Manganese- and cobalt-based coordination networks as promising heterogeneous catalysts for olefin epoxidation reactions

Article abstract of DOI:10.1021/ic502772k

We demonstrate the synthesis of Mn²⁺- and Co²⁺-based coordination networks using two Co³⁺-based metalloligands differing by the position of the appended arylcarboxylate groups. The structural analyses reveal topologically distinct networks with pores of variable dimensions allowing facile diffusion of substrates and/or reagents. All four networks function as heterogeneous catalysts for the olefin epoxidation reactions using tert-butyl-hydroperoxide without any requirement of solvent or additive. Control and optimization experiments illustrate recyclable network-based catalysts that make them attractive candidates for the "greener" oxidation chemistry processes.

Full text of DOI:10.1021/ic502772k

Article
pubs.acs.org/IC
Manganese- and Cobalt-Based Coordination Networks as Promising
Heterogeneous Catalysts for Olefin Epoxidation Reactions
†
†
Girijesh Kumar, Gulshan Kumar, and Rajeev Gupta*
Department of Chemistry, University of Delhi, Delhi 110 007, India
*
S Supporting Information
2
+
2+
ABSTRACT: We demonstrate the synthesis of Mn - and Co -based
3
+
coordination networks using two Co -based metalloligands differing by the
position of the appended arylcarboxylate groups. The structural analyses reveal
topologically distinct networks with pores of variable dimensions allowing facile
diffusion of substrates and/or reagents. All four networks function as
heterogeneous catalysts for the olefin epoxidation reactions using tert-butyl-
hydroperoxide without any requirement of solvent or additive. Control and
optimization experiments illustrate recyclable network-based catalysts that make
them attractive candidates for the “greener” oxidation chemistry processes.
1
7
18
INTRODUCTION
complexes are those based on porphyrin and salen systems,
although both manganese and cobalt complexes with nonheme
■
In recent time, research on crystalline porous materials has
19
ligands have also been utilized in oxidation reactions. An
important challenge is to heterogenize a homogeneous catalyst
as the conventional homogeneous catalysis is undesirable both
from an industrial-scale and from an environmental viewpoint
due to the excessive use of solvents and reagents. In addition,
aerobic and peroxidative oxidations of olefins are regarded
as the simplest yet efficient epoxidation methods rather than
the use of hazardous and inefficient oxo-transfer reagents.
Our research group has developed assorted coordination
complexes as the metalloligands, offering secondary func-
tional groups and/or coordination sites, for the construction of
These metalloligands on reaction
with secondary metal ions afford two- (2D) and three-
dimensional (3D) networks as well as layered materials.
Our metalloligand approach provides precise control over the
placement of Lewis-acidic secondary metal sites in the resultant
networks in an exclusively noncatenated manner.
approach has helped us in showcasing Lewis-acid-catalyzed
reactions in substrate-specific and regio-,
received significant attention due to their well-defined
1
architecture and potential applications in gas sorption and
2
3
4
5
storage, separation, sensing, ion exchange, molecula8r
6
7
transport and delivery, optics, molecular magnetism,
9
10
20
devices, and catalysis. In particular, their utilization as
heterogeneous catalysts in assorted organic transformation
reactions has shown noteworthy results due to the optimized
control of void space available within such crystalline porous
21
22
21,22
1
1
materials. Importantly, such catalytic properties are largely
possible due to the crystalline nature of networks and therefore
orderly arrangement of catalytic sites and the presence of well-
defined pores and channels for a better substrate and/or
12
23−25
1
0,11d
ordered architectures.
reagent accessibility.
For catalysis, it is imperative that the
catalytic sites are either coordinatively unsaturated or ligated by
easily dissociable solvent molecules while it is further
advantageous to design a material to circumvent the catenation
which prevents accessibility of the substrates and/or reagents to
the catalytic sites. Unification of such critical features is often
challenging to incorporate in a material via traditional
solvothermal synthesis involving organic ligands and metal
ions; however, a well-designed metalloligand promises to
include several of the mentioned parameters to afford a
desirable catalytic material.
25
25f
2
4,25
Such an
24
24,25a−c,e−g
ster-
12
2
5e−g
25e,f
eo-,
and size-selective
manner. Inspired by the
successful Lewis-acidic catalytic results, we were naturally
inclined to incorporate oxidation-sensitive secondary metals in
the coordination networks. Herein we report the syntheses and
characterization of {Co −Mn } and {Co −Co } coordina-
tion networks and their applications in the solvent-free
heterogeneous epoxidation reactions.
Metal-catalyzed oxidation reactions are one of the most
1
3
important reactions in chemistry. In particular, olefin
3+
2+
3+
2+
1
4
epoxidation has received considerable interest from both
research and industry as epoxides play an eminent role as
15
intermediates and buildings blocks in organic synthesis. In
this context, manganese and cobalt complexes have been
particularly studied as the oxidation catalysts for the
Received: November 18, 2014
1
6
epoxidation reactions. Some of the well-studied metal
©
XXXX American Chemical Society
A
DOI: 10.1021/ic502772k
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Article
Scheme 1. Synthetic Routes for Networks 3−6
them in a CH Cl solution containing 10 equiv of a substrate for 12 h
EXPERIMENTAL DETAILS
2
2
■
at 25 °C. The impregnated sample was washed thrice with fresh
Materials and Methods. All commercially available reagents were
of analytical grade and used as received without further purification.
CH Cl , dried under vacuum, and characterized.
2
2
General Procedure for the Olefin Epoxidation Reaction. In a
typical olefin epoxidation reaction, 100 μL of the corresponding olefin
was treated with tert-butyl hydroperoxide (TBHP) in a 1:3 ratio in the
presence of 2 mol % of catalyst (network 3, 4, 5, or 6) at 50 °C under
the solvent-free conditions. The progress of the reaction was
monitored by thin-layer chromatography (TLC) and/or gas chromato-
graph (GC). The reaction was quenched by adding 2 mL of EtOAc
that led to the precipitation of solid catalyst which was filtered off and
dried. The recovered catalysts were characterized by FTIR spectra and
X-ray powder diffraction (XRPD) patterns that showed identical
results to that of freshly prepared samples. The EtOAc layer was
washed with water and dried over anhydrous Na SO , and the organic
2
6
Solvents were purified using the standard literature procedure. The
p‑COOH
lignads, H L
(2,6-bis(4-benzoicacid-carbomyl)pyridine)) and
2
m‑COOH
H2L
(2,6-bis(3-benzoicacid-carbomyl)pyridine)), and their
p‑COOH
m‑COOH
metalloligands, Et N[Co(L
) ] 1 and Et N[Co(L
2
)2] 2,
4
4
2
5e,f
were synthesized as per our earlier reports.
Synthesis of [{(1) Mn (H O) }·11H O] (3). Network 3 was
2
5
2
11
2
n
synthesized by the reaction between Mn(OAc) ·4H O (61.52 mg,
.2510 mmol) and metalloligand 1 (100 mg, 0.1004 mmol) in
CH OH (10 mL). This reaction afforded a brown-colored product
which was dissolved in water, filtered, and layered with tert-butyl
alcohol at room temperature. After 4−5 days, deep green-colored
crystals of 3, suitable for diffraction studies, were collected by filtration.
Yield (based on 1): 0.224 g (81%). Anal. Calcd for
2
2
0
3
2
4
products were isolated after removal of solvent. The crude organic
products were purified by the flash column chromatography on silica
gel using 5% EtOAc/hexanes as the eluent. The products were
analyzed and quantified by the GC/GC-MS techniques as well as
proton NMR spectra in a few representative cases. For control
experiments, reactions were performed to evaluate the role of (a)
oxidizing agent, (b) solvent, (c) additive, (d) aerobic versus anaerobic
environment, (e) catalyst loading, (f) reaction time, and (g) reaction
temperature.
C H Co Mn N O ·11H O: C, 36.74; H, 3.23; N, 6.12. Found:
84
66
2
5
12 57
2
C, 36.16; H, 3.12; N, 6.05. FTIR spectrum (Zn−Se ATR, selected
−
1
peaks): 3412 (OH), 1602 (COO), 1569 (CO) cm . Absorption
spectrum (solid state, λ /nm): 605, 578, 542.
max
Synthesis of [{(2)Mn (μ-OH)(H O) }{Mn(H O) }·10H O] (4).
2
2
3
2
3
2
n
Network 4 was prepared in a similar manner with a similar scale as that
of network 3, however, using metalloligand 2 in place of 1. Reddish-
green-colored crystals of 4 were collected after a period of 5−6 days.
Yield (based on 2): 0.218 g (84%). Anal. Calcd for
C H CoMn N O ·10H O: C, 32.97; H, 5.07; N, 5.49. Found: C,
Physical Methods. The FTIR spectra were recorded with a
PerkinElmer Spectrum-Two spectrometer (Zn−Se ATR; 4000−600
−1
42
57
3
6
30
2
cm ). The diffuse reflectance spectra were recorded with a Lambda-
3
3
2.56; H, 5.17; N, 5.57. FTIR spectrum (Zn−Se ATR, selected peaks):
3
5 spectrophotometer. The elemental analysis data were obtained with
−1
212 (OH), 1555 (COO, CO) cm . Absorption spectra (solid
an Elementar Analysen Systeme GmbH Vario EL-III instrument. GC
and GC-MS studies were performed using a PerkinElmer Clarus 580
with an Elite-5 column and Shimadzu instrument (QP 2010) with an
RTX-5SIL-MS column, respectively. Thermal gravimetric analysis
(TGA) and differential scanning calorimetry (DSC) were performed
with DTG-60 Shimadzu and TA-DSC Q200 instruments, respectively,
state, λ /nm): 604, 579, 538.
max
Synthesis of [{(1)Co (μ-OH)(μ-H O)(H O) }·14H O] (5). Net-
3
2
2
4
2
n
work 5 was also synthesized in a similar manner with a similar scale as
that of network 3, however using Co(OAc) ·2H O in place of
2
2
Mn(OAc) ·4H O. Deep green-colored crystals of network 5 were
2
2
−1
obtained after a period of 5−6 days. Yield (based on 1): 0.225 g
at a 5 °C min heating rate under a nitrogen atmosphere. The XRPD
studies were performed either on a Bruker AXS D8 Discover
instrument or an X′Pert Pro from Panalytical (Cu Kα radiation, λ =
1.54184 Å). The samples were ground and subjected to the range of θ
= 5−50° with a scan rate of 1°/min at room temperature.
(
5
83%). Anal. Calcd for C H Co N O ·14H O: C, 34.32; H, 4.73; N,
42 41 4 6 22 2
.72. Found: C, 34.37; H, 4.61; N, 5.28. FTIR spectrum (Zn−Se ATR,
−1
selected peaks): 3335 (OH), 1602 (COO), 1563 (CO) cm .
Absorption spectrum (solid state, λ /nm): 620, 584, 547.
max
Synthesis of [{(2)Co (μ-OH)(H O) }{Co(H O) }·11H O] (6).
Crystallography. The single-crystal X-ray diffraction data for
networks 3−6 were collected on an Oxford XCalibur CCD
diffractometer using graphite-monochromated Mo Kα radiation (λ =
2
2
3
2
3
2
n
Network 6 was prepared in a similar fashion with a similar scale as
that of network 5, however using metalloligand 2. Deep brown-colored
crystals of network 6 were obtained after a period of 3−4 days. Yield
2
7
0.71073 Å). The frames were collected at 298(2) K for all four
networks. Data was processed with XCalibur, while the empirical
absorption correction was applied using spherical harmonics
(
1
based on 2): 0.238 g (90%). Anal. Calcd for C H Co N O ·
42 35 4 6 19
1H O: C, 37.05; H, 4.22; N, 6.17. Found: C, 37.15; H, 4.45; N, 6.22.
2
27
FTIR spectrum (Zn−Se ATR, selected peaks): 3350 (OH), 1560
implemented in the SCALE3 ABSPACK scaling algorithm. The
−1
28
(
6
COO, CO) cm . Absorption spectrum (solid state, λ /nm):
structures were solved by direct methods using SIR-97 and refined
max
2
09, 566, 538.
by full-matrix least-squares refinement techniques on F using the
2
9
General Procedure for the Exchange Studies. A batch of 100
mg of powdered sample was taken in a glass vial and heated at 150 °C
using oil bath) under vacuum for 8 h. Subsequently, the glass vial
containing sample was sealed in a bigger glass vial containing 2 mL of
program SHELXL-97 incorporated in the WINGX 1.8.05 crystallo-
30
graphic collective package. The non-hydrogen atoms were refined
anisotropically, whereas the hydrogen atoms were fixed at the
calculated positions with isotropic thermal parameters. For all four
networks, hydrogen atoms of the bridging hydroxide, bridging water,
and coordinated and uncoordinated water molecules could not be
located from the difference Fourier map. Although hydrogen atoms of
these moieties were not located, their numbers have been included in
the empirical formulas of every network. The solvent-accessible voids
(
D O (or any other substrate). The sealed sample was allowed to
2
equilibrate with D O vapors (or any other substrate) for 24 h. Finally,
2
the sample was dried under vacuum and characterized.
General Procedure for the Inclusion Studies. Batches of
sample ranging from 20 to 50 mg were impregnated by suspending
B
DOI: 10.1021/ic502772k
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Article
Figure 1. (a) Stick representation of a selected part of the crystal structure of network 3. Color code: orange, Co; yellow, Mn; blue, N; green,
Ocarboxylate; purple, Owater; red, Oamide; gray, C. (b) Packing diagram in a view along the a axis. (c) Space-filling view along the a axis showing pores and
2+
channels in the structure of network 3; the coordinated water molecules are omitted for clarity to show Mn ions (yellow spheres).
3
1
(
SAVs) were calculated using PLATON. The details of the single-
understand the network generation via the mediation of
metalloligands offering arylcarboxylate fragments. The metal-
loligand part in the crystal structures of all four networks is
crystal X-ray diffraction data collection and structure solution
parameters for networks 3−6 are provided in Table S1 (Supporting
Information).
3+
nearly identical and shows that the central Co2 ion is
3−25
meridionally coordinated by two tridentate ligands.
The
RESULTS AND DISCUSSION
3+
■
central Co ion is bonded to four Namide atoms in the basal
plane and two Npyridine atoms in the axial position with a
Synthesis and Cherecterization. The heterobimetallic
networks 3−6 have been synthesized by treating metalloligands
2
3−25
compressed octahedral geometry.
distances are longer than the Co−N
The Co−N
bond
Two
amide
23−25
p‑COOH
m‑COOH
Et N[Co(L
) ] (1) and Et N[Co(L
) ] (2) with
2
distances.
4
2
4
pyridine
the M(OAc) salt (Scheme 1). In each case, reaction with the
secondary metal salt resulted in a distinct color change followed
by precipitation of the product. All four networks exhibited
axial pyridine rings are trans to each other, whereas diagonal
2
3+
Namide groups make an angle with the Co3 ion larger than 160°
+ 2+ 3+
in all cases. The crystal structures of {Co −Mn } and {Co −
−1
2+
strong bands between 1561 and 1602 cm due to the νCOO
Co } networks from the metalloligands offering either p- or m-
2
5e,f,32
stretches in their FTIR spectra.
In addition, broad
arylcarboxylate groups have notable similarity and are therefore
discussed in that manner. Importantly, in all four networks, the
appended groups are present in their completely deprotonated
arylcarboxylate form and are involved in coordinating the
secondary metal ions. In the case of networks 3 and 5 (with p-
arylcarboxylate groups), however, one of the arylcarboxylate
groups remains uncoordinated. For all four networks, Figures
1−4 provide drawings of the crystal structures, whereas Figures
S7−S10, Supporting Information, display additional drawings
with complete numbering scheme. Further, Table 1 displays the
range noted for the bond distances, whereas Tables S2−S5,
Supporting Information, provide the detailed bonding param-
eters for all four networks.
−
1
features between 3212 and 3412 cm are indicative of the
presence of coordinated as well as lattice water molecules.
2
5e,f,32
Networks 3−6 displayed λmax in the range of 620−650 nm in
their diffuse-reflectance spectra which are tentatively assigned
to the d−d transitions (Figures S1 and S2, Supporting
Information). The XRPD patterns of the freshly prepared
samples of networks 3−6 are nearly identical to those simulated
from the single-crystal structures indicative of the phase purity
of the bulk samples (Figures S3−S6, Supporting Information).
However, certain differences observed in diffraction line
intensities can be attributed to the variation of the preferred
orientation of crystallites in the powdered samples.
3
+
2+
3+
2+
Crystal Structures. All four heterobimetallic networks were
characterized by the single-crystal diffraction studies to
Crystal Structures of {Co −Mn } (3) and {Co −Co }
(5) Networks with Metalloligands Offering p-Arylcar-
C
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Figure 2. (a) Stick representation of a selected part of the crystal structure of network 4. Color code: orange, Co; yellow, Mn; blue, N; green,
Ocarboxylate; purple, OH; red, O
; gray, C. (b) Packing diagram in a view along the b axis; coordinated water molecules are omitted for clarity to
amide
show Mn2 ions (yellow spheres). (c) Topological representation of the network. Color code: orange, central Co core as the node; yellow, Mn2
+
3+
cluster as the node.
boxylate Groups. Both networks 3 and 5 crystallized in a
triclinic cell with P1 space group. The asymmetric unit of
bridge. The μ -OH group acts as a bridge between Co1, Co2,
3
̅
and Co3 ions, whereas a μ -OH moiety additionally bridges
2
2
2
+
network 3 contains 1 metalloligand, 3 Mn ions, and 11
coordinated and 11 lattice water molecules, while 5 displays the
Co2 and Co3 ions. For all three cobalt atoms, the remaining
coordination sites are satisfied by Ocarboxylate, while both Co1
and Co2 ions are also coordinated by two terminal water
molecules. Network 5 displays two types of pores with
2
+
presence of 1 metalloligand, 3 Co ions, 1 bridging hydroxo, 1
bridging aqua, and 4 coordinated and 14 uncoordinated water
molecules. The structures of networks 3 and 5 illustrate that the
metalloligands are connected through the secondary metal ions
resulting in 3D architectures. For 3, every metalloligand offers
four p-arylcarboxylate groups, and three of them assist in the
2
dimensions of 11.15 × 7.46 and 11.83 × 11.69 Å , respectively.
3
+
2+
3+
2+
Crystal Structures of {Co −Mn } (4) and {Co −Co }
(6) Networks with Metalloligands Offering m-Arylcar-
boxylate Groups. The networks 4 and 6 crystallized in a
2
+
coordination of three different Mn ions, while the fourth one
monoclinic cell with P2 /n space group. Interestingly, both
1
2
+
remains free. Every Mn ion shows six-coordinated distorted
octahedral geometry ligated by a certain number of Ocarboxylate
atoms in addition to a few water molecules. Interestingly,
coordination of metalloligands to that of Mn1 and Mn3 atoms
results in the creation of one-dimensional chains, whereas Mn2
ions connect such chains to generate a network with large pores
networks show an identical unit cell as well as molecular
structure. The unit cells show the presence of 1 metalloligand, 3
secondary metal ions (Mn2+ or Co ), 1 bridging hydroxo, and
6 coordinated and 10 (for 4) or 11 (for 6) lattice water
molecules. In both 4 and 6, all four arylcarboxylate groups assist
in coordinating a pair of secondary M2+ ions that result in a 2D
architecture running in the crystallographic b axis. Both six-
coordinated secondary M2+ ions display a typical paddle-wheel
structure additionally bridged by a μ-OH group, however,
differing by the arrangement of terminal ligands. One of the
2+
2
with cross sections of 19.96 × 18.50 Å . In the case of 5,
arylcarboxylate fragments from a metalloligand hold a {Co }
5
2
+
cluster in which individual Co ions are bridged by a μ -OH
3
and a μ -OH synthon in addition to a typical μ-1,2-carboxylate
2
2
D
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Inorganic Chemistry
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Figure 3. (a) Stick representation of a selected part of the crystal structure of network 5. Color code: orange, Co; yellow, Co (secondary metal ions);
blue, N; green, Ocarboxylate; purple, Owater; magenta, Obridging water/hydroxide; red, Oamide; gray, C. (b) Packing diagram in a view along the b axis. (c) Space-
filling view along the c axis showing pores and channels in the structure of network 5; coordinated water molecules are omitted for clarity to show
accessible Co2 ions (yellow spheres). (d) Topological representation of the network. Color code: orange, central Co core as the node; yellow, Co5
+
3+
cluster as the node.
Figure 4. (a) Stick representation of a selected part of the crystal structure of network 6. Color code: orange, Co; yellow, Co (secondary metal ions);
blue, N; green, Ocarboxylate; purple, Owater; magenta, Obridging water;3red, Oamide; gray, C. (b) Packing diagram in a view along the b axis. (c) Topological
+
representation of the network. Color code: orange, central Co core as the node; yellow, Co cluster as the node.
2
E
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Article
a
Table 1. Selected Ranges for the Bond Distances (Angstroms) Observed for Networks 3−6
bond lengths
3
4
5
6
Co1−Npyridine
Co1−Namide
M−Ocarboxylate
M−Oamide
1.844(4)−1.858(4)
1.941(4)−1.945(4)
2.141(4)−2.193(4)
2.142(4)
1.866(9)−1.868(8)
1.931(10)−1.967(10)
2.099(9)−2.461(10)
1.860(6)−1.861(4)
1.951(5)−1.970(5)
2.026(4)−2.143(7)
1.850(6)−1.858(6)
1.926(6)−1.960(7)
2.016(7)−2.187(6)
M−Ohydoxide
M−Obridging water
M−Oterminal water
2.261(10)−2.273(8)
2.189(10)−2.217(10)
2.035(4)−2.088(4)
2.161(4)−2.200(4)
2.084(4)−2.145(4)
2.177(5)−2.178(5)
2.088(7)−2.145(8)
2.163(4)−2.222(5)
a
Where M = Mn2+ in networks 3 and 4 and Co in networks 5 and 6.
2+
metal ions is ligated with two Ocarboxylate atoms, one bridging
hydroxide, and three water molecules, while coordination sites
for the second metal ion are satisfied by five Ocarboxylate and a
bridging hydroxide group. Notably, in both cases, the third
25f
versus m-arylcarboxylate groups-based metalloligands. For
example, crystal structures of networks 3 and 5, with p-
arylcarboxylate groups, show SAVs corresponding to ca. 22.6%
and 23.7% of the unit cell volume, respectively. On the other
hand, networks 4 and 6 (with m-arylcarboxylate groups)
exhibited small or negligible SAVs. Such a striking difference is
due to a symmetrical nature of bonding between meta-
appended arylcarboxylate groups and that of secondary metal
ions in case of networks 4 and 6 that has resulted in little voids
2
+
metal is present as {M(H O) } cluster in the crystal lattice.
2
6
2
+
Such a {M(H O) } cluster is held in a network lattice by the
2
6
operation of assorted H-bonds between M-bound water
molecules, Oamide atoms, and lattice water molecules. Notice-
ably, when the grinded sample of either network 4 or 6 is
2
+
3+
2+
3+
suspended in various solvents the {M(H O) } cluster does
in the lattice. In fact, our earlier {Co −Zn } and {Co −
2
6
25f
2+
not come out from the crystal lattice. Further, no exchange
was observed when the powdered sample is suspended in a
solvent containing other divalent or trivalent metal ions. These
Cd } coordination networks with meta-appended arylcarbox-
ylate groups also showed very small or negligible SAVs
compared to large SAVs exhibited by networks with para-
2+
25e,f
experiments suggest the stable nature of {M(H O) } cluster
appended arylcarboxylate groups.
Furthermore, a very
2
6
3
+
within the network and the requirement of charge balance is
similar H-bonding-based packing was observed for the Co -
the primary driving force for their presence in a lattice. Both
based metalloligands appended with m-arylcarboxylic acid
groups offering very small or essentially negligible SAVs.
23b
networks 4 and 6 offer smaller pores with 8.79 × 8.34 and 8.06
2
×
7.98 Å cross-sectional area, respectively.
All four networks exhibit well-defined pores and channels
within the network. Such pores and channels were found to
contain lattice water molecules which were held together by
weak H-bonding interactions. Interestingly, networks 3 and 5,
with p-arylcarboxylate groups, offered pores with larger cross
Network Topology. The topology of all four coordination
networks has been examined in more detail using the TOPOS
3
3,34
software.
Although all four networks suggest a similar
35
architecture, they are topologically distinct. The networks
36a,b
2
offer secondary building units (SBUs)
that are composed of
sections of 19.96 × 18.50 Å (for 3) and 11.15 × 7.46 and
secondary Mn2 or Co ions coordinated by a certain number
+
2+
11.83 × 11.69 Å (for 5). Networks 4 and 6 with m-
2
of arylcarboxylate fragments: Mn(COO) (3), Mn (COO)
arylcarboxylate fragments displayed smaller pores of 8.79 ×
8.34 and 8.06 × 7.98 Å2 cross-section area, respectively.
Therefore, it could be concluded that p-arylcarboxylate groups
produced elongated networks that offered wider pores and
large SAVs. Importantly, such structural features are expected
to enhance the substrate and/or reagent accessibility. In
addition, all four networks showed an orderly arrangement of
secondary metal ions within the crystalline architectures.
Importantly, such secondary metal ions are not only oxidation
sensitive in nature but also coordinated with labile water
molecules. It was therefore presumed that the suitable
substrates may interact with such metal sites and replace the
labile water molecules without altering the network topology,
and such a situation may assist in possible catalysis.
Thermal Properties. It is a well-known fact that the
heterogeneous catalytic applications depend on the thermal
stability of coordination networks; therefore, thermal gravi-
metric analysis (TGA) and differential scanning calorimetric
(DSC) analyses were performed on all four networks (Figures
S11−S14, Supporting Information). All four networks dis-
played similar thermal profiles due to a comparable network
topology and coordination environment. For all four networks,
the first weight loss was observed in the temperature range of
25−90 °C corresponding to loss of coordinated as well as
lattice water molecules. For all four networks, the observed
weight losses were in close match to that of calculated ones for
the liberation of both coordinated as well as lattice water
2
2
4
(
4), Co (COO) (5), and Co (COO) (6). In all cases, such
5 6 2 4
3
+
SBUs expand through tetratopic Co -based metalloligands that
may be considered as nodes. Both networks 4 and 6,
generated via Co -based metalloligands appended with m-
arylcarboxylate groups, are topologically identical wherein
Mn (COO) (4) or Co (COO) (6) SBUs serve as the 4-
36
3
+
2
4
2
4
37
3+
connected cluster nodes by linking to four Co -based
3
+
metalloligands. Similarly, every Co -based metalloligand can
also be viewed as a 4-connected node which in turn links to
four Mn (COO) or Co (COO) SBUs resulting in a (4,4)-
2
4
2
4
connected network parallel to the ac plane (Figures 2c and
3
8
4
c). After applying the TOPOS simplification process the
resulting networks of 4 and 6 can be specified as the sql/
2
39
4
Shubnikov tetragonal plane nets with point symbol {4 .6 }.
3
+
Interestingly, network 5, generated via Co -based metal-
loligand appended with p-arylcarboxylate groups, is topologi-
cally distinct from the other two networks, 4 and 6. This
network can be topologically interpreted as a (3,6)-connected
network parallel to the ab plane wherein Co (COO) -based
5
6
37
SBUs serve as the 6-connected cluster nodes, while every
metalloligand stands for a 3-connected node (Figure 3d). The
TOPOS analysis of network 5 provides the kgd/Shubnikov
3
6 6 3
40
plane net topology with the point symbol {4 } {4 .6 .8 }.
2
Voids and Pores. Notably, all four networks show solvent-
accessible voids (SAVs) of different volumes, and considerable
differences were noted between the networks created by p-
F
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molecules. DSC studies supported the TGA finding by
displaying a broad feature in the exothermic region for the
loss of water molecules. The thermal analyses further exhibit
that all four networks are stable after the loss of water
molecules as no additional weight loss was noted up to ca. 350
networks during such studies. For example, ν
stretches
CC
−
1
for the impregnated substrates were red shifted by 8−10 cm .
In addition, diffusion and probable inclusion of such substrates
also caused noticeable perturbation to the νO−H stretches of the
coordinated/lattice water molecules (cf. Figures S28−S32,
Supporting Information).
°
C.
1
Exchange and Inclusion Properties. In order to evaluate
We finally carried out a H NMR spectral-based inclusion
possible diffusion of substrates and/or reagents through the
pores and channels of networks 4−6, sorption, exchange, and
experiment to ascertain the coexistence of both substrate and
reagent within the porous network and their reaction within the
confined environment. Therefore, styrene was selected as a
model substrate using TBHP as an oxidizing agent and network
3 as a representative catalyst. Network 3 was impregnated with
a 1:3 mixture of styrene and TBHP and investigated after
digesting the sample using DCl and extracting the products into
EtOAc at the following intervals: 1, 2, 4, 5, and finally at 6 h.
inclusion studies were carried out. The N sorption studies at
2
77 K for all four networks showed small uptake with low
Langmuir and BET surface area. Notably, our earlier metal-
25a−c,e,f
loligand-based networks
as well as related materials in
the literature have also displayed similar low-sorption
4
1
behavior. We, therefore, attempted to discern possible
exchange as well as inclusion of reagents and substrates within
1
The H NMR spectral results at 2 h display the peaks for
42
the pores and channels of the present networks.
substrate (styrene) as well as the epoxidation product (styrene
oxide) (Figure S35, Supporting Information). Notably, after 6
h, the peaks corresponding to substrate were negligible while
product formation was almost quantitative, whereas the
reaction showed ca. 70% completion after 4 h. Such an
experiment provides additional evidence that catalysis is
possibly occurring within the porous network and the reagents
not only have diffused but also reacted within the porous
network.
One of the significant challenges was to evaluate that the
coordinated water molecules are labile in nature so that the
substrates and/or reagents are able to replace them. We,
therefore, performed solvent exchange reactions using D O and
2
alcohol vapors. For such experiments, powdered networks were
first heated at 150 °C under vacuum in order to remove lattice
as well as coordinated water molecules followed by allowing
samples to equilibrate in a sealed environment of D O vapors.
2
3
+
2+
3+
2+
Such an experiment resulted in clean exchange of H O by D O,
Application of {Co −Mn } and {Co −Co } Net-
works in Olefin Epoxidation Reactions. Activation of
molecular oxygen is an important process in biology, chemistry,
2
2
−1
and the νO−D stretches were observed at 2480−2490 cm with
−1
a shift of ca. 900 cm from ν
stretches (Figures S15−S18,
O−H
1
3,14,21,22
Supporting Information). In order to confirm that the networks
and industry.
Such an activation ranges from the
retain their structural integrity and crystallinity during the
oxidation of C−H bonds, hydroxylation, epoxidation, and
13
solvent removal and D O exchange experiments, all four
oxidation of heteroatom-based substrates. Significant efforts
have been made to generate simple yet efficient methods to
2
networks were investigated by the XRPD studies after the D O
2
21
exchange. Notably, all four networks retain their structural
integrity and crystallinity as revealed by the XRPD patterns.
Importantly, several reflections displayed broad features with
activate molecular oxygen. Learning from the biological
17a 17b
systems, metalloporphyrins,1 metallophthalocyanins,
and
8
Schiff-base-based complexes have been effectively developed
but the activation process largely remains limited to
homogeneous reactions. In this regard, efforts have also been
made to heterogenize the molecular components in order to
minor shifts as a result of D O exchange which further support
2
our exchange assumption (Figures S19−S22, Supporting
Information).
We further attempted to understand if coordinated water
molecules could also be exchanged with few other similar
ligands. Thus, exchange studies were carried out using network
20
achieve heterogeneous activation of molecular oxygen. Out of
assorted heterogeneous catalysts, coordination networks
including metal−organic frameworks (MOFs) have shown
1
0,11
3
as a representative case with a few alcohols, such as CH OH,
remarkable results.
Our coordination networks offer
3
C H OH, C H OH, and tert-BuOH (Figures S23−S27,
interesting oxidative catalytic perspectives considering the
following points: (i) all four networks allow unhindered
diffusion of substrates and/or reagents, (ii) secondary metal
ions are coordinated by labile water molecules, (iii) secondary
metal ions are redox active and therefore likely to support
oxidation reactions, (iv) secondary metal ions line both external
as well as internal surfaces and are likely to enhance the
interaction with substrates and reagents, and (v) all four
networks are thermally stable up to 350 °C. We selected
epoxidation of assorted olefins due to the importance of
epoxides as the key starting materials for a wide variety of
2
5
3
7
Supporting Information). As an illustration, ν
stretch for
O‑Me
−1
the exchanged MeOH was clearly evident at 1060 cm . It was
observed that the coordinated/lattice water molecules were
replaced with alcohol in the following order: CH OH ≫
3
C H OH ≫ C H OH ≈ tert-BuOH. Importantly, both D O
2
5
3
7
2
and alcohol exchange experiments strongly suggest the
propensity of exchange of the coordinated/lattice water
molecules by the suitable solvents/ligands where the size of
the alcohol appears to be a limiting factor.
To further investigate the feasibility of possible diffusion of
reagents and/or substrates through the pores and channels of
1
4,15
industrial and chemical applications.
1
the networks, FTIR and H NMR spectral and XRPD studies
We noted that any of the four networks could act as a catalyst
for the epoxidation of styrene or substituted styrenes using
TBHP as an oxidizing agent. We, however, performed a series
of control experiments in order to optimize the reaction
conditions for a better catalytic efficiency and to avoid the
formation of undesirable side product(s). Of course, one of the
initial experiments substantiated that networks 3−6 are
essential in oxidation catalysis as without them only traces of
products were noticed in most cases (Table S6, Supporting
were carried out for the networks. Thus, when powdered
networks 3−6 were impregnated with a CH Cl solution
2
2
containing a few substrates (4-methoxystyrene, 4-methylstyr-
ene, 4-tert-butylstyrene, cyclohexene, and cis-stilbene), FTIR
spectra (Figures S28−S32, Supporting Information) displayed
42
noticeable differences when compared to neat samples
whereas XRPD patterns (Figures S33 and S34, Supporting
Information) ascertained the structural integrity of the
G
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Information; entries 1−19). The control and optimization
experiments are discussed in the following sections.
Oxidizing Agent. In order to find out the best oxidizing
effect of a polar solvent plays a crucial role in the displacement
of the coordinated water molecule(s) from the secondary
metals within the network and enhances the olefin accessibility,
thus resulting in better conversion. Subsequent control
experiments, however, established that the oxidation of styrene
to styrene oxide is maximum (92−95%; Table 3) under the
solvent-free reaction conditions with all four networks with
minimum production of benzaldehyde. This is probably due to
a better interaction between catalytic sites and the substrate
when its concentration is high under the solvent-free condition.
In fact, TBHP in decane (with negligible coordinating ability)
acted as the best oxidant, while TBHP in water (with very high
coordinating ability) only poorly functioned. Such an
observation further strengthens that the solvent-free reaction
conditions are the best for epoxidation with the present
networks.
agent, we screened the following reagents: H O , TBHP (in
2
2
decane), TBHP (in water), PhIO, N-methyl-morpholine-N-
oxide (NMO), trimethylamine-N-oxide (TMO), m-chloroper-
bezoic acid, and NaOCl. Notably, only TBHP was successful,
while other oxidizing agents did not result in the oxidation of
styrene (model substrate) or led to partial decomposition of the
networks. Interestingly, all four networks caused fast dis-
proportionation of m-chloroperbezoic acid, resulting in
vigorous generation of molecular oxygen. Importantly with
TBHP in decane, the product yield of styrene oxide was almost
quantitative with a very small amount of benzaldehyde, whereas
TBHP in water resulted in poor oxidation. It appears that the
presence of excess water had a detrimental effect on the
catalytic performance of networks. We believe that additional
water molecules possibly block the catalytic secondary metal
ions (also see next section, solvent). To prove such a fact, when
epoxidation of styrene was carried out in the presence of
molecular sieves, the reaction could be quantitatively completed
within 2 h rather than 4 h without sieves. Such an experiment
convincingly suggests that catalysis is occurring via displace-
ment of the coordinated water molecule(s) by the substrate,
and excess water potentially hampers such a vital step.
Additive. In porphyrin- and salen-based oxidation chemistry,
the presence of additive(s) plays a significant and sometime
43
decisive role in olefin epoxidation reactions. We, therefore,
attempted to understand the role played by the additives.
Importantly, there was no improvement in the oxidation of
styrene to styrene oxide when an additive was added during the
course of the reaction. For example, when pyridine was added
as an additive, the yield of the epoxide is 67% while the
byproduct benzaldehyde is 15% (entry 7, Table 2). However,
with t-BuOH, the yield of the styrene oxide dropped to 44%
(entry 8), whereas in the case of NaOAc and morpholine, only
53% and 67% epoxidation took place (entries 9 and 10, Table
2). Further, the presence of additives did not suppress the
formation of benzaldehyde. We speculate that the presence of a
better coordinating moiety (i.e., additive) rather blocks the
labile sites on secondary metal ions crucial for subsequent
catalysis. Therefore, it could be concluded that additive(s) do
not play any major role with our catalytic networks and
oxidation of styrene to styrene oxide is better in the absence of
any additive (92−95%, Table 3).
Solvent. To evaluate the effect of solvent in the catalytic
oxidation of olefins, styrene was selected as the model substrate
in the presence of catalyst 5 while screening various solvents
(
Table 2). Styrene oxide was the major product in most cases,
Table 2. Epoxidation of Styrene with TBHP in the Presence
of Catalyst 5: Effect of Solvents and Additives
Presence of O . We further investigated the effect of aerobic
2
b
conditions in the oxidation of styrene (Table S7, Supporting
Information). Notably, large amounts of benzaldehyde as well
as benzoic acid were formed as the byproducts along with the
desired product, styrene oxide. It is clear that the presence of
molecular oxygen resulted in free-radical-based oxidation of
styrene, resulting in the formation of benzaldehyde as well as
benzoic acid. In fact, network 4 predominantly produced
benzoic acid (70%) in addition to benzaldehyde (20%) and
styrene oxide (10%). Interestingly, out of four networks, 3 was
efficiently able to oxidize styrene to styrene oxide in 64% yield
even under aerobic reaction condition (entry 1; Table S7,
Supporting Information).
yield (%)
a
entry
substrate
solvent/additive
CH CH OH
E
A
1
2
3
4
5
6
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
80
74
60
60
60
24
67
44
53
67
20
26
40
40
20
67
15
14
20
14
3
2
CH OH
3
THF
14e
CH CN
3
DMF
dioxane
pyridine
t-BuOH
NaOAc
morpholine
c
7
c
8
c
9
c
10
Catalyst Loading. We also made efforts to understand the
optimal loading of catalyst in order to efficiently carry out the
epoxidation reactions. For such a purpose, catalyst loadings of
a
Conditions: Network 5 as catalyst (2 mol %); 4 h stirring at 50 °C.
Yield was calculated from gas chromatography. Under solvent-free
b
c
conditions.
0
.5, 1, 2, and 5 mol % were used for the oxidation of styrene
using TBHP. The product yield of styrene oxide varied in the
anticipated manner in the following order: 0.5 mol % (50) < 1
mol % (60) < 2 mol % (92) < 5 mol % (95). As the results
were closely similar to 2 and 5 mol % catalyst loading, we
decided to use only 2 mol % in an attempt to minimize the use
of catalyst.
Reaction Time. To evaluate the effect of reaction time on
epoxide yield, we performed time-dependent oxidation of
styrene using TBHP with catalyst 5 under the solvent-free
reaction conditions (Figure S36 and Table S8, Supporting
while benzaldehyde was also produced as the undesirable
byproduct in varying amounts. The highest yield of styrene
oxide (80%) was obtained in the case of C H OH (entry 1)
and lowest (24%) in dioxane (entry 6), whereas MeOH, THF,
MeCN, and DMF provided moderate yields. The results reveal
that the oxidation of styrene to styrene oxide is higher in highly
polar solvents than that of less polar solvents. Furthermore, less
polar solvents enhanced the yield of benzaldehyde (67% in the
case of dioxane; entry 6). We reasoned that the better solvation
2
5
H
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Table 3. Epoxidation of Substituted Styrenes and Other Olefins with TBHP in the Presence of Catalyst 3, 4, 5, or 6 under
Solvent-Free Conditions
b
yield (%)
3
4
5
6
a
entry
R/substrate
E
A
E
A
E
A
E
A
1
2
3
4
5
6
7
8
9
H
94
92
92
95
78
55
60
26
26
94
5
92
93
92
94
69
52
59
38
27
6
95
92
92
94
75
59
74
22
26
92
5
93
95
92
95
83
68
70
24
30
94
6
4-Me
8
7
7
5
4-OMe
6
7
6
6
4-Cl
5
6
6
5
4-C(CH3)3
1-vinylnaphthalene
2-vinylnaphthalene
9-vinylanthracene
cis-stilbene
8
7
8
10
10
12
6
11
12
4
9
8
10
5
10
3
4
4
3
5
1
0
trans-stilbene
5
93
7
6
5
a
b
Conditions: Networks 3−6 as catalyst (2 mol %); 4 h stirring at 50 °C. Yield was calculated from gas chromatography.
Table 4. Epoxidation of Assorted Cyclic Olefins with TBHP in the Presence of Catalyst 3, 4, 5, or 6 under Solvent-Free
Conditions
a
b
Conditions: Networks 3−6 as catalyst (2-mol %); 4 h stirring at 50 °C. Yield was calculated from the gas chromatography.
Information). As expected, the yield of the epoxide is directly
proportional to the reaction time, reaching the saturation after
Epoxidation Reactions. The aforementioned control and
optimization experiments allowed us to explore epoxidation
reactions of assorted substrates, and the results are presented in
Tables 3−5. In a typical epoxidation reaction, olefin and TBHP
were reacted in a 1:3 ratio in the presence of 2 mol % of catalyst
(networks 3, 4, 5 or 6) at 50 °C under the solvent-free reaction
conditions. Under these conditions, a smooth reaction took
place and the desired product, epoxide, was formed in good
yield along with a small amount of byproduct (respective
aldehyde). All four networks produced 92−95% styrene oxide
in addition to 5−6% of benzaldehyde (entry 1; Table 3). To
evaluate the impact of the electronic effect on the epoxidation
of olefin, several para-substituted styrenes were used. Notably,
4
5
h. The quantitative product conversion (95% styrene oxide;
% benzaldehyde) was observed after 4 h, whereas only 20%
conversion was noted after 1 h (Table S8, Supporting
Information, compare entries 1 and 6).
Reaction Temperature. To assess the optimal reaction
temperature, epoxidation of styrene was carried out using
network 5 at the following temperatures: 25, 50, 75, and 100
°
C. With 2 mol % catalyst loading, the best performance was
achieved at 50 °C while the epoxidation remained incomplete
at 25 °C. Notably, higher temperature resulted in the large
production of undesirable benzaldehyde.
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Table 5. Epoxidation of Assorted Linear Olefins with TBHP in the Presence of Catalyst 3, 4, 5, or 6 under Solvent-Free
Conditions
a
b
Conditions: Networks 3−6 as catalyst (2 mol %); 4 h stirring at 50 °C. Yield was calculated from the gas chromatography.
both electron-rich as well as electron-deficient styrenes were
equally effective with all four networks (92−95% yield; entries
methylcyclohexene a more electron-rich olefin. Cyclopentene
displayed a greater reactivity due to a smaller ring size and easy
accessibility (94−96%) than the sterically demanding cyclo-
octene which produces 74−78% of the corresponding epoxide
(entries 1 and 4). Cyclohexene and cycloheptene also exhibited
similar transformations (entries 2 and 3). It is important to
note that in all epoxidation reactions, except 1-methylcyclohex-
ene, the allylic oxidation product (-one; entries 1, 3, and 4;
dione, entry 2) was also obtained (5−28%) as the byproduct.
The formation of allylic product suggests the involvement of a
radical mechanism in the epoxidation of cyclic olefins (see
mechanistic insights). Notably, the quantity of allylic product is
quite small for the conformationally rigid molecules, such as
cyclopentene and 1-methylcyclohexene, but large for the
flexible cyclic olefins as noted for 6-, 7-, and 8-membered rings.
It is generally assumed that the oxidation of terminal alkenes
is difficult due to the presence of an electron-deficient double
bond. Importantly, our catalytic networks were equally effective
in the epoxidation of terminal alkenes (Table 5). All four
networks were able to oxidize 1-hexene (74−80%), 1-heptene
(66−82%), 1-nonene (58−64%), and 1-decene (>97%)
effortlessly. Interestingly, the yield of the epoxide was almost
quantitative for 1-decene but quite low for 1-nonene. The
reason for this behavior is probably related to the optimal
orientation of the substrate within the network channel in the
absence of other limiting parameters.
1
−4; Table 3). However, in the case of 4-tert-butyl-1-
vinylbenzene, the yield is dropped to 69−83% (entry 5;
Table 3), probably due to the steric hindrance of the tert-butyl
group which may have restricted its accessibility to the catalytic
sites. A similar observation was noted for the diffusion and
exchange studies using tert-butyl alcohol. The influence of steric
hindrance is further supported by the epoxidation of sterically
demanding substrates, such as 1-vinylnaphthalene, 2-vinyl-
napthalene, and 9-vinyl anthracene. In these cases, substrate-
size-dependent catalysis was observed. For network 6, the
conversion decreased in order of styrene (93%) > 4-tert-butyl-
1
-vinylbenzene (83%) > 1-vinylnaphthalene (68%) ≈ 2-
vinylnapthalene (70%) > 9-vinylanthracene (24%) in line
4
4
2
with their sizes: styrene (7.00 × 8.45 Å ), 4-tert-butyl-1-
2
vinylbenzene (7.00 × 9.93 Å ), 1-vinylnapthalene (7.00 × 8.81
2
2
Å ), 2-vinylnaphthalene (7.00 × 10.37 Å ), 9-vinylanthracene
2
(
8.42 × 10.34 Å ). A somewhat similar trend was observed for
the remaining networks. We, therefore, tentatively suggest that
the epoxidations are inversely proportional to the molecular
2+
size of the substrates and substrate accessibility to the Mn or
2+
Co sites controls the catalysis outcome.
In the epoxidation of cis- and trans-stilbene, stilbene oxide
was the major product along with a small amount of
benzaldehyde. Notably, with our catalytic networks there was
no evidence for the formation of benzil as frequently reported
Leaching and Control Experiments. In order to ascertain
whether the secondary metal ions are leaching out or not, after
3 h of epoxidation, solid networks were filtered off and the
reaction was allowed to continue. As illustrated in Figure 5
using network 5 as a representative case, there was practically
no epoxidation after filtering off network 5. Subsequently,
network 5 was readded after a gap of 2 h, which immediately
caused the epoxidation of styrene. An overall reaction profile
displays that the removal of network 5 practically ceases the
epoxidation while readding the filtered network 5 to the
reaction mixture restarts the epoxidation. This simple test
establishes the true heterogeneous catalytic behavior of network
5. Further, control experiments of using metalloligands 1 or 2
or metal salts (CoCl , MnCl , Co(OAc) , Mn(OAc) , or
4
5
in the literature. The epoxidation of cis-stilbene was poor and
afforded only 26−30% product yield (entry 9; Table 3), while
trans-stilbene was oxidized in excellent yield, ranging between
92% and 94% (entry 10; Table 3). It is believed that cis to trans
isomerization is one of the major factors for the observed
stereoselectivity and therefore poor oxidation of cis-stilbene.
Notably, only a trace amount of benzaldehyde was produced
both in cis- and in trans-stilbene epoxidation reactions.
The applicability of the catalytic networks was further
extended to the epoxidation of assorted cyclic olefins. All cyclic
olefins were oxidized in good to excellent yields (entries 1−5;
Table 4). In most cases, the amount of undesirable keto
products, K-1 and K-2, was limited to small quantities.
Interestingly, under identical reaction conditions, 1-methyl-
cyclohexene showed 100% epoxidation with all four networks
2
2
2
2
Mn(OAc) ) as the catalysts did not result in any product
3
formation. To confirm that the secondary metal ions, Co(II)
(
entry 5). This is most probably due to the presence of a
and Mn(II), are indeed required for the epoxidation reactions,
we tested our earlier Zn(II)- and Cd(II)-based networks
2
5e,f
methyl group, which shows a +I effect and makes 1-
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and selectivity throughout these numbers of cycles. Notably,
there was no special requirement of any purification or
regeneration for any of the networks, as they can be simply
filtered, washed with wet methanol, and dried at room
temperature and were ready for reuse. The FTIR spectra and
XRPD patterns authenticate that the crystallinity and structural
integrity of the recovered networks is preserved after the
catalytic reaction. A comparison of XRPD patterns of the
pristine samples to that of recovered networks unambiguously
supports this observation (Figures S37−S40, Supporting
Information).
Mechanistic Insights. The epoxidation reactions are
typically believed to operate either through a radical pathway
46
or via concerted metal−oxo species. In order to ascertain
which one, we decided to carry out epoxidations in the
presence of potential radical scavengers such as substituted
phenols. Importantly, in the presence of 2,4-di-tert-butylphenol,
2
,6-di-tert-butylphenol, and 2,4,6-tritert-butylphenol, epoxida-
tion was completely quenched whereas oxidized products
(diquinones 5−25%) and/or carbon−carbon coupled products
(2−10%) from the substituted phenols were observed. These
experiments convincingly suggest that the epoxidation proceeds
Figure 5. (A) Epoxidation of styrene with TBHP in the presence of
network 5 showing maximum oxidation at 6 h. (B) Catalyst was
filtered after 3 h, and epoxidation was ceased. (C) Catalyst was
readded after a gap of 2 h, and maximum epoxidation was observed
after 3 h. In all cases, the yield was calculated using gas
chromatography.
1
4e,46−48
via a radical-based mechanism.
On the other hand, the
same reaction in the presence of dioxygen displayed the
formation of benzoic acid and aldehyde products as a result of
dioxygen involvement (see Table S7, Supporting Information).
Thus, the proposed reaction mechanism is shown in Scheme 2.
with identical metalloligands 1 and 2 for the epoxidation
reactions. Satisfyingly, negligible epoxidations (<5%) were
observed for such networks, which we believe are due to
surface-based activation of TBHP. Collectively, such experi-
ments substantiate that the soluble and/or catalytic active
species are not leached out from the present networks under
the employed reaction conditions and networks 3−6 are acting
as genuine heterogeneous catalysts.
Recyclability Experiments. The solid heterogeneous
networks 3−6 can readily be separated from the reaction
mixture by simple filtration: a feature unique to the
heterogeneous and/or solid immobilized catalysts that
conveniently allows for studies of recyclability of the catalysts.
Indeed, all four networks are recyclable under the reaction
conditions up to several cycles, and Table S9, Supporting
Information, displays results with two representative networks.
It was noted that the amount of aldehyde was slightly increased
with reused networks when compared to pristine samples.
However, all four networks retain their crystallinity, reactivity,
n+
First, reaction between the secondary M ion and TBHP takes
n+1
n+1
place to form a M −peroxy adduct. The M −peroxy adduct
n+
then releases a tert-butoxy radical and regenerates M ion. This
is followed by a reaction between tert-butoxy radical and olefin
to form a tert-butoxperoxy species that further undergoes
49
migration of oxygen to form the epoxide as the product and
50
tert-BuOH as the byproduct. Furthermore, possible involve-
ment of dioxygen or tert-butoxy radicals with the tert-
butoxperoxy species will result in some acid and aldehyde
byproducts.
CONCLUSION
■
3+
2+
3+
To summarize, we synthesized {Co −Mn } and {Co −
2
+
3+
Co } heterobimetallic coordination networks using Co -
based metalloligands appended with arylcarboxylate groups as
the building blocks. We successfully demonstrated that the
immobilized catalytically active secondary Mn2+ and Co ions
efficiently carry out epoxidation of a number of olefins without
2+
a
Scheme 2. Proposed Reaction Mechanism for the Epoxidation Reaction Catalyzed by Networks 3−6
aM stands for Mn2+ ions in 3 and 4 and Co2+ ions in 5 and 6, respectively.
K
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any requirement of solvent or additive. The heterogeneous
networks demonstrate outstanding structural stability and
therefore good reusability. These examples display great
promise in the development of crystalline coordination
networks for catalytic applications.
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ASSOCIATED CONTENT
Supporting Information
■
(8) Kurmoo, M. Chem. Soc. Rev. 2009, 38, 1353−1379.
*
S
(9) (a) Zhu, Q.-L.; Xu, Q. Chem. Soc. Rev. 2014, 43, 5468−5512.
(b) Falcaro, P.; Ricco, R.; Doherty, C. M.; Liang, K.; Hill, A. J.; Styles,
M. J. Chem. Soc. Rev. 2014, 43, 5513−5560. (c) Furukawa, S.; Reboul,
J.; Diring, S.; Sumida, K.; Kitagawa, S. Chem. Soc. Rev. 2014, 43, 5700−
X-ray crystallographic data for networks 3−6 in CIF format;
figures for the diffuse-reflectance absorption spectra, XRPD
patterns, crystal structures, TGA-DSC plots, FTIR spectra,
NMR spectra; tables for X-ray data collection, bonding
parameters, catalysis, reusability, and size calculations of a few
substrates and/or reagents. This material is available free of
charge via the Internet at http://pubs.acs.org.
5
(
734.
10) (a) Liu, J.; Chen, L.; Cui, H.; Zhang, J.; Zhang, L.; Su, C.-Y.
Chem. Soc. Rev. 2014, 43, 6011−6061. (b) Dhakshinamoorthy, A.;
Garcia, H. Chem. Soc. Rev. 2014, 43, 5750−5765. (c) Corma, A.;
Garcia, H.; Xamena, F. X. L. I. Chem. Rev. 2010, 110, 4606−4655.
(
d) Lee, J. Y.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. B.
AUTHOR INFORMATION
Corresponding Author
Fax: +91-11-2766 6605. E-mail: rgupta@chemistry.du.ac.in.
■
*
T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450−1459. (e) Ma, L.;
Abney, C.; Lin, W. Chem. Soc. Rev. 2009, 38, 1248−1256.
(11) (a) Zhao, M.; Ou, S.; Wu, C.-D. Acc. Chem. Res. 2014, 47,
1
199−1207. (b) Horike, S.; Dinca, M.; Tamaki, K.; Long, J. R. J. Am.
Author Contributions
†
Chem. Soc. 2008, 130, 5854−5855. (c) Roberts, J. M.; Fini, B. M.;
Sarjeant, A. A.; Farha, O. K.; Hupp, J. T.; Scheidt, K. A. J. Am. Chem.
Soc. 2012, 134, 3334−3337. (d) Wang, C.; Zheng, M.; Lin, W. J. Phys.
Chem. Lett. 2011, 2, 1701−1709.
Girijesh Kumar and Gulshan Kumar have contributed equally.
Notes
The authors declare no competing financial interest.
(12) Kumar, G.; Gupta, R. Chem. Soc. Rev. 2013, 42, 9403−9453.
ACKNOWLEDGMENTS
(13) (a) Caron, S.; Dugger, R. W.; Ruggeri, S. G.; Ragan, J. A.; Ripin,
D. H. B. Chem. Rev. 2006, 106, 2943−2989. (b) Sheldon, R. A.; Kochi,
J. K. Metal Catalyzed Oxidation of Organic Compounds; Academic
Press: New York, 1981. (c) Meunier, B. Biomimetic Oxidations
Catalyzed by Transition Metal Complexes; Imperial College Press:
London, 2000.
■
R.G. gratefully acknowledges financial support from the Science
and Engineering Research Board (SERB), New Delhi, and the
University of Delhi. We acknowledge the CIF-USIC facility of
this university for the crystallographic data and analytical
facilities and AIRF-JNU for the GC-MS and PXRD studies. We
sincerely thank Dr. A. Sakthivel (DU) and Dr. Ahmad Husain
(
14) (a) Fei, H.; Shin, J. W.; Meng, Y. S.; Adelhardt, M.; Sutter, J.;
Meyer, K.; Cohen, S. M. J. Am. Chem. Soc. 2014, 136, 4965−4973.
b) Sen, R.; Saha, D.; Mal, M.; Brandao, P.; Rogez, G.; Lin, Z. Eur. J.
(
IACS, Kolkata) for their assistance in sorption and topological
(
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Inorg. Chem. 2013, 5020−5026. (c) Sen, R.; Saha, D.; Brandao, P.; Lin,
Z. Eur. J. Inorg. Chem. 2013, 5103−5109. (d) Rich, J.; Manrique, E.;
Molton, F.; Duboc, C.; Collomb, M. N.; Rodriguez, M.; Romero, I.
Eur. J. Inorg. Chem. 2014, 2663−2670. (e) Zhang, J.; Biradar, A. V.;
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