Enhanced Catalysis Activity in a Coordinatively Unsaturated Cobalt-MOF Generated via Single-Crystal-to-Single-Crystal Dehydration

Article abstract of DOI:10.1021/acs.inorgchem.5b00606

Hydrothermal reaction of Co(NO₃)₂ and terphenyl-3,2″,5″,3′-tetracarboxyate (H₄tpta) generated Co₃(OH)₂ chains based 3D coordination framework Co₃(OH)₂(tpta)(H₂O)₄ (1) that suffered from single-crystal-to-single-crystal dehydration by heating at 160 °C and was transformed into dehydrated Co₃(OH)₂(tpta) (1a). During the dehydration course, the local coordination environment of part of the Co atoms was transformed from saturated octahedron to coordinatively unsaturated tetrahedron. Heterogenous catalytic experiments on allylic oxidation of cyclohexene show that dehydrated 1a has 6 times enhanced catalytic activity than as-synthesized 1 by using tert-butyl hydroperoxide (t-BuOOH) as oxidant. The activation energy for the oxidation of cylcohexene with 1a catalyst was 67.3 kJ/mol, far below the value with 1 catalysts, which clearly suggested that coordinatively unsaturated Co^II sites in 1a have played a significant role in decreasing the activation energy. It is interestingly found that heterogeneous catalytic oxidation of cyclohexene in 1a not only gives the higher conversion of 73.6% but also shows very high selectivity toward 2-cyclohexene-1-one (ca. 64.9%), as evidenced in high turnover numbers (ca. 161) based on the open Co(II) sites of 1a catalyst. Further experiments with a radical trap indicate a radical chain mechanism. This work demonstrates that creativity of coordinatively unsaturated metal sites in MOFs could significantly enhance heterogeneous catalytic activity and selectivity. (Graph Presented).

Full text of DOI:10.1021/acs.inorgchem.5b00606

Article
pubs.acs.org/IC
Enhanced Catalysis Activity in a Coordinatively Unsaturated Cobalt-
MOF Generated via Single-Crystal-to-Single-Crystal Dehydration
Hai-Yun Ren, Ru-Xin Yao, and Xian-Ming Zhang*
School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, P. R. China
S
* Supporting Information
ABSTRACT: Hydrothermal reaction of Co(NO3)2 and terphenyl-3,2″,5″,3′-tetracarbox-
yate (H4tpta) generated Co3(OH)2 chains based 3D coordination framework Co3(OH)2-
(tpta)(H2O)4 (1) that suffered from single-crystal-to-single-crystal dehydration by heating
at 160 °C and was transformed into dehydrated Co3(OH)2(tpta) (1a). During the
dehydration course, the local coordination environment of part of the Co atoms was
transformed from saturated octahedron to coordinatively unsaturated tetrahedron.
Heterogenous catalytic experiments on allylic oxidation of cyclohexene show that
dehydrated 1a has 6 times enhanced catalytic activity than as-synthesized 1 by using
tert-butyl hydroperoxide (t-BuOOH) as oxidant. The activation energy for the oxidation of
cylcohexene with 1a catalyst was 67.3 kJ/mol, far below the value with 1 catalysts, which
clearly suggested that coordinatively unsaturated CoII sites in 1a have played a significant
role in decreasing the activation energy. It is interestingly found that heterogeneous
catalytic oxidation of cyclohexene in 1a not only gives the higher conversion of 73.6% but
also shows very high selectivity toward 2-cyclohexene-1-one (ca. 64.9%), as evidenced in
high turnover numbers (ca. 161) based on the open Co(II) sites of 1a catalyst. Further experiments with a radical trap indicate a
radical chain mechanism. This work demonstrates that creativity of coordinatively unsaturated metal sites in MOFs could
significantly enhance heterogeneous catalytic activity and selectivity.
the inorganic component of frameworks by direct synthesis of
the envisaged scaffold or by postsynthetic modification. For
example, the metal sites with unsaturated coordination
environments can be created via removal of terminal
coordinated water or other solvent molecules, which can be
utilized as catalytically active sites.5 However, creation of active
sites via external stimuli such as heat or mechanic force will
generally produce defects and damaged regions, which prevents
at the atomic level characterization of coordinatively unsatu-
rated metal centers via X-ray single-crystal diffraction. There-
fore, it is still a challenge to create coordinatively unsaturated
metal centers via single-crystal-to-single-crystal (SCSC) trans-
formation.6
In the ongoing research of porous catalytic frameworks,
cobalt based MOFs attract our attention, since Co(II) atoms
could display varied coordination geometries such as
octahedral, tetrahedral, square-pyramidal, trigonal-bipyramidal,
and square-planar. The flexible and diverse local coordination
geometries of cobalt make it easy to engineer coordinatively
unsaturated metal centers via SCSC transformation. Activated
Co(II)-MOFs were found to catalyze allylic oxidation reactions
of cyclohexene with O2 and/or t-BuOOH.7 However, 2-
cyclohexen-1-one, a kind of α,β-unsaturated ketones, only as
traces occurs in these oxidation reactions. Recently, Cr-MIL-
101 could behave as a heterogeneous catalyst to give main α,β-
INTRODUCTION
■
The radical oxidations of cyclic olefin are widely used in the
bulk chemical industry as well as the synthesis of value-added
fine chemicals.1 Among these cyclic olefins, oxidation of
cyclohexene is especially important since the oxidation
products, α,β-unsaturated ketones, are valuable intermediates
in the fragrance industry and organic synthesis. Although many
studies have revealed that the oxidation of cyclohexene could be
selectively catalyzed by homogeneous transition-metal (Fe, Cr,
Ru, Mn, Cu, Co, etc.) catalysts,2 their performance is still
limited by the great difficulty in separation and recyclable use.
From an environmental and an economic viewpoint,
corresponding heterogeneous catalysts are desired to improve
the processes by offering numbers of advantages over
homogeneous catalysts, including easy separation, efficient
recycling, minimizing metal trace in the product, and improved
handling and process control.3 However, it remains very hard
for heterogeneous catalysts to explain the catalytic mechanisms
because many more complex techniques are needed to study
the adsorbed active species on solid surfaces in a heterogeneous
catalytic system.
Recently, metal−organic frameworks (MOFs) based hetero-
geneous catalysts have been attracting the interest of the
material scientists because they precede general immobilized
catalysts in several important ways, such as high catalyst
loadings, more accessible and identifiable catalytic centers, and
the more flexible design of structural building units.4 The
catalytically active sites can be implemented at the organic or
Received: March 17, 2015
© XXXX American Chemical Society
A
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Scheme 1. (a) In Situ SCSC Dehydration from 1 to 1a. (b) 1a-Catalyzed Allylic Oxidation of Cyclohexene toward α,β-
Unsaturated Ketones
pink pillar crystals of 1 in a yield of 78.6% (based on H4tpta) were
recovered. Anal. Calcd (%) for C22H20Co3O14 (1): C, 38.56; H, 2.94.
Found: C, 38.35; H, 2.83. IR data (KBr, cm−1): 3566(s), 2960(b),
1584(s), 1410(s), 1150(w), 774(s), 586(s).
unsaturated ketones in the reactions, but its conversion only
achieved 16%.8 Additionally, little information is available about
the specific structures of coordinatively unsaturated metal
centers as active sites in these works,7−9 which made their
oxidation mechanisms feeble and futile.
Co3(OH)2(tpta) (1a). Complex 1a was obtained by heating the
crystal sample of 1 at 160 °C. Anal. Calcd for C22H10Co3O10 (1a): C,
43.24; H, 1.65. Found: C, 43.30; H, 1.68. IR (KBr, cm−1): 3608(w),
3563(s), 2989(w), 1591(s), 1396(s), 1266(w), 773(s), 590(m).
X-ray Crystallography. X-ray single-crystal diffraction data for 1
at 298(2) K and high-temperature X-ray single-crystal diffraction data
for 1a at 458(2) K under a N2 atmosphere were collected on an
Agilent Technologies Gemini EOS diffractometer using Mo Kα
radiation (λ = 0.71073 Å). The program SAINT was used for
integration of the diffraction profiles, and the program SADABS was
used for absorption correction. All the structures were solved by direct
methods using the SHELXS program of the SHELXTL package and
refined by a full-matrix least-squares technique with SHELXL.10 All
nonhydrogen atoms were refined with anisotropic thermal parameters.
Hydrogen atoms of the organic ligand were generated theoretically
onto the specific carbon and refined isotropically with fixed thermal
factors. Further details for structural analysis are summarized in Table
1, and selected bond lengths and angles are shown in Table S1 of the
Supporting Information.
In this Article, by in situ high-temperature SCSC
dehydration, Co3(OH)2(tpta)(H2O)4 (1) was transformed
into Co3(OH)2(tpta) (1a), during which the local coordination
environment of part of the Co atoms was changed from
octahedron to coordinatively unsaturated tetrahedron (Scheme
1a). Particular attention is given to investigation into the effect
of open CoII sites on the catalytic activity. We found the 1a
exhibits an excellent activity in the allylic oxidation of
cyclohexene using tert-butylhydroperoxide (t-BuOOH) as
oxidant and selectively catalyzes the oxidation reaction toward
formation of α,β-unsaturated ketone (Scheme 1b). The higher
catalytic activity and selectivity are rare, if not unique, in the
field. Furthermore, by the calculation of activation energy and
radical trap experiments, we proposed a plausible mechanism to
rationalize the high catalytic activation and selectivity for the 1a
catalytic oxidation.
Catalytic Oxidation of Cyclohexene. 1 and 1a were added to a
solution of cyclohexene (3.8 mmol), t-BuOOH (14 mmol), and 1,2,4-
trichlorobenzene (1 mmol; internal standard) at RT. The reaction
mixture was stirred at 70 °C. Aliquots of the reaction mixture (about
0.1 mL) were removed after time intervals indicated in the main text.
Each sample was diluted with cyclohexane (1 mL) and filtered through
EXPERIMENTAL SECTION
■
Materials and Methods. All chemicals were analytically pure from
commercial sources and used without further purification, unless
otherwise noted. 1,2,4-Trichlorobenzene was chromatographically
pure. Cyclohexane and cyclohexene were dried with anhydrous
CaCl2. t-BuOOH was dried with anhydrous MgSO4 in an ice bath.
Elemental analyses were performed on a Vario EL-II analyzer. FTIR
spectra were recorded from KBr pellets in the range of 4000−400
cm−1 on a PerkinElmer Spectrum BX FT-IR spectrometer. Variable-
temperature powder X-ray diffraction (VTPXRD) and powder X-ray
diffraction (PXRD) data were collected in a Bruker D8 advance
diffractometer. The purity of all compounds was confirmed by
comparison of experimental PXRD patterns with the simulated pattern
derived from the X-ray single-crystal data compound. The
thermogravimetric analyses (TGA) were carried out in an air
atmosphere using SETARAM LABSYS equipment at a heating rate
of 10 °C/min. N2 adsorption measurement was performed using a
Micromeritics ASAP 2020HD88 surface area and pore size analyzer,
which was conducted up to a relative pressure (p/p0) of 1.0 on 1a at
STP. UV−vis absorption spectra were recorded on a Cary 5000
(VARIAN) UV−vis−NIR spectrophotometer in a cell with a 1 mm
width. GC analysis was performed using an AGILENT 7820 gas
chromatograph with a flame ionization detector (FID) and a cross-
linked (95%)-dimethyl-(5%)-diphenylpolysiloxane (HP-5) capillary
column of 30 m in length. GC/MS analysis was performed using a
Thermo Fisher Polaris Q (Ion Trap) gas chromatograph/mass
spectrometer and a cross-linked (95%)-dimethyl-(5%)-diphenylpoly-
siloxane (DB-5) capillary column of 30 m in length. Atomic absorption
spectroscopy (AAS) was performed on a Thermo S2 atomic
absorption spectrometer.
Table 1. Crystallographic Data and Structure Refinement for
1 and 1a
1
1a
formula
fw
C22H20Co3O14
685.17
C22H10Co3O10
611.09
crystal system
space group
a (Å)
monoclinic
P21/c
monoclinic
P21/c
10.8471(5)
6.3389(3)
16.8839(8)
90
10.769(3)
5.8370(19)
17.706(5)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
105.592(5)
90
102.14(3)
90
1118.20(9)
4
1088.0(6)
4
ρcalc (g cm−3
)
2.035
1.865
F(000)
690
606
reflections
4443/2278
2278/184
1.052
4195/2000
2000/155
1.068
data/parameters
S
a
R1
0.0573
0.0790
b
wR2
0.1549
0.1548
Syntheses. Co3(OH)2(tpta)(H2O)4 (1). A mixture of Co(NO3)2
(0.116 g, 0.40 mmol), H4tpta (0.042 g, 0.10 mmol), HClO4 (0.15 mL/
3 M, 0.45 mmol), DMF (3 mL), and H2O (4 mL) was stirred and then
sealed in a 15 mL Teflon-lined stainless autoclave at 150 °C for 4 days.
After it was cooled to room temperature and subjected to filtration,
Δρmax/Δρmin (e A−3
)
1.211/−1.408
1.019/−0.636
a
b
2
2
R1 = ∑∥Fo| − |Fc∥/∑|Fo|. wR2 = [∑[w(Fo − Fc )2]/
2
∑[w(Fo )2]]1/2
.
B
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Figure 1. Views of the coordination environments of CoII atoms in 1 (a) and 1a (b); hydrogen atoms have been omitted for clarity. (Symmetry
codes: Compound 1 (a) −x + 2, −y + 2, −z + 1; (b) x, y + 1, z; (c) −x + 2, −y + 1, −z + 1; (d) x + 1, −y + 3/2, z + 1/2; (e) −x + 1, y + 1/2, −z +
1/2; (f) −x + 1, −y + 1, −z. Compound 1a (a) x, −y + 3/2, z − 1/2; (b) −x, y − 1/2, −z + 1/2; (c) −x, −y + 1, −z; (d) x, y + 1, z; (e) −x, y + 1/2,
−z + 1/2; (f) −x, −y + 2, −z; (g) x, y − 1, z; (h) −x + 1, −y + 2, −z + 1.
Figure 2. A color change in SCSC dehydration from 1 to 1a.
a 0.25 mm Acrodisc nylon filter. Then, the sample was identified by
GC−MS and quantified by GC. To investigate the influence of crystal
size on reaction conversion and yield, polycrystalline samples were
ground and sieved into the following particle size ranges: 0.2−0.3,
0.15−0.2, 0.125−0.15, 0.08−0.125, 0.063−0.08, 0.04−0.063, and
0.025−0.04 μm. Since catalytic activity was known to be strongly
dependent on the surface area of the catalyst, the curves of conversion
and yield as a function of particle size from the measurements made on
ground crystals for 1a are shown in Figures S7 and S8 (Supporting
Information). Different amounts of 1a were reacted with cyclohexene
(3.8 mmol) and t-BuOOH (14 mmol). For investigations on metal
leaching from the catalyst, the hot reaction mixture was filtered at the
reaction temperature after 3 h in the case of 1a. To investigate catalyst
activities in subsequent multiple runs, the catalysts were separated
from the reaction mixture by centrifugation and rinsed five times with
cyclohexane before reuse. All yields and conversions were based on
cyclohexene.
O angles are 180.000(1)°. The Co2 site also adopts a distorted
octahedral geometry, coordinated by two hydroxides (O5,
O5c), two carboxylate oxygen atoms (O1, O3d), and two
coordinated water molecules (O1w, O2w). The Co2−O
distances are in the range of 2.055(4)−2.184(3) Å. The cis
and trans O−Co2−O angles are in the range of 81.25(14)−
97.02(15)° and 168.48(16)−176.47(15)°, respectively. The
CoII ions are bridged by μ3-OH groups to form a zigzag
Co3(OH)2 ladder, in which three adjacent CoII ions are
arranged into an approximate isosceles triangle. Each isosceles
triangle consists of a symmetrically related pair of Co2 atoms
(Co2 and Co2c) and a Co1 atom with Co···Co distances of
3.118(2), 3.5145(6), and 3.5494(6) Å. The Co2−O−Co2c,
Co1−O−Co2, and Co1−O−Co2c angles are 98.62(15),
117.54(18), and 119.50(18)°, respectively. As observed and
confirmed in the cobalt hydroxyl derivatives,10 the Co2 atoms
sharing two μ3-OH groups are related by ferromagnetic
interaction, while antiferromagnetic exchanges might occur
between Co1 and Co2 and Co1 and Co2c. As shown in Figure
S1 (Supporting Information), 1 adopts a new 4-connected
binodal net. The Co3(OH)2 chains as a zigzag ladder are linked
by tpta4− groups which act as 4-connected nodes to furnish the
RESULTS AND DISCUSSIONS
■
Description of Structures. Single-crystal X-ray diffraction
analysis reveals that compound 1 is a 3D neutral framework
based on 1D zigzag chains Co3(OH)2. Compound 1 crystallizes
in the monoclinic space group P21/c with an asymmetric unit
consisting of two crystallographically independent CoII ions,
half tpta4−, one hydroxyl, and two coordinated water molecules
in Figure 1. The Co1 localizes special positions with a site
occupancy of 0.5. The dihedral angle of the two phenyls of
tpta4− is 121.2°, in agreement with the semirigid nature of
tpta4−. The Co1 site adopts an octahedral geometry,
coordinated by four carboxylate oxygen atoms (O2b, O2c, O
4d, O4e) in the equatorial plane and two μ3-OH groups (O5,
O5a) in the axial position. The Co1−O distances are in the
range of 2.054(3)−2.074(4) Å. The cis O−Co1−O angles are
in the range of 84.15(14)−95.85(14)°, and the trans O−Co1−
3D framework with a Schlafli symbol [42.63.8]4[62.84].
̈
In Situ SCSC Transformation. Thermogravimetric analysis
(TGA), in situ variable-temperature powder X-ray diffraction
(VTPXRD) studies, and in situ high-temperature single-crystal
X-ray in the single crystalline phase were all carried out to
investigate the SCSC transformations induced by removel of
coordinated water molecules in 1. As shown in Figure S4
(Supporting Information), the VTPXRD differences for 1
recorded in the temperature range of 25−260 °C suggest that
coordinated water molecules might be removed around 150−
C
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160 °C, which is well documented by the TGA of the first
weight loss of 10.2% (calcd 10.6% for two water molecules) in
the temperature range of 155−220 °C (Figure S5, Supporting
Information). Then, a crystal of 1 was in situ heated to 160 °C
to obtain the dehydrated single crystals of Co3(OH)2(tpta)
(1a), which is accompanied by a color change from pink to
dark violet (Figure 2). In situ high-temperature single-crystal X-
ray diffraction analysis of 1a shows that the 3D framework
remains intact upon dehydration. Compared with 1, the Co1
atom of 1a shows a squeezed octahedral geometry, in which the
equatorial Co1−O (O1, O1c, O5, O5c) bond lengths slightly
stretch about 0.130 Å, and the axial Co1−O (O1, O1c) bond
lengths are contracted by around 0.172 Å. The greatest
difference in metal coordination environments is that the
octahedral environment of Co2 in 1 has been changed into a
tetrahedron in the dehydrated phase 1a; the average Co2−O
length in dehydrated 1a is contracted by 0.117 Å. The changes
in the O−Co1−O and O−Co2−O angles are limited to 5.53°
and 38.68°, respectively. Compared to the zigzag Co3(OH)2
ladder in 1, the average Co···Co distances in 1a are contracted
0.300 Å, and the changes in Co−O−Co angles are limited to
21.70°. However, the unit cell contains no residual solvent
accessible void revealed by PLANTON.12 The lack of pores can
be also confirmed by the negligible N2 adsorption at cryogenic
temperatures (Figure S3, Supporting Information).
cyclohexane (10 mL) at 70 °C, giving a conversion (ca. 29.8%)
of the oxidant product in 24 h (Figure S6a, Supporting
Information). Recently, most reports about the catalytic
oxidation of cyclohexene by transition-metal CoII complexes
display high selectivity toward formation of tert-butyl-2-
cyclohexenyl-1-peroxide.7a,c,15 It is very interesting that 1a
exhibits a higher selectivity toward formation of 2-cyclohexene-
1-one (ca. 66.5%) in 12 h (Figure S6b, Supporting
Information). A further increase of reaction time to 24 h did
not bring about higher conversion and more excellent
selectivity. Absence of an accessible void in the unit cell
suggests that the open Co(II) sites on the surfaces should be
responsible for catalytic activity. To investigate the effects of the
particle size of 1a on conversion and selectivity, different
particle sizes of 1a were employed in the reaction (Figures S7
and S8, Supporting Information, and Table 2). When the
reaction was carried out using the particle size of 0.025−0.04
μm of 1a as catalyst, a higher conversion (ca. 73.6%) was
obtained. More intriguingly, the selectivity toward formation of
cyclohexenone was still up to 64.9% in 7 h. The TON was as
high as about ca.161 (entry 8). These results could be
attributed to increasing the surface area of 1a with more
catalytic CoII sites. To further prove the positive effects exerted
by open CoII sites, 1 (0.096 mmol) in the same size (0.025−
0.04 μm) was tested under the same reaction conditions as
well; 12.9% conversion of the oxidation reaction was obtained
(Figure S9, Supporting Information), which is probably due to
a trace of open CoII sites by grinding. However, much higher
conversion was not observed with increased mass of 1a.
Heterogeneous Nature and Recyclability. To confirm
the heterogeneous nature of the catalytic reaction, 1a was
removed from the hot solution by filtration after reaching
approximately 50% conversion (which took about 3 h for 1a).
It is clear from Figure 4 that no further catalytic conversion was
observed, indicating that cobalt ions (if any) leaching from 1a
are not responsible for the observed activity. The filtrate was
also analyzed by atomic absorption spectroscopy (AAS), which
indicated that no free cobalt(II) ions in the filtrate were
detected due to the concentration of cobalt(II) ions beyond the
lower detection limit of the AAS instrument. The results
indicated that 1a behaves as a truly heterogeneous catalyst. The
1a catalyst can be easily recovered and reused in the oxidation
three times with negligible loss of efficiency (see Figure S10,
Supporting Information), which indicated reusable and
efficiently catalytic activity of 1a. The crystalline structure of
1a catalyst was mostly retained after three catalytic cycles
(Figure S4, Supporting Information). The higher stability
toward degradation during the catalytic process correlates with
its higher thermal stability up to 260 °C.
UV−Visible Spectra. Electronic spectra of phase-pure
microcrystal samples of 1 and 1a are shown in Figure 3. The
Figure 3. Solid UV−visible spectra of 1 and 1a.
pink color of compound 1 is consistent with an octahedral
environment of divalent cobalt. Compound 1 shows two broad
absorption bands. One centered at 256 nm can be assigned to n
→ π* transitions, which may be mainly ligand-centered
electronic transitions; the other at 519 nm might be ascribed
4
to [4T1g(F) → T1g(P)] transitions, which are typical of high-
spin hexacoordinated octahedral CoII species, being well in
accord with the coordination environment of Co atoms in 1.13
Compared with 1, the absorption bands corresponding to n →
π* transitions for the dehydrated phase 1a are red-shifted to
303 nm. The absorption bands at 530 nm might be ascribed to
Calculation of Catalytic Activity. As shown in Figure 5,
the rate constant (k) for these reactions was determined at
different temperatures (50, 60, 70, and 90 °C), which was used
to calculate the energy of activation. According to the Arrhenius
equation
4
[4T1g(F) → T1g(P)] transitions, corresponding to Co1 in
octahedral coordination. The dual absorption peaks at 636 nm
[4A2 → 4T1] and 596 nm [4A2 → 4T2] might be due to Co2 in
the tetrahedral coordination geometry, in good agreement with
the literatures that the tetrahedron-containing cobalt com-
pounds exhibit typical peaks at lower energies.14
k = Ae−(E /RT
)
a
(1)
After the logarithm, the whole of eq 1 becomes
Catalytic Allylic Oxidations of Cyclohexene with 1a as
Catalyst. Herein, the liquid-phase oxidation of cyclohexene
was employed as a test reaction in the screening of catalytic
activity parameters over powder 1a. The reaction was first
tested on the oxidation reaction using t-BuOOH as oxidant in
Ea
RT
ln k = ln A −
(2)
where R is the gas constant; T, the absolute temperature; A, the
preexponential factor; and Ea, activation energy. The plot of ln
D
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Table 2. Allylic Oxidation of Cyclohexene with 1a as Catalyst
a
Reaction conditions: cyclohexene (3.8 mmol), t-BuOOH (14 mmol), 1,2,4-trichlorobenzene (1 mmol; as internal standard), 1a (based on cobalt),
b
c
temp = 70 °C, cyclohexane (10 mL). Solvent-free. TON(turnover number) = moles converted/mol of active Co2 site.
As shown in Figure 5 (inset), the conversion in the presence
of 1a increased with the reaction temperature from 50 to 70 °C,
but decreased beyond 70 °C, which may be due to the
decomposition of t-BuOOH without oxidizing cyclohexene.
The rate constant (k) for the oxidation of cyclohexene with 1a
catalyst was increased until 70 °C, but on further increasing the
temperature beyond 70 °C, the rate constant was decreased
(Figure 5). The initial increase in rate constant (k) for the
conversion of cyclohexene was an indication that the oxidation
of cyclohexene was an energy-activated process. The energy of
activation for the oxidation of cylcohexene in the presence of 1a
was 67.3 kJ/mol, far below the value with 1 as catalysts (129.6
kJ/mol) (Table 3), which are well documented that 1a catalysts
played a significant role in decreasing the energy of activation.
Oxidation Mechanism with 1a Catalyst. The radical
nature of the oxidation of cyclohexene (1a as catalyst) has been
confirmed by radical trap experiments. As shown in Table S2
(Supporting Information), the addition of radical scavengers,
2,4,6-tris-tert-butylphenole (TTBP) and 2,2,6,6-tetramethylpi-
peridine-1-oxyl (TEMPO), dramatically suppressed the for-
Figure 4. Allylic oxidation of cyclohexene with 1a as catalyst (black)
and filtration of 1a (gray).
Table 3. Conversion and Kinetic Parameters at Optimum
Molar Ratio of the Allylic Oxidation of Cyclohexene with 1a
and 1 as Catalyst, Respectively
Figure 5. Rate constant (k) vs temperature (°C) curves for the allylic
oxidation of cyclohexene with 1a as catalyst (Inset: conversion (%) vs
temperature (°C) curves for the allylic oxidation of cyclohexene with
1a as catalyst).
catalysts
temp. (°C)
con (wt %)
k (mol−1·L3·s−1
)
Ea (kJ/mol)
1a
50
60
70
50
60
70
42.4
60.4
73.6
4.8
9.01 × 10−7
1.90 × 10−6
3.89 × 10−6
1.54 × 10−8
6.23 × 10−8
2.34 × 10−7
67.3
1
129.6
k versus T−1 gave an activation energy (Ea) in units of kJ·mol−1
(Figures S11 and S12, Supporting Information).
8.2
12.9
E
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Scheme 2. Proposed Reaction Pathways of the 1a-Catalyzed Allylic Oxidation of Cyclohexene
mation of 2-cyclohexen-1-one, tert-butyl-2-cyclohexenyl-1-per-
oxide, and cyclohexene oxide, which strongly indicates the
dominant role of radical reactions in this oxidation.
decreasing the energy of activation. Studies extending the scope
of cyclic olefin that could be tolerated will be performed in the
near future. Our finding will help one to understand the
structure−activity relations of a catalyst, which might lead to
commercial exploitation of the highly active Co-MOF catalysts
toward formation of α,β unsaturated ketone.
On the basis of our results and previous mechanistic studies
of cobalt catalysis,7c,8,16 we put forward a plausible mechanism
to elucidate the observed selectivity pattern (Scheme 2). In the
initiation step, the flexibly assembled Lewis acid Co(II)2 ions in
1a might be preferentially coordinated by an oxygen atom from
t-BuOOH according to the soft and hard acids and bases
theory. Electrons flow from the Co(II)2 atoms with
unsaturated coordination environments to the peroxy bond of
t-BuOOH, causing the weak O−O bond to break, and then tert-
butoxy radicals (t-BuO·) might be created. The propagation
steps: The resulting t-BuO· radical might abstract quickly a
hydrogen atom from another t-BuOOH molecule. Conse-
quently, a t-BuOO· radical and t-BuOH would be created. A
majority of the t-BuOO· radicals might react directly with
cyclohexene to yield a 3-cyclohexenyl radical. The resulting
cobalt hydroxy species via fission of the Co−O bond could
further form a ·OH radical, which likely reacts with another t-
BuOOH molecule to produce a t-BuOO· radical. Then, the t-
BuOO· radical might combine with a 3-cyclohexenyl radical to
result in tert-butyl-2-cyclohexenyl-1-peroxide. However, the O−
O bond of tert-butyl-2-cyclohexenyl-1-peroxide most likely
thermally decomposes at 70 °C to give main product 2-
cyclohexen-1-one, which strongly depends on the nature of the
hydroperoxide.1,8 Meanwhile, the cobalt hydroxy species could
recover the initial catalyst 1a, and the catalytic cycle starts over
again.
ASSOCIATED CONTENT
* Supporting Information
■
S
CCDC 1031319−1031320 in CIF format, crystal structural
data for 1 and 1a, experimental details, and additional figures
and tables. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.inorgchem.5b00606.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zhangxm@dns.sxnu.edu.cn (X.-M. Z.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the 973 Program (2012CB821701), the
Ministry of Education of China (Grant IRT1156), and NSFC
(Grants 20925101 and 21401119) is greatly appreciated.
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■
In situ SCSC dehydration transformation of cobalt atoms from
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