Salen-based coordination polymers of manganese and the rare-earth elements: Synthesis and catalytic aerobic epoxidation of olefins

Article abstract of DOI:10.1002/chem.201203636

Treatment of N,N'-bis(4carboxysalicylidene)ethylenediamine (H ₄L), with MnCl₂·(H₂O)₄, and Ln(NO₃)₃·(H₂O)_m (Ln=Nd, Eu, Gd, Dy, Tb), in the presence of N,N-dimethylforma

Full text of DOI:10.1002/chem.201203636

DOI: 10.1002/chem.201203636
Salen-Based Coordination Polymers of Manganese and the Rare-Earth
Elements: Synthesis and Catalytic Aerobic Epoxidation of Olefins
Asamanjoy Bhunia,[a] Meike A. Gotthardt,[b] Munendra Yadav,[a] Michael T. Gamer,[a]
Andreas Eichhçfer,[c] Wolfgang Kleist,*[b, d] and Peter W. Roesky*[a]
Dedicated to Prof. Dr. Hartmut Bꢀrnighausen on the occasion of his 80th birthday
Abstract: Treatment of N,N’-bis(4-
carboxysalicylidene)ethylenediamine
(H4L), with MnCl2·A(H2O)4, and Ln-
(NO3)3·A(H2O)m (Ln=Nd, Eu, Gd, Dy,
Tb), in the presence of N,N-dimethyl-
formamide (DMF)/pyridine at elevated
temperature resulted (after work up) in
the formation of 1D coordination
ordination sites. All compounds were
used as catalysts in the liquid-phase ep-
oxidation of trans-stilbene with molec-
ular oxygen, which resulted in the for-
mation of stilbene oxide. Since the
choice of the lanthanide had virtually
no influence on the performance of the
catalyst, only the manganese–gadolini-
um was studied in detail. The influence
of solvent, catalyst concentration, reac-
tion temperature, oxidant, and oxidant
flow rate on conversion, yield, and se-
lectivity was analyzed. A conversion of
up to 70%, the formation of 61% stil-
bene oxide (88% selectivity), and a
TON of 84 were observed after 24 h. A
hot filtration test confirmed that the re-
action is mainly catalyzed through a
ACHTUNGTRENNUNG
CTHUNGTRENNUNG
A
CHTUNGTRENNUNG
polymers
{[Ln
(MnLCl)
N
heterogeneous pathway, although a
U
minor contribution of homogeneous
species could not be completely exclud-
ed. The catalyst could be reused with-
out significant loss of activity.
nation polymers the rare earth ions are
connected through carboxylate groups
from Mn–salen units in a 1D chain
structure. Thus, the Mn–salen complex
Keywords: coordination polymers ·
heterogeneous catalysis
· lantha-
nides · manganese · oxidation
acts as a “metalloACHTUNGTRENNUNGliACHTUNGTRENNUNGgand” with open co-
Introduction
net-based coordination polymers[2] and the concept of retic-
ular design initiated by the group of Yaghi,[3] infinite coordi-
nation polymers (ICPs) and metal–organic frameworks
(MOFs) have attracted a huge number of researchers.[2e,4] In
particular for MOFs, promising applications[4g,5] such as gas
storage,[6] catalysis,[7] and use as sensors for special classes of
molecules[6a,8] have resulted in a huge number of publica-
tions. In contrast to three dimensional MOFs one dimen-
sional coordination polymers (1D CP) have a much simpler
topological type of coordination array. The ease of forma-
tion of 1D CPs facilitates the incorporation of functional
sites at the metal centers or in the backbone of the organic
linkers.[9] Due to their fascinating structures[10] they have
been employed in the formation of gels, fibers, nanocrystals,
and microspheres.[11] The current progress on 1D CPs has re-
cently been reviewed by Leong and Vittal.[4f]
Coordination oligomers and polymers as well as metal
based polymers have been investigated for quite a long
time.[1] Inspired by the work of Robson and co-workers on
[a] Dr. A. Bhunia, M. Yadav, Dr. M. T. Gamer, Prof. Dr. P. W. Roesky
Institut fꢀr Anorganische Chemie
Karlsruhe Institute of Technology (KIT)
Engesserstrasse 15, D-76131 Karlsruhe (Germany)
Fax : (+49)721-608-44854
E-mail: roesky@kit.edu
[b] M. A. Gotthardt, Dr. W. Kleist
Institute of Chemical Technology and Polymer Chemistry
Karlsruhe Institute of Technology (KIT)
Engesserstrasse 18, 76131 Karlsruhe (Germany)
Fax : (+49)721-608-44820
Usually, rigid organic molecules are used as linkers be-
tween the metal centers in MOFs and ICPs but there are
also some examples in the literature in which organometallic
or coordination compounds are used.[4b,6b,7a,12] In this ap-
proach, the linker (L) is additionally functionalized by an-
E-mail: wolfgang.kleist@kit.edu
[c] Dr. A. Eichhçfer
Institut fꢀr Nanotechnologie
Karlsruhe Institute of Technology (KIT)
Hermann-von-Helmholtz-Platz 1, 76021 Karlsruhe (Germany)
[d] Dr. W. Kleist
other metal (M) forming a “metalloACTHNUTRGENNUGliACHTUGTNRENNUGgand” (ML). The as-
Institute of Catalysis Research and Technology
Karlsruhe Institute of Technology (KIT)
Hermann-von-Helmholtz-Platz 1
sembly of these coordination polymers proceeds mostly in a
two-step process: 1) synthesis of M1 L by reaction of well-de-
fined ligands and metal ions (M1) (mainly 3d metal ions)
and 2) reaction of M1 L with a second type of metal ion
(M2), which act as nodal units in the framework.[13] Popular
76344 Eggenstein-Leopoldshafen (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203636.
1986
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Chem. Eur. J. 2013, 19, 1986 – 1995

FULL PAPER
metalloligands among others are coordination compounds
based on “salen” (N,N’-bis(salicylidene)ethylenediamine) in
which additional functional groups[14] such as carboxyl-
alcohol oxidation and epoxidation reactions)[16a,23] have been
reported as promising target reactions in which MOF-based
catalysts show remarkable activity. We have chosen an epox-
idation as target reaction because the activity of not only
homogeneous Mn–salen complexes, but also of MOFs or
CPs in those reactions is known from the literature.[16a]
The microporous metal–organic framework STA-12(Co),
composed of N,N’-piperazinebis(methylenephosphonate)
linkers and Co2+ ions, is catalytically active due to free coor-
dination sites at the framework metal centers. The epoxida-
tion of stilbene with oxygen was carried out using STA-
12(Co) as catalyst and resulted in nearly complete conver-
sion after 12 h.[23b]
ACHTUNGTRENNUNG
ates,[13a,15] pyridyl groups[16] and benzoate groups[17] are locat-
ed in para or meta position to the OH groups on the aro-
matic ring.
Recently, we used the salen ligand N,N’-bis(4-carboxysali-
cylidene)ethylenediamine (H4L),[18] in which carboxylate
groups are attached in meta position to the OH groups, for
the synthesis of 1D CPs.[19] As a result of the different
stereochemistry compared with the established systems, the
metalloACHTUNGTRENNUNGliACHTUNGTRENNUNGgand is not a rigid linear rod but has a curved
shape conformation, in which the salen complex unit acts as
a flexible strut. Due to the bent conformation and the flexi-
ble nature of the linker, new structures of the resulting coor-
dination polymers were obtained.
Instead of using a complex MOF architecture we aimed to
incorporate a manganese–salen unit into a 1D CP to obtain
an easily accessible architecture. Based on this study we de-
We found that in the presence of a base, metal functional-
cided to examine the catalytic activity of {[Ln
2ACHTUNGTERN(NUNG MnLCl)2-
ized
{[Na4(LM)2]·9H2O}n (M=Ni, Cu) were obtained.[19] Then,
we introduced the salen-nickel complex as “metalloligand”
for the formation of rare earth element-based coordination
polymers {[Ln2A(LNi)3A(dmf)(H2O)3]·4DMF·10H2O}n (Ln=
Er, Lu) and {[Dy(LNi)(dmso)
(NO3)]·2H2O·DMSO}n.[20] In
addition, iron rare earth element coordination polymers
2A 2A 2A
Dy) were synthesized.[21] Besides their unique structures,
some of these compounds also showed interesting magnetic
properties.[20]
two
dimensional
coordination
polymers
U
CHTUNGTRENNUNG
A
A
G
C
ACHTUNGTRENNUNG
A
U
ACHTUNGTRENNUNG
Results and Discussion
{[Ln CHTUNGTRENNUNG(FeLCl) CHTUNGTRENNUNG(NO3) CHTNUTRGENN(GU dmf)5]·4DMF}n (Ln=Y, Eu, Gd, Tb,
Whereas previous investigations have focused on the mag-
netic properties of our compounds, possible catalytic appli-
cations will be the topic of the present contribution. MOFs
and coordination polymers combine advantageous proper-
ties of homogeneous and heterogeneous catalysts. Like ho-
mogeneous catalysts they contain mononuclear and well-de-
fined metal centers as active sites and therefore should fea-
ture high activity. On the other hand, separation by filtration
and reuse should be easily accomplished.
Therefore, we aimed to use manganese-based salen com-
plexes as linkers because it has been well established for
more than two decades that they are efficient homogenous
catalysts for epoxidation reactions of olefins.[22] Nguyen and
Hupp also reported on micro-
up) in the formation of 1D CP {[Ln
(dmf)5]·4DMF}n (Ln=Nd (1), Eu (2), Gd (3), Tb (4), and
Dy (5); Scheme 1).[24]
Compounds 1–5 have been characterized by standard ana-
lytical/spectroscopic techniques and the solid state structures
2ACHTUNGTRENNUG(MnLCl)2ACHTUNGTRENNUNG(NO3)2-
AHCTUNGTRENNUNG
were estabACTHNUTRGENUGlN ished by single-crystal X-ray diffraction (Figure 1
and Figure 2).
The IR spectra of compounds 1–5 show the asymmetric
and symmetric stretching bands of the carboxylate groups at
around 1644, 1476, and 1384 cmÀ1 for 1; 1645, 1476, and
1386 cmÀ1 for 2; 1644, 1464, and 1385 cmÀ1 for 3; 1645,
1472, and 1386 cmÀ1 for 4; and 1646, 1470, and 1404 cmÀ1
for 5. The differences between asymmetric and symmetric
porous metal–organic frame-
work compounds featuring
chiral manganese–salen struts
as highly effective asymmetric
catalysts for olefin epoxidatio-
n.[7a,16a] In this report 2-(tert-bu-
tylsulfonyl)iodosylbenzene was
used as oxidant.
There are already many ex-
amples for the use of metal–or-
ganic frameworks and porous
coordination polymers as cata-
lysts in liquid-phase reac-
tions.[7] Especially, oxidation
reactions (e.g., hydroxylation,
Scheme 1. Synthesis of 1–5 (simplified drawing of 1–5).
Chem. Eur. J. 2013, 19, 1986 – 1995
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W. Kleist, P. W. Roesky et al.
stretching bands indicate that the carboxylate group coordi-
nates to the metal atom in a bridging fashion. The absence
of a characteristic absorption band around 1700 cmÀ1 indi-
cates the complete deprotonation of the carboxyl groups of
the salen ligands[25] and, thus, coordination to the metal ions.
Moreover, characteristic stretching vibration bands (n1–n4)
of the nitrate group are observed (see the Experimental Sec-
tion) in compounds 1–5. The difference in wavenumber be-
tween n1 and n2 is about 200 cmÀ1, indicating that the nitrate
group coordinates to the metal in a bidentate chelating
mode.[25c] Compounds 1–5 are insoluble in common solvents,
thus, no NMR data could be acquired.
The solid state structures of compounds 1–5 were estab-
lished by single-crystal X-ray diffraction (Figure 1 and
Figure 2). Compounds 1–5, which crystallize in the triclinic
Figure 1. Solid-state structure of compound 3; the coordination arrange-
ment of a gadolinium dimer is shown, omitting hydrogen atoms. Com-
pounds 1, 2, 4, and 5 are isostructural.
¯
space group P1, are all isostructural to each other. They are
also similar to the earlier reported iron rare earth element
coordination
polymers
{[Ln2ACHNUTGRTEG(NNUN FeLCl)2ACHUTNRTGE(GNNUN NO3)2ACHTUNGTRENNUNG(dmf)5]·-
4DMF}n (Ln=Y, Eu, Gd, Tb,
Dy).[21] The asymmetric units
of compounds 1–5 consist of
two rare earth ions (Ln1 and
Ln2), two different Mn–salen
units ((Mn1LCl1)
ACHTUNGTRENNUNG
(Mn2LCl2)), two
groups, five coordinating DMF
molecules, and four non-coor-
dinating DMF molecules. In
the solid-state structure, two
neighboring rare earth ions
(Ln1, Ln1’ and Ln2, Ln2’) are
connected through four car-
boxylate groups from four
Mn–salen units (Figure 2, top).
Connecting the asymmetric
units results in a 1D CP along
the a axis, in which the two
Mn–salen units form two or-
thogonal wave-shaped scaf-
folds. The nods of these waves
are the lanthanide carboxylate
building units (Scheme 2 and
Figure 2). The solvent accessi-
ble void space in compound 3
calculated by the PLATON
program is 32%.[26] The guest
DMF molecules occupy the
void space, resulting in a regu-
lar
framework
structure
(Figure 2, bottom).
The two lanthanide atoms
Ln1 and Ln2 form different
secondary
(SBUs).[4d] Each SBU is based
on [(Ln)2A(m-O2CR)4A
(h2-
O2NO) (dmf)4] building block
building
units
a
2A
C
CHTUNGTRENNUNG
CHTUNGTRENNUNG
Figure 2. Solid-state structure of 3 omitting hydrogen atoms. Top: The SBU is highlighted. Bottom: cutout of
the polymeric structure. Compounds 1, 2, 4, and 5 are isostructrual.
that is formed around the lan-
1988
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Salen-Based Coordination Polymers
FULL PAPER
Scheme 2. Schematic setup of 1–5.
thanide atoms (Figure 2, top). These SBUs can be regarded
as a distorted square paddle-wheel built from two rare earth
ions bridged by four carboxylates. The coordination polyhe-
dron of Ln1 and Ln2 can be best described as a distorted
square antiprism. Thus, each Ln atom is ligated by eight
oxygen atoms (Figure 1): four oxygen atoms from metal
bridging carboxylate groups of the Mn–salen units (Ln1: O5,
O9, O6’, O10’; Ln2: O8, O11, O12, O11’), two oxygen atoms
from one nitrate group (Ln1: O14, O15; Ln2: O17, O18)
and two oxygen atoms from two DMF molecules (Ln1: O20,
O21; Ln2: O22, O23). Both SBUs are formed by two sym-
metrically and two asymmetrically coordinated carboxylate
groups. The difference between the SBUs is found in the co-
ordination of the asymmetrically bound unit.
Figure 3. TGA curve of 1–5 in the temperature range of 20 to 8008C at a
heating rate of 58CminÀ1 under N2 atmosphere.
13.50%, calcd: 14.27%). Finally, the coordination polymer
starts to decompose at a temperature of 3708C.
The experimental powder pattern of 3 shows a good
agreement with the calculated one based on the single crys-
tal data, which proves the crystalline purity of the com-
pound (Figure 4). Then, the samples were dried to remove
The two Mn–salen units ((Mn1LCl1)
(Mn2LCl2) differ by the amount of coordinated solvent.
The manganese ion in ((Mn1LCl1)(dmf)) is in the center of
ACHTUNGTRENN(UNG dmf)) and
AHCTUNGTRENNUNG
a distorted octahedral coordination polyhedron, which is
built by ONNO atoms of the salen ligand, one chloride ion,
and one DMF molecule. The chloride ion and DMF mole-
cule occupy the apical positions. The ONNO atoms of the
salen ligand form an equatorial plane around the Mn1 atom.
In contrast to Mn1, the five-fold coordinated Mn2 atom is
surrounded by the ONNO atoms of the salen ligand and
one chloride ion forming a square pyramidal geometry. The
equatorial plane around the Mn2 is built from two phenoxy
oxygen atoms and two imine nitrogen atoms of the salen
ligand. The apical position is occupied by a chloride ion.
Both five-fold-[27] and six-fold-coordinated[28] Mn atoms
within salen ligands have been reported earlier.
Figure 4. Powder X-ray diffractograms of compound 3 showing a) the si-
mulated pattern, b) the microcrystalline sample, c) the dried sample, d)
the catalyst after the reaction, and e) the catalyst resuspended in DMF,
and measured as a suspension of microcrystals in DMF.
Compounds 1–5 were investigated by TGA measurements
(Figure 3 and Figure S2, the Supporting Information). All
compounds show similar curves although in some cases, loss
of the coordinated and non-coordinated solvent is not well
resolved. Only the data of compound 3 are discussed in
detail here because it was used for most catalytic investiga-
tions. The TGA curves of the other compounds are shown
in the Supporting Information. For compound 3, a weight
loss is found due to the release of five DMF molecules in
the temperature range of 90 to 1638C (found: 19.50%,
calcd: 18.44%). The remaining four DMF molecules are lost
in the temperature range of 2008C to 3178C (found:
DMF from the lattice, which induces a structural rearrange-
ment. To show that this process is reversible we then sus-
pended the dried material in DMF to let the solvent diffuse
back into the lattice. We followed the slow diffusion by
powder X-ray diffraction. After about three weeks the
powder X-ray diffractogram of the sample (microcrystals
suspended in DMF) almost looked like the one of the single
crystals. Some small deviations were observed that could be
the result of a slightly different arrangement of the 1D CP
(Figure 4).
Chem. Eur. J. 2013, 19, 1986 – 1995
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W. Kleist, P. W. Roesky et al.
Table 2. Dependency of conversion, yield, and selectivity on the catalyst
The characterization of 1–5 shows that the Mn–salen-
based metalloligand in the 1D coordination polymer should
concentration (c).[a]
A
A
Catalyst c
AHCTUNGTRENN[GUN mol% Mn]
Conversion
[%]
Yield
[%]
Selectivity
[%]
TON
be accessible for catalytic reactions. Since Mn-based salen
complexes are known to be highly active catalysts in epoxi-
dation reactions,[22] we applied compounds 1–5 (dried at
1508C) as solid catalyst materials in the epoxidation reac-
tion of trans-stilbene with molecular oxygen. The reaction
resulted in the formation of stilbene oxide as main product
and benzaldehyde as the only side product (Scheme 3).
0 (blind test)
0.15
0.76
4
25
32
33
3
20
30
28
75
78
92
84
–
26
41
19
1.52
[a] Reaction conditions: stilbene (1 mmol), biphenyl (1 mmol) as internal
standard, 1508C, synthetic air (400 mLminÀ1), DMF (60 mL), 7 h.
from 25 to 32% by increasing the catalyst concentration
from 0.15 to 0.76 mol%. Moreover, the selectivity towards
stilbene oxide could be enhanced from 78 to 92% (Table 2,
entries 2 and 3). Higher amounts of the catalyst
(1.52 mol%) led to additional formation of the higher oxi-
dized side product benzaldehyde without increasing the con-
version. Therefore, the selectivity was clearly reduced from
92 to 84% (Table 2, entry 4). Evidently, using excessive
amounts of the catalyst is not favorable for the formation of
stilbene oxide.
As shown by TGA, compound 3 is stable up to 3608C
under N2 atmosphere (Figure 3). Therefore, catalytic tests at
up to 1508C should be possible without risking the decom-
position of the material. The stability of the material under
reaction conditions was checked by XRD characterization
of the used catalyst (Figure 4). It could be shown that the
diffraction pattern of the catalyst is not significantly altered
after using it in the reaction. Based on these observations,
we anticipate that the 1D CP structure of compound 3 is ba-
sically retained during the course of reaction.
Scheme 3. Epoxidation of trans-stilbene with O2 catalyzed by compound
3.
For the catalytic screening tests we used compound 3
only, but all five materials were later tested under the opti-
mized reaction conditions (see below). The reaction parame-
ters shown in Table 1 were varied to optimize the conver-
sion, yield, and selectivity; to do this, one parameter at a
time was varied, whereas the others remained unchanged.
Table 1. Varied reaction parameters.
Reaction parameters
T [8C]
100, 120, 140, 150
catalyst c [mol% Mn]
oxidant
0.15, 0.76, 1.52
O2, synthetic air, compressed air
200, 400, 600
It was discovered that by increasing the reaction tempera-
ture from 100 to 1408C, the conversion and yield could be
doubled (Figure 5). After 10 h at 1008C, the conversion and
yield were 13 and 10%, respectively, whereas at 1408C
these values reached to 29 and 24%. This observation is
common for catalytic reactions and can be ascribed to the
increasing reaction rates.
oxidant flow [mLminÀ1
]
First, the impact of different solvents (e.g., toluene, DMF
or N-methyl-2-pyrrolidone (NMP)), on the reaction progress
was probed. No conversion was detected in the presence of
toluene, whereas the use of NMP led to a large number of
different products, but none of them in excess. This observa-
tion might be a result of reactions of the solvent with the
substrate or intermediate products (e.g., ring-opening reac-
tions). Using DMF resulted in good conversion and the for-
mation of low amounts of benzaldehyde as single side prod-
uct. Thus, DMF was chosen as solvent for the oxidation re-
action, which has already been reported for stilbene oxida-
tions.[23b]
The catalyst concentration was varied from 0.15 to
1.52 mol% based on the Mn content (Table 2). Moreover, a
blind test was conducted under the same reaction conditions
without adding any catalyst to determine a possible blind ac-
tivity. Without adding a catalyst, there was hardly any prod-
uct formation after 7 h (Table 2, entry 1), whereas there was
noticeable conversion using compound 3. This observation
proved that the successful formation of stilbene oxide is
truly induced by the catalyst. Conversion could be improved
Figure 5. Dependency of yield on reaction temperature. Reaction condi-
tions: stilbene (1 mmol), biphenyl (1 mmol) as internal standard, O2
(200 mLminÀ1), catalyst (0.76 mol%), DMF (30 mL), 10 h.
1990
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Salen-Based Coordination Polymers
FULL PAPER
Pure oxygen, synthetic air, and compressed air were used
as oxidants for the epoxidation reaction. Surprisingly, it was
found that both synthetic air and compressed air were more
effective oxidants than pure oxygen (Figure 6). When using
Figure 7. Dependency on flow rate of the oxidant. Reaction conditions:
stilbene (1 mmol), biphenyl (1 mmol) as internal standard, 1408C, O2,
catalyst (0.76 mol%), DMF (60 mL), 7 h.
mation of stilbene oxide could be detected under the chosen
reaction conditions.
Combining these observations, the reaction parameters
shown in Table 3 were found to be ideal. Using the opti-
mized parameters, the conversion of stilbene after 24 h was
Figure 6. Dependency of conversion and yield on oxidant. Reaction con-
ditions: 1508C, oxidant (400 mLminÀ1), catalyst (0.76 mol%), DMF
(60 mL), 7 h.
pure oxygen, the conversion reaches up to 23% after 7 h,
whereas values of 33 and 28% were achieved with synthetic
air and compressed air, respectively. This observation might
be due to redox processes occurring at the catalytically
active center. The regeneration of catalytically active Mn
species through reduction at the end of the catalytic cycle is
most probably hindered in the presence of too high O2 con-
centrations. Furthermore, high O2 amounts might lead to ox-
idation of the organic scaffold molecules in compound 3
and, thus, result in a partial decomposition of the catalyst.
Using O2 as an oxidant also led to increased formation of
the higher-oxidized side product benzaldehyde, which means
that the selectivity was lower. The lower conversion and
yield when using compressed air (technical grade) compared
with the values obtained when using synthetic air (20.5%
O2 in N2) might be explained by impurities.
Table 3. Optimized reaction parameters.[a]
T
[8C]
Catalyst c
[mol% Mn]
Oxidant
Oxidant flow
[mLminÀ1
U
G
]
150
0.76
synthetic air
400
[a] Reaction conditions: stilbene (1 mmol), biphenyl (1 mmol) as internal
standard, DMF (60 mL), 24 h.
70% and the yield of stilbene oxide added up to 61% re-
sulting in a TON of 84 at 88% selectivity using catalyst 3.
Under these optimized reaction conditions, application of
the four other compounds 1, 2, 4, and 5 resulted in identical
TONs, proving that the choice of the lanthanide did virtually
have no influence on the performance of the catalyst
(Table 4).
In further experiments, the dependency of conversion and
yield on the flow rate of the oxidant was investigated
(Figure 7). With respect to high yield and selectivity, a flow
of O2 of 400 mLminÀ1 was found to be advantageous; under
these conditions, a conversion of 24% and a yield of stil-
bene oxide of 21% were achieved (88% selectivity) in con-
trast to 22% conversion and 19% yield at 200 mLminÀ1
(85% selectivity). Using a flow rate of 600 mLminÀ1 led to
a much lower selectivity of only 77%.
Benzaldehyde (which is also formed as side product
under the chosen reaction conditions) was reported in litera-
ture as an effective promoter of the target reaction. Due to
that fact, we performed experiments using benzaldehyde as
a possible beneficial additive. However, in the epoxidation
of stilbene, no positive influence of this additive on the for-
A hot filtration test was carried out to prove that the re-
action is catalyzed heterogeneously and not by homoge-
Table 4. Catalytic performance of compounds 1–5 under the optimized
reaction conditions.[a]
Catalyst
Conversion
[%]
Yield
[%]
TON
1
2
3
4
5
73
74
70
77
69
64
63
61
66
60
86
85
84
86
83
[a] Reaction conditions: stilbene (1 mmol), biphenyl (1 mmol) as internal
standard, synthetic air (400 mLminÀ1), catalyst (0.76 mol%), DMF
(60 mL), 1508C, 24 h.
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W. Kleist, P. W. Roesky et al.
ACHTUNGTRENNUNGneous species generated by dissolution/decomposition of
compound 3 or undesired residual Mn precursor species.
The catalyst was filtered from the reaction mixture after 3 h.
Then the reaction was continued with the filtrate under the
same reaction conditions. As shown in Figure 8, the conver-
Scheme 4. Homogeneous Jacobsen-type catalyst 6.
plex was already reported,[29] we established a slightly differ-
ent synthetic protocol. Compound 6 was fully characterized
by standard analytical/spectroscopic techniques and the
solid-state structure was established by single X-ray diffrac-
tion (Figure S1, the Supporting Information); although the
quality of the data set was low, the connectivity of 6 and its
composition were deduced (see the Supporting Informa-
tion). Under the same reaction conditions, the homogeneous
catalyst 6 showed a slightly higher activity for the first hour.
Thereafter, the activities of both the homogeneous and the
heterogeneous catalyst were nearly identical, but 6 gradually
lost activity after 5 h (Figure 9). At the end of the reaction
Figure 8. Yield and conversion of the leaching test compared to those of
the reaction. Reaction conditions: stilbene (1 mmol), biphenyl (1 mmol)
as internal standard, 1508C, synthetic air (400 mLminÀ1), catalyst
(0.76 mol%), DMF (60 mL), 24 h. In the hot filtration test the catalyst
was filtered from the reaction mixture after 3 h.
sion and yield further increased after the catalyst was re-
moved but at much lower rate compared with the reaction
in presence of the catalyst. That observation proved that the
reaction is mostly catalyzed heterogeneously although a cer-
tain degree of homogeneous contribution cannot be exclud-
ed completely. On the other hand, the observed activity
after catalyst filtration might be caused by very small solid
catalyst particles that could not be removed from the reac-
tion mixture by filtration.
It could also be shown that the catalyst was active for
more than one reaction cycle without a significant loss of ac-
tivity. To prove the reusability of the catalyst it was recycled
after the first run (decanted, washed with DMF (5 mL),
dried at room temperature) and reused under exactly the
same reaction conditions. It was found that the catalyst was
not deactivated under the optimized reaction conditions
after two runs (Table 5).
The reaction was also carried out using a homogeneous
Jacobsen-type catalyst 6 (Scheme 4) to compare the catalytic
activity of compound 3 with that of a similar homogeneously
dissolved Mn–salen complex. Although this Mn–salen com-
Figure 9. Conversion with compound 3 as catalyst compared with that of
the homogeneous Jacobsen-type catalyst 6. Reaction conditions: stilbene
(1 mmol), biphenyl (1 mmol) as internal standard, 1508C, synthetic air
(400 mLminÀ1), catalyst (0.76 mol%), DMF (60 mL), 24 h.
the conversion amounted to 53%, whereas the conversion
for a reaction with compound 3 is 70%. The loss of activity
might be due to the oxidation of the organic ligand, which
leads to partial decomposition and deactivation of the cata-
lyst. On the contrary, in compound 3 the ligand is stabilized
by the framework structure and therefore protected from
oxidation.
Besides the homogeneous Jacobsen-type catalyst 6, there
are no close related systems in the literature to compare
with. On the other hand, a similar oxidation was investigat-
ed by using STA-12(Co), which showed a slightly higher ac-
tivity.[23b]
Table 5. Conversion and yield in first and second run using compound 3.
Conversion[a] [%]
Yield[a] [%]
first run
second run
57
55
49
49
[a] Reaction conditions: stilbene (1 mmol), biphenyl (1 mmol) as internal
standard, 1508C, synthetic air (400 mLminÀ1), catalyst (0.76 mol%),
DMF (60 mL), 24 h.
1992
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 1986 – 1995

Salen-Based Coordination Polymers
FULL PAPER
mental analysis calcd (%) for C63H87Cl2Mn2N15Nd2O27: C, 38.69; H, 4.48
N, 10.74; found: C, 37.67; H, 4.37; N, 10.39.
Conclusion
AHCUTNGRETNN{GNU [Eu2ACHTUNGTREN(NGU MnLCl)2ACHTNUGRETN(GNUN NO3)2ACTHNGUTREN(NUGN dmf)5]·4DMF}n (2): Yield: 52 mg, 21% (based on
We have synthesized and structurally characterized
a
Mn). IR: (ATR): n˜ =2931 (w), 2878 (w), 1645 (s), 1614 (s), 1598 (s), 1528
(m) (n1), 1476 (m), 1386 (m), 1304 (w), 1278 (m) (n2), 1196 (m), 1138
(m), 1106 (m), 1091 (s), 1036 (m) (n3), 976 (s), 902 (m), 897 (w), 829 (m)
(n4), 810 (m), 797 (w), 776 (s), 739 (m), 673 (w), 639 (s), 611 (m), 555 (w),
504 cmÀ1 (w); elemental analysis calcd (%) for C48H52Cl2Eu2Mn2N10O22
(corresponds to loss of the five DMF molecules): C, 35.91; H, 3.26; N,
8.72; found: 35.23; H, 3.78; N, 8.56.
number of manganese- and lanthanide-containing 1D CPs
by using trivalent lanthanide nitrates and manganese chlor-
ide along with the salen ligand H4L. The new manganese–
lanthanide
compounds,
{[Ln2ACTHNGUTREN(NUG MnLCl)2ACHTUNGTREN(NUNG NO3)2-
ACHTUNGTRENNUNG
materials with defined structures. The polymers consist of
manganese–salen-based metalloligands having carboxylate
linkers connected to lanthanide atoms to form 1D CPs. The
manganese–gadolinium compound (3) was used as a catalyst
in the liquid-phase epoxidation of trans-stilbene with molec-
ular oxygen, which resulted in the formation of stilbene
oxide. In a test series, the influence of solvent, catalyst con-
centration, reaction temperature, oxidant, and flow rate on
conversion, yield, and selectivity was analyzed. High temper-
atures (1508C), a low catalyst concentration of 0.76 mol%,
and DMF as solvent emerged as optimized parameters. Syn-
thetic air with a flow of 400 mLminÀ1 proved to be the best
oxidant. In the presence of compound 3, the reaction result-
ed in a conversion of 70% and the formation of 61% stil-
bene oxide (88% selectivity) after 24 h.
AHCUTNGRETNN{GNU [Gd2ACHTUNGTREN(NGU MnLCl)2ACHTNUGRETN(GNUN NO3)2ACTHNGUTREN(NUGN dmf)5]·4DMF}n (3): Yield: 56 mg, 27% (based on
Mn). IR (ATR): n˜ =2940 (w), 2871 (w), 1644 (s), 1615 (s), 1599 (s), 1529
(m) (n1), 1464 (m), 1402 (m), 1385 (m), 1338 (m), 1305 (m), 1278 (m)
(n1), 1251 (m), 1196 (w), 1138 (m), 1107 (w), 1090 (m), 1037 (m) (n3), 976
(m), 902 (s), 865 (m), 829 (m), 816 (m) (n4), 798 (m), 777 (m), 740 (s),
675 (s), 638 (s), 614 (m), 588 (w), 504 cmÀ1 (w); elemental analysis calcd
(%) for C51H59Cl2Gd2Mn2N11O23 (corresponds to loss of the four DMF
molecules): C, 36.26; H, 3.52; N, 9.12, found: 36.05; H, 4.12; N, 9.25.
AHCUTNGRETNN{GNU [Tb2ACHTUNGTREN(NGU MnLCl)2ACHTNUGRETN(GNUN NO3)2ACTHNGUTREN(NUGN dmf)5]·4DMF}n (5): Yield: 53 mg, 25% (based on
Mn). IR: (ATR): n˜ =2939 (w), 2892 (w), 1645 (s), 1617 (s), 1601 (s), 1529
(m) (n1), 1472 (m), 1386 (m), 1337 (m), 1308 (s), 1278 (m) (n2), 1197 (m),
1106 (s), 1062 (s), 1038 (s) (n3), 977 (m), 903 (m), 830 (s), 815 (m) (n4),
798 (m), 776 (s), 740 (s), 672 (s), 638 (m), 614 (m), 595 (m), 540 (m),
503 cmÀ1 (w); elemental analysis calcd (%) for C54H66Cl2Mn2N12O24Tb2
(corresponds to loss of the three DMF molecules): C, 36.73; H, 3.77; N,
9.52. Found: 36.38; H, 4.15; N, 9.59.
AHCUTNGRETNN{GNU [Dy2ACHTUNGTREN(NGU MnLCl)2ACHTNUGRETN(GNUN NO3)2ACTHNGUTREN(NUGN dmf)5]·4DMF}n (4): Yield: 47 mg, 22% (based on
Mn). IR: (ATR): n˜ =2938 (w), 2894 (w), 1646 (s), 1617 (s), 1602 (s), 1529
(s) (n1), 1470 (m), 1404 (s), 1385 (s), 1337 (s), 1308 (m), 1278 (m) (n2),
1253 (m), 1196 (s), 1107 (s), 1091 (s), 1039 (m) (n3), 977 (m), 913 (s), 897
(s), 828 (s) (n4), 816 (m), 799 (s), 778 (m), 740 (m), 675 (s), 638 (m), 615
(m), 559 (w), 504 cmÀ1 (w); elemental analysis calcd (%) for
C63H87Cl2Dy2Mn2N15O27: C, 37.98; H, 4.40; N, 10.55; found: 37.06; H,
4.25; N, 10.09.
Under the optimized conditions, all five catalysts (1–5)
showed remarkable activity with a TON of 84 in the pres-
ence of only a low amount of catalyst.
A hot filtration test confirmed that the reaction is mainly
catalyzed through a heterogeneous pathway, although a
minor contribution of homogeneous species could not be
completely excluded. Compound 3 could be reused without
significant loss of activity.
This study is one of the first examples of a 1D coordina-
tion polymer used as heterogeneous catalyst in an epoxida-
tion reaction of an alkene with molecular oxygen or synthet-
ic air as green, cheap, and readily available oxidant, which
does not form any side products.
X-ray crystallographic studies of 1–5: A suitable crystal was covered in
mineral oil (Aldrich) and mounted on a glass fiber. The crystal was trans-
ferred directly to the À1238C cold stream of a STOE IPDS II diffractom-
eter.
All structures were solved using the program SHELXS-97.[30] The re-
maining non-hydrogen atoms were located from successive difference in
Fourier map calculations. The refinements were carried out by using full-
matrix least-squares techniques on F2, minimizing the function (FoÀFc)2,
2
2
in which the weight is defined as 4F0 /2ACHTNUTRGEN(UNG Fo ) and Fo and Fc are the ob-
served and calculated structure factor amplitudes using the program
SHELXL-97.[30] The hydrogen atom contributions were calculated, but
not refined. The final values of refinement parameters are given below.
The locations of the largest peaks in the final difference Fourier map cal-
culation as well as the magnitude of the residual electron densities in
each case were of no chemical significance. Positional parameters, hydro-
gen atom parameters, thermal parameters, bond distances and angles can
be found in detail in the Supporting Information. Crystallographic data
(excluding structure factors) for the structures reported in this paper
have been deposited with the Cambridge Crystallographic Data Centre
as CCDC-889177 (1), CCDC-889178 (2), CCDC-889179 (3), CCDC-
889180 (4), and CCDC-889181 (5). These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Experimental Section
General: IR spectra were obtained on a Bruker FTIR Tensor 37 through
the attenuated total reflection method (ATR). Elemental analyses were
carried out with an Elementar vario EL Vario Micro Cube. TGA meas-
urements were made on a Netzsch STA 429 instrument. N,N’-Bis (4-car-
boxysalicylidene)ethylenediamine (H4L) was prepared according to liter-
ature procedures.[18] All other chemicals were used as purchased from
commercial sources without further purification.
General procedure for the synthesis of complexes 1–5: H4L (39 mg,
0.11 mmol), MnCl2·ACHTUNGTRENNUNG(H2O)4 (27 mg, 0.1 mmol), LnAHCNUTTRGE(NGNUN NO3)3·ACHTNUGTREN(UNNG H2O)m
(0.20 mmol) and pyridine (0.1 mL) were combined in DMF (3 mL) with
stirring. The resulting solution was then stirred for further 3 h at room
temperature and then sealed in a 10 mL glass vial. The glass vial was
heated at 908C for 44 h in an oven and then cooled to room temperature.
The red block shaped crystals were collected and washed three times
with DMF followed by diethyl ether and dried in air.
X-ray powder diffraction patterns (XRD) for different samples of 3 were
measured on a STOE STADI P diffractometer (CuKa1 radiation, Germa-
nium monochromator, Debye–Scherrer geometry) in sealed glass capilla-
ries. The theoretical powder diffraction pattern was calculated on the
basis of the atom coordinates obtained from single crystal X-ray analysis
by using the program package Mercury 2.4 by ccdc.
ACHTUNGTRENNUNG{[Nd2ACHTUNGTRENNUNG(MnLCl)2ACHTUNGTRENNUNG(NO3)2ACHTUNGTRENNUNG(dmf)5]·4DMF}n (1): Yield: 52 mg, 25% (based on
Crystal data for 1: C63H87Cl2Mn2N15Nd2O27, M=1955.74, triclinic, a=
Mn). IR (ATR): n˜ =2919 (w), 2848 (m), 1644 (s), 1593 (s), 1528 (m) (n1),
1476 (m), 1437 (m), 1384 (w), 1332 (m), 1301 (m), 1277 (m) (n2), 1253
(m), 1140 (w), 1106 (m), 1088 (m), 1033 (m) (n3), 975 (w), 903 (w), 828
(m) (n4), 796 (m), 738 (s), 672 (m), 638 (w), 611 (m), 504 cmÀ1 (m); ele-
13.7670(4), b=14.7260(4), c=21.4859(6) ꢂ, a=88.189(2), b=74.150(2),
3
¯
g=76.573(2)8, V=4073.4(2) ꢂ , T=200(2) K, space group P1, Z=2, m-
AHCTUNGTRENNUNG
(MoKa)=1.706 mmÀ1, 41925 reflections measured, 21440 independent re-
Chem. Eur. J. 2013, 19, 1986 – 1995
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1993

W. Kleist, P. W. Roesky et al.
flections (Rint =0.0694). The final R1 values were 0.0503 [I>2s(I)]. The
final wR(F2) values were 0.1359 (all data). The goodness of fit on F2 was
0.994.
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¯
74.018(2), g=76.523(2)8, V=3999.3(2) ꢂ , T=150(2) K, space group P1,
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2s(I)]. The final wR(F2) values were 0.0653 (all data). The goodness of
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clinic, a=13.7251(3), b=14.7160(3), c=21.4508(4) ꢂ, a=88.2180(10),
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74.229(3), g=76.632(3)8, V=4045.8(2) ꢂ , T=200(2) K, space group P1,
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a typical experiment stilbene
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Acknowledgements
This work was supported by the DFG Center for Functional Nanostruc-
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Received: October 11, 2012
Published online: December 28, 2012
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