A highly efficient heterogeneous catalyst of cobalt-based coordination polymers for aerobic epoxidation of cyclohexene

Article abstract of DOI:10.1039/c8ce00899j

A novel heterogeneous catalyst of cobalt-based coordination polymers has been successfully fabricated using a tyrosine-based derivative. Heterogeneous catalytic experiments on allylic oxidation of cyclohexene indicate that the titled complexes present high catalytic activities using tert-butyl hydroperoxide (t-BuOOH) as an oxidant. The activation energy for the whole process of oxidation of cyclohexene has been calculated to be 25.5 kJ mol^-1, which indicates the important role of the selected ancillary ligands in the synthesis of the heterogeneous catalyst.

Full text of DOI:10.1039/c8ce00899j

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Tiddo J. Mooibroek, Antonio Frontera et al.
Towards design strategies for anion–π interactions in crystal engineering
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A Highly Efficient Heterogeneous Catalyst of Cobalt-based
Coordination Polymer For Aerobic Epoxidation of Cyclohexene
Fan Yua,*
Received 00th January 20xx,
Accepted 00th January 20xx
Abstract: A novel heterogeneous catalyst of cobalt-based coordination polymer with tyrosine-based
derivative had been successfully constructed. Heterogeneous catalytic experiments on allylic oxidation of
cyclohexene indicate that titled complex present highly catalytic activity by utilizing the tert-
butylhydroperoxide (t-BuOOH) as oxidant. The activation energy for the whole process of oxidation of
cylcohexene had been calculated as 25.5 kJ mol-1, which indicate the important role of the selected ancillary
ligands in the resulted heterogeneous catalyst.
DOI: 10.1039/x0xx00000x
www.rsc.org/
catalytic process.
Introduction
The research of Coordination Polymers ( CPs ) is undergoing
an explosive growth stage since CPs would exhibit the diverse
Conversion of alkenes into value-increased oxygenated
products is of a basic reaction in chemistry which has been
widely used in the bulk chemical industry and fine chemicals.1
Among the versatile alkenes, oxidation of cyclohexene is
especially important and extensively investigated because its
oxidation products are valuable intermediates. To some
structures, known spatial configurations and specific sites for
adsorbing the guest molecules4,5, which has been also utilized
as heterogeneous catalysts based on the several
considerations: 1) the catalytically active sites could be easily
implemented on the organic or the inorganic component of
CPs via different easy-ways as heat or mechanic force6; 2) the
separation and re-cyclic use of CPs based heterogeneous
catalysts could be easily achieved since the stable topological
structures of CPs7; 3) more importantly, CPs can explore the
mechanism of catalytic reaction at the micro level, which helps
us to further understand the subtle structural-activity
relationship between catalyst and catalytic reaction8. In
addition, as one unique advantage of CPs , the specific
configuration and active sites could be finetuned via crystal
engineering. The versatile choices of organic linkers would
change the configuration and functionality of the final
materials, which is very important for improving the
anticipated properties. Therefore, more efficient, green and
portable CPs-based catalysts are expected to boost the
development of related research fields via crystal engineering.
Through the previous investigation, the advantages of
catalyst must be originated from the coordination habits of
cobalt center and amino acid derivatives. Referred to the
related results, the adjustment of the final structures of CPs via
using different amino acid derivatives should obviously modify
the reaction pathway.9 Therefore, to further explore the
structural-activity relationship and catalytic mechanism of CPs-
based catalyst, the new type of L-tyrosine-based derivative
was adapted to adjust the corresponding catalytic effect
guided by the results achieved in crystal engineering. Tyrosine,
as one of the common amino acids, has shown widely
attractive applications in many fields, but rarely in catalytic
extent, traditional catalytic procedures produce
a large
number of environmentally undesirable wastes during the
reaction process. And therefore, how to attain the effectively
green reaction of catalytic oxidation of alkenes has been
becoming imperious demands.2
It has been revealed that transitional metal complexes
containing redox active transition metals are important
homogenous catalysts which can selectively oxidize alkenes
into the specific oxydate.3 However, the performance of these
catalysts is still limited, and the recovery of the catalysts from
the reaction systems is very difficult since the oxidation
reactions are usually performed in the liquid phase. Based on
the
environmental
and
economic
considerations,
heterogeneous catalysts are desired to improve the level of
environmental protection. Compared to homogeneous
catalysts, the heterogeneous ones possess specific advantages,
such as easy separation, efficient recycling and minimizing
metal trace in the final product. However, the mechanism of
the catalytic reaction between heterogeneous catalysts and
catalytic reactions is very difficult to understand, and more
systems are required to present the deep cognition of the
scope.
Herein,
a
novel
cobalt(II)-based
CP,
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{[Co(H2O)(L)2)2]
·
(H2O)}n (
1
, L = 3-(4-hydroxyphenyl)-2-(pyridin- with the Co(II) state.10 In addition, the L ligand was folded
interaction ( 4.028 Å ) between two
4-ylmethylamino) propanoic acid), constructed by tyrosine- caused by the weak π-π
based derivative had been successfully synthesized, and then aromatic rings ( center-to-center ). The adjacent cobalt centers
whose catalytic effect for oxidation of alkenes had been were further interconnected by one of the amino acid derivate
investigated.
via the decorated pyridyl-N atom (N1) to form the 1D helical
chain along b-axial direction. Each 1D chain binds onto
adjacent ones through hydrogen bonding interaction to
assemble the 3D supramolecular network. Viewed from the
packing mode, the hydroxyl of phenol and coordinated water
molecule locate toward the outside of chain structure, which
might be useful to improve its catalytic ability referred to other
CPs acted as heterogeneous catalysts. In addition, the
Results and discussions
Crystal structure of 1
Crystal samples of
1 were synthesized by slow evaporation at
room temperature, which were further characterized via single
crystal X-ray diffraction, elemental analyses and IR spectrum
( Figure S1 ). The existence of O-H, N-H and C-H groups could
be deduced from the broad peaks of stretching vibrations
around 3583-3532, 3445-3330 and 3103-2883 cm-1. The
asymmetric and symmetric stretching vibrations of
deprotonated carboxyl could be also validated via the peaks
around 1605 cm−1 and 1382 cm−1. In addition, the existence of
the pyridyl ring could be also verified via the stretching
vibrations at 1665, 1570, 1560 and 1470 cm-1. After heated,
the typical IR peaks could be still observed in the spectra,
indicating the reservation of specific function groups in
activated sample.
existence of hexa-coordinated Co(II) state in
1 could be also
validated by the absorption peaks around 510 nm in UV-Vis
( Figure S2 ) for the crystal samples10.
Fig. 2 (1): Conversion vs. Reaction time curves for the oxidation of cyclohexene in
the presence of
conversion curves at different temperature; (3) Simulation of lnk vs.
temperature; (4) Allylic oxidation of cyclohexene with as catalyst (circle) and
filtration of (square) at 55 . Reaction conditions: cyclohexene (12 mmol), t-
1 as catalyst at 35 , 45 , 55 and 65 ; (2): Simulation of the
1
1
BuOOH (18.5 mmol), 1,2,4-trichlorobenzene (1.5 mmol; as internal standard), 1
(0.075 mmol, based on cobalt).TON equal to 546, TON (turnover number) =
moles converted/mol of active Co site. TOF= 0.078 h−1
.
Fig. 1 Up: the view of asymmetric unit of
clarity. Asymmetric code: -X+2, Y+1/2, -Z+3/2; Down: The perspective view of 1D
helical chain in
1, hydrogen atoms were omitted for
Catalytic properties of 1
The purity of as-synthesized crystal samples had been
examined by PXRD, which presents the anticipated consistency
with simulated spectra from crystal structure ( Figure S3 ). The
minor difference between the simulated and experimental
patterns might be caused by the guest molecules in the lattice,
which was not fixed via the studies of X-ray diffraction.
Therefore, the activated samples obtained from the as-
synthesized crystal samples were directly utilized for catalytic
oxidation of cyclohexene with the oxidant of tert-butyl
hydroperoxide. When most of the reported cobalt CPs acted as
the catalyst in the oxidation of cyclohexene, 2-cyclohexen-1-
one is usually detected as the main product, and the extent of
catalytic effect is also not exciting. However, an exhilarating
1.
The investigation of crystal structure has been carried out at
room temperature. Crystal data and selected structural
parameters are listed in Table S1 and S2. Compound
1
crystallizes in the orthorhombic space group P2(1)2(1)2(1),
whose asymmetric unit contains two tyrosine derivates, one
cobalt ion and one coordinated water molecule ( Figure 1 ).
The central coblat ion adopts octahedral configuration and is
coordinated by one pyridyl-N atom, two carboxylate oxygen
atom, one aqua molecule and two aliphatic N atoms, while the
axial positions are occupied by two oxygen atoms from water
molecule and carboxylate group. Two adjacent de-protonated
amino acid derivates utilize their N-O chelate mode to occupy
the cis-coordination sites of center. The Co-N and -O bond
distances range from 2.088(4) to 2.201(3) Å, in accordance
catalytic effect could be achieved with the presence of 1 as the
catalyzer in the oxidation of cyclohexene, including not only
the different main product, but also the very fast reaction
2 | J. Name., 2012, 00, 1-3
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velocity. The catalytic results had been gathered in Table 1. catalytic results. Therefore, the titled complex could truly act
When the oxidation reaction was firstly monitored under the as the heterogeneous catalyst for oxidation of cyclohexene. In
routine conditions but without the the titled catalyst for addition, the cyclic test verified the reusable catalytic activity
twelve hours at 65
detected. Comparably, a very significant catalytic result could
be achieved after five hours under the same conditions in the the related chemical kinetics, the rate constant and activity
presence of as the catalyst. The maximum substrate energy, were further calculated. The catalytic experiments
conversion of 93 % could be given. Through the analysis by gas with the same conditions have been further carried out at 35,
chromatographic and mass spectrometric, the main product is 45 and 55 C ( Figure 2 ), which exhibit the excellent catalytic
tert-butyl-2-cyclohexenyl-1-peroxide ( P1 ), while the by- results even at 35 C with the maximum conversion of 65%
°C, no obvious catalytic result could be with negligible loss of efficiency.
To present the deep cognition for the catalytic activity of
1
,
1
°
°
products are 2-cyclohexen-1-one ( P2 ) and cyclohexene oxide after 5 h. The rate constants simulated from the experimental
( P3 ). In addition, compared to the usual catalytic reaction curve exhibit the increasing tendency along with the increase
with homogeneous catalyst based on cobalt CPs and the of reaction temperature, indicating the energy-activated
oxidant of alkyl hydroperoxides, the selectivity for P1 is also process for the oxidation of cyclohexene. The corresponding
very high, and calculated as 83% at 65
selectivity is higher than the reported heterogeneous catalysis the catalyst of
with the cobalt(II) complexes as catalyst, and manifest the according to Arrhenius equation ( Figure 2 ). Compared to
specific advantage of complex as the heterogeneous other reported Ea values of cobalt-based catalysts, the
catalyst.9,10 No additional increases of conversion and parameters for
was very small, validating the vital role in
selectivity could be detected since the reaction time promoting the process of related catalytic reaction.9,10
prolongated to twelve hours. In the previous work, it had been The selectivity of to the above description catalytic
°
C ( Table 1 ). Thus high activation energy (Ea) for the the oxidation of cylcohexene with
1
was finally determined as 25.5 kJ mol-1
1
1
1
concluded that even without obvious porous structure, the reaction had been further explored by substituting the
open metal sites on the surfaces should be also assigned to the substrate of cyclohexene with styrene, and different catalytic
corresponding catalytic results. Herein, for 1, the open cobalt results have been presented. Although with the same reaction
sites are more directly exposed to the periphery due to the conditions, styrene oxide as the main production had been
twistable chain-like structure. The more open configuration examined. The catalytic reaction was also carried out at 65 °C
could ensure the smaller spacial hindrance effect, and might for eleven hours, presenting the maximum conversion of 55%
be beneficial for the combination of cobalt center and hydroxyl ( Figure S10 ). However, the main reaction product is ascribed
radical for
1, which subsequently promises the excellent to styrene oxide, while the by-product is benzaldehyde with a
catalytic results.
very small amount. The different catalytic results for different
substrates manifests that the titled complex as heterogeneous
catalyst for olefin compounds has definite selectivity toward
different substrates.
Table 1. Conversion and Kinetic Parameters at Optimum Molar Ratio of the Allylic
Oxidation of Cyclohexene with 1 as Catalyst
tꢀBuOH
+
tꢀBuO
tꢀBuOOH
OOBu-t
+
+
Temp.
Total Con.
(wt %)
P1 Con.
P2 Con.
P3 Con.
k
Ea
( Kj mol-
1 )
Co(II)ꢀCP
tꢀBuOOH
HOꢀCo(III)ꢀCP
(
)
(wt %)
( wt %)
( wt %)
( L mol-1 s-1 )
tꢀBuOO
OH
+
+
H2O
Co(II)ꢀCP
35
45
55
65
67.58
79.16
83.98
89.62
77.22
86.69
84.71
82.97
10.93
6.88
7.04
6.27
11.85
6.43
4.28×10-5
5.68×10-5
6.80×10-5
1.08×10-4
25.5
Scheme 1. The proposed reaction mechanism of 1-catalyzed allylic oxidation of
cyclohexene
8.25
10.76
Oxidation Mechanism with 1 Catalyst.
By utilizing the 1 as heterogeneous catalysts, the main
production is tert-butyl-2-cyclohexenyl-1-peroxide. With the
consideration of final product and cobalt-based catalyst, the
mechanism of reaction pathway had been proposed as scheme
1. The diffusing peroxy radicals are firstly produced via the
reductive cleavage of t-BuOOH that coordinate onto cobalt(III)
centers in CP, and then induce the series of reaction.
The role of heterogeneous catalyst of
1 could be confirmed
by two additional experiments. Firstly, as shown in Figure 2, no
further catalytic conversion could be detected after removing
the solid catalysts from the hot reaction solution since the
reaction occurred for 3h. Secondly, no cobalt(II) ions could be
detected from the analysis of atomic absorption spectroscopy
(AAS), indicating the concentration of cobalt(II) ions in the
resulted filtrate was lower than the limitation of the
corresponding equipment. All of these two experiments
illustrate that rare cobalt ions leach from the solid catalyst.
And even there are some cobalt ions infiltrating into the
reaction solution, they would not be responsible for the
Initially, t-BuOOH molecules preferentially coordinate onto
cobalt centers by the utilization of oxygen atom, and occupy
the remnant Lewis acid site of cobalt centers.9,11 And then,
tert-butoxy radicals (t-BuO·) and HO-Co(III) matter tend to be
released due to the the breakdown of weak O−O bonds of tert-
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,
In conclusion, by utilizing the tyrosine-based derivative and
cobalt ions as the components of CP, a new type of highly
efficient heterogeneous catalyst for the oxidation of
cyclohexene have been successfully constructed, whose
detailed catalytic activity including activation energy,
selectivity, cyclic performances and versatile reaction
parameters had been investigated. By means of microscopic
structural analysis, we can understand the vital influence of
the spatial configuration of complexes on the final catalytic
effect, which further emphasize the important role of crystal
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We gratefully acknowledge the National Natural Science
Foundation of China ( nos. 51403079 ) and Jianghan university
for generous financial support.
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A novel heterogeneous catalyst of cobalt-based coordination polymer with tyrosine-based derivative had been
successfully constructed, which exhibits highly catalytic activity in changing the reaction pathway and lowering
the activation energy to 25.5 kJ mol-1.