A metalloporphyrin functionalized metal-organic framework for selective oxidization of styrene

Article abstract of DOI:10.1039/c1cc10461f

A functional metal-organic framework assembled from palladium-porphyrin building blocks and cadmium(ii) connecting nodes presents interesting topological network structure, high framework stability and interesting catalytic property for the selective oxidation of styrene.

Full text of DOI:10.1039/c1cc10461f

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COMMUNICATION
A metalloporphyrin functionalized metal–organic framework for selective
oxidization of styrenew
Ming-Hua Xie, Xiu-Li Yang and Chuan-De Wu*
Received 24th January 2011, Accepted 15th March 2011
DOI: 10.1039/c1cc10461f
A functional metal–organic framework assembled from palladium-
porphyrin building blocks and cadmium(II) connecting nodes
presents interesting topological network structure, high framework
stability and interesting catalytic property for the selective oxidation
of styrene.
of [Cd1.25(Pd–H1.5TCPP)(H O)]ꢀ2DMF (1), which showed
2
significant catalytic activity for styrene oxidation.
Dark brown crystals of 1 were synthesized by heating
a mixture of Pd–H
4
TCPP and Cd(NO
3
)
2
2
ꢀ4H O in a mixed
solvent of DMF, MeOH and acetic acid at 80 1C for ten days.z
Single crystal X-ray structural analysis has revealed that
there are two crystallographically independent cadmium(II)
atoms, one partially protonized ligand, one aqua ligand and
two DMF guest molecules in the asymmetric unit.y The first
Cd atom coordinates to eight carboxyl oxygen atoms of
four Pd-TCPP carboxylate groups, while the second partially
occupied Cd atom is surrounded by four carboxyl groups of
four Pd-TCPP ligands and two water molecules. Because the
second Cd atom is only quarterly occupied, the Cd atoms are
bridged by the carboxylate groups to extend into a pseudo
linear network. Alternatively, each Pd-TCPP ligand acts as an
octadentate ligand to coordinate eight Cd atoms of four
neighboring Cd chains to propagate into a 3D framework
structure (Fig. 1). The solid-state framework contains
Compared with traditional inorganic zeolites, the functionalities
of metal–organic frameworks (MOFs) have the advantage that
they can be easily modified by tailoring the organic ligands
and selecting the metal nodes, which can be subsequently
1
–3
realized into various applications in diverse fields. Because
metalloporphyrin molecules possess unique biological and
chemical functionalities, such as structural robustness, catalysis,
charge and energy transformation, the interrelated researches
4
have attracted considerable interest in diverse fields. There
are many metal–organic coordination networks assembled
from porphyrin or metalloporphyrin building blocks, which
present interesting topological network structures, great
5
–7
thermal and chemical stabilities.
However, limited works
2
˚
were focused on the investigation of the catalytic properties of
7
these interesting materials.
two kinds of channels with dimensions of 4.61 ꢁ 12.55 A and
2
˚
8.27 ꢁ 9.32 A (considering the van der Waals diameters) along
the a axis. The channels are filled with DMF solvent molecules.
During the course of our studies on the synthesis of
functional MOFs for catalytic applications, we think that
incorporating metalloporphyrin building blocks in porous
MOFs might generate some interesting properties for hetero-
geneously catalytic applications. Because the active metal
sites are fixed in the rigid porphyrin cores, the catalytic active
sites on the channel walls of MOFs should be accessible to
the included substrates. Most interestingly, the catalytic
functionalities can be simply tailored by adjusting the active
metal sites and the peripheral environments of the porphyrin
cores. Inspired by the relevant functionalities, we have
recently synthesized a series of robust functional porous
MOFs based on 5,10,15,20-tetra(carboxyphenyl)metal-
4
porphyrin (M-H TCPP) ligand. Herein, we report the synthesis
II
and characterizations of a novel Pd -porphyrin-based solid
Department of Chemistry, Zhejiang University, Hangzhou, 310027,
P. R. China. E-mail: cdwu@zju.edu.cn; Fax: +86-571-87951895;
Tel: +86-571-87953938
w Electronic supplementary information (ESI) available: Experimental
procedures, additional figures and crystallographic data. CCDC
Fig. 1 (a) A view of the 3D framework of 1 down the 1 1 0 direction,
showing the arrangement of the palladium-porphyrins. (b) The 3D
framework of 1 as viewed down the a axis, showing the 1D opening
7
15005 (1), 777026 (1a), 777027 (1b) and 806276 (1c). For ESI and
crystallographic data in CIF or other electronic format see DOI:
0.1039/c1cc10461f
II
channels and the accessible Pd sites. (c) Side view of the 1D channel
1
in the porous framework of 1.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5521–5523 5521

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a
Table 1 Selective oxidation of styrene catalyzed by solid 1
patterns are similar to that of the freshly prepared sample.
To eliminate the ambiguity of the solvent identities from X-ray
diffraction studies, we have monitored the solvent exclusion
1
processes by H NMR spectroscopy (supporting information
Fig. S6w). With a benzene internal standard and CDCl as the
3
c
solvent, we determined that a freshly prepared sample of 1
Select. (%)
contains two DMF molecules per formular unit. After a
1
sample of 1 was treated under vacuum at 90 1C for 12 h, H
b
c
Entry
Acid
Conv. (%)
I
II
1
2
3
4
5
6
7
8
9
1
0
100
0
75
100
0
91
0
82
88
0
52
86
95
78
0
49
57
89
9
0
NMR spectroscopy has showed that the DMF molecules are
almost fully evaporated, which confirmed the solvent free
framework of 1a. The above experiments indicate that the
porous frameworks were maintained after the removal or
absorption of the solvent molecules, which led to the solids
with permanent porosity and framework integrity. These
results suggest that the framework structure of 1 can be
maintained during potential heterogeneous catalytic reactions.
We are aware that the porous network of 1 comprises the
palladium sites in the channel walls along with the tailored
functional zeolitic periphery. Recent studies have shown
that such kind of metal sites within porous MOFs present
0.5
1.5
1 (HOAc)
1 (HCl)
1 (HNO
2 4
1 (H SO )
1
1
1
1
1
18
12
0
47
14
5
5
3
)
80
90
90
0
35
100
100
d
22
e
1
1
1
1
0
1
2
3
0
51
f
g
7
11
h
a
Solid 1 (0.01 mmol), styrene (0.2 mmol), HClO
(30%, 0.6 mmol) in CH CN (6 mL) were stirred at 55 1C for
2 h. Acid/catalyst ratio (the acids other than perchloric acid are
4
(70%, 0.01 mmol)
and H
2
b
O
2
3
7,9
interesting catalytic activities. Since the oxidation of styrene
1
c
has attracted considerable interest for the academic research
1
shown in the parentheses). Conversion% and selectivity% were
d
determined by GC on a SE-54 column. Catalyzed by Pd–H
0,11
and utilization in the industry,
we have evaluated the
4
TCPP.
e
f
g
Catalyzed by Pd/C. Catalyzed by Pd(OAc)
Catalyzed by PdCl
2
.
The additional product is benzoic acid (36%). The sixth cycle.
2
.
accessibility of the open channels in 1 to styrene molecules
by submerging the crystals of the evacuated sample of 1a
in styrene for 12 h. The resultant crystals of 1c can tolerate
such treatment without losing the crystallinity as confirmed by
PXRD. The single crystal X-ray analysis showed that the unit-
cell dimensions of the resultant crystals are not significantly
changed, and the styrene molecules are enclosed in the channels
h
It is worth noting that the porphyrin unit in 1 adopts a coplane
˚
conformation with the mean deviation of 0.0275 A, while the
palladium atom is fully coplane with four pyrrole nitrogen
atoms. In the crystal structure, all of the porphyrin units are
packed along two orientations with the dihedral angle of 95.31.
1
of 1c.y H NMR spectroscopy showed that the formular unit
of 1c contains two styrene molecules, as determined by H
3
1
˚
PLATON calculations indicate that 1 contains 38.9% (2218.5 A
per unit cell) void space which is accessible to the solvent
8
molecules. Thermogravimetric analysis showed a weight loss
NMR integrations with a methanol internal standard and
CDCl as the solvent.
3
of 13.8% occurred between 25 and 310 1C, corresponding to the
loss of DMF molecules and aqua ligands (expected 13.7%).
Whereas the catalytic application of MOFs requires that
the framework structures need to be maintained without
solvent guests or exchanged with other molecules, we therefore
examined the framework stability of 1 during solvent removal
and inclusion processes. After a sample of 1 was treated under
vacuum at 90 1C for 12 h, a PXRD of the resultant crystalline
solid showed a sharp diffraction pattern that is similar to that
of the pristine sample (supporting information Fig. S5w). This
result indicates that the porous framework was maintained
after the removal of the solvent molecules. Furthermore, the
crystals of the evacuated sample did not change in the
morphology, size and transparency that were observed under
a microscope. Single crystal X-ray analysis of 1a in a sealed
capillary showed that the robust solvent free framework
Recently, the palladium-catalyzed selective oxidation of
styrene to acetophenone has attracted considerable interest
because the practical importance of acetophenone and the
1
high selectivity of the palladium-catalysts. The styrene
1
oxidation by solid 1 was conveniently performed in CH CN
3
using H O as oxidant in the present of HClO with GC-MS
2
2
4
monitored through out the reaction. When the reaction was
performed at 55 1C for 12 h, styrene was completely oxidized
into a mixture of 91% acetophenone and 9% benzaldehyde
(Table 1, entry 1). The catalytic results are very sensitive to the
quantity and acidity of the additive acids (Table 1, entries
2–8). Addition of solid 1 alone, there is no detectable product
(Table 1, entry 2). When the additive quantity of HClO was
4
increased, the styrene conversion increases gradually. How-
ever, the acetophenone selectivity is depressed concomitantly
(Table 1, entries 3 and 4). To understand the different acid
additive effect on the catalytic transformation, we have at-
tempted the reactions using a variety of acids instead of
perchloric acid (Table 1, entries 5–8). HOAc did not give
any help for the catalytic transformation, while HCl prompted
the transformation less efficiently. Despite the acetophenone
selectivities of HNO and H SO which are comparable to that
6
presents almost identical structural features as the original.
Interestingly, upon exposure of the solvent free framework 1a
to the vapor of water for one day or in air for one week, 1a can
adsorb water molecules as guests enclosed in the channels to
form hydrated solid 1b, which was characterized by single
crystal X-ray analysis. The framework of 1b is essentially
identical to that of 1. The only difference between 1 and 1b
lies in the inclusion of different solvent molecules. PXRD of
the bulk sample of 1b showed that the sharp diffraction
3
2
4
of HClO , the styrene substrate cannot be oxidized completely.
4
These results indicate that the acids and their acidities play
critical roles for the styrene oxidation.
5
522 Chem. Commun., 2011, 47, 5521–5523
This journal is c The Royal Society of Chemistry 2011

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To make a comparison, a series of control catalysts were used
to conduct the catalytic experiments. Pd–H TCPP is less efficient
I
C
>
2s(I) with 275 parameters. Crystal data for 1b:
48 4 r
H47.5Cd1.25N O19Pd, M = 1231.30, monoclinic, space group C2/c,
4
˚
˚
˚
a = 7.1564(9) A, b = 25.080(4) A, c = 31.192(3) A, b = 96.29(1)1, V =
to prompt the catalytic transformation, while PdCl2 cannot
prompt the catalytic transformation (Table 1, entries 9 and 10).
Moreover, when Pd/C was used as catalyst, only a small amount
of styrene was oxidized with very low acetophenone selectivity
3
ꢂ3
,
˚
5
564.7(12) A , Z = 4, T = 293(2), Rint = 0.0783, D
c
= 1.470 g cm
m = 7.036 mm , F(000) = 2486, GOF = 0.788, 8511 reflections
measured, 3941 unique. The final R = 0.0938, wR = 0.2092 for
638 observed reflections with I > 2s(I) with 284 parameters. Crystal
data for 1c: C64 8.5Pd, M = 1248.91, monoclinic, space
ꢂ1
1
2
1
H
41Cd1.25
4
N O
r
(
Table 1, entry 11). Despite Pd(OAc)
fully oxidized, the acetophenone selectivity is highly decreased
Table 1, entry 12). These results suggest that the catalytic activity
of 1 is superior to its corresponding components.
After a mixture of solid 1 and CH CN in the presence of
HClO was heated at 55 1C for 12 h under stirring, H and
2
can prompt styrene to be
˚
˚
˚
group C2/c, a = 7.2751(3) A, b = 25.591(2) A, c = 30.775(1) A,
˚
3
b = 96.478(4)1, V = 5692.9(5) A , Z = 4, T = 293(2), R = 0.0417,
int
ꢂ3
ꢂ1
D
c
= 1.457 g cm , m = 6.754 mm , F(000) = 2508, GOF = 1.070,
346 reflections measured, 4037 unique. The final R = 0.0969,
wR = 0.2234 for 2852 observed reflections with I > 2s(I) with
80 parameters.
(
9
1
2
3
2
4
2 2
O
1
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styrene were subsequently added into the hot filtrate, which
was heated at 55 1C for another 12 h. GC result indicates that
the mixture is unreactive. Additionally, H NMR spectro-
1
scopy indicates that no detectable Pd-TCPP ligand was
released into the solution. Catalyst 1 can be simply recovered
by filtration, which was subsequently used in the successive
runs without deteriorating the catalytic activity (Table 1, entry 13).
A PXRD pattern of the recovered solid suggests that the
structural integrity of the catalyst was maintained after the
catalytic experiment (supporting information Fig. S5w). All
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heterogeneous in nature, and especially the catalytic activity
was highly depressed when homogeneous components were
used instead of solid 1.
2
3
˜
and J. R. Long, Angew. Chem.,
´
4
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In summary, we have synthesized a new porous MOF 1,
II
which incorporates the functional bridging ligand with Pd
(
active sites in the porous solid 1 for the catalytic oxidation
reaction. Compound 1 undergoes interesting transformations
from the solvent inclusion to solvent free to solvent inclusion
frameworks. The framework-immobilized solid 1 not only
conferred the advantage of higher stability, but also showed
significant styrene oxidation activity, easier separation and
recyclability for the catalytic application.
1
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(
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This work was financially supported by the NSF of China
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6
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Notes and references
z Yield: 38 mg (37.8%, based on Pd–H
4
TCPP). IR (KBr): n = 1605
7 (a) K. S. Suslick, P. Bhyrappa, J.-H. Chou, M. E. Kosal,
S. Nakagaki, D. W. Smithenry and S. R. Wilson, Acc. Chem.
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˜
(
(
m), 1586 (m), 1541 (m), 1401 (s), 1351 (w), 1177 (w), 1102 (w), 1014
s), 871 (w), 853 (w), 795 (w), 777 (w), 715 (w), 495 (w) cm
ꢂ1
.
6 r
y Crystal data for 1: C54H41.5Cd1.25N O11Pd, M = 1197.33, mono-
clinic, space group C2/c, a = 7.2953(1) A, b = 25.5761(7) A, c =
˚
˚
3
˚
˚
3
R
0.7597(6) A, b = 96.361(2)1, V = 5704.0(2) A , Z = 4, T = 293(2),
ꢂ3
ꢂ1
int = 0.0332, D
c
= 1.394 g cm , m = 6.756 mm , F(000) = 2406,
GOF = 1.024, 15 512 reflections measured, 4054 unique. The final
= 0.0776, wR = 0.1837 for 3395 observed reflections with I >
s(I) with 284 parameters. Crystal data for 1a: C48 Pd,
= 1033.12, monoclinic, space group C2/c, a = 7.217(2) A,
Soc., 2008, 130, 5854.
10 Y. H. Lin, I. D. Williams and P. Li, Appl. Catal., 1997, 50, 221.
R
2
M
1
2
´
11 (a) A. C. Bueno, A. O. de Souza and E. V. Gusevskaya,
H
25.5Cd1.25
N
O
4 8
˚
r
Adv. Synth. Catal., 2009, 351, 2491; (b) C. N. Cornell and
M. S. Sigman, J. Am. Chem. Soc., 2005, 127, 2796;
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2009, 11, 1317; (d) A. L. Miller II and N. B. Bowden, J. Org.
Chem., 2009, 74, 4834.
3
˚
˚
˚
b = 25.613(5) A, c = 30.660(3) A, b = 96.39(1)1, V = 5632.5(17) A ,
ꢂ3
ꢂ1
Z = 4, T = 293(2), Rint = 0.0585, D
F(000) = 2046, GOF = 0.908, 7339 reflections measured, 3942 unique.
The final R = 0.0886, wR = 0.1752 for 1498 observed reflections with
c
= 1.218 gcm , m = 6.713 mm
,
1
2
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5521–5523 5523