Polyoxotungstate incorporating organotriphosphonate ligands: Synthesis, characterization, and catalytic for alkene epoxidation

Article abstract of DOI:10.1021/ic502404m

A S-shaped organotriphosphonate polyoxotungstate, K₄H₆[H₄{(AsW₉O₃₃)Zn(H₂O)W₅O₁₁(N(CH₂PO₃)₃)}₂(μ₂-O)₂]·27H₂O (1), has been synthesized and characterized. Compound 1 contains a different geometry of [{Zn(H₂O)W₅O₁₉(N(CH₂PO₃)₃)}]^12- clusters, which forms a chiral conformation. The catalysis of 1 for alkene epoxidation was investigated with a hydrogen peroxide (H₂O₂) oxidant.

Full text of DOI:10.1021/ic502404m

Communication
pubs.acs.org/IC
Polyoxotungstate Incorporating Organotriphosphonate Ligands:
Synthesis, Characterization, and Catalytic for Alkene Epoxidation
Yu Huo, Zhiyuan Huo, Pengtao Ma, Jingping Wang,* and Jingyang Niu*
Key Laboratory of Polyoxometalate Chemistry of Henan Province, College of Chemistry and Chemical Engineering, Henan
University, Kaifeng 475004, P. R. China
*
S Supporting Information
8
1
8H O, while no triphosphonato-based polyoxotungstate has
2
ABSTRACT: A S-shaped organotriphosphonate polyox-
otungstate, K H [H {(AsW O )Zn(H O)W O (N-
been discovered. The amino trimethylene phosphonic acid
(ATMP) has our attention for the following advantages: (a) It
is stable over a wide range of temperatures and pH values. (b)
4
6
4
9
33
2
5
11
9
(
CH PO ) )} (μ -O) ]·27H O (1), has been synthesized
2 3 3 2 2 2 2
and characterized. Compound 1 contains a different
geometry of [{Zn(H O)W O (N(CH PO ) )}]
It is flexible in space morphology, which has benefits for
coordination. (c) It is a nontoxic and multidentate ligand,
which provides an impetus for the synthesis of diverse
structures and functional materials. In addition, the precursor
1
2−
clus-
2
5
19
2
3 3
ters, which forms a chiral conformation. The catalysis of 1
for alkene epoxidation was investigated with a hydrogen
peroxide (H O ) oxidant.
14−
2
2
dilacunary [As W O (H O)] should be an ideal building
2
19 67
2
block because of the bridging {WO(H O)} linker in the
2
precursor providing a fracture point where the molecule can be
disassembled and may rearrange during the course of the
reaction.
rganophosphonate-based polyoxometalate (POM) deriv-
atives have been growing in leaps and bounds in recent
10
O
years due to their great diversity in size, nuclearity, and shape,
as well as their promising applications in catalysis, electro-
Herein, we report the synthesis and characterization of a
zinc-containing ATMP-based POM derivative
K H [H {(AsW O )Zn(H O)W O (N(CH PO ) )} (μ -
1
chemistry, photochromism, magnetism, and medicine. To
4
6
4
9
33
2
5
11
2
3
3
2
2
date, the crystallographic structures of the reaction products of
O) ]·27H O (1), which presents the first example of
2 2
VI
V
Mo and V ions with bisphosphonates have been widely
discovered and fully explored on account of their potential
incorporating functionalized triphosphonates into polyoxotung-
state. Furthermore, giving the presence of both ATMP ligands
and Zn ions in the same polyanion, the catalytic oxidation of
olefin has been preliminary studied.
2
properties, which are further modified by transition metals
(
TM) combining with organic linkers to generate high
3
dimensional structures. Whereas, the development of organo-
phosphonate-decorated polyoxotungstates is sluggish, which
may be due to the inactivity of the reaction system of W and
organophosphonate. The first example of polyoxotungstate
Colorless block-shaped crystals of 1 were obtained by a one-
4−
pot reaction of the precursor [As
triphoshponate ligand ATMP, and ZnCl
W
O
67(H
O)]1 , organo-
2
19
2
. The phase purity of 1
2
was confirmed using X-ray powder diffraction (Figure S5,
Supporting Information). Single-crystal X-ray structural analysis
indicated that 1 crystallizes in the monoclinic space group C2/c.
containing a functionalized diphosphonate is Pope’s [(O P−
3
1
6−
4
CH −PO ) W O ] , which was reported in 1994. Since
then, only a series of the organophosphonic acid derivatives
were grafted on the Keggin tungstate anions [PW O ] and
γ-SiW O ] , as well as Lindqvist [NbW O ] . The only
2
3 4 12 36
As shown in Figure 1, [{(AsW
O
33)Zn(H
(1a) can be viewed as two Zn-
substituted trivacant Well−Dawson [{(AsW O)-
W O (N(CH PO ) )}(μ -O) ] building blocks in opposite
O)W O11(N-
9
2 5
9
−
14−
(CH PO ) )} (μ -O) ]
9
34
2 3 3 2 2 2
4
−
3−
[
O33)M(H
9 2
10
36
5
19
9
−
example of TM-substituted organodiphosphonate-based poly-
5
11
2
3 3
2
2
directions that are connected by two μ -oxo groups acting as
oxotungstate, [{(B-α-PW O )Co (OH)(H O) (O PC(O)-
2
9
34
3
2
2
3
1
4−
the “hinge”, giving the anion a S-shaped structure. The Well−
(
C H NH )PO )} Co] , 5was reported by Mialane, Dolbecq,
3
6
3
3
2
Dawson structure contains a trilacunary fragment [α-
and co-workers until 2012.
9
−
AsW O ] and a triangular ring-like cluster [{Zn(H O)-
A survey of the available literatures shows that tungsten
compounds are active, and selective catalysts for olefin
9
33
2
1
2−
W O (N(CH PO ) )}]
fused together by six corner-
5
19
2
3 3
6
sharing μ -O atoms. An intriguing structural feature of 1 is
epoxidation and the catalytic reactions can be significantly
2
the two ring-like fragments presenting a different geometry
between them, which has a chiral conformation (Figure 1c).
increased by the incorporation of organic and organometallic
7
groups. Therefore, we focus on the synthesis of novel
The fragment contains a central tetrahedral {PO C} group
polyoxotungstate structures based on organophosphonate and
hope to discover good efficiency and selectivity in the catalytic
oxidation of alkenes. Among the reported organophosphonate
ligands, the design and synthesis of triphosphonato-POMs are
extremely limited. Only two examples including the triphosph-
onate ligand are a two-dimensional trinuclear [V O (OH)-
3
encircled by a ring of five {WO } and an extra {ZnO }
6
6
octahedra sharing edges and corners alternately, which is similar
8−
11
to the well-known polyoxoanion [Mo O (O PCH PO ) ] .
7
16
3
2
3 3
The central P atom coordinates to three μ -O atoms, which
3
3
3
{
C H (C H PO H) }] cluster and one crown-shaped poly-
Received: September 30, 2014
6
3
6
4
3
3
oxomolybdate Na [H {N(CH PO ) }Mo O (OH)(H O) ] ·
5
7
2
3 3
6
16
2
4 4
©
XXXX American Chemical Society
A
DOI: 10.1021/ic502404m
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
5
.2, only one set of three peaks (δ = 2.68, 8.57, 10.86 ppm) was
observed (Figure S8, Supporting Information). In the presence
of Li and Na , only the other set of three single peaks
remained at 3.86, 8.74, and 11.13 ppm in the P NMR spectra.
In order to explore the electrochemical behaviors of 1, cyclic
voltammetry studies were carried out in a pH 3 sulfate medium
+
+
31
−1
(
0.2 M Na SO + H SO ) where 1 was stable at 150 mV s
2 4 2 4
scan rate. The electrochemical properties of the product were
investigated in the potential range of 0.3 V to approximately
−
1.0 V. Three W-based waves were observed with their
reduction peak potentials located at −0.39, −0.55, and −0.71 V
vs SCE, respectively. The redox peaks could be assigned to
13
three consecutive two-electron processes of W. The peak
potential differences of three couples of redox waves are 60, 62,
and 56 mV, respectively, indicating that they are one-electron
charge-transfer processes. In addition, the peak currents related
to the tungsten redox processes were proportional to the square
root of the scan rate in the range of 80−300 mV s , suggesting
that redox processes are diffusion controlled.
Figure 1. (a) Combined polyhedral/ball-and-stick representation of
1
a. (b) Combined polyhedral/ball-and-stick representation of the
−1
subunit. (c) Chiral conformation of the ring-like clusters. Color code:
13
As, dark blue; C, black; H, gray-50%; N, dark yelllow; P, pink; PO C
3
As known, use of organic−inorganic hybrid polyoxotung-
tetrahedral, yellow; WO octahedral, sky blue; and ZnO octahedral,
6
6
14
states has been recently proposed as a catalyst upgrade.
gray-25%.
Compared to the nonfunctionalized POMs, the phosphonate
derivative displays an improved thermal stability and a wide
substrate scope. Thus, we explored the catalytic reaction of 1,
the initial experimental procedure for the epoxidation reaction
by using cyclohexene as a model substrate with 2 equiv of
anchors to two M atoms (M = W, Zn). The other two P atoms
15
of ATMP separately connect with two {MO } octahedra via
6
two μ -oxo groups, which are exposed on the outside of the
2
8b
ring. As reported previously, ATMP plays an important role
in the stabilization of metal-substituted POMs. Additionally, the
C1 atom, center P atom, and As heteroatom are almost linear
H O . Under optimal conditions, cyclohexene was oxidized to
2
2
III
the expected 1,2-epoxycyclo-hexane with high conversion
(
94.1%) and excellent selectivity (>99%) (Table 1, entry 3).
with the C−P−As angle of 176.74° (Figure S2, Supporting
Information).
a
2
12
Table 1. Epoxidation of Alkenes with H O
According to the bond valence sum (BVS) calculations, the
oxidation states of the As, P, W, and Zn atoms are +3, +3, +6,
and +2, respectively, and the results of P and W are further
confirmed by XPS spectra (Figure S14, Supporting Informa-
tion). In the ring-like polyanion, the Zn atom is coordinated to
a single terminal aqua ligand and has two Zn−O−P with angles
of 128.0(8)° and 131.0(9)°, which also possesses three Zn−
O−W linkages with angles in the range of 89.8(5)−159.5(1)°.
The P atoms connect with M atoms through μ -oxo and μ -oxo
2
2
3
groups; thus, the O−P−C bond angles, O−P−O angles, and
P−O−W lie in the range of 99.4(9)−110.0(9)°, 109.3(8)−
III
1
15.9(8),° and 131.9(8)−138.8(8)°. As expected, the As
atoms appear with trigonal-pyramid coordination geometry
with the As−O bonds ranging between 1.776(13) and
1.808(15) Å. Furthermore, the structure of 1a is extended to
a two-dimensional layerwork structure by the coordination of
+
four additional K cations (Figure S4, Supporting Information).
NMR spectroscopy was studied on the compound in order
to investigate the solution stability of the polyanion. The 31
P
NMR spectrum of the fresh solution of 1 in D O (700 μL)
2
a
exhibits two doublets and two single peaks (Figure S6,
Supporting Information). The result may contain two sets of
signals, which is probably a mixture of isomers, that is, Λ−Λ,
Δ−Δ-type, and Δ−Λ, Λ−Δ-type. The single peaks belong to
the central P atoms, and the doublets are attributed to the
peripheral P atoms (Figure S7, Supporting Information). In
Reaction conditions: catalyst (1 μmol), substrate(1 mmol), H O (2
2 2
b c
mmol), and acetonitrile (5 mL). Filtered catalyst. Solution at the end
of a catalytic reaction. Determined by gas chromatography (GC)
analyses based on initial substrate.
d
As shown in Table 1, the conversions remarkably declined
(entries1−5), which demonstrated that both the reaction time
and temperature were the key factors that influence the
conversion of epoxidation products. A blank experiment
without 1 (Table S5, Supporting Information) indicated that
the epoxidation would not occur. In contrast to catalyst 1, the
epoxidation reactions with the precursor {As W O (H O)}
3
1
order to study this equilibrium in solution, P NMR spectra at
different pH values ranging between 1.0 and 5.8 were observed.
We observed that in the range of pH 1.0−1.6, the compound
has partially decomposed. When the pH values ranges between
2
.2 and 3.4, the presence of the two sets were observed. Upon
increasing the pH to 4.6, three resonances located at 3.86, 8.74,
and 11.13 ppm faded away. When the pH value increased to
2
19 67
2
and subunit {AsW O } indicated that the conversion was only
9
33
B
DOI: 10.1021/ic502404m
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
1.2% and 50.7%, respectively. After oxidation was completed,
the catalyst could be retrieved from the solvent/oxidant/
substrate system by filtration. When the reaction was carried
out with the filtered catalyst, the conversion of cyclohexene was
low (68.1%) after 8 h of reaction (entry 6). The epoxidation
reaction with the solution at the end of a catalytic reaction gave
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ASSOCIATED CONTENT
Supporting Information
X-ray crystallograhic files (CIF), experiment sections, synthetic
discussion, additional physical measurements, catalytic proper-
ties, and additional structural figures for 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
Putnis, C. V.; Rodriguez-Navarro, C. Cryst. Growth Des. 2008, 8,
*
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AUTHOR INFORMATION
Corresponding Authors
E-mail: jpwang@henu.edu.cn (J.W.) .
E-mail: jyniu@henu.edu.cn (J.N.).
■
*
*
R. N.; Keita, B.; Nadjo, L.; Nellutla, S.; Tol, J. V.; Dalal, N. S.; Kortz,
U. Inorg. Chem. 2010, 49, 4949−4959. (b) Nambu, J. I.; Ueda, T.;
Guo, S. X.; Boasc, J. F.; Bond, A. M. Dalton Trans. 2010, 39, 7364−
Notes
The authors declare no competing financial interest.
7
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ACKNOWLEDGMENTS
■
Long, D. L.; Cronin, L. Dalton Trans. 2012, 41, 10000−10005.
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We gratefully acknowledge the National Natural Science
Foundation of China, Foundation of Education Department
of Henan Province, and Natural Science Foundation of Henan
Province for financial support.
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DOI: 10.1021/ic502404m
Inorg. Chem. XXXX, XXX, XXX−XXX