A two-dimensional, hydrogen-bond-cross-linked molybdenum(VI) network polymer with catalytic activity

Article abstract of DOI:10.1002/ejic.200601054

The new hybrid inorganic-organic polymer [MoO₂Cl ₂(H₂O)₂]·(H₂dipy-pra)Cl ₂ (1), where dipy-pra = 1,3-bis(4-pyridyl)propane, has been synthesized and crystallographically characterized. MoO₂Cl ₂(H₂O)₂ and the [H₂dipy-pra] ²⁺ cation are cross-linked by Mo-H₂O...O and H ₂dipy-pra...Cl hydrogen bonds to form a two-dimensional layer structure. Complex 1 is an efficient catalyst, with H₂O₂ as the oxygen-source oxidant and NaHCO₃ as the cocatalyst, in the epoxidation of olefinic compounds under ambient conditions. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200601054

SHORT COMMUNICATION
DOI: 10.1002/ejic.200601054
A Two-Dimensional, Hydrogen-Bond-Cross-Linked Molybdenum(VI) Network
Polymer with Catalytic Activity
[a]
[b]
[c]
[b]
Yi Luan, Ge Wang,* Rudy L. Luck, and Mu Yang
Keywords: Hydrogen bonds / Crystal engineering / Hydrogen peroxide / Epoxidation / Molybdenum
The new hybrid inorganic–organic polymer [MoO
dipy-pra)Cl (1), where dipy-pra = 1,3-bis(4-pyridyl)pro-
pane, has been synthesized and crystallographically charac-
2
Cl
2
(H
2
O)
2
]·
is an efficient catalyst, with H
2 2
O as the oxygen-source oxi-
(
H
2
2
dant and NaHCO as the cocatalyst, in the epoxidation of
3
olefinic compounds under ambient conditions.
2
+
terized. MoO
cross-linked by Mo–H
bonds to form a two-dimensional layer structure. Complex 1
2
Cl
2
(H
2
O)
2
and the [H
2
dipy-pra] cation are
2
O···Cl and H
2
dipy-pra···Cl hydrogen
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
The design and synthesis of metal–organic coordination epoxidation of olefins with tert-butyl hydroperoxide
polymers that is based on the strategy of crystal engineering (TBHP).[
15,16]
From both an economic and an environmen-
is an emerging area, and a variety of novel supramolecular tal viewpoint, there is a definite need for catalytic oxi-
architectures have been obtained. Moreover, coordination dations employing hydrogen peroxide as the stoichiometric
[
1,2]
polymers are used in areas such as magnetism,
tronics, guest exchangeability,
optoelec- oxidant, because hydrogen peroxide is environmentally
[
3]
[4,5]
[6]
[17]
and catalysis. The con- friendly, cheap, and readily available.
To the best of our
cept of crystal engineering was first introduced as an ap- knowledge, there is no report regarding efficient epoxida-
proach to more efficient topochemical reactions in the early tions with H O by using dioxidomolybdenum(VI) halide
2
2
[
7]
1
960s. However, the synthesis of coordination polymers complexes. In the present work, the synthesis, crystal struc-
following the principles of crystal engineering was not ture, and characterization of [MoO Cl (H O) ]·(H dipy-
2
2
2
2
2
achieved until 1995.[
8,9]
Several recent reviews have de- pra)Cl is reported, as well as the potential of this complex
2
scribed how coordination polymers ranging from one-di- as an epoxidation catalyst with hydrogen peroxide as the
mensional to three dimensional can be attributed to the ra- oxidant.
pid development of crystal engineering research.[
10,11]
Mo-
The key to our approach was the utilization of aromatic
lybdenum- and tungsten-based supramolecular networks amines such as 1,3-bis(4-pyridyl)propane. It is known that
have also been investigated.[ Zubieta described a molyb- under more acidic reaction conditions protonation of the
denum-based network that employed protonated 4,4Ј-bipyr- aromatic amine provides an aromatic ammonium cation.
12]
[
13]
idine as a ligand serving to buttress the inorganic layers.
Thus, the aromatic ammonium cations of a linear bidentate
In light of recent successes in the construction of potentially ligand can serve as a structure-directing template by virtue
useful coordination polymers, we wanted to utilize a biden- of multipoint hydrogen bonding between the cation and the
tate ligand and a dioxidomolybdenum(VI) center to yield metal complex to form a new hydrogen-bond network
this type of metal–organic hydrogen-bond network by crys- structure. Compound 1 was synthesized by the reaction of
[
18]
tal engineering.
MoO Cl (H O) and protonated dipy-pra. Single-crystal
2 2 2 2
Organomolybdenum reagents have been used in a variety X-ray diffraction studies on the sample obtained from the
of oxidation reactions such as epoxidation and the oxi- reaction revealed its composition, [MoO Cl (H O) ]·
2
2
2
2
dation of alcohols and sulfides. In the past few years, diox- (H dipy-pra)Cl , and an extended structure composed of
2
2
idomolybdenum(VI) halide complexes have been found to the building units shown in Figure 1.
be efficient epoxidation catalysts.[ We reported a series of
14]
The supramolecular structure of complex 1 is shown in
dioxidomolybdenum(VI) halide complexes that catalyze the Figure 2. Each dipy-pra molecule is linked by H-bonds to
a H O molecule of two uncharged MoO Cl (H O) units
2
2
2
2
2
[
[
a] Beijing University of Chemical Technology,
Beijing 100029, P. R. China
through external chloride anions. The two-dimensional
polymeric structure was formed by H-bonded linkages
(O2W···Cl4 = 3.087, N2···Cl4 = 3.100, O1W···Cl4 = 3.059,
O2W···Cl3 = 3.125, O1W···Cl3 = 3.047, N1···Cl3 =
b] School of Materials Science and Engineering, University of
Science and Technology Beijing,
Beijing 100083, P. R. China
E-mail: gewang@mater.ustb.edu.cn
3.038 Å). The Mo–O(oxido) and Mo–Cl distances, and the
[
c] Department of Chemistry, Michigan Technological University,
Houghton, MI 49931, USA
cis-MoO(oxido) and trans-MoCl angles, are comparable
2
2
Eur. J. Inorg. Chem. 2007, 1215–1218
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1215

Yi Luan, Ge Wang, R. L. Luck, Mu Yang
SHORT COMMUNICATION
lengths would appear to be related to the trans effect of the
oxido ligand.
The catalytic property of compound 1 was then studied.
With complex 1 as the catalyst, NaHCO as the cocatalyst,
3
and H O as the oxygen source, we were able to catalyze
2
2
the epoxidation of olefinic compounds. Cyclooctene was se-
lected as a standard substrate for the optimization process
with H O in acetonitrile. A good yield (99%) of the epox-
2
2
ide was achieved with a catalytic amount of 1 (1 mol-%)
Figure 1. The building-block unit present in crystalline 1. Thermal and NaHCO
as cocatalyst (25 mol-%) after 1 h at room
3
ellipsoids are drawn at the 50% probability level. Selected bond
temperature (25 °C).
lengths [Å] and angles [°]: Mo(1)–O(2) 1.671(3), Mo(1)–O(1)
.683(3), Mo(1)–O(1W) 2.217(3), Mo(1)–O(2W) 2.244(3), Mo(1)–
Cl(1) 2.3739(13), Mo(1)–Cl(2) 2.3754(15), O(2)–Mo(1)–O(1)
To evaluate the scope of this procedure, the oxidation of
other alkenes was also studied (Table 1). As shown, carbo-
1
1
1
05.12(16), O(1W)–Mo(1)–O(2W) 75.33(12), Cl(1)–Mo(1)–Cl(2) cyclic alkenes were oxidized to produce the corresponding
62.13(6).
epoxy compounds in high yields (entries 1 and 2, Table 1).
However, oxidation of the aromatic and linear alkenes re-
quired longer reaction times than that of the carbocyclic
alkenes (entries 3 and 4, Table 1), especially 1-octene. Simi-
to those reported for other related MoO Cl L com-
plexes. There are slight differences in the Mo–O(W) dis-
2
2 2
[
19]
tances, and the shorter distance, i.e. Mo–O(1W) = larly, the allylic alcohols underwent oxidation without af-
.217(3) Å, correlates with a smaller O(1W)–Mo–O(2) an- fecting the hydroxy group (entry 5, Table 1). Kuhn reported
gle of 164.07(14)° and the longer Mo–O(2W) distance of that dioxidomolybdenum derivatives are able to catalyze the
2
2
1
.244(3) Å with
66.08(14)°. This significant difference in Mo–O(W) bond H O .
a
larger O(2W)–Mo–O(1) angle of olefin epoxidation reaction with TBHP but not with
[
14]
2
2
Figure 2. The two-dimensional layer structure of complex 1. For clarity, part of the H atoms have been omitted.
216 Eur. J. Inorg. Chem. 2007, 1215–1218
1
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A Molybdenum(VI) Network Polymer with Catalytic Activity
Table 1. Epoxidation of selected olefins catalyzed by molybde- for the conversion of a variety of alkenes to their respective
SHORT COMMUNICATION
2 2
num(VI) complex 1, with H O as oxidant and hydrogencarbonate
epoxides.
[
a]
as cocatalyst.
Acknowledgments
We thank the NSFC (Grant no. 2220406003) and the Program for
New Century Excellent Talents in University (NCET-05–0103) for
support. We also thank Dr. Zhan Shi at Jilin University for the
determination of the X-ray structure.
[
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[
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[
(
a] Reaction conditions: the substrate (10 mmol), catalyst 1
0.1 mmol, 1 mol-%), NaHCO (2.5 mmol, 25 mol-%) and 30%
aqueous H (40 mmol) dissolved in acetonitrile (10 mL) at 25 °C.
b] Determined by GC by using an internal standard technique. [c]
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3
1996, 35, 4449–4461.
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peratures. In the epoxidation of cyclic alkenes (such as cy-
clohexene) at 50 °C, 2,3-epoxycyclohexanone and 2-cy- [10] K. Biradha, M. Sarkar, L. Rajput, Chem. Commun. 2006,
clohexen-1-one were obtained in addition to the desired ep-
oxide product. When the reaction was carried out at reflux
temperature, i.e. 75 °C, the efficiency was significantly re-
duced. To confirm whether this reduction in efficiency was
4169–4179.
[
11] N. W. Ockwig, O. Delgado-Friedrichs, M. O’Keeffe, O. M.
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[
due to the conversion of NaHCO (which acts as a cocata- [13] P. J. Zapf, R. C. Haushalter, J. Zubieta, Chem. Commun. 1997,
3
321–322.
lyst) into Na CO , NaHCO was replaced by Na CO ; this
2
3
3
2
3
[
20]
[14] F. E. Kuhn, A. M. Santos, A. D. Lopes, I. S. Goncalves, E.
resulted in almost no oxidation products, even after 24 h.
Herdtweck, C. C. Ramao, J. Mol. Catal. A 2000, 164, 25–38.
15] G. Wang, G. Chen, R. L. Luck, Z. Q. Wang, Z. C. Mu, D. G.
Besides our system, Burgess and Bhattacharyya demon-
strated that MnSO and [MoO(O ) (saloxH)] can catalyze
[
4
2 2
Evans, X. Duan, Inorg. Chim. Acta 2004, 357, 3223–3229.
[
21,22]
epoxidation reactions efficiently with NaHCO /H O .
[16] G. Wang, L. S. Feng, R. L. Luck, D. G. Evans, Z. Q. Wang, X.
3
2
2
Duan, J. Mol. Catal. A 2005, 241, 8–14.
17] R. Noyori, M. Aoki, K. Sato, Chem. Commun. 2003, 1977–
The epoxidation of alkenes in the presence of hydrogencar-
[
bonate alone was also investigated.[
20,23]
Interestingly, the
1
986.
use of complex 1 or hydrogencarbonate alone gave much
[
18] Experimental details: MoO
3
(0.826 g, 5.74 mmol) was dis-
lower yields of epoxides than when they were used to-
solved in excess concentrated HCl by stirring at 40 °C for 4 h;
a pale yellow solution was obtained. The solution was cooled
in an ice bath, and a solution of a dipy-pra (1.138 g,
[
21,22]
gether.
Richardson showed that a key aspect of such
reactions is that hydrogen peroxide and hydrogencarbonate
combine in an equilibrium process to produce peroxymono-
5
.74 mmol) in ethanol (10 mL) was then added in portions. Af-
ter stirring for 6 h, a yellow solution was obtained, which when
left overnight at ca. 4 °C afforded the lemon yellow crystalline
complex 1 in 1.48 g, 51% yield. The characterization results
are as follows: C13
H 3.98, Mo 18.96, N 5.54, O
–
[24,25]
–
carbonate, HCO .
HCO4 may be more reactive than
4
hydrogen peroxide and can be expected to attack the metal
center or other more potent, activated groups at a much
H
20Cl
4
MoN
2
O
2
4
(506.66): calcd. C 30.85,
12.65; found C 30.71,
2
–
[
21]
faster rate. The mechanism for our epoxidation reaction
is under investigation.
2–
H 4.11, Mo 18.91, N 5.46, O
bands): 952, 936 (MoO , asym.), 906 (MoO
Crystal data for 1 at 293 K: C13 Cl Mo, M = 506.05,
plate crystal
8.6398(17),
2
12.87. IR (KBr disc; selected
–1
2
2
, sym.) cm .
In conclusion, a novel 2D network material with the ge-
neral formula [MoO Cl (H O) ]·(dipy-praH )Cl has been
H
P1,
20
N
2
O
4
4
¯
triclinic,
space
group
yellow
=
2
2
2
2
2
2
(
0.32 mmϫ0.28 mmϫ0.24 mm),
a
b
=
synthesized and successfully applied to epoxidation reac-
8
8
.6481(17), c = 13.944(3) Å, α = 85.33(3), β = 83.03(3), γ =
tions. Our current observations suggest that, under acidic
3
3
2.44(3)°, V = 1022.9(4) Å , Z = 2, Dcalcd. = 1.643 Mg/m , µ
2
+
–1
conditions, the protonated [H dipy-pra] cation serves as
= 1.182 mm , F(000)= 508, 10144 reflections measured, 4632
unique. CCDC-614792 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
2
a structure-directing template by using multipoint hydrogen
bonding in the formation of infinite frameworks. The epox-
idation of alkenes was studied with hydrogen peroxide in
the presence of complex 1 to give products in high yields.
This reaction provides a new environmentally friendly route
[
19] M. J. Taylor, J. R. Wang, C. E. F. Rickard, Polyhedron 1993, 12,
1433–1435.
Eur. J. Inorg. Chem. 2007, 1215–1218
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1217

Yi Luan, Ge Wang, R. L. Luck, Mu Yang
20] H. R. Yao, D. E. Richardson, J. Am. Chem. Soc. 2000, 122, [24] J. Flanagan, D. P. Jones, W. P. Griffith, A. C. Skapski, A. P.
SHORT COMMUNICATION
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Received: November 10, 2006
Published Online: February 14, 2007
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1218
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 1215–1218