Controllable synthesis, characterization, and catalytic properties of three inorganic-organic hybrid copper vanadates in the highly selective oxidation of sulfides and alcohols

Article abstract of DOI:10.1021/acs.cgd.5b00086

Three novel inorganic-organic hybrid copper vanadates α-[Cu(mIM)₄]V₂O₆ (1), β-[Cu(mIM)₄]V₂O₆ (2), and [Cu(mIM)₂)](VO₃)₂ (3) (mIM = 1-methylimidazole) have been synthesized by rationally controlling of the hydrothermal conditions and fully characterized by single-crystal XRD, powder XRD, elemental analyses, TGA, and FT-IR spectroscopy. Interestingly, compounds 1 and 2 were isolated as geometric isomers by tuning the solvothermal reaction temperature. Because of the different coordination modes between the tetradentate [V₄O₁₂]^4- and [Cu(mIM)₄]²⁺ subunits, the supramolecular structures of the two isomers show a 3D framework with an interpenetrating diamond topology for 1 and a 2D network for 2, respectively. Compound 3 was obtained by tuning the reaction temperature and the ratio of mIM/H₂O, which showed interesting left- and right-handed helixes with an identical pitch of ca. 5.388 ? in its 2D network structure. As heterogeneous catalysts, compounds 1-3 exhibit excellent catalytic performance in the oxidation of sulfides with H₂O₂ as oxidant. Among them, the catalytic activity of 1 (conv. up to 98.7%, sele. up to 100%) outperforms others and can be reused without losing its activity. The activity of 1 is also investigated in the oxidation of various alcohols, and excellent results (conv. up to 98.5%, sele. up to 100%) are obtained.

Full text of DOI:10.1021/acs.cgd.5b00086

Article
pubs.acs.org/crystal
Controllable Synthesis, Characterization, and Catalytic Properties of
Three Inorganic−Organic Hybrid Copper Vanadates in the Highly
Selective Oxidation of Sulfides and Alcohols
†
,‡
†
†
,†
,†
Jikun Li, Xianqiang Huang, Song Yang, Yanqing Xu,* and Changwen Hu*
†Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic,
School of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China
‡
College of Chemistry and Chemical Engineering, Taishan University, Tai’an 271021, Shandong P. R. China
*
S Supporting Information
ABSTRACT: Three novel inorganic−organic hybrid copper vanadates α-
[
(
Cu(mIM) ]V O (1), β-[Cu(mIM) ]V O (2), and [Cu(mIM) )](VO )
3) (mIM = 1-methylimidazole) have been synthesized by rationally
4 2 6 4 2 6 2 3 2
controlling of the hydrothermal conditions and fully characterized by single-
crystal XRD, powder XRD, elemental analyses, TGA, and FT-IR spectros-
copy. Interestingly, compounds 1 and 2 were isolated as geometric isomers
by tuning the solvothermal reaction temperature. Because of the different
4
−
2+
coordination modes between the tetradentate [V O ] and [Cu(mIM)4]
4
12
subunits, the supramolecular structures of the two isomers show a 3D
framework with an interpenetrating diamond topology for 1 and a 2D
network for 2, respectively. Compound 3 was obtained by tuning the
reaction temperature and the ratio of mIM/H O, which showed interesting
2
left- and right-handed helixes with an identical pitch of ca. 5.388 Å in its 2D
network structure. As heterogeneous catalysts, compounds 1−3 exhibit
excellent catalytic performance in the oxidation of sulfides with H O as oxidant. Among them, the catalytic activity of 1 (conv.
2
2
up to 98.7%, sele. up to 100%) outperforms others and can be reused without losing its activity. The activity of 1 is also
investigated in the oxidation of various alcohols, and excellent results (conv. up to 98.5%, sele. up to 100%) are obtained.
INTRODUCTION
organic subunits and the vanadium oxide. Therefore, the
rationally controlling synthesis of hybrid vanadates is essential
but remains challenging.
■
Inorganic−organic hybrid materials have recently become an
area of great interest, especially those hybrid materials
combining both metal-O clusters and organic ligands, not
only because of their intriguing varieties of structures but also
owing to their potential applications in many fields, such as
catalysis, hydrogen storage, molecular adsorption, magnetism,
Because of the enormous diversity, it is difficult to draw
straight correlations between the obtained crystal structures of
hybrid materials and the initial synthetic conditions. However,
it is possible to partially control the synthesis of hybrid
vanadates by adjusting the initial pH value, concentration,
temperature, solvents, etc., for the formation of the V O is
1
−5
and photochemistry. Polyoxometallates (POMs) are familiar
inorganic building blocks for their prominent catalytic proper-
ties along with their chemical stability, including oxidative
x
y
sensitive to these factors. For example, a recent research
reported by the Arriortua group of the Ni(II)/Bpe/V O and
6
−16
stability, thermal stability, and hydrolytic stability.
As a
x
y
Ni(II)/4,4′-Bpy/V O systems revealed the effect of the initial
subclass of POMs, vanadates are a promising candidate to
construct POMs-based hybrid materials due to their
unprecedented structural diversity, mixed valency, and variable
coordination geometries.
Hybrid vanadates with first-row transition metals show very
rich structural diversities.
exhibit a range of different dimensional structures due to
various polyhedral fragments built of VO units (n = 4, 5, 6).
x
y
concentration and stoichiometry on the vanadium coordination
environment and the vanadates subunits of the final 3D crystal
28
1
7
structures. Their excellent work prompts us to explore the
possibility to synthesize hybrid vanadates with potential
applications by tuning the initial reaction conditions.
1
8−25
Among them, V-O clusters
It is well-known that vanadium-based materials are active
heterogeneous catalysts for several reactions but remain
26
n
29
n−
3−
unexplored to a great degree. To our knowledge, only a
The structure of [V O ] ranges from monomeric [VO ]
x
y
4
4
−
4−
6−
units to polymeric [V O ] , [V O ] , and [V O ] , etc.,
2
7
4
12
n−
6
18
n−
27
from [VO3]n chains to [V O ] networks. Further, the
Received: January 20, 2015
Revised: February 22, 2015
x
y
hybrid vanadates are expected to exhibit improved properties
and functions by the synergetic interaction between the metal−
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.5b00086
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Table 1. Crystal Data and Structure Refinement for Compounds 1−3
1
2
3
formula
C H CuN O V
C H CuN O V
C H CuN O V
6 2
1
6
24
8
6
2
16 24
8
6
2
8
12
4
Mr
589.85
tetragonal
P4(2)/n
296(2)
13.106(3)
13.106(3)
13.634(6)
90
589.85
425.64
crystal system
space group
temperature (K)
a (Å)
monoclinic
P2(1)/n
296(2)
10.8254(3)
16.3599(4)
12.9484(3)
90
monoclinic
P2(1)/c
296(2)
7.7433(13)
5.3882(9)
17.039(3)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
90
95.073(1)
90
102.523(2)
90
90
V (Å3)
2341.8(13)
4
2484.21(1)
4
694.0(2)
2
Z
Dcalc.(g cm−3)
1.673
1.715
2.037
F(000)
1196
1196
422
R1 [I > 2σ(I)]
0.0421
0.1193
0.0596
0.1278
1.055
0.0266
0.067
0.0273
0.1322
0.0330
0.1587
1.034
wR [I > 2σ(I)]
2
R1 (all data)
0.0331
0.0703
1.019
wR (all data)
2
GOOF
CCDC No.
932116
1008975
932117
few researches focused on the catalytic properties of inorganic−
organic hybrid vanadates in the field of the oxidation sulfides,
the cyanosilylation reaction of aldehydes with trimethylsilyl
cyanide, the decomposition of methylene blue, splitting of
water, and degradation of pollutants. The hybrid vanadates
used in these catalytic reactions are the family of M(HAep)2-
EXPERIMENTAL SECTION
■
Materials and Methods. All reagents and solvents were
commercially obtained and used without further purification. All
syntheses were carried out in 20 mL Teflon-lined stainless steel
containers under autogenous pressure. The mixed reactants were
stirred at room temperature for 120 min before heating. The metal
content of the compounds 1−3 was measured by inductively coupled
plasma (ICP) on a JY-ULTIMA2 analyzer. The FT-IR spectra were
29
(
VO ) (M = Co, Ni, Cu; Aep = 1-(2-aminoethl)piperazine)
3 4
3
0
31
compounds, [{CoNi(H O) (Bpe) }(V O )]·4H O·Bpe,
−1
2
2
2
4
12
2
recorded from KBr pellets in the range of 4000−400 cm on a
[
{Ag(Bpy)} (V O )]·2H O, [{Ag(Dpa)} (V O )]·4H O
4 4 12 2 4 4 12 2
Nicolet 170 SXFT/IR spectrometer. The PXRD of all samples was
collected on a Bruker D8 X-ray diffractometer equipped with graphite
monochromatized Cu Kα radiation (λ = 0.15418 Å). Elemental (C, H,
and N) analyses were performed on a PerkinElmer 2400 II analyzer.
The TGA was performed on a DTG-60AH instrument under a N2
atmosphere with a heating rate of 10 °C/min. After the catalytic
reaction was completed, the resulting mixture was analyzed by GC-MS
and GC using naphthalene as internal standard substrate. The GC
analyses were performed on a Shimadzu GC-2014C with an FID
detector equipped with an Rtx-1701 Sil capillary column. The GC-MS
spectra were recorded on an Agilent 7890A-5975C at an ionization
voltage of 1200 V. Atomic absorption analysis was measured by an
inductively coupled plasma spectrometer (ICP) on an ICP-6000
analyzer.
and {Ag (2-Pzc) }(V O ) (Bpy = 4,4′-bipyridine; Dpa = 1,2-
4
2
2
6
32
bis(4-pyridyl)-ethane; Pzc = pyrazinecarboxylate), [Ag(Bbi)]-
{Ag(Bbi)} {Ag (V O ) }]·2H O (Bbi = 1,4-bis(N-
imidazolyl)butane), and Mn(Bpy)(V O )(Bpy). From
[
4
3
4
12
2
2
3
3
34
4
11
the above-mentioned point of view, there is much room for
expanding the catalytic application of inorganic−organic hybrid
vanadates.
Herein, to continue our solvothermal synthesis of poly-
35
oxovanadates (POVs), we select 1-methylimidazole (mIM) as
ligand/solvent and POV precursors to synthesize the
inorganic−organic hybrid vanadates. Fortunately, by the
rational controlling of the hydrothermal conditions, three
novel inorganic−organic hybrid copper vanadates α-[Cu-
Synthesis of α-[Cu(mIM) ]V O (1). A mixture of CuCl ·2H O
4
2
6
2
2
(
0.1705 g, 1.0 mmol), NH VO (0.1404 g, 1.2 mmol), mIM (2 mL),
4 3
(
(
mIM) ]V O (1), β-[Cu(mIM) ]V O (2), and [Cu(mIM) ]-
4 2 6 4 2 6 2
and H O (12 mL) was heated at about 160 °C for 72 h. After the
2
VO ) (3) were synthesized and fully characterized by single-
mixture cooled to room temperature, dark blue crystals were isolated
from the mixture. Yield: 58% (based on NH VO ); Anal. Calcd
3
2
crystal X-ray diffraction (SXRD), powder X-ray diffraction
PXRD), elemental analyses, TGA, and FT-IR spectroscopy.
Among them, α-[Cu(mIM) ]V O (1) and β-[Cu(mIM) ]-
4
3
(
found) for C H N CuV O (%): C, 32.58 (32.35); H, 4.10 (4.32);
(
16 24 8 2 6
N, 19.00 (18.75); Cu, 10.77 (10.55); V, 17.27 (17.02).
4
2
6
4
Synthesis of β-[Cu(mIM) ]V O (2). A mixture of CuCl ·2H O
4
2
6
2
2
V O (2) were isolated as a pair of isomers showing 3D
2
6
(
0.1705 g, 1.0 mmol), NH VO (0.1404 g, 1.2 mmol), mIM (2 mL),
4 3
interpenetrating frameworks and 2D network structures,
respectively. Further, the catalytic activity of 1 as heterogeneous
catalyst was probed in the oxidation of various sulfides and
alcohols using H O as oxidant. The hybrid copper vanadate 1
and H O (12 mL) was heated at about 120 °C for 72 h. After the
2
mixture cooled to room temperature, dark blue crystals were isolated
from the mixture. Yield: 55% (based on NH
VO ); Anal. Calcd
4
3
(
found) for C H N CuV O (%): C, 32.58 (32.37); H, 4.10 (4.35);
16 24
8
2
6
2
2
N, 19.00 (18.78); Cu, 10.77 (10.62); V, 17.27 (17.12).
showed high activity and selectivity in the oxidation reactions
toward corresponding sulfoxides and aldehydes. To our
knowledge, it is the first time that inorganic−organic hybrid
vanadate was used as catalyst in the conversion of alcohols to
corresponding aldehydes.
Synthesis of [Cu(mIM) ](VO ) (3). A mixture of CuCl ·2H O
2
3 2
2
2
(
0.1705 g, 1.0 mmol), NH VO (0.1404 g, 1.2 mmol), mIM (1 mL),
4 3
and H O (13 mL) was heated at 120 °C for 72 h. After the mixture
2
cooled to room temperature, pale blue block crystals were isolated
from the mixture. Yield: 65% (based on NH VO ); Anal. Calcd
4
3
B
DOI: 10.1021/acs.cgd.5b00086
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Scheme 1. Controlling Syntheses of Compounds 1−3
Figure 1. (a) The 1D zigzag chain structure in 1. (b) Polyhedral view of the 2D network in 1. (c) The 3D framework of 1. (d) The interpenetrating
diamond topological structure of 1. Color code: [V O ] cluster, green; [CuN O ] octahedron, sky blue.
4
12
4
2
(
found) for C H N CuV O (%): C, 22.57 (22.32); H, 2.84 (4.98);
graphic data are summarized in Table 1. For the selected bond lengths
and bond angles, please see Table S1 (Supporting Information).
8
12
4
2
6
N, 13.16 (12.95); Cu, 14.93 (14.73); V, 23.94 (23.85).
General Procedure for Compound 1 Catalyzed Oxidation of
Sulfides and Alcohols. Sulfides oxidation: Compound 1 (5.0 mg,
RESULTS AND DISCUSSION
■
8
.48 μmol), sulfide (0.25 mmol), H O (9.4 mg, 0.275 mmol), and
2 2
Syntheses. As vanadates can exhibit different dimensional
structures, in order to explore the control toward the forming
of the various architectures of hybrid vanadates with potential
applications, we chose the simple vanadium source NH VO as
the precursors to react with CuCl ·2H O and mIM under
hydrothermal/solvothermal conditions. By controlling the
methanol (1 mL) were added to a glass tube; then the catalytic
reaction was performed on a Wattecs Parallel Reactor at 40 °C for 4 h.
Alcohols oxidation: Compound 1 (6.0 mg, 10.17 μmol), alcohol (0.25
mmol), H O (34.1 mg, 1 mmol), and chlorobenzene (1 mL) were
4
3
2
2
added to a glass tube; then the catalytic reaction was performed on a
Wattecs Parallel Reactor at 120 °C for 8 h. After the reaction was
completed, the resulting mixture was analyzed by GC-MS and GC.
Crystal Determination. Crystal data for all compounds were
collected at 298(2) K on a Bruker APEX−II CCD detector with
graphite monochromatic Mo Kα radiation (λ = 0.71073 Å). Crystals
were mounted on a glass fiber and coated with oil. All absorption
corrections were applied using a multiscan technique. The reflections
collected were integrated and scaled using the APEX 2 software
2 2
reaction temperature, compounds α-[Cu(mIM) ]V O (1)
4
2
6
and β-[Cu(mIM) ]V O (2) with isomeric structures were
4
2
6
isolated, respectively, as shown in Scheme 1. Interestingly, the
2+
different coordination modes between [Cu(mIM) ] and
4
4−
[
V O ] subunits lead to the 3D interpenetration framework
4
12
structure of 1 and the 2D network structure of 2. Since mIM
may serve as coordination ligand, organic base and solvent,
3
6,37
package.
All structures were solved by the direct method and
2
compound [Cu(mIM)
](VO ) (3) with a 2D network
2 3 2
refined by full-matrix least-squares on F using the SHELXTL program
package (Bruker).
n−
3
6,37
structure is formed by the linkage of [VO ]
chains and
Cu(mIM) ] subunits, which can be prepared by reducing the
3
n
All non-hydrogen atoms were located by direct
2+
[
methods and were refined anisotropically. All hydrogen atoms were
placed at calculated position and refined as riding models. The
crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre (CCDC) as CCDC-932116 (1),
CCDC-1008975 (2), and CCDC-932117 (3). The detailed crystallo-
2
ratio of mIM and H O to 1/13 with other synthetic conditions
2
remaining unchanged with 2. Therefore, the ratio of mIM/H O
is an important factor in the synthetic procedure of compounds
2 and 3.
2
C
DOI: 10.1021/acs.cgd.5b00086
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
Structural Description. Bond valence sum calculations
show that all vanadium atoms are in +V oxidation states, and
copper atoms are in +II oxidation states in 1−3 (Table S2,
Supporting Information). This result is consistent with
structural analyses and charge balance.
skeleton in 1 and 2 are different. In order to distinguish them,
hereafter the [Cu N V O ] skeletons in 1 and 2 are labeled as
4
16
4
12
α -[Cu N V O ] and β -[Cu N V O ], respectively (Figure
1
4
16
4
12
1
4
16
4
12
2). In the α -[Cu N V O ] skeleton, the alternate [VO ]
1
4
16
4
12
4
Structure Description of Compound 1. The SXRD
2
+
analysis reveals that 1 consists of one Cu cation, one
2
−
[
V O ] anion cluster, and mIM ligands (Scheme 1a). The Cu
2 6
atom is six-coordination in an octahedral geometry, defined by
four nitrogen atoms from mIM ligands and two oxygen atoms
2
−
from two [VO ] tetrahedra. In the [V O ] cluster, there is
4
2
6
only one crystallographically independent V atom. The V atom
possesses a distorted tetrahedron achieved by a terminal oxygen
atom, three bridging oxygen atoms from two [VO ] tetrahedral,
4
and one [CuN O ] octahedron.
4
2
Compound 1 is a novel hybrid compound based on POVs
that displays an interesting 3D interpenetrating framework.
Such an intriguing structure can be described in the following
Figure 2. Coordination environment of [V O ] subunits in 1 (α -
4
12
1
[Cu N V O ]) and 2 (β -[Cu N V O ]).
4
16
4
12
1
4
16
4
12
three steps: first, the adjacent [VO ] tetrahedra are connected
4
4−
4−
by the corner-sharing oxygen atoms to form [V O ] clusters;
tetrahedra of the [V O ] subunit coordinate to two
4
12
4
12
4−
2+
then two alternate [VO ] tetrahedra of each [V O ] cluster
are linked by two Cu[(mIM)4] subunits, giving rise to a 1D
[Cu(mIM)4] ions by the terminal O atoms on the same
side of the pseudo-plane formed by the [V O ] . In the β -
4
4
12
2
+
4−
4
12
1
zigzag chain structure along the crystallographic c-axis (Figure
[Cu N V O ] skeleton, however, the alternate [VO ]
4 16 4 12 4
4
−
2+
1
a). The [V O ] cluster in 1 serves as a tetradentate ligand,
tetrahedra coordinate to two [Cu(mIM) ] ions on the
4
12
4
4−
4−
and in the second step, each [V O ] donates its third [VO ]
different side of the [V O ] pseudo-plane. It is worth
4
12
4
4
12
2
+
tetrahedra to coordinate to a Cu[(mIM)4] subunit by one
terminal O atom to connect the adjacent 1D zigzag chains into
a 2D network parallel to the [111] plane (Figure 1b). At last, as
shown in Figure 1c, a 3D interpenetrating-framework is formed
mentioning that the formation of the two isomers (compounds
1 and 2) can be controlled by the hydrothermal reaction
temperature only. In addition, the different coordination modes
4−
2+
between the [V O ] subunit and the [Cu(mIM)4] ions
4
12
by the coordination of the forth [VO ] tetrahedra of each
result in the different supramolecular structures; only a 2D
network is formed, as shown in Figure 3, instead of the 3D
framework of 1.
4
4−
2+
[
V O ] to other Cu[(mIM)4] ions. In order to better
4 12
understand the structure of compound 1, the topological
analysis of the 3D framework has been performed by
4
−
considering each [V O ] cluster as a 4-connected node;
4
12
thus the structure displays 2-fold interpenetrating diamond
6
̈
topology with a Schlafli symbol of {6 } (Figure 1d).
For the hybrid vanadates, the use of bridging ligands allows
the combination of the vanadium oxide subunits with
secondary metal−organic subnets to give rise to various
29
dimensional crystal architectures. Especially, the hybrid
vanadates with 3D frameworks are usually the extension of
z+
the 1D or 2D {M(L)n}m structures with the connection of the
4−
bridging ligands. For example, most of the reported [V O ]
4
12
containing inorganic−organic hybrid vanadates with 3D
4
−
frameworks are formed by the combination of [V O ] and
4
12
29
the secondary metal−ligand subunit. In 1, however, the 3D
interpenetrating framework was constructed by the mere
inorganic skeleton [Cu V O ] without the participation of
2
4
12 n
the bridging ligands. The mIM here only serves as a chelating
ligand to complete the coordination environment of the Cu2+
center and stabilize the crystal structure of 1. Such a type of 3D
4
−
interpenetrating framework is rare in the [V O ] containing
4
12
inorganic−organic hybrid vanadates.
Figure 3. Polyhedral view of the 2D network in β-[Cu(mIM) ]V O
6
(2).
4
2
Structure Description of Compound 2. Interestingly, by
lowering the hydrothermal temperature with other conditions
remaining unchanged, compound 2 (Scheme 1b) was isolated
as a geometric isomer of compound 1. Although 1 and 2 have
the same chemical formula, the coordination mode between the
Structure Description of Compound 3. When we reduce
the ratio of mIM and H O (ligand/solvent) and keep other
2
synthetic conditions the same with 2, compound 3 was isolated
when the reaction mixture was down to room temperature.
According to the SXRD and element analysis, the structural
formula of compound 3 is [Cu(mIM) ](VO ) , in which there
[
Cu(mIM)4]2 cations and the [V O ] anions is very
+
4−
4 12
4−
different. As in 1, the [V O ] subunits in 2 can also be
4
12
regarded as a tetradentate ligand and each [VO ] tetrahedra
4
2
3 2
2
+
coordinates with a Cu ion via one of the terminal O atoms.
is only one crystallographically independent vanadium atom
and it is in a tetrahedral coordination environment, as shown in
However, the connectivity patterns of the [Cu N V O ]
4
16
4
12
D
DOI: 10.1021/acs.cgd.5b00086
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
1, 805 and 665 cm− for 2, 836, 760, and 631 cm−1 for 3 can be
1
Scheme 1c. Each [VO ] tetrahedra is linked to other two
4
n−
tetrahedra through two of their vertex, forming [VO3]n chains
assigned to the bridging V-O-Cu vibrations.
n−
along the crystallographic b-axis. The adjacent [VO ] chains
Catalytic Activities of Hybrid Cu-vanadates. Selective
oxidation of organic compounds, such as sulfides, alcohols,
aromatic and aliphatic hydrocarbons, etc., is one of the most
vital transformations for upgrading raw starting materials into
high value added products. In these reactions, vanadium
peroxides are known as very effective oxidants, where the
terminal oxidant is normal, environmentally friendly hydrogen
peroxide or alkylhydroperoxide.
3
n
were further connected together by [Cu(mIM)2]2 subunits,
+
giving rise to a 2D network, parallel to the [100] plane (Figure
4
a). The Cu atom, residing in a square planar environment, is
n−
coordinated by two O atoms derived from two [VO3]n chains
and two N atoms from the mIM ligands.
Because of the importance of sulfoxides as intermediates of
4
1
42
biologic molecules, chiral auxiliaries, and oxo-transfer
reagents,
43,44
the high selective oxidation of sulfides to obtain
sulfoxides is more desirable. Using hydrogen peroxide as an
oxidant offers significant environmental and economic benefits
over traditional stoichiometric oxidants. Our initial oxidation
studies focused on the case of oxidation of sulfides, with the aim
to understand the catalytic property of the Cu-vanadates
(
compounds 1−3) for developing a new oxidation system in
organic chemistry.
The oxidation of methyl phenyl sulfide was selected as a
model reaction to evaluate the catalytic activities of compounds
1−3 (Scheme 2). Before the reaction, all catalysts were
Figure 4. (a) The 2D network of 3. (b) Stereoview of the left- and
right-handed helical chains in 3. Color code: V, green; O, red;
[
CuN O ] square plane, pale blue.
2 2
Scheme 2. Oxidation of Methyl Phenyl Sulfide Using Cu-
vanadates as Catalysts under the Preliminary Optimization
Conditions
Interestingly, compared with recently reported 2D vanadates
3
8
based on V-O chains such as [Zn(pyim)] V O , [Zn (bbi) -
(
2
4
12
2
2
3
9
40
V O )], [{Cu(bipy)(en)}{Cu(bipy)(H O)}{VO } ] ,
4 12 2 3 4 n
30
and M(HAep) (VO ) (M = Co, Ni, Cu), the V-O chains
2
3 4
in compound 3 show a pair of left- and right-handed helixes
with an identical pitch of ca. 5.388 Å along the b-axis connected
by [Cu(mIM)2]2 subunits (Figure 4b).
activated by stirring them with H O (mole ratio, ncatalyst/n
2 2
H O
+
2
2
=
1:2) in methanol at 40 °C for 30 min. Then, they were
Powder XRD and TGA Analyses. To further study the
purity and repeatability of all the crystals, the bulk products
were crushed to obtain a fine powder suitable for powder XRD
analysis. The powder XRD detection allowed us to confirm that
the bulk powder was a single phase material. All peaks are
perfectly indexed on the basis of the simulated parameters from
single crystal analysis. As shown in Figure S1 (Supporting
Information), the diffraction peak positions of the experimental
powder XRD patterns of 1−3 are in agreement with the
simulated patterns, which indicate that the structures of bulk
powders are the same with those of single crystals and the
phase purity is satisfactory. The TGA analyses show that all
compounds are stable under 150 °C and meet the conditions of
the catalytic reactions (Figure S2, Supporting Information).
FT-IR Spectra. The IR spectra of compounds 1−3 display
similar characteristic vibration patterns mainly resulting from
the mIM ligands and vanadate groups. As shown in Figure S3
separated from the solvent and dried for use. The methyl
phenyl sulfide oxidation catalyzed by Cu-vanadates (mole ratio,
ncatalyst/nsubstrate = 3.4%) was performed in methanol at 40 °C for
4
h. The results are illustrated in Table 2. As shown in Table 2,
Table 2. Conversion of Methyl Phenyl Sulfide to Methyl
a
Phenyl Sulfoxide with Different Catalysts
b
catalyst
conv. (%)
sele. (%)
reaction system
1
2
3
98.7
95.8
92.5
28.6
>99
100
100
98.5
100
95.5
heterogeneous
heterogeneous
heterogeneous
homogeneous
homogeneous
c
[Cu(mIM) (H O) ]Cl
4 2 2 2
VIV (mIM) ]V O
[
V
4
d
2
8
14
a
Reaction conditions: methyl phenyl sulfide (0.25 mmol, 1 equiv),
catalyst (8.48 μmol, 3.4 mol %), H O (1.1 equiv), methanol (1 mL),
2
2
b
c
4
0 °C, 4 h. Selectivity to sulfoxides. The byproduct was sulfone. Cu
(
Supporting Information), characteristic bands in the range of
d
content equivalent to 1. V content equivalent to 1.
−1
1
500−1600 cm can be attributed to the CC and CN
stretch modes of the mIM. Peaks associated with CC−H
−1
vibrational modes can be clearly observed at 3113 cm for 1,
compounds 1−3 all can efficiently catalyze the oxidation of
methyl phenyl sulfide to corresponding sulfoxide with the
conversion of 98.7−92.5% and selectivity of 98.5−100%.
Notably, compound 1 showed higher activity than the other
two compounds under optimized conditions. Indeed, using
compound 1 as catalyst, without any further optimization, the
conversion of methyl phenyl sulfide reaches to 98.7% with
methyl phenyl sulfoxide as the only product. Given the fact that
no channels exist in compounds 1−3, the different catalytic
1
−1
3
115 cm− for 2, and 3135 cm for 3, respectively. The
characteristic absorption bands of the methyl organic group
−1
appear at 1100−1290 and 2700−2900 cm . Bands below 1000
−1
cm are principally assigned to inorganic groups vibrational
modes and two types of oxygen atoms: O (terminal oxygen
t
atoms), and O (bridging oxygen atoms). Specifically, the
b
−1
terminal VO stretching bands appear at 923 cm for 1, 937
t
−1
−1
−1
cm for 2, and 967 cm for 3. Bands at 795 and 661 cm for
E
DOI: 10.1021/acs.cgd.5b00086
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
activity may be ascribed to the different supramolecular
structures. As heterogeneous catalyst, compound 1 with a 3D
interpenetrating framework may result in more exposure of the
active sites than the 2D network structures in 2 and 3.
The generality of 1 for sulfoxidation was examined on various
sulfides with different electronic and steric effects under
optimized conditions. As shown in Table 4, no obvious
To probe the role of each active center of compound 1 in the
selective oxidation of methyl phenyl sulfide, we further studied
Table 4. Results of Selective Oxidation of Various Sulfides
a
Catalyzed by 1 Using H O as Oxidant
2
2
45
the activity of [Cu(mIM) (H O) ]Cl
(Figure S4) and
(Figure S5) (Supporting Informa-
4
2
2
2
IV
V
35
[
V O (mIM) ]V O
2 2 8 4 12
tion). As shown in Table 2, the low conversion of methyl
phenyl sulfide (28.6%) and prominent selectivity toward
sulfoxide (100%) were achieved when using [Cu(mIM) -
4
IV
V
(
H O) ]Cl as catalyst, and using [V O (mIM) ]V O as
2 2 2 2 2 8 4 12
catalyst; however, the conversion of methyl phenyl sulfide
reached to 99% with barely satisfactory selectivity (95.5%).
From the above results, we can conclude that the combination
of [Cu(mIM)4]2 and the vanadate anion cluster can produce
the heterogeneous catalyst (Cu-vanadates) with satisfactory
conversion and excellent selectivity in the catalytic oxidation of
methyl phenyl sulfide (Table 2). Further, we speculate that the
cation part in compounds 1−3 possibly contributes to enhance
the selectivity and the anion part contributes to increase the
+
46−50
conversion of the substrates.
Generally speaking, the
cooperation between cations and anions in all Cu-vanadates
plays some synergistic role in the catalytic sulfonation process,
which can improve the catalytic performance.
Reutilization is one of the greatest advantages of heteroge-
neous catalysts and can also provide useful information about
the catalyst stability along the catalytic cycle. For the excellent
performance, compound 1 is selected to examine the long-term
stability in a heterogeneous system. After the completion of the
reaction, the catalyst can easily be separated from the reaction
mixture by filtration and further reused directly in the
subsequent oxidation reactions. The catalyst was recycled for
at least 3 runs and still maintained its activity and selectivity
a
Reaction conditions: sulfide (0.25 mmol, 1 equiv), catalyst (8.48
(
Table 3). The IR spectra (Figure S6) and PXRD (Figure S7)
μmol, 3.4 mol %), H O (1.1 equiv), methanol (1 mL), 40 °C, 4 h.
2
2
b
c
Selectivity to sulfoxides. The byproduct was sulfone. Reaction time,
d
1
0 h. Hydrolysis products of thiol and alcohol were found and
Table 3. Reutilization Data for Oxidation of Methyl Phenyl
Sulfide with H O over Catalyst 1
e
a
confirmed by GC-MS. Using urea hydrogen peroxide (UHP) instead
2
2
of H O aqueous solution as oxidant.
2
2
cycle
conv. (%)
sele. (%)
1
2
3
98.7
96.5
96.2
100
100
100
decrease of the catalytic activity is observed when electron
donor or acceptor groups with less steric hindrance are
introduced in the monophenyl sulfides (Table 4, entries 2−4).
Regarding sulfoxidation of diphenylsulfide and dibenzyl sulfide,
due to their large steric hindrance, a drastic decrease of the
catalytic activity is observed with excellent selectivity to the
sulfoxides (Table 4, entries 5, 6). It was found that the
prolonged reaction time could not efficiently increase the
conversion of diphenylsulfide and dibenzyl sulfide, but
sacrificed the selectivity to sulfoxide (Table 4, entries 5, 6).
When the aqueous solution of H O was used as the oxidant of
a
Conversion and selectivity toward the sulfoxide formation results
under the optimized reaction conditions.
(
Supporting Information) of 1 after the catalytic reactions
indicate that no significant changes were shown by comparing
with those of the freshly obtained catalysts. To check the
heterogeneity of 1, a hot filtration experiment (separation of
catalyst at the reaction temperature) was carried out. When 1
was filtrated off after 30 min (conv. 38.5%, sele. of sulfoxide
2
2
dipropyl sulfide, the hydrolysis of the substrate was observed
and the selectivity of dipropyl sulfoxide was lowered to 56.8%.
Therefore, we use urea hydrogen peroxide (UHP) instead of
H O , and dipropyl sulfide was converted to dipropyl sulfoxide
1
00%) and the supernatant was then allowed to react under the
same conditions, after 3.5 h, a slight increase in the conversion,
up to 45.8%, was detected, which was obviously lower than in
the presence of 1 (98.7%) (Figure S8, Supporting Informa-
tion). The results further indicate that 1 has good stability
during the oxidation reaction. The heterogeneity of the reaction
was also probed by a leaching test with 1 after the third reaction
cycle. Atomic absorption analysis of the liquid phase after
separation of the catalyst showed that there was no leaching of
the V ions from the catalyst to the reaction mixture (S9,
Supporting Information).
2
2
with high selectivity (Table 4, entry 7).
The excellent stability, high activity and selectivity for the
transformation of sulfides to sulfoxides of catalyst 1 prompted
us to explore the selective oxidation of other organic
compounds. Here, alcohols were selected to investigate the
selective oxidation properties using 1 as catalyst. The
exploratory experiments started using benzyl alcohol as the
model substrate. Under optimized conditions, benzyl alcohol
F
DOI: 10.1021/acs.cgd.5b00086
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
can be efficiently oxidized to the only product benzaldehyde by
selectivity in the oxidation of sulfides. Specifically, compound
1
can convert sulfides to corresponding sulfoxides efficiently
H O with high conversion of 98.5% (Table 5).
2
2
and can be reused by filtration without loss of its activity. The
catalytic reaction can also be expanded to the oxidation of
various alcohols with high efficiency using compound 1 as
catalyst, and this is the first time that inorganic−organic hybrid
vanadates are used as catalysts in oxidation of these substrates.
The rational controlling synthesis of other hybrid vanadates and
the use of them in other catalytic reactions are in progress.
Table 5. Results of Selective Oxidation of Various Alcohols
a
Catalyzed by 1 Using H O as Oxidant
2
2
ASSOCIATED CONTENT
■
*
S
Supporting Information
Crystallographic data in CIF format, PXRD for simulated and
as-synthesized samples, IR spectra, TG curves, bond valence
sum calculations, kinetics of the catalytic reaction, and atomic
absorption analysis. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
E-mail: xyq@bit.edu.cn. (Y.X.).
E-mail: cwhu@bit.edu.cn Fax: (+)86 10 68912631. Tel: (+)86
0 68912631 (C.H.).
*
*
1
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (21173021, 21231002, 21271025,
2
1276026), 973 Program (2014CB932103), the 111 Project
(
B07012), the Beijing Higher Education Youth Elite Teacher
Project, and the Natural Science Foundation of Shandong
Province (ZR2013BL012).
a
Reaction conditions: alcohol (0.25 mmol, 1 equiv), catalyst (10.17
μmol, 4.1 mol %), H O (4 equiv), chlorobenzene (1 mL), 120 °C, 8
2
2
b
h. Selectivity to aldehydes. The byproducts were corresponding
carboxylic acids.
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DOI: 10.1021/acs.cgd.5b00086
Cryst. Growth Des. XXXX, XXX, XXX−XXX