Synthesis and catalytic properties of trans-A2B2-type metalloporphyrins in cyclohexane oxidation

Article abstract of DOI:10.1139/cjc-2013-0400

A new efficient and mild protocol for synthesizing a series of trans-A ₂B₂-porphyrins through a TFA-catalyzed condensation reaction between various aldehydes and dipyrromethanes has been developed. Several trans-A₂B₂-metalloporphyrins (cobalt, nickel) were synthesized and used to catalyze cyclohexane oxidation C-H bonds with dioxygen in the absence of additives and solvents. The results show that the catalytic activities were relative to the nature of the substituted groups and the central metal ions of trans-A₂B₂-metalloporphyrin. Cobalt metalloporphyrin presents better catalytic performance in the conversion of cyclohexane than the nickel metalloporphyrin under the same reaction conditions.

Full text of DOI:10.1139/cjc-2013-0400

49
ARTICLE
Synthesis and catalytic properties of trans-A2B2-type
metalloporphyrins in cyclohexane oxidation
Yujia Xie, Fengyong Zhang, Pingle Liu, Fang Hao, and He’an Luo
Abstract: A new efficient and mild protocol for synthesizing a series of trans-A2B2-porphyrins through a TFA-catalyzed conden-
sation reaction between various aldehydes and dipyrromethanes has been developed. Several trans-A2B2-metalloporphyrins
(cobalt, nickel) were synthesized and used to catalyze cyclohexane oxidation C–H bonds with dioxygen in the absence of
additives and solvents. The results show that the catalytic activities were relative to the nature of the substituted groups and the
central metal ions of trans-A2B2-metalloporphyrin. Cobalt metalloporphyrin presents better catalytic performance in the con-
version of cyclohexane than the nickel metalloporphyrin under the same reaction conditions.
Key words: trans-A2B2-porphyrin, trans-A2B2-metalloporphyrin, cyclohexane oxidation, cyclohexanol, cyclohexanone.
Résumé : Nous avons établi un nouveau protocole efficace et doux pour la synthèse d’une série de porphyrines de type trans-A2B2
par une réaction de condensation de divers aldéhydes et dipyrrométhanes en présence de TFA comme catalyseur. Nous avons
synthétisé plusieurs métalloporphyrines (cobalt, nickel) de type trans-A2B2 dont nous nous sommes servi pour catalyser
l’oxydation des liaisons carbone−hydrogène du cyclohexane par du dioxygène en l’absence d’additifs et de solvants. Les résultats
montrent que l’activité catalytique dépend de la nature des groupes substitués et de l’ion métallique central de la métallopor-
phyrine de type trans-A2B2. La métalloporphyrine contenant du cobalt est un meilleur catalyseur de la conversion du cyclohex-
ane que celle contenant du nickel dans les mêmes conditions de réaction. [Traduit par la Rédaction]
Mots-clés : porphyrine trans-A2B2, métalloporphyrine trans-A2B2, oxydation du cyclohexane, cyclohexanol, cyclohexanone.
cyclohexane and have obtained some achievements by using met-
alloporphyrins as catalysts.5,10–11 However, the technology in
which cyclohexane was oxidized by air or dioxygen to produce
cyclohexanol and cyclohexanone has not been improved much up
to now.
Introduction
Functionalization of inert alkanes to more valuable products
(e.g., carboxylic acids, alcohols, ketones) has attracted much at-
tention.1 Generally speaking, the oxidation of alkane is problem-
atic because of the inactivity of C–H bonds of alkane chemically.
Although catalytic oxidation of C–H bond in saturated hydrocarbons
under mild conditions is a key step in the oxyfunctionalization of
organic compounds, much progress has been achieved in the se-
lective cleavage and functionalization of C−H bonds of alkanes.2–7
The cytochrome P-450 monooxygenase enzyme, which can cata-
lyze the oxidation of the inert C–H bonds under mild conditions in
organisms, is able to effectively and stereospecially catalyze the
hydroxylation and epoxidation of hydrocarbon in the metabolic
system. There has been significant interest in modeling of the
P-450 enzyme active sites and developing the biomimetic oxida-
tion catalysts. Therefore, progress has been made in the prepara-
tion of metalloporphyrin and subsequent application in catalytic
reaction of C−H bonds.
Oxidation of cyclohexane with air or dioxygen in the absence of
additives and solvents to ketone−alcohol (KA) oil (cyclohexanol−
cyclohexanone) or adipic acid is a very crucial industrial process
with regard to economic viability and from environmental points
of view.8 For instance, KA oil is used to make adipic acid, which is
required for the production of nylon-6.6, and cyclohexanone is
the building block in the large industrial scale manufacture of
␧-caprolactam, which is the intermediate for the production of
nylon-6. Currently, cyclohexane oxidation is carried out by using
the cobalt salt as the catalyst with a poor cyclohexane conversion
of 3%–6% and unfavorable selectivity of KA oil.9 People have made
much effort to discover better ways to increase the conversion of
In 1979, the first metalloporphyrin (iron tetraphenylporphyrin)
catalyzed oxidation was discovered by Groves12 in which cyclohex-
ane was oxidized to cyclohexanol by using iodosobenzene as the
oxidant. Subsequently, oxygenation of alkanes has been observed
with a number of metalloporphyrin systems. In general, the met-
alloporphyrin catalytic activity is related to its substituent group
and central metal ion (ionization potential and dioxygen-binding
ability). Research has shown that the adsorption of dioxygen is the
first step for oxygen reduction;13–14 then the interaction between
the catalyst and oxygen molecule and its effect of dissociation of
the O−O bond is important in understanding the reaction mech-
anism. These processes are considered to proceed by a mechanism
similar to the enzymatic systems. Hence, higher ionization poten-
tial and larger dioxygen-binding energy are associated with better
catalytic activity.
Most of the previous research only focused on the synthesis of
A4-type porphyrins and their catalytic activity studied. In this
work, trans-A2B2-metalloporphyrins (cobalt, nickel) were synthe-
sized and applied to the oxidation of cyclohexane inert hy-
drocarbon bonds. Several trans-A2B2-porphyrins and cobalt- and
nickel-trans-A2B2-porphyrins with different substituents on the
porphyrin rings were synthesized and are described in Scheme 1,
and their catalytic activity for cyclohexane oxidation with di-
oxygen, the relationship between the substituent structures and
the central metal ion of trans-A2B2-metalloporphyrins, and their
Received 3 September 2013. Accepted 5 November 2013.
Y. Xie, F. Zhang, P. Liu, F. Hao, and H. Luo. College of Chemical Engineering, Xiangtan University, Xiangtan 411105, China.
Corresponding author: Pingle Liu (e-mail: liupingle@xtu.edu.cn).
Can. J. Chem. 92: 49–53 (2014) dx.doi.org/10.1139/cjc-2013-0400
Published at www.nrcresearchpress.com/cjc on 7 November 2013.

50
Can. J. Chem. Vol. 92, 2014
Scheme 1. Synthesis of trans-A2B2-porphyrins and trans-A2B2-metalloporphyrins.
catalytic selectivity were studied. The structures of different trans-
A2B2-metalloporphyrin catalysts and the dioxygen oxidation of
cyclohexane to the KA oil catalyzed by these metalloporphyrins
are shown in Scheme 2.
Scheme 2. Cyclohexane oxidation catalyzed by the given catalysts
with dioxygen.
Experimental method
General
1H spectra were recorded on a Bruker Avance (400 MHz). Neutral
aluminum oxide (100–200 mm average particle size) and silica gel
(40 mm average particle size) were used for column chromatogra-
phy. Thin-layer chromatography was performed using SiliCycle
silica gel 60 F254 TLC plates and visualized with ultraviolet light.
All compounds were purchased from Alfa-Aesar and used without
further purification unless otherwise noted. Pyrrole was distilled
from CaH2 before use.
for 3 h. The reaction was terminated by addition of triethylamine
(1 mL). The mixture was passed through column chromatography
(alumina, CH2Cl2) and eluted with CH2Cl2 until the eluant was no
longer dark. The collected eluant was concentrated. The resulting
crude product was purified by column chromatography on silica
gel (CH2Cl2−hexanes = 3:1). Recrystallization (CHCl3−CH3OH) gave
a purplish black solid. The main products of trans-A2B2-porphyrin
were obtained in yields between 23% and 35%.
General procedure for the preparation of dipyrromethanes
Data for D(p-Cl)PP
Condensation of 5-(4-chlorophenyl)dipyrromethane (0.51 g,
2.0 mmol) and benzaldehyde (0.21 g, 2.0 mmol) in CH2Cl2 (200 mL)
with TFA (0.3 mL, 4.0 mmol) following the procedure described for
the general procedure gave a purple solid with a yield of 27%.
1H NMR (400 MHz, CDCl3) ␦: 8.83 (m, 8H, pyrole-H), 8.20 (m, 4H,
ArH), 8.16 – 8.05 (m, 4H, ArH), 7.74 (t, J = 7.8 Hz, 10H, ArH), −2.80 (s,
2H, NH). UV/Vis (CHCl3): 419, 515, 551, 591, 647 nm.
Data for 5-(4-bromophenyl)dipyrromethane (1c)
By modifying
a a mixture of
literature procedure,15–16
4-bromobenzaldehyde (3.7 g, 20 mmol) and pyrrole (60 mL, 0.87 mol)
was treated with InCl3 (0.2 g, 1.0 mmol) at room temperature
under argon. After stirring for 6 h, powdered NaOH (4.0 g, 0.1 mol)
was added and the reaction mixture was stirred for 45 min. The
mixture was filtered. Excess pyrrole was removed by distillation
under reduced pressure and the resulting yellow solid was treated
with hexanes. The solvent was removed under reduced pressure,
affording a precipitate. Recrystallization (CH3CH2OH−H2O) gave a
Data for D(p-Br)PP
Condensation of 5-(4-bromophenyl)dipyrromethane (0.60 g,
2.0 mmol) and benzaldehyde (0.21 g, 2.0 mmol) in CH2Cl2 (200 mL)
with TFA (0.3 mL, 4.0 mmol) following the procedure described for
the general procedure gave a purple solid with a yield of 25%.
1H NMR (400 MHz, CDCl3) ␦: 8.83 (m, 8H, pyrole-H), 8.20 (d, J =
5.8 Hz, 4H, ArH), 8.06 (d, J = 8.1, 4H, ArH), 7.88 (d, J = 8.1 Hz, 4H,
ArH), 7.75 (d, J = 6.2 Hz, 6H, ArH), −2.81 (s, 2H, NH). UV/Vis (CHCl3):
419, 516, 551, 592, 648 nm.
1
light-yellow solid (5.62 g, 93%). H NMR (400 MHz, CDCl3) ␦: 7.91
(br, 2H, NH), 7.42–7.44 (d, 2H, ArH), 7.07–7.09 (d, 2H, ArH), 6.70 (s,
2H, pyrrole-H), 6.15–6.16 (d, 2H, pyrole-H), 5.88 (s, 2H, pyrole-H),
5.43 (s, 1H, meso-H).
Data for phenyldipyrromethane (1a)
A sample of benzaldehyde (2.12 g. 20 mmol) was treated identi-
cally as for 5-(4-bromophenyl)dipyrromethane, affording a tan
solid (4.0 g, 90%). 1H NMR (CDC13) ␦: 7.89 (br, 2H, NH), 7.35–7.19 (m,
5H, ArH), 6.69 (q, 2H, pyrole-H), 6.15 (q, 2H, pyrole-H), 5.91 (m. 2H,
pyrole-H), 5.47 (s, 1H, meso-H).
Data for D(p-Cl)PD(p-Br)P
Condensation of 5-(4-chlorophenyl)dipyrromethane (0.51 g,
2.0 mmol) and 4-bromobenzaldehyde (0.37 g, 2.0 mmol) in CH2Cl2
(200 mL) with TFA (0.3 mL, 4.0vmmol) following the procedure
described for the general procedure gave a purple solid with a
yield of 30%. 1H NMR (400 MHz, CDCl3) ␦: 8.83 (s, 8H, pyrole-H), 8.12
(d, J = 8.1 Hz, 4H, ArH), 8.06 (d, J = 8.1 Hz, 4H, ArH), 7.89 (d, J = 8.1 Hz,
4H, ArH), 7.74 (d, J = 8.1 Hz, 4H, ArH), −2.85 (s, 2H, NH). UV/Vis
(CHCl3): 420, 516, 550, 590, 647 nm.
Data for 5-(4-Chlorophenyl)dipyrromethane (1b)
A sample of p-chlorobenzaldehyde (2.8 g. 20 mmol) was treated
identically as for 5-(4-bromophenyl)dipyrromethane, affording a
light-yellow solid (4.35 g, 85%). 1H NMR (CDC13) ␦: 7.97 (br, 2H, NH).
7.35–7.15 (m, 4H, ArH), 6.82 (m, 2H, pyrole-H), 6.20 (m, 2H, pyrole-
H), 5.87 (m. 2H, pyrole-H), 5.47 (s, 1H, meso-H).
Data for D(p-Cl)PD(p-OCH3)P
Condensation of 5-(4-chlorophenyl)dipyrromethane (0.51 g,
2.0 mmol) and p-methoxybenzaldehyde (0.27 g, 2.0 mmol) in
CH2Cl2 (200 mL) with TFA (0.3 mL, 4.0 mmol) following the proce-
dure described for the general procedure gave a purple solid with
a yield of 26%. 1H NMR (400 MHz, CDCl3) ␦: 8.85 (s, 8H, pyrole-H),
8.12 (t, J = 7.6 Hz, 8H, ArH), 7.87–7.61 (m, 4H, ArH), 7.42–7.13 (m, 4H,
ArH), 4.09 (s, 6H, OCH3), −2.80 (d, 2H, NH). UV/Vis (CHCl3): 419, 516,
551, 590, 648 nm.
General procedure for the synthesis of trans-A2B2-
porphyrin derivatives
By modifying a literature procedure,17 dipyrromethane (2 mmol)
and the appropriate aldehyde (2 mmol) was dissolved in CH2Cl2
(200 mL) under argon at room temperature. After a homogenous
solution was obtained, TFA (0.3 mL, 4.0 mmol) was added and the
reaction mixture was stirred at room temperature for 3 h. Chlo-
ranil (750 mg, 3.06 mmol) was added and stirring was continued
Published by NRC Research Press

Xie et al.
51
Data for D(p-OCH3)PP
1H NMR (400 MHz, CDCl3) ␦: 15.24 (s, 8H, pyrrole-H), 12.89 (s, 8H,
Condensation of phenyldipyrromethane (0.44 g, 2.0 mmol) and
p-methoxybenzaldehyde (0.27 g, 2.0 mmol) in CH2Cl2 (200 mL)
with TFA (0.3 mL, 4.0 mmol) following the procedure described for
the general procedure gave a purple solid with a yield of 35%.
1H NMR (400 MHz, CDCl3) ␦: 8.58 (s, 8H, pyrole-H), 7.97 (m, 8H,
ArH), 7.51–7.55 (m, 6H, ArH), 7.25 (m, 4H, ArH), 4.16 (s, 6H, OCH3),
−2.59 (s, 2H, NH). UV/Vis (CHCl3): 419, 516, 552, 591, 648 nm.
ArH), 9.96–9.53 (m, 8H, ArH). UV/Vis (CHCl3): 419, 518 nm.
Data for Ni-D(p-Cl)PP
The compound was prepared using the general procedure as
described above; a brick-red solid was obtained with a yield of
93%. 1H NMR (400 MHz, CDCl3) ␦: 8.73 (d, J = 11.6 Hz, 8H, pyrrole-H),
7.99 (s, 4H, ArH), 7.93 (d, J = 6.7 Hz, 4H, ArH), 7.67 (s, 10H, ArH).
UV/Vis (CHCl3): 415, 528 nm.
Data for D(p-Br)PD(o-Br)P
Data for Ni-D(p-Cl)PD(p-Br)P
The compound was prepared using the general procedure as
Condensation of 5-(4-bromophenyl)dipyrromethane (0.60 g,
2.0 mmol) and 2-bromobenzaldehyde (0.37 g, 2.0 mmol) in CH2Cl2
(200 mL) with TFA (0.3 mL, 4.0 mmol) following the procedure
described for the general procedure gave a purple solid with a
yield of 23%. 1H NMR (400 MHz, CDCl3) ␦: 8.82 (s, 4H), 8.70 (s, 4H),
8.25–7.95 (m, 8H), 7.89 (s, 4H), 7.69 (s, 4H), −2.75 (s, 2H). UV/Vis
(CHCl3): 420, 516, 551, 590, 647 nm.
described above; a brick-red solid was obtained with a yield of
1
89%. H NMR (400 MHz, CDCl3) ␦: 8.73 (s, 8H, pyrrole-H), 7.89 (m,
12H, ArH), 7.67 (s, 4H, ArH). UV/Vis (CHCl3): 416, 529, 617 nm.
Data for Ni-D(p-Br)PD(o-Br)P
The compound was prepared using the general procedure as
described above; a brick-red solid was obtained with a yield of
86%. 1H NMR (400 MHz, CDCl3) ␦: 8.71 (d, J = 4.0 Hz, 4H, pyrrole-H),
8.61 (s, 4H, ArH), 7.91 (m, 6H, ArH), 7.82 (s, 4H, ArH), 7.67 (d, J =
7.2 Hz, 2H, ArH), 7.61 (s, 4H, ArH). UV/Vis (CHCl3): 415, 529, 620 nm.
General procedure for the synthesis of trans-A2B2-
metalloporphyrin derivatives
A standard reaction was performed using a 100 mL round-
bottom flask in which trans-A2B2-diphenylporphyrin (0.25 mmol)
was dissolved in DMF (50 mL). Then, cobalt acetate or nickel ace-
tate (1.5 mmol) was predissolved in MeOH (5 mL) and was added to
the reaction mixture at room temperature. The reaction mixture
was refluxed and the evolution of the reaction was monitored by
thin-layer chromatography. Once no further progress of the reac-
tion was detectable, the solvents were placed into water. The mix-
ture was filtered and washed with water. The resulting solid was
vacuum-dried. The titled compound was obtained in yields be-
tween 85% and 97%.
Cyclohexane oxidation catalyzed by trans-A2B2-
metalloporphyrin with dioxygen
Except where a special explanation is given, cyclohexane oxida-
tion was performed according to the following procedures. It was
conducted in a 50 mL autoclave reactor with a magnetic stirrer in
the absence of solvent. Catalysts (1.8 mg) and cyclohexane (15.6 g)
were added. Then the reactor was sealed and heated to the setting
temperature. Molecular oxygen was then pumped into the auto-
clave until the system pressure reached the setting pressure. Then
the mixture was stirred. After the reaction, the reactor was cooled
to ambient temperature. The catalysts were removed by filtration.
The products of cyclohexanol and cyclohexanone were analyzed
by gas chromatography with the internal standard method using
chlorobenzene as an internal standard. The total acid in the prod-
uct was analyzed with a sodium hydroxide solution with the
chemical titration method. The total ester in the product was
analyzed with a solution of hydrochloric acid with the chemical
titration method.
Data for Co-D(p-Cl)PP
The compound was prepared using the general procedure as
described above; a brick-red solid was obtained with a yield of 97%.
1H NMR (400 MHz, CDCl3) ␦: 15.70 (s, 8H, pyrrole-H), 12.97 (s, 8H,
ArH), 9.87 (s, 8H, ArH), 9.70 (s, 2H, ArH). UV/Vis (CHCl3): 411,
528 nm.
Data for Co-D(p-Br)PP
The compound was prepared using the general procedure as
described above; a brick-red solid was obtained with a yield of
92%. 1H NMR (400 MHz, CDCl3) ␦: 15.74 (s, 8H, pyrrole-H), 12.96 (d,
8H, ArH), 10.01 (s, 4H, ArH), 9.90 (s, 4H, ArH), 9.70 (s, 2H, ArH).
UV/Vis (CHCl3): 411, 529 nm.
Results and discussion
The catalytic activities of different cobalt- and nickel-trans-A2B2-
metalloporphyrins in liquid-phase oxidation of cyclohexane was
investigated under 1 MPa for 1 h; the experimental results are
shown in Table 1. From Table 1, we could find that the structure of
cobalt- and nickel-metalloporphyrins affected their catalytic per-
formance. In other words, the catalytic activities of cobalt- and
nickel-metalloporphyrins in the oxidation of cyclohexane to
cyclohexanol and cyclohexanone are related to the variety of
peripheral substituents on the metalloporphyrin ring. For the
cobalt-trans-A2B2-metalloporphyrin catalyst, the different sub-
stituents in the phenyl ring of metalloporphyrin (Table 1, entries
1, 2, 3, and 6) showed that these trans-metalloporphyrin catalysts
with an electron-withdrawing group such as o/p-Cl or o/p-Br ex-
hibit higher conversions than those with an electron-donating
group such as p-OCH3 (Table 1, entries 4 and 5). This result is
similar to the reported result.18 Namely, the existence of electron-
withdrawing groups could also accelerate the reaction rate of
cyclohexane hydroxylation with the metalloporphyrin catalysts.
The reasons might be that the electronic density of cobalt ions in
metalloporphyrins decreased with an increase of the electron-
withdrawing degree of substituents from p-OCH3 to o/p-Cl or
o/p-Br, thus leading to the enhancement of the Co(III)−Co(II) reduc-
tion potential, and Co(III) could be easily reduced to Co(II) and the
catalytic cycle of metalloporphyrins could proceed successfully.
Data for Co-D(p-Cl)PD(p-Br)P
The compound was prepared using the general procedure as
described above; a brick-red solid was obtained with a yield of
95%. 1H NMR (400 MHz, CDCl3) ␦: 15.74 (s, 8H, pyrrole-H), 12.90 (s,
8H, ArH), 9.96 (m, 8H, ArH). UV/Vis (CHCl3): 419, 520 nm.
Data for Co-D(p-Cl)PD(p-OCH3)P
The compound was prepared using the general procedure as
described above; a brick-red solid was obtained with a yield of
90%. 1H NMR (400 MHz, CDCl3) ␦: 15.79 (s, 8H, pyrrole-H), 13.03 (s,
8H, ArH), 9.89 (s, 4H, ArH), 9.46 (s, 4H, ArH), 5.28 (s, 6H, OCH3).
UV/Vis (CHCl3): 413, 531 nm.
Data for Co-D(p-OCH3)PP
The compound was prepared using the general procedure as
described above; a brick-red solid was obtained with a yield of 91%.
1H NMR (400 MHz, CDCl3) ␦: 15.93 (s, 8H, pyrrole-H), 13.13 (s, 8H,
ArH), 9.92 (s, 6H, ArH), 9.71 (s, 2H, ArH), 9.43 (s, 2H, ArH), 5.24 (s,
6H, OCH3). UV/Vis (CHCl3): 411, 528 nm.
Data for Co-D(p-Br)PD(o-Br)P
The compound was prepared using the general procedure as
described above; a brick-red solid was obtained with a yield of 85%.
Published by NRC Research Press

52
Can. J. Chem. Vol. 92, 2014
Table 1. Results of oxidation of cyclohexane catalyzed by trans-A2B2-metalloporphyrins at 1 MPa.
Selectivity (%)
Ketone/alcohol
(mol ratio)
Turnover
number (×105)a
Entry
Catalyst
Conversion (%)
KA
Acid
Ester
1
2
Co-D(p-Cl)PP
Co-D(p-Br)PP
7.93
7.64
9.23
7.20
7.12
8.85
10.65
9.61
5.57
5.70
3.54
76.16
78.67
74.54
78.05
72.19
75.93
74.27
73.26
68.76
72.63
57.06
8.20
8.25
8.15
15.64
13.08
17.31
12.13
20.23
15.14
14.84
17.06
20.11
16.84
36.73
0.51
0.54
0.61
0.64
0.48
0.63
0.61
0.60
0.78
0.78
1.00
6.04
6.51
8.52
5.92
5.35
8.98
7.35
7.99
4.24
5.26
3.59
3
4
5
6
7
8
9
10
11
Co-D(p-Cl)PD(p-Br)P
Co-D(p-Cl)PD(p-OCH3)P
Co-D(p-OCH3)PP
Co-D(p-Br)PD(o-Br)P
Co-TPP
Co-T(p-Cl)PP
Ni-D(p-Cl)PP
Ni-D(p-Cl)PD(p-Br)P
Ni-D(p-Br)PD(o-Br)P
9.22
7.58
8.93
10.89
9.68
11.13
10.53
6.21
Note: Reaction conditions: cyclohexane 185 mmol, catalysts 1.8 mg, 155 °C for 1 h, oxygen pressure 1 MPa.
aThe turnover number is the value at 1 h of reaction time calculated by mole product (ketone + alcohol + acid + ester)/mol trans-A2B2-metalloporphyrin.
Table 2. Results of oxidation of cyclohexane catalyzed by trans-A2B2-metalloporphyrins at 2 MPa.
Selectivity (%)
Ketone/alcohol
(mol ratio)
Turnover
number (×105)a
Entry
Catalyst
Conversion (%)
KA
Acid
Ester
1
2
Co-D(p-Cl)PP
Co-D(p-Br)PP
17.43
17.30
18.57
14.10
14.10
17.49
18.99
19.64
10.51
14.06
2.10
67.24
60.92
57.21
57.81
57.81
58.15
55.98
56.36
61.94
60.60
45.24
14.74
14.68
17.63
18.77
15.43
15.32
17.59
18.28
14.65
15.93
5.23
18.02
24.40
25.16
23.42
22.87
26.53
26.43
25.36
23.41
24.47
49.53
0.81
0.84
0.85
1.26
1.26
0.90
0.94
0.99
0.63
0.82
0.98
13.27
14.75
17.15
11.60
10.60
17.75
13.11
16.34
8.00
3
4
5
6
7
8
9
10
11
Co-D(p-Cl)PD(p-Br)P
Co-D(p-Cl)PD(p-OCH3)P
Co-D(p-OCH3)PP
Co-D(p-Br)PD(o-Br)P
Co-TPP
Co-T(p-Cl)PP
Ni-D(p-Cl)PP
Ni-D(p-Cl)PD(p-Br)P
Ni-D(p-Br)PD(o-Br)P
12.98
2.13
Note: Reaction conditions: cyclohexane 185 mmol, catalysts 1.8 mg, 155 °C for 1 h, oxygen pressure 2 MPa.
aThe turnover number is the value at 1 h of reaction time calculated by mole product (ketone + alcohol + acid + ester)/mol trans-A2B2-metalloporphyrin.
Table 2 demonstrates the results of cyclohexane oxidation cata-
lyzed by cobalt- and nickel-trans-A2B2-metalloporphyrins; the
consequence of the various substituents of the cobalt- and nickel-
metalloporphyrins is similar to that of the cobalt-metalloporphyrin.
Obviously, the conversion of cyclohexane increased with an in-
crease of pressure; on the contrary, the selectivity of KA oil de-
creased significantly. More cyclohexanone and cyclohexanol was
oxidized into by-products (acid and ester) in the course of cyclo-
hexane oxidation catalyzed by trans-A2B2-metalloporphyrin.
The conversion of cyclohexane was compared with cobalt-
metalloporphyrin and nickel-metalloporphyrin in the same parent
under the same reaction conditions. From Table 1 (entries 1, 3,
6, and 9–11) and Table 2 (entries 1, 3, 6, and 9–11), we could find
that the different central metal ion influenced their catalytic per-
formance. Under the same reaction conditions, the catalytic activ-
ity of cobalt-metalloporphyrin in the conversion of cyclohexane
was higher than that of the nickel-metalloporphyrin. It is gener-
ally known that cyclohexane oxidation undergoes two steps: the
adsorption and desorption of oxygen from the catalyst. Thus, the
interaction between catalyst and oxygen molecule and its effect
on dissociation of the O−O bond can cause the unique conversion
of cyclohexane.
turnover number is higher. We also find that it is about a 3–10 min
inducing period for cobalt-trans-A2B2-metalloporphyrin, but a 30–
50 min inducing period for nickel-trans-A2B2-metalloporphyrin,
and that they had different turnover numbers. The phenomenon
may explain that different central metal ions had a different in-
fluence on the cyclohexane oxidation process.
The significant differences in the ratio of the ketone to alcohol may
be attributed to the redox property of trans-A2B2-metalloporphyrin.
That is, the difference of the substituents between trans-A2B2-
metalloporphyrin (Table 1, Entry 1–6) and A4-mettalloporphyrin
(Table 1, entries 7 and 8) led to those results under the oxidation
conditions. Possibly the substituent changes the adsorption and
desorption behavior of cyclohexane and oxygen toward the cata-
lyst.
Conclusion
The catalytic activities of trans-A2B2-metalloporphyrins in the ox-
idation of cyclohexane to cyclohexanol and cyclohexanone not only
change with the variety of metal atom ions in trans-A2B2-
metalloporphyrins but also change with the variety of peripheral
substituents on the trans-A2B2-metalloporphyrin ring. Trans-A2B2-
metalloporphyrins with electron-withdrawing groups were more
efficient catalysts than those with electron-donating groups
while using the same center metal ions. The cobalt-trans-A2B2-
metalloporphyrins show better catalytic performance than the
nickel-trans-A2B2-metalloporphyrins under the same reaction condi-
tions. Further studies on the synthesis of trans-A2B2-metalloporphyrins
and even supported catalysts are in progress in our laboratory.
Tables 1 and 2 show the differences in cyclohexane conversion and
turnover number with reaction pressure for cyclohexane oxidation
catalyzed by cobalt- and nickel-trans-A2B2-metalloporphyrins. It is
obvious that the cyclohexane conversion and turnover number cat-
alyzed by cobalt-trans-A2B2-metalloporphyrin was higher than that
of nickel-trans-A2B2-metalloporphyrin under the same reaction con-
ditions. The more quickly the activated species comes into being, the
more cyclohexane is oxidized, indicating that the corresponding
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Xie et al.
53
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Acknowledgements
This work was supported by the NSFC (21276218), the program
for New Century Excellent Talents in University (NCET-10-0168),
SRFDP (20124301110007), and the Scientific Research Fund of the
Hunan Provincial Education Department (13k043, CX2012B253).
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