Immobilization of Keggin polyoxovanadoniobate in crystalline solids to produce effective heterogeneous catalysts towards selective oxidation of benzyl-alkanes

Article abstract of DOI:10.1039/c7dt01122a

Two ionic organic-inorganic hybrid compounds, [Cu^II(C₂N₂H₈)₂]₄[Cu^II(C₂N₂H₈)₂(H₂O)₂]₂[PNb₁₂O₄₀V^VV^IVO₂]·(OH)₂·11H₂O (1) and [Co^III(C₂N₂H₈)₃]₂[Co^III(C₂N₂H₈)₂(H₂O)₂]_0.5[H_2.5PNb₁₂O₄₀ V^VV^IVO₂]·20H₂O (2), based on P-centered dicapped polyoxoniobates and organometallic cations were isolated and structurally characterized by routine techniques. The trivalent cobalt complex-containing compound exhibits a looser arrangement compared with its divalent copper complex-containing counterpart, with a space volume of 34.9% for the former and 17.0% for the latter. The two compounds were proved to be effective in facilitating the oxidation of benzyl-alkanes to ketone products in a heterogeneous manner, evidencing the feasiblity of the strategy of self-immobilization of catalytically active, readily soluble PNb₁₂O₄₀(VO)₂ species in crystalline solids.

Full text of DOI:10.1039/c7dt01122a

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DOI: 10.1039/C7DT01122A
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Immobilization of Keggin Polyoxovanadoniobate in Crystalline
Solids as Effective Heterogeneous Catalysts towards Selective
Oxidation of Benzyl-Alkanes
Received 00th January 20xx,
Accepted 00th January 20xx
Jufang Hu,†a Jing Dong,†a Xianqiang Huang,a,b Yingnan Chi,a Zhengguo Lin,a Jikun Li,a Song Yanga,
DOI: 10.1039/x0xx00000x
Hongwei Maa and Changwen Hu*a
www.rsc.org/
Two ionic organic-inorganic hybrid compounds, [CuII(C2N2H8)2]4[CuII(C2N2H8)2(H2O)2]2[PNb12O40VVVIVO2]·(OH)2·11H2O (1) and
[CoIII(C2N2H8)3]2[CoIII(C2N2H8)2(H2O)2]0.5[H2.5PNb12O40 VVVIVO2]·20H2O (2), based on P-centered dicapped polyoxoniobates and
organometallic cations were isolated and structrually characterized by routine techniques. The trivalent cobalt complexes-
containing compound exhibits a looser arrangement compared with the divalent copper complexes-containing
counterpart, with a space volume of 34.9% for the former and 17.0% for the latter. The two compounds were proved to be
effective in facilitating the oxidation of benzyl-alkanes to ketone products in a heterogeneous manner, evidencing the
feasible strategy of self-immobilization of catalytically active, readily soluble PNb12O40(VO)2 species in crystalline solids.
frameworks (MOFs), in which the polyanions are alternately
arranged as non-coordinating guests and exhibit charming
substrate size selective properties induced by confined
Introduction
The exploration of green catalysts working in a heterogeneous
manner is of considerable interest in industrial application due
to their ready recovery and recyclability from the reaction
environments.1 Polyoxometalates (POMs) are discrete anionic
clusters of transition metals primarily from Group VI (V, Nb,
and Ta) and Group V (Mo, W),2 and continue to gain attention
as a kind of promising heterogeneous catalysts in such
reactions as oxidation, hydrolysis, photocatalysis, and
electrocatalysis.3-6 Effective matrixes like silica/carbon-based
compounds4,5 and layered double hydroxides6 have been ever
studied to immobilize those soluble POMs. However, lacking of
finely-tunable disperisty as well as leaching of the catalytically
active POM species during the reaction and the subsequent
recycling remains the issues to be solved.
Another feasible strategy to the heterogenization of
molecular polyoxometalates is construction of POM-based
organic-inorganic crystalline hybrids with high tunability
intrinsic to molecular materials.7 In the POM-based ionic
solids, the two components are simply combined via H-
bonding and/or electrostatic interactions, with retention of the
catalytically active POMs at the molecular level. To this point,
one remarkable system is POM-based metal-organic
channel sizes.8 Another promising way to access to POM-based
ionic organic-inorganic catalysts is introducing organometallic
cations ionically interacting with the polyanions in the
crystalline lattices, leading to insoluble solids and hence
realizing the self-immobilization of catalytic POM species.9 For
example, Mizuno proposed that complexation of {SiW10O38V2}
with Ni(tacn)2 (where tacn = 1,4,7-triazacyclononane) gave rise
to an ionic compound that was effective towards epoxidation
of 1-octeneand cyclooctene.9a In the POM-based
heterogeneous catalysts mentioned-above, the POM species
are much restricted to Group V consisting of tungstates,
molybdates as the addenda atoms. Group V POMs, bearing
high negative charges, have proved to be much more suitable
building components for supramolucular crystalline solids,10,11
however, their potentials as catalysts are much less explored.
Polyoxoniobate, XNb12O40(VO)2, where X is heteroatom
and it can be Si/Ge11a or P/V,12 is one of the well-studied
polyoxoniobates (PONbs). In particular, TMA9PNb12O40(VO)2
(where TMA = tetramethylammonium) can be readily
synthezed in high yield as soluble organic salts, promising an
homogenously catalystic system in oxidative transformation,
as we have tested that it was effective towards catalytic
decontamination of chemical warfare agent simulants.13 We
envisage the combination of XNb12O40(VO)2 with
organometallic cations may lead to insoluble materials as
heterogeneous catalysts with ready recovery and separation,
which is of great significance with respect to the practical
utilization. Herein we successfully isolated two ionic organic-
inorganic hybrid compounds based on PNb12O40(VO)2 and
a Addres Key Laboratory of Cluster Ministry of Education of China, Department of
Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail:
cwhu@bit.edu.cn.
b Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell
Technology, School of Chemistry & Chemical Engineering, Liaocheng University,
Liaocheng 252059, China
† Jufang Hu and Jing Dong contributed equally.
Electronic Supplementary Information (ESI) available: micrograph of samples,
PXRD, IR, XPS. See DOI: 10.1039/x0xx00000x
copper/cobalt
complexes,
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[CuII(C2N2H8)2]4[CuII(C2N2H8)2(H2O)2]2[PNb12O40VVVIVO2]·(OH)2·1 5.02%, V, 3.62%, Nb, 38.94%; IR (KBr, cm-1): 342V4ie(sw)A,rt3ic2le1O1n(lsin)e,
1H2O
[CoIII(C2N2H8)3]2[CoIII(C2N2H8)2(H2O)2]0.5[H2.5PNb12O40
VVVIVO2]·20H2O
(2). The catalytically active polyanion, 1122(w), 1057(s), 1027(s), 973(w), 944(w), 876(m), 703(s), 632
(1
)
and 3101(m), 2970(w), 2900(w), 1619(m)D, O1I:5107.910(s3)9,/C71D4T60211(m22)A,
1384(w), 1366(w), 1325(w), 1300(w), 1250(w), 1156(m),
PNb12O40(VO)2, is solidified in the ionic crystalline solids, and (w), 580(m), 488(m).
thus a PONb-based heterogeneous system is established. With
selective oxidation of benzyl-alkanes to ketone products as the
Catalytic experiments
Benzyl-alkanes (0.125mmol, 1 equiv), catalysts (0.004 mmol,
model reaction, the two compounds were investigated as
effecient heterogeneous catalysts. The contaning
copper/cobalt-diamine complexes as the cation components
shall also contribute to the catalytic activity. Moreover, a
radical mechanism involving ·OOBu-t formed from TBHP
decomposition is proposed.
0.03 mol %), tert-butyl hydroperoxide (TBHP, 70% in water, 2.5
equiv), naphthalene (internal standard, 0.125mmol, 1 equiv),
and benzonitrile(0.5 mL) were added in a Schlenk Tube under
ambient atmosphere and stirred at 60 oC. After refluxing for 24
hours, the resulting reaction mixture was diluted with
acetonitrile
and
quantitatively
analyzed
by
gas
chromatography (GC).
Experimental section
Characterization
Materials
Elemental analyses (C, H and N) were measured on a
ELEMENTAR vario EL cube Elmer CHN elemental analyzer; Nb,
V and Cu/Co were determined with a Thermo iCAP 6000
atomic emission spectrometer. IR spectrum was collected in
the range 40000-400 cm-1 with KBr pellets dispersed with
sample on a Nicolet 170SXFT-IR spectrophotometer. Powder X-
ray diffraction (PXRD) data on samples were recorded on a
Bruker instrument equipped with graphite-monochromatized
Cu-Kα radiation (λ = 0.154060 nm, scan speed of 8 °/min, 2θ =
5−50°) at room temperature. The EPR analysis was conducted
on a Bruker ECS 300 spectrometer. Experimental parameters:
microwave frequency 9.068 GHz, microwave power 0.998 mW,
modulation frequency 100 kHz, and modulation width 0.2 mT.
The GC analyses were performed on Shimadzu GC-2014C with
a FID detector equipped with a HP-5ms capillary column.
All common laboratory chemicals were reagent grade,
purchased from commercial sources and used without further
purification. Hexaniobate precursors K7HNb6O19·8H2O and
VOPO4·2H2O were prepared according to the literature
method.14,15
Synthesis of 1
To a solution of K7HNb6O19·8H2O (0.15 g, 0.11 mmol) in 8 mL of
H2O, VOPO4·2H2O (0.1 g, 0.51 mmol), Cu2(OH)2CO3 (0.22g, 1.00
mmol) were added successively, followed by the addition of
0.5 mL 1,2-diaminoethane. The mixture was stirred for 30 min
at room temperature. The resulting solution was transferred to
a Teflon-lined stainless steel autoclave (23 mL), kept in an oven
at 160 °C for 24 h, and then cooled to room temperature at a
rate of 5 °C/h. Block-shaped dark purple crystals of
1 were
Complete crystallographic data for
1 and 2 were collected at
isolated and washed with deionized water. Yield: 0.10 g, ~
55.87 % based on Nb. Anal. Calc. for C24H128Cu6N24Nb12O59PV2:
C, 8.67%; H, 3.88%; N, 10.11%, P, 0.93%, Cu, 11.46%, V, 3.06%,
Nb, 33.52%; Found: C, 8.78%; H, 3.61%; N, 10.32%, P, 0.89%,
Cu, 11.62%, V, 3.02%, Nb, 33.43%. IR (KBr, cm-1): 3308(s),
3214(s), 3138(m), 2965(w), 2947(w), 2886(w), 1638(m),
1586(s), 1453(w), 1386(m), 1324(w), 1280(w), 1168(m),
1104(m), 1047(s), 1028(s), 988(w), 975(w), 953(m), 872(s),
810(w), 679(s), 625(w), 526(m), 474(m).
296(2) K on Bruker APEX- CCD detector with graphite
Ⅱ
monochromatic Mo Kα radiation (λ = 0.71073 Å). The structure
were solved and refined using the OLEX2 program suite,16
equipped with ShelXT and ShelXL program.17 All non-hydrogen
atoms were located from Fourier map directly by ShelXT and
refined anisotropically. Hydrogen atoms on all non-hydrogen
atoms were placed in calculated positions, and their
coordinates and displacement parameters were constrained to
ride on the carrier atoms. In addition, some restrains, like RIGU
and DFIX, were performed to get better structure models. In
both compounds, the structures contain large voids, and the
exact amount of solvent molecules located in the voids
couldn’t be identified due to their high disorder and small
residual peaks. Therefore, SQUEEZE in PLATON program was
Synthesis of 2
To a solution of K7HNb6O19·8H2O (0.15 g, 0.11 mmol) in 8 mL of
H2O, VOPO4·2H2O (0.1 g, 0.51 mmol), Co(CH3COO)2·4H2O
(0.14g, 0.55 mmol) were added successively, followed by the
addition of 0.5 ml 1,2-ethylenediamine. The mixture was
stirred for 30 min at room temperature. And the resulting
solution was transferred to a Teflon-lined stainless steel
performed
Crystallographic data and selected bond lengths are
summarized in Table S and Table S2-S5, respectively.
to
remove
those
solvent
molecules.
1
autoclave (23 mL) and was kept in an oven at 160
℃ for 72 h,
and then cooled to room temperature at a rate of 5 °C/h. The
reaction mixture was left undisturbed for ten days at room
Results and discussion
Synthesis and Structures
temperature, and block-shaped light yellow crystals of
2 were
isolated and washed with deionized water. Yield: 0.005 g, ~
3.48 % based on Nb. Anal. Calc. for C14H100.5Co2.5N14Nb12O63PV2: Following our previous work on vanadium-containing PONbs,
C, 5.86%; H, 3.53%; N, 6.84%, P, 1.08%, Co, 5.14%, V, 3.55%, herein we explored the potential combination of P-centered
Nb, 38.87%; Found: C, 5.73%; H, 3.71%; N, 6.59, P, 1.13%, Co, polyanion PNb12O40(VO)2 with organometallic cations to
2 | J. Name., 2012, 00, 1-3
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3-
construct ionic organic-inorganic hybrids, with hope to “fix” be directed by PO4 from the vanadyl phosphate to
View Article Online
DOI: 10.1039/C7DT01122A
the catalytically active polyanion in crystalline lattices. reassemble a Keggin type polyanion PNb12O40. Subsequently
Different organometallic cations were introduced to enrich the the two vanadyl ions released from the vanadium precusor cap
three-dimensional structures. Specifically, the reaction of the square windows to complete the construction of
potassium hexaniobates, vanadyl phosphate, and copper PNb12O40(VO)2 (Scheme S1). The introducing two vanadyl
carbonate in the presence of 1,2-diaminoethane under groups lower the charge of the polyanion {PNb12O40} from -15
o
hydrothermal conditions at 140 C gave rise to
1
. Following a to -9, and hence produces a structure-stabilizing effect on the
procedure similar to that of
was replaced with cobalt carbonate hydroxide,
The 1,2-diaminoethane molecule serves as both protective partial occupancy of 0.5.
1
, where the copper carbonate highly negative PONb polyanion. The central P atom lies on an
2
was isolated. inversion centre, and is surrounded by eight O atoms with a
ligand and reagent for the pH value adjustment. The digital
photographs of the two compounds are shown in Fig. S1.
There are two crystallographically distinct copper centers
1. One(Cu1) is tetra-coordinated with square geometry (Fig.
in
The phase purity of the crystalline products is veried by 2a) and the other(Cu2) is hexa-coordinated with octahedral
powder X-raydiffraction (PXRD) (Fig. S2). The oxidation states geometry. Both copper centers coordinate with four nitrogen
of V ions are evidenced by IR and XPS analysis. In the IR spectra atoms from two 1,2-diaminoethane ligands, and additionally
(Fig. S3), both compounds share peaks at 952 cm-1 (for
1) and two aqua ligands can be located for Cu2 to complete the
944 cm-1 (for
2 1 2), geometrical sphere. The bond lengths are 1.963－2.035 Å for
), 975cm-1 (for ) and 973 cm-1 (for
characteristic of vibration mode of of VIV=O18 and VV=O,12 Cu-N. The introducing coordinated unsaturated metal
respectively. We can further determine the oxidation states of sites(Cu1) provide potential Lewis acid sites, and are very
vanadium species in both compounds by XPS. In the XPS of the attractive in catalysis. In
1, the polyanion PNb12O40(VO)2 is
core level region of V2p, two well-resolved peaks at 515.81 ionically interacted with twelve copper complexes cations,
and 517.26 eV for 1, 515.84 and 517.28 eV for 2 can be found with each copper center located approximately 7.239－7.711
(Fig. S6), where the former is attributable to V4+, and the later Å from the central phosphorus atoms of the polyanion. The
can be assignable to V5+.19 The V5+ in the starting materials can copper complexes cations form a ten-membered ring (red
be partially reduced by organoamine to V4+ under broken circle in Fig. 3a), where the polyanion resides inside.
hydrothermal conditions.11b,20,21 In the XPS of the core level The specific arrangement as well as the relative small size of
region of Cu2p, Peaks at 934.96 and 954.36 eV with satellites these copper complexes leads to discrete polyanion in the
(Fig. S7, left) support the presence of Cu2+ in 1 22
noting that peaks at 933.32 and 953.19 eV are originated from As
.
It’s worth crystalline lattices in
result,
1
, in line with the previous reports.11a,11c
a three-dimensional (3D) supramolecular
a
Cu0,22a the fully reduced form of Cu2+ in the presence of framework is assembled. Lattice water molecules are filled in
organoamine. Accordingly, we assign the peak at 781.84 eV the voids of the 3D network, as supported by TG analysis (Fig
being arose from Co2p3/2 (Fig. S7, right) to Co ions in +3 S4, right).
oxidation state in
2
in the XPS of the core level region of
When introducing cobalt ions, cobalt-based
PNb12O40(VO)2 ionic hybrid is isolated as 2, and crystallizes in
a
Co2p.23 It’s reasonable for the assignment of Co3+ species in
2
,
which is isolated after leaving the baker undisturbed, while
exposure to air for 1 month, facilitating the oxidation of Co2+
ions to Co3+.
Single crystal X-ray diffraction reveals that
1 crystallizes in
the tetragonal P4/mnc space group and possesses a discrete
PONb cluster PNb12O40(VO)2 (Fig. 1), with copper complexes
compensating for the charge of the polyanion. The polyanion
PNb12O40(VO)2 is a surface-modified Keggin PONb, i.e., the
α-
Fig. 2 (a) Copper(Cu1) complexes in 1 and cobalt complexes in 2. Color codes:
PNb12O40 captures two vanadyl groups in two opposite sites.
Under hydrothermal conditions, the Nb precursor Nb6O19 is
likely to be broken up into fragments Nb3O13, which is then to
Cu (light green), Co (olive green), O (red), C (gray), N (blue).
Fig.
3 Stacking view of PNb12O40(VO)2 and copper complexes in 1: (a)
supramolecular layer along the b axis and (b) supramolecular arrangement in
the three-dimensional lattices. Color codes: Nb (cyan), V (pink), Cu (light
green), O (red), C (gray), N (blue).
Fig. 1 (a) Representation of polyanion PNb12O40(VO)2: (a) Ball and stick mode
and (b) polyhedron mode. Color codes: Nb (cyan V (pink), Cu (light green), O
(red).
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the triclinic P-1 space group. Similar to
1
, the central P atom Catalysis Investigations
View Article Online
Given the fact that vanadium-containingDcOoIm: 1p0o.1u03n9d/sC7hDaTv0e11t2h2Ae
resides on an inversion centre, and is surrounded by eight O
atoms with each site of them half-occupied. The two
crystallographically distinct cobalt centers are both hexa-
coordinated with octahedral geometries; each cobalt center
located approximately 7.264 － 7.367 Å from the central
phosphorus atoms of the polyanion. For Co1 ion, the
coordination mode is defined by six nitrogen atoms from three
potential to catalytically oxidize C−H bonds,27 we anticipated
that
1−2 could show catalytic activity in the oxidative
transformation of benzyl-alkanes. The oxidation of
diphenylmethane to benzophenone was selected as a model
reaction. In a typical reaction process, 1/2 with 0.03 mol %
catalyst loading was dispersed in benzonitrile (0.5 mL),
followed by addition of tert-butyl hydroperoxide (TBHP) as an
1,2-diaminoethane ligands. The Co-N bond lengths are 1.936
-
o
1.981 Å. Differently, Co2 center is equatorially coordinated by
four nitrogen atoms from two 1,2-diaminoethane ligands and
axially coordinated by two aqua ligands, resulting in a 4+2
mode with a slight Jahn-Teller-distortion (Fig. 2b,2c). The bond
lengths are 1.947－1.980 Å for Co-N and 1.988 Å for Co-Ow,
respectively. It’s worth nothing that the coordinated water
molecules can be removable and thus are potential Lewis acid
oxidant. The reaction was conducted under stirring at 60 C for
24 hours, and was quantitatively analyzed by GC. As shown in
Table 1, the conversions of diphenylmethane are 95.8% and
93.4% after 24h in the presence of
corresponding selectivity for benzophenone is 92.7% (for
and 91.3% (for ). Blank test without catalyst give a poor
conversion of 37.2%.
To gain insight into the heterogeneity of the oxidative
system, a hot leaching test was carried out with as the
catalyst. After the reaction has performed for 6 hours, was
1
and
2
, respectively. The
1
),
2
sites. In 2, the polyanion PNb12O40(VO)2 is surrounded by eight
cobalt complexes cations via electronic interactions and
hydrogen bonding interactions. The cobalt complexes cations
form a six-membered ring (red broken circle in Fig. 4a), as
observed in the supramolecular layer (Fig. 4a). The two-
1
1
o
filtrated off and the filtrate was subsequently stirred at 60 C
for another 18 hours. Result shows that the conversion of
diphenylmethanewas slightly increased by 6.1% (Fig. S12). No
V/Cu ions were detected in the filtrate solution as confirmed
by atomic absorption analysis. These results collectively
confirm that the catalytic system is heterogeneous in nature.
The catalyst can be easily separated by centrifugation or
filtration after the reaction, and subsequently used in the
successive runs for at least 3 times without obvious loss in
catalytic activity. The PXRD patterns and IR spectrum before
dimensional layer then stacks into the 3D network in
2 (Fig. 4).
The resulting voids of the 3D network are filled with lattice
water molecules, in accordance with the TG analysis (Fig S4,
left). Among the reported few Co-based PONb hybrids in
hetero-PONb chemistry,24
2 present another new organic-
inorganic hybrid based on hetero-PONb and cobalt complexes,
illustrating the generality of cobalt complexes to access to a
diverse set of new structures based on PONbs.
There are several capped PONb-based organic-inorganic
hybrids where the polyanion usually integrates with TM
complexes via covalent bonds or weak interactions, and the
introduced TM ions are copper,12,25 zinc,22 nickel,23 and very
recently cobalt.23 It’s worth nothing that all these transition-
and after the catalysis process demonstrated that
1
maintained its structural integrity during the reaction process
(Fig. S13 and S14).
When TMA9[PNb12O40(VO)2]13b is utilized under otherwise
metal ions share oxidation state of +2. In 2, the complexation
of a PONb polyanion with trivalent cobalt complexes is realized,
which shall make a difference in the three-dimensional
arrangement. Using SQUEEZE treatment and with PLATON
Table 1 Conversion and selectivity of diphenylmethane to benzophenone with
catalysts.a
calculations, the space volume of
of the crystal lattice, larger than that (17.0%) of
2
is determined to be 34.9%
. This is
1
expected in that cation-anion interaction decreases with an
increase in the charges of the cationic counterpart, leading to
Catalyst
Conv. (%)
Sele. (%)b
Reaction system
the looser packing of the ions in 2 26
.
1
95.8
93.4
87.7
39.7
68.3
37.2
93.7
91.3
93.7
68.3
92.6
67.6
Heterogeneous
Heterogeneous
Homogeneous
Homogeneous
Heterogeneous
-
2
TMA9PNb12V2
Na16SiNb12O40
Cu(en)2SO4
No catalyst
1
1
93.52
92.71
95.74
93.28
2nd run
3rd run
Fig. 4 Stacking view of PNb12O40(VO)2 and cobalt complexes in 2: (a)
supramolecular layer along the c axis and (b) supramolecular arrangement in
the three-dimensional lattices . Color codes: Nb (cyan) V (pink), Co (olive
green), O (red), C (gray), N (blue).
aReaction conditions: diphenylmethane (0.125 mmol, 1 equiv), catalyst (0.004
mmol, 0.03 mol %), solvent benzonitrile (0.5mL), TBHP (70% in water, 2.5equiv),
60oC, 24 h. bSelectivity to ketones.
4 | J. Name., 2012, 00, 1-3
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identical conditions, a moderate conversion of 87.7% is
achieved. In this case, however, the reaction proceeds in a
homogenous manner, being largely restricted in industrial
application where recovery and separation is a prerequisite. In
contrast, when the catalytically active species PNb12O40(VO)2 is
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a
DOI: 10.1039/C7DT01122A
Table 2 Results of Selective Oxidation of Benylic Compounds Catalyzed by 1.
Conv. Sele.
Entry
Substrates
Products
(%)
(%)b
93.7
“immobilized” as insoluble solids,
1 or 2 works in a
heterogeneous way, and can be separated from the reaction
mixture by simply filtration or centrifugation. Hence, the
heterogenization of catalytically active PNb12O40(VO)2 is
realized. We believe the counter-ions of organometallic
cations, where transition metal ions hold either unsaturated
1
2
95.8
>99
>99
>99
>99
>99
>99
coordinated sites (in 1) or the removable aqua ligands (in 1/2),
can also contribute to the reaction to some extent, given the
certain conversion by copper complexes Cu(en)2SO4. The PONb
component shall make no contributions to the reaction, as
indicated by the poor conversion as to that of blank test in the
control experiment using Na16SiNb12 as a catalyst. Hence, a
synergetic effect between the two components, the vanadyl
groups in PNb12O40(VO)2 and [Cu(en)2]2+/[Co(en)2(H2O)2]3+ in
3
4
1
/2
should be better illustrated for the enhanced activity.
To study the general applicability of 1, a wide range of
5
>99
>99
benzyl-alkanes were subjected to this catalytic oxidation
reaction under the same conditions. Table 2 summarizes the
6
7
>99
>99
results of the oxidative transformation. Compound
1 catalyzed
transformation of diphenylmethane, 9H-xanthenes, fluorene,
2-bromofluorene, 2,7-dibromofluorene and 2,7-di-tert-
butylfluorene to the corresponding ketones with excellent
conversions (95.8-99%) and selectivity (93.7-99%) (Table 2,
entries 1-6). That is, comparable performance is observed
78.3
98.5
towards substrates with different molecule sizes. Hence
1 was
8
9
63.3
92.1
95.1
90.3
not substrate size-selective, and might promote the oxidation
of benzyl-alkanes on its exterior surface. The alkyl-substituted
benzylichydrocarbons (ethylbenzene, 4-ethylnitrobenzene and
4-ethylanisole) were oxidized to the corresponding carbonyl
compounds in a more moderate conversion (63.3-78.3%)
(Table 2, entries 7-9). The different conversions for these
substrates are most likely due to the activation energy of the
phenyl groups.
aReaction conditions: substrates (0.125 mmol, 1 equiv), catalyst (0.004 mmol,
0.03 mol %), solvent benzonitrile (0.5mL), TBHP (70% in water, 2.5equiv), 60oC,
24 h. bSelectivity to ketones.
A radical process might be involved in the catalytic
reaction, as
1 can also smoothly promote the oxidative
transformation of 2,7-di-tert-butyl-9H-fulorene (Table 2, entry
6) bearing methane group with high steric hindrance and
hence should be not accessible to the surface of catalyst.28
A
radical probe reaction was thus designed and carried out to
understand the catalytic mechanism of the oxidation reaction.
Diphenylamine was used as an oxygen-radical scavenger.28b
The catalytic oxidation of diphenylmethane was totally
inhibited when diphenylamine was added in the reaction
o
system at 60 C for 24 h in the presence of
1
. Using 5,5-
dimethyl-1-pyrroline-N-oxide (DMPO) as spin trap, the
generated DMPO/·OOBu-t spin adduct were well detectable by
EPR spectra as indicated by g value of 2.0017 (Fig. 5),29
suggesting the formation of ·OOBu-t during the reaction. Thus
we propose that
1 promoted the oxidation reaction by
decomposing the oxidant into ·OOBu-t radical, which then
oxidized the benzylic substrates. As a result, the synergistic
effect between the cation and vanadyl groups in the polyanion
Fig. 5 EPR spectra with (line in blue) and without (line in black) 1, using
DMPO as spin trap. Reaction conditions: diphenylmethane (0.125 mmol),
solvent benzonitrile (0.5mL), TBHP (70% in water, 2.5equiv), 60 oC, 10min.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2017, 00, 1-3 | 5
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Page 6 of 7
ARTICLE
Journal Name
S. J. Wery, S. Reinoso, B. Artetxe, L. S. Felices, B.VEie.wBAartkickleaOlin,lGine.
components in 1 shall be responsible for the radical formation.
Trautwein, J. A. Monge, J. L. Vilas and J. M. G. Zorrilla, Inorg.
DOI: 10.1039/C7DT01122A
The insight into the role of cationic and anionic parts in the
oxidative transformation of benzyl-alkanes to the
corresponding ketone products is under study.
Chem., 2016, 55, 4970.
11 (a) Y. Zhang, J.-Q. Shen, L.-H. Zheng, Z.-M. Zhang; Y.-X. Li and
E.-B Wang. Cryst. Growth Des., 2014, 14, 110; (b) J.-Q. Shen,
Q. Wu, Y. Zhang, Z.-M. Zhang, Y.-G. Li, Y. Lu and E.-B Wang,
Chem. Eur. J., 2014, 20, 2840; (c) Z.-Y. Zhang, J. Peng, Z.-Y Shi,
W.-L. Zhou, S. U. Khan and H.-S. Liu, Chem. Commun., 2015,
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Conclusions
In summary, using organometallic cations with different
charge, two polyoxoniobate-based ionic organic-inorganic
compounds with diverse three-dimensional arrangements
were assembled, and assessed as efficient heterogeneous
catalysts towards the selective oxidation of benzyl-alkanes. A
heterogeneous system based on PONbs was established for
12 (a) J.-H. Son, C. A. Ohlin, E. C. Larson, P. Yu and W. H. Casey,
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the first time. This work takes a step toward polyoxoniobate- 15 N. Dielt, R. F. Hockendorf, M. Schlangen, M. Lerch, M. K.
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Beyer, H. Schwarz, Angew. Chem. Int. Ed. 2011, 50, 1430.
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a
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China 21671019, 21231002, 111 Project
(B07012) and 973 Program (2014CB932103).
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ActaCrystallogr., Sect. C: Struct. Chem.2015,71, 3.
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View Article Online
DOI: 10.1039/C7DT01122A
Immobilization of Keggin Polyoxovanadoniobate in Crystalline Solids as Effective
Heterogeneous Catalysts towards Selective Oxidation of Benzyl-Alkanes
Jufang Hu, Jing Dong, Xianqiang Huang, Yingnan Chi, Zhengguo Lin, Ji-Kun Li, Song Yang, Hongwei Ma
and Changwen Hu*
The catalytically active PNb12O40(VO)2 was immobilized in crystalline solids and worked as
effective robust catalysts towards selective oxidation of benzyl-alkanes.