Co(ii)-cluster-based metal-organic frameworks as efficient heterogeneous catalysts for selective oxidation of arylalkanes

Article abstract of DOI:10.1039/C8CE02151A

To explore metal-organic frameworks (MOFs) based on Co-clusters as heterogeneous catalysts to selectively catalyze the reaction of C-H bond oxidation of aromatic alkanes to their corresponding ketones, three MOFs {[Co₅(pmbcd)₂(μ₃-OH)₂(H₂O)₄(DMF)₂]·4DMF}_n (MOF 1), {[Co₂(pmbcd)(bpea)₂]·2H₂O·2DMF}_n (MOF 2), and {[Co₂(pmbcd)(dpp)₂]·3H₂O·2DMF}_n (MOF 3) (H₄pmbcd = 9,9′-(1,4-phenylenebis(methylene))bis(9H-carbazole-3,6-dicarboxylic acid), bpea = 1,2-bis(4-pyridyl)ethane, dpp = 1,3-di(4-pyridyl)propane) were successfully synthesized and structurally characterized. MOF 1 was constructed from a pentanuclear Co(ii) cluster and exhibited a porous framework with channels of 8 × 10 ?² along the b axis. MOF 2 was constructed from [Co₂(CO₂)₄] units and presented a porous three-dimensional (3D) framework with channels of 11 × 13 ?² along the b axis and of 10 × 12 ?² along the c axis. MOF 3 was a flat two-dimensional (2D) layer based on binuclear Co(ii) units when dpp as an auxiliary ligand was introduced. The Co₅-cluster-based MOF 1 exhibited excellent catalytic activity for the direct C-H bond activation of arylalkanes to ketones in H₂O under room temperature because of its high density of Lewis acidic sites within the frameworks and suitable channel size to access the catalytic sites. It also presented the spatial confinement effect and catalyzed the reaction with high regioselectivity, forming mono-ketones as the sole products. Easy product separation, simple reaction procedures, and recyclability of these catalysts make the catalytic system attractive. Our work highlights the superiority of the MOF-based materials as heterogeneous catalysts.

Full text of DOI:10.1039/C8CE02151A

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DOI: 10.1039/C8CE02151A
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ARTICLE
Co(II)-Cluster-Based Metal-Organic Frameworks as Efficient
Heterogeneous Catalysts for Selective Oxidation of Arylalkanes
Received 00th January 20xx,
Accepted 00th January 20xx
Yanru Fan, Xiao Li, Kuan Gao, Yu Liu, Xiangru Meng, Jie Wu* and Hongwei Hou*
DOI: 10.1039/x0xx00000x
To explore metal organic frameworks (MOFs) based on Co-cluster as heterogeneous catalysts to selectively catalyze the
reaction of C-H bond oxidation of aromatic alkanes to their corresponding ketones, three MOFs {[Co5(pmbcd)2(μ3-
OH)2(H2O)4(DMF)2]∙4DMF}n (MOF 1), {[Co2(pmbcd)(bpea)2]∙2H2O∙2DMF}n (MOF 2), and {[Co2(pmbcd)(dpp)2]∙3H2O∙2DMF}n
www.rsc.org/
(MOF 3) (H4pmbcd
= 9,9'-(1,4-phenylenebis(methylene))bis(9H-carbazole-3,6-dicarboxylic acid), bpea = 1,2-bis(4-
pyridyl)ethane, dpp = 1,3-di(4-pyridyl)propane) were successfully synthesized and structurally characterized. MOF 1 was
constructed from pentanuclear Co(II) cluster and exhibited porous framework with channels of 8 × 10 Å2 along the b axis.
MOF 2 was constructed from [Co2(CO2)4] units and presented a porous three-dimensional (3D) frame work with channels
of 11 × 13 Å2 along the b axis and of 10 × 12 Å2 along the c axis. MOF 3 was a flat two-dimensional (2D) layer based on
binuclear Co(II) units when dpp as an auxiliary ligand was introduced. Co5-cluster-based MOF 1 exhibited excellent catalytic
activity for direct C-H bond activation of arylalkanes to ketones in H2O under room temperature because of its high density
of Lewis acidic sites within frameworks and suitable channel size to access the catalytic sites. It also presented spatial
confinement effect and preceded the reaction with high regioselecticity, forming mono-ketones as the sole products. Easy
product separation, simple reaction procedures, and recyclability of these catalysts make the catalytic system attractive.
Our work highlights the superiority of the MOF-based materials as heterogeneous catalysts.
perform the reaction in a medium to high yield. Notably, the
atomistically well-defined isolated active sites with uniformly
fixed azimuth and distance throughout the solid in materials
can lead to unique spatial confinement effect and catalyze the
reaction with regioselecticity.4 Therefore, developing highly
selective, environmentally friendly, recyclable and stable metal
organic frameworks (MOFs)-based catalysts for the oxidation
of arylalkanes to ketones can offer the prospects for
addressing the selectivity issues.
On the other hand, as a new class of porous solid materials,
MOFs are a kind of atomically precise and permanently porous
materials composed of organic linkers and metal ions/clusters
(also known as secondary building units, or SBUs).5 The unique
features of MOFs, including high crystallinity, large surface
area, structural and functional diversity and tunability endow
their diverse potential applications such as gas adsorption,
drug delivery, chemical sensing, luminescence, and proton
conductivity, etc.6-11 In particular, MOFs have been regarded as
ideal platforms to develop single-site heterogeneous catalysts
or catalyst support.12 Compared with homogeneous catalysts,
MOFs with robust frameworks are well recyclable, and can
protect the catalytic species from aggregation or degradation.
Meanwhile, in contrast to the traditional microporous and
mesoporous materials, such as zeolite and activated carbon,
MOFs have advantages as heterogeneous catalysts in following
aspects: 1) the catalytic active sites in MOFs can be uniformly
dispersed at the molecular level, which may achieve high
selectivity and reactivity; 2) the pore environments in MOFs
Introduction
The oxidation of C-H bonds is a core technology for converting
petrochemical based materials to commodity chemicals in
chemical industry.1 In this field, selective oxidation of inert C-H
σ-bonds of arylalkanes to ketones is a valuable transformation
that leads to medicinally attractive oxygenated functional
molecules with high step- and atom-economy.2 Though a
number of methods have been developed, it is still challenging
because of the selectivity issues arising from the inertness and
the ubiquitous nature of the saturated C-H bonds. The
traditional oxidation of methylene group of arylalkanes using
KMnO4 often suffers from drawbacks such as negative
environmental impact and poor selectivity due to over-
oxidation. Over the past few decades, considerable research
efforts have been devoted to exploring transition-metal
catalyzed processes to catalyze C-H bond activation of
aryalkanes to form the homologous ketones.3 However, the
lack of selectivity is
a common problem in typical
homogeneous system for this reaction. Recent studies by our
group have demonstrated that some Cu(I)-based coordination
compounds can be used as heterogeneous catalysts to
The College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou, Henan, 450052, P. R. China
Electronic Supplementary Information (ESI) available. CCDC 1882631, 1882634,
and 1882637. Ligand synthesis, crystallographic data, structure figures, XRD, TGA,
and 1H NMR and 13C NMR spectra. See DOI: 10.1039/x0xx00000x.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Synthesis of {[Co5(pmbcd)2(μ3-OH)2(H2O)4(DMF)2]∙4DMF}n
can be tunable via either in-suit synthesis or post-synthetic
functionalization; 3) the crystalline nature of MOFs affords the
opportunity for molecular level characterization of the
catalytically active sites and favors for the mechanism study.
Therefore, MOFs combine the optimal features of
homogeneous catalysts (selectivity and ease of modification)
and heterogeneous catalysts (ease of purification and
recyclability).13
Especially, some metal-cluster based MOFs constructed
from multinuclear metal clusters as SBUs usually have some
unprecedented catalytic properties because of their high-
density activation sites, extraordinary stability, and synergistic
effect among ions, such as Zr clusters, Cu clusters and so on.14
Recently, our group has demonstrated that some Co-cluster-
based MOFs can be used as heterogeneous catalysts to
promote the acylation-Nazarov cyclization reaction with
benzene derivatives, which can’t be promoted by previous
used homogeneous and heterogeneous Lewis acid catalysts.15
Herein, we designed and synthesized three Co(II)-based
MOFs, {[Co5(pmbcd)2(μ3-OH)2(H2O)4(DMF)2]∙4DMF}n (MOF 1),
View Article Online
(MOF 1).
DOI: 10.1039/C8CE02151A
A mixture of Co(NO3)2∙6H2O (0.09 mmol, 0.026 g), H4pmbcd
(0.03 mmol, 0.018 g), DMF (4 mL), H2O (2 mL) and three drops
of HNO3 was placed in a 25 mL Teflon-lined stainless steel
container, which was sealed and then heated at 160 °C for
three days, followed by cooling to 60 °C at a rate of 5 °C h-1.
Purple crystals of MOF 1 were obtained in a yield of 78%
(based on Co). Elemental analysis calcd (%) for MOF 1
(Co5C90H92N10O28): C 52.56, H 4.52, N 6.81. Found: C 51.94, H
4.36, N 6.75. IR (KBr, cm-1): 3050.50 (s), 1663.31 (s),
1599.83(w), 1562.10 (w), 1140.49 (m), 1021.21 (m), 907.50
(m), 778.64 (s), 665.16 (s).
Synthesis of {[Co2(pmbcd)(bpea)2]∙2H2O∙2DMF}n (MOF 2).
A mixture of Co(NO3)2∙6H2O (0.09 mmol, 0.026 g), H4pmbcd
(0.03 mmol, 0.018 g), 1,2-bis(4-pyridyl)ethane (0.05 mmol,
0.009 g), DMF (4 mL), H2O (2 mL) and three drops of HNO3 was
kept in a 25 mL Teflon-lined stainless steel vessel at 120 °C for
three days. After the mixture was cooled to 60 °C at a rate of 5
°C h-1, pink crystals were collected with a yield of 70% (based
{[Co2(pmbcd)(bpea)2]∙2H2O∙2DMF}n
(MOF
2),
and
{[Co2(pmbcd)(dpp)2]∙3H2O∙2DMF}n (MOF 3) (H4pmbcd = 9,9'-
(1,4-phenylenebis(methylene))bis(9H-carbazole-3,6-
on Co). Elemental analysis calcd (%) for MOF
2
(Co2C66H62N8O12): C 62.07, H 4.90, N 8.78. Found: C 61.73, H
4.76, N 8.65. IR (KBr, cm-1): 3442.40 (s), 3063.14 (w), 2926.93
(w), 1670.46 (s), 1603.75 (s), 1564.05 (w), 1533.78 (w),
1402.70 (s), 1302.26 (w), 1142.89 (s), 1020.18 (m), 781.82 (s),
550.15 (m).
dicarboxylic acid), bpea = 1,2-bis(4-pyridyl)ethane, dpp = 1,3-
di(4-pyridyl)propane) under solvothermal conditions. We
demonstrated the catalytic potentials of MOFs 1-3 for the C-H
bonds oxidation of aromatic alkanes to ketones. Co5-cluster-
based MOF 1 showed more excellent catalytic activity and
selectivity in contrast to MOF 2 and MOF 3 because of its high
density of Lewis acidic sites within frameworks and unique
spatial confinement effect. In addition, factors influencing the
performance were investigated, and a possible catalytic
mechanism of oxidation C-H bonds of arylalkanes to ketones
catalyzed by Co-based MOFs was proposed.
Synthesis of {[Co2(pmbcd)(dpp)2]∙3H2O∙2DMF}n (MOF 3).
A mixture of Co(NO3)2∙6H2O (0.09 mmol, 0.026 g), H4pmbcd
(0.03 mmol, 0.018 g), 1,3-di(4-pyridyl)propane (0.1 mmol,
0.020 g), DMF (5 mL), H2O (1 mL) and three drops of HNO3 was
sealed in a 25 mL Teflon-lined stainless steel container and
heated at 120 °C for three days. After cooling to 60 °C at a rate
of 5 °C h-1, rose-carmine crystals of MOF 3 were obtained in a
yield of 67% (based on Co). Elemental analysis calcd (%) for
MOF 3 (Co2C68H68N8O13): C 61.72, H 5.19, N 8.47. Found: C
61.34, H 5.01, N 8.36. IR (KBr, cm-1): 3441.93 (s), 1661.11 (m),
1603.71 (s), 1569.67 (w), 1533.84 (w), 1403.87 (s), 1141.87
(m), 782.24 (s), 652.52 (m).
Experimental
Materials and physical measurements
All reagents and solvents were commercially available and
used as received without further purification. Ligand was
synthesized according to the modified procedures reported
previously.16 1H NMR and 13C NMR spectra were obtained with
Bruker Avance-400 and 600 spectrometers. Powder X-ray
diffraction (PXRD) analyses were performed on a Bruker D8
Advance X-ray diffractometer with Mo Kα radiation (λ =
0.71073 Å). The thermal decomposition characteristic of the
samples was analyzed by thermogravimetric analysis (TGA)
using a NETZSCH STA 409 PC/PG thermal analyzer at a heating
rate of 10 °C min-1 in air. Elemental analyses for C, H, and N
were performed on an Elementar Vario MICRO Elemental
analyzer. FT-IR spectra were performed on a Bruker Vector 22
FT-IR spectrophotometer using KBr pellets.
Crystal data collection and refinement
The structures of MOFs 1-3 were tested on a Bruker D8
VENTURE diffractometer with Mo Kα radiation (λ = 0.71073 Å).
The SAINT program was used to control the integration of the
diffraction data, polarization effects, and the intensity
corrections for the Lorentz.17 We used the SADABS program to
perform semiempirical absorption correction and solved
structures by immediate ways and refined by a full matrix
least-squares technique relied on F2 with the SHELXL-2014
software package.18 The hydrogen atoms were generated
geometrically and refined isotropically using the riding model.
The summary of crystallographic data for MOFs 1-3 is listed in
Table S1. Corresponding bond lengths (Å) and bond angles
(deg) are provided in Table S2. Crystallographic data for MOFs
1-3 have been deposited at the Cambridge Crystallographic
Synthesis of MOFs 1-3
2 | J. Name., 2012, 00, 1-3
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data Centre with CCDC reference numbers 1882631, 1882634,
and 1882637.
View Article Online
DOI: 10.1039/C8CE02151A
General procedure for the C-H bonds oxidation of arylalkanes to
ketones with catalysts
A mixture of arylalkanes (4a-4j, 1.0 mmol, 1.0 equiv), tert-butyl
hydroperoxide (TBHP; 1.5 mmol, 1.5 equiv), catalyst (0.05
mmol, 0.05 equiv based on Co ion), and 4 mL of water was
added in a 25 mL round-bottom flask. The system was
ultrasonically treated for
1 h, then stirred at room
temperature in the air. After 8 h, the mixture was filtrated and
extracted by EtOAc (3 × 15 mL). The organic phase was merged,
dried by anhydrous Na2SO4 and evaporated under reduced
pressure. The residue was purified by column chromatography
(silica gel, PE-EA, v/v = 10:1).
Figure 2. (a) The second building unit (SBU) in MOF 2. (b) Structure of
the H4pmbcd in MOF 2. (c) The 2D structure (in the ab plane)
constructed from Co(II) ions and ligands pmbcd4-, which contains the
binuclear [Co2(CO2)4] unit. (d) The 2D structure is connected with
bpea molecules along the b axis to form the 3D structure. The
solvent molecules and all hydrogen atoms are omitted for clarity.
Results and discussion
interesting coordination mode: (κ1)-(κ1-κ1)-(κ1)-(κ1-κ1)-µ6
(Figure S1b). Additionally, the presence of semi-flexible p-
dibenzyl group in the pmbcd4- ligand results in the formation of
dihedral angle between two carbazole rings approximately
9.17(2)° (Figure 1c), which further modify the surface of the
channel in the structure.
Crystal structure of MOF 1
The X-ray single-crystal diffraction analysis declares that MOF
1 is a three-dimensional (3D) framework and crystallizes in the
monoclinic system with space group P21/n. The asymmetric
unit of MOF 1 contains three independent Co(II) ions, one
completely deprotonated pmbcd4- ligand, one µ3-OH group,
two coordinated water molecules, one coordinated DMF
molecule, and guest molecules (Figure S1a). Interestingly, five
cobalt(II) ions are connected together by µ3-OH groups,
coordinated DMF molecules and H2O molecules to assemble
Crystal structure of MOF 2
MOF
2 was successfully obtained under the similar
solvothermal conditions as those of MOF 1 except with the
addition of bpea as an auxiliary ligand. The result of X-ray
single-crystal diffraction analysis reveals that MOF 2 shows a
3D framework and crystallizes in triclinic crystal system Pī. The
coordination models of Co1 and Co2 display similar
octahedron configurations (Figure 2a). The two carbazole rings
of the semi-flexible pmbcd4- ligands in MOF 2 are ladder-like
due to the introduction of auxiliary ligand bpea, leading to the
conformation difference between MOF 1 and MOF 2 (Figure
2b). In MOF 2, Co1 and Co2 are bridged by four carboxyl
groups to furnish binuclear [Co2(CO2)4] units, which are further
linked by completely deprotonated ligands pmbcd4- and bpea
ligands to yield a 3D open framework. The framework
possesses two kinds of channels, about 11 × 13 Å2 in the ac
plane (Figure 2d) and 10 × 12 Å2 in the ab plane (considering
van der Waals radii) (Figure 2c).
into
a
pentanuclear cobalt(II) cluster [Co5(CO2)8(µ3-
OH)2(H2O)4(DMF)2] (Figure 1a). Each pentanuclear Co(II) cluster
links with eight carboxyl groups from ligands pmbcd4- to form a
3D framework (Figure 1b). The structure of MOF 1 has
channels of 8 × 10 Å2 (considering van der Waals radii) along
the b axis. The pmbcd4- ligand acts as a µ6-bridge to link
clusters though its four carboxyl groups, and exhibits an
Crystal structure of MOF 3
When dpp as an auxiliary ligand was introduced into the
reaction, MOF 3 was obtained. The single-crystal diffraction
analysis indicates that MOF 3 crystallizes in the monoclinic
space group P21 and exhibits a binuclear two-dimensional (2D)
structure. As depicted in Figure 3a, four carboxyl groups and
two Co(II) ions construct
a [Co2(CO2)4] unit, which is
Figure 1. (a) The Co5 cluster second building unit (SBU) in MOF 1
(b) View of the 3D network along the b axis. (hydrogen atoms and
guest molecules are omitted for clarity). (c) Structure of the
H4pmbcd in MOF 1.
approximately identical to that of MOF 2. However, under the
effect of auxiliary ligand, dihedral angle between two
carbazole rings of a pmbcd4- ligand changes into 43°, which
indicates that the conformation of the ligand transforms from
This journal is © The Royal Society of Chemistry 20xx
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planarity of MOF 2 to distortion of MOF 3 (Figure 3b). Thus, result of MOF 1 showed that there were three steps of weight
View Article Online
the [Co2(CO2)4] units are connected by pmbcd4- ligands and loss. The first two steps of weight loDssOI:o1f0.12053.97/8C%8CE(0c2a1l5c1dA.
dpp ligands to produce a 2D framework (Figure 3c).
26.48%) before 365 °C were attributed to the release of water
molecules, coordinated and free DMF molecules. The third
step of weight loss at 365 °C was due to the collapse of the
main framework (Figure S1c). The curve of MOF 2 revealed
that the initiatory weight loss of 13.51% (calcd. 14.24%) in the
range of 30-255 °C was the result of the departure of two free
water molecules and two free DMF molecules. After that, MOF
2 began to collapse at 350 °C (Figure S2c). As for MOF 3, the
preliminary weight loss of 15.09% (calcd 15.13%) in the 30-
248 °C range was because of the release of three free water
molecules and two free DMF molecules, and then the 1,3-di(4-
pyridyl)propane (dpp) ligands and H4pmbcd ligands were
gradually released above 250 °C (Figure S3c).
The catalytic properties of MOFs 1-3
The PXRD analyses of MOFs 1-3 were carried out to test the
purity of MOFs 1-3. The PXRD patterns of as-synthesized
samples matched well with simulated ones from the single-
Table 1. Optimization of the Reaction Conditions.a
O
OOH
+
Figure 3. (a) The second building unit (SBU) in MOF 3. (b) Structure
of the H4pmbcd in MOF 3. (c) View of the 2D network. The solvent
molecules and all hydrogen atoms are omitted for clarity.
Entry
1
Catalyst
TBHP
yes
no
Atmosphere
Solvent
H2O
Yield (%)
10<
5<
none
air
air
N2
air
air
air
air
air
air
air
air
air
O2
2
1
H2O
Analysis of thermal stability of MOFs 1-3
3
1
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
yes
H2O
10<
16
To study their thermal stability, thermogravimetric analysis
(TGA) experiments of MOFs 1-3 were measured in air. The
4
CoCl2
H2O
5
1
2
3
1
1
1
1
1
1
H2O
92
6
H2O
66
7
H2O
60
8
acetonitrile
DMF
90
9
10<
10<
10<
47
10
11
12
13
toluene
DMSO
dioxane
H2O
95
aReagents and conditions: indane (1.0 mmol), TBHP (1.5
mmol), and catalyst (0.05 mmol), H2O (4 mL), air, room
temperature, 8 h.
Figure 4. (a) The PXRD patterns for the simulated and as-synthesized
samples for MOF 1. (b) The PXRD patterns for the simulated and as-
synthesized samples for MOF 2. (c) The PXRD patterns for the simulated
and as-synthesized samples for MOF 3.
4 | J. Name., 2012, 00, 1-3
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crystal data, suggesting that MOFs 1-3 were presented as pure
phases. Furthermore, the solvent stability of MOFs 1-3 was
examined by immersing crystals in water for about 24h
because it was chosen as the reaction solvent for the catalytic
reaction. The PXRD analyses revealed that MOFs 1-3 were
stable in water (Figure 4).
View Article Online
DOI: 10.1039/C8CE02151A
Table 2. The C-H Bonds Oxidation of Arylalkanes to
Ketones Catalyzed by MOF 1.a
O
H2
C
C
catalyst
OOH
X
n
X
n
+
H2O, r.t.
air, 8h
To research the potential of MOFs 1-3 as heterogeneous
catalysts in the C-H bonds oxidation of aromatic alkanes to
ketones, we chose indane as the model substrate to explore
the optimized reaction conditions, and the corresponding
results were recorded in Table 1. First, the reaction was
conducted using 0.05 mmol MOF 1, MOF 2 or MOF 3 (based on
Co ion), 1.5 mmol TBHP and 4 mL H2O in air at room
temperature for eight hours, and the corresponding product
was obtained in a yield of 92, 66, and 60%, respectively (Table
1, entries 5-7). In comparison, CoCl2 was used as homogeneous
catalyst for this reaction by keeping the same content of Co
(5%) and the low yield indicated that the salt couldn’t
effectively catalyze the reaction (Table 1, entry 4). The control
experiment showed that no product was obtained in the
absence of TBHP (Table 1, entry 2). When the reaction was
placed under N2 atmosphere instead, the yield decreased
sharply (Table 1, entry 3). The influence of solvent was also
studied. Replacement of H2O with organic solvents, such as
DMF, toluene, DMSO, and dioxane could decrease the yield
considerably (Table 1, entries 9-12).
X = C, N, O,Cl
n = 0,1, 2
Entry
1
Substrate
Product
Yield (%)
92
O
4a
5a
O
2
3
4
85
86
83
4b
5b
O
O
O
5c
4c
4d
O
O
O
5d
With the optimized reaction conditions established, we then
started investigating the scope of this oxidative
transformation. As shown in Table 2, MOF 1 showed excellent
catalytic ability for the C-H bonds oxidation of alkyl arenes (4a-
j) and generated the corresponding carbonyl compounds in
excellent yields (5a-j). More importantly, 4a-d with two
benzylic sites were employed to research the regioselectivity
of MOF 1 on this reaction (Table 2, entries 1-4), it showed that
the respective mono-ketones are formed as the sole products
by using MOF 1 as the heterogeneous catalysts. Furthermore,
alkyl benzenes, which were substituted by hydroxyl or chlorine
atoms, could take place oxidation reaction to have good yields
(Table 2, entries 8-10).
MOF 1 shows a porous 3D framework with channel
dimension of 8 × 10 Å2 along the b axis, which can facilitate the
transport of organic substrates and products. In addition, the
pentanuclear Co (II) clusters of MOF 1 provide high density of
well-defined isolated Lewis acidic sites within frameworks.
More importantly, the spatial confinement effect produced by
the nature of crystallinity, which provides atomistically well-
defined isolated active sites with uniformly fixed configuration,
geometry and symmetry, and suitable channel size and shape
throughout the solid, leads to extraordinary regioselectivity. In
comparison, due to the fact that the Co(II) ions in MOF 2 and
MOF 3 are coordinatively saturated, we speculated that the
relatively poor catalytic activity of MOF 2 and MOF 3 could be
ascribed to the catalytic reaction on the surface and defects of
the crystalline materials.
O
5
6
95
93
4e
5e
O
N
H
4f
N
H
5f
O
7
8
98
81
4g
4h
5g
O
5h
O
Cl
9
74
89
Cl
5i
4i
4j
OH
O
5j
10
aReagents and conditions: arylalkanes (1.0 mmol), TBHP
(1.5 mmol), and catalyst (0.05 mmol), H2O(4 mL), room
temperature, air, 8
decomposed into tBuO∙ and HO∙ radicals through homolysis by
ultrasonic treatment. Then, the HO∙ radical combined with
Co(II)-based MOFs to form HO-CoIIILn. Subsequently, the BuO∙
According to reported work, a possible mechanism for C-H
bonds oxidation of arylalkanes to ketones by the Co(II)-based
MOFs was proposed (Scheme 1).19 First, the TBHP was
t
radical abstracted one of allytic H atoms from the substrate to
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Scheme 1. Proposed Mechanism for the C-H bonds
oxidation of arylalkanes to ketones by the Co(II)-based
MOFs.
tBuO•
tBuO-OH
O
R1CR2
2R2
H
C
R1
LnCoII
HO-CoIIILn
H2O
Figure 5. (a) Recycling tests for the conversion of aromatics into
ketones with MOF 1. (b) Comparison of the PXRD patterns of MOF 1
before and after recycling catalysis PXRD.
tBuOH
O2
solvothermal conditions with a semirigid tetracarboxylate
ligand H4pmbcd and the introduction of rationally selected N-
donor ancillary ligands. Co5-cluster-based MOF 1 was used as
efficient and green catalysts to catalyze the C-H bonds
oxidation of arylalkanes to ketones under mild conditions,
yielding mono-ketones as the sole products. The excellent
catalytic activity and selectivity of MOF 1 could be ascribed to
its high density of Lewis acidic sites within frameworks and
spatial confinement effect of porous MOFs. In addition, it
could be easily separated and reused at least four times
without obvious loss. Therefore, this work highlights the
superiority of the MOF-based materials as heterogeneous
catalysts.
OH
•
R1CHR2
R1CHR2
I
II
t
produce the BuOH, and the substrate was changed into the
allytic radical I, which further reacted with HO-CoIIILn to
produce the corresponding peroxyether II. Finally, II reacted
with O2 to provide the expected ketone. To clarify the radical
nature of the mechanism, we first used indane as substrate
and butylated hydroxytoluene (BHT) as radical-trapping
reagent in the reation to give the 10% yiled, which was much
lower than entry 5 (Table 1). This result desmonstated that the
reaction process was a free radical process to control the
target products.
In order to study whether the catalytic reactions were
heterogeneous or homogeneous, we carried out atomic
absorption spectroscopy analysis of the filtrate. The
experimental result indicated that there was slight leaching
(<1ppm) of cobalt from the crystal samples of MOF 1 during
the reaction. And then a filtration experiment was conducted.
At almost 20% conversion of 4a in the presence of MOF 1, the
reaction was passed through a sand core funnel to remove the
catalyst, and the filtrate was allowed to react for another 3h. It
was found that the conversion was unchanged during the
time. The filtration experiment showed that the reaction was
basically heterogeneous. Furthermore, as a heterogeneous
catalyst, MOF 1 can be simply separated from the reaction
mixture through centrifugation and filtration. Hence, the
recycle performance of MOF 1 as a catalyst was explored, and
the corresponding results are recorded in Figure 5a. To our
excitement, excellent catalytic activity of MOF 1 still could be
observed after being employed four times, indicative that MOF
1 can be reused at least four times. Then, the PXRD pattern
was further obtained (Figure 5b), and the result remain
consistent with the simulated result of MOF 1. Likewise, the
powder PXRD patterns of MOF 2 and MOF 3 after catalytic
cycle also closely matched the simulated MOF 2 and MOF 3,
respectively (Figure S2d, Figure S2e, Figure S3d, Figure S3e).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was funded by the National Natural Science Foundation
(Nos. 21771163, 21671174 and 21371155), the Natural Science
Foundation of Henan province (Nos.162300410243 and
182300410008), Thousand Talents Program of Zhongyuan and
Project for the Leading Young Teachers in Henan Provincial
Institutions of Higher Education of China.
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DOI: 10.1039/C8CE02151A
Co(II)-Cluster-Based Metal-Organic Frameworks as Efficient
Heterogeneous Catalysts for Selective Oxidation of
Arylalkanes
Yanru Fan, Xiao Li, Kuan Gao, Yu Liu, Xiangru Meng, Jie Wu* and Hongwei Hou*
Three Co-cluster-based metal organic frameworks were designed to
explore the catalytic activity for selective oxidation of arylalkanes.