The Polyoxovanadate-Based Carboxylate Derivative K6H[VV17VIV12(OH)4O60(OOC(CH2)4COO)8]·nH2O: Synthesis, Crystal Structure, and Catalysis for Oxidation of Sulfides

Article abstract of DOI:10.1021/acs.inorgchem.7b02207

The high-nuclearity polyoxovanadate-based carboxylate derivative K₆H[V^V₁₇V^IV₁₂(OH)₄O₆₀(OOC(CH₂)₄COO)₈]·nH₂O (1) has been successfully synthesized by conventional aqueous methods and structurally characterized. The [V^V₁₇V^IV₁₂(OH)₄O₆₀(OOC(CH₂)₄COO)₈]^7- polyanion is built up from three cages: one {V^V₁₇(OH)₄O₄₄} cage and two identical [(V^IV₃O₆)₂(OOC(CH₂)₄COO)₄]^8- cages. Of the three cages, the {V^V₁₇(OH)₄O₄₄} is a purely inorganic polyoxovanadate cluster, whereas each of the [(V^IV₃O₆)₂(OOC(CH₂)₄COO)₄]^8- cages is a vanadium-based organic-inorganic hybrid cluster framed via four adipate ligands linking simultaneously to two triangular {V₃} units. The two [(V^IV₃O₆)₂(OOC(CH₂)₄COO)₄]^8- cages are covalently attached to the central {V^V₁₇(OH)₄O₄₄} cage via V-O-V bonds in a linear arrangement, resulting in a {V₂₉}-based hybrid cluster skeleton. The catalytic properties of compound 1 for the oxidation of sulfides by tert-butyl hydroperoxide were investigated, and the result indicates that 1 exhibits excellent catalytic activity for the oxidation of sulfides under mild conditions.

Full text of DOI:10.1021/acs.inorgchem.7b02207

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
The Polyoxovanadate-Based Carboxylate Derivative
K6H[VV VIV (OH)4O60(OOC(CH2)4COO)8]·nH2O: Synthesis, Crystal
17
12
Structure, and Catalysis for Oxidation of Sulfides
Ke Wang, Yanjun Niu, Dongyan Zhao, Yanxin Zhao, Pengtao Ma, Dongdi Zhang, Jingping Wang,*
and Jingyang Niu*
Henan Key Laboratory of Polyoxometalate Chemistry, College of Chemistry and Chemical Engineering, Henan University, Kaifeng,
Henan 475004, People’s Republic of China
S
* Supporting Information
ABSTRACT: The high-nuclearity polyoxovanadate-based carboxylate derivative K6H-
IV
[VV
V
12(OH)4O60(OOC(CH2)4COO)8]·nH2O (1) has been successfully synthesized by
17
conventional aqueous methods and structurally characterized. The
IV
[VV
V
12(OH)4O60(OOC(CH2)4COO)8]7− polyanion is built up from three cages: one
17
{VV17(OH)4O44} cage and two identical [(VIV3O6)2(OOC(CH2)4COO)4]8− cages. Of the
three cages, the {VV17(OH)4O44} is a purely inorganic polyoxovanadate cluster, whereas each
of the [(VIV3O6)2(OOC(CH2)4COO)4]8− cages is a vanadium-based organic−inorganic
hybrid cluster framed via four adipate ligands linking simultaneously to two triangular {V3}
units. The two [(VIV3O6)2(OOC(CH2)4COO)4]8− cages are covalently attached to the central
{VV17(OH)4O44} cage via V−O−V bonds in a linear arrangement, resulting in a {V29}-based
hybrid cluster skeleton. The catalytic properties of compound 1 for the oxidation of sulfides by
tert-butyl hydroperoxide were investigated, and the result indicates that 1 exhibits excellent
catalytic activity for the oxidation of sulfides under mild conditions.
would endow the POV-based organic−inorganic hybrids with
INTRODUCTION
■
additional functionality and promising applications.
Polyoxovanadates (POVs), as an important subclass of
polyoxometalates (POMs), have attracted interest not only
due to the different coordination geometries and versatile redox
activities of vanadium ions but also due to their interesting
molecular magnetism, good catalytic performance, bioactivity,
and importance in medical chemistry.1−8
In various kinds of organic components, the covalent grafting
of carboxylate ligands onto POV building units is a remarkable
approach for the construction of POV-based carboxylate
derivatives.29 To date, a number of POV-based carboxylate
derivatives have been reported, particularly based on mono-
carboxylate and aromatic multicarboxylates as structural
modifiers. The monocarboxylic-functionalized POV clusters
include the following examples that are based on {V2},34
{V3},35−39 {V4},36,37,40−45 {V5},40,44,46 {V6},47,48 {V8},49 {V9},46
{V10},49,50 and {V11}, {V13}, {V16}, and {V18}.51 POVs
incorporating aromatic multicarboxylate ligands have been
successively reported by the groups of Zaworotko and Su. In
their works, {V6O6(OCH3)9(SO4)}, {V6O6(OCH3)9(VO3)-
(H2O)}, and the aforementioned {V4} and {V5} clusters act
as secondary building units that assemble with rigid aromatic
dicarboxylate or tricarboxylate linkers as edges or faces to form
POV-based polyhedral cages.29,52−56 In comparison with
monocarboxylic acids or rigid aromatic carboxylic acids, the
number of POV-based aliphatic dicarboxylate derivatives is very
limited. To our knowledge, only one compound has been
reported, [(n-C4H9)4N]2[V8O8(OCH3)16(C2O4)], with an
oxalate ligand in the central cavity bridging eight vanadium
ions with a ring-shaped core.57 The reason for this is the
As is known, vanadium has the ability to form many
vanadium−oxygen clusters. The reported crystallographically
characterized isopolyoxovanadates [V2O7]4−,9 [V3O9]3−,10
11
12
13
14
[V4O12]4−
,
[V5O14]3−
,
[V10O28]6−
,
[V12O32]4−
,
[V13O34]3−,15 [V15O42]9−,16 and [V16O42]4−,17 form the class
of fully oxidized vanadium species.1 Likewise, the cagelike
18
19
20
clusters [V10O26]4−
,
[H12V13O40]4−
,
[V15O36]5−
,
[V16O38]7−,21 [V16O42]7−,22 [V17O42]4−,23 [V18O42]n− (n = 4,
10),24 [V18O44]6−
,
[V19O49]9−
,
[V22O54]6−
,
and
25
26
25
27
[V34O82]10− constitute the class of mixed-valent VV/VIV
species. Only the [V18O42]12− polyanion belongs to the “fully
reduced” POV species.28 Among them, larger clusters are
relatively few in number, probably due to the small size of the
central templates, which determine the shape of the POV
clusters.25,27 The introduction of oxygen-,29,30 nitrogen-,31 and
phosphorus-containing32,33 donor ligands to POVs allows
charge compensation via formal substitution of peripheral,
bridging, or terminal oxo ligands, thus stabilizing initially
unstable POV architectures, and further provides novel POV
building units for the assembly of larger systems. Moreover,
incorporation of organic groups into POV building blocks
Received: August 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02207
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Inorganic Chemistry
Article
atoms were refined anisotropically. The crystal data and structure
random winding structure of the aliphatic dicarboxylic acids
that are often present in the solvent and the O-donor groups of
the acids are rather difficult to coordinate to metal ions.
However, aliphatic dicarboxylic acids HOOC(CH2)nCOOH
acting as flexible multidentate O-donor ligands exhibit more
coordination modes in comparison to other carboxylic acids,
which benefits the construction of interesting vanadium-based
cluster systems.
refinement results revealed that the empirical formula was K6H-
I V
[ V V
V
1 2 ( O H ) 4 O 6 0 ( O O C ( C H 2 ) 4 C O O ) 8 ] · 2 0 H 2 O
1 7
(C48H109O116V29K6), which is composed of an adipate-functionalized
POV cluster and 20 water molecules. The SQUEEZE program
performed in PLATON was further used to calculate and evaluate the
possible numbers of the disordered solvent water molecules in the two
accessible voids of the crystal structure. A total of 43 extra disordered
solvent water molecules should be added to the chemical formula
I V
K6 H[VV
V
1 2 (OH)4 O6 0 (OOC(CH2 )4 COO)8 ]·63H2 O
On the basis of these facts, we employed aliphatic
dicarboxylic acids as ligands to construct POV-based
inorganic−organic frameworks and obtained an unprecedented
1 7
(C48H195O159V29K6) on the basis of the calculation results. However,
the TGA and CHN elemental analysis results showed that the number
of water molecules might be 26 for the empirical formula
{ V 2 9 } - b a s e d a d i p a t e d e r i v a t i v e , K 6 H -
I V
K6 H[VV
V
1 2 (OH)4 O6 0 (OOC(CH2 )4 COO)8 ]·26H2 O
IV
[VV
V
12(OH)4O60(OOC(CH2)4COO)8]·nH2O (1). As far
1 7
17
(C48H121O122V29K6). This might be attributed to the weathering of
crystals of 1. Gradually, most of the highly disordered water molecules
are lost in the process of drying and storing at room temperature.
Crystallographic data in this paper have been deposited at the
Cambridge Crystallographic Data Centre with CCDC number
1551856 for 1. Selected crystallographic data of compound 1 are
given in Table S1 in the Supporting Information.
as we know, it is the largest carboxylic-functionalized POV
cluster and first POV-based adipate derivative. The
IV
[VV
V
12(OH)4O60(OOC(CH2)4COO)8]7− polyanion can
17
be isolated as discrete potassium salts and has approximate
dimensions of 2.1 nm × 1.1 nm (Figure S1 in the Supporting
Information).
RESULTS AND DISCUSSION
■
EXPERIMENTAL SECTION
■
Synthesis. The following four aspects are worth mentioning
in the synthesis process of 1. (1) Compound 1 was synthesized
in aqueous solution, but almost all of the reported POV-based
carboxylate derivatives were prepared in organic solvent or
mixed solvent.34,46−49,51−57 (2) Due to the utilization of simple
and stable potassium metavanadate (KVO3) as a precursor, the
reaction was carried out under aerobic conditions rather than
rigorous anaerobic conditions. (3) In comparison with
monocarboxylic acids, long-chain dicarboxylic acids HOOC-
(CH2)nCOOH as multidentate O-donor ligands exhibit more
coordination modes and good flexibility and conformational
freedom and preferably link covalently to the hard vanadium
atom, thus forming interesting structures. Among the aliphatic
dicarboxylic acids HOOC(CH2)nCOOH (n = 1−4, 6), we
finally found that only HOOC(CH2)4COOH could covalently
link to the POV skeleton, resulting in an unprecedented
adipate-functionalized POV structure. Analogous reactions
using the dicarboxylic acids HOOC(CH2)nCOOH (n = 1−3,
6) as raw materials give unknown brown powders. (4)
Hydrazine hydrate serves as a reducing agent. The V/N2H4
molar ratio is crucial to the formation of 1. Single crystals of 1
can be obtained when the V/N2H4 ratio is increased from 3.0:1
to 3.8:1, but the crystallization of 1 becomes difficult with
further addition of hydrazine hydrate beyond the above range.
Structure of Compound 1. Single-crystal X-ray diffraction
analysis demonstrates that compound 1 crystallizes in the
tetragonal space group P4/mnc with the central V4 atom
Materials and Methods. All chemical reagents were purchased
from commercial sources and used without further purification. The
FT-IR spectra were performed in the range 2500−450 cm−1 using KBr
pellets on a Bruker VERTEX 70 IR spectrometer. Powder X-ray
diffraction (PXRD) patterns were recorded ranging from 5 to 40° at
room temperature on a Bruker D8 ADVANCE diffractometer with Cu
Kα radiation (λ = 1.54056 Å). The C and H elemental analyses were
obtained with a PerkinElmer 2400-II CHNS/O elemental analyzer.
Metal element analysis was conducted on a PerkinElmer Optima2100
DV inductively coupled plasma optical-emission spectrometer.
Thermogravimetric-mass (TG-MS) analysis of the samples was
performed using a NETZSCH STA 449 F5 analyzer heated from 30
to 900 °C under nitrogen at a heating rate of 10 °C min−1. Variable-
temperature magnetic susceptibility data were obtained in the
temperature range of 2−300 K using a SQUID magnetometer
(Quantum Design, MPMS-3) with an applied field of 1000 Oe. X-ray
photoelectron spectroscopy (XPS) measurements were performed on
a Kratos AXIS ULTRA XPS spectrometer with monochromated Al
(1486 eV) exciting radiation source.
IV
Preparation of K6H[VV
V
12(OH)4O60(OOC(CH2)4COO)8]·
17
nH2O (1). A solution of KVO3 (3.45 g, 25.0 mmol) in H2O (50
mL) was treated at 90 °C with hydrazine hydrate (N2H4·H2O; 80%,
200 μL, 3.30 mmol) and maintained for 1 h at this temperature with
stirring. Thereafter, adipic acid (3.00 g, 20.5 mmol) was added to the
brown solution at 90 °C and the mixture was stirred until the solid was
completely dissolved. The solution turned black. After addition of
N2H4·H2O (80%, 200 μL, 3.30 mmol) once again to the reaction
mixture, the solution was heated at 90 °C for 5 h with consistent
stirring and then filtered hot. The filtrate was left to slowly evaporate
at room temperature for 2 weeks, and then the precipitated dark green
rodlike crystals were filtered off. Yield: calculated 0.743 g (19.8% based
on vanadium). Anal. Calcd for C48H121O122V29K6: C, 13.22; H, 2.80; V
33.86; K 5.38. Found: C, 13.47; H 2.58; V, 34.90; K 5.40. FT-IR (KBr,
cm−1): 3431, 2927, 2856, 1579, 1545, 1455, 1426, 1361, 989, 969, 954,
815, 765, 681.
through the C2 symmetry axis of the polyanion. Compound 1 is
IV
composed of one [VV
V
12(OH)4O60(OOC(CH2)4COO)8]7−
17
polyanion, six potassium cations, and 63 lattice water
molecules. Two K+ cations are encapsulated by two identical
[(VIV3O6)2(OOC(CH2)4COO)4]8− cages and connect to the
interior oxygen atoms of the cages, respectively. The other four
K+ ions are situated on one side of the polyanion and link to
the adjacent cluster through O−K−O bridging bonds, leading
to the formation of a one-dimensional belted structure along
the crystallographic c axis (Figure 1).
The polyanion of compound 1 consists of one {VO4}
tetrahedron, four {VO5} trigonal bipyramids, eight {VO5}
tetragonal pyramids, 16 {VO6} octahedra, and eight adipate
ligands (Figure 2e). As shown in Figure 2a, two {V2O10} dimers
and two {VO5} trigonal bipyramids are linked alternately in a
X-ray Crystallography. A suitable single crystal of 1 was carefully
selected under an optical microscope and sealed into a capillary glass
tube. Crystal data were collected on a Bruker APEX-II CCD
diffractometer with graphite-monochromated Mo Kα radiation (λ =
0.71073 Å) at 296 K. The structures were solved by direct methods,
and non-hydrogen atoms were refined by full-matrix least-squares
methods on F2 using the SHELX program suite.58 The hydrogen
atoms of the organic groups were placed in calculated positions and
then refined using a riding model with a uniform value of Uiso = 1.2Ueq.
All H atoms of water molecules were directly added to the molecular
formula but not included in the structural model. Non-hydrogen
B
DOI: 10.1021/acs.inorgchem.7b02207
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Inorganic Chemistry
Article
{VV17(OH)4O44} cluster at a distance of 4.2458(14) Å from the
V4 atom of the central {VO4} tetrahedron of the cluster. Unlike
most known POV clusters that display a closed-sphere-like
structure, this spherical {VV17(OH)4O44} cluster contains four
openings at the surface. Thus, two kinds of vanadium ions, so-
called “cap” and “belt” vanadium centers, exist in the
{VV17(OH)4O44} cores (Figure S3 in the Supporting
Information). It is worth noting that three different {VOx} (x
= 4−6) units but four types of coordination modes around the
VV ions exist in the {VV17(OH)4O44} cages. The V1 center
adopts a five-coordinate {VO5} trigonal-bipyramidal config-
uration. V2 and V3 ions employ {VO6} octahedral coordination
geometries. A {VO4} regular tetrahedron is formed around the
central V4 atom. The other {VO5} unit is determined as a
tetragonal-pyramidal configuration for the capping vanadium
(V5) (Figure S4 in the Supporting Information). Therefore,
compound 1 is expected to have versatile activities due to its
rich coordination chemistry. Furthermore, the V−O framework
bonds define four distinct types of environments within the
{VV17(OH)4O44} cage for the five crystallographically in-
dependent vanadium atoms. One of these atoms (V1) is
bound to three μ3-O atoms (V−O = 1.743(4)−1.789(4) Å)
and one μ4-O atom (V−O = 2.435(5) Å). Two vanadium
atoms (V2 and V3) are linked to one μ4-O atom (V−O =
1.939(4) and 1.911(4) Å), three μ3-O atoms (V−O =
1.962(4)−2.281(4) Å) and one μ2-O atom (V−O = 1.809(5)
and 1.707(5) Å). The central V4 atom forms bonds to four μ3-
O atoms (V−O = 1.705(4) Å). Finally, V5 is bound to four μ2-
O atoms with V−O distances ranging from 1.904(5) to
1.958(5) Å. The terminal V−O distances for these metal atoms
all lie within the narrow range of 1.582(5)−1.608(4) Å. The
[VIV5VV12O42]4− anionic cluster, composed of a close-packed
distorted-supercubane {V13O14} core and four capping VO
groups, has been observed in the compound (n-
Bu4N)4[V17O42].23
Figure 1. Polyhedral/ball and stick view of the belted structure of 1
along the crystallographic c axis. {VOx} (x = 4−6) groups are shown as
orange polyhedra. Hydrogen atoms are omitted for clarity (as is the
case for subsequent figures).
The {VV17(OH)4O44} cage is further expanded by four {V3}
fragments via V−O−V bonds, forming an unprecedented
crablike 29-nuclearity POV cluster skeleton (Figure 2d).
Obviously, a 29-nuclearity purely inorganic POV cluster cannot
exist solely due to its high negative charge. Actually, two of the
four {V3} moieties are covalently bridged by four adipate
ligands via sharing oxygen sites to build up a cage with a
scaffold structure (denoted [(VI V 3 O6 )2 (OOC-
(CH2)4COO)4]8−) (Figure 3a). Four oxo groups of each
adipate anion acting as pillars within the scaffold structure
bridge three V centers from two {V3} moieties (Figure 3b). As
reported previously,60 the introduction of adipate ligands serves
to reduce the charge of the polyanion, thus stabilizing the rare
POV skeleton. Then the two identical [(VIV3O6)2(OOC-
(CH2)4COO)4]8− cages are covalently attached to the central
{VV17(OH)4O44} cage via V−O−V bonds in a linear arrange-
Figure 2. Polyhedral graphical representation of 1: (a) {V6} moieties
and VO4 tetrahedron; (b) {V13O40} fragment; (c) {VV17(OH)4O44}
cluster; (d) 29-nuclear vanadium-based cluster skeleton; (e) polyanion
of 1. Color codes: {VO4}, pink tetrahedron; {VO6}, orange octahedra;
{VO5}, gray tetragonal pyramids; {VO5}, blue trigonal bipyramids.
corner-sharing mode to form a cyclic {V6} moiety. Two {V6}
moieties are interlinked perpendicularly via sharing edges and
at the same time connect to the O13, O13A and O13B, O13C
atoms of the central {VO4} unit, respectively, thus facilitating a
wheel-shaped {V13O40} cluster. The {V13O40} unit (Figure 2b),
as a novel type of oxovanadate building block, has an
architecture similar to that of the {Mo8V5O40} unit except
that the sites of four {VO5} trigonal bipyramids and eight
{VO6} octahedra were formally exchanged for four {VO4}
tetrahedra and eight {MoO6} octahedra, respectively (Figure S2
in the Supporting Information).59 Additionally, we note that a
highly protonated [H12V13O40]4− anion with a pseudo-Keggin-
type structure was obtained by Wang’s group.19
The {V13O40} framework is capped above and below by two
{V2O8} units through sharing corners, leading to a spherical
{VV17(OH)4O44} assembly with C2v symmetry (Figure 2c). Of
interest in the spherical {VV17(OH)4O44} cluster are the two
capping {V2O8} units, which are rotated by 90° with respect to
each other and are arranged on the spherical surface of the
ment, resulting in a {V29}-based hybrid cluster structure.
IV
Therefore, the entire [VV
V 12(OH)4O60(OOC-
17
(CH2)4COO)8]7− polyanion can be regarded as a “cage to
cage” architecture constructed from three cages (Figure 2e).
However, the V−μ2-O(adipate) bond distances (2.007(6)−
2.236(6) Å) are longer than those of V−μ2-O(V) (1.707(5)−
1.958(5) Å).
The [(VIV3O6)2(OOC(CH2)4COO)4]8− fragments of the
polyanion present an unusual structure, the appreciation of
which is facilitated by a comparison with other structurally
characterized triangular trinuclear vanadium-based carboxylate
complexes. In the reported compounds [V3(μ3-O)(μ-
C
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Figure 3. Polyhedral/ball and stick views: (a) [(VIV3O6)2(OOC-
(CH2)4COO)4]8− moieties; (b) coordination mode of the adipate
ligand; (c) {(V3O6)(OOC)4} unit.
Figure 4. Temperature dependence of χM and χMT versus T plots of 1.
negative Weiss constant Θ = −110.9 K, which further confirms
the presence of antiferromagnetic coupling between vanadium-
OOCCH2 CH3 )6 (OH2 )3 ]Cl·2H2 O,36 [V3 (μ3 -O)(μ-
OOCCH3)6(OH2)3]Cl·3.5H2O,37 and V3(O)3(THF)-
(C6H5CO2)6,39 the mean V···V distances are 3.312, 3.332,
and 3.714 Å (Table S2 in the Supporting Information),
respectively. In contrast, the average V···V distance of the
[(V3O6)2(OOC(CH2)4COO)4]8− fragment is 3.259 Å, which is
significantly shorter than those of {V3O}-based compounds. In
particular, the V6···V7 distances are 2.841 Å (Figure 3c).
According to the reported values, the neutral {V4O4}O4 cubane
unit forms two covalent V−V bonds that are 2.65 Å in length;27
in comparison, a kind of strong metal−metal interaction exists
between V6 and V7 metal ions. Additional bond distances are
VO(terminal)(av) = 1.583 Å, V−μ3-O(centered)(av) =
1.913 Å, and V−O(adipate)(av) = 2.087 Å.
The oxidation states of all V centers were established by a
combination of bond valence sum (BVS) calculations and X-ray
photoelectron spectroscopy (XPS). BVS values reveal that 12
vanadium centers range from 4.213 to 4.272 with an average of
4.235 and the other 17 vanadium centers range from 4.668 to
5.222 with an average of 5.098, indicating that the respective
valences are +4 and +5 (Table S3 in the Supporting
Information), attributed to VV/VIV−POV species. In addition,
XPS measurements of 1 were performed to further prove the
above results. They exhibit two peaks of ca. 517.13 and ca.
516.15 eV in the energy region of V 2p3/2, which can be
attributed to V5+ 2p3/2 and V4+ 2p3/2, respectively (Figure S5 in
the Supporting Information).56 Furthermore, five protons
should be added for charge-balance considerations and the
BVS values of all oxygen atoms in Table S4 in the Supporting
Information suggest that one proton delocalizes over the entire
architecture, while O3, O3A, O3B, and O3C atoms are
monoprotonated.
(IV) centers. In addition, a Curie constant of 4.35 emu mol−1
K
is highly consistent with the expected value for 12 uncorrelated
V4+ ions at 300 K, further supporting the assignment of
oxidation states (Figure S6 in the Supporting Information).
Antiferromagnetic coupling between oxovanadium(IV) mag-
netic ions is common in mixed-valence vanadium(IV/V)
clusters.49,51,55,56 In addition, the χM value increases from
0.01 emu mol−1 at 300 K to 0.09 emu mol−1 at 25 K and then
exponentially increases to reach a maximum of 0.90 emu mol−1
at 2 K.
Catalytic Oxidation of Sulfides. The oxidation of organic
sulfide has gained increasing interest owing to the various
utilities of both sulfone and sulfoxide in organic synthesis.61 In
the reaction system, vanadium-based POMs present good
catalytic activity for desulfurization.62,63 Thus, we explored the
IV
oxidation of sulfides using K6H[VV
V
12(OH)4O60(OOC-
17
(CH2)4COO)8]·26H2O as a catalyst in dichloromethane
(CH2Cl2) with tert-butyl hydroperoxide (TBHP) as the oxidant
at room temperature/50 °C/60 °C.
The results of catalytic experiments with four kinds of
sulfides as substrates (diphenyl thioether, alkyl thioethers, aryl
alkyl thioethers, and thiophenes) along with blank experiments
are presented in Table 1 and Table S5 in the Supporting
Information. As shown in Table 1, TBHP dosage (entry 13),
reaction temperature (entries 2 and 6), time (entries 2, 4, and
5), and dosage of catalyst (entries 2 and 7) were the important
factors that influence the conversion of oxide products.
Specifically, under optimized conditions, compound 1 exhibits
extraordinary efficiency in converting diphenyl thioether to
diphenyl sulfoxide with much higher conversion and barely
satisfactory selectivity (entry 2, conversion 92%, selectivity
87%) in comparison to those reported under similar
experimental conditions (Table S6 in the Supporting
Information).61,64 Furthermore, alkyl thioethers were converted
into sulfoxides with the maximum 99% conversion and more
than 80% selectivity (entries 8−10) within 1/2 h at room
temperature, which demonstrated the excellent activity of
catalyst 1 at low catalytic dosage. Meanwhile the aryl alkyl
thioethers (entries 11−13) also show higher conversion in
comparison to diphenyl thioether and have high selectivity for
sulfoxide products. Moreover, 1 was used as catalyst for the
oxidation of thiophenes (entries 14−16) containing dibenzo-
thiophene (DBT), 4,6-dimethyldibenzothiophene (4,6-
DMDBT), and benzothiophene (BT), which reacted with the
Magnetic Properties. The variable-temperature magnetic
susceptibility of 1 was studied in the range of 2−300 K under
an external magnetic field of 1000 Oe (Figure 4). The room-
temperature value of χMT of 12 uncoupled V4+ ions (4.50 emu
mol−1 K with g = 2.0, S = 1/2) is much higher than the
experimental χMT value of 3.21 emu mol−1 K. Subsequently,
the χMT value of 1 decreases continuously with decreasing
temperatures and at 2 K approaches 1.81 emu mol−1 K,
indicating the presence of strong antiferromagnetic exchange
interactions within the clusters. The temperature dependence
of the reciprocal susceptibilities (1/χM) fits to the Curie−Weiss
law in the high-temperature region (160−300 K) with the
D
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investigate the stability of the catalyst, IR spectra of the catalyst
1 before and after the catalytic reaction have been studied. The
differences in the IR spectra are the extra peaks at 1703 cm−1 of
the samples after the catalytic reaction, which may be attributed
to the carbonyl stretching vibration of carboxyl (Figure S8 in
the Supporting Information). The results suggest that the V−
O(adipate) bonds might partially ruptured but the adipate
ligands remains intact within the framework, so that the
structure of POV is still retained. In order to authenticate this,
elemental analysis have been performed for compound 1 before
and after the catalysis experiment. The chemical formula of 1
indicates that the theoretical C/V molar ratio is 1.655.
According to the results of elemental analysis, the ratios of
carbon atoms to vanadium atoms before and after the catalytic
reaction are 1.636 and 1.642, respectively, which further
confirms that the adipate ligands do not fall off from the
framework. In addition, XPS measurements were performed to
identify the oxidation states of V centers in compound 1 after
the first run, second run, and third run. The XPS data of the
samples after each run give three peaks, all at about 517.17 eV,
which should be attributed to V5+ 2p3/2. The results confirm
that the samples after the first run have been completely
oxidized to the fully oxidized vanadium species (Figure S9 in
the Supporting Information). Future work will be dedicated to
improving the activity and selectivity for oxidation of organic
sulfide with nontoxic solvent and eco-friendly oxidant and
improving the stability of the catalyst.
Table 1. Results for Oxygenation of Different Sulfides with
TBHP Catalyzed by 1 in CH2Cl2
CONCLUSION
■
In conclusion, the high-nuclearity vanadium-based compound
IV
K6H[VV
V
12(OH)4O60(OOC(CH2)4COO)8]·nH2O has
17
been synthesized under reducing conditions in aqueous
solution. Interestingly, compound 1 features a “cage to cage”
architecture via a pure inorganic V-based {VV17(OH)4O44}
cluster, which is expanded by two identical [(VIV3O6)2(OOC-
(CH2)4COO)4]8− hybrid cages through V−O−V bridges in a
linear arrangement, yielding a 29-nuclearity V-based discrete
structure. Magnetic studies indicate that 1 shows antiferro-
magnetic behavior. Furthermore, compound 1 shows excellent
activity at low catalytic dosage for the oxidation of thioethers
with high sulfoxide selectivity. In comparison with the main
interest in the rational design and basal synthesis of novel POV-
based carboxylate derivatives, the exploitation of catalytic
performance of POV-based carboxylate derivatives has not
been explored until now, and it is still a challenging task for us
now and in the near future.
a
Conversion determined by using GC with dodecane as an internal
b
standard. The products were identified by GC-MS. Reaction
conditions for entries 1−5 and 11−13: 1 mmol of substrate; 3 μmol
of catalyst; 3 mL of CH2Cl2; 25 °C. Conditions as in footnote b but
c
d
reaction at 50 °C. Reaction conditions for entries 7−10: 2 μmol of
e
catalyst. Reaction conditions for entries 14−16: 0.1 mmol of
substrate; 3 μmol of catalyst; 2 mL of CH2Cl2; 60 °C.
oxidant TBHP in CH2Cl2 medium at 60 °C. The maximum
substrate conversions were 81% achieved after 2 h for DBT,
81% after 3 h for 4,6-DMDBT, and 38% after 3 h for BT. The
three substrates were finally oxidized into the corresponding
sulfones with 100%, 92%, and 100% selectivity, which was
superior to the aforementioned three kinds of sulfides in
selectivity of oxidation products. Most of the oxidation
reactions could happen without catalyst 1 (Table S5 in the
Supporting Information), but the conversion of the catalytic
oxidation reactions was very low; the highest conversion of the
blank experiment was just about 13%. In contrast, we
speculated that the catalytic reaction might be concentrated
on the POV-based organic−inorganic units.
The catalyst was recovered by simple centrifugal separation,
washed with CH2Cl2, dried, and reused directly without being
replenished or reconditioned. A comparative study of the
conversion and sulfoxide selectivity for the 1-catalyzed
sulfoxidation of diphenyl thioether in CH2Cl2 across three
runs is presented in Figure S7 in the Supporting Information.
The catalytic oxidation reaction in the final run gave 60%
substrate conversion. In comparison with the initial catalytic
activity, there was evident loss of catalytic activity. In order to
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02207.
Structural and catalytic figures, crystallographic data and
catalytic oxidation of sulfides, TGA, X-ray powder
diffraction patterns, IR comparison spectra, XPS
spectrum, and TG-MS curves (PDF)
Accession Codes
CCDC 1551856 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
E
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Article
(17) Marrot, J.; Barthelet, K.; Simonnet, C.; Riou, D. Synthesis and
Structure Determination of a New Ball-shaped Polyoxovanadate with a
Hexadecanuclear Core. C. R. Chim. 2005, 8, 971−976.
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
(18) Hayashi, Y.; Miyakoshi, N.; Shinguchi, T.; Uehara, A. A
Stepwise Growth of Polyoxovanadate by Reductive Coupling Reaction
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C4H7)Pd}2V4O12]2−, [V10O26]4− and [V15O36(Cl)]4−. Chem. Lett.
2001, 30, 170−171.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for J.W.: jpwang@henu.edu.cn.
*E-mail for J.N.: jyniu@henu.edu.cn.
(19) Hao, N.; Qin, C.; Xu, Y.; Wang, E.-B.; Li, Y.-G.; Shen, E.-H.; Xu,
L .
A
n e w p s e u d o K e g g i n - t y p e p o l y v a n a d a t e
ORCID
(NH4)4[H12VV12O36(VIVO4)]·11H2O: hydrothermal synthesis and
crystal structure. Inorg. Chem. Commun. 2005, 8, 592−595.
Jingyang Niu: 0000-0001-6526-7767
Notes
(20) Muller, A.; Krickemeyer, E.; Penk, M.; Walberg, H.-J.; Bogge, H.
̈
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Spherical Mixed-Valence [V15O36]5−, an Example from an Unusual
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C. J. Synthesis and Characterization of a Polyoxovanadate Cluster
Representing a New Topology. Chem. Commun. 2005, 4658−4660.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge support from the National Natural
Science Foundation of China (21573056).
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