Tris-chelate complexes of cobalt(III) with N-[di(alkyl/aryl)carbamothioyl] benzamide derivatives: Synthesis, crystallography and catalytic activity in TBHP oxidation of alcohols

Article abstract of DOI:10.1016/j.molcata.2011.11.019

New six coordinated tris-chelate cobalt(III) complexes of the type [Co(L)₃] (1-4) {where HL = N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives}were prepared from the reaction between CoCl₂· 6H₂O and N-[di(alkyl/aryl)carbamothioyl]benzamide in ethanol and characterized by elemental analysis and spectral data (UV/Vis, IR, ¹H & ¹³C NMR). The molecular structure of a representative complex [Co(L1)₃] (1) [where HL1 = N-(diisopropylcarbamothioyl)benzamide], was determined by single crystal X-ray diffraction method and reveals a distorted octahedral geometry and a facial configuration of S atoms around the Co(III) center. These complexes act as efficient catalysts for the oxidation of alcohols to their corresponding aldehydes or ketones in presence of tert-butyl hydroperoxide (TBHP) at 80°C.

Full text of DOI:10.1016/j.molcata.2011.11.019

Journal of Molecular Catalysis A: Chemical 353–354 (2012) 156–162
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Tris-chelate complexes of cobalt(III) with N-[di(alkyl/aryl)carbamothioyl]
benzamide derivatives: Synthesis, crystallography and catalytic activity in TBHP
oxidation of alcohols
N. Gunasekarana, P. Jeromea, Seik Weng Ngb, Edward R.T. Tiekinkb, R. Karvembua,∗
a Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India
b Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 August 2011
Received in revised form 13 October 2011
Accepted 16 November 2011
Available online 25 November 2011
New six coordinated tris-chelate cobalt(III) complexes of the type [Co(L)3] (1–4) {where HL = N-
[di(alkyl/aryl)carbamothioyl]benzamide derivatives}were prepared from the reaction between
CoCl2·6H2O and N-[di(alkyl/aryl)carbamothioyl]benzamide in ethanol and characterized by elemental
analysis and spectral data (UV/Vis, IR, 1H & 13C NMR). The molecular structure of a representative complex
[Co(L1)3] (1) [where HL1 = N-(diisopropylcarbamothioyl)benzamide], was determined by single crystal
X-ray diffraction method and reveals a distorted octahedral geometry and a facial configuration of S
atoms around the Co(III) center. These complexes act as efficient catalysts for the oxidation of alcohols
to their corresponding aldehydes or ketones in presence of tert-butyl hydroperoxide (TBHP) at 80 ◦C.
© 2011 Elsevier B.V. All rights reserved.
This paper is dedicated to Prof. Christian
Bruneau on the occasion of his 60th
birthday.
Keywords:
Cobalt(III)
Thiourea derivatives
Catalytic oxidation
tert-Butyl hydroperoxide
1. Introduction
considered as intermediates in these catalytic processes [15–18].
Thus, the direct use of Co(III) may enhance the rate of reactions
Selective oxidation of alcohols to the corresponding carbonyl
compounds plays an important role in organic synthesis and in
the fine chemicals industry, often being a key step for the prepara-
tion of important synthons or directly affording fine chemicals and
valuable specialty products such as fragrances, drugs, vitamins and
hormones [1–3]. The stoichiometric amounts of hazardous inor-
ganic oxidants, notably Cr(VI)/Mn(IV), used in these conversions
generate copious amounts of waste [4]. In terms of economical ben-
efit and environmental impact, catalytic oxidation processes are
extremely valuable. In recent years, there has been a growing inter-
est in the search for new transition metal catalytic systems for the
oxidation of alcohols that use hydrogen peroxide [5,6], tert-butyl
hydroperoxide (TBHP) [7] or dioxygen [8–14] as the ultimate sto-
ichiometric oxidant, due to their obvious economic and ecological
advantages.
and hence be beneficial. In addition, the use of Co(III) complexes
stabilized by suitable ligand environments leads to a greater cat-
alyst stability owing to the relative substitutional inertness of
Co(III) [19]. With this in mind, several Co(III) catalytic systems
including [Co(acac)3] [20,21], Co(III)-oxo cubanes [22,23] and o-
phenylenebis(Nꢀ-methyloxamidate) cobalt(III) complexes [24,25]
have been developed.
N-[Di(alkyl/aryl)carbamothioyl]benzamide derivatives are ver-
satile class of ligands which can form stable complexes
with
a large number of transition metals. In general, N-
[di(alkyl/aryl)carbamothioyl]benzamide derivatives are known to
coordinate in different ways [26]. The enormous increase in the
attention paid to these complexes has been mainly driven by their
applications in precious metal separation and extraction [26,27],
single source precursors for nanomaterials [28], biological activity
[29–33], etc. Though these complexes have been used in various
fields, their application in catalysis is comparitively unexplored.
We have utilized N-[di(alkyl/aryl)carbamothioyl]benzamide com-
plexes of Ru(II) [34], Ru(III) [35] and Cu(I) [36] as catalysts for the
oxidation of alcohols to the corresponding carbonyl compounds
in the presence of N-methylmorpholine-N-oxide (NMO)/hydrogen
peroxide as oxidant. All these catalytic reactions are effective
Cobalt(II) salts and complexes have been used as homoge-
neous catalysts in the oxidation of alcohols to the corresponding
carbonyl compounds. Cobalt(III)-peroxo/alkylperoxo species are
∗
Corresponding author. Tel.: +91 431 2503636; fax: +91 431 2500133.
E-mail address: kar@nitt.edu (R. Karvembu).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.11.019

N. Gunasekaran et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 156–162
157
Fig. 1. Structures of ligands employed in this study.
in terms of the yield of products and proceeded under mild
reaction conditions. In continuation of our ongoing research
on the catalytic applications of transition metal complexes of
N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives, we report
herein the synthesis, the crystal structure and the catalytic activity
of their tris-chelate Co(III) complexes. While some Co(III) com-
plexes with N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives
are known [37–39], this report is the first describing their catalytic
applications. The structures of the ligands used in this work are
shown in Fig. 1.
(C S), 139.57, 139.08 (C O), 130.90, 129.12, 127.67 (aromatic),
51.95, 49.31 (CH), 20.92, 20.35, 19.23, 19.03 (CH3) ppm. FT-IR (KBr,
cm−1): ꢀ(N–H) absent; ꢀ(C---O) 1476; ꢀ(C---S) 1201. UV [Ethanol,
ꢁ in nm (log ε)]: 206 (4.58), 281 (4.68), 373 (3.83). Anal. Calcd. for
C42H57CoN6O3S3 (%): C, 59.40; H, 6.94; N, 9.89; S, 11.32. Found
(%): C, 59.35; H, 6.88; N, 9.82; S, 11.30.
—–
—–
[Co(L2)3] (2) was prepared from CoCl2·6H2O (100 mg,
0.42 mmol) and HL2 (296.4 mg, 1.26 mmol). Yield: 85%, decom-
position point: 175 ◦C, 1H NMR (acetone-d6): ı = 7.28–8.15 (m,
15H, aromatic), 3.83 (m, 12H, CH2), 1.20 (t, 9H, CH3), 0.82 (t, 9H,
CH3) ppm. 13C NMR (acetone-d6): ı = 174.64, 173.78 (C S), 137.98
(C O), 130.33, 128.39, 126.92 (aromatic), 50.11, 50.01 (CH2), 12.68,
2. Experimental
12.50 (CH3) ppm. FT-IR (KBr, cm−1): ꢀ(N–H) absent; ꢀ(C---O) 1486;
—–
—–
ꢀ(C---S) 1209. UV [Ethanol, ꢁ in nm (log ε)]: 206 (4.49), 280 (4.41),
2.1. Materials and reagents
371 (3.73). Anal. Calcd. for C36H45CoN6O3S3 (%): C, 56.52; H, 5.92;
N, 10.98; S, 12.57. Found (%): C, 56.42; H, 5.85; N, 10.89; S, 12.49.
[Co(L3)3] (3) was prepared from CoCl2·6H2O (100 mg,
0.42 mmol) and HL3 (368.4 mg, 1.26 mmol). Yield: 80%, decom-
position point: 225 ◦C, 1H NMR (acetone-d6): ı = 7.34–8.21 (m,
15H, aromatic), 3.86 (m, 12H, CH2), 1.70 (m, 12H, CH2), 1.37
(m, 12H, CH2), 0.92 (m, 18H, CH3) ppm. 13C NMR (acetone-d6):
ı = 175.31, 174.45 (C S), 138.65 (C O), 131.01, 129.24, 129.05,
127.58 (aromatic), 50.79, 50.69, 26.10, 19.99, 19.87 (CH2), 13.36,
All the chemicals were obtained from commercial sources
and used as received. The solvents were purified and dried in
accordance with the standard literature methods. The precursor
[CoCl2(PPh3)2] was prepared by the literature procedure [40] while
CoCl2·6H2O was purchased from Loba Chemie (India). The ligands
HL1, HL2, HL3 and HL4 were prepared from benzoyl chloride, potas-
sium thiocyanate and the corresponding secondary amine in dry
acetone [41].
13.18 (CH3) ppm. FT-IR (KBr, cm−1): ꢀ(N–H) absent; ꢀ(C---O) 1491;
—–
—–
ꢀ(C---S) 1208. UV [Ethanol, ꢁ in nm (log ε)]: 208 (4.83), 282 (4.92),
2.2. Physical measurements
376 (4.03). Anal. Calcd. for C48H69CoN6O3S3 (%): C, 61.77; H, 7.44;
N, 9.00; S, 10.30. Found (%): C, 61.70; H, 7.34; N, 8.95; S, 10.20.
[Co(L4)3] (4) was prepared from CoCl2·6H2O (100 mg,
0.42 mmol) and HL4 (454.2 mg, 1.26 mmol). Yield: 90%, decom-
position point: 105 ◦C, 1H NMR (acetone-d6): ı = 7.15–8.28 (m,
45H, aromatic), 5.21 (m, 12H, CH2) ppm. 13C NMR (acetone-d6):
ı = 177.24, 175.93 (C S), 138.65, 136.85, 136.09 (C O), 131.59,
129.35, 128.51, 127.91, 127.53, 127.37, 127.14 (aromatic), 52.70,
Microanalysis was carried out with a Vario EL AMX-400 elemen-
tal analyzer. Melting points were recorded with a Veego VMP-D
melting point apparatus and were uncorrected. FT-IR spectra were
recorded as KBr pellets with a PerkinElmer Spectrum RX1 FT-IR
spectrophotometer in the range 4000–400 cm−1. Electronic spec-
tra of the complexes were recorded in ethanol solutions using a
PG Instruments Ltd T90+ spectrophotometer in the 800–200 nm
range. Magnetic susceptibility measurements were made with a
Sherwood Scientific auto magnetic susceptibility balance. 1H and
13C NMR spectra were recorded in Bruker Avance 400 MHz instru-
ment in acetone-d6. TMS was used as an internal standard for 1H
and 13C NMR spectra. The capillary gas chromatography was per-
formed on a Shimadzu GC-2010 gas chromatograph with a RTX-5
column (60 m length, 0.32 mm inner diameter).
51.83 (CH2) ppm. FT-IR (KBr, cm−1): ꢀ(N–H) absent; ꢀ(C---O) 1494;
—–
—–
ꢀ(C---S) 1206. UV [Ethanol, ꢁ in nm (log ε)]: 206 (4.36), 284 (4.34),
375 (3.48). Anal. Calcd. for C66H57CoN6O3S3 (%): C, 69.69; H, 5.04;
N, 7.38; S, 8.45. Found (%):C, 69.60; H, 5.01; N, 7.33; S, 8.38.
2.4. X-ray structure determination
Green crystals of [Co(L1)3] (1) were grown at room temper-
ature from a ethanol and dichloromethane mixture (1:3) by the
diffusion of diethyl ether vapour, and a plate having dimensions
0.05 mm × 0.20 mm × 0.25 mm was selected for the study. Data
were collected at 100(2) K on an Agilent Technologies SuperNova
Dual diffractometer with an Atlas detector using Mo-K␣ radia-
tion so that Âmax = 27.5◦. The data set was reduced and corrected
for absorption effects using CrysAlis PRO [42]. The structure was
solved by direct-methods with SHELXS-97 [43] and refinement
(anisotropic displacement parameters, hydrogen atoms in the rid-
ing model approximation and a weighting scheme of the form
w = 1/[ꢂ2(Fo2) + (0.030P)2 + 0.841P] for P = (Fo2 + 2Fc2)/3) was on F2
by means of SHELXL-97 [43]. The crystallographic data and the
final refinement details are given in Table 1. Figs. 2 and 3 were
2.3. Preparation of Co(III) complexes
All the complexes were prepared using the following general
procedure. CoCl2·6H2O (100 mg, 0.42 mmol) in ethanol (8 mL) was
added dropwise to ligand (HL) (296.4–454.2 mg, 1.26 mmol) dis-
solved in ethanol (8 mL) in the presence of a few drops of Et3N with
a constant stirring for 30 min. The precipitate formed was filtered,
washed with a small amount cold ethanol and dried in vacuo.
[Co(L1)3] (1) was prepared from CoCl2·6H2O (100 mg,
0.42 mmol) and HL1 (330.5 mg, 1.26 mmol). Yield: 88%, decompo-
sition point: 180 ◦C, 1H NMR (acetone-d6): ı = 7.36–8.23 (m, 15H,
aromatic), 3.86 (br s, 6H, CH), 1.60 (d, J = 8 Hz, 18H, CH3), 1.29 (d,
J = 8 Hz, 18H, CH3) ppm. 13C NMR (acetone-d6): ı = 175.66, 173.97

158
N. Gunasekaran et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 156–162
Table 1
Crystallographic details for 1.
Formula
C42H57CoN6O3S3
Molecular weight
T/K
Crystal system
Space group
a (Å)
849.05
100(2)
Triclinic
¯
P1
11.0327(4)
13.7430(6)
16.0505(7)
100.662(4)
93.721(3)
112.628(4)
2182.52(16)
2
b (Å)
c (Å)
˛ (◦)
ˇ (◦)
Scheme 1. Synthesis of [Co(L1)3] (1).
ꢃ (◦)
V (Å3)
Z
drawn with ORTEP [44], at the 50% probability level, and DIAMOND
[45], arbitrary spheres, respectively. The data manipulation and the
interpretation were done with WinGX [46] and PLATON [47].
Dcalc (Mg m−3
)
1.292
No. reflections measured
No. unique reflections
No. observed reflections [I > 2␴(I)]
R1 [I > 2␴(I)]
18,005
9632
7173
0.048
2.5. Procedure for catalytic oxidation
wR2 (all data)
0.104
To a solution of alcohol (1 mmol) in acetonitrile solvent (10 mL),
tert-butyl hydroperoxide (287 ␮L, 3 mmol) and the complex 4
(7 mg, 0.01 mmol) were added. The solution was stirred at 80 ◦C
for 24–48 h. At the requisite time, aliquots of the reaction mixture
were removed and the alcohol and aldehyde/ketone were extracted
with n-hexane. The n-hexane extract was then analyzed by GC.
Authentic samples were used to identify the reaction products and
determine their percentage yields by comparison of retention times
and corresponding peak areas under identical experimental condi-
tions. The area normalization method was employed to calculate
the percentage yields of the formed products.
3. Results and discussion
The reaction between [CoCl2(PPh3)2] and HL1 in 1:2 stoichiom-
etry was carried out in ethanol in the presence of a few drops of
triethyl amine in order to prepare a complex with the formula
[Co(L1)2(PPh3)2] but the analytical, spectral and crystallographic
data indicated the formation of [Co(L1)3] (Scheme 1). Reactions
were also conducted changing solvents as well as the molar ratios
of ligands in order to obtain the desired cobalt phosphine com-
plex, but in all the cases only [Co(L1)3] was obtained. The complex
[Co(L1)3] was also obtained by the reaction of CoCl2·6H2O with
HL1 (molar ratio of CoCl2·6H2O:HL1 = 1:3) in ethanol in the pres-
ence of triethyl amine (Scheme 1). Subsequently, CoCl2·6H2O was
used as the cobalt precursor for the preparation of complexes with
HL2, HL3 and HL4 (Scheme 2). The oxidation of cobalt(II) by air
(dissolved in solution) occurred during the synthesis. The Co(III)
complexes were isolated as green or brown solids and these were
highly soluble in acetone, acetontrile, DMSO, DMF, partially sol-
uble in ethanol, methanol, chloroform and dichloromethane and
insoluble in n-hexane. All the complexes are air-stable and hence
are suitable for catalytic oxidation reactions. The analytical data
Fig. 2. Molecular structure of [Co(L1)3] (1) showing the atom numbering scheme
and displacement ellipsoids at the 50% probability level.
Fig. 3. View of the unit cell contents of [Co(L1)3] (1) shown in projection down
the a-axis. The dashed lines represent C–H. . .␲ interactions. Hydrogen atoms not
involved in C–H. . .␲ interactions have been omitted for reasons of clarity.
Scheme 2. Synthesis of Co(III) complexes.

N. Gunasekaran et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 156–162
159
Table 2
obtained are in excellent agreement with the proposed molecular
formulae.
Selected bond lengths (Å) and angles (◦) for 1.
Co–S(1)
Co–S(2)
Co–S(3)
2.2050(6)
2.2161(8)
2.2012(7)
1.744(3)
1.741(2)
1.740(2)
178.30(5)
176.44(6)
87.27(2)
85.59(7)
86.98(7)
Co–O(1)
Co–O(2)
Co–O(3)
1.9087(18)
1.9320(17)
1.9174(15)
1.262(3)
1.268(3)
1.263(3)
177.18(5)
87.47(3)
89.75(3)
84.86(7)
3.1. Spectroscopy
C(8)–S(1)
C(22)–S(2)
C(36)–S(3)
S(1)–Co–O(3)
S(3)–Co–O(2)
S(1)–Co–S(3)
O(1)–Co–O(2)
O(3)–Co–O(2)
C(7)–O(1)
C(21)–O(2)
C(35)–O(3)
S(2)–Co–O(1)
S(1)–Co–S(2)
S(2)–Co–S(3)
O(1)–Co–O(3)
A very strong absorption band was observed around 3200 cm−1
in the FT-IR spectra of the free ligands which is characteristic of
the N–H group [41]. This band disappeared in the spectra of 1–4
suggesting the deprotonation of the N–H group during the com-
plex formation. In the FT-IR spectra of free carbamothioyl ligands,
a strong band in the region 1652–1690 cm−1 and a medium inten-
sity band in the region 1243–1314 cm−1 are attributed to C O and
C
S groups, respectively. In the FT-IR spectra of the complexes,
each of which coordinates in the bidentate S,O mode. Within the
octahedral geometry defined by the O3S3 donor set, the S atoms
have a facial arrangement, and the three O atoms define an octa-
hedral face. The ranges of Co–S and Co–O bond distances, Table 2,
are each narrow. In terms of angles, the deviations from the ideal
octahedral geometry are small with the cis angles ranging from
84.86(7)◦ to 93.45(6)◦. The S–Co–S angles are systematically wider
than the O–Co–O angles as expected on size considerations, Table 2.
The three phenyl rings lie to the one side of the plane defined
by the three O atoms whereas the i-Pr groups lie to the other
side. This is consistent with the NMR study that indicated the
inequivalent of the N-bound substituents in these non-labile Co(III)
complexes. The crystal structure of the HL1 ligand is known [51]
and available for comparison with the anions in 1. There are two
independent molecules in this structure and the C = S bond dis-
tances of 1.6687(15) and 1.6689(15) Å in these are significantly
shorter than the C–S distances observed in (1), Table 2, consistent
with the spectroscopic study. A similar observation pertains to the
C = O bond distances of 1.2304(18) and 1.2343(18) Å. The overall
coordination geometry observed in 1 is consistent with those seen
in related structures reported in the literature [37,39,52].
these bands were observed in the lower frequency regions, i.e.
1476–1494 and 1201–1209 cm−1, respectively, consistent with the
reduced electron density in these bonds and therefore, suggesting
the coordination of the O and S atoms to cobalt [37].
The new cobalt complexes were diamagnetic (␮eff = 0), indicat-
ing the presence of cobalt in the 3+ oxidation state. The electronic
spectra in ethanol solution showed three bands/shoulders with
absorption maxima values in the ranges of 206–208, 280–284 and
371–376 nm. High energy bands appeared in the regions 206–208
and 280–284 nm have been assigned to ␲–␲* transitions and a
shoulder appeared in the region 371–376 nm have been attributed
to charge transfer transition (LMCT) [48]. The UV–visible spectral
data of complexes 1–4 are analogues to the results described earlier
for facial tris-chelate Co(III) complexes [49].
The 1H NMR spectra of complexes 1–4 in acetone-d6 solu-
tion showed a complex multiplet around 7.15–8.28 ppm, which
has been attributed to the aromatic protons present in the [N-
di(alkyl/aryl)carbamothioyl]benzamide ligand. The characteristic
N–H signal in the region 10.40–10.93 ppm in the ligands was not
present in the 1H NMR spectra of any of the complexes indicating
deprotonation of N–H proton of the ligands upon coordination [50].
In addition, complex 1 showed two doublets at 1.29 and 1.60 ppm
and a broad singlet at 3.68 ppm which have been assigned to methyl
and methine protons of the ligand, indicating the methyl groups are
in distinct magnetic environments. Complex 2 exhibited a multi-
plet around 3.83 ppm corresponding to the methylene protons and
two triplets at 0.82 and 1.20 ppm corresponding to methyl protons,
again indicating distinct environments for these groups. A multiplet
was appeared around 3.86 ppm in complex 3 due to the methy-
lene protons proximate to nitrogen. Complex 3 also exhibited two
multiplets at 1.70 ppm and 1.37 ppm which are attributed to the
remaining methylene protons of aliphatic carbon chain. A multi-
plet (merging of two triplets) around 0.92 ppm in complex 3 has
been assigned to the methyl protons. Complex 4 showed a mul-
tiplet around 5.21 ppm corresponding to the methylene protons.
13C NMR spectra of all the complexes (1–4) have been recorded
in acetone-d6 solution. The 13C NMR spectra of the complexes
showed the expected resonances. The signals observed in the range
126.92–131.59 ppm for all complexes have been assigned to the
aromatic carbons of N-[di(alkyl/aryl)carbamothioyl]benzamide lig-
and. The resonances due to the C S and C O carbon nuclei in the
Co(III) complexes have been shifted significantly with respect to the
free ligands, and appeared in the regions of 173.78–177.24 ppm and
136.09–139.57 ppm, respectively, indicating the coordination of
the ligands to cobalt through the S and O atoms [37]. All complexes
exhibited resonances due to aliphatic carbons in the expected
regions.
There are a number intramolecular C–H. . .S and C–H. . .N inter-
actions within the complex molecule of 1 which preclude the S1
and S2, and N2, N4 and N6 atoms from forming significant inter-
molecular contacts. The most notable intermolecular interactions
operating in the crystal structure of 1 are of the type C–H. . .␲
[53]. These involve each of the phenyl rings and lead to a three-
dimensional architecture, Fig. 3.
3.3. Catalytic oxidation
The new Co(III) complexes (1–4) have been used as cata-
lysts for the oxidation of 1-phenylethanol (Table 3, entries 1–4)
in acetonitrile solvent with TBHP at 80 ◦C. Though all the com-
plexes exhibited good catalytic activity, complex 4 was chosen
for optimizing the reaction conditions and extending the scope of
substrates since yield of acetophenone (93%) was slightly higher
(Table 3, entry 4). Generally, oxidation of alcohols catalyzed
by Ru(II) [34], Ru(III) [35] and Cu(I) [36] complexes containing
N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives and triph-
enylphosphine was completed at room temperature with in
12 h. But in the present catalytic system with Co(III) complexes
containing only N-[di(alkyl/aryl)carbamothioyl]benzamide deriva-
tives, the reaction proceeded only at elevated temperature and
requires 48 h; this may be due to the fact that tris-chelate Co(III)
complex provides a coordination site for the formation of possible
active species [Co(III)-peroxo] by dangling one of the coordinated
N-[di(alkyl/aryl)carbamothioyl]benzamide ligands which normally
requires high temperature and more time [54]. With complex 4 as
catalyst, catalytic oxidation of 1-phenylethanol (Fig. 4) was car-
ried out in different solvents (CH3CN, (CH3)2CO, CH2Cl2, CH3OH,
C6H6 and DMF). When CH3CN was used, a significant increase in
the yield of acetophenone was observed. The catalytic oxidation of
3.2. Crystal and molecular structure of 1
The molecular structure of a representative complex, namely
[Co(L1)3] (1), was established by X-ray crystallography and is illus-
trated in Fig. 2. The Co center is tris chelated by three L1 anions,

160
N. Gunasekaran et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 156–162
Table 3
Optimization of catalytic oxidationa
.
O
OH
[Co] (0.01 equiv.),
Oxidant (3 equiv.), Solvent
80 oC, 48 h
1 equiv.
Entry
Catalyst
Solvent
Oxidant
Yield (%)b
1
2
3
4
5
6
7
8
1
2
3
4
4
4
4
4
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
NMO
H2O2
H5IO6
t-BuOOH
91
92
92
93
No reaction
No reaction
50
93c
a
Reaction conditions: 1-phenylethanol (1 mmol), oxidant (3 mmol), catalyst (0.01 mmol),. solvent (10 mL), 80 ◦C, 48 h.
b
Yield is determined by GC with area normalization; GC conditions: RTX-5 column, 60 m × 0.32 mm, 240 ◦C; FID detector, 270 ◦C; injector, 220 ◦C; carrier gas, N2; rate:
1 mL/min.
c
Reaction conditions: 1-phenylethanol (1 mmol), oxidant (3 mmol), catalyst (0.01 mmol), solvent (10 mL), 80 ◦C, 48 h, under nitrogen atmosphere.
1-phenylethanol (Table 3, entries 4–7) was studied with
the different oxidants such as tert-butyl hydroperoxide, N-
methylmorpholine-N-oxide, hydrogen peroxide and periodic acid
with 4 as catalyst and CH3CN as solvent. The oxidation of 1-
phenylethanol did not proceed at all when NMO or H2O2 was used
as an oxidant. When H5IO6 was used as the oxidant, the yield of
acetophenone was found to be only 50%. Among these four oxi-
dants, TBHP was found to be the best as it gave acetophenone in
93% yield. Hence, CH3CN as solvent and TBHP as oxidant were used
while extending the scope of substrates. It is further noted that
0.01 mmol of the Co(III) complex and 3 mmol of oxidant were suffi-
cient for effective oxidation process. The control experiments were
carried out under identical conditions with CoCl2·6H2O or HL4 as
catalyst in the place of complex 4. No oxidation was observed in
these reactions. The oxidation of 1-phenylethanol (Table 3, entry
8) with complex 4 under nitrogen atmosphere also gave 93% of
acetophenone, which indicates that only tert-butyl hydroperoxide
acts as oxidant in these reactions and not air.
2-chlorophenylethanol. 1-Phenoxy-2-propanone was obtained
in good yield (92%) from 1-phenoxy-2-propanol (Table 4, entry
5) after 30 h. 1-Indanol gave 1-indanone in 80% yield after 48 h.
The oxidation of primary benzylic alcohols (Table 4, entries 6–8)
afforded the corresponding aldehydes in moderate yields. Beyond
36 h, the formed aldehydes were converted into corresponding
acids in the case of primary benzylic alcohols. Remarkably, the
conversion of cyclic alcohols (Table 4, entries 9–11) to cyclic
ketones proceeded efficiently in the present catalytic system.
Co(III)-catalyzed oxidation of 1-cyclohexylethanol (Table 4, entry
12) and 1-cyclopropylethanol (Table 4, entry 13) lead to the
formation of corresponding ketones in yields of 51% and 83%,
respectively. Though the present catalytic system requires a
slightly longer reaction time, the efficiency in terms of the yield
of products is comparable or higher than the existing Co(III)/TBHP
and Ru(II)/TBHP catalytic systems [7,22,23].
It is proposed hypothetically that the catalytic oxidation pro-
ceeds through tert-butylperoxocobalt(III) species which is formed
from solvated intermediate, as shown Scheme 3. The FT-IR spectra
of the residue obtained after stirring complex 4 and TBHP at 80 ◦C
for about 24 h showed a strong band at 3371 cm−1 and a medium
intensity band at 850 cm−1 which might be attributed to O–H and
Having established the optimal conditions with 1-
phenylethanol, the attention was switched to other substrates. The
results of the oxidation of various alcohols in CH3CN solvent using
4 as catalyst and TBHP as oxidant at 80 ◦C are shown in Table 4.
Benzylic secondary alcohols (Table 4, entries 1–4) can be readily
converted to the corresponding ketones in good yield except
OH
S
t-BuOOH
NCCH3
O
100
80
60
40
20
0
Co
H CN
3
S
S
C
CH3CN
O
S
OH
O
But
S
O
S
O
S
O
O
S
Co
O
Co
O
S
O
OH
R1 R2
H2O t-BuOH
R1 R2
O
S
CH CN
CH Cl
C H
6 6
(CH ) CO
CH OH
DMF
monoanion of N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives
3
=
2
2
2
3
3
Solvent
Scheme 3. Possible mechanistic pathway for cobalt(III)-catalyzed oxidation of alco-
Fig. 4. Effect of solvents upon oxidation of 1-phenylethanol by 4.
hols.

N. Gunasekaran et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 156–162
161
Table 4
Oxidation of alcoholsa by 4.
[Co] (0.01 equiv.),
t-BuOOH (3 equiv.), CH3CN
O
OH
80 0
C
R1
R2
R1
1 equiv.
Entry
R2
Substrate
Product
Time (h)
48
Yieldb (%)
93 (91)c
O
OH
1
2
OH
OH
O
O
42
36
84 (81)c
3
83
Cl
Cl
Cl
OH
OH
Cl
O
O
O
4
5
6
42
30
24
25
92
51
O
O
OH
OH
H
H
O
O
7
36
74
H3CO
O2N
H3CO
O2N
OH
OH
H
O
8
9
36
48
64
80
O
OH
OH
10
11
36
30
96
90
O
OH
OH
O
12
48
30
51
83
O
13
a
Reaction conditions: alcohol (1 mmol), t-BuOOH (3 mmol), complex 4 (0.01 mmol), acetonitrile (20 mL), 80 ◦C.
Yield (average of two trials) is determined by GC with area normalization.
Isolated yield is given in parenthesis.
b
c

162
N. Gunasekaran et al. / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 156–162
O–O groups respectively present in Co(III)-peroxo intermediate
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ing structure factors) for the structure described in this paper
have been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 838004. Copies of the data can be obtained
free of charge on application to The Director, CCDC, 12 Union
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i
[53] C(4)–H(4). . .Cg(C29–C34) = 2.91 A,
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˚
Å,
with
angle
at
H(4) = 146◦;
C(18)–H(18). . .Cg(C1–C6)ii = 2.85 A,
˚
C(18)–H(18). . .Cg(C1–C6)ii = 3.685(4) Å, with angle at H(18) = 148◦;
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C(28). . .Cg(C29–C34)iii = 3.643(3)
˚
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−1 + y, z.
˚
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