Synthesis and characterization of cobalt(III) complexes containing 2-pyridinecarboxamide ligands and their application in catalytic oxidation of ethylbenzene with dioxygen

Article abstract of DOI:10.1039/b301963b

Four novel cobalt(III) complexes were found to have high catalytic activities and excellent selectivities in the oxidation of ethylbenzene to acetophenone using O₂ as oxidant without need of solvent.

Full text of DOI:10.1039/b301963b

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Synthesis and characterization of cobalt(III) complexes containing
2-pyridinecarboxamide ligands and their application in catalytic
oxidation of ethylbenzene with dioxygen†
Jian-Ying Qi,ab Hong-Xia Ma,a Xian-Jun Li,*a Zhong-Yuan Zhou,b Michael C. K. Choi,b Albert S. C.
Chan*b and Qi-Yun Yangc
a Department of Chemistry, Sichuan University, Chengdu, China; Tel: 86 28 5412904
b Open Laboratory of Chirotechnology of the Institute of Molecular Technology for Drug Discovery and
Synthesis and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic
University, Hong Kong, China. E-mail: bcachan@polyu.edu.hk; Fax: 852 23649932; Tel: 852 27665607
c
Department of Chemistry, Changsha University of Electric Power, Hunan, China; Tel: 86 731 2617338
Received (in Cambridge, UK) 19th February 2003, Accepted 8th April 2003
First published as an Advance Article on the web 7th May 2003
Four novel cobalt(III) complexes were found to have high
catalytic activities and excellent selectivities in the oxidation
of ethylbenzene to acetophenone using O2 as oxidant without
need of solvent.
The longer UV adsorption wavelengths of complexes 5–8
(284–298 nm) than the corresponding free ligands (304–360
nm) were consistent with the presence of the larger conjugation
systems in the complexes. For the complexes, the absence of
nN–H peak in the IR spectra and the disappearance of amido
1
The selective oxidation of organic compounds utilizing dioxy-
gen as a primary oxidant represents an environmentally friendly
technology for industrial application.1 Much effort has been
made in the past to activate dioxygen by vast number of
biomimetic catalysts or metal complexes for the catalytic
oxidation of organic substrates under mild conditions.2–6
However, the advancement for their industrial application is
slow. For example, even though the halogenated porphyr-
inatoiron(III) complexes greatly increased the catalytic activity
in the selective oxidation of alkanes,7 the practical application
of these catalysts is severely limited by the thermal stability of
the porphyrins under oxidative conditions.
proton peak in the H NMR spectra indicated that the ligands
had lost the amido proton. Besides, free ligands 1–4 exhibited
nCNO at 1670–1684 cm21 while the nCNO of complexes 5–8 were
found to shift to 1620–1636 cm21. The lowering of vibration
frequency was due to the delocalization of the higher electron
density at the deprotonated amide nitrogen.16 The molecular
structures of complexes 6–8 were determined by single crystal
X-ray diffraction†‡ and the results were consistent with the
corresponding spectroscopic data. It was also found that, for
every ligand in the complexes, the pyridine ring and the
carbonyl group were on the same plane. The coordinations of
these ligands were clearly through the pyridine nitrogen and the
negative amido nitrogen ion instead of neutral amide nitrogen or
carbonyl oxygen. The deprotonated amido nitrogen was a good
s-donor,17 which facilitated the aerobic oxidation of Co(II) to
Co(III) during the formation of the complex. The three anionic
ligands neutralized the trivalent Co3+. An ORTEP drawing of
complex 6 is shown in Figure 2.
The complexes were found to be highly active and re-
markably selective in the oxidation of ethylbenzene to acet-
ophenone using dioxygen as the oxidant, a reaction of high
scientific and commercial interest. Typical results of our studies
are summarized in Table 1. When the reaction temperature was
increased from 120 to 150 °C, the conversion of ethylbenzene
increased rapidly from 49.8 to 70.4%. The main product was
acetophenone with its selectivity up to 90.2% at 150 °C.
Acetophenone productivity was 635 mol per mol catalyst·h, 27
times higher than that of the best system reported previously
using Co(OAc)2 combined with pyridine hydrobromide as
Recently there has been an increased interest in the
development of clean and economical processes for the
selective oxidation of ethylbenzene to higher-value added
product acetophenone.8–13 The current industrial production of
acetophenone is via the oxidation of ethylbenzene with
molecular oxygen using cobalt cycloalkanecarboxylate or
cobalt acetate as catalyst in acetic acid solvent.12 This method
suffers from its corrosive and environmentally unfriendly
nature. Herein, we report for the first time the synthesis of four
cobalt pyridinecarboxamide derivatives 5–8 and their remark-
able catalytic activities and excellent selectivities in the
catalytic dioxygen oxidation of ethylbenzene to acetophenone
without the need of any solvent and reducing reagent.
Ligands 1–4 were obtained via a general procedure14 from
2-pyridinecarboxylic acid to 2-pyridinecarboxyl chloride15
followed by the reaction with an appropriate aniline in THF.
The oxidation of Co(II) chloride with aerobic oxygen in the
presence of ligands 1–4 gave complexes 5–8.
Fig. 1 Synthetic route of cobalt(III) complexes.
† Electronic supplementary information (ESI) available: full experimental
and spectroscopic data for all compounds. See http://www.rsc.org/suppdata/
cc/b3/b301963b/
Fig. 2 Molecular structure of complex 6.
1294
CHEM. COMMUN., 2003, 1294–1295
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catalysts in acetic acid solvent.12 Using Co(OAc) as a catalyst
under the same conditions as those used in our study, the
conversion of ethylbenzene and selectivity of acetophenone
were only 40.4 and 80.2%, respectively. These results clearly
showed the advantages of the new Co(III) complexes 5–8. When
the reaction temperature was raised to 160 °C, extensive
carbonization occurred. For this reason we chose 150 °C as a
standard reaction temperature for the rest of our study. On
extending the reaction time from 2 to 10 h, both conversion and
selectivity of acetophenone increased initially, then the conver-
sion had no obvious change due to the inhibition of the reaction
by the products and the selectivity of acetophenone decreased
due to the over oxidation to benzoic acid after reaching 90.2%
in 4 h. The effect of product inhibition in the catalytic reaction
was demonstrated by mixing acetophenone with ethylbenzene
(molar ratio 7:3) as starting material and no ethylbenzene
conversion was observed under otherwise identical reaction
conditions.
unsubstituted complex 5. This may be due to the outer spatial
hindrance of octahedral coordination sphere making it some-
what more difficult for the active species to form. On the other
hand, the electronic effects of the substituents may increase
selectivity to acetophenone. For example, 93.2% selectivity for
acetophenone was achieved on 63% conversion using catalyst 6
with methoxyl group on the phenyl ring of ligand 2.
In summary, we have synthesized and characterized trivalent
cobalt complexes containing 2-pyridinecarboxamide ligands
and have found them to be highly effective in the catalytic
oxidation of ethylbenzene to acetophenone with O2 in the
absence of solvent and reducing reagent. This economical and
environmentally friendly system showed excellent potential for
industrial application.
We thank the Hong Kong Polytechnic University ASD Fund
and the University Grants Committee Areas of Excellence
Scheme (AOE P/10-01) for financial support of this study.
On increasing the O2 pressure from 0.8 to 1.8 MPa, the
conversion levels increased from 60.0 to 70.7%. Further
increase of O2 pressure did not show enhancement in conver-
sion. The effect of pressure on the selectivity for acetophenone
was not observed. On increasing the catalyst concentration from
0.5 to 2.0 mmol L21, the ethylbenzene conversion increased
from 43.8 to 70.4% with the increase of acetophenone from 83
to 90%. When the catalyst concentration was higher than 2.0
mmol L21, the over oxidation to benzoic acid became more
significant and the selectivity to acetophenone decreased. Our
further investigation of different substituents on the phenyl ring
of the ligands indicated that the conversions of ethylbenzene
decreased using complexes 6–8 as catalysts rather than the
Notes and references
‡ Crystal data. 6: CoC39H35N6O7, M = 785.66, triclinic, a = 10.0802(16),
b = 13.104(2), c = 14.813(2) Å, U = 1739.1(5) Å3, T = 294 K, space group
P1, Z = 2, m(Mo–Ka) = 0.555 mm21, 11760 reflections collected, 7609
¯
independent reflections (Rint = 0.0395). The final R indices [I > 2s(I)]: R1
= 0.0572, wR2 = 0.0924, R indices (all data): R1 = 0.1039, wR2 = 0.1007.
7: CoC40H39N6O5, M = 742.7, triclinic, a = 13.756(2), b = 15.878(3), c
= 19.388(3) Å, U = 3754.5(10) Å3, T = 294 K, space group P1, Z = 4,
¯
m(Mo–Ka) = 0.509 mm21, 13115 reflections collected, 13115 independent
reflections (Rint = 0.0000). The final R indices [I > 2s(I)]: R1 = 0.0601,
wR2 = 0.1433, R indices (all data): R1 = 0.1270, wR2 = 0.1724. Cr. 8:
C36H28Cl3CoN6O5, M = 789.92, monoclinic, a = 15.0628(12), b =
14.7653(12), c = 16.8999(14) Å, U = 3693.6(5) Å3, T = 294 K, space
group P2(1)/c, Z = 4, m(Mo–Ka) = 0.732 mm21, 8381 reflections
collected, 8381 independent reflections (Rint = 0.0000). The final R indices
[I > 2s(I)]: R1 = 0.0512, wR2 = 0.1306, R indices (all data): R1 = 0.0826,
wR2 = 0.1467. CCDC 189734–189736. See http://www.rsc.org/suppdata/
cc/b3/b301963b/ for crystallographic data in .cif or other electronic
format.
Table 1 Results of the oxidation of ethylbenzene to acetophenone
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Change of
reaction
Sel (%)c
I
Entry
conditiona
Convb%
II
9.3
III
IV
2.2
3.6
3.9
7.0
7.1
7.0
10.4
11.8
16.5
5.8
7.0
7.6
7.2
2.1
1.9
7.0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
120 °C
130 °C
140 °C
150 °C
2 h
4 h
6 h
8 h
49.8
59.0
63.8
70.4
66.5
70.4
70.5
70.8
71.9
60.0
70.4
70.7
70.7
43.8
45.8
70.4
70.3
70.4
70.4
63.3
64.7
64.2
88.0
88.4
88.5
90.2
87.7
90.2
87.7
86.1
81.8
91.4
90.2
90.0
90.2
83.0
85.8
90.2
87.5
87.1
90.2
93.2
91.1
83.6
0.5
0.5
0.6
0.7
0.6
0.7
0.4
0.4
0.5
0.5
0.7
0.4
0.4
2.6
1.8
0.7
0.3
0.3
0.7
0.6
0.4
0.6
7.5
6.7
1.6
4.4
1.6
1.2
1.5
0.7
2.3
1.6
1.7
2.0
12.2
10.5
1.6
1.3
1.6
1.6
3.2
1.6
2.3
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1625.
5 T. S. I. Murahashi, N. Komiya, Y. Oda, T. Kuwabara and T. Naota, J.
Org. Chem., 2000, 65, 9186.
10 h
0.8 MPa
1.6 MPa
1.8 MPa
2.0 MPa
0.5 mmol/L
1.0 mmol/L
2.0 mmol/L
6.0 mmol/L
8.0 mmol/L
Catalyst 5
Catalyst 6
Catalyst 7
Catalyst 8
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11 S. Evans and J. R. Lindsay Smith, J. Chem. Soc., Perkin Trans. 2, 2001,
2, 174.
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Inorg. Chem., 1996, 35, 6282.
10.6
10.8
7.0
3.1
6.6
12.9
a For entries 1–18, catalyst 5 was used. Entries 19–22 were the comparison
of the effectiveness of the four complexes 5–8. Standard reaction
conditions: ethylbenzene = 8 mol L21, catalyst concentration = 2 mmol
L21; O2 pressure = 1.6 MPa; reaction time = 4 h; temperature = 150 °C.
b Conversion of ethylbenzene. c Products were identified by GC (HP 5890
or 4890, column AT-1 30m 3 0.25 mm) using authentic samples for
comparison.
17 T. J. Collins, Acc. Chem. Res., 1994, 27, 279; A. K. Patra and R.
Mukherjee, Inorg. Chem., 1999, 38, 1388.
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