Heterogeneous catalytic epoxidation of olefins over hydrothermally synthesized lanthanide containing framework compounds

Article abstract of DOI:10.1002/ejic.201000937

A series of novel complexes [Ln₂(N₃)(nic) ₂(OH)₃(Hnic)(H₂O)]_n (Ln = Y ^III 1 and Gd^III 2; Hnic = nicotinic acid) and [Sm ₄(N₃)₂(nic)₄(OH)₆(Hnic) ₂(H₂O)₂]_n (3) have been hydrothermally synthesized and structurally characterized by single-crystal X-ray analysis. Complexes 1, 2 and 3 have similar structures and consist of [Ln₂(N₃)(nic)₂(OH)₃(Hnic)(H ₂O)] units that are linked to neighboring units through end-on azido, μ₂ and μ₃ hydroxo, and bridging carboxylato groups to form 1D infinite chains. In all the complexes the 1D chains propagate parallel to crystallographic a-axis to give rise to a ribbon-like chain structure. All the compounds exhibit excellent catalytic performance in olefin epoxidation reactions, which is reflected in high turnover numbers. Good expoxide selectivity was obtained in the cases of cyclooctene and 1-hexene. The possibility of easy recycling makes these catalysts highly promising candidates for addressing industrial needs and environmental concerns. A series of novel compounds [Ln₂(N₃)(nic)₂(OH) ₃(Hnic)(H₂O)]_n (Ln = Y^III 1 and Gd^III 2; Hnic = nicotinic acid) and [Sm₄(N ₃)₂(nic)₄(OH)₆(Hnic) ₂(H₂O)₂]_n, 3, have been hydrothermally synthesized and structurally characterized. The overall structures of complexes 1, 2 and 3 are ribbon-like 1D infinite chains. All the compounds exhibit excellent catalytic performance in olefin epoxidation reactions. Copyright

Full text of DOI:10.1002/ejic.201000937

FULL PAPER
DOI: 10.1002/ejic.201000937
Heterogeneous Catalytic Epoxidation of Olefins Over Hydrothermally
Synthesized Lanthanide Containing Framework Compounds
[a]
[a]
[b]
[c]
Rupam Sen, Subratanath Koner,* Dipak K. Hazra, Madeleine Helliwell, and
Monika Mukherjee[
b]
Keywords: Heterogeneous catalysis / Hydrothermal synthesis / Lanthanides / Epoxidation / Framework solids
A
series of novel complexes [Ln
2
(N
(Ln = Y 1 and Gd 2; Hnic = nicotinic acid) and
2 n
(nic) (OH) (Hnic) (H O) ] (3) have been hydro- lent catalytic performance in olefin epoxidation reactions,
3
)(nic)
2
(OH)
3
(Hnic)-
propagate parallel to crystallographic a-axis to give rise to a
ribbon-like chain structure. All the compounds exhibit excel-
III
III
(
H
2
O)]
n
[Sm (N
4
3
)
2
4
6
2
2
thermally synthesized and structurally characterized by sin-
gle-crystal X-ray analysis. Complexes 1, 2 and 3 have similar
which is reflected in high turnover numbers. Good expoxide
selectivity was obtained in the cases of cyclooctene and 1-
hexene. The possibility of easy recycling makes these cata-
lysts highly promising candidates for addressing industrial
needs and environmental concerns.
structures and consist of [Ln
units that are linked to neighboring units through end-on
azido, μ and μ hydroxo, and bridging carboxylato groups to
2 3 2 3 2
(N )(nic) (OH) (Hnic)(H O)]
2
3
form 1D infinite chains. In all the complexes the 1D chains
Introduction
organic synthesis methods. Combinatorial chemistry, cur-
rently used in the pharmaceutical industry for the synthesis
Research in the area metal-organic framework (MOFs)
fabrication continues to be of interest because of the unique
topology and tunable properties of these materials. Inter-
est in lanthanide compounds is very much focused on the
of drugs, has recently been employed in the field of cataly-
[14]
sis.
The performance of transition metal complexes in
[
1]
[15]
homogeneous catalysis is well established.
Polynuclear
metal complexes, especially MOF solids, are receiving great
[
2]
preparation of homogeneous catalysis, molecule-based
[16]
attention in the field of heterogeneous catalysis. A variety
magnetic materials,[ artificial nucleases for the hydrolytic
3]
of reactions involving MOF catalysts have been studied.
The catalytic activity observed for these materials is directly
related to their metallic components, which may be isolated
[
4]
cleavage of DNA and RNA, contrast agents for magnetic
[5]
resonance imaging, and fixation media for atmospheric
[
6]
gases.
[17a,17b]
[17f]
[17c]
[17d]
metal centers, clusters
(dimers,
or sheets,
trimers,
tetra-
Lanthanide salts are used extensively in the synthesis of
organic compounds and for several organic transforma-
[17e]
[17g,17h]
mers,
etc.), chains,
that are con-
nected by organic linkers. Kitagawa et al. reported the prep-
aration of a pillared copper-containing layered MOF,
[
7]
tions. The oxidation of alkenes is catalyzed by lanthanide
[
8]
salts in homogeneous reaction media. Oxidative transfor-
[
Cu (pzdc) (pz)] (pzdc = pyrazine-2,3-dicarboxylate, pz =
2 2
[
9]
mations, and especially the epoxidation of alkenes, are key
chemical processes in synthetic organic chemistry, in the
pyrazine). The complex, [Cu (pzdc) (pz)], catalyzes the po-
2
2
lymerization of alkynes, which was possible because of the
[
10]
[11]
chemical industry,
and in biology.
Considerable ad-
[18]
marked Lewis basicity of the carboxylate groups.
Hupp
vances have been made in the development of atom-efficient
heterogeneous catalytic oxidation methods employing
et al. performed the oxidation of olefins that was catalyzed
by a bimetallic mixed ligand MOF under heterogeneous
[
12]
tBuOOH.
There is ever-growing interest in the applica-
[16f]
conditions with a substituted iodosylbenzene oxidant.
In
tion of reusable catalysis in the synthesis of fine chemicals,
including enantioselective reactions,[ which could reduce
the large amounts of waste products formed in noncatalytic
this catalytic compound the active center is a Schiff base
13]
III
Mn moiety, which is stacked in between paddle-wheel
carboxylate Zn dimers and 4,4Ј-biphenyldicarboxylate li-
2
gands to form square planar layers. The iron(III)-based
MOF containing benzene 1,3,5-tricarboxylate, Fe -
MIL100, which has a zeotype architecture has been synthe-
[
[
[
a] Department of Chemistry, Jadavpur University,
Kolkata 700 032, India
III
E-mail: snkoner@chemistry.jdvu.ac.in
b] Department of Solid State Physics, Indian Association for the
Cultivation of Science,
[16c]
sized hydrothermally.
The Friedel–Crafts reaction of
III
Jadavpur, Kolkata 700 032, India
benzene with benzyl chloride was catalyzed by Fe -
MIL100 at 343 K and showed excellent selectivity for di-
phenylmethane. Ravon et al. have successfully used zinc di-
carboxylate MOF solids as heterogeneous catalysts for the
c] Department of Chemistry, University of Manchester,
Oxford Road, Manchester, M13 9PL, UK
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000937.
Eur. J. Inorg. Chem. 2011, 241–248
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
241

R. Sen, S. Koner, D. K. Hazra, M. Helliwell, M. Mukherjee
FULL PAPER
alkylation of aromatics.[19] Alkylation of toluene with tBu two YIII cations, one azide ligand, and three nicotinate
III
chloride selectively affords the corresponding para-substi- anions (Figure 1), and each Y ion is eight coordinate, and
tuted product. The same reaction catalyzed by AlCl or the has anti-prismatic geometry. The coordination environment
3
acidic zeolite beta gives mixtures of ortho- and para-substi- of the metal center Y(1) in 1 is achieved by one nitrogen
#
2
tuted compounds and dialkylated products. Recently, we atom [Y(1)–N(1) 2.420(10) Å (#2: x + 1, y, z)] from an
successfully used layered metal carboxylates and hydrogen azide group, three oxygen atoms [Y(1)–O(1) 2.328(9) Å,
phosphate to catalyze olefin epoxidation reactions under Y(1)–O(3) 2.307(8) Å and Y(1)–O(5) 2.326(9) Å] from three
heterogeneous conditions.[
16e,16g]
Lanthanide-based MOFs nicotinic acid groups, one μ2 hydroxy group [Y(1)–O(8)
#
1
are being used in for the catalytic oxidation of organic sul- 2.504(8) Å], and three μ3 hydroxy groups [Y(1)–O(9)
fur compounds as well as alkanes.[
2d,2e]
However, to the best 2.362(8) Å, Y(1)–O(10) 2.388(7) Å, and Y(1)–O(10)#1
of our knowledge no attempts have been made so far to use
lanthanide carboxylate complexes for the catalytic epoxid-
ation of olefins in heterogeneous media.
Herein we report the synthesis and crystal structures of
three novel 1D lanthanide carboxylate complexes 1–3 that
show good activity towards olefin epoxidation in hetero-
geneous media.
Results and Discussion
Crystal Structures of Compounds 1, 2 and 3
Single crystal XRD analysis revealed that complexes 1
and 2 have similar structures and crystallized in the centro-
¯
symmetric space group P1 with Z = 2. Selected bond
lengths and angles are collated in Table 1. Since both of
the compounds have similar structures, complex 1 has been
selected to be the representative example for the following
Figure 1. ORTEP diagram of compound 1 with ellipsoids at the
structure description. The asymmetric unit of 1 contains 40% probability level.
Table 1. Selected bond lengths [Å] and angles [°] for complexes 1, 2 and 3.[a]
1
2
3
Y(1)–O(1)
Y(1)–O(3)
Y(2)–O(4)
Y(1)–O(5)
Y(2)–O(6)
Y(2)–O(7)
Y(1)–O(8)
2.328(9)
2.307(8)
2.330(8)
2.326(9)
2.301(8)
2.368(8)
2.504(8)
2.531(8)
2.354(8)
2.293(8)
2.362(8)
2.388(7)
2.342(8)
2.366(7)
2.420(10)
2.484(10)
108.1(3)
Gd(1)–O(1)
Gd(1)–O(3)
Gd(1)–O(4)
Gd(2)–O(3)
Gd(2)–O(4)
Gd(2)–O(5)
Gd(2)–O(7)
Gd(2)–O(9)
Gd(2)–O(10)
Gd(2)–N(4)
2.442(7)
2.264(6)
2.378(7)
2.382(7)
2.290(6)
2.503(6)
2.380(7)
2.471(7)
2.579(7)
2.403(8)
Sm(1)–O(1)
Sm(3)–O(1)
Sm(4)–O(1)
Sm(1)–O(2)
Sm(2)–O(2)
Sm(4)–O(2)
Sm(1)–O(3)
Sm(2)–O(3)
Sm(2)–O(10)
Sm(4)–O(10)
Sm(1)–N(1)
Sm(2)–N(4)
Sm(4)–N(4)
2.421(4)
2.436(4)
2.404(4)
2.406(4)
2.387(4)
2.426(4)
2.381(4)
2.390(4)
2.595(4)
2.569(4)
2.549(5)
2.530(5)
2.497(5)
Y(2)–O(8)#4
Y(2)–O(9) 3
Y(2)–O(9)#
Y(1)–O(9)#1
Y(1)–O(10)#
Y(1)–O(10) 1
Y(2)–O(10)
Y(1)–N(1)#2
Y(2)–N(1)
#
1
O(6)#1–Gd(1)–O(4)
O(3)–Gd(1)–O(1)
O(4)–Gd(1)–O(1)
O(1)–Y(1)–O(10)
79.6(2)
78.1(2)
109.4(2)
81.1(2)
66.3(2)
137.2(2)
69.0(2)
83.1(2)
71.5(2)
80.7(2)
75.3(2)
O(3)–Sm(1)–O(2)
O(3)–Sm(1)–O(1)
O(2)–Sm(1)–O(4)
N(1)–Sm(1)–O(4)
O(3)–Sm(1)–O(4)
O(1)–Sm(1)–O(4)
O(2)–Sm(2)–O(3)
O(2)–Sm(2)–O(9)
O(3)–Sm(2)–O(9)
O(3)–Sm(2)–O(5)
O(3)–Sm(2)–N(4)
O(5)–Sm(2)–N(4)
72.05(13)
113.2(12)
131.4(12)
64.48(14)
69.13(12)
152.5(13)
72.20(13)
142.3(13)
145.4(13)
71.37(13)
77.20(14)
146.1(14)
O(5)–Y(1)–O(10)#1 82.0(3)
O(3)–Y(1)–O(8)
O(1)–Y(1)–O(8)
O(1)–Y(1)–O(10)
72.4(3)
78.3(3)
71.0(3)
137.0(3)
140.7(3)
75.5(3)
#
1
#1
O(8) –Gd(1)–O(4)
O(1)–Gd(1)–O(4)#
1
#
3
#2
O(9) –Y(2)–O(6)
O(4)–Gd(1)–O(10)
#
3
#2
O(9) –Y(2)–O(4)
O(6)–Y(2)–O(4)
O(3)–Gd(1)–N(4)#
2
O(4)–Gd(1)–N(4)
#
3
#1
#2
O(9) –Y(2)–O(10) 112.0(3)
O(6) –Gd(1)–N(4#)
O(1)–Gd(1)–O(10)
O(4)–Gd(1)–O(4)#
2
O(6)–Y(2)–O(10)
O(9)–Y(2)–O(10)
O(4)–Y(2)–O(7)
83.9(3)
71.4(3)
75.2(3)
1
[a] Symmetry codes, #1: –x + 1, –y + 2, –z + 1; #2: x + 1, y, z; #3: –x, –y + 2, –z + 1; #4: x – 1, y, z.
242
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 241–248

Heterogeneous Catalytic Epoxidation of Olefins
2
.342(8) Å (#1 = –x + 1, –y + 2, –z + 1)]. Whereas Y(2) is
coordinated to an azide nitrogen atom [Y(2)–N(1)
.484(10) Å], and two oxygen atoms [Y(2)–O(4) 2.330(8) Å,
Y(2)–O(6) 2.301(8) Å] from two nicotinic acid groups, one
2
3
Figure 3. Skeleton of the 1D wave-like ladder (only μ -O atoms are
#
4
μ2 hydroxy group [Y(2)–O(8) 2.531(8) Å (#4: x – 1, y, z)] shown) in 1.
and three μ hydroxy groups [Y(2)–O(9) 2.354(8) Å, Y(2)–
3
#
3
O(9) 2.293(8) Å (#3: –x, –y + 2, –z + 1) and Y(2)–O(10)
.366(7) Å], and a water molecule [Y(2)–O(7) 2.368(8) Å].
ure 3) constructed from Y cations and μ hydroxy ligands,
and is reinforced between adjacent Y cations in two ways:
3
2
III
As the nicotinic acid ring atoms are disordered, the subse-
quent discussion of the structural parameters in 1 is limited
to atoms that do not display any disorder. As shown in
Figure 2, complex 1 forms a 1D chain with the azide li-
gands in end-on (EO) bridging mode, while the nicotinic
ligands and hydroxy groups bind in two modes: bidentate
μ2-(syn–syn) and monodentate for the nicotinic groups, and
μ and μ modes for the hydroxy groups. The 1D chains
(i) by being bonded to two carboxylate groups of the nico-
tinic ligand in syn–syn mode; (ii) by being bonded to both
an μ hydroxy and μ azide group.
2
1,1
The overall structure of compound 3, including the coor-
dination environment around the central metal ion, is the
same as that of 1 and 2. Compound 3 also crystallizes in
¯
the centrosymmetric space group P1 with Z = 2. However,
2
3
the crystallographic asymmetric unit of complex 3 contains
propagate parallel to the crystallographic a-axis. The skel-
eton of the ID chain has a wave-like ladder structure (Fig-
III
four Sm cations (see Supporting Information, Figure S2).
Selected bond lengths and angles for 3 are listed in Table 1.
The crystal packing of compounds 1, 2 and 3 is stabilized
by a combination of intermolecular C–H···O hydrogen
bonds (Table 2). In addition to the van der Waals interac-
tions, the molecular packing in 1 is influenced by strong
hydrogen bonding involving a nicotinic acid group C atom
and a water molecule, where the C(nicotinic)···O distance is
2.795(16) Å. The atom C4 of the nicotinic acid group in the
molecule at (x, y, z) acts as a hydrogen bond donor for the
water molecule O7 atom in the molecule at (1 – x, 2 – y, –z),
2
forming a centrosymmetric R (22) ring that is propagated
2
along the [001] direction (Figure 4), while in 2 a combina-
tion of strong intermolecular C4–H4···O9 (–1 + x, y, –1 +
z) and C17–H17···O9 (3 – x, 2 – y, 2 – z) hydrogen bonds
Figure 2. Propagation of the 1D chain in compound 1 parallel to
crystallographic a-axis.
Table 2. Intermolecular contacts for complexes 1, 2 and 3 [Å, °].
D–H···A
d(D–H)
d(H···A)
d(D···A)
Ͻ(DHA)
Symmetry
transformation
[
[
a]
b]
C(4)–H(4)···O(7)
C(4)–H(4)···O(9)
0.93
0.95
0.95
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
1.89
2.03
1.87
2.40
1.89
1.86
1.85
1.86
2.59
1.82
1.92
2.795(16)
2.937(13)
2.784(13)
3.304(8)
2.787(8)
2.774(8)
2.757(7)
2.761(7)
3.281(7)
2.740(7)
2.793(8)
165
160
160
164
161
166
163
163
132
169
154
1 – x, 2 – y, –z
–1 + x, y, –1 + z
3 – x, 2 – y, 2 – z
2 – x, –y, 1 – z
1 – x, –y, 1 – z
–x, 1 – y, –z
1 – x, 1 – y, 1 – z
x, –1 + y, z
–1 + x, 1 + y, z
x, 1 + y, z
[
b]
C(17)–H(17)···O(9)
[
c]
[
C(4)–H(4)···O(8)
c]
C(5)–H(5)···O(16)
[
[
[
c]
C(12)–H(12)···O(20)
C(19)–H(19)···O(16)
C(26)–H(26)···O(10)
C(32)–H(32)···O(6)
C(33)–H(33)···O(4)
C(40)–H(40)···O(20)
c]
c]
[
[
c]
c]
[
c]
1 – x, –y, –z
[a] Complex 1. [b] Complex 2. [c] Complex 3.
Figure 4. Hydrogen bonding in 1 forming a centrosymmetric R
2
2(22) ring that is propagated along the [001] direction.
Eur. J. Inorg. Chem. 2011, 241–248
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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243

R. Sen, S. Koner, D. K. Hazra, M. Helliwell, M. Mukherjee
FULL PAPER
inter link the molecules to form a supramolecular frame- any support to act as recyclable heterogeneous catalysts for
work (Figure 5). In 3, a pair of C(5)–H(5)···O(16) hydrogen olefin epoxidation. Aromatic and aliphatic alkenes react
bonds, with the C5 atom in the molecule at (x, y, z) acting with tBuOOH to produce epoxides with remarkable selec-
as a donor to the O16 atom in the molecule at (1 – x, –y, tivity and in good yields with compounds 1, 2, and 3, acting
2
1
– z), generates a centrosymmetric R (18) dimeric ring as heterogeneous catalyst in acetonitrile, a result which is
2
(
A). Similarly, a pair of intermolecular C(12)–H(12)···O(20) reflected by the high turnover numbers for all the olefinic
2
hydrogen bonds produce another R (18) dimeric ring (B). substrates in this study (Table 3). Alkyl-hydroperoxides are
The two R (18) rings are alternately linked into an infinite used on a large scale for industrial epoxidation reactions,
2
2
2
1
D ABABAB chain that propagates along the [001] direc- for example in Halcon-Arco and Sumitomo process-
[
25a–25c]
tion (Figure 6). Additional reinforcement within the chain es.
The recycling of co-products e.g. tBuOH has been
is achieved by other C–H···O hydrogen bonds that give rise realized in the Sumitomo process. The results of the cata-
to the supramolecular network in 3.
lytic epoxidation of different substrates are summarized in
Table 3 and Figures 7, 8 and 9. The oxidation of cyclooc-
tene proceeded smoothly, showing conversion (86%, 41%
and 40% for 1, 2 and 3, respectively) to cyclooctene oxide
with 100% selectivity. Oxidation of styrene occurred with
81–92% conversion, while epoxide yields were 41–47%;
along with styrene oxide, benzaldehyde was also formed.
In fact, in the epoxidation of styrene with tBuOOH over
transition metal immobilized zeolites, or molecular sieve
[
26,27]
catalysts, the epoxide yields seldom exceed 40%.
We
recently succeeded in improving the yield of styrene epoxide
to ca. 70% using layered metal carboxylate catalysts.[
The substituted styrene was converted (99%) successfully to
the oxide with up to 60% selectivity, the remaining conver-
sion product was the corresponding aldehyde. However, in
16e]
Figure 5. Formation through interlinking hydrogen bonds of the
supramolecular framework in 2.
Table 3. Epoxidation of olefins catalyzed by 1, 2 and 3.[a]
Figure 6. R 2
(18) rings alternately linked into an infinite 1D
2
ABABAB chain that propagates along the [001] direction in com-
pound 3.
Catalytic Epoxidation Reactions
Olefin epoxidation reactions catalyzed by lanthanide
containing catalysts under homogeneous conditions are
[
20]
well documented.
However, homogeneous catalysts suf-
fer from the problems of separation of catalyst from the
reaction mixture and recycling of the catalyst. Catalyst re-
cycling is a task of great economic and environmental im-
portance in the chemical and pharmaceutical industries, es-
pecially when expensive and/or toxic heavy metal complexes
are employed. To this end many attempts have been made
to resolve this problem such as intercalating or encapsulat-
ing the metal complex into layered compounds or within
[
a] (a), (b), (c) corresponds to the catalytic performance of com-
pounds 1, 2 and 3, respectively. Reaction conditions: alkenes (1 g);
catalyst (2 mg); tBuOOH (2 mL); acetonitrile (8 mL); T = 68–
[21]
the cavities of a porous solid (e.g., zeolites), binding the
[
22]
metal complex into a polymeric matrix,
steric hindrance
and employing 70 °C. The products of the epoxidation reactions were collected at
[
23]
to design recyclable catalysts. Polymer different time intervals and were identified and quantified with a
¯
Varian CP3800 gas chromatograph equipped with an FID detector
supported lanthanide-binol systems and Ln(O-iPr) systems
3
and a CP-Sil 8 CB capillary column. [b] Benzaldehyde. [c] 4-Meth-
ylbenzaldehyde. [d] 3-Methylbenzaldehyde. [e] 1-Hexanol. [f] Turn
over number (TON) is the ratio of (mol converted):(mol of active
have been used extensively in the catalytic oxidation of ole-
[
24]
finic substrates.
At variance with the above, the lantha-
nide carboxylate compounds reported herein do not require site).
244
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 241–248

Heterogeneous Catalytic Epoxidation of Olefins
the case of linear alkenes, 1-hexene was converted to the were performed with a simple carboxylate Gd(OAc) salt
3
corresponding oxide in a high yield of 85–96%. The forma- acting as the heterogeneous catalyst. The performance of
tion of side products was minimal in this reaction. Notably, Gd(OAc) in heterogeneous catalytic epoxidation reactions
3
in the epoxidation of 1-hexene catalyzed by 2 and 3 no side is summarized in Table 4. The results clearly indicate that
products were formed.
the reactions with lanthanide chain compounds are more
selective and the yields are much better than when simple
gadolinium carboxylate is the catalyst. Further comparison
of catalytic efficiency of 1, 2 and 3 with the corresponding
lanthanide oxides, clearly indicates that the 1D lanthanide
carboxylates perform better as catalysts than the metal ox-
[
25d]
ides (Table 5).
The temperature dependence of the cata-
lytic performance of compounds 1, 2 and 3 has been
studied. While the catalysts showed almost no conversion
at room temperature, they did exhibit the desired conver-
sion at 70 °C, which demonstrates the vital role of tempera-
ture for the activation of the catalyst (see Supporting Infor-
mation, Figure S3). Notably, the amount of catalyst in the
reaction mixture has no influence on the progress of reac-
tion. No induction period was observed for all the reac-
tions.
Figure 7. Kinetic profile for the conversion of olefins catalyzed by
compound 1.
Table 4. Epoxidation of olefins catalyzed by Gd(OAc)
3
.[a]
Figure 8. Kinetic profile for the conversion of olefins catalyzed by
compound 2.
[
a] Reaction conditions were the same as given in the footnote of
Table 3. [b] Benzaldehyde. [c] 4-Methylbenzaldehyde. [d] 3-methyl-
benzaldehyde. [e] Turn over number (TON) is the ratio of (mol
converted):(mol of active site).
Table 5. Epoxidation of styrene with tBuOOH catalyzed by a vari-
ety of Ln-based catalysts.[
a]
Catalyst
Conversion % Yield of product
TON[b]
References
[%]
oxide
others
1
2
3
Y
Gd
Sm
92
81
87
41
46
47
30
60
51
35
40
2886
3061
2337
887
74
this work
this work
this work
this work
Figure 9. Kinetic profile for the conversion of olefins catalyzed by
compound 3.
2
O
3
82
52
The catalytic reactions were performed with a variety of
oxidants to study the catalytic efficacy of 1, 2 and 3. The
best catalytic performance was observed with tBuOOH in
acetonitrile. Hydrogen peroxide and sodium hypochlorite
solution completely failed to activate the catalysts. We also
[28d]
2
O
3
15.7
9.8
54
39.8
51.3
23
[28d]
2
O
3
48.6
31
38
867
Gd(OAc)
3
this work
[
a] Reaction conditions were the same as given in footnote of
Table 3. [b] Turn over number (TON) is the ratio of (mol convert-
undertook a few control experiments; all of these reactions ed):(mol of active site).
Eur. J. Inorg. Chem. 2011, 241–248
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
245

R. Sen, S. Koner, D. K. Hazra, M. Helliwell, M. Mukherjee
FULL PAPER
Separation, Catalyst Reuse and Heterogeneity Test
bit excellent catalytic performance in heterogeneous olefinic
epoxidation reactions. As there are no pores in the com-
pounds, epoxidation reactions clearly take place on the sur-
face of the catalysts. This study affords new vistas for the
design of lanthanide compounds that can be used as recy-
clable heterogeneous epoxidation catalysts. Further investi-
gations employing this type of catalyst in other reactions
are currently in progress.
The major advantage of the use of heterogeneous cata-
lysts is the possible recovery of the catalyst from the reac-
tion mixtures and its possible reuse. To test if metal is le-
ached out from the solid catalyst during reaction, the liquid
phase of the reaction mixture was collected by filtration,
at the reaction temperature, after 30% of the epoxidation
reaction was complete, and the residual activity of the su-
pernatant solution after separation of the catalysts was
studied. To test for leaching, the organic phase of a first
run was separated from the catalyst and new reagents were
Experimental Section
added to the clear filtrate, and then the composition of the Materials and Physical Measurements: Gadolinium nitrate hexahy-
reaction mixture was determined by gas chromatography. drate, samarium nitrate hexahydrate, yttrium nitrate hexahydrate,
This homogeneous reaction mixture was then treated as a nicotinic acid, sodium azide, cyclooctene, styrene, 4-methylstyrene,
3
-methylstyrene, 1-hexene, and tert-butyl hydroperoxide (70 wt.-%
standard catalytic experiment. After 6 h, the composition
was of the mixture was redetermined, and no reaction prod-
ucts were observed, which excludes the presence of an active
catalytic species in the solution. These experiments clearly
demonstrate that metal is not leaching out of the solid cata-
lyst during the epoxidation reactions. In order to check the
stability of the catalysts, we have characterized them after
aqueous) were purchased from Aldrich and were used as received.
Solvents were purchased from Merck (India). The solvents were
distilled and dried before use. Fourier transform infrared spectra
of KBr pellets were measured on a Shimadzu S-8400 FTIR spec-
trometer. The powder XRD patterns of the samples were recorded
with a Scintag XDS-2000 diffractometer with Cu-K
products of the catalytic reactions were identified and quantified
α
radiation. The
the completion of the reactions in which they were involved. with a Varian CP 3¯ 800 gas chromatograph with a CP-Sil 8 CB capil-
After the catalytic reactions were complete, the solid cata- lary column. Other instruments used in this study were the same
lyst was recovered by centrifugation and washed with fresh as reported earlier.[
16e]
acetonitrile several times and dried in an air oven. The reco-
Synthesis and Preliminary Characterization of the Compounds:
vered catalyst was then subjected to IR and powder XRD Compounds 1, 2 and 3 were prepared by a hydrothermal process.
analysis. Comparison of the IR spectra and XRD patterns To obtained the desired product, Ln(NO ) ·6H O, sodium azide
3
3
2
of the pristine complexes and recovered catalysts convinc- and nicotinic acid were mixed in a 1:3:2 ratio, and maintained at
1
70 °C for 3 d at autogenously developed pressure in a teflon lined
ingly demonstrated that the structural integrity of com-
plexes 1, 2 and 3 remains unaltered during the epoxidation
reactions (see Supporting Information, Figures S4–S9). No-
tably the recovered catalyst can be reused several times for
epoxidation reactions with no considerable loss of activity
reaction bomb. After cooling to room temperature colorless needle
shaped crystals were isolated in high yield by filtration [Yield ca.
85%, 87% and 91% (based on metal) for 1, 2 and 3 respectively],
washed with water and dried in a desiccator. For preliminary char-
acterization of the compounds, elemental analysis and IR spectro-
scopic studies were undertaken. 1: calcd. C 33.12, H 2.14, N 12.88;
found C 33.05, H 2.10, N 13.0. Selected IR peaks (KBr disk): ν˜ =
(Table 6). The kinetic profiles for the epoxidation reaction
of 1-hexene with fresh and recovered catalysts are virtually
the same (see Supporting Information, Figures S10–S12).
–
–
1617, 1549 [υas
(CO
600–3200 [s. br., υ (O–H)] cm . 2: calcd. C 27.38, H 1.78, N 10.64;
found C 27.28, H 1.72, N 10.60. Selected IR peaks (KBr disk): ν
2
s 2 s
)], 1412 [υ (CO )], 1230, 1063 [υ (C–O)],
–
1
3
Table 6. Catalytic efficacy towards 1-hexene epoxidation of the re-
˜
covered complexes 1, 2 and 3 over successive runs.[
a]
–
–
=
1623, 1577 [υas (CO
600–3200 [s. br., υ (O–H)] cm . 3: calcd C 27.94, H 1.81, N 10.86;
found C 27.86, H 1.77, N 10.78. Selected IR peaks (KBr disk): ν˜
2 s 2 s
)], 1406 [υ (CO )], 1284, 1176 [υ (C–O)],
–
1
3
Cycles
Conversion
Yield of product
(wt.-%)
TON
(wt.-%)
–
–
=
3
1669, 1573 [υas (CO
2 s 2 s
)], 1449 [υ (CO )], 1284, 1178 [υ (C–O)],
1
2
3
first reuse
second reuse
first reuse
second reuse
first reuse
second reuse
95
93
90
88
96
95
85
84
90
88
96
95
3688
3572
4210
4116
4221
4375
600–3200 [s.br., υ (O–H)].
X-ray Crystallography: X-ray diffraction data for 1, 2 and 3 were
collected at 100(2) K on a Bruker SMART APEX CCD X-ray
diffractometer with graphite-monochromated Mo-K
=
α
radiation (λ
0.71073 Å). Determination of integrated intensities and cell re-
[a] Reaction conditions were the same as given in the footnote of
[28]
finement were performed with the SAINT
software package
Table 3.
with a narrow-frame integration algorithm. An empirical absorp-
tion correction (SADABS)[ was applied to the data. All the struc-
tures were solved by direct methods and refined using full-matrix
29]
2
least-squares technique against F with anisotropic displacement
Conclusions
parameters for all non-hydrogen atoms with the programs
[30]
SHELXS97 and SHELXL97. Since the nicotinic acid ring atoms
in 1 and 2 are disordered over two positions, anisotropic refinement
of these atoms was restrained with the DELU, SIMU and ISOR
options in SHELXL. Hydrogen atoms were placed at calculated
In summary we have successfully prepared three new 1D
lanthanide compounds via a hydrothermal route. Metal
centers in these compounds are bridged through end-on
azido, μ and μ hydroxo, and carboxylato ligands to form positions with suitable riding models and isotropic displacement
2
3
ribbon-like structures. Notably, compounds 1, 2 and 3 exhi- parameters derived from their carrier atoms. In the final difference
2
46 Eur. J. Inorg. Chem. 2011, 241–248
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Heterogeneous Catalytic Epoxidation of Olefins
Fourier maps there were no remarkable peaks except ghost peaks
surrounding the metal centers. A summary of crystal data and rel-
evant refinement parameters for complexes 1, 2 and 3 are given in
Table 7.
S1/IC-01/2009). R. S. thanks CSIR [Ref. No. 09/096(0551)2k8-
EMR-I] for a Senior Research Fellowship. We also acknowledge
the Department of Science and Technology (DST), Government of
India for financial assistance to procure a Gas Chromatograph in
an another project (to S. K.) (SR/S1/IC-23/2003).
Table 7. Crystal data and structure refinement details for complexes
1, 2 and 3.
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1
2
3
47, 4966; b) H. Furukawa, J. Kim, N. W. Ockwig, M. O’Keeffe,
Formula
Formula weight
Temperature [K]
Wavelength [Å]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C
652.17
100(2)
0.71073
triclinic
P1
7.925(4)
12.502(6)
12.474(6)
108.191(7)
103.893(7)
98.026(7)
1108.5(8)
2
18
H N O Y
14 6 10 2
C
786.84
100(2)
0.71073
triclinic
P1
8.050(2)
12.523(3)
12.581(3)
108.011(5)
98.218(5)
104.173(6)
1136.1(5)
2
18
H
12
N
6
O
10Gd
2
C
36
H
24
N
12
O
20Sm
4
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1546.07
100(2)
0.71073
triclinic
P1
8.1462(4)
¯
¯
¯
12.5647(6)
23.0020(11)
99.207(1)
90.245(1)
98.283(1)
2298.9(2)
2
γ [°]
Volume [Å ]
3
Z
Calculated density 1.966
2.300
2.234
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[
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
247

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Received: September 2, 2010
Published Online: December 9, 2010
[
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Eur. J. Inorg. Chem. 2011, 241–248