Four alkoxohexavanadate-based pd-polyoxovanadates as robust heterogeneous catalysts for oxidation of benzyl-alkanes

Article abstract of DOI:10.1021/ic502447b

Four alkoxohexavanadate-based Pd-POVs [Pd(dpa)(acac)]₂[V₆O₁₃(OMe)₆] (1), [Pd(dpa)(acac)]₂[V₆O₁₁(OMe)₈] (2), [Pd(dpa)(acac)]₂[V₆O₁₁(OMe)₈]·H₂O (3), and [Pd(DMAP)₂(acac)]₂[V₆O₁₁(OMe)₈]·H₂O (4) (POV = polyoxovanadate; dpa = 2,2′-dipyridine amine; DMAP = 4-dimethylaminopyridine; acac = acetylacetone anion) have been synthesized and fully characterized by single crystal X-ray diffraction and powder X-ray diffraction analyses, Fourier transform infrared spectroscopy, element analyses, and X-ray photoelectron spectroscopy. In 1-4, Pd complexes and hexavanadate anions are assembled through electrostatic interactions. Interestingly, the [V₆O₁₁(OMe)₈]^2- cores in 2 and 3 are a pair of isomers that can be isolated by controlling crystallization temperature. Moreover, to the best of our knowledge, the {V6} core in 3 represents a new octamethoxyhexavanadates cluster. It is notable that compounds 1-4 exhibit excellent heterogeneous catalytic performance in the oxidation of benzyl-alkanes with t-butylhydroperoxide as oxidant. Among them, the catalytic activity of 1 (conv. and selec. up to 99%, respectively) outperforms others and can be reused without losing its activity.

Full text of DOI:10.1021/ic502447b

Article
pubs.acs.org/IC
Four Alkoxohexavanadate-Based Pd-Polyoxovanadates as Robust
Heterogeneous Catalysts for Oxidation of Benzyl-Alkanes
Ji-Kun Li, Xian-Qiang Huang, Song Yang, Hong-Wei Ma, Ying-Nan Chi,* and Chang-Wen Hu*
†
,‡
†
†
†
,†
,†
†Key Laboratory of Cluster Science Ministry of Education, 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: Four alkoxohexavanadate-based Pd-POVs [Pd(dpa)(acac)] [V O (OMe) ]
2
6
13
6
(
1), [Pd(dpa)(acac)] [V O (OMe) ] (2), [Pd(dpa)(acac)] [V O (OMe) ]·H O (3), and
2 6 11 8 2 6 11 8 2
[
Pd(DMAP) (acac)] [V O (OMe) ]·H O (4) (POV = polyoxovanadate; dpa = 2,2′-
2
2
6
11
8
2
dipyridine amine; DMAP = 4-dimethylaminopyridine; acac = acetylacetone anion) have been
synthesized and fully characterized by single crystal X-ray diffraction and powder X-ray
diffraction analyses, Fourier transform infrared spectroscopy, element analyses, and X-ray
photoelectron spectroscopy. In 1−4, Pd complexes and hexavanadate anions are assembled
2−
through electrostatic interactions. Interestingly, the [V O (OMe) ] cores in 2 and 3 are a
6
11
8
pair of isomers that can be isolated by controlling crystallization temperature. Moreover, to
the best of our knowledge, the {V6} core in 3 represents a new octamethoxyhexavanadates
cluster. It is notable that compounds 1−4 exhibit excellent heterogeneous catalytic
performance in the oxidation of benzyl-alkanes with t-butylhydroperoxide as oxidant. Among
them, the catalytic activity of 1 (conv. and selec. up to 99%, respectively) outperforms others
and can be reused without losing its activity.
[VIV
1
VV
O (OR) ] (R = -Me, n = 0, 1; R = -Et, n = 0,
(2+n) 7 12
n+
INTRODUCTION
(
4−n)
■
2
, 2) have been synthesized and structurally characterized.
The increasing interest toward alkoxohexavanadate clusters
originates from their fascinating electronic, magnetic, photo-
Polyoxometalates (POMs) are an outstanding class of nano-
sized metal-oxo clusters with wide structural diversity and O-
1
n−
enriched surfaces. The Lindqvist [M O ] anion is one of
6
19
14,15
chemical properties and potential catalytic application.
the classical POM structures formed by Mo, W, Nb, and Ta,
but until now the theoretical [V O ] has never been
observed in aqueous solution. The instability of [V O ]
probably arises from the small ionic radius of V in
8
−
However, it is worth mentioning that although alkoxohex-
avanadates are expected to have excellent performance as
oxidation catalysts, to the best of our knowledge very few
6
19
8
−
6
19
V
16
2
examples about their catalytic property have been reported.
combination with the high charge of cluster. Besides using
3
Actually, alkoxohexavanadates, especially the monodentate
alkoxo ligands substituted hexavanadates, are commonly highly
reactive and extremely susceptible to hydrolysis by trace
organometallic groups, substitution of bridging oxygen atoms
8
−
in the Lindqvist [V O ] with oxygen donor-ligands is proved
6
19
to be another effective method to compensate the negative
charge and stabilize the cluster. Monodentate or multidentate
alkoxo ligands are promising candidates as they can serve as μ -
alkoxo bridging moieties in the [V O ] . After the first case
9
,14
4
amount of water,
but the introduction of water is inevitable
in most oxidation reactions. Therefore, exploring synthetic
methods for stable and highly active catalysts based on
alkoxohexavanadates is essential.
Generally, both the structure and magnetic and electronic
properties of alkoxohexavanadate-based compounds can be
controlled by the following three approaches (i) varying the
2
8−
6
19
[
(n-C H ) N] [V O {O NC(CH O) } ] contributed by Zu-
4 9 4 2 6 13 2 2 3 2
5
bieta, quite a few alkoxohexavanadate clusters stabilized by
6
multidentate trisalkoxo ligands are reported. Through selecting
proper trisalkoxo ligands, the obtained alkoxohexavanadate
clusters can be further functionalized and have satisfactory
2
,6a
number or the type of the substituted alkoxo ligands;
changing the V /V ratio in the Lindqvist cluster; (iii) using
(ii)
IV
V
13
7
properties.
different organic ammonium, alkali metal, or metal−organic
Compared with multidentate alkoxo ligands, the examples of
alkoxohexavanadate stabilized by monodentate alkoxo ligands
are limited. Methoxo as well as ethoxo are the most commonly
1
0,13
complexes as counterions.
Our previous work displayed
that the combination of Pd-complex cation and Keggin-type
polyanions could lead to excellent catalytic activity.
17
V
6
2−
8
used monodentate ligands and up to now, [V O (OMe) ] ,
13
6
V
−
9
I V
V
2 − 1 0
[
[
(
VIV
O
(OM e) ] , [ V
V
O 1 1 (OM e) ]
,
6
1 2
7
2
4
8
V O (OMe) ], [VIV V O (OMe) ] , [V V O -
V
11
V
+
11
IV
V
V
Received: October 6, 2014
3
3
8
11
2
4
8
11
4
2
7
1
2
IV
V
(4−n)
13
OEt) ], [V V O (OMe) ]
(n = 3, 4, 5, 6) and
12
n
6−n
7
12
©
XXXX American Chemical Society
A
DOI: 10.1021/ic502447b
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Inorganic Chemistry
Article
a
Scheme 1. Simplified Representation of the Controlling Synthesis of 1−4
a
Color code: Pd, pink; V, green; N, blue; O, red; C, gray.
With this in mind, the Pd-complex was selected as
countercation to couple with alkoxohexavanadate cluster
through electrostatic interaction and as a result four novel
Fourier Transform Infrared (FT-IR) Spectra. IR spec-
troscopy is a useful tool for the analysis of alkoxopolyoxovana-
dium clusters. In particular, the 500−1100 cm− region is of
interest, containing absorption bands due to metal−oxygen
stretching vibrations which are characteristic of POMs spectra
in general. As shown in Figure S1, Supporting Information, the
1
V
Pd-POVs [Pd(dpa)(acac)] [V O (OMe) ] (1), [Pd(dpa)-
2
6
13
6
IV
V
(
acac)] [V V O (OMe) ] (2), [Pd(dpa)(acac)] -
2
2
4
11
8
2
IV
IV
V
[
[
V
V
V O (OMe) ]·H O (3), and [Pd(DMAP) (acac)] -
V O (OMe) ]·H O (4) (POV = polyoxovanadate; dpa
2
4
11
8
2
2
2
V
2
4 11 8 2
IR spectra of 1−4 exhibit absorption bands in the range of the
=
2,2′-dipyridine amine; DMAP = 4-dimethylaminopyridine;
−1
9
1
50−1100 cm region corresponding to the O−CH (1000−
3
acac = acetylacetone anion) were synthesized in a simple and
straightforward method. The four compounds are not only
stable in air and aqueous solution but also show unexpected
catalytic activities in the oxidation of benzyl-alkanes.
−1
−1
11
100 cm ) and VO (950−1000 cm ) stretching modes.
10
−1
Further absorption bands located in the 550−650 cm region
are attributed to V−O−V bending. The bands in the 940−
−1
9
50 cm region are assigned to VO vibration, 750−785
−1
cm region to V−O−V bridging fragments, and 650−690
RESULTS AND DISCUSSION
−1
10,18
■
cm region to V-OCH3 fragments, respectively.
In
addition, the infrared spectra in the range 1100−1650 cm−
are associated with the CC and CN ring stretches of the
N-donor ligands.
1
Synthesis. Compounds 1−4 were synthesized by the
reaction of VO(acac) , Pd(OAc) , and N-donor ligands in
2
2
the presence of Et N and methanol. As shown in Scheme 1, all
3
+
compounds contain two [PdL (acac)] cations (L = dpa, n = 1;
n
Crystal Structure of 1. Single crystal X-ray diffraction
L = DMAP, n = 2) and one so-called Lindqvist type
+
(
SXRD) shows that 1 consists of two [Pd(dpa)(acac)] cations
alkoxohexavanadate anion. Because Et N might facilitate the
2−
3
and one [V O (OMe) ] polyanion (Scheme 1a). In the
6
13
6
deprotonation of MeOH, six methoxo substituted
2+
cation part, the four-coordinate Pd center is ligated by two N
2
−
[
[
V O (OMe) ] cluster (in 1) and eight methoxo substituted
−
6
13
6
atoms from one dpa and two O atoms from one acac anion
2−
V O (OMe) ] cluster (in 2, 3, and 4) can be prepared by
6
11
8
and adopts a nearly square-planar geometry. The structure of
tuning the amounts of Et N. Although the structures of
3
2−
[
V O (OMe) ] is very similar to the six methoxo substituted
6
13
6
compounds 2 and 3 appear to be similar, the locations of the
eight substituted methoxo ligands on the hexavanadate core are
different. A detailed investigation displays that the crystal-
lization temperature plays a key role in the formations of 2 and
hexavanadate in [{VO(bmimpm)(acac) }{V O (OMe) }]
2
6
13
6
(
bmimpm = bis(1-methylimidazol-2-yl)-4-methoxyphen-1-yl-
8a
methanol), in which six VO octahedra build up an {V }
6
6
octahedron of Lindqvist type. In the reported case, the
3
. Specially, compound 2 was obtained when the crystallization
2
−
[
V O (OMe) ] core and vanadyl complexes are connected
temperature remained at room temperature, and if the
crystallization process was performed at 5 °C, compound 3
was isolated. To further investigate the influence of ligand on
the structures of Pd-POVs, DMAP was adopted in the reaction
of VO(acac) and Pd(OAc) and compound 4 was obtained.
6 13 6
by coordination bonds forming a [V + V ] skeleton which is
6
2
different from the cationic−anionic mode in compound 1
through electrostatic interaction. The bond valence sum (BVS)
calculations show that the oxidation states of all the vanadium
atoms are +V, which are in good agreement with the charge
balance of compound 1 (Table S1, Supporting Information)
2
2
The experimental result suggests that the N-donor ligand has
no obvious influence on the structures of Pd-POVs, especially
the hexavanadate core.
19
and XPS analysis. (Figure S2, Supporting Information)
B
DOI: 10.1021/ic502447b
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
−
2−
Besides electrostatic interaction, Pd-complex and polyanion
are further linked by a strong hydrogen bonding interaction
α-[V O (OMe) ] and β-[V O (OMe) ] , respectively.
6 11 8 6 11 8
The two polyanions represent a pair of rare isomers in the
family of organic-functionalized POMs. Furthermore, to the
best of our knowledge, the β-[V
alkoxohexavanadate framework. It is worth mentioning that the
formation of the two isomers could be controlled by
crystallization temperature. As in 2, there are also two V
2−
between the μ -oxo of [V O (OMe) ] and the amino group
2
6
13
6
2
−
with the O···N distance of ca. 2.7 Å (Figure 1). Interestingly, a
D supramolecular structure of 1 is constructed by another
O
6
11(OMe)
8
]
in 3 is a new
3
relatively weak hydrogen bonding and π···π interactions (Figure
S3).
IV
V
and four V in the V core of 3, which has been confirmed by
6
BVS calculations (Table S1) and XPS analysis (Figure S2).
In addition, the different positions of substituted methoxo
ligands result in the different hydrogen bonding assembly. In 2,
two μ -oxo atoms in opposite positions connect with −NH
2
+
groups of [Pd(dpa)(acac)] by hydrogen bonding with the O···
N distance of ca. 2.7 Å (Figure 3a). However, in 3, the angle
Figure 1. Strong hydrogen bonds interactions in 1. Hydrogen atoms
are omitted for clarity.
Crystal Structure of 2. By increasing the amount of base,
compound 2 instead of 1 was obtained under the same
conditions. The SXRD analysis shows that 2 contains two
+
2−
[
(
1
Pd(dpa)(acac)] cations and one [V O (OMe) ] polyanion
6 11 8
Scheme 1b). Although the cationic part is identical with that of
, the number of the substituted alkoxo ligands on the
hexavanadate core is changed from six to eight. The eight μ2-
methoxo groups are symmetrically arranged around central
2
−
oxygen atom of [V O (OMe) ] core (Figure 2 left). To
6
11
8
Figure 3. Hydrogen bonds interactions in 2 (a) and 3 (b). Hydrogen
atoms and lattice water molecule are omitted for clarity.
Figure 2. Locations of the eight substituted methoxo bridging ligands
between two hydrogen bonds formed by μ -oxo atoms and
2
2
−
on the hexavanadate cores: (left) α-[V O (OMe) ] in 2 and (right)
6
11
8
−
NH groups is about 120° with the O···N distance of ca. 2.9 Å
2
−
β-[V O (OMe) ] in 3. Gray balls represent the symmetric methoxo
6
11
8
(
Figure 3b). The 3D supramolecular structures formed by weak
bridging ligands, and blue balls represent the unsymmetric methoxo
bridging ligands.
hydrogen bonds and π···π interactions of compounds 2 and 3
are shown in Figure S3, Supporting Information.
Crystal Structure of 4. In order to understand the
influence of the N-donor ligand on the structures of
alkoxohexavanadates core, DMAP was selected instead of dpa
in the synthesis Pd-POVs. Under the same reaction conditions,
balance the charge of the anion in 2, a mixed-valence state of
IV
V
vanadate anion containing two V and four V are needed,
which is also identified by both BVS calculations (Table S1,
Supporting Information) and X-ray photoelectron spectroscopy
compound 4 can be isolated from methanol. As shown in
+
(
XPS) analysis (Figure S2, Supporting Information). As far as
Scheme 1d, 4 contains two [Pd(DMAP) (acac)] cations and
2
2
−
we know, only one eight-methoxo substituted
one α-[V O (OMe) ] which is similar to the polyanion
6
11
8
2−
+
[
V O (OMe) ] has been obtained with [Co(NCS)L] (L
structure in 2 with only slight differences in the bond lengths
and bond angles. The results show that when using different N-
donor ligands, there is no obvious influence on the structure of
hexavanadate core. But due to the absence of the −NH group,
no obvious hydrogen bonding interaction is observed in 4. The
results of BVS calculations (Table S1) and XPS spectra showed
6
11
8
=
tris(1-(3,5-dimethylpyrazolylmethyl)amine) as the counter-
10
cation.
Crystal Structure of 3. Interestingly, by lowering the
crystallization temperature, compound 3 was isolated, which
+
2−
contains two [Pd(dpa)(acac)] cations, one [V O (OMe) ]
6
11
8
IV
V
polyanion, and one lattice water molecule. Although anion parts
in 2 and 3 have the same chemical formula, the locations of the
methoxo groups are very different. As shown in Figure 2, in 3
four substituted methoxo groups are symmetrical by taking the
central O of Lindqvist cluster as the center, while the other four
are asymmetrical, but in 2 all the methoxo groups are
distributed in a symmetrical mode. In order to distinguish
that there were also two V and four V in the α-
2
−
[V O (OMe) ] as in compounds 2 and 3 (Figure S2).
6
11
8
Catalytic Activities of Pd-POVs. Oxidation is an
important pathway to large-scale production of chemicals.
Selective oxidation of benzyl-alkanes is a powerful tool to
generate high value chemical feedstock from less expensive raw
materials. In this regard, the means to convert benzyl-alkanes
into valuable compounds have received considerable attention
2−
them, hereafter the [V O (OMe) ] in 2 and 3 are labeled as
6
11
8
C
DOI: 10.1021/ic502447b
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Inorganic Chemistry
Article
2
0
in recent years. Given the fact that both Pd complexes and
alkoxohexavanadate clusters in compounds 1−4 have the
potential to oxidize C−H bonds, we anticipated that
compounds 1−4 could exhibit catalytic activity in the oxidation
of benzyl-alkanes.
95.8% with benzophenone as the only product. By comparing
the oxidation state of V atoms in compounds 1−4, we speculate
that the existence of vanadium(IV) in 2−4 might be
responsible for the slightly decline of conversion and selectivity
in the reaction. Previous studies indicate vanadium(V) is the
21
The oxidation of diphenylmethane to benzophenone was
selected as a model reaction to evaluate the catalytic activities of
Pd-POVs (Scheme 2). The diphenylmethane oxidation
active site in many catalytic oxidation reactions.
Because of excellent performance, compound 1 was selected
to examine the long-term stability in a heterogeneous system.
After completion of the oxidation reaction, the catalyst can
easily be separated from the reaction mixture by filtration. The
recovered catalyst was reactivated by washing with methanol
and further reused directly in the subsequent oxidation
reactions. The experiment results display that after four runs
no obvious loss of activity [conv. 95.8% (first), 95.5% (second),
Scheme 2. Oxidation of Diphenylmethane using Pd-POVs as
Catalysts under the Preliminary Optimization Conditions
9
4.7% (third), 94.6% (fourth)] was observed (Figure 4). The
catalyzed by compounds 1−4 was performed in chlorobenzene
at 65 °C for 16 h using t-butylhydroperoxide (TBHP) as the
oxidant. As shown in Table 1, compounds 1−4 all can
efficiently catalyze the oxidation of diphenylmethane with the
conversion of 95.8−94.5% and selectivity of 99.8−92.5%.
Table 1. Conversion of Diphenylmethane to Benzophenone
a
with Different Catalysts
b
catalyst
conv (%) selec (%)
reaction system
1
2
3
4
5
95.8
94.8
94.7
94.5
70.3
45.5
99.8
93.5
93.2
92.5
61.4
78.4
heterogeneous
heterogeneous
heterogeneous
heterogeneous
homogeneous
homogeneous
Figure 4. Recycling of 1 for oxidation of diphenylmethane.
IR spectra (Figure S6) and X-ray powder diffraction patterns
(Figure S7) of 1 collected before and after the catalytic
reactions indicate that the structure is maintained under
turnover conditions. To check the heterogeneity of 1 in liquid
phase oxidation of diphenylmethane, hot filtering experiments
(separation of catalyst at the reaction temperature) was carried
out. When 1 was filtered off after 2 h, the reaction still
proceeded for 14 h under the same conditions but with a
obviously lower final conversion than in the presence of 1
(Figure S8). Atomic absorption analysis shows that there were
no palladium and vanadium ions in the filtrate of catalyst
(Supporting Information S9).
Pd(dpa)(OAc) + Hacac (1:1)
2
a
Reaction conditions: diphenylmethane (0.25 mmol, 1 equiv), catalyst
3.75 μmol, 1.5 mol %), TBHP (2.5 equiv), chlorobenzene (1 mL), 65
C, 16 h. Selectivity to ketones. The byproduct was benzhydrol.
(
°
b
To probe the role of Pd-complex in the diphenylmethane
17
oxidation, Pd(dpa)(OAc)2 and acetylacetone in a molar ratio
of 1:1 was used as catalyst, and the low conversion (45.5%) and
satisfactory selectivity (78.4%) were achieved under the same
conditions (Table 1). To further explore the role of
alkoxohexavanadate cluster in the reaction, a new compound
The mild reaction conditions, excellent stability, and high
yield for the transformation of diphenylmethane to benzophe-
none prompted us to extend the scope of 1 as heterogeneous
catalyst for other benzylic hydrocarbons. As shown in Table 2,
the oxidation of several substrates to the corresponding ketones
using compound 1 proceeded smoothly with moderate to high
yields. It is noteworthy that 1 exhibits excellent catalytic activity
for oxidation of diphenylmethane, 9H-xanthenes, fluorene, and
derivates of fluorene to the corresponding aryl ketones with up
to 99% yield (Table 2, entries 1−6). Moderate catalytic
activities for the oxidation of ethylbenzene (conv. 79.4%, selec.
92.2%) (Table 2, entry 7) and tertralin (conv. 87.5%, selec.
75.5%) (Table 2, entry 8) were observed under the same
reaction conditions. For the oxidation of tertralin, the
byproducts are 2,3-dihydronaphthalene-1,4-dione and α-
tetralol, and for the oxidation of ethylbenzene the byproduct
is 1-phenylethanol. The different activities observed for the
substrates are mainly attributed to the different C−H bond
energies (Table S3, Supporting Information). By comparison
with previous reports, we found that compound 1 out-
performed some heterogeneous catalysts such as polyoxome-
[
HNEt ] [V O (OMe) ] (5) was synthesized and character-
3 2 6 13 6
2
−
ized by SXRD. The structure of [V O (OMe) ] in 5 is
identical with anion of 1, in which six methoxo ligands
substitute six bridging oxygen atoms in Lindqvist {V O }
6
13
6
6
19
(
Figure S5, Supporting Information). The conversion of
diphenylmethane catalyzed by compound 5 reached 70.3%
with 61.4% selectivity (Table 1). Although Pd(pda)(AcO) and
2
compound 5 were used as homogeneous catalysis, their
conversions and selectivities are obviously lower than those
of heterogeneous catalysts 1−4. The combination of Pd
complex and alkoxohexavanadate cluster can dramatically
enhance catalytic activities. According to the above results, we
speculate that the cation part in catalysts 1−4 possibly
contributes to enhance the selectivity, and the anion part is
to increase the conversion of substrate. Generally speaking, the
intramolecular cooperation between cation and anion in 1−4
plays some role in the catalytic oxidation process.
Then, we note that the catalytic selectivity of 1 is a little
higher than 2−4. For example, using 1, without any further
optimization, the conversion of diphenylmethane reaches to
D
DOI: 10.1021/ic502447b
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Inorganic Chemistry
Article
a
Table 2. Results of Selective Oxidation of Benylic Compounds Catalyzed by 1 Using TBHP Oxidant
a
Reaction conditions: substrates (0.25 mmol, 1 equiv), compound 1 (3.75 μmol, 1.5 mol %), TBHP (2.5 equiv), chlorobenzene (1 mL), 65 °C, 16 h.
Selectivity to ketones. The byproducts are corresponding alcohols for entries 1−7 and 2,3-dihydronaphthalene-1,4-dione and α-tetralol for entry 8.
b
2
2
23
24
25
talates, MOFs, cage compounds, 2 solid porous materials,
under standard reaction conditions, and when 0.25 mmol
6
and even homogeneous catalysts in the oxidation of
diphenylmethane, fluorene, and 9H-xanthene (Table 3).
An interesting question is emerging: that is, how does
compound 1 promote the oxidation of benzyl-alkanes. First, we
assume that compound 1 plays a vital role in the activation of
C−H bond on its exterior surface. If that is true, very low yield
will be obtained in the oxidation of 2,7-di-tert-butyl-9H-fluorene
because the huge steric hindrance from two tert-butyl groups
might prevent methene group to approach the surface of
catalyst. However, in fact the oxidation of 2,7-di-tert-butyl-9H-
fluorene (entry 6 in Table 2) proceeded smoothly with quite a
high yield. This indicated that the catalyst in our case might not
facilitate the reaction through activating the C−H bond.
Then, according to previous investigations,
consider that a radical process might be involved in the
reaction. To prove our speculation, some free-radical
scavengers, such as Ph NH as oxygen-radical scavenger and
2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO) as carbon-
radical scavenger, were chosen and added in the oxidation of
diphenylmethane catalyzed by compound 1 at 65 °C for16 h
see Table S4). The results (Table S4, entries 2 and 3) show
that Ph NH could effectively retard the oxidation reaction
Ph NH was added into the reactor, the conversion of
2
diphenylmethane drops almost to zero (Table S4, entry 3),
instead of 95.8% in the absence of radical scavengers. However,
the inhibitory effect induced by TEMPO was not as obvious as
Ph NH (Table S4, entries 4−6). The conversion of diphenyl-
2
methane only decreases to 56.5%, even if 1.00 mmol of
TEMPO was used. Although both oxygen and carbon radical
scavengers could retard the oxidation at different degrees, we
conclude that oxygen-radical process may play a vital role, and
the function of catalyst is to activate and decompose TBHP
into t-BuOO· radical species which oxidize the substrates to
28
generate ketones and other byproducts. Our speculation was
further supported by electron paramagnetic resonance (EPR)
spectra of control experiments (Figure 5). When catalyst 1 was
added into the reaction system in the presence of DMPO (5, 5-
dimethyl-1-pyrroline-N-oxide) as spin trap, radicals arising from
DMPO trapping were observed. The g value of DMPO/·
OOBu-t spin adduct was measured to be 2.002, which was in
2
3b,24,27
we
2
(
2
4
29
good agreement with the reported data (Figure S10). In
contrast, the experiment under same conditions without
catalyst 1 gave a negligible EPR spectrum (Figure 5 red).
(
2
E
DOI: 10.1021/ic502447b
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Comparison of 1 and Several Reported Catalysts
to 99%) and can be reused by filtration. The EPR analysis
indicates that catalyst 1 can facilitate the generation of oxygen
free radicals which oxidize the benzylic hydrocarbons.
Investigations on the use of these Pd-POVs catalysts for
other potential catalytic reactions are in progress.
EXPERIMENTAL SECTION
■
General Methods and Materials. All reagents and solvents for
synthesis were purchased from commercial suppliers and used as
received unless otherwise noted. The metal content of the compounds
1
−5 was measured by inductively coupled plasma (ICP) on a JY-
ULTIMA2 analyzer. Mass spectra were recorded on an Agilent 6520
Q-TOF LC/MS. The FT-IR spectra were recorded from KBr pellets in
the range 4000−400 cm on Nicolet 170 SXFT/IR spectrometer.
The powder X-ray diffraction pattern was collected by using a Rigaku
D/max-2550 diffractometer with Cukα radiation. The XPS spectra were
Figure 5. EPR spectrum (blue) with catalyst 1 (3.75 μmol, 1.5 mol %)
and the EPR spectrum (red) without catalyst 1 were recorded at room
temperature in the presence of DMPO. Reaction conditions:
diphenylmethane: (0.25 mmol, 1 equiv), TBHP (2.5 equiv),
chlorobenzene (1 mL), 65 °C, 10 min.
−1
recorded on an ESAY ESCA spectrometer with an Mg−K achromatic
a
X-ray source. 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
Shimadzu GC-2014C with a FID detector equipped with an Rtx-1701
Sil capillary column. The GC-MS spectra were recorded on Agilent
The role of cationic and anionic parts in the selective oxidation
of benzyl-alkanes is under study.
7
890A-5975C at an ionization voltage of 1200 V. The C, H, and N
CONCLUSIONS
■
elemental analyses were conducted on PerkinElmer 240C elemental
analyzer. Atomic absorption analysis was measured by inductively
coupled plasma (ICP) on an ICP-6000 analyzer.
In summary, through using Pd-complexes as countercations, we
have successfully synthesized four novel methoxohexavanadate
compounds 1−4 in a facile and straightforward method. The
hexa- and octa-substituted methoxohexavanadate clusters could
be isolated through adjusting the amount of base and a pair of
Synthesis of [Pd(dpa)(acac)] [V O (OMe) ] (1). VO(acac) (0.6
2
6
13
6
2
mmol, 159.1 mg) and triethylamine (0.6 mmol) were added to 6 mL
of methanol. The resulting solution was stirred at room temperature
for 20 min, and then 2,2′-dipyridine amine (34.3 mg, 0.2 mmol) and
2
−
rare organic-functionalized [V O (OMe) ] isomers are
6
11
8
Pd(OAc) (44.9 mg, 0.2 mmol) were successively added. The reaction
formed by controlling crystallization temperature. As the
introduction of Pd-complexes, compounds 1−4 are not only
stable, but also can be used as heterogeneous catalyst in the
selective oxidation of benzyl-alkanes with moderate to high
catalytic activities. Specifically, under unoptimized conditions,
compound 1 exhibits extraordinary efficiency in converting 9H-
xanthene, fluorine, and its derivatives to corresponding ketones
with high conversion and selectivity (conv. up to 99%, selec. up
2
mixture was stirred for 12 h. From the resulting solution, yellow
crystals suitable for X-ray diffraction grew after leaving the solution to
stand for a week. Yield: 64.3%. Anal. Calcd (found) for
C H N O V Pd : C, 29.75 (29.58); H, 3.47 (3.69); N, 5.78
36
50
6
23
6
2
−1
(5.56); V, 21.03 (19.87); Pd, 14.65 (14.42). IR spectrum, ν (cm ):
3
185 (N−H, m), 3065 (Ar−H, w), 1639 (s), 1579 (CN, s), 1534
(s), 1466 (CC, s), 1247 (s), 1164 (s), 1025 (s), 953 (s), 946 (s),
906 (s), 782 (s), 668 (s), 468 (Pd−N, m).
F
DOI: 10.1021/ic502447b
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 4. Crystallographic Data for Compounds 1−5
compounds
formula
1
2
3
4
5
C H N O Pd V
C H N O Pd V
C H N O Pd V
C H N O Pd V
C H N O V
18 50 2 19 6
3
6
50
6
23
2
6
38 56
6
23
2
6
38 58
6
24
2
6
46 82
8
25
2
6
Mr
1453.26
triclinic
1483.32
1501.34
1665.63
904.24
crystal system
space group
temperature (K)
a (Å)
triclinic
monoclinic
P2(1)/c
296(2)
15.949(4)
24.934(6)
13.822(3)
90
triclinic
monoclinic
P2(1)/c
P1
̅
296(2)
P1
̅
296(2)
P1
̅
296(2)
296(2)
10.594(2)
10.709(2)
12.472(3)
89.615(7)
75.262(7)
84.946(7)
1363.0(5)
1
10.686(4)
10.790(4)
12.571(5)
89.876(7)
75.108(6)
85.398(7)
1396.0(1)
1
10.7283(8)
11.9106(6)
13.1328(9)
86.490(5)
73.385(6)
78.023(5)
17.2607(3)
10.4781(2)
19.6314(4)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
98.513(4)
90
95.1640(10)
90.00
γ (deg)
V (Å3)
5436(2)
4
1573.04(19)
1
3536.11(12)
4
Z
Dcalc (g cm−3)
1.77
1.764
1.834
1.758
1.699
F(000)
722
740
3000
844
1848
R1 [I > 2σ(I)]
0.0615
0.0805
0.0842
0.2286
0.1552
0.2849
1.044
0.0502
0.1380
0.064
0.0344
0.0848
0.0491
0.0927
1.067
wR [I > 2σ(I)]
2
0.1474
0.2246
R1 (all data)
0.1172
0.1171
wR (all data)
2
0.1783
0.2679
0.1503
1.057
GOF
1.008
1.053
CCDC No.
942425
935841
935840
1008976
973741
Synthesis of [Pd(dpa)(acac)] [V O (OMe) ] (2). VO(acac) (0.6
2921 (m), 2815 (m), 1576 (m), 1531 (m), 1453 (m), 1033 (s), 956
(s), 755 (s), 660(m).
X-ray Crystallography. Single-crystal X-ray diffraction data for
2
6
11
8
2
mmol, 159.1 mg) and triethylamine (0.8 mmol) were added to 6 mL
of methanol. The resulting solution was stirred at room temperature
for 20 min, and then 2,2′-dipyridine amine (34.3 mg, 0.2 mmol) and
compounds 1−5 were conducted on a Bruker-AXS CCD diffrac-
tometer equipped with a graphite-monochromated Mo-K radiation (λ
Pd(OAc) (44.9 mg, 0.2 mmol) were successively added. The reaction
a
2
=
0.71073 Å) at 296 K. All absorption corrections were applied using
mixture was stirred for 12 h. From the resulting solution, yellow
crystals suitable for X-ray diffraction grew after leaving the solution to
stand for a week. Yield: 62.5%. Anal. Calcd (found) for
C H N O V Pd : C, 30.77 (30.52); H, 3.81 (3.98); N, 5.67
multiscan technique. The structures were solved by the direct method
and refined through full-matrix least-squares techniques method on F
using the SHELXTL 97 crystallographic software package.
2
3
0,31
The
3
8
56
6
23
6
2
−1
hydrogen atoms of the organic ligands were refined as rigid groups.
Crystallographic data for compounds 1−5 are summarized in Table 4.
The crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre (CCDC) as entries 942425 (1), 935841
(
3
(
9
5.41); V, 20.61 (20.38); Pd, 14.35 (14.12). IR spectrum, ν (cm ):
177 (N−H, m), 3076 (Ar−H, w), 1649 (s), 1582 (CN, s), 1531
s), 1464 (CC, s), 1344 (m), 1244 (s), 1166 (s), 1024 (s), 951 (s),
45 (s), 760 (s), 651 (s), 465 (Pd−N, m).
(
2), 935840 (3), 1008976 (4), 973741 (5). The selected bond lengths
Synthesis of [Pd(dpa)(acac)] [V O (OMe) ]·H O (3). The syn-
2
6
11
8
2
and bond angles of compounds 1−5 please see Table S2.
thetic procedure was the same as for compound 2. After the reaction
was finished, yellow crystals suitable for X-ray diffraction were
obtained when the resulting solution was maintained at 5 °C for 10
days. Yield: 60.2%. Anal. Calcd (found) for C H N O V Pd : C,
ASSOCIATED CONTENT
Supporting Information
■
*
S
38
58
6
24
6
2
3
1
0.4 (30.17); H, 3.89 (4.12); N, 5.6 (5.36); V, 20.36 (20.12); Pd,
X-ray crystallographic files in CIF format, some structural
figures, X-ray powder diffraction patterns, IR spectra, XPS
spectra, EPR spectra and ICP analysis. This material is available
free of charge via the Internet at http://pubs.acs.org.
−1
4.18 (13.91). IR spectrum, ν (cm ): 3610 (O−H, m), 3280 (N−H,
m), 3067 (Ar−H, w), 1645 (s), 1582 (CN, s), 1534 (s), 1478 (C
C, s), 1365 (m), 1251 (s), 1152 (s), 1032 (s), 952 (s), 940 (s), 763
(
s), 684 (s), 466 (Pd−N, m).
Synthesis of [Pd(DMAP) (acac)] [V O (OMe) ]·H O (4). The
AUTHOR INFORMATION
Corresponding Authors
E-mail: chiyingnan7887@bit.edu.cn.
E-mail: cwhu@bit.edu.cn. Tel: +86-10-68912631. Fax: ++86-
2
2
6
11
8
2
■
synthetic procedure was same as for compound 2 except that
DMAP was used instead of dpa. From the resulting solution, yellow
crystals suitable for X-ray diffraction grew after leaving the solution to
stand for a week. Yield: 65.6%. Anal. Calcd (found) for
C H N O V Pd : C, 33.17 (33.02); H, 4.96 (5.12); N, 6.73
*
*
1
0-68912631.
4
6
82
8
25
6
2
−1
(
3
(
9
6.41); V, 18.35 (18.13); Pd, 12.78 (12.65). IR spectrum, ν (cm ):
629 (O−H, m), 3079 (Ar−H, w), 1639 (s), 1586 (CN, s), 1534
Notes
The authors declare no competing financial interest.
s), 1466 (CC, s), 1353 (m), 1247 (s), 1164 (s), 1021 (s), 953 (s),
43 (s), 760 (s), 651 (s), 466 (Pd−N, m).
ACKNOWLEDGMENTS
This work was financially supported by the NNSFC
21231002, 21276026, 21173021, 21371024), the 111 Project
B07012), and 973 Program (2014CB932103).
■
(
(
Synthesis of [NHEt ] [V O (OMe) ] (5). VO(acac) (0.6 mmol,
3
2
6
13
6
2
1
59.1 mg) and triethylamine (0.6 mmol) were added to 6 mL of
methanol (methanol was dried over 4 Å molecular sieves for a week
and distilled before use). The reaction mixture was stirred for 12 h.
From the resulting solution, light yellow crystals suitable for X-ray
diffraction grew after leaving the solution to stand for a week. Yield:
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DOI: 10.1021/ic502447b
Inorg. Chem. XXXX, XXX, XXX−XXX