec-
ond cycle. The sample thus formed was denoted as re-used CoL.
3. Results and discussion
To synthesize new cobalt complex we lead the way proposed by
Casella and Gullotti: we use the possibility of the template conden-
sation of the aldehydes with amine in presence of metal salt [16].
The template rout toward the cobalt complexes with chiral azo-
methines based on L-valine amino acid is typical method [20–
22]. The over used procedure for the preparation of the complexes
with Schiff bases is reaction of the metal salt with the formerly
prepared ligand, but this way is less popular and to the best of
our knowledge this procedure never used for preparation of chiral
compounds [23–30] probably due to racemization [31,32]. The 5-
chloro-2-hydroxy-3-nitrobenzaldehyde reacts easily with L-valine
and cobalt salt in anhydrous methanol at room temperature to
form the binuclear cobalt complex CoL (Scheme 1) as a product
of the template condensation.
For understanding the electrochemical behavior of the complex
and for separation of the possible electrochemical activity of the li-
gand itself from the potentials of the metal ion-concentrated redox
processes we need to compare the electrochemical behavior of the
complex thus formed with its ligand. For this purpose the sodium
salt of the corresponding Schiff base was synthesized according to
Scheme 2 and was denoted as NaL.
The CoL compound possesses two asymmetric centers at the C2
and C14 carbon atoms with the absolute configurations of 2S,14S.
The Co(II) cobalt atom is octahedrally coordinated to one car-
bonyl and five hydrate oxygen atoms. The cationic [Co(H2O)5]2+
fragment of the compound CoL is disordered over two sites relative
3.1. X-ray diffraction study
Single crystals for X-ray diffraction were obtained from a
CH3OH solution. A representation of the X-ray structure is shown
Fig. 1. X-ray structure of CoL. The disordered [OCo(H2O)5] fragment is depicted by dashed lines. The solvate water molecules are not shown.

224
L.I. Rodionova et al. / Inorganica Chimica Acta 392 (2012) 221–228
Fig. 2. A part of the crystal structure of CoL showing the disorder of the [OCo(H2O)5] fragment relative to twofold axis.
Table 2
Table 1
Hydrogen bonds for CoL.
Selected bond lengths (Å) and angles (°) for CoL.
D–Hꢂ ꢂ ꢂA
d(D–H) (Å) d(Hꢂ ꢂ ꢂA) (Å) d(Dꢂ ꢂ ꢂA) (Å) \ (DHA) (°)
Co1–O1
Co1–O3
Co1–O6
Co1–O8
Co1–N1
Co1–N3
1.906(4)
1.892(3)
1.895(4)
1.902(3)
1.896(5)
1.879(5)
176.6(2)
89.7(2)
89.8(2)
91.2(2)
89.4(1)
177.7(2)
84.1(2)
92.7(2)
88.0(2)
94.3(2)
90.8(2)
92.6(2)
84.5(2)
93.3(2)
170.9(2)
Co2–O2
Co2–O11
Co2–O12
Co2–O13
Co2–O14
Co2–O15
2.016(5)
2.201(4)
1.949(9)
2.078(9)
2.045(8)
2.098(8)
169.8(2)
90.5(3)
87.8(3)
76.4(3)
99.6(3)
86.7(2)
95.7(2)
93.8(3)
90.0(3)
175.3(3)
91.9(3)
85.3(3)
91.9(3)
90.7(3)
175.1(4)
O11–H11Aꢂ ꢂ ꢂO7a
O12–H12Bꢂ ꢂ ꢂO17
O12–H12Cꢂ ꢂ ꢂO6a
O13–H13Aꢂ ꢂ ꢂO1
O14–H14Aꢂ ꢂ ꢂO19b
O14–H14Bꢂ ꢂ ꢂO14a
O15–H15Bꢂ ꢂ ꢂO17
O16–H16Dꢂ ꢂ ꢂO8c
O16–H16Dꢂ ꢂ ꢂO9c
O16–H16Eꢂ ꢂ ꢂO3c
O16–H16Eꢂ ꢂ ꢂO4c
O17–H17Dꢂ ꢂ ꢂO16
O17–H17Eꢂ ꢂ ꢂO18
O18–H18Aꢂ ꢂ ꢂO1d
O18–H18Bꢂ ꢂ ꢂO19c
0.95
0.90
0.90
0.90
0.90
0.90
0.90
0.93
0.93
0.93
0.93
0.91
0.91
0.91
0.91
0.91
0.92
0.92
2.13
1.92
1.85
2.04
1.94
1.55
2.13
2.37
2.10
2.13
2.33
1.92
2.02
2.23
2.12
2.50
2.03
2.06
2.927(8)
141
155
161
134
147
146
115
131
155
150
123
143
154
151
133
122
158
139
2.765(13)
2.718(10)
2.749(10)
2.733(10)
2.344(10)
2.639(11)
3.058(6)
2.973(6)
2.973(6)
2.949(6)
2.703(11)
2.868(13)
3.060(10)
2.822(13)
3.078(13)
2.902(13)
2.822(13)
O1–Co1–O3
O1–Co1–O6
O1–Co1–O8
O3–Co1–O6
O3–Co1–O8
O6–Co1–O8
O1–Co1–N1
O3–Co1–N1
O6–Co1–N1
O8–Co1–N1
O1–Co1–N3
O3–Co1–N3
O6–Co1–N3
O8–Co1–N3
N1–Co1–N3
O2–Co2–O11
O2–Co2–O12
O2–Co2–O13
O2–Co2–O14
O2–Co2–O15
O11–Co2–O12
O11–Co2–O13
O11–Co2–O14
O11–Co2–O15
O12–Co2–O13
O12–Co2–O14
O12–Co2–O15
O13–Co2–O14
O13–Co2–O15
O14–Co2–O15
c
O18–H18Bꢂ ꢂ ꢂO190
O19–H19Aꢂ ꢂ ꢂO7e
O19–H19Bꢂ ꢂ ꢂO18f
Symmetry transformations used to generate equivalent atoms: (a) x ꢀ y, ꢀy, ꢀz + 2/
3; (b) x ꢀ 1, y ꢀ 1, z; (c) ꢀy + 1, x ꢀ y, z + 1/3; (d) y, x, ꢀz + 1; (e) ꢀy + 1, x ꢀ y + 1,
z + 1/3; (f) ꢀx + y + 1, ꢀx + 1, z ꢀ 1/3
to the twofold axis. Three water solvate molecules are statistically
disordered with the total positional occupancies of 0.5, one of
which is also disordered over two sites in the ratio of 0.3:0.2. The
O11 oxygen atom occupies a special position on the twofold axis
(Fig. 2). The Co2–O bond distances differ significantly and are in
the range of 1.949(9)–2.201(4) Å.
In the crystal of CoL there are numerous hydrogen bonding
interactions with participation of water molecules. Details of the
hydrogen bonds are listed in Table 2.
3.2. IR-spectroscopy
The IR spectra of NaL and CoL complex are presented in Fig 3a
and b, respectively. Both spectra show a strong band at 1648 cmꢀ1
,
which is typical for Schiff-base ligands [35–37]. The bands in the
CoL spectrum at 1533 and 1380 cmꢀ1 are assigned to asymmetrical
and symmetrical stretching C@O vibrations of carboxyl groups, and
the great difference between them confirms the strong covalent
binding between carboxyl groups and the cobalt ion [38], while
in the NaL spectrum this difference is smaller, which is in good
agreement with literature data [39]. The broad absorption band
at 3376 cmꢀ1 in the CoL spectrum can be attributed to free hydro-
xyl groups as well as to water coordinated to the cobalt ion [37,40].
The phenol ring skeleton vibrations appeared as shoulder at
1597 cmꢀ1 and phenolic C–C and C–O vibrations are overlapped
Fig. 3. IR spectra of NaL (a) and fresh CoL (b).

L.I. Rodionova et al. / Inorganica Chimica Acta 392 (2012) 221–228
225
Table 3
XPS peaks of fresh CoL and of re-used CoL.
XPS peak
Co 2p3/2
780.9
Co 2p1/2
796.2
C 1s
O 1s
N 1s
Cl 2p
199.6
Binding energy (eV)
Fresh CoL
284.4
287.1
284.3
287.3
531.4
533.5
531.3
533.3
398.9
404.9
399.0
404.9
Re-used CoL
780.7
796.5
199.8
to each other and cause a band at 1228 cmꢀ1 [35,41], while in the
spectrum of NaL very high intensity of the band at 1221 cmꢀ1, re-
lated to phenolic C–O vibrations, is due to the absence of bonding
between phenolic OH group and cobalt ion. Two intense bands at
1515 and 1287 cmꢀ1 for CoL and at 1510 and 1280 cmꢀ1 for NaL
are assigned to asymmetrical and symmetrical stretching N@O
vibrations of nitro groups in the phenol ring [42]. The presence
of amino acid in the sodium salt of ligand and the complex is con-
firmed by triple bands in the range 2840–2980 cmꢀ1 due to the
methyl C–H stretch [43].
Thus, composition and structure of the CoL complex, first syn-
thesized in this work, were confirmed by XRD, elemental analysis
and IR spectroscopy. It was found, that CoL can be represented as
binuclear cobalt complex with one cobalt bonded to a Schiff base
ligand and L-valine, and the other cobalt bonded to five water mol-
ecules and the oxygen atom in the carboxyl group of the amino
acid.
Fig. 5. XPS peaks in the O 1s and C 1s binding energy region for fresh CoL (a) and re-
used CoL (b).
3.3. XPS
second cobalt ion [46]. A Cl 2p signal can be attributed to the sub-
stituent at the benzene ring [47] (Fig. 4).
To determine the oxidation state of Co, the binding-energy dif-
ference between the 2p3/2 and 2p1/2 Co signals and the intensity of
the satellite of the Co 2p signal is more reliable than the binding
XPS measurements were carried out to determine the cobalt
oxidation state in the surface region of the crystals and to compare
fresh and re-used CoL.
The XPS results for the fresh CoL complex are given in Table 3
and Figs. 4 and 5. The N 1s spectrum of the complex has two com-
ponents with an intensity ratio of 1:1 (Fig. 4). According to the lit-
erature [44,45], the first band at 398.9 eV can be attributed to
azomethine nitrogen coordinated with the metal ion. The nitrogen
in the NO2 group, being more positive, has the higher binding en-
ergy (404.9 eV).
Two C 1s signals at 287.1 and 284.5 eV with an intensity ratio of
1:3 (Fig. 5) can be assigned to carbon connected with oxygen and
nitrogen and carbon in phenol ring and aliphatic groups [45].
The oxygen atoms in CoL cause by a broad peak at 532 eV
(FWHM – ca. 2.7 eV), which indicates the presence of different
types of oxygen. Deconvolution of this broad peak leads to appear-
ance of two bands at 533.5 and 531.5 eV (Fig. 5). The contribution
at 531.5 eV can be attributed to oxygen in the nitro group, phenolic
and carboxylic oxygen, while the band at 533.5 may arise from the
C@O group coordinated with second cobalt ion, the positive shift
contrary to literature data being due to the coordination with the
energy [48]. In CoL
D
(2p3/2 ꢀ 2p1/2) (15.23 eV) is close to that ob-
tained for Co(acac)3 [49] (Fig. 4). Also, the satellite peaks at the
Co 2p signals are very small [50,51]. These results quite clearly
demonstrate the predominance of Co 3+ in the near-surface region.
In re-used CoL, the O 1s and C 1s signals are quite similar to the
signals observed for fresh CoL (Fig. 5), while the intensity ratio of
the N 1s signals has changed to 2.5:1 for amino and nitro group,
which testifies a partial substitution of nitro groups in phenol ring
(Fig. 5). This is possible with simultaneous partial substitution of
chloride group also observed in XPS spectra (Fig. 4).
The
D
(2p3/2 ꢀ 2p1/2) binding energy difference for Co 2p in-
creased from 15.23 eV (fresh CoL) to 15.85 eV (re-used CoL), indi-
cating that cobalt was largely transformed from Co3+ to Co2+. The
broad satellite peaks observed for CoL after catalysis (Fig. 4), con-
firm this suggestion [50,51].
Thus, it was found, that the during catalytic experiment cobalt
atoms in CoL complex partially changed the oxidation state from
Fig. 4. XPS peaks (Co 2p, O 1s, N 1s and Cl 2p region) for fresh CoL (a) and re-used CoL (b).

226
L.I. Rodionova et al. / Inorganica Chimica Acta 392 (2012) 221–228
Epc6 = ꢀ1.85 V. The first two-electron irreversible reduction peak
presumably corresponds to the reduction of two non-connected
Co3+ ions to Co2+ [53,54], while irreversible one-electron reduction
peak at ꢀ1.85 V can be attributed to the reduction of Co2+ to Co1+
[55,56]. The irreversible reduction of Co3+ to Co2+ proceeds by
the mechanism ECE. Furthermore, the first ligand based reduction
peak displaces in more positive direction in the CV of CoL because
of the less ionic character of the bond between cobalt and the li-
gand in the complex than in the sodium salt of the ligand.
In the anodic region of CV of CoL only two oxidation peaks are
present, the complex is oxidized at more positive potentials com-
pared to the corresponding sodium salt of the ligand. An additional
oxidation peak at Epa1 = 1.00 V in CV of NaL can be attributed to the
Kolbe reaction [57], which is impossible in CoL.
3.5. Cyclohexane oxidation with air
The CoL complex was tested in the partial oxidation of cyclo-
hexane with air to produce a mixture of CyOH and CyO. In addition
to alcohol and ketone, CyOOH was formed, which is the evidence of
the radical chain mechanism of CyH oxidation in the presence of
the cobalt complex. Berezin and Denisov demonstrated, that such
cobalt-salt based catalysts have three functions in alkane oxidation
processes: an initiating function, an abscopal function and a prod-
ucts regulation function [58]. The initiation function is explained
by reaction of Co3+ with alkane under formation of free radicals
and Co2+
.
As reported on Fig. 7, cyclohexane oxidation without catalyst
has a considerable induction period, corresponding to the initiation
step shown in Eqs. (1)–(3).
RH þ Inꢀ ! Rꢀ þ In
ð1Þ
ð2Þ
ð3Þ
Rꢀ þ O2 ! ROꢀ
2
ROꢀ2 þ RH ! ROOH þ Rꢀ
The end of the induction period corresponds to the accumula-
tion of a significant amount of cyclohexyl hydroperoxide, which
decomposes under formation of cyclohexanol and cyclohexanone
(Fig. 7). The initiating function of CoL is clearly demonstrated by
a considerable decrease of the induction period: CyH conversion
reaches the maximum conversion (4.5%, limited by the amount
of oxygen available) after 50 min, as compared with 240 min in
the non-catalytic experiment.
Fig. 6. Cyclic voltammograms of NaL (a); CoL (b) in DMF solution containing 0.1 M
TBP. Scan rates = 0.200 V sꢀ1
.
Co3+ to Co2+ because of a transformation of the Schiff base ligand
with substitution of the nitro and chloride groups in the benzene
ring.
3.4. Electrochemical studies
The product selectivity as a function of reaction time is pre-
sented on Fig. 8. The amount of CyOOH decreases with reaction
time due to the decomposition under formation of CyOH and
CyO, and after 60 min the content of all products reaches a con-
stant level. Cyclohexanol and cyclohexanone undergo further oxi-
dation, forming a number of acids (butyric acid, adipic acid,
hexanoic acid), which are unfavorable products. After 4 h the total
selectivity for main products reaches 85%, with selectivity to cyclo-
hexanol and cyclohexanone of 53% and 32%, respectively (Fig. 9).
For comparison, the total selectivity for non-catalytic oxidation is
only 74%, with 30% for cyclohexanol and 43% for cyclohexanone.
Change of the CyOH/CyO ratio for catalytic and non-catalytic oxi-
dation (1.65 and 0.68, respectively) is the confirmation of the prod-
uct regulation function of CoL. In non-catalytic process there is also
a large amount of unreacted cyclohexyl hydroperoxide in the
The electrochemical investigations of CoL and NaL were carried
out to confirm the oxidation state of cobalt in the complex; the so-
dium salt of the ligand was used as a reference to determine the
region of cobalt oxidation and reduction processes. Cyclic voltam-
mograms (CV) obtained are shown in Fig. 6 and the electrochemi-
cal oxidation and reduction potentials are given in Table 4.
The sodium salt of ligand is electrochemically inactive in the
range from ꢀ1 to +1 V. In the cathodic region of NaL CV, two irre-
versible peak are shown at Epc2 = ꢀ1.48 V and Epc3 = ꢀ1.86 V.
Moreover, a low-intensity peak at Epc1 = ꢀ1.11 V indicates the
adsorption of the initial complex on the electrode surface [52].
In addition to the above ligand-based processes, the CoL com-
plex shows two irreversible reductions at Epc5 = ꢀ0.64 V and
Table 4
Electrochemical data of NaL and fresh CoLa.
Compound
Epc1
Epc2
Epc3
Epc4
Epc5
Epc6
Epa1
Epa2
Epa3
NaL
CoL
ꢀ1.11
ꢀ1.48
ꢀ1.36
ꢀ1.86
ꢀ2.05
–
–
–
1.00
–
1.16
1.23
1.43
1.52
–
ꢀ2.36
ꢀ0.64
ꢀ1.85
a
Potentials are vs. Ag/AgCl in 0.1 M, samples concentrations are 10ꢀ3 mol/l.

L.I. Rodionova et al. / Inorganica Chimica Acta 392 (2012) 221–228
227
Fig. 7. CyH conversion and CyOOH content without catalyst, with fresh CoL and with re-used CoL. Cyclohexane (0.36 mol), toluene as internal standard (0.09 mol), catalyst
(6.5 ꢁ 10ꢀ6 mol), 140 °C, 25 atm of air, stirrer.
The oxidation of Co2+ to Co3+, which is not observed in the elec-
trochemical investigation, takes place according the Eq. (5). This
process is probably faster than the chemical reaction in an electro-
chemical cell.
The cyclohexane conversion on the re-used CoL develops very
similar as on the fresh complex (Fig. 7), but the distribution of
the main products differs. With re-used CoL the alcohol to ketone
ratio is smaller (Fig. 9). This is due to partial transformation of Co3+
in the fresh complex to Co2+ in spent one (as was shown in Sec-
tion 3.3), which leads to changes in the product regulation function
of catalysts. At the same time, cyclohexane conversion and selec-
tivities to the main products for CoL after catalysis are still higher,
than obtained without catalyst or with catalysts on the basis of co-
balt salts.
Since the CyH conversion and main products selectivities
strongly depend on the type of the oxidant it is advisable to com-
pare different catalysts using the same oxidant. Taking this into ac-
count, it is possible to compare the activity of CoL with Au/SBA-15
[60], which demonstrates selectivity of CyOH and CyO mixture
about 73% with the time to reach 4% CyH conversion level about
90 min. Thus, the CoL is more active and selective catalyst for
cyclohexane oxidation with air.
Fig. 8. Product selectivities on fresh CoL complex. Cyclohexane (0.36 mol), toluene
as internal standard (0.09 mol), catalyst (6.5 ꢁ 10ꢀ6 mol), 140 °C, 25 atm of air,
stirrer.
4. Conclusions
A new binuclear cobalt complex with a Schiff base ligand and an
amino acid was synthesized and characterized by a number of
physical measurements. Structure, composition, oxidative proper-
ties of complex were fully investigated and the presence of cobalt
in the oxidation state 3+ was shown in this complex.
Fresh and used complexes were tested in partial cyclohexane
oxidation with air and high activity and selectivity for cyclohexa-
nol and cyclohexanone were observed. A radical chain mechanism
of cyclohexane oxidation over the CoL complex was verified by
detection of cyclohexyl hydroperoxide as intermediate reaction
product. It was clearly demonstrated, that cobalt in oxidation state
3+ is more preferable for catalytic cyclohexane oxidation. Transfor-
mation of Co3+ to Co2+ leads to a slight decrease in catalyst activity
and changes in main product distribution.
Fig. 9. Distribution of main products without catalyst, with fresh CoL and with re-
used CoL. Cyclohexane (0.36 mol), toluene as internal standard (0.09 mol), catalyst
(6.5 ꢁ 10ꢀ6 mol), 140 °C, 25 atm of air, stirrer, reaction time 4 h.
Acknowledgments
L.I. Rodionova thanks the German Academic Exchange Service
(DAAD) for a Leonhard Euler Grant.
product mixture unlike in the CoL catalyzed reaction, which indi-
cates that CoL also catalyzes the decomposition of cyclohexyl
hydroperoxide by Haber–Weiss reactions (Eqs. (4) and (5)) [59].
Appendix A. Supplementary material
ROOH þ Co3þ ! ROOꢀ þ Co2þ þ Hþ
ROOH þ Co2þ ! ROꢀ þ Co3þ þ OHꢀ
ð4Þ
ð5Þ
CCDC 855626 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The

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L.I. Rodionova et al. / Inorganica Chimica Acta 392 (2012) 221–228
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