Catalytic oxidation of alkene by cobalt corroles

Article abstract of DOI:10.1016/j.molcata.2016.11.019

Four cobalt (III) corroles bearing different number of pentafluorophenyl and phenyl groups were synthesized and characterized by elemental analysis, HR-MS, UV–vis, NMR, XPS as well as cyclic voltammetry. The first investigation of cobalt corrole catalyzed oxidation of alkene was conducted by using styrene as substrate. The best yield was obtained in acetonitrile solvent in the air with TBHP oxidant (96% yield based on oxidant, up to 96 TON). Benzaldehyde was detected as the main product by using PhI(OAc)₂, TBHP, KHSO₅, PhIO as oxidants. In contrast, styrene oxide was found to be the major product when using m-CPBA oxidant. Nearly no products could be found by using H₂O₂ oxidant. Possible catalytic oxidation pathway was also discussed based on the obsewrvations of UV–vis changes of the ctatalytic system in the absence of substrate and in-situ HR-MS.

Full text of DOI:10.1016/j.molcata.2016.11.019

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Journal of Molecular Catalysis A: Chemical
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Catalytic oxidation of alkene by cobalt corroles
Li-Ting Huang, Atif Ali, Hua-Hua Wang, Fan Cheng, Hai-Yang Liu∗
Department of Chemistry, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 May 2016
Received in revised form 8 October 2016
Accepted 14 November 2016
Available online xxx
Four cobalt (III) corroles bearing different number of pentafluorophenyl and phenyl groups were synthe-
sized and characterized by elemental analysis, HR-MS, UV–vis, NMR, XPS as well as cyclic voltammetry.
The first investigation of cobalt corrole catalyzed oxidation of alkene was conducted by using styrene
as substrate. The best yield was obtained in acetonitrile solvent in the air with TBHP oxidant (96% yield
based on oxidant, up to 96 TON). Benzaldehyde was detected as the main product by using PhI(OAc)2,
TBHP, KHSO5, PhIO as oxidants. In contrast, styrene oxide was found to be the major product when using
m-CPBA oxidant. Nearly no products could be found by using H2O2 oxidant. Possible catalytic oxidation
pathway was also discussed based on the obsewrvations of UV–vis changes of the ctatalytic system in
Keywords:
Corrole
Cobalt
Styrene
Catalytic oxidation
the absence of substrate and in-situ HR-MS.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Metal corrole has been proved one type of efficient catalysts for
oxidation [1]. In fact, lots of metal corroles such as chromium cor-
roles [1], manganese corroles [2–5], and iron corroles [6–9] have
been used in the catalytic oxidation of organic substrates. It was
found that catalytic oxidation reactions by metal corroles are signif-
icantly affected by oxidants, solvents and the electronic structure
of the catalysts. Cobalt corrole is a very important class of metal
corroles. It is a kind of very promising non-precious-metal catalyst
in fuel cell technology and a large number of investigations have
been devoted to the catalytic reduction of oxygen by cobalt corroles
[10–16].
Cobalt corrole has also been proved as efficient catalysts for
cyclopropanation [17], water oxidation [18–21], hydrogen evolu-
tion [22–25], CO2 reduction [26], and photocatalytic water splitting
reaction [27]. Cobalt corroles are also potential detection sensor for
CO [28–30]. It is well-known that Co(II) porphyrins are an efficient
catalysts for the oxidation of alkene [31–34]. To the best of our
knowledge, no studies about cobalt corrole catalyzed oxidation of
organic substrates has been reported so far. Herein, we wish to
report the catalytic oxidation of styrene by cobalt corroles bearing
different electronic features (Scheme 1). The effect of solvents and
oxidants on the catalytic reactions were also explored.
2.1. Materials and methods
All chemicals and solvents were purchased from Sinopharm
Chemical Reagents Co. Ltd. and used without further purification
unless otherwise mentioned. Pyrrole was redistilled prior to use.
Commercial analytical grade styrene (Aladdin) was passed through
short column of basic alumina prior to use. Iodosylbenzene (PhIO)
was synthesized by a reported method [35]. The supporting elec-
trolyte in CV experiments, tetrabutylammonium perchlorate, was
purchased from Fluka and recrystallized from ethanol at least three
times. The 1H NMR and 19F NMR spectra were recorded at room
temperature on a Bruker Avance III 400 spectrometer. X-ray photo-
electron spectroscopy (XPS) was performed using an Axis Ultra DLD
spectrometer. Mass spectras were taken on Bruker Esquire HCT plus
mass spectrometer (ESI/MS) and Bruker maxis impact mass spec-
trometer with an ESI source (HR-MS). All cyclic voltammograms
(CV) were performed in acetonitrile solutions containing 0.1 M
TBAP (tetrabutylammonium perchlorate) using an Ingens Model
1030 and cobalt corroles (1 × 10−3 M) under nitrogen atmosphere
at ambient temperature. A three-electrode system consisting of a
glassy carbon working electrode, a platinum wire counter electrode
and Ag/AgCl reference electrode were employed. The scan rate
was 100 mV/s. Half-wave potentials (E1/2) for reversible or quasi-
reversible redox processes were calculated as E1/2 = (Epa + Epc)/2,
where Epa and Epc represent the anodic and cathodic peak poten-
tials, respectively. The E1/2 value for the ferrocenium/ferrocene
couple under these conditions was 0.47 V. The freshly synthesized
cobalt corrole solution (∼3 × 10−3 M) was taken in the cuvette and
∗
Corresponding author.
E-mail address: chhyliu@scut.edu.cn (H.-Y. Liu).
http://dx.doi.org/10.1016/j.molcata.2016.11.019
1381-1169/© 2016 Elsevier B.V. All rights reserved.
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Scheme 1. Structures of cobalt (III) corrole complexes.
Fig. 1. a (left) X-ray photoelectron spectroscopy of the chemical composition of F15C-Co 1b. (right) Co 2p regions in F15C-Co.
placed immediately in a thermostat cell holder in a UV–vis spec-
fied by column chromatography on silica gel (200-300 mesh) using
hexene/CH2Cl2 as eluent, respectively. The yields of cobalt corroles
in all cases were about 75%. All reported 1H NMR and 19F NMR as
well as HRMS-ESI-MS spectra of cobalt corroles were given (see
Figs. S1–S11 in the Supporting information).
trophotometer. All experiments, unless specified otherwise, were
carried out at room temperature. Electronic absorption measure-
ments were performed on a Blue Star B UV–vis spectrophotometer
connected with a thermostat. The absorbance data was collected
over the range of 300–800 nm at 5 min interval. Each measurement
was repeated three times at 25.0 0.1 ◦C. The X-band CW EPR test
was measured on a Bruker, A300-10-12 Bruker EPR spectrometer
(microwave (mw) frequency 9.43 GHz). An mw power of 2.19 mW,
a modulation amplitude of 0.4 mT, and a modulation frequency of
100 kHz were used. Measurement was done at a temperature of
103 K.
2.4. 5,10,15-tris (phenyl) cobalt (III) corrole, F0C Co (III)
UV–vis. (CH2Cl2): ꢀmax, nm. (×10−3 ␧, L mol−1 cm−1). 385
(42.89), 562 (8.14). HR-MS (ESI): m/z found: 846.2273, calcd for
C55H38CoN4P: 846.2271; 1H NMR (400 MHz, CDCl3) ı 8.61 (d,
J = 4.3 Hz, 2H), 8.34 (d, J = 4.7 Hz, 2H), 8.16–7.97 (m, 7H), 7.68–7.49
(m, 11H), 7.39 (d, J = 7.5 Hz, 1H), 7.06 (t, J = 7.3 Hz, 3H), 6.71 (t,
J = 7.4 Hz, 6H), 4.84–4.66 (m, 6H). Elemental analysis: found (%):
C 78.58, H 4.78, N 6.16. calcd for (C55H38CoN4P): C 78.19, H 4.53, N
6.63.
2.2. Synthesis of freebase corrole
All other A3- and trans-A2B free base corroles were prepared
smoothly according to the reported procedures [6,36–38].
2.3. Synthesis of cobalt (III) corroles
Cobalt corrole F0C-Co was synthesized which follows a method
reported in the literature [15]; (5,10,15-tris(pentafluorophenyl)
cobalt corrole F15C-Co was synthesized according to the literature
[39]; New cobalt corroles F10C-Co, F5C-Co were prepared by 20 mg
corresponding free base corrole which was dissolved in 30 mL of
ethanol containing 141 mg CoCl2·4H2O, triphenylphosphine (5.0
equivalents vs. the free base corrole) and 30 mg anhydrous NaOAc
in 50 mL in a round bottom flask. The mixture was stirred for about
for 1.5 h under room temperature and the progress of the reaction
monitored by thin-layer chromatography until the starting corrole
was consumed. After the completion of reaction, the sample was
washed with dichloromethane and water for 2–3 times and was
collected. The red fraction was evaporated to dryness and was puri-
2.5. 5,15-bis (phenyl)-10-(pentafluoropheny) cobalt (III) corrole,
F5C-Co (III)
UV–vis. (CH2Cl2): ꢀmax, nm. (×10−3 ␧, L mol−1 cm−1). 386
(43.72), 556 (6.66), 557 (6.66). HR-MS (ESI): m/z found: 935.1768,
calcd for C55H33CoF5N4P: 935.1768; 1H NMR (400 MHz, CDCl3) ı
8.50 (d, J = 4.3 Hz, 2H), 8.34 (d, J = 4.6 Hz, 2H), 8.05 (d, J = 4.5 Hz,
2H), 8.00 (d, J = 4.8 Hz, 2H), 7.79 (s, 2H), 7.59 (t, J = 6.2 Hz, 8H),
7.49–7.37 (m, 15H); 19F NMR (376 MHz, CDCl3) ␦ -137.95 (dd,
J = 395.4, 24.2 Hz), -154.58 (t, J = 20.9 Hz), −162.19 to −162.74 (m).
Elemental analysis: found (%): C 71.04, H 3.76, N 5.54. calcd for
(C55H33CoF5N4P): C 70.67, H 3.56, N 5.99.
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2.6. 5,15-bis (pentafluorophenyl)-10-(phenyl) cobalt (III) corrole,
F10C-Co (III)
UV-Vis. (CH2Cl2): ꢀmax, nm. (×10−3 ␧, L mol−1 cm−1). 379
(43.19), 408 (38.17), 550 (7.99), 587 (6.70). HR-MS (ESI): m/z
found: 1047.1123; calcd for C55H28CoF10N4NaP: 1047.1116; 1H
NMR (400 MHz, CDCl3) ı 8.69 (d, J = 4.2 Hz, 2H), 8.25 (d, J = 4.9 Hz,
2H), 8.19 (d, J = 4.8 Hz, 2H), 8.00 (d, J = 4.5 Hz, 2H), 7.79–7.64 (m,
5H), 7.64–7.32 (m, 15H); 19F NMR (376 MHz, CDCl3) ␦ -137.04 (dd,
J = 220.7, 22.7 Hz), −153.71 (dd, J = 367.8, 346.9 Hz), −162.23 (dd,
J = 81.2, 59.1 Hz). Elemental analysis: found (%): C 64.86, H 2.77, N
5.42. calcd for (C55H28CoF10N4P): C 64.46, H 2.75, N 5.47.
2.7. 5,10,15-tris (pentafluorophenyl) cobalt (III) corrole, F15C-Co
(III)
UV-Vis. (CH2Cl2): ꢀmax, nm. (×10−3 ␧, L mol−1 cm−1). 378
(33.82), 410 (34.14), 550 (5.18), 585 (5.19). HR-MS (ESI): m/z
found: 1137.0646; calcd for C55H23CoF15N4NaP: 1137.0645; 1H
NMR (400 MHz, CDCl3) ı 8.74 (s, 2H), 8.38 (s, 2H), 8.29 (s, 2H),
8.14 (s, 2H), 7.29 (s, 4H), 7.13 (s, 11H); 1F NMR (376 MHz, CDCl3)
␦ −137.54 (d, J = 128.5 Hz), −153.72 (td, J = 20.9, 6.2 Hz), −162.12
(dd, J = 30.7, 15.3 Hz). Elemental analysis: found (%): 59.56, H 2.12,
N 4.99. calcd for (C55H23CoF15N4P): C 59.26, H 2.08, N 5.03.
Fig. 2. Cyclic voltammogram of F15C-Co in acetonitrile containing TBAP as support-
ing electrolyte. Scan rate 100 mV/s.
Table 1
Redox potentials (vs. Ag/AgCl) of cobalt (III) corroles in CH3CN containing TBAP as
supporting electrolyte.
Solvent
CH3CN
Corrole
15C-Co
F10C-Co
F5C-Co
F0C-Co
CoIII/CoII
CoII/CoI
Cor+/Cor
F
0.37a
0.44a
0.51a
0.73a
1.38c
1.42a
1.55c
1.57a
0.79b
0.71b
0.68b
0.60b
2.8. Catalytic oxidation of styrene
Cobalt corroles F15C-Co and F0C-Co have been reported in the
literature [15,39], F5C-Co and F10C-Co are new compounds. All cat-
alytic reactions were performed at room temperature in 10 mL
vessel equipped with a magnetic bar and loaded with catalyst
(F15C-Co, F10C-Co, F5C-Co, F0C-Co) (1 ␮mol), oxidant (0.1 mmol)
and styrene (115 ␮L, 1 mmol) in solvent (2 mL). After an appropri-
ate reaction time, chlorobenzene (5 ␮L) was added to this reaction
mixture as internal standard. The products were analyzed on an
Echrom A90 gas chromatograph equipped with HP-5 capillary col-
umn (30.0 m × 320 ␮m ID; 0.25 ␮m film thickness) coupled with
FID detector. The carrier gas was nitrogen and the chromatographic
conditions were as follows: the oven temperature was increased at
a rate of 10 ◦C/min from 60 ◦C to 250 ◦C; the injector temperature
was set 230 ◦C; the detector temperature was kept at 250 ◦C. The
injection volume of the filtrated reaction mixture was 1.0 ␮L and
the products were confirmed by the retention time using standard
samples at the same GC conditions. The yields of products were
reported with respect to the amount of oxidant used.
a
Irreversible reduction peak potential at a scan rate of 0.1 V/s.
A irreversible oxidation peak potential.
b
c
A reversible re-oxidation peak potential (E1/2). dFc/Fc couple were measured as
0.47 V under identical conditions.
with the reported data of Co 2P3/2 [40] and Co 2P1/2 in cobalt com-
plexes [41–43], revealing that central cobalt is formally in the Co
(III) state as reported with previous literature [44].
3.2. Electrochemistry
It is apparent that the redox property of cobalt corroles might
be the main factor influencing the oxidation of styrene. Thus,
to determine the effect of four cobalt corroles with different
electron-withdrawing or electron-donating substituents and an
axially bound triphenylphosphine ligand on redox property, the
electrochemical characteristics of F15C-Co(Fig. 2), F0C-Co, F5C-Co,
F10C-Co (see Figs. S15–S17 in the Supporting information) were
examined in CH3CN (V vs Ag/Ag+) with TBAP (0.1 M) as supporting
electrolyte, scan rate = 0.1 Vs−1. The CH3CN was chosen as a solvent
for electrochemical studies of cobalt corroles on the basis of good
yield which is based on the oxidant in catalytic oxidation of styrene
by cobalt corroles (Table 2).
As shown in Table 1, the first reduction is irreversible in all cases
which is in line with previously shown in other five-coordinate
cobalt (III) corroles with a bound triphenylphosphine (PPh3) axial
ligand and different substituents on the three meso-phenyl rings
in CH2Cl2 and DMF, which have been reported [15,45]. In the case
of CH3CN, a single one-electron reversible redox couple located at
E1/2 = −1.38 V and one irreversible reduction peak at -0.37 V, with
one irreversible peak at 0.79 V were seen for F15C-Co. Here, for
more electron-deficient cobalt corrole F15C-Co, it is more likely
that the half-wave potential (E1/2 = −1.38 V) is attributed to CoII/CoI,
the irreversible reduction peak at -0.37 V which may be assigned
to CoIII/CoII, the irreversibility was distributed to a partial loss of
the PPh3 ligand from [PPh3CoII(tpfc)]−, which was in agreement
with the electrochemical properties of cobalt corrole previously
3. Results and discussion
3.1. X-ray photoelectron spectroscopy (XPS)
The XPS survey spectras of the chemical composition of four
cobalt corroles are presented in Figs. 1a and S36–S38 in the Sup-
porting information. Fig. 1a shows C, N, P, Co, F in F15C-Co. To further
understand the electronic state of cobalt element, high-resolution
XPS spectras were examined. The XPS survey of Co 2P region for
cobalt corroles are presented in Figs. 1b and S12-S14 in the Sup-
porting information, the binding energy of Co 2P3/2 and Co 2P1/2
for F10C-Co, F15C-Co are much higher than that of F0C-Co, F5C-Co.
Investigation of the XPS spectras of all cobalt corrole revealed sim-
ilar behavior, Co 2P1/2 is less intense than Co 2P3/2, and the Co 2P
core-level spectrum shows two intense peaks. The Co 2P3/2 bind-
ing energy is 777.4 eV in F0C-Co, 777.3 eV in F5C-Co, 777.7 eV in
F10C-Co, 777.8 eV in F15C-Co, and Co 2P1/2 binding energy is 793 eV
in F0C-Co(Fig. S12), 792.1 eV in F5C-Co(Fig. S13), 792 eV in F10C-
Co(Fig. S14), 792.6 eV in F15C-Co(Fig. 1b). The result coincides well
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Table 2
Oxidation of styrene catalyzed by F15C-Co using PhI(OAc)2 in different solvents.
Solvent
Yield (%)b
BA
BAc Selectivity (%)
PA
SO
TON
Total
CH3CNa
DMF
DMAc
DCM
73
14
7
3
2
4
2
14
21
6
90
37
17
9
90
37
17
9
81
38
41
56
5
2
Note: reaction time: 10 h; a using 0.001 mmol catalyst, 0.1 mmol oxidant, 1 mmol styrene (The molar ratio of catalyst/oxidant/styrene was 1/100/1000); b Yields are based
on concentration of oxidant; c Selectivity of BA is the percentage of BA in total products.
3.4. Catalytic activity
In order to explore the catalytic potential of synthesized cobalt
corrole complex, we conducted a series of experiments concern-
ing catalytic oxidation of styrene by (triphenylphosphine) cobalt
(III) corroles under different systems. The effect of various solvents
and oxidants on the catalytic oxidation were investigated. Styrene
is often used as a substrate [51–53]. Using PhI(OAc)2 as oxidant
and CH3CN as solvent gave three products: (1) benzaldehyde, (2)
phenylacetaldehyde, and (3) styrene epoxide (Scheme 2).
3.4.1. Solvent effect
It has been reported that solvent exerts an effect in the catalytic
oxidation reaction [54] and solvent effect in chromium-oxo cor-
roles [55] and molybdenum-oxo corroles [56] has been previously
reported. In order to investigate the solvent effect on the reactivity
of the cobalt corroles, we used various types of solvents viz. CH3CN,
DMF, DMAc, and dichloromethane (2 mL each) on the oxidation
0.10414 g (1 mmol) of styrene using 0.0322 g (0.1 mmol) PhI(OAc)2
at 40 ◦C in 10 h. And one salient feature is that yield which is based
on oxidant of benzaldehyde using acetonitrile as solvent was much
higher than that of the other three solvents. It turned out that ace-
tonitrile was an ideal solvent with the maximum of 90% yield which
is based on oxidant, which is in accordance with previous work of
our group, showing that acetonitrile is the most suitable solvent for
manganese catalyzed oxidation of organic substrate [4,5,57], fol-
lowed by 37% yield which is based on oxidant of styrene using DMF
as a solvent. As the data depicted in Table 2, CH3CN was selected as
the solvent for the optimization of remaining reaction conditions.
Fig. 3. UV–vis spectra of F15C-Co in DCM, DMAc, DMF and CH3CN at 25.0 0.1 ◦C.
reported in the literature [26]. The oxidation potentials of four
cobalt corroles range from 0.60 V to 0.79 V in CH3CN may be safely
assigned as corrole oxidation (Cor+/Cor) to ꢁ− trianions radicals.
These results are consistent with the fact that the CoII and CoI forms
of the corrole which have been previously reported in the litera-
ture [15]. One important finding reflected in Table 1 was that the
redox processes were observed for these four cobalt complexes
with the CoII/CoI, CoIII/CoII and Cor+/Cor redox couples gradually
shifted positively, presumably due to the electron-deficient virtue
of the meso-subsitution by phenyl and pentafluorophenyl groups.
This was consistent with that the addition of one or more NO2
groups on the three meso-phenyl rings of the triarylcorrole would
be expected to shift all redox potentials toward more positive val-
ues [46,47]. Similarly, the phenomenon was observed by comparing
the cases of reduction potentials of Co complexes of tetrakis (per-
fluorophenyl) porphyrin [48] and octaethylporphyrin [49], as well
as a series of other metal complexes of tpfc and omc [50].
3.4.2. Oxidant effect
The catalytic data with all the cobalt corroles catalysts using dif-
ferent oxidants was summarized in Table 3A and 3B. Based on the
maxmium yield which is based on oxidant (Table 2), acetonitrile
was chosen as an ideal solvent. Therefore, we conducted a series
of catalytic oxidation of styrene reaction using 1 umol cobalt cor-
roles, 100 equilvalent of six different oxidants viz. PhI(OAc)2, TBHP,
H2O2, KHSO5, m-CPBA and PhIO, 1000 umol styrene, 2 mL CH3CN at
40 ◦C both in the presence and absence of air under the optimized
solvent CH3CN. As seen from Tables 3A and 3B, the yield which is
based on oxidant was very low in absence of catalysts. Benzalde-
hyde was main product along with formation of styrene epoxide
and phenylacetaldehyde in CH3CN when PhI(OAc)2 was used as
oxidant. The total yield which is based on oxidant of oxidation of
styrene range from 80% to 90% by four kinds of cobalt (III) corroles
in CH3CN using PhI(OAc)2 as oxidant in atmosphere, while it was
slightly lower in agron for the four cobalt (III) corroles (77%-85%).
Obviously, a significant feature was observed that the selectivity
of benzaldehyde was above 80% using four different cobalt cor-
rles as catalysts in the reaction system PhI(OAc)2/CH3CN at 40 ◦C
in 10 h in the presence of oxygen. It is clear that cobalt corroles
are effective for oxidation of styrene in the presence of PhI(OAc)2.
As shown in Table 3A, higher yield of benzaldehyde was obtained
in the presence of oxygen than performed in argon atmosphere
3.3. UV–vis spectroscopy
Four cobalt (III) corroles with different electronic stuctures were
synthesized and their spectral behavior in four different solvents
(DMF, DMAc, DCM, and CH3CN) were characterized by spectro-
scopic methods (Fig. 3). In case of these four solvents, the electronic
spectras show that F15C-Co and F10C-Co exhibited similar change
(two pronounced Soret bands at 350–450 nm and two weaker
absorption in Q-bands in the visible spectral region centered at
550–600 nm), whereas F0C-Co exhibited both single Soret band
around 400 nm and Q-band around 550–600 nm, which are in
accordance with previously reported literature [15,26]. Similarly,
UV–vis spectra of F5C-Co was almost the same as that of F0C-Co.
UV–vis spectras in various solvents imply that there is no solvent
effect on the valent of cobalt corroles.
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Scheme 2. Oxidation of styrene catalyzed by (triphenylphosphine) cobalt (III) corroles.
Table 3A
Oxidation of styrene catalyzed by cobalt (III) corroles using different oxidants in CH3CN.
Oxidant
Catalyst
Yield (%)c
BA
BAe Selectivity (%)
PA
SO
TON
Total
none
F15C-Co
none
1
12
0
1
0
1
1
14
1
14
b
PhI(OAc)2
F15C-Co
F10C-Co
F5C-Co
F0C-Co
none
73 (58)
66 (51)
65 (51)
64 (49)
4
3 (5)
3 (16)
5 (17)
5 (14)
0
14 (14)
11 (14)
11 (17)
11 (20)
0
90 (77)
80 (81)
81 (85)
80 (83)
4
90 (77)
80 (81)
81 (85)
80 (83)
4
81 (75)
83 (63)
80 (60)
80 (59)
TBHPa
F15C-Co
F10C-Co
F5C-Co
F0C-Co
none
F15C-Co
F10C-Co
F5C-Co
F0C-Co
79 (55)
90 (47)
85 (47)
78 (35)
1
1 (1)
1 (0)
2 (1)
2 (1)
1 (14)
4 (12)
2 (15)
3 (30)
0
0 (0)
0 (0)
0 (0)
0 (0)
3 (8)
2 (15)
2 (9)
1 (8)
0
0 (0)
0 (0)
0 (0)
0 (0)
83 (77)
96 (74)
89 (71)
82 (73)
1
1 (1)
1 (0)
2 (1)
2 (1)
83 (77)
96 (74)
89 (71)
82 (73)
1
1 (1)
1 (0)
2 (1)
2 (1)
95 (71)
94 (64)
96 (66)
95 (48)
d
H2O2
100 (100)
100
100 (100)
100 (100)
Note: reaction time: 10 h; an Except for 5 h; b using 0.001 mmol catalyst, 0.1 mmol oxidant, 1 mmol styrene (The molar ratio of catalyst/oxidant/styrene was 1/100/1000); c
Yields are based on concentration of oxidant; d Yields in parentheses were obtained in agron condition; e Selectivity of BA is the percentage of BA in total products.
Table 3B
Oxidation of styrene catalyzed by cobalt(III) corroles using different oxidants in CH3CN.
Oxidant
KHSO5
Catalyst
Yield(%)c
BA
BAe Selectivity (%)
PA
SO
TON
Total
none
10
1
0
11
11
F15C-Co
F10C-Co
F5C-Co
F0C-Co
none
64 (23)
78 (58)
46 (18)
27 (13)
5
4 (3)
6 (6)
7 (3)
3 (7)
0
0 (1)
0 (0)
0 (2)
2 (0)
68 (27)
84 (64)
53 (23)
32 (20)
5
68 (27)
84 (64)
53 (23)
32 (20)
5
94 (85)
93 (91)
87 (78)
84 (65)
0
PhIO
F15C-Co
F10C-Co
F5C-Co
F0C-Co
none
59 (43)
52 (37)
63 (49)
55 (36)
2
6 (5)
5 (10)
9 (10)
14 (15)
9
9 (13)
10 (10)
11 (13)
10 (12)
28
74 (61)
67 (57)
83 (72)
79 (63)
39
74 (61)
67 (57)
83 (72)
79 (63)
39
80 (70)
78 (65)
76 (68)
70 (57)
m-CPBA
F15C-Co
F10C-Co
F5C-Co
F0C-Co
4 (2)
5 (3)
8 (2)
4 (3)
4 (13)
11 (13)
19 (9)
7 (11)
37 (47)
38 (37)
32 (28)
40 (33)
45 (62)
54 (53)
59 (39)
51 (47)
45 (62)
54 (53)
59 (39)
51 (47)
9 (3)
9 (6)
14 (5)
8 (6)
(Table 3A, yields in parentheses), indicating that oxygen plays an
important role in formation of benzaldehyde in this reaction. Based
on the experimental fact and previously reported literature [4,5],
formation of benzaldehyde may be attributed to oxygen free radical
catalytic reactions. It is noteworthy that increasing trend in the total
yield which is based on oxidant using F15C-Co as catalyst is higher
than that of other three cobalt corroles in CH3CN which implies that
corrole F15C-Co exhibits higher activity in CH3CN using PhI(OAc)2
as oxidant. The most electron-deficient cobalt corrole gave higher
yield of benzaldehyde and higher total yield which is based on
oxidant. This was probably due to the electron-withdrawing effect.
It is noteworthy that using TBHP as oxidant, main product was
benzaldehyde as indicated by GC analysis, a significant increase
in the total yield which is based on oxidant of oxidation products
was observed (82%–96%) (See Table 3A) even in five hours, this fact
indicates that cobalt corroles work effectively in CH3CN according
to the control experiment with TBHP as oxidant.
However, all the four different cobalt corroles hardly worked
in CH3CN using H2O2 as oxidant, because only a small trace of
benzaldehyde could be traced, and no other products were found
(Table 3A), revealing that the four cobalt corrole catalysts exhibited
the least activity on the styrene oxidation in CH3CN.
During the oxidation of styrene using KHSO5 as oxidant, sim-
ilar result was obtained as using PhI(OAc)2. In other words, main
product was benzaldehyde and the selectivity of benzaldehyde was
above 80%. From the date reflected in Table 3B in parentheses, when
the reaction was performed in agron, the yield of benzaldehyde
which is based on oxidant was significantly decreased.
Similar to the catalytic system with PhI(OAc)2 and KHSO5 as
oxidant, an obvious finding was that benzaldehyde was the main
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product using PhIO as oxidant, with significantly higher total yield
which is based on oxidant in CH3CN (67%–83%) (Table 3B). This
result was contrary to that of catalytic oxidation of styrene by
N-confused manganese porphyrin [5]. Interestingly, benzaldehyde
was main product in the cobalt/PhIO/CH3CN system, whereas iron
corroles exhibit different activity on the styrene catalytic oxida-
tion using PhIO as oxidant in CH3CN [58], which greatly improve
the yield which is based on oxidant of styrene epoxide using iron
corroles as catalyst.
We can observe that there is a big difference in product profile
during oxidation of styrene with Co-corrole/PhI(OAc)2 and Co-
corrole/PhIO. PhI(OAc)2 has been used as a mild oxygen source
[59–61], thus, PhI(OAc)2 has advantage in enhancing corrole cata-
lyst stability against the oxidation degradation. This is particularly
beneficial for metallocorrole-catalyzed oxidations because metal-
locorroles are less robust and prone to oxidative degradation. PhIO
has greater oxidizing power, it may react with cobalt corrole to form
reactive metal-oxo species directly, which might accelerate the
degradation of the cobalt (III) corrole catalysts, and finally results
the decrease of total product yield which is based on oxidant.
Interestingly, as in the case of the m-CPBA/CH3CN system, with
catalytic data in Table 3B, styrene epoxide was the main product
rather than benzaldehyde. The formation of styrene epoxide may
consistent with the previous literature [62,63]. And compared with
other oxidants, this is a big difference.
Fig. 4. UV–vis spectral change upon addition of PhI(OAc)2 (8.5 equiv) to a solution
of F5C-Co in CH3CN at 25 ◦C.
For comparison, additional tests on simple Co (II) salts
(Co(OAc)2·4H2O) as catalyst for the homogeneous oxidation of
styrene, VCH3CN/Vwater = 3:1 as solvent because simple Co (II) salts
are not soluble in CH3CN and blank tests without any catalyst under
the identical conditions have been carried out. And the catalytic
data is listed in Tables 3A to 3C. From the data reflected in Table 3C,
the total yield which is based on oxidant range from 1%-14% at
atmosphere, which is much lower than that of using cobalt cor-
role as catalyst. In other words, Co(OAc)2·4H2O shows little activity
towards styrene oxidation. Here, we did not perform tests on simple
Co (III) salts because they are not stable in the room temperature,
which is not suitable in this catalytic system (the reaction temper-
ature is 40 ◦C).
When the substrate and oxidant are used in a 1:1 ratio (using
0.1 mmol oxidant, 0.1 mmol styrene) with 0.001 mmol F15C-Co as
catalyst under the identical experimental conditions, we found that
the total yield which is based on oxidant range from 1%-10% (the
data is not provided), which is much lower than using substrate
and oxidant are used in a 10:1 ratio.
Fig. 6. UV–vis spectral change upon addition of KHSO5 (8.5 equiv) to a solution of
F10C-Co in CH3CN at 25 ◦C.
3.4.3. Mechanistic considerations
To better understand the efficiency of the catalytic system and to
gain futher information about the role of high-valent cobalt corroles
on the oxidation of styrene, the UV–vis spectras of the four cobalt
corroles in absence of organic substrate with several equilvalents
amount of oxidant added were carried out. The UV-Vis spectras of
four cobalt complexes upon addition of large excess of oxidants
viz. PhI(OAc)2, TBHP, KHSO5, PhIO, m-CPBA were examined every
five minutes are shown in the following (Figs. 4 to 9). UV–vis spec-
tras of catalytic reaction mixture when the substrate and oxidant
is used in a 1:1 ratio (using 0.1 mmol oxidant, 0.1 mmol styrene)
with 0.001 mmol F15C-Co as catalyst in CH3CN under the identical
experimental conditions were performed (see Fig. S39-S44 in the
Supporting information).
Figs. 4 and S21–S23 in the Supporting information exempli-
fied that an obvious characteristic of weaker Soret band at around
350–450 nm and loss in intensity of Q bands at around 550–600 nm
when 8.5 equivalents of PhI(OAc)2 was added to solution with four
cobalt corroles in CH3CN at 25 ◦C. During the process, we could
observe that red cobalt corroles in CH3CN converted into other
Fig. 7. UV–vis spectral change upon addition of PhIO (8.5 equiv) to a solution of
F15C-Co in CH3CN at 25 ◦C.
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Table 3C
Oxidation of styrene catalyzed by Co(OAc)2·4H2O using different oxidants in CH3CN/H2O (3:1).
Oxidant
Yield (%)c
BAe Selectivity (%)
BA
PA
SO
TON
Total
none
0
0
0
0
0
b
PhI(OAc)2
3 (2)
3 (2)
1 (0)
1 (0)
4(2)
3 (2)
5 (3)
6 (0)
0 (0)
1 (0)
0 (0)
6 (3)
3 (4)
5 (6)
0 (0)
1 (2)
0 (1)
5 (6)
11 (9)
14 (8)
1 (0)
3 (2)
4 (3)
14 (11)
11 (9)
14 (8)
1 (0)
3 (2)
4 (3)
14 (11)
27 (22)
21 (25)
100
33 (0)
100 (67)
21 (18)
TBHPa
d
H2O2
KHSO5
PhIO
m-CPBA
Note: reaction time: 10 h; an Except for 5 h; b using 0.001 mmol catalyst, 0.1 mmol oxidant, 1 mmol styrene (The molar ratio of catalyst/oxidant/styrene was 1/100/1000); c
Yields are based on concentration of oxidant; d Yields in parentheses were obtained in agron condition; e Selectivity of BA is the percentage of BA in total products.
Fig. 5. Situ HR-ESI-MS spectrum of cobalt(III) corrole F10C-Co with KHSO5 in CH3CN.
species with distinct green color change in several minutes and
finally were bleached.
experiment of styrene oxidation by F10C-Co with KHSO5 as oxidant
in CH3CN (Fig. 5). It has been reported that manganese corroles
with m-CPBA or PhIO can form the reactive new specie Mn(III)-
oxo corrole [2,64,65]. Thus, we presumed that the new species
with reactivity is Co(V)-oxo. To further elucidate the state of the
new species, we conducted EPR spectroscopy experiment. How-
ever, X-band CW EPR Spectroscopy of F10C-Co in CH3CN solution
with KHSO5 (103 K) exhibited an electron resonance spectra sig-
nal at g = 2.0135 which similar to value reported in the literature of
Co(IV) corrole [66] (see Fig. S45 in the Supporting information),
which suggested the formation of Co(IV)-oxo. In the absence of
The UV–vis spectra of F10C-Co upon mixing with styrene in
CH3CN with KHSO5 is demonstrated in Fig. 6. The UV–vis spectras
of all four cobalt corroles exhibited similar behaviors in CH3CN (see
Figs. S24–S26 in the Supporting information). The red color changed
into green and slight change was observed in both Soret bands and
Q bands, but after minutes later both the Soret bands and Q bands
of the four cobalt corroles were significantly red-shifted, indicat-
ing that existence of an active species expected as the high-valent
cobalt corroles. Furthermore, we conducted the situ HR-ESI-MS
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Scheme 3. Plausible mechanism for catalytic oxidation of styrene by cobalt (III) corroles.
Fig. 9. UV–vis spectral change upon addition of m-CPBA (8.5 equiv) to a solution of
F0C-Co in CH3CN at 25 ◦C.
Fig. 8. UV–vis spectral change upon addition of TBHP (8.5 equiv) to a solution of
F0C-Co in CH3CN at 25 ◦C.
organic substrate, it is possible that the high reactive cobalt(V)-oxo
oxidant may react with unreacted cobalt(III) precursor to produce
the stable ␮-oxo dimer. 0ther possibility is that cobalt(V)-oxo may
react with unreacted cobalt(III) precursor in comproportionation
reaction to produce Co(IV)-oxo, which has also been reported in the
corrole-iron(V)–oxo species [67,68]. According to the data depicted
in Tables 3A and 3B, higher yield which is based on oxidant of ben-
zaldehyde was obtained in the presence of oxygen than in argon
atmosphere (see yields in parentheses) when using PhI(OAc)2,
KHSO5, PhIO as oxidants, which implies that the formation of ben-
zaldehyde may be attributed to the free radical mechanism with
the participation of oxygen catalytic reactions [4,5]. The plausible
mechanism is suggested using PhI(OAc)2, KHSO5, PhIO, m-CPBA as
oxidant was depicted (Scheme 3. Path 1).
the four cobalt corroles exhibited some similar phenomena as using
KHSO5.
Interestingly, when oxidant TBHP was added to a solution
CH3CN of cobalt corroles, we could observe slight loss of absorbance
in Soret band and Q band for F0C-Co (Fig. 8) and other three cobalt
corroles (see Figs. S30–S32 in the Supporting information). From
the data reflected in Table 3A, we can see that higher total yield
which is based on oxidant was obtained in the reaction system
cobalt corrole/TBHP/CH3CN in 5 h. Hence, four cobalt complexes
exhibited higher catalytic activity when using TBHP as oxidant.
When this reaction was carried out in agron, formation of main
product benaldehyde was significantly decreased (Table 3A, yields
in parentheses). A free-radical reaction that is progagated by the
involvement of oxygen led to formation of benzaldehyde, which
has been reported previously [4,5]. Thus, plausible mechanism is
shown (Scheme 3 Path 2).
As shown in Figs. 7 and S27–S29 in the Supporting information,
when oxidant PhIO was added to cobalt corrole/CH3CN system, all
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In case of cobalt corrole/m-CPBA/CH3CN system, as showed in
Figs. 9 and S33–S35 in the Supporting information, loss of intensity
of the absorption peaks could be observed, which clearly showed
that four cobalt (III) corroles were completely bleached upon excess
addition of m-CPBA to the solution. The intermediate could not be
traced in cobalt corrole/m-CPBA/CH3CN system. Thus, by combing
the data in Table 3B and Fig. 8, we made a conclusion that the stabil-
ity of four cobalt corroles was poor when m-CPBA was added to the
solution, which may account for the fact that m-CPBA inhibited the
activity of cobalt corroles. And this was consistent with the result
that total yield which is based on oxidant was much lower when
using m-CPBA as oxidant reflected in Table 3B.
Combined with the catalytic data shown in Table 3A, the total
yield which is based on oxidant was so low that among which ben-
zaldehyde was the major product and no phenylacetaldehyde and
styrene epoxide were found in cobalt corroles/H2O2/CH3CN sys-
tem. It is probably that cobalt corroles hardly work in H2O2/CH3CN
system, which means that cobalt corroles cannot effectively cat-
alyze styrene oxidation in this case. Hence, UV–vis spectra of the
four catalysts in absence of organic substrate with several equilva-
lents amount of H2O2 added was not carried out.
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4. Conclusion
In conclusion, we have prepared a series of cobalt corroles with
different electronic structure. They were characterized by UV–vis,
NMR, X-ray Photoelectron Spectroscopy and cyclic voltammetry.
The catalytic oxidation of styrene by using these cobalt corroles
were also carried out. This work revealed that cobalt corroles were
potential catalysts for the oxidation of alkene. The solvents, oxi-
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effect on the catalytic oxidation reaction. In CH3CN solvent, ben-
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TBHP, KHSO5 and PhIO as oxygen source. While styrene epoxide
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discussed. Further investigation on the active intermediate of cat-
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