Highly efficient activation of molecular oxygen with nanoporous metalloporphyrin frameworks in heterogeneous systems

Article abstract of DOI:10.1002/adma.201100974

A nanoporous metalloporphyrin framework exhibits high activity in the activation of molecular oxygen and excellent catalytic activity in the aerobic epoxidation of olefins, with high conversion, outstanding selectivity, and broad substrate applicability. The porous framework is reusable and allows large-scale transformation. These catalytic features are correlated with the structural characteristics of the porous framework and mark a breakthrough for this classic reaction. Copyright

Full text of DOI:10.1002/adma.201100974

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Highly Efficient Activation of Molecular Oxygen
with Nanoporous Metalloporphyrin Frameworks
in Heterogeneous Systems
Long Chen, Yong Yang, Zhaoqi Guo, and Donglin Jiang*
Activation of molecular oxygen is a subject of increasing
interest in relation to the bioinspired function mimicry of cyc-
tochrome P450 and the development of highly active catalysts
for oxidation reactions. In this sense, metalloporphyrin is a
well-established motif with a similar active center to that of cyc-
tochrome P450 and has been developed for various oxidation
reactions.[1] Studies of metalloporphyrin catalysts have mainly
focused on homogeneous systems with the aim to improve
activity in the activation of molecular oxygen via the molecular
design along with precise synthesis of porphyrin macrocycles.
In contrast, examples of metalloporphyrin-based heterogeneous
catalysts have been very limited to few metalloporphyrin-based
metal-organic frameworks (MOFs)[2] and porous polymers[3]. In
the case of MOFs, the framework is fragile to break down once
the crystalline solvents are lost during reactions, which dete-
riorates their further utility in catalysis. Therefore, the rational
design of metalloporphyrin-based heterogeneous catalysts is a
challenging subject in relation to the activation of molecular
oxygen.
Conjugated micro- and mesoporous polymers (CMPs) are a
class of porous frameworks with 3D polymer frameworks and
high surface areas.[4–8] CMPs are intriguing platforms for the
molecular design of porous skeletons because they allow elabo-
rate control of the frameworks and tuning of the pore param-
eters.[4] One of the remarkable characteristics of CMPs is their
crosslinked 3D molecular skeletons. These structural features
offer an opportunity for creating robust heterogeneous catalysts,
provided that the catalytic sites can be covalently integrated into
the CMP skeleton.[8b]
broad range of substrates including alkyl, aromatic, and cyclic
olefins. FeP-CMP is robust for recycling and allows the activa-
tion of molecular oxygen under ambient conditions. The porous
framework outperforms the monomeric metalloporphyrin and
nonporous metalloporphyrin polymer analogues. With these
outstanding features, FeP-CMP offers a breakthrough for the
activation of molecular oxygen.
FeP-CMP (Scheme 1a,b) was synthesized by Suzuki cross-
coupling polycondensation of iron(III) 5,10,15,20-tetrakis-(4’-
bromophenyl)porphine and 1,4-benzene diboronic acid in
the presence of Pd(PPh3)4 as a catalyst under alkaline condi-
tions. FeP-CMP was unambiguously characterized by various
instrumental analyses.[8b] As a control for FeP-CMP, FeP-LP
with a linear polymer structure was synthesized (Scheme 1).
The average molecular weight (Mw) of FeP-CMP was esti-
mated to be 490 000, which corresponds to a polymerization
degree of 570. From field-emission scanning electron micros-
copy (FE-SEM) images, FeP-CMP adopts flake-shaped mono-
liths with sizes of 1–2 μm (Figure 1a–e). Of further interest is
the fact that each monolith consists of layers of much smaller
flakes with a size of approximately 100–200 nm (Figure 1f).
Such a hierarchical structure from a molecular level frame-
work to nanoscale layers and further to micrometer scale
monoliths is highly interesting because it is common to bio-
logical catalytic systems. High-resolution transmission elec-
tron microscopy (HR-TEM) measurements reveal the pres-
ence of nanometer-scale pores (Figure 1g,h). The Brunauer–
Emmett–Teller (BET) surface area is as high as 1270 m2 g−1 ,
while the pore widths and pore volume calculated by nonlocal
density functional theory are 0.47 nm, 2.69 nm, and
1.18 cm3 g−1 , respectively. It is noteworthy that the surface
area is much higher than that of metalloporphyrin-based crys-
talline MOFs (typically ≈500 m2 g−1 ).[2] These results suggest
that the amorphous FeP-CMP possesses dense catalytic sites,
holds inherent nanopores, and has large surface areas. As
a high surface area material, FeP-CMP, which is comprised
largely of metalloporphyrin moieties in the skeleton, has an
exceptionally high loading of catalytic sites, which is in sharp
contrast to strategies involving porphyrin immobilization on
other frameworks such as silica.
Here, we report an excellent heterogeneous system using
iron(III) porphyrin-based CMP (Scheme 1, FeP-CMP) for the
activation of molecular oxygen. We highlight that FeP-CMP
can activate molecular oxygen for highly efficient epoxidation
of olefins[9] with large turn over number (TON) and turn over
frequency (TOF) values, high selectivity, and adaptability to a
Dr. L. Chen, Dr. Y. Yang, Dr. Z. Q. Guo, Prof. D. Jiang
Department of Materials Molecular Science
Institute for Molecular Science
National Institute for Natural Sciences
5–1 Higashiyama, Myodaiji, Okazaki 444–8787, Japan
E-mail: jiang@ims.ac.jp
To elucidate the porous structure, we first carried out sem-
iempirical calculations at the PM3 level and then performed fur-
ther calculations using hybrid density functional theory (DFT)
at the B3LYP/3–21G level using the Gaussian 03 program.[[12]]
We performed the calculations on two typical frameworks of
free-base porphyrin. The elementary closed tetragon gives a
nearly flat structure with a pore width of 2.4 nm (Figure 2a).
Prof. D. Jiang
Precursory Research for Embryonic Science and Technology
Japan Science and Technology Agency (JST)
Chiyoda-ku, Tokyo 102-0075, Japan
DOI: 10.1002/adma.201100974
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isobutyraldehyde (IBA) and O2.[6b,e] IBA/O2
has been well studied with various catalysts
for aerobic oxidations including epoxida-
tion, C–H oxidation,[10] and Baeyer–Villiger
oxidation.[11]
a)
Cl
Fe
Cl
Fe
Cl
Fe
N
N
N
N
N
N
N
N
N
N
N
N
FeP-CMP exhibited outstanding catalytic
performance for the selective epoxidation of
trans-stilbene with IBA/O2. Under ambient
pressure (1 atm) and temperature (298 K)
with 1 mg FeP-CMP (sub-ppm order Fe),
trans-stilbene oxide was produced in 98% con-
version with 97% selectivity after 9 h without
any sign of the formation of cis-stilbene oxide
(Table 1). Benzaldehyde (3%) was the sole
byproduct, and no diol or ester overoxidation
compounds were observed. Similar to the
typical systems,[9b−d] FeP-CMP also requires
the coexistence of IBA and O2; epoxidation
was completely halted in the absence of either
IBA or O2. On the other hand, in the absence
of FeP-CMP, the conversion dropped signifi-
cantly to only 30% after 24 h, which is much
lower than that in the presence of FeP-CMP.
This result indicates the catalytic nature of
FeP-CMP. We noticed that the catalytic per-
formance could be enhanced in the presence
of an excess amount of IBA. For example,
the conversion increased from 46% (24 h) to
96% (17 h) when the molar ratio of IBA/trans-
stilbene was increased from 1 to 2. Notably,
when the molar ratio was 3, 97% conversion
was achieved in the much shorter time of 9 h.
Cl
Fe
Cl
Fe
Cl
Fe
N
N
N
N
N
N
N
N
N
N
N
N
FeP-CMP
Linear FeP-polymer
b)
High Activity (99%), Excellent Selectivity (> 99%), Broad Range of
Substrates, Good Reusability, Large Scale Transformation (TON > 107)
O
In control experiments,
a monomeric
metalloporphyrin complex used as
a
homogeneous catalyst showed low activity
(Table 1).[9e] Therefore, the porous frame-
work outperforms the monomeric metal-
loporphyrin calalyst. We postulated that the
enhanced activity of FeP-CMP may be related
to its porous structure. Thus, we prepared
FeP-CMP samples with different surface
areas and pore sizes and utilized them in
the epoxidation under otherwise identical
conditions. Surprisingly, all FeP-CMP sam-
ples displayed catalytic activity with high
selectivity. This observation suggests that
the catalytic CMP skeletons are excellent
for the selective epoxidation of olefins. The
Aromatic, Alkyl and
Cyclic Olefins
Cooperativity of structural features endows CMPs with
superb performance in heterogeneous catalysis
Scheme 1. a) Structure of the microporous iron(III) porphyrin framework FeP-CMP and its
linear polymer analogue FeP-LP. b) Schematic representation of the aerobic epoxidation of
various olefins catalyzed by FeP-CMP under ambient conditions. The tetragonal FeP-CMP struc-
ture is a simple representation and is not meant to imply an infinite 2D structure (purple:
porphyrin; blue: phenyl; green: N; red: Fe).
This tetragonal structure accounts for the presence of mes-
opores in FeP-CMP. The eight-member porphyrin framework
significantly twists between the two flat tetramer counterparts
and this distortion eventually leads to the growth of the CMPs
into an amorphous and 3D network (Figure 2b). Such a porous
framework is permanent because the skeleton is crosslinked
and both the phenylene connections and porphyrin units are
conformationally rigid.
catalytic activity is highly dependent on the surface area. An
increase in the activity was observed as the surface area of the
catalyst increased. The pore size distribution had less of an
effect on the catalytic activity. This result is reasonable because
the epoxidation takes place on the skeleton, which consists of
highly dense active sites. Consequently, a large surface area
can facilitate the epoxidation reaction. The positive surface area
effect is encouraging for the development of porous hetero-
geneous catalysts and has implications for designing catalytic
frameworks based on CMPs. Along this line, we synthesized a
linear control FeP-LP without porous structure and utilized it as a
Epoxidation reactions were carried out under the same
conditions as those used for typical systems such as the
Mukaiyama Ni(II) catalyst, employing a combination of
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efficiently to afford nearly the same activity
and selectivity as the reaction with O2
(Table 1).
Epoxidation using IBA/O2 has been
reported to proceed via a free radical chain
mechanism.[9f ] We examined the presence
of radical species using hydroquinone as
a radical scavenger (Table S1, Supporting
Information). Addition of hydroquinone to a
reaction after 4 h resulted in the inhibition
of the epoxidation reaction and simultaneous
inhibition of the production of trans-stilbene
oxide (Figure 3;
᭿
symbols). More dramati-
cally, when hydroquinone was present in
the reaction system from the beginning, the
epoxidation reaction did not occur at all (
The sharp difference in the results obtained
in the presence ( ) and absence (•) of hydro-
᭿
).
᭺
quinone suggests that the reaction involves
radical species. The nature of the activa-
tion process is not yet fully clear but may
be related to the high-valent metal-oxo por-
phyrin complex, as suggested by the charac-
teristic green color observed for the catalyst
during the reaction. These features are
similar to the system catalyzed by a mono-
meric metalloporphyrin.[9e]
Based on the above findings, we next
examined the scope of substrates for the
epoxidation reaction. We chose three classes
of olefins as representatives: terminal ole-
fins 1-octene and 1-heptene; internal olefins
2-octene; and 2-norborene and cyclic olefins
cycloheptene, cyclooctene, and cyclododecene
(Table 2). Even in the case of the terminal ole-
fins, 1-octene and 1-heptene, which require
relatively high activation energies in the epox-
idation when compared with internal olefins,
FeP-CMP also displayed good catalytic per-
formance with conversion of about 60% and
nearly 100% selectivity after 24 h. The con-
versions of the internal olefins were much
better. For example, 2-norborene was nearly
quantitatively transformed to its epoxide
after 20 h. Similar good results were also
obtained for cyclic olefins with conversions
up to 99%. Notably, for all of these olefins,
FeP-CMP displayed excellent selectivity with
the highest value greater than 99%. These
results are superior to those of the recently
reported excellent epoxidation catalyst of Ru-
loaded SiO2 particles (40–90% selectivity) and other catalysts
using IBA/O2.[9b−e] We then investigated the epoxidation of
the aromatic terminal olefin styrene. Styrene was transformed
to styrene oxide with a conversion of 55% (TON = 550) and a
selectivity of 69% after 24 h. This result is again superior to that
of the Ru-loaded SiO2 particles under similar conditions (e.g.,
TON = 25, selectivity = 38% after 48 h).[9f ] All of these results
indicate that FeP-CMP is widely applicable for catalyzing the
Figure 1. a–f) FE-SEM images and g,h) HR-TEM images of FeP-CMP.
catalyst in the epoxidation. FeP-LP exhibited a similar low activity
to that of the monomeric metalloporphyrin complex. There-
fore, the porous framework again outperforms the nonporous
metalloporphyrin polymer analogues. This control experiment
supports the importance of porous structure in achieving high
catalytic activity. Aerobic epoxidation under an ambient air
atmosphere is very versatile. Thus, we replaced O2 with air in
the epoxidation reaction. Remarkably, the reaction proceeded
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Figure 2. Simulated structures of typical molecular frameworks. a) Close tetragon and b) eight-membered porphyrin framework simulated using hybrid
DFT at the B3LYP level and with the 3–21G basis set using Gaussian 03.
(1270 m2 g−1 ). Leaching experiments on the reused FeP-CMP
using an inductive coupled plasma (ICP) atomic emission spec-
trometer showed that the Fe content (3.27 wt%) was the same
as that present in the original catalyst (3.27 wt%). In addition,
upon centrifugation of the reaction mixtures, the supernatants
were monitored by electronic absorption spectroscopy and no
traces of metalloporphyrin or related decomposition products
were observed.
aerobic epoxidation of various olefins and features high effi-
ciency and excellent selectivity.
FeP-CMP is an excellent reusable catalyst. Simple centrifu-
gation allowed the separation of FeP-CMP from the reaction
solution. We collected the FeP-CMP catalyst after the reaction
and reused it in the next round of epoxidation reaction. Due to
the covalently crosslinked framework, FeP-CMP maintained its
catalytic skeleton structure and showed robust recycling capa-
bility with well-retained activity and selectivity after six cycles
(Table 3). FeP-CMP after six cycles displayed a BET surface
area of 1255 m2 g−1 , very close to that of the pristine sample
Based on above results, we investigated the capability of FeP-
CMP for large-scale synthesis. Using 0.41 mol trans-stilbene
in the presence of 6 × 10−9 mol [Fe], the epoxidation reaction
T a b le 1 . Catalytic epoxidation of trans-stilbene by FeP-CMP under different conditions.a)
FeP-CMP
CHO
O
+
IBA, 1 atm O2
CH2Cl2, 298 K
1
2
Entry
Catalyst
Surface area
Time
[h]
Conversion
[%]
Selectivity [%]
TON
[m2 g−1
]
1
2
3
1a)
2b)
3c)
FeP-CMP
–
1270
–
9
98
30
n.r
n.r
46
96
90
96
91
85
86
97
92
–
980
300
–
24
24
24
24
17
24
15
12
20
20
8
FeP-CMP
FeP-CMP
FeP-CMP
FeP-CMP
FeP-CMP
FeP-CMP
FeP-CMP
TPPFeCl
FeP-LP
1270
1270
1270
1270
1270
531
233
–
–
4d)
5e)
6f)
–
–
–
91
96
95
97
96
82
81
9
460
960
900
970
940
850
860
4
7g)
8a)
9a)
10a)
11a)
5
3
4
18
19
–
a)FeP-CMP (5.86 × 10−7 mol Fe), solvent (3 mL), 298 K, O2 (1 atm). Fe:trans-stilbene:IBA = 1:1000:3000 (molar ratio); b)in the absence of catalyst; c)under N2 (1 atm); d)in
the absence of IBA; e)Fe:trans-stilbene:IBA = 1:1000:1000 (molar ratio), f)Fe:trans-stilbene:IBA = 1:1000:2000 (molar ratio); g)in air (1 atm).
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T a b le 3 . Recycle of FeP-CMP for catalytic epoxidation of trans-stilbene.
100
FeP-CMP
CHO
O
+
IBA, 1 atm O2
CH2Cl2, 298 K
80
60
40
20
0
1
2
Run
Time
[h]
Conversion
[%]
Selectivity [%]
1
96
96
97
96
97
97
2
1
2
3
4
5
6
6
6
6
6
6
6
99
97
98
99
98
98
4
4
3
4
3
3
activity, excellent selectivity, broad substrate applicability, good
reusability, and enzyme-like large TON and TOF values. The
porous framework outperforms the monomeric metallopor-
phyrin and nonporous metalloporphyrin polymer analogues.
Our strategy also highlights a new approach for the design of
heterogeneous catalysts with built-in catalytic skeletons and
inherent nanopores; these structural features together with the
excellent catalytic performance were not accomplished by other
heterogeneous and homogeneous catalysts reported so far.
0
2
4
6
8
10
Reaction Time / h
Figure 3. Effect of hydroquinone on epoxidation of trans-stilbene cata-
lyzed by FeP-CMP. •: no hydroquinone; : hydroquinone added after 4- h
reaction; : hydroquinone present from the beginning)
᭿
᭺
yielded 78 g (0.4 mol) of trans-stilbene oxide after 120 h. There-
fore, the TON and TOF of FeP-CMP were extremely high at 6.7 ×
107 and 9.3 × 103 min−1 , respectively. These values are close to
these of the enzyme cytochrome P450. All of the above excellent
catalytic features mark FeP-CMP as a top-class catalyst.
In conclusion, we have developed a nanoporous metallopor-
phyrin framework for the activation of molecular oxygen and
demonstrated its utility in highly efficient aerobic epoxidation
of olefins under ambient conditions. FeP-CMP achieves high
Experimental Section
The epoxidation of olefins was carried out in 25-mL pyrex glass reactor
equipped with a mechanical stirrer. In a typical protocol, FeP-CMP
(1 mg, 0.586 μmol; the same sample as reported previously[5b]),
dichloromethane (3 mL), olefins (58.6 μmol), and IBA (175.8 μmol)
were added to the reactor and the resulting mixture was stirred under
an atmosphere of oxygen at 298 K. The solution was periodically
analyzed by gas chromotography mass spectrometry (GC-MS; Shimadzu
GC-QP2010plus) at appropriate intervals. All products were identified
by the comparison of GC retention times and mass spectra with
those of the authentic samples. Selectivity, conversion, TON, and TOF
were calculated using the following equations: selectivity = (produced
epoxide)/(consumed olefin) × 100; conversion = (consumed olefin)/
(initial olefin) × 100; TON = (consumed olefin)/(amount of Fe); and
TOF = TON/(reaction time).
For recycling experiments, the reaction mixture was centrifuged for
30 min after the reaction and the liquid layer was siphoned out. The
residual solid was washed with anhydrous dichloromethane and
centrifuged twice. Dichloromethane, trans-stilbene, and IBA were then
added to the reactor for the next reaction, which was run under otherwise
identical conditions.
T a b le 2 . Catalytic epoxidation of various olefins by FeP-CMP. Condi-
tions: 1 mg catalyst (5.86 × 10−7 mol Fe), Fe:olefin:IBA = 1:1000:3000
(molar ratio), CH2Cl2 (3 mL), 298 K, 1 atm O2.
Entry
Substrate
Time Conversion
Selectivity[%]
epoxide others
>99 <1
TON
[h]
[%]
12
13
14
15
24
24
24
20
62
58
76
99
620
580
>99
>99
>99
<1
<1
<1
760
>990
16
17
24
20
95
95
950
580
>99
<1
Supporting Information
97
3
Supporting Information is available from the Wiley Online Library or
from the author.
18
19
20
24
99
55
>99
<1
>990
Acknowledgements
69
31
550
L.C. and Y.Y. contributed equally to this work. This work was supported by
Precursory Research for Embryonic Science and Technology (PRESTO),
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Japan Science and Technology Agency (JST) (D.J.). D.J. thanks the
JSPS Asian core program and a Grant-in-Aid for Scientific Research (B)
(20350059) from MEXT (Ministry of Education, Culture, Sports, Science
and Technology, Japan). The computations were performed using the
Research Center for Computational Science, Okazaki, Japan.
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